DEPARTMENT OF THE INTERIOR

BULLETIN

UNITED STATES

GEOLOGICAL SURVEY

No. .63

THE GREENSTONE SCHIST AREAS OF THE MENOMINEE AND 

MARQUETTE REGIONS OF MICHIGAN

WASHINGTON

GOVERNMENT PRINTING OFFICE 

1 8«,) 0

LIBRARY CATALOGUE SLIPS.

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

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

States | geological survey | no. 62 | [Seal of the department] | 

Washington | government printing office | 1890

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

director | | The greenstone schist areas | of the | Menominee 

and Marquette regions of Michigan | A contribution to the subject of dynamic metamorphism | in eruptive rocks | by | George 

Huntington Williams | with au introduction by | Roland Duer 

Irving | [Vignette] |

Washington | government printing office | 1890

8°. 241pp. 16 pi.

Williams (George Huntington).

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

The greenstone schist areas | of the | Menominee and Marquette 

regions of Michigan | A contribution to the subject of dynamic 

metamorphiem | in eruptive rocks | by | George Huntington Williams | with an introduction by | Roland Duer Irving | [Vignette] | 

Washington | government printing office | 1890

8°. 241 pp. .16 pi.

[UNITED STATES. Department of the interior. (77. 5. fleological survey). 

Bulletin 62]. -

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

S? The greenstone schist areas | of the | Menominee and Marquette 

g regions of Michigan | A contribution to the subject of dynamic 

§ metamorphism | in eruptive rocks | by | George Huntington Wills' iams | with an introduction by | Roland Duer Irving | [Vignette] | 

£ Washington | government printing office | 1890 

~ 8°. 241pp. 16 pi.

S . {UNITED STATES. Department of the interior. (U. 8. geological survey).

5 Bulletin 62].

ADVERTISEMENT.

 [Bulletin No. 62.]

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

approved March 3, 1879, which declares that 

" Thepublications of the Geological Survey shall consist of the annual report of operations, geologic al 

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

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

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

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

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

shall be the property of the United States and form a part of the library of the organization: And the 

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

States."

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

by Congress:

'' That whenever any document or report shall be ordered printed by Congress, there shall be printed, 

in addition to the number in each case stated, the'usual number' (1,900) of copies for binding and 

distribution among those entitled to receive them." '

Exceptin those cases in which an extra number of any publication has been supplied to the Survey 

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

no copies for gratuitous distribution.

ANNUAL KEPOKTS.

. I. First Annual Keport of the United States Geological Survey, by Clarence King. 880. 8°. 79pp. 

linap. A preliminary report describing plan of organization and pablications.

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

8°. Iv, 588pp. 62 pi. 1 map.

III. Third Auuua.1 Report of the United States Geological Survey, 1881-'82, by J. W. Powell. 1883. 

8°. xviii, 564 pp. 67 pi. and maps.

IV. Fourth Annual Report of the United States Geological Survey, 1882-'83, by J. W. Powell. 1884. 

8°. xxxii, 473 pp. 85 pi. and maps.

V. Fifth Annual Report of the United States Geological Survey, 1883-'84, by J. W. Powell. 1885. 

8°. xxxvi; 469 pp. 58 pi. and maps.

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

8°. xxix, 570 pp. 65 pi. and maps.

VII. SeventhAnnualReportoftheUnitedStatesGeologicalSurvey,1885-'86, by J. W. Powell. 1888. 

8°. xx, 656 pp. 71 pi. and maps.

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

8°. 2 v. xix, 474, xii pp. 53 pi. and maps; 1 p. 1.475-1063 pp. 54-76 pi. and maps.

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

8°. xiii, 717pp. 88 pi. and maps. 

The Tenth Annual Report is in press.

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, 264 pp. 42 pi. and atlas of 24 sheets folio. Price $10.12.

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

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

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

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

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

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.

I
II ADVERTISEMENT.

Vfl. Silver-Lead Deposits of Eureka, Nevada, by Joseph Story Curtis. 1884. 4°. xiii, 200 pp.' 1& 

pi. Price $1.20.

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

241. 24 pi. Price $1.10.

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

by Robert P. Whitfleld. 1885. 4°. xx, 338 pp. 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. 501. 56 pi. Price $2.70.

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

Cook Russell. 1885. 4°. xiv, 288pp. 40 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 ol the Pacific Slope, with atlas, by George F. Becker. 

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

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

XV. The Potomac or Younger Mesozoic Flora, by "William Morris Fontaine. 1889. 4°. xiv, 377 

pp. 180 pi. Text and plates bound separately. Price $2.50.

XVI. The Paleozoic Fishes of North America, by John Strong Newberry. 1889. 4°. 340 pp. 53 pi. 

Price $1.00. 

In preparation:

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

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

C. R. Van Hiae.

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

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

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

 Sauropoda, by 0. C. Marsh.

 Stegosauria, by 0. C. Marsh.

 Brontotheridae, by O. C. Marsh.

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

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

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

 The Glacial Lake Agassiz, by Warren Upham.

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

BULLETINS. '

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

Geological Sketch of BuffaloPeaks, Colorado, by S. F. Emmons. 1883. 8°. 42pp. 2 pi. Price 10 cents.

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

3. On the Fossil Faunas of the Upper Devonian, along the meridian of 76° 30', from Tompkins County 

N. Y., to Bradford County, Pa., by Henry S. Williams. 1884. . 8°. 36 pp. Price 5 cents.

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

5. A Dictionary of Altitudes in the United States, compiled by Henry Gannett. 1884. 8°. 325 pp. 

Price 20 cents.

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

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

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

8°. 184 pp. Price 10 cents.

1 8. On Secondary Enlargements of Mineral Fragments in Certain Rocks, by R. D. Irving and C. R. 

Van Hise. 1884. 8°. 56 pp. 6 pi. Price 10 cents.

9. AReport of work done in the Washington Laboratory during the fiscal year 1883-'84. F. W. Clarke, 

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

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

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

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

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

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

12. A Crystallographic Study of the Thinolite of Lake Lahontan, by Edward S. Dana. 1884. 8°. 

34 pp. 3 pi. Price 5 cents.

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

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

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

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

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

33 pp. Price 5 cents.
ADVERTISEMENT. Ill

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

£6 pp. 3 pi. Price 5 cents.

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

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

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

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

19. Notes on the Stratigraphy of California, by George F. Becker. 1885. 8°. 28 pp. Price 5 cents.

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

21. The Lignites of the Great Sioux Reservation. A Report on the Region between the Grand and Moreau Rivers, 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°. 25pp. 5 pi. Price 

5 cents.

23. Observations on the Junction between the Eastern Sandstone and the Keweenaw Series on 

Keweenaw Point, Lake Superior, by R. D. Irving and T. C. Chamberlin. 1885. 8°. 124pp. 17 pi. 

Price 15 cents.

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

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

1S85. 8°. 336 pp. Price 25 cents.

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

1885. 8°. 85 pp. Price 10 cents.

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

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

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

28. The Gabbros and Associated Hornblende Rocks occurring in the Neighborhood of Baltimore, Md. 

by George Huntingtou Williams. 1886. 8°. 78 pp. 4 pi. Price 10 cents.

29. On the Fresh-water Invertebrates of the North American Jurassic, by Charles A. White. 1886. 

8°. 41 pp. 4 pi. Price 5 cents.

30. Second Contribution to the Studies on the Cambrian Faunas of North America, by Charles 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°. 23 pp. 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. White. 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 Laramie Flora, by LesterF. Ward. 1887. 8°.. 354pp. 57 pi. Price 25 cents.

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

39. The Upper Beaches and Deltas of the Glacial Lake Agassiz, by Warren Uphain. 1887. 8°. 84

 pp. 1 pi. Price 10 cents.

40. Changes in Riyer Courses in Washington Territory due to Glaciation, by Bailey Willis. 1887. 8°. 

10 pp. 4 pi. Price 5 cents.

41. On the Fossil Faunas of the Upper Devonian the Genesee Section, New York, by Henry S. 

Williams. 1887. 8°. 121pp. 4 pi. Price 15 cents.

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

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

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

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

44. Bibliography of North American Geology for 1886, by Nelson H. Darton. 1887. 8°. 35 pp. 

Price'5 cents.

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

 pp. Price 10 cents.

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

47. Analyses of Waters of the Yellowstone National Park, with an Account of the Methods of Anal- 

.syis employed, by Frank Austin Gooch and James Edward Whitfield. 1888. 8°. 84 pp. Price 10 

cents.

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

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

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

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 Ked 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°. 55 pp. 10 pi. Price 10 

cents.

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

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

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

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

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

7 pi. Price 10 cents.

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

Price 5 cents.

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

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

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°. 174pp. Price 15 cents.

61. Contributions to the Mineralogy of the Pacific Coast, by "William Harlow 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 metamorphism in eruptive rocks, by George Huncington "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.

64. A Report of work done in the Division of Chemistry and Physics, mainly dnring the fiscal year1888-'89. F. W. Clarke, chief chemise. 1890. 8°. 60 pp. Price 10 cents.

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

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

In press:

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

67. The relations of the Traps of the Newark System in the New Jersey Region, by N. H. Darton.

68. Earthquakes in California in 1889, by J. E. Keeler.

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

In preparation

 The Viscosity of Solids, by Carl Barus.

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

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

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

 A Bibliography of Paleobotany, by David White.

STATISTICAL PAPERS.

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

50 cents.

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

pp. Price 60 cents.

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

8°. vii, 576 pp. Price 40 cents.

Mineral Resources of the United States, 1886; by David T. Day. 1887. 8°. viii, 813 pp. Price 5ft 

cents.

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

cents.

Mineral Resources of the United States, 1888, by David T. Day. 1890. 8°. vii, 652 pp. Price 5ft 

cents.

The money received from the sale of these publications is deposited in the Treasury, and the Secretary of the Treasury declines to receive bank checks, drafts, or postage stamps; all remittances, therefore, must be by POSTAL NOTE or MONEY OKDER, made payable to the Librarian of the U. S. Geological 

Survey, or in CURRENCY, for the exact amount. Correspondence relating to the publications of the

Survey should be addressed

To THE DIRECTOR OF THE

UNITED STATES GEOLOGICAL SURVEY, 

WASHINGTON, D. C., September, 1890. WASHINGTON, D. C.
DEPAKTMENT OF THE INTEBIOR

BULLETIN

OF THE

UNITED STATES

GEOLOGICAL SURVEY

No. 63

WASHINGTON

GOVERNMENT PRINTING OFFICE 

1890

UNITED STATES GEOLOGICAL SURVEY

J. W. POWELL, IVIRECTOK

THE GREENSTONE SCHIST AREAS

OF THE

A CONTRIBUTION TO THE SUBJECT OF DYNAMIC METAMORPHISM 

IN ERUPTIVE ROCKS

GEORGE HUNTINGTON WILLIAMS

WITH. AN JNTHODUCTION BY

ROLAND DUER IRVING

WASHINGTON

GOVERNMENT PRINTING OFFICE

1890

CONTENTS.

Page.

Letter of transrnittal, by R. D. Irving ....................................... 9

Explanatory and historical note, by E. D. Irving............................. II

Introduction................................................................ 3.1.

CHAPTER I. Present state of our knowledge regarding the metamorphism of

eruptive rocks.................................................. .>.... 34

Value of the microscope in the study of metarnorphism................... 34

Historical outline of studies on tbe inetamorphism of eruptive rocks...... 40

CHAPTER II. Greenstone belts of the Menominee iron district ................. 64

Introductory and historical.............................................. 64

Sturgeon Falls.......................................................... 67

Lower, or Little Quinnesec Falls .............................. .......... 77

CHAPTER III. Greenstone bolts of the Menomiuee iron district (continued)... 96

Upper, or Big Quinuesec Falls .......................................... 96

The dark colored greenstones of the basin............................ 97

The light colored greenstones at Upper Quinnesec falls............... 102

The coarse grained diorites of the Horse Race........................ 106

The acid rocks of the Upper Quiunesec Falls and Horse Eace ......... 110

Four-foot Falls ......................................................... 123

The Twin Falls..................... .................................... 127

Lower Twin Falls................................................... 129

Upper Twin Falls....................... ............................ 132

CHAPTER IV. Greenstone belts of tlie Marquette district.................... 134

Introductory............................................................ 134

Rocks of the northern portion of the Marquette area..................... 138

Basic intrusives..................................................... 138

Acidintrusives...................................................... 146

Banded greenstone schists........................................... 154

CHAPTER V. Greenstone belts of the Marqnette district (continued).......... 163

Rocks of the southern portion of the Marquette area..................... 163

The aphanitic greenstones........................................... 163

Coarse grained dike rocks........................................... 168

Greenstones south of the quartzite.............................'...... 170

Rocks of the Negaunee area............................................. 171

Aphauitic greenstones............................................... 171

Coarsely crystalline greenstones..................................... 173

The stretched fragmental rocks on the Carp River.................... 175

Acid rocks.... ...................................................... 178

Rocks of the Northern area.............................................. 179

Unaltered basic iutrusives............................,,............. 180

Altered coarse grained rocks. ........................................ 180

Banded greenstones................................................. 184

Green schists and agglomerates, of Deer Lake............................ 185
6 CONTENTS.

Page. 

CHAPTER VI. General results and conclusions..!... ........................ 192

Original character of the Menomiuee and Marquette greenstone areas.... 192

Evidence of eruptive character...................................... 192

Different original rock types........................................ 197

Original mineral constituents....................................... 199

Conditions under which the greenstones were formed................ 200

Macrostructural metamorphism of the Menominee and Marquette massive

rocks .........................................v .................. 201

Macrostrnctnral uietauiorphism through compression, faulting, or 

crushing.......................................................... 202

Macrostructural metamorphism through stretching.................. 204

Microslructural metamorphism of the Menominee and Marquette massive

rocks. ............................................................ 204

Effects of dynamic action on individual minerals.................... 205

New structures produced by dynamic action ......................... 206

Mineralogical (chemical) metamorphism of the Menomiuee and Marquette

massive rocks..................................................... 208

Secondary minerals and their origin................................. 209

Progress of alteration in the original minerals....................... 214
ILLUSTRATIONS.

Page. 

PLATE I. Outline geological map of the Marquette region................ 14

II. Outline geological map of the Menoininee iron region........... 24

III. Map of Sturgeon Falls, Menominee River (after Brooks)........ 68

IV. Map of Lower Quinnesec Falls, Menomiuce River (after Brooks). 86

V. Map of Upper Quinnesec Falls, Menomiuee River (after Brooks). 96

VI. Map of Twin and Four-toot Falls, Menominee River (after Brooks) 124

VII.- Map of the environs of Marquette................................ 138

VIII. Fig. 1. Saussurite gabbro (first stage)......................... 222

2. Sauasurite gabbro (second stage) ...................... 222

I.. Fig. 1. Altered diabase or diorite.............................. 224

2. Schistose band in the last figured rock, showing the effect

of dynamic metauiorphism (stretching) upon it...... 224

X. Fig. 1. Saussurite diorite ..................................... 226

2. Typical aphanitic greenstone of the Marquette region.. 226

XI. Fig. 1. Chlorite-epidote aggregate in an altered diabase....... 228

2. Stretched greenstone.................................. 228

XII. Fig. 1. Coarsely crystalline diorite............................ 230

2. Epidiorite ............................................. 230

XIII. Fig. 1. Sphene crystals around ilmeni to........................ 232

2. Rutile originating from the alteration of ilmenite...... 232

XIV. Fig. 1. Sericite porphyry ..................................... 234

2. Stretched and brecciated greenstone................... 234

XV. Fig. 1. Structure of one of the " eyes" in an "Augengueiss" or

stretched granite porphyry .......................... 236

2. Stretched quartz porphyry ............................ 236

XVI. Fig. 1. Greenstone schists (tuffs) of the northern Marquette area 238 

2. Amphibole granite (?) forming a dike in the aphanitic 238 

greenstones ..... ...... ...... ...... .................'.

FIG. 1. Hypothetical section of the Menominee region in the vicinity of

Quinnesec (after Brooks) ............. ............................ 26

2. Hypothetical section of the Menominee region in the vicinity of Twin

Falls (after Brooks) .............................................. 27

3. Hypothetical section of the Menominee region in the vicinity of Quinnesec (by R. D. Irving).........'.................................. 30

4. Twinning lainelhe in secondary albite............. .................. 69

5. Zoisite crystals in saussurite ........................ ................ 69

6. Zoisite crystal showing position of optical constants with reference to

cleavage lines .................................................... 70

7. Compact and fibrous secondary hornblende.......................... 70

8. Helminth (vermicular chlorite) in quart/...... ....................... 71

(J. Hornblende altering to chlorite ..................... ................ 70

10. Cross gashes in greenstone produced by stretching .................. 81

11. Broken feldspar crystals in crushed and stretched greenstone ........ 105

7
8 ILLUSTRATIONS.

Page.

FIG. 12. Rutile needles (Tbonschiefernadeln)................................ 106

13. Broken tourmaline crystal proving the secondary fracture of the inclosing quartz.................................................... 112

14. Foliated granite (gneiss) intrusive in greenstone.................... 115

15. Foliated granite (gneiss) intrusive in greenstone.................... 115

16. Foliated granite (gneiss) intrusive in greenstone..............'...... 116

17. Orthite (allanite) in granite........................................ 117

18. Broken zircon crystal in schistose porphyry......................... 122

19. Compact surrounded hy fibrous hornhlende (secondary growth?) .... 126

20. Rhomboidal parting in greenstone (diagrammatic).................. 128

21. Acute rhomboidal parting iu greenstone (diagrammatic)............. 128

22. Schistose parting developed from that in Fig. 21 .................... 128

23. Greenstone made schistose by faulting.............................. 129

24. Greenstone made schistose by faulting.............................. 130

25. Anatase formed by the alteration of ilmenite in greenstone ........ 131

26. Spheroidal parting of aphauitic greenstone produced by brecciation.. 166

27. Same as in Fig. 26, drawn out into lenses by pressure................ 177

2b. Bent hornblende crystal. ........................................... 183

29. Surface of agglomerate from Deer Lake............................. 188
LETTER OF TRANSMITTAL

UNITED STATES GEOLOGICAL SURVEY,

LAKE SUPERIOR DIVISION, 

Madison, Wisconsin, February 10, 1888.

SIR : I have the honor to transmit herewith, for publication as a 

bulletin of the Survey, a paper, by Prof. George H. Williams, of Baltimore, Maryland, on The Greenstone-Schist Areas of the Marquette and 

Menominee Regions of Michigan.

The exact object and bearing of the work thus accomplished by 

Prof. Williams I have set forth in an explanatory and historical note 

prefixed to his memoir.

I am, sir, very respectfully, yours,

It. D. IIIVING,

Geologist in Charge. 

Hon. J. W. POWELL,

Director U. 8. Geological Survey.

EXPLANATORY AND HISTORICAL NOTE.

BY EOLAND DUEII p

At quite a number of points in the Lake Superior region 

liar schistose rocks, which combine with a prevalent fine grain a general 

tendency to greenish color. Besides occurring here and there more or 

less confusedly mingled with masses of granite and other rocks, these 

greenish schists occur also in large continuous areas, which they entirely 

occupy, except for certain relatively unimportant basic and acid iutrusives. Microscopic study has shown that these rocks are at times fully 

developed hornblende schists, and again glossy kinds, in which chlorite 

is so important an ingredient as to warrant our calling them chlorite 

schists. In other places again they are more massive, and present more 

or less distinctly the appearance of fine-grained basic eruptives; but 

the bulk of these areas is composed of nondescript fine-grained greenish schists, which appear to grade in to the more massive greenstone-like 

forms on the one hand, and into the more distinctly developed chloritic 

and hornblendic schists on the other. As a rule these various schists 

present no parallel structure other than that which seems referable 

directly to secondary causes; that is to say, they do not present such 

banded varieties as would suggest the action of sedimentation during 

their production. However, such banded varieties do occur iu subordinate quantity, presenting then very strikingly regular, rapid alternations of light and dark bands.

Again, here and there in these peculiar areas, in some places with a 

very considerable development, there occur kinds which present a more 

or less obscure fragmental appearance, this appearance being generally 

far more pronounced on a weathered surface than on a fresh one, on 

which latter the matrix and the parent fragments are apt to look much 

like one another, either from original similarity in character or from 

having undergone alterations which have led both fragments and. 

matrix, originally different, to similar results. At times there is a coincidence of occurrence between the banding above referred to and the 

appearance of the pebble-bearing forms, but so far as my experience 

goes this is very commonly not the case, the fragmental ph, »es being 

without any parallel structure other than the general second* y cleavage, which all of these rocks present, the apparent pebbles occurring 

more commonly in tumultuous assemblages of masses of all sizes.

11
12 GREENSTONE SCHIST AREAS OF MICHIGAN. (BULL. 62..

As prominent illustrations of such green schist areas, there may be 

mentioned that which runs directly westward from the shore of Lake 

Superior at Marquette, Michigan, for a distance of| some twenty miles, 

with a width ranging from two to six miles; 1 those which occur in the 

Menominee Valley, on the boundary between Wisconsin and Michigan; 2 

that of the Gogebic country of western Michigan, in T. 47 N., E. 43 W., 

and T. 47 N., E. 44 W., where there is a very great development of the 

fragmental phases; 3 that which has been recently outlined by Dr. W. 

S. Bayley to the south of the so-called Mesabe" Eange, and west of 

the Embarras Lakes, in T. 59 K, E. 15 W., and T. 59 N., E. 16 W., 

northern Minnesota, where there is again a considerable development 

of the fragmental forms; 4 and that which lies to the northward of the 

iron-bearing slates of Vermillion Lake, traversing the island in the 

northern part of that lake from west to east, and extending thence east 

and northeast, with an average width of from two to three miles, through 

townships 62, 63, 64, and 65 K, of ranges 14, 13,12,11, 10, and 9 W., in 

northern Minnesota, to the national boundary line at Carp Lake, whence 

it extends an unknown distance into Canada. This last-named area, 

which is the most extended continuous one with which I am acquainted, 

shows prevalently those kinds of rock which lack the fragmental appearance. Still, fragmental phases here and there present themselves, 

as, for instance, along the east shore of that island in Vermillion Lake, 

which lies in the northern part of Sec. 10 and southern part of Sec, 3, 

T. 62 N., E. 16 W. To these areas may be added also several smaller 

ones which lie southward and southeastward of the Vermillion Lake iron 

belt. One of these areas, lying south of Sea Gull Lake and east of 

Ogishkimauissi Lake, in T. 65 N., E. 5 W., presents an extraordinary 

development of the obscurely fragmental or brecciated phase. Finally, 

should be mentioned the large development of similar greenish schists, 

including also the fragmental or " agglomeratic" phases, which is 

described as occurring on the Lake of the Woods in the recent report 

of Mr. A. C. Lawson, of the Canadian Survey.5

My first acquaintance with any of these green schist areas was in the 

summer of 1883, when a number of days were spent in examining the 

exposures at Twin Falls, Upper Quinnesec Falls and Lower Quinnesec 

Falls, on the Menominee Eiver, the boundary between Wisconsin and 

Michigan. The very handsome detailed maps and the descriptions of 

these exposures by Maj. T. B. Brooks6 had already been published and

1 See map of T. B. Brooks, Geol. Survey Michigan, Atlas, PI. Ill) map by C. Rominger, accompanying Geol. Survey Michigan, Vol. 4,1881; PL I of the present volume.

2 See map by T. B. Brooks and C. E. Wright, PI. 28 of the atlas to the Geology of Wisconsin ; map 

by C. E. Wright. PI. 30 of the same atlas; map of the Menominee region, by C. Eominger, Geol. Survey Michigan, Vol. 4, 1881, PI. 2.

3 See maps by E. D. Irving and C. E. Van Hise, accompanying a memoir on the Penokee-Gogebic 

iron region, Mon. TJ. S. Geol. Survey, No. 18.

4 See map by R. D. Irving, Pi. XLI, of the Seventh Ann. Eept. U. S. Geol. Survey. 

6 See Geol. Nat. Hist. Survey Canada, Ann. Rep. (new series) Vol. 1, 1885, pp. 41-46 cc; also map 

accompanying same. 

6 Geol. Wisconsin, vol. 3,1880.
IKVINO.I EARLY IMPRESSIONS OF MENOMINEE GREENSTONES. 13

were used on the ground. These maps represent the exposures at the 

several points named as composed of a series of very regular alternations of various schistose and massive rocks, the conclusion being 

reached with regard to them by Brooks that they form, in all a regular 

series of layers originally of sedimentary origin, the present lack of fragmental appearance being taken as the result of rnetamorphism. To me 

it seemed at that time that the regularity of the alternations was far 

less than one would think to be the case from Brooks's maps, and that all 

of the several phases, schistose and non-schistose, were rather variations of one great mass of material, whose original structure was massive rather than schistose. This impression was gathered from the way 

in which the several phases graded into one another, and from the noncontinuity in the direction of the strike of the schistose kinds, single 

schist beds displayed on one side of tha stream being often absent 

where their continuations should lie on the opposite side of the stream. 

The impression was strengthened by the lack of regularity in the direction of the schist planes, and by the striking way in which schist 

bands inclose vaguely outlined areas of non-schistose massive rocks. 

The same conclusion as to the inseparable nature of all of these rocks 

had already been reached and published by Eominger,1 though he appears to have looked upon the parallel or schistose structure as representing an original condition of the rocks, all of which he seems to 

consider of sedimentary origin; while the impression made on me was 

iust the opposite, namely, that the schistosity is of a secondary nature, 

and the original structure of the rock a massive one. In order to test 

this question, a large collection of specimens, with quite full notes, was 

made at the time. In the next year, however, before the thin sections 

made from these specimens could be examined, the entire collection 

was destroyed by fire, and my work in other directions having 

become pressing, it was decided to put the study of this question in 

the hands of Prof. George H. Williams, of the John Hopkins University, whose qualifications for investigations in microscopic petroggraphy, such as this would mainly be, are well known to be of the first 

order. In the summer of 1885, therefore, Professor Williams made, at 

my request, a reexamination of all of these points in the Menominee 

valley, going into far greater detail in the field study than any one 

had done before, and collecting on a very large scale. During the 

summer of 1886, Professor Williams was asked to extend his field examinations into the Marquette greenschist area, and here similar detailed studies and large collections were made. The present publication is the result of Professor Williams's work, whicli has included the 

detailed examination of about 400 thin sections made from the specimens gathered.

While Professor Williams's investigations have been going on, the 

various other green schist areas named, so far as they lie within the

1 Geol. Survey Michigan, vol. 4, 1881, p. 214.
14 GREENSTONE SCHIST AKEAS OF MICHIGAN.

boundary of the Uuited States, have been examined in considerable detail, large collections and extensive field-notes having been gathered by 

Prof. Van Hise, Messrs. W. N. Merriarn and W. S. Bayley, and myself. 

These additional collections are now quite thoroughly sectioned, and it 

is hoped soon to study them in detail, and to offer a second contribution 

to our knowledge of this important class of rocks. The present publication, 'however, deals only with the Marquette and Menominee areas.

Prof. Williarns's investigations having been directed especially to a 

microscopic study of these peculiar greens chists, with the object of ascertaining whether such study would throw any light upon their original 

condition; it does not come within the province of his memoir to discuss 

the geologic relations of the green schist masses as wholes. Nevertheless, it is almost unavoidable that some reference should be made in this 

volume to these geological felatious, and, since there has been a good 

deal of difference of opinion as to them among the several geologists 

who have written upon the Menorniuee and Marquette areas, it seems 

necessary to explain briefly the different views that have been advanced, 

including the conclusions to which my own studies have thus far led me, 

though without attempting any general discussion of the subject for 

 the whole Lake Superior region. Such a discussion it is designed toleave 

until exhaustive microscopic studies, like that of Professor Williams here 

presented, shall have been extended over the other areas of similar rocks 

above named. We shall then be in possession not only of all available 

structural facts with regard to these peculiar rocks, but also of all that 

the most refined methods of study can give us with regard to their internal texture and mineralogical composition.

. The map (PI. I) shows, in a general way, the distribution of the different kinds of rocks in the vicinity of Marquette. In preparing this 

map, those by Brooks and Eominger, read in the light of my own studies 

in this region, have been used. It should be said that a detailed map 

of the Marquette district has not been attempted by the TJ. S. Geological Survey as yet, partly because other more pressing work was in progress, and partly because without a thoroughly accurate topographical 

basis for such a map it has not been thought that much advance could 

be made upon the several maps heretofore constructed. It is proposed, 

however, to begin such topographical survey immediately, and upon 

the basis thus prepared, to place everything exactly as it is seen, and, 

making use of all the latest mining developments, to attempt to work 

out accurately and in detail the structure of the region in such a fashion that what is inferred may be easily separated from what is actually 

known.

Upon the present map the large area colored for greenstone schists 

is the one which came particularly under Professor Williams's investigations. Northward this area is limited, in the eastern portion, by a great 

spread of granitic and gneissic rocks. The line of demarkation between 

the schists and the granites, however, is not a sharp one, since the
U.S.GEOLOGICAL SURVEY. BULLETIN No.62 PLATE I.

ARCHEAN. AGE UNKNOWN. ALGONKIAN. CAMBRIAN.

Granite, and Gneiss. Greenstone Schisls. Altered Peridotite. .Iron Bearimj Series. Lake Superior S.S.

(Delrilals, Limestones arid 

FeiTiujinous Srhists,with interbedded 

_____ ______ .Greenstones.)

OUTLINE GEOLOGICAL MAP OF THE MARQUETTE REGION. 

Compiled by U.D.Ii-viiuj li-oniinnps by T.RBroolts <nul ('.RonuTKjer.

Sf.ale of iidles, 

.o .3 ,6 ,9

l

19OO8O
GREENSTONE SCHISTS NEAR MARQUETTE. 15

two seem to mingle more or less confusedly ou each side of the somewhat arbitrary line indicated upon the map. Southward of this greenstone-schist area, aud again dovetailing into it on the north, are belts 

of country occupied mainly by detrital rocks, such as quartzites and 

various fragmeutal slates; with these, however, are large bodies of 

crystalline limestone aud several phases of ferruginous schist, all 

of which have iu common an entire lack of anything like a fragrnental texture. In addition to these rocks these areas include also 

sheets of diabasic greenstone, which are interbedded with the detritals 

and ferruginous schists alluded to. On the south of the southernmost 

of these detritai areas, in which are included all the well known iron 

mines of the Marquette region, is again a large area of granitic and 

gneissic rocks.

The greenstone schists are affected generally by a vertical or nearly 

vertical cleavage structure, to which the occasional banding of the 

rock, suggestive of a sedimentary origin, is also parallel. The layers 

of rock of which the detrital areas are composed are aifected partly by 

a similar cleavage structure, but, on the whole, far less markedly so, 

while the bedding of these rocks is as a rule very distinct, and indicates a more or less open or gentle bowing of the layers, the dips being 

at times southward and again northward, and usually at a much lower 

angle than is the cleavage structure of the greenstone schists. At 

times, however, particularly in the easternmost portion of the southern 

of these two belts, the inclinations of the detrital rocks are much higher, 

and then bedding and cleavage structure approach to parallelism with 

each other, and with the cleavage structure of the greenstone schists 

further north. While folded back and forth, these detrital areas show 

for the most part an evident general synclinal structure; that is to say, 

the layers of rock of which they are composed, as they appear along the 

edges of these areas, dip inward towards their middle portions. Thus 

the greenstone schists seem plainly to rise from beneath the higher detrital layers. As to the inferior position of these greenstone schists to 

the detrital iron-bearing rocks, all geologists who have examined this 

region would probably agree; at all events all those who have expressed 

themselves with regard to the matter seem to have no hesitation concerning it.

Foster and Whitney, whose classical work1 gives the first comprehensive account of the geology of this region, do not appear to have 

separated the greenstone schists from the other stratiform rocks iu such 

a fashion as to indicate their exact strati graphical relations. All of 

these stratiform rocks they appear to have looked upon as constituting 

one great series, whose crumpled condition they attributed to the subsequent eruption of the granite masses on either side of the single 

trough, which they regarded the stratiform rocks as occupying.2 They

1 Report of the geology of the Lake Superior land district, by J. W. Foster aud J". D. "Whituey, pt. 2. 

The Iron Region, Washington, 1851. 

=Ibid., p. 41, Fig. 5.
16 GREENSTONE SCHIST AREAS OF MICHIGAN. [BULL. 62.

appear, however, to refer especially to the greenstone schists in the 

following:

Many of the slates appear to be composed of pulverulent greenstone, as though they 

might originally have been ejected as an ash, and subsequently deposited as a sediment, and pass by imperceptible gradations, from a highly fissile to a highly compact 

state. * * * The slates are composed essentially of the same .ingredients as the 

trappean rocks with which they are associated, and the main difference between 

them may be that the one was the product of salses, ejected in the form of mud, while 

the other was the product of volcanoes, ejected in molten streams. 1

They appear also to refer especially to the greenstone schists when 

they speak of the stratiform series as exhibiting greater alteration in 

the vicinity of the masses of eruptive granite, than at distances from 

these masses.2

J. P. Kimball, who wrote in 1864, 3 considered the granitic and gneissic 

masses on either side of the belt of stratiform rocks, as corresponding to the Laurentiau series of Canada, while he parallelized the whole 

of the stratiform succession with the Huronian series of the north shore 

of Lake Huron, thus making the granitic and gneissic rocks the older 

basement upon which all of the stratiform Huronian was subsequently 

spread, wholly by aqueous agencies. None of the various rocks which 

Foster and Whitney looked upon as eruptive, are thought by Kimball 

to have had that origin, even the various greenstones being taken to be 

metamorphic sedimentary material. In the case below Kimball appears 

to refer particularly to the greenstone schists, which form our present 

subject, and which, it thus appears, he looked upon as the basement 

member of the Huronian succession. He says:4

The gneiss which marks the boundary of the granite belts, and accordingly characterizes the top of the Laurentian series in this region, the same as it is elsewhere 

represented, is succeeded by dark colored hornblendic schists, which consequently 

represent the base of the Azoic or Huronian series. These schists are followed by a 

series of augitic rocks and schists, interstratified with magnesian hydrous rocks and 

slates, the two kinds of rocks being represented on the one hand by hypersthene, pyroxene, and bedded diorite passing into dioritic slates, and on the other by talcose 

and chloritic schists. The former character of rocks prevails to such an extent as to 

impart to the lower members of the Huronian series a distinctive augitic aspect. 

The several rocks composing this augitic zone are commonly of a greenish color, and 

vary in this respect chiefly as to shade, resembling in this particular the Lower Slate 

Conglomerate, which marks the base of the series in Canada, and from which they 

seem to differ only in the absence of pebbles and bowlders from the subjacent Laurentian rocks, which there form a distinguishing feature.5

1 Keport of the geology of the Lake Superior land district, by J. W. Foster and J. D. Whitney, pt. 2. 

The Iron Kegion, "Washington, 1851, pp. 16,17.

2 Ibid., p. 14.

aAm. Jour. Sci., 2d series, vol. 39, 1865, pp. 290-303.

- Ibid., p. 294.

6 This supposed similarity of the greenstone schists of Marquetfco to the Lower Slate Conglomerate 

of Lake Huron is not borne out at all by later microscopic and field studios. Indeed, the two have 

nothing whatever in common, unless the occasional greenish color of the Lake Huron rocka be taken to 

indicate such a resemblance. Tho Lower Slate Conglomerate of Lake Huron, -with its granitic and 

gneissic pebbles, proves to be a most completely detrital and plainly water-formed mass composed chiefly 

of fragments of quartz and feldspar, with the alteration products of the latter, tho whole cemented by 

a silicious paste. This statement is made only after a careful examination of tbe Lake Huron region, 

and after the study of a very large number of thin sections from Logon's so-called Lower Slate Conglomerate. .
IKYING.] MARQUETTE GREENSTONES. 17

T. B. Brooks, upon his detailed map of the Marquette district 1 leaves 

blank the larger part of the area occupied by the greenstone schists 

which now especially concern us, indicating here and there upon it, 

however, the occurrence of belts of "diorite" and of "chloritic and 

dioritic schists." All of these rocks he evidently regards as belonging 

conformably beneath the detrital quartzites and slates which form his 

division No. V, of the iron-bearing series, which division appears in a 

bold ridge along the northern edge of the southern of the two detrital 

areas indicated on the outline map herewith. The diorites Brooks 

classes lithologically with the greenstones interstratified with the detrital rocks themselves, though apparently looking upon them, as belonging to much lower layers in the succession. In this lithological 

correlation, as will be seen in the sequel, Brooks was largely correct, 

since the diorites of the greenstone schist area prove to be diabasic 

greenstones in dike form, and to be entirely similar lithologically to the 

ititerbedded greenstones of the detrital or iron-bearing areas, with which 

they are also presumably wholly contemporaneous. All of these rocks, 

both massive and schistose, Brooks plainly regarded as metamorphosed 

water-formed sediments.

Wads worth, writing in 1830,2 advocates in the main the views 

presented by Foster and Whituey, as will be seen by the following 

quotation:

The general structure of the country would seem to be as follows: The schists, 

sandstones, etc., having been laid down in the usual way, were then disturbed by the 

eruption of the jasper and ore; this formed the knobs of jasper, the banding belonging to the fluidal structure, and not to sedimentation. Besides occurring in bosses, 

the jasper was spread out in sheets, and intruded through the rock in wedge-shaped 

masses, sheets, and dikes. Much of the original rock still remained horizontal, and 

new sedimentary deposits continued to be formed out of the jasper and the other rocka. 

Next came the eruption of "diorite," which completed most of the local folding and 

tilting of the strata. Finally, the granite eruption took place on both sides of the 

" Huronian," uplifting and contorting the strata near it, aud perhaps laterally compressing the inclosed iron-bearing rocks. 3

Dr. Wadsworth does not appear to express any particular opinion as 

to the stratigraphical position of the greenstone schists of the area now 

especially under consideration, speaking of them only in connection 

with all the rest of the stratiform rocks of the region as older than the 

adjacent granite areas. The greater part of these greenstone schists 

he would seem to have looked upon as certainly sedimentary,4 since he 

records a number of observations going to show that there is no gradation between them and certain dike-like masses occurring with them, 

using these observations in opposition to the ideas expressed by some 

of his predecessors to the effect that inasmuch as the massive rocks and

'Geol. Survey, Michigan, 187U, vol. 1, Atlas pi. ;!. See also pp. 99-104, same vol. 

* Notes on the Geology of tho Iron and Copper Districts of Lake Superior. By M. E. Wadsworth 

Bull. Mus. Comp. Zool. Harvard College, Whole Series, vol. 7 (Geol. Series, vol. 1 No. 1). 

8 Ibid., p. 75. 'Ibid.,- p. 37.

Bull. 62 2
18 GREENSTONE SCHIST AREAS OF MICHIGAN. t»utL.62.

the schists grade into one another, the former are of sedimentary origin 

as well as the latter. That there are genuine dikes intersecting these 

schists andhaving nogradations into them, Professor Williains'sobservations recorded in this volume, as well as those by Eominger and others, 

make very plain, although even in the case of these genuine later dikes 

there are at times partially schistose forms produced apparently by 

secondary dynamic agencies. On the other hand, it will be seen from Professor Williains's descriptions that there are other massive forms among 

these greenstone schists which do present every possible gradation into 

the schists themselves, and must, therefore, be believed to have had a 

common origin with them.

Bomiuger, in his account of the geology of the Marquette region, 

published in 1881,1 describes very fully the greenstone-schist area which 

Professor Williams has studied. Rouainger's mapping of this area has 

essentially been followed upon the small sketch-map accompanying the 

present volume. The rocks of this area he describes under the general head of the " dioritic group," which group he looks upon as forming the basement member of the Huroniau series within which he would 

include all the stratiform rocks of the region, considering it as placed 

conformably beneath the detrital strata. The origin of the structure 

of these greenstone schists he looks upon as having been a secondary 

one, although he recognizes distinctly their intrusion by later genuinely 

eruptive dikes. Inasmuch as in its more northern portion the greenstone-schist area presents numerous bosses and veins of granite, he 

considers these as proving the more recent origin of the great granite 

masses lying both to the north and south of all stratiform rocks, thus 

returning, as did Wadsworth before him. to the early expressed view 

of Foster and Whituey, with whom he agrees also in considering that 

the crumpling of all of the stratiform rocks, greenstone schists and detritals as well, has been accomplished by the protrusion of the later 

granite masses on either side. In the same volume, and upon his map, 

Dr. Kominger indicates numerous small areas of massive and schistose 

greenstones as occurring in the vicinity of Negaunee and Ishpeming, 

within the district represented on the outline map herewith, as occupied 

entirely by the detrital iron-bearing series. These masses he regards 

as forming portions of his dioritic group, although at the same time 

recognizing distinctly the fact that they occur at various horizons 

among the detrifcals, which he looks upon as lying wholly above the 

main mass of his dioritic rocks. This anomalous arrangement he explains by representing that the dioritic group, inasmuch as it lies at 

the base of the entire stratiform series, comes directly into contact 

with the granite protrusions, whose metamorphosing influence was so 

great as to cause the fusion of a portion of these basement layers. 

These fused portions, then, he imagines to have intruded themselves 

in various shapes among the higher detrital layers.2 This view, how-

'Geol. Survey Michigan, 1881, vol. 4, with map of Marquette region. 

2 Ibid., pp. 22-39, particularly pp. 37, 33.
IKVIKG.I MAKQUETTE GREENSTONES. 19.

ever, Dr. Eominger subsequently abandoned, as will appear from the 

following quotation from the manuscript of his last and yet unpublished report: 1

As from the massive form of the diorites a gradation exists iuto the schistose condition, and as schistose structure formerly appeared to me a positive proof of a 

former sedimentary origin, I resorted in my previous report, in order to explain the 

similarity in the composition of the schists with the massive diorites, to the hypothesis of a secondary fusion of the lower beds of sediments nearest to the focus of central heat, and subsequent injection of the fused part iuto the folds and fissures of 

the remainder of the strata, simultaneously also the molten mass to have been forced 

into the fissures and crevices of the adjoining granite. I have since lost .much of my 

faith in this supposition, since I have convinced myself that schistose structure is 

not necessarily the result of aqueous sedimentation, but that; cooling eruptive masses 

under circumstances can assume a schistose form. * * * I am inclined to suppose 

that these schists so intimately associated with massive diorite beds are a product of 

their decomposition, under circumstances favoring the schistose arrangement of the 

molecules, or, to speak iu more definite terms, are a modified form of those eruptive 

masses, and do not refer to former sedimentary deposits.

My examinations of the Marquette region, made at different times in 

the summers of 1883, 1886, and 1887, have served to convince me thus 

far of the correctness of the views of most of the geologists who have 

examined the region with regard to the inferior position of the rocks of 

the greenstone schist area to the remainder of the stratiform rocks. 

They have also served to impress rue strongly with the probable correctness of the view which would make at least some granitic rocks subsequent in point of time to the greenstone schists themselves, since the 

latter are so intricately penetrated by granitic bosses and dikes in their 

more northern portions.

I have also seen enough to make me confident that the dike masses 

which cut the greenstone schists of this area are of wholly subsequent 

date to the schists, and. indeed are equivalent in point of time to 

those intruded sheets and masses which lie within the overlying detrital iron-bearing series. On the other hand, I have seen some reason 

to suspect that nearly all previous geologists have been mistaken in 

considering the rocks of the greenstone-schist area as belonging within 

the same great geological period as that which holds the remainder of 

the stratiform rocks of the region. In other words, it thus far appears 

to me that there is good reason to believe that these greenstone schists 

along with the granites, gneisses, etc., form a portion of the basement 

upon which the overlying detrital iron-bearing series was once horizontally and uncontbrmably spread. That they do not constitute the conformably underlying basement beds of that series is suggested by a 

glance at the map (PI. I), from which it will be seen that they do not 

everywhere appear between the granite areas and the detrital rocks 

themselves, .the granite at times coming iu contact with them, and again 

with the higher stratiform horizons. But this anomalous arrangement

1 Geological Report on MIC Upper Peninsular of Michigan, exhibiting the progress of the work 

from 1881 to 1884.
20 GREENSTONE SCHIST AREAS OF MICHIGAN. [HULL. 62.

might possibly be explained by the eruptive nature of the granite, 

which, since eruptive, would come up indifferently at any horizon of 

the rocks intruded by it. However, that the granite is not newer than 

those stratified rocks which overlie the greenstone schists there appears 

very excellent proof. At a number of points where the detrital beds, 

whi3h form the basement members of the iron-bearing series proper, 

come in contact with the granite, they contain its debris in a very notable 

fashion, while they are entirely devoid of granitic veins, such as occur 

where the greenstone schists come into contact with the granite. If, 

then, the granites are of one general age, it appears manifest that they 

lire newer than the greenstone schists, but older than the overlying 

strata. Moreover, there are several points on the contact line between 

the detrital rocks of the iron-bearing series and the underlying greenstone schists, where a conglomerate is to be seen, in which are included 

not only numerous granitic fragments, but also large sized pieces of the 

greenstone schists themselves.

Since the occurrence of these basal conglomerates is a matter of so important a bearing on the geologic structure of this region, it is desirable that the points of their occurrence should be briefly indicated here. 

Beginning with the south side of the main, or southern area of the ironbeariug series, indicated on the accompanying sketch map, it may be 

noted in the first place that at several points along the contact line 

in Sees. 1, 2, 3, 4, and 5, T. 47 N., E. 25 W., the basal quartzite of the 

detrital series, which is in general made up almost completely of quartz 

fragments cemented by a siliceous matrix, takes on a peculiar character, containing at times a large quantity of pinkish orthoclase, along 

with a good deal of sericitic mica. This peculiar rock was regarded by 

Dr. Eominger as having been altered from the quartzite by its contact 

with the more recently erupted granite. A study of the thin sections, 

however, reveals the completely fraginental nature, not merely of the 

quartz, but of the feldspar pieces, while the sericitic ingredient appears 

to have arisen from an immediate alteration of the feldspar fragments. 

These feldspathic fragments, as also the quartz mingled with them, are 

of the kinds characteristic of granite, and, moreover, are entirely identical with those occurring in the granite with which this peculiar rock 

is in contact. Southwest of Goose Lake, however, in Sees. 21 and 22, 

T. 47, E. 26 W., there are much more obvious occurrences of granitic d6bris. In the lower detritals of the iron-bearing series near the 

southwest quarter of section 22 may be seen layers of fraginental quartz 

slate holding seams of granitic pebbles; while in the southeast quarter 

of the same section contacts are seen between the fragmental quartzite 

and masses of the granite, the qaartzite at the contact being crowded 

with bowlders and fragmeutal material of all sizes derived from the 

granite. It is not evident whether the granitic masses here seen are 

brought to view by a fold of the strata, or are directly connected at 

surface with the main granitic mass lying to the southeast of them, the
BASAL CONGLOMERATES NEAR MARQUETTE. 21

exact outlines of the areas occupied by different rocks in this vicinity 

having never been traced. These occurrences were noted by Dr. Rominger,1 who also describes similar ones as obtaining in the southwestern portion of the same township. It should also be said that, at 

numerous points along this line, the quartzite contains pebbly beds in 

which the pebbles are of a quartzite whose microscopic features prove 

to be those so well known as characteristic of the quartz of granite.

Turning now to the northern line of the same area we may note first 

the conglomerate to be seen on the side of the so-called State Road, 

running west from Marquette at a point near the west line of the SW. 

£ of the SE. £ sec. 29, T. 48 N., R. 25 W. This is exactly on the line 

between the greenstone-schist area and the area occupied by the detrital 

rocks, which just here are thinly banded slates, standing vertical or 

even overturned slightly so as to dip to the northward and to lie in general conformity to the cleavage of the greenstone schists immediately 

north. These slates, whose lithological character is one very commonly 

met with at the base of the iron-bearing series in many different portions of the Lake Superior region, are proved by the thin sections to 

be quite beyond question water-formed detritals and at the same time 

to present a most striking contrast with the hornblendic and chloritic 

greenstone schists lying immediately north of them. This contrast, 

however, though very distinct in the thin section, is by no means so 

pronounced in the hand-specimens which, though one sees that they are 

of two different natures, might nevertheless be readily taken to be all 

of the same origin. At the point indicated the fragmental slates become 

crowded with large and small fragments of granite, quartz, and green 

schist, the quartz pebbles usually being of rather small size, while those 

of granite, which are the most numerous, are well rounded and reach as 

much as two feet iu diameter. The green schist pebbles, on the other 

hand, are not so plentiful, but are still abundant and of all sizes, from 

the smallest particles up to pieces a foot across. The larger schist 

pieces are somewhat rounded, but in the main they are all far less so 

than the fragments of granite and quartz. The thin sections cut from 

the matrix of the rock show that it is completely of a detrital nature. 

On the north side of Teal Lake, at both eastern and western extremities, 

are found other conglomerates, iu one case holding only quartz pebbles, 

and in the other case pebbles not only of quartz, but of greenish schists 

as well.

Turning our attention now to the northern one of the two detrital 

areas indicated on the sketch-map herewith (PI. I), I may note the occurrence at a number of points along its southern margin of conglomeratic layers of quartzite and sandstone, iu which are contained pebbles of white quartz, which pebbles are at times identical in microscopic 

and macroscopic appearance with the quartz of certain seams in the 

greenish schists (as, for instance, on the west line of Sec. 20, T. 48 N., R.

1 G-eol. Survey, Michigan, vol. 4, 1881, pp. 62, 63.
22 GREENSTONE SCHIST AREAS OF MICHIGAN. [HULL. 82.

27 W.), or with that of the granite (as, for instance, near the middle of 

the south half of Sec. 17, T. 48 K, R. 26 W.), against which different 

rocks the quartzites rest in different places. Near the middle of Sec. 21, 

T. 48 N., R. 27 W., the quartzite, resting here against certain agglomeratic green schists, contains not only fragments of quartz like that of 

numerous seams in the green schists, but also pieces of the green schists 

themselves, which in some cases are several inches in diameter.

But the most striking occurrences of basal conglomerates upon the 

edges of this area of detrital rocks are those to be met with in the 

northern arid western portions of T. 49 K, R. 28 W. One of these places, 

to which my attention was first drawn by Mr. C. E. Wright, the state 

geologist of Michigan, is on-the northwestern shore of Silver Lake, in 

the SE. £ of the NW. ^ of Sec. 8. Rising abruptly all along the northern side of this lake is a bold ridge of granite with which are associated 

several kinds of schistose rocks, very prominent among which is a finegrained green schist entirely analogous in general appearance to the 

greenstone schists which form the special subject of this paper. At 

the particular point to which attention is now directed, this granite protrudes in places into the lake in low, shelving ledges, facing which, and 

lying within the cracks of which, is a black cherty slate. Following 

the shore along to the southwestward from these exposures, the cherty 

slate is found more largely developed, occupying a large hollow in the 

surface of the granite, and becoming crowded with its fragments of all 

sizes, from a fine detritus up to pieces two feet in length. These fragments, though prevailingly subangular to angular in outline, generally 

present some evidence" of water-wearing. Many of them are quite 

gneissoid in structure, while others are more granitic, two principal 

phases being thus presented. The thin sections made from the specimens selected on the ground as characterizing these two phases of pebbles, and also the granitic and gueissoid phases of the granite against 

which the conglomerate rests, show the entire indentity of the fragments and the massive granite. In addition to the granite fragments 

are rarer ones of white quartz, these occasionally being of some size, 

and others of a greenish schist entirely analogous to that green schist 

which occurs here in situ along with the granite. The matrix of this 

conglomerate is a dark gray to nearly black, carbonaceous, cherty slate, 

which in the thin section shows a predominating quantity of a cherty, 

chemically deposited silica, with which are mingled varying proportions 

of fragmental material, this fragmental material being composed of 

pieces of quartz and feldspar, and again, when the pieces become coarser, 

of a granite in which these same minerals are attached to each other. 

At times, as is so commonly the case among the cherty rocks of the 

iron-bearing series generally, throughout the Lake Superior region, there 

is a brecciated condition in this slaty matrix itself; that is to say, irregular, angular pieces of the carbonaceous cherty material are cemented 

together by a matrix of the same substance. Quite similar occurrences
JHVING.] AGE OF THE GRANITES. 23

are met with near the center of Sec. 19, of the same township, where 

black cherty slate and quartzite lie upon the east and south flanks of a 

bold hill of granite; both slate and quartzite, and particularly the latter, being crowded with fragments of the granite, which range from 

small pebbles up to pieces a foot or more in diameter, the smaller sized 

pebbles being usually quite well rounded, while the larger fragments 

are subangular to angular.

Such occurrences as these, when considered in connection with the 

manner in which the granite penetrates the greenish schists and is involved with them, seem to render necessary the belief that, while it is 

plainly younger than the green schists, it is nevertheless greatly older 

than the overlying xletrital rocks; and more than this, that when the 

latter rocks were spread, the granites and greenstone schists together 

had already suffered disturbance and deep denudation. It does not 

appear possible to escape this conclusion by supposing that, since 

granite and greenstone schists are eruptives, they may have furnished 

fragments to almost contemporaneous sedimentary deposits; for, in 

the, first place, both the greenstone schists and the gneissoid granite 

must have received their schistosity before yielding the fragments. 

Moreover, whatever may have been the depth at which the schistose 

rocks were first formed, the granitic masses which intruded them, according to all the later developments and doctrines of petrography, 

must have been crystalized in depth, and must therefore have had removed from over them great masses of materials before yielding fragments to wave action. In this connection attention should be drawn 

to the fact that there are evidently granitic rocks of two different ages 

in the great granitic areas of the Marquette region ; because dikes of a 

tine grained, reddish granite are frequently met with cutting the other 

granite, which may be either gueissoid or not. These later granites, 

which appear to be of relatively small extent compared with the main 

mass, may perhaps have been even later in time of formation than the 

detrital rocks themselves. It seems probable that to these later granitic 

eruptions we should refer certain rare quartz-porphyry dikes, and such 

very rare granitic dikes as that which is seen near Metropolitan, in the 

Felch Mountain district, intersecting a ferruginous schist of the ironbearing series itself.

On the whole, then, accepting Prof. Williatns's conclusions as to the 

surface origin of most of the greenstone schists of the Marquette 

region, I should suppose that, after the accumulation of these rocks to 

the thickness of several thousand feet, they were intruded by granitic 

bosses. These bosses perhaps may have been merely softened portions 

of the underlying gneissic basement, which indeed may be represented 

in an unaltered condition in portions of the granitic areas themselves, 

for all that has yet been determined to the contrary. Subsequent 

mountain-making movements brought about the folding and alteration of these enormous sheets of eruptive material, now represented by
24 GREENSTONE SCHIST AKEAS OF MICHIGAN. . [BULL. 62.

the greenstone schists. Following this was the great denudation 

which brought to light the previously buried granitic masses. This 

erosion was followed in turn by the accumulation in the usual horizontal position of the iron-bearing detrital series, whose folding and erosion 

were still later processes. And yet this folding and erosion all preceded the deposition of the horizontal Oambrian sandstones of the 

region.

The entire similarity which is shown by Prof. Williams to obtain between the great dikes of diabasic greenstone which traverse the greenstone schist area, and the sheets of eruptive greenstone which are included within the iron-bearing series, furnishes a further confirmation 

of these conclusions. For the latter greenstones are in large measure 

directly interstratitied with the sedimentary layers of the iron-bearing 

series, following the bending of its layers; so that even if these greenstones are in the nature of intruded sheets, it seems necessary to believe that their intrusion took place before the folding of the iron-bearing series. Now, the corresponding dikes in the greenstone-schist area 

were evidently intruded subsequent to the production of the schistosity of the intruded rocks. If, then, these are facts, the time when 

the iron-bearing series was folded was very much subsequent to that 

time at which the greenstone schists received their schistosity. It has 

been mentioned that the schistose structure of these greenstones at 

times corresponds with the bedding structure of the iron-bearing 

series. But quite frequently there is no such correspondence, the beds 

of the latter series being noticeably at relatively lower angles when 

compared with the vertical schistose structures of the greenstone 

schists themselves.

Such correspondence in some places and lack of correspondence in 

others are easily enough explicable upon the views that I have advaiiced. However, there is good reason to think that at the time of the 

folding of the iron-bearing series, the greenstone schists received a second squeezing, which developed further alterations and further schistosity. The later dikes just alluded to as penetrating the green schists 

are at times rendered somewhat schistose, though far less markedly so 

than are the rocks which they intrude. The same is true with the greenstones that are intercalated in the iron-bearing series, where somewhat 

schistose phases are found. The structure of these last-named schistose phases, as also the occasional slaty cleavage seen in the detrital 

layers of the iron-bearing series, where it is very strongly folded, correspond in general direction, as would be expected, with the schistose 

structure of the greenstone-schist areas.

The geology of that portion of the Menominee region which has been 

especially under study in the present connection, is indicated by the 

outline map, Plate IT. Here, again, we find a series of detrital ironbearing rocks, lying between great areas of granite and gneiss. The 

iron-bearing rocks are generally quite closely like those of the Mar-
ARCHEAN. ALGONKIAN.

Gratiite and Gneiss. Greenstone Schists. Ivoii Bearing Series. 

(I)eultaTs,Limestones ^Fcrnujir 

Schists.)

CAMBRIAN. 

Potsdam Sandstone.

OUTLINE GEOLOGICAL MAP OF THE MENOMINEE IRON REGION.

Compiled in'R.D. Irviruj from maps by T.B.Brooks, C.E.WrigKt arid C.Rominger and from original observations.

Scale of miles.

O __________ 3_______ ___ 6___________ __ 9

I90O8O
HIVING.] GREENSTONE SCHISTS ON THE MENOMINEE. 25

qnette region, the only important differences being the very great rarity of greenstone intrusions, and the very much more closely crumpled 

condition which the rocks here present. In the same region are two 

belts of greenstone schists closely analogous in general appearance 

with those of the Marquette region. The southern one of these borders 

fora long distance the southern granite area, separating the granite 

from the detrital rocks farther north. The inclination of the schistosity 

of the greenstone schists is almost vertical, there being generally a 

slight southern departure from vertically. Very high southern dips, 

often approaching vertical! ty, also prevail among the layers of the detrital succession itself, although here frequently occur reverse dips to 

the northward, often at a flatter angle.

The tirst geologists who appear to have noted the Menominee rocks, 

are Foster and Wliitney and Charles Whittlesey, as quoted in Foster 

and Whitney's work. 1 By these writers all of the Menominee stratiform rocks, so far as they were encountered in the rapid trips by canoes, 

which were at that time the only means of traversing the country, 

were classed together under the general name of Azoic slates. However, the exposures of greenstone schists, which are met with at Twin 

Falls, Upper (Big) and Lower (Little) Quinnesec Falls and Sturgeon 

Falls, appear to be directly referred to as intercalations of "Azoic 

slates and traps;" so that, while no opinion was expressed as to their 

stratigraphic relations to the other rocks of the region beyond what is 

indicated in general statements as to the greater recency of all the 

granitic masses ot the region as compared with the schistose and slaty 

rocks it appears plain that these geologists looked upon the rocks 

now included under the term of greenstone schists as partly eruptive 

and partly sedimentary:

The same conclusion as to the origin of these rocks appears to have 

been reached by Creduer, who published an account of this region in 

18G9,2 after having been some time on the ground as an assistant to 

Prof. E. Pumpelly in a private economic examination of the lands of 

the Lake Superior Ship Canal Company. On account of the slight departure from vertically towards the south; on account of the schistose 

structure of the rocks exposed at the several falls of the Menominee, 

which rocks he describes under the general term of " Dioritic Series' 

and "Talcky Clay Slate," and because of the prevalence of similar 

southern dips among the detrital iron-bearing rocks farther north, 

Credner considers the former rocks as constituting the highest portion 

of the entire succession of the region. That there existed to the south 

of these rocks another mass of granite does not appear to have been 

realized by Credner.

T. B. Brooks, who followed Credner in the Menominee region and

 Geological Survey of the Lake Superior Land District, vol. 2, pp. 24-31. Sen also general section 

of tbe Lake Superior region in atlas to tlio samo volume.

2 Die vorsiluriscbcu Gobildo der obereii Halbinsel vou Miebigau iu Nord-Amerika. Zeitscbr. 

Deutscb. geol. Gesell., Berlin, 1869, vol. 21, pp. 51C-554.
GREENSTONE SCHIST AREAS OF MICHIGAN. [BULL. 62.

who published a brief account in the reports of the Geological Survey of 

Michigan1 , appears to have agreed with that geologist with regard to 

the relatively high position in the series of the greenstone schists of 

the Menominee River. Subsequently the same geologist, assisted by 

C. E. Wright, was engaged in a more minute examination of this 

region on the part of the Wisconsin State Geological Survey, publishing a somewhat elaborate report in 18S0.2 In this report very many 

more facts are given than were ever known before, all exposures seen 

being very accurately and carefully indicated upon the accompanying 

maps. This detailed work, however, does not appear to have altered 

Brooks's opinion as to the relatively high position in the Menorniuee 

series of the greenstone schists now especially under consideration, 

although it appears that he would now correlate them with still higher 

layers of the Marquette series rather than with those to which he 

had previously referred them. As to the origin of the greenstone 

schists, Brooks would seem to have regarded them at first as all sedimentary; at all events when the Michigan report referred to was 

written.3 Later, however, he appears to have been disposed to separate 

from the rest certain diabases as eruptive, thus coming nearly to the 

same conclusion as that reached by Credner.4 To convey Brooks's conception of the structure of this region more clearly,"there are given here 

two sections, constructed from general sections given by him upon 

Plates 27 and 29 of the atlas to the Geology of Wisconsin. The granitic *

XIV. n VI V

MIDDLE LOWER

HURON I AN LADKENTM

FIG. 1. Hypothetical section of the Meuoininco region in the vicinity of Quiuuosec, according to 

Brooks; I, Sericitic slates; II, quartzite ; V, limestone; VI, the great iron horizon; XIV, quartzite ; 

XV to XIX, greenstone schists, etc.; XX, Hurouian granite; P, Potsdam sandstone. Scale 13,000 

feet to the inch.

and gneissic rocks to the north Brooks regarded as having formed part 

of the ancient Laurentiau basement upon which all of the slates and 

schists were subsequently piled. From this northern granite to the 

granite on the south side of the Menomiuee he looked upon the entire 

succession as an ascending one, though admitting certain bowings of 

the strata. The southern granite, as to whose sedimentary or eruptive 

origin he appears to have been doubtful, he places as the summit member of the Huronian series. The entire succession he divides into three 

portions, beginning below or to the north, as follows : Lower Huroniau,

1 Geol. Michigan, vol. 1, 1873, pp. 157-182. See also atlas to the same, PI. 4.

2 Geol. Wisconsin, vol. 3, 1880, pp. 430-663; also atlas. Pis. 28-30.

8 Geol. Michigan, vol. 1, 1873.

4 Geol. Wisconsin, vol. 3, 1880, p. 521.
1HVING.] OBSERVATIONS OF BROOKS AND ROMINGER. 27

5,200 feet; Middle Huroiiiau, 3,100 feet; Upper Hurouian, 10,700 feet; 

making' in all a maximum thickness of 19,000 feet beneath the upper or 

southern granite. The surf.ice distribution of the uppermost of these 

three divisions corresponds almost exactly with the areas of greenstone 

 schists indicated upon the outline map herewith (PL II). The more northJVJ3.

HUH

Fro. 2. Hypothetical section of the Menominee region in the vicinity of Twin Falls, according to 

Brooks. Scale 13,000 feet to the inch. Letters and shadings have the same significations as in Fig. 1.

ern area of greenstone schists, namely, that which is sectioned by the 

Menominee river at Twin Falls, is made on Brooks's structural sketches 

to be brought to view by a synclinal depression of the strata. For this 

view of the structure of the region there never was any other support 

than the very slight departure from vertically towards the south of 

the schistose structure of the southern greenstone schists and a prevailing similar inclination in the detrital iron bearing series proper. 

No contacts of the different kinds of rocks were observed to support 

Brooks's hypotheses as to the relatively high position of the greenstone 

schists and southern granite, between which and the other stratiform 

rocks there is indeed usually a wide belt of country without exposures. 

On the other hand, the extraordinary similarity of these greenstone 

schists to those which Brooks regarded as underlying the entire 

Marquette series appears to have been disregarded, as also was the 

complete similarity between the northern and southern granites, and 

again between the contact of the southern granite with the greenstone schists next to it, and the corresponding contact in the Marquette 

region.

Eominger. who followed Brooks in the Menominee region in 1880 

and published an account of his observations in 1881, appears 1 to have 

realized very vividly the difficulties met with in accepting Broobs's 

structural hypothesis. He represents all of the greenstone schists of 

the region as belonging at the base of the entire stratiform series, and 

as equivalent in position and nature with the similar rocks of the Marquette region. The northern and southern granites Eominger looks 

upon as the same, and as intrusives of a date later than the accumulation 

of the entire stratiform succession. As to origin, he evidently looked 

upon the greenstone schists as altered sediuientaries, assigning the 

relatively great alteration, which on this view they must be assumed 

to have undergone, to the action of the later granitic bosses;' in which

'Crcol. Survey Michigan, vol. 4, J881, pp. 157-241.
8 GREENSTONE SCHIST AREAS OF MICHIGAN. [HULL. 02.

view he fails quite to explain, as he does in the case of the Marquette 

region, how the southern granite of the Menominee area should have 

produced so great alterations, while that on the north conies in contact 

with wholly unaltered detrital rocks, to which moreover, as Brooks had 

previously shown, it furnishes an abundance of detrital material. Unfortunately, though describing carefully very many exposures not recorded on Brooks's map, Rominger gives no sections indicative of his 

views as to structure. Just what his ideas were in this respect it is 

not easy to make out from his descriptions, but it appears evident that 

with him the greenstone schists lie at the base of the succession; that 

they are next succeeded in order by those slates which lie about Lake 

Hanbury; that these are succeeded by the great ore horizon, and this 

by the great limestone bed of the region with a thickness of a thousand 

feet and more. Thus he reverses entirely the succession as given by 

Brooks, and as it appears in nature so far as the limestone and ore 

belts are concerned. What stratigraphical position Romiuger would 

give to those great masses of quartzite which lie adjacent to the northern 

granite in Menominee belt I have not been able to understand distinctly 

from his descriptions. The Twin Falls greenstone schists he would 

place along with the Quiunesec series at the base of the entire succession, bringing them to the surface by an anticlinal instead of a synclinal fold,' such as is resorted to by Brooks. The following paragraphs 

are quoted fromflominger, 1 as indicative of his views upon these points:

A superposition of the diorite formation on the Lake Haubury rock series, which 

adjoins in the whole length of the Menominee valley from the upper Qninuesec Falls 

to the Sturgeon Falls, asserted by Major Brooks, is not observable; the nearly vertical strata of both formations are even never seen in contact. There is always 

quite a large covered interval between them. The nearest exposures of the two groups 

are observable in Sec. 26, T. 39, E. 29, where, in the center of the section, a hill is formed 

of the vertical ledges of ferrugino-siliceous flagstones and slaty beds representing the 

Lake Hanbnry series, and about two or three hundred steps from these exposures we 

find on the south side of the road to Menomiuee small hillocks of diorite. * * *

My reasons for holding the dioiitic rocks south of the iron formation as older 

than the latter are based on the lithological similarity of this formation with 

the dioritic group of the Marquette district and on the degree of metauiorphism exhibited by the two groups, the dioritic and the iron-bearing. In,the great succession 

of strata commencing with the Haubury slate group and upward we rarely find a 

bed so much altered that its sedimentary structure is altogether obsolesced, and the 

majority of the strata shows it very plain, while in the dioritic rocks, considered to be 

the young,T, a stratified structure is also recognizable, but not one of these thousands 

offset of ledges exhibits its original sedimentary lamination with any degree of distinctness like the others ; they have evidently been transformed under cooperation 

of heat and partially brought into a plastic condition, which is shown by the extreme corrugation and mode of intermixture of those rock masses, of which effects the 

other rock groups do not exhibit near as high a degree. It would be very strange, 

 then, if the lowest beds nearest to the focus of the central heat should have been so 

much less affected by these altering influences than those pretended to be the higher 

upper strata of the rock crust. One might object: If the diorites are the older beds, 

why don't we find them just as well developed ou the north side of the upheaved beds 

between the quartzite and the granite ? The sandy and conglomeratic nature of

1 Geol. Survey Michigan, vol. 4, 1881, pp. 208-210.
- -s

IRVTNO.] OBSEKVATIONS OF ROMINGER AND IRV£NG. 29

-* many of the strata of the quartzite and iron formation proves them to be shore deposits, while the dioritic group consists only of the finer material of deep sea deposits, 

which explains the point in question. Moreover, the dioritic rocks are not altogether 

missing on the north side of the ore-formation, as we can see by the occurrence of the 

(i-mile-long chain of diorite extending eastward from the Twin Falls. A similar discrepancy between the rocks underlying the ore-formation on the two opposite sides 

of i-ts exposure is seen in the Negaunee district. On the south margin, at the Cascade and Palmer mines, it rests directly on the granite, while on the northern exposures the diorite underlies it in great thickness.

The equal dip of the strata to the south in these adjoining formations is not necessarily a proof of the younger age of the most southern beds. The whole succession is 

so near to a vertical position that in many instances it has to be left uncertain which 

way they dip, but suppose their dip is conformably to the south; the upheaval of the 

diorites by the eruption of the still more southern granite masses pushing the whole 

incumbent rock-series north until all tipped over is the hypothesis by which I explain 

the order in the succession of beds as an in verted one, the seemingly lowest beds being 

actually the youngest.

In my own studies in the Menoininee region, made iu the summers of 

1883 and 1885,1 became early impressed with the close similarity between the greenstone schists of the Meuorainee River and those which 

underlie the iron-bearing series of Marquette; with the entire similarity between the rest of the stratiform rocks of the region and those of 

the Marquette district; with the essential identity in character of the 

granite areas lying respectively on the northern and southern sides of 

the Meuoininee River; with the granitic intrusions met with in the greenstone schists bordering the southern granite, and with ftie striking contrast between the nature of this contact and that of the northern gran-

- ite with the detrital rocks which border it to the south. In the latter 

case the granite, instead of sending intrusions into the rocks w.hich 

rest against it, has furnished fragments to them, as may be most beautifully seen at the Falls of Sturgeon, Sturgeon River, on the eastern side 

of Sec. 8, T. 39, R. 28 W., Michigan.1 These considerations naturally

1 For previous descriptions of the striking occurrences at the Falls of Sturgeon, Sturgeon Eiver, 

see Credncr iu "Die vorsiluriscbon Ge.bilde der obereu Halbiusel von Michigan," Zoitschrift dor 

Dentschen geologischeu Gesellsohaft. vol. 21,1869, p. 521, et eeq.; also T. B. Brooks, in Geol. AVmcoiisin 

vol.3,1880, pp. 467, 468, and Atlas, PI. 38; also, see Rominger, Geol. Michigan, vol. 4, p. 192. Tbe 

granitic fragments at this place occur in a flue-grained slaty rock, in which there is a great deal of 

spricitio material, which at times gives the slate somewhat the look of a crystalline schist. This fact:, 

along with the slight inclination from the vertical towards the north, and therefore towards the granite, which is so largely exposed farther up the stream, led Creditor to include these conglomeratic layers, along with the granite, as Laurentian, the great quartzite mass farther south being taken by him 

as the basement member of the Huroninn. Brooks, however, objects to this conclusion, considering 

that the conglomerates to be seen at this place are genuinely basal conglomerates, and that they form 

the lowestportion of the Huronian region. To support this view, Brooks draws attention to the frequent arenaceous nature of the conglomeratic slates; their ripple-marked surfaces; their generally 

little altered appearance; their parallelism of strike with the admitted .Murouian bods fartlior south, 

and the lack of parallelism between this strike and such structures as is to be seen iii the granitic and 

gneissic rocks on the north. Kominger's description corresponds with that ol'Brooks, except that ho 

seems to describe a granitic mass or sheet as occurring interleaved with the conglomerates themselves, 

a statement for which, in my examinations, I could lind no support whatever. Kominger, however, 

to whom the greater recency of all the granite as compared with all the stratiform rocks was an accepted conclusion, explains the occurrences as ho saw them by the siugularsnppositioa that " we have 

here evidently a series of sedimentary beds, deposited on a granitic substratum, which, during the upheaval, became wedged in between the plastic granite mass, tilting and overlapping them locally HO as 

to appear as the lower beds," thus making the same granite yield fragments to the sedimentaries, and 

^ subsequently intrude them. It should, be aaid that the examinations of the several geologists were
30 GJREENSTONE SCHIST AREAS OF MICHIGAN. | BULL. 62.

led me to the conclusion that the whole structure iu this district is similar to that already described as obtaining iu the Marquette region, 

namely, that the granitic masses had intruded themselves in the shape 

of great bosses into rocks now represented by the greenstone schists, 

after which followed a protracted period of disturbance and denudation 

before the deposition of the overlying detrital and iron-bearing rocks 

of the region. Taking Major Brooks's detailed map of the Menoininee 

district, published in the atlas of the Wisconsin survey, I platted upon 

it all of the' exposures described by Romiuger and not mapped by 

Brooks, which exposures amount iu all to a large number. Examining, 

then, the more important of the exposures of the region, I encountered 

still others, which were also platted upon the same map. Two sections 

were then constructed across the district from southwest to northeast, 

upon which wereplatted all of these exposures, with their dips ; and it 

should be said that very many new facts in this direction have been 

developed of late years by mining.

HUR o A isr i

D c c c D ED c DE DCBCOED

G Sch Sch. G

FIG. 3. Hypothetical section of tbe Meuoininee region in the vicinity of Quinuesec Valley, according to R. D. Irving. A, basal sericitic quartz slates; B, quartzite; C, limestone; D, iron horizon ; E, 

slates and quartzitea; G, granite and Sch., schists of the Laurentiau. Scale, 13,000 feet to the inch.

It has thus become evident that a structure such as is indicated iu 

the accompanying Fig. 3 would not only coincide with the recorded 

facts as well as the sections of Brooks above given, but very much better than those.

made without the study of thin sections of the slaty rocks here exposed. When examined microscopically, these rocks show their completely fragmental nature, .all the alteration that they have undergone having been in the nature of a metasomatic development of mica flakes. Both hand specimens 

and thin sections of these slates, moreover, show a moat striking similarity between them and the 

slaty rocks which frequently lie at the base of the Penokoe series of northern Wisconsin and Michigan, where they are often in contact with granitic rocks, of which they hold fragments. Similar slates 

are met with iu a number of places .it the base of tbe iron-bearing series of the Marquette region. As 

seen at the Sturgeon Falls, these slates are at times fine grained and thinly laminated, arenaceous and 

sericitic, and again more closely grained and of a vitreous quartzitic appearance, in which cases the 

predominating quartz fragments have been cemented together by secondary enlargement. Their entire 

ackof anything like metamorphism is perfectly plaiii, even without the examination of the thin section, to any one who has studied many sections of £uch rocks from the Lake Superior country. The 

pebbles in these slates are arranged in bauds, with intervening less pebbly or non-pebbly portions, 

which portions are the ones which show particularly the ripple-marked surfaces. The pebbles are in 

the main of a pinkish granite and gneiss, wholly identical with the mass of granite and gneiss against 

which they lie, but include also a smaller number of pieces of white quartz and of a fine-grained greenstone or greenstone schist. Entirely similar quartz occurs in seams in the granite close by, as do also 

masses of greenstone and greenstone schist. Not ouly these pebbles, but the whole make-up of the 

finer portion of the rock, make it entirely evident to me that we have here to do with a detritus derived 

by water action from the granitic and gueissic area immediately to the north. The slight inclination 

;iway from the vertical towards the granite which these conglomeratic slates sometimes show is, of 

course, no argument'against their having been deposited upon that granite as a substratum.
THE GREENSTONE SCHIST AREAS OF THE MENOMINEE AND 

MARQUETTE DISTRICTS, MICHIGAN.

By GEORGE HUNTINGTON WILLIAMS.

INTRODUCTION.

This memoir is intended as a contribution to the subject of dynamic 

or regional metamorphisra. It contains the results of a careful study, 

both in the field and in the laboratory, of an extensive series of eruptive rocks, for the most part of basic character. Although the original 

character of these rocks is still evident, they are to a large extent 

cleavable, and the production of this secondary feature has been 

accompanied by more or less extensive chemical and structural alterations.

It is the aim of this paper to trace each of the rock types represented within the areas studied from its least altered to its most 

altered form, and to discuss what may have been tfie agencies which 

produced the changes noticed.

The rocks selected as likely to throw additional light upon the metainorphisrn of eruptive masses are so-called "greenstones" and "greenstone schists," which, in association with certain more acid types, cover 

extensive districts on the south shore of Lake Superior, where they 

lie immediately beneath the iron-bearing strata. Two distinct and 

sharply defined areas of these rocks were chosen as typical of the 

whole formation. Particulars regarding their geographical positions 

and their geological relationships have already been given by Professor 

Irving in his preliminary note. In the first area, situated on the 

Menominee River, the eruptive character of the greenstones and greenstone schists is plainly evident; within the second area, near the city 

of Marquette, the original character of the rocks is much less apparent.

The investigations, the results of whjch are embodied in this memoir, were first suggested to me in the spring of 1885 by the late Prof. 

R. D. Irving, while he was in charge of the Lake Superior division of 

the U. S. Geological Survey. Th'eir aim was to discover, if possible, 

the origin of the greenstone schists of the Lake Superior region, and 

at the same time to afford a contribution to our knowledge of the metamorphism of basic eruptive rocks in general. In the latter sense they 

form a continuation of the writer's earlier studies of the less altered 

gabbros occurring near Baltimore, Maryland. 1

1 Bull. U. S. Geol. Survey, No. 28, 1886.

31
32 GREENSTONE SCHIST AREAS OF MICHIGAN. [BULL.62.

Three weeks of the field season of 1885 were spent in the Menominee Valley, and four weeks of 1886 in the neighborhood of Marquette. 

During tliis time, with the assistance of Prof. W. S. Bayley, the rocks 

were examined in the field, and ample material was collected for their 

laboratory study, which was carried on at the Johns Hopkins University, Baltimore, Maryland, during the winters of 1885-'86 and 1886- 

'87, at such times as could be spared from regular university duties. 

The entire investigation has been conducted under the auspices of 

Professor Irving, for whose uniform kindness and constant readiness to 

supply every facility for the work I gladly take this opportunity of 

expressing jny sense of deep obligation. My thanks are also due to 

Maj. T. B. Brooks and to Prof. Raphael Pumpelley for the loan of 

microscopic sections of the Marquette and Menominee rocks, prepared 

during the course of the Wisconsin State Geological Survey; and to 

Mr. Andrew 0. Lawson, Ph. D., lately of the Canadian Geological Survey, for the opportunity to make a comparative study of closely allied 

rocks from the Lake of the Woods and the Eainy Lake regions.

I feel that no especial apology is needed for the constant use throughout this paper of the term " greenstone." An opinion prevails that this 

word is antiquated and not consistent with the sci.entific accuracy now 

obtainable. The very indefiniteness of this designation, however, constitutes its chief value. It is essentially a field term, such as it is not 

only desirable but absolutely necessary to employ. It is often impossible to state with certainty in the field whether a given basic massive 

rock is a gabbro, a diabase, or a diorite; indeed, where such masses have 

undergone extensive metamorphisra, as in the regions here studied, even 

the most careful microscopical and chemical investigation may prove 

inadequate to disclose what was the original form. Wherever accurate 

diagnosis was possible, correspondingly definite terms have always 

been employed; but for use in the field or where the processes of alteration have obscured the original character of the rock beyond recognition, an intentionally indefinite designation is-necessary. For such a 

purpose it is believed that no term is better fitted than the ancient and 

much abused "greenstone."

For convenient reference the specimen numbers belonging to the collections of the Lake Superior division of the U. S. Geological Survey 

are here appended.

1. Menominee Valley...................................... 11000-11196

Sturgeon Falls.........................:..-.....-.'.. 11153-11174

Little Quinuesec Falls .............................. 11000-11048; 11098-11103

Big Quiuuesec Falls....... .......................... 11049-11085

Horse Race ......................................... 11086-11096; 11182-11196

Four Foot Falls..................................... 11142-11152; 11175-11181

Twin Falls ................... ...................... 11120-11141

2. Marquette region....................................... 11610-11864

Marqnette area ....... .......... ...... .............. 11610-11734

Neganuee area ..................................... 11735-11812

Northern area ................ ............ .......... 11813-11864

3. Agglomerates of Deer Lake ..'o............................ 12.023-12058
WILLIAMS.] INTRODUCTION. 33

The material embodied iu the present paper will be arranged in three 

separate divisions.

1. In chapter one the importance of the service which the microscope 

is capable of rendering toward the solution of the questions of archtean 

geology will be indicated aud a summary presented of the chief results 

already obtained by various workers in this field.

2. In chapters two, three, four, and five the observations collected 

during an extended study of the rocks of the two areas selected will 

be given in detail.

3. In chapter six a connected discussion of these results will be attempted, and a comparison of them with those reached by other observers, as enumerated iu chapter one, will be made. 

Bull. 62 3
CHAPTER I.

PRESENT STATE OF OUR KNOWLEDGE REGARDING THE METAMORPHISM OF ERUPTIVE ROCKS.'

VALUE OF THE MICROSCOPE IN THE STUDY OF METAMORPHISM.

In comparison with the results which have been obtained during 

nearly a century of patient research among sedimentary deposits, 

the progress thus far made in the domain of archean geology appears small. Theories of the origin of the so-called crystalline schists 

are almost as numerous as the investigators who have examined them, 

and yet the more extensive and critical the studies of these rocks 

become the more obscure and anomalous do they appear. So varied 

are the facts which Nature presents to the observer in the oldest rocks 

of the planet that the conviction is soon reached that no single hypothesis or explanation can account for them. It is therefore from no lack 

of interest attaching to the study of pre-fossiliferous rocks that they 

have heretofore yielded so little fruit, but rather from the insurmountable difficulties which have stood in the way of their satisfactory in vestigatiou. That the study of fossils has made up so large a part of what 

has been known as geology is the result rather of necessity than of 

choice. The lack of exact and delicate methods of lithological research 

has confined the attention of investigators to those formations where 

lithological characters are of little value on account of the presence 

of other and more certain guides. The basement underlying all of the 

fossiliferous rocks has, therefore, remained well-nigh a terra incognita, 

because there seemed to be no available means of exploring it.

The recent multiplication of refined methods for the investigation 

of crystalline rocks, however, has opened an almost new field of geological inquiry. The difficult and obscure problems here presented 

may now be attacked by truly scientific methods. The prophecies 

which Hermann Vogelsang made in 18672 for the new departure in 

geology have been more than realized within the last twenty years. 

The almost new science of petrography may be said to have proved 

itself capable of rendering, in the study of the crystalline rocks, a service equal to that which paleontology has already given in the deciphering and correlating of the fossiliferous strata.

1 This chapter was written as it now stands, excepting an occasional foot note, in 1887. 

8 Die Philosophic der Geologic, Pt. 3, Moderne Geologie odor mikroskopische G-es'teinsstudien. 

34
WILLIAMB.] DEVELOPMENT OF MICROSCOPICAL PETROGRAPHY. 35

Petrography, in its rapid development since the introduction of the 

microscope, has already passed through two distinct periods and is 

now entering upon a third and most important stage. At first methods 

had to be perfected for identifying the various constituents of crystalline rocks. During this stage the science was purely mineralogical, 

although its methods were necessarily somewhat different, both in their 

nature and in their application, from those of ordinary mineralogy. The 

attainment of this end required the study of a vast amount of material 

wholly without reference to its geological significance.1

The second period in the development of petrography was occupied 

with the investigation of practically unaltered rock-types, i. e., of rocks 

in which the minerals and structure produced by their original solidification are still readily discernible. It has been found from such studies 

that the relationship of the component minerals to one another, or rockstructure, expresses, to a large degree, the circumstances under which 

the original solidification took place, and hence the importance of the 

second stage in the development of modern petrography.

The most important problems presented by an unaltered massive or 

igneous rock relate to (1) its chemical composition and (2) to the conditions under which it was formed. The composition expresses itself, 

in a general way, in the nature of the component minerals, while physical conditions attendant upon the formation of the rock may be traced 

in its structure. Each of these has therefore been, in turn, the particular object aimed at during the first two periods of petrographical research.

But if petrography were able to solve satisfactorily all the problems 

presented by the unaltered massive rocks, it would even then be prepared only to commence its most difficult and most important mission. 

Rocks are in reality far from being the dead, inert, stationary masses 

which they appear to the ordinary observer. The fascinating study of 

chemical geology, especially when aided by the microscope, shows them 

to be in a state of almost constant change. It is true that some of the 

oldest rocks seem to have suffered hardly any alteration since they 

were first formed, but most of them are ever active laboratories where 

old products are being pulled to pieces and new ones built up. The 

tracing out cf such changes is an important aim of petrography in its 

present stage.

The student at the microscope wonders at the minute bubbles which 

he so often finds inclosed in the little cavities in quartz. So delicately 

are they poised that they are sensitive to all the slight oscillations of 

temperature that constantly pass through the microscopic section, and 

the little prisoners keep up a never-ending movement, as though beatJ The writer lias given an .account of the development of these methods ia a small pamphlet, entitled 

"Modern Petrography," published as No. 1 of the Monographs of Education, by Heath & Co., of 

Boston.
36 GREENSTONE SCHIST AREAS OF MICHIGAN. [BULL. C2.

ing against every side of their narrow cell in the vain search, for an 

exit. Scarcely less delicate seems to be the equipoise between the various chemical compounds in the earth's crust and the surrounding 

physical conditions under which they exist. Like the constant changes 

of temperature which pass through the slide are the changes in physical conditions to which a rock-mass is subjected. With these comes a 

state of more or less unstable equilibrium to the chemical compounds, 

and, like the vibrations of the bubble, molecular movements and rearrangements result.

There are two distinct kinds of alteration which take place in a solid 

rock-mass, dependent, of course, on the nature of the changed physical 

conditions. These are:

(1) Metamorphism ; or the passage, under circumstances of high temperature or pressure, or both, of less crystalline into more crystalline 

compounds; or the change of minerals into others, not less crystalline 

or insoluble than themselves.

(2) Decomposition or weathering; the passage, under ordinary atmospheric conditions, of crystalline rock constituents into compounds 

less crystalline and more soluble than themselves. This is accomplished 

generally by hydration or carbonatization.

Both of these processes are frequently seen to have gone on in succession in the same rock-mass, the latter more or less completely effacing the effects of the former. While distinct, both processes agree 

in being atomic and molecular rearrangements in a solid mass, necessitated by some change in external conditions. The differences in these 

conditions, however, produce widely different results; and all of these 

again are essentially different from those produced by the solidification 

of a liquid magma.

The student of the crystalline rocks can distinguish in a general 

way four classes of constituent minerals, and this is true in spite 

of the fact that the same species may be represented in two or more of 

these classes.

(1) Original minerals of the acid rocks, formed by solidification of a 

magma in a state of aqueo-igueous fusion or by the aid of mineralizers; e. g., quartz, orthoclase, mica, zircon, etc.

(2) Original minerals of the basic rocks, formed from a state of dry 

fusion; e. g., plagioclase, augite, olivine, etc.

(3) Metamorphic minerals, formed, as above explained, from original 

minerals; e. g., hornblende, albite, biotite, zoisite, garnet, staurolite, 

andalusite, etc.

(4) Decomposition minerals; e. g., chlorite, quartz, carbonates, the 

hydroxides, etc.

Such a division, though necessarily not a sharp one, is still not 

unwarranted and shows how it is possible, with the aid of the microscope, not merely to study the composition of a rock in its component 

minerals, or the conditions under which it was first formed in its
WILLIAMS.] CONTACT AND REGIONAL METAMORPHISM. 37

structure, but also to trace out life histories. We may discover not 

only the conditions under which a rock-mass solidified, but we may 

learn as well something of all the conditions to which it has subsequently been subjected.

Metamorphism, or the recrystallization of rocks, whether massive or 

stratified, igneous or sedimentary, is as varied in its results as the 

materials acted upon and the agencies which produce it. How true 

this is may be seen where the cause of the change is local and well 

understood, as iu the numerous instances of contact-metamorphism 

which have been studied in detail during the last fifteen years. Here 

the alteration produced by the same eruptive mass is different in 

a shale from that in a sandstone, and in each the alteration is quite 

distinct from that produced in a limestone. If intrusive rocks fall, 

within the range of influence, the effects produced iu them are again 

different; while still wider variations are noticeable if the character of 

the active or metamorphosing rock be changed. This may be seen in 

the contrast between the effect produced by a mass of granite and a 

mass of diabase upon the same surrounding rock. Still other differences are traceable to the probable action of vapors, as in the case of 

a local development of tourmaline or topaz within the limits of a granite 

contact zone.

How much greater then might we expect the differences to be which 

all the complicated and imperfectly understood conditions of regional 

metamorphism produce. Many rocks occupying large areas exhibit a 

character very similar to that of rocks which have been produced by 

contact-metamorphism, without there being any cause to which the 

alteration can be so directly attributed. In some schists, like those of 

the Ardennes Mountains 1 and those of Bergen in Norway,2 and in 

many limestones, inetainorphic minerals have been extensively developed without the total obliteration of organic remains. In the case 

of other rocks, of essentially the same character though perhaps more 

crystalline, the internal evidence or the strati graphical relations are 

enough to prove that they are metamorphosed sediments.

Many of the so-called crystalline schists present features of both 

composition and structure which are strikingly similar to those of 

rocks of undoubted metamorphic origin, but nevertheless we have at 

present but few data for constructing a satisfactory explanation of the 

origin of these enigmatical formations, Theorizing in regard to them 

has done its best, and has succeeded in introducing only confusion and 

disagreement. As before remarked, the variety here is far too great to 

be accounted for by a single hypothesis, however broad. Many agencies 

have been at work, whose exact nature and importance only the most 

laborious and patient investigation can show. In working in Archeau

. 'A. Kenard: Les roches grenati feres et amphiboliques de la r6gion de Bastogno. Bull. Mus. Roy. 

Hist. Fat. Belgique, 1882, vol. i, pp. 1-54.

2 H". H. Reuscli: Silurfossiler og pressede Koujrlomorator i Bergensskifrene, 1882. Germ, trails!, by 

by E. Baldauf.
38 GREENSTONE SCHIST AREAS OF MICHIGAN. [BULL. 82.

geology tie only safe method is to free the mind completely from all 

traditions and theories to start with the idea that almost nothing is 

known with certainty and that everything is to be discovered. The 

facts must be most critically observed and considered, without too great 

a tendency to use them at once for the deduction of general principles. 

Only such conclusions as can not be doubted by any one who will take 

the pains to examine the facts are of real value to the advance of arclieau geology; and every careful student in the field must realize how 

slow and difficult such an advance must be. Detailed analyses of the 

workings of some well recognized agency, made where the action has 

been as little as possible disguised and complicated by the action of 

other agencies, must yield valuable assistance in the penetration of the 

mysteries which now everywhere surround the prefossiliferous formations of the earth's crust. Such work has been done in Europe and has 

been begun in America. The little that has already been accomplished 

in this manner is full of promise for the future. The most striking example of work of this kind is perhaps that of Lossen in the Hartz 

Mountains, and it is with the sincerest appreciation of his method and 

results that the present studies have been prosecuted.

If it be granted, as it must be, by every impartial observer that such 

a thing as regional inetarnorphism does exist that certain rocks, when 

they are subjected to enormous strains, and are upheaved, crushed, and 

crumpled do become more crystalline or have their crystallization altered then we have a well recognized agency whose particular results 

are worthy of patient and detailed study. What part regional rnetamorphism has had in the production of the archean rocks as a whole, 

future years must show. This question lies wholly without my present 

purpose; and yet it is hoped that an extended investigation of a particular phase of it may aid in the final solution of the problem.

The investigation was undertaken with a firm conviction of the peculiar advantages offered by eruptive rocks for the accurate tracing of 

progressive metamorphism, an advantage which Prof. A. K. Losseu. 

seems to have been the first to emphasize, if we may judge from the 

following extract from one of his papers. He says r 1

I attribute the extreme value of such metamorphosed eruptive rocks for the general 

theory of metamorphism to the fact that they are certainly known to have been derived from a solid rock of definite mineral aggregation, average chemical composition 

and structure. In the primary minerals and structures of the igneous rocks we possess a well known quantity, upon which we can base our conclusions a definite scale 

according to which the nature and amount of those secondary minerals and 

structures characterizing the metamorphic rocks can be measured. Frequently 

the certain, incontestable fact of pseudomorphism proves the secondary alteration of 

these rocks in a much more general way than even the very exceptionally preserved 

fossil remains are able to prove the origin of metamorphosed sediments.

1 Den hohcu Worth dieser metamorphischen Eruptivgesteine fur die Lohre vom Metamorphismus 

fand ich darin, dass ein von Haus anafestes Gestein vonganz bestimmter Mineralaggregation, chemischer 

Durchsehnittszusammensetzung wid Structur, zuverlassig als ihr Muttergestein angegehen werden 

kanii. Tu denfprhnarcn Mineralien undpriniarenStructurenderErstarrungsgesteinebesitzen wireine
WILLIAMS.] CHANGES IN ROCK-FORMING MINERALS. 39

There is no student of mineralogy who has not been struck with the 

great similarity which exists in the composition of so many of the 

rock-forming silicates.

We have after all a very small number of bases in these silicates and 

they are often combined in almost the same proportion in minerals 

which show the greatest disparity of crystal form, physical properties, 

and mode of occurrence. It seems to be oftentimes more a matter of 

external condition rather than of chemical composition, which determines what particular mineral is formed; and the equipoise between 

the existence of a certain silicate and the external conditions is often 

so delicate that a mere change in the latter is alone sufficient to destroy the mineral as such and to cause it to change to some other modification or compound. The recent discoveries of the alterations which 

dimorphous bodies, like leucite or tridymite undergo, show how this 

may be accomplished without chemical, change. The breaking up of a 

mineral into an aggregate of two or more is not less common ; as for 

example, the passage of plagioclase into zoisite and albite (saussurite), 

or of spodumene into albite and eucryptite (the y3-spodumene of 

Brush and Dana.) 1 This subject has lately been elaborated by Prof. 

J. W. Judd in his presidential address before the Geological Society of 

London, and in many others of his recent papers.2

Bocks whose component minerals are so delicately balanced to accord 

with the particular set of conditions under which they were formed, 

must be peculiarly subject to alteration when these conditions are 

changed; and for this reason the writer has before insisted that eruptive rocks must be even more liable to metamorphism than the sediments which contain them.3 '

The advantage, then, of eruptive rocks, especially of basic eruptive 

rocks, for the study of metamorphic processes is, as Lossen has remarked 

with so much force, because we have in them a set of minerals and 

structures which can with certainty be referred to eruptive conditions, 

i. e., conditions of fusion. These, at least for the basic rocks, may be 

easily reproduced in the laboratory, as has been so successfully done by

wohlbokannte Grb'aso, die wir uuserom TJrtheil zu Grundo logon kiinnen, einen festen Maassstab, an 

wolchem Art und Grad jencr secundarcu Mineralien und seemidiiren Structuron gemessen werdonkounen welche die Natur dos metamorphischen Gesteins mehr oder weniger ansmachen. Vielfach ist es 

goradezu die sichore, unanfqchtbare Thatsache der Pseudonlorphosenbildnng, die in solclien Gestoinon 

jii viol allgemeinercr Weiso bewoisend fiir die TTmbilduug ointritt, als dio nur nnter besonders giinstigcn Uinstanden erhaltenon Petrafacten in den metamorphisirten Sedimenten. Jahrbuch der kb'niglichen preussischen geologischen Laudesanstalt u. Bergakademie fiir 1883, Berlin, 1884, p. C20.

Ou page 619 ibid., Lessen in a note gives the follc.wiug list of references to his former remarks on this 

same important topic: Zeitschrift der dentschen geologischen Gesollschaft, vol. 21, p. 298,1869; vol. 

24, pp. 706, 707, 763,1872; vol. 27, pp. 451 and 969, 1875; vol. 29, p. 360, 1877; Sitzungsberichte der Gesollschaft naturforschender Freunde in Berlin, Mch., 1878; Jan., 1880, and Nov., 1883; Jahrbuch der 

kongl. preuss. guologischeu Landesanatalt fiir 1880, p, 12; fiir 1881, p. 43," and finally in tho Erliiutemngeu zur geologischen Specialkarte von Proussen uud den thiiringischen Staaten. Blatter: Harzgerodo (p. 79); Wippra (pp. 27,43); Schwenda (p. 34).

1 Am. Jonr. Sci., 3d sor., vol. 20, 1880, p. 257, and Zeitschr. Kryst. u. Mineral., vol. 5, p. 192.

8 Quar. Jour. Geol. Soc. London, vol. 43,1887. Proc. pp. 54-82.

8 Bull. U. S. Geol. Survey, No. 28, p. 9,10.
40 GREENSTONE SCHIST AREAS OF MICHIGAN. [BULL. 62.

Messrs. Fouque and Michel-Le'vy, of Paris.1 The exact mineral association and structure of diabase and other basic igneous rocks may be synthetically reproduced by simple dry fusion, and there is no indication 

whatever that they are ever produced in nature by any other means. 

The finding therefore of such characteristic structure, even where the,}7 

have been more or less disguised by subsequent changes, at once furnishes a definite and certain starting point. We know what the orig 

inal character of the rocks in question was and the conditions under 

which it was formed; and the careful minute study of the changes 

which the original component minerals have undergone, when taken in 

connection with the external physical conditions to which the rock mass 

has been subjected, can but yield useful results to geological science.

HISTORICAL OUTLINE OF STUDIES ON THE METAMORPHISM OF ERUPTIVE ROOKS.

Notwithstanding the great importance which has always attached to 

the idea of metamorphism, it is only within recent years that it seems 

to have been regarded as applicable to any but sedimentary rocks. 

Witli Hutton, who may practically be considered the originator of the 

idea, and with Lyell, to whom we owe the term which describes it, metamorpltism was the gradual consolidation, by means of the earth's internal 

heat, of sunken and buried strata, whose original parallel structure was 

not wholly obliterated by the change. So strong has the power of this 

tradition become, that it is not easy to overcome it even at the present 

time. Nor after the discovery and satisfactory explanation of secondary (slaty) cleavage in rock-masses, was the case much better. The 

distinction between foliation and stratification was accepted as radical, and it was generally coucede'd that the former was secondarily 

developed in solid masses as the effect of pressure, and yet we hear of it 

chiefly in sedimentary rocks in slates and shales. The eruptive rocks 

seem to have been regarded as too hard and unchangeable to be affected 

even by the enormous forces which upheaved a mountain chain. How 

untrue this is we are daily learning from the study of rocks undoubtedly 

eruptive, which possess a pronounced schistose structure.

As we now look back over a century of discussion and investigation 

of metamorphism, the total neglect of eruptive masses, save as an active 

agent in the change, seems surprising. Daubr6ejdn his admirable essay 

on the subject,2 has much to say of change of structure, and yet he 

makes no allusion to eruptive rocks as being subjected to it. Delesse, 

in the second portion of his lt Studies of Metamorphism" 3 which treats 

of regional or normal metamorphism, devotes considerable space to the 

metamorphism of eruptive rocks, but his conclusions are not such as 

at the present time deserve attention. He maintains that by some

1 Syntheso ties mineraux ct do roohes, Paris, 1879.

s $tiides et experiences s.yntti6l,iques sur le mfitamorphisme, etc., M6ra. presentes pnr. savants ft 

I'Aoadamiedes sciences, vol.17, ISG'J. Translated in thcSraithsonian Annual Report for .1.801, pp. 228-304.

! lSt.udes surlemetamorphisme tiesroches. Ouvrage couronue par TAoaderaio ties sciences. Paris, 

1869.
WILLIAMS.] METAMORPHISM OF ERUPTIVE ROCKS. 41

 vague process of "general metaniorphism " volcanic rocks are changed 

to plutonie. Thus a trachyte becomes a granite; and a trap, a diorite. 

The absence of volcanic rocks in the oldest formations is accounted for 

by the great length of time during which these have been subjected to 

metamorphosing agencies. The original plutouie rocks are considered 

to be capable of little or no change.

One of the first truly scientific studies of the metamorphism of eruptive rocks, based purely on careful observation of the facts, was that of 

Lossen in the Hartz Mountains. Here it was possible to trace the numerous diabase dikes and larger areas of more coarsely crystalline gabbros, along with the schists in which they were inclosed, from their 

unaltered to their most highly altered form. Ac early as 1872 he wrote 

as follows:

I shall begin with igneous rocks which have been altered in situ and with their 

(final) change into crystalline schists. The fact that massive rocks, which frequently

possess a parallel structure as an original feature, may, by rnetamovphic processes, 

be converted into foliated, but not on that account stratified, beds is incontestable. In 

this manner granular diabase is converted into leuticularly foliated (flaserig) by the 

more or less complete passage of the cleavable augite into a scaly aggregate of 

chloritic minerals. The rock thus assumes a kind of schistose structure, as is often 

the case in the southwestern Hartz. 1

The cause of the metamorphism, too, can be seen in some cases to 

be the eruption of large granite masses; in others, the orographicforces, 

which had crumpled and upheaved the entire district. Upon this subject 

Lossen says:

1 have often emphasized the significance of those metamorphic regions in which 

eruptive rocks interbedded in the schists and like them passively subjected to dislocation and mountain-making forces have undergone about the same substantial 

and structural alterations as the inclosing schists ; nor is it material whether this result has been accomplished within the contact-zone of some intrusive granitic mass 

or by dynamic (dislocation) forces properly so called. 2

Massive rocks may therefore be passively subjected to the influence 

of either contact or regional metamorphism.

Instances of the former seem never to have received their merited 

share of attention, and yet they have been mentioned in many different 

regions. Allport described dolerites which had been altered by adjacent eruptive granites in Corn wall.3 In France Michel-Le"vy has found

'Ich boginno mit den in situ umgewandelten Erstarrungsgesteincn und deren TJnrwandlung zu 

krystallinischen Schiefom. Die Tbatsache, dass durcli metainorphischo Processo mass!go Gestoine, 

die hiiuflg bereits eiue urspriingliche piano- Parallelstructur besitzen, in schiefrige, darum abernoch 

nicht ingeschichtete, umgewandelt werden, ist unbestreitbar. So gehcn die koriiigon Diabase dadurch 

hiiufig in fiasorige iiber, class das bliitterig brecheude augitischo Mineral ganz oder theilweise in oin 

seliuppiges Aggi'egat einoa chloritischen Minerals umgewandelt wi-rd, wobei das Gestein eine Art 

schiofrigo Structur annehraen kann, wie dies ira Siidost Ran nicht seltender Fall ist. (Zeitschr. 

Deutsch. geol. Gesell., Berlin, vol. 24, p. 763, 1872.)

2 Merfach boreitsbabe ichdie Bedeutung solohor inetamorphischer Gebiete hervorgehoben in wolchen 

zwischen den Sohichten eingeschaltete und nur passiv am Fiiltungs- und Gebirgsbildungsprocesso betheiligte Eruptivgestoine in anniihernd domselbeu Grade wio ilio dunebcn austohenden Schichtgesteino 

substautielle und structurello TJinwandliingen, sei es in dor Uoutaotsphare der in die Faltuug eingroifeudon eugranitischen Eruptivrnassen, sei os duroh den Dislocationsprocess sohlechthin, erlitten 

haben. (Jahrb.preus3.gool. Laudesanstalt fur 1883, p. 619.)

3 On the metamorphic rocks surrounding the Land's End mass of granite. Quarterly Journal Geol. 

Soc., London, vol. 32,1876, p. 422.
42 GREENSTONE SCHIST AREAS OF MICHIGAN. - [BULL. 62.

similar effects produced ia the Cambrian diabases, 1 and Barrois meirtionsdiorites ("diorite modifie'") which have been modified by the same 

cause.2 Still more recently Brogger has discovered the same class of 

phenomena in dikes of augite porphyrite which penetrate the strata of 

the classic Silurian region near Christiania.3

The most exact and satisfactory accounts of such rocks are, however, 

those given by Lossen of the diabases which fall within the limits of 

the Barn berg granite contact zone in the Hartz Mountains.4 The effect 

of the granite has been to produce alterations in the eruptive diabases 

quite analogous to those usually brought about by orographic forces. 

The augite has been changed to uralite, which surrounds the core of 

the original mineral in a double zone, the inner one being colorless and 

dotted with magnetite, while the outer one is composed of a more compact, green, and pleochroic hornblende.5 The labradorite of these 

diabases has been saussuritized; their ilmenite has been changed to 

sphene (leucoxeue) and their pyrite to pyrrhotite. Biotite and garnet 

have also been occasionally developed. The characteristic diabase 

structure is recognizable so long as any of the augite or labradorite 

substance remains.

If the results described by Lossen are in reality wholly due to the influence of the intrusive granite mass, then we see that the similarity 

between the effects produced by contact nietamorphism and regional 

metarnorphism is just as close in the case of eruptive rocks as it is in that 

of sedimentary beds. The connection which exists between regionally 

metamorphosed areas and the disturbances to which such areas have 

been subjected appears to be a constant one. Indeed, aside from the 

local influences of intrusive masses, the amount of metamorphism may 

be said to be in all cases proportional to the pressure or strain to which 

the rock in question has been subjected. The recent recognition of this 

fact has brought into favor the term "dynamic rnetamorpTiism, which, 

in a way, expresses the agency or process to which the change may be 

attributed.

Rocks are not metamorphosed by pressure alone, and yet the importance of this agency would appear to be greater than that which has 

commonly been assigned to it. Other factors, especially heat and 

moisture, have also played an important part in the changes, but their 

efficacy is in large measure due to the direct results of pressure upon

'Sur les rocliea 6rnptives basiqnes Cambriennes duM&connais et dn Beaujolaia. Bull. Soc. g6ol. 

France, 3 series, vol. 11, 1883, pp. 273-303.

2 Le granite de Rostrenen.-sos apophyses et aes contacts. Annales Soc. g6ologique du Nord, 1884, 

vol. 12.

3 Spaltenverwerfuugen in der Gegend Langesund-Skien. Nyt Mag. for Naturvidonslcaberne, vol. 

23, 1884, pp. 253-419.

4 Ueber den Ramberg Granit und seinen Contact-Hof. Erla'uterungen zu Blatt Harzgerode dor 

geologiscben Specialkarte von Proussen und der tbiiringiscbeb Staaten. Vidt> also Jabrb. kon. preuss. 

geol. Landesanstalt fur 1883, pp. 619-612 ; fur 1884, pp. 56-112, and pp. 525-545.

8 For an illustration of tbis .structure, see Bull. TJ. S. Geol. Survey, No. 28, 1886, PI. I. Fig. 2.

0 " Mecbanical metarnorphism " of Heiin andBaltzer, "Dislocation-M." of Lossen, "Dynamical-M." 

ofKosenbuscb, "Stauunga-M." of Credner, "Pressure-M." of Bonney.
WILLIAMS.] THREE TYPES OF METAMORPHISM. 43

the solid rock-mass. The crushing of rocks along fault or shearing 

planes allows here of the ready circulation of moisture and of a consequent increased distribution of heat. KocTic are hence most altered 

along such planes. The almost universal effect of such a crushing is 

made apparent by a microscopic examination of rocks which have been 

subjected to enormous pressure. We shall havo occasion to cite numerous instances of this in the sequel. For the present we will state the 

three different modes in which an eruptive rock is altered when it 

undergoes regional or dynamic metamorphisin, and mention the work 

of those who have been particularly successful in illustrating the action 

of all or any of them.

The three ways in which a massive crystalline rock may be modified 

by the action of orographic forces are:

L Macro-structural; i. e., it may have its external structure (morphology) changed so as to become schistose or foliated.

II. Micro-structural; \. e., it may have its internal or microscopic 

structure (histology) wholly changed, either with or without an alteration of the last-mentioned sort.

III. Mineralogicai ; i. e., there may be a change in the nature of one 

or more of the component minerals, either with or without a change in 

the chemical composition of the rock as a whole.

Of course, any one of these kinds of change may be produced in a 

rock-mass alone, or any two, or even all three, may exist simultaneously 

in any relative degree of intensity.

Macrostructiiral MeMmorphism. It had already long been recognized 

that transversal or slaty cleavage could be produced in sedimentary 

beds by mechanical force,1 when A. Heim, in his work on the " Mechanism of Mountainmaking," published in 1878,2 directed attention to 

an almost new line of geological work, by showing in a very general 

way how the orographic forces effect the deformation of rock-masses. 

He insisted upon three main points:

(a) That the rocks were solid and rigid when they were acted upon.

(&) That by a moderate pressure, they are torn asunder and the thus 

formed open fissures (" klaffeude Eisse") are subsequently filled by material segregated from the rock (" adern "). If the mass is plastic rather 

than brittle, the strain is relieved by numerous slicken-sides (" Eutschflachen") ("Umformung mit Brucli"}.

(c) That by the most intense pressure this action becomes infinitely 

small or molecular, i. e., the form is altered without rupture (" Umformung ohne Bruck").

Heim lays great stress upon the fact, already emphasized by Suess,8 

that the massive or crystalline rocks which form the center of the Al1 Oti slaty cleavage and allied rock-structures witli special reference to the mechanical theories of 

their origin. By Alfred Barker. Kept, fifty-fifth meeting Brit. Assoc. Adv. Sci., in 1885, pp. 813-855, 

188G.

2 Untersuchungen iVberflen Mechanismus der Gebirgsbildung iru Anschluss an die geologlscho Monographic der Todi-Windgiillen-Gruppe. Basle, 1878. Vol. 2.

3 Die Entstehung der Alpen. "Wien, 1875.
44 GREENSTONE SCHIST AREAS OF MICHIGAN. luuLL.62.

pine "massivs" were not themselves in any way instrumental in the 

elevation of the mountains. He gives an elaborate proof that these 

were solid long before the elevation ; and were only passively subjected to the upheaving force, in the same way as the overlying sediments. Heim does not, however, seem to apply his laws of mechanical 

deformation to these crystalline rocks, as he might well have clone, but 

regards their schistose or banded structure as probably an original 

character.

Other geologists were not long in recognizing the mechanical deformation of massive as well as sedimentary rocks. Daubre"e is struck by 

the frequent gradual passage of massive rocks into schistose varieties, 

of the same composition. He oays that, in spite of his own strong leanings toward the " metamorphic theory," he can not possibly regard 

such a schistose structure as any sign of original stratification but 

must consider it as secondarily produced in the eruptive mass by pressure. He is, however, inclined to believe that this result was attained 

before the eruptive rock had wholly solidified. 1 H. H. Eeusch, from 

his studies of the rocks near Bergen, in Norway, is also convinced that 

the frequent banding and gneissic structure in granite is secondary and 

a product of pressure, like slaty cleavage. Like Daubre'e, he too thinks 

that it was accomplished while the rock was still somewhat soft, atthe 

time of the faulting and dislocation to which the granite owed its own 

elevation, in a plastic state.2 This idea that such pressure deformation 

could only take place in softened or in unsolidified rocks was particularly dwelt upon by Carl Priedrich Naumann. How thoroughly the objections to the deformation of rigid rock masses, as far as sedimentary 

deposits are concerned, are answered by the facts observed by Heim, 

we have already seen. The assumption of an identical process for the 

highly crystalline massive rocks seems, at first, to present great difficulties, but observations in all parts of the world are daily proving 

its necessity.

In 1879, Eothpletz3' showed how the actinolite, or so-called greenschists near Hainichen, in Saxony, had been brecciated by pressure and 

the fragments shoved along more or less upon each other, so as often 

to produce an imperfect .schistose structure. This, which has a close 

analogy to certain rnacrostructural modifications of the Lake Superior 

greenstones to be described in subsequent pages, allowed, as Eothpletz 

remarked, a much increased chemical action, which tended to alter the 

character of the rock. A. Baltzer, in his monograph on the Finsteraafhoru,4 published in 1880. expresses himself as decidedly in favor of 

a 'mechanical metamorphism of the interior crystalline rocks. He says 

on page 244 of his work :

1 l?tu(les syntb6tiques tie g6ologie exp6rimentelle. Paris, 1879, p. 432.

2 Die fosajlien fiihrenden kvystalliniscben Schiefor von Bergen in Norwegeu. Deutsche Aufgabe vou 

E. Baldauf, .T883, pp. 129, 130.

3 TJeber mechanischen Gestein sum wand] ungen bei Hainichen in Sacbscii. II. Die Breccienbildungdes Aktinolithschiefers von Huiuicben. Zeitschr. Deutsch. gool. Gesell, 1879, Vol. 31, p. 374.

4 Die mechaniscbo Contact YOU Gneiss und Kalk im Beruer Oberland. Beitrage zur geologischeii 

£arte der Sohweiz. XX. Lieferung. Bern, I860.
WILLIAMS.] LEHMANN ON DYNAMIC METAMORPHISM. 45

I base ray assumption that the parallelism of the mica-scales in abnormally stratified gneiss is conditioned by pressure, upon this transverse foliation. Just as slatycleavage is generated in the younger sedimentary rocks by pressure and mechanical 

deformation, so is it also in the crystalline schists. This may be proved in the case 

of the last named rocks, if other evidence is lacking, by its parallelism with the undoubted cleavage in contiguous sediments. 1

Similar ideas have been more or less clearly expressed by Kjerulf, Brogger, Stapff, and other geologists, but oneof the most important contributions to our knowledge of this, as well as of other phases of mechanical metamorphism, was made in 1884 by Prof. Johannes Lehmaun in his 

work on the origin of the crystalline schists.2 That author has collected an immense number of observations from Saxony, Bavaria, and 

Bohemia especially in the "Granuiite" district of the first named 

country, upon the detailed study of which he has spent many years, 

From these observations Lehmann reaches certain general conclusions 

of importance with reference to the mechanical metamorphism of solid 

massive rocks. These are stated in chapters 16 and 17 of his work. 

He regards gneiss as simply a structure form of granular feldspathic 

rocks, and according to the composition of these there may occur granitic gneiss, syenitic gneiss, dioritic gneiss, gabbro gneiss, etc. He thinks 

that the essential parallel structure of gneiss may be, but very rarely 

is, original. He finds no indication that any true gneisses were once 

sedimentary deposits, but he considers them all as igneous rocks 

("Brstarruugsgesteine"), which, as a general thing, have acquired their 

present structure by stretching (" Streckung") in a solid state. Moreover, gneiss exhibits only the first stage of such a mechanical raetamorphisin. If the action of this process is more intense, finer grained 

and more evenly banded rocks result, which the author designates ;is 

granulite and felsite schist. According to Lehmann, therefore, all the 

massive crystalline rocks are subject to alterations in their structure 

through great pressure or tension. -They become banded or schistose in 

proportion to the intensity of this action.

The observations of many workers in various regions since the publication of Lehmann's conclusions have tended to substantiate them. 

Mr. Hatch3 has shown the secondary development of a schistose structure in the gabbros of the Tyrol, and the same thing has been done for 

the gabbros near Baltimore, Maryland.4 Results very closely resembling those of Lehmann have recently been secured by Mr. J. J. H. 

Teall in the gabbro region of the Lizard, Cornwall, England.5 This

1 Aufdiese transvorsalo Schiefernng griindo ich die Ajinahme dass dor Parallelitnus der Glimmerbliittcbeii ira anonnal gelagorten Gneiss ebenfalls von Druckaohieferung herriihrt. Wio in den jungoru Sediineiitgesteinen ilurch Druck und mechanische Umformungsprozease transvorsalo Sch lofty iing 

entstaud, so auch in don krystallinisclien Schiefern. Bei letztereu kann sio, in Ermangeluug anilorcr 

Anhaltspuuktc, durch den Parallelismus luit einer unzweifelbat'ten Schieferuugsriohtung iii dun angroDzeiulon Scdituonloii nacbgexnesen word en.

2 Unterstiohungen iibor die Entstoliung der altkrystallinischon Schieforgesteino, etc., with.Atlas of 

photographs. 4°. Bouu, 1884.

'Taobormak's mineral, u. potrog. Milthoil., vol. 7 (1885), p. 75.

4 Bull. U. S. Geol. Survey, No. 28.

6 Geol. Mag., London, November, 1886.
46 GREENSTONE SCHIST AREAS OF MICHIGAN. [BULL. 62.

investigator derives proof that the alteration in structure was accomplished after complete solidification from the relation of the rock to 

fault-planes. He says:

A rock must necessarily be solid before it can be faulted. Now, we find at Pen 

Voose near Landewednack, that massive gabbro passes over into gabbro scbist at a 

fault-plane, andtliat the foliation in the gabbro is such as would be produced by a 

shearing motion parallel with the fault-plane. Taking all the facts into consideration, we appear to be justified in concluding that the foliation in the Lizard gabbros 

is the result of pressure or regional metamorphism.

C. Schmidt has also recently given us the results of his studies of 

certain eruptive porphyries in the central Alps, especially in the Windgallen group, the district particularly studied by Heim.1 These, like 

the surrounding sediments, have been subjected to enormous pressure 

and strains and thereby have had developed in them a pronounced 

schistose structure, becoming in some instances veritable felsite schists. 

Still more recently H. EL Eeusch has communicated his observations 

in the regionally, metamorphosed district of Hardangerfjord, on the 

west coast or Norway.2 Here he finds dikes of all sorts of eruptive 

rocks, both acid and basic, all rendered secondarily schistose parallel 

to the planes of the inclosing schists, without reference to the direction 

of the dikes themselves. This, therefore, can not possibly be due to 

either flow or cooling, but must be the result of pressure. Ch. B. 

Weiss observed the same thing in the quartz porphyry dikes of Thai, in 

Thuriugia.3

Prof. T. G. Bonney, in his annual presidential address to the Geological Society of London (February, 1886), deals with the subject of 

metamorphism in general, and has much to say about the structural 

changes induced in holocrystalline massive rocks by pressure4. He 

distinguises between " stratification-foliation " and " cleavage-foliation," 

the latter being always secondarily produced by pressure.5 This same 

cause also in some cases may produce a false bedding parallel to the 

cleavage-foliation, almost exactly similar to the bedding of sedimentary 

rocks, although of a totally different origin, and to this he applies the 

name "pseudostrornatism." 6

Microstructural metamorphism. The statement of Heim that under 

sufficient pressure rock-masses became plastic, i. e., that they could be 

bent and crumpled by a true molecular movement within the mass, without rupture, seems to have been based entirely on a macroscopical study 

of exposures and specimens. Eocks, and even their constituent minerals, certainly do appear to the unaided eye to be stretched and contorted 

without any break in their continuity. Whether, however, this effect 

is accomplished by a truly molecular movement, as in a viscous body,

1 Neues Jahrbucli flir Hineralogie, etc., Beilageband 4, p. 388, 1886.

"Ibid., Beilageband 5, p. 56, 1887.

3 Zeit8chr. Dentsch. geol. Gesell., vol. 36, 1884, p. 858.

«Quar. Jour. Geol. Soc. London, vol. 42,1886, Proc., p. 95.

* Ibid., p. 64. - ._

6 Ibid., p. 65.
WILLIAMS.] DEFOEMATION OF ROCKS WITHOUT EUPTUKE. 47

or by a breaking or crushing of the component crystals with a movement in the mass before it is recemented, is something that can not 

be well decided by the unaided eye. Such authorities as Griimbel 1 and 

Pfaff2 at once took exception to the conclusions of Heim, and stated it 

as their conviction that, strictly speaking, there was no such thing as 

the deformation of rocks without rupture. Baltzer 3 could not decide 

between the conflicting views, but thought there was probably truth 

in each. .The microscopical study of such bent and contorted rocks 

seems to show that the explanation of Giimbel is generally the true one. 

This was emphasized by J. Lehinann.4 He calls attention to the fact 

that the conception of plasticity is not a simple one. We speak both 

of wax and of wet clay as "plastic," and yet the movement of the 

homogeneous body is different from that in the heterogeneous substance. 

In each the process of the deformation escapes the eye and the results 

appear the same; but in reality the first is molecular, the second 

caused by the slipping of the component particles (kaolin scales) over 

each other. In rocks the " plasticity " is of the second kind. Lehmann5 

says:

When rock-deformation has been accomplished by a breaking and sliding of the 

individual constituents upon each other, I think that I aiu still justified in calling it 

a deformation without rupture, provided the continuity of the rock-mass itself has 

been preserved and no fissures have been formed. * * * I recognize only that as 

rock-deformation with rupture when fissures, even though microscopic, traverse the 

rock independent of the individual constituents.6

Again he says :7

A plastic molding of the separate constituents, as is assumed by Heim, I have 

nowhere been able to discover. On the other hand, the maximum of resistance 

which a crystalline grain can offer to bending or crushing is very soon reached, and 

then ensues a pulverizing or chemical solution.8

Lehmanu also particularly emphasizes the importance of the very 

much increased chemical action which such a crushing of the rock 

components permits. This had heretofore been almost overlooked by 

all the students of dynamic or mechanical inetamoiphisin except Lossen. 

The ease with which a ready circulation and changed physical condition allow the destruction of certain chemical compounds, and the im-

'Das Verhalten dor Schichtgesteine in gebogcnen Lngeu. Sit'zuugsbcricMo <lcr kiin. bayrischen 

Akaderaie. Math.-phys. Claase, vol. 4, p. 590, 1880 (reviewed, by lloscnbusch in the Nones Jabrbucu 

fiir Mineral.. 1882, vol. 1, Eeferate, p. 221).

2 Dor Mechanismus tier Gobirgsbildung. Heidelberg, 1880, p. 140.

3 Die mechanische Contact von Gneiss und Kalk ini Berner Obeiiand, Bern, 1880, p. 240.

4 " Untersucbuugeu tiber die Eutstebung der altkrystalliniscbeu Scbiefergesteino, etc.." Chap. xvi.

*Ibid., p. 245.

6 Icli gluube sogar niit Eecht oiue Gesteinsfornmng, bei -welclier nur die eiuzelnen Gemengtheilo 

Mrspalten und sich an einander verschieben, als cine bruebloso bezuiclmeu zu lib'nen, wenn das 

Gestein seinen Zusainmenhalt bewahrt bat und keino das Geateiu flurcbsetzenden Risso vorliandeii 

siud. * * * Eine Gesteiusformung niit Brncli erkeuneioli orst dann an, wcnn Kisse, und inogoii cs 

auch rnikroakopischo&ein.unbektiniinert tun die EiiizolgomeugUioilodas Gostoinsgewobedurcbsetzou.

7 Ibid., p. 249.

"Euioplastiaohe Forraungder Eiussolgoruongthcile, wie sic Heim auneluuen zu iniiaseii glaubt, habe 

icb niemals gefunden ; vielmebr erreicbt die Biegung odor Zusammendruukung eines Krystallkorns 

solir bald das Maximum und erfolgt dann eine Zertriimmerung beziehungsweiseauch eine cneiniscko 

Auflosung.
48 GREENSTONE SCHIST AREAS OF MICHIGAN. [BULL. 62.

mediate formation of others out of their elements, is calculated to preserve the continuity of the rock-mass in spite of extensive stretching 

or compression.

Such chemical changes will form the subject of the following section. 

Here we are concerned only with the mechanical deformation of the 

elementary minerals.

The effects of pressure will naturally be first noticeable upon an 

elastic mineral like mica. A bending of the lamellae is well known to 

result frequently from even the slight movement of an eruptive magma 

before its complete solidification. Wherever, therefore, so sensitive a 

mineral presents no pressure effects when more brittle substances show 

them, it is fair to assume that the mica has been secondarily developed 

during the action of the pressure.

Harder minerals, like augite or feldspar, under the first action of 

strain, before their cohesion is overcome, appear to have twinning 

lamellae developed in them parallel to certain planes and most abundantly where the strain is the most intense. To this succeeds a cracking of the crystal, with often a considerable separation of the parts and 

a filling of the interstices by newly deposited substance.1 A still more 

intense pressure accompanied by a shearing stress seems to produce a 

regular granulation of the larger crystals, so that around their edge 

they pass into a mosaic-like aggregate of fine interlocking grains. 

This process in the case of augite, as in the Saxon " Plaser-Gabbros," 

is attended by uralitization, or the production of secondary amphibole. 

The resulting structure, in which larger grains are separated by a 

finer mosaic, has been called by Tornebohm, who observed it in' certain Swedish granites, "mortar-structure" ("Mortel-Structur").2 This 

granulation, with increasing strain, may extend farther and farther 

inward until finally the entire crystal is replaced by a fine mosaic of 

interlocking grains. When produced under the influence of sufficient 

pressure, it may, of course, give rise to a pronounced banded or schistose structure.

The action of strain upon the hardest and most brittle of the rockforming minerals quartz seems to result in cracking and breaking, 

rather than in bending it. Crystals which appear in hand-specimens 

to be stretched out into lenses, are shown by the microscope to be composed of smaller grains, more or less different in their optical orientation. Of this mineral Lehmann says:3

The phenomena which the quartz of stretched rocks exhibit manifest themselves 

under the polarizations microscope in the unequal extinction of different fields, and

1 See Lehmann's Atlas, PI. 6, Fig8. 3, 6, and PI. 21, Figs. 3, 6.

2 Wagra ord om grauit och gueis. Geol. Foreningens Stockholm Forhandl.,vol. 5, pp. 233-248, 1880- 

81 (review in the Neues Jahrbuch i'iir Mineral., 1881, vol. 2, Keferate, p. 50). Tornebohm, in this 

paper, considers this as an original structure. Kjerulf designates all such dynamic changes as are 

manifested in a movement among the fragments of broken crystals as " Kataklas-structur." He distinguishes three grades of intensity. See Grundfjeldsprofilet ved Mj0sens sydende. "Nyb Mag. for 

Naturvidenskaberne, vol. 29, p. 215, 1885 (review Neues Jahrbuch fur Mineral., 1886, vol. 2, Keferate, p. 244). See also Rosenbusch: Mikros. Physiog., vol. 2, 2d ed., p. 42,1887.

3 Untersuchungen iiberdie Entstehung der altkrystallinischen Schiefergesteiue, etc., p. 250.
WILLIAMS.] DEFORMATION OF QUARTZ. 49

then in a somewhat more exact separation of these fields. We imagine that we can 

detect sharp boundary lines without being able to certainly prove their existence 

(and in reality they are due merely to tension). Finally, however, the individual 

grain does really break up into separate interlocking parts, which at first are but 

slightly changed in their orientation. Nevertheless, toward the edge and in the 

more compressed portions these parts are quite irregularly orientated and participate 

in the composition of the ground-mass. True bending of quartz is unknown to me. 

It is shattered very easily, and, as it seems, is as readily subject to chemical solution. 1

Prof. Ch. E. Weiss, of Berlin, describes remarkably deformed 

quartzes in the schistose porphyries occurring in the almost horizontal 

mica schist near Thai in Thuriugia.2 This rock forms dikes which cut 

across the bedding of the schist and send apophyses in a horizontal 

direction between its layers. The schistose structure of the porphyry 

was formerly considered to be a flow structure, but Weiss has shown 

that this can not possibly be, as it runs in every case parallel to the 

bedding of the mica-schist without reference to whether this is 'parallel or 

perpendicular to the walk of the dike. The porphyritic quartz crystals in 

these rocks are drawn out into long, pear-shaped or tadpole-like ("kaulquappeuahnlich") forms, which follow the direction of the schistose 

structure. This is accomplished sometimes with, sometimes without, a 

rupture of the quartz substance, and produces what Losseu calls tailedquartzes. (" Schwanzchen-quarz.")3

It is remarkable that the porphyritic feldspar crystals of these 

rocks have suffered far less deformation than the much harder quartzes, 

an observation which, as we shall see, is abundantly substantiated in 

the schistose porphyries of the Lake Superior region. The phenomena 

exhibited by the Thuriugiau rocks, although left without explanation 

by Weiss, are referred without hesitation by Eoseubusch to the effects 

of dynamic metamorphism.4

Eosenbusch also remarks that, as a rule, the peripheral granulation 

("randliche Kataklase"), characteristic of the coarse-grained, granitic 

rocks, is wanting in the porphyries. This he attributes to the structure, 

for in the granular rocks under great pressure the grains rub against 

each other, while in porphyritic rocks the larger crystals are imbedded 

in a homogeneous matrix, so that they are either only optically disturbed or completely pulverized.5

1 Die Erscheinungen, welche der Quarz gestreckter Gesteino zeigt, bekunden sich unter dem Polarisationamikroskop in eiueiu ungloichuu Dunkelwerdeu veraclliodener Folder, danu in einer schilrfereii Abgrenzuiig deraelboii; man glatibt Begrenzungsliuien zu selien, ohue dass solche thatsiiclilich nachzuweisen wareii sie boruhen wohl nur auf Spamiuiigen endlich zerfiillt das einheitliche 

Korn aber wirklich in oinzolno, zaclcig begronzte Theile, die zunachst nur wonig aus ihror Lage 

gedrehterscheinen ; gegeu don Kami des Korna und in starker gepressten Theileu jedoch wirr durcheinanderliegon und'an der Zusammenaetzuugder feinkb'rnigou Grundmaasesich betheiligen. Eigentlicho Biegungen von Quarzen sincl mir nicht bekanut; er zerstiickt und zer'nillt sehr leioht uud 

orliegt \vio os scheint ebensoloicht eiuer cheinischen Auflosung.

2 Jaurbuch. pronss. geol. Landesaustalt far 1883, pp. 213-237, Berlin, 1884; and Zoitsch. Deutsoh. 

geol. Gescll., vol. 30, ]884, p. 858. Cf. J. G. Boruoiuann: Jahrbucu preuss. geol. Laudosaustalt far 

1883, pp. 383-400, Berlin, 1881.

3 Zeitsch. Duui.scli. gool. Gesell., vol. 31,1882, p. C78.

4 Die niassigoii Gosteiuo, 2d ed., 1880, p. 214. 

6 Ibid.,-p. 413.

Bull. 62 4
50 GREENSTONE SCHIST AREAS OF MICHIGAN. [HULL. 62.

Dr. C.Chelius describes compressed nnd schistose granite porphyries 

from the northern portion of theOdenwald (Hesse Darmstadt). In these 

the ground-mass is quite fine-grained, but where small areas of it have 

been protected, as it were, by large porphyritic crystals it has nearly 

twice as coarse a grain. 1

It would appear to be quite generally the case that porphyritic 

massive rocks, when subjected to great pressure, develop the so-called 

" Augeu structure." The larger crystals are only partially pulverized, 

and have on either side of them, in the direction of the schistose structure, a mosaic of their own debris, arranged like the " crag-and-tail" of 

a glaciated ledge. This has been observed and admirably described by 

Lehmann 2 in the Saxon granulites, by Teal I 3 in the gabbros of the 

Lizard district, Cornwall, and by Bonney.4

Mineralogical metamorpliism. This is undoubtedly the most important phase of metamorphism in eruptive as well as in sedimentary rock 

masses. It always accompanies such mechanical deformation whether 

macroscopical or microscopical as has been already described; but, 

' while such dynamic processes may greatly facilitate chemical action or 

molecular rearrangement, they are by no means always necessary to 

bring them about. The adjustment between the chemical combina 

tions and external conditions is so delicate in the inorganic world as 

to. make the difference, in this respect, between minerals and living organisms seem rather one of degree than of kind. This point has been 

elaborated by Professor Judd in his recent annual address before the 

Geological Society of London (February 18, 1887).5 Wadsworth 6 has 

also insisted that the changes in the mineral world are due to the passages from states of less stable to -those of more stable equilibrium ; 

but, as Judd justly remarks, the cycle of changes may be infinite because 

the stability of a compound depends upon its surrounding conditions. 

With a change of these, a state which was once stable becomes unstable, 

No student of mineralogy can fail to appreciate the extreme delicacy 

of the equipoise. Any one who has produced artificial twinning-lamelloe 

in feldspar or calcit^, or watched sulphur or boracite or tridymite or 

leucite pass from one modification into another, must acknowledge the 

importance of paramorphism in the formation of trocks. But changes 

of minerals accompanied by changes in the chemical nature of the compound are still more common. These may consist (1) in the breaking up 

of one molecule into two or more, with but little replacement of substance, as in the formation of saussurite from labradorite, or of /? spodumene (albite -f eucryptite) from spodurnene; or (2) in a reaction which 

takes place between two contiguous minerals, each supplying a part of

1 Notizblatt des Vereins fiir Erdkunde zu Darmstadt, 4. Folge, Heft 5, 1885, p. 29. 

2 Untersucbungen fiber die Entstohung der altkrystalliuischeu Schiefei'gesteine, etc. Bonn, 188J. 

3 Geol. Mag., London, Nov., 1886.

4 Presidential address to tbe Geological Society, Feb., 1886., Quart. Jour. Geol. Soc. London, vol. 42, 

Proc., p. 96.

6 See also the writer, Pop. Sci. Monthly, Sept., 188p. 

f Nature, March 3, 1887, p. 417.
WILLIAMS.] MINERALOaiCAL METAMORPHISM. 51

the substance necessary to form a new compound of intermediate composition, more stable for the then existing conditions than either. Such 

a case is the formation of a hornblende zone between crystals of olivine 

or hypersthene and plagioclase, ov of the so-called " kelyphite" zone 

between pyrope and serpentine 5 (3) in more complicated and less 

easily understood chemical reactions, like the formation of garnet or 

mica from materials which have been brought together from a distance 

and under circumstances of which it is at present impossible to state 

anything with certainty.

All such changes involving an actual change in the chemical composition are best designated in contrast to those produced by paramorphism, as metasomatic.1

Of course all of these metamorphic changes of a rnineralogical character whether paramorphic or metasomatic may go on in all conceivable proportions in a rock mass at the same time. They are often 

accompanied by changes in the original structure of the rock, but not 

of necessity, although such changes of structure can not be accomplished without chemical alteration.

Chemical changes in minerals and rocks have occupied the attention 

of mineralogists and chemists from the beginning of the century, and 

yet it is almost exclusively in sedimentary rocks that such studies have 

been carried, on. Only within recent years have the mineralogical metamorphoses which take place in eruptive rocks commenced to receive 

their proper share of consideration. Lessen, Tornebohm, and J. Lehmann were the first in Europe to recognize the importance of such investigations; while it will ever remain one of the most signal services 

which Dr. Wadsworth has rendered to petrography that he was the 

first in America to fully grasp their significance and to emphasize it. 

. It would be quite beyond the purpose of the present paper, even it1 

it were possible, to give a complete summary of all that has been done 

on the subject of chemical changes in eruptive rocks. In this place the 

attempt will be made only to trace out historically some of the more 

 important alterations which have a direct bearing upon the rocks forming the subject of this investigation. Important and wide-spread phases 

of the metamorphism of eruptive rocks such, for instance, as serpentinization which are not exemplified within the area studied will not 

be here considered. On the other hand, minor details or local modifications of well known and universally recognized processes will be elaborated in the course of the special petrographical descriptions (Chapters II-V) and summarized in Chapter VI.

The points which will be historically considered in this chapter are 

the following:

(l.)UraUtization, or the secondary origin of hornblende, both fibrous 

and compact, from pyroxene.

1 This term is preferred to inethylosis of King amlKowney ("An old chapter of the geological record 

with a now interpretation;" London, 1881), and Bonnoy (Quart. Jonr. Geol. Soc., London, vol. 42, 

.1886, Proc., p. 59), mid to mctachnmic of Daiia, (Am. ,1'our. Soi., 1886, 3d series, vol. 32, p. 70), because it

Jf) ftt proflont BQ. inuoli iiiovo wi<lolty wo<l wl honoo so nmoH nior«
52 GREENSTONE SCHIST AREAS OF .MICHIGAN. [BULL. 62.

(2.) Ghloritization.

(3.) Epidotization.

(4.) Formation of the viridite (chlorite) epidote aggregate.

(5.) Saussuritization.

(6.) Formation of the albite mosaic.

(7.) Sericitization.

(8.) Alterations of titanic-iron.

Uralitization. Ever since the classic record of observations made by 

Gustav Eose in the TJral Mountains in 1S30, 1 the fact has remained undisputed that pyroxene sometimes changes to an aggregate of amphibole needles which often preserve by their arrangement the original 

augitic form. It was not long before other similar occurrences were 

discovered, notably in Scandinavia and the Tyrol. Still, these localities 

were regarded as exceptional, and it was impossible that the full geological importance of Eose's uralite should be appreciated without the 

aid of the microscope. With the help of this instrument it was soon 

 discovered that the smaragdite of the saussurite gabbros or euphotides 

was a secondary hornblende which had originated from the diallage. 

As the number of observers and observations increased, instances of 

the undoubted passage of every sort of pyroxene into a fibrous amphibole grew constantly more and more abundant. It would be useless 

to attempt to trace out these discoveries in detail. It is enough merely 

to mention the names of Hose, Vom Eath, Fischer, Hagge, Zirkel, Von 

Lasaulx, Eosenbusch, Tornebohm, Svedmark, Sjogren, Becke, Kloos, 

Michel-L6vy, Allport, Phillips, Teall and Hatch, in Europe, and of 

Hawes in this country, to recall the accuracy and value of the facts recorded, while among the first to see further than mere facts and to grasp 

the full geological significance of this wide-spread change may be placed 

Lessen, Lehinann, Eeusch, Wadsworth and Irviug.

The alteration of pyroxene to hornblende is almost universally alluded 

to as paramorphism, and such, indeed, it may be in some instances. 

Still, many investigations go to prove that it is very often, perhaps always, accomplished by chemical change. The original uralite of Eose 

was too soft and hydrous to be compared with hornblende. The studies 

of Forschhammer, Eose,2 and Svedmark 3 showed that when augite 

changed to fibrous hornblende, magnetite and often calcite was separated out between the needles. A recent opinion by Eosenbusch on 

this point is as follows:

This uralitization can hardly be regarded as a simple act of molecular rearrangement; we should rather expect that a part of the lime contained in the augite would 

pass into other combinations, and, in fact, epidote is an almost constant companion 

of uralite.4

^oggendorff Annalen, vol. 22, 1831; vol. 31, 1834, p. 617; also Reise nach dem Oral, vol. 2, p. 342.

2 Zeitschr. Deutsch. geol. Gesell. Berlin, vol. 16, 1864, p. 6.

3 Nones Jahvbuch fiir Mineral., 1877, p. 99; Mikros. Pbysiog., vol. 2, 2d ed., p. 185; vol. 1, p. 472.

4 Diese TJralitisiruiig Icann uicht wobl oin eiufacber Act molecularer Umlagerung seiii, es istviel 

mehr zu erwarten, class eiu Theil des Kalkgehalts des Augits iu andero Verbindungen iibergebe uni 

thataacblich ist Epidot ein nahezu constanter BegleiLer des Uralitsj
tRALITIZATION 53

Dr. B. Harrington, of Montreal, has carefully analyzed three stages 

of alteration between pyroxene and a secondary fibrous hornblende 

resulting from it, and has found that during the change there has been 

a great loss of liine and a gain of magnesia, silica, and the alkalies.1 

The specimen investigated was one of the excellent examples of uralite 

from the apatite mines near Ottawa, Canada, similar to those named 

by Thomson " Eaphilite." 2

Mr. J. J. H. Teall has also recently discussed this point in connection with his studies of the alteration of the Scourie dike,3 and is of the 

opinion that the oxidation of iron from the ferrous to the ferric state is 

incident to the change of augite to hornblende.

While the alteration of almost any pyroxene into fibrous hornblende 

has now become a universally recognized fact, the possibility of the 

change of this mineral directly into compact hornblende is by no means 

so generally admitted. So admirable an observer as J. H. Kloos has 

recently based his distinction between primary and secondary amphibole in the Black Forest gabbros entirely upon their compact or thin 

fibrous character.4 Still, the work of many investigators shows that 

this is by no means a reliable criterion in all cases.

Svedmark found in 1876 that the pyroxene of the Vaksala porphyry 

passed into an aggregate of stout, compact amphibole needles.5 In 

1877 Streng described compact brown hornblende in the Minnesota 

gabbros, which he considered to have originated from the diallage; 6 

and in 1878 Dr. George W. Hawes described and figured similar occurrences in certain New Hampshire rocks.7 In 1880 appeared accounts 

bylrving, and in 1882 by both Irviug and Van Hise, of compact brown 

hornblende originating from diallage.8 In 1880 C. von John described 

the alteration of diallage to compact brown hordblende in the olivine 

gabbro of Bosnia.9 In 1883 Hj. Sjogren regarded the irregular spots 

of compact brown hornblende which occur in and around the pyroxene 

of the wernerite or dipyre-diorite of Bamle in Norway as produced by 

a molecular rearrangement of the pyroxene substance.10 In 1884 similar 

observations were made by myself in certain pyroxenites of the " Cortlandt Series," occurring at Montrose Point on the Hudson River j also 

an the so-called "black granite" (gabbro) from Adclison, Me., and in 

 rocks from the apatite regions of Canada.11 In the same year (1884)

I Gool. Survey, Canada. Keport of progress for 1878-79. Appendix G, p. 23. 

:2 Outlines of Mineralogy, Geology, etc., London, 1836, vol. 1, p. 153.

3 Quart. Jour. Geol. SOP. London, vol. 41, 1885, p. 137.

4 Nones Jabrbncb. fiir Mineral., Beilage-Band 3, 1884, pp. 32, 33.

:B Geol. Foreningens Stockholm Forbandl., 1876, pp. 151-164; Neues Jabrbnch fiir Mineral., 1877, p. 99.

10 Nones Jabrbuch fiir Mineral., 1877, pp. 133, 240; Am. Jour. Sci., Dec., 1884, p. 464.

* The Geology of New Hampsbire, vol. 3, 1878, part i v, Mineralogy and Litliology, pp. 57,206, pi. vii, 

figl. "

8 Geol. Wisconsin, vol. 3, (1880) p. 170; vol, 4, (1882) p. 602. See also Geol. Wisconsin, vol. 1,1883, p. 

321, and Am. Jour. Sci., 3d series; vol. 26, p. 29, 1883.

'J GrandlinienderGeologie von Bosnian-Herzegovina, Wien, J8SO. (Of. Neuea Jabrbucb fiir Mineral., 

]8'31, vol. 2. Eoferato, p. 353.)

10 Geol. Foreningens Stockholm Forbandl., vol. 6, p. 447, 1883. (Cf. Neuea Jahrbuch fiir. Mineral. 

I88i, vol. 1. Eeferate, p. 81.)

II Am. Jour. Sci., 3d series, vol. 28, Oct., 1884, p. 261.
54 GREENSTONE SCHIST AREAS OF MICHIGAN. [BULL- 62.

A, Schenck quotes in his Inaugural Dissertation a statement made by 

VOLI Lasaulx in 1878, that pyroxene passes first into fibrous and subsequently into compact brown hornblende. 1 Schenck himself observed in 

the Eimberg diabase from the upper Euhr Valley that the reverse of 

this was true, i. e., that the pyroxene passed first into compact brown, 

and this subsequently into fibrous green hornblende.2

The secondary hornblende, so well described and figured .by Teall in 

the Scourie dike, is decidedly compact in its nature,3 nor is that derived 

from the paramorphisin of the diallage in the Saxon " flaser-gabbros," 

according to the observations of J. Lehinann, less compact.4 Such 

eminent authorities as Lossen5 and Eosenbusch6 also allow that there is 

no doubt that compact basaltic hornblende occasionally results through 

the direct paramorphism of diallage, or even orthorhoin I >ic pyroxene. 

The chemical composition of the original mineral undoubtedly largely 

conditions the nature of the resultant product. This is shown by the 

observations of vou John in the Flysch-gabbros of Bosnia, where brown 

hornblende resulted from dark and green hornblende from pale diallage. 7

These facts are dwelt upon here somewhat in detail 'because, as will 

been seen in the sequel, they have possibly an important bearing on a 

large part of the Menomiuee Eiver greenstones.

It is not intended, of course, to imply by the above statements that 

true parallel growths of primary augite and primary brown hornblende 

may not occur, as held by both Lossen and Eosenbusch (loc. cit.). 

Such parallel growths would have the closest possible resemblance to 

alteration forms, and I can not escape the conviction that the tendency 

of future studies will be to attribute a secondary origin to more and 

more of such intergrown brown hornblende.

The derivation of fibrous or actinolitic hornblende, quite identical in 

all respects with uralite, from compact hornblende (whether brown or 

green) seems hitherto to have been much neglected. Inostranzeff in 

1879 described Eussian diorites in which the original hornblende had 

passed into secondary actinolite, as well as into biotite, chlorite and 

talc.8 Becke also,in 1882, mentions uralite formed from both pyroxene 

and compact, primary hornblende, in the kersantite of the "Niederoesterreichisches Waldviertel." 9

Descriptions have also been given by myself of compact brown hornblende changing into the green fibrous variety in the Baltimore 

gabbros.10

'Verb. d. naturh. Vereinsd. pr. Kheinl. und "Westf., 1878, p. 171, pi. iv, fig. 4.

2 Die Diabase des oberen Kuhrthals, etc., Inaugural-Dissertation. Bonn, 1884, p. 41.

3 Quart. Jour. Geol. Soc. London, vol. 41, p. 137, 1885, pi. n, fig. 2.

4 TJeber die Entstehung der altkrystallinibcheu Schiefergesteine, etc. Bonn, 1884.

5 Jahrbuclf preuss. geol. Landesanstalt fur 18S4, p. 537, Berlin, 1885.

6 Mikros. Physiog., 2d ed., vol. 2, 1887, pp. 141, 209.

7 TJeber kryst. Gest. Bosniens und der Herzegovina in Grundlinieu der Geol. von Bosnien-Herzegovina, Wieii, 1880 (ref. Neues Jahrbuch fiir Mineral., 1881, vol. 2. Eeferato, p. 353).

8 Studien iiber nietamorpbosirte Gesteiue im Gouvernement Oleiiez. Leipzig, 1879 (cf. Neues Jahrbuch fiir Mineral., 1880, vol. 2. Eeferate, p. 342).

9 Tschermak's mineral, u. petrog. Mittheil., vol. 5,1883, pp. 157-159.

10 Bull. 17. S. Geol. Survey, No. 28, p. 45.
WH.LUMB.] CHLO.KITIZATION. 55

The common alteration of rock-forming hornblende appears to be to 

chlorite or to epidote, or into an aggregate of both these minerals. 

This is almost the only change mentioned by Eosenbusch in his most re-, 

cent works, although in one place he says: "* * * bei Chloritisirung 

fasert sich die Hornblende aus * * * n etc.1 That this change of 

compact to fibrous hornblende is, however, of great importance, the rocks 

described in the sequel will abundantly prove.

Cliloritization. An admirable exposition of the present state of our 

knowledge relative to the so-called " chloritic constituent" of the diabases and allied rocks is to be found in the most recent work of Eosenbnsch.2 The secondary hydrous minerals of a green color, whose almost 

universal presence has brought to this whole class of rocks the general designation "greenstone," owe their existence to the processes 

known as weathering, rather than to such as may be strictly called metam orphic. There can be but little doubt that they have been formed 

mainly out of the components of the pyroxene, although these wander 

so during the process of the alteration that the new products by no 

means occupy the exact position of the original mineral. It seems possible to distinguish two classes of such secondary substances: First, 

such as are more or less fibrous in structure, without pleochroism, and 

have a decided action upon polarized light; and, second, such as are 

scaly in structure, with pleochroism and so weakly polarizing as to 

appear isotropic. The first class embraces substances allied to serpentine ; the second, those which more or less closely resemble chlorite. 

In certain cases they have been chemically identified with such indefinite species as delessite,grengesite (Hisinger), seladonite, chlorophaeite 

(Macculloch), epichlorite (Eammelsberg), diabantachrounyn (Liebe),3 

and diabantite (Hawes).4 All of these substances resemble each other 

more or less closely in both physical and chemical behavior, and I am 

inclined to agree with Eosenbnsch 5 in preferring the generic term 

"chlorite" or "chloritic substance" for them, to even the indefinite designations "viridite" of Vogelsang 6 or " cliloropite" of von Giimbel. In 

fact Keimgott has shown that the formula of diabautachronnyn may 

be brought quite into accord with that of typical chlorite.7

Liebe found the greenish coloring-matter of the dark titanic iron 

diabases of Voigtland and Frankenwald to be picrolite (serpentine), and 

it is not unusual to observe the curious vermicular aggregates of circular chlorite scales to which Yolger has given the name helminth.8

It is well known that the results produced by the weathering of born-

'Mikros. Physiog., 2d ed., vol. 2, 1887, pp. 468,469.

2 Ibid,, pp. 180-184.

8 Neue8 Jalirbuch fur Mineral., 1870, p. 2.

4 Am. Jour. Sci., 3d series, vol. 9,1875, pp. 454-457; Geol. New Hampshire, vol. 3, part 3; 1878, p. 120.

8 Mikros. Physiog.,2d ed., vol.2,1887,p. 183.

6 ArchivesN6erlandaiaes, vol.7, 1872; Zeitsch.Deutsch.geol.Gesell., vol. 24, 1872, p.529.

'Neuea Jahrbuch fiir Mineral., 1871, p. 51.

8 Studien zur Entwickelungsgeschichte dor Mineralien, 1854, p. 142.
o 

56 GEEENSTONE SCHIST AREAS OF MICHIGAN. [BULL. 62.

blende or mica are, in most instances, quite similar to those just described in the case of pyroxene. Such a change of hornblende may have 

an equal importance in those greenstones, like a large proportion of 

those to be considered in this paper, whose bisilicate constituent was 

originally augite, but which was subsequently changed by a truly metamorphic process (uralitization) to hornblende before the final weathering commenced.

The last result of the action of the atmospheric influences upon the 

chlorite is, as Rosenbusch long ago showed, to convert it into an aggregate of limonite, carbonate, and quartz.

Epidotization. The most careful and detailed observations on this 

process are to be found in the Inaugural Dissertation of Adolf Schenck 

on the Diabases of the upper Ruhr Valley and their Contact Phenomena. 1 This investigator finds that certain of these diabases are converted 

into "Epidosites," or aggregates of epidote and either quartz or calcite. 

He suggests the only three possible hypotheses for the formation of 

this epidote, viz: (1) That it was formed by the action of thedecomposition products of the bisilicate (mainly CaO and Fe2O3 ) upon the feldspar substance; (2) That the reverse was true, i. e., that the decomposition products of the feldspar (mainly A12O3 and CaO) acted upon the 

pyroxene; (3) That there was a mutual reaction between the decomposition products of both the original constituents. The first of these 

hypotheses is regarded by the author as the only tenable one for the 

rocks which he studied, because in all those diabases which contained 

much epidote the feldspar was remarkably fresh and the pyroxene decomposed, while in those whose feldspar was extensively altered epidote 

was almost wanting. Observations of the change of feldspar to epidote 

are cited from the writings of Dathe, Cohen, Eosenbusch, Liebisch, von 

Lasaulx, Inostrauzeff, Zirkel, Michel-Levy, Kiihn and Scheilitz; and even 

more authorities are quoted to show that this same mineral originates 

from augite, or more commonly from hornblende or uralite. If iron is 

not at hand, the place of the epidote appears to be taken in the feldspar by zoisite. An excellent example of the origin of epidote as a reactionary rim "between the substance of hornblende and plagioclase has 

been described and figured by myself in the Baltimore gabbros and their 

derivatives. Here the epidote forms a continuous border around the 

hornblende, iis crystals projecting from it outward into the feldspar 2

Viridite (chloritc)-Epidote Aggregate. The simultaneous action of the 

two last described processes (chloritization and epidotization) in an 

aluminous pyroxene or hornblende results in the production of an aggregate consisting of sharply defined, pale yellow crystals of epidote, 

imbedded in a green, scaly mass of chlorite. The appearance of this 

under the microscope is shown on PI. XI, fig. 1: The magnesia, together

1 Die Diabase des oberen lluhrthals und ihre Contaeterscheinungen mit dem Lenneschiefer. Bonn, 

1884. 

2 Bull. U. S. Geol. Survey, No. 28, 1886, p. 32. PI. in, Fig. 2.
THE CHLORITE EPIDOTE AGGREGATE. 57

with a proportion of the iron, alumina and silica, have here formed 

the chlorite; while the lime, instead of becoming a carbonate, as it often 

does, has become fixed, in combinatiou with the rest of the irom 

alumina and silica, as epidote.

Such aggregates have not escaped the 'notice of petrographical students, though curious mistakes have sometimes arisen as to their true 

nature and origin.

In 1874 Dathe described this association of chlorite and epidote with 

great correctness and precision. He says :

When secondary products become abundant and the leek-green viridite increases 

in amount and distribution, then there are often associated with this light yellow 

crystals, whose difference from the viridite is very.apparent under the microscope. In 

the progress of my studies, I have recognized these crystals as pistazite. 1

In 1876 Zirkel described the change of hornblende to an aggregate of 

epidote, viridite (chlorite) and magnetite, with which, in many instances, 

more or less calcite was associated.2

In the same year, Wichmann, in his paper on the iron rocks of the 

south shore of Lake Superior, which was not, however, published until 

1880,3 described this chlorite epidote quite correctly in some instances, 

although he says that it is beyond doubt that the epidote has been formed 

out of the viridite. In other cases he regards the mineral imbedded in 

the viridite as a secondary augite, which, as in the last instance, he 

thinks was formed from the chlorite. He says of this:4

The phenomenon of the transformation of viridite into augite has not been observed 

until recently. It is very interesting to have the fact established that this mineral, 

after, having been changed into another, has finally returned to its former state. * * * 

Such individuals are only present in viridite and do not seem to occur in any other 

part of the rock.

George F. Becker in the course of his microscopical studies of the 

rocks of the Oomstock Lode, Nevada, which were either identical with 

those investigated by Zirkel, or very closely allied to them, found the 

same change of bisilicates to an aggregate of chlorite and epidote which 

the latter observer had recorded. Mr. Becker is, however, very positive that the epidote has been developed after and at the expense of the 

chlorite, instead of simultaneously with it.5 This he attempts to explain 

upon chemical grounds.6 The improbability of Becker's supposition 

was brought out by Eosenbusch in a review of his work.7

In this, as well as in his more recent text-book,8 Bosenbusch clearly

1 Wenn die Neubildungsproducte sich iin Gestein hiiufen, wenn dor laachgrnno Viridit [CliloritJ an 

Masse und Verbreitung znniinrat, stellen sich oftmals neben letzteren lichtgolbliche Gebilde ein, 

rteren Verschiedenheit vom Viridit bei inikroskopiscber Betracbtung sofort in die Angen springt. Im 

Verlauf der TJntersuchuns: wurden diese Gebilde als Pistazit erkaiint. Mikroskopische Untor 

sucliungen uber Diabase. Zeitsch. Dentsch. geol. Gesell., vol. 20, 1874, p. 16.

3 Microscopical Petrography, Washington, 1876, p. 66, PI. in, Figs. 2 and 3.

3 Geol. Wisconsin, vol. 3,1880, pp. COO-656.

" Ibid?, pp. 62;!, 624.

8 The Geology of the Comstock Lode. Monograph TJ. S. Geol. Survey, vol. 3, p. 76.

« Ibid., pp. 211-2U.

7 Neues Jabrbuch fiir Mineral., 1884, vol. 2. Roferate, p. 189.

8 Mikros. Physiog., 2d ed., vol. 2, pp. 108,183.
58 GREENSTONE SCHIST AREAS OF MICHIGAN. [BULL. 62.

shows that the true deviation of the chlorite-epidote aggregate is to 

be found in a division of the bisilicates out of which it is formed, 

the magnesia passing into the chlorite and the calcium and ferric iron 

into the epidote. The tricliuic feldspar may often materially assist in 

this formation by furnishing the necessary lime.

Nor is this mistake of Mr. Becker's a new one. Francke in 1875 described the same mineral association and gave the same explanation for 

it; 1 while in 1880 Schauf,2 in 1882 Kiemann,3 and in 1884 Schenck corroborated the observation, although both Schauf and Schenck 4 raised 

unanswerable chemical objections to the derivation of the epidote from 

the chlorite.

Saussuritization. The hard, compact, whitish or greenish substance, 

without luster or cleavage, which forms one of the two main components 

of the certain coarse gabbros or euphotides, was called by H. B. Saussure, in 1780, "jade." 5 In 1806 Th. Saussure named this mineral, in 

honor of his father, "saussurite,6 but its true nature continued for over 

half a century to elude all mineralogists. As long as chemical analysis 

was the only means with which it could be studied, any satisfactory 

solution of the riddle seemed to be impossible. Such investigators as 

vom Bath,7 Klaproth,8 T. Sterry Hunt,9 and many others obtained the 

most diverse results; the composition in some cases approaching that 

of zoisite, in others that of garnet, scapolite, or feldspar. In 1866, Zirkel remarked, at length, upon the extremely imperfect state of knowledge regarding this geologically important mineral. 10 

. It remained for the microscope to disclose the composite nature of 

saussurite as well as to point out the true significance of its origin and 

occurrence.

Hagge was the first to correctly describe saussurite as an aggregate of 

colorless or greenish crystal needles, prisms or grains, irregularly scattered through a colorless, glassy-looking matrix.11 He, however, retained 

the name saussurite for the included grains, and did not attempt to determine their mineral nature. Hagge also clearly recognized that in all 

cases observed by him the saussurite was a product of the alteration of 

feldspar, and that the passage from one mineral to the other was a gradual one which the microscope could easily follow.12 These results were

1 Studien iiber Corillerengesteine. Inaugural-Dissertation. Leipzig, 1875.

2 Verhl. d. nat. Vereins d. pr. Kheinl. u. Westf., 1880, p. 6.

3 Ibid., 1882, p. 256.

4 Inaugural-Dissertation. Bonn, 1884, p. 45.

6 Voyages dans les Alpes, vol. 1, Neuchatel, 1779, p. 83.

6 Annales de chimie et de physique, 3d series, vol.-16, p. 469.

7 Poggendorff Aunalen, vol. 95,1855, p. 555.

8 Beitriige, vol. 4, p. 271.

9 Am. Jour. Sci., 2d series, vol. 27,3859, pp. 330-349.

'«Lehrbnch. der Petrographie, vol. 1, p. 27; vol. 2, p. 110,1866.

"Mikroskopiscke Untersuchungen iibcr Gabbro and verwandte Gesteiue. Kiel, 1871. On p. 51 he 

says: " Der Saussurit besteht aus kleinen Krystallnadeln, Prismen und Kornern, die farblos oder 

blassgriin sind und regellos in ein^r wie ein farbloses Glas aussehendeu Grundmaase, die aucb. vielf'acli klare Spalten in Uem Saussurit bilclot, verthielt liegen."

"Ibid., pp. 35 and 51.
WOLIAMB.] SAUSSUKITIZATION. 59

quoted by Zirkel in his text-book, published in 1873. 1 The conclusion 

that saussurite was "an aggregate and probably the end product of a 

molecular rearrangement in the feldspar" was also reached by Rosenbusch in the same year.2 In 1878 Becke showed that the saussurite of 

certain Grecian rocks was largely zoisite ; 3 and in 1833 appeared the important paper by Cathrein upon this subject.4 This author draws from 

his work the following six conclusions as to the nature of saussurite.

(1) The so-called saussurite, far from being a homogeneous mineral, 

is an aggregate of plagioclase, rarely orthoclase, with zoisite. Actinolite, chlorite, and other minerals may also occur as accessories.

(2) The chemical composition of saussurite resembles most that of 

the lime-soda feldspars; it, however, contains less silica, more lime, and 

has a higher specific gravity.

(3) The proportions of the saussurite elements can be calculated from 

the amounts of alkalies, lime, and iron, if we know the species of the

original feldspar.

(4) This proportion can also generally be calculated, even when this 

last factor is unknown, by a consideration of the relative difference of 

silica, alumina and lime in zoisite and auorthite.

(5) Saussurite is a product of the metamorphism of feldspar through 

an exchange of silica and alkali for lime, iron and water.

(6) The epidotization of feldspar is genetically the same process as 

saussuritization and differs only in the larger proportion of iron required.

H. Reusch considered the saussurite which composed a large part of 

the Bergen gabbros as partly epidote, partly zoisite.5 Traube found 

the3 structure and composition of the saussurite occurring in the gabbros of Lower Silesia to be like that described by Cathrein,6 while 

Brogger mentions neither epidote nor zoisite, but regards some member of the scapolite family as probably present in the saussurites studied 

by him.7 Paul Michael has recently investigated the saussurite-gabbros 

of the Fichtelgebirge in Bavaria and concludes that two main types of 

saussurite occur there. One of these consists mainly of zoisite, with 

more or less alkaline feldspar, and the other is composed of a pale lime 

alumina garnet together with serpentine.8

J. Lehrnaim 9 and Lessen 10 were the first to recognize in the saussuritization of feldspar a result of dynamic or regional metamorphism an 

idea which Rosenbusch has lately more clearly developed and elabo1 Die mikroskopiscbe Bescbaffenbeit der Mineralien uud Geateine. Leipzig, 1873, p. 143. 

2 Dio petrograpbisch wicbtigen Mineralien, 1873, p. 350.

3 Tacbermak's mineral, u. petrog. Mittbeil., vol. 1, 1878, p. 247.

4 Zeitschr. Kryst., vol. 7, 1883, p. 234.

"Die fosailien-fiibreuden krystalliuiscben Scbiefer von Bergen in iNorwegen. German translation by 

Baldauf, 1883, p. 40.

cBoitriige zur Kenntniss der Gabbros, Ampbibolite und Serpentine des nieder-scblesiscben Gebirges. 

Inang.-Dissor., 1884, pp. 7, 20.

'NytHag. for Naturvidenskaberne, vol. 28, 1884, p. 253.

8 Neues Jalirbuob fur Mineral., 1888, vol. 1, pp. 36-41.

9Uutersucbungen iiber die Entstebung der altkrystallluiscben Scbiefergesteine, etc. Bonn, 1884, 

pp. 197, 199.

10 Jabrbuch k. preuss. geol. Landesanatalt fiir p. 
60 GREENSTONE SCHIST AREAS OF MICHIGAN. [BULL. 62.

rated.1 This last-named authority shows that the new mineral developed in the feldspar is by no means always of necessity zoisite or epidote. It may also be garnet, or a mineral of the scapolite family. He 

remarks:2

The sum total of these new products, which are certainly not produced by normal 

weathering, but are due to a metamorphic process, we designate as the saussnritization of feldspar. 3

Michel-Le'vy was the first to show that the so-called " spotted-gabbro (gefleckter Gabbro) of Brogger and Reusch was a mixture of ainphibole and wernerite;4 and Sjogren has since demonstrated that the 

latter mineral is secondary to feldspar, and the first to pyroxene. 

Fouque* and Micbel-Levy tried the interesting experiment of fusing this 

rock, and obtained therefrom an aggregate of pyroxene and labradorite.6

Formation of the Albite Mosaic. Analogous and closely related to 

the saussuritization of feldspar are cases where, in the process of its 

alteration by metamorphism, the lime is more or less completely removed or crystallized as calcite, instead of forming a calcium silicate. 

In such instances the alkaline portion of the plagioclase molecule crystallizes in the form of a finely granular aggregate, or mosaic of limpid 

grains (albite). These may be wholly devoid of twinning lamella and 

often closely resemble quartz, with which they are not infrequently 

associated. Of course such mosaics are connected by every stage of 

transition with saussurite, according as more or less of the lime has remained as a calcium silicate (zoisite or epidote).

Such a granulation of the feldspar substance has already been 

alluded to under the head of microstructural metamorphism; but, inasmuch as a complete solution and recrystallization of clear feldspar substance often takes place, it belongs quite as.much to this division of the 

subject. The geological significance of this method of feldspar alteration, especially in certain basic eruptive rocks like diabase, has been most 

persistently emphasized by Lossen. He has shown how it may result 

in the total obliteration of the original and characteristic rock-structure, and how, therefore, it may prove most misleading, unless its nature and origin are clearly understood.7 J. Lehmann,9 Teall,8 and others 

have also observed and commented on such a secondary feldspar 

mosaic.

Sericitization. The soft, unctuous mineral characteristic of so many 

slates and schists was formerly regarded as talc. This gave rise to the

"Mikros. Pbysiog.,2d ed., vol. 2, 1886, p. 164. ,

2 Ibid., p. 137.

3 "Die Gesammtbeit dieser Eeubildungen, die sicher koine Artdornortnaler Verwitterung, sondern 

eiu metamorpuer Vorgang ist, bezeichnet man als die Saussuritisirung des Feldspatbs."

 'Bull. Soc. Mineral, de France, vol. 1, 1878, pp. 43, 79.

*Geol. ForeningensStockholm Forhandl., vol. 6, 1883, p. 447 (ISTeuesJahrbuchfur Mineral., 1884, vol. 

1, referatep. 81).

6 Bull. Soc. Min6ral. de France, vol 2, 3879, p. 105.

7 Jahrbucb preuss. geol. Landesanstalt fiir 1883, p. 640 PI. 29; ibid., 1884, pp. 525-530, PI. xxix, Figs. 

2 and 4.

8 tTntersuchnngen iibcr die Entstebung der altkrystallinischen Scbiefergesteine, etc., 1884, p. 207.

s Quart. Jour. Geol. Soc. London, vol. 41,1885, p. 139.
WILLIAMS.] SERICITIZATION. 61

common designation "soapstone" or "talcose slates," which was used 

very early in America by Amos Baton. 1 As early as 1819, however, 

Prof. Chester Dewey, of Williams College, made the statement that he 

was able to detect very little magnesia in the specimens which he examined, while alumina was abundant. He therefore preferred to call 

these rocks micaceous slates.2

In Europe, slates of this kind were first studied from the Taunus, 

where they were usually regarded as talcose until 1847, when Sandberger made the same discovery that Dewey had made with regard to the 

/ American rocks.3 List subsequently studied more carefully this micaceous constituent of the Tauiius schists, and named it, on account 

of its silky luster, kC Sericite." 4 Scharff attacked the conclusions of List 

and considered sericite to be only a mixture,5 while Losseu vigorously 

defended the individuality of the mineral.6

The sericite was soon identified by many investigators in the rocks 

of many regions.7 Prof. Dana united it with the species margarodite and damourite in the group of hydro-micas and called this class 

of slates the hydro-mica slates. Of these so-called hydro-micas Prof. 

Dana says:

The following species [margarodite, damourite, parophite, sericite, sericite schist, 

groppite, euphyllite, cookeite, voigtite, roscoelite,] are mica-like in cleavage and aspect, but talc-like in wanting elasticity, in greasy feel, and in pearly luster. They 

are sometimes brittle. Common mica, muscovite, readily becomes liydrated on exposure; but hydrous micas are not all a result of ^alteration. Hydro-mica schists 

form extensive rock-formations, equal to those of the ordinary mica-schists. 8

In 1880 Laspeyres made a very thorough examination of sericite 

and showed conclusively its identity with muscovite.9 It differs from 

the ordinary form of this species only in its compact structure and in 

its geological significance; but, since both of these are characteristics, 

there is a decided advantage in. retaining the name sericite, especially 

in petrography, to designate a peculiar form and occurrence of muscovite.

The alteration of orthoclase into kaolin or clay was very early known.10 

The corresponding change of the same feldspar into mica was first observed by Haidinger in 1841,11 but was subsequently found to be not

1 Index to the Geology of the Northern States, pp. 147,174, 280.

2 Am. Jour. Sci. 1st series, vol. 1,1819, p. 340. Thus the discovery, usually attributed to Sandberger, 

.was anticipated iu America l>y twenty-eight years. Cf. Dana's Manual of Geology, 3d ed., p. 72.

3 TJebemeht d. geol. Verhaltuisse d. Herzogth. Nassau,1847, p. 94.

4 Jahrb. d. Vereins f. Naturkunde ira Herzogth. Nassau, vol. 6,1850, p. 126; vol. 7, 1851, p. 260 ; vol. 

8, 1852, p. 128; Auualeu Cheni. und Pharni., vol. 81, 1852, p. 18K

5 Neues Jahrbuch 1'iir Mineral., 1868, p. 309; ibid., 1874, p. 271.

6 Zeitschr. Doutsch. mineral, geol. Gessell., vol. 19, 1867, p. 509; vol.21,1869, p. 281.

7 o. g.,byPichler in the Tyrol. Neues Jahrbuch fiir Mineral., 1871, p.56; by Tornebohm in Sweden, 

Neues Jahvbuch fiir Mineral., 1874,p. 141; by von Lasaulx in the Ardennes; by Credner in Saxony 

and by Wichmanu in the Lake Superior rooks.

8 Manual of Mineralogy and Petrography. 4th od. New York, 1887, p. 335.

9 Zuitschr. Kryst., vol. 4, 1880, pp. 244-250.

10 Blnin : Paeudomorplioson d. Miueralreiohes, 1843, p. 72.

11 Abb. d.k. bohru. Gesell. d. Wisaonschui't, 1841, p. 4. Bluiu : Pseudomorphoseu, Nachtr., I, 1847, 

p. 25.
-62 GREENSTONE SCHIST AREAS OF MICHIGAN. [BUM* 62.

uncommon. Since rocks have been studied with the microscope, the 

change of potash feldspar to muscovite or sericite has been found to 

possess a wide significance, especially in dynamic metamorphisin.

There seems every reason to believe that sericite always originates 

from potash feldspar and not from other micas. Its constant and very 

intimate association' with quartz shows, as first pointed out by Laspeyres,1 that it was derived from an acid silicate.

Any rocks, therefore, whether sedimentary or massive, which contain orthoclase, may give rise to sericite; but its formation, in any casewould seem to require the action of some great dynamic force. The 

close relation between dislocation and mica formation, so clearly illus, 

trated by J. Lehmann,2 is in an eminent degree applicable to sericite. 

It is now a well known and oft-recorded fact that in any rock mass, 

where the strains and stresses have been the most intense, there the 

micaceous minerals are most abundant; and if the rocks are ortboclastic these will be largely sericite. Sericitization is therefore a phenomenon of dynamic metamorphism. It is in a way, as Lehraann says, a 

retrogressive metamorphic process, since feldspar is by it not formed, 

but destroyed. It is also tbe most extreme manifestation of dynamic 

action in orthoclase rocks and may produce the same result a sericite 

schist from a clastic arkose on the one hand, and from a massive granite or quartz-porphyry on the other.3

In an admirable paper on the paragenesis of certain ore deposits, A*. 

von Groddeck has shown that the rocks, heretofore considered as talcose, which occur near Holzappel and Werlau on the Rhine, at Mitterberg in Salzburg and at Agordo in the Venetian Alps, are in reality 

sericite schists. These he considers to represent the extreme phase of 

metamorphism, in some cases of an eruptive rock (diabase), and in other 

cases of a normal clay state or graywacke.4 It seems most probable 

that these rocks owe their origin to dynamic agencies, with which the 

deposition of the ore also stands in close relationship.

The alteration of the feldspathic groundmass of quartz porphyries 

to muscovite or sericite, especially where these have been rendered 

schistose by pressure, is now well known. In this way porphyroids 

may be derived from massive eruptive rocks. In the more massive 

quartz porphyries studied by 0. Schmidt from the central Alps (Windgiillen), sericite is met with as an occasional pseudomorph after tbe 

feldspar, but in the schistose porphyries derived from the latter by pressure and stretching, this mineral is much more abundant; so abundant, 

indeed, as, in some instances, to make up, along with quartz, the whole 

mass of the rock.5

1 Zeitsclir. Kryst., vol. 4,1880, p. .

2 Untersucbnnjrcii iiber die Eutstehung tier alfckrystallinischen Schiefergostoine. Bonn, 1884. Cap, 

IX. Druckschieferung and Glimmerbildnng, p. 136.

8 Ibid., p. 101.

*Znr Kenntniss einiger Sericifcgesteine, weloho neben nnd in Erzlagerstatten auftreten. 

tTahrbnch fiir Mineral., Beilago-Band 2,1883, pp. 72-138. ' ' ' "''

Jahrbuch fijsr Mineral, Bpilage-Band 4,1886, p,438,
WILLIAMS.] LEUCOXENE. 63

Alterations of titanic iron. The element titanium exists in unaltered 

diabases almost exclusively in combination with iron as ilmenite or as 

titaniferous magnetite. In the processes of metamorphism, however, 

various other titanium compounds are formed. The most common phase 

of this change consists in the development of a gray rim around the 

the ilmeuite grains. This substance is also often seen along the rhombohedral cleavage cracks of the ilmeuite, giving rise to the well known 

gridiron structure so often figured.1 There can be no doubt that this 

gray material is an alteration product of the ilmenite, although Giunbel, 

who gave it its name, leucoxene, regarded it as a parallel growth of 

another mineral.2 Much difference of opinion formerly prevailed in* 

regard to the chemical nature of the leucoxene. Zirkel at first regarded 

it as iron carbonate,3 Cohen as titanic oxide,4 Kosenbusch as anatase.5 

Giimbel regarded it as probably a titanosilicate, while both Fouque" 6 

and Michel-Le'vy 7 thought that it was a form of sphene. The identity 

of both leucoxene and titano-morphite (another similar substance 

described by von Lasaulx around the rutile of an amphibolite from 

Lampersdorf in Silesia)8 with titanite or sphene was first conclusively 

proved by A. Cathrein in. 1882.9

The frequent passage of the dull gray leucoxene riin into clearly defined aggregates and crystals of sphene is mentioned by Cathreiu, 10 and 

the origin of sphene by the alteration of ilmenite is also described by 

W.» C. Brogger.11 Fine examples of this origin for sphene are also 

described and figured in the sequel.

Anatase has been described as an alteration product of ilmeuite, by 

Diller. 1 *

The rutile needles frequently seen about ilmenite, either with or 

without leucoxene, are regarded by Cathrein as an original intergrowth 

of this mineral with the ilmeuite. 13 Further reference will be made 

hereafter to needles of this character occurring in some of the Menotninee greenstones.

'Cf. Vall6e do la Poussin ct Ronard: Roclios plutoniennos do la Bolgique, etc., 1874, PI. VI, Figs 3t 

and 32. .

"PalaoolithischoErnptivgesteine des Fichtelgebrigoa, Munich, 1874, p. 35. 

3 Mikroskopischo Bescliaffonheit cl.Min. u. Geat., 1873, p. 409. 

4 Er]autcrndo Bomorluingen z. d. Koutonkarto, etc., 1875, p. 55. 

5M.ikros. Physiog., 1st cd., vol.2, 1877, p. 330. 

r'CoursdoColl6ffO do France, 3877. 

'Bull. Soc. Geol. France, 3d scries, vol.6, 1878, p. 103. 

"Xouos Jalirlmch fur Mineral., 1879, p. 068. 

"Zoitschr. TCryst. u. 'Min., vol. 0,1882, p. 244. 

'"Ibid.

" DTyt Mag. for Natiirvidonskaberne, vol. 27,1881, p. 359. 

"Neiies Jahrbuch fur Mineral, 1883, vol. 1, p. 193. 

'» Zeitschr. Kryst. u. Min., vol. 0, p. 256.
CHAPTEE II.

GREENSTONE BELTS OF THE MENOMINEE IRON DISTRICT. 

INTRODUCTORY AND HISTORICAL.

The exact geographical position of the two greenstone-schist areas 

of the Menoininee region are fully indicated in the explanatory and 

historical note by Prof. Irving at the beginning of this memoir 

and upon the accompanying geological map of this district (PL II). 

The same note explains and discusses the different views which have 

been held by various geologists in regard to the stratigraphical position 

of' these greenstone schists in the general succession of the stratiform 

rocks of the Menoininee region.

A brief mention of the opinions which have already been published 

regarding the petrographical character of these greenstone schists 

is, however, desirable.

The first attempt to describe the Menominee greenstones and greenstone schists lithologically was made by Hermann Credner in 1869, 

from observations collected while he was acting as assistant to Prof. 

Puinpelly on a survey for the Portage Lake and Lake Superior Ship 

Canal, during the years ISGT-'GS. 1 In his first paper entitled Die vorsilurischen Gebilde der oberen Halbinsel von Michigan in Nord-Amerika? 

Credner gives a section of the strata of the Menominee Iron Kegion, 

commencing with the oldest member, from which the following is abbreviated :

. Feet. 

a. Quartzite....................................... ........ 8,000

I. Crystalline dolomitic limestone. ......................... 3,500

c. Redlronore ............................................ 600-1,000

d. Chlorite schist .................... ......................1,000-1,500

e. Clay slate................... ............................ 8,500

/. Chlorite schist with diorite .................... .......... 1,200-1,400

g, Ji, i, k. Talc schist...................................... 150

1. Dioritic series .....................;...................... 2,300

m. Talcose clay slate ....................................... 1,500

The members / and I are evidently parts of the same formation, in 

which the beds g, h, *, and & are interstratified. In his description of 

/, Credner says:

In the upper horizon of this series there occur beds of fine and coarse grained 

diorite and aphanite, varying iu thickness from ten feet to several hundred. These

1 See Geology of Michigan, vol. 1,1873, p. 157, note. 

"Zeitschr. Deutach. geol. Gesell., vol. 21,1869, pp. 516-554. 

64
WILLIAMS.] WORK OF CREDNER AND BROOKS. 65

rocks consist mainly of dark greon hornblende and white or pale green oligoclase, 

with which granular or scaly chlorite is often associated. Crystals of pyrite and magnetite are also frequent. Indications of a lamellar parting are frequent, while rectangular jointing is rare. 1

In the same article, in speaking of the member I, the author says :2

Dioritic series 2,300 feet in thickness. Mainly a fine grained or aphanitic, rarely 

a coarsely crystalline aggregate of the constituents of diorite. All contain crystals 

of pyrite; the aphauite contains veins of calcite and quartz. 3

In a second article entitled Ueber nordameriJcanische ISchieferporphyroide, which also deals with certain rocks exposed on the MeuomineeBiver,4 Credner unites his beds /and I as "chlorite schist alternating 

with intercalated diabase;" while his g, h, i, and Tc are described as a 

series of " porphyroid-schists," 300 feet in thickness, between two beds 

of diabase.

In the year 1869 the geological survey of Michigan was inaugurated 

with the iron-bearing formations of the Upper Peninsula, in charge of 

Maj. T. B. Brooks. The results of his labors appeared in 1873,5 accompanied by two appendices, containing special petrographical descriptions of the rocks collected by Julien,6 and Wright.7 Several of the 

Menomiuee greenstones, both massive and schistose, come within these 

descriptions, but the descriptions are for the most part vague; as, for 

instance " diorite," "dioribic schist," " porphyritic diorite," etc.

In 1874 Major Brooks continued his studies of the Menominee region 

under the auspices of the Wisconsin Geological Survey. The results 

of this work appeared in 1879 8 and contained much more elaborate 

and accurate determinations of the greenstones and greenstone schists. 

Major Brooks gives a diagram9 to illustrate his general views of the 

observed transitions of greenstones (believed to be mostly metamorphosed sediments) into related rocks. Dr. Wichrnann, who furnished 

the systematic petrographical descriptions of these rocks in Brooks's 

report,10 however, regarded these rocks as for the most part eruptive 

aad to a large extent as derived from diabase. Special reference will 

be made to the descriptions of Wichmann and others who have worked

1 Zeitschr. Deutsch. geol. Gesell., vol. 21, 1869, p. 528.

2 Iin oberen Horizon to dieser Schichteureihe treten voii 10 bis inehrere bundert Fuss machtige Eiulagerungen von feiii- bis grobkornigein Diorit sowie von Aphanit auf. Sie beetebeu vomaltend aus 

dunkelgriiner Hornblende und weissem oder hellgriinem Oligoklas, wozu sicb an inauchen Puukteii 

viel kornig-schuppiger Chlorit gesellt. Einsprenglinge YOU Scbwefelkies und 'Magneteisensteiu siud 

in ihneu haufig. Andeutung von plattenforrniger Abaonderung iat gewohnlich, quarderformige Absonderung selten.

 Ibid., p. 529.

4 Dioritiscbe Gesteinsreihe von 2,300' Hachtigkeit. Vorwaltend ein feinkoruiges oder aphanitisches, 

eeltener ein grobkrystallinisches Geinenge der Bestaudtheile deu Diorites, alle mit Schwefelkies ein 

ein sprenglingen, die Apbanite mit Scbniiren vou Kalkspath und Quarz.

6 Heues Jabrbucb far Mineral., 1870, pp. 970-984.

6 Geol. Survey Michigan, vol. 1, Upper Peninsula. Part I. Iron-bearing rock, by T. B. Brooks. 1873.

7 Ibid., vol. 2, 1873. Appendix A, Litbological descriptions, etc., of 259 specimens of tbe Huroniau 

and Laurentian of the Upper Peninsula, by A. A. Julien, pp. 1-197.

8 Ibid., vol. 2, 1873. Appendix C. Microscopic determinations aud descriptions of 78 specimens of 

Hurouian rocks and ores, by C- E. Wright, pp. 213-231.

9 Geol. Wisconsin, Vol. 3, 1873-1879. The Geology of the Meuominee Iron Region, Oconto County, 

by T. B. Brooks, pp. 429-663. Ibid., Geol. Menominee Eegiou by C. E. Wright, pp. 665-741.

'"Ibid.. 519. "Ibid., pp. 600-656.

Bull. 62 5
66 GREENSTONE SCHIST AREAS OF MICHIGAN. [BULL. 62.

upon the petrography of the Menominee greenstones in the course of 

the following chapters of this work.

All observers seem to be agreed that the association of the massive 

greenstones with those which are more or less perfectly schistose is an 

extremely intimate one. Even Major Brooks, in spite of his idea that 

most of the greenstones are altered sediments, is constantly suggesting 

the possibility of a mechanical origin for the intercalated schistose layers. He says 1 of the Twin Falls exposure:

This slaty material may be simply a compacted form of the pulverized greenstone 

produced at the time of fisauring.

Again,2 in speaking of the Commonwealth and Eagle Mine rocks:

It will be observed that these rocks have some resemblance to aa, but it is believed that they are associated schistose and altered varieties of the great greenstone 

bed XVIII, so extensively developed to the north and east.

He says 3 also:

An unevenly splitting, gray-green schist associated with greenstones and apparently derived from them by alteration of the amphibole, is included in this family 

[chlorite schist], although its origin and associations are widely different from the 

above.

Again: 4

One of the largest, most generally distributed, and at the same time, obscure varieties embraces those chloritic rocks that are associated with greenstones.

Dr. C. Eoininger, in his Report on the Geology of the Menominee 

Iron Region,5 is equally impressed with the close association of the 

massive and schistose varieties of the greenstones. Like Brooks, he 

regards them as of sedimentary origin, and thinks that the massive 

portions are the result of extreme metamorphism, having recrystallized 

from a partially fused state.

There seem to be only three different ways of explaining the facts as 

they are plainly and abundantly manifested at each of the several points 

examined on the Menominee River.

We may suppose, as did Foster and Whituey, Whittlesey and Credner, that the schistose portions of the rocks exposed at these several 

points are more or less perfectly metamorphosed sedimentary material, 

while the more massive portions represent intercalated beds of eruptive 

origin.

We may imagine, on the other hand, as did Rouiinger and Brooks, 

that all of these rocks, including the most schistose and the most massive phases, are probably altered sedimentaries; supposing, with Rominger,that the more massive kinds are merely the same sedimentary material fused by the intensity of the metamorphosing action so as to lose all 

trace of the original stratification; or admitting, with Brooks, the possibility, though great improbability, that some 6T the more massive 

phases like the gabbro of Sturgeon Falls, are of eruptive origin.

1 Geol. Wisoouaiu, vol. 3,1873-1879. The Geology of the Menominee Iron Region, Ocorets County, by 

T. 15. Brooks, p. 477.

'Ibid., p. 484. 3 Ibid., p. 516.

«lbid., p. 518. 6 G-eol. Michigan, vol. 4, 1881, p. 209.
WILLIAMS.] STURGEON FALLS GABBRO. 67

Finally, we may imagine that all phases seen at these several places 

represent material of eruptive origin whose stratiform structure is due 

to secondary dynamic agencies. This last view has heretofore been 

advanced only by Irving, who, however, has presented it very briefly, 

and without descriptions or petrographic proof.1

It is the aim of the present portion of this paper to set forth all the 

evidence that a careful examination of these rocks can furnish as 

to which of these three views best explains their true origin and mutual relations. This material will first be presented in the form of a 

detailed description of each of the five localities selected for this purpose on the Menomiuee Eiver (see PI. II). The general results thus secured will be summarized, in connection with those obtained elsewhere, 

in Chap. VI. The Menominee localities will be treated of in regular 

order, commencing with the one furthest down the river, Sturgeon Falls.

STURGEON FALLS.

Kinds of rock. This exposure is situated a short distance below the 

mouth of the Sturgeon River, in Sec. 27, T. 39 N., R. 29 W., Michigan. Its 

general topography may be seen from the accompanying map (PI. Ill), 

which is copied from PL I, in Major Brooks's report with double his linear 

scale. Only the rocks exposed on the eastern or Michigan side of the 

river were examined; but those opposite could be seen to be of the 

same character.

The barrier which here forms the falls consists of two belts of massive rock,/and h, between which are softer schists, g. These are also 

intercepted by a harder and more massive band.

The massive rock is light in color and of a comparatively coarse grain. 

It is quite elaborately described in Major Brooks's report,2 and designated on the authority of Wapler, Rutley, and Pumpelly as a gabbro. 

Julien also examined this rock, but seems to have mistaken the diallage 

for hornblende, and the brown pleochroic hornblende for biotite. Rutley's opinion is given with hesitation. Pumpelly's description is brief 

and accurate. He says : 3 

Saussurite-gabbro (or hornblende gabbro): contains saussurite, diallage, bornblende. Identical under the microscope with the coarser crystalline rock of Upper 

Quinnesec Falls.

This rock was considered serpentine by Foster and Whituey4 and 

was provisionally named "porphyritic, speckled diorite" by Brooks, 

who separated it as his Bed XV of the Huronlan.

The massive occurrence at/is represented by Nos. 11154,11155, and 

11162 j that at li by Nos. 11167,11168,11169, and 1117o! Nos. 11156- 

11161, and 11163-11166 were collected from the softer schistose bands 

at g.

These rocks are particularly noteworthy as being the only ones discovered on the Menominee River which contained any trace of pyroxene.

1 Fifth Annual KeportU. S. Gool. Survey, p. 190. s Geol. Wisconsin, vol. 3, pp. 45C-663. 

Ibid8 ., p. 504. " lleport on the Q-eol. Lake Superior Land District Plate 2,, p. 25.
68 GREENSTONE SCHIST AREAS OF MICHIGAN. j BULL. 62.

We shall find that they agree perfectly with Pumpelly's description 

above cited, the hornblende being in part secondary, and they are hence 

to be designated as saussurite-gabbro. The least altered specimens were 

found at li (Nos. 11167-11170). The rock at/, although essentially the 

same as at /t, is more changed and somewhat schistose along certain 

bands, while some of the specimens found at g were clearly once the 

same as the others, but now represent a third and much more advanced 

stage of alteration. I shall first describe these three stages of the 

undoubted gabbro, and then trace their relation to the intermediate 

schists.

Saussurite-gabbro (first stage). The Color of this rock in the hand, 

specimen is rather light. On a fresh fracture it shows a finely mottled 

structure of white and dark greenish-gray. Examined with a pocketlens, the mottling is seen to be due to two minerals. One of these is 

opaque white, sometimes tinged with green, and only rarely exhibits 

the glistening cleavage surface of feldspar. The darker mineral has a 

brownish color and almost a metallic luster on its cleavage surface. It 

is, however, always tinged with green on account of incipient alteration.

No other constituents are microscopically visible. The structure of 

the rock is irregularly granular. The grain is as a rule moderately flue 

but is subject to sudden local variation which develops comparatively 

coarse-grained patches in the main mass.

Of this freshest type of the gabbro four thin sections were studied, 

viz, 11167, 11167%'11168 and 11170. All show profound alteration iii 

the original constituents, due to dynamic processes. These are, however, most intense in No. 11167, which has been subjected to such great 

pressure as to present a transition form to the second stage of this 

gabbro. Nos. 11167a, 11168, and 11170, are practically identical and 

may be described together,

Under the micros cope this rock is seen to be composed of plagioclase, 

almost wholly altered to a gray and opaque saussurite; diallage, of a 

very light gray color; hornblende, partly original, partly secondary, and 

a little titanic iron (ilmeuite). Certain alteration products, like quartz, 

calcite, and a colorless chlorite, are also present in varying quantity.

The feldspar has no crystal form of its own (i. e., it is allotriomorphous, in the sense of Eosenbusch). It seems to have crystallized simultaneously with the diallage, or if anything later. It is hardly ever 

so unaltered as to present its original twinning-larnellse, but when this 

is the case the high extinction angles and brilliant interference colors 

observed indicate a basic feldspar. No further determinations could be 

made on this mineral because of its almost complete alteration to a 

dull opaque white saussurite. Even in the thinnest sections this substance remains opaque for the lower powers of the microscope, being 

composed of such fine grains that it must be highly magnified before it 

can be resolved. Under higher powers, however, it is seen to be made
U. S. GEOLOGICAL SURVEY BULLETIN NO. 62 PL. Ill

MICHIGAN 

T.39JST.: R.29W.

 M. _ _L _ _ _Jw~ T"

"WISC ONS1BT 

T. 38N.;R..21.E.

V,*, ISaussuriteGabbra

MAP

of 

STURGEON FALLS

Menominee River .

(After Brooks)
WILLIAMS.] ALBITE AND ZOISITE IN SAUSSURITE. 69

up of small zoisite grains, embedded in a clearer matrix, which is probably albite. 1

This sanssurite substance is traversed in all directions by lightersometimes quite clear and transparent veins, which are composed 

largely of secondary albite. This in at least oue instance (No. 11168) 

shows the characteristic twinning striation of plagioclase (see Fig. 4). 

In other cases, however, these veins appear to be composed of quartz 

or chlorite (see PI. VIII, fig. 1).

FIG. 4. Veins filled with secondary albite in 

the altered gabbro of Sturgeon Tails (So. 

11168). Represented in polarized light in order to show the twinning striae. Magnified 

350 diameters.

FIG. 5 Zoisite crystals in a coarse sanssnrite from the altered gabbro of Stnrgeon 

Falls. Magnified 180 diameters.

In certain cases, notably in section No. 11167, the saussurite aggregate is coarser and the zoisite plainly visible and easily determinable.

Fig. 5 shows characteristic forms of this mineral. Its crystals are 

always without terminal planes, but they have a perfectly developed 

cleavage parallel to their vertical axis and a cross-jointing. Between 

crossed nicols they are seen to be twinned, sometimes polysynthetically, 

like albite. The zoisite is itself colorless, with a high index of refraction, but a weak double refraction; the interference colors are therefore 

dull, either gray or bluish, and sometimes a deep ultramarine. The extinction is parallel to the cleavage lines.

One unusually large crystal of zoisite, represented in Fig. 6, exhibits 

all of these characteristics, and also, when examined in converged polarized light, it gives a biaxial interference figure with the plane of the optic 

axes perpendicular to the cleavage. (Fig. 6.)

1 Zeitschr. Kryst. u. Min., vol. 7, pp. 234-249. Cathrein. in this article, calculates a number of 

saussnrite analyses to show the relative amounts of feldspar and zoisite or epidote present. He seems 

to think that the feldspar left after subtracting the zoisite gives the original constitution of the mineral before its change to saussurite, and attributes the alteration to the addition of CaO and A1S 03 and 

loss of silica and alkalies (p. 243). Another hypothesis would he that the original feldspar was much 

more basic (i. e., contained more CaO, A1203 and less Si02 and Xa-^O) than the one now forming the 

matrix of the saussurite, and that this compound broke up without material chemical change into two 

compounds, one a more acid plagioclase, like albite or oligoclase and the other zoisite, which would 

contain what CaO and superfluous AI203 was left after the change. A similar separation of one mineral into two has been traced by Profs. J. Brush and E. S. Dana in the alteration of spodumene into 

a.minute aggregate of eucryptite and albite. (Am. Jour. Sci., 3d series, vol. 20. p. 257 et seq., 1880.)
70 GREENSTONE SCHIST AREAS OF MICHIGAN. [BOLT.. 62.

The diallage of these rocks is quite colorless when seen in a thin section, although in the form of powder it is a light grayish green. The 

prismatic cleavage is well developed, but not as much so as the very 

perfect parting parallel to the oi'thopinacoid, which is the characteristic feature of diallage (PI. VIII, fig. 1). The extinction angle was found 

to be as great as 35° in the prismatic zone. The plane of the optic 

axes is perpendicular to the orthopinacoidal parting, and a single axis 

appears in such sections as show a prismatic angle of nearly 90°. This 

diallage is very sensitive to the action of pressure and is frequently 

much bent and twisted. The mineral is sometimes separated along 

cleavage cracks, the interstices being filled with a colorless, isotropic 

alteration product, which is probably some variety of chlorite. This 

substance becomes much more abundant in the more altered forms of 

this rock and will be again mentioned. The diallage is also often traversed by veins filled with clear quartz.

FIG. 6. Large crystal of zoisitein sanssurite 

of No. 11167. Altered gabbvo from Sturgeon 

Falls. Magnified 350 diameters.

FIG. 7. Border of both compact and fibrons 

hornblende around diallage in the Sturgeon 

Falla gabbro (No. 11G7»). Magnified 180 diameters.

The hornblende is an important and constant constituent of the 

Sturgeon Palls gabbro. It is for the most part compact, and brown in 

color, having the usual pleochroism. More rarely it is green, or apparently bleached to a lighter shade of brown. In a few instances 

this latter process has been carried so far as to make it quite colorless. 

The hornblende is in all cases most intimately associated with the diallage. At times it forms a continuous border of varying width around 

the diallage crystal, and in other instances it penetrates the latter 

mineral in irregular areas ( see PI. VIII. fig. 1). In some cases a delicate fringe of minute, colorless hornblende fibers is seen to lie on the 

outer side of the compact hornblende border and penetrate the saussurite substance (see Fig. 7). This is the same phenomenon that has 

recently been described by Lessen. 1

'Studien an metamorphischen Ernptiv- and Sedimentgesteine, etc., of. Jahrbnoh preuss. geol. Landesanstalt for, 1881,-p. 543, PI. XXIX, fig. 2. (Cf. Van Hise: Am. Jour. Sci., 3d aeries, vol. 33, 1887, pp. 

385 et seq.
WILLIAMD,! HORNBttENDE AND CHLORITE IN GABBRO. 71

It is impossible to say with certainty what the origin of the compact brown hornblende is. It may be an original product of crystallization in the magma, but its mode of occurrence and its general 

character indicate that it is probably of secondary origin, having 

originated by paramorphism of the diallage, as has been shown by myself1 and by Lessen2 to be sometimes the case.

Iso other constituents are important in this rock. Iron in any form 

seems to be quite rare in it. Occasional cloudy grains, resembling 

leucoxene, may represent some original ilmenite, while minute crystals 

of pyrite are still rarer.

The structure of the rock seems to be irregularly granular; none of 

the components being in any degree idiomorphic. Frequent ami 

abrupt changes in the coarseness of the grain are observable.

All specimens show the action of dynamic forces which have more or 

less profoundly affected the different constituents. These effects are 

particularly noticeable in slide No. 11167. Here the rock seems in 

places to have been crushed and a mosaic of the component minerals 

to have been formed. Hornblende, generally colorless, is unusually 

abundant. Colorless chlorite and zoisite are also developed, and all are 

mixed indiscriminately. In one part of the section a vein is seen to 

traverse the rock. This is filled with limpid quartz in long, wedgeshaped areas, which extend from one side of the small fissure to the 

other. This quartz is traversed by 

long, colorless fibers of the greatest 

delicacy, and it also contains a good 

deal of the colorless chlorite, both in 

solid masses and in those peculiar 

vermicular groups to which Volger 

has given the name helminth. These 

curious groups, which resemble piles 

of little coins, are sometimes straight, 

sometimes curved. They are so minute as to be visible only with a high 

magnifying power. Fig. 8 represents 

them as they appear when magnified 

350 diameters. Exactly the same min- Fl<T 8'-Vcrmicular cWmito (helminth) in

J quartz. Vein in altered gabbro, Sturgeon

eral has been described in the sec- Fails.

ondary quartz of diabase by Mohl, 3 von Lasaulx,4 and Schenck;5 and

in that of schalstein and syenite by Hussak.6

Saussurite gabbro (second stage). Representatives of this type are 

found at the lower end of the falls, at / on Major Brooks's map, and

'Am. Jour. Sci. 3d series, vol. 28, Oct., 1884, p. 262.

2 cf. Jalirbuch preuss. geol. Landesanstalt fur 1883, p. 632.

3 Neues Jahrbuch fur Mineral., 1875, p. 716.

4 Verb. d. naturh. Vereins d. prenss Ebeinl. a. Westf., 1878, p. 216.

5 Inaugural-Dissertation. Bonn, 1884, p. 22.

6 Tschermak's mineral, n. petrog. Mittheil., vol. 1, 1878, p. 275.
72 GREENSTONE SCHIST AREAS OF MICHIGAN. lnuu.-82.

at his e, at the lower end of the basin below the falls. Nos. 11153 and 

11154, from these two localities, are identical in mineral composition 

with the rocks of the first type above described. They are of a dirty 

greenish gray color and indications of a schistose structure have already 

begun to appear in them. Under the microscope they disclose the 

same constituent minerals as the rocks*of the first type; nor are these 

minerals much more altered, but they show the effects of a profound 

mechanical action. The feldspar is remarkably fresh and its twinning 

lamellse are quite distinct, but it is everywhere crushed, broken, and 

faulted. The crystals are often plainly seen to-be separated into a 

number of fragments which are removed a considerable distance from 

one another. Frequently a fine grained mosaic has been formed by 

the crushing of the larger feldspar crystals. In other cases (especially 

in section 1115-1) the feldspar is not so much broken, but it is altered 

around its edge to an opaque, gray saussuritic mass, while its interior is 

hardly changed. (See PI. VIII, fig. 2). The diallage is more altered 

than in the rocks last described, although it can still be seen to belong 

to the same species. The crystals are very much bent and twisted and 

frequently so changed to the light-colored chlorite that only a few 

minute remnants of the brightly polarizing mineral remain in this 

nearly isotropic base. (See PI. VIII, fig. 2.) Fibrous hornblende now 

becomes more abundant than the compact, and leucoxene patches are 

seen at intervals.

Kos. 11162, from the upper part of the point /, and 11172 from above 

the falls at «, present interesting varieties of this rock. They do not 

show the effects of crushing to such a degree as do those just described. 

Indeed the original structure of the rock seems well preserved, although 

the feldspar is almost completely changed to a fine grained, nearly 

opaque sanssurite, and every trace of the pyroxene (diallage) has disappeared. A pale green, fibrous hornblende occupies the place of this 

mineral, and there seems to be every reason for regarding it as derived 

from the diallage; although in No. 11172 the abundant remains of a 

compact, brown hornblende, evidently in the process of changing to 

the fibrous modification, suggests the possibility of this mineral having 

been the original bisilicate constituent. Still there is no warrant for 

assuming that what is essentially a diallage rockmass was locally developed as a hornblendic rock; and it may be that this is an instance 

of a remarkable passage of a pyroxene through a compact into a fibrous 

hornblende, for which we shall find much evidence in the studies that 

follow.

Saussurite gabbro (third stage). As has been already remarked and as 

may be seen on the map of Sturgeon Falls (PI. Ill), there is, between 

the two points of the more massive rock, h and/, an area g, composed 

of softer schists. These are easily eroded and have thus occasioned the 

small bay. The passage from the fresh and massive gabbro at h into
WMJAM8.] EXTREMELY ALTERED GABBRO. 73

these schists seems to be a gradual one. Along the lower side of h 

there are rocks which are without doubt essentially the same as those 

above described from this point but which have been profoundly altered 

by dynamic action, which has also occasioned considerable chemical 

changes. These rocks, while they show remains ot their original minerals, have become decidedly schistose; and by the alteration in their 

composition, they have developed certain other points of similarity 

with the schists at #, into which they appear gradually to pass. For 

this reason they have been selected to represent the third and most 

altered stage of the gabbro in which any sure signs of the original 

character of the rock still remain; at the same time they possess even 

more of the characters belonging to the schists, and thus afford just 

the evidence desired that the latter are the extremely modified form of 

the massive rock.

No. 11166 is.only indistinctly schistose, showing a soft grayish green 

matrix in which are imbedded altered grains of a reddish feldspar. 

Under the microscope the original coarsely granular structure of the 

rock may still be made out. Large but much broken areas of feldspar 

may be clearly seen where but very little of the unaltered substance 

remains. For the most part this has been changed to an aggregate of 

calcite and minute, brightly polarizing needles or plates of a colorless 

micaceous mineral (probably sericite) along.with occasional areas of 

secondary quartz. What was once the pyroxene or hornblende is now 

a colorless or extremely pale green, scaly mineral which an examination 

shows to be chlorite. Between crossed nicols it is isotropic or very 

feebly polarizing. It was separated by the Thoulet solution from the 

slightly heavier feldspar and as a powder appears pale green. In a 

closed tube it gives off water abundantly and becomes dark. With hot 

sulphuric acid it decomposes and gelatinizes immediately; with hydrochloric acid it decomposes slowly, giving a somewhat yellowish solution, 

from which ammonium-hydroxide precipitates alumina abundantly. 

There can therefore be no doubt that this mineral is a chlorite very poor 

in iron', like the nearly colorless pyroxene from which it is derived. 

Areas of leucoxene are also common through this rock.

No. 11165, taken from a place only a foot or two below the last and 

undoubtedly continuous with it, is decidedly more schistose. Under 

the microscope the minerals are essentially the same as in the last 

specimen, but the mechanical effects here exhibited are much more 

intense. The feldspar and the diallage are still recognizable, but the 

colorless chlorite is much more abundant than in any specimen before 

described. The feldspar areas have a brownish color, even in the thin 

section and they are pulled and torn asunder in a remarkable manner. Individuals are often pulled out into more than twice their original length, the separation taking place along a series of interlacing 

cracks which run approximately perpendicular to the schistose structure
74 GREENSTONE SCHIST AREAS OF MICHIGAN. tuuLL.62.

of the rock. The spaces between the separated fragments are filled 

with the chloritic mineral. This rock shows every indication of having 

been enormously stretched and this process, along with attendant 

chemical changes, has induced its schistose structure. Although it 

shows under the microscope traces of the original gabbro structure and 

of the original gabbro minerals, no one would hesitate from a microscopical examination to class it with the schists.

The schists above mentioned as occurring between the two masses 

of typical gabbro, li and/, at the Sturgeon Falls, possess an important 

relationship to these rocks. Taken by themselves, they offer as representative examples of fissile, silky, hydro-mica or sericite schists as 

could anywhere be found, and yet this narrow band presents such a 

complete series of transition forms that their origin as derivatives of 

the massive rock cannot be doubted. In fact, two such series were 

traced out in detail. One of these was collected on the lower side of 

h and represents the passage of the least altered and most massive 

gabbro into the typical schist, and the other was obtained near the 

center of the schist band where a narrow strip of the rock has happily been preserved from the extremest alteration and plainly 

shows its identity with the gabbro proper. This narrow layer, represented on the map at g, is of great value in conclusively proving the 

origin of the schists in the midst of which it lies and into which it 

passes on both sides.

No. 111(50, from this locality (0), was classified in the field-notes with 

the schists, and regarded as only a harder and more slaty form of these. 

Indeed, this rock is very schistose, cleaving readily in a direction parallel to the foliation of the more fissile schists. Under the microscope, 

however, an original gabbro structure is still distinctly seen. Triclinic 

feldspar, in broken and bent crystals, is plainly visible, while the other 

constituents have passed into colorless chlorite, quartz and calcite. 

The feldspar itself is mostly altered into the same compounds, though 

its form and occasionally its twinning strisB are still preserved.

No. 11161 is a hard, greenish slate, occurring beside the last described 

rock. Under the microscope, in ordinary light, crystal forms are seen 

outlined in a dark color, the matrix being colorless. In polarized light, 

however, these are no longer visible, and there is only a fine grained 

and schistose aggregate of colorless chlorite, quartz and calcite.

Nos. 11159 and 11158, collected just below the last, are typical schists 

of a light gray color and somewhat greasy feel. They contain just the 

same minerals as the rocks last described, but here every trace of original structure has disappeared. There is no indication of the form of 

any earlier constituents, but those now present are arranged in a finely 

parallel mosaic of somewhat varying grain. Calcite is much more 

abundant than in the previous instances. Indeed, in No. 11158 this 

mineral makes up a large share of the rock.

The other series of transition forms between the gabbro and the
WILLIAMS. J SCHISTS DERIVED FROM THE GABBRO. 75

schists occurs on the lower side of the gabbro mass 7t. This series embraces the specimens of the first, second, and third stages of the gabbro 

already described from this exposure, and terminates with two specimens of schist, Nos. 11163 and 11164. Any sharp line of demarkation 

between the typical and massive saussurite-gabbro at li, and the fissile, 

silvery schists, apparently so' different, lying below it, was sought long 

and carefully, but in vain. In fact no such line exists, and if one were 

to be drawn it must be drawn arbitrarily. The rocks already described 

from this locality as specimens of the gabbro in the third stage show 

this. No. 11165 is entered in the field-notes as belonging to the 

schists, but microscopical examination shows it to be even more closely 

allied than the almost massive No. 11166, to the gabbro. Traces of the 

original components and structure are by no means rarest in those 

rocks which appear macroscopically to have suffered the most profound 

alteration. ,,

Nos. 11164 and 11163 are light gray, silvery schists, with a somewhat 

greasy feel; such in fact as might ordinarily be classed as sericite or 

hydro-mica schists. Their lamination, however, is not in the least regular, but undulatory, and there is a tendency to cleavage not so much 

along a single plane as in almost any plane parallel to a line. In other 

words, these rocks have a well marked dip, but hardly any determinable strike. In 11164 especially, there is noticeable a jointing transverse to the strike, and these joint planes often gap open into wide, lenticular seams, as though the rock had been stretched in the direction 

of its strike.

Under the microscope these rocks are seen to be mostly composed of 

the nearly colorless chlorite, oalcite, and a little secondary quartz. 

These minerals are interlaced in narrow, wavy bands, producing what 

may be called after the German idiom, along"micro-flaser" structure. 

But even here every trace of the original constituents has not yet disappeared. Occasional battered and broken feldspar remnants are encountered, with their fragments widely separated, and for the most part 

changed to sericite or calcite. Around these curve and twist the silky 

chlorite bands developing a sort of "Augen" structure, which may be 

best seen by examining a section with an ordinary pocket lens. The 

areas of leucoxene also, before observed, are not lacking here, but 

these are pulled out in the direction of the schistose structure, until a single one may be followed nearly across an entire section. The 

bisilicates have evidently all passed into the colorless chlorite, and this 

mineral on account of its pliability has easily accommodated itself to 

the circumstances, and has developed the pronounced schistose structure which is now the most important feature of the rock.

These rocks are schists, indeed, of the most characteristic type, but 

in the light of their field relations and still more from the evidence 

which a microscopical study of the whole series has afforded, it is evident that they represent the most altered form of the massive gabbro, 

between two areas of which they are included.
76 GREENSTONE SCHIST AREAS OF MICHIGAN.

Chemical analyses. In order to ascertain what changes have taken 

place in the chemical composition of the Sturgeon Falls gabbro during 

the process of its alteration into the schists, the following analyses of 

specimens illustrating three successive stages of the alteration were 

made by Mr. E. B. Riggs :^

Si02 ..............

A120S ........ .....

Fe2O3 . ............

FeO...... ........

CaO...... ........

MgO. .............

Na-jO... ..........

KjO. .............

H«0...... ........

COZ . ..............

Total.......

I.

51.46

14.35

3.90

5.28

9.08

9.54

2.92

.24

3.30

.20

100. 27

jr.

38. 05

24.73

5.65

6.08

1.25

11.58

2.54

1.94

7.53

.93

100.28

ni.

45.70

16.53

4.63

 3.89

4.28

9.57

.55

. 3.82

4.70

5.95

99.62

Rock powder dried .it 105° C.

I. No. 11170, freshest gabbro from the point h.

II. No. 11166, gabbro in third stage from the south side of h. 

III. No. 11164, silvery schist from between g and h.

At first glance these analyses seem to present a curious anomaly. 

The intermediate rock, No. 11166, appears to differ more from the original type than the most altered specimen, No. 11164. A closer examination, however, shows that the nature of the chemical processes which 

have gone on in the two cases is essentially different. In the first case, 

No. 11166, this has been chiefly chloritization, while in the second case, 

No. 11164, the chloritization has been less, but sericitization and carbonatization have also been extensive.

The abundance of the colorless chlorite, above described as present 

in No. 11166, here manifests itself in the very low percentage of silica 

and in the correspondingly high percentage of alumina, together with 

the increase of both magnesia and water. The iron is unusually low 

in this chlorite because it did not exist in the mother rock; still there 

has been a gain rather than a loss of this ingredient. The lime has 

here been to a great extent removed, presumably in the form of the 

soluble bicarbonate, while the slight sericitization, above mentioned in 

the description of this rock, is indicated by the increase of the potash.

In the analysis of No. 11164, the less extent of the chloritization is 

shown by the less, although proportionate, increase of alumina and 

water and decrease of silica, while the iron and magnesia remain about 

the same as in the mother rock. Here the lime has been partly removed as before and partly retained as calcite, visible everywhere 

through the rock and indicated in the analysis by tbe 5.95 per cent of 

CO2 . The sericitization, so apparent under the microscope, is here 

shown by the great increase in potash and the exchange of the soda 

for potash.
WILLIAMS.] LOWER QUINNESEC GREENSTONE. 77 

LOWER OR LITTLE QUINNESEC FALLS.

The rock exposure of Lower Quinnesec Falls is not confined to the 

immediate vicinity of the falls but it extends, especially on the left 

bank of the river, for a considerable distance below. This portion 

forms a high, abrupt and homogeneous ridge, of uniform composition 

and structure, the description of which may be advantageously separated from that of the rocks occurring immediately at the falls.

We shall, therefore, consider under two successive heads:

(1} The greenstones of the so-called " Oabbro Ridge" of Major Brooks.

(2) The greenstones of the Lower Quinnesec Falls.

Greenstones of the so-called "Gabbro Ridge" of Major Brooks. For a 

mile below Lower Quinuesec Falls, the north batik of the river is skirted 

by an almost perpendicular wall of massive greenstone. This wall is 

sometimes over a hundred feet in height, and has every appearance of 

being a great dike. Although for the most part it is quite massive, it 

presents frequent and very instructive evidence of the effect of great 

pressure upon it. It is seamed and gashed, broken and torn, and contains schistose bands of varying width. Siuce the continuity of these 

bauds with the massive rock is established, their study is calculated 

to throw light on the subject of dynamical metamorphism.

Major Brooks designated the rock which composes this ridge as a 

4< massive gabbro," and correllated it with the above-described saussurite-gabbro of Sturgeon Falls. My studies have, however, failed to 

disclose in this rock any trace of pyroxene. In addition to its feldspathic constituent, which is generally altered to saussurite, it contains 

in abundance that peculiar pale green and more or less fibrous variety 

of hornblende which is quite universally conceded to be of secondary origin. What the primary form of all this green hornblende was, it is now 

impossible to ascertain with certainty. It is of a kind well known to 

originate from the alteration of pyroxene. The rock as a whole also 

bears decidedly the character of a diabase or pyroxene rock; and yet, 

not a trace of pyroxene has been discovered in any of the Menomiuee 

River greenstones, if we except the light colored diallage of the Sturgeon Falls gabbro. Whenever th.e pale green hornblende can be traced 

back to an original form, it is seen to be derived from a compact 

brown or basaltic hornblende. This fact was substantiated not merely 

for the rock composing the ridge here under discussion, but it is also 

true for the several other localities examined on the river.

This brown hornblende is seen first to turn green by a reduction of 

its iron to the ferrous state, or to become bleached by loss of its iron, 

and finally to break up into an aggregate of fine hornblende needles 

exactly similar to that commonly formed by the alteration of pyroxene 

to uralite.

It is, of course, impossible to prove that some of the secondary fibrous 

hornblende has not been derived from pyroxene. Indeed, it seems very 

probable that both augite and compact, brown hornblende may have
78 GREENSTONE SCHIST AREAS OF MICHIGAN. [BULL. 62.

existed side by side as original constituents of the rock, and that both 

finally succumbed to the same process of alteration, although the hornblende resisted this much longer than the augite., Such, in point of 

fact, has elsewhere been observed to be the case where both minerals 

have undergone uralitization in the same rock; 1 and this would account 

for the frequent survival of brown hornblende cores where every trace 

of pyroxene had disappeared.

If this is not the true explanation of the genesis of these rocks, there 

are but two other hypotheses possible either that the original rock 

contained no pyroxene, but was a diorite, composed of plagioclase and 

brown hornblende j or, that the pyroxene has passed in to uralite through 

an intermediate compact, brown hornblende. Opposed to the first of 

these suppositions is the structure of the rock, which is diabasic rather 

than dioritic. On the other hand, considerable evidence afforded by 

various of the Menominee rocks, as well as observations made elsewhere 

by the writer2 and by Lossen,3 indicate that the latter hypothesis may 

possibly be true. Inasmuch, however, as the rocks here under discussion afford no trace of pyroxene, it hardly seems justifiable to call them 

anything but diorite. We shall encounter still more distinct examples 

of this type farther up the river.

All the specimens collected from this greenstone ridge show an 

advanced stage of alteration. Some obtained from the western extremity of the ridge, near the Michigan bank of Lower Quinnesec 

Falls, have a coarsely porphyritic structure, and are among the freshest 

specimens that were procured. This variety of the rock is represented 

by Xos. 11034,11035, and 11098. Of these the last is perhaps the most 

typical. To the unaided eye it shows a light greenish gray matrix, 

thickly studded with irregularly shaped crystals of opaque white feldspar (saussurite) from one-half centimeter to two centimeters in diameter. A closer examination reveals many brightly reflecting cleavage 

surfaces of another and darker mineral. These are of about the same 

size as the feldspar crystals, but their continuity is interrupted by small 

-opaque spots, which produce the mottled structure recently termed by 

the writer pcecilitic.* The color of these cleavage surfaces, which with 

the aid of the microscope are found to belong to large individuals of a 

somewhat altered hornblende, is so nearly like that of the groundmass 

of the rock that unless their reflection is caught in just the right light 

they escape detection.

The large porphyritic crystals of feldspar are almost wholly changed 

to saussurite. This has a dull gray color, and is nearly opaque even in 

the thinnest sections. A high magnifying power resolves it into an aggregate of minute zoisite needles. Curiously enough, these feldspar crystals show a clear zone of unaltered substance around their outer edge.

1 Bull. U. S. Geol. Survey, No. 28, p. 45.

2 Am. Juur. Sci., 3d series, vol. 28, p. 262, Oct., 1884.

3 Jahrbuch preuss. geol. Landesanscalt fur 1883, p. C32, Berlin, 1885.

4 Prom JToi/ciAos, mottled. See A.m. Jour. Sci., 3d series 1886, vol. 31, p. 30.
WILLIAMS.] ALTERATION OF THE HORNBLENDE. 79

This fresh and finely striated portion presents a striking contrast to the 

dull gray saussurite. and it is difficult to imagine how it has so completely escaped the alteration which has attacked the rest of the crystal, unless it is the product of a more recent growth.

With the exception of groups of ilmenite grains, which are beautifully fringed with a leucoxene border, the remainder of this rock is composed almost wholly of hornblende. In its freshest state this mineral 

has a compact structure and coffee brown color, with the usual pleochroism and other optical properties of the species. Its first phase of 

alteration consists in turning green, and this is followed by a gradual 

bleaching. This process continues until the mineral has become quite 

colorless, without, however, losing its compact structure. In the larger 

individuals of compact hornblende, whether this is brown, green, or colorless, occur numerous small lath-shaped feldspars, which produce the 

mottled or ''poBcilitic" structure above alluded to. These seem to be 

less completely changed to saussurite than the larger feldspar crystals, 

but they often contain the zoisite crystals in a more perfect state of 

development.

It has already been mentioned that 

these hornblende crystals show an 

advanced stage of alteration. The 

hornbleude itself is bleached until it 

is nearly colorless. The final change 

seems to be the breaking up of this 

bleached hornblende into an aggregate of colorless needles, resembling 

tremolite. In other cases, however, 

it is changed in irregular patches to 

a colorless chlorite which is quite 

isotropic, and only visible between 

crossed nicol prisms. The peculiar 

manner in which this takes place deserves particular notice, and is represented in Pig. 9. The hornblende 

substance remains in interlacing veins between the chlorite areas. 

It is brightly polarizing and may be .often seen, as in the figure, to 

be continuous with an unchanged hornbleude individual. This peculiar 

mode of alteration is doubtless also conducive of the " pCBcilitic " effect 

above mentioned. The clearness, transparency, refraction, etc., of both 

hornblende and chlorite are so nearly identical that this structure is 

wholly invisible in ordinary light, and the effect of polarized light in 

transforming the apparently homogeneous and colorless mass into its 

two strongly contrasted elements is very striking.

No. 11034 is essentially the same as the specimen just described, 

except perhaps that it is more altered, and hence contains less of the 

original compact brown hornblende. Most of the hornblende, indeed,

FIG. 9. Hornblende undergoing alteration to 

colorless chlorite in a porphyritic diorite 

from Lower Quinneaee Falls. Nicola crossed. 

Magnified 80 diameters.
80 GREENSTONE SCHIST AREAS OF MICHIGAN. [BULL. 62.

e.i

in this rock is colorless, and much of it is fibrous. That which still 

retains any color is for the most part green.

No. 11035 was not sectioned, but hi the hand-specimen it exactly 

resembles the others.

No. 11032, collected with the specimens last described, is a beautiful^ 

fresh looking rock, without porphyritic structure, but with its hornblende 

in long, slender, greenish-black crystals. Under the microscope the 

feldspar of this rock is seen to be less altered than in those above 

described. Saussurite is abundantly developed in it, but not so extensively as in the porphyritic crystals of the other specimens. There is, 

however, here no brown hornblende. This mineral is all of a pale green 

color, or it is changed to chlorite, which sometimes contains sharp 

crystals of epidote. Ilmenite is abundant and is fringed with leucoxene.

The above described specimens, from the extreme western edge of 

the ridge near the falls, although they are the least altered form of the 

rock, hardly represent its most typical aspect, as the ridge is followed 

along the river bank, westward from Sandy Portage, its high and almost 

perpendicular front is found to be composed of a light green or grayishgreen, fine grained or aphanitic rock. This is compact and massive in 

structure, but everywhere profoundly seamed and jointed. It is cut by 

cross-gashes and parted joints, and gives every indication of having 

been pulled or crushed at all events, of having been subjected to 

enormous mechanical strains. The joints and seams often run in many 

different directions, producing a regular breccia without cement. The 

rock is also much slickensided, frequently so much so as to produce a 

schistose structure. The layers thus formed sometimes bend around 

more massive cores, which seem to have resisted the rubbing action.

The formation of what are above described as "cross-gashes" is 

very curious. At times the entire face of the rock wall is scarred with 

approximately parallel gaping seams, closely resembling the rents formed 

in moderately dry clay or putty when this is stretched. A single opening does not extend for any great distance, but a great number of them 

of all dimensions, closely crowded together, may produce an irregular 

sort of foliation. The general appearance of a rock thus gashed is represented in Fig. 10, which was drawn from a hand-specimen. This of 

course can give only an approximate idea of the appearance of this 

structure in a large rock-mass. These gashes seem to have been prodtfced by a stretching of the rock, or, what amounts to the same thing, 

by a bulging perpendicular to the action of some great pressure. They 

resemble the " Idaffende Risse? described by Heim in the Alps, and already mentioned in Chapter I (p. 43). The edges of the seams are ragged 

as though they had been formed by a forcible tearing asunder of the 

rock after it was solid. They are often filled with subsequent infiltrations of secondary minerals, like calcite or quartz, but more frequently 

they are open.
WILLIAMS.] STRETCHED GREENSTONE. 81

No. 11028 is a fair average specimen of the rock composing this ridge. 

It is a grayish green, compact, homogeneous mass, which is but little 

jointed. Tinder the microscope it is seen to be mainly composed of a

FIG. 10. "Cross-gashes" in a greenstone from Lower Quiimesec "Falls.

finely fibrous, pale green hornblende of secondary origin, of saussurite, 

quartz, and ilmenite. The structure of the rock has been almost entirely destroyed in the course of the uiineralogical changes which have 

taken place in it. As the new minerals have formed they have wandered from the position occupied by the older ones and have thus produced a fine grained aud confused aggregate in which remains of larger 

rectangular feldspars or hornblende crystals are only rarely discernible. 

The ilmenite is scattered about in minute dots, each surrounded by its 

own leucoxene border. The quartz is in irregular patches and bears 

 every evidence of being secondary in its origin.

No. 11033, from near the western end of the ridge, is almost identical 

with the specimen last described, but contains more quartz, and the 

remains of the feldspar imbedded in a dark gray, opaque matrix.

No. 11031 is a specimen collected from very near the spot where No. 

11028 was obtained, and the two, when taken in connection, show in a 

very clear and beautiful manner the effect of mechanical action in modifying both the macro-structure an4 the micro-structure of a solid rock. 

Bull, 62 6
82 GREENSTONE SCHIST AREAS OF MICHIGAN. [BULL. 62.

In spite of great present differences, there can be no doubt that the 

two specimens were once portions of a continuous mass. They were 

taken side by side from a single ledge which showed nothing like a line 

of contact between two different rocks. One is a typical massive greenstone, with hardly a joint-plane visible in it; the other, while agreeing 

closely with the first in color and texture, is deeply cracked, gashed 

and seamed, and possesses a pronounced schistose structure. The original form of both specimens was doubtless an igneous rock of the diabase type, which has been subjected in one case (No. 11028) to chemical 

(metasomatic) and in the other (No. 11031) to dynamic metamorphism.

Bocks may be altered by simple pressure, but the accumulated strains 

which are generated within them are relieved and adjusted by overcoming the force of cohesion along certain planes. Here there will be 

a shearing motion of greater or less extent, and a consequent crushing 

of the rock. The rent is soon healed by the crystallization of new compounds which cement the crushed fragments, and in this way a schistose 

band, of width varying with the intensity of the force, may be developed in the midst of an otherwise solid and massive rock; or a number 

of such bands may be formed parallel to one another, and together imparting to the rock the appearance of a foliated or even a banded 

schist.

Conclusive proof of this process might be difficult to discover without 

the aid of the microscope, but this instrument is happily able to aftord 

sufficient evidence to overcome all doubt. We shall have many instances 

of such action to describe in the sequel, each of which will exemplify 

some particular phase of the process.

The two rocks here under discussion, however, illustrate this general 

principle in an admirable manner. Plate IX represents the appearance 

of each of them in a thin section as seen under the microscope. The 

structure of the first, No. 11028, is, as above described, a granular aggregate of fibrous hornblende, chlorite saussuritized feldspar, quartz, and 

ilmenite, produced by chemical action alone in recrystallizing the elements of the original constituents in new compounds more in accordance with the altered physical conditions to which the rock was subjected. The component minerals of the second rock (No. 11031) are 

not so different from the last, but the story of its origin is told in its 

structure. The primary constituents seem to have been literally pulverized by crushing. The feldspar crystals have been pulled apart, 

the fragments being separated a considerable distance (always, however, in the same direction), but it is still possible to recognize fragments which once belonged together. The substance of this feldspar 

is remarkably fresh, another instance of the chemical action which takes 

place in this mineral being inversely proportional to the mechanical 

action which has affected it. The separation of the feldspar fragments 

is due to a stretching action and the spaces between them are filled 

with a pale green and almost isotropic chlorite. This mineral has
WILLIAMB.] PROCESS OF ALTERATION IN THE GREENSTONES. 83

evidently been developed at the expense of the hornblende, which is 

here only rarely to be found, and then always in bands where the mechanical deformation has been less intense. Where it is found, however, this hornblende is identical with that occurring in No. 11.028. The 

ilmenite and leucoxene are seen as before, but the former is far less 

abundant while the latter is proportionately more abundant. When 

the ilmenite has completely disappeared, the leucoxene forms bands and 

stringers, thus aiding in the production of a schistose structure.

This development of chlorite out of the hornblende substance seems 

to be a very important feature in the stretched basic eruptive rocks 

and many other instructive examples of it will be given beyond. It 

has a definite bearing upon the origin of the fine grained chlorite schists 

which accompany and are interstratified with so many of them.

Nos. 11041 and 11042 form an instructive pair of specimens, collected 

near the river bank toward the western end of the ridge. At this 

particular locality there is a well marked band of green schist traversing the here generally massive greenstone in a direction !S. 70° E. The 

schistose structure is irregular and undulatory, and on either side there 

is a gradual passage from the schist into the massive rock. No. 11042 

is from the surrounding mass immediately at the edge of the schist 

baud. In a hand-specimen it appears to be considerably altered, of a 

gray color, and without any cleavage. Under the microscope the original structure is easily recognizable. The rock is largely composed of 

stout rectangular feldspars, with a somewhat rounded outline, and internally changed to saussurite, though their periphery is mostly clear. 

Between these are the remains of former hornblende (possibly pyroxene) 

individuals now represented only by amphibole fibers and chlorite. 

Beautiful skeleton forms of leucoxene, composed of three sets of parallel bands, reproducing the rhombohedral parting of the original ilmeuite, are abundant.

No. 11041 is the same rock as the last in a much more altered form. 

The feldspar is mostly changed to calcite, and the hornblende to chlorite. The structure has wholly disappeared and there is a very fine mosaic of quartz and secondary albite substance. Still, the same skeleton 

forms of leucoxene remain and there is no doubt that the two specimens represent the same rock in different stages of alteration, the 

more changed form having become decidedly schistose.

Farther west, in the immediate vicinity of Lower Quinnesec Falls, the 

abrupt face of the diorite ridge proper retires a short distance' from the 

river bank. Along the shore the massive greenstones give place to slaty 

rocks, which cleave into rhomboidal prisms. They have no proper strike, 

but seem to break with equal readiness in all planes parallel to a line; 

and this line is here nearly perpendicular to the surface., No. 11043 is 

one of these rocks of a greenish color. Under the microscope it is seen 

to have been so profoundly altered that it is now impossible to assert 

;frpm internal evidence what its original character was. Still, its geo-
84 GREENSTONE SCHIST AREAS OF MICHIGAN. [BULL. 62.

logical position indicates that it was once a part of the gabbro or diorite 

ridge, and there is nothing in its present structure that directly contradicts this supposition. The rock has a pronounced, although irregular 

and undulatory, schistose structure. It consists largely of a fine grained 

matrix, composed of chlorite scales, arranged parallel to the direction 

of schistosity. With this mineral are associated quartz, feldspar, opaque 

black grains, probably magnetite, and occasionally sharply defined crystals of muscovite. Imbedded in this groundmass are larger feldspars, 

sometimes in well developed crystals, sometimes in fragments. The 

substance of these is but very little altered, and yet their outline is 

made indistinct by the penetration into them of the chlorite scales. 

They often appear to have been much squeezed and broken, the fragments being more or less separated. Grains of quartz are occasionally 

seen, which have also been fractured by pressure. Calcite, due to 

alteration, is also sparingly present.

Similar rocks, more or less schistose and varying from a dark green 

to a light gray, occur from this point along the river bank all the way 

to the falls.

No. 11046 is from the light silvery-gray, greasy-feeling baud, which 

projects into the basin at the foot of the falls. This is a typical sericite 

schist. Under the microscope it is seen to have a finely schistose structure, produced by the parallel arrangement (often in bands) of quartz 

grains, sericite scales, and chlorite, which is nearly colorless. Along 

with this is a large amount of calcite, also distributed in bands. There 

is no feldspar visible. The most interesting constituent is rutile. This 

mineral is sparingly distributed through the whole rock in very minute 

and sharply defined crystals. These have a pale yellow color, a high 

index of refraction, and the usual twinned forms. In certain portions 

of the section there are dull gray, opaque spots, which are yellow in 

reflected light. These are not continuous, but are more or less broken

up and pulled out in the direction of the schistose structure. They 

contain occasional dark red and irregularly shaped grains of rutile, but 

for the most part they are composed of innumerable small rutile needles 

(quite like the " Thonschiefernadeln" of the Germans) so closely crowded 

together as to form an opaque mass, only resolvable with a high power 

of the microscope. From these dense masses the other little needles 

in the other parts of the rock seem to have wandered.

This rock is almost identical with certain light colored, very fissile 

schists occurring on the opposite side of the river (of. No. 11011, described beyond), and the strike of these two exposures seems to identify 

them as belonging to a single band.

Greenstones of the Lower Quinnesec Falls. Passing now to the rocks 

exposed along the northern or northwestern shore of the basin below 

Lower Quinnesec Falls, we find alternating bands of lighter and darker 

colored greenstones, each accompanied by schistose layers^ which present instructive examples of dynamic inetamorphism.
WILLIAMS.] SCHISTOSE BANDS IN THE GREENSTONES. 85

The accompanying map (PI. IV) represents an enlarged portion of 

Major Brooks's map of Lower Quinnesec Falls. Commencing at the 

western corner of the basin there are, along its northern shore, three 

bands of dark and two of light colored rocks. The two kinds of structure, massive and schistose, are so intimately connected in both of them 

that it is impossible to escape the conviction that the latter has been 

secondarily developed.

If we begin at the extreme western end of this exposure, at the corner 

of the basin where a sharp bend in the river's course has produced a 

low, sandy beach, we discover that the rocks are of a darl? green color. 

The first specimen taken from this band (No. 11001) is massive and 

jointed. No feldspar is visible, except in large porphyritic crystals, 

which are not common. Even in this massive rock occasional narrow 

bands, having a wavy foliation, may be seen. These resemble slickensides, and represent planes of slipping or sliding in the massive rock 

(No. 11002). Tracing these rocks a short distance to the east, the wavy 

schistose bands become more and more numerous, until the whole mass 

becomes decidedly slaty, and has a greasy feel. This foliated rock 

strikes slightly south of east and dips steeply toward the north. The 

laminpe are frequently separated and bulged into lenses, which are 

either tilled with quartz or are still open and then coated with drusy 

quartz (No. 11003). These slaty rocks have a dark gray color, and, in 

some respects, resemble hydro-mica schists, but yet it is impossible to 

separate them in the field from the dark green massive rocks above 

described, as each passes by insensible gradations into the other.

Under the microscope, No. 11001 appears as a medium grained aggregate of feldspar and pale, fibrous hornblende with some ilmenite and 

leucoxene. The structure is still well preserved. The feldspar is in 

stout, columnar crystals, all distinctly striated and often considerably 

changed to zoisite or epidote. The hornblende is in irregular, ragged 

masses or in minute needles. The pale, fibrous sort is plainly seen to 

have been derived from a compact, darker green and pleochroic variety; 

but farther back than this (i. e., to a possible original pyroxene) it can 

not be traced. There is also a little pale green chlorite, looking much 

like the secondary hornblende, except between crossed nicols, where it 

is nearly isotropic.

No. 11002, taken from one of the narrow schistose bands, has plainly 

been produced by a shearing or sliding within the last described massive rock, and presents an interesting relationship to it when placed 

under the microscope. The two are plainly identical as far as the component minerals are concerned, but the structure of the latter has undergone a great change. The grain at first glance appears to be finer, but 

this is soon seen to be due to a crushing of the original minerals. The 

feldspars especially are very much broken and their parts more or less 

separated. This mechanical action has been attended by increased 

chemical activity. The original substance of the crushed feldspar crys-
86 GREENSTONE SCHIST ABBAS'OF MICHIGAN. IBUI.L. 62.

tals is hardly as much changed as in the preceding instance, but there 

has been a considerable recry^tallization of new feldspar substance 

(probably albite). The hornblende, because of its secondary nature, is 

more like that in the massive rock (No. 11001), but even this is sometimes sundered and torn. Chlorite is much more abundant, especially in 

the interstices between the fragments of the broken feldspar crystals. 

It is sometimes filled with very delicate and minute, though extremely 

sharp, epidote crystals. The ilmenite is more extensively changed to 

leucoxene than before, and, in some cases, shows a decided pulling out 

in the direction of the schistose structure. On the whole, many of those 

metasomatic changes which the greenstones most commonly show are 

here seen to have been accelerated and intensified by the mechanical 

force which has crushed the rock and so changed its structure. Thus 

the two forces dynamical and chemical may go hand in hand and 

together produce a rock which is both structurally and mineralogically 

very different from that out of which it has been derived.

Passing eastward along this exposure, we next come to a wide band 

of much lighter colored rock, which extends as far as the lower side of 

the small cove, situated on the right bank immediately below the falls. 

This band exhibits, in a most admirable manner, the secondary development of schistose layers in a massive rock, and is well deserving of 

careful study. The most typical and least altered form occurs just at 

and below the corner of the small cove. This is a medium grained, 

massive aggregate of opaque white saussurite and light green, fibrous 

hornblende, and it closely resembles, in its external aspect, the barrier 

rock at both Upper Quinnesec and Sturgeon Falls. Nos. 11008 and 

11009 represent this occurrence, but even the most massive specimens 

which could be obtained are seamed with cracks approximately parallel 

in direction and coated with a lamellar, soft green mineral like that in 

the adjoining schists. This is shown by examination to be chlorite. 

On both sides of the massive portion of this band the parallel cracks 

become more and more abundant (No. 11004 below, and No. 11010 

above), and on the lower side of the small cove the rock passes, without any break in its continuity, into silky schists (No. 11011), like those 

above described from the opposite side of the river (cf. No. 11046, p. 84).

Under the microscope, the least altered specimens of this rock (Nos. 

11008 and 11009) are found to bear a close resemblance to the light 

colored gabbro of Sturgeon Falls (see p. 68), except that in the present 

instance no trace of diallage is discernible. The structure of the two 

rocks is the same. The feldspar is here also replaced by the same gray, 

opaque saussurite, in which, with a high power, zoisite and calcite may 

be easily detected. The hornblende, as seen in a microscopic section, is 

practically colorless. Only occasionally are traces of a greenish tinge 

observable. Between crossed nicols this mineral generally exhibits a 

homogeneous extinction, so that it can not be regarded as truly fibrous, 

and yet there can be no doubt as to its secondary nature. Its edges are
U. S. GEOLOGICAL SURVEY BULLETIN NO. 62 PL. IV

 WISCONSIN

T. SBiN^R.SOE.

Dark Diabasef.£v

Light Ggbbro-diqi 

ana senate sch;

MAP 

Of

LOWER QUINNESEC FALLS

Menominee River

(AfterBrooks) 

SCALE
EFFECT OF CRUSHING ON THE FELDSPAR, 87

ragged and there is a decided tendency to separation into fibers, even 

when these are not actually present. It is noteworthy that this secondary hornblende can only be traced back (and that in rare instances) 

into a more compact greenish 'or brownish variety of the same mineral. 

In none of the thin sections is there any positive eyidence that diallage was ever present; and yet the close similarity of this rock to the 

Sturgeon Falls gabbro naturally suggests it. If the diallage were 

originally present it has completely changed to the' pale hornblende, 

while the brown hornblende longer resisted the bleaching action, and 

hence sometimes still remains. If this were not the case, we are compelled to assume one of the two other alternatives suggested in the 

case of the porphyritic diorite from the other side of the river, No. 

11098 (see p. 78).

The pale hornblende is often partially altered to colorless chlorite, 

and the ilmenite is more or less changed to leucoxene.

Nos. 11004 and 11010, taken from either side of the above described 

band, represent somewhat schistose varieties of it which have been 

produced by the action of pressure. That this is actually the case 

is admirably shown by a microscopic examination. The hornblende 

of the original rock seems to have passed completely into a very pale 

or colorless chlorite. Saussurite, on the other hand, is less abundantly 

developed than in the more massive rock above described, and it is disposed in narrow veins or in small, irregular spots. The ilmenite has 

given its place to a dark gray substance resembling leucoxene. This 

is drawn out in stringers following the direction of the schistosity, 

and in the center of these a yellow grain of rutile may sometimes be 

observed. Calcite is also present. The structure of the rock is 

completely different from that from which it is supposed to have been 

derived. It is now composed largely of a fine grained groundmass. 

This is made up of a microgranitic aggregate of minute grains of an 

unstriated substance resembling quartz, but which, from its high specific gravity, is probably a feldspar. These grains, together with 

chlorite scales, leucoxene, some calcite, and an occasional bit of epidote or sericite, are arranged so as to produce an indistinct schistose 

structure, the bands of which bend and wind around larger porphyritic 

individuals of feldspar. In this way a lenticular or " Flaser" structure 

results, which, however, is hardly discernible, except under the microscope. A very particular interest attaches to the porphyritic feldspar 

crystals of these rocks. In comparison with those in the more massive 

rock (11008 and 11009), they are but slightly altered to saussurite; indeed, for the most part, they are surprisingly fresh and glassy. Glistening cleavage surfaces may be readily detected with the unaided eye, 

and under the microscope this feldspar appears in clear, colorless grains, 

almost wholly free from inclusions of any kind. Occasionally these 

possess a light brownisli tinge, due to some evenly distributed pigment. Between crossed nicols this clear feldspar substance presents 

sharp twinning lamellae and a comparatively high extinction angle.
88 GREENSTONE SCHIST AREAS OF MICHIGAN. IBUU,. 62.

The individual crystals are invariably broken and the fragments are 

more or less separated. The intervening spaces are filled with a microgranitic groundmass, as shown in Plate VIII, figs. 1 and 2, of other 

similar, though more acid rocks from Upper Quinnesec Falls and from 

the vicinity of Marquette, Michigan.

' The clearness and freshness of this feldspar would at first suggest the 

probability of its being a secondary crystallization, as, for instance, 

albite is often found to be. This, however, is shown not to be the fact, 

first, by the broken state of all the crystals, indicating that they must 

have been in existence when the rock was pressed and crushed; and. 

second, by their physical properties which show that the feldspar is 

labradorite. Upon isolation this feldspar was found to have a specific 

gravity of 2.69; and in cleavage pieces parallel the basal plane, it gives 

an extinction angle of 12° to 20° on each side of the twinning-trace.

The survival of the original feldspar in an almost unaltered state, in 

a rock so profoundly changed by mechanical action, naturally occasioned some surprise. Examples of the same sort have already been 

cited (e. g. Nos. 11154 and 11165 from Sturgeon Falls, and No. 11032 

from the diorite ridge below Lower Quinnesec Falls), and so many cases 

of a like character were encountered from other localities that the occurrence of the freshest feldspar in the most crushed rock must be regarded as the rule.

I know of no way of interpreting this phenomenon, substantiated by 

so many instances, except by supposing that the pressure, which acts 

powerfully in stimulating chemical action in the solid rock, is relieved 

on the harder grains of a crushed band, since these are able to change 

their position by slipping among the softer materials.

The nearly vertical, silvery schists (No. 11011) which occupy the western corner of the little cove just below the Lower Qninnesec Falls, represent only a more highly metamorphosed state of the rocks above desrcibed. They are extremely soft, fissile, and schistose in a hand-specimen. Under the microscope they closely resemble the groundmass of 

the last described rocks (Nos. 11004 and 11010). The granular aggregate is so disposed as to produce a decided schistose structure. Both 

calcite and sericite have become more abundant, while the porphyritic 

feldspar crystals have completely disappeared. The leucoxene also can 

no longer be found, but the long lines of very sharp and minute rutile 

crystals, which are not present in any of the more massive related rocks, 

appear to have taken its place.

The perfect continuity of the massive rock (Nos. 11008 and 11009) 

with these schists, through the intermediate member (11010) is admirably shown in the exposure at the western end of this small cove.

The following chemical analyses of these three specimens have been 

made by Mr. R. B. Riggs.
WILLIAMS.J ANALYSES OB* LOWER QUINNESEC GREENSTONE.

I. No. 11008, Gabbro-diorite from Lower Quinnesoc basin.

II. No. 11010, Schistose form of the same rock. 

III. No. 11011, Seriate schist adjacent to last.

Si02 .... ..........

A12 03 - ............

Fe203 - ............

FeO.......... ....

UaO.......... ....

MgO...... ........

Ka20...... ........

K20. ..............

H20. ..............

C02 . ..............

Total..........

(I.)

47.96

10.85

4.33

4, 17

13.25

9.15

1.25

.30

2.89

.08

100. 23

(II.)

49.19

18.71

5.03

4.04

5.92

7.98

1.44

.77

5.05

1.82

99.95

(III. )

46.21

18.38

3.30

3.90

6.28

7.03

2.14

.35

3.82

8.32

99.73

TiO2 not determined.

The close agreement between these analyses is striking. The low 

percentage of iron sufficiently accounts for their uniformly light color. 

The most striking difference is perhaps the loss of calcium and the 

lesser loss of magnesium, both of which were undoubtedly carried off 

in the form of soluble carbonates when the rock was crushed. The 

water is contained in the pale chlorite, which first increases in .amount 

and then decreases, as its components are transformed in to carbonates. 

The sodium was first contained in the feldspar and the steady increase 

in its relative amount is probably due to the development of the sericitelike mica, which may be a paragonite.

It is interesting to compare the first of these analyses with that of 

the Sturgeon Falls gabbro (I on p. 76), the close resemblance of which 

to the rocks here under consideration has already been alluded to. 

The only noticeable difference is a slightly larger proportion of silica 

and a smaller proportion of alumina and lime in the Sturgeon Falls 

rock.

Passing now still farther to the eastward, we come to the second band 

of dark-colored greenstones (see map, PI. IV). These follow directly 

after the narrow exposure of the above-mentioned silvery schists in the 

western corner of the little cove and form a great part of its northern 

shore. They resemble the rocks occurring in the band first described, 

and present both massive and schistose varieties, which grade insensibly 

into each other in a manner not less instructive than, that in the two 

bands already considered.

Nos. 11012,11013, and 11021 (the first two from massive portions of 

this baud along the shore of the small cove; and the last from the Wisconsin side of the river directly opposite the falls) are essentially identical 

in appearance and composition. In the hand-specimen they form a 

compact, aphanitic mass, of a dark green color. Under the microscope 

the originally diabasic nature of the rock is at once apparent, although
90 GREENSTONE SCHIST AREAS OF MICHIGAN, [HULL. 62.

the extensive mineralogical changes which have gone on have greatly 

obscured its former structure. The components now present are almost wholly secondary. These are hornblende, chlorite containing epidote, quartz, and leucoxene. Ilmenite and occasional traces of feldspar 

are the only original constituents which remain. Still the disposition of 

the secondary minerals is such as to outline a diabasic or ophitic structure often with great distinctness. The feldspar is rarely well preserved j but a narrow zone of the unaltered substance of this mineral 

often outlines a stoutly lath-shaped crystal, even when its interior is 

wholly changed to an aggregate of quartz, chlorite, and epidote. The 

hornblende has a curious appearance. Its crystals are brownish and 

nonpleochroic with a somewhat granulated surface, so that it externally resembles diallage. Its cleavage and optical properties prove it 

to be undoubtedly hornblende, although this superficial likeness to diallage is so strong as to almost compel the conviction that it has originated by paramorphism from a pyroxene. This brown hornblende is 

seen with a high power to be gradually changing to a green variety, in 

which a pleochroism for the first time becomes apparent. This also 

frequently passes over into a fibrous hornblende. The chlorite epidote 

aggregate in these rocks is very finely developed. The chlorite is of 

an emerald green color and distinctly pleochroic. It appears between 

crossed nicols as isotropic or polarizes with a maroon tint. The epidote 

is in sharp, light yellow crystals, with the characteristic shape and optical properties of this species. The microscopic appearance of this 

aggregate is represented in PI. XI, fig. 1, drawn from a specimen 

collected below the Upper Quinnesec Falls (No. 11049). The literature 

relating to it has already been cited (Chapter I, p. 57). The epidote 

crystals have been described from the Menominee region by Wichmanii 1 

and others as secondary augite. This chlorite epidote aggregate covers 

considerable areas and occupies the place of both the feldspathic and 

the pyroxenic constituents. In addition to the minerals already named, 

ilmenite, with its leucoxene border, pyrite and secondary quartz are 

quite abundant in these rocks.

No. 11014, taken from a narrow, schistose band in the massive greenstone (No. 11013), shows in a remarkably clear manner how the dynamic metamorphism of this rock has been accomplished. In this instance there can not be the least doubt as to the continuity of the rockmass. The hand-specimen No. 11014 has a decidedly schistose structure, and under the microscope it shows the effects of mechanical crushing and attendant mineralogical changes with great distinctness. The 

whole rock, with the exception of certain remains of the larger feldsparcrystals, has been reduced to a fine grained mass, showing an aggregate polarization. Light green chlorite has been largely developed and 

has completely replaced all the bisilicate elements. The parallel arrangement of the scales of this mineral is what produces the schistose

1 Geol. Wisconsin, vol. 3, 1876, pp. 623,624,
WILLIAMS.] ANALYSES OF LOWER QUINNESEC GREENSTONE. 91

structure, and it surrounds and incloses the small grains in the manner which has already been explained in connection with No. 11031 

(p. 81) and illustrated in PI. IX, fig. 2. The clear, secondary grains forming this mosaic are certainly, to some extent at least, quartz, while a 

portion of them may be an uustriated feldspar. Calcite has been 

largely developed in the schistose rock, and the leucoxene is replaced 

by rutile, either in stout, yellow grains, or in minute, sharp crystals arranged in long, sinuous lines.

The mechanical action which produced this schistose band has therefore resulted in the crushing of the rock, and the almost total disappearance ,of all of the original components. The comparatively slight 

change in the chemical composition of the rock as a whole may be seen 

from the two following analyses of Nos. 11021 and 11014, made by Mr. 

E. B. Eiggs.

I. No. 11021. Dark massive greenstone, Lower Quinnesec Falls.

II. No. 11014. Dark schistose greenstone, forming a band in the last.

Si02 .....................

Ala03 . ...................

PejOg...... ........ ......

FeO. ....................

CaO.... ..................

MgO ....................

Na2 0 ....................

K20 .....................

H2O .....................

CO............. ..........

Total..............

(I.)

43.80

16.08

9.47

10.50

7.81

C.54

1.96

0.34

ii.99

0.08

100. 57

(II.)

44.49

16. 37

5.50

7.94

7.50

2.59

Q.56

4.99

5.38

100. 39

TiO3 not determined. 

Powder dried at 105° C.

The changes here are at once seen to be due (1) to the total removal 

of the iron ores (loss of iron); (2) to the production of carbonates 

(gain of CO2 , carbonatization) j and (3) to the increase in the amount of 

chlorite, (increase of H2O, hydratiou).

Toward the eastern end of the small cove the dark-green schistose 

rocks appear to pass by insensible gradations into exactly similar ones 

of a lighter color. No. 11015, one of these, is seen under the microscope to be a very schistose aggregate of extremely pale chlorite and 

quartz grains. There is a little calcite here present and occasional 

scales of sericite. The rutile, in very minute and sharp crystals, is here 

crowded into long bands and stringers which follow the schistosity of 

the rock and give the thin section a pronounced "Flaser" structure.

No. 11016, from the extreme eastern corner of this cove, represents a 

band of light colored brownish rocks with a perfectly developed schistose structure and a grain somewhat resembling that of wood. These
92 GEEENSTONE SCHIST AREAS OF MICHIGAN. fi«ur>i, 62.

stand nearly vertical, though some dip, from 65° to 70° N. 20° W. 

Their strike is turned from that of the other rocks here (S. 70° E) some 

30° to 40°, so that they now strike N. 70° B. Under the microscope 

this rock is seen to have essentially the same structure and composition 

as the last. The main difference between them is that the present specimen has a brown chlorite instead of a green one. This it is that imparts the peculiar color to the rock in the hand-specimen. There is 

also more calcite in this rock than in the other. It would seem, along 

with No. 11011 above described, to represent the most profoundly metamorphosed phases of the massive rocks at Lower Quinnesec Falls. 

The original constituents have undergone an alteration to chlorite, 

quartz, carbonates, and rutile. The iron has largely disappeared, and 

the crystallization of the new components under lateral pressure has 

produced a very pronounced schistose structure.

No. 11023, from the upper end of the breakwater, above the falls 

(Wisconsin side), is very closely allied to the two specimens last described. It is a grayish schist, with the prevailing strike, but everywhere seamed with transverse cross-gashes, which are, for the most part, 

filled with infiltrated quartz. There seems to be little doubt from a 

field examination, that this rock is continuous with the massive diabase 

(No. 11021) in the same way that the last mentioned specimens are continuous with Kos. 11012 and 11013. The massive green rock (No. 11021) 

is exposed a very short distance to the west of it, where it also is filled 

with seams and cross-gashes, which appear to have been squeezed open 

by lateral pressure. From this point the transition to the gray schistose rock is a very gradual one. The latter rock has no regular bedding 

but only a cleavage in one direction. This causes it to break out in angular prisms. The rock is everywhere fissured, slickensided, and filled 

with infiltration quartz, as evidence of the enormous pressure to which 

it has been subjected. Under the microscope it is much like No. 11016. 

There is the same quartz mosaic, pale chlorite and strings of rutile 

needles, arranged so as to produce a decided schistose structure. There 

is, however, in this rock considerably more calcite and an additional 

feature in the way of remains of former feldspar crystals. These are 

considerably altered, and appear to be changing to a mass quite like that 

which composes the rest of the rock. In fact, this has gone so far that 

it is almost impossible to distinguish these feldspars in ordinary light, 

although their outline and nature become very apparent between crossed 

nicols. They are arranged without reference to the present schistose 

structure of the rock, and evidently represent original constituents of 

the rock from which the present schist has been derived.

In place of the above-described transition from massive to schistose 

rocks along a line normal to the strike there is in many instances the 

still more conclusive proof to be found in this same transition around 

ellipsoidal or lenticular cores of massive rocks. Such masses grow gradually schistose around their periphery and pass into scliists which ap-
WILLIAMS.] SCHISTOSE LAYERS SURROUNDING MASSIVE CORES. 93

pear to wind around and enclose them, but which, in reality, have been 

formed out of their own substance. An admirable example may be 

seen just at the Wisconsin edge of the river at the foot of Lower Quinnesec Falls. This is a lenticular mass of massive greenstone about eight 

feet in its longest diameter. This grades insensibly.around its edge, 

into dark green schists, differing only in their structure from the central rock. Four specimens (Nos. 11017,1L018,11019, and 11020) were 

collected to illustrate this passage. The most interior and massive one 

(No. 11017), although to all appearances quite compact, is found upon a 

microscopic examination to be wholly altered niineralogically. The 

bisilicate constituents are now represented only by a light emeraldgreen, pleochroic mineral, which, in spite ot its comparatively strong 

action upon polarized light, is probably chlorite. This frequently contains sharp epidote crystals. The feldspar, while retaining much of its 

original form, is composed either of a gray, opaque saussurite or of a 

fine mosaic of quartz and albite grains and a great deal of calcite.

The extremely altered condition of the central portion of this core is 

perhaps rather advantageous than otherwise, for, while there can be no 

doubt as to its original character, the transition to the schists is seen to 

take place wholly by a change in structure, without further miueralogical alteration.

No. 11018, taken from a place where indications of a schistose structure 

are already quite apparent, shows just the same mineral components as 

the last. The chlorite scales, however, have assumed a decidedly parallel arrangement, which is followed, to a less extent, by the fine mosaic 

of quartz and albite grains. The calcite is better crystallized, and forms 

larger individuals. There are also present some minute scales of a deep 

red iron oxide, probably gothite, or limonite. The gray saussurite is 

likewise common.

No. 11019, in which the schistose structure is still more apparent, 

shows the same components as the others. The parallel arrangement 

of the chlorite is still more pronounced and the calcite still better crystallized. Strangely enough, we meet here, in a very schistose rock, 

better preserved feldspar crystals than^can be found in the more massive 

greenstone, with which it is so intimately connected. This is but one 

more instance of the remarkable rule set down on p. 88, and alluded 

to several times already. This thin section (No. 11019) exhibits feldspars 

so well preserved that only occasional epidote crystals are developed 

in them. There is also to be seen a remarkably good example of the 

change of pyrite to iron hydroxide.

No. 11020 represents the extreme member of this series. It is a typical 

chlorite schist, in which alternating and interlacing areas of pale green 

chlorite and a fine quartz albite mosaic may be seen. In both a highly 

developed schistosity is produced by the parallel arrangement of the 

components." The grain of this rock, in so far as it is due to these alternating areas, appears somewhat coarser than in the less altered form.s,
94 GREENSTONE SCHIST AREAS OF MICHIGAN. [BULL. 62.

The chlorite, if it be examined with a higher magnifying power, is seen 

to be filled with irregular spots of a reddish brown color, which feebly 

polarize, and seem to be the remains of former feldspar crystals. Eutile 

in grains or sharp crystal forms is also present.

The fine mosaic which composes the other areas, contains, in addition to the quartz and albite grains, calcite and chlorite in small, elongated scales.

A careful field examination shows conclusively that the four specimens here described (Nos. 11017 to 11020), represent only different 

forms of the same rock. In fact, the mineral composition is identical 

for all, although the most massive form is beyond all doubt an altered 

diabase, while the exterior bands, which surround this core are just as 

truly examples of a typical chlorite schist.

No. 11048 is a specimen of the massive, dark green rock which has 

been blasted out behind the shanty on the Michigan side of Lower 

Quinnesec Falls. A microscopic examination shows at once that this 

rock was originally a diabase, although its present constituents are 

for the most part secondary. The shapes of the original minerals are 

indistinctly outlined, and so the structure of the rock is preserved. 

There is now present a pale green hornblende, probably secondary to 

pyroxene, although no traces of this mineral are preserved j epidote, 

chlorite, saussurite and leucoxene in zones around the titanic iron. 

The feldspar has rarely changed to the opaque, gray saussurite, but is 

replaced for the most part by a mass of sharply defined epidote crystals. Where the feldspar and hornblende have jointly contributed to 

the formation of secondary products, we have the chlorite epidote 

aggregate as a result. A little secondary quartz is also observable.

The two remaining specimens examined from Lower Quinnesec Falls 

(Nos. 11101 and 11102) were collected on the Michigan side of the river 

above the fall, from two points designated by u and w respectively ou

Major Brooks's map. They exhibit in a marked degree the effects of 

stretching upon massive greenstones. If a rock mass were subjected 

to a great lateral pressure when it was free to expand in a direction 

perpendicular to that in which the pressure acted, it would appear as 

though it had been subjected to a strain or tension in the direction along 

which it was free to move. There would be a breaking and pulling out 

of the individual minerals into parallel bands along the line of expansion or apparent stretching. This phenomenon is very admirably displayed in these two specimens.

No. 11102 is the coarser grained of the two rocks and shows its structure best when the microscopic section is examined with a pocket-lens. 

Its. appearance, as seen uuder a low power of the microscope, is portrayed in PI. XI, fig. 2. One of the most important of the original 

constituents of this rock was a triclinic feldspar in large sized individuals. These are now fissured in a direction normal to the stretching and their fragments are separated in a most striking manner. The
WILLIAMS.] STRETCHING OF ILMENITE IN GllEENSTONE. 95

interstices are filled with long, chlorite scales arranged perpendicular 

to the walls of the fissures; more rarely calcite replaces this chlorite. 

Here again we have clear evidence that the chemical change in the 

feldspars of rocks which have been subjected to great mechanical action, 

is comparatively slight. The feldspars of this rock are scarcely altered 

except by being broken or crushed.

The grains of original quartz in this rock have suffered the usual deformation, being rarely broken but more frequently in a state of tension which imparts to them an undulatory extinction.

Very finely fibrous, dark green actinolite is quite abundant, representing some other bisilicate constituent (either pyroxene or compact 

 hornblende) all traces of which have now disappeared. The fibers are 

sometimes parallel over considerable areas, sometimes crowded together 

into a confused and matted mass. They themselves show no evidence 

of pressure and hence must have originated during the action of the 

pressure or subsequent to it. The former is the more probable and the 

pressure may have been the very cause to which the hornblende owes 

its origin, as I suggested in a former paper.1

None of the constituents displays the effects of the stretching action 

to which this rock has been subjected more distinctly than the ilmenite. This mineral is abundant in grains of considerable size, all of 

which have been irregularly fissured.in a direction transverse to the 

tension and the fragments separated, exactly like those of the feldspar.2 

(See PI. XI, fig. 2.) The absence of leucoxene around these grains may 

indicate that in this case also the chemical action is inversely proportional to the mechanical.

No. 11101 is a very similar rock of a finer grain, in which the effects 

of stretching are not so apparent. We therefore find the chemical alteration of the feldspar proportionately greater. This mineral is not 

so much broken as in the specimen last described, but it is filled with 

secondary crystallizations of chlorite and epidote. The ilmenite is 

broken and stretched in the manner described in the preceding specimen, though to a less degree. The separated ilmenite fragments, however, in this case present a contrast to the others, in having leucoxeue 

quite abundantly developed around them. The hornblende of this 

rock is both brown and green; each variety is dark colored and intensely pleochroic. Most of it is fibrous, the fibers following in their 

arrangement the general direction of the foliation.

1 Am. Jour. Sci., 3d series, Oct., 1884, vol. 28, p. 266.

2A. Schenck describes and figures exactly this phenomenon in the schistose diabases of the upper 

Ruhr Valley. Inaugural Dissertation, Bonn, 1884, p. 55. .
OHAPTEE III.

GREENSTONE BELTS OF THE MENOMINEE DISTRICT (Continued). 

UPPER OR BIG QUINNBSEG FALLS.

About 3£ miles above Lower Quinnesec Falls, described in the 

preceding chapter, the course of the Menominee Eiver is again 

interrupted by extensive rock exposures, which skirt the banks for 

nearly 2 miles. The upper mile of this distance is a high, rocky gorge, 

known as the "Horse Eace," through which the water rushes and 

tumbles in a foaming rapid. The current then flows more quietly 

through a broader, though still rocky channel, for half a mile, when 

it suddenly plunges over a fall of considerable height and beauty, known 

as Upper or Big Quinnesec. 1

Below this fall the stream broadens into a wide basin, which is also 

bordered by rocky shores.

The continuous exposure of Upper Quiunesec Falls is, therefore, 

separable into three naturally distinct divisions. These divisions are, 

moreover, each characterized by a different kind of rock; a fact which 

serves to distinguish them almost as sharply as their physical conformation. (See map, PI. Y.)

The rocks which are exposed along the basin belong, for the most 

part, to the class of dark, more or less schistose greenstones, similar to 

those occurring at Lower Quiiinesec Falls. These were once diabases

rich in iron, which have produced amphibolitic and chloritic schists of 

a deep green color.

The actual barrier at Upper Quinnesec Falls is, on the other hand, composed of the light colored type of greenstone with a distinctly granular 

structure, like that already described as occurring at both the Sturgeon 

and Lower Quinnesec Falls. This is also the commonest rock for the 

half mile above the falls, before the rapid is reached. It is deserving 

of careful study on account of the transition which it presents into 

light colored sericite schists.

The rock which forms the steep walls of the gorge known as the Horse 

Eace is of still a different type. It will receive a separate treatment, 

as it is quite unlike anything observed elsewhere in the Menomiuee 

Valley.

1 This name is a corruption of the Chippewa Bequenesec which is spelled by H. Creduer in his description of this locality aa lielenenesek. ("N'eues.Jahrbq.cl} fijr Mineral., 1870, p. 971.)

96 " 
U. 8; GEOLOGICAL SURVEY BULLETIN NO. 62 PL. V

 WIS-C.ONSI3ST 

T.38N..H.20E.

icit* Schist,

fd-beach

UPPER QUINNESEC FALLS.

Menominee River

I After Brooks)

Schistostitre0enne
WILLIAMS.] GREENSTONES BELOW UPPER QUINNES.EC FALLS 97

Finally bands of certain acid rocks granites, gneisses, and schistose 

porphyries occur in the greenstones both at the fall and along the 

Horse Race. They appear to stand in the closest genetic relation both 

to each other and to the great granite area which lies south of the river 

and approaches it at this point more nearly than at any other.

We may, therefore, consider the rocks of Upper or Big Quinnesec 

Falls under the four following heads:

(1) The dark colored greenstones of the basin.

(2) The light colored greenstones at the falls.

(3) The coarse grained diorites of the Horse Race.

(4) The acid rocks, both massive and schistose.

THE DARK .COLORED GUEEXSTONES OF THE BASIN.

At the lower (eastern) extremity of the basin, below Upjper Quinnesec 

Falls, is an exposure of greenstone, which juts into the river from the

Wisconsin side. It is of a uniformly green color and, for the most part, 

massive, although schistose portions are not infrequent, which have the 

prevailing strike, N. 70° W., and stand nearly vertical. These schistose bands have in the field every appearance of being secondary 

since they are only locally developed and grade imperceptibly into 

the massive rock. A specimen from this locality, No. 11049, when examined microscopically, appears to be very similar to a rock already 

described from the Lower Falls (No. 11048). The process of alteration 

there indicated, however, has here progressed much further. There is 

every reason to believe that this rock, like the other, was once a diabase; 

and yet, in this case, all traces both of the original structure and of the 

original constituents have disappeared. The feldspar substance is completely replaced by a confused network of epidote needles, or of these 

mixed with chlorite; some rather compact, green hornblende still remains, but this mineral is rapidly disappearing, and the manner in 

which, jointly with the feldspar, it furnishes the materials out of which 

the chlorite epidote aggregate is formed is here very instructively displayed. In fact, no other rock showed this characteristic form of alteration as well as this one, and it was therefore selected to furnish the 

illustration. (See PI. XI, fig. 1.) The titanic iron is here in the form of 

small grains, accompanied by feucoxeue.1

On the southern or Wisconsin side of this basin there are no more 

important exposures of the dark colored greenstones. On the Michigan side, however, such exposures are abundant and have been indicated on Maj. Brooks's map as the points #, aa, 66, and cc.z

Two suites of specimens collected at these points, one by Prof. Irving

1 Slide No. 931 of the collection of the Lake Superior Division of the U. S. Geological Survey was 

prepared from a specimen collected by Prof. B. D. Irving west of the above, among tin* light colored 

greenstones. It is of great interest in this connection because its feldspar is quito well preserved and 

shows in a most unmistakable manner the- opliitic structure of the diabases. In other respects especially in the character and abundance of tliu opidote-chlorito aggregate, it is quito identical with No. 

11049.

*Geol. Wisconsin, vol. 3, PI. 3, p. 474.

Bull 62 7
98 GREENSTONE SCHIST AKEAS OF MICHIGAN. [BULL. 02.

and the other by the writer, were studied. The rocks all exhibit the 

eftects of extreme mechanical action upon their constituents, which has 

so far obliterated their original structure as to render their true character doubtful. They agree in being composed of different proportions 

of a much crushed and altered, reddish brown feldspar and a pale green 

mass consisting of chlorite and quartz. The latter aggregate is of secondary origin and imparts a cleavage to the rock, since the chlorite 

flakes are all arranged in one direction.

No. 4162, from Prof. Irving's collection, was the only specimen studied 

microscopically from point z. It is composed mostly of feldspar, and 

has, therefore, a reddish color. This constituent is intensely crushed 

and its substance is so highly altered as to appear to grade imperceptibly 

into the quartz chlorite matrix, which is present in small amount between its fragments. Scattered through this mass in the direction of 

its foliation are long lines of some opaque iron oxide, accompanied by 

very minute though sharply denned crystals and twins of rutile (Thouschiefernadeln), like those represented in Fig. 12, p. 106. In some cases 

the iron oxide is absent, and these dark bands may be seen with a high 

magnifying power to be made up entirely of the closely clustered rutile 

crystals.

Nos. 4163 and 11070 are both from Brooks's point aa, which he describes (loc. cit.) as composed of a " chloritic greenstone schist, with 

pinkish grains of feldspar (Schalstein?)."

The first of these sections belongs'to Professor Irving's collection, 

and exhibits under the microscope the effects of the crushing and 

stretching in a very marked degree. Its structure resembles that of 

specimen No. 11102 from the lower Quinnesec Fall, described on p. 94, 

and represented in PI. XI, fig 1. White and reddish feldspar crystals, 

quartz grains, and opaque black spots of iron oxide are imbedded in a 

secondary matrix consisting of chlorite, quartz, and epidote. No trace

of aii original bisilicate now remains. All of the older components, 

especially the feldspar and iron oxide, are much broken and their fragments are separated in one uniform direction corresponding to the foliation of the rock; but yet the fragments which once belonged together 

can still be recognized. The feldspar substance, in spite of the intense crushing, is still remarkably fresh and its twinning striatiou distinct. Lath-shaped forms, like those of diabase, are present, though all 

traces of the original rock structure have disappeared. The opaque 

grains of iron oxide (magnetite) are broken and pulled apart like the 

feldspar and can be seen with a high magnifying power to be fringed 

with minute and imperfectly formed rutile crystals. Sharply defined 

octahedra of magnetite evidently of secondary origin, as they never 

exhibit any traces of mechanical action are also scattered through 

the chlorite, and in some cases these octahedra are crowded together 

with little rutile crystals, both looking as though they had resulted from 

the alteration of some preexisting mineral.
WILLIAMS.] EUTILE IN ALTERED DIABASE. 99

No. 11070, collected by the writer from the same locality as the last 

(Brooks's point aa), possesses a similar character, though with certain 

important differences. The hand-specimen is dark green in color and 

decidedly schistose. Under the microscope it is seen to be essentially 

the same rock as that last described, but in an even more advanced 

stage of alteration. The fragments of reddish feldspar, which can still 

be seen to belong together, are here, as before, separated in the direction 

of the foliation, while the spaces between them are filled with chlorite 

flakes and quartz grains. The feature of particular interestin this specimen is, however, the abundance of its rutile and the constant and close 

association of this mineral with the black iron oxide. This association, 

as well as the general appearance of this rock under the microscope, 

is represented in PI. XIII, fig. 2. The opaque, black grains, which in 

reflected light can be seen to be intergrowu with a considerable proportion of pyrite, are surrounded by a border of stout, brownish yellow 

needles, averaging from 0.1 to 0.05 mm in length by from 0.02 to 0.01mlu 

in width. They are crowded closely together and are often parallel to 

the external edges of the grains. Not infrequently, also, they exhibit 

a regularity of arrangement, crossing each other at angles approaching 

60° and 120°. It will be readily seen, however, that in a random section of the rock it is impossible to decide whether these are exactly the 

angles or whether they are the angles 54° 43' and 65° 35' (with their 

supplements, 125° 17' and 114° 25'), which voii Lasaulx has shown are 

produced by the twinning of rutile, and characteristic of its network 

growth called as sagenite.1 An isolation of these needles was made by 

dissolving all the other constituents of the rock in HOI and HF, for 

the purpose of obtaining material which might positively decide this 

point, but no definite results were obtained. The needles are for the most 

part irregular in their arrangement, and it still remains uncertain whether 

their occasional regularity is due to twinning or to some other cause.

A chemical examination of the isolated needles showed that they were 

composed of titanium oxide, while their crystallographic and optical 

properties observed under the microscope proved them to belong to the 

species rutile.

The fact that the black, opaque constituent of this rock is traversed 

by parallel cracks and fissures, often in sets intersecting at an angle of 

60°, like those observed in titanic iron, taken in connection with the 

close relationship of the rock itself to the family of diabases, naturally 

led to the assumption that the iron oxide was ilmenite. Moreover, the 

similarity of the distribution of the rutile about the black cores to that 

of leucoxeno about ilmenite crystals in the process of alteration its 

presence as a continuous border and in every crack, while the black 

substance itself is quite free from the needles 2 seemed further to indi1 Zeitschr. fur Kryst. u. Min., vol. 8, 1884, p. 56.

2 This was shown, not merely by the absence of the larger ueodles which, if they were present, 

must have appeared through the black substance of the irou oxide iti the thin section, but also by 

slowly dissolving away the oxide with HCI under the microscope and the failure thus to disclose any 

smaller crystals ot rutile, which might have been wholly inclosed within the black ore.
100 GKEENSTONE SCHIST AREAS OF MICHIGAN. [uuLL.62.

cate the secondary origin of the rutile out of the iltuenite. This is the 

view that was at first taken by the writer and published by him in a 

letter to the Neues Jahrbuch fur Mineralogie, etc., 1887, vol. 2, p. 

203. In a subsequent paper on the same rock,1 Prof. Cathrein of Karlsruhe showed, among other points which he considered as objections to the secondary origin of the rutile, that the black iron oxide 

was not ilmenite at all, but magnetite. This determination has since 

been confirmed by a careful chemical examination made at my request 

by Prof. S. L. Penfield, of the Sheffield Scientific School at New Haven.

A very similar occurrence of rutile needles, though inferior in size to 

those here described, has been observed by Cathrein in the leucoxeue 

border which surrounds both ilmenite 2 and titaniferous magnetite 3 in 

certain Tyrolese rocks. Dr. J. Blaas also encountered the same kind 

of a network of rutile crystals in a quartz phyllite from the neighborhood of Innsbruck, but in this case it was unaccompanied by any opaque 

ore.4 In none of the instances observed by him does Cathrein consider 

the rutile as even possibly a secondary product. On the other hand, 

he thinks that the little needles were formed simultaneously with the 

ilmenite or magnetite, and that they have always existed in their present form within the iron oxide, until more or less completely liberated 

by the solution and removal of the latter.

In his article on the Quiunesec rock above cited, none of the points 

insisted upon by Cathrein appear to militate against the secondary origin 

of the rutile, except the last. As has been stated above, it is impossible in a random section to determine with certainty whether the needles intersect at angles of exactly 60° and 120°, or at those so nearly 

like them produced by twinning. But whichever be true, it is certain 

that a large majority of the needles are quite irregular in their arrangement. The discovery, however, that the black iron oxide which accompanies the rutile is not ilmenite but a magnetite almost free from 

titanium, seems at first altogether incompatible with the idea that the 

rutileis a secondary derivative from ilmenite. Nevertheless, a continued 

and repeated study of a large number of thin sections of this rock have 

served to convince the writer more and more that the rntile was at 

least not a constituent of the rock in its original state. The rutile aggregates are never drawn out in the direction of the stretching, as are 

all of the original constituents, notably the feldspar, and as they must 

have been had they always been present in the rock. It is necessary, 

therefore, to assume that they originated subsequent to, or, more probably, at the time of the metamorphism.

The constant and intimate connection between the rutile and the 

magnetite clearly shows that they are in some way genetically related. 

Opposed to the supposition that they were originally parallel inter1 Neues Jahrbuch fur Mineral., etc., 1888, vol. 2, pp. 151-165. 

2 Zeitschr. fur Kryst. u. Min., vol. 6, 1882, p. 'J48.

3 Ibid., vol. 8, 1884, p. 321.

4 T8cherm«T,k'a min. u. petrog. Mittheil., vol. 4, 1882, p. 514, fig. 2,
WILLIAMS.] PROBABLE ORIGIN OF THE RUTILE, 101

growths, is, however, the fact that the rutile needles never occur within 

the magnetite, now present. The rntile forms a border around the black 

areas, which themselves frequently show a sharp crystal outline, difficult to reconcile with the idea that they have been partially dissolved 

so as to reveal the rutile which once existed within them. In one instance, indeed, an aggregate consisting of rutile needles and sharp octahedra of magnetite was observed, but this is exceptional. On the other hand, it may often be clearly seen that the magnetite has, to a 

greater or less degree, been removed by solution; but in all such cases 

the place that it once occupied exists within the fringe of rutile, but is 

itself wholly free from the needles. This space is often filled with calcite, or some similar carbonate, in slightly radiating tufts.

All of these observations seem to point to a contemporary origin for 

both the magnetite and rutile from some preexisting mineral, which in 

the present case may very probably have been ilmenite.

The change of hematite into magnetite is one which has been often 

observed. Roth ] quotes descriptions of this alteration from Breithaupt, 

Volger, Peters, vom Bath, Petersen, Doll, Gr-attarola, and Zeparovich. 

There seems to be no reason why the corresponding titaniferous compound, ilmenite, should not also pass over into magnetite with the 

separation of its titanium in the form which is most stable for the conditions then prevailing.

Rutile is well known to be widely distributed in microscopic crystals 

through schists which have resulted from the regional raetamorphism 

of both sedimentary and eruptive rocks. It must therefore be a compound well in accord with the conditions producing metarnorphism.

The common presence of rutile needles in diabase rocks which have 

been intensely altered by pressure, and its absence in their unaltered 

forms^ in connection with the fact that the original ilmenite disappears 

in proportion as the rutile is developed, h'as inclined such eminent 

petrographers as Lessen and Rosenbusch to the supposition that the 

latter mineral has originated from an alteration of the former. This 

hypothesis has derived much support from observations made in many 

of the Lake Superior greenstone schists which are altered diabases. 

In case of the particular rock under discussion, it is impossible to 

prove with the material now in hand that the magnetite and associated 

rutile have resulted from preexisting ilraenite, but in light of all that 

we know of this and other occurrences, it seems to the writer that such 

an origin is, to say the least, not at all improbable. Whether or not 

such an alteration as that here suggested really takes place in the 

ilmeuite of diabase when this rock undergoes dynamic metamorphism 

to chloritic schists must remain for future investigations to decide.

Nos. 4164 and 4105, from Prof. Irving's collection, show the same 

rock as the two specimens last described in a yet more advanced stage 

of alteration. Both have become typical chlorite schists, composed

° 'Allgenieine imd chemische Geologie, vol. 1, 1870, p. 97.
102 GREENSTONE SCHIST AREAS OF MICHIGAN. [BULL. 62.

mostly of chlorite, quartz, and iron hydroxide. . The structure is now 

finely parallel and schistose, not the least trace of the original structure 

remaining. In No. 4165, which is the darker colored of the two, leaflets of a colorless mica (sericite) are abundant.

No. 11072, collected from Brooks's point cc, is a dark green rock, which 

is seen with the unaided eye to be traversed by numerous fibrous veins 

of a paler green color. Under the microscope the section of this rock 

shows principally ragged areas of green hornblende, inclosing isolated 

crystals of a mineral, which agrees more closely in its physical properties with zoisite than with any other; i. e., there are colorless rectangular sections with a cleavage parallel to the longest axis, and a crossparting in the direction perpendicular to this. In polarized light these 

sections show pale, bluish gray interference colors, parallel extinction, 

and a biaxial figure. The fibrous veins, which compose a considerable 

proportion of,the rock, are made up of zoisite individuals arranged 

perpendicular to the walls of the vein, and of fibrous hornblende, which 

often penetrates the former mineral in fine asbestiform needles.

THE LIGHT COLORED GREENSTONE AT UPPER QUINNESEC FALLS.

The speckled rock which forms the barrier at Upper Quinnesec Falls, 

is a very typical representative of the light colored greenstones of the 

Menomin.ee region. It has a strong macroscopical resemblance to the 

rocks of this type already described from Sturgeon Falls and Lower 

Quinnesec Falls, but is more 'attractive in appearance than any of them. 

In the field it appears to be a medium grained aggregate of a white, 

opaque feldspar, in which, however, glistening, striated surfaces are by 

no means rare, and a grayish green mineral with a half metallic luster 

on its cleavage planes. The deceptive appearance of this mineral is the 

reason why Prof. Hermann -Credner calls this rock (which he described 

as early as 1870) a diabase with a decided resemblance to a gabbro. 1

This mineral does, indeed, show the most striking macroscopical similarity to the diallage of some of the Yolpersdorf and Italian gabbros; 

but in spite of this fact, the most careful microscopical examination of 

many different specimens the freshest that could be discovered 

failed to disclose a trace of pyroxene. This result is the more surprising as the rock appears even fresher than that at Sturgeon Falls in 

which considerable pale diallage still remains. The microscope, however, shows that the rock at Upper Quinnesec Falls is the more altered 

of the two.

No. 11054, collected on the Wisconsin side of the river, directly at 

the fall, may be regarded as a typical specimen of the perfectly massive

1 In speaking of this constituent he says: "Er [der Augit] bildet kurzsaulenformige, za krystalliuischen Partien verwacbsene Individueu, an deneii orthodiagonale Spaltungsflachen mit halbraet'allischcm Glanze besonders dentlich hervortreten. Dadurch erhalt das Gcstein eine Aehnlichkeit mit 

Gabbro, selbst rait Hypersthenit." (Neues Jahrbuch i'iir Mineral., 1870, p. 972.) Professor Pumpelly 

also says that this rock is identical with that of Sturgeon Falls. (Geol. Wisconsin, Vol. 3, p. 564.)
WILLIAMS.! GABBRO-DIORITE OF UPPER QUINNESEC FALLS. 103

rock. Its microscopical appearance is represented in Plate X, fig. 

1. The hornblende is mostly of a very pale color and more or less 

fibrous in structure. It has every appearance of being secondary, and yet 

in every case where it can be traced into a more compact mineral this 

is also a hornblende of a dark green or brown color. In many instances 

the hornblende is bent and curved, as diallage is so apt to be (see right 

side of the plate); but the most striking peculiarity of this hornblende 

and the one most like diallage is its very perfect parting, parallel to the 

orthopinacoid (100). This very unusual feature is apparently confined 

to the paler, and probably secondary hornblende. It stops abruptly on 

the edge of the compact, more deeply colored variety, even where this 

is continuous with the lighter kind (Plate X, fig. I). 1 This may indicate that the paler hornblende is secondary after diallage and therefore 

reproduces its orthopinacoidal parting, while the diallage was itself 

intergrown with an original compact hornblende which still remains. 

Aside from its characteristic prismatic cleavage, proof is furnished that 

this mineral is really hornblende by the fact that sections approximately 

perpendicular to the vertical axis, give, in converged polarized light, a 

bisectrix instead of an optical axis.

The feldspar of this rock is largely changed to opaque, gray saussnrite. Portions of it, however, remain in an unaltered state. These 

have, even in the thinest sections, a pale brown color (PI. X, fig. 1), 

and in thicker sections they are as dark as mahogany. This coloring 

is not due to inclusions from which the feldspar is surprisingly free, but 

to some unevenly distributed pigment whose nature it is far beyond the 

power of the microscope to reveal. With the exception of this color 

the feldspar is so remarkably fresh and clear as to make the impression 

at first glance that it must be of secondary origin. A slight examination, however, shows that the clear brown substance forms a peripheral 

zone about the saussuritized individuals; or where the whole crystal is 

unchanged, that it is bent, broken, or faulted in a manner inconsistent 

with any supposition of secondary origin. Here again we have additional, and, indeed, most convincing evidence that the greater the mechanical action to which the feldspar has been subjected, the less chemical change it has undergone. (See above, p. 88.)

The other constituents of this rock are ilmenite, surrounded by a leucoxene border, and some secondary quartz, which is penetrated by most 

delicate fibers of hornblende.

The analysis of this rock in column I of the folio wing table was made 

by Mr. E. B. Kiggs:

1 It ia interesting to note, in this connection, the recent observation of Cathreiu that the hornblende 

of Roda in the Tyrol has a perfectly developed parting due to very narrow twinn ing-lamellio which i n n 

parallel to the orthopinacoid. (Zeitschr. fur Kryst. u. Min., vol. 12, p. 12.) Whether the above-mentioned parting is due to the same cause or not, it is impossible, on account of the extreme narrowness 

of the parting-lines, to determine.
104 GREENSTONE SCHIST AREAS OF MICHIGAN^. [BULL. 62.

Silica (SiOj). ...........

Alumina ( A12 03 ) ....... 

Ferric oxide (Fe^Cy . . . . 

Ferrous oxide (FeO) ....

Soda (N%0). ..........

Potash (K20). ..........

Carbon dioxide (C02 ) . . .

Total..............

I.

48.35

15. 40 

4.04 

4.63 

10.38

11.61

1.87

.35

3.60

.'08

100.31

II.

51.46

14.35 

3.90 

5.28 

9ns

9.54

2.92

.24

3 3ft

.20

100. 27

III.

47.96

16.85 

4.33 

4.17

11 91

9.15

1.25

.30

2.89

.08

100. 23

Dried at 105°.

The analyses of the two analogous rocks, Nos. 11171 (II) from Sturgeon 

Falls, and 11008(111), from Lower Quinnesec Falls, are again cited from 

pp. 76 and 89, for comparison. The agreement between all three analyses 

is as close as could be expected for three different specimens of the 

same rockmass. In fact it'is difficult to escape the conviction that the 

rocks from both Upper and Lower Quinnesec Falls, like that from Sturgeon Falls, were once diallage-plagioclase aggregates or gabbros; and 

yet if this be true, we are compelled to assume either that the original 

diallage passed by paramorphism into compact brown hornblende before reaching its present state 5 or that the original rock was a hornblende gabbrp in which both bisilicate constituents passed finally into 

pale fibrous hornblende, although this process progressed much more 

slowly with the hornblende than with the pyroxene.

The feldspathic component of this rock (No. 11054) is determined by 

the analysis to be labradorite, as was also found to be the case in the 

corresponding specimens from the other localities.

Two other specimens of this same rock, No, 11061, from the Michigan side of the falls, and No. 11073, from a point above the falls, 

marked p on Major Brooks's map, are essentially the same as the one 

just described. Both are, however, more altered and both show, in a 

still greater degree, the effects of pressure upon the constituent minerals. 

In No. 11061 a good deal of quartz is present, always.penetrated by 

delicate needles and tufts of hornblende. The largest areas of this 

quartz form a rather coarse grained mosaic and look as though they 

had been deposited as a pseudomorph after some former constituent. 

Some colorless chlorite is also present in this section, having been derived from the alteratipn of the hornblende. The feldspar of No. 

11073 is colorless, and has been fissured, broken, and even crushed in a 

surprising manner. As a result, the saussuritization of this mineral has 

hardly more than commenced, in spite of the advanced state of alteration observable in the hornblende. One hornblende section in this 

slide, cut nearly perpendicular to the \ ertical axis, shows a very sharp 

line of demarcation between the interior, dark brown portion and the
WILLIAMS. 1 SHEARED GABBRO-DIORITK. 105

colorless peripheral zone. This resembles somewhat the secondary hornblende enlargements recently described by Van Hise, 1 but I can not 

regard this particular case as due to anything but exterior bleaching.

Nos. 11056 and 11057, from the Wisconsin side of the fall, illustrate 

the passage of this pale diorite into schistose varieties, as described at 

Lower Quinuesec Falls. The former specimen, though macroscopically 

still quite massive, has lost the distinctness of its structure. Under the 

microscope this is seen to be due to crushing and attendant chemical 

changes. Tne feldspar is almost pulverized, but has otherwise undergone but little change. A few of the larger crystals have been merely

cracked and faulted as shown in Fig. 

11. The hornblende is very largely 

changed to a colorless chlorite. 

Where one of the hornblende crystals has been broken and sheared, 

chlorite is seen to have been developed in parallel fibers. This furnishes an-admirable microscopic illustration of how the schistose bands 

are formed by pressure in the rockmass as a whole. (See also Fig. 22, p. 

128.) Just below the falls this rock 

becomes broken, gashed, and seamed 

with narrow, parallel veins of lamellar minerals. These gradually increase to bands, which finally compose the main portion of the rock. There can remain no doubt in the 

mind of anyone who will carefully study this exposure, that these schistose bands have been produced in the massive rock by the action of 

pressure, along lines where there has been a shearing motion.

No. 11057 is from one of the schistose bauds in No. 11056. The microscope shows it to be composed of a fine mosaic of quartz and feldspar (!) 

grains, made schistose by parallel fibers of serpentine and chlorite.

At and just below the engine house on the Michigan side of Upper 

Quinnesec Falls is another exposure which illustrates the gradual 

passage of these light colored, massive greenstones, into silvery, hydrous 

schists, similar to those described as occurring just below Lower Quinnesec Falls.

No. 11065 is a very light colored, although nearly massive, rock, seen 

in situ in a wall behind the engine house. It is composed of much 

crushed and faulted feldspar of the above described brown variety, 

together with colorless isotropic chlorite. There is also some of the 

quartz albite mosaic, though this is not abundantly developed, with a 

little iron oxide. There is no parallelism in the arrangement of the 

constituents which would tend to produce a schistose structure.

FIG. 11. Faulted feldspar crystal in crushed 

greenstone (No. 11056) from Upper Quinnesec 

Falls. Nicols crossed; magnified 30 diameters.

1 Am. Jour. Sci.. 3d series vol., 33, 1887, p. 385.
106 GREENSTONE SCHIST AREAS OF MICHIGAN. [BULL. 62.

Fro. 12. TtutiliMit'titllesin No. U05C magnified 

350 diameters.

ISTo. 11066 is from the same rock-wall as the last specimen and is undoubtedly continuous with it. It is decidedly schistose, and is composed wholly of a granular mosaic, with both sericite scales and a colorless chlorite. There are the finest microscopic rutile crystals in this

rock that were encountered in any 

specimens from the Menominee region. 

These crystals are simple individuals, 

twins and trillings showing a great 

variety of form. A few are represented in the accompanying figure. 

They are grouped together or arranged in lines or strings which follow the direction of the schistosity. 

As in other similar instances before 

described from the Lower Falls, they 

appear to have originated from the 

alteration of the ilmenite of the massive rock.

Just below the engine-house similar 

sericite schists are exposed (No. 11067 ] ) which contain green chlorite in 

spots and appear to grade into typical dark green chlorite schists (JSTo. 

11068). These rocks are mentioned by Major Brooks at the exposures 

marked on his map x and y 2. These .dark green schists are like those 

which originate from the alteration of the dark colored diabases, and 

their appearance probably marks the commencement of the belt of these 

rocks which skirts the basin below thie Falls.

THE COARSE GRAINED DIORITES OF THE HORSE RACE.

The half mile of comparatively quiet water, navigable by a small 

boat or canoe, immediately above the Upper Quinnesec Falls is bordered 

mostly by massive or schistose greenstones of the dark type similar in

all respects to the rocks of this kind which have been studied at several other localities on the Menominee Eiver. They are interspersed 

with bands of gneissose rock, identical with those yet to be described.

The narrow gorge known as the Horse Eace, through which the river 

flows for a mile 'or more, has, however, been excavated through rocks 

of a quite different character. These are for the most part coarse 

grained diorites, penetrated by bands of granitic and gneissic rocks 

which will receive special consideration in the following section.

Typical specimens of these coarse grained diorites areNos. 11077 and 

11093 from the Wisconsin, and Nos. 11182, 11183, 11185, and 11186 

from the Michigan side of the rapids.

The macroscopical appearance of these rocks is often that of an almost

'This rock was called by H. Credner "talc-schist" in 1809 (Zeitschr. Deutsch. geol. Gbsell., vol. 

21, p. 529). It was described in 1876 by Wichmaun, who gives seven reasons for regarding the sericite 

schists as of fVagmental origin. (Geol. Wisconsin, vol. 3, p. 638.)

'Ibid., 1, pp. 473,474.
WIU.UMS.) THE DIORITES OF THE "HORSE RACE." 1.07

uniformly gray mass, in which the large and frequently glistening hornblende crystals are not at first glance very apparent. This color is due 

to the alteration of the feldspar, in which new minerals are largely developed. An opaque, white feldspar, or rather saussurite, is, however, 

often visible. This is sometimes in the form of rounded or oval areas 

which represent the centers of crystals that have turned gray around 

their edges; sometimes in the form of the original feldspar crystals 

themselves, and again in sharp contrast with the dark green hornblende. The grain of these rocks varies extremely, but is rarely fine. 

The form of the different constituents is always visible to the naked 

eye; and, in some instances (No. 11192), these measure from one to 

two inches in 'diameter.

Aside from the unusual coarseness of their grain, the rocks of the 

Horse Eace differ from the other greenstones of the Menomiuee in frequently containing biotite. Another striking contrast is presented in 

the almost universal presence of talc. This mineral has without 

doubt originated from the hornblende, and has .wandered from it 

into the feldspar. It is scattered everywhere through the rock in 

minute, silvery, glistening scales, and it coats all cracks and fissures. 

The surface of the hornblende crystals is also frequently spotted with 

it. The nature of this mineral as talc is easily determined by both optical and chemical means. Under the microscope it is of a pale green 

color, resembling muscovite in its cleavage aud brilliant interference 

colors, but with a small optical angle and negative character. It is 

infusible aud not attacked by acids, while microchemical tests show 

that it is destitute of both alumina and alkalies.

The structure of the Horse Race rocks is that of the second kind 

mentioned by Kosenbusch as occurring in diorite, 1 i. e., that conditioned 

by the feldspar having crystallized before the hornblende. The latter 

component is plainly allotrimorphic, like the pyroxene in diabase, 

while the feldspar has a well developed form of its own. (PI. XII, 

fig. 2.) The hornblende, although comparatively compact and homogeneous over considerable areas, nevertheless conveys the impression 

of being of secondary origin. It has a pale green color, slightly darker 

toward the edge, and is externally frayed out into a fringe of radiating actinolite fibers, which wander along the cracks in the feldspar. 

(No. 11186; see PI. XII, fig. 2.) In some sections notably in No. 

11185 large portions of the hornblende have become wholly changed 

to a felt-like network of minute fibers. These characters, together 

with the shape of the hornblende individuals, convey the impression 

that they have been derived from pyroxene. While, therefore, it is 

impossible to prove it with certainty, it is difficult to escape the conviction that these rocks were originally coarse grained gabbros or 

diabases.

The feldspar shows in almost every instance a broad twinning stria1 Mikros. Physiog., 2d ed., vol. 2, p. 121.
108 GREENSTONE SCHIST AREAS OF MICHIGAN. [BULL.62.

tion, although this is more or less obscured by chemical changes. In 

one specimen (No. 11182) there is a brown feldspar which is perfectly 

preserved around the periphery of the crystals, while their interior is 

changed to saussurite. In other cases (Nos. 11093 and 11185) the feldspar is largely changed to saussurite, with which is mingled a good deal of 

actinolite and some biotite. In the latter case clear areas of brilliantly 

polarizing talc are quite abundant. In Nos. 11077 and 11186 the white 

or colorless feldspar is comparatively well preserved, but it is filled with 

pale green actinolite needles and some little biotite, which appear to 

have wandered into it from the hornblende. In these rocks the amount 

of sanssurite is inconsiderable.

Brown biotite is not infrequently an important constituent of these 

rocks. It is especially abundant in No. 11077. Although sparingly 

scattered through the feldspar like the actinolite, it is mainly to be 

found in close association with the hornblende, with which or out of 

which it seems to have been developed by secondary processes.

Other specimens represent exceptional varieties of the Horse Eace 

diorites.

No. 11091, from the Wisconsin bank of the stream, shows the feldspar wholly changed to a compact aggregate of epidote grains. 1 The 

hornblende, which is in sharp crystals, presents its usual appearance 

and characteristics. The epidote is so light colored that it might 

easily be mistaken at first glance for feldspar in the hand-specimen. 

The microscope, however, at once shows its true character. The interference colors of the closely interlocking grains present a brilliant 

appearance. No trace of the original feldspar remains. Only an occasional grain of calcite or quartz interrupts the continuity of the epidote. 

Epidote is not an unusual secondary product in feldspar, although such 

perfect pseudomorphs of this mineral as are here present are rare.2

Still other specimens show, in a marked degree, the effects of dynamic metamorphism upon the Horse Eace rocks. No. 11189, from the 

head of the rapid, on the Michigan side, has a pronounced gneissic 

structure. Its hornblende is of the usual character, sometimes in compact crystalloids of large size, sometimes in a fibrous aggregate, but 

almost always much twisted, bent, or broken. The feldspar, in some 

instances, is completely changed to an aggregate of epidote crystals, -as 

in the rock last described; in others, it merely has minute, highly refracting epidote grains more or less abundantly developed in it (saussuritization). These are often accompanied by rhombic clodecahedra 

of a colorless garnet.3 Other of the feldspar individuals show a mechanical rather than a chemical alteration. There is, in some cases, a

1 A not unusual alteration according to Eoth; Allgemeine und chemische Geologie, vol. 1,1879, p. 321.

2Cohen mentions such a perfect transformation of feldspar to epidote in the diorite which forms the 

base of the island Palma. Neues -Tahrbuch f iir Mineral., 1876, p. 751. Rosenbusch also mentions them 

in dioritos from the Vosges Mountains. Mikros. Physiog., 2d ed., vol. 2, p. 103.

8 Eosenbusch: Mikros. Physiog., 2d ed., vol. 2, p. 136. Paul Michael finds one of the two varieties of saussurite in the gabbro of Wojaleite, Ficbtelgebirge, composed of a lime-alumina garnet and 

serpentine. Nenes Jahrbuch flir Mineral., 1888, vol. 1, p. 39.
WILLIAMS.] THE DIORITES OP THE "HOUSE KACE." 109

peripheral granulation (randliche kataklase) producing the " mortar 

structure" (Mortel-Structur of Tomebohm). In other cases there is a 

complete reduction of the whole feldspar crystal to a granular mosaic, 

while iu still others there is only a separation into large, interlocking, 

and but slightly disturbed areas. Fine veins of epidote not infrequently 

traverse these feldspar crystals. A point of niiueralogical importance 

connected with this rock is the evidence it affords of the alteration of 

ilinenite into sphene. The usual alteration product of this mineral, 

leucoxene, is now generally believed to belong to this species, but it is 

rare to find the crystalline form of the spheue so typically developed \s 

in this section. The mineral is of a grayish brown color and nearly 

opaque, like the secondary leucoxene which surrounds ilinenite. It 

polarizes with dull colors, but there is no mistaking its nature on account of its characteristic form. (PI. XIII, fig. I.) 1

No. 11078, collected on the Wisconsin side of the river, just below 

the Horse Race, is perfectly continuous with No. 11077, above described, 

and represents a phase of this rock produced by the action of intense 

pressure. The hornblende is almost wholly changed to an aggregate 

of weakly polarizing fibers, which, according to the descriptions of 

Eosenbusch, are perhaps best regarded as of a serpentinous nature. 

These twine in and around the feldspar so as to produce a sort of 

" Flaser " structure. The feldspars themselves are sometimes reduced to 

a mosaic ; but sometimes they have only a peripheral granulation. Much 

of the biotite of the unaltered rock remains among the alteration products of the hornblende. The iron ore has been drawn out into sinuous 

stringers which follow the direction of the foliation.

No. 11191 was taken from a wavy schistose, chloritic band on the 

Michigan side of the Horse Kace. It has a strike 30° E. of N., and 

traverses a coarse grained massive greenstone, into which it passes by 

insensible gradations. In this rock there is considerable hornblende, 

which is seen in the process of transformation to a fibrous, yellow serpentine. The feldspar is of the reddish brown variety and has undergone granulation around the edge. This has been accompanied by the 

production of quartz and calcite. In a large area of the latter mineral 

thgre are crystals of a substance whose exact nature could not be determined, It has the bluish interference colors of zoisite, a parallel 

extinction and a biaxial interference figure. It is also changing to the 

same yellow serpentine as the hornblende. The sepentiue penetrates 

and traverses it as it does crystals of olivine. Chlorite is also a frequent 

component of this rock.

Nos. 11080 and 11195 are exceptionally fine grained greenstones, the 

former from the foot and the latter from the head of the Horse Eace. 

Both are composed essentially of actiuolite, feldspar, quartz, and epidote, and resemble iu their microscopical appearance and structure the

'Schenck observed the titanic iron entirely replaced by splioue in a, schistose diabase from the 

upper Ruhr Valley. luaugural Dissertation, Boun, 1884, p. 57.
110 GREENSTONE SCHIST AREAS OF MICHIGAN. [BULL. 62.

typical so-called " crystalline schists." Still the former is massive and 

cuts across the strike of schistose rocks like a dike, while the latter is 

decidedly schistose. No. 11195 contains minute and sharp, dark brown 

sphene crystals, which appear to have originated, like those iu No. 

11? 89, from the alteration of ilmenite.

THE ACID ROCKS OF UPPER QUINNESEC FALLS AND THE HORSE TtACE.

Among the rocks to be found in the neighborhood of the Upper Quinnesec Falls and Horse Eace are those of an acid type. They occur iu 

bands of varying width, which generally, but not always, conform to 

the foliation of the greenstones. These acid rocks are sometimes massive granites without a trace of schistose structure; sometimes gneisses 

with a " Flaser" or "Augen" structure, and sometimes finely banded and 

schistose rocks like the Saxon granulites. Whenever these bands show 

any indications of schistose structure this seems to be conformable to 

the cleavage of the adjoining greenstones, without reference to what 

the direction of the band itself, may be. Inasmuch, however, as these 

two directions generally coincide, it is not common that the foliation of 

the acid bands makes any angle with their sides. Such instances were 

nevertheless occasionally found, and they have a very important bearing on the genesis of these rocks and on their relation to the greenstones which surround them.

An inspection of the published geological maps of the region south 

of Lake Superiqr 1 will show that immediately south of the portion of 

the Menominee Eiver under discussion there is an enormous area of a 

typical granite. Major Brooks, in his report on the Menomiuee district, thinks that this granite may very possibly be of eruptive origin, 

but, whether so or not, that it certainly overlies and penetrates ("in 

veins") the upper members of the greenstone series.2 Since, however, 

he regards the greenstones as occupying the upper portion of the Huronian formation, he is compelled to assume that the granite represents 

its youngest member and hence he designates it as the "Huroniau 

granite." (See profile, Fig. 2.)

Without entering in this place into a discussion of the stratigraphical 

relationships of these rocks, it may at least be stated with certainty 

that this granite does penetrate the greenstones iu the form of dikes 

and £^>ophyses, especially where the main mass approaches nearest to 

the river. This point is about half a mile south of the Horse Eace, as 

may be seen by Major Brooks's section and map of this region 3. Here 

the typical, coarse grained and somewhat porphyritic granite appears 

in a high ridge.

The adjacent greenstones and greenstone schists do not here materially differ from those at Upper Quinnesec Falls. They strike nearly east

'Geol. Wisconsin, vol. 3, atlas, PI. XXVIII; Hon. TJ. S. Geol. Survey, vol. 5, PI. I; Fifth Ann. 

Kept. TJ. S. Geol. Survey, PI. XXII. 

*Geol. Wisconsin, vol. 3, pp. 452, 531, 7.10. 'Ibid., PI. in, p. 472.
WILUAMB.I GRANITE SOUTH OF THE "HORSE RACE." Ill

and west and dip to the south. Near the granite contact they are filled 

with dikes of all sizes, evidently offshoots from the main, granite mass 

(Nos. 11105 to 11107). These dikes, when small, are fine grained and 

felsitic, but when larger their texture is coarser and they have frequently a well developed schistose structure parallel to that of the adjoining schists. After a careful examination of this locality and of the 

exposures between it and the river, there is no doubt ^iii the writer's 

mind that the granite, "Augengneiss," biotitic gneiss, and schistose 

porphyry (or u por; thyroid," as Credner calls this rock) visible near the 

Upper Quinnesec Falls and along the Horse Eace^ are also dikes or 

aphophyses connected with the main southern granite area. The schistose or banded structure of these rocks, when such exists, is a secondary 

feature, produced by the same dynamic agencies which rendered the 

greenstones themselves schistose.

Aii intelligent study of these acid rocks should begin with an examination of the massive granite from the large area south of the Meuomiuee. 

No. 11104 was collected near the contact, about half a mile south of the 

Horse Kace, where, after appearing in numerous dikes in the greenstone, the solid mass of granite appears in an abrupt wall, trending N. 

80° E. This rock appears in a hand-specimen to be a typical, coarse 

grained grauitite, with a decided tendency to a porphyritic structure. 

The latter feature may, however, be due to the nearness of this locality 

to the contact. When examined under the microscope, the macroscopic 

diagnosis is found to be correct and several additional points of interest 

are brought to light. The oldest constituents are zircon and a,patite; 

both quite abundant in the form usual in granitic rocks. Iron oxide 

seems to be almost absent as an original constituent, though it is found 

in some of the altered micas. The biotite, the only mica present, is not 

abundant. It is invariably bleached to a green color by the reduction 

of its iron to the ferrous state. It contains abundant inclusions of apatite, zircon (around which are pleochroic aureoles), and some secondary 

magnetite. No trace of either hornblende or pyroxene was observed. 

Sphene, however, is present, as are also a few sharp crystals of a dark 

grayish blue tourmaline. The principal interest of this rock attaches to 

its feldspar and quartz. They together make up nearly the whole mass 

and exhibit, in a remarkable degree, the effects of pressure. Jhe feldspar 

is of three kinds: normal plagioclase (oligoclase), unstriated orthoclase, 

and microcline. The relationship of these species of feldspar is a suggestive one. Both the oligoclase and the orthoclase are always altered 

to a fine micaceous or kaolinitic product which is particularly abundant 

in the center of the crystals, a zone of the unaltered mineral being often 

preserved around the edge. The microcline, on the other hand, almost 

never shows any indication of alteration.

It is always clear and fresh in appearance, but its twinning lamellce 

are beut or curved and bear every sign of having been secondarily developed. 1 In other words, we have here again, in the writer's opinion,

 Bosenbusch, Mikros. Physiog., 2d od., vol. 2, p. 295.
112 GREENSTONE SCHIST AREAS OF MICHIGAN. [BULL. 62.

a striking example of the mechanical action apparent in the feldspar, being inversely proportional to the chemical action which has gone on in it. 

Original orthoclase has been converted partly into kaolin, partly into 

microcline, according to the way in which the action of the force manifested itself. The effect of the force which acted upon this rock is also 

apparent in other ways. The large original feldspar crystals show a 

peripheral granulation (see Chapter I, p. 49), and where they have been 

fissured their cracks are filled with a new crystallization of plagioclase, 

orthoclase, and quartz. None of these minerals shows any signs of 

chemical alteration and microcline is never to be found among them. 

Thus is produced a good example of what Tornebohm has called a mortar structure ("Mortel-Structur") 1 . In this secondary cement-like aggregate, a micropeginatitic intergrowth of quartz and feldspar is quite 

common, and calcite, in good-sized individuals, is by no means rare.

The original quartz of this granite also shows many indications of 

having been squeezed. The crystals or grains often have, an undulatory 

extinction, while larger grains are broken and the fragments are more 

or less displaced. Proof that these were not originally different grains is 

given in section No. 11104, where a small tourmaline crystal is broken 

and faulted just at the contact.2 (See Fig. 18). No. 11105 is a specimen of the prevailing rock near the 

granite contact. This is a hornblende 

schist, composed mainly of irregular 

grains of dark green hornblende and 

quartz. There are beside some striated and some unstriated feldspars, 

a little sphene and an abundance of 

extremely sharp and clear epidote 

crystals, The banding of this rock 

is caused by the preponderance of 

hornblende in some layers and of 

quartz in others. Its strike is nearly 

east and west and its dip 75° to 

80° to the south. Under the microscope all of the constituents by 

their extreme freshness and freedom 

from all inclusions convey the impression of being secondary crystallizations. The texture of the rock is granular, although the epidote 

is present either in exceedingly sharp crystals (which often pass 

unbroken from one quartz grain into another) or in much more 

minute, ovoid granules, forming the little, highly refracting particles 

which are, so common in the crystalline schists. No. 11107 was 

taken from one of the wider of the numerous acid dikes which pene1-N3,gra ord om granit och gneis. Geol. Foren. Stockholm Forliandl., 1880-'81, vol. 5, p. 233, (ref. 

Neues Jabrbuch fur Mineral., 1881, vol. 2, Referatc, p. 51; ami Rosonbuscli, Mikros. Physiog., 2d ed., 

 vol. 2, p. 42.)

2 Compare Dr. Chelius's remarks oil tbe Hessian granite-porphyries, Notizblatt des Vereins fur Erd. 

kimdc zu Darmstadt, 1883.

Flo. 115. Broken tourmaline crystal proving 

the secondary fracture of the inclosing quartz 

grain, "Horse Race" granite. No. 11104.
WILLIAMS.] GRANITE GNEISSES OF THE "HORSE RACE." 113

trate the last described hornblende schist in the neighborhood of the 

granite contact. It has a decidedly gneissic structure parallel to the 

banding of the inclosing schist, which is here also the direction of this 

band or dike. The microscope shows that the mineral composition of 

this rock is identical with that of the coarse granite (No. 11104). 

There is the same zircon, sphene, and tourmaline associated with the 

biotite, feldspar, and quartz. The differences are the gneissic structure, 

fineness of grain and absence of all porphyritic structure in the dike 

rock, while none of its constituent minerals show the chemical alteration which has progressed so far in the massive granite. Both the 

biotite and the feldspar are quite fresh, the rich brown color of the former contrasting with green and bleached mica above described.

After the foregoing review of the character of the acid rocks at and 

near the granite contact, we are in a much better condition to understand the occurrence of the bands of gneiss and porphyroid near the 

Upper Quinnesec Falls, which were observed by Major Brooks and Prof. 

Pumpelly.1 If we now examine some of the most typical of these exposures with care, we shall find that they are quite identical with the 

rocks which we have just been considering.

On the lower side of a small cove which indents the Wisconsin 

river bank immediately below the Horse Race there is a narrow band 

of acid rock well exposed on the shore. The center of this band is composed of a massive, gray colored, porpbyritic rock resembling a granite 

(No. 11082). This grades on both sides imperceptibly into a well characterized, fine grained gneiss (No. 11085), which in places is so finely 

banded as to resemble some of the Saxon granulites (No. 11084). The 

foliation of this exposure is nearly vertical and seems to be quite parallel to the sides of the baud, which itself makes only a slight angle with 

the cleavage of the adjoining greenstones.

No. 11082 was collected from the central and most massive portion 

of this acid band. Macroscopically the rock appears to be a typical 

granite porphyry, nor does the examination of a thin section with the 

microscope reveal anything opposed to this determination. The following analysis of the rock, however, made by Mr. R. B. Biggs, shows that 

this rock is in reality a diorite :

Silica (Si02 ) ................................................ 54. 83

Alumina (AL,O:1 ) ............................................ 25.4!)

Ferric oxide (Fe.2 0:J ) ........................................ 1.61

Ferrous oxide (FeO) ........................................ 1. 65

Lime (CaO) ................................................ 6.08

Magnesia (MgO).............. .... ...... .................... 1.96

Soda(Na2 0) ................................................ 5.69

Potash (K3 0) ............................................... 1.87

Water (H30)................................................ 1.18

Carbon dioxide (C0.3 ) ....................................... .18

Total............................... ..................100.54

1 Geol. Wisconsin, vol. 3, p. 474.

Bull. 62 8
114 GJKEENSTONE SCHIST AKEAS OP MICHIGAN. .[BULL.02.

The microscope discloses good-sized crystals of a striated arid zonally 

formed feldspar embedded in a granular mosaic of clear, colorless, and 

nnstriated grains associated with brown leaflet of biotite. Apatite, 

zircon, and a reddish pleochroic spheiie are also present in smaHcrystals. In spite of this rock showing no indication whatever of alteration, it contains calcite in considerable quantity. This mineral apparently penetrates and includes all the other constituents in a manner 

which has led Hawes,1 Tornebohm,2 and Chrustschoff,3 to regard it 

under similar circumstances as a product of the original crystallization. 

In one section of the present rock sharply denned sphene crystals were 

observed wholly surrounded .by the calcite. Nevertheless the presence 

of this carbonate in the fresh rock is to be explained, as Eosenbusch 

suggests, by the filling up of drusy or iniarolitic cavities.4

The rock appears both macroscopically and microscopically to be a 

granite porphyry with a micrograuitic groundmass. The analysis, however, shows it to be largely composed of a feldspar of the audesine 

series, and this is also indicated by a further examination. The colorless grains of the groundmass, in spite of their limpid character and 

superficial resemblance to quartz, give in converged polarized light the 

brush or hyperbola of a biaxial crystal. In the Thoulet solution the 

colorless portion of the powder fell between 2.683 and 2.650, a large 

proportion, however, being confined between the limits 2.668 and 2.659. 

A small proportion possessed exactly the specific gravity of quartz, 

so that it is impossible to say with certainty whether this mineral is 

present in the groundmass. The relative proportions of silica and 

alumina, however, as given by the analysis, make this=improbable. We 

must describe this rock as a very pure type of a mica-diorite porphyry 

whose grouudmass is composed of a mosaic of clear unstriated limesoda feldspar, probably andesine.

The two specimens (Nos, 11084 aud 11085), taken from the gueissic

portion of this exposure, when seen under the microscope, closely resemble the groundmass of the diorite porphyry just described, The 

chief difference is in the banded appearance, produced by the parallel 

arrangement of the grains, and especially of the mica plates. This is 

most pronounced in No. 11084, where the mica is collected in certain 

layers and where there is also an alternation in the grain, some layers 

being coarser and others finer. Occasional tourmaline crystals are 

here associated with the zircon and apatite. Calcite also is present, as 

in the last described rock, but now it is in stringers and arranged, like 

all the other components, in the direction of the schistosity. No. 11085 

has neither the calcite nor the tourmaline. Its most interesting1 feature 

is the presence of long strings of minute rutile crystals which appear to

1 Geology oi New Hampshire, vol.3, Part IV; Mineralogy and Lithology, 1878, p. 208, PI. XII, Fig. 1. 

2 OmKalkgvanit. Geol. Foren Stockholm Forhandl,, vol. 3, 1876, p. 210. Om Kalcithalt i graniter. 

Kong. Vcteask. ak. Forliandl., vol. 5, 1831, p. 233. 

3Bull. Soc. Mineral, do France, vol. 8, 1885, p. 137. 

4 Mikros. Pbysiog., 2d ed-, vol. 2, 1885, p. 34,
WILLIAMS.] GRANITE GNEISSES OF THE " HORSE RACE. 115

have originated from the alteration of the biotite. No. 11079, collected 

from a narrow acid band sligh ly east of this exposure, is quite identical in both composition and structure with the two last mentioned 

rocks. There are here occasional remains of former porphyritic feldspar 

crystals which have been mostly destroyed by granulation. Tourmaline 

is quite abundant, as are also flakes of 

colorless but brightly polarizin g sericite. 

It will be seen that these rocks are 

quite the same as those found farther 

south at the granite contact. Along 

the whole extent of the Horse Eace 

similar acid rocks are very abundant.

They occur iu bands varying from the

finest seams to such as are many yards 

in width. While these as a rule follow 

the direction of the foliation of the 

greenstones, they by no means always 

do so. Their mode of occurrence often 

bears strong testimony to their intrusive nature, as may be seen from the 

following figures sketched at the head 

of the Horse Race on the Michigan side 

of the river.

Fig. 14 shows two bauds of gneiss, one parallel to the strike of the 

greenstones, the other cutting across it. In each the lamination of the 

acid rock coincides with that of the greenstones, without regard to the 

sides of the band. This would seem conclusive evidence of the secondary nature of the gueissic structure.

Gneiss.

Fig. 14. Exposure on the edge of a small 

coveat the head of the Horse Kace. Direction of the shading lines indicates the 

strike.

Fig. 15. Foliated granite (gneiss) intrusive in greenstone. Head of the Horse Eace.

Fig. 15 shows a band of gneiss which comes to an abrupt termination in the greenstone and which sends out apophyses into this, as only 

an intrusive dike would do.
116 GREENSTONE SCHIST AREAS OF MICHIGAN. [BULL. 62.

Fig. 16 shows an irregular area of gneiss penetrating the greenstone. 

The lines in the drawing indicate the direction of the foliation. This 

may be a cross-section of a laterally intrusive arm.

FIG. 16. Foliated granite (gneiss) intrusive in greenstone. Head of the Horse Eaco.

Many other exposures might be selected to prove the intrusive 

nature of these acid rocks at the Upper Quinnesec Falls, but the ones 

already cited will suffice for the present purpose.

With respect to their structure, the acid rocks of the Upper Quinnesec Falls and Horse Race may be classified as follows:

I. Porphyritic:

Granite porphyry, represented by Nos. 11104, 11082, 11087,

11089, 11190.

Augen-Gneiss, represented by Nos. 11184, 11193 11196. 

Schistose porphyry, represented by Nos. 11050, 11051,11052,

11096.

II. Non-porphyritic: -

Gneiss, represented by Nos. 11079, 11084, 11085,11188. 

Felsite, represented by No. 11071.

The main points of interest will now be stated with reference to each 

of these types in succession, although no further mention will be made 

of those specimens which have already been described, viz, Nos. 11104, 

11082,11084,11085,11079. 

It is believed that the distinctive features of the schistose and 

banded rocks of this region have to a great extent been produced by 

secondary causes. An original porphyritic structure seems to have been 

very prevalent, if not universal, in all of these acid dikes. We are able 

to trace its gradual disappearance, and we find that this is proportional 

to the intensity of the dynamic action to which the particular rock has 

been subjected. The crushing or rubbing of the constituent minerals 

against one another has produced a peripheral granulation (" randliche 

Kataklase"). This has altered the porphyritic crystals into the eyes 

or "Augen" of the gneisses and schistose porphyries, as is shown in 

PI. XV, fig. J ; and there seems no reason why its extreme application may not have changed a porphyritic rock to a felsite.
WILLIAMS.] ORTHITE IN GRANITE. 117

Granite porphyry. No. 11190, from the Michigan side of the Horse 

Race, near its center, is from a small area of acid rock exposed at the 

water's edge, which seems to be completely surrounded on the three 

other sides by diorite. It is porphyritic, but contains a very large ptfoportion of feldspars (mostly striated) and comparatively little groundmass. This latter is so arranged as to produce a resemblance to the 

mortar-structure, and thus the rock approaches No. 11104, although all 

of its quartz is confined to the grouudmass. Biotite is quite abundant. 

It contains fine pleochroic aureoles, and is frequently bleached to a 

pale green color.

Nos. 11087 and 11089, both from the Wisconsin bank of the Horse 

Race, are decidedly porphyritic. A fine grained granular gronndmass 

is in sharp contrast to the well formed feldspar crystals. These are 

noticeable on account of their finely developed zonal structure, which 

is produced sometimes by a concentric arrangement of inclusions, sometimes by a variation of composition and extinction angles in different 

layers. The zonal structure of the feldspars in these rocks is as perfect as it is in the more modern andesites. The mica in No. 11087 is 

biotite of the usual character, but in No. 11089 muscovite is equally 

abundant, making this latter rock a true granite instead of a grauitite. 

Some of this muscovite, at least, may be plainly seen to have originated 

from the alteration of the orthoclase. In many cases it lies in brilliantly 

polarizing flakes of considerable size, clustered together in the center 

of the orthoclase crystals, which portion always appears to be the most 

subject to alteration.

These rocks show to a moderate degree the effects of pressure which 

are most manifest in the porphyritic quartzes. These are much distorted and often broken and granulated, as shown in PI. XV, tig. 2. 

It seems at first thought strange that 

the harder quartz should be more susceptible to pressure than the softer 

feldspar, and yet this is known to be 

a well established fact, as has been 

observed in Chapter I. The brittleness of the quartz causes it to break, 

where the feldspar is only bent or 

distorted, without rupture.

In some cases the feldspar, as well as 

the quartz, shows a slight peripheral 

granulation and passage into a mosaic, 

which can not be distinguished from 

the groundmass. This is in all instances, however, only incipient, while 

the main portion of the groundmass is an original feature of the rock.

In the granitites, Nos. 11087 and 11190 an unusual constituent is 

present. This is in sharp crystals of good size (2 by 3mm) and has the 

shape of epidote. Its color is yellowish brown or greenish yellow.

FIG. 17. Ortliite in granite. Horse Kace 

Rapid. No. ] 1087. Magnified 80 diameters.
118 GREENSTONE SCHIST AREAS OF MICHIGAN. fBULi,G2.

It seems to have undergone partial alteration, and agrees closely with 

the published descriptions of orthite (allanite).1 The two crystals in 

section No. 11087, although by no means entirely changed, appear to 

be completely isotropic, a peculiarity especially mentioned by Kosenbusch as not uncommon to orthite.2

Augen-Gneiss. The best example of this type is No. 11184, from the 

Michigan side of the Horse Eace. With reference to this rock the following is extracted from the field notes made in July, 1885:

A short distance above the. mouth of the Horse Race the ridge is cut by a ravine. 

Here, near the water's edge, are some beautifully porphyritic, red, acid rocks, occurring apparently as dikes in the greenstone. The direction of these dikes is quite 

irregular. Their width also varies continually. They contain a deep red feldspar, a 

white feldspar, and quartz in porphyritic crystals. When much of the first mentioned 

feldspar is present, the rock is very red ; when it is absent it is gray. Both varieties 

may occur in the same specimen. These rocks show the effectsof pressure very finely. 

The porphyritic crystals form "Augen," around which the groundmass bends. The 

direction of the handing when well marked appears to be parall 1 to the walls of the 

dikes. The " Augen " stand vertically.

Under the microscope the banded or gneissic structure of this rock is 

as well marked by the lenticular (" Flaser") arrangement of the mica as in 

the hand-specimen. Its most instructive feature is the formation of the 

eyes (" Augen ") and the gradual disappearance of the large porphyritic feldspar crystals by peripheral granulation due to pressure. A 

typical example of this is represented in PI. XV, fig. 1. Here it 

will be seen that a large porphyritic crystal of triclinic feldspar has been 

granulated on two opposite sides, corresponding to the direction of the 

schistosity, and that the material thus formed has been drawn out into 

a lenticular-shaped mass, with the remaining solid portion of the feldspar as its center. This forms the eye or " Auge" around which the 

groundmass is seen to curve. Such a structure has been described and 

figured by J, Lehinann 3 aud others. It is important to observe that the

secondary granulation has here produced a mosaic of much coarser grain 

than the surrounding grouudmass. This corresponds to observations 

of Dr. Chelius, made on certain Hessian granite porphyries.4

No. 11196, from the head of the Horse Eace, on the Michigan side, is 

described iu the field notes as " a typical biotite gneiss with a wavy 

structure, the biotite lamellae (secondary?) winding around the porphyritic crystals of feldspar and quartz." Under the microscope the groundmass is seen to be fine grained, and, to some extent at least, to have 

originated from the granulation of the porphyritic crystals, which exhibit this process in an admirable manner. Dark brown biotite and 

sphene are common. Zircon and apatite are also present. The quartz

'A. Sjogren, Geol. Foren. Stockholm Forhandl., vol. 3, 1876, p. 258, Tornebohm Under Vega-Expeditioneu insamlade bergarter. Vega-Exped. vetensk. jakttagelser, VI, Stockholm, 1S84, p. 124. Cross 

& Iddings, Am. Jour. Sci., 3d series, vol. 30,1885, p. 108.

2 Mikros. Physiog., 2d ed., vol. 1, p. 499.

3 Untersuchungen iiber die Entstehung der altkrystallinischnn Schiefergeateine. Bonn, 1884.

4 Notizblatt des Vereins fur Erdkunde zu Darmstadt, 4. Folge, 5 Heft., 1885, quoted by Rosenbusch, 

Mikros. PhyBiog., 2d ed., vol. 2, p, 294.
WILLIAMS,] SCHISTOSE PORPHYRIES OF THE "HORSE RACE.'* 119

is in large granular areas. Bows of fluid cavities traverse the grains 

and pass uninterruptedly from one to another. 

The following analysis of this rock is also by Mr. B. B. Biggs:

Silica (Si02 ) ............................................... 67.77

Alumina (A1 2O3 ) ...... ........................... .......... 16.61

Ferric oxide (Fe2 03 ) ....................................... 2.06

Ferrous oxide (FeO) ...... ... ............................. 1.96

Lime (CaO) ...... ........................................ 1.87

Magnesia (MgO) .......................................... 1.26

Soda (Na20)............................................... 4.35

Potash (K20) ............................................. 2.35

Water (H20) ...................................... ........ 1.69

Carbon dioxide (C02 ) ................'...................... .19

Total ...... ......................................... 100.11

This is much more like the analysis of a granitite than the last one 

given (No. 11082), but the preponderance of soda over potash indicates 

the presence of a large amount of soda orthoclase.

Schistose Porphyries or Porphyroids. Just below Upper Quinnesec 

Falls, especially on the Wisconsin side, are good exposures of finely 

schistose acid rocks. They contain small porphyritic crystals of feldspar and quartz, and a considerable amount of sericite or hydro-mica, 

which winds around these crystals so as to produce a lenticular or 

" leaser'" structure. Tn color they present every shade from flesh red 

to gray.

Bocks of this general type have a wide distribution and have arrested 

the particular attention of petrographical investigators. In some instances they have been conclusively shown to be the product of the 

raetamorphism of fragniental rocks either tuffs or true sediments 

while in other cases they have resulted from the action of great dynamic 

forces upon massive rocks of a corresponding composition, i.e. quartz 

porphyries. When it is possible to trace with certainty the origin of 

these rocks to stratified deposits they are usually designated Rsporphyroids, while in case their mother-rock can be shown to have been quartz 

porphyry they are usually called schistose porphyries. In their extreme 

development these two types are often petrographically indistinguishable and they present an instructive instance of the production of identical results by the action of the same physical forces upon matter of the 

same average composition, though differing widely in origin and its 

structure.1

The schistose porphyries occurring at Upper Quinnesec Falls were 

made the subject of an extended paper by Hermann Gredner, published 

in 1S70.2 This author concludes from a macroscopical examination that 

these rocks are altered sediments which are interpolated between the

' Bull. TJ. S. Geol. Survey, No. 28, p. 1.

z Uober Nordamorikaniache Schieferporphyroitle. Nenes Jahrbuch fiir Mineral., 1870, pp. 970-984.
120 GREENSTONE SCHIST AREAS OF MICHIGAN. [BULL. 62.

diabase beds of the Upper Huronian. He divides their whole exposure at this locality, which he gives as 300 feet, as follows:

	Feet.

(b) Weakly schistose orthoclase porphyroid ....................... 50

(c) Typical feldspar-paragonite schist............................. 10

(d) Light llesh-red orthoclasc-paragonite schist.................... 30

(e) Paragouite schist .. :.......................................... 15

(f) Lime-paragonite schist........................................ 15

(g) Light reddish gray schistose porphyroid....................... 30

(h) Lime-chlorito schist .......................................... 50

(i) Chlorite schist................................................ 100

Total:...................................................... 300

Of (b), (c), (d), and (e) he gives the following analyses:

SiOg. .......

A1203 ....... 

Fe203 .......

MnO. .......

CaO.........

Mo-0........

K2O.. .......

N^O ....:.

H2 0 ........

Total....

(b)

60.70

15.90 

4.70

tr.

tr.

8.06

5.50

100. 86

(c)

72.45

8.85 

6.20

tr.

9.24

3.70

100. 44

(d)

76. 505

7.950

S

onz

tr.

0.322

tr.

1.025

4.384

99. 061

(e)

75.5

8.6 

2.6

7.2

1.2

0

0

3.0 

1.5

99.9

In discussing these analyses Creduer finds sufficient proof that orthoclase alone is present, from the fact that the potash increases proportionately with the amount of feldspar, and in the absence of a twinning 

striatiou which is visible to the naked eye. The sodium he attributes 

entirely to the mica, which, in consequence, he names paragonite. 

The proof of sedimentary origiu he finds in the constant parallelism of 

the schist planes with the limit between these rocks and the adjoining 

diabase; and yet he cannot help being much struck with the alternation of such widely diverse rocks in a single and comparatively thin 

complex. 1

My studies of these rocks, both in the field and in the laboratory, 

have led me to results quite the reverse of those obtained by Credner. 

Nos. 11050 to 11053 were collected from the most typical band, which 

here stands nearly vertical and strikes 20° S. of E. They are all essentially the same in everything except color. They contain more or less 

rounded porphyritic crystals of quartz and a red or grayish feldspar, 

imbedded in a matrix apparently composed of a yellowish gray, greasyfeeling hydro-mica.

1 Die ebenerwahnte AVecbsellagerung vollkomraen verschiodenartiger Gesteine als zusammen' 

gehb'rige Glieder eiiier wenig macbtigen Schichteureihu ist bochst autiiillig. Zwiscbea zwei Lagern 

von Diabas, also einem namentlich aus Kalkfeldspatb und Augifc bestebenden basiscben Gesteine init 

etwa54 Proc. Kieselsiiure tritt zuuachst eiuo Zone von sauren quarzreicben Kalifeldspath-Natronglirn. 

mor-Porphyroiden mit iiber 70 Proc. Kieselsaure, aber obiie Kalkgebalt auf," etc. Loc. cit., p. 981.
WILLIAMS.] SCHISTOSE PORPHYRIES OF THE " HORSE RACE." 121

The following analysis of No. 11050 by Mr. E. B. Riggs shows its 

essential identity with Credner's specimen (b). Almost the only difference is the smaller amount of alkali, although the relative proportions remain about the same. In Credner's analysis the CaO and 

MgO, which make up this difference, do not appear to have been determined.

Silica (SL02 ) ................................................ 66.69

Alumina (A1203 ) ............................................ 1C. 69

Ferric oxide (Fe303 )......................................... 2.06

Ferrous oxide (FeO) ........................................ .93

Lime (CaO)................................................. 1.40

Magnesia (MgO) ............................................ 1.15

Soda(Na2O) ................................................ 2.46

Potash (K90)..~........................................... 5.23

Water (HS0).~............................................. 1.70

Carbon dioxide (C02 ) ............................... .. ...... 1.42

Total................................................. 99.73

This is evidently the analysis of an orthoclase rock, and yet the microscope shows that a considerable proportion of the porphyritic feldspars are striated and triclinic, in spite of the fact observed by Credner, that this character is not apparent to the unaided eye. We may 

therefore reasonably suppose that the potash exists largely in the 

groundmass and in the secondary hydro-mica, which is to be regarded 

as true sericite, and not paragonite as assumed by Credner.

The microscope discloses in all the thin sections of these rocks the 

typical structure of a quartz porphyry modified, however, by the 

action of great pressure. An idea * of it may be obtained from PI. 

XIV, fig. 1, which represents No. 11050. The fine grained, microgranitic groundmass is still present, inclosing the porphyritic quartzes 

and feldspars. The former often exhibit their characteristic dihexahedral form, but they are broken and possess anundulatory extinction. 

The feldspars are most instructive. They possess a sharply defined 

crystal form which, however, has been cracked and the fragments have 

been more or less separated in the direction of the schistosity, while 

brightly polarizing sericite scales have been abundantly developed 

in all the fissures. Sericite is also abundant in the groundmass, where it 

appears to have been formed from the orthoclase. Its scales are parallel throughout the rock, to which fact the schistose structure is almost wholly due. The original biotite of these rocks is now entirely 

represented by a pale green chlorite. During the process of this alteration, iron has separated in the form of opaque, black grains; and, 

in some instances (11050 and 11052), colorless but very highly refractive grains have been produced. These sometimes possess a sharp 

octahedral form and are probably anatase, formed from the titanic acid 

of the original biotite, as described by Stel/ner. 1 In and around this 

chlorite, tourmaline is also abundant either iu single individuals or in 

groups of crystals. Zircon and apatite are sparingly present. One

1 Neucs Jahrbuch fur Mineral., 1881., vol. 1, p. 273.
122 GREENSTONE SCHIST AREAS OF MICHIGAN. [HULL.62.

remarkably large (.4 x .04mm ) crystal of the former mineral, which has 

been curiously broken, is represented in Fig. 18, taken from No. 11052.

Galcite is not uncommon and is always of a secondary nature. In Ncs. 

11051 and 11053 it forms a more or 

less complete border around the porphyritic crystals.

As already stated, all four specimens collected from this locality are 

essentially identical. No. 11053 is 

more finely schistose, owing to its 

more advanced stage of alteration 

and the consequent greater production of sericite. In spite of this, however, the original structure is still

FIG. 18. Broken zircon crystal in schistose , .', . ., , -XT -, -, ,., , 

porphyry. TTpp.-r QumnesecFalls. No. 11052. plainly Visible. ISO. 11096 Was ObMognifled 180 diameters. taiiied ou the Michigan side of the 

river, at the foot of the Horse Eace. It has a very pronounced schistose structure, and, under the microscope, appears nearly identical 

with the rocks just described from below the falls, but is not so highly 

altered. Its biotite is in its original fresh state, its porphyritic crystals 

are more intact, and sericite is not so abundantly developed.

Gneiss. In addition to thenonporphyritic gneisses already described, 

Nos. 11079, 11084, and 11085 (see pp. 114-115) one other may be mentioned, No. 11188, from the head of the Horse Eace, on the Michigan side. 

This is apparently a typical biotite gneiss, but it is seen to pass by 

gradual transitions into the hornblende gneiss No. 11189 (see p. 108). 

The microscope shows that there are many points of resemblance between these two rocks. In the present case, although quartz is abundant, arnphibole and the same sphene observed in No. 11189 are frequent, 

and the feldspar is of such a nature as to be subject to saussuritization.

Felsite. True nonporphyritic felsite is not common among the acid 

rocks of the area about the Upper Quinnesec Fall aud the Horse Eace. 

The most typical specimen of this class is No. 11071, which occurs in 

narrow bands in the schistose greenstones below the falls on the Michigan side of the river. This is a very compact, flesh colored rock resembling jasper. Under the microscope it is seen to be essentially 

identical with the groundmass of the porphyritic rocks above described; 

and even here stretched and distorted crystals of quartz and feldspar 

are not entirely wanting, though they are rare.

There is, therefore, strong evidence in favor of the view that the acid 

rocks of the Upper Quinnesec region are of eruptive origin ; that they 

are to be regarded as apophyses which diverged from the granitic mass 

to the south; and that their schistose structure, when present, is to be 

attributed to secondary causes.
WILLIAMS.] ERUPTIVE ORIGIN OF THE ACID ROCKS. 123

This e%7 idence may be summarized as follows:

First. The structure of these rocks, as revealed by the microscope, 

in spite of its frequently being disguised by the action of secondary 

causes, is always that recognized as characteristic of massive or igneous 

rocks,

Second. There are present in these gneissoid bands granitic facies 

which agree exactly with the rock of the main granite area, and the 

bands themselves are identical with others near the contact, which can 

be plainly seen to be radiating apophyses or dikes.

Third. The schistosity of these bands is often independent of their 

direction and agrees with the prevailing strike of the surrounding 

greenstones.

Fourth. There is abundant microscopic evidence that the constituents have been fractured, stretched, and crushed since their solidifi 

ation.

The localities thus far considered on the Menominee Kiver, viz: 

Sturgeon, Lower and Upper Quinnesee Falls, contain all the important 

exposures of crystalline rocks,* belonging to what Major Brooks has 

regarded as the southern of the two belts of greenstone. If his location of these rocks is correct, the members of the northern belt appear 

upon the river at three points, viz: Four-foot Falls, Lower Twin, and 

Upper Twin Falls, of which, however, the two latter are connected by 

almost continuous rock exposures. These localities we shall now proceed to consider in detail.

FOUR-FOOT FALLS.

The exposures at this place are represented, together with those at 

both the Twin .Falls, upon Major Brooks's map, which is here reproduced in PI. VI. The fall is hardly entitled to be so called, as the 

rocks have formed nothing more than a rapid in the river. Just at 

the foot of this rapid the river is crossed by the Chicago and Northwestern Railroad bridge, immediately beyond which a cutting has 

disclosed some very instructive exposures of the greenstones. The 

rocks may be seen in position on either side of the river for some distance above the bridge. Brooks distinguishes upon his map many 

alternating beds of massive, schistose, and slaty rocks (a to n in his 

section). The beds here strike about S 80° E nearly at right angles 

to the direction of the river at this point and yet are mostly schistose 

on the Wisconsin side and massive on the Michigan side. This fact is 

indicated on Major Brooks's map and speaks strongly against the view 

that the present foliation is due to original bedding.

The rock of the exposure which occurs farthest down the river 

(Brooks's a) outcrops somewhat less than a quarter of a mile below 

the bridge. It was called by Wright a chloritic slate, 1 and by Wich1 Geol. Wisconsin, vol. 3, p. 712.
124 GBEENSTONE SCHIST AREAS OF MICHIGAN. [BUM,. 82.

mann a phyllite. Major Brooks himself designates it as a " light bluish 

gray, shining clay slate, with strong cleavage and no distinguishable 

bedding planes." 1 It does not belong to the rocks now especially under 

consideration, but since it was studied carefully it may be described 

here for comparison. This slate occurs in a bed, estimated by Brooks 

to be 550 feet in thickness, on both sides of the river and in its bed. It 

strikes nearly east and west and dips toward the north. In Brooks's 

collection this rock bore the number 2075; in the present collection it 

is represented by No. 11152.

Under the microscope there are visible in the thin section of this 

specimen unmistakable signs of sedimentary origin. Coarser and finer 

areas alternate, the former being made up of clastic quartz grains, 

either pure or mixed with iron hydroxide or chlorite, while the latter 

are composed of an argillaceous substance, filled with extremely minute 

muscovite or sericite scales and considerable carbonaceous matter. 

Tourmaline needles are also occasionally seen 5 and zircon too, either in 

minute, whole crystals or as irregular fragments of larger ones, is not 

infrequent. This rock belongs to the detrital iron-bearing series, the 

boundary betweien which and the greenstone schists lies just above 

this exposure.

The railroad which crosses the bridge at Four-foot Falls exposes this 

same rock again in a cutting about a mile west of Iron Mountain. A 

specimen from this locality, No. 11114, shows the same general character as the last described rock, except that it is more crystalline. 

Tourmaline is present as before, but anew crystallization of biotite has 

largely replaced the chlorite. The carbonaceous material is less abundant and what remains is in a much more finely divided state. The 

clastic origin of the rock is still very apparent in the shape of its quartz 

grains, which are here mingled with feldspar fragments, both plagioclase, microcline and orthoclase.

No. 11113, from a quartzite baud intercalated iu No. 11114, also contains feldspar grains mingled with the quartz. Zircon fragments are 

likewise present aod a little biotite, which is no longer quite fresh. 

This latter mineral shows in a beautiful manner the development of 

secondary rutile needles, as in the kersantites.2

Before speaking of the greenstones which occur along the river 

farther north, it will be well to examine those exposed in the railroad 

cutting at the western end of the bridge. These rocks are, for the 

most part, massive, but they nevertheless display evidence of extensive 

crushing and chemical alteration. No. .11178 represents the average 

type. This is of a light green color, and iu a hand-specimen quite 

aphanitic. Both macroscopically and microscopically it resembles the 

massive greenstones so abundant in and representative of the region 

south of Marquette (see PI. X, fig. 2). Miueralogically there is hardly 

a trace of the original rock left. Almost colorless hornblende, pale

1 Geol. Wisconsin, vol. 3, p. 475.

2 Bosenbnsch: Mikros. Physiog., 2d ed., vol. 2, p. 311.
U. S. GEOLOGICAL SURVEY BULLETIN NO. 62 PL. VI

c HI G

T.4ON. 5 R.3LW.

MAP

of 

T WI N AND TOU R FOOT FALLS

Menominee River

(After Brooks) 

. SCALE:

o 300 1000 isoo 2000 2500 Feet

sMile
WII-UAMB.] GREENSTONES NEA.R THE FOUR-FOOT FALLS. 125

green chlorite, zoisite, leucoxeue and a little calcite (all of secondary 

origin) are the present constituents; and yet the original structure of 

the rock is strikingly well preserved. When viewed with a comparatively low power, in ordinary light, the outlines of long, almost 

acicular, feldspar crystals are very apparent, in spite of the fact that the 

substance of the feldspar itself is changed to chlorite or zoisite. These 

outlines of former crystals make a confused aggregate, but each individual preserves its own proper form (idiomorphic in the sense of Eoseubusch). The angular spaces between the feldspars produce a typical 

example of the ophitic or diabase structure, although no trace of a diabase mineral remains.

But the exposure is not throughout as massive as the specimen just 

described. It is traversed here and there by schistose and wavy bands 

which show indications of having been much crushed and rubbed. 

Slickensides are abundant and lenticular fragments fit into one another so as to produce an imperfect sort of foliation, In other cases, 

where the crushing has been more intense, the schistosity is more perfect. These bands vail strike a few degrees north of west, being apparently conformable to the slates below and the greenstone schists above. 

This is a suggestive fact when it is remembered, as observed by Brooks, 

that these slates owe their lamination to slaty cleavage (a product of 

pressure), which seems to have obliterated the original stratification.

No. 11179, from, one of the schistose bands in the greenstone of this 

cut, shows under the microscope the effects of great mechanical action. 

Curving and interlacing areas of pale green chlorite and of a grayish 

substance (perhaps the remains of titanic iron) form the main mass of 

this rock. Thickly scattered through these are patches of a dark brown 

substance, often showing concentric zones of a clear, transparent character. JIhese look like opal, but their optical character shows them to 

be single individuals of crystalline quartz. Imbedded in this material 

of such pronounced secondary character, are fragments of feldspar, 

which have been crushed or broken. These, as has been observed so 

frequently before, are less changed chemically than those in the massive rock from which this schistose band has been derived.

Four specimens were selected to represent the series collected along 

the river north of the railroad bridge. Two of these came from each 

side of the river and illustrate both the massive and schistose bauds.

No. 11176 was found on the Michigan (eastern) side, just above the 

bridge. In color it is light green, and in structure it is massive. Under 

the microscope it shows a pale hornblende in ragged and irregular individuals, feldspar, zoisite, chlorite, and leucoxene. Traces of an original 

diabase structure are still abundant in this rock. The feldspars often 

perfectly preserve their lath-like form and their twinning striation, while 

the pale but compact hornblende occupies the intervening spaces, supplying the place taken by the augite in fresh rocks of this type.

ISfo. 11175, from the highest exposure on the Michigan side of FourO
126 GREENSTONE SCHIST AREAS OF MICHIGAN. IBULL. 62.

foot Falls, is quite schistose and of a much darker color than the last 

rock. Under the microscope it is seen to possess a much finer grain 

arid to be composed of hornblende, chlorite, feldspar, quartz, and leucoxene. The hornblende is for the most part pale green, but that it 

was bleached from the brown variety is shown by the numerous compact brown cores which occur in the centers of the paler crystals. The 

structure of this rock is purely granular, with no trace of the diabase 

type which characterized the specimen last described. This fact and 

tbe indication that the pale, fibrous hornblende was derived from a 

compact, brown variety of the same mineral, render it probable that 

this rock was originally a diorite, composed of feldspar and brown 

hornblende, in which little or no pyroxene was ever present. The hornblende of this rock exhibits a structure quite similar to that described 

by Becke 1 and Van Hise 2 as due to a secondary enlargement of the 

crystals in a solid rock.

The accompanying figure represents the compact brown interior of a 

hornblende individual, more or less sharply separated from a pale green 

or colorless border of hornblende fibers, which pdssesses exactly the 

same optical orientation as the core.

Evidence seems to be constantly accumulating that various minerals 

may continue their growth by regular 

accretions in solid rocks, whether massive or clastic. This fact is satisfactorily established at least for quartz, 

feldspar3 and hornblende. While not 

in the least doubting the correctness 

of the conclusions above cited in regard to the secondary growth.of hornblende, I must, however, confess that 

the present isolated instance appears 

to me hardly to offer conclusive proof 

that the fibrous hornblende border is 

a secondary crystalline enlargement. 

Such an explanation is possible, but 

the appearance can be equally well explained in my judgment, by assuming that the fibrous zone is the result of bleaching and fraying 

out of originally compact hornblende crystals. This process would 

naturally commence at the ends of the crystals and develop principally 

in the direction of the cleavage, as is here seen to be the case. The only 

difficulty with this explanation is the sharp line sometimes seen between 

the brown and the fibrous hornblende. But such a sharp boundary is 

rather the exception than the rule.

FIG. 19. Compact cores of hornblende surrounded by a (Ibrous border, No. 11175, a schistose greenstone from Four-foot Falls. Magnified 180 diameters.

'Tschermak's mineral, u. petrog. llittheil., vol. 5, 1883, pp. 158-59. (cf. Am. Jonr. Sei., 3d series, vol. 

33, p. 385,1887.)

'Am. Jour. Sei. 3d series, vol. 30, p. 231, Sept. 1885. A. Barker has also recently described similar 

secondary enlargements of hornblende in a. hoinblende-picrite from Anglesey. Geol. Mag., London, 

3d series, vol. 4, 1887, p. 550. -

"See Irving and Van Hise: Bull. "U. S. Geol. Survey, No. 8.
WILLIAMS.] GREENSTONES OF THE TWIN FALLS. 127

Nos. 11142 and 11143, taken from two contiguous bands on the Wisconsin side of the river, about midway between the two points which 

furnished the last described specimens, represent Brooks's localities I and 

If respectively. The first is schistose, the second massive. The first 

is a pale and schistose aggregate of nearly colorless chlorite, light 

green or colorless hornblende fibers, quartz and calcite. The second is a 

much darker green mixture of feldspar, hornblende, chlorite and leucoxene. The first named of these components has frequently its crystal form well preserved, which betrays the diabase character of the 

mother rock. The leucoxene in this section, as well also as in Nos. 11175 

and 11176, often possesses the sphene-like habit which has been figured 

from Ho. 11189. (PI, XIII, fig. 1.)

THE TWIN FALLS.

About a mile above the last described rock exposure at the so-called 

Four-foot Fall, the quiet course of the Menominee Eiver is again interrupted by a very considerable outcrop of greenstone. This is almost contin nous for half a mile, but at either end of it the river plunges over a 

barrier of somewhat harder rock than usual and spreads out below it 

into a small basin. The two little water-falls thus formed are known 

as the Twin Falls. In Major Brooks's report they are included in the 

same map that shows the topography of the Four-foot Fall,1 and that 

portion of this map which gives the outline of the Twin Falls is 

reproduced in twice the scale of the original on PI. VI.

All of these exposures belong to the northernmost of the two greenstone belts of the Menomiuee Iron Basin. This is .much more uniform 

in its composition than the southern belt, in which the Sturgeon and 

Quiunesec Falls exposures are located. Its rocks are altogether of a 

dark green, almost aphanitic type, which, when they become schistose, 

give rise to dark chloritic slates. Neither the light colored, gabbro-likc 

greenstone, nor coarse diorites, like those of the Horse Race, nor bands 

of acid rocks occur in the northern belt at least, not at the localities 

where this crosses the river.

The homogeneous character of the Twin Falls greenstones is largely 

due to an advanced state of chemical alteration, which has obscured 

its original grain.

The macrostructural changes, whereby the compact, dark green rocks 

are converted from a massive to a schistose state, may be admirably 

seen along the river banks at both the Upper and the Lower Twin 

Falls. The foliation of the massive and already chemically altered 

rock takes place in a somewhat different manner from that heretofore 

described. There are none of the ragged " cross gashes " and irregular, gaping seams (see Fig. 10, p. 81) which are so common at the exposures along the southern belt. These seem to be the result of stretching in a solid or very nearly solid mass, but in the present case this 

process is hardly apparent.

 . ' Geol. Wisconsin, vol. 3, p. 475, PI. IV.
128 GREENSTONE SCHIST AKEA.S OF MICHIGAN. [BULL. 62.

The first step toward the formation of a schistose structure in these 

Twin Falls greenstones (and this is hardly ever absent) is the division 

of the massive rock by two systems of joints, which stand about perpendicular to the surface and intersect at a varying but acute angle. 

These joint systems divide the mass into diamond, shaped or rhomboidal 

prisms, the cross sections of which are well displayed upon the frequent smoothly glaciated surfaces of the rock. The appearanceof such 

a surface is diagrammatically represented in Fig. 20.

(/AM\vA\/X V k j c / \ V \ >" X A^^ ^uv/A.Nx/v

//,.v4n

(XV)^)^^ V^'AWx X)v)/ok ^

/ W- y A / / ; . Vft M/y\y, ^ A

FIGS. 20-22 diagrams illustrating the transition from jointing to schistose structures in the green

stones at Twin .Falls.

As we approach the schistose band in the massive rock these interlacing rhombs become lengthened out more and more by an approximation to parallelism between the two systems of joint-planes. (Fig. 21.)
FOLIATION PRODUCED BY SHEARING. 129

These elongated prisms finally become very much extended lenses, 

which interlock and produce a well developed, wavy or even parallel 

schistose structure (Fig. 22). The almost slaty rocks thus produced, 

especially as seen at Lower Twin Falls, have a tendency to break, not 

so much along a definite plane as parallel to a line i. e., the direction, 

normal to the surface, parallel to which the original joint planes ran. 

It is difficult to obtain well shaped hand-specimens of these rocks, but 

narrow rhombic prisms of almost any angle are easily procured. There 

is an almost equal tendency to cleave along any plane which is parallel 

to the longest axis of these prisms.

If the prisms clue to the original joint planes were subjected to a lateral pressure which developed in them a cleavage that successively approached more and more nearly to the long axis as the prism was lengthened, this peculiar tendency to separate along a line rather than along 

a plane is precisely the structure which we might suppose would result.

The strike of these schistose bands follows the direction which bisects 

the acute angle of the rhombic prisms. This is for the most part from 

S. 70° to 80° E., agreeing with the prevailing strike of all the rocks in 

this system. There are, however, many exceptions, where these schistose 

bands, even where near together, follow different directions ; for instance, I observed, in the massive though jointed rock on the Michigan 

side of Upper Twin Falls, two schistose chloritic bauds quite near 

together, one having a strike 3ST. 180° E., and the other S. 73° E., while 

the dip of each was nearly vertical. Such cases are easily explicable on 

the supposition that these bands were produced by mechanical agencies, 

but it is quite impossible to reconcile them with the supposition that 

these bands are in any way the result of sedimentation.

This type of schistose structure was also noticed to a slight extent 

at some of the exposures of the lower greenstone belt notably jnst 

below Lower Quiunesec Falls on the Michigan (left) bank of the river 

(see p. 84). It is, however, there the exception, and is peculiarly the 

characteristic of the upper belt as exhibited at the Twin Falls.

LOWER TWIX FALLS.

The development of the foliation in the massive greenstone is also 

shown on a small scale, particularly on the left (Michigan) 

side of Lower Twin Falls, 

where there has been a slight 

araoiuitofslippiugalong aline 

of jointing. Fig. 23 gives a 

diagrammatic idea of a specimen observed at this locality. 

The area represented is about 

three feet square. Through 

the center runs a vein of white 

quartz which has been deposBull. G2 9

Massive diabase.

Schistose diabase. 

Quart/, vein. 

Schistose diabase. 

Massive diabase.

PIG. 23.
130 GREENSTONE SCHIST AREAS OF MICHIGAN. [HULL. 62.

ited by infiltration in the old joint seam. On the edges of this the rock 

is perfectly schistose and chloritic, but it passes gradually, in the space 

of a few inches, into the massive aphauitic greenstone, which composes 

almost all of this exposure. Fig. 24 represents a polygonal block, 

formed by the jointing of the massive greenstone, which has been 

frayed out into a perfectly foliated chlorite schist by a slipping movement against the adjoining blocks.

The extensive chemical alteration which has gone on in this greenstone seems to render it peculiarly subject to such a fraying out into a chloritic 

schist. This change takes place iu cases 

where a fresher and harder mass would 

have received only a slickensides.

The Twin Falls greenstones are interesting, in view of the macrostructural 

alterations which they exhibit. Mineralogically they are all so much decomposed that they present but little which 

is deteimiuable under the microscope.

The least altered specimen collected 

is No. 11140, from the east side of the 

Lower Fall. Together with the specimens shown iu Figs. 23 and 24, it came 

from a steep wall of massive and but little 

jointed greenstone, which rises abruptly 

from the water's edge. Diamond jointed 

and somewhat schistose rocks of the 

same sort occur on the north (No. 11139) 

and fissile chloritic slates (striking W. of 

N. and dipping 70° to the north) to the 

south of it. Under the microscope this 

massive rock is found to be a confused 

aggregate of pale green, somewhat fibrous hornblende, feldspar remnants, and 

leucoxeue, which possesses a sphene-like habit and rarely shows any 

trace of its original ilmenite. The clearer areas, which represent the 

former feldspar, are now so filled with actiuolite, zoisite, and epidote 

needles, that nothing regarding the former character of this mineral can 

be learned. In this rock secondary biotite is also quite abundantly 

developed, as was found so often to be the case in the Russian metamorphosed diorites described by Inostranzdt' 1 . The original structure of 

this rock is now disguised almost beyond recognition, but it seems to 

have partaken more of the granular nature of a diorite than of the 

ophitie nature of a diabase.

No. 11132, taken from the continuation of this mass on the opposite

1 Stuclien iiber rnetam. Goat., etc., 1879, pp. 81, 82, 109-112.

l'iu.24.
WILLIAMS. I ANATASE IN ALTERED DIABASE. 131

side of the river, is essentially identical with it, but in it the form of the 

original feldspars is better preserved. These may often be seen in 

well formed, lath-shaped crystals, which, in spite of their extensive saussuritizatior., still retain distinct traces of their plagioclastic twinning 

strise. This rock also, in distinction from the one last described, contains some chlorite but no biotite.

These rocks, like so many others of the Menominee Eiver greenstones, 

are representative examples of the type called by Giimbel " Bpidiorit," 

which is now generally conceded to be an altered form of diabase. 1

The rocks into which these least altered forms pass are representative 

chlorite schists and slates; and there can be no doubt that these latter 

have also resulted from the extreme effects of those metamorphic agencies which changed the eruptive diabase to the epidiorite.

No. 11130, collected just below No. 11132, and grading imperceptibly 

into it, is a schistose rock, composed of bright green chlorite, finely 

granular quartz and, perhaps, secondary albite. The leucoxeue in this 

rock is particularly interesting. It 

still retains small ilmenite cores in 

places, but it is broken and pulled 

out in the direction of the schistosity. 

A very considerable portion of this 

Jeucoxene, especially around the edge 

of the grains, is composed of highly 

refractive and brightly polarizing, although extremely minute, colorless 

crystals. These have an octahedral 

habit and resemble anatase (Fig. 25), 

which, as both Diller 2 and Kosenbusch 3 have shown, sometimes originates fiom the alteration of ilmenite 

(cf. also Nos. 11050 and 11052 p. 121). 

tities in this rock.

Pie. 25. Anatase derived from ilmeuite, No. 

11130, from the Lower Twin Fall. Magnified 

180 diameters.

Biotite also occurs in small quanAt the northwestern corner of the basin, below the Lower Twin Fall 

(see map PI. VI), the rock is rhomboidally parted and weathers to a 

brownish color. In spite of its massive appearance it possesses a latent 

tendency to cleave along the longest diagonal of the rhomboidal prisms, 

and this tendency is intensified by weathering. Toward the east this 

rock (No. 11128) passes gradually into corresponding slates (No. 11129). 

Under the microscope these two rocks are seen to be in all respects the 

equivalents of Nos. 11132 and 11130, above described. Their main difference consists in their being richer in iron, which has oxidized and

1 Cf. Rosenbuscli: Mikros. Physiog., 2d ed., vol. 2, p. 205.

2 Kenes Jahrbucli fiir Mineral., 1883, vol. ], p. 191.

3 Mikros. Physios-, 1st ed., vol. 2, 1877, p. 33Ci 2d ed., vol. 1, 1885, p. 332, cf. also Schenck; Inaugural 

Dissertation, Bonn., 1884, p. 25.
132 GREENSTONE SCHIST AREAS. OF MICHIGAN [BULL. 62.

stained the rocks brown. No. 11128 still retains much hornblende, feldspar, and ilmenite, and the diabase structure is still apparent in places. 

No. 11129 is, like its counterpart No. 11130, a chlorite schist with the 

hornblende replaced by chlorite, the feldspar by a quartz albite mosaic, 

its lencoxene drawn out, and its structure wholly changed.

Perhaps the most altered rock of any encountered at the Lower Twin 

Fall is No. 11139, which was one of the chloritic slates on the Michigan 

side of the river below the massive rocks. This* is a very fine grained 

aggregate of pale green chlorite, quartz, or quartz albite mosaic and 

biotite, the latter constituent being sparingly present.. Sharply defined 

crystals of tourmaline are also abundant in this rock.

UPPER TWIN FALLS.

At the Upper Twin Fall the same massive greenstones with a rhomboidal parting prevail, everywhere traversed by more or less schistose 

bauds. No. 11133 is the massive rock from the left or Michigan side 

of the fall. It is much altered chemically, but preserves its original 

structure almost perfectly. When examined by a low magnifying power 

there is seen a network of light colored, lath shaped feldspar forms, 

which are almost acicular, like those of a porphyrite. Between these 

is an allotriomorphic mass of a dark gray substance, representing the 

former augite, or possibly a glassy base. When examined more carefully with a higher power, the feldspar is found to be completely 

replaced by a mixture of pale green chlorite and brown biotite in the 

closest relationship, while in them an occasional crystal of zoisite is 

imbedded. The darker interstitial mass is composed largely of fibrous 

hornblende in shreds or confused, matted masses, but these are mingled 

with much impure and indeterminable matter.

Two schistose bandshave already been mentioned on p. 129 as traversing this rock in different directions. Of these, the one striking S. 73° E. 

is the less schistose, and is represented by specimen No. 11135. Though 

much more altered than the last, there are here also occasional unmistakable traces of the same ophitic structure. The representative of the 

other baud, No. 11134, is hardly more than a fine schistose aggregate 

of chlorite, quartz, and calcite, with only the faintest indications of its 

original structure, and yet here there are beautiful leucoxene borders 

around unaltered cores of iluieuite. Narrow veins filled with a mixture 

of calcite and chlorite traverse all these rocks.

On the opposite or Wisconsin side of the Upper Twin Falls the rocks 

are apparently the same, but they show, nevertheless, important microscopic differences. The massive forms represented by Nos. 11122, 

11125, and 11126, collected at different points between the falls and 

the bridge, are fine and confused aggregates of pale green, fibrous hornblende, chlorite, zoisite, quartz, leucoxene, and occasionally (No. 11122) 

a little biotite. All of these minerals are of secondary origin and in
WILLIAMS.] 'DIABASE TUFFS AT UPPER TWIN FALLS. 133

the process of their formation most of the traces of the former rockstructure have been obliterated. Such traces as still remain indicate 

that the mother-rock was a diabase, but there are seen none of the long, 

acicular feldspar forms, discovered in the rocks from the opposite side 

of the river.

The schistose bands interlainiuated with these rocks are quite like 

those already described. One of these, however, No. 11123, obtained 

directly beside the falls, shows almost the only evidence of fragmeutal 

origin anywhere observed in the Menominee greenstones. This contains irregular and angular fragments of quartz and a slightly altered 

feldspar of considerable size. These are imbedded in a matrix of irregular grain, composed of chlorite, calcite, quartz, and opaque iron oxide, 

which is accompanied by leucoxeue. The chlorite scales often have a 

radially divergent arrangement around the larger included fragments 

of quartz and feldspar.

Thin section, No. 11122,shows the contact of this schist with the massive rock. This contact is so sharp as to lend much additional probability to the idea that in this case the two rocks are of different origin.

Books of this kind are to be regarded as diabase-tuffs, similar to those 

which are so largely developed around Marquette, and which are to be 

described in the sequel. It is easy to see how essentially the same material, whether produced as volcanic ash or as a massive rock, when 

subjected for a long period to the action of the same metamorphosing 

forces, would give rise to masses which, in certain cases, could not be 

distinguished from one another. This is undoubtedly true of many 

occurrences on the Meuominee Biver, and hence we can speak with certainty only of those instances where sufficient of the original structure 

is preserved to do away with all doubt. Between undoubted massive 

rocks rendered schistose by pressure on the one hand, and fragments! 

tuffs which have been more or less completely solidified by the same 

agency on the other, there must therefore be rocks whose original form 

must always be uncertain.
CHAPTER IV.

. GREENSTONE BELTS OF THE MARQUETTE DISTRICT.

INTRODUCTORY.

Ever since the discovery of iron in the northern peninsula of Michigan, in 1844, by Burt and Houghton, the region around Marquette has 

received much attention from geologists. Although not eo early known 

as the famous copper district of Keweenaw Point, this iron region has 

divided the scientific interest whifch was before felt in the copperbearing rocks.

Douglass Houghton,1 Loeke,2 Foster and Whitney,3 Whittlesey,4 

Kimball,5 Credner.6 Brooks,7 Wright,8 Wadsworth,9 Roininger,10 and 

Irving 11 have studied the general relations of the deposits, while microscopical descriptions of some of the rocks have been given by Julien,12 

Wright, 13 Wichmann, 14 Pumpelly, 15 Wadsworth,16 and Irving. 17

The city of Marquette is situated on the south shore of Lake Superior, about midway between the Carp River and the promontory known 

as Presqu' Isle, between which points (a distance of about 4 miles) the 

lake shore runs nearly north and south. The city for the most part 

lies within the square mile designated as Sec. 23, T. 48 K., R. 25 W., 

Michigan.

An excellent idea of the topography of this region and of that extending for 18 miles back of it (i. e., westward from the lake) may be 

obtained from the large colored map prepared «by Dr. C. Romiuger. 18

1 Brooks, Geol. Michigan, vol. 1, 1873, p. 13.

2 Geol. Michigan, vol. 2,1873, p. 239.

3 Senate Docs., 1st session, 30th Congress, 1847-'48, II, Doc. 2. Keport on the Geology and Topography of the Lake Superior Lantl'Dist., Part IT, Iron Region.

"Proo. Am. Assoc. Adv. Sci., 1859, vol.xiii, part 2,pp. 301-308; ibid.,1875, vol.24, part 2,pp.60-72.

6 Ani. Jour. Sci., 2d series, vol. xxxix, 1865, pp. 290-303.

"Zeitschr. Deutsch. geol. Gesell, vol. xxi, 1869, pp. 516-554.

* Geol. Michigan, vol. 1,1873, part 1. pp. 1-319.

""Geology of the Lake Superior Iron District" in Swineford's History and Review of the Copper, 

Iron. Silver, Slate, and other Material Interests of the South Shore of Lake Superior, 1876, pp. 132-145.

9 Bnll. Mus. Comp. Zool., Cambridge, vol. 7, No. 1.

10 Geol. Michigan, vol. 4, part x, Marquette Iron Region, 1881, pp. 1-154.

11 Preliminary paper on an Investigation of the Archean Rocks of the Northwest; Fifth Ann. Rep. 

U. S. Geol. Survey, 1686, pp. 181-242.

12 Geol. Michigan, vol. 2,1873, Appendix A.

13 Geol. Michigan, vol. 2,1873, Appendix C.

14 "Microscopical Observations of the Iron-bearing (Huronian) Rocks from the region south of Lake 

Superior," 1876, Geol. Wisconsin, vol:3, pp. 600-656.

16 Am. Jour. Sci., 3d series, vol. x, 1875, pp. 17-21.

16 Bull. Mils. Comp. Zool., vol. 7, No. I.

"Bull. U. S. Geol. Survey No. 8, pp. 27-30.

I8 0eol. Michigan, vol.4,1881, in pocket. 

134
WILLIAMS.] THE MARQUETTE GREENSTONES. 135

The general, trend of both the drainage and of the geological formations within this area is toward the east. The two main streams follow 

somewhat irregular but approximately parallel (eastward) courses and 

empty into the lake about four miles apart. The more northerly of 

these is called the Dead River; the more southerly the Carp.

The general surface of this area rises by degrees from the level of 

the lake to an elevation of about 1,000 feet above the lake. This surface is, however, extremely broken and hilly, being traversed by east 

and west ridges composed of rows of rocky knobs. These elevations are 

never over two hundred feet in height from the base ; usually they are 

much lower (from one hundred to fifty feet or even less). They all have 

rounded outlines produced by the wearing action of the great glacier, 

evidences of which are everywhere apparent in the smoothly polished 

and frequently striated rock surfaces. These knolls rise from the even 

level of a plain produced by the glacial debris deposited between them.

All account of the general geological structure of tlie Marquette region

has already been given in Prof. Irving's explanatory note at the beginning of this paper, in which is included also a summary of the different 

views held by various writers with regard to the stratigraphical position and origin of the rocks of the greenstone-schist area, which forms 

the subject of this part of the present essay. In connection with this 

account has also been given a geological map of the Marquette region, 

compiled by Professor Irviug from the maps of Brooks and Rominger 

and from original observations of his own. (PI. I.) It will not therefore be necessary for me to present anything further as to these general 

matters in the present connection.

Special references to the petrographical work of others on the Marquette rocks will be made in the course of the following detailed descrip- 

. tions.

In some respects the greenstones of the Marquette area seem peculiarly suited to throw light upon the dynamic metamorphism of basic 

eruptives, since a portion of these rocks seem to have been extruded 

both during and subsequent to the action of the metamorphosing 

forces. Hence we find the same eruptives in different stages of alteration. Moreover, fragmental material (tuff) of the same nature and 

origin of the massive rocks is abundantly developed, and this too 

shows varying stages of consolidation into masses which closely resemble the solid rock. This great variety, while it is full of interest 

and suggestions of new possibilities, is frequently very confusing. It 

often becomes impossible to speak with certainty about the origin of a 

rock which, on account of its many analogies, we may regard as having been produced in several different ways.

The general similarity of the greenstone schists of the Marquette 

and Menominee areas will be brought out in the following pages. It is 

worthy of mention that the greenstone schists of both of these regions 

have a strong resemblance to some of the greenstones and aggloin-
136 GREENSTONE SCHIST AREAS OF MICHIGAN. [BUM..02.

erates of the extensile group of rocks occurring on the Lake of the 

Woods and on Bainy Lake in Canada, to which Mr. Andrew C. Lawson, of the Canadian Geological Survey, has given the name of Keewatin series. This series he regards as belonging immediately beneath the Huroniau, which is represented in those northern regions 

by the so-called Animik6 series. 1 This is also the position winch the 

greenstone schists of Marquette occupy, since they quite unmistakably underlie the iron-bearing detrital rocks, which, according to 

Irving, are the equivalents of the true Huronian. According to Irving 

also the greenstone schists here probably underlie the overlying ironbearing series unconforinably.

The Marquette greenstone-schist belt, which at the lake shore is 

between 2 and 3 miles wide, extends to the westward for about 6 miles, 

without materially altering its form or width. It then broadens suddenly by a wide extension toward the north, and at the same time is 

divided into two portions by the eastern extension of a narrow arm of 

granite (See map, PI. I). The southern of these portions continues a 

due westward course with an average width equal to that of the entire 

belt before its division. The northern portion is separated from the 

southern, first by granite, and then to a greater and greater extent by 

a large, wedge shaped area of the rocks of the overlying iron-bearing 

or Huronian series.

For convenience in arranging the following petrographical descriptions the ground covered has been divided into four areas, which will 

be considered in order: (1) The eastern area, near Marquette; (2) the 

icestern area, immediately north of Teal Lake, in the town of Negaunee; 

(3) the northern area, lying north of Dead River, and (4) the Deer Lake 

area, a short distance north of the town of Ishpeming. The last named 

area was not studied by the writer in person, specimens from it, with 

full descriptive notes, having been sent to him by Professor Irviug for 

comparison with the collections which the writer "had himself made in 

other areas.

In the eastern or Marquette area the whole width of the greenstone 

belt, including the granite contact on the north, was studied for a 

distance of 3 miles west from the lake shore. In the western or 

JSTegauuee area two sections were run northward across the greenstoneschist belt, one passing through Sees. 21, 28, and 33, T. 48 N., B. 26 

W.; the other through Sees. 13, 24, 25, and 36, T. 48 N., K. 27 W. 

In the northern area the examinations made were carried on from a 

camp situated in Sec. 9, T. 48 IS"., B. 26 W., and embraced the area 

covered by Sees. 3, 4, 5, 8, 9, 10, and 11 of this township.

The first of these areas is the most varied in its petrographical 

characters, and it was, therefore, the most thoroughly studied. It is

1 See General Report of Progress of the Geological Survey of Canada for 1885; also of Am. Jour 

Sci. (3d series), vol. 33, 1887, p. 473. I am indebted to Dr. Lawsou for the opportunity of examining 

the extensive suite of the .Rainy Lake rocks, upon the microscopical study of which lie was engaged 

in tlie petrographical laboratory of Johns Hoplcins University in the winter of lS8G-'87.
WILLIAMS.] DIVISIONS OF THE MARQUETTE AREA. 137

divided into two portions of nearly equal extent by the narrow east 

and west band of iron-bearing slates, to which Ron-ringer applied the 

name of Eureka series. The northern portion of the Marquette area, 

extending from Lake street, Marquette, northward to the granite, is in 

Jarge part composed of banded greenstone schists, having an east and 

west strike, and a steep northern dip. They are most conveniently seen 

in typical development on Lighthouse Point. The layers of these rocks 

are alternately of a darker and lighter shade of green, which gives these 

particular greenstones their characteristic striped appearance. In these 

banded rocks of the northern part of the Marquette area intrusions of 

comparatively little altered acid and basic matter are abundant. These 

are for the most part conformable to the bedding of the schists and 

embrace granites, gneisses, schistose porphyries, diorites, and diabases. 

Whenever, in these undoubtedly eruptive rocks, a schistose structure 

is apparent, this is conformable to the bedding of the banded greenstone schists.

The south era portion of the area about Marquette, on the other 

hand, is occupied by much more massive and homogeneous greenstones 

of a nearly uniform light green color, and an almost aphanitic structure. These are characterized by their division into oval or lenticular 

areas which interlace and which are separated by a finely schistose 

material of much finer grain. This peculiar parting, which, according to the writings of the Canadian geologists, appears to have a very 

widespread distribution through the greenstones of the Northwest, at 

first glance resembles the spheroidal weathering of many eruptive rocks. 

There is, however, better reason for regarding it as of mechanical 

origin, as will be more fully explained in Chap. V.

The schistose structure of the southern Marquette greenstones is of 

secondary origin, due, probably, in almost every case, to pressure, while 

anything like the banding or striping of the more northern rocks is here 

wholly wanting. Intrusive rocks are rarer than in the banded greenstones of the northern portion of this area. When such dikes do occur 

they are much more altered and hence approach closely to the enclosing rock a fact which renders their detection difficult.

The two portions of the Marquette greenstone area are therefore 

broadly distinguished in many of .their petrographical characters. The 

east and west band of iron shales, mentioned above as separating 

them, extends from the north side of Marquette harbor (Lake street) 

westward along the northern side of the Duluth, South Shore and 

Atlantic Eailroad. They seem to be identical with certain members of 

the iron series and at one point, about two miles west of Marquette, they 

actually contain quite an extensive deposit of hematite. It is here 

that the Eureka shaft was sunk at an early date in the history of iron 

mining in the Marquette region, and from this Dr. Eominger has taken 

his name for this series of Huronian beds. To make the references to 

localities in the Marquette area more intelligible, a sketch map of the 

environs of the city is here appended, PI. VII.
138 GREENSTONE SCHIST AREAS OF MICHIGAN. [nuu,62.

With reference to the other two areas examined it is here sufficient 

to say that the greenstones of the western or Negaunee area more 

closely resemble those exposed in the southern portion of the Marquette 

region 5 while the rocks encountered in the northern area show many 

points of likeness to those occurring north of Marquette.

ROCKS OF THE NORTHERN PORTION OF THE MARQUETTE AREA.

BASIC INTRUSIVES.

Diabase. Diabase in well defined dikes of various sizes is an important feature in the geology of the northern portion of the Marquette 

greenstone area. Tbis rock occurs in all stages of preservation from 

an unaltered condition to a hardly recognizable aggregate of secondary 

and decomposition products.

In all the freshest specimens examined oliviue is present, but with 

the commencement of alteration this mineral is the first to disappear 

In many cases, therefore, where the rock may still be recognized as a 

typical diabase, it is now impossible to say whether it was originally 

olivine-beariug or not.

The next stage in the alteration is marked by the change of the 

pyroxene to uralite, and the more or less synchronous passage of the 

feldspar into saussurite and of ilmenite into leucoxene.

Still later follows the development of such final decomposition products as chlorite, epidote, calcite, quartz, etc. The extent to which these 

chemical changes destroy the original diabase structure differs in different cases. As a rule this remains distinct during the first two stages 

of alteration, while it generally, though not always, disappears during 

the third. In some instances a pronounced schistose structure has 

been developed in undoubted diabase dikes by extensive alteration.

The Great Dike* The best material for the study of the fresh olivine 

diabase near Marquette was obtained from the great dike which forms 

the center of Lighthouse Point (see Pis. I and VII). Here its eruptive 

character may be admirably seen by the sharp line of contact which it 

presents with the adjoining banded green schists. This dike may be 

traced by several outcrops along Michigan street in Marquette, and 

then as a high, rocky ridge which extends for sorue three miles westward, 

through Sections 15 and 16, just north of the main road from Marquette 

to Negaunee.

From various portions of this dike nine specimens were collected. 

The four freshest of these specimens (Nos. 11617 and 11622, from Lighthouse Point; No. 11636, from Lighthouse Point quarry, and No. 11666, 

from a cutting on Michigan street) are not to be distinguished from 

one another.

The structure of this work is the hyp-idiomorphic diabase structure,1 which forms one of the most certain characteristics of this group

1 "Structure ophitique" of Fouqu6 and Miohol-L6vy, "Divergent-straliligkornige Strnctur" of 

Losseu, and " Diabasischkornigo Structur " of Rosenbusch.
U. 8. GEOLOGICAL SURVEY BULLETIN NO. 02 PL. VII

MAP or THE ENVIRONS OF MARQUETTE
WILLIAMS.) DIABASE OF LIGHTHOUSE POINT DIKE. 139

of eruptives. The grain is of medium coarseness. The lath-shaped 

plagioclase crystals measure from 1.5 to lmra by 0.25 to 0.15'nm . The 

interstices between these are filled with augite and ilrnenite. The mineral components observed under the microscope are, in order of their 

age, as follows:

Essential: Olivine, labradorite, augite, {Magnetite.}

Accessory : Apatite, hornblende, biotite, quart/.

Secondary: Serpentine, uralite.

The olivine is mostly in rounded grains or in irregular areas, although 

it sometimes shows evidences of a crystal form, especially the acute 

angle clue to the intersection of two dome faces (notably in No. 11666). 

Jtis commonly wholly changed to serpentine, but in a few instances 

remnants of the original transparent olivine substance remain.

The feldspar is always a plagioclase with the peculiar elongation of 

the crystal in the direction of the brachydiagonal axis, which is characteristic of diabase. The crystals are always striated, often minutely so. 

The extinction is generally sharp, although an undulatory extinction 

is sometimes observed.

The augite is quite normal for diabase. It is of a light, slightly 

reddish shade of gray, with a perfect cleavage parallel to the prism. 

Twins are very common, and the line of the lamellae may be seen in 

ordinary light traversing the basal sections and bisecting the cleavage 

angles. In polarized light the colors distinguishing the twin lamellae 

are very brilliant. No crystal form is ever recognizable, the mineral 

being typically "xeuomorphic," in the sense used by Kohrbach. 1

Quartz is rare in diabase, but where it occurs as it does in No. 11666 

we must regard it as an original constituent, on account of the extreme 

freshness of the whole specimen. z

The iron oxide is entirely opaque and shows a very irregular outline. 

This is mostly due to the penetration into it of the feldspar crystals 

whose form is clearly shown. Its crystallization must therefore have 

been subsequent to that of the feldspar, and was apparently about 

synchronous with that of the augite. The oxide itself is probably 

partly ilmenite and partly a titauiferous magnetite. In none of the 

freshest specimens does it show trace of leucoxeue. It is, however, 

sometimes partially surrounded by a border of biotite.

Original hornblende is rare in this rock, but traces of it were 

observed in a few instances.

Among the alteration products serpentine is abundant wherever 

olivine existed, but the rock is still loo fresh to show more than the 

most occasional uralitization of the pyroxene.

The five other specimens collected from this great dike show more

1 Tschermak's mineral. 11. pctrog. MitUieil., vol. 7, 1885. Uolirbach's two terms, "autlioinorphio'' 

and "xenomorphic," adopted by Kalkowsky (Lithologio, 1880, p. 14) were replaced by Kosenbusoh in 

the last edition of his Masaige Gesteino by " idiornoi-pliio " and " allotviomorpbic," tbongb for what 

reason is not apparent.

* Of. Roaenbasoh, Mikros. Physiog., 2d ed., vol. 2, I860, p. 188.
140 GEEENSTON.E SCHIST AEEAS OF MICHIGAN. [BULL. 62.

altered forms or abnormal varieties of the same rock. They are as 

follows:

No. 11621, glassy diabase. .

No* 11675, granophyre diabase.

No. 11672, uralite diabase (structure intact).

No. 11615, uralite diabase (structure intact).

No. 11610, epidiorite schist (structure gone).

No. 11621, collected near No. 11622 from the side of the great dike on 

Lighthouse Point, is a glassy modification of the diabase, produced by 

the rapid cooling of the molteu mass against the inclosing walls. The 

microscope shows this rock to be composed of a very dark browu glass, 

which can be rendered transparent ouly with the greatest difficulty. 

This opacity is due to the accumulation of minute globulitic bodies, 

which can be detected only with the highest magnifying power in the 

thinnest parts of the slide. Traces of the arborescent and fern-like forms 

common in basic glasses (tachylite) are easily recognizable. The only 

mineral constituents which have crystallized in this rock are olivine 

and feldspar. The former shows well defined crystals. These frequently contain inclusions of the brown glass, and are now wholly 

changed to a yellowish green serpentine. The feldspar shows the narrow, lath-shaped, and but partially completed crystalloids and skeleton 

forms which abound in all glassy rocks. These are filled with either 

penetrations or inclusions of the brown, glassy base.

The chemical constituents of the other diabase minerals augite, 

ilmenite, etc., are still present in the glass.

No. 11675 presents an unusual modification of this diabase. The specimen was collected from the high, rocky ridge which is formed by the 

great dike in the southern part of Section 16, about one and a half 

miles, west of l\£arquette. The hand-specimen is browner than is usual 

in this diabase and is speckled with reddish spots. Under the microscope the grain is found to be of the usual coarseness and the rock 

itself slightly more altered than the specimens above described.

The structure of the rock is still perfectly preserved. The ilmenite 

is'unaltered, and, as before, is frequently fringed with biotite. The feldspar is clear and its twinning striations are very distinct, although it is 

traversed by many interlacing veins of a green chloritic alteration product. There are no certain traces of olivine in this rock and the angite 

has undergone extensive change to a brownish yellow hornblende. 

This is partly fibrous, but also in part quite compact. Indeed, this rock 

offers a noteworthy instance of the apparent change of augite directly 

into compact brown hornblende. Much more of this mineral is here 

present than in any of the fresher specimens examined. It is impossible 

to assert positively that some of it did not exist as an original component of the rock, but the frequent cases of its undoubted derivation 

by parfimorphism of the augite makes it at least probable that all the 

hornblende has originated in this way.

The reddish spots spoken of as visible in the hand-specimen represent 

another interest ing feature of this rock. They are shown by the micro-
WILLIAMS.J ALTERED DIABASE OF THE GREAT DIKE. 141

scope to be areas of a regular intergrowth of feldspar substance and 

quartz. This is sometimes in the form of granophyre. i. e., in slender 

fibers, which group themselves* in radiating tufts about a clear feldspar 

crystal, and sometimes it shows the structure called micropegmatite. 

This -latter consists of small angular areas of quartz, each having the 

same optical orientation, which are embedded in the reddish globulitic 

feldspar substance.1

Quite recently Mr. Waldemar Lindgreu has described a similar structure in a diabase from the Big Belt Mountains, Montana.2

It was also observed by the writer some years ago in a diabase from 

Pleasant Valley, New Jersey, a specimen of which he received from 

Mr. J. F. Kernp, of Cornell University.

Eosenbusch 3 regards this structure as sufficient proof that the quartz 

is original, but the observations of Irving4 in the Keweeuawau felsiies 

and others make this conclusion doubtful.

No. 11672, collected at the corner where the road running west from 

Collins Furnace joins the road leading to Bancroft on the Dead River 

(center of Sec. 17, T. 48 N., K. 25 W.), is a uralite diabase, whose 

structure is still perfectly preserved.

The large crystals of idioniorphic feldspar are in part clear, but to 

a considerable extent clouded by the development of saussurite.

The allotriomorphic augite is in some cases intact or remains as a 

core in the center of the secondary hornblende which has been derived 

from it. More frequently, ho wever,thehornblende or uralite has wholly 

replaced the pyroxene. Its structure is oftener compact than fibrous, 

and its form reproduces exactly that of the original augite. Quartz 

areas are not uncommon. The ilmenite is mostly changed to leucoxene. 

Near one large grain of the latter is seen an area of brown mica, which 

is filled with a sageuitic network of minute rutile needles.

No. 11615, from an outcrop of this same great dike, at the bottom of 

the hill, which forms the eastern end of Arch street, Marquette, represents the next stage of alteration in the diabase.

The original structure is still plainly recognizable, although it is beginning to be concealed by alteration..

The feldspars exhibit their characteristic lath-like forms, but the 

sharpness of the outlines and more or less of the internal substance is 

clouded by secondary products. These seem to be for the most part 

small hornblende needles which have wandered in irom.the alteration 

of the pyroxene, but in some places epidote is also developed from the 

alteration of the feldspar itself. Still, in nearly every case enough of 

this unchanged feldspar is present to show plainly its original form and 

twinning structure.

1 lliis peculiar strnoturo was first observed in diabase by Tiirncbohm in 1877 (Neuos .Talirbiicli 

filr Mineral., 1887, p. 202), who (bund it in his Ivouga-diabaso type o(' southern Sweden, anil naim-d 

it " Schriftfeldspath."

"Tenth Census Eepts., TJ. S.. vol. 15,1887, p. 736.

3 Mikros. Phyaiog. 2d ed., vol. 2.

4 Cf. Moii., U. S. Geol. Survey vol. 5, p. 114.
142 GREENSTONE SCHIST AREAS OF MICHIGAN. [BULL. 62.

The augite is in the process of changing to hornblende. This alteration is sometimes.complete, but oftener cores remain in the center of 

what was the original pyroxene crystal. The border of hornblende is 

double, consisting of a wider internal baud of yellowish brown fibers 

which have apparently resulted directly from the augite substance, and 

a narrower, outer zone of bright emerald green, pleochroic needles 

which seem to project into the surrounding feldspar.1 The individuals 

of the inner zones are much smaller than those of the outer, and form 

a confused, felt-like mass, while the external needles as a rule have 

their longest axes parallel. These needles are most developed in the 

direction of the vertical axis of the augite. They may owe their different coloring to the influence of the feldspar substance which has contributed to their formation. In some cases the secondary hornblende 

appears to be homogeneous over considerable areas, exhibiting a uniform extinction and a comparatively compact character. This can not, 

however, be regarded as original in its nature.

The ilmenite still retains its form but is largely changed to leucoxene.

No traces of olivine were found in this rock, probably because of its 

altered condition.

No. 11616 is instructive when taken in connection with the specimen 

last described, No. 11615. Both were collected from the large and 

smoothly glaciated surface of trap; exposed at the east foot of Arch 

street. Through the_ massive rock run a number of schistose bands, 

produced by crushing and shearing along lines of weakness. One of 

these bands furnished the present specimen. There can be no doubt 

that these bands are a part of the massive rock. Both were the same 

before the schistose bands were produced by more intense mechanical 

and chemical action.

Under the iniscroscope the appearance of these two specimens is 

found to accord fully with their field relations. The original diabase

structure, so distinct in the massive rock, has wholly, disappeared in the 

schistose one. There are no traces of lath-shaped feldspar left. The rock 

is mainly composed of a confused network of fibrous, very pale green 

and slightly pleochroic hornblende needles. These are sufficiently parallel in their arrangement to produce the schistose structure of this 

rock. They are secondary formed from the alteration of the augite or 

the joint alteration of the pyroxene and feldspar as maybe admirably 

seen in section No. 116L5, where they are in the process of formation. 

Scattered among this hornblende, is considerable cloudy and nearly 

opaque saussuritic substance, mixed with more or less secondary quartz, 

both of which have been derived from the feldspar. The ilmeuite also 

has undergone an interesting alteration and is here represented by 

cloudy leucoxene, with only occasional opaque black dots of the original mineral. This substance is arranged in long stringers and shreds 

which follow the direction of the schistose structure.

'On the production of a similar double uralite zone around pyroxene. SeeLossen: Erliiut. z. geol. 

Specialkarte von Preussen u. d. thiiring. Staaten, Blatt Harzgerode,p.81,1882; and Williams: Bull. 

U. S. Geol. Survey, No. 28, p. 42, PI. I, Fig. 2.
WILLIAMS.] THE BROOK SECTION. 143

Here we have an undoubted instance of the dynamic metamorphism 

of a diabase, where every stage in the process can be followed. Every 

mineral of the schist can with certainty be traced to its original form, 

and yet the chemical alteration has been accompanied by such a change 

of position (migration) that the original structure has wholly disappeared 

and a new and different structure has been produced. This derived 

rock is quite identical with some of the most characteristic so-called 

hornblende or actinolite schists.

The chemical action has here been proportionately increased by the 

crushing. This seems true for all the constituents, as there are 110 

traces of the fresh fragments of broken feldspar crystals which were so 

often observed in analogous greenstones of the Menominee region.

There is an excellent section through the rocks of the northern portion of the Marquette area, exposed by a brook which flows northward 

almost along the Hue between Sectious 15 mid 1G (hardly over a mile 

west of the city) and empties into Dead Eiver. Between the road .leading from Marquette to Collins's Furnace and that forming the main highway toNegauuee, this small stream has excavated a deep ravine, upon 

the high, steep sides of which the relationship of the different rocks is 

admirably diplayed. This locality is described by Dr. Rominger, 1 and 

for convenience we will in future allude to it as the Brook section. 

(See PI. VII.)

Here were encountered several exposures of fresh olivine diabase. 

One of these, No. 11708, was taken from, the branching dike in the 

wall of greeuschist and granite, which Kominger has described and 

figured.2 This rock resembles the freshest specimens obtained from 

the great dike, except that its grain is finer, as might be expected in 

a mass of so much smaller size. No trace of alteration is here visible, except in the olivine. This constituent is not abundant, but it 

occasionally appears in well shaped crystals, which are completely 

changed to yellow serpentine. The iron oxide presents in its form a 

decided contrast to that above described. In this rock it was produced 

much earlier than in the other, and hence it has a well developed crystal 

form. This, as seen under the microscope, is square (octahedral section), 

which may be an indication that the mineral is magnetite instead of 

ilmenite, as before. The feldspar and pyroxene of this rock need no 

special comment.

No. 1L719, obtained near the edge of another dike not far from the 

last mentioned one, is only semi-crystalline. Its brown, glassy base resembles that of No. 11621, but crystallization is farther advanced. The 

sharply defined olivine and feldspar crystals are quite the same, but a 

vast number of smaller feldspar crystals are here developed in the base. 

Only a very little augite, and that in rounded grains, is present, This 

rock would be correctly termed a melaphyre.

'Geol. Michigan, vol. 4, p. 25. 

'ibid., p. 14&.
144 GREENSTONE SCHIST AREAS OF MICHIGAN. [BULL. 62.

No 11679 is from the center and No. 11680 from the edge of a dike 

of porphyritic greenstone, which intersects the banded greenschists on 

the western side of the Brook section, not far from its southern end.

The first qf these rocks has a coarsely porphyritic structure, which, 

on account of its altered condition, is only prominent on a weathered 

surface. A fresh fracture appears of an almost even gray color, resembling some of the finer greenstones. Under the microscope this 

specimen is found to correspond admirably to the descriptions of 

the rock-type, named by Giimbel " epidiorite." The augite, reddish 

brown cores of which still remain, is largely changed to fibrous hornblende. This mineral is mostly of a pale brown color, but often has a 

bright green edging where it adjoins the feldspar substance, as has 

already been noted in the case of No. 11615 (p. 141). The feldspar is 

still clear and fresh, showing the characteristic lath-shaped forms of 

diabase. It is only rarely clouded by small patches of a gray saussurite. 

Much more abundant in this feldspar are veins and small areas of green 

hornblende needles, which owe their existence mostly to the pyroxene, 

though the feldspar seems also to have aided in their formation. They 

are especially abundant in cracks, along which the substance appears 

to have wandered from the decomposing augite. This hornblende is 

finely fibrous and forms matted, felt-like masses. 'There is but little 

chlorite and secondary quartz in this rock. The ilmenite is largely 

changed to leucoxene and occasionally surrounded by a rim of biotite.

The second of these two specimens (No. 11680) from the edge of this 

dike, shows an extremely fine grained, greenish groundmass, whose 

components are very weakly polarizing and hardly determinable. This 

was perhaps once a glass which has become devitrified and viriditic. 

In it are imbedded sharp, lath-shaped plagioclase crystals, which are 

either unaltered or at most penetrated by veins of actinolite needles, 

and augite almost wholly changed to hornblende or chlorite,

The following five rocks are also good representatives of the epidio 

ritic type. They are largely composed of either light or dark greeu, 

fibrous, secondary hornblende. In none of them does any trace of augite now remain, and it is possible that some of them were originally 

true diorites, like the rocks to be described under the next head. However, the unmistakable evidences of diabase structure often apparent 

render it more probable that these rocks all belonged originally to this 

type.

No. 11663, from Pine street just north of Michigan street, Marquette, 

is most like a true diorite aud may be an altered form of the Picnic Island rock. The hornblende is of the general type represented in PI. XII, 

fig. 2 of No. 11186, from the Horse Race above Upper Qninnesec Falls 

(see p. 107). It is more or less fibrous, of a pale green color and often 

looks as though it had originated from the bleach in gaud disintegration 

of a compact original hornblende. Around the edge of large areas it 

has a darker green border and seems to ravel out into fine fibers which
Wl LI JAMS. EPIDIOEITES. 145

wander into the feldspar. Indeed, the feldspar is quite filled with such 

hornblende needles, which, together with its sausstiritic alteration products, almost conceal the original substance. Ilmenite in ragged areas, 

but without leucoxene, and a little of the chlorite-epidote aggregate (see 

PI. XI, h'g. 1) are also present.

Nos. 11712 and 11720, both from the Brook section, are rocks of the 

same general character as that last described. The former is decidedly 

schistose and bears much internal evidence of the action of great pressure. All of the constituents are arranged in interlacing and elongated 

bands. The feldspar is much crushed and shows the peculiar mosaic 

due to secondary crystallization. It is also often changed to a granular 

aggregate of brightly polarizing, pale green epidote grains. The hornblende crystals are broken and bent, and especially around their outer 

edge, are very frequently reduced to a confused mass of fine needles 

which have extensively wandered out into the feldspar substance. The 

ilmenite also is much crushed and elongated in the direction of the schistosity, but it shows no indication of being'changed to leucoxene.

The second specimen, No. 11720, is somewhat schistose, though less 

so than the last. It is extremely dark in color owing to the large proportion of iron which enters into its composition. Under the microscope 

it is a confused aggregate of very dark green and strongly pleochroic 

hornblende needles, a little feldspar, proportionately much ilmenite and 

quartz. Some of the larger hornblende crystals show the peculiar granulated center, lately described and figured by me from the gabbro-diorites 

occurring near Baltimore, 1 to which indeed this specimen presents many 

points of close resemblance.

The two remaining specimens are much more altered than those just 

described.

No. 11661, found beside the granite boss (the so-called "gold mine") 

just west of Pine street, Marquette, is coarsely granular. The feldspar 

is altered to saussurite or calcite. The hornblende is pale green and 

partly fibrous. The center of the crystals is frequently composed of a 

dark, yellowish green substance which hardly polarizes at all. This, at 

first glance, appears to be a core of compact brown hornblende but a 

closer examination shows that this is not its nature. A little sphene 

and considerable blood red iron hydroxide make up the remainder of 

this rock.

The last specimen of these epidioritic rocks, No. 11711, was part <>f 

a well marked dike of schistose greenstone which intersects a boss of 

gneiss near the northern end of the Brook section. This gneiss strikes 

B. and W., while the dike strikes N. 75° E. The schistosity of the 

latter is parallel to its walls, so that the foliations of the two rocks 

intersect at an angle. This rock shows the effects of pressure and 

crushing. It is now a finely granular aggregate of pale green fibrous 

hornblende, feldspar, quartz, calcite, and leucoxene.

' Bull. U. S. Geol. Survey, No. 28, p. 28, PI. Ill, ii;.'. 1.

Bull. 02 10
146 GREENSTONE SCHIST AREAS OF MICHIGAN. [HULL. 62.

Diorite. The occurrence of typical original diorite in the Marquette 

area still seems a little doubtful. No. 11654, which composes the Picnic 

Islands, a short distance north of Lighthouse Point, comes nearest to 

deserving this name, although this rock might with as much propriety 

be termed an amphibole granite. Julien called it a " quartzose porphyritic diorite," 1 and he was followed by Wadsworth.2 The microscope 

shows this rock to be a coarsely granular aggregate of green hornblende, saussuritized plagioclase, together with a less altered, unstriated 

feldspar, quartz, and sphene. The. hornblende is mostly quite compact, and shows in cross-section sharp crystalline outlines, which are 

often twinned. The plagioclase contains large zoisite individuals aside 

from its more usual sanssurite aggregate. The unstriated feldspar 

seems to alter to muscovite or kaolin, as is common in orthoclase. 

Seams in this rock are filled with epidote, calcite, or more rarely specular iron.

The Picnic Island rock seems to be most closely related to the anrphibole-grauites, gneisses, diorites, and massive hornblendite which occur 

in the granite farther north notably on Partridge or Middle Island, 

beyond Presqu' Isle. At this latter locality there is every evidence, 

from the intimate association of the granite and these hornblendic 

rocks, that they were both liquid, or at least plastic, at the same time. 

Such a perfect intermingling of the two magmas it seems impossible 

to explain on any other hypothesis ; nor, indeed, are indications of the 

same iuterniingliug wanting on some of the Picnic Islands.

ACID LNTHUSIVKS.

General character. The association of acid rocks of various types 

with the greenstones of the northern portion of the Marquette area is 

not less intimate than that already described as occurring at Upper 

Quiimesec Falls, on the Menominee River. (Chap. Ill, p. 110.) The 

general character of these rocks is very similar at both localities, and 

at both they offer equally valuable material for the study of dynamic 

metamorphism.

The passage from the Marquette greenstones to the granite lyingnorth of them is an extremely gradual one. There is no such sharp 

line of contact as is represented on Rouiinger's map, but, on the contrary, as Rorninger himself explains, there is a complete interpenctration 

of the two rock masses. The granite has intruded itself into the schistose greenstones, for the most part following their bedding and forcing 

apart their strata. The amount of the acid rock gradually diminishes 

as we go southward. At the Dead River it still composes over half 

of the entire mass, but between this and the city of Marquette the 

dikes and bosses of granite grow less and less frequent. They are,

'Geol. Mich., vol. 2, p. 163.

* Notes on the Iron ami Copper Rocks of Lake Superior. Bull. Mus. Coiup. Zool., Harvard Coll. 

Whole Series, vol. 7, Geol. Series, vol. 1, p. 39.
WILLIAMS.] THE GRANITE OF DEAD EIVER. 147

however, never entirely absent from the northern part of the greenstone area, but extend to the limits of the Eureka series. Around 

Lighthouse Point and near the Marquette water-works, on the north 

side of the harbor, bands of reddish and gray acid rocks may be seen 

along the lake shore in abundance. Some of these are but little altered 

granites and porphyries, while others are decidedly schistose and seem 

to owe their structure to dynamic agencies.

Granite. We will first examine specimens of the unaltered granitic 

type. These can be nowhere better seen than in the neighborhood of 

the powder mill on Dead liiver, near the center of Section 10. Near 

this place were obtained Nos. 11612 and 11613, both normal granites 

which penetrate the schistose greenstones. They are of a reddish 

color and consist principally of feldspar and quartz, both of which are 

allotrioiuorphic and form a granular aggregate of interlocking grains. 

The quartz areas are often composite, and are traversed by lines of 

fluid-cavities, containing movable bubbles: The feldspar is both inonoclinic and tricliuic. It is considerably altered to muscovite or kaolin. 

A little chlorite represents the original presence, in small quantity, of 

a third constituent, but its condition is now too much altered to allow of 

its exact determination. The second of the two specimens (No. 11613) 

is the coarser grained and contains both inicrocline and pyrite, which 

are lacking in the former.

No. 11668, also from near the powder mill, occurs where the granite 

and schistose greenstones are very intimately associated and it seems 

to be intermediate between them. Under the microscope it appears as 

a reddish granite, like those last described. Apatite crystals and minute, sharply defined zircons are abundant. The greenish color, which 

imparts to this rock an appearance intermediate between a granite and 

a greenstone, is due to the large amount of chlorite present. This 

sometimes traverses the feldspar in irregular veins and is sometimes 

massed together in larger areas, as though it had resulted from the alteration of some bisilicate, or, as is still more probable, of a micaceous 

constituent.

Toward the southern end and on the west side of the Brook section, 

the banded greenstones and granite are exposed in relations not less 

interesting than those to -be seen at Dead Eiver. A large, glaciated surface of the schists shows intrusions of both granite and diabase which 

cut directly across the strike. The former rock contains angular fragments of the schist and in one case it fills'the inequality formed by the 

faulting of the schist along a joint plane nearly perpendicular to its 

bedding. It also forms narrow veins and rows of bulging lenses in the 

schist. This granite shows no signs of foliation in the field, but a closer 

study of it in tlie laboratory discloses the effects of powerful dynamic 

action. The hand-specimen, No. 11678, proves upon microscopical examination to be an excellent example of peripheral granulation, producing whatTornebohm has called the " mortar-structure," The grains
148 GREENSTONE SCHIST AKEAS OF MICHIGAN.

have rubbed agaiust one another and/ormed a fine mosaic mostly a 

new crystallization which resembles a cement. The quartz has suffered so much by this process that hardly a trace of the original granitic 

quartz remains. The feldspar presents rounded grains, often bent or 

fissured. In the cracks thus formed the same mosaic-like cement is 

developed. Chlorite, epidote, muscovite, and iron hydroxide are present 

in this fine grained mass between the rounded feldspars.

 No. 11710 was obtained from the rounded knob of gneissoid granite, 

near the northern end of the Brook section, through which the dike of 

foliated trap described on p. 14.5 (No. 11711) passes. In this rock the 

gneissic structure is quite prominent through a parallel arrangement of 

the constituent minerals, but under the microscope it quite closely resembles the last specimen. The same peripheral granulation of the 

original grains by rubbing is even more apparent; and, although there 

is no marked tendency to parallel arrangement visible in the thin section, the effects of pressure are everywhere shown, as in the undulatory 

extinction of the quartz, in the production of secondary strain-lamella 

in the feldspar, and in the presence of inicroclihe.

No. 11614 was obtained from a gray granitic knob on the lake shore 

at the mouth of Dead River. This is penetrated by several sharply 

defined dikes of fine grained diabase, and, like the rock last described, 

is decidedly gneissoid in structure. The rock is composed mainly of 

quartz and orthoclastic feldspar, although both plagioclase and a green 

fibrous hornblende are also present. The gneissic or u flaser " structure 

is its most striking feature. This is plainly visible in the thin section 

and seems to be mainly due'to the elongation of the quartz areas in 

the direction of the foliation. This mineral has suffered greater deformation than the feldspar and is present in long, lenticular patches) 

whose axes are often sinuous. These are for the most part composite 

and always show an undulatory extinction. There is, strangely enough, 

no peripheral granulation visible in this rock.

No. 11660 is from a granite boss on the west side of Pine street, north 

of Ohio street, Marquette. A tine section has recently been laid open 

through the center of this mass by some eager gold seeker. The rock 

is a fresh, even grained aggregate of orthoclase and quartz. Any original micaceous constituent seems to be lacking, although good-sized 

plates of muscovite have been secondarily developed at the expense of 

the orthoclase. This feldspar possesses a beautiful zonal structure, 

which is brought out in unusual distinctness by the different degrees to 

which the alteration has progressed in different zones. The eruption of 

this rock must have taken place subsequently to the dynamic movements which metamorphosed so many of the Marquette intrusives. It 

shows no evidence of pressure in its microscopic structure, although 

No. 1.1658, collected but a few feet north of it, from a narrow band of 

acid rock, shows such evidence in a very marked degree.

.(Quartz porphyries. No rocks are better suited to exhibit the effects
WILLIAMS. I QUARTZ PORPHYRIES AT MARQUETTE. 149

of dynamic metamorphism than the quartz porphyries. Some of the 

more important results which this class of rocks has yielded to European students have already been stated in Chapter I. Near Marquette 

the narrower of the dikes and intrusions of granitic matter have, in 

consequence of their rapid cooling, assumed this form. Almost every 

variety of structure between a typical granite and quartz porphyry 

may be found by comparing specimens from the different exposures. 

Furthermore, the various dikes seem to have been subjected to different degrees of mechanical action, so that the successive phases of inetamorphism may be traced by a comparative study of them.

In No. 11629 we have a granitoid porphyry which has suffered much 

from chemical, but very little from mechanical alteration. This was 

collected from a branching dike of a massive, reddish rock, exposed on 

'the lake shore near the water-works in Marquette. It approaches a 

granite in being largely composed of porphyritio feldspar crystals with 

but comparatively little interstitial groundmass. What there is of this 

groundniass consists of a fine granular mosaic of quartz and feldspar 

grains with some sericite. The larger feldspar crystals are colored red 

by an abundance of fine globulitic dust. They also, show an advanced 

stage of chemical alteration in the development in them of a micaceous 

mineral (sericite). The only traces of the original biotite consist of 

chlorite areas dotted over with iron hydroxide. At first glance this rock 

seems to resemble No. 11678 (above described as a fine example of 

Tornebohm's mortar structure), but a closer examination shows that the 

structures, apparently so much alike, must have been produced by very 

different means. There is in the present instance no evidence of pressure, the feldspar crystals have their original outlines intact, and the interstitial mosaic, instead of being due to a grinding action between the 

grains, is here evidently a product of the original crystallization.

The next specimen, No. 11620, is a typical quartz porphyry which also 

has been but little modified by the effects of pressure. It is from a 

small dike at the eastern extremity of Lighthouse Point and is a much 

fresher rock than the last. The porphyritic crystals are both smaller 

and less frequent. They consist of orthoclase, plagioclase, and quartz, 

imbedded in an abundant microgranitic groundmass. The orthoclase 

often shows a twinning structure in accordance with the Carlsbad law 

and is extensively altered to muscovite, large plates of which occur 

around its crystals. The quartz does not here'possess its characteristic 

dihexahedral form, but occupies oval or irregularly shaped areas which 

are frequently composite.

These two specimens will serve as representatives of those acid dikerocks which have been least modified by pressure. They will serve as 

a starting point for tracing the successive stages of this action in the 

following specimens. The changes observed are: The deformation and 

elongation of the quartz crystals; the fracturing and separation of the 

fragments of feldspars; the parallel arrangement of the new crystalliza-
150 GREENSTONE SCHIST AREAS OF MICHIGAN. [BULL. 62.

tion products, giving rise to a gneissic or "flaser" structure; and the 

development of a cleavage. The intensity of these changes is of course 

proportionate to the amount of.force exerted.

No. 11633, obtained from the quarry on Lighthouse Point (A of map, 

PI. VII), is a quartz porphyry which differs only slightly from No. 11620, 

last described. Its groundmass is somewhat finer grained and sericitic. 

The porphyritic crystals of feldspar are nearly the same, but the quartzes 

are deformed and lenticular, sometimes with only an undulatory extinction, sometimes broken into several areas which are more or less displaced from their original position. Although the thin section shows 

no indications of schistose structure, this in the field is quite pronounced. 

The rock occurs in the green schist, in small bands, which wedge out 

very soon. It has a decided tendency to break along planes parallel to 

the schist-bedding, and upon these planes of parting sericite is abundantly developed.

No. 11653, from the northern edge of Lighthouse Point, just below the 

Lighthouse, is very similar to the last, except that here a schistose 

structure is microscopically, as well as macroscopically, visible. The 

groundmass and porphyritic crystals are quite identical with those 

above described. In the former there is a brown mica abundantly developed in aggregations of little plates, which form sinuous lines around 

the porphyritic crystals in a direction parallel to the strike of the surrounding greenstone schists. This fact is mentioned by Dr. Wadsworth, 

who was the first to discover these rocks.1

Nos. 11707 and 11717, were collected from two dikes of porphyry interlaminated with the green schists of the Brook section. In the hand specimens they exhibit no marked foliation, but under the microscope their 

evidences of mechanical alteration are most interesting and instructive. 

The groundmass is of the same character as that already described. 

In it, however, are sinuous bauds of green mica which bend and wind 

abou£ the porphyritic crystals so as to produce a decided " micro-flaser" 

structure. This feature is more pronounced in the second specimen, 

although the two are essentially the same. The microscopic appearance of 11707 is represented in PI. XV, fig. 2. The porphyritic feldspars have suffered little, or no change, unless it be by the production 

of microcliue which is quite abundant in the center of some of the crystals of No. 11707. The quartz has suffered much more deformation 

a fact which the observations of J. Lehmann 2 and Ch. E. Weiss 3 

would indicate to be the rule in rocks of this character. In the present 

case the quartz is drawn out into long, spindle shaped lenses which are 

often pinched at their ends into fine lines that bend around the other 

porphyritic crystals (see PI. XV, fig. 2). This deformation, however,

''Notes on the Iron and Copper Rocks of Lake Sftperior. Bull. Mus. Comp. Zool., Harvard Coll. 

Whole Series, vol. 8, Geol. Series, vol. 1, p. 38.

2 TJutersuchuiigen iiber dioEntstehung der altkrystalliuischen Sclviefergesteine. Bonn, 1884.

3 Zeitschr. Deutsch. geol. Gesell., vol.29, 1877, p. 418, and Jahrbuch preuss. geol. Landesanstalt fur 

1883, p. 213. Of. Rosenbnsch: Mikros. Physiog., 2d ed., vol 2, 1887, p. 412.
WILLIAMS.] TUFFS OF THE QUARTZ PORPHYRIES. 151

does not take place without the loss of continuity in the quartz substance. On the contrary a mosaic like aggregate of interlocking grains 

is formed whose optical orientation differs more and more in proportion 

as the original quartz crystal is elongated. In cases where the deforma. 

tion is slight, only a disturbed extinction results, as may be seen in No. 

11753. The presence of interlocking grains of different orientation, as 

in this instance, would indicate solution and subsequent deposition.

No. 11658, from.a narrow acid band immediately north of the granite 

boss near Pine street, Marquette, is a rock of precisely the same type as 

those just described but which shows a secondary schistose structure 

more perfectly developed than any of the preceding. The brownish or 

greenish mica is here arranged in nearly parallel bands which bend 

only slightly around what were once porphyritic feldspar crystals. 

These latter have undergone an almost complete change to muscovite 

and quartz aggregates, some of which are now drawu out into long lenticular areas. These patches are hardly to be distinguished from the 

groundmass, except by their coarser grain, and yet they can be traced 

with certainty back to the original orthoclase. Sharp crystals of pyrite 

and some epidote are also developed in this rock.

Tuffs of the acid rocks. Closely allied to these schistose porphyries 

which occur in dikes are certain banded acid rocks whose most important exposures are along the southern edge of the northern Marquette 

greenstone area, near the junction of this with the fissile argillaceous 

shales of the " Eureka series." These are hard, compact, fine grained, 

but decidedly schistose beds, locally known as novaculite. Indeed, 

these rocks have been worked to a limited extent as whetstone, whence 

the name of the small creek flowing through the southern part of Marquette.

The color of these so-called novaculites is sometimes reddish, sometimes pale greenish or yellowish. They are very frequently striped with 

different shades. Under the microscope they closely resemble the 

groundmass of the quartz porphyries described in the last section; No 

porphyritic crystals, however, are ever observed, and the structure is 

too fine grained to appear schistose in a thin section.

Three essentially identical sections of these rocks were studied from 

specimens collected at three of the most typical exposures.

No. 11627 is from near Burgess's saw-mill, at the east end of Lake 

street, Marquette. It has a reddish color produced by an abundance of 

iron hydroxide scales. The grain is for the most part quite regular, 

although larger and angular quartz fragments are occasionally seen.

No. 11684 was taken from the glaciated exposure of the novaculites^ 

occurring at the west end of Ridge street, Marquette. A fresh fracture 

in this rock shows a light gray color. It is parted by intersecting joint 

systems into sharp rhomboidal prisms which strike slightly south of 

west. Under the microscope this rock appears as a fine, even mixture 

of quartz grains and sericite flakes.
152 GREENSTONE SCHIST^ AREAS OF MICHIGAN. [BOLT, 62.

No. 11076 was collected at the south end of the "Brook section.,'1 

near Steinbrecher's house. (B of map, Plate VII.) It is quite similar 

to the others, but it is much seamed with infiltrated quartz, small veins 

of which appear in the microscopic section.

The regular banding of these anomalous acid rocks, as well as their 

interstratification with and gradual passage into the similarly banded 

greenstone schists, will not admit of the supposition that they wereever 

eruptive masses, however much we may imagine these to have been 

metamorphosed. On the other hand, their similarity, in both composition and structure, to the groundmass of-the quartz porphyries, is too 

striking to be overlooked

Apparently only two hypotheses are altogether reconcilable with the 

observed facts : First, that the rocks are true sediments, like those of 

the Eureka series, along whose northern edge they lie, which may have 

been largely derive*! from disintegrated quartz porphyry material; or 

second, that they are consolidated acid tuffs which accompanied the 

eruptions of the porphyries, in the same manner that some of the 

greenstone schists represent the fragmeutal diabase material.

Another possible supposition is that these compact, horustoue-like 

rocks resulted from the con tact-metamorphism of preexistent sediments 

by the diabase eruptions. Such an effect has been extensively produced by the Hartz Mountain diabase, as has been shown by the work 

of Lossen and others. The resultant rock is known as adinole, but 

it differs essentially from the Marquette novaculite in a "constant and 

high percentage of soda, due to the presence of albite. This, as may 

be seen from the following analysis, is absent from the Marquette rock. 

We further have no evidence of the existence of sediments when the 

greenstones were produced, as all the Huronian beds are younger.

Of the two first mentioned hypotheses, the second is the more probable, first, because of the very constant character of these banded acid 

rocks, and second, because of their almost exact identity in chemical 

composition with the massive acid eruptives. The following chemical 

analysis of specimen No. 11684, from the west end of Ridge street, Marquette, was made by Mr. W. F. Hillebrand :

Potash (K2O).... ............... 3.65

Soda (Na2 0).................... 0.56

Lithia (Li 2 0)................... Trace.

Water (B4O)................... 2.35

Phosphorous pentoxicle (P2O5 )... Trace.

Silica (SiOs)..----.............. 76.99

Alumina (A] 2 03 )..---. .-.-.. ---- 13.92

Ferric oxide (F<'2 0.i).. .......... 0.45

Ferronsoxide (FeO)............ 0.77

Muugaiions oxide (MnO) ........ Trace

"me (CaO)^ ............. 0.32 Total.................... 100.13

Magnesia (MgO) ............... 1.12

It will be seen that this is the composition of an aggregate of quartz 

and sericite, as the microscope shows our novaculite to be. The sericite 

is in all probability the result of the alteration of original orthoclase.

The occurrence of similar tuff deposits in connection with eruptive 

quartz porphyries is well known in Europe, especially in the German
WILLIAMS.] ACID TUFFS FEOM OTHER LOCALITES. 153

dyas. They have been carefully studied and described in Saxony, 1 the 

Vosges Mountains,2 the Black Forest,3 and Odemvald.4 The writer can 

speak from a very considerable personal acquaintance with the Black 

Forest and Odenwald tuffs, of their very close resemblance to the Marquette novaculites.

But perhaps the closest analogues of the Marquette tuffs are to be 

found among those described by Dr. ArchibaldG-eikie,- from St. David's, 

in Wales,5 a region which, as we shall see in the sequel, presents many 

points of resemblance to the Marquette district. The analysis of one of 

them, hereafter quoted (V), agrees very closely with that of our novaculite. Dr. Geikie finds that these acid tuffs accompanied superficial 

eruptions of quartz porphyries, which were intimately associated with 

more abundant and contemporaneous extrusions of diabase and diabase 

tuffs. The acid tuffs have been derived from true fine grained felsites. 

Some of them are conglomeratic (agglomerates); some of them fine ashy 

material, which has become consolidated into a sericitic schist. There 

are various intermediate varieties between the acid and basic types, 

due to the mingling in different proportions of the two kinds of debris.

Mr. J. S. Diller6 has also shown that the felsitic r.ock from Breakheart 

Hill, near Saugus, Massachusetts, is a silicified quartz porphyry tuff; 

and from the sharply angular shape of the quartz fragments, of which, 

it is now composed, he concludes that these are pseudoraorphs after 

fragments of an acid glass originally deposited as a volcanic ash.

The following analyses of well characterized acid tuff's from European 

localities are here quoted on account of their similarity to the Marquette novaculite:

Ferric oxide (Fej03 ) ...... > 

Ferrous oxide ( FeO) ...... J

Potash (K2 0) ...............

Soda(Na,0) ................

Water (HjO) ................

Total .................

I.

76.37

13.94

3.38

4.39

1.07

1.58

100. 53

II.

79.73

o 11. 34

0.99

0.27

3.81

0.17

2.12

98. 43

III.

82.47

9.55

f 0.43 

I 0.57 

0.53

4. 69

0.58

1.18

100. 00

IV.

84.12

y. 38

| 1.78 

0.08

0.01

0.85

0.25

' 3. 68

100.15

V.

80.59

11.29

c 0. 28 

< *1.41 

0. 52

0.95

2.98

0.72

tl.96

100.70

*Mangatious oxide trace. t.Loss on ignition ami water.

1 C. F. Xaumann: Geognosie, 2d ed., vol. 1, p. 671,1858. Also ErUiuterungeu v.nv geol. Specialkarte 

dos kiiii. Sachsens, Sect. Frobburg (59), Colditz (44), Diibeln (46), Leisnig-Lausigk (43), and Kochlitz 

-(CO).

2 D. Gerliard: Geognistisch-petrographischeMittheilungen ans dem Gebweiler-Thal, 1880. (Cf. Neues 

Jabrbucb fiir Mineral., 1881, vol. 1, Referate p. 374).

 <G. H. Williams: Neues Jabrbucb fiir Mineral., Boilage-Band 2, pp. 026-634.

4 Beuecke and Cohen : Geogn. Beschr. d. dmgegend von Heidelberg, 1881. pp. 221-235.

6 On the supposed pre-Cambrian rocks of St. David's. Quart. Jour. Geol. Soc. London, vol. 39, pp. 

298-301,1883.

°The felsites and their associated rocks north of Boston. Proe. Boat. Soc. Nat. Hist., vol. 20, pp. 

355-308; Bull. Mils. Coinp. Zool., Harvard Coll. Whole Series, vol. 7, pp. 165-180; also Science, 1884. 

vol. 3, p. 653.
154 GREENSTONE SCHIST AREAS OF MICHIGAN. [BULL. 02.

I. Thousteiu (tuff) from Zeisigwald, near Chemnitz, Saxony. 1

II. Same. 2

HI. Silicifled tuff, Oelberg, near Schriesheim, Odenwald. 3

IV. Silicified tuff, Kesselberg, near Triberg,J3lack Forest. 4

V. Felsitic tuff breccia, Clegyr Hill, St. David's, Wales. 5

If, then, these banded acid rocks occurring near Marquette, as seems 

most probable, really do represent solidified tuff material of the quartz 

porphyries, we might expect to find transitional forms, grading into the 

banded green schists. These do appear to exist, and indicate that 

some, at least, of the eruptions of both acid and basic rocks were nearly 

contemporaneous.

BANDED GREENSTONE SCHISTS.

The banded greenstone schists which form the prevailing rock over the 

northern portion of the Marquette area have been regarded by all geologists who have ever studied them, as originally sedimentary deposits, 

and a repeated examination of them in the field seems incapable of 

leading to any other conclusion. They are everywhere stratified with 

the greatest regularity in bands of lighter and darker shades of green. 

This structure is to be most advantageously seen in the woods just 

north of Marquette and near Lighthousa Point. Here glaciated areas 

of considerable extent often show a finely ribboned appearance, looking 

as though the sharp, parallel lines had been drawn with a ruler. The 

alternation in the color and composition of the layers is so frequent and 

so constant, and their parallelism to the east and west strike of all the 

rocks of this neighborhood is so exact that 110 hypothesis of their originally massive character will satisfactorily account for the observed 

facts.

On the other hand; the chemical and the microscopical characters of 

these schists agree closely with those of associated massive greenstones 

which are known to have been derived by the alteration of basic eruptive rocks. Both are composed of fibrous green hornblende, quartz, 

epidote, zoisite, and chlorite. There must, therefore, have been a close 

similarity in the original composition of these two classes of rocks, in 

spite of the wide difference in their structure.

Prof. M. B. Wadsworth, who has ably advocated the eruptive origin 

of many of the rocks occurring near Marquette, regards these banded 

schists as undoubtedly of sedimentary origin. He says: G

The schist from the same specimen (45) is composed of quartz, argillaceous material, chlorite, hornblende, magnetite, " leucoxene," and a little augite. It would seem 

that this had been formed from detrital material of the same nature a"S the dikes 

(basaltic). The close resemblance of the "diorite" and schist in mineralogical characters, but not in structure, is shown in another section contain ing the junction of the 

two rocks (48).

1 Neues Jalirbuen fur Mineral., 1864. 

2 Ibid., 1859.

3 Die zur Dyad gehorigon Gestoine des siidliclien-Odenwaldes, von E. Colieu, 1871, p. 57. 

4 Neues Jahrbuck fur Miueral., Beilage-Baud 2, p. 030. 

"Quart. Jour. Geol. Soc. London, vol. 39, 1883, p. 297.

"Notes ou the Iron and Copper Districts of Lake Superior. Bull. Mus. Cornp.'Zool., Harvard Coll., 

"Whole Series, vol. 7, Geol. Series, vol. 1, p. 37.
WILLIAMS.] SHEAF-LIKE HORNBLENDE IN THE BASIC TUP"PS. 155

Specimes of these banded greenstone schists which were collected 

from different localities within the northern Marquette area, although 

they are macroscopically nearly identical, present a great variety in 

their microscopical structure.

Nos. 11618 and 11619, from Lighthouse Point, Marquette, are comparatively coarse grained rocks composed largely of a green fibrous 

hornblende. The structure of this mineral, especially in No. 11619, is 

unusual. What at first glance looks like a confused mass of irregular 

and ragged hornblende areas, when more carefully examined, is found 

to consist of more or less radiating bundles of hornblende fibers, resembling sheaves. The center of these bundles is their most compact portion. Here the hornblende substance is often continuous and homogeneous, but toward the two ends of the mass it appears to be frayed 

out into separate fibers. These are so regularly divergent in their arrangement as to produce a black brash which sweeps across them as 

the stage of the microscope is revolved between crossed Nicol prisms. 

This peculiar, sheaf-like grouping of the hornblende is shown in PI. XVI, 

Fig. 1. It is very similar to that described by dela Vall6e-Poussin and 

Renard in an ainphibolite occurring near Laifour, in the valley of the 

Meuse, 1 and by Renard in the metamorphosed .Devonian slates of Bastogne in the Ardennes Mountains.2 Though it can still be traced in 

specimen No. 11618, this structure is by no means as distinct as in the 

case just described.

The groundmass in which this hornblende lies is a finely granular 

aggregate of albite, or quartz and albite grains. In it occur some, 

chlorite, epidote, or zoisite in good sized individuals, and, at least in 

No. 11619, some small ilmenite particles, surrounded by a highly refractive leucoxene border. There is comparatively little groundmass in 

either of these specimens, but of the two No. 11618 contains the more.

Nos. 11730, from the western part of Marquette, between Michigan 

and Ohio streets, and 11734, from the corner of Front and Ohio streets, 

are both banded greenstones of the same type as the. specimens last 

described. The former contains less hornblende, though this mineral, 

is, nevertheless, abundant in the same sheaf-like bundles as before. 

The groundmass is a fine grained albite mosaic, which contains some 

quartz and some epidote. The latter specimen, No. 11734, is largely 

composed of an unusually dark green and pleochroic hornblende, ,in confused ragged and fibrous groups. Along certain lines, wh,ere this component is less abundant, the rock assumes a lighter shade. The albite mosaic is present, but not to the exclusion of areas of gray saussurite, 

which sometimes show indications of a lath-like form, like that of the 

diabase feldspars. Narrow seams filled with epidote, or an aggregate 

of albite and epidote, traverse this rock parallel to its banding. Iluieuite grains, with their rims of leucoxene, occur in each of these specimens.

'Mom. snr los char. min. ct strut, lies roches rtites pliitouiennes, etc., Brussels, 1870, ]t. 252, PL V, 

Fig. 25.

a Lcs roches gr6natif6roHet aihpliiboliquea.ilu la region do Bn.itogno. Hull. Mils. roy. tl'Iiist. nut. tlo 

13u]gic[iie, 1882.
156 ' GREENSTONE SCHIST AREAS OF MICHIGAN. [BULL.62.

There is nothing in the microscopical structure of these slides to suggest a massive eruptive rock; on the contrary, the two instances cited, 

where a similar sheaf like hornblende has before been noticed, are cases 

of supposed metamorphosed tuffs.

No. 11630, from the quarry on Lighthouse Point (A of map, PI. YII) is 

also largely composed of hornblende in pale green, ragged-looking individuals, but without any of the sheaf-like structure. Amid the hornblende a clear transparent feldspar, evidently albite, has been abundantly developed in good-sized grains, which show a sharp twinning striation. The gradation of this well characterized albite into the granular mosaic lends additional force to the supposition that this also is composed of the same mineral, as, indeed, Lossen has shown to be the case 

in many of the regionally metamorphosed diabases of the eastern Hartz. 1

Nos. 11631 and 11637 are other specimens of the banded greenstone 

schists from the same quarry as the last on Lighthouse Point. The former is, perhaps, the most typical, although an abundance of calcite 

shows that it is much weathered. Its alternating light and dark green 

bands are due to the fact that hornblende is still present in the latter, 

while its place is supplied in the lighter colored bauds by a pale chlorite and epidote. An opaque iron oxide, often surrounded by a leucoxene rim, is found in small particles scattered through this rock, as 

is also a little biotite. The second of the above mentioned specimens, 

No. 11637, which was taken from just beside the great dike, shows what 

the last described rock was when it was in a fresher condition. The calcite is here absent, and the rock is, for the most part, a mass of ragged 

and raveled hornblende. Among this, is scattered some clear, transparent feldspar (probably albite) and zoisite, or epidote, which occurs in 

irregular crystalloids, arranged in long, parallel lines. The iron mineral 

and biotite are present as before.

No. 11623, found beside the great dike on Lighthouse Point, is only 

a confused aggregate of very pale and very fibrous hornblende needles 

with calcite. Lighter bauds are occasioned in this rock by a preponderance of calcite and chlorite over the hornblende.

No. 11626 is a gray schistose rock from the exposure near Burgess's 

saw-mill, on Lake street, Marquette. It is full of calcite, which is arranged in bands along with a very finely granular mosaic. Hardly any 

hornblende is visible in this specimen, but veins-of epidote and some 

chlorite occur in it.

Along the deep north and south exposure of the greenstone schists 

which has been described as the Brook section, these banded greenstones 

are also very admirably displayed. For the most part they are quite 

like those occurring in the neighborhood of Marquette. In one particular, however, some of them present an unusual feature which it is 

not altogether easy to explain. This consists in the presence of len1 Zeitschr. Deutscii. geol. G-cseli., vol. 24, 1872, p'. 730. Jahrbnch preuss. geol. Landesanstalt fiir 

1883, p. C40; ibid., fiir 1884, p. 528. Erliiufc. ziir geol. Specialkarte von I'r., Blatt Wippra, Berlin, 1383.
WILLIAMS.J WHITE AUGEN IN THE GREENSTONE SCHISTS. 157

ticular masses of a white feldspathic substance, which are so elongated 

in the direction of the bedding that while they generally present a 

lens-shaped form, their cross-section is nearly circular. The size of 

these lenses, which externally resemble what the Germans call "Augon" 

or eyes, is extremely variable. Some are smaller than peas, while others 

are more than two inches in length. The extent also to which they are 

elongated differs very much in different cases. Some are represented 

only by narrow white lines which are occasionally thickened or swelled 

out into oval spots. Between these and such as vary comparatively 

little from a spherical form we may find all intermediate stages.

These white lenses are by no means universally distributed through 

the green-schists of the Brook section; on the contrary they occur only 

rarely in bauds which transverse the greenstones parallel to their bedding. One of these bands consisted of only a few rows of large, oval 

masses, exactly resembling rounded pebbles, but in spite of its narrowness it could be traced for a considerable distance.

The white color of these lenses or "Augen" presents a decided contrast to the dark green of the inclosing schists. Their outline, too, is 

quite sharp, and yet, upon a close examination, they are seen to pass 

gradually into the substance of the surrounding rock. What their 

origin was, whether they were inclusions much flattened by subsequent 

pressure, or secretions of the rock itself, or infiltrations, it now seems 

difficult, if not impossible, to decide.

Sections 11703 to 11705&, show the microscopic composition and 

structure of these lenticular masses as well as their relation to the inclosing rock. This latter is in all cases a typical hornblende schist or 

amphibolite. It is composed of small, compact needles of strongly 

pleochroic, green hornblende, colorless grains of feldspar and probably 

quartz, and the minute, highly refractive epidote particles so characteristic of the crystalline schists. The feldspar is very fresh and free from 

inclusions, as though it were the product of a new crystallization. It 

rarely possesses an albite.or microcline twinning structure, but for the 

most part it is so homogeneous and limpid that it closely resembles 

quartz. Lossen has called particular attention to the difficulty of distinguishing a mosaic of secondary albite from quartz under the microscope,1 and it is very possible that quartz grains are actually present 

in this rock, although in certain favorable cases a biaxial character was 

substantiated.

No. 11703 is cut in the direction of the bedding and shows the hornblende in strictly parallel arrangement. Its feldspar "Augeu " are very 

much elongated, while in No. 11705, which is cut nearly perpendicular 

to the bedding, they are as broad as they are long. This substance 

was undoubtedly once some triclinic feldspar, but this has been changed 

to saussurite which now consists of epidote and sericite imbedded in a

1 Zoitsclir.Deutaoli. geol. Geself., vol.31, 1879, p. 441, ot seq. Jalii'bncli prouas. gooj. Landesanstalt 

fur 1884, pp. 528, o44, Tl. xxix, fin. 4-
158 GREENSTONE SCHIST AREAS OF MICHIGAN. [BULL.62.

new crystallization of albite. They contain no hornblende, but there 

seems to be a condensation of this mineral in the schist around their 

edge and enough of it penetrates for a distance into their substance to 

form a gradual transition rather than a sharp line of contact between 

them and the inclosing rock.

The composition of these "Augen " is not in every instance the same, 

although they all appear to have been derived from some form of 

feldspar. In No. 11704 they are composed wholly of epiilote, while in 

No. 11705a the original feldspar was more acid and sericite has been 

produced almost to the exclusion of epidote.

Other of the banded greenstones collected along the Brook section 

are quite devoid of the white "Augen" and agree closely with the rocks 

around Marquette.

Nos: 11706, 11713, and 11718 are essentially hornblende rocks, containing some altered feldspar and some quartz.' They are plainly 

banded, the alternate layers differing in both color and composition.

No. 11671, from an exposure very near the center of section 17, consists of sharp epidote needles and a little quartz, imbedded in a confused mass of pale green, fibrous hornblende.

Nos. 11610, 11611, and 11667, from near the powder mill on Dead 

Eiver, are all schistose greenstones in a much altered condition consisting now of chlorite, quartz, and carbonates.

As has been already remarked, the structure of these greenstoneschists is such as to necessitate a belief in the original nature of their 

stratification; while, on the other hand, their chemical as well as their 

inineralogical composition renders it impossible to separate them from 

the massive and highly altered greenstones (uralite diabases, etc.), 

with which they are most intimately associated. Their parallel bauding indicates a fragmental, but their chemical and their mineral composition indicate an igneous origin, The only satisfactory reconciliation 

of these opposite sets of characters is to be found in that group of 

rocks intermediate between sediments and lavas, known as volcanic 

tuffs.

In the opinion of the writer, then, the banded greenstone schists of 

the northern Marquette area are to be regarded as consolidated and 

highly inet.tmorphosed diabase tuffs. These are intimately associated 

with numerous contemporaneous flows of diabase and quartz porphyry, 

together with tuffs of the latter rock; while all have been broken through 

by much younger dikes, both basic and acidic.

From the preceding petrographical descriptions it will be seen how 

great is the similarity between the banded greenstones of Marquette 

and the older massive diabases. Mineralogically, they are now identical, but the fine parallel banding of the former necessitates the 

assumption that their origin differed from that of the eruptives.

In a former paper 1 the writer has maintained that two geological

1 Ball. U. S. Geol. Survey, No. 28, p. 1.
WILLIAMS.] OEIGIN OF THE BANDED GREENSTONES. 159

masses as different in their structure and origin as a clay-bank and a 

lava-stream, if they possessed the same chemical composition to start icith, 

when subjected for a long period to exactly the same physical conditions, would result in the same products. This is, of course, true only 

when these conditions allow of profound metasoraatic and structural 

changes. On the other hand, when we find two intimately associated 

rocks which for a long time have been subjected to exactly the same 

conditions, but which still bear evidence of dissimilar origin, in spite of 

practical identity of mineral and chemical composition, we may safely 

consider them as originally of the same, or of similar chemical composition. Now, it is difficult to imagine any stratified sediments of the 

composition of diabase, except diabase ashes ; nor could these have undergone transportation by water without suffering important chemical 

changes. We therefore conclude that the banded schists represent volcanic debris deposited at or near the point of its origin.

In order to obtain a clear idea of just how these ancient and much 

disguised tuff's acquired their present form and apparently dual character, it will be advantageous to ascertain what is known of analagous 

formations of comparatively recent date. Capt. 0. E. Button's descriptions1 of the fragmeutal rocks accompanying the Tertiary eruptives of the high plateaus of Utah, are well suited to this purpose. He 

says, in speaking of the extent of these deposits :2

Some of the most interesting lithological problems presented by the volcanic prodnets of the high plateaus are those relating to the origin and development of what 

may be termed the clastic igneous rocks, or rocks apparently composed of fragrnental 

materials of igneous or volcanic origin, but now stratified either as so-called tufaceous deposits or as conglomerates. These are exceedingly abundant in all of the 

great volcanic districts of »the world, and often enormously voluminous. How those 

of the high plateaus would compare, in respect to magnitude, with those of other 

regions, I do not accurately know, but absolutely their bulk is a source of utter astonishment. They cover nearly 2,000 square miles of area, and their thickness ranges 

from a few hundred feet to nearly 2,500 feet, the average being probably more than 

1,200 feet. Lavas are frequently intercalated, but much more frequently no intercalary lavas arc seen, and in general they seldom for n any large proportion of the 

entire bulk when they occur in conjunction with the clastic masses.

Again, in speaking of the peculiar liability of such deposits to metamorphism, the same writer says : 3

A very striking characteristic of these clastic volcanic rocks, both the tufas and 

the conglomerates, is their great susceptibility to metamorphism. Not only have the 

beds in many localities been thoroughly consolidated, but they have undergone crystallization. Those tufas and conglomerates which are of older date, and which have 

been buried beneath more recent accumulations to considerable depths, rareJy fail, to 

show conspicuous traces of alteration, and in many cases have been so profoundly 

modified that for a considerable time there was doubt as to their true character. The 

general tendency of this process is to convert the fragrnental strata into rocks having 

a petrographic facics and texture very closely resembling certain groups of igneous 

rocks. When wo examine the rocks in situ no doubt can exist for a moment that

' Report on the Gdology of tho High Plateaus of Utah. By C. E. Button ; Washington, 1880. 

3 Ibid., p. 09. . 3 Ibid., pp. 79, 80.
160 GREENSTONE SCHIST AREAS OF MICHIGAN. [HULL. 62.

\\

they are water-laid strata. The hand-specimens taken from the beds, which are extremely metamorphosed, might readily pass, even upon close inspection, for pieces 

of massive eruptive rocks were it uot that the original fragments are still distinguishable, partly by slight differences of color, partly by slight differences in the degree 

of coarseness of texture. Bnt the matrix has become very similar to the included 

fragments, holding the t>ame kinds of crystals, and under the microscope it shows 

a grouudmass of the same texture and composition. * * *

I see nothing at all incredible in the idea of metamorphism producing rocks so 

closely resembling some eruptive rocks that they can not be petrographically distinguished fro7ii them. It seems rather that we ought to anticipate just such a result 

from the alteration and consolidation of pyroclastic strata. The materials which 

compose them consisted originally of disintegrated feldspar, pyroxene, and the matter which constitutes the amorphous base of all eruptive rocks.

We are thus enabled to see that there is, at least, no geological difficulty in the explanation here offered of the origin and nature of the 

banded green-schists at Marquette. On the high plateaus of Utah, 

which form one of many similar districts, conditions are plainly seen 

to have prevailed quite similar to those here assumed for the more 

ancient rocks of Lake Superior; and yet the metamorphism which has 

taken place in the ancient Archean deposits is hardly greater than 

that described by Captain Dutton in the tertiary tuffs.

The most generally accepted type of diabase tuffs is the German 

" Schalstein ;" but, as Rosenbuseh well remarks, 1 in the course of many 

years so many totally different rocks, with only the most superficial resemblance to each other, have been described under this designation, 

that there is now no definite meaning whatever which can be attached 

to this term. The schalsteins are for the most part, green or greenish 

in color, earthy and schistose; but among them have been included 

altered and foliated diabases on the one hand, and true sediments on 

the other, while some of them are undoubtedly true diabase tuffs. In 

Germany their distribution is very wide in Nassau, Saxony, and Bavaria.

The microscopic sections of German Schalsteius accessible to me for 

comparison were those in the Stiirtz collection of typical rocks belonging to the petrographical laboratory of the Johns Hopkins University. 

They are from the localities of Eiibelaud in the Hartz Mountains, and 

Diez, Baldinnstein, Dillenburg, and Weilburg in Nassau. In spite of 

extreme alteration, resulting in several cases in the survival of'only 

chlorite, calcite, and iron oxide, it can be distinctly seen that the specimens were originally totally different. The rocks from Diez, Dillenburg, and two out of the three labeled Weilburg, are only very amygdaloidal diab;:ses. The structure in the base is well preserved, while 

the araygdules are filled with calcite surrounded by a rim of chlorite. 

The rock from Kiibelaud is apparently of the same nature, but the original structure has now disappeared. In the specimen from Baldinnstein 

and in one from Weilburg there is no indication of diabase structure, 

but there is a spheroidal aggregation resembling that seen in palagonite.

1 Mikros. Physiog., 2d ed., vol. 2, p. 245.
WILLIAMS.] FOKEIGN DIABASE TUFFS. 161

Theuiostextended descriptions of these rocks have been given by Gliuibel, who has treated the schalsteins of the Fichtelgebirge in Bavaria in a 

most careful manner. 1 Bothpletz has described the tuff's and agglomerates associated with the Silurian diabases in Saxony.2 In Great 

Britain the ancient volcanic areas of Cornwall and St. David's in Wales 

show the closest similarity, both in the character of their rocks and in 

certain features of their structure to the Marquette region. The slaty 

blue " elvans " of the former district were regarded by Be la Beche as 

altered ash-beds.3 J. Arthur Phillips arrived at the same conclusion.4 

He says:

The Cornish rocks afford numerous examples of ancient lava Hows so iuterbedded 

with slates and schists of the district as to lead irresistibly to the conclusion that 

they are contemporaneous ingeous deposits.

Also again: 5

The slaty blue el vans found between St. Erth and Sfc. Stephens (in Bramwell) have 

 A chemical composition identical with that of tho altered clolerites, and may he, as

was formerly suggested, highly metamorphosed ash-beds.

The basic tuffs, like the acid ones, occurring at St. David's have been 

described by Dr. A. Geikie.6 He finds that all varieties of texture can 

be traced, from large grained breccias to fine schistose mudstones. The 

finer material includes angular, subaugular, and rounded blocks or 

lapilli, consisting not alone of diabase, but also of the more acid felsitic 

rocks. These are true agglomerates, and their striking resemblance to 

certain other of the schistose greenstones of the Marquette belt will 

be alluded to in the sequel.

The eruptive rocks which Dr. Haus Eeusch has recently studied in 

the regionally or dynamically metamorphosed district on the west coast 

of Norway, present a close resemblance to those occurring near Marquette.7 He finds diorites, in part very schistose, which have originated 

through the metnmorphism (uralitizatiou and epitlotizatiou) of ancient

1 GcognostisclioBeschreibuug des Ficktelgebirges, mit dem Frankenwald iiud deru wcstliclieu 'Vorlumle, Gotha, 1879, pp. 222 ot soq.

* Erl. zuvgeol. Specialk. Sacbdtms, Sect. Fvaiilfenborg-Htiiuicheu(78), pp. 16 and 21; Sect. SchelleubergFluha (97), p. 05.

3 Geological Observer.

4 Quart. Jour. Gool. Soc. London, vol. 34, p. 493.

8 Ibid, p. 495.

6 Quart. Jour. Geol. Soc. London, vol.39, p.297,1883.

'Goologischo Beobacbtnugen in oinuin rogioual-metamorphosirton Gebiet am Hardungorfjord in 

!Nonvegen, Neues Jalirbuch fur Mineral., Beilage-Baud 5, pp. 52-07. On jfages54,55 tho author says: 

isricht selteii zoigt das dioritiscbo Gesteiu oiueu scbiclitciiwcisen Wiochsel von verscbiodenen Varielitteu. In diesoin Falle bin icli goucigt, auzuuebmon, dass das urspriiugliche Gestoin nachAi'tdor 

TufFiigebildet wurde. Dies geschicliteto dioritiscbo Gestoin bildet einon Uebergang zudeu gi-iitilicben 

feiukornigeu oder dicbtou, goschicbteteu, mehr odr.r wenigor schieforigeii Gesteinsarten, welcho die 

Hauptmasse vou dem ansniacbeu, was icb fur basischo Tuffo hnlto. Diese Gesteiiie fiilireii als wesentlicbo Bestandtlieile Hornblondo, Cblorit uud Epidot, danobeu gem roicblicb Cnrbonspatb. Sio outaproclieu oinigcrinassen deu Scbalsteinon dcr deutschon Petrograpben. Eingolagert koiuiaen Masseii 

vor, die mail ain obestou fiir gollosseuo Striinie linltcu muss. Eiuige wind wobl aucb intrusive G.;iuge. 

Uebrigeus ist ea bier, wie audorswo untor iilinliclien Verbiiltuissou, oft scbwierig z% uuterscbciden 

was masaig erstarrtnud was klastiscbo Bilduug ist. For thu detailed descriptions of tbese rocks by 

'Dr. Reuscb, see " Bonimeloon og Karmoen," Cbristiania, 1888 (in. Danish), p. 112, and tho appended 

English summary of contents, p. 402.

Bull 02 11
162 GREENSTONE SCHIST AREAS OF MICHIGAN. [BULL 62.

gabbros. These are most intimately associated with granites and quartz 

porphyries, while both acid and basic rocks are accompanied by abundant tuff deposits.

In America hardly anything reliable has been published on basic tuffs. 

Edward Hitchcock speaks of di;ibase tuffs in the trias of Massachusetts, but gives nothing specific respecting their nature. 1 References 

to this class of deposits are scattered through the writings of some of 

the Canadian geologists, especially in the recent report of Mr. A. C. 

Lawson on the Lake of the Woods region.2

1 Geolujiy of Massachusetts, 4°, 1841, p. G48. 

2 Geol. and JSTat.'Hist. Survey Canada, Aim. Kept, for 1885, Kept. CG., 1886.
CHAPTEE V.

GREENSTONE BELTS OF THE MARQUETTE DISTRICT Continued. 

ROCKS OF THE SOUTHERN PORTION OF THE MARQUETTE AREA.

On the south side of the narrow baud of Huronian slates which has 

been designated as the " Eureka Series," the greenstones of the Marquette area present a decided contrast to those which have been 

described in the previous section as occurring on its north side. They 

are, for the most part, massive, pale green in color, and apparently 

homogeneous in structure. Only in rare instances can individual minerals be detected with the unaided eye. A schistose structure is not 

uncommon in these rocks, but it is evidently a pressure foliation, while 

the parallel banding or striping, so frequent in the greenstones of the 

northern area, is altogether absent. A study of these pale green 

aphanitic greenstones seems to indicate that they were not originally 

to any great extent tuff deposits, but massive flows of diabase, which 

have since suffered profound chemical and structural change in consequence of having been subjected to intense dynamical action.

In addition to their foliation, they are fractured and brecciated in 

various degrees, and often exhibit a curious spheroidal or lenticular 

parting, for which no perfectly satisfactory explanation has been found.

The monotony of these prevailing aphanitic greenstones is interrupted by bands of more coarsely crystalline and less altered rocks, 

which represent basic dikes of more recent origin.

THE APHANITIC GREENSTONES.

The general character of these widely distributed rocks as seen under 

the microscope, is shown iu PI. X, fig. 2, drawn from a specimen collected near the town of Negaunee, where this type is developed even 

better than near Marquette. In the mass which appears to the 

unaided eye as quite homogeneous, the microscope discloses long, 

almost acicular feldspar crystals, embedded iu a confused aggregate of 

hornblende needles, chlorite scales, epidote, quartz, calcite, and cloudy 

spots, probably derived from leucoxene. The feldspar in these rocks 

is, as a rule, fresh, and their twinning lamellae are still distinctly visible. 

Some of the more acicular crystals seem divided at the ends, as is so 

often the case in semi-vitreous surface rocks whose feldspar microliths 

are not completely formed. Indeed, in many of their characters, these 

aphanitic greenstones strongly resemble porphyrites or melaphyres

163
164 GREENSTONE SCHIST AREAS OF MICHIGAN. lnuLL.02.

whose glassy base has been completely transformed into an aggregate 

of secondary products. Tneir structure, imperfectly preserved as it is, 

strongly indicates a superficial origin.

As typical representatives of these homogeneous pale green rocks 

may be mentioned: No. 11638, from the mouth of Whetstone Brook, 

just south of Marquette; Nos, 11725 and 11726, from the summit of 

the hill immediately south of the last locality; No. 11694, collected 

about an eighth of a mile south of the State road on the line between 

the southeast and southwest quarters of Sec. 27, T. 48 N., B. 25 W.; and 

No. 11721, from south of the township road, on the line between Sees. 

27 and 28, T. 48 N., B. 25 W.

These aphanitic greenstones appear to have been particularly sensitive to dynamic action. We find locally developed in them every degree of pressure foliation from a coarse, slickensided breccia to a chloritic slate. The cleavage, however, is developed parallel to a line rather 

than parallel to a plane, as is the case with a true bedding. There is a 

tendency to break into rhomboidal prisms or along almost any plane 

which is parallel to the average direction of dip.

No. 11696 is a much gashed and brecciated greenstone which adjoins 

on the north an uralite diabase dike, No. 11695 (see p. 169). It is both 

schistose and chloritic, but its microscopical structure shows plainly 

that it was once a diabase, and that its present character is due entirely to the action of some great mechanical force. The typical ophitic 

structure or irregular network of lath-shaped feldspar crystals is still 

readily discernible. The mechanical action seems to have been rather 

a stretching than a compression. The feldspar crystals, which are considerably altered, are broken and the fragments pulled aparr. The 

secondary chlorite, so common in all stretched rocks (cf. PI. XI, fig. 

2), is here abundant in the numerous longitudinal cracks, which traverse 

the rock irrespective of its component minerals. This is always arranged with its scales or fibers perpendicular to the walls of the fissure.

In addition to such brecciated rocks as the last specimen, very perfectly schistose varieties of these aphanitic greenstones are common in 

the southern part of the Marquette area, their strike conforming to that 

of all the other rocks of this region. These are typically exposed at 

the mouth of Whetstone Brook (Nos. 11639 to 11643); in the railroad 

cutting just west of it (No. 11645); on the high hill still farther westward (No. 11726); and in the Duluth, South Shore and Atlantic Bailroad, cutting just north of the Marquette sandstone quarries (No. 11691).

For the most part, these rocks are a fine chloritic mass, which, under 

the microscope, shows a pronounced schistose structure, but only rarely 

any certain indications of eruptive origin. Very faint but unmistakable traces of the ophitic structure are nevertheless sometimes visible 

even in these rocks, as, for instance, in No. 11726, which was entered 

in the field notes as a " slaty band traversing the massive greenstones 

above Whetstone Brook."
WILLIAMS.] LOSSEN'S WORK ON THE GERMAN GREEN SCHISTS. 165

The history of these schistose greenstones must be deciphered with 

the conjoint evidence afforded by the microscope and a study of their 

relations in the field. The occasional survival of the characteristic 

diabase structure even in some of the more foliated forms, taken in 

connection with their evident identity with and gradual transition into 

the massive varieties, appears to be sufficient proof that, with the 

exception of certain unimportant tuff deposits, these green schists have 

been derived from basic eruptives through the agency of intense mechanical and chemical action.

The closest analogy to the rocks of the southern Marquette greenstone area is to be found in the metamorphosed diabases of the eastern 

Hartz. These rocks have been minutely studied and for the first time 

successfully deciphered by K. A. Lossen, of Berlin. His descriptions 

could be applied almost verbatim to the Marquette greenstones; and 

each of these two similar areas can but have additional light thrown

upon its interpretation by the facts afforded by the other.1 Lossen distinguishes between the coarse grained or granular (kornig) and the 

fine grained (aphanitisch) diabase, as has been done in the .area here 

under discussion. He finds that both rocks, but especially the aphanitic type, are very sensitive to dynamic action, and that in the process 

of mountain making they have been to a large extent passively metamorphosed and converted into breccias, "Maser" diabase and green 

schists. The last named "griine Schiefer " are of particular interest 

to us on account of their resemblance to the schistose rocks of the 

southern Marquette area. In the Hartz they make a conformable member of the Wieder schists (hercynian) which are of very considerable extent arid are always associated with the aphanitic diabase. The minute 

and careful studies of Lossen have led him to the conclusion that these 

schists are not in any way sediments nor even diabase tuffs (" Schalsteiue"), but molecularly metamorphosed eruptive rocks, of a fine 

grained diabase type.2 The more massive rocks are considered to represent old lava flows which occurred while the accompanying sediments were being deposited, and the idea that the green schists of the 

Wieder horizon were once also massive diabases, is based both upon 

the field relations of these rocks and more especially upon the frequent 

remains of the diabase structure which they still contain.

The published descriptions of many other green schist areas in Europe notably in Bavaria, Saxony and the Taunus show that they 

possess a strong resemblance to those of Michigan and the Eastern 

'Hartz, and it is not improbable that they also may have had a similar 

origin.

* ' The descriptions of Lossen are contained in tlio explanations to the special geological map of Prussia and Thuringia (Erliiuterunzen zur geologischon Specialkarte von Prousson und den thiiringisehon 

Staaten) sheets Psuisfelde (1882), and Wippra (1883).

2 " Dio mikrosknpischt) Untursuclinug characteristischor Vorkommen dioser typisclien Diabasaphauiten vorgosnllschaftotou griiuoii Schiefer hat orsebon, dass sio, wenn iiiclit insgosamtnt, doch 

jjrossentheils als witer Drticktchiyfeninyjinolecular urnyewandeltc Diabase autVaifasseu sind." Ibid., 

BI. Wippra, p. 27; (cf. also p. 40 and Bl. Pansfelde, p. 52).
166 GREENSTONE SCHIST AREAS OF MICHIGAN. [BULL. 62.

A noteworthy feature of the aphauitic greenstones and greenstone 

schists of the southern Marquette area, and one which is exhibited in 

a still more marked degree by the similar rocks occurring near the 

town of Negaunee (see the following section), is their frequent division 

into spheroidal, ovoid, lenticular or more irregularly shaped masses. 

These differ very much in size, as well as form, and often show a tendency to fit together like stones in a mosaic, although they are always 

separated by interlacing bands of a softer, more schistose and generally darker material. These anastomosing bands seem to wind about the 

harder and more massive cores, becoming thinner or thicker according 

as the masses approach each other more closely, or are more widely 

separated by the roundiug off of their corners. This peculiar structure, which appears to be only local in its development, may be seen at 

the mouth of Whetstone Brook in Marquette, near Baldwin's kilns, near 

Negauuee (NE J of KW £ of Sec. 28 W., T. 48 K, E. 26 W.), and on the 

south side of the Carp Eiver along the logging road leading northward 

from the east end of Teal Lake. The accompanying diagram (Fig. 26), 

drawn from the last named exposure, illustrates the general character 

of this structure.

Fio. 26. Diagram to illustrate spheroidal parting in aphanitic greenstone. Carp Eiver. 

Scale, 3 feet to the iiich.

The cores generally seem to be quite massive, but in some cases they 

have a latent foliation which is developed by weathering. Where the 

rock has become decidedly schistose, as was observed -in one part of 

the Carp Eiver exposure, these cores are pulled out into long interlocking lenses, which have their longest axes parallel to the foliation. (See 

Fig. 27, p. 177.)

Such a structure as that above described appears to be not uncora-( 

inon in the greenstones and greenstone schists of the Northwest. It 

is well known to the Canadian geologists, who have designated the 

rocks exhibiting it as "concretionary traps." It is described by Dr. 

Andrew C. Lawson, in his report on the Lake of the Woods, in the following terms:

This structure consists of the rock being divided into moreor less irregularly spherical or ovoid masses, varying in diamater from 2 to 3 inches to as many feet. These 

ovoid masses are not in close contact, but are separated from one another by an in-
WILLIAMS.] SPHEROIDAL PARTING IN THE GREENSTONES. 167

terstitial material. Tho concretionary masses are at their points of nearest approximation to each other, generally about halt' an inch or an inch apart, no matter what 

may be their size, so that when the ovoid masses are largo, the interstitial material 

appears in section as thin, anastomosing sheets, in which is developed a schistosity 

parallel to the outlines of the ovo'id masses they inclose. The interstitial filling is generally of a darker color, more chloritic, 8ofter,andofafiuer, more homogeneous texture 

than the ovoid masses, and weathers out, often leaving the latter, in sections afforded 

by glaciated surfaces, surrounded by sharp little trenches.

The ovoid masses are uniformly arranged as regards the direction of their long axes, 

and each one is surrounded by a sharp border half an inch wide, of a dark greenish 

gray color, which has been more resistant to weathering agencies than the rest of 

the rock. The ovoid masses present as the result of weathering a rough or pimpled 

surface of porous aspect and bleached greenish white color. The interstitial filling 

is firmer iu texture and softer than either the ovoid masses or their border, and is 

intermediate between them in color, with a brownish yellow tinge.

In the hornblende schists this ovoid structure in the rock takes a somewhat different 

aspect, and presents the appearance of thin anastomosing sheets of dark green, soft 

cliloritic material, sometimes enveloping completely ovoid or irregularly shaped portions of the hornblende schist, and at others losing themselves in a tapering, disconnected fashion in the main mass of the rock. 1

It is evident upon the most superficial examination that this structure is in uo way "concretionary," as that term is commonly understood. Nor is it possible to suppose that the harder ovoid masses were 

originally ejected fragments, which have become imbedded in a finer 

matrix of volcanic material forming a true agglomerate. A sufficient 

objection to this hypothesis is the fact that these harder masses so 

frequently fit together like parts of a mosaic.

In many instances the spherical or ovoid form of the disjointed 

fragments is so perfect that the structure strongly resembles the 

spheroidal parting, developed in a latent form in eruptive rocks by 

cooling, and made more distinct by weathering. This, which is really 

a perlitic structure on a large scale, frequently extends through largo 

masses of eruptive material; and the descriptions and photographs 

kindly furnished the writer by Mr. Whitman Cross, who has studied it 

in Colorado, indicate many points of striking resemblance to the structure of the Lake Superior greenstones. It is quite possible that the 

phenomenon may in certain cases be due to contraction, although in 

others the irregular shape of the fragments, as well as their occurrence 

in fragmental rocks, indicates another origin.

A structure showing many points of resemblance to that here described is presented by the well known green schists (griine Schiefer), 

occurring near Hainicheu, in Saxony, and the explanation afforded by 

the study of their very favorable exposures may assist our understanding of the parting in the Lake Superior greenstones. The fragments 

of the Saxon green schists, in spite of-the great difference in their size 

(varying from that of a pea to that of a house), are so rounded that they 

were formerly regarded by C. F. Nauru ami as a conglomerate.2 Never-

'Geol. and Nat. History Survey of Canada, Animal Report for 1885 (now series, Vol. I); Report 

CO on the Lako of tho AVoods, by A. C. Laweou, pp. 52, 53. 

"Erliiutoniugou zur gcogn. Karte dosKoiiigi-eichs Sachseu, pt. 1, 1834, p. 69.
GREENSTONE SCHIST AREAS OF MICHIGAN. [HULL. 62.

tbeless their fitting together convinced subsequent observers that the 

structure must be due to brecciation in situ. 1 These rocks have more 

recently been carefully studied by A. Rotbpletz, who was able to trace 

the structure downward into a series of interlacing joints, and who explains the rounded character of the fragments and the production of 

much of the interstitial material by the rubbing together, under the 

action of intense orographic pressure, of a mass already finely subdivided by cracks. Fig. 4 of Rothpletz's plate gives a fair representation 

of some of the Lake Superior occurrences.2

The hypothesis of the mechanical origin of this curious parting in the 

greenstones and greenstone schists seems best to explain the following 

facts:

(1) Its occurrence in fragmental rocks.

(2) The often very irregular shape of the fragments.

(3) The interlocking of the same into a mosaic.

(4) The fact, noticed by Lawson, that the thickness of the interstitial 

layer is independent of the size of the fragments.

(5) The fact, also noticed by the same observer, that the interstitial 

layer inhorneblende schists often loses itself in the rock without completely surrounding a fragment.

(G) The identity in petrographical character of all the cores in the 

same mass.

(7) The similarity, except where it is more altered, of the interstitial 

material to that forming the ovoid cores. At the Carp River locality 

(see p. 17G) the cores contain small white feldspar crystals, which are 

also present in the layers which separate them.

COARSE GRAINED DIKE KOCKS.

The fine grained aphanitic greenstones and greenstone schists which 

prevail over the southern Marquette area are intersected by many bands 

of coarser material whose eruptive character is still evident in their 

microscopic structure. These represent later dikes of a basic character 

which have broken through the older rocks, and which on account of 

their more recent eruption are less altered. One of the most typical of 

these is exposed on the small island near the end of the Cleveland ore 

dock in Marquette. This small island consists of two portions connected by a narrow neck. The southern part is composed of a hard, 

compact, brown rock, which is sometimes quite coarse grained or porphyritic. Its color and the abundance of epidote which is macroscopically visible indicate extensive alteration. Under the microscope a 

specimen from this locality, No. 11649, is seen at a glance to be a typical diabase. Although every vestige of the original pyroxene has been

1 Naumann: Erlauterungen der geogn. Karte der TJmgegend yon Hainicbcn im Konigr. Sachsen 

1871, p. 11. Rud. Crednor: Das Grunscjliiefersystem von Hainiclien, Zeitsoln-. fur die gcsammten, 

Naturvriss., vol. 47, p. 127,1876.

2 "tJeber nicchanische Gesteinsumwandlnngen bei Haiuicheu in Sachsen, part 2. Zeitschr. Deutscb. 

geol. Geaell., vol. 31, pp. 374-397. PI. IX, 1879.
wiu,iAM8.1 DIABASES SOUTH OF THE MARQUETTE. 169

replaced, by a finely fibrous green hornblende (uralite), yet the characteristic ophitic or diabase structure is still perfectly preserved. The 

lath-shaped feldspar crystals are but slightly altered and the spaces 

between them are as sharply defined as when the rock first solidified.

The northern portion of this little island shows the sam^ diabase in a 

much more alteredform. Therock has a pale greenish color, and, though 

quite massive in the center, is schistose on either side.

Nos. 11651 and 11652, the former from the massive and the latter from 

the schistose portion of this rock, offer additional examples of the fact so 

frequently observed in the Menominee River greenstones, viz, that the 

feldspar of those rocks which have been subjected to the greatest pressure is more broken, but, as a rule, less altered than that in rocks which 

have experienced less violent mechanical action. Both of these specimens show positive evidence in their structure that they .were once 

diabases, In both the ophitic arrangement of the constituent minerals 

is still distinct, but in the more massive rock the chemical alteration has 

progressed much further than in the other. Here the lath-shaped feldspar crystals are preserved in outline by the various secondary products which supply their place. Fibrous hornblende and leucoxene are 

abundant and nowhere, in either the thin section or hand-specimen, is 

there visible any parallelism of arrangement among the components. 

In the schistose rock, on the other hand, chlorite has largely replaced 

the hornblende, but the feldspar is almost unaltered. The long, lathshaped crystals are broken and faulted the fragments of a single individual often being separated a considerable distance but in spite of 

this the substance is as fresh and the twinning-lamellce are as sharp as 

in a recently solidified rock. Few better examples could be found of the 

evident action of pressure upon a solid rock mass. There has been a 

crushing and a consequent development of schistose structure which 

the microscope reveals with great distinctness. Along lines where an 

actual slipping took pi ace, fibrous hornblende, chlorite, and calcite have 

been quite abundantly developed.

No more typical exposure of the greenstones of the southern Marquette area can be found than that seen in the high hill which rises 

between Whetstone Brook, the State road', and the railroad. (D on the 

map, PI. VII.) The freshest rock obtained from the summit of this hill, 

No. 11724, is one of the best preserved diabases found anywhere south 

of the Eureka series. The feldspar is perfectly fresh but is much 

broken and faulted, as in the specimen (No. 11652) last described. The 

interstices between the fragments are filled with chlorifce. The allotriotnorphic pyroxene is now largely replaced by paramorphic hornblende, although cores of the original mineral are not infrequent in the 

center of the hornblende areas. Apatite, ilmenite, and leucoxene are 

also present.

Nos. 11646, from the southwest corner of the railroad crossing on Front 

street, Marquette (see on the map, PL VII.), and 11695, collected just
170 GREENSTONE SCHIST AREAS OF MICHIGAN. [MULL. 62.

north of the state road (in the center of Sec. 27 T. 48 N., E. 25 W.), are 

also well defined uralite diabases. In both the structure is still intact 

and the feldspars are generally well preserved,though theaugite is wholly 

uralitized. One rock of exceptional character (No. 11687) which occurs within |he limits of tbo southern Marquette area, must not be 

passed over without notice. This outcrops in a small exposure at the 

end of the road leading westward from Marquette to the Eureka 

shaft, about two miles from the city (see map, PI. VII). It is of a light 

brownish color, and, in spite of an advanced state of chemical alteration, it shows plainly the effects of great pressure. It appears once 

to have been a gabbro, very similar to that occurring at Sturgeon 

Falls, on the Meuominee Eiver (see p. 67), but now its original components and structure have both disappeared. Interlacing, lenticular 

areas produce a decided u micron1 aser" structure, which, in places, 

even resembles the fluidal appearance of certain vitrophyres. The minerals now present are a very pale chlorite filled with sharp epidote or 

zoisite needles, saussurite, quartz, calcite, ilmenite, leucoxeue grains, 

and winding bands of a clear, brown, isotropic substance which looks like 

opal. This rock appears much shattered in the field and it is in immediate contact with the thin, fissile schists.iu which the Eureka shaft is 

sunk.

GREENSTONES SOUTH OP THE QUAUTZITE.

Some distince south of the Carp Eiver, in the west half of the NW. 

£ Sec. 6, T. 47 K, E. 24 W., below the limestones and quartzite, a few 

much contorted and altered greenstones appear just on the contact 

with the granite which incloses the Marquette basin on the south.

No. 11778 may still be recognized as a diabase, though it now contains only saussurite, chlorite, epidote, quartz, and leucoxeue.

Nos. 1L774 and 11775 are from the same greenstone band, the former 

massive, the latter schistose, They do not resemble any other rocks in

the Marquette area, but they find their analogues in certain remarkable 

rocks on the south bauk of the Carp River, where this is crossed by the 

road leading uoithward from Teal Lake (see succeeding section, p. 175). 

Sharply defined crystals of reddish feldspar are imbedded in a bright 

green mass of chlorite and serpentine. The feldspars, although not 

pulled apart as in rocks which have been subjected to a stretching, show 

evidences of theactiou of enormous pressure. Eernnants of hornblende, 

from which the serpentine was probably derived; ilmenite, surrounded 

by leucoxene; and round grains of bright red iron hydroxide are also 

present in this rock. In the schistose modification, the constituents are' 

pulled out into parallel bands. The feldspars are lath-shaped and 

smaller, and, without any loss of sharpness in their outline, they are replaced by calcite, which is surrounded by an opaque black rim. Certain bands in this rock are much finer grained and present the feldspar 

and hornblende in a much less altered condition. 

No. 11779, from this same locality, is one of the best specimens any-
WILLIAMS.] APHANITIC GREENSTONES NORTH OF NEGAUNEE. 171

where collected to show the effects of rock-stretching. It appears once 

to have been a granite porphyry, but is now very schistose and is colored green by the secondary chlorite. Its groundmass is a felt-like aggregate of sericite, quartz, and chlorite with a perfectly parallel arrangement of the constituents. The large porphyrifcic feldspar crystals are 

much broken and pulled apart, always in the direction of the foliation, as shown in PI. .XIV, fig. 2. Large calcite individuals are surrounded by bright green chlorite, and both of these minerals may represent original biotite or some bisilicate.

ROCKS OF THE NEGAUNEE AREA.

As has already been stated at the beginning of the preceding chapter, two points were selected for the study of the more western portion 

of the Marquette greenstone belt. One of these was the mining town

of Negaunee, situated at the southeast corner of Teal Lake, about 

twelve miles west of Lake Superior. The town itself is on the ironbearing Huronian rocks, south of the greenstone belt, whose southern edge skirts the northern shore of Teal Lake. Both east and 

west of the lake the greenstones are bounded on the south by the 

narrow ridge of white quartzite, whose termination is Mount Mesnard.

From this quartzite two sections, three miles apart, were run northward to the granite and Huronian rocks, which here divide the greenstones into two separate areas. The more southern of these, which we 

will designate as the Negauuee area, resembles in its general character 

the region south of the Eureka series near Marquette; while the region 

north of the Dead River, which will form the subject of the succeeding 

section, is much more like the northern Marquette area.

The greenstones occupying the region north of Negauuee do not 

occur in continuous exposures, but rise out of the plain composed of 

glacial and later deposits in the form of rounded knobs. Both the 

longest axis of these knobs and the approximately parallel rows in 

which they are arranged follow the east and west strike, common to 

all the rocks of this region. The greenstones are, for the most part, of 

the much fissured and brecciated, light green and aphanitic variety, 

described in the last section. They are also very commonly associated 

with the same coarser N and more crystalline eruptives, probably of a 

much later age, that we encountered south of Marquette.

As will be readily seen from an examination of Dr. Boiniuger's map 

the Huronian rocks around and south of Negaunee and Ishpeuiiug are 

also penetrated by numerous greenstones. These, as a rule, resemble 

the more crystalline and later intrusions of the greenstone belt proper, 

and they may have been contemporaneous with them.

APHANITIC GKKKNSTONES.

The main mass of the greenstone belt near Negaunee is composed of 

the fine grained or aphanitic rock which has been described in the last
172 GREENSTONE SCHIST AREA.S OF MICHIGAN. [HULL. 62.

section as characteristic of tlie region south of Marquette. They belong to Lossen's type of "dicbte Diabase" and always show more advanced chemical alteration than the coarser rocks. This is to be explained by their finer'grain and also from the fact that they are older 

and have been subjected to more dynamic action than the others. In 

the field these rocks are found to be much brecciated and not infrequently quite perfectly foliated. They also exhibit the peculiar spheroidal parting already described, p. 166.

The microscopical appearance of a fair average type of these aphanitic 

greenstones (No. 11747, from Baldwin's Kilns) is represented in PI. X, 

fig. 2, but they show considerable variation in both structure and composition. In none is any trace of pyroxene now to be found, and yet they 

were probably once typical diabases. Their mineralogical differences are 

mainly dependent on the relative proportions of hornblende and chlorite. 

The preponderance of one or another of these minerals may be attributed to a more or less advanced stage of chemical alteration, but certain differences in structure have probably always existed. The grain 

may vary considerably in its coarseness, while the long acicular feldspar crystals, so abundant in some specimens, are wholly wanting in 

others.

No. 11736 was collected on the road from Negaunee to Baldwin's 

Kilns, just beyond the Carp Eiver, from about the center of the SW 

£ of the NE i of Sec. 29, T. 48 IS"., E. 26 W. It is comparatively coarse 

grained and little altered. Under the microscope it appears as a confused aggregate of fibrous hornblende, a cloudy, opaque, indeterminable substance and some original feldspar whose crystals are much 

broken.

No. 11744, obtained from an isolated knob of greenstone, situated in 

the S W | of the NE. £ of Sec. 28, T. 48 N., E. 26 W., on the longer road 

to Baldwin's Kilns, about one rnile east of where the last specimen was 

collected, is much like it in appearance and microscopic character. 

The only differences are due to the more advanced alteration of the latter rock. Here chlorite is abundant and the feldspar hardly A'isible.

No. 11745, from the same road as the last, but much nearer the Kilns, 

shows the spheroidal parting or brecciation in a very perfect manner. 

The grain is very fine, and the incomplete skeleton-like forms of porphyritic feldspar crystals are numerous. The mass itself is now largely 

chloritic, but looks as if it might once have been a finely porphyritic 

rock.

No. 11747, found at the same locality, is the one from which PI. X, 

fig. 2, w.as drawn. It is similar in structure to the last specimen, but 

its grain is coarser, and fibrous hornblende is still abundant in its 

groundmass. The porphyritic feldspar crystals are broken, faulted 

and separated.

No. 11748 is a schistose modification of 11747. This rock has a much 

finer grain than the other, but no evidence of schistose structure is ap-
WILLIAMS.] COARSE GRAINED GREENSTONES. 173

parent under the microscope. The chemical alteration has progressed 

so far that hardly any of the substances remaining have much action 

on polarized light; nevertheless, faint traces of small acicular feldspar 

crystals are visible.

Oue massive greenstone, No. 11740, found just north of Baldwin's 

Kilns, iii the NW £ of Sec. 21, T. 48 N., K. 26 W., is striking on 

account of its being mottled with round white spots from 5inm to 6mm 

in diameter. The rock is somewhat darker than usual in a rim around 

these spots, and the whole effect is exactly that of a variolite. Under 

the microscope the rock itself is found to be composed almost wholly 

of green fibrous hornblende, together with some opaque grains and 

saussurite. The round whitish spots are very largely composed of 

saussurite or a very fine mosaic, in which sharply defined hornblende 

crystals are imbedded. They do not show the least trace of a radial 

or sheaf-like arrangement, such as is characteristic of a true variolite. 

These spots are not unlike those found in the banded greenstones of

the Brook section (see p. 157) in their composition and structure, but 

they are very much more regular and uniform in their shape.

Other specimens of these aphanitic greenstones, as for instance Nos. 

11793, 11795, 11796, and 11797, collected along the line of the old 

logging road, which leads from the eastern end of Teal Lake northward to Johnson's Camp, in the SE £ of Sec. 13, T. 48 N., It. 27 W., are 

quite like those above described from the neighborhood of Baldwin's 

Kilns.

COARSELY CRYSTALLINE GREENSTONES.

No. 11749, collected in the extreme northeast corner of Sec. 21 T. 

48 N., K. 26 W., northeast of Baldwin's Kilns, is an unusually beautiful rock. The structure is coarse grained and rather granular, in 

spite of the feldspar crystals being idiomorphic. The large individuals 

of hornblende also, to some extent, have their characteristic crystal 

form. They are frequently changed to chlorite or to fibrous hornblende, but still ofteuer, although bleached to a pale green color, they 

retain their compact texture and show a twinning structure. There 

are no present indications that this rock ever contained pyroxene, and 

it is therefore to be designated as a diorite. The hornblende shows, 

in the process of its alteration, a curious tendency to concentrate the 

deep green color around the periphery of the crystals ; also the fraying 

out of their edges into hornblende needles similar to those described 

by Becke and Van Hise as secondary growths, produced by regularly 

orientated accretions of hornblende substance. All of these features 

are shown in Plate XII, fig. 1, which represents the microscopic appearance of this rock. The feldspar is filled with epidote or actinolite 

needles. Large areas of bright green chlorite also occur which contain 

sometimes sharp epidote crystals, but more commonly fibrous hornblende. Leucoxeue after ikuenite is abundant.
174 GREENSTONE SCHIST AREAS OF MICHIGAN. [BULL.ee.

No. 11739, from the southeasterly portion of the SW £ of the NW J 

Sec. 21, T. 48 N., E. 20 W., just north of Baldwin's Kilns, is a dioritic 

rock much like the last, but it is considerably more altered. The hornblende is more fibrous, the feldspar more saussuritized, while the 

original structure is also beginning to suffer.

No. 1174G was taken from a well marked dike which intersects the 

spheroidally parted, aphanitic greenstones, near Baldwin's Kilns. .Here 

the diabase or ophitic structure is perfectly developed, and there is 

no reason to doubt that the pale green, fibrous hornblende, which now 

tills the allotrioinorphic spaces between the lath-shaped feldspar crystals is secondary after augite. The most interesting feature of this 

rock is its amygdaloidal structure which was observed in no other Marquette greenstone. Several small amygdules are filled with brightly 

polarizing epidote, while one large circular cavity, measuring four 

millimeters in diameter, has its sides coated with radiating epidote 

needles and its center filled with calcite.

No. 11741 is the rock exposed at Baldwin's Kilns. It is much jointed 

but quite massive. Under the microscope it appears like the specimen 

last described, except that it is finer grained and devoid of amygdaloidal cavities. The structure of this rock is perfectly preserved. 

It was once a typical diabase, although now no trace of pyroxene 

remains.

Another of the more coarsely crystalline dike rocks of the Negaunee 

greenstone belt was obtained near the granite contact in Sec. 13, T. 48 

N., R. 27 W., two and one-half miles north of Teal Lake. This (No. 

1179L) is a typical uralite diabase, as is shown both by the distinct 

traces of ophitic structure and by the frequent cores of pale reddish 

brown augite which still remain in the fibrous hornblende. Ilmenite, 

partially changed to leucoxene, is also present informs especially characteristic of diabase. The original structure has been considerably 

obscured by chemical changes, The feldspar is partly recrystallized to

a mosaic, and epidote, chlorite, and quartz have been secondarily developed.

As has been above remarked (p. 171), the greenstones which occur so 

abundantly in the iron-bearing Huroniau rocks both north and south of 

Negauuee are apparently identical with the coarse grained, younger 

rocks of the greenstone belt proper. Still they show the effects of having been subjected to enormous pressure whicli has frequently produced 

in them a schistose structure; and as this is, in every case, conformable 

to the general strike of nil the rocks of this region, we must conclude 

that these greenstones were intruded before the folding of the Huronian 

sediments was completed. 1

Specimens were collected from the prominent exposures of these 

greenstones near the town of Negauuee. They are all rather coarse 

uralite diabases of the type last described from the greenstone belt.

1 See sections by Brooks, Atlas to G-ool. Survey Michigan, Pis. in ami v.
WILLIAMS.] CARP RIVER TUFFS. 175

No. 11754, collected from the SW J of the SW £ Sec. 6, T..47 N., ft. 26 

W., near the mouth of the Jackson mine, has a well preserved diabase 

structure. Its pyroxene is wholly changed to pale secondary hornblende ; its feldspar is filled with epidote, while chlorite and leucoxeue 

are common.

No. 11755, from a high hill somewhat farther east, is essentially the 

same rock. It contains epidote and chlorite even more abundantly.

No. 11756 was collected from a small greenstone knob just east of 

the hotel (Breituug House) in Negaunee. This rock has been quarried 

and may be plainly seen to have been rendered partially schistose by 

pressure. Under the microscope it is like the preceding rocks except 

that alteration has here progressed further so much so as to have 

obscured, though it has not obliterated, the original diabase structure. 

Chlorite, epidote, and calcite are among the prominent constituents.

Nos. 11758 and 11759 are frotn the greenstone bluffs which skirt the 

southern Shore of Teal Lake near its eastern end. The former is the most 

massive, the latter a schistose variety of this exposure. The massive rock 

is a nralite diabase of the ordinary type, retaining its original structure 

and also occasioual cores of red augite in the masses of secondary hornblende. Besides the usual chlorite, epidote, and leucoxene, there is considerable quartz present in this rock. The second specimen, taken from 

the schistose band on the south side of this greenstone ridge, shows 

evidence of intense mechanical and chemical action. The original 

structure has almost wholly disappeared, while its component minerals 

are calcite and chlorite, together with a little quartz and leucoxene. 

Rarely these secondary products have preserved the outline of lathsi) aped feldspars, which show that they have been much broken and 

pulled apart by the stretching action to which the rock has been subjected.

THE STRETCHED FRAGMENTAL KOCKS ON THE CARP RIVER.

An enigmatical group of rocks was met with near the center of the 

Negaunee greenstone belt, where the old road leading to Johnson's 

logging camp crosses the Carp Eiver. This locality is in the NE £ 

Sec. 25, T. 48 N., E. 27 W. The first rock south of the river is exposed 

in a low wall which rises just west of the trail. (No.; 11798.) This is 

apparently a dark colored, schistose greenstone, standing nearly vertical 

and filled with red granite fragments of all shapes and sizes, though 

generally with well rounded outlines. The dark green matrix also contains much of the red material in a finely divided form, often resembling 

sharp crystals. The rock itself is much brecciated and cut by crossgashes, as if it had been subjected to a stretching tension.

Under the microscope the dark green portion of this rock is found to 

consist entirely of chlorite, the scales of which are arranged parallel to 

each other and to the cleavage planes of the mass. Its. color, when free 

from inclusions, is a clear, bright green, but it frequently contains con-
176 GKEENSTONE SCHIST AREAS OF MICHIGAN. [BULL. 62.

siderable black or brownish opaque material, which is arranged in long, 

sinuous lines following the foliation. The reddish inclusions are almost 

altogether orthoclase, colored by a tine, unevenly distributed, globulitic dust. A striated feldspar may also be occasionally observed. The 

orthoclase is partly in'sharply defined crystals, but more commonly in 

irregular and angular fragments. These range through all sizes down 

to the most microscopic dimensions. The smallest bits are locally 

scattered so thickly through the chlorite as to give the rock a decidedly clastic appearance.

The chlorite matrix is of that peculiar character which we have 

already had occasion to notice as characteristic of stretched greenstone, 

particularly in the Menominee Valley (see pp. 82,83, and PI. XI, 

fig. 2). This chlorite must have crystallized after the orthoclase fragments had come into their present relative positions, as may be seen 

from the relations of its scales to the feldspar, and also by the bending 

around these fragments of the sinuous bauds of opaque matter which 

the chlorite sometimes contains. We may go even further and assert 

that the chlorite could not have been formed prior to the stretching 

action. Proof of this is to be found in the fact that some of the orthoclase inclusions may be plainly seen to have been broken during this 

process^ and their fragments to iiave been separated but a short distance and always in the direction of the foliation; and yet the same 

chlorite which forms tks whole matrix Jills these late fissures.

It is impossible to state anything positive with reference to the origin 

of this chlorifce; and still its universal presence in the stretched greenstones would indicate that this particular form of dynamic action upon . 

a mass of the proper chemical composition is, in some way, necessary, or at least very favorable to its production.

Isro. 11799, from the same locality, is essentially like the last specimen, except that the included feldspar is lighter colored and perhaps 

more altered to kaoliu or Muscovite. The matrix is not so purely

chloritic, but contains much finely divided feldspar. The stretching 

action is here even more apparent, a single feldspar crystal being pulled 

out to over double its original length and its fissures being filled with 

the same green chlorite.

If we trace this ridge farther south it develops into a finer grained 

and more schistose greenstone, which is free from the larger granitic 

inclusions but is mottled with small white spots (No. 11800). These aie 

found upon closer examination to be feldspar crystals. Under the 

microscope this rock is quite like those above described, but it shows 

much more distinctly the effects of dynamic action. The grain is very 

uneven. In some places the feldspar crystals are thickly crowded and 

are cemented by the usual chlorite in which epidote is abundant j in 

other places the matrix is a very fine grained aggregate of chlorite, 

sericite, quartz, and feldspar, containing considerable opaque substance 

and possessing a micro-flaser structure produced by the winding of its
WILLIAMS.] EFFECT OF STRETCHING ON SPHEROIDAL PARTING. 177

parallel bands about the occasional feldspar inclusions. These latter 

are much pulled out in the direction of the foliation and have their 

interstices filled, sometimes by the chlorite, but oftener still by calcite. 

No. 11801 is a more weathered specimen of this same rock, which 

shows with great distinctness the spheroidal parting described above 

(see p. 1G6). The stretching action is here displayed by the pulling 

out of the spheroids into a system of interlocking lenses, as represented 

in Fig. 27. Under the microscope this rock shows the effect of the meFIG. 27 spheroidal parting of apbanitio greenstone produced by brecciatious; drawn out iuto lenses by pressure. Scale same as in Fig. 26.

chauical action in the extremest degree. The matrix is very schistose 

and consists of a gray, indeterminable, almost isotropic mass in which 

minute flakes of scricite are distinguishable. The feldspar crystals 

are so altered that they seem to grade imperceptibly into the matrix, 

leaving their original shapes but indistinctly outlined. The dark green 

chlorite occurs only in rare patches.

No. 11802 is from the interstitial mass between these" lenses. It is a 

finely fibrous, felt-like, sericitic mass, in which the scales are so strictly 

parallel that the entire slide extinguishes the light between crossed 

Nicol prisms almost like a homogeneous crystal. There are besides present only small patches of a light brown, isotropic substance, some little 

quartz in isolated areas, and long stringers of leucoxene, out of which 

minute but sharp, highly refractive octahedrons have been developed 

(auatase ? cf. No. 11130, p. 131).

No. 11803, also from this interstitial material, contains white feldspar 

crystals like the more massive cores. A microscopic examination shows 

that this specimen is in all respects identical with the rock which it surrounds. The schistose structure is due to a crushing accompanied by 

a tension in one direction. The effect of this is so striking on the included feldspar crystals that it has been selected for illustration. (PI. 

XtV, fig. 2.)

After a careful study of this suite of rocks, which is distributed over 

a considerable area, it is impossible to escape the conviction that 

they are of a fragmental character. Neither their structure nor their

Bull. 62 12
178 GREENSTONE SCHIST AREAS OF MICHIGAN. [BULL. 62.

mineral composition resembles that of any known eruptive rock while 

the sudden variations in grain, the irregular and angular shape of the 

feldspar inclusions, and, above all, the extreme differences in the size of 

even such of these inclusions as lie side by side can be reconciled only 

with a clastic origin. Still these rocks do not bear the stamp of true sediments, and the most probable hypothesis of their nature which the 

writer can offer is that they are deposits of volcanic ejectaffieuta which 

have subsequently been rendered schistose by dynamic agencies. The 

irregular and angular fragments of red granite must be regarded as 

material broken from off the underlying rock masses and hurled out by 

the violent explosive force, along with much finer rapilli, sand, and 

ashes. These rocks seem to agree very closely with the agglomerates 

described by Mr. A. C. Lawson, of the Canadian Geological Survey, as 

attaining an extensive development in the Lake of the Woods region. 

These are filled with angular fragments and have also been subjected 

to great pressure. Mr. Lawson's opinion regarding the origin of these 

agglomerates is the same as that above advanced. x

The presence, as here, of the spheroidal or lenticular parting in rock 

of fragmental origin is sufficient to show that it must be a mechanical 

and not a molecular phenomenon.

ACID ROCKS.

There were no acidintrusives encountered within the Negaunee greenstone area proper, nor do the granites which skirt the northern edge of 

this belt present any points worthy of especial note.

No. 11790 is the granite in immediate contact with the greenstones at 

the end of the section which was run northward from Teal Lake. It 

was found at the Johnson logging camp in Sec. 13, T. 48 K, E. 27 W. 

This is a typical granitite in which quartz is not very abundant. The 

biotite is quite altered and bleached. The feldspar is particularly distinguished by its beautiful zonal structure, which the incipient kaolinizatiou has brought out all the more distinctly. This feldspar shows the 

effects of pressure, and in one instance this can be plainly seen to have 

resulted in the production of a microcline structure. One small, sharp 

crystal supposed to be orthite was observed in this rock. Apatite and 

zircon also occur, and occasionally a few rutile needles are secondarily 

developed in the altered mica.

No. 11738 is from the granite in contact with the greenstone just 

north of Baldwin's Kilns. This rock is completely crushed, the constituents being in some cases so pulverized as to give the specimen, when 

seen under the microscope, almost the appearance of a fragmental deposit. The mica is here much altered, and among secondary products 

has given rise to an abundance of most delicate rutile needles, which. 

often form the characteristic twins.

1 Geol. and Nat. Hist. Surrey of Canada. Ann. Kept, for 1885, p. 49, CO.
WILLIAMS.] QUARTZITE NOVACULITES. 179

Here may conveniently be recorded some observations made on the 

qnartzite of the ridge which borders the greenstones on the south in the 

neighborhood of Negaunee. This quartzite mass is composed on its 

northern or lower side of compact, even grained, thinly laminated 

rocks, varying in color from black to the lighter shades of green, yellow, 

and drab. At one point, near the northeast corner of Teal Lake, these 

have been quite extensively quarried as novaculites, although they are 

of a different character from the novaculites occurring near Marquette. 

These rocks have the usual east and west strike and dip steeply to the 

south. These so-called novaculites are again underlaid by conformable 

beds of a coarse, white qnartzite conglomerate, which in its turn is in 

immediate contact with the greenstones. The best locality for seeing 

these contacts is on the wood road leading northeastward from the 

slaughter-house oii Teal Lake. (NW £ Sec. 31, T. 43 K, E. 26 W.)

No 11785 is a nearly black specimen of the novaculite. Under the 

microscope the fragmental character of this rock is at once apparent. 

It is composed of irregular quartz and altered feldspar grains in nearly 

equal proportions. In the finer cement biotite has abundantly crystalized. An occasional zircon crystal would seem to indicate that the rock 

is the stratified debris of an old granite. The present foliation of these 

novaculites is to be regarded as a slaty cleavage due to pressure, rather 

than an original stratification.

The conglomerate between the novaculite and greenstone, and forming the base of the Huroniau quartzite, is composed of large grains 

and pebbles of granitic quartz, imbedded in a fine, silky matrix of 

sericite. (Hos. 11787 and 11788.)

EOCKS OF THE NORTHERN AREA.

The second point selected for the examination of the western portion 

of the Marquette greenstone belt is near the center of Sec. 9, T. 48 

N., E. 26 W. Here a camp was established and from it were studied 

the greenstones represented upon Dr. Eomiuger's map as occurring 

north of the Dead Eiver. The rocks within this area present a much 

more varied character than those of any of the other areas examined. 

Indeed, although a considerable collection of specimens was made, it is 

not for a moment supposed that all the types of massive rocks which 

occur in this region were obtained. Enough, however, will be noticed 

to indicate the general character of the area and at the same time to 

illustrate the extreme variety offered by different, and often nearly 

contiguous, outcrops.

fn its chief features, this area more closely resembles the northern 

portion of the belt near Marquette, jnst as the southern branch near 

Negaunee, as stated in the preceding section, is more like the southern Marquette area. Still, the more recent eruptive rocks which penetrate the banded green schists far exceed, both in amount and variety, 

those to be found nearer Marquette.
180 GREENSTONE SCHIST AREAS OF MICHIGAN. [uuLL.62.

UNALTERED BASIC IXTRU8IVES.

At the falls of the Dead Biver, in the southwest corner of Section 9, 

the " arenaceous slate group" of Kominger is well exposed. This con^ 

sists here of thinly bedded slates which strike N. 75° W. and stand 

nearly vertical, with perhaps an inclination to the south. The upper 

part of the fall is a steep incline over the slates, but the lower portion 

is a clear fall oc from twenty-five to thirty feet, occasioned by several 

trap dikes which run nearly east and west. These are coarse grained, 

fresh, and sometimes porphyritic.

No. 11814, from one of these dikes, is found, under the microscope, to 

be a very fresh and typical olivine diabase, quite like that of the great 

dike on Lighthouse Point, Marquette. The structure is typically 

ophitic. The porphyritic appearance is occasioned, not by single individuals of feldspar, but by concretionary aggregates of this mineral. 

The olivine, which frequently presents crystal outlines of great sharpness, is sometimes completely, sometimes only slightly serpentinized.

No. 11826 was collected from an outcrop beside the road at the southwest corner of Sec. 10, T. 48 N., E. 26 W. It has every appearance of 

being in place, but the character of the rock, unknown elsewhere in this 

region, renders it much more probable that it is a portion of a huge glacial 

erratic which is nearly buried in the drift. Still, thepetrographical interest of this rock makes it worthy of a description. A close examination shows it to be an olivine gabbro, perfectly fresh and peculiar on 

account of the abundance and beauty of its olivine. This constituent 

makes up a large percentage of the entire mass. It is present in extremely sharp, and well formed crystals from 0.1 to lmm iu diameter. 

These show the usual hexagonal cross-sections, due to the presence of 

faces of the prismatic zone and domes.1 No signs of even incipient 

alteration are visible in this olivine. The augite and triclinic feldspar 

are both allotriomorphic, and form much larger individuals than the 

idiomorphic olivine which they inclose. Magnetite is abundant iu 

sharp octahedral crystals, showing quadratic sections ; while other irregular areas of an opaque black iron mineral may be ilinenite. These 

latter are often partially bordered by a dark brown mica. No other 

rock exactly like this is known, to me, but in its general character it is 

allied to the olivine gabbros which play such an important role on the 

north shore of Lake Superior.?

ALTERED COAHSE GRAINED ROCKS.

The coarser grained intrusive rocks, which penetrate the prevailing 

green schists of the northern area, have, as a rule, undergone a considerable amoun t of chemical alteration. This has more or less completely 

replaced the original minerals by secondary products; while, in some

1 Tliesecrystalsshowindicntions of twinning, as described by Kulkowsky (Zeitschr. Kryst. n. Mineral, vol. .10, p. 17) but these are not as distinct as tho sharpness of the crystals would lead us to expect. 

2 Cf. IiTing; Mou. U. S, Gcol. Survey, vol. 5.
DIORITES OF THE NORTHERN AREA. l8l

cases, it has also obliterated the original structure. Among these 

younger eruptive greenstones the same two types can be distinguished 

as in the Marquette area, viz, diorite and diabase.

Diorite Nos. 11828 and 11829 are from an exposure thirty paces in 

width, on the line between Sees. 5 and 8, T. 48 1ST., K. 26 W. This rock 

is sometimes parted into parallel plates so as almost to resemble a slate. 

In the hand-specimen it is seen to be composed of rather large, wellformed hornblende crystals, which are often hollow at the center, and 

a reddish feldspar. The microscope shows that the two specimens are 

identical. The hornblende is idiomorphic, though without terminal 

planes; the feldspar, on the other hand, is wholly allotriomorphic. 

This latter mineral is almost free from twining striations, so that the 

rock were perhaps more properly termed a syenite. The hornblende 

preserves much of its compact structure and brownish color. It is 

frequently changed at the center to a mass of fibrous needles, or even 

to chiorite, while the exterior remains compact and unaltered, Apatite

is very abundant; spheue and magnetite are not uncommon.

No. 11817, from a wide dike at the northwest corner-post of Section 

9, of the same township, is of the same type as the last specimens, but 

more altered. Macroscopically tufts of hornblende crystals may be 

seen imbedded in a white feldspar substance. Under the microscope 

this feldspar appears to be mostly altered to a brownish saussurite, in 

spite of which, however, traces of a stout lath-shape are discernible. 

The hornblende is much altered to a fibrous form. It frequently has 

the emerald green color of actinolite, especially around the outer edge, 

where it is always darker and more compact than at the center. II menito, accompanied by leucoxene, is abundant in the rock, though absent 

in the two last mentioned specimens. It will be noticed that this rock 

approaches much more nearly than the others to the diabase type, and 

yet there is no reason to believe that the hornblende is parainorphic 

after pyroxene. Although it is not compact, it still retains its twinning 

structure as one of its most common features.

A still more striking example of idiomorphic, lath-shaped feldspar 

in a diorite (a structure which Kosenbusch mentions as not at all uncommon in rocks of this class1 ), is to be found in No. 11831. This was 

obtained on the line between sections 5 and 8, just east of Nbs. 11828, 

11829, above described. It is entered in the field-notes as a coarse 

reddish diorite or syenite, having long, green hornblende crystals imbedded in a red feldspar. On account of its uustriated feldspar, this 

rock may perhaps be even more properly designated as an ampliibole 

granite,oalthough its structure is in many respects peculiar. (See PI. 

XVI, fig. 2.) The feldspar is present in two distinct generations. 

The earliest of these yielded stoutly lath-shaped crystals, which are 

wholly idiomorphic. These are mostly uustriated, but have a zonal 

structure. They are internally changed to a grayish mass, which a

1 Mikroa. Pliyaiog., 2d od., vol. 2, p. 121.
GREENSTONE SCHIST AREAS OP MICHIGAN. I BULL. 62.

high magnifying power shows to be muscovite or kaolin, while their 

periphery is composed of clear and fresh feldspar substance. Between 

these feldspar crystals is a granular aggregate of quartz and a younger 

feldspar. This latter mineral is also unstriated (though sometimes 

possessed of the gridiron microcline structure) and much fresher than 

the older feldspar. With the quartz it frequently forms beautiful 

micropegmatitic growths.

Hornblende was most probably the original bisilicate constituent of 

this rock, but it has undergone an unusual alteration, i. e., the alteration to mica. This mineral can be seen under the microscope in brown 

areas of irregular shape, which have evidently been produced at the 

expense of the hornblende. 1 They are unevenly colored, being a 

darker brown in some places than in others. In some cases the new 

mica crystallizes in sharply denned hexagonal plates. Certain of these, 

which on optical examination prove to be basal sections, show cleavage lines parallel to the prismatic faces. These are the so-called 

pressure figures ("Drucklinien" of the German mineralogists), which 

are rarely visible in microscopical sections and are strongly indicative 

of pressure.

This secondary mica to a considerable extent has turned green by a 

reduction of its iron to a ferrous state. More rarely the alteration has 

progressed still further, resulting in complete bleaching, or even the 

production of chlorite. Apatite is very abundant in this rock; sphene 

is also common, but its crystals always surround ilmenite grains in 

such a manner as to suggest that the titanite has resulted from its 

alteration (see PI. XVI, fig. 2). Chlorite also frequently surrounds 

the opaque iron mineral.

Of all the remarkable features which this rock presents, perhaps 

none is more striking than its hypidiomorphic structure and the 

presence of the feldspar in two distinct generations without tbe production thereby of a porphyritic structure. This is all the more noteworthy in a rock which from its mineralogical composition is to be 

strictly classed among the granites.

No. 11832, which occurs with the specimen last described, appears 

to the unaided eye as a compact, fine grained mass through which 

large glistening hornblende crystals are unevenly distributed. Under 

the microscope the groundrnass of this rock is seen to be composed of 

.lath-shaped feldspar crystals, which are altered sometimes to epidote, 

sometimes to sericite; allotriomorphic hornblende, frequently changed 

to chlorite; a little quartz and magnetite in grains and crystals. 

The sharp quadratic and hexagonal sections are sufficient to identify this species; but we must assume that it is a titaniferous 

magnetite on account of the very narrow leucoxene border which

"Rosenbusch mentions only parallel growths of hornblende and biotite. The present instance can 

hardly be so interpreted, while thoro is nothing surprising in an alteration which has been so frequently observed. (Cf. J. Roth: Allgeineine uud chemische Geologic, vol. 1, p. 333, 1879.)
WILLIAMS.] DIABASES OF THE NORTHERN AREA. 183

frequently surrounds it. The porphyritic hornblende of this rock is 

sometimes regular, sometimes irregular in outline. It is a later crystallization than the groundmass, because it is always tilled with the 

lath-shaped feldspars. This produces the structure which I have 

recently termed "poicilitic," 1 and which is characteristic of younger 

porphyritic crystals, especially hornblende and pyroxene.

No. 11834, from a ledge 640 steps north and 20 steps west of the southwest corner of Sec. 4, T. 48 N., H. 26 W., is composed of stout, lath-shaped 

feldspar crystals and large individuals of fibrous hornblende. The former mineral suggests by its form that the rock was once a diabase; the 

hornblende, however, is not allotriomorpliic, but is present in long, columnar crystals, which are generally twinned. One of these crystals is 

bent in the most unaccountable manner. It forms three-quarters of a 

circle (see Fig. 28), but no satisfactory explanation can be offered for 

this unusual shape.

On the whole, this rock is considerably altered. Epidote and hornblende needles are developed in its 

feldspar, while secondary quartz is 

quite abundant. In spite of the 

fibrous character of its hornblende, 

the structure of this rock is much 

more suggestive of a diorite than of 

a diabase.

Diabase. Among the altered intrusive rocks, those which represent 

the diabase type are all uralite diabases. In none was any trace Of the Flr- 28.-13 t hornblende crystal. Groon-

. . , . , stone No. 11834. Magnified 30 <liiinii>tora. original augite ooserved.

In No. 11827, from 485 steps west of the southeast corner of Seel ion 

5, the structure is best preserved. Here the fibrous and in part bright 

emerald green hornblende retains exactly the allotriomorpliic form of 

the diabase augite. The feldspar is largely altered to a brown, opaque 

saussurite, and leucoxene has rejflaced the ilmenite.

No. 11837, from 815 steps north of the southwest corner of Section 

4; No. 11848, from 500 steps west of the southeast corner of Sec. 10, 

T. 48 N., E. 26 W., are both much more altered than the specimen last 

mentioned. The ophitic structure is best preserved in the latter, 

although epidote is abundantly developed, while it is altogether wanting in the former.

Other of these uralite diabases are finer grained.

No. 11825, from a somewhat schistose greenstone ridge, exposed near 

the center of the southwest quarter of Section 10, admirably exhibits 

the effects of crushing. The lath-shaped plagioclase crystals are still

'Am. Jour. Sci., 3d aeries, vol. 31, 1886, p. 30.
184 GREENSTONE SCHIST AREAS OF MICHIGAN. [BULL. 62.

wonderfully fresh, but are broken, faulted, and pulled apart, while their 

interstices are filled with the secondary chlorite usual in stretched 

rocks. There is a delicate granophyre or micropeguiatite structure in 

this specimen undoubtedly an original feature, as in No. 11675 (see 

p. 140). The pyroxene is wholly changed to a pale green, fibrous horn, 

blende, which, to a considerable degree, produces the foliation. Large 

areas of brown leucoxene, of the variety most characteristic of altered 

diabase, are also drawn out in the direction of the schistose structure. 

In one instance this has altered to a network of dark rutile needles 

imbedded in a yellowish micaceous mineral, as described in No. 11070 

(see p. 99, and Plate XIII, fig. 2).

No. 11830, from 700 hundred steps west of the southeast corner of 

section 5, is a very typical uralite diabase. The structure is still well 

preserved by the feldspars, in spite of needles of the fibrous hornblende 

having freely wandered into them. Traces of augite cores remain in 

this rock.

No. 11835 is a schistose rock from 640 steps north and 20 west of the 

southeast corner of Section 5. It has been much altered, and now consists mainly of a fine aggregate of fibrous hornblende, feldspar, quartz 

and calcite. In this lie large porphyritic crystals of hornblende which 

are wholly altered, especially in their interior.

BANDED GREENSTONES.

Striped and banded greenstone schists, like those of Lighthouse Point, 

Marquette, are very abundant in the northern area.

No. 11856, from an exposure near the northeast corner of Section 9, is 

macroscopically identical with the Marquette rocks. Under the microscope it would be designated as a hornblende schist. It consists of minute crystalloids of compact green hornblende, together with feldspar 

which is mostly changed to sericite. Zoisite in crystalloids, like those 

represented in Pig. 2, p. 27, and about the size of the hornblende, is 

also present. The structure of this rock is schistose, caused by the 

parallel arrangement of all the constituents.

Other banded greenstones from tftis region, like No. 11818, from near 

the southeast corner of Section 5; 11821, 850 steps north of the southwest corner of Section 4; 11839, 170 steps south of west quarter post of 

section 4, and 11841, 180 steps north of the same quarter post, are 

essentially like the specimen just described mostly a mass of confused hornblende fibers with more or less feldspathic substance, quartz, 

and chlorite. The relative proportions of these components condition 

their different colors and cause the parallel striping.

These rocks are so similar to those occurring immediately north of 

Marquette, that we must apply to them the conclusion already expressed (p. 158) in regard to the origin of these green schists, and regard 

them as basic tuff deposits, which were contemporaneous with surface 

flows of diabase and subsequently profoundly metamorphosed.
WILLIAMS.] FIELD RELATIONS OF THE DEER LAKE ROCKS. 185 

 GREEN SCHISTS AND AGGLOMERATES OF DEER LAKE.

After the work on this paper was practically completed, my attention was called by Prof. Irving to a group of. greenstone schists and 

conglomeratic rocks which lie in the western extension of the southern 

branch of the Marquette greenstone-schist belt. I was unable to study 

and collect these rocks in the field, but Prof. Irving furnished me 

with notes regarding their distribution and mode of occurrence, together 

with a suite of hand-specimens and thin sections for laboratory study. 

I incorporate the results of such a study in this place the more willingly because the occurrence of these rocks does not in the least differ 

from that of others closely allied to them, which I carefully examined in the neighborhood of Negaunee. The distinct evidence, moreover, which these rocks afford as to their mode of origin is of great 

value in confirming the conclusions already reached in regard to certain other analogous, though less distinctly characterized, occurrences,

These peculiar rocks are best exposed on the south side of Deer Lake 

in Sees. 33 and 34, T. 48 K., E. 27 W., Michigan, but they may also be 

advantageously seen farther to the northwest, in Sees. 20, 21, 20, and 30 

of the same township.

The following field-notes were furnished me by Prof. Irviug:

On the road going northward from Ishpemiug to Deer Lake furnace a belt of 

quartzite dipping southward is crossed in the middle of Section 34. Immediately 

north of this quartzite follows a porphyritic greenstone of tolerably fresh aspect. 

This is exposed on the south side of the two knobs, which, on Dr. Romiuger'a map, 

lie to the southward of Deer Lake, to the east of the Carp River, and to the north of 

the road. To the northward of this greenstone succeed first some greenish schistose 

rocks without apparent agglouieratic character. These are represented by No. 12057. 

These schists soon grade, however, both across the strike toward the north and in 

the course of the strike toward the west, into a most strikingly conglomeratic rock, 

which appears to make up about all the hills in the northern half of Section 33, between the quartzite and the road leading to the Ropes mine.

Specimen No. 12058 is an obscurely conglomeratic rock, collected about 150 steps 

north of No. 12057. The conglomeratic character is best seen on a weathered surface. A particularly fine exposure may bo seen just west of Deer Lake furnace 

(No. 12031).

Specimens No. 12029 are pebbles from a bold knob somewhat farther t<> the west. 

From the same point came Nos. 12023 and 12024, representing the conglomerate 

itself; and Nos. 12025 and 12026, from two dikes two and one-half and ten feet wide 

respectively. The smaller of these dikes cuts the conglomerate in. a direction transverse to its foliation, while the larger runs parallel to this structure.

Throughout the conglomerate there is a tendency to schistose structure, which, 

however, is never very pronounced, and which varies cousidertlbly in its degree of 

development. It seems to be rather the result of the intersection of .close joints cutting each other at a small angle than a true parallelism of structure. This foliation 

stands about vertical and trends in an eastern and western direction.

Tho pebbles vary in size from such as are two feet in diameter down to minute 

fragments. Pieces less than six inches in length are most abundant. Occasionally 

they appear well rounded, and this seems to be more particularly the case with the 

larger sized pieces ; but more commonly they are sub-angular and flattened out in a 

direction parallel to the schistose structure. On an exposed surface these pebbles
186 GREENSTONE SCHIST AREAS OF MICHIGAN. [BULL. 62.

stand out by virtue of their whiter -weathering. On afresh fracture they are not 

nearly so apparent, but seem to differ from the rest of the rock by their finer grain 

and by their pinkish or greenish color; the body of the rock having usually a dark 

greenish gray tint.

No other kinds of pebbles were seen than those shown in the samples, which represent perhaps two phases: (1) A pinkish weathering felsitic (?) kind, and (2) a grayish 

weathering greenish and more schistose kind. But this distinction, written on the 

. ground, seems hardly to bo borne out by the specimens brought away. Certainly 

those of the first named kind are more abundant.

Occasional schistose bauds a few feet in width and quite free from pebbles may be 

seen in the conglomerate, trending east and west. These, therefore, stand at a considerable angle with the schistose structure. They are the only signs of a true bedding in the rock, and if they do actually indicate a bedding are of very great importance, both as showing a discordance between the bedding and schistosity and 

also a nonconformity between these and the true bedded rocks lying just to the 

south. Indeed, neither these doubtful bedding bands nor the schistose structure 

perceptible in this conglomerate is concordant with the dip of the qtiartzite, which 

is toward the south at a moderate angle.

North of the road leading to the Ropes mine the only rock noticed, for a short 

distance, is represented by No. 12032, which does not show any conglomeratic structure, but which on the ground appeared very much like the pebbles in the conglomerate further south.

Still farther north, in the same township (T. 48 N., R. 27 W.), as, for instance, in 

the middle and eastern part of Section 21, and again in the southwest quarter of Section 20, and in the northwest quarter of Section 29, to the north of Dr. Rominger's 

serpentine area (peridotite area of Plate I, herewith) are other very similar rocks. 

Specimens 12036, 12037, and 12039 are from the SW i of SVV i Sec. 20, while No. 12043 

is from the NW J Sec. 29. Among the exposures from which this last specimen 

was taken are some which seem to show the conglomeratic character of the Deer 

 Lake rocks. It is not positive, however, that the obscure markings which they 

show are really those of pebbles.

Both the macroscopical and the microscopical examination of these 

specimens show that they have been subjected to an intense strain or 

stretching1 , which has resulted in the elongation of the component pebbles, and in the modification and partial recrystallization of the matrix, 

after the manner shown in PI. XI, fig. 2, and in PI. XIV, fig. 2. Nevertheless, themetamprpliisin attendant upon this dynamic action fortunately has not been sufficient entirely to disguise the tuff nature of the 

rock. In most cases this is still very apparent, and this occurrence is 

therefore of great importance in confirming the conclusions reached with 

regard to analogous, but more metamorphosed rocks, in the Marquette 

and Negaunee areas.

We must regard these tuffs or agglomerates as originally composed 

of the fine fragmental material of diabase. This consisted mainly of 

triclinic feldspar and pyroxene, mingled with more or less amorphous 

base, and stratified either by gravity or by the agency of water. Included in such a matrix were numerous true bombs of solid diabase and 

ejected fragments of other rocks, mostly of a more acid'character.

As is well known, material of this kind is of all the most liable to undergo chemical alteration. This has been conclusively shown by the 

observations of Captain Button, quoted above (p. 159). In consideration
WILLIAMS.] AGGLOMERATES OF DEER LAKE. 187

of the great antiquity of the deposits here under discussion, therefore it 

need excite no wonder that little of the original material now remains. 

The rocks are largely composed of such secondary products as chlorite, 

sericite, epidote, quartz, and calcite. All traces of original pyroxene, 

or of possible olivine and glass, have wholly disappeared. The feldspar 

has, however, better retained its individuality. In some instances its 

substance is still quite fresh; and in others, where the substance is 

changed, the form is still easily recognizable. The same class of alterations has gone on in the solid portions of the agglomerates as in the 

finely fragmental matrix. The more basic fragments produce chloritic, 

the more acid sericitic, areas. Their original boundaries are, however, 

for the most part, very distinct, often more so in the field, or in the 

hand-specimen, than under the microscope. Where the dynamic action 

has been most intense a schistose structure has been developed in the 

fragments, which is quite continuous with that of the finer grained matrix. This is accompanied by a distortion or elongation of the included fragments, caused by crushing and recementing, whereby the 

sharpness of their outline is destroyed, and an apparent transition to 

the matrix is produced.

Specimen No. 12057, described in the notes as "a greenish schistose 

rock without apparent agglomeratic character," appears under the microscope at first glance as a confused aggregate of chlorite, calcite, and 

quartz, through which occasional large grains of yellowish green 

epidote are scattered. Characteristic forms of ilmenite, with leucoxene, 

some sericite, and delicate little rutile needles are also present A 

more careful study of this section reveals the indistinct outlines of lathshaped feldspars, which once formed the interlacing network of a 

granular diabase. These are so disguised by the new products developed in them chlorite, sericite, arid epidote that it is only by an 

attentive examination with a low magnifying power that they can be 

recognized. They are, however, undoubtedly present, and the rock 

must therefore be regarded, not as a tuff, but as an extremely metamorphosed form of a massive flow, in which the profound chemical changes, 

and the development of a schistose structure progressed together.

Specimen No. 12058, "an obscurely conglomeratic rock collected 

about 150 steps north of the locality of No. 12057," bears in the handspecimen a close resemblance to the one last described, but under the 

microscope its different character is at once apparent. The feldspar 

substance is here less altered, and the rock is mostly composed of 

minute crystals or fragments of crystals connected by that peculiar 

aggregate of chlorite scales common in stretched basic rocks, where the 

direction of the scales follows the schistose structure. (See PI. XT, 

fig. 2.) Occasional larger fragments of feldspar crystals are scattered 

through this finer mass, but these are always broken and irregular in 

shape. The chloritic substance has doubtless originated from the pyroxene or glass, while a vast number of the most delicate little rutile
188 GREENSTONE SCHIST AREAS OP MICHIGAN. [BOLL. 62.

needles, arranged iu long, sinuous bands, appears to represent the 

original ilmenite. Calcite areas also occur. A narrow white vein 

which traverses this hand-specimen possesses an interesting microscopic structure. The little fissure w;is ILrst coated with chalcedonies 

quartz, and then with a lajer of quartz crystals which projected their 

terminations into the opening. These possess a perfect zonal structure, 

due to the arrangement in successive layers of fluid inclusions. The 

remainder of the space was subsequently filled with carbonate.

Specimen No. 12031, from a point a short distance west of where the 

last mentioned pieces were collected, and Nos. 12023, 12024, 12029, and 

12030, from another point still farther west, represent the agglomerate 

in its most typical development, and leave no doubt as to its true 

nature and origin. As in the specimens last mentioned, the matrix of 

the rock is of a grayish green color aud the grain is mostly aphanitic, 

although frequent minute angular fragments may be detected by the 

aid of a pocket lens. There is always a more or less pronounced foliation developed in this matrix. The pebbles, which impart to the rock 

its agglomeratic structure, show most distinctly on a weathered surface. Here they present a white or pinkish gray color, which contrasts 

most sharply with the pale green of the inclosing matrix. On a fresh 

fracture the inclusions have a greenish gray color, but their compact, 

fine grain, like that of halleflinta, serves to identify them where the 

color fails. These massive felsitic fragments are of varying size and 

angular or subaugular in shape (rarely well rounded). They are distributed through the matrix indiscriminately, the longest axes being by 

no means parallel, as we might expect in a sediment. The foliation of 

the rock appears to have been produced by a great tension, which has 

stretched the matrix and sometimes elongated the pebbles, sometimes 

broken them, and sometimes forcibly torn the solidified matrix away 

from them. The latter case is represented in Fig. 29, which represents

l'io. 29. Weathered surface of a specimen of the Deer Lake agglomerate. 

Natural size. No. 12023.

the appearance of a weathered surface on specimen No. 12023. It can 

here be plainly seen that the longest axes of the pebbles stand almost 

at right angles and1 that the matrix has been forcibly separated by the 

stretching action, on each side of the central pebble.
WILLIAMS.] MASSIVE DIABASE OF DEER LAKE. 189

The microscopical study of these specimens adds much to our knowledge of the true structure of these agglomeratic rocks.

The matrix is fouud to be composed of very angular to subangular 

fragments, extending down to the minutest size. These generally still 

. retain their form, although their original substance has undergone 

change to chlorite, epidote, and calcite. Small feldspar crystals or 

broken fragments of larger ones are scattered about, the substance of 

which has been altered in varying degree. In. the chlorite especially, 

very sharp rutile needles abound. These seem to represent the titanium of an original ilmenite.

Unmistakable evidence of clastic structure is everywhere encountered; but this is not the clastic structure of a sediment. The angular 

shape, different substance, and enormous variation in the size of fragments lying side by side, all clearly indicate an agglomeration of volcanic detritus and one, indeed, in which many of the original features 

are admirably preserved.

Among the smallest of the included fragments, those resembling a 

massive diabase in structure are not uncommon; but to judge from 

the statement in the field notes quoted above, as well as from the 

hand-specimens submitted to me for study, the larger " pebbles'" or 

fragments are mostly composed of the lighter and felsitic rock. This is 

unfortunately too ranch altered to allow of a wholly satisfactory diagnosis. It may be best seen in section No. 12023, where it resembles a 

porphyrite. The grouudmass still retains its ophitic structure, and 

is filled with well formed porphyritic crystals of plagioclase. No 

original quartz nor orthoclase could' be detected ; one or two chloritic 

areas, which most probably represent former augite or hornblende 

crystals, were, however, observed.

Specimen No. 12025 represents a narrow, transverse dike of massive 

greenstone, and No. 12026 a broader one of the same material, parallel 

to the schistose foliation. Under the microscope both are typical, 

though highly altered diabases. The former is aphanitic and has 

only a very indistinct foliation in the hand-specimen. Under the 

microscope the original structure is nearly obliterated; still, traces of 

it remain. Chlorite, calcite, quartz, and iron oxide are almost the only 

mineral components.

The second, coarse grained diabase, is entirely massive in the bandspecimen. It has an even green color, but cleavage surfaces of feldspar are frequent. Pyrite is also common in it. Under the microscope 

the typical diabase structure is perfectly preserved. The texture is 

quite coarse, and the feldspars mostly retain their twinning structure. 

The rock has been stretched, and these feldspars are much broken and 

have their fragments separated. The pyroxene is wholly altered to 

chlorite, epidote, and calcite, and the ilmenite is accompanied by 

Jeucoxene, 

The rook occurring north of the road leading to the Ropes mine and
190 GREENSTONE SCHIST AREAS OF MICHIGAN. [BULL. 82.

directly west of Deer Lake, is devoid of conglomeratic structure. 

(No. 12032.) In the field it was noted to have a resemblance to the 

substance of the larger " pebbles," and this resemblance, to a certain 

extent, is borne out by the microscopical examination. The rock appears to be massive, and not a tuff; but it has been subjected to intense crushing and stretching action. It is full of porphyritic crystals, 

which are mostly altered to muscovite. The groundmassis principally 

composed of small rectangular feldspar crystals. All, however, -are 

much broken and separated, and the interstices are filled with green 

chlorite scales. The main difference between this rock and that forming the compact felsitic pebbles in the agglomerate lies in the apparent 

orthoclastic nature of the porphyritic feldspar crystals, and in the 

chlorite which has been so abundantly developed in the groundmass.

The specimens from the more northerly areas, numbered 12036, 

12037, 12039, and 12043, belong to the same category as those just 

described. No. 120.-.6 is a confused aggregate of feldspar fragments of 

various sizes, together with some quartz and chlorite. Nos. 12039 and 

12043 are almost too much altered to allow of any study. The former 

is a mass of nearly opaque decomposition products, and the latter is an 

almost continuous mat of sericite scales. No. 12037 alone appears like 

a massive rock most probably a fragment included in the tuff. It is 

a hornblende granite, with but little quartz, composed principally of 

idiomorphic feldspar. The minute green hornblende needles fill the 

interstices between these square or rectangular crystals.

We may, therefore, consider the pyroclastic1 nature of the Deer Lake 

conglomerates as beyond any question. They represent volcanic detritus, ejected by an explosive force at the earth's surface. In this way 

they form a most welcome but not unexpected support to the conclusions reached in regard to the other more highly altered rocks of 

the same class occurring in the Marquette region.

The term u agglomerate " here applied to these rocks is one in general usage among the English geologists * to designate a tumultuous 

assemblage of volcanic ejectameuta bombs, foreign blocks, etc., of all 

sizes and shapes', cemented by a fine grained paste of volcanic ash.

The closest analogues to the Deer Lake rocks are the breccias, agglomerates, and tuffs, described by Dr. A. Geikie from St. David's, in 

Wales.3 This author says, p. 93:

The rocks of St. David's present features of interest and importance, not only in 

the palaeozoic history of Britain, but in regard to general theoretical questions. 

They include, for example, perhaps the oldest group of lavas and tuffs, the relative 

date of which is known. They have been subjected to a process of metamorphism 

which has affected only certain beds or kinds of rock. They have been penetrated 

by masses of granite and quartz porphyry, around which another kind of metauiorphism has been manifested. At a later period they have been injected with diabase 

dikes, etc.

1 J. J. H. Teall, Geol. Mag., London, 3d series, vol. 4, November, 1887, p. 493.

2 Cf. JA. Phillips, Quart. Jour. Geol. Soc., vol. 24,1878, p. 470, and A. Geikie Text-Book of Geology, 

1882, p. 163. 

3 Quart. Jour. Geol. Soc.,London, vol.39,1883.
WILLIAMS.] AGGLOMERATES OP ST. DAVID'S, IN WALES. 191

All this might have been written to apply equally well to the rocks 

occurring near Marquette. 

With special reference to the tuffs, Dr. Geikie says : l

They vary likewise in texture from somewhat no arse breccias or agglomerates, 

through many gradations, into fine silky schists in which tho tufacoous character is 

almost lost. Generally they ai'o distinctly granular, presenting to tho naked eye 

abundant augular and subanglar lapilli, among which broken crystals of a white, 

somewhat kaoliuized feldspar and fragments of lino grained fclsito are often conspicuous.

The matrix of these Welsh agglomerates appears to be generally 

green, while the included lapilli or k < pebbles" are sometimes basic in 

character and of a greenish color; sometimes more acid, with a compact structure and pinkish color.

Their resemblance is thus seen to be very close to the Deer Lake 

agglomerates described in this chapter.

1 Quart Jour. Gcol. Soc. London, vol. 39, p. 295.
CHAPTER VI.

GENERAL RESULTS AND CONCLUSIONS.

ORIGINAL CHARACTER OF THE MENOMINEE AND MARQUETTE GREENSTONE AREAS.

In speaking of the geology of the Menominee and Marquette regions 

I refer, it must be understood, only to the greenstones and certain intimately associated acid rocks. No attention was paid to the quartzites, dolomites or shales of the younger, iron-bearing Hhronian. With 

a view of obtaining a clear idea of the primary nature of these rocks 

we shall consider in succession: The evidence of their eruptive character; their original types and inineralogical composition; the conditions 

under which they were formed; and, finally, their succession in time.

KVIDENCK OF EUUPTIVE CHARACTER.

Field evidence. This is more distinct in the case of the Meuominee 

greenstones on account of the simpler character of their exposures. 

Here we find them extending in-two comparatively narrow and nearly 

parallel bands for a considerable distance. Their color, weight, texture, and frequent massive structure are such as are associated with rocks 

of igneous origin, while in some cases, as just below the Lower Quinnesec Falls, they project out of the surrounding schists in the form of a 

high, precipitous ridge, suggestive only of .a great dike. Although 

successive layers of these rocks differ considerably in composition, and

though they possess within themselves well marked schistose varieties, 

yet they are sharply defined against the adjoining Htironian beds, into 

which they never pass by gradual transitions. Indeed, these rocks have 

been considered by many of the geologists who have examined them 

most carefully as of eruptive origin. Those, on the other hand, who 

have regarded them as either wholly or partially sedimentary have 

been able, as far as I know, to cite only two arguments in favor of their 

opinion. These are: First, the coincidence of the direction of these 

bauds, and of their alternating component members with the general 

strike of all the rocks of this region; and, second, the frequent conformity of the schistose structure in these bands to the bedding of the 

adjacent sediments.'

Let us, however, consider whether these two facts are not wholly in 

accord with the supposition that these greenstones are of igneous origin.

1 This conformity is by no means universal.

192
WILLIAMS.] EVIDENCE OF ERUPTIVE CHARACTER. 193

If they were intrusive they would follow in their course the line of least 

resistance, which would probably be the direction of the bedding of the 

sediments. If, however, as seems from various considerations much 

more probable, these greenstones belong to a formation older than the 

Hurouian, and if they were produced in the form of great igneous (and 

perhaps submarine) overflows which composed the bottom of the basin 

in which the iron-bearing sediments were subsequently deposited, then 

the great lateral compression, which so contorted these sediments and 

elevated them into their present almost vertical position, would have 

brought the underlying greenstones at the same time into just the position they now occupy. And if these greenstones had been formed as 

successive flows, differing more or less in structure and composition, in 

the process of upheaval they must have behaved like any other complex of horizontal beds and have produced conformable members of the 

series exposed in the Meuomiuee valley, as is actually the case.

Now, also in regard to the second point of objection, if we admit that 

massive rocks may have a schistose structure (like a slaty cleavage), 

developed in them by pressure, then, in this case, the direction of this 

cleavage, perpendicular to the action of the pressure, must agree with 

the stratification of the now almost vertical sediments.

Kor is it necessary to assume that the schistosity of these rocks was 

wholly produced at the same time with the crumpling and slaty cleavages of the overlying detritals; since the same crumpling force may 

have worked in the same direction at two widely separated periods. 

Indeed, there is excellent reason to believe iu two such periods, since 

the dikes, which iu the Marquette region penetrated the greenstone' 

schists subsequent to their being made schistose, have evidently themselves been subjected to further squeezing. These dikes appear to be 

contemporaneous with the sheets of greenstone interfolded with the 

detrital iron-bearing strata, which sheets themselves show a similar degree of schistosity with the dikes referred to. In the Marquette region 

there is generally a very noticeable difference as to the amounts of secondary cleavage in the greenstone-schist area, and in the strata of the 

overlying iron-bearing series, this being very much greater in the former 

case. Coincident with this difference is the frequent gentle bowing of 

the iron-bearing strata in this region, the dips of its layers being often 

quite flat, while the cleavage of the green schists always stands at a 

very high angle. In the Meriominee region, on the other hand, the 

folding of the iron-bearing series is very much closer and slaty cleavage 

is much more generally developed. This would seem to indicate that 

the second period of squeezing was more intense in the Menomiuee 

area, and that it was, therefore, probably more instrumental in rendering the greenstones schistose here than in the Marquette region.

The arguments used by the defenders of the sedimentary origin of 

the Menominee greenstones can not therefore be regarded as valid.

On the other hand we may cite, as positive evidence that the foliation 

Bull. 62 13
194 GREENSTONE SCHIST AREAS OF MICHIGAN.- [HULL.62.

of the greenstones is a secondary feature, the fact stated by Major 

Brooks, that at the Four-Foot Fall this foliation is conformable to the 

adjoining clay-slate, which has a " strong cleavage and no distinguishable bedding planes." 1 Thus the schistose structure of the greenstone 

now agrees with the secondary slaty-cleavage of the'sediment, which, 

of course, may or may not agree with its original bedding, since this 

is here obliterated. Furthermore, what foliation the greenstones possess may be traced in almost every case into a jointing, or into bauds 

produced by the crushing attendant upon faulting and slipping, which 

has taken place within the rock-mass. Finally, the abruptness with 

which the foliation of the greenstones commences and breaks off in the 

direction of the strike is wholly inconsistent with the theory that it is 

due to original stratification. At the Four-Foot Fall, for instance, the 

greenstones strike almost at right angles to the river, and yet they are 

very schistose on the Wisconsin and very massive 011 l.he Michigan 

bank. Such cases are not uncommon while even more difficult to explain are differences in the strike of schistose bauds which traverse 

a massive rock (like those observed at the Upper Twin Fall), unless we 

assume that they were produced by a force which acted unevenly and 

not in a constant direction.

The field evidence that the greenstones within the Marquette belt 

are of igneous origin is not so plain as that encountered in theMenominee valley. Although we find here unmistakable dikes of unaltered diabase and others of altered though still massive greenstone, 

yet some of the most important of these rocks, especially in the northern part of the Marquette area, are so regularly banded and stratified 

that we are obliged to seek the agency of water to account for their 

formation. Nevertheless, we are not obliged to separate even these 

greenstones altogether from the igneous rocks which accompany them. 

For reasons stated at length in Chapter IV, they may be most satisfactorily explained as tuff deposits, half volcanic, half sedimentary, 

which throw important light upon the physical conditions under which 

the more massive rocks were produced.

Strong collateral evidence respecting the original nature of the greenstones of both these areas may be obtained from a study of the acid rocks 

which are quite constantly associated with them. The indications of 

the eruptive character of these rocks are so unmistakable that it seems 

to have hardly ever been doubted by any one who has examined them. 

Large areas of granite occur on each side of the Huronian basins, both 

in the Menominee valley and near Marquette, and dikes and apophyses, 

presumably radiating from these main masses, penetrate the greenstones for a considerable distance (cf. the Upper Quinuesec Fall, Horse 

Race, and region just north of Marquette). Now, the composition of 

these acid rocks is such that they resist chemical alteration much more 

successfully than those of a more basic character. Hence we should

1 Gcol. AViaconsin, vol. 3, p. 475.
WILLIAMS.] FIELD EVIDENCE OF EEUPTIVE CHAEACTER. 195

expect to find their distinctively eruptive features, such as dike-form, 

line of contact, original structure, etc., much better preserved. This is 

the case, and yet these eruptive rocks often present phenomena of secondary foliation quite .identical with those seen in the greenstones.

Near the southern end of the Brook Section, just west of Marquette, 

this intrusive granite is filled with fragments of the surrounding schist 

which it tore off while yet in a molten state. At tho lower end of the 

Horse Bace Eapid, on the Menominee Eiver, the acid dikes show a 

marked diminution in the size of the grain toward the dike*walls, a 

structure eminently characteristic of rocks which have crystallized in 

fissures. Near Upper Quinnesec Falls also was discovered a rock closely 

resembling a granitic hornfels (No. 11064). But in spite of all the evidence that the acid rocks are eruptive, their dikes; for the most part, 

follow the strike of those through which they break, since this.is the 

line of least resistance, while many of them have a perfect foliation and 

even a gneissic structure. What we may regard as satisfactory proof 

that this foliation is a secondary feature may be seen at the head of the 

Horse Kace, where it conforms to the general strike of the surrounding 

greenstones, even where this does not coincide, as is exceptionally the 

case, with the sides of the acid'band (see p. 115). A similar structure 

has been observed by Prof. Ch. E. Weiss, of Berlin, near Thai, in Thuringia; 1 and, like the present case, it may be regarded as certain evidence that a schistose or even a banded structure may be developed iu 

acid dikes by secondary causes, independent of their own direction, 

but conformable with the strike of the surrounding rocks.

Microscopical evidence. The most convincing proof that the rocks of 

the Menominee and Marquette greenstone areas are of igneous origin 

is not to be derived, however, from their field relations, but rather from 

their microscopical structure. It is true that there are many cases 

where rocks of widely dissimilar origin resemble one another so closely 

that not even the minutest study of their internal structure is able to 

distinguish them with certainty; nevertheless there are in other cases 

well marked peculiarities of structure which may be regarded as unfailing indications that the rock possessing them has crystallized out of 

a molten magma.

An explanation for these facts is not difficult to find. The original 

structures of clastic and igneous rocks are characteristic and essentially 

distinct. Where these remain there is no danger of confusion. In the 

process of metarnorphism, however, there is a constant tendency to obliterate the original features and to substitute for them certain secondary features. But these latter depend altogether upon the chemical 

constitution of the mass and the nature of the metamorphosing forces, 

and are independent of both the mineral composition and the structure 

of the original rock. Hence it is that rocks fundamentally distinct in

1 Petrographisclie Beitriige aus dem nordli^ei} Thjjringer WaWe, Jalvrtucli precis, geol, L.a$d.ea," 

omtalt fur 1883, pp. 213-237, B§rtiiU884, . -. - 
196 GKEENSTONE SCHIST AREAS OP MICHIGAN. [BULL.62.

both origin and structure grow more and more alike when subject to 

metamorphism. They may finally become indistinguishable and thus 

their life histories may be lost, but so long as any trace of the original 

structure is recognizable it may be relied upon as a safe guide.

It is, therefore, a matter of prime importance in petrography to be 

thoroughly acquainted with original rock structures, and to understand 

their significance. As a rule, those found in igneous rocks are more unmistakable than those which characterize clastic deposits. For instance, 

universal observation, together with numerous synthetical experiments 

in the laboratory, has shown that the peculiar divergent radial structure, known as the " ophitic" or " diabase" structure, is always the 

product of crystallization from a molten mass of certain basic rocks. 

No single observation has ever been made to indicate that this structure 

can originate in.a sediment or by any metamorphism of a sediment; 

hence, we are justified, when we encounter the "ophitic" structure in 

a rock (no matter how much it may otherwise resemble a sedimentary 

deposit) in assuming that it is of igneous origin. Now, precisely this 

characteristic structure is what we do meet most frequently in the 

greenstones of both the Menorninee arid the Marquette area. Nor is it 

alone in the most massive and apparently least altered specimens that 

it is found; often the most perfectly schistose of these greenstones disclose it, and that, too, where none of the mineral components have 

escaped complete alteration.

Other structures, scarcely less typical of an igneous origin are also 

encountered in other of the Marquette and Menominee rocks. Some 

of these are: porpJiyritic structure, with well defined crystals and even 

a zonal growth; micropegmatite and grandphyre structure, in granite, 

diorite and diabase; poicilitic structure, shown by the hornblende in the 

diorite porphyry at Lower Quinnesec Falls. The skeleton forms of the 

acicular feldspar crystals, still recognizable in the aphanitic greenstones 

of the Upper Twin Fall, as well as in those of the southern Marquette 

and Negaunee areas, are also strongly indicative of igneous, and perhaps also of glassy rocks.

The evidence of eruptive origin afforded by the present mineral components of the Marquette and Menominee rocks is not so satisfactory 

as that based on their structure. Nor is this different from what we 

should expect. Chemical metamorphism precedes structural metamorphism. Instances are frequent in both regions of the disappearance of 

every original mineral, while the structure remains intact. If we except 

the comparatively recent and unaltered diabases near Marquette, pyroxene that most characteristic of volcanic minerals was hardly ever encountered except in the gabbro at Sturgeon Falls. We can, however, 

assert with certainty that it was once present in many other rocks, 

where it is now represented by the pale green hornblende into which it 

so readily passes. Other species typical of igneous rocks, suck as lath-
WILLIAMS. 1 GABBRO AND DIABASE. 197

shaped labradorite crystals and ilmenite fringed with leucoxene, on the 

other hand, are still met with in abundance.

With such evidence that the greenstones and associated acid rocks 

of the Menominee and Marquette regions are igneous in their origin, 

we may next inquire as to how many different types may still with certainty be recognized among them. In this section, whose effort it is 

to reconstruct the original form of the greenstones, no reference will 

be made to the secondary or metamorphic rock-types.

DIFFERENT ORIGINAL ROCK-TYPES.

BASIC ROCKS.

Olivine Gabbro. This type, containing abundant and perfectly crystallized olivine, was found only in the northern area above Negaunee 5 

(See Chap. Y, p. 180); and even here under circumstances which left 

dOllbt as to whether the exposure was in situ, or a part of a huge, buried 

glacial bowlder.

Oabbro. The only undoubted representatives of this type, i. e., gran, 

ular aggregates of allotrioinorphic diallage and plagioclase, were found 

at Sturgeon Falls on the Menominee River. Their diallage is of an 

unusually pale color and, like the whole rock, remarkably poor in iron, 

(cf. analysis on p. 76.) Their feldspar is almost wholly changed to 

saussurite. The rock exposed near the Eureka shaft, two miles west 

of Marquette, appears once to have belonged to this same type (p. 70); 

while the granular diorites so common at the Quinnesec Falls, although 

they now contain only pale green hornblende and no diallage, agree so 

closely with the Sturgeon Falls rock in appearance, structure, and chemical composition, that it is very probable that they were once gabbros 

also (pp. 86, 102).

Diabase. This is by far the most common rock-type and it is found 

in every variety and stage of alteration.

An unaltered olivine diabase is the youngest rock of the Marquette 

area. It intersects all the other greenstones as well as the granite, as 

may be seen at the mouth of Dead liiver, besides occurring in interbedded sheets in the iron bearing series itself. It forms the great dike 

which terminates at Lighthouse Point, as well as many other smaller 

ones exposed in the Brook Section, in the northern area and at other 

localities. As the olivine is the first of all minerals to disappear, this 

type can, of course, be recognized only in the freshest specimens.

It is not always possible to say positively whether the quartz in the 

diabase is an originalor a secondary constituent. The former, however, is probably the case in certain very fresh specimens and where, 

as in No. 11675 from the great dike, and No. 11646 from Front street, 

Marquette, it is present as a micropegmatitic intergrowth with the 

feldspar (p. 141).

No non-oliviuitic diabase was discovered in a perfectly fresh state, but 

this type is assumed for many occurrences in which alteration is more or
198 GREENSTONE SCHIST AREAS OF MICHIGAN. tnuLL.62.

less advanced. Such an assumption must, however, be doubtful, since 

any original olivine would have been the first mineral to disappear.

Diabase Porphyry. It is now impossible to say whether the porphyritic greenstones occurring at the western end of the ridge below the 

Lower Quiimesec Falls once belonged to this type or not. If they did, 

they contained considerable original brown hornblende as an accessory 

constituent; but it seems more probable that these rocks were diorites.

Glassy Diabase and Melaphyre. From the sides of the unaltered diabase dikes, glassy and half glassy rocks were obtained, which, of 

course, represent only a local facies of the main mass, due to more rapid 

cooling. They contain incomplete and skeleton forms of the minerals 

which were the first to crystallize, and in this respect they closely resemble some of the aphanitlc greenstones of the southern Marquette 

and Negaunee areas. Indeed, one of these half glassy rocks from the 

edge of a much altered dike in the Brook Section, near Marquette (No. 

11680, see p. 144), is quite identical with a certain widespread variety of 

the fine grained greenstones, and gives perhaps the clew for the deciphering of their original form (cf. p. 163).

Diorite. The readiness with which the pyroxene of eruptive rocks 

passes over into a corresponding amphibole, makes the number of diorites among the Menominee and Marquette greenstones appear much 

larger than it really is. A majority of these are undoubtedly of secondary origin, having been derived from pyroxene rocks. Nevertheless, 

others of them, in which the secondary hornblende can be traced back 

into a compact brown variety of the same mineral, must be regarded as 

true diorites,-unless we make the assumption that pyroxene has passed 

by paramorphism first into basaltic, and subsequently into fibrous green 

hornblende. The origin of other o'f the dioritic greenstones must 

always remain uncertain. Even at the risk of including some rocks of 

secondary origin, the principal dioritic types may be enumerated 38 follows : Gabbro type light colored, granular rocks, like those form ing the 

barrier at Upper Quinnesec Falls; these so closely resemble the Sturgeon Falls gabbro (even down to the orthopinacoidal parting of their 

hornblende) that there is strong reason to believe that they have been 

derived from a similar rock. Coarse grained diorite of the Horse Race, 

with idiomorphic feldspar and a pale green hornblende, which, though 

itself secondary, has probably been formed from a compact variety of 

the same species. Granular diorite, like No. 11175 from Four-Foot Falls 

and several dike-rocks in the Negaunee and Northern areas. Quartz 

diorite^ like the Picnic Island rock and No. 1183J, from the Northern 

area, west of Marquette. (See Plate IX, fig. 2.) Both of these are perhaps more properly amphibole granites, although the relative proportions of their orthoclase and triclinic feldspar leaves this in doubt. The 

so-called " epidiorites " are undoubtedly of secondary origin, and have 

been derived generally from augitic rocks.
WILLIAMS.] ORIGINAL MINERAL CONSTITUENTS. 199

Diorite Porphyry. The rocks containing large poicilitic hornblende 

crystals, collected from the western end of the ridge below the Little 

Quinuesec Falls, which have already been mentioned under the head of 

Diabase Porphyry, although they should perhaps receive the above 

designation.

Tuffs. The schistose, banded greenstones which compose so large a 

part of the northern Marquette area are found to be best explained as 

tuff deposits of the basic eruptives which accompany them.

ACID KOCKS.

Granite. Each of the greenstone areas is bounded on both the north 

and south by a large mass of granite. This rock is a typical granitite 

as can be best seen from the specimens collected south of the Horse 

liace in the Menorainee Valley and north of the Marquette greenstone 

belt. Muscovite granite occurs in some of the bosses near Marquette, 

notably at the so called ." gold mine " near Piue street. Amphibole 

granite is possibly represented in the specimens from Picnic Islands and 

the Northern area which have already been mentioned under the head 

of Quartz Diorite, although they appear to occupy a position midway 

between these two types.

Granite Porphyry and Quartz Porphyry are the forms assumed by the 

granite where it penetrates the greenstones in dikes or apophyses. 

They are abundant in both areas and are united by a continuous series 

of transitional forms. These rocks are among the most interesting ones 

anywhere met with on account of the perfection with which they show 

the effects of dynamic action. By either stretching or compression 

they are changed into Augen gneiss or schistose porphyry, both of which 

retain in their microscopic structure very plain evidence of the manner 

in which they were produced.

ORIGINAL MINERAL CONSTITUENTS.

Orthoclase occurs in all the acid rocks granites, and porphyries; in 

some of the fragmental rock, as, for instance, those near Iron Mountain and in the stretched tuffs on the Carp River north of Teal Lake, 

and probably in the Picnic Island rock north of Marquette.

Microcline. It is doubtful whether this form of potash feldspar is 

ever original. In some cases at least it is certainly the result of intense 

dynamic action on rocks already solid.

Oligoclas'e is, in all probability, the species of striated feldspar occurring in the granites.

Labradorice is the prevailing original feldspar in all the greenstones, 

as it is in the unaltered diabases occnring near Marquette.

Quarts; is an abundant component of all the acid types; and it occurs 

also in both diorites and diabases, although it is here not always possible to distinguish between what is original and what is secondary.

Muscovite seems to exist in one or two granites as a primary component, but in the majority of cases it is a derivative of the orthoclase.
200 GREENSTONE SCHIST AREAS OF MICHIGAN. [BULL.62.

Biotite is abundant in tlie granites. Ifc is also present in the diorites, 

especially in those from the Horse Race Rapid, although here it may 

be secondarily developed out of the hornblende. In the diabase it is 

sometimes seen forming a border around the ilmenite.

Hornblende. Compact brown, or basaltic hornblende is common in 

very mauy of the greenstones, though mostly as remnants of crystals 

which have become partly green or fibrous. It is also present in the 

Sturgeon Falls gabbro and in the Marquette olivine diabase. Compact 

green hornblende is also a very common mineral in the greenstones, but 

it impossible to tell to how great a degree this may be of secondary origin.

Diallage of a very pale color and very poor in iron is abundant in the 

gabbro of Sturgeon Falls. (See PI. VIII, fig. 1.)

Augite of the ordinary kind, having a reddish brown color in transmitted light, is an essential ingredient of all the fresher diabases; and 

often exists as a core in the center of partially, uralitized crystals.

Olivine, in fresh and perfectly formed crystals, was found in the olivine gabbro of doubtful origin which occurred in the Northern area 

above Negaunee. It also once formed a component of the younger diabases near Marquette, although it is now so altered to serpentine as to 

be only recognizable by its form.

Zircon is abundant in nearly all the acid rocks, and is present'in the 

sediments near Iron Mountain in the Meuominee valley. (No. 11113.)

Apatite is universal in all the rocks which have not been sufficiently 

altered to obscure it.

Tourmaline is common in the more acid rocks. It may be best seen 

in the granite and acid dikes near the Horse Race and Upper Quinnesec Falls. It is also present in the sediments exposed near Iron Mountain and Four-Foot Falls, as well as in the hornfels (No. 11064.)

Spliene occurs in both granite and diorite, especially in association 

with hornblende. In some instances it seems to have been derived 

from the alteration of ilmenite.

Orthite (Allanite) is present in certain granites, both near the Horse 

Race and in the Northern area above Negaunee.

Ilmenite and Magnetite. The opaque iron oxide minerals are present 

iu all the rocks in their usual abundance. Ilmenite is the most frequent, especially in the greenstones, but magnetite is also present.

CONDITIONS UNDER WHICH THE GREENSTONES WERE FORMED.

There is considerable evidence to show that the greenstones, both of 

the Menominee and the Marquette region, solidified at the surface, 

under subaerial or subaqueous conditions.

In the Menominee Valley this evidence consists (1) of the fine texture of the rocks; and (2) of the alternation of bands of different types, 

which probably in their original position represented successive flows. 

Fineness of grain is universal in the Menominee greenstones, and we may 

be certain that it was a primary feature in spite of the extensive alter-
WILLIAMS.] MACROSTKUCTUrUL METAMOEPHISM. 201

ation of these rocks. It is especially noticeable in the case of the gabbro, which is almost always a coarse grained rock when it has solidified 

at any depth. The succession of massive beds, like the pale gabbros 

and the dark diabases seen at Lower Quinnesec Falls, are difficult 

to account for except by supposing that they were once horizontal 

sheets which flowed one over another and which were subsequently elevated into their present nearly vertical position. Traces of tuff material are not as distinct here as in the Marquette region, although indications of their existence are by no means wanting. We might reasonably expect that any original scoriaceous or amygdaloidal structure 

would have disappeared in the course of the profound chemical changes 

through which these greenstones have passed.

In the Marquette area proof of the superficial origin of the greenstones may be found (I) in the fineness of grain, and (2) in the fragmental detritus or tuff, such as would accompany volcauic action, The 

so-called " aphanitic greenstones," which play so important a role in the 

southern Marquette and Negaunee districts, have traces of a structure 

like that of a porphyrite or melaphyre (see PI. X, fig. 2). At least 

there are incomplete forms of feldspar crystals apparently imbedded in 

a grouudmass which might easily have originated from the devitrification and decomposition of a cryptocrystalliue or even glassy base. The 

finely banded greenstone schists occupying the northern portion of the 

Marquette belt present such strong evidence of stratification and such 

peculiarities of microscopic structure (see PI. XVI, fig. 1) that they arebest explained as tuff deposits belonging to the massive basic rocks. 

The presence of fraginental material of this kind, of course, strongly 

indicates surface origin for all these greenstones. Still other signs 

point in the same direction. At least one well marked specimen of 

amygdaloidal structure (No. 11746) was found near Baldwin's Kilns, but 

in the present highly altered condition of these rocks it can only occasion surprise that even this was preserved.

MACROSTRUCTURAL METAMORPHISM OF THE MENOMFNEE AND MARQUETTE MASSIVE ROCKS.

This embraces all modifications in the structure of the massive rocks 

produced by dynamic agencies and plainly visible to the unaided eye. 

Such changes consist for the most part in the production of a banding, 

foliation, or schistose structure, which tend to make the eruptive rocks 

resemble stratified deposits. They are a secondary feature and must 

be correlated with the slaty cleavage, not with the original bedding of 

sediments.

Inthe process of upheaval and folding rocks may be subjected to great 

compression, with more or less attendant shearing or faulting, or the reverse maybe true and a tension result. Inthe first case foliation is 

produced ; in the second, the phenomena of stretching.
202 GREENSTONE SCHIST AREAS OF MICHIGAN. [BULL. 62.

 MACROSTRUCTURAL METAMORPIIISM THROUGH COMPRESSION, FAULTING, OR CRUSHING.

Simpl'e compression. There seems to be no doubt that a foliation or 

slaty cleavage may be produced by pressure in massive as well as in 

stratified rocks. This cleavage is developed in all cases at right 

angles to the direction of the pressure. It is particularly frequent in 

such basic eruptive rocks as, by their chemical composition, are more 

subject to alteration. In these the secondary development of such 

cleavable minerals as chlorite, hornblende, sericite, and biotite, which 

are given a parallel arrangement by crystallizing under pressure, 

strongly conduces to the production of a schistose structure. The 

pressure accelerates the chemical action, and may in this way develop a schistose out of a massive rock without movement in the mass.

A very general feature in the foliated greenstones of both the Menominee and Marquette regions is their tendency to cleave parallel to 

a line instead of parallel to a plane; in other words, they possess a 

dip but no decided strike. . The rock then breaks with readiness into 

long rhoinboidal prisms with almost any angle, but is severed with 

difficulty at right angles to this axis of foliation. Such a structure 

would seem to indicate the action of an unequal or shifting pressure. 

Examples of this method of parting are very common, especially at 

Sturgeon, Lower Quinnesec, and Twin Falls, in the Meiiominee Valley, 

and in the southern portion of the Marquette greenstone area.

One of the best proofs of the secondary origin through dynamic 

agency of the greenstone foliation is the fact, noticed by Major Brooks, 

that at Four-Foot Falls this is parallel to the slaty cleavage, but not to 

the bedding of adjoining sediments.

Another method by which a schistose structure is produced in the Menominee greenstones, is the gradual flattening of the rhomboidal prisms 

which are formed by the intersection of two sets of joint-planes. This 

is illustrated by Figs. 20, 21, and 22, on page 128. These prisms seem 

to be elongated by the action of intense pressure into a series of interlacing lenses. A sort of " Flaser " structure is thus produced which, in 

its extreme development, becomes a well characterized schistose struct, 

ure. Even in the broadest of these prisms a latent cleavage is often 

noticeable parallel to their longest diagonal, always following the 

general strike of the adjoining rocks. This structure, which I have 

designated as rhomboidal, or diamond parting, may best be seen at the 

Twin Falls or at the upper end of the great greenstone ridge below 

Lower Quinnesec Falls.

Faulting or crushing. A schistose. structure is always produced in 

massive rocks along lines where a great tension has been relieved by 

breaking and a consequeutdisplacement. Movementof this kind within 

a solid mass, must be accompanied by more or less intense crushing, 

and this allows increased circulation and hence of the rapid production 

of secondary crystallizations. Admirable instances of this phenoin-
WILLIAMS.] FAULTING OR CRUSHING. 203

euoii abound in both the Menominee and Marquette greenstones. At 

the Sturgeon Falls, along the basin just below Lower Quiunesec Falls^ 

below the Horse Eace, at the Twin Falls, at the foot of Arch street, 

Marquette, and at many other localities within this area, it maybe seen 

with remarkable distinctness. The exact effect of the crushing on the 

original mineral constituents is visible only under the microscope and 

hence will be considered in the succeeding chapter; but any one who 

will carefully examine any of the above named localities can not fail to 

be convinced of the continuity and original identity of the massive and 

schistose rocks.

Both the width of these schistose bands and the intensity of the 

crushing which they exhibit vary widely in different cases. Their di. 

rection almost always coincides with the general strike, although at 

Upper Twin Falls such bauds were observed to have a different direction, (Seep. 132.)

This pulverization of the rock is accompanied by greatly increased 

chemical action (except in the notable instance of the feldspar) and the 

abundant production of chlorite. Such crushed bands possess a more 

or less perfect lenticular or "Flaser" structure, which passes gradually 

into a lamination where the action has been more intense. This is almost entirely due to the parallel arrangement of the secondary chlorite 

scales, so that we have a basic eruptive rock developing into a typical 

chlorite schist.

Excellent examples of a massive, though much altered greenstone 

fraying out into a chlorite schist by the action of a shearing force along 

lines of slight displacement, were found at lower Twin Falls, and are 

illustrated in Figs. 23 and 24 (See pp. 129, 130.)

The great mechanical force generated in the course of orographic 

movements, aided by chemical action, sometimes develops schistose 

bauds in massive rocks, which wind around and encircle compact cores. 

These are spheroidal or lenticular in shape, and seem for some unknown 

reason to have better resisted the metamorphosing influences. A tine 

example of this kind may be seen on the Wisconsin side of the Menominee Eiver, just at the foot of Lower Quinnesec Falls. It is represented by specimens Nos. 11017 to 11020, described on p. 93. Other 

examples of the same kind occur in the railroad cut near Front street, 

Marquette.

This last described structure has much in common with the spheroidal 

or lenticular parting observed in the aphanitic greenstones of the southern Marquette and Negauuee areas. (See Figs. 26 and 27, pp. 106,177.) 

Near the mouth of Whetstone'Brook; in the neighborhood of Baldwin's Kilns, and between Teal Lake and the Carp JRiver, we see the finegrained greenstones broken up into circular or oval areas, which are 

separated by a finer schistose matrix. This structure is common 

through the Northwest, and has been termed by the Canadian geologists " concretionary trap." . It is, however, in no sense really concre-
204 GREENSTONE SCHIST AREAS OF MICHIGAN. [BULL.62.

tionary, but must be ascribed either to contraction which frequently 

produces a perlitic structure or spheroidal parting in volcanic rocks, or 

to a mechanical brecciation and rubbing together of the disjointed 

fragments, .as was found by Rothpletz to be the case in similar greenstone schists .of Saxony. (See p. 167.)

MACROSTRUCTURAL METAMORPHISM THROUGH STRETCHING.

The effect of stretching upon rock masses has only recently come to 

engage the attention of geologists. It is evident that in the process of 

the upheaval or folding some parts of the mass must be strained, while 

other parts are compressed. As Heim first pointed out, such a tension 

is relieved sometimes with and sometimes without a visible rupture. 

Both of these methods are well illustrated in the massive rocks of the 

Menominee and Marquette regions.

In the first case we find the formation of irregular, ragged seams, 

which Heim has designated as gaping rents (" klaffeude Risse"), and 

which I have termed " cross-gashes," from their being torn open in a 

direction perpendicular to that in which the tension was exerted. They 

are, therefore, approximately parallel, and may produce a rough sort 

of schistose structure. These seams sometimes remain open, but are 

sometimes filled with secondary crystallizations, like quartz, calcite, or 

chlorite, often with epidote. They are best exhibited in the great 

greenstone ridge which extends along the river's left bank below Lower 

Quinuesec Falls. Their appearance, as far as it can be shown in a handspecimen, is represented in Fig. 30, p. 81.

The manner in which the round cores of the spheroidally parted 

greenstones in some cases have been drawn out into interlacing lenses, 

has already been noted (p. 177).

Stretching action may sometimes produce a banding in massive rocks. 

This is especially the case with acid types, like granite and quartz porphyry. Examples of this are to be found among the acid dikes near 

the Horse Eace and in the vicinity of Marquette, but the most indubitable evidence of stretching in these rocks is contained in their microscopic structure, and must therefore be reserved for description in the 

following section.

MICROSTEUCTUEAL METAMOEPHISM OF THE MENOMINEE AND MAE' 

QUETTE MASSIVE ROCKS.

Where profound mechanical and chemical changes have gone on simultaneously in rock masses, the final product may be wholly different 

from the original starting point. Sedimentary and eruptive rocks seem 

to approach each other more and more nearly, the longer they are exposed to the same metamorphosing influences. In many cases the derivation of certain schists can never be definitely settled; in other cases, 

however, where the field evidence is insufficient to decide, the micro-
WILLIAMS.] EFFECT OF DYNAMIC ACTION. 205

scope may yield reliable data for settling the origin of doubtful crystalline schists. Characteristic minerals or structures may often be detected under the microscope after every original feature visible to the 

unaided eye has been obliterated.

In stating the results of dynamic action observed with a microscope 

in the Lake Superior rocks, we may speak first of the effect upon individual minerals, and then of new structures produced. As a rule, those 

minerals and rocks which are hardest and most brittle are ibund to 

exhibit these mechanical effects most perfectly.

EFFECTS OF DYNAMIC ACTION ON INDIVIDUAL MINERALS.

Quartz. We find corroborative evidence of the fact stated by others 

that quartz is frequently more sensitive to pressure than feldspar. In. 

the quartz porphyry, No. 11707, for instance (see PI. XV, fig. 2), 

which has undergone a violent stretching, the feldspar crystals apparently exhibit no resultant phenomena, if w( e except the presence in their 

center ofmicrocline as a possible product of strain; the quartz crystals, 

on the other hand, show an unusual amount of deformation. In some 

cases they are pinched out into pear or spindle shaped areas with an 

undulatory extinction j while in other cases they are much more 

elongated, but show that this result has not been reached without a 

crushing and displacement of their original substance (granulation). 

The fragments are converted into a mosaic of interlocking grains by the 

readiness with which silica is dissolved and deposited. Quartz that is 

more or less* optically disturbed is almost universal in the acid rocks 

examined, showing that hardly any of them have been entirely free from 

mechanical strains. That the quartz grains in the granite No. 11104 

are not all original is proved by the little tourmaline crystal, which is 

fractured and faulted at the junction of two of them, as depicted in Fig. 

13, p. 112.

Feldspar. The first effect of strain on feldspar is to produce an undulatory extinction. This cannot, however, be carried as far as it is in 

quartz, because the tension is here able to relieve itself by molecular 

movements and the consequent formation of twinning lamellfe. Several cases were encountered where the gridiron or microcliue structure 

appears to have been secondarily developed in a potash feldspar by 

this means, as in the granites Nos. 11104 and 11710 and in the schistose quartz porphyry No. 11707. Such a production of secondary or 

strain lamellae in the triclinic feldspars is by no means unusual.

Where the mechanical action is too intense to produce mere optical 

disturbance or molecular gliding without rupture, the force of cohesion is entirely overcome and the feldspar is fissured or crushed. In 

case the force is exerted as a tension, the fragments thus formed are 

separated, and the spaces between them are filled with secondary crystallizations, especially chlorite. In other cases the fragments are merely 

faulted against one another. Such examples of stretched and faulted 

feldspar are very abundant in the schistose greenstones and acid rocks
206 GREENSTONE SCHIST AREAS OF MICHIGAN. [BULL. 62

of the Menomiuee and Marquette districts. It is generally possible to 

recognize what fragments once belonged together, as may be seen by 

an inspection of PI. IX, fig. 2, PI. XI, fig. 2, PI. XLV, figs. 1 and 2, and 

Fig. 11 on p. 105. Even in very schistose rocks, which are in other respects chemically much altered, these broken feldspars are, as a rule, 

unexpectedly fresh; a fact which will receive particular attention in the 

succeeding section.

If the mechanical force is still greater, a pulverizing or granulation of 

the feldspar ensues. This is always accompanied by chemical action, 

consisting of more or less complete solution and recrystallization. In 

this way the substance is molded into new forms, which accord better 

with the existing strains.

Such a partial granulation of a large porphyritic feldspar crystal, producing one of the lenticular " Augen " of the metamorphic gneisses of 

the Hors.e Eace, is shown in PI. XV, fig. 1.

In the process of feldspar granulation the calcium is frequently removed, and the secondary mosaic is largely composed of albite. as was 

first shown by Lessen.

Pyroxene. Diallage is very subject to mechanical deformation. On 

account of its relative softness and -flexibility this consists mostly of the 

bending or twisting of the crystals, or in the occasional production of 

twinning lamellae. These phenomena may be well seen in the almost 

colorless diallage of the Sturgeon Falls gabbro. Pyroxene exists in no 

other of the rocks examined except in those which are too young to 

show any effects whatever of dynamic action.

Hornblende. Certainly in a large majority and possibly in all the 

rocks which I have studied from northern Michigan, hornblende is a 

secondary product; since, then, it is a result of inetamorphisrn, it is not 

strange that we rarely see in it the effects of that mechanical action 

which caused the metamorphism.

Mica. On account of its flexibility mica, like diallage, would be well 

fitted to exhibit mechanical phenomena; but the fact that, like the 

hornblende, it is mostly a secondary mineral in those rocks to which 

we have here devoted our attention, prevents such deformations from 

being common.

Zircon, Tourmaline, etc. These minute products of the first crystallization in the rock are often found to be broken or faulted by mechanical action, as is shown in Fig. 13, p. 112, and Fig. 18, p. 122.

Ilmenite. The iron minerals, on account of their opacity, of course 

can exhibit MO optical disturbance, but their grains are frequently torn 

apart just like the feldspar crystals. This has been described in numerous instances, and it is represented in PI. XI, fig. 2.

* NEW STRUCTURES PRODUCED BY DYNAMIC ACTION.

Whenever the crushing of rocks by dynamic agencies is accompanied, 

as it almost always is, by chemical action and the production of new 

minerals, these must arrange themselves in accordance with the exist'
WILLIAMS.] NEW STRUCTUKES BY DYNAMIC ACTION. 207

iug strains. These secondary minerals are suck as themselves possess 

a very perfect cleavage; their production, therefore, under circumstances 

of uniform and continued strain, will naturally impart a foliation to 

the originally massive rock. Nor is this all. The relief of the strain 

and the consequent crushing of the rock will take place along certain 

planes much more completely than along others; hence in these planes 

the secondary minerals will be more abundant, and a banding of a once 

homogeneous rock may result.

The final structures produced in this manner depend not altogether 

upon the force exerted, but also upon the primary structure of the rock.

The rubbing together of individual crystal grains, the adhesion between which is weaker thau their own internal cohesion, may produce 

* a pulverizing only around the edge, when the force is not too intense. 

This phenomenon, to which Professor Kjerulf has applied the term 

"peripheral granulation" (randliche Kataklase), 1 may bring about a 

sort of pseudo-porphyritic structure, in which, however the porphyritic 

crystals are only the larger remnants of the former grains without a 

crystal form of their own ; or, if the crushing has been less, the grains 

may appear to be held together by a fine granular cement, like the 

stones of a wall by their mortar. Hence Professor Tb'rnebohin has 

spoken of this as the "mortar structure" (Mortel Structur).2 Many 

admirable examples of these structures are to be seen in the Menomiuee 

and Marquette rocks, especially in those of an acid character. There 

may be particularly mentioned the granite from the great area south 

of the Menominee, No. 11104; Nos. 11189 and 11190 from acid dikes at 

the Horse Eace; Nos. 11678 and 11710 from the Brook Section, near 

Marquette. In the first instance, a distinct micropegmatitic structure 

is a noticeable feature of the granular cement.

If the original structure of the rock was porphyritic instead of 

granitic, we find corresponding differences in the result. The groundmass is made schistose by the development out of its feldspar of sericite 

or chlorite. The porphyritic crystals are more or less granulated, but 

the secondary mosaic thus formed is often coarser in grain than the 

grouudmass. This is arranged around the remnant of the original 

crystal iu a lenticular area known as the " eye," or " Auge," in rocks 

called by the Germans, on account of this structure, Augengneiss. (See 

PI. XV, fig. 1.) Unusually beautiful examples of this kind are to be 

found among the acid dikes.along the shores of the Horse Eace 

especially specimens numbered 11184 and 11196. In some of the 

schists which have been derived from the Sturgeon Falls gabbros a similar structure is present.

Where secondary micaceous minerals have been developed in sinuous 

bands which interlace and twine about abundant oval cores, a lenticu1 Grundfjeldsprofilet ved Mjesens sydende. Nyt, Mag. for Natnrvidenskaberne, vol. 29, p. 215,1885 

(Neues Jahrbiicli fur Mineral., 1886, vol. 2, Keferate, p. 244.)

a Nagra ord ora granit och gneis. Geol. Fb'ron. Stockholm FgrbaBdl, vol. 5, pp. 233-2,48. (Neues. 

JaUrbucb fiir Mineral., 1881, vol. 2, Ileferato, p. 50.)
208 GREENSTONE SCHIST AREAS' OF MICHIGAN. [BULL. 62.

lar structure is produced, in which there is no contrast between groundmass and porphyritic crystals. This is called by the Germans " Flaser 7 ' 

structure, a term which may advantageously be transferred to the English. It is often the case that a structure of this kind is so flue as to be 

visible only under the microscope, when the designation " microflaser 

structure" is applicable. Examples of this lenticular interlacing are 

abundant in the schistose greenstones of northern Michigan. It is especially typical in the Sturgeon Falls schists, and in those at Lower 

Quiunesec Falls; it may also be well seen in the granite at the mouth 

of the Dead Eiver, north of Marquette.

In rocks that have been subjected to a great tension, a peculiar microscopical structure has been developed, caused by the rupture of the 

original crystals and a pulling apart of their fragments. Between these 

chlorite is formed, the scales of which are parallel to one another and 

to the direction of the stretching. This chlorite appears to have resulted from the recrystallization of the substance of the original bisilicates, since no trace of these now remains, although the broken feldspar 

is always remarkably fresh. There must be something in the dynamic 

action of an intense tension particularly conducive to the formation of 

this chlorite, for exactly this modification of this mineral was not encountered in rocks produced in any other way. Its presence, of course, 

brings out a decided schistose structure. Excellent examples of this 

phase of structural metamorphisui abound in the Menominee and Marquette greenstones, although I do not know that anything exactly like 

it (particularly when taken in connection with the remarkable freedom 

of the broken feldspar from all signs of chemical change) has ever been 

described before. It is illustrated in PI. XI, fig. 2. The widely different results produced in the same greenstone by chemical action alone 

on the one hand, and by this agency combined with stretching on the 

other, are shown in PI. IX; figs. 1 and 2. The same thing is admirably seen in two specimens (Nos. 11651 and 11652) from the Cleveland 

ore dock in Marquette. (See p. 168.)

If we could desire any more certain proof that these "stretching 

structures " are of secondary origin, developed in an already solid rock, 

we should find it in No. 11803 (PI. XIV, fig. 2). Here the porphyritic 

feldspar crystals lie in the groundinass at every conceivable angle, showing that at the time they were formed there was no force to affect their 

orientation. Their breaking and tearing asunder, however, in every 

case, has taken place in the direction of the foliation or stretching, 

without the least reference to the position of the feldspar crystal itself.

MINERALOGICAL (CHEMICAL) METAMORPHISM OF THE MENOMINEE 

AND MARQUETTE ROCKS.

Chemical alteration may go on to almost any extent in massive rocks, 

unattended by any mechanical deformation; and yet certain chemical 

changes in rock-forming minerals seem to be dependent upon the phys-
WILLIAMS.] PRODUCTS OF METAMORPHISM. 209

ical conditions produced by great orographic strains. It is easy to 

understand how the crushing of a rock should, as a rule; accelerate 

chemical activity by increasing the circulation and possibly by raising 

the temperature by friction j but more than this, we find that of several 

different alterations to which a given mineral is subject, certain ones 

are never found except in those regions where dynamic action has been 

intense.

It is true that the same secondary mineral may be produced in. differ, 

out ways. It may result from the same original mineral under different 

conditions, or from different minerals under the same conditions, or from 

different minerals under different conditions. It may also sometimes 

be derived from a single substance, and sometimes from a reaction between two or more substances. On account of the small number of 

elements which enter into the composition of rock-forming minerals, 

their physical differences are due mostly to the different proportions in 

which these elements are combined. Very slight variation in conditions may modify these proportions and so produce one mineral or 

another. Even in exactly the same chemical compound, what may be 

a stable state under one set of conditions, may be an unstable state 

under another set. So delicate is this adjustment that the secondary 

minerals produced in a given case depend not merely upon the chemical composition of the original rock or of its constituents, but even to 

a greater degree upon the physical conditions obtaining at the time of 

their formation. Thus with every change of these, one generation of 

secondary minerals may give place to another.

Such a succession of alterations, each dependent upon its own set of 

conditions, tends to obscure the life history of a rock mass; and yet, in 

spite of its complexity, this study is full of promise. It is generally 

possible to separate the products of metamorphism proper from those 

of weathering, as described in Chapter I (see p. 36). All the massive 

rocks of the Menoinmee and Marquette regions offer admirable opportunity for the study of the first of these alterations, jwhile the more 

basic types often present the subsequent effects of weathering.

In reviewing and summing up the results of chemical alteration 

already described at length, it will be advantageous first to enumerate 

the minerals of secondary origin, stating briefly the circumstances under which each was probably formed, and second to trace out the different phases of alteration to which each of the more important original 

constituents has been subjected.

SECONDARY MINERALS AND THEIK ORIGIN.

FBODUCTS OF MBTAMOKPHISM.

Feldspar. This mineral is not common as a secondary product except in the form of a'lbite, which as Lessen has shown (see Chap. I. p. 60) 

is a very characteristic result of dynamic metamorphism. In the 

Sturgeon .Falls gabbro albite is a common secondary mineral, both as 

Bull 62 U
210 GREENSTONE SCHIST AREAS' OF MICHIGAN. [BULL. 62.

the base of saussurite and in the form of clear, transparent veins. Hero 

the twinning structure is not frequent, but it may be seen in some of the 

veins, as is shown in Fig. 4, p. G9. Albite in the form of a fine granular mosaic, with or without quartz also occurs in many of the altered 

diabases. Its substance is so clear and glassy as to leave no doubt 

about its being a secondary product.

Microcline as a secondary mineral is to be referred rather to the effect 

of pressure twinning than to chemical alteration.

Saussurite (Chapter I, p. 58) is not a simple mineral, but a mixture 

derived from the alteration of the lime-soda feldspars. It is abundant 

in the more basic rocks of the Menominee and Marquette regions in 

tho gabbros of Sturgeon Falls and in many diabases and diorites. The 

base is a clear, soda feldspar (albite) which contains zoisite needles, or 

more rarely epidote and colorless garnet. (No. 11189, p. 108.)

Zoisite occurs only as one of the constituents of saussurite above 

described. It is in the form of minute needles, without terminations, 

as shown in Figs. 5 and 6 (pp. 69, 70). These differ much in size, and 

between crossed nicols display dull bluish interference colors. When 

the zoisite needles are very small the saussurite is a gray opaque mass, 

which only the highest power of the microscope is able to resolve.

Garnet was observed only once, in minute colorless crystals, forming, 

along with epidote, one of the constituents of the saussurite in speci. 

men No. 11189. (see p. 108).

Quartz is a widespread secondary mineral. It often originates from 

the breaking up of the more acid feldspars, like orthoclase. It may also 

originate from the lime-soda feldspars. Primary and secondary quartz 

sometimes present an identical appearance. In some of the Lake Superior diabases quartz seems to be an original component, and in others 

of the more altered ones it is certainly of secondary origin. In a few 

intermediate cases its origin must always remain doubtful, Owing to

the ease with which silica is transported in solution, that which is set 

free by the decomposition of the silicates is often entirely removed from 

the rock or is deposited in seams. In at least one instance a brown 

isotropic substance resembling opal was observed (the gabbro from 

Eureka Shaft near Marquette p. 170).

Hornblende. Although there is reason for considering the hornblende 

in some of the Menominee and Marquette greenstones as a primary 

constituent, still there can be no doubt that this mineral is also the most 

important secondary component of these rocks. In a fibrous form as 

uralite, amianth, etc., hornblende is almost the sole representative of 

the former pyroxene in the more altered and schistose diabases, and the 

same mineral seems also to be the final metainorphic product of the 

compact hornblende in the diorites. Indeed, the .schistose structure 

of these rocks is largely due to the production of fibrous hornblende and 

lamellar minerals of the mica type, and to their arrangement parallel 

to a single plane. The components of the secondary fibrous hornblende
WILLIAMS.] SECONDARY MINEEALS. 211

are mainly derived from some bisilicate, like pyroxene or compact 

hornblende, but its position is by no means confined to the area formerly 

occupied by these substances. On the contrary, the material shows a 

tendency to wander, and to develop amphibole needles along the cleavage cracks of the feldspar, as seen in Plate XII, fig. 2. This may even 

become filled with the hornblende, as in the epidiorite, No. 11663, from 

Pine street, Marquette, and the whole rock be finally reduced to a fine, 

felt-like mass, as shown in Plate IX, fig. 1.

Compact hornblende also, both brown and green, appears to have resulted extensively in the greenstones from the direct alteration or 

molecular rearrangement of pyroxene. Such a change has often been 

recorded, as stated in Chap. I. Indeed, the probability that much of 

the pyroxene of the original rocks has passed into fibrous green hornblende through an intermediate stage of compact hornblende has already 

been alluded to (p. 72). The brown or basaltic variety is to be found 

associated with the diallage of the Sturgeon Falls gabbro, as shown in 

PI. VIII, figs. 1 and 2; also in the porphyritic .rock from the western 

end of the " gabbro ridge," near Little Quinnesec Falls, on the Menomiuee. The compact green hornblende of Nos. 11176 and 11178, from 

the Four-Foot Fall, and of many of the Horse Race diorites, has precisely the form of diabasic augite; and in spite of its compact structure 

shows in its pale color and darker green border clear evidence of its 

secondary origin. 1 The pale compact hornblende of the barrier rock 

at Upper Quinnesec Falls (No. 11054) looks exactly like diallage,2 and 

under the microscope its most exceptional orthopinacoidal parting 

bears testimony to its derivation from this mineral. (See p. 103.)

The manner in which the compact hornblende ravels out and becomes 

fibrous is often clearly seen in the Lake Superior greenstones, and is 

shown in PI. XII, fig. 1, and in Fig. 19 on p. 126.

Epidote is a common metamorphic mineral, and appears almost always in small but well formed yellowish crystals. It sometimes forms 

one of the constituents of saussurite, but far less frequently than zoisite, 

as more iron is necessary for its genesis. In one case, No. 11091, the 

feldspar of a coarsely granular rock from the Horse Race, seems wholly 

changed to large individuals of epidote, while the pyroxene has given 

place to corresponding areas of a rather compact greenish hornblende (p. 

108). In No. 11712, from the Brook Section, near Marquette, the change 

of the feldspar to ep*idote is less complete. Epidote more commonly 

results from the alteration (perhaps weathering) of some ferrous bisili- 

.cate. Here it is generally in association with bright green chlorite in 

the form of the characteristic aggregate described in Chap. I (p. 56). 

This is very common in many of the more altered greenstones of the 

regions studied.' (See PI. XI, fig 1.)

1 See PI. XII, fig. 2 ; and cf. Lossen: Erlaut. zur. geol. Specialkarte von Preassen, Blatt Harzgerode, 1882, p. 81; and Williams' Bull. TJ. S. Geol. Survey, No. 28, PI. I, flg. 2; PI. n, fig. 1,1886. 

2 cf. H. Credner: Neues Jahrbuch fur Mineral., 1870, p. 972.
212 GREENSTONE SCHIST AREAS OF MICHIGAN. IBULL. 62.

Biotite originates under certain circumstances from hornblende. This 

is an alteration well known to mineralogists,1 and may be admirably 

seen in several of the Horse Eace diorites and in the curious ainphibole granite, No. 11831, figured in PI. XVI, fig. 2.

Muscovite, ISericite. The potash micas hold a similar place in relation 

to orthoclase, as a product of dynamic metamorphism, that saussurite 

does to the lime-soda feldspars. In some specimens, as for instance in 

the granite No. 11089 from the Horse Eace and in No. 11660 from the 

so-called gold mine near Pine street, Marquette, broad plates of typical 

inuscovite are developed in the feldspar; generally, however, the alteration is due to the minute scales or curved folise called sericite. These 

two minerals are quite identical in composition and generally in origin, 

as has been explained in Chap. I. The development of sericite may be 

best studied in the schistose porphyries, which owe their cleavage to 

stretching. These are abundant in both the regions investigated, 

especially at Upper Quinnesec Falls (see Chap. Ill, and PI. XIV, fig. 

]) and near the city of Marquette (Chap. IV). While the production 

of sericite in the acid K)cks is for the most part the result of intense 

dynamic action, it would seem as though it were especially liable to result, like certain varieties of chlorite (see below), when this force acted 

as a tension rather .than as a compression.

Titanium minerals. The metamorphosed eruptive rocks of the regions studied offer unusual opportunities for tracing the varied transformations of the titanium compounds. It is doubtful whether this 

element existed in the original rocks in any other form than titanic iron 

(ilmenite) or as a component of certain biotites. In the metamorphic 

rocks, however, titanium minerals are numerous and their origin can 

be traced to different sources. The probable derivation of rutile and 

magnetite from ilmenite may be seen in No. 11070, from Upper Quinnesec Falls (PL XIII, fig. 2), and also in No. 11825, from northwest of 

Marquette (p. 184). The little " Thonschiefernadeln," shown in Fig. 12 

(p. 106), seem also to have been derived from ilmeuites. A sagenitic 

network of rutile produced by the alteration of biotite and the consequent freeing of titanic oxide is seen in No. 11113,from Iron Mountain; 

in No. 11672 from Marquette; and in No. 11738, from near Baldwin's 

Kilns, Negaunee.

Anatase, along with leucoxene, has resulted from the alteration of 

ilmenite in No. 11130, from Twin Falls (Fig. 26, p. 166) and in No. 

11802, north of Negaunee, near the Carp Eiver. Anatase seems also 

to have resulted from the decompositon of titaniferous biotite in Nos. 

11050 and 11052, from near Upper Quinnesec Falls.

Sphene in the form of leucoxene is a universal result of the alteration 

of ilmenite in the metamorphosed diabases. In rare' cases, like No. 

11014, from Lower Quinnesec Falls, it is intimately associated with

'See J. Roth: Allgemeine undchemiscke Geologie,vol. 1, p.333, 1879,and Inostranzeff: Metamorphosirte Gesteine im Gouvernment Olonez, 1879, p. 192.
WILLIAMS J PRODUCTS OF WEATHERING. 213

rutile, which may have been produced simultaneously with the alteration of the leucoxene, or have resulted from it. In other exceptional 

cases the product of the ilmenite appears with all the crystal form and 

physical properties of rock-forming sphene. Such an occurrence in No. 

11189 from the Horse Eace is shown in PI. XIII, fig. 1; and another 

in No. 11831, from northwest of Marquette, in PL XVI, fig. 2.

The secondary minerals thus far enumerated are products of metamorphism proper, as distinguished from weathering; that is, they are 

highly crystalline, and have been produced under circumstances very 

different from those ordinarily prevailing at the earth's surface. Here 

the tendency is for the more crystalline and less soluble compounds to 

become more soluble through union with water and carbon dioxide. 

Hydration and carbonatization are therefore characteristic results of 

surface action upon rocks. They bring about their disintegration and 

decay, as purely metainorphic processes do not. Both inetamorphism 

aud weathering are very apparent in the eruptive masses of the Menominee aud Marquette regions. The rocks of Keweenaw Point, on 

the other hand, show only the effects of weathering. In spite of their 

great antiquity they have not undergone inetamorphism in the strict 

sense. Much younger rocks in disturbed areas are often found to be 

intensely metamorphosed, which clearly shows that something besides 

mere lapse of time is necessary to accomplish this change.1

PRODUCTS OF WEATHERING.

The secondary minerals in the Menominee and Marquette massive 

rocks which owe their existence to atmospheric action or weathering 

belong mainly to the following species: Chlorite, talc, serpentine, carbonates (calcite, dolomite, etc.), iron-hydroxide, and pyrite.

Chlorite is generally a product of weathering, but its origin" is sometimes closely associated with dynamic agencies. For instance, it is a 

constant feature in the basic rocks which have been stretched (like sericitein the more acid ones), filling with its parallel folia the interstices 

between the broken and separated fragments, and thus imparting a 

degree of schistosity to the entire mass, (see PL XI, fig. 2.) The 

ordinary process of cbloritizatiou has been described in Chapter I, and 

frequently alluded to in the petrographical descriptions, since it is extremely common in the basic eruptives. If the original mineral was very 

poor in iron an almost colorless chlorite was the result, as in the Sturgeon Falls gabbro (PL VIII, fig. 2). In the highly ferruginous diabases, on the other hand, the chlorite has a deep green color aud is 

often pleochroic. This is the " viridite" of the older authors, which is

1 The efficiency of orographic movements to bring about this result has already been sufficiently 

emphasized. The observations of Losson that similar changes may he produced by the contact action 

of large masses of intrusive granite have been briefly referred to in Chapter I. To what extent the 

enormous granite areas lying on both sides of the Marquette and Menomineo greenstone belts Tnay 

have been instrumental in effecting their mineralogical motamorphism must for the present be left 

undecided, inasmuch as no evidence directly bearing upon this question has as yet been collected.
214 . GREENSTONE SCHIST AREAS OF MICHIGAN. [BULL.62.

frequently associated with sharply denned epidote (secondary augite of 

Wicttmann), as figured in PI. XI, fig. 1. Chlorite seems to be the 

common effect of weathering upon pyroxene, hornblende, and biotite.

The peculiar variety of chlorite known as " helminth " in vermiform 

aggregates occurs imbedded in secondary quartz in some of the Sturgeon Falls gabbros, and is represented in Fig. 8 on p. 71.

Talc results in some cases from the alteration of hornblende, as observed by Tschermak,1 Inostranzeff,2 Becke,3 and the writer.4 This is 

not a common phenomenon in the Michigan greenstones, but some admirable examples were observed in the coarse grained diorites of the 

Horse Eace above Upper Quinnesec Falls.

Serpentine. The scarcity of olivine in the rocks examined makes the 

ordinary derivation of serpentine uncommon. This, however, does occur in some of the least altered diabases of Marquette. Indeed, serpentinization is almost the only effect of weathering noticeable in some 

specimens from the great dike of Lighthouse Point (olivine diabase), 

certain weakly polarizing fibers resulting from the change of hornblende 

appear to be serpentine, which, according to Eosenbusch,5 frequently 

gives rise to this mineral. An instance of this may be seen in Nos. 

11078 and 11191 from the Horse Eace (p. 109).

Carbonates. These most frequent products of weathering abound in 

the most altered greenstones, but they are nevertheless not unfrequently 

absent on account of the readiness with which these compounds are 

removed in solution. Calcite and dolomite are not microscopically distinguishable without a chemical reaction, but this is hardly necessary 

for most cases.

Iron-hydroxide and Pyrite are both iron compounds universally produced in basic rocks under ordinary atmospheric influences. The Michigan greenstones present no exception in the amount of these minerals 

which they contain.

PROGRESS OF ALTERATION IN THE ORIGINAL MINERALS.

Feldspar gives alteration products depending very largely upon its 

original composition, whether alkaline or calciferous. By far the most 

common change in the more basic rocks is to saussurite. This varies in 

different cases from a comparatively coarse grained zoisite aggregate to 

a dull gray, almost opaque mass which, even in the thinnest sections, 

is beyond the highest power of the microscope completely to resolve. 

The saussuritization may be often seen to have proceeded from the 

periphery of the feldspar crystal inward. In such cases a comparatively unaltered center remains, as in No. 11168 from Sturgeon Falls ' 

(PI. VIII, fig. 2). Quite as frequent is the occurrence of a clear per1 Tschermak's mineral, u. petrog. Mittheil., vol. 4, p. 65, 1876. 

2 Metamorphosirte Gesteine im Gouvernment Olonez, Leipzig, 1879, p. 167. 

3 Tschermak's mineral, u. petrog. Mitthoil., vol. 4, 1882, pp. 339 and 349. 

4Bnll. TT. S. Geol. Survey, No. 28, p. 58, 1886. 

*Mikros. Physiog., 2d ed., vol. 2, p. 108.
WILLIAMS. 1 ALTERATION PRODUCTS OF FELDSPAR. 215

iphery around feldspar crystals which have been completely sanssuritized. (See No. 11167 from Sturgeon Falls (PI. VIII, fig. l)f No. 

11054from Upper Quinnesec Falls, and No. 11182 from the Horse Race.) 

The clear feldspar substance in such cases appears like- a new crystallization rather than an unaltered survival of the original individual (cf. 

Fig. 4, p. 69).

The inverse ratio existing between the mechanical and the chemical 

action visible in the altered feldspars of the basic rocks has been often 

alluded to on pp. 88,169. This seems to be the reverse of what might 

be expected, as well as of what is actually observed in the case of all 

the other constituents. It is, however, frequently the fact that, in a 

continuous rock mass, feldspar is most completely saussuritized which 

occurs in the least crushed portion, while the rock which is most broken, 

stretched, and foliated retains its feldspar fragments in a quite fresh 

and unaltered condition. That this fresh feldspar is not a new crystallization, is proved by the fact that fragments can often be seen to 

have once fitted together, as portions of one individual, before the 

crushing took place. Among many sections in which this phenomenon 

can be observed may be mentioned : The Sturgeon Falls gabbro; the 

rocks of the diorite ridge near the Lower Quinnesec Falls; Nos. 11004, 

11010,11019, and 11102 from just below the Lower Quinnesec Falls; 

Nos. 11054,11073, and 11056 (see Fig. 11) from Upper Quinnesec Falls; 

No. 11179 from Four-Foot Falls, and Nos. 11651 and 11652 from the 

Cleveland ore dock in Marquette.

Such observations, made at many widely separated localities within 

the Menominee and Marquette greenstone areas, would at first thought 

seem to indicate that there is a reciprocal relation between the chemical 

and mechanical effects produced by great pressure. In other words, 

that the force which in the m assive portion of the rock caused increased 

chemical action and alteration of the feldspar to saussurite, was expended, in another part, in the work of crushing. The fact, however, 

that the bisilicate constituents are more altered in the crushed bands 

than in the other part of the rockrnass is opposed to such an explanation, and seems rather to indicate that the more ready alteration of 

these and the production of such minerals as fibrous hornblende, chlorite, 

and talc, formed a soft matrix which protected the feldspar from the 

further action of pressure and chemical action.

A totally different alteration from saussuritization is sometimes observed in the feldspars of £he more basic eruptives. This consists of 

their change to green hornblende needles, and can of course occur only 

where a portion of the necessary elements is furnished by some bisilicate. These elements (i. e., magnesium and iron) wander along the 

cleavage cracks of the feldspar, causing the development of the needles 

as shown, in PI. XII, fig. 2. A very complete instance of this change 

is shown in the epidiorite, No. 11663, from Pine street, Marquette. v

Still another alteration of the plagioclase is to epidote. This is com-
216 GREENSTONE SCHIST AREAS OF MICHIGAN. * [BULL. 62.

plete in No. 11091 from tbe Horse Eace, and partial in No. 11712 from 

the Brook Section, near Marquette.

While thelime feldspars change to zoisite, epidote, or horn blende, the 

alkaline feldspars show a corresponding tendency to pass into mica 

(sericite). This process is most commonly central, the periphery of a 

crystal remaining intact after the interior is completely altered, as may 

be seen in No. 11104, the granite from south of Upper Qninnesec Falls. 

This same specimen exhibits an interesting phase of the inverse ratio 

existing between the mechanical and the chemical action in the acid 

feldspar. Microcline seems to have been abundantly developed from 

orthoclase by pressure but while the orthoclase itself is sericitized, the 

microcline never shoics a trace of this change.

Pyroxenes comparatively rare in the rocks studied, occurring only 

as diallage in the Sturgeon Falls gabbro, and as augite in the younger 

Marquette diabases; nevertheless, from the structure of many of the 

other rocks, we may affirm with certainty that they once contained pyroxene, and may hence trace the alterations which this mineral has undergone.

The most common, and indeed the almost universal change of the 

pyroxene in the Menominee and Marquette rocks is to hornblende. The 

compact brown hornblende in the Sturgeon Falls gabbro and in the 

porphyritic rocks from the west end of the ridge near Lower Quinnesec 

Falls, shows strong evidence of having originated from pyroxene, but 

this can not perhaps be regarded as beyond all doubt. Compact green 

hornblende appears to replace original diallage in the gabbro-like 

diorite which forms the barrier rock at Upper Quiunesec Falls (see 

PI. X, fig. 1), where it even retains the orthopinacoidal parting. The 

same mineral likewise seems to replace original augite in many of the 

Horse Eace diorites, whose structure is typically diabasic.

Fibrous green hornblende or uralite is too common and well known 

an alteration product of pyroxene to deserve especial description. It 

is universally distributed through all of the more basic rocks in the 

areas investigated.

A colorless chlorite may be seen originating from the almost colorless diallage of'the Sturgeon Falls gabbro, as shown in PI. VIII, fig. 

2. From the varieties of pyroxene which.are richer in iron, a deep green 

chlorite results which is often associated with epidote, as shown in PI. 

XI, fig. 1.

Hornblende. Without reference to the original or secondary origin of 

the hornblende itself, this mineral is found subject to several different 

alterations.

It passes from the compact to the fibrous form by fraying out at the 

ends, as shown in specimen No. 11175, from the Four-Foot Fall (see 

Fig. 19, p. 126). It also seems to give rise to biotite in many of the 

Horse Eace diorites and in the amphibole granite, No. 11831, obtained 

at the camp northwest of Marquette. (See PI. XVI. fig. 2.) Talc
WILIUMB.J BIOTITE, OLIVINE, AND ILMENITE. 217

frequently is produced by hornblende alteration in the Horse Eace 

diorites as described in Chap. Ill (p. 107). A colorless chlorite, closely 

resembling that produced from the Sturgeon Falls diallage, is derived 

from the pale hornblende (itself probably paraniorphic) of No. 11098, 

from the ridge near Lower Quinnesec Falls. This is produced in spots 

over the hornblende and is visible only by its isotropic behavior in polarized light, as shown in Fig. 9 (p. 79). The viridite (chlorite) epidote 

aggregate originates from hornblende just as it does from pyroxene. 

Finally serpentine is an exceptional result of hornblende alteration, as 

may be seen in specimens 11078 and 11191 from the Horse Eace (p. 109).

Bwtite shows the usual alterations and is principally the result 

of weathering rather than of metamorphic processes. Bleaching first 

takes place and then chloritization. Carbonates are sometimes separated in the form of interpolated lenses, while the titanium which 

entered into their composition separates out sometimes as anatase 

(Nos. 11050 and 11052 from Upper Quinnesec Falls) and sometimes as 

rutile.1 (Nos. 11113, from near Iron Mountain in the Menominee valley, 

No. 11672 from Marquette and No. 11738 from the Brook Section.)

Olivine occurs only in a few of the Marquette diabases, where its 

alteration is the usual one to serpentine.

Ilmcnite is the last of the important original constituents of the 

Menominee and Marquette massive rocks whose alterations can be 

clearly traced'. This gives rise to a variety of new titanium minerals

Its commonest change is that to leucoxene, which is everywhere 

encountered, see PI. IX, figs. 1 and 2.

Sphene or titanite, of which leucoxene is only a special form, sometimes originates from ilmeuite, as is shown in PI. XIII, fig. 1, and in 

PI. XVI, fig. 2.

Anatase also, in association with leucoxene, is a secondary product 

of ilmenite alteration. This is shown in Fig. 25 (p. 131) of No. 11130, 

from Twin Falls, as well as in No. 11802, one of the stretched agglomerates on the Carp Eiver. (See Chap. V, p. 177.)

'On the frequency of these oh.inges see Whitman CTOSH: " Petrography of the Leadville Jtocks" 

in Mon. U. S. Geol. Survey, vol. 12, 1887.

PLATES.

219

PLATE VIII.

221
PLATE VIII. 

FIG. 1. Saussurite gabbro (first stage).

From point h, PI. Ill, at Sturgeon Falls, Menominoe River, Michigan. Specimen 

No. 11167a. Ordinary light. (Seep. 68.)

The triclinic feldspar, which shows occasional idiomorphic forms indicating that 

it is in part, at least, older than the pyroxene, here appears almost completely altered 

to an opaque, gray saussurite. A high magnifying power resolves this into an aggregate of minute zoisite needles, calcite and albite. These are somewhat unevenly dis. 

tributed which produces the blotched appearance of the saussnrite.

The diallage, which is of a pale green color in the hand-specimen, is almost colorless when seen under the microscope. Its well marked orthopinacoidal parting is seen 

on the right of the figure. This diallage is surrounded by a border of compact hornblende, which is either brown, pale green or colorless. It frequently becomes fibrous 

on its exterior edge, as is shown in Fig. 7, on p. 70. Small patches of compact brown 

hornblende are also abundantly scattered over the entire surface of the diallage in a 

manner that would seem to indicate that it had originated from the latter by paraniorphism. There is some pale green chlorite visible in the upper part of the drawing and on the right is a bit of ilmenite and leucoxene.

FIG. 2. Saussurite gablro (second stage).

From point /, PL III, at Sturgeon Falls, Meuominee Eiver, Michigan. Specimen 

No. 11154. Represented between crossed nicol prisms. (Seep. 72.)

A large individual of the pale diallage is here seen in an advanced stage of alteration to a colorless chlorite. Because of the almost isotropic character of this mineral,

it is necessary to show its appearance in polarized light. Imbedded in the chlorite 

are remnants of the still brightly polarizing diallage, and around it is the border of 

hornblende, which here shows pale yellow interference colors.

The feldspar is less completely saussuritized than in the last figure. Its characteristic twinning lamellae are still visible in many instances, particularly at the center 

of the crystals.

222
U.S.GEOLOGICAL SURVEY. BULLETIN No. 62 PLATE VIII.

FIG. I 1X30)

Fl G.2 i X3O)

G. H.W. DEL.
PLATE IX.

223
PLATE IX.

FIG. 1. Altered diabase or diorile.

From the so-called " Gabbro Kidge," below Lower Quiunesec Falls, Menominee River,

Michigan. Specimen No. 11028. Ordinary light. (See p. 81.) 

The original structure of this rock has been completely changed, in spite of its still 

massive character. It is now composed of pale green, fibrous hornblende, saussurite, 

quartz, calcite, chlorite, ilmenite, and leucoxene. Most of these constituents are of 

secondary origin, and in the process of their formation the form of the primitive minerals has been so completely obliterated that it is now impossible to say whether the 

mother-rock was a diabase or a diorite. 

FIG. 2. Schistose band in the last figured rock, shotting the effect of dynamic melamorphism (stretching) upon it.

Locality same as that of last figure. Specimen No. 11031. Ordinary light. (See

p. 82.)

The feldspar in this rock is chemically less altered than in the last, but it has been 

subjected to a much more intense mechanical action. It is fractured, and the fragments are widely separated, although those pieces which once belonged to the same 

crystal may still often be identified. The interstices are filled with scales of a green 

chlorite, arranged parallel to one another., so as to produce a schistose structure. 

Even the little leucoxene zones around the ilmenite grains have been elongated in 

the same direction. 

224
U.S.GEOLOGICAL SURVEY. BULLETIN No. S2 PLATE IX.

FIG.l (X8O)

Fl G.2 ( xGO )

G. H.W. DtL .
PLATE X

Bulh 62 15 225
PLATE X. 

FIG. 1. Saussurite diorite.

This rock forms the barrier at Upper Quinnesec Falls, Menorniuee Eiver, Michigan. 

Specimen No. 11054. Ordinary light. (Seep. 103.)

In spite of the close external resemblance of this rock to the Sturgeon Falls saussurite gabbro (PI. VIII, fig. 1), it shows no trace of any pyroxenic constituent. Its 

only bisilicate is hornblende. This is sometimes compact (and then both brown and 

green in color), sometimes ragged and fibrous. The former variety is seen in the 

center and on the left of the figure, with its remarkably perfect orthopiuacoidal 

parting, like that recently observed by Cathrein in a hornblende from Eoda, in the 

Tyrol. This parting may be an indication that this hornblende has been derived 

from original diallage. A large individual of the more ragged and fibrous hornblende, 

filled with secondary quartz, is shown on the right of the figure.

The feldspar of this rock has undergone the usual alteration to opaque, gray saussurite, but in this are imbedded frequent grains of a fresh brown feldspar. Leucoxene rims around the iluieuite are abundant.

FIG. 2. Typical a,2)hanitic greenstone of the Marqnette region.

From Baldwin's Kilns, northeast of Negaunee, Michigan. Specimen No. 11747. Ordinary light. (See p. 172.)

This is a fair representative of the commonest rock type in the southern Marqiiette 

and Negaunee greenstone areas. Its grain must always have been very fine and compact. Long, slender feldspar crystals may still be seen, sometimes with an imperfectly developed form, such as is common in the serai-crystalline rocks (porphyrites). 

The groundmass is composed of a fine grained aggregate of fibrous hornblende,

chlorite, quartz, calcite, and ilinenite leucoxene. These minerals are all secondary, 

and may easily have resulted from the alteration of a cryptocrystalline, or even a 

partially vitreous base. 

226
U.S.GEOLOGICAL SURVEY. BULLETIN No 62 PLATE X.

FI'G. I < X3O)

FIG 2 (XilOi

G H.W. DLL .
PLATE XI.

227
PLATE XL 

FIG. 1. Chlorite-epidoie aggregate in an altered diabase.

From below Upper Quinnesec Falls, Menominee River, Michigan. Specimen No.

11049. Ordinary light. (Seep. 97.)

This is a very common alteration product in rocks of the diabase type, especially 

in those rich in iron. In a confused mass of deep green and somewhat pleochroic 

chlorite scales lie embedded extremely sharp crystals of a yellowish or colorless 

epidote. These latter are what the earlier students of these rocks mistook for 

secondary augite. In the lower part of the figure are seen ilmenite grains surrounded by rims of leucoxene and a portion of a large secondary hornblende crystal.

FIG. 2. Stretched greenstone.

From above Lower Quinnesec Falls, Meuominee River, Michigan. Specimen No.

11102. Ordinary light. (Seep. 94.)

The eifects of tension upon a solid rock mass are rarely seen more distinctly than 

in this specimen. Not only the feldspar, but also a large grain of ilmenite has here 

been forcibly torn asunder, and the fragments have been separated a considerable distance, always in one direction. The peculiar green chlorite so characteristic of all 

stretched basic rocks (cf. PL IX, fig. 2) fills the interstices between the mineral fragments. It must have crystallized after the breaking of the solid rock constituents 

and its scales, which are all arranged parallel to the direction in which the strain 

was exerted, produce a decidedly schistose structure in the rock. The fresh state of 

the feldspar in a rock which has been subjected to such intense mechanical action as 

this one is very noticeable.

228

0 ' 
U.S.GEOLOGICAL SURVEY. BULLETIN No. 62 PLATEXI.

FIG. I (X ISO)

Fl G. 2 ( X30)
PLATE XII.

229
PLATE XII. 

FIG. 1. Coarsely crystalline diorite.

From near Baldwin's Kilns, northeast of Negaunee, Michigan. Specimen No. 11749.

Ordinary light. (See p. 173.)

This one of the coarse grained dike rocks which penetrate the aphanitic greenstones of the Negaunee area. The hornblende is idiomorphic, and, as there are no 

certain traces of pyroxene, the rock is designated a "diorite." The feldspar is 

quite filled with secondary actinolite or epidote needles. The hornblende is often 

altered internally to chlorite or fibrous amphibole, while its exterior still remains 

compact and shows a curious concentration of the green coloring matter. There is 

also observable around the edge of the hornblende crystals a separation into fibers, 

as described by Becke.

FIG. 2. Epidiorite.

From the Horse Race above Upper Quinnesec Falls, Menominee River, Michigan.

Specimen No. 11186. Ordinary light. (See p. 107.)

This is one of the coarse grained dioritic rocks so abundant along the Horse Race 

Rapid. The pale green hornblende is entirely allotrioniorphic and fills the spaces 

between the idiomorphic feldspar crystals exactly as augite does in diabase. Still 

there is no certain trace of pyroxene now present. The hornblende shows a deeper 

color around its edge and a breaking up into fibers, which are arranged in irregular 

bundles and appear to wander off into the surrounding feldspar. 

230
U.S.GEOLOGICAL SURVEY BULLETIN No. 62 PLATE XII.

FIG. I ( X I IO 1

FIG.2 IXIIO)

G. H.W. DEL.
PLATE XIII.
\tf*

PLATE XIII. 

FIG. 1. Sphene crystals around ilmenite,

In a dioritic rock from the Horse Kace Rapid, above Upper Quinnesec Falls, Menominee River, Michigan. Specimen No. 11181). Ordinary light. (See p. 109.) 

These sphene crystals, although their minerplogical character is undoubted, are 

not so clear or so sharply defined as usual. They are gray and cloudy, resembling 

leucoxene in appearance; and, like this substance which is known to be identical 

with sphene in its composition, these crystals surround grains of ilmenite, as though 

they had originated from its alteration. If this be the case, we have here an interesting example of leucoxene assuming its characteristic titanite form.

FIG. 2. Rutile originating from the alteration of ilmenite.

In a greenstone from below the Upper Quinnesec Falls, Menominee River, Michigan.

Specimen No. 11070. Ordinary light. (See p. 99.)

The rock, which was probable once a diabase, now consists principally of green 

chlorite and reddish feldspar substance. The opaque iron oxide now present is 

magnetite. This occupies irregular areas and is intimately associated with a network of yellow rutile needles iu a manner which renders probable the origin of both 

of these minerals from an original ilmeuite.

232
U.S.GEOLOGICAL SURVEY. BULLETIN No 62 PLATEXIII.

FIG.I (XQO)

Fl G. 2 ( X I IO )
PLATE XIV.

233
PLATE XIV. 

1 FIG. 1. Sericite porphyry.

From just below the Upper Quinnesec Falls, Menominee River, Wisconsin. Specimen

No. 11050. Nicols crossed. (Seep.121.)

This rock, which was once an eruptive quartz porphyry, shows in an admirable 

manner the effect of stretching. The porphyritic quartz crystals are somewhat 

elongated and testify to the tension to which <hey have been subjected by an uudulatory extinction. The porphyritic feldspar crystals are fractured and pulled apart 

always in the same direction, without reference to the position which they themselves occupied in the rock. The fissures are cemented by a new crystallization of 

sericite. The micrograniticgrouudtnasshas been rendered schistose by stretching and 

by the abundant production of sericite, whose scales have a parallel arrangement.

FIG. 2. Stretched and brccciated greenstone.

From near the Carp River, north of Teal Lake, Michigan. Specimen 11803. Nicols

crossed. (See p.177.)

Some of the interstitial material, occurring between the massive cores of the 

spheroidally parted (brecciated) greeustoues, contains distinct crystals of white 

feldspar. Between the eastern end of Teal Lake and the Carp River some of these 

massive cores are themselves much elongated by stretching. (See Fig. 27, p. 177.) A 

section of the insterstitial material from this locality was selected for illustration because of the distinctness with which this stretching is apparent in the feldspar 

crystals. They are always broken and pulled apart in the same direction, while the 

smaller components of the rock show a corresponding elongation. The continuity of 

the original rock-structure in the interstitial portion of this exposure indicates that 

the spheroidal parting is in this case a result of cooling, rather than of mechanical 

brecciation in situ. (See p. 167.) 

234
U.S.GEOLOGICAL SURVEY. BULLETIN No. 62 PtATEXIV.

FIG I <X30)

FIG.2 i X3O)

G. H.W. DEL .
i PLATE XV.

235
PLATE XV.

FIG. 1. Structure of one of the " Augen" in an "Augengneiss," or stretched granite porphyry.

From a schistose dike in the greenstones of the Horse Race Rapid above Upper Quinnesec Falls, Menominee River, Michigan. Specimen No. 11184. Ordinary light. 

(Seep. 118.)

The center of the "Auge," or eye, is seen to consist of a portion of an old, brown 

colored porphyritic feldspar crystal. This is granulated on either side, and pulled 

out into a lens, whose longest axis agrees with the direction of the foliation. The 

granulation is composed of a new crystallization of the feldspar substance, which is 

clear, and although granitic in structure, it has a much coarser grain than the rest 

of the rock. This formation of an " Atige" out of a porphyritic feldspar crystal 

agrees with the observations of J. Lehinaiin, Chelius, and others.

FlG. 2. Stretched quartz porphyry.

From a conformable dike in the schistose greenstones of the Brook Section, west of 

Marquette, Mich. Specimen No. 11707. Nicols crossed. (Seep. 150.)

This rock is particularly noticeable on account of the wonderful manner in which 

it shows the effects of a stretching action upon quartz. The original crystals are 

elongated into lenticular and pear shaped forms, or even into long, narrow bauds. 

The first effect of the tension is to disturb the optical homogeneity and produce an 

undulatory extinction. If, however, the strain is great enough to overcome the force 

of cohesion, a mosaic of interlocking and differently orientated grains is the result. 

Similar phenomena have been observed and illustrated by J. Lehmaun in Saxony. 

The porphyritic feldspar crystals in this rock are not broken, nor do they show the 

evidence of great pressure. Their form is generally intact, but it is worthy of remark that their center is always occupied by an irregular area of inicrooline.

The grouudmass is finely microgranitic, containing sinuous bands of micaceous 

minerals, which bend around the porphyritic constituents, while they follow the 

general direction of the foliation. 

236
U.S.GEOLOGICAL SURVEY. BULLETIN No. 62 PLATE XV.

FIG.! (X30)

FIG. 2 (X30)
'IAX
PLATE XVI.

FIG. 1. Greenstone schist (tuff) of the northern Marquette area.

Lighthouse Point, Marqnette, Mich. Specimen No. 11619. Ordinary light. (See 

p. 155.)

A characteristic, although by no means universal structure, in the banded green 

schists which compose so large a portion of the region around Marquette, consists in 

the tufted form of the hornblende crystals. These, although quite compact at their 

centers, are fibrous at the ends and expand so as to resemble sheaves tied in the middle. The fibers are so fine and the radial arrangement at the ends of the bundles is 

so perfect that the arm of a black cross is seen to sweep across the field when the 

thin section is viewed between crossed nicol-prisms. Hornblende with a structure 

like this has been observed by Renard and others as a new crystallization in altered 

sediments, a fact which lends additional support to the hypothesis that these green 

schists are tuff deposits.

FIG. 2. Amphibole granite (?) forming a dike in aplianitic greenstones.

Northeast of Negannee, Mich. Specimen No. 11831. Ordinary light in part; polarized light in part. (See p. 181.)

This rock presents many points of interest, both in its structure and alteration 

products. The feldspar is in two distinct generations, of which the older variety is 

entirely idiomorphic, with a zone of clear substance surrounding a gray and more 

altered interior.

The hornblende exhibits the unusual alteration to biotite, which is sometimes 

brown, sometimes green. This mineral in rare instances has crystallized in sharp 

hexagonal plates which show pressure lines of cleavage ("Drucklinieu ") 

The groundmass of this rock is an aggregate of quartz and the younger generation 

of feldspar. These sometimes form a microgranitic mosaic and sometimes micropegmatitic growths, which care represented in the figure as they appear iu polarized

light. Apatite is abundant, and also ilmenite or magnetite surrounded by cblorite, 

and iu some cases by sphene, as though this had resulted, from the alteration of the 

ilmenite, as shown in PI. XIII, fig. 1.

238
U.S.GEOLOGICAL SURVEY. BULLETIN No. 62 PLATE XV(.

riG.i (X3O)

G.H.W. DEL .
r

INDEX.

A.

Page. 

Acid rocks, eruptive, origin of ........... 123

Actinolite ............................... 108,181

Adinole................................... 152

Agglomerates............................ 187

Albite................................... 69,209 i

Page. 

Cores,massive in schist.................. 93

Credner, H., cited ...................25, 64,102,119

Cross, W., cited .......................... 167

Cross-gashes (Klaffende Kisse) .........80, 92, 204

Crnshiug.......................... ...... 214

Allanito(aeeorthite)..................... 118

Amphibole granite ................ ..... 381

Amygdaloidal structure.. t .............. 174

Analyses ... .76,89,91,104,113,119,120,121,152,153

Auatase ...................63,121,131,177, 212, 217

Andesine................................ 114

Apatite ................................. 200

Aphaniticgreenstones................... 163,171

Augengneiss ............ ............... 118,207

Augen structure....................... 75,118, 207

Augen in banded greenstone schists ..... 157

Augite................................ 139,175, 200

BBaldwin's Kilns ......................... 172,178

Baltzer, A. cited ......................... 44,47

Banded greenstone schists............... 154,184

Bayley, Prof. W. S-...................... 14,32

Becke, Fr., cited......................... 126,173

Becker.G.F ............................. 57

Biotite..-....... -- ..........107,108,200,212,217

after hornblende ..................... 182

Bouney.T.G ........................... 46

Brogger, W.C., cited ..................... 42,63

Brook Section ............ ................ 143

Brooks, Maj. T. B ............... 17, 25, 65,110,124

C.

Calcitein granites ....................... 114,122

Carbonates .............................. 214

Carbonatization ......................... 36

Carp Paver .............................. 175

Cathrein, A., cited .............. 59, 63,69,100,103

Cheliusi C., cited ........................ 50

Chlorito.................................. 55,213

Chlorite, colorless.......................73,79,105

Chlorite produced by stretching.... ..... 83,164,

171,176,208 

Chlorite-epidote aggregate.... ......56,90,97,214

Chloritizatiou............................ 55

Clay slate................................ 124

Cleveland ore dock, Marquette .......... 168

Compression............................. 202

D.

6',

57

44

180

185

Dana, J. D., cited.........................

Dathe, cited .............................

Datibree, cited ..........................

Dead River, Falls of......................

Deer Lake, agglomerates.of..............

Diabase .............................. J38,183,197

glassy............................. 140,143,198

micropegmatitic. ................... 141

oliviue ......... ..................138,143,180

tuffs of............................... 133,158

Diabase Porphyry........................ 198

Diallage ............................70, 87,200,206

Diller, J.S , cited.................... .... 131,153

Diorite ... .........................146,181,197

Diorito Porphyry........................ 199

Dutton, C.E., cited....................... 159

E.

Epidiorite ............................131,144,145

Epidote (after feldspar).................. 108,211

Epidotization ........................... 56

Eureka Series, Harquotto................ 137

F.

Feldspar ................................. 214

brown ............................103,105,108

clear peri phery of...............79, 90,103,108

deformation of ................. ..... 48,205

freshest where crushed...... .72,82,88,93,103,

105,169,215 

zonal..............................117,148, 178

Felsite................................... 122

Flaser structure .................... 75, 87,119,203

Foliation from jointing............. ..... 128

from shearing ....................... 129,142

Foster and Whitney, cited ............... 15, 25

Four-Foot Falls.......................... 123

G.

Gabbro .................................. 197

olivine ............................... 180,197

saussurite............................. 67,170

239
240 INDEX.

Gabbro-diorite........................... 86

" Gabbro Eidge " of Brooks .............. 77

Garnet................................... 108,210

Geikie, A., cited..................... 153,161,190

Gneiss................................... 122

Granite.......................... 111,147,178,199

Granite gneiss ....................... 113,115,116

Granite porphyry........................ 117,199

Granulation, peripheral.. .48,112,117,147,148, 207 

Great Dyke, Marquette.................. 138

Groddeck, A. von., cited................ 62

H.

Hagge, cited............................. 58

HarringtonB , ci,ted...... ................ 53

Heim, A., cited.......................... 43,47

Helminth................................ 55,71

Hitchcock, E., cited ..................... 162

Hornblende.............................. 206

brown ...... ...................... 77,79,200

like diallage ......................... 102

surrounding diallage................. 70

sheaf-like............................ 155

secondary to compact brown ........

hornblende after pyroxene ..... 72. 78 87,211

fibrous, not a secondary enlargement. ....................... 105,126,173, 210

with granulated center.............. 145

changing to chlorite ................. 79

changing to serpentine............... 109,170

Horse Race Eapid ....................... 96.106

I.

Ilmenite ................................. 200

altered to sphene..................... 109,217

stretching of......................... 95,206

probable change of, to rutile and magnetite ..............i............... 99-101

InostranzefF, cited........................ 54

Irving, E. D .............................. 185

views of, regarding the Marquette region................................ 19-24

views of, regarding the Menominee 

region............................. 29-30,67

J.

Jointing ................................. 128

Judd, J. W., cited ........................ 39,50

Julien, A. A.,cited ....................... 146

Kimball, J. P., cited 

Kloos.J.H., cited ...

L.

Labradorite.............................. 104,199

Laspeyres,cited.......................... 61

Lawson, A. C., cited............12,136,162,166,178

Lehmann, !., cited.........45,47,48,59,62, ] 18,150

Leucoxene ............................... 63, 83

Lessen, K. A., cited........ 38,41,42,49, 59,152,161

Lower Qainnesec Falls .................. 77

M.

Page. 

Magnetite................................ 200

Marqnette. .............................. 134

Melaphyre............................... 198

Metamorphism, defined .................. 36

macro-structural..................... 43,202

micro-structural ..................... 46, 205

mineralogieal ........................ 50,208

Mica..................................... 206

Microcline........................111,199,210, 216

Microflaser structure ..............75,117,170,176

Micropegmatite.......................... 141,182

Mortar structure......................... 48,207

Muscovite ............................117,199,212

Naumann, C. F., cited.................... 167

Negaunee area........................... 171

Novaculite............................... 151,179

0.

Oligoclase................................ 111,199

Olivine................................180, 200, 217

Olivine diabase .......................138,143,180

Olivine gabro ............................ 197

Ophitic structure ........................ 196

Orthite ...........................117,118,178, 200

Orthoclase............................... . 199

Orthopinacoid parting in hornblende .... 103

P.

Parting, orthopinacoidal................. 70,103

rhomboidal .......................... 128

spheroidal........................137,166,203

Penfield, S. L., cited...................... 100

Phillips, J. A., cited...................... 161

Picnic Islands, Marquette................ 146,198

Poicilitic structure .................78,79,183,196

Porphyroids ............................. 119

Pseudo-porpbyritic structure ............ 207

Pumpelly, R., cited....................... 67

Pyroxene ................................ 206

alterations of ........................ 216

change of to hornblende not simple

paramorphism ................... 5.2

QQuartz ................................... 199,210

broken............................... 112

deformation of............. 48,117,148,150, 205

Quartz porphyry......................... 148,199

Quartzite ......... ...................... 179

Quinnesec, Lower Falls .................. 77

Upper Falls.......................... 96

E.

Eeusch, H. H., cited..................... 44,46,161

Ehomboidal parting in greenstones....... 83

Rominger.............................18,19,27,06

Rosenbusch..............................49, 52,55

Eo hpletz,A.,cited ... .................. 44,168
INDEX. 241

'Entile ..................................84,91,212

(tbonscliiefernadeln) ..............98,106,212

after biotite.......................115,124,178

after ilrnenite ........................ 99-101

S.

Saussurite .............................59,103,210

Saussurite gabbro........................ 67

Saussuritization ......................... 58

Schalstoin ............................... 160

Schenck.A., cited ....................... 54,56

Schistose porphyry ...................... 119

Schinidt,C., cited ........................ 46,62

Sericite .............................61,121,212

Sericito schists ......................74,84,88,105

Sericitization .................. ......... 60

Serpentine after hornblende .......... 109,170,214

Shearing..............................129,130,203

Sphene .......................... .111, 200,212, 217

afterilmenite ........................ 109,182

Spheroidal parting.................... 137,166,203

Stelzner, cited ........................... 121

Stretching, effects of............82,94,177,188,204

Sturgeon Falls........................... 67

T.

Talc ..................................... 107,214

Teal Lake................................ 175

Teall,J.J.H.,cited ...................... 46,53

Thouschiefernadeln (see rutile)........98,106, 212

Titanic iron (see ilmenite) ............... 200

Tornebohrn, A.E., cited .... . ........... 48

Tourmaline...............111,112,114,124, 200, 206

Page.

Tuffs of diabase ......................133,184,199

acid rocks ................ ...... 151

Carp Kiver .......................... 177-8

Deer Lake........................... 185

Twin Falls............................... 127

U.

UpperQuinnesecFalls .................. 96

Upper Twin Falls ...................... 132

Uralite................................... 52

double /one of, around pyroxene ..... 142

Uralite diabase .......................... 174,175

Uralitization............................. 52

V,

Van Hise, C. R., cited .............. 70,105,126,173

Variolite (?)........ ..................... 173

Virirtite......... . ...................... 56,97

Vogelsaud, H., cited...................... 34

W.

Wadsworth, M.E., cited......17,50,51,146,150,154

Weathering defined...................... 36

Weiss, C. E., cited ..................46, 49,150,195

Whetstone, see novaculite.

Whetstone brook, Mar quette ............ 164

Wicbraann, cited....................57,65, 90,123

Wright.C.E.... ........................ 26,123

Z.

Zircon....................Ill, 113,122,124, 200, 206

Zoisite.............................69,102,184, 210