. . LIBRARY . . Connecticut Agricultural College. VOL J...0..O ..S. 5 _._Z CLASS NO .5...5...Q. C OST S...^^. DATE .^.iM,s4L..(e. 19a.A.. DEPARTMENT OF THE INTEUIOR MONOGRAPHS OF THE United States Geological Survey VOLUME XLYI WASHINGTON GOVERNMENT PRINTING OFFICE 1904 Digitized by the Internet Archive in 2009 with funding from Boston Library Consortium IVIember Libraries http://www.archive.org/details/menomineeironbeaOObayl UNITED STATES GEOLOGICAL SURVEY CHARLES D. WALCOTT, DIRECTOR THE MENOMINEE IRON-BEARING DISTRICT OF MICHIGAN BY WILLIAM SHIRLEY BAYLEY CHARLES RICHARD VAN HISE, Geologist in Charge WASHINGTON GOVERNMENT PRINTING OFFICE 1904 0 O 8 5 CONTEJ^TS, Page. Letter of Transmittal 17 Outline op Monograph '..S 19 Chapter I. — Introduction 33 Scope and date of work done 33 Acknowledgments 33 Limits of the Menominee area 34 Relations to other iron-bearing areas 34 Shape and size of the Menominee tongue 35 Economic importance of the district 35 Previous work in the district - 36 Method of work 37 Classification of formations 38 Names of the formations 39 References to Marquette monograj)h 40 Chapter II. — Bibliography and Abstract of Liter.\ture 41 Chapter HI. — Physiography 125 Topography 125 Drainage 126 Origin of the topography 127 Pre-Cambrian topography 129 Chapter IV. — The Archean System 1.30 Section 1 . Quinnesec schists 131 Relations to overlying formations 131 The southern area 131 Distribution 131 Topography 132 Composition and structure of the rock series 133 Lithology 134 Greenstone-schists and associated greenstones 135 Coarse-grained varieties 136 Gabbros and their derived schists 136 Diabases and their derived schists ., 1.39 Diorites and their derived schists 139 Fine-grained varieties 141 Origin of the schistosity 142 Basic lavas and their derived schists 142 Basic tufts and their derived schists 144 Chlorite-schists 145 Amphibolites 146 5 6 CONTENTS. Chapter IV. — The Archean System — Continued. Page. Section 1. Quinnesec schists — Continued. The southern area — Continued. Lithology — Continued. Origin of the basic schists .; 148 Acid intrusives and their derived schists 149 Gneissoid granite and granite-gneisses 150 Porphyries and felsites and their schistose phases 153 Sericite-schists 154 Interesting localities 155 Sturgeon Falls 155 Little Quinnesec Falls 156 Big Quinnesec Falls 157 Horserace Rapids 158 The western area 159 Distribution 159 Topography 159 Lithology 160 Fine-grained greenstones and their derived schists 162 Coarse-grained greenstones and their derived schists 163 Fragmental schisfe 164 Origin of the rocks 165 Interesting localities 165 Upper Twin Falls 166 Lower Twin Falls 166 Fourfoot Falls 166 Section 2. Northern Complex 167 Distribution 168 Topography 168 Sequence of rocks 168 Lithology .■ 169 Gneissoid granites 169 Banded gneisses 1 70 Hornblende-schists 171 Intrusives 172 Acid intrusives 172 Basic intrusives 173 Interesting localities 174 Ch.^pter V. — The Algonkian System 175 General character and definition 175 Unconformity within the system 175 Section 1 . Lower Menominee series 176 Succession and distribution 1 76 Sturgeon quartzite 177 Distribution and topography 177 Lithology 179 Conglomerates 179 Arkoses and gray wackes 181 Quartzite '- 183 Dolomitic quartzite 185 Veins and dikes in the quartzite 185 CONTENTS. 7 Chapter V. — The Algonkian System — Continued. Page. Section 1. Lower Menominee series — Continued. Sturgeon quartzite — Continued. Folding 186 Thicliness 187 Relations to underlying formations 188 Interesting localities 189 The "rock dam" on Pine Creek 189 Black Creek 194 North half of sec. 7, T. 39 N., R. 28 W 195 West half of sec. 8, T. 39 N., R. 28 W 196 Falls of the Sturgeon 196 Randville dolomite 200 Distribution and topography 200 The northern belt 200 The central belt 201 The southern belt 203 Distribution 203 Topography 208 Lithology 209 Dolomite and dolomitic sandstone 210 Dolomite breccias and conglomerates 215 Talpose schists 221 Argillaceous rocks 225 Cherty quartz rocks 226 Conclusions from microscopical study, and comparison with similar rocks in the Marquette and Gogebic districts 229 Origin of the dolomites and cherty quartz rocks 230 Folding 232 Major folding 232 Cross folding 233 Minor folding 235 The northern belt _ 235 The central belt 235 The southern belt 237 Marginal folds : 237 Walpole fold 238 Pewabic fold 239 Quinnesec fold 239 Norway fold 240 Aragon fold 242 West Vulcan fold 245 Other secondary folds at southern margin 246 - Secondary folds at northern margin 246 Interior folds 247 Thickness 248 Relations to adjacent formations 250 Relations to underlying Sturgeon quartzite 250 Relations to overlying Negaunee formation 251 Relations to basal member of the Upper Huronian 252 Relations to other formations 254 8 CONTENTS. Chapter V. — The Algonkian System — Continued. Page Section 1. Lower Menominee series — Continued. Randville dolomite — Continued. Interesting localities 255 In the northern belt 255 Northeast quarter of sec. 3, T. 40 N. , R. 30 W 2.55 Northeast quarter of sec. 14, T. 40 N. , R. 30 W 255 In the central belt 256 Southwest quarter of sec. 22, T. 40 N., R. 30 W 256 Iron Hill 2.56 In the southern belt 260 Southeast side of Lake Antoine 260 Southeast quarter of sec. 32, and southwest quarter of sec. 33, T. 40 N., R. 30 W 261 North and northeast of Quinnssec 263 Northwest quarter of sec. 9, T. .39 N. , R. 29 W 267 Sees. 12 and 13, T. 39 N., R. 29 W 271 Negaunee formation 273 Distribution 274 Lithology 275 Relations to adjacent formations 277 Conclusions from foregoing study 277 Section 2. Upper Menominee series 280 Character and occurrence 280 Component formations 280 Separation from the overlying sandstones and the underlying Lower Menomi- nee series 280 Distribution 283 Folding 284 Vulcan formation 285 Distribution 285 Topography 291 Subdivision into members 291 Traders member 291 Distribution 291 Lithology 294 Slates 295 Macroscopical 295 Microscopical and chemical 297 ' Conglomerates and quartzites 300 Macroscopical 300 Microscopical 302 Jaspilites 307 Macroscopical 307 Microscopical _ 313 Brier slate 320 Distribution 320 Lithology 324 Macroscopical 324 Microscopical and chemical 326 CONTENTS. 9 Chapter V. — The Algonkian System — Continued. Page. Section 2. Ujjper Menominee series — Continued. Vulcan formation — Contiimed. Subdivision into members — Continued. Curry member 331 Distribution 331 Lithology 333 Macroscopical 333 Microscopical 339 Eelations between the members of the Vulcan formation 349 Genesis 352 Folding 356 Folds of lower orders 356 Folds of higher orders 356 Secondary structures resulting from folding 358 Thickness 359 Relations between the Vulcan and adjacent formations 36] Explanation of the distribution and relations of the Vulcan and Hanbury forma- tions to the underlying formations 37O The ores 373 Lithology 373 Physical characters 373 Chemical composition 378 Mineralogical composition 384 Mineral constituents of the ores 384 Minerals associated with the ores 386 Quartz, dolomite, and calcite 386 Pyrite and chalcopyrite 388 Other minerals in joint cracks 388 Serpentine 389 Talc 390 Efflorescence on ores 390 The ore deposits 399 Distribution and shapes 399 Development of the deposits 395 Topographic relations of the deposits 401 Time and depth of concentration 403 Illustrations of deposits, including geology of the important mines 404 Loretto-Appleton deposit 4O4 Traders-Forest belt 407 Traders mine 407 Cornell mine 410 Cuff mine 410 Indiana mine 4j 1 Forest mine 412 The southern belt 413 Chapin-Pewabic deposits 414 Walpole mine 414 Pewabic mine 418 Chapin mine 420 10 CONTENTS. Chapter V. — The Algonkian System — Continued. Page. Section 2. Upper Menominee series — Continued. The ores — Continued. The ore deposits — Continued. Illustrations of deposits, etc. — Continued. The southern belt — Continued. Old Keel Ridge mine 423 Keel Ridge mine 424 Quinnesec, Cundy, and Vivian mines 425 Norway and Cyclops mines 428 Aragon mine 432 Mines east of Aragon mine 434 Brier Hill and Curry mines 434 West Vulcan mine 439 Central Vulcan area 449 East Vulcan mine 450 Verona mine 453 Emmett and Breen mines 453 Summary 455 Other localities of the Vulcan formation 456 Sees. 25 and 26, T. 40 N., R. 31 W 456 Sec. 33, T. 40N., R. 30 W 457 Sees. 1 and 2, T. 39 N., R. 30 W 458 Sec. 6, T. 39 N., R. 29 W 459 Sees. 12andl3, T. 39N., R. 29 W 461 Hanbury slate 462 Distribution and topography 462 Lithology 462 Clay slates 463 Gray wackes and quartzites 464 Calcareous slates and dolomites 466 Cherts and ferruginous oxides 467 Igneous rocks and their contact deposits 468 Folding and secondary structures 469 Thickness 470 Relations to Paleozoic beds 471 Interesting localities 471 Typical localities 471 Sec. 13, T. 40 N., R. 31 W 471 Hanbury Hill 472 Sturgeon Mills 475 Sees. 29 and 30, T. 39 N, R. 28 W 476 Localities at which cherts occur 476 Sec. 15, T. 40N., R. 30 W 476 Sec. 11, T. 39 N., R. 30 W 477 Southeast quarter of sec. 7 and southwest quarter of sec. 8, T. 39 N.,R. 29W 478 Iron Hill, southeast quarter of sec. 32, T. 40 N., R. 29 AV 481 Sec. 17, T. 39 N., R. 29 W 483 CONTENTS. 11 Chapter V. — The Algonkian System — Continued. Page. Section 2. Upper Menominee series — Continued. Hanbury slate — Continued. Interesting localities — Continued. Localities at which cherts occur — Continued. Northwestquarterof sec. 26, T. 39N., R. 29 W 484 Sec. 21, T. 39N., R. 28 W 484 Sec. 19, T. 39 N., R. 28 W 484 Possible iron-ore deposits 486 Chapter VI. — The Paleozoic Sy.stem 489 Section 1 . Lake Superior sandstone 489 Character and relations 489 Age 493 Section 2. Hermansville limestone 494 Chapter VII. — Outline op Geological History 495 Resum^ of formations 495 Succession of events 495 Archean 495 Lower Menominee deposition 496 Inter-Menominee unconformity 497 Upper Menominee deposition 497 Folding and metamorphism 498 Post-Huronian unconformity 499 Paleozoic deposition 500 Post- Paleozoic history 500 Correlation with other iron-bearing districts of the Lake Superior region 501 Index 503 ILLUSTRATIONS. Page. Plate I. Geological map of part of the Lake Superior region, showing relative position of the Menominee with respect to other Huronian areas 33 II. General outline map of the region between the Menominee River and the north side of the Felch Mountain district, showing position of the Menominee trough with respect to other Huronian troughs to the north 34 III. Portion of the geological map of the ilenominee iron region, by T. B. Brooks, 1872 .. 60 IV. Portion of the geological map of the Menominee iron region, Ijy T. B. Brooks and C. E. Wright, 1879 66 V. Outline geological map of the Menominee iron region, by R. D. Irving, 1890 9-1 VI. Organic markings in the Lake Superior iron ores. From a photograph. After W. S. Gresley 118 VII. Organic markings in the Lake Suiierior iron ores. From a photograph. After W. S. Gresley 120 VIII. Topographical map of the Menominee iron district In pocket. IX. Geological map and sections of the Menominee iron district In pocket. X. A, View from brink of Sturgeon Falls, looking down stream; B, Menominee River above Sturgeon Falls 132 XI. A, Horserace Rapids during high water; £, Log jam in Horserace Rapids 158 XII. A, Basin below Vpper Twin Falls; B, Barrier rock at Upper Twin Falls 160 XIII. A, Brecciated band of fine-grained greenstone at Upper Twin Falls; B, Ridge of dolomite, south of Lake Antoine 162 XIV. Sketch map of exposures near the contact between the Sturgeon quartzite and the Archean complex, in T. 40 N., R. 30 W., and T. 41 N., R. 29 W 186 XV. .1, View of dolomite bluffs on north side of highway between Quinnesec and Norway; B, Nearer view of bluff shown in .1 208 XVI. A, Brecciated Rand villa dolomite on wall at east end of gorge, east of Aragon mine; B, Dolomite conglomerate at Iron Hill 218 XVII. A, Fold in chert at Iron Hill; B, Folds in Traders jaspilite, we.st side of Clifford pit. Traders mine 234 XVIII. Maps showing magnetic observations over strips of country bordering areas of Quinnesec schists and Randville dolomite 286 XIX. Photomicrographs of rocks of the Vulcan formation 316 XX. Folds in jaspilites in the Vulcan formation 356 XXI. A, Brecciated Brier slates in Norway pit; B, Band of brecciated Brier slate crossing definitely bedded slates transversely to their bedding, in Norway pit 364 XXII. A, Surface of ore breccia near contact of the Traders member with the Brier slates, No. 3 pit, Curry mine; B, Concentrating works at Pewabic pit 368 XXIII. Hypothetical geological map of the Loretto and Appleton areas , 404 XXIV. Geological map of Traders and Cornell mines and vicinity 406 13 . 14 ILLUSTRATIONS. Page. Plate XXV. Geological map of the country adjacent to the Cuff and Indiana mines 410 XXVI. ,1, View east from D shaft, Chapin mine, showing shafts on Walpole-Chapin fold; B, View west from A shaft, Chapin mine, showing distribution of shafts on Chapin property 416 XXVII. Outlines of ore bodies on fifth, sixth, seventh, and tenth levels, Chapin mine, showing dip and pitch of ore bodies 420 XXVIII. Geological map of the Chapin-Pewabic folds 422 XXIX. Geological map and section, southeast quarter of sec. 32, T. 40 N., P. 30 W., and portion of adjacent sec. 33 424 XXX. Geological map and sections of the Quinnesec area 426 XXXI. Geological map of the Norway and Aragon folds 428 XXXII. Vertical cross sections through the Norway and Aragon folds, illustrating geo- logical structure i 430 XXXIII. Geological map of north half of sec. 9, T. 39 N., P. 29 W., including the Aragon West Vulcan, Prier Hill, and Curry mines 434 XXXIV. Geological map of Central Vulcan and portion of West Vulcan areas 438 XXXV. Geological majj of East Vulcan and Verona areas 450 XXXVI. Geological map of the Emmett and Breen mines and vicinity, Waucedah 4.52 XXXVII. Geological map of portions of sees. 25, 26, 35, and 36, T. 40 N., P. 31 W 454 XXXVIII. Geological map of soiith half of sec. 33, T. 40 N., P. 30 W 456 XXXIX. Geological map of portions of sees. 1 and 2, T. 39 N., P. 30 W., and sees. 35 and 36, T. 40 N., P. 30 W 458 XL. Geological map of southeast portion of sec. 6, T. 39 N. , P. 29 W 460 XLI. Geological map of portions of sees. 12 and 13, T. 39 N., P. 29 W 462 XLII. Map of exposures at Iron Hill in sec. 32 and neighboring portion of sec. 33, T. 40 N.,P. 29W 480 XLIII. View of unconformity between the Traders jaspilites and the Lake Superior sandstone, west side Quinnesec open pit 492 Jig. 1. Geological section across the Menominee district from Little Bekuenesec Falls north- ward. After Foster and Whitney, 1851 46 2. Portion of the geological map of the Lake Superior land district in the State of Michigan. After Foster and Whitney, 1851 48 3. Geological section along the Falls of the Sturgeon Piver. After H. Credner, 1869 51 4. Geological section across the Menominee district. After H. Credner, 1869 '.. 52 5. Portion of geological map of the Upper Peninsula of Michigan. After H. Credner, 1869 54 6. Geological section through Sturgeon Falls. After T. B. Brooks, 1873 58 7. Geological section through Lake Antoine. After T. B. Brooks, 1873 59 8. Geological section through Quinnesec. After T. B. Brooks, 1880 68 9. Geological section through Sturgeon River. After T. B. Brooks, 1880 68 10. Structure section across the Menominee region through the west end of Lake Fumee. After T. B. Brooks, 1880 69 11. Structure section across the Menominee district in the vicinity of Twin Falls. After T. B. Brooks, 1880 69 12. Hypothetical section through the Menominee region in the vicinity of Quinnesec Val- ley. After P. D. Irving, 1890 93 13. Sketch map of exposures in north half of sec. 32, T. 41 N., P. 29 W., showing relation between conglomerates and gneisses 192 ILLUSTRATIONS. 15 Page. Fio. 14. Sketch map of exposures on Black Creek, showing relation of conglomerates to gneisses 194 15. Sketch map of exposures at and near the Falls of the Sturgeon River 195 16. Cross section through pits and shafts near the center of sec. 25, T. 40 N., R. 31 W 204 17. Sketch plan of Cuff mine and vicinity 236 18. Vertical north-south cross section of the Norway and Aragon mines 242 19. Longitudinal section through the Norway pit 243 20. Horizontal section of the Aragon mine at the fifth level 243 21. Horizontal section of the Aragon mine at the sixth level 244 22. Sketch map of exposures near Sturgeon Falls, sec. 26, T. 39 N. , R. 29 W 288 23. Sketch of contortions in jasper bands, west side of Clifford pit, illustrating production of breccias 302 24. Diagrammatic sketch illustrating folding in the iron formation on east side of pit at old Keel Ridge mine 357 25. Sketch illustrating puckering in jaspilite on stripped surface, west end of Clifford pit. Traders mine, 1899 358 26. Sketch illustrating folding in Brier slates on wall of trench from No. 2 pit, "West Vul- can mine 358 27. Sketch illustrating puckering in Brier slates, west side of cut at Curry shaft No. 1 358 28. Calcite crystal in ore of ^V'est Vulcan mine 387 29. Calcite crystal in ore of West Vulcan mine 387 30. Vertical north-south cross section of the Loretto mir e 405 31. Horizontal section of the Loretto mine at the first level 405 32. Vertical east-west longitudinal section of the Loretto mine 406 33. Sketch map of exposures in south half of sec. 25, T. 40 N., R. 30 W 413 34. Horizontal section of the Walpole mine at the third level 417 35. Horizontal section of the Pewabic mine at the third level 418 36. Vertical section through No. 1 shaft along north-south crosscut, first level, Pewabic mine 419 37. Vertical north-south cross section through shaft D, Chapin mine 421 38. Vertical north-south cross section through No. 2 and C^ shafts, Chapin mine 422 39. Horizontal section of the Aragon mine at the first level 432 40. Horizontal section of the Aragon mine at the eighth level 433 41. Vertical east-west longitudinal section of the Aragon mine, north fold 434 42. Section across Vulcan formation about 600 feet west of Brier Hill mine 435 43. Plan of No. 3 pit, Curry mine, and section along its north end 438 44. Horizontal section of the West Vulcan mine at the eighth level 441 45. Vertical north-south cross section through No. 2 shaft. West Vulcan mine 442 46. Vertical north-south cross section through Burnt shaft. West Vulcan mine 443 47. Horizontal section of the West V^ulcan mine at the twelfth level 444 48. Horizontal section of the AVest Vulcan mine at the thirteenth level 445 49. Vertical north-south cross section through the V/est Vulcan mine 2.50 feet east of the Burnt shaft 446 50. Horizontal section of the West Vulcan mine at the fifteenth level , 447 51. Horizontal section of the East Vulcan mine at the eighth level. No. 4 shaft 450 52. Vertical north-south cross section through shaft No. 3, East Vulcan mine 451 53. Vertical north-south cross section through shaft No. 4, East Vulcan mine 452 54. Sketch map of explorations at Turner's location, sec. 19, T. 39 N. , R. 28 W 485 LETTER OF TRAI^SMITTAL. Department of the Interior, United States Geological Survey, , Madison, Wis., April 20, 1903. Sir: I have the honor to transmit herewith the manuscript of a monograph on the Menominee Iron-bearing District of Michigan, by William Shirley Bayley. This monograph is the sixth and last one of a series treating of the iron-bearing districts of the Lake Superior region. Monographs on the Penokee-Gogebic, Marquette, Crystal Falls, Mesabi, and Vermilion districts have already been published. (See PI. I.) The first monograph on the Lake Superior region to be published by the United States Geological Survey was that on the copper-bearing rocks, by Prof R. D. Irving. The completion of the original plans of the old Lake Superior Division will be marked by the publication of a closing- monograph on the general geology of the Lake Superior region. Very respectfully, C. R. A^AN HiSE, Geologist in Charge. Hon. Charles D. AValcott, Director of United States Geological Survey. MON XLVI — 04 -2 17 OUTLINE OF MONOGRAPH. Chapter I. The Menominee district is situated on the Michigan side of the Menominee River. It occupies an area of 112 square miles, lying principallj' in townships 39 and -±0 north and ranges 28, 29, 30, and 31 west. It consists of a narrow tongue, widening to the west into the broad expanse of the Crj^stal Falls district, and merging to the northwest into the southwestern end of another ore- bearing district 'known as the Calumet area. The importance of the Menominee district as an ore producer ma}' be inferred from the fact that since the first regular shipments of ore were made in 1S77 the total quantity of ore raised from its mines has aggregated about 29,000,000 tons, nearly all of which was of Bessemer grade. The gross product in 1902 was over 3,000,000 tons. The rocks of the district belong to the Archean, the Algonkian, and the Paleozoic systems. The iron-bearing beds are Algonkian. These are bounded on the north by a complex of gneisses and schists and on the south by a series composed mainly of greenstone-schists cut by dikes of granite, porphyry, gabbro, and diabase. The Algonkian rocks are divided into a lower and an upper series, distinguished as the Lower Menominee and the Upper Menominee, separated by an unconformity. These correspond to the Lower Marquette and the Upper Marquette series in the Marquette district and to the Lower Huronian and the Upper Huronian on the north shore of Lake Huron. The Paleozoic rocks are represented by the Lake Superior sandstone and an Ordovician limestone. Chapter II. An abstract of the literature devoted to the discussion of the geology of the district is given in this chapter. It begins with a reference to a report by George N. Sanders, printed in 1845, and ends with a reference to a general article on the iron-ore deposits of the Lake Superior region by C. R. Van Hise, which was issued in 1901. Chapter III. This chapter treats of the physiographj^ of the district. The topography is simple. It consists essentially of two longitudinal ridges, with eleva- tions of about 1,500 feet, separated by valleys, the floors of which are about 1,000 feet above the sea. Both the tops of the ridges and the floors of the valleys slope gradually to the southeast, representing, it is believed, two plains. The ridges are thus remnants of a higher plain that once occupied the entire area under discussion. The plan of the topography corresponds closely with the geological structure of the district. The residuals of the high plain are composed of hard dolomites, while the vallevs are carved in soft slates. 19 20 OUTLINE OF MONOGRAPH. The drainage is mainly lonoitudinal. The main di-ainage course is the Menom- inee River. The smaller streams are branches of this. All the streams possess the characteristic features of antecedent streams. Their courses are arranged without regard to the geology. It is evident that the present topography could not have been produced by the present drainage. It not only antedates the Glacial epoch, at the close of which the present drainage was inaugurated, but it even antedates in great measure the latest epoch of the Cambrian period, during which the Lake Superior sandstone was deposited. During this time the entire district was under water and the sand deposit covered all the hills as well as filled all the valleys that had been made prior to this period. Later, the land was raised above the water surface and erosion swept away the sandstone, except that on the tops of the hills, and the old topography was again brought to view. The present topography of the district is therefore similar to that which existed prior to Upper Cambrian time. Chaptek IV. The Archean crystallines bordering the Algonkian tongue are greenstone-schists on the south and a complex of gneisses, granites, and various schists on the north. The southern schists — the Quinnesec schists — occur in two areas. A southern area lies along the Menominee River and stretches southward into Wisconsin. A western area constitutes a wedge entering the Huronian Ijeds from the west and extending for 6 or 7 miles along the middle line of the Menom- inee tongue. The Quinnesec schists of the southern area are coarse- and fine-grained basic rocks, characterized by a schistose structure of varying degrees of perfection. The coarser phases were originallj- gabbros, diabases, and diorites; the finer phases were basalts, diabases, and basic tuffs. Associated with these are chlorite-schists, amphibolites, gneisses, schistose porphyries, schistose felsites, and sericite-schists. The acid schists, except perhaps the sericite-schists, are apophyses from a great boss of granite which is intruded in the basic rocks south of the Menominee River. The basic schists are cut by dikes of various basic rocks and bj- granites. In the western area the rocks are more massive. They are dense, grayish green in color, and uniform in their features. Some of them are ellipsoidal. All are apparently fine-grained basic lavas that have suffered extreme alteration. Most of the rocks are schistose, some slightl^y so but others markedly so. Their schistosity, as well as that of the southern schists, is ascribed to pressure. The northern complex of gneisses and schists is of the usual character of Archean complexes. Banded gneisses and gneissoid granites, hornblende-schists, greenstone-schists, and a few mica-schists are intruded by dikes of diabase and by veins and dikes of granite. The gneissoid granites are of a pink and a graj' variety, of which the former appears to partake largely of the nature of pegmatite. It intrudes the gray variety in irregular stringers and in series of narrow parallel veins. When in the form last named, the two rocks together give rise to banded gneisses. A few localities are described at which the Quinnesec schists and the northern complex can be seen in good exposures. Chapter V. The Algonkian rocks comprising the Menominee tongue are almost exclusively sedimentary, and are mainly mechanical sediments. They are separated OUTLINE OF MONOGRAPH. 21 from the underlyiug granites and gneisses of the northern complex by conglomerates composed largely of the debris of the underlying rocks. Their relations with the Quinnesec schists are not known, since the two series are not in contact. It is believed, however, that the sedimentary series is much younger than the schist series, because of the lithological analogies existing between the two series and corresponding series in the Marquette district. Moreover, within the Algonkian series there is an unconformity which is revealed by the presence of a coarse quartzite containing pebbles of jasper, which must have been furnished by beds under the quartzite. No such l)eds in this strati- graphic position are now known to exist in the district, and hence it is assumed that thej' have been removed by erosion, and that a portion of their debris is now incor- porated in the quartzites. This unconformity corresponds with that Ijetween the Upper Marquette and the Lower Marquette in the Marquette district, and between the Upper and Lower Huronian in the Crystal Falls area. In this district the two series are called the Lower Menominee and the Upper Menominee. Section 1. The Lower Menominee series is sul)divided from the base upward into three formations— the Sturgeon quartzite, the Randville dolomite, and the Negaunee formation (iron bearing). The Sturgeon quartzite is found only along the northern side of the Menominee trough, where it forms a southward-dipping monocline bordering the south side of the Archean complex, and separated from it by an unconformity. Its topography is rugged. At its base the formation consists of conglomerates composed largely of the debris of granite and gneiss. These grade upward into quartzites through arkoses and graywackes. The conglomerates, the arkoses, and the graywackes are nearly always schistose, but the quartzites are practically alwaj^s massive. This difference in structure is explained as due to the fact that the conglomerates, arkoses, and graywackes are nearer the contact plane with the underlying Archean than the quartzites, and hence were nearer the zone of accom- modation in which movement occurred during the folding of the district. The quartzites are principally white vitreous or saccharoidal varieties, composed of plainly fragmental quartz grains that often are enlarged by the addition of quartz on their peripheries. A few schistose quartzites differ from the predominant massive varieties in containing much sericite. At the top of the formation the quartzites pass into dolomitic quartzites, and these in turn grade into quartzose dolomites at the base of the Randville dolomite. The major folding of the quartzite is simple. Within the formation a few divergencies of sti'ike and dip are noted, l)ut in the main tlie beds are nearly vertical. They constitute one limb of a synclinorium, the other limb of which should appear adjacent to the southern area of Quinnesec schists, and also around the western area. Its absence from these positions is supposed to be the result of the erosion which intervened between Lower Menominee and Upper Menominee time. At the western end of the district the quartzite belt turns northward around the end of the Archean 22 OUTLINE OF MONOGRAPH. anticline, separating the Menominee from the Calumet tongue. At this turn it is folded into a number of synclines and anticlines pitching west. The thickness of the formation is estimated to be between 1,000 and 1,250 feet. The most interesting occurrences of the quartzite are to be found at the rock dam on Pine Creek and at the falls of Sturgeon River. The Randville dolomite is identical with a similar dolomite series in the Felch Mountain and the Crystal Falls districts. It occupies three belts, called, respec- tively, the northern, the central, and the southern. The northern belt lies immedi- ately south of the Sturgeon quartzite, in the valley of Pine Creek. Few exposures have been seen, as the area underlain by the belt is covered with the sands distributed by the stream. The central belt is narrow, it occupies the axis of the trough extending from a point north of Lake Antoine eastward to the bluff known as Iron Hill, in sec. 32, T. 40 N., K. 29 W. The southern and most important belt stretches from a point near the Menominee River, west of Iron jNIountain, eastward to the end of the district, where it is lost under a covering of Paleozoic sediments. The most important mines of the district are just south of its southern border. On account of its resistant nature the dolomite in the southern belt, and, to a less extent, that in the central belt, gave rise to the elevations which stretch through the district in the form of the two ridges already mentioned, and which are explained as residuals of a plain once existing over the entire Menominee area. The dolomite formation comprises an interbedded series of dolomites, quartzose dolomites, dolomitic quartzites, dolomitic slates, cherty quartzites, and talcose schists. The dolomites predominate. At the base of the series they are more or less richly quartzose. The cherty quartzites are fine-grained cherty rocks that are usually brec- ciated. In color thej' varj' somewhat, but white and red shades are most prevalent. These rocks occur in but a few places, but always above the dolomites. Their absence from much of the region is accounted for by the erosion which removed them from over the most exposed portions of the surface during the interval between the Upper and Lower Menominee epochs. The slates are light-colored talcose and sericitic varieties. They are not very prominent. Occasionally they are found well down in the series, but usually they are limited to its upper portions, where they are in contact with closely similar slates belonging in the Upper Menominee series. The talcose schists have been observed only at the upper contacts of the Randville forma- tion, where the purer dolomites are immediately beneath the basal layers of the Upper Menominee rocks, and more laarticularly in places where severe folding has taken place. They are soft, dark, much-jointed rocks, composed largely of serpen- tine and talc. They were formed, in all probability, in consequence of the fact that the dolomites were in a zone of movement where the conditions were favorable to active chemical processes. In many places the dolomites were crushed into breccias, and at one place. Iron Hill, a well-defined dolomitic conglomerate occurs. The cherts of the Randville dolomite are identical in character with those in the Gogebic and Mai-quette districts and have the same stratigraphic position as these. In all three districts they are supposed to be of organic origin. OUTLINE OF MONOGRAPH. 23 The folding of the Randville dolomite "is the key to the knowledge of the folding of the entire series of Algonkian rocks in the district." The formation occui's as a monocline in the northern belt and as anticlines in the central and southern belts. The northern belt follows closely the distribution and the folding of the Sturgeon quartzite. The central belt is the top of an anticline which is connected with the northern and the southern belts by synclines. It is terminated at both ends by plunging beneath the overlying beds. At its east end the eastward plunging anticline is plicated into several minor folds. Thus the central belt is aflected by a broad anticline with a north-south axis, as well as by a narrow one with an axis trending a little north of west. The west end of the southern belt must likewise end in a plunguig anticline, as slates of the Hanbury formation are known to occur a few miles west of the Menominee River on the strike of its trend. Its east end disappears under Paleozoic beds. The south side of the syncline, which must exist to the south of this belt if its structure is anticlinal, would be exjjected on the north side of the southern Quinnesec schists. Its absence from this position and from the border of the western area is explained in the same way as is the absence of the Sturgeon quartzite from these stretches of country. The dolomite in all three belts is closely plicated by folds of high oi'ders. Those of the second order are important from an economic point oj: view, because they determined the positions of the great ore deposits. These folds express themselves in the interiors of the dolomite areas by causing variations in the strikes and dips of neighboring beds. On the margins of the areas they are exhibited as indentations in the boundaries of the belts. These are best seen along the borders of the southern belt and more particularly on its southern side. Beginning at the west, the most important of these marginal folds have been called the Walpole, the Pewabic, the Quinnesec, the Norway, the Aragon, and the West Vulcan folds, because within them are situated the great mines of the same names. There are other less important folds along this southern border, and in addition there are known to be several important ones on its northern border. Exposures are rare, however, on the north side of the southern belt, and there is therefore much difficulty in recognizing the folding. Each of the folds is described and the reasons for regarding them as folds are given in detail. In the western portion of the area all the marginal folds pitch west; at the east end of the district they pitch east, thus confirming the view that the district as a whole is affected by a broad cross anticline as well as by a more cotnpressed longitudinal syncline. The thickness of the formation is probably somewhere between 1,000 and 1,500 feet. The dolomite series is nowhere seen in actual contact with the underlying Stur- geon quartzite, but the gradation observed at the top of this formation, together with the gradation of the purer dolomites into quartzose phases at the base of the dolomite formation, indicates that the two pass into one another through dolomitic quartzites. Above, the dolomite may have passed into the Negaunee formation through the 24 OUTLINE OF MONOGRAPH. cherty quartzites, Init since no i-ocks belonging to the Negaunee formation remain in the Menominee district the exact nature of the transition is not known. In most places the uj^per contact of the RandviUe dolomite is with the members of the Upper Menominee series. When the overlying formation is the basal member of the series — that is, when the contact is with the Vulcan formation — the transition between the two is sudden. Thei'e is no recognizable structural unconformit}' between them, but at the base of the upper series there is usuall}- a conglomerate or coarse quartzite containing pebbles of chert and jasper that must have been derived from some formation beneath. Their existence is regarded as proof that there was once above the dolomite a jaspilite formation, like the Negaunee formation in the Marquette district. Thus, it is believed, there was an erosion interval preceding the deposition of the Vulcan i-ocks and at some time during the period when erosive agents were at work the RandviUe dolomite formed a land surface. In many jjlaces the dolomite is in contact with the Hanbury slate, which normallj' lies above the Vulcan beds. This is the case at one place on the southern side of the southern belt and very generally along its northern side. It is also the case at the east end of the central belt, and possibly along its northern side. The absence of the Vulcan beds from those places at which it would noruiallj' be expected to occur is explained as the result of overlap along a sinking shore. At a number of places along the contact, especially where the contact is between the dolomite and the Vulcan beds, the rocks on both sides of the contact line are severely brecciated. Both the underh'ing and the overlying beds are shattered and the line between them is often completelj' obliterated. The Negaunee formation is represented in the district only by the pebbles in the quartzite at the base of the Upper Menominee series. There are a few jaspilites near the Curry mine, however, that are slightly different from most of the corresponding rocks in the Vulcan beds. Since their jasper layers are identical in character with the jasper pebbles in the quartzite, these jaspilites are described as affording a fair idea of the nature of the Negaunee beds that formerly must have existed in the district. Section 2. The Upper Menominee series comprises all the beds between the top of the RandviUe dolomite and the bottom of the Lake Superior sandstone. It includes two formations — the Vulcan formation (iron bearing) and the Hanbury slate. The reason for the separation of this series from the Lower Menominee series is the presence of a stratigraphical lireak between the RandviUe dolomite and the bottom of the Vulcan formation. The rQcks of the Upper series occupy the synclinal areas between the anticlines of dolomite and those between the dolomite and the two areas of Quinnesec schists. From the distribution of the series it is clear that it must occur in three sj'nclines and two anticlines with east-west axes and the same number of similar folds with north-south axes. Since the Vulcan formation occurs immediately above the dolomite, it should surround the dolomite areas in a continuous belt under normal conditions. At many OUTLINE OF MONOGRAPH. 25 places, howevei', the rocks of the Vulcan formation are hacking- in this situation, and the Hanbury slate occupies the position thej' would naturall}- be expected to occupj'. Wherever found, liowever, the Vulcan beds always lie between the dolomite and the slate. The lack of continuitj' of the Vulcan belt is ascribed to overlap of the Hanbury slate. Lithologically the Vulcan formation is separable into three members, which are, in ascending order, the ore-bearing Traders member, the Brier slate, and the ore- bearing Curi'v member. The first comprises slates, conglomerates, quartzites, and jaspilites; the second is composed exclusively of slate, and the third consists of jaspilites and slates. Ore deposits occur in both the Traders and the Currj- beds. The Traders member is not as widelv distributed as the other two members of the formation. Where one member is absent, it is the Traders member. Although this is not as continuous as the other members, nearly all the large mines of the district obtain their ores from its deposits. The slates of the Traders member are always found in its liasal portions, where they are associated with quartzites and conglomerates. These are usually light- colored phases that are with great difficult}'' distinguishable from some of the talcose slates at the top of the Randville dolomite. In a few places the slates are black, heav}^ varieties that are merely verj' quartzose fragmental ores. The light-colored slates grade into the quartzites and conglomerates. The latter rocks contain abundant jasper and ore pebbles. Where these constitute the main portion of the deposits the rocks pass into jaspilites, which are banded rocks, composed of alternating lavers of I'ed jasper and black ore. When the structure of the jaspilites is fairly coarse, the small grains of jasper and ore composing them can be distinguished on their bedding surface as small oval areas, producing a distinct mottling. Where shearing has taken place the ore bands have been rendered schistose, or micaceous, producing specular ores, and the jasper bands have been mashed so that the tinj' grains of jasper have assumed lenticular shapes. In some places brecciation has occurred, and the rock is now a mass of jasper fragments in an ore matrix. Much of the material in the Traders jaspilites is thus of fragmental origin. In addition to the fragmental material in them, however, there is also much crystallized quartz and a good deal of newly deposited hematite. As the grain becomes finer the fragmental structure of both jasper and ore disappears, the quantity of secondarily deposited quartz and hematite increases, and the jaspilites become more like the typical jaspi- lites of the Marquette area, which were formed mainly by the decomposition of a cherty, ferruginous carbonate. Under the microscope a few nodular masses of jasper and ore are observed in thin sections of the Traders rocks, and these are thought to lie pseudomorphs of siderite or greenalite concretions. Their presence is evidence that some of the silica and hematite in the Menominee jaspilites was derived from an iron carbonate by metasomatic replacements. In all cases the ore bands differ from the jasper bands mainly in the greater abundance of their ferruginous component. 26 OUTLINE OF MONOGRAPH. The Brier slate is an even-banded, heavy, blacJs slate, occupying a belt of country adjacent to the Traders jaspilites. The rock consists of quartz grains and hematite crystals, embedded in a matrix composed of quartz, decomposed feldspar, kaolin, and a little chlorite. Here and there are a few large plates of brown biotite and white muscovite. Some specimens contain a great deal of dolomite. The Brier slate grades into the jaspilites of the Curry member through increase in the quantity of crystallized silica in the matrix and decrease in the amount of fragmental quartz present. The Curry member is probably more widely distributed than either the Traders or the Brier member. It is found in all places where any portion of the Vulcan beds have been discovered. Lithologically the member is an even-bedded series of jaspilites and quartzose slates, besides ore deposits. Of the jaspilites two varieties are recognized. In one the jasper is dark red or purple and very fine textured and'the ore a dense black hematite. These are very like the Traders jaspilites. In the other variety the jasper may be dark red, pinkish, or white. Both the jasper and the ore are sandy textured and look as though made up largely' of loosely cohering grains. When examined microscopicall}' the sandj' jaspers are found to contain manj' oval and round masses of cherty quartz, surrounded by narrow zones of hematite and embedded in a finely crystalline aggregate of quartz. In the ore layers the zones of hematite around the chert nuclei are very broad, and the interstitial quartz is small in quantity. The quartzose slates differ from the jaspilites in being more homogeneous. The}* con- sist of a series of very thin alternating siliceous and ore layers, so that there is little or no distinction between ore and jasper bands. These rocks are found at the base of the Currj' member, and are in a way gradation phases between the Curry jaspi- lites and the Brier slates. The oval masses in the sandy phases of the Curr}^ jaspilites are much more numerous than thej" are in the Traders jaspilites. As in the case of the Traders rocks, they are believed to be pseudomorphs of siderite or greenalite concretions. They are similar in all respects to the concretions that have been described in the Gogebic and Marquette jaspers and in the Mesabi and Gunflint Lake cherts. In the vicinitj' of the Curry mine all the rocks of the Curry member are cut by veins of red crystalline dolomite, and the ores are saturated with the same material to such an extent that their siliceous component has entirelj- disappeared, and in its place is a matrix of dolomite. Where no marked disturbances in their relations exist the members of the Vulcan formation grade into each other by transition forms. Where, on the contrary', the members are closely folded the contact between the Traders and the Brier members is often sharp, and the rocks on both sides of the contact line are severely brecciated. This is true at the Norway mine and in the Curry location. Subsequently hematite was deposited in this crushed zone, producing marketable ores. The Vulcan formation is a succession of beds laid down in water. The lower beds are largely fragmental, although intermingled with the fragmental material OUTLINE OF MONOGRAPH. 27 there must have been some cherty, ferruginous material that had been precipitated chemically. In some places the mechanical sediments were in great excess. In other places the chemical sediments appear to have predominated. In the course of time the latter were changed to crystalline quartz and hematite through the agency of descending meteoric waters, and the mass was enriched by deposits of hematite between the original grains. After the Traders beds had been laid down to a thickness of several hundred feet in some places the conditions of deposition changed. The cherty material ceased to be precipitated, and the Brier slates were laid down. At the end of Brier time the conditions that prevailed at the end of Traders time recurred, and chemical sediments were again precipitated. In this period they were less contaminated with fragmental material. The abundance of concretionary ore in the Curry beds shows that some of these must have consisted almost exclusively of the chemical precipitate. Jaspilites were produced from the carbonate and the greenalite in the same manner as in the Traders beds, and some of these, after enrichment, became ore Ijodies. The major folding of the Vulcan beds follows closely the folding of the subjacent dolomite. Within the formation, however, the beds are crumpled and crinkled into small folds, and upon these are superposed still smaller flutings. Wherever folding is observed it is best preserved in the jasper bands. The ore layers between these were sheared and made schistose. Where the folding was very severe both ore and siliceous layers developed a slaty cleavage. In some places the jasper was fractured and a breccia of jasper fragments in a micaceous ore matrix resulted. The total thickness of the Vulcan formation averages about 650 feet, divided as follows: Traders member, 150 feet; Brier slate, 330 feet; Curry member, 170 feet. The Brier and Curry members maintain an almost uniform thickness in all portions of the district where they have been encountered. The Traders member, however, varies widely in thickness, as would be expected of a series of beds deposited against a shore. The relations of the Vulcan beds to the underlying Randville dolomite are those of a younger series to an older series, where the two are not separated by a structural unconformity. That the Vulcan beds were laid down on a shore composed partly of the dolomite is shown by the presence of dolomite bowlders within the iron formation on the seventh and eighth levels of the Chapin mine. Their relations with the over- lying Hanbury slates are those of complete conformity. In a few places the Hanbury slate is against the dolomite, the Vulcan beds being nowhere present in the vicinity. This is true east of Quinnesec and at the east end of the central dolomite belt. It is also believed to be true at a number of other places where the slate and the dolomite series have not been seen in actual contact or in exposures very close to one another. Faulting of the slate beds over the iron-bearing beds will not explain the phenomenon, because faulting is of minor importance in the district. The only explanation that suggests itself to account for 28 OUTLINE OF MONOGRAPH. all the facts of distribution of the Vulcan and the Hanbury formations is that of unconformity between the Lower Menominee and the Upper Menominee sei'ies, with a gradual advance of the Upper Menominee sea, the deposits of which slowlj' overlapped the earlier deposits and gradualh" buried the higher lands composed of the Lower Menominee rocks. The Traders and the Curry ores are not very different. Practically all are of Bessemer grade, though some are highly siliceous and others contain but little silica. The former are especialh' rich portions of the jaspilites that have had their ferruginous component increased by processes of enrichment. These lean ores differ very little in appearance from the jaspilites, of which they are essentiallj^ a pail. They are banded, brecciated, and often specular. The brecciated ores may consist of jasper fragments in a mass of hematite, or of hematite fragments in a mass of dolomite, or they may be composed of fragments of ore. jasper, and slate in a mass consisting largely of slate debris that has been strongly ferruginized. The rich ores are usually liluish-black, porous, rine-grained aggregates of crys- tallized hematite, occurring in the troughs of pitching folds or in other situations toward which descending water is likely to be directed. Comparisons of analyses of all the ores of the district show them to consist principally of hematite, with addi- tional varying amounts of magnetite, silica, alumina, lime, magnesia, carbon dioxide, phosphorus pentoxide, and water. Most of the ores contain also manganese, potash, and soda, and a few of them titanium and carbon. The mininumi silica reported in the ores of 1900 is 2.75 per cent and the maximum 38.65 per cent. Twelve analyses of cargoes of typical ores are given and four complete anah'ses. The latter indicate that tlie richer ores are mixtures of hematite, magnetite, muscovite, serpentine, dolomite, apatite, pj'rite, quartz, and some manganese oxide. All the minerals occurring as constituents of the ores are found also as visible masses either in veins cutting the ore bodies or in vugs or pores within them. Dolomite, calcite, and pyrite sometimes exist in excellent crystals, and serpentine as large, white, almost pure masses. Talc also occurs in thick seams of almost ideal purity, and chalcop3'rite in small crystals associated with pyrite. The carbonates and sulphides are found near water coui'ses and the silicates mainly in the lower portions of the ore bodies. The ores when exposed to the action of the atmosphere become coated with a white elflorescence, consisting of a mixture of the sulphates of sodium, magnesium, and calcium, in which the ffrst named is gi'eatly in exces.s. The larger ore deposits all rest upon relatively impervious foundations, which are in such positions as to constitute jjitching troughs. Within this district such pitching troughs may be made by (1) the marginal folds in the Randville dolomite; {■2) the slate forming the bottom part of the Traders memlier; and (3) the Brier slate beneath the Curry l)eds. The dolomite is especially likely to furnish a suitable basin for the accumulation of ore bodies, where its upper member has been transformed into a talcose schist. OUTLINE OF MONOGRAPH. 29 Smaller ore bodies may occur at contacts between the different members of the Vulcan formation and at places within the iron-bearing- member where severe brecciation has occurred. The forms of the ore bodies var}' with their positions. While verj^ irregular in shape they nevertheless conform in a general way to the shape of the foundation on which they rest. The deposits in troughs have in general a U-shajied cross section, very thick at the bottom. Where much compressed, the arms of the U may unite at the center and produce a lens-shaped deposit. Contact deposits are usually broad and sheet like, with irregular pi-ojections extending from their upper surfaces. From the distribution, associations, and composition of the ores and the shapes of the ore deposits it is evident that the ores of the Menominee district, like those in the Gogebic and Marquette districts, were concentrated by descending waters flowing in definite channels. A portion of the iron oxide in the Traders member is of frag- mental origin, b^ing'the debris of an older jaspilite formation, and perhaps a portion of that in the Curry member had a similar origin. These ferruginous bodies, how- ever, were enriched by the addition of hematitic material from some overlying stx-atum, from which it was dissolved by meteoric waters and transported downward, finally being precipitated between the fragmental grains of the original sediments. The processes of concentration were the same as those worked out b\' Van Hise for the Gogebic and the Marquette districts. Oxj^genated meteoric waters descending through the rocks of the Hanbury and Vulcan formations dissolved iron carbonates and silicates and precipitated the metal as oxide in or near the position of the original compounds. Carbon dioxide was thus liberated and dissolved in the descending waters. These took up more iron salts. In their downward passage they were converged into trunk channels by plunging synclines, or were directed into definite courses by the contact planes between adjacent beds or by zones of brecciation. At these places the iron-bearing waters, which necessarilj' must have taken cii'cuitous routes, were intermingled with water which had descended more direct!}' from the surface, and which, therefore, had retained its oxygen, or most of it. Here the dissolved iron carbonate was decomposed and iron oxide precipitated. Thus are found pseudomoi'phs of hematite in place of original ferruginous concretions, and great deposits of ore in the troughs of sjmclines within the iron-bearing formation. Continued passage of water along the same channels purified the deposits by removing from them deleterious substances. Topographically the ores are usually found below the crests of hills, on their slopes or in valleys which once had below them lower valleys in which the descending- waters may have found an outlet. In the Menominee district, however, this relation between the position of ore bodies and the topography is not as clear as it is in the other Lake Superior iron districts. The beginning of the concentration of the ores must have been at the close of Upper Hui-onian time, that is, after the folding of the Huronian rocks. It was practically completed before the beginning of the Upper Cambrian. 30 OUTLINE OF MONOGRAPH. A critical study of the geological relations of the ore deposits of each of the mines in the district indicates that all deposits of any magnitude are situated in just such positions with respect to the surrounding rocks, as might have been prophesied, on the assumption that they were accumulated by the action of descending ground water. The Hanbury slate occupies nearlj- all the low ground within the Menominee trough. It occurs in three synclinal belts lying between the anticlines of dolomite and between the southern dolomite belt and the south area of Quinnesec schists. The slate belts widen toward the west because of the westward plunging of the entire synclinorium. The formation comprises clay slates, calcareous and graphitic slates, graywackes, thin beds of ferruginous dolomite, and small bodies of chert and hematite. The formation is cut by a few greenstones that are now great!}' decomposed. The clay slates are normal rocks. AVhen sheared and much weathered they become light-colored sericite-schists. Many specimens are stained red in irregular patches, producing a red and white mottled rock known locally as calico slate. The calcareous and graphitic slates appear to be limited to the lower horizons of the formation, the former being nearly always associated with ore bodies. The quartzites are very siliceous dolomites. The cherts and hematite are usually closely associated. The former are gray or white. The latter is a dense-black or dark-brown variety. Where the slates are cut bj^ greenstone they have suffered some contact metamorphism, with the production of a little biotite and actinolite. The folding of the formation is very complicated. Folds of high orders are common — practically universal. In the eastern portion of the area the pitch of these small folds is to the east and in its western portion to the west. The distribution of the folds and their varying pitches corresponds closely to the major folding of the district. No approximately correct estimate of the thickness of the slate formation is attempted, because of the difficultv of eliminating the effects of the close folding. It is safe to say, however, that the Hanburj' formation is the thickest of all the formations in the district. Like the other members of the Menominee series, the slates are unconformably beneath the Lake Superior sandstone. No workable ore deposits have thus far been discovered within the slate area, but there are seven or eight places at which lean ores have been obtained. These are widely distributed. Because of the interest naturally attached to the discovery of ores of any kind in the gi'eat slate areas, each of these localities is briefly described. It is possible that ore deposits of workable size occur in the slates in very favorable situations, though no indication of their presence has j^et been observed in this district. From the fact, however, that large deposits are known to exist in the Hanbury slates of the Florence, Crystal Falls, and Iron River districts, it is possible that similar deposits ma}' occur in the Menominee district. OUTLINE OF MONOGRAPH. 31 In exploring within the Hanbuiy area only the most favorable localities need be tested. These are the places where ferruginous dolomites occur, where the rocks are folded or brecciated, and where the folds involve au impervious stratum. Chapter VI. Above the folded Algonkian rocks lie the horizontal beds of the Paleozoic sediments, with a profound unconformity between the two series. The Paleozoic series comprises the Lake Superior sandstone below and the Hermans- ville limestone above. These once extended over the entire district. East of Waucedah they still cover all the older rocks, but west of this place they are now found capping only the higher hills. The Lake Superior sandstone is mainly a red sandstone. Its thickness is esti- mated to be 300 feet. In its lower portions are conglomerates, which, where thej' lie on the Vulcan beds, contain ore bowlders in such quantitj^ that they may occasionally become sources of ore. Fossils are extremely rare. A few fragments of trilobites and a few shells of brachiopods have been found in a few places. The former have been identified as Dicellocephalus misa and the latter as Lingulepis 2}innifarmis. They indicate the St. Croix horizon of the Upper Mississippian series. The Hermansville limestone is a sandstone with calcareous cement, interbedded with pure dolomite. Its maximum thickness is about 100 feet. The series is of little importance within the limits of the Menominee district, but is widely spread farther east. Rominger identified it as corresponding to the Chazy and Calciferous formations of the Eastern States. Chapter VII. This chapter contains an outline of the geological history of the district. Comparison of the succession of formations in the Menominee district with the succession in the Marquette and the Grogebic districts shows that the geological history of the three districts, while alike in its major features, was very different in minor features. The attempt to correlate the events that transpired in the various iron-bearing districts of the Lake Superior region is left for a succeeding publication. U, S. GEOLOGICAL SURVEY MONOGRAPH XLVI PL. I ULIUS BIENiCO LITH r Iron, ore -bea/uu/ districts for whiiJt dataU rruifis hoh-e d«gn published are cnclosfd. by r«tt lines 1 MenomUiee 2 Oyxtal FriUx 3 Mewiftiette 4 Gogebic 5 MesabL 6 VwnUion ARCHEAN algo:nkian- HURONIAN nvtn KEWEKI'iA.WAN I Ak . POST-ALGOXKrAN Including considerable Imiuding considerable areas of Algonkian granite areasofArchean (the iron-bearing series) GEOLOGIC AL\P OF PART OF THE LAKE SUPERIOR REGION SHOWING RELATIVE POSITION OF THE MENOMINEE AREA WITH RESPECT TO OTHER HURONIAN AREAS Compiled from Official maps of United States, Staie,and Canadian Surveys Sc ale HrHONIAN Original Huronum. Uartfliette iron.-beartnff serias Ct-vs(iil FdlL^ iran-b«arUu/ svfes Afaioniineir i/-an- bearing series Wl'iconstn Valt&'i' series PmokM-GoqrAlr ironb^orina series BlaiitRt.'i'er iron beai-inif scfitifs QiippeH'a. VtUie^' qaarrxtzts St Lotus series ifesabL irort-betiruif/sertes Vermilion tixm- bearing series AiiunUcte irorrbearuig series Folded sc/tiste of Canada. THE MENOMINEE IRON-BEARING DISTRICT OF MICHIGAN. By William Shirley Bayley. CHAPTER I. INTRODUCTION. Scope and date of ivorh done. — The present report is an account of the geology of that portion of the Menominee district bordering the Menominee River on the Michigan side. The district lies entirely within the State of Michigan, its western extension into Wisconsin not being discussed. A preliminary report of the district, accompanied by a geological map, was published in the year 1900 as a folio of the Geologic Atlas of the United States." The field work upon which the report is mainly based occupied the summer of 1896. In this I was assisted in the geological work by J. Morgan Clements and Samuel Weidman. Since this I have twice visited the district for supplemental work, first in 1899 and again in 1900. Acknoioledfpnents. — To the superintendents and engineers of the mines in the district the Survey is under many obligations for the numberless courtesies afforded the field parties and for the generous manner in which they have allowed the use of mine plats. Some of these have been reproduced as illustrations in the body of this report. All have been of value in working out the intricate structure of the district. «VanHise,C. R., and Bayley, W. S., Description of the Menominee district: Geologic Atlas U. S., folio 62; U. S. Geol. Survey, 1900. MON XLVI— O-t 3 33 34 ^ THE MENOMINEE IRON-BEARING DISTRICT. To Messrs. F. A. Janson, of the Penu Iron Company; G. Helberg, of the Aragon mme; V. S. Hillyer, of the Minnesota Iron Company; and L. M. Hardenbiirgh, formerly of the Pewabic mine, special thanks are due for copies of maps and plats which were prepared especially for this work and furnished to the author gratuitously. Limits of the Menominee area. — The Menominee district proper is bounded on the west by the Menominee River; on the south by the same river and the south line of the northern tier of sections in T. 38 N., Michi- gan; on the east by the east line of sees. 2, 11, 14, 23, 26, and 35, T. 39 N., R. 28 W., and their continuation north and south, and on the north by the north line of T. 40 N., Michigan. On the general map (PI. II) the area represented includes a region extending 8 miles farther north, to the north side of the second tier of sections north of the south line of T. 42 N., and as far west as the west line of Ts. 41 and 42 N., R. 30 W. The geological map (PL IX) includes only the area of the Menominee district proper. Relations to other iron-hearing areas. — The area designated above as the Menominee district proper constitutes a tongue of sedimentary deposits lying between a granite area to the north and a green schist area to the south. This tongue is the southernmost of five distinct tongues (see map, PI. II) which extend eastward from the great central area of Huronian deposits in Wisconsin and Michigan, described in part in the Crystal Falls monograph." The five tongues, beginning with the northernmost, are the Marquette tongue, discussed in the Marquette monograph; the Sagola and the Felch Mountain tongues, treated in the Crystal Falls monograph; the Calumet tongue; and the Menominee tongue. Each is structurally a trough of Huronian sediments lying between rims of Archean granites, gneisses, and schists. To the west they all widen out into the bi'oad expanse of Huronian sediments referred to above. To the east all except the Mar- quette tongue plinige beneath Paleozoic deposits. The Calumet tongue runs a little north of east and then east through the center of T. 41 N., R. 29 W., Michigan, as a narrow belt a mile or a mile and a half in width. At the east side of the township it widens rapidly, becoming broader and broader until, in T. 41 N., R. 27 W., where it disappears under Paleozoic deposits, its width measures 7 J miles. a Clements, J. M., and Smyth, H. L., The Crystal Falls iron-bearing district of Michigan, with a chapter on the Sturgeon River tongue by AV. S. Bay ley, and an introduction by C. R. Van Hise: Mon. ' U. S. Geol. Survey, vol. 36, 1899. U. S. GEOLOGICAL SURVEY MONOGRAPH XLV I PL. 'in£se GENERAI. OLTTLINE MAP OK THE RECilOX liE'HVl'KN TIIE MFL\0\nNEE RI\T':R AND THE NOUTH SIDE ()!• FEl.CH MOUNTAIN DISTmCT. MK'HIdAN ^""^ SIIOWINO I'dSITION OK THE MENOMINEE TROUGH WIT KESPECT TO OTHER HURONIAN TROUGHS TO THE NORTH Scale 5 miles R.30W. INTRODUCTION. 35 At its west end the south side of the Calumet tongue merges with the north side of the Menominee tongue. Farther east the two tongues are separated by an elliptical area of Arehean rocks. Shape and sise of the Menominee tongue. — In general the Menominee tongue is a spindle-shaped area about 17 miles long, trending about N. 55 "^ W. Its narrowest portion is in the middle, in the vicinity of Vnlcan, where it measures about 4 miles in width from its contact with the granite on the north to its contact with the green schists on the Menominee River on the south. To the east it widens gradually until, in the eastern 2:)ortion of R. 28 W., its width is about 7 miles. To the west also it gradually becomes wider and tinally loses its identity as a distinct trough at about the center line of R. 30 W., where it merges with the Calumet trough, and extends into the wide area of Huronian sediments to the west. At its eastern end the characteristic rocks of the tongue are so deeply buried beneath later sediments that they can not be traced. Lines of mag- netic attraction, however, have been obtained east of the eastern limit of the district, as given above, and these are taken to mean that the Huronian sediments continue for at least a short distance beyond the places where they are last seen on the surface. The area covered by the tongue measures about 112 square miles. The whole area studied, including the Calumet tongue, the Archeau area between the Calumet and Menominee tongues, and the narrow strip of green schists along the Menominee River, aggregates about 261 square miles. All the producing mines, however, are situated within the Menominee tongue, the three in the Calumet tongue having been idle for over fifteen years. The geology of the Calumet area will be discussed in another jjubli- cation. The present monograjih deals only with the Menominee tongue. Economic importance of the district. — In 1902 the mines of the Menominee trough shipped 3,001,189 long tons of ore, and since the first shipment of ore from the Quiunesec mine in 1873 the aggregate shi23ments of all mines to the close of 1902 have amounted to the large total of very nearly 29,000,000 long tons. Of this aggregate by far the larger proportion of ore has been of Bessemer grade. The total shijjments from the Marquette district to the end of 1902 were 66,686,502 long tons; those from the Crystal Falls, Iron River, and Felch Mountain districts in Michigan, and the Florence district in Wisconsin, taken together, have 36 THE MENOMINEE IRON-BEARING DISTRICT. amounted to about 13,400,000 long tons; those from the Gogebic range in Michigan and Wisconsin to 37,818,274 long tons; those from the Vermilion range in Minnesota to 19,061,506 long tons; and those from the Mesabi range in the same State to 53,747,807 long tons. It will thus be seen that in proportion to its area the Menominee trough has yielded as large a product as any other of the Lake Superior districts, with the exception of the Mesabi. Since the discovery and development of the Mesabi district the demand for low phosphorus and high silica ores to serve as mixtures for the Mesabi ores has so largely increased that many lean, low phosphorus ore deposits, formerly not marketable, now find a ready sale. The Menominee district can furnish an abundance of this grade of ore, so that it is probable that the importance of the district as a mining center will increase rather than diminish in the future. Previous work in the district. — The only detailed geological maps of the Menominee trough that have heretofore been published are those of Brooks" (PL IV) and Wright,'' in the Geology of Wisconsin. Irving" published a general map (PI. V) of the district in his introduction to Dr. Williams's bulletin on the origin of the Menominee green schists, but he made no claim that it exhibits more than the generalized structural features of the district. Wright's map shows the distribution of the green schists, some of the iron-bearing belts, and the pre-Huronian rocks of the district, while that of Brooks exhibits, in addition, the location of all the ledges of dolomite, slate, quartzite, and other sedimentary rocks met with during this author's explorations. Brooks also presents a structural sheet illustrating his views as to the sequence of the rock series and the character of the folding. This map was of great use to the field parties of the United States Geological Survey, since it enabled them to make systematic plans for the survey of the district and directed their attention to many rock ledges that might otherwise have been overlooked. In addition to the works and reports referred to above, another valua- ble report on the district is that of Rominger."* This report, like that of the a Geology of Wisconsin, Survey of 1873-1879, vol. 3, pt. 7, by T. B. Brooks, and pt. 8, by C. E. Wright, and Atlas, Pis. XXVIII, XXIX, and XXX. i Ibid. c Williams, G. H., The greenstone-schist areas of the Menominee and Marquette regions of Michigan, etc., with an introduction by R. D. Irving: Bull. U. S. Geol. Survey No. 62, 1890. ''Rominger, C, Upper Peninsula, pt. 2, Menominee iron region: Geol. Surv. Mich., vol. 4, 1881, pp. 155-241. INTRODUCTION. 37 same author on the geology of tlie Marquette district, consists mainly of a discussion of ledges and of detached statements concerning the relations to one another of the different "rock groups" met with. It, nevertheless, was ot great value in the prosecution of the field work on which the present volume is based, since it called attention to the most promising exposures in the district and in manj^ instances afforded clues as to the places at which relations could best be studied. The reports of Brooks, Wright, Irving, and Rominger are referred to more at length in the following chapter, and in this chapter also are given abstracts of all the other important papers in which the geology of the district has been discussed. A perusal of this chapter will show that many facts bearing on the subject have gradually been accumulated; but that in no other cases tlian those mentioned above did the facts known concerning- the distribution of the different formations warrant the construction of geological maps of the district. Method of work. — Most of the field work on which this monograph is based was done in the months of July, August, and a part of September, 1896. The entire area whose limits have been outlined above was cut by north-south traverses at intervals of one-tenth, one-fourth, one-half, or three-fourths of a mile, or 1 mile, according to the intricacy of the geology iu different parts of the district and the character of the surface exposed to view. In those portions of the district from which the forest and undei'- growth have been removed the traverses were at greater intervals than in those portions covered by dense thickets of young trees and brush. In the wide expanses of slate to the south of the Chicago and Northwestern Rail- road the traverses were 1 mile apart. In those portions of the district where the different rock belts are closely folded traverses were made every quarter of a mile. The iron-bearing belt was examined thoroughly, every ledge, so far as is known, and every mining pit having been studied in detail. The same careful examination was made of the contact between the quartz- ite at the base of the sedimentary series and the crystalline rocks to the north, and an almost equally careful search was made for a contact of the slates with the greenstones to the south. In areas where exposures are small and scattered, north-south magnetic lines were run every half mile. During the summers of 1899 and 1900 two other visits were made to the district, but the field work was limited to the study of relations, to the 38 THE MENOMINEE IRON-BEARING DISTRICT. ruimiug of a few additional magnetic lines, and to the investigation of the structure of small complicated areas and the study of the mines. The work was supplementaiy to that of 1896, and was intended simply to fill the gaps left by the earlier survey. Though the topography of the district is simple, much of the surface is covered with a thick mantle of glacial drift through which ledges of the softer rocks rarely penetrate. Other portions are covered by a sheet of sandstone which obscures some of the most interesting contacts. Moreover, thick growths of brush hide much of the surface, especially in the northern and eastern portions of the district. The detail maps show the character, the position, and the number of ledges investigated, and it is from the evidence afforded by these and by the mine plats that the structure of the district has been worked out. Classification of formations. — The rocks of the Menominee district belong to the Archean, Algonkian, and Paleozoic systems. The oldest series of rocks bordering the Menominee tongue comprises various schists, gneisses, and granites. These are regarded as Archean. Resting unconformably upon the Archean rocks is a succession of Algonkian sediments, which are divisible into a Lower Menominee and an Upper Menominee series, sepa- rated from each other by an unconformity. The Paleozoic rocks comprise horizontal Cambrian sandstones and Ordovician limestones. These occur in patches on the tops of the hills, capping the closely folded and trun- cated Huronian rocks. Both of the Menominee series are divisible into a number of formations, each representing a time during which the condi- tions of deposition were approximately uniform. Each of the pre-Cambrian formations has been named and is represented on the general map of the district (PI. IX) by a distinctive color. The following table gives a list of the formations, arranged in descending order according to age. The frac- tional formations, or members of the Vulcan formation, are represented on the detail maps and are separately characterized in the text, but they are not differentiated on the general map. INTRODUCTION. 39 Table showing the mccession of formations in the Menominee district and their relations to gener'ol geological systems. Formation. Paleozoic . [Ordovician /„ .^^f' JHermansville limestone. [Cambrian Potadam Lake Superior sandstone. Algonliian . Unconformity. Upper Menominee Hanbury slate. Vulcan formation, subdivided into the ore-bearing Curry member, Brier slate, and ore-bearing Traders member. Unconformity. {Negaunee formation Randville dolomite. Sturgeon quartzite. Unconformity. Aichean. Ciranites and gneisses, cut by granite and diabase dikes. Quinnesec schists, cut by acid and basic dikes and veins. Names of the formations. — The names of the Upper Menominee forma- tions and of the Archean schists are taken from locaHties in the district. The names of the Lower Menominee formations are those of formations in adjacent districts ah-eady reported upon, with which the Menominee forma- tions are beheved to be continuous. Beginning- at the bottom, the Quin- nesec schists are so named since they are typically developed at the Big and the Little Quinnesec Falls on the Menominee River. The Sturgeon quartzite is so called because this formation in the Menominee district has been traced almost continuously to a like formation in the Crystal Falls district, which has been called the Sturgeon quartzite." The Menominee dolomite is called the Randville dolomite because it has been practically connected with the Randville dolomite of the Crystal Falls district.* The assumed iron-bearing Lower Menominee formation is called the Negaunee formation because this is the Lower Huronian iron-bearing formation of the Marquette district. In the Menominee district, as will be seen, this formation has not yet been identified, although its presence is indicated by the character of the basal member of the Upper Menominee series. In the Upper Menominee the Vulcan foi'mation is so named since the iron formation occurs in typical development, with full succession and fine a Mon. U. S. Geol. Survey, vol. 36, 1899, pp. 398-405. i-lbid., pp. 406-411. 40 THE MENOMINEE IRON-BEARING DISTRICT. exposures, in the vicinity of West Vulcan. It is threefold, comprising a series of quartzites and fragmental ores at the base, called the Traders member; following these in upward succession, a series of slates known as the Brier slates; and above these, a set of ore beds, jaspilites, and quartzites, which has been called the Curry member, the names in each case being taken from the names of the mines near which the respective series is best exposed. The Hanbury slates are thus named because in the vicinity of Lake Hanbury this formation is better exposed than anywhere else in the district. References to Marquette monograph. — In the following pages references will be made repeatedly to the monograph on the Marquette district, espe- cially in connection with the discussion of the Archean rocks. These are so nearly like the corresponding rocks in the Marquette district that a minute description of them would be little more than a repetition of what has already been recorded in the account of the Marquette Basement Complex. In order to avoid this unnecessary repetition, only brief descrip- tions of these rocks will be given. Those who may be interested in their petrography are referred to the chapter on the Basement Complex in the Marqviette monograph," and to Dr. Williams's bulletin '' on the greenstone- schist areas of the Menominee and Marquette regions of Michigan. "Van Hise, C. R., and Bayley, W. S., The Marquette iron-bearing district of Michigan, including a chapter on the Republic trough"by H. L. Smyth: Hon. U. S. Gaol. Survey, vol. 28, 1897, pp. 149-220. 1) Williams, G. H., The greenstone-schist areas of the Menominee and ISIarquette regions of Michi- gan; a contribution to the subject of dynamic metamorphism in eruptive rocks: Bull. U. S. Geol. Survey No. 62, 1890. CHAPTER II. BIBLIOGRAPHY AND ABSTRACT OF LITERATURE. The geological literature relating to the Menominee district is much less voluminous than that relating to the Marquette district. Nearly all of it is concerned more particularly with the general problems presented by the district. Very little of the work done has been accomplished by geol- ogists working privately. By far the greater portion of it, including all that is of the greatest value, is the result of public enterprise. The earliest important publications are those of the United States geologists who were intrusted with the examination of the geological features of the "Chippewa land district." After these came the publications of the Michi- gan survey, followed by those of the Wisconsin survey, and, finally, by those of the U. S. Geological Survey. The authors who have done most toward familiarizing us with the broader features of the Menominee geology are J. W. Foster, J. D. Whitney, T. B. Brooks, C. Rominger, and R. D. Irving. Messrs. Foster and Whitney first recorded the existence of pre- Cambrian rocks within the limits of the district. Brooks separated these into the Laurentian and the Huronian groups, and published maps outlining the Huronian basin and the distribution of the principal formations repre- sented in it. This author and Rominger both give a great many details with reference to the relations of these formations to each other, and both worked out a general theory of structure for the bedded rocks. Irving- busied himself principally with a discussion of the relations of the iron- bearing formation to the overlying and the underlying series. Brooks's map (PI. IV) is the only detailed one of the district. Others that have been issued are mainly copies of this, except the map of C. E. Wright, which was constructed primarily for economic purposes, and which shows merely the outline of the Huronian basin and the distribution of the iron-bearing and the green-schist formations within it. 41 42 THE MENOMINEE IRON-BEARING DISTRICT. In the present chapter reference is made to all tlie articles that are known to treat of the district under discussion. These are abstracted in each case, and the conclusions reached are outlined. A knowledge of the contents of many articles that treat of the relations of the pre-Cambrian formations to one another in the Lake Superior region, but which do not refer specifically to the Menominee district, is of importance to the correct understanding of the history of the discussion of the Menominee geology, but they have been so fully referred to in the monograph on the Marquette district " that it has not been thought necessary to abstract them a second time. The ai-rangement of the abstracts is chronological, the dates of publication of the original articles being regarded as the times when they were first made public. This method of arrangement is unfair to a few authors, notably to Dr. Romiuger, a portion of whose work was first published, through no fault of his own, years after it was completed, and to certain others of the official geologists ; but it is the only method that is practicable, since it is impossible in most cases to learn when the various articles left the hands of their authors. The very first information given us concerning the district now under discussion was imparted through documents of Congress. The first author who left a record of his obsei'vations in the Menominee country was George N. Sanders. Following his report came the reports of the other early United States geologists in rapid succession. Then ensued a period during which little new work was done. In 1877 the discovery of ore at the Breen mine called renewed attention to the district. This is shown in the excellent reports of the Michigan and Wisconsin geologists, published between the years 1872 and 1881. The investigations begun by these surveys were continued without interruption by the United States Geolog- ical Survey, but in the Government survey the Menominee geology was studied as a portion of the broader problems relating to the entire Lake Superior region prior to the inauguration of the work on which the present volume is based. a Van Hise C. R., and Bayley, W. S., The Marquette iron-bearing district of Michigan, inckiding a chapter on the Republic trough, by H. L. Smyth: Men. U. S. Geol. Survey, vol. 28, 1897. (See Chapter I, Geological explorations and literature, pp. 5-148.) BIBLIOGRAPHY AND ABSTRACT OF LITERATURE. 43 1845. Sanders, George N. Report to J. J. Abert: Sen. Docs., 2d sess. 28th Cong., 18^14-45, vol. 7, No. 117, pp. 3-9. The first known reference to the Menominee district is found in the report of George N. Sanders, who made an examination of the country along the Menominee River with a view to determine the feasibiHty of con- structing a road from Grreen Bay on Lake Michigan to Copper Harbor on Lake Superior. In this report we find described the general features of the country traversed. The report is topographical rather than geological; nevertheless, the author refers to various veins of spar met with in his travels. Sanders, George N. Report to J. Stockton: Sen Docs., 2d sess. 28th Cong., 1844-45, vol. 11, No. 175, pp. 8-14. This report is a reprint of the preceding one. 18-49. Foster, J. W. Report to Dr. C. T. Jackson: Sen. Docs., 2d sess. 30th Cong., 1848-49, vol. 2, No. 2, pp. 159-163. Dated Sept. 28, 1S48. In this article the author prints an abstract of his report to Dr. Jack- son, published in full during the succeeding year. A brief description of Menominee geology is given, the reader being referred to the full report for details. 1850. Jackson, C. T. Report on the geological and mineralogical survej^ of the min- eral lands in the State of Michigan, etc. Sen. Docs., 1st sess. 31st Cong., 1849-50, vol. 3, No. 1, pp. 371-624. Dated Nov. 10, 1849. Dr. Jackson's report is devoted mainly to the region immediately bor- dering Lake Superior. In it, however, the author mentions having received a specimen of slightly magnetic iron ore from the Menominee River. It was given him by Mr. Barbeau, of Sault Ste. Marie, who had received it from an Indian. Tlie ore was reported as occurring in mountainous masses somewhere between the head of Keweenaw Bay and the Menominee River. An analysis of the ore yielded 89.70 per cent FcsOg; 12.20 per cent siliceous matter. Foster, J. "W. Notes on the geology and topograph}' of portions of the country adjacent to Lakes Superior and Michigan in the Chippewa land district, pp. 773-801. Dated May 26, 1849. 44 THE MENOMINEE IRON-BEARING DISTRICT. Messrs. Foster and Hill were sent by Dr. Jackson to make a section from L'Anse to the Menominee River and to search for the iron mountain referred to in the preceding article. These geologists report that "alternating beds of hornblende and argillaceous slates " occur on the Menominee River about 1 mile below the junction of the Brule and the Michigamme. Near the south line of T. 41 N., R. 30 W., they also report the existence of a high ridge of "argillaceous slate containing amygdules of calc spar." Between this point and the Upper Twin Falls the argillaceous slates and chloritic slates largely predominate. At these falls and at the Lower Twin Falls they present good sections. About 2 miles southeast of the lower falls, near sec. 30, T. 40 N., R. 30 W., large beds of specular ore are associated with talcose and argillaceous slates. The ore is similar to that of the "iron mountain," which is now identified as Republic Mountain, in the Marquette range. Among' the other rocks observed on what is now known as the Menominee ransre were ffreat blocks of limestone in the Menominee River at the mouth of the Misskos (T. 40 N., R. 31 W.), ledges of "hornblende" exposed on the banks of the stream, beds of talcose slate at the foot of the Great Bekuenesec Falls (now the Big Quinnesec Falls, in the northwest portion of T. 39 N., R. 30 W.), a similar bed at the foot of the Little Bekuenesec Falls, and a third bed of the same character at the foot of the Sturgeon Falls. At the latter place the slates are "wedged out between walls of sienite." At Chip- pewa Island the slates again occur. Here the authors constructed a section with drift on top, followed beneath by nearly horizontal sandstone, dark- colored basalt, and argillaceous and talcose slates. The sandstone rests upon the upturned edges of the slate. Since its deposition the former rock is said to have suffered no great alteration or disturbance, whereas the slates on which it rests are contorted and altered by the protrusion of igneous rocks. The iron ores referred to are reported to "bear upon their surfaces strong marks of their mechanical origin." The report continues: They are regularly stx-atified, * * * so that a specimen, on its cross fracture, resembles ribbon -jasper. The lines of stratification can readily be distinguished from those of lamination. Like the slates, thej' are often found contorted and wrinkled, and the same facts could be advanced in both cases to prove their common origin [p. 779]. This statement sounds strange in view of the author's later attempt, in conjunction with Whitney, to show that similar ores in the Marquette district are eruptive in origin. BIBLIOGRAPHY AND ABSTRACT OF LITERATURE. 45 In the systematic description of the rocks met with in the journey we find that a range of rock, supposed to be granite, was discovered running parallel to the Menominee River, in Tps. 39 and 40 N., Rs. 30 and 31 W., and crossing- the river at Great Bekuenesec Falls. On both sides of this range igneous hornblende rocks were found. 1851. Foster, J. W., and Whitney, J. D. On the age of the sandstone of Lake Superior, with a description of the phenomena of tlie association of igneous i-ocks: Proc. Am. Assoc. Adv. Sci., Fifth Meeting, pp. 23-38. 1851. In the course of a genei-al description of the Lake Superior sandstone the authors state that the belt of this rock is 14 miles wide where it crosses the Menominee, that it has a gentle dip to the southeast, not exceeding 3°, corresponding with the slope of the country, and that in the bed of the river it rests on vertical edges of slate rocks and of compact and igneous rocks intercalated with them. Foster, J. W., and Whitney, J. D. Report on the geology of the Lake Superior land district, pt. 2, the iron region, together with the genei-al geology. Dated Nov. 12, 1851. Sen. Docs., special sess. 32d Cong., 1851, vol. 3. No. -i. xvi, 406 pp., with map. In the general portion of this report the statements made to Dr. Jack- son by the senior autlior with reference to his observations on the Menomi- nee River are repeated. In addition it is stated that above the Big Quinnesec Falls and just above the lower falls the rocks consist of serpen- tine, and that at the head of the upper falls there is a protrusion of a rock like protogine, " composed of feldspar, talc, and quartz. * * * Occasion- ally hornblende replaces the talc, when it [the rock] passes into a well- characterized syenite" (p. 25). Slates are mentioned as occurring between the Little Quinnesec Falls and Sandy Portage, and serpentine between the latter place and Sturgeon Falls. On the portage a ridge was crossed in which the rock has the external character of granite, but the mineralogical composition of protogine. Slates and dark-green igneous rocks alternate as the Menominee is descended, the gradations between the igneous rocks being so numerous as to prevent their proper classification. At soine dis- tance below the portage are basaltic and other crystalline greenstones which at Chippewa Island are declared to be in contact with talcose slates. Near the south end of the island the slates are described as being porphyritic 46 THE MENOMINEE IRON-BEARING DISTRICT. with red phenocrysts, and with them are said to be associated large masses of serpentine. After describing the rocks occurring along the Menomi- nee the authors give a general account of the topog-raphical features of the Menominee Valley, and describe a geological section (see fig. 1) across the valley from sec. 35, T. 42 N., R. 30 W., to a point near the Little Quinnesec Falls, in sec. 14, T. 39 N., R. 30 W. The description is taken from the notes of Charles Whittlesey. The quartz shown in the sec- tion is said to pass into hornblende-slate, and to the north into gneissoid rocks. A large number of observations were made on the rocks occurring north of the river, some of which are of interest. On the north side of Lake Fumfee is a sharp and elevated ridge whose top consists of Potsdam and Calciferous sand- stone resting undisturbed on the Azoic rocks beneath. In sees. 34 and 35, T. 40 N., R. 30 W., is a compact marble belonging with the Azoic, and in sec. 30 of the same town is a "conspicuous iron mountain." Iron ores were also noted in a ridge south of Antoines Lake, in the southern portion of T. 40 N., R. 30 W. These are believed to be the south- ernmost ores in the district. They are specular in structure and are of a bluish-black color. Between sees. 28 and 29, in f «- a T. 40 N., R. 28 AV., is a great deposit of ore containing from ^ ill 63 to 68 per cent of iron. IQ ^ -*! S . 'f. The series to which the ores, schists, limestone, and a.|i.S quartz rocks belong occupies a belt whose broadest expan- ds I sion is not less than 80 miles in width. The rocks compris- nig the series are supposed to be flexed and folded, as measurement across their upturned edges would give a thick- I ness for the series, providing it is assumed to be unfolded, ^"Z'^ too o-reat to be reg-arded for an instant as correct. The entire "d o "a Igl series is considered to be metamorphic, even to the compact I'll "hornblende," which resembles an ig'ueous rock. Between ^~^: the granite that iinderlies them and the Sihman sandstones ^ that overlie them the rocks of the series — throughout their whole extent * * * are more or less metamorphosed, pre- a " to f^ w 1^ GO 3 O &4 CO a (-■ <;§ s s ^ " 6 .a g c c-3 ." BIBLIOGRAPHY AND ABSTRACT OF LITERATURE. 47 sentin^ a series of gradations represented at one extreme by crystalline gneiss and compact hornblende and at the other by bedded limestone and ripple-marked quartz. To the presence of granitic and ti'appean rocks this transformation is, in a great degree, to be attributed. Much of the compact hornblende presents the external characters of an igneous product; but, since it is found to occupj^ an almost invariable relation to the granite axes — flanking their slopes— and to assume a fissile structure as it recedes from the lines of igneous outburst, we can not but regard it as the more highly metamorphosed portions of the dark-green chlorite-slates. This compact hornblende is not to be confounded with those lenticular-shaped masses observed in the slates which, we doubt not, are trappean in their nature. We have seen that those igneous causes which produced numerous axes of eleva- tion, and folded the strata into a series of flexures, had ceased to operate before the deposition of the Silurian groups, since they are found to repose in a nearlj^ hori- zontal position upon the upturned edges of the slates, or to occupy the sinuosities in the granite, nowhere exhibiting traces of metamoi-phism or derangement of the strata. * * * From the local details above given, it will be seen that the igneous rocks of the Azoic period, though crystalline, compact, and occasionally porphj'ritic in their texture, are never amygdaloidal (like the traps on Keweenaw Point), and hence we infer that they were pi'oduced under widely difl^erent conditions. The lat- ter may have been consolidated beneath the pressui-e of a great ocean, while from the former a greater part of this pressure may have been removed; or it may be that both were, in the first instance, equally vesicular, but that the latter assumed a crys- talline or compact sti-ucture from long-continued exposure to heat, under immense pressure. All the phenomena would seena to indicate that the eruption of the trap- pean rocks of this period took place beneath an ocean of great depth; or, at least under conditions widely different from those which prevailed during the formation of the trappean belts of Keweenaw Point and Isle Royale [p. 32]. After making some general remarks upon the necessity of regarding the rocks below the base of the Silurian as composing a great system, which they call the Azoic, the authors proceed to describe each rock in detail and to note its occurrence in the region examined. They repeat many of the statements above referred to, and theorize as to the origin and the relative ages of the different rocks. Their conclusions with respect to the age of the Menominee rocks are not essentially different from those reached in the discussion of the Marquette rocks — results that have been freely described in the Marquette monograph." The most important of these con- clusions relates to the igneous origin of the iron ore, which in the previous a Mon. U. S. Geol. Survey, vol. 28, 1897. 48 THE MENOMINEE IRON-BEARING DISTRICT. year the senior author had argued to be sedimentary. (Cf. p. 44.) Two other igneous rocks are especially noted; one is a compact, dark, horn- blendic rock, the other a light-colored rock resembling a member of the granite family, and designated a "feldstone." ^ = Iron »-" = Beds of marble C.S.= Crystalline schists P = Potsdam sandstone CALr Calciferous sandstone ; Tragpean rocks '"!x.'JM- Granite Chippewa Island (Mw. Fig. 2.— Portion of the geological map of the Lake Superior land district in the State of Michigan. After Foster and Whitney, 1851. The squares are toiTnships, each embracing about 31; square miles. The map which accompanies the report is intended to show approxi- mately the distribution of the Azoic, the Paleozoic, and the igneous rocks. That portion of it wliich relates to the Menominee district is here reproduced without colors as fig. 2. BIBLIOGRAPHY AND ABSTRACT OF LITERATURE. 49 1855. Foster, J. W. Catalogue of rocks, minerals, etc., collected by J. W. Foster: Ninth Ann. Rept. Smithsonian Institution, pp. 384:-386. 1855. As its title indicates, this article is simply a catalogue of specimens. 1860. Whittlesey, Charles. On the origin of the Azoic rocks of Michigan and Wisconsin: Proc. Am. Assoc. Adv. Sci. , Thirteenth Meeting, pp. 301-308. 1860. In a somewhat general article on the "Azoic" rocks of the Upper Peninsula of Michigan, the author records the results of analyses of 15 rocks collected mainly from the Menominee drainage basin. By compari- son of these analyses with those of the Laurentian rocks of Canada, the conclusion is reached that the Menominee "metamorphic rocks," including the slates, etc., must be of a different age from the sediments which yielded the Laurentian rocks, if these are really metamorphic. The author, how- ever, is inclined to doubt their metamorphic origin. Igneous rocks are described as being in contact with Potsdam sandstone in the Menominee district, and the sandstone is said to have been metamor- phosed at the contact. The agent producing the change is nevertheless thought not to be heat. It appears that the author would regard the "metamorphic rocks" associated with the iron ores as igneous. 1861. WiNCHELL, A. First biennial report of the progress of the geological survey of Michigan, etc. Lansing, 339 pages. 1861. In this report the only allusion to the Menominee district is found in the statement that "On the State boundary the Azoic belt stretches from beyond Lac Vieux Desert to Chippewa Island, in the Menominee River" (p. 49). The rocks of the system are declared to be talcose, chloritic, and siliceous slates, quartz, and beds of marble. 1868. Credner, H. Die Gliederung der eozoischen (vorsilurischen) Formationsgruppe Nord-Amerikas: Zeitschr. f. die Gesammten Naturwissenschaften, vol. 32, 1868, pp. 353-405, and Habilitationschrift mit Genehmigung der phil. Fak. der Univ. Leipzig, Halle. 1869. In connection with a general discussion of the pre-Silurian rocks of MON XLVI — 04 4 50 THE MENOMINEE IRON-BEARING DISTRICT. North America, wliich are divided into the Laureutian and the Huronian systems, Credner describes briefly the geology of the Upper Peninsula of Michigan. The Laurentian system of IMichigan is made to consist of a series of gneisses, mica-schists, hornblende-schists, granites, and syenites, with a total thickness of over 20,000 feet. These I'ocks occur with many different variations of mineralogical composition, chloritic and talcose varieties being especially abundant. They all show their sedimentary origin in their present structure. Besides, at the Falls of the Sturgeon River there are typical conglomerates interbedded with micaceous and hornblendic rocks. At this point the author observed, in the_ midst of the gneiss series, several hundred feet of a complex consisting of thin-bedded talcose and sandj- ripple marked schists, a thin layer of protogine-gneiss, and three beds of conglomerate, each 30 feet in thickness. These conglomerates contain pebbles of gneiss, granite, and quartzite, varying in size from that of a hazelnut to that of one's fist, embedded in a talcose sandy grouudmass. The conglomerate is described as conformably overlain by gneiss. No eruptive rocks were discovered in the Laurentian of Michigan older than the coarse-grained and porphyritic granite that intrudes the gneiss. The Huronian series surrounds the Laurentian rocks and lies upon them unconformably. It is characterized as a series of sediments interme- diate in age between the Laurentian rocks below and the Silurian series above. It consists of a regular succession of quartzites, limestone, iron ores, chloritic and clay slates, and talc-schists, with a thickness of 18,000 feet, and, interlaminated with them, beds of diorite and aphanite. The series forms a major syncline, with a minor syncline extending into the embayments along the edge of the Laurentian areas. In tliat portion of the Menominee district where the Huronian system is most regularly developed, this system comprises, in order, beginning with the oldest, 2,000 feet of quartzite; 2,000 feet of white or red lime- stone; 700 feet of schistose hematite, varying in composition from a ferru- ginous quartzite to a granular hematite; 1,200 feet of chlorite-schists ; 8,000 feet of gray clay slates, interlaminated with layers of granular quartzite; 1,200 feet of chlorite-schists; 2,000 feet of coarse diorite; and a series of talcose clay slates and quartzose talc-schists. The limestone contains thin layers of siHceous clay slate, bands of BIBLIOGRAPHY AND ABSTRACT OF LITERATURE. 51 quartzite, and occasional inclusions of tremolite. On the south shore of Lake Antoine beds of coarse calcareous sandstone and of a conglomerate made up of limestone fragments in a quartzose groundmass are interlam- inated with the limestone beds. In the upper horizon of the upper chlorite- schists are about 100 feet of diorites, and in certain places about 100 feet of talc-schists. The Haronian series, like the Laurentian, is devoid of eruptives. 1869. Ceedner, H. Die vorsilurischen Gebilde der "oberen Halbinsel von Michigan" in Nord-Araerika: Zeitschr. Deutsch. geol. Ges., vol. 21, pp. 516-554. Map and four plates of sections. 1869. In a fuller account of the pre-Silurian geology of the Upper Penin- sula of Michigan Credner discusses the details upon which the conclusions expressed in his former paper (pp. 49-51) are based. Qiiarzit Gnei^. Cchiefer ti. cunglomerac Giieisse Flc. 3.— Geological section along the Falls of the Sturgeon River. After H. Credner, 1869. The Laurentian gneisses and schists are reported to exist as islands projecting through the Huronian sediments. The most important Lauren- tian rocks are mica-gneisses, hornblende-gneisses, hornblende-schists, and granites, with a measured thickness of about 10,000 feet at the Falls of the Sturgeon River. Here the succession, beginning at the north, is as follows (see fig. 3): (a) A great thickness of fine-grained, micaceous gray gneiss, interbanded with coarse-grained, feldspathic, red gneiss, and a few beds of hornblende-gneiss and liornblende-schist. (b) Chlorite-gneiss, with streaks of chlorite-schist. (c) Talc (protogine) -gneiss, through loss of talc passing into — {d) Chlorite-gneiss containing bands of chlorite-schist. (e) Fine-grained talc-gneiss, inclosing a mass of granular magnetite and hematite one-half foot in diameter. (/) Fine-grained mottled schists ( ' ' Fleekschiefern " ) , composed of fine plates of talc and mica and very small grains of sand and of feldspar. In this are lenticular masses or thin bands of pure feldspar or of flne-grained talc-gneiss. The schist is thin bedded, and its bed- ding surfaces are marked by ripple marks. It occurs in four zones measuring from 8 feet to 40 feet in thickness. Between these are — (g) Three beds of conglomerate, 15-.30 feet thick, composed of a matrix similar to the "Fleek- schiefern," filled with sharp-edged and rounded fragments of granite, gneiss, and quartz, varying in size between a hazelnut and a man's fist. Beds (/) and {g) dip vertically or steeply to the south. 52 THE MENOMI^JEE IRON-BEARING DISTRICT. '7 J- ^u^ \ Obsei'vations made a mile farther west show that these conglomerates are followed to the south by — (V) Gneiss-granite, and — (t) Fine-grained hornblende rock. After comparing this section with several other sec- tions observed farther north, the author summarizes con- cerning the Laurentian system as follows: It con.sists of predominating mica-gneisses in all possible varieties that may be formed bj' the variations in structure and in the proportions of constituents present, of hornblende-gneisses and hornblende-schists interbedded with these, and of chlorite- gneisses and chlorite-schists associated with zones of granite, sj-enite, and chlorite-granite. These constitute the surface rocks over extensive areas, strike with great regularity east and west, usually dip vertically, are here and there contorted, and are intruded by 3'ounger granite. Less broadly distributed are two series of talcose and chloritic forms. One series is composed of talc-gneiss, talcose mottled schists ("Fleckschiefern"), and conglomerates, with a sandy talcose groundmass. The other series consists of talc-chlorite- schists, with zones of crystalline, dolomitic limestone, chlorite- gneiss, chloritic hornblende rock, chlorite-schists with quartz peb- bles, and foliated talc-gneiss. Between the equivalents of these as described in Canada are three limestone zones, of which the upper- most contains the Eozoon canadense. In the Laurentian lime- stones of the Upper Peninsula of Michigan I have not been so fortunate as to discover this fossil [pp. 525-526.] The Laurentian areas are surrounded by a schist system composed of quartzite, limestones, iron-bearing rocks, and crystalline schists. Beginning at the bottom, the system consists of the following (see lig. 4) : (a) Dense, glassy, or sugary quartzite, thick bedded or thinly lami- nated, and possessing coatings of yellow mica in its foliation joints. Thickness, about 3,000 feet. (6) Crystalline dolomitic limestone. Rarely pure. Generally impure through admixture of silica. Its texture varies between coarse grained and very fine grained. Its color may be yellow, red. Its bedding is thick or thin, anil the planes between the layers are Sometimes there are interpolated between them thin beds of argillaceous Quartz veins penetrate the dolomite in thin and The 1^ ^SllOUtOlt^Jf^ brown, or white. sharply marked. chlorite-schist and siliceous clay slate. thick seams, which are more abundant in the upper portions than in the lower parts. variation in the amount of quartz present is exhibited in the topographic differences noted. Occasionally the dolomite contains tremolite. Thickness, 3,.500feet. BIBLIOGRAPHY AND ABSTRACT OF LITERATURE. 53 (c) Hematite rock. This varies from a ferruginous quartzite or a ferruginous clay slate to a pure steel-gray dense or granular hematite (Rotheiseustein). It is thinly schistose to thick bedded, but usually occurs as a series of beds about an inch thick, in which sili- ceous and iron-rich bands alternate. In a few zones the jasper layers are entirely lack- ing and beds of iron ore 30 feet thick replace them. The ore is free from phosphorus and sulphur, but it contains everywhere traces of magnetite. Thickness of the group, 600 to 1,000 feet. The cjuartzite underlying the dolomite is also ferruginous in places in its upper horizons and is consequently colored red. At several points the ferruginous material is thought to be sufficiently concentrated to constitute ore bodies. (d) Chlorite-schist, with spots and thin stains of red, ferruginous clay. Interlaminated with the schist are layers of quartz 3-4 feet thick. Thickness, probablj- 1,000-1, .500 feet. («) Clay slate, gray, thinly laminated and rusty brown on its schist planes; or blue, black, and very finely schistose. In the midst of the slate is a 150-foot bed of quartzite, which is very hard, granular, bluish gray, and is penetrated by veins of white glassy quartz and red orthoclase. Thickness, 8,500 feet. (/) Dark-green chlorite-schiist, often argillaceous. Thickness, 1,200-1,400 feet. In its upper horizons fine-grained and coarse-grained diorites in beds varying from ten to several hundred feet are associated with the schist. (g) Feldspathic talc-schist, light yellow or light brown in color. Thickness, 30 feet. (h) Greenish-gray talc-schist, flecked with emerald-green spots and containing rounded quartz grains. Associated with the schist are very thin lenticular laminae of crystalline dolomite. Thickness, 30 feet, (i) Flesh-colored feldspathic talc-schist, containing lenticular grains of quartz. Thickness, 40 feet. (k) Fine-grained rock consisting of a feldspathic groundmass, containing plates of talc, small reddish-brown orthoclase, and gray quartz grains. Thickness, 50 feet. (1) Diorite rock series, 2,300 feet thick. (»rt) Talcose clay slate and quartzose talc-schist, 1,500 feet thick. This series is the yoimgest in the Upper Peninsula of Michigan. Farther south in AVisconsin the same rock series occurs, but here it dips to the north, forming a basin with the northern schists. The beds (ff), [It), (i), and (A) seem to possess only a slight horizontal extension. They are local in their development. They are most fully developed at the Big Quinnesec Falls, but rapidly wedge out on botli sides along their strike. At the Little Quinnesec Falls a portion of the talc-schist series is replaced by chlorite-schist and a diorite bed 12 feet thick. The lowermost member of the Huronian, the quartzite, lies uncou- formably upon the gneisses. The other members of the iron-bearing series follow the quartzite conformably. In the neighborhood of Lake Antoine they are folded into two synclines, with limbs dipping very steeply toward the axes of the folds. South of the southern fold the entire iron- bearing series is repeated, with a steep southern di]), forming the north side of a third syncline with its south side in Wisconsin. 54 THE MENOMINEE IRON-BEARING DISTRICT. Over the Hiironian rocks in many places lie patches of horizontal Potsdam sandstone and Calciferous sandstone. Two sections showing the relations of the Potsdam to the underlying rocks are given. The entire district is supposed to have been covered formerly with Paleozoic sedi- ments, as is the district farther east at the present time, the patches on the tops of the hills being the remnants of this covering, which have thus far resisted the combined effect of weathering and the action of the ice. In a few words, the iron-bearing series as developed in the southern part of the Upper Peninsula of Michigan is characterized as follows: A conformable series of Fig. -Portion of geological map of the Upper Peninsula of Michigan. After H. Credner, 1869. The squares are town- ships, each embracing about 36 square miles. quartzites, limestone, hematite, clay slates, chlorite-schists, and talc-schists, the last two associated with beds of diorite and having a total thickness of 20,000 feet, overlies unconformablj' a gneiss series, and is in turn unconformably overlain by Silurian beds. This schist complex occupies the entire distance between gneiss and granite rims in long narrow folds. Organic remains have not been discovered anywhere in the series [p. 534]. The Menominee district is thought to have remained under water longer than the northern or Marqixette district, since in the former district above the quartzite a great thickness of rocks occurs which in the northern district is not represented at all. BIBLIOGRAPHY AND ABSTRACT OF LITERATURE. 55 The map accompanying the article shows the general distribution of the Archean, the Hnronian, and the Paleozoic formations in the Upper Pen- insula. That portion covering the Menominee district is reproduced in fig. 5. 1870. Crednek, H. Ueber Nordamerikanische Schieferporphyi'oide: Neues Jahrbuch fur Min., etc., pp. 970-98-i. 1870. At the Big Quinnesec Falls of the Menominee River, in the upper por- tion of the Huronian series, as defined by Credner, is a belt of porphyroid schists, about 300 feet in thickness, lying between two beds of diabase (the diorite of former articles). The schists comprise 50 feet of slightly schistose orthoclase-porphyroid, 10 feet of feldspathic paragonite-schist, 30 feet of orthoclase-paragonite-schist, 15 feet of paragonite-schist, 15 feet of calcareous paragonite-schist, 30 feet of schistose porphyroid, 50 feet of cal- careous chlorite-schist, and 100 feet of chlorite-schist. Above and below this schist series are diabase layers, which separate the porphyroids from an underlying series of chlorite-schists and an overlying series of siliceous talc-schists. The petrographical composition of the porphyroids is carefully discussed. All of the acid members consist essentially of quartz, ortho- clase, and paragonite in varying proportions. Analyses of four vai'ieties, the first two by Aarland, the third by Berghandler, and the fourth by Bornemann, resulted as follows: Analyses of porpkyroid cmd schists from Big Quinnesec JFalls, Menominee River. Orthoclase- porphyroid. Felds. para- gonite- schist. Ortlio. para- gonite- schist. Paragonite- schist. SiOj 66.70 15. 90 4.70 tr. tr. 72.45 8.85 6.20 tr. tr. 76.51 7.95 8.88 tr. .32 tr. 1.02 4.38 75 50 AI2O3 8 60 Fe,0, 2 60 MnO CaO 7 '^0 MgO 1 '>0 KjO 8.06 5.50 9.24 3.70 30 NajO 3.00 1 50 H,0 Total « 100. 80 « 100. 50 99.06 99.90 "These totals are as given in the original. The correct footings are 100.86 and 100.44, After examining the theories proposed to account for these and similar rocks, the author concludes that they are not eruptives, nor are they meta- 56 THE MENOMINEE IRON-BEARING DISTRICT. raorpliosed sediments. On the other hand he regards them, together with all the other Huronian rocks and all the Laiirentian series, as crystalline sediments, thrown down from the waters of the pre-Silurian ocean. Later they may have suffered some alteration tlirough the influence of mineral waters. 1873. Brooks, T. B. Iron-bearing rocks (economic): Geol. Surv. Michigan, Vol. I, part 1. Chap. IV, pp. 157-182. With atlas plates, three of which relate to the Menominee district. 1873. At the time Major Brooks published his report on the Menominee district but two mines, the Breen and the Ingalls, had been opened, so that the author, in his study of the district, was compelled to rely for his data almost exclusively upon surface exposures. Many of the facts he records in connection with the geology of the district were obtained largely from a survey made by Messrs. Pumpelly and Credner for the Portage Lake and Lake Superior Ship Canal Company. In the chapters preliminary to the one devoted to the Menominee district several facts are incidentally noted which throw considerable light upon the author's views concerning Menominee geology. The hills in the drainage basin of the Menominee River are reported to be capped with horizontal Silurian sandstone, which once also probably filled the valleys between them (p. 68). In comparison with the ]\Iarquette district the Menominee district is simpler in its geological structure, and it possesses a correspondingl}' less varied topography. The elevations trend nearly east and west. The south iron range is the Menominee range proper — the one discussed in the present monograph. The north range is that now known as the Felch Mountain or Metropolitan range. Its geology was discussed in the Crystal Falls monograph." The southern range can be traced 15 miles in a WNW. direction through T. 39 N., R. 29 W., and T. 40 N., R. 30 W. (p. 72). Its structure is so simple that "whoever identifies the upper marble in the Menominee region has a sure key to the discovery of any ore which may exist in the vicinity" (p. 74). In Chapter IV of the report the general geology of the district is discussed, many localities of the different rock types occun-ing within it are mentioned, and three structural sections across the iron-bearing series are described. «Mon. U. S. Geol. Survey, vol. 36, 1899. BIBLIOGRAPHY AND ABSTRACT OF LITERATURE. 57 The Menominee belt of ore-bearing rocks is separated from the more northerly belt . of similar rocks by a Avedge-shaped area of granite. The most easterly exposure of ore in the southern belt is at the Breen mine, in the north half of the northwest quarter, sec. 22, T. 39 N., R. 28 W. Travel- ing from this point toward the west, following a course running 16° north of west, several other exposures of ore are encountered before the last ore in Michigan is met with in the center of the southeast quai'ter, sec. 25, T. 40 N., R. 31 W. By magnetic observations the ore belt was traced across the Menominee River into Wisconsin, where its probable continuation is marked by outcrops between the Brule and the Pine rivers. Immediately north of this iron-bearing belt is a broad belt of impure marble, and north of this, in the Adcinity of the Sturgeon River, are local magnetic attractions and a few iron-ore bowlders that are believed to mark the position of a second ore belt, which outcrops as a siliceous ore north of Lake Antoine. North of this second ore belt, and underlying it, is an immense bed of quartzite. This quartzite, although believed to be geologically conformable with the ore formations, is not parallel with them, running more northwesterly, and dividing in T. 40 N., R. 30 W., into two and perhaps three ranges [p. 159]. North of the quartzite and underlying the whole series, which is Huronian in age, are the Laurentian granites, gneisses, and schists. South of the southern belt is a bed of chloritic schist that is well exposed on the south shore of Lake Haubury and on the Sturgeon River, and south of this is a second quartzite, very different from that to the north. South of the quartzite follows a broad exposure of argillaceous slate in a belt running nearly parallel to the ore belt. It is exposed at several points in T. 39 N., R. 28 W., and in T. 39 N., R. 29 W. Finally, south of the slates, is a broad, well-detined belt of chloritic, hornblendic, and dioritic rocks running parallel with the iron range, the harder members of which form the barrier rocks of all the falls in this part of the Menominee, and probably those of Pine River in Wisconsin [p. 159]. Above the iron-bearing series is the Silurian sandstone, which occurs as the capping of many of the hills of the iron-bearing area and completely covers both Laurentian and Huronian rocks a few miles east of the last known exjDOSure of ore at the Breen mine. 58 THE MENOMINEE IRON-BEARING DISTRICT. With regard to the structure of the district the author declares that, like their equivalents in the Marquette district, the Menominee rocks usually conft)rm in strike with the trend of the belts and dip at high angles, thus presenting their upturned edges to the observer. The conditions are thus not favorable for working out the structure of the district, especially in view of the difficulty in distinguishing between cleavage and bedding in the slates and chlorite-schists. The north and south ranges taken together constitute a great east-west anticline, of which the Laurentian area is the great backbone on and against which the iron series reposes. In order to picture the structure two sections across the district are described, one crossing the Menominee River at Sturgeon Falls in sec. 27, T. 39 N., R. 29 W., and the other passing just east of Lake Antoine. The lowermost rocks in the first section (fig. 6) are unmistakably Laurentian (D). They comprise granite, syenite, various gneisses and schists, and some chloritic and talcose slates. Separating these from the Menominee L. i. j \ I St. Mary 8 sandstone (Potsdam) . Keweenaw ( copper series ) Wanting. Granite (eruptive?), gneiss, liornblende, Huronian (iron bearing). Tipper.. actinolite, mica-, chlorite-, and quartz- scliists, iron ores, clay and carbonaceous slate, quartzite, and conglomerate. Middle . . .Clay slate and quartzite. - Lower Dolomite, iron ore, and quartzite. Laurentian (not subdivided) Granite, gneiss, and crystalline schists. 64 THE MENOMINEE IRON-BEARING DISTRICT. In a second table (facing p. 437) the Huroniau rocks are divided into two groups — an eruptive group, embracing diorite, gabbro, granite, and diabase, and a metamorphic group, including granite, greenstone, syenite, quartzite, limestone, and the various schists so abundant in the district. In the Menominee region the granite is declared to exist as dikes, which are said to be more frequent in the upper division of the series than in the lower divisions. The diorites and gabbros occur as conformable beds. Most of them are believed to be metamorphosed sediments. The Lauren- tian rocks are likewise separated into an eruptive group, which includes granite and greenstone, and a group composed of metamorphosed sediments. This latter group includes gray granites, gneisses, and schists, with the addition, probably, of granulite and quartzite. In the first chapter of the report the author describes the Huronian and Laurentian series in some detail. The Lower Huronian includes beds I-VII. Of these, beds I, III, and IV are little known. Bed II is the great lower quartzite and bed V the great marble bed, which in the Sturgeon River district is almost as prominent as the quartzite. Overlying the mar- ble is the great iron horizon. It is coextensive with the marble. This belt is best exposed along the line of the Chicago and Northwestern Railway. It is believed to be connected with a similar ore belt north of Lake Antoine by an anticline which pitches westward under Wisconsin. The Middle Huronian members include beds VIII-XIII. They consist of quartzites, clay slates, and schists. The Upper Huronian embraces mica-schists, gneisses, and granites. It comprises beds XIV-XX, represented best in the exposures on and near the Menominee River. The schists of the series dip at a high angle to the south, and apparently underlie the granite and gneiss observed south of the Big Quinnesec Falls. In consequence of the sharply folded character of the Menominee rocks, the total thickness of the series can not be estimated with a close degree of accuracy. Excluding the granite (bed XX), which the author is inclined to regard as eruptive, the thickness of the series is supposed to be from 10,000 to 15,000 feet. This estimate is from 4,000 to 9,000 feet less than that obtained by the measurement of the individual beds at different places within the limits of the district and the addition of the results thus obtained. BIBLIOGRAPHY AND ABSTRACT OF LITERATURE. 65 Approximate thickness of the Hwronian strata in the Menominee region. Bed. Where observed. Maximum thickness. XIX Michigamme River . .. Fed. 4,000 XVIII Brule River 1,900 XVII Pine River . 1,400 XVI Sturgeon Falls 1,700 XV Sturgeon Falls 900 XIV Pine River 800 XIII Fourfoot Falls 700 XII Fonrfoot Falls 200 XI Fourfoot Falls 1,000 X .300 (?) 400 (?) 1,000 (?) 700 IX VIII- VII .. Lake Hanburv VI South belt iron formation V Marble, south belt 1,700 IV-IIJ Pine Creek 1,000 (?) 1,000 (?) 300 II Sees. 7, 8, T. 39 N., R. 28 AV. Falls of Sturgeon River , Michigan I 19, 000 The character of the different formations met with in the Menominee district and their equivalency with the beds constituting tlie Marquette Huronian are exhibited in the following table. The correlation of the Menominee rocks with those of the Marquette, the Penokee, and the Sunday Lake series is so complete that the author thinks that it points to the fact that the rocks of these different districts were all formed in one basin under essentially like conditions. The Roman numerals affixed indicate to which beds of the Marquette series, as worked out by the author, the Menominee beds correspond. Table showing the character of the formations in the Menominee district and their equivalency with those of the Marquette district. XX. Granite, rarely gneissic, perhaps also including the rocks at Peminee Falls. XIX. Hornblende- and tremolite-schist, greenstone, gabbro, and diabase. The gabbro has the appearance of being eruptive. XVII. Gneisses and schists. XVI. Gabbro, diorite, diabase, and schistose greenstones. Sericite- , magnesian, and greenstone-schists, and serpentine. MON XLVI — 04 5 66 THE MENOMINEE IRON-BEARING DISTRICT. XV-VIII. Covered by drift in Menominee Vallej', in vicinity of Quinnesec. At moutti of Sturgeon Eiver covered with drift for a mile soutli of bed XYI. Lower portion of series consist of slates, greenstones, slates, quartzite and mica-schist, and greenstone. VII. Hydromicaceous schist, graduating into clay slate. YI. Iron ore. Y. Dolomitic marble. lY-III. Covered. II. Quartzite. I. Chloritic gneiss, hydrous magnesian schists, slate conglomerates, quartzite, and perhaps diorite (may belong with Laurentian). Nonconformable with Laurentian. Many details are given concerning the exposures of each of the formations, and a number of large-scale maps are published which exhibit well the distribution of these exposures in different portions of the district. The iron-ore rocks include magnetites, magnetic quartzose, and mag- netic amphibole rocks; specular hematites and martites; siliceous, jaspery, and argillaceous hematites; and limonitic quartzose ore. Tlu'ee beds have produced merchantable ore, viz, VI, XV, XIII. The magnetites and the specular hematites grade into each other through martite and the hematites into limonites, but the latter do not pass into magnetites. The quartzites are sometimes conglomeratic and sometimes schistose. The occurrence of conglomerates at the Falls of the Sturgeon is described in great detail. The author is now assured that they mark an unconformity between the Laurentian granites and g-neisses and the Huronian beds. The sequence of rocks beginning at the basin below the falls, i. e,, between the quartzites and the granite-gneiss complex, is given as follows: 1. On the south side of this point, hence forming the north shore of the basin, is a considerable bed of a soft, fine-gnxined rock, apparently a chloro-argillaceous, arenaceous schist. * * * The strong cleavage planes strike N. 80- W., dip 60° S. A somewhat distinct banding had a strike N. 75° W., and vertical dip. As no rock is exposed for some distance south, this schist may have considerable thickness, and in j^art underlie the basin. 2. North of the schist is S feet in thickness of a reddish-gray quartzite. 3. A thin bed of a schistose conglomerate holding pebbles of what appear to be Laurentian granite and gneiss and white quartz, loosely bedded in a matrix resembling 1. * * * 4. Five feet of schist similar to 1. 5. Eight feet of conglomerate similar to 3. 6. Three feet of schist similar to 1, which brings us to a narrow part of the river and ends the series, for on the opposite side is Laurentian granitic gneiss. * * * BIBLIOGRAPHY AND ABSTRACT OF LITERATURE. 67 The bedding- of the coiig-lomerate and schistose beds 1 to 6, above described, was unmistakable, being- N. 80° W., with vei'tical dip; hence essentially parallel with the great Huronian quartzite which overlies them on the south. * * ->^ The structural facts in connection with the strong- lithological affinities which the schist-conglomerate series bear to the Huronian, and the still more important fact that the pebbles contained in the conglomerate are unmistakablj^ Laurentian, leave no question in my own mind but that the rocks under consideration are Huronian, and form the base of the series at this point [pp. 467-468]. Tlie dolomitic marble is qnartzitic. It is associated with beds of uovacnlite and clay slates. The latter rocks are found also as beds inter- stratified with quartzites and actinolite-schists, as layers alternatino- with the ores and with mag-nesian schists, and as independent beds constituting a distinct fornnation. The chloritic rocks are so closely related to the argillaceous ones that the author finds it difficult to draw the line between the two. On the one hand the schists appear to grade through argillo-chloritic schists into clay slates and iron ores, through quartzose varieties into quartzites and through micaceous varieties into inica-schists ; and on the other hand they appear to grade into diabases and diorites, of which they seem to be altered varieties. The greenstones include diabases and diorites and a series of fine- grained undetermined rocks that appear to be connected with these. The special forms of greenstone that have been made out are diorite, diabase, gabbro, and serpentine. These grade into hornblende rock, hornblende- gneiss, mica-schist, mica-gneiss, chlorite-schists, and kersantite. The schists occurring- along the Menominee River are actinolite-schists, tremolite-schists, micaceous schists, and sericite-schists. The rocks formerly supposed to be talcose schists are now known to be sericite-schists. Of the maps accompanying the volume in which Brooks's report is published two are by the author, with the assistance of C E. Wright and others. The most important of these is the geological "map of the Menominee iron region," a portion of which is reproduced in PI. IV. This map illustrates the distribution of exposures within the district studied. It is accompanied by two sections, one along the line C-C passing- between Lakes Antoine and Fumee, and the other along the line D-D near the Sturgeon River. These are reproduced in figs. 8 and 9. The second map embodies the author's views concerning the folding of 68 THE MENOMINEE IRON-BEARING DISTRICT. the district. ^_ 0) ^ =s \ I c S On it are two ideal sections illustrating his conception of the folding in the neighborhood of Quinuesec and in the vicinity of Iron Mountain. (See figs. 10 and 11.) "WiCHMANN, Aethuk. Micr'oscoplcal ob- servations of the iron-bearing (Huronian) rocks from the region south of Lake Superior. Geology of Wisconsin, Survey of ] 8T3-1879, vol. 3, pp. 600-656. 1880. Many of the rocks of the Marquette and the Menominee districts were submit- ted to Dr. Wichmann for microscopical study. His results are embodied in the paper which constitutes Chapter V of Brooks's report. Among the rocks de- scribed from the Menominee district may be mentioned siliceous dolomite, quartz- ite, actinolite- magnetite -schist, serpen- tine, diabase, quartz-diabase, mica-gneiss, sericite-gneiss, chlorite-mica-schist, cal- careous mica- schist, sericite- schist, hornblende-schist, clay slates, and cal- careous and feiTuginous sandstones. The serpentine is said to occur in sec. 27, T. 39 N., R. 29 W. B £ 2 o i «.2 O ' 5 S ^ > -^ ■- i'g| he fao PS o ■« o Brooks, T. B. Sketch of the Laurentian rocks of Michigan. Geolog}^ of Wisconsin, Survey of 1873-1879, vol. 3, pp. 661-663. 1880. g»'f:i Mica-gneiss, hornblende -gneiss, I III hornblende-schist, chloritic gneiss, gran- M ll ite, and hornbleudic gi-anite are declared bv Brooks to be the characteristic rocks of the Laurentian in Michigan. Horn- blende-schists are said to be especially abundant in the Menominee district. By the alteration of the hornblende into chlorite, the horn- blende-gneiss passes into chloritic gneiss, which also is I ■■§ s I abundant in the Menominee district. The srranite is "that extreme massive BIBLIOGRAPHY AND ABSTRACT OF LITERATURE. 69 variety of gneiss in whicli all interior evidence of bedding is obliterated by metamorphic action. I assume it to be an altered sedimentary rock, as it apparently must be from its structural relations with the other beds, the granite dikes and certain great irregular red masses not being included" (p. 662). Among the varieties of granite observed in the Menominee dis- trict, a red, massive granite, a fine-grained, white variety, and a porphyritic. Upper -Sv^ Lower V* Lower / Granite / 1 1 1 1 / / / / / / / / / / // / \„ 1" Granite 0 (Laurentian) 3 milea (approx.) Fig. 10. — structure section across the Menominee region tlirough tlie west end of Lake Fiimee. After T. B. Brooks. 1880. gneissoid variety are most common. The hornblendic granite is not found in the Menominee district. The superposition of the beds in the Laurentian can not often be made out, owing to the complicated folding and the uniformity in the lithological Upp -^/Middle--. / ..-Ubwer-. '<."' Upper / / / I' / ( 1 / / / / V \ w \ \ / XiH. XVIII XV XIV IX VI II 0 V ' I 2 miles (approximate) Fig. 11.— Structure section across the Menominee district in the vicinity of Twin Falls. After T. B. Brook.?, 1880. character of the different rocks. Cutting these rocks are granite and g-reen- stone, among the latter of which are Some dolerites. Wright, Charles E. The geology of the Menominee iron region (economic resources, Hthology and westerlj^ extension). Geology of Wisconsin, Surve}' of 1S73-1S79, vol. 3, pp. 665-734, and atlas sheet xxx. 1880. In Brooks's report the chief interest centers in the scientific problems presented by the Menominee iron region. In the present report Wright deals with the economic geology of the district. He describes the progress of the work at each of the mines in operation in 1879, gives analyses of their ores, and mentions some interesting details concerning the relation of the ore bodies to the surrounding rocks in some of the mines. At the Vulcan mine an ore lens occurs in the midst of jaspery schists, into which 70 THE :\JEXOMINEE IRON-BEARING DISTRICT. the former passes without any break in the stratification. The author's impression regarding the Menominee ore deposits is that they are of a secondary nature. He thinks that the ores "were originally the same as the jaspery specular schists in which they occur, and have been brought to their present condition by the dissolving out of the silica from the lean schists" (p. 671). The author is inclined to the opinion that at the Saginaw mine, in the southwest quarter of sec. 4, T. 39 N., R. 29 W., there is a narrow syncline in the marble beds. If this is a fact, it points to the existence of a second ore belt to the south of the one on which the mine is situated, and it is on the eastward extension of this second belt that the Vulcan and Curry mines are opened. On the east side of the Norway property the formation is much disturbed and some of the beds are actually brecciated. At the Quinuesec mine the formation has a dip of 70° N. South of the ore belt the dip becomes steeper, then vertical, and then there is a southerly dip. From a consideration of other phenomena the author thinks there is here an indica- tion of an anticline dipping west, and that this again indicates a second ore belt farther south. Nearly all the ores are declared to be strictly first class. "Many of them contain quite a percentage of lime, magnesia, and alumina, all desir- able elements as impurities. * * * The sulphur in the majority of these ores is hardly worth considering, while the phosphorous is remarkably low" (p. 678). In Chapter II the author describes the lithology of the beds constitut- ing the Huronian series. He divides the rocks, on the basis of a microscop- ical examination, into calcareous rocks, quartzose rocks, including quartzites, mica-schists and various quartz-schistS, liornblende-schists, and hornblende rocks; greenstones, including diorite and diabase; and schists and slates, including chloritic, talcose, and argillaceous varieties. The chloritic schists are closely allied to the greenstones. Their manner of association indicates that they ma}- be greenstone tuff's, although it is possible that they may be metamorphic beds. The microscopical features of all the different varieties of all these rocks are briefly described. Among the greenstones one diabase is recognized, though it does not occur on the Michigan side of the river. The map accompanying the report exhibits only the distribution of the iron formation and the greenstones. It contains no information not on Brooks's map. BIBLIOGRAPHY AND ABSTRACT OF LITERATURE. 71 Numerical index to specimens from the Menominee region. Described by Messrs. Broolcs, Wicliman, Wright, and others. Geology of Wisconsin, Survey of 1873-1879, vol. 3, pp. 735-741. 1880. This is a list of the specimens collected by the above-named authors from various portions of the Menominee district. 1881. RoMiNGER, C. Menominee iron region: Geol. Surv. Michigan, vol. i, pp. 157-2il. 1881. In his report on the geology of the Menominee district Rominger does not attempt to classify the rock beds into groups, as they were arranged in the report on the Marquette district, but describes in minute detail a large number of exposures in the district and makes a few general remarks on the succession of the beds, comparing them with the succession in the Marquette district. At the Bi'een and the Emmett mines, the most easterly ones on the range, the ore-bearing beds are seen to dip south, the highest beds being white and red mottled hydrornica- schists. Below these is a large series of thin-bedded siliceous and argillaceous rocks impreg-nated with hematite and martite. Interlami- nated between them are seams of nonstratified, reddish-brown ore, which "are evidently a secondary product of lixiviation of the strata b}' per- colating water" (p. 158). In the pits of the Breen mine there is a contact of the Silurian sandstone with the neai-ly vertical strata. The lower ledges of the sand rock generally consist of a breccia containing angular fragments of ore and of the ore-bearing rocks. Cracks and cavities in the Huronian rocks are often filled with sandstone, so that in some places the younger sandstone appears to be beneath the older schisfs. After tracing the ore belt to the west, lie concludes that the ore- bearing series amounts to more than 1,000 feet in thickness. North of it are ridges of limestone, and north of the limestone is a large series of "flaggy rock beds, richly impregnated with bright specvilar iron-oxide granules." The rocks are sandy quartzose beds that are often so richly bespangled with hematite as to resemble the specular ore of the Marquette range. North of this ore belt no rocks are met with for a quarter of a mile, when a belt of quartzite ledges is reached having a thickness of not less than 1,000 feet. The quartzite dips noi'therly, sometimes northwest, and sometimes northeast, and rests unconformably on the granite at the Falls of the Sturgeon River. 72 THE MENOMINEE IRON-BEARING DISTRICT. South of the East Vulcan mine, on the west hne of sec. 13, T. 39 N., R. 29 W., crystalline diorites are apparently interbedded with slates and quartzites. They are regarded as intrusive. The rocks west of the East Vulcan are the same in character as those east of the mine, and their relations to one another are the same. lu the western of the Vulcan pits the ore beds dip only 20° to 30°, though in the eastern pits they are nearly vertical. The hills south of Lake Hanbury are formed of "a large succession of dark, blackish-colored clay slates, merging into various modifications of lighter gray-colored, pale silky -shining micaceo-quartzose and feldspathic schists, which contain a considerable proportion of carbonate of lime, or of sparry carbonate of iron, and of numerous interlaminated belts of dark-colored granular quartzites" (p. 164). The author agrees with Major Bi'ooks in supposing a repetition of strata in this belt of exposures, one-fourth of a mile wide, due to plication, although he could observe "no synclinal and anticlinal position of the ledges." On the north side of the ledges the dip is clearly southward, in the center of the ledge it is vertical, and on the south side in many places a northern dip is observed, but no significance is attach(;d to the phenomena. North of the Curry mine it is noticed that the upper layers of the liiTiestone formation are quartzitic and sometimes brecciated. Occasionally beds of conglomerate are interbedded with the series, whose thickness here amounts to about 400 to 500 feet. In the pits of the Saginaw mine in the southwest quarter of sec. 4, T. 39 N., R. 29 W., the ore belt is inclosed in well-laminated, thin-bedded, partly siliceous, partly argillitic beds rich in iron oxides. At the Norway mine the limestone appears to be underlain by light-colored, reddish, gray, or greenish slates, interbedded with which are arenaceous seams rich in mica scales. At the Cyclops mine is another xinconformable contact of the Silurian sandstone upon the Huronian schists. Here the latter rocks are hollowed out into a trough in which the sandstone is deposited. The creek draining Lake Fumee passes through the limestone forma- tion in sec. 35, T. 40 N., R. 30 W., exposing several successive synclinal and anticlinal arches, the measured thickness of the beds being 600 to 700 feet. The limestone here is dolomitic. It contains larg-e belts of calcare- ous breccia, and is often full of quartzose seams parallel to the stratification. Certain ledges consist exclusively of flinty quartz. The author also notes the existence of a range of limestone hills north BIBLIOGRAPHY AND ABSTRACT OF LITERATURE. 73 of Lake Fumee and an outcropping' of quartzite south of it. The quartzite is considered to be identical with the upper quartzose beds of the limestone formation, and not to be equivalent to the lower quartzite at the Falls of the Sturgeon River. This limestone ridge, surrounded on lioth sides by low, swampy lands, is directly north of the limestone belt which underlies the ore formation at the Norway and Stephenson mines, a little over a mile.apart from it, with a swamp valley between them. Both belts dip to the south. We must therefore either suggest a rupture of the rock belt in the intervening space, or an intervening synclinal trough connecting the two. The same correspondence in dip exists between the limestone bluffs on the roadside near Quinnesec and the equivalent quartzose belts on the north side of Lake Fumee; but in that case we can prove by natural exposures the occurrence of a repeated plication of the rock belt in the intervening space. Besides the locality already mentioned in which this plication of the beds is seen, there is another one handy for observation in the southeast quarter of the northeast quarter of sec. 34, on the roadside to Lake Antoine, where the limestones dip northward in anticlinal position with the more southern outcrops. An anticlinal position exists also between the limestones exposed in the south slope of the Quinnesec ore range and those on the north slope of the range dipping under the bed of Lake Antoine, and great probability exists for the occurence of a synclinal trough of limestone in the place where the basins of Lake Antoine and Lake Fumee are now [p. 181]. Recapitulating the so far ascertained facts, we have become acquainted with three distinct groups of rock, one succeeding the other conformably, or at least in direct superposition on the other. The most southern, seemingly uppermost, is a series of dark, gray-colored slaty or schistose beds, with interlaminated quartzose belts, amounting to a thickness of perhaps over 2,000 feet, which I will call the Lake Hanhury slate group. A second group next succeeding it consists in the upper part of light-red, or whitish, or gray-colored, hydromicaceous and argillitic strata: in the lower, of siliceous beds richly impregnated with iron oxide in the amorphous hematitic condition, or in the crystalline form of raartite, with metallic luster, which lower series incloses seams almost exclusively composed of martite granules, consti- tuting the economically valuable ore deposits. This group I will name the Quinnesec ore formation; it amounts to a thickness of not less than 1,000 feet, but locally, perhaps, it is much thicker. The third group is formed of a series of light-colored quartzite and limestone beds of a siliceous character, usually in part of a brecciated structure, and also amounting to at least 1,000 feet in thickness, which I will call the Norway limestone helt. All these strata are upheaved in a certain axial direction, which is about west-northwest, and dip southward, if we consider them as a body, and overlook folds of the strata and other local irregularities [p. 182]. The author is not certain that the succession here given and the succession as marked out to the north and to the south of this known belt 74 THE MENOMINEE IRON-BEARING DISTRICT. is correct, because of the difficulty of deciding properly as to the relative positions of the rock beds, on account of folding, faults, and the covering of critical areas by drift. North of the limestone, in the southwest quarter of sec. 7, T. 39 N., R. 28 "W., the author tinds an occurrence of siliceous specular flag ores dipping southward under the limestone, which also has a southern dip. Again, north of the Norway mine is a belt of micaceous slaty argillites over 200 steps in width, and in the southeast quarter of the southeast quarter sec. 32, T. 40 N., R. 29 W., is a series of siliceous flags, argillites, and quartzose beds, consisting of alternating narrow bands of a jaspery quartz and of a compact siliceous iron ore. North of this bed again is a bed of graphitic slate 8 feet wide, beds of banded micaceous quartz-schists, and again graphitic slates. This recurrence of beds to the north of the principal iron-bearing formation is explained as due to a synclinal trough, supposed to exist in the limestone. This supposition requires the turning over of the series as usually seen. To quote the author: The hitherto described rock series of the Menominee iron range allows a much more simjale and harmonious explanation of its structure if we suggest a synclinal trough of limestone in this place and revert the generally observed order in the superposition of the rock beds, considering the most southern, apparently highest rock beds of the Lake Hanbury series as the lowest, directly succeeding above the diorites south of the Menominee; next higher would be the iron formation, and highest or youngest the limestone formation and strata north of it. This order is actually exhibited on the east side of Sturgeon River * * * and in the Chapin mine and Quinnesec mine. In all other described localities, according to this theory, which I believe to be the fact, the strata have been placed in an overtilted position by the upheaval acting most powerfully from south to north, whereby the limestones came to lie beneath the others, and were by me at first mistaken for the oldest in the succession [p. 186]. Immediately north of Pine Creek, in T. 40 N., R. 29 W., are ridges of quar.zite, and north of them, and apparently in discordant contact with the quartzites, are bluffs of granite. This great quartzite, over 1,000 feet thick, begins in sec. 10, T. 39 N., R. 28 W., and runs westward in an almost contin- uous ridge to the Falls of the Sturgeon River and beyond for the distance of a mile or so. Of the limestone belt shown in Major Brooks's map south of the falls Dr. Rominger did not see the slightest evidence. BIBLIOGRAPHY AND ABSTRACT OF LITERATURE. 75 The relations between the quartzite and granite at the falls are thus described : The bed rock of the falls is granite, which evidently forms here an arched, bubble-like protrusion, dipping in all directions; the sedimentarj^ strata above this bubble have likewise been pushed aside with the same irregularity. Thej' have in places become entangled between the granite, and seem to dip under it, but the same ledges are seen in another place dipping in a different direction and to lie above the granite. * * * Interstratified with the granite are belts of dark diorite-like rock, consisiting of quartz, white, probably anorthic feldspar, and of a large pro- portion of black mica. * * * In intimate, seemingl}' conformable contact with the granite occurs a series of schistose beds, which are exposed at the foot of the falls in an almost vertical position, but they seem to be in an anticlinal position on the two sides of the river. On the west side are stratified red-colored feldspathic rocks, crowded with granules of magnetite and witli iron pyrites in small cubes; with them occur narrow seams of a rich granular magnetic ore contaminated with iron pyrites. This belt amounts to about 8 or 10 feet, and the strata dip under the granite. Next and below it is a rock belt, 10 feet wide, of schistose, feldspathic, and sericitic beds; then comes a seam of compact, finely granular, red feldspar 20 feet wide. * * * Under it succeed silky, shining gray sericite-schists, with a feldspathic groundmass; then a break in the formation occurs, and the same feldspathic sericite-schists dip in an opposite direction, away from the granite, and farther on in this direction we soon come to quartzite ledges, apparently incumbent on them. On the east side of the river we find similar hard sericite-schists, with interlaminated granular feldspar seams, and with several belts [of] a coarse conglomerate rock formed of red granite pebbles and of white quartz pebbles, some of them opalescent. The cement is the schistose sericite. Some of the granular feldspathic beds of the schists are distinctly ripple marked. Going across this schistose belt, which amounts to about 100 feet, we come again into granite, which seems to be in conformable contiguity with the schists. We have here evidentlj^ 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, so as to appear as the lower beds [pp. 190-192]. North of the falls for miles nothing but granite was seen, except where this rock is replaced by gneiss and is cut by "doleritic" and other basic dikes. Passing to the green schists exposed so abundantly on the Menominee River, the author recoimts their occurrence in order, beginning vvith the outcrops at Twin Falls. The greenstone-schist or diorite formation is rarely in contact with the iron formation, and even when in contact the relations between the two rock series are not clearly ascertainable. 76 THE MENOMINEE IRON-BEARING DISTRICT. * * * The dioritic rocks generallj' play the part of an intrusive rock with regard to the strictly sedimentary rock beds of the Huronian series. A superposition of the diorite formation on the Lake Hanbury rock series, which adjoins it the whole length of the Menominee Valley from the Upper [Big] Quinnesec Falls to the Stur- geon 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, R. 29, where, in the center of the section, a hill is foi'med of the vertical ledges of ferrugino-siliceous flagstones and slaty beds repre- senting the Lake Hanbury series, and about two or three hundred steps from these exposures we find, on the south side of the road to Menominee, small hillocks of diorite. * * * My reasons for holding the dioritic rocks south of the iron formation as older than the latter are based on the lithological similaritj" of this formation with the dioritic gi'oup of the Marquette district, and on the degree of metamorphism exhibited by the two groups, the dioritic and the iron-bearing. In the great succession of strata commencing with the Hanbury 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 j^ounger, a stratified structure is also recognizable, but not one of these thousands of feet of ledges exhibits its original sedimentary lamination with any degree of distinctness like the others; they have evidently- been transformed undei' coopei'ation of heat and partially brought into a plastic condition. * * * One might object: If the diorites are the older beds, why don't we find them just as well developed on the north side of the upheaved beds, between the quartzite and the granite? The sandy and conglomeratic nature of manj^ of the strata of the quartzite and iron formation proves them to be shore deposits, while the dioritic group consists onh^ of the finer material of deep-sea deposits, which explains the pojnt 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 6-mile- long chain of diorite extending eastward from the Twin Falls. * * * The equal dip of the strata to the south in these adjoining formations is not necessarily proof of the j'ounger age of the most southern beds. The whole succession is so near to a vertical position that in many instances it has been left uncertain which way thev dip. but suppose their dip is conformable to the south; the upheaval of the diorites bj^ 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 succession of beds as an inverted one, the seemingly lowest beds being actually the youngest [pp. 208-210]. The author describes the exposures along the Menominee River in detail as fine- and coarse-grained greenstone-schists, micaceous chlorite- schists, light-colored, grayish-green porphyritic schists containing well- BIBLIOGRAPHY AND ABSTRACT OF LITERATURE. 77 formed orthoclase crystals, inicaceo-feldspathic seams, and beds of pale-red schists composed of orthoclase, hydroraica, and quartz, etc. At the Big Quinnesec Falls are also raas.sive diorites. These and the diorite-schists have the same composition, and neither, according- to the author, should be classed with the diabases. The whole series of rock beds exposed at these falls "evidently represents one inseparable group of altered sedimentary deposits formed from bottom to top of the same material, in different molecular form and different proportion in the intermixture of the component mineral substance" (p. 214). The rocks at the Little Quinnesec Falls and at the Sturgeon Falls are similar to those above described. Some little distance below the mouth of the Sturgeon River are said to be exposures of serpentine, and below these are great outcrops of feldspar, porphyries, granites, etc., some of which are thought to be like the granite and porphyry dikes cutting the greenstone schists of the "diorite group" in the Marquette district. Reviewing what we have seen of the formations along the Menominee River, said to be the youngest of the Huronian group, I again point out the great similarity in the composition and structure of this very lai-ge series of rocks with the dioritic formation of the Marquette district; also its intersection bj' the serpentine group, which in the Marquette district is under similar circumstances associated with the diorite formation. Further in favor of this analogy is the intersection of the Menominee diorites by porijhyritic granite in dike form, as is the case with the diorite grovip of Marquette. These porphyritic grahites are in their part in close relationship with the felsite porphyrj^ of the Pemenee Falls, merging by insensible gradations with the granite, which is only a more completely crystallized form of the same lava mass. On the other hand, there exists not the slightest resemblance between the dioritic rock belt of the Menominee River and those rock beds of the Marquette district which represent subdivisions 15 to 20 of Major Brooks. The exact order in which the different rock masses composing the dioritic forma- tion succeed each other — whether the dark-green diorites of the Twin Falls and in other places are the lowest and the lighter-colored diorites at the Quinnesec Falls the higher ones — is at the present state of our knowledge uncertain, but it is most likely the case; so the dark-colored, coarsely crystalline hornblende rocks exposed a mile above the Pemenee Falls may be older beds than those at the Quinnesec Falls. The massive belts of this series of altered sedimentary rocks, interlaminated with the schistose members, can, as I think, not all be considered as regular links in the stratified succession; some of these, and particularly the larger masses, as they occur at the Quiver Falls, 1 believe to be intrusive, in the same qualified sense in which I have considered some of the massive dioritic rock belts of the Marquette district, and still in another sense they represent only a more altered portion of the stratified beds 78 THE MENOMINEE IRON-BEARING DISTRICT. connected with them. Considering the g-ninite on the soutli and west side of Menominee Valley as an eruptive rock, like the porphyiy of the Pemenee Falls, 1 can agree full}' with Major Brooks in this part of his chronological system; these rocks undoubtedly came to the surface after all the other Huronian strata of sedi- mentary origin were formed, as their eruption to the surface caused the upheaval of the others. I therefore have always represented the dike granites of the Marquette district as actually the youngest rocks in the group, but I suppose this was not the original meaning of Major Brooks's system. In all his stratigraphical descriptions he has not made a proper distinction between sedimentary succession and interstrati- fication, counting up the beds just as they came in a crosscut; his groups VII, IX, and XI are a proof of this assertion [pp. 221-222]. Ill conclusidii the author compares the rocks of the Menominee with those of the Marquette district, all of which are reo^arded as Huronian in age. 1883. Irving, R. D. Iron ores: Geology of Wisconsin, Survey of 1873-1879. vol. 1, pt. 3, pp. 613-636. 1SS3. After describing in general the iron ores found in Wisconsin, Irving describes briefly the geology of the difierent iron-ore districts. In the Menominee district the jjriucipal Huronian rocks are said to be hornblendic and micElceous schists, clay slate, chloritic schist, actiuolitic schist, limestone, diorite, diabase, and iron ore. The diorite and diabase are thought by the author always to be of eruptive origin, and to occur in part as interbedded contemporaneous flows and in part as intrusions. The schists are intri- cately folded. Any mapping of the folds must be in a large measure hypothetical, because of the numerous faults by which the beds are crossed and because of the existence in them of masses of eruptive material. The ore bodies of the range are irregular, lens-shaped masses or portions of the belt richer than the rest. These lenses, with one or two exceptions, differ from most of those met with in the Marquette region, in that the latter are distinctly inter- calated, the beds above and below them closing about them, while in this case the iron oxide simply impregnates certain areas of the stratum, whose subordinate layers continue undeflected through the ore bodies. The ores are of the peculiar '"soft specular" variety already noted as found only in the ^Menominee region [p. 621]. There are two ore horizons in the district according to Brooks, the lower of which was numbered VI in the series. This is the horizon on which all of the Michigan mines are located. The second horizon is higher in the series, but its exact position Is unknown. BIBLIOGRAPHY AND ABSTRACT OF LITERATURE. 79 On a later page in the same article several analyses of Menominee ores are given, most of them being copied from Wright's report. Crednee, H. Eleniente der Geolog-ie, pp. 400^10. Leipzig. 1883. Credner divides the pre-Gambrian formations into the Urgneiss for- mation (the Laurentian) and the Ur-Schiefer formation (the Huronian). In the latter he places the Menominee rocks, stating that the dolomitic limestone in this district is near the lower limit of the series. He gives a section across the district from north to sonth, which is a copy from his paper on this district. The Ur Schiefer formation in the Menominee, as well as elsewhere, is unconformable under Lower Silurian sediments. In this district the iron ore is overlain in places by sandstones and conglom- erates, which are found sometimes extending down into cracks in the ores. A section is published to illustrate this relation. A few other references are made to the Menominee rocks in the author's discussion of the Archean, but nothing new is recorded concerning them. Ikving, R. D. The copper-bearing rocks of Lake Superior: Mon. U. S. GeoL Surve}' , vok .5, pp. 392-400, with maps, including a general map of the Lake Superior region. 1883. In connection with his discussion of the Keweenawan rocks, Irving' briefly describes the characteristics of the lithology of the Marquette and of the Menominee Huronian. The rocks belonging to these series afford, at first glance, a strong contrast to the Penokee Huronian. Close study, however, shows that there is a general stratigraphical equivalence between the three series, and perhaps even a direct connection between them. The author compares the different series and shows their points of resemblance and of difference. The diorites of the Marquette and Menominee regions he suspects to be altered diabases. The granite of the Menominee district has not been satisfactorily shown to be Huronian. It may be eruptive. Many of the less common kinds of rocks in the Marquette and Menominee districts are thought to be due to metamorphic changes, which may be connected with the complex folding observed. In the Menominee, as well as in the other Huronian areas, the Huronian schists are limited by granites and gneisses. Usually the latter rocks are unconformably beneath the schists and in all cases, save that of the so-called Hiaronian granite in the Menominee district on the south side of the 80 THE MENOMINEE IRON-BEARING DISTRICT. Meuomiuee River, the granite and gneiss complex plainly rises from beneath the schists. Most of the crystalline schists associated with the granites and gneisses are older than the Huronian schists. 1884.. WiNCHELL, N. H. The crystalline rocks of the Northwest: Am. Nat., vol. 18, pp. 984-1000. 1884. In this article there is nothing new concerning the geology of the Menominee district. The anther simply classes all the crystalline rocks of the Lake Superior region into six groups and states his views as to the equivalency between these groups and those established by other w- iters on the different crystalline areas of the Northwest, among which is included the Menominee area. Irving, R. D., and Van Hise, C. R. On secondary enlargements of mineral fragments in certain rocks: Bull. U. S. Geol. Survey No. 8, p. .39. 1885. One of the specimens described by the authors to illustrate their con- tention that fragments of minerals in clastic rocks are often enlarged by the deposition of secondary material around them is from the bed of Cambrian sandstone immediately above an unconformable contact with Huronian iron ores at the Cyclops and Norway mines, near Norway, in the Menominee district. It is described as — a very much indurated, buff to brown sandstone — at times almost a vitreous quartzite. The thin section is composed almost entirely of interlocking grains of quartz each with its distinctly outlined f ragmental core. There is a little independently deposited interstitial quartz and a little f ragmental feldspar. Whitney, J. D., and Wadsworth, M. E. The Azoic system and its proposed subdivisions. Bull. Mus. Comp. Zool. Harvard College, vol. 7, pp. xvi and 331-565. 1884. One paragraph in this volume which is an argument in favor of the indivisibility of the pre-Cambrian formations criticises Rominger's views relating to Menominee geology. The authors declare that Rominger's — idea that the Marquette and Menominee schists are Huronian means nothing beyond this, that the^' appear to him to be lithologically similar to the rocks called Huronian in Canada; while so far as his actual work goes he reaches conclusions regarding the relations of the granitic and schistose rocks indentical with those advocated b}' Foster and Whitney thirty years ago. The result of Rominger's work is decidedlj' opposed to the division of the Michigan Azoic into two or more formations [p. 494]. BIBLIOGRAPHY AND ABSTRACT OF LITERATURE. 81 1885. Swank, James M. Iron ores in the United States: Mineral Resoui'ces U. S. for 1883-84. 1885. « In his summary description of the iron-producing districts of the United States the author declares that the Menominee district ranks second in productiveness to the Marquette district. About 1876 the Menominee Mining Company of Milwaukee obtained control of a large extent of country in this portion of Michigan and at once began active operation. In 1877 only 10,405 tons of ore were shipped, but thereafter the output increased rapidly, until in 1882 it rose to 1,032,611 tons, falling in 1884 to 698,047 tons. The Chapin mine was the largest producer in the entire Lake Superior region in 1883 and 1884, its product being 265,830 tons in 1883 and 290,972 tons in 1884. The ores are generally red hematites, partaking of the same general characteristics as the similar ores of the Marquette district, except that they are, as a rule, softer. Analyses of specimens of ore from the Vulcan, Cyclops, Norway, and Quinnesec mines are given (pp. 265-266). Irving, R. D. Preliminary paper on an investigation of the Archean formations of the Northwestern States: Fifth Ann. Rept. U. S. Geol. Survey, pp. 175-242, with general map of Lake Superior region. 1885. The problems that confront the investigator of the ancient rocks of the Lake Superior region are outlined by Irving in this paper and the progress made in solving them is stated. General descriptions are given of the series of supposed Huronian rocks in the different areas, among them descriptions of those occurring in the Marquette and Menominee areas, which are grouped together. Their rocks are highly folded and their structure is often very difficult to work out. Moreover, the metasomatic changes which the crystalline members of the series have undergone have often been extreme, added to which difficulties are fi-equent interruptions by drift covering. «" Mineral Resources of the United States" has been published annually by the United States Geologioal Survey since 1883. Up to and including the year 1894 it appeared as an independent pub- lication. Later it was published as part of the "Annual Report of the Director," and beginning with the year 1900 it is again issued as an independent report. It contains statistics of the ore production in all the mining districts within the United States and occasionally descriptive notes concerning these districts. In this monograph only those volumes will be referred to which contain new infor- mation of geological interest concerning the Menominee district. MON XLVI — 04 6 82 THE MENOMINEE IRON-BEARING DISTRICT. The greenstone-schists and the greenstones at the Big Quinnesec and the Little Quinnesec Falls are believed to be eruptive, the schists "repre- senting merely an extreme degree of metasomatic change, which in some ineasure has influenced all the greenstones." While declining to acknowledge the possibility of establishing a scheme of stratigraphy for these districts without the aid of good maps, Irving thinks that there is no question that Brooks's scheme is faulty with respect to the various greenstone layers included among the sedimentary beds. These greenstones -are believed by Irving to be altered basic eruptives, some of which are contemporaneous flows, and others intrusive masses. The jaspers and ores of the Menominee, like those of the Marquette district, are believed to be of sedimentary origin. In the Menominee district much of the silica in the jaspers is chalcedonic, and many great belts of ferruginous rocks seem to be mainly composed of it. The Menominee area is thought to connect westward with the area of the upper Wisconsin Valley, but detailed mapping of this westward extension had not been completed when the author wrote his paper. One plate in the report illustrates an unconformity between the Ores and the Potsdam sandstone at the Cyclops mine, Norway. In the petrograi)liical portion of the paper the author refers to the existence in the Menominee district of clay slates, some of which are fine- grained graywackes in which the feldspathic ingredients are largely represented by kaolinized material, while others present distinct gradations into true mica-slates and mica-schists, in which at least a considerable proportion of the material is of original crystallization. Other rocks are also described, but only the limestones are referred to as existing in the Menominee district proper. The cherts, however, are mentioned as being in part at least of direct chemical origin. All the quartzites in the Huronian are thought to be — merely sandstones which have received various degrees of induration by the interstitial deposition of a siliceous cement, which has generally taken the form of enlargements of the original quartz fragments, less corpmonly of minute independently oriented areas, and still less commonly of an amorphous or chalcedonic silica, two or even all three forms of the cementing silica occurring at times together in the same rock * * *. It appears that they have undergone no other alteration than that found to affect sandstones in the newer, undisturbed, and generally unaltered formations [p. 236]. BIBLIOGRAPHY AND ABSTRACT OF LITERATURE. 83 1886. Ikving, R. D. Origin of the ferruginous schists and iron ores of the Lake Superior region: Am. Jour. Sci., 3d series, vol. 32, pp. 255-278. 1886. Althoug'h the argument in this paper is based particularly on observa- tions made in other districts, the author, nevertheless, refers to the Menom- inee ores as having resulted from the alteration of an iron carbonate whose origin was sedimentary. The Menominee rocks are much folded, like those of the Marquette region, and in them the alteration of the carbonate has proceeded much further than in the unfolded series of the Penokee and other districts. In these districts all the phases of the altered carbonates are met with, but the quantity of unaltered material is much less than it is in the less folded rocks. The j^rocess by which the alteration has been effected and the proof that this alteration is responsible for the existence of the ores need not be referred to In this place. An account of these discus- sions is given in the literature chapter of the Marquette monograph." The author refers to the Menominee iron-bearing series as being so folded that the rock belts for the most part are in an approximately vertical position. The most abundant of the ferruginous rocks are cherty schists charged with varying quantities of magnetite, hematite, and brown oxide of h'on, and containing more or less iron carbonate. These schists graduate into graphitic and carbonaceous slates that are sometimes highly contorted and at other times are nearly free from contortions. These contortions have no parallel in the adjoining layers, and often seem to have little relation, in axial directions, to the general system of folding of the strata. They are taken to indicate the relatively great resistance to folding offered by these schists, on account of the siliceous induration they received prior to the folding process [p. 265J. The iron carbonate occurs in the schists as an original ingredient in varying proportions. Actinolite is present in some of them. The siliceous matter in them is more commonly cherty or flinty than jaspery, but at times it is jaspery over large areas. The iron ores lie in these schists, not as lens-shaped masses around which the schists bend, but as irregular bodies lying directly in the course of the schistose banding or as layers within the schists. The ores appear to have originated in one of two ways, « Mon. U. S. Geol. Survey, vol. 28, 1897, pp. 5-148. 84 THE MENOMINEE IRON-BEARING DISTRICT. or in a combination of them, viz, from direct oxidation of the bauds of car- bonate in place, and from deposition within the schist of oxide of iron from percolating waters. All the indications point to the derivation of both fer- ruginous rocks and iron deposits by a process of silicification from stratiform shales impregnated with carbon and iron carbonate. PuMPELLY, R. Report on the uaining industries of the United States (exclusive of the precious metals), etc. Rept. Tenth Census U. S., vol. 15. With maps. 1886. Putnam, Batard T. Notes on the samples of iron ore collected in Michigan and northern Wisconsin, Rept. Tenth Census U. S., vol. 15, p. 421. 1886. In his introduction to the discussion of analyses of Menominee iron ores Putnam gives a brief outline of the geology of the Menominee district. He recognizes two belts of iron-bearing rocks, as did Brooks before him. The Southern belt embraces the region now known as the Menominee district. Since the completion of the Chicago and Northwestern Railway to Quinnesec in 1877 the mines on the eastern portion of the Southern belt have been extensively wrought, yielding, as Pumpelly declares in his introduction to the general discussion of the iron ores, 491,347 tons during the census year 1880. The ores consist — principally of a soft, specular, blue hematite, which runs high in iron and low in phosphorus. Although quite soft, the ore usually resists hydration and rarely changes color except in handling. * * * When examined fresh from the mine the ore is seen to be made up of innumerable fine crystalline particles of specular hematite, which are somewhat loosely agglutinated. * * * The ore usually occurs in irregular pockets or lens-shaped masses in a banded quartzose ferruginous schist, which is often magnetic [p. 437]. Sketch maps of the Emmett and Breen mines, of the Vulcan, the East Vulcan, the Curry, the Saginaw and Stephenson, the Norway, the Quinne- sec, the Chapin, the Ludington, and the Cornell mines, and sections through many of them, illustrate the paper. At the Emmett mine the ores consist of an upper blue variety and a lower brown variety, both of which dip south- ward under a swamp. The blue ore is protected from the action of the swamp water by a covering of clay, while the brown oi'e is saturated with ground water which nses through the underlying schist. The blue ore contains but 0.008 per cent of phosphorus, while the brown ore contains 0.103 per cent of this element. At the Vulcan mine the ore lens dips BIBLIOGRAPHY AND ABSTRACT OF LITERATURE. 85 south and pitches west. One of the ore samples from this mine contains a small proportion of titanium, thus proving an exception to the Menominee ores in general, which are usually free from this metal. In one of the pits of this mine the ore is overlain by a ferruginous siliceous schist, which is so richly impregnated with iron near the ore body that it sometimes itself constitutes a lean ore. At the Norway mine the ore is between a foot wall of soap rock and a hanging wall of "conglomerate ore," composed of irreg- ular angular pieces of hard specular ore cemented by soft brown hematite. 1887, BiRKiNBiNE, John. The iron ores east of the Mississippi River: Mineral Resources U. S. for 1886, pp. 65-67. 1887. The author gives a biief history of the operations in the Menominee district since 1877, describes the work done at the Hamilton mine, and furnishes the usual statistics of the output of the principal mines on the range as well as of the district as a whole. Irving, R. D. Is there a Huronian group? Am. Jour. Sci., 3d series, vol. 34, pp. 204-216, 249-263, and 365-374. 1887. In this article Professor Irving gives his reasons for regarding the series of nonfossiliferous sediments lying between the top of the Archean crystal- line schists and the bottom of the Potsdam sandstone in the Lake Superior region as worthy of being considered a distinct system coordinate in importance with the Paleozoic, etc., with the Huronian as a distinct group in the system. The arguments made use of have been outlined in the Marquette monograph." In his description of a section through the Menomi- nee district, the author declares that the chief difference existing between the Marquette and the Menominee rocks lies in the much closer folding of the latter. All the arguments that apply in favor of the existence of the two formations in the Marquette district apply also in the Menominee district. A more detailed elaboration of the arguments is given in the paper on the classification of the pre-Cambrian formations, referred to below. 1888. Irving, R. D. On the classification of the early Cambrian and pre-Cambrian formations; a brief discussion of j^rinciples, ilhistrated by examples drawn mainly from the Lake Superior region: Seventh Ann. Rept. U. S. Geol. Survey, pp. 365-454. With maps, including a general map of the Lake Superior region. 1888. "Moil. U. S. Geol. Survey, vol. 28, 1897, pp. 110-112. 86 THE MENOxMINEE IRON-BEARING DISTRICT. The main portion of this paper is a discussion of the value of uncon- formities in classifying nonfossiliferous sedimentary beds. The illustrations taken from the Lake Superior region afford examples of great unconformities in all the areas that have been recognized as Huronian. In the Menominee district there are many outliers of the Potsdam sand- stone reposing horizontally on the upturned edges of the Huronian schists. A picture (p. 410) illustrates the appearance of a contact of sandstone on the steeply inclined beds of the iron-bearing schists near Norway. At the immediate contact of the two series the sandstone is filled with fragments of the schists. The iron-bearing rocks are thus demonstrably older than the sandstones. Beneath the Huronian is another series of rocks, comprising granites, gneisses, and hornblendic and micaceous schists. These rocks are closely folded, and upon them, in eroded basins, the Huronian schists are deposited. The upper series is a relatively little-altered iron-bearing series and the lower one a deeply altered series of gneiss and schists, cut by immense masses of intnisive granite. There is evidently a great discordance between the two series — a discordance that is further marked by the existence of a great basal conglomerate at the Falls of the Sturgeon River. The folding in this district is often so sharp that there is a seeming conformity between the upper and the lower series in many places. A generalized cross section of the Menominee rocks in the vicinity of Quinnesec illustrates the author's interpretation of the geology of the district (p. 435). Larsson, Per. The Chapin iron mine, Lake Superior: Trans. Am. Inst. Min. Eng., vol. 16, pp. 119-128. Map and cross sections. 1888. Larsson's article is devoted mainly to a description of the methods employed in mining at the Chapin mine, although in its introduction brief reference is made to the geology of its ore deposits. Up to July, 1887, the ore of this mine had been found in three lenses, conforming in dip and strike with the Huronian clay slates and jaspers associated with them, and pitching about 30° west. A cross section of the ore formation shows on the north or hanging side of the ore about 200 feet of clay slates, and farther north a heavy belt of magnesian limestone. The slates and the dolomite are generally separated by a conglomerate of broken dolomite and soft slates. BIBLIOGRAPHY AND ABSTRACT OF LITERATURE. 87 The foot-wall rock is also a soft slate, containing a higher percentage of iron and less magnesia than the hanging slate. Farther south occur alternate beds of slates and lean ore or jasper [p. 119]. The paper is accompanied by a geological map of the Chapin mine property, a longitudinal section of the workings, and two cross sections exhibiting the geological relations between ore, slate, and dolomite. Fulton, John. Mode of deposition of the iron ores of the Menominee ii-on range, Michigan: Trans. Am. Inst. Min. Eng., vol. 16, pp. 525-536, with 9 figures. 1888. According to this author the Menominee range is composed of three distinct groups of Huronian rocks. The "supporting group" consists of 1,200 feet of light-colored, siliceous limestones, called the Norway limestone belt. The next or flanking group which is estimated at a thickness of 1,000 feet is called the Quinnesec ore formation. It consists, in the portion next to the lime- stone, of siliceous or jasper slates, largely impregnated with iron oxide. These ai-e succeeded by argillaceous hydro-mica black and flesh-colored slates. This formation embraces the deposits of the iron oi*es. The third group consists of a series of dark-gray, slaty, or schistose beds, with occasional quartzose bands, having a thickness of 2,000 feet. It is called the Lake Hanbury slate group [p. 525]. The three groups succeed one another conformably, dipping at high angles to the south in that portion of the range east of Quinnesec and to the north in that portion west of this town. Their relative ages can not be determined from their relative positions, as "the limestone is the floor of the series east of Quinnesec, while west of this it is uppermost" (p. 527). There is a well-marked unconformity between the Huronian rocks and the overlying Potsdam sandstone, but the geology of the older rocks is com- plicated by "frequent and violent flexures and strangely postured dips" (p. 527). The rocks of the three groups are sedimentary. The iron ore at the Quinnesec mine is thin bedded, as are also the foot-wall jasper slates. The limestone is thick bedded. The ore occurs at two or three hori- zons, which, so far as is known, are not constant. At the East Vulcan mine one of the ore bodies is at the contact between the siliceous or jasper slates and the aluminous slates or "soapstone." A second is in jaspers south of the black slates. At the Curry mines there are also two horizons 88 THE MENOMINEE IRON-BEARING DISTRICT. at Avhich the ore occurs. The fii-st, most northerly, is in jasper slates and the second at the contact between the jasper and black slates. The Norway deposit is pecuHar, being separated from the Umestone by about 10 to 40 feet of brown sediment, locally termed "soap rocli." The ore body is mainly composed of a brecciated deposit of red and blue iron ore, occasionallymixed with jasper slates, and cemented in great masses [p. 531]. The underground workings reveal the existence of a fold in the limestone. The large open pits of C\'clops are very peculiar, as they are the residuum of a period of denudation and destruction of the ore body. These pits occur in the jasper slates, without any regularity. * * * The ore was evidently carried into them by some eroding current from the wreck of an original deposit. Just where it came from, or under what conditions it was swashed into these large pits can not be determined. * * * The example of a pool of ore at East Vulcan illustrates the supposed condition of these Cyclops deposits [p. 532]. At the Quinnesec mine a similar "pool" of ore is found infolded in the Potsdam sandstone. A little east of this mine a flexure occurs in the Huronian rocks, west of which fold the beds dip north. The blue ores of the Quinnesec and of the West Vulcan mines are in the form of lenses whose tops have been eroded. These are thought to represent the original form in which all the ore in the range once occurred. They are embedded in jasper and clay slates, and are stratified like these rocks. After thus describing the characteristics of many of the deposits the author proceeds to outline a theory to explain their origin. The most acceptable exposition indicates that their normal form was that of thinly bedded ferriferous carbonates, with some admixture of dusty magnetite, and that they have been wholly altered to hematite by heat in their lower geological horizon, together with the heat evolved in the pitching and folding of the rock- measures in which they are found. Chemical agencies also contributed to this result [p. 535]. The author concludes by declaring that the structure of the range can not be explained as "a continuous monocliual structure." "The more evident structure should consist of a series of crust flexures, repeating the iron-ore measures at intervals, as they rise on the crests or flanks of anticlinal waves" (p. 536). BIBLIOGRAPHY AND ABSTRACT OF LITERATURE. 89 The papei" is accompanied by a map of the district and nine geological sections through mines. Fulton, John. Methods of mining in the Menominee range, Michigan: Trans. Am. Inst. Min. Eng., vol. 16, pp. 891-906, with 8 figures. 1888. The introductory portion of this paper describes the position of the ore bodies with respect to the jaspers and slates in the Menominee district. These ore bodies are reported as being found in three or more horizons in the jasper slates and also on the contact between these and the clay slates that usually overlie them. The remainder of the paper is devoted to a description of the methods of mining on the range. 1889. Lawton, Chas. D. Mineral resources of Michigan, 1888, pp. 190-208. 1889. In this report on the mines and mineral resources of Michigan the author announces the discovery of a large percentage of gold in the rock from a pit near the bank of the Menominee River, south of Quinnesec. He also gives an account of the condition of the iron mines on the Menominee range at the close of the year 1888. In his introduction to that part of the paper which deals with the Menominee district he outlines the geology of the district as worked out by Brooks. WiNCHELL, N. H., and Winchell, H. V. On a possible chemical origin of the iron ores of the Keewatin in Minnesota: Proc. Am. Assoc. Adv. Sci., Thirty-eighth Meeting, pp. 235-242. 1889. Also Am. Geologist, vol. 4, pp. 291-300. 1889. Although this paper is devoted mainly to a theorj^ in explanation of the origin of the iron ores in Minnesota, Professor Winchell nevertheless argues against Irving's view that the ores of all the iron-ore districts in Michigan and Wisconsin are derived by metamorphic processes from an iron carbonate. Browne, D. H. The distribution of phosphorus in the Ludington mine. Iron Mountain, Michigan: Am. Jour. Sci., 3d series, vol. 37, pages i!99-310, with figures. 1889. The ore of the Ludington mine occurs in several lenses lying in clay slates. The main deposit, which is 700 feet long and about 60 feet wide, strikes N. 75° W. and pitches 45° W. Its dip is 70° to 80° N. The ore is of a soft, laminated hematite, whose layers alternate in places with thin seams of calcium-magnesium carbonate. The contact of the ore body 90 THE MENOMINEE IKON-BEARING DISTRICT. with the hanging wall is curved, while that with the foot wall is more nearly a plane surface. Upon mapping the phosphorus contents of the different samples of ore taken from the mine it was found that the foot- wall sides of the deposits are apt to contain less phosphorus than the hanging- wall sides, and that the western (lower) ends of the lenses contain less phosphorus than the eastern (upper) ends. The ore increases in phos- phorus from the foot wall to the hanging wall, and from the west to the east. Sometimes a streak of ore rich in phosphorus crosses the high-grade ore, but in general the relations just mentioned obtain. These relations are illustrated in the paper by a large number of sections through the different rooms in the mine from which ore has been removed. The explanation offered to account for the regularity in the distribu- tion of the phosphorus is as follows: The ores were aqueous deposits upon the hanging-wall slates. If we suppose that the ore was formed in hollows in the hanging wall, and was covered by the foot-wall slates, and that this bed has been tilted up from the north side through an angle of 100° to 110°, it will be readily' understood that the original trend of the deposit becomes the complement of the present pitch of the ore. This supposition exi^lains * * * the fact that what is now the hanging wall seems to have been the original bed of the deposit. It is improbable that the tilting has been from the south side upward through an angle of 70° to 80°; for if this had been the case the ore would pitch east at the same angle at which it now pitches west [p. 306]. The author does not think that Van Rise's explanation of the origin of the ores in the Gogebic district can be applied to the conditions in the Menominee district, because the slates associated with the Menominee ores contain no unaltered carbonate. The suggestion of Irving that the ore was washed into its present position from previously precipitated beds of carbonate appears to him more plausible, but he would modify the theory by making the original deposits hydrous oxides and carbonates of iron intermixed with calcareous materials. By the action of acidulated waters upon these the iron was dissolved and the solutions thus formed were evaporated in shallow lakes or valleys, yielding the ore deposits now worked. The theory of the aqueous deposit of these ore bodies, as drawn from chemical evidence, is then briefly as follows: From previously deposited beds of bog iron ore, by the action of acidulated water, iron, lime, silica, and phosphorus were dissolved. The first solution contained a large amount of phosphorus in propoition to the BIBLIOGRAPHY AND ABSTRACT OF LITERATURE. 91 amount of iron dissolved. On coming into hollows in the surface of the exposed slates the acid solution, losing acid by evaporation, deposited iron, as hydrated oxide, which carried down an amount of phosphorus proportionate to the amount of iron precipitated. As the acid became still weaker crystals of carbonate of lime and magnesia settled out, forming a layer of carbonates. A second inflow of water would tend to dissolve these crystals and precipitate another layer of iron. In similar man- ner, by successive inundations, the depressions became filled with alternating layers of iron ore and calcium-magnesium carbonates, each laj'er being as a rule lower in phos- phorus than the preceding one. * * * Moreover, as both calcium and iron phos- phate are of lower specific gravity, and more soluble than the hydrated oxide of iron, the tendency of the water was to carry these phosphates toward the lower end of the lake, and to deposit them in shallow water, along banks of previously precipitated silica and in places where precipitation was most rapid. * * * After the deposition was complete, further action of the water would stir up the upper layers of ore and mix them with suspended sand or clay, while the iron and phosphorus were carried farther along, to be deposited in other depressions to the northeast. As jasper occurs as vein matter, and in lamint« cleaving in the same line as the ore, it would seem either that the jasper has been produced by precipitation with the iron, or that subsequent action of water has eroded the beds of iron thus formed and substituted silica for the iron removed. A study of the vein map of the sixth level at the Ludington mine * * * seems to show that the jasper is a later formation than the ore. It will be seen by reference to fig. 17 [figure 22 in the article] that the jasper deposit widens toward the foot wall. * * * The greater width of the jasper at the foot wall also sug- gests an erosion of the original ore bed and a subsequent deposition of silica. Had the silica been the primary deposit the ore would be widest at the foot wall instead of at the hanging [pp. 307-308]. For the explanation of the silica deposits "bedded in the same plane with the ore," the author adopts Van Hise's hypothesis, "with the provision that subsequent erosion must be taken into consideration." The "surface deposits or washes of ore formed at Keel Ridge, Quinnesec, and Norway mines" are thought to be accumulations of the detritus worn away from outcropping edges of ore dui'ing the glacial erosion. After the deposition of the ore it was greatly modified in its chemical peculiarities by the action of surface water, removing phosphorus in one place and adding it in another. The theory of aqueous deposit will explain, as will no other, the marked regu- larity of isochemic lines [lines drawn through those portions of the deposit containing equal quantities of phosphorus] and their peculiar curves, the regular decrease of phosphorus from hanging wall to foot, the alternation of carbonate of lime and oxide 92 THE MENOMINEE IRON-BEARING DISTRICT. of iron, the ripple-marked hanging wall, the uniform lamination of the oi-e, and the hydrated muddj"- deposit next the foot wall. It also suggests explanation of the general features of the Menominee range, and the gradual change from high phosphorus and low iron ores resembling altered bog ores at its western extremity, through regular deposits of high iron and lower phosphorus, to the immense washes and surface deposits of exceedinglj^ low phosphorus ore which mark its eastern termination (p. 309). 1890. Irving, R. D. The greenstone schist areas of the Menominee and Marquette regions of Michigan; explanatory and historical notes: Bull. U. S. Geol. Survey No. 62, pp. 11-30. Map of Menominee district opp. p. 24. 1890. In this introductory note to Dr. Williams's study of the greenstone- schists of the Marquette and Menominee districts Irving gives an historical summary of the views held by his predecessors with respect to the relations existing between these schists and the bedded sedimentary rocks associated with them. The general problems connected with the subject have been outlined in the Marquette monograph." These green schists occur inter- mingled with granites and also form large continuous areas, which they entirely occupy, except for some unimportant dikes that intrude them. Some of these I'ocks are now hornblende-schists, others are c^dorite- schists, wdiile still others are massive greenstones. In the Menominee district the schists occur south of the ore belt. They are well exposed at the Big and the Little Quinuesec Falls and at the Sturgeon Falls on the Menominee River, and again in the vicinity of the Fourfoot Falls, north of Bass Lake. These schists, it will be i-emembered, were regarded by both Brooks and Rominger as layers of sedimentary material that have been metamorphosed since their deposition. By Brooks they were considered to be the uppermost beds of the Huronian series and by Rominger as the basal layers of this system. To Irving the regularity of the alternations in the schists seems less than one would think to be the case from Brooks's maps. The rocks seemed to him to grade into one another and into the massive beds. The schistosity appears to be of secondary orisrin and the orig-inal structure of the rocks seems to have been massive. In the Menominee district a series of detrital iron-bearing rocks lies between the green schists and great areas of granite and gneiss to the north. The iron-bearing rocks are similar to those of the Marquette district. They oMon. U. S. Geol. Survey, vol. 28, 1897. BIBLIOGRAPHY AND ABSTRACT OF LITERATURE. 93 are, however, rarely intruded by greenstone eruptives, and are mtiSi'mbre crumpled than are the Marquette rocks. In the same region are two belt.s of oreenstone-scliists closelj' analogous in general appearance with those of the Marquette region. The southern oub of these borders for a long distance the southern granite area, separating the granite from the detrital roclts farther north. The inclination of the greenstone-schists is ahiiost vertical, there being generally a slight southern departure from verticality. Very high southern dips, often approaching verticality, also prevail among the layers of the detrital succession itself, although here frequently occur reverse dips to the northward, often at a flatter angle [p. 25]. With respect to the age of the greenstone-schists, Irving writes : In mj' own studies in the Menominee region, made in the summers of 1883 and 1885, I became early impressed with the close similarity between the greenstone- schists of the Menominee River and those which underlie the ii'on-bearing series of Marquette; with the entire similarity between the rest of the stratiform rocks of Fig. 12.— Hypothetical section across the Menoniinee region in the vicinity of Quinnesec Valley. After R. D. Trying, 1890; A, basiil sericitic quartz-slates; B, quartzite; C, limestone; D, iron horizon; E, slates and quartzites; G, granite; Sch, schists of the Laurentian. Scale, 13,000 feet to the inch. 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 Menominee River; with the granitic intrusions met with in the greenstone-schists bordering the .southern granite, and with the striking conti'a.st between the nature of this contact and that of the northern granite and the detrital rocks which border it to the south. In the latter case, the granite, instead of sending intrusions into the rocks which rest against it, has furnished fragments to them, as may be mo.st beauti- fully seen at the Falls of Sturgeon, Sturgeon River, on the eastern side of sec. 8, T. 39, R. 28 W., Micliigan. These considerations naturally led me to the conclusion that the whole structure in this district is similar to that already described as obtaining in 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 Menominee district, published in the atlas of the Wisconsin survey, I platted on it all of the exposures described by 94 THE MENOMINEE IRON-BEARING DISTRICT. Rominger and not mapped by Brooks, which exposures amount in 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 were platted 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 j'ears by mining It has thus become evident that a structure such as is indicated in the accompa- nying fig. 3 [reproduced as fig. 12, p. 93] would not only coincide with the recorded facts as well as the sections of Brooks * * * but very much better than those [pp. 29-80]. With reference to the conglomerates at the Falls of the Sturgeon, the author says that the granitic fragments occur in a fine-grained, slaty rock, in which there is a great deal of sericitic material, which at times gives the slate somewhat the look of a crystalline schist. This fact, together with the slight inclination from the vertical toward the north, led Credner to include the conglomerates with the Laurentian granite. Brooks's view is regarded as the correct one — i. e., the conglomerate is at the base of the sedimentary series. The granite sheet described by Rominger as inter- leaved with the conglomerates could not be found. Irving also points out the fact of Rominger's inconsistency in making the same granite yield fragments to the sedimentaries and subsequently to intrude , them. The nature of the pebbles and the structure of the matrix which holds them are clear evidence to Irving that "we have here to do with a detritus derived by water action from the granitic and gneissic area immediately to the north. The slight inclination from the vertical toward the granite which these conglomeratic schists sometimes show is, of course, no argument against their having been deposited upon the granite as a substratum." (Footnote, p. 30.) The map accompanying the paper shows the distribution of the Archean, Algonkian, and Cambrian areas within the district. Although largely a compilation, it outlines definitely for the first time the limits of the iron-bearing series. A reproduction of it forms PI. V. Williams, G. H. The greenstone schist areas of the Menominee and Marquette districts, Michigan: Bull. U. S. Geol. Survey No. 62, pp. 31-131 and 192-217. 1890. Dr. Williams's work is purely microscopical. The aim of his paper is "to trace each of the rock types represented within the areas studied from its least altered to its most altered form, and to discover what ma}' have U. S.GEOLOOICAL SURVEY MONOGRAPH XLVI PL.V R. 28 W, MICH. -^^^K|- onMN-^-!>i.' Jl' :j-j.l ,^^ d : ■Cp -\ii iac;/l-i- Q\iiru\esec --?*?!!?; l-..-t:M.f.^, ^;^^fV;l!^y>E^^g:g^ ^^^oyver &iim{t€St gFoJIs V j;. Han bit ry' FaU.9 of (he . . 'SCiwr/ecffi "• -"4^- — £j..k.u J] £p US eiEN & co.h ARCHEAN ALGONKIAN CAM BRIAN Granite and gneiss m Greenstone schists Iron-beartag series Potsdam sandstone (Deiritals, limestones, and ferruginous schists) ■s- .■ ' Ai Cp OUTLINE GEOLOGIC MAP OF THE MENOMINEE IRON REGION Compiled byR.D.Irving from maps byT.B. Brooks, C.E.Wright, and C . Rominger, and from original observations Scale of miles 1890 BIBLIOGRAPHY AND ABSTRACT OF LITERATURE. 95 been the agencies which produced the changes noticed." The object of the author's investigations "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" (p. 31). The areas in the Menominee district selected for special study are five in number, embracing the vicinity of the Sturgeon Falls and the areas around the Little and the Big Quinnesec Falls, the Fourfoot Falls, and the Twin Falls. At the Sturgeon Falls the rocks on the Michigan side of the river consist of five bands of saussuritized gabbro and four bands of greenstone- schists. The gabbro constitutes Brooks's bed XV. In thin section it shows a gradation from an almost massive pyroxene-bearing saussuritized gabbro to schistose varieties composed of saussurite, quartz, and hornblende. The green schists associated with this gabbro are plainly the result of dynamic agencies. In some of them broken feldspar crystals may be detected, but others are now composed exclusively of chlorite, quai'tz, and calcite. These schists are believed to be derived from the gabbro by pressure and shearing. Associated with them are several bands of light-colored sericite-schists which the author considers as having been produced from the same gabbro by chemical processes that are essentially different from those that gave rise to the green schists. At the Little Quinnesec Falls the rocks are mainly diorites, diabases, chlorite-schists, and sericite-schists. The great gabbro ridge of Major Brooks, described as extending along the Michigan side of the river, is in composition a diorite, originally containing a brown hornblende that has been rej^laced by a green variety of the same mineral. It is possible, according to the- author, that the brown hornblende may in turn have been derived from diallage and the rock may have been a diabase. By further alteration the green hornblende passes into a tremolitic mineral and into a colorless chlorite. The schists are believed to be pressure-changed diabases. The slaty rocks recorded by Brooks as occin-ring here are thought to be altered basic eruptives, since they grade into massive diabases, and under the microscope their thin sections show evidence of the close relationship existing between them and the massive beds. The author gives a clear statement of the evidence from which he concludes that the massive rocks pass into schists by pressure action. He illustrates his 96 THE MENOMINEE IRON-BEARING DISTRICT. remarks by the picture of a hand specimen taken from the western end of the "diorite ridge" near the falls. It shows a rock traversed by cross gashes, which the author explains as due to the stretching of the rock after solidification. At the Big Quinnesec Falls the rocks below the falls are dark, more or less schistose greenstones that were once diabases. At the falls and for a half a mile above them are exposed light-colored granular greenstones which graduate into sericite-schists. At the Horserace Rapids the rocks forming the steep walls of the gorge are coarse-grained diorites. These rocks and those at the falls are cut by bands of granite, gneiss, and schistose porphyries that are closely related genetically with the granite south of the Menominee (Brooks's Huronian granite bed XX). The rocks below the falls show the effects of pressure in a striking degree. They were once diabasic, but at present they show only obscure traces of their ophitic structure. The barrier rock of the falls was regarded by Credner as very similar to a gabbro. Williams, however, finds it to be essentially a dioi'ite, although originally it may have been a hornblende-gabbro, or possibly a normal gabbro. The coarse-grained diorites of the "Horserace" (above the falls) are dioritic varieties containing talc, which has been derived from hornblende. The I'ocks appear to be much-squeezed diabases. The green schists are cut by dikes of granites and of quartz- porphyries that are usually foliated like the schists themselves. Usually their foliation is pai'allel to the foliation of the surrounding- rocks without respect to the direction in which the dikes run; thus it may sometimes be parallel to the sides of the dikes and at other times may be inclined to them. Most of the acid bands are regarded as apophyses of the gi-anite to the south. If they are offshoots of the granite, this rock is younger than the schists; and if the schists are the uppermost members of the Huronian series, the granite must be the youngest rock in this series. The writer does not attempt to decide as to the age of the green schists, however, and conseqviently he makes no supposition as to the age of the granite with respect to the sedimentary beds of the Huronian. With reference to the acid dikes, the author writes: The dikes when small are iiue grained and felsitic, but when larger their textui'e 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 in the writer's BIBLIOGRAPHY AND ABSTRACT OF LITERATURE. 97 iniiid that the g-ranite, " Augen-gneiss," biotitic gneiss, and schistose porphyry (or "porphyroid," as Credner called this rock) visible near the Upper [Big] Quin- nesec Falls and along the Horserace are also dikes or apophyses connected with the main southern granite area. The schistose or banded structure of these rocks, where such exists, is a secondary feature, produced by the same dynamic agencies which rendered the greenstones themselves schistose [p. 111]. The granite of the main southern granite area is a typical granite with a tendency to a porphyritic structure. Like all the rocks in the vicinity it bears the marks of having been crushed. At the Fourfoot Falls the greenstones are again Avell exposed. On the Wisconsin side of the river they are schistose and on the Michigan side on the strike of these schists the rocks are massive. A quarter of a mile below the Chicago and Northwestern Railway bridge is an exposure of black slate with the typical structure of a sedimentary rock. Above the railroad bridge a few steps begin the greenstone-schists, which are entirel}^ different from the slate, but are similar to the green schists already described. At the Twin Falls the greenstones are of a dark, aphanitic variety, and when schistose, resemble a chloritic slate. Transitions between massive diabases and greenstone-schists and between these latter rocks and chlorite- sehists are traced by the author step bj^ step, so that there can seem to be no question but that the schists are squeezed eruptive rocks. In summarizing the evidence as to the origin of the green schists derived from their study in the field and laboratory, the author shows that the foliation of the schists is no proof of their sedimentary character. The foliation of these rocks is parallel to the foliation of the sedimentary beds of the iron-bearing formation, but not always to the strike of their beddino-. Both the schistosity of the greenstones and of their associated granites is a phenomenon due to pressure, which probably acted in two different jjeriods, in one of which the genuine sediments received also their foliation. 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 theii- microscopical structure. It is true that there are many cases where rocks of widely dissimilar origin resemble one another so closelj^ 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 [p. 195]. MON XLVI — 04 7 98 THE MENOMINEE IRON-BEARING DISTRICT. The structures that are characteristic of eruptive rocks and that have been recognized in the Menominee greenstones are the ophitic, the porphy- ritic, the micropegmatitic and granophyric, and the poikiUtic. The original rocks still recognizable in the Menominee district and those from which the schists have been derived are gabbros, diabases, diabase-porphyries, diorites, and diorite-porphyries among the basic phases, and granites, granite- porphyries, and quartz-porphyries among the acid areas. The evidence of the microscope indicates that the greenstones solidified at the surface. In the Menominee Valley this evidence consists (1) of the tine 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 alteration 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 the Lower [Little] Quinnesec Falls, are difficult to account for except by supposing that they were once horizontal sheets that flowed over one another and which were subsequently elevated into their present nearly vertical ])osition. Traces of tuff' material are not as distinct here as in the Marquette region, although indications of its existence are b3^ 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 [pp. 200-201]. The author's jjaper is illustrated by maps of the districts studied and by 12 lithographic reproductions of thin sections of chai-acteristic rocks. 1891. Van Hise, C. R. An attempt to harmonize some apparently conflicting views of Lake Superior stratigraphy: Am. Jour. Sci., 3d series, vol. 41, pp. 117-136. 1891. The author believes that many of the difficulties that have arisen among geologists vrith respect to the correlation of the pre-Cambrian rocks is due to the neglect to note the existence of a physical break in the series of strata placed by Irving in his Hurouian group. He shows that such a break occurs in the Marquette district and that here there is a well- defined upper and a distinct lower series, separated from each other by a great unconformity and a basal conglomerate. In the Menominee district the evidence of this break is lacking, possibly because the knowledge of the relations existing between the Menominee rocks is less exact than it is BIBLIOGRAPHY AND ABSTRACT OF LITERATURE. 99 in the case of the Marquette sediments. It is thoug-ht probable, however, that ill the Menominee district the equivalents of both the lower and upper Marquette rocks occur. The Menominee proper — that is, the portion of the Menominee district east of the Menominee River — is thought to correspond in position with the Lower Marquette and that portion west of the river with the Upper Marquette. GoETZ, George W. Analyses of Lake Superior ores: Trans. Am. Inst. Min. Eng., vol. 19, pages 59-61. 1891. The author records the results of analyses of ores from 24 mines in the Menominee district on both sides of the Menominee River. 1892. Van Hise, C. R. Correlation papers — Archean and Algonkian: Bull. U. S. Geo!. Survey No. 86, pp. 72-208, 470-529. With general map of Lake Superior region. 1892. This volume contains a summary of all the work done on the pre- Cambrian rocks of North America up to within a few months of the time the volume was pubhshed. The evidence collated from the writings of those geologists who had investigated Lake Superior geology is discussed and conclusions are drawn from it by the author. With the abstract of the literature we have nothing to do in this review, nor with the matter dealing with the classification of the pre-Cambrian formations in the Lake Superior region. The latter subject is freely discussed in tlie chapter on literature in the Marquette monograph," where the discussion rightly belongs, since many of the conclusions arrived at with reference to the separation of the pre-Cambrian series were reached very largely by studies prosecuted in the Marquette district. There are, however, a few direct references made to the Menominee district which should be considered liere. Li 1890 the author examined the rock succession at Iron Mountain, finding above the ore formation at the Ludington and Chapiii mines a con- glomerate which bears fragments of ore and jasper. "It therefore appears that after this material reached its present condition in the ore-bearing series it was eroded and furnished debris for a newer series" (p. 156). In com- pany with Professor Pumpelly he again in 1891 and 1892 examined the ore formation. In a quarry east of the Chapin mine, and also in the deeper "Mon. U. S. Geol. Survey, vol. 28, 1897, pp. .5-148. 100 THE MENOMINEE IRON-BEARING DISTRICT. workings of this mine, it was discovered that the ores resting almost directly upon the limestone bear a considerable percentage of carbonates, and in the first-mentioned locality they grade directly downward into the limestone. It is therefore probable that the ore formation of these districts, in part at least, is but an upward continuation of the limestone formation, perhaps differing from it originally only in that the upper part contained a greater quantitj^ of original carbonate of iron. Above the ore formation at Quinnesec, test pits show the presence of a typical chert and jasper conglomerate, in every respect like the basement conglomerates of the Upper Marquette [p. 156]. The additional evidence with respect to the physical break in the clastic series lying between the green schists and the Potsdam sandstone is summarized as follows: In the Menominee district as evidence in favor of a physical break within the clastic series are the conglomerates described by Brooks at the Pine and Poplar rivers district, and in the Commonwealth section. * * * Also, the structui-al break indicated by these conglomerates is supported by Brooks's major divisions of the Menominee rocks. His inferior Huronian comprises the lower quartzite of great thickness, a great marble formation, and the great iron-ore horizon, consisting of magnetitic, hematitic, and jaspery schists, with deposits of iron ore. In this formation are the Norway, Quinnesec, Ludington, Chapin mines, etc. Brooks's middle Huronian, presumably above the unconformity, includes quartzites, clay slates and obscure soft schists. Within these soft slates is the upper iron-beai'ing horizon, including such mines as the Commonwealth, those at Crystal Falls, etc. [Pp. 180-181.] The author does not believe that a correspondence can be made out between the subordinate members of the Menominee and Marquette districts, and yet he thinks that ' the Menominee district in Michigan corresponds as a whole to the Lower Marquette series. Van Hise, C. R. The iron ores of the Marquette district of Michigan: Am. Jour. Sci., 3d series, vol. 43, pp. 116-132. 1892. Although this article deals mainly with the manner of occurrence of the ore bodies in the Marquette district, those of the Menominee district are referred to in a few words. The ores of this district are said to occur in Iwo formations, one of which belongs with the Lower Menominee and the 'Other with the Upper Menominee. From the general work done in the '"jj i ■'V ^^,: ~.-~ ' 1 / ■ ,. <. . _ .--" V- -. ,^~:: ->wd si^^^^g^^^^-:?^'-^: -aasx,^^.. - ■ -jj^^^^^L. ' -— ^^fei=^::t-":;- --p' iH >*«*3: ri r 1- = J :'^^ !*^ S Hid ' fi I 7)*. MENOMINEE RIVER ABOVE STURGEON FALLS, One of the quiet basins in the Quinnesec schist area produced by the backwater above the falls. ARCHEAN, QUINNESEC SCHISTS. 133 of the river is about 2G0 feet in the 20 miles of its course betweeu the point west of Iron Mountain, where it enters the southern area of schists, to the point where it crosses the east range Une of T. 38 N., R. 29 W., neai- the southeast limit of the map. When it is realized that only about 14 miles of this course is in the schists, and that in this distance nearly all the fall is made, some idea may be gained as to the roughness of the stream. The fact that more than half of this fall is made at Sturgeon Falls and at the Big and Little Quiunesec Falls, affords abundant opportunity for the development of immense power at these localities when economic condi- tions for its utilization are more favorable than they are at present. At the Little Quimiesec Falls, where about 10,000 horsepower is available for use, there is now a paper mill which employs less than half of this power, and at the Big Quinnesec Falls is an air-compressor plant. The greater portion of the power of the stream in this area is, however, not used for any purpose. Away from the ri\er the country underlain by the schists is very rough, though lofty eminences and deep valleys are not noticeable. The topography is made up of many ridges and elongated hills having steep slopes covered with thin layers of soil, or precipitous sides of bare rock. On the tops of the hills bare flat ledges are met with, here and there cov- ered with patches of soil an inch or so thick. On the Michigan side of the river a thick mantle of sand and glacial drift lies ujjon the schists and obliterates their characteristic topography, but very close to the river bank, where recent erosion has removed the sand, and along the Wisconsin shore, for a few miles from the bank, the rugged nature of their surface is well shown. COMPOSITION AND STRUCTURE OF THE ROCK SERIES. The Quinnesec schists of the southern area consist mainly of schistose basic and acid igneous rocks and a few basic tuff's." The former comprise "Althougli tuffs are only rarely found in the area included in this study, they are by no means rare in the extension of the schist area south of the Menominee River. About midway between this river south of Norway and the "Soo" Railroad in Wisconsin, there are great hillocks and bare bluffs that are composed exclusively of coarse greenstone conglomerates like those so abundant in the northern Archean complex in the Marquette district, and interlaminated beds of fairly well-banded greenstone tuffs. The area has not been studied, Ijut from the great magnitude of these deposits at the locality referred to, it appears probaVile that volcanic fragmentals play a much greater part in the constitution of the (^uinnesec-schist complex than would be inferred from studies confined to that portion of the schist area in the neighborhood of the Menominee River. 134 THE MENOMINEE IRON-BEARING DISTRICT. chloi-ite-schists, ampliibolites, schistose diabases, schistose diorites, and schistose gabbros. AVith these are large, ahnost massive la3^ers of gabbro aud diorite that ai-e supposed to be interbedded sills or flows, and dikes of gabbro, diorite, diabase, and granite. The basic schists constitute by far the greater part of the schists of the area, but in the vicinit}' of the Horse- race Eapids and of the the Big Quinnesec Falls, the basic rocks are asso- ciated with large quantities of acid ones. The acid rocks are in some places nearly massive granites, in other places they are gneisses, and in still other places they are finely banded and schistose rocks like the Saxon granulites. The last-named rocks occur in bands of different widths, nearly always striking conformably with the strike of the foliation of the basic schists with which tliev are associated. The schistosity of the bands is pai-allel to the schistosit}^ of the adjoining schists, irrespective of the direc- tion of the band itself Although rather irregular in their distribution, the different schists may be observed to occur in more or less well-defined belts extending for comparatively short distances in approximately straight lines. These belts strike a little north of west at the Sturgeon Falls, a little south of west at the Little Quinnesec Falls, and about northwest at the Big Quinnesec Falls. Tlieir schistosity strikes approximately parallel to the trend of the belts, but not ahvays exacth' so. The dip varies slightly in different ledges, but is never far from perpendicular. On the northern and eastern peripheries of the granite boss in Wisconsin the foliation of the schists is very well developed. Here the schistosity is nearl)^ parallel to the contacts with the granite, changing with the trend of this boundary and following in a general way its sinuosities. Tliis fact suggests that the schistosity of these rocks is due to pressure and that its direction is deter- mined by the directions of the lines along which the stresses acted, i. e., along lines at right angles to the boundary of the granite boss. LITHOLOGY. There is very little to add to the account of the lithological features of the Quinnesec schists given by Williams" in his bulletin on the Marquette and Menominee greenstone-schists. In this report the author presents excellent descriptions of all the various forms of schists met with along, the ff Williams, George Huntington, The greenstone schist areas of the Menominee and Marquette regions of Michigan, with an introduction by Eoland Duer IrWng: Bull. U. S. Geol. Survey No. 62. 1890. ARCHEAN, QUINNESEC SCHISTS. 135 Menominee River and describes in detail the character of the intrusion traversing them. There seems to be no need of duplicating these descrip- tions in the present volume. In the following pages a brief summary of Williams's discussion will, however, be given, and a few additions will be made to his descriptions in order that the great similarity between the Arcliean in this and other districts may be clearly perceived, and that the great contrast always existing between the Archean and the Huroniau rocks of the Lake Superior region may again be emphasized. For purposes of description the Quinnesec schists may be divided into greenstone-schists, chlorite-schists, amphibolites, gneisses, and sericite-schists. The massive rocks associated with them may be classed as greenstones and granite. The greenstones include gabbros, diorites, dialjases and basalts; the granites include granite and granite-porphyry. Because of the close genetic relationship existing between the greenstones and the greenstone- schists on the one hand and between the granites and the acid schists on the other hand, the former will be described together, as will also be the latter. «iBEE>'STO>E-Si;HISTS AXD ASSOCIATED tiREENST(^IVES. The term greenstone-schists is applied to those basic schists which show clearly by their composition and structure that they were derived from basic igneous rocks. They grade on the one side into massive gabbros, diorites, diabases, basalts, and basic tuffs, and on tlie other side into the chlorite-schists and amphibolites. Specifically they might be termed gabbro- gneisses, diorite-gneisses, etc., but their present mineralogical and chemical composition is so different from that of the corresponding massive rocks that it is thought best to refer to them under the generic term greenstone-schists. The term is a convenient one, partly because it is so comjjrehensive, and partly because some such term is absolutely necessary to the prosecution of field work in regions of ancient schist complexes. The term comprehends all dark-colored schistose feldspathic rocks derived from massive igneous rocks and their tuffs. In the Menominee district the greenstone-schists comprise schistose gabbros, diabases, diorites, basalts, diabase tuffs, and basalt tuffs. They vary in color from greenish gray to greenish black, in texture from coarse grained to aphanitic, and in structure from almost massive to perfectly schistose. The more massive phases are sometimes poikilitic. The finer-grained rocks are usually more schistose than the coarse-grained ones, though not always so. The most perfect schistosity, 136 THE MENOMINEE IRON-BEARING DISTRICT. amounting almost to perfect fibrosity, is met with only in the aphanitic phases. The rock of nearly every exposure, or at least of every group of exposures, possesses a characteristic appearance which differs from that of cori-esponding rocks of other ledges, so that a detailed description of the macroscopic features of all the different greenstone-schists of the district would be nothing more or less than a description of individual specimens. The same statement applies to the description of the massive greenstones. A detailed description of this kind does not seem necessary in this place, especially since Williams has so well described nearly all the several phases of the greenstones and their schists in the bulletin referred to above. Only the principal and predominating types of the schists will be described in the following pages, the unusual types being referred to only in the descrip- tions of interesting localities. COARSE-GRAINED VARIETIES. The material of the coarse-grained schists, which may represent dikes, or flows of coarse lavas in a series of finer-grained lavas, differs very little from the material of the coarse-grained, massive greenstone dikes cutting through the schist series. Most of these rocks are light-gray diabasic aggregates of a white feldspathic mineral and a greenish-gray hornblendic one, with occasionally here and there an irregular grain of magnetite or ilmenite. Others are darker in color and greener in tinge. In these the hornblendic mineral predominates and the feldspar is often epidotized. In most specimens the feldspar is in diabasic forms, in others it is in the irreg- ular grains characteristic of gabbro, and in still others it is in rounded phenocrysts. Near slickensided surfaces the characteristic gabbro or dia- basic structure disappears, and the rock becomes a typical chlorite-schist or amphibole-schist. Gahhros and their derived schists. — The barrier rock of Sturgeon Falls (see PI. X, A) is an excellent example of one of the most massive phases of the coarse-grained rocks. It is not known whether it is a dike or an immense flow. It exhibits many lithological features of the massive dikes, and at the same time it possesses phases that are identical in character with those of many of the coarser-grained schists." The most massive phase of the rock is light gray in color. On a fresh fracture it is mottled in white «Cf. Williams, op. cit., p. 68. ARCHEAN, QQINNESEC SCHISTS. 137 and dark-greenish gray, the consequence of the presence of a greenish- white feldspar and a brownish, lustrous amphiboloid. The texture is moderately fine, but is subject to sudden local variations which develop comparatively coarse-grained patches in the main mass. Under the micro- scope the rock is discovered to be a gabbro in which there still remain renmants of colorless diallage, but in which the plagioclase has been almost wholly altered to a gray and opaque saussurite. Most of the original diallage has been changed to a compact brown amphibole, which, togetlier with calcite, albite, chlorite, and quartz, are the principal secondary products present. All specimens exhibit the action of dynamic forces that have more or less profoundly affected the different constituents. 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 abun- dant. Colorless chlorite and zoisite are also developed, and all are mixed indiscrim- inately. In one part of the section a vein is seen to traverse the rock. This is tilled with limpid quartz in long-, wedge-shaped 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." In the more schistose phases of the rock the constituent minerals, while the same as those in its less schistose phases, have suffered much greater changes. 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. Frequentlj^ a fine-grained mosaic has been formed by the crush- ing of the largei- feldspar crystals. In other cases * * * the feldspar is not so much broken, but it is altered ai-ound its edge to an opaque gray saussuritic mass, while its interior is hardly changed. * * * The diallage is more altered than in the rocks last described. * * * 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 l)ase. * * * Fibrous hornblende now becomes more abundant than the compact, and leucoxene patches are seen at intervals.* a Williams, op. cit., p. 71. ''Ibid., p. 72. 138 THE MENOMINEE IRON-BEAKING DISTRICT. The compact massive or slightly schistose rock passes gradually into softer, more schistose, ones, that have lost all traces of their original structure and of their original mineral composition as well. The schists are now composed of broken pieces of feldspar which has been almost comjjletely 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, scalj' mineral, which an examination shows to be chlorite.. " The most schistose phases of the rock are examples of fissile, silky, chloritic schists, composed of a fine-grained schistose aggregate of colorless chlorite, quartz, and calcite. 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 studj^ 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. * The chemical changes undergone by the gabbro in its transition to a. schist are shown by the following three analyses made for Dr. Williams by Dr. R B. Riggs: Analyses of gahhro and schist from Sturgeon Falls. SiO^ AlA FeA FeO CaO MgO Na,0 K,0 H^O CO2 Total I. 51.46 14.35 3.90 5.28 9.08 9.54 2.92 .24 3.30 .20 100. 27 II. 38.05 24. 73 5.65 6.08 1.25 11.58 2.54 1.94 7.53 .93 100.28 III. 45.70 16.53 4.63 3.89 4.28 9.57 .55 3.82 4.70 5.95 99.62 Rock powder dried at 105° C. I. Freshest gabbro from barrier rock of Sturgeon Falls, II. Schistose galibro from south side of I. III. Silvery chlorite-schist from band between two bands of saussuritized gabbro. "Williams, op. cit, p. 73. nbid., p. 75. ARCJHEAN, QUINNESEC SCHISTS. 139 The most marked variations noted in the composition of the three rocks is with reference to the lime, alumina, soda, potash, and carbon dioxide. Williams explains the fact that the intermediate rock (II) differs more from the original type (I) than does the most schistose phase (III) by supposing that the processes of alteration have been different in the two altered phases. In the first case the main i)roduct of the alteration lias been chlorite; in the second case it is sericite and calcite. It is difficult to conceive a process which, acting upon two portions of the same rock close together, will cause chloritization in one part and sericiti- zation in the other. The production of sericite in a rock of the composition of a gabbro necessitates the addition of potash and alumina from some extraneous source, through the transporting agency of solutions. It is very difficult to understand how such solutions might be confined to a certain definite band in an altering basic rock and be practically excluded from other portions of the same rock. It is more probaljle that the three rocks whose analyses are given above originally possessed different com- positions, and that this accounts for their differences in composition at present. Diabases and their derived schists. — The coarse diabases and their derived schists are in all essential respects, except structure, like the gabbro and the gabbro-schists. Diorites and their derived schists. — The diorites and the schists derived from them are slightly different from the gabbros and their associated schists, but the processes which changed the massive rocks into schistose phases were practically the same in both cases. The rock forming the high bluff skirting the east side of the river below the Little Quinnesec Falls is the best example of the massive diorite met with in the district. It is described by Williams as being in all probability a great dike. Though for the most part quite massive, the rock presents frequent and 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. Since the continuity of these bands with the massive rock is established, their study is calculated to throw light on the subject of dynamical metaraorphism. Major Brooks designated the rock which composes this ridge as a "massive gabbro," and correlated 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 librous variety of 140 THE MENOMINEE IRON-BEARING DISTRICT. hornblende which is quite universally conceded to be of secondary origin. What the primar}' 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 Menominee River greenstones, if we except the light-colored diallage of the Sturgeon Falls gabbro. Whenever the pale-green hornblende can be traced back to an original form, it is seen to be derived from a compact brown or basaltic hornblende. ******* 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 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. * * * Inasmuch, however, as the rocks here under discussion afford no trace of pyroxene, it hardly seems justifiable to call them anything but diorite." Some of the pliases of the rock are porphyritic. These contain large crystals of saussuritized feldspar in a matrix composed wholly of hornblende in compact, brown and green, or colorless grains, and in light-green fibers, in which are embedded small laths of plagioclase. The hornblende by alteration passes into chlorite and tremolite. The schists derived from the diorite are largely chlorite-schists. They present no peculiar features distinguishing them from the schists derived from the gabbro, except in those phases where the structure of the original rock is still retained. At one place toward the west end of the diorite ridge a well-marked band of schist traverses the massive rock. The latter is — 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 ilmeaite, are abundant.* The rock of the schist baud is much more altered. Its feldspar is mostly changed to calcite, and the hornblende to chlorite. The original structure has wholly disappeared, and there is now a fine mosaic of quartz and secondary albite between the calcite and chlorite masses. The same skeleton forms of leucoxene remain, however, and " there is no doubt o Williams, op. cit., pp. 77-78. bJbid., p. 83. ARCHEAN, QUINNESEC SCHISTS. 141 that the two specimens represent the same rock in different stages of alteration, the more changed form liaving become decidedly schistose." " FINE-GRAINED VARIETIES. The finer-grained greenstones and greenstone-schists were originally diabases, basalts, and basic tnffs. In composition they are similar to the coarser-grained varieties described. Their structure is less well defined, and in some cases the original rock appears to have been essentially a glass. One of the best preserved of the fine-grained rocks is that on the Michigan side of the Little Quinnesec Falls. It is a massive, dark-green rock which, according to Williams — 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; 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 shai-ply 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.'' These massive or almost massive rocks, as has been said, arrade in many places into schists. Where they do not actually pass into schistose phases they are often associated with basic schists in which traces of the original structure of eruptives can be detected or which can be traced into rocks exhibiting this structure. In other cases, as stated by Wadsworth'^ in connection with the discussion of the origin of these rocks at the Little and the Big Quinnesec Falls, the basic schists may have been derived from volcanic tuffs or, as Wadsworth calls them, porodites. The predomi- nant rock on the Michigan side of the lower falls, for instance, is jolainly conglomeratic. It is either an old eruptive ash or an agglomerate. "Of the fragmental and conglomeratic nature of the rock," Wadsworth declares, "no one can possibly doubt after studying it on the Michigan side, below the falls, and then tracing it back into the finer grained and more compact part near the bridge." "Williams, op. cit., p. 83. 6 Ibid., p. 94. c Wadsworth, M. E., Report of State Board of Geological Survey for 1891 and 1892, Lansing, 1890, p. 125. 142 THE MENOMINEE IRON-BEARING DISTRICT. Origin of the schistosity. — The least-altered phases of the fine-grained greenstones are characterized by a rhomboidal parting. As the rocks depart more and more from their original character the rhombs produced by the parting become more and more elongated nntil finally their sides are approximately parallel and a well-developed schistosity results. At the same time the original components of the rock mass undergo change. In many of the less-altered schists that have been derived from massive eruptives the structure of the originals can still be recognized, though their mineral composition can only be surmised, since they consi.st of secondary products exclusively. Basic lavas and their derived schists. — In the field, dark- and light- colored schists are readily distinguished, and both are closely associated with nearly massive phases. On the east side of the basin, below the Little Quinnesec Falls, the dark schists are so intimately associated with massive greenstones that there can be little doubt as to the original identity of the two. In the hand specimen the least schistose rock is a compact, aphanitic mass of a dark -green color. Under the microscope the original diabasic nature of the rock is at once apparent, although the extensive mineralogioal 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 constit- uents 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; 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 granu- lated 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 cro.ssed 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. * * * This chlorite-epidote aggregate covers considerable areas and ARCHEAN, QUINNESEC SCHISTS. 143 occupies the place of both the feldspathic and the pyroxenic constituents. In addi- tion to the minerals already named, ilmenite with its leucoxene border, pyrite, and secondary quartz are quite abundant in these rocks. " Through this rock passes a schistose band so intimately related to the massive rock in both sides of it that there can not be the least doubt as to the continuity of the two. The hand specimen taken from the band is decidedly schistose. 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 feldspar crystals, 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 bisiiicate elements. The parallel arrangement of the scales of this mineral is what produces the schistose structure.* Tlie chlorite together with clear grains of quartz and probably of unstriated feldspar are embedded in a quartz-albite mosaic. Calcite is also abundant as little nests in the mosaic, and rutile either in stout yellow grains, or in minute sharp crystals traverse it in long sinuous lines. This rutile represents the ilmenite of the original rock. The mechanical action which produced this schistose band has therefore resulted in the ci-ushing 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 * * * made by Mr. R. B. Riggs: Analyses of greenstone from Little Quinnesec Falls. I. II. I. II. SiO 43.80 16.08 9.47 10.50 7.81 6.54 44.49 16.37 5.07 5.50 7.94 7.50 Na.p 1.96 0.34 3.99 0.08 2 .59 AI2O3 K.fl 0.56 Fe^O., HjO 4.99 FeO CO., . 5 38 Total CaO . 100. 57 100. 39 MkO TiOj not determined. Powder dried at 105° C. 1. Dark, massive greenstone, Lower [Little] Quinnesec Falls. II. Dark, schistose greenstone, forming a band in the last. "Williams, op. cit., p. 90. ''Ibid., p. 90. 144 THE MENOMINEE IRON-BEARING DISTRICT. The changes here are at once seen to be due (1) to the total removal of the iron ores (losy of iron); (2) to the production of carbonates (gain of CO^, carbonatization) ; and (3) to the increase in the amount of chlorite (increase of tl.,0, hydration)." These darker schists pass into hghter ones that are more typical chlorite- schists, composed of extremely pale chlorite and quartz grains, with a little calcite and occasional sericite shreds, and numerous minute sharp crystals of rutile. Whether these lighter schists are actually phases of the darker ones, or whether the two are schistose phases of rocks that were originally different, can not now be told, since the true nature of the con- tact between them has been obliterated by the changes through which the rocks on both sides of it have passed. Even if the latter be the case the two originals were not very different, both in all probability being basic lavas or tuff beds. Basic tuffs and their derived schists. — The schists derived from tuffs generally exhibit their origin in their structure when this has not been completely destroyed. Usually, however, the tufifaceous structure disap- pears so rapidly with the assumption of schistosity that nothing remains to distinguish the schists derived from tuffs from those derived from fine- grained eruptives. In a few instances sections made from the schists at the Little Quln- nesec Falls possess characteristics that point to a tuflaceous origin. In natural light tliese sections show irregular granular areas and black dots in a colorless, transparent, almost amorphous matrix. Between crossed nicols plagioclase, saussurite, kaolin, calcite, colorless or light-green chlorite, and bleached hornblende are noticeable. The plagioclase is largely altered into chlorite, saussurite, and kaolin. The greater portion of the mineral is often fresh enough ti) exhibit clearly its twinning striations, though not fresh enough to yield accurate measurements of extinction angles. This feldspar occurs in lath-shaped cr5^stals, in rounded and irregular-shaped grains, in sharji-edged fragments, and in some instances it appears as though it were a secondary deposit filling triangular spaces between the other components. Some of the crystals are fairly well formed, others are broken, and still others are castellated at their ends. A large number of skeleton crystals are also noticed scattered through the matrix. a Williams, op. cit., p. 91. ARCHEAN, QUINISESEC SCHISTS. 145 Only a few indications of the original structures can be detected. In a few small areas of some of the sections the structure appears to have been ophitic, the plagioclase laths occurring as radial groups in a matrix of chlo- rite, calcite, and bleached amphibole. In most of the sections, however, no traces of diabasic structure can be detected. The rocks, tliough much decomposed, look as though they were composed of crystals, fragments, and small, almost dust-like grains of feldspar in a grouudmass that yielded on decomposition new plagioclase, light-green amphibole, chlorite, and calcite. Most of the fragments are certainly portions of crystals that were crushed when the rock was made schistose, but there are many others which do not seem to be due to mashing. Some of these have concave outlines, like the outlines of splinters of minerals often found in volcanic dust. The structure thus appears to resemble more closely that of tuffs than that of lavas. CHLORITE-SCHISTS. The chlorite-schists differ from the very schistose greenstones mainly in the fact that they contain no original plagioclase nor any saussurite derived from this. They are finely schistose, sometimes fissile, dark-green or light-gray rocks containing abundant quartz The darker varieties are often so dark and so very fine grained that they look very much like greenish-black slates. The lighter-colored varieties resemble very soft, fine-grained sericite-schists or talcose schists. Under the microscope the dark-green chlorite-schists are seen to be aggregates of green chlorite, calcite, and an occasional flake of a sericitic mineral, in a mosaic of quartz and secondary albite. In tlie light-colored schists the sericitic mineral and calcite are more prominent than in the dai'ker phases. At the same time the chlorite has been replaced by an almost colorless variety. MON XLVI — 0-t 10 146 THE MENOMINEE IRON-BEARING DISTRICT. An analysis of a specimen of one of the lightest colored of these rocks, taken from "the western corner of the little cove just below the Lower [Little] Quinnesec Falls,"" resulted as follows: Analysis of schist from, Little Quinnesec Falls. [Analyst, R. B. Riggs.] SiO, - - 46.21 AI2O3 - 18. 38 FeA - :^-30 FeO - - 3.90 CaO - 6.28 MgO 7.03 NEjO 2.14 KjO - - - 35 HjO 3.82 CO2 8.32 Total 99.73 TiO., not determined. The small proportion of K2O present in the rock would seem to indicate that the sericitic minei'al is paragonite rather than sericite. Between the dark and the light varieties of the schists all gradations are met with, the color depending upon the relative proportions of the dark chlorite and the colorless sericitic mineral developed. Many of the chlorite-schists of all kinds occur as definite bands in the schist series. They are not so related to the massive rocks as to present undoubted evidence that they are derived from these. In composition and structure, however, they are identical with the most schistose phases of the greenstone-schists described in preceding pages (pp. 136-139) as the end products of the alteration of gabbros, diorites, diabases, and their tuffs, so that little doubt can be felt as to their origin from similar rocks. AMPHIBOLITES. The amphibolites are limited in their distribution to the neighborhood of the great granite mass in Wisconsin. They neaxdy always occur near the contact of the granite with the greenstone-schists, though they are occasionally met with afe narrow selvages along the side of granite or granite-porphyry dikes where these cut through the green schists. The only careful examination of any part of the contact between the "Williams, op. cit., p. 89. ARCHEAN, QUINNESEC SCHISTS. 147 granite and the suiTounding greenstones was made in 1894 by A. T. Lincoln. Lincoln followed this contact abont three-cjuarters of a mile in sees. 15 and 16, T. 38 N., R. 20 E., Wisconsin, a short distance sou.th- west of the Little Quinnesec Falls. In an unpublished thesis" he states that "near the granite the greenstone becomes quite schistose, and in some places becomes a typical hornblende-schist. Fragments of the greenstone were broken oif and included in the granite mass at the time of its intrusion. In some parts the greenstone is very intricately ramified by apophyses of granite." The hornblende-schists are described as fine-grained, lustrous, black rocks, traversed l^y small veins of feldspar, as well as by cracks ramifying in various directions. They all contain more or less altered plagioclase in considerable quantities. Some are porphyritic, others are devoid of phenocrysts. l^he porjihyritic schists, when examined in thin section, are found — to consist chiefly of actinolite crystals so arranged as to give the rock a decided schistose structure. Distributed throughout the hornblende are phenocrysts of much-altered feldspar. These have quite the appearance of amjrgdules. The}' have altered almost completely to zoisite, with which is found also some calcite. * * * Sphene is quite abundant, some of which still retains small cores of ilmenite. The n( mporphyritic schists differ from the porphyritic opes merely in the absence of the phenocrysts. They are aggregates of hornblende and comparatively fresh plagioclase. Since all of these schists contain large quantities of plagioclase they are more properly designated amphibolites than hornblende-schists. The latter are characterized by the possession of much quartz and but little feldspar. Typical hornblende-schists have not yet been recognized among the Quinnesec schists, though they are found in fair abundance in tlie gneiss-schist complex north of the Menominee trough. From the nature of the amphibolites and their intimate association witli the ordinary types of the greenstone- schi.sts, it may fairly be concluded that they were derived from the same kinds of basic igneous rocks as were the greenstone-schists and the chlorite-schists. Their exclusive occui-rence « Lincoln, A. T., On the greenstone area in the vicinity of the Lower Quinnesec Falls, Wisconsin side; a thesis submitted for the degree of B. S. in the group course in mineralogy, University of Wisconsin, 1894. (Not published. ) 148 THE MENOMINEE IRON-BEARING DISTRICT. near granite contacts would indioate that they owe their peculiar character to their proximity to the intrusive. In other words, they are greenstones that have been subjected not only to dynamic metamorphism, but to contact alteration as well. Types of rocks intermediate in character between the amphibolites and the greenstone-schists are found in profusion along the banks of the Menominee River at the rapids known as the Horserace. Here the basic schists are intruded by acid rocks, to which fact their peculiar features may be ascribed (see pp. 158-159). OKKJIA OF THE BASK: SCHISTS. Fi'om the field and laboratory study of the basic schists, one must conclude with Williams that they are squeezed igneous rocks, and their tuifs, which, on account of the squeezing to which they have been subjected, have lost in many instances all traces of their original structure and nearly all of their original mineral components. From the nature of the products that liave arisen from the alteration of these components we are led to believe that the original rocks were basic. In a few cases the original composition and the original structure of the schists can be recon- structed from the evidences still remaining, and in these cases it may be safely asserted that the original rocks were gabbros, diorites, diabases, and perhaps basalts among the crystalline varieties and diabase tuflp among the fragmental varieties. MaiiA' of the schists that are without a distinct igneous structure are so closely associated with basic rocks which are almost massive, and are connected with these by such intimate gradational phases, that no doubt can be felt as to their origin. These schists are unquestion- ably squeezed gabbros, diorites, diabases, and other basic rocks. They are identical in every respect with many of the schists whose field relations do not directly connect them with massive eruptives, and thus lend force to the view that these latter schists are likewise squeezed igneous rocks. The conditions under which the igneous rocks consolidated are not plain in all cases. Some of the more massive gi-eenstones were certainly originally dikes. Many of the others and most of the schists were prob- ably lavas or sheets, or tuffs. Dr. Williams thinks that there is considerable evidence to show that the Menominee greenstones solidified at the surface under subaerial or subaqueous conditions. ' ARCHEAN, QUINNESEC SCHISTS. 149 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 theii original position represented successive flows. Fineness of grain is universal in the Menominee greenstones, and we may be certain that it was a pT-imaiy feature in spite of the extensive alteration 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 [Little] Quinnesec Falls, are difficult to account for except by supposing that they were once horizontal sheets whicli 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 indica- tions 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." Since the above was wiitten undoubted tuffs have been discovered in the Menominee district in the vicinity of the Little and the Big- Quinnesec Falls, and in Wisconsin about a mile southwest of the lower falls, and over a large area in the latter State several miles south of the river, opposite the city of Norway. This discovery adds additional strength to Williams's view, which must therefore be accepted as probably true. ACID IXTKCSITES AND THEIR DEBITED SCHISTS. The acid rocks associated with the basic schists in Michig-an are limited principally to the neighborhood of the Horserace Rapids and the Big Quinnesec Falls. They include gneissoid granites, porphyritic gneisses or porphyroids, felsite-schists, sericite-schists, and probably paragonite-schists. The sericite-schists and paragonite-schists are found also associated with the greenstone-schists in other portions of tiie southern area. The gneisses, porphyroids, and felsite-schists are so similar in composi- tion to the Wisconsin granite, which is only about three-quarters of a mile from the Menominee River at the Little Quinnesec Falls, that they may be regarded as its apopliyses. Some of the larger of the gneiss bands may be traced practically continuously into the granite mass. The sericite- schists and the paragonite-schists in many places pass by a continuous series of gradational phases into the felsite-schists. Hence it seems probable that these are also extensions of the granite. Williams, op, cit., pp. 200-201. 150 THE MENOMINEE IRON-BEARING DISTRICT. Structurally the acid rocks are in the form of dikes or sheets. Some of them cut across the bands of basic schists in any direction. These are unquestionably dikes. The greater portion, however, occur in bands that trend parallel to the direction of the bands of basic schists suiTOunding- them. These may be either intrusive sheets or they ma,j be dikes which at the present surface happen to follow the bedding planes of the schist bands. Below the surface they may all cut across the schist bands, as some are known to do. The rocks of these two classes comprise gneisses, porphyroids, felsite-schists, and some of the sericitic schists. The finer- grained rocks are always associated with coarser-grained gneisses or gneissoid porphyries. A third class of acid schists may have been volcanic flows These comprise some of the sericite-schists and paragonite-schists. The}^ occur in definite bands running parallel to the bands of the basic schists. They occur sporadically in the midst of the greenstone-schists, and are not associated with any forms of rocks regarded as apophyses of the granite. All of the acid members of the Quinnesec schists are more or less schis- tose. The most massive granite-like dikes, and even the peripheral portion of the granite boss, exhibit distinct foliation. In the porphyritic varieties • the finer-grained groundmass is very strongly schistose, while the pheno- crysts are elongated into lenticular masses. The felsites and, in general, the fine-grained rocks are much more distinctly schistose than the coarser-grained ones, and many of them are fissile. The schistosity of all the acid rocks, like that of the basic schists, is clearly the result of pressure acting after the intrusive masses assumed their present position, since in some cases, at least, the direction of the .schistosity is inclined to the direction of the walls inclosing the intrusion. Gneissoid granite and granite-gneisses. — The mass of the rock constitut- ing the boss in Wisconsin is- — i\ typical coarse-^raiued granitite, with a decided tendencj' to a porphyritic struc- ture. * * * When examined under the microscope the macroscopic diag'nosis is found to be correct, and several additional points of interest are brouj^ht to light. The oldest constituents are zircon and apatite; 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 oiify mica present, is not abundant. It is invariablj' bleached to a green color by the reduction of its iron to the ferrous state. It contains abundant inclusions of apatite, zircon (around ARCHEAN, QUINNESEC SCHISTS. 151 which are pleochroic aureoles), and some secondary magnetite. No trace of either hornl)lende 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 degi'ee the effects of pressure. The 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 oligo- clase 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 lamella are bent or curved and bear eveiy sign of having been secondarily developed. * * * The large original feldspar crystals show a peripheral gi'anulation, '•' * * and where they have been fissured their cracks are filled with a new crystallization of plagioclase, orthoclase, and quartz. None of these minerals shows an}' 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"). In this sec- ondary cement-like aggregate a micropegmatitic intergi'owth 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." The coarser-grained gneissic granites, augen-gneisses, and porphyries incorporated with the green schists are identical in composition with por- tions of the great granite mass. The rocks have in many cases undergone considerable chemical change, and have been subjected to a great deal of crushing. Nevertheless they can all be recognized as having differed orig- inally from the granite only in structure. Nearly all appear to have been porphyritic. Many of them have lost their porphyritic character by the crushing of their original components and the crystallization of new material between the crushed fragments. The mashing, together with recrystalliza- tion, has changed the phenocrysts of feldspar into the augen of tlie augen- gneisses; it has produced well-characterized gneisses from quartz porphy- ries; and has changed the quartz phenocrysts of the latter into granular, lenticular aggregates of this mineral. Williams describes a band of granite-porphyry cutting the greenstone- schists on the south side of the basin, just below the Horserace Rapids. The "Williams, op. cit., pp. 111-112. 152 THE MENOMINEE IRON-BEAKING DISTRICT. center of the baud is a massive gray porpliyritic rock. This grades on both sides into a well-characterized fine-grained gneiss, resembling a o-rannlite. The central portion of the band consists of large crystals of a zonal plagioclase embedded in a granular mosaic of clear, colorless, untwinned, and probably newly crystallized grains of audesine, associated with browu leaflets of biotite. Apatite, zircon, and a reddish pleochroic sphene are also present in small crystals. Calcite is also abundant. Though the rock is fresh and appears to be unaltered, the presence of calcite indicates that it has suffered a change from its original character. The audesine is probably one of the new products formed." The gneissic portion of the band resembles the groundmass of the central, jjorphyritic phase. The chief difference is the banded appearance of the gneiss, produced by the parallel arrangement of the biotite and the alternation of layers of different-sized grains. Tourmaline and rutile are present in some sections. Remains of plagioclase phenocrysts are observed in others, but the porphyritic crystals have been nearly destroyed by granulation. An analysis of the most massive portion of the band was made by Mr. R. B. Riggs. It disclosed the fact, already intimated above, that the rock is more nearly a diorite than a granite in composition." Analysis of massive portion of hand of granite-porphyry near Horserace Rapids. SiO, 54.83 AlA - - - 25.49 FeA - - ^■''^ FeO - - - 1-65 CaO - - «-08 MgO . - 1-96 Na/) - - - - - 5. 69 K,0 -- 1-87 H,0 - ---- 118 CO, - ^ Total 100.54 Other specimens of the granite-porphyry are more like granite than the one just described. In these quartz and orthoclase both occur as phenocrysts. Biotite and nniscovite are present in the groundmass, the latter being derived partly, at any rate, from orthoclase. The quartz phen- oWilUams, op. cit., p. 113. ARCHEAN, QUINNESEC SCHISTS. 153 ocrysts are usually granulated — sometimes throughout their entire masses, often only peri[)heral]y. The augen-g-neisses difter from the porphyritic gneissic granite mainly in the extent of the mashing which they have suffered. They are well banded 01 gneissic through the arrangement of the constituents of the groundmass, notably biotite and muscovite, in parallel directions. Phenocrysts have disappeared by granulation, and have been deformed into flattened bodies, ajid new crystallizations of feldspar or quartz have been deposited in masses with triangular cross sections at the ends of the long axes of the flattened grains. The crystal forms of the phenocrysts have thus disappeared, and lenticular bodies have resulted. One of these rocks from a dike at the upper end of the Horserace Rapids, on the Michigan side of the river, was also analyzed by Mr. Riggs. Its composition is that of a granitite." Analysis of gr emit ite from uppei^ end of Horsefrace Rapids. SiO^. 67.77 Al^Os 16. 61 FejO,, 2. 06 FeO 1. 96 CaO 1.87 MgO 1.26 Na^O 4. 35 K^O 2.35 HjO 1 . 69 CO2 19 Total 100. 11 Por])hyries and felsites and their schistose ^ihases. — The quartz-porphyries are now to all intents and purjjoses like the groundmass of the augen- gneisses, with the addition of rounded or dihexhedral quartz grains and an occasional phenocryst of plagioclase. They are 3^ellowish-gray or reddish schists, with a greasy feeling. The components are the same as those of the augen-gneisses, but muscovite is much more abundant in them than in the latter rocks. The mineral that was originall}^ biotite is often now represented by chlorite. All the larger grains are fractured or granulated, and the smaller ones are flattened. Nests of calcite are often abundant. An analysis of one of these rocks by Riggs showed it to belong unques- tionably with the granite magmas.'' "Williams, op. cit, p. 119. ''Ibid., p. 121. 154 THE MENOMINEE IRON-BEARING DISTRICT. Analysis of quartz-po7'phyrt/ Jrom J3ig Quinnessec FaUs. SiOj - - •- 66. 69 AlA - - 16.69 FeA - 2.06 FeO 93 CaO - 1 • 40 MgO - - - - 1.15 Na^O 2.46 K,0 5.23 H^O - 1. 70 CO2 1.'12 Total 99. 73 The schistose felsites differ from the schistose quartz-porphyries only in texture. They are very fine-grained aggregates of the same minerals that characterize the matrix of the porphyries. Originally they may have been felsitic apophyses of the granite mass, or possible porphyritie microgranites, that were later so thoroughly crushed that their constituents have been completely granulated into minute flattened particles which are now cemented together by newly deposited material. The felsites as they now exist are very schistose. Muscovite is abundant in them and this is always so arranged in the thin section that the long axes of its shreds are parallel to the plane of flattening of the other components. SERICITE-SCHISTS. All the acid rocks thus far discussed are plainly younger than the green- stone-schists which they intrude. From their nature and the relations of some of them to the granite mass south of the Menominee River we are led to conclude that they are apophyses of this mass. A few other schists may also be acid rocks. These are so intimately incorjjorated with the green schists that they appear to be integral portions of the schist series. They always occur in bands parallel to the bands of basic schists, and never cross the latter. These rocks are the light-colored white or gray micaceous schists called by Williams "silvery schists." In composition they are mainly sericite-schists. Williams regarded them as specially metamorphosed basic schists. Some of the light schists associated with the dark-green ones may be of this character, but these are light-colored chlorite-schists. The sericite- schists are of a different character. Under the microscope they are seen to be composed of quartz grains, sericite scales, and chlorite shreds arranged in a parallel manner. No feldspar- is noticeable in them, but calcite is ARCHEAN, QUINNESEC SCHISTS. 155 abundant. As at present constituted the rocks are unquestionably acid. Though no analyses are at hand to substantiate tliis statement, the abun- dance of quartz and sericite noted in the slides leaves no doubt as to its correctness. These acid schists probably represent acid flows contemporaneous with the basic flows with which they are associated. Their source is not yet known. They are so rare that they constitute but a small fraction of the Quinnesec schist series, which is essentially a set of schists produced by mashing from a succession of layers of basic lavas and tufis. INTERESTING LOCALITIES. Grood exposures of the Quinnesec schist series and their intrusions may be seen at the Sturgeon Falls, the Big and the Little Quimiesec Falls, and along the Horserace Rapids. At the various falls only the basic mem- bers of the series are present, and a few minor bands of sericite-schists, except that at the Big Quinnesec Falls there are a few bands of gneiss. At the Horserace Rapids the basic schists are also mainly in evidence, but acid intrusions are also common. Moreover the basic schists are of a different character from those at the several falls. In appearance they are more crystalline than the latter. In composition they are more hornblendic and less chloritic. The peculiar character of the schists along this stretch of the river he been ascribed in previous pages (pp. 14(i-148) to the eff"ect of the granitic intrusions. Sturgeon Falls. — The most abundant rock at the Sturgeon Falls is a coarse-grained, light-gray massive saussurite-gabbro. It forms the first bar- rier over which the water plunges in the series of cascades that constitute the falls (see PI. X, A), and again it occurs at the lower end of the basin below the falls. The falls proper occupy a stretch of several hundred feet along the river. The stream here is narrow and its channel is gorge-like. Precipitous walls of the gabbro form the banks and these are practically continuous. At the lower end of the narrows the rocks are slightly schistose, but still retain the gabbroitic texture. About midway between the upper and the lower portions of the exposure several narrow bands of fissile, silky sericite-schist penetrate the gabbro and appear midwaj' of the east wall. Between two of these is a narrow band of the massive rock exactly like the gabbro of the barrier. Intimate gradations exist between the schists and 156 THE MENOMINEE IRON-BEARING DISTRICT. the massive gabbro. Although the gradation is much more rapid in some places than in others, no sharp line of demarcation can be drawn lietween the schistose and the massive rocks. From the field relations of the two phases it is evident that the schists represent mashed and altered forms of the gabbro. Little Quinnesec Falls. — At the l^ittle Quinnesec Falls a great width of basic schists and massive rocks, best seen in the west or Wisconsin side of the river, stretch up and down the stream for a distance of about 1,500 feet. The succession is made up of dark-green chlorite-schists, light- colored sericite-schists, light-gray schistose diabases, and several beds of fairly massive dark-green diabase. The light schists grade into the lighter- colored diabases, and the darker ones into the dark diabases So gradual Is the transition between the aiassive and the schistose phases of the rocks and so intimate is the relation of the two varieties that there is no escape from the conviction that the former are but schistose ])ortions of the latter. At the immediate foot of the falls on the east side is an exposure of sericite- scliist of the kind that is supposed to have been derived from an acid bed. It is a little more distinctly separable from the bands of diabase between wliicli it lies than are the light-colored schists farther downstream, but nevertheless it apparently grades into them. On the Michigan side of the river, near the water's edge, much of the rock is essentially like that on the Wisconsin side, but in addition there is present just below the falls and extending from this point to the bridge over the falls the thick band of conglomeratic greenstone refeired to by Wadsworth (p. 102). This is doubtless a tuif bed. Sericite-schists and bands of fairlv massive basic rocks traverse the tuft'. The former were regarded by Wadsworth as dikes and by Williams as special phases of the green schists. The bands of basic rock were described by Williams as dikes. By Wadsworth it was thought that they might be finer-grained and more massive phases of the tuft" beds. On both sides of the river the strike of the schistosity of the schists and of the trend of the more massive bands associated with them is a little south of west. The dip is about vertical. All the massive or nearly massive bands of this series consist of some form of diabase, which is usually saussuritized. On the Michigan side of the river, however, and a little distance back from its bank is a high, abrupt ridge composed of a dark coarse-grained gabbroitic rock, which is uniform ARCHEAN, QUINNESEC SCIHISTS. lo7 in composition and structure throughout the entire length of the ridge. Near the western end of the ridge the rock is composed of a light-greenish gray matrix in which lie porphyritic crystals of white feldspar and black hornblende. This is the type called by Williams "diorite." At its south- east end the rock is green and fine grained. It contains no phenocrysts. This is compact and massive in structure, but everywhere profoundly seamed and jointed. It is out l\y cross gashes and parted joints, and gives everj' 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 moderatel}^ 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. * * * These gashes seem to have been pi"o- duced b}- 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 " klaffende Risse," described by Heim, in the Alps. * * * Tj^g 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." Big Quinnesec Falls. — At the Big Quinnesec Falls and the Horse- race Rapids there is about a mile of continuous exposure on both sides of the river. The barrier rock of the falls is a medium-grained, gray, saussu- ritized gabbro, which is in places almost massive. On the Wisconsin side of the rivei' the pale gabbro can be seen grading into a light-green schist; and again, just below the engine house, on the Michigan side of the falls, it can be traced into the silvery hydro-micaceous or sericite-schists and into green chlorite-schists. At the lower extremity of the basin, below the falls, is an exposure that juts into the river from the Wisconsin side. The main portion of the exposure is composed of a green massive rock, which is a good illustration of a diabase that has suffered epidotization and chloritiza- tion. In places it is crossed by bands of schist, that grade imperceptibly "^VilIiams, op. cit., p. 80. 158 THE MENOMINEE IRON-BEARING DISTRICT. iuto the massive rock. Farther up, ou the Michigan side, is a series of exposures of practically the same rocks. Schistose phases are more couiiuou in these exposures than in the one on the Wisconsin side. It is in these rocks that the fine intergrowths of magnetite and rutile described by Williams" occur Horserace Rapids. — At the Horserace Rapids the rocks are markedly different from any others occurring in the Michigan area of the Quinnesec schists. They are, however, like many of those near the granite mass in Wisconsin. During times of high water the Horserace is a foaming, dash- ing cataract, rushing between two low but steep walls of dark-green rocks, crossed here and there by dark pink bands. When the water is low, shelving banks extend streamward from the bases of the cliffs, and along these the relations of the various rocks to one another may easily be studied. Between the Big Quinnesec Falls and the lower portion of the rapids is a short stretch of comparatively quiet water bordered by dark- green massive and schistose greenstones like those in the basin below the falls. In places these are cut by bands of pink gneiss. The rapids have excavated their channel through rocks of an entirely different character. Here the greater portion of the river's banks consists of a coarse, gray diorite, speckled by large, and often glistening, crystal faces of dark-green hornblende, which sometimes measure 1 or 2 inches in diameter. Occasionally biotite flakes are discernible in the hand speci- men, and minute silvery glistening scales of talc. This mineral coats the walls of all cracks and fissures in the rock and dots the faces of the horn- blende crystals. In many places the basic rocks have a distinctly gneissic structure. The schistose rock is dai'ker than the more massive phase and ^jresents a more crystalline appearance. Other exposures consist of fibrous dark amphibolite and others of dark-green chlorite-schists. The latter often traverse the massive diorites and grade into them on both sides. In general the basic rocks along the Horserace are coarser grained, darker colored, and more crystalline than elsewhere along the river. Another respect in which the Horserace exposures differ from those elsewhere is in the presence of numerous acid rocks. These occur in bands a Williams, G. H., The greenstone-schist areas of the Menominee and Marquette regions of Michi- gan: Bull. U. S. Geol. Survey No. 62, 1890, pp. 99-101; and Neues Jahrbuch fiir Jlin., etc., 1SS7, Cf. also A. Cathrein, ibid., 1888, vol. 2, p. 151. U. S. GEOLOGICAL SURVEY MONOGRAPH XLVi PL. XI ^-4. V ► X.^*^ '' 1 1 1 1 r 1 % j Li 1 1. jk^ n 1 1 ^E ^ ^ i^ 1 1 1 1 1 « ■' 1 A. HORSERACE RAPIDS DURING HIGH WATER. Looking downstream from Chicago, Milwaukee and St, Paul Railway bTidge, li. LOG JAM IN HORSERACE RAPIDS. Looking downstream fiom Chicago, Milwaukee and St. Paul Railway bridge. ARCHEAN, QUINNESEC SCHISTS. 159 of varying width, generally conforming to the foliation of the greenstones. The bands are sometimes composed of a massive pink granite. Usually, however, they are gneissic or schistose, and the rocks are gneisses, augen-gneisses, felsites, or other acid types already referred to in previous pages. In texture these rocks are usually coarse grained, but some of them are very tine grained. In color the}' vary between gray and red, pink shades predominating. Although, as has been stated, the acid bauds usually trend conformably with the foliation of the greenstones, there are enough exceptions to this rule to prove conclusively that many of the bands are intrusive. On the shore of the little cove in the Michigan side of the river at the head of the Horserace, the intrusive character of one of these bands is plainly shown. Two bands of gneiss cut the greenstones. One traverses the schists. At another point farther downstream a band of gneiss ends abrujjtly in the green schists and sends out short apophyses into them. A few otlier instances of similar character may be seen at sev- eral places on the Michigan side of the rapids, but tlie general tendency seems to have been for the acid intrusions to follow the planes of foliation of the schists. THE WESTERN AREA. DISTRIBUTION. The western Quinnesec schists occuj^y a triangular area of about 5 square miles, in Michigan, extending from about the center of sec. 15, T. 40 N., R. 30 W., Michigan, westward to the Menominee River. Where it crosses the river the belt is about 3 miles wide. After crossing the strean into Wisconsin, it gradually widens into a broad expanse, the boundaries of which are at present unknown. The Fourfoot Falls are at the southern side of the area where it crosses the Menominee River, and the old Indian village of Badwater is at its northern limit. The portion of the area away from the river is outlined by a few small, scattered exposures rising as little knobs above the soil and glacial sands. TOPOGRAPHY. The topography of the area underlain by the schists possesses no distinctive peculiarities. At its east end are several small rugged knobs with exposures of greenstones on their slopes, and at the western edge along the river are other knobs, some of which are rough and bare. The 160 THE MENOMINEE IRON-BEAKING DISTRICT. rest of the area is covered with glacial deposits that are so thick as to produce the usual assemblage of hills and hollows characteristic of drift topography. In the river channel, however, stretches of dead water are interspersed between rapids and falls in the manner described as charac- teristic of the channel tln-ougli the Quinnesec schists of the southern area. tSniooth water is more common than in the southern area, and rapids are less common, probably partly because of the thicker covering of drift, and partly because of the fact that the rocks are more uniform in character than in the southern district. The Upper and the Lower Twin Falls, situated in sec. 12, T. 40 N., R. 31 W., are about one-half mile apart. Each is a single cascade, with a plunge of about 20 feet (PI. XII, B). Between them is a stretch of quiet water (see PI. XII, A) flowing gently for the most of this distance between banks of gravel, sand, and clay. Below the Lower Twin Falls the stream is fairly rapid, but its banks are free from exposures for a distance of about a mile. Here the river leaves the schist area at a little falls and rapids known as the Fourfoot Falls. The final plunge is not above 1 or 2 feet in height, but back of this the stream flows over a rapids for a distance of 100 yards or more, and in this distance it makes a fall of about 6 feet. LITHOLOGT. The rocks of this area are grayish green in color, and so fine grained that in manv instances no texture can be discovered in their hand specimens. Most of them are schistose, but their schistosity is not particularly noticeable until an attempt is made to break them, when they tend to split easily along approximately parallel planes like slates, and only with difficulty can they be made to break in any other direction. In some instances, however, the schistosity is so perfect that the rocks cleave exactly like slates. On many of the exposures, especially of the more massive phases, a typical ellip- soidal structure is discernible. The ellipsoids vary in diameter from a few inches to 3 or 4 feet. There is no sti-iking contrast between the material of the ellipsoids and that of the matrix between them. In both cases the rock is a dense grayish greenstone without any distinct textural features The matrix is usually slightly more schistose than the ellipsoids, but other- wise it is like them. In a few exposures in the northern and eastern poi-tions of the area the rock is a little coarser grained. It possesses a structure that suggests U. S. GEOLOGtCAL SURVEY MONOGRAPH A. BASIN BELOW UPPER TWIN FALLS. The water surface is smooth and the current sluggish, The rocks are dense, fine-grained, and ellipsoidal greenstones. The bridge is on the highway between Iron Mountain and Florence, Wis. B. BARRIER ROCK AT UPPER TWIN FALLS, The basin below the falls into which the water plunges is about 20 feet below the crest of the cascade. The rock is a dense, jointed greenstone. ARCHEAN, QUINNESEC SCHISTS. 161 the diabasic structure. It is, moreover, distinctly schistose, but its schistosity is more hke that of the gneisses than Hke that of the shites. The rock probably represents a dike or a series of dikes in the fine-grained green- stone. At the Fourfoot Falls the exposures consist of alternating bands of massive, schistose, and slaty rocks striking about N. 80° W., almost at right angles to the course of the river, and yet these exposures are usually schistose on the Wisconsin side of the stream and massive on the Michigan side. At many jjlaces, both at this point and elsewhere in the area, the more massive phases alternate with schistose phases in the same manner as in the southern area. The schists grade into the massive rocks, and often no break of any kind can be discovered between them. This relation suggests the origin of tlie former from the latter. One peculiar rock of the Upper Twin Falls remains to be mentioned. It is observed in the smooth face of the exposure extending a little south of east from the barrier OA^er which the water plunges. It is apparently a bed of con- glomerate or breccia in the schists. At its extreme eastern end, where first met with, it is a band 6 feet wide composed of irregularly shaped and sharp- edged fragments of an aphanitic green rock in a matrix composed of small angular fragments of chlorite-schist cemented together by a mixture of quartz, calcite, and chlorite (PL XIII, ^-1). The large fragments often match together in such a way as to indicate clearly that they were originally portions of the same mass. This was crushed into fragments and these were thrust apart, and at the same time the matrix was forced between them. To the west this bed splits up into several smaller ones, the largest of which passes over the brink of the falls. This peculiar conglomerate or breccia band strikes approximately parallel to the schis- tosity of the rocks in its vicinity. It appears to be a breccia zone, produced in all probability by the crushing of the rocks wliich it traverses. Unlike the schists in the Menominee River, those of the western area are rarely cut by dikes. A few intrusions of diabase are met with in the area, but granitic intrusives are entirel}' absent. The microsco])ical features of most of the finer-grained rocks of this area do not afford much evidence as to their original character. Tie rocks have suffered so much alteration that in many cases all traces of the original structuies by which their nature might have been identified have disappeared. The composition of the alteration products, however, MON XLVI — 04: 11 162 THE MENOMINEE IRON-BEARING DISTRICT. and the gradation of the dense rocks into coarser-grained varieties which still possess traces of structure, admit of no interpretation other than that the original rocks were basic igneous i-ocks. Their fine grain and ellip- soidal structure proclaim them lavas. Fine-granted greenstones and their derived schists. — A few of the fine- grained greenstones still retain their original structure, and this is in accord with the character ascribed to them above. One of the more massive homogeneous light-green rocks from the Fourfoot Falls is described by Williams as follows: Mineralogically there is hardly a trace of the original rock left. Almost colorless hornblende, pale-green chlorite, zoisite, leucoxene, and a little calcite (all of seeondarj' origin) are the present constituents; and yet the original structui'e of the rock is strikingl}^ well preserved. When viewed with a comparatively low power, in ordinarv light, the outlines of long, almost acicular, feldspar crj'stals 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 cr3^stals make a confused aggregate, but each individual preserves its own proper form (idiomorphic in the sense of Rosen- busch). The angular spaces between the feldspars produce a typical example of the ophitic or diabase structure, although no trace of a diabase mineral remains." This rock is traversed by "schistose 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." Under the microscope this schistose rock exhibits the effects of arreat mechanical stresses. tj"^ Curving and interlacing areas of pale-green chlorite and of a grayish substance (pei'haps the remains of titanic iron) form the main mass of this rock. Thickh' scat- tered through these are patches of a dark-brown substance, often showing concentric zones of a clear, transparent character. These 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 * * * are less changed chemically than those in the massive rock from which this schistose band has been derived.* Other schists of this vicinity are composed of hornblende, chlorite, feldspar, quartz, and leucoxene. Most of the hornblende is a pale-green variety, surrounding compact cores of a brown variety of the same mineral, from which the lighter-colored phase is believed to have been derived by aWilliams, op. cit., pp. 124-125. ^Ibid., p. 125. u. «. r.EOLnr.irAi. curve' A. BRECCIATED BAND OF MASSIVE FINE-GRAINED GREENSTONE AT UPPER TWIN FALLS. The fragments are mainly dense, I'ght-coloied greenstone. The matrix is a schistose, chloritized greenstone containing streaks of chlorite, Ji RIDGE OF DOLOMITE SOUTH OF LAKE ANTOINE. One of the typical hills of the dolomite formation. View taken from north side of the Clifford open pit, Traders mine. The crest of the hil is about two miles south. ARCHEAN, QUINNESEC SCHISTS. 163 bleaching. In these rocks no diabasic texture was observed. This fact, together with the presence of the brown hornblende, seems to indicate that the original rock from which the schist was derived may have been a diorite. At the Twin Falls the rocks are so much altered that no trace of their original structure can be detected. They are usually dense, homogeneous, dark-green rocks, composed of a confused aggregate of pale-green fibrous hornblende, shreds of biotite, remnants of plagioclase, and grains of ilmenite. Around the feldspar remnants, and occupying the space originally occupied by this mineral, is now a mass of actinolite, zoisite, and epidote. These rocks are crossed by bands of chlorite-schists, consisting of bright-green chlorite, finely granular quartz, and perhaps some secondary albite. In addition to these components the schists also often contain grains of ilmenite and leucoxene, flakes of biotite, and occasionally crystals of tourmaline. The contacts between the schists and the massive phases are often very indefinite. The rocks grade into each other by infinitesimal variations. As described by Williams," the first step in the production of the schistose structure is the division of the massive rock by two systems of joints inter- secting at acute angles. These joints divide the mass into rhomboidal prisms, whose cross sections are well displayed on the smooth glaciated surfaces of many of the ledges. As the schistose phases of the rock are approached, the joints become more and more nearly parallel, with tlie result that the rhombs are all lengthened in a corresponding direction. The elongated prisms finally, by a more severe action of the lengthening process, become very thin and much extended lenses, which produce a well-developed wavy schistosity. The process is plainly dynamic, the schistosity being produced by pressure acting at right angles to the final direction of the foliation. Coarse-grained greenstones and their derived schists. — Although most of the rocks in the western Quinnesec schist area are of the indeterminable character described above, a few of them possess more definite characteris- tics. A few exposures are plainly porphyritic. The phenociysts were originally plagioclase, and the groundmass was a fine-grained ophitic aggre- gate that may have contained some glass. These rocks must be classed "Williams, op. cit., p. 128-129. 164 THE MENOMINEE IRON-BEARING DISTRICT. with the basalts. Their relations to the fine-grained homogeneous green- stones can not be seen, so that it can not be determined whether they are dikes or flows. In a few instances coarser-grained rocks have retained enough of their original structure to leave no doubt as to their character. Of course they are much altered, but they show plainly the divergent radial structure of diabases. Moreover, some of them still possess remnants of augite grains, surrounded by light-green hoi-nblende in the triangular areas between the plagioclase laths. Through the hornblende are scattered little grains of magnetite, ilmenite, leucoxene, a few crystals of rutile, and nests of calcite. The plagioclase is partly fresh, partly changed to zoisite, and partly changed to chlorite. These rocks are nearly massive, only traces of foliation being occasion- ally observed in them. Their constituents are much fresher than those of the schists aiid the dense greenstones. Although their relations to the lat- ter rocks can not be seen it is probable that they are younger than these, and that they are dike masses intruding them. Fragmental schists. — The only rocks in the area that appeal- to l)e frag- inental are several narrow bands at the Upper Twin Falls, just below the Cas- cade. The bands are only an inch or two in width. In the field they were supposed to be stringers from the breccia already referred to (p. 1 61) as occur- ring at this place. Under the microscope the rock is seen to be very differ- ent from anything else found among the western Quinnesec schists. It is plainly a very fine-grained aggregate of quartz and tiny grains and specks of some black, opaque substance. From the fact that the bands run irreg- ularly through the greenstones, it is believed that they represent cracks that were filled from above by material from some of the Huronian beds that must once have overspread the greenstones. A somewhat similar rock is described by Williams from the same place. It constitutes one of the schist bands alternating with the more massive greenstones. The thin section — 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 leucoxene. The chlorite scales often have a radially divergent arrangement around the larger included fragments of quartz and feldspar." "Williams, op. cit., p. 133. ARCHEAN, QUINNESEC SCHISTS. 165 The particular schist band from which this specimen was taken differs from most of the schists of the area in the fact that its contact with the massive rocks is very sharp and distinct. This leads Williams to the con- clusion that it is not simply a schistose phase of the massive rock, but that it represents a distinct bed in a volcanic series, the massive rocks and the schists derived from them being old lava flows and the schist just described being a tuff bed. ORIGIN OF THE KOCKS. The fine-grained texture of most of the rocks of tlie western area of Quinnesec schists, when considered in connection with their composition, indicates that they are old lava flows of the general character of basalts. Their ellipsoidal structure points to the same conclusion. In the Crystal Falls district, a few miles west of the Menominee district, the greenstones associated with the Huronian deposits exhibit this structure in a peculiarly fine manner. These rocks are plainly volcanic flows, since they occur in well- defined beds, many of which are amygdaloidal. The Quinnesec schists seem to have originated in the same kinds of rocks. They have, however, suffered so much more metamorphism than the Crystal Falls volcanics that their distinctive volcanic characteristics have disappeared." Tuff beds may have been associated with the old lava flows, but if so, they seem to have been in very subordinate quantity. At any rate, there is no certain evidence that they ever exisited in the area, though they may have done so. The volcanic beds were cut by basic intrusions, and then the whole complex was folded. Where the folding was severe the finer-grained lavas and tuffs were changed into fissile schists, and the coarse-grained beds into schistose greenstones. Where the folding was less severe all the rocks were rendered schistose, but the resulting schists still retained traces of their original structures. After the folding other basic intrusions took place. These are represented by a few massive diabases met with in the area. INTERESTING LOCALITIES. The best exposures of the rocks of this area are found at the Twin Falls and the Fourfoot Falls. Both the Twin Falls are easily reached by the wagon road from Iron Mountain to Florence, Wis. The two falls are o Clements, J. M., Mon. U. S. Geol. Survey, vol. 36, 1899, pp. 113-135. 166 THE MENOMINEE IRON-BEARING DISTRICT. about half a mile apart, but it is only in the vicinity of the cascades that rock exposures are abundant. Upper Twin Falls.— At the Upper Twin Falls (PI. XII, A and B), low flat exposures occur practically all the way from the highway bridge around the basin below the falls to a point a little above the cascade. The ledges here are fine examples of the dense, dark- colored massive green- stones exhibiting the ellipsoidal structure. This structure is best seen on the flat rocks near the water's edge on the Michigan side of the river just above the bridge. The rock is dense and presents a smooth, almost featureless surface. On it, however, by close inspection, can be detected round and oval areas separated from interstitial areas by fine lines. The rock within the lines and that without them is practially alike, except that the former is sometimes slightly different in tinge from the lattei*. The round and oval areas are cross sections of ellipsoids which, in this ledge, vary in size from those having a diameter of 6 or 7 inches to those having a similar dimension of a little over 2 feet. None of them have amygdaloidal peripheries. Near the falls, on both sides of the river, and at the lower end of the basin below the falls, on the Wisconsin side, several bands of chlorite-schist traverse the massive rocks, but they are rare. At the cascade, and on its east side, is the narrow band of fragmental greenstone referred to in a previous page (p. 161). Lower Twin Falls. — At the Lower Twin Falls the ledges are larger and rougher than at the more northerly fall. The rocks are practically the same at both places, but at the lower falls the schistose phases are rather more common. Here can be well seen the transition between the massive- jointed greenstones and the chlorite-schists, and here also, on a small scale, can be seen the actual passage of massive into schistose phases along shear- ing zones. Just below the falls, on the Michigan side, the dense rock is traversed by a seam of quartz occupjnng a crack along which there has been movement. The rock, at the distance of a few inches from the vein, is quite massive; nearer the vein it becomes schistose, with a foliation inclined about 45° to the direction of the vein, and very near the vein it is a typical chlorite-schist. Similar chlorite-schists may also be seen in this ledge coating surfaces of joint planes. Fourfoot Falls. — At the Fourfoot Falls the rocks exhibit greater ^-ariety. ARCHEAN, NORTHERN COMPLEX. 167 The falls themselves are not much more than a rapids over ledges of greenstone-schists. The Chicago and Northwestern Eailway bridge, on the line between Iron Mountain and Commonwealth, crosses the river at the foot of the rapids. For a few hundred yards above the bridge instructive exposures can be seen on both sides of the stream. Alternating bands of massive and schistose greenstones, 'striking about N. 80° W., outcrop on bare banks and form the rapids and several small islands in the river. On the Michigan side the prevailing rocks are massive. On the Wisconsin side schists predominate. At the western end ot tlie railroad bridge the rock is a light-green ajDhanitic, massive greenstone, traversed here and there by wavy bands of schistose greenstone, characterized by abundant slickensides. Beyond this to the north is a narrow band of a black graphitic-looking schist, and north of this another band of schistose greenstone best exposed near the water's edge. Immediately north of this exposure rises a high, sheer cliff, under- mined at the base by hollows produced artificially. This consists of a soft, black, slaty-looking, very schistose, and quite fissile rock. In the hand specimen and in the ledge it bears a strong resemblance to a black slate, but under the microscope it is seen to be a chlorite-schist. At the water's edge, beyond the cliif, the schist again appears, and north of this follow ledges of schistose greenstone. On the Michigan side the rock is fairly imiform in character. The southernmost ledge just under the bridge is a massive, dense, light-green rock. The northernmost ledge is a dark greenstone-schist. The intervening low ledges that dot the bank are massive and schistose greenstones that are a little coarser in grain than the rock under the bridge. In the rocks of some of these ledges the diabasic structure is plainly discernable, but in most ledges the grain is so fine that no well -characterized structure is observable except in thin section under the nucroscope. SECTION 2. NORTHERN COMPIiEX. The rocks constituting the Northern Complex are gneissoid granites, banded gneisses, hornblende-schists, and a few feldspathic green schists, identical with some of the squeezed basic schists among the Quinnesec schists. Mica-schists also occur in the complex, but they are rare. They have been found only in a few exposures in the interior of the Archean area 168 THE MENOMINEE IRON-BEARING DISTRICT. north of the limits of the map. The granites, gneisses, and schists are cut by dikes of coarse-grained and fine-grained basic rocks, by small dikes and veins of aplite and pegmatite, and by numerous quartz veins. DISTRIBUTION. These rocks occupy an elliptical area north of the Menominee trough of fragmentals, separating it from thj Calumet trough farther north. The complex is separated from the Huronian beds by unconformities and by basal conglomerates which will be referred to later. Onlj^ those rocks occurring along the southern edge of the area have been studied in detail. The rest of the area has been examined sufficiently closely, however, to warrant the statement that it does not diifer in any essential respect from its sovithern periphery. TOPOGRAPHT. The topography of the country underlain by the crystalline rocks is not unlike that of Basement Complex areas elsewhere. In the western portion of the area small and large, rounded, glaciated hummocks of rock, often with precipitous sides to the south, rise above the gravels and sands of the glacial deposits. In the eastern part of the district, where these deposits are thinner, the hills protrude higher above the drift, appearing as groups of knolls with bare, fairly smooth upper surfaces, but with ragged and precipitous sides. SEQUENCE OF ROCKS. The study of the relations of the rocks of the complex to one another has not aftbrded any more data for determining their exact sequence than studies in other Archean areas. In general it appears that some of the gneisses and some of the hornblende-schists are older than most of the granites. Others of the schists are plainly mashed intrusives that are younger than most of the granites. All of the greenstone-schists are of this nature. The aplites, pegmatites, and some of the basic intrusives are the youngest rocks belonging in the complex, but even these, since they are not known to cut through the Huronian beds, are thought to have taken their present position before the sediments were deposited. The latest of all the intrusives are certain coarse-grained massive diabases and gabbros. These rocks not only occur as intrusives in the complex, but they are foimd also in the lower member of the Hui'onian series overlying ARCHEAN, NORTHERN COMPLEX. 169 the Arcliean rocks. There is no reason to believe that any of the rocks of the complex are metamorphosed sediments. Most of them are clearly igneous in origin. LITHOLOGT. GNEISSOID GRANITES. The most abundant rocks of the complex are gneissoid granites and granitic gneisses. These rocks differ from one another in no essential respect. The former are merely less schistose than the latter. Both embrace a series of medium-grained to fine-grained gi'ay and jjink rocks with a granitic texture that sometimes approaches in appearance the texture of fine-grained quartzites. The pink or red granites are usually a little coarser grained than the gray ones. In the hand specimen red orthoclase is seen to be the principal constituent. In addition to this there can also be detected a few grains of white feldspar, a large number of quartz grains, and an occasional flake of mica. The gray granites appear to be almost homogeneous. Here and there through them are stringers and patches of pink granite, but most of the hand specimens are of a nearly uniform dark- gray tint. The gray rock passes into the pink rock by almost impercep- tible stages, the differences in the tints of the two end members of the gradation series being due mainly to the color of their feldspathic component. In some instances the pink stringers are very coarse-grained aggregates of feldspar and quartz, like pegmatites. The material of these pegmatitic veins, like that of the finer-grained ones, grades into the mass of the gray rock. There is nowhere any sharp contact between the two. The red granite does not seem to be of the nature of an ordinary intrusive, but it appears rather to have the character of an impregnation, which saturated the gray granite and crystallized in certain places as patches or in'egularly shaped stringers, and in other places, probably along cleavage planes, as more regular and definite veins. In the banded gneisses, to be referred to later, the pegmatitization followed approximately parallel planes, but in the gray and pink gneissoid granites it took place irregularly through the rock mass. Under the microscope all the granites and granitoid gneisses of this area, whether of the pink or the gray variety, are found to consist of quartz, orthoclase, plagioclase, and biotite, and large quantities of the decomposition products of the last-named mineral, and of the feldspars. The most abundant of the decomposition products are kaolin and chlorite, but sericite, calcite. 170 THE MENOMINEE IRON-BEARING DISTRICT. epidote, and ocher are nearly always present in small qnantity. Rutile, zircon, and apatite are also present as accessories. In some sections a little microcline is also to be found among the feldspars. It appears in all cases to be an original component and not a new product as in the Marquette granites. All the feldspars are altei'ed. Small flakes of sericite and spicules of kaolin are scattered through them somewhat uniformly. The microcline and the plagioclase have undergone less alteration than the orthoclase, which in many sections is so completely decomposed that no trace of the original material can now be detected. The biotite is a green variety with a pleoehroism in yellowish -green and emerald-green tints. It is present as small flakes and in aggregate of flakes between the feldspars and the quartz. The secondary chlorite and epidote are found mainly in the vicinity of the biotite, from which they seem to have been derived. At all events, when these products are abundant the biotite appears to be more or less bleached. The red granites differ from the gray varieties mainly in tlie fact that the feldspars of the former inclose large numbers of inclusions of hematite and reddish-brown ocher. They also contain but little biotite and almost no microcline. In other respects the two rocks are essentially the same. The red granite appears to be a little more decomposed than the gray rock and is apparently more crushed, but otherwise there is little besides difference in color to distinguish the two. All specimens of the gneissoid granites show plainly the eftects of mashing. Many of their components are broken and squeezed out into lenticular masses, which give the rocks their schistosity. The peripheral portions of many of the grains are granulated and in these granulated portions is found the greater portion of the secondary sericite. BANDED GNEISSES. The banded gneisses are composed of the same components as are the gneissoid granites. Secondary products are, however, more abundant than in the granites, and the primary constituents are usually less fresh looking. At fii-st glance these banded gneisses appear to be made up of alternate parallel bands of gray and pink schistose granitic material like that of the gray and pink gneissoid granite described above. A close inspection of the ledges, however, shows that while these bands are approximately ARCHEAN, NORTHERN COMPLEX. 171 parallel for short distances, they nevertheless wedge out when traced for distances of 18 or 24 inches, split up into several narrower bands, or coalesce with other bands, forming broader ones. Often, also, narrow bands of one color cut across broader bands of the other color, so that the rocks are actually made up of an interlacing network of veins with meshes of very unequal dimension. The long dimensions are parallel to the apparent banding of the rocks, which is also the direction of their schistosity, and the short dimensions are perpendicular thereto. The banded gneisses are thus like the mottled red and gray gneissoid granites in the fact that they consist of granitic impregnations in a granitic rock The impregnations in the banded rock, however, followed approxi- mately parallel courses, while in the mottled rocks they occun-ed irregularly. The source of the imjiregnating material may very well be the rock in which the impregnations took place. This niay have been partially dis- solved, yielding solutions which transported the dissolved material to situa- tions where the conditions were favorable to its crystallization. HORNBLENDE-SCHISTS. The hornblende-schists are usually lustrous greenish-black schists, with the normal characteristics of such rocks. They are cut by the gran- ites in some places. In other places large blocks are found included in granite. Plainly they are older than the granites, and probably they are the oldest rocks in the northern complex. A second kind of hornblendic schist exists in which the rocks are so related to the g]-anites and gneisses that they must be regarded as dikes. In some places they appear as bands cutting across the banding of the gneisses, and in others as bands conform- ing in strike and dip with the lighter-colored bands of these rocks. These schists are therefore looked upon as mashed intrusives. They differ from the schists of the first kind in being duller in luster and in having- a greener tinge. In their petrographical character the hornblende-schists vary between normal phases composed of hornblende, quartz, epidote, and perhaps occa- sionally a little feldspar, and greenstone-schists composed of amphibole, plagioclase and its decomposition products, quartz, epidote, kaolin, calcite, and sericite. As the proportion of quartz present increases, sericite, calcite, and kaolin gradually disappear, and the rocks approach more and more nearly the normal hornblende-schists in appearance and composition. This 172 THE MENOMINEE IRON BEARING DISTRICT. gradation from rocks that are unquestionably altered basic rocks to true hornbleude-scliists maj" not often be observed in a single outcrop, but the two end members of the series are united by so many gradational phases that there can be little question that they are genetically connected. It is very evident from the microscopical study of their thin sections that the feldspathic hornblende-schists are squeezed and altered basic eruptives. It is very probable that the nonfeldspathic hornblende-schists have the same origin and that they dift'er from the feldspathic schists only in the amount of masliing to which they have been subjected and the consequent amount of alteration they have suffered. INTKUSIVES. The granites, gneisses, and schists are cut by veins of granite, pegma- tite, aplite, and quartz, and by well-defined dikes of basic rocks. Acid intrusives. — The granite veins are of all sizes, from a few feet to a few inches in width. Their material is identical with that of the red granites refeiTed to a few pages back. The pegmatites are coarse-grained red rocks composed of red ortho- clases and white quartz. The feldspar is in grains that occasionally have rudely outlined crystal forms and the quartz in irregular areas between these. While the rock bears some resemblance to a graphic granite, the peg.natitic structure is not very pronounced. The aplite is a fine-grained purplish rock, in which can plainly be seen small lath-shaped crystals of red feldspar lying in a fine-grained matrix composed of irregular grains of red orthoclase and irregular dark- gray areas of a black micaceous mineral and white quartz. The rock is quite massive, not the slightest evidences of schistosity being observed in it. Under the microscope the latli-shaped crystals are found to be andes- ine. In addition to the lath-shaped crystals there are present also many quadrangular sections of a feldspar that extinguishes differentl}' in the four quadrants. These apparently consist of orthoclase twinned according to the Manebach and the Carlsbad laws. These feldspars are embedded in a groundmass made up of orthoclase, quartz, epidote, green chlorite, calcite, a little brown biotite, and some rutile. The orthoclase and the quartz compose the greater part of the groundmass. They are mainly in micropegmatitic intergrowths, the quartz apparently saturating the feldspar. Untwinned orthoclase is also often found ARCHEAN, NORTHERN COMPLEX. 173 surrounding- the quadrangular feldspar phenocrysts above referred to, while the quartz occupies the same position with respect to areas of micropeg- matite in which the quaitzose constituent predominates over the feldspathic one. Between the phenocrysts and the areas of micropegmatite is an aggre- gate of quartz, orthoclase, and the other minerals mentioned above. In this aggregate quartz and feldspar are in small grains, the former with rounded contours and the latter with very irregular ones. The other constituents are also in the aggregate, the chlorite as fairly large green flakes that look as thougli they may have been derived from biotite, the epidote as colorless irregularly shaped grains, the rutile as tiny crystals, and the calcite as nests in the interstices between the quartz and feldspar grains. The orthoclase of the phenocrysts and also of the groundmass is always more or less altered into kaolin-like decomposition products, the porphyritic cr3^stals, however, being much less decomposed than the grains of the matrix. Basic intrusives. — The basic dikes consist of rocks that are known comprehensively as "greenstones," dark-green rocks produced by the altera- tion of basalts, diabases, or gabbros. The dikes vary widely in size. Some are only a foot or two in width, while others measure as much as 300 feet. The smaller intrusions are usuallv dike-like in form, while the larger ones are more boss-like in chai'acter. Their intrusions follow the same general courses for short distances, but they vary in width and have ii-regu- lar contacts with the rocks through which they intrude. A few of the basic intrusives may be limited to the Basement Com- plex. The majority of them, however, cut both the rocks of this series and those of the Huronian. In several instances the large dikes may be traced step by step across the contact of the granite-schists series into the quartz- ites of the Lower Huronian without any break in their continuity, so that in these cases there is no question but that they are younger than the quartz- ites of the iron-bearing series. The macroscopic aspects of these rocks are those of the ancient basic dike rocks so common in Archean areas everywhere. Some of them are fine-grained green rocks without any peculiarly characteristic structure, others are darker-colored, medium-grained rocks with a distinct diabasic structure, and others are coarse-grained varieties with a granular structure. 174 THE MENOMINEE IRON-BEARING DISTRICT. These last mentioned are gabbros, the others are diabases, some of which are very fresh, while many are much altered. Under the microscope the diabases are identical in every respect with the diabases cutting the Quinnesec schists, and the gabbros with corre- sponding phases of the basic intrusives in the Huronian beds (see pp. 185-186). INTERESTING LOCALITIES. The rocks of this series are well exposed along the entire northern side of the Menominee trough. They may be seen at almost any place north of the quartzite belt in a number of small exposures separated from each other bv stretches of glacial drift. The best and largest exposures are in sees. 29, 31, and 32, T. 41 N., R. 29 W., and sees. 1, 2, and 12, T. 40 N., R. 30 W., where the rocks forms liuge bare cliffs that are almost in contact with equally large cliffs of quartzite (see PI. XIV). These exposures are easy of access from Iron Mountain by the road that leads from this city to the so-called gold mine that has been opened in the quartzite in their vicinity. They present the usual features characteristic of Archean complexes. CHAPTER V. THE ALGONKIAN SYSTEM. General character and definition. — The Algonkiau rocks constituting the Menominee trough, though strongh^ metamorphosed, are recognized as mainly sediments. The greater mass of tliese sediments is mechanical, clastic textures usually being plainly apparent in them. The iron forma- tions are largely mechanical, but with tlie mechanical material an impoi'tant amount of chemical and organic material was deposited, and some of the jaspers of the formation may be wholly chemical or organic. The dolomites ai"e principally chemical or organic sediments, but in their lower portions thei-e is an abundant admixture of mechanical debris. The sedimentary rocks have been intruded by a few coarse-grained and some fine-grained basic igneous rocks. The latter are now usually schistose. The lowest member of the Algonkian system has at its bottom basal conglomerates, which rest unconformably upon the Archean I'ocks of the Northern Complex. These conglomerates may be seen at a number of places along the north border of the trough. Their best-known exposures are those at the Falls of the Sturgeon River, made classic by Credner, Brooks, and Irving. Hence the Algonkian rocks are younger than the underlying schists. The members of the system are likewise separated from the overlying Cambrian sandstone by a profound unconformity. The Algonkian rocks are folded; the sandstone is practically horizontal. The latter thus lies across the truncated ends of the eroded folds. Its lower layers are formed largely of the debris of the more ancient rocks. Hence the Algonkian rocks formed a land surface for a vast period of time before the deposition of the Cambrian sandstones. Unconformity tvithin the system. — Within the Algonkian system there is an unconformity corresponding to that in the Marquette district between 175 176 THE MENOMINEE IRON-BEARING DISTRICT. the Upper Marquette and the Lower ]\Iarquette series. This unconformity is not so plainly marked in the Menominee as it is in the Marquette dis- trict, but it is similar in all essential respects to the unconformity between the Lower Huronian cliertv limestone formation in the Penokee disti'ict and the overlying quartz-slate member of the Upper Huronian. In the Marquette district the unconformity is indicated by a marked discordance between the upper members of the lower series and the lowest members of the upper series, and by the presence of a widespread basal conglom- erate in the upper seines. In the Penokee district it is indicated by an erosion contact between the limestone and the quartz-slate without dis- cordance of bedding, and by the presence of fragments of the lower for- mation in the lowermost beds of the overlying slates." In the Menominee district the direct evidence of the unconformity is the presence of a con- glomerate or of a coarse quartzite at the base of the upper series containing undoubted fragments of some of the rocks of the lower series, and the jjresence near the base of this series of an iron formation made up in part of the detritus of an older iron formation. There is also, in addition, indirect evidence of the unconformity in the absence from parts of the district of the lowest and most resistant members of the Algoiikian series — the Sturgeon quartzite and the Randville dolomite. The nonexistence of the iron-bearing Vulcan formation in parts of the district adjacent to the Randville dolomite and the overlapping of the Vulcan formation by the Hanbur}- slate at some of these places give further evidence of unconformity. The Algonkian system is therefore divided into a Lower Menominee and an Upper Menom- inee series, equivalent to the Lower Huronian and tlie Upper Huronian elsewhere in the Lake Superior region. The data upon which the foregoing conclusions are based are discussed in detail in connection with the descriptions of the several formations com- prising the different systems under the heading "Relations to adjacent formations." SECTION 1. liOWER MENOMINEE SERIES. Succession and distribution. — The Lower j\Ienominee series is divided into three formations. These are, in the order of upward succession, the Sturgeon quartzite, the Randville dolomite, and the Negaunee formation. o Irving, R. D., and Van Hise, C. R., The Penokee iron-bearing series of Michigan and Wis- consin: Hon. U. S. Geol. Survey, vol. 19, 1892, pp. 171, 443-444, 454-455, and 472. ALGONKIAN, STURGEON QUARTZITE. 177 The formatious belonging to the Lower Menominee are observed only in the center and on the northern side of the Menominee trough. On the southern side of the trough and around the borders of the western area of Quiunesec schists no evidence of the existence of these formations is obtain- able. This may possibly be due to the thick covering of drift that blankets the rocks in these portions of the district, bnt since ledges of the soft Hanbury slate are found at no great distance from the borders of the schist areas, it is probable that the two formations are actually absent. For it is hardly credible that two such resistant formations as the quartzite and dolomite could have been so completely planed down in these particular portions of the area as to leave no projecting ledges above the drift, while soft slate formations nearby resisted planation sufficiently successfully to yield ledges, especially since in the central and northern portions of the trough the quartzite and dolomite constitute the prominent elevations and the slates the valleys between these. STURGEON QUARTZITE. The Sturgeon quartzite is so called because the principal rock of the formation is a quartzite which is in the same position with respect to the Archean complex and the dolomite formation of the Menominee district as is the Sturgeon quartzite of the Felch Mountain district with respect to the Archean series and the dolomite of that district." DISTRIBUTION AND TOPOGRAPHY. The Sturgeon quartzite forms an almost continuous belt on the north side of the Menominee trough immediately south of the northern Archean complex, with which it is in contact. Its most easterly known exposures are in the center of sec. 9, T. 38 N., R. 28 W. From this point the belt continues in a general northwesterly direction to sec. 2, T. 40 N., R. 30 W., with an average width of a little less than one-fourth of a mile. In its eastern portion the belt is narrow, rarely reaching a breadth of one-fifth of a mile. As it joasses west, however, it gradually widens out, and in sec. 2, T. 40 N., R. SO W., it has a width of a mile. Here the belt turns north and the rock is folded into many pitching folds which cause repetitions of the same beds and a great increase in the apparent thickness and conse- "Mon. U. S. Geol. Survey, vol. 36, 1899, pp. ,398-405. MON XLVI —