])KI'A IITM UNT OK TIIK INTKUIOlt

BULLETIN

UNITED STATES

GEOLOGICAL SURVEY

No. 1O9

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LIBRARY CATALOGUE SLIPS.

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

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

States | geological survey | no. 109 | [Seal of tlie department] | 

Washington | government printing office 11893

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

director | | The | eruptive aud sedimentary rocks | on | Pigeon 

point, Minnesota | and | their contact phenomena | by | William 

Shirley Bayley | [Vignette] | 

Washington | government printing office | 1893

8°. 121pp. 16 pi.

Bayley (William Shirley).

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

The | eruptive and sedimentary rocks | on | Pigeon point, Minnesota | and | their contact phenomena | by | William Shirley Bayley | [Vignette] |

Washington | government printing office ] 1893 

8°. 121pp. 16 pi.

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

Bulletin 109.]

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

The | eruptive and sedimentary rocks | on | Pigeon point, Minnesota | and | their contact phenomena | by | William Shirley Bayley | [Vignette] |

Washington | government printing office | 1893 

8°. 121pp. 16 pi.

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

Bulletin 109.]

[Bulletin No. 109.1

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

approved March 3, 1879, which declares that 

"The publications of the Geological Survey shall consist of the annual report of operations, geological 

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

and economic "geology aiid 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 bo issued in uniform quarto series if deemed necessary by the Director, but otherwise in ordinary octavos. Three thousand copies of each shall be published for scientific exchanges 

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

shall be the property of the United States and form a part of the library of the organization; and the] 

money resulting from the sale of such publications shall be covered,into the Treasury of the United 

States." ' *

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' (1,734) 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. I

ANNUAL REPORTS.

I. First Annual Keport of the United States Geological Survey, by Clarence King. 1880. 86. 79 pp. 

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

II. Second Annual Keport of the United States Geological Survey, 1880-'8l, by J. W. Powell. 1882. 

8°. Iv, 588 pp. 02 pi. 1 map.

III. Third Annual Keport of the United States Geological Survey, 1881-'82, by J. AV. Powell. 1883. 

8°. xviii, 564 pp. 07 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 Keport of the United States Geological Survey, 1883-'84, by J. W. Powell. 1885. 

8°. xxxvi, 4C9 pp. 58 pi. and maps.

VI. Sixth Annual Keport of the United States Geological Survey, 1884-'85, by J. W. Powell. 1885. 

8°. xxix, 570 pp. C5 pi. and maps.

VII. Seventh Annual Keport of the United States Geological Survey, 188S-'8G, by J. W. Powell. 1888. 

8°. xx, G5C pp. 71 pi. and maps.

VIII. Eighth Annual Koport of the United States Geological Survey. 188C-'87, by J. "W". Powell. 1889. 

8°. 2pt. xix, 474, xii pp. 53 pi. and maps; 1 p. 1., 475-1003 pp. 54-70 pi. and maps.

I.. Ninth Annual Keport of the United States Geological Survey 1887-'8S, by J. W. Powell. 1889. 

8°. xiii, 717 pp. 88 pi. and maps.

.. Tenth Annual Keport of the United States Geological Survey, 1888-'89, by J. "W. Powell. 1890. 

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

.I. Eleventh Annual Keport of the United States Geological Survey, 1889-'90, by'J. W. Powell. 1891. 

8°. 2pt. xv, 757 pp. CO pi. and maps; ix, 351 pp. 30 pi.

.II. Twelfth Annual Report of the United States Geological Survey, 1890-'91, by J. W. Powell. 1891. 

8°. 2pt. xiii, C75 pp. 53 pi. and maps; xviii, 576 pp. 140 pi. and maps.

.III. Thirteenth Annual Report of the United States Geological Survey, 1891-'92, by J. W. Powell. 

1893. 8°. 3pt.

MONOGRAPHS.

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

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

1882. 4°. xiv, 204 pp. 42 pi. and atlas of 24 sheets folio. Price $10.00.
II ADVERTISEMENT.

III. Geology of the Comstock Lode and tlie "Washoe District, with atlas, by George F. Backer. 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,4C4 pp. 

151. 29 pi. and maps. Price $1.85.

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

Fontaine. 1883. 4°. xi, 144pp. 541. 54 pi. Price $1.05,

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

pi. Price $1.20.

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

241. 24 pi. Price $1.10. .

IX. Brachiopoda and Lamellibrancbiata of the Earitan Clays and Greensand Marls of New Jersey, 

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

X. Dinocerata. 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 Lahontan, 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,486pp. 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.

XVII. The Flora of the Dakota Group, a posthumous work, by Leo Lesquereux. Edited by F. H. 

Knowlton. 1891. 4°. 400pp. 66 pi. Price $1.10.

XVIII. Gasteropoda and Cephalopoda of the Raritan Clays and Greensand Marls of New Jersey, 

by Robert P. Whitfield. 1891. 4°. 402pp. 50 pi. Price $1.00. >,

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

and C. R. Van Hiso. 1892. 4°. xix, 534pp. 37 pi.

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

In press:

XXI. The Tertiary Rhynchophorous Coleoptera of North America, by Samuel Hubb'ard Scudder.

XXII. A Manual of Topographic Methods, by Henry Gannett, chief topographer.

XXIII. Geology of the Green Mountains in Massachusetts, by Raphael Pumpelly, J. E. Wolff, 

T. Nelson Dale, and Bayard T. Putnam. 

In preparation:

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

 Sauropoda, by 0. C. Marsh.

 Stegosauria, by 0. C. Marsh.

 Brontotheridse, 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. Eramons.

 The Glacial Lake Agassiz, by Warten Upham

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 value 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, 

New York, to Bradford County, Pennsylvania, 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. 3884. 8°. 325pp. 

Price 20 qents.

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

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

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. 

VanHise. 1884. 8°. 56pp. 6 pi. Price 10 cents.

9. A report of work clone in the Washington Laboratory during the fiscal year 1883-'84. F. W. Clarke, 

chief chemist. T. M. Chatard, assistant chemist. 1884. 8°. 40 pp. Price 5 cents.
^DVEETISEMENT. Ill

10. On the Cambrian Faunas of North America. Preliminary studies, by Charles Doolittle Wulcott. 

1884. 8°. 74 pp. 10 pi. Price 5 cents.

11. On the Quaternary and Eeceut Mollusca of the Great Basin; with Descriptions of New Forms, 

by 11. Ellsworth Call. Introduced by a sketch of the Quaternary Lakes of the Great Basin, by G. K. 

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

12. A Crystallographic Study of the Thinolite of Lake Lahontan, by Edward S. Dana. 1884.' 89. 

34 pp. 3 pi. Price 5 cents.

18. Boundaries of the United States and of the several States aud Territories, with a Historical 

Sketch of the Territorial Changes, by Henry Gannett. 1885. 8°. 135 pp. Price 10 cents.

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

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

15. On the Mesozoic and Ccnozoic Paleontology of California, by Charles A. White. 1885. 8°. 33pp. 

Price 5 cents.

10. On the Higher Devonian Faunas of Ontario County, Now York, by John M. Clarke. 1885. 8°. 

80 pp. 3 pi. Price 5 cents.

17. On the Development of Crystallization in the Igneous Hocks of Washoe, Nevada, with notes on 

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

cents.

18. On Marine Eocene, Fresh-water Miocene, and other Fossil Mollusca of Western North America, 

by Charles A. White. 1885. 8°. 26pp. 3.pl. Price 5 cents.

19. Notes on tbo 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. Hillcbrand. 1885. 8°. 114 pp. 1 pi. Price 10 cents.

21. The Lignites of the Great Sioux Reservation. A Eeport on the Eegion between the Grand and 

Moreau llivors, Dakota, by Bailey Willis. 1885. 8°. 1C pp. 5 pi. Price 5 cents.

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

Price 5 cents.

23. Observations on the Junction between the Eastern Sandstone and tbo Keweenaw Series on Keweenaw Point, Lake Superior, by 11. D. Irving and T. C. Chamberlin, 1885. 8°. 124 pp. 17 pi. 

Price 15 cents.

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

Localities between Cape Hatteras and Cape Boquo, including the Bermudas, by William Healy Ball. 

1885. 8°. 336pp. Price 25 cents.

25. The Present Technical Condition of the Steel Industry of the United States, by Phineas Bames. 

1885. 8°. 85 pp. Price 10 cents. 

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

27. Eeport of work done in the Division of Chemistry and Physics, mainly during the fiscal year

1884-'85. 1886. 8°. 80pp. Price 10-cents.

28. The Gabbros and Associated Hornblende Eocks occurring in the neighborhood of Baltimore, 

Maryland, 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. 1880. 

8°. 41 pp. 4 pi. Price 5 cents. -

30. Second Contribution to the Studies on the Cambrian Faunas of North America, by Charles Doolittle Walcott. 1886. 8°. 369 pp. 33 pi. Price 25 cents.

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

32. Lists and Analyses of the Mineral Springs of the United States; a Preliminary Study, by Albert 

C. Peale. 1886. 8°. 235 pp. Price 20 cents.

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

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

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

35. Physical Properties of the 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°. 58pp. Price 10 cents.

37. Types of the Laramio Flora, by LesterF. Ward. 1887. 8°. 354pp. 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 aud 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 Gcncsee Section, New York, by Henry S. 

Williams. 1887. 8°. 321pp. 4 pi. Price 15 cents.

42. Report of work done in the Division of Chemistry aud Physics, mainly during the fiscal year

1885-'86. P. "W. Clarke, chief chemist. 1887. 8°. 152pp. 1 pi. Price 15 cents. *

43. Tertiary and Cretaceous Strata of the Tuscaloosa, Tombigbee, and Alabama Eivers, by Eugene 

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

44. Bibliography of North American Geology for 1880, by Nelson H. Darton. 1887. 8°. 35 pp. 

Price 5 cents.
IV ADVERTISEMENT.

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. Penroso, jr., with an Introduction by N. S. Shaler. 1888. 8°. 143 pp. 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 "Whitfleld. 1888. 8°. 84pp. Price 

10 cents.

48. On the Form and Position of the Sea Level, by Robert Simpson "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.

60. Formulas and Tables to facilitate the Construction and Use of Maps, by Robert Simpsou "Woodward. 1889. 8°. 124 pp. Price 15 cents.

51. On Invertebrate Fossils from the Pacific Coast, by Charles Abiathar "White. 1889. 8°. 102 

pp. 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 Barns. 1889. 8°. 313 pp. 

iiicl. 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°. 96pp. Price 10 cents.

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

pp. 7 pi. Price 10 cents.

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

Price 5 cents.

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

Frederick Wright, with an introduction by Thomas Chrowder Chamberlin. 1890. 8°. 112 pp. iiicl. 

1 pi. 8 pi. Price 15 cents.

59. The Gabbros and Associated Rocks in Delaware, by Frederick D. Chester. 1890. 8°. 45 pp. 

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 Harlowe Melville and Waldemar 

Lindgren. 1890. 8°. 40 pp. 3 pi. Price 5 cents.

62. The Greenstone Schist Areas of the Menominee and Marquette Regions of Michigan; a contribution to the subject of dynamic metainorphism in eruptive rocks, by George Huutington Williams; 

with an introduction by Roland Duer Irving. 1890. 8°. 241 pp. 16 pi. Price 30 cents.

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

species and a systematic arrangement of genera, by Anthony W. Vogdes. 1890. 8°. 177 pp. Price 

15 cents.

C4. A report of work done in the Division of Chemistry and Physics, mainly during the fiscal year 

18'88-'89. F. W. Clarke, chief chemist. 1890. 8°. 60 pp. Price 10 cents.

65. Stratigraphy of the Bituminous Coal Field of Pennsylvania, Ohio, and West Virginia, by Israel 

C. White. 1891. 8°. 212pp. 11 pi. Price 20 cents.

66. On a Group of Volcanic Rocks from the Tewan Mountains, New Mexico, and on the occurrence 

of Primary Quartz in certain Basalts, by Joseph Paxson Iddings. 1890. 8°. 34pp. Price 5 cents.

67. The Relations of the.Traps of the Newark System in the New Jersey Region, by Nelson Horatio 

Darton. 1890. 8°. 82 pp. Price 10 cents.

68. Earthquakes in California in 1889, by James Edward Keeler. 1890. 8°. 25pp. Price 5 cents.

69. A Classed and Annotated Bibliography of Fossil Insects, by Samuel Hubbard Scudder. 1890. 

8°. 101 pp. Price 15 cents.

7fl. Report on Astronomical Work of 1889 and 1890, by Robert Sirapson Woodward. 1890. 8°. 79 pp. 

Price 10 cents.

71. Index to the Known Fossil Insects of the World, including Myriapods and A.rachnids, by Samuel 

Hubbard Scudder. 1891. 8°. 744 pp. Price 50 cents.

72. Altitudes between Lake Superior and the Rocky Mountains, by Warren TJpham. 1891. 8°.» 

229 pp. Price 20 cents.

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

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

cents.

75. Record of North American Geology for 1887 to 1889, inclusive, by Nelson Horatio Darton. 1891. 

8°. 173 pp. Price 15 cents.

76. A Dictionary of Altitudes in the United States (second edition), compiled by Henry Gannett, 

chief topographer. 1891. 8°. 393 pp. Price 25 cents.

77. The Texau Permian and its Mesozoic Types of Fossils, by Charles A. White. 1891. 8°. 51 pp, 

4 pi. Price 10 cents.
ADVERTISEMENT. . V

78. A report of work done in the Division of Chemistry and Physics, mainly during the fiscal year 

1889-00. F.W.Clarke, chief chemist. 1891. 8°. 131pp. Price 15 cents.

79. A Late Volcanic Eruption in Northern California and its Peculiar Lava, by J. S. Diller. 1891. 8°. 

33 pp. 17 pi. Price 10 cents.

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

Price 20 cents.

81. Correlation papers Cambrian, by Charles Doolittle AValcott. 1891. 8°. 447 pp. 3 pi. Price 

25 cents.

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

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

84. Correlation papers Neocene, by W. H. Ball and G. D. Harris. 1892. 8°. 349 pp. 3 pi. Price 

25 cents.

85. Correlation papers The Newark System, by Israel Cook Russell. 1892. 8°. '344 pp. 13 pi. 

Price 25 cents.

86. Correlation papers Archean and Algonkian, by C. R. Van Hise. 1892. 8°. 549 pp. 12 pi. 

Price 25 cents.

90. A report of work done in the Division of Chemistry and Physics, mainly during the fiscal year 

1890-'91. F. W. Clarke, chief chemist. 1892. 8°. 77 pp. Price 10 cents.

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

10 cents,

92. The Compressibility of Liquids, by Carl Bar us. 1892. 8°. 96pp. 29 pi. Price 10 cents.

93. Some insects of special interest from Florissant, Colorado, and other points in the Tertiaries-of 

Colorado and Utah, by Samuel Hubbard Scudder. 1892. 8°. 35 pp. 3 pi. Price 5 cents.

94. The Mechanism of Solid Viscosity, by Carl Barns. 1892. 8°. 138 pp. Price 15 cents.

95. Earthquakes in California in 1890 and 1891, by Edward Singleton Holden. 1892. 8°. 31 pp. 

Price 5 cents.

96. The Volume Thermodynamics of Liquids, by Carl Barns. 1892. 8°. 100pp. Price 10 cents.

97. The Mosozoic Echinodormata of the United States, by William Bullock C-ark. 1893. 8°. 207 

pp. 50 pi. Price 20 cents.

98. Flora of the Outlying Carboniferous Basins of Southwestern Missouri, by David White. 1893. 

8°. 139 pp. 5 pi. Price 15 cents.

99. Eecord of North American Geology fpr 1891, by Nelson Horatio Darton. 1892. 8°. 73pp. Price 

15 cents.

100. Bibliography and Index of the Publications of the U. S. Geological Survey, 1879-1892, by Philip 

Creveling Warman. 1893. 8°. 495 pp. Price 25 cents.

101. Insect Fauna of the Rhode Island Coal Field, by Samuel Hubbard Scudder. 1893. 8°. 27 pp. 

2 pi. Price 5 cents.

103. High Temperature Work in Igneous Fusion and Ebullition, chiefly in relation to pressure, by 

CarlBarus. 1893. 8°. 57pp. 9 pi. Price 10 cents.

104. Glaciation of the Tellowstone Valley north of the Park, by Walter Harvey Weed. 1893. 8°. 

41 pp. 4 pi. Price 5 cents.

105. The Laramie and the overlying Livingston Formation in Montana, by Walter Harvey Weed, 

with Report on Flora, by Frank Hall Knowlton. 1893. 8°. 68pp. 6. pi.. Price 10 cents.

106. The Colorado Formation and its Invertebrate Fauna, by T. AV. Stanton. 1893. 8°. 288 pp. 

45 pi. Price 20 cents.

107. The Trap Dikes of the Lake Champlain Region, by James Furman Kemp and Vemon Freeman Marsters. 1893. 8°. 62pp. 4 pi. Price 10 cents.

108. A Geological Reconnoissance in Central Washington, by Israel Cook Russell. 1893. 8°. 108pp. 

12 pi. Price 15 cents.

109. The Eruptive and Sedimentary Rocks on Pigeon Point, Minnesota, .and their contact phenomena, by W. S. Bayley. 1893. 8°. 121 pp. 16 pi. Price 15 cents. 

In press:

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

110. The Paleozoic Section in the vicinity of Throe Forks, Montana, by A. C. Peale.

111. Geology of the Big Stone Gap Coal Field of Virginia and Kentucky, by Marius R. Campbell. 

In preparation:

 Earthquakes in California in 1892, by Charles D. Perrine.

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

 On the Structure of the Ridge between the Taconic and the Green Mountain Ranges in Vermont; and On the Structure of Monument Mountain in Great Barrington, Mass., by T. Nelson Dale.

 A Bibliography of Paleobotany, by David White.

STATISTICAL PAPERS. 

MineralResources of the United States, 1-8SC, by Albert Williams, jr. 1883. 8°. xvii,813pp. Prjce.

50 cents,
VI ADVERTISEMENT.

Mineral Resources of the United States, 1883 ami 1884, "by Albert Williams, jr. 1885. 8°. xiv, 1016 

pp. Price 60 cents.

Mineral Eesources of the United States, 1885. Division of Mining Statistics and Technology. 1886. 

8°. vii, 57C pp. Price 40 cents.

MiueralEesourcesof the United States, 1886, byfDavidT. Day. 1887. 8°. viii,813pp. Price 50 cents.

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

Mineral Eesources of the United States, 1888, by David T. Day. 1890. 8°. vii. 952pp. Price 50 cents.

Mineral Eesources of the United States, 1889 and 1890, hy David T. Day. 1892. 8°. viii, 671 pp. 

Price 50 cents.

Mineral Eesources of the United States, 1891, hy David T. Day. 1893. 8°. vii, 630 pp. Price 50 cents.

In preparation: 

Mineral Resources of the United States, 1892.

The money received from the sale of these publications, is deposited in the Treasury, and the Secretary of the Treasury declines to receive hank checks, drafts, or postage stamps; all remittances, therefore, must he by POSTAL NOTE or MONEY ORDEB, made payable to the Chief Clerk of the U. S. Geological Survey, or in CURRENCY, tor the exact amount. Correspondence relating to the publications of 

the Survey should he addressed

To THE DIRECTOR OF THE

UNITED STATES GEOLOGICAL SURVEY,

WASHINGTON, L'. C. 

WASHINGTON, D. C., September, 1898.
DEPARTMENT OF THE INTERIOR

BULLETIN

OP THE

UNITED STATES

GEOLOGICAL SURVEY

No. 1O9

WASHINGTON

GOVERNMENT FEINTING OFFICE 

1893

UNITED STATES GEOLOGICAL SU1WEY

J. W. POWELL, DIRECTOR

THE

ERUPTIVE AID SEDIMEITARY ROCKS

PIGEON POINT, MINNESOTA

THEIR CONTACT PHENOMENA

BY.

WILLIAM SHIELEY BAYLEY

WASHINGTON

GOVERNMENT PRINTING OFFICE 

1893

CONTENTS.

^'ageLetter of transmittal.......................................... ............. 9

Outline of paper ........................................................... 11

Preface................................................................... 13

Introduction.............................................................. 15

Kelation of Pigeon point to the surrounding region.................... 15

Previous work on the geology of the point............................. 16

Limitations of the present paper ...................................... 18

Chapter I. The general distribution of the rocks on Pigeon point and their

relations to each other ........................................ 19

The fragmental rocks ................................................. 19

The eruptives and their contact phenomena ........................... 22

The contact between the fragmeutal and other rocks .................. 27

Chapter II. Petrographical description of the olivine gabbro-and its altered

varieties .................................................... 32

The fresh oliviue gabbro .............................................. 32

The altered varieties of the gabbro ..................................... 38

Chapter III. Petrographical description of the diabase..................... . 44

The non-micaceous diabases ........................................... 44

The micaceous diabases ............................................... 47

Chap jr IV. Petrographical description of the red rock .................... 49

The granular rod rock................................................. 50

The porphyritic red rock............................................... 53

Comparison of the granular and porphyritic red rocks with each other and 

with rocks of the same character occurring in the Lake Superior region 55

Chapter V. Petrographical description of the rocks on the contact between

the red rock and the olivine gabbro.......................... 60

Chapter VI. Petrographical description of the fragmental rocks............ 67

The unaltered quartzites.................'............................. 67

The unaltered si ates and conglomerates................................ 71

The spotted rocks.........................:........................... 72

Chapter VII. Petrographical description of the rocks of the contact bolt

between the sedimentary and other rocks................... 78

The least altered of the contact rocks.................................. 82

The mottled rocks of the middle contact zone.......................... 85

The red quart/ites and mottled rocks in the inner zone of the contact 

belt ................................................................ 92

The inclusions in the gabbro and the red rock ......................... 98

Comparison with other contact rocks............. i................^... 102

Chapter VIII./Origin of the red rock ....................................'.. 105

Summary .................................................................. 114

ILLUSTRATIONS.

Pago.

PLATE I. Pigeon falls of Pigeon river ................................... 14

 - II. Gorge of Pigeon river below the falls .......................... 16

III. Outline map of Pigeon point and vicinity....................... 18

IV. Polished surface of mottled rock, characteristic of the contact belt 28

V. a. Thin section of olivine gabbro, showing the effect of pressure

upon a piece of augite...................................... 36

b. Thin section of altered olivine gabbro, with green isotropic

mineral surrounding apatite ............................... 36

VI. a. Thin section of altered olivine gabbro, in Avhich the olivino

has been entirely changed to pilite.......................... 40

b. Fibrous green hornblende surrounding compact brownish

yellow cores of the same mineral in altered olivino gabbro .. 40

VII. a. Thin section of altered olivino gabbro, showing long slender needles of hornblende that have attached themselves to 

compact pieces of the same mineral........................ 42

b. Pseudamygdnle of hornblende in altered olivino gabbro...... 42

VIII. a. Amygdnle in diabase........................................ 46

b. Poecilitic structure in diabase ............................... 46

IX. a. Thin section of granular red rock............................ 51

b. Thin section of porphyritic red rock.....................'.... 54

X. a, I. Idiomorphic augite in diabase .................. , .......... 62

c. Quartz grains1 in altered quartzite in natural light ........... 62

d. The same in polarized light.................................. 62

a. Thin section of intermediate rock on the contact between the

olivine gabbro and the red rock ............................ 62

/. Entile in chlorite of intermediate rock; reflected light....... 62

(j. Plagioclase surrounded by zone of micropegmatitc, in porphyritic red rock; polarized light.......................... 62

h. Spot in thin section of spotted quartzitc......... '. .......... 62

XI. a. Thin section of indurated quartzite in polarized light....... 68

b. Thin section of altered quartzite from the outer zone of the

contact belt; polarized light............................... 68

Xli. a. Thin section of red mottling in mottled rock from the middle

zone of the contact belt.................................... 86

b. Thin section of ground mass of mottled rock from the middle

zone of the contact belt.................................... 86

XIII. a and b. Thin section of mottled red quartzite from the inner

contact zone, a, in natural light, b, in polarized light..... 94

XIV. Geological map of Pigeon point................................ 108

XV. Geological map of Pigeon point ................................ 110

XVI. Geological map of Pigeon point................................ 112

. . 7
8 ILLUSTRATIONS.

Page. 

FIG. 1. Sketch of the side of a cliff, showing crumpling of slates and qnartzite by the intrusion of an eruptive beneath them................... 20

2. Contact of olivine gabbro and quartzite, showing crumpling of latter. 20

3. Contact of porphyritic red rock with olivine gabbro at the base of a

bluff a little to the west of the Little Portage...................... 24

4. Sketch of faulted dike in olivine gabbro............................ 26

5. Sketch of portion of the contact belt, showing included fragments in

rock exhibiting apparent flow structure ........................... 29

6. Idiornorphic secondary magnetite in altered diabase ................ 45

- 7. Skeleton crystal of apatite in altered diabase....................... 47

8. Sketch of cliff side, showing regular parting produced in red rock by

weathering....................................................... 50

9. Grouping of hornblende crystals in one of the intermediate rocks.... 61

10. Shapes and groupings of spots on the surface of spotted rocks ...... 72

11. Epidote crystals in spots of spotted rocks .......................... 74

12. Broken crystal of quartz in red portion of mottled quartzite........ 86

13. Corroded quartz crystals in red quartzite on contact with granular

red rock........................................................... 93

14. Characteristic forms of zircon crystals occurring in the mottled rocks 95

15. Zircon crystal extending through portions of three quartz grains.... 95
LETTER OF TRANSMITTAL.

DEPARTMENT OF THE INTERIOR,

U. S. GEOLOGICAL SURVEY,

LAKE SUPERIOR DIVISION,

Madison, Wis., July 1,1892.

SIR : I have the honor to transmit herewith the manuscript arid illustrations of a paper upon " The Eruptive and Sedimentary Hocks on 

Pigeon Point, Minnesota, and Their Contact Phenomena," by Assistant Geologist W. S. Bayley, to be published as a bulletin of the Survey.

This work had been planned and in good part executed while the 

division was under the charge of the late Prof. Roland D. living. H-is 

wide knowledge of the Lake Superior region enabled him to pick out 

Pigeon point as a locality in which exceptional relations exist between 

eruptive and sedimentary rocks, and which was therefore worthy of 

detailed study.

I am, sir, very respectfully, yours,

C. E. VAN HISE, .

Geologist in Charge. 

Prof. G. K. GILBERT,

Chief Geologist, Washington, D. C.

OUTLINE OF THIS PAPER.

Pigeon point is the northeastern extremity of Minnesota. It is one of a series of 

parallel points extending from Minnesota and Canada eastward into Lake Superior. 

Its backbone is a great east and west dike-like mass of a gray, coarse grained rock 

that has always been called gabbro. This consists of phenocrysts of plagioclase in 

a diabasic grouudmass of the same mineral, olivine and diallag5, and, consequently, 

it is a diabase porphyrite.

Upon alteration the gabbro gives rise to phases that differ in their general aspect 

from the normal rock. The olivine and diallaige pass into chlorite, biotite and hornblende, of the latter of which three varieties may be recognized. When compact 

brown amphibole originates in this manner the resulting rock resembles Irving's 

hornblende gabbros. The plagioclase of the gabbro changes into chlorite, quartz, 

and a reddish feldspar^of which the last two mentioned minerals are often in 

micropegmatitic intergrowths. The change of the plagioclase into red feldspar is 

in all probability a contact phenomenon.

The rocks through which the. gabbro cuts are evenly bedded slates and indurated 

sandstones of Animikie ago. They dip sonth-sontheast at 15° to 20°, except at a 

very few places near the contact with other rocks, where they are more or less conr torted.

Small dikes also intersect these sedimentaries. Their width varies from an inch 

or two to 60 feet, and their material is a non olivinitie diabase that is often micaceous.

The most interesting features in the geology of the point relate to the series of rocks 

usually occurring between the gabbro and the clastic beds. Beginning on the gabbro side the series comprises in succession coarse-grained red rocks, a fine-grained 

red rock that is sometimes porphyritic and a well-marked belt of altered quartzites.

The fine-grained red rock has all the characteristics of an eruptive. It sends 

dikes into the contiguous bedded rocks, and consists essentially of a hypidiomorphic 

granular aggregate of plagioclase, anorthoclase, and quart/. The quart/ and 

anorthoclaso often form micropegmatite, while the plagioclase is in comparatively 

large grains, some of which have badly defined idiomorphic outlines. At a few 

places this red rock is porphyritic, v\ ith bipyramidal quartz crystals imbedded in 

a red grauophyric gronndmass. The rock is similar to many of the angite-syenites 

described by Irviug as occurring in the Keweeuawaii series, and is in structure and 

composition a quartz-keratophyre.

The coarse-grained rocks between the gabbro and the keratophyre are intermediate in character between these two. The variety nearest the gabbro differs but 

slightly from the basic eruptive. In addition to the gabbro components it contains 

a little quartz and red feldspar constituents derived from the keratophyre. As the 

latter rock is approached the augite, olivine, and plagioclaso disappear, while increased quantities of quartz, red feldspar, and brown hornblende make their appearance, and the rock becomes more and more like the fine-grained red rock. Finally 

the hornblende disappears and the keratophyre is reached. Since the intermediate 

rocks occur only between the gabbro and the fine-grained red rock, and since all

11
12 - OUTLINE OF THIS PAPER,*

gradations in composition .between the two end members of the series are represented, the coarse-grained red rocks are regarded as contact products formed by the 

intermingling of the gabbro and the keratophyre magmas. In general peculiarities 

they are identical with some of Irving's orthoclase gabbros.

In the undoubted contact belt between the keratophyre and the unaltered slates 

and quartzites three zones are distinguished. In the exterior zone (that nearest the 

unaltered elastics) the rocks differ but little from the corresponding unaltered forms. 

The enlargements of the quartz grains, that are so marked a feature in the latter, 

have been lost, and with them all traces of the original fragmental character of the 

grains. The quartzes now interlock by irregular sutures. Bleached biotite, sericite, and chlorite, are the only new minerals detected. The original regular bedding 

has not been disturbed.

In the middle zone of the contact belt the rocks are irregularly mottled in green 

and red. Sections made from the red portions of these rocks show rudely outlined 

bipyramidal quartz crystals in a matrix of globulitic red feldspar or of granophyre. 

The red feldspar, which is principally orthoclase,_is much more abundant than it is 

in the rocks of the outer zone. The green portions 'contain but little feldspar. In 

composition and structure they are like the members of the exterior zone.

In the inner zone the rocks are of a uniform bright-red color, or they are bright 

red, spotted here and there with large green circular spots; In the spots the quartz 

grains interlock as in the least altered of the contact rocks. In the red ground mass. 

on the contrary, nearly all the quartz particles have the form of crystals, whose con 

tours are more frequently than otherwise broken by embayments of the feldspathic 

mass in which they lie. Even in this, the zone of most intense action, but few new 

minerals are discovered. Long acicular zircons often occur traversing two or three 

quartz grains, thus showing plainly their contact origin, and bundles of sericite are 

not infrequently scattered through the red feldspathic matrix.

From the above-mentioned facts it is concluded that the contact belt represents 

Animikie slates and quartzites that have been altered near their contact with an intrusive rock. The metamorphism of the quartzites has resulted simply in the recrystallization of the quartz and feldspar of the fragmental grains, with the addition, 

perhaps, of'a little orthoclase.

Since, in several instances, the gabbro is in direct contact with the metamorphosed rocks, while the keratophyre is not to be found in the neighborhood, it is inferred that the former rock and not the Litter was the cause of the contact action.

Inclusions of fragmentals in the gabbro and the keratophyre have alike suffered 

the same alterations as have taken place in the various members of the contact belt, 

with this difference, that quartzite inclusions in. the basic rock are of ten surrounded 

by a rim of red rock, identical in all its properties with the keratophyre. This suggests that the keratophyre itself may be of contact origin.

The question that now arises is this: Is the keratophyre a genuine intrusive be- 

 tween the gabbro and its contact belt, or is it merely an extreme phase in the alteration of the clastic rocks by the gabbro?

The eighth chapter of the bulletin is devoted to the discussion of this question. 

The conclusion reached is that, in all probability, the keratophyre is of contact 

origin that is, it was produced by the -fusion of the slates and quartzites of the 

Animikie through the action upon them of the " gabbro." The magma thus formed 

then acted in all respects like any intrusive magma. It penetrated the surrounding 

rocks in the form of dikes, and solidified as a soda-granite under certain circumstances, and under others as a quartz keratophyre.
PREFACE

The preparation, of this paper is clue to the fact that the eruptive rocks 

occurring' on Pigeon point, Minnesota, may be taken as types of similar 

eruptives found so widely spread over the Huronian and Keweenawan 

areas surrounding Lake Superior, and to the peculiar nature of the 

'alteration these have superinduced in the fragmental rocks through 

which they have broken.

The materials for the study were obtained during two field seasons 

Jincler "the auspices of the U". S. Geological Survey. The first visit was 

made during the summer of 1885 in. company with Dr. George H. Williams, of the Johns Hopkios University, to whose carefully prepared 

notes the writer owes much for whatever success has attended his study 

of the region. The second visit occupied about three weeks in the summer of 1888.

The laboratory investigation of the specimens collected was begun 

at the Johns Hopkius University in the winter of 18S5-'86. It was continued at the University of Wisconsin during the winter of 18S7-'8S, aud 

has been completed at Colby University, during such time as could be 

spared from other duties.

Thanks are due first of all to Dr. Williams for the suggestions offered 

during the earlier part of the study; to the late Prof. E. D. Irving, 

United States geologist, for his kindness in patiently explaining the 

lines which the investigation should pursue in order to be of general 

value; and to Prof. 0. E. Yan Hise, of Madison, Wisconsin, for his 

liberality in affording every opportunity for the comparison of the Pigeon point rocks with those from other localities in the Lake Superior 

region, aud in providing everything that could be of use in advancing 

the work.

The writer would also acknowledge his obligations to Dr. Charles 

Barrois, of -Lille, France, for his examination of thin sections of some 

of the sedimentary rocks, aud their comparison with those of the altered 

quartzites 1 of Brittany; to Dr. K. de Kroustschoff, of Breslau, Germany, for his opinion in regard to the nature of the quartz of the altered quartzites; and to Mr. W. F. Hillebrand, Dr. E. B. Eiggs, Mr. 

J. E. Whith'ehl, and Mr. L. G. Eakins, chemists of the U. S. Geological 

Survey, for the careful analyses, which have been of great value in 

discussing the nature of the contact rocks.

w. s. B.

COLBY UNIVERSITY, Watermlle, Maine.

1 Sur Ie8 gr6s mfitamorphiques du massif granitique clu Gu6m6n6, Ann. d. 1. Soc. g6ol. du Nord., 

1884, vol. XI, p. 103.

13

U. 8. GEOLOGICAL SURVEY 

PIGEON FALLS, PIGEON RIVER MINNES

The stream plunges over a vertical cliff produced by joint cracks in gabbro, The rocks in the foregro
THE ERUPTIVE AND SEDIMENTARY ROCKS ON PIGEON 

POINT, MINNESOTA, AND THEIR CONTACT PHENOMENA. '

BY WILLIAM SHIRLEY BAYLEY.

INTRODUCTION. 

RELATION OF PIGEON POINT TO THE SURROUNDING REGION.

. Pigeon, point is the name given to the northeastern portion of Minnesota, embraced within sections 25, 26, 27, 28, 29, 30, 31, and 32, T. 

64 K, R. 7 E. of the fourth principal meridian. It extends from the 

main shore N. 77° E., into Lake Superior (PI. in). Pigeon river and 

Pigeon bay separate it from Canada on the north; on the south it is 

washed by Lake Superior, and on the west by the waters of Wauswaugoning bay. Measured from the eastern, side of Wauswaugoning 

bay to its easternmost extremity it is 5£ miles in length. Its width 

varies from a few hundred feet to as much as 1 mile in its western 

part, its average width being somewhere between a quarter and a half 

mile.

It is isolated from the mainland by a stretch of low swampy country, which runs from Wauswaugoning bay northeasterly to Pigeon river. 

The traces of former coast lines in the interior show that the ,swamp 

was at some earlier time covered with water, and that the point was 

consequently an island, or, rather, a group of islands, since all the lowlands on the point contain the remains of raised beaches.

One of these areas of lowland, formerly a strait, but now an isthmus, 

is situated about a mile and a half from the eastern end of the point. 

It is a narrow shingle beach, about 200 feet wide, over which the Indians are accustomed to portage in order to avoid the large stretch of 

Open water at the mouth of Pigeon bay, and to which they have given 

the name of il Little Portage," This isthmus affords a convenient 

means of dividing the point (for purposes of description only) into an 

eastern and a western portion. The eastern portion is heavily wooded 

In the interior, and is so thickly covered with surface material that it 

is only with the greatest difficulty that the relations of the different 

rocks to each other can be determined. In the western portion, on the

15
16 ROCKS ON PIGEON POINT, MINNESOTA. [BULL. 109.

other hand, the rocks are well exposed in some places, in consequence 

of the destruction of the forest by fire. In other places they are concealed by thick windfalls, through which it is almost impossible to 

penetrate, and in the extreme western part by a second growth of low 

scrnbby balsams and poplars. In this latter area, west of a line-drawn 

north from Fishermans point, the representation of the distribution of 

the rocks is only approximately correct.

In the preparation of the geological map accompanying this report 

(Pis. XIY, xv, and xvi) sections were made across the point at intervals 

of 100 and 200 paces1 in its eastern portion, and of 100, 200, and 300 

paces in its western part. This means that from 7 to 20 traverses have 

been made for each mile.

A glance at any good map 2 of Lake Superior will show that Pigeon 

point is one of a series of parallel points extending from the main land 

a distance of 1 or 2 miles easterly into the lake. These points consist 

in greater part of large dikes of a heavy, dark, basic rock, which is, as 

far as known,3 a fresh olivine-gabbro, very much like that occurring 

on Pigeon point. They are offshoots of an immense mass of the same 

gabbro, which, beginning on the north side of Grand Portage bay, 

runs northeasterly, crossing Pigeon river at the'falls and extending as 

a high ridge far into Canada. The junction of the Pigeon point mass 

with that of the main ridge can not be seen, since it is separated from 

the latter by Wauswaugomng bay and the lowland extending from the 

head of this bay to Pigeon river. From the nature of the prevailing 

rock -in this mass, however, it seeins to be beyond dispute that it is 

merely a branch of the main ridge, similar to those branches which 

give rise to the parallel points which project from the lake shore for 16 

miles northeast of the national boundary line.

PREVIOUS WOKK ON THE GEOLOGY OF THE POINT.

The first recorded description of the rocks of Pigeon point is that 

given by Maj. Eichard Owen,4 who, on the 17th of July, 1849, under 

instructions from Dr. Norwood, made a trip in a canoe from Grand 

Portage bay to the mouth of Pigeon river. Maj. Owen reports the 

point as consisting principally of a number of parallel trap dikes, with 

a trend a little north of east. These cut slates and sandstones, altering them in places, and are themselves cut by smaller dikes of a 

slightly different character. Many of Maj. Owen's observations are 

valueless. His recorded dips for the sedimentary rocks can not be 

verified, and he is mistaken in supposing the area covered by these to 

be so large.

1 A mile contains 2,000 paces.

2 Sketch map of the Thunder Bay mining region, accompanying report on mines and raining on 

Lake Superior, byE. D. Ingall, M. E. Ann. Rep. Geol. and Nat. Hist. Survey of Canada for 1887, partH.

3 Monograph, U. S. Geol. Survey, vol. v. The copper-hearing rocks of Lake Superior; B. D. Irving, 

pp. 371-372.

4 Keport of a geological survey of Wisconsin, Iowa, and Minnesota, by David Dale Owen, Philadelphia, 1852, p. 396.
U. S. GEOLOGICAL SURVEY 

GORGE OF PIGEON RIVER BELOW THE FAL

From the top of tne falis, looking southeast. The rock in the immediate foreground is gabbro, as is also that in the heav

horizontally bedded near masses of gabbro.
BAYLEY.] PREVIOUS GEOLOGICAL WORK. 17

In 1851 Messrs. Foster and Whitney, 1 on the authority of Prof. W. 

W. Mather, announced the occurrence of a large quantity of granite in 

the eastern portion of the peninsula, and consequently declared the 

age of the rocks there to be Azoic.

The next mention of the point was made by Prof. N. H. Winchell.2 

Prof. Winchell collected specimens along the coast from Wauswaugoning bay to the falls of Pigeon river, and also along the trail leading 

from the former place to Parkerville, a small settlement on the Pigeon 

river about three-quarters of a mile from its mouth. A second visit 3 

to the point was made in the succeeding year, but the relation of the 

rocks to each other was not definitely determined, although Prof. Wiuchell thought there was evidence of a metamorphic effect produced on 

the slates and qiiantzites by the eruptive masses intruding them. The 

rocks collected on these two trips were not studied in detail until some 

years later, when, in 1887, Dr. M. E. Wadsworth 4 investigated the 

specimens microscopically, and published his descriptions in a bulletin 

of the Minnesota survey. References to these descriptions will be 

made in other portions of this paper.

In 1881 Prof. Irving 5 examined the rocks under consideration during 

a trip along the north shore of Lake Superior, in connection with his 

discussion of the copper-bearing series. He describes the point as 

made up essentially of quartzites and slates cut by dikes of gabbro and 

diabase. The fragmeutal rocks he regards as belonging to Hunt's 

Aniuiikie" series, which occupies the stratigraphical position of Logan 

and Murray's 7 original Huronian. The granite described by Foster and 

Whitney he h'uds to be intrusive in the fragtneutals.

In 1888 Mr. E. D. Ingall 8 published a report on mines and mining 

on Lake Superior, and in it recorded some facts in relation to the geology of the region north a.nd east of the point. The report is accompanied by an appendix, in which the writer gives a brief account of the 

microscopical characteristics of some of the most interesting of the 

rocks occurring on the islands to the east of Pigeon point, a few of 

which show some of the peculiarities of the Pigeon point contact rocks.

Finally there have appeared two papers by the writer 9 treating of 

certain phases of the contact phenomena observed on the point.

'Report on tlie geology of the Lake Superior land district, Washington, 1851, Pt. H, pp. 12, 37.

2 The geological and natural history survey of Minnesota. Ninth Annual Report. St. Petor, 1881, 

pp. 03-71.

3 Ibid, Tenth Annual Keport, St. Paul, 1882, pp. 48 and 56-59.

4 Preliminary description of the peridotytes, gabhros, diabases and andcsytcs of Minnesota. Bulletin No. 2, pp. 29,53,87, 96,105,108, and 120.

 Monograph, TJ. S. Geol. Survey, vol. v. The copper-bearing rocks of Lake Superior, R. D. Irving, 

pp. 368-370.

"Trails. Am. Jnst. Min. Eng., Vol. 1, p. 33!).

' Geol. of Canada. 1863, pp. 50-60. For a full discussion of the ago of the Animikio series, see B. D. 

Irving, Am. Jour. Sci., Vol. 31, pp.259-262.

8 Geol. and Nat. Hist. Survey of Canada. A innial Report for 1887, Part H.

!l On some peculiarly spotted rocks from Pigeon point,-Minnesota; Am. Jour. Sci., May, 1888, p. 388; 

and. A quartz-keratophyro from Pigeon point and Irviug's augito-sveuit.o.3.- Ibid., 188i), j>. {vK

09 2
18 ROCKS ON PIGEON POINT, MINNESOTA. [BULL. 109. 

LIMITATIONS OF THE PRESENT PAPER*.

In the present paper attention will be confined to that piece of high 

land south and-east of the swamp referred to as separating Pigeon 

point from the main land. This area is isolated, not only in its position, but also in its geological features.' It exhibits throughout nearly 

its entire extent a series of slates and quartzites that have been altered 

by eruptive rocks which intrude them. The alteration which these 

fraginental rocks have undergone presents features which have not 

been observed in similar rocks elsewhere. Even in the Lake Superior 

region it is confined entirely, so far as known, to the rocks occurring 

on Pigeon point and on a few small islands lying some 10 miles northeast of it. 'No evidence of similar alteration has been discovered on 

the main laud, although large masses of eruptives have every where cut 

through clastic rocks of the same geologic age. In addition to the 

altered rocks there is'also on the point a large quantity of a red drusy 

granitic rock, which is different from anything in the Animikie area to 

the north and east (with the exception of a rock occurring in a small 

quantity on the islands referred to above), but which is very similar to 

many of the interbedded flows in the Keweenawan series.

For these, reasons it has been deemed best to treat only of those 

phenomena observed on the point, leaving the geology of the neighboring region for some futurc.publication.
U.S.GEOLOGICAL SURVEY. B

CANADIAN?

MAP OF PIGEON" POINT AND VICINITY.

Topography compiled from Lake Survey Charts and Land Office Plats

Scale

Contour Interval 50 feet. 

1891.
CHAPTER I.

THE GENERAL DISTRIBUTION OF THE ROCKS ON PIGEON POINT 

AND THEIR RELATIONS TO EACH OTHER.

The rocks on Pigeon point consist of a large mass of dark, heavy, 

olivine-gabbro; a light, clrusy, red rock5 diabase; fraginental rocks; 

and those which owe their origin to certain phases of contact action,

THE FRAGMENTAL ROCKS.

The fraginental rocks embrace, in the^ main, evenly bedded slates 

and quartzites, dipping at 15°-20° SSE., and striking nearly parallel to 

the axis of the point. They are cut by two systems of nearly vertical 

joint cracks at right, angles to each other, along which the parting is 

so'perfect that in many places the fragmental beds have the appearance of courses of masonry, in which the individual blocks have been 

squared and trimmed with the greatest nicety. These rocks are exposed on the south side of the point throughout almost its entire length. 

On its .north side they occur in but'two places (see map, PI. xiv), and 

at both of these they have the normal southerly dip and easterly strike. 

The northern dip recorded by Owen 1 as existing here could not be 

found.

The displacements caused in these strata by the eruptions through 

them appear to have been very slight. No such general disturbances 

as those reported by Maj. Owen Avere seen anywhere. At only three 

places was any variation from the noruia 1 position of these rocks 

observed, and in each of these the variation Avas found to be due 

entirely to local causes. The first place where a deviation from the 

usual strike and dip was noted is on the south side of the point, just 

east of the little isthmus about a mile and a half from its, extremity. 

-Here a large diabase dike, running parallel to the shore, about 100 feet 

from the water's edge, has broken through the slates and quartzites 

and crumpled them in a most remarkable manner. The rocks are bent 

into many short, sharp folds, which are crowded together in the vicinity of the dike, but gradually disappear as they recede from it.

About three-quarters of a mile west of the isthmus, on the same side 

of the point, is a beautiful exposure of gently bowed quartzites in the 

side of a cliff at the water's edge. Here layers of evenly bedded green 

slates and thicker beds of massive, jointed^ brown quartzites have been

. ' Geul. Survey of Wisconsin, Iowa, and Minnesota, p. 397.

19
20 ROCKS ON PIGEON POINT, MINNESOTA. [BULL. 109.

raised and thrown into gentle flexures by the intrusion under them of 

a mass of red rock, which has sent out apophyses into the overlying 

fragmentals, and has slightly altered them. (Fig. 1.)

The third point at which local disturbance was noticed is in the side 

of a cliff in the interior of the point, about 300 feet north and 125 feet 

east of the barite vein, whose position is indicated on the map (PI. Xiv). 

At this place the fragmental beds and the gabbro have come into conWa ier L ine]f

Slate\ \Intrusive RedRock

PIG. 1. Sketch of the side of a cliff, showing crumpling of slates and quartzites by the intrusion of

an eruptive beneath them.

tact, and in such a way that the former have lost their even bedding 

for a distance of about 3 feet from the latter. (Fig. 2.)

No other instances of contortion in the bedded rocks or of deviation 

from the normal southeasterly dip of 15° to. 20° were observed on the 

Point, although close search was made for the vertically tilted beds 

pictured by Maj. Owen in his report on the region.

FIG. 2. Contact of oliviue gabbro and quartzite, showing crumpling of latter.

On the top of the high hill on the Canadian side of Pigeon river, 

near the falls (see map, PI. in), there are twisted and contorted slales, 

some of which dip at a high angle to the north. But even here the 

northerly dip is maintained over only very small areas, for at the base 

of the same hill and along its southern side (see PI. u) the rocks are 

again found with a low dip to the southeast. In this case the disturbing cause was the large dike over which the river falls.
BAYLEY.] THE PRAGMENTAL ROCKS. 21

The dip of 15° to 20° to the southeast is not confined to the fragin en tal rocks on Pigeon point. It is the prevailing dip for all of the 

Animikie beds in this region, as was shown by living, 1 and is in nc 

way due to the Pigeon point eruptives, as was supposed by Owen.

Interbedded with the quartzites and slates are certain layers in which 

there are numerous light-colored and green spherical bodies, "which in 

cross section appear as spots on the surfaces of the rocks. 'In some 

areas these spots consist in large part of calcite; in others they are 

composed principally of epidote. The distribution of these spotted 

rocks with reference to the eruptives could not be precisely determined, 

because the beds in which they occur are exposed only on the lake shore, 

where they are found underlying unspotted beds, which spread over 

the former and cover them from view as we proceed from the water's 

edge toward the eruptive masses. These rocks have been designated 

u spotted rocks" to distinguish them from the "mottled rocks," which 

will be described later as characteristic of the contact belt occurring 

between the quartzites and the red rock. They are supposed to be due 

to the existence of calcite concretions in the beds, which have giveii 

rise to the quartzites upon silicification. Their origin will be explained 

in Chapter vi.

In addition to numerous dikes cutting the slates and quartzites 

there is a system of veins2 penetrating them in aNNW. direction across 

the prevailing trend of the dikes. These veins are filled with white 

barite, crystalized calcite, and amethystine quartz, with occasionally a 

little chalcoyrite or other compound of copper, which is never found 

in sufficient quantity to repay the expense of working. On Susie 

island (PI. in), however, there is an extension of one of the Pigeon 

point veins, from which a quantity of salable ore has been extracted. 

At present (1888) the mine is shut down, no work having been prosecuted for the last few years. On the point itself is a large vein exposed 

near the water's edge, on the south shore, about midway between Little 

Portage bay and Fisherman's point (see maps). This was worked for 

barite a few years ago, but evidently without success.

The Pigeon point veins form a part of a great system cutting the 

Animikie rocks throughout their entire extent. A careful description 

of this system has recently been given by Mr. E:D. Ingall3 of the Canadian Geological Survey, and by him the relations between the veins 

and the joint cracks occurring in the sedimentary beds have been ably 

discussed.

'Monograph, U. S. Geol. Survey, vol. v. The Copper-Bearing Rocks of Lake Superior. R. D. 

Irving, pp. 307-390.

Tor a, description of the independent veins occurring on the point, vide N. H. Winchell: Geol. and 

Nat. Hist. Survey of Minnesota, Seventh Ann. Rcpt. 1878, pp. 15-10; Ninth Ann. Rept., 1880, pp. 04- 

07. '

3 Annual Report for 1837, Part H.
22 ROCKS ON PIGEON POINT, MINNESOTA. [BULL. 109. 

THE EBUPTIVES AND THEIR CONTACT PHENOMENA. j

The heavy dark gabbro constitutes by far the greater part of 

the point. It is included in two principal areas (Pis. xiv, XV and 

xvi). In one of these it extends as a series of hills from the eastern 

side of the swamp between Wauswaugoning bay and Pigeon river, 

with few interruptions, to the eastern end of the point. It is best 

exposed on the north shore, where it forms perpendicular cliffs, 

some of "which are over 100 feet in height. Their perpendicularity 

is clearly the result of the existence of vertical joint cracks in the 

rocks composing them. Under the influence of even the slighest 

amount of weathering these cracks begin to open, and before the 

rock has undergone any considerable alteration, Jarge square or 

oblong blocks break off and tumble from the- sides of the cliff to its 

foot, thus exposing successively to the action of the waves new vertical 

faces of rock. At the tails of the Pigeon river the joint cracks have a 

direction S. 79° W. and N. 11° W., so that the loosening of large masses 

of the rock from the barrier of the falls results in the formation of a 

perpendicular wall over which the river plunges vertically to a depth 

of about 120 feet (see frontispiece), taking a second smaller plunge over 

a trap dike, which runs parallel to the larger dike at a distance of about 

50 yards east of it. The large dike has already been mentioned as the 

one which begins north of Grand Portage and extends in a northeasterly direction into Canada. In the interior of the point the gabbro 

occupies all the higher portions, and along its southern edge presents 

a steep face to the south.

The second gabbro area begins at Wauswaugoning bay and runs 

parallel to the first area with many interruptions for about a mile and 

a half, ending a few hundred yards west of Morrison's bay. Besides 

these are two or three isolated small areas, whose position may be 

learned by reference to the map.

The most prominent features of these gabbro masses are those of 

dikes. As has already been mentioned, the larger one in many places 

presents perpendicular walls both to the north and to the south. It 

occupies all the highest portions of the point, and these are in a straight 

line. It has'the appearance of an intrusive mass, and is like any one 

of those forming the numerous points to the ndr'th of the international 

boundary line. It has been regarded as a dike by both Irving l and N. 

JET. Winch ell.2 Its contact with the sedimentary rocks is only occasionally to be seen. At several of these contacts the eruptive has the 

appearance of having escaped from between the dike walls and thrust 

itself for a short-distance between the frag mental beds, or of having 

piled itself up around the dike orifice and overlapped the intruded rocks. 

The gabbro in the cliff pictured in Fig. 2 is a coarse-grainedporphyritic

'Monograph, vol. v, TJ. S. Geol. Survey. The Copper-Bearing llocks of Lake Superior, E. D. Irving, 

p. 370. 

2 Ninth Annual Report of the Geol. and. Nat. Hist. Survey of Minnesota, p. 69.
BAYLEY.I THE ERUPTIVE ROCKS. 23

rock in which the porphyritic crystals of feldspar are arranged in rnde 

layers parallel to the dip surfaces of the quartzite. Their longer axes 

are usually in the direction of the dip of the sedimentary rocks, and the 

general effect is such as would have been produced if the gabbro had 

flowed down upon the surface of the quartzite. Again, on the north 

side of the point, at the month of Pigeon river, are slates and qnartzites, 

capped by an overflow of a fine-grained variety of the gabbro. These 

two facts, coupled with the more general characteristics, seem to show 

that the larger mass of Pigeon point gabbro is in the form of a dike, which 

has broken through its walls at certain places and intruded itself between 

tha strata of the surrounding rocks. That it is not an interbedded or a 

surface flow, like those so numerous in the Keweenawan series and in 

the Animikie beds in the vicinity of Thunder bay, is clearly seen when 

a cross section of the point is viewed from the deck of a steamboat lying 

some distance off from the extremity. Moreover, if it were an interbedded flow the slates and quartzites, which are so abundant on the 

south shore of the point, would make their appearance in the face of the 

cliffs on its north side at some distance above the water line. This, 

however, is not the case. At only two places on the north shore do the 

fragmental rocks appear, and at these places they are far below where 

they should be'were they interbedded with the gabbro, and in neither 

case is the contact like that of interbedded eruptive and sedimentary 

rocks.

The view that the gabbro may be a surface flow on the eroded top of 

the Animikie beds, like the great gabbro flow at the base ! of the KeAveenawan series in the interior of northeastern Minnesota, it is not so 

easy to prove incorrect. The sedimentary beds in question are at the 

very top of the Animikie series; their contact 2 with the overlying 

Keweenawan beds being observable at Grand Portage island in Grand 

Portage bay, a few miles west of the western end of Pigeon point, 

nearly in the direct line of the strike of the beds. Here, however, the 

overlying rock is a diabase or diabase-porphyrite, very different from 

the rock occurring on the point. Moreover, there are no indications of 

the great erosion which would have been necessary to produce the relations now found, should we suppose the gabbro to have been a surface 

flow.

The red rock, to which reference has been made, is found in three 

distinct though indefinitely outlined areas (Pis. xiv, xv and xvi). The 

first is in the eastern portion of the point, where it occupies a position between the gabbro and the fragnieutal rocks. The second area begins 

at the Little Portage isthmus and extends about 3 miles westerly. For 

the first mile and a half, as in the case of the first occurrence, its position 

is between the gabbro and the slates and quartzites. Here it leaves 

the shore, near which it is found in the eastern half of the point, and

1 Am. Jour. Sci., Vol. 34. K. B. Irving, pp. 200-202.

2 Monograph, U. S. Geol. Survey, 'vol. v. The Copper-Bearing Hocks of Lake Superior. K. D. Irving, p. 297.
24 ROCKS ON PIGEON POINT, MINNESOTA. [BULL. 109.

bends inward, passing between the two gabbro areas for the remaining 

mile and a half. The third area begins at the eastern side of Wauswaugoning bay and runs about 2 miles easterly, on the south side of 

the second gabbro area, and between this and the fragmental rocks.

Throughout its greatest extent the rock possesses a fine to medium 

grained texture, and is non-porphyritic. At two places, however, it 

becomes porphyritic, through the development of quartz in two generations. The first of these places is on the top of a high bluff near 

the south side of the point, about a quarter, of a mile west of the 

isthmus referred to above. Here the porphyry overlaps the gabbro. 

At the base of the cliff, on its south side, the contact between these 

two rocks can be traced for some distance. The relations of the porphyry and the gabbro are not those of interbeclded flows, but rather of 

two irruptive rocks, of which the younger the porphyry has cut

Lake

!<' o. 3. Contact oi'porpllyritic red rock with olivinc gabbro at the base of a bluff a little to the west

of Little Portage.

through the other and slightly overlapped it« on the edges (see Fig. 

3). The second place where a porphyritic structure is developed in the 

red rock is at the eastern end of the western, or third, area. Here 

the relations between the gabbro and the red rock could not be clearly 

made out.

The relative ages of the rocks are difficult to determine. iNo undoubted dikes of either were observed in the other. On the north shore 

are small veins of a red rock cutting the gabbro, but these can not be 

traced directly to either one of the main masses of this rock, and although 

very similar in character to the material in the large masses, the identity 

of the two rocks can not be definitely proved, since a vein of this composition may easily have leached out of the surrounding gabbro. The 

relations of the diabase dikes (which are so prominent a feature in the 

. geology of the point) to the gabbro and the red rock afford an indica-
BAYLEY.] . GABBKO AND RED ROCK. 25

tion as to the relative ages of the two rocks, but lead to no positive 

conclusions. These dikes, as will be seen from the map, are most numerous in the fragmental rocks. A few occur in the gabbro; but none 

were found in the red rock. Moreover, several of the dikes which occur 

in the gabbro and clastic rocks, run up to and abut against the red 

rock, but do not enter it. By no amount of search could any kind of 

diabase dike be found in the latter rock. Other dikes when they approach the neighborhood of the red rock appear to fray out and to become impregnated with the red material.

The true relations between the gabbro and the red rock are obscured 

by the occurrence on their contact of a rock which has some of the characteristics of the gabbro and others of the red rock. Wherever these 

are found in close proximity, except at the porphyry bluff described 

above, and at one or two other places in the interior of the point, the 

intervening space is occupied by a dark-red rock that becomes darker 

as it approaches the gabbro, and gradually merges into it; and, on the 

other hand, takes on more of the aspect of the red rock until finally it 

can not be distinguished from the most typical variety of the latter. 

This rock when analyzed is found to have a composition which is intermediate between that of the gabbro and the red rock, and one which 

varies according to the distance from the one or the other of tliese rocks 

at which the specimen analyzed is taken. (See Chap, v.)

The persistent occurrence of this rock only on the contact between 

the gabbro and the red rock; its peculiar characteristics, which are 

intermediate between those of the two limiting rocks; and its gradual 

merging into the gabbro on the one side and the red rock on the other, 

mark it as a product of contact action between the two rocks on whose 

borders it is found. As a result of this action the line of demarcation 

between the gabbro and the red rock has entirely disappeared except 

at the two or three points to which reference has been made, where the 

conditions were such as to prevent the interaction of the tVo rocks. 

At the porphyry bluff (Fig. 3) there is a very sharp contact line, but at 

this place the red rock contains porphyritic quartz crystals, and is thus 

different from the greater portion of the red rock on the Jioint. If we 

may assume that the porphyry cooled more quickly than the nonporphyritic red rock, we have a ready explanation for the nonexistence 

of a zone of the contact rock at this place. The reason for the absence 

of the intermediate rock at the several other localities where a sharp 

contact line exists is not so apparent. In the interior of the western 

portion of the point is a very high cliff facing the west. At about twothirds the way up the side of this cliff there is a very sharp wavy line 

which marks the position where the red rock 1 and the gabbro come together. On each side of this line the rocks have all the characteristics 

of the same rocks at a distance from the contact, and no intermediate 

phases are present. A second place where the same phenomenon is 

noticed is in the eastern half of the peninsula. In every other instance
26 ROCKS ON PIGEON POINT, MINNESOTA. [BULL. 109.

/

more or less of the intermediate rock occurs along the border line of the 

gabbro and red rock.

The diabase dikes occurring on the point vary in width from an inch 

to more than 60 feet. They are most abundant in the slates and quartzites, where the majority of them have a trend in the direction of the 

strike of the sedimentary rocks, and a hade of 150-17° to the south. 

In the gabbro they are less numerous, and have no one general trend, 

but are found running in all directions. In the red rock none were discovered although earnest search was made for them. Even those dikes 

which have a trend that would carry them into the area occupied by 

the red rock do not enter in, but end abruptly. They fray out on the 

edges, breakup into many small dikes, are impregnated with red drusy 

material, and finally disappear.

The dikes in the quartzite are more numerous on the southern shore 

of the point than they are in its interior, and form many of the smaller

4. isUutcli of liiulicd (liko in olh'hif ^

capes extending southward into Lake Superior. They can often be 

followed for quite a long distance along their trend, but more frequently 

they disappear under the slates and quartzites, after exposing on the 

surface but a few rods of their length. In the gabbro th.e dikes are as 

a rule much shorter. They can not be traced with certainty for any 

great distance because of their irregular courses. In most cases it is 

impossible to determine whether two apparently independent dikes are 

really parts of the same mass, whose junction is hidden by the moss 

and other surface covering, or whether they are in truth the fillings of 

distinct and disconnected orifices. In one instance, at the extreme end 

of the point, the parts of a small dike have been displaced by light 

faulting of the gabbro through which the dike cuts (Fig. 4). If the 

same faulting has taken place on a larger scale in the interior of the 

point, it is easy to see how a single dike may have the appearance of 

several. In the mapping of the' dikes no attempt was made to deter-
BAYLEY.] THE CONTACT. 27

mine whether closely occuring dikes in the gabbro are independent of 

each other or not. They have been mapped as they were discovered 

on the'surface. It is believed that most of the dikes are independent 

geological bodies, since in no case, except in the one mentioned, was 

any evidence found that faulting has taken place in the gabbro. A 

glance at the map, however, will show that although independent of 

each other, most of the dikes are connected, in that they are nearly all 

dependent for their existence upon some great disturbance l that produced fissures in the existing rocks, whose prevailing direction was 

about ENE.

The largest of the dikes are quite coarse grained in the center and 

fine grained on the edges. They nearly all possess a well-marked cross 

columnar structure, which however is by no means as striking as is 

represented by Owen in his "Beport of a Geological Survey of Wisconsin, Iowa, and Minnesota." From their great number and their similarity to the diabases of the Keweenawan flows, living 2 is inclined to 

regard them as marking the orifices through which the material of the 

flows was extruded.

Although the number of large dikes cutting the slates and quartzites 

is great, no contact phenomena of any kind were noticed in the intruded 

rocks, unless certain spots in the quartzites may be regarded as such.

THE CONTACT BETWEEN THE FEAG-MENTAL AND OTHEE BOCKS.

The most striking feature in the geology of the point is the development of a series ot mottled rocks as a contact zone between the fragmentals and the eruptives. The character of the rocks comprising this 

zone, and their peculiar relations to the red rock so prominent on the 

point, render them a subject of great interest.

As one approaches the red rock the sedimentary rocks beeome slightly 

redder in color when quartzose, and lose to some extent their vitreous 

luster. A little closer to the eruptive they are mottled with dark 

green and light red spots. At times the inottlings appear as ill-defined 

red spots on a dark green background. At other times the spots are 

green, while the ground mass is red. In neither, case, however, are the 

spots as sharply defined against the ground mass as in the case of 

the " spotted rocks " referred to above. They appear merely as areas 

in which there are accumulations of some mineral, red feldspar in the 

case of the red spots, and green mica in the case of the green ones. 

In most cases these areas are sufficiently circular to merit the name of 

spots, but occasionally they are drawn out into long lenticular shapes, 

and threads so interwoven with one another as to produce a structure 

resembling the eutaxitic structure of eruptive rocks.

1 Cf. Geikic: The History of ATolcanic Action during the Tertiary period in the British Isles. 

Edinburgh, 1888, pp. 33 and 71.

^Monograph, II S. Gcol. Survey, vol. v. The Copper-Bearing llocks of Lake Superior, R. D. Irving. p. 

144.
28 ROCKS UN PIGEON POINT, MINNESOTA. [BULK 109.

Very close to the red rock appears a belt in which the various rocks 

are in the most complicated relations imaginable. In the eastern.portion of the point this belt is well seen on the southern shore, about 

one-third of a mile from the end of the point. (See PI. xvi.) Here 

the red rock is exposed in low cliffs, and in it are small, sharp slate and 

quartzite inclusions, into which the red rock penetrates in every direction. The exact line of contact between the red rock and the bedded 

fragmentals can not be detected, as they appear to. merge gradually 

into one another, the latter becoming redder and redder as they approach the former, which penetrates them in veins and dikes, and 

finally includes numerous pieces in such a way as to yield a good 

eruptive breccia.

A little to the north of this exposure the red rock is seen to pass 

imperceptibly into a coarse grained, gabbro-like red rock, similar to 

the intermediate rock mentioned a few pages back. Like the red rock, 

this also contains a large number of slate and quartzite inclusions, surrounded by narrow rims of pink feldspathic material, and occasionally 

by a bright red border of exactly the same character as that of the red 

rock. Many of the larger inclusions are distinctly mottled with red or 

green spots. In some of the quartzite fragments bright green circular 

spots, varying in size from a few millimeters to two decimeters in diameter, are scattered over a bright red ground mass, containing small 

dark green elongated spots faintly resembling some of the knots 

(Knoten) in the " Knoten Schiefer " of the Germans. The ground mass 

of these rocks, although of the same color as the typical red rock, differs from it a good deal in luster. The grouudmass of the altered 

quartzite is much more vitreous than is the red rock, and its structure 

is less crystalline. (See PI. iv.)

A little west of the point on the southern shore above referred to 

(PI. xvi) the red rock with its inclusions of slate exhibits to a fine 

degree the .effect of alteration in the latter. Some of the inclusions 

are very sharp and but little altered, while others are partially dissolved, and are surrounded by concentric zones, [resulting from the 

action of the red rock upon the material of the inclusion, and the 

reciprocal effect of the partially dissolved inclusion upon that portion 

of the red-rock magma immediately contiguous to it. The inner one of 

these contact zones consists of a dark-green chloritic slate, dotted with 

little crystals of red feldspar. The same dotting of slates by little 

crystals of feldspar is observed wherever these rocks are in contact 

with dikes or veins of the red rock, and also in the upper and lower 

layers of strata interbedded with altered quartzites, as on the cliff side 

represented in Fig. 1. Again on the island off the southeastern shore, 

about 2,000 feet from the end of the point, the slates are cut by a 

vein of quartz and red feldspar, and are dotted along its side by the 

same red crystals.

In the western part of the point the contact belt is well developed 

along the entire border of the red rock area south.of the northern gab-
U.S.GEOLOGICAL SURVEY. BULLETIN NO. IO9 PL. IV.

POLISHED SURFACE OF MOTTLED ROCK, CHARACTERISTIC OF THE CONTACT BELT.

NATURAL SIZE .
BAYLEY.] 1HE CONTACT BELT. 29

FIG. 5. Sketcli of a portion of the contact belt, showing included fragments in 

rock exhibiting apparent flow structure.

bro belt. Here the red rock occupies a low valley, to the north of which 

are the gabbro hills and to the south a series of less lofty hillocks coinposed of the material of the contact belt. The character of this contact 

belt differs but little from that of the one described above. Mottled 

inclusions of quartzite and slate are common in it, and these by solution in the inclosing rock have given rise to a large amount of a product resembling the red ground mass of the mottled rocks. Just north 

of the barite vein (for the situation of which see map, PI. xiv) there 

extends eastward a remarkable ridge, consisting of a mottled red rock 

full of inclusions. The inclusions are 

bent and twisted Into a great variety of 

shapes, while the cementing material has 

a well-marked flow structure (see Fig. 5). 

Near the inclusions the surrounding rock

is very markedly mottled with large green 

spots.

A third notable development of a belt 

of red rock holding inclusions is found 

along the southern side of the eastern 

end of the tongue of quartz-porphyry 

comprising the third area of the red 

rock. In tliis belt the phenomena met 

with are the same in character as those which distinguish the contact 

belt in other exposures.

A study of the contact belt throughout the entire extent of the point 

leaves no doubt in the mind that it is a constant accompaniment of the 

red rock, except in a few cases where this is present in the form of a 

porphyry. Thus it would seem to be a fact beyond controversy that 

the red rock is the immediate cause of the alteration noticed in the fragmental rocks and of the breccia observed along its contact with them. 

If, however, the contact belt is examined very closely it is found that 

although the red rock is always accompanied by a zone of this belt, 

there are localities in which the latter occurs without the presence 

of the former. In the description of a contact between the gabbro and the quartzites near that pictured in Fig. 2, the field notes 

state that " as the eruptive is approached * * * the regularly 

bedded slates and quartzites become reddened and much bent. They 

are succeeded by a band of pinkish rock filled with fine red inottlings 

on a green ground, which (baud) apparently runs under the gabbro. 

The inottlings are often so arranged as to give a distinct flowage structure. Inclusions of the fragmental rocks are frequent in the eruptive 

and the whole is penetrated in every direction by veins of drusy red 

granite." Again, at a point just west of the barite vein, the metamorphosing rock seerns to be the gabbro. Just as in the case of the contacts with the red rock, the quartzites become mottled as they approach 

the eruptive, and inclusions of the former in the latter are so frequent 

that there appears to bo a gradual transition between the two rocks.
30 ROCKS ON PIGEON POINT, MINNESOTA. * [BULL.IOO.

On the other hand, there are many places where the gabbro and the 

qitartzites are in contact without the least evidence of any action between them. On the north shore of the point slates and quartzites are 

found separated by a very sharp line, on each side of which the respective rocks preserve their distinctive characteristics without any trace 

of alteration in either.

From a consideration of thelacts above described, it is difficult to 

decide whether the red rock is the active agent in producing the phenomena observed in the contact belt, or whether the gabbro is the metamorphosing rock and the red rock merely one of the products of 

its contact action. The absence of contact rocks 'along the north 

shore of the point, where the conditions prevailing at the time of the 

eruption of the gabbro could not have been very different from those 

existing on its south side, would seem to prdve that the contact action 

did not have its source in the gabbro. Other apparent indications of 

the correctness of this view are found in the non-existence of contact 

rocks in connection with the large masses of gabbro occurring on the 

shore of Lake Superior to the northeast of Pigeon point. 1 All along 

this shore there are immense dikes of gabbro which cut slates and 

quartzites like those present on the point, but at no single instance 

have they affected the fragmental rocks through which they broke, 

except at one locality at the falls of Pigeon river.

At this place the slates are fissile and evenly interbedded with hard 

quartzites, the series dipping in the main at an angle of 17° to 20° to 

the southeast. On the top of the hill on the north side of the river the 

contact between the fragmental beds aiid the extension of the large 

dike forming the barrier of the falls is finely shown. The slates here 

dip to the north. They are jointed into small square blocks, and have 

become hard and black and apparently coarsely crystalline. They are 

mottled with fine dark spots, which are often arranged in lines resembling flowage lines. In them pieces of the harder siliceous beds appear 

as if included. It will be noted that the contact phenomena observed 

at this place differ in some respects from those observed on the point. 

No strongly marked mottled rocks are to be found here, nor is there 

present any of the vitreous red quartzites like those so prevalent.among 

the contact rocks of the point.

The view that the red rock is the metamorphosing agent finds confirmation in the fact that wherever the contact rocks appear in any 

quantity, there also is to be found more or less of the red rock. This 

is true not only of the occurrences.on Pigeon point, but it is true also 

of those on Spar, Jarvis, and Victoria 2 islands, four small islands on 

the Canadian side of the international boundary line, about 10 miles 

northeast of the extremity of Pigeon point. On these islands are large

1 E. D. Ingall: Rep. on Mines and Mining on Lake Superior. Geol. and N;it. Hist. Survey of Canada. 

Annual Report, 1887, Part H, p. 48.

2 W. S. Bay ley: Notes of Microscopic Examination of Rocks from tho Thunder Bay Silver Distriot. 

Rep. Geol, and Nat. Hist. Survey of Canada for 1887, Tart H, p. 116,
BAYLEY.] THE CONTACT BELT. 31

masses of gabbro cutting altered slates and quartzit.es, and between 

them there is everywhere present a quantity of the red rock.

The existence of the red rock in dikes and veins, its great similarity 

in appearance and composition (as shown in the chapter on the petrographical characteristics of the red rock) to the Keweenawan flow rocks 

on the islands south of Pigeon point, the flow structure observable in 

it at various places within the contact belt, all point to an original 

condition of plasticity, if not liquidity. Its comparative freedom from 

the chemical constituents of the gabbro indicate for it an eruptive 

origin.

If, however, the red rock is eruptive, and is the agent which has produced the alteration of the sedimentary rocks, its distribution with 

respect to the gabbro and the fragmental rocks is peculiar. As has 

been repeatedly stated, the red rock is found alniostexclusively between 

the gabbro on the one side and th'e fragmental rocks on the other. Its 

position is such as it would naturally have were it the end product in 

the alteration of the slates and quartzites by the gabbro. If it is original, its present position can be explained only on the supposition that 

a line of weakness extended the entire length of the point along the 

southern junction of the gabbro and the bedded rocks.

The distribution of the rocks under consideration and their relations 

to each other render it impossible to decide by field study alone whether 

the contact phenomena are to be ascribed to the action of the gabbro 

with the red rock as one of the alteration products, or whether they are 

due to the presence of the red rock. In opposition to the former view 

is the absence of a contact belt on the north shore of the point 

and along the sides of the large gabbro dikes on the shore of Lake Superior northeast of Pigeon point, as is also the composition of the red 

rock. In favor of the correctness of such a view is the distribution of 

the red rock.
OHAPTEEII.

PETROGRAPHICAL DESCRIPTION OF THE FRESH OLIVINE-GABBRO AND ITS ALTERED PHASES.

THE FRESH OLIVINE-GABBEO.

The rock of the large dike-like mass forming the axis of Pigeon point, 

and the smaller areas in its western part, is, as has already been stated, 

an olivine-gabbro, very similar in structure to the olivine-gabbros 1 

described as occurring so abundantly in other portions of the Lake 

Superior region. Since none of these has been subjected to a thorough petrographical examination, it is thought well to note in some 

detail the characteristics of the Pigeon point rock in order that it may 

serve as a type to which to refer'the other rocks of like character in 

this region.

The Pigeon point gabbro is a coarse to medium-grained gray rock, 

which weathers light where it has not been subjected to much internal 

alteration. The greater portion is apparently quite fresh, and has an 

oily luster due to the large amount of olivine in its composition. When 

examined microscopically the attention is immediately attracted by 

the glassy appearance of its feldspathic constituent. In many instances 

this is nearly as pellucid as the sanidine in the rhyolites and trachytes. 

It occurs both in large porphyritic crystals, and in smaller ones, which 

with olivine and augite form a groundmass in which the larger crystals lie. So far as was observed, there seems to be no general rule governing the development of the porphyritic structure in the rock, nor 

does there appear to be any sharp line between the truly porphyritic 

phases and those in which the feldspars are all large, but are not present in t\Vo generations.

The porphyritic crystals are frequently 1 to 3 inches long, and sometimes as much as 6 or 7 inches. Many are Carlsbad twins, in which*the 

composition plane and the oblique angle formed at the junction of the 

cleavage cracks in the two halves are strongly accentuated by superficial weathering. The specific gravity of one of these crystals, as determined by the Thoulet method, is XlYI. It is consequently a-pi-agioclase whose position in the series is between that of bytownite and labradorite. The specific gravity of typical bytownite 2 is 2-729; that of 

labradorite is 2-700.

1 Aug. Strong: Neues Jahrbuch fiir Mineral., etc., 1877, pp. 113-138. Julien: Geol. of Wisconsin, Vol. 

3, pp. 233-238. Irving, K. 1),: Geol. of Wisconsin, Vol. 3, pp. 168-183. Irving, R. D.: Tlie copperbearing rocks of Lake Superior: Monograph, Vol. 5, U. S. Geol. Survey, pp. 37-50.

2 Schuster: Tscbermaks mineral. Mittbeil., 1880, Vol. 3, pp. 181 and, 2Q3. 

32
BATLEY.] THE FEESH OLIVINE-GABBRO. 33

On flat, weathered surfaces the rock assumes a lighter color, as has 

been already stated. The feldspars become white and opaque. They 

project slightly above the general surface, which has a greenish gray 

tint and a rough, cellular aspect due to little projecting points of feldspar and knobs of chlorite, both of which appear as raised areas 

separated by depressions that have resulted from the easy decomposition of olivine. Pronounced as is this weathering, it extends but a fraction of an inch below the exposed surface. Large plates half an inch 

in thickness scale off, and leave the rock immediately beneath them as 

fresh as any specimens obtained from much greater depths.' Where 

there are fewer large feldspar crystals, and where the rock has been 

subjected to greater alteration, it first becomes nodular, then separates 

into rude spheroids, and finally disintegrates into a brownish yellow 

sand. In the nodular phases the interiors of the nodules are quite 

fresh, while the internodular substance is slightly altered.

The joint cracks which everywhere cross each other at right angles 

have already been referred to. They divide the rock into large rectangular blocks measuring about 6 feet on an edge. Fine specimens of 

these are to be seen at the base of the cliffs on the north side of the 

point, and at the falls of Pigeon river. The freshest specimens of the 

rock come from these blocks. Under the microscope the gabbros are 

seen to be composed essentially of feldspar, olivine, and augite.

A triclinic feldspar constitutes bj far the greater portion of the rock. 

It occurs in two forms: (1.) in large porphyritic crystals, and (2) in 

smaller crystals in the groundmass of the porphyritic varieties, and 

in the body of the nou-porphyritic varieties of the rock.

The porphyritic crystals are generally columnar in the direction of 

the G axis. The planes observed in them are the three pinacoids, and'a 

series of inacrodonies. Two series of cleavage lines are very well 

marked parallel to co P o> and OP. Those parallel to OP make an angle 

of G3°-64° with the c axis (angle ft). Under the microscope, twinning 

lamella} are found to be quite prevalent, although occasionally absent 

when large areas of the feldspar .show an undulatory extinction. Carlsbad twins composed of two individuals, each of which is polysynthetically twinned according to the albite law, are quite common.

The material of the crystals is usually quite fresh, except where 

slight kaolinization has taken place in the neighborhood of cleavage 

cracks. They contain but few inclusions, the most prominent ones 

being little negative crystals of devitrified glass, plates of aiigite and 

grains of magnetite. The augite and magnetite appear to have separated from magma included within the feldspar during its growth. 

The dust-like inclusions so abundant in the feldspar of the gabbros jof 

other regions are entirely lacking here. The specific gravity of a piece 

of one of the feldspar crystals is mentioned on page 34. Other pieces 

taken from different crystals, all of which were fresh and free from inclusions, varied in specific gravity between 2-711 and 2-717. These figBull. 109 3
34 ROCKS ON PIGEON POINT, MINNESOTA. [BULL. 109.

ures correspond to those obtained in the case of the lower members 

of the labradorites. Labradorite, corresponding to the composition 

Afy An 3, lias a specific gravity of 2-728 according to Tschermak1 , while 

Schuster2 gives 2-703 as corresponding to Ab;)An4 .

The feldspar in the groimdmass of porphyritic varieties of the rock, 

and that comprising the larger portion some 60 per cent of other 

varieties, occurs in lath-shaped crystals, varying in length from 1 to 

20mm . Each of these is composed of a number of smaller individuals, 

which by their union produce broad lath-shaped forms like those so 

characteristic of the gabbros from Volpersdorf, in Silesia, and from the 

Harz.3 Each of the small crystals is poly synthetically twinned, and 

two or more frequently unite in a Carlsbad twin.

Like the porphyritic feldspars, the feldspar of the groundmass is 

fresh and often glassy, and is comparatively free from inclusions. In the 

freshest varieties only a few dust-like particles and minute flakes of 

kaolin can be detected. When examined under high powers some of 

the dust-like inclusions appear to be devitrified glass. The extinction 

of two contiguous twinned lamellse, measured against their composition plane, is 24° on each side. The angle formed by the two series of 

cleavage lines on the macropinacoid is 95° to 96°. These results indicate a feldspar which belongs in the lower portion of the labradorite 

group.

An analysis of the feldspar which was separated by the Thoulet 

method from the powder of one of the freshest specimens (No. 11203)4 

yielded the figures in column I, in the table below. The specific gravity 

of the powder was 2-699. In column A is given the composition of a 

labradorite5 of the formula Ab;) An4 ; and in B that of a plagioclase6 of 

the specific gravity 2.700.

Si02 .... ......

AljOs.........

FO.A. - -.-- 

CaO ..........

NaaO .........

Total ....

O (Tr

I.

58-73

30-39

1-20

30-8-1

+ 3-70

100-00

A.

53-00

29-80

11-70

4-90

10000

27-03

B.

54-55

28-68

1-03

11-28

4-C2

100-16

27-00

*Differcnce.

It will be noticed that the specific gravity of this feldspar is lower 

than that of the porphyritic crystal, and that the latter is therefore

'LehrTmch der Minoralogio, 2d edition, n, Aufl.465. 

2 Tscvhermalvs mineral. Mittheil, 1880, ATol. 3, p. 203. 

3 JKoscnlmsch: Mikrosp. Physiographic, 1887, Vol. 2, p. 155.

4 The figures in parentheses refer to the numbers of the specimens in the series of rocks belonging 

to tho Lake Superior Division of the TJ. S. Geological Survey. 

6 Schuster: Op. cit., p. 153. 

6 Tschermak: Sitzungsbor. K. Akacl, Wiss., I, July, 1869.
BAYLEY.] THE PftESH OLTVINE-GABBRO 35

more basic than the former. That this difference was not a constant 

one was shown by powdering little chips taken from seven of the freshest specimens, and separating by means of the Thonlet solution. The 

greater portion of the plagioclase fell at 2-716, a value almost identical 

with that obtained for the porphyritic crystals.

Olivirie makes up about one-tenth of the entire rock. It is present 

in rounded grains of a light yellowish green color, and is usually free 

from inclusions other than the fine, dust like substances found in the 

feldspar. Occasionally lines of larger dark brown inclusions are met 

with. These are probably of secondary origin, since the lines pass 

uninterruptedly from the olivine over into feldspar or augite. The 

numerous black microlitic inclusions characteristic of olivine in most 

plutonic rocks (Tiefengesteine). were seeu in but one instance (No. 

11418). In no case is the oliviue entirely unaltered. In even the 

freshest varieties a-'little chloritization has taken place. Sometimes 

there is a development of actinolite needles in the otherwise unaltered 

olivine, and in still other instances a few flakes of biotite occur, mixed 

with chlorite on the outer edges of a grain. From its relations to the 

feldspar and augite the olivine must, in most cases, be regarded as 

older than either, although in some instances it includes a few lath - 

shaped crystals of the former mineral. Since the feldspar occurs in 

two generations it is probable that the crystallization of the olivine 

took place in the interval separating these, or that it began to crystallize before the complete separation of the feldspar of the groundmass. x

The augite is entirely allotriomorphic. It fills in the interstices 

between the other constituents, often including both olivine and labradorite. Where several crystals of the feldspars are included within a 

single augite the structure of the rock becomes poscilitic.1

The greater part of the augite is perfectly fresh, and, like the feldspar and olivine, it contains no inclusions other than a few of the dustlike particles already mentioned. Its color in the thin section is purplish pink or violet. A faint pleochroism is often apparent in deeply 

colored pieces, the a and .c rays being a deep purplish pink and the ft 

ray a yellowish gray. The two series of cleavage lines parallel to oop 

are very distinct on pieces cut parallel to the basal plane, where they 

make the usual angle of 87°. A parting parallel to the orthopinacoid 

is observable in only a few sections. That this is present, however, is 

proven by the fact that cleavage pieces frequently show an optical axis 

in converged light between crossed nicols. The extinction in plates cut 

parallel to the clinopiuacoid is 44°. No twins were observed, but in 

several sections the effect of pressure on this mineral is beautifully 

seen in the production of an undulatory extinction and a bending of 

the cleavage lines. This is particularly well shown in the section of

 Cf. Williams: Am. Jour. Sci., "Vol. xxxi, p. 30; and also Pumpelly: Proc. Am. Acad., Vol. xm, p. 

260; andlrving: Copper-Bearing Rocks, p. 42.
36 ROCKS ON PIGEON POINT, MINNESOTA. [BULL. 109.

No. 11555, where a large, wedge-shaped piece of augite included between plagioclase lias suffered a bending of its cleavage lines 13° and 

a twisting of its extinction plane an equal amount (PI. V, a).

Accompanying the diallagic augite in the section of one specimen 

(No. 11206) from near the eastern end of the point, is a highly refractive yellowish mineral, with many of the properties of pyroxene. Its 

extinction, however, never reaches above 7° 30', in most of the pieces 

being parallel to the cleavage; while the plane of its optical axes is also 

parallel to the cleavage. It is closely associated with the purple 

augite, from which it is easily distinguished by its yellowish tinge and 

dull polarization colors between crossed nicols. It is probably a monoclinic augite of a different composition from that of the purple variety, 

or a slightly altered form of this.

A partial analysis of the diallage isolated from/the powder of five of 

the freshest specimens of the rock was made by Dr. E. B. Biggs in the 

laboratory of the Survey. The result obtained was as follows:

n.

Si02 ........................................................ 48-34

A12 03 ....................................................... 2-90

Fe,03 ....................................................... 4-68

FeO....................................................... 14-15

CaO........................................................ 15-10

MgO......................................................... 11-34

Ti02 ........................................................ 1-98

Total ................................................. 98-49

\

The composition of this augites is that of a very pure diallage. An 

interesting feature is the large percentage of titanium in it a fact 

which may explain the purple color. Knop, l who made an exhaustive 

study of the augite of the Kaiserstuhl in Baden, found that all the varieties that are rich in titanium have a violet color in thin 'section, 

except when they contain large amounts of iron. Although investigations in other regions do not fully substantiate Knop's conclusions, it 

will at least prove of interest, if all the purple augite and diallage so 

very widespread throughout the rocks of the Lake Superior region is 

found to contain titanium.

In addition to the labradorite, pyroxene, andolivine, the fresh gabbro 

contains only apatite and titaniferous magnetite as.primary constituents. The apatite occurs in long, narrow, colorless crystals, with a parting parallel to the basal plane, and is sparingly present. The magnetite 

appears both in idiomorphic and allotriomorphic forms, with the latter 

predominating. Quite a large number of the grains are surrounded by 

rims of reddish-brown mica, and not a few show the beginning of an alteration into leucoxene.3 A little quartz and a few flakes of brown and

'Zeitschrift fur Krystallographie, Vol. x, p. 58.

2 At several places in tho eastern portion of the Point tho amount of titauiferous magnetite in the 

gabbro is BO great as to cause a deflection of the compass needle 95°.
EXPLANATION OF PLATE V.

V. a. Thin section of olivine gabbro (No. 11555), showing the effect of 

pressure upon a piece of augite. The purplish-pink mineral is augite, 

in which the cleavage lines have been bent, presumably by pressure. 

The green mineral to the left is olivine. The white, lath-shaped 

crystals are bytownite. The latter are represented as seen in polarized light, between crossed uicols, while the augite and olivine are 

shown in natural light. Magnified 35 diameters, 

b. Thin section of altered gabbro (No. 11222), with green isotropic 

mineral surrounding apatite. The yellowish-brown mineral to the left 

is biotite. The others are various decomposition products of olivine 

and augite. The. specimen was taken from a place where the gabbro 

approaches the red rock, and is a variety of the intermediate rocks, 

another variety of which is represented in a, PL x. Magnified 175 

diameters. Natural light.
U.S.GEOLOGICAL SURVEY. BULLETIN NO. IO9 PL. V.

a (X35)

OLIVINE GABBRCX
[BAYLEY. THE FRESH OLIVINE-GABBRO. °37

red biotite are evidently secondary. The former is found in micro-pegmatitic intergrowths with feldspar, and the latter as an alteration product of olivine, augite, and magnetite.

In order to obtain the average composition of the rock, pieces of 

equal size were taken from five of the freshest specimens. These were 

crushed, and the powders were thoroughly mixed. A portion of the 

mixed powders was analyzed by Mr. W. F. Hillebrancl, whose results 

are given in column III. In column C are the figures obtained by 

Streng and Kloos, 1 upon the analysis of a hornblende-gabbro from Duluth, Minnesota.

SiOs. .........

Ti08 . .........

'AlsO, ........

FeA---------

FeO ..........

MnO. .........

CaO ..........

BaO ..........

MgO..........

K20 ..........

Na20 .........

H20 ..........

PA-------.-.

Total . . . 

Specific gravIII.*

49 '88

1-19

18-55

2-00

8-37

 09

9-70

 02

 68

2-59

l-O-I

 10

100 -21 

2 -923-2 -070

C.

49-15

 18

" 21-90

6-60

8-22

3 -03

1-01

3-83

, 1 -92

 33

101 -31

* During the pulverization of the-rocks a small quantity of metallic iron became mixed with the 

powders, and tins may have reduced some of the ferric iron to the ferrous condition through the 

agency of the hydrogen generated in the course of solution. The above figures are corrected for the 

metallic iron present, but could not bo corrected for the amount of reduction produced, which was in 

any event very small, since the entire amount of metallic iron present did not exceed 0.3 per cent.

In referring this rock to its proper position among massive rocks, its 

composition, structure, and geological relations must be alike considered. Dr. Wadsworth2 has recently given a re'suine" of the opinions of 

different investigators on the value of the orthopinacoidal parting as 

a characteristic of diallage distinguishing it from augite. The results 

are to the effect that there is no real difference between the two minerals, and that the presence or absence of the parting should not be 

used as a basis for separating the gabbros from the diabases. The 

distinction between these two rocks must therefore rest upon their 

geological and structural relations. They both contain augite and 

plagioclase, with or without olivine, as essential constituents. The 

diabases are defined by Koseubusch 3 as rocks occurring in dike form 

or in intrusive layers and with a structure which varies within wide

1 Strong & Kloos: Nones Jabrbuch fiirMin., 1877, p. 31.

2 Bulletin No. 2, Gool. Survey of Minnesota, pp. 55-57.

3 Mikroskopsiche Physiographe, 1887, vol. 11., pp. 134 et seq. and 174 et seq.
38 ' ROCKS ON PIGEON POINT, MINNESOTA. [BULL. 109.

limits. The granular varieties have an ophitie structure, which is due 

to the development of feldspar in long lath-shaped crystals. The 

structure of the gabbros is described as more granitic than that of-the 

diabases in consequence of development of the feldspar in all directions 

equally (it is equidiinensional), and its geological position is sirnilarto 

that of granite.

All gradations between the typical gabbro and the typical diabase 

are, however, frequently met with, and nowhere are they met with in 

more profusion than in the Lake Superior region. The different rocks 

of this class are here so intimately related to each other, that seemingly the only structural distinction possible to make is one of coarseness of grain. The coarse-grained rocks of this region, where they 

consist of plagioclase and augite (or cliallage), have been called gabbros; the fine-grained kinds are known as diabases. The classification 

is not as artificial as it appears to be at first sight,, for Irving1 states a 

fact when he declares that the gradation of the line-grained kinds into 

the coarser varieties has never been observed in any one bed, and the 

differences in the external characteristics of the two are strongly 

marked.

According to this practice the Pigeon point rock must of necessity 

be classed with the gabbros, and such it has been called in this paper, 

because of its similarity to those rocks which have for so long passed 

under that name in the writings of those geologists who have done the 

most work upon them. If, however, we should adopt the suggestion of 

Prof. Judd,2 and limit the term gabbro to the granitoid forms, the Pigeon point rock would fall among the diabases or the diabase-porphyrites. Both its composition and structure accord well with those characteristic of diabase-phorphyrites, and its geological position is not very 

different from that of these rocks. It occurs in the form of a boss-like 

dike cutting quartzites and slates, and bears a strong resemblance to 

rocks of the same composition, which are probably, surface flows. 

This resemblance was so apparent to Irving :i as to lead to the remark 

that the gabbro dikes of Thunder bay (arid Pigeon point) "are noticeably much closer in character to the great gabbro (flow) at the base of 

the Keweenaw series than to those gabbros which are interleaved 

with the Aniniikie slates."

THE ALTERED VARIETIES OF THE GABBRO.

Although the fresh phases of the gabbro are so monotonous in character, its altered varieties present many peculiarities, some of which are 

quite interesting. These altered varieties are very much more abundant 

than is the fresh variety, and are found distributed indiscriminately

1 The Copper-Bearing Eocks of Lake Superior, E. D. Irving: Monograph, U. S. Geol. Survey, vol. v, 

p. 69.

2 Quart. Jour. Geol. Soc., 1885, pp. 844, 418. 

3 Is there a Hurouian group? E. D. Irving; Am. Jour. Sci., vol. xxxiv, p. 261.
BAYLBY.] ALTERED VARIETIES OF GABBKO. 39

throughout the gabbro area. Olivine and augite are the constituents 

of the gabbro most prone to decomposition. As the final product of 

their alteration, a rock is produced whose habit is very different from, 

that of the fresh rock described above. Its color is greenish and its 

structure fibrous. The feldspar crystals, so prominent a feature in the 

fresh rock, can not be detected in it without the aid of the microscope.

The first mineral .to undergo change is the olivine. On its edges and 

along its cleavage tracks slight serpentinization takes place, and at the 

same time considerable magnetite separates out: Serpentine, however, 

is not the usual alteration product. Chlorite seems to have been formed 

much more frequently, and in this case also magnetite is an attendant 

product. This chlorite is sometimes cloudy and grayish, when it shows 

no decided effect in polarized light. Sometimes it forms an amorphous 

mass, occupying the place originally held by the olivine; but most frequently it is arranged in radially fibrous groups, when it polarizes with 

a deep blue color, and is nearly always associated with biotite and hornblende. When biotite and chlorite occur together, the former usually 

occupies an external position with reference to the chlorite, indicating 

a reactionary origin.

The alteration of olivine to chlorite is well known as being widespread, but its alteration to biotite is much less common. Julien 1 describes the alteration of olivine to biotite in an olivine diabase or gabbro from the NE. ^ Sec. 31, T. 45 1ST., B. 2 W., Wisconsin, and gives a 

picture of it. Here the biotite is supposed to be due to the action upon 

the olivine of alkaline solutions emanating from the plagioclase. The 

same origin may be ascribed to the biotite in the Pigeon point rocks. 

The secondary origin of biotite from olivine has also been described by 

v. Foullon 2 in the melaphyres of the southern Alps; byBeckeMn a 

bronzite olivine rock from the Lower Austrian Waldviertel; by Irving 4 and Wadsworth 5 in the gabbros of the Northwest; by Sjogreu 6 

in a gabbro in Western Norway; by Schuste.r 7 in an olivine-gabbro 

from Birchville, California; by Lenk,8 and by Doss a in a minette from 

near Dresden.

Another prominent change of olivine is into hornblende. Little light 

green needles of this mineral extend from the edges of the olivine in to its 

interior. These needles increase in number as the alteration progresses, 

until they finally fill the entire space formerly occupied by the olivine, 

with the exception of small areas containing chlorite. Such an altera1 Geol. of Wisconsin, Vol. in, p. 225, aiul PI. xxn, Fig. 1.

2 Tschermaks mineral, Mitth., 1880, n, p. 481. Eef. Eosenlmscb: Mikroakopischo Physiographic, 

1885, 1 Ann1 ., p. 412.

3 Tschermaks mineral. Mitth., 1882, iv, p. 333.

4 The Copper-hearing rocks of Lake Superior, R. D. Irving: Mon. V, TJ. S. Gool. Survey, pp. 39,45-50.

6 Bull. Geol. Survey of Minnesota, No. 2, p. 66.

o Geol. Foren. i Stockholm Fiirh., 1883, vr, p. 447. Ref. Noues Jalirb. fiir Miner., etc., 1884, i, p. 81.

7 JTeues Jahrhuch fiir Min., etc., Beilago Band A7"., p. 519.

8 Zur geologischon Keuntniss dor siidlichen Hhiiii. Inaug. Diss. AViirzb., 1887, p. 50. 

9 Tscheruiaks mineral. Mitth., 1889, XI, p. 53'.
40 KOCKS ON PIGEON POINT, MINNESOTA. [BULL. 109.

tion of olivine was first mentioned by Becke 1 under the name of pilite. 

Its origin, like that of the biotite, is also referred to the action of alkaline solutions derived from theplagioclase. ' Specimen No. 11446 shows 

this alteration most beautifully. Here complete pseudomorphs of pilite 

after olivine are frequent. (PI. vi, a).

A. fourth alteration of olivine met with in tne xjigeon Point gabbro 

is to a brownish red homogeneous substance which is slightly pleochroic, but exerts no appreciable effect in polarized light between crossed 

nicols. It is probably a mixture of chlorite and serpentine stained 

brown by iron, oxide.

The next mineral to undergo alteration is augite. Its most usual 

change is into hornblende. Both the bluish green fibrous variety known 

as uralite, and dark green and dark brown compact kinds occur. Chlorite, biotite, and magnetite are also frequent products of its alteration, 

the biotite being developed more abundantly near the contact of the 

gabbro with the surrounding feldspathic quartzites and red rock. As 

alteration progresses the amount of these secondary products increases 

until finally no trace of augite remains. In these most highly altered 

phases the secondary hornblende is found in the compact and fibrous 

varieties in about equal proportions.

The more common type of the compact variety is a very strongly pleochroic mineral with a well marked cleavage, and an extinction of 24°. 

It is occasionally in idioniorphic crystals, but more frequently in fibrous 

inter growths with a second kind which is less strongly pleochroic and 

in different tints. The pleochroism of the first variety is a=light yellowish brown; fa and ,c=dark brown; that of the second kind is a=light 

yellowish green; ft=yellowish green; c=dark bluish green. The two are 

found in parallel growths, with the green variety surrounding the brown. 

Although the line between the two is quite sharp, there appears to be 

no break in the continuity of their substance. They extinguish simultaneously and the cleavage lines pass from one into the other without 

deformation. The little spots known as pleochroic halos (Hofe) are 

occasionally seen in the compact hornblendes. They are almost black 

when the prismatic cleavage of the mineral in which they occur is parallel to the vibration plane of the nicol, and a greenish brown in other 

positions. They sometimes surround little apatite crystals, and at 

other times exist in round spots without reference to any included mineral. These spots are often surrounded by circular rims of the same 

character as that of the interior portion, from which they are separated 

by narrow zones of ordinary hornblende or chlorite.

The fibrous hornblende is in long acicular crystals, with a dark bluish 

green color when their long axes .are parallel to the vibration plane of the 

nicol, and a light yellowish green when perpendicular to this direction. 

It occasionally fringes both the compact varieties, but more frequently 

forms with chlorite a matted mass in which the compact brown and 

green varieties, the biotite, and other secondary products are imbedded.

1 Minor. 11. Pei.ro;,'. MiUh.. 1882. Ill, pp. 330, 355, anil 450.
EXPLANATION OF PLATE VI.

(j 

VI. a. Section of altered olivine gabbro (No. 11446), in which the olivino has been entirely changed to pilite. The light-colored mineral 

is feldspar. That with the rectangular cleavage is augite, and most 

of the dark material is chlorite. From a photograph in natural light. 

Magnified 80 diameters.

b. Fibrous green hornblende surrounding brownish-yellow cores of 

the same mineral in altered gabbro (No. 11318). The dark acicular 

crystals in the lower left-hand corner of the figure are hornblende 

needles penetrating chlorite. The colorless mineral is plagioclase, 

and the black one pyrite. From a photograph in natural light. Magnified 80 diameters.
U. 8. GEOLOGICAL SURVEY BULLETIN NO. 109 PL. VI

a. ALTERED OLIVINE GABBRO.

b. FIBROUS GREEN HORNBLENDE.
BAYLEY.] ALTERED VARIETIES OF GABBRO. 41

This fibrous mineral is probably the youngest of the three hornblendes. 

It appears to have been derived both directly from theaugite and indirectly through the compact varieties and through biotite.

Jiocks from the gabbro areas of the Northwest containing large 

amounts of compact hornblende have been described by Kloos, Pumpelly, Irving, Wadsworth, and others. Pumpelly1 called them " augitediorites," and supposed the hornblende to be original. Kloos2 also 

appears to have considered it as original, and called the rock containing 

it " hornblende-gabbro." Irving3 and Wadsworth4 regard it as secondary, and as merely an extreme phase in the alteration of augite.

A second mode of alteration of the" augite is into biotite. This mineral is present in large-sized folia and flakes of a reddish brown color 

when seen in basal sections, but very strongly pleochroic in.sections 

showing the cleavage. The color of the light vibrating parallel to the 

cleavage is very dark brown; in a direction perpendicular to this it is 

light brownish yellow. It is usually associated with chlorite and hornblende, lu the latter case the two minerals are frequently so intergrown that the cleavages of both are parallel. Since the hornblende is 

in many cases fibrous, and is commonly found surrounding the biotite, 

it is probably to be regarded as an alteration product of this mineral. 

When associated with chlorite the biotite presents the same general 

features as it does when accompanied by hornblende. In this case, 

however, the biotite occurs in larger flakes, and in shreds disseminated 

throughout the mass of chlorite. Between the folia little lenticular 

masses of calcite and a brightly polarizing, highly refractive, sphenelike looking substance are quite common, as are also pleochroic halos. 

That this biotite is secondary and that much of it lias been derived 

from the augite is evident from the fact that this mineral, together 

with hornblende." and chlorite, make up areas which have the characteristic wedge shape of the augite in fresh specimens.

The presence of biotite in the Lake Superior gabbros has been reported by Irving, Wadsworth, and others. Most of these writers have 

apparently looked upon it as primary, Wadsworth5 alone stating positively that it is in many cases secondary after augite. In the rocks of 

other localities biotite has often been met Avith as an alteration product 

of augit«.G It has recently been found by Schuster7 in a gabbro from 

near Big Oak Flat, in California.

1 Geology of AVisconsiu, Vol. Ill, pp. 35-30.

2 Strong and Kloos: Neues Jahrbucli fur Min., etc.., 1877, pp. 27-31.

3 Geology of Wisconsin, Vol. Ill, p. 170, and Copper-bearing rocks of Lake Superior, p. 56.

4 Op. cit., pp. 57-58, 66-67, 84.

6 Bulletin No. 2, Geological Survey of Minnesota,p. 65.

6 Blum: PseudoinOrpliosen, Vol. i,p. 31; Vol, in, p. 93. Tschermak: Porphy rgesteino Oesterreiclis, 

18G9. p. 264. Ric.btbofen: Sitzb.d. Ak. d. Wiss. zu Wien, 1858, Vol. xxvn, p. 335. Lebmann: TJntersucliuug iiber die entstebung der Altkryafc. Scbiefergesteine, 1884, pp. 198, 231. Kobrbacb: Tscberinaks mineral. Mitth., 1885, Vol. vir, p. 27. Brauns: Neues Jabrb. f. Min., etc., Railage-Band V.,p. 

289. Bergt: Tscberniaks mineral. Mittb., 1888, Vol. x, pp. 320, 326. Doss: Tscherrnaks mineral. 

Mittli., 1889, Vol. XI, p. 41.

7 Nenes Jabrb. f. Miner., etc., 1887, Beilage-Band V., p. 515.
42 ROCKS ON PIGEON POINT, MINNESOTA. [BULL. 109.

It is evident that the change of augite into biotite is attended with 

loss of calcium and silica and a gain in. iron, aluminium, and the alkalies. In the gabbro from Pigeon point, aluminium and the alkalies 

could easily have come from the plagioclase, and the iron from the 

magnetite. It is noticeable that magnetite is nearly always very close 

to the biotite in those sections where this mineral is most plentiful. 

The calcium has probably passed over into the calcite, mentioned as 

present between the biotite folia.

The first stage in the alteration of the plagioclase is the loss of its' 

glassy luster. Little flakes of a brightly polarizing, colorless mineral, 

probably kaolin, are developed throughout its mass; chlorite accumulates in its cleavage cracks; and.between contiguous crystals there 

forms a micropegmatitic intergrowth of chlorite or a reddened feldspathic substance with quartz. This' micropeginatite almost always 

radiates from the edge of an altered plagioclase, and in a few instances 

its feldspathic component is optically continuous with that of the original crystal. As alteration progresses the plagioclase becomes clouded. 

It loses its poly synthetic twinning striations and takes on the appearance of the orthoclase( *?) in Irving's orthoclase-gabbro1 . The amount 

of quartz present becomes very great. The spaces between the other 

constituents are filled with radially arranged quartz aud.feldspar, and 

the original structure of the rock entirely disappears. This alteration 

of the plagioclase with the production of micropeginatite is especially 

noticeable near the contact of the gabbro with the red rock. It proceeds hand in hand with the alteration of augite into biotite as described 

above, and gives rise to a rock which resembles in some respects the 

intermediate rocks to be described in Chapter V.

Magnetite, pyrite, and apatite are much more abundant in the altered gabbro than in the fresh rock. The former is highly titaniferous. On its edges leucoxene or biotite is nearly always to be detected. 

The apatite penetrates biotite, chlorite, and even magnetite, 'and occasionally contains fluid inclusions, in which are little air bubbles.

In addition to the minerals mentioned above there was noticed in 

one specimen (No. 11222), taken from a place where the gabbro approaches the red granitic/rock, a green, transparent, isotropic'mineral 

with a hexagonal cross section. This mineral has a high index of 

refraction, and appears to form in little plates piled one upon the other 

in the manner of tridyrnite in the eruptive rocks, or of the plates of 

corundum so frequently seen in imperfectly developed crystals from 

North Carolina. Many of these little plates unite to form a group, 

which is always found associated with the alteration products of augite. 

Occasionally, as shown in b, PL v, they surround apatite. When occurring singly they resemble garnets with a diameter of 0.05""". Their 

quantity is so small that 'attempts made to isolate them in sufficient 

amount for chemical analysis proved futile.

1 The Copper-bearing Hocks of Lake Superior, li. D. Irving. Monograph, TJ. S. Geol. Survey, Vol. V, 

p. 51.
EXPLANATION OF PLATE VII.

VII. a. Section of altered gabhro (No. 11446), in which long, slender needles 

of hornblende are seen to have attached themselves to compact pieces 

of the same mineral, which have undoubtedly been derived from 

augite. Chlorite, quartz, kaolinized feldspar, magnetite, and sphene 

are the other minerals shown in the figure. From a photograph in 

natural light. Magnified 80 diameters.

b. Pseudamygdule of hornblende in altered gabbro (No. 11247). The 

circular area in the center of the figure is the pseudarnygdule, composed of green and yellowish-brown hornblende. Surrounding it are 

the altered remains of the original gabbro, in which the gabbro 

structure can still be detected. From a photograph in natural light. 

Magnified 80 diameters.
. S. GEOLOGICAL SURVEY BULLETIN NO. 109 PL. VII

a. ALTERED GABBRO.

b. PSEUDAMYGDULE OF HORNBLENDE IN ALTERED GABBRO.
BAYLEY.] ALTERED VARIETIES OF GABBRO. 43

At several places near the contact of the gabbro with the quartzites 

and the granitic rock, and at one point, on the top of a high hill north 

of Fisherman's point (see map, PI. in), are peculiar phases of the gabbro which merit a little attention. These peculiar varieties are remarkable for the extensive development of fibrous green hornblende in them. 

This hornblende occurs in irregular patches, in long, sinuous, and parallel ridges, and in circular areas. On a weathered surface it has disappeared and left in its place little hollows. The result is a rock 

presenting very muo,h the appearance of an amygdaloid. Instead of 

disappearing entirely the hornblende in some specimens has passed 

into chlorite, which stands out in ridges and irregular and circular 

patches, just as does the hornblende when present, and is of a bright 

red color where it has suffered alteration in consequence of forest fires. 

When examined under the microscope these rocks are seen to have 

retained to a certain extent their original structure. The decomposed 

feldspar crystals can still be detected in lath-shaped forms. Epidote 

in green grains is added to the secondary products mentioned above as 

characteristic of the feldspar. Augite is found in every stage of alteration in a single thin section. Intermingled with the fresh variety is 

another in which a fibrous structure is marked. From this fibrous 

variety there is a gradual transition through compact brown and green 

hornblende into fibrous hornblende, and finally into long acicular crystals of this mineral. These hornblende areas have at first the general 

wedge-shaped outline of the augite. As the alteration progresses, the 

wedge-shaped outline is lost, and the groups of hornblende become circular in shape, and thus give rise to the pseudoamygdaloidal cavities 

referred to above. Olivine can not be recognized in any of the sections 

of the rocks of this kind, although the abundance of chlorite, hornblende, and other alteration products of this mineral leave no doubt of 

its original existence. Biotite, leucoxene, apatite, and lozenge-shaped 

crystals of colorless sphene are also present in all the specimens of these 

planes examined.

PI. VI, b, shows a portion of a section of one of these rocks in 

which there are long needles of fibrous green hornblende, surrounding more compact cores of the brown mineral, and also slender acicular 

crystals penetrating chlorite. PL vn, a, represents an area in which 

augite has been replaced by quartz and hornblende. The quartz is 

without inclusions other than the long slender needles of light green 

hornblende which pass uninterruptedly from one quartz individual 

into another. Many of these hornblende needles, some of which are 

0-65mm in length, have attached themselves to more compact hornblende,1 and even to slightly altered augite. In b of the same plate 

one of the circular pseudo-amygdaloidal areas is seen. This is composed of brown and green compact hornblende, mingled with a little 

uralite and chlorite. The dark portion represents the green hornblende, and the light portion the brown.

1 Cf. Van Hise: Am. Jour. Sci., 1887, ATol. xxxni, p. 385.
CHAP TEE III.

PETROGRAPHICAL DESCRIPTION OF THE DIABASE.

The rock of the numerous small dikes on the point presents but few 

features to which attention need be drawn. Macroscopically it is a 

dark gray, heavy diabase, in the coarse varieties of which the diabase 

structure can readily be distinguished. It is more frequently altered 

than fresh. In the altered forms it possesses a slight greenish tinge, 

in consequence of the development of a small quantity of a fibrous 

green hornblende.

Under the microscope, the fresh, coarse-grained varieties are found 

to be composed essentially of plagioclase, augite, magnetite, apatite, 

pyrite, and chlorite. The plagioclase is in little lath-shaped crystals, 

with a maximum extinction of 27° on each side of their twinning bands. 

It is consequently a labradorite. The augite is in wedge-shaped areas 

between the plagioclase. When fresh it has a pale purplish pink color 

like the augite of the gabbro. The magnetite is in little grains and 

crystals in the coarser kinds, and in rods in the finer-grained varieties. 

Apatite is present in small colorless crystals, and chlorite in fibrous 

masses on the edges of the augite.

The altered varieties may be divided into two classes the micaceous 

and the non-micaceous kinds. In the latter the constituents are the 

same as those described above, with the addition of uralite. In the 

former a brown mica is also present.

THE NON-MICACEOUS DIABASES.

In the -least altered of the non-micaceous .varieties, the diabasic 

structure is preserved. The augite, however, has undergone decomposition, with the development of magnetite or ilmenite in irregular 

masses intermingled with -chlorite and a little uralite, or in little grains 

arranged in straight and curved lines. In some sections examined the 

mouoclinic augite is found, not in the wedge-shaped forms characteristic of this mineral in diabase, but in long, narrow crystals. These 

appear in some cases to be older and in others younger than the accompanying feldspar. Their mode of alteration is somewhat peculiar. In 

the first stage a few green fibers extend from the sides of the crystals 

44
BAYLEY.j NON-MICACEOUS DIABASES. 45

into the interior, and at the same time a cleavage is developed perpendicular to the long axes. Further alteration takes place, not along 

these cleavage cracks, but along the axes of the crystals. At this 

stage of their change the crystals appear as if perforated in this direction by cylindrical cavities, into which a quantity of dirty green chloritic substance has been forced. PI. x, a. &., represents two of these 

idiomorphic augites in this stage of their alteration.

In the final stages of the alteration of the rock all traces of augitic 

material have disappeared. Chlorite, uralite, and titaniferous magnetite have entirely replaced it. These minerals are not confined to the 

areas formerly occupied by augite, but are disseminated throughout the 

thin section and fill all the 

space between the plagioclase 

and its decomposition products 

and apatite. The magnetite 

(Fig. 6) is present in idiomorphic grains, in long rod-like 

forms, in little dots arranged 

in curved lines, and in fine dust 

scattered through all the other 

constituents with the excej)tio]i 

of the apatite. The apatite is 

much more abundant than in 

the fresh varieties, but possesses no peculiar features. 

The plagioclase in these rocks 

has undergone change to such 

an extent that its original outline has entirely disappeared 

and with it the diabasic structure of the rock. Quartz and a red feldspathic substance, both of which are probably derived from the plagioclase, fill the little corners between the altered labradorite. The amount 

of this feldspathic substance present is often so great as to give the hand 

specimen a reddish tinge. No olivinc was detected in any of the nonmicaceous diabases, although a few round areas composed of pilite and 

chlorite render it probable that this mineral was originally present in 

some instances. 

Round grains of a highly refractive, colorless, augitic-looking mineral 

are met with in a few sections of the least altered diabases. This 

mineral is older than the monoclinic augite, and has an extinction parallel to its cleavage. It gives rise upon alteration to a dirty green chlorite and a little uralite. Intermingled with these products are masses 

of magnetite, whose origin is evidently secondary. This magnetite is 

not only scattered through the chlorite and uralite, but it is also 

grouped around the exteriors of the highly refractive grains in such a 

way as to preserve their outlines even when all traces of their original

FIG. C, Icliomorphic secondary maguoUto in altered 

diabase.
46 EOCKS ON PIGEON POINT, MINNESOTA. [BULL. 109.

substance has entirely disappeared. From its optical properties and 

the nature of its alteration products the colorless mineral is probably 

to be regarded as an orthorhombic pyroxene.

The fine-grained phases of rock occurring On the edges of the dikes 

are all highly altered. Between feldspathic and magnetitic microlites 

is a dirty green interstitial substance, which polarizes slightly, and 

shows under high powers little fibers of chlorite and hornblende. No 

glass was observed in any one of these rocks, but the impression left 

by a study of their thin sections is that this did occur abundantly, but 

has been entirely replaced by secondary substances.

The material of one or two dikes contains large porphyritic crystals 

of plagioclase in a groundmass differing but slightly from those types 

already described. In one instance (No. 11265, from the center of the 

third shingle beach from the end of the point on its south side) the porphyritic crystals are surrounded by little rims of pyrite.

In other dikes the rock presents the appearance of an amygdaloid. 

In the midst of an altered groundmass it contains circular areas filled 

with calcite, chlorite, a little green hornblende, a few flakes of mica and 

a small quantity of an almost colorless, highly refractive, isotropic 

substance with the appearance of garnet. 1 Around these areas lie little plagioclase crystals with their long axes tangent to the circumferences of the circles, as if they had been shoved aside by the expansive 

force of some gas formerly contained in a bubble. At the same time 

there is a faint indication of a fluidal structure in the arrangement of 

the feldspar crystals in the body of the rock. This amygdaloidal 

character is not noticeable in the hand specimens of any of these rocks, 

except as it is brought out by weathering. On weathered. surfaces 

only a slight mottling distinguishes them from the nonamygdaloidal 

varieties. PI. Vli, b, shows the appearance of one of these areas when 

magnified 60 diameters.

A peculiarity characteristic of therock from several of the larger dikes 

is noted only when their thin sections are placed on sheets of white 

paper. It is then observed that they have a mottled appearance, which 

is due to the development of augite in large pieces in such a way as to 

include within itself many of the small plagioclase crystals. This 

augite is quite fresh and free from magnetite, while the interstitial substance is altered and contains magnetite in large quantity. This structure is quite common among the diabases of the Lake Superior region, 

and is known as luster-mottling.2 It has also been observed in the 

peridotites near Peekskill, New York, where it has been denominated 

"poicilitic." 3 (See b, PI. vm.)

'Cf. A. C. Lawson: Notes on some Diabase Dikes of the Bainy Lalco Region. Proc. Can. Inst., 

1887.

2Pumpelly: Proc. Am. Acad. Sci., 1878, Vol. xm, p. 200, and Geology of Wisconsin. Vol. in, p. 33. ITving: Copper-Bearing Rocks of Lake Superior. Monograph, U.S. Geol. Survey, Vol. V, pp. 69-72, 

and 370.

3 Or, H. Williams: Am. Jour. Sci., Jan., 1886, Vol. xxxi, p. 30.
EXPLANATION OF PLATE VIIL

VIII. a. Amygdule in diabase of dike (No. 11406.) Around the edges of the 

amygdule, which is filled with chlorite and other secondary minerals, 

plagioclase crystals are arranged with their long axes tangent to the 

surface of the cavity. From a photograph in natural light. Magnified 60 diameters.

b. Poscilitic structure in diabase of dike. The light-colored crystals 

are plagioclase, and the ground mass augite. This polarizes in two 

large areas, each of which incloses many feldspars. The black substance between the plagioclase at the right and left of the figure is 

clilorite. A narrow band of this crosses the center of the picture 

and separates the two augite individuals. From a photograph in 

natural light. Magnified 60 diameters.
U. 3. GEOLOQICAL SURVEY BULLETIN NO. 109 PL. VIII

a. AMYGDULE IN DIABASE OF DIKE.

b. PCECIUTIC STRUCTURE IN DIABASE OF DIKE.
BAYLEY.] MICACEOUS DIABASES. 47

THE MICACEOUS DIABASES.

The micaceous diabases a.re characterized by the possession of a 

strongly pleochroic brownish-red biotite. This is undoubtedly a product of the alteration of augite. It can be seen in parallel growths with 

this mineral, and also filling wedge-shaped areas from which the augite 

has disappeared. It includes all the minerals contained in augite, and 

in addition contains a large amount of secondary magnetite. The 

amount of the biotite present varies greatly. In some specimens only 

a few flakes are discoverable, while in others it constitutes the greater 

portion of the iron-bearing minerals present. Apatite is also abundant 

in these rocks. This mineral occurs not only in well developed forms, 

but also in skeleton crystals. Fig. 7 represents some of the forms 

assumed by the apatite in a rock from the large dike on the south shore, 

about three-fourths of a mile from the end of the point (PI. XIV).

Upon carefully noting the relations of the micaceous diabases to the 

surrounding rocks it is found that in every case in which mica is present in any considerable quantity the dike rock is either cut by veins or 

dikes of the red rock or is thoroughly impregnated with its 

substance. And, on the other 

hand,wherever the red rock approaches very close to a dike 

the rock of this latter is found 

to contain mica. From these 

facts it would appear that the 

biotite is a secondary mineral, 

produced in the diabase upon 

its contact with the red rock, 

just as it is in the case of the 

gabbro, where the biotite is 

found most abundantly developed on or near its contact

With the red rOCk. FIG. 7. Skolotou crystal of upatite iii altered diabase.

The properties of the rock present in the dikes cutting gabbro 

are not different from those of the rock occurring in the dikes penetrating the sedimentary beds. Both classes comprise dikes composed of 

comparatively fresh varieties of the same rock, as well as those consisting 

of decomposed material. As a general rule, the dikes in the gabbro are 

less fresh than those in the fragmental rocks, but otherwise no distinc. 

tion can be drawn. Porphyritic diabases are as common in the gabbro 

as in the fragmental rocks, and those containing red feldspar are as frequently found in the one as in the other.

A comparison of the freshest of the diabases with the coarse olivine 

gabbro of the point reveals scarcely any differences in the structure of 

the two rocks save tha.t due to the differences in coarseness of grain.
48 ROCKS ON PIGEON POINT, MINNESOTA. [BULL. 109.

In composition they differ at present only in the presence of olivine, 

which is a characteristic constituent of the gabbro, but is represented 

in the diabases only by its decomposition products. In method of alteration the two rocks are likewise slightly at variance. An abundant product of the alteration of olivine and augite in the gabbro is hornblende, while in the diabase the most prominent secondary product is 

chlorite. The two rocks are thus found to have, most of their 

characteristics hi common, while those in which they differ are of 

secondary importance. Coarseness of grain and freshness of the mineral components mark the gabbro, while fineness of grain and secondary products characterize the diabases. These differences, although 

of little geuetical value, are of importance in separating the two rocks 

practically. As has already /been mentioned, the coarseness of grain 

in the gabbro has been used by Irving as a means of distinguishing 

these from the diabases, and for this reason the coarse-grained rock 

has been designated gabbro in the present paper. The diabases are 

similar l to many of the diabases in the flows interstratified with the 

Keweenawan beds. Since no diabase dikes, so far as the writer is 

aware, have been found in the Cambrian rocks of the Lake Superior 

region, it is probable that the numerous dikes in the Aniniikie rocks 

mark the orifices 2 through which the Keweenawan flows above them 

were extruded.

1 Irvin^: Coppcr-Buaring Hooks of Lake Superior, pp. 61-91. 

'Cf. living: Op. cit., pp. 143,144.
CHAPTER IV.

PETROGRAPHICAL DESCRIPTION OF THE RED ROCK.

The red rock which is found so abundantly in the neighborhood of 

the gabbro has attracted the attention of all geologists who have visited 

Pigeon point.

Prof. W. W. Mather, according to Foster and Whitney, 1 regarded it 

as granite. i)r. Norwood,2 assistant United States geologist, in 1851, 

described it as a reddish syenitic rock, containing but a small amount 

of quartz. About thirty years later Prof. K H. Winch ell 3 of the Minnesota State Survey, saw a red rock associated with gabbro near the 

extremity of Pigeon point, and a rock red with orthoclase on its north 

shore,about a mile from its eastern end, the latter 4 of which he mentions as having probably originated by the fusion and recrystallization 

of the sedimentary beds through which the gabbro cuts. A microscopical examination of this rock has very recently been made by Dr. M. E. 

Wads worth,5 who regards it as an altered phase of some eruptive, the 

original nature of which he is unable to determine from the single section at his command. In 1881 Prof. E. D. Irving 6 examined a rock 

from Brick island (Little Brick island), one of the smaller group of 

the Lucille group of islands, about a mile south of Pigeon point. He 

describes it as "pink to bright brick-red in color, thinly and very distinctly stratified, dipping southeast S=p." He says further:

Its thin section reveals a rock very close to those red rocks of the Kewecnaw 

series, which I have described under the names of augite-syenite and granitic porphyry; that is to say, it is a mass of feldspar crystals, saturated with secondary 

quartz, arranged in the usual graphic form, while other larger quartz areas seem 

also to belong with the secondary quartz. Here and there is an. augito crystal to 

complete tho resemblance, and there is no trace of fragmental texture.

On Pigeon point Prof. Irving also found a red rock which "resembles 

in every particular the rock from Brick island." Eed rocks, similar in 

character to the red rock of Pigeon point, have been observed by various geologists at different places in the Lake Superior region, but have

1 Reporton the Geology of the Lake Superior Land District, Washington, 1851, Part 2, pp. 12 and 37.

2 Report of a Geological Survey of W isconsin, Io\va, and Minnesota. By D. D. Owen, United States 

geologist, Philadelphia, 1852, p. 399.

3 The Geol. and Nat. Hist. Survey of Minnesota for 188J. Ninth Annual Report, p. 70.

4 Ibid. for 1881. Tenth Annual Report, p. 57.

s Geol. and Nat. Hist. Survey af Minnesota, Bulletin 2. Preliminary description of the Pcridotytes, 

Gabbros, Diabases, arid Andesytesof Minnesota, 1887, p. 81.

«Tne Copper-Bearing Rocks of Lake Superior; Monograph, U. S. Geol. Survey, Vol. v, 1883, p. 3C9.

49

Bull. 109 4
50 ROCKS ON PIGEON POINT, MINNESOTA. [BULL. 109.

received comparatively little attention. MaoFarlane 1 seems to have 

looked upon them as eruptive in origin ; the other observers regarded 

them as altered sedimentaries. As these writers have mentioned them

only incidentally, references to their statements are unnecessary,

\

THE GRANULAR RED ROCK.

This red rock presents two phases differing markedly from one another. 

In its most typical aspect it is a fine grained, brick-red, drusy, granular 

rock, speckled with little dark green spots. Its color >is due to the 

prevalence of a red feldspar, which possesses at times a well-marked 

cleavage, but is rarely observed with crystal outlines. Scattered 

through this as a background are little particles of white quartz, and the 

green spots mentioned above, which microscopical examination proves to 

be chlorite. In a few cases a light-colored plagioclase can be. seen intermingled with the red feldspar. The drusy cavities are lined with crys% 

tals of red orthoclase and quartz. The latter contain only the planes 

iE and <x>B. The former have a specific gravity of 2-4-9, and are 

bounded by OP, <*>P, ooPdo and P<5E>. In some cases the interiors of 

the druses are partly filled with calcite, in which case the crystals project into its mass. x

When fresh the rock possesses a tendency to break most easily in a 

horizontal direction. In certain exposures on the sides of cliffs this

tendency is emphasized by weathering to such an extent as to produce 

a parting which is as regular as 

the parting along bedding planes in 

many sedimentary rocks. This parting is particularly noticeable in a 

cliff1 hear the eastern end of the 

point (PI. xvi), where the parting 

pla.nes are nearly parallel to the 

bedding planes of the surrounding 

quartzite (Fig. 8). On Little Brick

FIG. 8. Sketch of cliff side, showing regular part- island, aS announced by Irving (10C. 

ing produced in rod rock by weathering. {& o

, Under the microscope the coarser-grained specimens of this variety 

of the red rock are seen to be composed essentially of an hypidiomorphic granular aggregate of at least two feldspars, quartz, and 

chlorite, with a few subordinate constituents muscovite, rutile, leucoxene, hematite, and apatite.

The feldspars embrace a striated plagioclase, twinned according to 

the Carlsbad law, and in one instance according to the Mannebacher 

law, and a second less well-individualized feldspar, which is younger 

than the plagioclase, but slightly older than the accompaning quartz.

1 Geol. Survey of Canada, lleport of Progress, 1863-'6C, p. 142. ^
BAYLISY.] THE GRANULAR RED ROCK. 51

It surrounds the plagloclase and is intergrown with the quartz in micropegmatitic and granophyric forms, and is found also in rudely outlined 

crystals Avith a square cross section. Both the plagioclase and the 

granophyre feldspar contain little pliites of hematite, but the number 

of these in the latter is very much greater than in the former, so that 

the plagioclase appears lighter in color. It has also undergone less 

alteration than the red variety, and is therefore much more transparent 

in the thin section. In some instances the twinning lamellae were distinct enough to yield measurement indicating a composition near that 

of oligoclase. The specific, gravity is 2«61 to 2-65. The red feldspar is 

so much altered that no measurements of extinction angles could be 

made, although traces of twinning lamella can still be detected. These 

lamellce are broader than those in the plagioclase and frequently appear as if produced by Carlsbad twinning. In addition to the hematite plates there are present as inclusions in both varieties of the feldspar apatite, leucoxene, chlorite, and little shreds of muscovite or 

kaolin. The chlorite occurs principally in the plagioclase and the 

kaolin in the red feldspar. They are both secondary products, derived 

from the alteration of the minerals in which they occur.

The amount of the plagioclastic constituent present in the rocks of 

this type varies quite widely. Where it is most abundant the rock is 

lighter in color than are the varieties in which the predominant feldspar 

is the red variety. The micro-pegmatite is less common in the former 

and the quartz more often possesses the club-shaped outlines' indicative of a secondary origin. Where the red feldspar is the predominant 

one, it is found in the rudely outlined crystals mentioned above, and 

also in micropegmatitic intergrowths with quartz. The crystals often 

show traces of Carlsbad twinning, but, as has been stated, they are so 

much altered that their extinction could not be measured. Other apparent crystals consist entirely of intergrowths of quartz and red feldspar with the crystal outlines of the latter mineral.1 In these the structure of the rock is determined by thev abundance of these skeleton feldspar crystals, and is like that of the typical augite-syeuite, as represented in PI. xiv, Fig. 1 of Irving's " Copper-bearing rocks of Lake Superior." Between these crystals the interstices are usually filled with 

club-shaped masses of quartz and feldspathic substance without any 

definite shape. Occasionally the place of this irregular growth of 

quartz and feldspar is taken by a mosaic of quartz grains.

In order to determine the nature of the red feldspar, a separation of 

the constituents of one of the freshest of the granular rocks was made 

in the usual way. Most of the feldspar fell at 2-577. When examined 

under the microscope the powder was found to be free from quartz, 

though"slightly altered, and.filled with hematite inclusions. An analysis yielded Mr. Whitfield the figures in column vin. The composition

1 Jos. P. Iddiugs: Obsidian Cliff, Yellowstoue National Park. 7th Annual Eeport, U. S. Geol. Survey PI. xv, Fig. 5.
EOCKS ON PIGEON POINT, MINNESOTA. [BULL. 109.

of anortlioclase1 from the rliyolite of Pantelleria is given in column D. 

In E. is the calculated composition of an auorthoclase corresponding to 

Ab2Or.

SiO*. .........

AlA--- .----.

Fe2 03 .........

CaO ..........

MgO..........

K2 0 ..........

Na.20 .........

Igii. ...........

Total ....

Sn er

VIII.

65-00

18-22

2-64

 06

4-18

8-40

 46

100-32

9 >ei77

D.

66-06

19-24

 u

1-11

 u

5-45

7-63

100-14

2-381-2-592

E.

19-165

7-721

100-00

It will thus be seen that the red feldspar has a composition corresponding to that of anorthoclase. Its optical 2 and physical properties, 

so far as they could be determined, are not in opposition to this view. 

They are, however, not to be relied upon as distinctive for the mineral, 

because of its great alteration.

The quartz is in irregular areas filling the interstices between the 

other constituents, and is also intergrowu with the red feldspar in micropegmatitic and granophyric forms. The granophyre is almost always 

present in greater or less amount in the little angular areas between 

contiguous feldspar crystals, and it often surrounds a plagioclase crystal as a zone. In some instances the plagioclase is a comparatively 

fresh, clear mineral, with prominent twinning striations, while thezone

is composed of a fibrous intergrowtli of red feldspathic substance and

quartz, in which the fibres of the quartz are developed with their longaxes perpendicular to the surface of the internal plagioclase. The exterior of the zone is bounded by an outline parallel to that of the kernel. 

The most common inclusions.of the quartz are fluid cavities with little 

dancing bubbles, red feldspar, and tiny, black, dust-like particles of 

some substance whose nature could not be determined.

The chlorite is, in all probability, the result of the alteration of a 

previously existing biotite. Traces of this mineral can still be detected 

in many sections, and in one (No. 11541) the biotite is quite fresh. The 

chlorite occurs both in little radiating spherulites crowded close 

together, and in plates enclosing quartz and feldspar. Calcite, rutile, 

magnetite or ilmenite, and leucoxene are its most common inclusions, 

while the little pleochroic halos (Hofe),^characteristic of this mineral 

when derived from biotite, are not rare.

1 Forstner: Zeits, Chr. liir Kryst. 1883, p. 125.

1 For a careful description of the optical properties of anortlioclase, or soda-inicrocliue, see Dr. J. S. 

Hyland: on Soda-Microcline from Kiliniandscliaro. Geol. Magazine, III, vol. 6, No. 298, p. 160.

3For the discussion concerning the nature of these halos, see Neues Jahrb, fur Min., etc., 1888,1, p. 

165.
BAYI.EY.] PORPHYRITIC RED ROCK. 53

Associated with the chlorite is oftentimes a very light-green fibrous 

mineral in groups of sheaf-like bundles. It is easily distinguished 

from the chlorite by its bright polarization colors and its silvery luster 

between crossed nicols. It is probably a secondary muscovite or sericite, derived in part from the biotite. In one or two instances bleached 

biotites were observed whose edges presented the appearance of frayed 

fibers of muscovite.

Eutile forms quite a prominent accessory constituent in some specimens. It is found in irregular masses of a dark brown color, and also 

in long, rod-like forms. In both instances it is intermingled with 

Leucoxene, and frequently with chlorite. When present in the rodlike forms it is without doubt an alteration product of titanic iron. 

Eutile is also widespread as sagenite in the chlorite. Calcite is present as an alteration product of feldspar. It occurs in little nests in 

drusy cavities, and also in the interior of plagioclase crystals.

The structure of the rock, as has already been stated, is the hypidioinorphic-granular, in which the mica, the greater portion of the feldspar and quartz have separated in the order mentioned. PL ix, a, 

represents the structure of a specimen (No. 11541) collected in the interior of the western portion of the point.

THE PORPHYRITIC RED ROCK.

* N

The second phase of the red rock resembles quartz-porphyry. Budely 

outlined quartz crystals and occasional brick-red and greenish-white 

feldspars are scattered through a very fine-grained ground mass of a 

dark red or purplish color. Although a few of the light colored feldspars appear in the hand specimen to be homogenous throughout, a 

much larger number upon close inspection are discovered with a core 

of the brick-red mineral. The two feldspars are often united in Carlsbad twins, composed.of two zones, the inner one red and the outer one 

white. On weathered surfaces the latter is dark and the former an 

opaque white. The red feldspar is also occasionally found forming the 

center ,of an area consisting principally of a yellowish green chloritic 

substance.

When one of these quartz-porphyries is broken, the little quartz crystals are observed projecting above the fractured surface. Examined 

casually, they resemble pieces of pebbles whose exteriors have been 

worn smooth by tlie action of running water. All traces of interfacial 

angles have been lost, and the general shape of the bodies is that of a 

cylinder, the ends of which have been rounded. Under the lens their 

surfaces are found to be pitted with little depressions that remind one 

of the depressions produced in the roughening of glass by the sandblast.

Under the microscope these rocks are characterized by the beauty of 

their granophyre structure. All gradations between the granular 

structure described in the previous section and the typical porphyritic
54 ROCKS ON PIGEON POINT, MINNESOTA. [BULL 109.

structure have been observed, and in all there is more or less of the 

true grauophyre. In the porphyries the porphyritic crystals are quartz 

and feldspar.

In its less perfectly developed phases the quartz occurs in round, 

elliptical, and crescent shaped areas, and includes portions of the 

groundrnass. When more 'completely developed, these areas have a 

lenticular cross section resembling in general outline that of a di-hexhedral crystal, always extinguishing as they do in the direction of their 

longest axis. The quartz is clear and transparent, and contains no 

inclusions other than fluid cavities with movable bubbles.

A very few irregularly outlined feldspar areas represent the porphyritic crystals of this mineral in their earliest stages of development. 

Other areas which appear in the hand specimen as crystals are seen 

under the microscope to be composed of granophyre substance with, the 

outlines of crystals. Other apparently homogeneous crystals consist 

of a core of plagioclase, with an outer rim of granophyre, while others 

are composed of two kinds of feldspar in zonal growths. The red crystals in the midst of a green substance are cores of feldspar, that were 

originally surrounded by a zone of a different composition, which 

yielded chlorite upon alteration. The most interesting intergrowths 

are those in which the nucleus is a comparatively fresh striated plagioclase and the external zone a mass of secondary quartz and altered feldspathic substance. In these the outlines of the areas are always elliptical in shape. Figure g of Plate x represents one of these areas as 

seen between crossed nicols. The interior of the area is occupied by 

an irregularly-shaped piece of striated plagioclase,, around which is a 

zone of secondary quartz and red feldspar.

The groundmass in which these porphyritic crystals are imbedded 

has the structure of Rosenbusch's typical granophyres.1 It consists 

in large part of pseudospherulites (PI. ix, b) composed of fibers 

of quartz and red feldspar in granophyric intergrowths. Between 

these are little areas of micropegmatite and sometimes mosaics of 

quartz-grains. In addition to the pseudospherulites there is often a 

growth of granophyre substance around the distorted and corroded 

porphyritic quartzes and in the undeveloped feldspars mentioned above. 

The granophyre surrounding the quartz crystals is composed of fibers 

which radiate from the surfaces of the crystals and form a zone, which? 

in ordinary light, resembles the "quartz globulaire" of Levy, but in 

which, the quartz fibers, between crossed uicols, are seen to be optically 

independent of the .orientation of the substance of the mineral which 

they surround.

Chlorite, secondary brown biotite, and various undeterminable green 

alteration products of some constituent whose nature can no longer be 

recognized, but which, in some cases appears to have been augite, are 

about as abundant as the corresponding secondary products in the

1 Mikroskopische Physiographic, 1887, 2d ed., pp. 383-388.
EXPLANATION OF PLATE IX.

IX. a. Thin section of granular red rock (No. 11541), showing the structure. 

The'red substance is feldspar, whose color is due to alteration, 

whereby little plates of hematite have been formed. The white 

mineral is quartz. "It is in round granules, except in the upper portion of the figure, where it forms a micropegmatitic growth with the 

red feldspar. The green areas are chlorite that has been derived 

 from mica. In the small area at the upper portion of the figure the^ 

cleavage of the mica can still be seen, and its pleochroisrn is still 

marked. The black spots are magnetite and the white substance 

inclosed in the chlorite is leucoxene. Under crossed nicols the apparently single area of red feldspar on the right-hand side of the 

figure breaks up into an aggregate of granules, many of which are 

striated in the direction of their longer axes. The figure represents 

the section as seen in natural light, magnified 40 diameters, 

b. Thin section of porphyritic red rock (No. 11373). The large white 

body in about the center of the field is a quartz crystal whose extinction is parallel to a line drawn vertically from the apex of the 

figure. The irregularly shaped brown substance to' the left of the

quartz is some secondary product, perhaps chlorite, which has been

stained by iron hydroxides. The rest of the figure is occupied by 

spherulites of red feldspar and quartz in very fine granophyric intergrowths. A few spicules and little nests of chlorite are scattered 

through these spherulites, and between them (as in the extreme 

lower right-hand side of the figure) are small areas of granular 

feldspar and quartz. Natural light. Magnified 40 diameters.
U.S.GEOLOGICAL SURVEY. BULLETIN NO. 109 PI...DC.

a (X40)

GRANULAR RED ROCK.

 "' ¥p

^ -^* - '^^.oto . ^- -

b (X40)

PORPHYRITIC RED ROCK.
BATLEY.] COMPARISON OF RED ROCKS. 55

granular red rock. Some of these substances are arranged in long narrow lines, as if derived from microlites. Others are aggregated into 

clusters, or are concentrated around quartz crystals. Tiny grains of 

hematite and titaniferous magnetite, and little plates of a green chloritic mineral are scattered everywhere throughout the groundmass, but 

never in any large quantity. In the pseudospherulites they are arranged 

in a general way parallel to the fibers of quartz and feldspar.

Leucoxene and a highly refractive uniaxial colorless mineral, taken 

to be zircon, are occasionally met with. The former is mingled with 

biotite in the vicinity of magnetite, and the latter occurs in crystals 

with a rhombic cross section, which remains dark through an entire 

revolution between crossed nicols. The zircon forms little crystals 

whose cross section is rectangular. They are of a yellowish tinge, 

possess a, zonal structure, show brilliant interference colors between, 

crossed nicols, exhibit a shagreenous surface in natural light, and extinguish parallel to the sides of the rectangular cross sections. Calcite is 

quite abundant as an alteration product of some of the fibers intergro wu 

with the quartz, and also in the little cavities contained in the rock. 

In PI. ix, b, one of the pseudospherulites and a rounded quartz crystal 

are represented as they appear in one of the most typical quartz-porphyries. The rock in which they occur was obtained from the top of 

the quartz-porphyry bluff just west of the Little Portage.

COMPARISON OF THE GRANULAR AND PORPHYRITIC RED ROCKS 

WITH EACH OTHER, AND WITH OTHER ROCKS OP THE SAME 

NATURE OCCURRING IN THE LAKE SUPERIOR REGION.

The microscopical characteristics of the porphyritic and the granular 

varieties of the red rock indicate the probability of the identity of the 

two. Although "the most typical quartz-porphyry is quite different in 

structure from the typical granular variety, gradations between these 

two types can be recognized. Their mineralogical composition is the 

same and, as is shown below, their chemical composition is identical.

In order to obtain an average composition for the two types of the 

red rock, pieces of equal size were broken from the freshest specimens 

and powdered, and these powders were analyzed by Mr. W. P. Hillebrand. In column IX is the result of .an analysis of the powder of 

seven of the freshest specimens of the granular rock. In X is an 

analysis of the powder of three quartz-porphyries. P is an analysis of 

the granite from Bejby, Sweden, containing red orthoclase, gray, and 

brownish gray quartz, black mica, and a few flakes of golden yellow 

mica. 1

1 Gerhard: Neues Jalirb. fur. Hin., etc., 1887, ir, p. 271.
56 ROCKS ON PIGEON POINT, MINNESOTA. [BULL. 109.

Si02 ..........

TiOa..........

ALA---------

Fe A .........

FeO* .........

MnO. .........

CaO ..........

SrO ..........

BaO ..........

HgO.-.- ......

K2O ..........

Na.,0. ........

LijO. .........

H20 ..........

P205 - ---------

Cl ............

Total ....

IX.*

72-42

 40

13-04

 68

2-49

 09

 66

 1 f\

 ?&

4-97

3'44

1-21

 20

Trice

> 100-37

2-G20

X.*

74-00

 34

12-04

 78

2-61

 05

 85

^Crico

 19

 42

4-33

3-47

 80

 06

99-93

2-565

F.

73-32

14-25

2-60

 no

 83

4-96

3-21

1-22

100-48

*During the pulverization of the rocks ;i small quantity of metallic iroii became mixed with the 

powders, which may have reduced some of the ferric iron to the ferrous condition, through the agency 

of the hydrogen generated in' the course of solution. The above figures are corrected for the metallic 

iron present, but could not be corrected for the amount of reduction produced, which was iu any 

event very small, since the entire amount of metallic iron present was not greater than 0.35 per cent.

An inspection of the figures in columns IX and X shows that there 

can be but little doubt that the two rocks analyzed are parts of the 

same mass. The very slight differences noted in the amounts of silica, 

alumina and potassa are not greater than are frequently found in different hand specimens of the same rock.

The close agreement in composition between the Pigeon point rocks 

and the granite from Bejby (F) becomes a matter of interest, when we 

remember that the latter has been called a soda-granite because of the 

supposition, based on an earlier analysis, that the rock contained a 

feldspar rich in sodium. Gerhard, however, claims (loc. cit.) that the 

rock can not be regarded as a soda-granite, but makes no attempt to 

isolate the feldspars and subject them to examination. He states that 

the feldspars present in the rock are plagio.clase, orthoclase, and microcline.

A separation of the components of one of the freshest of the granular 

rocks from Pigeon point yielded the minerals in the following percentages :

Feldspar.................................................... 62. 76

Quartz ..................................................... 34.94

Chlorite, biotite, etc ........................................ 2.30

The feldspar fell in two stages; first, when the specific gravity of the 

heavy solution used was between 2.61 and 2.G5, and, second, when the 

specific gravity was 2.577. The first feldspar is a plagioclase with a 

composition near that of oligoclase, while the second is probably anorthoclase, as indicated by its analysis.
BAYLEY.] COMPARISON OF ROCKS. 57

Upon comparison of the composition of this feldspar with that of the 

rock in which it occurs, it will'be noticed that whereas in the former 

the quantity of soda present is much greater proportionately than the 

potassa, in the latter the potassa is in excess. A calculation of the 

composition of the residue (a) left after allowing for the quartz and the 

small amounts of plagioclase, chlorite, apatite, and biotite present in 

the rock, gives a result which agrees very closely wth the theoretical 

composition of a feldspar of the formula Abi Ort (&).

Si02 . .........

A1203 .. .......

K2 0 ..........

Na,0 .........

(I,

63-52

22-00

8-56

5-92

b.

6C-51

19-05

8-70

5-74

It appears probable from these figures that there is present in the 

red rock a series oi potassium-sodium feldspars, of which some members contain more potassa than does the one analyzed. Those members which are richer in potassa, if Lagorio's hypothesis l of the order 

of crystallization in eruptive rocks is correct, would probably have 

crystallized toward the last stages of the rocks' solidification, and thus 

would be apt to be intergrown Avith quartz in the micropegmatite, and 

would therefore have fallen wheu the specific gravity of the separating solution was somewhat above 2-577 in the neighborhood of 2-C5 

(the specific gravity of quartz).

From the descriptions of the thin sections of the two types of the 

red rock, and from the close agreement in their composition, it is evident that they are parts of the same rock mass, whatever may have 

been its origin. There can belittle doubt that the porphyry has solidified from a molten magma. It presents all the structural features of 

Rosenbusch's Vogesen gtfanophyres.2 It is very similar in macroscopic 

and microscopic appearance to the Keweenawan quartz-porphyries 

described by Irviug3 as flows in the copper-bearing rocks on both 

sides of Lake Superior, and which are undoubtedly of eruptive origin. 

The granular phase occurs in dikes and veins cutting slates and 

diabase, and is identical in structure with some of Irving's augite-syenites,4 which were regarded by that geologist 5 as altered forms of an 

eruptive rock. It is identical with the red rock of Little Brick island, 

which appears as a flow, and is not very different from the material of 

one of the numerous ribbon-like dikes which constitute the principal 

portion of Brick island, a member of the same group to which Little 

Brick island belongs.

JLagorio: Mineral, und Petrog. Mitth., 1887, vui, p. 422. 

2 Dio Steiger Schief'cr. Strassburg, 1870. 

3 Copper-Bearing Hooks, p. 95. 

 "Loc. cit., p. 112, et seq. and p. 369.

5 For a discussion"'of the nature of tho augite-sycnites anil their relations to the Pigeon point rock, 

see Am. Jour. Sci., Vol. 37, Jan., 1889, p. 54.
58 EOCKS ON PIGEON POINT, MINNESOTA. LBULL. 109.

An analysis of the Little Brick island rock was made by Mr. L. G-. 

Eakins for .ID other purpose. It is given in column XI below, and with 

it, in column X, is repeated the analysis of the quartz-porphyry of 

Pigeon point, as given on page 56.

Si02 ..........

Ti02 ..........

A1203 - - - 

Fe2 03 .- .......

FeO ..........

MnO .........

CaO. .........

SrO..... ......

BaO ..........

MgO..........

K20 ...........

Na2 0 ........

H20. .........

P

r\

Pi

Total ....

XI.

73-70

 12

12-87

3-7C

 31

 07

 14

 11

4-5C

3-63

 57

tr.

99-84

X.

74-Ou

 34

12-04

 78

2-61

 05

 85

 12

 42

4-33

3-47

 86

 06

99-93

The very close agreement in the chemical composition of these two 

rocks accords well with the agreement in their mineralogical composition and structure. The Little Brick island rock is a bright red porphyry in which the porphyritic' crystals are quartz and feldspar, and 

the interstitial substance is principally a.granophyric intergrowth of 

quartz and red feldspar. The few secondary products present in it 

are the same as those in the Pigeon point rock. Augite is also present 

in an occasional light-colored crystal.

The conditions which in one case gave rise to a granophyre on Pigeon 

point, and in another allowed the development of a holocrystalliue 

rock from the same magma, could not be determined. It appears that 

the normal development was the granular one, but that in certain 

localities where the magma cooled in contact with preexisting rocks, 

the porphyritic structure was assumed.

The classification of these rocks is a matter of no little difficulty. 

In a former paper 1 an attempt was made to show reasons for placing 

them among the quartz-keratophyres. A quartz-keratophyre, as defined 

by Eosenbusch, 2 is a paleovolcanic rock composed of quartz, a soda: 

potash feldspar, and usually a little augite. The Pigeon point granophyre contains quartz in porphyritic crystals. It contains also a sodapotash feldspar and decomposition products of a bisilicate which in all 

probability was augite. The characteristic structure of the quartzkeratophyres is porphyritic, like that of all other effusive rocks, while

1 Am. Jour. Sci., Vol. 37, Jan., 1889, p. 54.

2 Mikroskopische Physiographic, 1887, II, pp. 434-442.
BAYLEV.] COMPARISON OF ROCKS. 59

that of tlie predominant variety of the Pigeon point rock is granular. 

This deviation from the normal structure of the quartz-keratophyre 

can readily be accounted for on the supposition that the Pigeon point 

rock represents only the more deep-seated portions of a mass which 

near its upper surface may have been porphyritic. There is no absolute method by which we can determine how much of the material .of 

the Animikie beds upon the point has been removed by erosion. If we 

regard the rock upon Little Brick island as the bottom of a flow which 

once overspread the slates and quartzites in the Pigeon point region, the 

amount of material which was removed in reducing the point to its present altitude was over 1,100 feet. That this is by no means an exaggerated estimate is proven by the fact that the shortest distance between 

the red rock on the point and the southern limit of exposures of slates 

and quartzites on Susie island (see map, PI. in) is over a mile, measured 

in the direction of the dip of the fraginental rocks, which with a dip 

of 15° would give a thickness of over 1,370 feet of sedimentaries that 

must have been removed before exposing what is now the surface 

layer of these rocks on Pigeon point.

The most acceptable name for the porphyritic variety of the red rock 

is undoubtedly quartz-keratophyre. It describes both -the composition 

and structure of the rock and its general appearance. The granular 

variety corresponds most closely to the soda granites! It can not be 

regarded as an augite-syenite, since it contains a great deal of quartz 

that is not secondary. Eosenbusch 1 refers to the keratophyres as 

paleovolcauic equivalents of the augite-syenites. Following this view 

the quartz-keratophyres must be looked upon as paleovolcanic augite 

granites, and as we distinguish as soda granites those varieties of granite containing a sodium feldspar or anorthoclase, then the quartz 

keratophyre should be denominated augite-soda-grauite. The augite 

constituent is, however, so small in amount that it would seem best to 

neglect it as an essential component, arid to call the rock a soda granite. The designation of different portions of the same rock mass by 

different names is of doubtful propriety. In the present case it serves 

the purpose of calling attention to two of the most prominent types of 

structure derived by the cooling of a molten magma under different 

conditions.

1 Roseiibusch: Mikroscopische Physiographic, 2d ed., p. 442.
CHAPTEE V.

PETROGRAPHICAL DESCRIPTION OF THE ROCKS ON THE CONTACT BETWEEN THE RED ROCK AND THE OLIVINE GABBRO.

At the contact.between the granular red rock,and the gabbro is a 

series of rocks which partake partly of the nature of the former and 

partly of that of tbe latter. The most acid varieties differ but slightly 

from the typical red rock. They are a little darker in color and contain a little more plagioclase. The darker color is due to the increase 

in the number and size of the green spots, which have been described 

in the case of the red rock, as consisting of chlorite. In addition 

to these there is more or less altered hornblende, which can readily 

be detected in the hand specimen. As this rock approaches the gabbro it becomes still darker. The amount of plagioclase in it increases. 

It becomes much coarser grained and contains a much greater quantity of iron-bearing constituents. In this phase the structure is granitic, with a tendency to the ophitic. Still closer to the gabbro a rock 

is observed which is very dark in color. It contains a small amount of 

red feldspar and quartz, but otherwise is very little different from the 

gabbro itself.

These gradations between,the gabbro and the granular red rock are 

noticed wherever the two rocks are in contact, except in the two cases 

previously referred to. In most instances the gradation is so uniform 

that no line of junction can be detected between the stwo rocks. In 

other cases quite a sharp line exists between them. But even at these 

places the rock on either side of the line of contact is very evidently 

modified by the presence of the rock on the other side, and there are 

present at least two of the varieties mentioned above. Along the contact between the granophyrB and the gabbro, at the base of the quartzporphyry bluff, for instance, the intermediate rocks are wanting. There 

is here a sharp contact between the two rocks. On the gabbro side 

no change in the character of the rock seems to have been effected. On 

the granophyre side the rock is a little darker than the granophyre at 

some distance from the contact, but this change in color is due to the 

presence of inclusions of the gabbro.

The best development of the intermediate rocks is found on the 

60
BAYLEY.] PETROGRAPHIC DESCRITION OF ROCKS. 61

south side of the" point near its eastern extremity. At this place the 

gradation of the acid red rock into the basic 'gabbro can be seen very 

beautifully. All the rocks are well exposed, so that the change in their 

character can be traced without any difficulty. Here the red rock is 

the granular variety. As it approaches the gabbro it becomes quite 

coarse grained. In the hand specimen large cleavage surfaces of red 

feldspar and long acicnlar crystals of a dark hornblende are very 

prominent. The dark crystals have a bronzy luster on their cleavage 

surfaces, Avhich are very perfect. A little nearer the gabbro the rock 

is still coarser in grain. The feldspars now include not only the red 

variety, but also a light greenish yellow plagioclase. The bisilicate 

occurs in large lath-shaped crystals, a number of which are united into 

arborescent forms (Fig. 9). Very near the gabbro is a rock in which

but a little red feldspar can be detected. The predominant minerals

are a glassy, greenish white plagioclase with bright cleavage surlaces, 

and a dark augite. Its structure is like that of the finer-grained uonporphyritic gabbro. No line of contact between the gabbro and the 

red rock or between their intermediate varieties can anywhere be discovered.

The lighter colored of these intermediate rocks examined microscopically is found to be composed of broad lath-shaped crystals of red 

feldspar; irregular areas of the same mineral. intergrown with quartz in inicro-pegmatite; the remains of what were originally 

hornblende crystals; large irregular areas 

of a bright green chlorite; crystals and rodlike masses of a titaniferous magnetite; 

apatite, rutile and an occasional zirconC?).

The feldspar crystals are crossed by bands 

running in the direction of their long axes. 

These bands are broad, and in most cases 

are evidently due to Carlsbad twinning. 

All of this feldspar is more or less altered 

into kaolin and-chlorite, both of which occur 

in small flakes mingled 0 with tiny plates of ^ L

hematite.

The hornblende is much changed. Only occasionally can the original 

outlines of greenish brown idioniorphic crystals be detected. It is 

more frequently chloritized. Large irregularly shaped areas, composed 

of fibrous chlorite, are presumably a' secondary product derived from 

the alteration of this mineral. Scattered throughout the mass of chlorite are grains of magnetite, surrounded by rims of leucoxene, little 

crystals of apatite and rarely an irregularly shaped piece of rutile. As 

in the case of the red rock the most common occurrence of the rutile 

is as a sagenitic web in the chlorite derived from the hornblende (PI. 

x,/). Between these constituents, and more particularly in the angles

tula i rocks.
62 EOCKS ON PIGEON POINT, MINNESOTA.

between contiguous feldspars, is quite a large quantity of quartz 

and red feldspar in micro-pegmatitic intergrOwths. This micro-pegmatite sometimes extends out from the edges of the feldspar, but more 

frequently exists in areas which are independent of all the other constituents. In these the different quartz areas extinguish simultaneously, as does also the feldspar.

In the second stage of the transition the thin section shows that the 

amount of red feldspar present has decreased considerably. In its 

place there has been added a large quantity of a clear, colorless plagioclase, with two systems of twinning lamellae and cleavage cracks, cutting each other at angles of about 83£° and 96£°. The specific gravity 

of this plagioclase is about 2-65, and the maximum extinction of bands 

on each side of a twining plane in- the zone OP. o> P 55 is 9°. It is 

crossed by irregular fissures, in the crevices of which a green earthy 

substance has been deposited. In many places this plagioclase has 

undergone slight alteration with the production of flakes of chlorite and 

kaolin. The red feldspar is confined almost entirely to the micropegmatite, which occurs here, as in the former case, in the angles between 

the other constituents. The specific gravity of a powder consisting 

principally of this red feldspar was 2-583.

Hornbleade is found in well-formed, fresh, idiornorphic crystals in 

quartz, and also in particles which have undergone chloritization. It 

occurs also in fibrous masses, which have probably been derived from 

augite. A peculiar feature of the fibrous hornblende is the interposi-j 

tion of a brown substance (probably limonite or some other hydrated 

oxide of iron) between the fibers, that gives the mineral the metallic j 

luster noticed in the hand specimen. In addition-to these two varieties 

of hornblende, there is a third with the properties of uralite. It surrounds the second variety and occurs as isolated needles in the quartz.

Chlorite is less abundant than it is in the lighter rock described above. 

It is found in rosettes, intermingled with hornblende, magnetite, and 

apatite. The last mentioned mineral is very much commoner than in 

either the typical red rock or in the lighter phase of the intermediate 

rocks. It occurs as an inclusion in all the other minerals but the plagioclase, and is perhaps most abundant in the quartz and red feldspar. 

In both macroscopic and microscdpic appearance this rock resembles 

some of living's orthoclase-gabbros, 1 as well as some of the altered 

phases of the olivine-gabbro, -described in the earlier pages of this bulletin, as characteristic of this rock where it approaches the red rock. 

The main distinction between the altered phase of the olivine-gabbro 

and the transition rock consists in the presence of biotite in the former 

and its absence from the latter..

On PI. x, e. is the representation of one of the intermediate rocks, 

which is just midway in composition between the gabbro and the red 

rock. In it can be seen an idiomorphic hornblende crystal in quartz, 

and also a portion of a large plagioclase crystal. The difference in

1 Copper-Bearing Bocks, p. 50.
EXPLANATION OF PLATE X.

a, 1). Idiomorphic augito from diabase (No. 11265). The augite is in 

long, narrow crystals with terminations at one end. Through their 

centers run little canals filled with a dirty green chloritic substance, 

and the same material extends from the peripheries of the crystals 

into their interiors, until finally all of the augite is replaced by chlorite. a shows a crystal in which the peripheral alteration is but 

just beginning. In the crystal represented in & all of the original 

augite has been replaced by chlorite, with the exception of a very 

narrow band between the axial caual and the peripheral chlorite. 

Magnified 40 diameters. Natural light.

c, d. Thin sections of what Avere originally feldspar grains in sandstone (No. 11350). In c the grains are shown in natural light, where 

their outlines are clearly marked, d, in polarized light, shows how 

the different grains when changed into quartz give rise to differently 

orientated quartz areas. These by merging into the interstitial 

quartz produce a quartz mosaic between crossed nicols, in Avhich the 

-boundaries of the individual original grains are lost. Magnified

' about 40 diameters.

e. Thin section of intermediate rock on the contact between the oliviue 

gabbro and the red rock (No. 11209). The area represented in this 

figure shows the general structure of the rock. The irregular white 

mineral on the left is quartz, containing microlites of apatite and 

a large icliomorphic crystal of brown hornblende. The white substance in the upper right-hand corner is plagioclasc, and the lightgreen mineral just below this is uralitized augite. In the lower lefthand corner is a wedge-shaped- area filled with little rosettes of 

chlorito and a few flakes of hornblende. The remainder of the field / 

is occupied by a micropegmatitic Intel-growth of red feldspar and ~\ 

quartz. Natural light. Magnified 23 diameters.

/. Entile (sagenite) in chlorite of intermediate rock (No. 11356). The i 

red needles and areas are rutile and the inclosing green substance is 

chlorite, in which is a little magnetite. Reflected light. Magnified 

40 diameters.

g. Plagioclase grain surrounded by a zone of micropegmatite from 

porphyritic red rock (No. 11371) in polarized light. The small dark 

irregular grains in this zone are quartz, and the lighter inclosing 

material is feldspar. Large areas of feldspar, with its included 

quartz, polarize together. Magnified 17 diameters,

7i. Spotted quartzite, as seen in natural light in the thin section when 

this is placed against a white background. The green circular area 

consists of quartz grains and epidote crystals. The surrounding 

white circle is composed of quartz, and the inclosing matrix has the 

composition of the ordinary quartzite, i. e., it consists of grains of 

quartz and feldspar, a few very small grams of magnetite, and interstitial quartz and chloritic substances.
U.S.GEOLOGICAL SURVEY. BULLETIN NO. 109 PL.X.

C (X23)

THIN SECTION OF INTERMEDIATE ROCK ONTHE CONTACT BETWEEN THE OLIVINE 

SABBRO ANDTHE RED ROCK.

f (X40)

(X-f)
BAYLEY.] PETROGRAPHIC DESCRIPTION OF ROCKS. 63

structure between this rock and the typical red rock may be observed 

by comparison with PI. ix a. -

In the darkest of the intermediate rocks there is a large amount of 

augite,none of which is perfectly fresh. In the least altered pieces a 

fibrous structure is plainly evident. This structure becomes more emphatic as alteration progresses, and the individual fibers gradually pass 

over into hornblende. Ohlorite and a green earthy substance are 

abundant as additional alteration products of this mineral. The plagioclaseis very fresh, and is twinned according to two laws, the twinning 

lamellae crossing each other nearly at right angles. Extinctions measured against the combination edge of two contiguous lamellae are 

between 29° and 30°. It is therefore more basic than the feldspar in 

the next lighter-colored rock described above. With the increase of 

the quantity of plagioclase there has been a corresponding diminution 

in the amount of red feldspar and quartz present. This is now confined 

entirely to very small areas between the large plagioclase crystals. It 

is particularly noticeable for the large number of apatite crystals that 

cut through it in all directions. Apatite crystals are found also in the 

secondary products derived from augite, more especially in the chlorite, 

when they are sometimes surrounded by pleochroic halos.

Analyses of these rocks, when compared with those of the gabbro and 

the red rock, show conclusively that the former are transition phases 

between the two end terms of the series. '

SIC* ..........

Ti02 -. .......

ALA---------

FeA---------

FeO ..........

MuO. .........

CaO ..........

SrO...........

BaO ..........

MgO.... ......

K20 ..........

 N*.2 0 .........

Li 20.... ......

H2 o .....;....

T> n

ci...... v ....

Total...

Sy.Gr ........

III.

, 49-88

1-19

1S-5E

2-00

8-37

 09

0-72

.02

5-77

 68

2-59

1-04

 1C

trace.

100-12

2-923

XII.

50-69

7-94

XIII.

. 57-88

4-68

2-741

XIV.

57-98

1-75

13-58

3-11

8-68

2-01

2-87

3-44

2-47

 29

trace.

99-91

IX.

72-42

 40

13-04

 68

2-49

 09

 66

 15

 58

4-97

3-44

1-21

 20

2-620

G.

61-09

15- 34

5-74

3-69

3-10

1-33

3-65

3-41

1-80

III. Olivine gabbro, analyzed by Mr. Hillebrand.

XII. Intermediate rock (No. 11211) near the gabbro, analyzed by Prof. W. W. Daniells, of the University of Wisconsin.

XIII. Intermediate rock (No. 11209) midway between the gabbro and the red rock, analyzed by Prof. 

Daniclls.

XIV. Intermediate rock (No. 11210)"near the red rock, analyzed by Mr. Hillebrand. 

IX. Granular red rock, analyzed by Mr. Hillebrand.

G. Intermediate rock from North Shore of Pigeon Point, analyzed by M. E. Wadaworth: Geol. and 

Nat. Hist. Survey Minn., Bull. No. 2.
64 ROCKS ON PIGEON POINT, MINNESOTA.

In these results can be traced the gradual transition from the basic 

gabbro, rich in calcium and magnesium and poor in potassium, to the 

acid keratophyre, which is poor in calcium and magnesium and rich in 

potassium.

The relations of the intermediate rocks to the gabbro and thfr red rock 

are such as to leave no doubt that the former are the result of contact 

action between the two latter. The intermediate rocks are found only 

where the red rock and the gabbro approach very near to each other. 

In the case described above the line of contact between these can not be 

observed. In other cases their junction is marked by a well-denned line, 

on each side of which the rocks have undergone more or less change from 

their normal condition, and in this case the red rock usually has a granophyric or porphyritic structure. This contact action may be imagined 

as having taken place in one of two ways. It may have resulted from 

the passage of solutions from one rock into the other, or it may have 

been produced by the penetration of the molten magma of one into 

the other and a recrystallization of the mixed magmas thus formed. 

Messrs. Herrick, Clarke, and Deming1 regard the orthoclase-gabbros of 

Duluth as having been formed in the first way. Dr. Wadsworth2 thinks 

that all the quartz of a certain rock collected on the north shore of Pigeon 

point just west of the Little Portage is of secondary origin, and that 

the rock itself is an altered gabbro or an altered form of a more acidic 

rock. This rock is a very much weathered variety of what the writer 

has called an intermediate rock. Its analysis is given in column G, 

above.

The objections to this theory of the origin of this class of rocks are: 

First, the perfect freshness of the large plagioclase crystals in the darker 

varieties; second, the entire lack of even a trace of a line of contact 

between the more acid and the more basic phases; third, the occurrence 

of idioinorphic crystals of hornblende in some of the intermediate rocks; 

and, fourth, the perfect gradation in the composition of these rocks as 

revealed by the analyses. It is almost impossible to imagine the conditions under which a rock of the composition of the gabbro (III) could 

be changed into one of the composition of the intermediate rock No. 

11210 (XIV) by means of solutions emanating from the red rock, unless 

these solutions contained in them the materials of the red rock in about 

the proportions in which they are present in that rock a supposition 

which is not at all probable, since different minerals possess different 

degrees of solubility in any menstruum that may be employed to dissolve 

them.

According to the second theory, the production of the intermediate 

rocks is due to the intermingling of gabbro material with that of the 

red rock and a recrystallization of the magma thus formed. That the 

conditions prevalent on the point were favorable to this "kind of contact 

action is shown by the presence there of a large quantity of rocks which

1 Some American norites and gabbros. American Geologist, June, 1888, p. 343.

2 Bulletin No. 2, Geol. and Nat. Hiet. Survey of Minn., p. 82.
BAYLEY.] PETROGRAPHIC DESCRIPTION OF ROCKS. 65

it is believed have resulted from the action of an eruptive upon the 

preexisting quartzites. In the case of the formation of the intermediate rocks, the intermingling of the gabbro and the red rock must have 

occurred at some considerable depth below the then existing surface of 

the earth, where the conditions were such as to allow of the production 

of a holo-crystalline structure. Under these conditions the magma of 

one of the two rocks may have penetrated the other, or it may have 

absorbed pieces broken from it, and thus given rise to a new magma, 

which upon cooling yielded rocks which varied in composition in accordance with their nearness to the gabbro or the red rock, i. e., varying 

with the number and size of the fragments dissolved.

At the contact of the granophyre and the gabbro the conditions were 

slightly different. In this case it seems that the red rock broke oft' 

pieces from the gabbro, but before it could entirely dissolve them it 

solidified. On the granophyre side of the contact pieces of the basic 

rock can still be detected in a groundmass which varies slightly in composition from the typical granophyre, in consequence, presumably, of 

the partial solution of gabbro fragments, The rock consists almost 

solely of quartz and feldspar in micropeginatitic intergrosvths and a 

few spherulitic masses of chlorite. Large lenticular areas of quartz, 

with embayments of the feldspathic grouudmass, represent the porphyritic crystals of this mineral, which are common in the typical rock 

at a distance from the contact. On the gabbro side of the contact the 

augite is very pale in color and is almost nonpolarizing. The olivine is 

serpentinized and chloritized. It is often stained brownish-yellow, and 

contains magnetite grains, particularly ou its edges.

The features which are presented by the intermediate-rocks in the 

two places referred to above are found also, with little variation, at all 

other points where the two rocks are in contact. All phases of the intermediate rocks are not always to be found, but it is certain that in 

every locality in which are exposed the borders of either the gabbro or 

the red rock near their contact one or more phases of the intermediate rocks are to be detected. The microscopical examination of these 

shows them to have about the same composition as some one of the 

three varieties above described. In a few cases there is a large development of brown biotite in irregular-shaped plates and folia. These do 

not seem to have crystallized from a magma, but to have been secondarily developed from some constituent already present in the rock, since 

th e original gabbro structure of the rock remain s. Ilmenite is ab undan t 

in most varieties, and in some it gives rise to leucoxene or sphene in 

great quantity. Magnetite occurs in little grains and in rodlike bodies, 

and on its edges is fringed with dark-brown biotite.

The formation of a holocrystalline rock at the contact of two eruptives is not uncommon among geological phenomena. In 1883 MichelL6vyl described a rock formed at the contact of gabbro (or diabase)

'Sur les roches 6ruptives baaiques, cambrieiujea du Mftcounaia ot du Boaujolais. Bull- Soc- G6ol. do 

Fr., 1883, p. 273.

Bull. 109 5
66 ROCKS ON PIGEON POINT, MINNESOTA. [BULL.IOD.

and granite in the region of Maconnais and Beaujolais, in France. On 

the diabase side of this contact the angite of the diabase was changed 

to hornblende and the plagioclase crystals gradually disappeared. 

The granite, on the other hand, lost its biotite and assumed plagioclase. 

In the parish of Helsinge, in Finland, a red inicrocline-granite, according to Wiik, 1 loses its rnicrocline in the neighborhood of inclusions of 

diabase and has developed in its place white plagioclase. The granitite of Ehrenberg, Ilmenau, contains augite instead of hornblende at 

its contact with labrador-diorite.2 The more usual case, however, is 

the formation of a glassy or porphyritic rock by solution of broken 

fragments in the magma of the rock which produced the fracturing. 

This kind of action is most common on basalt 3 contacts. Whether the 

intruded rock is a sedimentary or an igneous one, the basalt 4 in many 

cases dissolves fragments of it, and thus gives rise to a rock which differs in character from the basalt itself and also from the unaltered intruded mass. The resulting product is usually a glass containing porphyritic crystals, which have been generated by the contact action.

1 Referate, Neues Jahrb. fiir Mineral., 1888, n, p. 74.

2 E. E. Sclimicl: Der Ehronberg bci Ilmcnau, 187(5. Kef. Kosenbuacli: Massigo Gesteine, 1887, p. 43. 

3Joh. Lehmann': Verh. cl. nieclerrh. Ver. f. Natur. -unrt Heilk., 1874.

'Chrustscboff: Bull. Soc.Hiu. d. Fr., 1885, vol. 8, p. 02. Job. Stock: Tsckermak's mineral, u. petrog. 

Mittb., vol. 0, p. 429. Otto Beyer: Ibid., vol. 10, p. 1.
CHAPTEE VI. 

PETROGRAPHICAL DESCRIPTION OF THE FRAGMENTAL ROCKS.

The sedimentary rocks include slates, quartzites, and conglomerates. 

That these rocks are really sediments is shown in the field by their 

occasional conglomeratic character, and more decisively by the lines of 

false bedding observed on the quartzites and in the beautiful ripple 

markings seen on many of the slates.

In their unaltered forms the quartzites comprise evenly bedded light 

gray, pink, and black varieties. These are all very compact and hard. 

When struck with the hammer they yield a sharp, ringing sound, and 

break with a conchoidal fracture. The light gray and the darker kinds 

have the vitreous appearance characteristic of indurated quartzites. 

In the pink varieties this is not so noticeable. The color of the latter 

is due to the large proportion of red feldspar in them. This feldspar 

has undergone alteration and has consequently lost its luster and 

assumed an earthy aspect, which it imparts to the rock. The slates? 

interbedded with the quartzites in thin -layers, are all black silicious 

rocks, differing from the dark quartzites mainly in texture. They are 

finer grained and break more readily along the bedding planes, but 

otherwise are npt to be distinguished from them. The conglomeratic 

beds form but a very small portion of the fragmental series. They are 

found in but two or three places, where they appear to be only locally 

developed. They consist merely of the ordinary slates and quartzites, 

with included pebbles of quartz and red .granite. Two series of vertical joint cracks, one running about north and south, and the other 

about east and west, divide the fragmental rocks .into rectangular 

blocks. The walls of the cracks are covered with druses of quartz or 

epidote.

THE UNALTERED QUARTZITES.

Under the microscope the darker quartzites are found to consist 

mainly of round and irregular quartz grains embedded in a mass of 

colorless quartz in which are also a few grains of red feldspar, a little 

green mica, some chlorite, and a very few flakes of brown biotite and 

grains of magnetite.

The quartz grains are filled with tiny fluid inclusions withN movable 

bubbles. These are arranged in straight lines in the interior of the 

grains, and are, besides, particularly abundant on their surfaces, when

67
68 ROCKS ON PIGEON POINT, MINNESOTA. [IOLL.IOO.

they define the original outlines of the grains with great distinctness. 

The feldspar grains are colored red by numerous inclusions of a red iron 

oxide. Although probably orthoclase, their true nature can not be 

determined because of the alteration they have undergone. This alteration has resulted in the production of green mica, chlorite, and quartz. 

The green mica is slightly pleochroic. In the neighborhood of magnetite it becomes brown and assumes the properties of biotite. The chlo - 

rite occurs in small quantity mingled with the mica and quartz. The 

latter mineral is the most abundant product. It is present in such 

large quantity as' to completely obscure the other minerals when the 

thin section is examined between crossed nicols. Under these conditions it appears as a mosaic of interlocking areas, some of which extend 

out beyond the original surfaces of the grains and are continuous with 

similar areas in the interstitial cement. PI. x, c, d, represents the 

appearance of two decomposed feldspar grains in one of these rocks 

(No. 11350). In c the grains are shown as they appear in natural light. 

In d they are represented in polarized light. The quartz areas within 

the grain are continuous with similarly orientated areas without it, so 

that its outline as seen in natural light is entirely obliterated between 

crossed nicols. In the figures the extensions of the areas beyond the 

original outlines of the grains are not pictured.

The quartz substance between the grains is broken up under crossed 

nicols into a mosaic of little areas interlocking by irregular sutures. 

Many of these little areas are optically continuous with the quartz grains 

they surround, while others are similarly orientated l with reference to 

the secondary quartz derived from the feldspar. All of the interstitial 

quartz is distinguished from the quartz of the grains by the small 

number of its inclusions. PI. xi, a, shows the appearance of one of 

these quartzites in polarized light when magnified sixty diameters. In 

the lower left hand corner are two large grains of quartz with their 

original contours marked by dark lines of inclusions. Beyond these 

lines there is additional quartz material, which is optically continuous 

with that of the grain it surrounds. The dark minerals in the section 

are chlorite and other alteration products of feldspar.

The pink quartzites contain a very much larger amount of feldspar 

than do the darker varieties. As in the more quartzose varieties, the 

feldspar has undergone alteration into chlorite, green mica, and quartz. 

The chlorite is inore'abundaut than in the dark varieties. It not only 

occurs scattered through the feldspar, but is found also around the 

edges of both this mineral and quartz. It thus seems evident that the 

alteration of the feldspar had begun before the silicificatiou process 

had been completed, and that some of the chlorite was derived from a 

cla'yey material which adhered to the grains of quartz. An additional 

mineral not found in the dark varieties is often present in the pink 

quartzites in the form of a light green, fibrous substance closely resem-

' Of. Irving and Van Hiae: Bull. TJ. S. Geol. Survey, No. 8,1884,
EXPLANATION OF PLATE XI.

XI. a. Section of quartzite (No. 11249) formed by the deposition of quartz 

around previously existing quartz grains in such a way that the 

added material has the same orientation as that of the original 

grain, \vhich it surrounds. In the lower left-hand corner are two 

large grains, with their original outlines marked by dark lines of 

inclusions. Beyond these lines the new quartz enlargement is discovered to be optically continuous with the substance of the grain to 

which it is contiguous. The dark portions of the figure represent 

chlorite and other secondary products of feldspar'. From a .photograph in polarized light. Magnified 60 diameters, 

b. Thin section of altered quartzite (No. 11213), from the outer zone 

of the contact belt. The light-colored quartz no longer shows the 

rounded outline of grains. The various individuals now interlock with 

irregular sutures and the enlargements have disappeared. The interstitial substance is a mixture of feldspar and chlorite. Polarized

light, Magnified 80 diameters,
U. S. GEOLOGICAL SURVEY BULLETIN NO. 109 PL. XI

a. QUARTZITE.

b. ALTERED QUARTZITE.
BAYLEY.] THE UNALTERED QUARTZITES. 69

bling the green biotite, from which, however, it is easily, distinguished 

by its silvery luster and bright polarization colors between crossed 

nicols. This same mineral is very beautifully developed in certain 

phases of the altered quartzites, and will be more fully described in 

their discussion. It is probably a muscovite or sericite. A few grains 

of plagioclase, sometimes fresh, but more frequently much altered, a 

little magnetite, rutile, and earthy iron minerals make up the balance 

of the rock.

In order to obtain some idea of the chemical composition of the two 

varieties of quartzites the following analyses were made by Dr. E. B. 

Eiggs. No. IV is the analysis of a dark vitreous quartzite from the 

little peninsula on the south shore of Pigeon point, about half a mile 

west of the isthmus connecting its eastern and western parts (No. 11399). 

No. V is the analysis of an iuterbedded lighter quartzite containing 

quite a large amount of altered feldspar (No. 11400). Under VI is 

given the percentage of silica and water present in one of the reddest 

of the quartzites occurring on the point. It was collected at the west 

side of the shingle beach east of the bai\te vein. (See map, PI. xiv, 

No. 11409.)

SiOi ..........

A1A-- - 

FoA---------

FeO ..........

Ti0.2 . .........

MnO.......... 

CaO ..........

MgO.... ......

K20 ..........

!NasO .........

H.,0 ..........

IV.

74-22

10-61

" 45

 85

 16

 56

1-48

1-08

2-12

1-79

100-32

V.

73-65

11-08

7-24

 77

trace. 

 40

1-52

1-65

 1-67

.1-88

90-8G

VI.

74-88

1-63

By a comparison of these figures it will be seen that the only appreciable differences in the composition'of the two rocks are found in the 

amounts of silica and the alkalies present; and even in the latter case 

the total amount of sodium and potassium, present in each is practically, the same (3.20 in No. 11399 and 3-32 in No. 11400). This near 

identity of composition in typical specimens of the two principal varieties of the unaltered quartzites 'render the discussion of their altered 

phases somewhat simpler than it would otherwise be, as will be seen 

in the sequel. The figures given above do not represent the differences 

in composition between the most quartzose and the most feldspathic of 

the quartzites, but they show the difference in the composition of specimens representing the two most prevalent types on the point. The 

most quartzose varieties consist almost exclusively of quartz, while the 

most feldspathic kinds contain nearly 75 per cent of feldspar.
70 ROCKS ON PIGEON POINT, MINNESOTA. [BOLL. 109.

Qnartzites similar to those described above have been mentioned by 

Irving and Van Hise1 as existing throughout the entire extent of the 

Huronian area, and quartzites produced in the same way, viz, through 

the enlargement ofquartz grains by the addition of new quartz material, 

have been found by these and other geologists2 in the fraginental beds 

of different horizons at widely different localities.

The silicification of the quartzites of Pigeon point may, therefore, be 

regarded as due to conditions which have no connection with the occurrence of eruptive rocks in their vicinity, bufwhich were the result 

of causes acting over large areas. In one case Irving3 found the source 

of the new silica to be solutions arising from the weathering of exposed 

portions of the quartzites themselves. Crosby4 ascribes it to the solution of siliceous organisms existing in the sediments from which the 

slates and quartzites were formed.

A peculiar feature of several of the darker quartzites found near 

contacts with diabase dikes, is noticed only when their thin sections 

are examined. This consists in the development of large quantities of 

epidote in plates and little granular masses. This epidote is pleochroic 

in bright yellowish green and light pink tints, and is intermingled with 

a brownish green viriditic substance, calcite and a little quartz. The 

nature of these minerals and their intimate association point to a secondary origin from plagioclase. 5 In section number 11426, a little plagioclase can still be detected, but in others no trace of this mineral is 

left. In the former case the plagioclase grains contain within themselves small grains of epidote. On their edges these grains extend out 

into the interstitial quartz cement and form plates. But very little 

chlorite is present in these rocks, while black and red iron oxides are 

absent.

It is impossible to arrive at any definite conclusion as to the conditions which gave rise to the formation of epidote from feldspar in these 

instances; whereas the ordinary alteration is into chlorite, quartz, and 

green micas. Solutions emanating from the dikes may have influenced 

this alteration to some extent. Other quartzites found in the same 

relation to dikes, however, contain no epidote, and since the epidotic 

varieties show no other evidence of alteration (the quartz grains present the same enlargements, and contain the same inclusions arranged 

in the same general manner as in tho non-epidotic varieties), it would 

seem more consistent with the facts to regard this alteration as due to 

the nature of the original feldspar grains, together with the general 

composition of the rock itself. When orfchoclase was the principal 

feldspathic constituent, and in addition there were present iron miner -

'Bnll. TJ. S. Geol. Survey, No. 8, 1884; and Irving: Preliminary Paper on an Investigation of the 

Archean Formation of the Northwestern States, 5th Annual Report TJ. S. Geol. Survey, pp. 218-242.

2 Of. T. G. Bonney: Geol. Mag., No. 289, July, 1888, p. 297. / v

3 Bull. U. S. Geol. Survey, No. 8, pp. 48-49.

*W. 0. Crosby: Quartzites and Siliceous Concretions. Technology Quarterly, May, 1888, p. 397.

6 Roth: Allgemeine und Cheinische Geologic Bel. I, p. 321.
BAYLEY.] THE UNALTERED SLATES. 71

als and a little clay, the alteration has been to chlorite, quartz/ green 

biotite and sericite (H); when the predominant feldspar was plagioclase, 

and the quantity of iron oxides and interstitial clayey substance present was small, epidote was formed.

Another peculiar variety of the unaltered quartzites was observed in 

one or two localities, where the" red quartzite was mottled with green 

. spots. These spots are sections of lenticular bodies, with the same 

composition as the dark quartzites, while the red groundmass possesses 

all the features of the pink quartzites. Neither the spots nor the 

groundmass show any evidence of alteration other than that due to 

weathering. Both contain enlarged quartz-grains, a little chlorite and 

green mica, and other decomposition products of feldspar. In some 

cases ttie spotted layers form thin beds with well-defined outlines between the beds of dark quartzites. In other cases the two rocks 

grade insensibly into each other, the green spots always having apparently the same composition as the unaltered dark rocks, with which 

the spotted layers are interstratified. The restriction of the spots to 

certain beds and their arrangement in planes with their longer elliptical 

axes parallel to the bedding plane indicate an origin connected in some 

way with the sedimentation of the rocks in which they occur.

THE UNALTERED SLATES AND CONGLOMERATES.

The slates differ from the quartzites in containing less quartz and 

more chlorite. The proportions of quartz and water in two interbedded 

rocks, one (No. 11409) a pinkish brown quartzite, and the other (No. 

11410) a compact bluish-black slate, are as follows:

Slate..............

Si02 .

74'88

53-45

H,0.

1-63

3'92

The slates are composed of enlarged quartz grains, red feldspar, and 

a little comparatively fresh, twinned plagioclase imbedded in a mass of 

chlorite, fine needles, and shreds of the brightly polarizing green mica, 

little irregular grains and dust-like particles of magnetite, some leucoxene, and occasionally a few plates of a brightly polarizing mineral 

not unlike sphene in general appearance. They may be looked upon > 

as fine-grained quartzites in which there was originally a considerable 

amount of interstitial clay.

Upon their contact with the diabase dikes the slates appear to have 

suffered a little more alteration than the quartzites. Little plates of 

brown pleochroic mica have developed in them in large quantity. In 

section number 11307, from the contact of the straight dike half a mile 

from end of point (PI. xiv), the mica makes up about a third of the 

entire rock, the other constituents being quartz, a very little chlorite,
72 ROCKS ON PIGEON POINT, MINNESOTA. [BULL.IOO.

and magnetite dust scattered throughout the rock and included within 

the mica. A narrow band of lighter colored material in the same section contains less biotite, a little more chlorite, and a large additional 

amount of quartz.

The conglomerates interstratified with the slates and quartzites differ 

from these only in containing pebbles of quartz, granite, chert or quartzite. The quartz pebbles are usually composite and show enlargements 

around the edges, the added quartz orientating itself to correspond 

with that portion of the pebble nearest it. The material in which the 

pebbles are imbedded presents the same features as do the quartzites 

and slates described above.

THE SPOTTED ROCKS.

. Associated with the unaltered quartzites in certain restricted areas 

on the south shore of the point, notably in the western half of section 25 

and in the SW. | section 27, are certain other quartzites on whose surfaces curious circular spots are developed. These spots vary in size 

from less than a quarter of an inch to more than two inches in diameter. 

On a weathered surface they appear as slight depressions surrounded 

by a raised rim of a light brick-red color. Their distribution is very

irregular. Frequently a single spot

;C5) /^fQk ©3 stands alone. Sometimes two or

more spots are united, and when 

this is the case one rim encircles the 

group. The accompanying figure

"^ *> *|^r (Fig. 10) represents the shape and 

O & ^ general appearance of some of these

FIG. 10. -Shapes and groupings of spots on the Spots as Seen OH a SlDOOtll Weathsurface of spotted rocks. (Natural size.) prpfi cm-fa PP

When one of these rocks is broken open it is observed that the bodies 

producing the spots are themselves either lenticular in shape or spherical. They are pistachio green in color and possess a sugary texture. 

When moistened with hydrochloric acid they effervesce sometimes with 

an abundant, sometimes with only a slight evolution of gas. Like the 

spots on a weathered surface, the spheroidal and lenticular bodies are 

surrounded by a narrow brick-red rim. These rims are here fairly well 

defined against the substance which they inclose, but on their outer 

edges gradually shade off into the body of the rock. PI. x, h, represents the macroscopic appearance of a thin section made through one 

of the spots. The central circular green area is a section through a 

spot. Around this is a colorless zone, which under the microscope is 

found to consist of only quartz and feldspar. Surrounding this is the 

rock mass composed of enlarged grains of quartz, feldspar, magnetite, 

etc.

In other parts of these same areas are quartzites which show no well 

defined spots on a weathered surface, but contain little concavities or
BAYLKY.] THE SPOTTED QUARTZITES. 73

irregularly outlined red areas with diameters of about the same magnitude as those of the spots described above. On a fresh fracture of 

these, instead of the green spheroidal -bodies, are oval and circular 

areas which reflect the light evenly, as from a smooth cleavage surface, 

and have a silvery luster like that of calcite. When treated with acids 

they effervesce briskly with the evolution of carbon dioxide. It is 

evident that these oval and spheroidal silvery spots are nothing more 

than morpholites and other concretions of calcite like those so common 

in the States of the Connecticut valley and in the rocks 1 of other localities. The existence of this calcite in a state so highly crystalline as 

to yield large cleavage surfaces is not so surprising when we recall the 

perfection of the rhombohedral crystals of this substance in the Foutainebleau2 sandstone. On the weathered surface the calcite dissolves, 

first producing a cream colored spot, which afterwards becomes red,

and finally disappears, leaving a little hollow. Epidote druses on the 

walls of the joint cracks traversing the spotted rocks and those associated with them are particularly noticeable.

From, the fact that the spotted quartzites are found only on the south 

side of the point near the water's edge, and further that the trend of 

this shore is about parallel to the strike of the rocks, it appears probable that they are all parts of the same bed, which has been exposed 

only where the water has washed away the overlying ordinary quartzites. The lateral extent of the bed could not be determined, nor was its 

continuity proven. Its thickness, where exposed, can not be greater 

than 4 or 5 feet.

Under the microscope the body of the rocks containing calcite concretions is seen to be composed of an aggregate of enlarged quartzgrains, feldspar, chlorite, and green mica, like the great mass of the 

unaltered quartzites already described. Much of the feldspar is triclinic and quite fresh. The green mica is slightly pleochroic, and appears to have crystallized in position. A few grains of magnetite are 

scattered throughout the sections, and small areas of calcite are occasionally found inclosing the other constituents.

The silvery spots consist of perfectly transparent calcite surrounding 

grains of quartz and feldspar, and flakes of the same minerals as are 

common in the main portion of the rock. The interstitial quartz 

substance is absent, and the amount of secondary minerals present is 

much smaller than in other portions of the same section. The calcite 

polarizes in large areas, several of which occur in a single spot.

The quartzites in the beds above and below the calciferous varieties 

are among the most typical of the unaltered kinds. They are usually 

highly vitreous, and show not'the least evidence of any alteration other 

than that attendant upon ordinary weathering.

1 Cf. Nuumann : Lchrbnch der Geognosie, Bel. I, pp. 419-421. Dana: Manual of Geology; pp. 85, 80. 

v. G-uiubel: Gruudzugoder Geologic, pp. 244-240. 

* Auger: Krystalliserte Samlsteiu YOU Foutainebleau, Tscbermaka mineral Mittbeil, 1875, p., IfiO.
74 ROCKS ON PIGEON POINT, MINNESOTA. [BULL. 109.

The rocks in immediate association with those in which the green 

spots occur, but which are themselves unspotted, appear to have 

undergone more or less alteration. Chlorite is much more abundant 

than in the former varieties. . It has been derived from biotite on the 

one hand and feldspar on the other. Closely intermingled with this, 

but more particularly in the neighborhood of little nests of calcite, 

irregularly shaped plates of a light green, pleochroic, and brightly 

polarizing epidote are frequent. In most cases, however, the epidote 

occurs in tiny rounded grains and rudely outlined crystals scattered 

everywhere through the feldspathic and chloritic ground mass in which 

the enlarged quartzes are embedded. These little grains are sometimes crossed by cleavage cracks, parallel to which extinction takes 

place. They have a faint greenish tinge and a very high refractive 

index. In addition to quartz, feldspar, epidote, and chlorite, the rocks 

of this class sometimes contain flakes of brown biotite, grains of magnetite, and a little calcite. As will be shown later, the development 

of biotite in the Pigeon point quartzites is a result of contact action. 

The occurrence of this mineral in the rocks now under consideration

is an indication that these have . 

been affected by the presence 

near them of eruptive massesThose in which the epidote is 

most abundantly developed are 

very near diabase dikes, or are 

cut by stringers and dikes of 

the red rock.

The groundmass of the rocks 

containing the green spots presents the same features as are 

observed in the epidote-bearing rocks just described, except that the epidote grains 

are much less common here and 

in some cases are entirely 

absent. The spots, on the contrary, are usually very rich in epidote. They consist of enlarged quartz 

grains, a little feldspar, and more or less chlorite, imbedded in a mass of 

calcite and epidote. The amount of calcite present varies within wide limits. It sometimes occupies nearly the entire space between the quartz, 

feldspar, and chlorite to the almost complete exclusion of the epidote. In 

other cases it occurs only sparingly, while the epidote is massed in little plates, grains, and crystals. Only rarely are well terminated crystals observed (No. 11463). These average aV"m in length. A few forms 

are represented in Fig. 11. Although but very slightly pleochroic, the 

true nature of these crystals can not be doubted. In color they are of

FIG. 11. Epidoto crystals in spots of" spotted rock. 

(Enlarged 300 diameters.)
BAYLEY.] ~ THE SPOTTED ROCKS. 75

such a light green tint as to'be almost colorless. They possess a cleavage and an extinction parallel to their long axes, have a high index of 

refraction, and are usually free from inclusions. They are distinguished 

from sahlite, which they closely resemble ^ by the fact that the plane of 

their optical axes is at right angles to their cleavage. The s,ahlite 

described by Irving and Van Hise l in a quartzite from Pigeon point is 

in all probability this variety of epidote.

Around the spots-are clear zones corresponding to the raised rims, 

mentioned as surrounding the hollow interiors of spots on a weathered 

surface. These zones are sometimes composed of grains and plates of 

epidote larger than those found in the interior of the spots, either with 

or without calcite. When epidote is present in the rims there is a 

scarcity of this mineral in the interior of the spots. This is the rule 

in those rocks containing a large amount of altered feldspar and chlorite. In these both the interior of the spots and the body of the rock 

are composed of quartz, feldspar, chlorite, bleached biotite, little plates 

of hematite, and grains of magnetite. In the spot the chlorite is better 

crystallized than elsewhere. The surroundingrirn contains only quartz, 

calcite, and epidote, except on its outer edge, where there is an accumulation of red iron compounds. In those cases in which the interior of 

the spots contains a large amount of epidote, the exterior zone is comparatively free from this mineral, consisting essentially of quartz and 

feldspar stained red by iron salts. In both cases chlorite is absent 

from the rim.

As has already been stated, the spotted rocks are found near eruptive masses of diabase or in the close vicinity of the red rock, and contain within themselves the evidence of partial alteration. The existence of these rocks in a well denned bed, between others which show 

no spots, would at once suggest that the origin of the spots is due to 

the occurrence in these beds of some substance which was absent from 

those which are unspotted. The shape of the spots and their grouping 

suggest concretions. Epidote is not known to occur in such forms in 

fragmental rocks, while concretions of calcite 2 are common. The existence of such calcite concretions in the unaltered quartzite of Pigeon 

point is established beyond question in the case of the rocks containing 

oval silvery) areas. In these no epidote is present. In others in which 

the calcite is no longer so well crystallized as to yield good cleavage 

faces, epidote is found in small quantity, while in those which are of 

the brightest green color there is no calcite, but instead there is always 

a large quantity of epidote, either in the interior of the spot or on its 

edge.

i Bull. U. S. Geol. Survey, No. 8, p. 3G.

2 Calcitic concretions in other parts of the Huronian area north of Lake Superior are described _by 

Mr. In gall, as characteristic of the upper division of the Animikio series in the neighborhood of Silver mountain. (Mines and Mining on Lake Superior, Part H of Gcol. and Natural Hist. Survey of 

Canada, p. 84.)
76 ROCKS ON PIGEON POINT, MINNESOTA. [BDLL. 109.

From a consideration of these facts it would seem that the epidote 

and calcite are genetically connected. Epidote is one of the most common ] of the new minerals produced when limestones have undergone 

change in consequence of the-presence near them of eruptive masses. 

In the Pigeon point rocks it occurs only where these are in close proximity to eruptive masses, and when they contain within themselves 

evidences of contact action. It would appear, then, that we are justified in regarding this epidote as the result of the action of the intrusive rocks upon the calciferous sedimentary beds through which they 

break. Where the calcite was scattered in little nests through the 

mass of the fragmental rocks, epidote'is now found in similar relations 

to the other constituents. Where it was present in spheroidal concretions inclosing quartz grains, feldspar, etc., little spheroidal bodies are 

now found consisting in large part of crystals of epidote. That the epidote rims give rise to spots whose interiors differ but slightly from, 

the main portion of the rocks in which they occur, may be due to the 

original deposition of the calcite in annular concretions. The hard 

envelopes around the epidote spheroids, and those forming the raised 

rims around the concavities on weathered surfaces, owe their characteristics to the lack of chlorite in their composition. This absence of 

chlorite is in all probability the result of the formation of the epidote, 

which, in the effort to build itself up, was compelled to draw from that 

portion of the rock immediately surrounding the original calcite concretions those constituents of which it had need, and which were at 

the same time necessary to the formation of the chlorite.

An analysis (VII) of one of the epidotic rocks (No. 11274) free from 

spots shows it to have practically the same composition as "that of the 

unaltered quartzites. The analysis was made by Dr. E. B. Biggs. In 

column IV is given the analysis of one of the typical quartzites (No. 

11399) for comparison.

SiO 2 .. ........

A1,0 S ........

FoO, .........

FeO ..........

Ti0 2 .........

MuO..........

CaO ..........

MgO..........

K 20. ........

NazO. ........

H 20. .........

vn.

73-14

12-60

7-57

1-31

 04

 43

1-67

I'OO

1-78

-83

100-37

IV.

74-22

10-61

7-45

 85

 1C

 56

1-48

1-08

2-12

1-79

100-32

1 Cf. J.Roth: Allgemeine mid cheniische Geologic, Bd. I, p.428, et seq.; and Koseiibusch: Mikroskopiscbe Physiographic, 1885. Bd. I, p. 498.
BAYLEY.] THE SPOTTED ROCKS. 77

The above table also -shows the close agreement in composition between two quartzites of very different appearance obtained from widely 

separated localities on the point. The small amount of calcium present 

in the epidotic rock is due to the entire lack of plagioclase fragment 

among its components. Analyses of the spotted rocks would undoubtedly show a comparatively high percentage of calcium.
CHAP TEE VII.

PETROGRAPHICAL DESCRIPTION OF THE ROCKS OF THE CON. 

TACT BELT BETWEEN THE SEDIMENTARY AND OTHER ROCKS.

Although a general description of the rocks of. the contact belt between the quartzite and red rock has already been given in Chapter I 

of this paper, a repetition of some of the statements made there may 

facilitate the clear understanding of the relations existing between the 

various rocks comprising this belt and between these and the red rock 

along whose southern border the former usually occur in their largest 

and best development.

The contact belt has been mentioned as compienending a series of 

rocks existing between the indurated vitreous quartzites and slates in 

the southern part of Pigeon point and the eruptive rocks to their north. 

Except in one or two localities, the rock lying immediately to the north 

of the belt is the granular red rock. In it are numerous fragments of 

the sedimentary rocks, in which effects have been produced varying 

with the nature of the inclusion. Pieces of quartzite are reddened and 

mottled with green spots, as represented in PL iv. When the spots are 

comparatively few in number, they are sometimes an inch or more in 

diameter. In other cases they are very numerous, with diameters of 

only a few millimeters. The bright red groundmass through which 

the spots are strewn is crypto-crystalline in texture. It possesses a 

vitreous luster, and contains scattered through it small elongated spots 

of the same color as the larger mottlings. Eb feldspar grains can be 

detected in it, nor can any cleavage pieces be observed, as in the case 

of the granular red rock.

' The slaty fragments are differently affected. It will be remembered 

that the unaltered slates are hard, black, vitreous rocks, with a finer 

grained texture than the quartzites and more thinly bedded, but otherwise of the same general character as these. When embedded m the 

red rock the fragments of slate take on an aspect somewhat resembling 

that of a chloritic mica-slate. On a fresh fracture, small shining folia 

of muscovite are prominent, and the rock' has a greasy feeling, in consequence of the large amount of chlorite that has been developed in it. 

Throughout this dark-green background are scattered little red crystals of feldspar, like those occurring at the contact of the slates with 

dikes and veins of the rock. On a weathered surface the contrast

78
BAYLBY.] ROCKS OF THE CONTACT BELT. 79

between the feldspar cystals and the groundmass of the rock is very 

striking-. The latter assumes a dark brownish-gray color, and the 

former have a pale red or pink color. Large areas with a smooth 

surface thus appear speckled with little light-colored dots. Around 

the inclusions of this kind are usually apparent two or more concentric 

zones composed of material differing slightly according to its greater 

or less distance from the center of the inclusions, but not distinguished 

by any striking characteristic features. In the first zone is an irregularly mottled red and green'micaceous rock, in which the niottlings are 

so indefinite in outline as to seem to grade imperceptibly into the substance surrounding them. In some cases the red material is less in 

amount than the green, when the former appears as mottling the latter. 

In other cases the green material is in smaller amount when this forms 

spots in the red. Surrounding this is a second zone in which the red 

rock occurs with an occasional dark spot in it having the composition of 

the darker portion of the first zone. Beyond the second zone a third is 

sometimes met with. In this no darker spots occur, but the entire 

bands consists of red-rock material of a little darker color than that of 

the typical red rock.

The arrangement of the three zones and their composition would 

seem, to indicate that the inclusion was impregnated with the magma 

of the red rock and was dissolved on its exterior, thus giving rise to 

a new magma, from which were separated upon its solidification certain dark minerals, to which is due the darker color of the surrounding 

red rock. A little farther within the inclusion there was an insufficient 

amount of the material of the red rock to completely dissolve the slate, 

and remnants of this remain in the dark portions of the second zone. 

In the inner zone the supply of red material was not large enough to 

melt any portion of the slate. It merely impregnated it and solidified, 

thus giving rise to the indefinite mottling observed. In the interior 

of the inclusion the molten magma may have penetrated to some extent, 

so that the feldspathic portion was enabled to separate out in indefinitely outlined crystals, but no thorough fusion resulted; or hot solutions carrying the constituents of feldspar may have permeated the 

slate and have deposited red feldspar in the little crystals which are 

now so prominent on fresh or' weathered surfaces of the included 

rocks. Quartzite inclusions have produced very much less effect in 

the rock surrounding them than have the slates, presumably because 

the former are nearer the red rock in composition than are thejatter.

The breccia, composed of these pieces of slate and quartzite cemented 

together by the red rock when it is present, forms quite a distinct 

zone immediately contiguous to the red rock. To the south of it are 

found the most altered members of the contact belt in very complicated 

relations to the red rock. As a rule no definite line of contact can 

anywhere be detected between them and the red rock. The original 

fragmental texture and bedded structure of the sedimentary rocks
80 ROCKS ON PIGEON POINT, MINNESOTA. [BULL. 109.

have entirely disappeared. They are now of a bright red color, and 

possess a highly vitreous luster. They are cut by innumerable veins 

and dikes of the granular red rock, and often include fragments of less 

altered slates and quartzites. -

Beyond this, at a greater distance from the red rock, is another zone, 

like the mottled zdne around slate inclusions. This zone is neither 

well defined against the former, nor is it to be clearly distinguished 

from the least altered zone of the contact belt nearest the unaltered 

quartzites. In it the rocks are neither shattered nor are they pushed 

from their original positions. The bedding can still be distinguished, 

but it is by no means as well marked as in the case of the unaltered 

rocks. The more slaty portions of this part of the contact belt weather 

more easily than the more quartzose portions. The former, when they 

overlie the latter, are eroded oft' in such a way as to leave patches adhering to the quartzites, so that they appear as if included in this rock. 

In some instances this was at first sight thought to be the case; but 

closer examination proved it to be otherwise.

In certain other instances undoubted fragments of both qnartzite and 

slate were observed in a rock resembling the altered slates.. Since 

these, however, are found, upon microscopical examination to be completely crystalline, they may have acted as an igneous rock, and thus 

have really formed a breccia with the fragments of the less altered 

slates and quartzites.

In the exterior zone of the contact belt the rocks are but slightly 

altered. They are less vitreous than the unaltered fragmental rocks, 

but are redder in color, and not so evenly divided by joint cracks. 

Their bedding remains very distinct, and they emit a slightly earthy 

odor when breathed upon.

At the contact of the fragmental beds with the olivine-gabbro there 

has been less deep-seated alteration. On the north shore of the point 

slates and quartzites are in direct contact with a large mass of the 

gabbro, and in most of these no traces of alteration can be detected. 

From one point, however, in the easternmost slate area a rock was 

obtained which, under the microscope, is seen to be slightly altered. 

At this -place a vein of red rock, a few feet in width, runs in a northeasterly direction across both the slates and the trap dikes that penetrate them.

On the southern border of the gabbro area there are two or three 

localities where the eruptive rock, appears to have exerted some influence upon the quartzites contiguous to it. In most cases, though, the 

metamorphosing rock has the character rather of one of the intermediate rocks described as occurring between the gabbro and the red rock 

than that of the oliviue-gabbro proper. Here altered fragments of 

quartzites and slates are embedded in a rock that is neither like the 

olivine-gabbro nor like the typical red rock. It is darker than the 

latter^ but contains a great deal of red feldspar. Its structure is quite
BAYLBY.] KOCKS OF THE CONTACT BELT. 81

different from that of either one of these rocks, and its microscopical 

features are those of an intermediate rock. . The slate inclusions that 

are abundant in this rock are all surrounded by a fringe, composed' of 

small feldspar crystals radiating from the exterior of the fragments* 

Outside of this is often a rim of red drusy material exactly like that of the 

red rock. Large qia.artz.ite inclusion s embedded in the same rock are also 

surrounded by a similar red rim. At this place, as at all others where 

there has been much alteration in the quartzites, both the included 

rock and the iuclosures are penetrated by red veins and dikes. In the 

western portion of the point, near the barite vein, fragments of quartz 

occur in a fine-grained altered gabbro. The pieces of quartz vary in 

diameter from a millimetre to 2 decimetres. They are very irregular in 

outline and are surrounded by rims of bright red material.

Beyond the zone described above, viz, that in which the olivine-gab - 

bro or one of the intermediate rocks forms a breccia with fragments 

broken from the sedimentary rocks, there is sometimes a second zone 

in which the rocks resemble those of the second zone of the red rock 

contact. This zone is, however, very narrow when present, and is not 

as well defined as the corresponding zone associated with the red rock.

The third zone, i. e., that in which the rocks are but slightly mottled, 

is usually absent, but is sometimes present when it has the same characteristics as the outer zone surrounding the contact of the slates and 

quartzites with the red rock.

From the above-mentioned statements relative to the contact phenomena observed in the neighborhood of the oliviue-gabbro and in that 

of the red rock, it will be noticed that the effects produced in the slate 

and quartzite fragments of the breccia are the same in character, but 

that they are distinguished by differences in intensity. Around the red 

rock the three indistinct zones are always present. They are usually 

quite broad and the rocks in each are evidently much altered. At the 

gabbro contact one or more of the three zones may be absent. When 

present they are always much 'narrower than the corresponding zones 

at the red rock contact, and they are never as well characterized as the 

zones around the red rock. On the north side of the gabbro mass, and 

at several places on its south side, when it is in direct contact with the 

slates and quartzites, there is no evidence of contact phenomena of any 

description, except in the single instance mentioned above, where a 

quartzite in the easternmost slate area on the north shore of the point 

is cut by a vein of material resembling the red rock.

From the uniformly holocrystalline structure of the olivine-gabbro, 

and of the red rock when this is accompanied by contact rocks, it is 

evident that -the surface of the point now exposed to view was not at 

the surface when the eruptions of the gabbro and the red rock took 

place. In other words, the structure of the two crystalline rocks indicates that those portions of these rock masses which at present appear 

Bull. 109 6
82 BOOKS ON PIGEON POINT, MINNESOTA. [BULL. 109.

on the surface were at the time of their solidification at some distance 

beneath the then existing surface of the earth, and that the contact 

phenomena now observed around their peripheries are the result of 

deep-seated contact action, under the influence of great pressure and 

probably under the influence of high temperature. The breccia found 

so widespread along the borders of the red rock, and the numerous 

veins and dikes of this rock ramifying the rocks of the contact belt, are 

further evidence of the pressure to which the former rock was subjected 

during the period of its solidification.

THE LEAST ALTERED OF THE CONTACT ROCKS.

In the outermost zone of the contact belt the rocks differ but slightly 

from the unaltered slates and quartzites in appearance. As has already 

been stated several times, they have a redder color than these, are less 

vitreous in luster, and break less readily along their bedding planes. 

Occasionally there is developed in some of the quartzites a slight mottling, but .since rocks of this kind are more characteristic of what has 

been described as the second zone of the contact belt, and since their 

microscopical features are the same as those of the members of this 

zone, their description is deferred until the mottled rocks of the sec(md 

zone of the contact belt are discussed.

Although the macroscopic appearance of the least altered of the contact rocks differs but slightly from that of the unaltered slates and 

quartzites, their microscopical features are quite different. All traces 

of the enlargements around the quartz grains, that are so prominent a 

characteristic of the unaltered quartzites, have disappeared in the rocks 

now under consideration. The quartz now forms a kind of mosaic in 

which the individual grains interlock by irregular outlines. It contains 

as inclusions numerous fluid cavities, with dancing bubbles, irregularly 

shaped glass enclosures, containing devitrification products, tiny transparent needles of some indeterminable mineral, and little masses of an 

earthy substance. The feldspar grains have lost their outlines. On 

their edges they merge gradually into the quartz (see PI. xi, B), so 

that no line of demarcation between the two minerals can be detected. 

They have undergone deep-seated alteration with the production of 

large quantities of fibrous chlorite, more or less of a brightly polarizing, slightly pleochroic, fibrous sericite, a few flakes of kaolin, sometimes a little calcite, a very little brown biotite, grains of magnetite or 

ilmenite, and a dirty green or brown earthy substance. Since the outlines of the original quartz and feldspar grains have in most cases 

entirely disappeared, it would be difficult to determine from a mere 

microscopical examination of these rocks that.they were once fragmental. Their true nature, however, admits of no doubt, since in the field 

they can be traced directly into rocks which, under the microscope, can 

not be regarded as anything but true slates and quartzites of fragmental origin.
THE LESS ALTERED CONTACT ROCKS. 83

The characteristics of all the rocks belonging to this zone are the 

same. Their differences are found only in the varying amounts of 

bleached biotite, sericite, chlorite, and earthy substances present in them, 

andthese seem to be dependent upon the various amounts of feldspar and 

earthy interstitial substance originally existing in the unaltered rocks. 

When the chlorite is in large quantity it appears to have resulted partially from the alteration of feldspar and partially from the alteration 

of biotite, which in turn may have been derived from the clayey substance 

originally present between the grains of sand that afterwards became 

compacted into slate or quartzite. The constituent that varies most 

widely in amount is the sericite. In the most quartzose rocks it occurs 

only as an occasional flake between the grains of quartz. In the more 

slaty members it makes up a very large portion of the thin section. 

Since the slates differ from the quartzites principally in fineness of 

grain and in the greater abnndance of interstitial clayey substance, it 

is probable that the sericite is in large part the result of the alteration 

of the interstitial material.

The formation of the secondary minerals discovered in these phases 

of the contact rocks, and the corrosion of the quartz-grains to such an 

extent that all traces of their enlargements have disappeared, are results that are not due to the ordinary weathering processes that have 

acted on the rocks of the point, since most of the slates.and quartzites 

occurring there are not affected in this manner. The changes, however, are such as may be ascribed to the action of water alone without 

the aid of any great amount of heat, as the secondary minerals most 

characteristic of these rocks are such as are known in many cases to 

be the-result of ordinary weathering processes. In the case under consideration the water may have emanated from the masses of eruptive 

rock, around whose borders the altered rocks are found. These waters 

may have held in solution some materials derived from, their eruptive 

sources, or small quantities of other substances dissolved from the 

rocks through which they percolated before reaching the outer zone of 

the contact belt, and these substances may have aided in the formation of the biotite and some of the other constituents of the altered 

rocks, although all of these might as well have resulted from, reactions 

set up within the quartzite upon the solution of its different components.

The result of the analysis of one of the quartzitic rocks collected 

just west of the cliff', represented in Fig. 1 indicates that little if any 

substance has been added to the original quartzite by the agency 

which effected the change in its structure and mineralogical composition. All the constituents necessary to the formation of the minerals 

characteristic of the altered rocks already existed in the original rock. 

The conditions requisite for their rearrangement into the minerals of 

the contact rocks would have been afforded by the presence of heated 

water. In column XV is the analysis of the quartzite referred to
84 ROCKS ON PIGEON POINT, MINNESOTA. [BULL. 109.

above (No. 11394). It was made by Mr. E. B. Bigg's of the Survey 

laboratory. In column XVI is the composition of an unaltered 

quartzite (No. 11421) obtained on the lake shore, at the first sin all point 

east of the barite vein. In column H is the average of the three 

analyses of unaltered quartzites, quoted in the present paper.

Si02 ..........

Ti02 ...........

A12O3 .........

Fe203 .........

FeO ..........

MnO. .........

CaO ..........

MgO..........

K20 ..........

Ka-jO .........

H2 0 ..........

XV.

71-00

 44

12-88

6-69

 65

 21

1-68

2-95

2-03

99-96

XVI.

73-64

11;25

6-24

1-04

.on

1-57

1-42

3- 04

1-98

100-54

H.

73-85

 05

10-91

6-98

 89

 44

1-52

1-39

1-88

100-19

From these analyses it is clear that there was no addition of material 

to the substance of the quartzites during the change of these to the 

condition in which they exist at present in this portion of the contact 

belt. The amount of potassium present in them has been increased, 

but the total amount of alkalies remains practically/ the same. The 

changes have probably been superinduced through the agency of water 

alone.

The analysis of a very slightly altered slate (No. 11552) collected at 

some distance from the contact points to the same conclusion. In it 

no constituents occur that can not be found in the unaltered slates, and 

none are present in abnormal quantities. The result of the analysis of 

this specimen, made by Mr. B. B. Biggs, is given in column XVII.

Si02 ..........

Ai263 -- .......

Fc203 -- .......

FeO ..........

Ti02 ..........

PoO

MgO..........

K20 ..........

Na,0 .........

H20 ..........

XVII.

59-71

18 -32'

8-11

 -85

1-05

3-43

1-93

3-24

100 -18
BAYLEY.] ANALYSES OF ROCKS. 85 

THE MOTTLED ROCKS OF THE MIDDLE CONTACT ZONE.

In the middle zone of the contact belt the rocks are possessed of an 

indistinct irregular mottling, consisting, sometimes, of indefinite red 

blotches on a dark green background, like that of the altered slates. 

At other times the red portions of the rock exceed the green portions 

to such an extent that the latter appears as blotches or spots on the 

former. In both cases, as has been indicated, the dark portion 

has the general aspect of the altered slates. It is dark green in color, 

contains minute scales of a glistening, silvery mica, and has a slightly 

greasy feeling. The red portions resemble somewhat certain phases of 

the red rock. When this is abundant large cleavage surfaces of red 

feldspar can frequently be detected in it.

Rocks of this class are found quite near the eruptive, whether this 

be the red rock or the oliviue gabbro. The nearness to the eruptive is 

indicated by the amount of the red portion present, it being greater in 

proportion as the mottled rock approaches the eruptive, until finally 

the dark portion disappears entirely and a rock colored uniformly red 

replaces the mottled rock. At a little greater distance from the eruptives the mottling on the altered rocks is so very faint and indistinct 

that it can, in many cases, be detected only when the thin section is 

placed against a dark surface. Here it is seen as a light cloudy mottling against a transparent background through which the dark surface 

shows.

A third type of this class of rocks occurs very near the red rock, 

when the rocks to be described later as characteristic of the inner contact zone are absent. In this type the mottling of red and green takes 

rather the character of an irregular intermingling of two distinct rocks, 

as if two pasty magmas had been stirred together and had cooled 

without mingling, producing a structure resembling the entaxitic structure of some volcanic rocks. Here, as before, the red portions contain 

red feldspar in pieces that exhibit cleavage surfaces, and their general 

appearance is like that of the granular red rock.

In this zone have also been placed those slates near the red rock that 

are spotted with little crystals of red feldspar. These are found on the 

contact of the slates with veins and dikes of the red rock, on th,e borders of inclusions imbedded in this rock, and on the upper and lower 

portions, and sometimes in the centers of layers of slate interbedded 

with altered quartzites. When viewed in the mass these slates have 

more of a purple color than those of the altered slates that have been 

described. Their bedding is not very distinct, although they always 

break with 'an uneven, nearly horizontal fracture, and when exposed 

in the side of a cliff weather into platy layers like the red rock.

Under the microscope the faintly mottled quartzites are seen to differ 

markedly from the unmottled quartzites belonging to the outer zone of 

the contact belt. In the red portions of these the quartz has lost all
86 ROCKS ON PIGEON POINT, MINNESOTA". [BULL. 109.

traces of its fragmental nature. It now appears in rudely outlined bipyrarnidal crystals in a groundmass that-for the most part consists of 

giobulitic red feldspar, or of a granophyric iutergrowth of this mineral 

and quartz. The quartz is sometimes in quite well-forme'd crystals. 

At other times it has the appearance of corroded crystals, containing 

embayments of the feldspathic substance. The extent of the corrosion 

varies in different specimens. In some instances the corrosion has 

merely destroyed the regular outlines of the crystals and has made 

them ragged, while in other cases it has resulted in the breaking up of 

the original grain or crystal into separate pieces and has moved them 

apart. The general outline of such parts taken together suggests that 

of a shattered bipyramidal crystal, whose different portions have been 

moved asunder and consequently extinguish in different positions. 

Fig.-12 represents one of these broken crystals whose parts extinguish 

nearly, but not exactly, simultaneously.

In the interstitial feldspathic substance is often a little chlorite, a 

few fibers of the brightly polarizing mica which has been called sericite, and an occasional flake of brown 

biotite. Figure a of PI. xn shows the 

structure of this portion of one of the 

mottled rocks (No. 11414).

Between the red mottlings, in that 

portion of the rock resembling a dark 

green slate, the predominant mineral is quartz. This is full of liquid 

inclusions, lines of which run uninterruptedly from one grain into 

another without the 3 least break or 

change in direction. It occurs in 

grains intimately interlocking byirregFiG.12.-Broken crystal of quartz in red portion ular sutureS. In isolated CaSCS a Single 

of mottled quartzite. °

grain appears to retain a portion of its

added silica, but usually no enlargements can be detected. In addition 

to the quartz there is present in this portion of all these rocks a few large 

flakes of strongly pleochroic, brown biotite, some chlorite in bundles of 

fibers, more or less sericite, and a very little feldspar, without well-defined contours. The sericite exists both between the,quartz grains^ in 

fan-shaped groups intergrown with chlorite, and in needles penetrating 

the quartz in all directions, regardless of the boundaries of the individual grains. A few dust particles are scattered through all the 

other constituents, but are apparently most common in the quartz. A 

few colorless, highly refractive, brightly polarizing acicular crystals of 

zircon are also occasionally present in the materials occurring in the 

interstices between the grains of quartz. In general, the appearance 

and the composition of this portion of the rock is not very different 

from that of the least altered quartzites at some distance from the
EXPLANATION OF PLATE XII.

XII. a. Thin section of red mottling (No. 11414) in mottled rock from -the 

middle zone of the contact belt. As in the other sections, the white 

mineral is quartz and the darker substance feldspar. The characteristic structure of the mottling is shown in the center of the figure, 

where there are granophyric iiitergrowths of quartz and feldspar and 

bipyramidal crystals of quartz. The most marked feature of section 

is the outline of the quartzes. Polarized light. Magnified 80 diameters.

t>. Thin section of groundmass of mottled rock (No. 11414) from the 

middle zone of the contact belt. This figure shows the structure 

of the groundmass in which the mottlings occur. The amount of 

feldspar here is much less than in the mottling (as may be seen by 

comparison with a) and the amount of quartz much greater. The 

latter have crystal outlines, but these are not as distinct as in the 

former case. Polarized light. Magnified 80 diameters.
U. S. GEOLOGICAL SURVEY BULLETIN NO. 109 PL. XII

a. SECTION OF MOTTLED ROCK FROM CONTACT BELT.

b. SECTION OF QROUNDMASS OF MOTTLED ROCK.
UAVLET.] THE MOTTLED ROCKS. 87

eruptive rocks. In PI. xii, b, is pictured the structure of the darker 

portion of the same rock whose red portions are figured in a of the 

same plate.

While the most universal features of the spotted rocks are those that 

-have been described abov#, there are many peculiarities characteristic 

of certain specimens that should be briefly considered before a correct 

idea as to the processes incident to the formation of these rocks can be 

gained. In one specimen (No. 11877), for instance, a portion of the 

quartz is in grains, while another portion is corroded. The sericite in 

this rock undoubtedly originated, on the one hand, from biotite and on 

the other from feldspar. In another section (No. 11288) the quartz is 

corroded on the edges so as to resemble the peripheral granulation of 

certain minerals in dynamometamorphosed rocks.

A study of the last two sections, and of very many others, seems to 

show that much of the corrosion to which the quartz grains have been 

subjected is a result of the action of solutions, which have attacked the 

outer envelopes of the quart/ grains, i. e., the enlargements, and removed them, and in many cases have begun to attack the inner nuclei.

The composition of one of the least mottled of these contact rocks, 

(No. 11414) as found by Dr. E. B. Biggs, is given in column XVIII. By 

comparison with the figures in column H it will be seen that the mottled 

rock has nearly the same composition as the unaltered quartzites. Nothing has been added to it during the processes which resulted in its mottling, except perhaps a little iron and potassium. The total amount of 

alkalies has apparently increased, but this increase is so slight that it 

would be unsafe to base any general conclusion upon it. If analyses 

of the red portion and of the dark portion of the rock were at hand 

for comparison, it is probable that the latter would be found to be 

much less rich in the alkalies than those specimens of the unaltered 

quartzites whose composition is given, and that the red portions would 

show a very high percentage of this constituent. A mixture of the 

two' in the p'roper proportions would yield a rock with the composition of the feldspathic quartzites. The high percentage of potassium 

in the red portions would indicate that these contain constituents 

derived from without from a source where a potassium feldspar is 

more abundant a fact which .may be regarded as sufficiently shown 

by the mineralogical composition of these portions of the mottled rocks, 

as revealed by the microscopic examination.
ROCKS ON PIGEON POINT, MINNESOTA. [BULL. 109. 7 >

Si02 ..........

Ti02 ..........

Al a 03 . .......

Fe2 03 . .......

iToO

MnO.....-..-. 

CaO ..........

MgO. .........

K2 0 ..........

NajO .........

H2 0 ..........

XVIII.

72 -25

10-73

8-01

 38

trace. 

 42'

1-85

2-56

2-03

2-05

100 -28

H.

77 &*,

' -05

10-91

'C-98

 89

 d4

1 -iW

1-39

2-28

1-88

100 -19

The major portion of the slates spotted with little red crystals when 

viewed under the microscope, resembles the material of the altered slates 

It consists of small interlocking quartz grains, in and between which 

are little fibres and bundles of sericite and chlorite. These two minerals are of a light green color and are slightly pleochroic. When 

examined in natural light they are very near alike, but between crossed 

nicols they can readily be distinguished. The chlorite polarizes with 

its usual bine color, and the sericite in bright blue and scarlet or crimson tints of a high order. Irregular grains of brown rutile always 

accompany the chlorite. A small amount of red feldspar occurs in the 

interstices between some of the quartzes and as a globulitic mass surrounding the other constituents. A little ilmenite in grains is changed 

on the edges to brown rutile. A few brown biotite flakes, partly 

chloritized, complete the resemblance of the rock to altered slates.

The red crystals scattered through this mass sometimes consist of 

large areas of red feldspar polarizing together, and including small 

quantities of chlorite, quartz, grains of magnetite, and a very small 

quantity of sericite. The outlines of the areas are not well defined, 

but are rendered indistinct by projections of the same substance extending for short distances into the surrounding material. At other times 

the apparent crystals are composed of granophyric intergrowths of 

quartz and feldspar, with the outlines of feldspar crystals. As in the 

former case, the substance of the pseudo-crystals includes grains of 

quartz and fibers of chlorite and sericite.

The cause of the mottling of slates by crystals of red feldspar can 

best Jbe learned by the study of an exposure in the little bay on the 

southern shore about a quarter of a mile from the end of the point. 

Here a mottled slate is in contact with a rock resembling the granular 

red rock. The description of this contact is given in the field notes in 

the following words:

The brick-red rock is most abundant below and under a darker altered slate, in 

which are patches of the same rock mottled with small red feldspar crystals. Both 

the-red/rock and the mottled slate weather in horizontal plates. Along the eastern 

shore of the bay the two rocks appear in very complicated relations. Each occurs 

as patches in the other, and each shows every possible gradation into the other.
IIAYI.ET.] THE MOTTLED BOOKS. 89

At one place, however, a distinct line of contact is observed. On one 

side of this line is a platy bright red rock, like the granular red rock 

in structure, but differing slightly from it in containing more dark spots 

scattered throughout its1 mass. On the other side of the contact line 

the rock is a purplish slate, containing many very small crystals of red 

feldspar and small glistening scales of mica. Occasional patches of a 

darker color than the main mass are observed in the rock, and these 

have the appearance of altered slate. A study of this exposure alone 

would indicate that the darker rock has been impregnated with material from the red rock, and that some of this material had crystallized in 

the form of the crystals' occurring so abundantly in the dark rock.

Viewed microscopically the red rock presents all the features of the 

granular red rock. It is composed essentially of red feldspar, occasionally in crystals, with a square cross-section, irregularly shaped 

quartz grains and a good deal of chlorite. Sometimes the red feldspar 

surrounds a core of fresh, clear plagioclase, with distinct twinning striations, and sometimes it is present with quartz in niicropegmatite 

between the other constituents.

The purple rock on the other side of the contact has all the properties of the mottled slates. Brown grains of rutile are more abundant 

in it than in the above-mentioned rocks, and much chlorite is present 

as an alteration product of biotite. Large quantities of sericite are 

everywhere scattered throughout the thin section, and a few small 

crystals of apatite appear in the feldspar and chlorite. Very near the 

contact round and irregular areas of red feldspar represent the feldspar 

crystals seen in the hand specimen. These are very numerous close to 

the contact, and decrease1 in number as the distance from it increases. 

The analyses of specimens of the two rocks throw some light on the 

origin of the dark rock and aid somewhat in understanding the processes which have changed the indurated slates and sandstones into 

mottled rocks, which have lost all traces of their fragmeutal nature. 

XIX is the composition of the purple rock (No. 11228) as found by Mr. 

J. E. Whitfield; XX is the analysis of the red rock (No. 11227) made 

by the same chemist; H is the average composition of the unaltered 

quartzites, as given on pages 84 and 88 ; XVII is the analysis of a very 

slightly altered slate.
90 ROCKS ON PIGEON POINT, MINNESOTA. [BULL. 109.

SiO...........

TiO,. .........

Al,0,. ........

FcA-- - - 

FeO ..........

MnO- .........

CaO. .........

MgO.. ........

K20 ..........

Na^O .........

SOS . ..........

P n

H,0 ..........

Total . . .

XIX.

63-82

2-66

14-65

3-16

5-12

.-70

2-08

2-81

1-95

 33

 19

2-62

100-09

XX.

68-36

1-^7

13-76

2-65

9-7<i

 70

 68

4-48

3-56

 66

 ^i

 98

100-48

H.

73-85

 05

10-91

0-98

 89

 44

1-52

1-39

2-2S

1-88

100-19

XVII.

59-71

Trace.

18-32

8-11

 8^

1-05

3-54

 3-43

1-93

3-24

, 100-18

A comparison of the amounts of silica and magnesia present in the 

purple slate with the amounts of the same substances in the slate and 

quartzite, shows the former to be somewhat intermediate in composition between these two rocks. The large amounts of titanic oxide and 

ferrous iron in the former, and the presence of sulphuric and phosphoric 

acids, seem to show the effect of the presence of the red rock in its 

vicinity. The composition of the red rock differs from that of the typical red'rock in the presence of a large amount of iron, and of a comparatively small quantity of silica, a result attributable to the solution 

of pieces of slate in the magma of this rock before its solidification. 

The reason that greater differences are not noticed is due to the fact 

that the slates and quartzites formed from the detritus of granitic 

rocks have very nearly the same composition as .that of the red rock.

Among the rocks of this character are a few that in small chips are 

only with the greatest difficulty to be distinguished, on the one hand, 

from some specimens of the transition or intermediate rocks on the contact between the red rock and the olivine gabbro and, on the other 

hand, from certain of the altered slates. The mottlings are very coarse 

and their contours very ill defined. In the dark portions there is much 

more of the light-colored mica than in any of the rocks that have been 

described thus far. In the red portions the red feldspar is in large crystals, and with them are mingled blebs of quartz and various dark ironbearing minerals. The crystals of feldspar reveal themselves by the 

reflection of light from large cleavage surfaces, many of them measuring -4mm in cross section. The reflection is not always equally brilliant 

from all parts of one of these surfaces, since a single crystal sometimes 

includes within itself numerous little dark bodies, which the microscope' 

shows to be chlorite. The green portions of the rock weather more 

readily than the red portions, leaving these standing out as globular 

masses from exposed surfaces. The bedding of the rocks is also difficult to detect, as they break with a very uneven fracture into rough
THE MOTTLED EOCKS. 91

plates, somewhat like the weathered plates of the granular red rock 

pictured on page 54.

Under the microscope the red portions of these rocks look so much 

like the granular red rock that it is impossible to conclude that they 

are anything but special phases of this, to which certain features have 

been added by the addition of small quantities of material from the 

slates that are represented in the darker portions of the same rocks. 

Large pieces of red feldspar with an indefinite crystal outline include 

bundles of fibers of sericitic mica, fan-like aggregates of fibrous chlorite, irregular grains of quartz, rods and grains of magnetite, an occasional flake of chloritized biotite, little brown grains of rutile surrounded by rims of magnetite or ilmenite, small flakes of kaolin and 

rarely a little calcite. Around its edges the feldspar contains so many 

of these inclusions, particularly chlonte, sericite, and quartz (which are 

the essential constituents of the groundmass of the rock), that it seems 

gradually to merge into the surrounding groundmass. The outlines of 

the feldspar crystals are indistinct in consequence of the inclusion of 

the minerals mentioned, or they are rendered very uneven through the 

addition to the nucleus of an outer rim of granophyre, which extends 

by little tongues into the body of the rock. The feldspar crystals with 

their zone of granophyre are imbedded in an irregular assemblage of 

quartz, feldspar, and all of,the minerals mentioned above as existing as 

inclusions in the feldspar crystals. The quartz appears generally with 

irregular jagged outlines as if corroded. Sometimes it has the shape 

of a bipyramidal crystal and at other times it unites with a neighboring 

quartz by irregular sutures. The feldspar is not clearly crystalline, 

but has rather the appearance of a .globulitic mass or a residual base, 

approaching in character the felsitic base of some quartz-porphyries. 

Between the different quartz grains, and indiscriminately throughout 

its mass, are the chlorite and sericite. The same minerals, together 

with a little biotite, magnetite, rutile, and an occasional crystal of zircon are found also in the feldspa'thic ground mass surrounding the 

quartz and crystals of feldspar. The biotite is in quite large flakes. 

It is partially chloritized, and around its edges is changed into sericite. It retains much of its original brown color and is weakly pleochroic in brown tints. ' Some of the magnetite is idiomorphic, but most 

of it is in irregular masses, surrounded by brown decomposition products, mixed with grains of leucoxene. The leucoxene is also occasionally in little octahedral grains, in which case it is evidently a pseudomorph after titaniferous magnetite. Nothing present in the sections 

examined points with clearness to the origin of the chlorite and sericite, but from the relative positions of these minerals and the feldspar, 

it is undoubtedly true that, in some instances, they have been derived 

from the latter mineral. In most cases they seem to be the result of 

the alteration of some other substance that has entirely disappeared 

and left no traces by which its true nature can. be detected.
92 ROCKS ON PIGEON POINT, MINNESOTA. [BUU..IOO.

The darker portions of the rock are in nowise different from the 

darker portions of the other mottled quartzites.

From the microscopical appearance of the red portions of these rocks 

it can not be doubted that the material composing them is almost identical with the material composing the granular red rock. It is probable that the rocks were originally slates, and that these were penetrated by the red rock magma, which dissolved portions of the slates and 

then cooled, yielding a rock resembling the granular red rock in its 

most striking features, but possessing also distinctive characteristics, 

which are due to the substances added to the magma by solution. Tbe 

portions of the slate which were not dissolved would, in this case, be 

altered like the slates on the contact with the red rock.

No analyses of these rocks are at hand for comparison with those of 

the slates that have been but slightly altered. The structure of the 

red mottlings, however, is such that no supposition other than that 

which has been outlined above will explain the appearances of the different minerals and their mutual arrangement.

THE RED QUARTZITES AND MOTTLED ROCKS IN THE INNER ZONE OF

THE CONTACT BELT.

The quartzitic rocks in immediate contact with the gabbro, and in 

very many cases those immediately in contact with the red rock, are 

the coarsely mottled rocks described in the last section. In certain localities, more especially in the contact belt in the western portion of 

the point, there are certain rocks with peculiarities quite different from 

those of any rock of which descriptions have been given. These are 

found, when they occur, between the red rock and the coarsely mottled 

slates, and as inclusions in the former. They have a bright red color, 

very much like that of the granular red rock, but are easily distinguished 

from this by their compact texture, their cryptocrystalline structure, their 

vitreous luster, and the little elongated green spots scattered over their 

surfaces. Among these are some that have throughout the appearance 

described. Others contain irregular circular green spots, varying from 

a few millimeters to several decimeters in diameter, as illustrated on 

PI. iv, p. 28. These spotted rocks are easily distinguishable from the 

spotted rocks in which the spots are the.result of the alteration of calcite concretions, and are as clearly to be distinguished from the mottled 

rocks described in the last section .of this chapter. They are easily 

recognized in hand specimens by the sharp contrast in color between 

the bright green of the spots and the bright red of the groundniass, by 

the distinctness of the outlines of the spots, and by the vitreous luster 

of both these and the grouudmass. They are also characterized by 

their compact texture and the lack of macroscopic crystals of red feldspar. They are cut by a few dark veins'containing much biotite, and 

their joint cracks are covered with druses of quartz crystals.
BAVJ.EY.] BED QUARTZITES AND MOTTLED ROCKS. 93

The microscopic appearance of these red ai)d mottled rocks is as distinctive as their macroscopic appearance. The description of a fewspecimens taken from several widely separated exposures will serve as 

a description for the entire class.

On the eastern side of the little bay on the southern shore, about a 

quarter of a mile from the end of the point, PI. xvi, and in the interior 

of-the point a few hundred steps north of the water's edge, a large 

quantity of the granular red rock occurs.

To the west of the place where this comes out at the shore is a large ledge of 

moss-covered rock, extending back into the woods. It is composed of a rock with 

a gray or pink, fine-grained base, curiously speckled with black elongated spots. 

The rock has a glazed appearance as if full of infiltrated quartz. Below, the ledge 

grades into a fine-grained red rock, generally more or less porous or drusy and 

closely resembling some of Eosenbusch's Vogeusen granophyres. This contains 

patches of a darker colored rock, but is otherwise quite uniform in character. It 

forms the lower portion of the high cliffs rising abruptly around this bay and shows 

a decided tendency to part into horizontal plates [the typical granular red rock]. 1

The vitreous rock is discovered in thin section to be composed of 

thickly crowded quartz grains or crystals, more or less perfectly bounded 

by crystallographic planes, in a globulitic red feldspathic groundmass, 

which appears to have forced its way into many of the quartzes and 

now occurs as embaynients in them. In many cases the quartz has the 

outlines of bipyramidal crystals; even when corroded its general outline is that of a porphyritic 

crystal of quartz, and the extinction is always in accordance with this form. Fig. 13 

represents some of these, as 

observed in several different 

sections. The quartz is always 

clear and colorless, and it contains no inclosures except lines 

of fluid cavities, which sometimes run uninterruptedly from 

one individual over into and 

through another.

The feldspathic base in which 

these quartzes lie fills the interstices : sometimes as mere 

threads between neighboring 

quartzes; sometimes in larger

masses, when it possesses traces of twinning striations; sometimes as 

granophyric intergrowths with quartz in the angles between larger 

grains of this mineral; and frequently as embayments within the 

border of the eroded quartz grains; but it is never found in crystals, as 

in the red rock. In all cases the feldspar is much decomposed and 

reddened, and it always incloses all of the other constituents.

Fio. 13. Corroded quartz crystals in red quartzite 

on contact with granular red rook.

1 Prom the field notes.
94 . ROCKS ON PIGEON POINT, MINNESOTA. [BULL. 109.

Eosettes of chlorite occupy some of the angles between the quartz, 

and occasionally large chloritized pieces of biotite inclose many grains 

of this mineral. An infrequent grain of magnetite, brown masses of 

rutile in the chlorite, numerous crystals of what is taken to be zircon, 

and a few crystals of apatite complete the list of components forming 

the matrix of the rock.

The little elongated spots scattered through this matrix are masses 

of chlorite, composed of green fibers well crystallized or of brown earthy 

amorphous material in which traces of a chloritic nature can frequently 

be detected. In both cases the chlorite appears to have resulted from 

the alteration, of biotite, which, like the chloritized biotite in the other 

portions of the rock mass, inclosed many different grains of quartz 

during its crystallization.

In the mottled rocks, spotted with large green spots the constituents 

are the same as those in the red quartzites, with the addition of sericite. 

The red groundmass of these rocks is composed of a globulitic red feldspar surrounding grains of limpid quartz, in which are numerous liquid 

inclusions bounded by the planes of negative crystals. The quartz, as 

before, is often bounded by crystal planes, and, when corroded, is often 

penetrated by embayments of the matrix. The sericite occurs as little 

slender needles cutting through the feldspathic groundmass. The other 

constituents present the same features as are observed in the red 

quartzites described above.

The green rnottlings are composed almost entirely of quartz and sericite. The quartz is in large interlacing grains, between which no lines 

of dema.rkation can be detected in natural light, so limpid and uniform 

in character are the different pieces. Between crossed nicols this almost 

uniformly appearing field breaks up into a number of different areas 

which interlock with each other by very irregular sutures. In a single 

instance (No. 11317) one of the quartz grains retained a trace of its enlargement.

The sericite is present in large quantity. It occurs as fan-shaped 

radial groups of very light green fibers, between some of the quartz 

grains, and also* as inclusions penetrating these in every direction in 

such a manner that a single large grain of quartz appears as if made 

up of many smaller ones. In polarized light, however, several of the 

small areas are found to extinguish together, so that they may be regarded as parts of a single grain. The light green fibers of sericite are 

very slightly pleochroic, being light green when their long axes are 

parallel to the vibration plane of the nicol, and almost colorless in other 

positions. They are insoluble in acids, aud may thus be distinguished 

from chlorite, which they much resemble in ordinary light. Between 

crossed nicols they polarize in bright blue, yellow and crimson tints of 

a high order, and extinguish parallel to the long axes of the fibers. A 

silvery luster is also pronounced in every group of fibers examined, and 

other properties of sericite were recognized. It was impossible to de-
EXPLANATION OF PLATE XIII.

XIII. a and b. Thin section of mottled red quartzite (No. 11323) from the 

inner contact zone. The left half of the figure shows part of one 

of the green spots. In this are interlocking quartz grains cut by 

/ spicules and flakes of sericite. In natural light the structure of 

the spot appears to be granophyric, but in polarized light the quartz, 

is seen to be an aggregate of grains. There is no interstitial substance between the grains, except in some places a very thin seam of 

feldspathic material. The structure of the red groundmass in which 

the spots He is shown on the right of the figure. Here there is much 

more interstitial substance than in the spot, and the quartzes have 

quite a well-defined porphyritio outline. The interstitial material 

in both the spot and the grouudmass is almost entirely feldspar. 

Now and then a flake of chlorite or a grain of magnetite is present, 

but these are rare. Magnified 80 diameters.' a is in natural light,

' and b in polarized light,
-U. S. GEOLOGICAL SURVEY BULLETIN NO. 109 PL. Xlli

a. MOTTLED RED QUARTZITE, POLARIZED LIGHT.

b. MOTTLED RED QUARTZITE, NATURAL LIGHT.
BAYLEY.] THE MOTTLED ROCKS. 95

FIG. 14. Characteristic forms of zircon crystals occurring iu the mottled rocks.

teriniue the position of the optical axes in any single instance, and 

attempts to separate the mineral for analysis failed. From the chemical 

composition of the raottlings, however, it is believed that there can be no 

doubt of the true nature of the mineral. It is often iutergrown Avith 

chlorite, when existing between 

the grains of quartz, and often 

forms intergrowths with the latter mineral, resembling the intergrowths of quartz and feldspar known as granophyre.

Zircon crystals are even more 

common in the mottlings than 

they are in the body of the rock. 

They possess a long acicular 

form, sometimes doubly terminated, sometimes with a single 

termination. They extinguish 

parallel to their long axes, have 

a rhombic cross-section, a very 

high index of refraction, show 

brilliant interference colors in 

polarized light, sometimes show 

traces of zonal growth, and nearly always contain inclusions with 

immovable bubbles. A few characteristic forms are shown in Fig. 14. 

Most of these were observed in the spots and the groundinass of the 

mottled rocks, although similar forms were also found in the uninottled 

red quartzites. Fig. 15 represents a crystal extending through one

quartz grain and projecting at both ends 

into two others. The figures of PI. xm 

illustrate the appearance of groundinass 

and mottling in one of these mottled 

quartzites (No. 11323).

As the mottled quartzites weather 

they become nodular. The red portion 

between the mottlings weathers more 

quickly than the green material of the 

no. i5.-zircon crystal extending through mottlings, so that-the latter are left as 

portions of three quartz grains. elevations on a depressed surface. The

final result of the weathering is a mass

of round marble-like bodies in a loose sandy matrix, which falls apart 

when disturbed, leaving the more durable mottlings as a pile of little 

spheres. A cross-section of one of the spheres, examined under the 

microscope, presents the characteristic appearance of the green mottlings in the fresh mottled red quartzite. The sericite is not quite so 

prominent a feature in the weathered rock as it is in the fresh one, but 

the quartz has all of the same peculiarities.
96 ROCKS ON PIGEON POINT, MINNESOTA. [HULL. 109.

In discussing the origin of these rocks it is important to recall the 

principal features of their structure, viz, the porphyritic appearance 

of the quartz grains in the red groundmass, the globulitic character of 

the feldspar, the long acicular crystals of zircon, extending sometimes 

from one grain of quartz over another, and the irregular dissemination 

of the sericite throughout all of the quartz of the spots.

The corrosion of the quartz grains and the embayrnents of feldspathic 

substances in them would seem to indicate that the matrix of the rock 

had once existed as a molten magma that had eaten into the quartz 

crystals that lay immersed in it. The existence of sericite fibers in the 

quartz and the presence of well formed zircon crystals running from one 

quartz grain into another prove that the original material of the fragmental rocks entirely recrystallized during the process which resulted 

in the formation of the new rocks. This recrystallization may have 

taken place either through the aid of material derived from the red 

rock or through the influence of solutions emanating from it.

The origin of the green mottlings is difficult to determine. The 

quartz of these, like the quartz of the red grouudrnass, is sometimes in 

apparent porphyritic crystals, with an extinction parallel to the long 

axes of their vertical sections. More frequently, however, it is in grains 

united by irregular sutures^ very like the quartz in some of the less 

altered rocks. In all the quartz, whether it be porphyritic or corroded, 

sericite is abundant, and is so arranged that its position can be ascribed 

only to an original crystallization. It seems necessary, in order to 

account for the structure of these mottlings, to assume that their material has crystallized in situ. The existence of an enlargement on one 

of the quartz grains referred to above would, however, seem to oppose 

the notion that this crystallization took place from a molten magma. 

It appears more probable that the crystallization of the material now 

forming the mottlings was in consequence of the continuation of a 

process like that which gave rise to the rocks in the outer contact zone. 

As the result of the solvent action of heated water under pressure, in 

most cases the outer envelopes, and in many cases the entire nuclei of 

the quartz grains were dissolved and the quartz substance was again 

deposited. In the redeposition, however, sericite separated contemporaneously with the quartz, and was included within the crystals of this 

substance as they grew. In this case we would expect to find the 

quartz possessing the appearance of the mineral when in the form of 

the well-known secondary quartz. And this, indeed, we do find, as 

reference to the figure illustrating the structure of one of these mottlings 

(PI. xni, Figs. 1 and 2) will show. The individual quartzes interlock 

with each other in the most intricate fa'shion, the sinuosities resembling the outlines of aggregated blebs of this mineral, which have been 

secondarily developed by the decomposition of silicates. It is possible, 

then, that the red groundmass of these rocks has recrystallized with 

the addition of feldspathic substance from the red rock, while the
UAYLEY.] THE MOTTLED ROCKS. 97

green inottlings are the recrystallizecl residues of the original quartzites 

that have not been penetrated by the red magma.

Analyses of the mottlings and of the red groundmass of one of these 

rocks show that they differ but slightly in composition, and that the 

red groundmass contains a little more of the constituents of the red rock 

than does the material of the mottliugs. Neither differs materially from 

unaltered quartzites, except in the proportion of ferrous and ferric iron, 

of lime, and of soda present. The greater amount of alumina in the red 

groundmass as compared with that in the mottling, and the increase in 

the amounts of lime and soda may be attributed to the absorption of 

the red-rock magna by the quartzite, and it is in all probability clue to 

this addition of extraneous substance that the peculiar structure of this 

groundmass is found. The portions of the rock represented by the inottliugs were not so thoroughly saturated with this substance, and consequently upon their recrystallization they have yielded different minerals.

The figures in column XXI wereobtained by Mr. W. F. Hillebrand from 

the red groundmass of the rock represented in PI. iv (p. 28), and those in 

column XXII by the same gentleman from the green mottlings of the 

same rock (No. 11317).

Si02 ..........

TiOs..........

AUOs..- .......

I'eA--- ------

FoO ..........

MnO- .........

GaO..........

SrO. ..........

MgO..........

K2O ..........

Na20 .........

Li2 0. .........

H2 0........'..

p A

XXI.

76-57

 42'

9-21

1-67

3-94

 05

 73

1-51

3-07

Trice

100-08

XXII.

77-70

 30

7-67

3-29

 04

 26

(?)

1-83

1-04

1-96

2-36

10-000

The composition of the mottlings, as indicated by the above analysis, 

seems to be a little unusual, when it is remembered that the principal 

components of this portion of the rock are quartz and sericite. The 

rock from which the material for the analyses was obtained is the most 

coarsely mottled one that was at hand at the time when the analyses 

were made. On the second visit to the point, however, a much more 

coarsely mottled variety was collected. This Avas crushed, and from 

the broken pieces enough of the green material was selected for another analysis. To prevent the possibility of the admixture of portions 

of the red ground mass with the green material, the pieces of the latter 

Bull. 109 7
98 ROCKS ON PIGEON POINT, MINNESOTA. [BULL. 109.

were broken into still smaller fragments, and from these the most homogeneous looking were picked. An analysis of these was made by MrL. Or. Eakins. His results are given in column xxin.

Si02 ..........

Ti02 ..........

A1203 . ........

F&A- ........

FeO ..........

MnO. .........

CaO ..........

MgO......... ;

KZ0 ..........

NajO .........

H20 ..........

Total ....

XXIII.

83-27

7-81

1-99

1-81

 20

1-59

1-11

 19

2-32

100-29

XXIV.

83-69

7-50

1-81

 38

 39

 35'

2-61

2-46

 72

99-91

A study of the figures here given indicates the correctness of the 

hypothesis outlined above. It will be noticed that the mottling contains, in addition to the quartz, but a little alumina, iron, magnesia, 

potash, and water. The small amount of chlorite present in the mottling would account for some of the alumina, iron, magnesia,,and water, 

leaving the potash and the remainder of the iron and alumina, and possibly some of the water to be accounted for by the brightly polarizing 

green mica. As these are the constituents composing sericite, and 

since the properties of the green mica are not unlike those of this mineral, there can be little doubt that the light green fibers in the mottliugs, and in certain portions of the other mottled rocks are fibers of 

sericite. If, further, the composition of these mottlings be compared 

with the composition XXIV of one of the most brilliantly red vitreous 

quartzites (No. 11425) obtained from an inclusion in the contact belt 

in the western portion of the point, it will be found that the two rocks 

differ principally in the percentages of the constituents of the red rock 

that are present in them. The amounts of silica and alumina in 

each are about the same, but the amounts of potash and soda in the 

two are quite different. The last two substances are the most characteristic components of the red rock, and they are present in the 

altered red quartzite in nearly the same relative proportions as those in 

which they exist in the red rock. If the mottlings represent very 

nearly the constitution of the quartzite when unaffected by the addition 

of foreign material, the red quartzite undoubtedly represents a mingling of the quartzite with the rich feldspathic magma of the red rock.

THE INCLUSIONS TN THE GABBRO AND THE RED ROCK.

The effect of the gabbro and of the red rock upon the quartzites and 

slates can be studied directly in the inclusions. These, as has been 

stated, are slates and quartzites in the red rock, and quartzites and 

quartz pebbles in the gabbro.
BAYLET.] INCLUSIONS IN THE RED ROCK. 99

The inclusions in the red rock present all the features of the contact 

rocks in one or the other of the three contact zones. Altered slate 

fragments consist of quartz, feldspar, chlorite, and sericite, with a little 

magnetite and rutile. Around these are zones of spotted slates, where 

the spots are little crystals of feldspar, including within themselves 

all the constituents of the slates. Sometimes these crystals are composed principally of granophyre substance, and sometimes they are 

more compact and possess occasional traces of twinning stria tions. 

The red rock surrounding the inclusions is very like the red rock near 

the contact with slates.

The quartzite pieces in the red rock have undergone the same changes 

that have been undergone by the quartzites on the contact with the 

red rock. Certain of them are reddened and mottled, others are 

spotted like the spotted red quartzites, while still others seem to be 

but very slightly altered. As the slates and quartzites which are now 

found as inclusions were probably once like the corresponding rocks 

outside of the contact belt, it is natural to regard the changes that 

have been induced in them as due to the action of the rock in which, 

they are imbedded. This rock, to judge from its general nature, must 

once have existed as a plastic mass under great pressure. When in 

this condition it was intruded into the quartzites and slates, penetrating them as veins and dikes, and breaking from them fragments which 

it afterwards dissolved. Under the influence of heat and pressure 

those portions of the fragments that resisted solution longest were 

permeated by the material of the red rock; they became softened and 

thus subject to the rearrangement of the chemical molecules in such 

a manner as to yield stable compounds, and in this way may have resulted the mottled, slates and the red quartzites with their sericitic 

and chloritic ingredients. Other portions of the fragments were, perhaps, not impregnated with the material of the red rock, bat were saturated with solutions emanating from the surrounding rock. These portions would have crystallized without the addition of new substances, 

and may have given rise to the fragments resembling the altered slates 

and mottled quartzites.

The red quartzitic inclusions spotted with circular green areas are 

so thoroughly intermingled with the material of the rock in which they 

are imbedded that no definite conclusion as to the origin of the spots 

can be reached. The red portion of these inclusions, like the substance 

of the red quartzites, appears to be a mixture of the quartzite and the 

red rock. If so, the mottlings may represent the portionsof the original 

fragment which escaped solution, but in which certain changes, amounting in most cases to an entire recrystallization, Avere effected. If the 

supposed enlargement of a quartz grain, observed in the mottling of 

one of these rocks, is really such, then at least a portion of the original 

quartzite has in one case entirely escaped change. The zircon crystals 

in other portions of the mottling and the bunches of sericite scattered
100 ROCKS ON PIGEON POINT, MINNESOTA. [BULL. 109.

through the quartz are, however, abundant proof that in the greater 

part of the mottling as well as in the groundmass of the rock a complete rearrangement of the constituents has taken place.

Inclusions in the olivine gabbro have been subjected to changes a 

little different in character from those that have taken place in the 

fragments imbedded in the red rock. The fragments of slate in the 

former differ from those in the latter in the development of small plates 

of biotite throughout their mass, but more particularly on their edges. 

The quartzites present much the same features as the slightly mottled 

quartzites in the red rock, but the phases representing the final stages 

of alteration in the latter case are absent. In their place are certain 

rocks differing from the mottled rocks in the possession'of a great deal 

of brown mica, which crystallized between and around the quartz 

grains, including many of them in a single one of its flakes. Around 

the edges of these fragments are also developed little, brightly polarizing, almost colorless grains of augite.

The first stages in the alteration of the quartzite fragments by the 

. gabbro are similar in most respects to the corresponding stages in the 

alteration of the same rocks by the red rock. The natural conclusion 

from these facts would be that in both- cases the same processes had 

acted to produce the same effects. From rocks as different in chemical 

composition as the olivine-gabbro and the red rock, solutions may have 

emanated which contained only traces of the constituents of the rocks. 

By the action of these solutions upon the material of the quartzites 

the same results would be produced. In the third stage, however, 

where material from the including rock was intermingled with that of 

the quartzite, a different product would be expected, and a different 

product has been found.

The effect of the solution of fragments of quartzite and slate on the 

two inclosing rocks has also given rise to new products, which, however, are as different as the two inclosing rocks themselves. The solution of slate fragments in the red rock has resulted in the formation of 

a dark, granular, crystalline rock, in which sericite and chlorite have 

been developed, but which otherwise is not unlike the typical red rock. 

Quartzite fragments upon dissolving added nothing new to the red rock, 

as the chemical composition of both is nearly the same.

The olivine-gabbro is quite different from the fragmental rocks, both 

in the constituents contained in it and in the proportions of those which 

are common to both. The solution of slate and quartzite fragments 

in thiSj therefore, changed the composition of the magma in the vicinity 

of the dissolved fragments, and a product resulted upon cooling that 

is different in composition and structure from either one of the two 

rocks by whose intermingling it resulted, but is not so different from 

the rock on the contact of the gabbro and the red rock.

Several thin sections across the contact between the gabbro and a 

quartzite inclusion reveal interesting relations existing between the
BAYLET.I INCLUSIONS IN GABBRO AND RED ROCK. 101

rocks. The quartzite is altered in a manner corresponding to that 

characteristic of other quartzite inclusions. The same new minerals 

are found in it, and in addition it contains flakes of biotite. The gabbro has suffered changes that are undoubtedly due to the solution of 

a portion of the inclusion. Quartz occurs in it in irregular and clubshaped masses. Olivine and augite have been changed to uralite, and 

a large quantity of strongly pleochroic brown biotite has been developed. 

The characteristic features of this product are not very different from 

those on the contact of the red porphyritic rock and the gabbro, at the 

foot of the quartz-porphyry bluff, west of the Little Portage. The red 

rock at this place includes pieces broken from the gabbro. It contains 

flakes of brown biotite and a few grains of colorless augite. In the 

included gabbro a great deal of strongly pleochroic brown biotite has 

been developed, as has also uralite and quartz. Many of the specimens which have been described as altered gabbros possess the same 

characteristic minerals. Therefore it is but natural to assume that 

these owe their origin in most cases to the effect of the material of the 

red rock upon that of the gabbro. Since the composition of the red 

rock is practically the same as that of the feldspathic quartzite, the 

results of the mingling of their material with the magma of the gab- 

' bro should in both cases be the same. Around the quartzite inclusion, 

between it and the gabbro is a rim in which there is a great deal of red 

feldspar. This is in rudely outlined crystals, in granophyric intergrowths with quartz, and in micropegmatite. With it is more or less 

quartz in club-shaped masses, and a great deal of pale hornblende and 

chlorite that has been derived from biotite. Magnetite and ilmenite, in 

grains and long rods, a little leucoxene, and a few crystals of apatite 

are the only other constituents found.

The mineralogical composition of the material forming the rim and 

its structure are both like the corresponding features of the lighter 

colored of the intermediate rocks on the contact of the gabbro and the 

red rock. It has been shown that the intermediate rock was produced 

by the intermixture of the magmas of the red rock and the gabbro. It 

has also been repeatedly stated that the quartzites 'differ from the red 

rock but slightly in composition. It is therefore to be expected that 

the result of the fusion of quartzite inclusions by the gabbro would be 

the same as that produced by the fusion of the gabbro and the red rock.

In other cases the same kind of a rim surrounds what appear to bo 

pebbles of white quartz. These apparent pebbles are corroded and are 

fringed with a bright red border, in which the microscope detects 

numerous small colorless grains and crystals of augite. In addition to 

the augite are the usual constituents of the red rock or of the lighter 

colored of the intermediate rocks.

In certain areas the rock cementing fragments of slate and quartzite 

is not the gabbro proper, but is rather like that phase of the intermediate rock in which the arborescent groups of hornblende occur. This
102 ROCKS ON PIGEON POINT, MINNESOTA. [BULL.IOQ.

cementing material may be due either to the solution of some of the 

fragments in the gabbro, or it may have had the same origin as the 

intermediate rocks on the contact of the gabbro and the red rock. In 

either case it afterwards acted as an eruptive, breaking off pieces from 

the fraginental rocks and altering them in the same manner as did the 

gabbro and the red rock.

The most striking feature in connection with the quartzite inclusions 

in this rock is the existence of a red rim around the inclusion, between 

this and the inclosing rock. Under the microscope the substance of 

this rim has very much the same appearance as the granular'red rock. 

It consists of red feldspar and quartz, much micropegmatite, and a 

very little chlorite. The sharp contrast in color and structure between 

this material and that of the surrounding rock, and its well defined 

position as a fringe around the quartzite fragment seem to prove its 

origin to have been due to the fusion of the exterior portion of the 

inclusion.

COMPARISON WITH OTHER CONTACT ROCKS.

It has already been stated incidentally that the contact phenomena 

just described differ in many respects from those which have been 

observed at other places. In. the usual case of the alteration of sedimentary by intrusive (plutonic) rocks, the latter appear to have acted 

through their mass and not through, their material, for, with the exception of the loss of organic matter and water and the addition of volatile 

substances like boric acid, the sedimentary rocks retain the same composition after alteration as they possessed prior to it.1 The rock resulting from their alteration differs according to the nature of the sedimentaries, and is independent of that of the metamorphosing agent, whether 

this be an acid granite or a basic gabbro. The character of the changes 

which take place in the fragmeutal beds is too well known to need 

description in this place. Prof. Eosenbusch has given an excellent 

review of the literature of the subject in his "Mikroskopische Physiographie der inassigeu Gesteine," and since the publication of this book 

a tew other petrographers2 have reviewed the work in special phases of 

the subject. . The changes that have taken place in the Pigeon point rocks are attended in some instances by changes in chemical composition, although 

the larger portion of the contact rocks differ but very slightly in composition from the original quartzites. In the latter case the change is 

analogous to the alteration effected in sedimentary rocks by intrusives. 

In the former case there has been an impregnation of the quartzites by 

the material of the red rock. Alteration of the latter kind has been noted 

in recent years in one or two localities where sandstone and gneisses 

are in contact with granite and gabbro. Barrois3 describes an instance

1 Cf. Roseiibuscli: Mikroskopiselie Pbyaiograpbie, 1887, vol. n, p. 44.

2 Cf. C. G-reim: Neues Jabrb. f. Miner., etc., 188'8,1, p. 1. 

> 3 Ann. Soc. Geol. <lu Kord, 1884, vol. 11, p. 103, and vol. 12, p. 1.
BAYLEY.] COMPARISON WITH OTHER CONTACT ROCKS. 103

of the alteration of Lower Silurian sandstone by granite at Gue'inene', 

Morbihau, and Rostrenen, Cdtes-du-Nord, France. This contact differs 

from those to which allusion has been made above, in the fact that the 

sandstones have been impregnated by the material of the granite, and 

have thus had new minerals developed in them. At the same time a 

recrystallization of the quartz has taken place whereby the grains of 

this mineral have assumed a round or hexagonal form and have included within themselves small flakes of biotite. The changes which 

have been effected here are clue to the intrusion of small veins of granite into the spaces between the grains of the sandstone. Barrois was 

able to trace these veins, as they break up into smaller and smaller 

veins until they finally become too small to be observed with the microscope. Nothing analogous to this has been observed in the Pigeon 

point rocks. Veins of the red rock penetrate the quartzites and slates, 

but they can not be followed into smaller and smaller veins. Moreover, 

the changes which have taken place in these rocks differ materially 

from those which have been observed in the French sandstones. In 

the latter biotite, sillimanite, cordierite and more or less muscovite in 

large plates have been developed, while in the Pigeon point rocks the 

only new minerals observed are sericite and chlorite in sheaf-like bundles. Moreover, there is nothing in the French rocks which resembles 

in the least the mottliugs in the Minnesota quartzites. The only similarity between the two occurrences is in the crystallization of the quartz 

grains. Barrois, who has examined thin sections of some of the Pigeon 

poiut rocks, observes in a letter to the writer that the characteristics of 

the Minnesota contact rocks are entirely different from those of the 

French rocks, and suggests that the differences may be due to the differences in the composition of the rocks effecting the metamorphism. 

Very recently Lacroix l has discovered the same kind of action at a 

granite contact in the Loire Inferieure, France, and a result which is 

ascribed to a similar cause at Pierrepont, N. Y. In these cases the surrounding rocks arepyroxene-scapolite-gneisses into which the granite 

has penetrated. Le"vy 2. and Lehmann 3 had already described the impregnation of gneisses by granite in the years 1879 and 1884, respectively. Lacroix4 further mentions the injection of gabbro into the 

gneiss of Pallet, Loire Euferieure, and the formation of a contact rock 

differing from the two rocks by whose combination it resulted.

A third class of contact phenomena, differing essentially from those 

above mentioned, has been observed on the contact of quartzites with 

basalt and other effusive rocks. Here the interstitial substance of the 

quartzite (or sandstone) has been so changed as to produce a glass, in 

which as a matrix, are scattered various porphyritic crystals and devitrification products. This glassy interstitial substance, according to Zir-

, > Bull. Soc. Prance d. Miner. 1889, vol. 12, pp. 117 and 275. ^

2 Bull. Soc. Geol. d. Fr. 1879, vol. 7, p. 801.

. "Untersuchung iiber die 'Eiitstehung der altkrystalliuisckeii Schiefer-gesteine, etc. Bonn, 1884. 

*Loc. cit., p. 247.
104 ROCKS ON PIGEON POINT, MINNESOTA. [BULL. 109.

kel, 1 Hussak 2 and Stecher,3 is due to the fusion of certain portions of the 

altered rocks without the addition of basalt material. Very close to 

the contact, however, some of the eruptive material is found to have 

penetrated a short distance into the quartzite and to have left its impress in the numerous inicrolites of augite and grains of magnetite 

that occur here.4 This accumulation of augite and magnetite on the 

borders of the quartzite is an indication that the latter rock acted as a 

filter,5 allowing some of the material of the basalt to enter it and arresting the balance at the threshold. Pieces of quartzite and sandstone 

that occur as inclusions m the basalt are frequently surrounded by a 

rim of augite that has been developed by interaction of the basalt 

magma upon the material of the inclusion.0

As the contact on Pigeon point resembles in some respects that of 

intrusive rocks, so does it also resemble in some other respects the contact between quartzites and eftusives. The development of augite 

around the edges of inclusions has been referred to as characteristic of 

quartz pebbles and quartzite fragments in the gabbro. The filtering 

action observed in. the basalt contacts has also its analogue in the 

Pigeon point contact. In this place the rocks nearest the red rock 

have become impregnated with red feldspathic substance, while those 

in the next outer zone have obtained almost nothing from the metamorphosing agent.

This contact then possesses some of the characteristics of the contacts between quartzites and flow-rocks, and some of those of contacts 

between sedimentaries and plutouic rocks. It resembles most closely 

the contact described by Barrois, in which are comprehended the characteristics of both classes of con tact phenomena. The Pigeon point contact differs from Barrois's contact in the smaller amount of new minerals developed, and in the character of these.

Further, if the red rock is a product of the contact action between 

the gabbro and the quartzites, then the Pigeon point occurrence represents.the first instance of the formation of a massive crystalline rock 

from a fragrnental rock through contact metamorphisin.

1 Neu es Jalirb. fiir Min., etc. 1872, pp. 7-12.

2 Tschermsiks mineral und petrog. Mittheil, 1883, vol. 5, p. 530.

* Ibid., 1887, vol. ix. pp. 159,160.

4 Zirkel: Pctrograpliie, n, p. 577.

6 Stecher: op. cit., p. 159.

c Mohl: Neues Jahrb. i'iir Miner., etc., 1874, p. 799. Trippe: Zeitscbr. d. deutscli.geol. Geselscli., 

1878, p. 155. v. Cbrustschott': Bull. Soc. Min. d. Fr., 1885, vol. vm, p. 02, and Tscuormaks Mineral u. petrog. Mittheil., 1882, vol. IV, p. 485.
CHAPTER VIII. 

ORIGIN OF THE RED ROCK.

From a perusal of the preceding pages it is evident that the relations 

of the gabbro and the red rock to each other and to the members of the 

contact belt are so complicated that it can not be determined positively 

whether the gabbro is the cause of the contact phenomena and the red 

rock a product of its action, or the latter an original imiptive and the 

agent producing the metauiorphism. Isolated facts bearing upon the 

solution of the question have been referred to in the chapters on the 

red rock and the contact belt, and arguments to be drawn from them 

have been stated in brief. In the present chapter are collected statements of all the facts which may serve to throw any light upon the 

origin of the red rock, and a discussion of their bearing is given at some 

length.

At first glance it would appear that the evidence is strongly in favor 

of the red rock as the active agent producing the contact phenomena. 

It is found in largest quantity where the contact belt is widest, and is 

absent from those places in which there are no indications of a metam orpine action in the slates and quartzites. When present it sends 

out apophyses into the gabbro on the one side of it, and into the outer 

members of the contact belt on the other side. It is almost identical in 

chemical and mineralogical composition with some of Irving's 1 augitesyenites, of which the rock on Little Brick island is a typical example. 

Its structure is holocrystalliue like that of an undoubted irruptive, or 

is porphyritic like that of an eruptive rock. It resembles undoubted 

eruptives like the quartz-porphyries of Irving 2 or the granophyres of 

Rosenbusch.3

Moreover, the gaboro has produced no very great alteration in the 

surrounding sedimentary beds in those localities where no traces of the 

occurrence of the red rock can be found. It has not affected the bedded 

rocks on the north shore of Pigeon point with the possible exception 

of a single instance, in which slight alteration in quartzite is accompanied by a small dike of the red rock, nor are immense dikes of similar 

rock known to have produced any change in the Animikie slates and 

quartsdtes at other points along the north shore of Lake Superior.

1 Copper-Bearing Rocks, pp. 112-125. 

"Loc.cit., pp. 95-111.

8 Die Steiger-Soliiefer uud ibre Contact Zone an den Granititen vou Barr-Andlau und Hohwald 

1877.

105
106 KOCKS ON PIGEON POINT, MINNESOTA.

If, however, we examine these facts critically, it will be found that 

none of them exclude the possibility of a secondary origin for the red 

rock or of the active agency of the gabbro in promoting the inetaniorphisin of the clastic beds.

The existence of the red rock in veins in the gabbro and in dikes and 

veins in the contiguous slates and quartzites proves merely that it once 

existed in a plastic condition, and not even necessarily in a molten 

state, since the material of a very well-marked bright red dike has 

been discovered under the microscope to consist of clastic material 

which has undergone quite a great deal of alteration, but in which the 

clastic structure can still be detected. The structure of the main mass 

of red rock, however, indicates that it did once exist as a molten magma 

or in such other condition as would allow of its complete crystallization, unless we assume, with Wadsworth1 and Judd,2 that the porphyritic and granophyric structures may arise from secondary causes. 

However, the nature of the alteration which has been effected in the 

contact rocks immediately beyond the red rock their impregnation by 

the material of this is further evidence that the red rock was once in 

a liquid state.

If, then, we assume the original molten or" liquid condition of the 

red rock all of its present characteristics are explained, whether its 

origin be regarded as igneous or whether it be considered as a final 

product of contact action. In either case its most characteristic features would be those of a plutonic rock, since the conditions under which 

it solidified are those which give rise to the distinctive features of plutonic rocks. The porphyritic structure noted in a few places must be 

due to local conditions that prevailed only at a few localities. Under 

this assumption are explained the intrusion of the red rock into the 

slates, the crumpling of the latter, and the alterations effected ill those 

near the sides of dikes of the former.

The microscopical and macroscopical structure of the rock and its 

geological features, then, have no bearing upon the source of the material of which it is composed, other than to show the conditions prevalent at the time of the rock's solidification. The similarity in chemical 

composition between the Pigeon point red rock and the rock on Little 

Brick island and its similarity in mineral composition with undoubted 

flow rocks in various portions of the Lake, Superior region, are likewise of no value in deciding as to the source of its material; for it is a 

well-known fact that rocks of the same composition may have originated 

in very different ways. For instance, rocks as different in structure

iGeol. anil Nat! Hist. Survey of Hiun., Bull. No. 2, pp. 12 aikl 68. 

' 2 Quart. Jour. Geol. Soc., London, May, 1889. pp. 175,178.
BAYLEY.] ORIGIN OF THE RED ROCK. 107

and origin as granite, gneiss, .and liparite are shown by Both1 to be almost identical in composition. The question of the origin of the rock 

under consideration must therefore be decided upon field evidence 

rather than upon the evidence obtained by means of the microscope.

In a former paper2 on the origin of the red rock, it was stated that 

the field relations seemed to point to an independent origin for it, and 

that u a more careful examination of the structure of the point would 

probably reveal facts which would place beyond doubt the conclusion 

reached by the microscopical examination (viz, that the red rock is an 

independent eruptive). Since the article in question was written 

another visit to the point has been made, but contrary to expectation 

the study of the relations of the rocks tends rather to the conclusion 

that the red rock is a product of contact action.

The distribution of the red rock,.its occurrence only between the 

gabbro and the altered quartzites, is the strongest argument in favor 

of its secondary origin. A glance at the map (Pis. xiv, xv, and xvi) 

will show that the red rock occurs only at those places where the gabbro 

and the members of the contact-belt in the quartxite approach each 

other. At no other places is it found, except in the form of dikes, 

which are directly connected with the larger masses of the rock. It 

will also be noticed that the gabbro and the altered quartzites are 

sometimes in direct contact without the presence near them of any of 

the red rock.

The existence of the rock between the gabbro and the members of 

the contact belt only indicates a close relationship between the three 

rocks. If it could be shown that the contact rocks of the quartzite owe 

their origin to the existence of gabbro near them, it would follow as a 

most natural probability that the red rock is likewise of contact origin. 

Whenever it is present, there is on its one side the gabbro, and on the 

other side are two or more members of the quartzite contact belt. Its 

position is that of a member of the contact belt. It follows the sinuosities of the gabbro,0 as no irruptive rock would be likely to do, and is 

wider or narrower, according as the contact belt in the quartzite beyond 

it exhibits contact phenomena in greater or less degree. Where this contact action has been most intense there is found the greatest amount of 

red rock; where the action has been less, the amount of the red rock

1 Cliemischo vnul allgemeiiiu Geologic, Bel. II, pp. CG-C7:

a....

b....

SiO,.

75.00

75.90

75. 65

A1.03 -

11. 70

12.90

11.52

FeA1.04

2.37

(Fo.Mu)O.

1.57

1.31

MgO.

.19

.16

CaO.

1.01

1.48

.76

NaaO.

2.56

2.39

2.91

K,0.

6.25

5.12

5.93

a. Granite, from Pyterlaks, Finland, 

b. Gneiss, from Crodo, Aiitigoriotbal, Piedmont, 

c. Liparite, from Hot Springs Hill, Nevada. 

2 Am. Jour. Sci., vol. 37,1889, p. 57.
108 ROCKS ON PIGEON POINT, MINNESOTA. [BULL. 109.

decreases until finally none is present. All this is also what would be 

expected if the red rock were regarded as an independent irruptive 

and the cause of the alteration of the quartzite. However, when the 

red rock is wholly absent contact action is yet often marked. One, or 

even more, of the contact zones may be present, but that which indicates the most intense action, viz, the zone of the spotted red quartzites, has disappeared. It is, then, plain that the red rock as an independent irruptive is inadequate to explain the whole of the quartzite 

contact belt.

The existence of the red rock at points where contact phenomena in 

the quartzite are observed is not confined to the Pigeon point locality. 

At a few other places where such phenomena are known to occur, viz, 

on the islands east of the point, there is also present a red rock in the 

same relative position as that of the rock on the point. The localities 

on Spar, Jarvis, and Victoria islands' have already been referred to. 

Here are masses of gabbro which present many of the features of the 

Pigeon point gabbro. Around their borders is a series of contact 

rocks, and between these and the gabbro is a mass of the red rock. 

Tuis red rock is exactly like the red rock on the point in every particular. If an independent irruptive, its position, always between the 

gabbro and the contact rocks, needs explanation; if a product of contact action, the explanation is given.

These phenomena of distribution are such as would be expected were 

the gabbro the cause of the existence of both the red rock and the 

quartzite contact zone. If the red rock is a result of contact action 

we would expect to find it between the gabbro and the outer members 

of the contact belt in greatest quantity where the other contact effects 

are best exhibited, and in small quantity where the other contact 

products are scant in amount. Where the effect of the metamorphosing rock upon the quartzites is very slight the red rock should be 

absent.

Since the distribution of the red rock with respect to the gabbro and 

the altered quartzites is such as to indicate the probable correctness of 

the view that it is a final product of the action of the former rock upon 

the latter, there remain to be considered the great differences in the intensity of the effects of the action of the gabbro as evinced by the 

varying amounts of the contact rocks on its periphery.

If the gabbro is the cause of the contact effects, the difference in the 

intensity of its action may easily be accounted for upon the supposition 

that a great mass of the eruptive immediately underlies those portions 

of the contact belt in which the greatest alteration has been produced 

and the greatest quantity of the red rock is present, or that just at 

these places were situated the orifices through which the rock was extruded. When the contact products are not abundant the gabbro may

'E. D. Ingall: Report ou Mines and Mining on Lake Superior, Part H. of Ann.Kept. Geol. and 

Nat. Hist. Survey of Canada, 1887, pp. 41-46 and 115-116.
U.S.GEOLOGICAL SURVEY. B

SHEET A.

GEOLOGY OF PIGEON POINT, MINN.

BY W. S. BAYLEY.

Scale 5/8 Inches = I mile

(Contour Interval 20feet) Topography from U.S. Lake Survey 

1890.

SPATES SfOTTEOSLATES CONTACT GRANULAR IMtEHHtWAIE OUVINE TRAP SURFACE

HRTZITES Q.UARTZITES BELT. BED BOCK ROCK. GABBRO DYKES. MATERIAI

' 'O'O

WAUSWAUGONING
ORIGIN OF THE RED ROCK. 109

extend to no great depth. It may here have intruded itself between 

the sedimentary beds and cooled rapidly, so that its action upon the 

latter was not appreciable. At the contact of the gabbro with the 

slates and quartzites on the north side of the point, where no contact phenomena are to be found, the junction between the latter rocks 

and the gabbro is visible. The appearance here suggests the contact 

of a flow upon the eroded side or top of a bed of slates, or is such as 

would be produced by the lateral flow of an irruptive between, and 

then obliquely across bedded rocks. Other contacts of the gabbro with 

the slightly altered slates and quartzites indicate the same relation 

' between the different rocks. 'The apparent nonexistence of altered 

slates and quartzites on the sides of the great gabbro dikes so abundant along the north shore of Lake Superior may be accounted for 

by the differences in the geological character of the gabbro masses. 

Along the north shore of the lake the dikes always have the features 

of simple dikes, while on Pigeon point and on the islands to the east 

the gabbro possesses other characteristics which cause its masses to 

resemble those of bosses.

The production of contact phenomena in the quartzite by the gabbro 

alone is Avell shown at the falls of Pigeon river. At this place the 

gabbro has intruded the slates and has broken from them fragments 

which it has altered much in the same way as slates have been altered 

by the granite of the Andlau region described by Eosenbusch1 and others. 

Since there is not the least indication of the presence of the red rock 

in the vicinity of the falls of Pigeon river, it is plain that the effects 

noted in the slates have been produced by the gaJbbro. As on,Pigeon 

point, this gabbro is not in the form of a simple dike,.but is rather like 

that of a dike from which the orifice-filling material has escaped and 

insinuated itself between the beds of the surrounding fragmental 

rocks.12 This intrusion of the dike material between the layers of the 

surrounding rocks is probably due to the enormous pressure under 

which the rock existed at the depth at which the contact effects were 

produced. Whether the rock mass possessed the shape of a dike or 

not, the conditions under which it solidified were more nearly like those 

which obtain during the solidification of plutonic rocks.

More direct evidence of the action of the gabbro upon the quartzite 

is found in the inclusions of the latter in the former. It will be remembered that the alteration of the fragments in the gabbro is in general similar to that of quartzite fragments in the red rock. In the 

latter in certain places the quartz fragments are surrounded by a rim 

of red material, which, under the microscope, presents all the appearances of the red rock, except in the presence of green flecks of chlorite. 

Quartzite fragments in the gabbro are bordered by a rim which is

1 Mikroskopische Physiographic, II Aufl., pp. 47-54.

s Of. N. H. AVincliell: Ninth Ann. Eept. Geol. and Fat. Hiat. Survey of Minnesota, p. 63, Nos. 261 and 

263.
110 ROCKS ON PIGEON POINT, MINNESOTA.

exactly like the material of the red rock. At a point on the southern 

shore, in the eastern portion of the peninsula, about ^ mile from the 

end of the point (PI. xvi), the rock cementing quartzite and slate fragmeutsis similar to one of the rocks intermediate between the gabbro 

and the red rock, and whose formation is supposed to be due to the 

interfusion of these two rocks. The origin of the cementing material of this breccia may be the direct solution of fragments in the 

gabbro. Most striking instances of the occurrence of red rims around 

inclusions are found in this breccia. One of the fragments embedded 

in the intermediate rock is a large rhornboidal block of pink quartzite, 

about 7 feet long and 4 feet wide. Surrounding this, between it and 

the including rock, is a bright red border 2£ or 3 inches wide. In the 

field this border looks 'as if it were a small dike of the red rock which 

had been formed by the filling of a crack produced by the shrinking of 

the fragment from the rock in which it is embedded. That this is not 

the case is shoAvn by the fact that there can nowhere be seen any connection between the rim and the mass of the red rock. The structure 

of the border is likewise different from that of most dikes. The red 

feldspathic material has a granophyric structure in which the fan-like 

groups of feldspar and quartz extend perpendicularly from the bounding planes of the inclusion. Since the rim is probably the result of the 

fusion of portions of the quartzite by the surrounding rock, and its 

structure and composition are identical with those of the red rock, it 

may fairly be concluded, in the absence of any evidence to the contrary, 

that the red rock itself has been produced by the fusion of quartzites 

by the gabbro that it is a product of the action of the gabbro upon 

the slates and quartzites, melting the latter and thus producing a 

magma from which the red rock solidified. Under the influence of great 

pressure this magma has acted to a certain extent as an irruptive, 

intruding and altering the fragmental rocks at a distance from the 

gabbro. Finally, solidifying slowly, it has for the'most "part crystallized with the structure of a plutonic mass, but with occasional effusive 

phases.

The idea that an eruptive rock may originate from a clastic one by 

fusion is by no means a novelty in geological literature. The well 

known metamorphic 1 view of the origin of granite supposes the fusion of 

sedimentary beds by the rise of the isogeotherms. The only instance 

known to the writer in which it has been shown conclusively that sedimentary rocks have been fused and have afterwards acted as eruptives, 

is the one described by BeuschMn B#mmel0eu, Norway, where granite 

conglomerates have been fused by dynamometamorphism and have 

afterwards acted as eruptives, intruding eruptive, and clastic rocks in 

the form of dikes.

'Cf. LeConte: Elements of Geology, 1879, p. 219. J.D.Dana: Manual of Geology, 3rd ed., pp. 754- 

762. Von Giimbel: Grnndzdge der Geologie, p. 382. ' 

z Bfteumelpen og Karm^en mod omgivelser geologisk beskrevno. Kristia'na, ]888. * 
U.S.GEOLOGICAL SURVEY. 

PIGEON BAY

Diabasedykes CROSS-SECTIO

/

SHEET B.

GEOLOGY OF PIGEON POINT, 

BY W. S. BAY LEY.

Scale 5/8 Inches = I mile

(Contour Interval 20feet) Topographyfrom U.S. L

1890.

SLATES 8, SPOTTED SLATES CONTACT GRANULAR INTERMEDIATE OLIVINE 

QUARTZ IT ES &aUARTZITES BELT. REDROCK. ROCK. GABBRO. 
BAYLEY.] ORIGIN OF THE RED ROCK. . Ill

The fusion of clastic rocks by basalt has been referred, to in the pre-' 

ceding chapter. In this case the molten material, thus produced 

without the addition of any material from the metamorphosing agent, 

cooled under the conditions which give rise to the structures characteristic of effusive rocks. It failed to act as the magma of ah. eruptive, 

only because it was under insufficient pressure. On Pigeon point the 

rocks were under great pressure, so that, if a magma was produced by 

the fusion of the slates and quartzites by the gabbro, this magma must 

have acted like the magma of any other rock under similar conditions. 

Exceptjn respect to this feature, the formation of the red rock of Pigeon 

point presents a complete analogy with the formation of glassy rocks 

at basalt contacts. It Avas produced without the addition to it of any 

substance from the gabbro, as is shown by the very small amounts of 

the characteristic gabbro constituents (calcium and magnesium) in its 

composition, except on its immediate contact with the latter rock, where 

the intermediate phases occur'. The new rock produced at the basalt 

contact, however, remained in its original position. It cooled there 

quickly under slight pressure, and as a consequence formed a glass. 

The magma of the Pigeon point rock, on the contrary, formed under the 

influence of considerable pressure, shifted its position, was thrust into 

the surrounding rocks in the form of dikes, was forced between the 

grains of quartz in the quartzites, and finally cooled, yielding a rock 

with the holocrystalline or the porphyritic structure according to the 

conditions which prevailed at different places.

One other point remains to be discussed. If the red rock is a fused 

' sedimentary, there should be intermediate stages in the transition from 

a clastic rock to a crystalline one. The red quartzites of the inner contact belt have a structure that can be explained by supposing them to 

be quartzites whose interstitial substance has been fused, or by regarding them as fraginental rocks into which the liquid magma of the red 

rock has been squeezed by pressure. In either case the apparently 

porphyritic and the irregularly outlined quartz grains are probably the 

corroded remains of the original grains of the quartzite. Other indications of a transition between the fragmental rocks and the red rock 

are found in the mottled rocks of the middle contact belt. Here are 

undoubted evidences of clastic origin on the one hand, and on the 

other well-outlined crystals of quartz and feldspar, indicating pyrogenous origin. ,

In the field there appears to be a gradual transition between the 

altered quartzites of the inner contact belt and the red rock. At the 

point on the west side, of Little Portage bay there is-a gradation 

between the typical red rock occurring at the base of the cliff, a 

platy red recrystallized quartzite in which are jagged and corroded 

quartz crystals, and ordinary spotted quartzites that are undoubtedly ' 

altered fragmentals. At many other places the same gradation is encountered. At no place is there a sharp line of contact between the
112 ' ROCKS ON PIGEON POINT, MINNESOTA.

red rock and the highly altered fragmentals, except where the former 

has cut the latter in dikes. In Fig. 1, on p. 20; where a sharp contact 

between altered quartzites and the red rock is pictured, the latter has 

broken through the rocks of the inner contact belt and has intruded 

those which belong to the outer zone, where the contact effects have 

been slight. Again, on the top of the quartz-porphyry bluff (see map) 

the porphyritic red rock is abundant on the west side of the cliff. As 

we proceed eastward, however, mottlings are discovered in it which 

remind one of the mottlings of the spotted quartzites. On the extreme 

eastern side the prevailing rock is the altered red quartzite. No line 

of demarkation can anywhere be discovered between the different 

types.

Although these gradations may not serve to prove that the quartzites by fusion gradually pass into the red rock, they afford at least a 

proof of the existence of transition phases between the rocks. Transitional phases would be expected, whether the red rock is a product of 

the fusion of the quartzites, or whether as an independent irruptive it 

has impregnated them .and thus altered their characters; but its universal presence bears in favor of the first explanation.

To complete the chain of evidence which links the red rock with the 

slates and quartzites, a comparison of the composition of the former 

with the latter is necessary. At present it is not possible to obtain 

a sufficient basis for this comparison, since it is impossible to discover 

the mean composition of the sedimentary beds. , The unaltered beds 

show interstratified slates and quartzites of various compositions. 

Some of them contain much more feldspar than any of the specimens 

whose analyses are given, while others are more quartzitic. The 

quartzites are in largest quantity, but their proportion with respect to 

the slates is unknown. It is evident, therefore, that no comparison 

that can be instituted will show accurately the relation between the 

composition of the red rock and the slates and quartzites. 

The nearest approach to an average composition for the fragmental 

beds is obtained by taking a mean of the results of the analyses of the 

three unaltered quartzites and the slightly altered slate, given in 

earlier chapters of this paper. In column I is given this mean, and in 

column IX the figures obtained by the analysis of the powder of seven 

specimens of the red rock.
U.S.GEOLOG1CAL SURVEY. B

SHEET C.

GEOLOGY OF PIGEON POINT, MINN

BY W. S. BAYLEY.

Scale 5/8 Inches = I mile

(Contour Interval 20feet) Topography from U.S. Lake Survey. 

1890.
BAYLEY.J OEIGIN OF THE RED EOCK. 113

SiOz ..........

Ti02 .... ......

A1A---------

re203 - - ------

Peo. ..........

MnO....-..--.

PoH

SrO...........

BaO... .......

MgO.. .....-- .

K,0. ..........

NazO .........

Li-iO. ......... 

H2 0 ..........

v r\

Cl ............

IX.

72-42

 40

13.04

 68

2-49

 09

 GO

 15

 58

4-97

3-44

trace. 

1-21

 20

100-33

I.

70-31

12-81

7-26

 88

 60

2-03

1-00

2-19

2-22

100-20

The principal differences to be noted are in the amounts of iron, magnesium, and alkalies present. But since none of the quartzites analyzed 

contain nearly as much feldspar as many of those on the point, it is 

probable that the average composition of the fragmental beds as given 

in column I, differs from the true average in containing much less potassium and sodium.

From the evidence at hand it would seem probable that the red rock 

is a product of contact action between the gabbro and the bedded rocks, 

rather than an independent irruptive which has altered the quartzite. 

No conclusive evidence bearing upon the question is available, but if the 

assumption of the secondary origin of the red rock is made, all the facts 

observed are readily explained; not a single one but is in accord with 

this supposition; whereas, if we assume that the red rock is the metamorphosing agent, there remain to be explained the contact phenomena 

around the gabbro in those places where the red rock is absent and the 

peculiar distribution of this rock. The view that the gabbro is the cause 

of the alteration of the slates and quartzites and the red rock is an independent irruptive between the former and the latter meets with the 

same objection as does the supposition that the red rock is the cause of 

the contact belt. The presence of the red rock between the gabbro and 

the contact rocks, and in this position when the contact phenomena in 

the adjacent quartzite evince intense action, is a mode of distribution 

for an independent irruptive rock which, to say the least, is peculiar.

Since, therefore, objections can be raised to any view that regards the 

red rock as an original irruptive, and since, on the other hand, every 

fact observed, either with reference to the gabbro or to the red rock, 

accords with the assumption that the latter rock is a product of contact 

action, and none can be found which contradict this supposition, it 

would seem gratuitous to regard the red rock as having originated in 

any way but through the fusion by the gabbro of the fragmental rocks 

which lie upon one side of it. 

Bull. 109 8
SUMMARY.

A recapitulation of the facts set forth in the present paper and of the 

conclusions that have been drawn from them may be of interest to 

those who find it inconvenient to follow the detailed description of the 

different rock masses and of the mutual effects which these have had 

on each other.

The oldest rocks on the point are the interbedded Animikie slates and 

indurated quartzites, which strike a little north of east and dip about 

15° to the southeast. They consist of-quartz grains, feldspar, and 

more or less green chloritic interstitial substance. The induration takes 

the form of the enlargement of the quartz fragments. In certain beds 

the quartzites contain lenticular and spherical concretions of calcite, 

that by alteration and replacement have given rise to epidote in grains 

and crystals. This change has taken place more particularly where the 

quartzitesare penetrated by diabase dikes. The resulting rocks are characterized by round green spots which are scattered oveivsurfaces with all 

the peculiarities of the surfaces of the indurated quartzites. Under the 

microscope the spotted rocks are found to have a groundmass exactly 

resembling the quartzites with enlarged quartzes. The spots consist 

of enlarged quartz grains, a little feldspar and tiny grains and crystals 

of epidote. The calcite concretions are like those so common in the 

sedimentary beds in the Connecticut valley and elsewhere; and the 

green spots have originated from these by alteration superinduced by 

the presence of eruptive Tocks in the vicinity of the calcite-bearing 

beds.

The irruptive rocks cutting the fragmental beds are an olivine gabbro, 

diabases, and a peculiar red rock whose origin is ascribed to the action 

of the gabbro on the surrounding slates and quartzites. The olivine 

gabbro is a heavy, dark, basic rock with a porphyritic structure. The 

porphyritic crystals are bytownite. They are of various sizes, sometimes measuring as much as 6 inches in length. The groundmass in 

which they lie is composed essentially of olivine, bytownite, and augite 

in the order of their age. Its structure is the hypidiomorphic granular, with a tendency to the ophitic. In accordance with the general 

usage of American writers, and for the purpose of distinguishing it 

from the diabase dikes on the point, some of which are porphyritic, 

the rock has been called a gabbro, although perhaps its true designation, following the classification of Profs, Kosenbusch and Judd, should 

be diabase-porphyrite,
BAYLEY.] ' SUMMARY. ' 115

This gabbro occupies all the higher portions of the point. It is in 

all probability the lower portion of a large dike whose upper part has 

been removed by denudation. In consequence of the depth at which 

that portion of the mass nowx exposed formerly existed, and as a result 

-1)f the pressure under which the rock cooled, its magma was thrust 

between the interstratified layers of the slates and quartzites. Its 

geological relations are very similar to those of plutonic masses.

The material of the gabbro is unvarying in character, differing from 

the type described only where it has Undergone alteration.' The alteration consists principally in the change of augite and olivine to hornblende and the production of more or less biotite. It has taken place 

more particularly near the contact with the red rock and with the fragmentals on its southern border.

The trap dikes offer but few interesting peculiarities. They are much

altered, nonolivinitic diabases, that cut the bedded rocks and the gabbro. 

They are occasionally porphyritic and sometimes they contain red feldspar. Usually they consist of plagioclase, uralitized augite, and chlorite, that may in some cases have been derived from olivine. They differ 

from the gabbro in their finer grain, their lack of fresh olivine and 

augite, and in their well-marked ophitic structure. Those in the gabbro 

are on the whole more decomposed than those in the quartzites and 

slates.

Between the olivine gabbro and the bedded rocks, in most places, are 

successively a coarse-grained red rock, containing large hornblende 

crystals, a fine-grained red rock, and a series of contact rocks.

The fine-grained, red rock is undoubtedly younger than the gabbro. 

It occurs as dikes and veins cutting thefraginental rocks, and is always 

found as stringers in those of the contact belt. It occupies two distinct 

areas on the point. One of these is south of the large mass of olivine 

gabbro, beginning on the north side of Wauswaugoning bay, and running east to the extremity of the point. The other large area is on the 

southern border of a second gabbro belt, beginning at the eastern side of 

Wauswaugoning bay and extending east about a mile and a half. The 

most typical rock in both of these areas is a fine-grained granular aggregate of a soda-potash feldspar, quartz, and a little chlorite. Between 

some of the rudely outlined feldspar crystals is occasionally a little micropegmatite. Granophyre fringes others. In certain restricted areas the 

' red rock is developed with a porphyritic structure. The porphyritic 

crystals are eroded quartzes and fairly well-defined feldspars in a 

groundmass made up essentially of pseudospherulites of grauophyre and 

a few secondary constituents. The structure of this phase of the rock is 

like that of Eoseubusch's granophyres. It possesses the same composition as the granular red rock, and is therefore regarded as identical 

Avith it. From the nature of the feldspar contained in both phases the 

rock has been called a soda granite, or quartz-keratophyre, in which 

the keratophyre structure is developed only in a few localities.
116 ROCKS ON PIGEON POINT, MINNESOTA. [BOLL. 109.

The structure of the porphyritic red rock proves it to have solidified 

from a molten inagina. And since the granular red rock is identical 

with it in composition, it is assumed that this also originally existed as 

a melted mass. The existence of dikes and veins of the latter, and the 

contortions that have been produced in the slates and quartzites where 

they have been intruded by the red rock, are further incontestable 

proof that the latter was once in a plastic condition.

The similarity in structure and miueralogical composition between 

this rock and the undoubted eruptives occurring as flows in the Keweenaw series would indicate a community of origin for them. The 

peculiar relations of the red rock to the gabbro, however, its distribution along the borders of this, and its nonexistence at other places, 

as well as the seeming gradual transition between it and the fragmental rocks lying along its southern edge would, on the other hand, 

appear to point to a contact origin, the red rock representing the 

most altered stages in the metainorphism of the quartzites by the 

gabbro.

Whether the red rock is an original eruptive, or a product of contact 

action, there are certain other rocks on the point which undoubtedly 

owe their origin to contact phenomena. Between the gabbro and the 

red rock is a series of dark red, coarsely crystalline rocks, through 

whose mass are arborescent groups of hornblende crystals. These 

rocks have been called intermediate rocks because their composition is 

intermediate between tha.t of the basic olivine gabbro and that of the 

acid soda granite, varying proportionately as the distance from one or 

the other of the end members of the series. They are found only where 

the gabbro and the red rock are in contact, and around inclusions of 

quartz or of quartzite imbedded in the gabbro. In the latter case a 

rim marking the outline of the inclusion, and possessing the same

character as the material of the intermediate rocks, has evidently been 

derived by solution of portions of the fragments in the inclosing rock. 

The same kind of origin is ascribed to the large masses of intermediate 

rock between the gabbro and the red rock. They are supposed to have 

originated by the interfusion of the two rocks between which they lie. 

Again, between the unaltered quartzites and the red rock is another 

series of rocks to which a contact origin must also be assigned. As the 

unaltered fragmental rocks approach the red rock they first assume a 

red tinge and lose their vitreous luster, and then become mottled with 

large red and green spots. The first class consists of those rocks 

in which the quartz grains have corroded outlines. In addition to the 

quartz they contain indefinitely outlined grains of feldspar, some 

chlorite and an e'arthy substance, and in addition more or less of a 

brightly polarizing light green mica, which is in all probability sericite. 

The amount of sericite increases as the rock approaches a slate in 

macroscopic appearance, and hence it is regarded as the result of the 

alteration of the interstitial clay that once existed between the grains 

forming the rock.
BAY-LEY.] SUMMARY. . 117

The rocks in the next zone of this contact belt are the raottled rocks. 

In the case of slates these consist of the altered slates in which rudely 

outlined red feldspar crystals have separated. These occur wherever 

the slates are in direct contact with dikes or veins of the red rock, or 

where the slates are very near large masses of the latter. The mottled 

rocks are, as their name implies, those in which there is an irregular 

mottling, usually of green and red. The green portions have the composition and structure of slightly altered slates and quartzites; that is, 

they consist of corroded quartzes interlocking by irregular sutures, a 

little feldspar, and more or less sericite and chlorite. The red portions 

differ materially from the green in the very large amount of feldspar 

present. This feldspar is occasionally in rudely outlined crystals, but 

usually it occurs as a globulitic groundniass in which are sericite, chlorite, and slightly corroded quartzes with the putlines of porphyritic N 

crystals.

The contact rocks nearest the red rock are bright red in color. They 

possess a vitreous luster, and are sometimes spotted with bright green 

spots. The red quartzites and the red portions of the spotted rocks 

are composed of somewhat corroded bipyrainidal quartz crystals in a 

globulitic red feldspathic base, in which is a little chlorite, crystals of 

zircon, and sericite. In the green mottlings sericite is abundant in 

and between quartz crystals, with bypyramidal contours. Zircon crystals are quite abundant, and these extend through two or more quartzes 

without interruption. Feldspar is entirely absent.

From the structure of the rocks of the contact belt and from their 

composition, it would seem that the red quartzites have resulted from 

the complete crystallization of a magma produced by the fusion of the 

quartzites alone, or from the intermingling of this fused substance with 

the material of the red rock. The spots in these may represent portions of the original quartzite which has escaped entire fusion or solution. The mottled rocks beyond the red quartzites have been impregnated with some of the material of the red rock, and in consequence 

have suffered recrystallization in parts without complete fusion. The 

recrystallized portions are represented by the red mottlings. The 

green portions of these rocks and the mass of the less altered slates 

and quartzites have been changed by solutions .emanating from the 

red rock, but which have been freed from most of their dissolved salts 

by nitration through those members of the contact belt that are nearest the red rock, and which would have to be passed through before 

the solutions reached the outer members.

If the red rock is an original eruptive, the changes which have been 

outlined owe their origin entirely to the presence of this rock. If the 

red rock is a product of contact action, it was produced by the fusion 

of beds of slates and quartzites and recrystallization underpressure. 

Itthen acted as an eruptive, intruding and altering the fragmental rocks 

in its vicinity. The composition of the red rock is such that it can in
118 ROCKS ON PIGEON POINT, MINNESOTA. [BUM,. ] 09.

no case have received any considerable amount of material from the 

gabbro, near which it exists, except in the narrow belt on its northern 

side where the intermediate rocks are found.

Further, in a few localities where the red rock is absent, there is a 

series of contactrocks between the gabbro and the bedded rocks. This 

series is composed of altered quartzites and slates whose characteristics are the same in kind as those of the rocks at the contact with the 

red rock.

Since the intensity of the action around the gabbro is much less than 

that in the neighborhood of the red rock, or in other words, since the 

red rock is present only where the contact effects are most pronounced, 

it is concluded that this rock is only the final stage in the alteration of 

the slates and quartzites by the gatobro.

That this latter rock is capable of producing contact alteration in the 

Aniinikie beds with which it comes in contact, is shown by the contact 

rocks around the gabbro mass at the falls of Pigeon river. That similar gabbro masses' in other places along the north shore, of Lake Superior have not produced alteration in the surrounding fragmental beds 

is explained by the difference in the geological relations of these dike 

masses as compared with those of the boss-like* dike on Pigeon point.

DESCRIPTION OF MAP. (PLATES XIV, XV, XVI.)

Geological map of Pigeon point, based upon the Lake Survey chart. The outline 

of the point east of Morrison's bay was mapped by pacing; that west of the bay is 

taken from the chart. The areal distribution of the rocks was determined by makingsections across the point at intervals of 250 feet in its eastern portion and 500 feet 

in its western part. The"rocks generally pass into one another so gradually that 

the mapping of their distribution must be to some extent arbitrary. In the eastern 

portion of the point the colors representing the different rocks and groups of rocks 

cover areas approximately occupied by the respective rocks. The color for the coiitact belt covers all of the altered quartzites and slates. No distinction has been 

made between those that are entirely recrystallized and those that have suffered but 

.little chauge. The former always occur nearer the gabbro and the red rocks than do 

the latter, and the latter pass by insensible gradations into the unaltered fragmentals. In the few places where the rocks do not pass into each other, their contact is 

indicated by a solid line. At A in the interior of the western part of the point, 

and at a few other places, the gabbro occurs in knobs, when the area occupied by it 

is represented as sharply denned. Many of the trap dikes have been exaggerated in 

size, in order that they might be represented on the scale of the map, as have also 

the spotted-rock areas. In the western port!on of the point the distribution is not 

as carefully worked out as it is in the eastern part, but is accurate enough to illustrate the features most prominent in the geology of the region. A large swamp 

occupies most of this part of the point, and in it no rock exposures were detected. 

A few other low areas have been covered with the color of the rock which undoubtedly underlies them. Scale: 5£ inches to the mile.
INDEX,

A.

Pago.

Analyses, anorthoclase ................... 52

diallage-augite........................ 36

epidotic rocks..._.................. 76

feldspar from gabbro................. 34

feldspar (red) ......................... 52

gabbro._..___....................... 37

intermediate rocks.._.............. 03

mottled rocks ..............._...... 88

mottled rocks, red ground mass of .. 97 

mottled rocks, green mottlingsfrom. 98 

quartzites ........._......___ 69

quartzite, unaltered...._....:..._. 84

red rock, showing average composition of the two types......._.... 36

red rock of Little Brick island....... 58

red rock, seven specimens...._._ 113

slate, slightly altered........._..... 84

Aniniikie beds, prevailing dip of......... 21

veins cutting.._._............_ 21

contact with Keweenawan at Grand

Portage island _................ 23

Anorthoclase, analysis of ................ 52

Augite,altered to biotite ....'... .......... 41,42

altered to hornblende......... _..... 40

character of...._..................... 35

B.

Barrois, Charles, acknowledgment to.... 13 

cited...............................102-10?,104

Basalt, contact between quartzites and

fusion of clastic rocks by .........103-104, ill

Bayley, W. S., publications by, on observed contact phenomena on the 

point..__..................... 17

Bejby, agreement in composition between Pigeon point rocks and the 

granite of................._.......... 50

Biotite, altered to chlorite ............... 53

Calcite, connection of epidote with ...... 70

California, biotite in a gabbro form ..... 41

Chlorite, alteration product of biotite .. 52

in intermediate rocks................. 62

Conglomerates, composition of .......... 72

Contact belt, analysis of qnartzite'from. 83

breccia in.............................. 79-80

breccia in mottled zone near......... 80

composition of least altered rocks of. 83

Page. 

Contact belt, formation of secondary

minerals in least altered rocks of. 83 

macroscopic appearance of least altered rocks of ..................... 82

microscopic characteristics of least

altered rocks of ................... 82-83

mottled zone around slate inclusions

in.................................. 79

mottling of rocks of middle zone. _. 85-87 

mottled rocks and red quartzites in 

inner zone of ...................... 92

rock series included in ............... 78

character of slaty fragments in.._. 78-79 

effect on slate and quartzite fragments in the breccia.. ......... 81

D.

Diabase, macroscopic appearance of.. 44 

microscopic character of........._. j 44

alteration of nonmicaceous variety

of ................................44,45-40

characters of micaceous variety of.. 47 

compared with olivine gabbro ....... 47.48

relations to surrounding rocks of 

micaceous variety of.. _........ 47

dikesof.............,....!......^...... 26

Diallage-augite, analysis of......._.... 36

Dikes, characterof the.................... 27

diabase................................ 26

in the quartzite..._................ 2G

nonmicaceous diabase in............. 46

micaceous,diabase in .........._.... 47

Duluth, Minn., analysis of hornblende

gabbro from........._........... 37

E.

Eakins, L. G., acknowledgment to ...... 13

analysis of the red rock of Little 

- Brick island by.................... 58

analysis of green mottliugs by .._. 98 

Epidote, in the dancer quartzites._.... 70

connection of, with calcite............ 76

Epidote rocks, analysis of..._ ........ 76

Eruptives, character of, at contact with

sedimentaries.................._ 22

F.

Feldsp*ar, analysis of ..................... 34

in the gabbro, specific gravity of .... 33

in the gabbro, characterof........... 34

119
120 INDEX.

Page.

Feldspar in the red rock, character of.. 51 

in the porphyritic red rock........... 54

in the intermediate rocks ........._ 61

inunalteredquartzites.... .._.... 69

red, analysis of..._.............._. 52

red, microscopic appearance of...... 51

Foster and Whituey, cited ............... 17,49

Fragmental rocks, localities of exposure................................. 19

dipof ,--._..__.....___.........-...-... 19

disturbances in........................ 19,20

contact with gabbro .................. - 20

spotted beds in......^................. 21

veins penetrating.....--.......-..----- 21

cause of alteration in................. 29

Fragmental beds, alteration at the contact of the olivine gabbro and ... 80 

average composition of _..........112-113

France, (Gue'me'ne", Morbinan, and Rost.

renen) sandstone altered by gran- ' 

ite in............................... 103,

G.

Gabbro, areas of.......................... 22

dikes in................................ ' 22

relations of the red rock to .......... 24,25

position of, among massive rocks.... 37

distribution of altered varieties of... 38 

alteration product of................. ' 39

peculiar phases of._................ . 43

gradations between red rock and.... 60

relations between quartzitic inclusions and .........................100-101

relations between red rock and.105,106,107 

alteration produced by ..............105,109

Gabbro and red rock, relative ages of... 24 

contact line at the porphyry bluff.... 25

intermediate rock at contact of...... 25

GrandPortage, dikenorth of............ 22

Grand Portage island, contact of Aniinikite and Keweenawan at -------- 23

H.

Hillebraud, W. P., acknowledgment to.. 13 

analysis of gabbro by................. 37

analysis of the two types of red rocks, 

showing average composition..... 55-56

analysis of red groundmass of mottled rocks by....................... 97

Hornblende, character of compact variety of............................... 40

character of fibrous variety of....... 40

in intermediate rocks, alteration of.. 61,62 

Hussak cited .............................. 104

I.

Ingalls, E. D., cited ....................... 17,21

Intermediate rocks, character of ........ 60

locality of best development of...... 60-61

alteration of the hornblende in ...... 61

microscopic characters of............ 61

feldspar crystals in................... 61

analysisof __....___.....__....._..>... 68

relations of gabbro and red rocks to. 64

Page.

Intermediate rocks, theories of the origin of._;__..........--.._--...---. 64,65

Irving, R. D., cited.17,22,30,41,42,49,50,51,57,70

J. 

Judd, cited.................................38,106

Julien, A., cited.............-... ..... - 39

Keweena wan, contact with the Animikie

at Grand Portage island.......... 23

Kloos, cited........_.._.._........ 41

Kroustschoff, K. de, acknowledgment to. 13

L. 

Lacroix, A., cited..................._... 103

Lehmann, cited....................... 103

Leucoxene, presence of, in porphyritic 

red rock......................... 55

Little Brick Island, dip of the red rock

on.................................. 50

red rock of................... ....... 57

analysisof the red rock of. _...... 58

M.

Mather, W. W., cited ..................... -ID

McFarlane, cited...................... .... 50

Metamorphism...........................10:2-103

Michel-L6vy, cited.....................65,66,103

Mottled rocks, character of ........ . . 27

position of..-.------.....-..-.-..-.. 27,85

microscopic appearance of........... 85

composition of ......_............... 87

analysisof .................-....... - 88

analysis of red rock and...............89,90

in inner zone of contact belt ......-.'- 92-94

green mottlingsin _............. . 9-1

sericite ingreenmottlingsof..... 94

zircon in green mottlings of...... 95

origin of green mottlings of.......... 66

composition of green mottlings and

red groundmass of....... ------ 97

analysis of green niottlings-of-.....-. 98

N.

Norwood, cited,

O.

Olivine, altered to biotite................

altered to chlorite...._..............

altered to hornblende.................

character of ........... .............

Olivine gabbro, composition of..........

macroscopical characters of..........

weathering of.........................

joint cracks in.........................

porphyritic crystals in................

primary constituents of --------------

alteration at the contact of fragniental beds and......^............

indications afforded by the structure 

of the red rock and................

inclusions in...................._....

Owen, D. D., cited .........................

Owen, Richard, cited....... ..............

49

39 

39 

39 

35 

3^

32

32,33

33

33

36

80

81-82 

100 

27 

16
INDEX. 121

P.

Page.

Pantelleria, analysis of anorthoclase

from the rhyolite of............... 52

Peekskill, N. Y., luster mottling in peridotites near.. ................... 4<*

Pigeon river, slates and quartzites on .

north side of.._..............---- 23

Pigeon river falls, joint cracks at ..". 22 

character of slates at..._ .......... 30

contact of gabbro and quartzite at.. 109 

Plagioclase, alteration of................. 42

in the red rock, character of ......... 51

Porphyritic crystals, groundmass of .... 54

Pumpelly, R., cited........................ 41

Q.

Quartz of the red rock.......... .... 52

Quartzite, dikes in ........................ 26

alteration of feldspar in.............. 68,69

analysis of....._............ ..... 69

contact of eruptlves on the southern

border of the gabbro area with .. 80-81 

effect of gabbro and red rocks on. 98,99,109 

cementing material between slate 

and................................ 101-102

contact of basalt and other effusive

rocks with........................103-104

transition between red rock and..... Ill

red, in inner zone of contact belt .... 92

unaltered, microscopic characters of. 67,68 

unaltered, feldspar in................. 68

unaltered, mottling of................ 71

unaltered, analysis of ................ 84

Quartz porphyries, macroscopic character of...........-..........--. .. 53

micrpscopic character of ............. 53

R.

Red rock, areas of......................... 23,24

character of...................'........ 24,25

relations of the gabbro to............ 24,25

character of contact belt along border

of, in the western part of the point. 28,29 

inclusions of slate and quartzite in. - 28 

position of............................. 31

similarity to the Keweenawan flow 

rocks........_...._............. 31

localitiesof............................ 49

opinions of geologists concerning... 49 

microscopic characters of granular

variety of.......................... 50

typical aspects of the granular variety of ............................ 50

quartz-porphyry variety of .......... 53

identity of porphyritic and granular 

varieties of........................ 55,57

analyses showing average composition of the two types of........... 56

classification of....................... 58,59

gradations between gabbro and...... 60

indications afforded by the structure 

of the olivine gabbro and......... 81-82

Page.

Red rock, analyses of mottled slates and. 89-90 

inclusions in .......................... 99

relations between gabbro and........ 105

. origin of.-.-...-..-- -- .......105-112

transition between quartzites and... Ill 

characterized.........................115-116

Red rock and gabbro, relative ages of... 24 

intermediate rock at contact of...... 25

contact line at the porphyry bluff.... 25

Reusch, H., cited ......................... 110

Riggs, R. B., acknowledgment to........ 13

analyses of quartzites by ..............69,84

analysis of epidotic rocks by......... 76

analysis of slightly altered slate by.. 84 

. analysis of mottled rooks by ......... 87,88

Rosenbusch; H., cited.....................37,102

Rutile of the red rock .......__........ 53

s.

Sericite, in green mottlings........ ..... 95

Shuster, cited ............................. 34,41

Slates, contact of diabase dikes and...... 71

cause of mottling of .................. 88

contact of granular red rock with.... 88,89

cementing material between quartzite and ............................101,102

slightly altered, analysis of .......... 84

unaltered, composition of _........ 71

Spar island, red rock at _............... 30

Spotted rocks, characteristics of......... 72,73

groundmass of...:..................... 74

rocks in association with............. 74

clear zones in .._..._...._....... 75

position of.............._........... 75

Stecher, cited.............................. 104

Strengand Kloos, analysis of hornblende

gabbro by.......................... 37

T.

Tschermak, cited.

V.

Van Hise, C. R., acknowledgment to .... 13

cited ................................... 70

Victoria island, red rockat............... ' 30

W.

Wadsworth, M. E., cited............17,37,41,106

Whittield, J. E., acknowledgment to.... 13

analysis of red feldspar by........... 51,52

analyses of red rock and mottled

slates by........................... 89,90

Whitney. See Foster and Whitney.

Wiik,cited ................................ 66

Williams, G. H., acknowledgment to.... 13

Winchell, N. H., cited ..................17,22,49

Z.

Zircon, in porphyritic red rock... _.... 55

in green mottliugs ............_._. 95

Zirkel, cited............................... 104