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LIBRARY CATALOGUE SLIPS.

United States. Department of the interior. ( U. S. geological survey.)
Department of the interior | — | Monographs | of the | United

Hague (Arnold).

Washington | government printing office | 1892

4°. xvii, 419pp. 8 pi.

[UNITED STATES. Department of the interior. (U. S. geological nuney).

Monograph XX.)

£ United States geological survey | J. W. Powell, director | — |

Arnold Hague | [Vignette] |
? Washington | government printing office | 1892

4°. xvii, 419 pp. 8 pi.
S [UNITED STATES. Department of the interior. (17. S. geological euney.)

Monograph XX.]

[Monograph XX.]

The publications of the United States Geological Survey are issued in accordance with the statute
approved March 3, 1879, which declares l hat —

"The publications of the Geological Survey shall consist of the annual report of operations, geo-
logical and economic maps illustrating the resources and classification of the lands, a.nd reports upon
general and economic geology and paleontology. The annual report of operations of the Geological
Survey shall accompany the animal report of the Secretary of the Interior. All special memoirs and
reports of said Survey shall be issued in uniform quarto series if deemed neceasary by the Director, hut
otherwise iii ordinary octavos. Three thousand copies of each shall be published lor scientific exchanges
and for sale at the price of publication; and all literary and cartographic materials received in exchange
shall be the property of the United States and form a part of the library of the organization : And the
money resulting from the sale of such publications shall be covered into the Treasury of the United
States."

The following joint resolution, referring to all government publications, was passed by Congress
July 7, 1882:

"That, whenever any document or report, shall be ordered printed by Congress, there shall be
printed, in addition to the number in each case stated, the 'usual number' (1,900) of copies for binding
and distribution among those entitled to receive them."

Except in those cases.in which an extra number of any publication has been supplied to the Sur-
vey by special resolution of Congress or has been ordered by the Secretary of the Interior, this office
has no copies for gratuitous distribution.

ANNUAL REPORTS.

I. First Annual Report of the United States Geological Survey, by Clarence King. 1880. 8°. 79
pp. 1 map. — A preliminary report describing plan of organization and publications.

II. Second Annual Report of the United States Geological Survey, 1880-'81, bv J. W. Powell.

1882. 8C. Iv, 588 pp. 62 pi. 1 map.

III. Third Annual Report of the United States Geological Survey, 1881-'82, by J. W. Powell.

1883. 8°. xviii, 564 pp. 67 pi. and maps.

IV. Fourth Annual Report of the United States Geological Survey, 1882-'83, by J. W. Powell.

1884. 8°. xxxii, 473 pp. 85 pi. and maps.

V. Fifth Annual Report of the United States Geological Survey, 1883-'84, by J. W. Powell.
1385. 8°. xxxvi, 469 pp. 58 pi. and maps.

VI. Sixth Annual Report of the United States Geological Survey, 1884-'85) by J. W. Powell.

1885. 8°. xxix, 570 pp. 66 pi. and maps.

VII. Seventh Annual Report of the United States Geological Survey, 1885-'86, by J. W. Powell.

1888. 8°. xx, 656pp. 71 pi. and maps.

VIII. Eighth Annual Report of the United States Geological Survey, 1886-'87, by J. W. Powell.

1889. 8°. 2v. xix, 474, xii pp. 5:! pi. and maps; 1 p. 1. 475-1063 pp. 54-76 pi. and maps,

IX. Ninth Annual Report of the United States Geological Survey, 1887-'88, by J. W. Powell.

1889. 8°. xiii, 717 pp. 88 pi. and maps.

X. Tenth Annual Report of the United States Geological Survey, 1888-'89, by J. W. Powell.

1890. 8°. 2 v. xv, 774 pp. 98 pi. and maps; viii, 123 pp.

XI. EleventhJAnnual Report of the United States Geological Survey, 1889-'90, by J. W. Powell.

1891. 8°. 2 v. xv, 757 pp. 66 pi. and maps ; ix, 3">1 pp. 30 pi. and maps.

XII. Twelfth Annual Report of the United States Geological Survey, 1890-'!U, by J. W. Powell.
1891. 8° 2 v. xiii, 675 pp. 53 pi. and maps ; xviii, 576 pp. 146 pi. aud maps.

MONOGRAPHS.

I. Lake Bonneville, by Grove Karl Gilbert. 1890. 4°. xx, 438 pp. 51 pi. 1 map. Price $1.50.

II. Tertiary History of the Grand Canon District, with atlas, by Clarence E. Button, Capt., U. S. A.
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III. Geology of the Comstock Lode and the Washoe District, with atlas, by George F. Becker.
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IV. Comstock Mining and Miners, by Eliot Lord. 1883. 4°. xiv, 451 pp. 3 pi. Price $1.50.

II ADVERTISEMENT.

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VI. Contributions to the Knowledge of the Older Mesozoic Flora of Virginia, by William Morris
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VII. Silver-Lead Deposits of Eureka, Nevada, by Joseph Story Curtis. 1884. 4°. xiii, 200 pp.
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VIII. Paleontology of the Eureka District, by Charles Doolittle Walcott. 1884. 4°. xiii,29S pp.
241. 24 pi. Price $1.10.

IX. Brachiopoda and Lamellibrancbiata of the Raritan Clavs and Greensand Marls of New Jersey,
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X. Diuocerata. A Monograph of an Extinct Order of Gigantic Mammals, by Othniel Charles
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XI. Geological History of Lake Lahontan, a Quaternary Lake of Northwestern Nevada, by Israel
Cook Russell. 1&85. 4°. xiv, 288 pp. 46 pi. and maps. Price $1.75.

XII. Geology and Mining Industry of Leadville, Colorado, with atlas, by Samuel Franklin Em-
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XIV. Fossil Fishes and Fossil Plants of the Triassie Rocks of New Jersey and the Connecticut
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XV. The Potomac or Younger Mesozoic Flora, by William Morris Fontaine. 1889. 4°. xiv,
377 pp. 180 pi. Text and plates bound separately. Price $2.50.

XVI. The Paleozoic Fishes of North America, by John Strong Newberry. 1889. 4°. 340 pp.
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XVII. The Flora of the Dakota Group, a posthumous work, by Leo Lesquereux. Edited by F. H.
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XVIII. Gasteropoda and Cephalopoda of the Raritan Clays and Greensand Marls of New Jersey,
by Robert P. Whittield. 1891. 4°. 402 pp. 50 pi. Price $1.00.

XX. Geology of the Eureka District, Nevada, with an atlas, by Arnold Hague. 1892. 4°. xvii,
419 pp. 8 pi. Price $5.25.

In press :

XIX. The Penokee Iron-Bearing Series of Northern Wisconsin and Michigan, by Roland D.
Irving and C. R. Van Hise.

XXI. The Tertiary Rhynchophorous Coleoptera of North America, by S. H. Scudder.

XXII. Geology of the Green Mountains in Massachusetts, by Messrs. Punipelly, Wolff, and Dale.

In preparation :

— Mollusca and Crustacea of the Miocene Formations of New Jersey, by R. P. Whitfleld.

— Sauropoda, by O. C. Marsh.

— Stegosauria, by O. C. Marsh.

— Brontotheridae, by O. C. Marsh.

— Report on the Denver Coal Basin, by S. F. Eminons.

— Report on Silver Cliff and Ten-Mile Mining Districts, Colorado, by S. F. Eiumons.

— The Glacial Lake Agassiz, by Warren Upham.

BULLETINS.

1. On Hypersthene-Andcsite and on Triclinic Pyroxene in Augitic Rocks, by Whitman Cross, with
a Geological Sketch of Buffalo Peaks, Colorado, by S. F. Einmons. 1883. 8°. 42 pp. 2 pi. Price 10
cents.

2. Gold and Silver Conversion Tables, giving the coining values of troy ounces of fine metal, etc.,
computed by Albert Williams, jr. 1883. 8°. 8 pp. Price 5 cents.

3. On the Fossil Faunas of the Upper Devonian, along the meridian of 76° 30', from Tompkins
County, N. Y., to Bradford County, Pa., by Henry S. Williams. 1884. 8°. 36pp. Price 5 ceuts.

4. On Mesozoic Fossils, by Charles A. White. 1884. 8°. 36 pp. 9 pi. Price 5 cents.

5. A Dictionary of Altitudes in the United States, compiled by Henry Gannett. 1884. 8°. 325pp.
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6. Elevations in the Dominion of Canada, by J. W. Spencer. 1884. 8°. 43 pp. Price 5 cents.

7. Mapoteca Geologica Americana. A Catalogue of Geological Maps of America (North and South),
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8. On Secondary Enlargements of Mineral Fragments in Certain Rocks, by R. D. Irving and C. R.
Van Hise. 1884. 8°. 56 pp. 6 pi. Price 10 cents.

9. A Report of work done in the Washington Laboratory during the fiscal year 1883-'84. F. W.
Clarke, chief chemist; T. M. Chatard, assistant chemist. 1884. 8°. 40pp. Price 5 cents.

10. On the Cambrian Faunas of North America. Preliminary studies, by Charles Doolittle Wal-
cott. 1884. 8°. 74 pp. 10 pi. Price 5 cents.

11. On the Quaternary and Recent Mollusca of the Great Basin ; with Descriptions of New Forms,
by R. Ellsworth Call. Introduced by a sketch of the Quaternary Lakes of the Great Basin, by G. K.
G'ilbert. 1884. 8°. 66 pp. 6 pi. Price 5 cents.

ADVERTISEMENT. Ill

12. A Crystallographic Study of the Thinolite of Lake Lahontan, by Edward S. Dana. 1884. 8°.
34 pp. 3 pi. Price 5 cents.

13. Boundaries of the United States and of the several States and Territories, with a Historical
Sketch of the Territorial Changes, by Henry Gannett. 1H85. 8°. 135pp. Price 10 cents.

14. The Electrical and Magnetic Properties of the Iron-Carburets, by Carl Barns and Vincent
Stronhal. 1885. 8°. 238 pp. Price 15 cents.

15. On the Mesozoic and Cenozoic Paleontology of California, by Charles A. White. 1885. 8°.
33 pp. Price 5 cents.

16. On the Higher Devonian Faunas of Ontario County, New York, by John M.Clarke. I88a. 8°.

86pp. 3 pi. Price 5 cents.

17. On the Development of Crystallization in the Igneous Rocks of Washoe, Nevada, with Notes
on the Geology of the District, by Arnold Hague and Joseph P. Iddings. 1885. 8°. 44 pp. Price 5
cents.

18. On Marine Eocene, Fresh-water Miocene, and other Fossil Mollusca of Western North America,
by Charles A. White. 1885. 8°. 26 pp. 3 pi. Price 5 cents.

19. Notes on the Stratigraphy of California, by George F.Becker. 1885. 8°. 28pp. Price5cents.

20. Contributions to the Mineralogy of the Rocky Mountains, by Whitman Cross and W. F. Hille-
brand. 1885. 8°. 114 pp. 1 pi. Price 10 cents.

21. The Lignites of the Great Sioux Reservation. A Report on the Region between the Grand and
Moreau Rivers, Dakota, by Bailey Willis. 1885. 8°. 16pp. 5 pi. Price 5 cents.

22. On New Cretaceous Fossils from California, by Charles A. White. 1885. 8°. 25 pp. 5 pi.
Price 5 cents.

23. Observations on the Junction between the Eastern Sandstone and the Keweenaw Series on
Keweenaw Point, Lake Superior, by R. D. Irving and T. C. Chamberlin. 1885. 8°. 124 pp. 17 pi.
Price 15 cents.

24. List of Marino Mollusca, comprising the Quaternary fossils and recent forms from American
Localities between Cape Hatteras and Cape Roqne, including the Bermudas, by William Healey Dall.
1885. 8C. 336 pp. Price 25 cents.

25. The Present Technical Condition of the Steel Industry of the United States, by Phineas Barnes.
1885. 8°. 85 pp. Price 10 cents.

26. Copper Smelting, by Henry M. Howe. 1885. 8°. 107 pp. Price 10 cents.

27. Report of work done in the Division of Chemistry and Physics, mainly during the fiscal year
1884-'85. 1886. 8°. 80 pp. Price 10 cents.

28. TheGabbros and Associated Hornblende Rocks occurring in the Neighborhood of Baltimore,
Md., oy George Huntingtou Williams. 1886. 8°. 78pp. 4 pi. Price 10 cents.

29. On the Fresh- water Invertebrates of the North American Jurassic, by Charles A. White. 1886,
8°. 41 pp. 4 pi. Price 5 cents.

30. Second Contribution to the Studies on the Cambrian Faunas of North America, by Charles
Doolittle Walcott. 1H86. 8°. 369 pp. 33 pi. Price 25 cents.

31. Systematic Review of our Present Knowledge of Fossil Insects, including Myriapods and
Arachnids, by Samuel Hubbard Scudder. 1886. 8°. 128 pp. Price 15 cents.

32. Lists and Analyses of the Mineral Springs of the United States; a Preliminary Study, by
Albert C. Peale. 1886. 8°. 235 pp. Price 20 cents.

33. Noteson the Geology of Northern California, by J. S.DHler. 1886. 8°. 23pp. Priceocents.

34. On the relation of the Laramie Molluscan Fauna to that of the succeeding Fresh- water Eocene
and other groups, by Charles A. White. 1886. 8°. 54 pp. 5 pi. Price 10 cents.

35. Physical Properties of the Iron-Carburets, by Carl Barns and Vincent Strouhal. 1886. 8°.
62 pp. Price 10 cents.

36. Subsidenceof Fine Solid Particles in Liquids, byC'arlBarus. 1886. 8°. 58pp. Price 10 cents.

37. Types of the Lanimie Flora, by Lester F. Ward. 1887. 8°. 354pp. 57 pi. Price 25 cents.

38. Peridotite of Elliott County, Kentucky, by J S. Diller. 1887. 8°. 31pp. 1 pi. Price 5 cents.

39. The Upper Beaches and Deltas of the Glacial Lake Agassiz, by Warren Upham. 1887. 8°.
84 pp. 1 pi. Price 10 cents.

40. Changes in River Courses ia Washington Territory due to Glaciation, by Bailey Willis. 1887.
8°. 10 pp. 4 pi. Price 5 cents.

41. On the Fossil Faunas of the Upper Devonian— the Geuesee Section, New York, by Henry S.
Williams. 1887. 8°. 121 pp. 4 pi. Price 15 cents.

42. Report of work done in the Division of Chemistry and Physics, mainly during the fiscal year
1885-'86. F. W. Clarke, chief chemist. 1887. 8°. 152 pp. 1 pi. Price 15 cents.

43. Tertiary and Cretaceous Strataof the Tuscaloosa, Tombigbee, and Alabama Rivers, by Eugene
A. Smith and Lawrence C.Johnson. 1887. 8U. 189pp. 21 pi. Price 15 cents.

44. Bibliography of North American Geology for 1886, by Nelson H. Dartou. 1887. 8°. 35 pp.
Price 5 cents.

45. The Present Condition of Knowledge of the Geology of Texas, by Robert T. Hill. 1887. 8°.
94 pp. Price 10 cents.

46. Nature and Origin of Deposits of Phosphate of Lime, by R. A. F. Penrose, jr., with an Intro-
duction by N. S. Shaler. 1888. 8°. 143 pp. Price 15 cents.

47. Analyses of Waters of the Yellowstone National Park, with an Account of tin- Methods of
Analysis employed, by Frank Austin Gooch and James Edward Whitfield. 1888. 8°. 84 pp. Price

10 cents.

48. On the Form and Position of the Sea Level, by Robert Simpson Woodward. 1888. b°. 88

pp. Price 10 cents.

IV ADVERTISEMENT.

49. Latitudes and Longitudes of Certain Points in Missouri, Kansas, and New Mexico, by Robert
Simpson Woodward. 1889. 8°. 133 pp. Price 15 cents.

50. Formulas and Tables tu facilitate the Construction and Use of Maps, by Robert Simpson
Woodward. 1889. 8C. 124 pp. Price 15 cents.

51. On Invertebrate Fossils from the Pacific Coast, by Charles Abiatliar Wbite. 1889. 8C. 102
pp. 14 pi. Pric-e 15 cents.

52. Subae'rial Decay of Rocks and Origin of the Red Color of Certain Formations, by Israel Cook
Russell. 18>9. ,- . 65pp. 5 pi. Price 10 cents.

63. The Geology of Nantncfcet, by Nathaniel SonthgateShaler. 1889. 8°. 55pp. 10 pi. Price
10 cents.

54. On the Thermo-Electric Measurement of High Temperatures, by Carl Barns. 1889. 8°.
313pp. iucl. 1 pi. 11 pi. Price 25 "cents.

55. Report of work done in the Division of Chemistry and Physics, mainly during the fiscal jcar
1886-'87. Frank Wigglesworth Clarke, chief chemist. 1889. 8°. '96pp. Price 10 cents.

56. Fossil Wood and Lignite of the Potomac Formation, by Frank Hall Ktiowlton. 1889. s .
72 pp. 7 pi. Price 10 cents.

57. A Geological Reconnaissance in Southwestern Kansas, by Robert Hay. 1890. 8°. 49 pp.
2 pi. Price 5 cents.

58. The Glacial Boundary in Western Pennsylvania, Ohio, Kentucky, Indiana, and Illinois, by
George Frederick Wright, with an introduction by Thomas Chrowder Chamberlin. 1890. 8°. 112
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59 The Gabbros and Associated Rocks in Delaware, by Frederick D.Chester. 1890. 8°. 45pp.
1 pi. Price 10 cents.

60. Report of work done in the Division of Chemistry and Physics, mainly during the fiscal year
1887-'88. F. W. Clarke, chief chemist. 1890. 8°. 174 pp'. Price 15 cents.

61. Contributions to the Mineralogy of the Pacific Coast, by William Harlow Melville and Wal-
demar Limlgren. 1890. 8°. 40pp. 3 pi. Price 5 cents.

62. The Greenstone Schist Areas of the Meuominee and Marqnette Regions of Michigan, a contri-
bution to the subject of dynamic metamorphism in eruptive rocks, by George Hmitiugton Williams,
with an introduction by Roland Duer Irving. 1>90. 8°. 241 pp. 16 pi. Price 30 cents.

63. A Bibliography of Paleozoic Crustacea from 1698 to 1889, including a list of North Amer-
ican species and a systematic arrangement of genera, by Anthony W. Vogdes. 18!tO. 8°. 177 pp.
Price 15 cent*.

64. A Report of work done in the Division of Chemistry and Physics, mainly during the fiscal
year 1888-'89. F. W. Clarke, chief chemist. 1890. 8°. 60pp. Price 10 cents.

65. Stratigraphy of the Bituminous Coal Field of Pennsylvania, Ohio, and West Virginia, by
Israel C. White. 1891. 8°. 21<i pp. 11 pi. Price 20 cents.

66. On a Group of Volcanic Rocks from the Tewan Mountains, New Mexico, and on the occurrence
of Primary Quartz in certain Basalts, by Joseph Paxson Iddings. 1890. 8°. 34 pp. Price 5 cents.

67. The relations of the Traps of the Newark System iu the New Jersey Region, by Nelson Horatio
Darton. 1*90. 8°. 82 pp. Price 10 cents.

68. Earthquakes in California in li-89, by James Edward Keeler. 1890. 8°. 25pp. Price5 cents.

69. A Classed and Annotated Bibliography of Fossil Insects, by Samuel Hubbard Scudder. 1890.
8°. 101 pp. Price 15 cents.

70. Report on Astronomical Work of 1889 and 1890, by Robert Simpson Woodward. 1890. 8°.
79pp. Price 10 cents.

71. Index to the Known Fossil Insects of the World, including Myriapods and Arachnids, by
Samuel Hubbard Scudder. 1891. 8°. 744pp. Price 50 cents.

72. Altitudes between Lake Superior and the Rocky Mountains, by Warren Upham. 1891. 8°.
229 pp. Price 20 cents.

73. The Viscosity of Solids, by Carl Barus. 1891. 8°. xii, 139 pp. 6 pi. Price 15 cents.

74. The Minerals of North Carolina, by Frederick Augustus Genth. 1891. 8°. 119pp. Price
15 cents.

75. Record of North American Geology for 1887 to 1889, inclusive, by Nelson Horatio Darton.
1891. 8°. 173 pp. Price 15 cents.

76. A Dictionary of Altitudes in the United States (second edition), compiled by Henry Gannett,
chief topographer. 1891. 8°. 393pp. Price 25 cents.

77. The Texan Permian and its Mesozoic types of Fossils, by Charles A. White. 1891. 8°. 51
pp. 4 pi. Price 10 cents.

78. A report of work done in the Division of Chemistry and Physics, mainly during the fiscal year
1889-'90. F. W. Clarke, chiet chemist. IS'.lt. 8°. 131 pp. Price 15 cents.

79. A Late Volcanic. Eruption in Northern California and its peculiar lava, by J. S. Diller.

80. Correlation papers— Devonian and Carboniferous, by Henry Shaler Williams. 1891. 8°.
279 pp. Price 20 cents.

81. Correlation papers — Cambrian, by Charles Doolittle Walcott. 1S91. 8°. 447 pp. 3 pi.
Price 25 cents.

82. Correlation papers— Cretaceous, by Charles A. White. 1891. 8°. 273 pp. 3 pi. Price
20 cents.

83. Correlation papers — Eocene, by William Bullock Clark. 1891. 8°. 173pp. 2 pi. Price 15
cents.

91. Record of North American Geology for 1890, by Nelson Horatio Darton. 1891. 8°. 88 pp.
Price 10 cents.

ADVERTISEMENT. V

In press:

84. Correlation papers — Neocene, by W. H. Dall and G. D. Harris.

85. Correlation papers — Tlie Newark System, by I. (J. Knssell.

86. Correlation papers — Algonkian and Archean, by C. R. Van Hise.

87. Bibliography and Index of the publications of the U. S. Geological Survey, 1879-1892, by
P. C. Wannan.

!»(). A report of work done in the Division of Chemistry and Physics, mainly during the fiscal
year 1890-'91. F. W. Clarke, chief chemist.

92. The Compressibility of Liquids, by Carl Barns.

93. Some Insects of special interest from Florissant, Colorado, by K. II. Scudder.
91. The Mechanism of Solid Viscosity, by Carl Barus.

95. Earthquakes in California during 1890-'91, by E. S. Holden.

96. The Volume Thermodynamics of Liquids, by Carl Barns.

97. The Mesozoic Echinodermata of the United States, by W. B. Clark.

98. Flora of the Outlying Coal Basins of Southwestern Missouri, by David White.

99. Record of North American Geology for 1891, by Nelson Horatio Dartou.

In preparation :

88. Correlation papers — Pleistocene, by T. C. Chamberlin.

100. The Eruptive and Sedimentary Rocks on Pijreon Point, Minnesota, and their contact phe-
nomena, by W. S. Bayley.

101. Insect fauna of the Rhode Island Coal-licld, by Samuel Hubbard Scudder.

102. A Catalogue and Bibliography of North American Mesozoic Invertebrata, by C. B. Boyle.

103. The Trap Dikes of Lake Champlain Valley and the Eastern Adirondacks, by J. F. Kemp.

— High Temperature Work in Igneous Fusion and Ebullition, chiefly in relation to Pressure,
by Carl Barus.

— Glaciatiou of the Yellowstone Valley, by W. H. Weed.

— The Laramie and the overlying Livingstone Formation in Montana, by W. H. Weed, with
Report on Flora, by F. H. Knowlton.

— The Moraines of the Missouri Coteau and their attendant deposits, by James Edward Todd.

— A Bibliography of Paleobotany, by David White.

STATISTICAL PAPERS.

Mineral Resources of the United States [1882], by Albert Williams, jr. 1883. 8°. xvii, 813 pp.
Price 50 cents.

MiniM-al Resources of the United States, 1883 and 1884, by Albert Williams, jr. 1885. 8°. xiv,
1016 pp. Price 60 cents.

Mineral Resources of the United States, 1885. Division of Mining Statistics and Technology.
1886. 8°. vii, 576 pp. Price 40 cents.

Mineral Resources of the United States, 1886, by David T. Day. 1887. 8°. viii, 813 pp. Price
50 cents.

Mineral Resources of the United States, 1887, by David T. Day. 1888. 8°. vii, 832 pp. Price
50 cents.

Mineral Resources of the United States, 1888, by David T. Day. 1890. 8°. vii, 652 pp. Price
50 cents.

Mineral Resources of the United States, 1889 and 1890, by David T. Day. 1892. 8°. viii. 671 pp.
Price 50 cents.

The money received from the sale of these publications is deposited in the Treasury, and the
Secretary of that Department declines to receive bank checks, drafts, or postage-stamps; all remit-
tances, therefore, must be by POSTAL NOTE or MONEY ORDER, made payable to the Librarian of the
U. S. Geological Survey, or in CURRENCY for the exact amount. Correspondence relating to the pub-
lications of the Survey should be addressed

To THE DIRECTOR OF THE

UNITED STATES GEOLOGICAL SURVEY,

WASHINGTON, D. C.
WASHINGTON, D. C., September, 1892.

DEPARTMENT OF THE INTERIOR

MONOGRAPHS

OF THE

UNITED STATES GEOLOGICAL SURVEY

VOLUME XX

WASHINGTON

GOVKKNMENT PRINTING OFFICK
1892

UNITED STATES GEOLOGICAL SURVEY

J. W. POWELL, DIRECTOR

GEOLOGY

OF THE

EUREKA DISTRICT, NEVADA

WITH AN ATLAS

WASHINGTON

GOVERNMENT PRINTING OFFIOK
1892

For Atlas

see

Map Collection
F847
.89
1883
.H13

|54I5

Bancroft Library

CONTENTS.

LETTER OF TRANSMITTAL ix

PREFACE xi

OUTLINE OF THIS VOLUME xv

CHAPTER I. — General description 1

CHAPTER II. — Geological sketch of the Eureka District 8

CHAPTER III. — Cambrian and Silurian rocks 34

CHAPTER IV. — Devonian and Carboniferous rocks 63

CHAPTER V. — Descriptive geology 99

CHAPTER VI. — General discussion of the Paleozoic rocks 175

CHAPTER VII. — Pro-Tertiary igneous rocks 218

CHAPTER VIII. — Tertiary and post-Tertiary volcanic rocks 230

CHAPTER IX. — Ore deposits 292

APPENDIX A. — Systematic lists of fossils of each geological horteon. By C. D. Walcott 317

APPENDIX B. — Microscopical petrography of the eruptive rocks. By .1. P. Iddings 335

INDEX 407

ILLUSTRATIONS.

Page.

PLATE I. Geological Map of Ruby Hill 115

II. Geological Cross-sections 174

III. Minerals in section 396

IV. Quartz grains in section 398

V. Feldspars in section 400

VI. Micropegmatite and andesite 402

VII. Andesite and basalt 404

VIII . Rhyolite 406

FIG. 1. Silurian quartzite west of Castle Mountain 56

2. Section across Lone Mountain 62

3. Nevada limestone, Modoc Section 66

4. Section across Pifion Range 201

5. Granite porphyry dikes 223

6. Vertical Cross-section — Phoenix mine 306

7. Carlsbad twinn of labradorite 351

8. Diagram exhibiting extinction angles of a Carlsbad twin of Inbradorite 351

9. Carlsbad twin of plagioclase 353

VII

ATLAS SHEETS.

Title Sheet I.

List of Atlas Sheets and Legends Sheet II.

Topographical and Index Map of the Kureka I >istrict Sheet III.

Geological Map of the Eureka District - Sheet IV.

Geological Map of Northwest Sheet Sheet V.

Geological Map of Northeast Sheet Sheet VI.

Geological Map of Northwest-Cent™ 1 Sh.'c-t Sheet VII.

Geological Map of Northeast-Central Sheet Sheet VIII.

Geological Map of Southwest-Central Sheet Sheet IX.

Geological Map of Southeast-Central Sheet Sheet X.

Geological Map of Southwest Sheet .' Sheet XI.

Geological Map of Southeast Sheet Sheet XII.

Geological Cross-sections Sheet XIII.

VIII

LETTER OF TRANSMITTAL.

DEPARTMENT OF THE INTERIOR,
UNITED STATES GEOLOGICAL SURVEY,

Washington, I). C., June 30, 1891.

SIR : I have the honor to transmit herewith a report on the Geology of
the Eureka District, Nevada.

To yourself and to the Hon. Clarence King, under whose direction the
field work was commenced, I am greatly indebted for the personal interest
taken in the investigation, and also for the generous facilities afforded me
both in the field and office.

Very respectfully, your obedient servant,

ARNOLD HAGUE,

Geologist in Charge.
Hon. J. W. POWELL,

Director, U. S. Geological Survey.

PREFACE.

The survey of the Eureka District was authorized by the Hon. Clarence
King, the first Director of the United States Geological Survey, and the
field work, for the most part, was done during his administration. The field
season was confined to the summer and autumn of 1880, and was limited to
five months, the work being brought abruptly to a close early in December
owing to the inclemency of the weather. Visits of a few days' duration
were made by different members of the party during the two following
years, but these were simply to verify previous observations or to correct
apparently conflicting statements.

This monograph is purely geological in its scope and is mainly a
careful study and survey of a comparatively small block of mountains,
which may be designated the Eureka Mountains, but which should not be
confounded with the Eureka mining district, as several other well known but
less important mining districts also lie wholly within this mountain area.

As it was unmapped and only occasionally visited by geologists, little
had been accomplished, except for the immediate purposes of mining, toward
investigating its structure or solving its many geological problems. The
Eureka region was known to occupy an exceptionally broad expanse of
mountains, affording fine geological sections if carefully worked out, and
of special interest for the purposes of comparative study in other regions of
the Cordillera. In this direction scarcely anything had been accomplished.

The field work, as planned, could not have been completed in the

Xil PREFACE. *

allotted time except for the untiring energy and interest of all those connected
with the survey. In the geological work I was fortunate in having the
cooperation of two thoroughly equipped assistants, both of whom have since
attained honorable distinction by published writings in their special lines of
research. To Mr. Charles D. Walcott was assigned the collection of the
paleontological material, while Mr. Joseph P. Iddings was engaged in work-
ing among both volcanic and sedimentary rocks.

The report appears in two parts, one a volume of text, the other an
accompanying atlas of topographical and geological maps and cross sec-
tions, and as the text is, in great measure, explanatory of the atlas, the two
can be considered only as parts of the same work.

A paper embodying the more important results obtained at Eureka
was prepared in 1882 and published in the Third Annual Report of the
Survey as an abstract of the final monograph. It was accompanied by
a geological map similar to sheet iv of the atlas. The volume of atlas
plates bears the imprint of 1883, but is now issued in complete form for
the first time. In its more essential features the present report was pre-
pared several years ago, but the completion of the manuscript has been
delayed from time to time for various unforeseen reasons, mainly by press-
ure of other duties. It presents, as concisely as is consistent with clearness
and completeness, the principal geological facts gathered in the field and
such general deductions as have been drawn from their study. I haye
endeavored to make each chapter complete in itself, and this has necessitated
the repetition of certain observations, as a large number of facts are more or
less related to the subjects discussed in the different chapters. It is an
advantage, however, to the special reader, to have such facts as he may
need brought together under one grouping, and not to feel obliged to
search through the volume for them.

The atlas consists of thirteen sheets. The preparation of the topo-
graphical map was intrusted to Mr. F. A. Clark, who employed three able
assistants in the field — Mr. G. H. Wilson, assistant topographer with the
plane table; Mr. Gr. Olivio Newman, in charge of triangulation, and Mr.
Morris Bien, assistant topographer.

A special paper by Mr. Iddings, upon the microscopical petrography of

I'liEFACE. XIII

the eruptive rocks of the Eureka District, appears as an appendix to this
report. It presents the results of a careful examination of several hundred
thin sections prepared from a large number of rocks, representing every
variety known to occur in the region. It is a concise statement of results of
a systematic study of the material and is of great interest, bearing directly
upon many geological questions connected with eruptive masses. Mr.
Iddiiigs's report is illustrated by six plates, four of which are reproductions
of photomicrographs, showing some interesting features in structure of fine
groundrnass, and two of drawings of minute crystals and microscopic objects
found in the rocks. At the time these photomicrographs were produced
they were superior to anything which had been done in this class of illus-
tration.

Mr. Walcott's report upon the "Paleontology of the Eureka District"
was published as Monograph VIII of the U. S. Geological Survey, in 1884.
It gives the results of a detailed study of the organic forms obtained
throughout a wide range of geological formations, the region having proved
an exceptionally rich one in paleontological material from Cambrian, Devo-
nian and Carboniferous rocks. In addition to the descriptions of many
forms new to science and the identification of over five hundred species,
the report contains notes, more or less full, upon many species which
presented in their characters or geographical distribution information not
heretofore published. The work is illustrated by over five hundred and fifty
accurate drawings of fossils, arranged on twenty-four plates. Four plates
represent the fauna of the Cambrian, two that of the Silurian, ten that of
the Devonian, and eight that of the Carboniferous. All specific identifica-
tions of organic forms from Eureka referred to in this work were made
by Mr. Walcott.

After the completion of the field work for the Eureka map, Mr. J. S.
Curtis began his investigations of the ore deposits found on Ruby Hill.
The surface maps published by Mr. Curtis were taken from the atlas sheets
accompanying this monograph. Mr. Curtis's report appeared in 1884 as
Monograph VII of the U. S. Geological Survey, and is entitled "Silver-Lead
Deposits of Eureka, Nevada." It is a valuable work and one which forms
an important part of the scientific memoirs relating to the Eureka District.

XIV PKEFACE.

The writer's acknowledgments are due to many gentlemen, superin-
tendents of mines and others, who rendered valuable assistance in furnishing
information in regard to the country, and who generously afforded every
facility in the prosecution of the work. Special thanks are due to Mr. R
Kickard, formerly superintendent of the Richmond Mining Company, and
to Mr. Thomas J. Read, superintendent of the Eureka Consolidated Mining
Company.

June 6, 1891. ARNOLD HAGUE.

OUTLINE OF THIS VOLUME.

CHAPTER I. The area covered by the geological survey of the Eureka Mountains embraces a
region of country 20 miles square. The mountains are situated on the Nevada plateau and form a
somewhat isolated mass, surrounded on all sides by the broad detrital valleys so characteristic of
the Great Basiu. These valleys which encircle the mountains have an average elevation above sea
level of 6,000 feet. Rising above them the highest peaks attain altitudes varying from 9,000 to 10,500
feet. In strong contrast with most of the Great Basin ranges, the Eureka Mountains present a rough
and rugged appearance, with varied topographical features. •

CHAPTER II. Sedimentary rocks belonging either to the Paleozoic or Quaternary age form the
greater part of the mountains and valleys. Quaternary beds present little of geological interest,
although they extend over wide areas, being mainly superficial accumulations composed of detrital
material brought down from the mountains and deposited along their flanks and out over the broad
plains. A great thickness of limestone, sandstone, and shale, which make up the Paleozoic series of
rocks, was laid down under varying conditions of depth of water and rapidity of deposition with
only one well recognized unconformity from base to summit. In this region the Paleozoic age was a
time of comparative freedom from dynamic movements. Eureka presents no direct evidence as to
the time mountain building took place other than that the region was elevated into a broad conti-
nental land mass after the deposition of the Upper Coal-measure limestone. Reasons are assigned for
supposing that all the Great Basin ranges owe their origin to a post-Jurassic movement. The folding,
flexing, and faulting which outlined the mountains broke up this mass of sediments into six sharply
denned orographic blocks, each with well marked structural peculiarities. These mountain blocks
have been designated as follows: Prospect Ridge, Fish Creek Mountains, Silverado and County Peak
group, Mahogany Hills, Diamond Mountains, and Carbon Ridge and Spring Hill group. Taken
together these six blocks present a compact mass of mountains, the result of intense lateral com-
pression and longitudinal strain. Profound longitudinal faults extend the entire length of the moun-
tains, showing a displacement of beds of over 13,000 feet. The Paleozoic sediments measure 30,000
feet in thickness, with Cambrian, Silurian, Devonian, and Carboniferous, all well represented by
characteristic fauna. In these four periods fourteen epochs have been recognized.

CHAPTER III. Cambrian rocks measure 7,700 feet, divided into five epochs, as follows: Pros-
pect Mountain quartzite, Prospect Mountain limestone, Secret Canyon shale, Hamburg limestone, and
Hamburg shale. The Middle, Lower, and Upper Cambrian are all exposed. On the crest of Prospect
Ridge, at the base of the Cambrian limestone, occurs the Olenellus shale, the oldest fossiliferous strata
recognized in the Great Basin. Hamburg Ridge carries a Potsdam fauna both at its base and summit.
Conformably overlying the Cambrian come the Silurian rocks, 5,000 feet in thickness. They
fall readily into three epochs, two limestones and an intervening body of quartzite. They have
been designated Pogonip limestone, Eureka quartzite, and Lone Mountain limestone. The qnartzite

xvi OUTLINE OF THIS VOLUME.

is easily distinguished from both the coareo sands and grits of the Cambrian below and the Carbon-
iferous conglomerate above. An unconformity of deposition exists between the Eureka and Lone
Mountain epochs. Both the Trenton and Niagara formations are included within the Lone Moun-
tain epoch.

CHAPTER IV. By imperceptible gradations limestones of the Lone Mountain epoch pass
upward into those of the Devonian period. Devonian rocks occupy a larger area in the District than
those of any other period, and present a greater thickness than either the Cambrian or Silurian.
They measure 8,000 feet, divided into two epochs : A bluish limestone — the Nevada limestone — and an
argillaceous black shale — the White Pine shale. The limestone carries a rich invertebrate fauna from
base to summit. The black shale is characterized by a flora which, though fragmentary, is suffi-
ciently well preserved to identify the genera as belonging to the Upper Devonian.

The Carboniferous rocks measure 9,300 feet, which, however, does not quite represent their full
development, the uppermost beds having undergone more or less erosion. They have been divided
into four epochs, as follows : Diamond Peak quartzite, Lower Coal-measure limestone, Weber con-
glomerate, aud Upper Coal-measure limestone. As the limestone is in general favorable to the preser-
vation of organic remains, fossil-bearing strata occur throughout the beds. Three salient features
mark the life of the Lower Coal-measures. First, the occurrence near the base of the limestone of a
fresh- water fauna; second, the varied development of the Lamellibranchiates a class which has here-
tofore been but sparingly represented in the collection of Carboniferous fossils from the Cordillera ;
third, the mingling near the base of the horizon of Devonian, Lower Carboniferous, aud Coal-measure
species in gray limestone directly overlying beds characterized by a purely Coal-measure fauna.

In the first range to the east of the Eureka Mountains Carboniferous rocks extend for miles
along the edge of the valley, in which well developed coal seams occur.

CHAPTER V. This chapter is devoted to the descriptive geology of the sedimentary rocks.
Each orographic block is described in detail, beginning with Prospect Ridge, where the oldest rocks
occur, followed by the other blocks according to the succession of strata. It gives a connected
description of the country and points out the relations of the different mountain masses to each other.

CHAPTER VI. A discussion of the Paleozoic rocks follows, based upon the facts presented in
the earlier chapters. It is shown that during Paleozoic time a pre-Cambrian continent existed in
western Nevada which furnished to an ocean lying to the eastward an enormous amount of detrital
material. It is pointed out that the Eureka region was situated not far from the eastern border of this
land mass, and that a large part of its coarse conglomerates aud mechanical sediments must have been
offshore deposits. . The geological record affords proof of elevation and depression th roughont Paleozoic
time with intervals of shallow water and proximity of land areas between periods of relatively deep
seas. Fresh-water life, plant remains, and coal seainb at different horizons furnish additional evidence
of shallow water and offshore deposits. A study of Paleozoic rocks in other parts of southern and
western Nevada exhibit nearly similar geological conditions as regards sequence of beds. This is
especially well shown both at White Pine and in the Highland and Pifion ranges. The sequence of
strata, both to the north and south, indicates a closer agreement with the conditions of sedimentation
at Eureka than the many exposures situated but a short distance eastward of the latter area. The
structural relations of the, different orographic blocks to each other and the outbursts of igneous rocks
are well brought out in cross-section. An instructive feature at Eureka is the close relationship
between the anticlinal and synclinal folds to the profound north and south faults.

CHAPTER VII. Pre-Tertiary igneous rocks play a very subordinate part. They may be classed
under three heads: Granite, granite-porphyry, and quartz-porphyry. The granite occupies a limited
area on Prospect Ridge. Both the granite and quartz-porphyries occur as dikes. Structural varia-
tions in the dikes are mainly dependent upon the chilling effect of cold contact walls upon a rapidly

OUTLINE OF THIS VOLUME. XVII

cooling molten mass. The width of the (like has much to do in determining the physical conditions
governing crystallization. A8 regards the age of the dikes little is known other than that they pene-
trate Siluriaii strata.

CHAPTER VIII. The Eureka District offers no direct proof of the age or duration of volcanic
energy, although evidence based upon observations elsewhere in the Great Basin points to the
conclusion that the lavas belong to the Tertiary period, and probably the greater part of them
to the Pliocene epoch. They broke out in four ways: First, through profound fissures along
meridional lines of displacement; second, following lines of orographic fracture, they border
and encircle large uplifted masses of sedimentary strata ; third, they occur as dikes penetrating the
sedimentary rocks; fourth, they occur in one or two relatively large bodies, notably Richmond Moun-
tain and Pinto Peak, along lines of displacement. The sequence of lavas was hornblende-andesite,
hornblende-mica-andesite, dacite, rhyolite, pyroxene-andesite, and basalt. The lavas display a great
variety of volcanic products in both chemical and mineral composition. They are all derived from a
common source, a homogeneous molten mass. They are due to a process of differentiation by molec-
ular change within the molten mass under varying conditions of pressure and temperature. Starting
with a magma of intermediate composition, the extreme products of such a differentiation are rhyolite
and basalt.

CHAPTER IX. In the Eureka District the ores occur in sedimentary rocks belonging to the
Cambrian, Silurian, and Devonian periods, and may be found in all horizons, except the Secret
Canyon and Hamburg shale, from the base of the Prospect Mountain limestone to the summit of the
Nevada limestone. Through 17,000 feet of strata ores have been deposited in sufficiently large
bodies to encourage mining exploration. The most productive deposits have been found in Cambrian
rocks, but this is owing to orographic and structural conditions rather than the geological age of
strata or chemical nature of sediments. Nearly all the more productive mines are included within
the beds which form the Prospect Mountain uplift between the Hoosac and Spring Valley faults. The
ore followed the rhyolite and is consequently Pliocene or post-Pliocene age. All the ores came from
below and were originally deposited as sulphides. They were subsequently oxidized by atmospheric
agencies, mainly surface waters percolating through the rocks.

In Appendix A, Mr. C. D. Walcott gives a systematic list of fossils from each formation found
at Eureka.

In Appendix B, Mr. Joseph P. Iddings discusses the microscopical petrography of the crystal-
line rocks. It is a thorough study of the mineral and structural character of the rocks and is illus-
trated by several plates.
MON XX II

GEOLOGY OF THE EUREKA DISTRICT.

BY ARNOLD HAGUE.

CHAPTER I.

GENERAL DESCRIPTION.

The Eureka District is situated on the Nevada plateau in the central
part of the state of Nevada, midway between the basin of Lake Lahontan
westward and the basin of Lake Bonneville eastward. The area covered
by the geological and topographical survey embraces a region of country
20 miles square, lying partly in the county of Eureka and partly in the
county of White Pine.

The meridian of 116° west from Greenwich passes just westward of
the center of the examined area, and the 39° 30' parallel of north latitude
crosses Ruby Hill, the seat of the present activity in precious-metal
mining.

Nevada plateau.— On the Nevada plateau the broad central north and
south valleys, lying between meridional mountain ranges, reach an aver-
age altitude of 6,000 feet above sea-level, the country falling away grad-
ually on both sides till at Salt Lake, in Utah, the altitude is 4,250 feet, and
at Carson and Humboldt Lakes, in Nevada, 3,800 feet above sea level.
These valleys, however, compared with those of the depressed areas adjoin-
ing the plateau, are relatively narrow, with few marked exceptions, seldom
measuring more than 10 or 12 miles in width. In general the broader
physical features of the Great Basin ranges are much the same all the way
MON xx 1 1

2 <1 EULOGY OF THE EUKEKA DISTRICT.

from the bold escarpment of the Sierra Nevada of California to the precip-
itous wall of the Wasatch Mountains of Utah, the distance across the
widest part in an east and west line being about 425 miles. These ranges
form long, narrow mountain uplifts with sharply defined limits, rising with
more or less abruptness above dreary intervals of desert. Their nearly
uniform trend and the remarkable parallelism of the lines of upheaval of the
older sedimentary ridges present the most marked feature of the region.
In width they seldom exceed 8 miles, but frequently extend in an unbroken
line for more than 100 miles in length, with serrated peaks and ridges rising
from 2,000 to 6,000 feet above adjacent valleys. For the most part they
possess a simple topographical structure and a simple drainage system.
They are characterized, more especially the lower ranges, by absence of
trees, and in many cases are nearly bare of all vegetation, presenting rough,
rugged slopes of naked rock.

On the higher parts of the plateau the ranges, reaching a greater alti-
tude, partake more of an Alpine or sub-Alpine character. Precipitation
of moisture is more abundant, as seen both in the more frequent rains of
slimmer and snows of winter. A greater precipitation produces larger and
more frequent streams, and a continued moisture favors a varied vegeta-
tion— the spurs and ridges being more or less covered with a dwarfed and
stunted forest growth, and the long slopes with nutritious grasses.

These salient features distinguish the ranges of the Nevada plateau
from those of Lake Lahontan and Lake Bonneville Basins, which present a
more arid and desolate aspect. A striking feature of nearly all these ranges
is their isolated position, only a few of them presenting outlying spurs or
low lines of rolling foothills. Occasionally inferior ridges of sedimentary
beds stretch diagonally across valleys from one range to another, com-
pletely shutting in the intermediate valley, and still more frequently out-
bursts of volcanic rocks in irregular flows serve to unite in confused masses
bodies of sedimentary formations otherwise distinct.

Midway between the Sierra and the Wasatch stand the East llum-
boldt Mountains, the most prominent range in the Great Basin. They lire-
sent, not only by reason of the greater number of rugged and commanding
peaks, many of them attaining an elevation over 11,000 feet above sea

EUREKA MOUNTAINS. 3

level, but by their broad, massive proportions, long, unbroken ridges, and
Alpine character, the boldest uplift on the Nevada plateau. Next west
from the Humboldt occurs the Diamond Range, followed by the Pinon
Range, with the broad Diamond Valley lying between them. Southward
the southern extremities of these two ranges enter the Eureka District and
form a part of its mountainous region.

On the plateau, among the more marked exceptions to the long narrow
ranges which rib the surface of the country, may be mentioned the Rob-
erts Peak Group, connecting the Wahweah with the Pinon Range, the
White Pine Mountains, and the subject of the present report, the moun-
tains of the Eureka District.

Eureka Mountains.— The Eureka District forms a rough mountain block
standing out prominently by itself, except for its narrow connections with
both the Pinon and Diamond Ranges, almost as completely isolated from its
neighbors as the longer parallel ranges. As a mountain mass, however,
although well deserving such a distinction, it has never received any definite
appellation which would include all its members, it being made up of por-
tions of several ranges and short uplifted blocks sp intimately connected
and inosculated as to form both topographically and geologically a single
group, hemmed in on all sides by the characteristic detrital valleys. To
the north Diamond Valley, which may be taken as a type of the higher
valleys of the Great Basin, extends for over 40 miles in an unbroken plain,
the lowest part of the depression being covered in winter by a broad, shal-
low sheet of water, which, upon evaporation, presents during the greater
part of the year a hard, level floor, strongly impregnated with salt. Con-
siderable quantities of salt for metallurgical purposes have been collected
from the shores of the small lakes at the northern end of the valley. To
the south of the district lies the broad basin of Fish Creek Valley, con-
necting with Newark Valley on the east side of Diamond Range, while
the Antelope Valley cuts off" the Eureka District on the west side from the
neighboring mountains. All these valleys stand at about the same elevation
above sea level, and offer to the eye a monotonous olive-gray color derived
from a vigorous growth of the Artemcsia tridenlata which covers all the low-
lands except the central portions of the broader basins.

4 GEOLOGY OF THE EUREKA DISTRICT.

It is doubtful if any urea of equal extent iu Nevada possesses more
varied physical features with such strongly marked contrasts than the Eu-
reka District. In close proximity may be seen long serrated ridges, broad
summits, gently inclined tables of nearly horizontal sedimentary beds, with
abrupt escarpments along canyon walls, and highly tilted strata in rough
irregular spurs. And, as might be expected in a country made up of indi-
vidual blocks and parts of ranges and so interlocked as to form one broad
mass, the region is characterized by broad shallow basins, long narrow
ravines, and winding valleys, presenting a more than ordinarily accidented
surface with an intricate structure. Above the broad base of the surround-
ing sage-brush valleys rise many prominent peaks from 2,500 to 4,500 feet.
Diamond Peak, in the northeast corner of the district, at the southern ex-
tremity of Diamond Range, is the culminating point, measuring 10,637 feet
above sea level, and, with the exception of the high summits in the East
Humboldt Range, is one of the loftiest peaks on the Nevada plateau.
Prospect Peak, on the central ridge, and the second point in the district,
measures 9,604 feet, while Atrypa Peak, to the southwest on the same
ridge, has an altitude of 9,063 feet above sea level. Other points are White
Cloud Peak, the highest point on a broad plateau-like ridge, 8,950 feet;
Alpha Peak, 8,985 feet; and Woodpecker's Peak, 8,598 feet; all of them
being formed of sedimentary rocks. Among volcanic mountains may be
mentioned Richmond Mountain, just east of the town of Eureka, which rises
to a height of 8,392 feet, and Pinto Peak, an isolated cone in the center of
the district, reaches an altitude of 7,880 feet above sea level.

Up to the time of the rapid development of the mining interests upon
Ruby Hill and Prospect Mountain, the slopes and ridges about Eureka
were exceptionally well supplied with an arborescent growth, a condition
which was due partly to the number of high peaks but in great part to
broad masses of mountains acting as condensers of desert moisture. To-
day, so great has been the demand for wood B,nd charcoal in the reduction
of lead ores, that the mountains are as bare of trees as any part of the
Great Basin. Several species of pines, dwarfed junipers (Juniperus occiden-
talis), and mountain mahogany (Cercocarpus Icedifoliits), which attains a
height of over 20 feet, are, or rather were, the prevailing trees, but are now

SOIL— CLIMATE. 5

found only in a few areas preserved by their owners for future use, at no
distant day. Not only have the Eureka Mountains lost their forests, but
the neighboring mountains for long distances have been devastated to fur-
nish fuel for the smelting furnaces. Some idea may be obtained of the
enormous consumption of wood from the statement that 10,000 bushels of
charcoal are required daily for the smelting furnaces when the works are
running their usual force, and that for five or six years the daily consump-
tion was rather over than under that amount.

Soil.— Nature presents a barren, arid appearance. Perennial streams in
the ravines are exceptional, other than those found on the slopes of Diamond
Peak. Fresh water springs lie scattered about the mountains and fur-
nish a scanty supply of water, barely sufficient to meet the wants of the
people. A few deep wells have been successfully sunk in the broader
valleys. Vegetation is everywhere limited, and is mainly confined to bunch
grasses on the mountain slopes and sage brush in the open valleys.

As the valleys are mainly filled with coarse detrital material from
mountain slopes, soils suitable for agricultural purposes occupy very
small areas, and are found only in the broader basins. In the favored
spots where water for irrigation purposes can be readily obtained, all the
more hardy vegetables grow well, and are of excellent quality, but nearly
all crops suffer from early frosts. In no sense can the country be regarded
as an agricultural one, and cultivation of the soil is remunerative to
the farmer only by reason of the very high prices received for his produce.

climate.— A rigorous winter, a long hot summer, a dry atmosphere, with
a light precipitation of moisture, are characteristic climatic features of the
Eureka District. In summer, rainfalls are limited to showers, frequently
very severe, but of short duration, and what are commonly known as cloud-
bursts are by no means uncommon during late July and early August.
The clouds, late in the afternoon, centering over Prospect Peak, break with
such force that many people caught without warning have been drowned.
In July, 1874, a severe storm and flood destroyed seventeen lives, and
carried off property to the value of many thousands of dollars.

During the period of our survey careful meteorological observations
were made throughout the summer. Snow fell in the month of May no

6 GEOLOGY OF THE EUEEKA DISTRICT.

less than eight times, and again on June 10 and 11 In summer the days
are warm, and for the most part cloudless; the nights cool. The daily
variation between the maximum and minimum thermometers was always
very considerable, frequently showing a difference of 40° F. For the three
summer months of June, July, and August, of 1880, the maximum ther-
mometer in the shade stood over 90° F. on eighteen days, or one day in
six. As the climate is very dry, the heat was seldom oppressive, except
in some inclosed basin or valley. As early as August 30, the thermometer
fell below the freezing point, and on October 9 a light fall of snow covered
both mountain and valley.

History.— In the summer of 1864 the first locations of mining property
were made in New York Canyon, on the easteni side of Prospect Mountain,
near the present " 76 " Mine. This property was known as the Eureka
Mine, and although it never fulfilled the expectations of its original owners,
it transferred its name to the very successful property on Ruby Hill and
subsequently gave a name to the town, to the mining district, to the county,
and finally to the neighboring group of mountains. The original property
gave so little promise that the district was finally abandoned. In mining
operations very little was accomplished until the spring of 1869, when im-
portant discoveries were made on Ruby Hill and active, intelligent work
was undertaken. The Champion and Buckeye claims on the south side of
Ruby Hill were the first properties located, and soon afterward the ground
was broken on the now famous Richmond and Tip Top Mines. From that
time forward mining operations on Ruby Hill have gone on steadily, and
to-day the Eureka District is the most successful mining region in the state
of Nevada. Success on Ruby Hill was quickly followed by active enter-
prise developing mining locations on both slopes of the ridge of Prospect
Mountain, in Secret Canyon, and in the Silverado Hills in the southwest
corner of the district.

Estimates of the value of the ore production of the district since the
first shipment of crude bullion in 1869 are as follows :

From 1869 to 1873 $10,000,000

From 1873 to January, 1883 50,000,000

Total 60,000,000

H1STOKY OF THE DISTRICT. 7

One-third of this amount, according to the best estimates, was gold,
and two-thirds silver. The product in lead is not so easily determined, but
it is not far from 225,000 tons, an amount sufficient to affect the market
price of lead in all the great commercial centers of the world.

Around this industry has grown up the town of Eureka, which is the
center of population and trade for this part of the state. It is a long, narrow
settlement, lying in the main northern drainage channel of the mountains,
and sheltered on the east side by Richmond Mountain. Here are located
the smelting furnaces of both the large companies.

The Eureka and Palisade Railway, 88 miles in length, connects the
town with the Central Pacific Road at Palisade. Branch tracks connect
with the Eureka Consolidated and Richmond furnaces, the former at the
lower, and the latter at the upper end of the town, and these again by
a somewhat sinuous course with the principal mines, which are situated
about two and one-half miles southwest of Eureka. There are an imposing,
well built court house, three or four churches, and several blocks of brick
stores and warehouses in the town. It supports two daily papers, which
have a considerable influence and a wide circulation throughout the state.

Ruby Hill, the only other town of any importance in the district, is a
flourishing place, nearly the entire population being actively engaged in
mining in the immediate neighborhood. It is built on the north and east
sides of an isolated hill which bears the same name, and on which are
located all the more prominent mines, including the Albion, Richmond,
Eureka Consolidated, Phoenix, and Jackson properties. On the slopes to
the north are situated the Bullwhacker and Williamsburg mines, while to
the southward of Ruby Hill, on Prospect Ridge, are found the Dunderberg
and Hamburg properties and others of more or less importance.

CHAPTER II.

GEOLOGICAL SKETCH OF THE EUREKA DISTRICT.

Sedimentary rocks, belonging either to the Paleozoic or Quaternary
period, form by far the greater part of the mountains and valleys of the
Eureka District. The beds of the Quaternary present but little of geological
interest, and although they extend over wide areas they are, in most
instances, superficial accumulations composed of detrital material brought
down from the mountains and deposited along their flanks, concealing the
underlying rocks of the foothills. Igneous rocks play a most important
part in the geological history of the region, but nevertheless do not form an
imposing feature of the individual mountain uplifts, appearing either as ex-
travasated masses along lines of faulting, or as larger bodies encircling and
lying outside the main blocks of sedimentary formations. The older crys-
talline rocks offer a still less marked topographical feature of the country,
occupying very limited areas in the older Paleozoic limestones, where they
appear as intruded masses exposed by erosion.

It is doubtful if within the province of the Great Basin there can be
found any region of equally restricted area surpassing the Eureka District
in its grand exposures of Paleozoic formations, especially of the lower and
middle portions.

The great thickness of limestone aiid sandstone of which the Paleozoic
is composed was laid down under varying conditions of depth of water and
rapidity of deposition, with only one well recognized unconformity from its
base to summit. In this region the Paleozoic age was a time of compara-
tive freedom from dynamic movements. Most geologists who have given
any attention to the history of the Great Basin ranges substantially agree
that the movements that finally built up the mountains began after the
close of Paleozoic time, and that between the Carboniferous and the close

AGE OF MOUNTAIN BUILDING. 9

of the Jurassic period took place the folding, flexing and faulting of the
beds which outlined the structural features of nearly all the meridional
ranges between the abrupt walls of the Wasatch and those of the Sierra
Nevada. At Eureka no direct evidence is offered as to the time when this
mountain building took place other than that the region was finally lifted
above the ocean after the deposition of the Upper Coal-measures. So far
as the mountains themselves are concerned, there is a total lack of evidence
that the blocking out of the ridges did not begin at the close of the Paleozoic
period, but, on the other hand, all observations tend to show that whenever
and by whatever causes the other Great Basin ranges were uplifted, the
same orographic conditions which prevailed elsewhere held true for the
Eureka Mountains. In other words, the Eureka Mountains were a part of
a more extended geological province.

According to the conclusions of Mr. Clarence King,1 based upon the
observations of the geologists of the Fortieth Parallel Exploration, the
mountains west of the Havallah Range and the meridian of 117° 30' belong
to a post-Jurassic upheaval, and to the west of this line there existed during
Paleozoic time an elevated continental area which fumished the material
accumulated in an ocean basin to the east. At the close of the Paleozoic
this oceanic area, stretching as far eastward as the Wasatch, was lifted up
into a broad laud-mass, and the former continental region sank below the
water and in turn became an ocean basin. From the Wasatch westward
to this ancient shore line the mountain ridges exhibit much in common in
their structural and physical features, being made up in great measure of
Paleozoic strata, whereas from this boundary westward the ranges show
a marked contrast in the nature of their sedimentation and bear ample
paleontological evidence of their Mesozoic age. Over this latter area, not-
ably in the West Humboldt, Piute, and Augusta Mountains, limestones
characteristic of the Triassic and Jurassic have been described in detail by
the geologists of the Fortieth Parallel Exploration,2 while to the east of this
shore line no Mesozoic rocks occur. Mr. King assigns excellent reasons for

1 Geological Exploration of the Fortieth Parallel, vol. i, Systematic Geology, p. 733. Washing-
ton: 1878.

"Geological Exploration of the Fortieth Parallel, vol. n. Descriptive Geology, pp. 657, 711,
ami 724. Washington. 1877.

10 GEOLOGY OF THE EUEEKA DISTRICT.

the opinion that all the Great Basin ranges across Utah and Nevada were
uplifted at the same time under identical dynamic influences, and conse-
quently owe their origin mainly to a post-Jurassic movement.

This indicates a marked unconformity between the Carboniferous and
Triassic, but it neither necessitates nor precludes the beginning of mountain
building over the Paleozoic area at the time of the uplifting of the conti-
nental laud-mass from beneath the ocean. Nowhere throughout this region,
any more than at Eureka, have the Great Basin ranges as yet offered any
direct evidence of folding accompanying this elevation, yet it would seem
highly probable that some crumpling of strata might have taken place before
the main blocking out of the mountain ridges at the close of Jurassic time.

Most of the Great Basin ranges are narrow, longitudinal ridges, and
while they present much in common as to their origin and primary struc-
ture, each possesses its own special physical features due to local dynamic
conditions. Most of them are formed by direct lateral compression result-
ing in anticlinal folds, occasionally accompanied by synclines. Some of
them are simple mouoclinal ridges, representing one side of an anticlinal
axis. Still others exhibit great complexity of structure with both folding
and faulting along the meridional axes of the ranges, with which are asso-
ciated transverse faults and folds striking obliquely across the topograph-
ical trend of the uplifted mass.

Orographic Blocks.— The Eureka Mountains lie near the western edge of
what was at one time the Paleozoic ocean. The nearness of these uplifted
beds to an older pre-Paleozoic continent is in some measure indicated by the
relatively great amount of disturbance of strata and plication of mountain
masses as compared with the more gently inclined strata, and simplicity of
structure found farther to the eastward. Unlike the ordinary type of nar-
row ridges, the Eureka Mountains exhibit a solid mountain mass over 20
miles in width, including several uplifted blocks whose length does not
greatly exceed their width. Taken together they present a compact mass
of mountains thrown up by intense lateral compression accompanied by
longitudinal strain. The forces which brought about the elevation of the
mountains produced an intricate structure with powerful flexures and folds
and broke up this immense thickness of sediments into individual blocks

PALEOZOIC SECTION. 11

accompanied by profound longitudinal faults, several of which extend the
entire length of the mountains, and have played a most important part in
bringing about the present orographic conditions.

Although these mountain masses stand so intimately related to each
other that it is frequently difficult to draw sharp topographical lines between
them, the Eureka Mountains may be divided into six blocks with well
marked structural and geological differences. These blocks may be desig-
nated as follows:

Prospect liidge.

Fish Creek Mountains.

Silverado and County Peak group.

Mahogany Hills.

Diamond Mountain.

Carbon Ridge and Spring Hill group.

Paleozoic Section.— As already mentioned, the Eureka Mountains lie just
eastward of the old shore line. In this and the following chapters the
evidence is presented, derived from the history of the rocks themselves, to
show the close proximity of a land area when the beds were laid down.
The nature of these off-shore deposits near the western border of an old
Paleozoic sea form one of the principal objects of this investigation. Much
of the material, such as the coarser conglomerates, must necessarily have been
off-shore deposits The sedimentary rocks which make up the mountains
present a great development of limestones, quartzites, sandstones, and shales,
comprising many thousands of feet of Cambrian, Silurian, Devonian, and
Carboniferous beds. From the lowest exposed members of Cambrian strata
to the top of the Coal-measures there are represented a series of sedimentary
deposits 30,000 feet in thickness. Nowhere within the limits of the Eureka
district can there be found any one exposure which shows the beds with-
out a break in their continuity, the longest unbroken section representing
about one-third of the entire sequence of strata, yet the region offers in so
many instances such continuous exposures of beds and so many in which
the series of strata overlap each other with such a constant repetition of
beds, that the reconstruction of the entire section is easily made out when
the individual parts are carefully compared and studied. The reason why
there is no one unbroken section may be readily understood by a glance

12 GEOLOGY OF THE EUREKA DISTRICT.

at the map which shows how the sedimentary strata have been broken up
into separate mountain blocks, each made up of a portion of the entire
thickness of beds.

In the four grand periods of Paleozoic time represented at Eureka, 14
epochs have been recognized : 5 in the Cambrian, 3 in the Silurian, 2 in the
Devonian, and 4 in the Carboniferous.

With a single exception local geographical names have been employed
to designate the different epochs into which the Cambrian, Silurian, and
Devonian have been divided. Heretofore, throughout the Great Basin the
division of the larger periods into epochs has not been deemed necessary,
the individual horizons not having been studied sufficiently in detail to
require it. The exception is made in favor of the Pogonip limestone, a
name first applied by the Geological Exploration of the Fortieth Parallel to
the belt of limestone which forms the base of the Silurian. In the Carbon-
iferous period a large quartzite body at the base of the series has been
designated the Diamond Peak quartzite, but for the remaining epochs the
well known names Lower Coal-measures, Weber conglomerates, and Upper
Coal-measures are retained, notwithstanding some serious objection to the
use of the term Coal-measures in this region.

Each of the six blocks expose several thousand feet of strata, and
while they frequently overlap each other no two of them represent precisely
the same horizons, although the Diamond Range includes within its strata
the beds which make up the Carbon Ridge and Spring Hill blocks. The
six blocks essentially correspond to the following periods :

Prospect Ridge : Cambrian and Siluriau.

Fish Creek Mountains : Silurian.

Silverado and County Peak: Silurian and Devonian.

Mahogany Hills: Devonian.

Diamond Mountain : Devonian and Carboniferous.

Carbon Ridge and Spring Hill: Carboniferous.

In the subjoined section, which may be best designated as the Eureka
section, the relative thickness and general lithological characters are given
for all the geological divisions which have -been made of the sedimentary
rocks. A plane of unconformity in the Silurian is indicated by double
dividing lines between the Eureka quartzite and Lone Mountain limestone.

EUEEKA SECTION.

Eureka Section, Nevada, 30,000 feet.

0>'

500

Light colored blue and drab limestones.

2,000

Coarse and fine conglomerates, with angular fragments of chert;
reddish yellow sandstone.

layers of

3,800

Heavy bedded dark blue and gray limestone, with intercalated
chert ; argillaceous beds near the base.

bauds ot

3,000

Massive gray and brown quartzite, with brown and green shales at the
summit.

DEVONIAN, 8,000 feet.

White Pine shale

2,000

Black argillaceous shales, more or less arenaceous, with intercalations of
red and reddish brown friable sandstone, changing rapidly with the
locality; plant impressions.

6,000

Lower horizons indistinctly bedded, saccharoidal texture, gray color, pass-
ing up into strata distinctly bedded, brown, reddish brown, and gray
in color, frequently finely striped, producing a variegated appearance.
The upper horizons are massive, well bedded, bluish black in color ; highly
fossiliferous.

SILURIAN. 5,000 feet.

Lone Mountain limestone 1, 800

Black, gritty beds at the base, passing into a light gray siliceous rock, with
all traces of bedding obliterated; Trenton fossils at the base; Haly sites
in the upper portion.

500

Compact, vitreous quartzite, white, blue, passing into reddish tints near
the base; indistinct bedding.

2,700

Inters tratitied limestone, argillites, and arenaceous beds at the base, pass-
ing into purer, fine grained limestone of a bluish gray color, distinctly
bedded ; highly fossiuferous.

CAMBRIAN, 7,700 feet.

350

Yellow argillaceous shale, layers of chert nodules throughout the bed, but
more abundant near the top.

1.200

Dark gray and granular limestone; surface weathering, rough and ragged ;
only slight traces of bedding.

1,600

Yellow and gray argillaceous shales, passing into shaly limestone; near
the top, iuterstratined layers of shale and thinly bedded limestones.

Prospect Mountain limestone. . .

3,050

Gray, compact limestone; lighter in color than the Hamburg limestone,
traversed with thin seams of calcite ; bedding planes very imperfect.

Prospect Mountain quartzite . . .

1.500

Bedded brownish white quartzites, withering dark brown; ferruginous
near the base; intercalated thin layers <»t arenaceous shales; bvds whiter
near lite summit.

NOTE.— I'lane of uucouforiuily indicated by double dividing line.

14 GEOLOGY OF THE EUREKA DISTRICT.

Longitudinal Faults.— The most profound faults, those which mark the
greatest amount of displacement and have exerted the most influence in
producing the present structural features of the region, cross the mountains
at varying intervals with an approximately north and south trend from Fish
Creek Basin to Diamond Valley. These faults constitute the principal
factors in outlining the individual orographic blocks, and probably from the
beginning of mountain building up to the present time, and certainly
through the Tertiary period, have played a most important part in their
development. The amount of displacement along those faults that extend
the entire length of the mountains is very great, measuring at some points
in their course as high as 13,000 feet.

The four principal lines of displacement are the Spring Valley and
Sierra fault, on the west side of Prospect Ridge; the Hoosac fault, separat-
ing Prospect Ridge from Spring Hill and Carbon Ridge; the Pinto fault,
lying between the Spring Hill and Carbon Ridge on the one side and the
County Peak and Silverado Mountain block on the other, and the Rescue
fault, on the east side of the latter block. These main faults will be de-
scribed here. Numerous other longitudinal faults, while they express
powerful orographic movements, are more restricted in their influence and
confined within the limits of one or the other mountain blocks into which
the country is broken up. They will be mentioned with more or less detail
when describing the particular region in which they occur.

Spring Valley and sierra Fault — The Spring Valley fault adheres closely to
the west base of Prospect Ridge and sharply defines the ridge both in
physical and geological structure from the Mahogany Hills on the opposite
side of the narrow valley which has given its name to the fault, and through
which the line of the displacement runs Along the base of Prospect Ridge
the oldest Cambrian strata yet recognized in the Great Basin come up
against the fault and are separated by it from the Silurian and Devonian beds
which form the mountains to the west. On the west side of the fault
and opposite Prospect Peak, the culminating point on the ridge, the Eureka
quartzite of Spanish Mountain is exposed against the fault line. The strati-
graphical position of the Eureka quartzite along the Hoosac fault on the
east base of Prospect Ridge, where it overlies the great development of

HOOSAC FAULT. 15

Cambriam strata and the Pogonip limestone of the Silurian, thoroughly
well establishes the fact that there occurs a displacement of over 1 1,000
feet along the Spring Valley fault at the west base of Prospect Peak. At
the southwest corner of Prospect Peak a fault runs up the steep slope of
the mountain with a somewhat irregular course till reaching the summit,
where it joins the Sierra fault on the south side of the peak. This cross
fault going up the side of the mountain has been designated the Prospect
Peak fault. By this fault the entire series of beds belonging to the Cam-
brian quartzite are abruptly cut off, and Silurian strata are found lying
unconformably against it. The Sierra fault resumes the longitudinal trend
and, with an occasional break in its course, continues southward until the
Cambrian ridge which it limits on the west gradually sinks below the plain.
Along the Sierra fault the Eureka quartzite for the greater part of the dis-
tance lies next the Prospect Mountain limestone, the Cambrian quartzite
not being exposed south of Prospect Peak; otherwise the Sierra fault
presents much in common with that of the Spring Valley, having the same
general trend, and with the Cambrian on one side and the Silurian on the
other. From many points of view these three faults, the Spring Valley,
Prospect Peak, and Sierra, may be regarded as a single line of faulting
making a sharp turn or fold in its course up the steep slope of Prospect
Peak and on reaching the summit of the ridge, swinging back again to the
normal north and south direction. The three faults taken together extend
the entire length of the mountains, from Diamond to Fish Creek valleys,
completely isolating the Cambrian strata from the Silurian and Devonian
lying to the westward. As evidence of the continuity of the faults, it may
be stated that along the course of the Sierra fault on the summit of the ridge,
no displacement of strata has been recognized north of its junction with the
Prospect Peak fault, the base of the Cambrian limestone resting conform-
ably on the summit of the Cambrian quartzite.

Hoosac Fault.— A sharp contrast between the Hoosac fault lying on the
east side of the Prospect Ridge and the Spring Valley fault on the west side,
is shown by the large amount of lavas that have broken out along the
former and that are wholly wanting along the latter. Indeed, the course of
the Hoosac fault can be traced only approximately, owing to the vast ac-

16 GEOLOGY OF THE EUREKA DISTRICT.

cumulation of these lavas poured out along the line of displacement, in
places concealing the underlying rocks for considerable distances on both
sides. Within certain limits, however, there is no great difficulty in de-
termining its main course, as on the one side only Silurian rocks occur,
while on the other all the beds known to be in their true structural position
belong to the Lower Coal-measures. At the southern end of the moun-
tains, where the sedimentary beds emerge from beneath the Quaternary,
the fault is completely obscured by rhyolite flows that flank the slopes of a
long ridge' of Eureka quartzite, the uppermost member of the Prospect
Ridge series just to the westward. Opposite Pinto Peak, where the rhyo-
lite flows are of exceptional width and of great thickness, no indications of
its trend are visible, and not until east of Hoosac Mountain do the sedi-
mentary rocks rise above the rhyolite. At Hoosac Mountain occurs the
only case of Silurian beds found on the east side of the fault line, and this
is more apparent than real, as it is rather an instance where a body of
quartzite has been thrust eastward by powerful volcanic forces and lies
superimposed either upon igneous rocks or a body of Carboniferous lime-
stone. It is probably only a thin capping of quartzite, and evidently out
of place, as just eastward of it the limestones may be seen in their true
position.

Proceeding northward the Eureka quartzite, at the base of Hamburg
Ridge, marks the fault on the west, and in direct contact with it lies the
Lower Coal-measures of Spring Hill Ridge, a contact which is maintained
nearly to New York Canyon, only here and there slightly obscured by
Quaternary accumulations. At New York Canyon the fault bifurcates, one
branch turning to the northeast and the other to the northwest, the easterly
branch being the main one and retaining the name, Hoosac fault. The
fault trending to the northeast still continues to mark the boundary between
the Silurian and Carboniferous, following the course of New York Canyon,
and from here northward the contact is nowhere obscured by outbursts of
lava, the Lone Mountain Silurian of McCoy's Ridge being found on the
northwest side of the displacement, with the Lower Coal-measures on the
southeast. A short distance beyond the entrance to New York Canyon,
near the Richmond smelting works, the fault ceases to be traceable toward

KUBY HILL AND PINTO FAULTS. 17

the north. No precise measurement of the amount of displacement along
the east base of Prospect Ridge can be given, but estimating it from the
known thickness of the strata lying between the summit of the Eureka
quartzite and the base of the Lower Coal-measures as given in the Eureka
section, we have a vertical movement of 12,800 feet. Now, if we suppose,
and it seems highly probable, that there are 300 or 400 feet of limestones
beneath the beds exposed at the surface, and that the upper portion of
the Eureka quartzite is also wanting, we have a displacement of over 13,000
feet. Probably the vertical movement at its maximum displacement
amounted to more than 2J miles, lying wholly within Paleozoic rocks.

Ruby Hill Fault.— The branch fault which leaves the main one just after
it enters New York Canyon from the south trends northwesterly across the
slope of Prospect Ridge, thence across Ruby Hill, probably connecting
with the Spring Valley fault although it has never been traced beyond the
Richmond and Albion mines. It has been designated the Ruby Hill fault.
On the atlas sheet its course is indicated only a short distance beyond the
Jackson fault, its true position on Ruby Hill not having been accurately
located until after the printing of the map. Although the Ruby Hill fault
possesses features of great economic importance bearing upon the ore de-
posits of the district, it is by no means so profound a displacement as the
Hoosac and is measured by hundreds instead of thousands of feet. The
dynamic movements which produced it have not influenced in any marked
manner the structural features of the country, presenting, in this respect,
the greatest possible contrast with the main Hoosac fault. There is some
reason for the opinion that the Ruby Hill fault is of later date than the main
fault, and belongs to the period of Tertiary eruptions. A more detailed
description of this fault will be found in the chapter devoted to the discus-
sions of the ore deposits.

Pinto Fault.— This fault is situated about 2 miles to the east and nearly
parallel with the Hoosac fault, which it closely resembles in structural
features. Like the Hoosac, its course can not be traced with precision, yet
the geological characters are so distinctive that there exist scarcely any
difficulties in the way of determining its main trend across the mountains
as it sharply defines the boundary between the elevated County Peak and
MON xx 2

18 GEOLOGY OF THE EUREKA DISTRICT.

Silverado block on the one side and the depressed Spring Hill and Carbon
Ridge block on the other. On the west side, wherever the volcanic and
detrital material fails to conceal the underlying rocks only Carboniferous
strata are exposed, whereas, on the opposite side Silurian strata every-
where rise above the fault line in bold and abrupt ridges.

Starting from the southern end of the mountains the fault follows up
Pinto Valley, with Carbon Ridge on the west and English Mountain on the
east, the intermediate valley being filled with pumices and tuffs. Not
until nearly opposite Dome Mountain do the sedimentary beds on both
sides of the fault come in direct contact at the surface, but here we find
the Lower Coal-measures limestone brought up unconformably against the
Lone Mountain limestone. From here a deep, narrow limestone gorge
extends northward, along which the limestones of the two different
epochs stand out boldly on opposite walls, the direction of the gorge coin-
ciding with the line of the fault. Where the drainage channel following
the gorge turns abruptly toward the west the Eureka quartzite comes in
beneath the Lone Mountain strata, but the fault, without deviating in the
least from its course, continues northward with the Carboniferous limestone
still on the west side. A short distance farther northward the sedimentary
strata are buried beneath the lavas of Richmond Mountain. The vertical dis-
placement along the Pinto is probably quite as great as that found along
the Hoosac fault; the same geological horizons are here brought into juxta-
position, although higher beds form the contact along the Pinto fault, and
at Carbon Ridge the Weber conglomerates come in as the uppermost beds.
The enormous development of Devonian strata and the Diamond Peak
quartzite, which, as shown by the section, have an estimated thickness of
11,000 feet, is wholly wanting.

Rescue Fault.— About 2| miles east of the Pinto fault, and on the east side
of the Silverado and County Peak block, runs the equally persistent but
less profound Rescue fault. It derives its name from Rescue Canyon,
which, in turn, owes its origin primarily to the fault. The canyon, a longi-
tudinal mountain valley nearly 2 miles in length, opening out into Fish
Creek Basin, is now occupied for the entire distance by rhyolite extrava-
sated along the course of the fault, At the head of the canyon the rhyolite

PROSPECT RIDGE. 19

gives out and the fault enters the Nevada limestone with a course a little
east of north, and follows along under the abrupt east wall of Sugar Loaf.
A short distance beyond Sugar Loaf the fault coincides with the contact of
the Nevada limestone with the White Pine shale, maintaining this course
until both the limestone and shale pass beneath the basalt tableland toward
the north. That the fault continues beyond this point beneath the basalt
is clearly established by geological structure, the Devonian strata of County
Peak passing under the tableland on the west side and the Weber con-
glomerate and Upper Coal-measures dipping toward it and passing beneath
it on the east. There can be no doubt that the Rescue fault sharply defines
a great physical break separating the County Peak from the Diamond Peak
block. After entering the region occupied by the basalt field, there is 110
means of determining the precise course of the fault, everything being
obscured by recent lavas. Upon leaving the basalt area the fault probably
follows along the east base of Richmond Mountain, but is hidden beneath
the andesitic rocks that, flowing eastward, rested against the base of the
gently inclined slopes of the Upper Coal-measure limestones of the Dia-
mond Range. Beneath the lavas the trend of the fault, while in a great
degree conjectural, can not vary far from the course of the contact between
the Nevada limestone and the White Pine shale as exposed to the south and
the line of the Carboniferous rocks to the north and east. In the region of
the volcanic rocks the displacement along the fault can not be measured,
although it must be very great, as is shown by the Devonian beds on the
one side and the upper members of the great development of the Carbon-
iferous sediments on the other. South of the basalt the fault runs wholly
within the limits of the upper portion of the Nevada limestone, or else at
the base of the White Pine shales. Nowhere along its entire course, from
Packer Basin to Fish Creek Valley, does the downthrow apparently exceed
3,000 feet of vertical displacement.

GEOGRAPHIC BLOCKS.

Prospect Ridge.-This ridge stands out as the most prominent orographic
feature of the Eureka Mountains. It is situated in the very center of the
mountains and presents a bold, serrated outline, extending with an approx-

20 GEOLOGY OF THE KUKKKA DISTRICT.

imately north and south trend from Diamond Valley to the Fish Creek
Basin. From Diamond Valley the northern slopes rise gradually out of the
plain to the summit of Ruby Hill, beyond which the mountains assume a
more rugged aspect, continuing southward in an unbroken ridge until cut
off sharply by eruptive masses or concealed beneath Quaternary accumula-
tions of the valley.

As already described, this orographic block is sharply outlined along it:?
entire eastern base by the Hoosac fault, evidence of which is shown in the
geological character of the opposite walls and in the extravasated rocks that
have broken out along the line of dislocation. The Spring Valley, Prospect
Mountain, and Sierra faults as clearly define it on the west, except that
along the entire length of these combined faults no lavas reach the surface.
The Sierra fault marks a more decided geological than topographical break,
since along the displacement an intricate and confused mass of mountains
unites Prospect Ridge with the country to the west of it, the Silurian and
Devonian rocks resting against the Prospect Mountain limestone high up on
the summit without any intervening valley or depression. With these
clearly defined boundaries the Prospect Ridge block measures 10 miles in
length and across its broadest development, in the region of Prospect Peak,
between 2 and 2^ miles in width. Topographically this mountain block is
quite simple — a longitudinal ridge rising abruptly on the west side with
Prospect Peak, the culminating point, descending for 2,500 feet toward
Spring Valley with an average slope of 30°, but on the east side falling
away much more gradually and with far less regularity towards the Hoosac
fault.

In structure Prospect Ridge is an anticlinal fold, and affords an admir-
able example of such structure, accompanied by profound north and south
faults approximately parallel with the strike of the beds. The axis of the
fold lies wholly on the western side of the ridge and is well shown on the
slopes of Prospect Peak, the beds on both sides of the axial plane standing
inclined at an angle of nearly 80°. While the crest of the ridge trends
north and south, the axis of the fold, striking west of north, follows obliquely
down the slope and is finally lost in the valley toward the west. The rocks
which constitute this great body of folded strata between the two lines of

FISH CREEK MOUNTAINS. 21

faulting present a conformable series of sediments inclined throughout their
entire thickness at angles seldom less than 75°.

From the axis of the anticline, near the summit, on the west side of
Prospect Peak, to the Hoosac fault along the eastern base of the ridge,
there is exposed a series of strata measuring nearly 10,000 feet in thickness,
and wholly made up of Cambrian and Silurian rocks. The axis of this
fold occurs in the Prospect Mountain quartzite, the underlying member of
the Cambrian, and is in turn overlain successively by the Prospect Moun-
tain limestone, Secret Canyon shale, Hamburg limestone, Hamburg shale,
Pogonip limestone, and Eureka quartzite. Along the Hoosac fault the
Eureka quartzite is well exposed at Caribou Hill, McCoy's Ridge, Hoosac
Mountain, and the narrow ridge east of Round Top.

Prospect Ridge affords the grandest section of Cambrian rocks yet
recognized in the Great Basin, and with the exception of one or two insig-
nificant exposures of slight importance east of the Sierra fault, the rocks of
this period are confined to this orographic block. Section CD-EF (atlas
sheet xni), constructed across the central portion of the Eureka Mountains,
intersects Prospect Ridge about 3,000 feet to the north of the peak at a
point well chosen to bring out the anticlinal structure of the uplifted block
and its relations to the fault lines. There is represented on PI. n, Fig. 4, a
geological section drawn at right angles to the strike of the beds across the
culminating point of Prospect Peak, from Spring Valley to the Hoosac
fault. The Prospect Mountain limestone is here shown capping the peak
and the entire east slope, and it is again exposed at the base of the ridge on
the west side of the anticline, rising above the detrital material of Spring
Valley. In Fig. 3 of the same plate will be found a section of the same
strata across Ruby and Adams Hills. Here the beds are inclined at a much
lower angle, otherwise the structural features and succession of strata are
nearly identical, Ruby Hill coi-responding to Prospect Peak and Adams
Hill to the Hamburg Ridge, with the intermediate Secret Canyon shale
occupying a depression between them.

Fish creek Mountains.— To the southwest of the Sierra fault the character
of the country changes, and a confused and intricate series of ridges come
in, presenting a strong contrast to the adjacent region. In place of the

22 GEOLOGY OF TIIE EUKEKA UISTEIGT.

single ridge structure, as seen toward the north, the configuration of
the country shows a broad, rough mass of mountains, from 4 to 5 miles
in width, of very diversified topographic forms and deeply scored by
narrow gorges. In the region of Atrypa Peak, Gray's Peak, and Lookout
Mountain a classification of the mountain masses becomes a matter of much
difficulty, the orographic structure being complex, and the resultant of
forces in some respects different from those which elevated Prospect Ridge
or the Fish Creek Mountains. Southward from Castle Peak the latter
mountains become a distinct range, and with a north and south trend stretch
off southward several miles beyond the limits of this survey. They are
situated in the extreme southwest corner of the Eureka District, and are
sharply defined by the broad valley of Fish Creek on the one side and An-
telope Valley on the other, which partially disconnects them from the
Eureka Mountains. They measure about 5 miles in width and rise over
2,000 feet above the adjoining Quaternary plain. They present the im-
pressive appearance of a solid mountain mass gently inclined to the west,
but falling off somewhat abruptly on the east, accompanied by a steep es-
carpment just beneath and parallel with the summit of the ridge. The
structure is that of an anticlinal fold whose axial plane coincides with the
escarpment along which there has been a downthrow of 600 feet. The
origin of the escarpment is due to the faulting. At the base of the cliff the

•

faulted strata are uniformly inclined toward the valley at an angle of
about 15°. Along the west side of the anticlinal axis the beds lie at much
lower angles, exhibiting first a slight synclinal fold followed by an equally
gentle anticlinal, beyond which for nearly 2 miles they fall away with a
nearly uniform dip toward Antelope Valley.

The Fish Creek Mountains may be considered as essentially made up
of Silurian rocks, in marked contrast with Prospect Ridge, which is, as has
been already shown, formed of Cambrian strata with outlying slopes of
Pogonip limestone and Eureka quartzite. Here are exposed the two lower
members of the Silurian in a manner which can hardly be excelled for sim-
plicity of structure elsewhere in the Great Basin. Nearly all the more ele-
vated portions of the mountains consist of Upper Pogonip limestone, the
axis of the fold occurring not far below the top of the horizon. The Eureka

MAHOGANY HILLS. 23

quartzite overlies the limestone on both sides of the mountains, but as the
dip of the strata coincides closely with the inclination of the western slope,
it comes to the surface only near the base of the ridge. As the strata dip
away both to the north and south from the central body of Pogonip lime-
stone, a belt of the quartzite may be observed encircling it on all sides.
Nowhere do the Fish Creek Mountains expose a section of the Pogonip
limestone for more than one-quarter of its thickness, as given in the general
section, although numerous excellent partial sections are shown of the
Upper Pogonip beds. Northward of Bellevue Peak, and in the region
of Castle Mountain, the Lone Mountain, limestone overlying the Eureka
quartzite comes to the surface, and again at the southern end of the range,
but beyond the limits of the map.

From this description, and by the aid of the map (atlas Sheet xi), a clear
idea may be obtained of the broader features of the Fish Creek Mountains,
and in the chapters devoted to the Silurian rocks and the descriptive geology
there will be found the evidences in detail for the conclusions presented
here as to their age and structure."

Mahogany Hills.— The Mahogany Hills are situated on the west side of the
Eureka Mountains. They occupy by far the largest area of any of the
mountain blocks into which the country has been divided, and are as sharply
denned as any of the others by natural physical outlines. Spring Valley and
Canyon serve as an excellent boundary between them and Prospect Ridge,
but everywhere else, except along the narrow belt which connects them with
the Fish Creek Mountains, the broad Quaternary plain rests against the
upturned edges of the outlying ridges. From Spring Valley the Mahogany
Hills extend westward, a mountain mass over 8 miles in width ; in a north
and south direction they present an unbroken body of limestone, 12 miles
in length. This broad mountain mass maybe divided into two nearly equal
parts, separated by the level plain of Dry Lake and the narrow gorge of
Yahoo Canyon, the lake at one time draining northward through the canyon
into Hayes Valley. The country to the east of the lake and canyon,
while it has much in common with the western side, is, in structural
features, closely related to the Pinon Range. This latter range, which is
made up of a number of longitudinal ridges extending from the Humboldt

24 GEOLOGY OP THE EUKEKA DISTRICT.

River to the Eureka Mountains, may be said to terminate at the deeply
eroded pass known as The Gate, as it there loses its distinctive features.
The monoclinal character of the uplifted ridges is, however, still maintained
nearly to Spanish Mountain, or until cut off" by the Spring Valley fault.

From Dry Lake westward the mountains rise abruptly, frequently in
steep cliffs, presenting a somewhat monotonous aspect of dark bluish gray
limestone covered with a scanty growth of mountain mahogany (Cercocarpns
laxlifolius), from which the region derives its name. A few culminating
points attain elevations above the general level, but these gradually fall
away to the westward in long uniform ridges, sharply denned by drainage
channels that cut down hundreds of feet into the limestones with nearly
vertical escarpments

Mahogany Hills are made up for the most part of Nevada limestone,
which everywhere forms all the more elevated portions. Silurian rocks
occur in one or two localities, but principally at Spanish Mountain, where
the Eureka quartzite is admirably shown, with all its peculiarities of struc-
ture, overlain by the Lone Mountain limestone, which in turn passes con-
formably into the Nevada limestone. For purposes of stratigraphical geol-
ogy, the position of Spanish Mountain is most fortunate, as its relation to
the overlying Devonian limestone is well brought out, while its relation to
the underlying limestones and shales of the Lower Silurian and Cambrian
is demonstrated beyond question in both the Fish Creek Mountains and
Prospect Ridge. Spanish Mountain happens to be the only area of Eureka
quartzite in the Mahogany Hills. On the southern slope of Comb's Peak
the upturned beds afford an excellent exposure of the limestones overlying
the Eureka quartzite, and give a section of Lone Mountain rocks lower than
found elsewhere, including a series of beds whose geological position is
determined by a characteristic Trenton fauna. The relationship of this fauna
just above the Eureka quartzite to the fauna found elsewhere immediately
below the quartzite offers an important link in the paleontological history
of the Eureka District. One of the best sections across the Nevada limestone
may be found on the ridge north of Modoc Peak, where the beds throughout
a great vertical thickness present a nearly uniform strike and dip, with but
little disturbance or dislocation. The Modoc section measures about 5,400 feet

SILVEEADO AND COUNTY PEAK GROUP. 25

in thickness. It is given in detail in the chapter devoted to a discussion of
the Devonian rocks, on page 66.

Silverado and County Peak Group.— This mountain block stands almost com-
pletely isolated from the others, being cut off by profound faults on all
sides, along which igneous rocks have reached the surface in enormous
masses. In this way it is clearly outlined from the Diamond Range on the
northeast by the broad basalt table of Basalt Peak and the Strahlenberg,
on the north by Richmond Mountain, and on the west in great part from
Carbon Ridge and Spring Hill group by the extravasated rocks along the
Pinto fault. A glance at the map will show how closely these lavas sur-
round the mountains and there is good reason to believe that if the Quater-
nary deposits along the foothills were removed this encircling belt of lavas
would be still more noticeable. Here and there a few isolated patches
of lava rise above the level of the plain in Fish Creek and Newark valleys,
but in most instances the exposures occupy too limited areas to permit
of their being located upon the map. The outlines of the knobs and knolls
of rock partially concealed by recent deposits indicate their probably vol-
canic origin.

The mountains are roughly broken up into three groups — northern,
southern and southeastern. Wood Valley, a relatively broad drainage
channel open to the west, and Charcoal Canyon, a narrow but deep ravine
south of Sentinel Peak on the east, separate the two former, while the latter
is somewhat isolated by the deep valley of Rescue Canyon and an arm of
Newark Valley. For convenience the northern region may be designated
as the County Peak Mountains, the southern as the Silverado group, and
the region to the southeast as the Alhambra Hills. Taken together they
stretch from Fish Creek Valley to Richmond Mountain and in an east and
west direction from the Pinto fault to the Quaternary plain.

Between the two great lines of displacement, the Pinto and Rescue
faults, the broad mass of limestone presents a gentle synclinal structure,
the beds dipping toward the center from both fault lines and away from
the lines of igneous outbursts. The mountains are almost wholly made up
of limestones belonging to the Silurian and Devonian periods, all the more
elevated portions being formed of characteristic strata of the middle and

26 GEOLOGY OF THE EUEEKA DISTEICT.

upper portions of the Nevada limestone. At the extreme northeast corner
the Eureka quartzites occupy a small area, but are of no special importance
themselves except in determining the basal rocks of this elevated mass and
the position of the overlying' strata. Numerous narrow gorges with mural-
like faces cut deeply into the limestones, affording excellent comparative
sections across the strata, datum points being readily established by the
brown, red and gray beds of the middle Devonian. Represented in this
uplifted mass occur between 6,000 and 8,000 feet of limestones. That the
upper beds of the Nevada epoch are represented here is shown just to the
east of Sugar Loaf and Island Mountain where the White Pine shales lie
conformably upon the uppermost beds of limestone.

Diamond Mountains.— This range is one of the best denned mountain up-
lifts on the Nevada plateau, extending 40 miles along the east side of Dia-
mond Valley. Only the southern end of it, however, in the northeast corner
of the map, comes within the limits of this survey, as the range properly
terminates with Newark Mountain. Its immediate proximity to the County
Peak limestones, from which it is separated only by an overflow of igneous
rocks, relates it in the closest possible manner with the Eureka Mountains.
Diamond Peak (10,637 feet), the highest and broadest in the range, lies
within the limit of this survey, and the geological structure and continuity
of beds exposed upon the flanks of both Diamond Peak and Newark Moun-
tain, add greath' to our knowledge of the sequence of Paleozoic sediments.
For the greater part of its length Carboniferous rocks flank both sides of the
Diamond Range, and, as is so often the case tliroughout Nevada, no beds
immediately underlying them had previously been recognized toward the
north. Here, however, Newark Mountain consists exclusively of Devonian
rocks passing beneath the east base of Diamond Peak, where they are con
formably overlain by an immense thickness of Carboniferous beds. New-
ark Mountain rises abruptly out of the plain and offers a typical example,
so common in the Great Basin, of an anticlinal ridge with one side of the
fold dropped down along the line of the axial plane. In this instance the
downthrow lies on the east side and the mountain presents along the summit
a bold escarpment 1,000 feet in height, facing Newark Valley. At the base
of the escarpment easterly dipping beds come in, and dark blue massive lime-

DIAMOND MOUNTAINS. 27

stones of the Upper Devonian form the remainder of the steep slope for
about 1,000 feet and then stretch far out into the valley in a line of low
hills and isolated Imttes, still dipping toward the east. The entire western
side of the mountain, including the summit of the ridge, dips uniformly
toward the west, and is in turn overlain by the White Pine shales through
which Hayes Canyon has been eroded. On the north side of Newark
Mountain these flexible shales curve around to the northeast and form the
east base of Diamond Peak, only the uppermost beds of the Nevada lime-
stone here appearing above the level of the valley, the remaining portion
of the Devonian beds upon both sides of the fold having dropped completely
out of sight.

Diamond Peak rises above Newark Valley over 4,000 feet, with an
exceptionally steep slope, the White Pine shales presenting smooth rounded
ridges along the base of the mountain. The shales are overlain by a great
thickness of rough and rugged Diamond Peak quartzites, followed by the
Lower Coal-measure limestones which for a long distance form the summit
of the ridge. In its structure the Diamond Range is in strong contrast with
the anticlinal structure of Newark Mountain, presenting a synclinal fold
whose axis lies in the Lower Coal-measures. The identical series of beds
found dipping into the peak on the east side come in again on the west
side, but with a reverse dip, except that the White Pine shales are not
brought to the surface, owing to a longitudinal fault which extends along
the west side of Diamond Peak, completely cutting them off and bringing
up still higher Carboniferous formations than those found near the summit.
From the axis of the anticline on the east slope of Newark Mountain diag-
onally across Diamond Peak there is exposed an admirable section, includ-
ing Nevada limestones, White Pine shales, Diamond Peak quartzites, and
Lower Coal-measure limestones. The geological importance of this section
lies in the fact that it offers, across the middle of the Paleozoic rocks, a con-
formable and continuous series of beds rarely found elsewhere, uniting the
upper Paleozoic with the great development of Silurian and Cambrian rocks
beneath. From Bold Bluff, at the southern end of Diamond Peak, the New-
si rk fault brings the Lower Coal-measures against the White Pine shales, the
entire development of Diamond Peak quartzite having been displaced along

28 GEOLOGY OF THE EUREKA DISTRICT.

the west side of Newark Mountain. North of Newark Mountain, however,
the limestones occupy their true geological position, overlying the quartzite
and dipping westerly.

Alpha Ridge for its entire length is made up of Lower Coal-measure
limestones uniformly inclined toward the west and in turn overlain by the
Weber conglomerates and Upper Coal-measures. In the Weber conglom-
erates there is a synclinal and anticlinal fold, the latter being well shown in
long narrow ridges stretching in north and south lines parallel with the
bedding. Of the Upper Coal-measures there occurs only a limited expo-
sure above the conglomerates, but they are admirably displayed with their
stratigraphical position well brought out and their geological age deter-
mined from ample paleontological evidence.

In the area north of Newark Canyon, stretching northward as far as
the limit of the map and west of the Alpha fault, a north and south fault
011 the west side of Alpha Peak ridge, occurs an inclined table "wholly
made up of Upper Coal-measure limestones. Its identity upon both lith-
ological and paleontological grounds, with the body of Carboniferous lime-
stones overlying the Weber conglomerates south of Newark Canyon seems
conclusive, and the finding of Carboniferous species unlike those known to
occur in the Lower Coal-measures at Eureka and characteristic of the Upper
Coal-Tneasures elsewhere establishes the geological position of these beds.

Carbon Ridge and Spring Hill Group.— This block OCCUpieS a far less COUSpicU-

ous position than any of the others, and seen from any commanding
point of view it would not be in the least likely to attract attention as a

prominent physical feature of the country. Unlike the adjoining uplifted
blocks which rise boldly out of the plain, this one has rather the appear-
ance of a depressed region without any persistent or distinctive character-
istics. Nevertheless, it is sharply defined, geologically, by parallel lines of
displacement, the Hoosac and Pinto faults. On the one side rises Prospect
Ridge and on the other rises the broad mass of County Peak and Silverado
Mountains. This relatively depressed block measures 6J miles in length,
but between the faults has an average width of only If miles. Estimating
from the thicknesses of the different epochs given in the Eureka section
both faults show profound vertical displacements of 12,000 to 15,000 feet.

rARBON RIDGE AND SPUING HILL GROUP. 29

Embraced within these lines of faulting only Carboniferous beds are exposed,
whereas the inclosing outer walls on botli sides consist of Silurian rocks
traceable the entire length of the mountains except where concealed by
volcanic overflows. Fissures along these fault lines have served as conduits
for extravasated lavas, through which have poured out, either upon one side
or the other, vast accumulations of volcanic material, for nearly the entire
length of the mountains. So extensive have been these flows over the
Carboniferous rocks that not oidy have the fault planes become obscured,
but large areas of the sedimentary beds lie concealed beneath the lavas,
while in the region of the Hoosac Mountain they have so spread out
over the country as to completely bury all the underlying rocks between
the two faults. Naturally such an amount of volcanic energy displayed
all along the line has broken and dislocated the strata, caused minor fault-
ings and displacements, and over much of the area rendered it difficult, if
not impossible, to work out the structural relations of the exposed beds.
Many fractures and breaks in the inclosed rocks, although not of any great
magnitude, are frequently sufficient to render any precise measurement of
the beds impossible, the amount of faulting being undeterminable. On
the other hand great blocks of strata have been tilted up at high angles
with only slight disturbances, affording fairly good cross-sections.

The volcanic rocks separate the sedimentary beds, which otherwise
would form a continuous body, into two or more distinct areas, the northern
known as the Spring Hill group and the southern as the Carbon Ridge,
while between them lies a much smaller area of limestones every where sur-
rounded bv eruptive rocks. The middle area serves in a measure to connect
the other two, the same beds found here occurring both north and south.
Across the southern end of Spring Hill, where the strata are less dis-
turbed than elsewhere, the limestones present a synclinal fold whose axis
lies on tlfe west side of the ridge east of Spring Hill. Adjoining the
Hoosac fault lies a low, narrow ridge separated from the main body of lime-
stone by a north and south fault, beyond which the limestones on Spring
Hill dip easterly at an angle of 30°, the beds on the opposite side of
the fold attaining angles as high as 60° westerly. Measured on the line
of the main section there are about 3,400 feet of limestones included

30 GEOLOGY OF TI1K EUREKA DISTRICT.

between the Hoosac and Pinto faults. This entire series of beds belongs to
the Lower Coal-measures, evidence of their age being found in the charac-
teristic fossils obtained at both the top and the bottom of the limestones. Car-
bon Ridge possesses a simple structure, a single block inclined uniformly to
the east, the beds varying slightly from 60°. Here, however, the position
of the uppermost beds of limestone is determined by the overlying Weber
conglomerates. Limestones form the west base and crest of the ridge, the
conglomerates coming in all along the east slope and stretching out toward
the Pinto fault until buried beneath the acidic pumices and tuffs. The
limestones afford about the same thickness of beds as developed on Spring
Hill, and the overlying Weber conglomerates measure 1,900 feet, assum-
ing a uniform dip and the absence of all faulting. This series of beds
of Lower Coal-measure limestones and Weber conglomerates is similar to
the section exposed on Alpha Ridge and Weber Peak in the Diamond Moun-
tains, the thickness being about the same. It is the sequence of strata most
commonly met with in the Great Basin ranges wherever we find a broad
limestone body overlain by one of sandstone.

TERTIARY ROCKS.

Tertiary Lavas.— Subsequent to the movements that folded and faulted
by powerful dynamic forces this great body of Paleozoic strata came the
pouring out of volcanic lavas, the only other rocks that play an important
part in the geological history of the Eureka Mountains. These lavas were
forced to the surface not only after the crumpling of the beds and blocking
out of the mountains, but after very considerable erosion had carved the
deepest canyons and brought about the configuration of the country much
as it is seen to-day. Evidence of this erosion before the pouring out of the
lavas is shown by the position of many extensive bodies of lava in the
bottoms of the largest canyons, and by the blocking up of ancient drainage
channels through the welling out of erupted masses, necessitating new outlets.
It is evident that a very long period of time must have elapsed subsequent
to the building up of the Paleozoic masses before the breaking out of the
lavas. Although no direct evidence of the age of these lavas can be found
in the Eureka District, they are regarded as belonging to the Tertiary

QI ATi;i!NAl;V I >K POSITS. 31

period. In many ways they bear the closest resemblance in their mode of
occurrence, to similar lavns elsewhere in the Great Basin, when; evidence of
their age has been determined by their relation to sedimentary strata carry-
ing a Miocene or Pliocene fauna or flora. In mineral and chemical composi-
tion the lavas show great variations, hornblende-andesite, dacite, rhyolite,
pyroxene-andesite, and basalt being well represented, with a wide range in
structural and physical features. A description of these different lavas and
their relations to each other, as well as their geologic-i 1 relations to the
orographic blocks, will be found in the chapter devoted to a discussion of
the Tertiary rocks.

QUATERNARY DEPOSITS.

Quaternary Valleys.— The Eureka Mountains rise out of a broad plain
everywhere covered by Quaternary deposits that stretch away in all direc-
tions far beyond the limits of the present survey. The atlas sheets accom-
panying this work fail to indicate the relative area occupied by the moun-
tains to that of the desert plains, but an extension of the map only a few
miles more on all sides would at least have shown how completely the
mountains were surrounded by a broad expanse of the so-called sage-brush
deserts. With a single exception these broad plains open one into the
other, the only barrier being the Diamond Mountains, which separate Dia-
mond Valley from Newark Valley.

Newark Valley and Fish Creek Basin are simply extensions of the
same great plain, the former situated on the east and the latter on the south
of the Eureka Mountains. The Fish Creek Basin connects, by means
of a narrow pass south of the Fish Creek Mountains, with Antelope Valley,
a few miles beyond the limits of the map. Antelope Valley may be re-
garded as a southern extension of the broad, desert-like expanse of Hayes
Valley, which stretches far toward the north on the west side of the Pinon
Range. Hayes Valley connects with Diamond Valley by the narrow
gorge known as The Gate, which is simply a low pass cut down to the
level of the plain through which the former valley at one time drained into
the latter.

Little time has been devoted to the investigation of the Quaternary
geology in the immediate region of Eureka, but so far as the deposits have

32 GEOLOGY OF THE EUKEKA DISTRICT.

been studied they resemble closely those found in the neighboring valleys,
and do not offer much of special or local interest.

During the Quaternary period vast accumulations of detrital material
were brought down from the mountains and transported far out upon the
neighboring plain or laid down upon the flanks of the outlying foothills.
These deposits have been classed under two distinct epochs — an upper and
a lower Quaternary.

Lower Quaternary.— The earlier deposits, or the lower Quaternary, are
for the most part lacustrine, made up of finely comminuted stratified sands
and clays carrying varying amounts of calcareous material. All the beds
have a prevailing light yellowish color. - They form the so-called alkali
flats of Nevada, and when dry resemble a hard tile pavement, but when
moist have all the disagreeable qualities of a plastic clay, well nigh impas-
sable. Nowhere within the neighborhood of Eureka have they been cut by
water channels for more than a few feet, and at the time of our investigation
no deep borings for water had been made. In consequence no reliable data
exist for a correct estimate of their thickness, which in places ma}- reach
several hundred feet. No recent shells have as yet been found in the few
exposures observed along the stream beds. On the map the line of de-
marcation between the upper and lower divisions of the Quaternary has
been drawn somewhat arbitrarily, it being by no means easy to separate,
sharply, the finer material of the upper series from the lacustrine deposits
underlying them.

Upper Quaternary.— The upper or mountain Quaternary is made up of
angular material varying in size from large bowlders to fine sand and
gravel. It is in all cases traceable to the neighboring mountains, the nature
of the coarser fragments depending upon the rock exposure above it. The
material is subaerial in origin. It everywhere fringes the flanks of the
mountains, encroaching upon the area of the underlying lacustrine beds for
shorter or longer distances, according to the configuration of the hill-slopes
or the transporting power of floods and freshets. The finer material is, nat-
urally, transported the greater distance, consequently it gradually becomes
mingled with and forms a superficial layer over the lower Quaternary de-
posits. South of Prospect Peak and opposite the entrance to Secret Can-

UPPER QUATERNARY. 33

yon, these upper Quaternary accumulations extend up the flanks of the
mountains for 1,500 feet above the lowest part of Fish Creek Valley, every-
where concealing- the nature of the underlying rocks.

Most of the intervening meridional valleys lying between the parallel
ranges of Nevada consist of narrow, trough-like depressions, in compaii-
son with the level plains bordering the Eureka Mountains. In western
Utah and eastern Nevada these valleys exhibit great similarity as regards
their physical and geological history. They have been described at great
length by Mr. Clarence King1 and Mr. G. K. Gilbert,2 both of whom have de-
voted much time to the study of the Quaternary accumulations and the cli-
matic conditions under which the material was laid down. Many local details
of these valleys may also be found in the volume devoted to the descriptive
geology of the Fortieth Parallel Exploration,3 and the reader who desires
to pursue the subject further is referred to the works quoted.

1 U. S. Geol. Explor. of the Fortieth Parallel, vol. I. Systematic Geology.

2U. S. Geol. Surv., Monograph I. Lake Bonneville.

3 U. S. Geol. Explor. of the Fortieth Parallel, vol. n. Descriptive Geology.

5ION

CHAPTER III.
CAMBRIAN AND SILURIAN ROCKS.

CAMBRIAN ROCKS.

Rocks of the Cambrian period, with the exception of two small expos-
ures, are confined to Prospect Ridge, forming all the more elevated por-
tions and the steep slopes of both sidels. Indeed, the ridge is almost
wholly made up of Cambrian sedimentary beds. Silurian rocks perfectly
conformable with the upper beds of the Cambrian come in only along the
outlying spurs and foothills to the east and north. All along the east slope
of the ridge these beds exhibit a nearly uniform thickness, but attain their
greatest development in the region of Prospect Peak, where the lowest
members of the group are best exposed. Here the Cambrian rocks measure
about 7,700 feet from base to summit. They have been divided into five
epochs, designated by local names, as follows: Prospect Mountain quartzite,
Prospect Mountain limestone, Secret Canyon shale, Hamburg limestone,
and Hamburg shale. The varied physical differences in the composition of
the sediments cause them to fall readily into these five epochs, each char-
acterized by its own distinctive geological and topographical features.
The fauna also agrees with geological divisions and adds its own evidence
to strengthen them. So far as known, nowhere else in the state of Nevada
do the Cambrian rocks afford as fine geological sections as at Eureka ; nor
have they elsewhere been subjected to as careful a survey. The great
thickness of the group, the simplicity of structure in the region of Prospect
Peak, the slight rnetamorphism of the strata, and the uniformity of dip
over wide areas and across many thousand feet render a study of the sedi-
ments a comparatively simple matter and far easier than most Cambrian
areas in other regions of the world.

PROSPECT MOUNTAIN QUARTZITE. 35

Prospect Mountain Quartzite.— This group lies at the base of the Cambrian
series at Eureka and is consequently the oldest sedimentary rock exposed.
It takes its name from the peak, the highest point along the ridge, where it
reaches its broadest development and forms the greater part of its western
slope. With one or two breaks in the continuity, the quartzite may be
traced along the base of the ridge northward to Ruby Hill, where, as the
footwall of the Richmond and Eureka Consolidated Mines, it becomes
of considerable economic interest. There can be no question that the
quartzite of Prospect Peak and that of Ruby Hill are identical. From
Ruby Hill the quartzite curves around the end of the mountain, following
the east side of the ridge, and stretches southward for more than a mile
until abruptly lost by a fault. The only occurrence in the district of this
quartzite is on Prospect Ridge. On Prospect Peak the strata have a thick-
ness of 1,500 feet and occur distinctly bedded, but in some localities all
lines of stratification appear to be wanting. At the base of the series the
beds are largely composed of conglomerates and brecciated masses firmly
cemented together with ferruginous material, with the weathered surfaces
deeply stained by iron. In the conglomerates quartz pebbles may occasion-
ally be seen, showing compression and flattening on their broader sides,
arranged in beds parallel to the planes of stratification. The upper beds
are usually finer grained, carrying less iron oxide. In the Charter Tunnel,
the only locality where they have been exposed by mining exploration,
they show highly metamorphosed beds derived from impure siliceous mate-
rial.

Interstratified throughout the quartzite are occasional bands of fine
grained arenaceous and micaceous shales only a few feet in thickness. No
organic remains have been found in this group, although diligent search
was made in the interstratified shales, as, if they occur, they would be of
the highest paleontological interest, extending the Cambrian fauna lower
than has yet been known in the Great Basin. The Prospect Mountain
quartzite differs from the Eureka quartzite, the next overlying siliceous
group, in being more ferruginous and in general less uniform in texture,
carrying throughout more or less clayey material, while the latter quartzite
is a nearly pure, highly altered sandstone.

36 GEOLOGY OF THE EUEEKA DISTRICT.

Prospect Mountain Limestone.— Directly over the Prospect Mountain quartz-
ite occurs the Prospect Mountain limestone, which forms the greater part of
the ridge and both slopes of the mountain all the way from Ruby Hill
southwai'd to the entrance of Secret Canyon. Beyond the limits of the
mountain these beds are unknown in the district. It is difficult to define
sharply the characteristic features of this group, changes are so frequent in
the deposition of the sediments, not only in the vertical, but lateral extension.
Secondary alterations caused by the intrusion of eruptive rocks and vari-
ations in color near the ore bodies tend to conceal the original nature of
the rock. Breccias firmly held together by calcite are of common occur-
rence, while throughout the group there is abundant evidence that the beds
have been crushed and broken and subjected to an enormous pressure. In
general, however, the group possesses a light bluish gray tint when observed
over large areas, although nearly all colors from white to black are found
in the limestone, which at the same time is characterized throughout the
entire thickness of beds by seams of calcite varying from one-half to 6
inches in width, and frequently forming a network of white bands.

In texture the limestone is crystalline and granular and over wide areas
is so highly altered as to obliterate all traces of organic life; and, while in,
places planes of bedding may be distinctly seen all the way from Ruby
Hill southward, they are Avholly wanting over the greater part of the ridge.
Stratification is well shown on the seventh level of the Richmond Mine and
in the Eureka and Prospect Mountain tunnels, where the beds are usually
bluish gray in color.

Coarse and fine white marbles, occasionally highly crystalline, are
found on the north end of the mountain, and white and light gray marbles
more than 600 feet in width are cut by the Prospect Mountain tunnel,
good varieties being observed at 750 feet and again at 1,700 feet from the
entrance of the tunnel. Analyses show them to be nearly pure carbonate of
lime. Characteristic black limestone is found near the Geddes and Bertrand
Mine, in Secret Canyon.

Numerous analyses of the rock from Ruby Hill, Prospect Mountain
Tunnel, and localities on both sides of the ridge prove that the beds
throughout the formation are a magnesian limestone. Nearly pure dolo-

ANALYSES OF LIMESTONE.

mites in thin layers have been recognized in several localities, but the per-
centage of carbonate of magnesia in most instances is too low to allow the
beds, for any considerable thickness, to be classed as dolomite, neither is
there any evidence that dolomitic rock is characteristic of any particular
portion of this great thickness of beds. Both dolomite and pure limestone
have been shown to occur near the large ore bodies, analyses demonstrat-
ing, however, that there exists no possible relation between the chemical
composition of the limestone and the occurrence of ore. Analyses of lime-
stone from the neighborhood of several large ore bodies situated in widely
separated localities along the ridge and from different geological horizons
throughout the epoch give the following results :

Mine.

Insoluble
residue.

Carbonate
ofmagnesia.

0-36

14-00

Geddes & Bertrand

13-83

1-09

5-79

1-84

0-20

26-32

An analysis of the stratified limestone from the seventh level of the
Richmond mine may be taken as a fair sample of the limestone body. It
yielded as follows:

Carbonate of lime 88-34

Carbonate of magnesia 4-98

Iron 1-59

Silica.. 4-83

Total 99-74

Mr. Thomas Price, of San Francisco, made a careful chemical study of
the limestones of Ruby Hill, collecting his samples for examination from the
most important points on the surface and from different levels in the mines.
Amono- the localities from which the rocks were selected, were the contact

beds between the limestone and the overlying Secret Canyon shale, strati-
fied beds on the seventh and eighth levels of the Richmond mine, the under-
lying rocks of Potts Chamber, the mouth of the Bell Shaft, and near the ore
body of the Tiptop Incline. In sixteen analyses the amount of carbonate

38 GEOLOGY OF THE EUEEKA DISTEICT.

of magnesia varies from 1-06 to 44 '35 per cent; three of them yielded less
than 2 per cent. In nine out of the sixteen the amount of the silica in the
limestone was less than 2 per cent.

Many of the beds, more especially the darker limestones, give evidence
of the presence of organic matter, even where no signs of fossils are seen.
Proof of this is found in the presence of phosphoric acid in the rock. Two
specimens yielded O13 per cent, evidently derived from the fossil remains
now almost wholly obliterated.

Sandstone layers are rarely seen in this group. Intel-stratified in the
limestone are irregular beds of shale, lenticular or wedge-shaped bodies
varying greatly in width. Indeed, throughout the entire thickness of this
group they are a characteristic feature of the beds, which pass by insensible
gradations from pure limestone to hard argillaceous shales. Occasionally
they may be traced interstratified in parallel bands for long distances, and
again the shale will develop considerable thickness, then rapidly thin out
in all directions For the most part they can be followed for no great dis-
tance. Two of these shale beds are quite distinctly marked on the top of
the ridge to the northward of Prospect Peak, but all traces are lost on the
surface to the south of that point. One of these shale beds on the east slope,
however, attains so great a thickness that it has been designated Moun-
tain shale, to distinguish it from the Secret Canyon horizon. Unlike the
larger body of overlying shale they are of slight geological significance, the
limestone both above and below presenting nearly identical physical fea-
tures, and so far as known carrying the same organic forms. The Mountain
shale comes to the surface on the ridge near the Industry mine and on the
steep slope of the ridge above the Eureka Tunnel, across its widest devel-
opment reaching over 300 feet in thickness. It differs from the Secret
Canyon shale in carrying alternate layers of argillaceous and calcareous
shales, the latter frequently passing into stratified shaly limestone. This
body of intercalated shale presents some features of economic interest bear-
ing upon the ore deposits, and may possibly be the same bed found in all
the deep mines on Ruby Hill. The thickness of the Prospect Mountain
limestone across its broadest expansion may be taken at 3,050 feet. On
Ruby Hill, owing to faulting, it never attains its full development.

HAMBURG LIMESTONE. 39

secret canyon shale.— The Prospect Mountain limestone passes by gradual
transition from shaly limestone into brown and yellow argillaceous shales,
which, with the exception of one or two thin calcareous layers, present a
very uniform character for the entire distance from the extreme southern
end of Secret Canyon, where they first crop out, northward until cut off by
a fault a short distance northwest of the Eureka Tunnel. Toward the
upper portion of the series the shale becomes gradually interbedded with
thin layers of limestone. The designation of the group is taken from the
name of the canyon where it appears most characteristically shown. These
beds are recognized only on Prospect Mountain ridge and north of Ruby
Hill. The topographical features of Prospect Mountain are largely modi-
fied by this shale body, which, eroding more readily than either the over-
lying or underlying limestone, has been largely instrumental in determining
the drainage channels of the ridge. There are few finer examples of the
wealing away of a soft, easily eroded body lying between two harder rock
masses than can be seen, in Secret Canyon, where the Prospect Mountain
limestone rises like a wall on one side and the Hamburg limestone nearly
as abruptly on the other, while the canyon for over 3 miles is carved out
of the shale in a deep, trough-like valley. In their broadest development
the shale measures 1,600 feet, although in places where they are encroached
upon by the Hamburg limestone they occur somewhat thinner. As yet no
organic forms have been found through the entire group, though diligent
search was made for them in the more promising calcareous layers.

Hamburg Limestone.-Transition beds of shaly limestone, varying in thick-
ness from 25 to 200 feet, pass gradually into the overlying Hamburg
limestone, which forms a prominent, bold ridge between the easily eroded
overlying and underlying shales, and, as it is cut through at regular inter-
vals by east and west drainage channels, presents one of the most striking
topographical features of the region, and a geological horizon most easily
traced in the field. On the surface this limestone is dark gray, frequently
grayish black, and throughout the greater part of the thickness presents a
gramilar texture. Layers of fine sandstone and hard cherty bands occur
at irregular intervals. In chemical composition it offers no essential differ-
ence from -the Prospect Mountain limestone, presenting quite as wide a

GEOLOGY OF THE EUREKA DISTRICT.

range, both in silica and magnesia. Two complete analyses were made of
this limestone, one from the summit and the other from the base of the
epoch, each representing a well denned and persistent bed, as follows:

Base of
Hamburg
limestone.

Summit of
Hamburg
limestone.

Silica . . ... .

24-00

3-94

Alumina . . . .. ..

•12

•64

Ferric oxide . .. ..

•12

•43

Ferrous oxide . .

•20

Manganese -. ...

•61

41-97

51-96

•80

•52

Water

•16

•37

32-62

40-71

Phosphoric acid...

•07

•50

Chlorine . . ...... . . ..

•01

Organic matter

•03

Alkalies ....

trace

Total

99-87

99-92

Aii examination made of a dark compact limestone from the base of
the Hamburg, collected on the north side of the ravine opposite the dump
of the Richmond shaft, gave

Silica -84

Carbonate of magnesia 1-18

A gray dolomite from the 350-foot crosscut in the Dunderburg mine
yielded

Silica -07

Carbonate of magnesia 40-04

lu general, this limestone is sharply contrasted in its lithological habit
with the Prospect Mountain body, as it is darker in color, carries siliceous
material in place of the clayey beds of the latter, and possesses a character-
istic rough and ragged surface produced by weathering. The thickness of
this limestone may be taken at 1,200 feet, and except in the shaly lime-
stones at the top and bottom of the series, 110 planes of bedding are trace-
able for any great distance. At Adams Hill, however, where the beds lie

CAMBRIAN FAUNA. 41

inclined at a much lower angle and have undergone much less movement
and compression, stratification may be frequently observed.

Hamburg shale.— This shale body in general resembles the one underlying
the Hamburg limestone, except that it is by no means as uniform in com-
position, showing very rapid changes in conditions of deposition, becoming
more or less arenaceous or calcareous throughout its entire thickness as
well as in its lateral extension It is characterized by cherty nodules, and
near the top by more or less persistent layers of chert and sand, followed
by calcareous shales which pass into the overlying Pogonip limestone of
the Silurian. Across its broadest development it measures 350 feet, yet it
rarely maintains a uniform thickness for any long distance. The best
exposures are seen opposite the Hamburg and Dunderburg mines, and
again in the • ravine north of Adams Hill, where it attains as great a thick-
ness as anywhere on the eastern slope, and is in every way as well shown.
This group is not as thick as the Mountain shale in its broadest develop-
ment in the Prospect Mountain limestone, yet its persistency, stratigraphi-
cal position, and its relations to the fauna of the Cambrian render it of
far greater importance.

Cambrian Fauna.— As has already been mentioned, no evidences of organic
remains have been observed in the Prospect Mountain quartzite, and the
conditions under which the beds were deposited could hardly be considered
favorable to life. In the overlying Prospect Mountain limestone obscure
fragments of fossils may be detected at various places throughout the
epoch, but localities showing any grouping of species or forms, sufficiently
well preserved for identification, are limited to three horizons. The lower
of these horizons occurs at the base of the limestone, in a narrow belt rest-
ing on the quartzite; the second is found in strata of calcareous shales
several hundred feet higher up, while the third horizon, which may be two
or three hundred feet in thickness, lies at the top of the limestones just
below the Secret Canyon shale.

Directly overlying the quartzite, in strata which may be regarded as tran-
sition beds between it and the Prospect Mountain limestone, occur the low-
est organic forms obtained in the district, and the equivalent of the lowest
Cambrian fossiliferous strata in the Great Basin. Along the east side of

42 GEOLOGY OF THE EUEEKA DISTEICT.

Prospect Peak, near the summit of the ridge, there may be traced for over
a mile a red arenaceous and calcareous shale, which is lost to the southward,
but which, followed to the northward, may be seen to pass gradually into a
dark gray shaly limestone. This arenaceous shale may be taken at 100 feet
in thickness, and, from the organic remains which it carries and from its
paleontological and geological importance, has been designated the OleneUus
shale. From this horizon the following species have been obtained :

Kutorgina prospectensis. Olenellus gilberti.

Ptychoparia sp.f Olenellus iddingsi.

About one-half mile northward of this locality, and in a bed of lime-
stone 100 feet in thickness, underlying the fossiliferous arenaceous shale,
and, in the same manner, resting directly upon the quartzites, species indi-
cating an identical geological horizon were found, as follows :

Olenellus gilberti. Olenoides quadriceps.

Olenellus iddingsi. Scenella conula.

Anomocare parvum.

These two groupings represent all that have as yet been identified
from this lower horizon.

The Olenellus shales pas? upward into a great thickness of bluish gray
limestone, with an occasional thin band of interstratified shale. The beds,
however, yield no well defined organic remains for nearly 500 feet, but at
that horizon they furnish forms which might belong both to the Olenellus shales
below and the next fossiliferous strata above. Although localities yielding
well defined fossils from this second horizon are seldom met with, indistinct
traces of life are seen in the limestone underneath the Mountain shale. The
best known locality is found at the head of New York Canyon on the long
sloping ridge south of the Fourth of July mine. Here were obtained the
following :

Olenoides quadriceps. Agnostus interstrictus.

Scenella conula. Ptychoparia prospectensis.

The species of Ptychoparia prospectensis has not as yet been found at
a higher horizon. Above this horizon the limestone is much metamorphosed
and altered to marble, and is so broken up that well defined beds favorable

FOSSILS FROM THE EICHMOND MINE. 43

to the preservation of fossils are rarely met with, even the calcareous shale
presenting but slight indications of them. Not till within 300 or 400 feet of
the summit of Prospect Mountain limestone and 2,000 feet higher up in the
strata was there any grouping of fossils observed. From this horizon, and
extending up to the base of the Secret Canyon shale, numerous localities
occur all along the east slope of Prospect Mountain, which present a fauna
with much the same grouping at each, and showing a mingling of both
Georgia and Potsdam faunas.

These organic forms occur both in compact limestone and shaly calca-
reous beds, and constitute the third and upper fossiliferous strata of Prospect
Mountain limestone. The following list contains most of the species col-
lected at this horizon in New York Canyon, many of them being found at
several localities :

Obolella (like O. pretiosa). Protypus senectus.

Lingula manticula. Dicelloceplialus nasutus.

Agnostus communis. Ptychoparia oweni.

Agnostus bidens. Ptychoparia occidentals.

Agnostus neon. Ptychoparia dissiniilis.
Agnostus richmondensis.

From the corresponding beds in Secret Canyon near Geddes and Ber-
trand mine, and in a compact black limestone a short distance above the
base of the Secret Canyon shale belt, were collected the following species :

Kutorgina whitfleldi. Aguostus neon.

Orthis eurekensis. Protypus expansus.

Stenotheca elongata. Ptychoparia oweni.

Agnostus communis. Ptychoparia haguei.

Agnostus bidens. Oleuoides spinosa.

In a well denned stratified black limestone exposed for several hundred
yards on the seventh level of the Richmond mine were obtained the following

forms:

Obolella . Agnostus neon.

Lingula manticula. Agnostus richmondensis.

Agnostus communis. Ptychoparia oweni.

Agnostus bidens.

The finding of this grouping of fossils in the mine is of some special
importance as it adds paleoiitological proof to structural evidence to show

44 GEOLOGY OF THE EUREKA DISTRICT.

the geological age of the limestone in which the great bodies of ore upon
Ruby Hill occur.

The Prospect Mountain limestone carrying this fauna passes by grad-
ual transition into the Secret Canyon shale, the passage beds being mainly
thin interstratified layers of limestone and calcareous shale. No fossils have
been obtained from the argillaceous strata of the Secret Canyon shale
throughout its development, but imperfect fragments more or less obliter-
ated have been observed in several of the more calcareous beds. At the
top of this group the calcareous shales appear, which must be taken as
forming the base of the well known Hamburg limestone, inasmuch as they
indicate new conditions of sedimentation. It is the coming in of these cal-
careous deposits that renders possible the development and preservation of
a higher fauna. These calcareous shales may be recognized readily all along
the line of contact. In places it is well characterized by its grouping of
fossils, the same species being observed from both the east base of Ham-
burg Ridge and the corresponding beds north of Ruby Hill, presenting a
higher Cambrian fauna. The following species have been determined from

this horizon :

Protospongia fenestrata. Dicellocephalus osceola.

Lingulepis msera. Dicellocephalus richmondensis.

Lingulepis minuta. Ptychoparia pernasuta.

Lingula manticula. Ptychoparia laticeps.

Ipbidea depressa. Ptychoparia bella.

Acrotreta gemma. Ptychoparia linnarssoni.

Kutorginr« minntissima. Ptychoparia oweni.

Hyolithes priiuordialis. Ptychoparia haguei.

Agnostus coininunis. Ptychoparia similis.

Agnostus bidens. Ptychoparia unisulcata.

Agnostus neon. Ptychoparia laeviceps.

Agnostus seclusus. Chariocephalus tumifrons.

Dicellocephalus nasutus. Ogygia problematica.

After leaving the calcareous shales, which form the base of the Ham-
burg limestone, the next fossil horizon occurs in the shales at the summit
of the same group, and in thin interlaminated limestones in the overlying
Hamburg shale.

OLENELLCS SHALE. 45

This horizon has yielded the following species:

Lingulepis inaera. Dicellocephalus angnstifrons.

Lingulepis miuutu. Dicellocephalus inarica.

Lingula manticula. Dicellocepbalus bilobatus.

Obolella discoidea. Dicellocephalns oswola.

Acrotreta gemma. Ptychoparia aflinis.

Kntorgiua minutissima. Ptychoparia oweni.

Hyolithes primordialis. Ptychoparia bagnei.

Aguostus coinmunis. Ptychoparia granulosa.

Agnostus bidens. Ptychoparia simulata.

Aguostus neon. Ptychoparia nuisulcata.

Aguostus prolongus. Ptychoparia breviceps.

Agnostus tumidosus. Arethusina americana.

Agnostus tuimfrons. Ptychaspis minitta.
Dicellocephalus nasutus.

The Olenellus shales lie not only at the base of the fossiliferous rocks
at Eureka, but are equivalent to the lowest fossiliferous strata as yet
recognized in the Great Basin. Their known stratigraphies! position
overlying the Prospect Mountain quartzite and at the base of a conform-
able series of limestone and shale of Cambrian and Silurian age, measuring
9,000 feet in thickness, renders the question still a matter of some doubt
whether older fossil bearing strata will ever be found in Utah or Nevada.
Wherever the Olenellus shale is known to occur, it is always found resting
upon siliceous beds, and in no single instance, where they occur -together,
is the thickness of the lower quartzite so great as at Eureka. Unfortunately
no sedimentary beds are known to come to the surface below the Prospect
Mountain quartzite, and of the latter we are wholly ignorant as to its thick-
ness. What is needed in working out the stratigraphy of the Great Basin
ranges is a locality exposing a section of Lower Cambrian rocks still lower
than those at Eureka, but at the same time showing their relations with
the Olenellus shale and Prospect Mountain limestone above. In the many
uplifts of quartzose strata which have been provisionally assigned to the
Cambrian upon theoretical grounds, investigation may yet furnish proof
that certain iuterstratified shale bands carry either a similar or still lower
fauna, and if their structural relations with the Olenellus horizon can be
shown, it will make a Cambrian section much to be desired. Organic

46 GEOLOGY OF THE EUREKA DISTRICT.

forms closely allied to the Olenellus grouping of species have been found
in four places in the Great Basin: in the Oquirrh Range, in Utah; in the
Highland and Timpah-Ute Ranges, and at Silver Peak, in Nevada. In all
these they are described as occurring in a similar arenaceous shale conforma-
ble to and overlying a body of quartzite, the base of which is not exposed.

As early as 1874, Mr. F. B. Meek', in a letter to Dr C. A. White, described
the two species, Olenellus gilberti and 0. howelli, from Pioche, Nevada. He
called attention to the relationship existing between them and Olenellus ver-
montana and 0. thompsoni, Hall, from the Georgia slates of Vermont, and
to him belongs the honor of first correlating these widely separated beds.

Quite recently, after a careful review of all the material at his com-
mand, and a comparative examination in the field of the well known New
York, Vermont, and Newfoundland regions with the more recently studied
Great Basin areas in Nevada and Utah, Mr. C. D. Walcott' suggests dividing
the Cambrian into three divisions, namely : Lower Cambrian, Middle Cam-
brian, and Upper Cambrian. These three primary divisions are recognized
in the Cambrian of Europe, and each of them has received local designa-
tions derived from the name of the region where the terrane is typical and
well exposed. Thus, in the Cordillera, the Lower Cambrian is designated
as the Prospect Mountain group, whereas in New York and New England
it is best known as the Georgia shale, from the well known locality in Ver-
mont. The Middle Cambrian has as yet no better typical locality than the
slates and shales of St. John, New Brunswick. The Upper Cambrian is
usually spoken of as the Potsdam so well recognized all the way from the
Atlantic coast to central Nevada. At Eureka the latter epoch is represented
by the Hamburg Ridge.

Wherever in the Great Basin, so far as known to the writer, the genus
Olenellus has been discove 'ed, the beds do not attain a development of more
than 400 feet; at least they pass from shale and shaly limestone to lime-
stone, in which as yet no organic forms have been recognized. Only at
Eureka and in the Highland Range are their structural relations with both
the overlying and underlying beds clearly made out. We have very little

1 U. S. Geographical Surveys, West of 100th Meridiau, vol. iv, Paleontology, 1877, p. 47.
2Stratigraphic Position of the Olenellus Fauna in North America and Europe. Am. Jour.
Sci., 3d ser., vol. xxxvn, May and July, 1889.

SILURIAN KOCKS. 47

knowledge of the structure at the other localities, and in the Oquirrh Range
the Olenellus shales are known to be cut off by a sharp fault from the Upper
Cambrian.

By reference to the Eureka section it will be seen that the Olenellus
horizon is nearly 2,500 feet below the top of the Prospect Mountain lime-
stone, where there comes in a fauna showing a mingling of Middle and
Upper Cambrian forms. At the base of the Hamburg limestone, 1,600 feet
higher in the strata, the true Potsdam fauna of Wisconsin and Minnesota
is abundantly represented by a characteristic grouping. By comparing
these lists of fossils from the different horizons, it will be seen that in this
group, at the top of the Hamburg limestone, there are found seven species,
which first occur at the top of the Prospect Mountain limestone. They
pass up through the beds at the base of the Hamburg limestone and, together
with five additional species obtained for the first time from the latter hori-
zon, come up to the close of the epoch, making in all twelve species common
to the top and bottom of the Hamburg limestone. Three species obtained
from both the base and the summit of the limestone are identical with
forms from the Potsdam sandstone of Wisconsin — Hyolitlies primordiaUs,
Dicellocephalus osceola, Ptychaspis minuta. Another, Lingula manticula, first
described by Dr. C. A. White,1 from the Schell Creek Mountains, Nevada,
has here at Eureka a wide range, extending from the Prospect Mountain
limestone through the Hamburg limestone and shale and well up into the
overlying Pogonip group of the Silurian.

SILURIAN EOCKS.

Rocks of the Silurian period at Eureka fall readily into three epochs.
From our present knowledge, it would be a somewhat difficult matter to
subdivide them still further, except upon fine distinctions founded upon
paleontological grounds, which might not hold good over any large area
of country. These three divisions correspond with the lithological character
of their sediments, two heavy masses of limestone with a sharply defined
intervening bed of quartzite. This quartzite is a highly altered sandstone,
much purer in composition than the Cambrian quartzite below or the sili-

1 U. 8. Geographical Surveys West of the 100th Meridian, vol. iv, Paleontology, part 1, p. 5-'.

48 GEOLOGY OF THE EUltEKA DIST1UCT.

ceous beds of the Carboniferous above. They have been designated as
follows: first, Pogonip limestone; second, Eureka quartzite; third, Lone
Mountain limestone. The division between the Cambrian and Silurian
rests mainly upon paleontological evidences and is by no means a well
defined line of separation. While the underlying Hamburg shales of the
Cambrian present a lithological distinction, the transition beds are of vary-
ing thickness and pass gradually into the overlying limestone. Moreover,
while at Eureka the argillaceous shales serve to separate the two periods,
the distinction would not hold good in other regions, particularly at White
Pine, where both the Upper Cambrian and Pogonip are well developed,
with a great thickness of strata and an abundant fauna, but without a well
recognized intermediate shale belt. Wherever in the Great Basin the
Silurian is exposed, conformably overlying the Cambrian, there occur at
the same horizon a commingling of species of both periods, but this con-
dition of things presents no valid objection against the division of any two
periods, for the argument holds with equal force between the limestones of
the Upper Silurian and Devonian, and between the limestones of the latter
and the Carboniferous.

Pogonip Limestone.— The name given to this epoch is taken from Pogonip
Ridge at White Pine, and was first employed by the Geological Exploration
of the Fortieth Parallel to designate the great belt of limestone at the base of
the Silurian period. At White Pine this epoch is remarkably well exposed
and of much greater thickness than at Eureka, although at the latter locality
it covers large areas and may be equally well studied, both in its structural
relations and faunal development. On the line of the Section E F (atlas
sheet xm) the transition between the Hamburg shale and Pogonip passes
gradually upward from argillites and fine grained arenaceous beds with
interstratified calcareous shales into purer limestones distinctly bedded. The
limestone is for the most part bluish gray, but near the top is of a darker
tint, in places becoming almost black. It is distinguished lithologically from
both the lower belts of limestone in its more massive bedding, fineness of
texture, and the smoothness of its weathered surfaces. This last feature,
however, holds true only in a broad, general way, as bands of chert fre-
quently produce roughness of texture resembling Hamburg limestone.

THICKNESS OF POGONIP LIMESTONE. 49

Chemistry shows no characteristic difference between this limestone and
the older masses, the beds being more or less magnesian throughout their
entire vertical range. A complete analysis was made of a siliceous variety,
taken from near Wood Cone, yielding the following result :

Silica 9-345

Alumina 0-309

Ferric oxide 0-289

Ferrous oxide

Manganese

Lime 50-011

Magnesia : 0-535

Water 0-130

Carbonic acid 39-111

Phosphoric acid 0-240

Chlorine 0-030

Organic matter Traces.

Alkalies Traces.

Total 100-000

To the east of Jackson Mine, where the beds are well exposed and lie
inclined at a nearly uniform angle, they measure 2,700 feet across their
greatest development. This thickness is probably surpassed by the beds
on the long spur southwest of Wood Cone, but there they stand nearly ver-
tical, in some places dipping eastward and in others westward, occasionally
showing evidences of faulting, which prevents any reliable estimate of their
thickness. It is probable they measure over 3,000 feet. An estimate of the
strata at White Pine gives over 5,000 feet of limestone. At first sight it would
appear as if there must have been some displacement of beds along Pros-
pect Mountain, but the succession of a rich fauna with the same character-
istic specific forms at the base and summit of the epoch at both Eureka and
White Pine would preclude such a supposition and the simplicity and uni-
formity of structure go to show that such is not the case.

Fauna of the Pogonip Epoch.— Throughout the entire thickness of the Pogo-
nip beds, organic remains characterize the epoch. At the base there is a
decided mingling of species, a number of Potsdam forms extending up-
ward for some distance into the limestone. Passing upward, however,
these species gradually diminish and there comes in rapidly a numerous
fauna representing higher and higher forms, till midway in the beds nearly
MON xx 4

50 GEOLOGY OF THE EUEEKA DISTRICT.

all the characteristic Cambrian fauna have passed away and genera equiv-
alent to the Chazy horizon of New York have taken their place, and near
the top a grouping of fossils comes in strongly indicating the Trenton hori-
zon. In the collections made from the Pogonip beds at Eureka, nearly
eighty species have been determined, a large proportion of them forms
found for the first time either at Eureka or White Pine, while many of
them are common to both localities and from the same stratigraphies! posi-
tion in the beds. Many of them are identical with species found in New
York and Canada and along the Atlantic border.

Fifteen species comprise all those forms which have been recognized
as common to both the Cambrian period, and Pogonip epoch of the Silu-
rian, and several of these present a wide vertical range extending downward
to the summit of the Prospect Mountain limestone.

The list is as follows:

Lingnlepis maera. Agnostus neon.

Lingulepis minuta. Ptychoparia affinis.

Lingula manticula. Ptychoparia oweni.

Obolella discoidea. Ptychoparia granulosus.

Acrotreta gemma. Ptychoparia haguei.

Leptajna melita. Ptychoparia unisulcatus.

Agnostus conununis. Arethusina americaua.
Agnostus bidens.

Only two species of the genus Dicellocephalus have been recognized as
yet in the Pogonip group at Eureka, D. finalis and D. inexpectans, both new
to science. They occur associated together several hundred feet above the
base, at a horizon where many of the Cambrian species have already dis-
appeared. Of the genus Dicellocephalus only two species are known from
the corresponding beds at White Pine. Near the base of the Pogonip in a
limestone northeast of Adams Hill, a decided mingling of both Cambrian
and Silurian occur, as seen by the following list:

Lingulepis maera. Agnostus neon.

Obolella discoidea. Ptychoparia (Euloma) affinis.

Acrotreta gemma. Ptychoparia oweni.

Leptsena melita. Ptychoparia haguei.

Triplesia calcifera. Ptychoparia unisulcatus.

Hyolithes vauuxemi. Illsenurus eurekeusis,
Asaphu.s caribouensis.

FAUNA OF THE POGONIP. 51

A number of localities southeast of Ruby Hill represent, in their fauna,
a somewhat higher horizon, the most favorable for collecting being found
on the first ridge southeast of the Jackson Mine, where the base of the
Pogonip beds are wanting, having been cut off by the Jackson fault.
These beds yielded the following species :

Lingulepis msera. Ptychoparia (Enloma) affinis.

Lingnla manticula. Arethusina americaim.

Acrotreta gemma. JJlaeuurus eurekensis.

Leptoeua melita. Asaphus caribouensis.

Orthis hamburgensis. Asaphus (sp. undt.).

Directly east of the Hamburg Ridge ,and several hundred feet above
the last locality, a grouping of fossils comes in which is characteristic
of a slightly higher horizon :

Lingulepis ina-va. Triplesia calcifera.

Lingula mauticula. Tellinomya! hamburgensis.

Disciua (sp. undt.). Dicellocephalus finalis.

Acrotreta gemma. Dicellocephalus inexpectans.

Schizambon typicalis. Ptychoparia annectans.

Obolella ambigua. Ptychoparia oweni.

Orthis hamburgensis. Amphiou (sp. undt.).
Orthis testudinaria.

This horizon may be easily identified by collections of fossils more or
less complete from numerous other localities in the district. From about
this point in the limestone the older persistent forms gradually disappear,
and the new species introduced in the above list become more and more
abundant, as is evidenced by the increasing number of localities where they
occur as higher strata are reached.

In a compact gray limestone southwest of McCoy's Ridge are the fol-
lowing :

Orthis perveta. Plumulites (sp. undt.).

Orthis testudinaria. Ceraurus (sp. undt.).

Triplesia calcifera. Elcenurus eurekensis.

Maclurea annulata. ' Asaphus caribouensis.

Midway in the Pogonip, the genera Meceptaculites, CJuefcff*, rimroto-
maria, Maclurea, Bathyurus, Asaphus, and Cyphaspis, make a decided change

52 GEOLOGY OF THE EUREKA DISTRICT.

in the fauna from the Hamburg limestone. Many of these genera gradually
give way and are replaced by others, until at about 800 or 1,000 feet below
the summit the fauna! development is shown by a grouping of fossils made
at two widely separated areas, which begin to foreshadow the strongly
marked fauna at the summit of the epoch. From the east slope of the
ridge east of the Hamburg Ridge there were collected —

Beeeptacolites ellipticus. Maclurea amiulata.

Cystidian plates. • Bellerophon ?

Orthis perveta. Ortlioceras (like O. mnlticauieratnm).

Triplesia calcifera. Cypliaspis brevimarginatus.

Raphistoma? Ilkeruirus eurekeiisis.

Pleurotomaria loueusis. Asaphus? curiosus.

And from the long, eastern slope of White Mountain, about 800 feet
from the top of the mountain and probably nearly the same distance below
the summit of the Pogonip, there were collected as follows :

Monticulopra. Pleurotomaria lonensis.

Orthis testudiuaria. Endoceras proteiforme.

Raphistoma nasoiii. Ortlioceras sp. ?

Maclurea annulata. Bathyurus similis.

Maclurea subaimulata. Asaphus cariboueDsis.

Throughout the upper 600 feet of the Pogonip, wherever organic
remains have been observed, the association of genera are much the same,
the horizon being well determined both by the fauna and the position of
the overlying Eureka quartzite. In many areas where the Eureka quartzite
forms the surface rock an underlying limestone several hundred feet in
thickness is frequently exposed, which carries paleontological evidences of
the upper Pogonip strata. Two localities in these upper Pogonip beds
have furnished a rich and varied fauna. From a dark limestone on the
summit of White Mountain the following species have been determined :

Receptaculites ellipticus. Tellinomya contraeta.

Receptaculites elongatus. Helicotoma sp?

Receptaculites mammillaris. Orthoceras inulticaineratum.

Cystidean plates. Eudoceras (like E. inultitubulatum).

Strophomena uemea. Leperditia bivia.

Orthis perveta. Leperditia sp?

Orthis testudiuaria. Beyrichia sp?

FAUNA OF THE POGONIP. 53

A similar grouping of fossils was procured in the Fish Creek Moun-
tains a short distance below the quartzite, numerous localities yielding
nearly identical lists:

Receptacotites ellipticus. Modiolopsis occidens.

Receptaculites elongatus. Modiolopsis pogouipeiisis.

Beceptacnlites mammillaris. Pleurotomaria sp?

Cystidean plates. Maclurea sp ?

Ptilodictya- sp? Orthoceras multicameratum.

Monticulopora sp ? Endoceras protciforme.

Ortliis perveta. Aini)hioii nevadensis.

Tellinouiya contracta. Ceraurus sp?

On the north slope of Surprise Peak, just below the quartzite, the lime-
stone supplied the following :

Receptaculites mainmillaris. Rapliistoma nasoui.

Cystidean plates. Pleurotomaria ?

Ortliis perveta. Maclurea annulata.

Ortliis tricenaria. Leperditia bivia.

A convenient locality to those visiting Eureka and wishing to exam-
ine the Upper Pogonip beds may be found on the west side of Caribou
Hill, which has furnished a few typical forms :

Ortliis perveta. Receptaculites maminillaris.

Ortliis tricenaria. Maclurea auimlata.

Asaphus cariboueusis.

Other localities which have presented evidences of the same horizon
may be found in Goodwin Canyon, at the head of Lamoureux Canyon,
and in the limestones not far from the line of the general section E F,
atlas sheet xm.

This grouping of fossils from the summit of the Pogonip limestone is
of special interest on account of the commingling of species and the position
of the strata. Ascending in the beds it will be found that the Cambrian
fauna entirely disappears, the life of the Middle Pogonip gradually
passes away, and new species come in until the grouping of the fauna
presents an aspect peculiarly its own. Two species of the genus Modiolop-
sis, and the characteristic fossil, Tettinomya contracta, foreshadow still higher
strata, indicating the coming in of the Trenton horizon. The summit of
the Pogonip is also marked by an increase in the number of species of

54 GEOLOGY OF THE EUREKA DISTRICT.

fri/rnaria, 0. t<-xti<tH>i(U-iK, and 0. perveta, characteristic forms in K
York and Wisconsin. A marked feature of this upper horizon is the pres-
ence of the genus Ecccptaculites, three species having been identified. Im-
mense numbers of specimens of one of them, R. mammiUaris, are found
throughout the beds with a vertical range of several hundred feet, and are
abundant where all other fossils are wanting. Oraptolites, in the Pogouip
epoch at Eureka, are represented by a single undetermined species, which,
according to Mr. C. D. Walcott, resembles closely G. bijid/ix.

Eureka Quartzite.— The name of the district has been employed to desig-
nate this formation, as during the progress of the survey the quartzite was
determined for the first time as a distinct geological epoch and its strati-
graphical position clearly defined. Up to this time the occurrence of a
broad belt of quartzite lying between two massive bodies of Silurian lime-
stone had never been recognized. Moreover, nowhere else in the Great
Basin has the formation been so carefully studied. It lies superimposed
directly on the Pogonip limestone, and where the upper beds of the latter
epoch are exposed they are frequently capped by a greater or less thickness
of the quartzite, as is well shown on Caribou Hill and McCoy's Ridge.
Again, the position of the Eureka quartzite is clearly brought out by the
patches of quartzite left by erosion upon the massive Pogonip beds of Fish
Creek Mountains. No horizon is more marked in its physical features
than the Eureka quartzite. Besides its frequent occurrence as a capping
rock, its snow-white color, and its tendency to fracture in mural-like escarp-
ments render it easily recognizable wherever it occurs.

The Eureka quartzite is made up almost entirely of siliceous grains firmly
compacted together. It possesses a granular texture and a vitreous luster,
and for the most part is free from partings parallel to the planes of bedding.
At the base of the formation the quartzite is colored red and gray by iron,
but it rapidly passes into white, with an occasional bluish or purplish tinge,
frequently presenting a mottled coloring. In general it is exceptionally free
from seams or patches of ferruginous material, its purity and uniformity of
composition and marble-like appearance being a marked feature of the hor-
izon. In one or two places it shows a brecciated appearance, with fine,
cherty masses, notably on Hoosac Mountain. In the neighborhood of

EUREKA QUARTZITE. 55

McCoy's Ridge it has been quarried for fluxing purposes at the smelting
furnaces, the rock yielding nearly two dollars in gold per ton, which paid
for hauling. Whether the gold is of primary origin in the quartzite or
whether it was derived from some vent carrying mineral matter in solution
has never been determined. The locality where the rock was quarried is
situated near the Hoosac fault, and in close proximity to ore bodies.

The i-idge extending southwest from Castle Mountain shows a fine
body of the Eureka quartzite, the southern escarpment of which exposes a
section 300 feet in thickness. Numerous specimens collected at intervals
across the quartzite were subjected to microscopic examination. All the
upper portion of the rock proved to be an exceptionally pure and fine
quartz, the grains averaging between 0-02 and 0'03 millimeters in size,
with a granitoid structure ; that is, the grains did not show rounded
outlines, but instead presented irregular shapes that fitted into each other
and firmly crystallized together without fine groundmass between them.

The quartzite is free from impurities but full of fluid inclusions with
moving bubbles, some of them evidently liquid carbonic acid. The minute
fluid cavities appear white in incident light. An examination of the quartzite
indicated that the entire rockmass had undergone a recrystallization of
the material and was not by any means a simple solidification and packing
together of quartz grains. In other words, it is a true quartzite and not a
compact sandstone, hardened by superincumbent rock. Even under the
microscope the rock appears to cany but little oxide of iron. Toward
the upper part of the formation the microscope detects increasing numbers
of needles and grains of iron oxide, accounting for the change of color
both in the unaltered rock and on the weathered surfaces of the larger
detached blocks. Particles of calcite also begin to appear some distance
beneath the Lone Mountain limestone, associated with the quartz grains,
while at the base of the quartzite there is a very decided increase in the
amount of lime present.

Although not differing materially from those observed elsewhere, the
most satisfactory section across the quartzite was made just west of Castle
Mountain. Here the quartzite presents a perpendicular cliff, 300 feet in
thickness, resting horizontally on the Pogonip Ihnestone. The subjoined

GEOLOGY OF THE EUREKA DISTRICT.

section is numbered from the top downward, the numbers inclosed in
brackets coinciding witli the specimen number in the collection. Through-
out the section the quartzite is for the most part vitreous without partings
parallel to the bedding, the coloring, however, being in nearly horizontal
planes, passing insensibly from one tint to another.

•0

a
(§

10
10

30
10

31)

Id
20

10
20

to
so

10
20
20

:v NO.

=¥ No.
V

igr i.:

J7 No.
-T- No.
\3 No.

3
4

7
7«
8
9

10
11

lib
12

14
15

(391)
(390)

(389)
(388)
(387) )
(386) \
(385)
(384) )
(383) ]
(382)
(381)
(380) )
(379) (
(378) )
(377) [
(376) }
(375)

(374)

(373)
(372)

No.

9 No.

No.

r NO.

.-'* No.
it
No

,4 No.

No.

10 feet of white vitreous quartzite.

10 feet grayish white, with segregation of fer-
ruginous material.

30 feet white and vitreous.

10 feet purple and white, vitreous.

30 feet purplish white, with three narrow bauds
of dark gray granular quartzite.

10 feet dark gray quartzite.

20 feet white, banded with steel grayj dark gray
quartzite bands in No. 7.

20 feet dark gray and white, banded and mottled.

20 feet light gray, fine granular.

40 feet white and pinkish white.

60 feet dark gray passing into light gray, with
bands more or less calcareous, weathering
red.

20 feet gray, having cross bedding brought out
by weathering.

20 feet dark steel gray quartzite, somewhat cal-
careous.

20 feet siliceous limestone. ) Pogonip

40 feet black compact limestone. ) limestone.

360 feet

Fio.l.— Eureka quartzite west
of Castle Mountain.

The junction between the quartzite and the underlying limestone pre-
sents a sharp line of demarcation and indicates an abrupt change in the
deposition of sediments.

Although the Eureka quartzite is probably not more than a few hun-
dred feet in thickness, it can be estimated only approximately, as an uncon-
formity exists between it and the next overlying group. Over the large
area covered by the exposures of the quartzite, evidences of denudation
prior to the deposition of the Lone Mountain limestone may be observed in
the mountains connecting the Fish Creek Range with Prospect Ridge, but
no satisfactory estimate of the amount seems possible. Again, not only

LONE MOUNTAIN LIMESTONE. 57

different horizons of the Lone Mountain limestone, but even of the Devon-
ian, are seen to repose directly upon and to overlap the quartzite. Under
any circumstances the quartzite would be difficult to measure, inasmuch as
over the greater part of the area stratification lines are wanting, and the
beds are frequently broken up by a succession of small parallel faults not
always easy to recognize, rendering the amount of displacement still more
difficult to estimate. These minor displacements, when the rocks lie nearly
horizontal, produce steps and mural faces wherever the quartzite occurs as
the surface rock. In nearly all such instances the Pogonip beds are exposed
in the more deeply eroded canyons. On the other hand, where the beds are
inclined at high angles, accompanied by numerous faults, the formation fre-
quently presents the appearance of a much greater thickness than is really
the case, as is seen on Hoosac and Lookout mountains.

The best estimates place the thickness of the beds at about 500 feet,
although no escarpment of the quartzite free from faulting presents quite
so broad a development. No fossils have been obtained from this horizon,
nor is it likely that they will be found. The microscope shows clearly how
complete an alteration has taken place since the original sand deposits were
laid down, so that all traces of fossils, if any existed, must have been
obliterated.

Lone Mountain Limestone. — Next above the Eureka, quartzite comes a body of
limestone without any transition beds, the change in the character of depos-
its being unusually abrupt. The designation of the epoch is taken from a
bold isolated mountain which rises out of the plain a few miles to the north-
west of the Eureka District, where it is seen in its full development better
than in the immediate area of the map. Not only is it well shown at Lone
Mountain, but in a continuous section its relations are clearly made out with
the other members of the Silurian period and with the overlying body of
Devonian limestone. The section at Lone Mountain is given in detail at
the end of this chapter.

The Lone Mountain epoch may be divided upon paleontological
grounds into two horizons, which, for convenience, are provisionally desig-
nated as the Trenton and Niagara. The lowest beds resting immediately on
the quartzite are a steel-gray, almost black, gritty limestone, in most places

58 GEOLOGY OF THE EUBEKA DISTRICT.

without traces of bedding, and so altered as to have obliterated all evidences
of organic remains. Ascending the strata these steel-gray beds pass up
into dark bluish gray limestone, which in one locality north of Wood Cone
yielded a small lot of fragmentary and poorly preserved fossils, but which
represent a characteristic Trenton grouping. These black and gritty beds
are recognized in but few places at Eureka, mainly in the southwest corner
of the district, along the southern base of the Mahogany Hills. It is quite
possible that the horizon covers a larger area than has been supposed, but
if such is the case the beds have undergone so great a lithological change
that their recognition seems impossible without paleontological evidence,
and that is wholly wanting. Moreover, the beds resting upon the quartzite
in other places resemble higher strata in the Lone Mountain epoch.

This limestone appears to be magnesian throughout ; a siliceous variety
from the fossiliferous beds north of Wood Cone yielded 8'41 per cent silica
and 2'55 per cent magnesium carbonate. The thickness of these lower
beds, in which the Trenton aspect of the fauna is so strongly marked, may
be taken at 300 feet, at least the black and blue limestone presents about
that development before passing into the upper strata.

Above the horizon with the Trenton grouping the rocks pass gradually
into light gray siliceous limestone, with a peculiar saccharoidal texture, in
places becoming almost white and wholly without bedding. On the surface
the limestones weather brown and buff, their light colors throughout a
great vertical range standing out in strong contrast with the other massive
limestone beds of the Paleozoic. It weathers in rounded outlines, breaking
with an irregular fracture and presenting a monotonous appearance weari-
some to the eye. Rock of this character makes up by far the greater part
of the horizon, and then by slow, imperceptible changes it becomes darker
in color, with more and more tendency to develop planes of stratification,
and gradually passes into the overlying limestone of the Devonian.

As already mentioned, an unconformity exists between the Eureka,
quartzite and the Lone Mountain limestone. There is therefore no direct
evidence in the district of the thickness of the limestone. The average
thickness of strata exposed has been taken at 1,800 feet, but it is probable
that this is under rather than over estimated, and at Lone Mountain they

TRENTON FAUNA. 59

attain a somewhat greater development, at least 2,000 feet being exposed.
In most localities at Eureka where the limestone rests upon the quartzite
the upper members of the epoch are wanting, and in others they pass under
the Devonian without any means of measuring their thickness. Another
difficulty arises from the impossibility, on our present knowledge, of de-
termining a line of separation between the Silurian and Devonian, as no sharp
lithological distinctions exist and there is no means of telling exactly how
far down in the limestone a Devonian fauna comes in. It is known, how-
ever, that Silurian corals extend up into the limestone about 1,500 feet from
the base, and the dark blue limestone which characterizes the Devonian
makes its appearance about 300 feet higher up in the series.

Fauna of the Lone Mountain Limestone.— The fauna obtained from the Lone

Mountain limestone, although meager and most of the material too poorly
preserved for specific ^identification, is of special interest, as it occupies a
most important position in the development of life in the geological record.
Not only are organic forms poorly represented, but the beds themselves over
large areas of the Great Basin have not as yet been recognized and over
other areas are known to be wanting. The collection indentifying the
Trenton fauna was found on a low ridge a short distance northeast of Wood
Cone. The list comprises several characteristic species : Leptcena sericea,
Orthis subqmdrata, 0. (like 0. plicatella), Trinucleus concentricus, and Asaphus
platycephalus, and representatives of the following genera : Streptelasnia,
Rhynchonella, Orthoceras, Cyrtoceras, Ceraurus, Dalmanites, and Ulanus. It
is worthy of special mention that in this small but representative collection,
all the more typical forms found in the beds immediately below the Eureka
quartzite, which indicated the coming in of higher horizons, are wanting or
at least have not as yet been found.

Above the Trenton no good grouping of fossils has as yet been dis-
covered until the Devonian rocks are reached. The upper portion of the
Silurian limestone presents a most forbidding aspect for the preservation of
organic remains, and although diligent search was made throughout the
horizon it was rewarded only by finding a few imperfect corals, belonging
to the species Hall/site* catrmilatus, which is so characteristic of the Niagara
of the East, and here found in what should be its true geological position.

60 GEOLOGY OF THE EUREKA DISTRICT.

They have a wide range and occur nearly 1,500 feet above the summit of
the Eureka quartzite. The same coral has been obtained from Lone Moun-
tain and White Pine, and in both these latter localities associated with the
genus Zaphrentis.

Lone Mountain.— This isolated mass rises abruptly out of the broad plain
lying between the Wahweah and Pifion ranges and about 15 miles north-
west of the Eureka Mountains, which shut in the plain to the south-
west. Its isolation, its great altitude as compared with the length of the
uplift in strong contrast with the neighboring ranges, and its steep slope to
the eastward make the mountain a most conspicuous object. In its geolog-
ical structure the mountain appears to be a monoclinal ridge of great sim-
plicity and uniformity, remarkably free from any great faults and folds and
presenting a block of strata about 4,000 feet in thickness and reaching- an
altitude nearly 2,000 feet above the plain. The beds have all the appear-
ance of being cut off by a sharp fault at the south end of the block, evi-
dence of which may be found in the body of Carboniferous limestone rest-
ing against the Devonian at the southeast base of the uplifted mass. The
dip of the strata upon Lone Mountain is uniformly to the east at an angle
of 30° to 50°, with a strike a little east of north. To the geologist a
series of beds like this at Lone Mountain would at all times command
attention, but in this exposure of 4,000 feet of strata is represented a sec-
tion of the Paleozoic rocks rarely seen in the Great Basin and so far as
known nowhere else so well shown as here. The value of the exposure
consists in the simplicity with which the three divisions of the Silurian are
brought out in the same continuous section. At the western base of the
mountain the upper members of the Pogonip come to the surface, but.
with an exposure of only about 375 feet of beds. Within this belt, how-
ever, a fauna strikingly characteristic of this horizon is found and almost
identical with that occurring in the corresponding Pogonip beds at Eureka.
A few hours' search yielded the following:

Receptaculites mammillaris. Modiolopsi.s occidens.

Monticulopora sp. ? Modiolopsis pogouipeiisis.

Cystidian plates. Hellicotoina?

Acrotreta (like A. subconica). Plenrotomaria loneusis.

Stropboineua neraea. Murchisonia sp. I

SECTION ACJKOSS LONE MOUNTAIN. (H

Orthis lonensis. Maclurea annulate.

Orthis perveta. Maclurea carinata.

Orthis tcstudiiiiiria. Maclurea sp.?

Streptorhynchus minor. Cyrtolites sinuatus.

Coleoprion minuta. llla-nus sp.?

Resting upon the Pogonip comes the Eureka quartzite, but with less
thickness than the corresponding beds at Eureka. Immediately above the
quartzite, with but little development of transition beds, occur the light
colored siliceous limestones, measuring at least 2,000 feet. These beds form
the greater part of the western slope of the mountain, and are so character-
istically shown as to make the local name of Lone Mountain an appropriate
one to designate the epoch. In the lower limestones, resting directly upon
the quartzite, the Trenton fauna appears to be wanting, and it is by no
means certain that the beds are represented. At all events the bluish gray
limestone characteristic of the Trenton at Eureka and White Pine has not
been recognized. On the other hand, throughout the entire epoch evi-
dences of organic remains are exceedingly meager and confined to silicified
corals imperfectly preserved. The Niagara coral, Halysites catemilatus, which
usually occurs several hundred feet above, is found here within 50 feet
of the quartzite.

The light colored siliceous limestone passes up gradually into the dis-
tinctly bedded Nevada limestone of the Devonian, which forms the summit
of the ridge, and as the strata dip eastward make up the greater part of the
eastern slope. It is by no means certain, however, that a displacement of
strata does not extend along the eastern face of the uplifted mass, the base
of the ridge not having been examined.

Mr. C. D. Walcott made the following section across Lone Mountain
(see Fig. 2):

Feet.

1. Dark gray limestone, with brown and variegated layers iuterbedded.

Typical Devonian fauna. (Nevada limestone.) 1, 500

2. Siliceous bluish gray limestone breaking up into shaly bands carrying

abundant fossils of the Lower Devonian. (Nevada limestone.) 200

3. Siliceous limestone, light brown, gray, and buft' in color, with Hull/site*

catenulatitit near the base; passing up into beds almost white, with blue
and gray tints, followed by alternating dark and light beds. (Lone
Mountain limestone.)

GEOLOGY OF THE EUEEKA DISTRICT.

4. White quartzite. (Eureka quartzite.)

5. Dark gray limestone, massive bedding, with intercalated slialy layers

carrying a typical Silurian fauna. (Pogonip limestone.)

6. Siliceous cherty limastoiie

Feet

200

300

4,275

^Pogonip
Limestone.

'F.ureKa. JuoneM1. .Devonian.

Quartzite Limestone Limestone

FIG. 2. — Section across Lone Mountain.

Iii the Nevada limestone at Lone Mountain the fauna is exceedingly
rich in species. A list of the fossils occurring here, together Avith some
remarks upon their geological significance, will be found in the following
chapter in the discussion of the Devonian rocks.

CHAPTER IV.
DEVONIAN AND CARBONIFEROUS ROCKS.

DEVONIAN ROCKS.

By imperceptible gradations limestones of the Lone Mountain epoch
pass upward into those of the Devonian period, and as no definite horizon
separating them has as yet been determined 110 accurate measurements of
their respective thicknesses can be given. Devonian rocks cover a far
greater area in the district than those of any other period; they are much
more widely distributed and present a thickness greater than either the
Cambrian or Silurian. In no part of the Great Basin are they better
exposed than at Eureka, and as nowhere else have they been so carefully
investigated the district must long remain a typical one for the study of
Devonian strata. Notwithstanding the beds present a rich fauna, only two
subdivisions of the Devonian have been made — first, Nevada limestone, and
second, White Pine shale — although taken together they have a thickness
of about 8,000 feet, This division is based upon a marked change in both
the fauna and character of the sedimentation.

Nevada limestone.— The name selected to designate this horizon is taken
from the name of the state where the epoch is so well represented by a
broad development of beds and the only state or territory in the Great
Basin where it has been recognized as attaining any great thickness and its
limits and geological relations studied. As the designation of the epoch
would suggest, the beds throughout the entire series are composed mainly
of limestone, although intercalated beds of shale, quartzite, and sandstone
occur. The Lone Mountain and Nevada limestones taken together present
an immense thickness of beds, lying between the Eureka quartzite and
White Pine shale. Together they measure about 7,800 feet in their broad-
est development. The division into Silurian and Devonian is based mainly
upon paleontological grounds. The transition in sedimentation from char-
acteristic Silurian to unmistakable Devonian is so imperceptible that a

64 <;EOLOGY OF THE EUKEKA DISTRICT.

boundary between them is impossible to establish, and, as is usually the
case where beds form a continuous, conformable limestone series, a line
of separation based upon faunal changes must always remain more or less
arbitrary. Lithologically, in their broader features, the Silurian and
Devonian limestones are quite distinct; it is only in the intermediate beds
that no line can be drawn. The light gray and white siliceous beds that
form the mass of the Lone Mountain present a wide vertical range, and
in these beds are occasionally seen obscure impressions of Niagara corals,
and in other localities, in similar rocks not much higher up in the series,
occur Atrypa reticularis and other forms foreshadowing the Devonian. It
is known that characteristic Lone Mountain beds carrying Hah/sites
catenulatus extend for nearly 1,500 feet above the Eureka quartzite, and
that beds easily identified by their organic remains bring the Devonian
down to about 6,000 feet below the summit of the great limestone belt
lying between the Eureka quai-tzite and White Pine shale. Hatysites and
Atrypa reticularis were never found associated together, although it can not
be definitely stated that the former fossil does not appear as low down in
the limestone as the highest occurrences of the characteristic coral.

The Nevada limestone presents broad elevated rock-masses character-
ized by bold escarpments and castellated summits. Profound orographic
movements have broken this great body of limestone into massive blocks
intersected by gorges and canyons, affording a mountain scenery both
grand and picturesque, and one rarely equaled in any limestone region of
the Great Basin. Although these uplifted blocks afford abundant geological
exposures across the greater part of the limestone, in no one instance is
there a complete or in every way satisfactory section from base to summit.
In many localities the exposures extend upward from the summit of the
Lone Mountain several thousand feet into the Nevada beds; in others the
strata are well shown from the top down till cut off by some line of faulting
which hides all the lower limestones. Frequently the lower beds of the
Devonian are buried beneath the Quaternary plain. The region, how-
ever, affords many excellent and overlapping sections exposing from 4,000
to 5,000 feet of rock; one continuous series of beds being estimated at 5,400
feet, which includes nearly the entire Nevada epoch. Throughout the

NEVADA LIMESTONE. 65

Nevada limestone, the physical features of sedimentation are sufficiently
characteristic to correlate the strata when comparing a large number of
sections across several thousand feet, although the details across any one
section are not persistent enough to determine with precision the horizons
over any extended area. Modoc Peak, Combs Mountain, Atrypa Peak,
Woodpeckers Peak, and Newark Mountain afford typical sections.

In general the lower limestones are indistinctly bedded, light gray in
color, and highly crystalline, passing up into brown, reddish brown, and
gray beds, which are distinctly stratified and finely banded and striped,
presenting a somewhat variegated appearance on the weathered surfaces.
This latter feature is very persistent throughout the middle portion of
the limestone. In the upper members the limestones are more massive,
usually well bedded, and possess a normal bluish black and bluish
gray color. In broad masses it is difficult to distinguish the upper
members of the Nevada limestone from the Carboniferous limestone,
and they closely resemble the great bodies of the Wasatch limestone
of Utah. The intercalated bands of argillaceous shale and quartzite
vary greatly in width, but do not especially mark any part of the
limestone, except that they occur more frequently in the middle portion
than elsewhere. Calcareous shales are found throughout the epoch. The
limestones are everywhere more or less magnesian, nearly pure dolomites
frequently occurring in narrow layers. At the base of the section north of
Modoc Peak (Fig. 3) the rock carries 4O62 per cent of magnesium
carbonate, with Ol per cent of insoluble residue. In band 15, of the same
section, the dark colored limestone carries T26 per cent of carbonate of
magnesia, while the light colored rock holds 26 78 per cent.

The Modoc Section.— A section in detail across the strata, extending from
the summit of the Nevada limestones nearly to the base, was made
by Mr. J. P. Iddings. It was constructed across the high ridge lying
between Signal and Modoc peaks, beginning with the lowest rocks exposed
at a point northwest of the latter peak just east of the Modoc fault, and
terminating at the eastern base of the hills where the uppermost beds pass
beneath the valley accumulations (atlas sheet vn). The section measures
5,400feet. The beds trend obliquely across the ridge, striking N. 50°-55° W.
MON xx 5

GEOLOGY OF THE EUKEKA DISTRICT.

Nevada limestone — Devonian.

MODOC SECTION.

(Dark gray to bluish black massive limestone poor in fossils ; quite well
bedded ; weathering partly smooth and dark colored ; partly rough
and pitted and of lighter color ; mostly compact and massive, also of
uneven texture ; with numerous calci te seams.

( Light and dark colored limestone with Stromatopora and Chcetetes;
ISO ) contains two layers thinly bedded (fissile).
so Compact light yellow sandstone.

Light and dark colored limestone in layers 10 to 20 feet thick, with
Stromatopora and Clustetes.

Dark colored limestone with Stromatopora and CJiaitetes.

t Alternating layers (about 10 feet thick) of dark and light gray lime-
900 \ stone, finely banded and lined ; weathering brownish gray ; in places
( bearing Chatetes.

so Compact yellow sandstone.

ISO Dark and light gray limestone; indistinct bedding.

jo ( Compact yellow sandstone.

) Dark and light colored limestone interbedded in layers from 4 to 10
250 feet thick.

270 Light gray siliceous limestone ; very siliceous near base.

< Alternating beds of dark and light gray limestone; at base 30 feet;
ISO j very siliceous limestone ; with cross bedding on weathered surface.
•3O Compact yellow sandstone.

f. I Dark and light gray limestone in thick belts of dark, lighter, and
"*• \ gray colors.

22.5 Dark dense limestone ; well bedded; bearing fossils.
iOO Shaly limestone rich in fossils.

( Light gray siliceous limestone, with fine lines of bedding; in upper
SSO \ portion weathering in almost rectangular fragments; growing
( less siliceous toward the bottom.

/4O Light gray highly crystalline, saccharoid dolomite ; not siliceous.

2400

FIG. 3.— Nevada limestone— Modoc
section.

LAMOUKEUX SECTION. 67

The Lamoureux Section. — The section along the limestone ridge northeast
of the head of Lamoureux Canyon (atlas sheet ix) exposes 4,300 feet of
strata, the lowest members resting immediately upon the Eureka quartzite of
the flat-top hill about three-quarters of a mile south of Atrypa Peak. It is
impossible to say just how great a thickness of these beds should be
assigned to the Lone Mountain epoch. Unquestionajbly the lower members
of the Silurian are wanting, and if a line be drawn placing the alternating
blue and light gray bedded rocks No. 6, in the Devonian, it would give
about 800 feet to the lower group. About 500 feet above this line a fossil-
iferous belt comes in, carrying a well known Devonian fauna. This fossil-
iferous belt may be traced around to the east slope of Atrypa Peak, where
a most abundant fauna occurs rich in generic and specific forms. Here at
Atrypa Peak, however, there are nearly 2,000 feet of strata below the fos-
siliferous belt as against 1,300 feet in the Lamoureux Section before reaching
the Eureka quartzite, but as the inclination of the beds can not well be deter-
mined no accurate measurement of the thickness can be given. Apparently
the lowest horizon at Atrypa Peak is below the one shown in the section,
although the character of the sedimentation is much the same.

The section is as follows :

Section Hast of Lamoureux Canyon — 4,300 feet.

Feet.

1. Brown and blue limestone, well bedded, with occasional

mottled beds 300

2. Brownish gray, finely striped, well bedded limestone, with

corals 1, 000

3. Dark blue, light gray, and brownish limestone 1, 000

4. Alternating dark and light limestone 500

5. Fossiliferous shaly belt 200

6. Light blue and gray bedded limestone 500

7. Light colored siliceous limestone, with indistinct bedding. . 800

g ( Thin layer of black siliceous limestone.
' ( Eureka quartzite.

4,300

County Peak Section. — On the east side of the Eureka District, in the region
of County Peak, the Devonian rocks offer still another section quite similar
in the character of its sedimentation to those already given. It includes a
portion of the Lone Mountain rocks exposed in the bluffs on the east side

68 GEOLOGY OF THE EUREKA DISTRICT.

of C. C. Canyon and extends eastward until the upper members of the
Nevada limestone are submerged beneath the great basalt flow of Basalt
Peak and the Strahlenberg.

County Peak Section — 5,200 feet.

Feet.

1. Evenly bedded, bluish gray limestone, with interbedded

bauds of dark limestone 600

2. Irregularly bedded, blue limestone, with intercalated seams

of quartzite 1, 600

3. Yellowish gray quartzite, with narrow bauds of gray silice-

ous limestone 100

4. Massive beds of siliceous limestone alternating with beds of

pure gray limestone and narrow bands of quartzite 700

5. Massive, gray vitreous sandstone 100

6. Siliceous limestone in massive beds more or less siliceous in

thin bands, carrying shaly limestone belts 800

7. Grayish white, vitreous sandstone 100

8. Gray and blue limestone well bedded 500

9. Light colored, compact quartzite changing from red to white 50
10. Massive, light colored limestone without bedding, more or

less siliceous . . 650

5,200

In this section the lower 700 feet are assumed to belong to the Lone
Mountain, giving 4,500 to the DeAronian. This leaves about 1,500 feet of
the Upper Devonian strata wanting as compared with the beds in the region
of Modoc Peak. These upper beds are again well shown at Newark Moun-
tain and Mahogany Hills.

white pine shale.— Conformably overlying the Nevada limestone occurs a
heavy body of black shale, which has been designated as above, it having
been first recognized as a distinct horizon in the White Pine mining district
to the southeast of Eureka. It occupies a clearly defined stratigraphic
position with a marked change in the character of sedimentation and a
fauna distinct from both the underlying and overlying horizons.

There are only two large bodies of White Pine shale at Eureka, but
they both offer excellent rock exposures, one west of Newark Mountain,
the other east of Sentinel Peak. The shale is best studied west of Newark
Mountain (atlas sheet vi), where it forms the entire rock mass through which

DEVONIAN PLANT EEMAINS. 69

Hayes Canyon has been eroded and where its geological relations with the
Nevada limestone below and the Diamond Peak quartzite above may be
easily recognized. The shale attains its greatest development east of
Sentinel Peak and Sugar Loaf, but as it is cut off from the Nevada lime-
stone by a north and south fault which passes up Rescue Canyon its
stratigraphical relations with the underlying strata are not as clearly shown
as at the first locality, while the overlying beds are buried beneath the
detritus of the plain. The thickness across the broadest part of the White
Pine shale east of Sugar Loaf may be placed at 2,000 feet. A marked
feature of the beds is the rapid changes which they undergo, both in their
lateral and vertical extension, passing abruptly from pure, argillaceous, black
shale into beds more or less arenaceous and frequently carrying interca-
lated beds of red, friable sandstone appearing as lenticular masses in the
shale. In Hayes Canyon the beds for the most part are brownish black
shale, with thin bands of red sandstone while opposite Sugar Loaf the inter-
calated red sandstone strata occasionally attain a thickness of 100 feet. Out
in the valley the lines between the shale and sandstone may be easily fol-
lowed for long distances, the former occupying shallow, trough-like depres-
sions and the latter low intervening ridges slightly elevated above the gen-
eral level. Cross sections made at no great distances apart differ widely
in the character of the sediments. All evidence indicates a shallow-water
deposit. The formations at Eureka and White Pine are identical in every-
way except in thickness of deposits, at the latter locality measuring not
more than 600 feet.

Plant Remains in white pine shale.— Impressions of plants which are exceed-
ingly rare in Paleozoic rocks of the Great Basin are very abundant and
form a distinctive feature of this epoch, notwithstanding that everything
which has been collected is of fragmentary nature. The most promising
specimens for identification were submitted to Sir J. William Dawson,
who, in his report, called attention to the poor state of preservation of the
plants. Under date of Montreal, June 11, 1889, he writes:

One slab contains a small ribbed stem referable to Goeppert's Anarthrocanna,
a doubtful Calamitean plant. The specimen is not unlike those found at Perry, in
Maine, and Bay de Chaleur. On the large slab is also a slender branch stem which I
suppose may be the stipe of a fern, and from its character and angle of ramification

70 GEOLOGY OF THE EUKEKA DISTRICT.

probably belongs to the genus Aneimites, but no trace of the pinnae can be seen. The
evidence, so far as it goes, would indicate the Upper Devonian (or Brian, as I prefer
to call it,) rather than the Middle Devonian or the Lower Carboniferous.

It will be seen that this determination as to the age of the plants is
quite in accord with the geological position of the beds above the Nevada
limestone of the Devonian and directly below the Diamond Peak quartzite
of the Carboniferous.

Notwithstanding the great development of the black shales they have
as yet been recognized only in the two localities already mentioned, Eureka
and White Pine. On the east side of the Eureka District, if they are repre-
sented at all, it is only by 100 feet more or less of dark shaly beds, highly
arenaceous, and passing into sandstones and quartzites of the Diamond
Peak beds. There seems to be no doubt that the Diamond Peak forma-
tion in the Pinon Range rests conformably upon the Nevada limestone,
without the interposition of any great thickness of White Pine shales,
although there are a few black sandstones and narrow chert bands which
apparently represent the intervening argillaceous epoch. The evidence in
favor of this correlation is strengthened by the presence of poorly preserved
fragments of vegetable life wherever the black belt comes in. These inter-
vening beds have yielded one single species, Discina minuta, which, accord-
ing to Mr. C. D. Walcott, corresponds closely with typical specimens from
the Marcellus shale of New York. The fact that the White Pine shales are
Avanting over large areas, where both the Devonian and Carboniferous are
found together, renders it highly probable that these shallow water deposits,
although developed to a great thickness, form exceptional occurrences,
and that the Nevada limestone passes over abruptly into sandstones of
Carboniferous age. On the map (atlas sheet v) these intervening beds on
both sides of The Gate are included in the Nevada limestone.

Fauna of the Devonian. — As already mentioned, no subdivisions in the Nevada
limestone have been made. Geology as yet fails to furnish sufficient evi-
dence for drawing any sharp demarcation, sedimentation having gone on too
uniformly under similar conditions to form any marked change in the char-
acter of the beds. From the sections already given it will be seen that this
epoch was essentially a limestone-making one, the amount of sandstone de-

DEVONIAN FAUNA. 71

posited being relatively small. Paleontology fails equally with geology to
point out any strong reasons for subdivisions ; moreover, it would be impos-
sible, from our present knowledge, to subdivide the epoch into horizons as
recognized in the Mississippi Valley and the Appalachians of the Atlantic
coast. The groupings of fossils at the base and those at the top show very
considerable difference in the fauna, but the mingling of species throughout
the beds has rendered it difficult to draw any line of separation. Many of
the species characteristic of a restricted horizon elsewhere have been identi-
fied in the Nevada limestone, but with a wide vertical range, and in some
instances have reversed their relative positions, as recognized in New York
state. At no distant day, when the epoch becomes still better known and
comparative studies have been made with other localities in the Great Basin,
it may be quite possible and even desirable that such divisions should be
drawn. At present, however, it will be quite sufficient to speak in general
terms of an upper and a lower horizon.

The Nevada limestone has yielded an exceedingly rich and well preserved
fauna; certainly no epoch in the Great Basin can surpass it in general in-
terest, either in the variety of its organic forms, in the number of species
determined, or in the commingling of species found elsewhere in widely
separated localities. This terrane alone has yielded more species than the
Cambrian and Silurian periods together, and surpasses the entire Carbonif-
erous, with its great thickness and wide areas, by more than one hundred
specific forms. From Eureka and White Pine together it has furnished
over two hundred species, of which one-third have been described for the
first time in the report of Mr. C. D. Walcott;1 while, a fact of great interest
as regards geographical distribution, one hundred and nineteen of them are
specifically identical with previously described forms from other well known
Devonian localities, and no less than seventy-nine of them have been identi-
fied with species occurring in New York. The Upper Helderberg, Hamil-
ton, and Chemung are all well represented so far as species are concerned,
although the vertical range of certain species by no means agrees with the
limits assigned to them in New York. In comparing the Nevada limestone
of the Great Basin with the Devonian of New York state, Mr. Walcott says:

'Paleontology of the Eureka District, Monograph VIII. Washington, 1884.

72 GEOLOGY OF THE EUREKA DISTRICT.

The Upper Helderberg horizon of the New York series is represented by thirty-eight
species common to it and the lower portion of the Devonian of the Eureka district;
the Cheinung group of the same by sixteen species; of the Hamilton species of New
York twenty-three are distributed through the lower portion of the Eureka Devonian
limestone and eighteen species in the middle and upper portions, but not in such a
manner as to distinguish a middle division corresponding to the Hamilton formation
of New York. Of strictly Hamilton species in New York, twenty-three are found, of
which eleven are in beds a little below the summit, and twelve just above the base of
the formation.

Eleven species not known in New York are common to both the
Great Basin and Iowa, thus emphasizing the faunal relations between the
corresponding horizons in the Cordillera, the Mississippi Valley and the
Appalachians.

While the fauna at Eureka is rich and varied, both in genera and
species, remains of Devonian fishes appear to be restricted to a single
ctenacaiithus-like tooth. Mr. S. F. Emrnons, while engaged on the Fortieth
Parallel Exploration, brought in a small tooth of the genus Cladodm
from the western entrance to Emigrant Canyon, in the Tucubit Moun-
tains north of Humboldt River. These two single specimens, collected
at widely separated points, are all that is known of Devonian fishes from
Central Nevada, although from Northern Arizona, in the Kauab Canyon,
Mr. C. D. Walcott1 obtained abundant evidence of the presence of placo-
ganoid fishes from Devonian beds, which were represented by only 100
feet of strata as against 8,000 feet in Nevada.

Corals occur throughout the Nevada limestone and certain species
present a wide vertical range. Among these Stromatopora are known from
base to summit, and in one or two horizons they are found in such profusion
as to characterize the strata by the peculiar weathering-out of the imbedded
silicified corals. In the siliceous limestone of the Upper Devonian, fragments
of Syringopora associated with Stromatopora are occasionally abundant when
all other species are wanting. The bedded limestone on both sides of the
Yahoo Canyon offer favorable conditions for the preservation of these
forms. Prior to the survey of the Eureka District the Lamellibranchiates
were poorly represented from the Great Basin. To a meager list almost

1 Am. Jour. Sci. Sept., 1880.

DEVONIAN FAUNA. 73

wholly collected by the Geological Exploration of the Fortieth Parallel,
Eureka has now furnished no less than twenty-three genera 'and thirty-five
species.

In the collections from Eureka, occur two species, first described by
Mr. F. B. Meek/ Orthis macfarleni and EhynchoneUa castanea, from the Mac-
kenzie River. Both of these important species were brought to this country
by the late Mr. Robert Kinnicut, and were found associated together on
the Lockhart River, a tributary of the Mackenzie, in latitude 67° 15' north,
longitude 126° west, while the Orthis was also obtained in a very similar
limestone 40 miles below Fort Good Hope, on the Mackenzie. According
to Mr. A. K. Isbister," who traveled extensively in Northern British Amer-
ica, along the base of the Rocky Mountains, and who published a sketch
map of its geology, the Devonian extends through the valley of the Macken-
zie from its mouth southward for 15° of latitude, nearly, if not quite, to the
headwaters of the Saskatchewan River. It certainly is of considerable
interest to find these two species, which occur together in the Arctic
regions, associated at Eureka in the upper members of the Lower Devonian
They are found near Woodpeckers Peak, about 3,000 feet above the base
of the limestone, while B. castanea was also obtained from the upper hori-
zon at Rescue Hill.

Within the area covered by the Nevada limestone collections of fossils
were made more or less complete from nearly forty localities. For the pur-
pose of this volume it seems hardly desirable to publish the lists in full,
and such only are made use of as may be necesesary to elucidate for
geological purposes the faunal development and also to point out clearly
upon what evidence the division into two groups is based. Of the 6,000
feet included within the epoch, 4,000 are provisionally assigned to the lower
and 2,000 to the upper horizon. About two-thirds of the species belong to
the lower and one-third to the upper, corresponding roughly to the relative
thicknesses of the two horizons. The upper portion of the limestone,
however, represents a fauna equally varied, although not so complete, as
the lower. So far as they have been studied the upper and lower horizons
furnish quite characteristic faunas, with only seventeen species which may

'Trans. Chi. Acad. Sci., vol. i, pt. 1, 1867-'69, p. 88.

3 Quarterly Journal, Geological Society, vol. xi, London, 1855, p. 497.

74 GEOLOGY OF THE ETTEEKA DISTEICT.

be considered as common throughout the epoch. The following list com-
prises the species common to both upper and lower horizons:

Stromatopora, ? Productus shumardianus, var. pyxidatus.

Syringopora perelegans. Productus subaculeatus.
Streptorhynchus chemungen- Spirifera pinonensis.

sis var. pandora. Spirifera (M.) maia.

Orthis tulliensis. Atrypa recticularis.

Strophodonta perplana. Bhynchonella castanea.

Chonetes deflecta. Nyassa parva.

Productus hallanus. Paracyclas occidentalis.

Productus shumardianus. Styliola flssurella.

A complete systematic list of all the genera and species known from
the Nevada limestone at Eureka and White Pine tabulated into an upper
and lower group, will be found as an appendix at the end of this volume.

At Eureka, above the light gray, crystalline strata carrying the
Halysites, and somewhere near the base of the Nevada limestone, the beds
begin to yield Atrypa reticularis, Spirifera, Stromatopora, and Edmondia,
which have a wide vertical range, all but the latter extending well up
nearly to the top of the limestone. The lowest well denned fossiliferous
belt carrying a decided Devonian fauna is found at Lone Mountain not
far above the Silurian line. The fauna is uncommonly rich in species, no
one locality having furnished quite as many forms. They occur in shaly
strata in belt No. 2 of the Lone Mountain section. No less than fifty-two
species were obtained from this horizon. The list of fossils is as follows:

Liiigula Ifena. Strophodonta perplana.

Liugula lonensis. Strophodonta puuctulifera.

Discina, sp. ?. Chonetes filistriata.

Pholidops bellula. Chonetes hemispherica.

Pholidops quadrangularis. Chonetes macrostriata.

Orthis inipressa. Productus shumardianus.

Skeuidium devonicum. Productus subaculeatus.
Streptorhyuchus chemungensis, var. Productus navicella.

perversa. Spirifera piiioneusis.

Strophomena rhomboidalis. Spirifera raricosta.

Strophodonta arcuata. Spirifera varicosa.

Strophodonta calvini. Nucleospira concinna.

Strophodonta pattersoni. Trematospira infrequens.

DEVONIAN FAUNA. 75

Atrypa desquamata. Paracyclas occidentalis.

Atrypa reticularis. Microdon macrostriata.

Meristella nasuta. Anadontopsis amygdalffifonnis.

Ehynchouella tethys. Sckizodus orbicularis.

Cryptonella circula. Platyceras nodosum.

Pentamerus comis. Loxonema nobile.

Pterinea flabella. Bellerophon pelops.

Mytilarca dubia. Tentaeulites gracilistriatus.

Plethomytillis oviforme. Orthoceras (2 sp.t).

Modiomorpha altiforme. Beyrichia occidentalis.

Modiomorpha obtusa. Phacops rana.

Goniophora perangulata. Dalmanites rneeki.

Megainbonia occidualis. Proetus marginalia.
Edmondia pifionensis.

About 500 feet above this belt, in the dark gray limestone, occurs a
group of fossils, mainly silicified corals, as follows:

Paleomanott roemeri. Cyathophyllum davidsoni. •

Stroinatopora. Cyathopliyllum rugosuin.

Favosites basaltica. Diphyphyllum simcoense.

Favosites hemispherica. Cystiphyllum americanum.

Favosites, n. sp. Zaphrentis, sp. !.

Syringopora perelegans. Atrypa reticularis.

Above this latter grouping only a few fossils were found, mainly species
like Atrypa reticularis and Styliola fissurella, which occur all through the
epoch. In this long list of species from the base of the Devonian at Lone
Mountain, only seven forms occur which are known in the Upper Devonian,
the list as a group being decidedly Lower Devonian in character. Skenidiwn
devonicum is the only species of this genera which is known above the
Silurian, while Atrypa desquamata, here associated with A. reticularis, occurs
only in the lower beds. The Devonian trilobites in this list occur in
nearly all the other fossil-bearing beds at the base of the Nevada limestone —
namely, Combs Peak, Atrypa Peak, Brush Peak — but are not found in the
middle or upper horizons. It will be noticed that the list includes quite a
number of species usually regarded as characteristic types of the Upper
Helderberg.

At Combs Mountain, Atrypa Peak, Brush Peak, Modoc Peak, and
several other localities occur fossiliferous calcareous shale bands well defined

76 GEOLOGY OF THE EUKEKA DISTRICT.

lithologically, which present much the same aspect at each place, with a
similar Lower Devonian fauna, many of the forms being specifically
identical. The evidence goes to show that they belong essentially to the
same horizon, although the estimated vertical distance of the beds above
the Eureka quartzite varies considerably in the different localities. This
difference is undoubtedly due in part to the varying thickness of the under-
lying Lone Mountain beds resting on the quartzite and partly to the more
rapid changes in some places than in others in the nature of the sedi-
mentation. In certain localities, under favorable conditions, the cal-
careous shale seems to have been deposited earlier than elsewhere. In
other words, the shale belts are not absolutely synchronous ; in some places
they are known to be wanting. They may be taken as representing char-
acteristic horizons in the Lower Devonian without at the same time occu-
pying a sufficiently definite position to be made a datum point in deter-
mining the thickness of the strata between the shale belt and the basal
member of the epoch.

The following list includes all species obtained from the calcareous
shale belts of Brush Peak, Atrypa Peak, and Combs Mountain. The
numerals affixed opposite the name of each species indicate from which of
the three localities they have been obtained. In this way it will be seen
at a glance which forms are common to more than one of these typical
localities.

— 3 Strom atopora. - 2 - Strophodonta demissa.

- 2 - Favosites basaltica. -23 Strophodonta inequiradiata.
1-3 Favosites n. sp. - 2 3 Strophodonta perplaua.

— 3 Pachyphyllum woodmani. - 2 - Strophodonta punctilifera.

- 2 3 Zaphrentis. 1 2 - Chonetes deflecta.

- 2 - Lingula whitei. - 2 3 Chonetes filistriata.

1 2 - Orthis impressa. 1 — Chonetes granulifera.

123 Streptorhynchus chemungen- 1 — Chonetes hemispherica.

sis. 1 2 - Chonetes niacrostriata.

- 2 3 Streptorhynchus chemungen- - 2 - Productus navicellus.

sis, var. pandora. - 2 - Productus subaculeatus.

- 2 - Streptorhynchus chemuugen — 2 - Productus truncata.

sis, var. perversa. 123 Spirifera pinouensis.

1-3 Strophodonta calvini. - 2 - Spirifera undifera,

DEVONIAN FAUNA. 77

- 3 Spirifera sp.?. l _ 3 Platyceras conradi.

- 2 - Atrypa desquamata. - 2 - Platyceras dentalium.
123 Atrypa reticularis. I - - Platyceras thetiforme.
1 2 - Rhynchonella horsfordi. - 3 Platyceras thetis.

- - 3 Rhynchonella occidens. 1 - - Platyceras undulatum.

- - 3 Rhynchonella tethys. 1-3 Platyostoma lineata.

- 3 Pentamerus comis. - 2 - Ecculiomphalus devonicus.

- 2 - Leipteria rafinesqui. - 2 3 Euomphalus eurekensis.

- 2 - Limoptera sarmentica. - 3 Calonema occidentalis.

- 2 - Mytilarca sp.!. - 2 - Cyclonema (like C. midtilera).

- 2 - Modiomorpha oblouga. - 2 - Loxonema approximatum.
1 — Modiomorpha obtusa. - 2 3 Loxonema uobile.

- 2 - Goniophora perangulata. - 3 Loxonema subattenuata.

- 2 3 Edmondia pifionensis. - 2 - Bellerophon neleus.

- 3 Sanguinolites combensis. 123 Bellerophon perplexa.

1 - - Sanguinolites gracilis. - 2 - Scoliostoma americana.

- 2 - Sanguinolites sauduskyensis. 1 - - Tentaculites attenuatus.
— 3 Conocardium iievadensis. - - 3 Tentaculites scalariformis.

- 2 - Posidomya devonica. - 2 - Hyolithes sp. ?.

- 2 - Posidomya laevis. - 2 - Orthoceras sp. ?.

- 2 - Microdon macrostriata. - 3 Goniatites desideratus.

- 2 - Schizodus orbicularis. 123 Phacops rana.

- 2 - Cypricardinia iudenta. 123 Dalmanites meeki.

- 2 3 Platyceras carinatum. - 3 Proetns marginalis.

[No. 1, from the south slope of Brush Peak. No. 2, from the shale belt of Atrypa Peak. No. 3, from

the west Bpur of Combs Mountain.]

The shale belt of Brush Peak promises to the collector a most varied
fauna of Lower Devonian species. It measures about 150 feet in thickness
and may be traced along the west side of both Brush and Modoc peaks;
thence still farther northward, where its connection is clearly made out
with shale belt No. 3, of the Devonian section, south of Signal Peak.
On the southeast slope of Atrypa Peak the shale belt crosses the spur
striking N. 30° E., dipping 40° W. The beds are of a light bluish gray
color about 150 feet in thickness. The horizon corresponds to the fos-
siliferous shale belt in the section east of Lamotireux Canyon (p. 67).

Combs Mountain presents upon its south side a fine display of massive
limestone beds dipping northward into the mountain. There is exposed
here between the base of the mountain and the summit of the ridge

78 GEOLOGY OF THE EUEEKA DISTRICT.

nearly 5,000 feet of strata. No line of demarcation can be drawn
here between the Lone Mountain and Nevada epochs. Fossils were
rarely met with except in well denned strata, separated by long ver-
tical intervals. The Trenton horizon, which is well represented, is esti-
mated at 300 feet in thickness, resting immediately upon the Eureka beds.
From the top of the Trenton the section across the beds is strikingly similar
to those observed at Atrypa and Brush peaks. Careful estimates place the
fossiliferous shale at 1,700 feet above the Trenton or 2,000 feet above the
Eureka quartzite. This is the same vertical distance above the quartzite
assigned to the shale belt at Atrypa Peak, although at the latter locality
the Trenton limestone is not recognized either by its physical features or
its organic forms. From the shale belt to the top of the ridge the only
species secured were corals having a wide vertical range or else fragments
too imperfect for specific description. A comparison of the species obtained
in the three shale belts, taken together with the stratigraphy of the beds,
proves without much doubt the equivalency of the Combs Mountain shale
with those at Atrypa and Brush peaks.

In the County Peak body of limestone the lowest organic remains
obtained occur midway in the siliceous limestone beds of No. 6, of the
County Peak section (p. 68). Here the gray and blue limestone of No.
8 is assigned to the base of the Devonian, which places the fossil-bearing
bed about 1,000 feet above the Silurian. The species recognized are
Edmondia pinonensis, Atrypa reticularis, Spirifera sp. ?. and Cladopora sp. I.

Passing upward for 2,000 feet above this last bed, or 3,000 feet above
the base, and in about the middle of the great limestone belt (No. 2), there
occurs in a thinly bedded bluish gray limestone an interesting grouping of
species characteristic of the middle Devonian, or rather a mingling of
species from both upper and lower horizons. The bed, owing to its
marked lithological features, may be traced by the eye for long distances
along the slope of the mountains. At Woodpeckers Peak, where the col
lection was made, the fauna is by no means as large or as varied as that
found in the lower shale belt. While many species are identical with those
found at the lower horizon, and present a decided Lower Devonian aspect,
the greater part of them are common to both Upper and Lower beds. It is

UPPER DEVONIAN FAUNA. 79

at this locality that the two Mackenzie River species are seen associated
together in the same matrix. The following is the list of species collected at
Woodpeckers Peak:

Orthis macfarleni. Productus truncatus.

Streptorhynchus chemungensis, var. pandora. Spirifera (M.) maia.

Streptorhynchus chemungeiisis, var. perversa. Atrypa reticulans.

Ehynchonella castanea. Nyassa parva.

Strophomena rhomboidalis. Edmondia piuonensis.

Chonetes deflecta. Paracyclas occidentalis.

Productus hallanus. Metoptoina devonica.
Productus subaculeatus.

On the south slope of Sentinel Peak, southeast of the last locality, at
about the same horizon as the grouping of fossils, a small collection
was obtained, all but two of them being identical with those observed at
Woodpeckers Peak, and all of them, without exception, forms recognized
from the Upper, as well as the Lower, horizon. The two species not known
at Woodpeckers Peak are Styliola fissurella and Lingula ligea, var. nevadensis,
the former common tliroughout the Nevada limestone, and the latter a
Hamilton species of New York state, collected also from Rescue Hill, of
the Upper Devonian.

Another 1,000 feet of limestone reaches the dark blue massive beds in the
upper part of No. 1 of the County Peak section. If the somewhat arbitrary
line, provisionally drawn between the Upper and Lower Nevada limestones,
is correctly placed about 4,000 feet above the base of the Devonian, these
beds would lie at the base of the upper series. In all probability they belong
to the Upper Nevada limestone, although there is nothing sufficiently dis-
tinct in the meager fauna obtained to determine the question definitely.
The only species observed which is at all restricted in its range is Spirifera
engelmanni, a form common to the highest members of the epoch, but no-
where as yet found lower down than these intermediate strata. Somewhat
higher beds give much the same grouping of fossils, and in several localities
Spirifera engelmanni has been recognized. The highest horizon in this
great mass of limestone from which fossils have been obtained is in a well
stratified blue bed near the mouth of Packer Basin, where the fauna has a
decidedly Upper Devonian aspect. Among the species collected here are

80 GEOLOGY OF THE EUEEKA DISTRICT.

Spirifera engelmanni and the two Chemung forms, Rliynchonella duplicate and
E. sinuata, both fcmnd at several localities in the Upper Nevada limestone.
Rescue Hill, on the east side of Rescue Canyon, is a faulted block of
Devonian limestone. Along the abrupt east slope of the hill the north and
south Rescue Canyon fault cuts off the limestone from that found on the
opposite side of the canyon, while an east and west fault, approximately
coinciding with the course of Silverado Canyon, intersects the Rescue Can-
yon fault, and separates the hill from the limestone body to the north.
The beds forming the summit of Rescue Hill belong to strata somewhat
higher in the series than those found on the summit of Sentinel Peak and
Island Mountain, but the lower limestones of the three localities may be
easily correlated. In a light bluish gray limestone just below the top of
Rescue Hill the following grouping of fossils occurs :

Lingula ligea, var. iievadensis. Mytilarca chemungensis.

Productus hallanus. Leptodesma transversa.

Productus shumardianus. Nucula rescueusis.

Productus stigmatus. Nucula (like N. niotica, Hall).

Productus subaculeatus. Grammysia minor.

Spirifera (M.) maia. Sanguinolites ventricosus.

Atrypa reticularis. Paracyclas occidentalis.

Rhynchonella castanea. Platyceras carinatus.

Rhynchoiiella duplicata. Bellerophon pelops.

Ehynchouella (L.) laura. Naticopsis, sp.f (like If. aequistriata, Meek).

Rhynchonella iievadensis. Tentaculites gracilistriatus.

Rhynchonella siuuatus. Styliola fissurella.

Cryptonella piuonensis. Proetus haldermanni.

The Rescue Canyon fault, as already described, is a profound displace-
ment. After crossing Silverado Canyon at the head of Rescue Canyon, it
extends northward until concealed beneath the great basalt flow. By
reference to the map (atlas sheets vm and x) the course of the fault will
be seen along the base of Sugar Loaf and Sentinel Peak. In the faulted
block to the eastward there occurs a wedge-shaped mass of Devonian
limestone lying north of Silverado Canyon and east of Island Mountain
and Sugar Loaf. It conformably underlies the great body of White Pine
shale and admirably shows the relation between the Nevada limestone
and the overlying shale. These beds directly underlying the shale are of

WHITE PINE SHALE FAUNA. 81

course the uppermost members of the Nevada limestone. The following
section gives the sequence of beds from the Quaternary plain westward
across the White Pine shale and the underlying limestone until the beds

are cut off by the fault.

i ... •

1. Shaly sandstone followed by 50 foot of dark argillaceous shale and a great thickneas of

arenaceous shale and thinly bedded sandstone ; occasional lied* of fine siliceous con-
glomerate ; constant changes from shale to sandstone 1,000

2. Black argillaceous shale passing into arenaceous shale and shaly sandstone becoming dis-

tinctly bedded and passing up into a fine siliceous conglomerate. Throughout the series

are occasional thin belts of argillaceous shale 400

3. Gray criuoidal limestone in layers of varying thickness and more or less sandy ; carries

Chonetea 50

4. Dark bluish black argillaceous and calcareous shale weathering yellow on the surface ;

fossiliferons 300

5. Blue limestone with alternating thin massive layers ; fossiliferous 250

6. Siliceous limestone passing into gray limestone with irregular seams and nodules of

calcite 150

The lower gray limestone carries no fossils.

In the massive blue limestone (No. 5) occur the following Upper
Devonian species:

Productus shumardianus. Ehynchonella duplicate.

Spirifera eiigelmanni. Leperditia rotundatus.

Atrypa reticularis. Styliola flssurella.

In the overlying 300 feet of clay shales (No. 4) the more calcareous
portions carry Spirifera engelmannl and Productus shumardianus, while in
the more argillaceous strata are numerous imperfect plant remains.

The gray limestone (No. 3) overlying the black shale is characterized
by typical Devonian forms: Chonetes mucronata, Spirifera ctiijclnuinni and
Beyrichia occidentalis. Above this latter limestone in the clayey and sandy
strata (Nos. 1 and 2) no invertebrate forms have as yet been obtained, but
numerous fragments of plant remains, some of which would doubtless
admit of generic determination, are abundant. A careful search for a
Devonian flora would yield important results. The evidence of the
Devonian age of the upper 1,400 feet of shales and sands is apparent,
from the identity of the plants with those obtained from the black shale
below the gray limestone as well as from the character of the sediments.
Another locality where the Nevada limestone and White Pine shale are
MON xx (i

82 GEOLOGY OF THE EUBEKA DISTRICT.

structurally well shown with a typical fauna in both horizons is found at
Newark Mountain. The mountain presents a bold impressive mass of
bluish gray limestone with the physical features of the Upper Devonian
strata. The section here is as follows:

Feet.

1. Black argillaceous shale more or less arenaceous and similar to the lower black shale 1,000

2. Compact fine grained sandstone with minute dark siliceous pebbles scattered through

the beds 100

3. Black argillaceous shale with fine intercalated beds of arenaceous shale. These shales

crumble. oil exposure to atmospheric influence 500

4. Reddish gray shaly calcareous beds 100

5. Dark gray heavily bedded siliceous limestone passing into bluish gray limestone in places

finely banded 3,500

Several hundred feet below the top of the Nevada limestone and cal-
careous shale the limestone yielded a small group of fossils, some of them
common to both the upper and lower horizons, but none of them character-
istic of the Lower Devonian.

Stromatopora. Spirifera pifiouensis.

Strophodonta perplana. Arrypa reticularis.

Producing shamardianns. Pterinea newarkeusis.

Spirifera disjuncta. Platyschisma maccoyi.

Immediately below the black shales, near the eastern end of Newark
Mountain, the folio wing species occur:

Ortliis tulliensis. Atrypa reticularis.

Spirifera disjuncta. Nyassa parva.

Spirifera engelmanni. Straparollus newarkensis.

Athyris angelica. Beyrichia occidentalis.

Reddish gray calcareous shales pass rapidly into the argillaceous
beds. Invertebrate remains wherever found in the black shale are imper-
fectly preserved so that specific determinations are in most instances out of
the question. From the lower beds were obtained Auiculopecten and a
species of Goniatites, while the upper and rather more sandy beds have
furnished a more varied material in which, according to Mr. C. D. Walc<>Tt,
the facies is Devonian with a foreshadowing of the Carboniferous period.
Among the genera found here are Fenestettu) Chonetes, MotKomorpha, sp. ?.
Cypricardinia, sp.f, Palaeoneilo, sp.f, Cardiomorpha, sp.?, Conocardium, sp. ?,
and Goniatites. In only two cases were specific determinations possible:

FAUNA. OF YAHOO CANTOM. 83

Productus hirsutiforme and Coholm Icevix. Plant remains occur here sim-
ilar to those found east of Sugar Loaf, but still less perfectly preserved.
The identification of the flora from the former locality places the age of
these beds without doubt at the top of the Devonian, in accordance with
their stratigraphical position. The corresponding horizon at White Pine
Mountain presents still stronger evidence of the Devonian age of the
shale; but here, as well as at Eureka, little has been accomplished by
investigating this ancient flora.

Passing to the Pinon Range and the Mahogany Hills in the northwest
corner of the district, the Upper Devonian limestone is well exposed in
massive beds lying beneath the Diamond Peak quartzite. It is easily deter-
mined by its lithological habit and fauna, as well as by its geological
position beneath the Carboniferous quartzite. Fossils are known in a num-
ber of places, but the localities which have furnished the largest and most
varied fauna and offer the most promising return are found on the east side
of Yahoo Canyon and north side of The Gate. Near the entrance to
Yahoo Canyon the beds have yielded a rich fauna characterized by silici-
fied corals. The grouping here is as follows:

Strornatopora. Pachyphyllum woodmuui.

Alveolites rockfordeiisis. Spirifera glabra, var. nevadensis.

Cladopora pulchra. Spirifera disjnncta.

Syringopora hisingeri. Atrypa recticulans.

Syringopora perelegaus. Bhynchonella c.astanea.

Cyathophyllura corniculuni.f Styliola fissurella.

On the north side of The Gate, at a little higher horizon and directly
beneath the quartzite, there is exposed a fine section, 500 feet in thickno-.
of massive blue limestone, passing into shaly beds, in places almost fissile.
Fossils characteristic of the Upper Devonian are abundant throughout the
beds. The limestone yielded the following species:

Stromatopora. Orthis tulliensis.

Syringopora hisiugeri. Productus lachrymosa, var. liina.

Syringopora perelegans. Productus schninardiauus.

Cyathophyllum corniculum. Productus speciosus.

Disciua iniuuta. Productus stiginatus.

Orthis impressa, Productus subaculeatus.

84 (iEOLOGY OF THE EUKEKA DISTRICT.

Spirifera disjuncta. Sanguinolites rigidus.

Spirifera engelmanni. Paracyclas occidentalis.

Athyris angelica. Euomphalus (P.) laxus.

Atrypa reticularis. Euomphalus, sp.?

Ehynchonella pugnus. Platyschisma 1 ambigua.

Bhynchonella (L.) laura. Naticopsis, sp.?

Ehynchonella (L.) nevadensis. Styliola fissurella.

Ehynchonella (L.) sinuata. Cytoceras nevadensis.

Grammysia minor. Orthoceras, sp. !

It is immediately overlying the limestone holding this fauna that the
argillaceous, cherty beds occur which carry poorly preserved fragments of
plant remains and the single species, Discina minuta. They probably
represent the great development of the White Pine shale found upon the
east slope of Newark Mountain, but they are not represented on the map,
as they are recognized only in a few localities lying between the Nevada
limestone and Diamond Peak quartzite.

CARBONIFEROUS ROCKS.

Although rocks of this period cover large areas and make up the
greater part of many mountain ridges in the Great Basin, few localities
offer better exposures of all the epochs into which they have been divided
than that portion of the Diamond Range which lies within the limits of the
Eureka survey. To the northeast and east of Eureka, Carboniferous rocks,
more especially the limestones, present a greater thickness of strata than is
shown here, but inmost cases the single, narrow ridges fail to expose in anv
continuous section the entire series of rocks from base to summit, At
Eureka the Carboniferous rocks have been estimated to measure 9,300 feet
in thickness, which, however, does not represent the full development of
the Carboniferous period, the Upper Coal-measures, the top of the Paleozoic
system having suffered a very considerable amount of erosion. This upper
limestone is by no means as thick as that found elsewhere.

The Carboniferous rocks have been subdivided into four epochs: First,
Diamond Peak quartzite; second, Lower Coal-measure limestone; third,
Weber conglomerate; fourth, Upper Coal-measure limestone.

DIAMOND PEAK QUAETZITE. 85

Diamond Peak Quartzite.-This epoch, the base of the series, takes its name
from Diamond Peak, where it is exposed on both flanks of the peak, dip-
ping into the range with a synclinal structure. On the west side of the
peak, where it attains its greatest exposure, it measures about 3,000 feet in
thickness. Beds of this epoch are found only at Diamond Peak and on
the opposite side of the valley in the region of The Gate. At the base of
the horizon fine conglomerates firmly cemented together lie next the argil-
laceous shale of the White Pine epoch, but quickly give place to a more
massive, usually vitreous, quartzite with a characteristic grayish brown color
and breaking irregularly with a flinty fracture. Intercalated black cherty
bands, carrying a more or less ferruginous matter, occur near the middle
portion of the horizon. Near the summit the beds pass into thinly
laminated green, brown and chocolate-colored schists and clay shales. The
Carboniferous age of the epoch is determined by a narrow belt of blue
limestone, which occurs iuterstratified in the quartzite about 200 feet above
its base, in which the widespread species Productus semireticulatus occurs
associated with an undetermined species of Athyris. As the fauna at the
top of the black shales foreshadows the coming in of the Carboniferous, the
presence of this characteristic Productus, with only a Carboniferous fauna
higher up in the series, determines without question the geological position
of the quartzite between the black shale and Coal-measure limestone.

Lower Coal-measure Limestone.— Beds of this epoch are found in a great
number of ranges in Utah and Nevada, stretching all the way from the
Wasatch to Battle Mountain, and the horizon has probably been better
studied than any other in the Great Basin. The beds cover large areas at
Eureka and offer better exposures than any other division of the Carbon-
iferous. In the Diamond Range they overlie conformably the Diamond
Peak quartzite, the transition beds passing rapidly from siliceous to cal-
careous sediments. In their lithological character and physical habit they
do not differ essentially from the same beds elsewhere, except, perhaj»,
at their base, where they carry intercalated beds of chert, argillite, and
gritty, pebbly limestone, with evidences of shallow water deposition. They
pass rapidly, however, into purer gray and blue limestone, for the most
part heavily bedded and distinctly stratified at varying intervals. In

86 GEOLOGY OF THE EUKEKA DISTRICT.

broad masses they resemble the Upper Nevada limestone, but are rather
lighter in color in distinction from the dark blue and black of the latter
horizon. No true dolomite beds of any considerable thickness have been
recognized, 9 '21 per cent being the largest amount of magnesium carbon-
ate obtained in any of the rocks subjected to chemical analysis. Across
their broadest development they measure about 3,800 feet in thickness,
which is much less than has usually been assigned to this horizon in other
mountain uplifts, more especially those lying eastward.

As the term Lower Coal-measure has been employed by most geolo-
gists to designate this epoch throughout the Great Basin, it has been
thought best to retain the name provisionally, although not exactly appli-
cable, as the epoch includes such a commingling of species from both the
Upper and Lower Coal-measures that a separation of the beds seems quite
impossible. Moreover, those distinctions which hold good in the Missis-
sippi Valley are by no means always applicable to the Cordillera. In the
present state of our knowledge of the Carboniferous limestone, it is impos-
sible to establish subdivisions in either of the Coal-measure epochs, based
upon faunal differences, owing to the fact that so many species extend
through a wide vertical range, and so few characteristic species occur within
restricted limits.

Lower Coal-measure Fauna.— As the limestones are in general favorable to
the preservation of organic remains, fossil-bearing strata are found through-
out the beds, and geologists are not so dependent upon definite horizons
as among Lower Paleozoic rocks. About 100 species have been collected
from this epoch, but most of those obtained from the upper and middle
portions have already been recognized as occurring elsewhere in the
Lower Coal-measures of the Great Basin. In comparison with the new
species obtained from the Cambrian, Silurian and Devonian, the Carbon-
iferous of Eureka offer singularly few forms new to science, but this, of
course, may be accounted for by the thorough researches which have been
made in this period elsewhere. At the base of the limestone the life is
more varied and presents certain facts that are of both geological and
biological interest.

Three salient features in the life of the Lower Coal-measures at Eureka

CARBONIFEROUS FRESH-WATER LIFE. 87

call for special mention, and each is worthy of still further investigation:
First, the occurrence near the base of the limestone of a fresh- water fauna;
second, the varied development of the Lamettibranchiates, a class which has
heretofore been but sparingly represented in the collection of Carboniferous
fossils from the Cordillera ; third, the mingling near the base of the horizon
of Devonian, Lower Carboniferous and Coal-measure species in gray lime-
stone directly overlying beds characterized by a purely Coal-measure fauna.

Fresh-water Life.— The lowest strata from which we have any record of
organic life from this epoch are found at the extreme northeast corner of New
York Mountain, and also near the railway cut immediately south of the
Richmond furnaces. Both localities lie just east of the Hoosac fault, which
brings up Carboniferous beds against the Silurian. But for the alluvial
deposits, which occupy the valley, the beds of the two localities would
probably be found to be continuous; the rocks in botli are similar. There
occur here 100 feet or more of fine clays and grits, interstratified with
arenaceous and argillaceous limestones passing up into pure limestone,
showing abrupt changes and rapid deposition. In these transition beds
were found abundant evidence of a varied fresh-water life, it being possi-
ble to determine several distinct species. The shells indicate a shallow
water fauna, as is also clearly established by the mode of deposition of the
sediments. Mingled with these shells are a few fragmentary bits of twigs
and stems of plant life, for the most part referable to a coniferous growth,
and showing signs of having been washed down from a land surface that
could not have been very far away. Mr. Walcott has briefly described
three species: one belonging to the genus Physa, named by him P. priscu;
another form is a pulmonate shell, allied to the genus Auricula, and to
which he has given the name Zaptychius carboiiaria; a third shell is related
to, if not identical with, Ampullaria, and is provisionally named after the
Director of the Geological Survey, A. powetti. The discovery of fresh or
brackish water shells so low down in the Paleozoic and so remote from any
known locality of similar beds renders their mode of occurrence one of
peculiar interest.

Lameiubranchiate Fauna.— From the horizon of the Lower Coal-measures
there have been collected over forty species of Lamellibranchiate shells, a

88 GEOLOGY OP THE EUREKA DISTRICT.

class which heretofore has been but sparingly represented ill the collections
of Carboniferous fossils from Utah and Nevada. Indeed, all told, there
have been but few species recognized from the Paleozoic of the Great
Basin. Most of those collected at Eureka are new species, described for
the first time, but allied to forms found in the Mississippi Valley and At-
lantic States, while others appear to be identical with well known species
A complete catalogue of the Lamellibranchiates will be found under the
lists of Devonian and Carboniferous species in an appendix at the end of
this volume.

Commingling of Carboniferous Species.— Prof. R. P. Wllitfield aild Dr. C. A.

White have frequently called attention to the commingling of Lower Car-
boniferous and Coal-measure species in New Mexico, Colorado and Utah
which, in the Mississippi Valley, are quite distinct and regarded as char-
acteristic of one or the other of the two horizons. So far as known to
the writer nowhere is this commingling of types more strikingly brought
out than at Eureka. Moreover, here they are associated with species
which, in New York and Ohio, are regarded as typical of the Devonian,
several of them being restricted within a very limited vertical range.
This grouping of fossils is found on a low hill on the west base of
Spring Hill, a long monotonous ridge lying just to the east of
the Hoosac fault and made up wholly of Lower Coal-measure strata.
The beds of Spring Hill Ridge, along the fault, for the most part dip
toward the east. On a small but prominent outlying hill on the western
slope of the ridge they lie inclined toward the west, the result of an anti-
clinal fold within the main body of limestone. In this outlying hill occurs
a well marked bed of arenaceous limestone dipping about 50° to the west
towards the Hoosac fault and cropping out both on the east and west slopes
of the hill; the same bed being recognized in the main ridge on the
opposite side of the anticline. This limestone, which has been traced for
short distances, both north and south, has furnished a most varied fauna.
Owing to its paleoutological importance, Mr. Walcott has given especial
attention to the group and has distinguished over fifty forms, most of which
he has specifically determined. About one-third of them he regards as ideu-

MINGLING OF CARBON J FERGUS SPECIES. 89

tical with species found in the Mississippi Valley in Lower Carboniferous
rocks, while many of them have usually been considered as restricted to
that horizon. Associated with them, in sufficient force to sho\v ;i comming-
ling of types, occur characteristic Coal-measure fossils like Athyrix subtilita
and Euomphalus subrugosus. Mingled with these fossils, in the same strata,
are the Lamellibranchiates, which present so striking a feature of the Carbon-
iferous fauna. Notwithstanding the fact that the Devonian, at Eureka,
furnishes an exceptionally rich fauna in Lamellibranchiates, nearly all the
species found in the Carboniferous occur for the first time at this horizon,
and but few, if any, specifically agree with the Devonian forms. This is all
the more noticeable because species, which are identical with those found
in New York and Ohio, are in the latter localities only recognized in restricted
areas xand inmost instances from horizons low down in the Devonian. This
is well shown by the species Grammysia arcuata and Macrodon hamiltonte,
both regarded as typical of the Hamilton group, while others like Sanguino-
lites ceolus is referred to the Chemung and to the Waverly sandstone of
Ohio.

The complete list of species from these strata is as follows :

Archseocidaris, sp. f Aviculopecten, sp. !

Fenestella (3 sp. !) Myalina nessus.

Discina newberryi. Pterinopecten hoosacensis.

Streptorhynchus crenistria. Pterinopecten spio.

Grthis resupinata. Crenipecten hallanus.

Chonetes grannlifera. Ptychopteria protoformis.

Chonetes verneuiliana. Pinna consimilis.

Productus prattenianus. Pinna inexpectans.

Productus semireticulatus. Modiomorpha ambigua.

Spirifera camerata. Modioiuorpha ! desiderata.

Spirifera neglecta. Nucula insularis.

Spiriferina kentuckiensis. Nucula, sp. ?

Athyris subtilita ? Solenomya curta.

Ehynchonella eurekensis. Macrodon truucatus.

Rhynchonella (Leiorhynchus type). Grainmysia arcuata.

Aviculopecteu affinis. Grammysia liannibaleiisis.

Aviculopecten eurekensis. Edinondia medoii.

Aviculopecten haguei. Sanguinolites a?olus.

Aviculopecten peroccidens. Sanguinolit«'s a'olus. var.

90 GEOLOGY OF THE EUREKA DISTRICT.

Sanguinolites naeuia. Euomphalus subrugosus.

Sanguinolites retusus. Pleurotomaria nodomarginata.

Sanguinolites salteri. Bellerophon textilis.

Sanguinolites simplex. Naticopsis, sp. ?

Sanguinolites striata. Dentalium, sp. f

Microdon connatus. Orthoceras randolphensis.

Schizodus cuneatus. Ortboceras, sp. ?

Schizodus deparcus. Gomphoceras, sp. ?

Cardiola filicostata. Griffithides portlocki.

Below this horizon there is a bed of bluish gray limestone interesting
on account of its grouping of Lower Coal-measure fossils without the
presence of any of those species which might be regarded as indicating a
lower stratigraphical position, but which are here found in the overlying
strata. The list is small, but characteristic of the Coal-measures. It is as
follows :

Fenestella, sp. f Productus semireticulatus.

Streptorhynchus crenistria. Spirifera camerata.

Chonetes granulifera. Rhynchonella eurekensis.

Productus prattenianus. Griffithides portlocki.

Richmond Mountain Fauna.— There is some reason to believe that the inter-
calated arenaceous and calcareous strata lying at the base of the great
limestone belt all the way from Richmond Mountain southward to Fish
Creek Valley represents a portion of the chocolate-colored clay shales
underlying the limestone of Diamond Peak, and referred to the upper
members of the Diamond Peak quartzite. From the base of the Lower
Coal-measure limestone along the Hoosac fault up to the capping of
andesite lavas of Richmond Mountain the highly inclined strata measure
about 1,800 feet. Fossils occur scattered throughout the limestones. From
highly fossiliferous strata favorable for their preservation, a grouping of
species was found which may be taken as typical of the entire epoch,
although only in a few localities is the life so full and well represented.
This list from the southwest base of Richmond Mountain is as follows :

Zaphrentis. Streptorhynchus crenistria.

Fenestella, sp. t Chonetes granulifera.

Lingula mytaloides. Productus longispinus.

Discina newberryi. Productus nebrascensis.

WEBER CONGLOMERATE. 91

Productus pratteuianus. Athyris hirsuta.

Productus semireticulatus. Rhynchonella eurekensis.

Spirifera annectans. Camarophoria cooperensis.

Spirifera camerata. Terebratula hastata.

Spirifera ieidyi. Aviculopecten afflnis.

Spirifera neglecta. Streblopteria similis.

Spirifera rockymontana. Myalina congeneris.

Spirifera atriata. Bellerophon, sp. f

Spirifera (M.) setigera. Metoptoma peroccidens.

Syringothyris cuspidatus. Griffithides portlocki.

Along Carbon Ridge the limestones are well developed but have as yet
yielded little calling for special comment as regards the life of the period.
The limestone forming the top of Diamond Peak and the long Alpha ridge
west of Hayes Canyon carry several fossiliferous strata at different hori-
zons, but all of them present much of the same grouping of species. Near
the summit of Diamond Peak a shaly limestone was found to contain

Polypora (like P. stragula). Spirifera (M.) setigera.

Orthis resupinata. Athyris roissyi.

Productus nebrascensis. Athyris hirsuta.

Productus semireticulatus. Griffithides portlocki ?

Spirifera trigonalis. Camarophoria cooperensis.

It seems hardly necessary to repeat nearly similar lists from neigh-
boring localities so long as there appears to be no marked change of fauna
with the development of the limestones. Most of the species obtained
proved to be specifically identical with those from the limestone body of
Richmond Mountain and Carbon Ridge east of the Hoosac fault. The
region of Diamond Peak does not offer as many species, but on the other
hand it has not been as diligently searched. The Lamellibranchiate fauna
was nowhere recognized in the region of Diamond Peak.

Weber Conglomerate.— Conformably overlying the Lower Coal-measures
comes the Weber conglomerate, one of the most persistent and well defined
horizons over wide areas of the Cordillera, stretching westward all the way
from the Front Range in Colorado to the Eureka Mountains. It varies in
the nature of the sediment with every changing condition, but it is nearly
everywhere easily recognized as a siliceous formation between two great
masses of Carboniferous limestone. In places it is made up of an admixture

92 GEOLOGY OF THE EUREKA DISTRICT.

of calcareous and sandy beds; in others, of fine grits and shales; and, again,
of nearly pure siliceous sediment, varying from fine to coarse grained, de-
pendent largely upon the distance from any land area and depth of water in
which it was deposited. Here at Eureka the material is exceptionally coarse
with abundant evidence of shallow water deposition and the existence of a
land surface not very far removed at the time the beds were laid down.

Two large bodies represent the Weber conglomerate at Eureka, one
directly east of Carbon Ridge and the other overlying the Alpha Ridge
west of Hayes Canyon. The former is not shown in its full development,
the upper members being cut off by the Pinto fault, but the geological
position of the latter is admirably brought out by the underlying and over-
lying limestones. Across their broadest development the beds have a
thickness estimated at about 2,000 feet. They are well shown in long par-
allel ridges inclined at high angles, with a synclinal followed by an anti-
clinal fold. For the most part the formation is made up of coarse material
of both angular and rounded fragments of red, brown and white grits,
together with jasper, brown horustone, and green cherty pebbles firmly
held together by a siliceous cement. Interstratified in the coarse material
are occasional beds of fine, yellow white sandstone, which has been used as
a lining for the large smelting furnaces at Eureka. In certain beds the
angular pebbles predominate, and in others the rounded, but in general there
is a fair admixture of both varieties. Near the summit of the horizon a
single belt of blue limestone comes in, which, however, in its lateral exten-
sion, may not be persistent. Considering the thickness and nature of these
conglomerates, they present an exceptionally uniform appearance through-
out, with almost no shale and but little limestone. No subdivisions need
be drawn. Although the formation has yielded no fossils, its structural
relations permit of its being easily correlated with the Weber conglomerate
of northern and eastern Nevada. With the coarse conglomerates of the
Weber at Agate Pass1, in the Cortez Range, there is the closest resemblance ;
both areas must have been near the shore line of the Paleozoic sea, in cen-
tral Nevada.

1 U. S. Geological Exploration of the Fortieth Parallel, vol. 2, Descriptive Geology, p. 574.

UPPER COAL-MEASURES. 93

Upper Coal-measures.— Beds of this epoch are found conformably overlying
the Weber conglomerate, their true geological position being admirably
shown at the head of Hunters Creek (atlas sheet vm) in a belt of lime-
stone about one mile in length, west of the Weber conglomerate horizon.
Both series of rocks dip to the west at high angles, the limestones,
however, being cut off by a body of basalt which forms the mass of Basalt
Peak and the Strahlenberg. A much larger body of this limestone is found
forming the long uniform slope of Diamond Peak, although there its true
position is obscured by longitudinal faults, which in places bring it in direct
contact with the Lower Coal-measures and in others it abuts unconforma-
bly against the Weber conglomerate.

The thickness attained by the rocks of this epoch is nowhere exposed
in the district, the overlying beds having either suffered removal by
denudation or else been concealed beneath flows of igneous rocks. West of
Diamond Peak a number of narrow valleys cross the limestone, but, as the
inclination of the ridge coincides closely with the dip of the beds, they
nowhere reveal any considerable thickness. The beds are estimated at 500
feet. In the northern and central portions of the state of Nevada the Upper
Coal-measure limestones attain a development of nearly 2,000 feet. At
Moleen Peak, 'just south of the Humboldt River, they are estimated at 1,800
feet in thickness where they conformably overlie a heavy deposit of con-
glomerates in their essential features quite like the Weber conglomerate of
Eureka. In the field the Upper Coal-measures may be distinguished readily
from the Lower Coal-measures by their lighter color and greater preva-
lence of fine grained beds. These colors are light bluish gray and drab,
the latter possessing a conchoidal fracture and compact texture. These
compact limestones frequently present forms of erosion quite different from
the coarse grained and granular limestones of the Lower Coal-measures.
Throughout the horizon the limestones are interstratitied with belts of grit

O .

and siliceous pebbles, held together by a calcareous cement, in which are
intercalated thin beds of purer limestone. One or two prominent beds are
apparently made up of quartz pebbles and fragments (if an older limestone.
carrying such fossils as Fiisilina riilindriru and Pi-«<ln<-tnx

>U. S. Geological Exploration of the Fortieth Parullol, vol. 2. Desrriptive ecology, p. (500.

94 GEOLOGY OF THE EUREKA DISTRICT.

as if indicating that they had been derived from the underlying Carbon-
iferous rocks. The fossils, however, which are all Coal-measure species,
might be derived quite as well from the Upper as from the Lower beds.
A chemical examination failed to detect any beds of dolomite in the lime-
stones, the highest amount of magnesium carbonate obtained being T33 per
cent. This is not without interest, as it is the only limestone horizon in the
Paleozoic series at Eureka free from dolomitic strata.

Upper coal-measure Fauna.— This epoch has not yielded as large a number
of species as the Lower Coal-measures and many of those found in the
middle and upper beds of the latter are known to occur in both divisions of
the Carboniferous limestone throughout the Great Basin. The following
list comprises all those species obtained from the Upper Coal-measures
which were not observed in the Lower Coal-measures:

Zaphrentis, sp. ? Ptilodictya carbonaria.

Polypora, sp. ? Ptilodictya serrata.

Orthis pecosi. Productus punctatus.

Retzia monnoni. Macrodon tenuistriata.

Terebratula bovidens. Pleurotomaria, sp. ?
Myalina subquadrata.

A further search of the Lower Coal-measures might show several of
these species, and in other localities outside the District it is by no means
certain that they have not been found lower down. Terebratula bovidens is
known to range throughout the Coal-measures; on the other hand Productus
punctatus, a common form, seems to be restricted to the Upper Coal-meas-
ures in the Great Basin.

A narrow belt of yellowish gray, somewhat shaly, limestone, near the
head of Hunter Creek, carries the following grouping of fossils :

Fusilina cyliudrica. Productus semireticulatus.

Fusilina robusta. Spirifera camerata.

Chaetetes, sp. ? Spirifera rockymoiitana.

Orthis pecosi. Spiriferina cristata.

Productus longispinus. Athyris subtilita.

Productus nebrascensis. Terebratula bovidens.

Productus prattenianus. Myalina subquadrata.

Productus punctatus. Pleurotomaria (like P. turbiiiitbrmis).

CAKBONIFEKOUS COAL. 95

At the extreme northern end of the district, on the west slope of Diamond
Peak and north of Garden Creek, in a very similar limestone, the beds
yielded as follows:

Fusilina cyliiidrica. Productus prattenianus.

Chaetetes, sp. f Productus semireticulatus.

Zaphrentis (fragments). Spiriferina cristata.

Ptilodictya (Stenopera) carbonaria, ! Athyris subtilita.

Ptilodictya (Stenopera) serrata, ! Eetzia mormon i.

A complete list of fossils from the somewhat restricted fauna of the
Upper Coal-measures will be found at the end of this volume.

Carboniferous Coal.— In the first range to the east of the Eureka District,
Carboniferous formations extend for miles along the edge of the valley
which in a study of Paleozoic rocks present some points of more
than ordinary interest. It is the only range in the Great Basin where coal
of Carboniferous age has been discovered in anything like a well defined
seam of sufficient thickness to encourage exploration, although beds carry-
ing small amounts of carbonaceous matter are known in one or two other
localities in central Nevada. Two outcrops of this coal are known and
considerable exploration has been undertaken in order to determine the value
of the coal seams ; one is situated on a low flat hill known as Pancake Ridge,
and the other on Bald Mountain, which stands out prominently at the southern
end of the Humboldt Range. Pancake lies about eight miles to the west-
ward of the Eureka District in a mass of low ridges connecting the
Humboldt Range with the White Pine Mountains. Rising above the plain
occurs a body of rhyolite, beyond which is a low ridge of coarse conglom-
erate followed by a second ridge somewhat higher than the first with an
intervening valley or shallow depression. Along the western base of this
second ridge an exposure of drab clay shales crops out only a few feet in
thickness, striking approximately north and south with a low dip to the
east rarely exceeding 10°. This clay carries a seam of lignite varying
from 10 to 18 inches in width which may be readily traced for nearly !.">(>
feet along the line of outcrop. Both above and below this coal seam are
alternating layers of bituminous shale and purer day shale ronformably
resting upon a bed of coarse conglomerate. Above tin- rlay shales comes

96 GEOLOGY OF THE EUREKA DISTRICT.

a bed of conglomerate about 25 feet in thickness made up mainly of rounded
quartz pebbles followed by another belt of shale quite like the one below,
40 feet in thickness. In both series of shale occur beds of carbonaceous
material and thin seams of impure coal, but nowhere on the surface are the
exposures more than three inches in width. Still higher up is another belt
of conglomerate carrying more or less lime and followed by buff colored
massive limestone changing to brownish gray limestone followed by a cherty
limestone, the latter extending to the top of the mountain. This series of
limestones has an estimated thickness of nearly 1,000 feet. Fossils charac-
teristic of the Coal-measures are common throughout the limestone, but are
more abundant in the lower beds, more especially in those immediately
above the coal, although no horizon presents any special faunal peculiarities.
Scattered throughout the limestone occur the following species:

Zaphrentis centralis, 1. Productus costatus

Diphyphyllum, sp. I. Productus semireticulatus.

Chaetetes, n. sp. Spirifera camerata.

Discina, sp. ?. Spirifera rockymontana.

Orthis pecosi. Spiriferina kentuckiensis.

Qrthis resupinata. Retzia mormcnii.

Streptorhynchus crenistria. Athyris roissyi.

Chonetes grauulifera. Athyris subtilita.

Productus cora. Terebratula bovidens.

This grouping may be said to present some distinctive features con-
taining forms regarded as belonging to the Lower Carboniferous, mingled
with others typical of the Coal-measures. Zapl/rattix mitm/ix, Dipliyphyllum
and Athyris roissyi give to the horizon a Lower Carboniferous aspect,
while the relatively large number of Coal-measure species would ordinarily
determine the position of the beds. Not only do the Coal-measure species
outnumber the others, but several of them happen to be those forms like
Orthis pecosi and Reteia mormoni, which have as yet been recognized only
in the Upper Coal-measures. Nevertheless, the evidence of the fauna is
strongly in favor of the lower horizon for these coal beds, as certain species
are elsewhere unknown higher up in the Carboniferous, whereas it is a
feature of the Coal-measure fauna of the Great Basin that" it presents a wide
vertical range.

Lithologically the evidence is not specially decisive. The series of

BALD MOUNTAIN COAL. 97

beds at Pancake bear some resemblance to the section found at the base
of Richmond Mountain, which, of course, indicates the base of the Carbon-
iferous limestone. Such evidence, however, is not conclusive, as the beds
also resemble and may be synchronous with the interstratified grits and lime-
stones of the Upper Coal-measures, with which by far the greater number of
the observed species are identical.

In exploring these coal seams for marketable coal considerable work
has been done, although all operations had been abandoned three years
previous to our visit. In places the vein was reported as 5 feet in
width, although much broken up and displaced. A vertical shaft, said to
be 180 feet in depth, had been sunk before the project was abandoned and
several tunnels and inclines run along the line of the coal. Examinations
could be made only in one tunnel, owing to the caving-in of the clay beds.
Sixty feet from the entrance, where the seam measures 20 inches, samples
of coal were collected. It closely resembles the lignites of the Green River
basin. On exposure the coal crumbles readily.

Bald Mountain lies to the north of Pancake and is situated in the
main ridge of the Humboldt Range. The coal or lignite outcrops are ex-
posed near the base of the range in clay shales inclined at low angles
toward the mountain. The mode of occurrence bears the closest resem-
blance to the strata at Pancake — interstratified conglomerates and shales
followed by massive, distinctly bedded yellowish brown and buff lime-
stones. At the time of our visit, in the autumn of 1880, the Bald Mountain
Coal Company had run a tunnel from the outcrop for 160 feet into the
mountain following the coal seam. At the head of this tunnel the coal
strata measured only from 2 to 7 inches in width, passing into black car-
bonaceous clays. At this point there was more or less displacement of
the strata, and this thin seam of coal was apparently cut off by a line of
faulting, which put an end to further explorations, the poor quality and
limited quantity of the coal discouraging any further outlay of money. A
search of the black shale beneath the coal was rewarded by the rinding of
a number of fossils all belonging to the species Athiiri* xxbtilita. In
the buff limestones, immediately above the coal, a small number of fossils

were found:

MON xx 7

GEOLOGY OP THE EUEEKA DISTRICT.

Orthis pecosi. Spirifera rockymontana.

Streptorhyuchus creriistria. Athyris subtilita.

Productus cora. Eetzia mormoiii.

Productus setnireticulatus.

They represent a distinctively Coal-measure fauna and are identical
with forms collected from beds at Pancake. On the other hand, none of
the species obtained at Pancake, indicating the horizon at the base of the
Lower Coal-measures, have as yet been found at Bald Mountain. In this
grouping at Bald Mountain there is nothing to prevent the horizon from
being considered as belonging to the Upper Coal-measures, but it is hardly
possible to suppose that the geological position of these beds differs from
the position of the coal at Pancake. The geological mode of occurrence
at both places and the sections across the beds indicate that the coal comes
from near the same horizon, and was deposited under similar conditions, with
the probabilities in favor of their having at one time formed a continuous
coal area.

The following analyses of samples of these coals, collected at the time
of our visit, are given here for the purpose of showing the character of the
deposits. They were made by Dr. W. F. Hillebrand, of the U. S. Geolog-
ical Survey.

No. 1,
Pancake.

No. 2,
Bald Moun-
tain.

Moisture

6-17

2-60

Volatile matter

31-88

30-97

Fixed carbou .. ..

55-59

44-60

Ash

6-36

21-83

Total

100-00

Sulphur in pvrites

0-73

5-44

Sulphur in soluble sulphates

0-79

0-14

The coals do not cake or sinter.

These coals, while they are of no commercial value, are of geological
importance from their exceptional mode of occurrence in the Carboniferous
rocks of the Great Basin. A further search would doubtless indicate whether
they belong to the base of the Lower Coal-measures or to the middle of
the Upper Coal-measures.

CHAPTER V.
DESCRIPTIVE GEOLOGY.

In the following pages will be found a detailed description of the sed-
imentary rocks in the Eureka District, the order followed being for the most
part the same as that adopted in the chapter devoted to the general
geological sketch. Each orographic block is described by itself, beginning
with Prospect Ridge, where the oldest rocks occur, followed by the other
blocks according to the geological succession of strata; the only changes
made in the order of treatment being for the purpose of bringing out more
forcibly the structural relations of the individual blocks to each other. This
chapter necessarily contains a repetition of many facts stated in other por-
tions of the volume, but at the same time there is an omission of many
details that, if not presented elsewhere, would properly find a place here.

The principal object of this chapter is to give a connected description
of the country and to place numerous details in permanent form for the
use of those who may wish to study the field in person, or who may desire
to investigate more fully the facts upon which the generalizations are based.
Certain portions of the country are described more fully than others, and
in a few instances the descriptions follow closely those given elsewhere in
the volume.

PROSPECT RIDGE REGION.

This region includes all the country lying between the Hoosac fault
on the east side and the Spring Valley, Prospect Mountain, and Sierra
faults on the west. These lines of faulting sharply outline a mountain
block which, in its geological structure, stands out on all sides clearly
defined from the adjacent country.

100 GEOLOGY OF THE EUREKA DISTRICT.

Along the east side of the Hoosac fault no sedimentary rocks are
known other than those belonging to the Lower Coal-measures, while on
the west side of the other three faults only Silurian and Devonian beds are
brought up against the fault line. In this uplifted mountain mass lying
between these great lines of faulting occur all the Cambrian rocks exposed
in the district, with the exception of two small patches of limestone, one of
Prospect Mountain limestone and one of Hamburg limestone, found on the
west side of Surprise Peak, one-half mile to the westward of the line of
the Sierra fault. They occur in a region of much local disturbance, not far
from the body of hornblende-andesite which occupies the bottom of Sierra
Valley, and are of no special geological interest otherwise than indicating
great displacement of strata. This uplifted mass of Prospect Ridge meas-
ures 10 miles in length by about 2\ milesin width across its broadest expan-
sion, in the region of Prospect Peak, in places narrowing to one-half that
distance. Within this block, evidence of minor fractures and dislocations
are everywhere to be seen, influencing in a greater or less degree the geo-
logical structure of the country.

jackson Fault.— Two faults, designated as the Jackson and the Ruby
Hill, profound in their displacement and of great economic importance,
deserve special mention ; both of them, however, lie within the limits of the
Prospect Ridge uplift. The Jackson fault starts in just north of the Eureka
tunnel, in Goodwin Canyon, on the east side of the ridge, and may be
traced northward along the line of contact between Prospect quartzite and
the Hamburg limestone (atlas sheet vin). It follows down the narrow ravine
past the Jackson mine to the east of Ruby Hill and Adams Hill, and is lost
near the body of quartz-porphyry just beyond the Wide West ravine. This
fault brings up the Pogonip limestone of the Silurian against the entire
series of Cambrian strata of Ruby Hill, from the lower quartzite to -the
Hamburg shales inclusive. On the east side of this fault exploration has
failed to bring to light any large and permanent bodies of ore, if we except
that of the Williamsburg mine ; the more valuable mining properties in the
immediate neighborhood of Ruby Hill being, for the most part, on the west
side of the fault in the Cambrian rocks.

PKOSPEOT KIDGE SECTION. 101

Ruby Hill Fault.— This fault starts in near the reservoir in New York
Canyon, branching out from the Hoosac fault and running in a northwest
direction. It cuts diagonally across the Pogonip limestone, and abruptly
terminates the Hamburg limestone and Hamburg shale, which form such
persistent topographic features of the country to the southward, and inter-
sects the Jackson fault near the American shaft, just south of the Jackson
mine (atlas sheet vm). For a short distance this fault apparently coincides
with the Jackson fault, then crosses it, following a northwesterly direction —
the same course it held before the intersection with the great north and south
fault. On Ruby Hill the fault may be traced in the underground workings
of all the principal mines through to the Albion. It has exerted a most
powerful influence upon the structure of Ruby Hill, and from its relations
to the ore bodies its importance from a mining point of view can not be
overestimated. Reference will be made to this fault in the discussion on
the geology of Ruby Hill.

From Spring Valley eastward across Prospect Ridge and Hamburg
Ridge to the Hoosac fault, the highly inclined strata offer an unbroken
geological section from the lowest beds of the Cambrian to the Eureka
quartzite of the Silurian. It offers the best section to be found in Nevada
of the Cambrian rocks, with all the epochs into which it has been divided
clearly denned. Sections across Prospect Mountain limestone may vary
greatly in details within a few hundred feet in the relative thickness of
compact limestone and calcareous shaly beds, but in general the sections
across the entire thickness of the horizon coincide fairly well.

Section CD-EF (atlas sheet xui), constructed across the central portion
of the Eureka Mountains, intersects Prospect Ridge about 3,000 feet to the
north of the peak, at a point selected to bring out the anticlinal structure of
the mountains. The underlying quartzite is overlain on both sides of the
fold bv the Prospect Mountain limestone, which on the west side extends
down to Spring Valley, while on the opposite side it forms not only the
summit, but the entire east wall of the main ridge. This is in turn over-
lain by the remaining subdivisions of the Cambrian, all of wlxich stand
inclined at a uniformly high angle to the east. As the section is drawn
across a high saddle at the head of New York Canyon, connecting Pros-

102

GEOLOGY OF THE EUREKA DISTRICT.

pect Peak and Hamburg Ridge, the erosion of the Secret Canyon shale is
not so well shown as it would be if the section had been drawn either to the
north or south of this point, but it is quite sufficient to bring out the promi-
nence of the Hamburg Ridge, which is everywhere parallel to the main
ridge. Overlying the Hamburg shale occurs the Pogonip limestone, in turn
followed by the Eureka quartzite, which occupy the long slope down to
the Hoosac fault. The entire series of beds dips to the east, with angles
varying from 75° to 85°. The section across these beds from the axis of
the fold is as follows :

Prospect Ridge Section.

Feet.

500

Com actvitreou whit t i ' d' ' bedd'

500 •

Massive siliceous dark pray limestone, occasionally black lime-

550

2,150

Fim- grained, evenly bedded, ash gray limestone, with more

1.250

Calcareous shales, passing into thin-bedded limestones; bands

350

{Yellow argillaceous shale, with thin layers of gray limestone;

350

1,200

( Dark gray granular limestone; only slight traces of bedding;

) in places highly siliceous; beds brecciated in the upper portion.
f Argillaceous shales, yellow and brown in color

1.200

750

1,600

100

750

Massive, light gray limestones, passing into bluish gray and
bluish black beds, with occasional bands of black limestone

1,250

Fissile calcareous shales, with a thin band of green and drab-

350

Prospect Mountain limestone

3,050

700

Argillaceous shales, ash gray in color; weathering red and yel-

350

Light gray compact limestone, with thin seams of ealcite

300

100

Prospect Mountain quartzite

1 000

{Bedded vitreous quartzite; weathering dark brown; inter-

1 000

Total thickness

9 350

Along the line of this section there has been less faulting, crushing
and local displacement than anywhere else on the ridge. Such local
disturbance as has taken place in the uplifted mass is more apparent in the
Prospect Mountain limestone than in the other horizons, partly owing to

PEOSPECT EIDGE SECTION. 103

frequent changes in the physical conditions of the alternating beds of shale
and limestone and partly to the fact that this series of beds forms the sum-
mit of the ridge and, lying nearer the axis of the fold, has been subjected
to much greater pressure and strain. The shales, yielding easily to pressure,
have folded and flexed under excessive strain, while the more compact
limestones, under the same force, were faulted and fissured. Evidence of
this is seen in the Mountain shale belt and the overlying limestone, the
former exhibiting a tendency to flatten out and the latter to recover the
normal dip by a sharp break, causing numerous fissures and faults. Since
the first uplifting of the mountain, intrusive dikes of rhyolite have filled
preexisting fissures and broadened lines of weakness, besides causing addi-
tional faulting and displacement. These intrusive masses, however, are
for the most part narrow and have produced no fundamental structural
changes, but much of the secondary alterations, such as local meta-
morphism of beds, the cementation of brecciated limestone, and similar
phenomena, are easily explained by their action.

Numerous tunnels, run for the purpose of mining exploration, vary-
ing from 50 feet to several hundred feet in length, penetrate the Prospect
Mountain limestone all along the ridge, at different elevations. Among
them may be mentioned the Fourth of July, Maryland, Lemon, and
Golden Era tunnels. Most of them, however, extend only for short dis-
tances, and, while they offer fair sections of portions of the great limestone
belt and may have subserved the purposes of the miner, are of but little
value for purposes of geological structure. Two tunnels, the Eureka and
Prospect Mountain, running at right angles to the strike of the beds and
from opposite sides of the ridge, give admirable sections across nearly the
entire thickness of the limestone belt.

Eureka Tunnel.— The entrance to the Eureka tunnel is situated near the
head of Goodwin Canyon, to the west of the Hamburg Ridge (atlas sheet
vm). The tunnel starts in near the base of the Hamburg limestone and
is driven in a nearly due west direction for 2,000 feet, passing several
hundred feet beyond the crest of the ridge and about 800 feet below.
The following is the series of beds encountered in the Eureka tunnel,
beginning at the entrance:

104 GEOLOGY OF THE EUBEKA DISTRICT.

Feet.
Black crystalline limestone (Hamburg limestone) 85

Argillaceous shale (Secret Canyon shale) 300

(Prospect Mountain limestone) :

Limestone 935

Calcareous shale 30

Brecciated limestone 51

Mountain shale 460

Stratified limestone 90

Brecciated limestone 50

The body of limestone near the entrance to the tunnel belongs to the
base of the Hamburg limestone and is a small mass left by erosion
upon the west side of Goodwin Canyon, the canyon for the most part having
been eroded along the line of contact between the Hamburg limestone and
the Secret Canyon shale. Where the tunnel enters the mountain the Secret
Canyon shale pinches out to a few hundred feet, and, a short distance to the
north, it is entirely cut off by the Prospect Mountain quartzite. At the
tunnel the shales are only 300 feet in thickness. Through the Prospect
Mountain limestone nearly all signs of stratification and bedding are want-
ing, the rocks everywhere showing evidence of crushing and local faulting.
Evidence of movement is seen in the brecciated appearance of the lime-
stone, which has been recemented by calcite. Fissures and seams
nearly vertical are common, dipping slightly both to the east and
west; the larger number of them being inclined toward the east. Dyna-
mic action has caused such frequent changes throughout the limestone
that it is difficult to recognize any belt by lithological distinctions. The
narrow bed of shale, 30 feet in thickness, is a well defined belt, calcareous,
and more or less argillaceous, but of little importance, simply fore-
shadowing the coming in of the broad belt of Mountain shale beyond.
Whether it would be found to be continuous on further exploration, either
to the north or south, is questionable. Beyond this narrow shale band
occurs another limestone belt, similar to the main body, in turn followed by
the Mountain shale, which, unlike the Secret Canyon shale, is character-
ized by intercalated limestone. It resembles the clay shale found on
the surface, but is less pure than the Secret Canyon body. It bears a
close resemblance to the shale belt found in the Prospect Mountain lime-
stone of 'Ruby Hill, but there is no direct evidence of their ever having

PEOSPECT MOUNTAIN TUNNEL. 105

formed a continuous bed. Nowhere else on the ridge do the Mountain
shales appear so broadly developed, 300 feet being the greatest thickness
observed on the surface. Beyond this shale belt the limestone is occasion-
ally stratified and then again occurs crashed and broken, showing that it
has undergone much pressure; the stratified rock in general lying next the
shale.

From a geological point of view the value of the tunnel lies in the evi-
dence of the crushing, faulting and fissuring which the entire series of beds
have undergone since the first uplift of the mountain, the changes in the
character of the limestone being far better studied in the tunnel than on the
surface. A marked fissure, slightly inclined to the east, occurs about 840 feet
from the mouth of the tunnel. Stringers of ore, or rather indications of
ore, are encountered all through the limestone, but few of them are of
economic value, being mainly filled with calcite, oxide of iron and man-
ganese and carrying but little lead and silver. At one point a nearly per-
pendicular pipe connects with the surface, but carries no ore. A small
amount of ore was discovered near by, however, just north of the tunnel.
The largest body of ore opened by the tunnel occurs nearly 1,200 feet from
the entrance, the metal-bearing fissure running approximately north and
south and standing nearly vertical. At the time of our visit this was the
only ore body encountered which was of sufficient economic value to be
profitably worked ; but since then a fair amount of good ore has been
extracted.

Prospect Mountain Tunnel.— This mining tunnel starts in at the west base
of Prospect ridge at an elevation of about 7,200 feet above sea level
(atlas sheet vn). It has been driven about 2,350 feet into the moun-
tain, with a course a little north of west, but does not penetrate quite to
the center of the ridge, the slope of the mountain being more gradual on
the west than on the east side; if prolonged it would pass the crest of the
mountain only a few hundred feet south of the Eureka tunnel. It lies
wholly in the Prospect Mountain limestone, which, being less fractured and
brecciated than the limestone toward the east, offers a more typical cross
section, although there is but little well defined bedding. For the first
100 feet from the entrance the tunnel passes through a dark gray rock,
beyond which it becomes much lighter in color and apparently uniform in

106 GEOLOGY OF THE EUREKA DISTRICT.

texture for 500 feet. From this point frequent belts of crystalline white
marble occur, alternating with compact light gray limestone. Specimens
in the collection show a very fair qiiality of marble. A marked change in
the limestone comes in about 1,500 feet from the entrance, where a fissure
is met at right angles to the tunnel, inclined a few degrees to the west from
the vertical; beyond this point the character of the limestone more closely
resembles the brecciated rock found on the east side of the ridge, as shown
in the Eureka tunnel. This resemblance is borne out by the appearance
of a belt of stratified limestone, followed by argillaceous shale like the
Mountain shale, but, as the latter occurs at the head of the tunnel and has
not been fully explored, its true position is unknown; it may simply be
one of the many lenticular shale bodies observed elsewhere in the Prospect
Mountain limestone. One or two fissures were cut by the tunnel, but little
ore was found, the most promising indication of an ore body being worked
for a short time without any profitable return. At 475 feet from the
entrance there is a well defined fissure connecting with the surface, suffi-
ciently large to admit light and air. It evidently at one time formed a
drainage channel for surface waters, as is shown by the smoothly rounded,
water-worn sides. The Eureka and Prospect Mountain tunnels nearly
pierce the ridge, the two taken together being over four-fifths of a mile in
length.

charter Tunnel.— The Charter tunnel lies mainly in the Prospect Mountain
quartzite. The entrance is situated in the drift deposits of Spring Valley,
just west of Mineral Hill, but soon after enters the quartzite, which here
forms the western base of the ridge as it rises above the valley. In
1882 it had a total length of 700 feet, with a trend of N. 64° W., affording
a good exposure across the beds. This tunnel, where it cuts the quartzite
south of Ruby Hill, exposes narrow bands of highly altered rock, com-
posed of fine siliceous material associated with monoclinic pyroxene and
pyrites. On the ridge above the tunnel, and not far below the overlying
limestones, occurs a band of exceedingly fine-grained rock, light green in
color and made up of an aggregation of quartz, monoclinic pyroxene,
white in thin section, probably diopside, and glossularite, a lime garnet
In the ravine immediately south of Ruby Hill is a small body of iron

MAGNETIC ORE. 107

ore, which analysis shows to be magnetite. It possesses some interest
from its position in the lower Cambrian rocks, but on account of the lim-
ited amount is of no economic value. Material dried at 104° C. yielded
Mr. J. E. Whitfield the following result:

Per cent.

Silica 5-29

Titanic acid None

Sulphuric acid -36

Alumina -18

Ferric oxide 64-69

Ferrous oxide 18-96

Mangauous oxide 1-16

Lime -88

Magnesia 5-X5

Water.. 2-Bx

Total 100-25

Prospect Ridge.— North of the Prospect and Eureka tunnels the main ridge
loses its simple anticlinal structure and a synclinal fold, much distorted and
broken, takes its place. From about the" line of these tunnels to the northern
end of Mineral Hill it is difficult to make out the structural features. The
Prospect quartzite, which is obscured for some distance by the overlying
limestone, reappears again along the west base of the ridge, curves around
on the north side of the small body of granite exposed at the north end of
Mineral Hill, and may be traced southward on the east side of Prospect
Ridge in a continuous body until terminating abruptly near the Eureka
tunnel, where it is cut off by a fault; its eastern extension is determined by
the sharp line of the Jackson fault. Overlying the quartzite comes the
Prospect limestone, forming the summit of Mineral Hill, with lines of
bedding, although much obscured, dipping into the ridge on both sides of
the hill. By reference to atlas sheet vu, the synclinal structure of Mineral
Hill may be readily understood, the quartzite coming in along the base of
the hill on both sides, with the limestone crushed and broken occupying the
crest of the ridge.

That the small granite body at the northern end of Mineral Hill, directly
opposite Ruby Hill, exerted an influence in determining the structure of
Prospect Ridge, seems evident, but in just what manner it is difficult to

108 GEOLOGY OF THE EUREKA DISTRICT.

say. The relation of this granite to the Prospect Mountain uplift will be
more fully considered in discussing the geology of Ruby Hill.

South of Prospect Peak the limestone maintains a fairly persistent
north and south strike and easterly dip, the angle of which seldom falls
below 60°. These highly inclined beds occur for a long distance north of
the Geddes and Bertrand mine. In the Irish Ambassador the beds lie
inclined at 40°. In general, lines of bedding have been obliterated, but
are found in sufficient number of instances to establish the structure, while
a meager fauna affords ample evidence of the age of the beds. Near the
Geddes and Bertraud mine in a compact limestone, the upper horizons of
the Prospect Mountain limestone are identified by the occurrence of several
species found also in the Richmond Mine on Ruby Hill, as well as by other
forms found in the same belt just below the Secret Canyon shale. These
beds yielded Kutorgina whitfieldi, Plychoparia oweni, and Agnostus bidcns.
Lenticular beds of argillaceous shale are by no means as broadly developed
as to the northward, but are of frequent occurrence and indicate the same
alternating conditions of deposition. On the other hand cherty beds and
highly siliceous dark limestones are very characteristic of the region.
Occasionally thin siliceous beds, from their superior hardness, withstanding
erosion better than the purer beds, rise like walls above the surrounding
hill slopes. This latter feature frequently gives the limestone body quite a
different aspect from that observed to the north and at the same tune aids
in determining the strike of the beds.

As already mentioned the Eureka quartzite on the west side of the
Sierra fault" lies unconformably against the Prospect Mountain limestone
from Prospect Peak nearly to Surprise Peak. At this latter locality a body
of Pogonip limestone abuts against the Cambrian limestone; the fault line,
which has maintained a persistent direction, swerves suddenly eastward and
then again turns and with a north and south course strikes across an easterly
spur of Surprise Peak. On a broad shoulder of this spur the Prospect
Mountain limestone again comes in contact with the Eureka quartzite of
Surprise Peak, the line of faulting passing about 200 feet below the summit.
Structurally the position of the Pogonip limestone is shown by its passing
conformably beneath the Eureka quartzite. Paleontological evidence con-

DRAINAGE OF SECRET CANYON. 109

firms this fact by the finding of a group of Silurian fossils which are
characteristic of the upper beds of the horizon. Among the species found
here on the north base of the Peak are Ortiiis perveta, 0. tricenaria, Itaphis-
toma nasoni, and Eeceptaculites mammillaris. The Prospect Mountain lime-
stone follows around on the south side of Surprise Peak, thence southward
until lost beneath the extravasated lavas, which encircle the ridge where it
falls away toward Fish Creek valley. From Surprise Peak southward
these limestones lie unconformably against Pogonip beds, the former stand-
ing at the usual high angles of 60° or more, and the latter also dipping
eastward, but at angles varying from 35° to 45°.

secret Canyon.— This canyon forms one of the most prominent physical
features of the district, a deeply eroded valley lying between two parallel
ridges, one of Prospect Mountain limestone and the other of Hamburg
limestone. The canyon lends its name to the intermediate body of argil-
laceous shales which are better exposed here than elsewhere. For more
than 2 miles in length the narrow valley is cut out of these easily eroded
beds, the harder limestones rising upon each side in abrupt walls several
hundred feet in height. There are few finer instances to be found any-
where of a valley carved out of soft friable material, the beds of which
lie highly inclined and conformable with overlying and underlying strata
of superior hardness, withstanding erosion better. No one overlooking
Secret Canyon from any high point in the country would understand the
appropriateness of the appellation; its true significance is recognized only
when approached from the south. The course of the present drainage
channel follows the trend of the shales until nearly opposite the southern
end oi Roundtop Peak, when, instead of maintaining its direction along
the line of the shales for a short distance further and thence out through
the Quaternary covered slopes to Fish Creek valley, it turns suddenly, fol-
lows a narrow defile obliquely through the ridge of Hamburg limestone and
shale, carves its way through the Pogonip and Eureka quartzite, cn>
the Hoosac fault, and is again deflected to the south only by Carbon Ridge.
The reason for its leaving the valley of Secret Canyon is to be found in
the rhyolite mass which probably underlies the hills of detritus near the
entrance to the canyon, blocking the former drainage channel. This is,

110 GEOLOGY OF THE EUKEKA DISTRICT.

however, only a partial explanation, as it is difficult to understand why the
stream should not continue on its course, cutting its way through the low
rhyolite barrier, rather than turn io the east and follow the present course,
which it finally took across the uplifted sedimentary beds. There seems
no doubt that, before the rhyolite eruption, the stream bed followed the
canyon and emptied directly into Fish Creek Valley.

Of the shale formation, little need be said in addition to the descrip-
tions already given of the beds. They show great uniformity of deposi-
tion and physical character, monotonous in outline and color, and, so far as
recognized, carry no organic remains. The sandy, limy transition strata
into the Hamburg limestone generally offers better lines of stratification
than either the shales below or the limestones above, and the dip and strike
may be determined at a number of points along the base of the overlying
horizon.

Hamburg Ridge.— Along the east side of Secret Canyon the Hamburg
limestone and shale and the Pogonip limestone horizons form a single ridge,
which, although of less elevation and of less rugged aspect, is singularly
like Prospect Ridge in its salient topographical features. With the excep-
tion of the summit of Roundtop, all the more elevated portions are found
in the Hamburg limestone. Although evidences of bedding are for the
most part obliterated in the Hamburg limestone, they are by no means so
exceptional as to leave any doubt that the ridge dips easterly with
great uniformity. Occasional beds are found with a dip and strike not in
accordance with this general structure, but in such instances they can be
shown to be the results of local disturbance produced by the action of in-
trusive rhyolites. In studying the district, care has been taken to discrim-
inate between such local disturbances, which may be very considerable
within limited areas, and the structure due to the primary upheaval and the
blocking out of the great mountain masses. At the southern end of the
ridge the Hamburg limestone has been a good deal broken up under the
influence of the rhyolites of Gray Fox and the numerous small dikes
of the same intrusive rock. Here the beds are seen standing nearly
vertical, sometimes inclined westerly, and again resuming the normal
dip to the east. The limestone beds throughout are highly siliceous.

ROUNDTOP MOUNTAIN. HI

Black cherty bands and beds of black quartzite form a characteristic
feature of the horizon. One of these siliceous beds on the crest of the
ridge may be followed for a long distance without any break in the con-
tinuity and is sufficiently well marked to form a characteristic feature of
the ridge.

Evidence of the age of these beds, based upon their organic remains,
rests mainly upon the material obtained from the limestone immediately
overlying the Secret Canyon shale. Fossils are known to occur, more or
less well preserved, in a number of places, but the most satisfactory locali-
ties are found just north of the Geddes and Bertrand dike and immediately
west of the divide separating Secret Canyon from New York Canyon. All
the species obtained are identical with those collected from the same hori-
zon north of Ruby Hill. Midway up the west slope of Hamburg Ridge,
and nearly due west from Roundtop, several species with much the same
grouping occur in a dark, compact limestone — a locality which, if thor-
oughly examined, might possibly yield a rich fauna. The Hamburg shale
forms a well marked horizon, but, being harder and more compact, yields
less readily to erosion, and, in consequence, is less easily determined by
topographical features than the same horizon northward. It may be
traced from the extreme southern end of the ridge northward across the
broad west spur of Roundtop, until abruptly cut off by the rhyolite body
which occupies Glendale Valley. The Pogonip limestone has much the
same north and south limits, rising gradually out of the rhyolitic tuffs at
the base of Gray Fox Peak on the south, and terminating in a high wall
which forms the west side of the upper Glendale Valley.

Roundtop Mountain.— Roundtop Mountain is almost wholly made up of
Pogonip limestone, and offers the best exposure of the series of beds
characteristic of this horizon to be found in the southern part of the Eureka
District. On the spur running out to the west from the top of the moun-
tain, and in an arenaceous limestone immediately above the Hamburg
shale, a few organic remains were obtained, belonging to a characteristic
grouping which marks the transition from Cambrian to Silurian, found in
several other localities at the base of the Pogonip. On the southern spur
of Roundtop, in beds dipping from 65° to 70° eastward, a small but

112 GEOLOGY OF THE EUEEKA DISTRICT.

characteristic fauna occurs, in which were found Lingula manticula, Orthis
hamburgensis, 0. testudinaria, Tiplesia calcifera, and Ptychopatria liaguei. To
the north of Roundtop the beds are much broken up by volcanic masses,
the structure being most difficult to make out and the beds impossible to
follow, but beyond this again the beds recover their normal position, strik-
ing north and south and dipping at a high angle to the east, until the entire
series of beds is lost beneath the rhyolite. Along the east slope of Round-
top the Eureka quartzite dips generally eastward, an exception being the
block lying between Glendale Valley and the ravine coming down from the
north slope of Roundtop. Here it has been thrust violently forward toward
the south and dips with a high angle to the southwest, in marked contrast
to the main body.

Along the west slope of Hoosac Mountain both the Hamburg shale and
the Pogonip limestone again come to the surface, the latter rising within
200 feet of the top of the mountain, the line between the two limestones
being defined as elsewhere by the occurrence, although poorly preserved,
of a grouping of species characteristic of the border line between the Cam--
brian and the Silurian.

Hoosac Mountain.— This bold mountain mass, situated to the east of the
Hamburg Ridge, attains an elevation several hundred feet higher than any
point along the ridge, rising prominently above the immediate country with
an altitude of over 8,500 feet above sea level. The broad summit for
nearly one-half mile in length maintains approximately the same elevation,
a few points here and there rising slightly above the general level. With
the exception of the narrow strip of Pogonip limestone upon the west slope,
the Eureka quartzite forms the entire mountain. The mountain falls off
gradually to the north and south, but more or less abruptly to the east,
where the quartzite, broken down by a series of small parallel faults,
presents numerous low walls and cliffs toward the Hoosac fault. The
quartzite body, where it is possible to determine any structure, trends inva-
riably north and south and dips easterly, but nothing can be made out as to its
thickness, owing to the great amount of local displacement. The quartzite
resembles the horizon as seen elsewhere, except that it is more or less altered
by solfataric action and by the intrusive rocks, which penetrate it as narrow

HOOSAC MINE. 113

dikes. There occur here some curious bands of a dark brecciated quartzite
made up of chert and jasper, in fragments firmly cemented together and
brilliantly colored by secondary alteration. The cementation probably fol-
lowed the infiltration of silica, which took place during the volcanic period.
Both hornblende-andesite and rhyolite penetrate the mountain, but mainly
in narrow dikes, the surface exposures of which are much decomposed and
in most instances so altered as to render a study of them impossible ; no
dikes of perfectly fresh rock were observed. Miners searching for ore
bodies along the outcrops of these decomposed rocks have explored them in
a way to permit of their general course and mode of occurence being made
out. From underground exploration there is reason to believe that but a
small part of the andesite dikes reach the surface, and these only in stringers
and offshoots from some parent body. Mapping the hornblende-andesite
exposures along the mountain, they are seen to follow a common course
approximately north and south, coincident with the lines of faulting and
the trend of the mountain uplift, following the direction of the main
Hoosac fault. Although much decomposed, the andesitic character of
these rocks can be readily made out from a study of their hornblendes and
glassy feldspars; the latter under the microscope are found to be all tri-
clinic. The rhyolite exposure just east of the Hoosac mine appears to be
a remnant left by erosion from the main body of the Hoosac fault outburst.

The Hoosac mine, situated on the east slope of the mountain, is one
of the oldest mining properties in the district, having been located in 1869
and opened early the succeeding year. As it is the only mine in the dis-
trict found in the Eureka quartzite, it has much geological interest, and its
development has served at least to furnish data bearing upon the structure
of a singular mountain. A vertical shaft 200 feet in depth has been sunk
through the quartzite, from the bottom of which a level 300 feet in length
runs westward into the mountain. All the mine workings lie in quartzite,
the ore bodies encountered being found in connection with the intrusive
rocks. It is reported that the owners of the property took out in a short
time precious metals to the value of $500,000. Continued exploration
failed to maintain the high hopes first entertained of the mine.

Northward of Hoosac Mountain the Pogouip limestone maintains, as
MONXX 8

114 GEOLOGY OF THE EUEEKA DISTRICT.

far as New York Canyon, its uniform and simple structure, while the
Eureka quartzite, on the other hand, occurs only here and there in irregular
patches cropping out from beneath heavy flows of hornblende-andesite,
which come to the surface along the line of the Hoosac fault. This profound
fault coming up from the south may be said to bifurcate at New York Can-
yon, the main branch swerving off to the northeast, retaining the name of
Hoosac fault, the other, trending to the northwest, being designated as
the Ruby Hill fault. Between these two lines of faulting lies a block of
uplifted beds, which in structure is in some respects quite independent of
the Prospect Mountain Ridge, a result probably brought about by the
dynamic forces which produced both the Ruby Hill and Jackson faults
and the rhyolite outbursts of Purple Mountain. This block is wholly made
up of Silurian strata, all three periods being represented. The Ruby Hill
fault may be traced on the surface from New York Canyon to its junction
with the Jackson fault by the numerous outbursts of rhyolite, whereas
northward along the Jackson fault no rhyolite has anywhere been observed.
As far north as Shadow Canyon the strata incline southwest toward
McCoy's Ridge, but beyond this canyon the dip and strike of the beds is
most irregular, in general dipping away from the Jackson fault and under
Purple Mountain and Caribou Hill. The greatest thickness of limestones
anywhere represented in this belt is about 2,700 feet, measured across the
strata from Shadow Canyon to McCoy's Ridge. The age of the limestone
underlying the quartzite of McCoy's Ridge is determined by the presence
of a Pogonip fauna characteristic of the upper horizons, serving also to
identify the quartzite which here forms such a persistent ridge along the
north side of New York Canyon. The trend of the ridge is determined in
part by the direction of the Hoosac fault and in part by the outbursts of
the lavas of Purple Mountain. The limestones overlying the quartzites can
be no other than the Lone Mountain beds. Although they cany no organic
remains, their geological position and lithological habit, quite like the Lone
Mountain strata immediately over the Eureka quartzite elsewhere, leave no
doubt as to their true correlation. It is the only exposure of Lone
Mountain limestone found in the uplift of Prospect Mountain Ridge, but
owing to the want of well denned lines of stratification no reliable estimate

GEOLOGY OF RUBY HILL. H5

can be made of the thickness. There are, however, only 200 or 300 feet
of beds before the horizon is sharply cut on" bv the Hoosac fault bringing
in the Carboniferous in juxtaposition with it.

Caribou Hill, separated from McCoy's Ridge by Purple Mountain, stands
out as a prominent topographical feature. It is capped by the same
Eureka quartzite. There are only 200 feet of beds and consequently the
Lone Mountain limestones are wholly wanting. It is this cap of quamite
which has protected from erosion the underlying limestones. Here, again,
in a narrow ravine at the west base of the hill, in the underlying limestone
immediately beneath the quartzite, the Receptaculites beds occur, with several
characteristic species, offering additional proof, if any was needed, as to their
geological position. From Caribou Hill northward no outcrops of the
Eureka quartzite were recognized. The Pogonip limestones present low,
flat-topped ridges inclined northward, gradually passing beneath the recent
deposits of Diamond Valley.

RUBY HILL REGIOX.

Ruby Hill and Adams Hill together occupy a small but clearly denned
area which may be considered simply the northern extension of Prospect
Ridge. The Jackson fault sharply outlines this area on the east side, while
the recent accumulations along the line of the Spring Valley fault limit it on
the west side. The geological importance of the region is mainly derived
from the enormous ore deposits found in the limestones of Ruby Hill, which
had yielded, up to the time of this investigation, over 860,000,000 in
precious metals. In general the orographic structure is simple, and only in
detail in the immediate neighborhood of Ruby Hill is it in anv way complex.

On Plate i will be found a geological map of Ruby Hill and the adja-
cent country, prepared from the large atlas sheets for more easy reference
to the text. Unfortunately the line between atlas sheets vn and viu runs
directly across this area, interfering greatly with the clear understanding
of the structural relations of the beds of Prospect Ridge with those of
the Ruby Hill as well as with those lying east of the .Jackson fault. Hy
referring to the map it will be readily seen that the Jackson fault cuts off
the Cambrian strata and brings the Pogonip up against the entire series.

116 GEOLOGY OF THE EUEEKA DISTRICT.

Granite.— North of the granite exposure at the end of Mineral Hill the
strata all dip northward, curving gently around the crystalline rock which
apparently has acted as a center of upheaving forces. The beds present a
broad anticlinal arch, less and less disturbed as they recede from the granite
and with a slightly decreasing angle of dip. The granite body occupies but a
small area on the steep slope of Mineral Hill. It is quite obscure in its sur-
face exposure, and might readily be overlooked but for its probable influ-
ence in producing the present geological features of the country. Fortu-
nately, it gives some clue to the peculiarities of structure. The age of
this granite is by no means easily determined, but the evidence seems 'to
show that it was a portion of an Archean island, around which the sedi-
ments were deposited. At some later period there was a movement of the
entire region, and the beds were uplifted and arched into their present
position around the granite. The exposure of the granite is wholly due to
erosion, and up to quite a recent date was covered with quartzite. There is
reason to believe that at the time the quartzite was deposited, a land surface
existed at no great distance, and this granite may have been connected with
it. Evidence in favor of such a supposition was found near the bottom of
the Richmond shaft, 1,200 feet below the surface. The vertical shaft, after
passing through limestone as far as the seventh level of the mine, pene-
trates the quartzite for 500 feet. In a white, fine grained quartzite, small
pieces of rock were obtained, closely resembling granite. Although some-
what decomposed, the rock was found to be made up of quartz, mica, and
an altered highly kaolinized mineral, probably feldspar.

Encircling the granite and resting directly upon it, occurs the Prospect
Mountain quartzite, followed in turn by the Prospect Mountain limestone,
Secret Canyon shale, Hamburg limestone, Hamburg shale, and Pogouip lime-
stone, the entire series of sedimentary beds exposed on Prospect Ridge. That
the Ruby Hill series of beds were once continuous with those of Prospect
Ridge there is no reason to doubt, ample evidence being found in the char-
acter of their sedimentation and the sequence of strata. The continuity
was broken only by profound faulting in much later times. As the quartzite
lies next the granite it is much broken up in the sharp tunis which it is com-
pelled to make as the underlying rock of the arch. No dips or strikes can be

0 S GEOLOGICAL SURVEY

ytJATEKNAKY

jfiKffiMk

GEOLOGICAL MAP OF RUBY HILL

EUREKA. MINING DISTRICT, NEY

CAMBRIAN

Pojjotup

H«nbui< 3«HT«lCA. Prospect IT

ShaU- Iim«uone ShM*

GEOLOGY OF ADAMS HILL. H7

made out except on the slopes of Ruby Hill, where the beds are distinctly
seen to pass beneath the limestone which caps the hill. Owing to this
abrupt curve, and the consequent breaking up of the strata, erosion has
cut a deep ravine in the quartzite. It is this ravine which separates Ruby
Hill from the main ridge. Overlying the quartzite comes the Prospect Moun-
tain limestone forming the summit, the isolation of the hill being made com-
plete by the erosion of a broad, shallow ravine in the Secret Canyon shale
on the north side.

Adams Hill, a flat topped mass of Hamburg limestone, lies between
two nearly parallel ravines, one of which is eroded in the Secret Canyon or
underlying shale, and the other in the overlying Hamburg shale. On the
south side the Secret Canyon shale passes beneath the limestone, the line
of contact being well determined at the base of the hill, the dip and strike
of the beds agreeing closely with those found on Ruby Hill. On the north
side of Adams Hill the Hamburg shales appear and are sharply denned by
the limits of the Wide West ravine. Beyond this latter ravine the Pogonip
limestone comes in, gradually falling away beneath the deposits of Diamond
Valley. On PL n, Sec. 3, will be found a geological section drawn across
the strata from the Prospect Mountain quartzite on the south slope of Ruby
Hill to the Silurian limestone, the two Cambrian limestones forming the
summits of the two hills, the underlying one capping Ruby Hill and the
overlying one forming the mass of Adams Hill. The section is drawn
across a body of quartz-porphyry which breaks through the Pogonip lime-
stone. It is quite unlike any other crystalline body known in the district,
but it is of no special value as it has exerted little influence upon the
limestone, the latter being very little disturbed and showing but few signs
of alteration. The age of the quartz-porphyry is unknown, as it penetrates
Silurian rocks only, but it is probably older than the rhyolites, which it in
no way resembles except in mineral composition.

A comparison of the section referred to with the one across Prospect
Ridge (atlas sheet xm) brings out the complete correlation between the two
series of beds, and the great similarity in the configuration of the two areas,
Ruby Hill and Adams Hill to the north corresponding with Prospect Ridge
and Hamburg Ridge of the east and west section of the main mountain.

118 GEOLOGY OF THE ETJEEKA DISTEICT.

On PI. n, Sec. 4, there is shown for comparative purposes a section
across the highest point of Prospect Peak where the quartzite reaches the
very summit of the ridge. On Prospect Peak the strata stand at an angle
of nearly 70°, whereas on Ruby Hill and Adams Hill they lie inclined at
about 40°.

Paleontological evidence that the Ruby Hill series of beds are the
precise equivalent of those found 011 the east side of Prospect Ridge is
ample for all purposes of identification. Three well defined horizons are
recognized yielding the same organic forms which characterize identical
strata elsewhere. The lowest of these three horizons is found not far below
the summit of the Prospect limestone, the middle one near the base of the
Hamburg limestone and the upper one near the base of the Pogonip.

Fossils in Richmond Mine.— In a compact stratified limestone on the seventh
level of the Richmond Mine a sufficient number of organic forms were
found to identify the beds with the upper members of the Prospect Moun-
tain limestone, and locating beyond all question the geological position of
the ore bodies. The species collected were:

Lingula manticnla. Agnostus neon.

Agnostus communis. Agnostus richmoiidensis.

Agnostus bidens. Ptychoparia oweni.

At the base of the Hamburg limestone opposite the Richmond dump,
and again north of the Albion mine, species have been identified correspond-
ing to those obtained in New York Canyon and Secret Canyon just above
the great shale body. North of the Wide West ravine a small grouping of
forms correlates the limestone just above the shales as the base of the
Pogonip, showing the mingling of the Cambrian fauna with a grouping of
fossils which higher up in the beds becomes characteristic of the Pogonip.
The two species Obolella discoidea and DiceUocephalvs marica, occurring in the
Pogonip elsewhere, have been collected from the limestones north of the
Wide West ravine.

FISH CREEK MOUNTAINS.

Fish Creek Mountains.— These somewhat isolated mountains lie to the south-
west of Prospect Ridge. They are surrounded on three sides by the ever-
present sagebrush valleys of Nevada, but to the northward maintain their

ANTICLINAL STRUCTURE. H9

connection with the Eureka Mountains by a complicated system of ridges
which closely unites them with both Prospect Ridge and the Mahogany
Hills. Although their northern limit is very ill denned, they stretch in a
north and south direction for 10 or 12 miles and measure about 5 miles in
width, with an elevation above the surrounding valleys of over 2,000 feet.
Bellevue and White Cloud Peaks are the two most prominent points in the
mountains, the former with an altitude of 8,883 feet, the latter of 8,850 feet
above sea level, while between them is a still higher table-topped summit,
having an elevation of 8,951 feet above the sea.

In structure the main body of Fish Creek Mountains consists of an
anticlinal fold, whose axis lies along the eastern edge of the broad, slightly
inclined table which forms the top of the range. A north and south line
of faulting coincides with this axial plane and is accompanied by an escarp-
ment, nearly 600 feet in height, showing a downthrow at least equal to
that amount. The displacement may be traced readily for a considerable
distance along the mountain. The fault is not laid down on the map, but
the escarpment itself is indicated by the contour lines being thrown close
together. At the base of this cliff" the rocks are much broken up, as there
appears to be a series of small faults rather than one sharp displacement.
The anticline is nevertheless sharply brought out by the limestone dipping
in opposite directions with a marked difference in the angle of inclination.
The beds of the cliff incline at low angles into the mountains, whereas the
slopes upon the east side, with an average dip of 15°, fall away abruptly
for about 1,500 feet or until buried beneath the Quaternary deposits of Fish
Creek Valley. On the west side of the main axis the limestones assume a
gentle synclinal roll, followed by a low, broad anticline, the westerly dip-
ping beds of which extend for nearly two miles, with a monotonous uni-
form dip, rarely exceeding 5° or 6°, till lost beneath the detrital accumula-
tions of Antelope Valley. The geological structure is that of a faulted
anticline, gentle on one side and relatively steep on the other, a
structure typical of many ranges in the Great Basin. Besides the north
and south anticlinal fold there is a gentle quaquaversal dip from the central
mass about Bellevue Peak, the beds to the northward, however, dipping
away steeper than in the other directions.

120 GEOLOGY OF THE EUREKA DISTRICT.

All three divisions of the Silurian are found here — the Pogonip lime-
stone, Eureka quartzite, and Lone Mountain limestone. This orographic
block is one of the few mountain ranges made up wholly of Silurian rocks.
Nearly all the more elevated portions are formed of Pogonip beds, which
gradually pass under the overlying Eureka quartzite, which forms continu-
ous bodies to the west and north. The drainage channels running out from
the summit are narrow ravines, and, although cutting hundreds of feet into
the Pogonip, never, so far as is known, expose the underlying Cambrian
strata. It is probable that only the higher Pogonip beds are represented.
Abrupt walls of nearly black limestone, characteristic of the upper mem-
bers of this horizon, form the sides of these ravines, in many instances the
dark rock being capped by overlying beds of white Eureka quartzite,
showing that these upper beds were in place. This is especially noticeable
to the northwest of White Cloud where the heads of nearly all the ravines
occur in the quartzites. Near the summit of the range they cut through
nearly vertical walls of quartzite from 200 to 400 feet in thickness. Out-
lying patches of quartzite, remnants of erosion, are still to be seen capping
the ends of the ridges on both slopes of the mountains. These isolated
patches are seldom more than 50 feet in thickness; they lie scattered all over
the slopes, many of them being so small and obscure as to be unrepresented
on the map. Over the long western slopes detached blocks of quartzite
may be found resting on the limestone, showing that while the quartzite has,
for the most part, been carried away, the uppermost beds of limestone still
remain in place. The Receptaculites beds extend in all directions under the
quartzite, paleontology confirming structural evidence of their geological
position. All three species of the genus Receptaculites known in the Great
Basin have been recognized here, associated with a varied fauna typical of
this horizon elsewhere, with the same foreshadowing of Trenton species.
The same specific forms occur here that are found underlying McCoy's
Ridge and Caribou Hill. A list of the species obtained at Bellevue and
White Cloud Peaks will be found on page 53.

Bellevue Peak is capped with Eureka quartzite which, from here north-
ward, stretches in a continuous body to Reese and Berry Canyon. Over
this intermediate country it presents much the same general features, a

GEANITE-PORPHYET. 121

white vitreous rock inclined at angles seldom exceeding 10° and frequently
horizontal. The country offers, in places, broad table-topped masses, and
; i gain in others is roughly accidented, caused by numerous minor faults
and small displacements, producing picturesque mural-like cliffs that serve
to break the otherwise monotonous scenery. A measurement of the thick-
ness of the quartzite is impossible. These displacements, although fre-
quent, are seldom sufficient to bring the underlying limestones to the surface.
The greatest thickness observed in any vertical wall is about 300 feet, which,
however, fails to take into account the amount carried off from the surface
by denudation. A section across the vertical cliff just west of Castle
Mountain will be found on page 56. Near the base of the quartzite cross-
bedding has been detected in one or two localities, indicating shallow water
deposits; it appears, however, to be wanting in all the higher beds that
present a singularly uniform body of quartz grains free from impurities.

Castle Mountain is capped by 200 feet of Lone Mountain limestone
overlying the quartzite, and from here extends in a narrow belt in a south-
east direction for over 2 miles. Here, as in many other localities, the Lone
Mountain limestone is devoid of fossils, and not until Stromatopora, Chcetetes,
and Atrypa reticularis appear in beds generally regarded as Devonian, have
organic forms been recognized. The country is monotonous in the extreme,
dazzling to the eyes, waterless, and for the most part treeless. The lime-
stone shows no lines of stratification.

Granite-porphyry.— To the northwest of Bellevue and White Cloud Peak,
in the region of the granite-porphyry dikes, the simple structural features
of the Fish Creek Mountains are lost by the intrusion of large bodies of
granite-porphyry. It occurs in two distinct masses with a few outlying
smaller dikes and knolls, the two principal bodies being separated by a
belt of limestone scarcely 300 feet in width.

The largest exposure of granite-porphyry presents an irregular body
lying between Fish Creek Mountain and Mahogany Hills on the extreme
western edge of the District. The smaller body occurs as a prominent
north and south dike, which, breaking through Pogonip limestone, appears
at the surface as an offshoot from the larger mass. From this massive dike

122 GEOLOGY OF THE EUEEKA DISTEICT.

several lesser ones branch off, nearly all of them lying approximately
parallel with the same northeast trend.

On the summit of the Fish Creek Mountains, midway between Belle-
vue and White Cloud Peaks, occurs a vertical dike of granite-porphyry only
a few feet in width. It is made up of feldspar, hornblende and mica
imbedded in a groundmass of quartz and feldspar, possessing typical
microgranitic structure. Apparently this dike itself exerted little, if any,
influence on the adjoining country, and the only geological interest at-
tached to the occurrence consists in its being closely allied to the larger
bodies of coarse granite-porphyry, from which it is most likely an offshoot.
It is quite possible that the quaquaversal dip of the strata from White Cloud
Peak, of which mention has already been made, may be due to an under-
lying mass of intruded crystalline rock, of which the dike is the only
evidence upon the surface.

Coinciding in direction with the secondary off-shoots from the main
dike occur narrow dikes of granite-porphyry penetrating the Lone Moun-
tain limestone of Castle Mountain. They are exceptionally fine grained,
with a characteristic microgranitic grouudmass. In their mode of occur-
rence they resemble the dike near Bellevue Peak, and doubtless have the

same common ongm.

As the geological and petrographical features of the granite-porphyry
are discussed with some detail in chapter vu, devoted to the discussion of
the pre-Tertiary crystalline rocks, it is needless to enter more at length
into the subject here. By reference to the map (atlas sheet xi) the
position of the main body of granite-porphyry and its relations to the
primary and secondary offshoots from the parent mass may be readily seen.

Ridge west of Wood Cone.— In many respects the best locality to study the
Pogonip of the Eureka District is the long, narrow, monotonous ridge which
stretches westward from Wood Cone. Here the beds abut against the
southern end of the main granite-porphyry body, standing invariably at
high angles, in most places nearly vertical, but sometimes inclined westerly
and again easterly. Just west of the limestone saddle, which separates the
two bodies of porphyry, there is a fault in the limestone which brings up
the lower beds. There is apparently a synclinal fold, to the west of which

THICKNESS OF POGONIl' BEDS. 123

comes in a sharp anticline, beyond which the beds dip uniformly to the
west. At the western end of this ridge occurs a small knoll or hill of Eureka
quartzite, its geological position being determined by the Eeceptaculites fauna
immediately underlying it.

At the eastern end of this ridge, just west of Wood Cone, a fauna was
obtained which indicated a horizon not far above the base of the Pogonip,
being largely made up of species found near the summit of the Cambrian,
associated with others never as yet recognized below the Pogouip. It is a
fauna characteristic of the lower portions of the epoch and quite like a
grouping found on the east side of Hamburg Ridge. In other words, they
may be correlated with the transition beds just above the Hamburg shale.
Many of the species also characterize the Pogonip of White Pine. Among
the species identified were the following:

Lingulepis mj«ra. Orthis hamburgensis.

Lingula manticula. Triplesia calcifera.

Leptajna inelita. Bathyurus congeneris.

Illaenurus eurekeusis. Bathyurus tuberculatus.

No accurate measurements of the Pogonip along this ridge can be
made, owing to the great irregularities of dip and strike, but it is probable
that the beds exceed 3,000 feet in thickness. From the fauna obtained
just below the Eureka quartzite, and that from the base of the limestone
west of Wood Cone, it is evident that the entire development of Pogonip
is represented in this ridge. This gives a somewhat greater development
for the epoch than has been recognized east of the Prospect Ridge, but, on
the other hand, it does not reach the very great thickness found on Pogonip
Mountain at White Pine, estimated at 5,000 feet.

REGION BETWEEN FISH CREEK MOUNTAINS AND PROSPECT RIDGE.

This region possesses some distinctive features unlike either of the
mountain blocks that adjoin it, yet at the same time it shows the influence
of the forces that uplifted Prospect Ridge on the northeast and Fish Creek
Mountains on the southwest. It is sharply denned from Prospect Ridge in
geological structure by the Sierra fault, which brings tin- Silurian up
against the lower Cambrian of Prospect Ridge. The anticlinal structure

124 GEOLOGY OF THE EUREKA DISTRICT.

of the latter ridge has disappeared, in place of which there is a complicated
and confused mass of mountains without any well denned characters. The
same dynamic forces that produced the great longitudinal faults extending
across the Eureka Mountains, on both sides of Prospect Ridge, may still
be seen westward of the Sierra fault in a series of north and south' fractures,
approximately parallel with the more powerful displacements. Such lesser
faults as the Lookout Mountain, Pinnacle Peak, and Lamoureux Canyon
faults, are by no means as persistent as the Hoosac and Pinto, and nowhere
indicate such profound displacements. The forces that caused these dis-
placements died out gradually to the west of the Sierra fault.

From Fish Creek Mountains the line of demarcation is by no means
as easily denned, being unaccompanied by great physical breaks of any
kind or abrupt changes in geological structure. The simplicity of the Fish
Creek Mountains as they approach Prospect Ridge gradually gives way to
a more intricate structure, the north and south displacements being compli-
cated by numerous minor cross-fractures and faults. North of Castle Moun-
tain, the configuration of the country gradually assumes new forms, and
from here to Prospect Peak it suggests little in common with the ordinary
type of Great Basin ranges. This intermediate region is the resultant of
varying forces not always easy to define.

The Eureka quartzite forms the surface rock over the greater part of
this area, stretching in an almost unbroken line from Spring Valley to the
Sierra fault, although faulting or erosion has exposed the underlying Pog-
onip limestone in a number of places. Overlying the Eureka quartzite
comes the Lone Mountain, usually passing into the Nevada limestone of the
Devonian, the latter in the neighborhood of Atrypa Peak offering an
exposure several thousand feet in thickness. Everywhere the Eureka
quartzite serves readily as a datum point to determine the position of the
faulted strata, and in most instances the age of the underlying beds may be
identified by the Receptaculites fauna. Where the thickness of overlying
limestone admits of it, the Devonian age is shown by characteristic
organic forms. By these two groupings of fossils and the intermediate
broad belt of quartzite, the stratigraphical position of beds in this highly
disturbed region may generally be determined without difficulty.

ATEYPA PEAK. 125

Castle Mountain may, for sake of convenience, be taken as the northern
limit of the Fish Creek Mountains. From Castle Mountain to Reese and
Berry Canyon no beds come to the surface other than the quartzites. Here,
however, a sudden change takes place, the canyon occupying a line of
southeast and northwest faulting with the quartzite on one side dipping at a
low angle to the west, and the Lone Mountain limestone on the opposite
side, but without any distinct line of bedding. From the head of Reese
and Berry Canyon the limestone crosses over a low saddle to the head of
Lamoureux Canyon, following the latter ravine until it makes an abrupt
bend to the south. The limestone may be traced eastward around the base
of Atrypa Peak, thence westward again with an irregular course as far as
Spring Valley. In this area the underlying limestone belongs, for the most
part, to the Silurian, but in one or two places the beds assigned to the
Devonian on lithological grounds rest directly upon the quartzites abutting
against them almost at right angles. The division between the Silurian
and Devonian in this region is an arbitrary one, but in most instances the
passage from the white saccharoidal limestone of the former into the strati-
fied gray beds of the latter is the same here as elsewhere in the District.

Atrypa Peak.— Nowhere in this area is there any place which permits of a
measurement of the Silurian rocks, but the region of Atrypa Peak, the cul-
minating point, affords excellent sections across the Nevada limestone, the
beds presenting nearly uniform dips and strikes. This imposing mountain
is formed almost wholly of Devonian limestone, the name of the peak
being derived from the abundance of Atrypa reticularis found on its slopes.
Two sections for comparative purposes were made : one, directly across the
strata on the southeast slope of the peak, the other on the high ridge
extending westward lying between the peak and the head of Lamoureux
Canyon. The latter section will be found on page 67.

Where the sections include the same geological horizons they agree
closely in details, but the one taken across the slope of the peak gives a
much greater thickness of Silurian rocks, whereas the ridge section ex-
tends higher up into Devonian strata. The fossiliferous shaly belt (No. 5),
in the section east of Lamoureux Canyon, is easily traceable across the
ravine to Atrypa Peak and may be taken as a base for comparing the

126 GEOLOGY OP THE ETJEEKA DISTRICT.

two sections. In the ridge section there are 1,300 feet of strata below this
shale belt before reaching the quartzite, and about 3,000 feet above the
shale. The Atrypa Peak section gives 2,000 feet from the shale to the
quartzite at the base, and nearly the same thickness from the shale upward.
This shale carries an abundance of characteristic species and, although a
larger number were obtained on the slope of Atrypa Peak, there is no
question that the fauna is identical in both.

At the head of Lamoureux Canyon there is a ridge of limestone,
striking northwest and southeast, which rests unconformably against the
quartzite. Not far above the quartzite a small collection of typical fossils
was made, amply sufficient to prove that the beds belong to the Devonian.
On the summit of the high peak east of Jones Canyon is another excellent
locality for the collection of Devonian species, but no specific forms were
found here not recognized elsewhere. Owing to local faulting, the exact
position of these latter beds could not be determined other than that they
belonged to the lower Nevada limestone. They are well bedded, strike
across the ridge and dip westerly.

Jones Canyon lies wholly in the Devonian limestone and offers some
good exposures of rock, but no continuous section at all comparable to those
described in the region of Atrypa Peak.

white Mountain.— The country between Atrypa Peak, and the Prospect
Peak fault culminates in White Mountain (9,941), the highest point west
of Prospect Ridge, with which it is connected on the northeast by a high
ridge of quartzite. From Spring Valley a fairly uniform slope of 1,500
feet extends to the summit of White Mountain, made up wholly of Pogonip
limestone, which stretches eastward and falls away gradually for about 800
feet to a high saddle in the range, beyond which it descends in a narrow
belt for another 300 feet to Mountain Valley. Here it is cut off by a fault
bringing up a narrow strip of Nevada limestone lying between the Pogonip
on the one side and the Eureka quartzite on the other. It is possible that
this fault may be only an extension northward of the Pinnacle Peak fault.
In the neighborhood of the saddle the quartzite encroaches on the lime-
stone. The structure of the mountain is difficult to make out, but the
limestone is everywhere surrounded by the quartzite, long belts of the

WHITE MOUNTAIN KEGION. 127

latter rock stretching down on both the north and south sides of the moun-
tain to Spring Valley. Patches of quartzite resting upon the limestone on
the summit give stratigraphical evidence of the age of the beds. It is proba-
ble that the quartzite passed over the top of the limestone, east of the
mountain, and that the patches of the former, found near the summit, are
mere relics of erosion. As regards stratigraphic position of beds, we have
here conditions nearly identical to those in the Fish Creek Mountains.
Characteristic Pogonip fossils, sufficient to determine the position of the
beds, have been secured from a number of localities, proving the age of the
limestone, while the beds forming the summit have furnished a typical
fauna of the upper portions of this horizon. About 800 feet below the top
of the mountain and not far from the same distance below the quartzite
bodies an interesting grouping of fossils occurs, and immediately beneath
the quartzite on the summit the Eeceptaculites beds are well shown. The
student of structural geology in this region owes much to the genus Recep-
taculites, which is very abundant within a restricted vertical range. A list
of the principal groupings of fossils collected on White Mountain will be
found on page 52.

South of White Mountain, and separated from it by a belt of Eureka
quartzite not over 1,000 feet in width, an irregular shaped body of lime-
stone is exposed from beneath the quartzite. If any evidence of its age is
needed beyond its stratigraphical position, it will be found in the typical
Pogonip fossils which occur scattered throughout the beds which, like the
corresponding beds on the east slope of White Mountain, possess a south-
east dip and a northeast and southwest strike. This limestone, like the
main body, is nearly everywhere encircled by the quartzite, the only ex-
ception being on the south side, where it abuts against the Nevada lime-
stone, which forms a part of the east ridge of Atrypa Peak. The two
limestone bodies are unconformable, of different lithological character, and
dip in opposite direction.

North of White Mountain the Eureka quartzite terminates abruptly
against the Prospect Peak fault, the Cambrian and Silurian quartzites
being placed in juxtaposition. These quartzites resemble each other
closely in their upper strata, being simply indurated sandstones, and it is

128 GEOLOGY OF THE EUREKA DISTRICT.

only after long study of them that they can be readily distinguished;
along the line of contact it is by no means easy to separate them. Evi-
dences of geological position come in, however, and the limestone, both
above and below the Eureka horizon, usually determines the age of
the beds. As the country is much broken up by profound faults, and
the Eureka quartzite is not over 500 feet in thickness, either the Pogonip
below or Lone Mountain horizon above, frequently both, are apt to come
to the surface near the exposures of the Silurian quartzite. Wherever the
Cambrian quartzite is found it is overlain by Cambrian limestone.

On the summit of the ridge along the line of the Prospect Peak fault
occurs a small patch of highly altered limestone, without any structural
indications of its relationship to either of the quartzite bodies. Its position is
difficult to explain satisfactorily, but it has been referred to the Pogonip,
since it more closely resembles the limestone of White Mountain than that
of Prospect Ridge.

From Prospect Peak southward the Eureka quartzite forms the west
side of Prospect Ridge, following the line of the Sierra fault. The ridge
falls away steadily to the south for 1^ miles, with a descent of over
1,500 feet to Sierra Valley. A series of minor longitudinal faults pre-
sents a much more abrupt slope on the west side and prevents the
underlying formations from coming to the surface, notwithstanding that a
narrow ravine is eroded in the quartzites for nearly 700 feet in depth. Not
till descending the slope for nearly 1,000 feet do the Pogonip beds come
to the surface, and then only a small patch of this underlying rock is
exposed. This interesting body of limestone crops out to the northeast of
Lookout Mountain, where it presents an obscure exposure of slight area
and thickness. The fauna obtained here is strikingly Pogonip in aspect,
and resembles the fauna found on the face of White Mountain for 500 to
1,000 feet below the summit. Associated with other more common forms
are Raphistoma nasoni, Maclurea annulata, and Leperditia livia, all recog-
nized as belonging to the Pogonip of White Pine. The interest in this
identification lies in the fact that only a few hundred feet to the southward
the Cambrian limestone comes to the surface in Sierra Valley, while just to

FAULTED LIMESTONE BLOCKS. 129

the westward the Devonian limestone is exposed in Mountain Valley, the
three horizons being determined by characteristic species.

Lookout Mountain.— This isolated mountain stands out prominently from
the surrounding country, cut off on three sides by faults. On the east runs
the Lookout fault, and along the west base the persistent and profound
Pinnacle Peak fault brings up the Nevada limestone against the Eureka
quartzite. The mountain is wholly made up of quartzite, inclined eastward
at low angles, the beds of which are for the most part darker in color and
more ferruginous than those of the same horizon found elsewhere. At the
east base of the mountain occurs a small patch of limestone, in part
obscured by surface accumulations of Sierra Valley and in part by
andesitic lavas. As this limestone lies on the east side of the Lookout

•

fault its age can be determined only by its fauna, but fortunately this is
sufficiently typical to admit of its reference to the Cambrian.

Northward of this last exposure and separated from it by only 300 feet
of acidic lavas, occurs a larger body of limestone, which forms a narrow
ridge, cut by the stream bed which comes down along the north side of
Lookout Mountain. The ravine affords a fair exposure of the beds. This
second body of limestone presents no structural evidence of its position,
the fauna alone determining its age, but fortunately it yielded a small num-
ber of fossils. These two groupings are not quite identical, but the beds
from which they were obtained can not be wide apart. The outcrop east of
Lone Mountain indicates clearly the horizon of the Hamburg limestone,
carrying certain species which extend downward into the Prospect Moun-
tain beds, mingled with others occurring as high as the middle portion of
the Pogonip. The larger exposure at the northeast base of the mountain
has been assigned to the Prospect Mountain limestone, without any decided
evidence as to the correctness of the reference otherwise than that it belongs
to the Cambrian.

Pinnacle Peak.— This summit lies about one and one-quarter miles due
south of Lookout Mountain and presents much the same general features
in the character of the beds and mode of occurrence, the two mountains
being connected by a continuous mass of quartzite. The beds strike
invariably north and south and incline eastward at angles si-Mom
MON xx 9

130 GEOLOGY OF THE EUREKA DISTRICT.

exceeding 20°, forming the entire slope as far as the Lookout fault. There
is little doubt that this quartzite is correctly referred to the Silurian,
although no direct evidence exists. Nearly everywhere else the Eureka
quartzite may be determined upon structural grounds alone, but here the
entire body from Lookout Mountain to Pinnacle Peak has been uplifted
between two longitudinal faults, with limestones of different age brought
to the surface on opposite sides of the displacements and lying unconform-
ably against the quartzite. In contrast with the quartzite on the west side
of the Lookout fault, limestones form the east wall stretching southward
until beds on both sides of the fault are buried beneath volcanic lavas.
This body of limestone extends eastward until cut off by the fault, bring-
ing up the basal members of the Cambrian limestone of Prospect Ridge.
Between these two faults the beds are broken by irregular outbursts of
andesites and in places have undergone considerable alteration, due to sol-
fataric action, the beds being frequently intersected by calcite and quartz in
naiTOW seams and veins. So much disturbed are the beds that structural
features are of little value, although it may be well to add that the general
dip is eastward. These limestones have been referred to the Pogonip,
although evidence of their position is not in all respects satisfactory.
Obscure fragments of fossils may be obtained in a number of places, but
only in one was anything like a grouping of forms observed. This fauna
was collected on the west side of Sierra Canyon, nearly due south from
Surprise Peak and just west of the Prospect Mountain limestone, in dis-
tinctly bedded strata inclined at an angle of about 40° eastward. All the
species obtained have been found in the Pogonip limestones elsewhere, but
singularly enough they are all known in the Hamburg limestone, every
species having a wide vertical range. They probably represent beds not
far from the base of the Pogouip and possibly should be referred to the
same horizon as the beds east of Lookout Mountain, although at the latter
locality the fauna distinctly indicates the Hamburg period. This refer-
ence to the Pogonip, however, is justified by the occurrence of undoubted
Silurian beds underlying Surprise Peak; a further search would certainly
determine the question.

SUKPK1SE PEAK. 131

Surprise Peak.-No mountain in this part of the district affords a more
commanding view than Surprise Peak. It is situated between the Sierra
fault on the east side and Sierra Valley on the west. It is capped by
Eureka quartzite, which is underlain by the Pogonip, the limestone being
distinctly seen to pass beneath the quartzite. On the north side of the
peak, and on the opposite side of the fault, in beds unconformable with
the Prospect Mountain limestone, was found a small but characteristic
Pogonip fauna. Its occurrence here is so important that it is given in full,
as follows:

Keceptaculites mainmillaris. Kaphistoma nasoni.

Cystidean plates. Pleurotomaria?

Orthis perveta. Leperditia bivia.
Orthis tricenaria.

Sierra Valley, along the west base of Surprise Peak, has been the
center for the eruption of considerable masses of andesitic pearlites and
hornblende audesites, which, in the form of small irregular knolls and dikes,
have penetrated the limestone on the south side of the peak. Associated
with these dikes are others of rhyolite, while still farther southward, where
the sedimentary rocks pass beneath the valley, occur large accumulations
of pearlites, pumices, and tuffs. Details in regard to these igneous rocks
will be found on page 234 et seq.

Grays Canyon.— The Pinnacle Peak fault lies on the west side of the
peak of the same name, at the southern end of the mountains. The
line of the fault is obscured by broad lava flows, but where these give out
it is easily traceable northward nearly to Prospect Peak with the Eureka
quartzite on one side and the Nevada limestone on the other.

West of the Pinnacle Peak fault the Nevada limestone extends from
Mountain Valley southward till the sedimentary beds pass beneath Fish
Creek Valley. Through these limestones Grays Canyon cuts a narrow
ravine, which offers a few good exposures, but nowhere exhibits a continu-
ous sectioa across any great thickness of beds. Only the lower portions of
the Nevada limestone are exposed, and over the greater part of this area
bedding planes are wanting. The best locality observed tor the collection
of fossils was found on the low, flat-topped ridge west of (ir;iys Canyon

132 (1KOUH1Y OF THE EUltEKA DISTRICT.

and southwest of Pinnacle Peak, the beds dipping to the southeast at a low
angle and striking northeast and southwest. These beds yielded the fol-
lowing forms:

Thecia ramosa. Dystactella iusularis.

Aulopora serpens. Conocardium nevadeusis.

Chonetes deflecta. Loxonema subattenuata.

Spirifera piiioneusis. Bellerophon perplexa.

Atrypa retieularis. Tentaculites scalariformis.
Rhynchonella occidens.

Nearly all these species occur in the shale belts of Atrypa Peak, Brush
Peak, and Combs Mountain, the exceptions being the three species, Thecia
ramosa, Aulopora serpens, and Dystactella insularis, which are, however,
characteristic of the upper Helderberg in New York and Ohio; Thecia
ramosa and Dystactella insidaris have only as yet been found at this one
locality at Eureka. A smaller but somewhat similar grouping of fossils
occurs in the limestone just west of Lookout Mountain, where they are
associated with Strophodonta canace, a species found by the writer in the
limestone at Treasure Hill, White Pine.

On the west slope of Pinnacle Peak the beds dip toward the fault at
an angle of 10°, reaching to within 150 feet of the summit and lying un-
conformably against the Eureka quartzite of the peak. Following the line
of the fault the beds trend off to the southeast, the quartzite belt gradually
narrowing until lost beneath the pumices, the Nevada limestone, on the
other hand, continuing southward in a low ridge bounded on the east and
west sides by igneous rocks. The beds exhibit much the same habit as
those to the northward, usually light in color and highly siliceous, but show-
ing more distinct lines of bedding. By reference to the map (atlas sheet xi)
the structure will be seen indicated by strikes and dips. South Hill, the
most prominent point on this southern extension, has a marked anticlinal
fold, the axis of the fold striking N. 40° to 45° east, with a dip of 15°.
The brownish gray limestones are distinctly bedded and probably belong to
a somewhat higher horizon than any of those exposed in Grays Canyon.
South of the road, which traverses the ridge near its southern extremity, a
well defined but gentle synclinal fold may be seen crossing the ridge

GRAYS PEAK. 133

obliquely, with approximately the same strike as the strata on South Hill.
In this southern extension the only fossils obtained were ChaeMes and as-
sociated corals so abundant in the Lower Nevada limestone.

Grays Peak.— This name has been given to the flat topped summit which
forms the eastern limit of the broad quartzite plateau. It offers a command-
ing1 view, as the country falls off rapidly to the south and east. On the
summit the beds lie nearly horizontal, but break away abruptly and dip
off iu every direction accompanied by mural-like escarpments produced by
a series of small parallel faults lying wholly within the quartzite. On the
eastern side the slope descends for nearly 1,000 feet, with an average dip
of 20°, the angle of the slope and the inclination of the beds coinciding
within 1° or 2°. South and east the quartzites are overlain by the Nevada
limestones which dip away from the peak with varying angles. On the east
side the line of contact between the two formations is strongly marked by
a deeply eroded ravine draining into Grays Canyon. While these lime-
stones have been referred to the Nevada period, it is by no means definitely
ascertained that beds which in other places have been assigned to the Lone
Mountain series may not here, in some instances, rest upon the quartzite.
In many instances there is an entire absence of bedding, and in others the
strata rest unconformably upon the quartzite. Apparently the underlying
limestones belong to the transition series between well recognized Silurian
and Devonian, but pass rapidly into limestone which has everywhere else
in the district been assigned to the Nevada epoch. These limestones stretch
away to the south in insignificant monotonous hills and ridges of lower
Devonian age and have as yet yielded only a few obscure corals of wide
vertical range. North of Grays Peak on the plateau where the beds lie
either horizontally or at low angles, there are several patches of limestone
still left in place as remnants of erosion. These exposures resemble the
beds of the Lone Mountain series and serve to show by their geological
position that the quartzites on the ridge belong to the upper members of
the Eureka epoch. To the westward of these Silurian limestone patches
the quartzites break down in abrupt walls and cliffs toward Lamoureux
Canyon much in the same way as seen on the east side of Grays Peak.
Along Lamoureux Canyon, however, the wall is most persistent, continuing

134 GEOLOGY OP THE EUREKA DISTRICT.

northward nearly to Atrypa Peak, and is an excellent locality for studying
the Eureka quartzite. A longitudinal fault line follows up Lamoureux
Canyon, but the amount of movement is by no means as great as along the
Sierra and Lookout faults ; the orographic movements apparently dis-
playing less and less force to the westward of Prospect Ridge. Passing up
to the head of Lamoureux Canyon, there is an interesting occurrence of an
exposure of the underlying limestones brought up by faulting. Here the
Pogonip beds are surrounded on all sides unconformably by the quartzite.
The hill in the middle of the canyon formed of these limestones is capped
by about 100 feet of quartzite resting conformably upon the underlying
beds. A careful search in this locality reveals the Receptaculites fauna,
associated with Orthis and Maclurea, immediately beneath the quartzite.

Between Lamoureux Canyon and Castle Mountain the country presents
the appearance of a shallow trough or basin with a northwest and southeast
trend. This basin is for the most part filled with Nevada limestone, between
which and the Eureka quartzite the Lone Mountain beds generally come
to the surface, forming a narrow belt around the edge of the basin and in
places extending up on to the top of the quartzite rim. Over this area the
beds dip east and southeast except immediately next the quartzite of Lam-
oureux Canyon, where, conforming with it, they show a westerly dip. But
few fossils have been recognized in this area other than an occasional Atrypa
reticularis and corals characteristic of the Devonian, but without indicating
any special horizon.

MAHOGANY HILLS.

Spring Valley extends the entire length of the Eureka Mountains and
sharply distinguished Prospect Ridge and the Fish Creek Mountains from
Mahogany Hills, all that region lying on the west side of this valley being
included within the Mahogany Hills. Strictly speaking, it is not one
continuous valley, but rather two valleys, with a low dividing grassy
ridge between them, the water draining both to the north and to the
south. From the broad plain of Diamond Valley, Spring Valley, only a
few hundred yards in width, rises gradually for 1,200 feet to the divide,
following the course of a remarkable fault, which brings both the Lone

COMBS PEAK. 135

Mountain and the Nevada limestones in juxtaposition with the Prospect
Mountain quartzite, recent accumulations, however, obscuring the precise
line of the displacement. The water-shed lies nearly opposite Prospect
Peak. Southward from this dividing ridge the valley becomes a more im-
portant physical feature, in places opening out to more than a mile in width,
finally draining into Antelope Valley southwest of the mountains. The
southem end of the valley is arid and covered with sage-brush, closely
resembling the broader longitudinal valleys of the Great Basin.

Mahogany Hills occupy by far the largest area of any mountain block
in the Eureka District, measuring 12 miles in length by 8 miles in width.
Nevada limestones constitute by far the greater part of this orographic
block, four epochs of the geological section — Eureka quartzite, Lone
Mountain limestone, Nevada limestone, and Diamond Peak quartzite — are
all represented and their structural relations well shown. In presenting
some of the more important details of the region, it will be well to begin at
the southern end, where both in geological and topographical structure
Mahogany Hills are closely connected with the Fish Creek Mountains
through Wood Cone and the granite-porphyry region.

Combs Peak. On the north side of Wood Cone, resting uncomformably
upon the Eureka quartzite, lies a body of bluish black and dark gray lime-
stones dipping beneath the limestones of Combs Peak. These dark lime-
stones everywhere form the southern slopes of the Peak, and westward of
the quartzite rest directly upon the granite-porphyry body. The hillsides
are scored by frequent ravines and water-courses showing the inclination of
the strata northward into the mountain, but lines of stratification are
exceedingly rare, nowhere affording, for any considerable distance, con-
tinuous dips and strikes. The best locality for observing these beds was
found just north of Wood Cone, on the end of the long spur coming
down from Combs Peak. From their dark steel-gray color and their
uniformly fine grained appearance, it is easy to see that they differ essen-
tially from the characteristic Lone Mountain beds observed elsewhere. This
is all the more noticeable, as they are found to pass into beds possessing
the peculiar habit of the latter horizon. This striking contrast in the lime-
stones led to a diligent search for paleontological evidence of their geologi-

136 GEOLOGY OF THE EUKEKA DISTRICT.

cal position, a search which was rewarded by finding a limited and imper-
fect fauna, characteristic of the Trenton period. The finding of this group-
ing of fossils is important, as it carries the comformable Silurian limestones
overlying the Eureka quartzite down into beds generally regarded as lower
Silurian, whereas, elsewhere in the district there is no paleontological evi-
dence of strata older than the Niagara or Hall/site* beds above the quartzite.
Some description of this fauna will be found on page 59.

The dark limestones which have been referred to the Trenton at this
point measure, according to the best estimates that can be made, about 300
feet; that is to say, this is approximately the thickness from the Eureka
quartzite on Wood Cone to the strata having the characteristics of the
horizon found elsewhere and regarded as of Lone Mountain age. These
dark limestones extend northward to the low saddle over which the wagon
road passes, beyond which the light colored, pearly limestones come in.
Westward and northward of the granite-porphyry a second locality was
found yielding a similar fauna, proving the extension of the horizon in
that direction. Here the Trenton beds, or those assigned to that epoch
upon lithological grounds, appear somewhat thicker than those obtained near
the first mentioned locality. Passing up the slope of the peak over the Lone
Mountain beds, north of Wood Cone, the strata generally referred to the
Nevada limestone make their appearance at the base of the first abrupt
slope of the long spur from Combs Peak, and from here to the top of the
prominent hill south of the peak the ridge offers an excellent section across
the limestones. The beds strike across the ridge and dip toward the peak,
with varying angles. A number of the observed strikes and dips will
be found recorded on atlas sheet ix. On the top of the hill a few fossils
may be found, indicating that the beds at the top of the northerly dipping
rocks still belong to the Lower Nevada limestone. Between this hill and
the summit of Combs Peak occurs a sharp syncline, the axis of the fold
lying in the saddle at the base of the steep slope of the peak. The lime-
stones on both summits strike about N. 55° west; those 011 the peak dipping
25° southwesterly, and those on the spur 35° northeasterly. The amphi-
theater of Combs Canyon has been eroded out of the beds lying within
the synclinal fold.

RHYOLITE OP MAHOGANY HILLS. 137

OH the west spur of Combs Peak, in beds dipping to the northeast,
occurs a belt of calcareous shales about 150 feet in width, carrying a rich
and varied fauna quite similar to the fossil-bearing shale belts of Atrypa
and Brush peaks and with a nearly identical fauna. On page 7<> will be
found a list of the Combs Peak fauna, together with those of the other
peaks, showing the strong parallelism in the life from the three localities.
The precise locality from which this fauna was obtained is designated on
the map. All the beds on the north slope of Combs Peak belong to the
east side of the synclinal fold, dipping into the mountain and passing
beneath the beds which form .the summit.

Browns Canyon, at the base of the mountain, lies in the axis of an
anticlinal fold, the beds on the north side dipping to the northeast at angles
seldom exceeding 20°. At the head of this canyon, along the axis of the
fold, occurs a body of compact rhyolite, which has for the most part been
extravasated on the south side of a local line of faulting. It forms a hill
about 250 feet in height, whose outlines are sharply denned by drainage
channels which almost completely surround it on all sides. The slopes of
the hill are strewn with fissile, sherdy fragments of rock characteristic
of the entire mass. The rhyolite has a microcrystalline groundmass, with
but few microscopic crystals of gray quartz, brilliant biotite flakes, and
occasional dull orthoclases. In the middle of this rhyolite is an irregular
exposure of Nevada limestone about 100 feet in thickness, indicating that
the greater part of the lava is only a thin flow over underlying limestones.
It is the single instance of a rhyolite exposure observed in Mahogany Hills
east of Yahoo Canyon.

Temple Peak.— From this rhyolite body the limestone hills rise gradually
to the northeast in gentle, flat topped spurs, culminating in Temple Peak
(8,398 feet), the highest point between Browns and Denio canyons.
Across this limestone body, from Browns Canyon to Dry Lake, the strata
dip persistently to the northeast, with a northwest and southeast strike.
The limestones at the summit lie inclined at angles seldom exceeding 5°,
but are distinctly bedded, and in physical habit and sequence of strata
resemble those about midway in the Nevada limestone epoch. The same

138 GEOLOGY OF THE EUEEKA DISTRICT.

limestones cross Denio Canyon and continue northward to Bnrlingame
Canyon, invariably dipping slightly to the northeast.

About 150 feet above the bottom of Browns Canyon, in beds near the
base of the Nevada limestone, a small number of fossils were procured,
most of them like Atrypa reticularis, common forms having a wide vertical
range. Associated with them was the coral Acervulariapentagona. This was
found also by the writer in the Nevada limestone of Treasure Hill,1 White
Pine, the only other locality where it has been observed in the Great Basin.

Table Mountain.— South of Browns Canyon the beds of Combs Peak con-
tinuing westward gradually curve around until the limestones of Table
Mountain strike north and south and lie nearly horizontal, but with a slight
dip to the east. Table Mountain is made up of dark massive beds, the
upper strata occupying about the same geological position as the summit of
Temple Peak. From Table Mountain westward to Antelope Valley, the
long spurs afford a fair opportunity to study the beds of the lower and
middle portions of the Nevada epoch, which is here represented by 2,500
to 3,000 feet of limestones.

Devon Peak.— The culminating point of the northwest part of Mahog
any Hills is known as Devon Peak (8,537 feet), although it is simply the
highest point in a broad, plateau-like body of nearly horizontal limestones.
To the west and north the beds incline gently toward the sage brush plain
of Antelope Valley and the broad plain west of the Pinon Range. One or
two of the more deeply eroded canyons offer partial exposures of the beds,
but nowhere any continuous sections more than 500 to 700 feet in thick-
ness ; yet they serve to show similar conditions of sedimentation over a wide-
spread area. All over this area, at several horizons, a few scattering
fossils may be found, such as Atrypa reticularis, Strophomena rkontboidaUs,
Spirifera pinonensis, Stromatopora, and Chcetetes. In the first ravine running
up to Mahogany Hills from Hay Ranch Valley, the limestones afford
such large numbers of corals, partially weathered out, that the locality
would well repay a visit by anyone specially interested in the study of
Devonian fauna.

Yahoo canyon.— This canyon has its source at the northern end of Dry

'U. S. Geol. Explor. 40th Par., vol. ii, Descriptive Geology, ].. 511.

YAHOO CANYON. 139

Lake and is the only one of the principal drainage channels of the Mahogany
Hills that follows a north and south course. At one time it drained the
depressed basin of Dry Lake. At the head of Yahoo Canyon a small out-
burst of rhyolite forms a low obscure hill, around which the wagon road
passes on the west side. A few hundred feet to the south of the hill is a
dike of similar rock about 100 feet long by 25 feet wide. This rhyolite
is a light gray rock, weathering brown, and carrying a few macroscopic
secretions of biotite, sanadin, and quartz; it closely resembles the rhyolite
of Browns Canyon. Yahoo Canyon presents some interest as being the
dividing line between two quite different types of orographic structure; on
the west side the plateau-like body of limestones in the neighborhood of
Devon and Temple Peaks lies gently inclined to the westward, while on the
east side the limestones have been uplifted into longitudinal ridges with the
structural peculiarities of the Pifion Range. In general the canyon may be
said to have been eroded along the axis of an anticlinal fold, although this
is not strictly correct, as on the east side near its lower end a sharp anti-
clinal ridge exists, which, however, dies out toward the head of the canyon.
The structural details are rather intricate and were by no means carefully
worked out, but the dips and strikes indicated on the map (atlas sheet v.)
show this anticlinal structure with the trend of the ridges agreeing with the
course of the canyon. The main ridge of limestones east of Yahoo Canyon
inclines invariably to the eastward with an average dip of about 35° and
with a strike a little west of north, maintaining this position till passing
beneath the Carboniferous rocks which everywhere seem to overlie them
conformably. The ridge is made up of monotonous blue massive lime-
stones characteristic of the Upper Nevada epoch as seen elsewhere, espe-
cially in the neighborhood of Signal Peak on the west side and Newark
Mountain on the east side of the district. On the east side of Yahoo
Canyon a most interesting collection of characteristic species was made,
consisting largely of Upper Devonian corals. Associated with them occurs
such distinctive species as Spirifera disjuncta and the widely distributed
Spirifera glabra ; a fauna indicating a higher horizon than any of the ex-
amined beds in the Mahogany Hills to the west. A list of the fauna
obtained is given on page 83. Between this locality and the Diamond

140 GEOLOGY OF THE EUKEKA DISTRICT.

Peak quartzites, paleontology again supports structural evidence, the organic
forms being such as are only found in the upper horizons or mingled with
those having a wide vertical range.

Spanish Mountain.— This broad, elevated mass of Eureka quartzite, nearly
two and one-half miles in width, lies due west of Prospect Peak. Its struc-
tural features differ from those of any other area of the Eureka Mountains,
.but at the same time bear some resemblance to those of Grays Peak, both
being formed of strata of the same geological age, with the Lone Mountain
beds resting upon their slopes. On Spanish Mountain the quartzites dip
away in every direction from the summit, but without any clearly defined
lines of bedding, presenting the appearance of a great dome-shaped body
falling away on all sides. This quartzite is fractured by local displace-
ments, but they fail to bring to the surface any underlying Pogonip beds,
and the few drainage channels, which have cut one or two narrow gorges,
still lie wholly within the quartzite. Over this dome-shaped body the Lone
Mountain beds undoubtedly passed at one time; erosion, however, has
worn them off the summit, with the exception of two small patches, which
are sufficient to establish the fact that the upper members of the quartzite
are still in place on the top of the mountain. Surrounding the quartzite
on all sides occurs the Lone Mountain limestone, except along Spring Valley,
where it is probably obscured by recent accumulations.

Isolated patches of limestone in the valley confirm the opinion that
the Lone Mountain beds extend down to the Spring Valley fault. These
limestones cross the divide connecting Spanish Mountain with Swiss
Mountain and come within 200 feet of the summit of the former. Wherever
observed, the limestones rest unconformably upon the quartzite, but, as they
are for the most part devoid of bedding plane, no determination can be
made of their thickness. Moreover, the line between the Silurian and
Devonian is arbitrarily drawn and rests, as elsewhere in the district, on
lithological distinctions and the absence of evidence of life in the lower
rocks. As shown on the map, the thickness ascribed to the Lone Mountain
beds varies greatly at different localities, but there is no doubt that the
vertical distance between Eureka quartzite and limestone characterized
by a Devonian fauna actually does exhibit great variations in thickness.

SPANISH MOUNTAIN. 141

The horiiblende-andesite body on the edge of Dry Lake Valley, at the
southwest base of Spanish Mountain, will be discussed in the chapter
devoted to igneous rocks, which form a most important group, not only in
themselves, but in connection with similar outbursts in Sierra Valley and
elsewhere. Here at Dry Lake they present a marvelous variety in color,
density and texture, but on careful study they are shown to be closely
related, with a marked similarity in mineral and chemical composition.
The small body designated on the map as dacite is simply an extreme form
of the larger mass, being characterized by considerable free quartz and
biotite, and has much the nature of a pumice, while the main body might
be designated more concisely as an andesitic pearlite.

North of Spanish Mountain, as elsewhere, the Lone Mountain lime-
stones pass gradually into those of the Nevada epoch, and with this change
the structural features of the region assume new aspects, quite different
from the rest of Mahogany Hills or Fish Creek Mountains. From Brush
Creek northward the structure is that of a simple monoclinal ridge,
trending about north 40° west, with a dip invariably to the east. Rising
above the Quaternary accumulations along the east base of the ridge in
Spring Valley, at sufficiently frequent intervals to prove the continuity of
strata, occur exposures of quartzite beds, conformably overlying the lime-
stones. As the latter beds bear ample testimony of their Devonian age to
the very summit, the siliceous strata have been referred to the Diamond
Peak horizon of the Carboniferous. Brush, Modoc, and Signal peaks are
the culminating elevations along this limestone ridge, which stretches north-
ward all the way to The Gate. Along the west base of these peaks
runs the Modoc fault, extending, southward from Hay Ranch Valley, near
The Gate, till lost in the Lone Mountain limestones west of Brush Peak.
This fault brings up the Diamond Peak horizon in juxtaposition with the
Devonian, leaving the limestone ridge between two nearly parallel belts of
quartzite of the same age, conformable on the east side, but unconfonuable
on the west. As the line of the fault follows the contact between two dis-
similar rocks it is easily traced. North of Signal Peak erosion has worn
out a deep ravine along the contact, and still farther southward the east
drainage of lieilley Creek also owes its origin to erosion along the same

142 GEOLOGY OF THE EUEEKA DISTRICT.

fault line. The Diamond Peak beds may be represented in their full
development near The Gate, but they gradually die out to the southward
in a wedge-shaped body and finally disappear altogether, beyond which the
fault may be followed for a considerable distance, with Nevada limestone
walls upon both sides. This long body of Diamond Peak quartzite rests
conformably upon the Nevada limestone to the westward, both series of
strata dipping uniformly to the east, We have here, then, a duplication of
strata made up of the Upper Nevada limestone, overlain by the Diamond
Peak quartzite. Small drainage channels, branches of Reilley Creek traverse
the quartzite, affording fair cross sections. Numerous minor dislocations,
at right angles to the Modoc fault, trend easterly across the ridge, dying
out in the plain beyond, but, while they tend to break up the uniformity
of structure, do not cause any very decided dislocation in the Nevada lime-
stones.

Perhaps the best' section, the one showing the greatest vertical thick-
ness across the Nevada limestone, may be found on the ridge north of
Modoc Peak. This section is given on page 66. Starting in at the Modoc
fault in Reilley Canyon, nearly due west of Modoc Peak, it crosses the
strata nearly at right angles and terminates at the base of the hills in Dia-
mond Valley. The beds strike N. 50° to 55° W., measuring about 5,400
feet in thickness. Just north of Modoc Peak a fossiliferous shaly limestone,
200 feet in thickness, crosses the ridge. It is the belt designated No. 3 of
the section, and is the equivalent of the rich fossiliferous shale which has
yielded such an abundant Devonian fauna at several localities in the Dis-
trict, notably, at Brush Peak, about 2 miles southward. Higher up in the
strata, corals of the middle and upper horizons were obtained, but nowhere
immediately along the line of the section was any special fossiliferous zone
recognized.

Both north and south of the line of the section the strata are easily
traceable, striking obliquely across the ridge, the upper horizons being
developed on Signal Peak and the lower on Modoc and Brush Peaks. Just
below the summit of Brush Peak the fossiliferous shale belt, which is here
about 150 feet in width, determines the position of the beds without ques-
tion. It is at this locality that the shales have furnished such an excellent

METAMORPHOSED SANDSTONES. 143

opportunity for the collection of a Devonian fauna. The few hours spent
here gave promise of an abundant harvest if time would permit of a dili-
gent search. From this shale belt the limestones pass down into the Lone
Mountain series, the hill lying between Brush Peak and Spanish Mountain
being formed of the latter beds.

Metamorphosed sandstones.— Interstratified in the Nevada limestone of this
ridge occur numerous bands of fine grained sandstones with their bedding
planes parallel to the inclosing rock. Some of them may be traced for over
a mile without interruption, rarely exceeding 50 feet in thickness, but most
of them only a few feet in width. They are shown in the section north of
Modoc Peak occurring at varying intervals throughout nearly 1,000 feet of
limestones.

Instances of sandstones in limestones are common enough and would
call for no special comment but for the fact that here they have undergone
considerable alteration, and as the original material was more or less
impure, they have developed under dynamic influences a crystallization
and structure of a micro-granite. All of these sandstones show alteration,
but at the same time exhibit remarkable transitions from a normal sand-
stone to a rock closely resembling a cryptocrystalline granite. The quartz
grains are granitoid in structure, and do not show the action of water usually
seen in a compact sandstone made up from the disintegrated material
derived from an older rock. Accompanying these quartz grains are flakes
of muscovite with some ferrite and calcite. It is evident that the beds have
undergone a marked change since they were originally laid down. That
these rocks are of sedimentary origin no one would question, yet they are
associated with others which have undergone so great an alteration that they
present many structural features of igneous rocks. The transition from
undoubted sandstone to the highly metamorphosed beds shows every stage
of gradation and it is impossible not to see the close relationship existing
between them. In the more highly altered rocks may be observed well
developed feldspars, both orthoclase and plagioclase. Most of the feldspars,
however, have undergone decomposition, and are accompanied by calcite
and other secondary products. Singularly enough, some of the more crys-
talline bodies exposed along the west sides of Signal and .Modoc peaks attain

144 GEOLOGY OF THE EUKEKA DISTRICT.

a much greater width. In one instance the rock measures about 200 feet
across its broadest development, but diminishes rapidly to only a few feet.
Here it loses its distinctive features as a sedimentary bed, and, on the con-
trary, appears to cut across the limestones, suggesting an intrusive dike.
That these nearly identical rocks should, in some cases, have the charac-
teristics of sedimentary deposits, and in others those of an intrusive dike,
is, to say the least, most remarkable ; but, after a study in the field of their
mode of occurrence, no other conclusion seems reasonable than that they
are similar rocks which have undergone various degrees of metamorphism.
These occurrences have no special bearing upon the history of the sedimen-
tary strata, as they occupy very limited areas in the limestones, and perhaps
still less upon the history of the Tertiary volcanic outbursts of the Eureka
region. They are well worthy an investigation, and Mr. Iddings, in his
chapter on the microscopical petrography of the crystalline rocks, has
devoted considerable space to a discussion of the phenomena which these
rocks exhibit.

signal Peak.— On this peak the limestones belong exclusively to the Upper
Nevada horizon, being massive grayish black rocks, distinctly bedded.
They dip northeast about 35°. The fauna is characterized by Upper
Devonian corals, associated with species found all the way through the
Nevada epoch. North of Reilley Canyon the beds dip eastward at a .still
lower angle, throwing the overlying quartzite to the east, out toward
the valley. On the summit of the ridge north of the last named canyon
occur Syringopora hisingeri, Bellerophon mtera, and other more common
forms, the beds carrying occasional corals, without being confined to any
special horizon.

The Gate.— At The Gate occurs a marked change in the structure of the
region. The ridge, which from Brush Peak northward maintains a fairly
uniform course, here undergoes an abrupt break, trending off more to
the west, and at the same time the entire mountain mass north of The
Gate has been thrust eastward, bringing the beds on opposite sides of
the break unconformably against each other. The Gate is a deep, narrow
gorge, cutting completely through the ridge along the line of the disloca-
tion. It cuts down to the very base of the range, draining the broad

ABSENCE OF WHITE PINE SHALE. 145

desert region of Hayes Valley out into Diamond Valley. On the south
side of The Gate the beds strike N. 20° W., dipping 20° easterly, but on
the north side they strike N. 55° W., with a dip increased to 30° easterly.
Owing to the thrust which forced the beds toward the east the walls on the
south side belong mainly to the Diamond Peak quartzite, while those on the
north side are formed of a bold cliff of Nevada limestone. The sections
across the strata on opposite sides of the gorge are readily correlated by
structural features confirmed by paleontological evidence. Fortunately,
just beneath the Diamond Peak beds south of The Gate a fauna character-
istic of the Upper Nevada limestone occurs in the low ridge near the west
entrance to the pass. There is exposed here a thickness of 1,000 feet of
the upper limestones. The underlying beds are dark gray in color, with
poorly preserved fossils, followed by a black band bearing many large
Stromatopora and other corals. Interstratified in these limestones are several
quite shaly beds, seldom more than 1 foot in thickness. These gray beds
are followed by a belt of distinctly stratified black limestones, weathering
a light color, and yielding numerous corals. Above this, again, are thinly
bedded, dense limestones, extending up to the overlying quartzites. In
these latter beds occur the Upper Devonian fauna already mentioned.
Conformably overlying the limestones occurs a broad belt of Diamond
Peak beds, forming the wall along the south side of The Gate and extend-
ing in low, round, monotonous hills out to Diamond Valley- The cliffs on
the north side of The Gate expose about 500 feet of massive, dark lime-
stones, passing into shaly and fissile beds 2 or 3 feet in thickness. A rich
and varied fauna from this locality will be found published in full on
page 83. The locality would well repay a more diligent and careful
search.

Absence of white Pine shale.— On both sides of the gorge the overlying
siliceous beds are much the same, the base of the series being made up of
quartzites, interbedded, impure sandstones, compact, dense argillites, fine
conglomerates, and black cherty layers, rapidly passing into purer quartz-
ites. On the south side the black cherty belts present a greater thickness
and are not confined to the base of the horizon.

It will be noticed that no mention has been made of the White Pine
MON xx 10

146 GEOLOGY OF THE EUKEKA DISTEIOT.

shale, which on the east side of the Eureka District exposes such an enor-
mous thickness. There is but little doubt that these lower beds represent
the White Pine horizon, but, as they are so poorly developed as compared
with the shales at Newark Mountain and so difficult to trace along any
definite horizon, they have been omitted on the geological map. No
exposure of these beds was seen more than 100 feet in thickness and in
places they are entirely wanting. It would seem that after the deposition of
the limestones the conditions here were more favorable for purely siliceous
beds than at Newark Mountain, and that the transition was more or less
rapid. It must be remembered that the White Pine shale, although of great
thickness at White Pine and on the east side of the district, is of local
occurrence, never as yet having been recognized in other parts of the
Great Basin. The occurrence in the argillites just south of The Gate of a
few obscure plant remains and the species Discina minuta is strong evi-
dence, taken in connection with their stratigraphical position, that these
beds represent the White Pine shale.

The Diamond Peak beds which overlie the limestones on the north
side of The Gate form the great mass of Anchor Peak, showing a greater
thickness of strata than the same horizon exposes in the Diamond Range;
the explanation being found in the argillites of the White Pine shale giving
out and being replaced by a greater development of siliceous material.
After the coming in of the siliceous beds north of The Gate the quartzites
stretch for nearly a mile beyond the limits of the map. At the west base
of Anchor Peak there is a small exposure of Devonian limestones dipping
under the quartzites, probably extending northward along the west base
of the Pinon Range.

SILVERADO AND COUNTY PEAK.

This mountain block is mainly outlined by profound faults, along which
igneous rocks of varied composition have burst forth in vast quantities,
almost completely isolating it from adjoining sedimentary regions. On the
south and east the Quaternary accumulations of Newark and Fish Creek
valleys rest against the base of the hills and probably in a large degree
conceal eruptive rocks which broke out along the edge of the uplifted

COUNTY PEAK REGION. 147

mountain mass, but nowhere attained any considerable elevation. This
mountain block is, for the most part, made up of sedimentary beds belong-
ing to the Silurian and Devonian. In the chapter devoted to a sketch of
the general geology of the district the principal features of this region are
given, and in the chapter on the Devonian rocks a description will be found
of the Nevada limestones, together with some discussion upon the develop-
ment of the Devonian fauna, as shown upon Sentinel Mountain, Woodpeckers
Peak, and Rescue Hill. Only such additional facts are here presented as
may be of value in a detailed study of the region in the field and for com-
parative purposes in distant areas of the Great Basin.

County Peak Region.— The Pinto fault, which trends approximately parallel
with the Hoosac fault, sharply defines this block on the west, and, like the
latter fault, is probably deflected to the east at its northern end. The lowest
rocks exposed by the fault are two bodies of Eureka quartzite, one imme-
diately at the base of Richmond Mountain, the other near by, but separated
from it by the tuffs of Hornitus Cone. The first exposure is so completely
surrounded by igneous rocks that there is nothing to indicate its geological
position but Hthological habit and proximity to the second and larger bod}',
the age of which is clearly determined by overlying Lone Mountain
beds. At its northern end the quartzite of this larger body forms a broad-
topped hill nearly 500 feet in height, with the beds inclined a few degrees
to the east. As regards their Hthological habit, they could not be distin-
guished from the corresponding beds along the Hoosac fault or those in the
region of Grays Peak.

Along the Pinto fault the quartzite is exposed for nearly a mile, thin-
ning out in a wedge-shaped body, and replaced by the Lone Mountain
limestone, which, in tuni, gives way to the Nevada limestone, the latter
forming the fault wall opposite Dome Mountain. Erosion has worn out
a deep, narrow ravine along the displacement, with the Carboniferous lime-
stone, admirably shown on one side, dipping westerly, at angles never less
than 60°, and the Lone Mountain limestone of the Silurian equally well
shown on the other side, dipping easterly, but inclined at low angles,
seldom, if ever, exceeding 20°.

The canyon wall is cut out of the Lone Mountain beds, but on the

148 GEOLOGY OF THE EUEEKA DISTRICT.

steep lull slopes they give way to the Nevada limestones, which continue
eastward across the entire width of the mountains till they are lost beneath
the lava beds of Basalt Peak. County Peak (8,350 feet) forms the culmi-
nating point of this broad, elevated mass of limestones, all the beds of which
strike north and south and dip easterly, affording an excellent cross-section
over 5,200 feet in thickness, with the Lone Mountain beds at the base. The
sequence of rocks shown here may be taken as a typical one of the
Nevada epoch and will be found on page 68, in a chapter devoted to the
Devonian rocks. The cross-section E-F, atlas sheet xm, is drawn across the
summit of County Peak, and gives at a glance the structure of the moun-
tains, which is shown better here than to the south, where it is diffi-
cult to obtain a continuous section for anything like the same distance across
the strata at right angles to their strike. Midway on the ridge connecting
County and Woodpeckers peaks, about 200 feet below the summit and 3,000
feet above the base of the limestone, occurs an important grouping of fos-
sils exhibiting the most complete mingling of both upper and lower Devo-
nian species yet found in the district. Radiating from County Peak iu all
directions occur numerous narrow gorges scored deeply into the mountains,
frequently exposing 1,000 or 2,000 feet of strata and offering excel-
lent opportunities for detailed studies across the middle Devonian strata.
These gorges are the source of the two drainage channels that encircle
Richmond Mountain, finally running out into Diamond Valley. North of
County Peak toward Richmond Mountain, the limestones are characterized
by a development of siliceous beds, aggregating a thickness of over 100 feet
and rising in bold, rugged outcrops above the otherwise even hill slopes.
Nowhere else were similar rocks recognized in the Devonian, the siliceous
material apparently increasing in amount toward Richmond Mountain,
although the higher horizons maintained their normal character. It is only
directly west of County Peak that the upper members of the Nevada lime-
stone are exposed, the basalts concealing more and more of the beds as they
approach Richmond Mountain. In this area north of County Peak, scarcely
any fossils were collected, and nowhere any grouping of species; conse-
quently no locality indicating the presence of organic remains is marked
upon the map. It is proper to say, however, that very little time was allot-

SILVERADO HILLS. 149

ted to their search, but it seems hardly possible that they are absent, as occa-
sional evidence of poorly preserved corals was noted in the purer limestones.
Silverado Hiiis.— South of Dome Mountain the Lone Mountain strata again
come in along the Pinto fault, and with the exception of occasional breaks
caused by overflows of both rhyolite and basalt continue to form the base
of the sedimentary beds until the ridges pass beneath the deposits of Fish
Creek Valley. These rhy elites and pumices, with the glassy basalts break-
ing through them, present identical features with those found in the basin
south of Richmond Mountain, while the basalts in the limestone do not
differ essentially from those occurring as dikes in pyroxene-andesite.

The drainage from the slope of Hoosac Mountain follows a southeast
course until it meets the upturned Silurian ridge on the east side of the
Pinto fault, then runs south across the Pinto Basin, where, instead of con-
tinuing southward following the natural grade along the line of the fault
and across the soft, easily eroded pumices, it turns abruptly and follows a
deep channel cut clear through the hard rocks of English Mountain, finally
running southward to Fish Creek Valley. The divide between this water
course and the broad drainage channel running southward along the Pinto
fault and also emptying into Fish Creek Valley, lies only a few feet above
the level of the two stream beds. So far as can be made out the barrier
between the streams is wholly formed of recent lavas. It is similar to the
case mentioned in describing the drainage of Secret Canyon, where the
stream, after following the course of the canyon for a long distance, sud-
denly crosses the upturned ridge of Cambrian and Silurian rocks, avoiding
the low and insignificant ridge of volcanic material which blocks the
entrance to the canyon. The cause of this sudden turn in the course of
these stream beds is difficult to understand, but it is worthy of note that the
drainage channel bi-eaking through English Mountain lies nearly due east
of the one cutting Hamburg Ridge.

The Lone Mountain beds are not so uniformly made up of limestones
as the corresponding horizon elsewhere. Many of the intercalated strata
resemble the underlying Eureka quartzites, but, as the latter nowhere carry
any considerable layers of calcareous material, siu-h a reference is out of
the question. That they correspond to the Lone Mountain horizon there

150 GEOLOGY OF THE EUREKA DISTRICT.

can be no doubt, the only difference being that the siliceous beds occur
here more prominently developed than on the west side of the Hoosac fault
with the friable sandstones altered to compact quartzites. Moreover, they
are seen to pass into Nevada limestones, except where their continuity is
broken by outbursts of basalt. In the region of English Mountain this
connection is in no way disturbed by intrusive material and the transition
into the Nevada beds maybe readily made out. Nevertheless, there occurs
along the Pinto fault one or two exposures of siliceous beds whose geolog-
ical position it is difficult to determine. One of these is found east of the
Pinto Mill, where a long, narrow ridge, largely made up of quartzites, dips
from 25° to 35° to the east. A ravine, which cuts through this ridge, gives
a fair idea of the beds, and it is not improbable that they belong to the
Eureka horizon. Another instance may be found southeast of Pinto Basin,
near the place called The Wells, where a small isolated hill occurs, appar-
ently a faulted mass composed of white vitreous quartzite with intercalated
bluish gray limestones. Except for these limestones the evidence would
point quite as much to the Eureka quartzite as to the overlying Lone Moun-
tain beds. English Mountain offers the best locality for a sttidy of these
Lone Mountain beds to be found on the east side of the district, as they
show a gr.eat thickness of strata dipping uniformly eastward, overlain by
the lower beds of the Nevada limestones. The base of English Mountain
is formed of quartzites and sandstones, followed by gray limestone, in turn
capped by brownish red, vitreous quartzite. The latter is a rough and
jagged rock, full of nodules and water-worn cavities.

On the south side of the Silverado Hills the Silurian rocks rise above
the pumices and tuffs that follow the base of the hills and in a large degree
conceal the sedimentary beds. Here the limestones have gradually changed
their strike and dip and lie inclined to the northward with the great body
of Devonian limestone that forms the bold escarpment of Red Ridge resting
upon them. Continuing eastward, the limestones gradually swing around
until they assume a westerly dip, forming a synclinal fold, with those of
English Mountain. This Red Ridge escarpment offers excellent vertical
sections of the middle portions of the Nevada limestones, and the variegated
red, gray, and brown belts, with the interbedded sandstones, may be traced

PACKEK BASIN. 15]

for long distances from one mountain to another. In this way it becomes
an easy matter to correlate strata in such blocks as Island and Leader
mountains and Sugar Loaf. The deep gorges penetrating the limestones
afford grand exposures. Sugar Loaf offers one of the best points of view
for gaining a clear understanding of the synclinal structure of the Silverado
Hills, the characteristic belts of sandstones and mottled limestones being
readily traceable from an easterly to a westerly dip. The summit of Sugar
Loaf is formed of Upper Devonian strata, with abrupt escarpments on all
sides. At the east base of this isolated mountain, the Rescue Canyon fault
may be traced crossing the ridge between the head of Rescue Canyon and
the faulted block of White Pine shales at Charcoal Canyon. From Sugar
Loaf northward to Packer Basin all the limestones on the west side of the
fault dip westerly, the fault following the line of contact between the
Nevada limestone and the White Pine shale. Opportunities for observing
these westerly dipping beds may be found in Charcoal and Ox Bow canyons,
the streams which cut the ravines crossing the strata nearly at right angles
to their strike.

Packer Basin.— Packer Basin is a small depressed block of Nevada lime-
stone lying between the northern end of the main ridge and the broad
basalt table, the abrupt wall of the latter shutting in the basin on the north.
As the basin lies on the very edge of a broad volcanic field, it has naturally
undergone a good deal of dislocation, and is much broken up by pumices
and tuffs, which partly fill the basin, having poured out along a fissure
on the west side of the faulted block. It is interesting to see here the same
association of pumices and tuffs, followed by a later outburst of basalt, in
all respects similar to those occurrences seen in so many other places
bordering the uplifted block. The limestone still maintains the north and
south strike and westerly dip of the main ridge to which it really belongs.
Its chief interest lies in the finding in a massive blue limestone a fauna
characteristic of a somewhat higher horizon than those observed at Wood-
peckers and Basalt peaks. Additional interest is derived from the disappear-
ance of the Rescue fault and the accompanying White Pine shales beneath
the basalts.

152 GEOLOGY OF THE EUREKA DISTRICT.

Rescue Hiii.— Scarcely any mention need be made here of this locality as
the essential structural features and the list of species obtained in the upper
beds have been given in the chapter describing the Devonian rocks. The
hill is a block of limestone faulted over 1,000 feet below its true strati-
graphical position. It lies in the angle formed by the intersection of the
Rescue and Silverado faults. The beds lie inclined at a very low angle
presenting an excellent section for comparative purposes with beds found
elsewhere. Owing to the faulting of this block the variegated beds of Red
Ridge can not be followed on Rescue Hill, but to the north they are
easily traceable on Island Mountain and Sugar Loaf.

Century Peak Ridge.— Rescue Canyon severs the Century Peak Ridge from
the main body of Silverado Hills, a separation which is intensified by the
rhyolitic outbursts along the line of the canyon. Structurally the country
east of the canyon differs in a most striking manner from Red Ridge and
Rescue Hill, the horizontal, plateau-like character of the former giving way
to a narrow ridge with steep slopes. This ridge, of which Century Peak is
the highest point, presents a sharp, anticlinal fold, the beds dipping away
from the axis at angles varying from 70° to 80°. The axis of the fold
follows closely the crest of the ridge, with a strike approximately north
and south. On the summit of Century Peak occurs one of the many
intercalated beds of quartzite found in the Nevada limestone, and here
forms the greater part of the west slope, extending down the ridge nearly
to the line of rhyolite. Just where this quartzite belt belongs in the lime-
stone was not determined, but the entire uplift is of Upper Devonian age,
as is shown by the lithological character of the beds. No fossils identify-
ing any special horizon were obtained, but those found were forms having a
wide vertical range, such as Atrypa reticularis. The corals belong to the
upper portion of the limestone and, although too obscure for specific iden-
tification, closely resemble the forms found in the limestones at the northern
end of the Mahogany Hills. Along the line of the Silverado fault the
rocks give evidence of considerable disturbance and folding with abrupt
flexures and breaks. For the greater part of the distance along the canyon
the Nevada limestone may be seen south of the fault, with the White Pine
shale on the north or opposite side of the gorge resting unconformably

ALHAMBEA HILLS. 153

against it. On the north side of the Silverado fault, between the White
Pine shale and the rhyolites occurring at the head of Rescue Canyon, is a
triangular block of limestone inclined to the east. This block of limestone
lies on the east side of the Rescue fault, conformably underlying the White
Pine shale and offering ample structural evidence that it belongs to the
highest beds of the Nevada horizon. The amount of faulting along Silver-
ado Canyon has never been determined, but probably does not exceed a few
hundred feet, which is additional evidence that the Century Peak beds
belong to the upper portion of the Devonian. South of Century Peak there
is a decided break in the strata and the entire limestone ridge dips off
toward Fish Creek Valley, with a northeast and southwest strike.

Aihambra Hiiis.— The low ridge of limestone designated as the Alhambra
Hills lies to the east of the Century Peak ridge and is connected with the
latter by a continuous body of limestone. North of this connecting ridge
Quaternary deposits lie between these hills and Century Peak ridge,
but they are of no great thickness and undoubtedly overlie a depressed
area of limestone. Alhambra Hills rise but a few hundred feet above
the plain. They present a dull, monotonous, arid aspect, with but few
scattered trees and without soil. The limestones belong to the upper
members of the Nevada horizon and are massive, distinctly bedded, grayish
blue rocks. But little time was devoted to the search for fossils, but such
as were found denoted the upper beds of the Nevada and were mostly
corals similar to those found in the neighborhood of Century Peak, associ-
ated with the ever present Atrypa reticularis. Beyond this identification of
the age of the beds the Alhambra Hills present no special geological in-
terest. A few mineral veins penetrate the limestone, but so far as known
are unaccompanied by rhyolite intrusions. The latter rock, while it prob-
ably encircles the Alhambra Hills, does not appear to enter the limestone
body.

White Pine Shale Area.— There is little that need be said about this area in
addition to the observations presented elsewhere in discussing the geological
position and the paleontological evidence of the age of the White Pine
shale. On page 81 will be found a description of the strata across the
entire thickness of shales and sandstones, at least until they are overlain

154 GEOLOGY OF THE EU11EKA DISTRICT.

by Quaternary deposits. They measure over 2,000 feet. This section was
made east of Sugar Loaf, where the underlying limestones are exposed,
passing conformably beneath the broadest expansion of overlying shales.
The occurrence of this limestone is exceedingly fortunate, as upon it
rests the evidence of the position of the overlying shales, whereas, north of
Charcoal Canyon no limestones occur beneath the shale, and as the beds
trend to the northwest with a greater angle than the course of the Rescue
fault, the lower strata are cut off along the line of the displacement. Direct
evidence is wanting of the precise position of the beds lying next the
fault. From Silverado Canyon northward to Packer Basin the strata dip
uniformly eastward. Charcoal Canyon, Ox Bow Canyon, and the other
drainage channels traversing the formation, fail to give any good sections
across the beds, as the valleys, though broad, are extremely shallow, with
the underlying rocks more or less covered with soil and gravel, derived from
the disintegration of the friable interbedded sandstones. The stream bed
coming from Packer Basin has eroded somewhat more deeply into the shale
formation, the beds lying more highly inclined, but shortly after leaving
the mountains it enters the tuffs which overlie the shales.

cuff Hills.— South of Silverado Hills, and separated from them by the
broad expanse of Fish Creek Valley, lies a low ridge designated Cliff Hills
on account of the mural-like escarpment which they present to the Quater-
nary plain. These hills have no direct topographical connection with the
Eureka Mountains and are referred to here only because they happen to
come in on the southeast corner of the map. By reference to atlas sheet
xn, their relations to the Eureka Mountains may be seen at a glance. Geo-
logically they are of great interest, as the White Pine shale, which has
been recognized over such limited areas, occurs here under conditions simi-
lar to those found east of Sugar Loaf. Low undulating ridges of shale and
sandstone formed of westerly dipping beds pass beneath a broad, flat-topped
body of pyroxene-andesite. It is this andesite which gives the cliff-like
appearance to the hills, the dark bare rocks presenting a forbidding aspect
as they rise above the desert valley. In then* mode of occurrence and
petrographical habit these andesites closely resemble those of Richmond
Mountain, and show the same modification in color, density, and chemical

NEWAEK MOUNTAIN. 155

composition; in mineral composition they are identical. These resemblances
are borne out by microscopical investigation, the differences in structure in
Richmond Mountain finding their counterpart in Cliff Hills.

Cropping out beneath the andesites at the north end of the hills are three
small exposures of gray limestones, only one of which is represented on the
map. It dips westerly at an angle of 15° and strikes nearly north and
south. No evidence of the age of these limestones could be obtained, but
from their proximity to the White Pine shale and their general resemblance
to the Devonian rocks of the Silverado region, they have been referred to
the Nevada limestone. In the White Pine shale a few fragmentary plant
remains were procured, none of which were sufficiently well preserved to
admit of identification, although they bear the closest resemblance to the
plants found elsewhere at this horizon.

DIAMOND RANGE.

Few of the narrow longitudinal ridges in central Nevada form so prom-
inent a physical feature as the Diamond Range. Only the southern end,
however, comes within the limits of the Eureka District, but here it is so
intimately connected with the County Peak and Silverado uplift as to form
a part of the same geological region.

Diamond Peak, the highest elevation in the range, is situated just
within the limits of the survey, although the north and east slopes lie
beyond the boundaries of the map. In a study of the sedimentary rocks of
the Eureka district, this peak is of the highest interest, showing the rela-
tionship between the Devonian and Carboniferous beds in a manner unsur-
passed elsewhere in the Great Basin, and at the same time carrying the
Paleozoic section nearly, if not quite, to the top of the Upper Coal-meas-
ure limestone.

Newark Mountain,— As seen from the east, Newark Mountain present* a

bold front of blue limestone rising nearly 2,000 feet above Newark Valley,
the upper 1,000 feet an abrupt cliff, followed by a highly inclined slope
to the plain. Along the summit it is a narrow ridge 3 miles in length, fall-
ing off gradually toward the west in strong contrast with the opposite side.
In structure, Newark Mountain is an anticlinal fold whose axis may be
traced all along the base of the cliff, the eastern side of the arch having

156 GEOLOGY OF THE EUREKA DISTRICT.

dropped about 1,000 feet, causing a picturesque escarpment. It is a fine
example of a limestone wall formed by a displacement. The easterly
inclined beds, begining at the base of the cliff with a dip from 15° to 25°,
gradually fall away with a less and less angle, stretching in low broken hills
and knolls far out toward the plain. Along the face of the cliff on the west
side of the anticline the strata incline into the mountain, arching over from
an angle of 25° on the crest of the ridge to 55° along the western base in
Hayes Canyon. At the southern end of the ridge the beds rise steeply
out of the Quaternary plain along the line of an east and west fault.
They strike a few degrees east of north, gradually curving more and more
to the east, coinciding approximately with the trend of the i-idge until at the
northern end they fall away toward Newark Valley and pass beneath the
east base of Diamond Peak. The limestones of Newark Mountain belong
to the upper portion of the Nevada Devonian. They are usually dark blue
and gray in color and distinctly bedded. It is estimated that there are
exposed on the mountain about 3,500 feetof these upper Nevadalimestones,
which would carry the beds down nearly to the middle of the formation
They may be correlated readily with the limestones of Silverado Hills by
the sequence of strata and by their physical habit. Their stratigraphical
position is determined without doubt by the overlying White Pine shale in
Hayes Canyon, the contact between the two formations being easily trace-
able for miles, all the way from the entrance to the canyon around to the
northern base of Diamond Peak. Paleontological evidence confirms other
evidences by the finding of upper Devonian species in several localities in
two distinct horizons, one, near the summit of the limestones along the west
base of the mountain, the other, several hundred feet lower down in light
gray, somewhat shaly beds on the south side of Milk Canyon. Fossils
may also be obtained near the summit of the mountain. A list of the
species obtained from both horizons will be found in the chapter devoted to
the discussion of the Devonian rocks, and, while they both contain specific
forms having a wide vertical range, they are characterized by types found
only in the upper Devonian. The species Beyrichia occidentalis, obtained just
below the White Pine shale in Hayes Canyon, occurs on the east side of
the mountain 1,000 feet or more below the summit; it has also been identi-

DIAMOND PEAK. 157

fied from the top of Telegraph Peak at White Pine, where it also occurs
not far below the base of the shale.

At the summit of the Nevada beds a reddish gray, impure limestone
passes gradually into the black, argillaceous shales of the White Pine
series, the contact between the two formations being admirably shown all
along Hayes Canyon at the base of Newark Mountain. The drainage
channel marks closely the line of contact. Hayes Canyon lies wholly in
the shales, erosion having carved out of them a broad valley, similar in
topographical structure to Secret Canyon, between the Prospect Mountain
and Hamburg limestones. Upon one side of Hayes Canyon rises a wall of
dark blue, Devonian limestone, and on the other light blue and gray Car-
boniferous limestone. At the summit of Hayes Canyon the shales follow-
ing the course of the limestones of Newark Mountain trend off to the north-
east and rapidly pass under Diamond Peak. The relationship between the
shales and the Diamond Peak quartzite may be best studied along the base
of Bold Bluff, the former being seen to dip conformably beneath the
quartzites at an angle of 30°.

Diamond Peak.— The summit of Diamond Peak attains the highest eleva-
tion of any point within the limits of this survey, reaching an altitude
above sea level of 10,637 feet. From Newark Valley it rises for over 4,000
feet with an almost unbroken slope to the summit. No peak commands
a more favorable view for a study of the relationship between the topo-
graphical configuration and geological structure of the country. The
structure of the peak is that of a sharp, synclinal fold, the axis of which,
striking northeast and southwest, lies along the crest of the ridge. The
westerly dipping beds form the entire eastern slope of the peak, exhibiting
a great thickness of Devonian and Carboniferous rocks. At the base of
the peak, just outside the limits of the map, the Nevada limestone comes
in, overlain by a broad belt of black shales, which form the lower slopes,
but, as denudation has worn them smooth, they present rather a monoto-
nous aspect. Following the shales are the Diamond Peak quartzites, in rough
and rugged ridges and bold walls, extending within 1,200 feet of the sum-
mit, over which come the massive Coal-measure limestones forming the top
of the peak.

158 GEOLOGY OF THE EUREKA DISTRICT.

The following section gives the broader divisions of the beds from
base to summit, including those exposed on Newark Mountain, as the
Nevada limestones on Diamond Peak are shown only to a very limited
extent:

Feet.
1. Bluish gray distinctly bedded limestones 1, 000

2. Green and brown and chocolate colored clay shales, with interbedded

siliceous bands and cherty beds 500

„ ~\ O T\, —

i
I

3. Dark gray quartzites, compact conglomerates, with interbedded layers
of jasper and siliceous grits. Near the base narrow belts of blue
limestone, carrying Products semireticulatug 2, 500

'4. Black argillaceous shale, more or less arenaceous and similar to the

lower black shale 1, 000

5. Compact, fine grained sandstone, with minute dark siliceous pebbles

scattered through the beds 100

6. Black argillaceous shale, with fine intercalated beds of arenaceous

shale. These shales crumble on exposure to atmospheric influence. . 500

7. Reddish gray shaly calcareous beds . . 100

8. Dark gray heavily bedded siliceous limestone, passing into bluish gray

limestone, in places finely banded 3, 500

Total 9, 200

The importance of this section lies in the fact that it gives over 9,000
feet of conformable limestones, shales, and sandstones of Upper Devonian
and Lower Coal-measure strata, the best section as yet recorded from
this portion of the Paleozoic series in Nevada. It will be noticed that at
the base of this series of beds less than one-half of the thickness of the
Nevada limestone is represented, and at the top only about one-quarter of
the entire thickness assigned to the Lower Coal-measures is exposed on the
summit of Diamond Peak.

Along the summit of the range occupying the axis of the fold the
Coal-measure limestone extends for a long distance, and on Diamond Table,
at their southern limit, they present a bold body of nearly horizontal beds,
300 feet in thickness, resting directly upon the quartzites. In Water
Canyon, which drains the southern end of Diamond Peak, the position of
these two formations is well brought out, erosion having carved a mag-
nificent amphitheater, with abrupt walls, 2,000 feet into the quartzite. In
the bottom of the canyon the White Pine shale comes out beneath the
quartzites, all three formations being shown in the canyon walls.

Scattered throughout these limestones may be found Coal-measure

ALPHA AND FUSILINA PEAKS. 159

fossils, the best locality noticed being on the summit of the ridge about
one-third of a mile south of the peak and 150 feet below the highest point.
Ten species were obtained here, the list being given on page 91. The two
most interesting species are Spirifera trigonalis and Camarophoria cooperensis,
the latter identical with the Missouri form. Both of them, as pointed out
by Mr. Walcott, are characteristic of the lower Carboniferous in the Mis-
sissippi Valley. It is these two species that serve to correlate the low lime-
stone ridges south of Newark Mountain with the base of the Lower Coal-
measures.

Immediately northwest of the crest of the ridge the strata dip easterly,
and at about the same distance below the summit, as observed on the
opposite side of the peak, the quartzites come in conformably beneath the
limestones, dipping easterly into the ridge. No considerable thicknesses of
quartzites are exposed, as they are abruptly cut off by the profound Alpha
Peak fault, which brings the Upper Coal-measure limestones unconform-
ably against them. Following the quartzites southward, they are seen to
be much broken up and dislocated, and southwest of the peak again dip
westerly, with an angle of about 15°, a dip which they maintain as far
south as Bold Bluff, where they terminate abruptly against the Newark
fault. By reference to atlas sheet vi the position of the quartzites may be
readily made out, completely encircling Diamond Peak on all sides.

Newark Fauit.-This line of faulting, starting in at Bold Bluff, trends
southward along the abrupt west wall of Hayes Canyon, following the
contact between the two dissimilar formations — the gray Lower Coal-
measures and the black White Pine shale. It is easily traceable for nearly
3 miles. At the southern end it gradually trends off to the southeast, com-
pletely cutting off the shales, as well as the Diamond Peak quartzite, and
at the mouth of Hayes Canyon brings the Lower Coal-measures directly
against the Nevada limestone of Newark Mountain.

Region of Alpha and Fusiiina Peaks.— The Lower Coal-measure limestone
overlying the Diamond Peak quartzite forms an unbroken narrow ridge,
extending southward for over 9 miles, and falling away gradually until
it passes beneath the Quaternary of the valley. This ridge presents great
simplicity of structure and monotony of appearance, the beds exhibit-

160 v GEOLOGY OF THE EUKEKA DISTRICT.

ing much the same lithological habit throughout and everywhere lying
inclined toward the west at high angles.

At Bold Bluff, where the quartzite gives out, the Newark fault brings
the lower members of the limestone next the White Pine shale. Along the
west side of Hayes Canyon both formations dip into the ridge, but it is
somewhat difficult to recognize the unconformity along the contact, owing
to the amount of debris, in spite of the fact that the angle of dip between
the two horizons varies from 20° to 30°. Several observations, taken at
different points along the canyon wall, gave about 25° as the angle of
unconformity. The evidence of the unconformity is strengthened by the
absence of the entire thickness of quartzite, the true position of which,
between the limestone and shale, is so well exhibited both on the east side
of Diamond Peak and in the neighborhood of Bold Bluff and Water
Canyon. Again, the wedging out of the White Pine shale, which is
completely lost at the mouth of Hayes Canyon, gives additional evidence of
the unconformity.

The upper members of the Lower Coal-measures are quite as sharply
defined on the west side by the Alpha fault, which for a short distance
follows along the steep northwest slope of Diamond Peak, bringing the
Upper Coal-measures unconformably against the quartzite. Nearly due west
of the summit the fault trends off to the southwest and the Lower Coal-
measures come in next the quartzite, the line of fault marking the contact
between the two bodies of Carboniferous limestone. The Alpha fault con-
tinues southward along the base of Alpha Peak, but terminates abruptly
on reaching the north slope of Weber Peak. It is rarely that an uncon-
formity in Carboniferous limestone strata is more strikingly shown than by
the two Coal-measure formations along the Alpha fault. There may be
seen here on one side of the fault, the underlying limestones dipping west-
ward at angles varying from 65° to 85°, and on the opposite side, the over-
lying limestones inclined at angles rarely exceeding 10°.

At Weber Peak, where the Alpha fault terminates, an east and west
fault brings up the Weber conglomerate, and from here southward the beds
of the latter epoch are found in their true geological position conformably
overlying the Lower Coal-measures. This east and west fault does not

WEBER PEAK. 1(}1

cross the Alpha fault, at least the limestones appear to have undergone no
displacement, West of the Alpha displacement the course of the east and
west fault after passing Weber Peak is lost, being buried beneath the accu-
mulations of igneous rocks.

The thickness of the Lower Coal-measures may be best estimated
south of Fusilina Peak, where the upper members of the epoch are deter-
mined by the position of the Weber conglomerate, and, although there ex-
ists no positive evidence that the beds resting on the White Pine shale are
the equivalent of the lowest members found elsewhere, they probably do
not belong far above the base. It is estimated that the limestones measure
about 3,800 feet in thickness.

Organic remains may be found scattered throughout the limestone, but
nowhere were any grouping of species obtained which were of special in-
terest or which could be regarded as the equivalent of the Spring Hill
fauna. At the head of Newark Canyon, which starts in near the base of
the limestone immediately resting on the White Pine shale, were found
Producing longispinwSj P. semireticulatus, and Spirifera camerata, while south of
Fusilina Peak, at the top of the horizon, the same species occur associate! 1
with Fusilina cylindrical and other Coal-measure forms. On the map will be
found a number of localities designated where fossils were procured but
they indicate only a few of the horizons where they are known to exist.

Weber Peak and Pinto Springs Region.— Under this heading may be designated
the area of the Weber conglomerates lying between the two great bodies
of Carboniferous limestone. From Weber Peak southward they overlie
conformably the Lower Coal-measures, although not extending southward
out into the open valley quite as far as the limestone, being buried beneath
either basaltic flows or the alluvial deposits of Pinto Creek. Along the
line of contact the Weber conglomerates form a well defined series of ridges
parallel with the Alpha and Fusilina ridges, the two formations standing out
sharply contrasted by their surface forms, atmospheric agencies acting quite
differently on the fine crystalline limestones and the coarse conglomerates.
In like manner erosion acting upon the more easily disintegrated conglom-
erates has worn out a number of narrow drainage channels along the con-
tact which serve still more sharply to define tin- boundaries. The conglom-
MON xx 11

162 GEOLOGY OF THE EUKEKA DISTRICT.

erates stretch out toward the west until cut off by the broad basaltic table-
land of the Strahleuberg, which, concealing- everything over a wide area,
leaves to conjecture the probable structural relations of the Carboniferous
rocks of the Diamond Range to the immense block of Devonian limestone
of the County Peak uplift. East of Strahlenberg, the highest point on the
eastern rim of the basaltic field, the conglomerates present a broad, high
ridge, with a strike of N. 30° W. and an easterly dip of 75°. It is against
this ridge that the basalts have been piled up, the ridge acting as a barrier
to their further progress in that direction. Between the basalt and the
Lower Coal-measures of Alpha Ridge the conglomerates are plicated into
a broad syncline followed by a sharp anticline, the axes of both folds being
traceable the entire length of the conglomerate area. The conglomerate
ridge lying next to the Lower Coal-measures forms the east side of the
syncline, the beds coming up again on the opposite side of the fold in a
ridge nearly parallel with the first one. The anticlinal fold presents a much
sharper axis, the beds 011 both sides of the arch dipping at angles varying
from 55° to 65°.

One of the most fortunate occurrences in working out the structural
geology of the region is the belt of light gray Upper Coal-measure lime-
stone lying between the westerly dipping beds of the anticlinal fold and
the basalts. It furnishes within the district evidence of the position of
the Weber conglomerate between the two great belts of Coal-measure lime-
stone and although ample proof could be found elsewhere, it makes the
chain of evidence complete for all the divisions of the Paleozoic series
of rocks in the Great Basin. It is a narrow strip of limestone, in places
only a few hundred feet in width and about one mile in length, being cut
off both at the north and south by igneous rocks. It strikes nearly north
and south and dips between 55° and 60° to the west, coinciding with the
inclination of the underlying conglomerates on the west side of the anti-
clinal fold. In a yellowish gray bed occurs a characteristic fauna of the
Upper Coal-measures ; a list of the species procured here will be found
elsewhere. The continuity of this body of Upper Coal-measure limestone
with the larger body north of Newark Canyon is broken not only by
igneous flows, but the connection is completely severed by a line of fault-

WEST SLOPE OF DIAMOND RANGE. 1C,.;

ing along the canyon. The distance between them measures only about
one-half mile and is mainly occupied on the surface by rhyolitic pumices
and tuffs.

No special mention need be made of the physical characters of the
Weber conglomerate, as it has been described in sufficient detail in the chap-
ter devoted to the Carboniferous rocks, nearly all the observations there
given being taken from this region.

west slope of Diamond Range.— From Newark Canyon northward and west-
ward of the Alpha fault, the country, both in topographical features and
geological structure, presents much the same general aspect over the entire
area. It is the most monotonous and least disturbed region within the
limits of the survey. The opposite sides of Newark Canyon offer marked
geological contrasts; on the one side folded and distorted beds of coarse
conglomerates, on the other a uniformly inclined slope of limestones. Along
the lower end of the canyon the contact of the two rocks is broken by
overflows of pumices, tuffs, and basalts, but higher up and north of the
drainage channel the relations between the two horizons are strikingly shown
on the north slope of Weber Peak about 150 feet below the summit. Here
the conglomerates lie inclined about 18° to the west, with the limestones
resting against them at an angle of only 6°, but without any essential
difference in their strike, both rocks following the trend of the Alpha and
Fusiliua ridge. This change is all the more strongly marked by the con-
trast in topographical features and unconformity of strata between the two
bodies of limestone on the opposite sides of the Alpha fault. This region
is sharply denned by the Alpha fault on the east. From the fault to the
Quaternary deposits of Diamond Valley there is a nearly uniform slope
three miles in width, with a fall of over 1,200 feet. It is crossed by fre-
quent drainage channels at fairly regular intervals, all of them having a
course a little north of west. Nowhere have they cut down into the under-
lying limestones more than a few hundred feet, the bottoms of the valley-,
as a rule, being shallow ravines with narrow strips of meadow land along
the stream bottoms. All the intervening slopes present much the same
superficial features, for the most part smoothly worn down, with here and

164 GEOLOGY OF THE EUREKA DISTRICT.

there an occasional elevation, seldom rising more than 100 feet above the
average height of the surrounding country.

Over this entire area the only rocks which have been recognized are
the Upper Coal-measures, inclined toward the west at low angles agreeing
closely with the slope of the country. This prevents any considerable
thickness of strata being exposed, and it is doubtful if there can be seen
here a greater development of beds than those found south of Newark
Canyon, where it is estimated that 500 feet are shown in the ridge
which rises above the basaltic flows. At the latter locality the base of the
horizon is unquestionably exposed, but along the line of the Alpha fault
there is no structural evidence that the basal rocks come to the surface.
Almost anywhere" scattered through these limestones organic remains
characteristic of the Coal-measures may be found, but the most promising
field for collection is on the summit of the ridge just north of Garden
Canyon. Nearly all the forms obtained here are common enough elsewhere,
with the exception of Ptilodictya (Stenopera) carlonaria and P. serrata. Far
to the south of this latter locality, north of Weber Peak, and just above the
Alpha fault, occurs a nearly similar grouping without the latter two forms,
but with the addition of Macrodon tenuistruita.

Perhaps the most important geological feature of this inclined table of
Upper Coal-measure limestone is the occurrence of an interstratified bed
of conglomerate varying in thickness from 15 to 20 feet. It is exposed in one
or two of the long ridges stretching out toward Diamond Valley, and in one
instance occupies a low depression on the top of the ridge. This conglomer-
ate is made up of pebbles of chert, jasper and quartz such as are found
throughout the Weber epoch, firmly cemented together into a hard sand-
stone. Mingled with these siliceous pebbles occur rounded fragments of
limestone carrying organic remains such a&Syringopora and Fttyilhta cylindrica
and other forms common to the Carboniferous limestones below the Weber
conglomerate, but in no instance are specific forms obtained other than those
previously recognized in the underlying limestones. This implies that after
the deposition of the lower portion of the Upper Coal-measures the country
underwent some slight changes in elevation, subjecting the Weber con-
glomerate and Lower Coal-measures to the influences of erosion, the mate-

NEW YOEK HILL. H;;>

rial being redeposited. All indications point to the fact that this material of
the interbedded conglomerates, was derived from some land mass in close
proximity to the present beds, as it seems hardly possible from the size and
nature of the easily disintegrated limestone that it could have been exposed
for any great length of time to subaqueous influences.

CARBON RIDGE AND SPRING HILL GROUP.

The area embraced within this block is situated in the center of the
Eureka Mountains and stretches in a narrow belt from Diamond Valley to
Fish Creek basin. It lies hemmed in between Prospect Ridge and the
County Peak and Silverado uplift, presenting somewhat the appearance of a
depressed and broken region bounded by two elevated and well defined
mountain masses. This appearance is, in part, due to its relatively slight
elevation, and in part to the struggle for supremacy between sedimentary
strata and the volcanic lavas spread out over them concealing large areas
and breaking the continuity of strata. At Pinto Peak the rhyolites have
been piled up until they have attained an elevation higher than any point
reached by the upturned limestones. These igneous rocks divide the sedi-
mentary beds into two areas, one a northern, of which Spring Hill is the
center, the other to the south, designated as Carbon Ridge. Both regions,
however, present much the same geological conditions and consist wholly
of Carboniferous rocks, the only two epochs represented being the Lower
Coal-measures and Weber conglomerate.

New York Hill.— The direct contact between the Silurian and Carbonifer-
ous rocks on opposite sides of the Hoosac fault may be best seen where the
Lower Coal-measures of New York Hill rest against the Lone Mountain
limestones of McCoy's Ridge, as along the fault between these two ridges
no lavas have reached the surface to obscure. the sedimentary beds. New
York Hill is in some measure isolated from the rest of the Carboniferous
rocks, being completely surrounded by lines of faulting. On two sides the
Hoosac fault outlines it from the Prospect Ridge uplift while a secondary
fault of but slight displacement breaks the continuity of strata between the
hill and the beds underlying Richmond Mountain on the east and Spring
Hill on the south. The limestones of New York Hill strike approxi-

166 GEOLOGY OF THE EUREKA DISTRICT.

mately parallel with the trend of the canyon, which in turn coincides with
that of the Hoosac fault. The beds dip uniformly to the southeast with an
average inclination of 30°. There is no direct means of determining the
base of the Lower Coal-measures anywhere in the Spring Hill block,
although the lower beds of New York Hill are probably not far from the
base of the epoch and occur as low down in the series as any strata to be
found along the east side of the Hoosac fault. Between the base of the
Lower Coal-measures at Diamond Peak and those of New York Hill some
resemblance may be traced, but lithological evidence is not of much
value, as the beds change rapidly in the character of their sedimentation.
On the west slope of New York Hill, Coal-measure fossils may be found
scattered through the beds and in one locality in a shaly limestone near
the summit the following species were collected :

Fusilina cylindrica. Productus nebrascensis.

Fusilina robusta. Productus prattenianus.

Productus semireticulatus.

At the extreme northeast end of New York Hill the drainage channel,
instead of following closely the line of the fault between the Silurian and
Carboniferous rocks, deviates to the northward, cutting through, for some
unexplained reason, the Lone Mountain strata, leaving a portion of the
latter limestone resting upon the slope of New York Hill on the south side
of the canyon. At the northeast end of New York Hill, but east of the
Silurian limestone, occurs 100 feet or more of thinly bedded clays, grits,
and argillaceous limestones, passing into purer beds, which are apparently
unconformable with the main body of limestones as they dip to the north-
ward, toward the fault, at an angle of 30°. They occupy only a small area,
but it seems difficult to tell just how they are related to the main body of
limestone, or to connect them in the section with the southeasterly dipping
beds. That they are low down in the limestones is evident from the fact
that they can be con-elated with the beds on the east side of Eureka Can-
yon which lie near the base of the uplifted strata, dipping under Richmond
Mountain. Their geological position would be of no importance except
that it is in these beds that the fresh and brackish water shells occur which
have already been described in the chapter devoted to the Carboniferous

FRESH WATEE FAUNA. 167

rocks. Their mode of occurrence everywhere shows evidence of shallow
water, but they rapidly pass into beds indicating much deeper water.
Associated with these fresh-water shells are numerous fragments of plant
remains, proving, without doubt, the existence at no great distance of a
land surface. The specific characters of these shells will be found described
elsewhere by Mr. Walcott.1

Limestone of Richmond Mountain.— Between Eureka Canyon and Richmond
Mountain lies a body of limestone uniformly inclined to the east until it
passes beneath the andesites of the latter mountain. It rises nearly 700
feet above the valley, with a fairly regular slope, except where trenched by
short drainage channels which have cut deeply into the rock, giving the
ridge a somewhat ribbed appearance. The beds strike N. 16° E. and
dip from 40° to 50° under the lavas. The Richmond Smelting Works are
situated near the northern end of this limestone body. Just back of the
smelters the base of the limestones are well exposed, and near the rail-
way cut there may be seen a good exposure of strata. At the base of
the cliff occurs a series of dark argillaceous shales of unknown thickness
weathering 011 exposure to blue and gray clays. In these clays may be
found indications of plant remains associated with the Physa prisca and
Ampularia obtained on the opposite side of the ravine in New York Hill,
the shells serving to correlate the beds. It is to be regretted that their
strike and dip could not be determined with precision, but they give every
appearance of passing conformably beneath the overlying strata.

The following section was made across the strata extending from the
top of the series down to the clay beds at the base:

Feet.

1. Coarse conglomerate cemented in fine arenaceous grains 50

2. Compact gray and yellow sandstones carrying a little calcareous material,

and occasional thin belts of limestone 200

3. Fine smooth pebbles in a yellow matrix 100

4. Brownish white sandstone

5. Fine conglomerate, with an admixture of calcareous material throughout- 100

6. Gray limestone, passing into a light gray and yellowish sandstone . 75

7. Cherty limestone, passing into fine siliceous limestone 73

8. Light colored and banded vitreous quartzite 25

1 Paleontology of the Kureka District, Mon. vin, U. S. Geolo>tH'»l Survey, p. 261.

168 GEOLOGY OF THE EUEEKA DISTRICT.

Feet.

9. Cherty bluish gray limestone, carrying Griffithides portlocki, Productus

semireticulatus, P. lonffispinvs, P. prattenianus, Fusilina cylindrica. . . 300

10. Blue limestones in massive layers, with thin iuterbedded calcareous shales

carrying Pleurotomaria conoidea, Metoptomia peroccidens, Macro-
cheilus, Nucula, Ortkocefas, Leperditia 400

11. Dark argillaceous shales, weathering to blue and gray clays, carrying

fresh water shells and plant remains Unknown thickness.

1,525

Throughout the entire series of beds above the quartzite band (No. 8)
occurs a grouping- of characteristic Coal-measure fossils from which
twenty-eight species have been determined. The list will be found in the
chapter devoted to the Carboniferous rocks. Overlying the limestones the
andesitic rocks rise in precipitous walls for over 800 feet.

Spring Hiii.— The uppermost members of the Richmond Mountain beds
are traceable across Eureka Canyon, the conglomerates standing out
conspicuously along the west slope of Spring Hill dipping into the
ridge. A line of displacement runs along the Secret Canyon Road valley,
and, as it approaches the Hoosac fault, the continuity of strata becomes
more and more difficult to follow, showing signs of displacement under the
influence of the outpouring of lavas near the centers of volcanic activity
About a mile up the valley a complete change in structure takes place and
a low hill, somewhat isolated from the ridge, stands out between the main
body of Spring Hill and the Hoosac fault. It rises about 400 feet
above the level of Secret Canyon Road and from its peculiar outlines, the
result of erosion, it has been designated as Conical Hill. It presents a
small block of Lower Coal-measure strata which, instead of dipping easterly
in conformity with the rest of Spring Hill, forms an anticline with
the main ridge, the beds dipping westerly directly toward the Hoosac
fault. On Conical Hill the strata strike from N. 20°-25° E., parallel with
the Canyon Road valley, and dip 30° W.1 On both sides of the axis of
the fold the series of beds are easily traced, consisting of limestones, cal-
careous shales, arenaceous layers, with a well defined bed of coarse con-
glomerate about 75 feet in thickness. This conglomerate appears on the

1 Owing to an error in the proof-reading of tho map, the beds on Conical Hill are represented as
inclined steeply to the east, whereas the dip of 30° to the west, as given in tho text, is correct.

SPRING HILL. 169

west side of Conical Hill and again near the summit of Spring Hill,
standing out prominently on both sides of the fault as a well denned body,
serving as an excellent datum ledge in determining the position of the beds.
The transition from the calcareous to the siliceous beds is rapid, both above
and below the conglomerate. This description of Conical Hill is given
somewhat in detail, as it is here that the Lamellibranchiate fauna of the
Carboniferous occurs. On the east slope of this hill, near the saddle which
connects it with Spring Hill, there is found in a shaly limestone a small
but most typical Coal-measure fauna. Above these shaly beds, about 200
or 300 feet, occur the limestones carrying the Lamellibranchiate fauna, asso-
ciated with Coal-measure species, as described in the chapter on Carbon-
iferous rocks. Ovei lying the Lamellibranchiate beds, on the east side of
the fold, on the east side of Spring Hill, characteristic Coal-measure fossils
come in, but without the mingling of the fauna found below.

These limestones are in turn overlain by a belt of fine conglomerate
100 feet in thickness, in places altered to an indurated sandstone, which
forms the lower slope of the ridge on the west side of Eureka Canyon south
of Spring Hill. It crosses the canyon near the toll-house, with a strike
of N. 16° E. and is traceable on the opposite hills without difficulty. At
the east base of Spring Hill, along the bottom of the Eureka Canyon
and underlying these conglomerates, occurs a thin band of black, fissile,
argillaceous shale, from which were collected Spirifera lineata and a small
Discina not unlike D. minuta. This shale varies somewhat in thickness, but
was estimated at 50 feet. The origin of the canyon is in part due to a
fracture in the quartzite and in part to the nature of the easily eroded
shales, but it does not appear to be accompanied by any considerable
amount of displacement of strata, as is the case with nearly all the other
principal longitudinal drainage channels; in this respect, however, it re-
sembles Secret Canyon. Overlying the conglomerates blue and gray lime-
stones continue on up to the summit of the section, with occasional thin
bands of chert and arenaceous layers, but with less and less siliceous mate-
rial. On the top of the ridge east of the toll-house the gray limestones
carry a typical Coal-measure fauna, and in a thin bed on the west side,
about 100 feet below the summit, there were collected:

170 GEOLOGY OF THE EUEEKA DISTEICT.

Fusilina cylindrica. Productus longispinus.

Chonetes verneuiliana. Productus punctatus.

Productus costatus. Productus semireticulatus.

Spring Hill and the limestone ridge lying on the west side of the Pinto
fault form a synclinal fold whose axis is situated on the western side of the
high hill east of the toll road. The strata dip away from the Pinto fault
into the ridge at high angles, but on the opposite side of the fold they
lie more regularly inclined at a much lower angle. The synclinal structure
here does not differ essentially from that shown southward along the
geological section E-F, atlas sheet xui.

On the south side of Conical Hill a fault coincides with a narrow ravine
separating it from the next hill to the south. Both the ravine and fault
trend to the south and the latter is finally lost beneath the audesites. On
this second hill the beds are still in accord with those of Conical Hill and
dip westerly, but to the southward of it runs a cross fault connecting the
Hoosac fault with the Conical Hill fault. To the south of this cross fault
the limestones again dip easterly in conformity with those of Spring Hill.

A short distance south of this latter fault the geological section E-F,
atlas sheet xni, crosses the Carboniferous rocks lying between the Hoosac
and Pinto faults. The entire block of limestones west of the Conical Hill
fault dips easterly at about 30°. With apparently only a slight break in
the strata along this displacement the beds on the east side of the fault-plane
still dip easterly at about the same angle followed by a synclinal fold, the
westerly beds of which attain angles as high as 70° and both north and
south of the cross-section reaching even 80°. Taken as a whole, the Car-
boniferous rocks included within this block consist of limestone strata more
or less arenaceous with interstratified belts of both fine and coarse con-
glomerate and carrying from base to summit characteristic Coal-measure
species. It is estimated that the Lower Coal-measure beds along the line
of this section have a thickness of about 3,400 feet, but it is evident that
the base of the series is not reached, and that there are at least 300 or 400
feet of beds, and probably more, on New York Hill and Richmond Moun-
tain unrepresented here. Measurements of the Lower Coal-measures in the
Diamond Range calculated from observed strikes and dips give 3,700 feet

LOWEE COAL MEASURE FAUNA. 171

of beds. From this data the development of the Lower Coal-measure
epoch at Eureka is placed at 3,800 feet ; this thickness is probably rather
under than over estimated.

Region South of Spring Hiii.-Along the divide which separates Spring Hill
from Carbon Ridge vast accumulations of andesites, rhyolites, pumices, and
basalts have poured out, submerging over a large area all sedimentary
beds. An exception is found in the broad, deeply eroded basin just north
of Pinto Peak, Carboniferous rocks again coming to the surface.

Within this basin occurs several exposures of limestones, and on the
north side there is a short narrow ridge nearly 200 feet in height in which
the beds are seen to strike N. 24° W. and dip steeply to the east. At the
western end of these exposures there occurs a well denned belt of sand-
stones, beneath which crops out an area of clay shales. The latter are so
obscured by Quaternary accumulations that but little could be made out of
them. They resemble, however, similar shales to the west of Carbon Ridge.
The sequence of beds indicates their close relationship to those of Spring
Hill, but their geological age is still more strongly shown by the grouping
of fossils obtained from the limestones. The complete list is given here, as
it is rather a characteristic grouping of the Lower Coal-measures of Eureka
and carries with it a number of species found elsewhere in the district at
both lower and higher horizons. The list is as follows :

Stromatopora, sp. f Crenipecten hallanus.

Zaphrentis, sp. ? Pterinea pintoensis.

Syringopora. Pinna consiinUis.

Ptilodictya. Myalina subovata.

Lingula mytaloides. Myalina congeneris.

Orthis resupinata. Modiomorpha? pintoensis.

Cbonetes granulifera. Sanguinolites retusus.

Productus seniireticulatus. Microdon coimatus.

Productus prattenianus. Schizodus cuneatns.

Spirifera camerata. Schizodus pintoensis.

Spirifera striata. Belleroplion inajusoulus.

Bhynchonella eurekensis. Orthooeras randolphensis.

Aviculopecten pintoensis. Orthoceras, sp. !

Aviculopecteu peroccidens. Leperditia, sp. T

Streblopteria similis. Griffithides portlockL

172 GEOLOGY OF THE EUEEKA DISTRICT.

Carbon Ridge.— The area included under this designation is almost com-
pletely encircled by volcanic rocks, and nowhere does it come in direct
contact with sedimentary beds of adjacent regions. The nearest approach
to such contact occurs just northeast of Gray Fox Peak, where a body of
rhyolite about 700 feet in width separates the Carboniferous rocks from the
Eureka quartzite situated on the west side of the Hoosac fault. If the
superficial detrital material along the southeastern slopes of Carbon Ridge
were scraped away it seems highly probable that the isolation of this block
would be still more noticeable, as there is good reason to believe that igne-
ous rocks lie just beneath the surface. This is indicated by the configura-
tion of the drift-covered hills, the superficial drainage channels, and the
nature of the detrital material itself. The exposures of the andesites, rhy-
olites, and pumices which are shown in the narrow ravine draining the
southern slopes of Carbon Ridge are portions of much more extensive
bodies bordering the southern end of the mountains. Not only is the con-
tinuity of sedimentary beds destroyed by volcanic overflows, but nowhere
are the Carboniferous rocks of Carbon Ridge recognized immediately along
the lines of the two great displacements — the Hoosac and Pinto faults. On
both sides of Carbon Ridge the precise trend of these faults is obscured
by igneous rocks, although at several localities it is possible that they
may form only superficial layers over the sedimentary beds. Carbon
Ridge measures about 2| miles in length, but varies in width, owing to
irregularities in the volcanic flows. Across its widest expansion, as seen
on the surface, it measures 1J miles. Along the summit of the ridge the
beds strike nearly north and south and maintain an average dip of 70° to
the east, presenting a fairly regular uplifted block of limestones and con-
glomerates. Along the west base of the ridge runs a baud of gray granular
sandstone, beyond which to the westward lies an area of fissile clay shales,
exhibiting no good exposures and without reliable dips and strikes, as they
are much broken up and disturbed, owing to their proximity to the Hoosac
fault. Apparently they lie unconformable with the limestones of Carbon
Ridge, but their relationship with the latter is by no means satisfactorily
made out. A line of faulting of which little is known cuts them off from
the main body of limestones, the shales lying at a much lower angle than

THICKNESS OF WEBEE CONGLOMERATE. 173

the highly inclined beds of the ridge. It seems probable that they are
identical with the shales observed underlying the limestones in the expos-
ures north of Pinto Peak. On Carbon Ridge the beds exhibit much the
same sequence of sediments as are found in the Spring Hill region,
the limestones being more or less siliceous and carrying interbedded con-
glomerates. On the summit of the ridge there is a considerable develop-
ment of thinly bedded calcareous shales, in places fossiliferous. Unlike
this horizon at Spring Hill, abundant structural evidence exists here to
show that the uppermost members of the Lower Coal-measure series are rep-
resented, as the Weber conglomerates overlie them conformably. Between
the beds of the two epochs a peculiar structural feature may be noticed in
the narrow ravines which have been worn out by erosion along the contact
of the limestones and conglomerates. These ravines, which start in with
approximately north and south trends, invariably curve to the east and cross
the conglomerates at right angles to their strike, breaking up the formation
into individual blocks, which are united to the main body of Carbon Ridge
by low, connecting saddles of conglomerate.

Everywhere the conglomerate is seen to overlie the limestone conform-
ably. Estimating from the observed dips and strikes, the Lower Coal-
measures of Carbon Ridge show a thickness of 3,500 feet, which does not
vary essentially from the development found on Spring Hill and is within
the measurement obtained for the horizon in the Diamond Range, where the
structural relationships with both the upper and lower beds are much better
determined. The Weber conglomerate has been regarded as dipping
uniformly, throughout the entire development, at 70°, and upon this assump-
tion is assigned a thickness of 1,900 feet. This allows the conglomerate
100 feet less than the estimated thickness in the Diamond Range, but
here the uppermost beds are known to be buried beneath a greater
or less accumulation of tuffs and pumices. That there is about
the same thickness of beds and great similarity in the nature t.f
the sedimentation, is evident from a comparative study of the two
regions. No specially favorable locality for the collection of fossils was
recognized in the limestones, mainly because none were sought, but through-
out the entire series of beds Coal-measure forms may be found. Such

174 GEOLOGY OF THE EUliEKA DJLSTKICT.

species as Productus semireticulatus, P. lonyispinus, AtJiyris subtilita, and
Spirifera camerata are sufficient to establish the Carboniferous age of the
limestones, and their position beneath the Weber conglomerate assigns
them, beyond question, to the Lower Coal-measures.

On PI. ii will be found two cross sections drawn across the volcanic
rocks that stretch between the Hoosac and Pinto faults, separating the Car-
boniferous strata into distinct areas. Both sections lie between the two gen-
eral sections E-F and I-K. They are drawn on due east and west lines
and measure a little over 2 miles in length. Section i, atlas sheet vm, passes
just south of the Spring Hill limestone body and crosses the hornblende
andesite nearest its broadest expansion. At the extreme western end occurs
a small exposure of Carboniferous limestone, only a few hundred yards in
length, completely surrounded by andesite. As shown in the section these
andesites extend with a very irregular outline for a long distance, beyond
which a body of basalt comes in, followed by limestone, in turn followed
by pumices overlain and buried beneath other basalts. These latter basalts
give out on the steep slopes of Dome Mountain, which is made up of Ne-
vada limestone, lying on the west side of the Pinto fault.

Section n, atlas sheet x, is drawn so as to show the great body of Pinto
Peak rhyolite, and passes just south of the summit of the peak. Along this
section, between the two great meridional lines of displacement, none other
than volcanic rocks reach the surface, the pumices all along the east slope
resting against the upturned Silurian rocks of English Mountain. In this
section the Pogonip limestone is seen beyond the line of the Hoosac fault,
but its direct connection with the fault is wholly lost by outbursts of lava.
By reference to the atlas sheets the position of this Pogonip limestone on
the west side of the fault and the Carboniferous limestone on the east side,
will be readily understood.

U.S. GEOLOGICAL SURVEY

GEOLOG< OF EUREKA DISTRICT PLATE I!

Base-. 7000 fr.

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RubyHill

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Prospect P««k .

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SCALE: 1600 ft - I INCH

GEOLOGICAL SECTIONS.
EUREKA DISTRICT, NEV.

CHAPTEK VI.
GENERAL DISCUSSION OF THE PALEOZOIC ROCKS.

Paleozoic shore-line.— Between the Wasatch Range, which incloses the
Great Basin on the east, and the western border of the Paleozoic area in
central Nevada, the sedimentary beds which make up the greater part of
the meridional mountain ranges may, as regards their broader divisions, be
fairly well correlated with each other. In most instances paleoutological
evidences are sufficient to determine at least the age of one or more of the
great bodies of limestone usually found in these mountain uplifts, and the
sequence of strata correlates the geological position of overlying and under-
lying beds. Differences in the character of these sediments exist, but they
are mainly those dependent upon distance from land areas and depth of
water in which the material was originally deposited. Along the Wasatch
the sequence of strata exhibits much the same physical conditions of deposi-
tion, and the horizons may be recognized and their positions determined in
great measure by similarity of sedimentation. Over a large part of central
Utah and easteni Nevada the beds at many geological horizons indicate
deep water or off-shore deposits quite unlike those of corresponding age
found both to the east and west. Here and there over this region some
evidences of ancient land areas may be found. In central Nevada, how-
ever, there occurs throughout the beds abundant evidence of deposition
in shallow seas. The western limit of this Paleozoic ocean across the
broadest expansion of the Great Basin was not far from longitude 117° 30'.
In width it measured along the line of the fortieth parallel nearly 300
miles. It is by no means definitely established that the waters rolled un-
broken, from shore to shore, across this broad surface free from all land

175

176 GEOLOGY OF THE EUREKA DISTRICT.

barriers. In the neighborhood of the "East Humboldt Range a pre-Cam-
brian barrier may have existed, but the evidence seems rather in favor of
islands rising out of an ocean, which stretched across the entire area between
broad continental regions.

The evidence of this ancient shore-line rests, as has been pointed out
elsewhere, upon the complete and unmistakable differences in the charac-
ter of the beds which now lie uncomformably upon the older rocks of
the continental area and the deposits upon the ocean bottom to the
east. Upon this pre-Cambrian continent no Paleozoic rocks have as yet
been recognized in western Nevada, while the enormous thickness of
uncomformable strata laid down since the uplifting of the Paleozoic area
bears ample evidence of a Jura-Trias fauna, as shown in the Piute and
West Humboldt ranges. Again, the Mesozoic rocks of the Wasatch and
those of western Nevada bear but slight resemblance to each other, either
in the nature of the material deposited or in the character of the life repre-
sented. Across this broad intervening area all evidences of Mesozoic sedi-
ments are wanting and the opposite sides are sharply contrasted by their
physical and faunal distinctions. In all probability the Paleozoic ocean in
Nevada presented an indented shore-line with a general northeast and
southwest trend, accompanied by outlying islands stretching far eastward
and rising high above the water level. This ancient coast line has never
been traced, consequently its outlines are most indefinitely determined. It
is obscured by enormous quantities of erupted material, in places literally
mountain high, burying for long distances all traces of preexisting rocks.

The course of this eruptive action was in great part determined by the
profound displacements which accompanied the elevation and transformed
an ocean bottom into an area of dry land. Measured at right angles to the
supposed trend of the shore are broad areas 40 miles in width across which
no other rocks are exposed than Tertiary lavas or the recent Pleistocene
deposits of the valleys. That these lavas followed the trend of the old
shore is indicated by their parallelism with the mountain uplifts, and that
they conformed to the course of the preexisting continental ranges is shown
by the frequent outcrops of Jura-Trias rocks projecting above the vast accu-

PALEOZOIC SHORE-LINE. 177

mulations of volcanic material. Ranges situated eastward of the supposed
shore-line expose above flows of rhyolite long ridges of quartzite which
have been referred to the Paleozoic age. They are at all events quite
unlike the rocks of the region to the west. In a study of the geological history
of continental development it is important to know that it was along this
ancient shore-line that volcanic activity has displayed its greatest energy
in Nevada. Upon one side of these accumulated lavas is found an enor-
mous thickness of Paleozoic strata with no rocks younger than the Upper
Coal-measures, and on the opposite side a great development of alpine
Trias and Jura is seen with an absence of the Paleozoic beneath it. These
facts furnish strong evidence for belief in the existence of a continental
area in western Nevada during Paleozoic time. To the south the shore-
line probably ran out toward the California boundary; to the northward it
may be traced well up into central Nevada. This old coast line is a region
of great interest and one which would well repay careful investigation
and yield valuable geological results.

If this interpretation of observed facts is correct, the degradation of the
land surface during Paleozoic time should have supplied an enormous mass
of detrital matter to the ocean to the east. Now, by a study of the Eureka
Paleozoic strata, this is precisely what is found to be the condition of things.
Along this coast line coarse conglomerates aud mechanical sediments derived
from the neighboring land areas attest the fact that this detrital material
must have come not only from the west, but from a land area at no great dis-
tance. Along the shore the conglomerates form heavy masses of material,
indicating littoral deposits, but to the east these same formations gradually
pass into fine grained sandstones, the beds in general becoming more
uniform in character. Exceptional occurrences of coarse and rapidly chang-
ing material can be found in eastern Nevada, but for the most part they
occupy restricted areas, and may be accounted for by their nearness to pre-
Cambrian islands. All evidence goes to show that Eureka was situated not
far from this western boundary, and its geological record affords ample
proof of elevation and depression throughout Paleozoic time, with inter-
vals of shallow water and nearness of land areas between periods of rel-
atively deeper seas.
MOW xx 12

178 GEOLOGY OF THE EUREKA DISTRICT.

Nature of Material.— The 30,000 feet of sediments at Eureka between the
base of the Prospect Mountain quartzite and the summit of the overlying
Upper Coal-measure limestone are made up of well denned bodies of sili-
ceous, calcareous, and argillaceous strata, each representing a distinctive
epoch in the geological history and development of the region. These
rock masses grouped according to the character of their sediments show
clearly the relative amount of the three classes of deposits into which sub-
aqueous sediments may be divided.

SILICEOUS.

Feet.

Prospect Mountain (Cambrian) 1, 500

Eureka quartzite (Silurian) 500

Diamond Peak quartzite (Carboniferous) 3, 000

Weber conglomerate (Carboniferous) 2, 000

Total 7, 000

CALCAREOUS.

Prospect Mountain limestone (Cambrian) 3, 050

Hamburg limestone (Cambrian) 1, 200

Pogonip limestone (Silurian) 2, 700

Lone Mountain limestone (Silurian) 1, 800

Nevada limestone (Devonian) 6, 000

Lower Coal-measure limestone (Carboniferous) 3, 800

Upper Coal-measure limestone (Carboniferous) 500

Total • 19, 050

ARGILLACEOUS.

Secret Canyon shale (Cambrian) 1, 600

Hamburg shale (Cambrian) 350

White Pine shale (Devonian) 2, 000

Total 3, 950

For the most part the siliceous formations are composed of pure quartz-
ites, sandstones, or conglomerates, interstratified beds of foreign material
occupying very inferior positions. Oil the other hand the calcareous and
argillaceous deposits are more or less interrupted by occasional belts of
other material or impure layers of mixed sediments occurring as transi-
tion beds.

CHANGES IN OCEAN LEVEL. 179

In the Prospect Mountain limestones occur narrow belts and lenticular
bodies of clay shales, in contradistinction to the Pogonip, which is charac-
terized by a series of grits and sandstones. Throughout the 6,000 feet of
Nevada limestone the pure sandstone beds, taken together, would scarcely
measure more than 300 feet, varying from 25 to 100 feet in thick-
ness, while argillaceous strata are exceptional occurrences. In the White
Pine shale, which is mainly argillaceous, occur several beds of reddish
sandstone near the summit, amounting in the aggregate to several hundred
feet. Deducting from the limestone epochs the beds that are decidedly
siliceous and argillaceous, but leaving the impure strata, which are mainly
calcareous, we find the aggregate thickness of the three classes of sedi-
ments as follows : Argillaceous, 3,000; siliceous, 9,000; calcareous, 18,000;
or as 1 : 3 : 6.

The Prospect Mountain quartzite throughout, at least so far as it is
exposed at Eureka, can hardly be otherwise than an off-shore deposit. The
base of the formation is largely composed of coarse conglomerates, made
up of a great variety of unassorted siliceous pebbles, while the finer beds
nowhere present any considerable thickness, and show evidence of strong
currents. It is certain that such material could not have been transported
for any great distance in deep water. At the top transition beds of
siliceous sands pass rapidly into the limestones of the Prospect Mountain
series, which, across a thickness of 3,000 feet, carries but one heavy, per-
sistent belt of clay shale.

The Secret Canyon shale, a remarkably uniform horizon throughout
its great thickness, presents a wholly different character of deposits from
the two underlying formations. Nowhere are there any evidences of rapid
deposition. Following the latter formation comes a second belt of lime-
stone, with occasional beds of siliceous material in place of the clayey beds
found in the Prospect Mountain limestone. Above this second belt of lime-
stone occurs the Hamburg shale, indicating a complete change in the sedi-
mentation and reproducing conditions like those of the Secret Canyon
period, although only 350 feet in thickness. In the character of its deposits
it changes more rapidly and shows unstable conditions in its mode of sedi-
mentation. Next in order is found the widespread Pogonip limestone, the

180 GEOLOGY OF THE EUREKA DISTRICT.

•

base of the Silurian system. The transition beds show a constant change
in the deposit, all of them presenting more or less mixed material, develop-
ing into argillites, grits, and arenaceous schists, finally passing into distinctly
bedded uniform limestone. Overlying the Pogonip rests the Eureka
quartzite, about 500 feet in thickness, but singularly uniform in its material
and wholly unlike all the other siliceous formations, being formed of pure
white siliceous grains completely metamorphosed to quartzite. Through
this formation, except at its base, there is a marked absence of mixed sedi-
ments and off-shore material. All the conditions of deposition suggest deeper
water and a quieter sea bottom. Immediately above the Eureka quartzite
comes an immense development of limestone, with occasional interbedded
sandstones at varying intervals, but comparatively free of earthy matter.
Taken together, the Lone Mountain and Nevada formations which make up
these limestones measure nearly 8,000 feet in thickness, apparently laid
down without any decided break in the conditions of deposition, although
the accumulation of such a vast amount of calcareous sediment must have
occupied a long period of time. That it was sufficient, notwithstanding its
uniformity, to bring about marked changes in the life of the sea is shown
by its faunal development. There exists no greater break in the character
of the sediments than that found between the Nevada limestone and the
overlying White Pine shale. The change from the calcareous deposit of a
quiet ocean with a marine fauna to an argillaceous deposit full of carbon-
aceous material with evidence of cross-bedding, rapid currents, and shallow
water recalls a retreating sea and the proximity of a land surface.

Remains of vegetable life are of rare occurrence in the Paleozoic
rocks of the Great Basin and eastward of the East Humboldt Range are
quite unknown. The White Pine shale, both at Eureka and White Pine,
carries innumerable fragmentary bits of twigs and stems throughout the
entire formation, although for. the most part too poorly preserved for
specific determination, yet indicating land areas throughout a long period
of time. Over the White Pine shale was laid down the Diamond Peak
quartzite, a uniform deposit of fine grained siliceous material without any
special evidence of the proximity of land, either in the life or mode of
deposition. An interstratified bed of limestone carrying Productus semire-

LAND AREA IN THE CARBONIFEROUS.

ticulatus establishes the Carboniferous age of the quartzite. After the Dia-
mond Peak quartzite attains a thickness of 3,000 feet, a change sets in with
alternating beds of coarse shales and conglomerates more or less mixed
with calcareous sediments, the entire series being admirably exposed at the
base of the Lower Coal-measures on the southwest slope of Richmond
Mountain. Here we find positive evidence of the existence of fresh-water
shells associated in the same beds with plant life fairly well preserved,
although specific determinations are impossible. This is the only instance
yet discovered of the existence of fresh-water species in the Paleozoic
rocks of the Great Basin, and points conclusively to the existence of a land
surface at no great distance and long after the White Pine shales had been
buried beneath 3,000 feet of sands. Even if they had been deposited in
an estuary and washed into their present position by rapid currents the
land area could not be far away. This group of rocks carrying a fresh-
water fauna soon becomes submerged beneath the limestones of the Lower
Coal-measures, which occupy such widespread areas of the Great Basin and
which, so far as the physical conditions of deposition are concerned, closely
resemble the Nevada limestone.

Additional evidence of land areas during the Carboniferous is found at
Pancake and Bald Mountain, a somewhat similar series of strata occurring
in both localities, with well developed coal seams, bituminous shales, and
evidences of plant life both above and below the coal.

Next in turn overlying the Lower Coal-measures occurs the Weber
conglomerate, a formation 2,000 feet in thickness, of coarse siliceous mate-
rial made up of pebbles varying in size, composed of quartz, jasper, chert,
and hornstone, unquestionably an off-shore deposit in shallow water. Such
coarse material could not have been transported any great distance.
The conglomerates of Agate Pass, in the Cortez Range, and at Moleen
Peak present identical physical conditions with evidences of the same
off-shore deposits. To the eastward, removed from the continental area,
the sediments of this epoch become finer grained and arenaceous in tex-
ture. How long a time was occupied in the accumulation of this great
thickness of siliceous pebbles it is of course impossible to say. Finally it
was followed by a submergence accompanied by a deposition of limestone

182 GEOLOGY OF THE EUREKA DISTBICT.

characterized by a fauna which did not differ in any marked degree from
the underlying limestone of the Lower Coal-measures.

The Upper Coal-measures indicate once more that a deeper sea swept
over the siliceous beds as the limestones of this upper horizon were laid
down over a wide area, presenting great uniformity in composition and
marine life. Of these overlying rocks we have only 500 feet at Eureka,
and with them the geological record of Paleozoic time comes to an end.

From this recapitulation of the record of the Eureka rocks, it is evident
that they were laid down under very varying physical conditions. In its
broader outlines this sequence of strata may be correlated with the rocks
of the Great Basin stretching as far eastward as the Wasatch, with this
difference, that at Eureka there may be seen immense thicknesses of shal-
low water sediments derived from a continental area to the west, whereas
in going eastward evidences of deeper waters are met with, the material
being more uniform in character and deposited in a quieter sea. In the
latter rocks there is, so far as yet recognized, a marked absence of argil-
laceous and calcareous deposits enriched with plant remains and fresh-
water shells.

Naturally, it is the siliceous formations that exhibit the greatest litho-
logical contrasts, and in going eastward from off-shore to deep-water de-
posits it seems highly probable that Cambrian quartzite in places approxi-
mates the Eureka quartzite in appearance, and may possibly have been
mistaken for it. In the same manner the widely deposited Weber formation
passes from the coarse conglomerate as represented at Eureka into fairly
uniform beds of quartzite or sandstone. Owing to its geological position
between the Upper and Lower Coal-measures, it becomes a comparatively
easy matter to correlate the Weber quartzite of Eureka and Agate Pass
with sandstones farther eastward or the Weber shales of still other localities.
Again, the Weber epoch, when composed of fine siliceous grains and car-
rying a considerable amount of calcareous material, may be difficult to sep-
arate from the overlying and underlying limestones.

Paieontoiogicai Divisions.— As already mentioned, the 30,000 feet of strata
have been divided into the four great periods of Paleozoic time, and at
Eureka they have again been subdivided into epochs mainly based upon

PALEONTOLOGICAL DIVISIONS. 183

the lithological character of their sediments. For the advancement of geo-
logical science it is necessary for the geologist and paleontologist to agree
upon some broad principle governing the division of Paleozoic time and for
the purpose of correlating the strata of one locality with those of another.
From long experience it is found that a division based upon paleontological
data is the only one which will meet the requirements over widely separated
areas of the globe. Structural breaks, based upon unconformities of deposi-
tion or lithological distinctions, determined by manner of occurrence, may
meet the needs of local geological provinces far better than a paleontologi-
cal classification, but for broader continental areas they are far too restricted
for the purposes of correlation. The broad divisions at Eureka are based
upon paleontological evidence. In the 6,000 feet of Cambrian sediments
above the base of the Olenellus shale, the Lower, Middle, and Upper Cam-
brian horizons are all well represented by characteristic faunas. The line
between the Cambrian and Silurian is drawn just above the Hamburg shale,
and is determined wholly by faunal development. In the interstratified
grits and limestones, which bring in the Pogonip, animal life undergoes
a gradual change with the extinction of an old fauna and the coming in of
a new one. Without any marked physical disturbance and a continuance
of limestone strata, a commingling of forms is to be expected. A few of the
more persistent Potsdam types are found at the base of the Pogonip, but a
characteristic Chazy fauna rapidly takes the place of the life found below
the Hamburg shale. At the top of the Pogonip the Trenton epoch is fore-
shadowed by the presence of a number of species characteristic of that hor-
izon on the Atlantic border. The Eureka quartzite affords no evidence of
animal life, but immediately upon the renewal of conditions favorable to
limestone deposition several of the same Trenton species reappear, strongly
reinforced by a group of forms decidedly Trenton in its aspect, while by
far the greater part of the life observed below the quartzite has passed away
forever. The Trenton is followed by a monotonous limestone 2,000 feet in
thickness, carrying a few scattered corals, Hali/sitcx catcnulatus being suffi-
ciently characteristic to identify the beds as belonging to the Niagara. The
Silurian of Eureka consists, then, of two heavy bodies of limestone with dis-
tinct faunas, separated by a dense white quartzite.

184 GEOLOGY OF THE EUEEKA DISTRICT.

Without any discordance in deposition or chemical change in composi-
tion of the limestone, the next great period of Paleozoic time as deter-
mined by its life comes in, marked by the appearance of Atrypa reticwlaris
and associated species, followed by an abundant and strikingly character-
istic Devonian fauna, which is maintained to the top of the Nevada lime-
stone. A decided change in the nature of the sediments brings in the
White Pine shale with its peculiar fauna and flora, but still characterized by
its Devonian aspect.

The Diamond Peak quartzite carries the first evidence of an unques-
tioned Carboniferous life, as shown by one or two Coal-measure species in
an interbedded limestone. From the summit of this horizon upward the
easily recognized Carboniferous fauna, as seen in so many ranges over the
Great Basin, continues to the summit of the Paleozoic sediments, through
two heavy masses of limestone and an intermediate quartzite or sandstone.

Physical Divisions.— There can be no question that a geologist making
a division of the Paleozoic rocks of Eureka, if he were guided solely by
the physical conditions found there, would carry the first period up to the
Eureka quartzite. In doing this he would be drawing a line in accordance
with the most important break to be found in the faunal development of the
lower Paleozoic rocks, the greatest change occurring between the life found
just below the Eureka quartzite and that coming in a short distance above it.
It would be a line in agreement with a sharp lithological change which
brought about conditions detrimental to life, and at the same time would
make a separation which would coincide with the one unconformity by
deposition as yet recognized in the record of the Lower Paleozoic rocks in
the Great Basin. The conformable series of limestones and shales between
the Prospect Mountain quartzite and the Eureka fall naturally into one
grand period. As already pointed out, the Hamburg shale separates the
Cambrian from the Silurian at Eureka, but in other localities this shale is
entirely wanting, the Hamburg limestone passing up into thePogonip with-
out any lithological break. Even at White Pine, only 40 miles away, the
shale is wanting. In working out the structural relations of the entire series
of Paleozoic sediments over much of the Great Basin, one of the chief diffi-

PHYSICAL DIVISIONS AT EUEEKA. 185

culties has been to connect the Cambrian and Lower Silurian rocks below
the Eureka quartzite with the Upper Silurian and Devonian above it.

Again, from a physical point of view there are obvious reasons for
linking together in one group the enormous development of limestones
lying between the Eureka quartzite and the Diamond Peak quartzite. So
imperceptible is the transition in physical characters between the Lone
Mountain beds of the Silurian and the Nevada limestone of the Devonian
that no line can be sharply drawn, and while the fauna slowly undergoes
change there are hundreds of feet of sediments that might as well be placed
in one as the other of the two formations. Atrypa reticularis and other species
found near the base of the Devonian have at other localities been obtained,
associated with species regarded as belonging to the Silurian.

With the coining in of the Diamond Peak epoch another great change
takes place in the physical conditions, and with it a marked faunal break,
which brings in the Carboniferous period. From the Diamond Peak quartz-
ite to the summit of the Paleozoic rocks the beds form one natural group,
no matter from what point of view they may be considered. The litholog-
ical distinctions hold good over wide areas.

From the standpoint of physical geology the record shows three grand
divisions: first, one with the Prospect Mountain quartzite at the base, fol-
owed by a series of limestones and shales; second, beginning with the
Eureka quartzite, followed by another series of limestones to the top of
the Devonian; third, a great quartzite belt, followed in turn by a lime-
stone, a conglomerate, and a second limestone.

LOWER PALEOZOIC IN ADJOINING REGIONS.

After the completion of the field work, upon revisiting several of those
ranges in the Great Basin where the descriptions given of the lower Paleo-
zoic sections differed essentially from the sections exposed at Eureka, it
was found that, as regards sequence of beds, they stood singularly in accord
and could easily be correlated with those of the latter locality. A knowl-
edge of the sedimentary beds at Eureka serves to unravel in neighboring
ranges several knotty problems previously not clearly understood, and to
show that similar physical conditions existed over a wide area of ocean

186 GEOLOGY OF THE EUKEKA DISTRICT.

bottom. For comparative purposes, therefore, it may be well to introduce
here, with more or less detail, some descriptions of the geological structure
and occurrences elsewhere in the Great Basin of those portions of the lower
Paleozoic horizons that happen to be well represented at Eureka. The sec-
tion at Eureka may be taken as a standard.

The Oquirrh Mountains.— In the_ Oquirrh Mountains, the first range west of
the Wasatch, the Olenellus horizon has been identified in a thin bed of
yellow shale conformably overlying a reddish white quartzite of unknown
thickness, above which occur from 3,000 to 4,000 feet of limestone carry-
ing Lower Carboniferous and Coal-measure fossils. The geological posi-
tion of the shale bed is determined by the following species : Lingulella ella,
Olenellus gilberti, and Bathyuriscus prodmta. Mr. S. F. Emmons,1 while
engaged upon the Geological Exploration of the Fortieth Parallel, exam-
ined this range and shrewdly suggested, from orographic evidences, that a
fault existed between the shales and overlying limestones — a structure
which would be in accord with the observed facts brought out at Eureka.

The Highland Range.— The Highland Range, about 125 miles south of
Eureka, presents a geological structure in many respects similar to Pros-
pect Mountain, although by no means as simple or furnishing an unbroken
section of equal extent. At Pioche, at one time a nourishing mining town,
situated on an eastern spur of the main range, Mr. E. E. Howell obtained
two species of the genus Olenellus, which Mr. F. B. Meek' described as
0. gilberti and 0. howelli. According to Mr. Howell3 they occur in a red-
dish yellow arenaceous shale about 400 feet in thickness, overlying 1,200
feet of quartzite. In the published section the shale is represented as con-
formably overlain by gray limestone, which he regarded as of Carbonifer-
ous age, although no paleontological evidence was presented. If these
strata are conformable it would seem highly probable, from the known
sequence of beds at Eureka as well as elsewhere in the Highland Range,
that the overlying limestone belongs to the Prospect Mountain epoch.

'U. 8. Geol. Explor'n 40th Par., vol. 2, Descriptive Geology, p. 444.

3 Geographical Surveys West of One hundredth Meridian, vol. 3, p. 182.

3 Op. oit. vol. 3, p. 258.

HIGHLAND RANGE. 187

Subsequently, during the summer of 1885, Mr. C. D. Walcott studied
the structure at Pioche, and also made an extended examination of the Cam-
brian and Silurian rocks of the main uplift of the Highland Range. He
estimated about 7,000 feet of strata between the summit of the Prospect
Mountain quartzite and the base of the Eureka quartzite, as against 9,000
feet at Eureka. Between Bennett Spring and Stampede Gap the lower mem-
bers of the group were carefully measured, the section presenting the beds
more in detail and showing greater variation of sedimentation, with more
interbedded siliceous material than observed in the corresponding horizons
at Eureka. Detailed lithological sections, however, across the Cambrian
rocks are perhaps of little value, owing to the rapid changes in the charac-
ter of the beds. Sections made across Prospect Mountain show considera-
ble difference in detail, but agree substantially in general features. The
section at Bennett Spring is as follows :'

• Feet.

1. Dark reddish brown quartzite, evenly bedded and ripple-marked in places . 350

2. Bluish gray limestone 35

Fossils : Olenellus gilberti.

3. Buff argillaceous and arenaceous shales, more or less solid near the base

and laminated in the upper portions 80

Fossils: Annelid trails and fragments of Olenellus in the lower part;
higher up the heads of Olenellus gilberti and 0. iddingsi occur in
abundance.

4. Light colored gray limestone and bluish black limestone 16

5. Sandy, buff colored shale 40

Fossils : Annelid trails, Cruziana, sp. ?

6. Dark bluish black limestone 46

7. Finely laminated buff argillaceous shale 80

Fossils : Hyolithes billingsi and Ptychoparia piochensis.

8. Gray to bluish black compact limestone 18

9. Buff arenaceous shales 64

10. Compact cherty limestone

11. Compact shaly sandstone in massive layers 40

12. Hard siliceous gray limestone, almost quartzite at base 12

13. Yellow to buff sandy shales . , 70

14. Bluish black limestone

'Second Contribution to the Studies on the Cambrian Faunas of North America. U.S. G«oL Surv.

Bull. No. 30, 1886, p. 34.

188 GEOLOGY OF THE EUREKA DISTRICT.

Feet.

15. Yellow to buff sandy shales 40

16. Bluish black, hard, compact limestone 12

Fragments of fossils.

17. Shaly sandstone in massive layers 52

18. Gray arenaceous limestone 2

19. (a) Buff sandy shale 40

(b) Gray arenaceous limestone 30

20. (a) Massive bedded bluish gray limestone 200

Fragments of fossils.

(b) Compact gray siliceous limestone, almost quartzite in places 400

606
Strike K 30° W., dip 10° E.

21. Buff to pinkish argillaceous shale, with fossils, and a few interbedded

layers of limestone from 3 to 15 inches thick 125

Fossils: Eocystitesf? longidactyltis, Lingulella ella, Kutorginapannula,
Hyolithes billingsi, Ptychoparia piochensis, Oknoides typicalis,
Bathyuriscws howelli, aiid B. producta.

22. Massive bedded siliceous limestone, weathering rough and broken into

great belts 200 to 300 feet thick by bands of color in light gray, dark
lead, to bluish black ; on some of the cliff faces the weathered surface
is reddish 1, 570

23. Bluish black limestone in massive strata that break up into shaly layers

on exposure to the weather. The latter feature is less distinct 850 feet
up, and the limestone becomes more siliceous, with occasional shaly

beds 1, 430

Fossils : Near the summit specimens were found that are referred to
Ptychoparia minor.

The upper limestone on the line of the section is not favorable for the
preservation of organic remains, but the same horizon a short distance to
the southward yielded a fauna similar to that from the Upper Cambrian at
Eureka, two species being identically the same, while two others, Beller-
ophon antiquatus and Dicellocephalus pepinensis, occur in the Potsdam sand-
stone of Wisconsin.

Timpahute Range.— Mr. G. K. Gilbert reports from the southern end of
Timpahute Range a section over 2,300 feet in thickness, which, taken

8ILVEE PEAK. 189

tog-ether with paleontological evidence, may be readily correlated with the
lower part of the Cambrian of Eureka and the Highland Range. The
thicknesses are estimated. The section is as follows r1

South end of Timpahute Range. Eastern Nevada.

Feet.

1. Heavy bedded gray limestone, light and dark 400

2. Yellow argillaceous shale:

(a) Yellow shale 350

(6) Yellow sandstone 75

(Conocoryphe) 500

925

3. Purple ripple-marked vitreous sandstone, with bands of siliceous shale. . . 1, 000

Total 2,326

The lower bed corresponds to the Prospect Mountain quartzite. In the
overlying yellow shale he collected a few fossils, determined by Mr. Wal-
cott as Olenellus gilberti and 0. iddingsi.

silver Peak.— Still farther west, in a bed of yellowish brown limestone
with intercalated gray argillaceous shales at Silver Peak, a small collection
of fossils was made, which Prof. J. D. Whitney2 placed before the Cali-
fornia Academy of Sciences as early as 1866. At that time he regarded
them as Upper Silurian or Devonian. Quite recently Mr. Walcott3 has ex-
amined the collection and determined the following species:

Archseocyathus atlanticus. Kutorgina (like K. cingulata).

Archseocyathus, undt. sp. Hyolithes princeps.

Ethmophyllum whitneyi. Olenellus gilberti.
Sti ephochetus ? sp.?

A number of species proved to be identical with those found on the
coast of Labrador and the horizon is evidently the equivalent of the Georgia
or Lower Cambrian formation of Prospect Mountain. He also determined
Olenellus gilberti as closely resembling Olenellus tltowpsoni from L'Anse au
Loup.

'Geographical Surveys West of One hundredth Meridian, Washington, 1875, vol. 3, p. 169.
J Proc. Cal. Acad. Sci., vol. 3, p. 270.

3 Second Contribution to the studies on the Cambrian Faunas of North America. U. S. GeoL
Snrv. Bull. No. 30, 1886, p. 38.

190 GEOLOGY OF THE EUREKA DISTRICT.

Silurian and Devonian.— Exposures of Silurian and Devonian rocks present-
ing a development of strata at all comparable with the Eureka section are
known in but few localities in the Great Basin, and nowhere are the struc-
tural relations of the Cambrian, Silurian, and Devonian so clearly brought
out as here. Nevertheless, numerous mountain uplifts present so many
partial exposures of the Eureka section that the evidence is sufficient to de-
termine the same succession of strata over a wide area of the Great Basin.

Geological Section, white Pine.— In the year 1872 the writer1 visited White
Pine and Eureka and established the identity of a great thickness of the
Pogonip beds at both places. The fossils collected at that time from Pogo-
uip Mountain, White Pine, and the hills east of the Jackson Mine at Eureka,
were submitted to Messrs. Hall and Whitfield and shown by them to be
specifically identical. At that time, however, neither tliu. base nor the
summit of the epoch was clearly defined and not until after the thorough
survey of the Eureka district were their exact limitations known, nor could
they be determined until after the collection of a large amount of paleonto-
logical material. Topographically, Pogonip Ridge holds much the same
relation to the White Pine Mountains that Prospect Ridge does to the Eu-
reka Mountains. It forms the most prominent uplift in that district, occur-
ring as a sharp longitudinal ridge, the highest point attaining an elevation
of nearly 10,800 feet above sea level. Across this ridge the oldest sedi-
mentary beds of the district lie inclined at high angles to the east, the
northern end being made up almost wholly of Silurian rocks. The structure
is simple, the beds which trend obliquely across the ridge being easily fol-
lowed from the summit of the peak to the northern base.

After the completion of the work at Eureka Mr. C. D. Walcott made a
careful examination of the Silurian rocks at White Pine where, according to
him, the Pogonip strata measure over 5,000 feet in thickness. The oldest
strata identified by their organic remains are the beds at the base of 'the
Hamburg limestone, several species being identical with those from the
corresponding horizon at Eureka. These Hamburg limestone beds, several
hundred feet in thickness, are cut off by a fault bringing them in contact

1 U. S. Geological Exploration of the Fortieth Parallel, vol. 2, Descriptive Geology, pp.
542 and 547.

SILURIAN AT WHITE PDTE. 191

with a heavy bed of quartzite that forms the western central spur of Po-
gonip Ridge. As the Hamburg shales are wanting, the Hamburg limestone
and the included fauna continue to the base of the Pogouip. Here, as well
as at Eureka, the base of the Pogonip is determined by a commingling of
Cambrian and Silurian species, the line of demarcation resting wholly upon
paleontological evidence. Mr. Walcott examined the beds from the lower
quartzite across the Pogonip, Eureka quartzite, and Lone Mountain, until
the upper beds of the latter epoch were lost beneath the detritus of the
plain.

The section is as follows:

NIAGARA.

Feet.

1. Yellowish shaly limestone 50

2. Light colored, massive bedded siliceous limestone, with plates of cri-

noids, etc 650

3. Light blue siliceous limestones with impressions of corals, Halysites caten-

ulatus, Stromatopora f 150

4. Light gray siliceous limestones 50

TRENTON.

5. Evenly bedded pure bluish gray limestones 50

Fossils: Cystidian plates, Bryozoa 3 sp., Rhynchonella capax, Trinu-
cleus concentricus, Streptorkynchus filitexta, Orthis subquadrata,
Pterinea.

6. Dark colored siliceous limestone in massive beds 500

EUREKA.

7. Light vitreous quartxite, ferruginous near the base 350

POGONIF.

8. Dark blue and black limestones, with numerous shaly belts, characterized

by the fossils of the Upper Pogonip as seen at Eureka, nearly all the
genera being recognized, with the exception of Receptaculites 900

9. Dark evenly bedded limestones, with more or less siliceous bauds 4, 300

Fossils: Acrotreta gemma, Ill&nus eurekensis Triplesia calcifera in
the lower portion, followed higher up by the siiine forms as found
at Eureka east of the Jackson mine and east of Hamburg Eidge.

192 GEOLOGY OF THE EUKEKA DiSTEICT.

HAMBURG.

Feet.

10. Dark bluish black limestone, carrying Hamburg limestone fossils 800

7,800

Divided according to the epochs adopted at Eureka we have :

Lone Mountain limestone 1, 450

Eureka quartzite 350

Pogonip limestone 5, 200

Hamburg limestone 800

The Hamburg limestone yielded the following species:

Protospongia sp.! Conocephalites sp.f

Lingulepis niinuta. Crepicephalus nitidus.

Orthis sp.f Crepicephalus unisulcatus.

Agnostus bidens. Chariocephalus tumifrons.

Agnostus communis. Illaenurus sp. f

While at White Pine the relationship between the Cambrian and
Silurian is well shown, the Devonian has not been recognized directly over-
lying the Lone Mountain Silurian. Between Pogonip Mountain and the
next ridge to the eastward a displacement brings up the Nevada limestone,
forming the massive beds of Mount Argyle and Treasure Peak.1 This
limestone is here overlain by the black argillaceous shale, which passes
into sandstone, followed by Carboniferous limestone. The black shale is
the counterpart of the corresponding terrane at Eureka, a comparison of
the two sections showing the greatest resemblance. The coarse yellow
sandstone above seems to be the equivalent of the Diamond Peak quartz-
ite, although here at White Pine it is represented by only a few hundred
feet, while the black shale attains a development of 1,000 feet. From the
Nevada limestone there has been collected an abundant fauna characteristic
of the middle and upper beds. It was for the most part obtained by the

'U. 8. Geological Exploration of the Fortieth Parallel, vol. 3, Mining Industry, p. 409, and

accompanying atlas sheet 14.

DEVONIAN AT WHITE PINE. 193

writer from Mount Argyle and Treasure Peak, and represents thirty -three
genera and forty-nine species, as follows:

Cyathopliyllum sp. ? Eetzia radialis.

Feuestella (2 sp. ?) Atrypa reticularis.

Thainniscus sp. ? Bhynchonella duplicata.

Liugula alba-pinensis. Ehynchonella emmonsi.

Discina lodensis. Ehynchonella occidens.

Chonetes sp.? Ehynchonella (L) quadricostata.

Strophodonta canace. Cryptouella circula.

Strophodouta inequiradiata. Pentainerus lotis.

Strophodonta sp.? Terebratula sp.?

Orthis macfarlani. Aviculopecten catactns.

Orthis impressa. Pterinopecten sp. ?

Productus hirsntiforme. Lunulicardium fragosmn.

Productus subaculeatus. Cardiomorpha missonriensis.

Productus sp. ? Nuculites triangulus.

Spirifera alba-pineusis. Paracyclas peroccidens.

Spirifera disjuncta. Gonocardium sp.?

Spirifera engelmaniii. Platyostoma sp. ?

Spirifera pinonensis. Euomphalvis laxus.

Spirifera strigosus. Euomphalus sp.?

Spirifera subuuibona. Loxonema sp. ?

Spirifera sp. ? Platyschisma sp. ?

Cyrtina davidsoni. Bellerophon neleus.
Ambocffilia unibonata.

A more characteristic White Pine fauna is preserved in the black shale
than has yet been obtained in the corresponding beds at Eureka, and a
belt of intercalated limestone in the shale similar to that found east of
Sugar Loaf at Eureka bears equal evidence of its Devonian age. Here
the limestone appears as a lenticular body in the shale, with beds identical
in composition both above and below. While there is much in the group-
ing of forms foreshadowing the Carboniferous, the shales maintain their
Devonian aspect by carrying certain characteristic species up nearly to the
top of the series, and in this respect resemble the black shales found at
Hays Canyon west of Newark Mountain.
MON xx 13

194 GEOLOGY OF THE EUKEKA DISTRICT.

From Applegarth Canyon the White Pine shales yielded the following
species:

Cyathophyllurn sp. ? Athyris (of type of A. plano-sulcata).

Penestella (2 sp. ?) Bhynchouella (L) quadricostata.

Thamniscus ? sp. 1 Aviculopecten catactus.

Lingula alba-pinensis. Nuculites triangulus.

Discina lodensis. Cardiomorpha missouriensis.
Chonetes (of type of C. illinoisensis). Lunulicardium fragosum.

Productus hirsutiforine. Hyolithes sp. ?

Productus subaculeatus. Pleurotomaria sp. !
Productus (of type of P. semireticu- Goniatites kingii.

latus). Goniatites sp.l

Spiriferina cristata. Prcetus sp. f

Ambocffilia umbonata. Cytoceras cessator.
Eetzia radialis.

With the exception of some indeterminable fragments of crinoid col-
umns and a few impressions of stems and twigs, the sandstones have yielded
no life. The few vegetable remains, however, are important, as they are
of rare occurrence in Paleozoic sandstones of Nevada. The Carboniferous
limestones overlying this belt of sandstones have been but little studied
since the explorations of the fortieth parallel, and no additional material
throwing light upon the life of the period has been obtained.

Silurian and Highland Range.— In the Highland Range the Silurian rocks have
not been as carefully studied as the Cambrian. Indeed, it is by no means
certain that in the area covered or in the exposure of beds that the Silurian
is as well represented as in a number of other ranges, although, as has been
already shown, the Cambrian compares favorably with the same epoch in
the Eureka Mountains. Both the Pogonip and Eureka quartzite, however,
are well exposed on the west side of the range in a hill just north of the
road leading from Bennett Spring to Hyko, where the fauna in the lime-
stone immediately below the quartzite is so characteristic that both forma-

FOSSIL BUTTE. 195

tions are readily determined. At this locality, in beds below the Eureka
quartzite, Mr. Walcott made the following collection:

( )rthis perveta. Subulites sp. ?

Orthis tricenaria. Orthoceras sp ?

Orthis pogonipensis. Ortlioceras (Aiiiiulated species).

Strophornena fontinalis. Leperditia bivia.

Modiolopsis occidens. Ceraurus sp. !

Modiolopsis pogonipensis. Illamus crassicauda.

Raphistoma acuta. Bathyurus pogonipensis.

Murchisonia, 2 sp.* Pleurotomaria lonensis.

Fossil Butte.— At Fossil Butte, 10 miles north of Hyko, on the east side
of Pahranagat Valley, the Pogonip is again seen overlain by the Eureka
quartzite. The butte stands out as an elevated ridge, but presenting an
exactly similar succession of strata as seen along the east side of Prospect
Mountain, Pogonip Ridge, and the western base of Lone Mountain. In the
limestone occur the following species:

Receptaculites mammillaris. Metoptoiua phillipsi.

Orthis tricenaria. Ecculiomphalus, like E. distans.

Strophornena fontinalis. Orthoceras rnulticameratum.

Triplesia? sp.? Endoceras multitubulatum.

Leperditia bivia. Modiolopsis occidens.

Maclurea subannulata. Modiolopsis pogonipensis.

Maclurea, 2 sp., undet. Illaenus crassicaiida.

Taken together these two groups from Bennett Spring and Fossil
Butte carry the more marked fauna of the Upper Pogonip. Overlying the
quartzite occur some light gray limestones, without organic remains, but
resembling the Lone Mountain beds. Along the east side of Pahranagat
Valley limestone ridges extend for several miles. The beds have been
much disturbed and have undergone considerable faulting, preventing accu-
rate sections^ but it is estimated that there are from 2,000 to 3,000 feet of
limestones exposed. They are more or less siliceous, weathering reddish
brown and brownish gray. The lower members may possibly belong to the
Lone Mountain series. Near Hyko there is an exposure of shaly limestone,
overlain by massive beds of dark arenaceous limestone, carrying a Devonian

196 GEOLOGY OF THE EUKEKA DISTRICT.

fauua. The specimens, although poorly preserved, allowed of the following
determinations:

Stromatopora sp. t Modiomorpha sp.

Spirifera sp. ! Holopea sp. ?

Atrypa reticularis. Euomphalus (P) laxus.
Pentamerus lotus.

Pahranagat Range.— On the west side of the valley the Pahrauagat Range
forms a long, continuous ridge, for the most part made up of sedimentary
strata faulted and broken into massive blocks by outbursts of acidic lavas.
Quartz Peak, the culminating point, affords a fine exposure of Silurian
strata, with much the same series of beds as seen in the central part of the
State, with this exception that neither the Upper Pogonip at the base nor the
Niagara at the summit are represented in their full development. Upon the
south side of the peak, extending from the base to the summit, there is an
unbroken exposure of strata 2,000 feet in thickness, striking N. 30° E.,
with an average dip of 20° N. The summit of the peak is formed of the
Niagara limestone. The section, with the accompanying fossils, is, accord-
ing to Mr. Walcott, as follows:

LONE MOUNTAIN — NIAGARA.

Feet.

1. Massive bedded gray siliceous limestone, with occasional layers of sand-

stone and chert 535

LONE MOUNTAIN — TEENTON.

2. Massive bedded dark siliceous limestone, with a stratum 30 feet thick,

almost made up of a species of Pentamerus like P. galeatus. These

occur not far above No. 3 335

3. Bluish black and bluish gray thin bedded limestone, with numerous fossils . 30

Zaphrentis, sp. ?, Bryozoa, 3 sp., Streptorhynchus filitexta, Orthis tes-
tudinaria.

4. Massive bedded dark iron -gray siliceous limestone : 150

EUREKA.

5. Hard, vitreous white quartzite, becoming tinged with a reddish color toward

the base 400

PAHBANAGAT KANGE. 197

POGONIP.

Feet.
6. Evenly bedded layers of a dark bluish black and bluish gray limestone, thin

layers making more massive beds that break up on exposure to the

influence of the atmosphere 150

Beceptaculites mammillaris, Orthis pogonipensin, Orthis tricenaria,
Porambonites obscurutt, Bellerophon, spJ, Hyolites, sp. undet., Endo-
ceras multitubulatum, Leperditia bivia, Illcenus crassicauda.

1. Thinner bedded bluish gray limestone that is shaly in places 400

Fossils numerous.

8. Massive bedded gray limestone, in layers from 1 to 4 feet in thickness .... 200
Orthis, Murchisonia, and Orthoceras are seen in the lower layers, and
Beceptaculites mammillaris and B. elongata 150 feet higher up.

2,200
This section gives for the different horizons as follows:

Niagara 535

Trenton 515

Eureka 400

Pogonip 750

On the northeast slope of Quartz Peak there is a heavy mass of light
gray siliceous limestone, roughly estimated at 1,000 feet, without fossils,
which, by its stratigraphical position and lithological habit is easily
referred to the upper beds of the Lone Mountain, a continuation of the
beds upon the summit of the peak as given in the section. To the south of
Quartz Peak occurs a great development of limestone. A section across
the beds is of special interest, owing to the thickness of the limestones
from the Lone Mountain to the Carboniferous, which is unbroken by the
presence either of Diamond Peak quartzite or White Pine shale, as in both
the Eureka and White Pine sections. The section is as follows:

CARBONIFEROUS.

Feet.

1. Siliceous limestone, sandstone, and quartzite 500

2. Cherty siliceous limestone 250

3. Shaly limestone in massive layers 55

4. Massive bedded gray limestone, hard and compact ; it passes into gran-

ular dark gray limestone and then into more thinly bedded bluish

black limestone 1, 260

198 GEOLOGY OF THE EUREKA DISTRICT.

Feet,

5. Bluish black limestone in tbiu layers, overlain by shaly limestone, with

intercalated beds of bluish black limestone. The limestone in layers
gradually replaces the shaly limestone until the latter disappears
from the section. Upper beds characterized by Lower Carboniferous

fauna. (List follows the section.) 390

At 95 feet from the top, Spirifera lineata and Spirifera cristata were
observed.

DEVONIAN.

At 140 feet from the top, numerous fragments of crinoids, crinoidal
columns were seen for the last time, and Atrypa reticularis began to
appear.

At 350 feet from the top, buff shaly limestone predominates and a more
evenly grained, smoother, harder limestone begins to appear as thin
layers in the shaly limestones. Fossils few in number and badly pre-
served.

6. Hard, compact, yellowish sandstone in thin layers 25

7. Calcareous sandstone overlain by arenaceous limestone, and above that

bluish gray thin bedded limestone 175

8. Gray siliceous limestone with shaly limestone partings and bands of bluish

black limestone 340

At 125 feet from the top, a band of bluish black thinly bedded limestone

carries numerous fossils of the Upper Devonian age. (List follows

the section.)
At 240 feet from the top, Stromatopora and coralline markings appear.

9. Hard buff colored sandstone 25

10. This is almost a repetition of No. 8, the three grading into each other in

places 225

Seventy-five feet down from the top, Stromatopora and a small slender
coralline stem crowd the darker siliceous layers.

11. Light gray siliceous limestone, almost a sandstone in places, passing up

into a dark siliceous limestone, and then into thinner bedded bluish

black and bluish limestone 1, 920

The upper layers contain Strophomena pvrplana, Atrypa reticularis, Cyr-
tina, sp.f Pleurotomaria.

12. Gray quartzitic sandstone in massive layers 100

13. Gray siliceous limestone 110

14. Quartzitic ferruginous sandstone 85

15. Light gray and dirty brown siliceous limestone in alternating bands of

color, of varying degrees of hardness. The siliceous and calcareous
matter varies considerably in the different layers. Toward the lower
portion many layers are almost made up of a species of Stromatopora
and slender stems of a branching coral one-eighth to one-fourth of an
inch in diameter 2, 100

DEVONIAN AND CARBONIFEROUS FAUNA. 199

SILURIAN.

Feet.
16. Light gray siliceous limestone 1,000

8,560

In the bluish black limestone at the top of No. 5 the following Lower
Carboniferous fauna comes in :

Amplexus, sp. ? Spirifera (M) lineata.

Syringopora, sp. ? Cyrtina, sp?.

Acervularia pentagona. Athyris subquadrata.

Fenestella, sp.? Rhynchonella.

Chonetes, sp. I Terebratula, sp. ?

Chonetes granulifera. Platyceras, sp.?

Chonetes, sp.? Bellerophon, sp.?

Productus nebrascensis. Euomphalus, sp. ?

Productns punctatus. Euomphalus laxus.

Productus tenuicostatus. Euomphalus (Straparollus) ophinea.

Productus semireticulatus. Straparollus, sp. ?

Productus, sp! Holopea, sp.?

Productus, sp ? Loxonema.

Orthis resupinata. Loxouema.

Streptorhynchus cremstria. Pleurotomaria, sp. ?

Syringothyris cuspidatus. Pleurotomaria, sp. ?

Spirifera pinguis. Edmondia, 2 sp. ?

Spirifera pulchra. Leperditia, sp. ?

Spirifera striata. Prcetus peroccidens.

The following Devonian fauna was collected from No. 8 :

Lingula (like L. ligea). Rhynchonella sinuata.

Orthis impressa. Athyris ? sp. ?

Productus shumardianus. Pentamerus lotus.

Productus (like P. lachryinosa). Modiomorpha, sp?

Strophodonta, sp. ? Euomphalus, sp. ?

Spirifera, sp.? Platyostoma lineata?

Nucleospira concinna. Orthoceras, sp. ?

Cyrtina hamiltonensis. Orthoceras, sp. ?
Amboco3lia (like young of A. umbonata). Leperditia, sp:?
Rhynchonella duplicata.

In this section we have about 8,000 feet of nearly continuous lime-
stone strata, broken occasionally by thin beds of yellow sandstone, the

200 GEOLOGY OF THE EUREKA DISTEICT.

heaviest not over 100 feet in thickness. It is not singular that in a massive
bed of limestone it becomes a difficult matter to divide the Silurian,
Devonian, and Carboniferous with any degree of precision. Between the
Silurian and Devonian at the base of the section it is impossible to draw
any line of demarcation. It is safe, provisionally, to place 1,000 feet of the
light gray limestone in the Lone Mountain period, leaving strata carry-
ing Stromatopora and branching corals included in the Devonian. In the
390 feet of bluish black limestones (No. 5 of the section) a marked
Devonian fauna occurs at the base and Lower Carboniferous fauna at the
summit, without any change in the lithological character of the beds. Pro-
visionally the line is drawn so as to include in the Carboniferous all beds
carrying Spirifera lineata and Spirifera cristata, and leaving Atrypa reticularis
in the Devonian. By this division we have the following thicknesses for
the different periods:

Feet.

Carboniferous 2, 160

Devonian 5, 400

Silurian 1, 000

Pinon Range.— To these sections south and southeast of Eureka may be
added still another, constructed across Pinon Range about 60 miles north-
ward. This range is a long, narrow ridge, stretching from the Humboldt
River southward until it joins the Eureka Mountains at The Gate, the
southern end of the range coming within the area of this survey.

The Pinon Range attains its greatest elevation just south of the Hum-
boldt, where the best continuous sections of the Lower Paleozoic rocks
occur. The range was crossed at several points by the geologists of the
Fortieth Parallel Exploration, the Devonian rocks being traced by the writer
for nearly their entire length from The Gate to the Humboldt River. At
the request of the writer, Mr. Walcott visited the northern end for the pur-
pose of a comparative study of the section exposed at Ravens Nest with
the corresponding rocks at Eureka. Here the beds strike obliquely across
the trend of the range from the northwest base of Ravens Nest to the base
of Pinto Peak, the course of the range being approximately north and
south.

PlSfON RANGE.
The following ideal section was made by Mr. Walcott:

201

fcF^
fc-^

J^tmestone-.

JEiiretta.

limestone. "Limestone.

fv\. 4. — Section across Pinon Range.

Carboniferous

Across the range from west to east along the line of the section a dark
blue limestone, carrying a few fossils of the Lower Devonian, rises above
the plain. Beyond the limestone a sharp oblique fault brings up a broad
mass of quartzite, conglomerates, and black siliceous pebbles. These, in
turn, are conformably underlain by blue limestone, from which a sufficient
fauna was secured to identify Upper Devonian beds. The limestones are
again cut off by a profound fault, apparently along a line of an anticlinal
axis. To the west of this fault the beds all dip westward, but beyond this
point present an easterly dip, at least as far as the base of Pinto Peak.
Directly eastward of the fault a dark ferruginous quartzite stands out
prominently, followed by light gray siliceous limestones, the age of which
is determined by the presence of Halysites. The beds gradually assume the
habit of the Devonian and carry a fauna sufficiently characteristic to estab-
lish the horizon of the Lower Devonian, and still higher up in the series
yielded Upper Devonian species. Overlying the limestones occurs a great
thickness of quartzites and sandstones, with occasional argillaceous bed.
Near the junction of the Devonian limestones with the overlying quartzites
the beds upon both sides of the anticline are identical, showing the corre-
sponding horizons without the evidence of the fauna. If the White Pine
shale of Eureka is at all represented in the Pinon Range it is found in the
argillaceous and finely siliceous beds immediately overlying the Devonian
limestones on both sides of the fault. These beds resemble those observed
at the same horizon at The Gate, already described, and the continuance
northward of similar sediments is not without interest, especially when
taken in connection with the great thickness of White Pine shale to the
southeast as developed in the Diamond Range, Cliff Hills and White Pine.

202 GEOLOGY OP THE EUREKA DISTRICT.

Roughly estimated, the thicknesses of the beds in the above section
are as follows:

Feet.

Eureka quartzite 400

Silurian and Devonian limestones 5, 500

Carboniferous quartzites and sandstones 7, 000

Tucubit Mountains.— In the Tucubit or Wild Cat Mountains, north of the
Humboldt River, occurs a long stretch of massive limestones that have evi-
dently undergone much faulting and disturbance. In a black calcareous
shale belt overlying yellow calcareous shales there were found Atrypa reti-
cularis, Spirifera vanuxemi and Orthis multistriata, and other species, show-
ing the extension of Devonian beds into the northern part of the State.
These limestones in the Tucubit Mountains have been estimated by Mr. S.
F. Emmons as from 4,000 to 5,000 feet in thickness.1

Unconformity in the Silurian.— In the descriptive chapters references have fre-
quently been made to an unconformity existing in the Silurian, between
the Eureka quartzite and Lone Mountain limestone. This unconformity is
also indicated in the accompanying atlas sheets, where different horizons of
the Lone Mountain and Nevada limestones are seen to rest directly upon
the underlying quartzite. The Lone Mountain beds may frequently be
seen to wedge out in places ; in others, Devonian beds, determined as such
by their associated fossils, come down nearly if not quite to the top of the
Eureka quartzite. In only one locality along the southern base of Comb's
Mountain do the beds show the lithological characters or the fauna of the
Trenton limestone, but here they are overlain by a broad development of
the Niagara, the upper member of the Lone Mountain epoch, before the
coming in of the Devonian. Evidences of uncomformity by erosion have
been recognized in a few localities, but the nature of the quartzite is such
that they could hardly be expected to be conclusive, the amount of
erosion being slight. In most instances in the more elevated regions where
the Eureka quartzite lies horizontally the overlying limestones have been
eroded, but this, however, is not the case on the -plateau in the region of
Grays Peak, where isolated patches of limestone occupy depressed areas
and shallow basins in the undulating surface of quartzite.

1 II. S. Geological Exploration of the Fortieth Parallel, vol. 2. Descriptive Geology, p. 524.

MOVEMENT IN CARBONIFEROUS. 203

Movement in Carboniferous.— In the chapter devoted to the Upper Coal-
measures descriptions have been given of the coarse conglomerates inter-
bedded in the limestone, containing siliceous pebbles and worn fragments
of limestone carrying Coal-measure fossils, evidently derived from neigh-
boring land areas undergoing denudation. Many of the siliceous pebbles
have the appearance of coming from the Weber conglomerate, but this
can not be positively stated. The Productus semireticulatus and other
species found in these rolled fragments may have been derived from either
the Upper or Lower Coal-measures. The change in sediment from lime-
stone to conglomerate is abrupt, and no indications of erosion in the under-
lying rocks were observed. The evidence of movement rests wholly upon
the lithological character of the material forming the conglomerate, which
was in turn covered by deposits in every way similar to the limestone
below it. It seems evident that fragments of fossil-bearing limestone could
not have withstood the disintegrating action of water for any great length
of time or have been transported for any great distance.

Distribution of Upper Silurian and Devonian.— Over large areas of the Great

Basin, Upper Silurian and Devonian sediments are either wanting or have
not as yet been recognized. Several longitudinal ranges have been described
in their geological structure as tilted blocks formed either exclusively of
Carboniferous rocks, or else made up of the Pogonip of the Lower Silu-
rian overlain conformably by Coal-measure limestones, the intervening
horizons, which at Eureka are known to measure over 12,000 feet in thick-
ness, being entirely unrepresented. Sediments of Upper Silurian and
Devonian age, while they occupy limited areas, nevertheless play a most
important part in the ranges which rib the central portion of Nevada. To
the north of the Humboldt River, as already pointed out, the Nevada lime-
stones have been recognized in the Tucubit Mountains ; they form the
greater part of the Roberts Peak Mountains west of the Pifion Range,
where they probably overlie a considerable but unknown thickness of Lone
Mountain Silurian, and the writer has traced the Devonian beds all along
the Pinon Range, connecting them with the grand exposures of the Eureka
district. At Ravens Nest, at the northern end of this latter range, that
portion of the Eureka series of beds lying between the Silurian quartzitr
and the Coal-measures is strikingly reproduced, the structure being a faulted

204 GEOLOGY OF THE EUKEKA DISTRICT.

anticline with corresponding beds upon both sides of the axial plane.
Between the ancient shore-line to the west and the Humboldt Range
on the east, there appears to have been a deep meridional trough-like
depression in which all the beds from the Eureka quartzite up to the top of
the Devonian were deposited. How far north Avard they can be traced con-
tinuously is still a matter of conjecture, but we know that the Devonian
beds occupy large areas along the valley of the Mackenzie River. To the
south this narrow channel or trough apparently widens out into a broad
bay or open sea.

To the southeast of Eureka, in the White Pine Mountains, the Eureka
quartzite, and both the Trenton and Niagara members of the Lone Moun-
tain epoch are well developed on the northeast end of Pogonip Ridge, and
the Devonian on Treasure and Babylon hills. The sequence of strata,
together with the associated fauna from the base of the Pogonip to the
Diamond Peak quartzite, may be easily correlated in the two localities.
Still farther southward, on the east side of Pahranagat Valley, both the
Upper Silurian and Devonian are exposed in a great thickness of limestones
bordering the valley. In the uplifted block at Quartz Peak, in the Pahran-
agat Range, we have Pugonip, Eureka, Trenton, and Niagara all well
exposed, but neither the upper nor lower horizon is shown in its full devel-
opment.

Special mention should be made of the grand exposure of limestone
found south of Quartz Peak. Here we have over 8,000 feet of conform-
able beds starting in with the Niagara at the base, passing through a great
thickness of Devonian, and continuing on up into beds characterized by
a rich fauna of the Lower Carboniferous. In this section the White
Pine shale and the Diamond Peak quartzite are wholly wanting. While
this series of beds shows in some respects the closest resemblance to the
Eureka section, it is also significant as indicating an equally strong resem-
blance to the massive body of Wasatch limestone carrying Silurian, Devo-
nian, and Lower Coal-measure limestones without intervening siliceous
belts of any considerable thickness.

Such exposures on a grand scale are sufficient to show a very great
development of Silurian and Devonian rocks stretching for long distances
from southeastern Nevada well up toward the northern part of the State,

DISTRIBUTION OF THE PALEOZOIC. 205

if not far beyond. Throughout this entire distance, wherever they have
been studied, these limestones maintain a great thickness of strata. At the
southern end of this belt near Quartz Peak, the Silurian and Devonian
limestones are estimated at G,40() feet, and at Ravens Nest, just south of
the Humboldt River, the estimate gives 5,500 feet, while at Eureka they
present even a greater thickness.

As yet we know but little about the occurrences and distribution of the
Diamond Peak quartzite. According to the section at Quartz Peak it is
wholly wanting. In the Diamond Range at Eureka, it attains a thickness
of 3,000 feet ; on the opposite side of the valley in the Pinon Range it can
not measure less, and at Ravens Nest it attains a development of 7,000
feet. In the northern part of Nevada we see an enormous development of
arenaceous beds separating a Devonian from a Carboniferous fayua. This
material thins out to the south, and in place of it we find a continuous
limestone body extending all the way from the Eureka quartzite well
up into the Carboniferous, without any well defined intervening sili-
ceous horizon. Too few observations have "been made to determine
the geological history or geographical distribution of the Upper
Silurian and Devonian rocks. As to the character of their sedi-
mentation eastward, the thickening or thinning out of strata, or their
lithological transitions, we know but little. It is a most significant fact,
and one by no means easy to explain, that the entire series of beds included
within the second period in which the Paleozoic rocks of Eureka have been
classed, based upon their physical history, should apparently be wanting
over such large areas in Utah and eastern Nevada. In other localities, while
they may not be wholly wanting, the}' appear to be represented by thin
beds of Lone Mountain strata, identified by a stray Halysites, and the Devo-
nian by an occasional Atri/pa.

Another interesting fact as regards the position of these rocks is this :
Notwithstanding the enormous thickness of the Upper Silurian and Devo-
nian beds at Eureka, the same relative position which has been observed in
so maiiv plac.es elsewhere, with the Coal-measures resting upon the under-
lying Pogonip, may be seen here, the Upper Silurian and Devonian being
absent. The occurrence of the two limestone bodies lying in juxtaposition
may be seen all along the east base of Prospect Ridge, where the Iloosac

200

GEOLOGY OF THE EUREKA DISTRICT.

fault brings the Lower Coal-measures up against the Pogonip, organic
remains characteristic of both epochs being found within a few hundred
yards of each other, with the intervening space occupied mainly by
igneous extrusions along the fault-line. Here, however, they have been
brought into their present position by profound orographic displacement.

The Wasatch and Kanab Sections.— The Wasatch Range, which shuts in the
Great Basin on the east, combines, in a marked manner, many of the geo-
logical characters of both the Rocky Mountains and the Basin ranges. In
structure, however, it is closely related in its essential features to the ranges
of Utah and Nevada. There is exposed in the range a very remarkable
section of conformable beds, extending through 30,000 feet of sediments
and exhibiting nearly every geological period from Lower Cambrian to
Permian. ^ For the purpose of comparison a section constructed by the
Geological Exploration of the Fortieth Parallel is reproduced, as it shows
not only certain resemblances, but also striking differences in the sequence
beds from the section as exposed at Eureka :

Wasatch section, Utah: 30,000 feet; conformable.

PERMIAN, 650 feet .

CARBONIFEROUS,
14,000 feet

DEVONIAN, 2,400 feet. <

SILURIAN, 1,000 feet ...
CAMBRIAN, 12,000 feet .

Permian

Upper Coal-measure limestone .

Weber quartzite

Lower Coal-meas- "1
ure limestone . . I Wasatch

Waverly f limestone .

Nevada limestone J

Ogden quartzite.
Ute limestone . .

Cambrian

650
2,000
6,000

7,400

1,000
1,000

12,000

Clays, marls, and limestones; shal-
low.

Blue aud drab limestones; passing
into sandstones.

Compact sandstone and quartzite;
often reddish; intercalations of
lime, argillites, and conglomerate.

Heavy bedded blue and gray lime-
stone, darker near the base, with
siliceous admixture, especially
near the top.

Pure quartzite, with conglomerate.

Compact, or shaly, siliceous lime-
stone.

Siliceous schists and slates, quartz-
ites.

In the Wasatch section the 12,000 feet of metamorphosed schists,
slates, and quartzites probably occur below the Cambrian beds as exposed
at Eureka, except so far as they may be represented in the upper members
by the Prospect Mountain quartzite, while the great thickness of Cambrian
limestones and shales of the Eureka section is included within the 1,000
feet of Ute limestone in the former section. Again, at Eureka the Permian
at the top of the section is wholly wanting and the Upper Coal-measures,
which in other parts of Nevada attain a development of nearly 2,000 feet,

KANAB SECTION.

207

the thickness which has generally been assigned to them in the Wasatch,
are limited to 500 feet. It will be seen, therefore, that the upper and lower
portions of the section as exposed in the Wasatch, on the edge of the
Great Basin, are wanting in the Eureka section. Taking out the 12,000
feet of Cambrian at the base and 2,000 feet of Permian and Upper Coal-
measures from the summit of the Wasatch sections, there remains 16,000
feet of strata, which, from the base of the Prospect Mountain limestone to
the top of the series, are represented in the Eureka section by the enor-
mous development of 28,500 feet of sediments.

Mr. C. D. Walcott1 constructed a section across the entire series of
Paleozoic rocks as exposed in the Kanab Valley of the Lower Colorado
in the plateau province. This section presents 5,000 feet of beds from the
Cambrian to the Permian inclusive, and is republished here as it offers so
much that is of interest in a study of the Paleozoic rocks of the Cordillera.

Kanab section, Arizdha: 5,000 feet.

PERMIAN, 855 feet i

710

145

835
1, 455

970

100

235
550

Gypaiferous and arenaceous shales
and marls with impure shaly lime-
stone at base.

CARBONIFEROUS, 3,260 feet.. I

•
DEVONIAN, 100 feet

Same as above, with more massive
limestone.

Upper Aubrey

Massive cherty limestone, with gyp-
siferous arenaceous bed, passing
down into calciferous sandrock.
Friable, reddish sandstone, passing
down into more massive and com-
pact sandstone below. A few fil-
lets of impure limestone interca-
lated.
Arenaceous and cherty limestone,
235 feet, with massive limestone
beneath. Cherty layers coincident
with bedding near base.

Red Wall limestone

Sandstone and impure limestone.

CAMBRIAN, 785 feet . . . . )

Massive mottled limestone, with 50
feet sandstone at base.
Thin-beddi'd, mottled limestone in
massive layers. Green, arenaceous
ami micaceous shales, 100 feet at
base.

NOTE. — Planes of unconformity by erosion denoted by double dividing lines.
: Ain. Jour. Sci.. Sept., 1880.

208 GEOLOGY OF THE EUREKA DISTRICT.

Paleozoic Rocks in British America.— In this connection attention should be
called to the remarkable sections across the Paleozoic rocks of British
America exposed along the line of the Canadian Pacific Railway where it
crosses the grandest parts of the mountains. A description of the geolog-
ical structure of the country, accompanied by maps and diagrams, will be
found in a paper of Mr. R. Gr. McConnell, published in the reports of the
Geological Survey of Canada.1 The region described embraces a belt of
country about 70 miles in width, and for the most part lies just north of
the fiftieth parallel of north latitude. Within this belt several transverse
sections have been run across the Bow River Valley so as to include the
mountains on both the east and west sides. Sections constructed across
Mt. Stephen, Cathedral Mountain, and the Castle Mountain range present
an instructive sequence of strata for the Cambrian rocks, while those in the
vicinity of Cascade Mountain and along the Devil's Lake Valley offer
equally good exposures for tbe Silurian, Devonian, and Carboniferous.
The upper members of the Cambrian are exposed in both series of strata,
serving to connect the lower with the upper Paleozoic rocks. From the
standpoint of this work the chief value of the Canadian section consists in
its close agreement in many of its details with the sequence of strata found
at Eureka. According to Mr. McConnell2 the thickness of the Paleozoic
rocks in the region explored by him measures 29,000 feet. This, it will be
seen, is not far out from the thickness given for the corresponding rocks at
Eureka, where the best estimates place the thickness at 30,000 feet. The
fossiliferous Cambrian limestone, together with the underlying quartzite,
may be correlated with the Prospect Mountain quartzite and limestone.
Beds carrying the Olenellus fauna have been identified in the Canadian
rocks, although there they occur far below the limestone, the under-
lying quartzite having a much greater thickness than is exposed at Eureka.
It is difficult to determine how great a thickness should be assigned to the
Pogonip, although it is evidently well represented. Limestones carrying
Halysites are in many ways similar to the Lone Mountain beds, and have a

1 Report on the Geological Features of a portion of the Rocky Mountains. Accompanied by a
section measured near the fifty-first parallel. Geological Survey of Canada. Annual Report. New
series, vol. 2, 1886, pp. 24-30.

- Op. cit., p. 15.

STRUCTUBAL FEATURES. 209

thickness of 1,300 feet as against 1,800 feet assigned to them in Nevada.
In the Canadian section the Devonian exposes only 1,500 feet of strata as
against 5,000 feet of Nevada limestone, but on the other hand the Carbon-
iferous limestone immediately overlying the Devonian exhibits a much
greater development than the corresponding horizon at Eureka.

The sequence of strata in the Canadian localities shows a closer agree-
ment with the conditions of sedimentation at Eureka than do many expo-
sures of Paleozoic rocks situated but a comparatively short distance east-
ward of the latter area. In some respects the Canadian section more closely
resembles the Wasatch than it does the Eureka, as is shown in the great
thickness of Cambrian rocks below the OUnellm horizon. On the other
hand, there is no such development of Silurian and Devonian rocks in the
Wasatch as is shown both at Eureka and in Canada. Changes in sedi-
mentation appear much more sudden and varied in passing eastward from
Eureka than when followed northward. In structural and orographic
features the two regions present much in common, great lateral com-
pression, with anticlinal and synclinal folds, accompanied by north and
south lines of profound displacement.

STRUCTURAL FEATURES.

For a clear understanding of the relation of the different orographic
blocks to each other and to the numerous outbursts of igneous rocks, a
number of cross sections have been constructed across the central part of the
Eureka District. In one very marked way these transverse sections across
the mountains are of more than ordinary interest, as they bring out the geo-
logical structure connecting a number of distinct and, at the same time,
interdependent mountain masses, whereas in most instances in the Great
Basin sections are drawn across single uplifted ridges, isolated by broad
valleys whose recent deposits conceal everything beneath them. As these
valleys are frequently from 5 to 10 miles in width without rock exposures,
it is largely a matter of conjecture to say what the geological structure is
which underlies them. The most impressive orographic feature at Eureka
is the close relationship between the anticlinal and synclinal folds to the
north and south faults. It is these great meridional faults, at points attain-
MON xx 14

210 GEOLOGY OF THE EUKEKA DISTRICT.

ing a displacement of 13,000 feet, that have determined the orographic
blocks. A study of the structural details as presented in this work shows
that the folding and flexing of the beds are largely due to lateral com-
pression. The grandest effects of this lateral compression are seen along
the central portions of the mountains between the Spring Valley and
Rescue faults. It is here that the greatest energy has been displayed.
Within these lines lie the abrupt anticlinal fold of Prospect Ridge, the
sharp synclinal fold of Spring Hill between the Hoosac and the Pinto, and
the broad syncline in the Nevada limestone between the Pinto and the
Rescue faults. Westward of Spring Valley the structure stands out in
marked contrast with the mountain blocks included between these great
faults. Receding from the fault, the plication of strata becomes more and
more gentle to the west without any violent orographic disturbance, the
limestones falling away in broad sweeping rolls with relatively low angles
of dip. East of the Rescue fault a powerful compression of strata is shown
by anticlinal and synclinal folds in the region of Diamond Peak.

Faulted anticlines. —The structure whicli, in the opinion of the writer, is
most common in the Great Basin ranges, that of a faulted anticline with a
downthrow along the axial plane, is not brought out in the sheet of geolog-
ical sections, but is nevertheless well represented in the district, both in the
Fish Creek Mountains and at Newark Mountain. In both these uplifts
the displacement is accompanied by an escarpment along the fault which is
coincident with the axial plane. In the Fish Creek Mountains we have a
broad, gently inclined, westerly dipping limestone body, with the axis of
the anticlinal near the eastern edge of the uplift, the downthrow measuring
about 600 feet. On Newark Mountain, which is a sharp single ridge, the
downthrow also occurs along the eastern crest of the uplift, the escarpment
measuring, approximately, 1,000 feet. In both instances the easterly dip-
ping beds extend down the mountain slope until lost beneath valley accu-
mulations. On the west side of Newark Mountain the flexible White Pine
shale affords an excellent example of plication without fracture. Details in
regard to the structural features of both the Fish Creek Mountains and
Newark Mountain will be found in the chapter devoted to the descriptive
geology of those areas.

GEOLOGICAL CBOSS-SECTIONS. 211

On Prospect Peak we have a sharp anticlinal fold, with beds on both
sides of the fault standing at an angle of 70°, but without any great amount
of faulting.

In certain of the Great Basin ranges the axial plane occurs along the
center of the ridge, as seen at Ravens Nest in the Pinon Range. In others
it follows along the edge of the uplift, an escarpment usually facing the
valley. Of this latter structure, Newark Mountain is an excellent example.
In some of these ranges there may be 110 faulting of strata along the axis of
the anticline ; in others it may be confined to a few hundred feet, or the
faulted block may have suffered a downthrow measured by thousands of
feet. In the latter instance it is easily seen that the strata along the down-
throw side may be lost to sight, being carried down below the present level
of the Pleistocene deposits. Where this is the case only one side of the
fold is exposed, leaving a simple monoclinal ridge. This is what has
actually occurred in several of the narrow uplifts in the Great Basin.

GEOLOGICAL CROSS-SECTIONS.

These sections, atlas sheet xin, will be readily comprehended when
examined in connection with the geological map, and the detailed descrip-
tions of each block of Paleozoic sediments as given in the text may be fol-
lowed easily without much additional explanation. As the sections have
been constructed, as far as possible, across the strike of the beds, and for
the most part at right angles to the principal meridional lines of faulting,
they measure with a considerable degree of accuracy the amount of displace-
ment and indicate approximately the compression of strata. All sections are
carefully drawn on a scale of 1,600 feet to the inch, with a base line taken
at 6,000 feet above sea level, the height of the adjacent valleys on all sides.
So far as practicable they have been selected to show the average thickness
of all sedimentary rocks, from the base of the Cambrian to the summit of
the Carboniferous. Of course all underground structure is based upon
observed dips and strikes taken at the surface, but these have been
obtained, so far as possible, at frequent intervals and with every precaution
which could be exercised. In a country so broken by faults, dislocated by
igneous outbursts, and where bedding planes are so frequently wanting, the

212 GEOLOGY OF THE EUREKA DISTRICT.

construction of sections must necessarily, to a great extent, be based upon
theoretical reasoning.

On the geological maps, lines of cross-sections are laid down by nar-
row black lines designated at the borders of the sheet by block letters.

Section A-B.— This section is drawn only halfway across the Eureka
Mountains, and is confined to the northeast comer of the District, atlas
sheet vin. At the extreme western -end of the map the section exhibits 200
or 300 feet of the Hamburg limestone just west of the Jackson fault, fol-
lowed on the east side of the fault by the Pogonip limestone, which in turn
is capped by Eureka quartzite shown on the summit of Caribou Hill. On
the east slope of Caribou Hill the rhyolitic ashes and tuffs conceal the
quartzite and stretching eastward across the valley rest against the steep
wall of Richmond Mountain. These ashes and tuffs underlie the town of
Eureka, although nowhere of any great thickness, and probably over much
of this area overlie solid rhyolite not far below the surface. In the section
the Eureka quartzite is represented as underlying the valley as far eastward
as the Hoosac fault or the prolongation of the fault as recognized south of
the Richmond Smelting Works. The representation of a narrow strip of
Lower Coal-measure limestone is a theoretical deduction based upon strong
evidences observed at the latter locality. The basic andesites of Richmond
Mountain stretch eastward for 15,000 feet, followed by 7,000 feet of basalts,
completely cutting off all evidences of any continuation of either the Spring
Mountain or County Peak blocks. The depression of Hunter's Creek and
the gentle inclination of the basalt table toward it are well brought out in
cross-section. Nowhere are two epochs of Carboniferous limestone with
the intervening Weber conglomerate better shown than along the line of the
section. Rising above the basalt the Upper Coal-measures exhibit 500 feet
of beds dipping 45° to the west, resting upon Weber conglomerate. The
conglomerate is inclined at the same angle, but soon develops into an anti-
cline standing at 50° east, followed by a syncline varying from 50° east to
30° west. It is an admirable example of the effects of lateral compression.
The thickness obtained for the complete series of conglomerates measures
2,000 feet. Underlying the conglomerates come the Lower Coal-measures,
the section crossing Alpha Ridge just south of Fusilina Peak and showing

GEOLOGICAL CROSS-SECTIONS. 213

a uniform dip to the west at an angle of 25° or 30° with the horizon. With
the observed dips and strikes they measure 3,700 feet. As described in the
chapter devoted to the descriptive geology of the Diamond Range the
Lower Coal-measures rest unconformably upon the White Pine shales, the
dip of the beds in the latter epoch reaching an angle of 50° along the
Newark fault. These latter shales are shown to lie conformably on the
Nevada limestone along the line of Hayes Canyon. The anticlinal struct-
ure of Newark Mountain is not brought out in section, as the axis of the
fold only comes in near where the border of the map cuts off the easterly
dipping strata. The latter stretch far out into Newark Valley.

Section CD-EF.— This section is constructed across the central portion of
the Eureka Mountains (atlas sheets vn and vin), and stretches in an east
and west line from Antelope to Newark valleys. It presents more of the
salient structural features of the region than is shown in either of the other
sections, as it passes through the broadest part of the Mahogany Hills, the
flat-topped summit of Spanish Mountain, the steep anticline of Prospect
Peak, the syncline of Spring Hill, and the easterly dipping strata of
County Peak, and crosses the Spring Valley, Hoosac, and Pinto faults.
The section starts in at the western end of the Mahogany Hills and runs
obliquely across the strike of the beds, which lie nearly horizontal or
inclined at very low angles. In order to bring out the structural features
of the country, the Lone Mountain beds are represented as underlying the
Nevada Devonian above the base line of the section. At Dry Valley there
is a considerable but unknown thickness of valley accumulations with the
Nevada limestones on the west side and the Lone Mountain beds resting
against the flanks of Spanish Mountain on the east. These Lone Mountain
beds lie against the Eureka quartzites which come to the surface on the
slopes of Spanish Mountain long before reaching the summit. The moun-
tain presents in general a broad anticlinal fold, although broken by m;mer-
ous cross-faults and dislocations. One of these dislocations is represented
in the transverse section, giving a small exposure of Lone Mountain beds
overlying the quartzite. On the east slope of Spanish Mountain, where the
beds dip away steeply to the east, there is a small exposure of highly
siliceous limestones without bedding partially concealed by Quaternary

214 GEOLOGY OF THE EUREKA DISTEICT.

accumulations. These, from their position resting directly upon the quartz-
ites, have been referred to the Lone Mountain beds. The Spring Valley
fault brings these Silurian beds up against the Cambrian of Prospect Ridge.
The section intersects Prospect Ridge just to the north of the summit of
Prospect Peak and brings out the anticlinal structure in the quartzite on
the west slope overlain on both sides of the fold by the Prospect Mountain
limestone which on the west side comes down to the line of the Spring Val-
ley fault. Prospect Mountain limestone here forms the summit of the
main ridge. This is in turn overlain by Secret Canyon shale, Hamburg
limestone, and Hamburg shale, the remaining subdivisions of the Cambrian,
all of which stand inclined at about 70° to the east. As the section is
drawn across quite a high saddle at the head of New York Canyon, con-
necting Prospect Peak with Hamburg Ridge, the erosion of the Secret
Canyon shale, which is so marked a feature of the region, is not so well
shown as it would be if the section were drawn either to the north or south
of this point, but it is sufficient to bring out the prominence of the Ham-
burg Ridge, which is everywhere parallel to the main ridge. Overlying
the Hamburg shale occurs the Pogonip limestone and the Eureka quartzite,
the latter occupying the slope down to the Hoosac fault.

At the base of the long uniform slope of Pogonip limestone, which is
well shown in the surface outline, the line of the section has been moved
northward 700 feet, in order to illustrate to better advantage several struc-
tural features. By thus moving this line the Eureka quartzite is shown on
both sides of the homblende-audesite body, which in breaking out along
the Hoosac fault has shattered the quartzite all along the fault. Immedi-
ately along the line of the section only a small outcrop of the quartzite
occurs to the east of the andesite, but by reference to the geological map it
will be readily seen that the exposure forms a part of a continuous body of
considerable extent. East of the fault the Carboniferous limestones come
up dipping easterly, but separated from the main body by a minor fault,
which, so far as can be determined, is accompanied by only a slight dis-
placement. This is followed by a synclinal fold, described elsewhere,
lying between Spring Hill and the Pinto fault. Continuing along the
line of the section east of the Pinto fault the Lone Mountain limestones rise

GEOLOGICAL CROSS-SECTIONS. 215

as a precipitous wall, abutting against Carboniferous limestones, followed
by a great development of Nevada Devonian. The former are estimated
at 1,700 feet and the latter at 4,500 feet across the lower and middle mem-
bers of the Nevada series. At Basalt Peak igneous intrusions spread out east-
ward for 9,400 feet, concealing the sedimentary beds and completely obscur-
ing the structural features produced by the Rescue fault. Beneath these
basalts the section is constructed wholly upon observed data, both to the south
and east, and will be understood by reference to the map. From here to the
end of the atlas sheet basalt flows or Quaternary accumulations cover every-
thing with the exception of two outcrops — one, easterly-dipping beds of
Weber conglomerate, almost wholly encircled by igneous rocks, and the
other, a low, gentle swell of Carboniferous limestones, rising out of the
valley deposits not much above the base level of the sections.

section GH-iK.— This section (atlas sheets ix and x) is drawn across the
southern end of the mountains in a continuous line from west to east, pass-
ing through Grays Peak, Gray Fox Peak, Carbon Ridge, and Century Peak,
and crossing at right angles the Hoosac, Pinto, and Rescue faults. It
crosses the mountains about 4£ miles south of section CD-EF, and for the
most part runs along the extreme southern end of the different mountain
blocks, touching, however, the Fish Creek Mountains at their northern end,
but passing far to the south of the Diamond Range. The section passes
along the base of the Mahogany Hills, following the Lone Mountain lime-
stones approximately parallel with their strike, the beds dipping northward
into the hills beneath the Nevada Devonian. Across Spring Valley, for a
width of 6,000 feet, all Paleozoic strata are concealed beneath the Quater-
nary, but with the rising of the hills on the east side the Pogonip lime-
stones come in capped on the summit of the ridge by the Eureka quartz-
ites. Here the broad anticlinal structure of the Fish Creek Mountains is
clearly brought out, but without the fault recognized on the east side of the
higher portion of the mountains. East of Castle Mountain there occurs a
shallow basin or depression in quartzite occupied by Lone Mountain and
Devonian beds, beyond which there is a second anticline, with the Eureka
beds arching over the summit. At Lamoureux Canyon occurs a slight dis-
placement, the walls on both sides where the section crosses exposing the

216 GEOLOGY OF THE EUEEKA DISTRICT.

quartzite in abrupt escarpments. Between Lamoureux Canyon fault and
Pinnacle Peak fault the mountain mass presents another broad anticlinal
mass, of which Grays Peak forms the summit. The beds on the peak lie
nearly horizontal, falling away on both sides at relatively high angles.
Toward Pinnacle Peak the Nevada limestones come in, resting unconform-
ably against the uplifted block between Pinnacle Peak and Lookout Moun-
tain faults. Between these two latter faults, measuring on the surface but
scarcely more than 2,000 feet, the only beds exposed are the Eureka
quartzites, dipping eastward and capping Pinnacle Peak. East of the
Lookout Mountain fault there is a block of Pogonip limestone, beyond
which comes the Prospect Mountain Ridge uplift. The only Cambrian
rocks found on the surface are the Prospect Mountain limestones, the over-
lying horizons, together with the Pogonip limestone and Eureka quartzite
of the Silurian, being either buried beneath flows of rhyolite or Quaternary
accumulations. About a quarter of a mile to the north of the line of this
section the entire Cambrian series is exposed, and the beds are introduced
here very much as they are found beyond the line of the rhyolites and
pumices. Between the Hoosac and Pinto faults the section again crosses
the Carboniferous block, which here includes nearly all of the Weber con-
glomerate, as well as the Lower Coal-measure limestones, both members
lying at angles inclined from 60° to 70° to the east. Along the Pinto fault
the line of contact is obscured by tuff's and pumices. Along the southern
extremity of the Silverado Mountains no structural evidences were obtained,
as the underlying rocks are for the most part concealed by tuff's and puma-
ceous material, but in the section the lavas are represented as overlying
Lone Mountain beds, as the latter are found higher up in the foothills above
the line of igneous rocks. At the entrance of Rescue Canyon the rhyolites,
which break through the Nevada limestone along the line of the Rescue
fault, are represented with a width of nearly 3,000 feet. The section
crosses the summit of Century Peak and brings out clearly the anticlinal
structure of the limestone ridge on the east side of Rescue Canyon. On
the east side of the peak the beds gradually fall away toward Newark
Valley and are lost beneath the Quaternary deposits.

PLATE SECTIONS. 217

On PI. ii of this volume will be found a geological section drawn
across the summit of Prospect Peak, exhibiting the relations of the great
body of Cambrian limestone to the crest of the ridge, the limestone here
rising to the top of the peak. The position of Prospect Ridge to the Ham-
burg Ridge is also more clearly shown than in the general section. The
same series of rocks at the northern end of the ridge across Ruby Hill and
Adams Hill are also shown on this plate. On the same plate a section is
given across Pinto (Peak, and about 1 £ miles to the northward of the latter
section is another east and west section, drawn from the Pinto to the
Hoosac fault.

CHAPTER VII.

PRE-TERTIARY IGNEOUS ROCKS.

Igneous rocks have played a most important part in the development
of the geological history of the Eureka District. They may be separated
into two distinct groups : first, those which reached their present position in
pre-Tertiary times; second, a younger and much more extended series of
eruptions, those of Tertiary and post-Tertiary age. Not only do they be-
long to distinct geological periods, but their mode of occurrence is quite
unlike and their petrographical characters in every way different.

Granite, granite-porphyry, and quartz-porphyry are the types of the
pre-Tertiary rocks. Their surface exposures are very restricted, being
quite insignificant as compared with the more recent volcanic lavas, and
only to a very limited degree have their extrusions influenced the present
physical features of the country.

Granite.— Between the Sierra and the Wasatch there are probably few of
the many longitudinal ranges which rib the Great Basin, other than those
made up entirely of volcanic lavas, that do not show one or more bodies of
granite or crystalline schists of greater or less extent. Along the lines of
upheaval of one or two of these ranges, the accumulations of recent lavas
have been on so vast a scale that all direct evidences of an older preexist-
ing range are to-day wholly wanting. In some instances granite and gneisses
cover large tracts of country and occasionally culminate in peaks rising
high above the surrounding regions, but so abrupt are the changes in the
Archean topography that they occur for the most part only as subordinate
exposures over limited areas. The granite is found cropping out along the
base of the foot-hills beneath the Paleozoic sediments, occasionally occupy-
ing low passes through breaks in the ranges, or, as is frequently the case,
they are associated with extrusions of volcanic lavas and accidentally left
bare or else uncovered by recent erosion. Westward of the Salt Lake Basin,

218

GRAOTTE. 219

granite is found in isolated patches in such uplifts as the Ombe, Gosiute, and
Peoquop ranges. The East Humboldt Mountains present the grandest mass
of the older crystalline rocks, stretching with the trend of the range over
sixty miles in a nearly north and south direction, and is the most extensive
area of pre-Cambrian rocks to be found in central Utah and Nevada along
the country examined by the Fortieth Parallel Exploration. Immediately
westward of this latter range in the country occupied by the Diamond and
Pinon ranges, no exposures of granites occur, and it is one of the
largest areas known in the Great Basin, in which all evidences of granite
and of an Archean body are wanting. There are good reasons for believ-
ing that in early Paleozoic time this area east of the Humboldt Mountains
was a deep trough of the sea, in which Cambrian, Silurian and Devonian
sediments were deposited. At all events, the mountain ranges within this
region offer excellent sections of the lower Paleozoic strata, which over large
areas of the Great Basin are unknown.

At Eureka, where the Diamond and Pinon ranges unite in a broad ele-
vated mass of sedimentary beds of great thickness, singularly broken up by
great faults into bold mountain ridges, only one obscure outcrop of granite
is known. It is, however, not without considerable interest, and when the
geological position of these granites and allied rocks of the Great Basin
come to be studied in detail in their relation to the Archean masses and the
uplifts of the parallel ranges, it may be found to throw much light upon
some complex structural problems. Here at Eureka the outcrop does not
occur rising above the Quaternary plain along the base of a ridge, nor in
the bottom of some deeply eroded canyon. On the contrary it is found
1,000 feet above the level of Diamond Valley, at the extreme northern end
of Prospect Ridge, on the steep south slope of the ravine which separates
Ruby Hill from the main ridge. It is so insignificant and so covered with
deTjris derived from the limestone hills above that it might easily escape
attention, especially as it presents no inequalities of surface. This granite
is best seen on comingup the ravine from the west, and is exposed just above
the path, some miners having cut into it, attracted by the red color of the
decomposed rock. It extends from the footpath up the steep slope for 300
feet to within 50 feet of the top of the hill.

220 GEOLOGY OF THE ETJEEKA DISTEICT.

Although upon the surface this granite body is limited in extent, there
is reason to suppose that it represents a much larger mass and has exerted a
considerable influence on the geological structure of the ridge. Prospect
Mountain quartzite, the oldest sedimentary beds in the District, surrounds
the granite upon the northwest and northeast slopes and dips away from it
in irregular broken masses. To the south the Prospect Mountain lime-
stones, which form the main ridge, occur resting directly upon the granite.
That this exposure of granite along the steep slopes of the ravine is due to
the erosion of the crushed quartzite can not well be questioned, and the
ravine itself probably owes its existence to the fracturing of the quartzite
from some upward orographic movement of the granite. The farther away
the quartzite lies from the granitic center, the less disturbance is seen in
the beds.

The age of the granite is unknown. Quite possibly it formed a
portion of an Archean body around which the sands were deposited,
subsequent orographic movements disturbing and breaking tip the sedi-
mentary beds. If the granite is later than the sedimentary beds it only
broke through the quartzite at the northern end of the range and failed to
cut the limestone. No intrusive dikes of granite are known penetrating
the overlying beds. At the time of the sinking of the Richmond shaft the
workmen found, at a depth of 1,200 feet from the surface, near the bottom
of the shaft, fragments of a decomposed crystalline rock. They were so
highly altered that the microscope failed to detect with certainty the nature
of the rock, though it carried quartz and mica and kaolinized feldspar.
This evidence, though slight, would indicate an off-shore deposit for the
quartzites.

A peculiarity of this granite is the varied texture it offers over so
small an area, presenting, however, a true granitic habit. The essential
minerals are quartz, orthoclase, plagioclase, hornblende, and mica, the lat-
ter being very abundant. The quartz is grayish white in color, in irregular,
pellucid grains. Hornblende is apparently more abundant in the fine than
in the coarse grained varieties.

Quartz-porphyry.— Only two small exposures of quartz-porphyry have been
observed, both occurring on Mineral Point, north of Adams Hill, and,

GRANITE-PORPHYRY. 221

although separated by limestone, it seems probable that below the surface
the two bodies have a common origin, inasmuch as on top they present
nearly identical features. Neither body rises above the general level of the
country, erosion wearing away equally both limestone and dike.

The larger outburst is an irregular body with an east and west trend;
the other, a few hundred yards northward, and a short distance beyond
the Bullwhacker mine, occurs as a dike from 300 to 400 feet long, with a
direction N. 20° W. Both quartz-porphyry exposures are much decom-
posed and discolored by oxide of iron. Under the microscope they show,
however, orthoclase, quartz and mica, with characteristic isotropic glass.
The feldspars are altered into kaolin, with a secondary formation of potash
mica; the quartz grains carry both liquid and glass inclusions. In the
Bullwhacker mine the porphyry is seen as a dike, striking north 18° W.,
with a dip 35° to the east. In one place, at least, on the main level it
occurs as a white rock, differing from the surface exposure not only in
color, but by a relatively large amount of quartz grains and by a develop-
ment of numerous modified cubes of pyrites. The pyrites in the dike, in
distinction from that carried by the surface body, is perfectly fresh and
may account to some extent for difference in color. It is probable that the
pyrites is a secondary product, formed after the cooling of the original
magma. Careful assays show that the pyrites carries both gold and silver.
The quartz-porphyry differs structurally from all other igneous rocks in the
region, being distinct from granite on the one hand and from rhyolite on
the other. None of the rhyolites at all resemble it. It is possible that it
occurs as an offshoot from a large body of granite, the structural features
being due to conditions of cooling. No direct evidence of the age of the
quartz-porphyry is afforded other than that it penetrates the Pogonip lime-
stone of the Silurian.

Granite-porphyry.— The important granite-porphyry bodies are confined to
the country lying between the Fish Creek Mountains and the Mahogany
Hills. They occur in two large masses separated by a narrow belt of lime-
stone with a few smaller outlying exposures, probably offshoots from the
parent mass. The principal exposure of this porphyry lies immediately to
the west of Wood Cone along the southern base of the Mahogany Hills,

222 GEOLOGY OF THE EUEEKA DISTRICT.

and with an undulating surface gradually falls away to the broad valley
beyond the limits of the map. Its extension westward is lost beneath the
Quaternary. It presents an oval-shaped mass one mile and a quarter long
by three-quarters of a mile wide, partially hemmed in on both the north
and south sides by ridges of Silurian limestones which rest against it. For
the most part it is a coarse grained rock disintegrating readily under atmos-
pheric agencies. Much of it might easily pass for granite, but other por-
tions possess a decidedly porphyritic structure, especially along the line of
contact with the Pogonip limestone on the south. Geologically, a more
important body is the bold prominent dike trending approximately north
and south for over 2 miles, with a varying width, measuring across its
broadest expansion nearly 1,000 feet. At the surface no direct connection
can be traced between the main granitic area and the dike, the continuity
being broken by an arch or saddle of Pogonip limestone which passes
beneath the Eureka quartzite of Wood Cone. This limestone saddle is
scarcely more than 300 hundred feet in width. There can be no reason-
able doubt that the granite-porphyry dike is an offshoot from the main
body in much the same way as the branch dikes described farther on are
related to the larger one. A strong- proof of this connection is seen in an
isolated exposure of the crystalline rock worn bare by erosion on the sum-
mit of the limestone saddle.

The southern end of the main dike contracts to one hundred feet or
less in width and splits up into numerous small dikes ramifying in various
directions, becoming finally lost in the limestone. Between the surface
outlines of the two walls of the dike there exists a marked contrast. On
the one side the western wall curves slightly and regularly in sharply
defined lines. On the other the eastern side presents a most irregular
outline, caused by numerous branch dikes, offshoots from the main dike,
trending in nearly parallel courses in a northeastern direction. They break
the regularity of outline, besides fracturing and displacing the limestone
beds. Fig. 5 shows the position of the secondary dikes to the primary
one and is a reproduction on a reduced scale of a portion of the map.
The conventional signs indicate approximately the strike and dip of the
adjoining limestone.

GRANITE-POKPHYEY.

223

Of these branch dikes from the parent stock the longest has been
traced on the surface for nearly 2 miles, gradually narrowing to a few feet
and breaking up into stringers and veins of granite-porphyry at the north-
east end in the' same manner as the main dike. Other members of this sys-
tem of parallel dikes may be equally
as persistent in their trend, without
appearing on the surface, erosion hav-
ing failed to lay them bare. They vary
from 25 to 250 feet in width, but all
present much the same physical con-
ditions as regards their occurrence and
relation to the limestone. On the same
general course two narrow dikes pene-
trate the limestone just west of Castle
Mountain, and it is quite possible that
they belong to the same system as the
others if not actually connected with
them beneath the surface. The absence
of offshoots on the west side of the

main dike and their frequency and per- GmruleporpJiyry

sistency on the east are not without Fia- s.-Granite-porphyry dike,

geological interest. By reference to atlas sheet xi the position of the dikes
to the main granite-porphyry body and the Fish Creek Mountains and
Mahogany Hills may be readily seen.

Limestone adjoining Porphyry.— The main granite-porphyry dike, like the
quartz-porphyry, breaks through the Pogonip, and the smaller dikes on
Castle Mountain break through the Lone Mountain, but all other evidence
as to their age is wanting. Their mode of occurrence and their relations
to the orographic blocks are quite unlike the extrusions of the Tertiary
lavas. Although both sides of the main dike lie in limestone of the Pogo-
nip epoch, the beds have undergone considerable displacement; those on
the east side belonging to a higher horizon than those found in contact with
the dike at the north end on the west side. Immediately adjoining the

224 GEOLOGY OF THE EUEEKA DISTEICT.

dike at the north end the limestone stands at angles between 70° and
90°, and farther south only 35° to 45°, but for the greater part of the dis-
tance along the contact crumpling and metamorphism have so altered it as
to have obliterated all signs of stratification. Owing to the frequency of
the secondary dikes this metamorphism of strata is much more noticeable on
the east than on the west side. Instances may be seen where the lime-
stone is completely altered into a fine crystalline white marble. Nowhere,
however, does the alteration of strata produced by heat penetrate the lime-
stone for any great distance from the dikes, not even between those which
run parallel to each other only a few hundred yards apart. The effects of
heat are shown far more on the cooling and crystallization of the intrusive
molten mass than on the cold contact rocks. As the beds recede from the
main dike the dip becomes less and less steep, the stratification less
obscure, and toward the southern end of the dike the limestones lie nearly
horizontal.

The main porphyry dike in cutting the limestone follows closely
the strike of the beds, whereas the branch dikes trending approximately at
right angles to the main one run across the strata. The position of the
main dike is determined in part by a line of faulting and in part by an anti-
clinal fold. South of Wood Cone, where the dike first makes its app'ear-
ance, it trends off to the southwest along the fault line, maintaining this
course until near the summit of the limestone hill which lies midway
between the two wagon roads that cross the dike. Here meeting the anti-
cline, it curves slightly and follows the axis of the fold to the southeast.
This anticline may be traced southward beyond the surface outcrop of the
granite-porphyry, as is indicated by tlie dips and strikes on the map.
Along the northern end of the dike the amount of displacement can not
be determined beyond the fact already stated, that both sides of the fault
lie in the Pogonip. Why the secondary dikes should break out approxi-
mately at right angles to the main one and not parallel with it, it is diffi-
cult to say. It would seem as if lines of least resistance would have been
formed parallel with the line of the axial fold and the line of displace-
ment along which the main dike reached the surface. On the east side the
beds are a pure crystalline limestone, uniform in texture and bluish gray

VARIATIONS JN GRANITE PORPHYRY. 225

in color. The lower beds on the west side are darker in color, more sili-
ceous in composition, and rich in black cherty nodules, characteristics which
everywhere define the upper from the lower beds of the Pogonip lime-
stone.

That the strata near the southwest end of the dike belong- to the upper
portion of the Pogonip epoch is evidenced by the patches of Eureka
quartzite still in place and by a number of loose bowlders and quartzite
debris scattered over the hill slopes.

In the middle of the main dike, just north of the road which follows
the Spring Valley drainage channel, there occurs a curious bit of cherty
limestone. It is several hundred yards in length, but only a few feet in
width, and lies completely surrounded by granite-porphyry, which may be
seen penetrating and filling up the irregular outline in the limestone. In
places the molten mass appears to have eaten into the sedimentary body,
although only to a very limited extent. Along the contact both the por-
phyry and limestone present the same phenomena of cooling as seen near
the outer walls of the main dike. Even this narrow body of limestone
does not appear to have undergone much metamorphism, except along the
contact.

Structural Variatinos in Granite-porphyry.— The chief interest attached tO the

granite-porphyry lies in the very variable structural differences produced
in the erupted material of the dike, differences which are mainly dependent
upon the chilling effect of cold contact walls upon a rapidly cooling molten
mass. The width of the dikes has much to do in determining these physical
conditions governing crystallization. In other words, development of crys-
tallization is dependent upon rate of cooling, and in narrow dikes a molten
magma is more rapidly chilled than in broader bodies. There are probably
few localities in the Great Basin where the results of rapid chilling and
crystallization of a granite magma in narrow dikes along several miles of
contact walls can be studied to better advantage or are more worthy of a
detailed petrographical investigation. For petrographical details the reader
is referred to the paper by Mr. Joseph P. Iddings.

The large oval-sha.ped area to the north and the broader central por-
tions of the main dike are quite similar rocks, presenting the characteristics
MON.XX 15

226 GEOLOGY OF THE EUliEKA DISTRICT.

of a coarse grained granite composed of pellucid quartz, dark brown biotite,
and orthoclase in Carlsbad twins, with varying proportions of plagioclase
and strongly pleochroic hornblende. It everywhere weathers in rounded
masses with rough surfaces, disintegrating like many varieties of granite.
It is, however, only a limited portion of the dike which possesses anything
like a granitic structure, the greater part of the main dike and all the
secondary branches having a decidedly porphyritic structure. All through
the central part of the dike the rock is formed of large ill-defined crystals,
porphyritically imbedded in a groundmass of the same composition, which
under the microscope is seen to possess a micro-granitic structure. This
porphyritic structure can be traced for miles along all the dikes parallel with
their course. From the normal type in the central portion of the dike
toward the outer limestone wall there is a gradual and at first an almost
imperceptible transition to the finer grained rock, with more and more of
the porphyritic and less of the granitic structure. Across these transitional
rocks the mineral components remain the same, the differences consisting in
the size of the grains and their relative proportions and structural relations.

All thin sections show the rock to be entirely crystalline without
isotropic glass. At a distance of from 20 to 30 feet from the limestone, vary-
ing with the width and position of the dike,the rock shows a marked por-
phyritic habit, though the larger crystals are still in excess. In the nar-
rower dikes the transitions are not so well shown as the coarser portions and
are less characteristically developed and the changes far less gradual. In
the latter the quartz is very abundant and frequently occurs in well devel-
oped dihexahedrons, in strong contrast to the quartz, with irregular outlines,
as seen in the granitic structure. From here to the limestone contact the
change is more rapid, the larger crystals becoming less and less abundant,
being replaced by more and more micro-crystalline goundmass.

Nearly everywhere along the immediate line of contact the rock pre-
sents the habit of a quartz-porphyry made up of a crystalline groundmass,
with well developed crystals of quartz and orthoclase, and still accompanied
by some plagioclase. Both mica and hornblende usually fall off in amount
toward the edge of the dike, in the more porphyritic rocks, the hornblende
being present only in the granitic types, and the first to disappear with

VARIATIONS IN GRANITE-PORPHYRY. 227

structural changes. With this change in structure the rock becomes more
compact and weathers in angular blocks with smooth surfaces, the contact
products offering the greatest possible contrast with the central portion of
the dike, which weathers in rounded masses with rough surfaces, disintegra-
ting easily under atmospheric influences. Yet in the wider dikes these
changes can be traced so readily, step by step, from the one rock into the
other, that the evidence is clear that they are but different structural
developments of the same erupted material, but not necessarily identical in
chemical composition at the time crystallization took place. In crossing
the dikes one passes within 100 yards, over excellent quartz-porphyry, on to
normal granite-porphyry, and then on to a rock which can not be told from
many varieties of granite ; so that one is forced to believe that the only differ-
ences between granite and granite-porphyry is in many cases purely one of
structure, dependent upon conditions in cooling rather than upon any
differences in age or chemical constitution of the original magma.

A study of the dikes makes it evident that there could not have been
any forcing of lava into the middle of a dike already partially occupied
by an earlier crystalline rock. As the branch dikes are mostly narrow, the
granitic and normal granite-porphyry structures are less fully developed
and are frequently wanting, the effects of chilling and rapid cooling from
both walls toward the interior producing only the types of quartz-porphyry
developed along the walls of the broader dike. Another striking feature of
the rapid cooling of the magma is seen in the marked tendency of the
crystalline rock to develop a jointed structure near the lines of contact, in
planes parallel to the walls.

In places the porphyry contacts present a fissile, sherdy structure,
lines of parting becoming wider and wider apart toward the center of the
dikes and gradually disappearing. In the jointed portion the rock is always
fine grained, and frequently possesses an aphanitic structure, the mineral
components, however, remaining the same. The rock frequently undergoes
marked changes in color in passing from the coarse grained granitic structure
to the contact rock. In these changes it will frequently pass from light
gray into dark gray, blue, and along the contact becoming almost black.

228

GEOLOGY OF THE EUREKA DISTRICT.

Chemical Composition of Granite-porphyry.— The original magma injected into

the limestone through the various openings was, so far as can be told, much
the same in its ultimate composition. It is probable that the variation in
chemical composition between rocks of different dikes is no greater than
that between different parts of the same dike. While structural peculiar-
ities across the dike are strongly marked, the mineral constituents remain
much the same, although the walls are more acid than the center and carry
less ferro-magnesian minerals. As regards tenure of silica the variation
between one rock and another would not exceed 5 per cent, with corre-
sponding variations in their essential ingredients. It is probable that the
silica variations in the bulk of the granite-porphyry would fall within 4
per cent.

The following two analyses of granite-porphyry were made by Mr.
Andrew A. Blair:

No. 1.

No. 2.

Silica

Percent.
68-68

Percent.
72-01

Alumina. . ..

16-28

15-51

Ferric oxide. ..

•66

Ferrous oxide

2-55

1-36

Lime

2-24

1-35

Magnesia

•81

•51

Soda ...

2-88

2-36

Potash

4-07

4-71

Titanic acid

•05

Not det.

•17

•33

Water

•68

1-24

Total

99-07

99-38

Analysis No. 1 may be taken as representing the composition of a
large area of the rock west of Wood Cone possessing a granitic structure,
the essential minerals having no crystallographic outline. The quartz and
feldspar are of medium size and are accompanied by nearly all the accessory
minerals recognized in these rocks, including biotite, zircon, titanite, and
allanite.

Analysis No. 2 is made from granite-porphyry obtained from the mid-

COMPOSITION OF GRANITE-PORPHYEY. 229

die of a dike about 30 feet in width. It is probably slightly more acidic
than the mass of the rock. The groundmass of the rock is made up of an
aggregation of quartz and feldspar, the former crystallized in regular dihexa-
hedrons. Biotite is present, but no hornblende, and in the hand specimens
there is only a slight development of ferro-magnesian silicates. Both rocks
analyzed carried a trace of chlorine. It will be seen that the rock from the
central mass carries a higher percentage of all bases, except potash, than
the narrow dike. The latter probably represents fairly well the contact
rocks of the larger dikes.

It is possible that in a careful study of these rocks with reference to
the development of crystallization it might be shown that the ferro-magne-
sian minerals exhibited a tendency to segregate in the central or less rapidly
crystallizing portion of the dike, due to differentiation in the chemical
composition of the molten lava. It may be well to mention here that in
connection with these miles of porphyry dikes there are no evidences of
any recent volcanic action. The granite-porphyries and the rhyolites seem
to be wholly independent of each other as regards their mode of occurrence
and their loci of eruption.

CHAPTEE VIII.
TERTIARY AND POST-TERTIARY VOLCANIC ROCKS.

Eureka a Volcanic Center.— In the Eureka District the recent volcanic erup-
tions play a far more important part than the granites and porphyries just
described. They occur in much larger masses, cover more extensive areas,
and are more widely distributed over the district. While the older crystal-
line rocks have exerted little influence upon the surface features of the
country, the volcanic rocks have greatly modified its topographical outlines,
have built up isolated mountains, broad table-lauds, and numerous small
hills, and in coming to the surface have disturbed and broken up sedi-
mentary formations, greatly complicating geological structure. Moreover,
the volcanic rocks are of special interest from an economic point of view,
owing to their intimate geological connection with the argentiferous lead
deposits occurring in the adjoining Paleozoic rocks. As the Eureka Moun-
tains are surrounded on nearly all sides by the characteristic broad valleys
of the Nevada plateau, this volcanic region occupies a somewhat isolated
position with reference to the neighboring ranges, constituting a region
quite apart from all other centers of similar eruptions, yet at the same time
bearing the closest resemblance in the nature of its extra vasated material
to many other localities in the Great Basin. Nowhere in the district have
the accumulated lavas attained any great elevation above the surround-
ing mountains, but as regards mode of occurrence, peculiarities of distri-
bution, and varieties of modification they offer a wide field for investigation.
There is no such piling up of enormous masses of erupted material as in
the Washoe District, and no such opportunity for an investigation of the
more coarsely crystalline rocks, but, on the other hand, the relationship
between the extrusive lavas and the uplifted blocks of Paleozoic strata is
better shown than in any other carefully studied area in the Great Basin.

230

AGE OF ERUPTION. 231

Distribution of Extrusive Lavas.— In regard to the distribution of volcanic
rocks in the Eureka District it will be noticed by reference to the map that
there are none in that part of the Diamond Range which comes within the
area of the map. Although the southern end is completely cut off by recent
lavas from the mountain block of County Peak, nowhere do they seem to
penetrate into the range itself. In the southern end of the Pinon Range on
the opposite side of Diamond Valley, there is the same absence of lavas, if
we except a small outburst northwest of The Gate, which has but little to do
with the main body of the Eureka Mountains. The Mahogany Hills west
and north of Dry Lake furnish one or two small exposures of rhyolite along
lines of faulting 111 limestone, notably at the head of Brown's Canyon, but
they are of 110 special geological significance. In the Fish Creek Mountains
no outbreaks of extrusive rocks are known and the same holds true of the
mountains bordering the Spring Valley fault. In the extreme southeast
comer of the map the Cliff Hills come in, formed of basic andesites similar
to those of Richmond Mountain, but they lie wholly beyond the borders of
the Eureka Mountains. This confines the area of the principal volcanic
extravasations either to the region east of the summit of Prospect Ridge or
to the country encircling the southern end of that ridge, where it sinks below
Fish Creek Valley. Nearly every type and many of the varieties of vol-
canic rocks found upon the Nevada plateau have been erupted within these
restricted limits of the Eureka District. Indeed, the region furnishes many
rocks which may be taken as typical of a broad area of country lying
between the Wasatch and the Sierra Nevada ranges.

Age of Eruptions.— It should be clearly understood that the Eureka Dis-
trict, like many other regions of central Nevada, offers no direct evidence
as to the age of volcanic eruptions, as there occur no sedimentary forma-
tions between Upper Coal-measure limestones and recent Pleistocene de-
posits. While positive evidence may be wanting as to their precise age,
there can be no doubt that the eruptions took place subsequent to the
dynamic movements which brought about the flexing, folding, and mountain-
building, and these, while it may not have been demonstrated, have been
assigned upon excellent grounds to a post-Jurassic upheaval, already dis-
cussed in Chapter II. of this work. Moreover, that these orographic

232 GEOLOGY OF THE EUEEKA DISTRICT.

movements were followed by a long period of time before the outbursts of
lavas is evident by the amount of erosion which took place before the ex-
trusion of the latter rocks. This is shown by the position of the lavas in
the bottoms of deeply eroded canyons, by the blocking up of old drainage
channels and the cutting of new ones, all of which must have required
considerable time to be accomplished by slow geological processes. On
the other hand, based upon much the same kind of evidence, the amount
of erosion since the volcanic period seems relatively slight. Although evi-
dence of geological age of these lavas may be difficult to obtain over the
greater part of the Great Basin, wherever such proof is observed it always
points to the conclusion that the eruptions took place since the coming in
of Tertiary time and for the most part during the Pliocene period. Nearly
all geologists who have examined the volcanic areas of Nevada and Utah
are in accord with the opinion that they belong to Tertiary time. In the
region of the Montezuma and Kawsoh ranges, in western Nevada, the geol-
ogists of the Fortieth Parallel Exploration1 have described the cutting by
intrusive acid lavas of upturned Miocene strata carrying fresh-water mol-
luscan shells and the overlying of the latter beds by basaltic flows. Evi-
dence has also accumulated to show that several of the great rhyolite flows
that preceded the basalt belong to the Pliocene epoch. From unques-
tioned evidence in other parts of the Great Basin as to the geological posi-
tion of identical lavas, it is assumed that those of the Eureka District were
also poured out during the Tertiary age. As regards the duration of vol-
canic activity there is scarcely any evidence. From the earliest outbursts,
consisting of homblende-andesites, to the latest eruptions of highly glassy
basalts, volcanic activity may have extended over the greater part of late
Tertiary time. That this activity continued at varying intervals through-
out a long period seems clear from the amount of erosion, which, though
relatively not excessive, must have required considerable time. There is
no evidence to show that volcanic energy with varying intensity may not
have extended through Pliocene well on into Quaternary time, although
there is no reason to suppose that outbursts of basalt have taken place
within what may be called historic periods.

1 U. S. Geol. Explor., 40th Par. Descriptive Geology, p. 771.

CLASSIFi CATION OF LAVAS. 233

Classification of Lavas.— On the geological map all volcanic rocks have been
classed under the following heads : Hornblende-andesite, dacite, rhyolite,
pumice and tuff, augite-andesite,1 and basalt. All the more important bodies
of lava belong to some one of these sharply denned mineralogical groups.
It must be borne in mind, however, that several of these types of igneous
lavas pass by insensible gradations from one into another and it is not
always easy to decide to which group an isolated exposure in the field or a
hand specimen in the laboratory properly belongs. All division lines are
more or less arbitrary. They are necessary for purposes of classification,
although they may not exist in nature.

To inform those readers who have not kept up with recent ad-
vances in the classification of igneous rocks and at the same time to pre-
vent all misunderstandings as to the use of terms, a brief description
will be given of the physical characteristics of each group of lavas
which have been recognized as occurring in the district. Those who
desire a more detailed description of the special petrographical features
of the rocks are referred to the report of Mr. J. P. Iddings, which accom-
panies this work. As he has presented the results of a most thorough
microscopical investigation of the material collected in the field, in order
to prevent a duplication of facts much that might properly find place in
these pages is omitted, and only such data are employed here as are deemed
necessary to make this chapter complete in itself and to bring out more
clearly the important facts bearing upon the geology of the region.

Hombiende-andesite.— Under homblende-andesites are classed those vol-
canic rocks which are composed mainly of triclinic feldspars and horn-
blendes as essential constituents. Here at Eureka the fresh unaltered

1 Since the printing of the atlas (1883) accompanying this Monograph, the pyroxenic minerals
of the andesito of Richmond Mountain, which at that time were considered to be augites, have beeu
shown to consist of both hypersthene and augite, the former mineral being in most instances largely in
excess. The rock, therefore, should more properly be designated as pyroxeue-andesite, a designation
more in accord with the nomenclature now generally adopted by lithologists for similar rocks else-
where.

Throughout this volume the term "pyroxeue-andesite" will be used to designate the rocks of
Richmond Mountain and Clift' Hills, the two localities colored as augite-andesite on the atlas sheets.

See Notes on the volcanic rocks of the Groat I '.a sin. Hague and Iddiugs: Am. Jour. Sri.. June,
1884, p. 457.

234 GEOLOGY OF THE EUEEKA DISTRICT.

rock .is of a light reddish-purple color, for the most part holocrystalline in
structure, with well developed secretions of the two principal minerals,
accompanied by varying amounts of biotite. The black bordered horn-
blende is almost exclusively confined to macroscopic individuals which are
for the most part decomposed into chloritic material, frequently giving the
altered rocks a green color. Magnetite, though not sufficiently abundant
to be regarded as an essential ingredient, appears to be evenly disseminated
throughout the groundmass, and quartz, wanting in most varieties, serves as
an accessory mineral in certain localities, especially in the more acid types.
The mica gradually comes in as the rock becomes more and more acidic
and as the acidic variety of this group predominates over the basic the
mica occurs as a frequent if not constant constituent. Hornblende-mica-
andesite covers much larger areas than normal horublende-andesite. A
characteristic and important feature of the hornbleude-andesite of this dis-
trict is the absence of all pyroxene. Very little of the horublende-andesite
is perfectly fresh and most of it has undergone a considerable decomposi-
tion, changing the color of the rock to light shades of red and yellow,
while those portions which are most altered appear nearly white. Opal
and chalcedony as secondary products are by no means uncommon in the
more altered varieties.

Andesitic Peariiteu.— Nearly every occurrence of hornblende-andesite is
accompanied by more or less extensive outflows of andesitic pearlites,
which so far as their mineral composition is concerned are quite similar to and
in many instances identical with the crystalline types. They are in general
more acidic in composition than the hornblende-mica-andesites, carry fewer
well developed crystals, and in place of the holocrystalline structure are
rich in glass base with microcrystalline secretions disseminated through it.
It is the almost infinite varieties of this glass base which give to these
pearlitic rocks their varying physical habit, color, luster, and density.
Owing to their more acidic character, quartz becomes more frequent, but is
shown rather in macroscopic secretions than in minute grains scattered
through the groundmass. Sanidine, wanting in the normal varieties,
may occur, although as a nonessential mineral, while biotite comes promi-
nently to the front and to the eye appears as the most abundant macro-

ANDESITIC PEAELITES. 235

scopic mineral, and relates the pearlites more closely to the hornblende-
mica-andesites than to the more basic group. A variety of these pearlites
exceptionally rich in glass is characterized by the appearance of hyper-
sthene. As it is one of the most basic of this group of andesites it will be
discussed farther on in this chapter. Many of the varieties possess a dense
vitreous texture, breaking readily under a hammer-blow, while others are
more or less pumiceous, crumbling easily under atmospheric agencies.

The largest body of hornblende-andesite occurs as a fissure eruption
along the Hoosac fault, the lavas coming to the surface just to the south of
the junction of the Ruby Hill branch with the main fault and extending
southward till lost beneath rhyolitic flows. Like most acidic rocks these
extravasated lavas have not spread out over large areas, but have piled up
in irregular rounded hills, the highest reaching an elevation of 500 feet
above the base of the limestones along the fault line in the valley. For
the greater part of the distance along this fissure these lavas have under-
gone more or less alteration, due to solfataric action, kaolinization taking
place with the formation of secondary minerals. Comparatively fresh rocks
not far from the fault are still found northeast of Hoosac Mountain in the
larger and least altered bodies. Associated with the more crystalline types
occur excellent exposures of andesitic glasses and pearlites, products ot
more rapid cooling of the same magma under slightly different physical
conditions. Other localities of hornblende-mica-andesite with the accom-
panying pearlites closely resembling each other in manner of occurrence
and mineral composition are found at the southern end of Carbon Ridge,
in the neighborhood of South Hill, at Spring Valley, and near Dry Lake.
In all of the four latter localities hornblende-andesite and the horn-
blende-mica-andesite occupy positions quite inferior to the glassy varieties,
so far as the amount of extravasated lava is concerned, but it is by no
means easy, owing to insensible gradations, to draw a sharp line between
the crystalline and glassy types. At the first two localities they occur,
breaking out at the southern base of the upturned longitudinal ridges
of sedimentary strata. In Spring Valley and along tKe Lookout fault,
where the lavas penetrate the mountains on the west side of Prospect Ridge,
the glassy varieties have poured out in relatively large masses, pearlites

236 GEOLOGY OF THE EUREKA DISTRICT.

being tne prevailing rock. They form gentle spurs 200 feet in height, rest-
ing against the sedimentary ridges. An interesting transition rock occurs
here which unites the characteristic microcrystalline hornblende-mica-
andesite with fine examples of pearlite. At Dry Lake, while the petro-
graphical features are much the same, with similar transitions and variations,
the geological occurrence is somewhat different, the locality being quits
remote from the others and lying far to the west of Prospect Ridge and
the principal lines of north and south faulting. A suite of specimens from
any one of these localities could hardly be distinguished from those of
others either in physical features or composition. Identical transition prod-
ucts, both as regards degree of crystallization and mineral variation, are
recognized from all of them. They show the same order of succession and
the same sequence of geological events. The petrographical features of
these nearly identical series of lavas will be found described by Mr. Iddings
in the chapter already referred to.

Dacite.— This rock is a variety of andesite, in which secretions of quartz
play the part of an essential mineral, and is intimately related in mineral
composition and structural habit to hornblende-mica-andesite. All the
occurrences at Eureka are very similar in physical characteristics, possess-
ing a light ash-gray color, a hackly fracture, and a rough, pumice-like
texture which relate them closely to certain varieties of rhyolite with which
in the field they are frequently associated and from which they are not
easily separated, either in their mode of occurrence or lithological appear-
ance. The feldspars are nearly all plagioclase and probably largely oligo-
clase. Together with the larger secretions of quartz occur numerous thin
lamina; of biotite, in amount greatly in excess of that usually found in horn-
blende-andesite. Although of less frequent occurrence, occasional hyper-
sthene crystals, similar to those found in the andesitic pearlite, may be
recognized in the dacites and serve to show the dying out of ferro-mag-
nesian minerals in the more acidic lavas. These glassy dacites are closely
related to the andesitic pearlites, but occupy much smaller areas, their chief
interest lying in the fact that they represent transition products from the
andesitic to the rhyolitic lavas, sometimes associated with one, sometimes
with the other, and not infreauently as lava flows connecting them

DACITE. 21)7

both, in much the same way as the more crystalline dacites occur
as transition products between the crystalline varieties of hornblende-mica-
andesite and rhyolite. Near the entrance to Sierra Canyon, 011 botli
sides of the road, there is exposed a characteristic variety of gray
dacite, and at Dry Lake and South Hill they occur with andesitic eruptions,
but only in obscure low ridges and knolls, breaking through the Nevada
limestone of the Devonian. Again some varieties of dacite are closely
associated geologically with rhyolitic pumices and tuffs, but differ from
them petrographically in having a predominance of triclinic instead of mono-
clinic feldspars.

Rhyolite.- The essential components of this natural group are restricted
to orthoclase and quartz. Usually they carry more or less triclinic feld-
spars, in some cases almost equaling the monoclinic form, but they are
rarely developed in as large individuals as the orthoclase. Biotite as an
accessory mineral may be present in varying amounts, but is quite as likely
to be wholly wanting. In chemical composition they form the most acid
of all natural groups into which the lavas have been divided. In color and
texture no rock surpasses the rhyolite in the endless modifications which it
undergoes even within very limited areas. In crystalline structure it may
vary from a rock possessing a holocrystalline groundmass, with or without
large macroscopic secretions of the essential minerals, to one almost wholly
made up of glass. Whether the rhyolite is crystalline or in large degree
composed of glass, the sanidines occur in well developed crystals, frequently
presenting the brilliant iridescent hues so often observed elsewhere through-
out the Great Basin: The quartz occurs both as dihexahedral crystals and
dark gray and black angular grains which stand out in strong contrast to
the prevailing light tints of the inclosing groundmass.

At Eureka, where acid lavas are singularly well developed, among
the many extrusions of rhyolite occur two principal varieties which cover
large areas and embrace the greater part of the outbursts, and for the pur-
poses of the present chapter may be designated by local names: one, the
Rescue Canyon rhyolite, the other, the Pinto Peak rhyolite. In mineral
and chemical composition they are closely allied. The Rescue Canyon
rhyolite when fresh has a decidedly red color due to a considerable amount

238 GEOLOGY OF THE EUEEKA DISTE1CT.

of iron oxide iu the groundmass. It occurs only as a fissure eruption along
the Rescue fault, the brilliancy of coloring causing it to stand out promi-
nently in contrast with the inclosing dark gray limestones. It is largely
composed of glass base in which are porphyritically imbedded exception-
ally brilliant grains of dark quartz and tabular crystals of sanidine. The
Pinto Peak rhyolite, on the other hand, is characterized by a more crystal-
line groundmass, much of it being holocrystalline. It is iu general lighter
in color, more varied in tint, carries less iron, and is almost wholly
free from ferro-magnesian secretions. In places it disintegrates readily
into loose quartz grains and feldspathic fragments. The name is taken
from Pinto Peak, a prominent elevation, made up wholly of rhyolitic
accumulations, lying between Spring Hill and Carbon Ridge, just east of the
Hoosac fault. Similar rhyolites, singularly uniform in composition and

crystalline structure, extend without a break in their continuity the entire
distance from Pinto Peak to Gray Fox Peak. Nearly all the lesser out-
bursts of rhyolite scattered over the district and breaking out along fault
planes belong to the Pinto Peak variety. It is characteristic of many vol-
canic centers in the Great Basin as well as Eureka, and may be considered as
a typical rock over large areas.

Pumice and Tuff.— All rocks placed under this head are closely allied to
the rhyolites in mineral and chemical composition and belong to the same
natural group. The rhyolites and pumices break out under very similar
geological conditions and frequently pass from one into the other, even
more readily than the transition between the crystalline andesites and
glassy pearlites. They cover much larger areas than the corresponding
andesitic rocks and in their field occurrence offer such striking contrast to
the more compact rhyolite that it is thought best to separate them, more
especially as they exhibit definite characteristics sufficient to group them on
the map by themselves, and in several localities the erupted material con-
sists wholly of pumices and tuffs. Transitions from normal rhyolites into
pumices are admirably shown along the base of Purple Mountain. The
mass of the mountain rises 400 feet above the valley and is formed of a
characteristic rhyolite, while the base spreads out in a great variety of

PYKQXESE ASDES1TE. 239

pumices and tuffs, which stretch eastward beneath the town of Eureka as
far as Richmond Mountain.

Similar transitions may also be seen in the neighborhood of Pinto
Peak, although on a less extensive scale. It seems impossible for any
region to exhibit a finer display of pumices and tuffs than those occupying
the basin between County Peak and Richmond Mountain. Here, in the
neighborhood of Hornitos Cone, a symmetrical hill of tuff 400 feet in
height, these pumices are shown with every possible variation in color and
texture, the results of alteration produced by the breaking through of
basaltic masses. The color and density have undergone marked changes,
but the mineral development remains much the same as in the normal
rhyolite. Nothing could surpass the abrupt changes in physical habit
which these rocks undergo.

Pyroxene-andesite (Augite-andesite).— Under pyroxene-aiidesite are included
all those volcanic rocks whose essential constituents consist of triclinic feld-
spars and pyroxenes, the rock differing fundamentally from the hornblende-
andesite in having the hornblende replaced by some form of pyroxene,
usually a mingling of both hypersthene and augite. Hyperstheue in most
rocks of this group surpasses the augite in amount and in certain localities
predominates to such a degree that the rock might properly be classed as
hypersthene-andesite. Whether there exists any large body of extrusive
lava in the Great Basin retaining the andesitic habit, in which the augite is
the prevailing mineral, to the exclusion of hypersthene and without the
accompaniment of olivine, is a matter of some doubt. The rock is rich in
magnetite, disseminated throughout the mass, the mineral playing a far
more important part than in hornblende-andesites. In addition to the essen-
tial mineral constituents which make up the rock, both biotite and black-
bordered hornblende have been identified in the pyroxene-andesites from
several localities in considerable quantity. Richmond Mountain is the only
body of pyroxene-andesite in the Eureka District. It is well represented
in the Cliff Hills just south of the Fish Creek basin, the northern end of
which is shown on atlas sheet xn. It covers such an extensive area, present-
ing not only an important feature of the lavas of this region, but is such a
typical rock of many other localities, that it requires to be described in detail.

240 GEOLOGY OF THE EUREKA DISTRICT.

Richmond Mountain.— The mountain lies in a region of profound disturb-
ance and dislocation. Immediately to the west the depressed block of
Spring Hill sinks beneath the plain, while to the south the broad elevated
mass of County Peak rises abruptly above the lavas. The Pinto fault passes
beneath Richmond Mountain and apparently connects with the Rescue
fault, the great, line of displacement which separates the County Peak
block from the Diamond Range, but all structural features are obliterated
by the pyroxene-andesite lava flows. The culminating point of Richmond
Mountain, situated near its southern end, attains an elevation of nearly
2,000 feet above Diamond Valley. An abrupt wall, 800 feet in height,
forms the southern end, and from its summit the mountain falls away to
the north for nearly three miles, with an average slope of about 14°.
Across its broadest expansion, in an east and west direction, the mountain
measures three miles. For such an accumulated pile of lavas it presents a
uniform, monotonous appearance, relieved by occasional shallow drainage
depressions flowing northward, inclined with the natural slopes of its lava
ridges. Its geological position with reference to the Carboniferous beds
of the Diamond Range on the east, the Devonian on the south, and the
Silurian and Cambrian beds on the west is shown in cross-section A-B,
atlas sheet xm.

Richmond Mountain is almost wholly made up of pyroxene-andesite,
the prevailing colors of which are dark grayish purple varying to
bluish black. In crystalline structure the rock varies from a micro-
crystalline groundmass to one rich in glass base, porphyritic crystals
of light colored feldspars characterizing the rock through all degrees of
crystallization. This range in crystallization produces marked variations
in physical features, the lavas changing within short distances from a
highly vesicular rock with angular fracture to a compact one weathering
with rounded outlines. Varying proportions of the porphyritic constitu-
ents are found in all the rocks from the holocrystalline to those rich in
glass base. Hypersthene is the prevailing pyroxenic mineral, always
accompanied, however, by more or less augite. The feldspars are anorthite
and labradorite. In addition to the essential minerals, well developed
porphyritic crystals of hornblende occur in the less basic lava, but nowhere

RICHMOND MOUNTAIN. 241

as shown by the microscope do they enter into the composition of the
groundmass. Associated with the hornblendes are a few flakes of biotite,
the latter mineral occasionally appearing scattered through the rock with-
out the presence of the former. It is this association of hornblende and
biotite in the pyroxene-audesite that relates it to the earlier hornblende-
audesite. These relatively acid lavas are well shown in the neighbor-
hood of Trail Hill. In all these rocks of Richmond Mountain the
groundmass is made up of innumerable lath-shaped feldspars and micro-
lites of pyroxene, producing that peculiar felt-like structure first described
by Prof. Zirkel and since recognized by others as a characteristic of
pyroxene-andesites. Two typical varieties occur here, extreme forms
of the same lava : one, a rough, porous rock, dark purple in color and
having what has frequently been called a trachytic texture ; the other, a
compact rock, bluish black in color and possessing an oily resinous luster
which has been described as a characteristic of many pyroxene-andesites
elsewhere. Both of these sharply contrasted rocks pass by insensible transi-
tions, the one into the other, preventing the tracing out of separate flows
in the field. There is, however, this marked peculiarity between them:
the former has a laminated and fissile appearance, whereas the latter
nowhere exhibits any tendency to such lines of parting.

Owing to the geological and petrographical importance of the Rich-
mond Mountain rocks, Mr. Iddings has devoted much time to an investi-
gation of the lime-soda-feldspars and ferro-magnesian-silicates, the results of
which will be found in detail in his chapter on microscopical petrography.
Mr Iddings has determined anorthite by its extinction angles and other
optical properties as one of the prevailing feldspars which at the time of his
work was the first recognition of this species as an essential constituent of
the volcanic rocks of the Great Basin.

Within recent years investigations have demonstrated that hypersthene
is the prevailing ferro-magnesian silicate of many pyroxeue-andesites in
volcanic regions throughout the world. The importance of hypersthene
as an essential and controlling constituent of pyroxene-andesites in Colorado
was first shown by Whitman Cross.1 Soon after these observations were

1 Bulletin of the U. S. Geological Survey, No. 1, 1883.
MON XX 10

GEOLOGY OF THE EUREKA DISTRICT.

confirmed and extended by an examination of the lavas from the volcanoes
of the Pacific Coast' and those of the Great Basin.2 In the investigations of
these latter, the pyroxene-andesites of Richmond Mountain played an in-
teresting part, all the more important as a large suite of rocks from one
locality whose field relations were known were subjected to a most careful
mineralogies! study. The isolation of the hypersthene from augite was
accomplished by means of a solution of cadmium-boro-tungstate having a
specific gravity of 3 '3 9. Repeated treatment with the solution yielded a
brown hyperstheue carrying a small amount of green augite. Under the
microscope the former proved to be orthorhombic in form and strongly
pleochroic, the latter monoclinic and without pleochroism. In the greater
part of the pyroxene-andesites of Richmond Mountain the hypersthene was
always found to be in excess of the augite, the prevailing minerals being
hypersthene and anorthite, accompanied by labradorite and possibly other
plagioclase feldspars, together with varying amounts of augite and
magnetite.

Basalts.— Under basalts are included those volcanic rocks which have for
their essential ingredients plagioclase augite and magnetite. Olivine, which
occurs as a common accompaniment in varying proportions, in many vari-
eties, is, however, too frequently wanting to be rigidly regarded as an
essential constituent. The basalts form the most basic of all natural groups
into which the volcanic lavas have been divided. At Eureka they present,
for the wide field which they cover and the great number of their extru-
sions, a uniform appearance, and, although characterized by a large amount
of glass base, may be regarded as typical of many localities in Nevada.
It is fine grained and compact, frequently passing into vesicular forms, with
but few macroscopic secretions, and by far the greater part of it grayish
black in color. Olivine occurs in large grains and in such quantities as
occasionally to modify the external character of the rock ; yet over broad
areas it is wholly wanting, the microscope failing to detect its presence in

1 Notes on the Volcanoes of Northern California, Oregon, and Washington Territory. Hague
and Iddings. Am. Jour. Sci., September, 1883, vol. 26, pp. 222-235.

'Notes on the Volcanic Bocks of the Great Basin. Hague and Iddiugs. Am. Jour. Sci.,
Jane, 1884, vol. 27, pp. 453-463.

DISTEIBDTION OF LAVAS. 243

many thin sections. Hypersthene is wanting in the normal basalts, and if
present is only recognized in the intermediate rocks between typical pyrox-
ene-andesite and basalts. An exceptionally fine display of these intermedi-
ate rocks makes this group of special geological importance at Eui-eka. A
discussion of these transition rocks is reserved till later in the chapter,
when treating of the relations of the different groups to each other.

Manner of Occurrence of Volcanic Lavas.— In the Eureka District there are 11O

grand craters through which the greater part of the lavas reached the surface
and from which volcanic energy receding from centers of igneous action
gradually decreased in intensity and finally died out altogether. On the
contrary, the igneous rocks consist for the most part of extrusive lavas that
have poured out through numerous vents scattered over the volcanic area,
many of the outbursts being very limited in extent. At first sight it might
seem impossible to recognize any order in their distribution, so irregularly
do they appear to break out in most unexpected places. Further observa-
tion, however, shows how dependent these outbursts are upon the pre-
existing orographic structure a knowledge of which is absolutely necessary
to a thorough understanding of the volcanic phenomena.

As regards their mode of occurrence, all the lavas may be classed under
four heads: first, and most important, they break out along the three great
meridional and approximately parallel lines of displacement, the Hoosac,
Pinto, and Rescue faults; second, they border and almost completely
encircle the large uplifted masses of sedimentary strata, like the Silverado and
County Peak block; third, they occur in numerous narrow dikes penetrat-
ing the limestones, but for the most part confined to Prospect Ridge, and,
fourth, they occur in one or two relatively large bodies, notably Richmond
Mountain and Pinto Peak, along lines of displacement already mentioned.
Richmond Mountain is situated at the junction of the Pinto and Rescue
faults, while the lava of Pinto Peak has been piled up along an oblique
fault, which runs from the Hoosac to the Pinto fault, and which separates
Spring Hill from Carbon Ridge. It is along the lines of these latter
faults that the most powerful volcanic activity has been displayed. As
described elsewhere, these two faults are situated respectively on the east
and west sides of the depressed block of Carboniferous rocks lying between

244 CKOLOGY OF THP] EUEEKA DISTRICT.

the Cambrian and Silurian of Prospect Ridge on the one side, and the
Silurian and Devonian of County Peak and Silverado Mountain on the
other. These profound faults, as already described, show nearly 2£ miles
of vertical displacement. Fissures accompany these faults and through
them vast masses of lavas have reached the surface and poured out along
both sides of the fault planes. In places the lavas are found only in narrow
belts without any great accumulation of material and in others they are
piled up in rounded hills and knolls of irregular outlines, concealing every-
thing beneath them for long distances. In general acid lavas accumulate
near their source of eruption while basic lavas, owing to their greater fluid-
ity, show a tendency to flow from their vents in broad masses. Along the
Hoosac fault from Fish Creek Valley to its junction with the Ruby Hill
fault, a continuous body of acid lavas, either rhyolites or hornblende-aude-
sites, follow the course of the fault, but beyond their junction no lavas come
to the surface along the Hoosac, although they persistently follow the course
of the Ruby Hill fault as described in detail in the chapter devoted to the
discussion of the ore deposits of the District. Along the latter fault the
rhyolites do not form a continuous surface overflow, but break out in
isolated knolls all the way from its junction with the Hoosac to the Jackson
fault, beyond which, on Ruby Hill, the lavas never reach the surface,
although their presence is shown by underground mining galleries. West-
ward from the Hoosac fault volcanic energy slowly died out.

Following the trend of the Pinto fault from the southern end of the
mountains, rhyolitic pumices and tuffs define the general course of displace-
ment all the way to Dome Mountain. This light porous material has, by
gradually welling out along the fissure, heaped up bosses and knolls of vol-
canic products, but owing to their peculiar physical habit they have eroded
more easily than the denser rocks and present a more broken, undulating
surface. Isolated outbursts of pumices occur, penetrating the sedimentary
strata on both sides of the fault, and along Wood Valley extend far back
into the Devonian limestones. Northward of Dome Mountain the pumices
and tuffs again come in, but are finally lost beneath the imposing mass of
pyroxene-andesites of Richmond Mountain. Along the Pinto fault only

FISSUKE ERUPTIONS. 245

one occurrence of andesite is known, but on the other hand numerous
rounded bosses of basalt occur on both sides of the fault line.

The Rescue fault, as regards the amount of displacement, is a less
profound one than the others just mentioned, but it is as sharply outlined,
and owing to the pre-Tertiary erosion of Rescue Canyon exhibits quite as
striking an occurrence of volcanic outbursts. The erupted material does
not follow the entire line of the fault, but is confined to its southern end,
from Island Mountain southward, crossing Silverado Canyon and following
along the east side of Rescue Canyon. It presents a most remarkable body
of erupted material nearly 2 miles in length, seldom exceeding 200 or 300
feet in width. Starting in at the base of the escarpment upon the east side of
Island Mountain, along which runs the fault plane, it descends gradually
for 750 feet to the open country of Fish Creek Valley. The extravasated
material is wholly composed of rhyolite so uniform in appearance and com-
position, and so characteristic of the region, as to well deserve the designa-
tion of the Rescue Canyon rhyolite. Rhyolitic pumices and tuffs, which
occupy the valley near the base of the mountains, conceal the denser rock
and obscure all structural features.

Subordinate to the eruptive outbursts along these three great fissures
and to the west of the Hoosac fault, occur two other narrow belts of
igneous rocks similar as to their geological position, but far less important
as lines of eruptive energy. They are found on the west side of the
southern end of Prospect Ridge and penetrate into the mountains from Fish
Creek Valley. The most easterly outbreak occurs along Sierra Valley.
Just west of it in Gray's Canyon, on the west side of South Hill, lies the
second line of lava extrusions. The lava thrown out along these secondary
faults is restricted in amount, bearing some relation to the importance of the
orographic displacements.

Closely related to these north and south lines of eruption occur extrav-
asated masses, completely surrounding the uplifted blocks. It is evident
that they follow lines of orographic fractures, more or less profound,
although the amount of displacement can seldom be determined. In
some instances it is quite possible to estimate the faulting, but as a rule
these lines of east and west orographic fractures are completely obscured

246 GEOLOGY OF THE EUREKA DISTRICT.

by recent Quaternary accumulations or else buried beneath broad masses
of lava. These latter overflows of lava, breaking out along the base of
the escarpments, follow a somewhat sinuous course, yet cling most per-
sistently to the border line of the uplifted area, and vary greatly in the
amount of extravasated material and the manner in which they pile up at
the surface along Ikies of fracture.

The most striking illustration of this mode of occurrence may be
seen in the lavas surrounding the County Peak and Silverado block.
Along the Pinto fault, which defines these mountains on the west, the lavas
closely follow the fault, except for the short distance, already mentioned,
northeast of Dome Mountain. Richmond Mountain encircles the block on
the north, followed on the northeast by the broad basaltic flows of Basalt
Peak and the Strahlenberg, which lie between County Peak and the Dia-
mond Range. To the east every indication points to the occurrence of lavas
beneath the alluvial deposits of Newark Valley, while the Rescue Canyon
overflows continue southward to the open valley, completing the circuit in
this direction. Facing Fish Creek Valley, bosses and knolls of both acid
and basic lava stretch westward in sufficient number to plainly suggest a
continuous outbreak of igneous material along the southern base of the
uplifted block of Silurian and Devonian limestones.

In the case of the depressed Carboniferous area the bordering lines
of lava following the Hoosac and Pinto faults are clearly made out.
Between the Spring Hill and the Carbon Ridge bodies there is a sharp
break in the Carboniferous strata, along which acidic lavas have broken
out, crossing obliquely from the one great fault to the other. On the north
side of the Spring Hill body, Richmond Mountain cuts off everything to
the northeast. The volcanic pumices and tuff stretch westward under the
town of Eureka and terminate finally in the rhyolites of Purple Hill. To
the south of the Carboniferous area pumices and tuffs abut against the base
of Carbon Ridge, skirting the foothills, unless concealed beneath recent
deposits. South of Gray Fox Peak and Carbon Ridge there is a long line
of secondary ridges, now completely covered by Quaternary deposits. It
is easily seen that their trend is wholly out of accord with the line of the
Paleozoic uplifts, but is precisely what we might expect to find if the lavas

OCCUKRENCE OF DIKES. 247

had poured out along the foothills and encircled the terminal spurs of the
upturned Paleozoic beds. Pumices and tuffs again come to the surface
along the southern end of the Pogonip beds, skirt Devonian limestone upon
the south side of South Hill, and thence, penetrating the mountains, follow
up Grays Canyon on the west side. By reference to the atlas sheets it will
be readily seen that the lavas border the depressed areas of Carboniferous
rocks lying between the two great faults in as forcible a manner as they do
in the case of the elevated County Peak and Silverado orographic block.

intrusive Dikes.— Dikes of andesite, rhyolite, and basalt penetrate the strata
in a number of localities, for the most part, except in the case of rhyolites, in
close proximity to the principal lines of volcanic activity. That they
possess the same deep-seated origin with the larger bodies seems evident
from their position and similarity of petrographical characters, their mode
of occurrence clearly suggesting that they are merely offshoots from parent
magmas. The erupted material was forced upward into narrow fissures
and fractures, following lines of least resistance. In their geographical
distribution they present some striking differences, andesitic dikes being
found only to the west of the Pinto fault, and for the most part confined
to Cambrian and Silurian rocks of the Prospect Ridge uplift, whereas
basaltic dikes arrange themselves around the County Peak and Silverado
Mountain body. Rhyolite dikes, while they may break out anywhere
along lines of displacement, offer a marked geological feature of Prospect
Ridge, the eastern slope being cut by a network of intrusive bodies.
They vary from thirty feet to a few inches in width, and trend at all angles,
some of them agreeing with the strike of the beds, while a few, notably the
Geddes and Bertrand dike, cross the strata nearly at right angles to the
course of the main ridge. The Ruby Hill fault-plane is coincident with a
narrow fissure, into which the rhyolitic magma has forced an entrance for
the greater part of its length, forming the most persistent dike of any in
the region. In the neighborhood of the Dunderberg and Hamburg mines
numerous outbursts of rhyolite have reached the surface. Notwithstanding,
however, the great number of these dikes, none appeal- to have penetrated
the strata along the top of the main ridge, and in no single instance have
lavas built up any considerable knob or hill on the surface. It is quite

248 GEOLOGY OF THE EUREKA DISTRICT.

impossible that such knobs should have been formed and later have been
removed by erosion without leaving some evidence of overflow along the
line of the dikes.

This system of dikes upon Prospect Ridge presents certain geological
characteristics of interest bearing upon the mode of occurrence of erupted
material. Throughout they show a great similarity in mineral composi-
tion and petrographical habit, and when fresh in every way resemble the
unaltered rocks along the Ruby Hill fault. These latter lavas have been
shown elsewhere to have been erupted at the same time and under similar
conditions with the rhyolite of the Hoosac fault. Indeed, the Ruby Hill
fault is simply a prolongation of the main fault. Evidences of alteration
and metamorphism of the limestones and shales through which the erupted
material passed are by no means easy to detect, the encasing walls showing
scarcely any evidence of the effects of heat derived from ascending lava
currents. These dike rocks being narrow bodies have cooled rapidly and
imparted little heat to the limestones. Mining exploitations have frequently
encountered these intrusive bodies hundreds of feet below the surface, but
neither at the top nor underground do they exhibit structural features in
any way different from the larger bodies. The only marked feature in
which these dike rocks differ from the extrusive lavas of Pinto and Gray
Fox peaks is shown by the absence of flow structure due entirely to their
manner of occurrence, and in no way dependent upon either their chem-
ical or mineralogical composition. As regards the degree of ciystalliza-
tion, they exhibit characters identical throughout and similar to the material
erupted at the surface along the principal lines of faulting.

On all the great lines of orographic fracture along which both acid
and basic lavas have emanated, the amount of volcanic material reaching
the surface has varied greatly at different points. In certain localities
they have piled up to such an extent as to form prominent hills and
landmarks, but their mode of occurrence is precisely the same as those
where the lavas have only accumulated in narrow belts along the fissures.
Such masses as Pinto Peak, Purple Hill, and Gray Fox Peak are similar
piles of lava, uniform in character, only varying in size according to the
amount thrown out at each locality. In the same way Richmond Moun-

RELATIVE AGE OF LAVAS. 249

tain is a vast accumulation of pyroxene-andesite similar in its geological
occurrence to the smaller hills of basalt which have broken out at numer-
ous points along the fractures caused by the elevation of the County Peak
and Silverado block.

•

Mr. Clarence King, in summing up the observations of the geologists
connected with the Geological Exploration of the Fortieth Parallel upon
the mode of occurrence of the rhyolites between the Sierra Nevada and
Wasatch ranges, makes the following concise generalization:

Where a great mountain block has been detached from its direct connections
and dropped below the surrounding levels, there the rhyolites have overflowed it and
built up great accumulations of ejecta. Wherever the rhyolites, on the other hand,
accompany the relatively elevated mountain blocks, they are present merely as bor-
dering bauds skirtiug the foothills of the mountain mass. There are few instances
in which hill masses were riven by dikes from which there was a limited outflow over
the high summits; but the general law was, that the great ejections took place in
subsided regions.1

Nowhere within the Great Basin does this description hold true with
greater force than in the Eureka District. It holds true, however, for the
entire hornblende-andesite and dacite groups, as in their mode of occurrence
they can not be separated from the more acidic lavas. It holds equally
well for the pyroxene-andesites, since such broad masses as make up Rich-
mond Mountain are simply relatively large accumulations of lavas at cen-
ters of great dislocation in highly disturbed regions in every way similar to
those of other lavas. In the case of the hornblende-andesites and rhyolites
they have poured over and nearly submerged a depressed sedimentary
region, whereas the rhyolites, pyroxene-andesites, and basalts, which have
broken out in proximity to the Silverado and County Peak region, appear
more as an encircling belt to a relatively elevated country.

Relative age of Volcanic Rocks.— In the Eureka District the hornblende-
andesite and the closely related hornblende-mica-andesite are the earliest
of the Tertiary lavas, all others with which they are associated being found
either to break through or overlie them. Hornblende-andesite, wherever it
occurs in the district, is a crystalline rock and forms a central body, which,
by insensible transitions, passes into a rock with a more and more glassy

'U. S. Geol. Explor. 40th Par., 1878, vol. I, Systematic Geology, p. 6W.

250 GEOLOGY OP THE EUREKA DISTRICT.

base until it becomes a characteristic audesitic-pearlite. As the andesites
and pearlites become more and more acidic the rock gradually passes over
into dacite, the eruptions of which usually occur in obscure hills and low
ridges, and although covering comparatively restricted areas are clearly
seen to overlie the hornblende-mica-andesite in all the local centers of
eruption wherever the two rocks are observed together. In the neighbor-
hood of South Hill, where the largest exposures of dacite have been
observed, they rest superimposed against the andesite, and at Dry Lake,
where, however, only a small body of dacite is known, it is evident that
a similar sequence of flow was maintained.

In low hills near the entrance to Sierra Canyon northeast of South
Hill instances may be seen of finely banded rhyolite lying in direct super-
position upon good exposures of dacite. This dacite, though a moderately
compact rock, possesses in places a pumiceous texture and in a marked
degree strongly resembles many forms of rhyolite, but especially the variety
with which it is here associated. Both rocks are highly acidic, but the
dacite is richer of the two rocks in mineral secretions and is character-
ized by a great abundance of laminae of biotite. In the few areas where
both rocks occur together in such a way that their relations can be made
out, the rhyolite has been the last to reach the surface.

The district affords abundant and frequent evidence of the relative
geological position of andesite to rhyolite. Not only is this shown by
the relationship between the rhyolites and dacites, but over much more
extended areas the rhyolite encircles and overlies the andesite, filling in and
smoothing out the accidented surface of the older rock, which in turn may
occasionally be seen in isolated exposures rising above a broad expanse of
superimposed rhyolite. Further and conclusive evidence is found in the fre-
quent dikes of rhyolite penetrating the hornblende-mica-andesite in several
places adjoining the Hoosac fissure. The rhyolitic pumices, tuffs and allied
rocks appear in many instances to have preceded the more highly crystalline
compact rhyolites represented by the typical Rescue Canyon and Pinto
Peak rocks.

While it is by no means evident that all the overflows of pumice
broke out before the denser rock, yet there is ample proof that long and

RELATIVE A(JE OE LAVAS. 251

continuous bodies spread out over wide areas of country, especially along
the line of the Pinto fault, before the great bodies of the latter were forced
to the surface. Rhyolites occur breaking through the pumices, overflowing
and occasionally concealing them from view, except where the softer rock
is exposed by deep cuts along drainage channels. In some instances the
pumices lie superimposed upon denser rock, evidently of later age. It
seems most probable that throughout the duration of rhyolitic eruptions
conditions were at all times more or less favorable for the pouring out of
pumices and tuffs, and that outbursts of similar material began and closed
the rhyolite period. The conditions governing the physical characteristics
of the erupted material seem in a great measure to have been dependent
upon their relations to certain local centers of volcanic activity.

Along the .Pinto fault, wherever the acidic lavas have piled up,
pumices occur as the prevailing rock, and the same holds true along the
lines of displacement bordering the elevated mountain masses. Normal
crystalline rhyolite, on the other hand, characterizes the Hoosac fault and
breaks out wherever these lavas penetrate into the interior of the mountains
along fissures and lines of least resistance. They frequently reach the
surface in small isolated bodies in the most distant and unlooked-for places-
The Rescue fault is an instance of rhyolite penetrating into the very
center of the mountains, and the pumices and tuffs on the south side of the
Silverado Mountains offer a fine example of the pouring out of the latter
along the outer edge of an uplifted orographic block. Following lines of
least resistance they connect the rhyolites of the Rescue with those of the
Pinto fault.

When it comes to determining the geological relations of pyroxene-
andesite to hornblende-andesite and allied lavas, no direct superposition can
be found, nor are there any instances of dikes of one rock breaking through
an earlier body of the older rock. The main bodies of hornbleiide-andesite
and pyroxene-andesite are, as regards geological position and geographical
distribution, quite distinct.

Absence of direct evidence as to the relative age of the two large
groups of andesite may be explained satisfactorily by the fact that only one
body of pyroxene-andesite occurs in the district and this one, although

252 G-EOLOGY OF THE EUREKA DISTRICT.

covering an extensive area and of great thickness, has no outlying
exposures. The Cliff Hills which lie beyond the limits of the Eureka
Mountains, present a grand exposure of pyroxene-andesite, but as they stand
alone afford no evidence as to the relations of the different lava flows to
each other. Along the base of the escarpment, which forms the south side
of Richmond Mountain, occurs a contact over a mile in length, between
pyroxene-andesite and rhyolitic pumices, yet nowhere along this line has
the sequence of eruption been definitely determined by actual contact.
Evidence fails to show whether the pyroxene-andesite broke through the
pumices, which, on account of their friable nature have suffered more or less
erosion, or whether the latter banked up against a preexisting wall of the
former. For the greater part of the distance the junction of the two rocks
is completely obscured by both large and small blocks of. andesite, which
have fallen from the cliff above, and wherever these are wanting the con-
tact is hidden by fine friable pumice and ash, which has accumulated in
considerable thickness along the base of the escarpment, piled up by the
prevailing westerly winds. Although no actual superposition is seen, all
indirect evidences point so strongly to the true order of succession that the
fact seems well established that the pyroxene-andesite followed the rhyo-
lite.

Between the pyroxene-andesites and basalts there exists the closest
possible relationship, so much so that it is by no means an easy matter to
establish a sharp line between them, either in mineral composition or field
occurrence. Unlike pyroxene-andesite, however,- the outbursts of basalt
present a considerable diversity in their mode of occurrence and distribu-
tion, forming broad table-like masses and numerous small extrusions in dikes
and rounded knolls. Although the two rocks are closely related by transi-
tion products, extreme typical forms may easily be distinguished from each
other by both geological and petrographical features of rock masses, and
as to their order of succession there exists, fortunately, abundant proof to
show that the pyroxene-andesite preceded the basalt. Evidences of their
relative age may be seen on the summit of Richmond Mountain, where
several dikes of dense glassy basalt cut the andesite in sharply defined lines

SEQUENCE OF LAVAS. 253

of contact, and at several localities near the outer edge of the andesitic body,
notably just east of the town of Eureka.

Now, the relationship as regards age between the basalts and rhyolites
is placed beyond all question, numerous dikes of the former cutting the
latter both along the Pinto fault and in the pumice basin southwest of Rich-
mond Mountain. Hornitos Cone, about 400 feet in height, an isolated hill
rising abruptly out of the basin, is an excellent instance of the cutting of
rhyolite by basalt dikes. The cone is composed of light colored pumices,
broken through and ribbed on all sides by black basaltic dikes, which have
altered the siliceous rocks all along the lines of contact. Crater Cone, on
the east side of Richmond Mountain, affords an equally good example of
the relative position of the two rocks, the basaltic lavas which here form
the Cone flowing for long distances over the earlier pumiceous beds. Mag-
pie Hill, near the entrance to Rescue Canyon, affords still another equally
as good an illustration of the relative position of the two rocks.

If the pyroxene-andesite overflows preceded the rhyolite it would
hardly have been possible under the conditions of eruption for them not to
have broken out along some of the hornblende-andesite centers before the
appearance of the rhyolites. Again, if the rhyolites followed the pyroxene-
andesite there should be found some field evidences of such eruptions
between the pyroxene-andesite and basalt, whereas, on the contrary, there
exists not the slightest evidence of an overflow of acidic lava intervening
between the closely related basic lavas. It has already been pointed out
that the acidic lavas hold the same close relationship to each other.

Field observations clearly show that the order of succession of these
natural groups into which the lavas have been divided was as follows :
First, that the hornblende-andesite was the earliest of all the erupted
material ; second, that the homblende-mica-andesite followed the hornblende-
andesite; third, that the dacite followed the hornblende-mi ca-andesite;
fourth, that the rhyolite closely followed the dacite; fifth, that the pyroxene-
andesite succeeded the rhyolite; sixth, that the basalt was the most recent
of all volcanic products.

TWO Magmas of Eruption.— A study in the field of the geological distribu-
tion and mode of occurrence of the igneous rocks, shows that they all belong

254 G KG LOGY OF THE EUREKA DISTRICT.

to one or the other of two well defined groups, in each of which the lavas,
although possessing a wide range in chemical composition, are so intimately
related and so interdependent as to suggest that they must necessarily have
been derived from some common source. In other words, all lavas at Eureka
may be divided into two sharply contrasted groups, the one acid, and the
other basic.

A microscopical examination in the laboratory of a large amount
of material collected in the field in the opinion of the writer
lends support to this view of two magmas. It is brought about by
a study of the gradual transition in mineral composition and by cer-
tain peculiarities of structure and crystallization characteristic of each
magma. The acid magma was the earlier in age, the eruptions begin-
ning with hornblende-andesite and closing with the extreme acidic forms
of rhyolite. In general the lavas of this acid series are light in color,
the microcrystalline groundmass being composed for the most part of an
aggregation of feldspar and quartz grains without the accompaniment of
ferro-magnesian silicates. The hornblendes play no part in the composi-
tion of the groundmass, being present as porphyritic secretions, whereas the
pyroxenes, in the few instances where they have been recognized, at the
basic end of the series do not occur as porphyritic minerals, but only in
minute microlitic forms developed in the groundmass. The glass is always
highly acidic.

Sharply contrasted with these acidic lavas the basic lavas are char-
acterized by a predominance of the pyroxenic minerals, the prevalence of
lime-soda feldspars and the structural features of a groundmass peculiar either
to pyroxene-andesite or to basalt. The basic magma came in with pyroxene-
andesite and closed with numerous outflows of basalts. The two magmas
so sharply defined by mineralogical and structural distinctions may be
designated respectively as the feldspathic and pyroxenic magmas. A dis-
cussion as to their nature will bring out still more clearly their diagnostic
points of difference and the importance of this division in its bearing upon
the origin of the sequence of lavas. Further on in this chapter it will be
maintained that both these magmas are simply differentiated products of
an earlier homogeneous molten mass.

FELDSPATHIC MAGMA. 255

Feidspathic Magma.— Up to this point the composition of the rocks has
been but little considered except as regards the mineral constituents of inde-
pendent lava flows ; it is necessary now, however, to look at them from the
standpoint of a series of successive eruptions in order to understand their
interdependence and geological relations. Normal hornblende-andesite, the
earliest and most basic portion of the feldspathic magma, passes over with-
out any recognizable physical break into hornblende-mica-andesite by the
coming in of hexagonal plates of biotite which gradually increase in amount
until they become the most prominent of the ferro-magnesian minerals and
at the same time by insensible gradations the hornblendes decrease. Grad-
ually the lava grows more and more acidic and quartz grains are developed
in the groundmass, but at first not in sufficient force to be regarded as an
essential constituent. The presence or absence of quartz is also governed
in great measure by the degree of crystallization of the magma, a highly
crystalline structure carrying more individual secretions than one where
silica is largely absorbed in glass. With the increase of quartz the horn-
blende continues to diminish and the rock passes over into dacite, the biotite
apparently holding its position with an occasional hornblende.

In dacite, quartz has become an essential mineral. With a still larger
increase of the silica percentage orthoclase appears in broad and well devel-
oped crystals. Hornblende disappears entirely and in the normal varieties
of rhyolite the biotite is rarely seen and then only as an accessory min-
eral ; the feiTO-magnesian minerals are wanting. At the basic end of the
feldspathic series of lavas, labradorite and anorthite have been determined
by their optical pi-operties, but the predominating feldspars are apparently
oligoclase. By insensible gradation the lime-soda feldspars pass away.
Orthoclase, in most of the basic rocks, is entirely wanting, making its
appearance by degrees until at the acid end of the series it occurs as the
prevailing feldspar, although some species of plagioclase is nearly always
present. At one end of this series of eruptive material the essential min-
erals are hornblende and one or more species of lime-soda feldspars; at the
other, quartz and orthoclase.

Pyroxenic Magma.— The basic or pyroxene lavas began by the pouring out
of large masses of the Richmond Mountain pyroxene-andesite. Successive

256 GEOLOGY OF THE EUREKA DISTRICT.

changes in the mineral and chemical composition of this magma are by no
means as easy to follow through the different flows as in the sequence of
outbursts of the acidic products. Nevertheless, investigation shows as com-
plete a range in composition of the erupted material, even where it is
impossible to determine the relative age of the flows accompanying such
changes. In some instances in the more crystalline acidic varieties it has
been pointed out that both hornblende and mica occur as porphyritic secre-
tions, although as accessory constituents, hypersthene being the predom-
inant mineral. In lavas slightly more basic the former minerals are want-
ing; hypersthene still plays the part of the prevailing ferro-magnesian
silicate, accompanied by relatively small amounts of augite, while in rocks
still more basic augite is recognized as the predominant pyroxenic mineral,
accompanied by an increasing development of magnetite. By insensible
gradations a series of hand specimens and rock sections show that so far as
mineral constituents are concerned the pyroxene-andesites pass over into
basalts. While the rock masses of both lavas may be readily distinguished
in the field by marked differences in physical aspect, it is by no means
easy on a superficial examination to refer correctly from hand specimens
certain varieties which approach each other in structure and composition.
Mineralogically no sharp distinction can be drawn between intermediate
varieties, but careful investigation of the Eureka rocks brings out certain
differences which not only hold good for this region, but probably for other
areas in- the Great Basin. While observation, as already mentioned, offers
abundant evidence as to the position of the pyroxene-andesite to the basalts
and divides these closely connected rocks upon geological grounds, based
upon their relative age, the microscope in a marked manner corroborates
the distinctions made in the field. Mr. Iddings, who has submitted a large
number of thin sections of both pyroxene-andesite and basalt to micro-
scopical investigation, is able to substantiate by structural peculiarities of
the groundmass the geological divisions observed. He finds that all those
rocks which have been classed as pyroxene-andesite possess their own micro-
structure, characterized by the felt-like structure of the groundmass which
has been so frequently noticed elsewhere. The typical basalts present in

OLIVINE IN HAS ALT. 257

their structure a uniform grotmdmaaa made up of coarse-grained aggrega-
tions of feldspar and augite, imbedded in a globulitic glass base.

Nearly all the rocks of intermediate mineral composition possess the
basaltic habit. Hypersthene is wanting in the normal basalts. Augite and
magnetite, although essential minerals in the composition of both rocks, occur
much more abundantly in basalts. With one exception the microscope has
failed to detect olivine in any thin section of the lavas classed as pyroxeue-
audesite, the exception, however, furnishing quite a remarkable rock, and
one that might with some reason be placed among the basalts. It occurs
in an obscure exposure or knoll in Fish Creek Valley just west of Cliff
Hills, and from its association, and still more from the fact that its ground-
mass structure bears the closest relation to adjoining rocks, it has been
referred to the pyroxene-andesites. Although olivine is absent from the
pyroxene-andesites of the district, it will not serve, as has been suggested,
as a mineralogical distinction to separate the two natural groups, inasmuch
as over large basaltic areas it is wholly wanting. Moreover, within limited
areas, and apparently in the same flow, it may be present at one point and
wanting in another, occurring so irregularly disseminated through the rock
that any attempt to separate the basalts themselves into two divisions on a
basis of olivine seems futile. In an abstract of the geology of the Eureka
District published in 1883 this relationship between the olivine and basalt
was clearly pointed out.1 Since then it has been shown that olivine is
absent in numerous basaltic lavas of the Great Basin.2 Mr. George F.
Becker3 has recently arrived at the conclusion that olivine can not be used
as a basis of division for the basalts along the sierra of California.

characteristic Basalts.— It is well to mention two other marked peculi-
arities of these basalts — one, the very varying amount of silica which
they carry; the other, the very high percentage of silica contained
in the rock as compared with the most basaltic flows elsewhere. In
their chemical composition nearly all these rocks possess far more silica

1 Third annual report of the Director of the U. S. Geological Survey, 1881-'82.

a Arnold Hague and Jos. P. Iddiugs: Notes on the volcanic rocks of the Great Basin. Am. Jour.
Sci., June, 1884, p. 157.

"Geology of the quicksilver deposits of the Pacific Slope. Monograph XIII, I'. S. Geol. Survey.
1888, p. 157.

MON XX 17

258

GEOLOGY OF THE EUKEKA D1STKICT.

than is ordinarily supposed to occur in normal basalt, the amount reaching
as high as the percentage found in many andesitic rocks, and in some
instances equaling the amount in the pyroxene-audesite of Richmond
Mountain.

oiivine in Basalts.— In order to determine the amount of silica present in
these rocks and its relationship to oiivine, a number of chemical analyses
were made from specimens which field observation and a study of thin
sections had shown to belong to basalt. The subjoined table gives the
result of ten such chemical examinations, arranged in order according to the
silica percentage obtained. The presence or absence of oiivine in the thin
sections of the same rocks, as determined by the microscope, is also given
in the table.

Number.

Silica.

Oiivine.

49.23

Rich in macroscopic secretions.

51.86

Rich in microscopic secretions.

57.42

Abundant in microscopic secretions.

58.06

Easily recognized under the microscope.

58.26

Only a trace.

58.60

None detected.

58.64

Detected under the microscope.

59.51

None detected.

59.64

None detected.

60.11

None detected.

No. 1. South of Alhambra Hills. — This rock occurs as a low hill rising out of the Quaternary
plain, and isolated from all other volcanic outbursts. It is a highly crystalline rock.

_Yo. 2. Dike northeast of summit of Richmond Mountain. — An intrusive body penetrating the pyr-
oxeiie-andcsiti-.

No. 3. East of Basalt Peak. — A vesicular black basalt.

No. 4. liasalt Cone. — A compact dark rock characteristic of a large area of country.

No. 5. Basalt Peak. — A compact rock passing into vesicular varieties.

A'o. 6. West base of Richmond Mountain, near the town of Eureka. — A grayish red vesicular rock
lying between pyroxeuc-andcsite and earlier rhyolitic tuffs.

No. 7. West of Basalt Peak. — It occurs on the broad saddle just west of the peak, not far from
the groat body of Devonian limestone, and is a characteristic rock rich in glass base, black in color,
mottled with gray.

No. S. West bate of Richmond Mountain, nut far from the town of Eureka. — In its geological rela-
tions it is quite similar to No. 6. It is found breaking through rhyolitic tuffs and is a compact dark
rock with a characteristic basaltic habitus.

No. 0. A dike from the summit of Richmond Mountain. — Under the microscope the rock resembles
No. 2, which occurs not far distant, penetrating the same body of andesite under precisely similar
geological conditions. This rock, however, is much richer in glass and correspondingly richer in
silica. It is black in color, without macroscopic secretions, and has a decidedly conchoidal fracture.

OUYINK IN P.ASALT. 1>,V.)

No. 10. Went of Tiill llnad, went of Dome Mountain. — It ooeun ac one of the largest extniitlonB of

basalt along the, Piuto fault. The bcoad mass lies iu contact with hornblende -andesite, and Hows
from the same body arc tteeii to directly overlie rhyolitie tuffs. It in exceedingly rich iii glass, "nd so
mottled as to present a gray color. Although the highest on the list in the percentage of silica, if
possesses a Strongly marked basaltic habitus, quite as characteristic under the microscope as in the
hand-specimen.

It will be seen, with the exception of numbers one and two, that the
silica percentage in all the rocks is higher than is usually found in basalts ;
they show between the two extremes on the list a variation in silica of 10'88
per cent.

Although olivine is not an essential constituent in the basalts, the above
table shows how close a relationship exists between the olivine bearing
and olivine free varieties, and a study of the localities and their mode of
occurrence demonstrates how futile an}' attempt would be to try to sep-
arate them on the presence or absence of this mineral. In the hill south
of Alhambra Hills, the silica is low, while the olivine is present in com-
paratively large secretions. In the dike from the summit of Richmond
Mountain, the second in the table, there is an increase in the amount of
silica of over 2'5 per cent, with a large falling off in olivine. From the
rocks with 58 to 59 per cent of silica, there is only a small and varying
quantity of olivine, while in the three specimens which gave over 59 per
cent of silica the microscope failed to detect its presence.

Sufficient facts have been adduced to indicate how intricately the
entire series of pyroxenic rocks are related to each other throughout a wide
range in their composition. Throughout this entire group of extravasated
lavas the essential minerals remain the same, the differences consisting
for the most part in their relative proportions and the accompanying
modifications of groundmass structure. This holds true in a still more
striking manner if we exclude the extreme acidic end of the series where
the hornblende and mica play the part of accessory minerals. Some of
the basaltic masses determined as such by geological position and structural
peculiarities have been found in several instances, usually the more glassy
varieties, to be more acidic than the pyroxene-andesites, the two natural
groups overlapping each other as regards their composition. The sudden
changes which all these pyroxeuic lavas apparently undergo from crystalline
to glassy varieties is one of the marked peculiarities of the Eureka District

260 GEOLOGY OF THE EUKEKA D1STKIOT.

and with these changes occur more or less variation in both mineral and
chemical composition.

Age of Pyroxene-andesites Elsewhere.— Similar Surface flows of pyTOXeiie-

andesites occur at numerous localities in the Great Basin, all the way from
the Sierra Nevada Range to the Salt Lake Desert, although not always in
as large bodies as Richmond Mountain, nor always associated with basalts.
They are best shown along the TruckeeCanyonin the Virginia Range, and
in the Augusta, Cortez, and Wahweah ranges. In the Wahweah Range lavas
which were considered by Prof. Zirkel as augite-trachytes can not be dis-
tinguished from the Richmond Mountain rock in any of their petrograph-
ies] features. In the opinion of the writer many bodies of lava which
formerly were classed as augite-trachytes, augite-andesites, and basalts,
properly belong- to this group of pyroxene-andesites, and in some instances
rocks which had been determined as rhyolite from the fact that they were
supposed to cany large amounts of sanidine have within recent years been
shown to belong to this same natural group.

The Eureka District offers no positive direct evidence from super-
position of the relative age of the hornblende-andesite and pyroxene-
andesite, but this apparent break in the chain of evidence is more
than made good elsewhere, inasmuch as pyroxene-andesites of the
Richmond Mountain type have been observed breaking through
hornblende-andesites not unlike those found along the line of the
Hoosac fault. Similar volcanic rocks, as regards porphyritic secre-
tions and groundmass structure, have been described by Mr. S. F.
Emmous' as cutting through and overlying the homblende-andesites in the
Augusta Mountains, both in the region of Crescent Peak and Antimony
Canyon. In the Truckee Canyon, rocks which have been called augite-
andesites can not be distinguished from those of Richmond Mountain.
They were observed by the geologists of the Fortieth Parallel Exploration
to break through sanidine-trachytes (hornblende-mica-andesites) and were
regarded by them at that time as an exception to the natural order of
succession, all andesites being supposed to be older than the so-called
trachytes. Along the walls of the same deep gorge and in its lateral

•U.S. Geol. Explor. 40th Par., vol. 11, p. 654.

ANDESITE LATER THAN RHYOLITE. 261

branches pyroxene-andesite is exposed overlying rhyolite1 and for the same
reason was regarded as an anomalous occurrence, whereas it is now evident
that it belongs more properly to that group of pyroxene-andesite which
is found associated with and passing over into basalt. Inasmuch as
it distinctly overlies the adjoining rhyolite it was designated on the
geological maps of the Fortieth Parallel Exploration as basalt, although
in the text mention was made of its andesitic character. At Jacob's
Promontory, in the Shoshone Range, a body of lava which had been
determined as rhyolite has also proved on further examination to be
allied to pyroxene-andesite, and here, as at Eureka, it is found associated
with basaltic flows, although of earlier age but overlying typical rhyolite.
Numerous localities might be mentioned where similar pyroxene-andesites
occur, but their relationship with neighboring rhyolites is obscure. Nearly
similar pyroxene-andesites occur throughout California, according to the
descriptions given by Mr. George F. Becker,' who has also identified these
lavas from the west side of the Sierras with similar andesites in the neigh-
borhood of Steamboat Springs, Nevada, which closely resemble those of
Truckee Canyon. Quite recently Mr. H. W. Turner3 has reported the
occurrence of basic andesite overlying rhyolite at a number of localities
along the western Sierra foothills.

These instances suffice to show that this type of rock occurs over
widely separated areas, but it should, however, as regards its geological
position, in 110 way be confounded with an older body of pyroxene-andesite
of somewhat similar composition, such as is well represented in the Washoe
District on the slopes of Mount Davidson, in the Virginia Range. The
latter in general present a high degree of crystallization, carrying more
porphyritic secretions and consequently less glass. On the other hand, the
former present all those characters which ordinarily characterize surface
flows, and are for the most part darker in color, as they cany fewer well
developed feldspars. The hornblende and pyroxene-andesites of Washoe
have been well described elsewhere in numerous publications upon that
much discussed region. In the opinion of the writer the geologists of the

1 U. S. Geol. Explor. 40th Par., vol. n, p. 830.

2Geologyof the quicksilver deposits of the I'arific Slope, Mon. IT. S. Oral. Surv. vol. XIII.

'Mohawk Lake Beds. Phil. Soc. <•{ Wash., Rull. xi, pp. 385-410.

262 GEOLOGY OF THE EUREKA DISTRICT.

Fortieth Parallel Exploration were led into error in supposing that all the
rocks classed as pyroxene-andesite in the Great Basin belong to the same
time period and were identical as regards their geological position in the
order of succession, whereas there are two distinct periods, the earlier of
which is represented by the pyroxene-andesites of Washoe and preceded
the hornblende-mica-andesites, dacites, and rhyolites, and the latter bv the
pyroxene-andesites which followed the rhyolites, as developed on so grand
a scale at Richmond Mountain.

Accessory Minerals.— Disseminated through the lavas at Eureka four
minerals have been recognized, which in all cases occur simply as accessory
constituents, as in no single instance do they enter largely into the compo-
sition of the rocks. These minerals are apatite, zircon, garnet, and allanite.
Apatite and zircon iu a perfectly unaltered condition have been determined
in every type rock of both feldspathic and pyroxenic magmas. The apa-
tites are much like those described in volcanic rocks elsewhere, with well
developed terminations and a characteristic basal cleavage. Zircons in
both long, slender prisms and short, stout, colorless crystals are by no
means uncommon, and, judging from their distribution, occur apparently
uninfluenced by the nature of the lava, notwithstanding their high specific
gravity. They are found especially well developed in the andesitic pearl-
ites, the crystalline forms, as drawn by Mr. Iddings, having already been
employed as illustrations of microscopic zircons in recent text-books.

The presence of apatite is indicated by analyses in the determination
of phosphoric acid, but the amount of zirconia present has not yet been
estimated in any of these lavas. Judging from the analyses, the phosphoric
acid increases with the basicity of the lava, starting in with only "06 per
cent in the rhyolite from Rescue Canyon and reaching '29 per cent in the
basalt from the summit of Richmond Mountain. The two silicates, garnet
and allanite, have been detected only in the acidic magmas, but both of
them have apparently been developed in the same type of rocks. The
garnets, although minute, may be easily recognized by the naked eye,
standing out as brilliant dark red crystals in contrast with the light colored
pumices, tuffs, and pearlites which cany them. They occur in both the
Rescue Canyon and Pinto Peak rhyolites. They are well developed at

ACCESSORY MINERALS. 263

Gray Fox and in the porous white tuffs soutli of Richmond Mountain.
Microscopic individuals of brown and reddish brown allanite have been
determined, almost invariably in an unaltered state, in andesitic pearlite,
Rescue Canyon rhyolite, and in other very glassy varieties of rhyolite.
The determination of allanite by its optical and crystallographic properties,
its separation by chemical analyses, and its occurrence in widely separated
localities prove that the mineral may claim recognition as an accessory
constituent in recent volcanic rocks.1

In addition to the above minerals it mav be well in this connection to

«/

mention two nonessential constituents occurring in the pyroxenic lavas —
tridymite and quartz — which, although of interest from a petrographical
point of view have almost no bearing upon the ultimate composition of the
original molten mass. Tridymite is easily recognized under the microscope
in the vesicular rocks of Richmond Mountain in thin tabular crystals lap-
ping over each other in the manner so frequently observed elsewhere.
These leaf-like crystals arrange themselves in clusters lining the cavities.
Identical occurrences of tridymite may be observed in similar pyroxene-
andesites from other localities in the Great Basin, notably in this type of
lava in the Wahweah Range northwest of Richmond Mountain.

Quartz as an accessory constituent has been recognized in the basalts
from a number of localities and apparently bears no relation to the chemi-
cal composition, being quite as apt to be developed in the normal olivine
basalts as in the more siliceous flows. It is as characteristically displayed
in the basic rock of Magpie Hill as in any other, occurring in isolated irregu-
larly shaped grains encircled on all sides by minute augite crystals. Under
the microscope they have all the appearance of being of primary origin.
Similar quartz grains have been described by Mr. Iddings2 from New
Mexico and Arizona, their origin being referred by him to physical causes
attending an earlier stage of the magma. He regards the exceptional devel-
opment of the quartz in these basic rocks as comparable to the crystalliza-
tion of fayalite in the lithophysse of rhyolitic obsidian. Similar quartz
grains in basalts have been described by Mr. J. S. Diller, from the base of

'Joseph P. Iddings and Whitman Cross: Widespread occurrence of allanite as an accessory
constituent of many rocks. Am. .lour. Sei., Aug., 1885, vol. xxx, pp. 108-111.
'Bull. U. S. Oeol. Survey, No. 66, 18!K).

GEOLOGY OF THE EUREKA DISTRICT.

Lassen Peak in northern California and are also regarded by him as of
primary origin.'

chemical Composition.— During the progress of the investigation upon the
erupted material, analyses were made of several of the more characteristic
rocks, which are presented here in tabular form arranged in the order of
their basicity.

Silica.

75.69

73.91

73.09

67.83

67.03

65 13

61 58

56 54

50 38

12.26

15.29

14.47

15. 02

16.27

15 73

16 34

14 75

19 83

2 24

6 05

2.93

0.89

2.99

5.16

3 97

1 86

6 42

9 29

2 00

0 3g

Nickel

0.07

1 13

0 77

1 13

3 07

3 42

3 62

5 13

7 80

10 03

0 29

1 19

1 49

9 85

6 51

5 36

Soda

3.01

3.62

2.77

2.40

2.71

2.93

2.69

2.07

2 15

Potash

4.74

4.79

5.07

3.20

3.60

3.96

3.65

2 96

1 76

Lithia

0.06

0.07

0.26

0 23

0 28

0 29

1.04

1 07

0 58

0 68

0 55

0 83

1 19

1 11

1 56

2 43

0 64

1 37

Total

99.82

100.53

99.52

99.38

100 72

100 27

100 26

100 76

100 14

1. Coll. No. 163.— Rhyolite from Rescue Canyon. Analysis by R. W. Million. 1883.

2. Coll. No. m.— Rhyolite from top of Pinto Peak. Analysis by Dr. Edward Hart, of Lafayette
College. 1883.

3. Coll. No. 17a. — Rhyolite overlying daeite from northeast of South Hill. Analysis by R. W.
Mahon. 1883.

4. Coll. No. 35. — Hornblende-mica-andesite from hill northeast of Hoosac Mountain. Analysis
by R. W. Mahon. 1883.

5. Coll. No. 69. — Daeite, small canyon northeast of South Hill. Analysis by R. W. Mahon.
1883.

6. Coll. No. 71. — Andesi tic-pearl ite, south of Carbon Ridge. Analysis by W. H. Melville. 1890.

7. Coll. No. 79. — Pyroxene-andesite, Richmond Mountain. Analysis by Dr. Thomas M. Drown,
Institute of Technology. 1883.

S. Coll. No. 284.— Basalt from saddle east of Hasalt Peak. Analysis by Dr. Edward Hart. 1883.
0. Coll. No, 269.— Basalt, summit of Richmond Mountain. Analysis by J. Edward Whit-
field. 1886.

These nine analyses of carefully selected material represent the com-
position of the entire mass of extravasated lavas at Eureka and show a
range in their tenure of silica of over 25 per cent. Lavas from 1 to 6,
inclusive, belong to the feldspathic magma, and those from 7 to 9,

'Am. Jour. Sci., 3d ser., 1887, vol. xxxm, pp. 45-50.

CHEMICAL COMPOSITION OF LAVAS. 265

inclusive, to the pyroxenic magma. Analyses numbered 2, 4, 7, 8, and 9
give the composition of typical rocks from different natural groups and of
the most extensive bodies of rhyolite, hornblende-mica-andesite, pyroxene-
andesite, acidic basalt, and normal basalt. Each of these five rocks carries
about 6 per cent more silica than the one standing next below it in the
series.

All the vast accumulation of lavas may be regarded either as belong-
ing to, or as variations from, these main types, or else as transition products
between two closely related natural groups.

Along the Hoosac fault, where the most basic unaltered rocks of the
feldspathic magma are best developed, solfataric action has so decomposed
them that it becomes a matter of much difficulty to determine even approx-
imately their original basicity, as they all show more or less evidence of
infiltration of siliceous material. The oldest lavas occurring in any exten-
sive body and still preserved in a fresh condition consist almost wholly of
hornblende-mica-andesite, represented by the rock northeast of Hoosac
Mountain, carrying, according to analysis, 6 7 '83 per cent of silica. The
fine rhyolite from Pinto Peak, free from ferro-magnesian silicates and rich
in well developed orthoclase, is typical as regards chemical composition of
the acidic end of the feldspathic magma along the same great line of dis-
placement.

It will be noticed that the dacite from northeast of South Hill carries '8
per cent of silica less than does the hornblende-mica-audesite, whereas on
theoretical grounds it would be expected to show an amount somewhat in
excess, owing to the presence of quartz secretions. The rock was selected
on account of its well recognized geological relations with an overlying
rhyolite body, an analysis of which, for comparison, will be found in the
table. Normal dacite of the Great Basin usually carries about 70 per cent
of silica, whereas this rock stands as an intermediate variety between it
and the andesite. A study of the chemical analysis explains the mineral
composition. The large amount of iron and magnesia in excess of that
found in the rhyolite and the falling away in the percentage of potash are
sufficient to account for both the predominance of biotite and the absence
of sanidine. The plagioclastic nature of the prevailing feldspar assigns the

266 GEOLOGY OF THE EUREKA DISTRICT.

rocks to the andesites, while the presence of quartz as an essential con-
stituent .places it more correctly among the dacites. For the erupted
material of Eureka it stands as one of the most basic rocks of the feld-
spathic magma, rich in porphyritic quartz secretions.

The most basic of the feldspathic lavas analyzed is an andesitic pearl-
ite, very limited in extent, containing 65'13 per cent of silica, the complete
analysis of which will be found in column 6 of the table. It carries well
developed feldspars, with some hornblende and biotite, but is especially
noticeable for the numerous pyroxene microlites which enter into the
structure of the very glassy groundmass. The rock, although belonging
to the acidic lavas, is allied to the basic magma by the coming in of these
microlites of pyroxene, which more or less modify the nature of the glassy
groundmass and relate it in structural habit to the rocks of Richmond
Mountain. It is doubtful if any fresh rock of the feldspathic magma would
fall much below 65 per cent in silica. An analysis of a typical rock from
Richmond Mountain, given in column 7 of the table, yielded 6T58 per cent
of silica. The most acidic rocks derived from the pyroxenic magma, as
shown by a series of silica determinations in partial analyses, is 62'41 per
cent. As these analyses are only partial, they are not published. They
show variations from 49 to 62 per cent of silica, with a gradual falling off
in soda and potash as the rocks develop more and more magnetite and
olivine. The most basic basalt examined yielded about 49 per cent
of silica.

By reference to the table of complete analyses it will be seen that the
lime, magnesia, and oxides of iron increase from the acidic to the basic end
of the series. Of these bases, lime is the most regular in its behavior and
presents the widest range, starting with less than 1 per cent in the rhyolite
of Pinto Peak and reaching over ] 0 per cent in the dike of intrusive basalt
which cuts the pyroxene-andesite near the summit of Richmond Mountain.
It should be borne in mind that the Pinto Peak rock carries no ferro-
magnesian minerals and the feldspars are for the most part sanidine. Mag-
nesia stands second in this uniform increase, but is wholly wanting in the
rhyolites, coining in with the first appearance of the ferro-magnesian-
silicates and increasing rapidly with the development of pyroxene and

COMMON SOURCE OF LAVAS. 267

olivine. In general both alkalies :nay be said to decrease from the acidic
toward the basic end, and, except in the more basic basalt, the potash
exceeds the soda in amount.

There is a much gi eater range throughout the entire series of lavas in
the percentage of potash than in that of soda, the former showing a varia-
tion of over 3'25 and the latter of only 1/50 per cent. The greatest inter-
ruption in the regularity of the potash is shown along the line where the
sanidine disappears and some one or more of the lime-soda feldspars become
the predominant species, whereas with the soda no such break is noticeable.
In the liquid mass, under influences very little understood, the material
forming ferro-magnesian minerals draws apart from the alkalies and excess
of soda, the result of which is to produce separate magmas differing widely
in chemical composition.

Common Source of Lavas.— In the preceding pages all the extravasated lavas
have been considered as belonging to one or the other of two distinct
magmas, yet it is impossible, notwithstanding they are so sharply con-
trasted in certain fundamental structural characters, not to recognize the
fact that both magmas stand in the closest relationship to each other. The
similarity in mineral development as they approach each other in chemical
constitution, the gradual changes in the relative proportions of the oxides of
the different elements throughout the entire range of lavas, show how close
a connection exists between them. An equally strong argument is found
in their geological distribution, where the rhyolite occurs closing up the
vents occupied by the feldspathic magma and at the same time breaking
out as the earliest eruptions along fissures which later served as channels
for the pyroxenic magma. The loci of eruption of both magmas have been
shown to be in close proximity to each other, and some of the most acid
and most basic lavas, so far as external evidence can determine, not only
reached the surface along the same great fractures, but actually used the
same conduits at a number of localities.

To the writer, after studying all the facts, it seems impossible to regard
these differentiated volcanic products otherwise than as belonging orig-
inally to one and the same body of molten material; in other words, they
were derived from a common reservoir. To conceive of such a separation

268 GEOLOGY OF THE EUREKA DISTRICT.

from an earlier primordial molten mass is no more difficult than to conceive
of the breaking up of the feldspathic magma into a homblende-mica-
andesite and a rhyolite group, and the latter has been shown to take
place, so far as it is possible to demonstrate it from surface evidences,
along fissure planes through which the lavas issued. The original magma
separated into a heavier and a lighter portion, the groundmass structure
of the two being fundamentally different. It will be borne in mind
that the earlier magma consisted of a groundmass made up of an aggre-
gation of feldspar and quartz grains, through which were disseminated
porphyritic secretions of hornblende and mica, but no pyroxene, except
in a few instances of pyroxene microlites in the groundmass of some
varieties of audesite. The later magma consisted of a groundmass com-
posed of lath-shaped lime-soda feldspars and pyroxene microlites, so intri-
cately interwoven as to form the so-called felt-like structure characteristic
of pyroxene-andesite, through which were scattered the heavier ferro-
magnesian minerals already described.

History of Volcanic Action.— The geological history of volcanic action at
Eureka during Tertiary time is in many respects simple and, after a careful
study of its details, easily deciphered. There are among the lavas no masses
of coarsely crystalline rocks slowly cooled beneath the surface under
physical conditions different from those usually found accompanying extru-
sive flows. No powerful displacements have brought into juxtaposition
igneous rocks of different ages, crystalline structure and mineral composi-
tion, and although faulting attending extravasation doubtless did occur it
was not of a kind to obscure geological structure. Again, the sequence of
events was not complicated or broken by long intervals of activity and
rest through successive geological epochs during which an older and a
younger series of eruptions took place; but on the contrary the lavas were
apparently poured out under very similar physical conditions from the
beginning to the end of volcanic action. In coming to the surface these
lavas were not forced upward as one continuous eruption or rapid series of
eruptions, but were the result of a succession of overflows accumulating
slowly, although at times spasmodically, along lines of volcanic activity
coincident with lines of orographic displacement. The material thus poured

HISTORY OF VOLCANIC ACTION. 269

out gradually underwent changes in mineral composition offering a great
variety of volcanic products of which the relative age and order of succes-
sion of typical lava flows have been clearly established. It has also been
demonstrated that throughout this entire series of lavas the range in silica
amounts to about 25 per cent, a range which is quite as wide as is ordinarily
found in most centers of eruption, even where the volume of lavas thrown
out has been vastly greater and the duration of volcanic energy far longer.
The succession of events throughout the volcanic period presents a con-
tinuous chapter of geological history complete in itself with the rise, cul-
mination and dying out of eruptive energy. So far as ultimate chemical
composition of both acid and basic rocks is concerned it furnishes a com-
plete cycle of volcanic products.

Probably the feldspathic and pyroxenic lavas do not approach each
other in their tenure of silica within 2-25 per cent, at least no body of rock
or lava stream is known which indicates a closer coming together of the
two magmas. In chemical composition and mineral development the earli-
est eruptions of both magmas resemble each other closest, but from this
common ground they differentiate steadily until the feldspathic lavas reach
the extreme acidic and the pyroxenic the extreme basic end of their respec-
ive series. The former and earlier magma exhibits in the overflows a con-
stantly increasing acidity through a range of 11 per cent of silica, and the
latter an increasing basicity with a falling away in silica of 13 per cent, the
point of separation of the two magmas being nearly midway between the
extremes in composition.

Exceptional lavas in other localities may carry somewhat more silica
than those thrown out at Eureka, but it is doubtful if flows of any consid-
erable size exceed those of Rescue Canyon in acidity by more than 2 per
cent unless accompanied by secondary alterations or infiltration products.
Obsidians are reported as carrying 78 per cent of silica, but for the most
part these highly acidic glasses fall within the limits assigned to normal
rhyolites. Basalts somewhat richer in oliviue and magnetic iron are by
no means uncommon elsewhere, but these extreme basic varieties have not
as yet been recognized within the Great Basin. Not only as regards the
range in silica, but for all other essential elements entering into the original

270 GEOLOGY OF THE EUREKA DISTRICT.

composition of magmas, this series of lavas may be taken as representative
of many others in widely separated regions throughout the world. To the
lavas of Hungary they show very close resemblance.

Beginning with the hornblende-andesite the feldspathic magma became
gradually more siliceous until the close of the rhyolitic eruptions without
any abrupt break in the outpourings or the intervention of any percepti-
ble change in geological conditions. It seems impossible, therefore, to
consider these lavas in any other light than as a continuous succession of
flows, interrupted only by time intervals of longer or shorter duration.
Notwithstanding these gradual transitions, certain type rocks prevail to a
far greater degree than others, both as regards bulk and distribution, nota-
bly the hornblende-mica-andesite and the Pinto Peak variety of the rhyo-
lite, the two standing out prominently as the principal eruptions of the
feldspathic series. The dacites are greatly limited in their bulk, and the
same is true of all rocks of intermediate composition, the greater part of
them being easily classed under one or the other of the natural groups.

The earliest outbursts along different profound fissure planes have
not necessarily been identical in composition or synchronous in time. Along
some of these the first overflows observed are hornblende-mica-andesite,
in others highly siliceous andesitic pearlites, in still others dacites, and in
several of them rhyolites, but in no single instance, whatever may have
been the nature of the earliest lava poured out, has a more basic member
of the feldspathic series been recognized as breaking out along the same
fissure. It is as if certain of these fissures were opened by the forcing
upward of the lavas at different periods of eruptive energy and the vents
filled by a magma of definite composition at that time coming to the surface
simultaneously through all the fissures. It is also worthy of note that
along the meridional faults the andesitic material for the most part broke
out at the northern ends, the lavas in general growing more acidic toward
the south. Furthermore, certain fissures becoming filled and choked by
cooling and crystallization have prevented the more acidic lavas from find-
ing an outlet at the surface along the same line where the earlier portions
of the molten mass broke out.

HISTORY OF VOLCANIC ACTION. 271

When it comes to the pyroxenic magma it is found to break out and
follow the sinuous lines of fracture previously followed by rhyolitic lavas.
In some instances they present the appearance of actually employing the
identical conduits used by the feldspathic magma. In this way the rhy-
olite plays a most important part, not only as a connecting link between
the feldspathic and pyroxenic magmas in respect to sequence of flow, but
still more as regards geological distribution and mode of occurrence. Too
much stress can not be laid upon the fact already mentioned, that the rhy-
olites were the last to break out along the vents occupied by the hornblende-
andesite and the first to reach the surface along the same lines of fracture
which were afterward used by the basalts of the pyroxenic magma. That
these basic lavas may have occasionally forced open new vents for them-
selves is quite possible, but the greater number of outbursts followed the
same grand fractures as the earlier highly acidic magmas which border the
elevated orographic block of Silverado and County Peak. Richmond
Mountain, as already pointed out, may have reached the surface through a
separate and wholly independent vent, but it is so vast and its overflows
cover so large an area that it is impossible to determine the position of its
vent or vents and their precise relation to the earlier rhyolite. It must be
borne in mind, however, that it breaks out at the junction of two grand
lines of faulting, coming up from the south on opposite sides of a great
uplifted mountain mass. The earliest flows of the pyroxenic magma
resembled those of the feldspathic magma, in so far as they carry the same
ferro-magnesian silicates as porphyritic secretions. On the other hand, they
are sharply contrasted by an andesitic habitus of the groundmass, which,
however, had been slightly foreshadowed by a groundmass carrying pyrox-
ene microlites, shown in the basic pearlite from the south end of Carbon
Ridge, where the rock occurs as the earliest eruption at that locality, fol-
lowed by a series of feldspathic lavas, closing with rhyolite.

Following the great body of pyroxene andesite came lavas intermedi-
ate in composition between them and basalt, breaking through and over-
lying the less basic varieties. Some of these are allied to the earlier flows,
while others show a decided tendency to transition into basalt. Most of
them are related geologically either with the later basaltic eruptions or

272 GEOLOGY OF THE EUREKA DISTRICT.

stand alone, having broken through rhyolite. A large portion of the rock
masses designated as pyroxene-andesite would hardly be classed as typical
rock of that natiiral group, and the same may be said of many of the
basaltic flows which are far too rich in silica and wanting in olivine to be
regarded as normal basalt. It is probable that many modern volcanoes
would show the same wide range iu basic lavas as is developed in the region
of Richmond Mountain.

Throughout a wide range in composition and over an extended geo-
graphical area the pyroxenic magma fails to show the tendency, so strongly
marked in the feldspathic magma, to separate into well defined natural
groups, nor is the evidence by any means clear that during the period of
extravasation a steady increase in the basicity of the lava took place with-
out occasional oscillations in composition. Nevertheless, it is evident that
whatever oscillations there were must have been confined within very narrow
limits -and restricted to lavas of intermediate composition between pyroxene-
audesite and basalt. No pyroxene -andesite dikes have been observed pen-
etrating either the basalts or the intermediate lavas.

It seems evident from field observations that there were no abrupt
alterations of feldspathic and pyroxenic lavas after the appearance of the
earliest pyroxene-andesite.

Speculative Theories.— It does not come within the scope of this chapter,
which is mainly devoted to a presentation of observed facts, to enter upon a
full discussion of the speculative theories advanced by geologists to account
for the condition of the molten masses beneath the surface, nor the physical
causes leading to their Reparation into the varied products found either as
interbedded sheets and laccolites within the superficial crust of the globe,
or poured out upon the surface as extrusive lavas. Yet, at the same time,
after having devoted so much study to the constitution of the different
lavas and their order of succession, this chapter would be incomplete with-
out calling attention to the importance of the phenomena presented at
Eureka and pointing out the bearing of the observed facts upon the prob-
lems offered in volcanic regions elsewhere. Without entering upon a review
in detail or a critical discussion of the opinions held by others who have
considered these speculative matters, it is necessary to recall, briefly, the

BTINSEN'S VIEWS. 273

views expressed in the more important contributions to the literature on the
subject.

Bunsen's views.— Bunsen, after a visit to Iceland, where he laboriously
studied the volcanic phenomena displayed on a grand scale, conceived
the idea of two distinct bodies of lava, one acid and the other basic, the
former of which he designated as the normal trachytic, the other as the
normal pyroxenic magma. He was disposed to regard all volcanic products
intermediate in composition between these types as admixtures in varying
proportions derived from two distinct foci of eruption, the relative propor-
tions of each depending in great part upon the intensity of eruptive
energy. He sought to apply his views to all other volcanic regions, citing
as an identical mode of occurrence the table-land of Armenia.1 The grand
division of volcanic products into acid and basic lavas has been received
by most vulcanologists, but his theories to account for the very varied con-
stitution of volcanic rocks has not obtained the same general acceptance.
In this chapter the writer adopts the views of Bunsen as regards two great
groups of lavas, but differs with him as to the origin of the varied transi-
tion products of eruption.

The writer has used the expression feldspathic magma in preference to
trachytic magma, as the former is a mineralogical term contrasting sharply
with the expression pyroxemc magma. This is rendered all the more neces-
sary since the word trachytic now possesses a different signification from
what it did at the time when it was first employed by the German scientist.
Typical trachytes are somewhat rare and confined to restricted areas, since
many of the rocks formerly considered as trachyte have been found to be
characterized by plagioclastic feldspars, and hence more properly come under
the head of andesite. This is the case with the feldspathic rocks of Ice-
land, which Bunsen investigated and upon which he bases his conclusions.

Durocher's Theories.— Durocher,3 after studying the composition and petro-
graphical characters of a large number of crystalline rocks, endeavored by
ingenious and somewhat complex theories to establish universal laws to
account for the variations observed in crystalline rocks of all ages and

1 Ueber die Proeesse AVI vulkanischen Gesteinsbildung Islands. Poggendorf s Anualen, 1851,
Band 83, pp. 197-272.

3 Essai do petrologie compare. Ann. d. mines, Paris, 5th ser., 1857, Tome xi, pp. 217-259.
MON XX 18

274 GEOLOGY OF THE EUKEKA DISTRICT.

of every possible mode of occurrence. He followed Bunseii in accepting
the theory of both an acid and basic magma, but regarding them as
parts of the same body of lava. In an appendix to his paper1 he
admits the possibility in certain cases of a mingling of both types, but
objects to the hypothesis of Bunsen as altogether too broad a general-
ization. That part of Durocher's hypothesis which possesses the most
originality and upon which he places the most stress to account for the
differences in the mineralogical character of lavas has been designated the
liquation process applied to igneous rocks. His conclusions, based largely
upon chemical analyses, were not substantiated by any array of facts or
observations from any one center of volcanic energy. Durocher was dis-
posed to regard certain lavas as differentiated products obtained by the
breaking up of a magma by processes comparable to the separation and
segregation of metals in a bath containing several metallic substances in a
state of fusion, the theory being based upon well recognized processes em-
ployed in metallurgical establishments for the concentration of gold and
silver in molten lead. The views enunciated by Durocher have met with
slight recognition, but, although containing much that with the advance-
ment of knowledge has been shown to be based upon error, they are, in
the opinion of the writer, full of the most valuable suggestions bearing on
the origin of lavas, and entitled to far more consideration than has generally
been accorded them.

Roth's views.— In 1861 Justus Roth2 published his hypothesis of "Spal-
tung und Differeiizirung," in which he elaborated similar views, although by
no means identical with those held by Durocher. For the purposes of this
chapter it is sufficient to say that the two authors are in accord so far as
believing in the power of a magma to split up during crystallization into
secondary magmas of different mineralogical composition. Roth regarded
large bodies of crystalline rocks as "Spaltungsproducte," the result of the
separating out of certain groups or association of minerals from and de-
pendent upon the composition of a primary liquid lava, but governed by
varying conditions of pressure and temperature. His views are derived

1 Op. cit., p. 677.

2 Tabellarische Uebersicht <ler Gesteins Analyseu und mit kritischen Erliiuterungen. Berlin, 1861.

RICHTHOFEISPS VIEWS. 275

from a careful study and comparison of a large number of chemical analyses
of crystalline rocks gathered from all parts of the world, differing widely in
their mineralogical development and structural habit.

Besides these references to original contributions the reader who de-
sires to pursue the subject still further will find an excellent summary of
the views of Bunsen, Durocher, Roth, and others, published by Ferdinand
Zirkel in his text-book of petrography.!

Waitershausen's Conclusion.— Sartorius von Waltershauseii, after a careful
investigation of the lavas of Sicily and Iceland, published the results of his
researches in an elaborate memoir in which he presented his conceptions of
the physical condition of the interior of the earth. His conclusions, so far
as they relate directly to subjects considered here, stated briefly, were that
between a superficial cool crust and a solid interior there existed a broad
belt of fused material of undetermined thickness which furnished the source
of supply for the lavas poured out upon the surface. This material
arranged itself approximately according to its density. The most acid lava,
being the lightest, was situated nearest the surface, followed by that of
intermediate composition characterized by minerals of somewhat higher
specific gravity, and terminating finally with the heaviest, and consequently
most basic, lavas — basalts — carrying large amounts of magnetite and other
iron minerals. He concludes that in most instances the lavas were ejected
in the order of their position, the lightest being first thrown out, imperfect
separation by specific gravity being sufficient to account for all exceptional
occurrences. This simple and regular order of succession met nearly all
the requirements of Waitershausen's personal observations and were in
accord with his theories.2

Richthofen's views.— Baron von Richtliofeii accepted the main conclusions
of Waltershausen regarding the physical conditions of the globe, agreeing
with him as to the evidences of a liquid mass lying between a solid
interior and a superficial outer crust. This liquid mass was acid
near the surface, basic beneath, with the intermediate transition lavas
between them. He traveled extensively in the volcanic regions of Europe

1 Lehrlmch der Petrographie. Bonn, erster Band, 1866, pp. 453-473.

'- Uebev die vulkanischen Gesteine in Sicilieu und Island und ihre submarine Umbildung, Qot-
tingt-n, 185i<.

276 GEOLOGY OF THE EUEEKA DISTRICT.

and western America, studying the development of volcanic rocks. He
devoted special attention to the laws governing the mode of occurrence of
the different natural groups into which he divided all igneous rocks, and
the relations of these groups to each other, being more interested in the
geological problems than in the precise chemical composition of the
extruded products. As a result of his observations in the field, he was
impressed by the great similarity in the nature of lavas in widely sepa-
rated regions and the uniformity in the order of their succession. He found,
however, that this succession was by no means as simple as suggested by
Waltershausen, nearly every volcanic region which he visited presenting
abrupt, but similar, alternations from acid to basic rocks, at first sight not
readily explained. Richthofen's final conclusions were published in an
admirable and remarkably suggestive memoir presented to the California
Academy of Sciences,1 in which he gives what he considers to be the
natural law of the sequence of massive eruptions applicable to all centers
of volcanic energy. As his conclusions were based largely on observations
made in California and the western edge of the Great Basin, they are of
more than ordinary interest for purposes of comparison with results since
obtained by the investigations at Eureka.

The natural order of succession of massive eruptions as laid down
by Richthofen is as follows:2

1. Propylite.

2. Andesite.

3. Trachyte.

4. Ehyolite.

5. Basalt.

This law of succession as enunciated by Richthofen is far more com-
plex than the simple regular order suggested by Waltershausen, as it
supposes the breaking out, first of all, of intermediate lavas represented by
propylites and andesites, followed by others of varying composition,
but more acidic than the latter and belonging to the order trachytes. The
trachytes were succeeded by a still more acid series of lavas, and then the
closing of eruptive energy by an abrupt change from the most acidic of all

1 Natural system of volcanic rocks. Memoirs of the California Academy of Sciences, 1867, vol. I, p. 36.
a Op. cit., p. 29.

KING'S VIEWS. 277

lavas, the rhyolites, to the most basic of all, the basalts. From this order
he nowhere recognized any deviation. Accepting the hypothesis of Walter-
shausen as regards a liquid interior and the nature of the molten mass, he
seeks to account for the remarkable alternations observed in lavas upon the
surface of the globe by supposing changes to take place in the physical
conditions governing the emission of lavas which would from time to time
elevate or depress the loci of eruption. These changing conditions were
universal, producing similar results in volcanic centers all over the world,
but not necessarily contemporaneous in time. He says:

It appears that after the ejection of the chief bulk of andesite, when other pro-
cesses ending in the opening of fractures into the basaltic region were being slowly
prepared in depth, the seat of eruptive activity ascended gradually to regions at less
distance from the surface.1

clarence King's views.— As a part of the report upon the Geological
Exploration of the Fortieth Parallel, Mr. Clarence King published in 1878
the results of his researches upon the genesis of lavas as shown by their
occurrences in the field of his observations in the Great Basin. As regards
the law of succession, his views are for the most part in accord with those
of Richthofen, he going, however, still further and finding a much more
intricate system in the alternations from acid to basic rocks. He finds an
acid, a neutral, and a basic member in each natural group or order which
he designates by specific names, each member having a definite mineral
composition and a fixed place in the order of succession. To these modifi-
cations proposed to Richthofen's order he adds another still more radical,
in respect to classification, uniting rhyolite and basalt under one head, to
which he applies a new designation, "Neolite," these two types of lava
constituting the acid and basic subdivisions of this natural group, having
the same relative value as andesite and trachyte. The sequence of lavas as
recognized by Mr. King is as follows :2

1 Op cit., p. 58.

*U. S. Geol. Explor. of the Fortieth Parallel, vol. i, Systematic Geology, p. 690.

278 GEOLOGY OF THE EUREKA DISTRICT.

Natural succession of volcanic rocks.

Order. Subdivision.

1. Propylite «. Hornblende-propylite.

b. Quartz-propylite.

c. Augite-propylite.

2. Andesite a. Hornblende-andesite.

b. Quartz-andesite (Dacite).

c. Augite-andesite.

3. Trachyte a. Hornblende-plagioclase-trachyte.

b. Sanidine-trachyte (quartziferous).

c. Augite-trachyte.

4. Neolite a. Rhyolite.

b. Basalt.

This presents a much more complex system and could hardly be
accepted upon the simple conditions of a uniform and widespread liquid
mass, as held by Waltershausen and modified by Richthofen, in requiring
frequent elevation and depression of the loci of eruption hi accord with the
changes in the composition of the lava thrown out at the surface. Mr.
King is fully aware of the many physical obstacles encountered, and
explains the many oscillations and abrupt alternations in the volcanic
products which his system calls for by a carefully considered hypothesis of
his own, quite at variance with the views advanced by his predecessors.
In place of a broad belt or magma of liquid lava encircling the earth
beneath the sedimentary crust, he holds to the opinion of local reservoirs of
molten matter within the superficial crust, each of his orders being the
product derived from one of these reservoirs, or, as he calls them,
" extremely localized and only temporarily existing pools of fusion."
He says:

Under my hypothesis, by which fusion is the temporary result of erosion, each
one of Richthofen's orders, with its acidic and pyroxenic members, would be considered
as the product, of a single ephemeral lake. A period of erosion under this conception
would result in the formation of a lake. The cessation of erosion, either from climatic
causes or from the degradation of centers of erosion, would place a limit to the
expansion in depth of fusion; in other words, would define the time limits and the
vertical expansion of the lake.1

'Op. cit., p. 716.

ABSENCE OF PROPYLITE. 279

His views, which can not well be abridged here, will be found admira-
bly stated in his chapter devoted to a discussion of the genesis of volcanic
species, in which he treats of geological causes leading to the formation
of local lakes of lava.

Later observations.— Since the publication of King's memoir the study of
volcanic rocks has progressed with rapid strides, and nowhere have they
been investigated with more untiring energy than in the Cordillera of
North America. Notwithstanding our knowledge of the rocks of the
Washoe District and the Coinstock Lode, derived from the works of Rich-
thofen and King, later study of them, aided by methods of microscopical
research, has developed fresh points of interest bearing upon their order of
succession and mutual relations. After a thorough examination of the
propylites, Mr. George F. Becker1 has shown that they can not be separated
from the andesites as an independent rock species based upon any mineral-
ogical distinctions, since the peculiar habitus of the propylite is due to
chemical change and decomposition of the constituent minerals. Moreover,
the propylites and andesites are found to pass into each other by gradual
transitions.

Hague and Iddings,2 in the course of their examination of the Washoe
rocks, confirmed the results of Mr. Becker so far as the identity of the
propylite and andesite is concerned, and also failed to see any geological
evidences of a preandesitic eruption.

Similar views as regards the independence of propylite are now main-
tained by nearly all petrographers who have given much thought to the
subject or who attempt to classify volcanic rocks upon either a structural or
mineralogical basis.3

1 Geology of the Comstock Lode and the Washoe District, Washington, 1882.

*On the development of crystallization in the igneous rocks of Washoe, Nevada; with notes on
geology of the district. Bull. U. S. Geol. Survey, No. 17, Washington, 1885.

3 Since this chapter was written Prof. J. W. Judd has published an admirable paper on " The
Propylites of the Western Isles of Scotland, and their Relation to the Andesites and Diorites of the
District." He revives the use of the term propylite, but in the strict sense suggested by Rosenbusch,
regarding it simply as a "pathological variety" of andesite. His detailed descriptions identify the
Scottish rocks with similar rocks found iu Hungary. From his description it would be difficult to
distinguish thorn in any particular from the altered andesites of the Washoe District in the Virginia
Range. They even show the development of metallic sulphides. Quart. Journ. Geol. Soc., vol. XLVI,
pp. 341-382. London, 1890.

280 GEOLOGY OF THE EUREKA DISTRICT.

The same writers have demonstrated the nonexistence of trachyte as
one of the natural divisions of volcanic lavas in the Great Basin, the
occurrence of orthoclase rocks free from quartz secretions being almost
unknown in that region. These recent advances in our knowledge of vol-
canic rocks tends to simplify the law of sequence so far as their occurrence
in the Great Basin is concerned, since two of the groups, the propylite and
the andesite, as laid down by Richthofen, have been merged into one,
and the trachytes either relegated to some variety of andesitic lavas or
placed among quartz-bearing rocks, either dacite or rhyolite. The impor-
tance of these observations lies in the fact that there is no interpolation of a
strongly alkaline magma in the series of lavas, and that andesitic lavas pass
over directly into rhyolite. Not only in the Great Basin but in many other
regions as well, rhyolite is far more closely related to andesites derived
from a feldspathic magma than to trachytes.

Having thus briefly reviewed the literature bearing upon the genesis
of lavas and their order of succession, it becomes a matter of much interest
to see how far the facts observed in a carefully studied and surveyed region
like Eureka are in accord with the Adews expressed by the eminent writers
quoted, since it is only by the accumulation of vast amount of evidence
from many widely separated fields that we can hope to attain anything like
definite laws governing the mutual relations of igneous rocks.

In only one other i-egion of the Great Basin have volcanic phenomena
been investigated in a manner at all comparable with Eureka and that one
the much discussed area of the Washoe District. At Washoe the conditions
are in some respects very different, volcanic activity having extended
through a longer period of time. The coarse crystalline rocks which form
the long slopes of Mount Davidson do not make their appearance at Eureka,
and for that matter are wanting over the greater part of the Nevada plateau.
They belong to an earlier period, forming a distinct chapter in the Tertiary
history of volcanic action.

The earliest eruptions at Eureka may be correlated with the horn-
blende-mica-andesite of Washoe (trachytes of Richthofen and King and
later hornblende-andesites of Becker) in mineral composition and structural
features. They may be regarded from the point of view of this chapter as

EUEEKA AND WASHOE. 281

synchronous in age, since the succession of all subsequent lava-flows for the
feldspathic rocks in both localities may be said to be the same — hornblende-
mica-andesite, dacite, rhyolite. Analyses show the homblende-mica-ande-
site rocks of Eureka to carry slightly more silica than the corresponding
rocks at Washoe, the most acid members of this group from the latter locality,
coming just within the range of the basic members of the series at Eureka.

When it comes to the pyroxenic rocks following the rhyolite the
sequence of events does not appear so clearly established at Washoe, as
there no such grand exposui-es occur as at Eureka. In the immediate
region of the Comstock Lode only a few isolated patches of basalt are
exposed. Small outbursts of pyroxene-andesite, similar to those of Rich-
mond Mountain, have broken out only a short distance from Mount David-
son, but the relations between these two pyroxenic lavas are unknown. A
few miles northward in the same range of mountains large flows of
both pyroxene-andesite and basalt may be seen superimposed upon horn-
blende-mica-andesite and rhyolite. Taken together the Washoe District
and the region of Truckee Canyon present a sequence of lavas and a
geological history of volcanic events similar to that found at Eureka.

The subjoined table presents a seiies of twelve chemical analyses rep-
resenting the volcanic rocks of Washoe arranged according to their
basicity :'

1 On the development of crystallization in the igneous rocks of Washoe, Nevada; with notes on the
geology of the region. Bull. U. S. Geol. Survey, No. 17, p. 33.

282

GEOLOGY OF THE EUEEKA DISTRICT,

Samuel I,. Pentteld.

K. W. Woodward.

Gideon E. Moor.'.

K. W. Woodward.

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SEQUENCE OF LAVAS. 283

Analyses Nos. n to vn, inclusive, represent Tertiary rocks older than
any found at Eureka, but from Nos. vinto xn, inclusive, together with Xo.
i, they correspond fairly well to similar lavas at the latter locality. In this
table, however, pyroxene-andesites similar to those of Richmond Mountain
and of the same geological position, associated with basalts and later than
the rhy elites, were not shown, for the reason already stated: that they lie
beyond the limits of the mining districts.

Nowhere else between the Wasatch and Sierra have the lavas been so
carefully mapped, and only in a few places do they appear so varied and
complete. In many centers of eruption, even where the amount of lava
poured out is large, certain types of rock are wanting, and in others their
relative position can not well be determined owing to frequent breaks in the
continuity of exposures.

The history of volcanic action may be fragmeutal and only partially
recorded in any one locality, but throughout the Great Basin, where the
physical and geological conditions were much the same during the volcanic
period, it is probable that the sequence of lava will be found to be in accord
in many places with the observed facts at Eureka. As a center of eiopptive
energy in Tertiary time the Great Basin stands out as a geological unit.

The earliest lavas erupted at Eureka carry from 65 to 67 per cent of
silica and are of intermediate composition, in accordance with the broad
generalization of Richthofen and the facts observed by others else-
where. From this middle ground, however, the lavas increase in acidity
until they attain the composition of the extreme acid types. The latter are
in turn followed by lavas that are also intermediate in composition, but
which increase in basicity until they attain the extreme basic type found in
the later basalt.

Starting from a magma closely related in composition, they differ-
entiate in opposite directions from this common ground until they reach the
extreme type. It will be borne in mind that the existence of both an acid
and a basic magma at Eureka have been clearly established, and to this
extent conform to the views held by Bunsen. Nowhere are the two
magmas better exhibited, as shown in their distribution, mode of occurrence,
and even in the outlines of the lava masses, both types of rock being sharply

284 GEOLOGY OF THE EUKEKA DISTEICT.

contrasted in their surface features. In the opinion of the writer, however,
there are too many insurmountable physical obstacles and too few estab-
lished facts to warrant the acceptance of any theory which attempts to
account for the varied products of eruption by supposing them to be admix-
tures from wholly distinct reservoirs. The observed geological phenomena
at Eureka tend to controvert such a theory where the two magmas, although
in close proximity, fail to show any mingling of products from separate
reservoirs.

Furthermore, there are no evidences of any alternating flows of
feldspathic and pyroxenic magmas, nor of oscillations in relative acidity
within any acid magma, which would certainly be the case had there been
any basic material injected into the feldspathic lava. Within limited range
any large outburst of lava doubtless may display slight variations in com-
position, but this also holds true for different parts of the same flow, and is
still more noticeable in pyroxenic magmas owing to the greater liquidity of
basic lava streams and the consequent tendency of the basic mineral secre-
tions to lag behind. The first violent explosions after cessations of activity
might readily throw out a lava slightly different in composition from the
regular even flow of the mass, and again the last portions might vary some-
what in character from the great bulk of molten material.

Evidence is wanting at Eureka that the lavas were tin-own out, geolog-
ically speaking, from great distances below the surface or from very vary-
ing depths; at least the lavas themselves do not indicate that there were
any profound orgraphic movements during the eruptions. Nor is there any
evidence of oscillation in depth from which the material was derived, even
if we accept differences in specific gravity as evidence of increase of dis-
tance from the surface. There was one, and only one, great break in the
mineralogical character of the lava. Changes in specific gravity were
gradual, but at the same time they covered nearly the entire range of varia-
tion ordinarily found in volcanic lavas. Such heavy minerals as zircon,
allanite, and garnet occur in the rocks of the lowest specific gravity, and in
the case of zircons they are nowhere found better developed than in the
glassy rocks which must have cooled near the surface. As these heavy
infusible minerals were the first to crystallize out, they should have sunk

BASALT AND KHYOL1TE. 285

to the bottom if their position in the molten mass was mainly a question of
specific gravity. The writer can not but regard the lavas as derived from a
local reservoir, all the ejected material having had a common source in
some primordial magma. The order of succession is governed by far-
reaching physical forces which may vary greatly in different volcanic areas,
dependent on conditions of heat and pressure. A powerful orographic
movement such as frequently happens during a period of volcanic action
may be sufficient to affect the entire geological conditions in any eruptive
center. In widely separated parts of the world the extravasated products
are singularly alike, yet the sequence of lavas within restricted limits show
very considerable variation.

Supposing the products of eruption and order of succession to have
been much the same over the geological province of the Great Basin, it
does not follow that the same succession of events took place in another
region where the geological conditions were obviously different. Within the
observations of the writer instances are known outside the Great Basin
where such an order of events not only did not take place, but where the
mutual relations of nearly identical lavas exhibit a succession strikingly ar
variance with the sequence of flow as found at Eureka. The Yellowstone
Park may be cited as an instance where the succession of lavas is some-
what different. In the latter locality the earliest eruptions were of inter-
mediate composition, consisting of hornblende-andesite and homblende-
mica-andesite. While the sequence of lavas may vary owing to geological
conditions, the laws governing the differentiation of lava hold good
everywhere.

Basalt and Rhyoiite.— The writer accepts, with some important modifica-
tions, the views of Mr. Clarence King regarding rhyolite and basalt, not
only as geologically closely related rocks, but also as extreme members of
the same primordial magma. He differs from Mr. King as to the manner
in which these extreme products were derived from an earlier molten mass.
It is nothing against this view of their common origin that rhyolitic out-
bursts frequently occur unaccompanied by basalt, or that basaltic exposures
abound without any evidences of the presence of acid lavas. Both rocks
break out in the closest proximity and not infrequently through the same

286 GEOLOGY OF THE EUKEKA DISTEICT.

fissures, under precisely similar geological conditions, in too many localities
not to realize their mutual relations. Such occurrences appear far too
common the world over to permit us to suppose them to be derived from
wholly independent reservoirs, yet everywhere occupying the same relative
positions with the basalt superimposed upon the rhyolite. Basalt and
rhyolite may be the final products from the same common source, but not
necessarily differentiated by a simple process of specific gravity separation
as demanded by Mr. King.

Within the area of the Great Basin there does not appear to be any
rock whose composition is due to a mingling of minerals characteristic of both
basalt and rhyolite. Both rocks, while they exhibit considerable range in
chemical composition, always remain sharply contrasted as regards mineral
constituents. Variations from normal rhyolite carrying orthoclase and
quartz in most instances show a transition toward hornblende-mica-
andesite through dacite, and never toward a pyroxenic magma, which
could hardly be the case if the process was due wholly to the dropping
out of the heavier minerals. Plagioclastic feldspars may be developed in
large numbers, but they belong to less basic species than those which char-
acterize normal basalt. In like manner variations from normal basalt tend
toward pyroxene-andesite and do not carry orthoclase. The process by
which the two magmas are formed is more in the nature of a differentiation
by molecular change and changes of density in the molten mass under
varying conditions of pressure and temperature than by a separation of
minerals during crystallization based upon differences of specific gravity.
In the Great Basin and probably all through the northern Cordillera con-
ditions were favorable in many localities for a complete differentiation of a
normal magma to its final products, rhyolite and basalt.

Now, if we suppose a magma of intermediate composition, from
which the necessary material to form rhyolite has been withdrawn, the
chemical constitution of the residue will depend largely upon the quantity
of rhyolite produced. If the quantity of rhyolitic magma thus formed is
relatively large, the remaining basaltic magma may be correspondingly
small and necessarily basic in composition. Again, if the bulk of acid or
feldspathic magma which separated out is small, there will remain a rela-

DIFFERENTIATION OF LAVAS. 287

tively large quantity of pyroxenic magma, but less basic;. If the lava
which crystallized out from this latter magma upon cooling is forced
upward to the surface, it may consist of both pyroxene-andesite and basalt,
as at Eureka. It may be wholly a normal basalt, as shown in a number
of localities in the Great Basin, or it may be largely made up of magnetite
and other iron minerals, forming a basic rock not yet recognized in the
Great Basin, but known elsewhere at several widely separated places in the
world. It is a matter of observation in many localities that where the bulk
of rhyolite is excessive the basalt outflows frequently occur in small bodies,
and it will probably be found that where there are relatively large basic
flows a portion of them will at least show an andesitic habit.

Differentiation of Lavas.— The existence at Eureka of two groups of lavas,
differing primarily in structure and the chemical nature of their transition
products, has been clearly demonstrated and evidence has been advanced
to show that they were derived from a still earlier molten mass. Processes
of differentiation similar to those by which the molten material beneath the
surface is supposed to be capable of breaking up into rhyolite and basalt,
are sufficient not only to account for the breaking up of a primordial
mass into a feldspathic and pyroxenic magma, but also to account for the
existence of partial magmas and an entire series of transition lavas such as
found at Eureka. The first products of such a molten mass would naturally,
but not necessarily, be a lava of intermediate composition, such as are
often seen as the earliest eruptions in volcanic centers. The first eruptions
at Washoe being earlier than those at Eureka were consequently more
uniform in composition. Differentiation in the magma had taken place
only to a limited degree, and it is by no means easy to distinguish
homblende-audesite from pyroxene-andesite. The splitting up of both
the feldspathic and pyroxenic magmas, the former into hornblende-
mica-andesite, dacite, and rhyolite, and the latter into pyroxene-andesite
and basalt, has already been described. It is difficult to conceive a con-
trolling physical force acting upon one magma which could not under
similar conditions of heat and pressure exert the same influences upon
fractional magmas, the differentiated products of a primordial molten ma».

288 GEOLOGY OF THE EUKEKA DISTRICT.

In applying this hypothesis of differentiation to molten masses the
question naturally arises, What would have resulted at Eureka if the slow
processes of differentiation going on in a magma before final crystallization
had either terminated earlier or progressed still further? On the one
hand, supposing a separation less complete than that at Eureka, a stage in
the development would be reached when a feldspathic magma would form
consisting of hornblende-mica-anclesite or dacite, or more probably both,
followed by pyroxene-andesite withoiit the interpolation of any body of
rhyolite. On the other hand, if the segregation of feldspathic magma had
gone on more completely than we find it at Eureka, there might have been
formed the same sequence of feldspathic lavas, only with a much larger
extravasation of rhyolite, in turn followed by basalt, -without the inter-
polation of pyroxene-andesite. Again, the earliest rock might have been
hornblende-mica-andesite of the feldspathic magma, succeeded rapidly by
pyroxene-andesite. If this series of lavas had been followed by a cessa-
tion of volcanic energy and a long interval of rest, and then by a renewal
of activity, the final product, after a still further separation of the magma,
would result in the extravasation of rhyolite and basalt. This latter
sequence of lavas gives the order of succession so frequently met with
throughout the Great Basin. At Eureka, as already described, no long-
time interval, geologically speaking, is recognized between the andesites
and rhyolites, while the dacites and rhyolites frequently present the appear-
ance of continuous flows.

In considering these phenomena it is important to bear in mind the
facts so frequently observed elsewhere in the Great Basin, that a crystalline
lava derived from a feldspathic magma of intermediate composition is, in
many instances, as shown by Richthofen and King, followed by a pyroxene
lava, and the latter is almost invariably of intermediate composition; a lava
still more acid by one correspondingly basic, and the extreme acid type by
the extreme basic type. A rhyolite may be followed by pyroxenic lavas
varying in composition, but the writer knows no instance in the Great
Basin where a rhyolite is succeeded by a more basic feldspathic rock, nor
where a basalt is followed by a less basic pyroxenic lava.

DIFFERENTIATION OF LAVAS. 289

This hypothesis of the progressive differentiation by molecular changes
in a fluid or a molten mass under varying conditions of temperature and
pressure is offered to explain the variations in chemical and mineralogical
composition of lavas. It i.s offered tentatively and with much hesitation, the
writer knowing the many difficulties involved in the problem. It is based
upon and is in accord with the facts seen at Eureka and confirmed by
observations in many volcanic areas elsewhere. It at least has the merit of
accounting for nearly all variations in the sequence of lavas which have
from time to time been noted in the Great Basin. It offers a rational
explanation for the recurrence of lavas in certain localities and accounts for
their absence in others. The pyroxene-andesite furnishes a marked instance
of such a recurrence. Occurrences of lava which have been regarded as
exceptional and difficult to explain by any general law are now seen to
fall within the prescribed limits of variation as laid down here. Nothing
seems more clear than that there are certain laws determining the sequence
of flow that govern the extravasation of lavas in every great volcanic cen-
ter, notwithstanding the fact that we may still be a long way from the cor-
rect interpretation in all its details.

Summary.— The Eureka District presents a most instructive volcanic
region standing quite apart from all other centers of similar eruption, yet
typical in the nature of its extravasated material of many localities in the
Great Basin.

The region offers no direct proof of the age of volcanic energy, yet
all evidence points to the conclusion that the eruptions belong to the Ter-
tiary period and for the most part to the Pliocene epoch. They may have
extended well on into Quaternary time, although there is no reason to sup-
pose that eruptions took place within historic periods.

As regards their mode of occurrence the principal eruptions may be
classed under four heads : First, they broke out through profound fissures
along the three great meridional lines of displacement, the Hoosac, Pinto,
and Rescue faults, and to some extent along the lesser parallel faults;
second, following the lines of orographic fracture, they border and almost
completely encircle the large uplifted masses of sedimentary strata like the
MON xx 19

290 GEOLOGY OF THE EUREKA DISTRICT.

Silverado and County Peak block and the depressed Carboniferous block
between the Hoosac and Pinto faults ; third, they occur in numerous dikes
penetrating the limestones ; fourth, they occur in one or two relatively
large bodies, notably Richmond Mountain and Pinto Peak, along lines of
displacement already mentioned.

All the lavas may be classed under the heads : hornblende-andesite,
hornblende-mica-andesite, dacite, rhyolite, pyroxene-andesite, and basalt.
They pass by insensible gradations from one to the other. All division lines
are more or less arbitrary ; they are necessary for the purposes of classifica-
tion, although they may not exist in nature.

Field observations clearly show that the order of succession of these
natural groups into which the lavas have been divided was as follows : First,
that the homblende-audesite was the earliest of all the erupted material ;
second, that the hornblende-mica-andesite followed the hornblende-ande-
site ; third, that the dacite followed the hornblende-mica-andesite ; fourth,
that the rhyolite closely followed the dacite; fifth, that the pyroxene-ande-
site succeeded the rhyolite ; sixth, that the basalt was the most recent of all
these volcanic products.

In chemical composition this entire series of lavas shows a range in
silica amounting to about 25 per cent, a range which is quite as wide as is
usually found in most centers of eruption even where the volume of lavas
thrown out is vastly greater and the duration of volcanic energy far longer.
Analyses show endless transition products between the extreme basic and
acidic lavas, with a tendency of the alkalies and silica to accumulate at the
acidic end and the material forming the ferro-magnesian minerals at the
basic end.

It is maintained in this work that all the varied products of eruption
are derived from a common source, a homogeneous molten mass. Under a
process of differentiation this earlier mass split up into two magmas, desig-
nated as a feldspathic and a pyroxenic magma. The lavas at Eureka are the
result of the same process of differentiation derived from one or the other
of these magmas. Beginning with hornbende-andesite, the earliest lava, the
feldspathic magma became more siliceous until the close of rhyolitie erup-
tions. The rhyolite was followed by pyroxene-andesite and the eruptions

SUMMAEY. 291

became more and more basic until the close of the volcanic period. The
feldspathic and pyroxenic lavas do not approach each other in their tenure
of silica within 2-25 per cent. In chemical composition the earliest erup-
tions of both magmas resemble each other, but from this common ground
they differentiate steadily until the feldspathic reaches the extreme acidic,
and the pyroxenic the extreme basic end of their respective series. The
extreme products of differentiation in any volcanic center in the Great
Basin are rhyolite and basalt.

CHAPTER IX.
ORE DEPOSITS.

Geological History.— It is not the iuteiitioii to enter into a detailed descrip-
tion of the various ore deposits of this region or of their mode of occur-
rence. An excellent monograph upon the mines and ores of Ruby Hill
has been published by Mr. J. S. Curtis,1 in which he gives in much detail
the results of his studies of the silver-lead deposits of the Richmond and
Eureka mines.

This report, however, would be incomplete if the writer, after devoting
much time to an investigation of the structural features of the Eureka
Mountains, constantly keeping in mind the relationship between the ore
bodies and the sedimentary and igneous rocks, should fail to state his con-
clusions as to the geological position of the ores, their age, and origin.
Moreover, as many geologists do not care for the details of mining develop-
ments, but feel a keen interest in all questions relating to mineral, deposi-
tion, it seems desirable to state here, for the use of the general reader, such
facts as bear directly upon the geological occurrence of the Eureka ore
bodies.

It has been demonstrated beyond all question, from the facts presented
in the preceding chapters, that the Eureka Mountains are formed of
orographic blocks of Paleozoic strata made up of quartzites, limestones,
and shales. These blocks, strongly contrasted by their orographic struc-
ture, are separated from each other by profound north and south faults.
Along the lines of these displacements east of Prospect Ridge enormous
masses of igneous rocks have been poured out, which have tended still
more sharply to intensify the lines of demarcation between the individual
blocks. The entire thickness of Paleozoic sediments can not be far from

1 Silver-lead deposits of Eureka, Nevada. Mon. U. S. Geol. Survey, VII, Washington, 1884.
292

RHYOLITES AND OKE DEPOSITS. 293

30,000 feet. Between the close of the Carboniferous and the close of the
Jurassic period dynamic action folded and faulted the strata, producing the
present complex structure and outlining the configuration of the mountains
much as they are found to exist to-day except such changes as have
been produced by denudation. Soon after the coming in of Tertiary
time the volcanic period began in the Great Basin, and probably not
long after it volcanic energy manifested itself in the Eureka Mountains.
Evidence seems to show that the profound displacements were augmented
by intrusive rocks, and in many instances fissures were formed along the
fault planes. Accompanying the fissuring of these faults by volcanic lavas
was the forming of lateral and oblique secondary faults, cross fissures, and
fractures, complicating the already disturbed sedimentary beds.

After the outbursts of andesites and rhyolites, and possibly in part
subsequent to the basalts, the deposition of the ores took place. The basalt
is known to follow the rhyolite. As regards the relative age of the ores and
basalt, there is no direct evidence other than that in the region of ore
bodies the audesites and rhyolites show the action of steam and solfataric
agents, whereas the basalts are for the most part comparatively fresh and
unaltered. In a number of instances it is clearly evident that the ores
followed the rhyolite intrusions, the former being found to lie wholly undis-
turbed upon the latter rock. It is true that over the greater part of the
region the ores do not come in direct contact with the rhyolites, but, on the
other hand, all evidences of pre-rhyolite ore deposits are wholly wanting,
and it is hardly conceivable that there could have been such deposits with-
out some evidence of movement at a time when the region was undergoing
strain and dislocation on all sides. Furthermore, nowhere, so far as known,
does the network of dikes on Prospect Ridge cut any earlier ore body.

Since the intrusion of the innumerable rhyolite dikes there is no
evidence of any orographic movement of sufficient intensity to disturb or
dislocate them by faulting of the strata, and the same may be said of
the ore deposits. This gives both to the dikes and ores a comparatively
recent origin in the geological history of the region. As regards the ores
it should be stated that they have undergone alteration and oxidation since
their deposition, and, as much of the loose, friable material occurs in lime-

294 GEOLOGY OF THE EUKEKA DISTRICT.

stone chambers and cavities, it is quite likely to have undergone some
movement by earthquake shocks during Quaternary time, but this is quite
another matter from profound orographic displacement of beds. To-day
there is absolutely nothing positively known as to the source of the rhyolite
material nor the deep-seated centers from which it originated, and this
is equally true as to the source of the heavy metals. With our lack of
knowledge on these matters it seems out of place to speculate in the present
volume as to their ultimate source. Probably no geologists, however,
would question the statement that the volcanic products came from below.

The writer, after a careful study of the facts observed at Ruby Hill
and Prospect Mountain, as well as of the entire Eureka region, is forced to
the conclusion that there exists the closest relationship between the rhyo-
lites and the formation of the ore deposits, although they have been
observed in actual contact in only one or two localities in the larger mines.
In almost all cases where mineral deposits are found, rhyolite intrusions are
known to penetrate the limestone in close proximity to the ores, and it is
presumable that in many instances the presence of such ore bodies might
be detected without the discovery of any intrusive rock. No theory of the
ore deposits seems applicable to this region that does not carry with it the
fundamental proposition that the ores came from below, as the result of sol-
fataric action which accompanied volcanic energy. Evidence shows that
the centers of greatest deposition of ores were not the same as those of
greatest eruptive energy, but that the latter are associated with the secondary
dikes and offshoots rather than with the great lines of volcanic outbursts.
Solfataric action may have continued and probably did continue for a long
period after the rhyolite eruptions had altogether ceased, during which
metallic sulphides filled certain preexisting fissures, cracks, chambers, and
crevices in the limestone. After the deposition of the sulphides came the
period of oxidation which, so far as can be told, lasted throughout the
greater part of Quaternary time.

Ores of the Cambrian.— In regard to the distribution and geological position
of the ores in the Paleozoic strata all evidence shows that although the
most remunerative mines and those explored to a great depth occupy some-
what restricted limits, ores of similar mode of occurrence and composition

ORES OF THE CAMBKIAN. 295

as those so successfully worked on Ruby Hill, are found throughout a wide
vertical range of sedimentary beds. No ore deposits are known below the
contact between the Prospect Mountain quartzite and the overlying lime-
stones upon Ruby Hill. As will be shown later these limestones on Ruby
Hill carry deposits of ore throughout their entire thickness from the quartz-
ite to the overlying Secret Canyon shale.

Along the slopes of Prospect Mountain from Mineral Hill southward
to Surprise Peak, the crushed and brecciated limestones have undergone
considerable local disturbance and are honeycombed throughout by fissures,
seams, and irregular crevices of various width and length. Many of these
openings lie parallel with the stratification ; others cut across the beds, occur-
ring in the limestone anywhere between the quartzite and shale without any
recognized order. Oxidized ore bodies occupy these openings, many of
them being connected by narrow channels and seams more or less filled
with mineral matter. The William.sburg mine on the west side is a good
example of the ore found deposited in characteristic chambers, while on the
east side at the extreme southern end of the ridge the Geddes and Bertrand
mine appears to be a well defined north and south fissure carrying much
rich ore. Among others of the larger bodies of ore may be mentioned those
of the Silver Connor and Banner mines, the latter a good example of a fissure
which occurs on the summit of the ridge. In but few of these ore bodies,
at least on the surface, have any rhyolites been recognized. A marked
instance, however, may be seen in the case of the Geddes and Bertrand
mine, where a powerful east and west dike cuts the limestone and overlying
shale in close proximity to the north and south ore channel.

Nowhere along the grand exposures of Secret Canyon shales have the
ores penetrated to the surface, the pliable, argillaceous clays flexing and
folding instead of fissuring, and everywhere serving as an impervious
barrier to the ascending currents. Fine examples of dike cutting are
shown near the Greddes and Bertrand Mine and again on the summit of the
watershed between New York Canyon and Secret Canyon shales, but at
the latter locality, so far as known, wholly unaccompanied by important
mineral matter.

The beds of the Hamburg limestone are similar in their structural

296 GEOLOGY OF THE EUREKA DISTRICT.

features to those of Prospect Mountain, the resemblance holding equally
good for the ore bodies. On Adams Hill the Price and Davies mine lies in
this formation in close contact with the Secret Canyon shales, whereas the
Wide Wide West occurs near the summit just below the Hamburg shales.
Other localities where more or less work has been done were sufficient to
indicate the existence of mineral deposits across the intervening belt of
limestone from one shale belt to the other. Along Hamburg Ridge the
limestones are not so much disturbed as on the steeper mountain slopes, and
fissures and seams of ore are by' no means as common, but on the other hand
mines like the Duuderberg and Hamburg have produced large bodies of ore,
second in quantity to none in the district outside of Ruby Hill, and these
stand in the closest connection with intrusive masses of rhyolite. Dikes
and irregular shaped bosses of rhyolite along the summit of Hamburg Ridge
indicate a network of eruptive rocks between the two great shale belts.
Like the underlying Secret Canyon shale horizon, the Hamburg shales,
although of much less thickness, are impervious to ascending mineral cur-
rents, and neither along the front of the mountain or north of Adams Hill
is there the slightest evidence of ore bodies penetrating it.

Ores of the Silurian.— Coming to the Pogonip horizon, ore bodies occur all
the way from the north end of Adams Hill southward to Roundtop
Mountain, at the extreme southern end of the region, with, however, con-
siderable intervals where none have been exposed near the surface. Numer-
ous mining claims have from time to time been recorded, but most of the
ground proved unprofitable and unproductive. On the other hand, such
mines as the Bullwhacker and Williamsburg, northwest of the town of
Eureka, and the Page and Corwin, southwest of Pinto Peak, have yielded
large quantities of mineral matter and may be said to exhibit well its
mode of occurrence in the limestone of this horizon. In the Williams-
burg Mine a well defined quartz-porphyry dike penetrates the limestone,
and dikes of similar rock come to the surface near the Bullwhacker. The
Page and Corwin was not being worked at the time of the writer's visit,
and it is impossible to say whether any intrusive dikes have broken
through the strata in close connection with the ore, but the limestones are

ORES OF THE DEVONIAN. 297

much disturbed and faulted and rhyolite has reached the surface only a
short distance from the mining property.

In the Eureka quartzite the only instances known of mineral deposi-
tion are those found on Hoosac Mountain, a description of which is given
elsewhere. They have been worked extensively and have yielded consider-
able ore. Here they are intimately associated with intrusive dikes of both
andesite and rhyolite offshoots from the great bodies which forced their
way upward along the Hoosac fault.

Throughout the Eureka Mountains the Lone Mountain horizon has
here and there shown evidences of mineral deposits when found in the
neighborhood of rhyolite outbursts, but over the greater part of the area
they exhibit no surface signs of ore-bearing material. An interesting
example of ore in the Lone Mountain horizon may be found at the
Seventy-six mine, in hard, flinty limestone on the northwest side of
McCoy's Ridge. While it can not be looked upon as remunerative property,
from the point of view of the present description it serves as an instructive
link in the chain of facts bearing upon the geological position of the
Eureka ore bodies. This is the only body of Lone Mountain limestone
lying in close proximity to the Hoosac fault, and, in consequence, partially
explains the occurrence of ore.

Ores of the Devonian.— Passing upward, without any intervening lithological
break, the Nevada limestones are in like manner frequently found to carry
oxidized, argentiferous lead ores in fissures and crevices in the regions of
profound faults. It by no means follows that rhyolites necessarily accom-
pany the ore at the surface, and still less that the latter occurs wherever
rhyolite penetrates the Nevada limestone through fissure planes. Instances
may be cited in the case of the Reese and Berry mine, just north of the
canyon of the same name, and again on the summit of Newark Mountain,
both localities indicating disturbances of strata without any .assignable
cause on the surface. The dislocation of beds may be due to intrusive
rocks which failed to penetrate the surface, the fissuring being filled by
mineral matter.

Along Rescue Canyon, where there is such a continuous and powerful
mass of rhyolite under geological conditions similar in many respects to

298 GEOLOGY OF THE EUREKA DISTRICT.

those observed along the Hoosac fault, mineral deposits are found identical
in mode of occurrence with those found on Prospect Mountain, although
less productive. A line of ore deposits follows Rescue Canyon on the east
side in the highly inclined strata of Century Ridge. The Rescue mine is
the most important property, exploration having developed several small,
but rich, chambers of ore in following down the shaft between 400 and 500
feet below the surface. Other mines on Century Ridge are the Queen and
Maryland, both of which resemble the Rescue mine.

In the Alhambra Hills a shaft has been sunk in the Fairplay mine, 85
feet in depth, following down a clay seam between well denned walls of
limestone. It carries a good deal of galena. The White Rose mine closely
resembles the Fairplay and lies in nearly the same geological horizon.

Crossing to the Mahogany Hills, on the opposite or west side of the
Eureka Mountains, we find mining properties on the southeast side of
Brush Peak at localities designated as the Mountain Boy and Kentuck mines.
They show that mineral matter was deposited under geological conditions
similar to those found elsewhere. Again, at the head of Browns Canyon
there is a very decided break in the limestone, accompanied by a sharp
anticlinal fold, along the axis of which occurs an outflow of rhyolite pre-
senting geological conditions that might readily lead one to look for ore.
Indications of mineral deposits were found at the surface sufficient to war-
rant mining exploration, and an ore channel followed for considerable
distance into the limestone. A study of the geological position of these
different ore bodies makes it clear that they occur throughout the Nevada
limestone, being found near the base of the epoch and again not far below
the summit. With the coming in of the White Pine shales all the charac-
teristic oxidized and unoxidized ores of the district cease, and they fail to
reappear in any of the higher geological horizons.

NO ores in the Carboniferous.— Nowhere within the district have ores been
recognized in any of the grand divisions of Carboniferous time. In the
Diamond Range northward and westward of Newark Mountain the strata
seem to be entirely free from mineral matter. It is possible that mining claims
may have been recorded along some superficial outcrop or some segregation
of mineral matter, but these are so obscure and unpromising and usually

KANGE OF ORE DEPOSITS. 299

without any indication of the precious metals that they may be wholly
discarded. The same may be said of the entire area of the Carboniferous
block lying between the Hoosac and Pinto faults. It is somewhat remark-
able that in this latter block, which lies in the very center of volcanic
action, no mineral occurrences of any importance are known. Along the
two great meridional faults enormous masses of igneous rocks have been
poured out, notwithstanding which no ore deposits have been reported
either on the east side of the Hoosac fault or on the west side of the
Pinto fault.

Geological Range of Ore Deposits.— It will be seen from these facts that the
ore deposits of Eureka are found throughout a wide vertical range, extend-
ing from the base of the Prospect Mountain limestone to the summit of the
Nevada limestone, occurring in every grand division of the Cambrian,
Silurian, and Devonian periods, with the exception of the two great shale
belts — the Secret Canyon and Hamburg shales. From the base of the
Prospect Mountain limestone to the top of the Hamburg shale it is esti-
mated that there are 6,200 feet of strata; the Siluiian rocks measure 5,000
feet and the Nevada limestone of the Devonian 6,000 feet. This gives
from the base to the summit of the included strata over 17,000 feet of
sedimentary rocks, through which argentiferous lead ores have been
deposited on a sufficiently extensive scale to encourage more or less
expensive outlays for mining exploration.

From the rapid review of these facts it is evident that within the area
of the Eureka District the ores are by no means restricted to any definite
geological horizons and have been deposited in siliceous as well as calcareous
strata. Notwithstanding that the ore . bodies occur through a great thick-
ness of rock, it still remains true that the greater part of the mineral depos-
its and probably all those which have proved remunerative to the investor,
lie within restricted limits. The most productive mines, those carrying
the largest and richest bodies of ore, are found in Cambrian strata. This
is owing to orographic and structural conditions rather than to the
geological age of strata or the chemical nature of sediments. A study of
the structural features of the mountains together with the mode of occur-
rence of the rhyolite eruptions shows that the age of the rock has but little,

300 GEOLOGY OF THE EUREKA DISTRICT.

if anything, to do with the occurrence of the deposits. They depend more
upon the fissuring and fracturing of the mountain uplifts and the relations
of the accompanying faults to the outbursts of rhyolite.

A study of the mountain blocks and distribution of ores brings out the
fact that what has been designated the Prospect Mountain uplift, lying
between the Hoosac fault on the east side and the Sierra and Spring Valley
faults on the west, embraces pretty much all the valuable mineral deposits
which have as yet been successfully developed. Within the limits of these
lines of faulting are embraced all the mining properties extending from
Adams Hill southward to Surprise Peak, including those on the west side
of Prospect Mountain, together with the Dugout mine at the southwest base
of the peak on the west side of the anticlinal fold. In preceding chapters
the structural features of Prospect Mountain Ridge and relations between
the sedimentary beds and intrusive dikes have been described with some
detail. As has already been shown, the strata between these faults belong
to the Cambrian and Silurian periods up to and including the lower portion
of the Lone Mountain horizon exposed on the north side of McCoy's Ridge.
The principal overflows of rhyolite have been along the line of the Hoosac
fault, the two most powerful centers of extravasation being located at Pinto
Peak and Purple Mountain. Purple Mountain lies in the angle between the
Hoosac and Ruby Hill faults, and it is along this latter fault that rhyo-
lites come to the surface all the way from New York Canyon to the Jack-
son fault, thence crossing the latter fault, fill the fault-fissure for a consid-
erable distance along the north slope of Ruby Hill, but without building
up any accumulation of rhyolite on the surface.

While it can not be absolutely demonstrated, all evidence bears out the
assumption that the dikes penetrating Prospect Mountain Ridge have a
deep-seated connection with the source of the rhyolite material which has
furnished the surface outflows all along the line of faulting. It can hardly
be doubted that both forms of the same eruptive rock mass have had an
identical deep-seated origin. It should be also borne in mind that it is only
in exceptional instances that dikes and off-shoots from any parent body of
lava can be traced to their source step by step in the field without any
break in continuity.

EUBY HILL DEPOSITS. 301

Ruby Hill Ore Deposits.— Mr. J. S. Curtis, in his elaborate monograph upon
the ore deposits of Ruby Hill, has embodied the results of much careful
investigation of the underground exploitations of the mines. His work is
accompanied by numerous vertical and cross sections, compiled from the
original mine surveys, indicating the positions of the different ore bodies
and their mutual relations. It is unnecessary, therefore, to enter into the
details of the economic geology, and only such facts will be given as will
enable the reader to form a correct conception of the geological position
and mode of occurrence of the ore bodies, not only upon Ruby Hill but
those found throughout the district. Ruby Hill, from a geological point of
view, may be taken as typical of the deposits in what has been designated
as the Prospect Mountain uplift. For the details of the mines, and their
extensive underground workings the reader is referred to Mr. Curtis's report.
In chapter v, of this report, upon the descriptive geology, there will be
found an account of the geological structure of Ruby Hill and Adams
Hill.

By reference to the accompanying map (PL i,) and section (Fig. 3, PL
ii,) it will be seen that Ruby Hill is made up of the tliree underlying forma-
tions of the Cambrian. They possess a fairly uniform dip, although pre-
senting occasional abrupt changes due to faults and fractures. This dip
along the surface may be taken at 40°, and in the lowest workings of the
Richmond mine, which have reached a depth of over 1,200 feet, this angle
of inclination is still maintained. At the surface the line of contact between
the quartzite and limestone is easily made out all the way from the Jackson
fault to the west base of Ruby Hill. Near this latter fault the contact is
first observed just west of the American shaft and the Jackson mine. It
crosses the summit of the spur on which the Phoenix mine is situated and
follows along the southern slope of Ruby Hill, the quartzite at one point
rising to within 160 feet of the summit. In the underground workings flu-
plane of contact between the quartzite and limestone has been exposed in
all the mines and at very many of the levels, the latter frequently running
along the contact of the two formations and occasionally cutting the
quartzite where it projects to the north beyond the course of the drifts.
Numerous cross-cuts have also been run into the underlying rock.

302 GEOLOGY OF THE EUREKA DISTRICT.

These workings, although they do not offer a continuous exposure, are
sufficient to give the course of the quartzite all the way from the Jackson
to the Albion. Beginning with the Jackson mine, the plane of contact has
a course a little east of north, gradually turning more and more to the west
until at the Albion it curves slightly south of west. In the lower levels of
the mines this contact plane, as mapped by the underground surveys, pre-
sents roughly a concave outline curving outward toward the north or away
from the granite mass of Mineral Hill. This curve, however, is by no means
symmetrical, the tendency of the quartzite in making so sharp a bend
being to break abruptly and irregularly and for short distances to be forced
outward, assuming directions quite at variance with the general course ;
the dip frequently changing with the strike. This tendency of the quartz-
ite in fracturing to be forced outward beyond the line of the curve is well
shown just west of what is known as the compromise line between the
Eureka and Richmond mines. It may be seen all the way from the sur-
face down to the ninth level of the Richmond. Mr. Curtis has carefully
mapped the underground contacts, not only between the quartzite and lime-
stone but for all three formations. By reference to his map1 both the con-
cave outline of the beds and the irregularities of strike and dip may be
seen at a glance.

The overlying limestone and shale conform in their general outline
with the quartzite, the shale, however, exhibiting a decided tendency, as is
usually the case with argillaceous strata, to bend and fold rather than to
break abruptly.

Across the limestone on the east slope of Ruby Hill runs a profound
fault which, on account of its bearing upon the geology of the region, has
been designated the Ruby Hill fault. It is a continuation of a line of
faulting coming up from the southwest. From New York Canyon, where
the Ruby Hill fault leaves the Hoosac fault, to its intersection with the
Jackson fault the nearly straight course is easily followed by a chain of
rhyolite outbursts as well as by the conformity of strata Where it
crosses the Jackson fault its direction is somewhat disturbed and is not so
readily made out, but near the American shaft it reappears, with a course a

'Op. Cit., PI. ni.

RUBY HILL FAULT.

few degrees west of north, passing just west of the Jackson shaft. From
this point westward it is difficult to follow the Ruby Hill fault on the
surface, as it lies wholly in limestone more or less concealed by soil and
de"bris, and the rhyolite which to the southeast of the Jackson fault
materially aids in tracing the displacement nowhere comes to the surface
after crossing the latter fault. At the time the accompanying map was
printed the line of the Ruby Hill fault had not been followed west of the
Jackson mine, but since then Mr. Curtis has traced it through the under-
ground workings of all the mines as far as the extreme limit of exploration
in the Albion.

According to the investigations of Mr. Curtis the fault after leaving the
Phoenix mine runs in a nearly northwest direction, agreeing closely with its
course on the east side of the Jackson fault. It passes just to the north-
east of the KK shaft and southwest of the Richmond office. It persistently
cuts all formations, quartzites, limestones, and shales alike, scarcely devia-
ting from a straight line and apparently uninfluenced by the physical
conditions of the rock. In like manner the fractures and displacements
produced by the earlier orographic changes whicli elevated the region have
exerted but little influence on the course of the Ruby Hill fault. A study
of the disturbances and dislocations of the strata point to the conclusion
that this fault, with its accompanying fissure, was the last dynamic move-
ment in the history of Ruby Hill. Wherever underground explorations
admitted of observation the average dip of the fissure plane was found to
be about 70° to the northeast. Southeast of the Jackson fault the width of
the fissure and the dip of its plane are unknown.

Subsequent to the formation of the fissure and probably nearly coinci-
dent with it was the filling of the wider portions with intrusions of rhyolite,
notwithstanding the fact that they nowhere quite reach the surface on
Ruby Hill.

Evidence goes to show that the volcanic energy displayed along the
fault line expended the greatest activity near its junction with the great
Hoosac fault, the powerful extravasations of rhyolite gradually dving out
toward the northwest, and beyond the intersection with the Jackson fault
failed to overflow the top of the fissure walls. The rhyolites exposed in

304 GEOLOGY OP THE EUEEKA DISTRICT.

the mines rarely attain an average width of more than 15 to 20 feet across
the broadest expansions, although instances of much greater width occur in
the Phoenix. Decomposed rhyolite is recognized along the fissure in botli
the Jackson and Pho3iiix mines. It is intersected by the Jackson shaft
above the third level, and the cross-cuts from the old Jackson shaft on both
the third and fourth levels expose the rhyolite body oil the main fissure.
In the Phoenix, rhyolite is found on all the lower levels wherever they inter-
sect the fissure. Proceeding westward the fissure narrows, but the rhyolite
may still be detected on the sixth level of the KK, although so thoroughly
altered as to have lost the distinctive characters of a lava. In the Rich-
mond mine no rhyolites nor rhyolitic clays are recognized, nor have they
been observed anywhere along the fissure to the northwest. The fissure
gradually narrows and finally dies out and the fault is lost where the rocks
pass beneath Spring Valley, a short distance beyond the Albion mine.

Transition products from unaltered lava to highly kaolinized rhyolite
are found along the fissure in every stage of decomposition. In the Jack-
son mine the rhyolite origin of much of the filling of the fissure is deter-
mined by the presence of mica flakes and quartz grains imbedded in blue
clay. These transition products grade off into nearly pure clays holding
grains of quartz still unaltered, whereas the feldspars and glass base have
undergone such complete kaolinizatiou that the volcanic origin of much of
this material could not be made out but for its association with fresher
rock. In places the entire filling between the walls of the fissure, which
may be only a few inches in width, is composed of rhyolite clays, the
extreme product of the action of steam and solfataric fumes upon injected
volcanic rock. They have all the physical properties of and behave like
ordinary clays. Between the Phouuix and the Richmond occur bodies of
clay which are undoubtedly derived from the rhyolite, with an admixture
of more or less calcareous material. Such material abounds where the
fissure walls stand only a few inches apart, and a movement has pulverized
the limestone along the fault plane, producing an admixture of rhyolitic
clay with comminuted siliceous limestone. In the Richmond the filling
of clay between the fault planes is derived solely from attrition of the
walls; at least, no rhyolite can be detected. It is possible that the crack

SECONDARY FISSURE. ;j(i;)

became too narrow to permit of the forcing upward of the liquid lava
without sufficient power to widen the space between the inclosing walls.
Here the volcanic quartz grains are wanting, the calcareous nature of the
material determining its origin.

In following its northwest course the main fissure crosses the entire
width of the limestone of Ruby Hill, which, by means of the network of
underground workings, may be easily studied and compared from base to
summit with the same horizon on Prospect Ridge. Near the American
shaft the distance from the main fissure to the underlying quartzite is only
a few feet. This distance increases in the Phoenix and Jackson mines as
proved by the crosscuts on different mining levels, the limestone belt
gradually becoming wider toward the west. Near the Richmond mine the
main fissure, having traversed the limestone, follows the contact between
Prospect Mountain limestone and Secret Canyon shale for a considerable
distance, beyond which it is lost. In the Jackson mine a shale belt is
exposed which, although fairly persistent in the underground workings of
the KK, Eureka, and Richmond mines, never reaches the surface, owing to
the fault across the limestone. Without much doubt the shale corresponds
with the broad irregular shale belt found on Prospect Ridge and designated
the Mountain shale. To the south of this main fissure, along the contact of
the Prospect Mountain quartzite and the Prospect Mountain limestone,
occurs a line of faulting which, although of less magnitude than the Ruby
Hill fault, is, on account of its relation to the ore bodies, quite as important
from an economic point of view. Like the Ruby Hill fault it was formed
subsequent to the minor displacements connected with the earlier orographic
movements. Evidence seems to show that this faulting took place contem-
poraneously with that of the Ruby Hill fault and has been named the sec-
ondary fissure. This secondary fissure possesses an average dip of 40°,
coinciding with the contact plane between the two formations, and as the
ano'le of inclination of the main fissure uniformlv stands at 70°, the two

O •»

faults might naturally be expected to come together at 110 great distance
from the surface. Exploitation confirms this supposition and in the lower
workings of the mines the secondary fissure is easily traceable into the
Ruby Hill fault, but has nowhere been observed to cross it. Indeed, this

MON XX 20

30fi

GEOLOGY OF THE EUREKA DISTRICT.

secondary fissure may be considered as an offshoot from the more per-
sistent and profound Ruby Hill fault. Where the quartzite and limestone
show a tendency to curve to the south and southwest, following around the
spur of the mountain, the secondary fissure abandons the contact plane and
with a northwest course enters the limestone, leaving a block of the latter
rock between it and the quartzite.

HHv Phoenix Mine

Quartzite Crushed JJimes'tone SfatLel '.Rlyolitc
.Litnestone

Flo. 6 Cross-section in PboPnix mine.

Within the wedge-shaped limestone body included between the Ruby
Hill fissure and the secondary fissure have. been found all the deposits of
ore which were of sufficient value to repay extraction. Up to the time of
the present investigation all exploitations by crosscuts from the main levels

CAVES AND CREVICES. 3<)7

outside these limits have failed to discover any accumulations of ore. The
limestone on the north side of the Ruby Hill fault presents a fairly com-
pact uniform appearance occasionally well stratified. Between the two
fissures the limestone is crushed and broken, everywhere showing the effect
of great pressure accompanied by movement. Much of this rock indicates
alteration by chemical process since the fracturing and displacement. The
limestone south of the secondary fissure is for the most part black in color,
siliceous in composition, and in distinction to the limestone between the
fissures uniform in structure. It is more easily recognized than the other
belts and resembles the lower strata of limestone on Prospect Ridge. By
the miners the limestone beneath the secondary fissure is known as the
back limestone ; that found between the two fissures is called either the
crashed or mineral limestone, while the beds overlying the main fissure are
referred to usually as the front limestone.

Figure 6 represents the relative position of the Ruby Hill fault, along
which the main fissure has been formed, to the secondary fissure as shown
by a vertical cross-section in the Phoenix mine. It will be seen that the
two fissures come together just below the sixth level of the mine. The
rhyolite dike follows the Ruby Hill fault, and nowhere deviates to the
southward in its upward course. In the ground shown by the section the
secondary fissure adheres closely to the line of contact between the quartzite
and limestone. The ore body is cut by the shaft extending from the surface
nearly down to the point of contact between the formations. Near the
third level the shaft enters the underlying quartzite and has been sunk only
a short distance below the fifth level, the sixth being reached by an incline.

Preexisting Caves and Crevices.— It has 1)6611 Stated that the fissure which

accompanies the main fault on Ruby Hill has been the principal channel
through which the intrusive rhyolites have been forced upward to within a
short distance of the surface, if, indeed, they have not accumulated on top and
subsequently been removed by erosion. On the other hand, the secondary
fissure carries no rhyolite, but, accompanying it, especially along the contact
of rhyolite and limestone, are large and valuable bodies of ore. Between
these two fissures the crushed limestone shows the evidence of faulting
approximately parallel with the Ruby Hill fault, and due to forces acting at

308 GEOLOGY OF THE EUREKA DISTRICT.

the time the main ridge was uplifted. Other faults indicate lateral thrust, but
they are less effective than the former and may be of later origin, due to sub-
terranean forces connected with the period of volcanic energy. Accom-
panying these are innumerable small fissures, seams, crevices, chambers
and channels of varied shapes and sizes. Many of these owe their origin
simply to the dynamic effects of upheaval. Others are best explained on
the theory of surface waters percolating downward along lines of least
resistance, widening fissures and enlarging cavities. If these waters were
charged with carbonic acid, chambers and irregular shaped galleries and
drainage channels must necessarily have been dissolved out of the limestone.
A study of these channels and their intricate connections tends to the belief
in the theory of preexisting caves and underground water courses before
the introduction of ore.

Fining of Fissures.— The coming in of the volcanic period would be quite
likely- to disturb and dislocate any previous system of subterranean drain-
age, in some places completely closing and in others opening new channels
by the formation of fresh cracks and crevices. Subsequent to the penetra-
ting of the main fissure by rhyolite came the filling of minor fissures and
other openings in the limestone by the ascending mineral solutions and
gaseous currents. Wherever the narrow fissures admitted of it these open-
ings and chambers were more or less filled with mineral matter precipitated
from solution, the passage ways in many instances being left neai'ly ban-en
or only carrying stringers and slight indications of earthy, ocherous
material probably deposited before the dying out of the active mineral
currents. In some instances the narrow connecting channels between the
larger openings are richer in mineral matter than the chambers themselves.
It would be useless to speculate on the reasons why certain fissures and
chambers earned ores and others were left barren. The freaks of deposi-
tion from ascending currents — in some places rapid, in others slow — and the
varying conditions of temperature and pressure brought about by the vary-
ing intensity of solfataric action would produce endless differences in the
mode of occurrence. Channels which at one time presented conditions
most favorable for deposition might at a later period become entirely cut
off from ascending currents. Anyone who has observed carefully the

AGE OP ORE DEPOSITS.

apparent freaks in the deposition of mineral matter in such centers of
thermal activity as the Yellowstone Park realizes how little it takes to
deflect the course of ascending aqueous or gaseous currents and how, under
varying conditions, mineral deposition is liable to undergo change within
restricted areas. The occurrences of ore bodies on Ruby Hill and Pros-
pect Mountain are variable and uncertain, but such as one might anticipate
from their mode of formation.

Relative Age of Rhyolite and Ore.— The best example Oil Ruby Hill showing

direct contact between the ores and rhyolite bodies was observed in the
Jackson mine, but probably a still finer illustration of the relationship
between the two with the ore lying undisturbed along the under side of a
highly inclined dike was seen in the Dunderburg mine on Hamburg Ridge.
It was exposed on the third level of the mine near the main shaft which
had been sunk all the way in hard limestone. The rhyolite dike varied
from 1 to 8 feet with an average width of a little more than 2 feet. The
strike of the dike was approximately east and west with a dip to the north,
whereas the course of the ore channel stood nearly at right angles to it.
The ore never penetrated the rhyolite, its course being deflected on
approaching the intrusive dike. At the shaft house the ore body
measured 50 feet in thickness. Opportunity for examining the contact
was excellent as much of the ore still remained in place, while over
other areas along the contact the ore had been stripped off, rendering it
possible to observe the relationship between it and the dike, as well as the
position of both to the inclosing limestone. Nowhere did the ore penetrate
the rhyolite, and nowhere had any ore been found inclosed within the dike.
In like manner the ore was wholly free from rhyolite. Nothing could seem
more clear than that the mineral matter had been quietly deposited from
solution along the under side of a highly inclined dike, neither could any-
thing be seen suggesting a replacement of rhyolite by ore, although imme-
diately along the contact there is considerable kaolinization of rhyolite.
Such instances as the Jackson mine on Ruby Hill, the Dunderberg on
Hamburg Ridge, and the position of the rhyolite and ore at the Geddes
and Bertrand mine in Secret Canyon, furnish strong proof corroborating
other evidence that the ore followed the rhyolite.

310 GEOLOGY OF THE EUREKA DISTRICT.

Kaoiinization of Rhyoiite.— A careful study of the transition products of
kaolinization of the rhyolite shows how coinplete the decomposition has
been along the Ruby Hill fissure west of the Jackson fault. Equally com-
plete and impressive are the evidences of similar kaolinization in the Dun-
derburg and on the summit of Hamburg Ridge above the Dunderburg and
Hamburg mines wherever the rhyolites offer good exposures on the surface.
Along the line of contact on the summit of the ridge between the
thoroughly whitened rhyolite and the dark limestone there has been con-
siderable prospecting for ores, but without success. Perhaps the best
instance of the alteration of the rhyolite is found where the drainage chan-
nel of New York Canyon in coming down from Prospect Ridge has worn
a deep passage through the Hamburg limestone ridge. It is seen in the
limestone bluff on the south side where an exploring tunnel was run into
the hill following the contact between the nearly vertical rhyolite dike and
the inclosing limestone. There is exposed here a fine example of com-
pletely kaolinized rhyolite possessing all the properties of an ordinary clay,
except that the quartz grains of rhyolite still remain unacted upon with
here and there a little unaltered sanidin. This is an instance of thoroughly
kaolinized rhyolite without the presence, so far as known, of any ore body
as far as the tunnel was run. Finding no indication of ore, the tunnel had
been abandoned after running a long way into the hill along the contact
of the two formations.

Ores Deposited as Sulphides.— Solfataric action which accompanied the filling
of the intricate net-work of openings in the limestone may have continued
throughout a long period of time, the mineral matter accumulating slowly.
That the ores were originally deposited as sulphides there seems no good
reason to doubt, an opinion probably held by all geologists who have exam-
ined the district and who believe that the ores came from below.

The enormous amount of oxidized products indicates that the original
ore was mainly galena and pyrites. Evidence that such was the case on
Ruby Hill is shown by the discovery of fragments of galena and pyrites
found in a perfectly fresh state scattered throughout the ore bodies near
the surface as well as at great depths. These fragments are frequently sur-
rounded by partially oxidized material showing a nucleus or kernel of still

NATUKE OF ORES. 311

unaltered sulphide. Assuming it to be correct that the ores were originally
deposited as galena and pyrites, it is most difficult to see how such vast accu-
mulations of these sulphides could have been formed in any other way than
in the preexisting caves and openings. Any theory with which we are
acquainted of chemical and physical replacement of the limestone or dolo-
mite seems wholly inadequate to meet the necessary conditions. Pseudo-
morphs of galena and pyrites after calcite have been described as minera-
logical curiosities and possibilities, but nowhere have they been found in large
quantities in any mine, and so far as the writer is aware they have never
been recognized at Eureka. On the other hand, underground drainage
channels probably existed before the deposition of the ore bodies, and with
the coming in of the ascending mineral currents it is most natural that they
should have followed these channels in their upward course.

The Ores.— After the deposition of the metallic sulphides came the period
of oxidation, which probably continued throughout the greater part of
Quaternary time and was due to atmospheric agencies, mainly percolating
surface waters. On Ruby Hill this oxidation may be said to be nearly
complete, unaltered galena and pyrites being exceptional occurrences above
water level. It has produced a great variety of secondary minerals, but
such as it might be anticipated would follow the complete alteration of an
admixture of argentiferous galena and auriferous pyrites accompanied by
compounds of arsenic and molybdenum. Mr. Curtis has devoted consider-
able time to an investigation of the miueralogical character of the ore and
has published a catalogue of the minerals known to occur on Ruby Hill.
The secondary products of oxidation include a long list of carbonates,
sulphates, arseniates, molybdates, and chlorides. The ore is exceptionally
rich in gold. Wulfenite occurs in brilliant transparent crystals, varying in
color from lemon-yellow to bright orange, and is found in large clusters
filling cavities or incrusting other minerals. Since the opening of the
mines the wulfenite of Eureka has been much sought after by mineral-
collectors both in this country and in Europe. It appears to be the only
species which Ruby Hill has developed that has any exceptional value from
a mineralogical point of view.

The following analysis of a sample of all the ores smelted at the

312 GEOLOGY OF THE ECliEKA DISTRICT.

Richmond furnace tor the year 1878 was made for the company by Mr.
Fred Clandet, of London:

Lead oxide 35-65 Lead 33-12

Bismuth

Copper oxide -15 Copper.. -12

Iron sesquioxide 34-39 Iron .24-07

Zinc oxide 2-37 Zinc 1-89

Manganese oxide -13

Arsenic acid 6-34 Arsenic 4-13

Antimony -25 Antimony -25

Sulphuric acid 4-18 Sulphur 1-67

Chlorine

Silica 2-95

Alumina -64

Lime 1 -14

Magnesia -41

Water and carbonic acid 10-90

Silver and gold -10

99-60

Silver, 27'55 troy ounces per ton of 2,000 pounds. Gold, 1'59 troy
ounces per ton of 2,000 pounds.

The analysis is taken from the records of the Richmond company.
It is reproduced here, as it gives the average composition of a large quantity
of ore probably derived from the same ultimate source, and is therefore not
without scientific value, although it cannot be considered as representing any
definite deposit or the product of any special mode of formation.

The method of stating the present composition of the ore is somewhat
misleading. All the lead is estimated as lead oxide, whereas a very appre-
ciable amount of lead sulphide must have been present, as is shown by the
examination of any ore pile. It indicates, however, how completely the
ore body has undergone oxidation since deposition. No determination was
made of the molybdic acid, yet it is hardly possible that none was present
when it is easily detected in almost any ore sample. The low percentage of
base metals other than lead and iron shows the great uniformity and sim-
plicity of the original sulphides. That the ores vary in composition within
certain limits, dependent upon the position of the ore chamber and their

LOUD BYKON AND KELLY MINES.

313

connection with the fissures and pipes, is admitted by all who have care-
fully studied the deposits. Occasionally the ore contained in small pockets
in the limestone will present a fairly uniform composition, but differing
widely from that found in adjacent bodies, and in some of these zinc and
copper accumulate in relatively large quantities as compared with the
entire mass of ore. Such variations appear to be much more common on
Prospect Ridge than on Ruby Hill.

Two of the most singular and interesting of these isolated deposits
were found near the summit of Prospect Ridge, on two adjoining mining
properties, known as the Lord Byron and Kelly mines. They resemble
each other so closely that they may very properly be considered as having
a common origin and possibly filling the same fissure, the connection
between them being concealed beneath the surface.

The following analyses of these complex ores were kindly made for
the. writer by Dr. W. F. Hillebrand, of the U. S. Geological Survey:

Lord Byron.

Kelly.

RisTmit.hnns oxide ... .....

29-54

40-023

Lead oxide

1-97

Tin oxide

0-273

Telluric acid

12-69

1-077

A n t iinoii ions oxide

Not determined.

0 -752

Copper oxide ...

1-00

0-559

Ferric oxide

15-30

0-713

1-350

Alumina

0-23

0-259

Zinc oxide

5-54

0-350

Uranium oxide

Not determined.

0-081

Lime

2-77

20-650

Magnesia

Trace.

5-691

Carbonic acid

2-56

25-002

Chlorine .

Not determined.

0-047

Phosphoric acid . .

0-24

0-079

Sulphuric acid .. ..

2-41

0-520

Silica .

15-31

1-402

Water

3-39

0-913

Silver

1-01

0-034

Gold

0-001

0-002

Total

93-961

99-777

314 GEOLOGY OF THE EUltEKA DISTRICT.

Dr. Hillebraud regards the greater part of the tellurium as occurring
in the state of telluric acid, because in boiling with hydrochloric acid much
chlorine is evolved, although there is 110 manganese present, and on reducing
it with sulphurous acid the tellurium is immediately precipitated. Dr. Hille-
brand notes the absence of all sublimation products of tellurium, antimony,
and arsenic on heating the ore in an open tube, indicating the previously
complete oxidation of these substances. The telluric acid is nearly sufficient
in the Lord Byron ore to combine with the bismuth, leaving enough over
to combine with the small amount of silver present. Some silver in both
ores probably exists in combination with tellurium as a telluride. These
ores possessed no commercial value on account of the exceedingly small
amount of them obtained. In the case of the ore from the Kelly mine
it will be seen that there is a large amount of carbonate of lime present,
but it is not possible to say whether this occurred as calcite, a secondary
product, or as limestone derived from the country rock.

Occasional pockets or crevices in the limestone are characterized by
the deposition of wad and other compounds of manganese. Silica is rarely
found in the fissures and chambers in the Richmond and Eureka mines, but,
on the other hand, most of the deposits on Prospect Ridge carry more or
less quartz, and associated with it there appears to be, judging from the
assay reports, an increase in the amount of gold. Silica also characterized
the mines of the Hamburg Ridge, as is shown not only in some of the
larger deposits, but in such properties as the Connolly and California mines.
Variations in silica present no greater range than other mineral matter
deposited under conditions of solfataric action. It is known as one of the
most common products from deep-seated sources. The noticeable feature
about the silica is not its occurrence on Prospect Ridge, but rather its
absence from the Ruby Hill fissure and connecting chambers.

As has been previously mentioned, the mines of Ruby Hill have
yielded up to the time of the present investigation gold and silver to the
value of $60,000,000, accompanied by over a quarter of a million tons <>t
commercial lead. The large amount of iron contained in the ores has
never been estimated. These enormous products of the heavy metals,
deposited in small openings in the Ruby Hill limestone, within a very

RECENT CHANGES. 315

limited area, through narrow fissures, from some deep-seated source, cer-
tainly present, in their scientific aspects, most interesting problems to the
geologist.

Recent changes.— Following the oxidation of the sulphides and in some
degree associated with it, came the partial rearrangement of the oxidized
material under the influence of percolating surface water, in some instances
removing the ore from narrow passageways and fissures and sweeping it
into larger receptacles, where, together with ore already deposited, it was
piled up on the limestone floors This rearrangement of the oxidized
material, seen upon opening several chambers on Ruby Hill in the course
of mining exploration, appeared so self-evident that no other theory of their
formation seems adequate to account for all the observed facts. Such ore
receptacles, although more frequent near the surface, were opened at differ-
ent depths, but usually along lines that gave every appearance of having
been ancient water courses. On opening a chamber the tops of the ore
piles would be found covered by an accumulation of dust, fine sand, and
material foreign to the ore body. The stratification of material shown in
cross section and the settling of the heavier particles under the action of
water were too convincing to admit of any other mode of formation. Many
of the pipes coming down from the surface would be found wholly barren
of ore, yet carrying fragments of limestone rounded and worn smooth by
he action of percolating subterranean waters charged with carbonic acid.
Such recent drainage channels and pipes are by no means restricted to
Ruby Hill, but occur equally well preserved on Prospect Mountain, and
may be seen with more or less distinctness in some of the tunnels cutting
the ridge. They are well shown in both the Eureka and Prospect Moun-
tain tunnels and are so large as to serve the purposes of ventilation, and
in one instance, at least, so straight as to admit light from the surface.
Many of the tunnels suggest the existence of subterranean water courses
at a time when the country was less arid than at present

Dr. J. S. Newberry1 has cleverly suggested that climatic changes, with
alternating wet and dry periods, within Quatemary time in the Great Basin,
may have had much to do in determining water levels in deep mines. If

1 School of Mines Quarterly, March. 1880.

316 GEOLOGY OF T11E EtJltEKA D1STEIOT.

this is true, the action of percolating waters in underground drainage chan-
nels would be influenced in like manner. During the present period of
excessive dryness these channels in the limestone carry no water, and con-
sequently exert but little solvent power. If, however, subterranean cham-
bers can be worn in the limestone since the deposition of the ore; it seems
but logical to assume that on the identical ground, under nearly similar
conditions, caves should have been formed before the deposition of
sulphides.

Since the formation of these more recent water courses nothing of any
moment occured 011 Ruby Hill until historical time, when man, in his eager
search for wealth, excavated in a few years, by means of modern mechan-
ical appliances, the enormous mineral product which required untold ages
to deposit by natural process.

Conclusions.— The conclusions reached after an investigation of the
Eureka Mountains with regard to the geological position, age, and origin of
the ore deposits may be briefly stated as follows:

The rocks in which the ores occur are sedimentary beds belonging to
the Cambrian, Silurian, and Devonian periods.

The ores were deposited after the eruption of the rhyolite, and conse-
quently they are of Pliocene or post-Pliocene age.

In their mode of occurrence the ores are closely associated with the
dikes of rhyolite, although there is no evidence to show that they were
derived from them.

The ores came from below.

They were for the most part deposited as sulphides in preexisting caves
and cavities.

They were oxidized by atmospheric agencies, mainly surface waters
percolating through the rocks.

APPENDIX

SYSTEMATIC LIST OF FOSSILS

EACH GEOLOGICAL FORMATION IN THE EUREKA DISTRICT, NEVADA.

CHARLES DOOLITTLE WALCOTT.

317

APPENDIX A.

SYSTEMATIC LIST OF FOSSILS FOUND AT EUREKA, NEVADA.

BY CHARLES D. WALCOTT.

In the monograph of Mr. Arnold Hague the grouping of the different genera
and species and their stratigraphical succession and relations throughout the great
thickness of sediments at Eureka are given with considerable detail in the discussion
of the Paleozoic, rocks. For the student of general geology the vertical range of
species and their geographical distribution are (dearly brought out.

For the purpose of bringing together in tabulated form all the genera and
species of each important group into which the rocks have been divided, the following
systematic list was originally published in the Paleontology of the Eureka District,1
and is reproduced here in order that the student may see at a glance the life of each
geological horizon.

Invertebrate life is well represented throughout the entire series of rocks from
the base of the Prospect Mountain limestone to the summit of the Upper Coal-
measure limestone, a thickness of over 28,500 feet of sediments.

In the list all the Cambrian fauna is included under the head of Prospect
Mountain group, embracing the Prospect Mountain limestone, Secret Canyon shale,
Hamburg limestone, and Hamburg shale.

Since the publication of the Paleontology the Cambrian fauna has been divided
into three subfaunas: Lower, Middle, and Upper. Under this classification the
Lower Cambrian or OleneUiot fauna is included in the quartzites and immediately
superjaceut shales beneath the Prospect Mountain limestone; the Middle Cambrian
fauna in the Prospect Mountain limestone and Secret Canyon shales, and the Upper
Cambrian fauna in the Hamburg limestone and Hamburg shales. As the monograph
of Mr. Hague does not deal with these fauna! subdivisions in detail no further refer-
ence will be ma.de to them.

A number of generic references will be chauged in a forthcoming review of the
Middle and Upper Cambrian faunas, but it is not thought best to anticipate these
changes in the present systematic list.

1 Paleontology of the Eureka District, Nevada. Hon. XT. S. Gool. Surv., vol. vm, 18M.

318

320

GEOLOGY OF THE EUREKA DISTRICT.

The Lower Silurian embraces the fauna of the Pogonip limestone beneath the
Eureka quartzite, and the Silurian fauna above the, quartzite is represented by the
meager collections obtained from the Lone Mountain limestone.

Under the grouping of Carboniferous fossils are included the fauna from the
great belts of limestone both above and below the Weber conglomerate.

Included in the list is the collection obtained from the White Pine Mountains,
situated about 40 miles southeast of Eureka. The fauna from the base of the
Pogonip to the summit of the Devonian is so closely related to that of the Eureka
District that they may well be studied together, and in the tables are arranged in
parallel columns.

The reader who desires more detailed information in regard to the specific
character of the fauna is referred to the descriptions given in the Paleontology of the
Eureka District, above mentioned.

SYSTEMATIC LIST OF FOSSILS OF EACH GEOLOGICAL HORIZON.

CAMBRIAN.

PROSPECT MOUNTAIN TEBRANE.

[The clonlile multiple (XX) denot«a that the species passes to the group above.]

Genera and species.

Eureka.

White
Pine.

Remarks.

Porifera.

•

Type from the Cambrian of Wales.

Fragment.
Types from Eureka and White Pine dis-
tricts, Nevada.
Type from Eureka District.
Type from Schell Creek Range, Nevada.
Type from Eureka District.
Like O. pretiosa Billings.

Type from Calciferous formation in New-
foundland.

Type from Gallatin River, Montana.
Type from Eureka District.

Type from St. Croix (Potsdam) sandstone
of Wisconsin.

sp. ?..

Brachiopoda.

X
XX

XX
XX
XX

X
X
XX

X
X
X
XX

X
X

? minuta H. & W

Obolella discoidea H. & W

Acrothele f dichotoma Walcott

Kutorgina prospectensis Walcott

sculptilis Meek ..

whitfieldi Walcott ..

Orthis eurekensis Walcott

Pteropoda.

Hyolithe8 primordialis Hall (sp.) ... .

Scenella? conula Walcott. .

SYSTEMATIC LIST OF FOSSILS.

321

CAMBRIAN"— Continued.
PROSPECT MOUNTAIN TKRRANK — Continued.

Genera and species.

Eureka.

White
Pine.

Remarks.

Pcecilopodu.

X X

Type from White Pine District NevaU'i

neon H. & W

howelli Meek

Type from Pioche, Nevada

iddingni Walcott

Olenoides? expausus Waleott . . .

DicellocephaluH? angustifrons Walcott

? bilobatus H. & W .

Type from Eureka District

flabellifer H. & W

Geol. Expl. Fortieth Par vol IV p 227

iole Walcott.. . .

inurica Walcott

Ptychoporia (f) angulatus H. & W

(ieol. Expl. Fortieth Par., vol iv p 220

frichmondensis Walcott

anytus H. & W

Type from Schell Creek, Nevada

(S.) breviceps Walcott

? grauulosns H. &. W

Type from Eureka District.

hiK'uei II. & W. (sp.)

Type from White Pine District Nevada

la'.viceps Walcott

( ?)liunarssoni Walcott

?niaciilosus H. & W (sp.)..

Geol. Expl. Fortieth Par., vol. iv, p. 215.

nitidus H. & W (sp.)

Type from Eureka District.

occidcntalis Walcott

oweui M. & H (sp. ) ..

X X

Tvpe from Big Horn Mountains, Montana.

S?) prospectensis Walcott ..

?) similis Walcott

( ?) similis, var. robustus,

Walcott.
(?) unisulcatus H. & W. ..

Type from Eureka District.

(Euloma?) affinis Walcott.

(Euloma?) dissimilis Wal-

cott.
(P.) laticeps H. & W

X X

Type from White Pine District, Nevada.

(P.) occidens Walcott .....

8p. ?

? pernasutus Walcott .

uasutus Walcott

osceola Hall ..

Type from St. Croix (Potsdam) sandstone

f quadriceps H. & W ....

of Wisconsin.
Tvpe from Ute Peak, Was.iteh Range, Utah.

Tvpe from St. Croix (Potedani) sandstone

pustiilosa H. & W

of Wisconsin.
Geol.Expl. Fortieth Par., vol. iv, p.223.

Chariocephalus ? tumifrons H. & W

Type from White Pine district, Nevada.

X X

IlUenurus sp.? .. .

NOTE.— Sjiccies from White Pino District occur »t the base of tlie Pogonip group and are doubtfully referred I" the Cambrian.
MON XX 21

322 GEOLOGY OF THE EUREKA DISTRICT.

LOWER SILURIAN (Ordovician).

POGONIP TERRANE.

[The letter C in the first column denotes that the species also occnrs in the Cambrian.]

Genera and species.

Eureka Lower.

Eureka Upper.

<S V

« b P.

sss

Remarks.

Rhizopoda.
Receptaculites ellipticus Walcott

elongatus Walcott

Hydrozoa.
Graptolitlms sp. ? . . .

Like G. bifidus HalL

Actinozoa.
Monticulipora sp. ? ..

I'olyzoa.
Ptilodict va, sp. ?

Brachiopoda.
Lingulepis mjpra H. & W

fminuta H. & W

districts.

Lintnila ? uiauticula White

sp. ?

Obolella ? ambigua Walcott

discoidea H. & W .

Type from Eureka District

Acrotreta gemma* Billings ....

sp. ?

fbundland.
Like A. subconica Kutorga.

Schizambon tvpicalis Walcott

Lfpta'na melita H. & W

Strophomeua nemea H. & W

Type from White Pine District Nevada

Orthis haniburgensis Walcott

lonensis Walcott

perveta Conrad .

Trenton of Wisconsin Chazy of Canada

pogonipensis H. &, W. ..

Geol Expl Fortieth Par vol iv p 23

testudinaria 1 )alni;ni

tricenaria Conrad

Canada, etc.
Trenton group species of New York

sp. ? .

Canada, etc.

Sp. !

Streptorhynchus minor Walcott

I'orambonites obscurus H. & W

Geol Expl. Fortieth Par vol IV p 234.

Triplesia calcifera Billings

Lamellibrancli in In .
Telliuoinya contracta Salter ? . . .

? hamburgensis Walcott

Modiolopsis occidens Walcott. .

pogonipensis Walcott

sp. ? . .

tr.

* Identified by Mr. Meek, from Malade City. Hayden's Report for 1872, p. 464.

SYSTEMATIC LIST OF FOSSILS.

LOWER SILURIAN (Ordovidan)— Continued.
POGONIP TERRANE — Continued.

Genera and species.

Eureka Lower.

Eureka Upper.

« *

- •- --

- y,

sis

Remarks.

Gasteropoda.

Trenton group species in New York,
Canada, etc.
Like B. allegoricus White.

Trenton group of Wisconsin.

Trenton group species in New York,
Canada, etc.

Typo from the White Pine District, Ne-
vada.

A sp. of the Lower Trenton in New York.

Species of Lower Trenton in New York
State.
Trenton group sp.

Type from the Schell Creek Range, Ne-
vada.

Type from Gallatin River, Montana.
Type from White Pino District, Nevada.

Gcol. Expl. Fortieth Par., vol. iv., p. 231.
Geol. Espl. Fortieth Par., vol. iv., p. 226.

sp. ?

Straparollus sp, ?

Rapkistoma uasoni Hall (sp.)

X
X

sp. ?

MurctuHonia milleri Hall

HP. ?

v :
Sp. T . . . .

u.
u.

PI euro toni aria lonensis Walcott .

X
X
X
X
X
X
X

sp. ?

Helicotoma sp. ?

Miiclurea annulata Walcott

carinata Walcott

subannulata Walcott

sp ?

Metoptoma ? analogii Walcott

u.
u.

phillipsi Walcott . . .

C vrtolites siuuatus H. & W

X
X

I'teropoda.
t'oleoprion minuta Walcott

Hyolithes vanuxemi Walcott

sp.?..,

u.
u.

Cephalopoda.
Orthoceras multicameratum Hall

4 sp.?

Endoceras (like E. multitubulatum)

X
X
X

proteifonne Hall

Crustacea.
Leperditia bivia White

sp. ?

Hey richia sp. ?

Plumulites sp ?

Poecilopoda.
A^nostus bidons Meek

coiiimuiii.s H. & W .

X
X

Dicellocephalus inexpectans Walcott
tiualis Walcott

multiciuctus H. & W... x

324

GEOLOGY OF THE EUEEKA DISTRICT.

LOWER SILURIAN (Ordovician)— Continued.

POGONIP TERBANE — Continued.

Genera and species.

Eureka Lower.

Eurekal'pper.

"•d
***

^S£

- •: £~

-e o S

Remarks.

l'u til opoda — Continued.

granulosus H. & W. (sp. )

Type from Eureka District.

huguei H. &. W. (sp.)

Type from White Pine District.

maculosus H. & W. sp

Geol. Expl. Fortieth Par., vol. iv., p. 215.

oweui Meek ...

Type from Big Horn Mountains, Mon-

uniKiilcatus H. & \V (sp.)

tana.

(Eulomal) affinis Walcott..
Art'tlnisinu aniericana Walcott

Bathyurus ? congeiieris Walcott

rogonipeiisis, H. & W ....

Geol. Expl. Fortieth Par., vol. iv., p. 243.

si in i ]] ini us Walcott

? tuberculatns \Valcott

fsp.f

Cyphaspis ? breviiuarginatus Walcott

Amphion uevadensis Walcott . .

so. f .

sn.f

C'eraurus sp. ?

Symphysurus ? goldfussi Walcott

Barraudia ? maccoyi Walcott

? sp

sp f

Asaphus caribouensis Walcott .

curiosus Billings ..

Type from Quebec group of Canada.

3 gp. 1 . .

LONE MOUNTAIN SILURIAN.

Genera and species.

GO o-

j°'C
o

S3 d

White Pine.

Remarks.

Actinozoa.
Stroptelasnia Walcott ... .. . ..

sp.f

/aphrentisf sp.f

Hal vsites catenulatus Linn (sp. )

Mouticulopora, sp f . . ..

Echinodermala.
Cvstid..

Separate plates.

SYSTEMATIC LIST OF FOSSILS.
LONE MOUNTAIN SILURIAN— Continued.

325

Genera and species.

33 •

|l
o

a
S ^
fe 3

Remarks.

Brachiopoda.
Leptsena sericea Sowerby

Orthis

T ., „ ,. . „

Cephalopoda.
Orthoceras sp.?

Cyrtoceras sp. f

Pcecilopoda.
Ceraurus

Dalmanites

Trinuclens concentricus Murch . . .

».

llla'iius

Asaphns platycephalus Stokes

DEVONIAN.

[Abbreviations. — Up. Held. — Upper Helderborg. Ham. = Hamilton group. Ch. = Chemung group of the Devonian
eries of the Geological Survey of New York. fl. & W.— Hall & Whitneld, Geol. Expl. Fortieth Parallel, vol. IV, 1877.]

Genera and species.

.Port/era.

Palicomanon roemeri Walcott

Astylospongia sp ?

•Stromatopora, sp ?

Actinozoa.

Fistulipora, sp?

Favosites hemispherica Y. & S

basaltica, var

X
X

&
B
p

.-s a

Alveolitos multilamellataMeek .

rockfordensis Hall f

Cladopora prolifica Hall

pulchra Rominger i

sp.? X

Thecia raniosa Rominger? x

Syringopora hisingeri Billings

perelegans Billings X

Aulopora serpens Goldfnss ; X

Cyathophyllum corniculnm M. 1

, Ed.

rugosumM. Ed. A Haiue.

Remarks.

Up. Held, and Ham. of New York and
Canada; Up. Held., Falls of Ohio.

Geol. Expl. Fortieth Par., vol. iv,p. 25.
Type from Devonian of Iowa.
Up. H<-ld. of New York and Falls of Ohio.
Up. Held, of New York and Kails of Ohio.
Up. Held, of New York and Falls of Ohio.
Up. Held, of New York and Falls of Ohio.
Up. Held, of New York and Falls of Ohio.

Hani, of Michigan.

Up. Held, of New York. Canada. Fall*

of Ohio, etc.
Up. Held, of New York, Falls of Ohio,

etc.

826

GEOLOGY OP THE EUEEKA DISTRICT.

DEVONIAN— Continued.

Genera and species.

Actinosoa — Continued.

Cyathophyllum davidsoni M. Ed .

2n. sp

sp.?

Acervularia pentagona Goldfuss. .

Smithia hennahii Lousdale (sp) '

Pachyphyllnm woodman! White, sp j

Diphyphylluui simeoense M. Ed I X

I'tychophyllum ? infuudibuluni Meek I

Cystiphyllum ainericanum M. Ed x

2 n. sp

Polyzoa.

Fenestella, 2 sp. ?
Thamniscns ? sp ?

Brack iopoda.

Lingtila alba-pineusis Walcott

liena Hall

ligea Hall

ligea, var. nevadensis Walcott

lonensib Walcott

melie Hall

whitei Walcott

sp. f

Discina lodensis Hall

minuta Hall?..

sp.

Pholidops bellula Walcott

quudrangularis Walcott

Orthis impressa Hall

nmcfailanei Meek

X
X
X

tulliensis vanuxem

Skenidinm devonicum Walcott

Streptorhynchua cbemungensis Conrad

(sp.)
chemungensiSjVar. pan-

dora Billings,
cheinungensis, var. per-

versa Hall.

Strophomena rhomboidalis Wilckens,
(sp.)

Strophodonta arouata Hall

calvini Miller

canace H. & W

deraissa Conrad (sp.)

ineqniradiata Hall

patersoni Hall

perplnna Conrad (sp.) ^

punctulifera Conrad (sp.).i X

x
X
X

X
X

X
X
X

X
X

Remarks.

Ham. of Iowa.

Identified by Meek from White Pine

District, Nevada.

Geol. Expl. of Fortieth Par., vol. iv., p. 32.
Devonian of Iowa.

Up. Held, of New York, Falls of Ohio, etc.
Geol. Expl. Fortieth Par., vol. iv, p. 28.
Up. Held, of New York and Falls of

Ohio ; Ham. of New York, etc.

Ham. of New York.
Ham. of New York.

Pal. of New York, vol. iv, p. 14.

Ham. of New York.
Ham. of New York.

Ch. of New York.

Type from Mackenzie River, British

America.
Ham. of New York.

Up. Held., Ham., and Ch. of New York.
Up. Held, of New York and Canada.
Up. Held, and Ham. of New York.
Up. Held. New York, Falls of Ohio, etc.

Devonian of Iowa.

Geol. Expl. Fortieth Par., vol. iv, p. 246,
1877.

Devonian of Iowa, New York, etc. ; Mac-
kenzie River, British America.

Up. Held, of New York.

Throughout the Devonian of New York.

SYSTEMATIC LIST OF FOSSILS.
DEVONIAN— Continued.

327

Genera and species.

Brachiopoda — -Continued.

Cnonetes deflecta Hall

filistriata, Walcott

hemispherica Hall

macrostriata Walcott . .
mucronata Hall

setigera Hall.
HP.

X
X

Productus (P. ) hallanus Walcott

(P.) hirsutiforme Walcott

(P.) lachrymosns Conrad (up.)
(P.) laehrymosus var. limus

Conrad (sp.)
(P.) lachrymosus, var. stig-

matus Hall.

(P.) navicellus Hall X

(P.) shuuiardianus Hall X

(P.) shumardianus var. pyxi-

datus Hall.

(P. ) speciosus Hall

(P.) subaculeatus Murch

(P. ) truncatus Hall

sp.?

Spirifera alba-pinensis H. & W

disjuucta Sowerby

englemanni Meek ? .

parryana Hall f

pifionensis Meek

raricosta Conrad (sp.)

strigosus Meek

varicosa Hall

sp. uudt

(M.) glabra, nevadensis Wal-
cott.

(M.) maia Billings

(M. ) undifera Roemer

Spiriferina cristata Schlotheim (sp. )

Amboccelia umbonata Conrad (sp. )

Cyrtina davidsoni Walcott

hamiltouensis Hall

Nucleospira concinna Hall

Trematospira infrequens Walcott

Retzia radialis Phillips (sp.)

Athyris angelica Hall

sp.?

Meristella (W.) nasuta Conrad (sp.)

At ry pa desquamata Sowerby

reticularis Linn, (sp.)
Uhynchonella castanea Meek

duplicata Hall

emmonsi H. & W.
horstbrdi Hall ..

X
X

X
X
X
X

X
X

•

a
£ ri

X
X
X

X
X

•

Remarks

Ham. of New York.
ITp. Held, of New York.

Up. Held, of New York, etc. ; Great Bear

Lake, British America.
Ham of New York.

Devonian of Iowa.

Ch. of New York.
Cn. of New York.

Up. Held, and Ham. of New York.
Devonian of Iowa.
Devonian of Iowa.

Ch. of New York.

Up. Held, of Falls of Ohio.

Ham. of New York.

Of the type of P. scmireticulatns.

Geol. Expl. Fortieth Par., vol. iv, p. 25.~>.

Ch. of New York, Mackenzie River, Brit-
ish America, etc.

Type, Eureka and White Pine Districts,
Nevada.

Devonian of Iowa and Canada.

Type, Pinoii Uungr. Nevada.

Up. Held, of New York, Falls of Ohio, etr.

Geol. Expl. Fortieth Par., vol. iv. p. 43.

Up. Held, of New York, Falls of Ohio. eto.

Up. Held, of New York and Canada.

Also in Carboniferous.
Ham. of New York.

Up. Held, and Ham. of New York.

Ch. of New York.

Of the type of Athyris planosulentn.

Up. Held, of New York.

Devonian of England.

Devonian of America, Europe, etc.

Type from Mackenzie River, British

America.
Ch. of New York.

Geol. Expl. Fortieth Par., vol. iv, p. 217.
Ham. of New York.

GEOLOGY OF THE EUREKA DISTRICT.

DEVONIAN-Continued

Genera and species.

Jiracliiopoda — Continued.
Rliynchonella pugnus Martin

? occidens Walcott

quadricosta Hall

tethys Billings

(L )'lanra Billings

(L.) nevadensis Walcott.

(L. ) siuuatus Hall

Leptocoelia sp. ?

Pentamerus comis Owen (sp.)

lotis Walcott

Tropidoleptus carinatus Hall

Cryptonellaf circula Walcott -.

piiioucnsis Walcott

Terebratula, sp. ?

Lamellibranchiata.

Aviculopecten ? catactus Meek

Ptcrinopecten, sp. f

Glyptodesma, sp. ?

Pteriuea newarkensis Walcott

rlabella Conrad

Actinopteria boydi Conrad (sp.)

Leiopteria raiinesquii Hall

Leptodesma trausversa Walcott

Limoptera sarmenticia Walcott

My tilarca dubia Walcott

cheuiungeusis Conrad (sp.)

sp. f

(Plethomytilus) oviforinis,
Courad (sp.).

Modiomorpha altiforme Walcott

obtnsa Walcott

Goniophora perangulata Hall

Palii'oneilo, sp. ?

Nucula rescuensis Walcott.-.

sp.

Nuculites triangulus H. & W

iusularis Walcott

Megamboiiia occidnalis Walcott

Ny assa parva Walcott

Grammy sia minor Walcott

Edmondia pifionensis Meek

iSauguiuolites? coinbensis Walcott

? gracilis Walcott

(Spathella) ventricosus
White & Whitfield (sp.).
(Spathella) oblonga Wal-
cott,

Glossitcs ? sandusky ensis Meek

Sphenotus contractus Hall

Conocardium nevadensis Walcott

Lunulicardium fragosum Meek (sp.).

X
X

x
x
x

X
X

X
X
X

X
X

X
X
X

X
X

X
X
X

X
X

X
X
X

X
X

X
X
X

X
X

Remarks.

Devonian of Ohio, New York, and Eng-
land.

Genesee Slate of New York.

Up. Held, of Canada and Falls of Ohio.

Ham. of Canada and New York.

Ch. of New York.

Not unlike L. acutiplicata Hall.

Ham. and Ch. of Iowa.

Found in Piiioii Range, Nevada.

Geol. Expl. Fortieth Par., vol. IV, p. 93.

Upper Held, to Ch. of New York.
Ham. group of New York.
Ham. group of New York.

Ch. of New York.
Ham. of New York.

Schoharie Grit of New York.

Geol. Expl. Fortieth Par., vol. iv, p. 248.

Geol. Expl. Fortieth Par., vol, iv, p. 46.

Ch. of New York; Burlington sand-
stone of Iowa.

Up. Held, of Ohio.

Ch. of New York; Burlington sand-
stone of Iowa.

Geol. Expl. Fortieth Par., vol. iv, p. 92.

SYSTEMATIC' LIST OP FOSSILS.

DEVONIAN— Continued.

329

Genera and species.

Lamellibranchiaia — Continued.

Low

Paracyclas occidentalis

peroccideiiH H. & W

Posidonomya devonica Walcott

Isevis Walcott

Cypricardella macrostriatus Walcott

Cardiomorpha missouriensis, Swallow . . .
Anadontopsis amygdalasformis Walcott.. X
Schizodus(Cytherodon) orbicularis Wai- X

cott.
Cypricardinia indenta, Conrad (sp. ) X

Gasteropoda.

Platyceras carinatum Hall I X

conicum Hall X

couradi Walcott

dentalium Hall

nodosum, Conrad

thetiforuie Walcott .

thetisHall

iindulatum Walcott .
Platyostoma lineatuni Conrad. .

X
X
X
X
X
X
X

sp.?

Euculiompbalus devonicus Walcott X

Euomphalus eurekensis Walcott x

(P.) laxus Hall

sp. ? X

sp. ?

Sratparollus newarkeusis Walcott

Plaurotomaria, sp. ?

Platyschisnia ? ambiguum Walcott

? maccoyi Walcott

sp. ?

Calonema occidentalis Walcott X

Loxoueuia approximatum Walcott X

eurekensis Walcott X

nobile Walcott X

? subattenuatum Hall X

(2sp. ?) X

sp. ?

Bellerophon combsi Walcott ! X

ledaHall '....

lyraHall

ma?ra Hall

neleus H. & W

pelops Hall

perplexa Walcott

Scoliostoma americana Walcott

Naticopsis (like N. asquistriata)

sp.?

sp. ?

Metoptonia ? devouica Walcott

M.
M.

X
X
X

Remark*.

Geol. Expl. Fortieth Par., vol. iv, p. 248.

Gi-ol. Expl. Fortieth Par., vol. iv, p. 277.

Upper Held, of New York and the Falls
of the Ohio.

Up. Held, and Ham. of New York.
Up. Held, and Ham. of New York, and
Up. Held, of Falls of tin; Ohio.

Up. Held, of New York.
Up. Held, of New York.

Up. Held, and Ham. of New York.

Up. Held, and Ham. of New York, Can-
ada, etc.

Up. Held, and Ham. of New York.

Up. Held, of New York.

M. ' Ham. of New York.

M. ! Ham.' of New York.

X ! Ch. of New York.

I Geol. Expl. Fortieth Par., vol. iv, p. 250.

M Up. Held, of New York.

330

GEOLOGY OP THE EUREKA DISTRICT.

DEVONIAN— Continued.

Genera and species.

Remarks.

Pteropoda.

Tentacnlites attenuatus, Hall

bellulus, Hallf

gracilistriatus Hall
scalariformis Hall . .

Styliola fissurella Hall

var. intermittens Hall . .

Conularia (sp. f )

Coleolus lievis Walcott

Hyolithes (like H. aclis Hall

Cephalopoda.

Orthoceras (5 sp. ?)

Gomphoceras suboviforme Walcott
Cyrtoceras cessator, H. & W

ne vadense Walcott

Gouiatites desideratus Walcott

kingi,H. &W

sp.?

Crustacea.

Beyriehia occidentalis Walcott .
Leperditia rotuudata Walcott . .

Poxilopoda.
Phacops rana Green (sp.) . .

Dalmanites meeki Walcott .

sp. ?

Proetus nevadae Hall . .

marginalis Conrad (sp. )

sp.f „

Phillipsia coronata, Hall f

X
X

x
x

•

Ham. of New York and Canada.
Ham. of New York and Canada.
Ham. of New York and Canada.
Up. Held, of New York.
Ham. of New York and Canada.

Geol. Expl. Fortieth Par., vol. iv, p. 278.

Geol. Expl. Fortieth Par., vol. iv. p. 279.
Like (G. discoidus Hall).

Up. Held, and Ham. of New York, Can-
ada, etc.

Ham. of New York, Pal., N. Y., vol. vii.

p. 129, 1888.
Up. Held, of New York.

Ham. of New York.

CARBONIFEROUS.

Genera and species.

Lower.

Upper.

Remarks.

RHzopoda.
Fnsilina cylindrica Fischer

robusta Meek . .

Port/era.
Stromatopora sp.f

Adinozoa.
Zaphrentis sp. ?

Syringopora multattenuata ?

ChiEtetes 3 sp. ?

SYSTEMATIC LIST OF FOSSILS.

CARBONIFEROUS— Continn.-d.

331

Genera and species.

Lower.

Upper.

Remarks.

Echinodermata.
Arohiitocidaris 2 up. ?

Polyzoa.
Polypora, sp. ?

sp. ?

Ptilodictya (S.) carbonaria Meek? .

(S.) serrata Meekf

(sp. f

Fenestella 3sp. ?

Brachiopoda.
Disciua connata Walcott

newberryi Hall

nitida Phillips (sp. )

sp. ?

Lingula mytiloides Sowerby?.

Chonetes granulifera Owen .

U S Geol Snrv Nebraska p 170 1872

verneuiliana N. & P

U. S. Geol. Surv., Nebraska, p. 170. 187L'.

Productus costatus Sowerby ?

Geol. Expl. Fortieth Par., vol. iv., p.69,1877.

elegans McCoy

longispinus Sowerby

Geol. Expl. Fortieth F.ir., vol. iv., p. 78,1877.

longispinus, var. niuricatusN.

&P.
nebrascensis Owen

Expl. and Surv. W. 100th Merid., vol. 'v..

prattenianus Norwood

Pal. p. 116, 1875.
Geol. Expl. Fortieth Par., vol, i\ ., p. 72, 1877.

punctatus Martin (sp.)

Expl. and Surv. W. 100th Merid., vol. iv,

semireticulatus Martin (sp. ) . .

Pal. p. 114, 1875.
Geol. Expl. Fortieth Par., vol. iv, p. 69. 1S77.

snbaculeatus Murch

Strophomena rhomboidalis Linn (sp.)

Streptorhynchus crenistria Phillips (sp. )

Orthis pecosi Marcou

Expl. and Surv. W. 100th Merid., Pal., vol.

resupinata Martin (sp. )

iv, p. 125, 1875.
Geol. Expl. Fortieth Par., vol. iv, p. 265. 1X77.

Spirifera annectans Walcott

cumerata Morton

Geol. Expl. Fortieth Par., vol. iv, p. 91, 1877.

desiderata Walcott

leidyi N. & P

neglecta Hall

rockymontana Marcou

Expl. and Surv. W. 100th Merid., vol. iv,

striata Martin

Pal., p. 134. 1875.
Geol. Expl. Fortieth Par., vol.iv, p. 269.1877.

trigonalis Martin (sp.)

(Martinia) setigera Hall

Geol. Expl. Fortieth Par.,vol. iv, p.270.1877.

Syringothyris cuspidatus Martin (sp.)

Davidson's Monograph. Carb. Brachiopoda.

Spirifera cristata Schlotheim (sp. )

Also in Devonian.

lii' 1 x ia radialis Phillips

veneuiliana Hall. . .

L. Carb., Eureka District, Nevada.

Athyris royssii L/Eveille (sp. ) .

Geol. Expl. Fortieth Par., vol. iv, p. 82, 1877.

liirMita Hall

subtilita Hall (sp. ) .

Rhyuchouella enrekeusis Walcott

thera Walcott

sp. f

Leiorhvnohus-like.

Camarophoria coopereusis Shum . . .

Terebratula bovidens Morton

Expl. and Surv. W. 100th Merid., vol. IV,

kastata Sowerby . .

Pal., p. 144, 1875.

GEOLOGY OF THE EUREKA DISTRICT.

Systematic list of fossils of each geological horizon — Continued.
CABBONIFE ROUS— Continued.

Genera and species.

Lower.

Upper.

Lamellibranchiata.

X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X

Wi
Co

peroccidens Walcott

2 sp f

Leptodesma (2 sp. ?) - - .-

Nucula insularis Walcott

levatiforme Walcott- .

Macrodon hamiltona) Hall...

truncatus Walcott

teuuistriatus Meek & Wortlieii .
Grammysia arcuata Conrad (sp.)

' XX X XXX X XXX XX XXX

haunibalensis Shnmard (sp.).
Edmondia ? circularis Walcott .

medon Walcott .

Pleurophorus meeki Walcott

Sphenotus seolus Hall ....

retusus Walcott

salteri Walcott

simplex Walcott .... ...

Spathella ? u;ena Walcott

Cvpricardella striata Walcott . .

connatus Walcott

Cardiolaf filicostata Walcott

Schizodus cuueatus Meek

curtifonne Walcott

deparcus Walcott

pintoensis Walcott

X
X
X
X
X
X
X
X

Gasteropoda.
Platyceras occidens Walcott . .

piso Walcott

Platyostoma inornatum Walcott .

Euomphalus (S.) snbrugosns M. & W.. .
Loxonema bella Walcott

MacrochelluS; sp.f

Pleurotomaria nevadensis Walcott

nodomarginata McChes-
ney.

81). f

sp.t

Remarks.

Ham. of New York.

Geol. Ill, vol. v, p. 576, 1873.
Ham. of New York.

Waverly of Ohio.

Coal measures of Ohio.

SYSTEMATIC LIST OF FOSSILS.

Systematic list of fossils of each geological horizon — Continued.
CAKBOKIFEROU8 — Continued.

333

Genera and species.

Lower.

Upper.

Remarks.

Gasteropoda — Continued.

Not unlike N raua M & W.

sp ?

Like B. ellipticus Mc.Cheevey.

sp. ?

....::.:

Like B. sublievis Hall.

Pulmonifera.

Pteropoda.

Cephalopoda.

(2 8D ?)

Nautilus 'like N digonis M &W.)

Crustacea.

Pcectlopoda.
Griffithides portlocki M & W (sp )

MICROSCOPICAL PETROGRAPHY

OF THE

ERUPTIVE ROCKS OF THE EUREKA DISTRICT, NEVADA.

JOSEPH PAXSON IDDINGS.

336

APPENDIX B.

MICROSCOPICAL PETROGRAPHY OF THE ERUPTIVE ROCKS OF THE EUREKA

DISTRICT, NEVADA.

BY JOSEPH PAXSON IDDINGS.

CHAPTEE I.
GRANITE AND PORPHYRY.

The representatives of this division of eruptive rocks from the Eureka District
are but few in number, ami bear a very close resemblance to one another, being all
quartz- orthoclase rocks. They are composed of the same minerals, having in addition
to the quartz and orthoclase a triclinic feldspar with biotite and hornblende in varying
quantities. They are granite, granite-porphyry and quartz-porphyry.

Granite.— Of the many varieties of crystalline rocks found within the small area
of the Eureka District, granite plays but an insignificant role, and is represented
by only four thin sections from the exposure south of Ruby Hill; of these, 1, 2, and
3 show a fine grained rock of uniform texture, with the characteristic granitic struct-
ure. None of the individuals of quartz and feldspar have crystallographic outlines,
but are irregularly shaped by reason of the mutual penetration of adjacent grains.
The essential components of the rock are quartz, feldspar, hornblende, and biotite.
the accessory minerals being titanite, iron oxide, apatite, zircon, and allanite, besides
secondary minerals resulting from the decomposition of the first, which are chlorite,
calcite, quartz, epidote, and hydratcd oxide of iron. The rock is, therefore, an
amphibole granitite. The most abundant primary constituent, quartz, occurs in irreg-
ularly shaped grains which, together with its inclusion of portions of all the other
primary minerals, shows it to have been the last to crystallize. It occasionally occurs
in porphyritical grains. The only characteristic inclusions are minute fluid cavities
with very small moving bubbles. It shows the phenomena of irregular optical orien-
tation resulting from mechanical deformation. The feldspar is for the most part altered,
but the fresher sections show it to be both orthoclase and plagioclase in nearly equal
proportions. They both have a fine zonal structure; the former is frequently in Carls-
bad twins, the latter in multiple twins, after the albite and sometimes also after the
MON xx 22 337

338 GEOLOGY OF THE EUREKA DISTRICT.

pericline law. The decomposition commenced at the center, resulting in some cases
in a cryptocrystalline aggregate like kaolin, with calcite; in others filling the crystal
with shreds of colorless mica, and minute, pale yellow grains, traceable to larger
aggregations of epidote. Feldspar and quartz" form the main mass of the rock,
through which is scattered mica and hornblende in varying amounts. The hornblende
is in poorly defined crystals, except some of the smaller individuals, which are well
developed in the prism zone. The prismatic faces are much larger than the clinopina-
coid, and the cleavage parallel to the former is strongly marked. It is in simple
crystals and twins, twinned parallel to ooP do. The color is dark green, with strong
pleochroism, most noticeable in sections parallel to the clinopinacoid and base. The
colors are: c = dark green, b = brownish green, a = light brown, c = b > a. The angle
of extinction read from the vertical axis is mostly from 17° to 19°, but in two instances
is 21° and 25°. It incloses magnetite, apatite, and biotite, having been formed
after the latter in every case. It is quite fresh, though the mica is almost com-
pletely decomposed. The biotite, with which the hornblende is intimately asso-
ciated, occurs in comparatively thick crystals of irregular outline, of a deep brown
color, with nearly uuiaxial interference figure, and has occasional inclusions of iron
oxide, apatite, zircon, and rarely feldspar. It is especially interesting from its mode
of decomposition, which takes place along the basal cleavage and results in a dark
green pleochroic chlorite, which must be formed of an aggregation of -minute scales
parallel to the lamination of the mica, for basal sections remain dark when revolved
between crossed nicols and show no interference figure and no pleochroism, while
transverse sections exhibit a marked fibration parallel to the mica cleavage and are
pleochroic ; being green, parallel, and yellow at right angles to the line of fibration.
This chlorite, in turn, alters into epidote and possibly quartz. The epidote, in irregu-
lar grains, is pleochroic between intense greenish yellow and pale yellow. That it
does not result directly from the decomposition of the biotite is evident from the fact
that it never occurs in it unassociated with chlorite, while the latter occurs constantly
alone, and also because lenticular masses of epidote are seen to have disturbed the par-
allelism of the chlorite scales, proving its subsequent crystallization.

Titanite, in narrow rhombic sections and less regular grains is sparingly present.
The iron oxide appears to be magnetite for the most part. Colorless apatite is
abundant both in short, stout prisms, and long, slender, jointed needles, penetrating
everything in all directions. Apatite and sharply crystallized zircon appear to be the
first minerals formed in the rock. In thin section 2 there are three comparatively
large crystals of allanite, dark brown, with strong absorption; two are twinned. An
irregular grain of allanite is found in 4. Thin section 3 is highly decomposed and
stained with hydrous oxide of iron. Thin section 4 is of a porphyritic variety, having
a fine grained, microgranitic groundmass of quartz and feldspar. Though the feldspar
of this rock is still mostly fresh, and the hornblende entirely so, the biotite is com-
pletely altered to green chlorite, epidote, quartz, and calcite.

GEANITE-POEPHYltY. 339

Granite-Porphyry.-The microscopical study of the granite-porphyry of this district,
though somewhat limited, is of great interest as showing the modifications produced
in the final crystallization of a granitic magma through the chilling caused by the
inclosing rocks, and consequently the relation of the quartz porphyries to the coarse
grained granite; and also as pointing out the correspondence in microscopical structure
of the metamorphosed sandstone of the district to certain forms of micro-granite. The
most important occurrence of this rock is in the granite-porphyry dike and its
apophyses south of Wood Cone, near Fish Creek Wells, which is represented by thin
sections 10, 11, 12, 16, 19, 20, 21, 22, 24, 25, 26, 27, 28, 29. It is found to be a wholly
crystalline rock of most varying structure, from coarse grained granite and porphyritic
granite to dense porphyry with an aphaiiitic groundmass. It is composed of quartz
and feldspar, both orthoclase and plagioclase, with a small amount of biotite and
hornblende; and since the character of these minerals is the same throughout the
different thin sections, and only their relative abundance and structural combination
vary, it seems best to give a general description of each of the minerals first, and
afterwards the special features which characterize the different modifications of the
rock.

The most noticeable component is quartz, occurring both in macroscopic pheno-
crysts and in microscopic grains. In the former instance it is usually well developed in
the form of dihexahedral crystals, sometimes having short piism faces; but it also
occurs in rounded and irregular grains of varying size, the largest being about as large
as a pea. In the granitic portions of the rock the grains are wholly irregular in form.
The quartz substance is colorless and perfectly fresh, and is filled with minute fluid
inclusions, mostly with a single gas bubble, sometimes in motion. Frequently there are
double bubbles, the inner of which is sometimes briskly moving; the fluids in this case
are water aud liquid carbon dioxide. There are also rounded bays of grouudmass pene-
trating the crystals, and more rarely minute portions of groundmass in dihexahedral
cavities. The habit of the quartz differs from that of the quartz in quartz-porphyry
by the abundance of liquid carbon dioxide and the absence of any isotropic glass, but
corresponds closely to it in other respects. The microscopic grains of quartz which
form a large part of the groundmass, have a granitic habit, being in part irregularly
outlined, in part conjointly crystallized with the feldspar, producing micropegmatitic
structure, to be described later on.

The feldspar is mostly orthoclase, but a triclinic species also is always present,
the large phenocrysts of the former are often well crystallized with the ordinary faces,
ooP^, OP, »P,2Pa>, the cliuopinacoid being the most strongly developed, forming
tabular Carlsbad twins. The cleavage parallel to the base is very perfect, that paral-
lel to the cliuopinacoid less so, and in numerous individuals a fine striping is noticed,
which is remarkably regular, but occasionally deviates from right lines and loses its
parallelism. It at first suggests the polysynthetic twinning of plagioclase, but on
closer examination appears to be an iuterla mi nation of albite iu orthoclase parallel to

340 GEOLOGY OF THE EUREKA DISTRICT.

the orthoptnacoid, as in perthite; iu one section nearly in the plane of the clinopina-
coid the striping crosses the basal cleavage parallel to the direction of the other
piuacoid, and the angles of extinction for the main crystal and the included lamellae
are about 7° and 18°, respectively, on the same side of the basal cleavage ; angles which
correspond to orthoclase and albite in such a section. A zonal structure is com-
mon to many individuals, and may be observed in the fresher crystals, even in the
hand specimens, without the aid of a lens. There are no characteristic inclusions, „
but particles of the associated minerals are frequently met with, especially near the
margin of the crystal. The plagioclase is very similar iu occurrence to the orthoclase,
being characterized by the abundance of striations produced by multiple twinning,
mostly in one direction, like that in albite, but also in a second direction nearly at right
angles to the first, like that in pericline. Exactly how many species are present has
not been determined optically, but it is certain that labradorite is one of them, as the
highest symmetrical extinction angles reach about 30°. It is in general quite free
from inclusions; nevertheless, in some of the plagioclase crystals from widely different
parts of the dike there are minute, colorless, rectangular bodies always parallel to
the twinned lamell<e, that at once remind one of the glass inclusions characteristic of
andesitic plagioclase. Their nature, however, is doubtful, for though without influence
on polarized light in most instances, they appear in others to affect it slightly, and
besides are without a gas-bubble, from which it seems probable that they are not
glass, but possibly feldspar. The substance of the feldspar is sometimes perfectly
fresh and transparent, at others clouded by minute, irregularly shaped particles, that
reflect incident light and appear white. The orthoclase when further altered is filled
with brilliantly polarizing shreds of colorless potash-mica, arranged parallel to three
directions in the crystal. Calcite is noticed iu the partially decomposed plagioclase,
the decomposition in general setting in from the outside of a crystal and traversing it
in the most irregular manner.

The next essential mineral to be mentioned is biotite. It is universally present,
but in varying quantities. In the thin sections from this body it is mostly altered.
The fresh mineral is in poorly denned, six-sided crystals of a dark brown color,
with strong absorption parallel to the basal cleavage, the basal sections yielding
apparently uniaxial interference figures, with a negative character, but sometimes
showing a small angle between the optic axes. It is quite free from inclusions, but
occasionally carries small crystals of zircon and apatite, and more frequently titanic
iron; in one instance (12) it is surrounded by grains of iron oxide, and in another (22)
by hornblende. The alteration that has taken place iu most of the sections appears
to be a bleaching out of the brown color, leaving a yellow or light green, brilliantly
polarizing mica, with faint pleochroism, which is generally filled with slender acicular
crystals of a yellowish brown color arranged in Hues intersecting at 60°, besides
larger and stouter crystals very perfectly developed, which have a high index of
refraction and seem to belong to the tetragonal system. From their close association

GRANITE-PORPHYRY. 341

with partially decomposed titanic iron, which, is characterized by strongly marked,
rhombohedral cleavage, it is most likely that these minute, secondary crystals are
rutile or anatase. The decomposition starts from the surface of the crystal, sections
of partially altered mica being found with portions of the mineral still fresh in the
center. A further stage of alteration produces a green, fibrous chlorite, and in one
instance (12) quartz and epidote. The colorless potash-mica, scattered through the
groundmass in shreds and fan-like aggregations, which appears brilliantly colored
between crossed nicols, and shows a small angle between the optic axes, is undoubt-
edly of secondary origin, arising from the decomposition of orthoclase, as already
mentioned, or from that of the brown mica ; for in every thin section where it occurs
both the brown mica and feldspar are more or less altered, and in those where they
are both perfectly fresh it is wanting.

The hornblende is by no means a constant ingredient, being absent from all the
more porphyry-like varieties and present in only part of the granitic ones. It is of a
dark brown color, sometimes green, with strong absorption and pleochroism, and is
seldom in fully developed crystals, though some cross sections with the characteristic
cleavage show the presence of the prism and both the pinacoidal faces. The crystals
are short and stout, their outline broken by intruding grains of the surrounding
groundmass, which are also abundantly included in the hornblende, together with
apatite, iron oxide, and more rarely mica. It would seem to be one of the later crys-
tallizations, contemporaneous with that of the ground mass.

There are a great number of accessory minerals, which are not all present,
however, in any one thin section, and are more abundant in the granitic than in the
porphyry-like forms of the rock. The most exceptional of these is augite, found in
only one thin section (22). It is of pale green color, with high index of refraction and
characteristically great extinction angle. Titanite or sphene in wedge-shaped crystals
and irregular grains is common to the hornblende-bearing varieties, with which min-
eral it is usually in close association. The iron oxide appears to be for the most part
titaniferous, many of the larger grains showing a most pronounced rhombohedral
cleavage, the decomposition in several cases resulting in lencoxene and a chemical
test giving the reaction for titanic acid. Apatite and zircon in sharply defined
crystals are everywhere present in small quantities, and garnet is found in a thin sec-
tion from a closely related dike. Allanite is present in those sections rich in hornblende
and biotite; it is especially abundant in No. 11, where ten grains of it were noted.

The groundmass of this rock, in all of its modifications, is wholly crystalline,
no isotropic glass being found anywhere in it. It is composed of quartz and ortho-
clase feldspar, very little plagioclase having been recognized. To these is sometimes
added hornblende, biotite, and titanic iron, besides the colorless potash -mu-a of
secondary origin. The quartz and feldspar are either in an aggregation of irregularly
outlined grains of nearly uniform size, which is the ordinary structnie of granite, or
they form orderly arranged groups of triangular or rhombic figures, and others clou-

342 GEOLOGY OF THE EUREKA DISTRICT.

prated and feather-like. This structure is noticeable in ordinary light from the fact
that the quartz remains pellucid after the feldspar has become clouded by partial
alteration. Between crossed nicols, however, the appearance is very distinct. In the
coarser grained varieties, especially at the spot represented in Fig. 1, PL vi, the
field is covered with blocks of similar geometric and cuneiform figures — parallel-
ograms, trapezoids, and variously shaped triangles — the sides of all those forming any
single group being respectively parallel, besides which are long and narrow parallel
strips, two sets of which, meeting obliquely, produce a feather like appearance. It is
further seen that all the figures in any one group extinguish light in the same azimuth
or have the same optical orientation, and that the inclosing block is a single individual
with a different orientation, the one being quartz and the other feldspar. In this par-
ticular case the small figures are of quartz and throughout the field have the same
extinction as a central grain, the inclosing blocks being of differently oriented feld-
spar. This tendency to crystallize around grains of quartz or feldspar is more
noticeable in the finer grained varieties, where the nucleus is incrusted with a shell
that in section appears as a frame-like border, having a radiating structure composed
of variously oriented sectors, though the portions formed by the same mineral as the
nucleus have their axes of elasticity parallel throughout. Of the phenocrysts, quartz
seems to be the only one around which this special crystallization takes place.1
A very fine example of this structure is found in thin section 28, where it is seen
to have formed after the primary crystallization of the phenocrysts, but previous to
the final consolidation of the micrograuitic groundmass. This distinctive structure,
which is characteristic of many European granite-porphyries, has been described
by Eosenbusch and called by him "Granophyr."2 It is the "structure pegmatoide"
of the French petrographers, and is becoming generally termed micropeginatitic.

Having described the characters of the minerals composing this granite-
porphyry, it remains to notice their structural combination, whose variety is the strik-
ing feature of this occurrence. Thin section 10, from the large area north of Wood
Cone, is of a porphyritic granite, with little groundmass of fine grained granitoid
structure. The large phenocrysts have no crystallographic outline, but pass by
increasing abundance of inclusions into the groundmass, which contains biotite,
hornblende, titanite and titanic iron, apatite, zircon, and allanite. Thin section 11,
from southwest of the Wood Cone, is a local modification of slight importance; it is
a fine grained mass without phenocrysts, with granitoid structure and composed of the
same minerals as the previous section, but with a greater percentage of biotite and
hornblende. Thin section 12, from the bottom of a gulch on the east side of the dike
and north of Spring Valley road, is of porphyritic, coarse grained granite. The
larger phenocrysts of feldspar have more or less well defined outlines. The orthoclases

'This constitutes the quartz aureole of French petrographers.

!H. Rosenbusch. Die Steiger Schiefer, etc., pp. 347, 352, Strassburg, 1877. Mikroskopisehe Physi-
ograpie. p. 31, Stuttgart, 1877. Mikroskopisehe Physiograpie, vol. n, p. 383, Stuttgart, 1886.

GRANITE PORPHYRY. 343

have a marginal zone of included quartz grains, the inner limit of the zone being
sharply defined. In places the inclosed grains are so numerous that the feldspar
crystals merge in the groundmass and their outline is confused. The biotite, sur-
rounded by grains of iron oxide, is partly altered to green chlorite and epidote. The
hornblende is scarce and the crystallization of the groundmass granitoid. The next
three sections — 16, 19, 20 — should be considered together, since they are from the
same portion of the west side of the dike north of the last named road. No. 16 is
from a distance of 30 feet from the plane of contact with the limestone and is rich in
sharply defined porphyritical crystals, the quartz being in perfect dihexahedrons.
There is an abundance of biotite and titanic iron with titanite, but no hornblende.
The groundmass has the coarse grained micrOpegrnatitic structure illustrated in Fig.
1, PI. vi. No. 19, from a distance of 10 feet, and No. 20, from the contact, are still
more porphyry-like, having much more groundmass with finer grained micropeg-
matitic structure, of very homogeneous texture, the only phenocrysts being feldspar
and quartz; oiotite, hornblende, and the associated minerals are wanting. The feld-
spar is more altered than that of 16 and colorless potash-mica is more abundant.
Thin sections 21 and 22 are from the bottom of a gulch on the south side of the same
road. The first from a distance of 1 foot from the contact with limestone, shows a
porphyritic granite, rich in large, well defined crystals of feldspar and quartz, with
much biotite and hornblende. The groundmass forms but a small part of the whole
and is granitoid, the grains averaging 0-1 mm in size. The second thin section is from
immediate contact and differs from the first in having much more groundmass with the
same microgranular structure, the grains being only one- third as large as those at a
foot distance. The phenocrysts are smaller and more sharply outlined, the fresh
orthoclase having a satin-like sheen in thin section. Hornblende is more abundant in
minute crystals, and augite occurs sparingly.

Thin section 24, from a local modification of the porphyry near its contact with
limestone on the north side of the road, requires special notice. The rather small
phenocrysts of quartz and feldspar have the same characteristics as those from other
portions of the same body, but the groundmass is very diiferent. It is bluish gray iu
thin section, spotted with minute black specks; under the microscope it is seen to be
composed of irregular grains of quartz and feldspar having a granitic structure. The
black specks are found to be angular microcrystalline patches, crowded with black
particles, and bearing shreds of colorless uu'ca, and appear to be remnants of a base less
highly crystallized than the groundmass. Thin section 25 is from a breccia of por-
phyry and dark colored quartzite, the phenocrysts are angular fragments, the quartz
is rich in fluid inclusions, and the opaque, black portions produce a very prominent
flow-structure. Both sections are free from biotite or hornblende. Thin sections 26 aud
27 are interesting because they come from the middle and side of a branch dike
not 30 foot wide. The former has a dense greundmass, bearing large quartz dihexahe-

344 GEOLOGY OF THE ETTBEKA DISTRICT.

drons fairly flooded with fluid inclusions of water and liquid carbon dioxide; the
groundmass is a comparatively coarse grained aggregation of quartz and feldspar,
the latter more highly developed but entirely decomposed, there is a little completely
altered biotite, much colorless mica and some epidote ; the second thin section, from
the side of the dike, is a dense gray mass, poor in quartz pheuocrysts, but rich in
small crystals of feldspar and biotite, the latter partially altered to chlorite; the
groundmass is finer grained and shows in places an incipient micropegmatitic struc-
ture, which is noticeable around the quartz crystals and also in pseudospherulites ;
there is a little titanite, but no hornblende.

Thin section 28, from a narrow dike farther up the Spring Valley road, has
been already alluded to as presenting a most beautiful example of micropegmatitic
structure. It is a very fine grained rock, having a few small quartz dihexahedrons
containing fluid inclusions of both kinds and portions of groundmass, besides a
few crystals of feldspar, but no biotite or hornblende. Around the quartz and
smallest feldspars are frames of feather or fernlike aggregates of intercrystallized
feldspar and quartz, producing the effect of a flowered pattern on the microgranitic
groundmass of quartz, feldspar and colorless mica. Thin section 29 from a small dike
west of Castle Mountain is somewhat similar; the extremely fine grained groundmass
bears numerous quartz crystals, with bays of groundmass, some inclusions of color,
less glass and a few of water with moving bubbles, also crystals of feldspar com-
pletely altered to a cryptocrystalline aggregate, probably kaolin, besides calcite and
hydrous oxide of iron; and a little decomposed biotite. The groundmass is micro-
granitic with an incipient, micropegmatitic structure developed around the quartzes.
Garnet occurs iu well formed rhombic dodecahedrons, having long slender needles
radiating from their centers, which exert no influence on polarized light and are of
an indeterminable nature.

Another variety of granite-porphyry is found near the summit of the Fish
Creek Mountains, thin section 30. It is a fine grained rock, rich in biotite and horn-
blende. TL e sections show it to be composed of long rectangular feldspar crystals,
six-sided mica plates and rather stout hornblende crystals cemented together by
quartz and feldspar, with well developed micropegmatitic structure. Except in this
last respect the rock closely resembles the fine grained micaceous modification of the
large granite porphyry dike near Wood Cone, No. 11. The feldspar is much altered,
chiefly at the center of each individual, the product being partly potash-mica, partly
calcite. Of the fresher crystals many are triclinic. An estimate of the relative
abundance of the two feldspars, however, is impossible under the circumstances.
Quartz does not occur iu large pheuocrysts, but forms the greater part of the ground-
mass, where, with a little feldspar, it assumes the peculiar structure already alluded
to. It carries a small number of minute fluid inclusions with moving bubbles. The
biotite, in quite sharply outlined six-sided crystals, is reddish brown in its fresher por-

QUABTZ-POEPHYEY. 345

tions and shows a slight angle between the optic axes, but it is mostly altered to a
light yellow chlorite, through which are scattered grains of a yellow, highly refracting
mineral, resulting from the alteration of ilmenite and corresponding to leucoxene,
besides very small, sharply defined, colorless crystals, apparently epidote. The
crystals of hornblende are very well developed, showing the prismatic and both
pinacoidal faces, together with the base and pyramid. The individuals are compara-
tively large and broad, with the characteristic cleavage. The color is light brown,
frequently green along the margin. The pleochroism is strong from brown to yellow,
c= b> a. The highest extinction angle measures 19°. Along the cleavage crack red
oxide of iron is sometimes deposited, and, though for the most part fresh, a few are
completely altered to an irregular aggregate of fibrous chlorite and hydrous oxide of
iron, through which run colorless needles with an extinction angle of 17°, which are
probably actinolite. The accessory minerals are magnetite, with some ilmenite partly
altered to leucoxene, a very little titanite, and a large amount of apatite, both in
short crystals and also in extremely long, slender, colorless, hexagonal prisms, occa-
sionally broken and bent, but generally perfectly straight, although one measures
0-44 mm long by 0-0075 mm wide, or is sixty times as long as it is broad, which indicates
that the mass commenced to crystallize after all motion in it had ceased.

Quaru-porphyry.-Unfortuuately the only body of quartz-porphyry found in the dis-
trict is completely decomposed. It occurs in the vicinity of the Bullwhacker mine
and is represented by thin sections 31, 32, and 33, which have essentially the same
structure, though the first is full of pyrite and the second and third are discolored by
hydrous oxide of iron. It is closely related to granite-porphyry, having apparently a
microgranitic groundmass; but a thin film of isotropic glass is detected between the
grains along the thinnest edge of section 31, and colorless glass is found included in
the macroscopic quartz grains, whose quartz-porphyry habit is further evinced by intru-
sions of groundmass, small amount of fluid inclusions, some of which have salt cubes,
and by the absence of liquid carbon dioxide. The quartz shows a well developed
rhombohedral cleavage, especially in section 32, and is the only primary mineral except
apatite and zircon remaining unaltered. A small amount of feldspar is indicated by
patches of a colorless, aggregately polarizing substance, probably kaolin. The mica
occurs in comparatively large crystals, much elongated in the direction of the vertical
axis, which have been altered to a mass of confused lamina1 of colorless potash-mica,
calcite and red oxide of iron. The groundmass also is crowded with shreds of potash-
mica, but it seems probable that in both of its occurrences it is of secondary origin.
Sections that have the outline of hornblende crystals are filled with calcite and ferrite,
and quite large deposits of calcite with very distinct rhombohedral cleavage have tilled
cavities in the rock. Iron is present as magnetite and the hydrous oxides and as
ilmenite and pyrite, the latter in comparatively large crystals, including portions of
the groundmass. Apatite and zircon occur in very small quantities.

346 GEOLOGY OF THE ETJEEKA DISTEICT.

Appendix— Metamorphosed Sandstone— A micaceous fine grained rock occurs in several
localities in the vicinity of Modoc Peak, which is traceable to thin beds of sandstone,
which, however, are never so full of mica, and though the true nature of its occurrence
is somewhat in doubt it is safe to consider it a highly altered forms of the same
quartzose deposit, since a series of thin sections from the bedded sandstone and the
very micaceous rock grades imperceptibly from one extreme to the other, the coarsest
grained variety having the mineral composition and structure of a microgranite. Of
the thin sections prepared, three, 437, 440, 450, are from dense cryptocrystalliiie sand-
stone of a yellowish pink color, bearing a few quite perfect crystals of muscovite and
quartz. Under the microscope the rock is seen to be formed of minute quartz grains,
shreds of potash-mica and patches of a colorless cryptocrystalliue substance, through
all of which is scattered much calcite and ferrite, and occasionally long, slender, beau-
tifully terminated crystals of zircon. The quartz grains range from 0-1 to 0-05mm in
diameter, and have the granitoid form, in no instance suggesting waterworu fragments.
They contain extremely minute fluid inclusions, which literally swarm in the micro-
scopic quartz dihexahedrons of section 440. The form in which the calcite is found
suggests its alteration from feldspar, or its deposition by infiltration in the place of
decomposed feldspar, which is undoubtedly the case in one or two quite large sections.
Thin section 451 is very similar to those just mentioned, and with 452 came from a body
closely connected with the bed of sandstone represented by 450. The grouudmass of
451 is the same in every respect as those just described, but there are numerous
macroscopical individuals of mica and feldspar, the latter still showing in some cases
the striping of twinned plagioclase, though mostly altered to a cryptocrystalline mass
like that occurring in the groundmass, which may probably have the same origin.
The poorly denned mica is completely replaced by calcite and ferrite. Sharply crys-
tallized zircon and apatite are present in stout prisms with very uneven outline.
The three remaining thin sections, 452, 466, and 463, exhibit the highest development
reached, and might be considered micaceous microgranite. The groundmass is com-
posed of granitoid quartz and partially altered feldspar in grains about 0-lmm in diame-
ter together with shreds of potash-mica and calcite. In this lie porphyritically imbed-
ded well developed feldspar crystals and mica and occasionally quartz. The feldspar
is much altered, but shows that it is partly Carlsbad twins, and is partly striped
plagioclase. The mica is somewhat altered and is of brownish yellow color, with
strong absorption. A large individual of quartz in 466 has a multitude of minute
fluid inclusions arranged in planes parallel to the prism or rhombohedral faces. Ferrite,
apatite and zircon also occur. The whole is a thoroughly granite-like rock, without
signs of foliation.

It is interesting to note in this connection how an apparent granitoid form of
quartz grains may sometimes arise from an entirely different cause. The phenomenon
is exhibited in a quartz conglomerate of small grain, thin section 501, where it is

METAMORPHOSED SANDSTONE. 347

observed in polarized light that between the coarser waterworn fragments of colored,
cryptocry stall iue quartzite lies a mass of colorless quartz in angular, closely fitting
grains, the salient angles of one corresponding to reentrant angles of those surround-
ing it. Upon close examination in ordinary light each angular crystal is seen to
inclose a large round grain of quartz, frequently full of fluid inclusions and contain-
ing inicrolites and trichites, the narrow border being perfectly pure quartz. This
is illustrated in Fig. 3, PI. iv. From this it is evident that the rock is an ordinary
quartz conglomerate of rounded pebbles cemented together by silica that has crystal-
lized around the fragments of quartz crystals, taking the same crystallographic orien-
tation as the nucleus and thus extending the individual until obstructed by the
surrounding bodies.

The same observations were first made and published by Tornebohni1 in 1876
and subsequently were observed by H. Clifton Sorby2 and published by him in an
address before the Geological Society of London, February 20, 1880. The same phe-
nomenon was described by A. A. Young in the American Journal of Science for July,
1881; and still later, in 1883, R. D. Irving published in the same journal for June
a paper on the similar enlargement of quartz grains in the St. Peters and Potsdam
sandstones and in certain Archean quartzites in Wisconsin, and in 1884 Irving and
"Van Hise published a bulletin "on secondary enlargements of mineral fragments in
certain rocks," 3 in which, in addition to quartz, the enlargement of feldspars by the
same process of accretionary crystallization is described.

The same thing has been observed by T. G. Bonney and Mr. J. A. Phillips in
England.4

'A. E Tornebohm, "Ein Beitrag zur Frage der Qnarzitbildung." Geol. Foren Stockh, 1876, vol.
Ill, p. 35. Reviewed in Neues Jahrbuch fur Min., etc., 1877, p. 210.
'Quart. Journ. Geol. Soc., London, 1880, vol. xxxvi, p. 62.
3 Bull. 8 of the U. S. Geol. Survey, 1884.
••Quart. Journ. Geol. Soc., London, vol. xxxix, p. 19.

CHAPTER II.
VOLCANIC ROCKS.

For so small an area the variety of volcanic rocks is great, yet there is a
marked similarity between the individual crystals of the same mineral species
wherever they occur, with some few exceptions, which links the various kinds of rocks
together and suggests the possibility of a common source. Nevertheless, the differ-
ence between them in composition, structure, and physical appearance is sufficient
to establish their individuality. They have been divided into three groups — anclesite,
rhyolite, and basalt — and have been considered in the order of their relative impor-
tance in the field.

ANDESITE.

pyroxene-andesite (augite-andesite). —(a.)1 The rock forming Richmond Mountain isa dense
porphyritic lava, for the most part with a reddish purple homogeneous groundmass,
rich in macroscopic crystals of flesh colored feldspar, the largest 4 or 5""" long, without
distinct cleavage, and having a inierotine habit; long black prisms of hornblende, with
very perfect prismatic cleavage and less noticeable pyroxene crystals. The dense
purple variety is in most every case parted or jointed in nearly horizontal planes.
A dark bluish black variety, with a resinous oily luster, occurs in compact masses
without fissile structure, and appears to pass insensibly into the purple rock. At
Trail Hill, the most northern spur of Richmond Mountain, the same rock traced
continuously from the main portion is vesicular and is rich in triclymite. A few
hundred yards to the south a compact fissile exposure shows a more crystalline
development and is exceptional.

Under the microscope thin sections from various parts of the body have essen-
tially the same character — a gray, also yellowish to reddish gray, grouudmass, com-
posed of colorless or yellowish brown glass, very rich in feldspar microlites, augite
prisms, and magnetite grains, with marked flow structure; abundant phenocrysts of
zonally built plagioclase feldspar, with and without polysyuthetic twinning, the

1 Since the first determination of these rocks was made, a separation and optical and chemical
analysis of the pyroxenic constituent of the Richmond Mountain anaesite have been made and pub-
lished in the "Notes on the volcanic rocks of the Great Basin," by Arnold Hague and J. P. Iddings
(Am. Journ. Sci., Vol. xxvn, June, 1884, p. 458). This showed that the greater portion of the pyox-
ene belongs to the orthorhombic species and has the composition of hyperstheue. It is therefore
more correct to place them under the head of pyroxene-andesites, though they were first termed
"augite" andesites, those from Richmond Mountain belonging to the hornblende-bearing variety,
348

PTEOXENE-ANDESITE. 349

largest of which are so crowded with inclusions of foreign matter, with only a narrow
border of pure feldspar, as to appear decomposed in the hand specimen. The smaller
individuals of feldspar are quite free from like inclusions. Besides these are well-
developed crystals of pale yellowish green, strongly pleochroic pyroxene; dark brown
hornblende with the characteristic black border in not so sharply outlined forms;
and, as accessory minerals, magnetite and apatite, with very rarely quartz, mica,
zircon, and tridymite.

The phenocrysts of feldspar, including all that do not take part in the ground-
mass, are plagioclase. The largest individuals, reaching 4mm in length, are in
crystals nearly equally developed in the direction of the three axes, and show in the
sections, besides crystal faces, rounded outlines. They are not abundant in the rock
sections and can not be so carefully studied optically as the smaller feldspars, but
from those that are met with it appears that they are not more basic than labradorite,
and because of the great amount of glass included in them their separation and
chemical analysis would be both difficult and uncertain. The smaller macroscopic
individuals have well-defined crystal forms. Their sections are four, five, six, and
eight sided and correspond to those cut from crystals with OP, ooPa, «'P, ooP',
2 'Fob, 2 P'<5fc faces. They are for the most part prisms, lengthened in the direction of
the brachydiagonal, though some appear tabular in the plane of the brachypinacoid.
Irregularly outlined fragments are seldom met with. A very marked, sharply defined
zonal structure is common to most all the larger crystals, but is wanting in the more
minute ones of the groundmass. The cleavage parallel to the base and brachypinacoid
is not very generally present nor very perfect, the feldspar having the irregular frac-
ture and glassy appearance of sanidine, a resemblance still more striking because of
the nearly total absence in half the individuals of polysynthetic twinning, though in
almost every instance an apparently simple individual or Carlsbad twin is found to
contain one or more thin lamellae of feldspar twinned according to the albite law or
to that of pericline. The medium sized individuals seen in the thin sections, which
correspond to the smallest feldspars noticed in the hand specimens, from 0-5 mm to
l-0mm in length, show the characteristic polysynthetic twinning of plagiodnse
and give angles of extinction symmetrical to the composition plane as follows:
15°-15°, 30°-31°, 33°-33°, 33°-34°, 36°-39°, which, from the table of extinction-angles
published by MM. Fouque et Michel-Levy,1 correspond to those of anorthite or a
feldspar more basic than labradorite. The smaller individuals are twinned after the
Carlsbad law, with very few exceptions, and are characterized by having but few
lamella;, of short length, lying in two directions at nearly right angles, twinned the
one after albite parallel to the brachypinacoid, the other after pericline parallel to the
basal cleavage when present. lu many instances the lamella? are entirely wanting,
as just noticed. A careful study of all the sections that showed cleavage, or were

' Fouqu^ et Michel-L6vy. Mim-ralogie Micrographique, p. 228. Paris, 1879.

350 GEOLOGY OF THE EUREKA DISTRICT.

nearly rectaugular in outline, or extinguished symmetrically with respect to the trace
of the brachypinacoid, gave from more than fifty measurements the following results
in sections where the basal cleavage varied not more than 5° from being at right
angles to the trace of the brachypinacoid; and in sections without cleavage, almost
rectangular, the angle of extinction varied from 30° to 43°, in most cases being about
40°. In sections with symmetrical extinction it was 25°, 34°, 36°, 38°, 40° — that is,
in the zone perpendicular to the brachypinacoid the angles of extinction measured
from the trace of the latter plane reached 43° and were mostly greater than 31°,
showing a part of the feldspar to be anorthite.

The frequent occurrence in this andesite of nearly rectangular sections of twinned
crystals yielding both very high and widely varying angles of extinction led to
an investigation of the position of the axes of elasticity in the two halves of sec-
tions cut from Carlsbad twins of plagioclase in a zone at right angles to the brachy-
pinacoid (aoP*). From the nature of a Carlsbad' twin it is evident that the plane of
the optic axes in the two parts, being oblique with respect to the vertical crystallo
graphic axis in plagioclase feldspars, would be symmetrically disposed only with
respect to the vertical axis, considered as its axis of revolution; hence the extinction
angles for the two parts of the twin, that is, the angles on a cutting plane included
between the trace of its intersection with the brachypinacoid or composition plane
and the traces of its intersection with the planes of the optic axes, respectively,
would be symmetrical only for sections in the zone parallel to the vertical axis, that is
in the zone a>P<x, ooPdo; consequently in the zone at right angles to the brachy-
pinacoid there will be only one position where the extinction angles are symmetrical,
and that is in the section parallel to the vertical axis, while in a plane perpendicular
to it the extinction angles will be complementary, or equal when measured in the same
direction: that is, the axes of elasticity in the two parts will be respectively parallel,
but in all other sections of this zone they will be unequal. These relations, together
with the degree of variation in the extinction angles throughout the zone may be
graphically represented by the following diagram (Fig. 8), derived from the curves of
extinction angles of feldspars in the zone at right angles to the brachypinacoid pub-
lished by MM. Fouque et Levy.1 The case of labradorite will serve as an illustra-
tion. Fig. 7 represents the projection of a Carlsbad twin of that species on the plane
of the brachypinacoid; let (a) be the half in the normal crystallogaphic position, and
(b) that in the twinned position, then it is evident that in considering a series of sec-
tions perpendicular to the plane of the brachypinacoid, if we pass from the position of
a normal to the edge OP, <xP* of the first half (a) in the direction of the obtuse angle
of that half, we at the same time pass from a position 52° from the normal to the edge
OP, ccPdo of the second half (b) in the direction of the acute angle of that half. In
Fig. 8 the heavy line is the curve of the extinction angles for sections of the first half

1 Min^ralogie Micrographique, etc.

PYKOXENE-AXDES1TE.

351

(«) ; commencing at the normal, X, with a value of abont 30°, the direction to the left
corresponds to that which passes over the obtuse angle of that half (a); the light line
is the curve for the second half (b), its corresponding normal, X', being 52° 18' to the
right of the former, and starting at this point with the same value, 30°, but with oppo-
site sign; its passage to the left corresponds to that over its acute angle. From the

Flo. 7.— Carlsbad twill of labradoritc.

resulting figure it is readily seen that in the plane 26° 9' to the right of the normal,
X, which is the section parallel to the vertical axis, the angles of extinction in the two
halves are equal and opposite, that is, symmetrical with respect to the brachypinacoid ;
and that in the plane 63° 51' to the left of the same normal, X, which is the section
at right angles to the vertical axis, the extinction angles are equal, but have the same

.X n -y m X » Y

00-

z<£

-.- *

fc-l-*

-=^r

-^TTT

— - -

— — 1

—

--,.

--.

10"

•V

9.1

-t

f'l'

10° -
20°

,-'

3O° ,

„— ^-

4O'-

Y' X'

X* Plane 0.1 right angtcato toP. ofiPdb) or<oJ .X'JVan* at, "tght angles to (oP.ooPdb)of(bt
Y Plan? containing foP.ooPtib)of ta), ~Y'Plan,e conta ntng /oPooPdbJo/"'W

n Plant; parallel to vertical axis

m Plane at right angles to vertical axt*

Flo. 8. — Diagram of extinction augleg.

sign, which agrees with the conclusions previously arrived at. The greatest differ-
ence in the size of the angles in any one section appears to be about 20°. That these
variations occur in a great number of nearly rectangular sections is understood ujxui
comparing with the diagrams the following table of angles made by the basal cleavage
and the trace of the brachypinacoid in sections in the zone in question. The figures

352

GEOLOGY OF THE EUREKA DISTRICT.

in the second column denote the degrees to be added to or subtracted from 90° to give
the required angle for labradorite in sections taken every 5° from the normal to the
edge OP, ooPdb.

Table showing the angle between OP cleavage and the trace of ooP<X> in
planes in the zone perpendicular to ocP ob for labradorite.

Inclination of
plaue to that
perpendicular
to the edge
OP, »P<5b.

Angle to be
added to
or subtracted
from 90°.

Angle between 0 P cleavage and
the trace of ooP &>.

Obtuse angle.

Acute angle.

0°

3° 20'
3 20
3 23
3 26 40"
3 32 20
3 40
3 50
4 2 48
4 20 6
4 42 21
5 13 20
5 46 40
6 36 40
7 46 40
9 36 40
12 36 40
18 28 42
33 37
48 27
69 22
90

93° 20'
94 42 21"

180

86° 40'
85 17 39"

From this it will be seen that the variation in the angles made by the cleavage
is only 1° 20' for 45° of rotation each side of the normal, or for a whole quadrant, but
for 70° on both sides the variation is only about 6°. Applying this to Figures 7 and
8 it will be readily seen what combinations may occur. Thus in a section in ttis
zone 45° to the left of X the cleavage angle will be 94° 42' in the half (a) and about
120° in the half (6), and whilst the extinction angle in the first half (a) is 20° that in
the second half (b) is 0°. If in conjunction with this Carlsbad twinning we have the
polysynthetic twinning of albite, as generally happens, we shall find sections in the
zone under discussion one side of which will show striations having symmetrical
extinction angles differing from the symmetrical extinction angles of the striations in
the other side by as much as 20° in some cases, a phenomenon which might lead to
the erroneous conclusion that two species of plagioclase feldspar had formed together
along the plane of the brachypiuacoid. It is possible that instances of such an occur-
rence, which have been mentioned by other observers, may be sections of Carlsbad
twins of a single species.

In the thin sections of this pyroxene-andesite occur many examples of twinned
feldspars, in nearly rectangular sections, that exhibit optical phenomena similar to

PYKOXENE-ANUESITE.

353

those just described for labradorite, but which differ greatly in degree, the extinction
angles being very much larger than those of labradorite for this zone as given by
MM. Fouque" et L6vy, and which must upon this ground be referred to anorthite. An
especially fine example of such a feldspar, in which are combined the three sorts of
twinning most common to plagioclase — albite, pericline, and Carlsbad — is seen in thin
section 79. It has been made the subject of a series of careful measurements, which

are indicated on the accompany-
ing diagram (Fig. 9). It con-
sists of two nearly equal halves,
twinned after the Carlsbad law,
one having well-marked cleav-
age, which is absent from the
other. Each shows striations
due to albite twinning, which
give symmetrical extinction
angles that are n«»t the same
for the two halves. Near the
middle of the first mentioned
half is a portion twinned after
the law of pericliue, as the
cleavage and extinction angles
and position of the axes of elas-
ticity show. The section appears
to be in the zone perpendicular
to ooP& and nearly parallel to
the base of the second half.
There is also a marked zonal
structure and variation of ex-
tinction of about 10° from the
center outward, being greatest
at the center. In the left-hand
\lndicates^h&direcuon of the pace of the plane half the symmetrical extinction
of.the opcic ax.es. angles in the marginal zone

^tnOtcatesthepoaittwi of the interference figure, reach 30° and 33°, while the
F,O. 8-Carisbad twin of piagiociase. extinction in the central portim.

is 40° and 44°. In the second half the symmetrical extinction angles are 11° and 14°
in the marginal zone and 24° at the center. This variation is due to a change in tin-
position of the axes of elasticity, which is shown by the fact that near the margin of
the unstriated end of the first half the hyperbolas of the interference figure meet in
the center of the field, but near the center of the same portion they come together on
the edge of the field.
MON xx 23

354 GEOLOGY OF THE EUBEKA DISTRICT.

The phenomenon of zonal variation in the angle of extinction of feldspars
indicates that the chemical composition of the crystals varies from the center out-
wards. And as the extinction angle, so far as observed in the feldspars of the ande-
site of this district, is usually greater at the center of the crystal than toward the
margin, generally passing through a series of distinctly marked zones, which in rare
instances have been found to differ by 20°, yet passing frequently by imperceptible
gradations from one extreme to the other, it seems likely that during the growth of
such feldspars changes have occurred in the chemical composition of the successive
shells of enlargement, tending toward greater acidity, which, though often sharply
denned or interrupted, have sometimes taken place in the most gradual manner possi-
ble, a process only conceivable by admitting the correctness of Tscherinak's theory.
The particular section of twinned feldspar described and illustrated in Fig. 3 has been
treated with hot hydrochloric acid. The central portion of both halves was decom-
posed and clouded and the zonal structure more strongly emphasized. The marginal
zones appeared to resist the attack of the acid completely. This proves that the cen-
tral portion of the first half, with extinction angles as high as 40° and 44°, is anorthite
or bytownite, and that the central portion of the second half is of the same species,
but was cut in a position in which the extinction was only 24°. The outer zones are
probably labradorite. The difference of their behavior toward hydrochloric acid is
more striking than their optical difference.

The occurrence of auorthite in the volcanic rocks of western America has not
been previously noticed, partly because no very thorough investigation of the nature
of the plagioclase feldspar in them has been undertaken und also from the fact that
all simple crystals showing no stria? between crossed nicols, were classed with ortho-
tomic feldspar. Thus the simple crystals and Carlsbad twins of sauidine mentioned in
Prof. Zirkel's report on the rocks of the 40th Parallel Survey,1 as occurring in such
abundance in the " augite-andesite" at Basalt Creek, Washoe, and near Chirks Station
and Wadsworth, near the Truckee River, give in the zone perpendicular to the brachy-
pinacoid angles of extinction ranging from 0° in a few instances to 40°, thus 3!?°, 34°,
35°, 36°, 38°, 39°, 40°, most of the reading being over 30°, corresponding to those of
anorthite. One section cut at right angles to 'an optic axis showed tin- plane of the
optic axes at an inclination of 43° to the trace of the brachypinacoid. Similar auor-
thite is found in the -closely related andesites in the Cortez Range, head of Annies
Creek, and on Emigrant Road, Palisade Canyon, and also from the Traverse Mountain,
Utah. It occurs in the " augite trachyte, "* from the neighboring Wall weah Range,
in the " trachytes" ' from Emigrant Road and the south bank of Palisade Canyon,
Cortez Range, and in the rock from Jacobs Promontory, Shoshone Range, erroneously
determined as rhyolite,1 which is almost identical with the andesite from Richmond
Mountain. It will thus be seen that auorthite has a very wide geographical distri-

'F. Zirkel: Micro. Petro., U. S. Expl. 40th Par., vol. vi, Washington, 1876.

PYKOXENE-ANDESITE. 355

bution in the West, though the rocks containing it can be shown to be of the same
character throughout.

The largest individuals are characterized by a great abundance of glass inclu-
sions, which extend from the center outward, always leaving a border of feld-
spar free from inclusions. They are very irregular in outline and form a net-work so
thick in many instances as to equal in amount the feldspar which forms the meshes.
The glass is colorless and filled with opaque grains and transparent globulites, besides
colorless microlites, whose high index of refraction and similarity to other more
determinate ones in the groundmass suggest their pyroxenic nature. There also
occur inclusions of the groundmass developed to the same degree as that surrounding
the feldspar crystal. The smaller individuals are freer from inclusions, but contain a
greater variety, the glass ones having sharp outlines, either round or nearly rectan-
gular, with a comparatively large gas-bubble and fewer microlitic secretions; liquid
inclusions are less frequent, with a briskly moving bubble, besides needles and stouter
prisms of apatite, magnetite grains and rarely augite. The feldspar substance is
entirely fresh, without the slightest trace of decomposition; in some instances it is
intersected by cracks, in which hydrous oxide of iron has been deposited, and which
have led to the devitrification of part of the included glass, converting it into a yellow
cryptocrystalline aggregate. One single individual contained calcite deposited along
Hues of fracture. There is also present among the phenocrysts feldspars with quite
perfect cleavage, splinters of which parallel to the base give an angle of extinction of
0° and are probably oligoclase, their separation from anorthite by optical methods is
not possible in the thin section.

The microscopic lath-shaped feldspar crystals of the groundmass, averaging
0-03lnm in length by O-OOS0"" in breadth are slender prisms elongated in the direction
of the brachydiagonal, irregularly terminating in two or more needles of different
lengths and are in every case twinned with two or three lamelhe. The angle of
extinction measured from the direction of their length varies from 0° to 26° and cor-
responds to labradorite or a less basic feldspar. Small square sections, not very
abundant, prove by their diagonal extinction to belong to plagioclase.

The second most essential component is pyroxene, which occurs in macroscopic
crystals averaging lmm in length, a few reaching 2mm from which they diminish in
size to -03inm, having sharply defined outlines, well developed faces in the prism zone,
of which the pinacoidal are much the larger, and occasionally showing the pyramid
P and rarely the base OP. The larger number of individuals, however, are not
crystallographically outlined, but appear as imperfectly developed crystals in more or
less rounded forms. It is without the black border that surrounds the hornblende,
but has a narrow granular margin of pale yellow transparent grains, without doubt
augite of final crystallization, formed at the time of solidification of the gmundinass
about the primary larger individuals and to a lesser degree around the black bordered
hornblendes and magnetite grains, but in no instance altont the feldspars. Its

356

GEOLOGY OF THE EUltEKA DISTKICT.

presence is not universal, some pyroxenes being entirely free from. it. In a very few
instances an uncompleted black border has been added to the primary augite, in every
case projecting beyond the crystal outline of tlie remainder of the surface, Fig. 3, PL
in, and being inclosed in the narrow margin just described. This black border appears
to be an aggregation of magnetite grains. A zonal structure is occasionally noticed.
The prismatic cleavage parallel to ocP is quite perfect in some crystals, but in others
it is nearly lost in irregular fractures. The crystals are mostly simple individuals; a
few are twinned parallel to the orthopiuacoid and show three or four alternating bands
between crossed nicols.

At the time when these rock sections were studied it was considered probable
that all the pyroxene individuals observed in any one rock belonged to the same
species, and that those sections with the axes of elasticity parallel to their cleav-
age or to the trace of the faces in the prism zone were sections cut iu the zone at right
angles to the cliiiopinacoid of augite, when they were accompanied by other sections
with inclined position for these axes. Hence all the pyroxene in this case was thought
to be augite. But the observations of Cross1 on the hypersthene-andesites of Colo-
rado and oth.er localities, and our own observations on the andesites of the volcanoes
of northern California, Oregon, and Washington Territory,2 and on the volcanic rocks
of the Great Basin,3 and the studies of many other observers, in different parts of the
world have demonstrated the joint occurrence of an orthorhombic and a monoclinic
pyroxene in a great variety of rocks. Moreover, the pyroxene of this particular ande-
site from Kichmond Mountain has been separated from the rock by means of the
cadmiumborotuagstate solution, as already described iu the paper on the volcanic
rocks of the Great Basin just mentioned. The pyroxene was found to consist of green
augite and brown hypersthene; the latter was isolated with a small admixture of the
augite and analyzed. From the composition of the whole, analysis I, a theoretical
composition for the hypersthene and augite was calculated, resulting as follows :

Mixture.

II.
Hypers-
thene.

III.

Augite.

SiO2

51-16

51-39

49-02

A12O3

3-50

3-26

5-64

TiOj

•73

FeO

15-46

16-45

6-45

MnO

•56

MgO

19-22

19-75

14-37

CaO

8-84

7-31

22-60

IKH . .

•42

99-89

99-87

99-79

1 Am. Jour. Sci., 1883, vol. xxv, pp. 139-144.

2 Am. Jour. Sci., Sept., 1883, vol. xxvi.

3 Am. Jour. Sci., June, 1884, vol. XXVH,

PYROXENE-ANDESITE. 357

The percentage of FeO being greater than 14 per cent the orthorhombic pyroxene
may be classed as hypersthene. The optical character was determined in the isolated
crystals and corresponded to hypersthene.

A review of the thin sections of the andesite from Richmond Mountain shows that
the two pyroxenes resemble one another closely in thin sectioir, but the hypersthene
is pleochroic to a greater or less extent, the augite not at all so. The pleochroism of
the hypersthene is, of course, stronger in the thicker sections, but varies among the
individuals in a single section and in some instances differs zonally in a single crystal,
being stronger in the central portion of some individuals and in the marginal portions
of others. It is green parallel to the c axis and light brown parallel to a and b with a>b.
In some cases they are nearly colorless. The augites are very light yellowish green to
colorless. Cleavage parallel to the prism and more rarely to the pinacoids is observed
in cross sections cut perpendicular to the positive bisectrix; but in many longitudinal
sections there is no trace of cleavage.

The slight border of augite grains surrounding many of the pyroxenes is almost
exclusively confined to the porphyritic augite crystals. This is most noticeable where
both varieties of pyroxene have grown together in parallel crystallographic orienta-
tion, the hypersthene being the older secretion in most every case; the granular
augite border extends around the augite crystal, but ceases at the hypersthene. The
orthorhombic pyroxene is more readily altered than the augite, a fibration parallel to
the c axis sets in from the surface and along the cracks, resulting in a light green,
highly refracting mineral with an inclined extinction angle which reaches 15°, and is
evidently a fibrous hornblende (actinolite). The crystals are sometimes coated with
brown oxide of iron (limonite), which also coats the pyroxene micro! ites and porphy-
ritical hornblendes. Though generally free from inclusions some individuals bear
numerous magnetite grains, and irregularly shaped, colorless glass inclusions with a
gas bubble, besides apatite needles and, rarely, imperfectly formed brown hornblende.

The pyroxene microlites of the groundmass, varying from 0-04 or 0-05 mm in
length to microscopically minute proportions, are long slender prisms parallel to the
vertical axis, terminated by a pyramid. They are of a pale greenish color and con-
tain numerous magnetite grains, which are in no case associated with the feldspar
microlites. Their augitic nature is shown by their crystalline form, color, and high
index of refraction, taken in connection with their angle of extinction, which varies
from 0° to more than 35°, being indeed directly traceable, through occasional larger
individuals, to those of unquestionable augitic nature. A part, however, may be
hypersthene. The parallel, fibrous decomposition product is in one instance, No. 90,
colored red by oxide of iron, producing small prisms of a reddish yellow color, pre-
cisely similar to those mentioned by Prof. Zirkel as of an indeterminable nature in
the "trachyte" from the south bank of Palisade Canyon, Cortez Range, previously
referred to, which are there also traceable to augite. This microscopic angite <>t' tinul

358 GEOLOGY OF THE EITKEKA DISTRICT.

crystallization appears more readily altered than the macroscopic primary crystals,
and when discolored by iron oxide forms dark red, narrow borders around the still
fresh larger augites and black bordered hornblendes, suggesting the characteristic
black border of the latter mineral, from which, however, it is easily distinguished.
Aggregations of augitfe crystals around a foreign nucleus are occasionally met with.

The hornblende of this rock is quite abundant in crystals, which are not very
well developed, except in the prism zone, where, besides the ordinary faces, ooP and
ooP &, there is occasionally the orthopinacoid, ooP db. The terminal faces are not
recognizable iu the thin sections studied, but, judging from the macroscopic crystals
in the hand specimens, they appear to be those usually developed. The majority of
individuals seen under the microscope are irregularly outlined. The largest reach
4 to 5""" in length, but the greater number average less than lmm. They do not take
part in the composition of the grouudmass. The cleavage is very perfect, parallel to
the prism, forming a very sharp network of parallel lines, thus differing from the
pyroxene, in which the less perfect cleavage is combined with irregular cracks. There
is in some instances a second cleavage, parallel to the clinopinacoid, never well devel-
oped. Some of the individuals are twinned in the usual manner parallel to the
orthopinacoid. The hornblende is dark reddish brown in color, with a strong absorp-
tion. In the dark and more resinous varieties of the audesite (Nos. 77, 78, 79) the
color is reddish brown, being dark brown parallel to the axis of least elasticity (c),
nearly the same shade of brown parallel to the axis of mean elasticity (b), and light
yellowish brown parallel to the axis of greatest elasticity (a); that is, c = dark brown,
b = dark brown, a = yellowish brown, and c = b>a, possibly c > b > a. In the lighter
colored, purple and fissile varieties of the audesite (Nos. 85, 86) the pleochroism is
greater, but the absorption less, the brown color having a greenish tinge and the
pleochroism being as follows: Parallel to c browish green, parallel to b reddish brown,
parallel to a yellow, and c = b>a. In the specimen from Trail Hill (No. 90) the color
parallel to c is brownish red, parallel to b brown, parallel to a light brown, c > b > a.

The hornblende individuals are surrounded by an opaque black border that
bounds the whole outline of each section, the fractured or eroded portions in the same
manner as the crystal faces ; its width varies somewhat, and is not constant for any
one individual. It is quite sharply denned, both on the outside and inside, though
occasionally it is seen shading into the hornblende substance as minute opaque dust.
It appears to be magnetite, having the same luster in incident light and the same
products of decomposition, hydrous oxide of iron. Spots of similar magnetite dust
occur inclosed in the hornblende, besides the inclusions of coarser grains and crystals
of magnetite, sometimes arranged in lines parallel to the clinopinacoidal cleavage.
The fact that the black border does not occur between the hornblende and feldspar
or augite when they are in contact, but always between hornblende and the ground-
mass, together with the fact that it surrounds the fractured portions and lines the

PYROXENE-ANDESITE. 359

intruding bays of grounduiass in crystals, with more or less rounded angles, and that
the outline of the border is generally that of the crystal, while that of the hornblende.
substance within is mostly irregular, suggests its being the result of a change in the
condition of the molten magma when hornblende ceased to crystallize out and pre-
viously formed .hornblende crystals "may have been partially melted, or replaced by
magnetite. This has m some instances proceeded so far as to form pseudomorphs of
magnetite after hornblende (thin sections 90, 91, 87, 88), as noticed by other observers.
Some oi the pseudomorphs (thin section 91) show minute grains of augite uniformly
mingled with the magnetite,1 suggesting more strongly that there has been a melting
of the hornblende, followed by recrystallizatiou, under conditions which led to the
production of augite in place of the hornblende. This corresponds to the results
obtained in the artificial reproduction of hornblende, in which augite has always
been formed instead of hornblende. The hornblende is very free from inclusions, for
besides magnetite, only a small amount of colorless apatite is found, and in one or
two cases feldspar and augite. It is absolutely fresh in all the sections made from
Richmond Mountain ; as remarked before, it is not a constituent of the grounduiass.

Magnetite is less abundant than the minerals just described, and of much less
importance in the composition of the rock, yet at the same time it is a constant ingre-
dient. It occurs in crystals and irregularly shaped grains, the largest about 0-2 mm in
size, from which they range to almost indistinguishable grains in the groundmass;
it is very evenly disseminated, but not very .abundant.

Apatite is another constant factor, though of little importance; it occurs in com-
paratively large, stout crystals, 0-2 Inm long by 0-05 mm broad, giving sharp hexagonal
cross sections and showing in longitudinal sections the pyramidal termination, P. It
is colorless, but in some instances is crowded with opaque microlites arranged parallel
to the vertical axis of the crystal. These give it a brown or gray dusted appearance
and exhibit an absorption parallel to the longest axis. One cross section shows these
microlites arranged parallel to the longest axis and in planes parallel to the prism
faces (section 79). The apatites also contain a few inclusions of glass with gas bubbles,
which are in negative crystal cavities. The apatite is found closely associated with
the pheuocrysts and seldom alone in the groundmass.

As accessory minerals biotite ranks first in importance, being of special interest
on account of its scarcity in this pyroxeue-andesit< of Richmond Mountain and in the
similar pyroxene-andesite of Cliff Hills, as compared with its great abundance in the
hornblende-rnica-andesite and andesitic pearlite of the district. It is found in only two
thin sections from Richmond Mountain (Nos. 79, 78), and in each of these there is only
a single individual of rounded form with intruding bays of groundmass. The mineral
is brown, with strong absorption, and is filled with minute grains of magnetite deposited

'The sarno .ilw.-rvation has been made by Dr. K. Oebbeke: Beitrftge »nr Petrographie der Phillpplnen and dor
Palau-Insel. Neues Jahrbuch far Min., etc., 1881. B. B. I, p. 474.

360 GEOLOGY OF THE EUREKA DISTRICT.

along the lines of cleavage. An exceptional occurrence of mica is found in the andesite
exposed southeast of Trail Hill (No. 91). It does not form macroscopic crystals, but
occurs in small, irregular patches, closely associated with the macroscopic augite, and
also in more or less regular plates, quite uniformly disseminated through the groundmass.
It is brown and has a strong absorption, showing a large angle between the optic
axis, and appears in so fresh a rock to be undoubtedly of primary origin. A similar
occurrence is noted in the exceptional ''augite-andesite" from Palisade Canyon, Cortez
Range, described by Prof. Zirkel.1 The two rocks, however, appear quite different
both in the hand specimen and under the microscope. The latter is coarsely crystal-
line and contains plagioclase, quartz, hypersthene, and brown mica, while the former
has a microcrystalline groundmass with porphyritical crystals.

Quartz phenocrysts are very rare. Two rounded grains of very pure quartz
without inclusions are found in thin section 87. An irregular grain containing some
fluid inclusions, with briskly moving bubbles, in thin section 86, exhibits a varying
optical orientation, plainly arising from unequal tension throughout the individual.
It is found in the groundmass of the holocrystalline varieties (91-97), as the last min-
eral to crystallize, forming a cement for the other constituents. It can be determined
optically as a positive uniaxial mineral. It contains numerous glass and gas inclu-
sions. Its outline is very irregular, as the quartz individual extends among the
neighboring feldspar grains for some little distance, producing an irregular patch of
quartz substance, which becomes alternately dark and light throughout its whole
extent, as the thin section is rotated between crossed iiicols — a micropoikilitic structure.

Tridymite is very abundant in the vesicular forms of this andesite, thin sections
90, 87, 88. It occurs as microscopic aggregates of hexagonal plates about 0-02 mm in
diameter, filling small amygdaloidal cavities and incrusting the walls of larger ones
with easily recognizable macroscopic crystals. Tridymite has been found by Prof.
Zirkel in the precisely similar rock froin the south bank of Palisade Canyon, Cortez
Range,2 and in the rock from the same locality,3 before noticed in connection with the
occurrence of anorthite.

The groundmass of these andesites has the "felt-like" structure noticed by
Prof. Zirkel as characteristic of " augite-andesite." It consists of a colorless glass
base crowded with microlites of feldspar and augite, with minuter crystals of magi.et-
ite associated with the augite, besides more or less dark colored globulites of an
indeterminable nature, the whole generally showing a marked flow-structure. The
proportion of glass base to microlites varies in different localities on Richmond
Mountain. It is most abundant in the dark resinous variety (Nos. 77, 78, 79),
where it is nearly equal to the microlites in amount. The gray color in these
thin sections appears to be due to minute magnetite grains, together with augite

T. Zirkel. Micro. Petro.. U. S. Kxpl. 40th Par., vol. vi, p. 227, No. 527.
*Op. cit. specimen No. 311.
>Op. cit. specimen No. 310.

PYROXENE ANDESITE. $61

microlites, the reddish tint of the other varieties (Nos. 85, 86, 87, 88, 90, 91)
arising from the presence of a higher oxide of iron incrusting the magnetite. In the
first mentioned variety the number of augite microlites exceeds that of the feldspar.
In the lighter colored fissile forms (Nos. 85, 86) the feldspar is in excess and the glass
base is not so abundant. In the vesicular andesite the composition of the groundmass
is not homogeneous throughout, for besides the amygdules of tridymite are light
colored spots where the augite, magnetite and globulites are almost wholly wanting
(No. 88). Glass base is altogether absent from the mica-bearing groundmass of thin
section 91, which is nricrocrystalline, with grains and lath-shaped microlites of feldspar
cemented together with quartz. An exceptional red variety is found in which the
colorless glass base is so thickly crowded with red oxide of iron as only to be detected
in the thinnest possible section (No. 92).

(&.) The pyroxene-andesite of Cliff Hills is identical with that of Richmond
Mountain; it shows the same modifications in the field as the latter, corresponding to
which are the same microscopic characters. Thin section 102 is from a resinous blue-
black variety similar to Nos. 77, 78, 79 of Richmond ; section 107 is from a reddish
purple form, and corresponds to No. 90 from Trail Hill. Thin section 108 is like No.
92, and the remaining two sections, 104 and 109, are slightly modified varieties. Under
the microscope the typical andesite has a gray groundmass of glass with microlites of
feldspar and augite and an abundance of magnetite. It bears phenocrysts of feldspar,
augite, hypersthene, and black bordered hornblende.

The feldspar is triclinic without any admixture of recognizable orthoclase, the
individuals are all striated by multiple twinning. Their outline is mostly rectangular,
some with the angles truncated or rounded, indicating their form to have been prisms in
the direction of the brachydiagonal, having the faces OP, oo Pdb, coP', GO 'P, 2 PS. The
largest phenocrysts are developed more equally in the direction of the three axes; the
feldspar microlites in the groundmass are wholly lath shaped. The angles of extinction
of the porphyritical crystals reach 35°, 40°, and 44° in the zone at right angles to the
brachypinacoid, which correspond to anorthite, as does also the high light they exhibit
between crossed nicols in very thin sections. Optically it can not be determined
whether other species of triclinic feldspar are at the same time present among the
larger phenocrysts, unless the great divergence of extinction angles in the zonally
built individuals, which reaches in one instance 32° (102), be taken as evidence of
difference in chemical composition between the different zones. The zonal struc-
ture is beautifully developed in some individuals, especially so in the crystal just
referred to, and also in another in the same thin section, Fig. C, PI. III. Where the
inner zone has a sharp crystallographic outline, while the outer one is rounded at
the corners, the angle of extinction for the former being 38° and for the latter only
18°, narrow strips of twinned feldspar pass through the different zones, without tak
ing part in the zonal structure!, and having the same angle of extinction throughout

362 GEOLOGY OF THE EUREKA DISTRICT.

The individuals show both the polysynthetic twinning of albite and of pericline,
besides the simple Carlsbad twinning, which is often shown by the outline of the
sections, but the striae are in many cases few in number, and are sometimes altogether
wanting.

The larger feldspar crystals are especially rich in inclusions, which are massed
in the center or arranges in concentric zones, or are scattered irregularly through the
crystal. A good example of the zonal arrangement is seen in thin section 107. The
zone of inclusions in every case consists of minute particles of glass carrying globu-
lites and possibly gas bubbles, so densely crowded as to exceed in amount the inclos-
ing feldspar substance; when occurring scattered their form is seen to be in some
cases very irregular; in others rectangular, with the edges parallel to the outlines of
the feldspar crystal. In thin section 102, there are brown and gray globulitic glass
inclusions bearing augite inicrolites, besides which are isolated colorless glass inclu-
sions with gas bubbles, and an occasional microlite. There are also iuclosures of the
groundmass and of the associated inicrolites. The smaller crystals are much freer
from inclusions. The lath-shaped feldspar microlites forming the groundmass are
unevenly terminated and twinned in two or three stripes; the angle of extinction is
in general low, sometimes reaching the limit of labradorite, to which species they seem
to belong in part, though it is probable that a less basic species is also present.

Pyroxene is abundant both as macroscopic crystals and as microlites in the
groundmass, its crystals are prisms, frequently very long and slender, with the prism
zone well developed ; the pinacoidal faces are much larger than the prismatic ; the cleav-
age is poor, and there are many irregular fractures. The twinning is that ordinarily
met with. The pleochroism of the hypersthene is strong, but varies greatly among
the individuals in one and the same rock section, in some cases being scarcely per
ceptible. The absorption and pleochroism are green parallel to c, light reddish brown
parallel to a. In sections at right angles to the vertical axis the colors are, yellow
parallel to a and grayish purple parallel to b, that is c=green, o=light reddish brown
to yellow, b= grayish purple. Sections apparently in the same cry stallographic posi-
tion vary greatly in their degree of coloring. They are poor in inclusions, of which
the most characteristic are magnetite grains, apatite needles and glass. There is around
most of the augite crystals a narrow border of augite grains of final crystallization,
which also surrounds the black border of hornblende and magnetite as previously
described; some individuals are entirely free from it, and a very few have a partial
black border like hornblende, Fig. 2, PI. in. It is especially noticeable in thin
section 104, where of two pyroxene crystals almost in contact one, an augite, has a com-
plete border of magnetite, partially altered to red oxide, while the other, a hyper-
sthene, has no border whatever. The decomposition of the pyroxene results iu the
same yellow fibrous mineral mentioned under the Richmond Mountain andesite. The
granular augite border and the smaller augite crystals and microlites in the ground-

PYROXENE ANDESITK. 363

mass of thin section 107 are similarly decomposed and colored with red oxide of iron, as
in the corresponding variety of andesite from Trail Hill. The same is true of thin sec-
tion 108, the excess of red oxide rendering the slide nearly opaque. The augite micro-
lites in the groundmass are very abundant and are traceable directly to the larger
crystals; they are in stout prisms or irregular grains and in most every case have one
or more magnetite grains attached. The pyroxene in these rocks, like that in the
andesite of Richmond Mountain consists of pleochroic hypersthene and nonpleochroic
augite, with the same characteristic differences throughout.

The hornblende is much less abundant than the pyroxene and qpcurs only in
larger phenocrysts, with poorly denned outline, being frequently rounded and also
irregular, as though corroded. The cross sections are six and occasionally eight sided,
and show the prism and piuacoids. They are surrounded by a heavy black border,
the substance of which sometimes penetrates nearly to the center of the crystal. A
zonal arrangement of the minute .magnetite particles is seen in some individuals, thin
section 107. The hornblende is brown, with strong pleochroism : c = dark reddish
brown, b = brown, a = light brown, c>b>a. Inclusions are few, except grains of
magnetite, beside which there are a few prisms of apatite having a sharp hexagonal
cross section.

Biotite phenocrysts are present in small amount, always with rounded outlines
and crowded with magnetite grains. Magnetite and apatite occur as in the Richmond
Mountain andesite. Quartz, though quite noticeable in macroscopic grains in the
hand specimens as an accessory mineral, is not found in the thin sections studied,
except one small particle, 0-25 nun in diameter, which carries both glass and fluid inclu-
sions (107).

The groundmass is composed of feldspar and augite niicrolites, with much
minute magnetite associated with the augite, crowded together in a colorless glass
base, the whole showing a distinct flow- structure. The proportion of augite and
feldspar is about equal, but the size of the microlites is not so uniform as in the
Richmond Mountain andesite, and numerous crystals, from 0-05 to O-l""" long, arc
scattered through the mass, giving it a much less homogeneous texture. The funda-
mental structure, however, is felt-like, which completes the correspondence between
the two pyroxene-andesites of the district, which are indeed but 15 miles apart.
They represent, however, a rock of very wide occurrence in the West, judging by the
collection of the Exploration of the Fortieth Parallel, which, with a constant micro-
scopic habit of groundmass and of phenocrysts, varies only in macroscopic habit;
that is, in compactness, structure, and color, and in the relative size or abundance of
the phenocrysts, and in the absence or presence of hornblende and biotite, an excess of
which is generally accompanied by a modification of the groundmass, resulting in
difficulty deterimnable forms intermediate between pyroxeue-audesite, hornblende-
andesite, and hornblende-mica-andesite. From the foregoing description it is evident

364 GEOLOGY OF THE EUREKA DISTRICT.

that the rocks forming Richmond Mountain and Cliff Hills are pyroxene-andesites,
with a very considerable percentage of hornblende as an essential ingredient and
biotite as an accessory one. They might in fact be termed hornblende-pyroxeue-
andesites.

A very striking correspondence between the different varieties in each of the
two pyroxene-andesite occurrences in the district will be seen on comparing together
thin sections 102, 104 with 77, 78, 79; 107 with 90, and 108 with 92. The other sec-
tions from Richmond Mountain have corresponding varieties at Cliff Hills, which,
however, were not made into thin sections. Several thin sections remain, which need
a brief mention. No. 109 is from a modification of the Cliff Hills rock, which in some
respects resembles the basalt of Magpie Hill and that on the south slope of Alhambra
Ridge, but which is found under the microscope to be a much finer grained pyroxene-
andesite, rich in magnetite, with phenocrysts of the same feldspar and pyroxene, but
without hornblende or mica, and bearing some small red altered crystals of olivine,
whose presence might throw considerable doubt over the determination were there
not frequent patches, not inclusions, of coarser grained groundmass free from magnet-
ite, identical with the grouudmass of the neighboring andesite. Thin section 110 is
a variety from a limited exposure in the tuff northwest of Devils Gate, which is poor
in large phenocrysts.

Hornbiende-[Mica]-Andesite.— The hornblende-[mica]-andesite of the district, though
closely related to the pyroxene-andesite in many respects, has sufficient strongly
marked points of contrast to constitute a separate rock. The areas of exposure of
the two in the field are nowhere in contact, and no transition of one into the other is
detected under the microscope, except in the andesitic pearlites to be described later.
The hornblende [micaj-ande^ite has a light purple and reddish purple groundmass rich
in macroscopic crystals of feldspar, hornblende, and biotite, of which the feldspar pre-
dominates; it is further characterized by the total absence of pyroxene. Several mod-
ifications which occur in separate and limited exposures will be noticed in their proper
connection. Under the microscope the rock (thin sections 38, 41. 35, 37, 42, 42a, 39)
is seen to consist of a microcrystalline feldspathic groundmass, in some cases entirely
free from glass base. It is rich in macroscopic pheuocrysts of feldspar, black bordered
hornblende, and reddish brown biotite. The accessory minerals are apatite and very
little magnetite, and zircon; quartz is an accessory mineral in some occurrences, espe-
cially in that east of Pinto Road (39).

The feldspar is wholly triclinic, being for the most part striated, and the
unstriated sections giving angles of extinction belonging only to plagioclase. The
porphyritical crystals are beautifully developed, yielding sharply outlined sections,
one or two millimeters in length, of the usual form. They show remarkably fine zonal
structure, well illustrated in Fig. 1, PI. v. In this feldspar, besides the successive
stages when the crystal had rectilinear outlines, there were three periods when its

HORNBLENDE MICA -ANDESITE. 365

form must have been quite rounded as i t partially fused. The cleavage in these feld-
spars is very imperfect, and is for the most part wanting, the crystals being irregu-
larly cracked like sanidine. The polysynthetic twinning after albite and i>ericline is
very unevenly developed. The latter, never repeated to any great extent, is present
in many individuals, the lamella; seldom traversing the entire width of the crystal;
those produced by the, former twinning vary greatly both in breadth and length in
the same individual, as well as in different ones, the feldspars in general being char-
acterized by a paucity of striations. This is well shown iu Figs. 3 and 4, PL v, and
Fig. '2, PI. vi, the two figures on PI. v, also illustrating the characteristic difference
between the largest of the phenocrysts (Fig. 3, PI. v), and the medium sized ones
(Fig. 4, PI. v), both being magnified to the same extent, 35 diameters. The largest
have quite irregular outlines and an abundance of stria;, while the medium sized feld-
spars are very sharply crystallized and are poorly striated. Besides the multiple
twinning, nearly every individual is twinned in halves, either after albite or in a
manner corresponding to that of Carlsbad in orthoclase; and frequently several indi-
viduals have formed in parallel orientation with the brachypinacoid as the plane of
contact (Fig. 2, PI. vi). The angle of extinction averages about the same in each of
the thin sections studied. By far the larger number of readings give angles ranging
from 15° to 31°, some being lower and a very few being higher; for example, in thin
section 35 the observed angles in the zone perpendicular to the brachypinacoid are
7°, 150, 20°, 200, 210, 210, 280, 300, 31°, 320, 350, 350, 4<P. In the other thin sections
the higher angles are even scarcer and belong to very perfectly rectangular sections
with few striations. Anorthite is probably present only in small amounts, the greater
number of the porphyritical feldspars being labradorite. The lath-shaped microlites
of feldspar in the groundmass are fibrous and twinned, and have angles of extinction
varying only a few degrees from zero and are for the most part oligoclase; the species
of the ill defined patches or grains of feldspar in the groundmass is optically indeter-
minable. The large crystals of feldspar contain numerous small colorless glass inclu-
sions, each with a single gas bubble ; what sometimes appear like particles of dust
are found under a high power to be aggregates of these sharply defined inclusions
0- . 02mm in diameter ; they are occasionally arranged iu systematic order, but more com-
monly are scattered irregularly through the crystal; some are attached to needles of
apatite. There are also inclosed a few microlites of apatite, some of which in turn
contain glass inclusions, and rarely small crystals of zircon. The substance of the
feldspar in the thin sections studied is absolutely fresh.

The hornblende is found only in macroscopic crystals affording the character-
istic six sided cross sections and having in every case a black bonier, except in the
green variety of hornblende-[mica|-andesite at the east base of Hoosae Mountain i4_'.
42«). The substance of the hornblende is entirely decomposed, not a single unaltered
fragment having been found in any of the thin sections. The resulting product is a

366 GEOLOGY OF THE EUEEKA DISTRICT.

fluely fibrous, yellowish green mineral, faintly pleochroic and with a rather high index
of refraction, whose angle of extinction is over 20° in some sections, and which must
therefore be a fibrous amphibole. Its fibers start from transverse fissures and run
parallel to the vertical axis of the crystal for a short distance, thus forming a net
work, the meshes of which are filled with a colorless substance without noticeable
action on polarized light, who ;e granular texture and botryoidal form suggest amor-
phous quartz. Scattered through these minerals are numerous opaque and transpar-
ent globulites of indeterminable nature, together with ferrite and red oxide of iron
directly traceable to the black border, from which it projects into the crystal in thin
branching plates (35). A more intimate mixture of the two principal decomposition
products has taken place in thin section 41, most of which has fallen out in grinding.
In thin sections 42, 42«, the hornblende sections are without black borders and the
alteration has advanced farther, resulting in a yellowish green, fibrous chlorite, with
extinction parallel to the fibers, which has spread through the groundmass of the
rock, imparting to it a green color.

Biotite is less abundant than the hornblende, though in much larger crystals.
It occurs in six-sided crystals with very marked pleochroism and strong absorption,
being deep red when the section is parallel to the base, and in oblique sections in
ordinary light orange, yellow, and green (35). The interference figure shows a small
optical angle, which varies somewhat in different thin sections, the plane of the optic
axes being perpendicular to that of symmetry in the crystal. Twinning parallel to
<x>P (110), where the composition plane is the base OP, is frequent. It is recognized
by bands with different angles of extinction in sections slightly inclined to the base,
and by the different positions of the interference figures in such sections, and also by
difference in the pleochroism in some sections nearly perpendicular to the base.
There are numerous black microlites arranged in lines perpendicular to the six faces
of the mica crystal, besides irregularly scattered prisms of apatite, and more rarely
zircon. In thin section 39 there are portions of the groundmass, each containing one
or more apatite crystals. The biotite has remained perfectly fresh in most of the thin
sections, though the hornblende, has been entirely decomposed. In section 37 the
biotite, though bleached out and stained yellow by iron oxide, still retains its optical
properties.

Quartz appears as a very inconstant accessory ingredient, being wholly wanting
in the form of primary phenocrysts in the typical crystalline hornblende-[mica]-aude-
site 41, 35, but occurring in abundance in the glassier variety from east of the Pinto
|{<i;nl (39), where it is in rounded grains and fragments, the largest 3mm in diameter.
They carry numerous dihexahedral glass inclusions with gas bubbles, around which
in polarized light the quartz shows the phenomena produced by strain. Other small
grains and fragments are found sparingly in thin sections 42« and 38. Microscopic

HORNBLENDE MICA-ANDES1TE.

quartz grains form a constituent of the groundmass in the crystalline forms of t In-
rock and are determiuable as such in thin section 41; they are more numerous in thin
section 35. They also occur in small aggregates around the sides of cavities resem-
bling chalcedony. The abundance and intimate association of this modification of
quartz with the groundmass of the rock, and the abundance of macroscopic quartz
in the rock near Pinto Road makes it an intermediate variety between andesite and
(Incite.

Magnetite in macroscopic grains and in microscopic crystals is very evenly dis-
seminated through the groundmass, but is not nearly so abundant as in the pyroxene
andesite. It is everywhere coated with red oxide and in thin section 37 it has been
converted into the yellow hydrous oxide.

Apatite is especially well developed in stout hexagonal prisms with a pyramidal
termination and occasionally the base, a beautiful example being found in thin sec-
tion 37, Fig. 4, PI. in. They are dusted gray in the center and show the customary
pleochroism. Cross sections show inclusions parallel to the sides of the prism.
There are also glass inclusions in negative crystals, Fig. 1, PI. in. Apatite is asso-
ciated with hornblende and biotite and also occurs isolated in the groundmass; it is
specially noticeable in thin section 37. It has a flue red color in thin sections 35, 39.
Zircon is a constant ingredient, though in very small quantities. It is in microscopic
crystals of a yellow color easily recognizable by their sharp outline, high index of
refraction, and consequent brilliant display of interference colors between crossed
nicols. They are rather more frequent in thin sections 35, 42.

The groundmass of the typical hornblende- f mica] -andesite of this district (35,41)
is microcrystalline without glass. It is composed of microlites of plagioclasc, largely
oligoclase, in an aggregate of feldspar and quartz grains of irregular outline, that are
nearly free from microlites ac the center, especially in thin section 41. Besides these
minerals are minute crystals of magnetite and in thin section 35 opaque microlites,
which are seen to be made up of opaque aud transparent yellow grains and correspond
to the shreds of brown mica that occur in thin section 41. This is more abundant in
the fine grained audesitic breccia (38), where it also occurs in well defined hexagonal
plates. A flow structure is evident in the arrangement of the lath-shaped feldspar
microlites. The groundmass in thin sections 42, 42a, 39 presents a less advanced
stage of crystallization, the lath-shaped microlites being accompanied by smaller and
fewer faintly polarizing feldspar grains in a relatively small amount of colorless glass.
Through this in the green variety from the east base of Hoosac Mountain (42, 42a) is
disseminated yellowish green fibrous chlorite, resulting from the decomposition of tin-
hornblende.

Thin sections 45, 46, and 48 are from highly decomposed rock, \yhose original

368 GEOLOGY OF THE EUREKA DISTRICT.

character has wholly disappeared, but which still retains, besides some partially altered
mica, apatite and zircon in a perfectly fresh condition.

An interesting example of altered hornblende-mica-audesite and at the same
time of local accessions of porphyritical quartz is the occurrence west of Glen Dale
Valley, south of Hoosac Mountain (49, 50). The feldspar of the rock is completely
replaced by calcite, quartz and colorless potash-mica, which are also the residual
products of the decomposition of the grouudmass. Hornblende with dark border is
very abundant in thin section 49. It is altered to yellowish green, coarsely fibrous
chlorite, which polarizes strongly and extinguishes parallel to the fibers, and contains
small, yellow, highly refracting grains, possibly epidotc. The mica has become color-
less, but retains its negative, apparently uniaxial character, and is rich in the most
beautifully developed microscopic crystals, that are in part tetragonal pyramids of a
colorless mineral with high index of refraction, apparently anatase, in part slender
prisms of epidote ( ?) and thin plates of hematite, besides smaller grains in lines perpen-
dicular to the sides of the mica plate and apatite prisms with glass inclusions (49).
Quartz is sparingly present in rounded macroscopic grains in the variety rich in horn-
blende, but is very abundant in the purple variety (50), poor in hornblende, which in
the field appears as a local modification of the former. The quartz occurs both in
rounded fragments and in dihexahedral crystals and contains glass inclusions".

Andesitic Pcariite and Dacite.— The third and most interesting form of andesite found in
the district unites under its numerous and varied forms characters both of the
pyroxeue-andesite and hornblende-mica-audesite, and presents, as an extreme
variety, daeite. Its connection is most intimate with the hornblende-uiica-andesite,
which indeed is found passing into it in an outcrop back of the windmill pump east
of Secret Canyon road (74), and also east of Hoosac Mountain (73, 75a, 75b, 76). It
is again found in association with liornblendevinica-andesite in Sierra Canyon and in
the gulch south of Carbon Ridge. Its resemblance to pyroxene-andesite will be seen
to be confined to a variety with inicrolitic, felt-like groundmass, and to the presence
of pyroxene, which is lacking in the hornblende-mica-andesite just described. In the
two principal localities where it has come to the surface, at Dry Lake and in the
vicinity of Sierra Canyon and South Hill, it presents so great a variety of form that
the extremes of the series would scarcely be suspected of belonging to the same geo-
logical body, but this is evidently the case in the occurrence south of Dry Lake. The
following able of comparison is arranged to show the parallelism in the forms from
which thin sections have been made. The series commences with the most crystalline
variety and that most clossly allied to the horublende-mica-andesite, and finishes
with the most glassy, porous, and quartzose form, or dacite

ANUESIT1C PEARLITE AND DACITE.

Dry Lake.

Sierra Canyon.

South of Carbon
Ridge.

Kast of Hiin-:if
Mountain.

53
54

62 63

73
74 75 a 75 b

64
66

The rock in question is a pearlite with a variously modified glassy grouudinass
and abundant phenocrysts of feldspar, hornblende, hypersthene, augite, biotite and
quart/. The character of these minerals is constant throughout and similar in most
respects to those found in the more crystalline andesites. They may be here described
then for the whole series. The feldspar is triclinic, eight-tenths of the individuals
being striated and the remainder giving angles of extinction for twins in the zone at
right angles to the composition plane too great for orthoclase. In only a very few
individuals was the nature of the feldspar in doubt and its orthotomic nature possi-
ble, but even here the evidence was entirely negative. It is probable that orthoclase
is present to a very limited extent — one macroscopic; crystal, a carlsbad twin in hand
specimen 51, a hornblende-mica-andesite from Sierra Canyon like 52, having a
brilliant unstriated basal cleavage, proved optically to be sanidiue. The crystal form
of the plagioclase is like that already described, but a greater number are in angular
fragments. The zonal structure is marked and the polysyuthetic twinning irregular;
in many instances the stria? are scarcely perceptible, and in a few cases they are
altogether wanting. The angles of extinction range from a few degrees to between
25° and 31°. The feldspars are therefore labradorite in part, though a large propor-
tion are probably andesine. In several thin sections they correspond to anorthite,
which will be more fully noticed under the description of the different varieties. The
inclusions of glass and of the grouudmass are the same as those in the feldspar of
the pyroxene-andesite and hornblende-mica-audesite.

The hornblende is without a dark border of any kind. It is in all other
respects like that found in the pyroxene-andesite and the fresher hornblende mica-
andesite of Sierra Canyon (52). It occurs in well developed crystals of a greenish
brown color with very strong pleochroism. There are no characteristic inclusions,
it being for the most part quite free from them. It appears to have withstood decom-
position better than the hypersthene, as it shows no signs of alteration even in juxta-
position to almost completely altered hypersthene. Fig. 5, PI. in. The hyi>ersthen«'
MON XX 24

370 GEOLOGY OF THE EUREKA DISTRICT.

is strongly pleochroic, green and ligbt reddish brown, similar in all points to that of
the pyroxene-andesite. Its decomposition, which is the same as that already described,
has advanced farther than in the pyroxene-andesites and is illustrated in Figs. 5 and
!>, PI. in. Augite is found only in two thin sections from this locality, Nos. 56 and 57.

Biotite is macroscopically the most prominent mineral in the quartzose members
of this series. It is in hexagonal plates of a dark brown color with strong absorption,
and is optically negative with a very small angle between the optic axes. It is
twinned as in the hornblende-mica-andesite. Quartz is not so abundant in the thin
sections as in the hand specimens and is always in rounded grains or angular frag
ments with a few glass inclusions. Magnetite, apatite, and zircon are common to all
the varieties of this pearlite. The apatite is like that found in the other audesites; a
fine example showing the terminations and a basal cleavage is represented in Fig. 8,
PI. in.

The zircon crystals are not more numerous in this than in many other rocks
where there are found to be three or four crystals to a rock section, but their occur-
rence here in unaltered feldspars or isotropic glass renders them more than usually
favorable for study, and so a number have been drawn to show their crystal faces,
which were recognized by careful study in all possible lights and were drawn with the
aid of a camera. Owing to the high index of refraction of zircon the marginal faces
can not be as accurately determined as those near the center of the figures, and the
terminal planes of Figs. 15 and 20 being extremely minute could not be made out for
the same reason. It should also be remarked that the drawings are not mathematical
projections, because with the high magnifying power employed, in one instance 900
diameters, only a small part of a crystal is in focus at any one time, and a certain
amount of distortion necessarily follows. The figures represent, however, the sharp-
ness of the crystallization and will indicate the forms taken by the crystals. Besides
the short, stout crystals, from 0-05 to ()•! """ long, more usually met with, there are
sometimes long, slender prisms reaching a length of 0-37 mm and terminated at one or
both ends, Figs. 15 and 16, PI. in. The form of Fig. 15 appears to be the two prisms,
ooP, ooPoo , the double pyramid or zirconoid 3P3, and the pyramids P and Pec ; that
of Fig. 16 ooP, cePx. , and 3 P3; and Fig. 17 o>P, ooPoo , 3 P3, with P or .P» . Fig.
18 represents a very simple form, combining a prism with a pyramid of the opposite
order. Fig. 19 seems to present both prisms xP, -»Poo , the double pyramid 3P3,
and the two pyramids P and P-« ; and Fig. 20 ooP, ooPao , 3P3, and two pyramids P
and Pec . The occurrence of similar microscopic zircons has been observed by the
writer in most all kinds of rocks, except the very basic, but more especially in the
mica-bearing varieties, with which mineral it is frequently in close association.

In noticing the different varieties of this audesitic pearlite the description will
be confined to the series found in the vicinity of Dry Lake and the correspondence or
points of difference in the similar forms from the other localities will be mentioned in

ANDES1T1O PEAKL1TE AND DACITE. 371

their proper connection. At the top of the table stands the quartz-bearing horn-
blende-mica-andesite (52) found in Sierra Canyon, forming the connecting link that
unites by its microscopic structure the hornblende-mica-andesites and andesitic
pcarlites. The groundinass of this rock is completely crystalline, exactly as in the
typical hornblende-mica-andesite of the district (35). In the thin section besides the
plagioclase there are two or three unstriated sections which may possibly belong to
sauidiue. The fresh hornblende is without dark border, a few individuals having a
slight aggregation of magnetite grains around them, which is also noticeable around
the biotite. There is no pyroxene present, but some well developed quartz crystals.
The nearest approach to crystalline andesite in the Dry Lake series is thin section

53, whose gray grouudmass is microspherulitic. The spherulites are composed of
radiating colorless needles, besides which are multitudes of transparent globulites
and trichites, straight and curved, some black and opaque, others red and referable
to mica, and some formed of a string of transparent grains which are also found in
short, stout, interpenetrating microlites, which appear to belong to augite. The
whole shows a marked flow structure and bears phenocrysts of labradorite, biotite
and hornblende crowded with magnetite grains and no longer fresh; besides com-
pletely altered pyroxene [hypersthene] ; zircon occurs in good crystals. There is no
macroscopic quartz, but small aggregations of colorless plates appear to be tridymite.
Thin section 61 is more highly crystalline and illustrates the first stages of the forma-
tion of the feldspathic grains in the groundmass of the hornblende-mica-audesite; they
are seen forming around the phenocrysts as centers, which are the same as those
in 53 with the addition of macroscopic quartz.

A modification common to four separate localities is represented by thin section

54, and approaches closely to the pyroxene-andesite of the district; the silver gray
grouudmass has a satin-like sheen in transmitted light, produced by fibrous feldspar
microlites in nearly parallel arrangement in a colorless glass base, having a marked
flow structure, with a felt-like appearance in the thicker parts of the section ; there
are also grains of magnetite and a little hyperstheue. The larger phenocrysts are
well developed and the inclusions are very fine. Feldspar is in excess of the other
constituents, and hornblende and hyperstheue occur in about equal proportions,
biotite being scarce. The corresponding varieties (62, 63, 71, 73) are almost identical.
In 62 the feldspar microlites are more delicate, biotite is wanting and quartz occurs
in macroscopic grains; 63 is richer in glass and poorer in large crystals and has a
little brown mica in the groundmass.

In 71 the glassy grouudmass is richer in augite microlites, and also contains some
of hornblende and biotite. It very closely resembles the pyroxeue-andesite of Rich-
mond Mountain; 73 is remarkable for the abundance of biotite in hexagonal plates in
the groundmass. This variety of the pearlite is further characterized by the presence
of feldspars with very high angles of extinction, several of which reach 40 and 4f>°,

372 GEOLOGY OF THE EUREKA DISTRICT.

indicating auorthite, which is the feldspar so abundant in the pyroxene-andesite.
The next variety is a still more glassy rock, 55; it is a colorless glass with a pearlitic
fracture, with scattered microlites, which are beautifully developed, some, in long
prisms with pyramidal termination and transverse jointing, appear to be apatite;
others, shorter and stouter, are more doubtful, but resemble those in 53, which are
probably augite. There are also curved and tapering microlites and strings of grains
apparently of the same mineral. Larger microscopic crystals scattered through the
groundmass are hypersthene, hornblende and biotite. There is but little magnetite.
Of the macroscopic crystals, feldspar is very abundant as labradorite, with possibly a
little anorthite; biotite is also abundant, and hornblende and hyperstheue are scarce.
There is one rounded grain of quartz with good rhombohedral cleavage. Thin section
75 is like 55; 75« is taken from the ^ame specimen and shows a slight modification
caused by streams of opaque particles and hair-like trichites, which lie scattered or
aggregated in the most delicate dendritic forms. A small part is black in incident
light and may be magnetite, but the greater part is bright red and is hematite. Thin
section 74 is similar.

The remaining varieties differ from the preceding in having the glass base tilled
with opaque and more or less transparent, ill denned microlites and flocculent matter,
imparting to it a black, led, yellow or white color. Thin section 56 is from a brecciated
pearlite rich in angular fragments and crystals of labradorite, hypersthene, augite,
hornblende, and magnetite, with no mica. The groundmass is glass, probably o
itself colorless, but so crowded with microlites and more or less opaque grains as to
appear in the section dark brown, yellow, and bluish. In some places it is brown and
globulitic, in others it is tilled with flocculent matter, which is brown in transmitted
light and white in incident; in other places it is colorless, with few microlites. The
transition from one kind to another is generally sudden and the flow structure is well
^narked, being especially beautiful in thin section 57, which is similar to 56, as is also
64, though of a lighter color. -Mica and quartz are both wanting in these last three
thin sections. The varieties represented by thin sections 58 and 66 are very similar
to the last, much more so than their appearance in the hand specimen would indicate.
Their secretions are the same — labradorite, hyperstheue, and hornblende, with a little
augite and no mica or quartz. They are not brecciated, however, and the groundmass
is lighter colored, the opacite being red and white in incident light and the flow
structure very striking. There is a tine example of partially altered hypersthene
shown in Fig. 9, PI. in.

Variety 59 has a more pumice-like groundmass, the glass having numerous gas
bubbles. It is much lighter colored, with more white and less yellow opacite, and is
in part cryptocrystalline. The pheuocrysts are labradorite,, eight-tenths of the feld-
spars showing stria} and the rest probably belonging to plagioclase. There is a very
little partially altered hypersthene, considerable hornblende, and much biotite. The

ANDES1TIC PKAHLITK AND DACITE. 373

quartz, so abundant in the hand specimen in large rounded grains, is scarce in the
thin section. The two varieties from Sierra Canyon (68, 69) are denser than that just
described. The glass groundmass of 68 is without gas bubbles and is crowded with
yellowish translucent particles, which reflect incident light and appear white. It is
in places spherulitic and abounds in angular fragments of plagioclase, nine-tenths of
the feldspars being striated. There is, besides, quartz, with fine glass inclusions, a
very little pyroxene, more hornblende, and much biotite. Thin section 69 is identical
with the last under the microscope. Thin section 76, from east of Hoosac Mountain,
is similar to the foregoing, but has a cryptocrystalline gronndmass and is somewhat
decomposed. Some portions of the groundmass of 70 are crystalline and bear feld-
spar microlites, but the whole is the same as 08 and 69. Thin section 60 is more
porous, but has the characteristics of the last four sections. Its feldspar is all plagio-
clase and gives angles of extinction corresponding to labradorite. This last quartz-
bearing group (59, 60, 68, 69, 70) appears to be true dacite. and as such is very
interesting.

It may be well to note at this point some of the characteristic features distin-
guishing these closely allied rocks as they are found in this district. The gronndmass
of the hornblende-mica-andesite is in general microcrystalline, without glass, having,
besides lath-shaped feldspar microlites, which are probably oligoclase, interpenetrating
grains of quartz and feldspar. It is freer from magnetite and contains no pyroxene.
The 'groundmass of the pyroxene-andesite, on the other hand, is very glassy, with a
felt-like structure produced by feldspar and augite microlites, the feldspar being
labradorite, with an abundance of magnetite. The pheuocrysts of the former rock
are labradorite, dark bordered hornblende in every case decomposed, considerable
biotite, and sometimes quartz, but no pyroxene or the remains of any. The pheno-
crysts of the pyroxene-andesites are auorthite, hypersthene, augite, dark bordered
hornblende, with very little biotite and only an occasional quartz. The andesitic
pearlites hold an intermediate position between the two, some of the varieties being
quite like the horublende-mica-andesite, while others approach closely to pyroxene-
andesite, yet all have features differing from both. The groundmass is a glass more
or less full of microlites, and in the greater number of cases is crowded with indeter-
minable globulites and pai-ticles. Besides the feldspar phenocrysts, which are for the
most part labradorite and possibly a very little orthoclase, with some anorthite, there
are hornblende .crystals without dark border, hypersthene, a little angite, biotite, and
quartz. The dacites are a modification in which the macroscopic quartz has greatly
increased, together with the biotite, while pyroxene has nearly disappeared. They
are also the most pumice-like.

CHAPTER III.

RHYOLITE.

There are three distinct varieties of rhyolite iii the Eureka District, more notice-
ably distinct iii the hand specimen than in thin section, since their essential constitu-
ents are the same throughout. The difference arises from a change in the relative
proportion of the phenocrysts and in the nature of the groundmass. That from
about Pinto Peak which covers the greatest area has a light colored groundmass,
for the most part white, also gray and purplish gray, partly vitreous and partly
crystalline in appearance, with numerous porphyritical crystals of quart/ and feld-
spar and a few scattered bits of mica. A second variety, from Rescue Canyon, has a
reddish purple, vitreous groundmass, crowded with large crystals of quart/ and bril-
liantly reflecting sauidine; and the third, from south of Carbon Ridge, has a dense,
reddish purple groundmass, often finely banded, having few phenocrysts except those
of copper-colored mica. Upon a superficial examination of these rocks in the field it
would seem natural to separate the three varieties into the classes suggested by Von
Richthofen in 1867.' That from Rescue Canyon has all the appearance "at a dis-
tance" of granite, and might be said to be "granite-like," while that from Pinto Peak
is certainly " porphyry-like," and the variety from south of Carbon Ridge, being
quite poor in macroscopic crystals and having a beautifully banded structure, answers
to the description of rhyolite proper; but under the microscope the granite-like variety
is found to have an almost wholly glass groundmass, and to correspond, therefore,
more or less closely to quartz-porphyry. The groundmass of the porphyry-like kind,
on the contrary, is found to be microcrystalline in most cases, or microgranitic, and
the third to vary from a quite glassy to an entirely crystalline rock. Hence no sys-
tematic classification has been undertaken, the varieties receiving local designations
sufficient for the purposes of the present report.2

pinto Peat Rhyolite.— Under the microscope thin sections from a great number of speci-
mens of this variety present an extremely monotonous appearance ; a fine grained,
more or less wholly crystalline groundmass rich in large crystals and fragments of

1 Von Richthofen Natural System of Volcanic Bocks, San Francisco. 1867, p. 16.

2Since this was written u study of the rhvolites of the Great Basin led to more definite conclusions regarding von
Kichthofen's classification of rhyolites, which were expressed in a paper on the volcanic rocks of the Great Kasin by Arnold
Hague and J. P. Iddings. Am. Jour. Sri., vol. xxvn 1884, p. 461.

374

RHYOLITK. :>,7;,

quartz jintl feldspar, with occasionally a little biotite. The microscopical habit of the.se
porphyritical crystals is so constant in all the thin sections of this group as to permit
of a single detailed description, the different modifications of the groundmass only
requiring special notice. The feldspar present is sauidine, with which plagioclase is
associated to a greater or less extent. The latter is in some cases entirely wanting,
but in others is almost as abundant as the sanidine. Sometimes both occur in very
small quantities in the thin sections and hardly ever outnumber the quartz. Sani
dine occurs in well developed crystals and also in angular fragments. Sections of
the former are mostly rectangular, with the corners rounded; others show more than
four sides and indicate that their crystal form is made up of OP, ooPoc, ooP, 2P*
Zonal structure is rarely observed. Many of the individuals are in Carlsbad twins.
The cleavage is frequently very perfect, though often entirely wanting, but there
are always concoidal fractures, and the resemblance to quart/, is often very striking,
requiring an optical test to distinguish between them. It is characterized by a much
lower double refraction, which in these extremely thin sections causes it to remain
generally dark or but faintly lighted between crossed nicols. Quite a number of
sections happen to be nearly at right angles to the optical bisectrix and exhibit very
small angles between the optic axes, the interference figure being almost a cross and
showing the bisectrix negative. There are several of these in thin section 112. A
fortunate section parallel to the clinopinacoid occurs in thin section 142 and is at
right angles to the optical normal, which is found to be positive, the interference figure
being hyperbolas that unite in the center of the field. The inclination of the plane of
the optic axes is about + 7° to the basal cleavage, and the angles of the sides of the
feldspar section correspond to those cut from 0 P, ocP, and 2P oc. Besides the basal
cleavage, which in this section is very perfect, is a second less regular cleavage
parallel to the trace of the orthopinacoid. The plane of the optic axes in these sani-
diues is sometimes in the plane- of symmetry, sometimes at right angles to it. The
substance of the sanidine is very pure and free from inclusions of foreign matter.
Numerous minute gas cavities, however, occur irregularly scattered, some of which have
their sides wet with fluid, but the gas has always the greater volume. A notable
exception to this freedom from inclusions occurs in thin section 141, Fig. 2, PI. v,
where two sharply outlined crystals of sauidine grown together with different orien-
tation about a fragment of plagioclase are filled with quartz in orderly arranged
forms, with constant crystallographic orientation throughout certain portions of the
feldspar crystals, which is shown by the extinction of light and the parallel position of
numerous small dihexahedral glass inclusions with gas bubbles, found only in the
quartz, whilst irregularly shaped gas cavities occur in the feldspar substance. This
is a most interesting fact from its relation to the subject of fluid and glass inclusions
in volcanic rocks, for it would appear from this instance that quartz and feldspar crys-
tallizing out at the same time and under the same conditions have inclosed, the one

37B GEOLOGY OF THE EUitEKA DISTRICT.

glass with gas, the other gas without glass, which gas appears in the larger cavities
to be associated with water, suggesting that its condition at the time of its inclosure
was that of highly expanded steam. This crystal is further interesting as a sporadic
development of micropegmatitic structure.

The plagioclase is in crystals very similar to those of sanidine, but is not nearly
so abundant, being almost entirely wanting in all of the thin sections from the rhyolite
dikes (Nos. 138, 140, 141, 142, 144, 143, 146, 153, 155, 148, 150, 134, 137). The twinned
lamellae vary considerably in length, breadth and frequency, and in most all the indi-
viduals are twinned both after the law of albite and that of pericline, besides which
the composite crystals are also twinned in a manner corresponding to the Carlsbad
twins of orthoclase, which can be seen from the outline of the section and inequality
of the sets of angles of extinction in the two halves, Fig. 7, PI. in. The investi
gation of the extinction angles was not very satisfactory, owing to the scarcity of
favorable sections; the majority of readings were low, the highest being 17°, leading
to the conclusion that the triclinic feldspar is, for the most part, oligoclase. It is also '
of very pure substance, with few gas cavities and more rarely small glass inclusions; it
is without zonal structure and has poorly marked cleavage. The feldspar is extremely
fresh in the thin sections from the region of Pinto Peak and in those from most of the
dikes, but is partially replaced by calcite and kaolin in thin section 146. In 138 it is
entirely altered to calcite and kaolin, the latter appearing in the thin section as a
colorless aggregate of fibrous, faintly polarizing particles.

The most abundant and constant of all the ingredients is quartz, the pheno-
crysts of which are well developed dihexahedrons and angular fragments, less fre-
quently rounded grains. It is irregularly cracked and of very pure substance, free
from inclusions, except an occasional "bay" of gronndmass and a few colorless glass
inclusions with single gas bubble, around which, in some cases, is seen in polarized
light the phenomena of strain or unequal tension, the effects of which are still further
shown by small cracks that pass through the center of the dihexahedral glass inclu-
sions and extend a short distance into the quartz crystal, constituting three planes
corresponding to three of the planes of symmetry parallel to the vertical axis. These
appear in longitudinal section as a straight line or an inclined fracture, and in cross
section as a six rayed star. A flue illustration is found in thin section 111, Figs. 1
and 2, PI. IV, where a cross-section and longitudinal section occur within 1""" of
one another. In the cross- section of quartz is a minute fluid inclusion with moving
bubble, a very rare occurrence, though quite numerous fluid inclusions are found in
the fine dihexahedrons of quartz in thin section 127. Quartz in irregular grains forms
a large part of the ground mass. Small phenocrysts of biotite are found sparingly in
some of the thin sections, but are wholly wanting in others. The biotite is for the
most part free from magnetite grains or other inclusions when fresh. It is altered in
some cases to a colorless, brilliantly polarizing mica, crowded with yellow, opaque

HHYOLITK. 377

grains. This group of rhyolites is very poor in accessory minerals, there being only
two, which are of exceptional occurrence. Zircon in fragments and minute crystals
is occasionally met with in association with biotite. Garnet in well developed
dodecahedrons, and also in irregular grains of a light red color in thin section, occurs
in NOB. Ill, 112, 122, and 123.

The most striking feature ot this variety of rhyolite is its groundmass, which
presents the micrograuitic structure, not frequently met with. The remarkable thin-
ness of the sections prepared from this rock offers a highly satisfactory field for study
and leaves no reasonable doubt of the entire absence of glass in the composition of
the grouudmass of most of the thin sections. Besides the granular crystalline develop-
ment there are those that are partly cryptocrystalline and others that are spherulitic
and glassy. A microcrystalliue structure is common to thin sections 111, 112, 113,
114, 115, 116, 120, 119, 123, 140, 141, 153, 127, 134, 137. The groundmass of 112 may
be taken as representing that of all the first nine sections. It is composed of micro
scopic interpenetrating grains of quartz and feldspar, through which are scattered
larger grains, averaging 0-06 """ in diameter, for the most part quartz, with gas cavities
like those in the phenocrysts of feldspar. A small portion is deterininable as ortho-
clase and striped plagioclase. The quartz is often in aggregates of half a dozen or
more grains and is accompanied by irregular fragments of light red garnet. There is
also a little biotite in microscopic crystals, more abundant in thin section 116.
Through it all are innumerable dust-like particles, dark in transmitted light, but
reflecting incident rays and giving a whitish gray color to the section. They are
probably minute gas cavities. In addition to this are patches of yellow, ill defined
grains, corresponding to Vogelsang's ferrite, which is only in small quantities and
alone indicates the flow structure, best seen in the thin section without the aid of a
lens. The groundmass in this section (112) is porous and is filled with small, irregularly
shaped cavities. In the others it is more or less dense and varies somewhat in the
size of the grains.

Still more interesting are the changes of structure in the groundmass of the
rhyolite from the dikes. Thin sections 140, 141, 127, 134, and 137 represent the most
crystalline variety, being coarser grained than that just described. They are without
any sign of flow structure and carry larger grains, which are micropegmatitic in tliiu
section 140. The grains are composed of a colorless grain or crystal of quartz with
hexagonal outline, inclosing semi-opaque particles, which are white in incident light,
and are sometimes arranged radially. The same structure appears as a narrow
border around the quartz phenocrysts. The grains in the groundmass of thin section
141 are also mottled in polarized light, but in 127, where the similarly clouded grains
attain a diameter of 0-O.V"1", one in the thinnest edge of the section shows a beauti
fully developed micropegmatitic structure, which near the center of the grain is in tri
angular figures only 0-002 """ in size, and near the edge is in long, narrow strips. The

378 GEOLOGY OF THE EUREKA DISTRICT.

groundmass of 138 and 146 consists of somewhat larger grains of quartz in a crypto-
crystalline matrix, which is identical with the substance occupying the sections of
decomposed feldspars, already described as kaolin. Here, also, it is possibly the
alteration product of feldspar in the groundmass and not a devitrified glass. The
abundance of calcite is undoubtedly due to infiltration from the surrounding limestone.
Thin sections 142, 143, and 125 present less coarsely crystallized varieties, the first
two being similarly decomposed.

A lower stage of crystallization, in which the groundmass is largely or entirely
glass, is found in thin sections 118, 155, 122, 117, 126, and 130, some of which are
partly crystalline. The glass is spherulitic. There are also narrow bands of fibers,
the fibers lying at right angles to the direction of the bands, which make the flow-
structure very pronounced, Fig. 1, PI. viu. Thin section 155 is interesting as containing
round and oval spherules of colorless glass, with a few concentric inclusions, remind-
ing one strongly of leucite crystals. They polarize faintly in radiating rays. An
entirely glassy modification, which occurs in a small chimney about ten feet wide, is
shown in thin sections 144, 145, and is a pale green glass rich in feldspar microlites,
some of which are striated. They are partly rectangular, with the four corners pro-
longed like a "skate's egg." The corners of others are fringed, but the majority
appear like bundles of colorless fibers, the larger of which are compact in the middle
and extinguish light as a single individual, Fig. 14, PI. in. One can thus trace
the connection from the single microscopic fiber to the dense, sharply crystallized
feldspar, that is large enough to be seen without the use of a lens. In the thin sec-
tion, from the buff-colored, porcelain- like portion of the same flow (145), the microlites
are more numerous and are accompanied by clouds of yellow spots with aggregate
polarization.

Thin sections 129 and 132 are from rhyolitic pearlites, poor in microlites, with some
gas cavities and globulites of an indeterminable nature. The pearlitic structure,
consisting of spherical fractures which inclose one another like the imbricated scales
of an onion, is very well marked in thin section 129. The rhyolite at the head of
Yahoo Canyon (157) is similar to the eryptocrystalline forms of this variety and is
poor in phenocrysts. That from the saddle northeast of Combs Mountain (158) cor-
responds to the more coarsely microcrystalline kind and is very poor in macroscopic
crystals, which are quartz, feldspar, and biotite, the only inclusions noticed being
glass.

A completely decomposed rhyolite, thin section 148, is worth mentioning. In the
hand specimen it is seen to be kaolin, with numerous quartz crystals. In thin section
it is colorless, the groundmass having no action whatever ou polarized light and
being filled with minute grains which are white in incident light, also larger yellow
and red grains of iron oxide, resulting apparently from the decomposition of magnet-
ite, besides other small transparent yellow globulites with high double refraction,

HHYOLITK. 379

whose nature is indeterminable. The macroscopic quartz crystals have colorless
glass inclusions, which are for the most part spherical, a few having the form of nega-
tive crystals. There are no fluid inclusions and some of the quartzes show distinct
rhombohedral cleavage.

Rescue canyon Rhyoiite.-The second variety of rhyolite found in the district, thin
sections 162, 165, has many points of resemblance in microscopical habit to that just
described. It is, however, richer in phenocrysts, which under the microscope are
found to be angular grains and fragments of quartz, which are very free from inclu-
sions except a few glass dihexahedrons, the dark color of the quartz not being trace-
able to noticeable inclusions. There is also faintly polarizing sanidine, sometimes
indistinguishable from quartz except by optical tests. In this, also, there are no
inclusions to account for the slight opalescence seen in the crystals on surfaces at
right angles to the base. Besides sanidine there is a comparatively large amount of
striated plagioclase, some with angles of extinction corresponding to labradorite. In
addition to these abundant and larger phenocrysts and a small amount of biotite, in
which this variety of rhyolite resembles that first described, there is a small per-
centage of pyroxene in fragments and crystals, partly altered; one or two fragments
of brown hornblende without dark border, and some larger magnetite grains. There
is also an irregular grain of garnet and one of allanite. The groundmass is partly
crystalline, partly glassy and axiolitic, with much ferrite in fine particles which mark
its fluidal structure and give it a red color.

Banded Rhyolite.— The third variety differs from both the others and is in some
instances of rather doubtful nature, owing to the abundance of plagioclase and
scarcity of macroscopic quartz. The four thin sections prepared, 174, 173, 169, 1(58,
have numerous points of resemblance and, though differing somewhat, maybe classed
as the same rock and described as rhyolites. Thin section 168 is of a wholly crystal
line rock, in which the phenocrysts are quartz (with a few glass inclusions and less
frequently gas cavities) and feldspar, the greater part of which is sanidine, which is with
difficulty distinguished from quartz except by optical tests. Several sections of saui-
dine, with quadratic form and right-angled cleavage, remain dark when revolved
between crossed nicols, and give interference figures like crosses that are optically
negative. They have numerous irregularly shaped gas cavities, which are especially
abundant near the margin of the crystal. Some of the cavities have a thin coating
of fluid around their walls, and a few contain more liquid than gas. In these the
bubble is movable. There is also plagioclase and a little biotite, the latter tilled with
magnetite and red oxide of iron. The groundmass is composed of quartz grains, un-
striated feldspar, and microscopic spherulites, with many curved microlites which con-
sist of strings of transparent grains with a rather high index of refraction. Besides
these there is a little mica and magnetite. Thin section 169 is similar in the char
actor of its groundmass, which, however, is less coarsely crystalline and has a more

380 GEOLOGY OF THE EUREKA DISTRICT.

marked flow structure, produced by variations in microstractnre and in the minute
particles of coloring matter. It is poor in phenocrysts.

The two remaining thin sections (173, 174) are of somewhat doubtful character.
The groundmass is but partially crystalline, with yellow and colorless glass. It is
rich in grains of iron oxide, both black and red, and has a markedly banded structure,
as shown in Fig. 2, PI. viii. It is poor in phenocrysts, the greater number being
plagioclase, with marked zonal structure. They carry more glass inclusions than are
found in the plagioclase of the other rhyolites of the district. Biotite filled with red
oxide of iron is next in abundance, besides which there is a little apatite in compara-
tively large crystals, and one crystal of augite. There are numerous groups of color-
less grains of irregular shape, which appear to be tridymite. There is, indeed, a
close resemblance in some of its microscopical characters to certain forms of andesite,
while at the same time it seems closely allied to some forms of rhyolite.

Rhyoiitic Pumice.— Before describing the pumices it will be interesting to notice
the rhyolite of Purple Hill because of its easily traced connection with the adjoining
pumice and pearlite into which it is seen to pass. Thin section 176 is from rhyo-
lite on the summit of the hill ; No. 177 is from the same at the northeast base of the
hill where it passes into pumice. No. 180 is from denser pumice, almost pearlite, and
178 is from the dark compact pearlite. The first is light gray in thin section and has
a glassy groundmass filled with faintly polarizing particles and larger feldspar micro-
lites, together with numerous amygdules of tridymite. The phenocrysts are quartz
and feldspar, of which sanidine predominates over the plagioclase. There is a little
impure biotite and a fragment of pyroxene and some magnetite. In the next thin sec-
tion (177) the phenocrysts are much scarcer and the groundmass is a colorless glass
filled with gas cavities, some of which are spherical, but the majority are elongated,
spindle shaped, and drawn out to long tubes, that are much twisted and bent. There
are numerous six-sided microscopic mica plates and a smaller number of feldspar and
hornblende microlites. Much, if not all, of the opaque grains that are scattered in
patches through the groundmass is foreign to the rock and has filled cavities during
the grinding of the thin section. A more advanced stage is seen in thin section 180;
the phenocrysts are the same in character, but are more abundant, with a noticeable
amount of pleochroic hypersthene. The glassy groundinass is rich in spherical gas
bubbles and microlites of feldspar, hornblende, and biotite, with a small percentage of
trichites, which reach a greater development in the more perfect pearlite, thin section
178. The colorless glass of this rock bears a multitude of the most beautiful micro-
lites, consisting of colorless rectangular crystals of feldspar, brown hexagonal plates
of biotite, dark green prisms of hornblende, and curved trichites which appear opaque
under a low magnifying power, but are found to consist of a transparent fiber with
serrated edges or to be a string of disconnected globulites. They are grouped about

RHYOLITIC PUMICE. 381

an opaque grain from which they radiate in all directions, frequently resembling the
down of a thistle and suggesting in some instances a bunch of ravelings. There are,
besides, other indeterminable, smaller microlites and a few gas bubbles.

Closely related to the rhyolite of Purple Hill, both in their field occurrence and
mineral composition, and in the latter respect allied to the purple rhyolite of Rescue
Canyon, are the tuffs and pumice found in the vicinity of the town of Eureka, on the
west of Richmond Mountain, and also on the south slope of the same mountain.
These are specially interesting because of numerous alteration products which have
resulted from outflows of basalt that have broken through them. Thin sections from
a scries of specimens representing different stages of alteration naturally exhibit the
same character of phenocrysts, which have not been affected by the remelting and
may therefore be considered in one general description, the modifications of the pumice
having been confined to the glass base. Thin sections 185, 188, 189, 193, 191, 192, 196,
are from the quarry and hill slope east of the town. The phenocrysts consist of augu
lar fragments, seldom of perfect crystals of quartz and feldspar with a small amount
of hypersthene, hornblende, and biotite, together with magnetite, apatite, zircon, and
garnet as occasional accessory minerals. The quartz is of very pure substance carry-
ing only glass inclusions, one of which, in thin section 192, is brown, a neighboring
inclusion being colorless. There are two instances in the same thin section of quartz
and feldspar grown together with micropegmatitic structure. Of the feldspar, sani-
dine is the predominating species, many of the unstriated sections being optically deter-
minable as such. Triclinic feldspar is always present in greater or less amount.
A zonal structure is frequent and some individuals bear inclusions of glass in the form
of the most beautifully defined negative crystals; the feldspar is everywhere per-
fectly fresh. The strongly pleochroic hypersthene is in places crowded with apatite
and glass inclusions. It is precisely similar to that in the pyroxene andesite and ande-
sitic pearlites; while the dark green hornblende without black border and the biotite
correspond exactly to the same minerals in the pearlite. The accessory minerals ha ve
also similar characters to those found in the pearlites. Small fragments of allanite
are abundant in Nos. 189, 191, 192.

Thin sections 185 and 188 are from unaltered portions of the pumice breccia;
185 is from the quarry back of the engine house and 188 from a spot 6 feet distant
from the plane of contact with the basalt a little to the north. They are essentially
the same rock, 188 being the better section. It consists of a fine grained mixture of
colorless pumice fragments full of elongated fluid inclusions with variously sized gas
bubbles, sometimes looking like welded glass threads, together with a projwrtionately
smaller amount of crystallized minerals, in a matrix of yellow glass that appears to be
made up of minute glass particles held together by glass, in which are much fewer
gas cavities, and which is partly cryptocrystalline. There are also occasional frag-
ments of glass of other kinds, some brown and others microfelsitic and in part crypto-

382 GEOLOGY OF THE EUREKA DISTRICT.

crystalline, the outline of the brecciated fragments being sharp and well defined.
Thin section 189, taken from a spot 18 inches distant from the line of contact, shows
the effects of partial remelting, the character and composition of the breccia being the
same as in the last thin section. The greatest change is noticed in the colorless
porous fragments, where the size of the fluid and gas cavities has been greatly
reduced, the whole seeming to be contracted and crumpled together; there begin to
appear also in the place of the cavities minute black grains and microlites in small
numbers. The definition of the pumice fragments is no longer marked, and they
commence to merge in the surrounding matrix.

In thin sections 193, 191, and 192, from immediate contact with the basalt, where
the fusion has been complete, the resulting body is a compact glass almost free from
gas or fluid inclusions, which have been driven out by the heat, since the mass was
under little or no pressure. The glass in some instances, as in section 192, has retained
its former brecciated character, preserving the outline of its component fragments, but
has so contracted as to present many more phenocrysts to the same area of thin section
and has become of very dark, blue-black color. This color seems to be due to innumer-
able black hair-like trichites, opaque grains, and a smaller number of transparent
microlites, both short and stout and long and curved. There are in this thin section
portions of the neighboring basalt having an exceptionally dark brown glass base. In
193 and 191 the evidence of a former brecciation has almost entirely vanished ; the glass
of the different fragments in- some places has been very uniformly mingled, especially
in 193, though occasional fragments have offered greater resistance to fusion. The
lighter color of 193 is due to the reflection of light from mist-like clouds of gas bubbles
of the minutest dimensions, which appear at first to be opaque particles, but are found
under a power of 850 diameters to be transparent globules, with a heavy dark border.
They are especially abundant around two small cavities in thin section 191 and prob-
ably cause the yellowish white lining of the larger cavities in the hand specimen,
which is peculiar to several occurrences. Thin section 196 is from another form of
alteration of the same pumice ; it is rather more crystalline and is filled with opaque
particles that are red and yellow in incident light and give the rock its color; it is
also very porous.

The same effects have been produced in the pumice by the numerous outbreaks
of basalt along the south slope of Richmond Mountain, and the thin sections from
this locality present in many instances the same characters as those just described.
They will therefore need but a brief mention and will serve rather as evidence of the
identity of the two bodies of pumice and of the uniformity of the alteration arising
from the same cause. Thin sections 199, 200, 204, 205, 206, 207, 208, and 209 are from
rocks on the small spur south of the summit of the mountain, and occurring under
different conditions they vary somewhat in character. Thin section 19!) is of a fine
grained altered pumice not in immediate contact with basalt, and resembles thin

EHYOLITIC PUMICE.

section 196. There is inucb opaque coloring matter in the base and an abundance «»f
phenocrysts consisting of much quart/ and nearly equal quantities of sanidine and
triclinic feldspar; in one Carlsbad twin one half exhibits an interference cross that is
optically negative, while the other half gives a bar parallel to the clinopinacoid, in
which case the section must be perpendicular to the negative bisectrix of the first
half, having an angle between the optic axes of about 0°, and at the same time at
nearly right angles to one of the optic axes in the other half having a large optical
angle and the plane of the axes parallel to that of symmetry. Still another Carlsbad
twin shows the plane of the optic axes normal to that of symmetry. There is rather
more hypersthene than is common to these pumices. It is partially decomposed and
displays a very striking pleochroism, owing to the thickness of the section.

Thin section 200 is the most interesting of all the alteration products, on ac-
count of its undoubted relations to the basalt and its higher degree of metamor-
phism ; it is traceable directly to the same deposit of pumice as 199, and lies in apparently
undisturbed layers directly over basalt, which did not in this instance reach the sur-
face, but thoroughly altered the overlying pumice, breaking through it lower down
the slope. In thin section it is a whitish gray, fine grained breccia of about the same
grain as 199. Under the microscope the porphyritical crystals are seen to be angular
fragments of quartz, sanidine, and plagioclase of the same size and abundance as
those in the last named section; pyroxene, however, is wanting and only a little
biotite is present, besides a single grain of garnet. The groundmass has retained its
brecciated character, though the pumice fragments have lost their original form and
appear to merge into one another; but the degree of crystallization is far more
advanced, hardly any portion of it being without influence on polarized light. As a
natural result of its brecciated character the structure is most varied, which is the
more pronounced between crossed nicols. It is partly sphernlitic and axiolitic and
partly cryptocrystalline and in places it is microcrystalline in irregular grains.

Thin sections 204 and 205 are from a small outbreak of rhyolite on the south
side of the spur about 100 yards from the locality of 199, which, though not traced in the
field to unaltered pumice, exhibits under the microscope so close a resemblance in many
respects to the last described form as to leave little, if any, doubt that this small flow
of porcelain-like rhyolite is a highly altered pumice breccia that has escaped from its
place of confinement, probably having been heated under pressure to a greater degree
than the breccia met with in situ on the surface. In thin section it is whitish gray.
204 having a glassy groundmass strongly resembling that of 199, which is filled with
faintly polarizing particles, and shows as great a diversity of structure, which indi-
cates its once brecciated condition. It is in places spherulitic, cryptocrystalline, and
microcrystalline. There is a marked flow structure and a smaller amount of frag-
mentary crystals, consisting of quartz and feldspar, with very little biotite and one
fragment of greenish brown hornblende. Thin section 20-"> has a gronmlmass of more

;;s4 (UROLOGY or THE EUKEKA DISTRICT.

uniform structure, composed of microscopic grains of varying size, which pass into
cryptocrystalline portions. The flow structure is most noticeable in the thin section
without the aid of a lens. The phenocrysts are quartz and feldspar in fragments. A
cross section of zircon, oi1""' broad, shows only one set of prism faces and a good
cleavage parallel to the other, which is seldom met with in microscopic zircon crystals
(Fig. 10, PI. III.) This thin section is similar to those from the Pinto Peak rhyolite.

Thin sections 206, 207, and 208 from contact with basalt on the slope and at
the base of the same spur show exactly the same kind of alteration as 193, 191, and
192. which have been already described. The glass, however, is brown and red, with-
out the black trichites, and there is only a trace of the bisilicates and of biotite. No.
209 is a beautiful section of a reddish brown breccia formed of fragments of brown
glass almost free from microlites cemented together by a dark red ferrite-bearing
glass, rich in microscopic shreds of biotite, which is very abundant, together with
green hornblende in small fragments. Pyroxene is scarce, there is comparatively
little quartz, and there are about equal amounts of sanidine and plagioclase, besides
which are magnetite garnet and zircon.

Of the remaining instances of altered pumice one from contact with basalt on
the end of the east spur of Hornitos Cone, 210, is of purplish brown glass, containing
portions with very different structures, being itself an intimate mixture of brown and
gray glass with numerous grains of magnetite and a great abundance of brown horn-
blende in fragments, and with more perfect crystals of strongly pleochroic hypers-
thene with a narrow dark border; besides biotite, quartz, and feldspar, of which
plagioclase is in excess. The relative amount of the bisilicates and mica is much
greater than in any of the pumices previously described, and with an excess of
triclinic feldspar approaches nearer to the composition of an andesite. The altered
breccia from the summit of the cone presents in thin section 211 a reddish gray matrix,
bearing yellow, orange, and red fragments, which are found to vary greatly in micro-
structure. There are comparatively few and small phenocrysts, principally of quartz
and feldspar, with still less biotite. The groundmass is a glass, in places microfelsitic,
also spherulitic, and passing from cryptocrystalline into microcrystalline. Among the
fragments are several that appear to belong to basalt.

Similar to the last is the coarse breccia from the east side of Black Canyon, three
sections of which exhibit the changes wrought by the adjacent basalt. In general
they are poor in phenocrysts, plagioclase being the most abundant, together with a
little biotite and pyroxene, and besides the variously modified glassy portions are pieces
of the same basalt. The glass of thin section 224 is filled with irregularly shaped fluid
inclusions with stationary bubbles, besides patches of gray polarizing particles and
numerous magnetite grains. In thin section 225 the fluid inclusions have diminished
both in size and number and the contorted flow structure of the individual glass frag
meuts has been reduced more nearly to straight lines and to a general parallelism

EHYOLITIC PUMICE.

throughout the whole mass, except in the case of the less fusible pieces. The fluid
inclusions have wholly disappeared from the glass of thin section L'L'G, which is both
colorless ;i nd bright yellow, and is full of opaque red particles, without doubt red oxide
of iron. It is rich in trichites and microlites of feldspar, some of which are colored
yellow.

Prom the foregoing it appears that the richly quartzose, rhyolitic pumice in tin-
vicinity of Richmond Mountain, containing, as it does, a large percentage of triclinic
feldspar, which is, however, subordinate in amount to the monoclinic, and at the same
time carrying a varying amount of biotite, pyroxene, and green hornblende, holds
an intermediate position mineralogically between the dacite and the rhyolite of Res-
cue Canyon.

A thin section of pumice, 241, altered to a compact glass by the rhyolite of Pinto
Peak, is interesting as containing only a little mica in addition to the quartz and
feldspar, and therefore closely resembling in composition the surrounding rhyolite.
In addition to these phenocrysts, which are few, is garnet. The glassy groundmass
is nearly colorless, and contains only a small amount of black particles and starlike
groups of trichites. Another altered pumice, 242, from the basin west of Secret
Canyon road, is like the last in composition, the light brown glass being in places
filled with rectangular microlites of feldspar.

Differing greatly from the foregoing pumices is a tuff of fine grain occurring over
a small area on the east slope of Hornitos Cone, where it appears as a bedded deposit
of dark gray volcanic sand, altered by an outflow of basalt to a blue black, basalt-
looking mass. Thin section 223 shows it to consist of a purplish brown glass crowded
with fragments of feldspar, hypersthene, and augite, with some black bordered horn-
blende and large grains of magnetite. The feldspar is wholly tricliuic, the angle of
extinction in several instances exceeding that of labradorite and corresponding to
anorthite. It also contains a multitude of colorless glass inclusions and a few large
ones of brown glass. The hypersthene has the pleochroism common to that of the
neighboring andesite, and the greenish brown hornblende fragments are all sur-
rounded by a black border. There is no doubt that this tuff belongs to pyroxene-
andesite. though it is the only occurrence of the kind met with in the district. The
brown glass is in places globulitic, with more or less feldspar microlites and black
grains and trichites in very beautiful aggregations.
MON xx 25

CHAPTER IV.

BASALT.

The basalt that has been erupted in the vicinity of Richmond Mountain and to
the east, forming Basalt Peak, Strahlenberg, and Grater Cone, and also that found in
the neighborhood of Pinto, though varying much in macroscopical habit, that is, in
color, density, and compactness, and in its occurrence in large masses or thinly fissile
plates, exhibits in thin sections under the microscope the greatest uniformity in struc-
ture and in the microscopical character of its component minerals. It has many
points of similarity to pyroxene-audesite and the grounds for its determination as
basalt will be considered when the nature of its elements has been described. In
general it consists of a very homogeneous mixture of lath-shaped feldspar microlites
and angite crystals and grains, the latter being in excess, with a smaller amount of
hypersthene, besides which are minute crystals of magnetite in a more or less abundant
glass base. The whole is of very even grain and macroscopic phenocrysts are almost
never met with. Olivine, which is considered an essential constituent in most basalts,
plays the part of a very inconstant accessory mineral in this variety. Since the micro-
scopical habit of the different minerals in the basalts from the above mentioned
localities is constant throughout the series of thin sections, a single detailed descrip-
tion of them will be sufficient.

The feldspar is triclinic, in lath-shaped crystals for the most part well developed ;
their size varies considerably within certain limits, the average length being between
0-1 and 0-0.">""", a few reaching 0'25mm, and a much greater number being microscopic-
ally minute. They have a sharp outline along the base and brachypinacoid, but are
less regularly terminated, partly squared off as if by a pinacoidal face; they are gener-
ally notched or pronged, appearing as if made up of several prisms of unequal length;
frequently the halves of a twin are separated for a short distance at either end of the
crystal by a film of globulitic glass. The smaller individuals show but two twinning
stripes, but in the stouter crystals more are present. They are of different lengths,
sometimes wedging out in the middle of a crystal. A second twinning at nearly 90°
to the first is seldom seen, except in some of the stouter individuals. In all the thin
sections where the feldspar microlites are of sufficient size, the angles of extinction
reach those of anorthite for many individuals, which also show a very high light in
extremely thin sections between crossed nicols, indicating that a portion of the feld-
spar belongs to that species. There are besides many more faintly polarizing crystals
386

BASALT. 387

with low extinction angles, which may most likely belong to a less basic feldspar.
The long narrow crystals are without zonal structure and are free from inclusions of any
kind. In several thin sections of somewhat more coarsely crystalline structure tin-
shorter, thicker crystals have both zonal structure and numerous globulitic glass inclu-
sions.

The pyroxene constituent consists of both augite and hypersthene. The angitc in
thin section is almost colorless with a slight tinge of yellowish green. The hyper
sthene is colored light green and light reddish brown with the same pleocbroism as that
already noticed in the andesites, a phenomenon more common in the larger crystals,
though not of constant occurrence in any one thin section and frequently confined to
the inner portion of a crystal. The hypersthene is of older growth than the augite,
which frequently incloses slender prisms of the former. The crystals of angitc arc
not sharply outlined, except in a few of the larger individuals, but have an uneven,
jagged outline and are in the form of irregularly terminated prisms and grains, with
an octagonal cross-section, which is well defined in many cases, with the pinacoidal
faces more highly developed than the prismatic. It has a good cleavage parallel to
the latter, with an occasional less perfect jointing parallel to the former; there are also
irregular transverse fractures across the long slender prisms. The larger crystals are
sometimes twinned one or more times in the ordinary manner parallel to the orthopin-
acoid, and are often rich in glass inclusions with a g;is bubble and sometimes a color-
less microlite; apatite needles are not met with, but grains of magnetite are abundant.
A curiously curved crystal of augite occurs in thin section 260, one half being bent
without fracture through an angle of 40°. Augite is the most abundant mineral com-
posing these basalts and is considerably in excess of the feldspar: the size of its grains
is variable, the majority ranging from 0-01 to 0-05"""; many are smaller and a large
number evenly scattered through the groundmass average 0-1 """ in diameter, while a
small number of porphyritically developed crystals measure 0-75""" in length and are
frequently associated in groups of half a dozen or more. Augite is also found in
aggregates of radiating prisms encircling macroscopic grains of quart/. 1 1 is in nearly
every instance perfectly fresh, but in thin section 202 a fibration parallel to the verti-
cal axis has taken place, accompanied by a red coloration around the margin of the
crystal ; the fibers polarize brilliantly between crossed nicols and extinguish light

parallel to their length. Bancroft Libratf

The olivine, which appears to be only locally developed in this group of basalts
and is found in only a few thin sections, is in porphyritical crystals and fragments,
the largest not more than 0-7ranl long and some as small as (M»5""n. The sections are
in symmetrical figures of four and six sides, and also in irregular shapes; the outline
is not sharply defined, but notched. The substance of the olivine is eolorless in thin
section and very pure. There are in most cases t\vo or more straight cracks parallel
to the plane of the optic axes and the usual cleavage, besides numerous fractures in

;>,SS GEOLOGY OF THE EUBEKA DISTRICT.

various directions ; frequently curved, presenting the appearance common to pearlite
structure, a beautiful example of which is to be seen in Fig. 11, PI. in. Inclu-
.sions are very rare, the only kind noticed being of colorless glass with a fixed gas
bubble. The olivine is more or less decomposed in every instance, the alteration pro-
ceeding in two different ways, which are not found in association in the same thin
section. One is the characteristic alteration into serpentine, in which a green fibrous
aggregate is formed, the fibers projecting normally from the fractures, in which is
frequently deposited iron oxide. The resulting product has the appearance of a
network with meshes of variously oriented fibers, which are at times so intimately
mixed as to produce aggregate-polarization. In thin section 292 the color is green,
with only a small amount of reddish yellow ; but in thin section 286 its color is
brownish green. The other kind of alteration may be seen in thin sections 282,
284, 269, and also in 295, 296, and takes place in a different manner. There com-
mences from the surface and fractures as in the ordinary process a fibration, not
in directions always normal to the surfaces of fracture, but in lines parallel through-
out the entire crystal, and parallel also to some direction in the plane of the more
perfect cleavage. The fibers have a light yellow color at first, which deepens
into a reddish brown or blood red as the decomposition proceeds; they polarize
light brilliantly and show a parallel extinction and sometimes a faint pleochroism.
In some cases there appear reddish yellow scales and thin plates and a general
lamination, and less frequently the lamination or fibration is altogether wanting,
when the section yields a nearly uniaxial, negative interference figure, the plane of
the optic axes in the other cases being found to be perpendicular to the direction of
the fibers. The alteration in some individuals has started from the center, leaving
the outer portion still fresh. It is represented by Figs. 11, 12, 13, PI. in. The ordi-
nary serpentine alteration product is sometimes colored the same orange or blood red,
but is easily distinguished by its internal structure, which is that of irregularly
aggregated fibers, not of uniformly parallel fibers. A distinction between the two
has not been made by Prof. Zirkel, for in his Basaltgesteine he describes a reddish
serpeutinization of olivine from the basalt of Kotzhardt in the Eifel, and afterwards
a form of decomposition of the olivine in the basalt from Steinheim near Hanau, that
corresponds exactly to the second process, just described, and says in conclusion that
it is still doubtful whether the " reddish yellow " originates immediately from the fresh
mineral or first from the "green."1 And again in his report on the microscopical
petrography for the Exploration of the Fortieth Parallel he remarks that, in the excel-
lent basalt from east of Spanish Spring Station, in the Virginia Range, "olivine
occurs, its larger crystals altered along the borders and cracks, and its smaller ones
filled with a brownish red, somewhat fibrous substance, which is, without doubt, of a
serpentiuous character."2 The thin section of this rock has been examined and the

'!•. Zirkel. Basaltgeateine. Bonn, 1870, p. 65.

'F. Zirkel. Microscopical Petrography. Washington, 1876, p. 230.

BASALT. 389

red alteration found to belong to the second kind of decomposition, one section of
dark red altered olivine yielding a negative interference cross with OIK- dark ring.

Prof. Eosenbuscli describes a similar occurrence in the melaphyre from Asweilen,
among the crystalline ingredients of which, he says, "lie large grains having the
appearance of specular iron. They have partly the form of oliviiie and show by well
preserved remnants of this mineral that they are pseudomorphs after the sa inc. In
other cases, however, such an origin is not demonstrable; the blood red substance is
then either very compact and faintly or not at all translucent (basal section) or else
it shows a perceptible, monotonous cleavage, strong pleochroism, and a position of
the axes of elasticity parallel and at right angles to the cleavage. One can scarcely
consider this body as anything else than a blood red mica, for 1 know of no such
pleochroism in specular iron." ' In the melaphyre from Reidelbacher Hof nea r Wadrill
and the olivine-diabase from Eckelhausen and trotinesweilen on the left bank of the
Rhine, the decomposition of the olivine has resulted in the same red micaceous mineral.
It is very common in the basalts of the Fortieth Parallel collection, as noticed by Prof.
Zirkel ; but after a careful search through all the thin sections of basalt from that
region, with one rather doubtful exception, it appears that the two different proce-M-
are never found to have taken place together in the same thin section. The resultant
mineral from its optical properties is evidently not a confused aggregate, but a crystal-
lographic individual, with parallel orientation of all its parts, for the extinction of light
is the same throughout and the interference figure that of a doubly refracting crystal.

In order to arrive as nearly as possible at its actual nature, fragments of a similarly
altered, porphyritic olivine in the basalt from Truckee Valley, Truckee Range,2 were
subjected to hot concentrated hydrochloric acid, and afterwards placed under the
microscope, when they were seen to have lost their intense red color, which was due
to red oxide of iron, and to remain light yellow. The tabular fragments gave for interfer-
ence figures hyperbolas, which parted only a short distance, indicating a small angle
between the optic axes and showing a negative bisectrix. One plate was marked
by lines intersecting at 00°, leaving no reasonable doubt that the substance in
this case is a nearly colorless, mica-like mineral, colored by red oxide of iron, which
latter is occasionally seen in well crystallized hexagonal films in the cracks of less
altered olivine. That this mineral is a foliated, crystallized form of serpentine seems
probable from the fact that most of these basalts are so fresh, with the decomposi-
tion of the olivine frequently confined to the weathered surface, that a very radical
change is not likely to have taken place, and that a simple hydration and oxidation of
a very ferruginous olivine would supply all the chemical elements necessary to trans-
form it into anhydrous unisilicate of magnesia and ferric oxide; besides which is flu-
fact that the optical properties of the mineral in question correspond to those given

'H. liosenlmsi'h. Mikroskopi*. -In- rii\»i<i|;ro]iliii<. Stuttgart, 1877, p. 400.
'Forth-Ill paniUcl nilli-.-timi, No. 2X129.

390 GEOLOGY OF THE EFKEKA DISTRICT.

by Miller for thermophllite, a foliated mineral naviug the composition of serpentine,
concerning which Prof. Dana remarks that it seems probable both that this is "truly
crystallized serpentine" and that "the crystallization of this species is actually mica-
ceous, like that of chlorite and talc." ' The red, completely altered, macroscopic olivine
is seen in the hand specimen to have a glistening, mica-like cleavage surface. There
remained in the portion subjected to add well developed, nearly opaque octahedrons,
most likely picotite. The crystallization of the olivine appears to have preceded that
of the other minerals by only a short period, stopping just as they began, for no
inclusions of them are found in the olivine except around its border, where augite and
more rarely feldspar are seen penetrating its surface, causing it to have an uneven and
broken outline.

A less important, though ever present, component is magnetite, which is very
abundant in well crystallized octahedrons of from 0-03 to 0-01 mm and less. It is very
uniformly scattered through the rock, and in some cases, thin sections 285, 282, 283,
is closely associated with augite, attaching itself to the surface of the crystals and
being included in them, but it is seldom seen penetrating the substance of the feldspar
crystals. It is everywhere fresh and there is no evidence under the microscope of the
presence of titaniferous iron. As accessory minerals the only one to be mentioned is
quartz, which is found in one or more macroscopic grains in nearly every hand
specimen, but which is rarely met with in the thin sections. There is a fragment in
thin section 261, which is l-4mm long, with an angular outline and conchoidal fracture.
It is cut exactly at right angles to the optic axis, yielding a slightly distorted inter-
ference cross and proving to be a single individual grain. It contains a crystal of
zircon, several hair-like trichites, and a few irregularly shaped fluid inclusions with
very broad dark borders, carrying extremely active gas bubbles, which disappear at
a very slight elevation of temperature, thus indicating the fluid to be liquid carbon
dioxide. There is besides these inclusions a dihexahedral cavity filled with glass
and a comparatively large crystalline grain which crowds the bubble of gas out of its
usual spherical shape. The character of its inclusions, the unity of the whole quartz
as a single individual, together with the surrounding shell of augite crystals, leaves
no doubt of the primary nature of this quartz, which corresponds in a measure to
that of quartz-porphyry. There are besides in some thin sections amygdules of a
green or red, radially fibrous, delessite like mineral, and in others microscopic aggre-
gates of one of the zeolites, whose action on polarized b'ght and apparent polysyn-
thethic twinning suggest the characteristics of chabazite as given by MM. Fouque
and Michel-Levy.

Glass is more or less abundant in all the basalts from the localities mentioned. It
is for the most part colorless in thin section. In a few instances it is brown, and in
the darker varieties it swarms with seemingly black globulites, which, however, are

1 J. D. Dana. A System of Mineralogj, 5th ed., p. 465.

BASALT. 391

found to be transparent with very dark borders. This glass base and the micro-
structure of the rock as a whole are the only variable factors in this otherwise monot-
onous group of rocks. The variations in these features will be separately mentioned
in the following notes. Thin sections 253, 260, 261, 256, 257, 258, and 262 are from the
flow of basalt cast of the town of Eureka, at the western base of Eichmond Mountain.
No. 253 is vesicular, with relatively great irregularity in the size of the crystals, a small
amount of glass base, and, what is generally noticeable, a marked flow structure. Nos.
260 and 261, which are closely related in the field, have a more even grain and compar-
atively little glass, with few globulites and acicular microlites. The first is vesicular,
with some cavities filled with the delessite-like mineral and possibly the trace of
olivine. The second is compact; the mottled appearance of its groundmass is not
traceable to any modification of the crystalline structure and must lie in the isotropic
base. Nos. 256 and 258, from compact and vesicular portions of the same flow some
distance from the last two, are alike in structure, being of uneven grain with numer-
ous larger augite crystals similar to No. 253. The glass is partly brown, which is
also the case in iMii section 257, which is exceptionally beautiful, having a coarser
and more even texture with a larger amount of glass (Fig. 2, PI. vii). Thin section 262
differs from all the rest in having little or no glass and, besides the lath-shaped feld-
spar microlites, possessing ill defined feldspar grains approaching a microcrystalline
structure. The basalt on the summit of Richmond Mountain, thin sections 264, 265,
267, 268, has the same modifications of its structure. Nos. 264 and 265 are rich in
colorless glass filled with the minutest globulites, which give it a brown color. The
crystalline constituents are very small and almost wholly augite and magnetite. The
crystals of 267 are larger and include more feldspar, which is also true of 268, where
the mottled appearance of the rock is due to the unequal distribution of the augite
grains. In these four thin sections augite is greatly in excess of the feldspar. A
fine thin section with rich brown globulitic glass and minute augite crystals, closely
resembling No. 264, is from a bowlder on the foothills to the north of the mountain
(270). No. 269 is a red, finely porous variety from near the summit of Richmond
Mountain. Its red color is the result of the partial decomposition of abundant olivine.
The rock is more highly crystalline than the neighboring basalt. It is rich in feldspar
and poor in slightly globulitic glass. Of the basalt exposures in the pumice at the
south base of Richmond Mountain, the most westerly dike exhibits tine columnar
structure. Its thin section, 271, shows it to be quite like No. 260, in being well
crystallized, with little glass and considerable feldspar, and a very little olivine.
That from immediate contact with pumice on the spur south of the summit, 273, is
finely vesicular, poor in crystals, and full of ferrite, resembling those portions of
basalt found included in several altered pumices. Thin section 274 is from one of
the coarser grained, more highly crystallized varieties, which is poor in glass, but
very rich in augite, and having a trace of olivine in the form of a small number of
characteristically decomposed sections.

392 GEOLOGY OF THE EUREKA DISTRICT.

The basalt covering the large area east of Richmond Mountain is represented by
three thin sections from Basalt Peak, 283, 282, and 284; one from the light colored
vesicular variety from Strahlenberg, 285; one of compact rock from Basalt Cone, 286,
and another, 288, from back of the Toll House on Newark Valley road. The first
three are characterized by relatively large crystals, and a coarsely globulitic glass,
which in 284 is nearly opaque, being of a dark, rich brown in the thinnest places.
They all three contain olivine, which is completely decomposed in the ordinary manner
in 283, but is only partially altered in 282 and 284 to the red, laminated sub-
stance, already described. The basalt of 285 is without olivine and is in a less per-
fectly crystallized state. The colorless glass is almost free from globulites; the feld-
spar inicrolites are small, and the augite is in much larger individuals with nearly all
the magnetite attached. The structure of 286 is identical with that of 282, the glass
is coarsely globulitic and there is a small amount of serpentinized olivine present.
The globulitic base of 288 is crowded with feldspar microlites, with many micro-
scopic porphyritical augite crystals, and numerous grains of perfectly fresh olivine.
Of the two thin sections from the neighborhood of Pinto, 290 is in every respect like
that from the summit of Richmond Mountain (264), and contains no olivine, while 292
is identical with the basalt of Crater Cone (286), and abounds in altered olivine.

The foregoing detail is necessary in order to emphasize the fact of the unity of
the somewhat scattered outflows of this rock, as it shows the slight and nonessential
character of the variations, the fluctuating percentage of the olivine, and that its
presence or absence is without influence on the microstructure of the rock. The
grounds, then, for separating this rock from the andesites and classifying it as a basalt
may be summed up as follows : (a) The great difference in microstructure between the
andesite of the district and this rock, which is not porphyritically developed and
which is very glassy, with extremely small crystals of nearly uniform size, having no
macroscopic phenocrysts, with rare exceptions; (&) that while the feldspar is in well
developed lath-shaped crystals, the augite is mostly in less regularly outlined crystals,
and in much greater abundance, occurring frequently in larger individuals, in addition
to which there is a small percentage of hypersthene; (c) the presence of oliviue, though
in variable quantities. It is, however, not a normal basalt, and may be considered
more properly an intermediate rock between basalt and pyroxene- audesite.

Another variety of basalt is found in the southeastern corner of the district, at
Magpie Hill and on the southern slope of the Alhambra Ridge. Thin section 295 is
from the former and shows it to be a very homogeneous mixture of feldspar, augite,
olivine, and iron oxide, with no isotropic glass ; but there are irregularly scattered
patches of a light purple, cryptocrystalline substance, which may be the remains of a
glassy matrix. The rock is thoroughly crystalline, and as such is very different from
the first described variety. The body of the rock is made up of rather broad, inter-
penetrating, lath-shaped feldspar crystals, in which are many small augite prisms and

BASALT. 393

grains of magnetite, together with an abundance of larger and better developed
olivine crystals. There are a couple of grains of porphyril ical quartz, and calcite is
pretty generally disseminated through the whole mass. The feldspar is probably
labradorite with some little anorthite. It is not well outlined, but is in prisms, elongated
in the direction of the brachydiagonal, with multiple twinning after albite, a few rare
individuals having a second twinning at about 90°. There are no characteristic
inclusions, some crystals being free from any, others containing glass and small
portions of the associated minerals, besides long, slender jointed apatite needles. The
augite is not abundant and occurs in irregular grains and in short prisms with pyra-
midal termination. It is of a light green color without pleochroism, and is intimately
associated with the magnetite. The olivine is in larger, quite characteristic crystals,
the largest lmm long. It is almost completely decomposed to the blood-red micaceous
mineral already described, and is in every respect similar to that found in the basalt
from the vicinity of Richmond Mountain. The magnetite, which is very abundant, is
in irregular grains and crystals, with some long, narrow forms, that suggest titan-
ifei'ous iron.

The two quartz grains in 295 are very interesting. They are angular fragments
about 4mm long, with distinctly corroded outlines and several conchoidal fractures;
they are surrounded by shells of radiating augite prisms and calcite. One of them
is represented in Fig. 4, PI. iv. The only inclusions are a few dihexahedral cavities
containing at low temperatures liquid carbon dioxide. The bubbles, which have
very narrow dark borders, are motionless at a moderately low temperature, but
become greatly agitated at about 60° or 70°, and at a few degrees higher tem-
perature disappear, reappearing on being cooled again. The quartz is evidently
primary; that is, antedates the final consolidation of the rock; but the calcite,
which surrounds it and penetrates numerous small cavities, and also occurs in
irregular patches through the groundrnass of the rock, is secondary. In the augite
border surrounding the quartz grains occurs a strongly pleochroic mineral .in short,
stout, well developed crystals. It is biaxial and monoclinic, and two cross-sections
show the characteristic form and cleavage of epidote, with the strong absorption
parallel to the clino-diagonal, but the pleochroism is unusual for that mineral, being
reddish purple parallel to the axis c, light yellow to colorless parallel to a, and strong
yellow parallel to b. This is the pleochroism of pieduiontite or manganese epidote,
which is probably the mineral present. It is not found in any other part of the thin
section.

The basalt from the south slope of the Alhambra Eidge, thin section 296, is simi-
lar to the last in inicrostructure, but is full of phenocrysts. The groundmass is
formed of the same interpenetrating crystals of feldspar, in this instance labradorite,
with much more augite, magnetite, and red altered olivine, and with no glass. The
larger phenocrysts are not sharply outlined, the feldspars, by including more and

394 GEOLOGY OP THE EUEEKA DISTRICT.

more of the other minerals, pass gradually into the surrounding groundmass; they
have numerous gas cavities, some containing a fluid, besides very few glass inclu-
sions. The augite is in part so clouded with dust-like particles as to be almost
opaque. Hypersthene occurs among the larger phenocrysts, having a very irregular
form, and the same pleochroism that it exhibits in the andesite of this region. Asso-
ciated with the phenocrysts is light reddish brown mica, which has a small angle
between the optic axes; it is apparently of primary origin. There are, besides, large
grains of iron oxide and considerable calcite.

DESCRIPTIONS OF PLATES. 395

PLATE in.

1. APATITE FROM HOKNBLKNDE-MICA-ANDE8ITB.

Thin section 37, magnified 240 diameters. Longitudinal section of an apatite crystal, showing
prismatic, pyramidal, and basal faces, and bearing glass with a gas bubble in a negative crystal
cavity, as well as a multitude of needle-like inclusions arranged parallel to the principal axis of
the crystal.

2. AUGITE FROM PYROXENE-ANDESITK OF CLIFF HILLS.

Thin section 102 a, magnified 55 diameters. Section showing a partial black border of magnitite,
wholly external to the augite crystal.

3. AUGITE FHOM PYROXENE-ANDESITE OF RICHMOND MOUNTAIN.

Thin section 77, magnified 55 diameters. Section showing a similar black border.

4. APATITE FROM HORNBLENDE-MICA-ANDESITB.

Thin section 37, magnified 240 diameters. Crystal of apatite with prismatic, pyramidal, and basal
faces, showing a basal cleavage and needle-like inclusions parallel to the principal axis of the
crystal.

5. HORNBLENDE AND PARTLY ALTERED HYPERSTHENE FROM ANDES1T1C PEARLITE.

Thin section 58, magnified 75 diameters. Section of a fragment of hornblende and hyperstheue in
juxtaposition, with magnetite and glass inclusions. The hornblende has remained fresh while the
hypersthene has been partly altered into fibrous actinolite.

6. FELDSPAR FROM PYROXENE-ANDESITE.

Thin section 102, magnified 25 diameters, iu polarized light with crossed nicols. Section of a triclinic
feldspar showing a marked zonal structure, the extinction angle at the center being 25° greater
than that in the outer zone. There are very narrow strips twinned after albite that do not show
zonal variation. At the center is an abundance of glassy inclusions that appear black between
crossed uicols.

7. FELDSPAR FROM PINTO RHYOUTE.

Thin section 122, magnified 31 diameters, with crossed nicols. Section of a triclinic feldspar, showing
by its outline that it is a Carlsbad twin, each half has also the multiple twinning of albite, and
each a different set of extinction angles. It is cracked transversely.

8. APATITE FROM ANDESITIC PEARLITK.

Thin section, 75 a, magnified 230 diameters. Longitudinal section of apatite, showing both ends ter-
minated by the base and pyramid and having well defined basal cleavage, with a small amount of
needle-like inclusions.

9. HYPERSTHENE FROM ANDESITIC PEARIJTE.

Thin section 58, magnified 40 diameters. Partly altered hypersthene with inclusions of magnetite,
glass, and small apatite crystals. It shows the fibrous decomposition taking place from the
extremities and fractures of the crystal and proceeding in a direction parallel to the principal
axis of the crystal.

396 GEOLOGY OF THE EUKEKA DISTRICT.

10. ZIRCON FROM RIIYOL1TK.

Thin section 205, magnified 100 diameters. Cross section of a zircon crystal, showing only one set
of prism faces and a cleavage parallel to the other.

11. OLIVINK FKOM BASALT OK BASALT PEAK.

Thin section 282, magnified 78 diameters. Section of oil vine, showing spheroidal fractures and the
irregular outline caused hy the crystal losing itself in a multitude of inclusions of augitc and
feldspar. The red decomposition is seen taking placo from the cracks, giving the effect of a
libration that is parallel throughout the whole crystal.

12, 13. OLIVINK FROM ISASALT OF BASALT PEAK.

Thin section 282, magnified 75 diameters. Sections of olivine, showing the same cieavage and decom-
position as Fig. 11, in one the alteration setting in from the outside, in the other beginning at
the center.

14. FELDSPAR KKOM KHYOLITE.

Thin section 145, magnified 550 diameters. Form of feldspar microlitcs, probably oligochisc, round
crowded together in a glassy rhyolite. They show various stages, from rectangular compart
crystals with sprouting cornel's to the finest libers.

15. ZIKCON FROM ANDKSITIC PKARLITK.

Thin section 54, magnified 175 diameters. Long, slender zircon crystal, O'S?1""1 long, terminated at
both ends, lying in groundmoss ; appears to have the form ooP, ooP», 3P3, with P and Poo .

16. ZIRCON FROM ANDKsVriC PEARLITE.

Thin section 61, magnified 455 diameters. Long, slender crystal, 0-13 •'"" long, lying isolated near tin-
edge of the rock section, terminated at one end and broken at the other, with the faces ocp,
ooPac, and 3 P 3.

17. ZIRCON IN ANDESITIC PKARLITK.

Thin section 75 b, magnified 470 diameters. Stont crystal, 0-09 """ long, lying in glassy groundmass,
with apparently ooP, ooPoo , 3 P3, and P or Pao .

18. ZIRCON FROM RHYOLITE.

Thin section 168, magnified 540 diameters. Simple crystal of zircon, 0'08nlm long, inclosed in
feldspar, formed of a single prism and the alternate pyramid.

19. ZIRCON FROM ANDKSITIC PKARLITE.

Thin section 58, magnified 900 diameters. Crystal of zircon, 0-044""" long, occurring in feldspar
and appearing to have the faces ooP, ooPoo, 3P3, P, and Poo.

20. ZIRCON FROM AXDESITIC PEARLITE.

Thin section 73, magnified 570 diameters. Crystal of zircon, 0-06mn> long, in feldspar, having ooP,
ooPoo, 3P3, and two pyramids.

U.S. GEOLOGICAL SURVEY.

GEOLOGY OF EUREKA DISTRICT PLATE III

-*--..

MICROSCOPIC PETROGRAPHY.

398 GEOLOGY OF THE EUREKA DISTRICT.

PLATE IV.

1. FRACTURES ABOUT GLASS INCLUSIONS IN QUARTZ OF RHYOLITE (CROSS SECTION).

Thin section 111, magnified 300 diameters. A cross section of a quartz crystal that bears glass with gas
buhhles in dihexahedral cavities. About each inclusion the quartz is cracked fora short distance
in three planes, corresponding to three of the planes of symmetry passing through its vertical
axis. The cracks appear as six -rayed stars that are parallel to each other throughout the section.
In this figure a number have been brought together from different parts of the same quartz sec-
tion, in order to show the different appearances when the inclusions are cut through the middle
or near one end, or when the section passes just above or below them. From the inclusion nearest
the top of the figure it will be seen that the section is slightly inclined and not exactly at right
angles to the principal axis of the quartz crystal. The illustration shows the upper end of this
last-named inclusion and the lower end of the one just below it to the right.

2. FRACTURES ABOUT GLASS INCLUSIONS IN QUARTZ OF KHYOLITE (LONGITUDINAL SECTION).

Thin section 111, magnified 300 diameters. A longitudinal section of a quartz crystal only 1 millimeter
from that in Fig. 1, showing the same kind of glass inclusions with vertical fractures. These
are drawn as they occur, without any change of position. Those lying at the surfaces of the
quartz section have had the gas bubbles cut in grinding and filled with balsam.

3. QUARTZ-CONGLOMERATE.

Thin section 501, magnified 33 diameters. Section of fine grained conglomerate with siliceous cement,
showing that the apparently granitoid quartz grains are rounded, water-worn grains, about
which the silica of the cement has crystallized with the same crystallographic orientation as the
nucleus, thus extending the individual until obstructed by the surrounding fragments.

4. QUARTZ FRAGMENT IN BASALT OP MAGPIE HILL.

Thin section 295, magnified 28 diameters. Section of crystalline basalt, showing an irregularly shaped
fragment of primary quartz, surrounded by a shell of angite crystals and patches of calcite.
The somewhat darker, broader grains in the augite shell are piedmontite. The rock is composed
of feldspar, minute augite, and magnetite crystals, with larger crystals of red altered olivine.

US GEOLOGICAL SURVEY

GEOLOGY Or EUREKA DISTRICT PLA

MICROSCOPIC PETROGRAPHY.

400 GEOLOGY OF THE EUltEKA DISTRICT.

PLATE V.

1. PLAGIOCLASE FELDSPAR IN HORN'BLENDE-MICA-ANDESITE.

Thin section 42a, magnified 45 diameters. Between crossed nicols, exhibiting xona' structure and
several rounded contours due to partial corrosion at different stages of its growth. Also a net
work of irregular cracks.

2. A MICROPEGMATITIC PHEN'OCRYST IN RHYOLITE.

Thin section 141, magnified 19 diameters. Three sanidine crystals surrounding a plagiochiso, most of
which has fallen out, leaving a hole. The sanidine is filled with irregularly shaped shreds of
quartz arranged as in pegmatite. Between crossed nicols. The quartz is dark, the feldspar
light.

3. PLAGIOCLASE FELDSPAR FROM HORNBLENDE-MICA-ANDESITK.

Thin section 35, magnified 35 diameters. Between crossed nicols, exhibiting polysyuthetic twinning,
zonal structure, and microscopic inclusions of glass.

4. PLAGIOCLASE FELDSPAR ADJACENT TO FIG. 8.

•

In same thu section, magnified 35 diameters. Between crossed nicols, exhibiting polysyuthetic
twinning and microscopic inclusions of glass.

u 5 GEOLOGICAL Si i

LOGY OF EL

MICROSCOPIC PETROGRAPHY.

402 GEOLOGY OF THE EUKEKA DiSTltiOT.

PLATE VI.

1. MICROPKGMATITIC STIUTCTTKK IX GKANITK-POKI'HYRY.

Thin section 16, magnified 100 diameters. Intergrowth of quartz and feldspar. The quartz is the
lighter colored portion in the form of triangles and rhombs.

2. PLAGIOCLASK FELDSPAR IX IIORXHLF.VDK-M1CA-ANDK8ITE.

Thin section &5, magnified 32 diameters. Between crossed uicols, exhibiting the parallel growth of
twinned individuals and zonal structure.

S GEOLOGICAL SURVEY

MICROSCOPIC PETROGRAPHY

404 GEOLOGY OF THE EUREKA DISTRICT.

PLATE VII.

1. PHENOCRY8TS OF BLACK BORDERED HORNBLENDE AND PLAGIOCLASE FELDSPAR IN HORNBLEXDK-

BEADING PYROXENE-ANDESITK.

Thin section 77, magnified 50 diameters. The glass inclusions in the feldspars and tho feldspar micro-
lites in the groundmass are shown.

2. BASALT.

Thin section 257, magnified 225 diameters. The lath-shaped plagioclase and magnetite grains are dis-
tinctly shown, but the augite is not well defined.

QGY OF EUHEK , PLATE

MICROSCOPIC RETROD

406 GEOLOGY OF TUB EUKEKA DISTRICT.

PLATE VITI.

1. KIIYOI.ITK; MicKospiiKiiri.rnc WITH A MAKKKII FLOW sTurcTrRK.

Thin section 130, magnified (ili diameters. 'I'lic angular and irregular I'orin of the phenocrysts of
c|iiart/ and t'clilg|iar sbo\v« tin- fractured cliara( tci.

•1. KIIYOI.ITK; CILASSY AND IIANDKD.
Thin section 174, magnified 14 diaiuetcr.s. Small phenocrysts of feldspar.

U S GEOLOGICAL SU

OGY OF El

MICROSCOPIC P

INDEX.

Pago.

Accessory minerals in granite 338

granite-porphyry 341, 345

hornblcmle-nih-a andesite 364

lavas 262

Acid and basic magmas 254

Adams Hill, description 117

mines 290

Aliatc Pass 181

Weber conglomerate 92

Alhambra Hills 25

basalt :!G4, 392

description 153

Allanite in granite 337, 338

granite porphyry 341

lavas 262

rhyolite 379

rhyolitic pumire 381

Alpha fault 158,160

I Vak, altitude 4

region of 159

Ridge, description 28

Amphitheater in Water Canyon 158

Analyses of Washoe rocks 282

amlesitic pcarlite 264

basalt. Basalt Peak 264

Richmond Mountain 264

Washoe 282

< 'ambrian iron on' 37

limestone, Bell shaft 37

Pott's Chamber 37

Tip Top incline 37

coal 98

dacite ^ 264

Washoe 282

granite-porphyry 228

Hamburg limestone 40

hornblendf-amlfsitc, Washoe 282

liorublendr-niira-aiiili-sito 264

Waslioc 282

bypersthcne 356

Kelly ore 313

Lone Mountain limestone 58

Lord Byron ore 313

I'oiionip linifstoiir 49

Prospect -Mountain limestone 37

pyroxene-amh-.sitc 264. 266

rhyolite 264

\V ashoe 282

Ruby Hill ore all.'

Ancient coast line 176

Andesite, alteration products • 234

Andesites at Washoe 2«2

relative age of 278

Andesitic pearlite 234,368

and dacite, modifications 871-373

Anorthite in pyroxene-andesite 240

volcanic rocks 354

Anticline in Fish Creek Mountains 210

Newark Mountain 156, 210

Pinon Range 201

Prospect Ridge 20

Weber conglomerate 162

west of Wood Cone 123

Antimony in Kuby Hill ore 312

Apatite in andesitic-pearlite and dacite 370

granite 337, 338

granite-porphyry 345

hornblende-mica-andesite 367

lavas 262

pyroxene-andesite 359

Argillaceous beds at Eureka 178

Arsenic in Kuby Hill ore 312

Artemesia tridentata 3

A t rypa Peak, altitude 4

section across 65

structure 22, 125

Augite-andesite (pyroxene-andesite) 233, 239

Augite in audesitic pearlite and dacite 370

basalt 387, 394

pyroxene-andesite 356

Bald Mountain Coal Company 97

coal seams 95, 97

Banded rhyolite 379

Basalt and Rhyolite 285

Basalt, anjiitc in 387, 394

cutting pyroxene-andesite 252

distinguishing characteristics 392

feldspar in 386, 393

hypersthene in 387, 394

magnetite in 390

mica in 394

microscopical characters 386

mineral composition 242

olivine in 387, 393

penetrating rhyolite 253

piedmont ite in 393

pyroxene in ' 387

407

408

INDEX.

Page.

Basalt, quartz in 390,393

relative age 276

Basalt Peak, basalt 386

Basaltic glass 259

Becker, G. F., on olirino in basalt 257

propylite 279

p\ roxene-audesites in California 261

Bennett Spring, section 187

1 lit';. real ion of Hoosae fault - 16

Biotite in andesitic pearlito and dacite 370

granite 337,338

granite-porphyry 340

hornblonde-mica-audesite 366

pyroxenc-andrsite 359, 363

rbyolite 376

Bismuth on 1'rospect Kidge 313

Blair. Andrew A., analysis of granite-porphyry

Bold Bluff, description 160

Bouncy. T.O.. secondary enlargement ol' quartz, 347

British Ainrricu. Paleozoic rocks of 208

Browns Canyon 13"

Bullion product "

Buusen. Robert, origin of lavas 273, 275

Calcareous beds at Eureka 178

Hhalr. base of Hamburg limestone 44

Cambrian and Silurian Hocks !{4 -<i-

species. commingling of 48, 5U

fauna 41-47

on Koundtop Mountain 112

in British America 208

rocks. Prospect Kidge : 34

st-clioiis. comparisons of 117

Carbon Kidue 172

audesitie pearlitc and dacit'e south of ... 368

and Spring Hill group 28, 165

slructurc 30

Carboniferous at l^uartz Peak 197

block, depressed body 28

coal 95

fauna at Quartz Peak 199

movement iu 203

Kocks 84-98

species, commingling of 88

thickness 13, 84

Caribou Hill 21

Kureka quart /He 54

Pogonip fauna 53

Carlsbad twins of plagioclase, optical properties of. ..360, '•'•'>'.',

Carson Lake, altitude 1

Castle Mountain 23

Silurian limestone 121

Century Peak Kidge '•'>-

I Vrcoearpns licdilblius 4, 24

Chalcedony in andesite 234

('barter Tunnel 35, 100

Chazy fauna 50

Chemical composition of granite. porphyry 226, 228

lavas 290

< 'helming fossils at Kurcka 71, 80, 89

I 'l.issilicat ion of lavas 233, 290

Claudet, Fred., analysis of ore 312

Cliff Hills 154

and Richmond Mountain compared 155

plant remains in "White Pine shale 155

pyroxeue-audesite 23(1, 361

Page.
Coal in Carboniferous ................................. 95, 18!

Coal-measure fauna overlying coal .................... 98

Combs Peak, section across ........................ .' . . . 65

struct ure ................................ 135

thickness of Devonian ................... 78

Conical Hill ........................................... 168

Copper in Ituby Hill ore .............................. 312

Corals in Century 1'eak Kidge ......................... 152

Cortex Kangc .......................................... 181

County Peak region ................................... 147

section .................................. 67

across ............................ 68.78

Crater Cone ........................................... 253

basalt of .................................. 386

Cross, Whitman, allauitean accessory mineral ....... 263

on hypersthenc-andesite ............ 241

Cross-scction in Phcenix Mine ........................ 306

Curtis. J. S., course of Ruby Hill fault ............... 303

minerals on Ruby Hill .................. 311

silver-lead deposits .................... 292. 301

D.
Dacite ................................................ 368,373

- a t Dry Lak e .................................... 237

South Hill ................................... 2.!7

mineral composition ............................ 236

physical features ............................... 236

relation to rhyolite ............................. 250

IJana, J. D., cited ..................................... 390

Uawsou, Sir J. William, on Devonian plants .......... 69

Descriptive Geology .................................. 99-1 74

Devon Peak, structure ................................ 138

Devonian and Carboniferous rocks .................... 63-98

County Peak group ..................... 26

at Jones Canyon ............................ 125

Lone Mountain .......................... 74

\VhitePine... ............... 192

Yahoo Canyon ........................... 83

corals in Yahoo Canyon .................... 139

distribution of ............................. 203. 206

fauna ....................................... 70

Atrypa Peak .......................... 76, 125

Brush Peak ........................... 76

Combs Peak .......................... 76

Mt. Argyle ........................... 193

Newark Mountain .................... 156

Quartz Peak .......................... 199

Rescue Hill ........................... 80

Woodpeckers Peak ................... 78

in British America ............ . ............ 209

Devonian, Lower Carboniferous, and Coal-measure spe-

. cies, commingling of ..................... 87

plant remains .............................. 69

rocks ...................................... 62-84

thickness ............................ 13, 63

Diamond Mountains .................................. 3, 26, 28

Peak, altitude ............................... 4

quartzite at Anchor Peak ............. 146

The Gate ................ 142,145

description .................. 85

evidence of age ............. 85

thickness ................... 13,85

structure ............................. 157

thickness of beds ..................... 158

Range, west slope ........................... 163

Differentiation of lavas .................. 267, 269, 287, 289, 290

Dillur, J.S., quartz in basalt ......................... 263

INDEX.

409

Page.

Dolomite in Hamburg limestone 40

Drown, Thomas M., analysis of pyroxene-andesite 2fl4

Dry Lake, andcsitic pcarlite and dacite of 368

Valley 141

Durocher, J., theory of lavas 273, 275

East Humboldt Range 2,176

granite and gneiss in 219

pre-Cambrian barrier 176

Emmons, S. F., Devonian in Tucubit Mountains 202

fault in Oquirrh Kange 186

fisli remains in Tucubit Mountains ... 72
pyroxene-andesite cutting hornblende-

andesite 260

Erian plant remains 70

Erosion of Secret Canyon shale 39, 109

White Pine shale, Hayes Canyon 157

Eureka a volcanic center 230

and Washoc Districts compared 230

District, bullion product 6

general description 1-7

history 6

lead product 7

timber 4

Mountains, description 3

quartzite, base of Combs Peak 24

Caribou Hill 212

County Peak region 147

description 54

forfluxing 55

Hoosac Mountain 112

Lookout Mouiitaiu 130

nature of material 180

Pahrauagat Range 196

section across 56

section at Castle Mountain 56

Spanish Mountain 24

Surprise Peak 108

thickness 13,57

WhitePine 191

yielding gold 55

Tunnel 38,103

Faulted anticline. Fish Creek Mountains 119

Newark Mountain 155

anticlines 210

block in Lamoureux Canyon 134

Faults-
Alpha 159,160

Hoosac 14, 15

Jackson 51, 100, 300, 303

Lamoureux 134

Lookout 129

M in I MI- 141

Newark 159

Pinnacle Peak 126,129

Pinto 14,17,149

ProspectPeak 15

Rescue 14,18,151.154

Ruby Hill 17,101,302,307

Spring Valley and Sierra 14

Fauna of the Cambrian 41

Devonian 70-84

Lone Mountain limestone 59-61

Lower Coal-measures 86

Page-
Fauna of the Pogonip 49-54

Upper Coal-measures 94

Feldspar in andcsitic pearlitc and dacitc 369

basalt 386.393

granite 337

granite-porphyry 339

hornblende-mica-andesite 364

pyroxene-andesitc S49, 361

rhyolite 375

microlites in rhyolite 378

Feldspathic magma 254, 255

Fish Creek Mountains 21-28

granite-porphyry iikes in 122, 344

Pogonip fauna 53

structure 118

Fissure eruptions 245, 270

Fissures, filling of 308

in Prospect Mountain Tunnel 106

Fossil Butte 195

Fossils—

Acervularia pentagona 138, 199

Acrotreta (like A. subconica) 60

gemma 44,45,50,51,191

Agnostus bidens 43, 44, 45, 50, 108, 118. 192

communis 43,44,45,50. 118,192

interstrictus 42

neon 43,44,45,50,118

prolongus 45

richmondensis 43, 118

seclnsns 44

tumidosns 45

tumifrons 45

Alveolites rockfordensis 83

Amboco3lia umbonata 193, 194, 199

A 1 1 1 1 1 h i M 1 1 51

nevadensis 53

Amplexus 199

A in pi il l.u i;i 167

powelli 87

Anadontopsis amygdaUeformis 75

Anarthrocanna 69

Aneimites 70

A uomocare parvum 42

Archreocidaris 89

Archffiocyathus 189

atlanticus 189

Arethusina americanai 45,50,51

Asaphns caribouensis 50, 51, 52, 53

curiosus 52

platycephalus 59

Athyris 85,199

plano-sulcata 194

angelica 82,84

hirsuta 91

roissyi 91. 96

subquadrata 199

subtilita 89, 94, 95, 96, 97, 98, 174

Atrypa desquamata 75, 77

reticularis 64, 74, 75, 77, 78, 79, 80, 81,

82, 83, 84, 120, 125, 132, 134, 138, 152,
153, 184, 185, 193, 196, 198, 200, 202, 205

Aalopora gerpens 132

Auricula 87

Aviculopecten 82, 89

afflnis 89,91

catactas 193.194

410

INDEX.

Fossils — Continued. Page.

A viculopetiten eurekensis 89

haguei 89

peroccidens 89, 171

pintoensis 171

Kathyrns 51

Bathyuriscus howelli 188

producta 186,188

Bathynrus congeneris 123

pogonipensis 195

slinilis 52

tuberculatus ' 123

Bellerophon 52,91,197,199

antiquatus 188

Hurra 144

majusculus 171

neleos 77,193

pelops 75,80

perplexa 77, 132

textilis 90

Beyriehia ; . . . 52

occidentalis 75,81.82,156

Bryozoa 191

Calonema occidentalis 77

Caniarophoria eooperensis 91, 159

Cardiola fllicostata 90

Cardiomorpha 82

missouricnsis 193, 194

Ceraurus 51,53,59,195

Chastetes 51. 66, 94. 95, 9ti, 120, 133. 138

Chariocephalus tnmifrous 44. 1(12

Chemung 80

Ohonetes 81,82.193,199 \

deflecta 74.76,79.132

fllistriata 74, 76

granulifera 76, 89, 90, 96. 171, 199

hemispherica 74, 76

illinoiscn.-iis 194

macrostriata 74. 76

mucronata 81

verneuiliana 89, 170

Cladodna 72

Cla<lopora 78

pnlchra 83

Coleolus Icevis 83

Coleoprion minuta 61

Conocardium 82, 193

nevadensis • 77, 132

Conocephalites 192

Conocoryphe 189

Crenipecten hallanus 89, 171

Crepicephalus nitidus 192

unisulcatus 192

Cruziana 187

Cryptonella circula 75, 193

piuonensis 80

Ctenacantbns 72

( ' Yuthophylluin 193, 194

corniculum 83

davidsoni 75

rugosum 75

Cyclonema (like C. multilera) 77

Cypbaspis 51

brevimarginatus 52

Cypricardinia 82

indenta 77

Cyrtina 198,199

davidsoui 193

Fossils— Continued. Page.

Cyrtina hamiltonensis 199

Cyrtoceras r>9

<-*-ss;itor 194

nevadeusis 84

Cyrtolites sinuatu« 61

Cystidian plates 52, 53, 60, 131, 191

Cystipbyllum americaimm 75

Dalmauitcs 59

tueeki 75, 77

Dentalium 90

Dicellocephalus angustifrons 45

bilobatus 45

flnalis 50, 51

inexpectaus 50, 51

marica 45, 11 8

nasutng 43, 44, 45

osceola 44, 45, 46

pepinensis 188

ricbmondeiisis 44

Diphypbyllum 96

sinicoense 75

Discina 51,74,96,169

lodensis 193. 194

minuta 70, 83, 84, 146. 169

newberryi 89, 90

Dyatactella iusularis 132

Ei -•culioinpbalus (like E. ilistaus) 195

devonicus 77

Edmondia 74, 199

medon 89

[lifioneusis 75, 77, 78, 79

Endoeeras multitiibulatuni 52, 195. 197

proteiforme 52, 53

Eocystit«8 longidat'tyliis 188

Etbinoiihyllnm whitneyi ]8il

Euoniphalus 84, 193, 199

eurekensis : 77

laxus 84, 193, 1%, 19!)

(Straparollns) opbinea 199

subrugosus 89, 90

Favosites 75,76

basaltica 75, 76

hemispherica 75

Fenestella 82, 89, 90, 19:1, 194, 199

Fusiliua cyliudrica 93, 94, 95, 161, 164, 166, 168, 170

robusta 94, 166

Gomphoceras 90

troniatites S2. 194

desideratus 77

kingii 194

Goniophora perangulata 75, 77

Grammysia arcuat:i 89

hannibalcnsig 89

niinur 80,84

Graptolites 54

Grapt<»lithus bilidus 54

Griffithides portlovki 90, 91, 168, 171

Halysites 1:1. 74. 136. 201, 205, 208

catenulatus 59, 61, 64, 183. l!ll

Heliootoma 52, 60

Holopea 196, 199

Hyolithex 77, 194, 197

blllingsi 187, 188

primordialis 44, 46

princeps 189

vanuxemi 50

lllccuus 59,61

INDEX.

411

Fossils— Continued. Page.

I ! hi • 1 1 us r ra s- i ra i n I a 195, 197

Illrenurus 192

enrekensis 50, 52, 123, 191

Iphidea depressa 44

Kiitor<.'ina cingulata 189

in iniitis.siina 44, 45

pan n u la 188

prospectensis 42

wliitfleldi 43, 108

Lamellibranchiates 72, 87. 88, 89

Leipteria rafinesqui 77

T.eperditia 52,168,171,199

bivia 52. 53, 128, 131, 195, 197

rotnndatus 81

Leptfflna melita 50,51,123

sericea 59

Leptodesma transversa 80

Limoptera sarmentica 77

Liiigula alba-pinensis 193, 194

lama 74

ligea 79, 80, 199

] ( mm sis 74

manticula 43, 44, 45, 46, 50, 51, 112. 118, 123

mytaloides 90, 171

white! 76

Lingulella ella ...186,188

Lingulepis inrera 44, 45. 50, 51, 123

minuta 44, 45, 50, 192

Loxonema 193,199

approximatum 77

nobile 75,77

suhattenuata 77, 132

Lunulicardiiim fragoauni 193, 194

Marlurea 51.53,61,134,195

annulata 51, 52, 53, 61, 128

rarinata 61

suhanmilata 52, 195

Macrocbeilns 168

Macrodou hamiltonae 89

temiistriata 94, 164

t riiiii'atus 89

Megambonia oeeidualis 75

Meristella nasuta 75

Metoptoma devonica 79

peroceidens 91, 168

phillipsi 195

Microdon connatus 90, 171

Tiiacrostriata , . . 75, 77

Modiolnpsis ueeideus 53, 60, 195

pogonipcusis 53, 60, 195

Modiomorpha K2, 196. 199

altiforme 75

ambigiia 89

desiderata 89

oblonga 77

obtusa 75, 77

pintoensig 171

Monticulopora 52. 53, 60

Murchisonia 60, 195, 197

Myaliua congeneria 91, 171

uossus 89

subovata 171

si i In i nad rat a 94

Mytilarea 77

chemungensis 80

dubia 75

Naticopsis 84,90

Fossils— Continued.

Naticopsis scquistriata 80

Nucleospira concinna 74, 199

Nucula 89,168

inRularis 89

niotica 80

rcscuensis 80

Nuculites triaugulus 193, 194

Nyassa parra 74. 79, 82

Obolella 43

ambigua 51

discoidea 45,50,118

pretiosa 43

Ogygia problematica 44

Olenellus 183,186,187,208,209

gUberti 42,46,186,187,189

howelli 46,186

iddingsi 42,187,189

thompsoni 46, 189

Vermont ana 46

Olenoides qnadriceps 42

spinosa 43

typical!* 188

Orthis 134,191,197

eurekensis 43

hamburgensis 51, 112, 123

impressa 74,76,83,193,199

lonensis 61

macfarleni 73,79,193

multistriata 202

pecosi 94,96,98

perveta 51,52, 53, 54, 61, 109, 131, 195

plicatella 59

pogonipensis 195, 197

resupinata 89,91,96,171,199

subquadrata 59,191

testudinaria 51, 52, 54, 61, 112, 196

triccnaria 53, 54, 109, 131, 195, 197

tulliensis 74,82.83

Orthoceras 52, 59, 75, 77, 84, 90, 168, 171, 195, 197, 199

multicameratum 52, 53, 195

randolpheusis 90, 171

Pachypbyllum woodmani 76, 83

Paleomanon roemeri 75

Palseonello 82

Paracyclas occidentalis 74, 75, 80, 84

peroccidens 193

Pentamerus comis 75, 77

galeatus 1%

lotis 193,196,199

Phacops rana 75, 77

Pholidops bellula 74

quadrangular!* 74

Physa prisca 87, 167

Pinna consimilis 89, 171

inezpectans 89

Platyceras 199

carinatiim 77

carinatus 80

conradi 77

ili • n t :il i n 1 1 1 77

in >. li isii 1 1 1 75

thetiforme 77

thetis 77

nndulatnm 77

rial yeti isnia 193

ambigna 84

maccoyi K

412

INDEX.

Fossils — Continued. Page.

Platyostoma 193

lineata 77, 199

Plethomytillis oviforme 75

Pleurotomaria 51, 53, 94, 131, 194, 198, 199

conoideu 168

lonensis 52,60,195

nodoinarginata 90

* urbinifc >nnis 94

Plnmulites 51

Polypora »4

stragula 91

Porambonites obscurus 197

Poaidomya devomca 77

Iffivis 77

Productus 193,199

cora 96,98

costatus 96,170

hallanus 74.79,80

hirsutiforme 83,193,194

lacbrymosa, var. lima 83

longispinus 90, .94, 161, 168, 170, 174

navicella 74, 76

nebrascensi 90,91,94,166,199

prattenianus 89, 90, 91, 94, 95, 166, 168, 171

punctatus 94, 170, 199

semireticnlatus. . . 85, 89, 90, 91, 93, 94, 95, 96, 98,
158, 161, 166, 168, 170, 171, 174, 180, Ifl4, 199, 203

Bhnmardianus 74,80,81,82,83,199

shumardianus, var. py xidatns 74

speciosus 83

stigmatns 80,83

subaculeatus 74, 76, 79, 80, 83, 193, 194

tenuicostatas 199

tmncata 76,79

Prcetna .-. 194

haldennani 80

marginalia 75,77

peroccidens 199

Protospongia 192

fenestrata 44

Protypus expanses 43

senectns 43

Pterinea 191

flabella 75

newarkensis 82

pintoensis 171

Pterinopecten 193

hoosacensis 89

spio 89

Ptilodictya 53,171

carbonaria M

serrata 94, 168

(Stenopera) carbonaria 95, 164

(Stenopera) serrata 95

Ptychaspis miunta 45, 46

Ptychoparia 42

affinis 45, 50, 51

annectang 51

bella 44

breviceps 45

disBimffis 43

granalosa 45, 50

haguei 43,44,45,50,112

Iffiviceps 44

laticeps 44

linnarssoni 44

Fossils — Continued. Page.

Ptychoparia minor 188

occidentals 43

oweni 43, 44, 50, 51, 108, 118

pernasnta 44

piochensia 187, 188

prospectensis 42

similis 44

simulata 45

unisulcata 44,45,50

protoformis 89

Raphistoma 52

acnta 195

nasoni 52, 53, 109. 128, 131

Eeceptaculites 51 ,115, 120, 123, 124, 127, 134, 191

ellipticus 52, 53

elongatus 52, 53, 197

mammillaris . .52, 53, 54, 60, 109, 131, 195, 197

Retzia mormoni 94,95,76,98

radialis 193, 194

Rhynchonella 59, 199

(Leiorhyucbns type) 89

capax 191

castanea 73, 74, 79, 80, 83

dupiicata 80, 81, 193, 199

emmonsi 193

eurekcnsis 89, 90, 91, 171

horsfordi 77

laura 80,84

nevadenais 80, 84

occidcns 77, 132, 193

pugnus 84

qnadricostata 193, 194

sinuata 80, 84, 199

tetbys 75,77

Sanguinolites scolns 89

combensis 77

gracilis 77

nsenia 90

retnsuB 90, 171

rigidus 84

salteri 90

sanduskyensis 77

simplex 90

striata 90

ventricosuB 80

Scenella conula 42

Schizambon typicalis 51

Schizodus cuneatuB 90, 171

deparcus 90

orbicularis 75, 77

pintoensis 171

Scoliostoma americana 77

Skenidium devonicum 74, 75

Solenomya curta 89

Spirifera 77,78,193,190,199

alba-pinensis 193

annectans 91

camerata 89, 90, 91, 94, 96, 161, 171, 174

cristate 198, 200

disjuncta 82,83,84,139,193

engelmanni 79,80,81,82,84,193

glabra 83,139

leidyi 91

lineata 169. 198, 199, 200

(M.) maia 74, 79, 80

neglecta 89,91

INDEX.

413

Fossils— Continued. Page.

Spirifera pinguis 199

pifioncnsis 74, 76, 82, 132, 138, 193

pulchra 199

raricosta 74

rockymoutaua 91, 94, 96, 98

(M.) setigera 91

striata 91, 171, 199

strigosus 193

trigonalis 91, 159

nndifera 76

vanuxemi 202

varieosa 74

Spiriferfna cristata 94, 95, 394

kentuckiensis 89, 96

Stenotheca elongata 43

Straparollus 199

newarkensis 82

Streblopteria similis 91,171

Strephocbetus 189

Streptelasma 59

Streptorhynchus chemungensis 76

chemnngensis, var. pandora. . .74, 76, 79
chemungensis, var. perveraa . .74, 76. 79

crenistria 89, 90, 96, 199

filitexta 191, 196

minor 61

Stromatopora. .66, 72, 74, 75, 76. 82, 83, 120, 138, 145, 171, 191,

196, 198, 200

Strophodonta 193, 199

arcnata 74

calvini 74,76

oanace 132,193

demlssa 76

inoquiradiata 76, 193

pattersoni 74

porplana . . . 74, 76, 82

punctulifera 74, 76

Strophomena fontinalis 195

nemea 52, 60

perplana 198

rhomboidalis 74, 79, 138

Styliola flssurella 74, 75, 79, 80, 81, 83, 84

Subulites 195

Syringopora 72, 164, 171, 199

hisingeri 83,144

perelegans 74,75,83

Syringothyris cuspidatus 91, 199

Tellinomya contracta 52, 53

hamburgensis 51

Tentaculitea attenuates 77

gracilistriatus 75, 80

scalariformis .77, 132

Terebratula 19.!, 199

bovidena 94, 96

bustata 91

Thamniscus 193) 194

Thecia ramoaa 132

Trematospira infrcquens 74

Trinucleus concentricus 59, 191

Triples!* 195

calcifera 60,51,52,112,123.191

Zaphrentis 60, 75, 76, 90, 94, C5, 171, 196

centralig 96

Zaptychius carbonaria 87

Fossils in Itichmond Mine 43, 118

Fossils, systematic list of 817-831

Page.

Fouqufi and Levy, cited 350,353

Fresh water shells in Carboniferous 87,166,181

Fusilina Peak, region of 159

.thickness of Lower Coal-measures 161

Galena 3jo

Garnet in granite- porphyry 341

lavas 262

rhyolite 377,379

Geddes and Bertrand dike 111,247

Geological cross-sections 211-21 7

A-B 212

CD-EF 21,101,213

E-F 148

GH-IK 215

position of Eureka quartzite 54

range of ore deposits 299

section, White Pine 190494

sketch of the Eureka District 8-38

Georgia fanna 43, 46

Gilbert, G. K-, Quaternary valleys 33

section across Timpahnte Range 188

Glendale Valley, hornblende-mica-andesite of 368

Gold in Eureka quartzite 55

Ruby Hill ore 312

Gooch, F. A., analyses of dacite and rhyolite 282

Granite 218,337

absence along Diamond Valley 219

age of. 116,220

allanitein 337,338

apatite in 337,338

biotitein 337,338

bottom of Richmond abaft 116

feldspar in 537

hornblende in 337, 338

in Great Basin ranges 219

magnetite in 337, 338

Mineral Hill 116,219

quartz in 337

titanite in 337, 338

zircon in 337

Granite-porphyry 221

allanite in 341

apatite in 345

biotite in 340

chemical composition 228

description 121

dike, Castle Mountain 122

dikes, Fish Creek Mountains 122

feldspar in 339

Fish Creek Mountains 344

garnetin 341

hornblende in 341, 345

microscopical characters 339

mineral composition 226

quartz in 339

quartz-porphyry along contact 226

Graptolites in Pogonip fauna 54

Gray's Canyon 131,133

Peak, region south of 133

structure 22

Great Basin Ranges, description 2, 10

structure 10

Gronndmass structure of pyroiene-andesite 241

414

INDEX.

Page,

Hall, James, Pogonlp fauna 190

Hamburg limestone, description 39

dolomite 40

thickness 40

WhitePine 192

Ridge, ageof Ill

description 110

shale, description 41

thickness 41

Hamilton fossils at Eureka 71, 89

Hart, Edward, analysis of basalt 264

rhyolite 264

Havalah Range 9

Highland Range 186,189,194

Olenellus shale 46

Hillebrand, W. F., analysis of coal 98

ores 313

Hoosac fault 1*, 15

bifurcation of 16

displacement 17

Mountain 112

andesitic pearlite and dacite of 368

hornblende-mica-andesite of 365

Hornblende-andesite and pyroxene-andesite, relative

ageof 251

Hornblende in andesitic pearlite and dacite 369

granite 337, 338

granite-porphyry 341, 345

hornblende-mica-andesite 365

pyroxene-andesite 358, 363

Hornblende-andesite along Hoosac fault 244

cutting Hoosac Mountain 1 13

description of. 233

Dry Lake Valley 141

Hornblende-mica-andesite 364

apatite in 367

biotite in 366

feldsparin 364

hornblende in: 365

magnetite in 367

microstrueture 367

quartz in 366

zirconin 367

HornitosCone 253

tufl's and pumices '. . . 147

Howell, E. E., Olenellus shale at Pioche 186

Humboldt Lake, altitude 1

Hypersthene and augite, isolation of 242

in andesitic-pearlite and dacite 370

basalt 387, 394

dacite 236

pearlite... 235

pyroxene-andesite 356, 362

wanting in normal basalts 257

Iddings, J. P., on granite-porphyry 225

lime-soda feldspars 241

metamorphosed sandstones 144

Modoc section 65

petrographical features of pearlite.. 236

quartz in basalt 263

structural peculiarities in ground-
mass 256

zircon crystals 262

report on petrography 233

Page-
Igneous rocks. Sierra Valley 131

Inclusions in feldspar 355, 362, 365, 375

Interstratified conglomerate in Upper Coal-measures. . 164

Intrusive dikes 243,247

Prospect Ridge 103

Irving, R.D., secondary enlargement of quartz 347

Isbister, A. K., Devonian in Mackenzie river valley . . 73

Jackson fault 51.100.300,303

Jones Canyon, Devonian limestone 126

Judd, J. W., propylite of Scotland 279

Juniperus occidentals 4

Kuu.ii. section 207

Kaolinization along Hamburg Ridge 310

of rhyolite 310

Kawsoh Range 232

age of lavas 232

King, Clarence, geological position of rhyolite 249

origin of igneous rocks 277

post-Jurassic upheaval 9

Quaternary history 33

relation of basalt to rhyolitt- 285

Kinuicut, Robert, Devonian in British America 73

Labrauorite in pyroxeiie-andesite 240

Lacustrine deposits 32

Lamellibranchiatas in Carboniferous 87, 169

Devonian 72

Lamoureux Canyon 134

section east of 67

fault 134

Lateral compression at Eureka 210

Lavas, age of '. 231

along faults 29

lines of displacement 243

Pinto fault 149

CarbonRidge 172

common source of 267

decomposition along fissures 235

differentiation of 267, 289. 287. 289, 290

distribution of 231

ciicircliiii; County Peak and Silverado mountain

uplifts 246

following lines of least resistance 245

in Grays Canyon 245

Great Basin, succession of 288

Sierra Valley 245

magmas of eruption 253

mode of occurrence 243,249.289

of intermediate composition 283. 288

sequence of 25:1, 276, 283, 290

transitions in 255

welling out along fissures 244

Lead in Ruby Hill ore 312

product 7

Limestone, Richmond Mountain 167

underlying Cliff Hills 155

Liquid carbonic acid in quartzite 55

Lone Mountain, beds at base of Combs Peak 24

description 60

Nevada fauna 62

limestone, description 57

fauna 59

INDEX.

415

Page.

Lone Mountain limestone, Niagara 57

rnlmmagat Range 196

thickness 58

Trenton 57

Longitudinal faults 14, 209

Lookout Mountain 22,129

Eureka quartzite 130

Lower Coal-measure fauna 86

fresli water fossils 87

limestone 85

thickness 85,161,170

Spring Hill 170

Lower Paleozoic in adjoining regions 185

Quaternary, lacustrine deposits 32

Magnetite in basalt 390

Cambrian quartzite 107

granite 337, 338

hornblende-mica-andesite 367

pyroxene-andesite 359, 363

Magpie Hill, basalt 364,392

quartz in basalt 263

Mahogany Hills 23,134

Mabon, R. W., analysis of lava 264

Marble in Prospect Mountain tunnel 106

McConne.ll. II. G., structure of Rocky Mountains 208

McCoy's Ridge 21

Kureka quartzite 54

Lone Mountain limestone 114

Pogonip 114

Meek, F. B., fauna from Mackenzie river 73

Olenellus fauna 46, 188

Melville, W. H., analysis of andesitic pearlite 264

Metamorphosed sandstones in Devonian limestone.. .143,346

Meteorological record 5

Mica in basalt 394

Micropegmatitic structure in granite-porphyry.. 341, 342, 344

rhyolite 377

Mirropoikilitic structure 360

Microscopical Petrography ot Eruptive Rocks 335-394

Microstructure of basalt 391, 393

bornblende-mica-andesite 367

pyroxene-andesite 360, 363

rhyolito 377

Mineral constituents of rbyolitic pumice 381

Mineral Hill, granite body 107

Mines-
Albion 7, 302, 304

Banner 295

Bull whacker 7, 221, 296

California 314

Champion itiul Iluekcye 6

Connoly :)14

Dugout 37,300

Duntlurbcrg 7, 296, 309

Eureka Consolidated 7, 302. 305

Fairplay 298

Fourth of July 42

Geddes and Bertrand 36. 37. 43, 108, 295. 309

Hamburg 7, 296

Hodgson 37

Hoosac 113

Industry 38

Irish Ambassador 108

Jackson 7,37,49,51,190,301,302,304,309

P»ge.

Mines — Continued.

KK 304

Kelly 313

Kentuck 298

Lord Byron 313

Maryland 298

Mountain Boy 298

Page and Corwin 296

Phoenix 7, 301, 303, 304, 306, 307

Price and Davies 296

Queen 298

Reese and Berry 297

Rescue 298

Richmond 6,7,36,108,302,304,305

Seventy-six 297

Silver Connor 295

Tiptop 6

White Rose 298

Wide Wide West 296

Williamsburg 7, 295

Mining tunnels on Prospect Ridge 103

Mixter, W. G., analyses of lava 282

Modoc fault 65, 75, 141, 142

Peak, section across 65, 66

section 16,24.141,142

Molecular changes in molten masses 289

Molybdic acid in Ruby Hill ore 312

Montezuma Range 232

Moore, Gideon E., analyses of lava 282

Mountain Quaternary 32

shale 38.104

thickness 38

Mountains. (Set Peaks.)

Ncolite. a natural group of lavas 277

Nevada limestone, Atrypa Peak 125

Century Peak Ridge 152

description 63

Newark Mountain 156

The Gate 145

thickness 64

White Pine 138

plateau, climate 2

description I

Newark Canyon 28

fault, description .' 159

Mountain and Diamond Peak, section across. 158

section across 65

structure 26, 155

Ncwberry, J. S., varying water levels in mines 315

New York Hill 165

Niagara in Lone Mountain limestone 57, 59, 183

1'iibranagat Range 196

White Pine 191

Wood Cone 136

Obsidian, composition 169

Oil -shore deposits 177

Olenellus fauna in British America 208

shale 42,45

Highland Range 46,186-

Oquirrh Range 46. 186

t hickncss 42, 46

OHvine in basalt 242. 2", 258. 387, 393

relation to silica 258

416

INDEX.

Page.

Opal in andesite 234

Oquirrh Range, Olenellus shale 46, 186

Ore, age of 300,316

bodies, age of. 293

Deposits 292-316

Enby Hill 301, 306, 311

in Carboniferous, absence of. 298

Eateka Tunnel 105

on Hoosac Mountain 113

Ores deposited as sulphides 310

earlier than basalt 293

later than rhyolite 293

of Prospect Kidge 300

theCambrian 294

Devonian 297

Silurian 296

Origin of lavas 272

Orographic blocks 10, 19

Packer Basin 131

Pahranapat Range 196-200

Paleontological classification 182

Paleozoic ocean, western limit of 175

Rocks, Discussion of 175-217

in British America, thickness 208 i

Great Basin 208 !

section, Eureka 11-18 j

thickness, Cambrian 13

Carboniferous ... 13

Devonian 13

Silurian 13

shore-line 175, 177

Pancake Ridge, coal seams 95

Partial magmas 287

Peaks and Mountains —

Alpha Peak 159

Anchor Peak 146

Argyle Mountain 193

Atrypa Peak 65,75,76,125

Basalt Peak 25 j

Bellevne Peak 23,119,120

Combs Peak 65.75,76,78,135

Devon Peak 138

Diamond Peak 155. 157

Dome Mountain 147. 174, 244

English Mountain 149

Fusilina Peak 159,161

Gray Fox Peak 110. 2:«

Grays Peak 133

Honsac Mountain 112

Island Mountain 80, 151

Leader Mountain 151

Lone Mountain 60

Lookout Mountain 22. 129

Modoc Peak 65, 75, 141, 142

Moleen Peak 181

Newark Mountain . .65, 1 55

Pinnacle Peak 126,129

Pinto Peak 238

Purple Mountain 238

Quartz Peak 199, 204

Ravens Nest 200.203

Round top Mountain Ill

Sentinel Peak 69

Peaks and Mountains— Continued. Page.

Sign al Peak 141, 144

Silver Pe.ik 189

Spanish Mountain 140

Strablc. il* rg 25, 162, 386

Sugar Loaf 69,79,83,151

Surprise Peak 108, 131

Table Mountain 138

Temple Peak 137

White Cloud Peak 119

White Mountain 126

Woodpeckers Peak 65, 73, 78

Penfleld, S. L., analysis of basalt 282

Phillips, J. A., secondary enlargement of quartz 347

Phosphoric acid in Cambrian limestone 38

Physical classification of formations 184

Piedmont ite in basalt 393

Pinnacle Peak fault 126,129

PinonKange 200,323

section across 201

The Gate : 83,144

Pinto Basin, basalt 386

fault displacement 18

Peak rhyolite 238,374

Piute Range 9

Plant remains in Carboniferous 87,167

Great Basin 180

Hayes Canyon 82

White Pine shale 69

Pliocene age of lavas 289

ores 316

Pogonip fauna 49

CaribouHill 53

Fish Creek Mountains 53

Graptolites in 54

Lone Mountain 60

Surprise Peak 53

White Mountain 52,127

limestone, description 48

Pahranagat Range 197

west of Wood Cone, thickness 123

White Pine 48, 191

White Pine, thickness 49

Wood Cone, thickness 49

Potsdam fauna 43,46,49,183

Pre-Cambrian barrier in East Humboldt Range 176

Pre-Tertiary erosion 232

Igneous Rocks 218-229

Price, Thomas, analyses of limestones 37

Propylite, ageof 276

in Great Basin, absence of. 279

Prospect Mountain limestone, breccias in 36

description 36

dolomite in 36

horizons of fossils 41

marble in 36

Olenellus shale 42

Sierra Valley 129

stratification in 36

thickness 38

quartzite, absence of fossils 35. 41

anticline 21

description 35

thickness 35

tunnel 105

fissures in 106

Peak, altitude 4

INDEX.

417

Page.

Prospect Peak, fault 15

Ridge, Cambrian rocks 21

description 19

section 102

structure 20, 99, 107

Pumico and Tuff 238

Pumices at Hornitos Cone 239

Purple Hill, rhyolite 380

Pyrites 310

Pyroxene in basalt 387

pyroxene-andesite 355, 362

Pyroxcno-andcsite and basalt, relative age of 252

apatite in 359

augite in 356

Augusta Range 260

blotite in 359, 363

Cliff Hills 154, 239, 361

Cortez Range 260

feldspar in 349, 361

hornblende in 358, 363

hypersthene in 356, 362

in Great Basin, age of 260

magnetite in 359, 363

mineral composition 239

Mt. Davidson 281

oli vine in 364

pyroxene in 355, 362

quartz in 360, 363

Richmond Mountain 233, 239, 348

Shoshone Range 261

tridymite in 360

Truckee Canyon 260

tuff 385

Wahweah Range 260

Pyroxenic magma 254, 255

Quartz and sanidine in rhyolite, intergrowth of 375

conglomerate, with secondary enlargement of

quartz grains 346

grains, recrystallization of 55

in andesitic-pearlite and (Incite 370

basalt 390,393

basic lavas 263

granite 337

granite-porphyry 339

hornblende-mica-andesite 366

pyroxene-andesite 360, 363

rhy iilite 376

Peak, Pahranagat Range 197

Quartz porphyry 220,345

Adams Hill 221

Bullwhacker Mine 221

Ruby Hill 117

Quaternary valleys 31

Ravens Nest, Pinon Range 200

Ravines in Nevada limestone : 148

Red Ridge, Silverado Hills 150

Rescue Canyon rhyolite 237, 379

fault 14, 18, 151, 154

displacement 19

1 1 ill 73, 79, 80, 152

Rhyolitr. absence of ferro-magnosian minerals 266

age of 276

MON XX 27

Page.

Rhyolite, allanite in 262,379

along Hoosac fault 244

Jackson fault 300

Ruby Hill fault 244,:(04

and basalt, differentiation of 286, 291

ore, relative age of 309

hiotite in 376

Brown's Canyon 137

clays 304

cutting Hoosac Mountain 113

dike in Dunderberg Mine 309

dikes on Prospect Ridge :M7

encircling Alhambra Hills 153

feldspar in 375

feldspar microlites in 378

garnet in 282, 377, 379

Mahogany Hills 137

microscopical character 374

mineral composition 237

penetrating homblende-andesite 250

physical features 237

plagioclase in 376. 37fl

Purple Mountain 238.300

quartz in -176

sanidine in 375,379

zircon in 377

Rhyolitic pearlite 378

pumice 380

allanite in 381

in contact with basalt 381-385

Richmond Mine, Cambrian fossils 43.118

Mountain, altitude 4

and Cliff Hills compared 155

andesite 240,348

basalt 252,386

description 25. 240

fauna 90

junction of fault lines 271

limestone 167

pyroxene-andesite 212

rhyolitic pumice 381

source of lava 271

Richthofen. F. von, classification of rhyolite« 374

origin of igneous rocks 275

Roberts Peak Mountains 3.203

Rosenbusch, H., cited 342.389

Roth, Justus, origin of igneons rocks 274. 275

Roundtop Mountain Ill

Ruby Hill fault 17,101,302,307

ore deposits 301

quartz-porphyry 117

region 115

underground drainage channels 315

Salt Lake, altitude

Sanidine in andesitic pearlite 234

rhyolit* 375. 379

Schell Creek Mountains 47

Secondary dikes, granite-porphyry 222

fissureon Ruby Hill 305

Secret Canyon, description 109

shale W

description 39

thickness 39

Sentinel Peak 68.79

418

INDEX.

Page.

Serpentinization of oliviue 388

Shah1 beds in Prospect Mountain limestone 38

Shallow water deposits, White Pine shale 69

Sierra Canyon, andesitie pearlite and dacite of 368

Pogomp fauna 130

fault 20

Valley, a center of eruption 131

Prosjicct Mountain limestone 129

Signal Peak, stmcture 144

Silica in basalt i'.!l

Siliceous lii'da at Eureka 178

Silurian and County Peak group 26

at Highland Range 194

Quartz Peak 199

in Kritisb America 208

Pahranagat Range 196

Valley 195

rocks 34.47

Silverin Ruby Hill on- 312

Peak. Olenelltts .shale 46

Silverado and County Peak 25, 148

Silverado Hills 149

Soil 5

Sorby. If. ('., secondary enlargement of quartz 347

Spanish Mountain 24

structure 1 40

Spherulites in andesitic pearlite and dacite 371

Spring Hill, grouping of fossils 88

region of 171

structure 168

synclinal fold 29

Valley 134

and Sierra fault 14

St. John fauna 46

SI ampede G ap 187

St rahlenberg 25

basalt 386

Stratified limestone, Richmond Mine 37

Structural modifications of granite-porphyry 342. 343, 344

features 209-217

variations in granite-porphyry 225

Structure of Timpahute Range 188

Weber conglomerate 162

Sugar Loaf 69.83.151

Sulphides filling fissures 294

on Ruby Hill 310

Surprise- Peak 108

Pogonip fauna 53

Synclinal fold, Spring Hill 29

Syncline in Weber conglomerate 162

on Combs Peak 136

west of Wood Cone 122

Table Mountain, structure 138

Tellurium on Prospect Ridge 313

Temple Peak, structure 137

Tertiary age of eruptions 232

and post-Tertiary Volcanic Rocks 280-291

lavas 30

The Gate, Piflon Range 83, 144

Thickness of beds at Diamond Peak 158

Carboniferous 13, 84

Devonian 13, 63

Combs Peak 78

Diamond Peak quartzite 13,85

Eureka quartzite 13,57

I'age.

Thickness of Hamburg limestone 13, 40

shale 13,41

Lone Mountain limestone 13, 58

Lower Coal- measure limestone. . . .13.85, 161, 170

measures, Carbon Ridge 173

Spring Hill 170

Mountain shale 13, 38

Nevada limestone 13,64

Olenellus shale 42. 46

Paleozoic rocks in British America 208

Pogonip limestone 13, 49

west of Wood Cone 123

WbitePine 49

Wood Cone 49

Prospect Mountain limestone 13, 38

quartzite 13, 35

Secret Canyon shale 13,39

Upper Coal-measures 13. 93. 164

MoleenPeak 93

Weber conglomerate 13, 92

Carbon Ridge 173

White Pine shale 13, 154

Timber growth , Eureka Mountains 4

Tinipahute Range 188

Olenellos shale 46

Tin on Prospect Ridge 313

Titanite in granite 337,338

Tiirnebohm, A. E., secondary enlargement of quartz.. 347

Trachyte, age of 276

Trachyte in Great liasin, absence of 280

Trenton at White Pine 191

fauna 24,50,57,59,183

horizon 136

in Lone Mountain limestone 57

Pahranagat Range 196

Tridymite in pyroxene-andesite 263, 36U

Tucubit Mountains 202, 203

Turner, H. W., andesite in California 261

Unconformity between Carboniferous and Triassic. ... 10

in Silurian 202

Upper Coal-measures 164

Upper and Lower Devonian, commingling of 148

comparisons of 73

Upper Coal-measure fauna 94

limestone, area of 164

Upper Coal-measures, Moleen Peak, thickness 93

thickness 164

Upper Helderberg fossils at Eureka 71, 132

Upper Quaternary 32

sub-aerial deposits 32

Upper Silurian, distribution of 203, 206

Uranium, Prospect Ridge 313

Van Hise, C. R., secondary enlargement of quartz 347

Variations from normal basalt 286

rhyolite 2X6

in lavas 274

Volcanic action, history of 268-272

activity, duration of 232

rocks, age of eruptions 231

microscopical characters 348

natural succession of 278

relative age of 249

INDEX.

419

Page.

Walcott, C. D., Carboniferous fauna at Diamond Peak.. 159

fauna of Spring Hill

White Pine shale 70, 71, 82

fresh-water fauna in Carboniferous 87,167

Graptolites in Pogonip 54

Olencllus fauna 189

placoganoid fishes, Kanab Canyon . .

section across Lone Mountain 61

In Kanab Valley 207

Silurian in Highland Range 195

at White Pine 190.191

structure of Highland Range 187

Systematic list of Fossils 317

Waltershausen, Sartorius von, origin of igneous rocks . 275

Wahweah Range

Wasatch Range, section in

sequence of strata 175

section 206

Washoe and Eureka Districts compared

Waverly fauna at Eureka 89

Weber conglomerate. Agate Pass 92

Alpha Ridge

Carbon Ridge 92,173

deposits in shallow water 181

description 91

structure

thickness 92

Peak and Pinto Springs region 161

conglomerate 30

West Humboldt Range 9

White, C. A., Cambrian fauna, Schell Creek Mountains 47

commingling of Carboniferous species . - 88

White Cloud Peak 119,120

altitude 4

White Mountain, altitude 126

description 126

Pogonip fauna 52

White Pine, geological section 190

Mountains 3

shale 153

CliffHUls 154

description 68

Hayes Cany on 157

Page.

White Pine shale, Newark Mountain, section across. . . 82

section across 81

thickness 69. 154

WhitePine 194

thickness of Pogonip limestone at 49

Whitneld, J. E., analysis of basalt 264

iron ore 107

Whitneld, R. P., commingling of Carboniferous species 88

Pogonip fauna 190

Whitney, J. D., fossils from Silver Peak 189

Wide West ravine eroded in Hamburg shale 118

Wild Cat Mountains 202

Wood Cone, ridge west of 122

thickness of Pogonip 49

Woodpeckers Peak 73

altitude 4

section across 65

Woodward, R. W., analyses of lava 282

Wulfenite 311

Taboo Canyon 139

Devonian 83

rhyolite 378

Yellowstone National Park, hornblende-andesite ear-
liest eruptive rock 285

thermal activity 309

Young, A. A., secondary enlargement of quartz 347

Z.
Zinc In Ruby Hill ore ... 312

Zircon in andesitic pearlite and dacite 370

granite 337

hornbleiide-micu-audesite 262. 367

lavas 262,284

rhyolite 377

Zirkel, F., alteration of olivine 388, 389

felt-like structure in andesites 360

igneous rocks of Fortieth Parallel Explora-
tion 354,357

lavas in the Wahweah Range 260

olivine in basalt 390

structure of pyroxene-andesite 241

Zonal structure of feldspars 354, 362, 364

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