IDDIHGS, J.P.

...On a group of volcanic 

rocks from the Tewan Mountains, 

New Mexico... 1890.

i

Lfc.

«** "

.70(272} 

Idl

.«!, . ;.: , *

DEPARTMENT OP THE INTEBIOB

BULLETIN

OF THE,'

UNITED STATES

GEOLOGICAL SURVEY

JSTo: 66

ON A GROUP OF yOLGAXlC ROCKS' FEdJ

MOUNTAINS, NEW MEXICO,' AND ON 'TH 

/. 'OCCUREENpE OF PRIMARY QUARTZ 

' T' IN CERTAIN BASALTS.

-WASHINGTON '

GOVERNaiE.NT PRINT!N,G OFFICE 

1890 . V

LIBRARY CATALOGUE SLIPS.

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

Department of the interior | | Bulletin | of the | United 

States ! geological survey | no. 66 | [Seal of the department] | 

Washington | government printing office | 1890

Second title: United States geological survey | J. W. Powell, 

director | | 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 | [Vignette] |

Washington | government printing office | 1890

8°. 3lpp.

Iddings (Joseph Paxson)

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

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 | [Vignette] |

Washington | government printing office | 1890

8°. 34 pp.

[UNITED STATES. Department of the interior. (V. S. geological survey). 

Bulletin 66].

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

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 | [Vignette] |

Washington | government printing office | 1890

6°. 34 pp.

[UNITED STATES. Department of the interior. (77. S. geological survey). 

Bulletin 66.]

[Bulletin No. 66. J

The publications of the United States'Geological Survey are issued in accordance with the statute 

approved March 3, 1879, which declares that 

'' Thepublications of the Geo'logicat Survey shall consist of the annual report of operations, geological 

and economic maps illustrating the resources and classification of the lands, and reports upon general 

and economic geology and paleontology. The annual report of operations of the Geological Survey 

shall accompany the annual report of the Secretary of the Interior. All special memoirs and reports 

of said Survey shall be issued in uniform quarto series if deemed necessary by the Director, but otherwise in ordinary octavos. TWee thousand copies of each shall be published for scientific exchanges 

and for sale at the price of publication; and all literary and cartographic materials received in exchange 

snail 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."

On July 7, 1882, the following joint resolution, referring to all Government publications, was passed 

by Congress:

'' 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 Survey 

by special resolution of Congress or has been ordered by the Secretary of the Interior, this office has 

no copies for gratuitous distribution.

ANNUAL REPOETS.

I. First Annual Report of the United States Geological Survey, by Clarence King. 1880. 8°. 79pp. 

1 map. A preliminary report describing plan of organization and publications.

II. Second Annual Report of the United States Geological Survey, 1880-'81, by J. W. Powell. 1882. 

8°. 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. 1885. 

8°. xxxvi, 469 pp. 58 pi. and maps.

VI. Sixth Annual Report of the United States Geological Survey, 1884-'85, by J. W. Powell. 1886. 

8°. xxix, 570 pp. 65 pi. and maps.

VII. Seventh Annual Report of the United States Geological Survey, 1885-'86, by J. W. Powell. 1888. 

8°. xx, 656 pp. 71 pi. and maps,

VIII. Eighth Annual Report of the United States Geological Survey, 1886-'87, by J. "W. Powell. 1889. 

8°. 2 v. xx, 1063 pp. 76 pi. and maps.

I.. Ninth Annual Report of the United States Geological Survey, 1887-'88, by J. "W. Powell. 1890. 

8°. xiii, 717 pp. 88 pi. and maps. 

The Tenth Annual Report ia in press.

MONOGRAPHS.

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

II. Tertiary History of the Grand Caiion District, with atlas, by Clarence E. Dutton, Capt. U. S. A. 

1882. 4°. xiv, 264 pp.' 42 pi. and atlas of 24 sheeto folio. Price $10.12.

III. Geology of the Comstock Lode and the "Washoe District, with atlas, by George F. Becker. 1882. 

4°. xv, 422 pp. 7 pi. and atlas of 21 sheets folio. Price $11.00.

IV. Comstock Mining and Miners, by Eliot Lord. 1883. 4°. xiv, 451 pp. 3 pi. Price $1.50.

V. The Copper-Bearing Rocks of Lake Superior, by Roland Duer Irving. 1883. 4°. xvi, 464 pp. 

15 1. 29 pi. and maps. Price $1.85.

I
II ADVERTISEMENT.

VI. Contributions to the Knowledge of the Older Mesozoic Flora of Virginia, by 'William Morris 

Foutaine. 1883. 4°. xi, 144 pp. 54 1. 54 pi. Price $1.05.

VII. Silver-Lead Deposits of Eureka, Nevada, by Joseph Story Curtis. 1884. 4°. ' xiii, 200 pp. 16 

pi. Price $1.20.

VHI. Paleontology of the Eureka District, by Charles Doolittle Walcott. 1884. 4°. xiii, 298 pp. 

241. 24 pi. Price $1.10.

IX. Brachiopoda and Lamellibranchiata of the Raritan Clays and Greensand Marls of New Jersey, 

by Robert P. Whitfield. 1885. 4°. xx, 338 pp. 35 pi. 1 map. Price $1.15.

X. Diuocerata. A Monograph of an Extinct Order of Gigantic Mammals, by Othniel Charles Marsh. 

1886. 4°. xviii, 243pp. 561. 56 pi. Price $2.70.

XI. Geological History of Lake Lahoiitan, a Quaternary Lake of Northwestern Nevada, by Israel 

Cook Russell. 1885. 4°.' xiv, 288 pp. 46 pi. and maps. Price $1.75.

XII. Geology and Mining Industry of Leadville, Colorado, with atlas, by Samuel Franklin Emmons. 

1886. 4°. xxix, 770 pp. 45 pi. and atlas of 35 sheets folio. Price $8.40.

XIII. Geology of the Quicksilver Deposits of the Pacific Slope, with atlas, by George F. Becker. 

1888. 4°. xix, 486 pp. 7 pi. and atlas of 14 sheets folio. Price $2.00.

XIV. Fossil Fishes and Fossil Plants of the Triassic Rocks of New Jersey and the Connecticut Valley, by John S. Newberry. 1888. 4°. xiv, 152 pp. 26 pi. Price $1.00.

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. 53 pi. 

Price $1.00. 

In preparation :

 Gasteropoda of the New Jersey Cretaceous and Eocene Marls, by R. P. Whitfield.

 The Penokee Iron-Bearing Series of Northern "Wisconsin and Michigan, by Roland D. Irving and 

C. It. Van Hise.

 Mollusca and Cretacea, of the Miocene Formations, by R. P. "Whitfield.

 .Description of New Fossil Plants from the Dakota Group, by Leo Lesquereux.

 Geology of the Eureka Mining District, Nevada, with atlas, by Arnold Hague.

 Sauropoda, by O. C. Marsh.

 Stegosauria, by O. C. Marsh.

 Brontotheridaa, by 0. C. Marsh.

 Report on the Denver Coal Basin, by S. F. Emmons.

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

 Flora of the Dakota Group, by J. S. Newberry.

 The Glacial Lake Agaasiz, by Warren Upham.

 Geology of the Potomac Formation in Virginia, by "W. M. Fontaine.

BULLETINS.

1. On Hypersthene-Andesite and on Triclinic Pyroxene in Augitic Rocks, by "Whitman Cross, with a 

Geological Sketch of Buffalo Peaks, Colorado, by S. F. Emmons. 1883. 8°. 42pp. 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°. 8pp. 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 cents.

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. 

Price 20 cents.

6. Elevations in the Dominion of Canada, by J. W. Spencer. 1884. 8°. 43pp. Price 5 cents.

7. Mapoteca Geologica Americana. A Catalogue of Geological Maps of America (North and South), 

1752-1881, in geographic and chronologic order, by Jules Marcou and John Belknap Marcou. 1884. 

8°. 184 pp. Price 10 cents.

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. AReport 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 "Walcott. 

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. 

Gilbert. 1884. 8°. 66 pp. 6 pi. Price 5 cents.

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. 1885. 8°. 135pp. Price 10 cents.

14. The Electrical and Magnetic Properties of the Iron-Carburets, by Carl Barus and Vincent 

Strouhal. 1885. 8°. 238 pp. Price 15 cents.
ADVERTISEMENT. Ill

15. On the Mesozoic and Uenozoic 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. 1885. 8°. 

86 pp. 3 pi. Price 5 cents.

17. On the Development of Crystallization in the Igneous Bocks of "Washoe, Nevada, with Notes on 

the Geology of the District, by Arnold Hague and Joseph P. Iddings. 1885. 8°. 44pp. 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°. 28 pp. Price 5 cents.

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

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

22. On New Cretaceous Fossils from California, by Charles A. White. 1885. 8°. 25pp. 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°. 124pp. 17 pi. 

Price 15 cents.

24. List of Marine Mollusca, comprising the Quaternary Fossils and recent forms from American 

Localities between Cape Hatteras and Cape Roque, including the Bermudas, by William Healey Ball. 

1885. 8°. 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. The Gabbros and Associated Hornblende Rocks occurring in the Neighborhood of Baltimore, Md., 

by George Huntington Williams. 1886. 8°. 78 pp. 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 Dooittle Walcott. 1886. 8°. 369 pp. 33 pi. Price 25 cents.

31. Systematic Review of our Present Knowledge of Fossil Insects, including Myriapods and Arach 

nids, 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°. 235pp. Price 20 cents.

33. Notes on the Geology of Northern California, by J. S. Diller. 1886. 8°. 23 pp. Price 5 cents.

34. On the relation of tho Laramie Molluscan Fauna to that of the succeeding Fresh-water Eocene 

and other groups, by Charles A. White. 1886. 8°. 54pp. 5 pi. Price 10 centa.

35. Physical Properties of tho Iron-Carburets, by Carl Barus and Vincent Strouhal. 1886. 8°. 62 

pp. Price 10 cents.

36. Subsidence of Fine Solid Particles in Liquids, by Carl Barus. 1886. 8°. 58 pp. Price 10 cents.

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

38. Peridotite of Elliott County, Kentucky, by J. S. Diller. 1887. 8°. 31 pp. 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 in 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 Genesee 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 Strata of the Tuscaloosa, Tombigbee, and Alabama Rivers, by Eugene 

A. Smith and Lawrence C. Johnson. 1887. 8°. 189 pp. 21 pi. Price 15 cents.

44. Bibliography of North American Geology for 1886, by Nelson H. Darton. 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 Introduction by N. S. Shaler. 1888. 8°. 143pp. Price 15 cents.

47. Analyses of Waters of the Yellowstone National Park, with an Account of the 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 Sitnpson Woodward. 1888. 8°. 88 pp. 

Price 10 cents.

49. Latitudes and Longitudes of Certain Points in Missouri, Kansas, and New Mexico, by Robert 

Simpson Woodward. 1889. 8°. 133 pp. Price 15 cents.
IV ADVERTISEMENT.

50. Formulas and Tables to facilitate the Construction and Use of Maps, by Robert Simpson Wood- 

 ward. 1889. 8°. 124 pp. Price 15 cents.

51. On In vertebrate Fossils from the Pacific Coast, by Charles Abiathar White. 1889. 8°. 102pp. 

14 pi. Price 15 cents.

52. Subaerial Decay of Rocks and Origin of the Red Color of Certain Formations, by Israel Cook 

Russell. 1889. 8°. 65 pp. 5 pi. Price 10 cents.

53. The Geology of Nantucket, by Nathaniel Southgate Shaler. 1889. 8°. 55pp. 10 pi. Price 10 

cents.

54. On the Thermo-Electric Measurement of High Temperatures, by Carl Barus. 1889. 8°. 313pp. 

incl. 1 pi. 11 pi. Price 25 cents.

55. Report of work done in the Division of Chemistry and Physics, mainly during the fiscal year 

1886-'87. Frank Wigglesworth Clarke, chief chemist. 1889. 8°. 96 pp. Price 10 cents. .

56. Fossil Wood and Lignite of the Potomac Formation, by Frank Hall Knowlton. 1889. 8°. 72pp. 

7 pi. Price 10 cents.

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

Price 5 cents.

59. The Gabbros and Associated Rocks in Delaware, by Frederick D. Chester. 1890. 89. 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 Waldemar 

Lindgren. 1890. 8°. 40 pp. Price 5 cents. 

64. A Report of work done in the Division of Chemistry and Physics, mainly u>a'ing the fiscal year

1888-'89. F. W. Clarke. chief chemist. 1890. 8°. 60 pp. Price 10 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. 

In press:

58. The Glacial Boundary in Western Pennsylvania, Ohio, Kentucky, Indiana, and Illinois, by George 

Frederick Wright.

62. The Greenstone Schist Areas of the Menominee and Marquette Regions of Michigan, by George 

H. Williams; with an Introduction by R. D. Irving.

63. A Bibliography of Paleozoic Crustacea from 1698 to 1889, including a list of North American 

species and a systematic arrangement of genera, by A. W. Vogdes.

65. Comparative Stratigraphy of the Bituminous Coal Rocks of the Northern Half of the Appalachian Field, by I. C. White.

67. On the relations of the Traps of the Newark System in the New Jersey Region, by Nelson H. 

Darton. 

In preparation:

 Natural Gas Districts in Indiana, by Arthur John Phinney.

 The Viscosity of Solids, by Carl Barus.

 Mesozoic Fossils in the Permian of Texas, by C. A. White.

 A Late Volcanic Eruption in Northern California and its Peculiar Lava, by J. S. Diller.

 Altitudes between Lake Superior and the Rocky Mountains, by Warren Upham.

STATISTICAL PAPERS.

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

50 cents.

Mineral 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°.

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 remittances, 

therefore, must be by POSTAL NOTE or MONEY ORD^R, made payable to the Librarian of the U. S. Geological Survey, or in CURRENCY, for the exact amount. Correspondence relating to the publications of 

the Survey should be addressed

To THE DIRECTOR OF THE

UNITED STATES GEOLOGICAL SURVEY,

WASHINGTON, D. C.

WASHINGTON, D. C., June, 1890.
DEPARTMENT OF THE INTERIOR

BULLETIN

OF THE

UNITED STATES

No. 66 ,

^S^'-fil::^

29886

WASHINGTON

GOVERNMENT PRINTING OFFICE 

1890

UNITED STATES GEOLOGICAL SUKVEY

J. \V. FOWELL, DIKEOTOli

A GROUP OF VOLCANIC ROCKS

FJiOM THE

TEWAN MOUNTAINS, NEW MEXICO,

AND ON

BY

JOSEPH PAXSON IDDINGS

^i^m^^l^ 

29,886 *'

WASHINGTON

GOVERNMENT PRINTING OFFICE

1YS90

CONTENTS.

Pago.

Letter of trausmittaL..-........... ---..-.--...---..-.--..-...-----.----- 7

Introduction.............................................................. 9

Petrographical description ................................................ 10

Ehyolites............................................................. 10

Normal porphyritic varieties ...................................... 10

Obsidian ............................. ............................ 11

Lithoidite ........................................................ 11

Mica-bearing rbyolite................................... .......... 11

Tufa.............................................................. 12

Ash .I........................ .................................... 12

Audesifces ............................................................ 12

General characteristics.... .......................................... 12

Class I. Mica-andesiie........................... .................. 13

Class II. Hornbleude-niica-audesite.......... ...................... 13

Class III. Hornblende-pyroxene-audesite........................... 14

Class IV. Pyroxeue-andesite....................................... 15

Basalts............................................................... 16

Normal basalts..............'...................................... 16

Quartz-bearing basalt............................................. 16

Miueralogical gradations.................. ................................ 17

The occurrence of primary quartz grains in basalts.....................J.. 20

Basalt from Rio Gratide Canon........................................ 20

Basalt from Arizona................................................... 21

Basalt from Colorado .................................................. 22

Possible origin of porphyritical quartz.................,.,,............ 23

Its exceptional occurrence......................................... 23

Variation of conditions ........................................... 23

Influence of absorbed water....................................... 24

Comparison of exceptional occurrences ...... ..................... 24

Changes of physical conditions............ ........................ 25

Influence of water vapor...... .... ...... ...... .................... 2G

Application to quartz-bearing basalts.............................. 28

Confirmatory observations ............................................ 29

Porphyritical quartz in other volcanic rocks........................ 29

Chemical similarity of basalts with and without quartz............ 30

Chemical differences between basalts with quartz.................. 31

Different mineral development of chemically similar magmas. ...... 31

Summary.................................................................. 32

Index.................................... ................................ 33

5

LETTER OF TRANSMITTAL.

DEPARTMENT OF THE INTERIOR, 

UNITED STATES GEOLOGICAL SURVEY,

Washington, D. (7., May 24, 1888.

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

Iddings, "On a group of volcanic rocks from the Tewan Mountains, 

New Mexico, and on the occurrence of primary quartz in certain 

basalts."

It is the result of an investigation of a suite of rocks collected by 

yourself in northwestern New Mexico in the autumn of 1887.

Its special value consists in showing a gradual transition in mineralogical composition from acid to basic lavas and adds one more to the 

volcanic areas of the Far West which have been investigated by the 

searching analytical methods of modern science. Such studies as this 

of Mr. Iddings will enable us finally to elucidate some of the more important geological problems connected with the crystallization of eruptive rocks.

I recommend that the paper be published as a bulletin of the Geological Survey.

Very respectfully, your obedient servant,

ARNOLD HAGUE,

Geologist. 

Hon. J. W. POWELL,

Director U. 8. Geological Survey.

r

UN A GROUP OF VOLCANIC ROCKS FROM THE TEWAN MOUNTAINS, 

.NEW MEXICO, AND ON THE OCCURRENCE OF PRIMARY QUARTZ IN 

CERTAIN BASALTS,

BY JOSEPH P. IDDINGS.

INTRODUCTION.

Maj. J. W. Powell, Director of the U. S. Geological Survey, and Mr. 

William H. Holmes, in the course of their explorations in the Tewan 

Mountains, in New Mexico, during the field-season of 1887, collected a 

suite of rocks, which exhibit petrographic features of special interest.

The collection embraces a number of varieties of volcanic rocks and 

presents a series of transitions from acid rhyolite, through audesite, to 

quite basic basalt. This series, however, is interrupted in two places 

by the absence of varieties which occur in neighboring regions and, 

possibly, may exist in this one in places not yet visited. The greater part 

of the collection is made up of rhyolite, obsidian, and rhyolitic tufas 

and ashes, which cover a large area of the country explored. But the 

small number of andesites and basalts accompanying the rhyolites, by 

the variations in their mineral composition and physical habit, emphasize the fact that the rocks of the region present a series which grades 

in composition from one extreme to the other, rather than a number of 

well-defined types. It is the object of this paper to call attention to the 

iniueralogical relations existing between the rocks of this locality, as 

another example of that which has been found to exist throughout the 

series of volcanic rocks occurring in the regions of the Great Basin, 1 

including the Washoe district,2 and of the volcanoes of the Pacific 

coast,3 and the Kepublic of Salvador,4 Central America ; and also to

1 Hague and Iddiiigs. Notes on the volcanic rocks of the Great Basin. Am. Jour. 

Sci., 3d series, vol. 27, 1884, pp. 453-463.

2 Ibid. Notes on the volcanoes of Northern California, Oregon, and "Washington 

Territory. Am. Jour. Sci., 3d series, vol. 2G, 1883, pp. 222-235.

3 Ibid. On the development of crystallization in the igneous rocks of Washoe, 

Nevada, with notes on the geology of the district. Bull. U. S. Geol. Survey, No. 17, 

1885.

4 Ibid. Volcanic rocks of the Eepnblic of Salvador, Central America. Ani. Jour. 

Sci., 3d series, vol. 32, 1886, pp. 26-31.

9
10 VOLCANIC EOCKS OF TEWAN MOUNTAINS. IBULL.GG.

describe the occurrence of porphyritical secretions of quartz in various 

members of the series, particularly in the basalt, and to offer some 

suggestions as to their probable origin.

PETROGRAPHICAL DESCRIPTION.

In order to bring out more clearly the connection between the varieties of these rocks it will be necessary to describe as briefly as possible 

their mineral composition and general characters.

RHYOLITES.

Normal porpliyritic varieties. The fhyolites are light-colored rocks, 

varying from white and gray to buff, flesh-colored and light red. They 

are mostly porous and earthy in texture, and grade into pumice on the 

one hand and into iine-grained ash on the other. They are characterized 

by abundant porphyritical crystals of quartz and sanidine having a 

brilliant, glassy habit. The quartzes are mostly crackled and rounded, 

but in some of the rhyolitic tufas they have a perfect crystal form, and 

may be separated from the rock as dihexahedral pyramids, with very 

short prism faces. The sanidines frequently exhibit the intense blue 

iridescence or sheen, common to many occurrences in the Great Basin 

and the Kooky Mountains, and which is probably occasioned by a submicroscopic parting along planes parallel to an orthodome.1 Sauidines 

exhibiting strong blue iridescence occur in the rhyolites from the following localities in New Mexico: right fork of Coyote Creek; Middleton's Eanch ; seven or eight miles above Jemez Pueblo; and from the 

vicinity of Jemez Springs. The rhyolitic tufas range from fine grained 

earthy masses, without macroscopic crystals or fragments, to aggregations of rhyolitic ash with various sized fragments of pumice and dense 

rhyolite.

In thin sections, under the microscope, nearly all of the rhyolites 

present the same characters: a more or less glassy groundmass with 

numerous phenocrysts of quartz and sanidine. The quartz seldom 

exhibits crystal boundaries, being usually in rounded grains or angular 

fragments. It is poor in inclusions, many individuals showing none at 

all, others carrying a few glass inclusions. The sanidine is of very 

pure substance, frequently exhibiting crystal boundaries, though often 

in angular fragments, and is occasionally twinned according to the 

Carlsbad law.

Most of the rhyolite sections are free from plagioclase; nearly all 

contain a few scattered fragments of rather dark green augite; a smaller 

number carry a little brown hornblende, and rarely magnetite and 

zircon.

The groundmass is glassy and colorless, with a variable structure, 

being micro-cryptocrystalline to microcrystalline in patches, as well as

1 Whitman Cross. Contributions to the Mineralogy of the Rocky Mountains. Bull. 

U. S. Geol. Survey, No. 20 1885, p. 75.
IDDINGS.] KHYOLITE. 11

axiolitic. In most cases it is marked by irregular, short, curved streaks, 

shaped like the minute fragments of glass in the tufas. In a few 

instances it is spherulitic, and in others, pure glass.

A highly inflated and fibrous pumice is found at the old ruins, Te-yi, 

west of Espanola. It is light; gray with a satin luster on the sides of 

the fibres, and carries phenocrysts of quartz and sauidine.

Obsidian. A. very beautiful jet-black obsidian occurs at Obsidian 

Hill. It is free from porphyritical secretions, except a few small 

spherulites and lithophysre, the latter being abundant in a specimen 

from Eio San Diego. The spherulites and lithophysae resemble those 

from Obsidian Cliff, Tellowstone National Park,1 except that no fayalite 

has been observed in them. In the obsidian from six miles above San 

Antonio Springs there are curious flattened, dish-like spherulites. The 

obsidian from these localities is remarkably transparent, even in quite 

large flakes, being gray to smoke-colored. The color appears to be due 

to minute black specks, recognizable with a pocket-lens, the glass itself 

being colorless. The glass is hydrated, and upon being fused before an 

oxyhydrogen blowpipe, it boils up iuto white pumice through the 

escape of its water of hydration.

In thin section the glass is found to be almost free from microlitic 

secretions. There are scattered black grains, probably magnetite, and 

a few microscopic feldspars, mostly in minute groups of crossed crystals. 

The few small spherulites are like those just mentioned. Occasionally 

there are microscopic crystals of apatite and zircon.

Lithoidite. In this connection may be mentioned a lithoidal, vesicular rhyolite without porphyritical secretions, from the peak near 

Pela,do camp. The cavities are coated with glassy crystals of tridymite. 

In thin section it exhibits a spherulitic groundmass with radiating 

groups of feldspar microlites and branching spherulitic growths, quite 

similar to the microstructure of the lithoidal portion of Obsidian Cliff, 

Yellowstone National Park.

Mica-bearing rhyolite. Another variety of porphyritic rhyolite is 

represented by specimens from the head of San Antonio Valley, from a 

conical hill in the Valle Grancle, and from San Diego Cafiou, east side 

of Jemez Springs. These rhyolites are light gray and white, with 

abundant porphyritical quartz and feldspar crystals and numerous 

small plates of biotite. Two of them are fall of irregular cavities, the 

third is pumiceous. The quartz is in crackled grains and has a light 

amethystine color.

In thin section, the phenocrysts are found to be quartz, large sanidines and numerous plagioclases withosome red mica. One thin section 

contains a little brown hornblende. The groundmass of the first 

two forms is microcrystalliue and spherulitic, while that of the pumi1 J. P. Iddings. Obsidian Cliff, Yellowstouo National Park, Seventh Annual Report 

U. S Geological Survey, 1889. The nature and origin of lithophysoo and the lamination of acid lavas. AMI. Jour. Sci., 3d series, vol. 33, 1887, pp. 36-45.
12 VOLCANIC ROCKS OF T,EWAN MOUNTAINS. [uuLL.66.

ceous form is pure glass filled with gas cavities. The latter carries a 

small amount of microlites, which are abundantly included in some of 

the feldspars. Both the quartz and feldspar crystals of this rock arc 

rich in glass inclusions. The plagioclase probably belongs to the oligoclase series. Its presence together with that of mica in this variety of 

rhyolite gives it a dacitic character ; for it approaches the composition 

of the dacites of the Great Basin, already referred to, which are characterized by abundant biotite, besides plagioclase and quartz, with less 

sanidiue.

Tufa. The rhyolitic tufas appear to be composed of the variety of 

rhyolite first described. They bear similar porphyritical crystals: 

quartz and sanidine, without plagioclase; and also carry small fragmentary crystals of augite. The groundmass is mostly glass, showing 

some micro-cryptocrystalline and microcrystalline portions, and the 

irregular markings which suggest minute fragments of glass. The 

groundmass is brecciated and exhibits various structures, the larger 

fragments being pumiceous. The tufas also contain fragments of other 

rocks which are mostly audesitic.

Ash. The rhyolitic ashes are made up of minute angular fragments 

of glass and crystals. In some instances they contain a variable amount 

of gueissic fragments, consisting of angular particles of quartz, microclineaud small fragments of schist. This archsean material sometimes 

occurs in such quantities as to constitute a whole layer of the rock.

ANDESITES.

The andesites of the collection number twenty-three different specimens, and may be divided into four classes according to their mineral 

composition. But these classes grade into one another so that no sharp 

line of clemarkation can be drawn between them. They are: 

I. Mica-andesite.

II. Hornblende-mica-andesite.

III. Hornblende-pyroxene-andesite.

IV. Pyroxene-andesite.

GENERAL CHARACTERISTICS.

The first class is characterized by the presence of mica, without 

hornblende or pyroxene.

The second class may be subdivided into two varieties, one containing mica and hornblende without pyroxene, and one containing mica, 

hornblende, and pyroxene.

The third class, also, may be subdivided into two varieties, one containing hornblende and pyroxene with a little mica, and one containing 

hornblende and pyroxene without mica.

The fourth class is characterized by the presence of pyroxene; some 

specimens containing a very little hornblende and some being free 

from it.
ANDESITE. 13

Within the last three classes there is a great variation in the physical 

habit of the rocks, but the microscopical characters of the minerals 

composing them are quite uniform. Thus the colors of the hornblendemica-andesites range from light to dark shades of gray, brown, and red, 

some being mottled bluish gray and red. The nature of the groundmass varies from lithoidal and porous to glassy and compact. Mostof the 

varieties are characterized by large porphyritical feldspars, 5 to 10""" 

long, with smaller hornblendes and micas, and more or less numerous 

rounded grains of quartz. Two of the specimens have no large porphyritical crystals, but carry a multitude of small ones.

The colors of the hornblende-pyroxene-audesites are darker and 

include red, brown, gray and black. The groundraass varies from lithoidal to glassy. The porphryritical crystals consist of abundant feldspars about 5""" long, with less noticeable ferro-magnesiau silicates.

The five specimens of pyroxene-audesite differ considerably from one 

another. One is light purplish gray, full of small cavities, with numerous porphyritical feldspars and smaller pyroxenes. Another is a light 

red tufa inclosing darker red and black fragments. The remaining 

three specimens are bluish-black dense rocks, one of which is very 

vesicular. They carry porphyritical feldspars and smaller pyroxenes, 

and one of them bears a few small- grains of brilliant crackled quartz. 

The last three forms closely resemble basalt.

Class I. Mica-andesite. Of this class there is but one representative, 

from one-half mile below Sulphur Spring, west of Pelado Peak. It is a 

gray pearlite lull of dull white feldspars, with small inconspicuous 

micas. In thin section it is found to be a colorless glass rich in microlites, and having a well-marked pearlitic structure; there are numerous 

porphyritical plagioclases and brown biotites; several plagioclases appear to be surrounded by orthoclase, which does not occur in separate 

individuals. Magnetite, apatite, and zircon are accessory minerals.

Glass II. Hornblende-mica-andesite. There are two specimens of 

horublende-niica-andesite which contain no pyroxeue, and eight which 

carry more or less hyperstheue and augite besides the mica and hornblende.

In thin section they all agree in having a glassy ground mass, with 

one exception, in which it is holocrystalline with numerous aggregates 

of tridymite. The glassy groundmass in most cases is full of microlites 

of feldspar and augite with grains of magnetite, which give it the socalled felt-like structure. The glass is colorless except in one specimen, 

where it is brown. The grouudniass carries microscopic crystals of 

pyroxene, which are often colored red and sometimes are quite opaque, 

besides feldspars and occasionally small crystals of hornblende and 

mica. The porphyritical crystals are plagioclase, mica, hornblende, 

hypersthene, and augite. The plagioclase forms the largest individuals 

and carries a great number of glass inclusions; in some cases the 

amount of glass exceeds that of the feldspar substance. At times the
14 VOLCANIC ROCKS OF TEWAN MOUNTAINS. [BULL. 66.

feldspar appears to be made up 6f a group of small individuals with 

nearly parallel orientation, between which there is more or less glass. 

From their optical behavior the plagioclases appear to belong to the 

andesiue-labradorite series.

The mica is biotite; its color is brown and reddish brown, sometimes 

with a greenish tinge in sections inclined to the base. It has a very 

strong absorption and a small optic angle. Occasionally it is almost 

entirely replaced by iron oxide.

The hornblende is dark brown with strong pleochroism, in some cases 

being greenish brown, but more frequently reddish brown. The color 

is sometimes different at the two ends of a crystal. The hornblende 

also is occasionally replaced by an opaque iron oxide.

The hypersthene and augite are usually in smaller crystals than the 

hornblende. The hyperstheue is distinctly pleochroic, being green parallel to the longitudinal axis, c, and light reddish brown at right angles 

to it. It has quite the same character as the hypersthene found in the 

audesites of the Great Basin and Pacific Coast volcanoes. The augite 

is light greenish and is not pleochroic.

The iron oxide occurs in small irregular crystals whose form indicates 

magnetite. Apatite and zircon are found in almost all of the thin sections. The apatite is often colored reddish brown and exhibits a decided 

absorption parallel to the longitudinal axis. The quartz grains which 

are observed in most of the hand specimens are found in only four of 

the thin sections, partly because they are not very abundant and partly 

because they fall out during the preparation of the section. The quartzes 

observed are of the purest substance with perfectly uniform optical 

orientation throughout each individual grain. They are almost entirely 

free from inclusions; in some cases they contain one or two dihexahedral glass inclusions, and in one instance a small prismatic crystal of 

apatite.

Class III. Hornblende-pyroxene andesite. Of the rocks referable to 

this group five specimens carry a little inica, while two contain none, 

They are quite similar to the horublende-mica-andesites, but are somewhat darker colored with less prominent porphyritical crystals. In 

thin sections they exhibit the same miscroscopical characters as the 

rocks just described, differing from them only in the relative proportions of the mineral constituents. Three specimens, however, deserve 

special mention. Two are andesitic pitchstoues from the cliff near the 

Warm Spring on the north side of the canon, east branch of San Diego 

Creek. They are black, resinous glasses, with abundant white phenocrysts of feldspar and some rounded grains of crackled quartz.

In thin section they are found to consist of colorless glass, rich in 

trichites and inicrolites, which are not uniformly distributed through 

tbe glass, but are arranged in layers and streams, which curve around 

the crystals and produce a strongly marked flow structure. The 

porphyritical crystals are: plagioclase crowded with glass inclusions,
IDDINGS.J ANDESITE. 15

brown hornblende, hypersthene and augite, and in one section a little 

biotite. The quartz grains are not found in the thin sections.

The third rock is from about the same locality; it is red, streaked 

with black peaiiitic glass, with small fragments of light purple rock. 

It bears numerous porphyritical feldspars, and considerable quartz in 

brilliant, crackled grains, none of which are found in the thin sections 

made i'rom it. Under the microscope the rock appears to be a microscopic breccia of different kinds of glass: colorless, yellow and brown, 

in some places trichitic, in others clouded. The porphyritical secretions 

are like those of the previous varieties, with a little mica.

Glass IV Pyroxene-andesite. Within the group of pyroxene-andesites there is a range from a variety quite similar to the horubleudepyroxene-audesite to one which resembles andesitic basalt without the 

olivine.

The first variety is represented by a bluish-black rock with irregular 

cavities, from a coulee two miles u ortheast of Polvadera Eancho. The 

phenocrysts consist of medium-sized feldspars and small pyroxenes, 

with a few small grains of brilliant quartz. In thin section the 

grouudrnass is found to be glassy with a felt-like structure, produced 

by the microiites of feldspar and augite*. The porphyritical plagioclase 

has the same character as that of the hornbleude-pyroxene-andesites, 

being full of glass-inclusions. The hypersthene and augite have the 

same characters as those already described. There is a very small 

amount of reddish-brown hornblende present.

The light red audesitic tufa from Santa Clara Canon has the same 

mineral composition as that just described, but shows no quartz grains.

Another variety of pyroxene-andesite occurs eight miles from the 

head of Coyote Canon. It is light purplish gray with numerous irregular cavities, and bears porphyritical feldspars and inconspicuous 

pyroxenes. In thin section the groundmass is found to be glassy, and 

full of microiites of feldspars and augite with grains of iron oxide. 

The pheuocrysts are plagioclase, hypersthene, and augite, with accessory 

iron oxide and apatite. The plagioclase crystals differ from those in 

the aridesites just described. They are sharply rectangular, with fewer 

glass inclusions, and appear to belong to more basic species, probably 

to the labradorite-bytownite series. Hypersthene is more abundant 

than augite, both having the same characters as those in the other 

andesites.

The dense, bluish-black variety, from seven or eight miles above 

Jernez Pueblo, bears numerous porphyritical feldspars, and in thin 

section is found to have a groundmass of dark brown, globulitic glass 

with microiites. The porphyritical feldspars are rich in brown glass 

inclusions. The small porphyritical pyroxenes are mostly hypersthene.

The vesicular variety of pyroxene-andesite taken from an Indian 

metate, or griuding-mill, found five miles above Jeniez, is similar in
16 VOLCANIC ROCKS OF TEWAN MOUNTAINS. [BULL.OG.

composition and structure to the previous rock, the brown globulitic 

glass being particularly fine.

Owing to the preponderance of orthorhornbic pyroxene or hypersthene 

in these pyroxene-andesites they belong to the variety hyperstheneandesite.

BASALTS.

The basalts of the collection are mostly compact, fine grained rocks, 

differing in color from light gray to greenish black, in two cases being 

dark red.

Normal basalts. The basalt from a dike on the south side of Kio 

Chama, one and one-half miles west of Abiquiu, is light gray, very fine 

grained and dense. In thin section it is found to be glassy with abundant 

lath-shaped plagioclase, and long augite crystals with grains of magnetite and some trichites. There are almost no porphyritical secretions. 

There is considerable iron oxide and nearly opaque secondary matter, 

which appears to replace the oliviues that have been decomposed. 

There is no hypersthene in this nor in any of the other basalts in the 

collection.

A dike four miles northeast of Abiquiu is formed of another variety 

of basalt. It is a dense black rock with a few small phenocrysts. In 

thin section it is seen to be composed of microscopic crystals and grains 

of violet-brown augite and magnetite, with a small amount of lathshaped plagioclase and much colorless olivine in small porphyritical 

crystals; the whole is cemented by a colorless glass bearing black 

trichites.

The basalts which form the second and fourth ledges from the top of 

the wall of the Rio Graude CaGon are very much alike; they are lightgray dense rocks with small phenocrysts. In thin sections they are 

found to be holocrystalline, with much lath-shaped plagioclase and less 

augite in grains and stout crystals, besides magnetite and much colorless olivine in crystals and grains. There are a few porphyritical 

crystals of olivine and augite and very abundant microscopic needles 

of apatite.

Quartz-bearing basalt. Of the seven remaining specimens of basalt 

from the neighborhood of Eio Grande Cailou, two are light-gray dense 

rocks, three are greenish black, dense and fine grained, and two dark 

red and vesicular.

The light gray basalts are somewhat like those just described, but 

bear more porphyritical secretions of olivine, besides abundant small 

grains of crackled quartz. These quartz grains are surrounded by light 

green shells, composed of microscopic augites. In some cases the shells 

cover the quartz grains on the surface of the handspecirnen ; in others, 

especially on weathered surfaces of the rock, the quartz has dropped 

out, leaving the augite shell firmly adhering to the rock. The quartz 

grains are distributed through the rock quite as uniformly, though not 

so abundantly, as the crystals of olivine are.
IDDINGS.l BASALT. 17

In thin section these two basalts are alike and resemble those forming the second and fourth ledges on the canon wall, except that they 

are slightly coarser grained. They are holocrystalline, and are composed of lath-shaped and tabular plagioclase, light violet-brown augite 

in crystals and grains, with magnetite and much olivine in grains and 

porphyritical crystals. None of the quartz grains appear in the thin 

sections of these two basalts, but the augite rings and clusters indicate 

where they were located before the grinding of the sections. The three 

specimens of greenish black basalt also exhibit microscopic secretions of 

olivine and grains of quartz like those just described, being even more 

abundant. The quartz is sonietimes quite compact and free from 

cracks.

In thin section these basalts have the same microstructure as those 

just mentioned, but are finer grained and contain rather more augite. 

The olivine is partly altered to dark green serpentine, which is also 

scattered through the rock and gives it its green color. In one thin 

section there are seven quartz grains or indications of their former presence, in another there are two, and in a third there are ten.

The red vesicular basalts also bear many porphyritical grains of 

quartz; on the surface of one side of a small specimen 7.5cm long by 

6cm wide (3 by2£ inches) there are seventy-five grains, and they are no 

more abundant on this surface than on the others. In thin sections 

these red basalts resemble the general structure of the gray, compact 

varieties, but they do not appear to be holocrystalline; there is probably a little glass present. The rocks are filled with red oxide of iron, 

which also colors the margins of the porphyritical olivines. They contain a little more augite than the gray basalts do. Most of the quartz 

grains have dropped out in grinding, but fragments remain, and in most 

cases the -angite rings still indicate where they were formerly located. 

From these indications it appears that in one thin section which is 2cm in 

diameter there were once five grains of quartz.

MINERALOGICAL GRADATIONS.

From the foregoing descriptions it is evident that the volcanic rocks 

from this portion of New Mexico present a number of varieties grading 

from one extreme of mineral composition to another, which, judging 

from other localities, corresponds to a variation in chemical composition. 

This transition may be expressed by the following table, which gives the 

essential mineral constituents of the different varieties. 

Bull. 66 2
18 VOLCANIC ROCKS OF TEWAN MOUNTAINS. [BULL. C6.

"cj

a

P,

ao

c.

o

o

J3 

O

o 

M

c

c

p: 

<

C

P bo

ododdodoS

i : :::::::

cj * 8 . . (3 a! ^ GJ

: : : : a a a a

; ; « a a a 

 ;' 3323

O 0 O O O O O

a e ,=1 ^a j j3 c3

's g a a 'a § i

'o'o'o'o'o'o'o'o 

oooooooo

60 0> t( So bt bO bC bO

p.a.p.p.p.p.p.p,

.9 .2 :::::::

'5 "a ' ' ' \ '. '. '. a <* « o .....

00S .OOOOO

v^w ; i "^T" ^^^^ ^

: : i i

: : : :

 a a 

 ' a o «,

 ' A £ |

: : .2 2 i

i « a t*a <1

 _S « £4 *c

: =s « i P . S « -S

rrt ' ^ O a fl -2 a "^ S e 

a « S 3 3 S 5 ° -^ d a a c ' & s ° fe h > -^ ,§ B ° .3 b 

> fq w 3 H ?H P2 « <B

-M l£3 ^ 

CD u (D

a s a

a s a

« » «

bo o> ba

a 80

« 8 c8

a « 

* s

00 "SJ 

03 «

1 1

« 8 J2 

« "u O

o o o

bo Oi be

p< &. p.

i : :

. ! ' 

' a

8

8

.C

1

'I

-8

'Olivine-bearing an salt*........... TBaaalt...........
1DD1HC8.J MINERALOGICAL VARIATIONS. ' 19

In this series the most persistent constituent expressed in general 

terms is feldspar, but this passes through a series of variations. Thus 

it is wholly orthoclase in the first variety of rock j orthoclase and oligoclase, possibly with albite, in the second. But in the following varieties, since the particular species of the plagioclase has not been definitely determined, 110 attempt has been made in the table to separate 

them. It can be said, however, that the plagioclase becomes more basic 

as basalt is approached, passing through andesine to labradorite and 

possibly bytownite and auorthite. The composition of dacite is indicated in italics; it is similar to that of the second variety of rhyolite 

but the relative proportions of the minerals are different. It is possible 

that the glassy inica-andesite may have the chemical composition of 

dacite.

Mica, as an essential constituent, makes its appearance before hornblende does, and disappears sooner. Hornblende bears a similar relation 

to hypersthene and augite, which make their appearance together in these 

rocks, with the exception of the accessory augite in the first variety of 

rhyolite, which differs slightly from the other augites. Hypersthene 

runs its course before augite, which reaches its greatest development 

in the basalts, where olivine appears.

Theintermediateformsofoli vine-bearing andesiteorhyperstheue-bearing basalt, which are so abundant in other regions of the West, are not 

represented in this collection; their composition is indicated in italics 

in the table.

Magnetite is an accessory constituent of the acid rocks of the series, 

and with the gradual increase of basicity assumes the role of an essential constituent in the more basic members of the series. The occurrence of quartz as an accessory constituent of various members of this 

series will be fully discussed later.

Owing to the limited size of the collection it would be unwise to offer 

any generalizations as to the preponderance of one variety over another 

in this region. And, except for the abundance of rhyolite, the collection illustrates the variable character of the rocks, which is still more 

noticeable in larger collections from other regions, and becomes still 

more apparent when large bodies of volcanic rocks are studied in the 

field, for the mineral composition of a rock-mass varies within certain 

limits in nearly every case, and the greater the number of occurrences 

the more difficult it is to select a particular combination of minerals as 

a typical one. Moreover the predominance of any one variety in a given 

region may be an actual predominance or only an apparent one due to 

geologic conditions, such as the covering up or the denudation of other 

varieties; and a predominant variety of volcanic rock in one locality 

may be a subordinate variety in a neighboring district.
THE OCCURRENCE OF PRIMARY QUARTZ GRAINS IN BASALTS. 

BASALT FROM RIO GRANDE CANON.

Turning now to a consideration of the occurrence of quartz in the 

form of porphyritical secretions in the basalt from Rio Grande Caiion, 

it is to be observed that:

The quartz is not an alteration product of other minerals, for the 

rocks are quite fresh, and exhibit only a very slight alteration on the 

surface of the oliviues. On the contrary, it is evident that the quartz 

grains existed in the rock-mass in their present form previous to the 

final consolidation of the magma. For each grain is closely surrounded 

by a shell of augite crystals intimately connected with the inclosing 

rock-mass. This augite shell forms a narrow border or ring in thin 

section, and is composed wholly of short crystals of augite radiating 

from points along the side of the rock-mass toward the quartz. The 

augite crystals crowd closely against the surface of the quartz grains, 

but between them and the rock-mass there is no line of demarkation; 

the outside augite crystals lie among the feldspar and magnetite individuals, and take part in the general structure of the rock. In the 

holocrystalline and coarser-grained varieties the feldspar sometimes 

incloses a number of the augites situated on the outside of the shell, 

and the feldspar is thus proved to be a later crystallization than the 

shell.

The substance of the quartz composing these grains is perfectly pure, 

without inclusions of gas, fluid or glass; iu one instance there was a 

minute crystal of zircon. Each grain is a single individual, with uniform optical orientation throughout its entire extent; occasionally two 

individuals are in juxtaposition. But there is nothing like aggregates 

of quartz, the form in which secondary quartz usually appears. The 

grains are rounded or subaugular, in only one instance did the shape 

and optical orientation indicate a bipyramiclal form.

The substance and shape of the quartzes are like those of the porphyritical quartz grains in other volcanic rocks when they are free 

from inclusions, which sometimes happens in the rhyolites. They are 

not like the quartzes of granites and gneisses or of their derivatives, 

the sandstones, which are more or less filled with inclusions of gas and 

fluid, and frequently with individualized inclusions.

It is evident, therefore, that the quartz grains are primary constituents of the rock; that they are more like the porphyritical quartz 

secretions of other volcanic rocks in their microscopical habit than like

go
IDUINGB.] DISTRIBUTION OP QUARTZ-BASALT. 21

fragments of quartz from granitic or gneissic rocks or sandstones. 

Moreover, their mode of occurrence in these particular basalts militates 

agaiust the theory that they may be fragments of foreign rocks which 

have been caught up by the basalts during their eruption; for they are 

uniformly and intimately distributed through each of the seven handspecimens, which were collected from different parts of a large basalt 

flow, at least four miles long, and which represent portions of it differing in crystallization and general habit. Further, the grains are 

isolated individuals, and not clusters, and never exhibit indications of 

having been crystallized together with other minerals. They have 

every appearance of being primary secretions, or crystallizations from 

the rock magma.

The occurrence and distribution of very abundant quartz grains 

throughout the recent basalt flow near the Cinder Cone, ten miles 

northeast of Lassen Peak, California, has been described by Mr. J. S. 

Diller,1 and as claimed by him leaves no reasonable doubt of the 

primary and essential nature of the quat tz in that particular basalt.

Two new occurrences of quartz-bearing basalt may here be described, 

since they show that such basalts are scattered over a large area of 

country, having been found in California, Nevada, Arizona, New Mexico, 

and Colorado.

BASALT FROM ARIZONA.

That from Arizona is a fine example of quartz-bearing basalt, it is a 

red, compact rock from the canon near granite tanks, in the vicinity of 

Santa Maria Basin, Arizona, and was collected by Mr. Clarence King. 

In the hand-specimens the only porphyritical secretions are rounded 

grains of glassy quartz from 1 to 8mm in diameter, together with somewhat smaller olivines. The quartz grains are so numerous that one 

small specimen about 6cra (less than two and one-half inches) square 

shows as many as thirty grains on all its surfaces. In thin sections the 

basalt is glassy with abundant lath-shaped plagioclases and red oxide 

of iron scattered through it, which obscures the other constituents. 

The groundmass bears small porphyritical crystals of light-green augite 

and olivine. In the dense specimens the olivine is partly fresh and 

colorless, partly colored orange and red. In the porous specimen the 

olivine has been entirely removed, leaving only its characteristic outlines marked by iron oxide, and the augite is more or less reddened. 

The quartz is the same in both varieties, of which seven thin sections 

were made showing quartz. It forms rounded and subangular grains, 

of very pure substance, almost free from inclusions; the few inclusions 

observed consist of colorless glass, in one instance gas, besides a small 

zircon and an apatite. One large quartz grain has two inclusions of 

red glass with microlites like that on the outside of the same grain,

1 Am. Jour. Sci., 3d series, vol. 33, pp. 45-50, 1887.
22 VOLCANIC ROCKS OF TEWAN MOUNTAINS. [BULL.CG.

and it is probable tbat tbese apparent inclusions are cross-sections of 

pockets or pipes of the outer glass.

Only in a few instances do the quartz grains possess a continuous 

shell of augite crystals, which in these cases is very narrow and close 

fitting. Most of the grains are bordered directly by the groundmass of 

the rock, or have a fragmentary augite shell, which is sometimes 

separated from the quartz grain by red glass, sometimes by a strip of 

groundmass whose flow-structure indicates that it has forced its way 

between the augite shell and the quartz grain. Fragments of augite 

shell are observed at some distance from the quartz, or even entirely 

isolated in the rock-mass, from which it appears that the quartz grains 

were at one time surrounded by a shell of augite, as in most other occurrences of quartz-bearing basalt, but that in this instance the subsequent 

movement of the magma broke the shells and dislocated them.

BASALT FROM COLORADO.

The second occurrence to be described is that of a dark-colored, finegrained basalt from. Elk Head Creek, at the southeast base of Anita 

Peak, fifteen miles northeast of Haydeu, Colorado, in which basalt 

porphyritical quartz grains are very abundant. Dr. Whitman Cross 

kindly furnished me with the thin sections of this rock for study and 

description. They represent two modifications of the basalt, and show 

that the rock is partly altered, the olivines having been converted into 

serpentine, which is disseminated generally through the rock. The 

mineral composition and structure of this basalt are about the same 

as those of the basalts from Bio Graude Canou, New Mexico, but the 

quartz grains, which are equally abundant in both basalts, have a 

somewhat different microscopical character.

In the basalt from Elk Head Creek the quartz grains are surrounded 

by an augite shell or ring in every case. But the shell is quite thin 

and its connection with the quartz substance is more intimate. In 

places crystals of augite penetrate the quartz or are entirely inclosed 

within it, but they are always located along the margin of the quartz 

grain. In some instances the quartz exhibits sharp-edged crystal 

boundaries, which are usually adjacent to an area of serpentine or 

calcite, which may now occupy what was formerly glass or a cavity. 

At times the straight, crystal edge is bordered by a narrow rim of 

augite. The quartz grain is sometimes made up of two or three individuals crystallized together. The inclusions are numerous, and consist 

of zircon and occasionally apatite, with frequent gas cavities and less 

abundant glass inclusions. The microscopical character of the quartz 

resembles that of certain porphyries.

Its primary nature is shown by the presence of glass inclusions and 

the encircling shells of augite and by its uniform distribution through 

the rock. The existence of crystal boundaries to the quartz grains 

indicates that there was little, if any, resorption of the quartz by the
PHYSICAL CONDITIONS. 2

inclosing inagina. And the inclosure of augite crystals near the 

margin of grains containing dihexahedral glass inclusions suggests 

that the crystallization or secretion of the quartz took place in a molten 

magma in the presence of augite crystals similar to those forming the 

inclosing shell. It should be observed in this connection that partially 

corroded quartz and quartz with crystal boundaries occur together in 

rhyolites and related quartz-bearing rocks.

Quartz grains occur in the same manner but to a much less extent in 

some of the basalts near Eureka, Nevada, especially in those forming 

Basalt Peak and covering the country southeast of Eichmond Mountain.

They are very abundant in certain basaltic rocks from near Crescent 

and Whitehead Peaks and Camel Mountain, Colorado.

POSSIBLE ORIGIN OP THE PORPHYRITICAL QUARTZ.

Its exceptional occurrence. The occurrence of primary quartz in 

rounded grains in basaltic rocks is exceptional, and is contrary to the 

laws which appear to govern the development of the mineral constituents of volcanic rocks, so far as our present knowledge goes. The 

minerals which crystallize in a rock magma result from the chemical 

affinities inherent in a complex solution of silica, alumina, alkaline 

earths and alkalies, including iron, lime, magnesia, potash, and soda, 

between the various demands of which there must be a mutual accommodation. Hence it is evident that the component minerals of a given 

rock must be those which, under the conditions attending their crystallization, satisfy all the chemical demands imposed by those conditions.

Variation of conditions. That changes in the conditions under which 

crystallization takes place in a mixed solution affect the nature of the 

crystallization is well known, and has been demonstrated experimentally. From the generally uniform nature of the crystallization of most 

volcanic rocks the correspondingly uniform conditions attending their 

solidification are indicated. Where variations in the products of crystallization occur the condition affecting them may often be recognized. 

Hence variations from the general order and nature of crystallization 

within certain limits have become generally accepted facts, consequently the mineral composition of rocks is not expected to be in rigid 

accord with their chemical character within these limits.

While in the great majority of cases the mineral composition of volcanic rocks indicates a generally uniform range of condition which 

must have attended their consolidation, yet there are exceptions 

to the ordinary grouping of minerals in these rocks that emphasize the influence of certain attendant conditions, which are evidently physical. An instance of this is the mineral association in the 

lithophysse of some rhyolitic obsidians, especially in that of Obsidian 

Cliff, Yellowstone National Park, and of Cerro de las Navajas, Mexico. 

These holocrystalliue portions of the acid lavas are composed of alkali
24 VOLCANIC ROCKS OF TEWAN MOUNTAINS. [BULL.60.

feldspars, quartz, tridyruite and fayalite, an iron-olivine, that is, an orthosilicate of iron. This latter mineral is in very small amount compared 

with the free silica, quartz and tridymite. The chemical composition 

of the lithophysae is essentially the same as that of the remainder of 

the rock, and is like that of normal rhyolites. Hence we are forced 

to seek the cause of the development of the fayalite in something besides the chemical character of the magma.

Influence of absorbed water. In a paper on this occurrence 1 I have 

discussed the matter at length, and after calling attention to the experiments of M. Daubr»§e on the action of superheated steam on acid 

glass, and to experiments of others who have undertaken to produce 

these minerals artificially, I arrived at the conclusion that the most 

reasonable explanation of what in petrology seems an anomalous 

association of primary igneous minerals is the reference of their occurrence to aqueo-igneous action, which was induced by the presence of a 

considerable amount of water-vapor absorbed iu the molten magma. 

The strong mineralizing influence of the water-vapor under such conditions is apparent both in the structure and mineral composition of the 

lithophysse and in the alternating crystalline and glassy la.yers in the 

laminated lithoidal portion of the same flow of lava. For in the latter 

case the thin horizontal laminae of the magma in any particular portion of the mass must have been subjected to like rates of cooling and 

the same pressure, yet these laminae solidified alternately holocrystalline and glassy.

Comparison of exceptional occurrences. The occurrence of iron-olivine 

in a rhyolite with 75 per cent of silica and less than 2 per cent of iron 

oxide is as remarkable and as exceptional as that of quartz in the form 

of porphyritical crystals in basalt. They are both of the same kind, 

in that they are the occurrences of extremely acid and basic silicate 

minerals together in rocks, where we are generally accustomed to see 

silicate minerals of intermediate or of more closely related composition. 

In the first instance, however, most of the conditions under which the 

minerals crystallized can be surmised, but in the second case there 

must necessarily be great uncertainty, for the quartz was evidently 

crystallized iu some early period in the history of the rock, when the 

conditions then existing made its separation necessary. Whether the 

quartz was the only mineral crystallized at that time or whether there 

were others associated with it can not be determined in most of these 

basaltic occurrences. But in the basalt from Elk Head Creek, Colorado, 

the small augites which penetrate the margin of the quartz crystals 

were evidently contemporaneous with the close of the quartz crystallization.

From the rounded form of the quartz grains in most of its occurrences it is probable that the quartzes were in process of resorptiou by

1 Am. Jour. Sci., 3d series, vol. 33,1887, pp. 36-45. Also Seventh Annual Report, 

U. S. Geological Survey, 1889.
IDDINGS.) CHANGES OF PHYSICAL CONDITIONS. 25

the basic magma when tbe final solidification of the rock took place. 

A similar resorptiou of porphyritical minerals which Lave crystallized 

in an early period of a rock's existence is frequently observed iu all 

 varieties of volcanic rocks, as, for example, partly resorbed hornblendes 

and feldspars, and the rounded quartz grains of rhyolites and porphyries. The idea that these phenomena indicate changes in the 

physical conditions experienced by the magma and not in its chemical 

composition is a generally accepted one.

Changes of physical conditions. Let us consider what maybe some 

of the changes of condition experienced by rock magmas within the 

crust of the earth. In the case of a molten, viscous magma situated 

at some depth beneath the earth's surface the two physical factors 

which at first appear to exert the greatest influence on its existence as 

a plastic or fluid magma are temperature and pressure.

Other things remaining constant, the magma will be more plastic the 

higher the temperature, and with a decrease of the temperature below 

a certain point consolidation will take place. Consolidation under these 

circumstances, that is, at considerable depths within the earth, would 

be ordinarily accompanied by crystallization, as a rapid chilling is only 

possible when small bodies of magma are brought in contact with cold 

rocks. The nature of the crystallization will undoubtedly vary within 

certain limits with the rate of cooling.

If, on the other hand, the temperature of the magma is supposed to 

remain constant and the pressure to vary, and if we assume that an 

increase of pressure diminishes the mobility of the molecules of the 

magma by condensing them, we should expect that an increase of 

pressure would tend to solidify the mass. The latter supposition 

appears to be the most rational, and has received support from the experiments of Dr. William Hallock.1 This is also the conclusion reached 

by Reyer in his work on the physics of eruptions and of eruptive 

rocks.2 Dr. Hallock's observations show that cubical pressure unaccompanied by an increase of temperature simply increases the rigidity 

or viscosity of a body, but that in cases where pressure is allowed to 

produce motion by the crushing or yielding of the molecules of a substance the internal friction may generate heat enough to fuse the substance, arid thus reduce its rigidity.

The solidification of a molten, amorphous mass by increasing its 

viscosity might convert it into a solid glass, siuce Lagorio 3 has observed 

that solid glass is denser than the same glass when molten, or it might 

convert it into a crystalline mass, according to whether the viscosity 

was suddenly increased or slowly increased 5 the changes being analogous to the rapid increase of viscosity by chilling or the slower increase

1 William Hallock. The Flow of Solids. Am. Jour. Sci.,3d series, vol. 34, 1887, 

pp. 277-281. Also, Bull. U. S. Geological Survey, No. 55,1890.

3 E. Keyer. Beifcrii}; zur Fysik der Eruptiouen mid, der Erupfciv-Gesteine, Vienna, 

1877, p. 119.

3 Tschemiak's Mineral, u. petrog. Mittlieil., new series, vol. 8,1887, p. 510.
26 VOLCANIC ROCKS OF TEWAN MOUNTAINS. [BULL.66.

by gradual cooling. The latter permits the molecules to arrange themselves in obedience to certain chemical and physical affinities into crystalline minerals, while the former prevents such an arrangement.

Moreover, if we consider rock magmas as saturated solutions of 

silicate salts, as Lagorio 1 has done in the paper already alluded to, we 

may apply to them the law which Sorby deduced for aqueous solutions 

of salts, namely, that the solubility of those salts, which, like the 

silicates, expand upon solution and condense upon crystallization,2 is 

decreased by increasing pressure. In other words, in such solutions 

an increase of pressure would tend to crystallize the salts from solution. 

Hence in a molten rock magma an increase of pressure alone would 

tend to induce the crystallization of certain silicate minerals from the 

magma, or might lead to the crystallization of the whole magma.

From the foregoing argument concerning the effect of increasing 

pressure on the viscosity of magmas and on their crystallization it 

appears that an increase of pressure might lead to the consolidation of 

rock magmas at temperatures above their melting point for lower 

pressures. It might therefore happen that a highly heated magma 

exists within the earth under such great pressure that it is a solid mass^ 

which may be either amorphous or completely crystalline, or partly 

amorphous and partly crystalline. Such a condition would be one of 

unstable consolidation, for upon a relief of the pressure the temperature 

would be sufficient to melt the mass.

Influence of water-vapor. It is becoming more and more evident that 

a small amount of absorbed water-vapor in molten rock magmas plays 

an important part in their liquifactioii as well as in their crystallization. 

The influence of water-vapor upon the viscosity of lavas has been suggested long ago by Scrope and others, who asserted that its presence 

in molten lavas increased their fluidity. Its part as the explosive 

agent in volcanic eruptions is well established, and its presence in larger 

or smaller amounts in every volcanic rock has often been demonstrated.

The increased fusibility of hydrated glass has been noted by M. Daubr6e,3 and may be easily recognized in the case of hydrated obsidian 

like that from Obsidian Cliff and from New Mexico. These obsidians, 

containing about 0.5 per cent of water upon being fused before an 

oxyhydrogen blowpipe, melt at something less than white heat to an 

inflated glass, which, at first, is quite fluid, and flows away rapidly from 

the strong current of the blowpipe flame. It is so fluid that it curls 

over the edge of the unmelted portion of the rock and sometimes forms 

pendent drops. Upon heating this glass until it loses its water and contains no more gas bubbles, and allowing it to cool, a colorless or pale 

yellowish glass is obtained, which melts before the same blowpipe flame 

only at an intense white heat, and then forms a very viscous glass

1 Tsckermak's Mineral, n. petrog. Mittkeil., new series, vol. 8, 1887, p. 421.

2 J. Roth. Allg. n. Chem. Geol., vol. 2,1885, p. 52.

3 Etudes Syntke'tiques de Gdologie Exp6riinentale. Paris, 1879, p. 161.
IUUIHOB.] INFLUENCE OF WATER VAPOR. 27

which moves sluggishly before the same blowpipe current, and presents 

a decided contrast to the obsidian glass, first melted.

The bearing of Dr. Guthrie's experiments with chryohydrates or 

eutectic substances upon this problem has been pointed out by Prof. 

J. W. Judd l in a paper on The Natural History of Lavas as Illustrated by the Materials Ejected from Krakatoa. Dr. Guthrie demonstrated that the melting point of certain compounds is considerably 

below that of either of their components; they therefore behave like 

alloys. He experimented chiefly with hydrated substances, though not 

entirely. Prof. Judd has taken niter as an example. Under ordinary conditions this substance melts at 320° C.; upon the addition of 

4.89 per cent of water its melting point is reduced to 262° C., and when 

it contains 29.07 per cent of water it melts at 97.6° 0. Hence a mass 

of niter within the earth would be solid at 300° C., but at the same temperature upon the accession of 15 or 20 per cent of water it would be 

molten or in a condition to become molten if the pressure did not prevent it. Inversely, some substances may exist as fluids at temperatures 

considerably below their ordinary point of solidification by being combined with water; there is a point, however, at which solidification sets 

in. Mr. J. J. Harris Teall, who has been studying some of Dr. Guthrie's 

eutectic compounds, finds that upon solidification they separate into 

their component parts, which interpenetrate one another like quartz 

and feldspar in granophyre or micropegmatite. Prof. Judd, in his 

paper, suggests that the mixed silicates in rock magmas may be entectic 

substances, and Prof. Lagorio, in the paper already mentioned, states 

that the feldspars of the lime-soda feldspar series behave like eutectic 

substances. Prof. Judd also calls attention to the power of the water 

of hydration to lower the melting point of zeolites and siliceous glasses 

such as tachylites, hydrotachylites and palagonites.

While we are ignorant of the actual influence of absorbed water-vapor 

upon deep-seated molten magmas, it seems reasonable to assume that 

it is of the same kind as upon molten magmas at or near the surface of 

the earth. For the most apparent difference between the condition of 

a deep-seated and a surface magma is the pressure under which each 

exists. Hence it is probable that the presence of water-vapor tends to 

increase the liquidity of the mass. But since the accession of watervapor would increase the tension of the magma, a point may be reached 

when the resistance of the inclosing rocks is overcome, and the absorbed 

vapor becomes an active agent in the eruption of the magma.

If we consider the action of water-vapor in increasing the liquidity of 

molten magmas as simply a physical one, it must consist in a tendency 

to increase the mobility of the molecules of the mass among themselves. 

One effect of absorbed water, then, would be to retard an increase of 

viscosity, which was being produced in a magma either by cooling or 

an increase of pressure upon it. And this retardation might permit the

1 Geol. Mag., new series, vol. 5, 1888, pp. 1-11.
28 VOLCANIC ROCKS OF TEWAN MOUNTAINS. [BULL.CC.

crystallization of the more hydrated portions of the mass, while the less 

hydratecl parts became too viscous for crystallization, a condition of 

affairs which undoubtedly existed in the obsidian magma at Obsidian 

Cliff, where holocrystalliue portions occur irregularly scattered through 

amorphous ones.

It is to be remarked that the crystallization which was in this instance specially induced by the influence of superheated water-vapor 

differs from that which usually takes place in acid lavas upon cooling, 

and that the result was the production of extremely basic and acidic 

minerals by the side of one another, or the production of chemical extremes; a result in a measure analogous to the dissociation of the base 

and acid of a chemical compound by heating. In this case the absorbed 

water-vapor apparently weakened the affinity between the bases and 

the silica, which separated into basic silicates and quartz and tridyinite. 

Hence, we may reasonably expect that in deep-seated magmas conditions may obtain by which the water-vapor in them influences their 

crystallization, and produces minerals of extreme chemical character.

Application to quartz-bearing basalts. The application of the foregoing general considerations to the occurrence of porphyritical quartz 

in basalts is at once apparent. The crystallization of the quartz would 

seem to have been occasioned by the supersaturatiou of the magma 

through changes of pressure or temperature accompanied by the special 

mineralizing action of absorbed water-vapor. The rounded or corroded 

form of the quartzes indicates that the conditions under which they 

were crystallized were not the same as those under which the subsequent crystallization of the rock took place, and that they were to 

that extent abnormal. Reyer (loc. cit.) suggests that pressure and different degrees of saturation with absorbed water may lead to inetaineric processes, and also states that the development of quartz in rock 

magmas requires a considerable saturation of the magma with water.

Prof. Lagorio, in the paper already quoted, discusses the question of 

the formation of minerals in molten magmas, when they are considered 

as supersaturated solutions of silicates. He states that they may become supersaturated with silica, as well as with the oxides of the heavy 

metals and the alkalies; and that upon cooling the elements dissolved 

in excess crystallize out; and that in this way the concurrence of such 

minerals as olivine and quartz is to be explained.

Accepting the idea of supersaturation and its accompanying laws of 

crystallization, we still find its power to explain the phenomena of rock 

structures and mineral combinations limited. For all rock magmas 

that we have yet become acquainted with have been saturated or supersaturated solutions, whose degree, as well as kind, of supersaturation is 

dependent on both temperature and pressure, as well as on their 

chemical composition. Their kind of supersaturation, that is, the nature 

of the chemical compound which supersaturates the magmas, varies 

with the physical conditions under which the magmas exist. And the
IDDINGS.] PORPHYRmCAL QUARTZ IN OTHER ROCKS. 29

laws governing the solubility of salts under such varying conditions 

have not yet been ascertained, as Lagorio himself states. Thus the 

formation of fayalite in a rhyolitic obsidian with 75 per cent of silica 

and less than 2 per cent of iron oxide could scarcely be referred to 

supersaturation as generally understood.

We are still confronted by the fact that magmas of similar chemical 

composition have crystallized to rocks, which differ both in mineral 

composition and in structure, and which have in each case obeyed the 

laws governing the crystallization of saturated or supersaturated 

solutions. Therefore, we are forced to consider such variations of 

mineral combination and of structure as more especially the results of 

the variable physical conditions attending the consolidation of the 

magmas.

CONFIRMATORY OBSERVATIONS.

If the foregoing explanation, which refers the production of the 

quartz in these basalts to physical conditions apart from chemical ones, 

is correct, we should expect to find such anomalous associations of 

minerals in other varieties of volcanic rocks, and should not expect to 

find a necessary correspondence in the chemical composition of all 

basalts which carry porphyritical quartzes. Nor should we expect to 

find the quartz-bearing varieties, which are exceptional, necessarily 

holding a definite relation in point.of age to the other volcanic rocks 

with which they are associated. These expectations, I think, are realized 

by the following observations.

Porphyritical quartz in other volcanic rocks. The suite of rocks in the 

collection from New Mexico shows that similar quartz grains occur in 

most varieties of volcanic rocks from this region, and that their occurrence is not uniform throughout the series.

Thus in most of the rhyolites rounded or partially resorbed grains of 

quartz are very abundant, but in some specimens they are absent 

(obsidian and lithoidite). In the glassy mica-andesite, of which there 

is but one specimen, there are no quartz grains. In the horublendernica-andesites, of which there are ten different specimens, six show a 

considerable number of quartz grains, one shows a few, and three show 

none. Duplicate specimens show similar amounts of grains. In the 

hornblende-pyroxene-andesite, of which there are seven different specimens, one shows much quartz in grains, two considerable, one shows a 

few, and three show none; of the five different specimens of pyroxeueandesite, one shows a few grains of quartz, and four show none; of the 

twelve specimens of basalt, seven show much quartz in grains, and five 

show none. In other respects the specimens within each group resemble 

one another.

Another group of volcanic rocks, specially characterized by abundant 

rounded grains of quartz, is found in the vicinity of Crescent Peak, 

C/olorado. The collection of the U. S. Geological Exploration of the
30 VOLCANIC ROCKS OF TEWAN MOUNTAINS. [HULL. CO.

fortieth parallel contains thirteen specimens from this group, which 

range from basalt through andesite to what is probably trachyte. They 

all bear rounded grains of quartz, and some of the olivine-bearing varieties also carry hornblende paramorphs, which furnish additional evidence of a change of physical condition from one which induced the 

crystallization of certain minerals to a later one which partially resorbed 

them.

In these two localities, then, it would appear that the physical conditions which are supposed to have produced the crystallization of quartz 

in the basalts were also imposed on the magma or magmas from which 

the other varieties of rock were derived.

Chemical similarity of basalts with and without quartz. That the 

chemical composition of quartz-bearing basalts is not characteristic of a 

particular modification of rock magma will be seen from the accompanying analyses.

Ti02, titanic acid..... ..................

I.

52.57

1.49 

17.68

2.51

5.00

0.23

8.39

C. 05

0.06

1.58

4.19

0.82

100. 27

II.

52.37

l.GO 

17.01

1.44

5.89

0.32

7.59

6.86

0.06

1.59

3.51

1.29

0.37

99.90

III.

51.57

1.43 

17.72

6.24

1.78

0.45

8.82

4.91

0.16

1.93

3.59

0.64

0.58

99.88

IV.

52.38

1.22 

18.79

2.88

4.90

0.18

7.70

4.91

0.11

1.76

. 3.99

0.53

0.56

99. 91

V.

57. 25

0.60 

16. 45

1.67

4.72

0.10

7.65

6.74

0.00

1.57

3.00

0.00

0.40

0.20

100. 35

VI.

50.28

O.S4

14. '«!

4.69

4.05

0.10

6.37

1 OO

2 OR

0.93

T 

0.40

.17

300.-J8 

.04

100. 24

I. Quartz-bearing basalt (70), Rio Grande Canon, New Mexico. (L. G. Eakins.)

II. Quartz-bearing basalt (PI), Rio GraiideCaiion, New Mexico. (L. G. Eakius.)

III. Quartz-bearing basalt (71), Rio Grande Canon, New Mexico. (L. G. Eakius.)

IV. Basalt without quartz (64), Rio Grande Canon, New Mexico. (L. G. Eakins.)

V. Quartz-bearing basalt, Cinder Cone, Lassen Peak, California. (W. F. Hillebrand.)

VI. Quartzose diorite (2669). east slope of Electric Peak, Yellowstone National 

Park. (J.E.Whitfield.)

The first three are of three forms of quartz-bearing basalt from Kio 

Grande Canon; the first is a light gray dense basalt; the second, a 

greenish-black dense basalt; and the third, a dark red vesicular
IDDINGS.] CHEMICAL CONSIDERATIONS. 31

They have practically the same composition, with slight variations, 

which correspond to slightly different mineral composition. The higher 

oxidation of the iron in the red rock (III) is indicated by the high percentage of ferric oxide.

The fourth analysis is of a gray, dense basalt from the same locality, 

Rio Grande Canon, which resembles the basalt from which the first 

analysis was made, except that it exhibits no quartz in either macroscopic 

or microscopic grains. These four are normal basalt analyses, resembling one another as closely as analyses of similar rocks ordinarily do. 

There are no greater differences between the analysis of the variety 

without quartz and those of the quartz-bearing varieties than exist 

between the analyses of the three quartz-bearing varieties, so that the 

occurrence of the quartz in this instance can not be ascribed solely to 

the chemical composition of the magma.

Chemical differences between basalts with quartz. The fifth analysis is 

that published by Mr. Diller in his paper on the quartz-bearing basalt 

from near Lassen Peak, California. It shows a different chemical composition for this form of quartz-bearing basalt, which is more acidic than 

typical basalt, and corresponds closely to some andesitic forms of volcanic rocks.

It is probable that additional analyses of other quartz-bearing basalts 

will show as great a variation in their chemical composition as exists 

between that of basalts free from quartz grains.

Different mineral development of chemically similar magmas. The sixth 

analysis is presented for comparison with analysis V. It is that of a 

magma of very nearly the same chemical composition, slightly more 

basic, which has consolidated under different conditions. It may serve 

to illustrate two points: First, the mineralogical extremes to which chemically similar magmas may be developed. Second, the possibility that 

a basaltic magma existed at some previous period in a condition in 

which quartz might have been crystallized out. Analysis VI is of a 

coarse-grained quartzose diorite, perfectly fresh and unaltered, of quite 

recent geological age, and which is composed of plagioclase-feldspar, 

quartz, hornblende, biotite, pyroxene (hypersthene and augite), with 

accessory magnetite, apatite, and zircon. The quartz is in considerable 

quantity, very much more than the amount of quartz observed in quartzbearing basalts.

For a more extended discussion of this question and a fuller description of the diorite just mentioned the reader is referred to a recent 

paper on The mineral composition and geological occurrence of certain 

igneous rocks in the Yellowstone National Park, 1 in which the writer 

has shown that chemically identical magmas crystallize into mineralogically different rocks under the influence of different physical conditions.

i J. P. ladings, Phil, Soo, of Washington, 1890. .]Bull. ; vol. 11, pp. 191-220,
32 VOLCANIC ROCKS OF TEWAN MOUNTAINS. [HULL.66.

SUMMARY.

Iii conclusion, the principal points brought out by this study may be 

briefly stated as follows:

The collection of rocks from the Tewan Mountains, New Mexico, 

though small, shows a gradual transition in mineral composition from 

rhyolites through andestic rocks to basalts.

The whole series is characterized by a variable amount of porphyritical quartz in rounded grains, which is very noticeable in some of the 

basalts.

These quartzes are primary secretions or crystallizations from the 

molten magma, and exhibit no definite relation to its chemical composition, being present in or absent from rocks of similar chemical composition.

Their production is to 'be referred to certain physical conditions attending some earlier period of the magma's existence.

From analogy with the occurrence of iron olivine in rhyolitic obsidian 

it seems probable that the formation of primary quartz in basalt took 

place through the influence of water-vapor while the magma was under 

considerable pressure.
INDEX.

Pngo. 

Abiquiu ................................... 16

Andosite. ........... ..........:............ 9-12

pearlitic structure in................... 13-15

Andesite hornblende-mica ................. 12,13

Audesito horublende-p.yro.xeno............. 12,14

Andesite hypersthene...................... 10

Aiidesito mica ............................. 12,13

Audesite pitchstono........................ 14

Audesite pyroxene......................... 12-15

Andesito tufa .............................. 13

Arizona, quartz-basalt in................... 21

Ash, rhyolitie .............................. 12

Angite in andosito ......................... 14,19

Augito in rhyolito ....................... 10,12,19

Atigite shells about quartz............... 16, 20, 22

Basalt...................................... 9,10

quartz-bearing ...... .................. 10

chemical composition of................ 30

Basalt Peak................................ 23

Biotitoin anilesito.......................... 14

California, quartz-basalt in................. 21

Camel Monutain, Colorado ................. 23

Corro do las Navajas, Mexico .............. 23

Chryohydrates .--- -..-.-.................. 27

Cinder Cone, Lasseu Peak ...............21, 30, 31

Colorado, quartz-basalt in .................. 21,22

Coyote Canon .............................. 15

Coyote Creek .............................. 10

Crescent Peak, Colorado ................... 23, 29

Cross, Whitman............................ 10,22

Dacito .................. .................. 19

Daubr6o,M., cited.......................... 24,20

Dillor. J. S., cited...........................21,31

Diorite of Electric Peak ................... 30

Eakius, L. G., analyses by.................. 30

Elk Head Creek, Colorado.................. 22,24

Espafiola................................... .11

Euroka.Nev ............................... 23

Entectic substances........................ 27

Fayalite in obsidian........................ 24

Geographical distribution of quartz-basalt. 21 

Gueiss fragments in rhyolitie nsh.......... 12

Outline's experiments on chryohytlrates... 27 

Halloek,W.,citod.......................... 25

Hillebrand, "W. F.,analysis by.............. 30

Horu blende in andcsito .................... 14,19

Hornblende in rhyolito .................... 10,11

Hornblende-mica-andesite.................. 12,13

Horublende-pyroxene-andcsite............. 12,14

Hypersthenoin nndesite .................13,14,19

Bull. 06 3

Pago. 

Hyporsthene-andesite...................... 1C

Hyperstheno-bcariug basalt................ 19

Jem ez...................................... 15

Jemez Pueblo.............................. 10,15

Jemoz Springs ........................ ..... 10,11

Judd, J. W., cited .......................... 27

King, Clareuco.............................. 21

Lagorio, A., cited ......................25, 20, 27, 28

Lithoidito.................................. 11

Lithophysa)................................ 23,24

Mica in audesite .........................12,14,19

Micaiu rhyolite............................ 11,19

Mica-andesite.............................. 12,13

Middleton's Ranch......................... 10

Miueralogical gradations in the rocks...... 17

Miueralogical gradations, table of.......... 18

Mineralizing influence of absorbed watervapor .................................... 24, 28

Nevada, quartz-bearing basalt in........... 21

Obsidian..................................9,11, 23

Obsidian Cliff, Y.N. P.................11, 23, 20, 28

ObsidianHill .............................. 11

Olivine-bearing andesite ................... 19

Pearlitic structure in andesito ............. 13,15

PeladoPeak ............................... 13

Pitchstono, andesitic....................... 14

Polvadera Rancho ......................... 16

Pressure, effect of changes in .............. 25

Primary quart/- in basalt................... 20

Primary quartz in other rocks.............. 29

Pumice .................................... 11

Pyroxene-audosito ......................... 12,15

Quartz, porpbyritical 

in rhyolito ........................... 10,11, 29

in andesito........................13,14,15, 29

in basalt.......................1C, 17,20, 21,22

in trachyte............................. 30

Quartz, primary in basalt 

character of............................ 20,32

possible origin of .................... 23, 28, 32

its exceptional occurrence.............. 23

resorption of........................... 24

Eesorption of quartz phenocrysts .......... 24

Resorption of other phenocrysts ........... 25,30

Reyer, E., cited ............................ 25,28

Rhyolite..................................9,10, II

Rhyolitie ash .............................. 12

Rhyolitie tufa............................. 9,10,12

Rio Chama........... ..................... 16

Rio Grande Cafion ....................16,20,30,31

33
34 INDEX.

Page.

...... 10

...... 11

...... 11

...... 11

...... 14

...... 15

...... 21

Saturated solutions ........................ 26,28

Spherulites ................................ 11

Spherulitic rhyolite........................ 11

Sulphur Springs ...... .................. 13

Teall, J. J.H.,cited ........................ 27

Temperature, effect of changes in ......... 25

Te-yi....................................... 11

Sanidine with blue iridescence. 

Sail Antonio Springs...........

San Antonio Valley............

San Diego Canon...............

San Diego Creek...............

Santa Clara Canon.............

Santa Maria Basin, Arizona.

Page 

Tridymite with fayalite.................... 24

Tufa, andesitic............................. 13

rhyolitic ..............................9,10.12

Valle Grande .............................. 11

Viscosity affected by absorbed water vapor. 26 

Viscosity affected by physical changes..... 25

Warm Spring .............................. 14

Water vapor in obsidian ................... 11

Water vapor, influence of absorbed ........ 24,26

influence on viscosity.................. 26,27

influence on fusibility.................. 26

Whitehead Peak, Colorado................. 23

Wbitneld.J.E., analysis by ................ 30

Tridymito in andesite...................... 13