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DEPARTMENT OF THE INTERIOR MONOGRAPHS OP THE United States Geological Survey VOLUME XIX WASHINGTON GOVERNMENT PRINTING OFFICE 1892 QEp vol./? UNITED STATES GEOLOGICAL SURVEY .1. W. I'OWKI,!, lilKWri'OK THE PENOKEE IRON-BEARmG SERIES 01' MICHIGAN AND WISCONSIN BY KOLAND DUER IKVING AND CHAELES KlOHAliJD VAF HISE WASHINGTON ' GOVERNMENT PRINTING OFFICE 1893 CONTENTS. Letter op Transmittal -^'n Outline of this I'aphu xvu lNTROnnCTI()X 1 CllAPTEU I. — (iEOI.OCUAL KXI'LUKATIOXS AXI> I^ITEUATUliE O Geoldf^iciil oxploratiipii in thr district, p. 5. Barnes (1817), |). 5. Whitney and Barnes (1817), p. 6. Kandall (181-8), p. 7. Whittlesey (1849 and 1860), p. 7. Liiphani (1858), p. 7. BrooksaudPnnipelly(1871),p. 8. Irving (1873, 1876. 1877, 1885), p. 8. Wight, Sweet and Wriglit (1875), p. 9. Wriglit (1876), p. 9. Chaniberliu (1877), p. 9. Con- over (1878), p. 10. Rominger (1882), p. 10. Van Ilise (1884-1887), p. 10. Object of the work nnder the IT. S. Geological Snrvey, p. 10. Annotated list of the literatnre of the snlijeet, p. 13. * Chapter II. — The Soitiieux Complex 103 General, p. 103. Geographical distribution, p. 104. The W^eateru granite, p. 106. The Western green schist, p. 107. The Central granite, p. 111. The Eastern green schist, p. 116. The Eastern granite, p. 122. Wnmuiary, p. 122. Chapter III.— The Cherty Limestone 127 Relation of the limestone and chert, p. 127. Geographical distribntiou, p. 128. Pos- sible former greater continnity, p. 129. Thickness, p. 130. Petrographlcal character of the limestone, p. 130. Petrographical character of the chert, p. 132. Change to the overlying Quartz-slate, p. 134. Tabulation of petrographical observations, p. 134. Prominent exposures,' p. 138. Origin of the limestone and chert, p. 140. Sumnuvry, i>. 141. (Chapter IV. — The Quartz-Slate Member 143 Applicability of the name, p. 143. Geographical extent, p. 144. Topographical fea- tures, p. 145. Thickness, p. 146. General petrographical character and stratigraphy, p. 146. Microscopical character of the feldspathic (piartz-slates, p. 149. Micro- scopical character of the biotitic and chloritic quartz-slates, ji. 151. Microscopical character of the vitreous quartzite, p. 153. Microscopical character of the sand- stone, novaculite, and argillaceous slates, p. 154. Tabulation of petrographical observations, p. 155. Contacts with the Cherty limestone member, p. 171. Con- tacts with the Southern Complex, p. 172. Change to the Iron-bearing member, p. 175. Prominent exposures, p. 175. Mode of deposition and source of material, p. 179. Summary, p. 180. Chapter V.— The Iron-bearing Member 182 Section I. — Details 182 Applicability of the name, p. 182. Abruptness of transition from the underlying Quartz-slate member, p. 184. Geographical extent, p. 185. Topographi(tal features, p. 188. ■ Thickness, p. 189. General petrographical character, p. 190. Distribu- tion of the three types of rock, p. 198. Microscopical character of the cherty iron carbonates, p. 200. Microscopical character of the ferruginous slates and ferruginous cherts, p. 202. Microscopical character of the actinolitic slates, p. 210. Tabulation of petrographical observations, p. 215. v VI CONTENTS. Chapter V — ContiTiiied. Section II. — Origiu of the rocks of the Iron-bearing member 245 The original rock, p. 246. The ferruginons shites, p. 253. The ferruginona cherts, p. 254. The aotinolitic slates, p: 257. Section III. — The Animikie iron-hearing series 260 The olierty iron carbonates, p. 262. Tlie ferruginons slates, p. 264. The ferruginous cherts, p. 264. Tlie actinolitic slates, p. 266. General, p. 267. Section IV.— The iroil ores 268 Position of the ores in the Iron-bearing member, p. 268. Dikes in Iron-bearing member, p. 271. Position of ore in reference to the dikes, p. 274. Rock above the ore, p. 275. Practical deductions to be applied in prospecting and mining, p. 276. Nature of the rocks of the Iron-bearing member adjacent to the ore bodies, p. 279. The character of the ore, p. 280. A particular occurrence of iron ore, p. 283. Chemistry of the process of concentration, p. 283. Time at which concentration of the main ore bodies occurred, p. 284. Process of conceuti-ation, p. 285. Exceptional localities, p. 290. Probable extent in depth of ore bodies, p. 292. Emmons on ore deposits, p. 293. Iron ores in other parts of Lake Superior country, p. 293. Summary of more important conclusious, p. 294. Chapter VI. — The Upper Si.ate Member 296 Section I.— Details -' - 296 Name and basis of separation, p. 296. Transition from Iron-bearing to Upper-slate member, p. 297. Geographical distribution, p. 298 TopograiJhical features, ji. 301. General petrographieal character, p. 302. Petrographical characters of the four types of rock, p. 304. Tabulation of petrographical observations, p. 309. Section II. — Origiu of the upper slate rocks 332 (1) Quartzose graywacke, p. 333. (2) Muscovitic and biotitic graywacke, p. 336. (3) Biotitic graywacke, p. 337. (4) Muscovitic biotite-slate, p. 338. (5) Nearly crystalline muscovitic biotite-schist, p. 339. (6) Crystalline muscovitic biotite- schist, p. 340. Black mica-slates, p. 341. Source of material, p. 343. Summary, p. 344. Chapter VII. — The Erxiptives 346 Structural relations, p. 346. General character of the rock, p. 348. Comparison of Pouokee greenstones with greenstones of the Southern Complex and Keweenaw series, p. 349. Microscopical characters of the diabases, ji. 350. Eruptives in the Iron-bearing member, p. 355. Summary, p. 3.58. Chapter VIII. — The Eastern Area ^ 360 Introduction 360 Section I. — The Irou-bearing member 361 Distribution, p. 361. Petrographical character, p. 362. Mingled fragmental and uonfragmental sedimentation, p. 362. Probability of ore deposits in the Eastern area, p. 365. 'i'^abnlation of petrographical observations, p. 366. Section II. — FragmentarTocks soutli of the Greenstone-conglomerates 368 Geographical distribution, p. 368. Petrographical character, p. 369. Tabulation of petrographical observations, p. 371. Section III. — The Greenstone-conglomerates 374 Distribution, p. 374. General characteristics, p. 374. Origiu of the Greenstone-con- glomerates, p, 377. Tabulati(ui of petrographical observations, 381. Section IV. — Fragmental and ferruginous rocks north and east of the Greenstone-conglom- erates 387 CONTENTS. vii I'age. CHAI'TIM! Vlll^i'onliiiiioil. Section l\' — (Idiiliimcd. Geof^i'iiiihiciil (list li 1)11 (ion, 1 1. :iS7. SunDiiiiiliiif; rocks, ji. ;i8S. C()iitiini:itii)ii of thi! liolt eiisiaiiil \vcst,)i. :WS. Slnicf.iirc nl' Mir, hiOl, p. liKlt. (inicnil |>(',triif;r;iiilii(:il i-liara(H(!r, ]). :i90. .Mini;li'(l l'nif;iii<'iil;il and iKiiid'anniciitnl Nrdiiiicnts, ]). ;i!U. (Jiiarsely I'nit;- lll(^IItaI rocks, ]). 3itl. Tabulation of iic'li-of;ra|iliical oliscrvalions, ]i. lilMi. Seotion V.— Tlic (irci'iistonns 410 Tlio main area, \>. 110. Tlic area in seotion.s 20, 29, anil 39, townsliij) 47 N., raiif^e 43 W., Micliiyan, p. 411. The area in sections 13, 24, 14, aue 44 W., Miohifjaii, \t. 415. Section VI. -Stratigraphy ., 419 Litliologieal evidence as to equivalence witli the main Peuokee .area, p. 419. Strati- graphical evidence as to equivalence with the main Penokee area, p. 420. Belations of the belts of the Eastern area to one another, ]>. 423. Great width of parts of the Eastern area, p. 423. The southern dips, p. 426. Sequence of events, p. 428. Min- gled fragmental and uoufragmentai sediments, p. 432. . Sunnnary, p. 433. C'HAl'TEK IX. — GeNKRAL GeOLOGY OP TlIK DISTRICT 437 Section I. — Flexures and faults 437 Curving of the layers, p. 437. Fault at Bad river, p. 438. Fault at Potato river, p. 440. Fault in the Eastern area, p. 441. Section II. — Structure 441 The Southern Complex, p. 441. The Cherty limestone and Quartz-slate members, p. 443. Unconformity between the Southern complex and the overlying Cherty Ijime- .stoue and Quartz-slate, p. 444. Unconformity between the Chei'ty lime.stone and the Penokee series proper, p. 454. The Iron-bearing and Upper slate members, p. 455. The unconformity at the base of the Keweenaw series, p. 456. The Eastern sand- stone aud the unconformity at its base, p. 461. Rfisum^ of geological history, p. 463. Why the district is given a sep.arate memoir, p. 466. Depth and metamor- phism, p. 467. Section III. — Correlation ,.... 468 Equivalency of Penokee series proper with Animikie series, p. 468. Equivalency of Penokee and Marquette aeries, p. 470. Comparison with other series, p. 472. Table showing relations of Penokee succession to that of other Lake Superior districts, p. 473. ILLUSTRATIONS. Page Plate I. Preliminary geological map of the Northwest , xx II. General geological map of tlie Penokee district - 2 III. liejjrotluctiou of Whittlesey's cross section of Penokee range ' . 18 IV. Reproduction of Whittlesey's geidogical map of the Penokee range 20 V. Detailed geology, Sheet 1 * ' 128 VI. Detailed geology, Sheet 2 130 VII. Topography, Sheet 3 - 132 VIII. Detailed geology. Sheets 134 IX. Topography, Sheet 4 136 X. Detailed geology, Sheet 4 ■ 138 XI. Topography, Sheet 5 - - - 1^0 XII. Detailed geology, Sheet 5 142 XIII. Detailed geology. Sheet 6 144 XIV. Thin sections from the SoutUeru Complex -'- - - 476 Fig. 1. Biotite-grauite. Fig. 2. Biotitic granitoid gneiss. Fig. 3. Hornhlende-schist. Fig. 4. Hornblende-schist. XV. Thin sections from the Southern Complex 478 Fig. 1. Hornblende-granite. Fig. 2. Hornblende-biotite-syenite. Fig. 3. Biotite-gnelss. Fig. 4. Hornblende-gneiss. XVI. Thin sections from the Cherty limestone member 480 Fig. 1. Tremolitic dolomite. Fig. 2. Cherty limestone. Fig. 3. Concretionary chert. Fig. 4. Chert. XVII. Thin sections from the base of the Quartz-slate member 482 Fig. 1. Chert containing fragmental quartz. Fig. 2. Quartzose chert. Fig. 3. Chert-conglomerate. Fig. 4. Green schist and conglomerate. XVIII. Thin sections from the Quartz-slate member. - - 484 Fig. 1. Graywacke-slate. Fig. 2. The same, in polarized light. Fig. 3. Cherty slate. Fig. 4. Sericitic and chloritic slate. rs X ILLUSTRATIONS. Page. Plate XIX. Thin section-^ from the Quartz-slate member 486 Fig. 1. Biotitic clilorlte-slate. Fig. 2. Biotite-slate. Fig. 3. Sandstone. Fig. 4. Argill.iceons sliale. XX. Thin sections from the upper horizon of the Quartz-slate member 488 Fig. 1. Quartzite. Fig. 2. The same, in polarized light. Fig. 3. Ferruginous quartzite. Fig. 4. The same, in polarized light. XXI. Thin sections of sideritic rocks from the Iron-bearing member and from Law- rence county, Ohio — :. — 4:90 Fig. 1. Sideritic chert. Fig. 2. The same, in polarized light. Fig. 3. Sideritic slate. Fig. 4. Sideritic and ferruginous chert. XXII. Thin sections of ferruginous cherts from the Iron-bearing member 492 Fig. 1. Concretionary clie.rt. Fig. 2. The same, in polarized light. Fig. 3. Brecciated chert. Fig. 4. Ferruginous and brecciated chert. XXIII. Thin sections of ferruginous cherts and actinolitic slates from the Iron-hearing member ^"* Fig., 1. Ferrnginous chert. Fig. 2. The .same, iu polarized light. Fig. 3. Actinolitic schist. Fig. 4. The same, in polarized light. XXIV. Thin sections of the actinolitic .slates from the Iron-bearing member of the Penokee series, and cherty iron carbonates from the Vermillion series 496 Fig. 1. Actinolite-magnetite-sohist. Fig. 2. Actinolitic slate. Fig. 3. Cherty iron carbonate. Fig. 4. The same, in polarized light. XXV. Thin sections of sideritic slates from the Animikie series 498 Fig. 1. Cherty iron carbonate. Fig. 2. Sideritic chert. Fig. 3. Actinolite-siderite-slate. Fig. 4. The same, in polarized light. XXVI. Thin sections of ferruginous clierts and iron carbonates from the Animikie 300 Fig. 1. Concretionary chert. Fig. 2. The samej in polarized light. Fig. 3. Ferruginous chert. Fig. 4. Sideritic chert. XXVII. Thiu sections .showing formatiou of «oucretions of Iron-bearing member 502 Fig. 1. Sideritic chert. Fig. 2. Sideritic chert. Fig. 3. Another part of the same section. Fig. 4. Ferruginous chert. series . ILLCSTlfATIONS. XI f'l.A'lK XXVI II. 'I'liiii srrlioiisdl' iiKiniictilic Miuhii'l iiHilil ic si. -lies rroiii Ihi^ I r<>n-lie;iriiin- ini'rn l"i 504 l'i;j;. 1. Act.iiiolitic slalo. riji;. 2. MuKiiotitic roiiciotioniiiy c.hi'rt. I'Mfj. S. MiukNmI lUiifiiiotilic jiisper. I'M;;'. 1. Ai'l'iiKilil.ii', 8l;it:i\ XXIX. Thin .scctiDiiH fniiu tlio ii'dii I'Dnnatioii of the AuiiiiikiH sp.rie.s, mid IVnin Law- rriic<' coniity, Ohio 506 Kij;. 1. Concrctiduary (•hert.* Fig. 2. Actiiiolitic slati^ Fig. 3. Actiiiolitic ami sideritic slate. Fig. 4. Clierty iron carboiiatc. XXX. Ore deposits : 508 Fig. 1. Longitudiual sectiou of south dbpoNit, Moutieal iiiinc. Fig. 2. Longitudinal sectiou of north deposit, Montreal mine. Fig. 3. Cross section of south deposit, Moutreal mine. Fig. 4. Cross sectiou of south and ^rth depo.sits, Moutreal mine. Fig. ~y. Longitiulinal sectiou of Pence mine. Figs. 6, 7, and 8. Cross sections of Pence mine. XXXI. Ore deposits 510 Fig. 1. Longitudinal sectiou of south dcposih, Colliy miue. Fig. 2. Longitudin.al sectiou of north deposit, Colby mine. Fig; 3. Cross section of north ami south deposits, Colby miue. Fig. 4. Longitudin.at section of Trimble mine. Fig. 5. Cross section of Trimble mine. Fig. 6. Cross section of Minnewawa mine. Fig. 7. Theoretical'sectiou to show variation from unaltered carbonate to ferruginous chert and ore bodies. XXXII. Thin sections of gray waokes from Upper-slate member 512 Fig. 1. Micaceous graywaeke. Fig. 2. Biotitic and mviscovitic gr,aywacke. Fig. 3. Biotitic graywacke. Fig. 4. Biotitic and chloritic graywacke. XXXIII. Thin sections showing development of mica-slates 514 Fig. 1. Biotitic and muscovitic graywacke. Pig. 2. Biotite-slate. Fig. 3. Biotite-slate. Fig. 4. Biotite-slate. XXXIV.. Thin sections showiug development of mica-schists and mica-slates 516 Fig. 1. Muscovitic biotite-schist. Fig. 2. Biotite-schist. Fig 3. Biotite-slate. Fig 4 Biotite-slate. XXXV. Thin section., from the Eastern area 518 Fig. 1. Ferruginou." chert and quartzite. Fig. 2. Greeustoue-conglomerate. Fig. 3. Greenstone-conglomerate. Fig. 4. The same, in polarized light. XII ILLUSTRATIONS. Page. Plate XXXVI. Detailed geology in tlie vicinity of Penokee gap 520 XXXVII. Geologicnl map of Gimflint lake anil vicinity, Animikle series 522 Fiu. 1. Reproduction of Barnes and Whitney's geological map of region between Agogebic lake .and Montreal river - - 13 2. Reiirodnction of a portion of Brooks .and Pumpelly's geological map of the npper peninsnla of Michigan - - 31 3. Schist cut by massive granite, NW. i Sec. 4, T. 46N., E. 2 E., Wisconsin 117 4. .Schist cat by granite, NE. corner Sec. 28,.T. 47 N., R. 45 W., Michigan 117 5. Map of exposures at Pot.ato river - - 172 6. Junction of quartz-slate and green schists at Potato river 173 7. Large-scale drawing of junction of quartz-slate and green schists at Potato river ... 174 8. Map .and section showing position of rock exposures at Tyler's fork 177 9. Map of exposures at West branch of Montreal river -- - 178 10. Hornblende enlargement of augite in diabase - - - .112 11. Hornblende enlargement of augite in di.abase - - - - - 413 12. Basal conglomerate in contact with granite in Sec. 28, T. 47 N., E. 42 W., Michigan.. 450 LETT-HR OF TRANSMITTAL Departmen'I' ok thk Interior, U. S. Geological Survey, Lake Superior Division, .Madison, Wis., May 1, 1890.. Sir: I transmit lierewith the nianuscrii)t of a memoir ujiou tlie Peiio- kee iron-bearing series of Michigan and Wisconsin, by tlie late Prof Rohmd D. Irving- and myself. Prof. Irving's death occurred so early in life that, while his contribu- tions to geology are large, his later works show that he had but fairly entered the period which would have been the most fruitful in scientific results. The report submitted was designed by Prof Irving to be the first of a series which should treat each of the important iron-producing districts adjacent to Lake Superior. To him who built up the division and who plainied this investigation is very largely due whatever excellence this volume may have. I must necessarily assume, for the most part, the responsibility for the present form of the memoir. The field survey for the present report began- five ' years ago. The seasons of 1884, 1885, and the larger part of that of the following year I gave to this work. For a time in 1885 and in 1886 Prof Irving accom- panied the party in person. When the work began the district was one which explorers had but fairly entered and no railroad reached any part of the range except the Wisconsin Central, which crosses it at Penokee gap. The district has since developed into one of the most important iron-pro- ducing areas of tlie country, its product in the lake Superior region being exceeded by that of the Marquette district only. Before the beginning of this investigation Prof Irving had done a large amount of field work" upon a portion of the range for the Wisconsin XIV LETTER OF TRANSMITTAL. Geological Survey and had prepared a systematic report upon this part of it. He was thus able to direct the more detailed examination of the whole area so that no loss of time should occur. The plan of the work and of the present book is very largely his. The descriptions of the formations and thin sections are, for special reasons, given in greater detail than is intended with "any subsequent area. This is the first of the iron-producing districts of lake Superior in which the geology has been worked out in detail, and the fundamental conclusions reached are in opposition to those expressed by some geologists ; so that it was thoiight necessary to make the facts fully accessible to those who desire to have them. The succession of formations is so clear also that it is believed that the results contained in this report will serve as a key to unlock, in a measure, the more complicated geological structure of the adfaceut iron-producing districts. In order to enable the general reader to avoid details, the descriptions of particular rock exposures and their thin sections for each of the formations are placed together in small type. To Mr. J. Parke Channing our especial thanks are due for a large amount of gratuitous work, and in particular for his detailed examination and drawings of many of the mines, and for the facts contained as to the structural relations between the dike rocks and ore bodies. As mining inspector of Gogebic county, he resided in the district and has from time to time given us the results of the latest developments. Mr. J. M.. Long- year, of Marquette, Michigan, has also given us nuich assistance, including access to his very large collection of specimens of the Gogebic end of the range, made as the agent of the Lake Superior and Portage Ship Canal Company. To Mr. B. N. White and Mr. Charles Oley, woodsmen, we are indebted for all the assistance which could be given by skilled men some- what familiar with the district. To numerous miners and mine superin- tendents we are indebted for many courtesies. With a few exceptions, information with referertce to mining properties has been freely granted to us by all. The original design was to publish the whole book as a joint produc- tion, but the death of Prof Irving occurred in the midst of the preparation of the volume, so that umch of it has wholly devolved upon roe; conse- LKTTEIJ OF THANRl\riTTAL. XV quently, I luivc not I'cll \v:ni;mlcil in iisiuj^- his lumui in this general way. Chapter i lie prcpiircd, wilh the cxceijtiiin of a lew pages, and this is credited to him; cliaptci-s in, iv, and v wt^-o jointly pre[)ared; the remain- ing ehai)ters have fallen upon me. In order to lUJike it pert'eetly clear to what extent the woi'k can have the shelter of Prof. Irving's authority, each chapter is headed by the name of its author. Very respectfully, yoiu- obedient servant, C. R. Van Hise, Geologist in charge. Hon. J. W. Powell, Director U. 8. Geological Survey, Washington, D. C. OUTLINE OF THIS PAPER. The Peuokcc scries jjiopei- is ii .siiccessiou of formations extendiag, with some bvealcs, from Like Gogebic, Michigau, to lake Numakagon, in Wisconsin, a distance of abont 80 miles. It is a mouoclinal series, its dips being universally to the north. Tlie three formations making nj) the Penokee succession are the Quartz-slate member, the Iron-bearing member, and the IJpj)er-slate member, and below these is the Cherty limestone formation. The series is sharply separated geographically from a crystalline complex to the south, called the Southern Complex. It is separated with equal shar^mess from the Keweenaw series to the north. Cha])ter I gives a history of tlie geological explorations iu the Penokee district and a full summary of previous literature. Chapter II treats of the Southern Complex. This consists of two main types of rocks — light colored, coarse grained granites and granitegueisses, and dark colored, flue grained, iiuely laminated schists. In passing from west to east arc found in order the Western granite, the Western green schist, the Central granite, the Eastern gi-een schist, and the Eastern granite areas. The rocks of the Southern Complex are always comi^letely crystalline. If any of them are of fragmeutal origin their present constitution gives no evidence of this. The contacts between the granite and the schists are eruptive, the granite being the intrusive rock. The schists arc consequently older. Certain of the most laminated schists grade iuto rocks which are of distinctly eruptive ty])es, and hence the only rocks in the Southern Complex the origin of which is known are igneous. Chapter III treats of the Cherty limestone below the Penokee series proper. This formation, instead of being continuous, is fouUd only at intervals, and varies in thickness uii to 300 feet. It consists of cherty dolomitic limestone alternating with layers of chert. The Cherty limestone is a water-deposited sediment, but wluithei' of chemical or organic origin, or of both, is uncertain. Chapter IV treats of the Quartz-slate member. This member rests directly upon the Cherty limestone or upon the Southern Complex. It is a persistent, well charac- terized horizon, having an, average thickness of about 500 feet. The Quartz-slate is always plainly clastic, and quartz is its prominent constituent, although other minerals, and especially feldspar, are not unimportant, Its uppermost horizon is a layer of pure vitreous quartzite, MOH^ XIX — -II ^VH xviir OUTLINE OF THIS PAPER. Chapter V treats of the Iron-bearing member. This persistent formation, aver- a"-iug about 800 feet in thickness, rests upon the vitreous qnartzite of the Upper-slate member. It now consists of three main types of rock, cherty iron carbonates, ferru- ginous slates and cherts, and actinolitic and maguetitic slates. The first of these is the original type of rock and from it, by means of chemical changes, the second and third types, as well as the ore bodies, have been produced. Theore bodies are found in the loAvest horizons of the formations and are secondary concentrations. They occur in V-shaped troughs, one side of the V's being the upper quartzite of the Quartz- slate, and the other diabase dikes. In the Animikie series, on the opposite side of lake Superior, is an iron-bearing formation which has'the same types of rock, derived from the same original form as in the Penokee series. Chapter VI treats of the Upper-slate member. This formation rests upon the Iron-bearing member. It has a maximum thickness of 12,000 feet, and has an extent east and west for many miles, although it is not so extensive as the Iron-bearing and Quartz-slate members. The formation is of clastic origin and consists mainly of gray- wackes and graywacke-slate. It is now locally altered by metasomatic changes so that it has become a crystalline mica-schist. Chapter VII treats of the eruptives. The Penokee eruptives are diabases, which structurally are of two classes, dikes cutting the formations, and interbedded sheets which are probably intrusives of the same age as the dikes. The eruptives are fresh in the slate members, but are much or completely altered in the Iron-bearing member, showing that environment, not age, is the important factor in the preservation of these rocks. Chapter VIII treats of the Eastern area. In the eastern part of the district, as a result of contemporaneous volcanic action, the Penokee succession is disturbed, and associated with the ordinary detrital rocks are surface basic volcanic flows, and also greenstone-conglomerates and agglomerates. Consequent upon this volcanic distirrb- ance the regular alternation of clastic and nonclastic members of the Penokee vsuc- cession is much modified, so that the number and order of the formations here found differ from those in the remainder of the district. , Chapter IX treats of the general geology of the region. While the oiitcrop of the members of the Penokee series as a whole are gently curved, sharp flexures and faults are not common. One fault occurs at Bad river, another at Potato river, and per- haps one in the Eastern area. The base of the Quartz-slate member contains frag- ments derived from the "Cherty limestone member, showing that between these forma- tions there was an erosioiiiuterval. How great the time gap represented by this is there is no means of judging, except that the chert of the limestone was certainly in its present condition at the time of the deposition of the Quartz-slate. The Quartz- slate, Iron-bearing and Upper-slate members form a conformable succession. Between tl*B Penokee series proper and the Southern Complex there is a very great uncon^ UUTLINE OK THIS PAPER. xix fonnity. Before flic dcpositicin nl' (he Peiiokec series the .SouIIhtm (Jomplex Lad reaclictl its |)rcs('iit coiiiplclcly cryslalliiic condition and was reduced nearly 'to a ))a,se level. Between the J'lMioUee series and the oveilyinfi' Kcweeiiawau is a second very considerable uneoiitt)rniity, siilliciijnt to ha-ve removeil in places tlui entire Penokee succession. After the erosion and depositiun of the Keweenaw series the whole was tilter f\ Cam BRIAN Eastern Sandstone Keweenawan r^ \jl] r*^ GENERAL GEOLOGICAL MAP OF THE BY R D IRVINO AND C R VAN Scale of Map I in - 3 miles Algonkian PENOKEE DISTRICT HISE Penokee Series ARCH EAN SOOTHERN COMPLEJI BEAniNG UEMBEff qUARTZ- SLATE MEMBER CHERTr LIMESTONE ^m \jK] IB ;"STONE 4)NG10MERATC5 FEftnuGINOUS SLATES ERUPTIVCS ^ 6»»H(TE ( OOmrrOiO CNI'SS SCmiSTIum CMixEOeNtlSS CMB [23 ■■ Qn cm INTRODUCTION. 3 (listribution tVom the series of parallel belts northward. These belts, four iu uuiubi'r, follow in reg'ular succession, with the exception of a few miles at the east end of the area, 'l^hey are separated from each other upon the principle of frag'mental and uonfragmental character. This memoir treats of the rocks between the gneiss-granite areas and the Keweenaw series, and to theiu is applied the name Penokee series. The Wisconsin geologists, as will be seen by the literature, (tailed the iron-bearing rocks and associated slates the Penokee series. On the Michi- gan side of the boundary the course of travel was largely by the way of Agogebic, or as it is now contracted, Gogebic lake, and the geologists, explorers, and miners gave to that part of the area the name Gogebic district. As the areas are parts of continuous geological series, one or the other of these two names must be accepted for the whole, or else a new name be coined by their combination. The latter course would be perhaps the less objec- tionable if the resultant name Penokee-Gogebic were not so awkward. The only systematic geological treatment of any part of the district is that by Irving and Wright in the Geology of Wisconsin; hence, under the law of priority, the term Penokee series is in this volume used to cover the whole area. The southernmost of the belts of the Penokee series is called the Cherty limestone. This name sufficiently indicates its character. Whether it is of direct chemical or of organic clastic origin, it now gives no evidence of having been fragmental. The next belt to the northward is called the Quartz-slate member, because quartz is the preponderating constituent and a slaty structure the normal one. It is sharply separated from the under- lying Cherty limestone member l^y the fact that it everywhere reveals in thin sections its essential fragmental character, and also by the fact that it bears debris from the Cherty limestone. Next to the north is the Iron- bearing member, so called because all the known ore-bodies occur within it. Whatever its origin, it, like the Cherty limestone, never gives any evi- dence of a fragmental character. North of this belt is the Upper slate mem- ber. This is in places. several times as thick as the three lower members combined, but the whole width is included within a single belt because it is everywhere substantially alike, It is a graywacke, graywacke-slate, mica- 4 THE PENOKEE IRON BEAEING SERIES. slate, or mica-schist, which is chiefly composed of quartz and feldspar. Like the Quartz-slate member, in thiu section it usually reveals its fragmental character. The area of rocks situated at the same geologic horizons as these four belts, between Grogebic lake and the middle of T. 47 N., R. 44 W., Michigan, does not make up so plain a succession, so that this part of the district is separated from the four belts just spoken of as the Eastern area, and under this title is given separate treatment. CHAPTER T. By R. D, Irving. GEOLOGICAL EXPLORATIONS AND LITERATURE. Geological exploration in the district— Barnes (1847); Whitney and Barnes (1847); Randall (1848); Whittlesey (1849 and 1860); Laphara (1858); Brooks and Piimpelly (1871); Irving (1873, 1876, 1877, 1885) ; Wight, Sweet, and Wright (1875) ; Wright (1876) ; Chamberlin (1877) ; Conover (1878) ; Rominger fl882); Van Hise (1884-1887). Object of the work under the U. S. Geological Survey. Annotated list of the literature of the subject. As in the case of most other reo-ions on the south side of lake Superior, the first geological explorations made in the Penokee country date quite far back, but no attempt at detailed work was made before that done west of the Montreal river by the Wiscon.sin survey, 1873-1878. East of this river, in Michigan, the only approach to a detailed study prior to that by the authors of the j^n-esent volume was by Dr. C. Rominger, then state geologist of Michigan, in 1882. Dr. Rominger's results have not yet been published, but he has been kind enough to send me a manuscript copy of that portion of his last report which covers this district. While this report is unaccompanied by maps, and while Dr. Rominger's locations of specimens are not closer than the quarter section, it yet contains much valuable material. I add a few notes with regard to each of the geologists who have personally investigated any portion of the Penokee district, arranging these notes chronologically, the date after each name being the time of exploration. Barnes (1847).— Mr. George 0. Barnes was one of the assistants on the U. S. Geological and Mineralogical Survey of the lake Superior land district, then under the direction of Dr. C. T. Jackson, and subsequent to 6 THE PENOKEB IRON-BEARIiSTG SERIES. 1848 under the joint charge of Messrs. J. W. Foster and J. D. Whitney. Mr. Barnes appears to have been the first geologist to enter any portion of our district.^ In the summer of 1847 he accompanied the township land surveyors, noting and locating all rock exposures met with on the lines surveyed, from a point on the Ontonagon river where it crosses the east line of T. 49 N., R. 41 W., Michigan, southward for 28 miles; then westward 12 miles, north 12 miles, west 6 miles, nqrtli 6 miles, and east 6 miles to the northeast corner of T. 47 N., R. 43 W., Michigan; and thence northward to Grogebic lake. Mr. Barnes's exploration was thus chieiiy in the granitic and gneissic region in the southern part of the southern end of our district. One of his lines, however, crossed the iron-bearing .slates, but without affording him any indication of their existence.^ Whitney and Barnes (1847).— Later in the Same scasou Mr. Barnes accom- panied Mr. J. D. Whitney, then one of the chief assistants under Jackson, in a second trip into the country between Gogebic lake and the Montreal river.^ Their coui'se was from the mouth of Black river to the northwest comer of T. 48 N., R. 46 W., Michigan ; thence along the range and town- ship lines (then the only surveyed lines in the region), south 12 miles, east 6 miles, north 12 miles, and west 5 miles to the Black riAJ^er, on the north line of T. 48 N., R. 46 W., Michigan. This route, as also that previously traversed by Mr. Barnes, is indicated on Fig. 1. Thus Messrs. Barnes and Whitney made in 1847, in all, three traverses of oiu- district, the easternmost one lying 6 miles west of lake Grogebic, and the westernmost 2 to 5 miles east of the Montreal river. Whitney also saw nothing on his trip to sug- gest to him the existence in the region of any other formation than the granites to the south and the eruptive greenstones of the copper-bearing series to the north. Accordingly this portion of their district was mapped by Messrs. Foster and Whitney with the granites to the south and the erup- tive greenstones to the north coming directly into contact with one another, without any intervening slates. This mapping was rej^roduced on ail later ' Diary of lielfl work for the summer of 1847, in Report on the Geological and Mineralogienl Sur- vey of the Mineral Landa of the United States in the State of Michigan; by G. T. .Jackson, United States Geologist. Senate Docs., 1st sess. 31st Cong., 1849-50, vol. iii, No. 1, pt. 3, pp. 371-605, also 627-801.. = See p. 34. ' Whitney's diary, in same report as that of Mr. Barnes .just referred to, pp, 33, 34. • GEOLOCrlCAI. EXPLORATIONS AND LiTEKATURE. 7 g-eolofiMciil maps ol' flic Inkc Suporior roiiutry until after the exploration li\- Puiiipclh ;iii(l Hrooks in 1872. Randall (1848).— In 1848 l)r. A. Randall, one of the assistants of Dr. D. D. Owen on the U. S. Geological Survey of Wisconsin, Iowa, and Mimie- sota, accompanied the linear surveyors along the fourth principal meridian from lake Superior southward.' Dr. Randall appears to have been the first to note the existence in this region of any of the rocks of the iron- bearing series, he having observed exposures of lean magnetic ore. Whittlesey (1849 and i860).— Col. Charles Wliittlcscy was one of the "heads of sub-corps " on the survey above referred to'-^ under Dr. D. D. Owen. After Dr. Randall's discovery of iron ore on the fourth principal meridian in 1848, Col. Whittlesey followed the iron-bearing slates from the meridian westward to the vicinity of English lake, or Lac des Anglais, as it was then called by the French voyageurs. ' Col. Whittlesey's report covers all of the region drained l^y the Bad and Montreal rivers in northern Wiscon- sin. It is quoted at some length below. Eleven years later Whittlesey made a somewhat more thorough study of the same region for the Wiscon- sin state survey then organized under James Hall.'' His earlier work, however, was, of course, no more than a very rough reconnaissance. It could not indeed have been more, considering the difficulties of travel in the region and the fact that there existed in it at the time but a single sur- veyed line, that of the meridian above mentioned. These difficulties con- sidered, we must credit Whittlesey with having accomplished a good deal. In fact, to his earlier work for Owen's survey and later work for the Wis- consin survey was due all really definite information at hand with regard to the geology of the Penokee country, previous to the investigations of the Wisconsin survey of 1873-79. Lapham (1858).— In September, 1858, Dr. I. A. Lapham, afterward chief of the Wisconsin Geological Survey, made rapid trips along the Penokee range from Bad river at Penokee gap eastward as far as the fourth ' Report of a Geological Survey of Iowa, WIsconsiu, aud Miimesota, David Dale Owen, 1852 pp. 151, 153, 155, 444; see also C. AVhittlesey in an article on the Penoliie Mineral Eange, Proc. Bost. Soc. Nat. History, Vol. ix, 1862-'63, pp. 235-244. 2 Op. cit. p. XV. 3 The Penokie Mineral Range, Proc. Bost. Soc. Nat. Hist., vol. ix, 1862-63, pp. 235-244. 8 THE PENOKEE lEOlSr-BEARINa SERIES. meridian, and westward to the end of the range in the vicinity of English lake.' Dr. Lapham's object was an economic one mainly, his investigation being made in behalf of a Milwaukee iron company. Dr. Lapham located a number of the exposures of the lean magnetic ore that makes up so much of this range and some of the exposures also of the associated rocks. These exposures he indicated on a simple map accompanying his report. His map was thus the first attempt at a definite platting of any portion of this interesting belt. Brooks and Pumpeiiy (1871).— In the fall of 1871 Messrs. T. B. Brooks and R. Pumpeiiy, then engaged for the State of Michigan in a geological survey of the so-called upper peninsula of that State, made a rapid reconnaissance along the iron belt from the passage of Bad river through the Penokee range in Wisconsin eastward to the Ontonagon river in Michigan. No definite mapping was attempted, but some very interesting new conclu- sions were reached, as announced shortly afterward in publications below quoted from. Irving (1873, 1876, 1877, 1885).— The writer's studies in the Penokee region began in June, 1873, under the auspices of the Wisconsin Geological Sur- vey, then recently revived under the direction of Dr. I. A. Lapham.^ Sub- sequently this work was continued under the same anspices during the seasons of 1876 and 1877, under the direction of Dr. T. C. Chamberlin, the successor of Dr. Lapham as state geologist.^ The attempt was made to do such detailed work as would furnish the necessary material for the construction of maps and the determination of the true order of succession of the rocks of the region. The results obtained were finally published in 1880 in full, with numerous maps and other illustrations, in the third volume of the Greology of Wisconsin. This report covers not only that portion of our present district which is included within the State of Wis- consin, but also all of northern Wisconsin lying north of T. 43 N. and east of R. 6 W. Many of the results contained in this report are made ' The Penokee Iron Range, Trans. Wis. State Agr. Soc, vol. v, p. 391-400; also Report of the Directors of the Wisconsin and L. S. Mining and Smelting Co., Milwaukee, 1860, pp. 22-37. ^Geol. of Wis., vol. II, pp. 7, 8. 3 Annual Report of Progress and Results of the Wisconsin Geological Survey for the year 1876, pp. 13-18. Annual Report of the Wjaconsin Geological Survey for the year 1877, pp. 17-25. (JKOLOGK^ATi EXPLORATIONS AND LITERATURE. 9 use of ill the prcscnr voluiiic. The routes tbllowetl, so tar iis tliey traverse our jivesciit district, and the exposures located iin^ all indicated on tlic detailed maps herewith. 'I'he writer's visit to this district in IS.S,'') was made parth' tor the purpos<^ of" supervising the field work then going- on uu- to the Ponokee-Gog-eljic district. Hciiifr arrang-ed chron- ologically, the notes and (juotations attached to the name of each work will serve to show more fully than has been done in previous 2)ages the history of our subject, up to the time of the special investigations upon which the present volume is particularly based. The dates are the years of publi- cation. 1S49. Whitkey (J. D.). Letter to Dr. C. T. Jackson, in Jackson's Eei)oit on the Mineral Lands of Lake Superior, Senate Documents, 1st session, 30th Congress, vol. II, No. 3, 1847-'48, p. 223-230. Whitney's two traverses of the Gogebic iron belt have already been mentioned and indicated on Fig. 1. In this report we find the first men- tion of the results of his observations on the two lines followed, as also S''™ ^« " xn w? ~~SM -—an Fig. 1.— Eeproduction of Barnes and Whitney's geological map of region between AgogBbic lake and Montreal river. of the observations of Mr. Barnes along a line 12 miles farther to the east. The exact courses of these traverses are given in a volume subse- quently referred to, from which also is taken the geology of the map of Fig. 1. From this map it will be seen that no suspicion was raised in Whitney's mind as to the existence in this region of his "Azoic slates," these slates appearing under this name over a large -part of the very map of a portion of which Fig. 1 is a reduced copy. The trappean rocks which belong to what we now designate as the Keweenaw or copper-bearing series, are made to lie directly against the ''granite and syenite," without 14 THE PENOKEE IRON-BEAEING SERIES. intervening slates. The same conclusion is indicated in the following remarks quoted from this letter (p. 230) : On crossing the soiitliern edge of the trap range, we no longer find, on proceeding soiith, a belt of sandstone similar to that on the north, as is the case between the Portage and the Ontonagon. The only rocks which we have found in place are granite and greenstone; not, however, arranged in regular order, but scattered con- fusedly without order of superposition or direction. The whole country to the south, as seen from any one of the high points of the trap range, as far as the eye can reach, seems to be almost an unbroken plain. On traversing it, it is found to be made up of alternate swamps and low ridges of granite and greenstone, which are sometimes liiddeu by soil and covered with pine, hemlock, and especially sugar maple. Some- times, again, the granite rocks rise with almost vertical walls, yet never to any con- siderable height, from the midst of the swamps, and are covered only by thick moss. It will be impossible to draw any line of demarcation between the granite and green- stone rocks, since they occur together constantly, neither being confined to *any particular portion of the district. The Avhole of this country is almost inaccessible from its swampy nature; neither does it promise to be of aily value for its mineral contents, since we have never found any other ores than a iaw scattered magnetic masses of iron ore and iron pyrites; neither, however, in any considerable quantity. 1850. Baknes (George O.). Diary of field work for the summer of 1847, in Report on the Geological and Mineralogical Survey of the Mineral Lands of the United States in the State of Michigan, by C. T. JacksSn, XJ. S. Geologist, Senate Docs., 1st sess., 31st Cong., 1849-'50, vol. in, No. 1, pt. in, pp. 371-605, also pp. 627-801. Although a further and final report was published by Dr. Jackson in 1852, this one is essentially the closing report of his work, covering the period up to the time of his resignation of the survey into the hands of Messrs. Foster and Whitney. Mr. Barnes's diary of field work is given in this report. The larger part of the region west of Gogebic lake was at the time without surveyed lines. His course of travel and his failure to discover a series of iron-bearing slates have already been noted. Mr. Barnes crossed the Grogebic belt once and made an extended trip in the granite region farther- south. Later in the same season (see diary pages 738-740) Mr. Barnes accompanied Whitney in his traverses of the Gogebic district, and designated the positions of a number of rock exposures ou the • lines followed. C.EOLOdlCAL EXPLORATIONS AND LITERATFTRE. ]5 Whitney (,I. I ).). I )i;iiy in .saiu.' loporl will, tliaL of Mr. Btirues just referred to. In Whitney's diary ..f (iuM work (pp. 738-740) an ax;c.(.unt is given of the journey made hy him in company with Mr. Barnes. His course of travel has ah-eady been given and mapped, Fig. 1. The observations made on three Hues of travel by Barnes and Whitney in the country between Gogebic lake and the Montreal river are all the basis for any state- ments with regard to this region made by Jackson and by Foster and Whitney in their respective reports. All later published statements with regard to the geology of this region, up to the time of the rapid trip made by Brooks and Pumpelly in 1871, depend also upon these few observa- tions by Whitney aud Barnes. It is interesting to note that although these gentlemen found nothing in the region but granite and greenstone, they must have passed on at least two of these lines within a short distance of large exposures of slate and jaspery iron ore. FosTEK (J. W.) and Whitney (J. D.). Synopsis of Explorations of the Geolog- ical Corps iu the Lake Superior Land District in the Northern Peninsula of Michigan, Senate Docs., 1st sess., 31st Cong., 1849-'50, vol. in, No. 1, Pt. iii, pp. 605-626. This is the first report made by Messrs. Foster and Whitney after they had superseded Dr. Jackson in the control of the Geological Survey of the Mineral Lauds of the United States in Michigan. In it are embraced the results of work done by these gentlemen in the capacity of assistants to Dr. Jackson. Among other results thus obtained were those by Whitney and Barnes in the summer of 1847, in the country west of lake Gogebic. It appears evident that no further work was done in that district for this report, or for their final report below noticed, by either of these gentlemen or by any of their assistants. Accompanying this report are several geological maps, one of which, entitled "The District between Portage Lake aud Montreal River," drawn on a scale of 3 J miles to the inch, includes that portion of the district which forms the subject of the present volume as far west as the Montreal river. This is the map referred to above in connection with Whitney's letter of 1847 to Dr. Jackson. 16 THE PENOKEE IRON-BEARmG SERIES. 1851. PoSTEK (J. W.) and Whitney (J. D.). Report ou the Geology of the Lake Superior Laud District, Part 2, The Iron Rejjiou, together with the General Geology, Senate Docs., special session, 32nd Cong., Washington, 1851, vol. iii, No. 1, 406 pp., with maps. This is Foster and Whitney's final report on tl3.e iron regions of the upper peninsula of Michigan. In it several brief references are made to the region w^est of Grogebic and south of the trappean beds of the Copper range. We quote as follows (p. 39): Farther west another granite belt starts from the head waters of the Ontonagon river, and thence extends to the western limits of the district, intersecting the head of Agogebic lake and crossing the Montreal river about 15 miles from its mouth. Southward it forms the watershed between the rivers of lake Superior and the Mis- sissippi and passes beyond the limits of this district into Wisconsin. It is probable that this belt is a continuation of that first described, but we have not been able to trace the continuity. There is an interval of 20 miles where the surface of the country becomes nearly horizontal and is strewn with accumulations of clay and gravel, burying up the subjacent rocks. In the extreme western portion of the district, west of range 40, granite is the predominating rock below the southern boundary of township 47 and is associated with a hornblende rock which sometimes assumes a slaty structure. The granite is mostly a binary compound of feldspar and quartz, the former largely predominating and giving a reddish tinge to the whole rock; mica is present only in very small quantity, while hornblende and chlorite are occasionally scattered in minute i)articles through it. Nearly the whole of the granitic region in this part of the district pre- sents the most forbidding and desolate aspect. Though it forms the most elevated portion of the country, being the watershed between lake Superior and the Missis- sippi, it is low and swampy and filled with numerous lakes, of which over fifty were crossed by Mr. Burt in surveying the boundary line between Lac Vieux Desert and the Montreal river. There are occasional elevations, which are dry and wooded with sugar maple and which undoubtedly are covered with a good soil, but the larger por- tion of the region presents almost interminable cedar swamps, in the midst of which the granite and hornblende ridges rise, with precipitous walls, rarely to more than 50 feet in height above the surrounding country. These ridges are generally very nar- row, and their sides are covered with a thick coating of moss and lichens. Nothing can exceed the desolate solitude of this region. Not even the Indian traverses it; it is destitute of game and its stillness is never broken except by the crashing of the tornado through the dense forest, tearing up the trees and piling them together so as to present an almost impassible barriei', as if still further to repel the intrusion of man into a region so little fitted for his reception. GEOLOGICAL EXPLORATIONS ANT) LITEKATTTRE. 17 The {>raiiitr nl l lie whnlc of this pmtiiiii of I lie district is vory (M)arse grained aud crystalline and is cliaraili'i i/.cd by a, predomiuance of feldspar and an almost, entire absence of mica. ( I'p. 47. 4.S.) The .statuiueuts in the ab()ve quotations rest, of course, upon the few obser,vations by Messrs. Wiiitney aud Barnes. It is manifest that no thought of the existence of iron-bearing slates between hike Gogebic aud the Montreal river was entertained. This is the more singular since the discoveries of iron-bearing slates by Mr. Whittlesey west of the Montreal river (subsequently noted) were known to Messrs. Foster and Whitney, as appears from their statement (page 51). 1853. Whittlesey (Charles). Geological Report on that portion of Wisconsin bor- dering on the south shore of Lake Superior, surveyed in the year 1849, under the direc- tion of David Dale Owen, U. S. Geologist, by Charles Whittlesey, liead of subcorps. In Eeport of a Geological Survey of Wisconsin, Iowa, and Minnesota, by David Dale Owen, U. S. Geologist, Philadelphia, 1852. It seems that Dr. Oweu himself must have crossed the westernmost end of the district with which we are concerned near Nimaakagon lake. This appears to have been in 1848 or 1849, but no observations of impor- tance were made (pp. 159-160). Dr. Randall, one of Owen's assistants, appears to have made the first discovery of iron-bearing slates in this district while following the Fourth Principal meridian in 1848 (p. 444). Col. Whittlesey's own exploration was made in 1849, in which year he followed the belt of iron-bearing slates from the vicinity of Montreal river to English lake. The following quotation will serve to show what were his views as to the general structure of the whole Bad river country in Wis- consin : The accompanying map and sections are intended to represent at one glance the principal features in the geology of this region. The extent, elevation; and relative thickness of the various formations, as well of solid rock as the looser earthy deposits, will there appear in a more compact and intelligible form than I could give them by written descriptions, however elaborate. There are four formations or great classes of rocks shown on each section. These all appear in the same order of succession, reckoning from the lake southerly, and may be grouped thiis : MON XIX '^ 18 THE PENOKEE IKON-BEAEHSTG SEEIES. 1. Sedimentary. a. Eed sandstone. h. Black slate. ('. Conglomerate. 2. Tra])pous liocTcs, or those of volcanic origin. a. Black and red amygdaloid and greenstone trap . b. Augitic, hornblendic, and feldspathic rocks, embracing syenite and granites of the same age. 3. Metamorphosed Rocks. a. Hornblendic slates. b. Iron slates. c. Black slates, iu large, thin, rectangular sheets. ^7. Talcose slates, with quartz. c. Slaty quartz. 4. Granitic. a. Syenite, and b. Granite, occupying the country south of the mountain range or uplift, are the oldest rocks seen. (P. 425.) The Sedimentary and lyneous Rods. — The relative a tje of the rocks beneath the clay and drift is a subject upon Avbich a prolonged discussion would be in place if theoretical consideration might be introduced here at large. The granites and syenites of the interior are no doubt the most ancient rocks of the district. After the protrusion of those extensive interior granitic masses many successive changes have occurred, but in what precise order is a question not easily determined. The immense sandstone deposits of the basin of lake Superior must have been subse- quent to the granites of Wisconsin, Chippewa, and Montreal rivers, and probably rested on them. Since that era a prolonged and intense internal igneous action has taken place, and the trap, hornblendic, and greenstone masses have been ejected, and also with them irregular protrusions of recent granite and syenite. The metamorphic slates have been elevated during these convulsions, and the sedimentary rocks thrust away to the northward and tilted uj) at high angles. The old granites and syenites have been rent, and fluid matter, suck as quartz and hornblende, inserted iu the fissures and between the beds. Along the northern por- tion of the Penokie range an outburst has taken place, as it were, between the sedi- mentary rocks and their ancient basis, on a line from the Montreal to Lac des Anglais; but the overflows have not been confined to one volcanic effort. The black and red trap, against which the conglomerate abuts, is doubtless due to a different effort from that which produced the greenstone trap-rocks that rise between the East fork of Bad river and the Montreal. The augitic, hornblendic, and syenitic mountains between the East fork aud the main stream differ in form, in chemical constitution, and bedding or stratification from either the greenstone or black trap, Semef-nfoyic Coriff7artierez^ I ami compact Trap 9 FclspatTiic JBdrnhJerule Jlochs Qiioj'tz, .3-ort ari£l "Ihlv X offBi xd^e Gr- MocTc Of'. JRedf^enz^ 3&frtfu/ortz7(, nTsfeet ^37 feet ?oo feet soiif^et asoiket^ 73ofeet 7ra3 fce^t 7s^ee6 73 of eat (J. Jh'j'zt 87/ fe^ S^siefeet' SciZsccrrvSli/yc. jQ7at'eet J)£a-7o flrnty ,Auffitic JRocTos iSom^Zenetic MjOcTcs ffoTTthJ^n^zc Tblcose, jBoTTtMem^'c fvitJt 7n^f^^/ro7T^ Siervttic rr??^ ist josfi.,2,71^ eeft. w. Tyler-'sFork , «5 ■^S4'feet S77 feet 7osofeet TylersF<7rk 914' feet lyier^sFoT-h oS'f'feet uoofeet ^ ct7i€i3:Rt7^-s?ves Conglomercde. Sletck,- tzn^- G-T-eyTTzzp Siervite .SbrnbTeTJ^Szc RocJts &ra7zzte BtrribZern^iC RocTts Slaclv Slate SurrMendeSlate Wagneticlrcfn, Altered-T/ilcose Elates, aae i'eei Che^womi^on^try JiIasJikBgFcrr'7t, iEastFo-rh I'dOs ofiyierilW-h Pewabic Itange naei'ket GEOLOGICAL EXPLORATIONS AND LITERATURE. 19 Prof.eediiif;' almit; I lie iikiihiIiijii ihI^cs (if I lie iiorl liciii |i!irl of tin' range, between the main stioain and llic oiitk't (tf Ijacdcs Anji'lais, we cni-ouiilcr dtlior varieties (»f rocks, t'cldsiialliic, jjrauit-ic, and iKii'nblcndic in their coiiipositiun, a|)i)iircntly an independent ii|iiilt or ontbnrst. Along- this whole line, however, the nietainoiphie rocks of I lie sunthern I'idgos of the range arc eontinnons IroMi near the Montreal to I'M', des Anglais. They have, at ditt'ercnl times, been i)ushcd over tlu^ granites at the sonth, distorted, broken, and tilted np in diflercnt degrees, bnt always in the same direction. The northern portion of the range exhibits to my mind evidence of /o«r periods of igneous action, producing /(>«/• formations of I'oeksof a trappose cast, which I have rei)resented sej)arately on the map. They are: 1st, black iind red trap; 2d, greenstone trap, embracing or gradu- ating into massive hornblende and syenite at the west; M, angite and hojiiblcjidc rocks in mass, also embracing granite and syenite; 4th, granite, syenite, and coarse hornblende rocks, north of Lac des Anglais. But how to decide the order or relative age of these protrusions! It appears that the same materials under difl'erent circumstances of fluidity, pressure, and rapidity of cooling may take all these forms. At present I can only place these four varieties in one ffroup, filling a geological epoch of no great duration, and place it between the era of the red sandstone depos- its and the metamorphic uplifts; for it is by the appearance of this group that both those systems have been j)ushed aside, one to the north, the other to the south. Whether the schistose rocks, before their upheaval and metamorphosis, were older or newer than the sandstone I do not decide; but both the schists and the unaltered sedimentary rocks are more ancient than the above group numbered from one to four (pp. 429-430). The geological map mentioned in the above quotation was never printed ; but its main features were embodied in Owen's general geologi- cal map of the northwest.^ PI. iv is a reproduction, save as to the omis- sion of colors, of the original. In a similar manner Whittlesey's four cross- sections of the Bad river country are reproduced in PL iii. In Whittlesey's classification of the formations of the Bad river country, above given, the red sandstone (Iff) includes horizontal sandstones belonging, as our belief is, to the Potsdam sandstone, and also vertically placed sandstones belonging to the Keweenaw or Copper series. To the latter series belong Whittlesey's formations from Ih to 2b, inclusive. His group of naetamorphosed rocks (3) is about equivalent to the iron-bearing series Avhich forms tlie sul)ject of the present volume. His group of granitic I See volume of iUustrations aooompanyiug tlie Report, 20 THE PENOKEE lEOISr-BEARING SERIES. rocks (4) iucludes the granitic, gneissic, and schistose rocks which lie to the south of the Penokee range and form, as he says and as we also believe, the oldest division of the region. We are not sure that we fully under- stand Whittlesey's statements at the close of the quotation above, as to the relative ages of the different rocks of tlie region. It seems that he would say that the various eruptive rocks of the Keweenaw series Avhich occupy the belt of country between the red sandstone on the north and the iron- bearing slates on the south are all of them of an intrusive nature, having been thrown to the surface after the formation of the red sandstone, which they have thrust northward, at the same time pushing the slaty series to the south. His position as to the more recent origin of these trajipean rocks relatively to the red sandstone is one which, of course, can not be maintained. It has been abundantly proved by the work of the Wis«onsin Survey (1873-1879) that these eruptive rocks antedate the red sandstones. In large measure they originated as surface flows; and, moreover, certain of them have yielded the most of the fragmental material, coarse and fine, of which the sandstones are built. The following quotations have more especial reference to the iron- bearing formation: There is a continuous mountain chain from the Montreal river to Bladder lake, the xnolongation of the Porcupine mountain range in Michigan. I have called it the PenoMe range, tliis being the Indian word for iron, which is found in its westerly- portion in great force (p. 434). The most easterly appearance of magnetic iron which I obseiVed was in fissile black slate, about 4 miles west of the Montreal trail, along which the section No. 4 W. is made. The bed lies back of the trappose range, about 16 miles from the lake, in a protrusion of metaniorphic slates, the argillaceous portions merely tinged with iron. About 4 miles along the strike of the beds, southwest by west, the bed was seen by Mr. Eandall in -1848, in the fourth principal meridian, in township 44 north, 18 miles from the lake. From thence I and my assistant, Mr. Beesly, an active woodsman and faithful and acute observer, traced it at moderate intervals along the uplift to the west end of "Lac des Anglais," or about 15 miles, to where the rango terminates. Here the metamori>hic slates that first show themselves between the Montreal river and the Montreal trail on the east sink beneath the level of the country and are replaced by syenitic rocks. By examining the sections ]Sro>s. 1,2, 3, and 4 W., attached to this report, the position of the iron-bearing rocks will be found to be the same in each, and the (iK()L()(!l»'Al. KXI'LOKATIONS AND LlTKi; ATIUIR 21 details of the i-i»cky beds iil)()vt' and liclow Mu- iniii arc also lite same, so that wc. may with coiilidtMKM^ l. a (Miiitiiuioiis bed IVoiii tlic meridian wcsiwardto Lat' des Anglais. Its thickiu'ss, richness, and value vary very much, but wc loniid it more or less developed whenever we cross(Ml the ranse and could get a view of the rock. The geolofificiil relations of the ironbeaiinj;- strata are exhibited in the two fol- lowing sections, the first taken near the trail that passes over the Pewabic range between the forks of the Tyler branch of Bad river, the second south of Lac des Auglais. On the Pewabic range the strike of the beds is east by north; the dip north by west 80° to 850. The beds of quartz are of great thickness— 2(»(l to 250 feet. Near the junction of the cpiartz and talcose slate the latter assumes the aspect of novacu- lite. The iron bed is schistose in its structure and is composed of magnetic oxide, sometimes alternating with beds of (juartz. The total thickness of the talcose slate is not seen; it must be very thick and is traversed by numerous veins of quartz. Its dip and strike are variable. The bed of magnetic iron ore south of Lac des Anglais is of extraordinary thick- ness 25 to GO feet. The dip here is northeasterly, and the layers variable in thickness that alternate with quartz, which latter repose upon hornblendic slate, running down- ward into talcose slate. Here, as well as on the Pewabic range, the dip and strike of the beds are variable. The metamorphic strata are very much disturbed throughout this range, but agree in having the mural faces of the uplifts to the south and southeast, and the dip northerly and northwesterly at various angles of from 5° to 60°. The effect of this irregular action is to make detached ridges and crests, sometimes 2, 3, and 5 miles long, thrown up at different elevations and inclinations. Sometimes the iron stratum is composed of laminte of quartz and magnetic oxide, alternating, as at the crossing of the trail between the forks of the Tyler branch of Bad river ; also south of Lac des Anglais. The proportion of iron and quartz is very variable, but the separation of them by mechanical means would in general not bo difficult. The bands of ore vary from mere thin lamim* to a thickness of 12 and even 18 inches, presenting sometimes a black surface, contrasting with the white and gray color of the quartz, and sometimes a bright metallic gray color. The thickness of the m etal lifer (uis portion varies in the extreme from 5 and 10 feet up to 50 and 70 feet, and at the passage of the main portion of Bad river through the range reaches 250 feet. These exposed faces fre- quently extend beneath the surface, where, of course, no estimate can be formed of their entire thickness. There are many places in the mountain, west of Bad river, which present more than 50 feet of quartz and iron, in about equal proportions. In the wild and deep ravmes where the Bad river breaks through the range there is a cliff of slaty ore. 22 THE PEjSTOKEE IRON-BEARING SERIES. most of wliicli comes out in thin, oblique prisms, with well defined angles and sti'aight edges, probably 300 feet thick, including what is covered by the talus or fallen por- tions. I estimate more than one-half of this face to be ore, and in places the beds are from 10 to 12 feet in thickness, with very little intermixture of quartz. There are portions of it not slaty, but thick bedded. The dip of the laminae is mostly north and by east, 80° and 85°. The convulsions that have occurred at this point have thrown a part of the range beyond the rest of it, to the northward, so that in crossing the river and passing along the mountain to the eastward for several miles the fer- ruginous bed, as well as many of the associate strata, were not visible above the general surface of the ground. It should, however, be borne in mind that the whole region is not only covered so thickly with timber that no distant views can be had without climbing trees, but the drift often conceals the rocks over a large proportion even of the elevated ridges. In addition, the rocks themselves, previous to the era of the drift, have been the sport of giant forces, which tossed and tilted them about at various angles and elevations, realizing the table of Atlas (pp. 444-440). It will be noted tliat the two names " Penokie " and " Pewabic" are used in the above quotation for the bold range which runs from the Mon- treal river to the vicinity of English lake, near the southern boundar}^ of the iron-bearing formation. Whittlesey tells us in a later piiblication, sub- sequently noted, that the word "Penokie" here used is a misprint for "Pewabic" (more properly "Biwabik"), which latter term is the Chippewa word for " iron." However this may be, the former term in its more iisual form of" Penokee" has since become thoroughly fixed by general usage. Whittlesey's early work in this region was, of course, no more than a very rough i-econnaissance. However, considering- the difficiilties of travel in the region and the fact that there existed in it at the time but a single surveyed line, the fourth principal meridian, we must give Whittlesey the credit of having achieved a good deal. In fact, including the further examinations made by him some ten years later, and below noticed, he sup- plied all the information of any value obtainable at the time of the inaugur- ation of the Wisconsin State Survej^, in 1873. 1859. Lapham (I. A.). The Penokee Iron Range. Wis. State Agricultural Society Transactions, 1858-'59, vol. 5, pp. 391-400, with map. This is a very brief and general account of the Penokee Iron range, based on trips made along it by Dr. Lapham in September, 1858, from Bad CKOLoc.HAi, i;\im,()i;ati()NS and litkkatimM':. 23 riv(M', at I\'ii(tk(H' '^-a]), eastward lit tlio fourth principal meridian, and west- wanl to the end ol'tlic ranj>(' south ot" Euylisli lake. The map acconipauy- inji' tlic ])ap('r is topoji'raphical only and is vor\' siniplo, having- Ix'on coni- pilcd from the V. S. Land OfHcc^ township plats ; tlicf only addition on it to information atiordcd by these plats being the course of the crest of the range. It is ti) be noted that Lapham's publication is the first giving any geological facts based on an examination of the region subsequent to the completion in it of the Land Office Survey. The following quotation includes all of geological interest given in this paper : This remarkable inountain range has been traced from a little east of the fonrth principal meridian in township 45, in a direction a little south of west, across three ranges of townships; its length being about 20 miles, as shown on the accompanying map. At the west the range appears to slope down and terminates, but toward the east its extent is not known. The highest summits are about 1,200 feet above lake Superior, or 1,800 feet above the sea; the mean height is 100 or 200 feet less. Tyler's fork crosses the range at a place called "Tlie Grorge,-' and Bad river crosses at Peno- kee, through a gap cut down to a depth of about .'500 feet ; the river here having an elevation above lake Superior of 668 feet. On the north side the slope of the range is moderate, and covered with "drift;" but on the south it is quite abrupt, and steep, rocky precipices occur, looking as if they had at some remote period of the past formed the shores of some great body of water. What gives this great ridge its peculiar interest and importance is the immense stratum or bed of magnetic iron ore which it contains, extending, with varying thick- ness and value, throughout its whole length. It is not, therefore, an Iron mountain simply, like those heretofore known in Missouri and elsewhere, but, as its name imports, an Iron range; as if mountain masses of iron had been passed between gigantic rollers and drawn out for a length of 20 miles. The ore is found in a very ancient chloritic slate, so ancient that it is supposed to have been deposited long before the existence of vegetable or animal life upon the globe. The slate rests upon a light colored quartz-rock, which usually extends to the base of the range on the south side. The ore is laminated, like the .slate, and apparently has had the same origin; for, as we ascend from the qnartx,-rock the slate becomes more and more ferruginous until it passfes into pui-e iron ore. This change is so gradual that it is often difficult to deter- mine where the slate ceases and the ore begins, or how much should be classed as iron ore and how much as ferruginous slate. We noticed places where the ore had a thickness of 00 feet, at other places 10, and wherever we could get access to the rock at the proper place the ore was found. 24 THE PENOKEE lliON-BEAElNG 8EKIES. Above tbe ore, that is north of it, the slate has been hardened, probably bj^ some volcanic agency, into a compact mass, but still showing traces of its original laminated structnre. This highly indurated rock is the nucleus of the ridge, usually ibrming the crest or highest part; and it forms the north slope, except where covered with the bowlders and other coarse materials of the Drift formation. If we may judge from the polished and grooved surfaces, we may suppose that this excessively hard rock has resisted the action of the powerful currents and icebergs that once flowed over the very top of the ridge, which, with its invaluable beds of iron ore, was thus saved from destruction. All the rocks, including the ore, have a considerable dip toward the north, or toward the great basin of lake Superior; and they are always found in the same relative position in regard to the ore. If we, at any new locality, could find either of the rocks in place, we at once knew which way to turn to find the ore. The magnetic ore of the Penokee Iron range contains a notable and much varying proportion of silica in its composition, but is free from sulphur and other deleterious qualities, corresponding in this respect with most of the iron ores of this remote geological epoch. It is in some localities so highly magnetic that particles adhere to the hnumier when struck, like iron filings to a magnet; and the compass needle as often pointed toward the east or west as to the north, in one instance being entirely reversed, the north end pointing to the south. At Penokee, where Bad river crosses the range, the ore exists in such abundance that it may be obtained from the face of the hill, much as stone is taken from an ordinary stone quarry. Large masses that have fallen from the cliiis now lie loose upon the surface, and will supply a fur- nace for many years before it will be necessary to resort to the original bed (pp. 394-396). I860. Lapham (I. A.). Eeport to the Directors of the Wisconsin and Lake Superior Mining and Smelting Company, in the Penokee Iron Eange of Lake Superior, with Reports and Statistics, showing its Mineral Wealth and Prospects, Charter and Organization of the Wisconsin and Lake Superior Mining and Smelting Company, Milwaukee, 1860, pp. 22-37. This is a pi-ivate economic report based on the same exploration as the paper immediately preceding. It contains, however, some further general- izations of interest. Dr. Lapham's investigations appear to have been con- fined almost entirely to the ferruginous belt and its immediately adjacent layers, Avhich form the Penokee ridge proper. The higher members of the series received only very shght and incidental attention. The following quotation gives the more important generalizations reached: (IKOLOdlCAl. K.VIM-OIIATIONS AND MTllKATUlM-:. 25 1 will now |in>ceo(lt()t'X|>l;nii the }i'ool()fri«"il rebitiiiiis i>\' this vast, lied of magnetic iron (ire, sliow how it is associated witli other rocks, and its mode of oeenneniM' an 1)1' ii liifjli clilf, and of niiiiiiii;' it ht'low tlii' siulaci' of tlid j;r()iiml, will he sullii'iiMit to (lissi[»att' all tlif prolits that can be inadt! IVoiii Mie use of ores so obtaiued. (I'. ;!.{.) ISO.'}. Whittli;skv ((!harles). The Peuokie Mineral liaiige, Wisconsin, Proc. Bost. Soc. Niit. Hist., vol. IX, l.S62-'(;.{. pp. 235-244. The work doue l)y Col. Whittlesey in 184!) in coimectiou with Dr. Owen's general survey of the northwest antedated the linear sm-veys of this region. Eleven years later, August to October, 18G0, he made a further examination of the Bad river country, under the auspices of the Wisconsin Geological Survey, then organized under James Hall. This time he had the advantage of the linear survej^s. His report was never published, the stirvey being very short-Hved. He afterwards published a few brief details in reports to iron companies and the summary contained in this pamphlet, from which we quote quite fully. The map accompany- ing Whittlesey's report to the Wisconsin Greological Survey was not jjub- lished until 1880, Avhen it appeared in an appendix to the third volume of the Geology of Wisconsin, but it is properly reproduced here, having been prepared in 1860. (PI. iv.) The copper-bearing strata of point Kewenaw (lake Superior'! extend south- westerly across the boundary of the state of Michigan into Wisconsin. These strata constitute a. long, narrow, and bold mountain range from Copper harbor to Long lake, a distance of 100 miles. There are no stratigraphical breaks along this line, the order of the rock being everywliere the same. The dip of the beds is always north- erly or northwest and the strike to the northeast or east, the general line of outcrop being northeast by east. On i)oint Kewenaw and as far southwest as the Akogebe lake, on the west fork of the Ontonagon river, the copper veins have been found valuable. Beyond the waters of the Ontonagon, in the same direction, veins have been discovered, but, after limited workings, have lieen abandoned. The Montreal river forms the boundary between Mk-hUjnn and Wiscomin, and as early as the year 1845 mining locations were made on its waters where they pass the range. Locations were also made upon the waters ot the Bad or Mauvaise river, a stream with numerous branches, draining the country from the Montreal to the head waters of the Chip- peway and St. Croix rivers. 28 THE PBNOKEE IRON-BEAEmG SERIES. Historically considered, the exploration of this region commenced iu the year 1840, when Dr. Houghton, as a commissioner of the State of Michigan, accompanied Capt. Cram, of the United States Topographical Engineers, who was then surveying the Menominee and Montreal rivers. In 1840 and 1841 Dr. Houghton examined the rocks on both these streams and the country between their sources. I am in possession of a transcript of his field notes during these explorations. In 184u-'4C I made examinations along the range across the Montreal to the westward, as far as the main branch of Bad river. Uj) to this time the public lands in this part of Wisconsin had not been surveyed. The fourth i)rincipal meridian was extended northward through Wisconsin to lake Superior iu 1848. Dr. A. Randall, one of the assistants of Dr. Owen upon the survey of the Ghippeway land district in reference to mines and minerals, accompanied the linear surveyors along this line. In T. 44 N., Dr. Randall discovered an outcrop of magnetic iron ore and brought in a siiecimen. The next season, as a member of Dr. Owen's corps, I made an exph)ratiou on the western branches of Bad river, crossing southerly to the head waters of the Chippeway. Near Lac des Anglais, and thence easterly across the middle or main fork of the Bad river, I found clifts and liluts of siliceous magnetite. The results of this examination may be seen in the final report of Dr. Owen, published at Washington in the year 1850. In the Chippeway language the name for iron is imrabiJc, and I thought it proper to designate the mountains where this nietal exists in quantities that surprise all observers as the "Pewabik range." The compositor, however, transformed it to Penokie, a word which belongs to no language, but which is now too well fastened upon the range by usage to be changed. Soon after the publication of Dr. Owen's report the excitement of 1845-'46 in reference to copper was repeated in reference to iron. The government was at last induced to make surveys of the region. Preenaptors followed the surveyors, erect- ing their rude cabins on each quarter section between the meridian and lac des Anglais, a distance of 18 or 20 miles. The iron belt is generally less than one- fourth of a mile in width, regularly stratified, dipping to the northwest conformable to the formations, and having its outcrop along the summit of the second or southerly range. Viewing this mountain region from La Pointe, or from the open lake, it has the appearance of a single crest. Its outline against the sky on a clear day is very dis- tinct and regular. Along the range this crest line is nearly level, its elevatiou above the lake being 1,000 to 1,100 feet. But there are two ranges, known in the country as the first and second, or the "Copper" and the "Iron" range. There is not much difference in their elevation. The copper range, being nearest the coast, covers the iron range, which, at the distance of 30 miles, is visible only through gaps and notches, the whole forming one blended line in the horizon. To the south, beyond the iron range, the country is lower and swampy. GEOLOGICAL EXPLORATIONS AND LITEKATUKE. 29 Two roads were soon constructed tn the mineral deposits through the dense ever- green forests of tiiis hititudc. One of them conimcnccd at tlic^ lake, near the mouth of tlie Montreal river, and near the terniinalioti of tlic fonvth principal meridian, extending thence south and not far fioiii I lie meridian line. The other began on Chegoimegon bay, at Asiiland, pursuing also a southerly eonrse, and, after reaching the second range, coiinecteriferous series and the Huronian, and into the Lauren- tian, so that at a short distance west of the la^ke these rocks end in steep and high OE()T.<>(lI('AL KXI'LOKATIONS AND LITKKATURE. 31 declivitie.s, at Mic l>a,sc of wliicli lies the level (■(•iiiilry ol' tlie Siliiiiau saudstone, in which is cut the hasiii of tlie hiivc. From tliis point e:isl ward this iiiicient erosiou had made giM-af inroads upon the continuity of the (,ln|)riferous and okler rocks before the deposition of the Sihuian sandstone. The nudapliyre ridges are broken into knobs, or arc wanting, and no Iluroidati was found as far as tlic Ontonagon river, 7 miles away, and the linut of onr observations. (I'j). JtoO—i'il.) 1S73. Bkooks (T. B.). Geological Survey of Michigan, Upper Peninsula, 1869-1873, vol. I, New York, 1873, with an Atlas. Scale 13 rrUles =J inch . Fig. 2 Reproduction of a portion of Brooks and Purapclly's geological map of the upper peninsula of Michigan. Ill Part I of this volume, chapter vi (pp. 183-I86)is entitled "Lake Gogebic and Montreal River Iron Range." This gives a brief outline account of the occurrences in the iron belt between Montreal y'whv and Gogebic lake. The account is somewhat fuller than that given in the paper quoted immediately above, but is based on the same rapid examina- tion, and contains no details. On the general map of the Northern penin- sula, given in tlie atlas which accompanies this reixtrt, the Michigan end of the Penokee series is first mapped, Fig. 2 is copied from the western por- tion of this atlas map. 32 THE PENOKEE lEON-BEAEIl^G SERIES. The ironrauge under consideration may be regarded as the eastern prolongation of the Penokie range of Wisconsin, as well as the western extension of the Marquette series, the whole being Huroniau. The position of the range is tolerably well defined by magnetic observations and notes on the U. S. Land OflBce plats. On these we find mention of iron and magnetic attractions on Sees. 7 and 8, T. 47 N., R. 45 W., as also in Sees. 13 and 14 of the town west. The belt of Huroniau rocks, as made out by us, extends nearly east and west through the north part of T. 47, ranges 44, 45, 46, and 47, crossing the Montreal river in Sees. 16 and 21 of the last named township. Going east, the range was lost before it reached lake Gogebic. The geological boundaries of this range are fortunately of the most irnmistak- able nature and render a detailed description of its position unnecessary. On the north is the high, broad, irregular ridge, or series of ridges, constituting the South Copper range, the rocks of which are greenish and brownish, massive and amygdaloidal copper-bearing traps, their bedding being exceedingly obscure, with occasional beds of sandstone and an imperfect conglomerate. The strike of these rocks, so far as it could be made out, was east and west, with a dip to the north at a high angle, thns .conforming with the Huroniau rocks underneath. Against and over the copper series on the north abut the horizontally bedded lower Silurian sandstones, which are beautifully exposed on the west branch of the Ontonagon river, in Sec. 23, T. 46, R. 41. These sandstones form the surface rock, and occupy the broad belt between tlie two copper ranges from the region we are describing to KeweenaAv bay, but taper to a point before reaching the Montreal river in going west. On the south of tiie Iron-bearing rocks are a series of granites, chloritic gneisses, and obscure schists, Avhich, except the latter, are unmistakably Laurentian in their lithological character, and are nonconformably overlaid by the Huroniau rocks. The general structural relations of the four great systems here enumerated are shown in the accompanying diagram. (Pp. 183, 184.) The best locality in which to study the character of the iron series in the West region, is on Black river and its tributaries, especially on the outlet of Sunday lake, ■ T. 47, ranges 45 and 46. Here will be found banded ferruginous jaspery schists, chloritic greenstones, brown ferruginous slates, black and gray banded siliceous slates, siliceous flag ores, several varieties of quartzites, and clay slate. The whole series strike east and west, and dip north away from the granites and gneisses and under the copper rocks at an angle of from 40° to 90°. (P. 185.) In Part ii of the same volume (being Prof. Pumpelly's report on the Copper-Bearing Rocks), Chapter i contains the same statements, reproduced almost verbatim, with regard to the ..Montreal-Gogebic iron belt, as are above quoted from a paper pubhshed jointly by Brooks and Pumpelly. GEULOCiUJAL EXPLORATIONS AND LITERATURE. 33 JuLiEN (Alexis A.). Lithological Dcsfrii)tious, etc., of ^51) specimens of the niiroiiiiin iiiiil Lauiciitiari Rocks of tlic Upper Peiiiusula, appendix A, Geological Survey of Michigaii, New York, 1873, vol. ii, i)p. 1-197. This report (•(mtaius iiouinicroscoj)!*'. lithological descriptions of a few- rock specimens collei'ted by T. B. Brooks and R. Punipelly in the Gogebic region in 1871, during the trip already several times referred to. 187er cent up to 60 and 80 per cent of the whole. The entire series has a nearly uniform dip we-stof north, generally at a very high angle. The thickness never varies far from 4,000 feet, a figure obtained by actual measurement. (P. 48.) The estimate thus given for the thickness of the iron series falls far short of the truth, as the writer himself was able to show after further work. The mistake arose from a misconception during the first field season's work, of the nature of the iip2>er inica-schist or micaceous quartz- ite member of the iron series. This member was not at the time examined closely and was supposed to belong with a higher series. Ikving (R. D.). On Some Points in the Geology of Northern Wisconsin. Trans. Wis. Acad. Sci., Arts and Letters, vol. ii, 1873-'74, pp. 107-119, with map and section. (Published 1874.) This is essentially the same paper as the last referred to. isre. Whittlesey (Charles). Physical Geology of Lake Superior. Proc. Am. Assoc. Adv. Sci., 1875, 24th Meeting, part 2, pp. 60-72. (Published Salem, Massachu- setts, 1876.) This paper is published only in abstract. It gives in outline some of the author's views as to lake Superior geology. It contains only a few very general and incidental references to the Penokee-Gogebic district. Brooks (T. B.). On the Youngest Hurouian Rocks South of Lake Superior, and the Age of the Copper-Bearing Series. Am. Jour. Sci., 3d series, vol. xi, 1876, pp. 206-211. This paper maintains that the large development of granite south of the Menominee river in Wisconsin is the youngest member of the Huronian or Iron-bearing series of that region, and then proceeds to draw a parallel between this Menominee granite area and certain granites that lie north of the Penokee range in the vicinity of Bad river, Wisconsin, which he main- tains are in the right position to be again the uppermost member of the Iron-bearing series of this region. We quote from his argument: A careful consideration of all the facts to be observed in the Menominee region confirms me in this hypothesis, which is further supported, as it seems to me, by observations iu the Penokie Iron region (Bad river), Wisconsin, (iKOLUClCAli EXl'LDIiATlONS AND LlTKltATUItE. ' 35 Col. Whittlesey's maps aiul sections, given in Owen's rep(nt, 1S53, represent a belt (tC gi'iinite, syenite, and liornlileiule rocks us dividing the I'enokie series (Flnron- lan) from the overlying (Jopperbearing amygdaloidal traps and sandstones, which lie to the nortli and nearer the lako. I observed tiiesc rocks at several points in 1871, and noted their general litho. logical resemblance to the Lanrentian, as well as the almost insurmountable structural ditliculties in assigning to them tliat age, and recorded in my notes the probability of their being Upper Iluronian. Kowlaud Irving mentions these rocks as being coarsely crystalline aggregates "chiefly of labradorite and orthoclase feldspar, hornblende, and some variety of pyroxene," with occasional evidences of bedding, which points toward their entire conformability with the underlying Huroniau. He regards them as of the period of the (Jopper-bearing series, constituting its lowest and oldest portion. Having been, so far as I know, but little studied, it is perhaps impossible at this time to determine their age; but wliat is known can here be briefly surveyed, and an inference drawn, which will not be without value in directing further investigations. 1. The general lithological similarity of this granitoid belt to the Laurentian has been remarked. It has quite as much similarity, if not more, to several members of the Huroniau, and is, I believe, not identical with any rock known to belong to the Copper series. 3. Its geographical extension is peculiar in this, it wedges out rapidly to the east from the vicinity of Penokie gap, entirely disappearing at the Montreal river, which divides Michigan and Wisconsin. Prof. Pumpelly and myself traced the boundary between the Copper and Huronian rocks 30 miles farther eastward beyond lake Gogebic, without again observing it, which we should certainly have done if it had existed there, for we often found the two series very near together, although the actual contact was not seen. 3. Not only does this- granitoid formation thin out and disappear in its eastward prolongation, but the same is true of the whole Huronian series, the belt of which becomes narrow as followed east, and finally disappears in the neighborhood of Gogebic, where the Laurentian is seen very near the Copper series. 4. The fact that the granite mass does not cross either the Copper or Huronian series, or, so far as observed, give off dikes in either, renders it improbable that it came into its present position as an eruptive mass subsequent to the formation of both series of rocks. 5. The various ores of iron, which are so generally and abundantly diffused in the Lower and Middle Huronian, are entirely absent so far as observed from the upper three or four members as developed in the Marquette and Menominee regions, and also in the Penokie series if the following hypothesis is true; but they occur in aU forms, although it is believed not abundantly in the uppermost exposed member 36 THE PENOKEE IRON-BEARING SERIES. on Black river. If we suppose this irou to liave been mostly precipitated as a car- bonate, then we might expect it would be more generally dift'iised through the rocks of certain epochs than those materials derived frc^m the erosion of adjacent coasts. There is evidently but one hypothesis which will reconcile these facts, which is, that the granitoid formation in question is of the Huronian jieriod, and probably the youngest member, which series are here noncoiiformably overlaid by the Copper-bear- ing rocks. I conceive that this view is supported by the observations iu the Menom- inee region above recorded, and suppose this Penokie granitoid formation may be the equivalent of granitic bed XX of the Huronian series as developed in that region. On this hypothesis it is possible that the valley dividing the Penokie range proper from the granitoid belt may be underlaid by a soft slate, the equivalent of the micace- ous schist, bed XIX. (Pp. 207-208.) We are not in accord with Brooks as to the views here expressed. We can not allow that the granite of the Menominee is Huronian at all, inasmuch as we think it plainly belongs to the older or gneissic series; nor can we agree with the statements of Brooks as to the granites north of the Penokee range. As to the Menominee granite, we do not now need to present any arguments in defense of our belief. As to the Penokee granites, we may merely repeat the substance of what one of us has already said iu several publications, viz, (1) that the belt of "granite, syenite, and hornblende rocks" outlined by Whittlesey really has no existence; (2) that the granites occur- ring iu the vicinity of Bad river, and north of the Penokee range, lie in no continuous belt, but are veins and masses intersecting the gabbros at the base of the Copper-bearing series, and the micaceous quartzites at the sum- mit of the Iron-bearing series; (3) that these granites are manifestly but a phase of the intrusive reddish, granitic porphyries which mark this horizon both north and south of lake Superior; (4) that being so manifestly intru- sive, there is nothing like bedding about these granites; (5) that the "wedging out" to the eastward, which Brooks speaks of as characteristic of his supposed granite belt, is really a characteristic of a great gabbro area here occurring, and not of a granite area; (6) that this gabbro mass does traverse the Huronian beds in a most noticeable manner, in places even cutting them out at the surface altogether; and (7) that at the southern margin of this gabbro, or along its contact with the Iron-bearing slates, it is plainly involved with and intrusive in those slates, as the granite is in both gabbro and slates. (;E0L0(1I(L\L EXrLORATIOXS AND LITERATURE. 37 In short, we seo n()tliin<>- hero, nor indood anywhere in tlie lake Superior country to warrant the view tli;it the lron-hearin<<- or Huroniau series has an npperinost granitic member. Tlie Menominee granite belt we look on as belong-iug to the great basement gneissic formation; the Penokee granite belt is really a great gabbro area, in which occur isolated and limited intiiisions of granite, granitic por[)hyry, and allied acid rocks. This gabbro, with red acid eruptions, finds its exact equivalent as to stratigraph- ical position and associated intrusions in tlie great gabbro belt which forms the bold range of hills at Duluth, and stretches thence far to the northeast- ward into the interior of that portion of Minnesota which lies north of lake Superior.^ Brooks (T. B.). Classified List of Rocks observed in the Huronian Series south of Lake Superior. Aui. Jour. Sci., 3d series, a^oI. xii, 187G, pp. 194-204. This paper, as its title indicates, is an attempt at a lithological classifi- cation of all the varieties of rocks observed by Brooks in those formations on the south side of lake Superior, which he regarded as the equivalents of the original Huronian of the north shore of lake Huron. It also o'ives a table showing the "sequence of Huronian strata at several points near lake Superior, with hypothesis of equivalency." The districts referred to in this table are (1) the north shore of lake Huron, (2) the Marquette Iron district, (3) the Menominee Iron district, (4) the district of Black river, Michigan, and (5) the district of Bad river, Wisconsin. The last two dis- trict are within the area now under description, and we therefore quote the tah\e so far as it gives the succession of strata for Black and Bad rivers. We do not now discuss Brooks's hypothesis of equivalency for these sections with those of the other districts referred to. We need merely to say that we can not accept his scheme in many respects. Indeed, we find too many things that we can not agree with in his succession for the Penokee-Gogebic series itself to allow us to accept any scheme of equiva- lency based upon it. His Bad river section is particularly imperfect, there being great thicknesses of rock omitted entirely, while the granite belt ■Am. Jour. Sci., 3d series, vol. xi, 187G, p. 493; Geol. of Wis., vol. in, 1880, pp. 10, 13, 22, 3.5, 44-46, 145-149, 167-183, 193-195, 233-237; and atlas plates xxi, xxii, xxlv, xxv, xxvi, xxvii; Monograph U. S. Geol. Survey, vol. v, 1883, pp. 37-57, 56-58, 144-145, 155-157, 158, 230-238, 266, 268-275. 38 THE PENOKEE lEON-BEAEING SEEIES. placed at the top of the series is really as we have already insisted, but part of the great development of gabbro which lies at the base of the Copper-bearing series. The facts given in this tabulation for the Black and Bad river sections are based on the observations made by Brooks on his rapid trip through the region in 1871, above referred to. In a later publication he accepted the Bad river section given by R. D. Irving.^ The following is the portion of Brooks's tabulation referred to- Black river series. Bad river series. Compact greenstone, with green cherty(?) layers. Bright red specks of jasper and crystals of pyrite. Greenstone ; holds grains of glassy quartz and appears chloritic. Hematitic and magnetic quarizt'se flag (like Mar- quette flag ores). Gray, green, and brown banded ferruginous, silice- ous stale, with strong rhombohedral cleavage. Grayish and greenish handed schist, weathering brown, apparently cfeZoHtic, -with, jaspery layers Contains pyrites. In places apparently felsitic and again aphanitic. [Covered.] Ferruginous, handed (purple and green) cherty so/list (magnetic). Banded ferruginous ^'as^er schist. Ferruginous, siliceous flags (not magnetic). Arenaceous qiiartzose schist. Eeddisli quartsite. Hydrous magnesian or argillaceous schist. Greenstone. Banded cherty schist and schistose cherty hreccia, more or less ferruginous. 84, 85. Compact, hard greenstone. Very soft, apparently chloritic greenstone. Gray, banded, slaty schist. [Covered 100 steps.] Greenstone. ^anAeA ferruginous, siliceous schist, strongly mag- netic. Greenstone. Banded ferruginous slate. Massive greenstone, apparently chloritic. Banded ferruginous _/asi)er schist. Red, gray, coarse and flue grained granitic rock, rarely schistose. Gi-eenstone or hornblende rock, apparently chlo- ritic (somewhat soft, but tough). [Covered about i mile.] Clay slate? Magnetic amphiholic quartzose flags and quartzose magnetic ore. 148. Heavy bed forming crest of ridge. Black clay slate without oblique cleavage. Gray quartz schist, banded with occasional 1am- inse of magnetic ore. Grayish and reddish quartzose schist. Gi-eenstone (thin bed). Soft, light, gray-green slate, probably chloro- areillaceous. Gray quartzose schist, faintly banded. Gray to white, massive quartzite, (Thin bed.) Calcareous rock? Amphibole schist and perhaps hornblendic gneiss in heavy beds. [Covered.] 1 Geol. of Wis., 1880, vol. iii, table opp. p. 450. (IKOT.OdlOAL EXPLORATIONS AND LITI-niATUKH. ,S9 Sweet (E. T.). Notes on tlio Goology of Noitliern Wisconsin. Trans. Wis. Acad, of Sci., Arts and Letters, vol. in, lS7.^»-'70, pj). 4()-.'J.'>. This is an outline account of" observations in northern Wisconsin made in 1873 and 187.^) for tlie Wisconsin State Geological Survey. The quota- tion below gives certain facts with regard to tlie Bad river and Penokee gap section. It should be said that I afterwards measured this section in detail for the Wisconsin Survey and the results were published in the third volume of the Geology of Wisconsin. The thickness of the Iron-bearing" series is more than twice as great as supposed by Mr. Sweet, the uppermost beds being far above the uppermost mentioned by him. Two magnetic belts, moreover, do not exist, as supposed by Mr. Sweet, the supposed two belts being the same belt faulted apart. The junction between the Laiu-entian and Huronian is in the southern part of Sec. 14, T. 44, R. 3 W. At this point Bad river passes through a narrow gorge having nearly vertical walls on either side. In the left or northern wall of the gorge, fine grained white quartz with a vitreous coating and slaty siliceous schist occur, showing a strike nearly east and west, and dip of 60° to the north. The quartz represents the lowest member of the Penokie system examined by the party in 1873. Upon examining the opposite wall of the gorge siliceous marble was discovered for the first time to be one of the beds of the Penokie system, lying below the iron-bear- ing beds. A similar arrangement has long been known to exist in the Huronian of the Marquette district, which has led to the suspicion of its existence in Wiscon- sin. The thickness of the siliceous marble is about 50 feet. It is usually fine grained and grayish in color. Small crystals of calcite and dolomite, however, can be observed irregularly disseminated. An analysis of a specimen taken from the ledge afforded me the following result : Per cent. Carbonate of lime 50-52 Carbonate of magnesia 33"41 Insoluble matter , - - 13-85 Oxide of iron 1'70 Undetermined "52 Total 10000 The analysis shows that the prcJper name for the rock is siliceous dolomitic mar- ble. In the Marquette region the Morgan furnace limestone, but very little purer than this, has been extensively used as a flux. One hundred feet southeast from the expo- sure of siliceous marble there is a large ledge of gneissoid granite showing a weU 40 THE PENOKEE lEON-BEARING SERIES. defined dip of 77° to the south, and strike of north, 75° west. In following the strike west, one passes within 25 feet of the outcrop of siliceous marble which has a northerly dip. Between 100 and 200 feet south, on the line of the railroad, other large expo- sures of gneissoid granite are found having essentially the same bedding as that men- tioned above. When the railroad cut is completed at this locality the absolute June tion of the Laurentian and overlying Huronian will doubtless be exposed. There can be no doubt of the unconformability of these formations, approaching each other as they do with a persistent opposite dip and somewhat different strike. Unconforma- bility has been shown to exist between the Laurentian and Huronian in Michigan, but this is the first time that it has been proved in Wisconsin. Northward from the granites the section has been completed for over 1,600 feet. In this space are included two " magnetic ore " beds, the southern 130 and the northern over 500 feet thick. Directly above or north fi'om the northern " ore " bed there is a space of 1,400 feet upon which exposures have not been found. Above this blank recent railroad excavations enabled Mr. Wright and myself to subdivide and extend the belt of 400 feet, supposed to be the uppermost member of the Penokie system, into: a, siliceous schists, 100 feet; 6, blank (Bad river), 75 feet; c, contorted black slate, 250 feetj d, diorites, 75 feet; and e, black porphyritic slates, 50 feet. Owing to the heavy deposits of drift we were unable to find exposures for 1,300 feet north from the black porphyritic slates. We then found what are probably the latest beds of the Huronian formation: g, black slate, 40 feet; h, quartzite, about 250 feet; i, slaty amygdaloid, 75 feet. The thickness of the formation I estimate at something over 5,000 feet. The dip is about 66° to the north, showing entire conformability throughout. (Pp. 42-44.) 1877. iRViNGr (E. D.). Report of Prof. Irving. In Annual Report of Progress and Re- sults of the Wisconsin Geological Survey for the year 1876, by T. C. Chamberlin, pp. 13-18. This report contains a brief account of the progress of the work under R. D. Irving in northern Wisconsin, but nothing that is not much more fully developed in the third volume of the Geology of Wisconsin, published in 1880. Wright (C. E.). Mr, Wright's report. In same publication. as the preceding, pp. 18-23. Contains a brief preliminary statement of results obtained in the Penokee region in 1876. The same results are given more fully by Mr. Wright in the third volume of the Geolog}^ of Wisconsin. We may mei-ely quote the following generalization : (IKOUXilCAL EXT'LOHATIONS AND LITERATURE. 41 It has been my constant aim, and still ia, to correlate the Penokee series of rocks with those of Micliifian, and there exists in my own mind no reasonahle doubt that tlie rock Ibrniations of thesi; two districts are the, equivalents of each other. In the Peuokee we have the limestone and quartzite members; tlie belts of maf,nietic schist interlamiuated with the greenstones; also the black slates and mica-schists, all o(!cupying relatively the same stratigraphical position as in the Michigan series. (Pp. 22-23.) Irving (E. D.). Note on the Age of the Crystalline Eocks of Wisconsin. Am. Jour. Sci., 3d series, vol. xiii, 1877, pp. 307-309. The object of this note is to oppose Bradley's view, then recently expressed, and indicated also on his geological map of the United States, that the crystalline rocks of Wisconsin and Michigan may be altered Lower Silurian. It is a general outline statement of the succession of pre-Potsdam formations in northern Wisconsin, as the following quotation will indicate: The crystalline rocks of Wisconsin include unquestionably two distinct terranes, the one Ij-iug unconformably upon the other, as is beautifully shown at Penokee gap, on Bad river, in the lake Superior coimtry. Here a white siliceous marble of the Huroniau, overlain by hundreds effect of distinctly bedded slaty rocks, and dipping northward, is to be seen within 20 feet of large ledges of dark colored amphibolic gneiss, whose bedding planes dip southward and strike in a direction diagonally across that of the more northern beds. Tliere are no doubt instances where the two series are difficult to separate, similar rocks occurring in both groups, but the exist- ence of the two is incontestable, and their uuconformability with the unaltered Potsdam equally so. The facts proved thus far with regard to the older rock series of Wisconsin may be briefly summarized as follows: The oldest (I) are gneisses and granites with other rocks; these are overlaid unconformably by (II) a series of quartz- ites, schists, diorites, etc., with some gneiss and granite; these in turn are overlaid — probably also unconformably, but this is not certainly proven — by (III) the Copper series, which includes greenstones and melaphyres, and also great thicknesses of inter- stratified sandstone, melaphyres, amygdaloids, and shales, the whole having a thick- ness of several milps; these finally are unconformably covered by (IV) a series of unaltered horizontal sandstones including numerous fossils, many of which are closely allied to those of the Potsdam sandstone of New York, and all of which have a marked Primordial aspect. I and II are referred to the Laurentian aiid Huroniau systems of Canada, because they bear the same relations to one another and to the Copper series that these systems do. (P. 308.) XS78. . Irving (E. D.). Eeport to T. C. Chamberlin, State Geologist, of work done in the Penokee region in 1877, dated December 24, 1877. In Annual Report of the 42 THE PENOKEE IRON-BEAEING SERIES. Wisconsin Geological Survey for tlie year 1877, by T. C. Chamberlin, Chief Geologist, pp. 17-25. Madison, "Wisconsin, 1878. The principal work of the season of 1877 included (1) the extension of a detailed geological section begun in the vicinity of Penokee gap during the previous year, northward 5^ miles to the railroad track near the cross- ing of Silver Creek, Sec. 10, T. 45, R. 3 W.; (2) the making of a similar section along Bad river, somewhat farther east ; and (3) a detailed map- ping and magnetic survey of the Penokee range from Bad river to Potato river, T. 45 N., R. 2 E., Wisconsin. The plan adopted for this work was to cross the iron belt, which, although quite sinuous in its course, preserves still a general east and west direction, curving more and more toward the north as it is followed eastward — fi-om north to south at distances of about half a mile, using the section lines as much as possible. Ou each of the crossing lines stations were established at every hundred steps, and at every station the aneroid barometer, the variation of the magnetic needle, and the time were care- fully observed, a simultaneous series of barometrical observations being carried ou at Ashland. The lines were begun at points far enough to the south, on the Laureutian rocks, to be out of the influence of the irou or magnetic belt of the Huronian, and were extended northward far enough, not only to be out of the influence of this belt in that direction, but also to determine the presence or absence of any other similar belt or belts. Some of the lines, moreover, were extended farther than the rest, so as to pass on to the next series of rocks, allusion to which has been made above. Other subordinate lines of observation were frequently run across the sections in an east and west direction, and all the lines were controlled by constant reference to section corners and quarter posts. All outcrops were of course examined and located, and specimens were taken for subsequent study, particular attention being given to the magnetic belt traversing the center of the Penokee range. The largest outcrops are found where the several branches of Bad river break through the range from the southward. At each one of these gorges the work was carried into greater detail, in order that the true succession of the various layers might be made out. Many interesting facts were developed during this detailed work, one or two of which may be mentioned here. The exact junction of the Huronian and Laurentian series was found at the gorge of Potato river, where a cliff- side over 100 feet in height and over half a mile in length is traversed near the middle by the highly inclined contact line between the "siliceous slate," one of the lower members of the Huronian, and a greenish chloritic gneiss of the Laurentian. The siliceous slate inclines at a high angle to the north, whilst the gneiss layers dip to the south and strike in a direction oblique to that of the slate layers. It is worthy of note that the two lowermost layers of the Huronian, as seen at Penokee gap and for many miles to (IIOOLOCICAL KXI'LOUATIONS AND IJTKK ATIJIIK. 43 the eaistwaid, the •• white (|uait/." and ''siliceous doloinitit' inaii)]«'," iire here ciitiiely alvsciit; Imt this fact is ([uite in accord witli the rehitioiis everywhere to be observed between these two widely distinct rock series. Another t'ac^t of iinportanc^e is the steady lessening; of the disturbing iniluenee exerted on the magnetic needle by the iron belt of the Iluroniaii, as it is followed eastward. In its more western extension the variations observed on and near this belt are eonimonly as much as OQo to IStP, tlu^ disturbing influence extending, moreover, for a long distance to the north and south of the line of greatest disturbance. By the time the Potato river is reached the variations never approach 90o, and are to be observed along a very narrow belt only. Still farther east the attraction lessens yet more rapidly, and on the Montreal river you have yourself observed that it is essentially lost. This lessening in mag- netic attraction does not necessarily indicate a corresponding decrease in the amount of iron present in the rocks of the iron belt, but rather that the magnetic oxide is giving way more completely to the nonmagnetic, or sesquioxide, which is always present, in greater or less quantity, even Avhere the magnetic attractions are greatest. The outcrops observed bear out this conclusion ; for a considerable quantity of very highly manganiferous red hematite is to be seen at points all along from the passage of Tyler's fork eastward. Yet another point of interest brought oiit by this year's work is the apparent demonstration of the nonexistence of otlier magnetic belts in tlie more northern or upper portions of the Huronian series. Hematite, or specular ores, may exist here, but the gaps in the series of layers have now been so largely filled up that it appears probable that any discoveries of ore which may be made in the future will 1)6 on the already known magnetic belt. (Pp. 19-21.) Chamberlin (T. C). Annual Eeport of the Wisconsin State Geological Sur- vey for the year 1877. Madison, 1878. • Pages 25 to 28 of this report include a brief account of an examina- tion made by Prof. T. C Chainberlin of tliat pai't of the Penokee range which lies between the Potato and Montreal rivers. The following, as to the contact of the basal member of the Iron-bearing series with the scliists south of it, is of especial interest: At the falls of the Gogogashngun a most interesting section may be made out. The falls themselves are due to the barrier imposed by the siliceous schists that here form the lowest exposed member of the Huronian series. By going back from the falls a short distance, guided by the indications of the loose blocks of rock on the surface, the party were fortunate enough to uncover, at their first attempt, the exact junction between the Latirentian and Huronian series. The Laurentian membei' con- sists of a peculiar gneissoid rock, altogether like that which occupies a similar 44 THE PENOKEE IRON-BEARING SERIES. relation at Penokee gap. Its strike is N. 07° W., and its dip 49° NE. Tlie Huroiiian' rock lies in absolute contact with this, not even being separated by a fissure. Indeed, at one poiut the siliceous material that formed the Huronian rock had, at the time of its deposition, so insinuated itself into the irregularities of the surface of the gneiss that the two formations are interlocked, and n hand specimen Avas obtained, one portion of which is Laurentian gneiss and the other Huronian schist, the two being, of course, unconformable. It is doubtful whether a similar specimen has ever previously been secured. The base of the Huronian series as here exposed is formed by gray and purple siliceous schists, interleaved with which are occasional purplish layers of a clay-like texture. Some of these approach a pipestone and raise the question — which, of course, they are not competent to answer — whether they are not the approximate equivalents of the pipestones of Barron county, which sustain a somewhat similar relation. The general strike of these schists is N. 55° E., and their average di^J about 60° NW. By comparison with the Laurentian strata it will ]be seen that the two formations strike across eacli other at a large angle and dip in opposite directions. (Pp. 20-27). Hunt (T. S.). Special Report on the Trap Dikes and Azoic Rocks of South- eastern Pennsylvania. Part i, Historical Introduction, Second Geological Survey of Pennsylvania, volume E. Harrisburg, 1878. Contains in a general historical review several'brief references to cer- tain of the accounts of the Penokee-Gogebic district previously published and above noted. Dr. Hunt had not himself been in the district. • 1879. Chambeelin (T. C). Annual Report of the Wisconsin Geological Survey for the year 1878. .Madison, 1879, pp. 5-7. Contains a l^rief account of the woi'k then still in progress under R. D. Irving in northern Wisconsin, and particularly a statement of the route followed by Mr. A. D. Conover in making certain additional exjilorations. Irving (R. D.). Note on the Stratigraphy of the Huronian Series of Northern Wisconsin ; and on the Equivalency of the Huronian of the Marquette and Penokee Districts. Am. Jour. Sci., 3d series, vol. XVii, 1879, pp. 393-398. This article calls into question the scheme of stratigraphy for the Penokee district, above quoted from Brooks, and gives also a preliminary (iliOLOdlCAL KXI'LOltATlONa A^'l) LlTEUATintE. 45 statement of certiuu ix^sults, further noted below, in connection with tlie final report ot" the Wisconsin Survey. 1880. Irving (R. D.). The (k-ologiciil Structure of Northern Wiscousiii. In the Geolosy of Wiscousiii, vol. iii, pt. i, pp. 1-25, with outline luap auil plate of soctiouis. Madison, ISSO. As the title indicates, this portion of the final report of the Wisconsin Survey gives a general summary of the conclusions reached as to northern Wisconsin by the various members of the survey corps who had worked in this region, including the author. The following quotation shows the more general conclusions reached as to the two formations, gneissic and slaty, with which we are particularly concerned in the present volume: In a former volume of this report, I have shown how the Silurian limestone anil sandstone formations of the southern, eastern, and western portions of Wisconsin curve concentrically around three sides of the Laurentian nucleus of the northern part of the state. On the northern or lake Superior side, however, we find an alto- gether different structure; and it is evident at once that the Laurentian nucleus has constituted a barrier between the lake Superior and Mississippi valley regions since pre- Silurian times. Laurentian system. — The rocks of the crystalline nucleus itself are referred to the Laurentian of Canada, because (1) they sustain precisely the same structural rela- tions to the Huronian, Keweenawan, and Lower Silurian, as observed in the case of the typical Laurentian of Canada, and (2) because they have the same general litho- logical peculiarities that characterize the Canada series. There can, indeed, be no reasonable doubt that they are directly continuous with the Canada Laurentian. They extend to the shores of lake Superior in the vicinity of Marquette, Michigan, and appear again on the eastern or Canada shore of the lake. The separation be- tween the Wisconsin Laurentian and that of Canada is therefore only a superficial one, the connection being concealed by the waters of the lake, and by the overlying Silu- rian depositions in the eastern extension of the upper peninsula of Michigan. In Wisconsin, the northern limit of the Laurentian approaches most nearly to lake Superior on the Montreal river, which is here the state boundarv — the distance to the lake shore in a direct line being only 13 miles. From the Montreal river the north- ern limit trends about southwest by west, and on Bad river, 25 miles farther west, it is 25 miles from the lake. From Bad river the course is in general but little south of west, to Numakagon lake, T. 44, E.. 6 W. Here a rapid change to a more southerly direction comes in, and we find ourselves following the western side of the Laurentian nucleus, soon to be bounded by the regular Mississippi valley formations. The south- 46 THE PENOKEE IRON-BEARING SERIES. ern rim of the lake Sui^erior trough, at an elevation of 1,000 to 1,100 feet above the lake, lies but a few miles soiith of the northern boundary of the Laureutiau area, for about 50 miles westward from the Montreal, after which it passes on to the more north- erly and newer formations. The rocks of the Laurentian nucleus have already been partly described in former reports. Where they approach lake Superior they are almost wholly gneiss and gran- ite. The prevailing rock along the northern border is a dark gray to black, often greenish-black, hornblende-gneiss, in which the hornblende has usually been more or. less completely altered to chlorite. This alteration, when carried to any considerable extent, gives a greenish tinge and greasy feeling to the rock, and, in cases of extreme alteration, there is a passage to a green chloritic- schist. The associated granites are usually light pinkish- tinted to gray, and highly quartzose, a frequent gneissoid ten- dency showing their sedimentary nature. These I'ocks have a nearly due E.-W. strike, and, near the northern border, a high southerly dip. They are, however, beyond question greatly folded, and have as certainly an enormous thickness. Muronian system. — Lying immediately against the Laurentian, and very sharply defined from it, we find, extending from the Montreal river westward for 50 miles to lake Numakagon, a belt of schistose rocks which we refer unhesitatingly to the Can- ada Huronian, and Avhich are beyond question the direct westward extension of the iron-bearing series of the upper peninsula of Michigan. This belt has a width, in general, of from 1^ to 2J miles, and includes an aggregate thickness of strata of nearly 13,000 feet, with a number of well marked subdivisions, several of which are persistent throughout the entire length of the belt. These subdivisions may be briefly summa- rized as foUows, beginning below : (1) Crystallinetremolitic limestone, at times overlain by a band of white arenaceous quartzite, and at times absent, the next formation above them coming into contact with the Laiu*entian, 130 feet; (2) straw-colored to greenish quartz-schist, and argillitic mica-schist, often novaculite, 410 feet; (3) tremolitic mag- netite-scliists, maguetitic and specular quartzites, lean magnetic and specular ores- forming the " Penokee Iron range," ISO feet; (4=) alternations of black mica slates with diorite and schistose quartzites, and unfilled gaps, 3,495 feet; (5) medium-grained to aphanitic, dark gray mica-schists, with coarse intrusive granite, 7,985 feet — in all, 12,800 feet. These rocks all dip to the northward, the angle being usually high, but lessening toward the west, and trend with the course of the belt, which has numerous minor corrugations, while preserving one general direction. The strike direc- tions are always oblique to the trends of the underlying Laurentian gneiss, proving the unconformability of the two systems, the actual contact of which may indeed be seen in several places. Westward from lake Numakagou the Huronian belt is lost sight of, the Lauren- tian gneiss and Keweeuawan gabbro and diabase coming apparently into direct con- tact wiifch each other. GKOLOGIOAIi EXPLORATIONS AND LITKJiATURE. 47 Tlif rcffiriKr of tlic scliistosi' scries of the I'eiioUee region to tlie Iluroiiiaii is jiustitied by tlie tollowing coiisideratioiis: (1) Tlicre iippciirs to bo a direct eoiitiiuuitiou with the irou-boaring system of the Maniuotto region of Michigan; ('2) as shown on a subseciuent page, tlie grand snbdivisions of tlio Bad river and Marcinetto systems both show the same, relation to the Laurentian and Kewcciiawau systems as fonud in the Iluronian of Canada; i. e., are newer than the foinier and older than the latter; (4) the Mar(inette system is found in unconformable contact with the Lower Silurian red sandstone of lake Sl^pel^or. The Marquette Huronian, in its southerly extension in the IVIeuominee region, is also found in unconformable contact with the fossilifer- ous primordial sandstone of the Mississi])pi valley, a fact which, even if tbe evidence were not amply sufficient without, would demonstrate the futility of the attempts made by some to refer the whole series of lake Superior crystalline rocks to the Silurian. (Pp. 5-7.) The general succession of formations in northern Wisconsin, from above downward, is held to be as follows: (1) Lake Superior Sandstone; unconformably followed by (2) the Copper-Bearing or Keweenaw series ; unconformably followed by (3) the Iron-Bearing or Huronian series; un- conformably underlain in its turn by (4) the Gneissic or Laurentian. In support of the belief that there is an unconformity between the Kewee- nawan and Huronian series, Brooks's arguments, already noted above, are quoted and indorsed, after which the author proceeds as follows : I conceive that further evidence of nonconformity is afforded in the Wisconsin region by tbe following facts: (1) In the Penokee country the uppermost beds of the Huronian aregraclually cut out, as we trace them westward, by the gabbro tbat forms the base of the Keweenawan series, a fact which appears to me best explained by the supposition that the gabbro covers and conceals these missing beds. (13) There is not an absolute uniformity in dip between the Huronian and Keweenawan rocks in this region, the latter standing commonly at a higher angle. (3) West of lake Numakagon the diabases and otker eruptive rocks of the Keweenaw series appear to completely cover the Hiu^onian in a great overflow. Nevertheless, the approach to conformity in Wisconsin is close, and were we to draw our conclusions fiom this region only, the nonconformity could hardly be regarded as proved. There are no such undulations in the Hui'onian of the Penokee district as in Michigan, the subor- dinate members making long and regular bands conforming to the general trend of the formation, and also, in a general way, to the trend of the several belts of the Keweenaw series. Moreover, the lessening in dip toward the west, already noted as affecting the latter rocks, is observed also in the underlying Huronian so far as these can be traced westward. (P. 22). 48 THE PENOKEE IRON-BEABING SEKIES. Irving (R. D.). Geology of the Eastern Lake Superior District. In Geology of Wisconsin, vol. ill, Pt. 3, pp. 51-238, with 6 atlas plates, 18 volume plates (including 7 colored microscopical plates), and 4 figures. This is a detailed account of the geology of that portion of northern Wisconsin which is drained by the waters of Bad and Montreal rivers and, of course, covers all that portion of the belt now to be studied, which lies west of the Montreal river. The chapter titles are as follows: Introduc- tion, Topography, The Laurentian System, The Huronian System, The Keweenawan or Copper-Beaiing System, The Lake Superior or Potsdam Sandstone, The Quaternary Deposits, Appendices A and B. The chapters on the Huronian and Laurentian are of chief interest in the present con- nection. From the former chapter we quote: The Huronian rocks Ije together in a narrow belt ft'om half a mile to 3 miles in width, which stretches entirely across the district, ft'om the west line of T. 41, R. 5 W., to the Michigan boundary at the Montreal river, in Sees. 24 and 13, T. 46, R. 2 E. The total length of the belt in this distance is about 46 miles. The wider portions are toward the east, the western part narrowing in places to as little as a mile or even half a mile in width, as in the sections just south of Bladder lake. The total area underlain by these rocks is just about 100 square miles. (P. 100). Then, after giving in brief the course of the Huronian belt through the various townships and sections, the report proceeds: The southern boundary of the Huronian, or its line of junction with the Lauren- tian, is a very sharply defined line, on account of tlie bold topography and frequent rock exposures of the Penokee range. Even in that portion of the Huronian belt where the Penokee range disappears, and the rocks are entirely concealed by drift and swamps, the magnetic attraction exerted by the iron-bearing member of the formation, one of its lower layers, serves to fix very closely the southern boundary. On the accompanying atlas plates this boundary line is laid down so accurately and the facts upon which it is based are there detailed so faUy that no further explanation is needed here. In addition to the facts given on the maps and in the details of the following pages, it is merely necessary to say here, with regard to the northern bouudaryj that it does not follow the strike of the Huronian beds, but cuts across them in a more or less irregular way. The width of the Huronian does not vary on account of the thickening or thinning or disappearance of any of its subordinate layers, but the wider portions include higher layers, which are wanting in the narrower portions. The irregularity, then, of the northern boundary is due t(3 a nonconformity of the overlying rocks with the Huronian. It should also be said that these overlying rocks, GEOLOGICAL EXri-OI.'ATIONS AND LITEl'.ATUKE. 49 cliit'lly i;;il)l>i-iis, iirc. (tf iiiii iKiioous (irijjiii, iiml linvc. ccrtiiinly, in some places, and (luite possibly, also, in oMiers not ye(< Tei -oj^nized, pciictralcd fciio Uui'ouia.u, producing peculiar ineguhuities in (he line of Junction. The main lopograjthical leatuicss of the lluroiiiaii belt have already been given in another connection. It is only necessary to notice here somewhat more definitely the relation existing between the geological structure of the series and the topography yf the strip of country underlaid by it. The Iluronian series includes a succession of bi'ds, always markedly schistose and at times highly slaty, which are, for the most part, inclined at a high angle to the northward. At the base or southern side of the belt are luirrow layers of crystalline limestone and (luartzito, succeeded by a broad band of siliceous slate, some 400 feet in width, above which there is again a much broader band, generally as much as 800 feet wide, of magnetic and specular schists. Above these again is a series of alternating layers of mica slates, diorites, quartz slates, and quartzites — the latter comparatively ineouspicuous — which reaches a thick- ness of several thousand feet. A close connection may be traced between the nature of these beds and the features of the surface. The existence of the Penokee range, which marks the lower side of the Huronian belt for the greater part of its extent, and which has already been described in some detail, is determined by the broad bed of magnetitic quartzites and siliceous, mag- netic, and specular schists above referred to. These, by virtue of the superior hard- ness and power of resisting chemical action conferred on them by their siliceous ingredient, have remained standing, while the softer beds to the north have been worn, for the most part, into deep valleys, in which streams run parallel to the trend of the Penokee range, being impelled to their courses by the strike of the under- lying rocks. In places the more massive diorites and quartzites of the northern portion of the series rise from the valley in abrupt ledges, but they never constitute a continuous ridge like the Penokee range, on account of their smaller breadth and inferior resisting power. On the northern side of this valley the Huronian beds often extend well up the river on to the Copper range, being i^rotected here by the massive rocks of the Keweenaw series, which bound them on the north. The south slope of the Penokee range, again, is made up of the siliceous schist which underlies the harder rocks that form the body of the range, and, being itself generally a quite soft and easily eroded material, the southern slope is often iirecipitous, or at least very bold. This is esijecially true of the middle portions of the range, from a few miles west of Bad river nearly to Tyler's fork. Further east this layer becomes more quartzitic and harder and forms itself the body of the ridge, the overlying beds at the same time losing their comparatively great resisting power by a change in com- position. In some places in the eastern extension of the Huronian belt both the siliceous schist and the overlying beds are softer than the Laurentian below, and the crest of the ridge is made up of rocks of the latter series. IfON XIX — —i 50 THE PBlSrOKEE IRON BEAEING SERIES. A variation in the degree of northerly dip of the beds of the Huronian has also very measurably affected the surface features. From a point on the ridge a few miles west of Bad river to the Montreal the angle of dip is always very high, 55° to 750, while farther west it lessens to 45°, 35°, 25°, and even to 20° or 15°. In these places the result is a longer front slope to the ridge, and a very steep, frequently bold and precipitous southern face, made up usually of heavily bedded quartzitic iron ore overlyiug the siliceous schist, which now loses its prominence and forms only the foot of the southern face. The entire absence of the Penokee range in T. 44, E. 4 W. is perhaps to be attributed in part to a lessening in dip, though probably chiefly to a change in the character of the lower layers of the series. (Pp. 101-103). The Huronian series consists of a succession of more or less highly schistose to slaty beds, which reach a total thickness in the widest part of nearly 13,000 feet. These layers all stand inclined at a high angle to the north, and stretch across the country in outcrops generally parallel to the southern limit of the formation, some, of the more prominent ones preserving their characters across the whole width of the district, a distance of about 45 miles. Inclining, as they always do, to the north, these beds are without folds, and the series is only limited in that direction by the overlapping of masses of igueous rocks belonging to an unconformable system — the Copper-bearing or Keweenawan series — the unconformity being one, however, recognizable only on a comprehensive survey of the region, and not by any observed contact between the two formations. The absence of any folds in so highly altered and inclined strata is easily explained, if we regard them as forming part of a great bend underneath the trough of Lake Superior and reappearing on the north side of the lake with a reversed inclination. These points have been brought out on a prev- ious page, and need only to be alluded to here. The degree of northward inclination of the Huronian beds is, for most of the course of the formation, fi'om 55° to 75° ; most usually between 65° and 75°. To the west of Sec. 16, T. 44, E. 3 W., however, the degree of inclination is nearly always much less, becoming at times as low as 20°. The bends in- the course of the forma- tion have already been noted in a general way. Some of these are exceedingly abrupt, as, for instance, on Sec. 10, T. 44, E. 2 W. ; at the crossing of the creek in sections 8 and 17, T. 44, E. 2 W., and at several places in the western extension of the formation. These bends are well marked out in the rock exposures and are noted in detail on the accompanying maps. At the i^assage of Bad river, the strata are crossed by a fault, trending about N. 170 W., the layers on the west side of the fault being thrown 800 feet to the north- ward of those on the east side ; or, regarding the throw as a vertical one, the western side has been elevated or the eastern depressed a vertical distance of over 1,700 feet; the apparent lateral throw, on this supposition, being explained by the inclined posi- tion of the strata. This fault is marked in the topography by a corresponding set-off GEOLOGICAL EXI'LOKATIONS AND LITEUATUKB. 51 in the ronokcc riiUK'Oi wliicli on the west side of the river is well to the northward of its position on the east side. It is also well marked by large rock exposures on either side, and also by an abrupt break in the line of magnetic attraction caused by the magnetic belt of the formation. The facts with regard to it are all detailed on Atlas Plate xxiii, and also in the descriptions of the following pages. The following tabulation indicates the succession of layers of the Hurouian series in the vicinity of Bad river, so far as made out, with the average thickness, surface breadth, and other prominent points of each layer. Several of the lower layers, inchiding a total thi(;kness of some 1,500 feet, have been traced across the entire width of the district. The higher layers, on account of their comparative softness and susceptibility to chemical changes, have been for the most part deeply eroded, and are, moreover, largely buried beneath accumulations of drift materials, so that the exposures are comparatively few and distant, and the task of making out the succes- sion becomes much more difficult. (Pp. 103-104.) . . . Synopsis of the Straligraphij of the Hiironian of the Penokee region, Wiscousiti. Average . thickness. Formation. Feet. I. Tremolitio crystalline limestone 90 II. (A) Arenaceous white quartzito, often brecciated, 35 feet; (B) magnetitic quartz- schist, 5 feet 40 III. Siliceous slaty schists; including quartzito, " argillitio " mica-schist, and novacu- lite; all having much quartz, and none ever showing any amorphous material.. 410 IV. Magnetic belt; including: (a) banded magnetic quartzite — gray to red quartzite, free from or loan in iron oxides, banded with seams, from a fraction of an inch to several inches in width, of pure black granular magnetite, only rarely mingled with the specular oxide; (b) magnetitic quartzite, the magnetite in varying j)roportions, pretty well scattered throughout, and mingled with the specular oxide in proportions varying from nothing to a predominating quan- tity; (c) magnetitic quartz-slate, the magnetite pervading the whole, and mingled with the specular oxide as before; {d) slate like (c) but largely charged with tremolite or actiuolite ; (e) arenaceous to compact and flaky quartzite, free, or nearly so, from iron oxides ; (/) thin-laminated, soft, black magnetitic slate ; (g) hematitic quartzite, the iron oxide the red variety; (/i) garnetiferous actiuolite-schist, or eologite; (i) diorite, which is restricted to the western end of the Hurouian belt. Kinds, (a) to (d), all carry much pyrolusite, or other manganese oxide. These varities have no persistent stratigraphical arrangement, and are named here in order of relative abundance. Total thickness, about.. . 780 V. Black feldspathic slate; consisting of orthoclase grains imbedded in a paste of biotite, pyrite, limonite, and carbon 180 VI. Unknown, always drift-covered 880 VII. Dark gray to black, aphanitic mica-slate, having a wholly crystalline base of quartz and orthoclase, with disseminated biotite scales 120 VIII. Unknown, but probably in large part the same as VII 290 IX. Chloritic, pyritiferous, massive diorite 150 X. Black, aphanitic mica-slate, like VII 25 52 THE PENOKEE lEON-BEAEING SEEIES. Si/nopsis of the Stratigraphy of tlie Haronian of the Pcnokee region, Wisconsin — Continued. Average thickness. Formatiou. ■ Feet. XI. Covered, but iirobablj^ mica-slate 280 XII. Black mica-slate; ajiliaiiitic; at times chiastolitic 225 XIII. Cliloritic diorite-schist 35 XIV. Black mica-slate, like XII, often cliiastolitic 375 XV to XVIII. Alternations of black mica-.slates, with qiiartzites and ijuartz-schists 675 XIX. Greenstone-schist; aplianitio; the hornblende and plagioclase much altered 260 XX. Covered, but probably like XXI - 525 XXI. Mica-schist; from aphanitic to medium grained ; including bands of light gray (luartz-schist, the mica becoming subordinate; all varieties having a back- ground of quartz ; the mica wholly biotite ; penetrated by veins and masses of very coarse, pink to brick red biotite granite; total on Bad river, 4,960 feet. Seen farther east, higher layers, 2,500 feet ; in all .7, 460 Total 12,800 (Pp. 104-105.) The numerous local details of this volume are made frequent use of in the following ])ages and on the accompanying maps. JuLiEN (Alexis A.). Microscopic Examination of Eleven Eocks from Ashland County, Wis('onsin. In the Geology of Wisconsin, vol. iii, pt. 3, Appendix B, pp. 224-238. With one colored microscopical plate. This paper includes detailSd descriptions of the following rocks : (1) Chloritic gneiss, from the lower gneisses at Penokee gap ; (2) crystalline limestone ; (3) tremolitic magnetite-slate ; (4) black feldspathic slate ; (5) chloritic diorite ; (6) magnetitic mica-slate ; (7) chiastolitic mica-slate ; (8) diorite greenstone-schist ; (9) magnetitic mica-schist — all from the Bad river section through the Iron-bearing slates; and (10 and 11) two chryso- litic diabases from the overlying copper series. Wright (C. E.). The Hurouian Series West of Penokee Gap. In the Geology of Wisconsin, vol. ili, pt. 4, pp. 239-301. With one atlas i)late and ten volume plates. This report, the result of a special exploration with reference to the occurrence of iron ores in the slaty series west of Bad river, embraces three chapters, which are entitled, respectively, " Dipping Needle and Solar Dial Compass, and the Method of Employing Them," " Greological Cross Sec- tion of Penokee Iron Range," and "Special Examination of Penokee Iron Range West of the Grap." The first of these chapters calls for no consid- eration from us. The third chapter is occupied with local details, aixanged (iEOLOGK^AL KX I'LOKATIONS AND LITEIIATURE. 53 ill the form of a diary, with the results of a microscopical study of thin sec- tions iirterspersed. These details have often been used in the compilation of our maps herewith, most of the important outci-ops described by Mr. Wright having, however, been revisited for our work. Wright's route of travel also is laid down on our maps. The second of Mr. Wright's chapters is of a more particular interest, because in it he gives his views as to the general stratigraphy of the Iron- bearing series, and as to its relations to the adjoining formations. The fol- lowing quotations from this chapter will serve to shoAv Mr. Wright's con- clusions ; the omitted portions are mainly microscopical details: lu this geological section it has been my endeavor to correlate the Penokee Iron- bearing series — Lower Hnronian — as near as possible with those of Michigan, assumed to be of the same age, as they uudonbtedly are. We will adopt, for convenience of reference, the numbering first employed in the Geological Report of Michigan, 1873, to designate the Lower Hnronian beds. The first rocks we wiU .consider, however, are those of th^ Laureutian series, found outcropping on the south side of the Penokee range. These rocks are granites, gneissoid granites, and gneisses. At the Gap, they have a strike a little to the south of west, and dip from 65otb 80° to the south. The granites are dark gray to reddish, depending on the predominance of the mica or feldspar. They are fine to very coarse grained, the medium grained varieties being, however, the most common. The essential mineral ingredients are usually easily recognized. The bedding i^lanes, or "grain" of the granite, even in the massive varieties, may, generally, after careful examination, be made out. ... Orthoclase is the prevailing member of the feldsijar family. . . . The mica is chiefly biotite and of a dark color. . . . The gneisses, like the granite, vary from dark gray to reddish. Usually they are distinctly laminated with the layers of quartz, feldspar, and folia of mica. It is some- times slaty, but more generally it is heavily bedded, and passes almost imperceptibly into a gneissoid or a massive granite. The dark, fine grained, slaty varieties resemble hornblende-gneiss and hoi'nblende-schist; in fact, I have been obliged to make micro- scopic sections before being able to decide which it was. . . . Nonconformably overlying the Laureutian rocks are those of the Huroniau series. We have at the Penokee Gap, where the Wisconsin Central railroad crosses the range, one of the best opportunities I have ever chanced upon for observing this interesting fact. This nonconformability on the Penokee, I believe, was first noticed by myself in 1875. The Huroniau series at the Gap are plainly bedded, and have a strike of a little north of east, and dip very uniformly 06° to the north. 54 THE PENOKEE lEON-BEAEmG SEEIES. The lowest member which I have seen on the Penokee range is a. marble, or dolomitic limestone, which we will consider as No. IV. There may be other members below the marble, as is the case in the Hnronian rocks of Michigan, but I have never found them. At the Penokee gap the marble is siliceous; in color light drab to grayish white, with shades of green; also light red and pink not uncommonly. This diversity of colorsis often observable in a hand specimen. The marble is flue grained, and strongly bedded, or massive, depending on the degree of metamorphism. When massive, it is usually jointed, and weathers to a light drab. Some portions of the rock contain pale greeu to almost white, radiated bunches of actinolite, resembling, in the latter instances, clusters of arragonite. . . . No. V. Immediately overlying the marble at the Gap is a quartzite, varying in color from grayish white to white, and from saccharoidal to vitreous in texture. It is massive and highly jointed. ... No. VI. ISText in order above the qiiartzite we have a chloro- siliceous schist. This member was notable for preserving its individuality wherever we found it out- cropping, and was therefore easily and quickly recognized. It has a dark grayish green color, fine grain, and is jointed, and usually more or less slaty. Along the jointing planes it is often finely corrugated, and has an unctuous feeling. It cleaves readily into thin parallel and wedge-shaped plates. Examined with the lens, it is difficult to distinguish any of the chlorite. ... Nos. VII, IX, and probably No. XI, are represented in the near vicinity of the Gap by argillites or black slates, while west of the Gap, from 10 to 14 miles, as has been already noted, the slates are replaced by diorites and hornblende rocks. My impression is that these slates and greenstones do not vary greatly in their chem- ical composition, the texture and structure being due to different conditions or degrees of metamorphism. The argillites are brownish to bluish black and have a micro-granular texture. Some of these are quite slaty and cleave freely; others are more compact. The slates are thicker and the fracture uneven to conchoidal. On the cleavage planes they have a bright, lively luster, but a fracture across the cleavage is of a dull brownish black. Disseminated throiigh the slate is considerable carbon, which appears ragged under the microscope. Numerous slender blades resembling microlites of feldspar are visible. Small angular grains of silica are present; also brownish and slightly dichroitic leaves resembling biotite. The diorites, above alluded to as constituting in part Nos. VII and IX of our scheme, are massive and strongly jointed, rarely ever showing any signs of bedding. They are fine to coarse grained in texture; the cleavage planes of the hornblende in coarse varieties being unmistakable ; also grains of magnetite. The other mineral ingredients are not so readily distinguished. . , . r.EOLOOICAI, F,XrL( )ItyVTIONS ANT) IJTKTIATUEE. 55 Rptiiniiii j;' now to Xn. I'/Z/oI'mir cniss section we liiid it and No. X rojire- sciitcd liy actiiiolo-iiiiifiiu'fic scliists. Tlicsc iiiciiiltcrs ai« more or less iiiapietic, (leiieiidiiij;- on tlu^ iierceutafto ol' magnetite anlaces where the relations can be seen, they prove to be eruptive (pp. 66-68). (2) As to an unconformity between the so-called Laurentian and Huronian, referring to my statement as to this relation in Geology of Wis- consin— He states that a perfect case of nonconformability exists at "Penoka gap," Wisconsin, to which we have before referred; but, if we remember correctly Mr. Wright's personal statement to us, neither was the junction seen nor the kind of junction known that the two made with each other. It is too fast to assume, as has been done by Messrs. Brooks, Irving, and Wright, that the strike and dip of a foliated rock is the strike and dip of its stratification. This is especially the case when the view that they were ever stratified is still a disputed point (p. 25). We have heretofore seen that the view that the " Huronian" unconformably over- lies the "Laurentian" has been only supported by the fact that the foliation of the latter did not conform in its dip to the lamination of the former. This proof is of no value unless it can be shown that both rocks are stratified and in situ. That the latter is not so, we have seen in numerous localities. Heretofore the two systems have not been observed in contact, but recently statements have been published that their junctions have been seen in other regions. ' The statement is made that both rocks are stratified, but no proof is adduced to show on what the conclusion is founded, and although the contacts were said to show beautifully, nothing was pubMshed indi- cating that the kind and manner of the junction was observed. It would seem that even here the decision concerning the unconformability was based on the foliation only (p. 70). Ikving (R. D.). The Mineral Resources of Wisconsin, Trans. Am. Inst. Min. Eng., vol. VIII, 1880, pp. 487-508, accompanied by a geological map of Wisconsin, and adjoining portions of Michigan, Minnesota, Iowa, and Illinois. As the title indicates, the object of this paper is to give an account of the mineral resources of Wisconsin, as developed to date. Prefacing this, however, is given an outline account of the geological structure of the state. In this are included a number of references to the Penokee district, but nothing is given of interest in the present connection that is not included in vol. Ill of the Geology of Wisconsin, already referred to at length. Weight (0. E.). Annual Report of the Commissioner of Mineral Statistics of the State of Michigan for 1880. Lansing, 1881. ' Geol. of Wis., vol. in, 1880, pp. 98, 108, 117. r.EOT.OdTOAL EXPLOITATIONS AND LITERATURE. 59 Tliis report, ;is also latcrdiicw by the same autlior (for ISSI ami 1882), and l)y his successor, A. P. Swincford (lor 1883 and 1884), contain brief references to the results of private explorations for ii-on in tlie Gogebic region. They contain no geological information of significance. Actual mining develojjnumts first began in the Gogebic conntr)^ in tlie fall of 1884, and the first shi])inents of ore of imj)ortance were made in the summer of 1886. 18S3. Ohamberlin (T. C). Geology of Wisconsin, vol. i, 1883, ]>t. i. General Geology, with general geological map of Wisconsin. In chapters iv, v, and vi of this volume Prof. Ohamberlin gives a general account of the older rocks of Wisconsin, which, in accordance with previous publications of the Survey, are regarded as divided into three distinct formations — Laurentian, Huroniau, and Keweenawan. While in a considerable measure this account is a summary of conclusions already announced in previously published volumes of the Geology of Wisconsin,^ it still embodies some later conclusions, as also the most com- prehensive statement of Prof. Chamberlin's own Adews on these formations yet published. The following extracts will serve to show what these views are: (1) As to the Laurentian: Synoptical notes on Laurentian formation. — Name derived from Laurentide liills of Canada. Rocks of metamorphic class, mainly gneisses. Thickness undetermined, but gTeat. Strata much folded and contorted. Occupies a large area in northern Wisconsin. ... General character of the rocks. — We have already referred to this as the granitic foundation upon which the rocjc structure of our state is builded. The rocks as we now find them consist of a series of granites, . . . gneisses, . . . syenites, . . . hornblendic, micaceous, and chloritic schists and allied rocks. With these are associated igneous diabases . . . and similar rocks, together with diorites . . . of undetermined origin. Among these rocks the gneissoid granites vastly predominate, so that the whole series in a general view is conveniently termed granitic. Sedimentary origin. — But throughout the series evidences of sedimentary accumulation abound: (1) in the foliations and stratification, (2) in the alternating ' Vols. II, III, aud IV of this series antefLite vol. i In time of publication. 60 THE PENOKEE lEON BEARING SEEIES. bands of varying chemical constitution, (3) in the verging of one kind of rock into another laterally, and (4) in kinds of rock not known to be produced by igneous agencies. The ^yhole series has been distorted, folded, and crumpled in a most intri- cate manner, and the rocks, as the above names imply, are in a highly crystalline condition. It is manifest that the series was not so formed originally (pp. 64-66). Thickness. — The thickness to which these sediments accumulated was something enormous. In their present crystalline state the current estimate of 30,000 feet is probably not too great for the exposed portion, though the original Canadian measurement on which it is based included beds now referred to the Huronian series. How much may lie below is not known, since the base is not exposed. So great an accumulation could only have taken place on a subsiding bottom (p. 69). Distortion of the heds. — The long period of Laurentian subsidence and sedi- mentation at length drew to a close and the accumulated material underwent a most extraordinary transformation. The sands and clays lay originally in essentially hori- zontal beds, but at present we neither find horizontal beds nor sands nor clays. The strata are crumpled and folded in the most intricate manner. Not only have the great series of beds been arched and compactly folded iipon themselves, but even the thin laminations have been contorted and crumpled in the most remarkable man- ner. The axes of the folds in the region of northern Wisconsin run mainly north- east and southwest, varying several points in either direction. On the southwestern margin, however, there is a tendency to a more westerly and northwesterly trend, somewhat parallel to that margin of the area (pp. 72-73). Attending metamorphism. — The crystallization of the material is strikingly in harmony -with this hypothesis of its heated condition. The sediments, while still in their horizontal position, doubtless became solidified into somewhat firm rock (1) by their own weight, (2) by their tendency to cohere, and (3) by the agency of cementing infiltrations. But there is no reason to suppose that this induced any notable degree of chemical or crystalline change. But in their j)resent metamorphosed condition, instead of compacted sand and clay, we find thoroughly crystallized rock, in the form of granites, gneisses, syenites, hornblendic, chloritic, and micaceous schists. These ~ show that a profound chemical change has taken place, wherein the matter assumed new combinations. At the same time, compounds of like kinds collected together, under the control of crystalline forces, and assumed the form of definitely crystallized minerals. Sediments that may originally have been a sandy clay, composed of silica, alumina, and potash, mainly, formed granites, gneisses, or mica-schists. The potash, alumina and quartz united in part to form orthoclase feldspar, or, in different propor- tions, together with magnesia, to form a mica, while the excess of silica took the form of crystalline quartz. The minor incidental constituents of the sediments entered into these minerals as replacement elements, or as impurities, or formed distinct accessory minerals. When, as in some cases was true, there was a larger proportion of the GEOLOr.IOATi lOXPLOKATIONS AND LITEKATUEE. fi] hiisic iiiiiteriiil, as lime, iron, etc., luniihlciidc ami allied iniiicials were formed, giving ris(^ to syi'iiKic rocks. Where lliesc basic cleiiieiils existed in stJll larger projiorfioiis, and (lie silica was relatively less abundant, hornblendic and allied rocks were, Ibrnicd, and in similar ways ot liei' variations in the constitution of the sediments gave rise to other variations in the crystalline results. The changes wore not carried so far, in most cases, a,s to destroy all traces of the original bedding of the sediment, or to mix the material of adjacent layers in any nota- ble measure. There are certain massive portions, however, in which nearly all dis- tinct trai-es of original sediinentation are obliterated. To conceive in detail of the exact method by whicli these remarkable transfor- mations took place, lays a heavy tax upon the scientific imagination, and certainly transcends the limits of demonstrable science. In general terms, however, the meta- morpliism may quite safely be said to be due to combined chemical and molecular forces, acting under the conditions of (1) pressure, (2) heat, and (3) moisture. Beyond reasonable doubt the strata in qirestion presented these conditions, while undergoing the distortions ah'eady described (pp. 74-75). Igneous phenomena of the Laur.entian. — The Laurentian rocks are frequently traversed by dikes, veins, or irregular masses of intruded rock. These are most commonly composed of granite, but are sometimes of the darker basic classes. It has not been determined how far the i^henomena may be due to true igneous penetration from below, and how far to the rendering of the rock of certain portions of the series sufficiently plastic by heat and moisture to be forced into cracks and fissures of adjacent x)ortions. In either case the essential nature of the action was the same, the difference being in degree of liquefaction and the source of material (pp. 77-78). (2) As to a separation between Laurentian and Hnronian : We have Said that Laurentian sedimentation drew to a close, but it was only because the elevatory forces just described forced the beds up from the ocean, and prevented further accumulation upon them. But sedimentation elsewhere, did not cease. The wash of the land, the wear of the waves, and the settling of silts beneath the sea continued ceaselessly. Even while the great elevation was in progress, the land was being worn and beds were accumulating in the adjacent sea, and as soon as it reached its loftiest height it began at once to be cut down and carried back to the sea by the agency of the great leveler, water. Of the sediments formed during the elevation and immediately after — for a time whose limits are yet unknown— we know nothing. They are deeply buried from sight in our region, and if their equivalents elsewhere have been seen they have not yet been determined to be such. So far, therefore, as the details of the history are con- cerned, it is an unrevealed chapter. The record is not destroyed, as are certain pages of human history, but it has not yet been reached and read (p. 78). 02 THE PENOKEE IRON-BEARING SERIES. (3) As to the Huronian : Synoptical notes on Huronian formation. — Name derived from lake Huron, on the north side of which the formation is well developed. Known in Wisconsin and Mich- igan as the Iron-bearing formation. Probably embraces also the great iron deposits of Missouri, New York, and Canada. Consists of a variety of metamorphosed sedi- ments, embracing quartzites, limestones, clay slates, micaceous, hornblendic, carbona- ceous, and magnetic schists, and diorites and porphyries of doubtful origin. Thick- ness 13,000 feet, more or less. Strata arched and sometimes folded, but not usually closely crumpled and compacted like the Laurentian. Constitutes the Penokee, Menominee, and Black river iron ranges, the quartzite and porphyry outliers of central Wisconsin, and the quartzites of Barron and Chippewa, and probably of Marathon and Oconto counties. Existence of life probable. ... . Huronian geography.— At length the unrevealed interval gave place to a known era. In the progress of erosion and subsidence the sea advanced upon the Lauren- tian lands, and separated from them a large island within our northern boundaries, and two or three smaller ones, as it would seem, in the adjacent territory of Michi- gan. ... Local characteristics— PenoTcee region.— AXonfs, the Penokee range the Huronian beds are found abutting against a wall of Laurentian rock, which formed the ancient shore line, and definitely marked the southern limit of the primitive Superior sea.' Here we find a series of Huronian beds nearly 13,000 feet in thickness. These are now upturned and metamorphosed, but the history of their formation remains for the most part legible. The Penokee series— 1. Limestone.— The low«st member exposed to view is a crystalHne magnesian limestone 130 feet in thickness; the earliest limestone known in our series. Its bedding and its association with aqueous sediment show that it was deposited beneath water as a calcareous sediment. The source of its material deserves special consideration. The student will perceive, on a moment's reflection, that neither the simple decay nor the wear of the adjacent Laurentian rocks would give a material made up almost wholly of lime and magnesia, for the Laurentian rocks contain these ingredients only in very subordinate quantity, and, furthermore, these are among the ingredients removedr— not left— by decay. The ordinary sedi- ments resulting from decay and wear are clays and sands, not limestone. In later ages there is the clearest evidence that the great limestone formations were made from the calcareous remains of marine life in ways that will appear more clearly as we proceed. It is probable that the ancient bed of limestone under con- sideration was formed in a similar way, although no distinct traces of fossils have 1 This was not then a vertical wall as it now appears, because it has since been disturbed in common ■with the Huronian strata. But if the latter be depressed *o their original position, the Laurentian slope where observed would be about SO'^, which may be taken as the declivity of the Laurentian shore. (iK()L(Hil(JAL KXi'LOlfATlONS AND LITEKATITRB. 63 yet beiMi (liscovorcd in it. It is higlily inagncsian, and is a dolomite rather than a limestone proper. It is also impure, from the presence of siliceous and aluminous material. JDetrital beds. — Overlying this formation at some points is a bed of white gran- ular qnartzite, which indicates that the deposition of calcareous sediment was followed by an accumulation of ([uartz sand. Upon this lie beds of quartz-scliist and argillaceous mica-schist, having together a thickness of about 400 feet. These were probably originally a deposit of sand and sandy, calcareous, and magnesian clay, derived mainly by ordinary wear and decom- position from the adjacent land. Above these is a thick series of beds of iron-bearing and siliceous schists and quartzites, which now form the crest of Penokee Iron range. These have together a known thickness of about 800 feet. They appear to have consisted originally of beds of fine imiiure sand, with lenticular layers of iron ore thickly sandwiched throiigh the mass. Origin of the iron ore. — The origin of the siMceous material can be confidently referred to the atmospheric decomposition and the wearing and assorting work of streams and waves acting upon the granitic and other siliceous rock of the adjacent Laurentian land. To account for the iron ore is less easy. It occurs (1) in thin layers, or (2) more frequently in lenticular masses a few inches in thickness inserted irregularly among the laminations of the schist, and (3) in scattered particles dissem- inated through the rock. In its present form it is largely magnetic ore, though the specular variety is present. In some places both theee forms have been reduced to hematite and limonite by subsequent changes. The manner in which the iron is associated with quartzose material bears a somewhat close resemblance to the way in which magnetic-iron sands are distributed through the quartz sand of certain beaches, as may be seen at many points on the shore of lake Michigan ' at the present time, and as is reported to be the case on the coast of Labrador, where the ocean is now acting upon the same formation that the ancient Huronian sea did in its day in the Penokee region. This similarity suggests a like derivation — an explanation applicable to many of the features of the deposit — but it does not very satisfactorily account for other characteristics. It certainly seems inapplicable to some of the great iron deposits that occur in the Huronian series. The most probable explanation of the massive iron-ore beds in general refers their origin to organic agencies. Meteoric waters charged with decomposable organic matter, percolating through the soil and surface rock, change its iron ingre- dient from the insoluble to the soluble form and bear it onward, and at length out into some adjacent body of water, into which the drainage is discharged. Here it is ' Vol. II, p. 239. 64 THE PENOKEE IRON-BEARING SERIES. reoxidized by free contact with the ntuiosphcrc and precipitated in the insohibleform, and thus accumulates in beds. Bog ore is now being- deposited in this mauuer, and the ores of the Cliuton and Goal periods are gcQcrally attributed to similar action. Little licsitaucy would be felt iu refcrriug the Hurouian deposits to the same agency if there were any independent evidence of the prevalence of land vegetation. There is, as we shall ^ee, independent evidence of life, but it has not usually been thouglit to have been terrestrial. Lowland or marsh vegetation would probably furnish tlie requisite conditions, and there is no reason for doubting its existence, except the want of direct evidence of it in this and the succeeding formations. Notwithstanding this doubt, no equally satisfactory explanation of the origin of the massive iron ores has been proposed. tSlute.^, nchistn, and diorites. — Upon the magnetic schists there repose a series of black, niica-bearing slates, alternating with diorites {plagi-horn) and schistose quartz- ites, including several horizons which are concealed by superficial maferial and whose character is therefore unknown. Among these there appear to be included those horizons which in the Marcpiette region bear the rich iron ores. They are here doubtless concealed because of their softness, owing to which they have been more deeply eroded by denuding agencies. Whether these horizons are iron-bearing here remains to be determined by actual removal of the drift. The mica-slates were origijially clay beds, probably containing some carbonaceous matter. The schistose quartzites were siliceous sandstones or quartzose clays. What the diorites were originally is yet an open question, it being maintained on the one hand that they are metamorpliosed basic clays, and on the other that they are ancient lava flows, modified by long-continued chemical action. This series reaches a total thickness of about 3,500 feet. Mica-schists. — Above this is found a still thicker series of mica-schists, which were probably once mixed clayey sediments. This series now measures nearly 8,000 feet in thickness, making the entire group of the region embrace, as above stated, about 13,000 feet of strata. It will be observed, in glancing over the whole, that the great mass of the series was formed from the ordinary sediments arising from rock disintegration, and that they were un([uestionabIy derived from the adjacent Laurentian land. The excep- tions to this statement are found (1) in the limestone, probably derived from the remains of marine life; (2) in the iron ores, a portion at least of which probably arose through organic action ; and (3) possibly the diorites, which may have had an igneous origin. (Pp. 80-84.) Succeeding the period of Hurouian sedimentation, whether immediately or somewhat delayed, there was an era of upheaval and metamorphism, analogous to that which occurred at the close of the Laurentian era. It produced analogous, but less extreme, effects. OEOLOdlCAIi KXI'LOlIATlONa AND LITEUATURE. 65 Metamoiyhinm. — None of (lie oiiginal (Icpiosits now renuiin precisely in tluMr primitive contlition, tlion.uli only ;i p(n'tion of (lieiii liiive been so trinislbiincd that the orij;iual static is not dearly (lisccrnilile. The limestone was soiuewliat eompaetedand remlored more crystallini', and seiitterod erystals of tremolito were Ibrmed by the uuiou of lime and magnesia with siliea^ — in other words, were j;eneratcd from a some- whi^t silieious portion of the limesoonc. The great sand deposits were transformed into (piartzite, but, for the most part, the original grains and pebbles still remain unobliteratcd, while in some instanees line laminations and beautiful ripple and rill marks are excolleutly preserved, bearing the most uneipiivoeal testimony to their aqueous origin. The iron ores associated with the quartzites and silieious schists are now found largely in the form ot magnetite or derivations from it. If they did not originally exist iu that state (and they probably did not), they were doubtless trans- formed into it at this time of general metamorphism.' Probably some of the more massive iron deposits iu association with clay and cai'bonaeeous schists, as those of the Commonwealth and Florence mines, were only compacted and dehydrated. Certain substances that accumulated incidentally with the sand of the series now constitute accessory minerals scattered through the (juartzite, as pyrolusite, novac- ulite, mica, and others. The various finer silts, clays, and mixed sediments were changed to slates and schists. Iu short, the whole series was hardened, compacted, and in some measure chemically transformed aud crystallized. The changes in these respects, however, were rarely equal to those of the preceding Laurentian revolution. Bisturhance of heels.— In respect to attitude, great changes took place. Beds which lamination, ripple marks, and other characteristics show to have been essen- tially horizontal when formed are now found arched and tilted at high angles. In the Peuokee region the strata stand at angles varying from 20° to upwards of 80°. In the Menominee region they were warped and folded in a stillmore striking manner, and stand at various angles, according to situation. In ceutral Wisconsin, instead of close folds, immense arches were formed. The Baraboo quartzite ranges are but the insignificant remnant of the north side of an arch of gigantic dimensions, which swept upward to an altitude approaching, if not surpassing, the highest existing ele- vatious.i Similar broad arches were formed on the western side of the Laurentian island. (Pp. 89-90.) (3.) As to an interval between Huronian and Keweenawan :• Between the Huronian and Keweenawan periods an interval. of moderate extent appears to be iudicated by the fact that the beds of the latter repose unconformably upon those of the former. The amount of this unconformity is, in Wisconsin, but ' For Fig-., see vol. ii, p. 506. MON XIX 5 QQ THE PENOKEE IROISr-BEAEING SEEIES. slight, though it appears to be more considerable elsewhere.' This interval was probably entirely occupied by the disturbance and nietamorphism of the Huroniau strata above described. Indeed, there is reason to think that this was only partially accomplished when the Kew^eenawan eruptions began. Sedimentary deposits must, however, have' been in i)rogress while the slow upheaval was taking place. If we could reach these deposits we should doubtless find them in no very essential respect difterent from those which preceded and followed. Prof. Selwyu, director of the Canadian Geological Survey, as the result of his studies upon the equivalent forma- tion at the east, does not recognize any interval between the two series, and it may be that what is but a moderate break in Wisconsin is bridged by what seejns to be an essentially continuous series in the eastern region. (Pp. 94-95.) Ieving (E. D.). Lithology of Wisconsin, Geology of Wisconsin, vol. i, pt. 2, chapter iii, pp. 340-361. A general account of the various rock spedes of Wisconsin know^n to date (May, 1882). So far as the Penokee district is concerned the newer material here included is the same as that given in the volume next referred to. Irving (11. D.). The Copper-Bearing Rocks of Lake Superior, Monograph U. S. Geol. Survey, vol. V, 1883, 29 plates and maps. This volume is of course chiefly devoted to the Keweenawan or Copper- Bearing 'Series, but incident to a discussion of the relation of these rocks to the older formation is given a brief summary of the results obtained in the Penokee district by the Wisconsin Sm-vey (pp. 391-392). There is also given a map (PI. xxii. Monograph v) of the region extending from lake Gogebic, in Michigan, to Numakagon lake in Wisconsin, and northward to the shores of lake Superior, on which the entire length of the Penokee-Gogebic belt is shown, the Michigan end of the belt having been platted from the notes on the U. S. Land Office plats and from statements in the papers of Pumpelly and Brooks, above referred to. The belt is also shown in* its entire extent on the general maps (Plates i and xxviii. Monograph v) which cover the entire lake Superior region. Other points discussed having bearing on our present subject are the relations of the Penokee series to the Animikie series of the north shore of lake Superior (pp. 386, 392), the general relations of 1 Fuller data than are given in the Wisconsin reports, relating to the unconformity of the Huronian and Keweenawan series, may be found in the forthcoming Monograph of Prof. Irving, on the Kewee- nawan or Copper-Bearing Series, issued under the auspices of the U. S. Geol. Survey. See also the earlier jiapur of Major Brooks, Am. Jour. Sci., 3d series, vol. xi, 1875. GEOLOGICAL EX TLOKATIONS AND LITERATURE. 67 Keweeuiiwau and llurouiau (\)\). •402-40!)), and the nature (pp. 37-58), (trigin (p. 144), distribution (pp. 231-233), and relations to the Peuokee series (p. 16()) of the coarse grained g-abljros which are so largely devel- oped in the Had river country of Wiscousin. 18M-1-. RoMlNGKR (Dr. (!.). Geological Report on the Upper Peninsula of Michigan, exhibiting the Progress of the Work from 1881 to 1884. Manuscript copy of Part i of vol. V (unpublished) of the reports of the Geological Survey of Michigan. Through the kindness of Dr. Romiugcr, I have been funiished with a transcript of that portion of this volume which refers to the pre-Keweena- wan rocks. In it are included the results of certain studies made by the author in the Gogebic district, between lake Gogebic and the Montreal river. No maps or other illustrations accompany the volume. The head- ings under which the subject-matter of the report is classified are the names of certain of the several rock groups, into which the author had jireviously divided the pre-Keweenawan rocks of the Upper peninsula (vol. iv, Geol. Survey, Mich.), viz : the Granitic, Dioritic, Iron Ore, Arenaceous Slate, and Mica-Schist groups. Under each of these headings is given the addi- tional information obtained since the publication of the previous volume (iv) of reports, for the entire extent of the Upper Peninsula of Michigan. The following are full extracts from those portions of the report which apply to the Gogebic district : Granitic groiq). — . . . The granites bordering the south side of the Gogebic iron range and of its continuation into Wisconsin, the Penokee range, came under my observation during the progress of the survey. I found them in every respect analogous to the granites of the Marquette country. The rocks of that part are not so excessively corrugated ; the upheaval lifted the strata more in continuous sheets, and belts of granite intrusive into the incum- bent strata could not often be observed, although several granite seams cutting. across dioritic schists were seen about 4 or 5 miles west from the shore of lake Gogebic. Following the range the granite is not always found iu contact with the same kind of strata. Locally heavy quartzite strata are iu contiguity with it, the lower layers of the quartzite being often represented by a couglomerate tilled with rounded granite pebbles or by brecciated quartzose beds crowded with orthoclase crystals and cemented by a wax-colored hydromicaceous interstitial mass, which rocks resemble granite so much that it is difficult to distinguish the contiguous beds. These rocks correspond accurately with the rocks I have described in the previous report as 68 THE PENOKEE IRON-BBAEmG SEEIES. occurriug ou the contact Hue between the granite and quartzite formation in the north part of T. 47, E. 25, which I then supj)osecl to be quartzite altered by its con- tact with the granite into a granite-like rock. Now I am more inclined to consider the rock as a recemeiited mixture of granite fragments mingled with the arenaceous material which formed the overlying quartzite beds. Still it is very singular that the orthoclase crystals copiously iiubedded in the mass have all sharp outlines and look as fresh, as if they had been formed where they are and were not debris of a dis- integrated granite. At the above mentioned locality, in the NW. ^ of NB. ^ of Sec. 24, T. 47, E. 43, this singular rock in contact with the granite contains locally an abundance of brown spar, which on exposed faces of the rock weathers out, leaving behind ochraceous matter, which tills the spaces formerly occupied by the spar. , . . Only a short distance from the above described locality, in the adjoining Section 23, the granite is seen in contiguity with dioritic schists of a brecciated character, which inclose large angailar blocks of massive diorite of various qualities. The granite comes there also in contact with massive diorite belts and intersects them in dike form. . . . In Sec. 13, T. 47, E. 46, the granite is found in close proximity to cherty banded ferruginous beds, inclosing seams of good iron ore. Below these iron-bearing beds are light colored kaolinitic strata, which are in du'ect contact with the granite. Farther west, in Section 15 of the same town, the granite comes very close to the ore-bearing quartzite formation, in which extensive exploring pits are opened, but I had no opportunity to observe in this jjlace which sort of rock came in con- tiguity with the granite. The exjilorers informed me that diorite formed the foot- wall of the quartzites in which the ore deposits are found, and that the diorite joined the granite on the south side. Onward to the west the explorers made to me the same statements, always speaking of dioritic rocks intervening between the granite outcrops and their exploring pits. Near the Montreal river, in the NW. ^ of Sec. 27, T. 47, E. 47, 1 found granite in immediate contiguity with the ore-bearing quartzite and banded jaspery beds. In Wisconsin, above the island in the Gogogashugun river, a belt of schistose dioritic rocks intervene between the granite and the large succession of light colored slaty rocks which form the island. Above these slaty rocks are quartzite strata partly brecciated and interwoven with seams of limonitic iron ore. From here to Penokee gap I did not make any observations regarding the contact line of the granite with other rocks, but at the Gap, in the bed of the river under a railroad bridge, the dire«t superposition of crystalline limestone inclosing tremolite fibers on the granite could be seen. Above the limestone succeeded some beds of quartzite, and then a large series of light colored silicio-argillaceous schists, which most likely are identical with those composing the island in the Gogogashugun river. . . . C.IOOLlXilOAL K\l'L()i;.\TI()N'S AND LITKRATURB. 69 Dioritir ijfoup. — . . . 'I'lic cxtciisimi of I lie smvcy into Mk; (Jogcbic dis- trict siiowod to me ii ix'vl'cct :iiiiiii)j;y in tlio slnictnrc of tlic lliiroiiiiin sorios with tlic iM;ir(liU'tte or the Mt'iioniiiioc icf^ioii. Xortli of the s'm"'ti! range previously men- tioned, in many, l)iil not in ;ill, iociilitics a large body of schistose and massive dioritic rocks overlies it, dipping to the nortii, nnd forms the l)as<^ (Mi which the iion-liearing rocks repose. iMore rarely the dioritic I'ock Ix'lt is fonnd missing imd the iron rocks follow immediately above the granite. The dioritic rock grcmp tliere amounts to a considerable thickness. Most of the diorites are fine grained, and some of them very light colored, almost totally composed of granular phigioclase. Kocks of this kind are largely exposed along the north lino of Sec. 23, T. 47, E. 44, associated with singular compact rock belts of coarsely brecciated structure, composed of large and small ang-ular blocks of various kinds of diorite cemented by a seme interstitial mass very similar in composition to the inclosed dioritic fragments. In this cement numerous milky plagioelase crystals of large size, or also rounded concretionary needles of ieldspar, have segregated. (Jalcspar likewise sometimes enters freely into the composi- tion. The cementing groundmass exhibits a distinct fluidal structure, as if the rock fragments had been stin^d into it while it had the plasticity of dough. The fresh- fractured rock resend)les a compact porphyritic diorite, as the color of the rock frag- ments and the cement does not differ much, but on the weathered faces of the rock the brecciated composition and the fluidal structure of the cement become very obvious. Large bluffs of the same kind of breccia are also exposed on the side of the trail near Mr. Gillis's camp in T. 47, R. 43, along the north line of Section 23,- it forms there the foot- wall of the galena-bearing quartzite formation, whose lowest beds are a coarse conglomerate of quartz pebbles of various color. South of these outcrops a large succession of massive and schistose dioritic beds follows, then granite follows in close contact with them. Iron Ore group.— . . . The eastern portion of the range, extending from lake Gogebic to the Montreal river across the center part of the ranges 44, 45, 4G, and 47, of the Town tier 47, did not prove to be much charged with iron ore this side of Sunday lake, but west of it so far as Montreal river it was found to be richer, and in a number' of localities iron ore of a very good quality and in paying quantities is so far demonstrated to be ]n-esent; but no actual mine has yet been opened, as these remote places must first be brought into communication with the outside world by the construction of a railroad, which is surveyed but only partly built at this time. . . . The ore-bearing strata displayed in the Menominee region, on the north side of the Quinnesec ore range, are in all probability a perfect counterpart of those of the Gogebic region. Here, as well as there, a large belt of limestone forms the base of the series; the ore in both localities is to a great extent limonitic ore; in both places are graphitic schists associated with the ore deposits; and in the Penokee region 70 THE PBNOKEE IRON-BEARING SERIES. the immediate succession of the mica-schist formation above the ore formation is a further indication of the younger age of this group. ... ■ Starting from the landing on the west shore of Gogebic lake, situated in the center of Sec. 17, T. 47, R. 43, on an old Indian trail, we meet for the first two miles no rock exposures; thence repeatedly in the hillsides to the left of the path bluft's of rock are seen to project, which on examination are either granite or the brecciated diorite rock mentioned in the previous chapter; farther on qnartzite beds are seen to underlie the surface, on the right hand side of the trail, as we approach the mining camp of Mr. Gillis, situated in the SW. 4 of Sec. 14, T. 47, R. 43, where, by natural and artificial denudation, we are enabled to see a cross section of about 800 or 900 feet of strata, which dip at a high angle to the north. Tlie aforesaid brecciated dioritic schists are seen in the hillside south of the camp as the lowest rocks ; on them sncceedsa belt of dark conglomerate, composed of quartz pebbles of various color, and of granite pebbles cemented by an arenaceous gronndmass, in which is a considerable amount of feldspar grains besides the quartz sand. Then follow thick-bedded gray qnartzite layers; on them rest a flesh-red colored compact granular quartz belt, which by exposure weathers and becomes porous and absorbent, like an ordinary sandstone. Higher still, are brecciated qnartzite layers, composed partly of chalcedonic quartz masses, and intersected by irregular fissure seams filled with galena. On this brecciated qnartzite belt follow thinly lamlTiatcd quartz layers of very uneven surface, with interposed narrow wedge like seams of black shaly material, which causes rapid disintegration of this belt into shelly fragments. Within this series occur streaky, interrupted concretionary seams parallel. with the stratification, which are filled with galena. Higher beds, likewise mainly of quartzose nature, are even-bedded, delicately striped or lineated in the direction of the bedding by the alternating intermixture of linear graphitic seams, with the granular ([uartzose feldsjjathic gronndmass, which besides liolds a good proportion of the carbonates of lime and of iron. Weathered surfaces of the white and black striped rock are therefore rusty brown. In some of these "layers the shaly graphitic material predominates over the quartzose, which causes them to be softer and more pliable; the upheaving pressure therefore folded them throughout their substance into innumerable small wrinkles, as we often observe this same phenomenon of corrugation in still softer sericitic or micaceous schists in other geological horizons. North of these beds follow uniformly black fine grained slate- rock layers, which are from time to time interlaminated with seams of harder siliceous ledges, likewise black colored by carbon. The aggregate thickness of this uppermost graphitic slate-rock belt amounts to about 500 or 600 feet. It comes, on the north side, iu direct contact, with the diabases of the copper-lsearing rock group, which appears to be conformably superimposed on it. . . . The galena-bearing quartz formation and the graphitic slate series above it are traceable by extensive exposures in the hillsides along the trail until they cross the (}|'X)U)(il(;AI. lOXI'LOlIATIONS AMJ IJTEKATURE. 71 Presiitio Islo river, in SI'l. \ ol' Sw. IT, (he bt'tl of which strciiin is (sarved tiiore diajjo- iiiilly across the stratilicatioii into thi' hhwk graphitic slah's, which form here also a ver\' thick succession of lieds. A inih- lartlierwcst. in tiie SE. :^()t'.Sec. 18, T. 47, M. 4.i, I mot for the lirst time with outcrops of (lark puriile-cqlored banded (luartzite. beds, formed of alternating seams richly impregnated with specular ore grains, and others of a more purely (juartzose composition. The rock belt to which these strata belong is exi)osed in the bed of a small creek, but the exposures are too limited to otter a cross section giving informa- tion of the thickness of this belt and of tlie rock adjoiiiing it; but as the trend and dip of the beds are iu couforniity with the grai»hitic slates and the galena-bearing quartzites, it is probabU' that they belong approximately to the same geological hori- zon. West of this creek the ti-ail follows the south line of Section 18, and t'len of 13 and 14 in the adjoining township as far as the Little Presque Isle river. The quotations thus given from Dr. Roniinger under the head of the Iron Ore group, refer essentially to those slaty quartzitic and partly ferru- ginous rocks which lie to the east of the Presque Isle river and north of the o-reat area of brecciated greenstone-schist subsequently described. (See Plates II and x.) On account of the difficulties in reading the structural relations of this belt the quotations are made quite full. The remainder of Dr. Rominger's remarks under this heading consist only of unimportant descriptive details with regard to the exposm-es of ferruginous rocks between the Little Presque Isle and Montreal rivers. In addition to the foregoing, the following general remarks are woi'thy of quotation here, since they define Dr. Rominger's position as to the rela- tive ages of the granitic rocks and the iron -bearing slates which rest upon them. As already noted. Dr. Rorainger had previously published the opin- ion (Geological Survey of Michigan, vol. iv, p. 6) that the gTanitic rocks of the Upper Peninsula of Michigan are all newer than the various schistose and slaty rocks of the region into which he conceived them to have erupted — as did also Foster, Whitney, and Wadsworth before him — and to have produced the crumplings and alterations these schists now present us with. Subsequently to the publication of these views certain consider- ations were urged on Dr. Rominger by the present writer, going to show that these granitic and gneissic rocks are really older than a large portion of the schistose and slaty rocks ; for, while they have invaded a portion of 72 THE PENOKEE IRON BEAEING SEEIES. these, the}" have yet furnished to another portion an abundance of frag- mental material. The remarks quoted below are in no part a reply to these considera- tions. It Avill be seen that Dr. Rominger, while retaining his position in the main, has yet modified it so far as to accept an older granite in addition to the areas which he regards as newer tlian the schists adjacent to them. In a previous report,' which commences with a description of the geological structure of the environs of Marquette, I have spoken of the occurrence of large areas of granite some tlistance north and south of the city and of the intermediate space from 4 to 5 miles in width as being occupied by a large body of massive and schistose dioritic rocks. These in turn are succeeded upward by argillitic, chloritic, and hydromicaceous schistose layers, inclosing lenticular seams of hematitic iron ore, ■which on their part are overlain by a large quartzite formation and l)y still higher beds of siliceous limestones iuterstratified with argillitic or hydromicaceous schists of various color, some of them intensely impregnated with hematitic iron oxide. All these strata I described as being steeply upheaved in a constant axial direc- tion from east to west, and as excessively folded and corrugated, suggesting as the principal cause of these disturbances the uprising of the granite into a synclinal trough compressing the incumbent sedimentary layers. I farther stated, that particularly the lower dioritic portion of the rock beds inclosed within this trough was found intermingled with belts of granite, partly parallel to the stratiiication, partly transverse to it, from which circumstance I inferred the intrusive nature of these belts, and suggested that this intrusion occurred con- temporaneously with the upheaval of the granite into a trough, and that part of it at least must have been then in liquid or plastic condition. Generally, a solid crust of granite probably served as a substratum on which the Huronian sediments were laid down, but occasion is not often offered to see the rocks in contiguity well enough exposed to allow a discrimination as to whether such con- tact is an original primary one or resulted from subsequent dislocation. The existence of granite as a surface rock at the time the Huronian sediments- formed is proved by the occurrence of belts of granite conglomerate and breccia in different horizons of the series. A large belt of conglomerate formed of rounded, water-worn granite pebbles and schistose rock fragments, cemented by a matrix of similar schistose material is seen in contact with a granite belt in the south half of Sec. 2, T. 48, 11. 26, but this instance is not a satisfactory example of the deposition of sediments inclosing debris of the underlying rock, as the granite pebbles in the conglomerate are totally diifereut from 1 Geol. Survey of Mich., vol. iv, 1881, pp. 13-19, 22-39. (IKOLOCICAL HXI'LOUATKJNlS AND r.lTIOItATlTRE. 73 tlio iiii(U'rlyiiif;j;r;init(', wliicli is a porithyritic kind largely coiiiixtsed of iicryi)lo(Tys- tiillinc R^lsitic ^Liioimdiiiass iiurlosiiit;- (|iiai't./. f;raiiis and oitlioclasc crystals of larjjp.r si/.c. This ix'cuiiar variety orjiiaiiite is tyjtical for tlic sinallci' intrusive belts, and most likely the .i;ianit<' in this casiM-ann^ in contact with the coiiiiloineratc lielt by intiusion. Better proof tor the deposition of Ilnronian sediments on a base of t;niniteis furnished by another locality in the SH. \ of Sec. 32, T. -17, K. 2fl, where several knobs, (ientrally coini>osed of massive granite, are surrounded by a mantle of coarse granite breccia, with a well laminated quartzose material as a cement. This breccia is (lonformably succeeded by a. series of steel gray color(>d shiiung liydro-micaceons slate rocks inter- laminated with heavy belts of ligiit colored compact ipiartzite. . . . The upheaval of the granite and its intrusion into the overlying strata occurred in all probability near the termination of thenuronian period, as we And the granite in contact with any of the Huronian strata, up to the youngest, and these always in a dislocated position. . . . The dislocation of the Huronian beds is not exclusively due to the upheavaland intrusion of the granite, as numerous other intrusive rock belts, dioritic or diabasic, intersect the granite as well as the incumbent beds. Whitney (,I. D.) and Wadsworth (M. B.). The Azoic System and its Proposed Subdivisions. Bull. Mus. Oomp. Zool., Harvard Coll., whole series vol. Vii (Geological series vol. i), 1SS4, pp. 505. Part I of this work is a critical "Synopsis of the Evidence on which the Rocks of the Azoic System have been varioiisly grouped into Distinct Divisions by American Geologists," and occupies most of the volume, taking up in a geographical order all that has been written to date on the Archeau or Azoic geology of Canada and the several states of tlie United States within wliich these ancient rocks come to the surface. Part ii, which is a "R(^sum(i and General Discussion", in which the Authors offer "a brief synopsis of the conclusions at which we have arrived in the study of these older rocks." To any geologist who had concerned himself with the study of the pre-Cambrian formations in the years immediately preceding the appear- ance of this volume, it must have become very evident that sonae sort of a critical review of all that had been written with regard to these ancient rocks was quite necessary to future progress in investigation. For ^aore than one reason, what had been written prior to this time had rep.iited in the greatest of confusion. In the first place the very nature of the rocks themselves were such as to baffle most of the attempts of the older geolo- 74 THE PENOKBB lEON-BEAEING SERIES. gists at reaching an understanding of even their mineral composition. Most of those who had written upon them up to this time had been unac- quainted with the newer petrographic methods, which indeed are only recently reaching any very satisfactor}- development so far as the- study of these difficult rocks is concerned. Then again, the structural problems pre- sented by these rocks ai-e always among the most difficult the geologist has to deal with. Exposed as they have been to the disturbing forces of all the enormous lapse of ages since their first production, their occurrence in anything like their normal position is the rare exception. Faulted, folded, squeezed, internally altered in every possible way, and intruded in every fashion by every sort of eruptive material, the difficulties in the way of a correct understanding of even a small area of these ancient rocks, are often well nigh insurmountable. Certainly they are to be • overcome, only by the most minutely accurate structural work. Approaching them from different points of view, the geologist who has acquired his experience among the unaltered sediments and he whose ideas have been developed mainly among the more modern eruptives, unrestrained by the accurate knowledge obtainable only through the modern petrographic methods, and thi'ough exact structural investigation, have amved at the most opposite conclusions with regard to the structural relations and genesis of these ancient rocks. A careful sifting of all that has been written upon this subject was certainl)-^ very desirable — a sifting which should not attempt to destro}' all information gathered, but which should trj-, so far as possible, to separate what is inference only from what was a certain result of accurate observa- tion. In the case of the present work, however, it becomes very evident within the first few pages that the authors have a distinct theory to advo- cate with regard to the pre-Cambrian formations, a theory, in fact, for many years past advocated by the older of the two authors. This theory is simply that the pre-Cambrian rocks constitute a truly Azoic, confused intermingling of sedimentaries and eruptives, which, though covering in their production an immense lapse of time, are not diAasible on any . correct geological principle into chronologically distinct terranes. All evidence found that had "been jjresented in opposition to this view is criti- (lEOLOGlCAL KXl'LOliATlOISIS AND LlTKliATUJlE. 75 cised out of cxisteiicc, aiul tliis ol'teii with pungency and lack of a scientific spirit, or if it docs not \i((l(l so easily W) destructive criticisms is disinisj:ied with derision. On tlie other hand, all statements of previous authors which seem in an}' way to support tlic* \i('w aih'ocated are accepteil with a readiness which at times amounts to credulit}-. Tliat very much was found among the older writings which could be rejected goes without saying, ])articularl}' so since these older writers had not become possessed of any of the modern petrographical knowledge, aiid, having only done the best that the development of the science enabled them, ver}- frequently and naturally fell into serious mistakes. So generally, indeed, has the opposi- tion of writers to the view (jf the authors of this volume, or an occasional unavoidable mistake, been seized upon for their condemnation that nearly all geologists who had previously written upon the pre-Cambrian forma- tions, save the authors themselves, are included in the general censure. A satisfactory critical review of all that has heretofore been published on the Archean formations of America therefore remains still to be written. Unfortunately, however, it doQs not now seem probable that such a review can be prepared at an early date, for the author must not only be unbiased and read}' to go where trutli leads him, but, in ord(Jr that the review niay have any considerable value, he must have had an unusually wide experi- ence and must have the time to verify on the ground the statements of the various writers.^ The following quotations from Part i include all that has direct refer- ence to the Peuokee-Gogebic district (pp. 495-497): Passing now to the Azoic rocks of Wisconsin, we tiud tluit in 1876 Mr. E. T. Sweet pointed out a supposed unconforraability between the Laurentiau and Huroiiiau at Penokee gap, stating (Trans. Wis. Acad., vol. ill, 1S75-'7G, pp. 4'5-44): "When the railroad cut is completed at this locality the absolute junction of Laurentiau and overlying Huronian will doul)tless be exposed. There can be no doubt of the unconformability of these formations, approaching eacli other as they do with a jjersistent opposite dip aiid somewhat dift'erent strike. Unc(uiformability 'The above paragraph is left es.seiitially as it was written hy the late Dr. Irving. I know that now Dr. Watlsworfch, a.s .state geologist of Michigan, is working on the an<-ieut rocks of lake Superior in a systematic anil careful manner. This later Avork has led Jiim to modify or abandon many of liis earlier views concerning them as well as the pre-Cambriiin of other regions. — C. R. Van Hise. 76 THE PENOKEE IRON-BEAKING SERIES. has been shown to exist between the Laiirentian and Huronian in Michigan, but this is the first time it has been proved in Wisconsin." Of the same supposed unconformability at Penokee gap Prof.. R. 1). Irving remarks (Am. Jour. Sci., 3rd ser., vol. xiii, 1877, p. 308) : "The crystalline rocks of Wisconsin include unquestionably two distinct ter- ranes, the one lying unconforniably upon the other, as is beaixtifully shown at Penokee gap, on Bad river, in the lake Superior country. Here a white siliceous marble of the Hm^onian, overlaid by hundreds of feet of distinctly bedded slaty rocks and dipping northward, is to be seen within 20 feet of large ledges of dark colored amphibolic gneiss, whose bedding planes dip southward and strike in a direc- tion diagonally across that of the more northern beds. There are no doubt instances where the two series are difficult to separate, similar rocks occurring in both groups, but the existence of the two is incontestable." In the third volume of the Geology of Wisconsin (pp. , 94, 98, 108, 116, 117, 248-250) accounts of the unconformability of the Lau.rentian and Huronian are given, but the kind of contact when seen was not observed. But if the Laurentian rocks are eruptive, then of course there would be unconformability. The proof advanced was that the foliation of the granite and gneiss dipped at a different angle from that of the Huronian rocks. Here, as in the case of the Keweenaw series, tlie Wisconsin geologists failed to take into account the conditions necessary to prove their points, while Prof. Irving, without giving any evidence of value, made out a beautiful fault — on paper — at the Penokee gap. So far as can be judged from the evidence presented by these geologists, it ai)pears that they have in Wisconsin the same structure as exists in the Azoic of Michigan, namely, a series of mixed sedimentary and eruptive rocks. Prom the following extracts it will be readily seen that there are no other tlian lithological grounds for assigning these rocks to the Huronian and Laurentian; that they are two distinct formations they entirely fail to prove. . . . In 1880 Professor Irving gives as the reasons for assigning the rocks which are placed in the Laurentian in Wisconsin to that system, their " close lithological sim- ilarity— the only marked difference being the absence of crystalline limestones in the Wisconsin area — of similar structural relations to the Huronian, Keweenawan, and Lower Silurian systems, and of probable direct continuity with the Canada Lauren- tian through the upper peninsula of Michigan and underneath the waters of lake Superior." (Trans. Am. Inst. Min. Eng., vol. viii; 1880, pp. 480, 481.) Of the Huronian in the same article it is stated (p. 483) : "The rocks of this series have been called Huronian by Brooks, and, in the writer'sjudgment, correctly so, on account of their sinularity to the Canada Huronian, with which they not improbably have a direct connection underneath the Silurian of (;i:()lA)(il<'AI> KXI'LOUATIONS AM) MTKKATIIKK. 77 tho e;i.storii piii't of the pcniiisul;i, Iml nim-c especiiilly bccai.'si' Miey evidently 0(!cupy the siiiiie ^'eological interval as the ty[>ical (Jaiiadiaii .scries, exhiliitinj;' the same noii- coulorniit^y with an uiuleilyiTin- {^neissie and granitie system." It appears, then, thali tlie only evideuec that the Wisconsin f;colof;ist.s have that llie. Lanreiitiaii and lluronian are what Miey pnrjiort to be is Iitlio]o};ieal; and they have advanced no sonnd aryiimeut showing- that they form distinct ages in the Azoic system. The relation of the two snpposed series is net that which is seen when the Paleozoic comes in contact with the Azoic, or what it would be naturally were the Uuroiiian laid down on tlie i»reexistiug Laureutian. The contacts — when these con- tacts have been tigured — appear rather to be those made by eruptive rocks with prior existing ones. The geologists before mentioned have assumed, not proved, the sedimentary metamorphic origin of all the rocks in question, and on the correctness of that assumption depends their argument. They have failed to observe the phe- nomena of the contact when seen beyond the mere fact of a different dip to the foliation observed. In fact, they have failed to prove any of the points essential to establishing their conclusions. Since the questions of the unconforraabiHty and fauh at Penokee gap and of the general separability of the Penokee Iron series from the more southerly gneisses are fully discussed in subsequent pages of the present volume, it will not be needful to consider here 'at any length the criticisms above quoted. I may merely say in the first place that the Penokee fault appears to us entirely demonstrated by the facts presented on the maps and in the text of vol. iii of the Geology of Wisconsin; so that any question as to the existence of this fault becomes a question as to the presentation of facts and not as to the correctness of the conclusions drawn from them. Again, I may say that in case the stratiform arrangement of the gneiss at Penokee gap is a foliation (pressure result) its discordance with the sedi- mentation plane of the overlying slaty series is sufficiently good evidence of an unconformity; and, finally, that the general unconformable position of the Iron series of this region with regard to the more southern rocks seems completely estabhshed by the facts presented in the following pages. Irving (R. D.) and Van Hise (C. E.). On Secondary Enlargements of Mineral Fragments in certain Eocks. Bulletin of the U. S. Geol. Survey, No. 8, 1884, 5,6 pp. Among the special rocks with which the general conclusions of this pamphlet with regard to the origin of quartzite are fortified, are from the Penokee region vitreous quartzite, quartzite-schists, mica-schists, and gray- 78 THE PENOKEE IBOl^r-BEARING SERIES. wacke, the quartz fragments of these several rocks bemg shown to have received secondary enlargements in optical contiguity with the original grains, subsequently to the aggregation of the rocks. Since the publica- tion of this pamphlet, which was the first announcement of the existence of this very general and widespread mode of induration of rocks, our experience has only confirmed the conclusions presented, showing us not only these enlargements are to be widely met with in the Penokee district, but more than that, that quartzite fragmental rocks of all ages are found to be much more rarely without these enlargements than with them. 188S. WiNCHELL (N. H.). The Crystalline Rocks of the Northwest. Address before Section E., Am. Assoc. Adv. Sci., at Philadelphia, September, 1884. Proceedings, 33d Meeting, pp. 366-379. In this address Prof Winchell "calls the attention of Section E to' some of the interesting problems that beset the geologist who undertakes to study the crystalline rocks of the Northwest, and especially that part of the Northwest which is included in the state of Minnesota," and aims at a concise review of " the broad stratigraphic distinctions of the crystalline rocks that have lately been studied in Michigan, Wisconsin, and Minnesota by the aid of the published results of the surveys of Brooks, Wright, Irving, Rominger, Pumpelly, and others," who undertook to formulate a generalized statement. To this he adds also "such published results and unpublished field observations from Minnesota as may be furnished by the survey of that state, in order that the scheme may cover coiTectly the crystalline rocks of the entire Northwest." The following table indicates the six groups into which Prof Winchell would di^dde all of the rocks of the Northwest which belong below the Copper-beaiing series: (iEOLOGlCAL EXl'LOKATION.S AND LlTIOIIATIHiK. 79 Groups. Knuivalonts in MicbiKan. I'lipiivalonts in Wisconsiu. Equivalents in Minnesota. • irniiii 1, Granite anil Syiiiito with r.:it)l)n). XX 1 anil la at -lilackriver. Duhith. lirnlo Mountain. Misi|uah hills. Keavor bay. (ironp II, Mica-Schist. XIX at MnniucMr. XVII-XIX at Menomi- nee. XX-XXIIatl'<'nokec. Litth'. falls. I'ikc rapids. Outlet of V(^rmilion lake. Grou]> III, Carbouaceons and Arenaceous Black Slate. XIV-X VII at Marquette. XV and XVI at Menom- inee. VI-XVI at Penokee. Aniniikie. Black slates. Grand portage. Group IV, Hydrouiicaand Maguosiau Slate. VI-XIV at Marquette. VI-XI at Menominee. IV-VI at Penokee. At " The Mission." Vermilion lake. Verndlion Iron mines. Group V, Quartzite and Marble. V at Marquette. II-V at Menominee. I-III at Penokee. Ogishke-Muncie lake. Group VI, Granite and Gneiss with Honhlendic Gneiss. Laurcntiau. Laureutian. Laureutian. This table shows also what members of the different series that have been described as occurring in Michigan, Wisconsin and Minnesota, cor- respond with each one of these groups. It does not appear that Prof Win- chell would regard these several groups as necessarily separated by time-gaps from one another, but merely as certain lithologically distinct horizons, to which all of the rocks of the lake Superior region may be referred. In the original tabulation, of which the above is a partial copy, Prof. Winchell gives also four other columns, one of which indicates the equivalency of these six groups to the several divisions of the pre-Potsdam rocks of New England as recognized by Emmons. The other columns headed respect- ively Hunt, Brooks, and Irving, give the relations of these six groups to the divisions recognized by the geologists named. As to the relation of any of these rocks to any of the several groups recognized by Emmons and others in the East, we do not propose now to say anything, but merely to restrict oiirselves to a few remarks with regard to equivalencies indicated in the table for the lake Superior region itself To begin with, we can by no means 80 . THE PENOKEE lEON-BEAEING SREIES. accept the six groups of tlie first column even as a mere general strati- graphic succession. Group i, called " Granite and Syenite with Gabbro," shoiild rather read gabbro with some reddish acidic rocks. Here belong the enormous masses of gabbro which appear in the Bad river country of Wisconsin, and over a large area in northern Minnesota at the base of the Keweenaw series. That these rocks belong rather to the Keweenaw series itself than a separate group, is indicated both by their lithological similarity to that series, and the very striking unconformity which they present with regard to the lower slaty rocks, both in northern Minnesota and the Peuokee district. The second, or mica-schist group, is hardly a valid one unless the term mica-schist be made to include an enormous mass of rocks Avliich is not ordinarily covered by such a name. It is true that mica-bearing schistose rocks, quite different, however, from those completely crystalline mica- schists which belong at a much lower horizon in the lake Superior region, are met with in certain places in the lake Superior region at the summit of what has ordinarily been called the Huronian series ; but these micaceous schistose rocks are after all a mere phase and by no means the predominant one of a great thickness of genuine fragmental rocks, which merge, both horizontally and vertically, into those occurring farther below, called the carbonaceous and arenaceous black slate group. The fourth group, the hydromica and magnesian slate group, has, so far as we are aware, no distinct existence; certainly none at any such stratigraphical horizon as here .indicated. Hydromica-slates and magnesian slates are here and there stratified among the Iron-bearing rocks of different parts of the lake Suijerior region, but they never constituted, so far as we are aware, any continuous or well marked horizon. The fifth group, which, however, has certainly no separate and dis- tinct existence, is fau-ly well represented in the Marquette, Menominee, and Penokee districts, though Prof. Winchell's reference to this one horizon of the Huronian series of Canada has, we think, no basis. The sixth group correctly enough includes those granites and gneisses which, with certain schists, also belong throughout the lake Superior country beneath all the stratiform rocks. GEOLOGICAL EXPLORATIONS AND LITERATURE. 81 Turning' now to tlir cDliimns slmwinj;- tlid e([iiivalent.s of tliese several gi'oups in Michigan, Wisconsin, and Minnesota, \v<' note in tlie first place that Brooks's horizon No. xx, which is iiuuh' especially for the granite south of tlie Menominee rivei' in northern Wisconsin, appears to us, on the contrary, to belong- to the lowest one of Winchell's groups; in other words, to be nothing more or less than a part of the great basement series upon which the great thickness of stratiform rocks was originally spread; that numbers 1 and 1 a of the Black river succession, Wisconsin, belong simi- larly ^to this low liorizon ; that while we would put tlie Dulutli gabbro and the Brule m()untain red rocks at the horizon given (noting, however, the total dissimilarity of tliese red rocks to the granitic rocks just referred to), the reddish rocks of Beaver bay, on the other hand, are at a very much higher horizon, well u^j in the Keweenaw series; that numbers xx and xxii of the Penokee series are, as they are followed eastward from the gap, found to consist mainly of but little altered fragmental rocks, the mica- schist being in fact an alteration phase of a merely fragmental graywacke; and that the Aiiimikie series, taken as a whole, appears to us to have a thickness several times as great as indicated by Prof. Winchell, and to cor- respond to the whole thickness of those rocks, which on the south shore of lake Superior intervene between the gabbro and the Lower Huronian rather than to be equivalent to so small a portion of theni. Irving- (R. D.). Divisibility of the Arcliean iii the Northwest. Extract from Address as retiring President of the Wisconsin Academy of Sciences. Delivered Decembet 30, 1884. Published iii the Am. Jour. Sci., od ser., vol. xxix, 1885, pp. 237-249. Tliis paper presents in brief certain arguments for a belief in the divisibility of all of those rocks which in the Northwest lie beneath the base of the Keweenaw series into two wholly distinct groups, separated from one another by a great unconformity, the uppermost of the two being in its turn separated by a great discordance from the Keweenaw or copper- bearing series. These unconformities indicate the intervention between the several groups of periods during which prolonged denudation of land sur- face was being carried on. These two groups are, beginning below: (1) The great basement complex of gneiss, granite, and various schistose rocks; • MON XIX 6 82 THE PENOKEE IRON-BEARING SERIES. (2) the Iron-bearing slate series, in large measure composed of little altered fragmental rocks. The uppermost of these successions, it is main- tained, is plainly enough the equivalent in general of the original Huronian sei'ies of Canada, and should be so called. To the lower series we may for the present apply the Canadian term, Laurentian. This lower group may or may not be further divisible into subordinate numbers ; and indeed there are here and there indications of subordinate breaks in the upper series, but for the present the only distinct and widely applicable classifica- tion to be made is that above indicated. The proofs cited in favor of the conclusions presented in this paper are drawn in very large measure indeed from the Penokee district. Since the same facts are presented much more fully in the pi-esent volume, it is not desirable to repeat them in this connection with any, detail. We merely copy from the paper the tabulated statement that proofs of unconformity in the Penokee district were found. 1. lu the iniiuuei' in which the regularly succeeding belts of the higher series traverse the courses of those (jf the lower. 2. In the strong coutrnst between the two series as to rock kinds, the bedded members of the lower series having arrived at a nearly complete recrystallization, while th(jse of the higher are but little altered. 3. In the highly folded and contorted condition of the lower series, as contrasted with the unfoliled condition and simple stratigraphy of the higher. 4. In the striking contrast between the contacts of the granite with the lower schists and with the higher slates, the former being invaded by it in an intricate manner, the latter never when the two come together. (Granite iu veins and inter- secting masses occurs among the upper mica-schists of the Penokee series (see map), but this always of a different character from the granite at the southern contact, ■which has, as yet, never been found to intersect the slates.) 5. In the discordant laminations of the two sets of rocks when seen iu contact or close i3roximity. 6. In the occurrence iu the upper series, not only at horizons above the base, but also at iioints on the contact line, of abundant detrital material derived from the lower series. Irving (R. D.). Preliminary Paper on an Investigation of the Archean Forma- tions of the Northwestern States, Fifth Annual Report U. S. Geol. Survey, 1885, pp. 175-243. GKOLOGIOAI. EXPLOIIATIONH AND LITERATUEE. 83 This paper, as the title indicates, is merely preliminary to a general study ot" the pre-Kevveeuawan formations of the lake Superior country, recently begun by the writer, it g•i^■es a general outline review of what was known with regard to these formations at the time of the beginning of the investigation, and also of the new material gathered to the date of the paper by the author and his assistants. Although the Penokee series is several times referred to in this publication, there is nothing that needs especially to be quoted here. 1886. Wright (Charles E.). The Ajiogebic Iron Range. In Mineral Resources of Michigan, 1SS5, by Charles D. Lawton, Commissioner of Mineral Statistics, Lansing, 18S6, pp. 131-147. The late Mr. Wright gives a brief account of the history of exploration in the range and a detailed statement as to the amount of development at each of the mining properties at the end of the year 1885. For the most part these details have little geological interest, but the irregular character of the ore-bodies and their association with the soap rock and the under- lying quartzite are brought out. The only general points mentioned are given by the following quotations: In 1879 F. H. Brotherton, Esq., and party located, for the Canal Company, very closely, the Huronian belt across ranges 44, 45, 46, and 47, T. 47, and the ore vein within this belt; in fact all the discoveries of iron ore made in the above towns are within a hundred feet or so of the line determined by Mr. Brotherton. (P. 131.) Speaking of the Ashland mine, he says : The quartzite on the foot wall side is in places more like a hard-pressed sand bank, caused apparently from the decomposition of the matrix or cementing material of the quartzite. This is not to be wondered at, as the water forcing its way down- ward naturally follows the junction of the strata. Assures, and joints, dissolving a portion of the mineral ingredients of the rocks it traverses and again replacing it with others. It is highly probable that the purity of some of the soft hematites is due to this very iirocess, as has been noted in previous numbers of this report Many of the soft-ore veins were originally, no doubt, very siliceous, but " alkaline" water filtering through them under pressure, especially if by any means they had Itecome broken or shattered, would in time carry away the silica in solution and leave the iron oxide and other bases behind. 84 " THE PENOKEE IRON-BEARING SERIES. Further, of the Ashland mine : One interesting- feature is the presence of occasional rounded bowlders of quartzite in the ore. Whether these bowlders will disappear in depth is a problem that may throw some light on the origin of these hematite veins (pp. 141-142). Ill his account of the Noirie mine he further states : As at the Ashland mine, there are large rounded bowlders of quartzite in the ore, and the quartzite next to the ore is frequently disintegrated into common sand (p. 144). The accuracy with which the iron-bearing belt was located, by means of magnetic attractions, by the Wisconsin Greological Survey between the West Branch of the Montreal and the Montreal river, where there are no exposures, has been indicated by the fiuotations, from its reports. It appears that Mr. Brotherton, in 1879, located this belt in the same manner on the Michigan side of the line, although no report of his work has, so far as I know, been published. The suggestion which Mr. Wright made as to the ore of the Ashland mine being enrich' I by percolating waters is inter- esting as being in conformity with the conclusions which a detailed study has developed. The appearance of rounded bowlders of quartzite in the ore at the Ashland and Norrie mines appears to indicate that liere, before the beginning of the deposition of the layers of the iron-bearing formation, the nnderlying quartzite was broken by erosion. Van Hise (C. li.). Upon the Origin of the Mica-Schists and Black Mica- Slates of the Penokeo-Gogebic Iron-Bearing Series. Am. Jour. Sci., 3d series, vol. XXXI, 1880, pp. 453-459. This paper presents facts and arguments going to show that certain mica-bearing slates and schists, which in the Bad river country occur in the upper horizons of the Penokee series, are merely alteration forms of the plainly fragmental rocks which make up the greater part of these upper horizons. The process by which this alteration has been brought about, and by which in the extreme cases the original fragmental texture of the rocks has been nearly (_)r (juite obliterated, has consisted mainly in the development of muscovite and biotite from the feldspar fragments, a sepa- ration of silica taking place at the same time. Besides this main mode of (iKOI.OGlCAL KXI'LOUATIONS AND LITKItATHUK. Hi) chango, the autlior sliows tli;it the pmccss included also tlic sccoiidaiy enliirf^'enu'iit of (|ii:irtz traiiiuciits, and occasionally of feldspar fra<>ni(^nts. ^riic facts which arc sunuMarizcd in this paper arc j^'ivcn in detail in sid»se- (pient pages of the present vohnne, of wliich, in fact, the ])aper was merely an a cov cr tliis genei-al geolog-iciil intcrxal, ii;tiiLrl\-, tluit lying iH'tween tlic ( ';iinlii'i:ni iind the basement i-rystallines, t.) w liicli it is proposed that the term Archean should be resti-icted, the paper advocates the use of tlie new name A(fUoi()zoi(\ of e(|ual rank ^itli I'dlfozoir, MpsozoIc, etc. B[RiciNi5iNE (John). Tlie Irmi Ores Kast of tlie Mississi])!)! River. U. S. Geological Survey, J. W. Powell, Director. Mineral Resources of tlie United States for 1886, David T. Day, Chief of Division of Mining Statistics and Technology, pp. 39-103. ■ Mr. John Birkinbine contributes the article upon "The Iron Ores East of the Mississippi River" for this volume. In it he gives quite a full and accurate account of the development of the Gogebic range, from which the following somewhat extended extracts are made (pp. 67-72): In order of importance as shippers the various districts comprising the lake Superior region ranked in 1886 as follows: (1) Marquette; (2) Menominee; (3) Gogebic; (4) Vermilion; and this order will be maintained in 1887, with the possibility of the Gogebic and Menominee ranges changing places, but it is probable that these two districts will not vary greatly in their outputs for 1887. Geographically the Gogebic iron range may be described as running nearly parallel with the southern shore of lake Superior, and about 15 miles distant from it. The Montreal river (which is the boundary between the State of Wisconsin and the upper peninsula of Michigan), flowing northward into the lake, cuts through the range nearly midway between the extremes of the present exploitations, about one- half of the ore strike, as now believed to be determined, lying in Ontonagon county, Michigan, and the other half in Ashland county, Wisconsin. The occurrence of ores similar in character to those of the Gogebic iron range in lenses or pockets in the Marquette and Menominee ranges naturally points to like deposits in this newer district, and there seems good reason to believe that the ores lie in lenses of greater or less width and depth throughout an ore-bearing stratum confined by the quartzite hanging wall of what is believed to be the north vein, and the foot wall of what is usually known as the south vein, with a greater probability of finding the ore in this newer region, owing to the apparent persistence and regularity of the foot wall. Jiocal opinions favor, however, and not without reason, the existence of two veins, although the presence of two apparently distinct ore bodies is shown, so far, in but few instances. The belief in the existence of two veins is based upon their positions relative to the foot wall, and also to a greater percentage of mapganese in the south vein. 88 THE PBNOKEE lEOl^ir-BEAEING SEEIES. The miues alreadyopeued and worked sbow a high grade of red liematite ores, most of which are .strictly of the Bessemer class, the balance of the ore prejiared for shipment being rich in iron and close to the Bessemer limit in jdiosphorus. While some ores high in manganese are inined, none can be said to be high in phosphorus, and it is doubtful if the run of any of the developed properties would show 2 parts of phosphorus in 1,000 of iron. It may be asserted as a rule that where the ore lies in large masses but little of it will require sorting, and even in mining the jffoportion of lean ore and foreign nuvterial is iiisigniticant, except near the confining walls or where "horses" of rock occur. These "horses" are by no means uncommon, and are ftmnd in most of the mines now extensively oi>ened, but they are not a cause for discourage- ment; for already after .jiassing through a "horse" ore has been found below it, or the projection of a "horse" into the ore body has apparently forced the ore in front of it. A fact of apparent similarity to tlie oldei- i-egions is in the grouping of the large pro- ducers along a. coniparatixely limited strike, and it is i>robable that the (iogebic range will show the great jiroportioii of its future shipments made from a few large mines. . . . The appearance of tlie ores from the various mines and in some cases from the same mine differs materially both as to color and hardness. The colors are nearly black, blue black, bi'own, and almost brick red; the hardness varies from a soft, mass of finely comminuted ore to compact lumps, and occasionally grape, needle, or kidney forms, with l)rilliant surface. . . . From tlie Gogebic mines the following amounts were shipped in the first and second years of development : Production of iron ores from the leading Gogehic mines in 18S5 and 1886. 1885. 1886. Colby Tons. 84, 302 15, 419 6,471 5, 634 Tons. 257, 432 124, 844 74, 015 20, 069 94, 553 29, 184 18, 424 18, 497 17, 688 16, 388 Norrie Aurora - - - - Kiikagon .- Pabst 1,103 T*iii*itaii Total for 1886 671, 094 d Between the eastern and western extremes of ])ractical exploration upon the Gogebic iron range the distance is fully 30 miles, but the properties held as iron lands extend to the east and west of these extremes. The real work of development to date is covered by a distance of about 20 miles along the ridge. (ik()I,(h;i('al KXi'LoifATioNs AXi> ij rin; \Tiri;K. SiJ Tlic iiiosi (Mslcrii scctidii w liiclMii;i\ l)c liiirl y ciiiisidcird as open is locally kli"Wli as till' Sunday lake disd'icl, liaxiii^; Wakclicld, .Michij;iui, as its Imsiiicss (■enter. Mere sfVfral mines, notably tlie I'.idtlieiton, Sunday lake, and Iron Cliief, sliipix'd in the iifi'fiTejj'ate about .'U,(>()() tons ol'ore in lS.S(i from underijroiind workings, and as in tliis \ icinity the ridn'e is less delined and the .t;round al a lowei' le\-el, liic ex|)l()itation has been more exi)ensive and ditlienlt anrojiress less niaiked than elsewlKue. The ov(^-beariiiy roeks iii)i)arently strike M'n-oujili Sunday lake. Followinj;' \\(\st 3 miles the Black I'iver cuts tbi'ougii the ridge, and tlie oic has been found in this gap. Three miles still farther, the highest ele\ati(ni of the ridge is found (reporteil as 1,100 feet above lake Superior or 1,700 feet abo\'c tide) at the Colby mine, which overlooks the town of liessemer, Michigan, built on the northern sIojjc of the ridge. Just west of the Colby mine the strike of the ore-bearing rocks crosses a valley about half a mile wide, and then follows the ridg-e for about .^ nnles to where the main branch of the Montreal river cuts through it, and in the west(M'n half of this section are found three mines, the Aurora, the Norrie, and the Ashland, which, next to the Colby, have been the ])rincipal producers, and whose aggregate output for the year 1886 was about 320,000 tons. In addition otheis of smaller capacity are operated. After rising from the valley of the Montreal i'iver into Wisconsin the ore mines are ou elevated ground for about 3 miles, iu which there are several producers. After crossing- the west branch of the Montreal i'iver there are few more produc- ing mines, but exploitation has been carried beyond this stream for 3 miles, and farther west large syndicates o\vn land on which prosjiecting is fairly active. The Colby mine, the best known and largest developed working iu the Gogebic iron range, is at present operated under a lease which has less than two years to run. The time limit has undoubtedly encouraged the large outputs of 8sl:,302 tons in 1885, the first year of actual working, and 248,810 tons in 188C. The mine, being located at the most elevated point in the region, is reached by a switch-back railway connection which formerly ran directly into the open workings, and the ore was dug and loaded onto the cars which carried it to Ashland for shipping. While the open pitwork of the Colby and Aurora mines are the features of the Gogebic region, the deposit as found in the Norrie and Ashland mines indicates what may be considered as specially good mines, and if one-half of those already operating can reach the output of these the district will be an enormous producer. As underground workings must eventually be generally adopted, the operations of these two mines will prove a guide as to the future possibilities of others according to the width of the ore bodies. In them are also found the "horses " which add to the uncertainty of the mining enterprises. . . . TVTost of the mines that were on the shipping list in 1886 have been sinking and opening up ground for the season's work. No large deposits have been found during winter development except possibly at the Anvil and Ryan. A great many ex- plorations are being cari-ied on, but the most productive portion of the range seems 90 THE PENOKEE lEOlSf-BEAEIiSfG SERIES. to be in Michigan, from the Montreal river to the Colby open qnarrj' (11 miles). Th6 greatest depth \duch has been attained is 275 feet at the Ashland mine. The ISTorrie mine comes next, being 250 feet deep. The Ashland mine has not opened iip much ground on the fourth level, so that while it cannot be affirmed that the vein widens as it increases in depth, still the lens is wider at the third than at the second level and much longer. The third level of the Ashland showed a width in the fall of 1S86 of 145 feet when "soap rock " was struck. In May, 1887, a cut made through about 15 feet of this (the supposed hanging wall) was made, and ore again found that analyzed 63 per cent of iron and 0-011 of phos- phorus. A cross-cut 35 feet in this ore at last reports showed no signs of a hanging wall. Analyses of some of the ores of the Gogebic region. Iron King mine : North vein South vein Norrie mine : From stock pile Aurora mine : From cars Germania mine :* From stock pile No. l... Numher 2 Pabst mine (check sample) : Searle, of Hurley Haldeman, of Colby Joilet Steel Works Camp, of Pittsburg. ..... Iron. Phos- phorus. W. J. Olcott- Superior mine : 1 2. _ 3 4... Anvil mine - Eyan mine : 1 2 3 Hoppenjar mine (magnetic ore) Ashland mine (average for season's ship- ments, 48 cargoes) Per cent. 60.85 55.74 62.83 62.93 59.38 59.70 58.47 58.46 58.38 64.83 65.18 64.25 ! 59.30 j 60.00 j 61.78 j 64.15 57.67 ' 51.49 I j 64.50 Per cent. 0.027 .034 .0474 .0278 .058 .056 .040 .044 .031 .035 ft. 037 I t.036 [ .036 .047 .040 .054 .079 .035 .050 .064 .044 .0.59 .047 Silica. Per cent. 5.44 3.47 5.18 3.65 Man- Perceut. 1.30 12.28 4.00 3.88 3.73 3.65 ■ Messrs. Kerr & Olcott, analysts. t Volumetric. OEOLOmCAL EXPLORATIONS AND LITEIiATURE. 91 Tlif occiiriciicf (if iiiaiif^aiii'st' ciiii not Irorii incscnl cxiildilations he considered as lu'iiiy at all it-Kular, hut its ai)iK'ai'an<'i' in what is ('ailed the south vein is more general than in tlie ore of the so-ealled north vein. Tlie in-ojjortions of this metal vary from a traee to 33 per cent in quantities of ore, and si)ecimens of pyrolusite are found. Alumina is found in most of the ores, the am(mnt varying from 0-5 to 5 per eent, while the sulphur is from 0-()3 to -13 i)er cent. Water to the extent of 5 per cent exists in the hard ores and to a greater amount in the softer varieties. Traces of sulpliur, magnesia, and lime are also determined by analysis. The Colby mine produces an ore wliieh carries more manganese tlian most of those on the Gogebic range, the average composition of the ore as shipped being as follows : Average composition of shipments of iron ore from the CMlby mine, Gogebic range. Iron Manganese . Phosphorus Per cent. 58.5 3.5 .04 Some of the ore taken from what was termed the south vein carried as much as 33 per cent of manganese. Other analyses of the ore from this mine show the following : Additional analyses of iron ore from the Colby mine, Gogebic range. Iron Silica Phosphorus Ahimina .. . Mansauese . No. 1. Per cent. 58.67 5.87 .049 1.05 3.49 No. 2. Per cent. 59.59 6.13 .05 .51 8.45 Mr. Birkinbine naturally at this time sljared in the current opinion that there was in this region two well defined horizons at which the ore occurred in lenses. Suffice it here to say that, as explained at length later, the ore-bodies do not occur in lenticular but in peculiar shaped bodies . which have been ascertained to have remarkable relations Avith the under- lying quartzite and the associated soapstones. The only mention made in this report of the association of the ore and soap rock is an occurrence in the Ashland mine. Mr. Birkinbine notices that the manganese is upon the 92 l^HE PENOKEE IRON-BEAEmrT SERIES. whole more prevalent in the deposits near the foot-wall quartzite. This peculiarity is subsequently used in the explanation of the origin of the ores. Lawton (Charles D.). Tlie Gogebic Iron Range. Annual Report of the Com- missioner of Mineral Statistics of the State of Michigan for 1886. Lansing, 1887. pp. 125-165. This report contains a full account of the rapid development of the mines on this range during the year 1886. It describes in detail each of the important ore-producers, and also refers to the more promising prospects. Alread)' several of the mines have been developed sufficiently to show that they contain large deposits of ore. While this report is of great economic value, it adds comparatively little to the previous knowledge of the geology of the district. The fact that the dike-rocks have a tendency to undercut the ore-bodies is noticed, but the generalization as to their genetic relations was not reached. 1888. Lawton (Charles D.). The Gogebic Iron Range. Annual Report of the Com- missioner of Mineral Statistics of the State of Michigan for 1887. Lansing, 1888, pp. 90-114. Mr. Lawton's report for 1888 records the collapse which followed? the great speculative excitement of the previous year. Notvsithstanding the abrupt termination of speculation, the product of iron ore from the range is as large as in the previous jear. Detailed accounts are given of additional developments in each of the iron mines, and their condition. The fact that the dike-rocks form the northern basement of the iron-ore deposits is noted of quite a number of the mines. In the description of the Norrie mine it is said, "the foot-wall flattens out and extends a great distance north, carrying the ore-body with it." Whether this is due to a fault in the foot- wall at this point, or only represents the irregular configuration of the surface of the country at the end of the accumulation of the Quartz-slate member is not explained. BiRKiNBiNE (John). Iron Ore Mining in 1887. U. S. Geological Sm-vey, J. W. Powell, Director. Mineral Resources of the United States for 1887; David T. Day, Chief of Division of Mining Statistics and Technology, pp. 30-57. GEOLOGICAL KXl'LOIJATIONR AND LITEKATUKE. 93 The report (HI "Iron Ore Mining' in ISM?" is again by John Birkin- bine. In his account <<( tlic (lcvch)|)nient of the Gogebic range for this year he adds some points of interest to his former nccount of the region, from which the foHowing (piotiitions iu-e niiide: The Goftfbic ranyc in i\u- tliiid .><'ar of its (knelopinent outstripped the Me- nominee district by .58,001 long tons, the figures being: LoDg tons. Product of the Gogebic range in 1887 1,237 704 Product of the Menominee range in 1887 1,198 743 This is accounted for by tlie mistaken policy of over capitalization, which trans- formed tlie Gogebic range into a center for stoclv speculation, rather than for legiti- mate iron-ore mining enterprises. The result was that, with the desire to realize on the money invested, developments of some mines were made in advance of actual requirements and without studying judicious methods. Each organization 'strove to get its ore to market and to be recognized as a shipping mine, and the competition for lake freights forced them to rates ruinous to the shippers. These rates also enc(mraged all-rail shipments, and the Gogebic range in 1887 increased its output 63-0 per cent over that of 188(5, taking precedence of the Menominee range. But it is probable that in 1888 it will go behind its older rival, for the "bubble" which floated so many mining companies into prominence has collapsed and some Gogebic mines have suspended operation; the leases of others have reverted to the owners of the fee on account of defaults on royalties, and others which have been opened by imperfect methods must practically be developed anew. But the Gv>gebic range will continue as a very important factor in the lake Superior region, and will be a large producer of ores; in fact, under management which seeks to win ore cheaply and maintain the mines, the success of the district is more assured than Avhen the operations were largely so regulated as to bolster the stock shares above their intrinsic value. The four large mines, which up to the close of 1887 had produced almost 70 per cent of the ore nuned, give promise of continuing to add to the ore supply of the country and to maintain the Gogebic range as an important center of iron-ore mining. The products of these four mines in 1887 are given as follows : OtUpiil of prominent mines in Ike Goijebiv district in 1SS7. Quantity. Colby . . Norrie. . Ashland Aurora . Long Ions. 258, 518 217, 254 175, 561 159, 252 94 , THE PENOKEE IRON-BEAEING' SERIES. Concerning the iron-ore deposits of the Gogebic range, Mr. Richard A. Parker, m. e.,, indorses the opinion that there are not two veins. He says of the ore- bearing strata coafined by the red slates and jasper hanging wall that "there are not in any sense two veins; in tweaty or more miles of developmeat there are but three well established lenses of ore lying to the north of the strong foot- wall deposits, which have been called north veins, and their interrupted occurrence is so rare, compared to the continuity of more southerly deposits on the Laurentian schists, as to be scarcely suflcient to warrant the use of the significant term 'vein,' which was adopted and widely advertised by those interested in stock operations. As for manganese being made the basis of distinction between two veins, there may be instanced the continued occurrence of quite a regular percentage of it in the Kakagon and Bessemer mines, while the developed properties upon either side upon the same strike (Nimikon and Superior mines) are entirely free ft-om it." . . . Mr. Parker notes another point of interest in the frequency with which sheets of talcose matter, locally known as "soap rock," penetrate the ore bodies. At some period of development of all the large mines these sheets have been found, varying from 1 to 25 feet in thickness. One of the cleanest and most easily observed sheets is seen as the floor of the open pit at the Aurora mine. It has a pitch about the same as that of the ore lenses, but cuts across the deposit at right angles to the dip, ending when it reaches the foot-wall quartzite. It is soft, of smooth, even grain, and com- paratively free from iron or iron stain. Where the ore came in contact with it the former was decomposed for a foot or so, and the analysis showed that it contataed a higher percentage of phosphorus than usual (pp. 35-38). Mr. Birkinbine by this time clearly sees that there is not in any sense two continuous veins of ore in the region, as is shown by his quotation . from Mr. Richard A. Parker, In this the southerly deposits are explained as occurring on the Laurentian schists. This is in no case correct, the most southerly ore deposits always being found upon a fragmental quartzite which belongs to a group of rocks of which the iron-bearing member is one formation. What Mr. Parker says sbout the soapstone in the Aurora mine terminating when it reaches the foot- wall. quartzite seems to me improbable. He does not give any evidence that it does not cut the foot- wall quartzite, and, considering that it is now known that these soapstones are dike-rocks and in many cases do not cut the foot-wall quartzite, it seems probable that the same thing occurs here. Birkinbine (John). The Resources of the Lake Superior Region. Transac- tions American Institute of Mining Engineers, vol. xvi, 1888, pp. 168-203, GEOLOGICAL EXPLORATIONS AND LITERATURE. 95 This additional account by Mr. Birkinbine of tlie development of the range is accurate, and from it the following- quotations are made: If tho develoi)iii(Mit n\' t he Vcrinilinii inmuic district is startling, that of the Gojiebic iron laiige in Wisconsin ;in(l MicliigaJi is even ni(M'e so; for, although the existence of visible outcrops of ore was long known, am] considerable amounts of money were exi)ended by some of our large iroji comi)anies, no actual exploitation can be considered as having been made until the year 1885, when railroad connections were completed to Ashlaiul and dock facilities provided. During 1884 1,022 gross tons were sent from what is now the Gogebic region, but in 1885 this amount was increased enormously, and the shipments amounted to 119,766 gross tons; and in 1886, 756,281 gross tons were sent to market. . . . The development along the apparent strike of the ore covers nearly twenty miles in length, and active exploration is in progress for as great a distance both east and west of this territory, and also upon a parallel ridge 12 miles south, the latter being for magnetic ore. The center of the present developed iron-ore properties is near to the Montreal river, which forms the boundary between Wisconsin and Michigan ; the largest producers up to the present time are, however, chiefly in the State of Michigan. The ores in the Gogebic range differ from those of the Vermilion range in being softer, and therefore more easily mined, jdelding less iron and also less phosphorus, but carrying a greater percentage of manganese, and more moisture. The dip of the Vermilion ore is nearly vertical ; that of the Gogebic ores approximately 70°. The ores of the Gogebic range evidently lie in a series of lenses, often connected or in echelon, and the region has attained considerable notoriety from the fact that ore indications or actual deposits have been found u2:)on nearly every property along the apparent strike of the vein matter. The general geology of the district is explained by Mr. J. Parke Channing, of Bessemer. He says: "The chief characteristic of the Gogebic range, and that which makes it so easy to explore, is the regularity and persistency of the formation and the strongly marked character of the footwall, which dips from 45 to 70 degrees to the north, being flattest near Sunday lake. The sinking of a shaft on the North Aurora, which it is thought will reach the continuation on the Aurora vein at a depth of about 1,200 feet, will illustrate the faith in its persistency. "Their magnitude (aside from the occurrences of horses and fluctuations in their width) has been aft'ected in at least one instance by a dike which cuts clear across the vein. In a few cases dikes of this description have been sunk through, and the ore found under them. Faults of the entire formation have been suspected in one or two localities, but are not directly proven." . . . The Gogebic ores carry, on an average, about 60 per cent of iron when dry, some of the mining being above that figure. Carelessness in mining, particularly in open 96 THE PENOKEE lEOIf-BEAEING SERIES. pits, is almost invariably followed. by a decline in the iron-contents. Althougli the ore from a few mines is almost too high in phosphorns, the average product of the district is of Bessemer grade. The ore <'arries about 13 per cent of moisture in the winter, sometimes running as high as 15 per cent, while in the summer it is from 4 to 5 per cent less, averaging 7 to 10 per cent (pp. 184-187). Irving (R. D.). On the Classification of the Early Cambrian and pre-Gambrian Formations: A Brief Discussion of Principles, Illustrated by Examples Drawn Mainly from the Lake Superior Region. U. S. Geological Survey, J. W. Powell, Director. Seventh Annual Report, pp. 365-454. Ill this paper Prof. Irving gives a full account of the "Unconformities of the Penokee-Gogebic region of noi-thern Wisconsin and Michigan." The following is the first paragraph of the discussion : No other so striking example of unconformity between a series of liighly tilted but unfolded strata above the break, and a deeply folded series below, as that afforded by the Peuokee region is known to the writer. Indeed, there are in this region two notable stratigraphical breaks: one between the iron-bearing series and the folded gueissic formation to the south of it; another between the unfolded but inclined iron-bearing series and the equally higlily inclined Keweenaw series to the north. These breaks and the terranes which they separate are the counterparts of those just described as obtaining north of lake Superior (p. 423). The proof of the positions here taken are not given, as the whole question is gone over in a subsequent chapter of this monograph. Wtnchell (N. H.). The Gogebic Iron Region. In the Geological and Natural History Survey of Minnesota. Sixteenth Annual Report, for tlie year 1887. St. Paul, 1888, pp. 54-60. A cursory examination was made of some of the mines in the Gogebic iron region, in order to be able to compare the features of the rocks and the manner of occurrence of the ore with the iron-bearing rocks of the Vermilion iron range, and some rock-samples Avere collected for future lyicroscopic comparisons (p. 54^. At the Colby mine . . . there is apparent no hanging wall or footwall except the rock of the country, and that is a thin-bedded siliceous rock which itself is almost ore in some places, because of the high degree of ferruginization. This siliceous material is jasperoidal and distinctly bedded exactly like the bedding of stsdimenta- tion. The base of it all is apparently a fine "chalcedonic" silica, the same as that of the jaspilite, though in some, or many, of the beds it is a softer material, whicli may be earthy. . . , The south wall of the south Colby mine ccmsists of a crumbling, (!EOI;Or.l('AL KXIM-OKATTONS AND LITERATTTRE. 97 coarsely gra I Hilar, siliceous saiidsl one, (lie jjraiiis hcinfisubaiiguliir after disintegration. In some plafcs it is lirni and correctly hears the name of (|nartzit(^ It is stained, locally, with niucU or little iron, and witli manganese. Tlu; " sllicilicatiou" process seems here certainly to have produced fragmental silica, and sHt)se(piently, being interrupted, to have been followed by the process of ferruginization, which stained this sandstone with iron and manganese, sometimes almost constituting it an iron ore. It is uut possible to say this ferruginization is a secoiulary, or rather a third, step, later than the general ferriiginizing i)rocess, and that this (luartzitc has aeipiired the iron by reason of the accidental contiguity since, for the pit, where the iron has been worked out superficially, and timbered shafts are being prepared for deener mining. On the spur track from the Colby to the Valley mine is a short cut in siliceous greenish and yellowish slate, now largely ferruginous. This is sometimes earthy, like some seen at the Colby mine, but it is very siliceous with "chalcedonic" silica, some of it being almost Avholly white, although the prevalence of hydro-mica and perhaps of other flue grained mineral particles intimately disseminated through the siliceous parts gives a greenish color to even the hardest and most quartzose parts of this rock. This dips so as to conform to the rock in the Colby mine, and probably lies under the foot wall of the South Colby. The full thickness of the bedding here involved, making allowance for the oblique direction in which the road goes across it, is about 250 feet (pp. 54-55). At the Valley mine . . . in a ditch beside the railroad track at Bessemer, is a small exposure of tlie bottom conglomerate of the Cupriferous. It contains numerous pebbles of quartz and of fine porphyry like the copper -bearing conglomerate at the Calumet and Hecla mine. The dip can not be made out, as the exposure is small and the pebbles are rather evenly distributed throughout the whole. This conglomerate must lie in unconformity on the Gogebic iron-bearing rocks, and if it be the equivalent of the conglomerate at Negaunee (Cascade), as it appears to be, it indicates the greater age of the Hurouian quartzite (the New York Potsdam sandstone) than the Cupriierous formation, i. e., than the greater ])art of the Cupriferous, since the Cupriferous strikes east and west near Bessemer, forming a conspicuous range of hills, of eruptive rock but about a mile farther north, the dip being such as to cause it to overlie this con- glomerate. The Aurora mine, at Ironwood, has a sandstone or a soft quartzite, rather coarse, identical with that on the south wall of the South Colby mine, for its south or foot wall. HON XIX 7 98 THE PENOKEE lEOJSrBEARING SERIES. At the Aurora mine au interesting observation was made oii the "granite" which forms a low hill range a short distance south of the mine, and its manner of contact with the rocks lying just north. The low granite range, Avhich has been mapped as Laurentian by the Wisconsin geologists, rises about 50 feet above the mine and lies south from the mine about 600 feet. The section of strata intervening between the mine and the granite range is made up about as follows, in southward (and descending) order: 1. Iron ore, soft hematite, 100 to 150 feet. 2. Sandstone (sometimes a quartzite), about 15 feet seen. 3. Gray and greenish slates and quartzites, in beds from half an inch to 4 and 6 inches ; distinctly sedimentary, 580 feet. i. Gray quartzite like that of No. 3. 5. Granite, hornblendic and massive. The sandstone (No. 1) forms the south wall of the Aurora mine. The quartzites and slates (No. 2) are not all exposed at the mine, but at the Colby mine a section of 250 tVet, iu a connected exposure, can be seen along the spur track west of the niine. About 100 feet of similar strata are visible at the Aurora mine. It is partly assumed, therefore, that the whole interval of 580 feet consists of the same rock as seen at the northern and southern limits. It seems to be a part of the Animike slates and quartzites (pp. 55-56). The hornblende-granite contains bowlder forms of different rock from the mass of the granite, some of them being of some dark-coh)red greenstone-like rock and others of some earlier granitic I'ock. The great mass of the granite is mainly liomo- geneous, and these bowlder forms appear most distinctly on the weathered surface of the bluff. When these bowlder masses were not originally of greenstone they are apparent by a blotched aspect which the granite presents, the blotches being caused by some patches of rounded outline, much liner grained tlian the i.est, or by a marked difference iu the relative amounts of feldspar and ipiartz compared to the same min- erals in the most of the granite. The bowlder forms are, when distinguishable, from two or thi'ce inches in diameter to twelve incites. Their longer axes lie prevailingly in the same direction, but sometimes they have lost their lirst shape and have been drawn out into jjoints in one or more directions or have been a little distorted by unequal i^ressure.' This granite also has a uniform rift or grain, brought out by the constant or prevalent elongation of the hornblende crystals in the direction about east and west. The bowlders have their longer axes in the same ixjsition. It ajipears, therefore, that originally this granite was some conglomerate, which has been plasticor fluid-like. At the most southerly of the three low bluffs of this granite, each of which faces ' Similar changes from conglomerates to gneiss aro mentioned by Dr. E. Hitchcock in Vermont, in Am. Jour, Sci., 2d ser., vol. xxxi, p. 372. GEOLOGICAL lOXl'LOltATIONB AND LITERATURE. 99 toward (lie noidi. (lie jrniiiilc ciiii hv. seen, on tli.' north side, to lie upon, i»r at least ale ascertained. The third low outcroj) of tlie f^ianite is about 10 feet lower than the last and about 30 feet farther north. In the north face of this little bluff, not more than'6 feet hiyh, is observable the contact seen in Fi«-. 2 of Pig. S. Here the grainte is uncon- formable on and embraces i)icces and tongues and slabs of the gray quartzite and quartzite slates. This (luartzite slate is line grained, banded by sedimentation, and light greenish weathering. It is greenish gray within. At one point in this little bluff is a small area of granite surrounded by crumpled and broken portions of the slate. So at least it appears on the face of the bluff, but this isolated granite area is only superficial and doubtless was, and perhaps is still, united with the main granite mass. These three little bluffs, running, so far as they have apparent extension, in the same direction, do not conform in their trend with the direction of strike of the bedded rocks that intervene betweeu them aud the Aurora mine. They vary from it about 20°, as illustrated in Fig. 1 of Fig. 8. The interpretation of these facts and their bearing on the stratigraphic problems that relate to the horizon of the iron ore of the Gogebic range seem to warrant tlie following conclusions : (1) The granite acts the role of an eruptive rock, but was originally a conglomer- ate. It was so far molten or plastic that it flowed over the adjoining sedimentary strata, but not so completely fused as tg render the resultant grauite entirely homo- geneous. (2) The accompanying beds of sedimentary rock being a perfect lithologic repre- sentative of the quartzites and slates of the lower part of the Auimike, this con- glomerate can be parallelized, stratigraphically, with the Ogishke conglomerate of Minnesota, in which have been seen (Fifteenth Annual Report aud later in this report) similar semifuf.ed conditions, producing porphyries, syenite, and porphyritic conglomerates. (3) The horizon of the ore of the Gogebic range is probably that of the Animike rocks. (4) The granite is' not of Laurentian age, but is younger in its present condition than the Animike slates, though (jriginally a conglomerate older than those slates (pp. 56-59). WiNCHELL (Alex.). The Gogebic Iron Belt. In the Geological and Natural History Survey of Miuuesota; Sixteenth Annual Report, for the year 1887; St. Paul, 1888, pp. 185-195. 100 THE PENOKEE IRON-BEAEING SERIES. At the south vein of the Colby mine the ore is limited on the south by a com- pact hematitic slate, which is light-colored and sandstone-like in places. South of the mine is an outcrop of siliceous argillite, which is interbedded with quartzite- scliist. By the railroad near Bessemer, coming from the Valley mine, is an outcrop of conglomerate, with slight dip north. It is a small exiiosiu-e, but in place, com])osed mostly of pebbles of dark red sandstone, hematite, granulite, and diabase. It appears like a conglomerate at the base of the Keweenian (p. 186). The section at the Aurora mine, in descending order, is: Broken and mixed ore. Main deposit of ore dipping north at an angle of about 65o. Quartzite forming the foot wall in the Colby and Aurora mines. Siliceous argillite south of Colby mine — seen 250 feet; this perhaps occupies part of the concealed space south of the Aurora. Quartzite and syenite gneiss interbedded in the hill 633 feet south of the north face of the quartzite in the Aurora; thickness, 595 feet. Syenite gneiss (502) on the hill south of the Aurora. This syenite is a heavy outcro]). It contains some fragments, mostly of _green- stone, and only partly rounded. It reminds me of the Seagull and Saganaga regions, Minnesota. The rock weathers light colored. It is granular and porous, varying to compact. The feldspar is very pale pinki.sh, and the hornblende is grayish greenish. . It presents all the characters of true syenite, showing no trace of bedding within the area subject to observation. But it is evidently a fragmental rock, since it contains many rolled fragments. It furnishes us ocular evidence that real syenite, with all its proper crystalline characters, m^y be a rock of sedimentary origin. Looking around, we discover other evidence of its close affinity with ])roducts of sedimentary action. Close by, it overlies a true, fine-grained quartzite (503). The contact is apparent in several places. Four rods south of this syenite, a considerable mass of quartzite is imbedded in syenite. Down the hill a few rods farther, we find a vertical ledge of gneiss including warped and broken sheets of a siliceous schist (pp. 186-187). The succession in the vicinity of Penokee gap is given. Here at the south is found hornblende-schist and similar rocks 988 feet in thickness, which dip to the south, and then the Penokee series with 3,480 feet of strata dipping to the north, including, from south to north, viti'eous quartzite, siliceous slates, magnetitic slates, and argillites. The firmly accepted conclusion of the Wisconsin geologists in reference to the equivalency of the magnetitic and carbonaceous slates of the Penokee gap makes them a continuation of the hematitic schists of the Gogebic range. That is, they hold the formation in both regions to be Huronian. That the Gogebic iron-bearing strata are not Huronian, I feel prepared to afi&rm. And I can not resist serious doubts of (;E()L()(il('AL KXI'LOKATIONS AND LITERATURE. 101 tlioir (Miuivalt'iwo with the Pciiokcc strata, whicli stroiio-ly impross me as holding characters strikingly siniiliar to those of the Ilnronian slates of lake irnron and the Aniniike slates of (Jnnllint lake and Thnnder bay. This resemblance impressed me from the besinninfi'; bn( I feci rclnctant to controvert the judgment of the Wisconsin geologists. But while I hold the decision in abeyiince, I take the lilterty to offer a few points for consideration : (1) We discover the strong litbological resemblance referred to. (2) The litbological characters are unlike those of the ore-bearing strata of the Gogebic, Manjuette, and Vermilion regions. (3) The ore also is magnetic instead of hematitic. (4) It is diffused through the laminated sheets of the formation, as at Guuflint lake, and not segregated in lodes, as in the other regions mentioned. (5) A higher system of black slates, apparently unconformable 07i the hematitic schists, appears to exist in the Gogebic and Marquette regions, as it certainly does in the eastward prolongation of the Vermilion schists. (6) At the distance of 15 to 18 miles in a direct line SSB. of the Gogebic range, is a well known line of magnetic attractions, such as are exerted by the magnetitie schists of Penokee gap. » (7) These lines of attraction, though as far as I know they lie too far south, may nevertheless, when more accurately located, be found in the strike of the Penokee schists, especially if'the great exposure a mile north of Penokee station affords a reliable indication of the strike; for that is S. 67° E. But this is probably disturbed somewhat by the great fault. Should the Penokee slates be identified with the Animike (true Huronian) then the Marquette or Kewatin system will be found underlying, and the juxtaposition of the Penokee slates with the (sup]iosed) Laurentian schists on the south, may be due to au overslide accompanying the formation of the great fault. . If the dislocation resulted from a horizontal movement, instead of an upthrow, then the strata on the east side must have slijjped southward over 900 feet; and, if the movement was con- fined to the Penokee slates, they may thus have concealed a thin representation of the Kewatin which, on the identification assumed, is at present, wanting at this point (pp. 194-195). Our own observations as to the occurrences of the rocks south of the Aurora mine differ radically from those of the Professors Winchell. The granite south of the Aurora, described by N. H. Winchell as consist- ing of three parallel knobs interstratified with quartzite, was found to be granite cut by fine grained dike-rocks. Winchell's quartzite-slate is then a dike which cuts the granite. These dikes are separated from the gran- 102 THE PENOKEB lEON-BEAEING SEEIES. ite by perfectly sharp lines, and their intrusive character is unmistakable. The granite is described as at once an eruptive rock and a conglomerate. This description is doubtless due to the fact that in the granite are dark colored obscure roundish areas which have a pseudo-fragmental appear- ance. These areas, composed largely of dark-colored minerals, are very common in the ancient granites of the Northwest and may be either actual fragments of the more ancient crystalline schists which have been caught in the granite at the time of its eruption and only partially absorbed, or they may represent segregations. The north face of the granite ledge is brecciated to a certain degree and its joints are filled with cherty and sili- ceous slates belonging to the base of the Penokee series. The granite surface is plainly one of erosion, and there is absolutely no evidence that the granite has been plastic subsequent to the formation of the slate. The phenomena are precisely those that are always present when a frac- tured and jointed granite constitutes the surface upon which a fragmental slate formation begins to be deposite.d. If the above observations are correct, Prof. N. H. Winchell's conclu- sions that the granite acts the role of an eruptive rock, but was originally a conglomerate which flowed over the sedimentary strata, and that the granite is not of Laurentian age but is younger than the Animikie slates, are erroneous. Van Hisb (C. E.). The Iron Ores of the Penokee-Gogebic Series of Michigan and Wisconsin. Am. Jour. Sci., 3d series, 1889, vol. xxxvii, pp. 32-48. This article is a condensed advance account of what is contained in the present volume with reference to the position of the ore-bodies in the Iron-bearing member and their genesis. As the subject is treated fully in a subsequent chapter, an abstract of this paper will not here be given. CHAPTER TI By C. 11. Van Hise. THE SOUTHERN COMPLEX. General. Geographical Distribution. The Westeru granite. The Western green schist. The Central granite. The Eastern green schist. The Eastern granite. Summary. General — The rocks south of the Penokee series (PI. IT) are exceed- ingly complex, both as to theii- lithological character and structural relations. They comprise, first, unmistakable eruptives, fresh and in vai-ious stages of alteration, including diabases, syenites, gneissoid granites, granites; and, second, many different vai'ieties of gneisses and schists. There are large areas which contain only massive rocks, and other large areas which con- tain only schistose rocks, except for infrequent cutting basic eiiiptives; but between the different areas are zones in which are found mingled massive and schistose kinds and apparent gradations between the two. The folia- tions of the schists vary widely in dip and strike. More often than other- wise they are in rough conformity with the members of the overlying Penokee series; but this is true only in a very general way, the strike frequently being almost or quite at riglit angles to the strikes of those rocks. Further, the strikes vary widely within short distances, presenting a strong contrast in this particular to the rocks to the north. If this variation in strike is noticeable, the variation in dip is still more remarkable, inclinations in opposite directions frequently occurring within a short distance of each other. These abrupt changes in strike and dip clearly indicate that the series is one which has been closely crumpled. This report is not primarily designed to cover the complex basement series. What follows' is therefore of an incomplete and somewhat general 103 104 THE SOUTHERN COMPLEX. character. It is chiefly a Hthological treatment, and even as such is but an outline, the only reason for entering into the subject at all being to give a basis for comparison with the rocks of the Penokee series. Beginning at the west, each of the areas is taken up in order and the rocks contained briefly described. Geographical distribution (PI. II). — In Sec. 20, T. 43 N., R. 7 W., Wisconsin, as discovered by Mr. Charles E. Wright, late state geologist of Michigan, the rocks of the Copper-bearing series and a gneissoid granite belonging to the complex system under consideration, are found upon opposite banks of the Numakagon river. To the west of this point no rocks, aside from these, are known for a considerable distance; so that it may be considered that here is the westernmost point at which exposures belonging to the Penokee series proper will be foimd. The next exposures in the Southern Comjjlex east of this locality are in Sees. 21 and 22, T. 44 N., R. 5 W., Wisconsin, this rock being a gneissoid granite. From here eastward to Bad river, in Sees. 14 and 23, T. 44 N., R. 3 W., Wisconsin, occasional outcrops of gneissoid granites are found. The Western granite then is one which extends east and west for more than 25 miles, and north of which is found the Penokee series. It may be that within this area rocks other than the gneissoid g-ranites occur ; for the known exposures are comparatively few, and if within it softer schistose rocks were contained, it is probable that they wovild not be exposed. At Bad river, veiy close to the Penokee series, is a schistose rock. The fine-grained crystalline schists which here first appear continue without a break, so far as known, to the West Branch of the Montreal i-iver, in the northwest part of Sec. 3.5, T. 46 N., R. 2 E., Wisconsin, another stretch of more than 25 miles. As in the gneissoid granite area to the westward, the exposures are very few, but in at least two places, at the Potato river and the West Branch of the Montreal, the basement hxyer of the Penokee series is seen in contact with the schists to the south. How far the green schists of this area extend south of the Iron-bearing series is not known, but the traverses have in places extended as far as two miles. At Penokee gap and eastward for some distance, a little way south of the Iron-bearing series, the granites and schists are found in the immediate vicinity of each other, the line TUK SOUTIIEKN COMPLEX. 105 separating- them ^-i-adually swinging- away from the I'enokee rocks, leaA'ing a wedge of gi-ecii scliist between the latter and th(( gneissoid granites. Tlie eastern end of tiiis green selii.st area also nms to a sliar]) ])oint, the granite being found south of it live or six miles west of the point at which it reaches the Penokee rocks. The distribution of the granite east and west of the schists suggests that the two areas are connected. If this is the case, the Western green schist is an isolated area bounded on the south by tlie granite and on the north by the Penokee series. The second granitic area extends immediately south of the Penokee series, from Sec. 35, T. 46 N., R. 2 E., Wisconsin, to about the middle of T. 47 N., R. 46 W., Michigan. At the latter point the schists again appear, and like those to the westward, widen out in a wedge-shaped area, leaving the granite as the southernmost laiown rock for quite a distance south of the second schist area. This granite area has many exposures in such close proximity to those belonging to the Iron-bearing series for the distance mentioned as to leave no doubt that it is here the basement upon which the former rests. While it is called a granite area, it contains large masses of rock of a more basic character, including both syenites and altered gabbros, the relations of which to the granites will be referred to subsequently. East of this Central granite follows another schist area, running from near the middle of T. 47 N., R. 46 W., Michigan, to within a mile and a half of the east side of T. 47 N., R. 43 W., Michigan. The schists are in places nearly three miles wide, but throughout the entire distance, with the exception of two or three miles in the east half of T. 46 N., R. 44 W., Mich- ig-an, g-ranitic I'ocks are known to lie to the south. East of this Eastern green schist again appear granites and gneissoid granites which south of the Penokee rocks extend to Gogebic lake. What has been said about the connection of the Central granite and Western granite applies even more strongly to the Central and Eastern granites, which would perhaps have been connected if the country had been traversed a little farther to the south. The boundaries between the Eastern g-reen schist and the adjacent granites are of a most irregular nature, and quite often in the same exposure both granitic and schistose rocks are found. The significauce of these facts will be discussed later. 106 THE PENOKEB IRON-BEARING SERIES. _ The Western granite.— The granites and gneissoid granites of the western area are classed together, as they differ but little from each other. The gneissoid granites vary in character from those so coarsely schistose as to be with difficulty distinguished from the granites to those which are extremely contorted and quite finely foliated. Although the area is large, the rocks included within it are petrographically of essentially the same char- acter. The massive phases are relatively infrequent, and are medium grained granites. Those phases which are most foliated are somewhat finer grained. "The rocks of the area vary from almost white to quite a deep red, depending upon the coloration of the feldspar, which mineral is the preponderating constituent, the iron-bearing silicates and quartz being usually in subordinate quantities, but not infrequently plentiful enough to give the rocks a strongly mottled appearance or even a dark gray color. The normal rocks of the area (PL xiv, Fig. 1), when examined under the microscope, are seen to be composed very largely of alkaline feldspars, including the species orthoclase, microcline, and acid plagioclase. The feldspar is usually in indi^^duals of nearly uniform size which show more or less perfect idiomorphic outlines. With most individuals the average greatest dimensions are less than J mm., although occasionally individuals are found which run up to 4 mm. Frequentl)^ the feldspars interlock with each other, in which case the crystal outlines are more or less broken, although the regular forms are to some extent maintained. The only important iron-bearing silicates present are biotite and chlorite. These minerals occur at times included within the feldspars, but more frequently are found in the spaces between them. The chlorite often occurs in well defined blades having a rectangular extinction, and appears at times to be secondary to biotite, although often it is in part secondary to feldspar. Occasionally a little muscovite is found included within the feldspar. Quartz has been the last mineral to crystallize and has filled up the spaces left by those previously mentioned. It occurs in small rovmdish or irregular areas between the feldspars, giving the rock a completely interlocking crystalline character. In one or two cases the quartz and adjacent feldsjmr have a pegmatitic structure. This may signify that the quartz and feldspar in the final stages of crystallization were forming simultaneously, or that THE SOUTHERN COMl'LEX. 107 this is secondary saturatino- quartz. Tf tlu' lattor is truo it becomes not impossible, altlinii<.li hardly probable, that tluf independent (piartz in the interstices of the feldspar is a secondary material, in which case the original rock would have been a syenite. All of the minerals are more or less altered. A kaolinitic decom- position has widely affected the feldspar. Less frequently chlorite and mica have developed within it, and the chlorite has further partly changed to epidote. Jiut the most interesting decomposition shown by the feldspar is an alteration into quartz and biotite. This alteration has occurred to some extent in quite a number of the rocks, and is very clearly shown by the gneiss a short distance west of the south quarter post of Sec. 23, T. 44 N., R. 5 W., Wisconsin (PL xiv, Fig. 2) on Marengo river. Here the large individuals of feldspar have each decomposed in a great measure to the more basic mineral biotite, the excess of silica apparently separating as finely crystalline quartz, so that a single feldspar contains many score folia of biotite and grains of quartz. In certain pai'ts of the section this decompo- sition has gone on until little or no feldspar remains, the result being a finely crystalline interlocking aggregate of quartz and biotite in place of a single grain of feldspar. In other words, a somewhat coarsely crystalline strong'ly feldspathic rock — the normal phase of granite — has changed to a finely crystalline gneissoid biotitic qfuartz rock. It is interesting to note in this connection that these are the identical changes which have, in the upper belt of the Penokee series, changed a feldspathic fragmental rock to a crystalline mica-schist. The Western green schist. — Tlie exposures within this area are com- paratively few in number, particularly in its western half The rocks are quite finely laminated, dark colored, fine grained, crystalline schists. They are, then, in strong contrast to the coarse grained granites and gneis- soid granites to the westward. In different parts of the area they have very different characters, so that they can not be described together. At Bad river the schistose rocks are technically all gneisses. They have a strike which is nearly conformable to the strike of the overling Penokee series ; but their dip is south instead of north, and hence they 108 THE PENOKEB IRON-BE AEING SERIES. are not in conformity with that series.^ The chief constituents of the gneisses are feldspar, quartz, hornblende, biotite and quite frequently mag-netite, epidote being an accessory. In certain of the specimens bio- tite is absent and in others hornblende. The relations of the minerals are those ordinarily found in typical crystalline schists, which give little or no indication of their original condition. Some of the giieisses, however, show that the biotite has in a large measure been derived from feldspar. The alteration of each grain of the latter mineral to many folia of mica, with the simultaneous separation of quartz, presents exactly the appearance sub- sequently described (chapter vi) . as characteristic of the fragmental feld- spar rocks of the Upper slate belt, which there carried to the extreme has resulted in forming from a fragmental rock a completely crystalline mica- schist. There is this difference, however : In the Upper slate the crys- talline schist was traced back into an unmistakable fragmental rock, while here the origin of the rock is unknown. In these gneisses much of the quartz and feldspar has a ge^ieral roundish appearance, which suggests a fragmental origin. The hornblende-gneisses, upon the other hand, associ- ated with these biotite-gneisses contain comparatively little or no quartz, a good deal of hornblende, and plentiful magnetite. The last are, then, rocks which are presumably schistose eruptives." Between Bad river and Potato river the country south of ttie Penokee series is low, swampy, and almost without exposure, so that little is known of the character of the rocks. One exposure of an obscure chlorite-gneiss is found in Sec. 5, T. 44 N., R. 1 W., Wisconsin, the onl}^ 'noticeable thing about which is the occurrence of a considerable quantity of tourmaline, a rather uncommon accessory in the crystalline schists of this district. At Potato river, in contact with and just south of the Penokee rocks, large exposures of the underlying schistose rocks are found. They are here fine grained; some of them are nearly massive and others highly schistose. They are seen to contain, microscopically, many small grains or crystals of feldspar, roundish areas of chlorite, and sometimes. large areas ' For iletaileil descriptions of the exposures here see Geol. Wis., 1880, vol. in, pp. 93-96, aud p. 224. 2 A. Gelkie: Recent work of the Geological Survey in the Northwest Highlands of Scotland. Quart. Jour. Geol. Soc. Loudon, vol. xliv, 1888, pp. 387-399. Tin-: souTiiKiiX ('oimpi.fa'. 109 of calcite. They are of ;i mottlcil dark ynnMi color, and ard more hig'hly altered near (lie jiiucliou with the overlying' rocks than t'artlier to tlie soutli- ward. They can hardly he said to lia\'e any true strike and dip, hut the fibers of the schist abut almost perpendicularly a a. The chlorite of the rocks, where it is found instead of the hornblende, bears the same relation to the feldspar that the horn- blende does. In some of the sections the chlorite is in part secondary to hornblende and it may all be of this derivation. • Constituting an exception to the granites just described is an exposure of microgranite found in the NE. i of Sec. 27, T. 47 N., R. 47 W., Michigan. Macroscopically it has an aphanitic background which contains simple and complex areas of varying sizes, consisting of coarse individuals of feldspar, a dark colored mineral, and quartz. In other words the background in- cludes many areas which have a ti'ue granitic texture. These complex areas of greatly varying sizes are included in the most irregular fashion. More abundant than these are single porphyritic crystals. In. thin section the background is very fine grained, and apparently consists of intricately inter- locking quartz and feldspar. As indicated by the hand specimen it contains numerous roundish areas consisting of large individuals of quartz and feld- spar, with quite a quantity of chlorite, each mineral occurring separately and together in complex, intricately interlocking areas. Quite a number of the single individuals of quartz show plainly an interrupted growth, which gives each grain an appearance like that of enlarged quartzes met with in fragmental rocks. Here this doubtless means that the cores belong to a first generation and the enlargements to a second generation of crystals. The porphyritic feldspars include orthoclase, microcline, and plagioclase. This rock difiPers from any which I remember to have seen. In most cases in which crystallization has thus occurred in two generations only detached porphyritic crystals are contained in a finer grained, matrix. Here, how- THE SUUTHEHN COMPLEX. 113 ever, larC> «• Differing from these hornblende-syenites is a mica-pyroxeue-syenite, the pyroxene iu this case replacing for the most part the hornblende. This rock is like the other syenites in all other THE SOUTHERN COMPLEX. 115 points, and the relati 1)1 IS nrtlH' iiiic;i ;iii(l pyroxene tn tlie f'eklspar are the same as those of llic liiiriil)l(Ui(le antl biotite in the niicii-lioi-iihlende-sye- nites. This rock also carries some liornblende, hut in siihonUnate qun-ntity. In some cases the biotite seems to be forming' as a secomlar)^ pnnhict from it. The pyi'o.\ene is coh)rless, stronj>'ly (h»ubly refracting-, ;ui(l from its oj)- tical properties is taken to be malacohte. It is (generally in ronnihsh gran- ides in cross-section, and in hhides several times h)ng'er than broad in lon- gitudinal section. Many of the biotite blades are nmch distorted. All of the syenites contain very numerous, large, well formiul crystals of apatite, which are included in all of the other minerals. Not very fixr in basicity from these syenites are coarse, altered gabbros, winch in large exposures are closely associated with them. In the held and in hand specimen they do not dififer greatly in appeai-ance from some of the syenites just described. Their feldspars have a reddish cast, as in them, and their structure is much the same. However, when they are examined in thin section they are seen at once to be altered Ijasic eruptive rocks of the ordinary type. The chief original constituents are plagio- clase, magnetite, anddiallage, which have relations characteristic in a rcjck in- termediate in structure between a diabase and a gabbro. The plagioclase has altered very extensively to chlorite and kaolin, and is in many places replaced to some extent by saturating quartz. The magnetite occurs in well defined areas and crystals, which give no evidence of alteration, it being the only min- eral within the rock which is unaffected. The pyj-oxene has very largely decomposed, the resultant jjroducts being hornblende, biotite, and chlorite, more largely the first. It is a very noticeable thing that the secondary horn- blende is not paramorphic but several or many blades have formed which are in crystallographic position independent of the original augites, and much of this hornblende in its forms is like the hornblende contained in the syenites just described. As a result of the comjdete decomposition of the diallage, hornblende and biotite are formed with their axes arranged quite often in a common direction. In this case these minerals, in their appear- ance and relations, are much like those of the same minerals in the associ- ated syenites. It is further noticeable that these rocks also contain numer- ous large, well formed crystals of apatite. Considering all of the foregoing 116 THE PEWOKEE lEOX-BEAEING SEEIES. facts, it is quite probable that the gabbros, syenites, syenite-scliists, and possibly the granites are parts of the same continuous rock-mass. The Eastern cjrecn schist. — The shape of this area will be seen by examining PI. ii. The exposures contained are very numerous, and the rocks within a certain narrow range have great variety. While quite large subareas contain only a single phase of rock, they grade into other areas containing rocks of a different character, and even a single exposure fre- quently contains quite dissimilar rocks. This confusion is so great that no attempt will be made to subdivide the area into smaller ones, each contain- ing a detinite phase of rock, nor will any attempt be made to explain the field relations of the different })hases. The rocks here included may all be technically classed under one general term — gneiss — although perhaps some of them might better be called syenitic schists and most are fine grained green, crystalline schistsi In these gneisses, aside from the quartz and feld- spar, any one of the minerals hornblende, biotite, chlorite, sericite, or ejiidote may be the additional chief constituent, ^diile often two or more are equally abundant. Consequently the area contains hornblende-gneiss, biotite-gneiss, sericite-gneiss, chlorite-gneiss, epidote-gneiss, and various intermediate phases. The rocks vary greatly in coarseness, running from those that are so exceedingly fine grained that it is with extreme difficulty that the minerals are determined, to rocks which approach a granitoid gneiss. It is noticeable that the coarser grained phases are mostly found near the granitoid gneiss areas. In fact the line forming the boundary between this schist area and the surrounding granite and gneissoid granite areas is arbitrarily drawn. As the granite area is neared the schists become coarser and coarser, grading into a coarse gneiss. The change is so gradual at the east end of the area that tlie boundary between the gneisses and granites would probably not be put by another observer in the same place as mapped. The principle followed in our mapping is to include within the schist area "all rocks which in hand specimen do not have a strong granitic appearance. While there is this change, it is not a passage which is made out in any case by actually tracing a finely schistose rock-mass in contin- uous exposure into a granite, but the various phases are presented by numerous detached exposures. xVside from this apparent passage, the fiiae TIIK S()IT[Ii:itN CO.MI'lilOX. 117 grained scliists adjaccut to tlic ;4raiiit(' area not iitilr('(|ii('iitl\- contain massive granite. This uranitc is somctinics t'ouiid as a boss upon wliicli tlio sirliist . rests, liut more ot'tcii <'onstitutos one part ol'an ('Xjxisin'c, the sdiists cut )»y it coniposiiiij;' the other Dart (Fig". 3). lu sonic phiccs the niannei' in whicli FiQ. 3.— Schist cut by massive gi-.inite, NAV. ?„ Sec, 4, T. 10 N., li. 2 E., "W'isennsin. the granites cnt the schists is sucli as to leave no (h)ul)t of the eruptive character of the latter (Fig. 4). Schist. ' "■'- ':^ --■-- Fio. 4.— Schist cut by granite, NB. cor. Sec. 28, T. 47 N., E. 40 W., Micliigan. Macroscopically the gneisses of the area are light gray to very dark green. They vary in coarseness from aphanitic to the texture of a gneissoicl granite. In all of the rocks a schistose structure is very strongly developed. In the coarser gneisses the foliation is marked in liand specimen and in exposure, but in the finer grained varieties it is often onlj^ exhibited by a readier cleavage in one direction. The different phases described under the microscope differ from each otlier but little macroscopically. tSome of the syenite-schists are an exception to this statement in that they approach in structure to a basic eruptive. One of the most characteristic and abundant phases of schist, as seen in thin section (PI. xv, Fig. 3), is that which has a white granular background composed of quartz and feldspar. The proportions of these two minerals vary widely in the different specimens, running from those in which quartz is preponderant to those in which the feldspar almost entirely excludes that mineral. The individuals of each of these minerals in this backscround are 118 THE PENOKEE- IRON BEARIISrC=^ SERIES. approximately of uniform, size in each section, although varying widely in different sections ; in general they have roundish or oval forms. The parti- cles of the two minerals fit each other very closely, but do not intricately interlock. The eifect of this even grained background composed of round- isli ])articles is to suggest a fragmental origin, but nowhere is there any conclusi^'e evidence that any of the material is of fragmental character. If the grains represent fragments they could not have thus perfectly fitted each other as originally deposited, and nowhere is there any })roof of enlarge- ment in either the quartz or feldspars, and there is no space between them for a fine grained interstitial material ; so this granular structure can not be taken as proof of fragmental origin. The feldspar is very largely of the species orthoclase, although micro- dine and plagioclase are often abundantly present. It has usuall}' largely altered, the resultant products being kaolin, sericite, chlorite, epidote, and at times biotite and hornblende. This alteration is sometimes certainlj^, and probably often, attended with the simultaneous separation of quartz. Tliis decomposition in the cases when it has extended far enough results in producing in the place of a feldspar a complex interlocking mass of finely crystalline material. In a few cases in which the gneiss was originally strongly feldspathic the alterations have caused it to closely resemble some of the felds})athic graywackes of the Upper slate member of the Penokee series (chapter vi). As mentioned in the case of a similar occurrence in the Western granite there is, however, the great difference that no e^'idence can here be found that the original feldspathic rock was' of a fragmental character. The abimdant iron-bearing minerals of the gneiss are hornblende, bio- tite, and elilorite. Less frequently epidote, sericite, and calcite occur. In a single section any one of these minerals may be predominant; two or more, or all of them may occur together. There seems to be absolutely no regu- larity as to their occurrence, either in the field or in the sections themselves. Hornblende is, however, the preponderant mineral in many sections, although chlorite and biotite are hardly less frequently so. In the coarser grained gneisses (PI. xv. Fig. 4) the hornblende often occurs in large, well defined crystals, which in transverse sections show either the planes of the prism Tllk SOTTllKKN CO.MI'IJ'A. 119 dl' the |il;iiit's 111' the pi'isiii (•(iniliiiicd with the iii;ici'o|iiii;ic(ii(ls. The tci'- niiu;il |)l;iiH's iirc luit oftoii well (IcNrlopcd. 'I'lic |)l('()clii'i)isiii oi' the Ikmu- hlcndc is Ncry uiiitonii; c C>U; <• : C varies from 1,'}" to If)'^. Tlic liorii- Idciiilc individuals always include many grains of tlie other minerals present, usually more (juartz and feldspar than of chlorite and 1)iotite. Tt a])parently shows by its inclusions, combined witli its idiomorpliic forms, that it was the last mineral to crystallize. Its growth within these gneisses may be compared to the growth of crystals of garnet and staxn-olite in staurolitic and garnetiferous mica-schists, which frequently include large quantities of foreign materials. In its develo])ment, if a late mineral, it took within itself such material as it could not force aside. More often in the gneisses the hornblende is in small blades free from inclusions, located between the particles of quartz and feldspar or else penetrating them The biotite and chlorite occur in well developed folia, and in small fibers and irregular areas. Each of these minerals is in turn in quite a number of the exposures the chief constituent aside from the quartz and feldspar. Each frequently contains numerous sinaller particles of what is faken to be oxide of iron, which are arranged within their parts in the same regular manner. A portion of the chlorite and l^iotite is certainly secondary to feldspar and hornblende. This is particularly ti-ue of the chlorite. However, there is no evidence that their abundant, well defined blades are secondary. The epidote, so plentifully present at times, is found alike in the feldspar, chlorite, hornblende, and biotite, although most com- mon in the first two. It occurs in numerous small granules and large irregular areas. A large part of it is certainly derived from feldspar, chlo- rite often being an intermediate stage in its formation. It also has formed from hornblende. In some of the rocks the epit' the, mineral changes which explain this variation has been given. The coarser grained syenite-schists are found, by examining them in thin section, to hav(! a background composed of closely interlocking feldspar precisely like the massive syenites. The foliation of these rocks is due to the biotite and hornblende. The large blades of these minerals, arranged with their axes in a common direction, are within the feldspars, many blades often being included withhi a single individual of that mineral. The relation is such in many cases as to make it almost certain that these minerals have developed within the feldspar as secondary products. The pressure to which the rocks were doubtless sub- ject during the time of the growth of these minerals i)robably influenced their arrangement. The alteration of a massive feldspathic rock has pro- duced a foliated micaceous or hornblendic one. When this alteration of the acid feldspar to the more basic minerals, Ijiotite and hornblende, has been accompanied by the simultaneous separation of (quartz, the result is a crys- talline gneiss. Phases of strongly foliated rocks are found, from those which contain no quartz — that is, a syenite-schist — to those in which quartz is as abundant as the feldspar, when the rock is a gneiss. In the fine grained syenites and schists, in which the feldspar occurs in small roundish grains, there must have been a recrystallization of the feldspar itself if these rocks are transformed massive eru})tives, which is by no means certam. The question as to whether any of the crystalline schists and gneisses of the region are of fragmental origin can not be answered. The roundish, evenly granular appearance of the quartz and feldspar in many of them strono-ly suggests upon casual examination a clastic origin. However, the more closely they are examined the more clearly it is apparent that this is not sufficient evidence of a fragmental character. It has been noted that in the gneisses of this class the individuals fit each other perfectly. The grains as fragmental ones could not possibly have thus been deposited. If fragmental, either they must have recrystallized or else have been enlarged until they interlocked. Each of these processes and botli combined are now known to transform fragmental rocks into crystalline ones; but in 126 THE PENOKEE lEON-BEAEIFGr SERIES. order to prove that such a transformation has occurred, it is necessary to show that the rock in which they have been found to occur was a frag- mental one. The rounded cores in a quartzite are sufficient to show this, but in the case of a recrystaUization of feldspar it is necessary actually to trace the crystalline rock back to its fragmental state. The modified frag- mental rocks of these kinds in the Penokee series reveal themselves as such with little difficulty. Upon succeeding pages it is remarked that whenever quartz is present in them as a fragmental constituent, even if associated with quartz not of this origin, it is easily discovered. Its original outlines are so strongly marked by particles of gas, iron oxide, and other inclusions that all subsequent changes seem inadequate to obliterate them. While this is the case with these known fragmental rocks, the complete absence of anything which suggests that there have been cores in any case in the sections of the crystalline schists loses largely its force as negative evidence against a fragmental origin; because the fragmental rocks men- tioned are nowhere foliated, while the schists are strongly foliated. Frag- mental rocks in other districts which have been subject to dynamic forces, and have therefore become foliated, have rapidly lost evidence of their original fragmental character. To conclude, if the massive, coarsely crystalline rocks are assumed to be of eruptive oi-igin, it uecessarilj^ follows that a large number of the schistose ones are certainly derived from them ; also, it has not been possi- ble to trace any of these crystalline schists back to a fragmental rock. CHAPTER III. By R. D. Ikving and (J. R. Van Hisb. THE CHERTY LIMESTONE. Relation of tho liuiostouo aud olicrt. Guogniiiliical distributiou. Possible former greater coutinuity. Thickness. Petrographiciil character of the liiuestone. Petrographical character of the chert. Change to tlie overlying Quartz-slate. Tabulation of i)etrographical observations. Prominent exposures. Origin of the limestone and chert. Summary. Relation of the limestone and chert. — In the third volume of the Greol- ogy of Wisconsin (pp. 104, 106-108), and on Pis. xxiii to xxvi of the atlas accompanying that volume, the lowest member of the Iron-bearing- series of the Penokee district is represented as being a tremolitic crystalline limestone, with an average thickness of 90 feet, above which is said to succeed a peculiar white t^uartzite, with an ' average thickness of 40 feet. These two layers, though occasionally wanting altogether, as indicated on the maps referred to, are represented as having a considerable longitudinal extent and as being separate from one another ; that is to say, all of the white quartz is taken as lying above all of the limestone.^ That this is the case was concluded particularly from the exposures seen at Penokee gap, where such a succession is very manifestly the true one, as indicated on PI. XXVI of the Wisconsin atlas. Our later investigations, and more par- ticularly our experience on the Michigan side of the Montreal river, have shown us, however, that in fact these two rocks form portions t)f (jiie and the same layer; that is to say, that the white quartz, (jr chert, as we now think it should more properly be called, occurs as a rule interstratified and thinly interlaminated with the limestone. Taken together, these two ma- ' However, a doubt was expressed (Geol. of Wis., vol. m, 1880, p. 110) as to whether the limestone exposures in the western part of the Penokee district might not represent both limestone and cljert as seen at Penokee gap. X27 128 THE PEiSrOKEE lEON-BEAEIJiTG SEEIES. terials constitute a well marked belt, which is sharply separated, not only from the granites and schists to the south of it, but also from the fragmen- tal slates immediately above, from which it ditfers in being almost free from an interjnixture of fragmental material. Geofjraphkal distribution. — It is difficult to define very accurately the geographical distribution of the limestone member. Certainly in a number of places it is entirely wanting, since the strongly marked slate belonging immediately above it is found directly in contact with the granites and schists which belong below it. When the atlas sheets of the Geology of Wisconsin were drawn, this belt was represented as having a greater degree of continuity than is indicated on tlie maps that accompany the present volume, exposures distant from one another having been relied on to indicate a continuity, so that, substantially, this member was repre- sented as continuous except where it was certainly known to be wanting. Our later experience having taught us that this member may thin out and disappear quite suddenly, and that it is lacking more often than was for- merly supposed, we have pursued just the opposite course in mapping it; that is to say, we have mapped it as occuring only in those places where exposures demonstrate its existence. The detailed maps of Pis, v, vi, viii, x, xii, and xiii of the present volume, which show the exact positions of all of the exposures belonging to the various belts of the Penokee series, will serve to indicate to the reader the exact facts which we have had at command in deciding as to the degree of continuity possessed by this member. This member then is indicated on our maps as appearing in six detached portions in the distance between Atkins lake, T. 44 N., R. 5 W., Wisconsin, and the Little Presque Isle river, T. 47 N., R. 44 W., Michigan. Beginning with the west- ernmost of these areas, it is to be said that, so far as our knowledge goes, it may extend some miles farther west than the maps indicate, since we are without exposures to prove either its continuity or its absence. The eastern end of this westernmost area, however, is sharply defined by actual expos- ures, as indicated on PI. v, the granite and siliceous slate member of the iron series coming directly into contact with one another along the course of the Marengo river, in the SW. \ of Sec. 15, andNW. \ Sec. 23,' T. 44 N., SOUTHERN COMPLEX. Eruptives >J^ Exposures of eruptives Souih limit -North limn South limit ; 9Mt Exposures of ferruginous schist. Limits of surface distribution SI. Exposures Q. Exposures South limit of slaty phases rf quartzitic phases = Exposures showing sTnhe and dip. '^ Exposures showing noslructure Li Scale of Map I inch = l mile Sc Exposuresof limestone and chert /iW5^ Exposures of greenstones ^/<}:-- Exposures of granite - South limit nes of exploration by U 3 Gaol Survey — — Lines of exploration by Wis Geo! Survey ale of Sections! inch = 1320 feet DETAILED GEOLOGY OF THE PE;N0KEE DISTRICT. SHEET I. THE (;ili:iiTV LIMESTONE. 126 R. 5 W., Wisconsin. Farther east wc do not meet with any exposures of the hniestone until Sec. 24, T. 44 N., R. 4 W., Wisconsin, southeastward from Hhidder hike, is reached. In tlie interval the only known rock is a ts of surface d.str.bution : WW Q Quartt.te exposures E SW Gw GraywacKe exposures Quartz-Slate Member SI.Exposures of slafy phases 0 txposures of (^uartiiTic phases. Cheny Limestone Member ^ Exposuresof limestone and chert South limit. Eruptives. y/tfi Exposures of greensfom SOUTHERN COMPLEX. ^=^ W Exposures of hornblende schist. ■?>:' Exposures of granite xposures sho».ng sinke and d.p W Exposures sho«,ng no structure Lines of exploration by US Seol Survey Lines ofexplorationby Wis Geol, Survey Scale of Map I inch -I mile. Scale of Sections I inch = 1320 feet DETAILED GEOLOGY OF THE PENOKEE DISTRICT, SHEET 2. THE CIIKUTV lilMESTONK. Analysen of Hmcstonen, Vol SlOa TiO, Al.O;, F0;O:, FcO MuO CaO MrO HjO CO, PcO. SO, CI FeSj Organic matter. No. 1. 3-07 OOil 0-86 0'15 20 -72 19-95 0'30 45-31 Trace. 99-45 No. 2. 0-63 0-03 0-75 0-08 30-94 20-68 0-27 46-27 Trace. 99-85 Dolomite and calcite are not readily distinguished in the thin section. So far, however, as appearances go, the carbonate entering into the compo- sition of these rocks seems to be always the same, and, judging from the content of magnesium carbonate, it is rather to be counted as dolomite than as a mixture of dolomite and calcite or as a magnesian calcite, though the assertion that calcite is always absent is not ventured. Under the micro- scope the dolomite individuals, which constitute the larger part of each section, present the usual appearance met with in sections of crystalline limestone, being without crystal outlines, but fitting together closely along irregula* curved lines. The interlocking of the grains is however, usually somewhat greater than in the case of statuary marble. In addition to the carbonate, most of the sections show more or less tremolite, which varies greatly in quantity. Occasionally only a few minute flakes are discoverable, while in other cases broad single blades of radiating clusters make up a large portion of the thin section. The tremo- lite blades often penetrate the carbonate in every direction, and in the case of the larger blades the dolomite appears to be included within the tremo- lite. In yet other cases the tremolite in aggregated blades appears to make up the whole of certain narrow belts in the thin section. The tremo- 132 THE PENOKEB IROISr-BEAEING SERIES. lite is known to be such by its usual microscopical characters, which need not be here enumerated. Fig. 1 of PL xvi is from a section of one of these limestones magnified sixty diamaters, in which there is shown one of the larger tremolite blades, through whi(;h in places are dimly seen the out- lines of the dolomite individuals. Many of the sections of these limestones show more or less of a sili- ceous ingredient, which is found in varying quantities up to an amount which very largely predominates over that of the cax'bonate. In fact, these siliceous varieties furnish us with a complete gradation into the chert rock, which, as already indicated, at times excludes the limestone completely. Now and then there are apparent in the thin sections of these siliceous limestones a few grains of quartz, whose fragmental nature is demonstrated by their rounded contours and by the secondary enlargements they have occasionally received. But these fragmental particles are relatively sparse and unimportant, the most of the silica having plainly solidified in situ. Of the latter silica there may be distinguished two varieties, which, however, grade into one another. The first of these presents itself in the shape of an interlocking mass of quartz individuals of finer or coarser grain. As these become finer and finer, there is found intermingled with them more or less of a fine spotty and chalcedonic silica with the characteristic aggre- gate polarization and radiating structure ; and finally they pass into kinds containing a good deal of a completely amorphous opaline material. In Fig. 2 of PI. XVI the thin section of one of the more siliceous varieties of limestone is represented as seen in polarized light and magnified sixty diameters. In the middle band of this figure the dolomite predominates greatly over the silica. On the upper right-hand side of the figure is a band comjDosed of the finely crystalline silica, and at the lower left-hand corner is an area of the more coarsely crystalline quartz. Petrographical character of the chert. — -As already indicated, the cherty material of this limestone belt is often in layers of considerable thickness, at times apparently making up the greater part of the whole belt. At Penokee gap, for instance, it has a thickness of some 45 feet. In other cases it is scattered through or is interstratified with the limestone in narrow seams. As in the case of the siliceous material just described a-s occurring THE UIIEltTY LIMESTONE. 133 at times closely intermiiifrlod \\'itli the dolomite, so also in the case of this chert rock the silica presents itself in completely crystalline, half crystal- line, and amorphons conditions, these several phases occurring at times in- termingled with one anothin-. In some cases, however, as for instance in the rock at Penokee gap, all of the silica is completely individualized, and the individuals furnished more or less thoroughly with crystal outlines. As seen microscopically, this peculiar quartz rock is perfectly white, stud- ded with minute crystal facets, and of a saccharoidal texture, being often so crumbly as to be readily mistaken for a fine grained sandstone. This imiH-ession is confirmed by the crystal facets, which one takes at once to be in the nature of the. enlargements of quartz fragments commonly met with in sandstone, but the examination of the thin section fails to substantiate the impression, since the outlines of the original grains are not perceptible. On the whole, then, on account of the gradation varieties between this peculiar rock and those phases in which there is more or less chalcedonic and amorphous silica, we conclude that here also the whole of the silica has separated out in situ. The accessory constituents in this chert are few and unimportant in quantity. They include sericite, brown iron oxide, and occasionally mag- netite and dolomite. The usual snow white color of the rock is due to the general absence or sparseness of the iron oxide ingredient. Only rarely is the iron oxide present in sufficient quantity thoroughly to redden the rock. In these rare cases, however, the chert resembles a jasper. In PI. XVI, Fig. 3, we have represented one of the finer grained phases of chert as seen in polarized light. The concietionary and semichalcedonic arrangement of the silica is plain, the finer grained portions tending to form the centers of areas the outer portions of which and the interspaces between which are composed of the coarser grained quartz. PI. xvi, Fig. 4, shows a section of the chert in which a band of the coarser grained quartz is seen adjacent to one of finer, but still completely crystalline, material. Our photo- graphs of these sections of the chert and limestone containino- the finest grained and amorphous silica have not proved sufficiently successful for re- production, largely on account of the confused appearance of the objects themselves. 134 THE PENOKBE lEON-BEAEING SEEIES. One of the most notable peculiarities of this cherty rock is its tendency to assume a brecciated form, in which angular pieces of the chert, ranging from microscopic sizes to fragments two or three inches across, are buried in a chert of a character wholly similar to that of the fragments or differing from them only in carrying a small quantity of other ingredients, such as magnetite and chlorite. These brecciated phases occur at times wholly within the horizon of the Cherty limestone member itself, and in other cases are very near to its junction to the overlying slate, when it is not always easy to draw the line between the two members. Change to the overlying Quartz-slate. — As already indicated and subse- quently further explained, the limestone or basal member of the sedi- mentary series is directly overlain by a very considerable thickness of com- pletely fragmental rocks, whose main constituent is quartz, which mineral is, however, accompanied by a large proportion of, feldspar fragments and by various alteration derivatives from the feldspars. The change from the limestone member to this quartz-slate is very sharp, the fragmental rock often carrying at its base pieces from the cherty material belonging directly beneath it. As this detritus in the quartz-slate is precisely like the material of the cherty limestone as it now exists, there must have been a considerable lapse of time between the deposition of the two formations. Tabulation of petrographical observations. — Although, in the present de- velopment of the knowledge of petrography, it is rather unusual to include a description of individual sections in a volume like this, it is given in this and succeeding chapters because the essential unitj^ and continuity of each formation, as well as the contrast between it and the following one, thus appear with a clearness which can be enforced in no other way. This detailed observation is desirable for the further reason that, as explained in the Preface, the Penokee district is dealt with in greater detail than is intended with any subsequent iron-bearing district, the reason for the great elaboration being that tliis is the first of the iron-producing areas of lake Superior in which the geology has been fully worked out. The numbers of specimens and slides are usually those of the collec- tion of the lake Superior division. Specimens with Wr. after the numbers are from the collection of the late Mr. Chas. E. Wright. Specimens with KEWEENAW SERIES PENOKEE SERIES. SOUTHERN COMPLEX. Upper Slate Member Iron-bearing Member Quartz-Slate Member, Cherty limestone Member Eruptives = »MC Exposures of ferruginous schist. SI. Exposures of slaty phases ^Exposuresaf hmestoneand ch«rt «* Exposiires of greenstones jJJJi i^'^.^^^P^'^-'.^,^^^^^^ ' - ' '-- ' -•^' - Q Exposures 0* .-..- ->- =;-....k i.«r* t- =• *p South limit )SiW Exposures of erupt ives = ^WfiJ M S Mtca-slote exposuiea — "w. i..|ju3ij.c» u. .^. , L.g,M«ua a^...*.. ^,. ,.„f^w^_.>,w >., , , 1 — - . u jg^ ^ „ fxdosi -South limit ^ ^aw Bl SI Black slate exposures Limits of surface distribution Q Exposures of quartzitic phases South limrt "^ of granite ^= iWK Q QuaHzile exposures ^= Afr'Gvv (jraywache exposures ^Exposures showrng sTnke and dip >!?» Exposures showing no structure Li ne s of exploration by U S Geol Survey Lines of exploration by Wis Geol Survey Scale of Map I inch=l mile Scale of Section I inch = 1320 feet DETAILED GEOLOGY OF THE PENOKEE DISTRICT, SHEET 3. Tril'; (MIKKTV LIMESTOXE. 135 Wis. after tlic imnil)ci-s ;irc iVdiii tlic collection of the Wisconsin Geological Survc}-. Locations arc f^ivcii from southeast corner of the section, in steps of 2,000 per mile. 1. Treiiiolitic dolomito. Speciiucn 9078 (slide 3105). From 1850 N., 1C75 W., Sec. 22, T. 44 N., K. 5 \V., Wis. A iiuitbrmly granular, grayish white limestone; in appearance very close to marble. The thill section is composed chiefly of interlocking grains of dolomite, which often show the characteristic cleavage and twinning. Mingled with the dolomite are quite large l)lades of tremolite, which in many places are more or less decomposed (Pl.xvi, Fig. 1). 2. Tremolitic dolomite. Specimen 189 Wr. From 0 N., 1900 W., Sec. 15, T. 44 N., E. 5 \V., Wis. The mass of this rock is like 1, except that it is of a dark gray color. Cutting through it are veins of tremolite in very coarse, radiating blades. The greater part of the section is like that of 1, but a portion of it is cut from the vein of tremolite, which material occurs iu radiating clusters of quite large sized brilliantly jpolariziug blades. 3. Quartz rock. Specimen 9670 (slide 31G3). From 1900 N., 1300 W., Sec. 22, T. 44 N., E. 5 W., Wis. A finely granular, very friable, suow-white quartz rock, which in the sunlight exhibits immmerable glittering crystal facets. The thin section is comj»osed of a minutely crystalline mass of quartz. The quartz individuals are in large part crystal outlined, and appear to fit against each other x)erfectly, face to face, with little or no interlocking. There are usually no vacant spaces between the crystals, but the fragility and ijseudo-arenaceous texture of the rock are explained by its peculiar make-up. No amorphous silica is present, nor is there any evidence that the crystals have been produced by the enlargement of quartz fragments, as in ordinary quartzites. 4. Tremolitic dolomite. Specimen 9654 (slide 3161). Prom 1400 N., 1000 W., Sec. 24, T. 44 N., E. 4 W., Wis. This rock differs from 1 in that a portion of it is of a decided greenish color. The section differs from that of 1 in that the tremolite is almost as abundant a? the dolomite. 5. Dolomite. Specimen 9653 (slide 31G0). Prom 1300 N., 1000 W., Sec. 24, 1 44 R., E. 4 W., Wis. The rock is fine grained, uniformly granular, and of a gray color. The thin section is composed wholly of a finely and evenly granular aggregatioi of dolomite individuals. 6. Tremolitic dolomite. Specimens 9531 (slide 3138), 1421 Wis. (slide 251): Fron 1440 N., 1200 W., Sec. 14, T. 44 N., E. 3 W., Wis. 136 THE PENOKEE mON-BEAEING SEEIES. A finely granular, uniformly textured, light gray rock, containing quite numer- ous small grains and crystals of pyrite. The rock contains: calcium carbonate, 50-52 ; magnesium carbonate, 33'41 ; iron, 1'19; insoluble Ingredients, 13-85; undetermined 1-03=100.1 The sections are composed almost wholly of small, closely fitting particles of dolomite. Scattered through them are quite a good many blades of tremolite. Quartz and pyrite are sparse accessories. A small portion of the dolomite exhibits the char- acteristic cleavage and twin lamellation. 7. Sericitic quartz rock. Specimen 9533 (slide 3139). From 1460 N., 1200 W., Sec. 14, T. 44 N., R. 3 W., Wisconsin. A rock much like 3, but of a coarser grain. The thin section is composed of fine grains of quartz, which include many minute flakes of sericite. The individuals of quartz are coarser than in 3, and while occa- sionally showing the crystal outlines, the greater number of particles form junctions which are more or less irregularly curved. The sericite is equally distributed throughout the section, being included in each of the quartz grains. Often a single flake of sericite is seen to penetrate two or more grains of the quartz. No evidence of enlargement of the quartz can be detected. 8. Quartz rock. Specimens 4526 (slide 1112), 4520 (slide 1110). From 1460 N., 1200 W., Sec. 14, T. 44 N., 11. 3 W., Wisconsin. The thin section differs from that of 7 in being coarser grained, in not having the quartz particles very thoroughly interlocked, and in containing relatively little sericite. Chlorite also occurs as a sparse accessory. 9. Sericitic qirartz rock. Specimen 4534 (slide 1465). From the extreme NE. :i of Sec. 16, T. 44 N., E. 2 W., Wisconsin. The thin section resembles closely that.. of 7, the only difference of importance being the occurrence in it of magnetite in a few small crystals. 10. Flinty cherts. Specimens 7511 (slide 2056). From 1925 N., 1^34 W. ; 9430 (slide 3129), from 1900 N., 1900 W.; 9434 (slide 3131), from 1923 N., 1940 W., Sec. 14, T. 47 N., E. 45 W., Michigan. A fine grained to aphanitic, light gray or pinkish gray chert with a couchoidal , fracture. The sections are composed essentially of a minutely crystalline silica, the parti- cles of which, however, vary a good deal in size ; some portions looking as though the rock might be in part amorphous. In other parts' one sees that there is a vague concentric arrangement, the amorphous and more minutely crystalline silica tending to lie in the middle portions of certain areas whose exterior portions are made iip of more coarsely crystalline particles. This is an arrangement which approaches to the texture of true chalcedony, the resemblance to which mineral is also very evident in ' Wisconsin Geol. Survey, vol. iii, p. 107. T46N. C W O m O r O - o > r ui c < m o O z O Ci > ■D I >< X •-Hit' A TllK CllKKTV LIMESTONE. l;37 the hand s])eciiii<'ii of tlics<' rocks. In i)laoes a very small (|uaiitity of hiown iron oxide is ijrcscnt in niinnto iiarticlcs. (IM. XVI, EIk'- •^■) 11. Ohwt. Spcic.inicns !»4li;! (slide .'{(Mi.'i), !I424 (slide .•iO" jWHfMMMPWWif ^1 7m 38^ KEWEENAW SERIES Exposures of eruptives S Sandstona. -South limit - 1000 feet above theSea -- PENOKEE SERIES. Upper Slate Member ^^ JSSW Si Black slate exposures ^^ 'XSSSk^'" GrayvvacKe exposures Iron-bearinb Member ^^= -ggjS Exposures of ferruginous schist, Ltcnits of surface distribution Quartz-Slate Member SI. Exposures of slaty phases 0 Exposures ot quarlzitic phases South limit SOUTHERN COMPLEX. ^KKH Hb.S Exposures of hornblende-schist. ^g^^ C S Exposures of chlorire-schist >IHS9< Exposures of granite ,Xtf^ Exposures of greenstone. *E»posures shoeing str.ke and d.p liW E; O O JO > ■D X -< o X m TJ m z o m m D C/J H o H 0) X m m H 01 01 ■*• - ^ \5- fe' QQDI □no nnn DOD 3IlL -^fo i» w en m o r- O O n > r C/) c < m 2 O z o o > I X >< Tl Nit-i THE CHERTY LIMESTONE. 141 initely known to ho organic deposits. Tlie origin of similar cherty deposits in tlic Iron-luiaring- formation is discnssed, in cliapter v, section 2. What is said there with reference to the "original rock" applies equally well here. It apjiears that tlie chert of the limestone belt, whether original or s(!condar}-, had in the main reached its present condition before the accu- mulation of the immediately overlying Quartz-slate formation, since, as already stated, very numerous fragments of this chert are found included within the slate at its base, and even in its middle and upper parts, and if the concentration of this material in layers is due to secondary causes, there must have*been sufficient time between the deposition of the Cherty lime- stone and the Quartz-slate to accomplish this. That the chert has been rearranged to a greater or less extent since its deposition, and that in the cracks infiltrating solutions have brought additional silica, is more than probable. At the time of this subsequent rearrangement and introduction of silica, doubtless the tremolite was formed, although even this mineral may have developed very early. In the formation of the tremolite, the silica in solution had but to unite with a portion of the bases present, cal- cium and magnesium. The origin of actinohte from an analogous rock, except that it bore iron, is discussed in chapter v. What is there said applies equally well to the tremolite in the Cherty limestone. Summary. — The cherts and limestones are placed together because as a rule the chert occurs interstratified and thinly interlaminated with the lime- stone. This member, instead of being continuous, often thins out and disap- pears quite suddenly, so that it is mapped as occurring only where actually found. It is probable that • the Clierty limestone member had a much greater former continuity than at present. This is particularly probable because fragments of it are abundantly contained in the overlying Quartz- slate member. Its maximum thickness is 300 feet, and from this it varies to nothing. In petrographical character the limestone is close to a dolomite which is frequently tremolitic. J 42 THE PENOKEE lEON-BEARING SERIES. Chert occurs in the dolomite from minute particles through thin layers, which protrude on the weathered surface in leaf-like forms, up to belts which are 45 or 50 feet thick. The change from the Cherty limestone to the overlying Quartz-slate member is usually abrupt, while in many places between the two there is certain evidence of an erosion interval. The chert and limestone are water-deposited sediments. Whether chemical or organic is uncertain, but it is not improbable that they are partly or wholly the latter. However, if this is the case, the silica has subse- quently changed to the mineral form and has been extensively rearranged, while the limestone has become dolomitized. U. S. GEOLOGICAL SURVEY MONOGRAPH XIX- PL. XII. N. r TTB^^^sasr^ s. / ■- —T^^W N ^ ni -a-^jg, / s -'■sKi&Jv II SI Gn Gn ^— — s?-- G ~TS5!r~— W'.'.y.' ».'V.- jRf''^ ~~Tanmrr ^SCi! 7 '**''' "T*" KEWEENAW SERIES i Exposures of erupuves I S Sandstone -South limit. PENOKEE SERIES. Upper Slate Member. Iron-bearinb Member Quartz-Slate Member Cherty Limestone Member GraywacKe exposui K Exposures of ferruginous schist. - Limrts of surface distnbirtion SI.Exposures of slaty phases 0 Exposures of quartzitic phases South limit Eruptives. (j Exposures of greenstone SOUTHERN COMPLEX. ■-;■:■ Hb S Exposures of hornblende schist. :.■;.-.■ MS Exposures of mica-schist. ■jrv C 5 Eicposures of Chlonie-schtst. Jiv'^'i'^ Gn. Exposures of granitoid gneiss *» E> posun i of oranile ll Exposures of greenstone. = Exposures showtnd strike and dip. J Exposures showing no structure Lines of exploration by U S Geo I Survey Scale of Map I inch = I mrle Scalei of Sections i inch = 1320 feet DETAILED GEOLOGY OF THE PEINOKEE DISTRICT, SHEET 5. CHAPTER IV. By R. D. Irving and C. R. Van Hisb. THE QUARTZ-SLATE MEMBER. Applicability of the namoi. Geographical extent. Topographical features. Thickness. General petrographical character and stratigraphy. Microscopical character of the feldspathic quartz- slates. Microscojjical character of the biotitie aud chloritie quartz-slates. Microscopical char- acter of the vitreous quartzite. Microscopical character of the sandstone, novaculite, and argil- laceous slates. Tabulation of petrograiihical observations. Contacts with the Cherty limestone member. Contacts with the Southern Complex. Change t) a light gray novaculite. Besides these there are certain phases of relatively rare occurrence. These are (7) red, green, and purple clay-shales or clay-slates; (8) a conglomerate in which fragments of white cliert are "imbedded in a greenish chloritic matrix; (9) a magnetitic conglomerate, like the last, but containing a large pro})ortion of magnetite (these last two occur where the formation is in contact with the clierty limestone); and (10) a peculiar greenish con- glomerate-slate found only on the Potato and West branch of the Montreal rivers, immediately at the contact with an unconformably underlying greenish schist. These distinctions are based entirely upon the macroscopic appearances, which appearances, however, are quite well borne out by the study of the thin sections. The second phase mentioned includes kinds which prove to have, in addition to the jn-edominant quartz, a considerable proportion of a fragmental feldspathic ingredient, to whose presence the characteristic whitish to straw-colored weathering is undoubtedly due. The gradations fi'om this phase towards vitreous quartzite arise from a lessening in the amount of the feldspathic constituent ; those towards novac- ulite from an increase in the proportion of this feldspathic constituent, accompanied by a great decrease in coarseness of grain. The rocks of the 148 THE PENOKEE IKON-BEAEIKG SEEIES. first phase pi'ove microscopically to differ from those of the second in con- taining a relatively large proportion of fine micaceous particles, these in- gredients, including biotite, chlorite, and sericite, occurring either singly or together. A still further increase in these ingredients, accompanied by an increasing fineness in grain, gives i"ise to the third phase; while a still greater increase in fineness of grain, along with an increase in the amovint of clayey material from the decomposition of the feldspar, leads to the argillaceous slates or shales of the seventh variety. The microscopical study shows further that the difference between these several ])hases, so fai as they are not the result of unusual conditions, as in the case of the cherty and niagnetitic breccias above mentioned, are almost entirely dependent upon the original proportions and degrees of fineness of the two main frag- mental constituents ; that is, the quartz and feldspar. Some of these mica- ceous ingredients are taken to be of fragmental origin. This is particu- larly true of some of the large flaked sericite or muscovite, but in the main these materials, including also kaolinite, appear to have resulted from a decomposition of the feldspathic particles. This decomposition was accom- panied by the separation of a secondary silica, which is now apparent in most of the thin sections in the shape of a minutely crystalline quartz. Disregarding the special and rarer phases, the important kinds may be microscopically listed under the following heads: Chloritic and biotitic quarts-slates; feldspathic quartz-slates ; vitreous quartzite; sandstone; novacidite; argillaceous slates. These are variously interstratified with one another. However, the vitreous quartzite layer is a persistent element in the stratig- raphy, composing the uppermost part of the foraiation wherever it is exposed. The conglomerates referred to (Nos. 8, 9, and 10,) lie always at or near the base of the formation, their peculiarities having been caused by proximity to the underlying rocks ; but exposures of this kind are not suffi- ciently numerous to demonstrate that such rocks are a persistent element in the stratigraphy, although this is quite probable. At one place a vitreous quartzite is at or near the base of the series. However, while no further definite subordinate arrangement is observable in the cross-section, a change in character is to be noted as the belt is followed from west to east, the biotitic phase (Nos. 1 and 3,) predominating in the western portion of this Till': (.ilAlvTZ SLATK M KM ISKI;. 149 belt, thonj^-h steadily hisscniuy- in relative aiiuiuiit as one moves eastward. East of Sec. ;U, T. 45 N., K. 1 \V., AViscousin, altlumgli tlie uppermost (piartzite rontiimes well marked, the cross section l)efonies varic^d, t'eld- spatliic (piartzites, feMspathie ([uartz-slates, sandstones, and clay-shales alter- natinj;' with one another. Microsfopical rhardctcr of the fcldspathk (puii-t^-dak-'i (Phase 2, see PI. xviir, Figs. 1, 2, 3, 4). — In thin sections, from the typical specimens of this phase, Avhich is the prevailing- rock of all that portion of the quartz-slate belt to tlie east of Sec. 34, T. 45 N., R. 1 W., Wisconsin, a single glance through the microscope generally suffices to show that it is composed of two parts, a coarser plainly fragmental portion and a finer interstitial • material. The relative proportions of these materials vary greatly, the coarser portion at times sinking to quite a subordinate position, and again nearly excluding the matrix. Between the finer and coarser portions thei-e is often a material of an intermediate fineness, and the whole appearance suggests that the two portions are in large measure f»uly fined' and coarser particles of the same minerals. The coarser portion in these sections always comprises fragments of both quartz and feldsjjar. With these is very often more or less rather coarse gi'ained mica, which appears also in a finer condition in the inter- stitial material. This coarser grained mica is taken to be in the main frag- mental also, although, as will be seen subsequently, secondary micaceous minerals are plentifully developed in the rocks of this 'member. These coarser grained micas are the ones which appear as biilliant flakes to the naked eye on the surfaces of the laminae. There is considerable variation in size among the coarser pieces, which, as already said, grade down- ward into the matrix material itself. This mingling of coarser and finer material, since it is made up of fragments of difi'erent minerals, is taken to indicate that the detritus of which this rock is composed had received relatively little sorting before deposition. The fragments of quartz are for the most pai-t portions of single indi- viduals, but not unfrequently they are minutely complex, having been derived from some chert}- or fli'nty rock. These particles vary greatly as to the degree of rounding which they have received. In general the 150 THK PENOKEB lEOK-BEARING SERIES. amount of rounding- appears to be in a direct relation to the size of the particles, the more minute pieces having- remained quite angular. In speaking of tliese pieces as rounded, however, -vve refer ahvays to the original frasrments, whose outlines for the most part still remain distinct; but as they now stand, a large proportion of them are built out by second- ary enlargement; the added portions varjdng greatly in width, but often extending beyond the original fragments a distance equal to a fifth or sixth of their diameters. These enlargements are, as usual, optically continuous with the original fragments, and have frequently interlocked with one anotlier in such a fashion as to produce very irregular outlines. It is noticed that these secondary enlargements are narrowest in those sections which have a considerable quantity of brown iron oxide among the interstitial materials. • The outlines of the original .quartz fragments where they have received enlargements are emphasized, as is usual in such cases, by particles of brown iron oxide, and by the presence of minute cavities. In a few cases, mingled with this brown iron oxide, and at times almost excluding it, are films of a greenish chlorite. The feldspar fragments include three distinct kinds. The most abundant is unstriated, and its appearance is, in every respect that of the ordinary orthoclase of the granitic rocks, and there can be no reasonable doubt that it is of this nature. The difficulty of separating tliese supposed orthoclase particles from the other ingredients of the rock, particularl}' from the microcline, would be so great that it is not thought worth while to make the determination any more certain than this. A second variety is the ordinary cross twinned microcline, and the third is a striated plagio- clase. On making many measurements of the extinction angles of the last named variety by Pumpelly's method, we fail to find any angles which would suggest the presence of a plagioclase more basic than one belonging in the oligoclase series. We may therefore, with some confi- dence, say that the feldspar particles of these rocks include pieces of ortho- clase, microcline, and albite or oligoclase, or both, an association which is that of the ordinary granites. Often these feldspar fragments are very fresh, but in other cases many of them are much altered or decomposed, a gradation in this respect being found to obtain between those kinds in TIIH QUAKTZ-SliATl'; iM KIM liKi;. 151 wliicli the |)arti(4os ure still very fresli and tlic cliloritic slates or the argilla- ceous shales; the most plentiful ])ro(hie,ts of the (leronn)osition of the feldspars hein{:f ehlorite and kaolinite. In the decomposition of the feld- spars to chlorite, the particles of the latter mineral "are found to form in the iirst place in the nei<>'hl)orhood of the edges of the feldspar fragments, the alteratictn having progressed regularly from the outsides of the grains ; in the case of the kaolinitic alteration, the kaolinite particles appear as usual in nuuierous minute flakes throughout the entire feldspar grains. The I'arge particles oif white mica which are supposed to be of a frag- mental nature are taken to he an altered muscovite.' The fine interstitial material in these rocks, aside from the minute grains of quartz and feldspar, is a mixture of a minutely di^nded silica, flakes of kaolinite, chlorite, and a fine mica, Avhich is taken to be sericite and muscovite, in proportions which vary between very wide limits, although in nearly every case all of these minerals appear to be present in the matrix, except perliaps the sericite. The siliceous portion of this matrix is at times completely though very minutely crystalline, ])ut in other cases it takes on a chalcedonic or even an amorphous form. While some of the larger particles of the quartz in the matrix are doubtless of a fragmental nature, much of this interstitial silica is evidei^tly an original crystallization. The kaolinite and chlorite are in minute flakes intimately mingled with the fine silica ; the brown iron oxide occurs in irregular In-own patches, being usually in a rather minute quantity, though at times deeply staining the entire matrix. The sericite particles of the matrix are usually of somewhat larger size than the particles of the other minerals, on which account it is perhaps a question whether this sericite should not rather be regarded as belonging with the larger mica flakes which we have already mentioned as of a fragmental nature. All of these interstitial minerals are ones which, as is known, result from the alteration of feldspars and micas, and it is supposed that they are the result of metasomatic changes cari-ied out particularly in the finer detrital material subsequently to the original deposition of the rock. MicroscopicaJ character of the hiotitic and cMoritic quarts-slates. — (Phases 1 and 3, see PL xix, Fig. 1.) The thin sections of these slates, which are, as a 152 THE PENOKEE IRON-BEAEING SERIES. whole, mucli finer grained tlian the slates just described and which do not show the same separation into a coarser fragmental portion and a finer interstitial portion, show a background which is composed mainly of quartz, but also contaiifs usually more or less feldspar. In some sections the feldspar particles are quite abundant, while in otliers they are almost, or qtiite, wanting. Scattered through this background in vary- ing quantities in the different sections are green flakes of chlorite and brown ones of biotite, the chlorite predominating in some sections, the biotite in others. The quartz particles as now constituted are minutely angulai'. How- eA'er, manj- of the larger have very plainly marked fragmental cores whose outlines are emphasized by films made up of minute. flakes of chlorite and biotite. It is exceedingly difficult to say how much of the finer grained quartz is of a fragmental origin. Judging from the sections of the coarser grained varieties of phase 2, described immediately above, we conclude that part of this finer silica is fragmental, but that part of it is also an original crystallization. Tlie flakes of chlorite and biotite are usually quite small, but very Avell defined, though occasionall}' large sized scales of uniaxial chlorite are seen. It is taken as probable that all of these two minerals, along with the nonfragmental silica, have resulted from the decomposition of a detrital feldspathic material. At all events, in some sections all three of the minerals occur within the outlines of a single original feldspar fragment in such a manner as to show their derivation from the feldspar. This is a process of alteration, particularly as regards the biotite, which, as shown on a subsequent page, has been carried out in a very striking manner among the slates of certain portions of the Upper slate member of this district, fragmental feldspathic rocks having been thus altered to rocks that would ordinarily be taken as crystalline mica-schists. If this is also the origin of the micaceous ingredients of the rocks now especiallj^ under consideration, we should expect feldspar fragments, recognizable as such, to be ])resent in an abundance standing in an inverse ratio to the amount of chlorite and biotite present. The thin sections show that this is actually the case. In the most highly chloritic and biotitic varieties there is recognizable little or no feldspar. THE qi;ai{T/-slatk imkmi'.kk'. I'yp, ll()\vc\cr tliis may bo, that all of these roeks are oC a f'ragmental ori-^iii is rendered siitticiently evident by tlie fi;radation of those kinds, in wliieli tile fraj>-niental textui'e is lost into those ])hases in which it still remains distinct. It should be said that these gradations occur constantly within short distances, and that those j)ortions in which no distinct trace of frag-niental origin is now perceptible are all relatively of very small extent and rare occurrence. It is a noticeable cohicidence that, the most thor- oughly changed varieties met with in this slate belt occur in its more western portions, this being at the same time true of the uppermost member of the series. A reason for *this coincidence is later suersrested. Microscopical character of the vitreous quartzite. — (Phase 4, PI. xx, Figs. 1, 2, 3, 4.) The vitreous quartzites^ prove, as one would expect from their macroscopic appearance, to be composed almost entirely of relatively large sized quartz fragments, each one of which has received an enlargement, the several secondary enlargements interlocking with one another in a more or less intricate manner. P'Sldspathic quartzites are interstratified in thin seams at various horizons in the quartz-slate formations, particu- larly so in the more eastern portion of the belt; but the only occurrence of pure quartzite in this formation, as already noted, is that persistent band which forms throughout the entire extent of the formation its uppermost horizon. The specimens brought from this particular laj^er furnisli some of the handsomest illustrations which we have ever met with of a transformation of a sandstone to a vitreous quartzite by the enlargement process.^ The outlines of the original fragments of these rocks are in many cases emphasized by a brown iron oxide, in which case the rock has usuiilly a more or less distinctly brownish tinge ; but in many cases the emphasizing mineral is chlorite in minute flakes. In the latter case the rock is either of a light gray color, or, if the chlorite is ' As used in this memoir, the word "quartzite" i.s restricted to rocks which have been derived from Iragmeutals. The timdameutal dift'erence wliit^h exists between roclcs of this kind and tliose iji which the qnartz is an original crystallization, we believe, is in this volume for the first time fully recog- nized. However quartzite-like the nonfragmeutal (juartzose rocks of the Iron-bearing and Cherty limestone members are, they are always designated by some other name than quartzite. -For a more complete explanation of this enlargement process, with illustrations drawn from very many localities and from different geological horizons, see Bull. U. S. Geol. Survey No. 8, by E. D. Irviug and C. R. Van Hise. 154 THE PEiSTOKEP] lEON-BEAEING SERIES. somewhat plentiful, of a distinct greenisn tinge. The entire, or nearly entire, absence V)f particles of any other mineral than quartz in these rocks is taken to indicate a greater amount of sorting than has been received by materials which have composed the other phases of this forma- tion. This conclusion is borne out by the generally uniform size of the quartz fragments of the vitreous quartzites, the sorting having been carried so tar as to remo^'e not only the particles of other minerals, but the smaller particles of quartz. Microscopical character of the sandstone, novactilite, and argillaceous slates. — (Pliases 5, 6, and 7.) These phases of the Quartz-slate member, because of their relatively small importance, may be more rapidly dismissed. The sandstone phase (PI. xix. Fig. 3) is an unusual one, having been so far observed only in three localities, viz: In the vicinity of the Ashland mine. Sec. 27, T. 47 N., R. 47 W., Michigan; in the Aacinity of the Aurora mine. Sec. 23, T. 47 N., R. 47 W., Michigan, and at a point about three-fourths of a mile east of Sunday lake. Sec. 10, T. 47 N., R. 46 W., Mich- igan. In the first two localities it occurs at a high horizon immediately beneath the overlying vitreous quartzite; but in the last place the sandstone occurs on the contrary at a low horizon. Macroscopically these sandstones are of a rather fine grained arenaceous texture and from red to white in color, these colors being often very irregularly blotched. An examination of the hand specimens, without reference to their source, would certainly suggest that they came from some modem sandstone formation rather than from so ancient a terrane as that we are now concerned with. Microscopical examinations show, however, that thej'^ are merely less consolidated phases of the quartzites or feldspathic quartz-slates above described, these rocks being in fact no whit less frag-mental in texture than the sandstones them- selves. The novaculite or whetstone-like phase (PI. xviii, Fig. 4) of the Quartz- slate member occurs here and there in thin seams in the eastern half of the belt. One of the most noteworthy localities for this phase is the gorge of Tyler's fork where the novaculite shows on the west bank of the river at about 200 paces north of the quarter post on the east line of Sec. 33, T. 45 N., R. 1 W., Wisconsin. This novaculite is nothing more than a very fine and even grained variety of the feldspathic quartz-slates, above described. Tino (H'AKTZ-WLATE MEMHKH. 155 Tlio aro-illiU'cous sLitcs (I'l. \i\, F"'io\ 4) occiir in ,i iiiiiuIxt of |(lii('(^s, l)at iicai-ly always in i-clatixcly tliiii scams, iiiter.strati(ic(| witli the coarser vari- eties iiitii wliicli tliey li'rade. 'I'liese slates a|)])eai' tii diH'er IVoiii the teld- spathic (|uart/,-slates merely in ha\in00 N., 0 W., Sec. 24, T. 44 N., R. 4 W., Wisconsin. A tiiielj' grannlar, dark gray) uniformly textured material, of almost qnartzitic eoinpaetuess, is interbanded with a finely laminated material of a greenish gray color, whicli upon the snrface of the lamina^ shows the sheen of mica. The sections consist chiefly of quartz, feldspar, kaolin, chlorite, and biotite, the first t\yo minerals being fragmental. In the more qnartzitic phases the quartz in rather small nniform sized fragments, some of which have received a secondary en- largement, constitutes two-thirds of the mass of the rock. The feldspar fragments include both orthoclase and plagioclase. Scattered through the mixture of quartz and feldspar, and composing the greater part of the remaining third, are small flakes of chlorite and biotite, the former the more abundant of the two. The chlorite is in well defined flakes which extinguish rectangularly. The schistose part of the rock con- tains a greater proportion of feldspar, kaolin, and biotite than the more qnartzitic por- tion. The biotite and chlorite both appear to be secondary developments from the feldspar fragments. Magnetite is an unimportant accessory. 2. Hornblendic and chloritic quartzites, from the uppermost layers, above 9645 and 9646, and immediately underneath the Iron-bearing member. Specimens 9647 (slide 'The numbers of specimeus and slides are usually those of the collection of the lake Sujierior division. Specimens with Wr. after the numbers are from the collection of the late Mr. Charles E. Wright. Specimeus with Wis. after the numbers are from the collection of the Wisconsin Geo- logical Survey. Locations are given from the southeast corner of the sections, in steps of 2,000 per mile. 156 THE PENOKEE IRON-BEAEING SERIES. 3157), from 1575 K, 0 W., 9648; (slide 3158) from 1635 K, 0 W., Sec. 24, T. 44 K, R. 4 W., Wisconsin. A medium grained, massive, semi-vitreotis quartzite, mottled light gray and dark green. Fragments of a very limpid qnartz compose three-fourtlis of the thin sections. These fragments are often partly or wholly separated from each other by an interstitial material, composed mainly of chlorite in aggregates of pale green flakes, and horn- blende in small greenish needles and blades. However, over the greater part of the sections tlie quartz grains lit closely, or even interlock, and at times the original out- lines of the fragments are quite obliterated. Here and there a little interstitial car- bonate 'is seen. From the section in Sec, 17, T. 44 N., R. 3 W., Wisconsin. 3. Chloritic quartzite. Specimen 4521 (slide 1402). From SW. i, Sec. 17, T. 44 N., R. 3 W., Wisconsin. Rather large interlocking quartz grains compose the greater part of the section. These are believed to be enlarged fragments of quartz, mainly because in juost of the similar rocks of this belt they are manifestly so; but taking this section alone, the proof of the fragmental origin is not conclusive. In masses and Alms between the grains are aggregates of chlorite, the section as a whole being closely allied to those of 2. 4. Biotitic slate. Specimen 9644 (slide 3154). From near the center of Sec. 17, T. 44 N., R. 3 W., Wisconsin. This is for the most part a finely laminated slate, but occasionally bands of greater width and coarser grain occur. It is of a greenish gray color and shows strongly the sheen of mica. The thin section is composed mainly ofvery fine grained quartz and mica, the two minerals being about equally abundant and evenly mingled. The fragmental char- acter of this fine matrix is not entirely plain, but a coarser band running across the section shows the usual large fragments of qnartz, each with a wide secondary enlarge- ment, while occasional quartz grains of the same character are scattered through the fine grained groundmass. The line of demarkation between these quartz fragments and the enlargements is commonly marked by minute flakes of mica. There are also occasionally present large flakes of a rectangularly extinguishing chlorite, and some fragments of feldspar. It is not improbable that much of the fine grained quartz has developed from the alteration of the feldspar. (PI. xix. Figs. 1 and 2.) 5. Chloritic quartzite. Specimen 164 Wr. From 1000 N., 1000 W., Sec. 17, T. 44 N., R. 3 W., Wisconsin. This is a medium grained, gray, vitreous quartzite, which resembles closely 2, but differs in lacking the peculiar mottling shown by those rocks. Intricately interlocking clear quartz grains compose nearly the whole section, the only other minerals present being chlorite and ferrite, unless some of the greenish THE QUARTZ SLATE MEMBER. 157 interstitial inimn-al he aiiipliiholc. Most of tlie quartz individuals have, plainly been enlai-jjcd, aiul tlic lock is certainly an onliiiary fia},nncnlal (|uaitzit('. In tlie case of a few ol' till' iari^c (|uartz individuals the outliiifs of the i)rif;inal tra^iincntal cores arc iHit visihlc; hiil llicse areas arc so coniplctcly like the otiicrs, in wliich the cores arc pcrccplihic, dial all nuisthc taken as of the same nature. IJoth (Mdarf^enicnts and ori;;inal cores are composed of a. sin};ularly pure limpid (inartz ; whence the occasional invisibility of the outlines of the cores. This section, then, furinshcs us abundant proof of the possibility, under favorable cirtHimstancies, of the development, from an ordinary sandstone by enlaij;-cnicnr, of the, (piartz fragments, of a quartzite whose fragmenta! character would never be sus])ccted from the thin section. In this same connection reference should be made to U and li, above described, whose fragmental oi'igin is less clear than in the section now under consideration. From the section, in Sec. 16, T. 41 N., R. 3 W., Wisconsin. G. Hornblendic and chloritic quartzite, from the ui)permost layers and imme diately beneath the Iron-bearing member. Specimen 9tJJ:o (slide 31.33). From 1850 N., 1050 W., Sec. 10, T. 14 :N., E. 3 W., Wisconsin. A rock similar to 2, both macroscopically and microscopically. From the Fenokce (/up section. 7. Magnetitic chert-breccias, from the base of the Quartz slate niendjer. Speci- mens 9534 (slide 3140), 9535 (slide 3141), 1424, Wis. (slide 252). From 1470 N., 1200 W. ; 1455 Wis. (slide 265). From 15D0 X., 1635 W., Sec. 14, T. 44 K, R. 3 VV., Wisconsin. In these peculiar rocks a greenish to black schistose uuitrix contains numerous angular pieces of white chert, and fewer smaller ones of clear (piartz. The groundmass of the thhi sections consists mainly of a minutely crystalline silica of nonfragmental appearance. This is explained by the fact that this rock rests directly upon the friable (juartz i-ock (Nos. 7 and 8, ]>. 136) of the cherty lime- stone and has derived most (>{ its material from this underlying formation. It approaches closely to a recomposed quartz rock, the particles of which are separate grains. With the quartz are mingled more or less chlorite and actiuolite, some brown and red iron oxide, and a large ([uantity of magnetite. In 3140 and 3141 this magnetite plays a very ^subordinate part. In 252, how.wer, it is very abundant, while in 265 it predomhiates over the silica of the cement, occui-ring in most beauti- fully outlined crystals, often of a very considerable size. There is little iit first sight in the matrix of any of these rocks to suggest a fragmental origin. However, there are contained in it, in quantity sufficient to compose from one- third to three-fourths of the area of the thiu sections, fragments of quartz and of a minutely crystalline silica or chert. The quartz fragments are well rounded, and the pic:'cs of chert are only partly SO, being often quite angular, The chert pieces reach as much as a quarter of an jnc^ ]^58 THE PENOKBE IKON BEAEING SERIES. in diameter. The quartz fragmeuts are both simple and complex, and have often received secondary enlargement's. The most notable thing about these rocks is the fact that the crystals of magnetite, and also numerous minute needles of actinolite, besides being contained in the matrix, are included abundantly in the enlargements of the fragmental grains of quartz. These minerals have developed in situ. These very singular rocks come from a belt lying above the chert, which at Penokee gap forms all of the upper part of the limestone member, and the fragmental slates of the member now uader consideration. (PI. xvii, Pigs. 1 and 2.) 8. Biotitic and chloritic slates, from low horizons. Specimens 9562 (slide 3143), 9563 (slide 3097), from 14S0 N., 1700 W.; 9564 (slides 3144, 3145), 9565 (slide 3098), from 1530 N., 1160 W., Sec. 14, T. 44 X., R. 3 W., Wisconsin. These are fine grained, thinly laminated rocks, breaking at times with a sub- conchoidal fracture across' the planes of lamination. In color they range from dark brown or black, through gray, to various shades of green. The thin sections present a ground mass which is always chieily quartzose, the grains being of small but uniform size and generally of roundish or oval shapes. In detail, however, the outlines of the grains are angular, and the projections of the different particles commonly interlock. In many of these minute particles the out- lines of the original cores are perceptible, the enlargements being narrow and pro- ducing the angular projections referred to. The line of division between the enlarge- ment and the original core is generally emphasized by secondary developments of particles of chlorite and biotite. Many more quartz particles, however, do not show any traces of the original cores, and it is not certain that a portion of them may not have crystallized in situ. Small fragments of feldspar make up a part of the ground- mass, scattered through which are particles and clusters of particles of chlorite and biotite. The single flakes of these two minerals are larger than the particles of the groundmass itself, but are still very minute. In some slides tlie biotite almost excludes the chlorite, wliile in others the reverse of this is true. A considerable portion of these two minerals seems plainly to be due to the decomposition of the feldspar fragments. Accessory constituents are sericite, kaolin, tourmaline, and magnetite. 9. Quartzite, from the uppermost part of the slate, and immediately beneath the Iron-bearing member. Specimens 9560 (slide 3192), 9561 (slide 3193). From 1675 N., 1280 W., Sec. 14, T. 44 N., 11. 3 W., Wisconsin. A flue grained, quite grayish quartzite, having a conchoidal fracture. Quartz in rather small but uniform sized particles composes three- fourths of the sections. The i»articles of this mineral, although they very often closely fit and inter- lock, are plainly in the main of a fragmental nature. Biotite in 3192 and chlorite in 3193 occur sparingly between the (luartz particles. Numerous fragments of feldspar, flakes of sericite, and a few particles of limonite are seen. The sections are closely allied to those of 8. . THE CHJAKTZ-SLATH MEMBER. 159 10. Chloiitic- Mild hiotilic sl:itc. S|.i'(im«'n l.V^i (slide UOr,). From NE. J Sec. 17, T. 44 N., li. li \V., Wisconsin. Till' tliiii section is cDiiipiiscd ol' c\<-cc(lMi.;;ly line |>;nliclcs of iiuartz, feldspar, biotite, chlorite, and scricite. Tlic general ai)|icarauce is a fnij-iiieutal oue, but the grain is so minute that it would he ditliciilt lo |Mdvr a fragiueiital nature from this section aloue. From the Tykys fork nccfion. 11. (Jhloritic and biotitic shitc. Specimen Olil.j (slide .ilMtfJ). Prom 1135 N., 1980 W., Sec. 34, T. 45 N., R. I W., Wi.scoQsiii. A lino grained, slaty rock, breaking with eoucihoidal fracture across the lamina- tion. Upon cleaved surfaces are seen numerous flakes of white mica of a light gray color. The chief constituents of the thin section are (piartz and feldspar in plainly frag- mental particles, the former being very plentiful, and having the fragments ordinarily enlarged. Between the grains of (piartz and feldspar, and also included in the feld- spar fragments as secondary products, are numerous small folia of chlorite biotite and sericite. '^ From tlic I'utato river section. 13. Magnetitic quartz-slate and conglonierate, from the base of the Quartz-slate member. Specimens 0094 (slide 3297), 9()9G (slide 1!S<»5), 9175 (slides 2994, 2995). All from 888 N., 35 VV., See. 19, T. 45 K, R. 1 E., Wisconsin. The lowest portion of the Quartz-slate member ou Potato river, at and near the junction with the underlying green schists, diffei's from the body of the member above described only in the presence of mucjh magnetite, along with numerous pebbles and bowlders derivedfrom the lower schists, also fewer small tiaginents of jasper and chert. The background of the sections differs from the slates before described (mly in that they contain a very considerable (luantity of magnetite in well defined crystals, and aggregates of crystals, these being at times so large and numerous as to make up juost of the sections. As indicated in the macroscopic description, these rocks contain peb- bles and bowlders derived from the green schist immediately below, and also fewer of quartz, chert, and jasper. In sections 2994 and 2995 is seen a large clierty area in which are numerous magnetite particles. The occurrence in these rocks of numerous fragments ofcherty and jaspery material, associated with an abundance of magnetite, caUs to mind the peculiar magnetitic breccia whicli lies at this same horizon on Bad river, where, however, there lies between it and the lower schists a thickness of som(> 90 feet of limestone and white chert. The Cherty limestone member is entirely lack- ing at Potato river, but the occurrence just described suggests that it and a jasper formation may have formerly been here. (PI. xvti, Fiy. i.) 13. Feldspathic quartzites, or gray wackes, ehloritic (piartz-slates, and graywacke- slates, from middle and lower horizons. Specimens 9083 (slide 2781), from 950 N., 180 IQQ THE PENOKEE lEON-BEAEING SERIES. W.; 9084 (slide li782), from 970 N., 200 W.; 9086 (slide 2783), from 9(30 N., 185 W.; 9088 (slide 2784), from 960 K, 185 W.; 9089 (slide 2785), from 910 N., 105 W.; 9099 (slide 2788), from 888 I^., 36 W.; 9100 (slide 2896), from 889 N., 37 W.; 9101 (slide 2897), from 890 N., 38 W., Sec. 19, T. 45 N., R. 1 E., Wisconsin. Except at the extreme north, where the quartzite 14 is found, the exposures of • the Quartz-slate member on Potato river consist of interstratiflcations of thinly- bedded pale pinkish quartzitic layers, and greenish (chloritic) gray and brown argil- laceous slates. These grade into one another and are all described here together. The thin sections from the more quartzitic portions of the Potato river exposures show fragments of quartz as the predominant ingredient, but mingled with the quartz fragments are plentiful ones of feldspar, including orthoclase, microcline, and plagio- clase. The quartz fragments have nearly always received enlargements which have produced an interlocking of the (quartz areas. The feldspar fragments on the whole are singnlarly fresh. Chlorite, ferrite, and calcite are present as accessories in some of the sections, occurring as narrow films and areas between the fragmental particles. From the quartzitic phases there is found a gradation to those vai'ieties which are entirely aphanitic in the hand specimens, and are so exceedingly fine grained and clayey that in the thin section little is to be made out save that minute particles of (piartz and feldspar are scattered through the clayey background, which is taken to be composed of pulverized and kaolinized felds])ar, mingled with chlorite, sericite, etc. Slide 2788 is at the extreme as to fineness of grain and small number of recognizable quartz and feklspar particles. In the intermediate varieties half or more than half of each section is composed of minute but still distinctly recognizable frag- ments of quartz and feldspar, the quartz grains being almost always provided with plainly visible enlargements, even when widely separated from one another by tlie intermediate clayey matrix. This matrix in many sections, in additi(m to the minerals mentioned above (namely, chlorite, sericite, kaolin, and ferrite), contains calcite and occasionally crystals of magnetite, which last mineral is particularly met with in sec- tions from specimens which come from not very far from the underlying green schists. 14. Quartzite, from the uppermovst horizon of the Quartz-slate member, imme- diately beneath the Iron-bearing member. Specimen 9082 (slide 2780). From 1000 N., 210 W., Sec. 19, T. 45 N., R. 1 E., Wisconsin. A coarse grained, vitreous (quartzite, of a pale pink color. The section is almost wholly composed of clear quartz in large intricately inter- locking areas. Each one of these areas, however, is furnished with a very distinctly outlined fragmental core, and occasionally the secondary enlargement lias received ■crystal outlines. The cores and enlargements are separated from one another by films of ferritic material. This section furnishes one of the very finest illustrations tliat we have met with of the production of a vitreous quartzite from a completely fragmental .sandstone by the eulargenieut process. (PI. XX, Figs. 1 and 2.) THE QUAKTZ-aLATlO MEMBER. 161 From the section in- Sec. li, T. IS N., li. 2 E., Wiscomm. 15. Chloritio slato, from a low horizon underlying 9145 (Hi). Specimen 9133 (slide 3300). From 1000 N., KJOO W., Sec. 6, T. 45 N., K. 2 E., Wiscousiu. A line grained, thinly ch^avalile, grayish green slate. Minute fragments of (juartz and feldspar, the former enlarged in the usual manner, arc mingled with a liner material composed of kaolin, sericite, and ferrite. 10. (Jhloritic quartzite, from the uppermost lioi'izon. Specimen 9145 (slide 2800). From lOOO N., 1535 W., Sec. 0, T. 45 N., R. 2 E., Wisconsin. A line grained, vitreous, greenish gray quartzite. The thin section is composed mainly of rather small quartz fragments, which are provided, however, with wide and deeply interlocking enlargements. Chloritic flakes with a little ferrite occur in the interstices of the grains. From the West branch of the Montreal section. 17. Chloritic slate and conglomerate, from the basal portion of the Quartz-slate member, and iu contact with the underlying greeu schist. Specimens 9149 (slide 2912), 9171 (slide 2914). From 175 N., 1035 W., Sec. 27, T. 46 K, R. 2 E., Wisconsin. Specimen 9171 contains large pebbles of green schist, cemented by a dark green, uniform grained matrix, in which are recognizable numerous grains of quartz, feld- spar, and calcite. Specimen 9140 is like the matrix of 9171. They are both from the junction with the underlying greenish schists. The slides are like those described in 12, except that they carry numerous frag- nients of the greeuish schist beneath them, and much secondary calcite and chlorite, presumably derived from the secondary alteration of the schist fragments. IS. Sericitic and chloritic graywackes and graywacke-slates, from the middle horizons. Specimens 9035 (slide 2919), from 190 K, 1130 W.; 9038 (slide 2771), from 210 N., 1080 W.; 9040 (slide 2772), from 210 N., 1020 W.; 9043 (slide 2773), from 270 if., 1000 W. ; 9044 (slide 2774), ti-om 320 N., 1010 W., Sec. 27, T. 46 K, R. 2 E., Wis- consin. The rocks are flue grained, compact, of a uniform texture and subconchoidal fracture when broken across the lamination; are often readily cleavable parallel to the lamination, into large thin plates, the cleavage surfaces showing the sheen of mica. The colors vary through shades of green, pink, gray, and purple. The slides differ somewhat widely iu the quautityof fragniental material which is recognizable as such. The finer interstitial material, varying from a quite subordinate quantity to a predomiiuiting amount, is composed of flakes of sericite and chlorite, along with some minutely crystalline quartz and also kaolin and brown iron oxide. The quartz and feldspar fragments occur, as in thin sections of similar rock previously described. The feldspar particles, while fresh in the inner portions, are commonly altered upon the exterior to chlorite and kaolin, which minerals, along with the seri- cite and minutely crystalline quartz, are taken to be due to metasomatic changes. MON XIX 11 162 THE PENOKEE lEON-BEARmG SERIES. 19. Ferruginous and feldspathic quartzites, from the upper horizons. Specimens 9151 (slide 2802), from 310 N., 1120 W.; 9152 (slide 3104), from 310 N., 1130 W.; 9153 (slide 2803), from 310 N., 1140 W.; 9154 (slide 2804), from 350 N., 1132 W.; 9045 (slide 2885), from 395 N., 1055 W., Sec. 27, T. 46 N., E. 2 E., Wisconsin. The specimens vary from those of a medium grain, vitreous luster, and con- choidal fracture, to those which are very line grained and lusterless. They are stained from a light to a dark brown by hydrated iron oxide. Kather small and well rounded fragments of quartz, now provided with inter- locking enlargements, predominate in the thin sections. Fragments of orthoclase and plagioclase, commonly quite fresh, occur in some abundance. In some sections large films and areas of hydrated oxides of iron are plentiful between the fragments; in other cases (2802) chlorite flakes occur mingled with the iron oxide. (PI. xx, Figs. 3 and 4.) 20. Quartzite, from the uppermost layers of the Quartz-slate member, being the foot wall at the Bessemer mine. Specimen 9054 (slide 2920), from 1000 N., 1838 W., Sec. 26, T. 46 K, E. 2 E., Wisconsin. Both macroscopically and microscopically this quartzite is exactly like 14. From the Germania mine section. 21. Slaty feldspathic quartzite, from a high horizon and immediately beneath 9013. Specimen 9011 (slide 2766). From 133 N., 1500 W., Sec. 24, T. 46 N., R. 2 E., Wisconsin. The rock is composed of interstratified seams of a coarse pinkish quartzite and a flue grained green material. Fragments of quartz and feldspar make up most of the section, the particles being quite angular. In the case of the quartz this angularity is pla,inly due to a secondary enlargement. Between these fragments is an interstitial material, com- posed ill part of a minutely crystalline quartz, with limonite, kaolin, and sericite in minute flakes. The section resembles closely 2802 in 19, above described. 22. Quartzite, from the uppermost horizon, immediately in contact with the Iron- bearing member, being the foot wall of the Germania mine. Specimen 9013 (slide 2767). From 200 N., 1560 W., Sec. 24, T. 46 N., R. 2 E., Wisconsin. Both macroscopically and microscopically this rock is like 14. From the Ashland mine seetioti. 23. Slaty and cherty quartzite, from a low horizon. Specimen 7621 (slide 2017). From 1805 N., 1910 W., Sec. 27, T. 47 N., R. 47 W., Michigan. This rock is composed of thinly interstratified seams of a greenish or yellowish gray vitreous quartzite, and of a dark green aphanitic material. The thin section. appears to be cut wholly from one of the quartzitic seams. It TMK QUAKTZ SLATE MEMHIOK'. ] 63 shows a mixture of tViiginciits oC (iiuutz and of a niiimlcly crystalline cherty silica, iiiiboddcd in a cement which composes about one-third of the section, and consists of a mixture of very minutely crystalline silica, with particles of chlorite and ferrite. The (|uart/, fratinu^nts at times show very distinct secondary enlargements. A few feldspar fragments are seen. '2i. Quartzites, from a high horizon. Specimens 9005 (slide 3i01i), 7620 (slide 201(i). From 18S0 N., 1910 W., Sec. 27, T. 47 N., li. 17 W., Michigan. These are compact, coarse grained, vitreous (luartzites, of ai gray color. Quartz fragments, with the usual interlocking enlargements, and mingled with some pieces of feldspar, make up most of the sections. The enlargements of the quartz fragments are unusually wide. The outlines of the cores within these enlarge- ments are marked in a distinct manner by films of chlorite and brown iron ore, which minerals also occur along the line of junction between the enlargements of difl'erent grains. A number of instances are noted of the enlargement of complex quartz frag- ments, each individual area withiu the fragment having received its own enlargement. 25. Sericitic ferruginous sandstone, from a high horizon. Specimens 9004 (slide 2880), 7(>18 (slide 2015). From 1880 N., 1910 W., Sec. 27, T. 47 N., E. 47 W., Michigan. Specimen 7018 is massive, medium grained, dark reddish brown, and of a sub- vitreous luster; 9004 is nearer a slate, the quartzitic parts being interstratified with narrow green belts. These slides are intermediate between the fragmental quartzites and the argil- laceous slates of this member. The fragmental quartz particles still predominate, along with some fragmental feldspar, the quartz grains being either without enlarge- ments or with only very narrow ones. Contained between the fragments and serving as a matrix to them is a very fine nmterial composed of browm iron oxide, kaolin, minutely crystalline quartz, and chlorite. Numerous flakes of sericite occur also, ap- pearing rather as if fragmental, or altered from the fragmental mica, than as if sec- ondary to some of the other fragmental constituents of the rock (PI. xix, Fig. 3). From the Aurora mine section. 26. Slaty and cherty quartzite, from the uppermost layers, being the foot wall of the Aurora nnne. Specimen 7614 (slide 2014). From 624 N., 1000 W., Sec. 23, T. 47 N., E. 47 W., Michigan. The thin section shows an interlamination of coarser and finer materials. The coarser bands are composed in about equal quantity of fragments of fine quartz indi- viduals and of a chloritic chert. The finer grained bands are made up of fragments of quartz and feldspar, along with particles of probably secondary silica, chlorite, and iron oxides. 164 THE PENOKEE lEON-BEAEING SERIES. From the section at the Mount Hope mine. 37. Ferruginous quartzite, from the uppermost layers, being the foot-wall at the Mount Hope mine. Specimen 7611 (slide 2307). From 1140 N., 0 W., Sec. 23, T. 47 N., E. 47 W., Michigan. The thin section is composed of large interlocking quartz areas, at the junctions of which are often films of brown iron oxide. Each quartz area is furnished with a very distinct, well rounded, fragmental core. From the section between the Blue Jachet and First National mines. 28. Cherty quartzite, from a low horizon. Specimen 7602 («lide 2010). From 1900 N., 1150 W., Sec. 19, T. 47 N., E. 46 W., Michigan. Both in the hand si)ecimen and in the thin section this rock is like 23, which occurs at a similar horizon. 29. Ferruginous quartzite, from a middle horizon. Specimen 7603 (slide 2303). From 1980 N., 900 W., Sec. 19, T. 47 N'., E. 46 W., Michigan. The specimen and thin section are as in 27. From the Colby mine section. 30. Sericitic and chloritic slate (iiovaculite), from a low horizon. Specimen 9523 (slide 3091). From 775 N., 770 W., Sec. 16, T. 47 K, E. 46 W., Michigan. This rock in specimen and thin section resembles closely 9038 aud 9044 in 18, from the West branch of the Montreal river. (PI. xviir, Fig. 4.) 31. Quartzite, from a high horizon. Specimen 7591 (slide 2006). From 1000 N , 548 W., Sec. 16, T. 47 N., E. 46 AV., Michigan. This rock in specimen and thin section is lik* 7620 in 24 from the Ashland mine section. From the Tilden mine section. 32. Quartzite, from an upper horizon. Specimen 12524 (slide 5333). From 1228 N. to 1153 N., 1166 W., Sec. 15, T. 47 N., E. 46 W., Michigan. The rock is a fine grained, massive, greenish gray vitreous quartzite, having a conchoidal fracture. The thin section consists chiefly of rather small interlocking quartz areas, each with its fragmental core. A considerable quantity of fragmental feldspar is also present. Outlining the original fragments of quartz are Alms of chlorite and ferrite, the greenish color of the rock being produced by the former. The chlorite and fer- rite particles also occur where the eidargements of the different grains meet each other. From the Palms mine section. 33. Conglomerate. Specimens 12522 (slide 5388), 12893 (slide 5511). From SE. 4 of XE. 4, Sec. 15, T. 47 N., E. 46 W., Michigan. TniO QKAltTZ-SLATE MEMUKH. 105 A breccia composed chietly of wliilc, ^ray, black, and red chert, Imt contaniiiif;- also rrat-iiiiMits of other niiiierals aii444 (sli.lc .'U;!;?), 7.507 (slido 1!>52). From 1985 N., 750 W., Sec. 15, T. 47 N., K. 45 W., Micliifian. Tlic siK^ciiiit'iis and tliiii sections rcscnihlc, (-loscly those oC 24. 40. (Jliertcoiif>loiiu'i'ate, IVoni near the base of the (Jiiartz-shite member, aucl immediately beneath 7505 (41). Specimens 0440 (slide 31;M), 7.500 (sli(h^ 10.51 ). From 1908 N., 750 VV., Sec. 15, T. 47 N., K. 45 \V., Michigan. The matrix of this rock is fine grained, compact, and of a gray color. In this matrix ai'c very numerous rounded pebbles of chert, which are sometimes several inches in diameter. lu the thin sections the matrix of this conglomerate is seen to be composed of materials of two degrees of fineness. The finer appears, as is general with the slates of this member, to be composed of minutely crystalline, and, at times, even an amor- phous silica, with which are mingled some kaolin and ferrite. The coarser i)ortion of the matrix is distinctly fragmental and is made up wholly of quartz pieces, nearly all t)f which show the usual secondary enlargements. The pebbles of the conglomerate are wholly of a chert identical with that characteristic of the limestone member, as previously described. They are seen to be made up of an intimate mixture of a minutely crystalline and an amorphous or opaline silica. 41. Sandstone, from a very low horizon. Specimen 7.505 (slide 1948) From 0 N., 750 W., Sec. 10, T. 47 N., R. 45 W., Michigan. This rock in specimen and thin section resembles 35 from the Black river section. 42. Argillaceous slates or shales, from a low horizon. Specimens 7502 (slide 1944), from 2 N., 177 W.; 7503 (.slide 1945), from 10 N., 177 W., Sec. 10, T. 47 N., R. 45. W., Michigan. Aphanitic, highly laminated slates or shales having a strong clayey odor, and colors varying from reddish brown to greenish gray. The thin sections differ from those of 43 in that the fragmental particles of quartz and feldspar are now insignificant in quantity, the clayey matrix assiiming a relatively important role. 43. Argillaceous slates or shales. Specimens 7504 (slide 194C), from 1985 N., 450 W.; 7505a (slide 1949), from 0 N., 750 W., Sees. 10 and 15, T. 47 N., E. 45 W., Michigan. Compact clayey slates or shales, not always showing the hues of sedimentation ; in color varying from greenish gray to reddish brown, some specimens presenting both colors. The thin sections show small fragments of quartz a^d feldspar, the former the more abundant and commonly provided with secondary enlargements, imbedded in a groundmass composed of ferrite, chlorite, kaolin, and minutely crystalline silica. (PI. XIX, Fig. 4.) 44. Sericitic graywackes, from a low horizon. Specimens 9432 (slide 3130), from 1925 N., 1915 W.; 9435 (shde 3132), from 1935 N., 1940 W.; 9436 (slide 3068), from 168 THE PENOKEE IRON-BEARINa SERIES. 1935 N., 1940 W.; 7510 (slide 1966), from 1925 K, 1890 W., Sec. 14, T. 47 N., R. 45 W., Michigan. Ihese are fine grained slaty rocks, of a uniform texture, and greenisli to pinkish gray colors upon the cleavage surfaces. Except in 9432 the sheen of mica is seen. The thin sections of these rocks, with the excei)tion of 3130, which is peculiar in containing a relatively small quantity of sericite, are very close to 2771 and 2774 in 18. 45. Quartz-slate and cUoi'itic slate. Specimens 9453 (slide 3074), 9452 (slide 3073). From 200 N., 800 W., Sec. 10, T. 47 N., R. 45 W., Michigan. These slates consist of alternating bands of coarse quartzitic and aphanitic greenish material. • The aphanitic portions of these rocks, as seen niacroscopically, show in the thin section an intimate mixture of minute particles of quartz, feldspar, chlorite, kaolin, and biotite. The coarser seams show a groundmass of the same natui'e, in which are carried an abundance of rounded fragments of quartz, with which are associated numerous fragments composed of an exceedingly minutely crystalline silica, mingled with flakes of kaolin, or of a micaceous material. It seems not improbable that each of these fragments may be due to the decomposition of a single feldspar. 46. Quartzite and slate, from an upper middle horizon. Specimens 7495 (slide 1940), from .300 N., 0 W.; 7496 (slide 1941), from 140 N., 0 W., Sec. 10, T. 47 N., R. 45 W., Mic^higan. These specimens represent an interstratiflcation of greenish gray, semivitreous, medium grained quartzite, and argillaceous slate. The thin sections of these rocks are from the more quartzitic portions of the specimens. They closely resemble those of 37. 47. Chloritic and sericitic quartzite, from a high horizon. Specimen 94.54 (slide 3075), from 370 N., 800 W., Sec. 10, T. 47 N., R. 45 W., Michigan. A fine grained, greenish gray, massive quartzite, with a conchoidal fracture. The thin section shows large fragments of quartz and feldspar buried in a groundmass of minutely crystalline silica, mingled with cldorite and sericite flakes. The feldspar fragments are nmch altered, the resulting products being chlorite, seri- cite, and quartz. The manifest production of these thi'ce secondary minerals from the lai'ger fragments of feldspar makes exceedingly probable a^ similar origin for the same minerals in the matrix. 48. Quartzite, from immediately beneath the Iron-bearing member at the top of the Quartz-slate member. Specimen 7513 (slide 1954), from 420 N., 873 W., Sec. 10, T. 47 "N., R. 45 W., Michigan. A medium grained, massive, semivitreous quartzite. The thin section of this rock closely resembles 3069 in 37. TlIK CiUAUTZ-HLATE MKMIJER. 109 From tht; sectlou iti norlltrm part of Sec. If, T. 47 N., R. 4.'> W., Michigan. W. Obert-conglouio.riite, IVoiii the bottom of the Quartz-slato iiicinbei-. Speci- iiit'iis !tns (slide ;5(r)i)), ittlO (slides ;ill>(J, ;iiul 3127), Irom 1775 N., 1075 VV., Sec. 14, T. 47 N., R. 45 \V., Mi<^iiiKiiii. The matrix of this eouf'loinerate is tine gi'ained aiid of a dark green eolov. In the matrix are buried very plentiful rouuded. IVagments of translucent oijalinti cht^rt, which vary in size from three-fourtlis of an inch to two inches in diameter. The matrix of this conglomerate is seen in the thin section to be composed of rather small fragments of (puirtz and feldspar, the former pi'edominating, mingled, with a considerable proportion of interstitial cherty and amorpiious silica, along with some cldorite, brown iron ore, and kaolin. Large particles of mica, apparently frag- meutal in nature, are also seen. The pebbles are wholly composed of a ehcu't identical with that of the limestone member below, being in part a minutely crystalline and in part an amorphous silica. The slides closely resemble 3134 and 1951 in 40. (PL XVII, Fig. 3.) 50. Chert-breccia or conglomerate, from the bottom of the Quartz-slate member. Specimen 94L'0 (slide 3000), from 1775 N., 1225 W., Sec. 14, T- 47 N., R. 45 W., Mich- igan. This rock differs from 49 in that its matrix aijpears to be a chloritic and ferru- ginous chert, while in the latter the matrix is entirely fragmental. The pebbles and matrix in thin section differ from each other only in pvirity; the pebbles being composed of a semicrystalline cherty silica, while the matrix is composed of the same silica mingled with much ferrite and chlorite. A few large fragments of quartz are seen. 51. Quartzite, from near the bottom of the Quartz-slate member. Specimen 9421 (slide 3061), from 1775 N., 1200 W., Sec. 14, T. 47 N., R. 45 W., Michigan. A dark reddish, semi vitreous quartzite. , In the thin section well rounded quartz fragments with small enlargements are buried in a fine interstitial material composed of minutely crystalline and amorphous silica, kaolin, chlorite, and toown iron oxide. From the exposures on the Peninsular Mining Gompany^s property. 52. Ferruginous quartzite. Specimen 12789 (slide 547G), from 1910 N., 1040 W., Sec. 13, T. 47 N., E. 45 W., Michigan. A medium grained, vitreous quartzite, stained a dark brown color by iron oxide. About .one-half of the thin section is comj)osed of widely enlarged fragments of quartz; the main part of the remainder of the section consists of minutely crystalline silica, mingled with chlorite and ferrite particles, with a few fragments of orthoclas^e, microcUne, aud plagioclase. 170 THE PENOKEB IRON-BEARING SERIES. From the exposures in Sec. 17, T. 47 N., R. 44 W., Michigan. 53. Feldspathic quartzites and quartz- slates, from near the bottom of the Quartz- slate member. Specimens 9410 (slides 3125 and 3271), 9411 (slide 3052), from 525 N., 1925 W.; 9389 (slide 3043), from 350 N., 1550 W.; 9390 (slide 3044), from 350 N., 1550 W., Sec. 17, T. 47 N., R. 44 W., Michigan. Some of these specimens are slaty, while others are more quartzitic and vitreous. The color varies from gray to black, being in places mottled with red spots. The thin sections are made upof fragments of quartz, feldspar, chert, and jasper. The quartz fragments are usually widely enlarged ; the cherty and jaspery pieces seem in the main to represent the chert of the limestone belt immediately below. There are, however, a good many fragments which appear to be intimate mixtures of chlorite, minutely crystalline quartz, ferrite, and a koalin-like mineral. Tliese may possibly be altered feldspars. The sparse matrix is very heavily stained with brown iron oxide. 54. . Chloritic slate, from near the base of the Quartz-slate member. SiDCcimen 9409 (slide 3051), from 525 N., 1925 W., Sec. 17, T. 47 N., R. 44 W., Michigan. An aphanitic light green slate or shale, cleavable into tliin plates parallel to the lamination. The thin section shows an intimate mixtui'e of an exceedingly minutely crystal- line quartz, with flakes of pale green chlorite. 55. Clay-shales or clay-slates, from near the base of the Quartz-slate member. Specimens 9391 (slide 3045), from 350 N., 1550 W.; 9412 (sbde 3053), from 525 N., 1925 W., Sec. 17, T. 47 N., Rj 44 W., Michigan. These are flue grained and finely laminated slates or shales ; 9412 is gray, and 9391 reddish purple. The latter shows plentiful flakes of sericite. The thin sections of these rocks are closely like those of 42. 56. Graywackes and chloritic slates, from a low horizon. Specimens 12067 (slide 5391), 12608 (slide 5392), from 15 N., 05 W.; 12669 (slide 5393), from 78 N., 42 W.; 12670 (slide 5394), from 100 N., 105 W., Sec. 17, T. 47 N., R. 44 W., Micliigan. These specimens range ft'om slaty to more massive kinds, varying in color through dark green, reddish, dark brown, gray, and greenish shades. All are aphanitic. The thin sections show that they are of the typical slate of the Quartz-slate member, being composed of fragments of quartz and feldspar, buried in the usual argillaceous matrix, in which chlorite, kaolin, sericite, and a minutely crystalline quartz are recognizable. 57. Feldspathic quartzite or graywacke, from a middle horizon. Specimen 9394 (slide 3122). From .470 N., 975 W., Sec. 17, T. 47 N., R. 44 W., Michigan. A coarse grained, greenish gray, vitreous quartzite. The thin section of this rock is closely similar to 2016 in 24. 58. Chloritic slate, from a high horizon. Specimen 12682 (slide 5404). From 477 N., 790 W., Sec. 17, T. 47 N., R. 44 W., Michigan. THE (^UAUTZ-SLATE INFEMBEK. 171 An olive }i:reeu rock of nearly aphanitit! toxturo, and slaty structure. TIk^ tiiin section shows a ijredoniiuatiiif;' line i^raiiuxl background conii)oscd of flakes of clilorit(^, sericite, and kaolin, witli sonic ininutc^ly crystalline (juurtz, tliron<'ii wliicli aie scattei'ed rather plentifully small fragments of quartz and rcldsjtar. The section is cut across the lamination of the slate, and shows flakes ofcldoritc, sericite, etc., arranged with their longer axes in a comnion direction. .">!>. (inart/.iles, from a high horizon. Spc^cimens lli(>SO (slide 5402), ll'tilSl (slide 54();$). From 477 N., 795 to 825 W., Sec. 17, T. 47 N., R. 44 W., Michigan. These si)ecimens, taken from the same pit, ditt'er in character; 120S() is a massive, medium grained, ordinary quart/Jte; 12(i.Sl, a massive, strongly magiietitic (juartzite. The thin section 5402 is that of a typical (juartzite, wliose induration and vitreims appearance are due to the wide enlargement of each grain. Broad films of chlorite and ferrite separate cores and enlargements and occur again along the con- tacts of the eidargements themselves. There are some irregular areas com])osed of hematite, magnetite, and a minutely crystalline quartz. The quartz areas are all arranged with their longer axes in a common direction parallel to the structure, which perhaps indicates subjection to pressure. The thin section 5403 differs from 5402 iu tliat the fragmental ([uartz grains compose only about half of the mass of the rock, there being a very plentiful interstitial material consisting mainly of magnetite and containing also a considerable quantity of hematite, chlorite, and minutely crystalline quartz. The magnetite in this matrix is for the most part in very regular outlined crystals. These crystals occur also within tlie enlargements of the (piartz fragments, and occasionally appear to be even within the cores themselves, although this appear- ance may be due to the positions at which these grains are cut. They also occur somewhat jdentifully along the bounding line between fragmental cores and enlarge- ments. That portion of the magnetite which occurs within the cores themselves appears to be very distinctly more plentiful in the neighborhood of their outlines. As to this singular occurrence, see the description of 7. Contacts ivith the Cherty limestone member. — As a rule the contact of the Quartz-slate member with the more southerly rocks is concealed. At several points, liowever, it may be seen, either in contact with or very close indeed to exposures of the white chert or of the limestone itself of the underlying formation, and in other places again with the granite, gneiss, or schist of the Southern Complex. The contacts with white chert are at Penokee gap, Sec. 14, T. 44 N., R. 3 W., Wisconsin ; in the NW. | Sec. 1 6, T. 44 N., R. 2 W., Wisconsin ; in the northern part of Sec. 14, T. 47 N., R. 45 W., Michigan; and in the SW. \ Sec. 17, T. 47 N., R. 44 W., Michigan. It has already been intimated that in these places there are found in the 172 THE PENOKEE IRON-BEAEING SERIES. lower layers of the quartz-slate numerous fragments of the chert which immediately underlies it, and sometimes the rock becom.es a recomposed quartz rock, disting-uishable from the underlying formation onl}^ by the presence of a small portion of plainly recognizable fragmental material. These peculiar occurrences are taken to indicate that between the two formations was a more or less extended erosion interval. The chert fragments in these places occur both in basal conglomerates and in thin bands interstratified with a nonconglomeratic slate. As the contact is receded from the chert fragments become very soon of smaller size, although pieces of the same chert are occasionally microscopically recognizable even in the higher liorizons of tlie fomtiation. . Contacts with the Southern Complex. — At Potato river and West branch of Montreal river the Quartz-slate m'^ember is seen in contact with greenish schists on the south. The Potato river junction is illustrated by Fig. ^^ %FerruffmoiLsC?iert. Xr"^ Fig. 5. — M.ip i:ai.tz-slath mi:mi!i:u. 175 Clian/fr Id the fniii-lirdrinf/ inniihcr. — 'I'lie c-oiitiu^t l)etweeu tlio Quartz- slate ami file Inni-luiariiif^- iiieiuber wliicli imiiicdiately overlies it is exposed in scores of places, as a result of mining- operations. Nowhere where it lias been seen, however, does tliis contact sugf^est anything but the most abru})t cliange from one formation to Ihe other. In places the upper part of the (piartzite appears to be no more tlian a coarse sand, and occasionally blocks of it are contained in the basement layers of the iron formation. This would seem to indicate that here and there the quartzite was some- what broken before the beginning of the deposition of the overlying mem- ber or else by dynamic movements. Also, the upper part of the quartzite is often heavily stained with iron oxide which has been carried down along the cracks by leaching action. Nevertheless, the change from one forma- tion to the other is astonishingly abrupt, it often being possible to locate to a fraction of an inch the plane between the two formations. Upon one side of this plane is the coarse fragmental quartzite; upon the other the nonfrag- meutal varied rocks of the Iron-bearing member. Prominent exposures. — As stated at the beginuing of this chapter, the Quartz-slate is one of the best and most continuously exposed members of the entire Penokee sjeries. This will be realized from an inspection of the detailed maps herewith (Pis. v to xiii), upon which, however, are placed only the more accurately located exposures. Further detailed work and measurements would undoubtedly enable us to locate about as many more. In the following notes we refer only to those exposures which are particu- larly prominent, eitlier on account of their size or because they show some noteworthy peculiarity. Beginning at the west, the first exposures worthy of mention are. the large ones running along the north side of the Marengo river in the extreme southwestern portion of Sec. 14, and northwestern portion of Sec. 23, T. 44 N., R. 5 W., Wisconsin. The river runs nearly along the strike of the slate, and also apparently along its contact with the more southern granite, which shows in bold exposures all along the south side of the river. The slate is one of the biotitic and chloritic varieties. In the extreme northeastern part of Sec. 24, T. 44 N., R. 4 W., Wisconsin, several exposures of the quartzite, which forms the uppermost horizon of the quartz-slate, are to be seen. One of these lies on the eastern line of 176 THE PENOKEE IRON-BEARING SERIES. the section at about 400 steps south of the northeast corner. Immediately - to the north of this quartzite, but not directly in contact with it, are expos- ures of the Iron-bearing member, and immediately to the south larger ones of a biotitic and cldoritic slate of the Quartz-slate. Still larger exposures of quartzite and biotitic quartz-slate are shown along the crest of the Penokee range in its course across Sec. 17, T. 44 N., R. 3 W., Wisconsin. The east- ernmost one of these exposures shows a contact on the north with a mag- netitic schist. This contact lies 550 steps south and 400 steps west of the northeast corner of the section. Similar large exposures show in the northern part of Sees. 16 and 16, but much more striking and complete exposures are those met with in the vicinity of Penokee gap, where Bad river passes the Penokee range from south to north. The position and extent of these exposures will be best understood by an inspection of PI. xxxvi. The m6st striking of them is that which forms a bold hill at the southern end of Penokee gap itself Here the entire width of the Quartz- slate is seen, from its contact with the chert of the underlying limestone formation on the south, northward to its contact with the iron formation. The latter contact is between the usual uppermost vitreous quartzite of the Quartz-slate and an actinolite-magnetite-slate of the Iron-bearing member. Contacts are also seen at two points respectively 300 and 400 paces farther west. Except in the uppermost horizons where we have the usual viti'eous and relatively massive quartzite, the greater portion of the rock hei'e exposed is quite thinly slaty, seams of light and dark gi'ay and light and dark greenish slate alternating rapidly with one another. On the whole these slates are more generally of the chloritic and biotitic phases above described. Proceeding farther eastward the next very noteworthy exposures are those at mount Whittlesey on the south side of the Penokee range. This bold south-facing cliff lies at the corner of Sees. 8, 9, 16, and 17, T. 44 N., R. 2 W., Wisconsin. The northward angle of inclination of the slate here varies from 56^ to 65°, the edges of the inclined layers projecting on the face of the cliff in such a manner as to make an exceedingly jagged and irregular precipice. The greater part of the slate here belongs to the chloritic and biotitic quartz-slate phase. Quite a large portion of the thickness of the formation is exposed at the gorges of Carrie's creek, NE. | Sec. 11, T. 44 N., THE (iUAKTZ-SLATK MEMBER. 177 K. 2 W., Wisf-oiisiu: iiiid ajriuu in the X Iv ' oftlic SK. ', Sec. 1, T. 44 X., R. 2 W., ^^'isl•()Ilsi^, \\liure are to Ix' iiidcil aiiiono- the iiidic usual phast^s of the slate a iimiil)ci- of laxcrs of a pink to ln'ick i'c(| and luatl j;'i';iy shale, a [)liase wliirli lias tluis far not been noted to the west of this point, though oerurring somewhat fretiueutly further to tlu^ i^ast. 'I'hi^ greater part of the thickness of the Quart/,-slate formation is exposed again at tlie gorge of Tylers fork, SK. \ of tiie XE. \ Sec. 33, T. 45 N., R. 1 W., Wisconsin, the contact with the overh'ing iron formation Ijeing finely exposed on the Avest bank of the river, as indicated on the foUowhig sketch map (i^'ig. 8). The "■f' Itonbeannrfinen ' i Quat Iz date Tnember Fio. 8 Map and sectiou ahowui!; iiusituin of rock exiiosiircs at lylers loik. (Frmii Plate MV, toI. hi, Wiaconsin Geologi<-al Survey.) Scale: 1 in. =^ 310 feet. uppermost vitreous quartzite shows here very plainly, while the body of the formation is distinctly nearer in character to the feldspathic quartz- slate phase above described, although there are interleaved with the pre- MON xis- -12 178 THE PENOKEE IEO]S"-BEARmG SERIES. dominating variety some of the biotitic and ^cldoritic slates, novaculite, and a little red clay shale. At the gorge of the Potato river, in the NE. \ of the SE. \ Sec. 19, T. 45 N., R. 1 E., Wisconsin, the exposures are again large, showing the entire width of tlie quartz-slate. The positions of these ex- posures afe shown in Fig. 5. Tlie contact on the south between this slate and the underlying greenish schist hasf already been described. The usual vitreous quartzite is found as the uppermost horizon and the body of the formation is made up of thinly laminated feldspathic (juartz-slates. The exposures at the gorge of the West branch of the Montreal (Fig. 9) are Quarti Southern. CompUx.. Fig. 9.— Map of exposures at West branch of Montreal river. quite similar to those on the Potato, showing the lower junction but not the upper. From here eastward to the vicinity of Sunday lake the natural exposures of the Quartz-slate member are much rarer -and smaller than those already described. However, in a very large number of places the mining or exploring operations of the district have uncovered this forma- tion for small distances. Naturally the most commonly uncovered portion of the formation is its uppermost quartzite member, which is ordinarily THE QUARTZ SLATE MEMBER. 179 the foot-wall of the mines of tlu^ (Jo^^ciljic district; while tlie lower portions of the foninitioii h.-nc often been exposed in test-pitting operations, so that the course and width of the belt can l)e (juite accurately laid down. It may be noted that tliere ju-e larg-e exjwsures on the west side of Sec. 14, T. 47 N., 1\. 4(! W,, Michio-an, where the several railroad spurs running to the I'ahus and Anvil mines liave cut deeply into the slate. On the Black river, in the northern part of Sec. 13, T. 47 N., R. 47 W., ]\Iichigan, are natural exposures of the formation. To the east of' Sunday lake, in Sec. 10, T. 47 'N., R. 45 W., Michigan, where the (:iuartz-slate formation has an unusual thickness and its outcrop belt an unusual width, there are again very large and continuous exposures, displa}'iug the entire thickness of the formation, which here reaches something like 800 feet. The Quartz-slate here makes up the bulk of two bold bluifs, one in the soathwestern portion and the other in the southeastern portion of Sec. 10, and extending thence into Sees. 11 and 14. About all the phases that are characteristic of the Quartz- slate formation, with the exception of the chloritic and biotitic slates, are displayed in these exposures. The last exposures of this formation met with before reaching the Presque Isle river are those in the southern part of Sec. 17, T. 47 N., R. 44 W., Michigan. Mode of deposition and source of material. — It has already been made evi- dent that the various rock phases of which the Quartz-slate member is com- posed are of a detrital nature. Even the finest grained phases and the^ finer interstitial material of the coarser phases, although at times not now plainly showing their fragmental character, are taken, notwithstanding this, to be wholly of detrital origin, being composed of the same materials as the coarser phases, either in the original unaltered condition, or, as is often the case, somcAvhat changed by metasomatosis, the chlorite, kaoliuite, micas, and finely crystalline quartz being in the main secondary derivatives from feldspathic detrital material. The nature of the most of the detritus of which this formation is mainly composed is such as to suggest very strongly the derivation of its material from some sort of granitic or gneissic rock. The pieces of quartz, orthoclase, microcline, plagioclase (probably albite and oligoclase), and mica, are in their association and in their peculiarities just what we would expect in a 130 THE PENOKEE IRON BEARING SERIES. granitic detritus. And more than this, they are entirely similar to the par- ticles of these same minerals as they appear in granite and gneiss which is so largely exposed immediately to the south. One of the most noteworthy things about the formation (excepting the upper horizon) is the relatively small degree of assortment that the detrital material of which it is composed has received. The detritus, which, while quartz predominates, is still largely composed of feldspar and mica particles, can not have been carried to any great distance from its source. The quartzite, however, which forms tlie uppermost horizon of the formations, represents, of course,' a more thoroughly assorted detritus, and it is very interesting in this connection to note the extraordinary persistency of this horizon. At those places in which the Quartz-slate is in contact with the Cherty- limestone formation, or outcrops of it are not distant, a considerable por- tion of the detritus is often derived from this formation, and close to the contact the cherty detritus sometimes becomes predominant, when we have true basal conglomerates or recomposed rocks. In the higher horizons of the Quartz-slate the cherty detritus is usually sparse or absent. The pres- ence of red jasper fragments in certain of the basal conglomerates suggests the former presence of an earlier Iron-bearing formation. Suinman/. — The Quartz-slate member is given this name because of its slaty character, and of the fact that quartz is its prominent constituent. In geographical extent it is the most continuous belt of the Penokee range, extending from the westernmost exposures of the series nearly to Gogebic lake. The thickness of the greater portion of the belt varies from 300 to 4' 0 feet, but its maximum thickness is as great as 800 feet. Petrographically the Quartz-slate has many varieties; the most persist- ent of these is a vitreous quartzite, which is found at the uppermost horizon of the member. The quartz-slates are always fragmental. Their induration is due (1) to the enlargement of quartz fragments, rarely to enlargement of the feldspars ; and (2) to the alteration of the feldspars to other minerals, the most abundant of which are biotite, chlorite, and quartz. THK (,)|tai;tx-slatk memuei:. • i8l At sevorul plucc^s tlio Quiirtz-slati^ is in cniitact with tiic (Micrty lime- stone, and here a lui-<>-e part of its dc'hris is from this Idwcr mcmlx-r. It is more tVeciueiitly in contact witli tlu* Sonthern (Jomph-x. At these ((nitacts trne basal conglomerates are found. In one or two jdaces at those contacts unconformities are nicely shown. Tiie change t(^ the overlying Iron-bearing nieni])er is always alirujit, its nonfragmental rocks being found upon the vitreous (piartzite. The greater part of the material for the Quartz-slate member is derived from the underlying rocks of the Southern Complex, although locally it contains material froui the Cherty limestone member. CHAPTER V. By E. D. Irving and 0. E. Van Hise. THE IRON-BEARING MEMBER. Section I. Details. Applicability of tbe name. Abruptness of transition from the underlying Quartz-slate raember. Geographical extent. Topographical features. Thickness. General petrograpbical character. Distribution of the three types of rock. Microscopical character of the oherty iron carbonates. Microscopical character of the ferruginous slates and ferrngiuons cherts. Microscopical char- acter of the actinolitic slate.s. Tabulation of petrograpbical observations. Section II. Origin of tbe Rocks of the Iron-bearing Member. The original rock. The ferruginous slates. Tbe ferruginous cherts. The actinolitic slates. Section III. The Animikie Iron-bearing Series. The oherty iron carbonates. The ferruginous slates. Tbe ferruginous cherts. The actinolitic slates. General. Section IV. The Iron Ores. Position of the ores in the Iron-bearing member. Dikes in Iron-bearing member. Position of ore in reference to the dikes. Kock above tbe ore. Practical deductions to be applied iu lu-ospeetiug and miuiug. Nature of the rocks of the Iroii-be.iring member adjacent to the ore bodies. Tbe character of the ore. A particular occurrence of iron ore. Chemistry of the process of ccmcen- tratiou. Time at wbicb concentration of the main ore bodies occurred. Process of concentra- tion. Exceptional localities. Probable extent in depth of ore bodies. Emmons on ore deposits. Iron ores in other parts of lake Superior country. Summary of more important conclusions. SECTION I. DETAILS. ApplicahUity of the name. — The name given to this member is justified by its large content of iron. Certain phases of the belt contain little, or no iron, being wholly made up of silica in one form or another, but such phases have no very great extent, there being nearly always present at least a little iron oxide, while throughout the greater portion of the belt the content of metallic iron certainly exceeds 10 per cent. Very consider- able thicknesses are met with in which the amount of iron reaches 20, 30, 182 Till'. li;(»N-HKAl{IN(i MICMHKH. lB3 40, ami even ;"»(» |)i'i- cent. Carct'iil iiiciisiirciucnt ami saiiipliiif^' oi'tlie west dirt" of Feiiokee g-ap at tlic passajic oi" Had river tliroiioli tlic I'euokee range showed that tliis clitV is made u|) oi' the inllowiiif^' dixisioiis — these divisions beiiij^' measured at riglit angles to tlie strike, he<^-inuing- with the h)\vest parr of the clirt': 1!) feet, containing 44-94 i)er rent of nietallic iron; 7 feet G indies, containing 171(1 ))er cent; 18 feet, containing 49-4 per cent; fi inches, nearly free from iron; (i feet, containing 3(j-64 per cent; 36 feet, containing 45-87 per cent. Above and below the layers shown in this cliff are others nearl}^ or equally rich in metallic iron, alternating with layers in which the iron is present, but in smaller quantity. The following figures indicate the richness of various other thicknesses at Penokee gap: 7 feet, containing 43-29 per cent; 2 inches, included within the preceding measurement, 62-21 per cent; 10-incli rich seam from a different place, 67-52 per cent. The following are other figures, indicat- ing the content of iron in considerable thicknesses at various points on the Penokee range from the Potato river westward: (1) 5 feet, containino- 11-23 per cent; (2) 15 feet, containing 12-99 per cent; (3) 15 feet, contain- ing 25-81 percent; (4) 75 feet, contaimng 26-64 per cent; (5) 25 feet, con- taining 36-14 percent; (6) 10 feet, containing 37 percent; (7) 40 feet, con- taining 37-75 pel- cent; (8) 20 feet, containing 37-88 per cent; (9) 20 feet, containing 37-93 per cent; (10) 3 feet, containing 38-75 per cent; (11) 58 feet, containing 41-93 per cent; (12) 5 feet, containing 44-43 per cent; (13) 150 feet, containing 45 per cent; (14) 3 feet, containing 45-07 per cent; (15) 75 feet, containing 48-12 per cent; (16) 15 feet, containing 49-73 per cent; (17) 50 feet, containing 53*46 per cent.^ These figures, of course, 'These figures are taken from taWes of analyses given iu the third volume of the GeoloK-y of Wis- , cousin, pp. 156-160. The samples were mainly selecti'd hy R. D. Irving ; in a few cases hy E. T. Sweet. They were all made by taking a large numher of small pieces across the entire thickness indicated in each case. The places from which the samples were selected are as follows: (1) Exploring trench, NE. i Sec. 15, T. 44 N., R. 3 W., Wisconsin; (2) exposure SW. i Sec. 1, T. 44 N., R. 2 W., Wisconsin; (3) exposure SE. i Sec. 35, T. 45 N., R. 1 W., Wisconsin; (4; exposure NW. i Sec. 21, T. 44 N., R. 5 VV., Wisconsin; (5) is from the west side of the passage of the Potato river through the Penokee range; (6) exposure NE. i Sec. 18, T. 44 N., R. 2 W., Wisconsin; (7) exposure SW. i Sec. 17, T. 44 N., R. 3 W,, Wisconsin; (8) exposure SW. i Sec. 1, T. 44 N., R. 2 W., Wisconsin; (9) exposure on the fourth principal meridian, ,Sec. 19, T. 45 N., R. 1 E., Wisconsin; (10) exposurenearcenter of Sec. 18, T. 44 N., R. 3 W., Wisconsin; (11) trench SW. j- Sec. 10, T. 44 N., R. 2 W., Wisconsin; (12) exposure NE. i Sec. 14, T. 44 N., R. 3 W., Wisconsin; (13) exposure NW. i Sec. 23, T. 44 N., R* 5 W., Wiscon- 184 THE PENOKBE lEON-BEARING SERIES. represent the richer portions of the belt, the layers from which they come alternating with others which run down to only a small percentage of iron. In the more eastern portion of the belt the total amount of metallic iron may perhaps be nearly as great as at Penokee gap, but it tends to greater concentration, the result of which is the production of genuine ore bodies and of considerable thicknesses of rock, in which relatively little iron is present, though in these cases iron oxide is usually contained in bands, seams, and finely disseminated particles. Abruptness of transition from the vmderlying Quarts-slate member. — As in- dicated alread}" in Chapter II, the fundamental distinction between the Quartz-slate and Iron-bearing members lies in the fact that the former is wholl}" of clastic origin, being still made up mainly of fragmental material readily recognizable as such, while the latter not only contains no frag- mental material whatever, but presents us with no evidence at all of ever having accumulated in a detrital condition. Upon the other hand, the slaty rocks which overlie the Iron-beai'ing member, though occasionally so nmch changed by metasomatic processes as to have lost their fragmental character, have in the main preserved it thoroughly, and are plainly of a detrital origin. The transition, then, from the Quartz-slate member to the Iron- bearing member is one from a detrital to a noridetrital formation. The contact between-these two formations is to be seen at numerous points, and at all of them, the change is abrupt. This 'conclusion has been reached, not merely by an examination in the field, but as a result of a careful study of thin sections of specimens collected on each side of the contact with the verjr object of obtaining light upon this point. In the more eastern portion, of the district, and so far west as the vicinity of the passage of Tylers fork, in T. 45 N., R. 1 W., Wisconsin, this contact has been brought to \\eyv by mining operations at many points, the rule being, as further explained hereafter, that the principal deposits of ore lie at the base siu; (14) trench NE. i Sec. 15, T. 14 N., E. 3 W., Wisconsin; (15) exposure SW. i Sec. 17, T. 44 N., E. 3 W.', Wisconsin; (16) exposure NW. i Sec. 16, T. 44 N., R. 3 W., AVisconsin ; (17) exposure NE. J Sec. 14, T. 44 N., K. 3 W., Wisconsin. The samijles from which these analyses were made were all selected with reference to determining the value of these schists as iron ores, and therefoi-e, while fairly lepresentino; the percentages of iron in the thicknesses named, were of course selected from the richer portions of the exposures. TIIK llJON-ltEAIMNC .M i:.M liKli. l8il (it' the lnm-l)('ariiiji- inciuhfi- iiud iiuinediatcly iipnii llic \ itrciuis (|iiarty.ito wliicli t'oi'iiis the iipiicnnost la\cr nt' llic (Quartz-slate mcnil»cr. In tlicsc iiiiiics till- ciintact is lictwceii tliis ([luutzitc on the unc side and- citlicr th« ore (ir a liciiiatitic clicrtx- niattM-ial of an eutirel}' iiDidVajiincntal cliaractci- on the other. I)nt tlie coiitaet between the two foriiiatious is also visible at a uuinl»(M- of natural exposures, as, for Instance, at the passage of 'r}lers fork, in T. 4;') N., H. 1 W., Wisconsin (Fig-. H), and the passage of Had river at I'enokee gap, in T. 44 N., H. o W., Wisconsin. (See PI. xxxvi.) In these cases on one side is the niagnetitic cherty and slaty rock, and on tlu' other the ^•itreons quartzite; and, as usual, the two classes of materials come directly against each other without transition jjhases. West of Penokee gaj) but few contacts were seen. In a number of places, however, the exposures of the two formations were found in very close proximity to one another. (Pis. v, vi.) As already stated, the uppermost jjortiou of the Quartz-slate is everywhere a vitreous quartzite, i. e., a sandstone in which the interstices of the quartz fragments have been filled with a secondary silica, which has in the main coordinated itself with the original fragments. However, as is usually the case in such quartzites, some of the silica has deposited in a more or less minute mosaic in the interstices. This mosaic, itself of direct chemical origin, in a few sections is not far different from the coarser grained phase of the nonfragmental silica, which forms the groundwork of most of the rock of the Iron-bearing member. But in any case the chemically deposited silica is of a secondar}' nature, and can not be taken as indicating a transition between the modes of deposition of the two formations. Geographical extent. — Longitudinally the Iron-bearing member is co- extensive in distribution with the underlying Quartz-slate; that is to say, is continuous from the western end of the Penokee range in the north half of Sec. 24, T. 44 N., R. 4 W., Wisconsin, to the NE. \ of Sec. 21, T. 47 N., R. 44 W., Michigan, a distance of more than 50 miles. To the west of the western end of the Penokee range, as indicated heretofore, the entire succession of the Penokee series is lost for a distance of 6 miles, in which distance exposures of the itnderlying gTanitic and gneissic rocks on the one hand and of gabbros on the other, come in close proximity to each other 186 THiS PENOKEE mON-BEARmG SEEIES. and make it a question whether the entire Iron-hearing series is here cut out. This statement is at variance with the maps of the Geology of Wis- consin, wliich indicate a continuity across R. 4 W., Wisconsin, not only of the Iron-bearing member, but of the underlying and overlying members of the Penokee series/ It was known at the time these maps were made that the Penokee range, with all its characteristic rock exposures, ends abruptly in the eastei-n part of T. 45 N., R. 4 W., Wisconsin, being succeeded to the westward by a low, marshy country, with but rare exposures, the Penokee range rocks not reappearing until in the vicinity of the bold ridge in the NE. 4 of Sec. 24, T. 44 N., R. 5 W., Wisconsin. The late Mr. C. E. Wright, however, traced through the intervening low country a line of rather feeble magnetic attractions which he considered to establish the con- tinuity of the Penokee range rocks beneath the drift covering. Accepting Mr. Wright's conclusion, the surface maps were drawn accordingly. As to the magnetic attractions, it is to be said that they were too feeble to liase an)" certain conclusion upon, being pei'haps explicable by the considerable quantity of magnetite occurring in the gabbro, Avhich appears here to usurp the place of the Penokee rocks. Still, it is not impossible that this line of attractions may be due to the Penokee iron lielt itself, buried here under- neath a considerable drift covering. In the north half of Sec. 23, T. 44 N., R. 5 W., Wisconsin, as already stated, outcrops of the Penokee series reappear, and from here for a distance westward of 4 miles exposures of the Iron-bearing and Quartz-slate members are sufficiently frequent to indi- cate their continuity through this distance. The last exposure on this line is met with in the S. .] of Sec. 20, T. 44 N., R. 5 W., Wisconsin, beyond which point no exposures have yet been found until those met with in the SE. 4 of Sec. 26, T. 44 N., R. 6 W., Wisconsin, where the characteristic rocks of the iron belt are seen again. Still farther west Mr. Wright traced a line of feeble magnetic attractions as far as the north side of lake Numa- kagon. In the northern part of T. 43 N., R. 6 W., Wisconsin, beyond lake Numakagon, no further exposures of the Penokee series have been met with, while in Sec. 20, T. 43 N., R. 7 W., Wisconsin, the occurrence in close ' See Atlas of the Geol. of Wis., Pis. xxil and xxvii. See particularly the latter plate for the position of all exposures known at that time. TIIK lliON IJKAIMNC AlKMIiKlt. 187 pi'oxiniit \ (p| cxiiosiircs ol rocUs clKir.ictcristlc dl tlic Keweenaw series ami dl' tli<' iifaiiitfs heloiiyiii}'- beneath the I'enokee series ajjpear to iiiilieatt^ tiie teniiination in tliat dii-ection, lor sonic distance at h'ast, of tlic Tcnokee rocks. To the east of the easterinnost point indicated as reaciicd l)\- the continnons iron l)elt of tlie I'enokee raii"e, tli(^ NE. I of Sec. 21, '\\ 47 X., U. 44 \\ ., Micliigan, exposures of ferrnjiinous rocks allied to those of tlie Iron-beai'ini>- niend)er are met witli for a distance of G or 7 miles, and to witiiin 3 or 4 miles of Gogebic lake, but these occair in a peculiarly disturbed and difficult area, whose geology, whatever be the true interpretation of it, is unlike that of the Penokee belt proper. The iron-])earing rocks of this area receive special attention in C'hapter VIII. The width of the belt of country occupied by the Iron-bearing mem- ber is surprisingl)^ uniform from the westernmost exposure as far east as the central porti (in of T. 47 N., R. 46 W., Michigan. Throughout this very considerable distance this widtli rarely falls below 800 feet and as rarely exceeds 1,000 feet; its variations being frequently explicable by changes in the dip, though apparently some part of the variation may be due to actual difference in thickness. Still, through most of this distance the thickness luust be taken as more nearly constant than the width of the belt occupied. When R. 46 W., Michigan, is reached, however, a distinct widening of the belt becomes perceptible, and in the eastern part of that township this becomes so rapid that when Black river is reached it has become as much as 2,400 feet. From Black river eastward there is a still more rapid increase, the width on the east side of Sees. 7 and 1 8, T. 47 N., R. 45 W., Michigan, being fully 4,700 feet. A part of this great increase : in width is plainly due to a very iinusual flattening iu the degree of north- ward dip, but this will not serve to ex^ilain all of tlie Avidening, a. part of which may be due to an actual increase in thickness, but is owing in part at least to the presence of interbedded greenstones. Immediately east of the last named point the overlying Keweenaw rocks, whose southern boundary has been for some time rapidly ajjproached by the northern edge of the iron-bearing belt, are reached by it. Continuing to the east, the iron belt is in part cut off by the Keweenaw series, so that at one place its surface width is not much more than 500 feet. Not far east of Sunday lake a 188 THE PENOKEE IKON-BE AEING SERIES. widening comes in again. This is evidentl)' due to' a cliange in course of the iron belt, which now trends southward, and thus diverges from the Keweenawau beds, the divergence, however, not being so great as to allow the reappearance of the slates belonging above the Iron-bearing member as far east as the middle of T. 47 N., R. 44 W., Michigan. Topograpliical features. — The Penokee iron range, save for several trans- verse cuts made through it by the northward flowing streams, is a contin- uous ridge from the northern half of Sec. 24, T. 44 N., R. 4 W., Wisconsin, eastward to beyond Sunday lake in Michigan. To the west of the western termination of this range, as indicated in previous chapters, are again other detached ranges, which, being made up of like strata dipping in the same northerly direction, may be looked upon as forming portions of the same P-dneral line of elevation. The same is true east of the eastern termination indicated as far as the vicinity of the Little Presque Isle river. In places the ridge rises from 100 to 300 feet above the elevated swampy area south of it, and from 100 to 600 feet above the lower area north. In its more western portions this range is wide and has a rather narrow serrated crest, while eastward from Tylers fork it becomes more and more of a gentle swell until a point west of Sunday lake is reached, where there is again a broader range. In much of this distance the ridge forms the most prominent feature of the topography of the country, being visible from the waters of lake Superior in the vicinity of the Apostle islands as a blue line against the horizon. On a preceding page the relations of the Quartz-slate member to this ridge have been indicated, and incidentally those of the Iron-bearing member also. The points of principal interest, however, may conveniently be • repeated here. Along that portion of the Penokee range which is west of Bad river, and also in the detached ranges above referred to as occurring still farther west, in T. 44 N., R. 5 W., Wisconsin, the Quartz-slate member forms the foot of the bold south face of the ridge, the upper portion of the southern face, along with the crest and the upper portion of the northern slope, being all made up of the steeply inclined layers of the Iron-bearing member. The passage of Bad river through the range has been determined by the existence of a fault, which has caused a discordance in the layers of some 800 feet. Eastward from Bad river the THE IKON-HEAKING MEMBEK. 189 Qiiartz-slalc mcmluT cliinljs hin-licr mid lii^licr ini tlic snutlicni fare of tlie ridrre, all of which it makes up \)y the time iikkiiiI Whittlesey is reached. Oorrespoudiuu^ly, the Irou-beariii<'- ineinber creeps i'arther and farther down the northern slope. At Tylers fork this change has gone on so far that the crest of the ridsj'e is now entirely within the (piartz-slate, while east of that stream and all the way to the West branch of Black river the crest of the ridfje is in the granitic rocks belonging- to the Southern Complex. In this area the Iron-bearing member continues to creep down the northern face of the ridge until it lias the greater portion of its width in the low ground t(» the northward. East of the West branch of Black river for a short distance the ridge again lies within the iron Ijelt, which here contains an unusually large amount of resistant jaspery material. The changing relation thus indicated as obtaining between the positions of the ridge and that of the outcrop belt of the Iron member has been explained on a previous- page as a result of a variation in the mineralogical character of the member, which, where resistant, forms the upi)er jiortions of the ridges, while in other places the rocks to the south Ijeing more resistant the com-se of the iron belt lies in the lower ground to the north. Thkkmss.— It has already been said that the outcrop belt of the Iron- bearing member has a singularly constant width from 800 to 1,000 feet for all of the distance between the western extremity of the Penokee ranffe to the middle of T. 47 N., R. 46 W., Michigan. The thickness must be yet more constant than is indicated by these figures, the variations between the limits indicated being generally explicable by a somewhat changing degree of northward inclination. Throughout this distance the actual thickness of the formation can not vary much on either side of 850 feet. How far the great increase in surface width, which has already been indicated as obtaining in the eastern portion of T. 47 N., R. 46 AV., Michigan, and farther to the east, is due to an increase in actual thickness is .exceedingly difficult to tell. Some of the increased width is plainly the result of an unusually low angle of northward inclination. More of it is evidently due to the intercalation of eruptive greenstone sheets; but after these two causes of widening have been considered, how much remains to be accounted for by an increased thickness of the Iron-bearing member we 190 THE PElSrOKEE IEOIn-BEAEING SEEIES. have no means of determining accurately. Throughout much of this area, moreover, the uppermost portion of the iron belt is missing, but how much is gone there is no means of teUing. It is only possible to say in a general way that it would seem that in the eastern part of the formation its thick- ness may be very materially increased. General petrograpkical character. — Three main types of rock make up the Iron-bearing member. Between these three types there are various gradation phases, while each main type presents itself in a number of forms between which there are minor differences. In certain rare instances a little detrital material has been introduced during the original deposition of these rocks, but ordinarily this is completely lacking. The three types referred to may be briefly characterized as (1) slaty and often clierty iron carbonate, (2) ferruginous slates and ferruginous cherts, and (3) actinolitic and magnetitic slates. The first type is a very well marked one, and is present in very con- siderable thickness. It is characterized by the invariable presence of iron carbonate as a chief constituent. In some cases the iron carbonate con- stitutes the only important mineral in the rock, but usually it is mingled with more or less of calcium carbonate, magnesium carbonate and cherty silica, the latter ranging in character from minutely crystalline to amorphous. Other minor ingredients, one or more of which may be present, are hema- tite, limonite or other brown iron oxide, magnetite, carbonaceous or graph- itic matter, iron pyrites, a chloritic or viriditic ingredient, an excessively fine clayey substance, and, very rarely, pieces of a fragmental quartz. In texture these rocks are commonly earthy and aphanitic, but in some cases the iron carbonate is sufficiently coarsely crystalline for thfe cleavage sur- face of the minute crystals of siderite to be perceptible to the naked eye. There is very commonly a regular and thin lamination produced by an alternation of lighter and darker gray shades of the carbonate. Frequently, however, the gray carbonate is interlaminated with seams of black graphitic matter, red jasper, red hematite, black flint, greenish black or viriditic car- bonate, or with seams of carbonate in which magnetite particles are partic- idarly abundant. While the platy or thinly stratiforuA habit is very char- acteristic of these rocks, these laminee often become irregular, presenting THE IHON-HEARING MEMISEH. 191 tlie Ji])pearanco of liiiviiii^- licrii hrokeii aparl and rccpniciifi'd, in which case there is usually a (•(>iisi(lerat)U^ quantitx' nf the clicrtN' silica present. Tlie observations thus t'jir made have aj)[)H(Ml particuhirly to fresh surfaces. On exposed surfaces there is very apt to he a prevalent brownish or reddish iron stain from peroxichition of the iron of the carbonate. In specific g'ravity these rocks range generall}- between tlie two precedino- tyj)es. The specific gravity of jnxre iron carbonate is given in the mineralogies as between 3 1 and ;V!l. The rocks now considered fall below this figure because of the presence of other lighter substances, particularly the calcareous, siliceous and clayey ingredients. The following figures are the results of specific gravity determinations: 2-97, 3-04, 3-20, 3-22, 3-24, 3-29, 3-40, and 3-50. Of the following analyses the first five represent the composition of iron carbonates from the Penokee-Gogebic district. The remainder are of similar carbonates from other districts about lake Superior, and are inserted here for the sake of comparison. No. I (specimen 9191), analyzed by Mr R B. Riggs, of the U. S. Geological Survey, is of a rock exposed in a test-pit in the NE. | of the NE. ^ of Sec. 6, T. 45 N., R. 2 E., Wisconsin. No. II (specimen 9472), made by Mr. W F. Hillebrand, of the U. S. Geo- logical Survey, is of a specimen from the large precipitous exposure on the south side of the outlet of Sunday lake, NE. J of Sec. 13, T. 47 N„ R. 46 W., Michigan. No. Ill (specimen 12885), made by Mr. Thomas M. Chatard, of the U. S. Geological Survey, is of a carbonate occurring near the base of the Iron-bearing member on the Miner & Wells option, Sec. 13, T. 47 N., R. 46 W., Michigan. No. IV (specimen 12887), made by Mr. Hil- lebrand, is of a specimen representing a large natiu-al exposure on the Palms property, Sec. 14, T. 47 N., R. 46 W., Michigan. No. V (speci- men 12543), made by Mr. Chatard, represents a carbonate occurring at a low horizon in the member, in the NW.^ of Sec. 18, T. 47 N., R. 45 W., Michigan. Of the remaining analyses. No. VI (specimen 9264), bv Mr. Hil- lebrapd, represents a peculiar carbonate occurring in the SE. 4 of Sec. 20, T. 47 N., R. 43 W., Michigan, in that confused area to the east of the Presque Isle river, which is considered in the present volume separateh' from the regular Penokee succession. No. VII (specimen 10575), by Mr. Chatard, is an iron carbonate from the so-called gunfiint beds exposed on the eastern 192 THE PEJfOKEE lEON-BEAEmG SBEIES. side of the outlet of Gunflint lake, situated on the national boundary between Minnesota and Canada. No. VIII (specimen 10598), by Mr, Chatard, is from the same beds, but from an exposure on the northern side of Gunflint lake. No. IX (specimen 10588), also by Mr. Chatard, is a ferrif- erous carbonate from another part of the north side of Gunflint lake. No. X (specimen 10157), by Mr. Riggs, is a black slaty and carbonaceous iron carbonate, exposed at Kakabikka falls on the Kaministiquia river, Canada. And No. XI (specimen 10160), also by Mr. Riggs, is from a less carbona- ceous phase, exposed at the same place as No. X. The rocks represented by Nos. VII and XI, inclusive, of these analyses are all from the so-called Animikie series of northeastern Minnesota and the adjoining portions of Canada. Analyses of iron-bearmy carhoiiates. I. II. III. IV. V. VI. VII. vni. IX. X. XI. Silii-a 15-62 28-86 0-20 1-29 101 4601 0-12 0-83 1-35 46-47 0-10 ■ 0-70 0-86 36-73 0-19 0-38 0-98 3-16 0-08 0-93 68-23 trace 0-06 5-01 46-46 trace 0-24 0-64 23-90 none 0-07 0-44 37-73 54-26 4-27 8-14 3-41 6-42 2-57 3-62 Iron protoxide 32-85 37-37 ■ 26-00 28-57 34-74 15-18 18-41 26-28 10-66 22-93 19-6.1 Maiiiianoiis oxide 5-06 0.97 2-09 0-40 0-52 1-15 0.25 0-21 0-28 0-40 0-19 0-81 2-66 0-74 ;)-64 0-63 2-86 0-49 2-,30 0-48 2-74 26-66 11-01 0-38 9-59 1-87 3-10 22-25 8-52 1-26 3-98 1-07 2-93 ilajiUfsium oxide Cai'ljon dioxido. 30-32 ■^5-21 17-72 19-24 22-44 41-10 5-22 19-96 32-42 18-01 14-93 Plioaphoric arid 0-07 0-06 013 0-11 0-12 0-34 0-14 0-11 0-13 "Water at, 10S° 0-12 none. 0-07 0-07 ■ AVatiT at red heat 0-68 0-68 1-71 0-60 1-40 0-54 2-01 1-15 0-99 1-20 Water at red heat, partly 2-74 (*) (*) (*) (*) (*) n (*) :.-54 0-45 1 100-41 99-97 99-50 99-73 100-84 100-20 99-40 100-32 99-66 100-41 lOU-85 ' Undetermhied. The rocks of the second type, which we have collected under the gen- eral designation of ferruginous slates and ferruginous cherts, have in common a prominent siliceous constituent, which is always of a nonfragmental nature, and ranges in crystallization from a wholly though minuteh' crystalline con- dition, through partly crystalline and chalcedonic phases to an entirely amor- phous phase, the several phases being frequently associated in the same thin TJllO 1IH)N liEAKING aiEMBEE. 193 section. Compared witli the siliceoius ingredient of the rocks of the third type, })etvveen which and those now nnder consideration there are distinct eneral very nuich liner in <>rain. Indeed, those phases in whicli the silica is either in so minute individuals that they can only with extreme difficulty be sepa- rated laider the microscope, or in which it is mingled with more or less chalcedonic or even amorphous fonns, g-reatly predominate. In addition to this siliceous g-rouuchnass, to all of the phases of which we apply the general term of chert, the principal ingredients of these rocks are the several iron, oxides: that is, magnetite, hematite, and a brown hydrated oxide. These are generally greatly subordinate in quantity to the silica, at times sinking almost completely out of sight. In other cases they are present in consid- erable quantity, and frequently occur so plentifully as to furnish interme- diate phases between the cherts and the iron ores of the district. The principal one of these oxides is hematite. When magnetite is present in any considerable quantity, it is generally accompanied by more or less actinolite in minute needles, and phases of gradation between the cherts and the actiuolitic rocks of the third type, subsequently described, are thus produced. On the other hand, more or less iron carbonate is found in remnants in the many sections of these cherts, and by its increasing quantity leads us through phases of gradation into the rocks of the first type above described. In color these rocks vary greatly, presenting red, brown, gray, and white colors, depending upon the amount of iron oxide present. Perfectly white nonferruginous phases are occasionally met with; also. light to dark gray kinds, and grayish kinds mottled irregularly with black, in which phase the iron oxide is mainly magnetite. When hematite is present in large quantity the rock may have a uniform red color. Occa- sionally the red iron oxide is present in just such quantity and condition as to make of the chert a genuine jasper, but such phases are not common. A rather unusual kind is quite black, apparently from the presence of carbonaceous matter. In structure these rocks vary from the regular lamination of the first type to those that are much less regular, being either without any uniform banding, or, if the banding is present, the laminae present the appearance MON xix 13 194 THE PENOKEE lEON-BEARmG SERIES. of having been disrupted and the broken fragments recemented. Both the regular and irregularly laminated phases are found in large areas. The uniformly laminated phase to which the name ferruginous slate is given has a uniform texture and even lamination, and varies in color from yellow to deep red, the change in color being due to different oxides of iron. This phase might be taken for a slaty iron ore, but its low specific gravity and its large content of silica, as shown by analysis, exclude it from the iron ores. The silica and iron oxide are so uniformly .mingled and the silica is in such small particles that it is not macroscop- ically visible. The second phase, the ferruginous cherts, is distinguished from the first phase by the greater .concentration of the iron oxides. They occur in irregular bands aiid areas, interlaminated with chert layers of greater or less purity. There are all gradations in exposure, from the regularly laminated slates of the first phase through the ferruginous cherts to areas in which the iron oxide is concentrated in such large masses as to be workable deposits of iron ore. In general the texture of the chert is aphanitic ; occasionally it presents a chalcedony-like appearance. In many instances there is present a more or less porous texture, minute cavities occurring tlu'oughout the specimens. Also a minute brecciation of the rock is not unfrequently perceptible to the naked eye, though this peculiar characteristic comes out more prominently in the thin section, as subse- quently explained. When perceptible to the naked eye, it shows itself as an irregular mottling of dark and light gray, or of black and gray; the appearance being that of more or less angular fragments imbedded in a lighter colored matrix. The following are the results of specific gravity determinations of the ferruginous cherts: 2-65, 2-67, 2-69, 2-76, 2-90, 2-92, 2'95, 3"21, 3"25, 3"26, 3"39. Of these amounts those above three represent the more ferruginous kinds. The rocks belonging to the third type, the actinolitic and magnetitic slates, are in thq main very dark colored, being often black. Occasion- ally, however, in bands the color is lighter, in which case the light and dark colored bands present a striking striped appearance in natm-al exposure. The grain is usually a very fine one, being at times quite aphanitic. Occa- sionally each of the main individual constituents may be detected with the THE 1KON-J5EAKING MEMBER. 195 naked eye. More commonly, however, the use of the magnifying glass is required. These main constituents are shown by the microscope to be in most instances quartz, actinolite, liematite, and magnetite. The first of these is occasionally lacking, but such pliases are unusual. The four min- erals vary greatly as to relative proportion, each in turn predominating over the others, the lighter colored phases being of course the most highly quartzose ones, while the darker kinds are richer in iron oxide, which is often present in sufficient quantities to give the rock a more or less distinct metallic luster. Except in certain thin seams, however, which are at times even pure hematite and magnetite, the content is not so great but that the powder of the rock is of a light color. In the more detailed description given below it will be seen that in one unusual phase garnet is a prominent constituent, and that chlorite and biotite occur frequently as somewhat important alteration products. The lamination of these rocks is usually more or less strongly marked by variations in color, the individual laminae more commonly running from an insignificant thickness to about one-fourth or one-half an inch, only occasionally exceeding the latter figure. Parallel to the lamination there is generally a distinct tendency to cleavage. Obliquely transverse to this bedding cleavage is a join;ting which renders it exceedingly difficult to get from the ledge large sized pieces of the rock, they usually coming out in small lozenge-shaped slabs. The large amount of magnetite and hematite which nearly all phases of this rock contain render it very noticeably heavy. The following are specific gravity deter- minations made upon samples selected with a view to illustrating the sev- eral pliases of these actinolitic rocks: 3-06, 3-37, 3-42, 3-43, 3-46, 3'50, 3-53, 3"91, 4'31, 4'54, 5"01. The last three numbers represent thin seams unusu- ally rich in hematite and magnetite. The other numbers represent large bodies of rock, the lowest one being given by one of the most quartzose phases met with. The large amount of magnetite contained in the rocks of this kind produces extraordinary attractions upon the magnetic needle. In this connection reference should be made to the magnetic charts given in the third volume of the Geology of Wisconsin, and in the atlas accom- panying that volume.^ While the magnetic oxide of iron is the greatly ' Volume Pis. xxiii, xxiv, xxv, xxvi, xxvii, xxviii, xxix, and xxx. Atlas Pis. xxiii, xxiv, XXV, and xxvi. . 196 THE PENOKEB IRON-BEAEING SERIES. predominating one in all this class of rocks, nevertheless analyses show- that some of the sesquioxide is usually mingled with it, the presence of which oxide indeed is not infrequently evident to the naked eye in the more ferruginous varieties, the luster of specular iron and a red- dish or purplish tint in the powder both testifying to its presence. In the less ferruginous portions of these phases the sesquioxide is less plentiful, but at times the siliceous seams present a dull reddish or jaspery appearance from the presence of hematite. In one vicinity in the eastern portion of the district a bright red jasper is thinly laminated with a typical actinolitic magnetitic slate, but this is tmusuaT. The ordinary occurrence in the lake Superior region of the bright red phase of nonfragmental silica, commonly spoken of as jasper, is in direct association with the more brilliantly lustered, steely, specular iron ores. Such bright colored jaspers only rarely occur in immediate association with the actinolitic and magnetitic slates.^ The following analyses of the rocks of this type are taken from the Greology of Wisconsin.^ They are of samples selected by the senior author with direct reference to the richness of the rock in metallic iron; they therefore represent in the main the more ferruginous portions of the belt, or rather the more ferruginous portions having any considerable thickness. The original samples, having been selected for an economic purpose, were made by breaking small pieces from all across the thickness sampled. Had the analyses been made more especially ^The more highly siliceous jihases of this class of rocks are spoken of by Irving iu the Wis- consin Reports as quartzites (Geol. of Wis., 1880, vol. iii, pp. 118, 119, 120, etseq.), the adjectives magnetitic, hematitie, etc., heing prefixed to the word quartzite to indicate the special phases. The name quartzite was thus used in ignorance of the fandamental distinction which we now know holds between these rocks and the genuine quartzites, the latter having been shown to be always in the main composed of original fragmental material, while the siliceous constituent of the rock now especially under consideration is always of a nonfragmental nature, having been solidified in situ. The term quartzite is used, then, throughout this volume and all other later writings of the authors to designate only a genuine fragmental rock indurated by means of interstiti.al deposition. On PI. XXII of the Atlas of the Geology of Wisconsin, and on page 119 of vol. iii of that work, the Iron-bearing member is represented as made up of three subdivisions, a basement quartzite 50 feet thick, a series of magnetitic schists and quartzites 800 feet thick, and a garnetiferoiis actinolitic schist 10 feet thick. The quartz at the base of the series, although highly vitreous, we now know to be of a completely fragmental character, and to belong to the underlying Quartz-slate member rather than to the Iron-bearing member, the so-called quartzites of which prove to be wholly nonfrag- mental rocks. 3 Vol. Ill, pp. 156-160. Till!; IRONBEAHING MEMBER. 197 for petroyrapliic purijciscs, the .samples would of cours(} have been selected in a flifferent fashion. Nevertheless, as taken, they represent quite fairly the more ferruginous portions of this phase of rocks, one (No. IV) repre- senting- a less ferruginous portion. (If these analyses, I represents a thickness of 40 feet exposed in the SW. \ of Sec. 17, T. 44 N., R. 3 W., Wisconsin ; II, 41 inches in the NE. I of Sec. 15, T. 44 N., R. 3 W., Wis- consin; III, 19 feet exposed on the west cliff at Penokee gap, NW. ^ of Sec. 14, T. 44 N., R. 3 W., Wisconsin; IV, 7 feet 6 inches immediately overlying the rock represented by III ; V, 10 feet from the same cliff; VI, 50 feet exposed in the NE. 4 of Sec. 14, T. 44 N., R. 3 W., Wisconsin; VII, 58 feet in the SW. 4 of Sec. 10, T. 44 N., R. 2 W., Wisconsin ; VIII, 25 feet in the SW. i of Sec. 1, T. 44 N., R. 2 W., Wisconsin. Analyses of magnetiUc slates. I. n. ni. IV. V. VI. VII. VIII. Iron sesquioside Iron protoxide Silica 36-414 15-767 39-532 0-110 1-516 2-516 3- 120 tr.ice. 0-421 0-543 49-435 8-460 33-894 1-151 3-160 2-403 0-337 none. none. 1-500 42-897 19-173 31-838 0-384 1-373 1-293 1-126 none. none. 0-378 15-335 7-851 34-770 15-819 42-896 none. 1-.330 2-623 1-726 tr.ice. trace. 0-471 67-064 8-332 18-472 0-305 2-483 2-280 1-050 0-127 none. 0-450 41-241 16-797 40-420 12-331 1-025 1-139 Magiieaium oxide Mangauous oxide Phosphoric acid 2-150 0-193 trace. 0-160 1-078 .1-890 0-553 trace. none. 2-559 Total 99-939 100-336 98-462 99-635 100-613 100-541 99-436 It will be seen that several of these analyses show a proportion of iron sesquioxide beyond that which is required by the protoxide contents indi- cated to make up the mineral magnetite. In fact the excess is even slightly greater than is indicated at first sight, since a certain amount of the iron protoxide must be assigned to the amphibolic mineral which is always • present. Disregarding the small correction to be made on this account and considering all of the iron protoxide indicated by the analyses as contained in magnetite, we find that the following respective proportions of magnetite and hematite are indicated for the several samples. I, 50'80 per cent and 1-37 per cent; II, 27-26 and 30-63 ; III, 50-56 and 11-40; IV, 23-38 per cent of magnetite and no hematite; V, 50-38 per cent of magnetite and no 198 THE PENOKEE IRON-BEARING SERIES. hematite; VI, 27 per cent of magnetite and 48 "43 per cent of hematite. For VII the figures are 54-12 and 3-91; for VIII, 39-73 and 13-01; for IX, 16-27 and 28-83. Distribution of the three types of rock. — In all that portion of the Iron- bearing member which lies to the west of the passage through the Penokee range of Tylers fork, in Sec. 33, T. 44 N., R. 1 W., Wisconsin, the rocks of the third type or actinolitic and magnetitic schists prevail ; in fact, except that the rock of certain exposures, much more highly quai-tzose than usual, approaches the cherts of the second type, it may be said that these rocks are the only ones met with. At the gorge of Tylers fork, however, the actinolite has almost, though not quite, sunk out of sight, while the siliceous constituent is altogether the most prominent one. At the same time hema- tite and brown iron oxides begin to prevail over the magnetite. To the east of Tylers fork the first and second types greatly pre- dominate, though phases carrying minute quantities of actinolite continue to occur as far east as the Potato river. Nevertheless, these are very rare, and nowhere in this portion of the range has any highly actinolitic rock yet been met with. Just where these actinolitic rocks cease it is difiicult to tell, but they can not contiime far to the east of Tylers fork, not far from which stream the cherty and carbonated rocks of the first and second types have increased to such an extent as to occupy the whole of the iron belt, the frequent occurrence of bodies of hematite ore at the base of the member beginning at the same time with this change. For some 30 miles now to the east, or as far as near the east side of T. 47 N., R. 4.5 W., Michigan, the cherts and carbonates, with iron ore bodies at or near the base- of the formation, continue neai'ly to the exclusion of the actinolitic rocks. In this distance is included with few exceptions all of the working mines of the Gogebic districts. The somewhat magnetitic and actinolitic rocks just west of Tylers fork include one mine, and the somewhat actinolitic and magnetitic rocks in T. 47 N., R. 45 W., Michigan, include several. A rude subordinate stratigraphic ai'rangement in the iron belt appears to hold for this distance ; i. e., the purer carbonates are characteristic of the lipper horizons, the fer- ruginous slates of the middle horizons, while the ferruginous cherts and ore bodies lie within the lower horizons. It is not meant to indicate by this Tin; IKONBHAUlNCi MKMIJEH. 199 stiitonu'iit that tlic tniusition Ix'twocii the clicrty phases unci the car- bonates is a reg-ulai- or sharp one, as the carbonates sink to lower horizons in sous gronndmass of these rocks varies from very minutely thougli completely crystalline to jjartl}' amorphous (PL xxii, Fig. 2), often presenting the intermediate condition characteristic of chal(;edony. In some sections'none of the chalcedonic or amorphous phase of silica is present, but in others all three of these phases occur together. In general there is in these rocks a much greater tendency towai'd the crystalline kind of silica than in those of the first type, the chert}' iron carbonates ; but as compared with the silica in the rocks of the third tjqoe, or actinolitic slates, there is a greater tendenc}^ toward the noncrystalline kinds. It has been shown that the actinolitic slates have their main distribution in the western portion of the iron belt, occurring- again in considerable develop- ment far to the east, while in the intervening space the ferruginous cherts and carbonates have their great development. Correspondingly those phases of the ferruginous cherts, which in tlieir content of small quantities of magnetite and actinolite present us with a gradation into the actino- litic slate type, are fouiid , particularly toward the west as one approaches the region of actinolitic slates proper, and again at the extreme east as the actinolitic rocks of that area are reached. Accompanying the presence of magnetite and actinolite in these gradation phases is usually found the most completely crystalline condition of the siliceoixs groundmass. The ferruginous cherts, as indicated in the general description, pre- sent two widely different phases ; the ferruginous slates in which tlie iron oxides and silicates are quite uniformly mingled, and rocks in which the iron oxide is concentrated to a greater or less degree in bands, rings, and shots,' leaving the silica comparatively or almost wholly free from ii-on oxide. . The rocks of the first phase are composed of intimately mingled chert and brown, somewhat hydrated hematite, red Hematite, and occa- sionally magnetite. In the ferruginous slates there has also been a concen- tration of the iron oxide to a small extent in layers. At times these layers are very regular ones, between which alternate layers of chert, containing comparatively little oxide of iron. From these perfectly laminated phases, 204 THE PBNOKBB IKON BEARING SERIES, wliicli are as regular as any of the slaty iron carbonates of the first type of rock, the specimens vary to those in which the thin section shows appar- ently no proper lamination, although in hand specimen there is always some evidence of stratification. In these laminated phases the chert may be a background for the iron oxide or. the reverse, depending upon which is predominant. The chert in the ferruginous slates varies from finely crystalline to the very fine spotty quartz mingled with amorphous silica, characteristic of the first type of rock. When the quartz is of the more coarsely crystalhne kind the sections are often cut by veins of silica. In these cases the sec- ondaiy nature of a portion of the silica at least is indicated by the fact that it does not always lie directly parallel to the lamination, but breaks across the more ferruginous bands in little veinlets, while various singular depar- tures from the regularity of the lamination indicate the same thing. Usually the quartz shows little or no indication of a concretionary or brecciated nature. The iron oxide is generally of the brown somewhat hydrated hematite. Occasionally the hematite is bright red, when the rock becomes a jasper. These jaspery portions are not usually in any great thicknesses. Sometimes the rocks are quite reguilarly laminated, but often the jaspery parts are in the shape of noncontinuous fine laminaj. Less frequently the oxide of iron is in part magnetite. The iron oxide is present in irregular areas, and frequently is in suffi- cient quantity to form continuous ramifjdng areas in which the chert is buried. In a portion of the specimens little or no iron carbonate remains, or the iron oxide, either hematite or hydroxide, may present itself as mere stains in the carbonate, replying the carbonates in varying degrees, until finally an entire crystal or bunch of crystals of that mineral is changed to the oxides. Rhombic crystal sections, composed of oxides of iron, are to be .found in nearly all of the sections. In quite a good many cases all of the iron oxide of a sectit)n, or nearly all of it, will present itself in these rhombic shapes. More often the rhombs will be perceptible only on the edges of the iron oxide areas, the middle portions of these aggregates being too compact to allow of their recognition. The carbonate itself is found in more than half of the sections examined, and in nearly all of the remainder its former THE IRON-HEARING MEMMER. 205 presence is indicated by these rhombic crystal sections. Tlie inag'netite, wlion present, is commonly rhombic in outline. Such rliombic outlines may, of course, ])e produced by random sections of the ordinary niagnetite octahedra, l)ut the (piestion has su<>-<^ested itself as to whether it is not ])ossible that these rhombic magnetite sections express in their shape the outlines of carbonate crystals. In some cases such magnetite crystals with similar shaped sections of carbonate and hematite are found in the same rock. In one direction these rocks may, then, be traced into those of the first type. In the reverse direction by concentration of the iron oxides and by the development of a concretionary and brecciated character this phase passes into the second phase of the second type of rock. In the ferruginous cherts the iron oxides are often concentrated more or less into regular bands, but besides these bands there are many oval or spherical bodies of iron oxide, so that the specimens are best described as cherts containing bands and shots of ore. These shots occur in cavities in the cherts and often they do not entirely fill them. In such cases the iron oxide is usually lined with crystals of quartz. The structure is, upon a -small scale, that of a geode, the cavity of which has a layer of iron oxide, and within this quartz crystals. Even when the cavities are completely filled with iron oxide the same similarity to a geodic structure is apparent. It would seem that the cavities formed at some stage in the development of the rock (perhaps by a solution of a part of the iron carbonate at the time another part was oxidized, or else by solution of silica), were subse- quently partly or completely filled with iron oxide, after which, if space remained, followed silica. (PI. xxiii. Figs. 1 and 2.) In this phase of rock the arrangement of the constituent particles is often closely similar to that which has been described on a preceding page as characterizing the chert of the Limestone member; tliat is, it often pre- sents a more or less perfect concretionary arrangement, but in the limestones' and cherts this is at times exceedingly vague. However, the concretions are so numerous as to be one of the most important characteiistics of this phase of rock. (PL xxii, Figs. 1 and 2.) The concretionary structure affects both the iron oxide and chert, although it is most clearly made out by 206 THE PENOKEE IRON-BEARING SERIES. means of the iron oxide. In some cases the iron oxide which marks these concretionary areas is so plentiful as to render them nearly or quite opaque. From such extreme cases there is every gradation to those sections in which only an exceedingly minute amount of iron oxide remains to separate these areas from the interstitial silica, while there are not unfrequent cases in which even a minute amount of iron oxide is absent. The outlining of the areas is then perceived only in the polarized light, its silica being either nearer to or farther from the amorphous condition than that portion of the matrix immediately in contact with it. The iron oxide which designates the concretions from the matrix pre- sents itself in the shape of a general stain, composed of minute particles, distributed in the shape of a mere border or in concentric rings. It may be any of the three oxides — magnetite, hematite, or a brown, somewhat hydrated oxide, or a mixture of two or three of them together. In cases where the iron oxide is magnetite, as in the cherts which occur between Tylers fork and Sec. 10, T. 45 N., R. 1 E., Wisconsin, it may present itself either in the shape of an exceedingly fine dust or as sharply outlined crys- tals of some little size, and these crystals are not unfrequently arranged around the edges of the concretionary area, their sharp angles projecting from its outline. (PI. xxviii. Fig. 2.) It is not to be understood that the iron oxides are completely lacking in the interstitial material; on the con- ' trary, they are often present either in minute stains or aggregations of par- ticles; but the rule is that they are more plentiful in concretions than in the matrix, while in many cases the matrix appears to be almost wholly devoid of them. A concretion will often be sharply defined only along a portion of its outline, the remainder being exceedingly vague. This arises at times from lack of sufficient iron oxide stain to diff'erentiate the concretions from the matrix, while the silica of both may be so closely of the same degree of crystallization as not to help in the definition when the section is examined with the polarized light. In some sections the outlines of the concretions are of such a nature as to suggest very strongly their having been partly dissolved away, while in many cases veinlets of purer and differently crystallized silica of the matrix enter into the mass of the concre- tions. (PI. xxii, Fig. 3; PI. XXIII, Figs. 1 and 2; PI. xxv, Fig. 3.) These THE IRON-BE AKING MEMBER. 207 veinlots have very varying extents, and have often severed concretions into several frag'uients. In the concretions the iron oxide is arranged in concentric oval or spherical bands, and this is efjually trne whether the iron oxide is limonite, hematite, or magnetite. This iron oxide is arrang-ed without the slio-htest reference to the individuals of quartz cuttiaig tln-ouyh them in the most indiscriminate manner. The concretionary structure aifects the silica both in concentric arrano-ement and coarseness of individuals. It is quite common for the individuals of quartz to be larger in the concretions than in the cherty matrix. (PI. xxii, Figs. 1 and 2.) Rarely these concretions have nuclei from extraneous sources, as, for instance, small particles of fragmental quartz, of a comparatively large individual or a cluster of individuals of quartz, or else of iron oxide, but ordinarily they have none. Some- times the concretions are so closely clustered that in their growth they interfere, and in such cases two or more concretions are used as a nucleus about which the bands of iron oxide arrange themselves concentrically, thus forming compound concretions. (PI. xxvii, Fig. 3.) A less prevalent characteristic of these rocks than the concretions consists in the extraordinary brecciated appearance which they present. (PI. XXII, Figs. 2 and 3 ; PI. xxiii. Figs. 1 and 2.) A similar appearance has already been noted as characterizing the chert of the limestone. The out- lines of the fragmental areas are very commonly more or less sharply angular, while frequently convexities and projections in the outline of a detached fragmental area correspond to concavities and recessions of out- line of another fragment in such a manner as to demonstrate a former continuity of the two. The concretionary areas, in which, it should be remembered, silica is still the main ingredient, occiu- along with the frag- ment-like areas, while between the two there is found such a complete series of gradations that it is impossible to resist the conclusion that in many cases both are of the same origin, only the brecciated phases have been shattered by dynamic movements. The great vai-iety of forms presented by the different sections fortu- nately relate the history of these concretionary and brecciated areas. (PI. xxvii.) In the first place, entirely unaltered areas of iron carbonate are found associated with these concretions. Other iron carbonate areas lying 208 THE PENOKEB IRON-BBAEING SERIES. within the usual sihceous grounclmass may be cut by ramifying veinlets of sihca. Again, areas are seen in which curving Unes of iron oxide have been developed at or near their edges. These lines may or may not complete a loop. If the outline of the original carbonate area is more or less irreg- ular and angular, the iron oxide curve cuts off the irregularities. In other cases, again, several such complete or partial iron oxide lines, concentric with one another, have been developed. Subsequent tct or alternate or sinml- taneous with this process silica appears within the concretion, filling the spaces between the rings of iron oxide, the iron carbonate being previously removed. At other times the space occupied by the iron carbonate is left vacant, or else only partially filled, whence arise the frequent geodic arid other cavities found in this phase of rock. Finally, no iron carbonate remains, its place being taken by the concentric rings of iron oxide and the minutely crystalline to amorphous silica. Between the unaltered iron car- lionate areas and the perfectly formed concretions there are at times, often in a single section, every possible gradation. If the iron oxide developed from the carbonate was very plentiful but little room was left for the entering silica, and these areas appear now nearly or quite opaque with oxide, but generally the iron oxide has been developed in the partial or complete ovals referred to, and the substitution of silica for the remaining carbonate left these lying within a siliceous background. Quite similar has evidently been the process by which the pseudo- fragmental areas, such as shown in PI. xxiii, Figs. 1 and 2, have been produced. In such cases the silica of the background has plainly also been rearranged, so that it resembles and merges into the silica of the concre- tionary areas, thus giving the vague, irregular outlines which make these forms so closely resemble fragments. More rarely the fragmental areas are tabular in form, or else show a subordinate parallel lamination which appears to be lines of original deposition. The laminae of these areas frequently abruptly terminate at the exteriors of the apparent fragments. A number of such areas sometimes occur in a single section, now lying within the siliceous groundmass in such positions as to have the laminae of the different areas make all sorts of angles with each other. In such cases THE IRON-BEARIXG :\rE^rBER. 209 the ureas must Ix- lidicvcd lo \h; real iVaoinciUs wliidi Ii.lvc IoihumI citlifr 111 siiii liy ail actual local brecciation of tlii^ rock caused hv clicinical or inwJiHuicMl action, or tlicy may he cousiderod as liaviii' layer of nonfraj^-iiieiital rock by a tem))orary miii.iiliii.i-- of fra^iiieiital and iioufrag-iucutal de[)o.sition. Tliat tlie former is the true ex[)lauatioii of tlu'ir oriyiu we liave litth- d<)\\\)t. It is exeeediui^ly diilicult to convey in any general description a o'ood idea of the nniltituihnous phases presented by this second type of rock, ))ut some ftn-ther and more accurate conception may be obtained Ijy an exami- nation of the accompanying carefully described plates and study of the detailed descriptions of individual sections given below. In connection with the second type of rock, it is necessary to allude to some peculiar phases which resemble on the one hand the breceiated rocks described above, and upon the other the true fragmental rocks of the underlying Quartz-slate member. In them there has evidently been a mingling of mechanical and chemical sedimentation. They contain simple grains of quartz which have been enlarged, the fragmental character of which can not be doubted. These same sections also contain rounded chert areas, which include sometimes little iron oxide, sometimes abundant red . hematite, making them jasper, and sometimes both hematite and magnetite. Often the areas containing little or no iron oxide and those containing it abundantly are in juxtaposition. Simple fragmental grains of quartz and the chert areas are generally arranged with their longer axes in a common direction. The fragmental character of the simple quartzes, the likeness to them in form of the chert areas, and their an-angement with longer axes in a common direction, are almost conclusive proof that these parts of the rocks are genuine mechanical sediments. Tlie fragments are cemented by a matrix, which is a ferruginous chert in every respect similar to the matrices of the ferruginous cherts of the second type of rock. These rare occurrences of semifragmental rocks are interstratified with others which are typical forms of the Iron-bearing member. The meaning of this interlamination of the two classes of sediments will be con- sidered later. MON XIX 14 210 THE PENOKEE IKON BEARING SERIES. Microscopical character of the actinolitic slates (PI. xxiii, Figs. 3 and 4; PL XXIV, Figs. 1 and 2 ; PI. xxviii). — As already stated, the actinolitic rocks of the third type above macroscopically described have as their four main constituents, quartz, hematite, magnetite, and actinolite. In the large exposures all of these minerals are invariably found. In hand specimens any one of them may be found, to the almost complete exclusion of the others, and also all possible combinations of any two or three of them, and of all together. The order — quartz, iron oxide, and actinolite — expresses their usual i-elative abundance, taking the exposures as a whole. However, in the thin section, as would of course follow from the foregoing, any one of the three may at times be the predominant constituent, while also any one or two of them may be almost or (juite wanting. In o;-der of time of crystallization, the iron oxide is always the earliest of these constituents, next the actinolite, and finally the quartz. Yet while this is true as a general statement, it is not meant to imply that the crystallization of any one constituent is finished before that of the second one begins, for they are, to a certain extent, simultaneous. The areas of iron oxide are the freest from inclusion of the other two chief constituents. The actinolite contains large quantities of iron oxide, but everywhere penetrates the quartz, a single needle of actinolite commonly cutting two or more indi- viduals of quartz, and finally the quartz individuals include much magnetite and actinolite. However, each one of the minerals is, in certain cases, found to include both of the other two. In addition to these three main con- stituents, chlorite and biotite are found as extensive alteration products of the actinolite. The alteration products, chlorite and bio,tite, are always in minute flakes, several or many individuals of these minerals forming from a single needle or blade of actinolite. The magnetite and hematite are directly associated and probably formed at the same time. Occasionally the hematitic ingredient is manifest macroscopically by the joeculiar steely luster characteristic of specular iron, or by a purplish tint to the powder of very highly ferruginous varieties; but the analyses quoted above show that this ingredient is at times contained when neither microscopic nor macro- scopic characters would suggest its presence. It appears to be intimately associated with the magnetite, and occurring in aggregates of very minute THE IKON-BEARING MEMBER. 211 metallic lustered |iai'ticli's, ii fails lV('(|iicnlly to <>;'\vi' the reddish translu- cency ordinarily dcscrilxMl as a characteristic of it.' Of the tlircc i)rinci|)al mineral constitnents named, tjie ((nartz, or silica, presents itself in a, j^'cneral way as a very minntely crystalline i;ronndinass or hackf^^-round in which the other minerals are contained. Only rarely is the crystallization of this (^nartz so coarse as indicated in PI. XXVIII, Fig-. 4, in which, however, the individuals are still small, being greatly magnified. On the other hand, it only rarely sinks to that exces- sive degree of fineness which is cliaractei'istio of the chalcedoiiic form of silica. Still, a few sections have been found in which it does reach this excessively tine condition, and even becomes quite amorphous; but tliese conditions are, as seen, far more cliaractei-istic of the cherty rocks of the type previously described. Since these two types, however, manifestly grade into one another, it is evideiat enough that all of the silica of the three types' is of the same origin; that is to say, is all water deposited. Even where least minutely crystalline, as in some of the actinolitic slates, the individuals of quartz interlock with one another in such a fashion as to place their deposition in situ quite beyond question. Nowhere is there any indication of a fragmental origixi for any of the siliceous groundmass. Cases have been noted of the occurrence of genuine fragments of quartz within the nonfragmental mass of the rocks of the first and second types, but these fragments are always sharply defined and wholly distinct in character from the prevalent nonfragmental silica. * The magnetite and hematite occur partly in the form of an excessively fine dust contained within the quartz and actinolite individuals, partly in 'A single individual of hematite is cliaracterized in the thin section ordinarily by a reddish color in transmitted light, and lack of metallic luster in reflected light; but wheu the individuals are exceedingly small, an aggregate of hematite may readily present the metallic luster, and unless thin- ner than the ordinary thin section is, may at the same time appear perfectly oiiaque in transmitted light, in which case, if mingled with magnetite, its presence is difficult to detect with (lie microscope. In order to test this question a number of thin suctions were made of specular hematites from various places in the lake .Superior region. Some of these, after powdering, yielded nothing at all to the magnet, while others contained more or less magnetite intermingled with the hematite. In all cases, however, where the grain is sufficiently fine, it was found that the heiuatite presented the same metallic luster in the reflected light and opacity in transmitted light as the magnetite. Many of these thin sections, when the ore contained no magnetite whatever, presented in the transmitted light a mixture of red trauslucent and black opaque material, tMs opaque niaterial giving the metaj- ]ic luster in the reflected light, 212 THE PENOKEE IKON-BEAEING SERIES. the shape of single crystals, or groups of crystals, of moderately large size, and partly in the irregular, oval, opaque bunches of crystals above men- tioned. The outlines of the single crystals are usually very sharply defined, square, hexagonal, and rhombic outlines presenting themselves constantly. On the whole the rhombic outline is the most common. In the most regularly slaty phases of the actinolitic schists the iron oxides are found to be aggregated much more plentifully in certain bands than in others, the individual laminse of these slaty rocks presenting in turn each of the three main constituents as the predominant one. PI. xxviii. Fig. 4, shows the arrangement of the minerals in one of the more quartzose laminse of such a slaty rock, while PL xxiii, Figs. 3 and 4, from, photographs, show, with a nuich smaller degree of enlargement, portions of tlu-ee lamiufe, the central one composed maiidy of quartz and actinolite, while the other two are richer in magnetite, but tliese last two figures represent by no means a highly magnetitic phase. The rocks of the second" or ferruginous chert type have been described as presenting very commonly a most singular occurrence of the oxides of iron, the particles of these oxides being distrib- uted upon the quartz background in rude circles or ovals, or in two or more concentric circles or ovals. In these cases the curving lines, made up of particles of one or more of the three oxides, present no relation whatever to the silica individuals, traversing a number of these indi- viduals indifferently, and looking as though painted upon the silica background. While this occurrence is far more common among the cherty rocks of the second type, it is also met with among some of the highly actinolitic slates, in which case the actinolite is usually associated with the curving lines of iron oxide. In some cases actinolite and mag- netite are associated in a bimchy fashion, as, for instance, in PI. xxiv, Fig. 2, and are at times dissevered from one another by the intru- sion of silica, this silica, however, presenting the appearance of passing about the several detached areas where these are sufficiently trans- lucent to allow such a relation to be seen. These relations are still better shown by certain rocks from the Animikie series. (PI. xxix. Fig. 2.) The existence of this peculiar structure in the highly actinolitic rocks is of very considerable interest, since it furnishes one more link between them and 'rilK lUON-HKAltINC iMIO.M i:i:i{. '2U] tln' I'ciTiiii'iiKiiis clifrts ol' the last t\ |)c, (iiic iikiic tiling \^> indicate that all ai'f ilci'i\ril t'loiii ('ss('iitiall\ the saiiir (irjoinal material. Tlic actiiKilitic injiTcdifiit nt' tlicsc i-dcks iircscuts itscll' iisiialK in liin\clics of iniiuitc radiatiuji' i)lades oia pale greenish tint. OccasionalK- the iiidixidiial l)ladcs ;iiv largo enough, as indicated previoush', for them to be seen with the naked eye or with the luagnit'ying g-lass, but more usually they are micro- scopic. Kven these radiating bunches of actinolite are aggregated into certain laiuiuiv to the approximate exclusion of the other mineral ingredi- ents. They occur also in single blades scattered through the thin section, many minute blades being not infrequently inclosed in a single grain of quartz, while other Ijlades or bunches of blades often traverse a number of quartz individuals. One unusual ^ihfise of the aetinolitic rocks deserves especial description. At Penokee g-ap, both on the west and on the east side of the fault which there occurs (see PI. xxxvi), the uppermost layers of the Iron-bearing jnember are composed of a thin belt of peculiar slate, in which garnet is a prominent constituent. This slate is nearly aphanitic, of a dark color, banded vaguely with lighter streaks, and cleaves readily parallel to the lamination. Minute individuals of garnet may at times be detected with a strong ■ magnifying glass in the lighter colored streaks, but usually the)' require the microscope for their detection. Under the microscope this rock is seen to be almost wholly composed of actinolite, magnetite, and garnet. The garnet is in small and very numerous individuals, presenting a pale pinkish tint. In the ordinary light many of these garnets are to be seen provided with crystal out- lines, but in the polarized light this becomes less evident, because many of the garnets are penetrated by numerous needles of actinolite, which are frequently very plentiful in the outer portions of the garnet individtials. The actinolite also makes up most of the interstitial portions of the thin sections, while the magnetite, in fine particles, at times provided with sharp crystal outlines, is scattered uniformly throughout all portions of the sec- tion, being included within each of the other minerals. Numerous minute flakes of biotite occur always in such relation to the actinolite as to suggest their possible derivation from it. Chlorite occurs also in some sections 214 THE PENOKEE IftON BEARING SERIES. quite plentifully as an alteration product of the actinolite. In some cases brown iron oxide stains the actinolite blades. A separation of the garnet (specimen 9553) by W. S. Bayley having been made, the following analysis^ of the separated material was made by Dr. Thomas M. Chatard, of the laboratorj^ of tlie U. S. Greological Survey: Silica, 39-31; alumina, 12-86; iron sesquioxide, 10-21; iron protoxide, 32-81; manganous oxide, 1-03 ; calcium oxide, 1-88 ; magnesium oxide, 1-90 = 100-00. Dr. Chatard observes -that this analysis is close to that of an average almandite. At one time in our study of this rock it was supposed that the actinolitic ingredient might be the result of a secondary alteration of the garnet, but the abundance of the same actino- lite in the underlying inagnetitic and actinolitic schists with which this rock is so intimately associated, and in which the actinolite is surely not an alteration product of garnet, seemed to render such an origin improbable. A further study of the sections appears to show that the actinolite, even in the garnetiferous rock itself, is independent of the garnet, having been in part a simultaneous crj^stallization and in part an earlier crystalliza- tion. This conclusion is borne out by the analysis above quoted, from which it appears that the composition of this garnet is such that the actino- lite could not readily have l^een derived from it. Immediately over- lying this garnetiferous rock is found a black fragmental slate.^ At the junction between the two the garnets are found to occur within the frag- mental material, an occurrence which suggests the possibility that this frag- mental material furnished some of the ingredients which subsequently made up the garnet. ' Witli regard to thisi analysis Dr. Chatard says tliat the amount of material furnished was only 1.2 grams, and that owing to the loss of a portion there remained but 0.54 gram for the determinatioa of the absolute amount of iron protoxide. This reiuuant lieing too small for the deter- mination, the irou protoxide was calculated from the excess over 100 in the summation of the entire analysis after estimating all tlie iron as the sesquioxide. Dr. Chatard also says that this method of calculation "gives a result close to the aver.age analysis of almandite garuet, which the mineral undoubtedly is." ''For an early description of this garnetiferous slate, which, thou.gli not frequently met with in the Penokee region, is closely allied to certain garnetiferous actiu(ditic schists occurring moi-e widely in the Marquette region of Michigan, see Geol. of Wis., vol. HI, pp. 123, 124; see also for rude colored drawings of the thin sections of the same rocks, Figs. 1 and 2 of PI. xva of the same volume. Till-; llfON r.K.MMNd MKMl'.IOi;. 215 111 tilt' (letailod (Icscrlptioii ol' tlic nctiiiulitic mikI iii:iosed of vaguely defined lighter and darker colored bands, the latter so rich in magnetite as to give a metallic luster. The lighter colored ones are seen with the magnifying glass to contain more quartz. Sp. gr., 3'46. The groundmass is comiiosed of finely crystalline quartz. Magnetite particles are scattered generally through the groundmass, but are also often grouped in irregular outlined clusters. Actinolite, with its alteration product, chlorite, occurs as in 2. TlIK lliON-15RARrN(} IsriOMI'.KK'. 217 From the Marenno rircr iron runtji; Sit. SI, '/'. // .V., A'. ■; IT., Wisconsin. 7. Magimtitic iiftiiiolitic scliist. Siu^ciiiicn ISC \Vr. ; IVoin l!t(i(l X., 13lir» VV., Sec. 2:5, T. 44 N., It. r> W., VVis( sin. An aplianitic, dark gray, slaty rock, sn excessively line grained that the mag- nifying- glass detects none of its constituents. The thin section is mainly made up of interlaced, often radiating, blades of actiuo- lite, within which are inclnded niimei'ous crystals and particles of magnetite. The particles of the latter mineral are also at times aggregated into bunches of some size. 8. Magnetitic actinolite-scbist. Specimen 180 Wr.; from 500 N., 500 W., Sec. 23 T. 44 N., R. 5 W., Wisconsin. A dark gray, nearly aplianitic rock, showing on the weathered portions very thin and regular lamination. The magnifying glass can barely detect the magnetite particles and minute needles of actinolite. The thin section is similar to that of 7, save that the actinolite is in larger blades and that the rock contains a larger pi'oportion of magnetite. From the exposures in See. 24, T. 44 m, R. 4 IF., Wisconsin. 9. Actinolitic and magnetitic quartz-schist, from a low horizon. Specimen 9649 (slide 3159); from 1637 N., 0 W., Sec. 24, T. 44 N., R. 4 W., Wisconsin. This rock closely resembles 2 and 4, being composed of dark colored, strongly magnetitic bauds and light colored, quartzose, ones. The thin section of this rock is similar to that of 4. From the exposures in Sec. 9, T. 44 If., R. 3 W., Wisconsin. 10. Actinolitic and magnetitic quartz-schists, from a middle horizon. Specimens 9641 (slide 3151), from 0 N., 400 W., and 9642 (slide 3152), from 0 N., 500-600 W., Sec. 9, T. 44 N., R. 3 W., Wisconsin. These rocks, again, are closely like 2 and 4, having tlie same interlamination of lighter and darker materials. The darker colored bauds are very rich m magnetite, quite aphauitic, and have a conchoidal fracture. Sp. gr. of magnetite seam in 9042, 4-54. The thin sections show a groundmass of minutely crystalline quartz, the indi- viduals for the most part niterlocking with one another more or less deeply, thouo'h some of the more minute ones are provided with crystal outlines. This groundmass in slide 3152 composes two-thirds of the rock; in 3151 less than half. Magnetite is scattered through the groundmass in numerous minute grains and crystals, which are included within the quartz individuals. These are so abundant in 3152 as to make u]) ahalf of the whole section. Actinolite occurs in a few blades which are heavily stained with iron oxide and traverse the groundmass in such a manner that each blade cuts a number of quartz individuals. 218 ^HE PEI^OKEE lEON-BEARING SEHIES. From the PenoJcee gap section. 11. Magnetitic actinolitic quartz-schists, from middle horizons. Specimens 9556 (slide 3142), from 1900 N., 1500 W.; 9557 (slide 3096), from 1850 N., 1500 W.; 9558 (slide 3191), from 1800 N., 1500 W., Sec. 14, T. 44 N., E. 3 W., Wisconsin. These specimens represent the composition of the west cliff' at Penokee gap. This cliff is made up of alternating bands of lighter and darker colors, individual bands varying very greatly in width and in the amount of distinctness of definition. The darker bands, often in themselves thinly laminated, are very rich in magnetite, at times so much so as to present a distinct metallic luster. The lighter colored por- tions are highly quartzose and carry relatively little magnetite. The bands which are richest in magnetite are usually found running from a fraction of an inch to several inches in thickness, but thicknesses of a number of feet are met with in which there is as much as 40 per cent of metallic iron. The darker colored, very highly magnetitic phases are quite aphanitic, but the lighter color.ed portions, though still very fine grained, show distinctly under the magnifying glass a mixture of minute actinolitic blades and quartz. Sp. gr. of 9556, 3-37; of 9557, 4-31; of 9558, 3-91. The thin sections are composed of magnetite, actinolite, and quartz, named in order of time of crystallization. The only difference between the thin sections lies in the varying coarseness of grain and the varying proportions of the three constituents, each one of which is in turn the preponderating ingredient, and again insignificant in quantity. The quartz forms a groundmass of closely fitted or interlocked grains, the smaller ones of which are often provided with crystal outlines. Nowiiere do these grains show any traces of fragmental cores, the whole appearance forbidding any thought of a detrital origin for any portion of the section. The actinolite is in color- less to pale green, feebly dichroic needles and blades, at times arranged in radiatiug aggregations which vary very greatly in size. Numbers of the more minute needles occur often within single quartz grains, while the larger needles and blades extend through a number of quartz individuals. The arrangement of the actinolite has plainly been withoxrt any reference whatever to the quartz, which appears therefore to be of subsequent origin. The magnetite occurs in minute dust-like particles within both quartz and actinolite, and also in quite large aggregations of irregular outline. At times these aggregates of magnetite include particles of actinolite and quartz, but such cases seem rather to be esxjlicable on the idea that the actinolite and quartz have entered into cavities within the magnetite aggregations subsequent to their solidification. N"o evidence is found that these two minerals ever are included within or traverse single individuals of magnetite. (PI. xxiv. Fig. 1.) 12. Magnetitic and actinolitic quartz-schists, from near the summit of the Iron member. Specimens 9555 (slide 3190), from 0 N., 1625 W., Sec. 11; and 9567 (slide 3195), from 1675 N., 1100 W., Sec. 14, T. 44 N., E. 3 W., Wisconsin. TIIK llt()N-i;i;Ai;lN(! I\lKMltKI{. 219 These specimens are from very ncur thr siiiiiniil dl' ilio lidii 1(c;irino- Imrizdii, haviiii;- lu'cii lakfii immf W.; 1444, Wis. (.slide 263), from 0 N., 1600 W., Sec. 11, T. 44 N., \i. 3 W., Wisconsin. These are dark colored aphanitic rocks, banded vaguely with lighter streaks, having a pronounced coiichoidal cross fracture, but cleaving readily parallel to the lamination. In some places, with a strong magnifying glass minute individuals of garnet may be detected. These apiwar most abundant in the light colored streaks. Sp. gr. of !t5.53, 3-42. The thin sections are almost wholly composed of magnetite, actinolite, and garnet. The garnet is in rather small and very numerous individuals, which present a pale pinkish tint in the thin sections. In the ordinary light many of them are seen to be provided with crystal outlines; iu the polarized light this becomes less evident, because many of the garnets are penetrated by numerous needles of actinolite, which are ft'equently more plentiful in the outer portions of the garnet. The actinolite makes up most of the interstitial portions of the sections, but also penetrates and is included within the garnet individuals. It was at first supposed that this actinolite is an alteration product of the garnet, but the fact of the abundance of the actinolite in the immediately underlying maguetitic and actinolitic schists with which this garnetif- erous rock is so intimately associated, and in which the actinolite is surely not an alteration product of garnet, seeming to render snch an origin improbable, this view ■was abandoned. A further study of the section seems to show that the bulk of the actinolite is rather independent of the garnet, having been in part of a simultaneous crystallization with it and iu part of a x^revious erystallization. The magnetite, in fine particles, at times crystal outlined, is scattered uniformly through all portions of the section, being included within all of the other minerals. Biotite occurs in numerous mimrte flakes, which appear in all cases to be secondary to the actinolite individuals. 14. Altered garnetiferons actinolite-schists, from the summit of the Iron member on the west side of Penokee gap. Specimens 1501, Wis. (slide 276), from 1670 N., 1100 W.; 1502 Wis. (slide 277), from 1670 N., 1100 W., Sec. 14, T. 44 N., R. 3 W., Wisconsin. Macroscopically these rocks resemble 13. 220 THE PENOKEE lEON-BEAEING SERIES. This resemblance holds iu thethiu sections, save that the garnet is less abund- ant and that the actiuolite has been much more largely altered. The alteration ■ products are biotite and chlorite, but more or less brown iron oxide accompanies the biotite. Frotn the Penokee range, Sec. 9, T. 44 N., R. 2 TF., Wisconsin. 15. Maguetitic and actinolitic quartz-schist, from a low horizon. Specimen 2064 Wis. (slide 300), from 0 N., 1000 W., Sec. 9, T. 41 IST., R. 2 W., Wisconsin. The thin section of this rock does not dift'er iu any essential respect from those of the Penokee gap actinolitic schists, 11 and 12. The actinolite is arranged in the usual radiatiug bundles and is more or less stained witli brown and red iron oxides. The quartz, as usual in these rocks, is very minutely crystalline, and wholly without any appearance of a fragmental origin. From ihe (jorge of Tylers forh, Wisconsin. lis. Ferruginous quartz schists, from a low horizon. Specimens 9620 (slide 3147), from 1170 N., 160 W.; 9624 (slide 3149), from 1235 N., 193 W., Sec. 33, T. 45 N., R. 1 W., Wisconsin. The exposures of the Iron-bearing member at the gorge of Tylers fork present rocks having a geueral resemblance to those of the Penokee gap section, including darker colored, more maguetitic, and lighter colored more quartzose phases; but one is imjiressed at once with the relatively small proportion of magnetite contained in the Tylers fork section where the light colored, more quartzose kinds very greatlj' predominate, and where the thin banding which characterizes the Penokee section is only developed in one or two places. Sj)ecimens 9620 and 9624 are mainly made up of the lighter colored phase, which, it should be said, is still lighter colored than any of the Penokee gap rocks. jSTo actinolite is perceptible with the magnifying glass, m while the appearance of the quartz is quite peculiar, suggesting some sort of a con- cietionary arrangement. The lean ferruginous seams are exceedingly irregular in thickness and in course, branching and running in all sorts of irregular fashions instead of lying i^arallel to the general bedding of the rock. Sp. gr. of the quartzose portion of 9620, 2-93; of maguetitic banding seams, 5-01. The thin sections are composed of quartz, actinolite, and magnetite, and so far are analogous to the actinolitic and magnetitic schists of Penokee gap above described, but there are some important differences. In the first place, the actinolite iu the Tylers fork rocks is very unich less plentiful, and as a rule is in much more minute blades. A much m,ore])ronounced difference, however, consists in the striking tendency toward a concretionary development that these Tylers fork rocks show. This is brought out particularly in the arrangement of the individuals of the greatly predominating quartz of the groundmass and comes out with special prominence in the polarized light, when there is seen a tendency of the coarser individuals of the quartz to TFIE IK'OX-I'.KAlv'rXd MEMRETt. 221 arrange tliemsclvos intlio cenkMS of areas whoso outer portions are made up of more iiiimitc piirticlcs. The iiiaHiu^titc, liowcvor, also shows some sliglit tciidcMicy to a cou- crctioiiarv arraiij;ciiicut, wliilc the tiiiiiiitc actiiiolitc blades arc af^yrcgated into little dusters, which appear in the main to lie in tiic spaces between the concretionary areas or in the outer portions of these areas. TIk^ niaf^netite o(!(Mirs in bunches of crystals, and in curvilinear ajij-rcjuations, and also in single individuals scattered in- discriminately, thongh sparsely, throughout the section. Another noticeable difler- eiice between these rocks and those from Penokee gap (ionsisLs in their lack of the highly developed lamination which the latter rocks show, both in the thin section and in the hand specimen. There is no trace of evidence that the quartz of these rocks is in any measure of a fragmental uatuie. In fact, its concretionary arrange- ment and the interlocking of the diflerent individuals puts such an origin entirely out of the question. In slide 3147 stains of brown iron oxide occur here and there, usually mingled with the clusters of the minute actinolite blades. lu the section of this specimen there is seen also a single rhombic crystal section, which, on examina- tion with a. high power, proves to be made up chiefly of a number of actinolite blades, which on the border of the crystal are deeply stained by brown iron oxide. The shape of this piece suggests the i)robability that it was originally a single individual of iron carbonate. (PI. xxviii, Pig. 1.) 17. Ferruginous quartz-schist, from a low h(n-izon. Specimens 9617 (slide 3100) 9623 from 1175 N., I'M W., 9618 (slide 3148), from 1245 X., 185 W., Sec. 33, T. 45 N.' E. 1 W., Wisconsin. These si)eoimeus resemble those of 16 in all respects, except that numerous spots of reddisli jasper are seen upon them. These stand out in a prominent fashion and in such a way as to suggest a fragmental origin for them. In the same ledge, however, are large bands of the same jasper (9618), directly interlaminated with the other materials; Specimen 9617 shows the peculiar concretionary, or partly fragmental character described under 16. Sp. gr., of 9617, 2-76; of 9618, 2-92. The thin sections show that these rocks differ from 16 in containing no actinolite- but a more interesting difference lies in the very nuich more striking deveioi)ment which they show of a concretionary structure. This appeaiance shows m)t merely in the quartz background itself, as seen in the i)olarized light, but in the arrangement of the particles of iron oxides, which include both hematite and magnetite, and in smaller quantity the brown oxide. In some cases the magnetite and liematite are aggiegated into opaque, round or oval areas, the edges of which show the projecting corners of numerous individual crystals of magnetite; but there are more plentiful aieas in which these oxides are seen to be disposed in concentric bands, made u]> of niore or less sep- arated particles of the oxides, and having between them the ordinary quartz back- ground of the rock. When these areas are exainined in the polarized light they are found to lie in the quartz background in sueh a fashion as to suggest their haying 222 THE PEl^OKEE lEOK-BEAEING SERIES. been painted upon it, a single band of the iron oxide traversing multitudes of quartz individuals, to whose arrangement it bears little or no relation. As usual, the quartz in these rocks is plainly the last separated ingredient, and it seems that its concre- tionary form had something to do with the concentric arrangement of the iron oxide also. From what follows later, it is probable that the bands of iron oxide are the remains of iron carbonate areas whicli have altered to iron oxide in part, but have partly been replaced by silica, these processes produciug the concretions. Besides these concretionary areas, there are other areas whose outlines in the polarized light suggest their being fragments. These are mainly made up of silica like that of the groundmass, except that in some cases they are possibly finer grained and have the iron oxide particles scattered through them irregularly. This is particularly noticeable in slide 3148, which appears with a low power, as if it might have been a fragmental rock whose fragments had all been silicified, the same silica tilling the spaces between the fragments. In this sectiou also some of the apparent fragments are jaspery — that is to say, a mixture of red iron oxide and quartz — and appear therefore quite different from the rest of the rock. Of particular interest is the occurrence in this slide of quite an amount of iron carbonate. This substance, which is usually more or less stained with brown iron oxide, but is very i)lainly recognizable as a carbonate, appears both in single individuals scattered through the groundmass and in clusters of individuals. These clusters often lie in groups, between which are areas of the groundmass, in such a fashion as to render it evident that they were once connected with one another and have since been separated by the insertion of the quartz. In a few cases some of the apparent fragments above alluded to are mainly made up of this brown stained car- bonate, little veins and streaks of the quartzose groundmass entering into them or even entirely traversing them. This occurrence of the carbonate is of great interest, because it seems to indicate that all of the apparently fragmental and concretionary areas which are now made up of quartz and uon oxide were originally composed of this substance. Whether the fragmental appearance which these rocks have is due to a real fragmental character, or is caused by the same processes which have formed the rounded and oval concretions (i. e., is of a secondary nature), will be discussed later. 18. Ferruginous quartz-sclust, from a middle horizon. Specimen 9625 (slide 3150); from 1285 N., 230 W., Sec. 33, T. 45 N"., R. 1 W., Wisconsin. A nearly black aphanitic flinty rock in which the only ingredient recognizable with the magnifier is hematite in minute metallic lustered scales. Sp. gr., 2-90. The thin section of this rock, examined with a low power and iu the ordinary light, presents a much more striking irregular appearance than those immediately above described (except slide 3148), being made up of round or oval areas, whicli differ from the whiter quite sparse matrix in being dotted over with a fine black dust. These fireas are usually outlined by a border of magnetite crystals, but otherwise there is THE IHON-BEAHING MEMBER. 223 none of the concretionary appearance nlxiiit llicni tliat characterizes the apparent frag- ments of 17. Examined with a higher power in tlic polarized li;;li(, some of these areas are found (o lie nnule up mainly of (puut/,, whicli for the most pait is (|iiiteliiie that of the interspaces, though tlie latter is occasionally coarser than is seen within the apparent fragments. Tlie black particles alluded lo appear to be wholly iron o.xide and in the main magnetite, which mineral, however, occturs also in (piitc large sized crystals, arranged in bauds, as already stated, bordering the apparent fragments, but also sometimes scattered Ihrough them, and again in clusters within the interstitial quartz. (PI. xxvui, Pig 2.) ^ From tlie Pcmlcee range in Sec. 34, T. 45 N., R. 1 W., Wisconsin. 19. Maguetitic (luartz-schist. Specimeu 9081 (slide 4206) ; from 140 N., 0 W., Sec. 24, T. 45 N., R. 1 W., Wisconsin. The rock is a very highly inagnetitic, fine grained, dark gray schist, in which the magnifier reveals otdy quartz and magnetite, certain irregular bauds being much more highly maguetitic than the rest of the rock. The thin section of this rook is very closely like those of the Tylers fork rocks, particularly 17, presenting the singular concretionary and apparent fragmental appearance. In the case of the present rock there is more of the red and brown oxides of iron than of the magnetite. In many places the iron carbonate is found in the concretions. Here at times the iron carbonate extends beyond the outlines of the concretions and apparent fragmental areas in such a manner as to make it probable that the iron carbonate was the original material, and that the minerals contained ill it — iron oxide, actinolite, and quartz, making up concretionary and apparently fragmental areas — have formed from in part and replaced in part the original iron carbonate. In places the rhombic shai)es of the brown and red iron oxides make it evident that these are the direct results of oxidation of siderite individuals. (PI. xxiii. Figs. 1 and 2.) From the Potato river section. 20. Maguetitic quartz-schist. Specimen 9104 (slide 4214); from 1050. K., 250 W., Sec. 19, T. 45 N., R. 1 E., Wisconsin. A fine grained, nearly white, cherty rock, dotted with magnetite particles, and irregularly banded with thin seams of the same mineral. Sp. gr., 3-21. The thin section of this rock resembles those of the actinolitic and maguetitic quartz-schists of Peuokee gap (11 and 12), i>articularly those in which there is the minimum of parallelism in arrangement of the several ingredients. The arrangement of the quartz individuals and the bunchiness of the magnetite and actinolite suggest a vague concretionary arrangement, The actinolite is scattered about through the 224 THE PENOKEB lEOI^-BEAEHSTG SERIES. quartz grouudmass, but occurs very plentifully also among the magnetite aggrega- tions. As compared with the last rocks described, and others like it, this rock lacks the distinct separation into a relatively pure quartzose matrix and dark colored areas in which the iron oxides are particularly abundant, and which are outlined so dis- tinctly as to suggest their having been fi-agments. However, after having seen the sections in which this arrangement is very pronounced, one realizes that even in a case like the present one there is a faint indication of the same structure. More or less of the iron oxide occurs in the hematitic and hydrated forms. 21. Magnetitic quartz-schist. Specimen 9183 (slide 4213) ; from 1050 N., 270 W., Sec. 19, T. 45 N., R. 1 E., Wisconsin. The rock is a fine grained, dark gray magnetitic schist, closely resembling 19. Sp. gr., 3-39. The thin section resembles the last described as to its constituents, but differs from it in having the concretionary and pseudo-fragmental structure more largely developed. Since the two come from the same rockmass, it is evident that the faint indication of this peculiar structure seen in slide 4214 (20) points to a similar origin for the two rocks, Quite a little quantity of carbonate remains in some of the apparent fragments. Minute actinolite blades occur here, as in other similar sections, aggre- gated in minute needles about the clusters of magnetite. From the PenoJcee range in Sec. 16, T. 45 W., B. 1 E., Wisconsin. 22. Ferruginous quartz-schist. Specimen 12831 (slide 5495) ; from 513 N., 1647 W., Sec. 16, T. 45 N., R. 1 B., Wisconsin. The rock is line grained, dark gray schist, banded with seams of black iron oxide. The thin section of this rock resembles those of the several peculiar concretion- ary or pseudo-fragmental rocks above described, particularly 20. As seen in the ordi- nary light, it shows a colorless background, which is thickly studded witli distinctly outlined areas that differ from the background merely in containing numerous minute dark colored particles. Some of these areas, however, are surrounded by a rim of the several iron oxides and occasionally these iron oxides, particularly the red and brown oxides, cover the whole of one of these areas. In the polarized light the spotted areas lose much of their deiiniteness and are seen to be mainly made up, like ttie matrix, of a very minutely crystalline cherty quartz. Some of the areas are quite angular in outline, but most of them are more or less rounded, while at times a number of the areas are so related to one another as to seem to have been once continuous, having been severed by insertion of the matrix material along irregular rifts. Here and there these areas are seen to have not merely a single ring of iron oxide, but to have a,lso one within and concentric to the outer one, TllK I KOX-BEARING MEMBER. 225 From the Pemkce range in Sec. 10, T. 15 N., Ji. I K., Wisconsin. 23. Fernij;iiioii,s iiiid actiiiofitic vhart, or quartz aiid rC-hert. Specimen 9185 (slide 4-*ll); lioiii 0 N., 1200 W., Sec. 10, T. 45 N., K. I E., Wisconsin. Tilt! r(H!k is a lino sriii"Cs not appear to have been much affected by concretionary action, except that here and there the larger indi- viduals are bunched in irregular oval areas. Two or three slender veins of quartz traverse the section. In these the individuals of quartz are larger than in the general background of siUca. Here and there the nuclei of the concretionary areas referred to are seen to be composed of a number of unusually large sized quartz individuals (PI. XXII, Figs. 1 and 2.) 30. Ferruginous chert, from a low horizon. Specimen 9047 (slide 2775) ; from 405 F., 1095 W., Sec. 27, T. 46 K, E. 2 E., Wisconsin. ' The rock is a brown chert similar to 29, but having a darker color and contain- ing some spots which are bright red and jaspery. The thin section is composed of a finely divided but still whoUy crystalline silica, the larger sized individuals being much more plentiful than in 29. Irregularly blotching this background are areas of nearly opaque hematite, which are without any perfect concretionary arrangement, which arrangement is also only faintly indi- cated in a portion of the siliceous background itself. From the Germania mine. 31. Ferruginous chert or flint, from a very low horizon. Specimen 9015 (slide 3103) ; from 200 N., 1575 W., Sec. 24, T. 46 N., E. 2 E., Wisconsin. A dark brownish gray, aphanitic, cherty rock, carrying irregular seams of hematite. This section shows an almost pure chert, quite closely resembling that which forms the background of 29. A few concretionary areas only faintly marked with iron oxide are seen. From the Montreal river section. 32. Ferruginous chert or flint, at the base of the Iron-bearing member, Ashland mine. Specimen 7619 (slide 2308) ; from 1965 N., 1925 W., Sec. 27, T. 47 N.. E. 47 W., Michigan. The thin section is made up almost wholly of silica in an exceedingly finely divided and even amorphous state, at times showing faintly the radial chalcedonic arrangement. Brownish and reddish iron oxides occur here and there in irregular patches and spots. They also appear in v^ry distinctly outlined rhombic sections, which suggest a derivation from the oxidation of an iron carbonate; a suggestion which is borne out by the presence in some of the rhombic sections of areas of unal- tered carbonate. 33. Black banded flint and siderite, from a high horizon. Specimens 9007 (slide 2916), 9009 (slide 2765) ; from 160 N., 100 W., Sec. 21, T. 47 N., E. 47 W., Michigan. A fresh fracture of these rocks shows an interbanding of dark gray and black layers ranging from about one-eighth to one-half inch in width. The black bands are com- Till'] IK'ON-lJEAlMNt} MEMHKit. 229 l)oso(l ofiui ai)1iiuiiti(' Hint ; llic dnrk j;r;iy l);ni(ls, wliilc contaiiiint^- more or li^ss oCtliis Mint, apjjoar to hc^ iiiiult' ii|)(>('a crystiilliiic ag^jrc^'iiUoii of soiiHM-arhonate, Mie crystal- lizatioii heiiigf coarsti oiioujili for tlic iiakod eye to detect tlio'cleiivaROS of tlie indi- vidual crystals. S]t. f;r. of !t(»Oi), .'$-24. Ill thin sections the black bands of the rock represented by tliese specimens prove to bo made up mainly of an amorphous and an exceedingly finely crystalline silica. In this siliceous background arc included numerous minute black particles, to which evidently the black color of the bands is due. These pai'ticles, when examined with a high power, are found to be exceedingly iiregular in sha]ie, though often they are aggregated into thin belts, which are so identifiil as to give a strongly laminated ap]iearance to these i)ortions of the sections. They ap]iear to be quite without crys- tal outlines, and from the macroscopic a.i)pearance of the rock and its close resem- blance to others in which carbonaceous matter has been detected on analysis it is supposed that these particles are of that nature also. Very possibly they may be mingled witli more or less of pyrite and iron oxides. A few pieces of what ajipears to be a fragmental quartz are met with in those portions of these sections which represent the black bands, and also a few irregular areas of iron carbonate which are analogous in structure and alterations to those spoken of under .32. The gray bands seen microscopically have a background or groundmass similar to the nmterial which composes the bulk of the black bands. But in this case there are contained in this groundmass, so plentifully as to constitute the larger portion of these parts of the sections, irregular areas of the iron carbonate in various stages of alteration and others less plentiful of a greenish chlorite. These greenish areas have often an oval form. The viridite or chlorite wliich makes them up is arranged in fan-like aggre- gations. The areas of iron carbonate vary greatly in size, the smaller ones being almost perfect single rhombohedra; while the larger ones, although aggregates of a number of individuals, show around their borders the projecting edges of rhom- bohedra. Much of this iron carbonate has altered more or less thoroughly to iron oxides, including both the brown oxide and hematite. In many cases where the alteration has been complete the secondary iron oxides are plainly perfectly jiseudo- morphous after the siderite, i. e., they preserve still the rhombic outlines of the siderite crystals. More usually, however, the alteration has been only partial, and in such cases the secondary oxides are either in irregular bunches or (and this is very much more commonly the case) are arranged in very irregular concentric spherical rings, of which there are generally several in one area, though m other cases a single ring is seen or a single ring with the beginnings of one or more others. These rings of iron oxide, which are so plainly the result of a secondary oxi- dation of the carbonate, occur quite without any reference to the individuals of the latter mineral; that is to say, a single ring or a set of rings traverses an area made up of a number of carbonate individuals. In some of the most comi)letely 230 THE PBNOKEE lEON-BEAEING SEEIBS. altered areas of carbonate there is contained more or less of the finely divided silica of the matrix, and in Sl^ch cases we appear to have an intermediate phase between the unaltered carbonate and those cases above described as characterizing other sections where simple rings of iron oxide are within a siliceous background. In other words, the iron carbonate remaining after formation of rings of oxide has been more or less removed and silica deposited in its place. As has been noted above, these concen- tric areas of iron oxide and iron oxide and silica are found in every stage of forma- tion until the iron carbonate disappears. (PI. xxvii, Fig. 4.) 34. Ferruginous and sideritic chert, from a higli liorizon, interstratified with 33. Specimen 7622 (shde 2072); from 160 N., 100 W., Sec. 21, T. 47 K, R. 47 W., Michigan. The rock is from another layer of the ledge from AVliich 33 comes. It has a most striking and jjeculiar character, being grayisli, mottled with black, and suggests even to the naked eye a fragmental texture, blackish angular fragments being imbedded in a grayish cherty mass. - In thin section, a minutely crystalline to amorphous silica forms a grouudmass which occupies a relatively small i^ortion of the whole area. Contained in tliis ground- mass are small crystals and aggregates of crystals of siderite, oval or sjilierical concretions of mingled flinty silica and Iron oxides, particles of a blackish, probably carbonaceous, material, with or without accompanying siderite, and also irregularly outlined and even sharply angular areas or fragments composed of a mixture of ilinty silica, iron oxide, and carbonaceous material with or without siderite. The iron car- bonate, except that it is more often fresh, appears as in 33. Where altered, its alter- ations are the same' as in those sections, the peculiar beautiftil concretionary forms there described occurring here also and are evidently of the same origin. The more irregular outlines, and especially the angular fragment-like areas that appear in this groundmass, are entirely similar in composition and structure ^o the jnaterial which has been described as composing the blacker bands in 33. These are at times plainly fragments, as may be seen from their angular outlines, sharp definition from the matrix of the rock and from the fact that the lamination lines abruptlj- terminate ■ at the extremities of the areas. The lines of lamination in these fragments, when they are perceptible, are never parallel for any two fragments. It seems evident that the history of the rock exposed where Nos. 9007, 9008, 9009 and 7622 were obtained has been about as follows: It was first a stratiform car- bonate of iron, including apparently more or less carbonaceous matter, as such strati- form carbonates so generally do, and perhaps more or less of silica. By a pr(#ess of subsequent silicification, accompanied by oxidation, the structures now apparent were produced. At times the substitution of the silica and the solution and oxidation of the carbonate went on so as not seriously to break up the continuity of the original layers; but in other cases the rock had become shattered and the silica entered into the minute cracks and interstices of the rock, and this shattering was THE IKON-lJEAUINd MEMBEH. 231 often produciul after a certain amount of alteration liad been elfectert, inasmuch as tlu^ fragments thoniselvos have evidently been in part altered before beingf torn ajiart. The prDcess continniiig, tiiese disrupted pieces, so far as they were still pretty pure carbonate, h.id developed in them by oxidation the concretionary structure, and liiially were often replaced more or less completely by siliceous material. luto many of the fragmental areas ramifyiuf;' veinlets of the siliceous groundmass extend, so that the various appearances iJicsented by the section seem all expli(!able by a contin- uing process of solution, oxidation, and silicification, accompanied by a certain amount of dynamic movement. While the above process explains a part of the fragmental character of the rock, it seems probable that it has been to some extent actually shattered by erosion. The layers here contain some fragmental quartz; they are at a high horizon, which represents probably the beginning of the change from nonclastic to clastic sedimentation. The nonclastic sediments immediately after deposition were perhai^s broken to a greater or less extent, forming detritus, which was mingled with the same kind of sediments which continued to form at favorable times. (PI. xxii, Figs. 3 and 4; PI. xxvn, Figs. 2 and 3.) From the section on the Mount Hope property. 35. Henatitic flint, from a middle horizon. Specimen 7610 (slide 2306); from 1150 N., 0 W., Sec. 23, T. 47 N., R. 47 W., Michigan. The thin section shows seams composed entirely of a very minutely crystalline to qtiite amorphous silica, interbanded with others in which the red and brown iron oxides are mingled with more or less of the silica. There is no brecciation apparent in the section. Here and there are minute iiatches of iron carbonate from whose oxidation it may be supposed the iron oxides came. From the section on the Puritan property. 36. Ferruginous cherts or flints, from lower and middle upper horizons. Speci- mens 7604 (slide 2304); from 70 N., 1000 W.; 7605 (slides 2011, 2069); from 150 N., 1050 W. ; 7006 (slide 2305) ; from 276 N., 1175 W. All from Sec. 18, T, 47 K., R. 46 W., Michigan. The rocks are grayish to whitish chferts, banded by reddish seams. The sections of these rocks are made up of minutely crystalline chalcedonlc and amorphous silica, mingled with bauds and irregular patches of brown and red iron oxides. Occasional remnants of iron carbonate may be detected, and at times the background presents vaguely the concretionary appearance described in foregoing sections. 37. Hematitic flint or chert, from a low horizon. Specimen 7601 (slide 2302); from 275 N., 0 W., Sec. 18, T. 47 N., E. 46 W., Michigan. The prevailing siliceous grotindmass is in large proportion of exceedingly finely crystalline spotty quartz mingled with amorphous material. In irregular areas and 232 THE PENOKEE IRON-BEAEING SERIES. bands in this groundmass are opaque aggregations of red iron oxide. They are in part plainly derived directly from an iron carbonate, since the rhombic portions of the crystalline sections are plainly visible, particularly around the borders of the opaque aggregations. There is also abundantly in the' groundmass vaguely outlined areas representing probably the silicilied areas of the carbonate. These are analo- gous to those occurring in above described sections, but are of rather small size. 38. Hematitic ilint or chert, from a low horizon. Specimen 7600 (slide 2009); from 300 N., 1500 W., Sec. 17, T. 47 K, E. 46 W., Michigan. In thin section the chert background is completely though minutely crystal- line. It otherwise closely resembles the last section described. The hematite aggre- gates, however, show somewhat larger remnants of unaltered iron carbonate. 39. Hematitic flint or chert, from a middl6 horizon. Specimen 7596 (slide 2301); from 453 N., 1300 W., Sec. 17, T. 47 N., R. 46 W-., Michigan. This section differs from the two last described only in the very much larger proportion of iron oxide contained and that the silica is intermediate in crystalliza- tion between the silica of them. The iron oxides appear in the same sort of aggre- gates in which are often to be seen the rhombic crystal outlines of the original carbon- ate. The texture of the chert in this case also is somewhat open, numerous cavities being contained by the section. From^ the Colby mine section. 40. Siliceous iron carbonate, from a high horizon. Specimen 12508 (slide 5522); from 1125 N., 175 W., Sec. 16, T. 47 N., E. 46 W., Michigan. A very fine grained rock, with an irregular division into light and dark gray laminffi. Scattered along these laminae are minute shining facets of a carbonate, which show however, more plentifully in much larger individuals irregularly blotched all over the specimen without any reference to the arrangement of the bands. The appearance of this carbonate alone is sufficient to identify it as siderite, an identifi- cation which is confirmed by the deep brown weathering that the specimen shows. This section differs from many of those previously described in the very large amount of fresh iron carbonate which it contains. Tliis mineral occurs not only in single intUviduals, but in compact aggregations of individuals, which sometimes occuijy quite extensive areas in the section. Ii'regularly interwoven with the carbon- ate areas are areas of silica which are largely amorphous. In this siliceous back- ground are seen, however, in i)laces the peculiar concretionary areas described as characterizing some of the above sections. These areas very plainly originate from a change of the carbonate aggregates. Dotted throiigh the section are particles of quartz of varying size. These are partly in the background and partly within the masses of carbonate, the larger ones often appearing as if of fraguiental origin. (PI. XXVII, Fig. 1.) TllK lltON-l'.KAl{IN(l MKMKKlt. 233 From the seefioii on I be Tilden mine property. 41. ITciiiiititic Hint or choit, from a middle horizon. Si)ocinuMi 7571 (slide 19!>;j); from 15(M» N., 121;! W., Sec. 15, T. 47 N., It. Ki W., Mi.hjoai,. Tiie thill section is ii clicrt containinj-- much liiiely divided iiematite. 12. lEematitic flint or chert, from a middle horizon. Specimen 7505 (slide 2064); from 1450 N., 1185 W., Sec. 15, T. 47 N., R. 46 W., Michigan. The section is a hematitic Hint analogous to that of 41. The hematite in the larger part of the section is, however, more uniformly contained and in more minute particles. It is arranged so as to show faint but perfect concretions, lu parts of the section are concentrated large areas of hematite. From the section on the Palms property, 43. Cherty and altered iron caibonate, from a middle horizon. Specimen 7573 (slide 1996) ; from 1600 IST., 1955 W., Sec. 14, T. 47 N., R. 40 W., Michigan. About half the area of the thin section is a light gray and the other half a red- dish brown. The light colored portion is composed of uniformly mingled minutely crystalline quartz and gray siderite. The siderite occurs within the siliceous ground- mass both in detached individuals and in complex areas, the single individuals showing usually very distinctly the rhombic outlines. The brown portion of the slide is like the other portion so far as the siliceous groundmass is concerned, but here the brown and red oxides of iron take the place of the siderite of the lighter colored part of the slide. Since these iron oxides show often the same rhombic outlines as seen in the siderite; since they occupy precisely the same relation to the chert; since there is a gradation and not a sharp definition between the two iiortions of the section ; and, finally, since the single individuals of siderite may be seen partly changed to iron oxide, it is evident that the latter mineral is a secondary product of the former. From the section on BlacTc river. 44. Jasper and ferruginous chert, from very near the base of the Iron-bearing member. Specimens 9508 (slide 2983), from 1665 N., 1915 W.; 9509 (slide 3137), from 1600 N., 1900 W.; 9510 (slide 3087), from 1580 N,, 1900 W. All in Sec. 13, T. 47 N., R. 46 W., Michigan. Specimen 9508 is a bright red jasper, banded with very thin seams of brilliantly metallic lustered hematite. Specimen 9509 is a dark brownish gi'ay chert. Specimen 9510 is similar to 9508 but is mottled with irregular blotches of red iron oxide. The thin section 2983 consists of finely crystalline, mingled with some amor- phous silica, in which are included very numerous minute particles of bright red hematite, which are aggregated more thickly along certain bands. Many of the hematite particles have rhombic crystal outlines, and appear therefore to have orig- inated from the oxidation of iron carbonate. 234 THE PENOKBE lEON-BEARING SERIES. In slides 3137 and 3087 tlie quartz is more coarsely crystalline, and the iron oxide is mainly the brown hydrated variety. The iron oxides in them are arranged in concretionary and brecciated forms, and the silica is affected, although less plainly, by the same arrangement. Some of the larger concretions are complex, one of them perhaps including several smaller concretions. The areas in 3137 are very irregular, and suggest a mechanical brecciation or a brecciation caused by the processes of alteration. 45. Ferruginous chert, from a low horizon. Specimens 7554 (slide 2298), 7555 (slide 1983); both from 1680 N., 1900 W., Sec. 13, T. 47 N., E. 46 W., Michigan. Slide 2298 is an exceedingly finely and regularly laminated ferruginoiis chert, quite analogous in general character to the black bands of 33; while 1983 is a brec- ciated chert closely analogous to 34, which in turn is directly interstratifled with 33. In other words, we have here a representation of the phenomena presented by the exposure oh the Montreal river, from which 33 and 34 came, except that here all of the iron carbonate has been removed. 46. Siliceous siderite, from a lower middle horizon. , Specimen 9504 (slide 3186), from 1725 N., 1900 W.,^ec. 13, T. 47 N., E. 46 W., Michigan. A very fine grained, dark gray, evenly and finely laminated earthy rock. Sp. gr.,2-07. The thin section shows an intimate mixtiire of minutely crystalline, with perhaps some amorphous silica, with minute grayish rhombohedra of siderite only slightly altered here and there by oxidation. These rhombohedra are generally single, but are at times aggregated closely into bunches. 47. Ferruginous chert or jasper, from an upjjer middle horizon. Specimen 9500 (slide 3182), from 1840 N., 1975 W., Sec. 13, T. 47 N., R. 40 W., Michigan. The rock is an aphanitic chert or jasper, the colors varying in irregular blotches through dark gray to bright red. The thin section is a brecciated concretionary chert like others above described; rounded areas of a finely crystalline and amorphous silica, mingled with more or less brown and red iron oxides, being embedded in a silica which is usually more coarsely crystalline. Occasionally, instead of the rounded areas, there are long tabular pieces showing the lamination of the original chert. In both kinds of areas there are numerous places where the rhombohedral outlines of the original carbonate reproduced in the iron oxide may be seen. Besides this are to be seen within the chert frag- ments rhombic areas, often of large size, comitosed of silica similar to the rest, but outlined distinctly by brown iron oxide. These again are taken to be substitutions for original carbonate crystals. 48. Ferruginous chert, from an upper middle horizon. Si^ecimens 9501 (slide 3183), 9502 (slide 3184); both from 1840 N., 1975 W., Sec. 13, T. 47 N., E. 46 W., Michigan. TIIK IKON-HliAKINC MEMHKII, 235 The rocks arc diirk red cluirts or jaspi-rs, wliii'li lor flic must part luck, however, tilt' apliaiiitic lliiity appcaiance of tine jasjier. Tliey arc Iciiiilcd (|iiifc lesulaily witli lifilitci' colored seams. Tlic Ihiii section ;>ls;! shows a backiii-oiiiid of exceedingly liiiely crystalline, min- gled probably with iiniorplious silica, t liiMtnj^li whicli arc scattered a few small appar- ently fragnn'iital iiurticles of the same material and nnmerons minute stains of brown iiydrated iron oxide which occasionally present riiombic ontlines. The thin section 3184 difl'ers from MKi simply in containing much larger and even ])redominiiting quantities of iron oxide, mucli of which is hematite. Rhond)ic outlines to the parti- cles of iron oxide are frequently to be seen. The iron oxiih^ is aggregated especially into certain bands, so as to give the section a laminated appearance. •49. A ferruginous chert, from an upper middle horizon. Specimen 950.'1 (slide 3185), from 1775 N., 1925 W., Sec. 13, T. 47 iST., R. 40 W., Michigan. A light gray, evenly laminated earthy rock, banded with seams of- dark red hematite. The thin section is a ferruginoirs chert very closely resembling 3183 in 48. 50. Iron carbonate or siderite rock, from an upper horizon. Specimens 9481 (slide 3177), 9482 (slide 3178), 9483 (slide 3348). ' All from 250 N., 1000 W., Sec. 12, T. 47 N., R. 40 W., Michigan. An aphanitic, dark gray, earthy looking, stratiform rock, banded in some por- tions with nearly black bands, which are at times Ijroken and the detached portions imbedded in a lighter colored material. Sp. gr., from 3-22 to 3-40. The thin sections are composed mainly of a felted mass of iron carbonate, which in some of the darker bauds is mingled with a dark colored material in line particles, presumably of a carbonaceous nature. Certain bands, as seen in section 3177, contain a finely divided silica which has a distorted lamination, the carbonate being separated into more or less detached areas. At the same time these bands show large sized iiatchjes of a greenish chlorite. The carbonate is also altered to a consider- able extent to iron oxide, tbe latter being largely arranged in rings. This is another good illustration of the formation of concretions. 51. Ferruginoirs chert, from an ui^per horizon. Specimen 9485 (slide 3179) ; 295 N., 980 W., Sec. 12, T. 47 N., E. 40 W., Michigan. A very dark colored, nearly black aphanitic rock, banded irregularly with ill defined dark red seams. The thin section is evidently from one of the dark reddish brown seams. It presents a confused admixture of chlorite, brown iron oxide, and magnetite, the latter mineral being in distinctlj^ outlined crystals, all in a minutely crystalline and amor- phous siliceous background. Many of the brown particles also present rhombic out- lines, and are taken to have arisen from an oxidation of the carbonate. Siderite is not, however, recognizable in the section,'biit may be present plentifully in the dark colored earthy looking bauds mentioned above. 236 THE PENOKEE IRON-BEAEING^ SEEIES. 52. Black chert, from the siiinniit of the Iron-bearing- member. Specimen 7534 (slide 19G8), from 500 N., 1075 W., 7535 (slide 1969), from 535 N., 1050 W., Sec. 12, T. 47 ]Sr., E. 40 W., Michigan. The thin sections are concretionary, brecciated, and ferruginous cherts, analo- gous to a number above described. Slide 19G8 shows a predominating groundmass of exceedingly finely crystalline and amorphous silica, in which are strewn small parti- cles of quartz which are certainly fragmental. The fragmental grains of quartz stand out in the background in a wonderfully distinct way. They vary from well rounded to angular; some of them are distinctly enlarged. This section illustrates well the great difterence in appearance between fragmental quartz and the nonfrag mental quartz of the iron formation. Here, as in previously described rocks, as an upper horizon, is a mingling of chemical and mechanical sedimentation; the beginning of the transition to the upper fragmental member of the series. There are also present magnetite, hema- tite, and irt>n carbonate, all of which are arranged in a semiconcretiouary fashion, and in such a way as to suggest the derivation of the whole from an original carbonate. Slide 1969 differs from 1968 only in containing large areas of what seems to be a secondary calcite. From the section on the Miner & Wells property. 53. Oherty iron carbonate, from near the base of the Iron-bearing member. Specimens 12885 (slide 5507), 12886 (slide 5508) ; Sec. 13, T. 47 N"., E. 46 W., Michigan. An aphanitic ro(;k, showing a very thin and for the most part regular lamination, though in certain layers these lamiuie are somewhat bent. The laminiB range in thickness from that of a sheet of jiaper to as much as a quarter or half an inch. They range in color from black thi-ough various shades of brownish gray and greenish gray to a very light gray. All save the black bands show a very earthy, compact look, and the whole appears at first sight as that of some banded or earthy limestone; but the high specific gravity of the rock proves at once the presence of much iron. The surfaces of some of the black laminae glisten brightly as though con- taining a carbonaceous or graphitic material. Composition of 12885: silica, 4G'01; titanic oxide, 0-12 ; alumina, 0-83 ; iron .sesquioxide, 1-35 ; iron protoxide, 26-CO ; man- ganous oxide, 2-09; calcium oxide, 0-63; magnesium oxide, 2-86; carbon dioxide, 17-72; phosphoric acid, 0-07; iron sulphide, 0-11; waterat red heat, 1-71 = 99-50. In the thin sections the light colored bands are seen to consist of an almost solid aggregate of minute rhombohedra of iron carbonate, whose outlines are particularly well observed on the borders of the bands where separated slightly from the rest of the mass by the silica, which, while constituting the main constituent of the darker colored bands, penetrates the siderite in irregular tongues and seams. This silica is exceedingly finely crystalline and perhaps in part amorphous. Mingled with it in the darker colored bands are films of chlorite, detached rhombohedra of iron carbonate, and dark colored seams lying parallel to the geueral lamination of the rock, but non- TilK IK()N-liEAliIN(i MEMIJEU. 237 continuous. Those consist mainly of chlorite, l.u( miiy iirobably also include some, carbonacemis material. Similar films ajjpear also in the Ii};lit colored bands. The, section is cut by small veins running in various directions, which are lilled as often with siderite as (luartz, and sometimes a single vein contains both minerals; also occasionally chlorite is contained. ol. Cherty iron carbonates, from middle horizons. Specimens 0473 (slide .'5135), 9473 (slide 3081), 0-174 (slide 30S2); 0475 (slide .'iOSo), 047arts of 63. The amount of simple fragmental qufirtz iu slide 5400 is less than that iu 5408 and iu 63. The iron oxide iu the fragments aiul matrix is mostly hi'iuatite, although mag- netite is i)resent. The grains of simple tpiartz have been well rouiuled, and are often widely enlarged. They quite often contain within their cores crystals of hematite and magnetite. In this respect they are closely like the ferruginous ([uartzitc 12680, described page 171. In fact, the chief difference between the two rocks is that in 12680 the fragmental material is more abundant, jviON XIX 16 242 THE PENOKBE lEON-BE A.RIIirG SERIES. 66. Jaspery and cherty siderite, fiom nu upi:)er middle horizon. Specimens 12683 (slide 5405), 12684 (slide 5406), 12685 (slide 5407). All from 1260 N., 1350 W., Sec. 17, T. 47 ]Sr., E. 44 W., Mieliigaii. This is a laminated rock, composed of an ai:)hanitic to dark gray earthy looking, material, which is miuntely handed in itself with lighter and darker shades, and alternates with bands of very bright red jasper, which range from the thickness of a sheet of pai)er to an inch in width. Sp. gr. of 12685, 2.9'< . The jaspery bands in thin section 5405 are seen to be mainly composed of a uniform intermixture of minutely crystalline quartz and bright red liematite, with which are mingled soiiie nuignetite particles. Occasionally the hematite shows rhombic outlines, but these are not generally iierceptible, i^erhaps because of the close aggregation of the particles. The dark colored seams prove in the thin section to be in part a mixture of crystals of magnetite and minutely crystalline quartz. Such bands as this are found to be directly interlaminated with the biight red jasper. In other cases the darker colored bands owe their dark coloi' to a luixtnre of minute particles, which aio iu part probably of a carbonaceous nature. Such bands as this are found interlaminated more directly with those portions in which carbonate of iron is abun- dant (5407). Tlic lighter colored portions of 5407 are in part a mixture of iron carbonate, brown and red iron oxides, magnetite particles, actinolite Jieedles, and minutely crystalline silica, the latter mineral being tLic least plentiful. Still other bands are made uj) mainly of a- silica which r-anges from very minutely crystaJliue to nearly o" quite amorphous. In this flint or chert are found remains of bands of iron carbonate, single rhombohedra of the same, and scattering crystals of magnetite. 07. Actinolitic magnetite-slate, from a high horizon. Specimens 12703 (slide 5420), from 1180 IST., 655 W.; 12704 (slide 5421); from 1200 N., 655 W., Sec. 17, T. 47 N., R. 44 W., Michigan. A very heavy aphanitic slaty rock, made up of )uinute alternating laminaj of darlc gray and black shades. Portions of the si^ecimens sh(jw a distinct metallic luster, particularly on the black lamiu;e, and (xuite large ineces are lifted by the mag- net. Interlaminated with this material are bands of a much lighter color. In thin section, the lighter colored bands (5420), last referred to, are seen to have a minutely crystalline siliceous groundmass, in which are included numerous minute actinolite needles and particles of iron oxide, including the magnetic oxide. The actinolite is on the whole quite as plentiful as the silica, and in portions of the section is aggregated into felted masses. The earthy black i)ortions of the rock present a section which differs from that just described mainly iu the relatively great abundance of iron oxides, particularly magnetite; this last mineral being more especially aggre- gated into irregular lanuiue. Actinolite is very abundant, constituting in considerable portions of the section an iron-stained felted mass of minute needles. THE IKON-BEARING MEMBER. 243 68. Magnetitif iictiiHililc schist, tVimi a vciy lii^li iK.ii/.on iiuiiicdiatcly be.iuMtli the Kewi-eiia Willi fiiccnstniic. Siu'cinHMi 10402 (slide r,:;-2l); t'nmi l.>10 N., KiOO W., Sec. 17, T. -17 N., K. 41 W., .Micliif;an. .V ft'Ited mass of actinolilc needles eoiii|i(ises tlie backf;roiuid of the section. This mass is eoiiiiiioiily stiiiiied red and hrowii liy iioii oxides, and joiitaiiis besides, opaque aggregations ol' Mia.uiiclite crystals which iiiak.' iiii as much as half the section. From the expomircs tind trst-pifs in IScck. I',, id and :.>1, T. 17 N., U. J J W., Michigan. 60. Actiiiolitic magnetite-schist, from ;i low horizon. Specimens 1278(i (slide 5474), 12787 (slide 5475); from 1625 N., 650 W., Sec. 21, T. -17 N., R-. 44 W., Michigan. An aphaiiitic, laminated, very dark gray rock, analogous to those described under OS. Sp. gr. of 12787, o.53. The thin sections are typical actinolitic magnetite schists, such as have been described already as occurring at Penokee gap, etc. The gronndmass is a finely crys- talline (juartz, throughout which are contained minntt' blades of actinolite. Tlie mag- netite occurs as usual in bunchy aggregations of sharply outlined crystals, aud also in single crystals scattered throughout the gronndmass. In slide 5475 there is a gen- eral tendency towards a concretionary arrangement, which in portions of the section is very strongly developed; aud after having seen the various concretionary develop- meuts of the foregoing rocks one has no hesitation in saying that these are of the same origin with all the rest. 70. Actinolitic magnetite-schist, from a low horizon. Specimen 12781 (slide 5470); from 70 N., 1790 W., Sec. 15, T. 47 N., R. 44 W., Michigan. A very heavy, slaty, dark gray rock, analogous to that last described, except that certain vaguely defined bands have a pale reddish or jaspery appearance. The content of magnetite is evidently great, as large sized ijieces are easily lifted by the magnet. The section is again one of the ty^jical actinolitic magnetite-schists. The back- ground, as usual, is a minutely crystalline quartz. In this are contaiiied fan-like ag- gregates of unusually large actinolite blades, and crystals of magnetite, partly aggre- gated in certain bauds, in which are contained a large proportion of red and brown iron oxides. 71. Ferruginous cherts, ft'om a middle horizon. Specimens 12671 (slide .5395), 12672 (slide 5396), 12675 (slide 5398) ; all from 450 N., 675 to 729 W., Sec. 16, T. 47 N., K. 44 W., Michigan. The rocks are brown and red stained, highly ferruginous cherty schists, made up of alternating laminiB of black, red, and brown colors. The thin sections are composed essentially of a minutely crystalline to amor- phous silica with the red and brown and magnetic oxides of iron. These oxides are aggregated more especially into certain lamime, other laminie being almost jmre 244 THE PENOKEE lEON-BEAEING SEEIES. chert. On the eilges of the iron oxide huniiuB the individual particles of hematite and brown oxide are seen veiy frequently to have sharp rhombic outlines. The magnetite is quite subordinate in quantity to the otlier oxides, being, however, more plentiful in some bands than in others. It occurs in sharply outlined crystals. The hematite particles that occur in these bauds, which are mainly made up of flinty silica, are often arranged in a radiate manner. 72. Ferrugimnis chert-schists, from a high horizon. Specimens 12691 (slide 5412), 126!)2 (slide r.il3), 12G93 (slide 5414); all from 1380 N., 1960 W., Sec. 15, T. 47 N., E. 44 W., Michigan. These specimens represent alternating lamina* of gray and red stained chert and hematite ii^on ore. Tlie iron oxide seams show a more distinct subordinate kmina- tion than is perceptible in the cherty portions. Sp. gr. of irony layers (12693), 3.25. The cherty layers in the thin section are seen to be made up almost entirely of a minutely crystalline to nearly amorphous silica. Tlierc is also throughout this silica a general ^.endency to a concretionary structure, which is brought out by a vague concentric arrangement of the more or less completely crystalline particles. The whole appearaiuje of this chert is very strikingly like the cherts which have been above described and ttgured as characteristic of the limestone member of thePenokee series. In these cherts occur irregular bunches of hematite and brown iron oxide, the particles of which often show most distinctly rhombic outlines of the carbonate crys- tals, from whose alterations they are taken to have originated. Some of these rhombic crystals are of unusually large size; and in one or two places appear to still retain portions of the original carbonate. The more highly ferruginous laminaj differ from the cherty phases mainly in ihe large proportion of the oxides of iron, which now preponcerate greatly over the siliceous matrix. The particles of iron oxide iire here * arranged in regular lines, which, without much doubt, mark the original lamination of the rock. The rhombic outlines to the hematite particles arc frequent, and some of the silica shows the radiating structure characteristic of chalcedony. 73. Actinolitic ferruginous schist, from a high horizon and immediately in contac*; with overlying Keweenawau greenstone. Specimens 9381 (slide 3269), 9382 (slide 3039). From 670 K., 1020 W., Sec. 16, T. 47 N., E. 44 W., Michigan. The specimens present an alteruatiou of brown cherty, red jaspery, dark green and black aphauitic lamime, the whole rock having a distinct slaty or parallel struc- ture. Sp. gr. of the black laminic (9382), 3.90. The thin sections of these rocks are particularly interesting and instructive, in that they generally show in a single section all of the characteristic minerals of this class of rocks; i. e., brown and red oxides of iron, magnetite, actinolite, and cherty or flinty silica. These diflcrent minerals occur more or less intermingled, but the vary- ing appearance of the lamime as seen macroscopically is due to the preponderance of diflerent minerals in the diflerent bands, The black bands are particularly rich Till': ii;(>N-i!i:,\iMN(; miimiuh;. 245 ill iiiiijiiictitc. Tlic led luid lnowii InniiiKc ;iit' iic;iily oiiatiuc a^srciint ions (if particles of lieiiiutitc aiul liydro-scstiuioxide. Tlic greciiish bands are iiarticularly ricli in actinolite. Tlio siliceous snmiidmass is relatively not al)undant, but runs tliroujjliont the section. The bandiuj;- of this rock is talctMi to be d('i)cndent ujion an orifiinal sedimentary lamiiiation, but the \va.\ in wiiicii tiio bands are seen now to hv swollen, niinuiely contorted, or abruptly broken otV is a strong indication of the sec(»nd- ary orif>in of a i)ortion of the minerals whicli arc now jnesent. Besides followiuy the orifiinal lamination, the various groupings of tlic minerals lia\'e often traversed this laniinatiou. One traversing seam of actinolite and silica is particularly note- worthy. This is a structure whicli can hardly be described, but is of importance in its bearing upon the origin of tliese singular ferruginons rocks. SECTION II.— ORIGIN OF THE ROCKS OF THE IRON-BEARING MEMBER.' Knowiug" the exact facts as to the nature and method of occurrence of the iron-bearino- rocks, we are now prepared to present some consistent account of their origin. In the detailed tabulations and in the general account of their macroscopic and microscopic cliai-acters, it has been necessary to anticipate, to some extent the origin of certain phases. To the degree that their genesis has tluis been anticipated it is not in the nature of theory, but fact, because the stages of their development to this extent have actually been observed. Before attempting to give a histor)' of the rocks, it will perhaps be well to recapitulate the more important of the observed facts. (1) The Iron-bearing- member throughout most of its area gives abso- lutely no evidence of a fragmental character. This characteristic is one of the greatest importance. The quartz rocks of the iron-bearing belt have been confused with the underlying qtiartzites. By most observers the}' have been taken to be fragmental, and have been supposed to have reached their present condition b"\' one of the various mysterious processes of metamor- phism. It has, however, been seen that the fragmental character of the undei'lying quartzites and the overlying slates is manifest when their thin sec- tions are examined with a microscope. Now, this Iron-bearing member lies between these two fragmental belts, and yet nowhere in its typical rocks is 'Section I was nearly completed when Prof. Irving's sudden death occurred. From the begin- ning of this section the junior author is alone responsible for the form, although much of the substance of Sections II and III is Prof. Irving's work. 246 THE PENOKEE IRON-BEAIllN(J SERIES. there any indication of a fragmental character. Occasionally, it is true, in narrow transition bands between the iron-bearing and the fragmental belts, there is mingled with the nonfragmental material a small quantity of frag- mental quartz and feldspar. Also at the eastern extremity of the district clastic and nonclastic sedimentation have alternated to some extent. How- ever, this very appearance of fragmental material l)ut more strongly empha- sizes the fact of the nonfragmental character of the belt as a whole, by bi-ine'ing' into the same thin.sections clastic and nonclastic material. The clastic particles are recognized at a glance as entirely different from tlie mass of the finely crystalline or partly amorphous completely interlock- ing material which composes the i-ocks of the Iron-bearing member. (PI. XXXV, Fig. 1.) (2) Tlie rocks of the Ir(in-bearing member are laminated. Through laro-e parts of its area the lamination is as perfect and minute as is possible in any sedimentary stratified rock. These regularly laminated portions are found at all horizons, and nowhere is a fair degree of i-egularity of stratification absent. The most perfectly laminated parts of the belt are of the first type of rock — the cherty iron carbonates. As the lamination becomes less regular, rocks of the second and third types — the ferruginous cherts and the actinolitic slates — appear. (3) There have been observed at many places actual stratigraphical transitions of the regularly bedded carbonates into the remaining rocks of the belt. This gradation sometimes occurs in passing from east to west, as, for instance, the cherty carbonates and the ferruginous cherts grade into the actinolitic slates, or the gradation may be a transverse one ; that is, the cherty iron carbonates at a higher horizon grade into the ferruginous cherts at lower horizons. This stratigraphical gradation of the three types of rock into each other is alone suflicient to make very probable for all of them a common origin. The gradation is repeated in thin sections, nearly all stages of the various transitions being clearl}^ worked out. The original rock — The foregoing facts all point unmistakably in the same direction; that is, to the conclusion that these rocks at one time Avere cherty iron carbonates. There may be differences of opinion as to whether at some earlier stage they did not have another form ; there may be differ- TIIK IliON-liliAlUNd MM.MIlKi;. 247 encos of opinion as to the luninicr in wliicli, iVoni tlic clicrtx' cin-honatc s, some of the nuiltitndinous plia.scs of rock now fonnd haxc liccn foi'ni('(l ; but the conclusion can not he escaped that tliese rocks, wliicli still make u]) so kirye a proportion of the l)eU, were the oi'i^inal rocks ot the nieinl)er. It is extremely iniprol)ahle that, in a narrow heU of rej^nlarly stratilied rocks about 800 feet thick, iuterstratitied witli otlicr Ixdts of sedimentary rocks, there should be deposited in patches here and tliei'e, large and small, at nearly all horizons, cherty iron carbonate, and a short distance east or west ferruginous cherts and actinolitic slates. The c[uestion arises whether the cherty iron carbonates, the least altered rocks now observable, have been derived from an earlier and different rock. Thej^ are apparently unaltered, and that they Avere origi- nally deposited in the condition now found is probable enough from analo- gies presented by later geological thnes. The cherty ironstones from the Carboniferous, which occur so plentifull)^ in our own country in Ohio and Pennsylvania and so extensively in other countries, are like tliese thinly bedded carbonates, except that they are generally more argilla,ceous. Certain Ohio ores are so remarkably like some of the iron carbonates of the Penokee-Gogebic series that they can hardly be distinguished in hand specimen or thin section from one another. This essential likeness is shown by PI. XXVII, Figs. 1 and 2, and PL xxix, Fig. 4. The last is an Ohio cherty carbonate; the first two are Penokee carbonates. The only difference betweeir the two is the unimportant one as to the nature of the inclusions. The clayey character of the Carboniferous carbonates is no more than an accident, and to a certain extent is also characteristic of the older carbon- ates of the Northwest. It simply means that the nonfragmental sedimen- tation was accompanied by mechanical sedimentation to a greater extent in the Carboniferous than in the Iron-bearing series of the Northwest. In Mississippi, in the Claiborne formation of the Tertiary,' there are extensive beds of cherty iron carbonate, which, according to analyses pub- lished, are almost identical in composition with the cherty iron carbonates of the Penokee series. A third analogy presented by the formations of more ' A New Discovery of Carbonate Iron Ore at Enterprise, Mississippi, by Alfred F. Brainerd. Trans. Am. Inst. Min. Eng., vol. xvi, 1888, pp. 146-149. 248 THE PENOKEE lEON-BEAEING SERIES. recent times is in the ehert}^ limestones, which are so widely found at all horizons. It may be a question as to these limestones whether the chert was originally in nodules and layers or was scattered through the limestone as disseminated particles of silica, but from later investigations it appea,rs certain that in some cases the chert was originally deposited in the position in which it is now found. There is in these limestones a close association between the cherts and calcium and magnesium carbonates. In the cherty iron carbon- ates of the Penokee-Gogebic series calcium and magnesium are present as in the limestones, and we need onl}- to replace a portion of these elements by iron to have rocks -which are the exact analogue of the cherty limestones of later times. As has been seen also, such chertj- limestones occur in deposits of considerable thickness at tlie base of the Penokee-Gogebic series itself From analogy it is then extremely probable that the chei-t and iron car- bonates were simultaneously deposited, althotigh it may be a possibility (as has been maintained with reference to some of the cherty carbonates of later time) that the chert entered very early in the history of the rock as a pseudomorph, replacing carbonates. At any rate, it is certain that a large portion of the silica now present in these rocks was there vei-y early in their history. In other districts in the lake Superior country, notably at Gunflint lake, in the Animikie series, a similar cherty carbonate is found in extensive beds. That the chert was here present at a very early day can not be doubted. The chert}' bands, where there is no folding, are evenly interlaminated with the carbonates, but in folded areas the brittle cherts have been frac- tured in every direction, so that the rock, instead of being a regularly laminated one, is a breccia, which contains angular fragments of chert of greatly varyiirg sizes. Such a brecciated' rock frequently runs into the regularly laminated kinds in the space of a few inches. It is then certain that the chert of these beds was present befor.e the folding of the rocks. Further, there is no evidence that most of this chert was not deposited simultaneously or alternately with the iron carbonate. The i-elations of a portion of the chert and the iron carbonate are, however, such as to show that either the chert has entered by subsequent solution, or that the silica originally deposited with the carbonate was subsequently, to a greater or TIIK ll{ON-l!KAltlN(i MKMlIKIf. 249 less oxtont, r('iiiTanf>-e(l. This is indicjited l)\ the l;icts tliat tlic silicii is f|iiit(' coiiipletcfly crystallized, and that tlic silica belts, instead of heino- laminated with the carbonate, at tin\es break aiToss them in the most irreg- uhir manner. This tissnring must have been subsequent to the deposition of the carbonate, and may imply the entrance ot' silica from an extraneous source, but also may mean no more than that silica originall}' present has been taken into solution and recrystallized. In either case there is no proof « that the g-reater part of the silica was not an original deposition. It is assumed that such a cherty carbonate is water-deposited, as a direct eruptive origin has never been maintained for a rock of this char- acter. Such an origin has, however, been asserted for some of the ferru- ginous cherts and jaspers which in other parts of the lake Superior country are found associated witli the carbonates.^ Taking it for granted, then, that this cherty carbonate is a water- deposited sediment, the questions arise, in what manner the iron carbonate and silica were originally dissolved and how they were precipitated. We ma}', without varying too far from the law of uniformity, believe that in very ancient times the atmosphere was more highly charged with carbon dioxide than at present. We may also believe that the rocks composing the crust of the earth were then at a somewhat higher temperature. An increase in heat of but a few degrees would be a powerful assistance in the process of solution of the iron, and this would be especially true if the atmosphere at this time was still richly charged with carbon dioxide. The atmospheric waters would absorb this acid and carry it into the rocks which bordered the ancient sea, would decompose them, and take in solution ferrous carbonate. Such Avaters escaping into the shallow ocean at hand would bear the material for these irony deposits. While it is believed that these conditions may possibly have been present, their assumption is not necessary to account for the solution of the iron;- for it is well known that thick beds of iron ore have formed in recent times by virtue of the solubility of iron as a car- bonate under ordinary conditions. It is also not impossible that its solu- 'Notes on the Geology of the Iron and Copper Districts of lake Superior, by M. E. Wadsworth. Bvdl. Mus. Comp. Zool. Harvard Coll., whole series, vol. vii, Geological series, vol. 1, No. 1, 1880, pp. 62-68. 250 THE PElSrOKEE lEON-BEARlNG SERIES. tion was greatly assisted by means of terrestrial life! That there was marine life at this time, as will be seen, we have strong evidence. Of terrestrial life we have no such proof, but it is highly probable that life existed on the land, and if so, tlie organic acids would be of assistance in decom- posing the rocks and taking iron carbonates into solution. The iron is a carbonate rather than a hydroxide, for the same reasons that the bedded * carbonates of Carboniferous times are so. It is usually taken for granted that in the Carboniferous deposits the presence of a large amount of organic matter explains the presence of the iron as a carbonate. Whether the iron was originally precipitated as a carbonate, or was decomposed and precipi- tated as a hydrated sesquioxide, just as limonite now forms from iron car- bonate in places where bog ore is depositing, is uncertain. If the latter is taken to be the case (and it is perhaps the more probable supposition), it is necessary to believe that the organic matter with which the limonite was associated later reduced the latter to the protoxide, and by its decomposition furnished the carbon dioxide to unite with this pi'otoxide and thus rej^roduce iron carbonate. Analyses of the carbonates of the Penokee series show conclusively that there still remains in these rocks quite a large percentage of organic matter. Also in the thinly bedded argillaceous slates above them the percentages of hydrocarbons are at times quite lai'ge. Some of the black slaty carbonates and black slates of the other iron-bearing series in the Northwest remarkably resemble the black carbonaceous slates of the Carboniferous. That carbon in the form of graphite could be pro- duced in other ways than by life may be conceded ; but it will hardly be urged that the finely disseminated carbon and hydrocarbons in these slates is other than of organic origin. . Text-books commonly explain chert contained in limestones as of organic origin More recentl)" it has been maintained by a number of writers that this chert is a chemical sediment, which has entered the carbon- ates as a pseudomorph shortly after its deposition. The evidence that an}" of the chert is not of organic origin is of a negative character; but, as has been said, it is not at all impossible that the crust of the earth had a higlier tem- perature at the time of the formation of these rocks than at present. If so, we need not necessarily go to life to account for this silica. What a power- THE IRONHEAllING RIEMBEK. ' 251 fill assistance an increase of teinpei-jiturc is in the solnlioiidf silica is slmwu by the geyserite deposits so well described by Hague.' While it may thus be possible that a pai't of the silica is a direct cliciiiical precipitate, it is cer- tain that life is sufficient alone to collect from sea waters silica in solution and form extensive deposits. So far as we know, this was first shown of the Trinuningham chalks, the chert of which seems without question to be the remains of life." Later it has been shown by Dr. G. J. Hinde that ex- tensive deposits of chert in Ireland, England, Wales and Spitzbergen are largely, and possiblj- wholly, accumulations of sponge spicules. The deposits of Ireland are described,' as follows: They consist of uodular masses of irregular form, inclosed in beds of bard, bluish limestones, and following the planes of bedding much in the .same way as the flints in the XTpper Chalk; but, unlike the flints, these nodular masses are not sharply delimited from the limestones in which they are interbedded, but there is a gradual passage from the chert to the limestone. More frequently, however, the cheit exists in definite beds from 1 to 5 inches (.025-.12m.) in thickness, which intervene at irregular intervals between beds of limestone. These beds sometimes occur also as well marked layers in the central ]3ortions of beds of limestone. Both the nodular aggregations and the horizon- tally bedded chert usually occur in the same series of rocks. The i)articular mode of deposition probably depends on the extent to which the sponge remains (ot which it will be shown the chert consists) are present in the resijective areas. It will be seen tliat this description conforms in a remarkable manner to the occuiTence of chert and carbonate in the Penokee series. At Spitz- bergen, Axels island, Yorkshire, and North Wales there are alternations of chert and limestone, the pure cherty layers of which are here, however, often quite a number of feet in thickness. The most remarkable occm-- rence is that of Spitzbergen and Axels island. The beds of cherty material ' Geological History of the Yellowstone National Park, by Aruokl Hague. Trans. Am. Inst. Min. Eng., vol. XVI, 1888, pp. 783-803. - On the Flint Nodules of the Trimmingham Chalks, by Prof. W. J. Sollas. Auntils Nat. Hist. 1880, pp. 384 395, 437-461. 'On the Organic Origin of the Chert in the Carboniferous Limestone Series of Ireland, and its similarity to that in the corresponding strata in North Wales and Yorkshire, by George Jennings Hinde. Geol. Mag., London, New Series (Decade III), vol. rv, pp. 435-446. On the Chert and Siliceous Schists of the Permo-Carboniferous Strata of Spitzbergen, and on the characters of the sponges therefrom, ■which have been described by Dr. E. von Dunikowski. Dr. Hinde. Ibid., vol. v, pp. 241, 251. 252 THE PBNOKEE lEON-BEAEIlsTI SERIES. here aggregate 870 feet in thickness. That the cherty layers of Wales and England are almost wholly, if not wholly, of organic origin seems to be conclusively shown. Also, it is certain that a part of the chert of Ireland, Spitzbergen, and Axels island is of organic origin. Dr. Hinde concludes that — It is true that the number of specimens of chert available for examination are very few, and they might be regarded as insuflScient of themselves to warrant the conclusion that this gTeat thickness of rock, which at one locality on Axels island reaches 870 feet, is due to the accumulation of the skeletal debris of siliceous sponges; but taking into consideration the fact that beds of similar cherty rock, which in Yorkshire have an estimated thickness of 90 feet and in North Wales of 350 feet, can be proved to be due to sponge remains, there is nothing extravagant in the supposition that this much greater thickness of rock has had a similar origin. These cherts in Great Britain are Carboniferous ; those of Spitzbergen and Axels island are Permian. It is of interest to note that these are the terranes in which the most extensive beds of iron carbonate in Paleozoic time are found. Further, the sponge remains pass into a "pure translucent chert." If, as Dr. Hinde maintains, all this silica is derived from sponge spicules, it must now in some cases be extensivel}- rearranged; for a large part of the chert appears from the descriptions at the present time to be cherty or chalcedonic silica in forms independent of organic remains. In the Penokee cherts no evidence of organic origin for any of them has been found. This fact does not seem, however, to be auj proof that the chert was not originally deposited in the form of sponge spicules; for if the sihca in deposits so late as the Cretaceous has been so extensively rear- ranged as has the chert in the Triramingham chalks, it would be strange if similar deposits, so far back in geological time as the Penokee series, had not lost the evidence of organic origin. Our conclusion is, then, that we have no satisfactory proof as to whether the chert of the Penokee series is an original chemical sediment or the remains of life, but the latter is considered more probable. The quan- tity of chert which is contained, supposing the whole Iron-bearing belt to be as rich in silica as are the upper horizons, could not have been beyond 300 or 400 feet. It appears clear that original formations of chert have occurred which have more than twice this thickness, so there is no improb- THE IKON-BEARING MEMBER. 253 ability in the statement ni;i(le, tliat this material was deposited simulta- neously with the iron carbonate with which it is so closely associated. The ferruginous slates.— The microscopical description of the first phase of the second type of rock, tlie fen-u<;in(>us slates (pp. 203-206) has so fully indicated its origin that but little more need be said here than to bring together the actual facts of observation. Into the genesis of this phase of rock little or no theory enters, as all of the stages of its growth have been seen. The only conclusion which goes beyond the observed facts is the comparatively safe one as to the nature of those exposures which cannot be directly traced into the cherty carbonates. The more important of the facts which bear upon their origin are as follows: The stratification of these slates is precisely like that of the original carbonates; the mineral composition of the two rocks is the same, except that iron oxide in the ferruginous slates takes the place of siderite ; the change from sider- ite to the various iron oxides of the slate has been repeatedly noted in all its phases in thin section and is everywhere found in the slides when in the field the change from the cherty carbonate to the ferruginous slate could be traced. The background of the slates is at times more coarsely crystal- line than in the cherty carbonates ; also, the rocks are cut by veinlets of silica to a greater extent. These differences imply a partial rearrange- ment of the silica which they originally contained, and also perhaps the introduction of a small additional amount of silica. The quantity contained is not materially greater than in the unaltered cherty carbonates, and it seems a sufficient explanation of its variation in character to suppose that it is mostly due to a rearrangement; that is, the silica has been taken into solution to some extent and subsequently crystallized (other silica perhaps at the same time being added), thus becoming more perfectly quartzose and forming the veins which cut across the laminse of the rocks. The only chemical change implied in the above transformation of the cherty carbon- ates into the ferruginous slates is the decomposition of the iron carbonate and the peroxidation of the iron which it contained. In the transition forms which occiir between these slates and the ferruginous cherts there have been doubtless other chemical changes, but the}^ can best be consid- ered in connection with the origin of this rock. 254 THE PENOKEE lEON-BEAEING SERIES. The ferruginous cherts. — ;Iu explaining the genesis of the second phase of the second type of rocks it is necessary to acconnt for its concretion- ary and brecciated character; for the production of brown hydrated hema- tite, red hematite, and magnetite; for the concentration of the iron oxide in shots and bands; for the large quantity of silica which it contains; for the somewhat coarsely crystalline character of the chert as compared with that of the original carbonates; and for the presence of numerous ramify- ing veins of finely crystalline quai'tz. In the microscopical description of this phase of rock (pp. 205-209) the concretionary and brecciated character has already been accoxxnted for. These peculiar areas have been actixally observed in all stages of their formation, so that their history is not theory, but definitely observed fact, and will not be repeated here. The oxides of iron have been produced from the iron carbonate just as are the oxides of the first phase of rock, the only difference being that here occasionally magnetite is formed. This mineral, however, is readily accounted for by the decomposition of iron carbonate under conditions not favorable to complete oxidation. It is not t(i be supposed that these rocks were highly heated, but it may be remarked that a low degree of heat is sufficient to change iron carbonate into maa-netic oxide of iron with the liberation of carbon monoxide and dioxide. It is not believed that any carbon monoxide has been liberated, for the oxygen needed to change the protoxide of iron into the proto- sesqxiioxide was doubtless supplied by oxygen in percolating water. The manner of concentration of the iron oxides in bands and shots will be fully discussed in considering the origin of the iron ores, but it is here necessary to mention the causes which have produced such concen- tration. Percolating waters bearing oxygen in solution have decomposed a part of the iron carbonate, the carbon dioxide ])assing into the water and oxygen passing from the water into the rock, thus simultaneoush' forming sesquioxide of iron and solutions capable of taking up other iron carbonate. Sflch iron-bearing waters would after a time reach some crack or channel in the rock. This opening in many cases would serve as a passage for other waters more directly from the surface bearing oxygen in solution, and as a result of this mingling the iron in solution would be precipitated, THE IRON BEARING MEMBER. 255 and thus concentrations of ihv. iron oxide in bands and shots would occur. This expUmatiou is not a wholly suppositious one, hut accords with the facts of observation as seen in lunncrons exposures, one of which, the cliff bor- derin expluiu the differences between these rocks and the ferruginous clierts wo need, then, only to suppose one different con- dition— the absence of a sufKcient amount of oxygen at the time of their alteration to change all of tlie iron to the peroxide. As has been sug-' gested before, it is possible that those parts of the iron formation in which the actiuolitic slates occur were more highly heated than the other parts. The more coarsely crystalline character of the silica so prevalent in them may also be explained by this supposed increase in temperature; for, as has been shown by laboratory experiments, a temperature considerably above the normal is favorable to the production of coarsely crystalline quartz.' This supposed increase of temperature is rendered plausible by the frequent if not univiersal association of large quantities of basic erup- tives with the iron formation at the places where the actinolitic rocks occur. Also these cutting or interlaminated eruptives may have excluded to some extent surface water and thus caused the supply of oxygen to be deficient. The details by which the particular phases of the actinolitic slates have been produced cannot be satisfactorily given, for in so few of them is there any remaining iron carbonate. However, taken in connection with their position and intermediate links, there can be little question of their derivation from a clierty iron carbonate. If there is any doubt upon this point it is set at rest by considering the occurrences in the Animikie series in northeastern Minnesota. That these two iron-bearing series are stratigraphically equivalent can hardly be doubted. It has been seen that in the Animikie series all the transition phases between cherty iron carbonates and the most crystalline actinolitic slates are as perfectly illustrated as are the intermediate phases between the cherty iron carbon- ates and the ferruginous cherts in the Penokee series. The origin of these actiuolitic slates is analogous to that of the tremo- litic limestones, so widely present in the iron-bearing series of the North- west and in later geologic times. Tremoliteis calcium-magnesium-silicate; actinolite is calcium-magnesium-iron-silicate. The tremolitic limestones are, aside from the tremolite, mostly composed of calcium-magnesium-car- bonate. The original material from which the actinolitic slates were iRoscoe and Schorlemmer, Treatise on Chemistry, vol. i, p. 569. 260 THE PENOKEE lEON-BEAEING SERIES. derived is a siliceous iron carbonate bearing calcium and magnesium. In this case the silica had but to unite with these bases to produce the actino- lite; like solutions in the limestones had only to unite with the bases there present to produce tremolite. SECTION III.— THE ANIMIKIE IRON-BEARING SERIES. Before leaving this part of the subject it is necessary to make some allusion to like formations in other parts of the lake Superior country. The Animikie and Vermilion lake series of northeastern Minnesota and Ontario, the Marquette and Felch mountain series of Michigan, and the Menominee series of Michigan and Wisconsin, all contain large develop- ments of rocks which in their characters are almost exact reproductions of the iron-formation rocks in the Penokee series. In each of the districts mentioned all the types and most of the varieties described in the latter series are quite widely found. Cherty iron carbonate is not extensively known in some of them, but in one is more largely developed than in the Penokee series itself, while in the most folded and altered series men- tioned this rock typically occurs, as is shown by PI. xxiv, Figs. 3 and 4. A general condensed account of the iron formations and ore bodies of these various districts, including the Iron-bearing rocks of the Penokee series, has been published by us.^ While a continued study has made some modifi- cations of detail necessary, the main conclusion of the papers has been rendered more certain; i. e., that the many phases of peculiar rocks asso- ciated with the iron ores in these various districts have been derived mainly from a clayey or cherty iron carbonate. In the first paper refen-ed to an exception was taken in the case of some of the coarser grained actinolitic slates which are found in the Marquette country, "as to whose relations to the other ferruginous materials we feel now unwilling to speak." Later investigations make it not improbable that some of these rocks have been derived from eruptives; but the origin of this class of rocks has not yet 1 R. D. Irving : Origin of the Ferruginous Schists and Iron Ores of the Lake Superior Region ; Am. Jour. Sci., 3tl series, vol. xxxii, 1886, iip. 255-272 C. R. Van Hisc: The Iron Ores of the Mar- quette District of Michigan; Am. Jour. Sci., 3(1 series, vol. XLiii, 1892, pp. 116-132. C. R. Van Hise: The Iron Ores of the Lalie Superior Region; Trans. Wisconsin Acad, of Sci., Arts, and Letters, vol. viii, 1892, pp. 219-228. THE IKON liEAKING MEMBER. 201 been certainly detenuined. Nothing will be here added to what was said as to the character of tlic ferruginous rocks of the Marquette, Menominee, and Vermilion lake districts, except to state that not infrequently mingled with the nonuiechanical detritus are considerable quantities of mechanical material. Wliile nuich additional work has been done upon them, it has not gone far enough to warrant -, and wliicli lias ri'sult(Hl, in coinuictioii with erosion, in ""ivinji^ the series its present surface distribution. Topofiraphical features. — Tlie topof^raphy of the Penokee district (Pis. VII, IX, and xi) shows in a general way the boundaries of the; Upper slate membex*. The ridge known as the Penokee range has been described in detail, pp. 145-146. It is always found within the Quartz-slate or Iron- bearing members, or in the granite just to the south. North of the ii'on formation the ground slopes rapidly in most cases to a nearly level plain, which ordinarily has a gentle slope northward, as is shown by the direction of the flow of the streams (PL ii). A considerable portion of the ground is of a swampy nature, and the part not a swamp is largely low, rich, heavily . timbered and thick with underbrush. The northern boTindary of the Upper slate is for much of the distance defined by the trap range, which constitutes the base of the Keweenaw series. In traveling from the iron range north across the slate belt to determine its northern limit, the approach to this line is indicated by a rise in the ground. Ascending the swell for a short distance, one is pretty sure to find the eruptives of the Keweenaw series, and oftentimes in the form of a series of bold bluffs, which are a prominent topographic feature of the region. In general, then, the Upper slate member lies in a valley, bounded by the Penokee range on the south and the trap range on the north. At many points on both of these ranges the valley may be overlooked. The tree tops are seen stretch- ing as an almost level mass of forest for miles both east and west. This forest is now rapidly disappearing under the necessities of the neighboring mines. Within the belt the exposures are usually low and small. This results from the soft character of the slate. It is to be presumed that the judg- ment made up from the known exposures, conveys a somewhat erroneous impression with reference to the character of the member as a whole ; that is, the rocks which have a certain amount of the more resistant massive graywacke or quartzite are more apt to be exposed than the softer slates. In the vicinity and a short distance west of Bad river the exposures are quite numerous ; also a little way west of this locality, east of English lake, . there are again frequent exposures. This unusual immber is here probably 302 THE PENOKEE lEON BEARING SERIES. due to the fact that the rock in large measure becomes a somewhat resistant mica-schist, which is interstratified with quartzites and massive graywackes. East of this locahty the exposures are not numerous until Tylers fork is reached. Here, while they are large, they are in a good measure due to the cutting action of the river. No bluffs and hills of slate are found, as west of Bad river. Numerous large exposures are in the south part of Sec. 12, T. 45 N., R. 1 E., Wisconsin, just south of a small lake. The slates and graywackes here are the north slope of a steep bluff which rises rapidly from the water of the lake. The only remaining large exposures in the belt are those along the railroads in the west part of Sec. 28, T. 46 N., R. 2 E., Wisconsin. The slates and graywackes are here a prominent landmark, rising in a bluff of some size in the valley between the iron and trap ranges. General petrograpMcal character. — It has already been said that the rocks of the member under consideration are distinctly clastic, or at least are rocks which can be shown to have been once fragmental. While they are alike in this fundamental point there is great variety in minor charac- teristics. The varieties may be grouped under the heads: (1) Mica-schists and mica-slates; (2) Grayivackes and graywache-slates; (3) Clay-slates or phyllites; (4) Quartzites and conglomerates. Each of these main types has the various phases shown by the following tabulation: ' Muscovitic. Mica-schist and mica-slate. Micaceous i. Biotitic. 1, Muscovitic and biotitic. \ Cliloritic and biotitic. Micaceous and chloritic < > Cnlontic and sericitic or muscovitic. •i Biotitic. Micaceous < ( Biotitic and muscovitic. Graywacke and graywacke-slate. Micaceous and chloritic Cliloritic and biotitic. (Cliloritic. Magnetitic and chloritic. Ferruginous and chloritic. \ Chloritic. J Chloritic and magnetitic. V Clayey. Quartzite and conglomerate ' Feldspathic, TllK Ul'PKK SLATK M KM HER. 303 In the subsequent (letiiiled (lesc'ri[)tioiis of tlii^ rocks of this ineiiiber they 'AYG grouped into several ureas geographically. '^Phe (listriljution cor- responds in a loose way to the classification of the rocks ahove given. vVt the west end of the member the mica-schists and mica-slates are the only rocks found. At Bad river and vicinity these rocks greatly predominate. In i)assing eastwai'd micacecnis graywackes and graywacke-slates, chloritic graywackes and graywacke-slates, and clay-slates are each in turn the pre- ponderating rocks. In the transition from one type of rock to the next one there are cross sections in which two or more of these kinds may be found. Before passing to the petrographical characterization of the types of rocks of this group, it will perhaps be well to give the kinds of rocks foimd in the geographical divisions as given in the tabulations. All the rocks of the English lake, Penokee gap, and Mellan junction sections, and also the exposure in Sec. 18, T. 44 N., R. 3 W., Wisconsin, have as their chief con- stituents quartz, feldspar, and mica. The rocks in Sec. 1 8 and those in the English lake section are all mica-schists and mica-slates. At Penokee gap are some graywackes and thin belts of quartzite, but both are micaceous. At the Tylers fork section the mass of the rocks are graywackes and graywacke-slates. However, in exposure all show lamination with suffi- cient plainness to enable one to readily determine strike and dip. The difference, then, between graywackes and graywacke-slates is of minor importance, and is used only for convenience to show relative degrees of massiveness. The rocks at the section between Tylers fork and Potato river and at the section in the Adcinity of Potato river differ in no respect from those at the Tylers fork section, with the exception that biotite is less and chlorite more plentiful. Also, at one exposure in the extreme northern part of the member are found some conglomerates and quartzites, which will be further referred to. The rocks in the section located in the north part of T. 45 N., R. 1 E., and along the west range line of R. 2 E., Wisconsin, are mostly chloritic graywackes, although there are also found magnetitic clay-slates. The rocks, then, differ from those found at Tylers foKk and Potato river in that biotite has almost entirely 304 THE PBNOKEE lEON BEARING SERIES. disappeared and chlorite has become the predominant mineral aside from the quartz and feldspar. At the extreme • east end of this area appear the first exposures found of the clay-slates. These clay-slates or phyllites differ chiefly from the finer grained graywacke slates, into which they grade apparently oidy in that a portion of their mineral con- stituents are so fine grained as to make exact determinations impossible in many cases. The rocks in the section between the east range line of R. 1 E. and West branch of the Montreal, the section between the west branch of the Montreal and Montreal river, and "the section at the Montreal river and vicinity extend over a distance east and west of about 6 miles. The westernmost exposure placed in these areas is about 3 miles west of the west branch of the Montreal, while the easternmost exposure is just east of the Montreal river. In this locality the exposm-es are more numer- ous than in any of the previous ones except those at Bad river and Tylers fork. Also, some of the exposures are of large size. The rocks here included are wholly graywackes and graywacke-slates, which are always chloritic. Only in one or two of them is any biotite found, and then only in subordinate quantity. Some of them are ferruginous in appearance. The slates differ but little from those of the Potato river and Tylers fork sections. The areas west of Black river and at Black river include, with a single exception, all the exposures found in Michigan. They are widely scattered, few in number, and are, with one exception, small. While graywackes and graywacke-slates are found among them, they are very fine grained and approach closely to clay-slates, of which the greater number of the exposures are composed. Mica is found only in a single exposure and in subordinate quantity. Petrographical characters of the four types of rock. — In giving a charac- terization of the rocks of the Upper-slate member the following order will be followed: (1) The quartzites and conglomerates; (2) The clay-slates or phyllites; (3) The graywackes and graywacke-slates; (4) The mica-slates and mica-schists. This order is that of alteration, the first mentioned being nearest its original condition of deposition. By following this order the sections will in the main come from exposures in a direction from east to west. THE III'PKU SLATK MEMBEK, . 305 Tlie only expusui-c of tiaartzite and confjlomerate of any inii make the determination of their mineral com- position in part uncertain. A portion of each thin section is sufficiently coarse grained to certainly show that it contains small fragmental yax- ticles of quartz and feldspar. It can also be seen that it contains much chlorite, kaolin or sericite, finely crystalline quartz, and iron oxides. As the minerals thus known to be present are the same that ordinarily con- stitute clays, and as they are those whicli compose the graywackes and graywacke-slates with which the clay-slates are closely associated, there is little doubt but that the greater number of them are thus chiefly consti- tuted. One of the slates contains a small quantity of biotite. The iron oxide in the slates along the line of Sec. 1, T. 45 N., R. 1 E., and Sec. 6, T. 45 N., R. 2 E., Wisconsin, is so abundant as to be one of the chief constit- uents of the slate. This iron oxide is very largely magnetite in minute MON XIX 20 306 THE PENOKEK IRON-BEAEING SERIES. crystals. One of the slates has a peculiar mottled appearance when viewed under the microscope, resembling that presented by raindrops on shale, but the spots are indefinitely smaller. They seem to be due to the relative proportions of the mhierals which compose them. This an-ange- ment may be caused by the decomposition of feldspar, each one of the hghter spots perhaps representing a rounded fragment of that mineral. In order to reenforce the microscopical determination of the minerals in these obscure slates, analyses of two of the raagnetitic ones were made in the U. S. Geological Survey laboratory by Mr. L. G. Eakins, with the result of confirming fully the observations made. The first specimen is from NW. J Sec. 6, T. 45 N., R. 2 E., Wisconsin; the second from NE. J Sec. 1, T. 45 N., R. 1 E., Wisconsin. The amount of magnetite would appear in both cases to be as much as 15 per cent. *■ Analyses of slates. SiOj AI2O3 Fe^Os TeO MnO CaO MgO KjO NojO U-fi H2O P.O5 Total 53-44 19-62 11-118 5-35 trace -42 1-58 1-73 2-61 trace. 4-07 trace. 100-20 n. 52-58 20-76 12-17 408 -21 •30 1-33 4-87 -37 trace. 3-43 100-10 The graywackes and graywacke-slates cover much the largest territory of any class in the Upper slate member. The graywackes^ have always as chief constituents fragmeutal quartz and feldspar. The strength of the ' The term gray wacke is here used in a lithological sense, in accordance witli the deiiuitiou of the term given by Gcikie, Text Book of Geology, 2d ed., p. 162: "A compact aggregate of rounded or sub- angular grains of quartz, feldspar, slate, or other minerals; or rocks, cemented by a paste, whicli is usually siliceous, but may be argillaceous, fcldspathic, calcareous, or anthracitic. Gray, as its name denotes, is its prevailing color; but it passes into browu, brownish purple, and sometimes, -where anthracite predominates, into black. The rock is distinguished from ordinary sandstone by its darker hue, its hardness, the variety of its component graius, and above all by the compact cement in -which the grains are embedded," THE UPPER SLATE MEMHKlt. 307 roi'k i.s usually <>-ivcui hy "siliruous [lasU;," althouj^li laucli of thu cuiuoiit is trequeutly hi the t'orui of (|uartz added to the rounded fraynieutal grains. There is frequently " arg-iUaceous aud calcareous" and occasionally car- bonaceous material present as accessories. Tliere is also found in all of these graywackes abundant chlorite or mica, or both. The mica is chiefly biotite, although muscovite or sericite, or both, are plentiful. In impor- tance, these minerals are secondary only to quartz and feldspar. The class of rocks in this district — graywackes and graywacke-slates — has been divided into three phases — micaceous, micaceous and chloritic, and chlo- ritic. These divisions have a geographical significance as well as a litho- logical one. All of the graywackes and graywacke-slates in Michigan and Wisconsin as far west as the- Potato river are chloritic ; the Potato river rocks are mostly micaceous and chloritic, these two minerals being in about equal quantity ; about half of the rocks in the vicinity of Tylers fork are both chloritic and micaceous ; the remaining half micaceous only; west of Tylers fork and vicinity tlie graywackes and graywacke-slates are mica- ceous oidy. We have thus a series of graywackes which at the east end are wholly chloritic ; in passing to the west mica appears with the chlorite, becomes more and more plentiful, and ultimately entirely replaces the chlo- rite. How gradual and complete this transition is will be readily seen by turning over the tabulations of these rocks on subsequent pages. The graywackes and graywacke-slates are so closely associated, both strati- graphically and lithologically, with the mica-slates and mica-schists, that a more detailed characterization of them will be deferred until the general character and distribution of the latter are given. The mica slates and mica-schists^ (Pis. xxxiii and xxxiv) of the district have always as a chief constituent quartz, and usually tliis mineral is the most abundant of all. Feldspar, as in the graywackes, is almost always present, but its quantity is much less, and in the typical slates and schists is almost wholly absent. Biotite, musco-si's, sericite, and rarely chlorite, or two or more of these minerals, are always plentiful, the individuals generally being in well defined folia. The accessories present are the same ' Although in one place chlorite Is, aside from the feldspar, the chief c.oustituent, they will all be called niicft-schists aud mica-alates, to distinguish tliein from the graywackes and graywacke-slates. 308 ^ THE PElN'OKEE lEON-BEAEING SEEIES. as in the graywackes — ferrite, pyrite, some cai'bouate, and rarely carbona- ceous matter. As biotite is usually the prevalent mica, the slates and schists have as a whole a darker color than the graywackes, ranging from very dark gray to black. The fundamental differences, however, which separate them from the graywackes are their crystalline appearance and absence of feldspar. Very many of them when viewed under the micro- scope, unless closely examined, taken by themselves, show no trace of a clastic origin ; although the greater number, upon closer examination, are' seen to have something of a fragmental character. The distinction made between mica-schists and mica-slates is based upon structure and coarseness of grain. Most of the coarser grained upper member mica rocks have a well developed schistose structure parallel to the bedding of the rock — a structure which could fairly be called foliated in some places. They never become very thinly foliated, or contorted with a brilliant sheen upon the foliation surface, as do some of the most crystalline mica-schists. A few exposures have a dark and light banded appearance, suggestive of a fine grained gneiss, which in fact they are when the feldspar is a chief con- stituent. The rocks called mica-slates are generally finer grained than the mica-schists ; they cleave with a . very smooth slate-like parting parallel to the bedding. Many of them have a black color, due to abundant dark biotite, particles of ferrite, and in some places to carbonaceous material. Like the graywackes and graywacke-slates, the mica-slates and mica-schists have been divided into three divisions : those in which chlorite is the chief constituent aside from the quartz and feldspar ; those in which mica takes this place ; and those in which both chlorite and mica are abundant. The first division is represented by but a single exposure. The chloritic and micaceous schists and slates are quite j)lentiful, but by far tlie greater num- ber of the slates and schists are micaceous only. As with the graywackes and graywacke-slates, tlieir classification corresponds to geographical dis- tribution, and the distribution is of the same sort as that of the graywackes and graywacke-slates. The one exposure of sericitic chlorite-schist is but a short distance west of the Montreal river, in Sec. 14, .T. 46 N., R. 2 E., Wisconsin. In passing to the west, the mica-slates at the Potato river first appear. At this locality only one exposure is known which falls under TIIH riM'EK SLATE iMK.Ml'.KH. 309 this lieiul. At Tylers fork iiinl \iciiiity tlicrc nrc quite a miniber of expo- sures of cliloritic and liiotitic slates jind hut a single one of pure cliloritie, shites. West of Tylers fork the schists jiud slates are almost wlioll}- luiea- ceous, although chlorito is yet occasionally found. There are, then, precisely the same stratigraphical relations with reference to chlorite and mica in the slates and schists that there are in the graywackes and graywacke-slates. The two classes of rocks are also interlaininated with each other. In Sec. 14, T. 46 N., R. 2 E., a chlorite-schist is but a short distance from an expo- sin-e of typical graywacke. At Potato river again the one biotite-slate there found is interstratified with graywackes and graywacke-slates. At Tylers fork the graywackes and graywacke-slates and the mica-schists and mica-slates are interstratified in the most intimate manner, both rocks occur- ring at times in the same exposure. Beginning at the top. of the Upper slate member, the order of succession of the different phases of rock as here found, as taken from the tabulations, are biotite-slate, chloritic graywacke, chloritic biotite-slate, biotitic graywacke, biotitic graywacke-slate, chloritic graywacke-slate, chloritic graywacke. At Bad river, the order, beginning at the top, is as follows: mica-schists, micaceous graywacke, biotite-schists, micaceovis graywacke, mica-slates. West of Bad river, in the vicinity of English lake, mica-schists and mica-slates only are found. TABULATION OF PETRO GRAPHICAL OBSERVATIONS. Exposure in Sec. 18, T. 44 If., E. 3 W., Wisconsin. 1. Biotite-slate. Specimen 167 Wr., from 1,000 N., 1,250 W., Sec. 18, T. 44 N., E. 3 W., Wisconsin.! The rock contains, in an aplianitic background, very numerous small crystal sur- faces which feebly reflect the light, is black, and cleaves somewhat irregularly parallel to the bedding. The thin section is composed of a finely crystalline background and individuals and clusters of individuals of feldspar, in about equal proportion. The background consists of finely crystalline quartz, small brown folia of biotite, very numerous black particles of ferrite or biotite, or both, and probably also some carbonaceous material. ' The numbers of specimens .and slides are usually those of the collection of the lake Superior division. Specimens with Wr. after the numbers are from the collection of the late Mr. Charles E. Wright. Specimens with Wis. after the numbers are from the collection of the Wisconsin Geological - Survey. Locations arc given from the southeast corner of the section in steps of 2,000 per mile. 310 THE PENOKEE IRON-BEAlimG SERIES The feldspar areas are rounded and are decomposed to a greater or less extent, the secondary products being biotite and quartz. This alteration is more extensive upon the outside than in the interiors of the particles. These feldspars also include ferrite. From the section at east end of English lake. 2. Biotite-slates from a middle horizon. Specimen 126 Wr., from 1,300 N., 200 W., and specimen 128 Wr., from 1,000 IS., GOO W., Sec. 9, T. 44 N., E. 3 W., Wisconsin. These rocks are dark gray to black, flue grained, quite massive, mottled, the mottling being due to numerous small cleavage surfaces. In the thin section the cleavage areas are seen to be well rounded, altered grains of feldsijar. They are set in the grouudmass, which consists of intimately mingled small grains of quartz and small, brown jparticles of biotite, with a considerable quantity of ferrite. The partial decomposition of the clastic feldspars has resulted in the formation of very numerous small folia of biotite and a few larger ones of muscovite, the transition to these minerals being beautifully shown. In places also the feldspar is replaced by saturating quartz. In each of the feldspar areas the secondary biotite and muscovite are found most plentifully at or near the exterior ol the grain, although in almost every case the alteration has proceeded in a greater or less degree quite to the center. In the matrix it is impossible to determine which part, if any, of the quartz is fragmental. The biotite of the matrix is precisely like that found in the feldspar; it is all deep brown, very strongly dichroic, and therefore probably bears a large percentage of iron. Tliis biotite has doubtless been furnished its iron by the abundant oxide present. The peculiar spotted appearance of the section when held up to the light, taken in connection with its appearance under the microscope, gives a clear idea of the manner in which the rock reached its present condition. (PI. xxxiii. Figs. 2, 3, and 4.) 3. Muscovitic biotite-schist from an iipper middle horizon. Specimen 73, Wis. (slkle 24), from south shore of English lake, Sec. 9, T. 44 IST., E. 3 W., Wisconsin. The thin section is a rather fine grained, apparently completely crystalline, typical mica-schist. The groundmass consists chiefly of quartz, mingled with which is feldspar, both orthoclase and plagioclase. Biotite in rather small fine folia of uniform size is very plentiful ; muscovite is much less abundant. That all the mica is a secondary alteration of feldspar can not be proven, but a portion of it is certainly of this nature. Many grains of feldspar are partly surrounded and cut by folia of mica, while many of the larger particles of feldspar contain numerous flakes of mica, which in magnitude and appearance are precisely like the great mass of the mica in the section. Quite numerous black grains and crystals of a mineral which is taken to be pyrite are included alike in the quartz, feldspar, and mica. 4. Miiscovitic biotite- schists from an upper horizon. Specimen 78 Wis. (slide 26), specimen 79 Wis. (sUde 27^, from SW. I of Sec. 4, T. 44 N., E. 3 W., Wisconsin. T 1 1 1: UPPER SLATE M EM BEH. 3 1 1 Tlipse sections arc like .'{, excopl tliut Miey are somewhat (uiarser giained iiiul contain more ninsc((vite and apijroacli nearer (especially Klide 27) to a typical mica- schist. 5. Riotite-acliist, IVoiii an upper horizon. Specimen IRJ Wr., 335 N., 1,050 W., Sec. 4, T. 44 N., K. :i W., Wisconsin. ;; The rock is dark gray, of a rather fine, nniform grain, finely laminated, yet so compact as to break quite readily aca-oss the plane of lamination. The rather large black abundant flakes of mica give the specimen tlie appearance of a typical mica- schist. The thin section shows this I'ock to be a mica-schist. It has an interlocking quartzose background, mingled with which in subordinate (piantity are both ortho- clase and plagioclase. Contained in this background is nuich biotite in tolerably wide, long, well defined blades, which cut through both ((uartz and feldspar. No grains of quartz are found which are plainly enlarged. This rock is the most com- pletely-crystalline of any mica-schist in the formation. It contains the merest trace of old feldspar grains, no quartz which can be shown to be fragmental, and a consid- erable quantity of fresh feldspar. In short, if this schist has formed, as from its posi- tion we are obliged to bglieve, from a fragmental rock, no trace of the alteration is discoverable. The fresh feldspars in all probability, in this, as in other cases where they are present, are of secondary origin, not original fi-agmental particles. (PL XXXIV, Fig. 2.) 6. Biotite-schist, from an uppermost horizon. Specimen 154 Wr., 500 N., 1,000 W., Sec. 4, T. 44 N., R. 3 W., Wisconsin. This rock differs only from 153 Wr. in that it is finer grained. In section the chief constituents are quartz, biotite, and feldspar, the first being preponderant, and the last comprising both orthoclase and plagioclase. The section contains also quite a quantity of black ox^aque material. Many of the grains of quartz have undergone secondary enlargement. That all the biotite is secondary is probable, although only a portion of it can be shown to be of this nature. From the section at Bad river and vicinity. 7. Garnetiferous biotite- slates, from west side of fault at base of formation. Specimen 9552 (slide 3187), 0 K, 1,800 W.; Specimen 9554 (slide 3189), 0 N., 1,750 W., Sec. 11, T. 44 jST., R. 3 W., Wisconsin. These rocks are of a reddish or greenish black color, aphanitic, finely laminated, and readily cleave along the plane of lamination. The cleavage surfaces are lustrous and covered by many small protuberances, which are taken to be due to contained garnets. The sections are mostly composed of small flakes of biotite with particles of a black substance set in a fine quartzose groundmass. The quartz is in fine and closely 312 THE PBNOKEE IRON BEAlilNG SERIES. fitting grains. Whether any of it is fragmental is difficult to say. A few grains are found which are larger than the remainder of the mineral, and these are clear and appear to be clastic. Biotite in very small flakes, with the greatest diameters ordi- narily in a common direction, compose fully one-half of the section. Mingled with this biotite, and giving the rock its color, are large quantities of the black or very brown material before mentioned. This is not magnetite, but appears to be a very dark colored ferrite, or such ferrite mingled with pyrite or carbonaceous material, or both. In each section there chftnces to be but one or two garnets. Included in each of the garnets" are all of the remaining constituents. In the main the garnets are not crystal outlined, but in places they are, and here biotite blades often abut sharply against the garnets and abruptly terminate. These rocks are plainly intermediate between the Iron-bearing and Upper-slate members. 8. Biotite-slate, from east side of fault and near base of formation. Specimei\ 9568 (slides 4497 and 4498), 1700 N., 1000 W., Sec. 14, T. 44 N., R. 3 W., Wisconsin. This rock is black, very fine grained, massive, breaks with hackly subconchoidal fracture, contains many grains of colorless and flesh-colored feldspar, which upon the broken surface of the rock show well developed cleavage planes. The thin sections are composed of comparatively coarse fragmental particles of feldspar set in a much more abundant matrix, consisting largely of biotite, quartz, and feldspar. The larger fragments are mostly more than 1 mm. in diameter. They vary in magnitude from this to those so fine as to be lost in the matrix, their numbers increasing as their magnitudes diminish. These sections differ from any of the other biotite-schists and biotite-slates in the remarkable freshness of the larger feldspars. They are in their interiors generally clouded but slightly, and the plagioclase gives sharp twinning bands. The gTeater part of the feldspar is, however, orthoclase. The smaller feldspars have all largely and some of them wholly altered to biotite and quartz, and even the larger feldspars are often affected more or less deeply upon their exteriors by this biotitic decomposition, and sometimes complex areas of biotite are found quite a distance in the feldspars. Every stage of this change is seen from per- fectly fresh unaffected feldspar to that in which but a trace of feldspathic material remains in a fine aggregate of biotite and quartz. The amount of fragmental quartz is very small, not more than from a tenth to a twentieth as plentiful as the feldspar. The biotite is in small dark brown folia intimately -mingled with finely crystalline quartz, and all the particles of these two minerals have Avithout doubt formed by the decomposition of the feldspar. The absence in many cases of large fragmental grains of quartz in the more altered mica-schists and mica-slates is explained by this section. Here are abundant fresh large fragmental feldspars and but few small frag- mental quartzes. It is plain that this rock was once mainly a feldspathic sediment, the metasomatic alterations of which have formed a dark colored mica-slate. Doubtless the greater mass of the fragmental material was finer grained than the unaltered TlllO i;iM'i:i.' SLATK jMKMIiKi;. 313 largo grains of feldspar. It, would simmu from (liis socrtioti that tliP fin(> grained niica- scliists in wliicli tliii feldspar is deeouiposed to a inncrli greater extent than in tiiis rock, and which contain no hirge fragments of quartz, must originally have been I'eld- spathic sediments. it. Bhiick biotitie slate, from east side of fault and near base of formation. Speci- men itr)(i!) (slide .'5378), from 17UU N., lOUO W., Sec. 14, T. 44 N., 1!. ,". W., Wisconsin. A black fine grained coTupact rock, which breaks with a conchoidal fracture. Very small clastic particles of (piartz and ferdsi)a.r eomi)ose one-half of the area of the section. The quartz grains are ordinarily distin(;tly enlarged, while quite a good many of the grains of feldspar are tolerably fresli. The greater nuiid)er of tbeui are altered to some extent to biotite, while freciueutly they are almost completely thus altered. All stages of this process are seen, and while the particles of feldspar and secondary folia of biotite are very small, the transformation is distinctly made out. The interstitial material composing tlie other lialf of the section coiisists of exceedingly fine crystallized qnartz, of minute folia of biotite, and of an abundiyit black opaque material which is doubtless ferriferous and possibly also carbonaceous. All of the biotite in the section is believed to be of secondary origin. 10. Black biotitie slates, from west side of fault and at the lower middle horizon. Specimens 9550 (slide 3322) and 1480 Wis,, (slide 207), fi'om 500 N., 1400 W., Sec .11, T. 44 N^., E. 3 W., Wisconsin. Fine grained and ftnely laminar rocks, which cleave readily along the plaues of lamination. Contained in the fine material are numerous roundish black lustrous cleavage areas, which are taken to be large fragmental particles of feldspar. A lens shows very uumerous minute crystals of pyrite or pyrrhotite, probably the latter, for the rock gives, finely pulverized, a maguetitic powder, which may, however, be due to magnetite. The thin sections show the rooks to consist of two parts, a finely crystalline mati'ix and coarse well rounded fragmental feldspars. These feldspars have always altered to a greater or less degree, the alterations resulting in the formation of the biotite, many small folia being always found in a single individual of feldspar. Every gradation of the change is seen from grains of feldspar which contain but few folia of biotite to those in whicli the remaining feldspar is just sufficient in quan- tity to enable one to perceive that the detached areas are parts of one individual. Doubtless also the alteration to biotite and quartz has completely taken place in many cases, when the outlines of the original clastic areas would be entirely lost. Accompanying the biotite secondary to the feldspar is a large amount of black opaque material in minute particles. This is in all probability also secondary, as its amount increases as the quantity of biotite increases. The black roundish spots spoken of under the maitroscopic descriptions are evidently these altered fragmental feldspars. The matrix of the rock is composed of intimately mingled quartz, feld- 314 THE PENOKEE IKON-BE AKING SERIES. spar, biotite, and black opaque material,, which is mostly somewhat altered pyrite or pyrrhotite, but which may also contain carbonaceous material. A portion of the matrix is certainly frag'mental, as is shown by secondary enlargements of the quartz grains. The biotite is all believed to be due to the alteration of feldspar ; much of it is certainly of this nature. 11. Black chiastoliti(; biotitic slates, from west side of fault at lower middle hori- zons. Specimen 9576 (slide 3381), 650 N., 1350 W.; specimen 9574 (slide 3380), 670 N., 1350 W.; specimen 9572 (slide 3379), 680 N., 1350 W., Sec. 11, T. 44 N., E. 3 W., Wisconsin. The rocks are black, and exceedingly fine grained to aphanitic. Specimen 9572 breaks with a conchoidal fracture and 9574 and 9576 are finely laminar. In 9576 are large cleavage areas like those described in 10. lu the specimens numerous crys- tals of pyrite are contained. The thin section 3381 is almost precisely like tlie thin section 3322 in 10, above described, the only difference being that the decomposition of the large fragmental feldspars has proceeded somewhat further. Thin section 3380 differs from 3381 in that the large feldspars have been less numerous apparently and the alteration of those present has not gone so far. The appearance of the rocks as seen in hand specimens corresponds with their apiiearance under the microscope, the extent of the alteration of the feldspars as determined by the thin sections corresponding exactly with the condition of the large feldspars as seen macroscopically. In specimen 9550, in which the areas of feldspar are generally well developed and show distinct cleavage surfaces, the biotitic alterations characteristic of these rocks have taken place to but a comparatively small extent. In specimen 9574 the obscurely outlined feldspars and irregular cleavage surface are accompanied by extensive biotitic alteration. The section of 9572 is much like the finer parts of 9550 (in 10) and 9576. Scattered sparsely through the finer material of 9572 and 9574 are quite large crystals and intersecting clusters of crystals of chiastolite. These chiastolites often include such a quantity of the other minerals of the section as to be almost indistinguishable in ordinary light. The cloiids of inclusions are arranged in parallel lines, which correspond to the some- what obscure slaty cleavage of the rock. Pyrite is an abundant accessory. 12. Biotite-slate, from west side of fault at a lower middle horizon. Specimen 1440 Wis., (slide 261), 700 N., 0 W., Sec. 10, T. 44 N., E. 3 W., Wisconsin. The thin section is very fine grained, and consists of intimately mingled quartz, feldspar, biotite, and minute particles of dark opaque material a portion of whicli is doubtless pyrite. The biotite composes about one-third of the rock. A portion of this mineral is certainly secondary to feldspar, and all of it is probably of this origin. 13. Biotitic and muscovitic slates, from west side of fault, at a lower middle hori- zon. Specimens 129 Wr. and 130 Wr., 850 JST., 200 W., Sec. 10, T. 44 N., E. 3 W., Wisconsin. THE UlTEU SLATE iMEMl'.EH. 315 8i)00iiiipu 11'!) Wr. is likciinTfi in U, silxtvo described. Specimen I'M) Wr, differs from ll'it iiibeiiiy- of a dark j^ray color, and in tlmt itsliows hut few clea\'iifre surfaces. Tlie tlim section of 12!) Wr. resembles very closely ;5.')L'li in 10 and .t.tSl in 1 1 ; tlie oidy point 111' (Urierciuu' beinj;- tiiat pyrite and black material in ininnte specks ai'e particularly abundant. In K5(t Wr. the alteration of larj-e fragmental feldspar to mnscovite and biotite is nicely shown. The Hakes of mus(!ovite, which are of a greater magnitude than tlio.se of biotite, are clustered about and i>onctra-te the parti- cles of feldspar. The section is nimsually free from black material. In other resi)ects it does not differ from the other mica-slates of the vicinity. (PI. xxxiv, Fig. 3.) 14. Black biotite-slates, from west side of fault, at a middle horizon. Specimens 9549 (slide 3321), 1050 N., UKtO W.; 954S (slide 33130), 1070 N., 1000 W.;,9547 (slide 3094), 1090 N., IGOO W., Sec. 11, T. 44 N., R. 3 W., Wisconsin. The rocks are black, exceedingly fine gi'ained, finely laminar, and cleave readily along the plane of lamination. As in 9550 in 10, and 957(! in 11, which they are almost precisely like, nnmerous rounded cleavage areas are contained in the Hue material. Pyrite in small crystals is an accessory. The thin sections are almost i^recisely like 3322 in 10, the only difference of importance being that in them the biotitic alteration of the fragmental feldspars has taken jilace more largely than in 3322. The correspondence between the appearance of hand specimens and tWn sections described as applying to 10 and 11, due to feld- spar decomposition, is here equally well seen. 15. Muscovitic graywacke, from west side of fault, at a middle horizon. Speci- men 9540 (slide 3093), 1100 N., 1000 W., Sec. 11, T. 44 K., R. 3 W., Wisconsin. The rock is a light gray, medium grained one, with uniform texture, which cleaves irregularly along a schistose plane. The thin section is composed of coarse grains of quartz, having generally oval or roundish forms set in a fine matrix, which also appears to be chiefly quartzose, although with it are feldspar, chlorite, and mnscovite. While there is a wide grada- tion in the size of the quartz grains^ there is an approximate separation of the coarse and fine quartz. At first sight the section apjiears to have but a small amount of coarse fragmental feldspar. Ilowever, upon closer examination it is seen that the remnants of many such feldspars are present. In some cases these grains are honey- combed by saturating quartz, many detached areas often being in a single individual. Again a feldspar area contains many small individuals of quartz mingled with chlorite and mnscovite, and occasionally a feldspar particle is found which is tolerably fresh. Evidently this rock was originally mostly composed of rounded fragmental ijarticles of quartz and feldspar. The fragmental quartz grains are enlarged, and thus are now sharx)ly angular. The x>i^rticles of feldspar are largely replaced or decomposed, as above described, and consequently the rock has now become at first sight an almost completely cirystalline one. Had the i^rocess continued until all .the feldspars had 316 THE PENOKEE IRON BEARING SERIES. been thus decomposed, the only trace of any original fragmental material would be the general oval or roundish character of the larger particles of quartz. IG. Biotitlc and muscovitic graywacke, from west side of the fault, at a middle horizon. Specimen 9544 (slide 3092), 1105 N., 1600 W., Sec. 11, T. 44 N., R. 3 W., Wisconsin. A gray, coarse grained, massive rock, having a conchoidal fracture. Large fragmental grains of quartz and feldspar, with the alteration and replac- ing products of the latter, make up the mass of the thin section. The areas of quartz are enlarged, and consequently minutely angular, although still retaining their general roundish form. The feldspar is nuich fresher than in 15, many individuals showing no alteration influence except a little kaolinization. Other individuals, however, include many grains of quartz or large reticulating quartz individuals with numerous flakes of muscovite. Here these minerals are plainly alteration products of the feld- spar. In many cases this alteration has proceeded so far as to leave irregular cores, which are entirely surrounded with the secondary quartz, biotite, and muscovite. Again in other cases the original rounded outlines of the feldspar are distinct, the alterations having occurred in spots through the grains. The finer portions of the rock are composed of quartz, biotite, and muscovite, often without any feldspar. These portions are also probably alteration products of feldspar, the individuals per- haps being small, or at any rate are alteration products of. feldspar mingled with detrital quartz. Through the finer grained portions of the rock are numerous small particles of black opaque material. (PI. xxxii. Fig. 2; PI. xxxiii. Pig. 1.) 17. Biotitic graywacke, from the west side of fault at a middle horizon. Speci- men 9545 (slide 3319), from 1100 K, 1600 W., Sec. 11, T. 44 N., R. 3 W., Wisconsin. The rock is gray, coarse grained, massive, and has a conchoidal fracture. It contains numerous particles of limpid quartz and less plentiful ones of white or pale pink feldspar, which are so large as to enable one to easily see their rounded forms. The rock in appearance approaches that of a gray quartzite. fn the thin section very large, rounded, often complex areas of quartz are very numerous. These grains as to general form are well rounded, but their outlines are minutely irregiilar, and these irregularities are doubtless due to enlargements.- The feldspar also is found in rounded areas. It is often more or less decomposed, the method of alteration and the products produced being precisely as in 16. Contained in the section are large areas composed almost entirely of finely crystalline quartz, brown folia of biotite mingled with small remnants of felds])ar, each of which with- out doubt represents a single fragmental feldspar. 18. Biotitic schists, from west side of fault at middle horizon. Specimens 9543 (slide 3318), and 1494 Wis. (slide 273), from 1110 N., 1600 W., Sec. 11, T. 44 N., R. 3 W., Wisconsin. A dark gray mottled, fine grained, and quite massive rock. THE UJM'KK SLATK MKMl'.Elt. 317 At first sij^lit the sections appear like ordinary liiotite-schists. They contain grouiidmasses of quartz,* through whi(;li arc [deiitifully and unil'oinily scattered brown fohui of biotite and less numerous tlalces of inuscovitc. On closer examination numy of the larger grains of this (luartz are ])lainly fragmental, and they have often been enlarged. Tiic biotite is to a large extent secondary to feldspar, many Hakes of this mineral being found mingled with a small qiumtity of feldspalhic material, wliich throughout a considerable area acts as a unit. • In a few places fragments of ortho clase and plagioclase are found, in which the mica-schist decomposition is but begun, but in the greater part of the sections but little of the original feldspar remains. We have now the explanation of the vaguely outlined spots seen in the hand specimens. They represent original fragmental feldspars, which once composed a very large pro- portion of the rock; but now in most cases they have so completely altered to biotite, muscovite, and quartz that only here and there in the thin section is an area well out- lined. This rock was then originally like 2, but now apin'oaches very closely to a typical mica-schist. In 2 there is everywhere a plain distinctiou between the feld- spathic and nonfeldspathic areas. The larger portion of the sections resembles the fine grained matrix of 2. Oxide of iron in small jiarticles is an abundant inclusion in the biotite, feldspar, and much of the ([uartz. The quartz containing such quanti- ties of this oxide of iron is taken to be secondary, the larger plainly fragmental grains being free from this feixite. 19. Muscovitic and biotitic graywacke, from west side of fault at an upper mid- dle horizon. Specimen 9540 (slide 3316), from 1600 N., 0 W., Sec. 10, T. 44 N., K. 3 W., Wisconsin. A light gray, line grained rock, which is quite massive, but which breaks most readily along its obscure schistose i>lane. The section is chiefly composed of particles of quartz and feldspar of greatly varying sizes and quite deeply interlocking. At first sight the only indication of a fragmental nature is the general rounded contours of the larger particles of quartz and feldspar, these grains being minutely and sharply angular upon their borders. Upon close examination many of the grains of quartz show indications of enlargement. The lines between the sui>posed nuclei and their enlargements are in no case so distinct as in the ferruginous fragmental quartzites, but consist of small detatched particles of chlorite, kaolin, air bubbles, and sometimes iron oxide. In a few cases the cores of the quartz grains are clearly made out. Oftentimes there are considera- ble breaks in the lines, and the whole line in most grains might be taken to be ordi- nary inclusions. However, the constancy of the oval or rounded forms of the inclu- sions and their positions near the exteriors of the large grains lead to the conclusion that they mark the outlines of true fragmental cores which have become eidarged until they interlock. The irregularity of the exterior of the grains of feldspar is due to another cause, their partial alteration to chlorite, muscovite, and biotite, with con- 318 THE PENOKEE lEON-BEAEmG SERIES. sequent separation of silica. These minerals are not plentifully present, but are so situated witli respect to the feldspars as to leave little doubt of the connection between them. A portion of the finer grained quartz is certainly secondary. 20. Biotite-schist, from west side of fault at an upper luiddle horizon. Speci- mens 9539 (slide 3315), 1442 Wis. (slide 2G2), frorn^ 1650 N., 0 W., Sec. 10, T. 44 N., E. 3 W., Wisconsin. The rocks are like 2. The thin sections are like those of 2. 21. Muscovitic biotite-schists, from west side of fault at a middle horizon. Speci- mens 9537 (slide 3314), 50 N., 1950 W.; 2001 Wis. (slide 281), 30 N., 1950 W.; 2002 Wis. (slide 282), 130 N., 1950 W.; 9536 (slide 4223), 150 N., 1950 W., Sec. 2, T. 44 N., E. 3 W., Wisconsin. The rocks are gray or green, fine grained, quite massive, yet showing a more or less obscure schistose structure. The thin section 4223 resembles very closely those of 3 aiid 4. It differs from them in that the alterations have not gone articles of quartz and feldspar. The quartz grains are mostly enlarged. Secondary interstitial quartz has apiieareil. The feldsi)ars have largely decomposed, their places now being occupied by chlorite, inuscovite, saturating quartz, and to a small extent biotite. Section at and west of Tylers forU. 30. Black biotite-slate, from a low horizon. Specimen 12770 (slide 5461); short distance east of north quarter post. Sec. 1, T. 44 N., R. 3 W., Wisconsin. The rock is black, aphanitic, finely laminated, very soft, and apparently carbo- naceous. The thin section shows an exceedingly fine grained background, comjiosed of flinty and perhaps amorphous quartz, of kaolin, and of black material which may be carbonaceous. Contained in this background are numerous small fragments of quartz and feldspar and dark brown folia of biotite. 31. Bioti tic graywacke slate, from a low horizon. Specimen 12771 (slide 5462), SE. i of the SW. J of Sec. 31, T. 45 N., E. 1 W., Wisc^)nsin, on the Wisconsin Cen- tral Eailroad. The rock is dark gray, very fine grained, cleavable, and shows here and there lustrous flakes of white mica. The thin section consists of very small particles of quartz, feldspar, biotite, chlorite, brown iron oxide, and apj)arently some kaolin. Much of the quartz is frag- niental. The feldspar fragments are much altered, the resultant products being chlorite, biotite, and quartz. 32. Chlorite graywacke-slates, from a middle horizon. Specimens 12772 (slide 5463), 12773 (slide 5464), NE. J of the ]SW. \ Sec, 32, T. 45 'S., E. 1 W., Wisconsin. MON XIX — ^21 322 THE PENUKEE IRON-BEARING SERIES. The two specimens differ in appearance. The first is almost blapk, fine grained, and finely laminated. The second is gray, coarser grained,- and massive. Both contain shining flakes of white mica. In slide 5463 rather small fragmental particles of quartz and feldspar, with a few flakes of white mica, compose two-thirds of the section. The quartz grains are often enlarged. The feldspar comprises orthoclase, microcline, and plagioclase, the latter very plentiful. All the feldspars have altered considerably to chlorite, seiicite, biotite, and quartz, chlorite being the most abundant. The remaining one- third of tlie section is apparently composed of the same minerals as the coarser parts, with the addition of pyrite, opaque iron oxide, and a little of some carbonate. Slide 5463 diff'ers from 5463 in that its fragmental quartz and feldspar are in much f-maller particles, in that the latter is more altered; and in containing more ferru- ginous and perhaps some carbonaceous material. 33. Chlorite graywacke-slate, from a middle horizon. Specimen 12776 (slide 5466), near center SW. ^ of SE. i Sec. 29, T. 45 N., R. 1 W., Wisconsin, on- Wisconsin Central Railroad. The rock is precisely like 12773 in 32. The thin section is in no important point difterent from 5463 in 32. 34. Chloritic graywackes, from a middle horizon. Sj)ccimens 2108 Wis. (slide 387), 1000 N., 784 W.; 9611 (slide 3341), 1000 N., 800 W.; 2104 Wis. (slide 306), 1060 K, 784 W.; 9613 (slide 3342), 1150 N., 775 W., Sec. 28, T. 45 N., R. 1 W., Wisconsin. The rocks are light gray, rather fine grained, and cleave most readily along the plane of stratification, although easily breaking across this plane with a conchoidal or subcouchoidal fracture. In thin sections, well rounded clastic particles of quartz and feldspar are set in a fine matrix, consisting generally of quartz and chlorite, but containing some kaolin, biotite, and brown and black material which is taken to be ferrite, mingled i)erhaps with pyi'ite and organic matter. The fragments of quartz are frequently enlarged. The feldspars are often quite fresh, but much of this mineral is altered to chlorite, kaolin, and biotite, with simultaneous separation of silica. 35. Biotitic graywacke-slate, from a middle horizon. Specimen 9627 (slide 3343), 1550 N., 0 W., Sec. 29, T. 45 N., R. 1 W., Wiscousin. A dark gi'ay aphanitic banded rock. The thin section is exceedingly fine grained. It consists of a confused mass of quartz, feldspar, chlorite, biotite, and ferrite, with perhaps some sericite. 36. Chloriticbiotite-slates, from an upi^er middle horizon. Specimens 9609 (slide 4428), 100 N., 1525 W.; 9610 (sUde 4429), 140 N., 1540 W.; 2098, Wis. (sHde 304), south line Sec. 21, at point where intersected by Tylers fork, Sec. 21, T. 45 N., R. 1 W., Wisconsin. TIM'] UPl'KU SLATE MEMBER. 323 The rocks are black, fine s'ni'netl to apluinitic, aud break readily across the slaty clcavajjo with a subconchoidal fra(^ture. Tlie thill sections consist of intimately mingled minute particles of quartz, feld- spar, biotito, chlorite, and dark opaque patches of ferrite or altered pyrite. In these, as iu the previously described sections, the biotite and chlorite are in part at least plainly secondary to feldspar. These slates are somewhat coarse grained, and there- fore show more plainly their fragmeutal character than do most of the slates of the district. 37. Chloritic biotite-slate, from an upper middle horizon. Specimen 9608 (slide 3340), 475 N., 910 W., Sec. 21, T. 45 N., E. 1 W., Wisconsin. The rock is like 36. The thin section is iiner grained than those in 36, but is otherwise like them. 38. Biotite-graywacke, from an upper middle horizon, interstratified with 9608. Specimen 9607 (slide 3339), 550 E"., 910 W., Sec. 21, T. 45 K, R. 1 W., Wisconsin. The rock is dark gray, of a uniform medium grain, massive, and breaks with a subconchoidal fracture. In thin section this rock differs from 387 in 34, in that the chlorite is less and biotite more abundant. They were originally of the same composition. The differ- ence at the present time is due to the fact that the feldspai-s underwent different alterations in the two cases. Pyrite is present in abundant small grains and crystals. 39. Ghlpritic graywacke, from an upper middle horizon, interstratified with 9607 and 9608. Specimen 9606 (slide 4427), 715 IST., 885 W., Sec. 21, T. 45 K, E. 1 W., Wisconsin. The rock is like 38. The thin section resembles closely 3341 and 3342 iu 34. It differs from them chiefly in containing a smaller proportion of tinely crystalline interstitial material, and in that the fragmental quartz aud feldspar are in larger grains. The almost complete decomposition of the feldspar and the resultant formation of chlorite, biotite, muscovite or sericite, and finely crystalline quartz are nicely shown. 40. Biotite-slate, from an upper middle horizon. Specimen 9605 (sUde 3338), 740 ]Sr., 885 W., Sec. 21, T. 45 K, E. 1 W., Wisconsin. The rock is dark gray, exceedingly fine grained ; contains numerous minute par- ticles of pyrite, and is given a mottled apxiearance by cleavage surfaces of feldspar. The thin section consists chiefly of biotite, quartz, and feldspar, the latter min- eral now being much less abundant than the other two. The little remaining feldspar is in "the last stages of alteration, its resultant products being mainly biotite aud quartz. The decomposition of each grain of feldspar produced many particles of quartz and folia of biotite. In mauy cases polarized light is necessary to discover the remain- ing feldspar of a i)artly decomposed individual, so closely do its alteration products resemble iu appearance the remainder of the section. It is quite probable also that in • 324 THE PENOKEE IRON-BEARING SERIES. most cases the decomi)osition of the feldspars has been complete, and that all of the biotite and much of the quartz are secondary products. Scattered uniformly through the section are rather plentiful grains and crystals of pyrite. 41. Chloritic sericite-schist, from an uppermost horizon, mingled with Keweenaw greenstones. Specimens 9629 (sUde 4430); 9630 (slide 3345), 120 N., 150 W., Sec. 17, T. 45 N., R. 1 W., Wisconsin. The rock is light gray, moderately fine grained, banded, and breaks with a sub- conchoidal fracture. The thin section has a groandmass of small blades and fibers of pale green chlo- rite and colorless sericite, Avith a little biotite, which contains numerous small frag- mental grains of quartz and fewer of feldspar. The individuals of the latter mineral are usually badly decomposed, the alteration products being chlorite, sericite, and bio- tite. In some cases cores of feldspar surrounded by these secondary products are comparatively fresh. Section between Tylers forh and Potato river. 42. Ohloritic graywacke-slate, from a lower middle horizon. Specimens 12777 (slide 5467); 12778 (slide 5468), the SW. J Sec. 24, T. 45 N., R. 1 W., Wisconsin, on the Wisconsin Central Railway. These rocks are almost precisely like 32, the only differences being that 12778 is slightly schistose, while 12773 is massive and white mica flakes are plenMful. In thin section these rocks are almost precisely like those of 32. The only dif- ferences are that fragmental white mica (sericite) is more, biotite less, and pyrite more plentiful than in that number. 43. Chloritic graywacke-slate, from a lower middle horizon. Specimen 12779 (slide 5469), NE. \ of the SW. J Sec. 24, T. 45 N., R. 1 W., Wisconsin, on the Wis- consin Central Railway. This rock is like 12773 in 32, except that it is of a darker color. The thin section, is like 5463 in 32, except that the amount of black material, either impure iron oxide or iron oxide, iiyrite, and carbonaceous material is greater. 44. Biotite-graywacke, from an upper horizon. Specimen 9598 (slide 3334), 0 N., 600 W., Sec. 15, T. 45 N., R. 1 W., Wisconsin. ■ The rock is of a dark gray color, medium grained, uniform texture, and breaks wTth conchoidal fracture. This section is coarser grained than that of any of the previously described bio- titic graywackes, but resembles them in essential points. Clastic particles of quartz and feldspar, well rounded, compose two-thirds or more of the mass of the rock. A few of the quartz grains are finely comjilex, and nearly all of them are distinctly enlarged. The alteration of feldspar to biotite is very nicely shown. The freshest of the feldspar grains are surrounded by and more or less deeply intersected with sec- THE UPPER SLATE MEMBER. 325 *> ondiiry biotito. Those {•niins yet retain their well loiiiided form, but in luauy cases the (iiifiiuiil exteriors of the fehlspar {grains are h)st. Often the entire surfaces of the fehlspars inehule very numerous jjartich-s of the biotite, there romaininj>- throufthout such areas iiere and tliere little spots of feldspar, wiiicli tof^etlua' polarize as units. The rather sparse matrix of the rock consists of finely crystalline (pxartz, of feldspar, and of biotite. Mucih of this fine quartz is doubtless a secondary infiltration material and the biotite is believed to be secondary to feldspar. This section is one of the finest yet described to show the alteration of feldspathic areas into numerous small folia of biotite. (PI. xxxii, Fig 3.) 45. Biotitic graywaclce, from near the top of the series. Specimen 9595 (slide 3332), 700 N., 1940 W., Sec. 14, T. 45 N., R. 1 W., Wisconsin. The rock is dark gray, rather fine grained, massive, and breaks with a. subcou- choidal fracture. In thin section medium sized clastic particles of quartz and feldspar are set in a finer matrix, which consists of particles of these minerals mingled with much biotite in small folia and with less plentiful white mica (sericite). As in the mica-scMsts previously described, many of the feldspar j)articles are altered to biotite to a greater or less degree. Section at Potato river and vicinity. 46. Chloritic graywacke-slate, from a lower middle horizon. Specimen 9107 (slide 2789), 1925 N., 1990 W., Sec. 19, T. 45 N., E. 1 E., Wisconsin. The rock is dark gray, exceedingly fine grained, finely schistose, and shows upon the cleavage surface numerous minute particles of pyrite. In thin section small fragmental particles of quartz and feldspar are set in an exceedingly fine matrix, consisting chiefly of quartz and chlorite. Mingled with the chlorite are flakes of sericite or kaolin. Throughout the section are many small areas of dirty somewhat altered pyrite. 4'7. Biotitic and chloritic graywacke, from an upper middle horizon. Specimen 9109 (slide 4418), 1800 N., 0 W., Sec. 13, T. 45 K, E. 1 W., Wisconsin. The' rock is dark gray, medium grained, massive, and breaks with conchoidal fracture. The thin section is almost precisely like that of 38. The particles of quartz are distinctly enlarged; many of them are notable for the very numerous partly liquid filled cavities which they contain. (PI. xxxii. Pig. 4.) 48. Biotitic and chloritic graywacke, from an upper horizon. Specimen 9110 (slide 2902), 390 K, 1980 W., Sec. 7, T. 45 N., R. 1 E., Wisconsin. The rock is like 47. In thin section the rock is almost precisely like that of 47. 326 THE PENOKEE lEON-BEAEING SERIES. 49. Biotitic graywacke-slate, from an upper horizon. Specimens 9111 (slide 2790); 9112 (slide 2923), 35 ^., 940 W., Sec. 12, T. 45 N., E. 1 W., Wisconsin. The rock is dark gray, fine grained, and varies from massive to finely laminar. In all essential points the thin section is like 3329 in 27. As in it, there is much biotite, which is certainly secondary to feldspar. 50. Biotite-slate, from near top of series. SiDCcimen 9113 (slide 2903), 300 N., 940 W., Sec. 12, T. 45 N., E. 1 W. Wisconsin. The rock is dark gray to black, aphanitic, finely laminated, and mottled by very numerous black cleavage areas. The thin section, with the exception that black opaque material and pyrite are sparsely present, is exactly like 3322 in 10. (PI. sxxiv. Fig. 4.) 51. Feldspathic quartzites and conglomerates from top of series, mingled with Keweenawan greenstones. Specimens 9115 (slide 4419), 9116 (slide 2905), 470 N., 30 W.; 9118 (slide 2906), 9119 (sUde 3298), 470 N., 65 W., Sec. 11, T. 45 N,, E. 1 W., Wisconsin. The rocks vary from coarse grained gray vitreous quartzite to a conglomerate containing numerous pebbles several inches in diameter. In thin sections large fragmental grains of quartz compose a good portion of these rocks. Mingled with the quartz is a considerable quantity of fragmental feld- spar, also in large individuals. The quartzes are usually enlarged, this fact generally being easy to discover. Between the fragmental particles is finely crystalline quartz and other accessories in large quantity. In slide 2905 the finely crystalline quartz is mingled with actinolite, being cut through and though in every direction by it. The actinolite needles are also always included in the enlargements of the clastic quartz grains, but never in the cores. It is, then, plainly a secondary mineral, and about it the infiltrated quartz has crystallized. Some of the feldspars have decomposed, and in them is found, as secondary and replacing product, actinolite and quartz. In slides 2906 and 3298 the feld.^pars are badly, decomposed, the secondary products being kaolin, large brilliantly polarized flakes of muscovite, and many smaller ones of biotite. These minerals are also included in the interstitial quartz and in slide 2906, and also in the enlargements of the old quartz grains. This section nicely illustrates the micaceous alteration of feldspar, by the decomposition of which, accom- panying the micas, saturating quartz "has been formed. Many of the pebbles in the conglomerate are large, interlocking, complex fragments of quartz. (PI. xxxii, Fig. 1.) Section in the north part of T. 45 N'., K. 1 U., \7isconsin. 52. Chloritic graywacke, from a lower middle horizon. Specimen 9123 (slide 2794), 300 K, 0 W., Sec. 8, T. 45 N., E. 1 E., Wisconsin. The rock is dark gray, fine grained, massive, and breaks with conchoidal fracture. THE IJPrEK 8LATE MEMHEH. 327 111 thill section siiiall i)iirticIo.s of (juiirtz, generally enlarged and sometimes finely i'oiui)lcx, with fragments of feldsiuir, iwtli of greatly varying iiiagiiitudes, com- pose from ouebalf to two-thirds of the rock. The feldspar iiicliuh^s orthoclase, microcliue, and plagioelase. Many of the feldspars are inucli altered, while some of them are almost or quite decomposed, the resulting products being maiuly (ihlorite and quartz, although with these minerals are found sericite, or kaolin and ferrite. The matrix consists chiefly of finely ('rystallinc (piartz, with some amorpho.us silica, mingled with Avhich are a considerable quantity of chlorite and a little kaolin or seri- cite and ferrite. In the matrix are also numerous irregular patches of opafpie material, which is in part gray and in part bhick. These patches are probably mix- tures of ferrite aud partly altered pyrite. The section is that of a flue grained typical giaywacke. 53. Chloritic graywacke, from a middle horizon. Specimen 9124 (slide 2908), 1200 K, 0 W., Sec. 8, T. 45 N., R. 1 E., Wisconsin. The rock is like 52. The thin section is like that of 52. 54. Chloritic graywacke, ft'om an upper horizon. Specimen 9126 (slide 2909), 1000 K, 55 W., Sec. 5, T. 45 N., E. 1 E., Wisconsin. The rock is coarser grained than 52 and 53 and contains crystals of pyrite, but otherwise is like them. The tliin section differs from those of 52 and 53 only in being coarser grained. Section near east range line of B. 1 H., Wisconsin. 55. Magnetitic clay-slate, from base of formation. Specimen 9148 (sUde 2924), 1100 N., 40 W., Sec. 1, T. 45 K, E. 1 E., Wisconsin. The rock is gray, flne grained, finely laminated, and easily cleavable along the lamination. Composition: Silica, 52'58; ahimina, 20'76; iron sesquioxide, 12*17; iron protoxide, 4*08; manganous oxide, "21; calcium oxide, -SO; magnesium oxide, 1'33; potassa, 4-87; soda, .37; lithia, trace; water, 3-43: =100-10. In thin section the groundmass is very finely divided. It appears to consist of quartz, feldspar, kaolin, with perhaps chlorite aud biotite. In this groundmass is contained very numerous small crystals of magnetite. The analysis well bears out the appearance of the section. 56. Magnetitic clay-slate, from near base of formation. Specimen 9139 (slide 2911), 1200 N., 1975 W., Sec. 6, T. 45 N., E. 2 E., Wisconsin. The specimen differs from 55 only in being of a darker color. Composition : Silica, 53'44 ; alumina, 19-62; iron sesquioxide, 11-38; ii-on protoxide, 5-35; manganous oxide, trace; calcium oxide, -42; magnesium oxide, 1-58; potassa, 1-73; soda, 2-61; lithia, trace; water, 4-07; phosphoric acid, trace: = 100-20. 328 THE PENOKEE lEON-BEARING SERIES, The thin section differs from that of 55 only in that its matrix and the contained crystals of magnetite are both somewhat coarser. 57. Ohloritic graywacke, from a lower middle horizon. Specimens 9078 (sUde 2778); 9079 (slide 2779), 0 N. to 100 N., 0 W., Sec. 36, T. 46 N., R. 1 E., Wisconsin. The rocks are like 52 and 53. As in 52, 53, and 54, in thin section, fragmental particles of quartz and feldspar compose a large portion of the rocks. The quartz grains are usually enlarged. The feldspars comprise both orthoclase and plagioclase and are frequently quite fresh. The abundant matrices consist of finely crystalline quartz, of chlorite, of seri- cite or kaolin, and of opaque brown or black ferrite. In a few places small flakes of biotite are seen. In fragmental material and siUceous paste, as in other particuhxrs, these rocks are typical graywackes. 58. Ohloritic graywacke, from a middle horizon. Specimen 9077 (slides 3296 and 3377), 700 N., 0 W., Sec. 36, T. 46 N., R. 1 E., Wisconsin. The rock is like 57. The thin section is like those of 67. Section betiveen the east range line of B. 1 U., Wisconsin, and the west branch of the Montreal. 59. Ohloritic graywackes, from a middle horizon. Specimens 9068 (slide 2922), 1115 N., 1560 W., Sec. 28; 9066 (slide 2889), 1040 N., 160 W., Sec. 29, T. 46 N., R. 2 E., Wisconsin. The specimens are almost exactly like 54. The thin sections in no essential points differ from that of 54. 60. Ohloritic graywackes, from an upper middle horizon." Specimens 9071 (slide 3295), 1380 N., 180 W.; 9072 (slide 2890), 1400 N., 605 W., Sec. 29, T. 46 N., R. 2 E., Wisconsin. The rocks resemble closely the previously described graywackes. The thin sections are in essential particulars like the chloritic graywackes pre- viously described. The fragmental grains of quartz are always distinctly enlarged. While many of the feldspar areas are quite fresh, others are badly kaolinized and chloritized. The abundant matrix is of the same composition as in the previous chloritic graywackes. It, however, contains a little biotite and muscovite. Here and there are black opaque areas which appear to be ferrite, but which may contain pyrite or carbonaceous material. 61. Ohloritic graywacke, from an upper middle horizon. Specimen 9070 (slide 4415), 1950 N., 1540 W., Sec. 28, T. 46 IST., R, 2 E., Wisconsin. The rock is precisely like 60. The thin section is rather finer grained than that of 60; otherwise it is like it. THE Ul'l'Elt SLATD MEMBER. 829 Section ea^t of the went branch of the Montreal and in the vicinity of the Montreal river. 62, Graywiiclce-slate, near base of ronnation. Siu'c.iiiicii !K)30 (slide 2917), 530 N., 1470 W., Sec. 24, T. 40 N., K. 2 E., Wisconsin. The rock is dark brown, tine grained, finely banded, and cleaves readily along the plane of lamination. In tliin section fragmental (juartz and feldspar in grains ol' small size compose perhaps oue-half of the mass of the rock. The grains of feldspar arc mostly mnch chloritized or kaoliuized. The particles of qnartz have frecjucntly been enlarged. The abnndaut tilling material is chlorite, finely crystalline (piiirtz, dark brown oxide of iron, and black iron oxide, a portion of which appears to be magnetite. This rock resembles very closely the fragmental rocks found at the base of the Upper slate at Black river. 03. G-raywacke-slate, from a lower horizon. Specimen 9100 (slide 2928), 71.5 N., 1700 W., Sec. 24, T. 40 N., R. 2 B., Wisconsin. The rock differs from 02 only in being of a dark green color. The thin section differs from that of 02 only in that some of the iron oxide is magnetite. 04. Chloritic graywacke-slate, from a lower middle horizon. Specimen 9020 (slide 2882), 1350 N., 1000 W., Sec. 22, T. 47 N., R. 47 W., Michigan. The rock is dark greenish-gray, very fine grained, and finely laminated. In thin section the rock differs from the graywackes just described chieHy in being much finer grained. There is much fragmental quartz, and feldspar appears, but it is very fine grained, while the matrix is extremely so. 05. Chloritic graywacke, from a lower middle horizon. Interstratified with 04. Specimen 9027 (slide 2883), 1350 N., 1000 W., Sec. 22, T. 47 K, R. 47 W., Michigan. The rock is gray, medium grained, massive, and breaks with conchoidal fracture. In thin section the rock is a typical graywacke. It differs from that of 54 only in being slightly finer grained and in containing calcite. 00. Chloritic graywacke, fi-om an upper middle horizon. Spe(;imen 9103 (slide 2925), 1150 N., 1100 W., Sec. 22, T. 40 H"., R. 2 E., Wisconsin. The rock is like 54. In most points this section is like the previously described graywackes. Many of the fragmental feldspars, however, are large and fresh. They comprise both ortho- clase and plagioclase, a portion of the latter being microcline. The alteration of feld- spar to kaolin or inuscovite is nicely illustrated, some of the feldspar areas contain- ing very numerous flakes. 07. Chloiitic graywacke, from an upper horizon. Specimen 9034 (slides 2884 and 2918), 555 N., 1445 W., Sec. 14, T. 40 N., R. 2 E., W;isconsiu. The rock is gray, medium grained, massive, and breaks with conchoidal fracture. 330 THE PENOKEE lEON-BEARING SERIES. The thin sections closely resemble that of 54. Very plainly fragmental quartz and feldspar in grains of medium size compose two-thirds of the sections. The qviartz grains are often slightly enlarged. The feldspars are frequently fresh and often also much altered, the alterations resulting in the formation of sericite or muscovite, chlo- rite, and perhaps a little biotite. The matrix is like those of the previously described chloritic graywackes, except that sericite or muscovite is here present. 68. Chloritic and sericitic graywacke, from near top of series. Specimen 9032 (slide 2770), 720 N., 1280 W., Sec. 14, T. 46 N., E. 2 E., Wisconsin. The rock is like 67, except that it is finer grained. The thin section is finer grained than that of 67. It also differs greatly from it in that it has a much more crystalUne appearance, equaling in this respect many of the chlorite-schists west of the south end of Gogebic lake. The crystalline appear- ance is made still stronger by the fact that while the quartz particles are exceedingly angular, it is with the greatest diflflculty that enlargements can be seen to cause this angularity — in most grains quite impossible. The mineral constituents are identical with those of 67, but sericite is much more plentiful than in it, and both the sericite and chlorite are mostly in rather small, well defined leaflets. That all of these two minerals are secondary to feldsjiar can not be shown, although a large portion of them certainly is of this nature. However, from the association with and likeness to 67, it can not be doubted that both are of like origin — that is, altered mechanical sediments. It is also certain that the crystalline appearance of 67 is due to the prev- alence of the sericitic and chloritic alteration of feldspar, combined with the separa- tion of quartz in the interstices and the enlargement of the quartz particles. Section at and west of Blach river. 69. Chloritic clay-slate, from a middle horizon. Specimen 9194 (slide 2932), 1200 N., 1140 W., Sec. 13, T. 47 E"., E. 47 W., Michigan. The rock is gray, aphanitic, and shows a slaty cleavage which cuts across the bedding plane. The section is exceedingly fine grained. It appears to consist of a confused mix- ture of chlorite, kaolin, quartz, ferrite, mingled with which are here and there a few larger grains of quartz which appear to be clastic. 70. Pyi-itic clay-slate, from a lower horizon. Specimen 12530 (slide 5334), 1000 N., 1300 W., Sec. 17, T. 47 E., E. 46 W., Michigan. The rock is light gray, very fine grained, and contains a large amount of pyrite, both disseminated through the rock and in large irregular masses. The thin section consists of a confused mass of minute particles of quartz, feld- spar, chlorite, kaolin, ferrite, and perhaps other clay-forming minerals. Contained in this material are abundant crystals and areas of pyrite. ■* 71. Chloritic graywacke, from an upper horizon. Specimen 10417 (sMe 4021), 800 N., 1340 W., Sec. 10, T. 47 N., E. 46 W., Michigan. TIIK UPl'Kli SLATK I\l KM HKIt. 331 Till' rock is ji'i'ii.Vt ••''!• I'alluM- I'hk^ imiforiii fj^raiii, and iiiassivc. Ill tiic tliiii section, tV;iray\vaeke, from n lower middle horizon. Specimen '.)517 (slide 2980), 325 N., 715 VV., Sec. 10, T. 17 N., Ji. 40 W., Michigan. The rock is gray, rather flue grained, massive, breaks with subconchoidal frac- ture, and contains large fragments of black cherty material. In thin section, rather large fragmental particles of quartz and feldspar, the two minerals being in about ecpial abundance, compose the greater part of the rock. The grains of (piartz are usually quite widely enlarged. A ])ortion of the feldspar is relatively fresh, but the greater part of it is much chloritized and kaoliuized. The interstitial material consists of quartz, chlorite, kaolin, ferrite, with some biotite and muscovite. 73. Micaceous graywacke-slate, from a lower middle horizon. Specimen 9518 (sUde 3090), 325 N., 715 W., Sec. 10, T. 47 N., R. 40 W., Michigan. The rock is dark gray, fine grained, finely laminated, and the cleavage surface shows numerous glittering flakes of mica. In thin section, rather small clastic particles of quartz and feldspar compose about one-lialf the bulk of the rock. The grains of quartz are generally enlarged and the feldspars are usually much decomposed. The interstitial material consists of quartz, chlorite, biotite, muscovite, and ferrite. Folia of muscovite and biotite are arranged with their longer axes in a common direction, as though the rock had been subjected to squeezing. 74. Ohloritic clay-slate, from a Ioav horizon. Specimen 9515 (slide 2^5), 250 N., 1750 W., Sec. 11, T. 47 N., E. 40 W., Michigan. The rock differs from 09 only in that its cleavage and bedding correspond. The thin section is like that of 09. 75. Clay-slate, from base of formation. Specimen 9493 (slide 44132), 700 N., 1015 W., Sec. 12, T. 47 N., E. 40 W., Michigan. The rock is dark olive green to black, aphanitic, finely laminated, cleaves par- allel to the bedding and also in another direction, cutting across the first at an obtuse angle. Included are particles and nests of pyrite. The thin section is excessively fine grained. It appears to consist of finely crys- tallized quartz, partly amorphous silica, chlorite, feldspar, ferrite, and crystals of pyrite. Quite a x^roportion of the quartz and feldspar are, however, coarse enough to show that the rock is a fragmental one. 332 THE PBNOKEE lEON-BEARlNG SEEIES. SECTION II.— ORIGIN OF THE UPPER SLATE ROCKS. From what has preceded, it is evident that the association of the mica- schists and mica-slates of the Upper-slate member with the graywackes and graywacke-slates is of a most intimate nature, both as to corriposition and occurrence. The same set of minerals occur in both classes of rock; they are closely interstratified with each other. There is an identical change in each class in mineral composition in passing from east to west, and in any one region the change occurs simultaneously in both classes. Finally, there is every gradation between them, and the placing of many specijfnens in one class rather than in the other is somewhat arbitrary. It becomes, then, probable that the original condition- of the graywackes and graywacke-slates and the mica-schists and mica-slates must in the main have been the same. As some of the graywackes are completely frag- mental, others somewhat crystalline, others still more crystalline, thus grad- ing into ' the crystalline mica-schists and mica-slates, we will begin with those which are nearest their original condition and trace the processes step by step in which the original rock has been changed into a fully crystal- line mica-schist. A general notion of the graywackes and graywacke-slates has already been given. Before proceeding to trace these jjrocesses of alteration it will be necesaftry to describe in more detail the graywackes which are near their original condition. Macroscopically the graywackes vary from tolerably coarse grained to aphanitic. Many of them are apparently completely massive in hand specimen, and such break with conchoidal fracture, although even these in exposure show more or less of a cleavage along the bedding planes. Specimens of the graywacke-slates show a decided tendency to cleave parallel to the bedding. In color these rocks vary from light gray or light green, through various shades of gray and green, to almost black. Under the microscope the only difference between the graywackes and graywacke- slates is one of fineness of grain. The least altered varieties of them may be divided into two classes, one being composed mostly of tolerably large well rounded particles of quartz and feldspar (PL xxxii), and the other THE UPPER SLATK MEMBER. 833 having' feldspar prcdoiniiiant, little or no (|iiartz being present (PI. xxxiii, Fig. 1). riie feldspars conunonly comprise orthoclase, niicrocline, and plagioelase. Usually some of the feldspars are altered to a greater or less extent to chlorite, sericite, muscovite, biotite, and kaolin. In the inter- stices there may be a small quantity of finely crystalline quartz. In many cases, however, the graywackes and graywacke-slates are not so simple in composition. Mingled with the larger particles of fragmental quartz and feldspar are finer particles of the same sort, with other minerals (PI. XXXII, Fig. 4). When this finer silt is preponderant the graywackes and graywacke-slates grade into the clay-slates or phyllites. Naturally, in a belt in which the above described simple graywackes and clay-slates both occur there would be found gradations between them. Freqiieiitly in the same specimens we find mingled coarse and fine material. As tlie quantity of finer material increases it becomes increasingly difficult to trace the exact changes which have taken place in the minerals. Unless all of the material is excessively fine, i. e., unless the rock passes into a clay-slate of the finest possible sort, the processes of change subsequently described are seen to have taken place with the larger particles of quartz and feldspar in these fine grained slates. The graywackes, besides showing great variation in appearance, due to the mingling of coarser and finer material, occa- sionally contain so much ferrite — brown iron oxide — as to make this sub- stance a chief constituent. Pyrite, some carbonate (calcite, dolomite, or siderite), and rarely carbonaceous material are also quite often present as more or less plentiful accessories. (1) Quartzose grayivacke (PI. xxxii). — In the freshest of the quartzose graywackes many of the particles of feldspar are as unaltered as in ordinary granite, but generally they have decomposed to a greater or less extent. This decomposition has ordinai'ily taken place to a greater degree near the exteriors of the feldspar particles than in their centers. In the particles in which the decomposition has been somewhat extended the alteration has atfected the regularity of the original fragmental oval out- lines. Upon the other hand, even when alteration has progressed quite to the centers of the feldspars, the original outlines may at times be quite sharp. The minerals which have resulted from the partial alteration, and 334 THE PENOKBE lEON-BBAlUlsrG SB1{1ES. which therefore are iiiduded by or closely encircle the feldspar, are chlorite, sericite or muscovite (or both), biotite, probably kaolin, and quartz. In the grains which have decomposed to the greatest extent, the original feldspar, remaining and the mica and quartz most intricately interlock, so that an examination with a moderately high power which covers only one original individual of feldspar or a part of one gives the appearance of a com- pletely crystalline rock in which the interlocking is of the most intricate sort; yet when the same grain is examined with a low power its rounded character is evident, and that the area is but an altered feldspar is manifest, while the completely fragmental character of the rock as a whole is plain at a glance (PL xxxii. Figs. 2 and 3). The fragmental gVains of quartz, although now unusually sharply angular, often show with perfect clearness well rounded cores, their present angularity being due to a renewed growth subsequent to their deposition in their present resting place. Commonly the majority of the grains of quartz are simple, but at times many of tli^m are more or less finely com- plex, or even of a chalcedonic character. The quantity of this kind of quartz varies greatly, rarely becoming almost or quite as abundant as the simple quartz fragments. In the interstices of the rock is found usually a little finely crystalline quartz, which is of a secondary nature, as plainly so as are the enlargements of the fragmental quartz grains. In thQ simplest quartzose graywackes and gray wacke-slates little else is present. In these specimens it is evident that the induration which has often occurred is caused almost wholly by the eidargement of quartz fragments and the separation of finely crystalline quartz in the interstices, thus completely filling the spaces which originally existed between the loosely piled frag- ments and making the rock as compact and strong as tliough it were a granite. The green and gray colors of the rock (in the absence of fen-ite) are due to the secondary products of the feldspar, chlorite, biotite, etc.; -green when chlorite, gray when muscovite and biotite are preponderant. The original conditions and secondary changes of the simplest of the gray- wackes are thus clear. They were quartz-feldspar sediments, mingled in places with a little clayey matter, perhaps also with a small quantity of fragmental mica and some ferrite. They reached their present condition TlIK Ul'lMiU 8LATE MKMliJOK. ' 335 by a secondary enlargement of quartz fragments; tlic deposition or forma- tion in situ of interstitial finely crystalline quartz, accompanied with a micaceous or chloritic alteration of the feldspar — the first two being pro- cesses already fnlly described by us.' Before following further the series of changes which explains the met- amorphosis of fragmental sediments to mica-slates and mica-schists, it is important to recall the chemical changes^ which occur in the alteration of orthoclase, raicrocline, and oligoclase to chlorite, muscovite, andbiotite. The average content of silica of the following minerals is taken from Dana's System of Mineralogy : Orthoclase and raicroline, 65 per cent ; oligoclase, 62 per cent; muscovite, 45 per cent; biotite, 40 per cent; chlorite, 25 to 30 per cent. Evidently where the alterations of orthoclase, microcline, and oligoclase to muscovite, biotite, and chlorite have taken place so extensively as in the rocks under discussion, it is not difficult to explain the presence of the silica which lias enlarged the fragments of quartz, replaced those of feldspar, and separated as independent interstitial quartz. One of these alterations is stated by Tschermak' to occur as follows: "Wenn man die dreifache Formel des Feldspathes 3 (Kp-AiP;6SiOJ mit jener des daraus entstandenen Ghmmers Kp-Aip^.2SiO,+ 2 (H 0-Aip;2SiOJ vergleicht, so ergibt sich, dass von der urspriinglichen Menge 6810^ nur 2SiO^ in die neue Verbindung libergehen und 4SiO^ iibrig bleiben." In further speak- ing of the alteration of orthoclase to muscovite, Tschermak also observes : " Der neue entstandene Muscovit ist ofters auch von Biotit (Magnesiaglim- mer) begleitet." For the iron of the biotite and chlorite in the rocks under considera- tion, it is not difficult to account. Pyrite, marcasite, and ferrite are quite widely present in these rocks as accessory constituents. Often the relations of the pyrite or marcasite and biotite (folia of the latter surrounding particles of the former) are such as to lead to the supposition that the former min- ' On Secondary Enlargements of Mineral Fragments In Certain Rocks. E. D. Irving and C. E. Van Hise. Bnll. U. S. Geol. Survey, No. 8. '^I insert freely, without quotations, here and in the following pages, such parts of an article bv me on this subject, already published — Am. Jour. Sci., 3d series, vol. xxxi, 1886, pp. 453-459 — as can be used. ' Lehrbuch der Mineralogie, zweite Auflage, p. 462. 336 THE PENOKEE IRON BEAEING SEEIES. erals have furnished the iron necessary for the transformation from feldspar to biotite. At all events, they indicate a sufficient supply of iron. For a part at least of the magnesium of the biotite and chlorite, it seems that we must look to some source extraneous to the. feldspar frag- ments; i. e., we must regard it as having been supplied by some other mineral or by percolating waters. That calcium-bearing and magnesium- bearing waters have been present in these rocks is evident from the occa- sional presence of secondary calcite and dolomite. Partial analyses of three of the biotite-schists gave an average content of MgO of 2.22 per cent, which if entirely contained in the biotite would correspond to a probable proportion of that mineral of from 10 to 20 per cent.^ (2) Muscovitic and hiotitic graywacJce (PL xxxiii, Fig. 1). — Macroscop- ically, this rock is gray, medium grained, and massive. It breaks with a conchoidal fracture. Under the microscope large fragments of feldspar, with the alteration and replacement products of the latter, compose the rock. Most of the feldspar is orthoclase, although both microcliue and plagioclase are present. Much of it is quite fresh, many individuals showing no altera- tion further than a slight kaolinization. Other feldspar fragments, however, include in each many grains of quartz, or a single large reticulating quartz individual and numerous flakes of muscovite and biotite. Here the quartz, muscovite and biotite are plainly replacements and alteration products of the feldspar. In rare cases this alteration has proceeded so far as to leave but irregular,' partly replaced and altered cores of feldspar which are entirely surrounded with the secondary quartz, muscovite, and biotite. The finer grained parts of the rock are composed of finely crystalline quartz, a portion of which may be clastic; of feldspar, the proportion being smaller than in the coarser parts, probably on account of the more extended alterations in the small particles; and of biotite and muscovite. The mica is here clearly also to a very large extent secondary to feldspar, while there is little doubt that the small r^emaining fraction of the mica is of the same origin. Scattered through the finer portions of the section are ■ Lehmaim, in his work on the "Entsteliung der altkrystallinischeu Schiefergesteine," demon- strates the formation of abundant secondary biotite and other minerals as accompanying metamor- phoses by folding. His work does not state, however, from what the biotite developed. THE UPPER SLATE MEMBER. 337 numerous small particles of ii liliuk substance which is taken to be partly altered pyrite or maicasite, and perhaps partly carbonaceous material. The induration in the rock is due to the formation in situ of ([uartz, or in- filtration of silica, from which finely cry.stalline quartz has formed, assisted by the partial transformation of the feldspars into intei'locking complex areas of quartz, mica and feldspar, which also interlock with the material of the matrix: (3) Biotitic f/mywacke (PI. xxxiii, Fig. 2). — Macroscopically, this rock differs from (2) only in being" of a darker gray color; and under the inicro- scope also it is much the same, except that micaceous alteration of the feld- spars has been carried farther. Fragments of feldspar, with its secondary products, compose most of the section. The alterations of feldspar to bio- tite and quartz are beautifully shown. The freshest of the feldspar grains are surrounded and more or less deeply .penetrated by secondary biotite. These grains yet retain their well rounded forms. However, in many cases, the original outlines of the grains of feldspar are lost, although often the complex aggregate of resulting biotite folia ai^d quartz, mingled with the re- maining feldspar, retain very perfect general roundish or oval forms. Often the entire surfaces of the feldspars include very numerous particles of the biotite and quartz — the former usually much the more plentiful — there re- maining through such areas here and there little spots of feldspar which act as a unit in each area. With a low power such areas appear to be roundish aggregates of biotite. It is only with a higher power that, the remaining feldspar and its true relations to the biotite are discovered. In this rock all the stages of the ])rocess of alteration of the feldspars are nicely shown from those areas in which the secondary biotite forms but a film around the feldspars to those in which no feldspar remains, there being in the place of the fragmental feldspars interlocking aggregates of biotite and quai'tz. Even these areas often so perfectly retain their general oval or roundish character that, taken by themselves, they would be regarded in the sections as peculiar complex fragments, which in this condition had become worn and deposited. Taken in connection with these other grains present, there can be no doubt of their formation by the alteration of feldspar. As from a single large particle of feldspar many individuals of quartz and mica are 338 THE PENOKEE IRON-BEAEmG SERIES. formed, the result of the alteration is to make the rock a finer grained one. The rather sparse matrix of the rock does not differ materially from that of (2), except that it contains more mica. The induration is caused by the same processes as in (2). (4) Muscooitic biotite-slate (PI. xxxiii, Figs. 3 and 4). — Macroscopically, this phase of rock is mottled dark gray and black, quite massive. The mottlina' is due to more or less distinct roundish areas, which show in a greater or less degree cleavage. In some of the specunens the roundish areas are distinctly outlined, and in these cases the cleavage is eminent. In others, these areas are indistinctly outlined, and in such the cleavage is less plain. Under the microscope the grains showing cleavage macro- scopically are found to be well rounded, partly altered feldspars, mostly of the species orthoclase. These feldspars are set in a groundmass which consists of intimately mingled minute grains of quartz and brown folia of biotite with a considerable quantity of ferrite. In some specimens the quantity of matrix is considerable, even equal or greater than the known feldspar fragments and the material coming from them. In other cases, however, the feldspar areas were set very close together with room for little other material. The partial decomposition of the frag- mental feldspar has resulted in the formation of very numerous small folia of biotite, fewer larger ones of muscovite, and also quartz. In the fresher feldspar areas the secondary mica is found most plentifully at or near the exteriors of the grains, although in almost every case the alteration has proceeded to a greater or less degrees quite to tlie centers. In some speci- mens most of the original feldspar grains are yet cliiefl}^ unaltered, and in these the mottling and cleavage, seen macroscopically, is most distinct. In other specimens nearly every feldspar grain has almost wholly altered to mica and quartz, and in such specimens, those which are most completely altered and now therefore consist of a complex interlocking aggregate of quartz and mica, with little or no remaining feldspar, are with great diffi- culty separated from interstitial material. Many also are doubtless so completely changed as to be indistinguishable from the matrix. In the matrix of the rock it is usually quite impossible to determine if any of 'the quartz is fragmeutal. In some specimens some grains are certainly so, as THE UPPER SLATE MEMBER. 330 evidenced by rouuded cores avid secondary enlarfi^einents. The biotite of tlu' matrix is in part plainly secondary t«i feldspar and is iircciscl)- lik(! that t'ound in the laro-er feldspars The folia are deep l)n)\vn and very strongly dicliroic, and therefore probably bear a, hirg-e percentage of iron. Donbt- less much of this matrix is due to the decomposition of fragmental feldspars, which were smaller than those which yet remain partly unaltered, and which have therefore completely altered to mica and quartz. The feldspar plainly shows this rock to have been fragmental, and the alteration of feldspar to both biotite and muscovite upon a large scale is most beautifully shown. The large quantity of dark brown and black ferrite has doubtless furnished the iron required for the formation of the biotite. The peculiar spotted appeai-ance of the sections, the distinctness of the spots varying with the freshness of the feldspar, viewed without the microscope, gives a clear idea, when taken in connection with their appear- ance under the microscope, of the processes by which the rock reached its present condition. This 'rock must as deposited have been a tolerably coarse grained one, many of the larger, fresher feldspars averaging about 1 mm. in diameter, but the alteration of each of these most changed feldspars in the specimens to a vast number of mica folia and quartz grains has caused the rock to become exceedingly fine grained, there remaining, however, ^Jerfect proof even in these specimens of the original rouuded fragmental character of some of the feldspar. (5) Nearli/ cnjstalline muscovitic biotite-schist (PI. xxxiv, Fig. 1). — Macro- scopically, this rock is fine grained, grayish, and quartzose, with small mica flakes visible.^ Under the microscope the thin section shows a fine grained groundmass of quartz and feldspar in which are abundant muscovite and biotite. Many of the quartz grains include numerous folia of mica. The feldspar areas include quartz and both muscovite and biotite. This section by itself if not examined closely would be taken to be of a completely crystalline mica-schist in which the interlocking and mutual inclusions of the different minerals are of the most intricate kind, but, like the other mica-schists of the Penokee series, it is an ordinary clastic rock in which the metasonaatic changes have gone very far. The large areas of quartz, 340 THE PENOKEE lEON BEARING SEEIES, iiiclu(Jing folia of mica, are in the places of feldspar fragments. As the feldspar has altered to mica the excess of silica has separated as quartz. Frequently the alteration of a single feldspar has resulted in the formation of a single ramifying individual of quartz with several or many included folia of mica, mingled with which are detached remnants of the feldspar. In this rock in such cases the mutual interlocking of these four minerals, muscovite, biotite, quartz, and feldspar, could not possibly be more inti-i- cate in any schist, gneiss, or granite. In other cases the decomposition of a feldspar has resulted in the formation of many grains of quartz as well as numerous folia of mica. In yet other cases the feldspar areas have not altered to such an extent as described above. In almost every case the rounded exteriors of the clastic grains are lost, but irregular areas of con- siderable size often remain which include but few folia of biotite or little replacing quartz, or both. The folia of mica in this rock are the largest anywhere found in tlie mica-schists, many of them being 1 mm. in length. The biotite has a remarkably strong dichroism, and both biotite and mus- covite have well defined cleavages. (6) Crystalline muscovitic hiotite-schist (PI. xxxiv, Fig. 2). — Macroscopic- ally, this rock is rather fine grained. It has a finely laminated structure, along which the rock most readily breaks, although it is massive enough to break quite easily across the direction of lamination. The individuals of quartz and feldspar are too small to be seen with the naked eye, but are distinguishable mth a lens. The very abundant black, glittering flakes of mica are large enough to be readily seen. A lens shows the rock to con- tain a considerable quantity of pyrite. Under the microscope with a medium power the sections show a rather fine grained, apparently completely crystalline, typical mica-schist. The groundmass consists chiefly of quartz, mingled with which is con- siderable feldspar, both orthoclase and plagioclase. The grains of quartz vary considerably in size, but none are minute and none large, the largest being not more than ^ mm. in diameter. Some few of the largest, which are now very irregular in outline, contain rounded cores, proving them to be fragmental, and these cores do not include folia of mica. Much of the feld- spar present is quite fresh, many of the plagioclases exhibiting sharply TllK UITKE yLA'JK IMiaiBEK. 341 defined t\vinniiij>- hands and these may he secondary developments, liiotite in well ddined folia of var)inTains and into those of the feldspars, just as in a typical cr}stallin(; schist, "^riuit much of the mica is a secondary product can not be proved from the sec- tions from the locality of English lake, taken by themselves. A portion of it is, however, certainly of this nature. Many grains of feldspar are so cut by folia of mica as to make it probable that the latter is a secondary product, while some of the larger particles of feldspar contain throughout their areas folia of mica and grains of quai'tz which, with the remaining feld- spar, give an appearance of most intricate interlocking, as in no case do the grains of feldspar now show the rounded outlines which they once doubtless had. This mica included in the feldspar is precisely like that of the remainder of the rock — the word matrix for remainder can hardl}^ be used, for in coarseness of grain the parts of the rock which show no trace of fragmental origin are about the same as the quartz and mica included in the feldspars, and which, taken in connection with the rocks previously described, are regarded as proving that this rock, like them, was once altered sediments. Quite immerous crystals of pyrite are seen. The rocks (2) to (6), above described, thus constitute a graded series from the fresh feldspathic graywackes to the crystalline mica-schists. The gaps here left in these typical descriptions may be filled in by follow- ing the detailed descriptions of the graywackes and graywacke-slates and mica-slateS and mica-schists (pages 309-326). Here the wide inter- val between the most plaiidy fragmental graywackes and the most crys- talline mica-schists is completely filled by almost imperceptible stages, and the processes which caused the alterations are clearly indicated; the result heing the production from a completely fragmental arkose rock, by meta- ■ somatic changes only, of a rock which presents every appearance of complete original crystallization, and ivhicJi tvotdd be ordinarily classed as a genuine crys- talline schist. Black mica-slates (PI. xxxiv, Figs. 3 and 4).— The series of alterations above described has also been very important in the production of various 342 THE PENOKEE IRON-BE AEING SERIES. black mica-slates of the Penokee series. Macroscopically, these slates are all exceediiig-ly fine grained and finely laminated, cleaving readily along the planes of lamination. In color they vary from dark gray to black. A lens shows many of them to contain numerous small particles of pyrite. Very numerous roundish black areas are contained in the fine grained, gray material in many of the specimens. These areas in sontie cases show distinct cleavage surfaces and are taken to be large fragmental particles. In other cases they are dull and bi-eak without giving cleavage surfaces, while in yet other specimens these darker spots are not found at all. Under the microscope the rocks which have the mottled appearance described above consist of two parts, a finely crystalline matrix and coarse, well rounded fragmental feldspars, which are always altered to a greater or lesser extent. This alteration is to biotite and quartz, many small folia of biotite and grains of quartz always being found in a single feld- spar indi^ndual. Every gradation of this change is seen, from grains of feldspar which contain but little biotite and quartz to those in which the remaining feldspar is just sufiicient in quantity to enable one to perceive that the detached particles are parts of a common individual. Doubtless also the alteratio'n to biotite and quartz has often been carried out com- pletely. Accompanying the biotite, thus secondary to the feldspar, is much black, opaque, partly altered pyrite in minute particles. The black, roundish spots seen macroscopicall}- are evidently the partly altered feld- spar fragments. The degree of this alteration as determined by micro- scopical study corresponds exactly with the appearance of the rock as seen in the various hand specimens. In the specimens in which the feldspar areas are well outlined and show clearly marked cleavage siu'faces, the biotitic and qviartzose alteration has taken place to but a small extent. In the specimens in which the feldspars are obscurely outlined and which lack cleavage the alteration has extended very far. The matrices of these rocks and the sections of those hot containing large grains of feldspar are com- posed of intimately mingled quartz, feldspar, biotite, and pyrite, with perhaps a little organic matter.^ A portion of the quartz is certainly frag- 'Geol. AVis., vol. iii, p. 136. Tin-] UIM'HR SLATK MlCMl'.Klt. 343 mental, as is evidenced l)y secondaiy onlar<>-enient. Tin- hiotitc is all believed to he due to the alteration of feldspar, iiiucli of it certainly being of this natnrc. The matrices of the ditterent sections vary in coarseness and in the relative proportions of iho viirions niinerid constituents, but are alike in all essential jjoints. We have here plainly a series of locks which are parallel to (4), the chief differences between the two sets of rocks being that the black mica-slates are much finer grained, that they contain more pyrite, and also contain carbonaceous material. As the unaltered or partly unaltered feld- spars are as large as in (4), it is probable that the greater fineness of g-rain in the black mica-slates at present is simply due to the fact that the feld- spars altered to smaller individuals of mica and quartz in them than in (4). Source of material— It will be later shown that between the Penokee series and the Southern Complex there is a great structural break, the clastic series having been laid down b}^ water action upon the older crys- talline formations. From the description of the Upper slate, it is evident that the most of its material has been derived from a set of acid rocks, its two predominating constituents being quartz and acid feldspar in various degrees of comminution. There is no reason to believe that the Southern Complex, which stretches a great distance southward, was ever covered throughout the Avhole of its extent by this newer series ; and as the mate- rial of which this upper belt is composed is of precisely, the character which, would be expected if derived from its granite, gneiss, and schist, it may be considered as probable that this is the source of the material of the Upper slate. This hypothesis ]-eceives reenforcement when the distribu- tion of the granitic and schistose areas is considered, and the material which the}' are able to supply is compared with that of the material of the Upper slate member at adjacent points. With the exception of a few miles the newer series of rocks is directly in contact with areas of green schists as far west as Penokee gap. Without doubt the contiguous granite in the we.stern part of Michigan and south of the schist areas has also furnished its quota of material ; so that it has come, not from the schist alone, or the granite or gneiss alone, but from both of them. In passing westward toward Bad river, as has been seen, there is a 344 THE PBNOKEB lEOI^-BEAllING SERIES. gi'adual change in the nature of the original material of the Upper slate, it becoming more and more strongly feldspathic ; west of Bad river prac- tically all of it was feldspar of an acid character. By reference to chapter i it will be seen that the Western granite, which reaches the newer series in the vicinity of Penokee gap, extends west and south for a long distance. The granite contains an unusuallj^ small quantity of quartz, being either a true syenite or a granite in which tlie quartz is usually not abun- dant There is clearly a connection between the character of this part of the Upper slate and the underlying rock. The degradation of the latter, accom- panied by natural sorting, has furnished the nearly pure feldspar material which originall}^ constituted the Upper-slate member and made it of such a composition that it was possible by metasomatic processes to produce from it a fine grained crystalline mica-schist. Summary . — 1. The rocks of the Upper-slate member are mechanical sediments, which have been everywhere altered to a greater or less extent by metasomatic changes, and at times the alterations have extended so far as to produce crystalline schists. 2. In general, the eastern part of the formation is less altered than the western part. • Here the pre\^ailing rocks are clay-slates, graywackes, and graywacke-slates. In passing to the westward the rocks become more crystalline in character, and at the extreme west end only mica-slates and mica-schists are found. 3. While this is true in a general waj^, in the same vertical section there may be found all phases of the transformation from completely frag- mental rocks to crystalline schists. As far east as Sec. 14, T. 46 N., R. 2 E., Wisconsin, a rock Avhich is very close to a crystalline schist is found. At the Penokee gap section the most completely fragmental gray- wacke of the whole member occurs, and in this section there are all grada- tions to that but one degree removed from the most crystalline phases found in the formation. Even in a single exposure in this section are found plainly fragmental graywackes and mica-slates, and micaceous graywackes which approach very close to a mica-schist. 4. The parts of the Upper slate which have received large fragmental particles of quartz are those in which the clastic character is easiest to rec- Till': UPPKK SLATE MRMHER. 345 ognizp, for the grains of (jiiartz iilways roiuaiu in tlioir entirety. It may be, and intleed usually is, the case that they have undergone a second growth and have thus become angular; l)ut generalh' tlic original cores are easily discovered. In the nc!;irl}- pure feldspar sediments, upon the other hand, when the feldspar has changed to other minerals, it is more difficult, and, taking a specimen of the most crystalline mica-schist by itself, impossiljle to make out the original fraoinental character of the rock. 5. The more crystalline mica-schists are derived from nearly pure arkoses and in all cases the resultant schists are nuich tiner grained than were the original sediments, for from each fragment of feldspar there has been produced several or many individuals of mica and quartz. 6. The material for the Upper slate has been derived from the Southern Complex, and there is a dii'ect connection between the character of the rocks to the south and those of the slate belt adjacent. The greater part of the belt has received its material in part from the granitic and in part from the schistose areas ; while the part of the belt west of Penokee gap has received nearly all of its material from the syenitic granite to the south and west. CHAPTER VII. By C. E. Van Hise. THE ERUPTIVES. structural relations. General character of the rock. Comparison of Penokee greenstones with greenstones of the Southern Complex and Keweenaw series. Microscopical character of the diabases. Eruptives in the Iron-hearing member. Summary. Structural relations. — The eruptives of the Penokee series are structur- ally of two classes. Some are sheets, while others are dikes. The field relations of many ledges, however, are not well enough exposed to show to which class- they belong. The exposures in T. 44 N., R. 5 W., Wis- consin, are large, and there seems to be every indication short of dem- onstration that these rocks are really interleaved with the sediments of the Iron-bearing member. In the east part of T. 47 N., R. 46 W., Michigan, and the west part of T. 47 N., R. 45 W., Michigan, the iron- bearing belt has an unusual width. Here the eruptives are particularly abundant, as shown by natural exposure and by test pitting, and the rela- tions of the greenstones and the iron-bearing rocks are such as to indicate that to some extent the greenstones are interleaved, but there is not suffi- cient evidence of this to enable one to make the assertion without qualifica- tions. There is no absolute proof whether these interleaved greenstones are contemporaneous volcanic outflows which interrupted the deposition of the iron belt, or subsequent intrusions. All the evidence at hand is of a lithological character, and it points to subsequent intrusions rather than to contemporaneous flows. The rocks are medium grained, holocrystalline, nonamygdaloidal, and do not contain minerals in two generations ; proper- 346 THE ERUPTIVK8. 347 ties which are ordinarily takou as indicating- that the rocks possessing them have solidified at depth. The presence of very numerous dike-rocks in the iron-bearing belt has become apparent through exploring aiid mining operations. These dikes are much altered and soft, and therefore they do not outcrop, so that their existence would never have been otherwise suspecte a. The hornblende is usually paramoi'phic, a single large individual of hornblende resulting from one of pyroxene. The rare diallagic variety of pyroxene has generally altered to a green fibrous amphibole, which is taken to be smaragdite. But the most interesting fact in connection with the change of augite to amphibole is that the areas now found are at times larger than were the original augites; that is, a growth subsequent to the MON SIX 23 354 THE PENOKEE lEONBEAEING SERIES. consolidation of the rock has occun-ed. This growth is much better iUus- trated by the diabases of the eastern area and its description is deterred to chapter viii. The series of alterations has not ended in the change from pyroxene to amphibole. The secondary amphibole has to qviite an extent altered to biotite. The biotite is as clearly secondary to amphibole as the amphibole is to augite. In certain rocks in the Northwest the alteration of horn- blende to biotite occurs, definite crystallographic relations obtaining between the two; but in the Penokee rocks the alteration of each of the amphibole individuals has resulted in the formation, not of a single biotite, but in many small folia. All of the biotite is, however, not derived from the amphibole, for a considerable quantity of that mineral is found sur- rounding the magnetite areas, and, when the latter occurs in grate-like forms, filling the spaces between the bars. This biotite is also plainly secondary and is in the nature of a reaction mineral, the magnetite furnish- ing the oxide of iron necessary for its formation, while the surrounding minerals have furnished the remaining constituents. Magnetite areas inclosed wholly within feldspar are in some cases surrounded by such secondary biotite, and it wofild then seem that the plagioclase has largely furnished the material for the formation of the biotite. All the minerals of the diabases have now been considered, except the rare rhombic pyroxene. This has its usual fine fibrous rectangular cleav- age, and characteristically includes very immerous minute black particles along its parting. It is somewhat feebly pleochroic and is probably bronzite or hyperstheue. It has a tendency to occur in idiomorphic six-sided forms, showing apparently two macropinacoids and the four planes of the domes. It is quite frequently surrounded by reaction- ary or intergrowth rings, which consist in part of minute, brilliantly polariz- ing particles. The rhombic pyroxene is in strong contrast with the more abundant colorless or pale brown augite. The augite has no black inclu- sions, does not ordinarily show rectangular extinction, and has a coarsely prismatic, nearly rectangular cleavage, instead of a finely fibrous rectan- gular one. TTir: KRT^PTivES. 355 Eruptiovs in the [ron-haarinr/ member. — The oruptives contaiued in tlie iron formation ari' mostly iii tlie tonii of dikt'S. Tlii« is particularly true of its ceiiti-al part, but in T. 47 N., U. 45 W., Midiigau, there arc eruptives which are a])pareutly iuterleaved. Tlie great abundance of these dikes in the parts of the iron formation which have been cut by mining, and their relation to each other as well as to the containing formation, are shown by Pis. XXX and xxxi. Macroscopically, the dikes vary in extent from a coarse diabase to a very fine grained rock. The fact that the greenstones included in the iron formation are as a whole altered to a much greater extent than the eruptives elsewhere in the Penokee-Gogebic series has been noted in a previous chapter. Even those which are very much altered retain the characteristic structure of a greenstone, although in some cases the decom- position has gone so far, or else the rocks were originally so fine grained, that they are now aphanitic. The greenstone structure is usually so pronounced that the writer in the field had little doubt as to the real nature of these rocks, even before they were traced into the less altered phases, and of course before a microscopical study of them was made. In several mines the same dike in different places is a comparatively little altered rock, which is prop- erly a diorite, and a completely altered one, which is a typical soapstone. The great Colby dike presents a good instance of this, its major portion being, however, extremely altered. The color varies, depending upon the degree of alteration, from the dark greenish gray of the ordinary diabase of the Peiiokee-Gogebic series, through various shades of dark green and light green, to almost snow white. The more common phases of the rock are dirty greenish white. All are colored various shades of brown and red upon their surface and along the cracks by iron oxides, and oftentimes these stains have penetrated the solid dikes to a considerable distance. As a result of the decomposition the diabases have become very soft, so that they can readily be scratched by the finger nail, and a specimen may be broken to pieces in the hands. This softness has not resulted, however, in making them porous or less compact than when unaltered. Their non- penetrability by water is clearly shown by the fact that the interiors of the dikes are in the main unstained by iron oxide, which has colored red 356 THE PENOKEE IEOjST BEARING SERIES. everything else adjacent. The rocks have a strong soapy feel, and because of this, and the fact that they are somewhat hke rocks in other districts asso- ciated with iron ore, the miners have given them the name of soapstones. The^ soapstones in this district never have the schistose structure and seri- citic appearance presented by the soapstones of the Marquette, VermiHon lake, and Menominee districts. This difference may be and, indeed, proba- bly is, due to the fact that in the latter the rocks have been subject to pow- erful dynamic forces. A microscopical study shows that in the less decomposed phases of rock the character of the alteration is not materially different from that already described as occumng in the diabases of other parts of the series ; that is, the augite is merely altered to amphibole and the feldspar has been to some degree affected by gray decomposition. In other cases the alteration has resulted in the formation of green chlorite in the place of the augite, the feldspar being affected the same as in the previous case. When the decomposition has proceeded farther there appears quite often, in consider- able quantity, a brilliantly polarizing material which is taken to be a zeolite. When the decomposition has gone very far even the magnetite is mostly or wholly pei'oxidized. The place of the augite and feldspar is largely taken by a colorless or pale yellow material. Between crossed nicols this substance is often wholly dark or dull gray and does not change its appearance by rotation. At other times it polarizes as an obscure aggregate whose color and appearance are somewhat like those of serpentine, although analyses indicate that that mineral is not present. In all but the most altered phases some chlorite and apparently a little secondary quartz are present. Abundantly scattered through most of the sections are particles of limon- nite, hematite, and sometimes magnetite. When the rock is so altered that none of the original minerals remain, the diabasic structure frequently is imprinted upon the homogeneous almost amorphous material. In the some- what rare extreme degree of alteration, however, all definite structure has been obliterated, and such material, if examined by itself, gives no indication whatever of its origin. That these soapstones are altered diabases is shown by the facts: that in certain cases the same dikes exhibit little altered and THE EltUl'TlVEti. 357 extremely altered phases; that the great majority of the soapstoues retain a distiuet diabasic structure ; and that the alterations of the minerals from theij" fresh condition to comi)lete decomposition are traceable in all its stages. With the decay of the rock there has come about, as would be expected, a very considerable change in composition. The analyses below are by Mr. Thomas M. Chatard of the chemical laboratory of the United States Geological Survey. No. 1 (specimen 12880) is a fresh diabase, from Sec. 13, T. 47 N., R. 46 W., Michigan. The felds- par of this diabase is labradorite, No. 1 of page 352 being from this rock. No. 2 (specimen 12878) is from the same dike, where the alteration has ex- tended to a middle stage ; and No. 3 (specimen 12966) is typical soap rock from the Aurora mine, in the NE. 4 of the SW. ^4 Sec. 23, T. 47 N., R. 47 W., Michigan. Analyses of diabases and soapstone. HjOatioso H2O at red lieat. COj S03» P20S SiOj TiOj ALiOs Fe^Oa .^.... CrjOs- FeO NiO(CoO) MnO BaO CaO MgO K2O Na,0.. •15 2-34 •38 •03 •13 47-90 •82 15^ 60 3-69 trace. S^41 •10 ■17 ■05 9^99 8^11 •23 2^05 100-15 3-12 8-25 1-89 -06 -16 46-85 1-12 22-62 5-12 100-21 ■29 13-54 ■38 •14 41^60 3^79 f 37-20 3^21 1^58 -30 •08 2^54 -08 •10 trace. 1^25 -23 2-01 -02 2-66 100-85 * SO3 calculated from BaO foand,«3 this latter protaWy exists as BaSoj. t AI2O3 is probably a little liigh owing to alkali retained by titanic acid. The analyses indicate that in the decomposition of these rocks the minerals become hydrated; that the silica lessens in quantity; that the rela- tive proportion of alumina is largely increased ; that the calcium, magne- sium, and iron protoxide are almost wholly removed ; and that the relative proportion of titanic oxide is increased, this probably being due to the fact 358 THE PENOKEE lEON-BEAEING SERIES. that all of the titanium which was originally present in the magnetite re- mains in the decomposed rock. The percentage of silica which is taken away is really larger than is indicated by the difference between 1 and 3, since in the latter so large a proporton of water is present. Pure kaolinite, H4AI2 Si, Og contains SiOo, 46-50; AI0O3, 39-66; H^O, 13-94. Disregarding the small amount of impurities in No. 3, its composition corresponds very closely with this mineral. It seems plain that, as a result of the leaching action to which the lower part of the iron formation is subjected, the dikes — that is, augite-plagioclase-magnetite rocks — are so changed that their compo- sition is very close to that of the mineral kaolinite ; although it is possible that this average composition is due to several important minerals rather than to a single one. That the diabase dikes high up in the Penokee series are really con- tinuations of the dikes which cut nearly at right angles the underlying iron formation there can hardly be a doubt. The contrast between the two is a striking instance of the influence of environment upon the decomposition of a rock. The diabases inclosed by the impervious Upper slate have been kept in a well preserved condition through the ages which have elapsed since their intrusion, and some of them are remarkably fresh. Other parts of the same dikes in a formation which contains evidences of having been long subject to the action of percolating waters have been completely decomposed. It thus appears that in this case environment has been a far more important element than age in the preservation of the rock. Summary. — The Penokee eruptives are of two classes. Dikes cutting the formations, and interbedded sheets, which are probably intrusions of the same age as the dikes. The eruptives are diabases, which occasionally pass over into gabbros. Diabases in every respect like those of the Penokee series are found both in the Southern comj)lex and in the Keweenaw series. These are all presumably of the same age; that is, Keweenawan. The Penokee diabases are in all respects tj^pical rocks of their class. In one case a rhombic pyroxene of some interest is present. While the diabases of the Upper slate member are often quite fresh, these rocks have generally undergone an extensive series of alterations ; TIIK EKUPTIVES. 359 the feldspars lia\ ing altered to or been replaced by kaolin, chlorite or sma- ragdite ; and the pyroxene havinir passed over into liornblende, biotite or chlorite. The alterations have extended farthest in that part of the Iron-bearing member containing- the g-reat bodies of ore ; that is, in tliose parts of the formation which have been subject to the action of percolating waters. This alteration has gone so far that often none of the original minerals remain. All traces of the original structure of the rock may even be lost, although frequently the diabasic structure remains imprinted upon the homogeneous almost amorphous material which results from the complete alteration of the rock. The strong contrast in the characters of the diabases in the Upper slate and Iron-bearing members shows that environment may be a more important element than age in the preservation of a rock. CHAPTER VIII. By 0. E. Van Hise. THE EASTERN AREA. Introduction. Section I. The Iron-bearing member. Distribution. Petrographical character. Mingled fragmental and nonfragmentai sedimentation. Probability of ore deposits in the eastern area. Tabulation of petrographical observations. Section II. Fragmental rocks south of the Greenstone-conglomerated. Geographical distribution. Petrographical character. Tabulation of petrographical observa- tions. Section III. The Greenstone-conglomerates. Distribution. General characteristics. Origin of the Greenstone-conglomerates. Tabulation of petrographical observations. Section IV. Fragmental and ferruginous rocks north and east of the Greenstone-conglomerates. Geographical distribution. Surrounding rocks. Continuation of the belt east and west. Struc- ture of the belt. General petrographical cliaracter. Mingled fragmental and nonfragmental sediments. Coarsely fragmental rocks. Tabulation of petrographical observations. Section V. The Greenstones. The main area. The area in Sees. 20, 29, and 30, T. 47 N., R. 43 W., Michigan. The area in Sees. 24, 13, 14, and 15, T. 47 N., R. 44 W., Michigan. Section VI. Stratigraphy. Litholoidcal evidence as to equivalence with the main Penokee area. Stratigraphical eviclenc as to equivalence with the main Penokee area. Relations of the belts of the eastern area to each other. Great width of parts of the eastern area. The southern dips. Sequence of events. Mingled fragmental and nonfragmental sediments. Summary. INTRODUCTION. The area east of the center of T. 47 N., R. 44 W., Michigan,^ or roughly east of the Little Presque Isle river, differs from the simple suc- cessions described in the previous chapters in many important points. As will be seen later, the differences are due to the fact that this area was the center of o-reat contemporaneous volcanic activity. Consequently the suc- cession includes large thicknesses of volcanic tuffs and lava flows. These 360 THE EASTERN AREA. 361 beds are not itaralloli'd hy niiy tlmt an! found in the western area. P^'ur- tlier, this voU-anic material has greatly disturbed the normal succession of belts in the district, so th.it it is difficult to certainly correlate the forma- tions east of the Pres(|ue Isle with those west of it. Another point in which this area differs from the western area is that in one place the relations of the horizontal Eastern sandstone to the Penokee series can be made out. The subject is divided into the following sections: The Iron- bearing member; the fragmental rock south of the greenstone-conglomer- ates; the greenstone-conglomerates; the fragmental and ferruginous rocks north and east of the greenstone-conglomerates; the greenstones; stratig- raphy. SECTION I.— THE IRON-BEARING MEMBER. Distribution. — The rocks naturally and artificially exposed east of the Little Presque Isle river which can certainly be referred to the Iron-bear- ing member, .although quite numerous, do not form as continuous a belt as do similar rocks to the westward. Through the east part of T. 47 N., R 44 W., Michigan, and the west mile of T. 47 N., R 43 W., Michigan, the gaps between the exposures are in each case about a mile, while in one instance a gap of 2 miles occurs. East of this latter point prospecting has shown rocks of the Iron-bearing member to be practically continuous for about 4 miles; i. e., from the southwest part of Sec. 20 to the northwest part of Sec. 23, T. 47 N., R. 43 W., Michigan. East of the latter point no rocks are found which certainly can be refeiTed to the Iron-bearing member. ^ In the easternmost section of the western area of the Iron-bearing belt the exposures spread over a horizontal distance greater than at any other locality in the whole Penokee series, extending as they do from the north- east part of Sec. 21 to the northwest ^jart of Sec. 15, T. 47 N., R. 44 W., Michigan, a distance of nearly a mile. The most of the exposures of this section are south of a huge ridge of basic eruptives, some of them being in contact with this rock. North of this eruptive, and about a half mile north of the nearest exposure of iron-bearing rocks to the southward, are ex- posed by a test pit rocks which undoubtedly belong to the Iron-bearing member. The only other locality where the thickness of the belt ap- 362 THE PENOKEE IRON-BEAEIIfG SERIES. proaches that hei'e attained is west of Sunday lake, and here again the apparent thickness is in part and perhaps largely due to eruptives. For the distance between these two points the Keweenaw series directly overlies the Iron-bearing membei'. In the eastern area the rocks belonging to the Iron-bearing member constitute, as far as at present known, a narrow belt, narrower than anywhere to the west. The causes of this change from extreme width to extreme narrowness will be discussed later. Petrograpliical character. — The locations of the exposures belonging to the iron formation are found upon PI. xrii and indicated in the tabulations to follow. The kinds there found include nearly every phase of rock characteristic of the Iron-bearing member west of the Presque Isle. This likeness of the rocks east and west of this stream is such that no question can be entertained as to their identity of character and origin. A general discussion of the original nature aiid subsequent modifications of the rocks of the Iron-bearing- member has been given in another place and need not be here repeated. It is, however, worthy of note, that in some of the ferruginous cherts in T. 47 N., R. 43 W., Michigan, are found very numer- ous small geodic cavities which are lined with quartz crystals. That this quartz is of a secondary nature, or at least has been rearranged since the rock was originally formed, can hardly be doubted. The close association of siderite, magnetite, hematite, actinolite, and quartz is finely shown by one exposure. The relations here are such as to indicate that the mag- netite has formed directly from siderite, as has hematite so extensively in the iron-bearing belt to the west. Further, the actinolite appears where- ever quartz is found, while the quartz present in the section is in irregular veinlike forms cutting across the lamination. The conclusion is that this quartz is secondary, and that at the time it formed a portion of the silica in solution united with the bases present — calcium, magnesium, and iron — to form the actinolite. We have here, then, another reenforcement of the argument given for the derivation of the actinolitic slates from an original clierty carbonate. Mingled fragmental and nonfragmental sedimentation. — The one impor- tant point in which the iron-bearing rocks east of the Little Presque Isle differ from those to the westward is that thej^ are interstratified with a TIIK EASTEIJN AREA. 3(53 greater or less (luaiitity of mechanical sediments. This minf^lino- of fni'j;- mentiil and nctnfragmental sediments has occnrrcfl so extensively in the eastern area, that a, new color is us(h1 npon Pis. ii and xiii to desi'mental material. East and west of Sec. 21, T. 47 N., R. 43 W., Michigan, are fragmental rocks, which are, however, not a separate belt below the iron formation, but are intercalated with its nonfragmental sediments. Petrofjrajiliical cliaracter. — It is not practicable to separate these frag- mental rocks into sharply detined divisions, as the different phases grade into each other by insensible degrees. They can be somewhat arbi- trarily divided into quartz-slates including quartzite, and ferruginous feld- spathic quartz-slates. As to this first division nothing more need be said, as they are precisely like rocks of the same name in the Quartz-slate mem- ber to the west. The second division comprises the following varieties: Ferruginous and feldspathic (juartzite, sometimes conglomeratic, jasper- conglomerate, and ferruginous chlorite-slates. The ferruginous feldspathic quartzites differ from tlie quartzites found in the main quartz-slate area to tlie west in that tljey contain a very large amount of iron oxide, which is mostly hematite, but mingled with limonite and some magnetite. In some specimens the hematite is so abundant as to form a continuous ramifying slieet in which is buried tlie worn fragments of quartz and feldspar. These fragments at several exposures are so large as to class the rock as a conglomerate. The exposures shown by the test pits in the soutli part of Sec. 20, T. 47 N., R. 43 W., Michigan, are of peculiar interest because of their relations to the underlying eruptive, and the great likeness under the microscope of some of them to a large part of the greenstone-conglomei'ate, to be later MON XIX 24 .370 THE PENOKEE lEON-BEAEING SEEIES. described. Just south of the place of their occurrence is a high east and west ridge of diorite-porphyrite, which is a part of a flow outcropping at various places for a distance of two miles east and west. At "the northern foot of this hill, dipping to the north, is the jasper-conglomerate, having a slaty matrix. The specimens from the test pits a few paces to the north are the nonfragmental sediments of the iron belt. In the matrix of this "jasper-conglomerate are very numerous irregular compact frag- ments, which contain tabular plagioclases and which ajjpear to be frag- nients of the fine grained basic eruptive just to the south. These fragments are often vaguely defined; they are extraordinarily irregular in form; they are very much altered. Besides these complex .fragments there are found quite numerous angular pai-ticles of feldspar of moderate size. These may have been furnished by the porphyritic plagioclases of the tmderlying porphyrite. These two varieties of fragments are cemented by a groundmass which consists largely of cherty silica. This silica is around and between tlie fragments in narrow belts, just as it is found in a wide- spread phase of the greenstone-conglomerate. Contained in the above matrix are very numerous angular blood-red jasper pebbles of varying sizes, some of them being large enough to be classed as bowlders. These bright red jasper pebbles give the rock a very striking appearance. The only essential difference between this conglomerate and certain phases of the greenstone-conglomerates is in the presence of these jasper pebbles, and whether it ought to be classed here or with the greenstone-conglom- euates is a somewhat doubtful question It is placed here' because it is certainly a water-deposited fragmental rock, and is also certainly below the rocks of the iron-bearing belt. The third phase of rock is the chloritic and clay-slates, which are plentifully interstratified with the nonfragmental iron-bearing sediments in Sees. 20, 21, 22, and 23, T. 47 N., R. 43 W., Michigan. Macroscopically, these rocks are soft, green or brown, aphanitic, finely lannnated ones. Their constituents are difiicult to make out with certainty. Some of the chief ones are quartz, chlorite, sericite, brown ii-on oxide, pyrite, and perhaps kaolin. How far these rocks are fragmental and how far nonfrag- mental sediments it is difficult to determine, so fine grained are they. In THE EASTERN AREA. 871 a ixM-tion of them some of tlic (|uai-t/. is in small roimdisli areas wliieji have an unmistakable tVaoineiital character. Furtlier, all of the character- istics of the rocks, both in liaiid specimen and thiu section, are those of compact clayey sediments, which in all probability they are. TABULATn)N OF PKTROGRAPHICAL OBSERVATIONS.'- 1. Ferruginous and feldspatliic conglomerate. Specimen 0205 (slide 3012), 150 N., 1750 W., Sec. 10, T. 47 N., K. 4;5 W., Michigan. The rock is dark gray and massive, and the matrix is iiiedium grained. The contained pebbles are Jasper, white quartz, and green schist. They are mostly small, although occasionally one 10 inches in diameter is found. A thin section from the matrix shows fragmeiital particles of quartz aud feld- spar of quite uniform size, the former composing perliai)s four-flfths of the section. The grains of quartz are often enlarged, and some of them are liuely complex. The feldspar is orthoclase, nucrocline, and plagioclase. Its grains are In part fresh, and in part also much kaoliuized or partly altered to chlorite. In the interstices are finely crystalline quartz, dark brown iron oxide, and greeu chlorite. • 2. Ferruginous and feldspatliic quartzites. Specimens 7430 (slide 1801), 7437 (shde 1802), 1050 N., 1420 W.; 0285 (slide 2062), 0286 (slide 2063), 1075 N., 1350 W.; 0287 (slide 3008), 1040 N., 1300 W., Sec. 30, T. 47 N., R. 43 W., Michigan. The rocks are dark gray to black, medium grained, and vary from massive to schistose, the darker ('olored specimens containing much oxide of iron. In each of the thiu sections the mineral constituents are the sa ae as in the pre- vious number, the only difference between the various sections being relative propor- tions of the minerals contained. Slides 1891 and 2962 have darlv brown ferrite in a continuous ramifying sheet in wliich the other minerals are buried. In slides 2063 and 3008 the oxide of iron is muclr less in quantity, while the fragments of feldspar and quartz in them are abundant. The grains of fragmental (j[uartz are frequently enlarged. 3. Sericitic graywacke. Specimen 9284, (slide 2961), 300 N., 500 W., Sec. 19, T. 47 N., E. 43 W., Michigan. The rock is dark gray, fine grained, schistose, and cleaves readily along the plane of schistosity. Rather small clastic particles of quartz and feldspar, the former preponderant, compose three-fourths of the thin section. The interstices are filled with finely crys- talline quartz, kaolin or sericite, chlorite, dark brown ferrite, and iilentiful grains of black lustrous galenite. 'The mimbers of speciiiieus aud slides are those of the collection of the Lake Superior division. "Locations are giveu from the southeast corner of the sections in steps of 2,000 per mile. 372 THE PENOKEE INON-BEARING SEEIES. 4. Jasper-conglomerate. Specimens 9261 (slide 3005), 9263 (slide 4486), 7418 (slide 1877), GOK, 600 W., Sec. 20, T. 47 N., R. 43 W., Michigan. The matrix of the rock is dark green, fine grained, finely laminated, has a greasy feel, and contains uuineroiis medium sized grains of a cleavable mineral. This matrix is quite thiclcly studded with fragmeuts of red jasper, some of which are from 6 to 8 inches in diameter. The thin "sections are composed of finely crystalline and coarsely fragmental parts. The fragmental portions comprise large, somewhat rounded areas of a finely crystalline basic eruptive and medium grains of feldspar, the former variety of frag- ments being more abundant. These complex basic areas contain cldorite, biotite, tabular feldspars, ferrite, and epidote, and are almost precisely like the basic erup- tive which is developed upon a large scale just to the southward. The feldspar frag- ments are in part ortlioclase and in part plagioclase. The abundant fine grained cementing material consists of clierty silica, sm ill flakes of chlorite, a.nd few of biotite. The pebbles of the conglomerate are typical red-banded jaspers. The rocks are apparently intermediate between fragmental ones and those of the nonfragmental iron-bearing belt. jSTanfragmental sedimentation has began, but it is yet acc'om- panied with mechanical sedimentation. 5. Olay-slate. Specimen 12580 (slide 5340), 460 :N'., 1010 W, Sec. 20, T. 47 K, E. 43 W., Micliigan. The rock is a. light green, aphanitic, finely laminated, soft slate. The thin section consists of intimately mingled finely crystalline quartz, chlorite, sericite, and iron oxide. Much of this quartz is fragnreiital. The sericite is a light greenish-yellow, somewhat brilliantly polarizing, and is arranged in parallel rows of flakes which extinguish rectangularly. If other constituents are present, they are too obscure to be recognizable. 6. Feldspathic (luartzites. Specimens 7383 (slide 1849), 900 N., 460 W.; 9244 (slide 4479), 9245 (slide 4480), 1100 N., 460 W., Sec. 21, T. 47 N., R. 43 W., Michigan. The rock varies from flesh color through greenish-gray to dark brown ; is rather fine grained, almost massive, and breaks with a subconchoidal fracture. The thin section is composed largely of clastic particles of (juartz and feldspar, the former much the more abundant and often enlarged. Tlie feldspars are orthoclase, microcline, and plagioclase. The rather abundant interstitial material is finely crys- talline quartz, chlorite, kaolin, and iron oxide. 7. Quartzite. Specimen 7385 (slide 1851), 900 N., 750 W., Sec. 21, T. 47 N., R 43 W., Michigan. The rock is a greenish-gray, medium grained, compact vitreous quartzite. The thin section is mainly composed of enlarged fragmental grains of quartz. The induration is, however, mostly due to finely crystalline interlocking interstitial quartz, mingled with which is chlorite and brown oxide of iron. THE EASTKIiN AIMCA. J^73 8. FiMTiiR-inoiis and clildritic slate. Spcciiiiciis 7100 (sliilc l,S(i;;), 1 1 10 N., ".;;() W.; ims (slide USI), 1170 N., r>10 W., See. L'l, T. 17 N., If. l:: W., Miehi-aii. The roek is of a peculiar liislroiis lnowiiisli fipeeii coloi-, iiuiltled witli irrcj;iilar pat<-lies oCa dull hrielv-ied eiilor, is finely laiiiiiiaied and very soil. In thin section, a quart/, harkgromid e(Mitains abundant |»ale j^reen ehlmite, liniwu iron oxi(h', red hematite, and numerous scales of a luillianMy ])ohiiiziufi- min- eral which istakeu to be serieite. it. dhloiitic and biotitic slate. Six'cimeu 73S(; (slide :i045), 050 N., S(Mt \V., See. 131, T. 47 N., U. 43 W., Miehijiiiu. Ill tliin section, a tinely crystalline (|iiartzose iiioundmass contains a felted, mass of fibrous somewhat iron stained ehloiiteand biotite, the libers of which are arranged in a common direction, and give the rock a strong schistose character. Scattered through this fine material are a few small grains of jdainly fragmental quartz. 10. Ferruginous and chloritic slates. Specimens 7;!!>!) (slide 1S(;2), 1 l.jO N., 520 W.; 7401 (shde 1S(!4), 1200 N., 4S0 W.; 0249 (slide 4482), 1210 N., 450 W., Sec. 21, T. 47 N., R. 43 VV., Michigan. The rocks are dark'greeu, thinly foliated, and have a soft greasy feel. In thin section the rocks differ from S chieHy in that they have no liydromica, and contain (juite plentifully small grains of plainly fragmental (juartz, wbicli stand out sharply from tlie groundmass. Both hematite and limonite are plentiful. 11. Ferruginous quartzite. Specimens 9252 (slide 4484), 9253 (slide 4485), 1315 N., 570 VV., Sec. 21, T. 47 N., li. 43 W., Michigan. The rocks are greenish to reddish gray, massive, and vitreous. The thiu sections are almost wholly composed of interlocking large grains of quartz. Films of iron oxide are found in the interstices and Itetweeu the cores and enlargements of the quartz grains. 12. Feldspathic quartzite. Specimen 12588 (slide 5343), 1312 N., 1790 W., Sec. 22, T. 47 ]Sr., R. 43 W., Michigan. The rock is gray to pink, fine grained, vitreous. The thiu section is chiefly composed of rather small originally well rounded particles of quartz and feldspar of remarkably uniform size. The quartz is several times as abundant as the feldspar. The grains are sometimes iiuely complex and often plainly enlarged. The feldspar comprises ortlioclase, micro(;line, and plagioclase, all quite fresh, although a few of the grains have been affected by decomposition. Between the clastic jjarticles are tound tinely crystalline silica, numerous small well defined brilliantly polarizing flakes of kaolin or serieite, and a few particles of ferrite. The section also contains a few grains of each of the minerals zircon and tourmaline, the former at times showing its characteristic zonal structure. These minerals are rounded, and must be classed as fragmental constituents rather than as indigenous in this rock. 374 THli PENOKEE IROH-BEARING SERIES. SECTION III— THE GREENSTONE-CONGLOMERATES. Distribution. — The greenstone-conglomerates are restricted to tlie area represented in PI. xiii, occurring nowhere except in T. 47 N., R. 43 W., and R. 44 W., Michigan. It will be seen that their westernmost appearance is in the extreme northeast part of Sec. 16, T. 47 N., R. 44 W., Michigan.. In the northwest part of Sec. 14, T. 47 N., R. 44 "W., Michigan, are areas which are mapped as detached from the main mass of conglomerate because they are not known to be connected with it. From the exposure in the south part of Sec. 15, T. 47 N., R. 44 W., a belt of the conglomerate continues eastward, rapidly ,widening, and in Sees. 24 and 25, T. 47 N., R. 43 W., reaches its maxium width, 1| miles. Continuing eastward, the belt quite rapidly narrows, and its last appearance is in the northwest part of Sec. 23, T. 47 N., R; 43 W., Michigan. The exposures of eruptive rocks in Sees. 13 and 14, in the west part of Sec. 23 and in the northeast part of Sec. 24, T. 47 N., R. 44 W., Michigan, have all the characteristics of surface flows; that is, they contain minerals of two generations, are often amygdaloidal, and have a groundmass which is frequently parti}' amorphous and is always finely crj^stalline. These exposures are believed to be more closely allied to the o-reenstone-comfflomerates than to the diabases of the Penokee series. General characteristics. — The term "greenstone-conglomerate" is applied to the rock of this area because it is a fragmental one, in which nearly all the fragments are from basic eruptives, as is also the major portion of the matrix in which these fragments are set. The term "agglomerate" would convey a. false impression. The rocks covered by the term greenstone- conglomerate include agglomerates and water-deposited elastics, the detritus of which is chiefly /rom greenstones,' as well as gradations between these two extremes. Macroscopically, the rocks vary from an aphanitic slate- to a coarse conglomerate. At times the fine grained clay-slates and the con- glomerates are intimately mingled, but in general the slate exposures are 'The woi'd jrreenstone is used here in its old sense, to cover all the. basic eruptives of the district. 'A rock remarkably similar to many ot these conglomerates is f the pel)liles and bowlders are of a single variety, pale greenish-gray, aphanitie, and massive. In some localities aphanitie, dark reddish-brown, felsitic-looking pebbles are quite numerous, although they never become the predominant varietv. Very rarely worn white quartz pebbles are found. The pebbles are usually more resistant then the matrix, and consequently proti'ude upon the weathered sur- face in n(Klnlar or maminillar)^ forms. In the greater number of the expo- sures fracture takes place through matrix and pebble with about equal ease. In those exposures in which the matrices are much altered, and therefore schistose, fracture occurs around the pebbles to a greater or less extent, depending upon the degree of alteration. The lines of separation between the pebbles and matrices in the fresher rocks are quite sharp, but in those which are more altered they are vague ; and as there is often but a slight difference in the color and texture of the pebbles and matrices, when the, rocks are much altered it is difficult to separate one from the other on a fractured surface, although the difference is clearly seen upou the weathered surface. A stud}- of some fifty thin sections shows that the matrices are of many phases, which vary into each other by imperceptible gradations. At times the matrix appears to be a fine grained eruptive, which contains fragments of essentially the same material. This phase of matrix is described under the greenstones of the eastern area and the description will not l)e here repeated. 'An important fragmental kind is composed of the fine debris from material like the contained pebbles, combined with finely crystalline quartz ; that is, it is a recomposed 'greenstone, and is often schistose. The minerals most frequently found in this phase of matrix are quai-tz, tabu- lar plagioclases, chlorite, epidote, titanite and leucoxene, actinolite, and, as accessories, oxide of iron, a carbonate, and occasionally renmants of augite. 37G THE PENOKEE IRON -BEAEmG SEKIES. The proportions of these minerals vary widely in the different sections. In one variety a quartzose backgTound subordinate in quantit}- contains fine debris from the basic eruptives; that is, tabular plagioclases, actinolite, and chlorite, all of the minerals being mingled in the most confused manner, but generally showing a laminated structure. In another variety of frag- mental matrix- of about equal importance to the last the background is composed almost wholly of quite pure finely crystalline and chalcedonic quartz, in which are well outlined sharp areas of intermingled chlorite and epidote. Between these two main varieties there are various gradations. In some of the sections the matrix and fine fragmental material have a pretty well defined stratiform arrangement; in others, they vary from this regular stratiform character to a most extraordinary irregular arrangement, the fragments being extremely angular, and the matrix between running around and through them in the most lawless fashion. Words fail to give any proper conception of this strange appearance, but some idea of it will be obtained by reference to PI. xxxv, Figs. 2, 3, and 4. Of less impor- tance than the foregoing are the black chlorite-slates and black calcareous slates. These phases are usuall)' nonconglomeratic, and are precisely like the slates of the fragmental belt north and east of the greenstone-cc^ng-lom- erates. Some of them contain unmistakalole fragmental quartz mingled with fine grained clayey material. Others contain a good deal of some carbonate, oxides of iron, and finel}' crystalline quartz ; in other words, are like the mingled nonfragmental and fragmental water-deposited sediments of the eastern area. The fragments of the conglomerates vary from large bowlders to single individuals of one mineral. The pebbles are usually immerous, and often so thickly set as to give little room for a matrix, although the}' vary in abundance and are sometimes absent. It has already been stated that the great majority of the pebbles are of two well defined types. •. The com- mon light green or grayish-green pebbles are orjiinarily fine grained fo aphanitic porphyrites, which are quite often amygdaloidal. Their back- ground varies from glassy to holocrystalline. When glassy, they have been almost wholly devitrified, being changed into a pale green or light gray nonpolariziug or very feebly polarizing aggregate. The common re(;og- Till', i;ast1':i;x ai;ka. :^77 nizablc iniiicrals ctmlaiiUMl in tlic L;lass\ \ari('tics arc tahiilar plai^'ioclascs, Ifiiciixciic, and litaiiitc, cliliiritc, ami ciiidntc, Tlic IioIoctn stallinc \arict>' of pcWhlc is soiiH'tiiiu's fresh ciKHi^li In lie distiiictl \- rcicogniizcil as a- diabase- |)(ir|)liNi'it('; tlic more altered ones would he classed as porphyrites. These ha\e as cliief coustitueiits the iniuerals aliove mentioned with tli(^ a. 4o W., ]\Iicliifr}ni, and by the lar^i-e eriiptive exposures in Sees. 11, If), and IC, 'V. 47 N., K. 44 W., Ariclijo-jui, as widl as l)y tlic ;4Teat mass of oi-ccnstonc-coiij^'lomenite itself Tliis voleanie activity is an exceptional tiling- in tlic Tenokee sue oessir>i) W., Sec. 2:?, T. 47 N., K. 44 W., Michi.i;:Mi. Tlie rocks are like 2. Upon the weatliered surface t lie more resistiiiit i)ebbles Itrotrude with oval outlines. When tlie rock is freshly fractured tlie jiale yrayish- fireeu, rounded pebbles are well delincd in their darker colored matrix. In thin section the smaller frai^inents and (heir relations to the backj;j;round are precisely as in 2. This background is liner grained, ami a])pears to consist of very finely crystalline quartz, mingled with the finest sort of debris from material like that composing the fragments. This background and the smaller fragments are arranged in a roughly stratiform May, which must indicate stratification or a Howage structure. The sections contain abundant larger fragments, which vary in character from those having an almost wholly devitrified glassy background containing much altered tabular plagioclases to those which have a holocrystalline base in which is found many comparatively fresh augites and jxirphyritic crystals of plagioclase. The plagioclase has often wholly altered to white uuca or has been rejilaced by (-alcite. Since the above pebbles contain augite, they are properly augitei)()ipliyiites. 7. Greenstoue-conglonierates north of (i, near middle of belt. Specimens '.1337 (slide 3022), 75 N., 1500 W.; 7465 ; slide 1910), 7400 (slide 1917), 7407 (slide lOlS), 140 ]Sr., 1500 W.; 9330 (slide 3021), 270 N., 1500 W.; 7408 (slide 1919), 290 N., 1500 W., Sec. 14, T. 47 N., R. 44 W., Michigan. In places the conglomerate is free from pebbles, and is then dark grayish- green, fine grained, and schistose. The pebbles, when present, are all of the green massive variety described in 2. They vai;y in size from those so small as to be lost in the matrix to those several inches in diameter. The conglomerate is thickly studded ■with the pebbles, which are best seen upon the weathered surface. The matrices of the sections are of two varieties; in one, finely crystalline^ quartz is the predominant constituent, in the other it is subordinate in quantity. In this latter phase with the quartz there is mingled very abundant chlorite, much aiiiorphous gray material, and many partly altered tabular plagioclases. The phase of the ma- trix rich in (juartz also carries these minerals, although in less quantity. Contained in both varieties of matrix are, sciattered somewhat sparsely, large well rounded fragmental grains of feldspar and quartz, the latter being sometimes bunched into areas of some size, the constituent grains of which are usually enlarged. Tlie peb bles, as in the previously described conglomerates, are fragments of altered basic eruptives. (Jommonly this alteration has extended very far, so that they are now com- posed of chlorite, epidote, and partly decomposed tabular plagioclases as chief con- stituents, and with these rather abundant titanite and some secondary quartz. The pebbles vary from quite well rounded to those as irregular in form as in 2. ^hiny of them are cut into or even apparently dissevered by the ramifying quartzose matrix. The well rounded character of the (piartz grains in a matrix of nonfragmental quartz would seem to be evidence that these rocks were formed under water. 384 THE PENOKEE IRONBEAEING SEETES. 8. Greenstone-conglomerate uortli, of 7. Specimen 9331 (slide 2977), 1040 K, 1500 W., Sec. 14, T. 47 ISl., "R. 44 W., Michigan. The matrix of the conglomerate i.s dark mottled green, fine grained, and S(;his- tose. It is studded with pebbles and bowlders, some of the latter being of large size_ The thiu section is cut from the matrix. A continuous rauiifyiug mass of non- polarizing green chlorite and cherty and chalcedonic quartz contains numerous roundish com])lex areas of epidote, many small areas of titanite, few much altered crystals of feldspar, and occasionally large areas of calcite, these latter apparently replacing feldspars. There is nowhere any evidence of fragmental material. Parts of the section are quite like an altered eruptive, and it thus appears probable that the matrix, as well as the fragments of this rock, is almost wholly or wholly a volcanic product. Section along and near the east line of Sees. 14, 23, and 26, T. 47 N., B. 44 W., Michigan: 0. Greenstone-conglomerates. Specimens 9358 (slide 3029), 400 IST., 1525 W.; 9357 (slide 4932), 450 j!^., 1700 W., Sec. 25, T. 47 N., E. 44 W., Michigan. The rocks are in nowise different from 6, except that they show a more decided schistose structure. The thin sections show no difference between matrix and pebbles. The back- ground appears to consist of amorphous gray material, in which are included exceed- ingly tinely crystalline chlorite, ([uartz, epidote, perliaps a little actinolite, and here and there nnich altered porphyritic crystals of feldspar. A few large roundish areas of quartz or calcite, or both, of an amygdaloidal character arc seen. So far as the sections go, one would call this rock a schistose greenstone, and yet the conglomeratic appearatu'e of the exposure is marked. 10. Chlorite-slate, north of 9. Specimen 10413 (slide 4018), 800 K, 1625 W., Sec. 25, T. 47 N., E. 44 W., Michigan. The rock is dark green to black, aphanitic, very finely laminated, and readily cleavable. In thin section an exceedingly finely crystalline quartzose background contains abundant chlorite and small particles of black and dark brown ferrite. The arrange- ment of the chlorite and iron oxide corresponds with the lamina', of the rock. Con- tained in the fine grained material are some larger grains of quartz which are plainly fragmental. 11. Greenstone-conglomerate, north of 10. Specimen 7459 (slides 2050 and 2051), 345 N., 0 W., Sec. 23, T. 47 N., E. 44 W., Michigan. The thin sections of this rock do not differ essentially from those of 6. 12. Greeustone-cougloraerate; north of 11. Specimen 9312 (slides 2970 and 2971), 9313 (sUde 4929), 190 N., 20 W., Sec. 14, T. 47 N., E. 44 W., Michigan. THE lOASTEHN AREA. 385 The matrix of tlic rock is daiU iiiottiiMl ;,^■^^(•Il, fine, };iaiiic(l, mid sciiistose. The prcviiiling pebbles are the Siitne as those in 7. OiH'iisionally a wliite (luartz pebble is seen. The thin sections are in essential respects like those of 7, liavinj;- a (jnartzose matrix. Large frajjiuents of extraordinary irregularity are contained in a reticulating groundniass consisting mostly of finely crystalline (juartz. The (juartz runs around and through the fragments in such a manner as to suggest tluit it is, in large pait at least, secondary and has dissevered the contained-fragments. This is rendered prob- able by the fact that the irregular outline of one fragment is often the reverse of the outline of the adjoining one. The fragments are composed of green nonpolariz- ing chlorite, much altered tabular plagioclases, roundish granules of titanite, and abundant epidote, the latter being sometimes concentrated into large irregular com- plex areas. (PI. xxxv, Fig. 2.) Section east and west of the line between Sec. 19, T. 47 JV., E. 43 W., and Sec. 24, T. 47. K, B. 44 W., Michigan. 13. G-reenstone-conglomerate. Specimen 9350 (slide 4931), 20 N., 475 W., Sec 24, T. 47 N., E. 44 W., Michigan. The sj)ecimen does not differ from those of 2. The tWn section combines the characteristics of those of 2 and 12. The pebbles vary in size and are closely packed together, so that the matrix is very sparse. It consists of the line debris of the pebbles mingled with finely crystalline quartz. A portion of the j)ebbles have the gray amorpnous background so common in 2, con- tained in which are many somewhat altered crystals of plagioclase. Pebbles of another class are similar to those in 12, iu that they contain a large amount of epidote and chlorite. 14. Greenstone-conglomerate. Specimen 9349 (sMde 3027), 425 'N., 500 W., Sec. 24, T. 47 N., E. 44 yi., Michigan. The specimen is not different from the previously described greenstone- conglomerates. The thin section is cut from a schistose portion which .was taken to be the matrix, and is plainly a much altered, very fine grained, amygdaloidal porphyrite. The background now consists of gray amorjihous material, much altered plagioclases, and, as secondary materials, chlorite, epidote, and small, brilliantly polarizing flakes. The amygdules are chiefly chalcedonic quartz, but frequently associated with it are chlorite and calcite. 15. Greenstone-conglomerate. Specimen 9300 (slide 3013), 850 N., 1750 W., Sec. 19, T. 47 N., E. 43 W., Michigan. The matrix of the conglomerate is dark green, rather coarse grained, and schistose. The pebbles do not differ from those of the previously described con- glomerates. MON XIX 25 386 THE PENOKEE lEON-BEAEING SEEIES. The thin section is like those of 13, except that the qiiartzose background is finer grained. 16. Greenstone-conglomerate. Specimen 7451 (slides 1905 and 2153), 900 N., 850 W., Sec. 34, T. 47 N., E. 44 W., Michigan. The rock varies from dark grayish green to dark green, is schistose, and weathers to a dirty yellowish brown color. In thin section the matrix of the conglomerate is very fine grained and schistose. It consists of finely crystalline quartz and the debris of basic eruptives, including chlorite, epidote, and brilliantly polarizing needles arranged in parallel lines. The fragments vary from those of large size, which are very plainly altered porpliyrites, to minute altered particles which are lost in the contained matrix. Epidote is plentiful in all parts of the section. 17. Greenstone-conglomerates. Specimens 7447 (slide 1900), 1935 N., 1000 W.; 9311 (slide 4928), 1950 K, 1350 W., Sec. 24, T. 47 K, E. 44 W., Michigan. The rock is dark mottled green, roughly schistose, and contains numerous small, vaguely outlined pebble-like areas. These sections do not difter materially from those of 12. With the naked eye the well rounded outlines of the pebbles are clearly distinguished fi'om the matrix in wlii(;h they are contained. The background microscopically consists of quartz (much of which is chalcedonic), chlorite, epidote, and gray amorphous material. The large pebbles have a background consisting in about equal proportion of gray material and pale green nonpolarizing viridite, which contains greatly altered plagioclases and finely crystalline secondary quartz. Exposures in the east part of Sec. 20, T, 47 N., R. 43 W., MicMgan. 18. Greenstone-conglomerates. Specimens 9242 (slide 2949), 950 N., 625 W.; 9239 (slide 4925); 9241 (slide 2948), 920 N., 920 W., Sec. 20, T. 47 K, E. 43 W., Michigan. The rocks are like 2, except that the matrix is distinctly schistose. The thin sections are like those of 2 so far as the fragments are concerned, but the matrix consists, aside from finely crystalline quartz, of the fine debris derived from the basic fragments. 19. Greenstone-conglomerate. Specimen 7415 (slide 1874), 1075 N., 35 W., Sec. 20, T. 47 N., E. 43 W., Michigan. The specimen is like 2. The section is from a pebble of the conglomerate. It is a fine grained, amygda- loldal j)orphyrite, having a gray, nearly amorphous background, which contains greatly altered plagioclases, epidote, kaolin, and some calcite. Frequently the epidote is contained in the plagioclase and clearly is secondary to it. A large amygdule con tains a core of epidote, which is surrounded by an aggregate of jiale green chlorite. THE ^;ASTE1{^• AKEA. 387 ETposurcs nvar conur of Sees. 11, ]'>, ^^, So, T. 17 h'., R. tS TV'., Miehigan. liO. CiiTciistiiiie-conaloim'iiitcs. 8pccimc'ii {yj,21 (slide :iy44), 1985 N., 170 W.; 7377 (slide L'OW), IIIIK) N., ISO W., See. lili, T. 47 N., R.43 W., Mieliigaii. The matrix of the rock is dull jjiay, in ])laces heavily iron stained, aphanitic, aiid sometimes slaty. The apparent i)ebbles resemble very closely the nuitrix in whieli they are contained. The thin sections are very fine grained and obscure. They" appear, however, to contain largo illy deliued fragments, derived from a line grained jrorphyrite, M'hich are mostly composed of gray material containing minute tabular plagioclases. These fragments are set in a matrix, the chief constituent of which is quartz, but which also contains abundant material like that composing the tragments. 21. Greenstone-conglomerate. Specimens 9222 (slide 2942), 9223 (slide 3002), 9225 (slide 2943), 7375 (slide 1843), 1970 N., 1640 W., Sec. 23, T. 47 N., R. 43 W., Michigan. The matrix of the rocks is dark green and thinly foliated. The i)ebbles are in part of the pale green sort found in the ])reviously described conglomerates, while some of them are coarser grained than usual and have the characteiistic ajipearance of massive basic eruptives. The matrix consists, as in several cases before, of finely crystalline quartz, mingled with the debris derived from the basic fragments. The fragments vary in size fi'om this tine material to large bowlders. Some of the smaller fragmental areas are almost completely altered to chlorite and gray material, but the roundish forms which they still retain probably represent original fragments. The larger pebbles are plainly from basic eruptives. The coarser ones consist of greatly altered plagioclase, gray leucoxene, chlorite, and blades of actinolite or hornblende, the latter being plainly of secondary origin. At times these hornblende blades are so large as to include many particles of the other minerals of the pebbles, even con- taining so much foreign material at times as to make a single individual of horn- blende appear in section as detached areas. SECTION IV.— FRAGMENTAL AND FERRUGINOUS ROCKS NORTH AND EAST OF THE GREENSTONE-CONGLOMERATES. Geographical distribution. — North and east of the greenstone-conglom- erates occurs a continuous wide belt of fragmental rocks, which extend from near the north quarter post of Sec. 14, T. 47 N., R. 44 W., Michigan, to the center of Sec. 28, T. 47 N., R. 42 W., Michigan, a distance of about 11 miles. Through T. 47 N., R. 43 W., Michigan, this belt runs in a nearly east and west du-ection, and' probably has a surface width through this town- ship of about 1^ miles. West of T. 47 N., R. 43 W., Michigan, the few 388 THE PENOKEE lEON-BEAEING SERIES. exposures found indicate a northern trend to the belt. In the other direction — that is, east of T. 47 N., R. 43 W., Michigan, the belt bends southward, entering T. 47 N., R. 42 W., Michigan, in Sees. 18 and 19, and from this place describes approximately the arc of a circle, the easternmost point of which is in the center of Sec. 28. Surrounding r.ocks. — The rocks south of the belt vary in their charac- ter. In Sec. 14, T. 47 N., R. 44 W., Michigan, this rock is a porphyrite, of such a nature as to indicate that it is a surface flow. Through the greater part of the distance, from the west line of Sec. 13, T. 47 N., R. 44 W., Michigan, to the northwest part of Sec. 23, T. 47 N., R. 43 W., Michigan, the greenstone-conglomerate is the nearest known rock. It is to be observed that here is a strip of country, in some places as much as half a mile wide, in which no exposures are known between the fragmental rocks and the green- stone conglomerates ; therefore there may be between these two formations another belt. From the north quarter post of Sec. 23, T. 47 N., R. 43 W., Michigan, to the eastern end of the belt, its southern boundary is the under- lying complex of hornblende-schists and mica-schists, gneisses and granites. Here there is little doubt that these rocks are the immediately underlying ones ; for in several places in the fragmental series are basal conglomerates and recomposed granites, which are chiefly composed of debris derived from the crystalline rocks immediately to the south, while in one sec- tion the actual contacts between the fragmental and nonfragmental rocks are seen. The rocks north of the fragmental belt, from its western end to near the center of Sec. 18, T. 47 N.,. R. 42 W., Michigan, are the greenstones of the overlying Keweenaw series. In two places the fragmental rocks are found very close to the greenstones, and it is therefore probable that these rocks lie immediately to the north of the fragmental belt. East of the center of Sec. 18, T. 47 N., R. 42 W., Michigan, no exposures are found northeast of the belt except in Sec. 28, where the Eastern sandstone is found in hori- zontal position, unconformably overlying the fragmental belt of rocks under discussion. Continuation of the helt east and west. — Whether this belt continues east and west of the area outlined for it upon PI. xiii is an open question. West .TIIK HAHTICUN AltlOA. 389 of the westernmost exposure of the belt, in tlie nortli part of Sec. 14, T. 47 N., R. 44 \V., Michigan, is tlie valley of the Little Presque Isle, in which there are no exposures. About a mile west of the Presque Isle, and 1.^ miles west of the exposure referred to, is developed, b}- a test pit, rocks which are almost purely nonfraymental sediments, but which have mingled with them some fragmental sedimentation. It is doubtful whether the belt under discussion ought not to be carried west to this pit, and from here be continued westward until it merges into the iron-bearing belt of the main area. The eastern «nd of the belt, in Sec. 28, T. 47 N., R. 42 W., Mich- igan, is very narrow. Between the gneisses and granites and the horizontal Eastern sandstone are but a few score of feet, or at most one or two hundred feet. There is, then, but little room at surface for the belt under considera- tion. Whether southeast of the center of section 28 it is entirely covered by the Eastern sandstone or not we have no means of knowing'. In case the Eastern sandstone does thus overlap, we have here the eastern end of the Penokee series. Struchire of the belt. — The exposures in T. 47 N., R. 44 W., Michigan, and most of those in Sec. 28, T. 47 N., R. 42 W., Michigan, are without structure; but the most of the exposures in T. 47 N., R. 43 W. and 42 W., Michigan, are clay-slates or graywacke-slates, which have well defined strikes and dips. In the few outcrops in T. 47 N., R. 42 W., Michigan, in which this dip is known, it is northeast. These dips are not indicated upon the map because they are not so accurately known as could be desired. However, at one exposure in the southeast part of Sec. 20, the dip is clearly northeast, while the statement of the explorers who have done the test pitting generally agree with the above statement as to a northeasterly dip. In T. 47 N., R. 43 W., Michigan, however, where the exposures are the most numerous of anywhere in the belt, a portion of them dip north and a portion south, the greater number of exposures hav- ing the latter inclination. These apparent southern dips are of great impor- tance, because these are the only known ledges with a southern dip which unquestionabl}^ belong to the Penokee series. The more ferruginous of these rocks — that is, the few exposures which approach a jasper — have a northern dip. The rocks which show a southern 390 THE PENOKEE lEOlSr-BEARING SERIES. inclination are rather laard, green and black slates. All ledges in wliich a southern dip was found were closely examined, in order to ascertain whether this apparent southern dip is true bedding, or is due to slaty cleav- age. It was found impossible to determine this point in most cases. In one or two ledges there is a decided banding with northern dips trans- verse to the southern cleavage. In these cases, at least, it seems probable that the southern dips are cleavage and not bedding. Taking all the facts into consideration, it is probable that all of these apparent southern inclina- tions are due to cleavage and the true dip of the whole belt is north- ward, as a part of it certainly is, and as are all of the remaining rocks of the whole area belonging to the Penokee series. This probability is rendered greater by the fact that all rocks which do not readily take on a slaty cleavage, like the jaspers, have a northern dip ; while the clayey rocks, kinds which are known to most readily take a slaty cleavage, are the only ones which exhibit a southern inclination. General petrographical character. — The rocks of the belt, as before stated, are essentially fragmental, although the amount of nonfragmental sedi- ments is not inconsiderable. Many of the exposures are simple frag- mental rocks, but in numerous places, mingled with the fragmental, is a greater or less quantity of nonfragmental material ; either a carbonate and the products of its alteration, or chert, or both. Grenerally this nonfrag- mental material is subordinate in quantity to the fragmental, but in a num- ber of places, in narrow belts, nonfragmental sedimentation has built up the larger part of the rock, while a considerable thickness in several places is formed by nonfragmental and fragmental sediments in about equal proportions. The pure fragmental kinds, those which are both fragmental and nonfragmental, and those which are purely nonfragmental, can not be separated from one another in any stratigraphical order. In some parts of the belt its whole section, so far as known, is partly nonfragmental ; while in others fragmental sediments exclude altogether nonfragmental sediments; and in yet other sections both classes of rocks are found. The only possible classification of the rocks of the belt is then a lithological one, and as their phases are exceedingly numerous, and the various phases merge into one another, any classification would be to a large extent arbitrary. THE EASTERN AREA. 391 Below is •^■ivcn ii list of kinds, luit even this doos not give a full idea of the iiuuiy pliiiscs of i-(ifk whicii iin- nliiutst iis luiiiieroiis as the ledges found, and whieh if the wliole tiMitli were knowii, would doubtless comprise all possible pradations between tlic \arities mentioned: Fragmental Mingled fragmental and nonfragmental . . Quartzitos. Foldspatliic (piartzites. Recoiiiiioscd fjiaiiitc — often (onglomeratio. Graywacke. Graywacke-slate. Clay-slato. Sericite-slate. Chlorite-slate. Hematitic and magnetitic quartzite. Henuititic and magnetitic graywacke. Hematite-schist. Hematitic and magnetitic graywacke-slate, Ferro-doloniitic slate and graywacke. Chert-breccia. L Cherty quartzite. f Chert. Nonfragmental sediments ^ Cherty ferro-dolomite. [ Ferro-dolomitic chert. The rocks above included among the fragmental sediments are given names which have been before used in the descriptions of the Quartz-slate and. Upper slate members in the main Penokee area. These names are here used with the same significance as before and no characterization need be given of them as a whole. In the tabulations following the rocks are described in detail. The points of general interest shown by the belt are the mingled fragmental and nonfragmental sediments ; the change in the nature of the nonfragmental sediments in following the belt from west to east; and the presence of chert-conglomerates and basal conglomerates at the east end of the belt. Mingled fragmental and nonfragmental sediments. — The westernmost exposure of the belt is a hematitic schist and quartzite in the north part of Sec. 14, T. 47 N., R. 44 W., Michigan. The proportion of iron oxide is so great as to constitute a continuous ramifying background in which the frag- mental quartz and feldspar is stuccoed. In no other ledges of as large size in the belt is the proportion of iron oxide so great. ■ Rocks almost identical 392 THE PENOKEE IROi^-BEAEING HERIES. iu character with these are found in the fragmental belt south of the green- stone-conglomerate, in the southwest part of Sec. 19, and the northwest part of Sec. 29, T. 47 N., R. 43 W., Michigan. So like are the exposures from' these different localities that one is inclined to explain their similarity by supposing the area to have a structure which makes them contempo- raneous in formation. It can as well be explained, however, by consider- ing one of these belts as older than the other, the similarit}' being due to the recurrence of like, or nearly like conditions at two difiPerent times. In passing to the eastward, all the exposures in Sec. 13, T. 47 N., R. 44 W., Michigan, and in Sees. 18 and 19, T. 47 N., R. 43 W., Michigan, are ferru- ginous— most of them heavily. The larger part of the iron contained in the rocks is in the form of hematite, although in some places magnetite is found. In the northeast part of Sec. 19, T. 47 N., R. 43 W., Michigan, layers from mere films to several feet thick are so largely composed of hematite as to resemble somewhat an iron ore. This amount of iron oxide is so great as to have encouraged explorers to follow these beds to some depth in the hope of obtaining merchantable iron ore. In Sees. 17 and 20, and 16 and 21, T 47 N., R. 43 W., Michigan, the exposures are much less heavily ferruginous than farther to the westward. The amount of hematite and limonite remains considerable, and this is particularly true iu the south half of the belt. The most of the iron is in the form of oxide as before, but with this iron oxide a considerable quantity of ferriferous carbonate is associated. The ferro-dolomite or siderite is seen in all stages of alteration to iron oxide, the areas of the latter at times being beautiful pseudomorps. This car- bonate is also accompanied by a good deal of finely crystalline or cherty quartz. In the cases of some exposures in Sees. 1 6 and 20, T. 47 N., R. 43 W., Michigan, the amount of nonfragmental material is fully as great as the fragmental quartz and feldspar mingled with it. From the east side of Sec. 15 to the east end of the area in Sec. 28, T. 47 N., R. 42 W., Mich- igan, the quantity of iron oxide contained in the belt is small. Upon the other hand, the amount of ferriferous carbonate and of cherty silica is far greater than to the westward. The exposure in the eastern part of Sec. 15, T. 47 N., R. 43 W., Michigan, is in almost equal quantit}^ fragmental and nonfragmental material. In places, in narrow bands, cherty silica and THE EASTERN AKEA. 393 carbonate are almost free from any fragmentxil material, but the rock com- jjosinginost of the exposure has a background of chert and carbonate which contains abundnnt fragniental material. Almost in contact with the crys- talline rocks to the soutli, in the north part of Sec. 23, is a band a few feet thick of nearly pure cherty ferro-dolomite, and the fragmental rocks to the north contain quite a quantity of chert and carbonate. In the exposures along the east side of Sec. 14, T. 47 N., R. 43 W., Michigan, feiTO-dolomite is a chief constituent. From the west side of Sec. 13, T. 47 N., R. 43 W., Michigan, to the center of Sec. 28, T. 47 N., R. 42 W., Miclijgan, chert and feiTO-dolomite (chiefly the former, except locally) constitute the background in which the abundant fragmental material i's contained; while in some of the quartzites and conglomerates the n( mfragmental material sinks to an insignificant quantity. It will thus be seen that there is a marked change in the nonfrag- mental sediments, in passing from west to east. Throughout its eastern portion this matei'ial is cherty silica and a ferriferous carbonate; in its central part it is cherty carbonate and iron oxides, and here in most of the sections the iron oxides are seen in actual process of formation from the carbonate; in the western part of the belt the iron present is almost wholly in the form of oxides. Considering these facts by themselves, it seems a natural conclusion that it is probable that all of the iron was originally present as a carbonatearti(^les of (iiiartz anil fcldspiii-. The-graiiis of qnaitz have received secondary enlar},a'nient. Those of feldspar liav<\ larj;ely altered to aggregates of green chlorite, this mineral now heitig one of the most i)lentifiil in the section. Uniformly distributed are luiinerons crystals of magnetite. •4. Ilematitic graywackes and henuititic schists. Specimens D.'JUliA (slide 3015) 140 N., 1000 W.; 7138 (slide 1S!)3), 150 K, 1000 W.; !);!03 (slide tL'liO); 0301 (slide 4L'31); 0305 (slide 3109), 175 N., 975 W.; 7439 (slide 1S94), ISO N., 1000 W.; 7440 (slide 1895), 190 N., 1000 W., Sec. 18, T. 47 N., E. 43 W., Michigan. The rocks vary from rather fine grained, banded, dark greenish gray gray wackes to a dark brown hematitic schist. The belts of heavily ferruginous material vary in thickness from mere iilms to layers several feet across. In thin sections, the least ferrnginous phases are mostly composed of small frag- mental particles of quartz and feldspar. The enlargements of the qnartz are often relatively wide. The feldspar is altered to a considerable extent to kaolin and chlorite. In the interstices are found, in varying proportions, tinely crystalline quartz, chlorite, hematite, crystals of magnetite and liakes of white mica. In the more ferruginous phases, the hematite, with some magnetite, constitutes a continuous ramifying sheet which contains fragmental material. 5. Perrugiuuous graywacke. Specimen 9302 (slide 3014), 1850 IsT., 325 W., Sec. 19, T. 47 K, E. 43 W., Michigan. The rock is dark green, fine grained, massive, and includes a few crystals of pyrite. The thin section ditfers from the least ferruginous phases of 4 only in that in the interstices a considerable quantity of ferro-dolomite is present. 6. Chloritic graywacke-slate. Specimen 9238 (slide 4478), 1750 N., 1250 W., Sec. 20, T. 47 N., 43 W., Michigan. The rock is greenish gray, of a medium uniform grain and feebly schistose. Fragmental quartz and feldspar in about equal quantity, the latter being a mixtirre of orthoclase, microcline, and plagioclase, comj)ose nine-tenths of the thin section. The grains of quartz are enlarged and the interstices are filled with finely crystalline quartz and chlorite. Iron oxide is an accessory. The rock -is a typical graywacke-slate. 7. Chloritic and magnetitic graywacke-slates, from south part of belt. Specimens 9237 (slide 4477), 1625 N., 750 W.; 9236 (slide 4476), 1750 N., 750 W., Sec. 20, T. 47 N;, E, 43 W., Michigan. The rocks are like 6. The thin sections differ from that of 6 in that the feldspars have altered more extensively to chlorite and kaolin ; in that there was originally present a considerable amount of clayey material, and in that they contain some quantity of magnetite which is mostly in crystals. Sericite is an accessory. 398 THE pen6kee ieon bearing series. 8. Ohloritic graywacke-slate, uortU of 7. Specimen 93'55A (slide, 4475), 300 iST., 375 W., Sec. 17, T. 47 K, R. 43 W., Michigan. The rock is dark green and fine grained, but contains numerous small grains of quartz and altered fi'agments of feldspar of sufficient size to be perceptible to the naked eye. The thin section shows a rather abundant grouudmass, consisting of finely crystalline quartz, chlorite, kaolin, and iron oxide. Contained in this matrix are abundant grains of both quartz and feldspar of widely varying sizes, the fornu^r often being slightly enlarged, and in such cases merging gradually into the clayey matrix. 9. Clay-slate, north of 8, at top of belt. Specimen 9235 (slide 3003), 400 N., 800 W., Sec. 17, T. 47 N., R. 43 W., Michigan. The rock is dark gray to black, aphanitic, and has a well developed slaty cleavage. The thin section is exceedingly fine grained, but appears to consist of an inter- mingled mass of chlorite, quartz, kaolin, and ferrite, with a little ferro-dolomite. Section mostly in the west half of Sees. 16 and 21, T. 47 N., B. 43 W., Michigan. 10. Chloritic graywacke-slate, from south part of belt. Specimen 7416 (slide 1875), 1830 N., 100 W., Sec. 20, T. 47 K., E. 43 W., Michigan. The rock closely resembles 7. The thin section differs from those of 7 only in containing in the interstices much more chlorite and apparently a little magnetite. 11. Perro-dolomitic graywackes, north of 10, at south part of belt. Specimens 7397 (slide 1860), 9234 (slide 2947), 1900 N., 210 W., Sec. 20, T. 47 K, R. 43 W., Mich- igan. The rocks are dark gray, rather fine gTained, and contain very numerous dark brown areas of mingled ferro-dolomite and iron oxide, and white areas of ferro- doloniite. The sections have an exceedingly ffne grained matrix, consisting of crystalline and amorphous silica, of chlorite, and of black opaque iron oxide, a little of which is magnetite. ' Scattered plentifully through this matrix are fragmental particles of quartz- and feldspar. The areas which give the rocks their mottled appearance are the ferro-dolomite or limonite or the two combined. All of these areas were plainly once ferro-dolomite, and the alteration of this mineral has produced limonite. All stages of the change may be seen, from those areas which are pure ferro-dolomite to those which are wholly limonite. 12. Chloritic graywacke-slate, from lower middle part of the belt. Specimen 7387 (slide 1852), 230 N., 1850 W., Sec. 16, T. 47 N., R. 43 W., Michigan. The rock is dark gray, fine grained, schistose, but weathers to a dull pale yellow. The thin section differs in no important respect from that of 8. 'I'lll'; i: ASTERN ARK A. 399 13. Cliiy-slato, from iiiiddli^ dC hell. Spcciiiicii T.'WS (slide 1853), 380 N., 1!)80 \V., Sec. 1(5, T. 47 N., K 43 W., Micliif;iUi. The rock is jji'ayish green, apliuiiilie, and iinely sidiiptose. The thill section has a Iinely crystalline iinitrix consisting of (iiiartz, chlorite, kaolin, and iron oxide, and contains many small Iraginental particles of (jnartz ajul feklispar. 14. Clay-slate, from upper i)art of belt. Specimen 73S0 (slide 1854), 650 N., 50 W., Sec. 17, T. 47 N., E. 43 W., Michigan. The rock is black, aplianitie, and easily clcavable. The thin section is like that of 0. 15. Ferro-dolouiitic slate, from lower middle part of belt. Specimen 9233 (slide 2946), 175 N., 1610 W., Sec. 16, T. 47 N., R. 43 W., Michigan. The rock is dark grecnisli gray, fine grained, scliistose. Weathering gives a thi!' chert, exce|)t that it coutaius many small fragmental particles of (piartz as though it were a non fragmental sediment wliicii had received a considerable (luau- tity of mecluuiically deposited (piart/,. The interstitial material of the section consists of cherty silica, which contains large brilliantly polarizing Hakes of scricite and also areas of brown hydrated iron oxide. This matrix includes many large, well rounded, simple grains of quartz and few of feldspar. 48. Eecomposed granite. Specimen 12626 (slide 5375), 1730 N., 1480 W., Sec; 28, T. 47 N., E. 42 W., Michigan. The rock is chiefly composed of the coarse granite detritus, but contains some seams of fine quartzite-like material and few white quartz pebbles. The granite detritus is often in large complex fragments, but also there are present numerous large, apparently simple grains of quartz and feldspar. The section is cut from the fine grained quartzite-like part. It consists of a matrix and contained fragments in about equal quantity. These fragments are almost wholly quartz, rather small, mostly well rounded, and often enlarged, the enlargements fading into the fine grained matrix. Flakes of muscovite of some size appear, which are taken to be fragmental. The matrix is mostly cherty quartz, but mingled with it is a good deal of sericite, hematite, and limonite. 49. Sericite-schist. Specimen 12629 (slide 5378), 1748 N., 1462 W., Sec. 28, T. 47 N., R. 42 W.^ Michigan. The rock varies from dirty yellow to brown, is finely foliated, and upon its cleavage surface has a micaceous sheen. The thin section is essentially like that of 44, the only difference being that the fragmental particles are larger, the folia of the sericite is not in short wavy folds, and the section is heavily stained with iron oxide. 50. Recomposed granite. Specimen 12818 (slide 5492), 1,507 N., 1404 W., Sec. 28, T. 47 N., E. 42 W., Michigan. In hand specimens this rock would be taken foran ordinary granite unless it were closely inspected. The eye recognizes coarse pink feldspar, translucent quartz, and a green material, all being arranged so as to show a somewhat banded appear- ance like a coarse gneiss. The only indications that it is different from an ordi- nary crystalline rock are its somewhat nodular weathering, as though comjiosed in part of i^ebbles, and a roundish appearance of the larger white quartz areas. A large part of the section consists of complex granite fragments, composed of quartz, microcline, orthoclase, chlorite, iron oxide, and other accessoi-ies. In the inter- stices are finely crystalline quarts, (ihlorite, and some ferrite, included in which are large simple particles of quartz and feldspar. 408 THE PENOKEE lEON-BEARING SERIES. 51. Cherty qnartzites. Specimens 12620 (slide 5371) ; 12820 (slide 5493), IGOO K, 13C0W.; 12621 (slide 5372), 1600 N., 1300 W.; 12912 (slide 5516), 1615 K, 1360W.; 12917 (slide 5519), 1628 N., 1360 W.; 12627 (slide 5376), 1730 K, 1480 W.; 12812 (slide 5488), 1712 ISr., 1462 W., Sec 28, T. 47 N"., E. 42 W., Micliigan. Tliese quartzites are tolerably coarse grained and massive. Some of them con- tain rather large pebbles of white and cherty quartz. They all have a rough fracture, due to the fact that when broken the large quartz fragments are torn from their sockets. In color they vary from greenish gray to grayish brown or red. Specimens 12620 and 12621, besides being reddened are somewhat honeycombed, as though some constituent, possibly a carbonate, had been leached out. The sections of these rocks consist of two parts, a matrix and coarse material. The latter is chiefly quartz in large, simple, or coarsely complex grains, which are often well rounded, but also quite as often more or less angular, while part of them are very angular. There are a few fragments of very finely crystalline and i^erhaps partly amorphous cherty silica. Many of the simple quartz grains are slightly enlarged, the enlargements merging gradually into the matrix. Fraginental feldspar is quite plen- tiful in one of the sections, and sparse in the others. Also in one section are nu- merous large altered leaflets of biotite. The predominant constituent of the matrix is cherty silica, mingled with which is much sericite or kaolin, in small brilliantly jjolar- izing flakes. In one or two sections this sericite is so plentiful as to be comparable with the silica in abundance. In such sections some of the complex seri(Mte-quartz areas have a roundish appearance as though they were complex fragments. They perhaps represent altered feldspar detritus, as indicated by the fact tJiat in one of the sections many undoubted feldspars have undergone alteration to quartz and seri- cite. The cavities seen in hand specimen are found in thin section to be mostly bor- dered by iron oxide. They are of very irregular- form, resembling in this respect areas of iron oxides found in the matrix; and it seems probable that the cavities were either entirely filled with iron oxide, or more jirobably ferriferous carbonate, which largely went into solution, but from which the iron oxides now remaining were pro- duced. This probability is further strengthened by the fact that in other rocks pre- cisely similar to these and associated with them is found a good deal of carbonate. The sections differ from those of the slates and graywackes in one important partic- ular. The fragmental particles are all large, none nearly aj)proaching in minuteiiess the particles of the matrix, while in the graywackes there is every gradation from the coarsely fragmental material to the fine matrix. The minutely crystalline matrix in these rocks is taken to be a nonfragmental or recomj)osed sediment. The sericite in the matrix is largely arranged with its blades in a common direction. 52. Cherty and ferro-dolomitic quartzites. Specimens 12915 (slide 5517); 12916 (slide 5518), 1628 N., 1360 W.; 12628 (slide 5377),' 1730 N., 1480 W.; 12922 (slide 5520), 1744 N., 1466 W., Sec. 28, T. 47 N., R. 42 W., Michigan. THE KASTEim AREA. 409 TliC! rocks vary troiii Hiccnisli gray to inotllcd fiicenisli gray or red, aro inetlixiin {jraiiu'il, massive, and break witli a soiiiewlial rouKli fVaeture, resemliliii};- in tliis res])wt 51. ill must respects liie tliiii sections are similar to those of "d, tlie essential (lif- ferencii between the two beinj;' tliat the matrices .te, closely resemble belts in the Penokee series to the westward. So far as the Iron-bearing- member is concerned there is identity. The ferruginous and fragmental rocks north and south of the g-reen.stone-conglonierates differ from the fragmental rocks to the- westward, in that nonclastic is mingled with the clastic material. Aside from this dif- ference, the above belts of the eastern area are the exact parallels of belts in the main area. The lithological evidence, then, indicates that the eastern area belongs to the same great period of time as the Penokee series to the Avest. The only question, then, which needs here to be discussed is whether the greenstone-conglomerates belong- to this same series. It appears to me that all the facts indicate that they do. The data upon wl'ich this opinion is based are as follows : The conglomerates are inextricably mingled and interlaminated with black, cleaved, unmistakably fragmental slates. That these slates and greenstone-conglomerates have a common age can hardly be doubted, for there appears to be every gradation between the two classes of rocks. This fact is strong lithological evidence for placing these rocks in the Penokee succession, where similar slates are so largely developed, and not in the Southern Complex or in the Keeweenaw sei-ies to the north, where no such rocks are known, nor in the cherty limestone series, with which they have no apparent coiuiection. It may be asserted that the greenstone-conglomerates are more altered and obscure than any other i-ock in the Penokee series. This argument loses its force when the unusual nature of the materials of which they are composed is considered. The great mass of the fragmental rocks of the 420 THE PENOKEE IRON-BEARING SERIES. series is derived from granites, gneisses, mica-schists, hornblende-schists, etc.; in other words, from rocks which are always strongly quartzose and which contain a large qu.antity. of acid feldspar. From materials of this sort quartzites, graywackes, and slates are naturally formed. That the quartz and acid feldspars are often in a very fresh condition is not strange, but it has been seen that even these acid feldspars have exten- sively, in the Upper slate member, altered to the more basic minerals, mica and chlorite, with tlie simultaneous separation of quartz and the resultant formation of a mica-schist or chlorite-schist. These mica-sclnsts are unmistakably parts of ■ the Penokee series, and are as much changed from their original condition as is the basic eruptive material of the greenstone-conglomerates, notwithstanding the fact that the latter ax'e composed of materials very readily alterable. Basic eruptive rocks are well known to have undergone exteirsive changes, even when occurring in solid masses which belong to geologic periods long subsequent to that of the Penokee series. It thus appears that the widespread alteration of the basic detritus in these greenstone-conglomerates is no proof that they are not of the same age as the rocks of the typical succession." Also in the greenstone-conglomerates themselves is found a small quantity of well worn fragmeiital quartz and feldspar, which in every respect resembles like material in the graywackes and quartzites. It is thus plain that the litho- logical evidence points toward a classification of the greenstone-conglomer- ate with a series subsequent to the Southern Complex, a series of clastic rather than crystalline rocks. Finally, the association and relations of the greenstone-conglomerates to the other classes of rock adjoining, which unquestionably are the equiv- alent of the main Penokee succession, are such as to make it scarcely conceivable that they could be a part of the eruptives of the overlying Keweenaw series. StratifjrapMcal evidence as to equivalence with the main Penokee area. — The stratigraphical evidence that the eastern area rocks belong to a series separated both from the schists, gneisses, and granites to the south of them and from the great range of greenstones to the north is precisely like that which proves that the clastic formations to the west belong to a dis- THE KASTEUN AIJEA. 421 tinct series. Without reiH'iitin<>- tlu^ Mr^-uinciit in dctnil wliich is given in anotlier place (Chap, ix, 8ec. ii), it is only uecussary to iiiciition the salient tacts which show that tlie same relations ))revail l)etween the three great series of rocks east of the Little Prescpie Isle that arcf fonnd to the w- important points. Tliis area was the center of ^reat volcanic activity ; conse(piently the sedimen- tary succession includes large thicknesses of interstratified lava flows and volcanic tutfs, which are not paralleled by any rocks that ai-e found in the western area.. Further, this volcanic material has greatly disturbed the water-deposited sediments in the district, so that it is difficult to certahdy correlate the formations east of the Little Presque Isle with those west of it. Another point in which this area differs from the western one is that in one place the relations of the horizontal Eastern sandstone to the Penokee series can be made out. No exposures certainly belonging to the Cherty limestone member are found within the area. The Quartz-slate member is not known to be con- tinuous in this area. Typical exposures of the formation, which have near the base variegated slates, and for the upper horizon the vitreous quartzite, are found at one or two localities. The lowest belt which certainly is continuous for some distance is the Iron-bearing member. Even in this wide gaps occur in which no exposures are known. Its typical rocks are almost exactly like those of the iron for- mation to the west. There is here, however, closer relations between the original siderite and the actinolite and magnetite than to the westward. In this respect the rocks of this belt resemble very closely those of the Animikie iron-bearing formation. The one important point in which this part of the Iron-bearing member differs from that to the westward is that mingled Avith it is fragmental material. There are all gradations between the pure nonfragmental sediments of the iron belt and the fragmental material of the slate formations ; also there are in places interlaminations of the two. It thus is clear that during the time of the formation of the Iron-bearing member there were interruptions, as a result of which the belts of pure non- fragmental, water-deposited sediments are always narrow. Probably result- ing from this is the fact that within this area no workable ore deposits have MON XIX 28 434 THE PENOKEE lEON BEAEmG SERIES. been found, the belt always being narrow, so that the amount of iron in the carbonate present was insufficient when concentrated to form large ore bodies. The greenstone-conglomerate area varies from a material which is a basic eruptive amygdaloidal flow, through various volcanic or semivolcanic elastics, to rocks which are- wholly water-deposited. North and east of the greenstone-conglomerate is a continuous wide belt of fragmental rocks, which extends nearly to Gogebic lake. The rocks south of this belt vary in their character, being in the western part a por- phyrite of such a nature as to indicate that it is a surface flow; through the central part of the area, greenstone-coilglomei'ate, and in the eastern part of the area, the gneisses and granites of the Southern Complex. North of the belt are the eruptives of the Keweenaw series. At one place, just west of Gogebic lake, the Eastern sandstone is the overlying rock. It seems prob- able that this belt of fragmentals is the equivalent of the northern part of the Iron-bearing member with perhaps the lower part of the Upper slate member to the westward. If it extends farther east than Gogebic lake, it probably passes under the Eastern sandstone. The rocks of this belt are very largely clay-slates which have an apparent southern dip. In places, however, they become strongly ferrugi- nous, when a northern dip is usually found. The latter are believed to represent true bedding and the southern dips to be secondary cleavage induced by pressure. The rocks have a Avide variation in lithological character, running from those which are purely fragmental to those which are purely nonfragmental sediments. • The phases intermediate between these two extremes are those most abundant. The fragmentals are very nmcli like those north of the iron-bearing belt in the main area. The iningled nonfragmental materials are the various oxides of iron, siderite, and chert. The rocks, then, vary from those which lithologically belong to the Iron- bearing member to those which are typical Of the Upper slate. The iron varies in its combination from an oxide in the western part of the area to a siderite in the eastern part. This is another case in which the widespread occurrence of iron oxide is due to the alteration of a ferriferous carbonate. The basal portion at several places in the eastern part of the belt are THE EASTERN AREA. 435 recoraposed <;'riiiutic rocks, whicli consist of liiinl\' rt'cemented granitic debris and arc separable tVoiii tlic orij^-iual rocivs only lt\' an exctminatioii of the thin sections. The eruptives of the Eastern area are in several detached exposures, some of tlieni behiy of hirge size. A jutrtion of them are porplnrites con- temporaneous witli the rocks of the series in which they are fotuid; otliers have the structure of a diabase, or even tliat of a gabbro, and these are taken to be deep seated rocks. Between these two extremes there is found in various parts of the area intermediate i)hases. The augite of the diabases has often altered to hornblende, and this secondary hornblende has undergone a new growth. Also, the unaltered augite is not infre- quently surrounded by a sheatli of smaragdite, the crystallographic rela- tions of the original and secondary minerals being those known to occur between augite and amphibole paramorphic after it. The rocks of the various belts of the Eastern area, with the exception of the greenstone-conglomerate, correspond closely in lithological characters with those of the Penokee series proper to the westward, the only differ- ence of importance being that in the Eastern area there is more fre- quently found mingled nonfragmental and fragmental sediments. The greenstone-conglomerates bear such structural relations to the sediments whicli certainly belong- to the Penokee series, are mingled with them so intricately, and are so certainly surface volcanic accumulations, that there is no siifificient reason for placing them elsewhere. The stratigraphical evidence that the Eastern area rocks belong to a series separated both from the Southern Complex and from the Keweenaw series is precisely like that which proves the clastic formations to the west to belong to a distinct series. Notwithstanding the exceptional width of the Eastern area in certain parts and the southern dips which are found in the slate belt north of the greenstone-conglomerates, it is believed that the succession here is probably a simple conformable one. The sequence of events in the district seems to have been as follows: As in the main area, the complex of schists, gneisses, and granites, and probably the series to which the Clierty limestone belongs constitute a base- ment upon whicli the Penokee succession was deposited. In certain places rocks which are the equivalent of the Quartz-slate member were deposited. 436 THE PElSrOKEE IRON-BEARING SERIES. Above the Quartz-slate begins tlie iionclastic sedimentation of the Iron- bearing member. The deijosition of this belt did not go on undisturbed, but alternated with clastic sediments; and long before the whole of t! : iron-bearing member of the west was built up came the great volcanic outbreak which resulted in piling up the series of flows, tuffs and inter- stratified elastics of the greenstone-conglomerate area. At or near the ces- sation of volcanic activity the mingled clastic and nonclastic sediments which overlie them began to form. At this time apparently the Iron-bearing member of the western area had not yet wholly formed. Above the latter followed probably fragmentals which were the equivalent of the Upper slate member; but if this were the case they were wholly or nearly wholly removed by erosion. After a long period of degradation began Keweenawan time. Before the Eastern sandstone at the east end of the area was depos- ited the three series of the district were tilted into their present inclined position, and during this time and subsequently underwent enormous denudation. CHAPTER IX By C. E. Van Hise. GENERAL GEOLOGY OF THE DISTRICT. Section I. Flexures aud faults. Curviug of the layers. Fault at Bad river. Fault at Potato river. Fault in the Eastern area. Section II. Structure. The Southeru Complex. The Cherty limestone aud Quartz-slate members. Unconformity between the Southern Comjilex and the overlying Cherty limestone and Quartz-slate. Unconformity between the Cherty limestone aud the Peuokee series proper. The Iron-bearing aud Upper slate members. The unconformity at thebase of the Keweenaw series. The Eastern sandstone and the unconformity at its base. R^sum** of geological history. Why the district is given a sepa- rate memoir. Dejith aud metamorphism. Section III. Correlation. Equivalency of Penokee series proper with Animikie series. Equivalency of Penokee and Mar^ quette series. Comparison with other series. SECTION I.— FLEXURES AND FAULTS. Curving of the layers. — The cnrving' of the layers of the Penokee series subordinate to its uplifting as a whole, has in the main been of so gentle a nature that they have not been broken. At the west side of R. 3 W., Wis- consin, there are several very sharp bends in the Quartz-slate and Iron- bearing members. There is a somewhat sudden turn in the trend of the layers near the middle of R. 2 W., Wisconsin. West of Sunday lake the nearest known rock in the iron formation is about one-fourth of a mile south of the north quarter post of Sec. 17, T. 47 N., R. 45 W., Michigan. East of the lake the southern boundary of the iron formation is one-fourth mile north of the southwest corner of Sec. 10 in the same township. (PL xit.) Consequently the iron belt suddenly swings from an east and west course north more than a half mile in a distance of 1^ miles. In the northwestern 437 438 THE PENOKEE IKON BEARING SERIES. part of Sec. IG is an exposure of green schist belonging to the basement complex ; so that most of the northern swing occurs before this point is reached. Tlie relations are such that it is possible to explain this sudden change in direction by a sharp curve, but it is possible tliat it is wholly or in part due to a fault. Sunday lake and the surroimding low ground make it not easy to settle this question. However, at none of the places men- tioned is it known that there has been any actual faulting. Thi'oughout the whole length of the series, which has been subjected to two great oro- graphic movements, no dislocations great enough to make the belt difficult to follow have occurred ; but at three places there is pretty decisive proof that faulting has taken place to some extent. Fault at Bad rwer.— The most considerable fault is that at Bad river, in R. 3 W., Wisconsin. The relations of the exposiires and the line of fault have been accurately mapped and described by Pi'of Irving. ^ PL xxxvi is wholly from data furnislied by this map. The explanation is also largely taken from Prof Irving, it only being modified to correspond with the present understanding of the succession of belts. West of the line marked "Supposed position of fault line " the layers of the Penokee series follow in the regular order. Near the base of the left hand side of the plate are ]napped large exposures of granite and gneiss. North of these on Bad river, and very close to the gneiss are exposures of the Cherty lime- stone member. Next follows the Quartz-slate, which is exposed through- out its whole thickness, having, as usual, as its upper layer a vitreous quartzite. Just above this quartzite follow the exposures, which rise in high cliffs above the river, belonging to the Iron-bearing member. They are so numerous and large that they practically show the whole thickness of the formation. Upon the north side of this bluff is a thin layer of garnetiferous magnetitic slate, which passes into a garnetiferous black slate, plainly con- stituting the base of the Upper slate member. North of this point are a number of large exposures belonging to this belt. On both banks of Bad river is shown a lai'ge exposure of diabase. East of the "Supposed line of fault " the succession of belts in the Penokee series is found to be identical with that just given, the only difference being that but a small part of the 'Geol. of Wis. vol. iii, pp. 150-152, with aoeoinpanyiiig atlas, PI. xxiil. (IKXF.lJAl. C.KOUKlY OK Till'; DISTUIOf. 43V> Quartz-slate is exposed ; linwever, tlie cliai-actefistic (juartzite at its upper liorizoii is seen. Tlie ii-on I'oniiation shows a pi-actically coiitiiiuous expos- ure and at its uppermost iioi-izcn is found the peculiar o,arnetit'erous slate, wiru-h ])asses into the i;arnetit'erous hlack slate at tlu^ hasi^ oC the Upper slate member just as on the west side of the fault. The occurrence of these characteristic layers both north and south of the iron formation — east and west of the supposed fault line — is most foi-tunate, enabling one to determine the exact limits of that formation and thus to give data for a satisfactory explanation of the distribution. This order is in such per- fect accordance upon both sides of this line that there could be little doubt that the belts represented a once continuous set of layers, eveir if we knew nothing about the series farther to the east and west. But when it is remem- bered that these several belts stretch in the same order for many miles in both these directions, this former continuity may be considered demon- strated. The distribution of exposures at Penokee Gap can then only be explained as having been caused by a fault between the two sets of expos- ures, for there is not room between them for accordance to be produced by a. bend of the layers, however sharp. The space between the quartz-slates on the west side of the fault and the upper part of the iron formation on the east side is not one-half the thickness of the latter, yet between these two the whole of it must pass if there is not here a fault. The Quartz-slate on the west is seen to l)e farther north than the northernmost exposure of the magnetite schists, toward which it strikes, whereas it ought to be con- tinuous with the belt of the same kind on the east side of the line marked "supposed line of fault." The amount of discordance is even more strik- ingly shown by the positions of the exposures of the iron formation. It inay be that a part of the discordance can be accounted for by a sudden bowing of the layers, although the strike of the rock gives little indication that this is the case. If the entire dislocation is taken to be due to a fault, the throw must have been at least 900 feet if at right angles to the direction of the strike of the rocks; and if it is diagotial, as is probably the case, must have been more than this. That this fault extends south of the southernmost exposure of the Penokee series and north of the exposures of the basement layer of the black slate is plain, but the layers of the upper slate are so 440 THE PENOKEE IRON-BEARIK^G SERIES. closely like one another, and so little is known of the relations of the rocks of the Southern Complex to one another, that its direction can not, beyond these limits, be made out. If the greenstone upon Bad river is taken to be an interbedded flow, it appears that the fault line jjasses between this and the black slates east of this place on the railroad; but if it is an eruptive dike cutting across the formation, as is probable, it gives no indication of the location of the fault line. Fault at Potato river. — At Potato river, near the east side of Sec. 19, T. 45 N., R. 1 E., Wisconsin, there is again strong evidence that a fault exists, althousrh here the throw is not so great as at Bad river. The relations of the expostires are exhibited by Fig. 6. Upon the east side of the river is a large exposure of green schist belonging to the Basement Complex. The Quartz-slate is in contact with the schist and extends northward in contin- uous exposure, with only a very slight break throughout its whole thick- ness. North of and immediately adjacent to the vitreous quartzite, con- stituting the uppermost member of this slate, are exposures of the Iron- bearing member. The strike of the slate east of the river is west 20° south. On the west side of the I'iver large exposures of the underlying green schist are noted. North of these follow smaller exposures of quartz-slate, and still farther north the vitreous quartzite, and above this the rocks of the iron- bearing belt; so that east and west of the river we have the exact location of the junction between the quai-tzite and iron-bearing belt, Avliile east of the river is known the exact contact of the quartz-slates and the under- lying schists. Now, it is manifest that the large exposures of green schists west of the river lie directly athwart the course of the quartz-slates. The same is true of the quartz-slate and quartzite on the west side of the river as compared with the rocks of the iron belt on the east side. Here, again it may be suggested that a sharp bowing will explain the structure,- and hence a fault is not necessary to explain the facts; but between the slates and the nearest schistose rock there are but 200 feet of space, in which a much greater thickness of slates must pass. Further, there is absolutely no indication by a change of direction of the strike of the slates that an}' such bowing has here occurred. If the whole of the discordance here is taken to be due to the fault, the horizontal throw amounts to 280 feet, as determined by GENKIJAL (ii:«)LUi;v OF Tllh: J)18Ti:iCT. 441 a transit survcv made 1)\- Mr. J. Parke CliaiiiiiiiL;', takliin' as tlu' points of ineasun'iiu'iil tlic jiiiictioiis between tlic vitreous (juartzitcs and the over- lying {'erruginous rt»cks on tlie east and west sides oftlie rivci-. Fault ill the Juisfmi urea. — In elia])ter viii, p. 424, it has been suggested that there is a fault in tiie east side of T. 47 N., R. 44 W., Michigan, run- ning in a diagonal direction from the southwest corner of Sec. 26 to a short distance west of the east quarter post of Sec. 13. The rocks west of this line have probably been thrown south of the corresponding layers to the east, thus explaining part of the discordance in the rocks of the iron formation, as Avell as the difference of one-half mile in the southern bound- ary of the Keweenaw rocks in the eastern part of the township, and also lessening the thickness of the Penokee rocks in this locality, but as the probability of this fault and its structural relations have been fully treated in the chapter referred to, the subject will not be further discussed here. At all of these places the ground between the exposures is covered on the east and west sides of the fault lines, so that it is not possible to accu- rately locate them. As is common, the broken layers have readily yielded to erosion and the fault lines in two cases have been utilized by the drain- age system of the district. SECTION II.— STRUCTURE.' The Southern Complex. — The subject proper of this memoir is the con- formable succession of rocks which contains the Iron-bearing member, and has therefore often been called the iron-bearing series. It has often been necessary- to allude to the series south and north. The latter series has not been given a separate chapter, for the varied rocks there found are parts of the great Keweenaw series which has already been treated in a separate memoir.^ In chapter ii the former rocks have been described in sufficient detail to compare them in their structural features and litho- ' The separability of tlie Penokee series from the Southern Complex and the Keweenaw series by means of vmconformities has been maintained by Prof. Irving in several of his publications. (See Literature, Chap, i.) As to these general relations, the only thing added is new evidence supporting these positions. '^Copper-bearing rocks of Lake Superior; by R. D. Irving, U. S. Geol. Survey, Monograph v. 442 THE PBNOKEE lEON-BEARIlSG SERIES. logical cliaraeters with the various rock belts of the Penokee succession. It has been seen that these rocks are of two general types: first, massive granites and gneissoid granites, and, second, fine grained schists, which technically are gneisses. None of them can properly be said to have a sedi- mentary strike and dip. All but the granites have a foliation. In the case of the gneissoid granites this foliation is of the coarsest character, but the foliation of the rocks varies from this to that so fine that it is only possible to detect it in the hand specimen by the direction of readiest cleavage. In these latter cases, however, in thin section the schistose structure is just as distinct as in the coarser phases in which it is so easily recognizable. While, then, these rocks have no stiike and dip in a proper sense, they have a foliation to which the terms may be applied. It has been said that more often than not this foliation is approximately east and west. How- ever, it has wide variations within narrow ranges (as will be seen by look- ing at the detailed maps. Pis. v to xin), and at times the foliation is almost directly perpendicular to the strike of the rocks of the Penokee series. If there are abrupt and sudden changes in the direction of strike, the varia- tion in dip is still more marked, frequently within a few rods varying from a dip in one direction to that in the reverse. Further, while on the maps there are sharp lines of separation drawn between the fine-grained gneisses and the granites and granitoid g'ueisses, in the field there is at times an apparent transition rather than an abrupt change (pp. 123-125). More often, along the border of the schists are granitic intrusions, and these become more and more numerous in passing toward the granite, until this rock becomes pre- dominant. The great variation in the direction of foliation in the gneisses is readily understood when some of the larger exposures are examined, their banding being extremely contorted and consequently varying greatly in dip within a single exposure. From the foregoing it will be seen that if the rocks included within this belt have a distinct succession it is an extremely complicated one, and that at present we have no data which will enable us to reach even an approximate notion of it. The only structural fact that can be certainly stated is that some of the massive granites and gneissoid granites are intrusives of later age than the green schists. When the rocks of the Southern Complex are examined in thin section ( ! !•: N !■; It A 1 . ( i !':< ) I .( x ; v ( » !•' twd i ) i st ii i < ;t. 443 they are seen to be of completely crystiilliiic kinds. Sucli massive rocks as the granites and syenites are now regarded as ernpti\'e. 'I'lie origin of many of the thinly foliated schists and gneisses is not known. It is enongh here to say that, as indicated (pp. 12f)-\2G), nowhere in tliis area is there any suflicient evidence to show that any of them are of a fragmental char- acter. Certain of the gneisses of tliis area have such relations with the *massive syenites and granite^ that they may be considred as of eruptive origin; so that all that can be said as to the original condition of these rocks is that a large proportion of them are eruptive while the origin of others is unknown. The rocks to the south of the PenokQe series are, then, a set of massive rocks and crystalline schists having a peculiar com- plex contact and neither certainly possessing a sedimentary structure of any kind. This Southern Complex, as a whole, may be taken as the base- ment group of the district. , The Chertjj limestone and Quarts-slate members. — To the north of this complex are the belts of the Penokee series and the Cherty limestone. In chapters in to viii these rocks have been described in detail. Here it will be necessary to recapitulate such of the facts as bear upon their relation to another and their relation to the rocks to the south and north of them. The lowest formation is a cherty limestone. This rock is found in large exposures at various places from near Atkins lake, T. 44 N., R. 5 W., Wis- consin, to T. 47 N., R. 44 W., Michigan. In its greatest development it is about 300 feet thick. It appears to be a tolerably continuous formation throughout its western portion, bur in the eastern half of the district it is found only here and there. Next in order to the north is the Quartz-slate member. This formation is a fragmental one in which the particles are chiefly quartz and feldspar. In the vicinity of the cherty limestones it fre- quently carries a considerable quantity of material from this member, even becoming at times a conglomerate. This cherty limestone and the slate belt are, however, in apparent conformity with each other, so that while there certainly was an erosion interval between the two there was found no evi- dence that the Cherty limestone was closely folded. The case is analogous to that of the relations of the Penokee and Keweenaw series. It will be shown that such an erosion interval implies a considerable physical break. 444 THE PENOKEE IRON-BEARING SERIES. The narrowness as well as the permanence of the Quartz-slate mem- ber is strongly brought out by PI. ii. Here is a belt of rocks which in surface width is upon an average not more than 400 to 450 feet, and the thickness of which is riot more than 350 to 400 feet. Its maximum thick- ness east of Sunday lake is not more than 800 feet. In chapter iv the re- markable essential likeness of the various parts of this belt is indicated. Its southern part is a layer of green, brown, gray and red quartz- slates.* T!ts uppermost layer is a vitreous quartzite. In whatever part of the range a section is made across the belt, its essentially fragmental character is at once discoverable, the rocks being comparatively little changed since they were originally deposited. Tlie induration of both the quartzite and the slates has been explained to be due to the enlargement of quartz-grains and to metasomatic changes of the feldspar. Unconformity between the Southern Complex and ilie overlying Clierty limestone and Quartz-slate. — There is, then, between the Quartz-slate and the rocks to the southward the great fundamental difference between rocks which are easily provable to be of clastic origin and those which are com- pletely crystalline schists and massive eruptives. There can be nowhere a wider lithological difference in the characters of two sets of rocks tlian this. But more remarkable than the lithological simplicity of the Quartz-slate are its straightforward field relations. This slate once seen is easily recognized. It always dips northward, varying within narrow limits. Take the strike of any ledge and follow the direction either to the east or to the west with a little latitude of movement and almost invariably another ledge of like rock is found within a short distance if exposures are at all plentiful. The only part of the area in Avhich natural exposures within the belt were not found has been penetrated and exposed by mining operations, so that its continuity can be with certainty asserted. It is true it bows locally from its general course, and in two or three places it is somewhat faulted, but the variation is never sufficient to lose the belt. As one traverses this range north and south at short intervals, the complexity and variety of the litholog- ical characters and of the strikes and dips of the rocks of the Southern Com- plex and the likeness, simplicity, directness, and uniformity of this slate belt now just north of the granites and now in contact with the crystalline GENERAL C.EOUMiY OF THE DISTKICT. 445 schists are most strikiiij;-. Tliese facts iinj)ress oim so stroiifi'ly tliat it is safe to say that no on*' can pass thnmyli this ('X|KM'i('ii('u without having- the conviction foircil upon liiin that thciv is a great structural break between the quartz-shites and the coniplex series to the soutli. Such a general rela- tion, it seems to me, is more significant than the direct abutment of the schistose structure of one rock against another; more signiticant than actual structural breaks seen by the eye in any single exposure; more sig'- nihcant than an\' basal conglomerate; yet, as will be seen, all of these subordinate proofs of unconformity have been found at various places in this district. The above paragraph contrasting the lithological charactei's and strati- graphical relations of the Southern Complex and the Quartz-slate is applica- ble with almost equal force to the relations of the Southern Com])lex and Cherty limestone. The only modification that need be made is that the Cherty limestone is not so continuous as the Quartz-slate, so that one is not so strongly impressed with the absolute necessity for a discordance between the two in order to adequately explain their field relations. The relations of the eruptive massive syenites and granites to the adjacent schists and to the Cherty limestone and Quartz-slate are such as to show that the schists and massive rocks are vastly older. That the granites should intricately intersect the schists and yet for many miles be immedi- ately adjacent to the Cherty limestoue and Quartz-slate, yet never cut them, is inconceivable on any other hypothesis than that they were in this position before the deposition of that belt of rocks. It is, further, all but universally believed that the coarse grained granites and syenites are a class of rocks which have crystallized under great pressure and therefore at depth. If this be true they must have been subjected to great erosion in order that the Cherty limestone and Quartz-slate could have been deposited directly upon them, and they are therefore immensely older than the latter rocks. The independent relations, also, of the green schists to the quartz- slates are such as to show that between them is a great interval of time. It has been said that the fibers of the schist run in every possible direction and that in certain cases they abut dii-ectly against the slates. These schists may be eruptive or fragmeutal, but in either case their schistose structure 446 THE PENOKEE lEON-BEARING SERIES. was induced before the deposition of the slates upon them, for it is impossible that rocks could be so profoundly altered and yet the slates of much the same mineral composition in contact with them be unaffected. If they are eruptives the time required for their change from fresh rocks to extremely foliated altered ones must have been very great. If they are fragmental their metamorphism to completely crystalline rocks must have taken as great a time. So that on either hypothesis they are immensely older than the Quartz-slate which overlies them, for the rocks of this formation have neither a schistose structure nor a crystalline character. West of Potato river, T. 45 N., R. 1 E., Wisconsin, no actual contacts between the Southern -Complex and the Penokee rocks are known. At Penokee gap the green schists are found very close to the overlying Cherty limestone. The latter, as usual, dips to the north and strikes approximately east and west. The green schists to the south also have a nearly east and west strike, but their dip is southward. This being the case, the fibers of the gneisses, unless there is here an exceedingly sharp fold of which there is no evidence, abut sharply against the overlying beds, and thus there is here strong, although not conclusive evidence, of xmcon- formity. At Potato river, fortunately, the underlying schists and the overlying Quartz-slate are found in direct contact with each other. Here the Quartz- slate, rising in low cliffs, is exjjosed throughout its entire thickness upon the east side of the river (Fig. 5). For some distance the green schists to the south are also exposed, and the actual contact between the two is seen on the bank at intervals for a vei'tical distance of 75 feet, while the contact is continuous for the lower 25 or 30 feet. The slates are all of the normal character, dipping to the northward at an angle of about 70°. The green schist has no proper dip and strike, and yet it has a strongly schistos.e character. Its fibers abut almost perpendicularly against the layers of the slate, as shown by Fig. 6, reproduced from a carefully made drawing on the ground. So sharp is the junction line between the two rocks that it can usually be located to the fraction of an inch, and hand specimens were obtained in part from both formations. The surface of the green schist at the beginning of the formation of the conglomerate is seen GENERAL CEOLOGY OF THE inSTTJKT. 447 to have been somewhat irreg-ular (Fig. 7), and bcttwucii ii ;iiiil tlie ordinary slates is a layer of basal coug'lomerate varying- from m few iiiclies to several feet in thickness. It, however, quickly grades into tlie ordinary slate of the district, as though this place were at the bottom of a comparatwely level shallow sea, rather than adjacent to a shore. '^Diis conglomerate contains a few white quartz and jasper pebbles, the former sometimes being ten inches in diameter ; but the great mass of the fragments, and especially all of the large bowlders (which reach occasionally five feet in greatest length), are from the underlying green schist. Moreover, the conglomerate proves that the schistose character of the underlying rock had been fully attained before the deposition of the slates ; for the schistose structure of the frag- ments is as well developed as in the ledge below. A fragment broken from such a schist is naturally longer in the direction of its fibers than transverse to them. That the material does break in this manner was proved in obtaining specimens. The green schist fragments of the con- glomerate ai-e generall}' longest in the direction of schistosity. Now, frag- ments of this sort, when broken from the basement rock and laid down as a part of the conglomerate, would naturally lie with their longer direction parallel to the horizon; and this has been the case, for, as shown in Fig 7, nearly all of the larger fragments lie with their greatest length parallel to the then sea floor, that is, the schistose structure of the fragments is at right angles to that of the ledge from which they were derived. Now, if this schistose structure had been developed subsequently to the formation of the conglom- erate, it would have been parallel in both the fragments and basement rocks. So far as a basal conglomerate and unconformity in structure at a single place can indicate a time gap between two formations, this locality indicates it. If the underlying rock was a sedimentary one it must have been folded and extensively eroded to change it to a crystalline schist and bring its foliation in a vertical position at surface. If it was an eruptive rock it has been most profoundly altered, and its alteration, as just shown, must have taken place before the formation of the conglomerate. Such an alteration would hardly take less time than its transformation from a clastic. So in whatever .light it is regarded this basement rock is vastly older than the conglometate and slates which rest upon it, 448 THE TENOKEE IRON-BEAEING SERIES. At the west branch of the Moiiti'eal, T. 46 N., R. 2 E., Wisconsin, green schists are again found in contact with the rocks of the quartz-slates. The contact^ hei-e exposed is of much smaller size than at Potato river and to that extent is less satisfactory, but even here all the essential facts are the same. A basal conglomerate is found, the fragments of which are very largely from the green schist ; this conglomerate quickly varies into the normal slate of the regior , as at Potato river ; the fibers of the green schist abut against the conglomeratic slates. Just south of the Aurora mine the Quartz-slate is exposed in direct con- tact with the granite. The basal horizon is here a cherty slate which does not take on a conglomeratic aspect. The granite surface is plainly one of erosion and into its pre-Penokee joints and cracks the detritus of the Quartz-slate has sifted. A short distance north and west of the east quarter post of Sec. 15, T. 47 N., R. 46 W., Michigan, on the railroad spur of the Palms mine is another contact between the Southern Complex and the Penokee rocks. Here the junction is between granite and a conglomerate belonging to the Quartz- slate member. A ledge of massive granite is faced by a conglomerate, which penetrates the clefts and hollows of the granite. In the granite fresh microcline and chlorite are abundantly contained. The fragments of the conglomerate are mainly granite, but chert, jasper and quartz are also found. These fragments are in part well rounded, but mostly are angular, as is common when near their source. Much of the feldspar of the fragments is beautiful, fresh microcline. Green chlorite in well defined masses is also plentiful. It is certain that this debris was derived from the granite. The coarse grained granite at this place plainly consti- tuted tlie surface rock when the conglomerate was formed and contributed material to it. North and east of the conglomerate, after a short covered space, are found the ordinary phases of slates of the Quartz-slate member, dipping north at their normal angle. The time gap here between the conglomerate and granite must be taken to be great if it is conceded that the presence of normal granite at the surface is proof that it is an old rock. About 700 steps south and 75 steps east of the northwest corner of iTWs contact -was first described by Dr. T, C. Chamberlin. See Literature, pp. 43, 44. GENi:i{AI- (iKOLOCY OF THF, DISTKHT. 449 Sec. 13, T. 47 X., U. 4(1 W., Mic^liigau — i. e., jilxnit '1 miles cast of tlie cdii- g'lotiieratc just Sec. 28) a chert-conglomerate is found in several places but a few paces from the granite. These chert-conglomerates are in all essential respects like that found near the Palms mine, already described. While it includes a large quantity of granite debris, including quartz, feldspar, biotite, and complex areas composed of these minerals, it has a cherty background which contains abundant angular chert fragments. It appears that here, as near the Palms, the Quartz-slate member has derived material from both the Southern Complex and the Cherty limestone. From the foregoing it appears that there are definite proofs of a strati- graphic break between the Southern Complex and the Penokee series at ten places. Two of the localities (see PI. ii), both being actual contacts accompanied by basal conglomerates, are above the Western schist, the lowest layer of the upper series here being the Quartz-slate. Two of the localities, both again being contacts, one accompanied by a basal conglom- erate, are above the Central granite, the lower rocks of the upper series here being again the Quartz-slate. One of the places is above the Eastern schist, the contact being found, but no basal conglomerate appearing. The lowest layer of the upper series is here the quartzite, whicli occasionally appears at the base of the Quartz-slate. Finally, five of the localities are above the Eastern granite, basal conglomerates or recomposed granites occurring at each locality, while at two of the 2)laces are actual contacts. 452 THE PENOKEE IKON-BEARING SERIES. At these contacts the lowest member of the upper series is the Quartz-slate in different phases. When it is remembered that it is exceedingly difficult and unusual to find actual contacts between unconformable series, even when there is the same certain general evidence of the unconformity that is found in this district, the above number must be considered extraordinary. It is also noticeable that the contacts are widely distributed, and in about an equal number of cases the schists and the granites are the underlying formations. No actual contact between the Southern Complex and the Cherty lime- stone has been found. However, at Penokee gap and in Sec. 23, T. 47 N., R. 45 W., Michigan, the two are exposed close together. At the first place the rock of the Southern Complex is a schist; at the second it is a granite gneiss. At the former the schist and the limestone dip in opposite direc- tions, the- schist inclining 30° to the south and the limestone 65° to the north. But there is yet further evidence as to the magnitude of this uncon^ formity. It is certain that the complex basement upon which the newer series was laid down was nearly horizontal. When it is remembered that this Soutliern Complex is here resistant massive granite and there soft foliated schistose rocks, it is plain that the forces of erosion had nearly exhausted themselves, i. e., that a '"base level" of erosion had been nearly attained, before the newer series began to form. When we consider what is involved in this, it strongly reenforces what has gone before as to the great time gap which must have intervened between tliese older formations and the lower fragmeiatal rocks. The proof of the existence of this nearly level plain lies in the small variation in thickness of the Quartz-slate and the Cherty limestone. The difference of elevation in the basement throughout its whole extent east and west probably was not more than a few hundred feet at the time the Cherty limestone began to form. This formation is found here and there in detached expostires for a distance of many miles. Its maximum thickness is now 300 feet. In the sea floor there was probably at the time when tjie limestone was deposited no greater variation in elevation than the thickness of this formation as it then existed. (JElStKKAL (JEOLOCJV OK TIIK DISTRKX 453 At the beginning- of tlic (lc[)(isiti()n of tlie Quartz-slntu iiioinhor of the Peuokee series proper tlie ciis*? \v;is even more strikiiij^-. 'I'liis layer, avei'- ag'iiig 300 or 400 feet in tliickiu'ss, Wiii in one pl;u;e reaching" twice this amount, at no [lo'mt is known to l)e much U-ss than 300 feet in thickness. It forms a continuous belt throughout most of the extent of the series, and where missing has been swept away by erosion, as have the higher mem- bers at such places. It is certain, then, that the variation in elevation of this ancient land surface at this time was little or no more than 300 feet, except, perhaps, in tlie eastern part of the Eastern area, where it is not clear that the slates there occurring as the lower layers of the Penokee series are the equivalents of the Quartz-slate to the westward. A rock basin composed of diversified crystalline schists and massive rocks 90 miles in diameter and having no elevations greatly exceeding 300 feet is rare, and it is universally believed that the complex areas in which this is the case have been subjected to enormous denudation. The amount of erosion in such regions is often compared to that necessary to reduce lofty mountains to mere stumps. Before the beginning of the deposition of the Penokee series proper it is clear that the Southern Complex was thus brought nearly to a plane. To summarize, the proofs of unconformity above the Southern Com- plex are as follows: First, belts of sedimentary rocks strike across the coimtry, being now in contact with one variety of underlying rock, now in contact with another, always keeping their course, never being penetrated or interfered with by any of the southward lying rocks (excluding, of course, the later basic dikes), whether schistose or griinitic intrusions in the schists. Second, these underlying rocks are either massive ones which are presumably igneous or are schists in which the extreme of foliation and crystalline character is found, while the overlying Penokee rocks are plainly water deposited sediments. Third, in ten places above the Base- ment Complex are basal conglomerates or recomposed rocks. Seven of these places show the actual unconformable contacts. The detritus is mainly identical in character in each case Avith the material of the rock upon which it rests, showing, that the basement rocks must have reached . their present condition before the formation of the lowest overlying 454 THE PENOKEE IROH-BEAKING SERIES. member. When the basement rocks are green schists their fohatioii had been developed and they had been truncated ; when they are granite, if formed at depth, it had reached the surface by ei'osion. Fourth, the fact that the horizon at which the underlying complex is in contact with the Penokee series proper does not vary more than 300 or 400 feet at the out- side is the clearest sort of evidence that the underlying rocks had nearly reached a "base level" before the beginning of the deposition of the Peno- kee series.' The unconformity between the Cherty limestone and the Penokee series proper. — In the description of the Quartz-slate (Chapter iv) it was seen that at a number of places this member contains in its basal j)ortions abundant debris derived from the chert of the Cherty limestone, becoming in several places a genuine chert-conglomerate. • This derivation is indicated by the lithological likeness of the pebbles of the conglomerate and the chert of the limestone, as well as by the fact that these conglomerates are usually found near some of the exposures of the limestone. At Penokee gap the Quartz-slate rests directly upon the Cherty limestone, being here a recom- posed quartz-rock nearly all of the debris of which comes from the imme- diately subjacent member.^ Beginning at the west and passing east these recomposed rocks or conglomerates have been found at Penokee gap, mount Whittlesey, Potato river, near the Palms mine, and east of Sunday lake at several points in Sees. 10, 14, and 15, T. 47 N., R 45 W., Michigan. In all these cases the chert pebbles derived from the Cherty limestone are well rounded and in exactly the same condition as the chert now is in the limestone. These pebbles appear to be clear evidence that the limestone had become cherty before the deposition of the lowest member of the Penokee series proper. ' The time gap implied l)y such an unconformity as the above is fully discussed by Prof. Irving, U. S. Geol. Survey, 7th Annual Eeport, pp. 390-443. Suffice it here to say that it is clearly shown that an nnconformity of this sort indicates "a lapse of time long enough to cover (1) the folding of the lower series, (2) its elevation into a land surface, (3) a long continued denudation," .and (4) its depression under the sea. "In other words, it indicates an interval of more or less extended orogr.iphic movement, with its accompanying » * « denudation." In this case it is plain that the orographic movement is of a most extended character. 2 Geology of the Eastern Lake Superior District, R. D. Irving. Tlie recomposed rocli is Irving's IIB, Geol. Wis., vol. iii, pp. 109, 110. GENERAL tlEOUKiV OF THE DISTKHT. 455 '^m he parallrl (listrilmtiipii of llic (k^tiiclicd t>ntcr()|)s of the CliL-rty lime- stone and the coiitiniious belt ot"Quartz-slat<' is evidence tliat tlie oroyrapliic muveinent between the two in the Penokee district was not of a compli- cated character. Also tending- in the same direction is the similarity of the dip of the limestone and the Peiiokee series proper. While in some places the limestone appears to have a flatter (hp than the Penokee series, this is not so marked as tct sug-g-est an unconformity between the two were it not that other data point in this direction. Taking- all the facts into account, it is concluded that there was a considerable time interval between the Cherty limestone and the Penokee series proper. In this inter\al the limestone was consolidated, and if the chert is a segregation formed from scattered (irganic remains, or was introduced, from outside, this also occurred. The Cherty limestone and Basement Complex were raised above the sea and ei'osion began. The amount of material removed it is impossi- ble to estimate. It is only known that the Cherty limestone in some places is 300 feet thick, in others is absent. The presence of occasional pebbles of jasper in the conglomerates of the Quaitz-slate suggests that upon the limestone was once a higher formation of a different character which was subsequently wholly carried away. After this time of erosion the land was again depressed below the sea and the lower part of the Penokee series proper began to be deposited. Tlie lack of marked discordance in the bedding of the Cherty limestone and the Quartz-slate is no evidence that the time gap between the two was not long enough to have produced a most prononnced discordance, for this Penokee area may have been a part of a plain removed from zones of important folding and thrusting which may have occurred simultaneously in other districts. The Iron-hearing and Upper slate members. — Thus far the relations of the Southern Complex to the two lower members of the Penokee series and the relations of the latter to each other have been spoken of Above the quartz-slate follows m perfect conformity the Iron-bearing formation. This member, hke the major part of the Cherty limestone, has been shown to be of a nonfragmental character. The change from the fragraental Quartz- slate to the Iron-bearing member is always abrupt. The uppermost mem- ber of the former has been said to be a coarsely crystalline quartzite. It is 456 THE PENOKEE IRON-BEAEmG SERIES. plain that at the final stage of the formation of this member there was a clear sea, only well rolled quartz grains being deposited. This implies sorting of the material and therefore less rapid accumulation. It is prob- able that this change of condition was accompanied by a' gradual sinking of the sea bed. Naturally following the clearing up of the water have come the nonfragmental sediments of the iron belt. As has been shown (pp. 247- 248) tliey are analogous to limestone formations in many respects. The uniformity in width of this belt is noticeable throughout its western three- fourths. In its eastern part, where there are considerable variations in thickness, the irregularities have been explained to be due to contempora- neous volcanic activity. Above the Iron-bearing member next succeeded the great thickness of fragmental Upper slate. At Tylers fork these slates show their present maximuni thickness, nearly 13,000 feet. East and west of this point the belt gradually narrows until it is cut off, at the Avest near Numakagon lake and at the east near Sunday lake, by the overlying Keweenaw rocks. The relations between the Iron-bearing formation and the upper slates are the same as those between the latter and the underlying quartz-slates; that is, they are two formations which are in perfect comformity, as is shown by the strike and dip of many exposures in both belts. The change from this iron formation to the mechanical sediments of the Upper slate was rather the abrupt, although not so sharp as the change from the Quartz-slate to the Iron-bearing member. The three members mentioned with contempora- neous eruptives constitute the Penokee series. By an examination of PI. ii, it is seen that they are not all continuous throughout the district traversed by them; bui wherever the series is present one formation follows another, except in the Eastern area in the order mentioned in conformable succes- sion, so that they are properly placed together as a group of formations. The unconformity at the base of the Keweenaw series. — North of this Penokee series are found the eruptive and fragmental rocks of the Kewee- naw series. The rocks of the latter immediately north of the former are usuall}^ bedded surface eruptives ; but from Black river, in the east part of T. 47 N., R. 46 W., Michigan, to near the Montreal river, in the basement layers of the Keweenaw series, has been seen at various points a red sand- (IKNEKAL (lEOLOCiY Ol" TIIK DISTKICT. 457 stone <»r (juiirtzitc. Tliat tluTc is a (jTont tiiiic 'j;i\\) hctwccii tlic S(iiitli(irii Complex and the Peuokee scries lias just been slmwii. That tlicrc is also a time interval, altlioug-h not so vast a one, hctwccii the intii-l)earing' and the overlying Ke\>'eena\v series, is equally clear. In anv single section this discordance is not plain, for the Keweenaw rocks nortli of the Penokee series, like the latter, have as a whole a northern dip at a. high ang-le.^ The eruptive character of the basement member of the Keweenaw series for most of the distance, and its northern inclination, make it more difficult to prove an unconformity between the tMo than l)etween the Penokee series and the Southern Complex. The p>oof here rests entirely upon broad field relations ; iinless the cong-lomerate, placed provisionall}' at the top of the Upper slate Qjp. 305, 326), -belongs witli the Keweenawan. This conglom- erate lies but 200 steps south of the Keweenawan greenstone. It is sep- arated from the typical biotitic slate (p. 326) of the Upper slate member by an unexposed interval of 280 steps. The pebbles, ranging up to eig'ht or ten inches in diameter, are mainly of white quartz, Hut flint and black homstone are also abundant. The most probable source of the last two are the Cherty Limestone and Iron-bearing- members of the Penokee series. While it can not be asserted whether this conglomerate belongs to the Upper slate, or at the base of the Keweenawan,' the latter alternative is the more probable, and if this be true, the rock may be considered a basal conglomerate. Beginning at the west end of the Penokee succession, and following the contact of the two series of rocks to the eastward, it becomes evident that there is a considerable break between them. Near the northeast corner of Sec. 20, T. 43 N., R. 7 W., Wisconsin, characteristic eruptives of the Keweenaw series, as found by Mr. Charles E. Wright,^ are upon the north side of Numakagon river, and a gneissoid granite of the underlying complex upon its south bank. It is not certain that rocks belonging to the Peuokee series are not between, but there is no proof of this or of their existence to the westward for a long way ; so this point may be taken as the western- ' Coppei'-beariug Rocks of Lake Superior, pp. 225-234, R. D. Iiviug, IT. S. Geol. Survey, Mono- graph, vol. V. 2 Geol. of Wis., vol. Ill, p. 300. 458 THE PENOKEB IROiN-BEAEING SERIES. most one at, which it is jjrobable that rocks of this series will be found. As a matter of fact, the westernmost known exposure is in the SE. J of Sec. 24, T. 44 N., R. 6 W., Wisconsin, about 11 miles north of east of this point. In Sec. 16, T. 44 N., R. 5 W., Wisconsin, the eruptive rocks of the Keweenaw series at on* point are immediately north of the rocks of the Iron -bearing member, Avhile between them there is little space in this township for the rocks of the Upper slate member. Proceeding east- ward the exposures found in the NE. |- of Sec. 23, T. 44 N., R. 5. W., Wis- consin, are the last known in the Penokee series for fullj- 6 miles. For this distance the countrj^ is low and swampy. The next exposures to the eastward are found in Sec. 24, T. 44 N., R. 4 W., Wisconsin. No attempt is made to map the Penokee series in this interval, as the only indications of its existence are somewhat feeble magnetic attractions,^ which may be due to the magnetite of a basic eruptive. It is not even certain but that the entire succession is here cut oif by the overlying series. There are numerous exposures of coarse gabbro in the southern part of Sec. 24, T. 44 N., R. 5 W., Wisconsin (see PI. v), a short distance east of the granites in the south part of Sec. 23. In Sees. 22 and 23, T. 44 N., R. 4 W., are again exposures of gabbros a short distance from the granite to the south and east. We have no evidence which will enable us to locate these greenstones as a part of the Keweenaw series or as intrusive rocks in the Penokee or underlying series.' If they are regarded as Keweenaw rocks, that series must here be conceived not only to take the place of the Penokee succession, but to occupy at least half a mile of the area where one would expect to find the underlying complex. This struc- ture would imply an erosion not only sufficient to remove all of the rocks ■* of the Penokee series, but to have cut a valley of very considerable depth into the Southern Complex in the interval between Penokee and Keweenaw time. This unusual amount of erosion is not incredible, but the relative locations of the ledges in the south part of Sec. 24 and in the northeast part of Sec. 23 are such as to give an almost incredible steepness to this hypo- thetical gorge. Another possible explanation of the relations is, that the Penokee rocks are absent by erosion and that there has been here a double iGeol. of Wis., vol. Ill, pp. 278-281. GENEHAL CiKOL()(;V OF TUK DISTKIOT. • 459 ff fault witli a soiitliwiu-d tlimw, l)\ incniis dt' wliicli ;i scctiim r, Mud it seems, in the lack of deKnite evidence, tli;it it is probable that the Penokee series is continuous and that the greenstones adjacent to the granite in this area are intrusive within them, rather than a part of the Keweenawan basement flow. , About half a mile west of the east range line of R. 4 W., AVisconsin, numerous exposures of tlie iron formation are again found. The known thickness of the Penokee series is here about half a mile, while it ma}' be considerably thicker. At the center of Sec. 18, T. 44 N., R. 3 W., Wiscon- sin, the series is at least three-fourths of a mile wide. Just east of English lake this width has become more than a mile. This widening of the series goes on continuously until Tylers fork is reached, T. 45 N., R. 1 W., Wis- consin, where its maximum thickness is found. In passing eastward from this stream tlie series gradually narrows with great uniformit}' until a short distance west of Sunday lake, in the west part of T. 47 N., R. 45 W., Mich- igan, where the Upper slates have disappeared. From this point to the middle of T. 44 W., Michigan, the overlying Keweenaw rocks are in direct contact with the Iron-bearing formation. At one place just east of Sunday lake this formation can not be exjDosed for more than a third of its width. Through townships 44 W., 43 W., and 42 W., Michigan, the greenstone ridg'e constituting the basement of the Keweenaw series passes nearly in an east and west direction, with the exception of 1 J miles in the east part of T. 47 N., R. 44 W., Michigan, and here the discrepancy is explained (pp. 424, 425) as being probably due to a fault. In these townships the immedi- ately underlying rock is a mingled fragmental and nonfragmental one which, as explained (p. 431), is probably the equivalent of the upper part of the Iron-bearing member to the west. That the Penokee series could have been deposited to a thickness of about 14,000 feet at Tylers fork, and at other points be only 1,000 feet thick, or entirely absent, is not at all probable. This is especially true, as it is the lower 1,000 or 1,500 feet of the series which is found whenever any part is present. As already explained, the persistence of the Quartz- 460 ' THE PENOKEE lEON-BEAEING SERIES. slate and Iron-bearing- members in nearly uniform thicknesses shows that at the time of their deposition there was for this distance an approximately level basin. If no part of the upper slates and iron formation have been swept away by erosion, we must believe that there was inaugurated before the end of the latter period an orographic movement which formed a syn- clinal trough, the center of which was Tylers fork, and thus a great thick- ness of upper slates -there accumulated, while to the east and 'west the thickness became less and less until it finally disappeared, as well as a part or the whole of the lower members of the series. That a great earth move- ment of this sort could occur without disturbing the perfect conformity of the formation of the series is improbable. The discordance in the strikes of the layers would indicate the change, but as all the members are in appar- ent perfect accordance, one layer has followed upon another without dis- turbance. This being the case, the Penokee series was originally probably of rather unifomi thickness, and doubtless this thickness continued both east and west of the points at which the series can now be traced; for there are many reasons for believing that this district was a part of a great basin which extended to the Marquette district on the east and to the quartzites of the Chippewa valley on the west. The relations just sketched can, then, have but one meaning — that a great unconformity separates the Penokee and Keweenaw series. This is the only possible explanation of the facts that the Keweenaw series is above an Upper slate member 13,000 feet thick at Tylers fork, and east and west not a great distance is above a belt much thinner, while a little far- ther east and west this slate disappears altogether, and yet a little farther to the west the lower members are hidden, and west of Numakagon lake the Keweenaw series apparently rests directly upon the Southern.Complex. During this intervening period the rocks of the Penokee series were raised above the sea, and then suffered long continued denudation "until in places the entire succession was carried away, exposing the underlying rocks. How thick the series as a whole was at the beginning of this erosion there is no available evidence, but the amount of material swept away at Numa- kagon lake was 14,000 feet more than at Tylers fork, while the difference in amount of erosion between the last place and Sunday lake is scarcely GENERAL CiKOLOdY OF TUE DISTRICT. 461 1,000 feet less. Probabl) tliu series must have been everywhere thicker than at the present time at any place. Tlie amount of" erosion during this period was, then, in some places measured perhaps b\' twice 10,000 feet. During- the progress of this erosion it is possible that the series was gently bent into a synclinal trough, the center of which was in the ^^cinity of Tylers fork and the eastern end near Sunday lake. This suggestion is made as an explanation of the diiference in the amount of erosion, those parts naturally being eroded most which were at highest elevations. The time gap, then, between the Penokee apd the Keweenaw series must have been sufficient for a widespread orographic movement and deep denudation — a vast lapse of time ; so that while the unconformity between these two series is not nearly so great as between the Penokee series and the Southern Complex, yet it stands as one of the greater time gaps in geological history. The Eastern sandstone and the unconformity at its base. — The only remain- ing teiTane in which we are concerned in this memoir is the Eastern sand- stone, which (as shown by PI. i), extending from Keweenaw bay to a long distance west of Gogebic lake, conceals a broad strip of the older forma- tions. The low ground north of the trap range, in the north part of T. 47 N., R. 44 W. and 43 W., Michigan, is the southern boundary of this sand- stone. As T. 47 N., R. 42 W., Michigan, is reached, near Gogebic lake, this low ground swings rapidly to the southward, and it was early in this study suspected that here the Eastern sandstone is the surface formation, but no natural exposure was found which would either prove or disprove this suspicion. However, test pitting in the NW. -\ of Sec. 28, T. 47 N., R. 42 W., Michigan, has shown the sandstone to there exist. The basal con- gloifterates there found at the junction of the Penokee series with the underlying granites have already been described, pp. 394-395, 409. In this interesting locality is still another basal conglomerate which belongs to the Elastern sandstone. In the test pits close to the outcrops of the Peno- kee series and the Southern Complex the conglomerate is coarse, and its matrix is somewhat indurated, it being necessary to resort to blasting in sinking. Upon exposure to the air the matrix of the conglomerate dis- integrators, as a result of which a heap of sand, pebbles, and bowlders of 462 THE PENOKEE lEON-BEAEHSTG SERIES. various sizes is found about each test pit, although parts of the rock hold together with sufficient firmness to yield large specimens of the conglomer- ate. The pebbles and bowlders are well rounded. The character of the pebbles indicates that they have been derived from the Southern Com- plex, the Penokee and the Keweenaw series. From the last are coarse red indurated sandstones, amygdaloids, coarse grained basic eruptives, quartz- porphyries, and other varieties of rock. From the Penokee series the pebbles are more numerous than from either of the others, as would be naturally the case since the exposed conglomerate is in contact with these rocks. The pebbles belonging here are lean ore, banded jasper, magnetitic schist, chert, chert-breccia, recomposed granite, and quartzite, the induration of which is due to the enlargement of quartz-grains. From the Basement Complex pebbles and bowlders are also very numerous, comprising white vein quartz, gneiss, granite, and many vai'ieties of crystalline schist. The pebbles from all of these som-ces are characteristic of the series from which they are derived. To describe them in detail here would be but fo repeat the litho- logical descriptions given of them in the treatment of the formations of the district. It is, however, to be remembered that in this area no crystalline schist, gneiss, or granite has been found anywhere except in the Southern Complex; no jasper, magnetitic schist, chert-breccia, lean m-e, or recom- posed granite has been found in other than the Penokee series, while quartz- porphyry and certain phases of the basic eruptives have been found nowhere but in the Keweenaw series; so the evidence is as clear as any lithological evidence can be that the various pebbles have been derived from the sources assigned them. The test pits show, so far as tlie rock has any stratifica- tion, that it lies in a horizontal position, and the character of the basal con- glomerate alone would be sufficient to prove that we have to deal with the Eastern sandstone. Further, the relation of the underlying rock to the sandstone is shown in one test pit. After passing through a thickness of 24 feet of the conglomerate just described, the shaft penetrated the chert-brec- cia of the Penokee series (p. 406) clearly proving that this rock is earlier than the conglomerate. Again, a short distance northeast of these test pits are others in sandstone which in every respect is like the ordinary red Eastern sandstone, and which show, so far as it is possible in test pits, the GEN?]RAL (JEOLOGY OF THE DISTRICT. 463 rock to ho in a horizontal position. The Penokee series is not absolutely cut oil' b)' the sandstone, but its breadth at one point in the northwest part of Sec. 28, can not be more than 200 or 300 feet, and it is probable that farther south the entire succession is overlain by it. It is clear, then, in the upward passage to this Eastern sandstone that another great unconformity has been passed. First, the sandstone is in a horizontal position, lying directly iipon the upturned edges of the Penokee series and presumably having the same relations to the Keweenaw series. Second, this sandstone contains abundant fragments derived from the Southern Complex, Penokee-Gogebic, and Keweenaw series. From what has gone before it is manifest that in order that this should be the case the whole Keweenaw and Penokee series must have been tilted and have suffered vast erosion in order that the base of the latter and the iinderlying series — once probabh' buried under many thousands of feet of sediments — could reach the surface and furnish these fragments. Since the sandstone is hori- zontal it is plain that the two upper series were tilted to their present inclination and eroded before. the deposition of this sandstone. The final part of the erosion furnished the material of which the sandstone is made. This unconformity is one, then, which in magnitude is probably equal to if not greater than the geological break between the Penokee series and the Southern Complex.-' Resume of geological Mstory. — We are now prepared to give a r^sumd of the geological history of the district of which the Penokee series is a part. The oldest rocks are the great Southern Complex. Of the origin of a part of them little is known, but, whatever their genesis, before the beginning of the deposition of the Penokee series there was a long period during which earth movements and erosion acted upon them. This ' The discordance between the eastern sandstone and the Keweenaw series lias heen long main- tained by eminent geologists, though denied by others. This position was iirst taken l)y Brooks and Pumpplly in 1872 (On the Age of the Copper-Bearing Rocks of Lake Superior. Am. Jour. Sci., 3d series, aoI. in, pp. 428-432). The last exhaustive and convincing treatment of this question is by- Profs. R. D. Irving and T. C. Chaniberliu (Bull. U. S. Geol. Survey No. 23). Upon Keweenaw point it has been maintained that the pebbles in the sandstone do not imply a great break because of eruptive origin. At Gogebic lake are nearly all the phases of pebble.s of the three great series of rock of the region. That they should be abnudnnlly coutaiiicd in the saudstinic in t^ic condition which they are now found i«si(« can not be explained upon any other hypothesis thau a great unconformity, 464 THE PENOKEE IROE^-BEAEING SERIES. erosion coptinued until they were reduced nearly to a plain throughout the distance from Numakagon lake to Itike Gogebic. The district was then submerged. The conditions prevailing were quiet ones, for almost immediately there began forming at the bottom of the sea the nonfrag- mental rocks of the Cherty limestone membei'. Between the Cherty limestone and Quartz-slate there was an ei'osion interval which ma}^ mark a time break of considerable magnitude. It is even probable that above the Cherty limestone member were deposited other formations which have been entirely removed. While in the lower part of the Quartz-slate member a considerable amount of material— even sufficient to form basal conglomerates — ^lias been derived fromthe Cherty limestone, the great mass of the materif^l came from the gneiss-granite Basement Complex. Where the Cherty limestone is now absent the basal conglomerates found contain fragments which are almost wholly from the granite and gneiss. These conglomerates have been discovered at a num- ber of localities and may be nearly continuous. The character of the sediments was very uniform for some time, although there were to a certain extent variations in conditions, thin beds of feldspathic sandstone being interlaminated with beds of shale. After a time there was a clearing up of the waters and an assorting of materials, as a result of which well rounded quartz grains only were deposited, and this layer now constitutes the pure vitreons quartzite com- posing the upper 50 feet of the member. After the deposition of this thin layer of sandstone there was again a change of conditions by which fragmental sedimentation ceased and the nonfragmental chemical or organic sediments of the iron belt began to . form. During this period the conditions were again uniform, the material everywhere, except in the Eastern area, at all horizons appearing to be a cherty ferriferous carbonate. During the time of the accumulation of the 800 feet of the Iron-bearing member it is probable that the bed of the ocean continued steadily to subside. Again a change of conditions came about, resulting in the deposition of fragmental layers. The passage from the nonclastic iron formation to the clastic slates was not so abrupt as the change from the quartz-slates to GENKRAL (;EOL()(iY OF Till'; DISTRICT. 465 the iron formation, but in most localities it took place within a compara- tively short distance. From this time onward the accumulatintj- beds of the Penokee series were very uniform in character. The material depos- ited constitutes the Ujjper slate member. This member, in places more than 10,000 feet in thickness and making up at least six-sevenths of the whole series when at its maximum thickness, is in turn graywacke and graywacke- slate, mica-schist and mica-slate, with less frequent clay-slates. The original material for all of these phases of rocks was very largely quartz and feldspar, the only differences being the relative proportions of the two minerals and their fineness of comminution. Thick beds of fine grained black clay-slates and gray wacke-slates show tlmt through a large proportion of the area the material was very finely pulverized. From Penokee gap and westward the original rock has been metamorphosed into a mica-slate or mica-schist. Here the material furnished by the underlying gneiss and granite was almost wholly feldspar. This peculiarity is explained by the fact that the area of granite and granitoid gneiss stretching to the south and west is veiy strongly feldspathic. After the deposition of at least 12,000 feet of these materials, and perhaps other kinds of sediments which have been subsequently swept away, came the end of Penokee time. The district was then elevated above the sea, gently folded, and suf- fered a long period of atmospheric denvidation. The erosion was suffi- cient at the west and east ends of the district to entirely remove the series. What fraction of it has been removed at its place of present maxinuim thick- ness is unknown. Following the Penokee series came the great succession of eruptive and fragmental layers which built up the Keweenaw series. This series has been already treated in a monograph of the Geological Survey,^ and nothing will be here said as to the conditions which prevailed or as to the succession. But the probable connection between the eruptives of this series and the dikes of the Penokee series is to be noted. The little altered diabases of the Southern Complex and those of the Penokee series have been described in detail and their relations to the containing formation given (chapter vii). It has been seen that these same dikes have been pro- foundly altered in the iron formation, where they have been subjected to 'Copper-Bearing Rooks of Lake Superior; by R. D, Irving, u. S, Geol, purvey, Monograph v, MON XIX 30 466 THE PENOKEE IRON BEAEING SERIES. long continued leaching. In that part of the Penokee series which has been the seat of mining operations, the large number of these dikes and the fact that they cut the containing formations perpendicularly are shown by the descriptions, pp. 271-275 and Pis. xxx and xxxi. That these diabases are the pipes through which has passed, from deep within the earth, the vast amount of material which formed the basic volcanic flows of the Keweenaw series can hardly be doubted. The trap range north of the Penokee series is a set of rocks varying, it is true, greatly in structural character, being here amygdaloids and there diabases and gabbros, but m chemical composion these rocks are practically the same as those that are found in the form of dikes within the Penokee succession. ■ After or during Keweenaw time began the orographic movement, accompanied and followed by erosion, which made the synclinal trough of lake Superior, and which upturned and truncated the whole great tlnckness of formations constituting the Keweenaw and Penokee series. We have no measure by which to estimate the time required for this work, but it was sufficient to bring to the surface a continuous succession of beds more than . 50.000 feet thick. Prior to this- time, during it, and subsequent to it went on the alter- ations which changed the rocks of the Penokee series from their original condition to their present somewhat metamorphosed one. There is no means of placing the time at which the change from the feldspathic fragmental rocks to the mica-schists occurred. There seems to be tolerably clear evidence ■that the transformation of tlie cherty iron carbonates to the many phases of rock now found in the formation took place during the uplifting and erosion or subsequent to it. After the Penokee and Keweenaw series had assumed their present inclined position and had suffered vast erosion, there was de230sited upon their upturned edges the Eastern sandstone, which now cuts them both off just west of Gogebic lake. Why the district is given a separate memoir. — The reason is now more apparent than at any time before for giving this Penokee district a separate memoir. It stands out in the lake Superior country unique in its simplicity and isolation. It is a great series of water deposited sediments, the origin GENEKAL CiEOLOGY OV TIIK DISTRICT. 467 of which has been for the most part deterininud. TIk; locks liave simply been tilted to the northward at lui angle most convenient to determine the succession of l)i'hs. It is without any subordinate fold whatever, so that traversed anywhere, excluding- the Eastern area and Cherty limestone, if sufficient exposures are found, the succession of its belts can be made out, one following- the other in conformity. Tliis series is terminated on the east by the uuconformably overlying horizontal Easteni sandstone; it is terminated on the west by its being entirely swept away by erosion, the Keweenaw series directly underlying the Southern Complex. It is marked off from the underlying granitic and gneissic rocks by one of the greatest unconformities of geology. The proof of this unconformity, as evidenced by broad relations and by numerous localities in which actual contacts are found, is of the clearest possible sort. To the north of the Penokee rocks is a third independent set of formations, the Keweenaw series, which is separated from the former by a time gap only inferior to that just men- tioned. Depth and metamorphism. — The Penokee series furnishes an instructive lesson as to the depth to which rocks can be buried and still be slightly affected by metamorphosing processes. The series itself is 14,000 feet thick. It was covered before being upturned by a great thickness of Kewee- naw rocks. This series at the Montreal river is estimated^ to be 50,000 feet thick. Adding this to the known thickness of the Penokee series we have a thickness of 64,000 feet, or more than 12 miles. The Peno- kee rocks were, then, buried to a great depth, the exact amount depending upon their horizon and upon the stage in Keweenaw time when the tilting and erosion which brought them to the surface was inaugurated. That the synclinal trough of lake Superior began to form before the end of the Keweenaw period, and consequently that the Penokee rocks were not buried under the full succession, is more than probable. However, they must have been buried to a very great depth — at least several miles— and thus subjected to high pressure and temperature, notwithstanding which they are comparatively unaltered. In the quartz-slates near the bottom 1 Copper- Bearing Kooks of Lake Superior; by E. D. Irving, p. 230, U. S. Geol. Survey, Mon- ograph V. 468 THE PENOKEE IRON-BEAEING SEEIBS. of this pile the feldspars have, it is true, in large measure decomposed to chlorite, mica, aud quartz. Also the quartzite of the upper part of this for- mation has been indurated by the enlargement of the quartz-grains. The alteration in parts of the Iron-bearing and Upper slate formations has locally been more extensive. But the clastic character of the fragmental belts is usually seen at a glance under the microscope. The pressure to which these rocks have been subject, the amount of which one dares hardly to estimate in figures, has not been sufficient to distort the quartz-grains in any degree ; nor has it been sufficient to give a schistose structure to the quartz-slates. It is a probable deduction from these facts that the weight alone of any ordi- nary amount of superincumbent rock is not sufficient to develop schistose structure. The schistosity found in the various fragmentals of the Mar- quette, Menominee, and Vermilion lake districts, and in some places in the Eastern area of the Penokee series, must have been developed in connection with the dynamic action of folding to which they have been subjected. SECTION III.— CORRELATION. Throughout this memoir the terms Archean, Laurentian, Keewatin, Huronian, etc., have been avoided. Certain of these words have been so differently used by different writers that it was thought best to free the dis- cussions involved in this book from misapprehensions which would result from their use. The structural relations of the series considered are so per- fectly distinct that no evidence or use of names from other localities is necessary in order to make out the succession of belts within the Penokee district itself, or the relations of the series to one another. Before closing, however, it is necessary that some reference be made to other series, which have been designated by these and other terms, and an indication be given of our judgment as to their proper use, and where the series considered would fall under that usage. Equivalency of Penokee series proper with Animikie series. — As the open- ing step a comparison will be made with the Animikie series (see PI. xxxvii), the equivalency of which with the Penokee series proper is as plain as the equivalency of any two areas of detached rocks in a single geological basin can possibly be in which clear paleontological evidence is lacking. * GENERAL GEOLOdV OK THE DISTUIOT. 469 It luis been set'u tluit jibttve tliL' Cliorty liinostoiio of" the Peiiokee dis- trict is an erosion interval and perhaps a considerable structural break. In the Aniniikie district we know of no equivalent to this member, and in what follows it is excluded ft-om the discussion. The Penokee and the Animikie rocks have a parallelism in lithological characters which is remarkable. This parallelism has already been discussed, but the main facts are here repeated. Not only is there a general likeness between the specimens from the two districts, but almost every phase of rock from the Animikie series can be matched by specimens from the Penokee series. In the Animikie district the formations underlying the iron-bearing belt are not extensively exposed, and consequently little is known of the Animikie equivalent of the Quartz-slate of the Penokee series. But along the lower Current river, near Port Arthur, Ontario, occur quartz-slates underlying the Iron-bearing member which resemble certain phases of the Penokee quartz- slate. Beginning with the iron formations, the parallelism between the two series is almost exact. The irony beds upon Gunflint lake, whei'e are found the best known exposures of the' formation, are in their lower parts jasper, magnetite-actinolite-schist, and cherty ferruginous rocks containing more or less iron carbonate. Higher up are thick deposits of thinly bedded cherty iron carbonate. All these varieties of rock are found in the iron formation of the Penokee series, and at many places the order of succession is the same. Above the iron-bearing- belt in both districts is a great thickness of fragmental clay-slates and graywacke-slates which are again practically identical in character. It is true that in the western part of the Penokee district mica-schists have developed from these slates, but the original con- dition of these rocks was essentially like that of the unaltered phases. Underlying both the Animikie and the Penokee series is a complex of granites and schists, the unconformity between which and these series is of the most pronounced character. That the Animikie series is thus separated from the underlying rocks has been seen by all who have studied it, and, considering this general agreement, the proof of this unconformity will not be here repeated. Above both series follow the Keweenaw rocks. In both districts, in passing at any place from the underlying rocks to the Keweenaw series in section, the two are in apparent conformity; but when 470 THE PENOKEE lEON-BEAEING SEEIES. the lines of contacts between the iron-bearing and the Keweenaw series are followed for some distajice, both with the Animikie and Penokee series, this apparent conformity is found to be illusory ; that is, the Keweenaw series is now in contact with one member of the iinderlying series and now with another, until in both districts at one or more places the entire iron-bearing series is entirely cut oflP, the Keweenaw rocks com- ing directly in contact with the Basement Complex.-' This means that between the deposition of the Penokee and Animikie series and the outflows of Keweenaw time there intervened a period of erosion which was sufficient in places to entirely remove the two former and to cut in some places deeply into the older rocks themselves. There is, then, an immense time gap between these series and the overlying Keweenaw rocks, although this unconformity does not approach in the length of time involved to that separating them from the underlying schists and granites. The Animikie series, in its most typical development, extends from Gunflint lake, along the national lioundary between Minnesota and Ontario, to Thuilder bay, lake Superior. The Penokee series lies upon the opposite side of lake Superior. - The latter is a simple imfolded succession, dipping to the northward under the lake. The Animikie is another such siiccession, dipping to the southward under the same body of water. There is, then, little doubt, considering all the facts, that the two series represent a single period in the history of the synclinal trough which forms the basin of lake Superior.^ The relations and likeness of the Penokee and the Animikie series have been dwelt upon at length as showing the breadth of the geological basin in which the deposition of like rocks Avas taking place simultaneously. The equivalency here shown is a long step in under- standing the equivalency of other rocks in the lake Superior basin. Equivalency of Penokee and Marquette series. — A comparison of the ' For full discussion of the proof of the uncpnformity hetween th'e Animikie antl Keweenaw Beries see E. D. Irving : On the Classification of the Eai'ly Cambrian and pre-Cambrian Formations, 7th Auunal Report U. S. Geo]. Survey, pp. 417-423. If the Keweenaw series simply were in contact with the gneisses and granites it might be hehl that we have only to do with an overlap, but its con- tact— now with one horizon of the Animikie, now with another — can only be explained by an uncon- formity. ^E. D. Irving: Copper-Bearing Rocks of Lake Superior, U. S. Geol. Survey, Monograph v, 1883, pp. 410-418. GENERAL (ilCOl.OCiV <)K TIIK DISTKICT. 471 Penokoe aiul- .M,ii-([iu'tte .successions shows that hctwccu the iwo tlierc is u very close correspoudeiice. Uiicouforuiably below the elastics of tliosc districts is a crystalline Basement (!()ni[)lex composed of schists, g-neisses and granites. Witliin the pre-Keweeuavvan cla.stics in (ia-cli district is a, second physical bn^ak. In the Peuokee district the series below the break is known to be represented only by a single formation, the Clierty limestone. That other higher formations once here existed is indicated by the presence of fragments of jasper and quartzite in the lowest horizon of the Quartz-slate. These jasper frag'ments occur in the basal conglomerate of the Quartz-slate at Potato river, at two localities near the Palms mine, and at one place in the Eastern area. Usually the pebbles are not of large size, but occasionally they are several inches in diameter. Quartzite pebbles are even more abundant than those of jasper. It is, therefore, probable that the three sedimentary formations of the Lower Marquette series were once represented by equiva- lent members in the Penokee district. The correspondence of the members of the Penokee series j)roper with those of the Upper Marquette is complete. The Upper Marqnette and Penokee series, looked at broadly, are great slate formations, both of which contain, near the base, an iron-bearing horizon. In the Penokee series that portion of the slate overlying the ore formation has been called the Upper slate member, and that below it the Quartz-slate member. The lower part of the Quartz-slate is a quartzite and conglomerate which cor- responds to the quartzite and conglomerate forming the base of the upper Marquette series. The upper most horizon of the Penokee Quartz-slate is a narrow layer of persistent quartzite which does not appear to be repre- sented in the Marquette district. The character of the ore-bearing mem- ber is identical in both districts, being unquestionably derived from a lean cherty carbonate of iron. The characteristic rocks of both are now the iron carbonates, cherts containing bands and shots of ore, and the iron ores. The chief differences between the two are that in the Penokee district the actinolite-magnetite-schists are more prevalent, that the iron-bearing for- mation is more persistent, and that its ore bodies are more abundant. Con- nected with these facts is perhaps the presence of the upper horizon of 472 THE PENOKEE lEON-BEAEING SEEIES. quartzite in the Quartz-slate, which shows that a clearing up of the waters occurred before the beginning of deposition of the iron-bearing sediments. A still further analogy between the Penokee and Upper Marquette series is the presence in both of abundant surface volcanics. We have, then, in the two districts the following parallel descending pre-Keweenawan suc- cession: Fenohee. Upper slate, locally mica-scMst. Iron-bearing formation. Quartz-slate; upper liorizou persistent quartzite; central mass a slate; lower part often conglomeratic, bearing frag- ments of lower series, either Cherty limestone or Basement Complex, and locally a quartzite. Unconformity. Eroded away. Limestone. Unconformity. Basement complex. Marqiiette. Upper slate, rather extensively mica-schist. Iron-bearing formation. Lower slate; lower part quartzite or quartzite conglomerate, bearing frag- ments of lower series, either lower Mar- quette or Basement Complex. Unconformity. Iron-bearing formation. Limestone and lower quartzite. Unconformity. Basement comiilex. Comparison with other series. — As is well known, in the region about lake Superior are other areas containing limestone, quartzites, graywacke, gray wacke-slates, mica-slates, mica-schists, volcanic elastics, and the peculiar phases of rock of the iron-bearing formations. The positions of these various areas as shown upon PL i, are designated, respectively, by H, H2, H3, H^, etc. These have been known in the past as the original Huronian district, the Marquette, Felch Mountain, Menominee iron-bearing districts, the St. Louis slates, the Chippewa valley quartzites, the Black river Iron-bearing schists, the Baraboo quartzites, the Sioux quartzites, and the folded schists of Canada, including the Vermili on series. The relations of these series to one another and to the Penokee series, less closely related with the Penokee district than are the Animikie and the Marquette, are fully considered in a bulletin of the United States Geologi- cal Survey on the Algonkian and Archean.^ As a result of the discussion there sfiven, the relations of the series of these difFerent districts are tabu- lated as follows : iC. E. Van Hise: Correlation papers, Algonkian and Archean. Bull. 86, U. S. Geol. Survey. GENEUAL GEOLOGY OF THE DISTRICT. 473 <>5 "S" q52 Hi ■S3 ■3 a ^1 e.2 w « ^ 3 g •O iil 2 a 1 3 a 2 S .. . ^ ?d o « s 1 S S M I g I .9 M H B |2 1-1 s W ill H -I S fl S § *i '3 -^ a « .9 a 3. 1 g Fl g ^ Q fl a w p > a ^ •M .13 y *— (D 2 nl « 1 ■M fl a o O ^ S " — aj S S ■ft -s g =» & 2 2 •fl B q:l 00 ^ isD '-t^ iS n a tj « 5S o 3 o" -■a ^B M-9 s ^s ■gggl 9:5 -9 1^1 .-^1 ■o-s a 4ip.Sf •« B.3 ^ D a «^" ■32 fl o'S u ^'iS.a ^ 0+3 £■02 •^ S.;3 .931 ^?»>- «9r3 p a d .o bi^B B a " .S.-.9 5 2° a 0.2S m "^ ft — f> ft a»=^ 474 THE PENOKEE lEON-BBAEING SERIES. To give in full the reasons for the positions assigned to these various, series and the underlying and overlying rocks would be but to repeat the discussion before referred to. In the above table the Penokee series proper, including the Quartz-slate, Iron-bearing, and Upper slate members, is placed as the equivalent of the Upper Huronian, Upper Marquette, Upper Ver- milion, and the quartzites of Wisconsin, Minnesota, and Dakota, primarilj^ on structural and secondarily on lithological grounds, while the Cherty limestone at the base of the series, being separated by a considerable erosion interval from the Penokee series proper, is regarded on grounds of a similar character as equivalent to some part — probably a lower part — of the Lower Huronian, Lower Kaministiquia, Lower Vermihon, Lower Mar- quette, and equivalent series. The position assigned to this formation is warranted because of the very considerable time break between the Cherty limestone and the Penokee series proper. The Penokee series and those series equivalent to it constitute a part of the great Algonkian system, and the Southern Complex is Archean. PLATE XIV. 475 Plate XIV. — From the Southern Complex. Fig. 1. Biotite-granite. Specimen 9639, slide 4228. From the NE. i of the NE. i Sec. 20, T. 44 N., E. 3 W., Wiscousin. In polarized light, x 25. The section shows the strongly feldspathic character of the rock and the abundance of nonstriated feldspar, although striated feldspar, both microcline and plagioclase, are seen. The dark areas are largely biotite. Fig. 2. Biotitic granitoid gneiss. Specimen 9674, slide 3394, 0 steps N., 160 steps W., of the south- east corner of Sec. 23, T. 44 N., E. 5 W., Wisconsin. Upon the Marengo river. In polarized light, X 25. The section shows the alteration of feldspar into biotite and quartz. Large, irregular, much altered areas of orthoclase and plagioclase are contained in a fine ground- mass composed of quartz and biotite. The aggregates of these minerals, besides filling the interspaces, cut into the larger feldspar areas, so that there is a gradation from the fine grained background into the feldspar. In certain cases this alteration has extended quite to the centers of the feldspar individuals. This figure suggests that the rock was once very much more strongly feldspathic than at present — perhaps as strongly feldspathic as the rock in the previous figure — and that the alterations have changed it into a strongly biotitic and quartzose rock. The foliation which is now apparent in the rock may also be one of the results of this alteration. Fig. 3. Hornblende-schist. Specimen 9050, slide 2776. From near the northwest corner of Sec. 35, T. 46 N., R. 2 E., Wisconsin. In polarized light, X 25. The groundmass of the section is com- posed of finely crystalline, closely interlocking quartz and hornblende. Within this ground- mass are nimierous large roundish areas of feldspar, which are now on their outer parts apparently altering into quartz and hornblende. The appearance of the feldspar suggests a fragmental character, but however closely examined, nothing else in the section gives additional light upon this question. If any of the hornblende schists of the Southern Com- plex are clastic (but this is doubtful), this is probably one of them. Fig. 4. Hornblende-gneiss. Specimen 9060, slide 2921. From the south part of the SW. i Sec. 33, T. . 46 N., E. 2 E., Wisconsin. In ordinary light, X 25. The light background is composed of finely crystalline quartz mingled with a good deal of feldspar, the latter comu only being in larger areas than the quartz and often having roundish outlines. These facts are not apparent in the figure. The slide is reproduced to show the peculiar character of the horn- blende individuals. They run from minute fibers up to tolerably large blades which have extremely ragged outlines. They cut the quartz and feldspar through and through. Their appearance is such as to suggest that they are now in the process of grovrth, and in thin section the relations of the hornblende and .quartz to the feldspar farther suggest that from the feldspar these minerals have developed. If all of the minerals now present are original, the hornblende must certainly have been the first to crystallize, so thoroughly does it pene- trate the others ; but this is very much less probable than that it has developed as tl e last mineral of the rock and subsequent to its consolidation. 476 U. S. CeOLOOICAt. fiunvEY MONOORAPH XIX PLATE XlV Biotite-granite, Fig. 2.— Biotitic granitoid gneiss. Fig. 3. — Hornblende-schist. Hornblende-schist. THIN SECTIONS FROM THE SOUTHERN COMPLEX. PLATE XV. 477 Plate XV. — Feom the Southern Complex. Fig. 1. Hornblende-granite. Specimen 12873, slide 5504. From the NE. i of the SW. i Sec. 23, T. 47 N., R. 47 W., Michigan. South of Aurora mine. In polarized light, X 25. The strongly feldspathic character of the granite of the Southern Complex, shown by PI. xiv, Fig. 1, is again brought out, although more quartz is present than in the previous figure. A larger proportion of the feldspar is microcline. The pegmatitic structure of quartz and feldspar is nicely shown iu one place. The dark areas are mostly hornblende. In one individual twin- ning is seen. Fig. 2. Horublende-biotite-syenite. Specimen 7615, slide 2070. From near the northeast corner of Sec. 27, T. 47 N., R. 47 W., Michigan. In ordinary light, X 25. A coarsely crystalline feldspathic l)ackground is apparent. The individuals of feldspar are large and fit in much the same manner as does the feldspar in the massive syenites and granites from this vicinity ; but in this rock it is cut through and through with hornblende and biotite, which are arranged approximately with their longer axes in a common direction and thus give the rock its foliation. The relations suggest the secondary development of the hornblende and biotite within the feldspar by dynamic metamorphism. Fig. 3. Biotite-gneiss. Specimen 7529, slide 1963. From a short distance south of the north quar- ter post of Sec. 18,T. 47 N., R. 45 W., Michigan. In polarized light, X 25. The section is composed of small particles of nearly uniform size, consisting of quartz and feldspar min- gled with biotite. The roundish and yet closely fitting character of the quartz and felds- par is nicely shown. There are also seen a few larger roundish grains of quartz. This is one of the rocks of the Southern Complex which has a strong fragmental appearance, and yet there is no certain evidence that it is clastic. The particles now perfectly fit one another, therefore they could not thus have been mechanically deposited. None of them show evi- dence of enlargement, so the crystalline appearance can not be accounted for iu this way. Fig. 4. Hornblende-gneiss. Specimen 9458, slide 3077. From near the south quarter post of Sec. 16, T. 47 N., R. 45 W., Michigan. In ordinary light, X 25. The background of the section is composed of small, perfectly fitting, roundish granules of quartz and feldspar in nearly equal quantity. Contained within this groundmass are numerous large crystals of horn- blende, which in transverse sections often have well developed crystal outlines, the forms being usually the pinacoid and unit prism. Each individual of hornblende includes many grains of quartz and feldspar. The hornblende must have been here the last mineral to develop, since it includes so large a proportion of the other minerals. Its relations to the other minerals taken in connection with its crystal forms make its occurrence analogous to such metamorphic minerals as garnet and stauiolite. 478 U. 8. OEOLOOICAL SURVEY MONOGRAPH XIX PLATE XV Fig. 1. — Hornblende-granite. Fig. 2.— Hornblende biotite-syenite. Fig. 3. — Biotite-gneiss. Fig. 4.~Hornblende-gneiss. THIN SECTIONS FROM THE SOUTHERN COMPLEX. PLATE XVI. 479 Plate XVI. — From the Cherty Limestone Member. Fig. 1. Tremolitic dolomite. Specimen 9678, slide 3165. From the NW. i of Sec. 22, T. 44 N., E. 5 W., Wisconsin. lu ordinary light, x 60. The finer grained part of the section shows the evenly granular appearance characteristic of the massive limestones and dolomites. In one part of the figure is coarsely crystalline carbonate and a broad blade of tremolite. The sec- tion chances to be so cut that this mineral appears in part as a mere film, below which the vague outlines of the particles of dolomite are seen. (See p. 135.) Fig. 2. Cherty limestone. Specimen 7485 A, slide 1934. From the SE. J of Sec. 18, T. 47 N., R. 44 W. , Michigan. In polarized light, X 60. The central band of the figure is largely granular dolomite, in which is, however, a considerable quantity of chert. This dolomite is inter- laminated with two layers of nearly pure chert, one of which is much more finely crys- talline than the other. (See p. 187.) Fig. 3. Concretionary chert. Specimen 9434, sUde 3131. From the NW. i of Sec. 41, T. 47 N., E. 45 W., Michigan. In polarized light, X 60. A fine grained chert. The minute mosaic char- acteristic of chert when viewed in polarized light is nicely shown. The material is arranged in a vague concretionary fashion, areas composed of very finely crystalline silica being surrounded by borders of more coarsely crystalline quartz. Although imperfectly shown in the figure, the latter has to some extent the radial fibrous arrangement of chal- cedony. (See p. 137.) Fig. 4. Chert. Specimen 9424, slide 3064. From the NW. i of Sec. 14, T. 47 N., E. 45 W., Mich- igan. In jjolarized light, X 60. The section shows very well the variations in fieneness of grain of the more coarsely crystalline varieties of chert. It is wholly composed of com- pletely individualized quartz, the intricate interlocking of which is well shown and which is in strong contrast to the appearance presented by quartzites in which the interspaces have been filled by the enlargements of fragmental quartz. (See p. 137.) 480 U. 6. QEOLOQICAU SURVEY MONOGHAPH XIX PLATE XVI Fig. 1. — Tremolitic dolomite. Fig. 2. — Cherty limestone. Fig. 3. — Concretionary chert, pjg. 4 Chert. THIN SECTIONS FROM THE CHERTY LIMESTONE MEMBER. PLATE XVII. 4gi MON XIX 31 Plate XVII. — From the Base of the Qtjartz-Slatb Member. Fig. 1. Chert containiug fragmeutal quartz. Specimeri 9534, slide 3140. From the NW. i of See. 14, T. 44 N., R. 3 W., Wisconsin. In polarized light, x 30. The background of the section is composed of finely crystalline quartz. In it are contained large well rounded grains of quartz, oup of which has plainly received a second growth. In the lower part of the figure is a complex rounded area of chert not greatly different from the matrix in which it is set. (See pp. 157-15&.) Fig. 2. Quartzose chert. Specimen 953.5, slide 3141. From the same place as Fig. 1. In polarized light, X 60. The center of the figure shows in the cherty background a large quantity of magnetite. In the exteriors of the large fragmeutal grains of quartz^re very numerous minute needles which are taken to be actinolite. In certain cases these actinolite needles are included only in the enlargements of the quartz grains, but in others appear to penetrate rather deeply into the cores. This occurrence strongly suggests that actinolite may develop in such a manner as to penetrate quartz the latter, of course, being simultaneously removed. (See pp. 157-158.) Fig. 3. Chert-conglomerate. Specimen 9419, slide 3127. From the SW. i of Sec. 14, T. 47 N., E. 45 W., Michigan. In i)olarized light, X 60. The background of the conglomerate is a chloritic quartz-slate, the (luartz-grains of which at many places distinctly show the enlargements. The large roundish area with the spotty appearance is a part of one of the smaller of the chert fragments which, iu the form of pebbles and bowlders, are very abundantly contained in the rock. (Seep. 169.) Fig. 4. C4reeu-schist conglomerate. Specimen 0175, slide 2994. From the SE. ^ of Sec. 19, T. 45 N., R. 1 E., Wisconsin. In polarized light, X 60. The figure is from the Potato river basal conglomerate. For the most part it consists of three large fragments of green schist, which in (me case very distinctly shows the scJiistose structure. These fragments are set in a u»atrix consisting very largely of small roundish quartz-grains, mingled with which are quite numerous well defined crystals of magnetite. (See p. 159.) 482 U. S. OEOLOGtCAL SURVEY MONOGRAPH XIX PLATE XVII Fig. 1. — Chert containing fragmental quartz. Fig. 2.^Quartzose chert. Fig. 3.— Chert-conglomerate. Fig. 4.— Green schist and conglomerate. THIN SECTIONS FROM THE BASE OF THE QUARTZ-SLATE MEMBER. PLATE XVIII. i83 Plate XVIII.— From the Qtjautz-Slate Member. Fig. 1. Graywaoke-slate. Specimen 9442, slide 3071. From the SW. J of Sec. 10, T 47 N., E. 45 W., Michigan. In ordinary light, x 60. The figure represents a typical average grained gr.iywaoke-slate. The rounded fragments are in about equal quantity quartz and feldspar. Chlorite is the chief insterstltial mineral. (See pp. 165-166.) Fig. 2. The same in polarized light. The orthoclase is in most cases separable from the quartz, in that it lacks the perfect clearness and uniformity of color which each grain of that mineral shows. The striated feldspars are nicely shown. (See pp. 165-166.) Fig. 3. Cherty slate. Specimen 9641, slide 3310. From the NW. i of Sec. 13, T. 47 N. , K. 46 W., Michigan. In polarized light, X 60. The background of the section is an ordinary chlo- ritic slate. It contains, however, in certain layers numerous large well rounded frag- ments of (luartz and chert, both of which are well shown in the figure. (See pp. 164-165.) Fig. 4. Sericitic and chloritic slate. Specimen 9523, slide 3091. From the SE. i of Sec. 16, T. 47 N., R. 46 W., Michigan. In polarized light, X 60. The .section illustrates one of the rather uni- form fine grained fcldspathic quartz-slates. The small roundish areas are in part quartz and iu part feldspar. The dark material is mostly chlorite and grains of quartz and feld- spar, which chance to be near the point of extinguishment. (See p. 164.) 484 U. 8. QEOLOQtCAL SURVEY MONOGRAPH XIX PLATE XVIII Fig. 1.— Grayv»acke-slate. Fig. 2.— Graywaclie-slate in polarized light. Fig. 3.— Cherty slate, Fig. 4.— Sericitic and chloritic slate. THIN SECTIONS FROM THE QUARTZ-SLATE MEMBER. PLATE XIX. 485 Plate XIX.— From the Quaktz-Slate Member. Fig. 1. Biotitic chlorite-slate. Specimen 9565, slide 3098. From tlie NW. J of Sec. 14, T. 44 N., E. 3 W.; Wisconsin. In polarized light, X 60. Many small oval and angular particles of quartz are contained in a matrix consisting very largely of chlorite, with which is, however, a small amount of biotite. The angularity of the quartz-grains is A'ery noticeable. This is in some cases due to the enlargement of the quartz-grains, but often they were thus angu- lar when deposited. The figure has a more crystalline appearance than the section from which it was taken, from the fact that the cores of the quartzes which have undergone a second growth are not strongly marked off from the enlargements, although in thin section they are easily distinguished. (Seep. 158.) Fig. 2. Biotite-slate. Specimen 9644, slide 3154. From the NE. i of Sec. 17, T. 44 N., R. 3 W., Wis- consin. In ordinary light, x 60. A few grains of quartz with general roundish forms, although now minutely angular by enlargement, are contained in a background which consists almost wholly of biotite and quartz. In the light irregular areas quartz is pre- dominant, and elsewhere biotite. While the fragmental character of this rock is not evident in the figure, it is plain in thin section. The rock, however, is one of the most crystalline of the biotite-slates which are found in the Quartz-slate member. It is wholly possible that the irregular whitish, roundish areas now composed of quartz and mica rep- resent original fragmental grains of feldspar. (See p. 156.) Fig. 3. Sandstone. Specimen 9004, slide 288a From the NW. i of Sec. 27, T. 47 N., R. 47 W., Mich- igan. In ordinary light, x 60. Quartz in roundish and irregular fragmental grains, is the predominating constituent, although feldspar is important. The abundant interstitial material is so heavily stained with oxide of iron that it is difficult to determine what other luineruls ;ire present. (Seep. 163.) Fig. 4. Argillaceous shale. Specimen 7504, slide 1946. From the NE. i of Sec. 15, T. 47 N., R. 45 W., Michi'4:m. In ordinary light, x 60. The section shows one of the finer grained and more clayey phases of the Quartz-slate member. On one side of the figure recognizable frag- ments of quartz and feldspar are the chief constituents. Upon the other side extremely finely divided clayey minerals are preponderant. (Seep. 167.) 486 U. S. OEOUOQICAL SURVfY MONOCBAPH XIX PLATE XIX Fig, 1. — Biotitic chlorite-iilate. Fig. 2.— Biotite-slate. Fig. 3,— Sandstone. pig, 4,_Argillaceous slate. THIN SECTIONS FROM THE QUARTZ-SLATE MEMBER, PLATE XX. 487 Plate XX. — Pbom the Upper Horizon of the Quartz-Slate Member. Fig. 1. Quartzite. Specimen 9082, slide 2780. From tlie east side of Sec. 19, T. 45 N., R. 1 E., Wisconsiu. lu ordinary light, X 25. The rock is a vitreous quartzite, and jet the figure shows with perfect distinctness the outlines of each of the rounded grains of sand just as they were origiually deposited. (See p. 160.) Fig. 2. The same, in polarized light. The cause of the present strong and vitreous character of the quartzite is clearly shown by this figure. Each of the grains of quartz has added to itself other quartz until the grains have met and interlocked. This is a fine instance of the induration of a sandstone liy simple enlargement of the original grains. (See p. 160.) Fig. 3. Ferruginous quartzite. Specimen 9154, slide 2804. From the SW. i of Sec. 27, T. 46 N., R." 2 E., Wisconsin. In polarized light, X 25. The rock is again a vitreous quartzite, which has, however, a Lrown color. The outlines of the original grains»are distinctly seen as in Fig. 1. The cohu' of the rook is due to the oxide of iron located in the interstices. (See p. 162.) Fig. 4. The same, in polarized light. As in Fig. 2, each of the rounded grains of quartz has been enlarged. The interlocking of these enlargements has been interfered with by the included iron oxide, so that the Vock is not so vitreous and strong as that from which Figs. 1 and 2 are taken. (See p. 162.) U. S. QEOLOOICAL SURVEY MONOOHAPH XIX PLATE XX Fig. l.-Quartz(te Fig. 2 — Quditzite, in polarized light. Fig. 3 — Ferruginous quartzite. Fig. 4.— Ferruginous quartzite, in polarized light. THIN SECTIONS FROM THE UPPER HORIZON OF THE QUARTZ-SLATE MEMBER. PLATE XXI. 489 Plate XXI. — Sideritic Eocks, prom the Iron-Bearing Member and prom Lawrence County, Ohio. Fig. 1. Sideritic chert. Specimen 9814, slide 3880. "Limestone ore." Lawrence county, Ohio. In ordinary light, X 25. The section has a cherty background, which contains large oval and roughly rhombohedral areas of iron carbonate. It is introduced, here in order to com- pare this carbonate with those of the-Penokee and Animikie districts. Fig. 2. The same, in polarized .liglat. The fine mosaic character of the cherty background is here exhibited. The outlines of the carbonate areas become less distinct than in the previous figure. Fig. 3. Sideritic slate. Specimen 9191, slide 29S6. From the NE. i Sec. 6, T. 45 N., R. 2 E., Wis- consin. In ordinary light, x 60. The section is cut transversely to the lamination of the rock. The hand specimen is very thinly and regularly laminated. The laminae are seen to be somewhat irregular. The obscure dark and light portions are both iron car- ■* bonate, which differ chiefly in the inclusions which they contain. The minute white particles are chert and amorphous silica. (Seep. 237.) Fig. 4. Sideritic and ferruginous chert. Specimen 9474, slide 3082. From Sec. 13, T. 47 N., R. 46 W., Michigan. In ordinary light, x 60. The section shows the alteration of iron carbon- ate into iron oxide. The background is chert. Upon one side of the figure are rhombo- hedra of little altered siderite. Upon the other are black pseudomorphous areas which are composed of somewhat hydrated hematite. Between the two are gradation phases. (See p. 237.) 490 U. 8. OEOLOOICAL SURVEY MONOGRAPH XIX PLATE XXI Fig. 1.— Sideritic chert. Fig. 2. — Sideritic chert, in polarized light. %;;;^^SS^^'^i»^'' *' .^-^■^; '..r-^-y'^- ^;:-'.. ^^'mi--^^'f' .T^^f^a^:-- :;''%^ Fig. 3.— Sideritic slate. Fig- 4.— Sideritic and ferruginous chert, THIN SECTIONS OF SIDERITIC ROCKS FROM THE IRON-BEARING MEMBER AND FROM LAWRENCE COUNTY, OHIO. PLATE XXII, 491 Plate XXII.— Ferbu&inous Cherts prom the Iron-Bearing Member. Fig. 1. Concretionary cliert. Specimen 9048, slide 2886, From the SE. i Sec. 27, T. 46 N., E. 2 E., Wisconsin. In ordinary light, X 25. In a cberty background are beautiful concretions, wlilch are composed of concentric rings of iron oxide and chert. One concretion particu- larly is very fine, showing many closely packed concentric rings. Silica is seen breaking across these rings in a few places. (See pp. 227-228. ) Fig. 2. The same, in jiolarized light. Here the cherty backgrotind appears as a fine mosaic. The quartz in and about the concretions is more coarsely crystalline than the average of that in the matrix. One concretion has as a nucleus comparatively closely crystalline quartz. This variation in the character of the silica is suggestive that the flue spotty silica is perhaps original. The concretions, as shown by subsequent plates, have been produced from iron carbonate, and as the iron oxide formed from the carbonate, the remaining space was occu- pied by silica, which crystallized in larger particles than the supposed original silica. (See pp. 227-228.) Fig. 3. Brecciated chert. Specimen 7622, slide 2072. From the Montreal river, between Michigan and Wisconsin. In polarized light, x 25. The background is again cherty, and contains within it many small, rather perfect rhombohedra of siderite, which have altered to a greater or less degree to iron oxide. Contained in this groundmass are irregular areas which do not have a concretionary structure, but appear to be true fragments, in this matrix. One of the areas is severed in every direction liy numerous ramifying veinlets of silica. The same thing is to a less degree noticeable of others of these areas. That this cutting silica is secondary can hardly be doubted. (See pp. 230-231.) Fig. 4. Ferruginous and brecciated chert. From the same locality as the last. In polarized light, X 25. The background is chert, as in the previous figure. In this background are found perfect concretions and brecciated areas. One of the latter shows plainly its fragniental character. It is built up of laminse which are approximately parallel and have evidently been broken from a regularly laminated rook and here deposited. In this figure and the previous one we have clearly a mingling of fragmental and nonfragmental material, the fragmental portion of which is possibly derived from the immediately underlying iron- bearing beds. (See pp. 230-231.) 492 U. S. GEOLOOICAU SURVfv MONOGRAPH XIX PLATE XXII Fig. 1. — Concretionary chert. Fig. 2. — Concretionary chert, in polarized light. Fig 3.— ceiaecd trBchert. Fig. 4. — Ferruginous and brecciated chert. THIN SECTIONS OF FERRUGINOUS CHERTS FROM THE IRON-BEARING IVIEMBER, PLATE XXIII. 493 Plate XXIII.— Fekeuginotjs Cherts and Actinolite Slates from the Iron- Bearing Member. Fig. 1. Ferruginous chert. Specimen 9081, slide 4206. From the SE. i of Sec. 24, T. 45 N., E. 1 W., Wisconsiu. lu ordinary light, X 25. The cherty background contains .areas which are in part roughly oval or roundish, but are more largely exceedingly irregular. These areas are composed of quartz .and iron oxide, the latter being mostly hematite, the remainder magnetite. The regular areas suggest concretions, the history of which has been given. The irreo-ular areas resemlile fragments, but are probably of chemical and dynamic origin and have formed within the rock itself. They doubtless represent original iron carbonate areas. This carbonate has largely changed to oxide, but has also to some extent been leached out, thus leaving cavities. After or before the comiiletion of this process silica has entered and filled the cracks and cavities. The result of this oxidation, solution, and silicification, com- bined with movement, has been to put in the place of well defined areas of iron carbonate the exceedingly irregular forms presented by the figure. (See p. 223). Fig. 2. The same, in polarized light. The relations just mentioned are here again observed. It is further seen that the background, instead of being finely spotty and perhaps partly amor- phous, as in the previous plate, is completely cr^ystalline. That there has been an exten- sive rearrangement and entrance of silica, is shown, even more plainly than in the previous £gure. (See p. 223.) Fig. 3. Actinolitic schist. Specimen 9555, slide 3190. From Peuokee gap ; NW. i of Sec. 14, T. 44 N., R. 3 W., Wisconsin. In ordinary light, X 25. A quartzose background contains very numer- ous minute needles of actinolite and many particles of magnetite, the latter being roughly concentrated into bands, one of which with a part of another is shown in the figure. The figure represents a typical rock of this kind. (See pp. 218-219). Fig. 4. The same, in polarized light. The completely crystalline character of the quartzose back- ground appears, and the intricate maimer in which this material is cut by needles of actino- lite is perceived by comparing this figure with the preceding. (See pp. 218-219). 494 U. 8. GEOLOGICAL 8URVEV MONOGRAPH XIX PLATE XXIII Fig. 1. — Ferruginous chert- Fig. 2.— Ferruginous chert, in polarized light. Fig, 3— Actinolitic schist. Fig. 4.— Actinolltic schist, in polarized light THIN SECTIONS OF FERRUGINOUS CHERTS AND ACTINOLITIC SLATES FROM THE IRON-BEARING MEMBER. PLATE XXIV. 495 Plate XXIV. — Actinolite Slates from the Iron-Beauing Member of the Penokee Series and Cherty Iron Carbonates from the Vermilion Series. Fig. 1. Actinolite-magnetite-schist. Spocimeu 9558, slide 3191. From Penokee gap; NW. J of Sec. 14, T. 44 N., R. .3 W., Wisconsin. In polarized light, X 25. Magnetite and quartz are the most abundant constituents, but minute needles of actinolite are seen radiating from the magnetite areas. The rock is a regularly banded one, and while this is not strongly marked in the thin section its distinct lamination is apparent. The rhombic form of many of the magnetite areas is seen. Theseprobablyrepresent sections of octahedra. (Seep. 218.) Fig. 2. Actinolitic slate. Specimen 9680, slide 3167. From the SE. J of See. 20, T. 44 N., R. 5 W., Wisconsin. In polarized light, X 25. This is an actinolitic slate in -which the con- cretionary arrangement of the various constituents is shown. The close association of actinolite and magnetite is apparent, the minute actinolite needles frequently radiating from the magnetite particles. The actinolite is manifestly prior to the quartz in crystal- lization, as the individuals of tlie latter are penetrated in every direction by the actinolite needles. This figure is particularly noticeable on account of the very coarsely crystal- line ciiaracter of the quartzose background. It runs between the concretions iu such a manner as to make it evident that it must have crystallized subsequent to their formation. We have here, then, an illustration of the complete rearrangement of the silica originally present and the probable introduction of a good deal of silica from an extraneous source. (See p. 216.) Fig. 3. Cherty iron carbonate. Specimen 8726, slide 3367. From Vermilion lake, Minnesota. In ordinary light, X 25. This shows very finely the alternation of bands of chert and iron carbonate. The darker belts are nearly pure siderite, and the lighter ones almost jiure chert. The cherty belts are seen to contain minute rhombohedra of carbonate, and they cut across the carbonate belts in such a manner as to imply a rearrangement of silica originally present and the introduction of additional silica, or both. The figure is here introduced in order that the cherty carbonates of other districts may be compared with those from the Penokee series. (See p. 260.) Fig. 4. The same, in polarized light, x 50. It is seen that the cherty background is completely crys- tallized; that is, contains no amorphous material and is made ui> wholly of quartz. The intricate manner in wliich the quartz anastomoses, cutting the iron carbonate, shows conclusively that to some extent it is later than the siderite, while it has probably been extensively rearranged. (See p. 260.) •» 496 U. S. GEOLOGICAL SURVEY MONOGRAPH XIX PLATE XXIV Fig. 1. — Actinolite-magnetite schist. Fig. 2, — Actinolitic slate. Fig. 3. — Cherty iron carbonate. Fig. 4. — Cherty iron carbonate, in polarized light. THIN SECTIONS OF THE ACTINOLITIC SLATES FROM THE IRON-BEARING MEMBER OF THE PENOKEE SERIES, AND CHERTY IRON CAR- BONATES FROM THE VERMILLION SERIES. PLATE XXV. MON XIX 32 497 Plate XXV. — Sideritig Slates prom the Animikte Series. Fig. 1. Chert.y iron carbouiite. Speciiiieu 10172, slide 3422. Froui Diuvsoii'.s road, Port Artlmr, Canada; Auimikic. series. In ordinary liglit, x 60. The figure well illustratis tlie apjiearance ol" one of the ricliur Animikie chcrty carbonates. Throughont most of the sec- tion iron carbonate is the cliief constituent, and the manner in whicli the individuals are packed together is well shown ; wheu surrounded by chert they show perfect rlunnliohedral forms. Alter.ation of iron carbonate to iron oxide has begun. (See p. 264.) Fifi. 2. Sideritie chert. Specimen 6138, slide 1173. From north shore of Gunflint lake, T. 65 N., R. 3 W., Minnesota; Aniraikh; series. lu ordinary light, X 2.5. The iigure illustrates the for- mation of iron oxides, iisoudomorphous after siderite. A background of chert contains numerous small roundish ami rhombohedral areas of siderite and iron oxide. Between tfe little altered and wholly altered siderite a complete gradation is seen. (See ji. 264.) Fig. 3. Actinolite-siderite-slate. Specimen 10579, slide 5188. From the east side of north arm of fJuutlint lake, Minnesota; Animikie series. In ordinary light, X 25. This section is from one of the typical Animikie slates. The thinly laminated character, of the roik is well showu. Its background consists in about equal jjroportions of actinolite and siderile, mingled with a little chert. The dark colored material comprises all three of the oxides of iron. (Sec p. 263.) Fig. 4. The same, in polarized light, X 25. The figure is from another part of the sectioi], which shows the termination of a thick layer of nciirly pure chert. Such lozenge-shaped chcrty areas within the Animikie sla.tes are very common and are frequeutly of large size, lu the clu^rty background are seen rhombic outlines of siderite. The light colored border between the slaty lamina' and the chert is mostly actinolite. This rel.ation is suggestive that the actinolite has resulted fVom a reaction between the silica and siilerite. The lamina- of the slate are seen to curve about the chert area. A short distance from this nodule the lam- ina) are parallel, as represented in the previous figure. (See pp. 263, 265.) 498 U. 8 QFOLOOICAL SURVrv MONOGRAPH XIX PLATE XXV \:mJ . y Fig. 1, — Cherty iron carbonate. Fig. 2. — Slderitic chert. Fig. 3.— Actinolite-siderite slate. Fig. 4.— Actinolite-siderite slate, in polarized light THIN SECTIONS OF SIDERITIC SLATES FROM THE ANIMIKIE SERIES. PLATE XXVI. 499 Plate XXVI. — Ferruginous Cherts and Iron Carbonates from the Animikie Series. Fig. 1. Concretionary chert. Specimen 10577, slide 4997. From north side Gunflint lake; Animikie series. In ordinary light, x 25. The complex concretionary structure so characteristic of the ferrnginous cherts is here beautifully shown. The concretions are closely jiacked together, the amount of material between them being relatively small. This interstitial substance is cliiefly chert, but it contains numerous rhombohedra of siderite. The oxides of iron are mostly limonite and hematite, but with them is mingled some magnetite. The arrangement of these iron oxides with reference to one another is usually somewhat irregu- lar, but occasionally the magnetite and hematite are in alternate layers. (See p. 264.) Fir. 2. The same, in polarized light, X 60. Another part of the same section is here shown, the enlargement being greater. One of the concretions is seen to be compound ; that is, the larger belts of iron oxides inclose two smaller concretions. Within the concretions the quartz, like the iron oxide, is seen to have a banded arrangement. The background is very finely crystalline, but the silica is mostly or wholly individualized. (See. p. 264.) F'iG. 3. Ferruginous chert. Specimen 10576, slide 5186. From the Gunflint beds. In ordinary light, X 25. The section illustrates the sharp alterations which sometimes occur between tine grained evenly laminated cherty carbonate and ferruginous chert, with a well devel- oped concretionary structure. The fine grained part is composed of exceedingly crystalline and amorphous silica, and of siderite in minute rhombohedra. In the concretionary part of the section no carbonate remains. Within the concretions the quartz is somewhat coarsely crystallized. (See p. 264.) Fig. 4. Siderltic chert. Specimen 6138, slide 1173. Also from the Gunflint beds. In polarized light, X 25. The semiamorphous and chalcedouic phases of silica which are found iu the fer- ruginous rocks are nicely shown. The. darker colored part of the tigure consists chiefly of ■siderite and iron oxide, pseudomorphous after it. (See p. 264.) 500 U. S. QEOtOOtCAL SURVEY MONOGftAPH XIX PLATE XXVI Fig. 1 .—Concretionary chert. Fig. 2.— Concretionary chert, in polarized light. Fig. 3. — Ferruginous chert. Fig. 4. — Sideritic chert. THIN SECTIONS"OF FERRUGINOUS CHERTS AND IRON CARBONATES FROM THE ANIMIKIE SERIES. PLATE XXVII. 501 Plate XXVir. — Formation op Concretions in Iron- Bearing- Member. Fi6. 1. Sitleritic ohe.rt. Specirapu 1250S, slide 5522. From Sec. 16, T. 47 N., R. 46 W., MichisiUi, In ordiuary liglit, X 25. A clie.rty background coutaiii.s iniinerons riniTidisb and rboralio- liedral areas oT siderite. This siderite lias Iipgun to alter to opaipie lilai'k oxide of iron. In pLaces in the section this alteration lias gone far, and ronndisli forms are prodnced which are imitative of the shape taken by the original carlionate, althongh to a consideralile extent this carbonate has been replaced in its alteration by silica. (See p. 232.) Fig. 2. Sideritic clicrt. Specimen 7622, slide 2072. Fi'om tlie Montreal river, between Wisconsin and Michigan. In ordinary light, X 25. A l)aclcgronnd of cliert ciintains nnmeroiis rhoinbohedi'a of siderite of greatly varying sizes. The siderite ha.s altered to n consider- able extent to oxide of iron, and illustrates the formation of concretions as in the ]>revioiis tig'ire, but the stage of growth is more advanced. In oni' case a nearly solid oval area of oxide of iron has beeji produced by the alter.ation of one of siderite. In several other areas the oxide of iron forms a ring about the siderite inclosed. In still others the oxide of iron, while it is scattered somewhat irregularly througli the areas, 'as a whole retains the form of the original siderite areas, altliongh thospacc once occupied by thi^ siderite is taken in part by silica. In the large comidex concretion the series of alteration thus illustrated has been com])leted. It contains at present no siderite, liut consists of a. series of concen- tric rings of iron oxide which have a silice(ms background. This concretion is cut by a vein of silica, as are also three other areas on one side of the figure. These veinlets have clearly formed after the development of the concretionary areas. (See p)). : 30-231). Fig. 3. Another part of the same section, x 25. The figure again illustrates the form:.ition of con- cretions of iron oxide from original siderite areas. ■ This concretionary arrangement of the iron oxide is shown in nearly all of the siderite individuals, but is shown in a particu- larly fine manner by the Large area in the middle of the figure. Enongli siderite remains so that its rhombohedral cleavage is nicely shown. Upon the outer part of the area, is a tolerably continuous ring of bl.ack oxide of iron, beyond which areotlu'r imperfect larger rings of red and black oxides of iron. The alteration in the interior of the a-rea has to some extent followed the cleavage lines, and we tlius liave a.n (explanation of tlie irregular forms in the interiors of the concretions of tlie iirevious figures. It is to be furtlier noted that the forms of tlie rings in the concretions do not conform to those of tine original sider- ite area., but form regular ovals. (See pp. 230-231.) Fig. 4. Fernigiuons chert. Specimen 9009, slide 2765. From the same exposure as Figs. 2 and 3. In ordiuary light, X 25. This figure illustrates the rcsulti of an almost complete oxidation of the siderite areas, only a trace of that mineral remaining. The whole space that i t once occiiijied is taken by reddish brown hematite and a small amount of silica. The greenish areas are probably chlorite. (See pp. 228-230.) 502 U. S. GEOLOGICAL SURVEY. MONOGRAPH XIX. PL. XXVII SscicetuWilhclmsLilhoCc N Y PLATE XXVIII, 503 Plate XXVIII. — Magnetitio and Actinolitic Slates from the Ikon- Beaeing Membee. Fig. 1. Actinolitic slate. Specimen 9620, slide 3147. From Tylers fork, NE. i of Sec. 33, T. 45 N., R. 1 W., Wisconsin. In ordinary light, X 165. The figure shows the rhombohedral shape of a complex .irea of actinolite and magnetite and the close association of these minerals. The exierior of the area in the center of the figure is mostly magnetite, mingled, however, with some .actinolite, while the interior is pure actinolite. That this area represents an original rhomliohedron of iron carbonate is very probable. (See pp. 220-221.) Fig. 2. Magnetitic concretionary chert. Specimen 9625, slide 3150. From Tylers fork, NE. i of Sec. 33, T. 45 N., R. 1 W., Wisconsin. In ordinary light, x 60. A cherty background contains roiindish, oval, and roughly rhombic outlined areas. The iron oxide in these areas is mostly V magnetite, in the form of crystals. The magnetite is often concentrated upon the exteriors of the areas, projecting somewhat into the cherty background. The forms of the areas at once suggest the carbonate areas of PI. xxi, Figs. 1 and 2, and PI. xxvii, Fig. 1, and each doubtless now occupies the place once taken by siderite. In the alteration to magnetite the individuals grew beyond the outer borders of the siderite areas, but the forms as a whole were maintained. A considerable amount of silica must' have entered, as the magnetite occupies but a small part of the space once taken by the carbonate. (See pp. 222-223.) Fig. 3. Banded magnetitic jasper. Specimen 12791, slide .5477. From Sec. 11, T. 47 N., R. 45 W., Michigan. In ordinary light, X 25. The white background is a completely individualized but finely crystalline chert. The part containing the abundant red hematite is in hand specimen a brilliant red jasper. These jaspery bands alternate with the black ones, which, instead of hematite, consist of magnetite, mostly in well defined crystals. The change from magnetite to hematite is gradual. (See pp 239-240.) Fig. 4. Actinolitic slate. Specimen 9555, slide 3190. From Penokee gap, NW. J of Sec. 11, T. 44 N., R. 3 W., Wisconsin. In polarized light, x 165. The section is a typical actinolitic slate. The quartz is completely crystallized. The magnetite has mostly ■well defined crystal out- lines and is manifestly the first mineral to crystallize, being scattered uniformly through the section without any regard to the actinolite and quartz, and therefore included by both of them. The actinolite is in its characteristic blades and sheaf-like forms, having a radial arrangement of its fibers. It is as plainly the second mineral to crystallize, as needles of actinolite everywhere penetrate the quartz, but never the magnetite. The quartz constit- utes a background for the magnetite and actinolite, and includes them in such a manner as to make the conclusion certain that it must in the main have crystallized subsequently to the formation of the magnetite and actinolite. (See pp. 218-219). 504 U. S. GEOLOGICAL SURVEY. MONOGRAPH XIX, PL. XXVI 1 1 Fig. FiG.2. FiG.3. FiG.4-. THIN SECTIONS OF MA6NETIT1C AND ACTINOLITIC SLATES FROM IRON-BEARING MEMBER. Shc1c«ii » Wl(l«IlTlsLll^clCo N Y PLATE XXIX. 505 Plate XXIX. — Prom the Iuon-Fokmation op the Animikib Seeies and FROM Lawrence County, Ohio. Fig. 1. Conoretionarj' chert. Specimen 10577, slide 4997. From north side Gunflint lake; Auimikie series. In ordinary light, x GO. The photographs. Figs. 1 and 2, PI. xxvi, are repro- ductions from the same section as this figure. It is drawn, however, to show more exactly the relations of the various oxides of iron to the cherty hack'ground. In the first place veins of chert intricately intersect the oxides of iron in such a manner as to sliow that the silica entered after the iron oxides h.ad formed. The magnetite in the concretions is sonie- timos in nearly continuous hands, butin other parts is mingled intimately with the liem- atite. If the large area takes the place of one of iron earhouate, at difi'erent times tlie circumstances wer<' favorable for the production of each of the iron oxides iu prepohderat- iiig ([uantity. (.See pp. 2i;r)-2GG. ) Fig. 2. Aetiuolitic slate. ■■Specimen 7012, slide 2081. From the SW. i of .Sec. 23, T. 65 N., R. 4 W., Minnesota; Aniiuikie series. In ordinary liglit, X 25. The reliitiuns ol' the magnetite, actinolite, and quartz are here nicely shown. Tlie magnetite is plainly the first mineral to crystallize, fre(inently liaving crystal outlines, and being included within botli the other minerals. Thl^ close assoi'iation of a(^tiuolite and magnetite is again illustrated, the two combiniMl having usually roundish or oval ibrms. Tlie silica cuts the .section in such a maimer as to give it in places vein-like forms, which must have developed subsequently to the formation of the magnetite and actinolite. (See p. 266.) Fig. 3. Aetiuolitic and sideritic slate. Specimen 10.580, slide 5189. From the Gunllint beds;,Animi- kie series.' In ordinary light, x 60. The section illustrates a phase of the iron formation in which all of the minerals, quartz, siderite, magetite, -and actinolite are together. The colorless part is the cherty background, which is completely, although finely crystallized. The siderite is represented by the vague rhombohedral areas, which inchule numerous minute particles of gray and black material, presumably oxides of iron-. This mineral is the most abundant one iu the section. The magnetite occurs as usual with its cliaracteiistic crystal outlines, and in places is included in such a maniu'r in the siderite as to sug- ge.st its fcn-mation from that mineral. The actinolite is scattered here and there iu miiinte blades .and neeiUes, being particularly abundant upon one side of the figure. (See p. 265.) Fig. 4. Cherty iron carbonate. Specimen 9814, .slide 3880. From Lawrence county, Ohio. In ordi- nary light, x 25. The photographic Figs. 1 and 2, PI. xxi, are from this section. The beginning of the alteration of siderite to limonite and hematite, and also the beginning of the formation of concretions, are here better seen. The background is fincdy crystalline and ainoii)hous silica, the nature of which is better shown .by the figures referred to. This cherty iron carbonate from Ohio is remarkahly like that from the Pouokee series shown in PI. XXVII, Figs. 1 and 3, thc^ only dirt'erence being that the alteration of the siderite is of a diflercnt character, in one case black oxide of iron being ])rO(lueod, and in the other red and brown oxides. (See p. 247.) 506 U. S. GEOLOGICAL SURVEY. MONOGRAPH XIX. PL. XXIX. Fig. FiG.2. 'i^^.:^%^^lMffs^ FiG,3. FiG.4-. THIN SECTIONS FROM THE IRON FORMATION OF THE ANIMIKIE SERIES,AND FROM LAWRENCE COUNTY, OHIO, SackeluWilhilmsLjL'ioCoN ^ PLATE XXX. 507 Plate XXX.— Ore Deposits. Fig. 1. Longitudinal section of soutli deposit, Montreal mine. Above the main dike rock rich ore is found, while between ih'in and some smaller dikes is mixed ore. Fig. 2. Longitudinal section of nnrtli deposit, Montreal mine. Ore is seen above each of two dikes at different depths. The fact is noted that ore which was origin.illy fonnd at the surface . and mined in an open pit has in both cases been carried under the rock surface. In this case two parallel dikes, very close to each other, have, in both cases, above them a moder- ate sized ore deposit. Fig. 3. Cross section of south deposit, Montreal mine, at No. 8. shaft. The relations of the foot wall ^ quartzite, dike rock, ore and drift, are well illustrated. Fig. 4. Cross section of south and north deposits, Montreal mine, on line of No. 7 shaft of south deposit. The quartzite, both dike rocks of the south fti> ^liSj- OloNo,4 No3 Open pit _SuRFflcrMflTEmAL REMOVED No2 N0.1 No I "". '^ % ^ "'^^ r- '# No 7 Wo 3 ~i?^^s^g^i r^' ^ XjongituxicnaZ Section^ FflTHE^HENNEPIN r-SHAFT S^ ORE DEPOSITS. Fig. 1. — Longitudinal section of south deposit, Montreal mine. Fig. 2.— Longitudinal section of north deposit, Montreal mine, Fig. 3.— Cross section of south deposit, Montreal nnine. Fig. 4 — Cross section of south and north deposits, Montreal mine Fig. 5.— Longitudinal section of Pence mine. Figs. 6, 7, 8. — Cross sections of Pence nnine. PLAICE XXXI 509 Plate XXXI. — Ore Deposits. Fig. 1. LougitiUdinal suctiou nl' south deposit, Colby niiuo. This iigure shows a very large deposit of ore, Nvhieii was found at tlic siirlare. but whleli, in following eastward, was found to pass ■ under the ferruginous chert. So far as tliis figure goes, no indication of a dilce is seen, but a drill liole put down from.oue of its deeper levels has passed through a thicli dike reel;. Fig. 2. Loiigituilniiil section of north deposit, Colljy luiue. This deposit, before it had beea devel- oped to sucli a. deptli a.s tlie south deposit, readied a dike, and a little fartlier north this same dike reaches tlio surface of the ground. Fig. 3. Cross section ol' luuth and south deposits, Colby mine. The perpendicular line running tlirough tlie"ligurc is drawn liecause the two parts of the section are not exactly upon tlie same plane. Tluiy dilier Irom this so slightly, however, that the true relations of the ore bodies to tlie suriouniling rocks are shown by the combined hgnrc. Fig. 4. Longitudinal .section of Trimble miue. This is a case in which the main deposit of ore is above three parallel ICZDEZD ^SiHi FiG.3. FiG.6. fbrruffLnoutChcr^. Fig. 4. ,/ FiG.5. ORE DEPOSITS. Fig. 1. — Longitudinal section of south deposit, Colby mine. Fig. 2.— Longitudinal section of north deposit, Colby mine. Fig. 3. — Cross section of north and south deposits, Colby mine. Fig. 4. — Longitudinal section of Trimble mine. FlG.7. Fig. 5. — Cross section of Trimble mine. Fig. 6. — Cross section of Minnewawa mine. Fig. 7. — Theoretical section to show variation from unaltered carbonate. PLATE XXXII. 511 Plate XXXII. — Graywackes prom the Upper Slate Member. Fig. 1. Micaceous graywncke. Specimen 9118, slide 2906. From the SE. i of Sec. 11, T. 45 N., R. 1 W., Wisconsin. In polarized light, x 25. The section shows the general rounded character of the quartz grains, some of which are now minutely angular by enlargement. The com- plex, minutely crystalline areas represent single fragments of feldspar, which are now largely altered to mica and (juartz and thus appear more like fln^y crystalline interstitial material than simple Iragmental grains. (See p. 326.) Fig. 2. Biotitic and muscovitic graywackc. Specimen 9544, slide 3092. From NW. i of Sec. 11, T. 44 N., R. 3 W., Wisconsin. In polarized light, X 60. The figure illustrates a more quartzose part of the section. It shows nicely the alteration of feldspar. In the center of the figure was a simple fragment of feldspar, which has largely altered into mica and quartz, as a result oC which the rounded area is now a completely crystalline mass of mica, quartz, and feldspar. Its trno nature is, however, shown by its rounded appearance and by the fact that the many detached, small, irregular areas of feldspar extinguish together. (See p. 316.) Fig. 3. Biotitic graywacke. Specimen 9598, slide 3334. From SE. i of Sec. 15, T. 45 N., R. 1 W., Wisconsin. In polarized light, a 60. The figure illustrates the same thing as Fig. 2. The large roundish area in the center of the figure was ouce a single fragmeutal feldspar, but it has almost wholly decomposed, and thus forms an interlocking mass of mica and quartz. In this case detached areas of quartz thus formed are seen to be a unit over a con- siderable area. This figure and the previous one illustrate the crystalline character which can result from the alteration of fragmeutal feldspar. Many individuals of mica and (juartz have been produced from a single feldspar, and, as they form simultaneously by the altera- tion, they interlock as completely as if they were original crystallizations, and also with the residual feldspar in case there is any. (See pp. 324-325.) Fig. 4. Biotitic and chloritic graywacke. Specimen 9109, slide 4418. From the NE. i of Sec.l2, T. 45 N., R. 1 W., Wisconsin. In polarized light, X 25. A typical fine grained biotitic and chloritic graywacke. The fragmeutal character of the quartz is plain, although many of the smaller particles are quite angular. The fragmeutal feldspar was originally as abun- dant as the quartz, but the figure docs not well show tliis, as most of the grains of this mineral are extensively altered to biotite and chlorite. (See p. 325.) 512 U. 8. OEOLOOICAL SURVtY MONOGRAPH XIX PLATE XXXIl Fig. 1.— Micaceous graywacke. Fig, 2. — Biotitic and muscovltic graywacke, F'g. 3.— Biotitic graywacke. Fig. 4.— Biotitic and chloritic graywacke. THIN SECTIONS OF GRAYWACKES FROM THE UPPER SLATE MEMBER. PLATE XXXIII. HON XIX 33 513 Plate XXXIII.— The Development of Mica-Slates. Fig. 1. Biotitic and muscovitic graywacke. Siiecimen 9544, slide 3092. From the NW. i of Sec. 11, T. 44 Jf., R. 3 W., Wisconsin. In polarized light, X 60. The .section is one of the least altered feldspathic fragineutal rocks. Large rounded areas of feldspar make njp most of the section, although smaller iiarticles of fragmeutal feldspar aiid ([uartz are contained. The incipient alteration of the feldspars to the micas is ohserved, and the smooth out- lines of the fragmental grains have already been lost by alteration. This is the original phase of rock, the alterations of which carried to the extreme liave produced the crystalline mica-schists. (See p. 316.) Fig. 2. Biotite-slatc. Sitecimen 126, Wright. From the NE. i of Sec. 9, T. 44 N., R. 3 W., Wisconsin. In polarized light, X 25. The section is fr MONOGRAPH XIX PLATE XXXlV Fig. I.— Muscovitic biotite schist. Fig. 2,— Biotite schist. Fig. 3.— Biotite slate. Fig. 4.— Biotite slate THIN SECTIONS SHOWING DEVELOPMENT OF MICA-SCHISTS AND MICA-SLATES. PLATE XXXV. 517 tLATE XXXV. — Feom the Easteen Area. Fig. 1. Ferruginous chert aud quartzite. Specimen 12937, slide 6547. From near the center of Sec. 23, T. 47 N., E. 44 W., Michigan, In pohirized light, x 60. The section shows the sharp tran.sition which frequently occurs between noufragmental and fragmental sediments in the iron-bearing belt of the Eastern area. U})0u one side of the figure is seen a small part of a hanfl of pure nonfragmental chert. This abruptly changes into fragmental quartz set in a matrix consisting of nonlragmental chert and iron oxide. A portion of the frag ments of quartz are simple aud two grains are plainly enlarged. Other grains are of chert. In the change from nonfragmental to fragmental sedimentation the underlying nonfrag- mental material has been broken up to some extent and has yielded fragments which are associated with the sim])le grains of quartz. (See p. 363.) Fig. 2. Greeustoue-conglomerate. Specimen 9313, slide 4929. From the SE. i of Sec. 14, T. 47 N., E. 44 W., Michigan. In ordinary light, x 25. The figure shows the extraordinary struc- ture so constant in tlie greenstone-conglomerates. It is from one of the phases in which chlorite aud quartz are the i)reponderatiug coustituents. The anastomosing jiarts which contain the irregular areas ant largely compt)Sed of (|uartz, while tlie irregular areas are chiefly made U]) of quartz, chlorite, and epidote. This represents a pliase of rock in which the extreme of alteration has taken place. The products are minerals which, with the exception of chlorite, are ultimates in the series of transformations of I'ocks. (See pp. 384- 385.) Fig. 3. Greenstone-conglomerate. Specimen 9369, slide 3036. From the SW. i of Sec. 15, T. 47 N., E. 44 W., Michigan. In ordinary light, X 25. The figure shows another phase of the remarkalile structure characteristic of these rocks. It contains apparent fragments which are exceedingly angular and which are set in an anastomosing matrix, consisting largely of finely ci-j'stallinc quartz. The fragments in this sectiou are gray amorphous material, in which are numeroiis minute tabular crystals of plagioclase. (See pp. 381-382.) Fig. 4. The same, in polarized light, x 25. This figure shows the reticulating quartzose background. The almost completely a.niori)hous character of the major portion of the fragments is sharjily brought out. "Within them, as in the previous figure, the tabular crystals of plagioclase stand out. (See pp. 381-382.) 518 U. 8. CEOLOGICAl. SURVEY MONOGRAPH XIX PLATE XXXV Fig. 1.— Ferruginous chert and quartzite. Fig. 2. — Greenstone conglomerate. Fig. 3. — Greenstone conglomerate. Fig. 4. — Greenstone conglonnerate. THIN SECTIONS FROM THE EASTERN AREA. PLATE XXXVI. 519 U. S.GEOLQGrCAL SURVEY. 400 feel above L Superioi Qr. L Ha.. SI '^Q- MgSct. GSchBISI. tt General Sereccia South limit Diiecttons of coiorecJ hnes show stnke of layers Scale of Map and Sections I inch- 900 feet Contours 20 feel vertical distance MAP SHOWING THE DISTRIBUTION OF EXPOSURES AT PENOKEE GAP, WISCONSIN. PLATE XXXVII. 521 m o r O o n > r > V o Q X z I/) n 6' D (n n 3- > m > z o < o z H -< < ^ ^ z o ■ c (Ds 01 (D 3 (T> > r Q 0 Z > z > z (5. 3 (D cr m a Q. c "2-- < (t) o 3 cn n 3" o 01 3 Z 0 n ^^ m J r • m X > INDEX, A. Actiuolite, alteration of, 210, 21S-'>14. from feldspar. 305. of actiuoliri^' slate. 193, 198. 210-215, 257, 362, 364. of chlorite lui-k, 111. of ferrngiiuMia chert, 203, 26U. of greenstom'-coiislomenite. 375, 376,377. of irou-liejiring meiiiher, 141, 2GS. of iron r-arltnriHte, 266. origin of, 258, 2.')9. 26li, 267. jiseudoniorphs. 258. relations tn magnetite, 258. relation.s to sitleritc, 433. Actinolite-rock. descrihed, 366. Aetinollte-sehist, 469, 471. dese.rib(Hl, 215, 21G, 217, 219. 220. 243, 366, 367, 382, 494, 4fl6. (See Actinolitic date.) Actinolitic Mhite, 190, 195, 198. 199, 279, 297, 364, 365. analysis (tf, 197. concretions iu, 212. • crystallization of, 267-268. described, 194-198, 210-215, 266-267, 496, 498, 504, 506. grades into iron carhonate, 246,257,258,260, 262.266,267. 294.362. origin of, 257-260. relation.s to eruptives, 259. relations to ferruginous chert, 200, 202 203,212,213,246, 257, 258. 259, 266. relations to limestones, 259, 267. . relation.s to npper-.slale. 214. (See AotiitoUte-schist .) Agglomerate, use of term, 374. Agnotozoic. proposed by Irving, 87. Agogebic lake. (See Gogebic lake.) -Agogehic district, Wright on, 83. Albite of quartz slate, 150, Algonkian, 472, 473, 474. Alteration of actinolite, 210. 213, 214. of amphibole, 354. ofaugite,S56,414,435. o'f chlorite, 353. of diabase, 348, 355, 356, 357, 358, 359. of diallage, 115,353. ofdik:es,255,27],290. of eruptives, 294. of feldspar, 107, 108, 110, 118, 122, 125, 148, 150, 151, 155, 179 180, 305, 306, 333 334, 335, 336, 337, 338, 339, 340, 342, 343, 353, 356, 444. of hornblende, 122. of iron carbonate, 199, 201, 204, 205, 254, 283, 284, 286-290, 291, 292,294.295,393,434. of magnetite, 350, 358. AltiTiiIitui ut iiieiiiiccanite,414. of microcliiie. 335. of oligoclase, 335, of orfclioclase. 335, 338. of plagioclaae, 115, 353, 413. of pyroxene, 115, 353, 354. of siderite, 201, 20^ 253, 283, 362. Ampliiliole, alteration of, 354. from angite. 356, 414. from diallage, 353. from ]iyroxcne, 354, included in feldspar. 412. of actinolitic slate, 197. of diabase, 356. (See llornbleiide and Sinaraijdite.) Amygdaloid, 410,434,462,460. described, 418. of Keweenaw an, 349. relations to gTeenstou« eimgloraerate, 380. 418, 419. Analysi."^, actinolitic slate, 197. audesine, 352. anorthoclase, 352, biotite-schist, 336. clay-slate, 306. feldspar, 352. garnet, 214. Iron carbonate, 192. iron ore, 281. labradorite, 352. liinestones, ISO-lSl. Andesino of diabase, 352, 357. Animikie district,81. 192, 212, 248, 259. 260-267, =^94, 433. 472, 473, 498, 500. 506. Animikie series, relations to Keweenawan, 261, 469-470. relations to Huronian, 281. relations to Peuokeo series, 66. 261, 262, 468-170. unconforniably above granite, 261 , 262. uu conformably above schist, 261, 262. Anterite, described, 367. of actinolitic slate, 364. Anorthoclase, analysis of, 352. of diabase, 352. Anvil mine, 179. Apatite of diabase, 350. of gabbro, 115. of syenite, 115. Apostle islands, 188. Arcbean, 81-83, 87, 468,472, 473, 474. Ashbiud mine, 83-84, 91, 154, 1G2, 1G3, 277, 281. Atkins lake, 128. 216, 443. Atigite, alteration of, 356, 414, 415, 417, 435. enlargement of, 86, 353, 411. of au^ite-ijorpbyrite, 415, 417. 523 524 INDEX. Augite of diabase, 350, 354, 856, 358, 410-411, 412, 435. of gabbro, 115. of greeiistDne-cougloTiiera,te, 375, 377. relations to siuaragdite, 413, 435. Augite-porphyri'te, described, 414-419. of Keweenawan, 349. relations to diabase, 415. relations to greenstone-conglomerate, 416. Aurora mine, 94, 101, 102, 144, 146, 154, 163, 174, 281, 357, 448. Axels island, chert of, 251, 252. Azoic slates, Whitney on, 13. Azoic system, 73-75. Bad river, 36, 107, 108, 129, 138, 139, 176, 183, 185, 188, 301, 302, 303, 304, 309, 343, 344, 438, 440. Bad river area, 7, 8, 9, 10, 17-19, 22, 28, 30, 33, 34-36, 37, 38, 39-40, 42, 48, 67, 84, 104, 107-108, 144, 145, 146. Baraboo district, 1, 473. Barnes referred to, 5, 6, 13, 14, 15. Basal conglomerate. (See Conglomerv^te, basal.) Base level, 452, 454. Basement complex, 428, 438, 455, 471, 472. relations to Hnronian, 81-82, 86. relations to Keweenawan, 470. (See Southern Complex and Laurentian.) Bayley referred to, 214. Beaver bay, 81. Becke referred to, 86, 411, 412. Bedding, 26, 95, 29(i. Bessemer, 200. Biotite from actinolite, 210. from amphibole, 354. from aiigite, 414, 417. from diallage, 115. from feldspar, 118, 125, 180, 305, 333, 334, 335, 336,337,338, 339, 342, 343, 353, 413, 414. from pyroxene, 115. of actinolitic slate, 195, 210, 213. of biotite-schist, 33(j, 339, 340, 341. of biotite-slate, 338, 339. of chert-conglomerate, 451. of clay -.slate, 305. of diabase, 351, 359. of gneiss, 108, 116, 118, 119, 120. of granite, 106, 107, 112, 113, of gray wacke, 304, 333, 334, 335, 336, 337. of hornblende-gneiss, 110-111. of mica-schist, 307, 308, 309. of mica-slate, 307, 308, 309, 342, 343. of Quartz-slate, 148, 152. of syenite, 114, 115. of syenite-schist, 125. of Upper slate, 303. relations to hornblende, 119. relations to magnetite, 354. relations to plagioclase, 354. Biotite-gneiss, 108, 116, 470, 478. Biotite-graywacke, described, 323, 324^325. Biotite-schist, 309. analysis of, 336. described, 310-311, 318, 330-321, 339-341, 516. origin of, 339-341. (See Mica-schist.) Biotite-slate, described, 309, 310, 311, 312, 313, 315, 319-320, 321, 322-324, 326, 338-339, 486, 514, 516. Biotite-slate, origin of, 338-339. (See Mica-slate.) ^ Birkinbine referred to, 87-92, 92-96, 280. Black river, 81, 165, 179, 187, 233, 297, 298, 299, 304, 330, 456. Black river area, 37-38. east branch, 145. ■west branch, 145, 189. Black river district, Wisconsin, 1, 472, 473. Bladder lake, 129. Blue Jacket mine, 164. Bonney mine, 269. Bradley referred to, 41. Brainerd referred to, 247. Breccia. (See Chert-breccia.) Breeciation of actinolitic slate, 266. of ferruginous chert, 194, 207, 209, 254, 264, 265. of iron carbonate, 263, 264. of limestone, 207. Bronzite of diabase, 35-1. Brooks referred to, 7, 8, 15, 30-31,31-32,33,34-38,44,47,56,66, 78, 79, 81, 463. Brotherton mine, 269. Brotberton referred to, 9, 84. Brule mountain, 81. C. Calcite amygdules, 377, 418. of chlorite rock. 111. of gneiss, 109, 118. of gray wacke, 307, 333, 336, of limestone, 131. of porphyrite, 418. Cambrian, Irving on, 86, 87. Canada, 2, 261, 472. Carbonaceous material,-190, 193, 200, 203, 250, 307, 308, 333, 337, 343. Carbonate. (See Iron carbonate.) Carboniferous. 140, 247, 250, 252. Carries creek, 176. Chalcedony amygdules, 418. Chalcedony of actinolitic slate, 211.' of ferruginous chei-t, 192, 194, 203. of veins, 202. oi-gin of. 252. Chamberlin referred to, 8, 9-10, 43-44, 59-66, 463. Chanuing, indebtedness to, xiv. referred to, 12, 270, 271, 279, 441. Chatard, analysis by, 351, 357. referred to, 191, 192, 214. Chert, 32, 363, 379, 392, 423, 471. described, 132-134, 136-138, 225, 226, 227, 228, 230, 233, 239, 240, 241, 243, 244, 367, 368, 399, 400, 403, 480, 482, 506. character of, 132-134. of limestone, 139-140, 205. of actinolitic slate, 266. of clay-slate, 434. of conglomerate, 174, 180, 448. of ferruginous chert, 205, 209. of Iron carbonate, 190, 200, 201, 254, 262, 263, 264. of iron ore. 283. of quartzite, 395. of veins, 202. origin of, 140-141, 142, 248, 249, 250, 251, 252, 253. relations to limestone, 127-128, 132, 139, 141. relations to Quurtz-slate, 171, 172. replaced by iron oxide, 283. INDKX. 525 Clint bnoriii. i:m. 1 III. :i91. :(9-t, 39o. 462. ili'sorilud. ir>7. Ifiit. io;i. 40r>-407. (MuM't coiicrot inns. 205. CluTt-con^'lnmrnitc, I39,:iyi,-I54. aesrrilH'd, UiT. 109.482. Clicrt-sclii.st, 22">. (lesiTibcd. 226, 24.5. (Si'o ('hertij limcstom\ Fi-miijuioiis fhtrt, Flint, linna- titic c/u-rt, Jaspci\ Limrstoiif, Silica, Quartz.) Chorty carbimalo. (See Imn carbonate.) Cherfy iron rarbouate. (Soo Iron carhouate.). Cborty limrstono. 3, 137-142. 2IS. 420, 4:t:!, 4:15, 4HS, 443-444, 449, 451. 452, 457. 4G4. 467, 469. 471, 472 -47;i, 474. fragments of, 423. relatious to Easteru area rtx'.ks, 422, 42.'I. n-latioiis to gneiss, 446. relations to granite, 127-128, 445. relations to Quartz-slate, 130, 134. 139, 141. 142, 143. 144. 147, 171,180,181,443,454,464. relations to schist, 127-128, 446. relations to slate, 128, 134, 139, 445. thicknessof, 130, 141. iiu conformably below Penokee seri('s proper, 454-455. uucontbrniabie above Sontberu Complex, 444-454. (See Chert, Limestone.) Chippewa valley district, 1, 460, 472, 473. Chlorite, alteration of, 353. from actiuolite, 210:213-214. from augite, 356,414, 415, 417. from feldspar, 118, 122, 151, 179, 180, 333-336,353,413,414, 415, 41S. from hornblende, 322. from plagioclase, 115, 417. from pyroxene, 115. of actinoUtic slate, 196, 210, 213, 364. of aagiteporphyrite, 415. of chert, 134. of clay-slate, 305. of coii^lomerate, 448. of diabase, 356. 359. of ferruginous chert. 203. of ferruginous slate, 203. of gneiss. 108-109, 111, 116, US. 119. 120. of grauite, 106, 107, 113, 123, 123. of graywacke, 307, 333, 334, 335. 336. of greenstone-conglomerate, 375, 376, 377. of iron carbonate, 190. 200. of mica-schist, 307, 308, 309. of mica-slate, 307, 308, 309. of microgranite. 112. of porpliyrite, 418. of quartzite, 153-154. of Quartz-alate, 148, 151, 152. 468. of schist, 121. of slate, 364, 365, 370. of syenite, 122. of Upper slate, 303. of veins, 202, 418. relations to hornblende, 119. Chlorite amygdtiles, 418. Chlorite-gneiss. 108, 116. Chlorite-rock,113. Chlorite-schist, 308, 309. Chlorite-slate, described, 382, 384, 391, 486. Easteru area, 369, 391. Claiborne iron carbonate, 247. Clay-shale, 147, 149, 170. Clay-slate, 147. 302. 303, 3(I4. 344. 363, 365. 389, 391. 465. 469. analyHJH of, 306. cleavage of, 434. described, 305-306. 327-328, 330, 331 , 372, 398, 399, 400. dip of. 434. relatioiiM to gray wackoa, 305. 333. relafions to grecn.Htonc-r(inglonn.'rate, 374, 375. I'olatlonsto iron on-. 285, 286. (Seo.S7ff/c, Phyllitc.) (Meavage of actin()lili(! slate, J95. CU^avage of slate, 296. 390, 426, 427, 434 . C(dby mine, IGl, 232, 269. 270. 273, 281, 3.55, .510. Concenlration of iron oxide, 254. ( 'oncentral ion of silica, 255. Concretions, 205-209, 268. of actinolitic slate, 212, 258, 266. 267. of ferruginous chert, 212. 254, 255, 257, 265. (.'onglomerate, 147, 148, 302, 303, 304-305, 364, 393. 394, 395. 454. 455. basal, 172, 173, 174, 180, 181, 388, 391, 443, 447-451. 4.57, 461- 462,464,471,472. basal, relations to ^athern Complex, 422. described, 159, 161. 164-165. 326, 371, 400, 401. 409, 482. (Sec J'asper-eoiiglomerato, Vhcrt-congloinvrate.) Conglomerate- slate, unconformably above schist, 147. Conover referred to, 9-10, 44. Copper-bearing series, 47, 48, 66-67. relations to grauite, 104. (See Keweenatvan.) Coutchiching, 473.' Credner referred to, 56. Cretaceous cliert, 252. Crystalline rocks of "Wisconsin, 41. Crystallization of grauile. 113. of microgranite, 112-113. of syenite, 114. Currant river, 469. D. Dakota, 473, 474. Dawson's road, 498. Decomposition of rocks, effect of enTironment on, 358, 359. Devonian, 86. Diabase, 103, 115, 122, 261, 268, 346, 347, 425, 431, 435. 465. 466. alteration of, 348, 355, 356, 357, 358, 359. analysis of, 357. description of, 348, 354. 410-414. grades into gabbro, 348, 350, 3.58, 410. of Kcweonawan. 52, 349, 377. of Southern Complex. 349, 358. ophitic structure of, 350, 351, 410. relations to augite-porphyrite, 415. relations to greenstone conglomerate area, 410. Diabase-por]3hyrite, described. 416,417. of Keweenawan, 377. Dial compass, "Wright on, 52. Diallage, alteration of, 115, 353. of g.abbro, 115, 350, 353. Dikes, 346, 347, .3^8, 358, 359. alteration of, 255, 271, 290. dip of, 279. of iron formation, 35.5-358,359. of Southern Complex, 349. of Upper slate, 358. pitch of. 278. relations to Iron-bearing member, 12, 271-274, 465, 466. relations to Keweenawan, 465, 526 INDEa Dikes, relations to quaitiite, 272, 374. relations to ahutt. 277, 278. thickness of, 273. Dike-rock, relation to iron ores, 92, 274-275, 276, 277, 287, 289. 290, 291, 292, 295, 3S,5. Diorite, 12, 52. I'rom diabase, 355. Irom Iron-bearing member, 271. of Keweenawan, 349. Diorite-porpliyrito, 370. Dioritic rocks. 07, 09. Dip of clay -slate, 434. of (likes, 272, 273, 276, 279. of quarbzite. 270. of slate, 427-428. ♦ Dipping needle, 52. Dolomite, described, 13.0-137, 480. of chert, 133. of gray wacke, 333, 33G. of limcstolae, 130, 131,132. Drifts in iron ore,, 278. Dulnth gabbro, 37, 81. v Dnmkowski, von, referred to, 140, 251. Eakins, anal.ysis by, 306. Eastern area, 300-436, 450, 467, 518. iron ore of, 305, 366. Eastern area rocks, relations to cherty limestone, 422, 423. relations to eastern sandstone, 431. relations t(t greenstone range, 422. Eastern sandstone, 434, 430, 407. relations to eastern area rocks, 431. rolati(nis to gneiss and granite, 389. relatiorrs to Keweenawan, 47, 461-463, 460. relations to Penokeb series, 12, 301. 401-403, 466. relations to Sonthern Complex, 461-403. (See Lake f^uperior sandstone.) unconformity below, 388, 461-463. Emmons (E.) referred to, 79. Emmons, S. E., on lead ores, 293. England, chert of, 231, 252. English lake, 7, 300, 301, 303, 309, 459. English lake area, 8, 17-19. Enlargement of angite, 80, 353, 411. Enlargement of feldspar, 180, 395, 409, Enlargement of hornblende, 80, 353, 410, 411. Enlargement of quartz, 112, 132,150,152, 153, 180, 185, 209, 5 288, 305, 33 1, 335, 339. 343, 345, 377, 395, 396, 444. 468. Enrironment, effect of, on decomposition of rock. 358, 359. Epidosito, 119. Epidote aniygdnles, 377. Epidote, from cidorite, 353, 414, 415. from feldspar, 118, 413, 414, 417, 418. of augite-porphyrite, 415. of gneiss, 108, 109, 111, 1 16, 118, 119. of greenstone-conglomerate, 375, 376, 3T7. of porphyrite, 417. Epidote-gneiss, 116. Epidote veins. 418. Eruptives, 340. 359, 304, 305, 374-387, 410-419. of Iron bearing member, 271-274. relations to actinolitie-slate, 259. relations to greenstone conglomerate, 374. relations to Iron-bearing memlier, 346, 361. relations to Upper slate, 347, 348, 358. Y. Eault at Bad river, 26, 77, 188, 438-440. at Potato river, 440^141. in Eastern area, 424-425, 441. Eaultiug along dikes, 347. Federal mine, 273. Felch mountain district, 300, 472, 473. Feldspar, alteration of, 107, 108, 110, 118, 133, 125, 148, 150, 151, 155, 179, 180, 305, 306, 333, 334, 335, 336, 337, 338, 339, 340, 342, 343, 353, 356, 415. 418, 444. analysis of, 352. enlargement, 180, 395, 4U9. growth of, 110. included in hornblende, 119, 413. ., of augite iiorphyrite, 415. of biotite-slate, 338, 339. of biotite-schist, 339, 340, 341. of chert-cimglomerate, 451. of clay-slate, 305. of conglomer.ate, 305, 305, 448. of diabase, 350, 351, 353, 354, 356, 3.V7, 359. of gabbro, 115. of gneiss, 108, 109-110, 111, 110, 119, 120, 121, 123, 125. of granite, 100, 107, 111, 112, 122, 123. of gray wacke, 300, 333, 333, 331, 336, 337. of greenstone-conglomerate, 379. of mica-schist, 307, 308, 345. of mica-slate, 307, 308, 343, 343. of microgranite, 112, 113. of novaculito, 147. of porphyrite, 417. of (luartzite, 153, 305, 369, 391. of Qnartz-slate, 134, 143, 147, 148, 149, 150, 151, 1.12, 443. of schist, 117, 118, 121, 122, 125. of slate, 380. • of syenite, 11 4, 115, 133, 125. of ijpper slate, 303, 343, 344, 345, 465. porph.vritic, 418. (See Plaf/loclatie, Ortkoclase, MicroeUn<^, Labyadorite, Atioythlte, Oligoclase, Ajldesine, Anorthoclase.) Ferrite of biotite-slate, 338. of gray wacke, 335. (See Iron oxide.) Ferro-doloraite, 300, 393. described, 399, 400. Perrnginons chert, 190, 192, 198, 379,280,294,362,364. breociation of, 207, 209. concretions in, 205-209, 212. crystallization of 207-268. described. 194, 202-209, 264-205, 490, 492, 494, 500, 502, 504, 518. grades into ferruginous slate, 194, 205. grades into fragmentalrock. 209. grades into iron carbonate, 199, 203. 246, 256, 259. 260, 294. rel.ations to actiuolitic slate, 200, 203. 2:i i, 2 12-213, 246, 257, 258, 259, 266. relations to iron ore, 303, 203, 275, 270, 283, 287, 295. origin of, 249. 254-257. Ferruginous schist, origin oi", 85-80. Ferruginous slate, 190, 198, 203, 279. described. 192-194, 202-205, 264. grades into ferruginous cliert, 194, 305. grades into iron carbonate, 193, 201-202, 205, 253, 294, origin of, 253. First National mine, 164, Flint, 256, INDEX. 527 Flint, tloHcrihed. 228-230, 231. 233. (Set- Clifit.) Foliation oCschisI. in:;. III. 117. 125, 30K. 442. I'tMH -« nil of iron ore. 'J70-'J71. 274, 276, 277. Fo«tor rrtiM-ml toA^. lH. 14. 15-17. '.H, 71. Fni;;mcntal ami uonrraf.!;inont!U rocks, t^radationa lictwiu'ii, 209. 24(5. Fra;;uiuntal rofU. grceuatoiio- i-onj^loiiieratf ;;iaih;^ iiit.o.;i7!)- rolutioDS to j^rimitu, ll'J."). Cf. Gabbro, 67, 105, 111,113, 116-^116, 261,262.208,348,351,358,410, 425, 435, 458, 466. grades into diabase, 348. 350, 358. 410. of Kewtt'Uawan, 3t", 37. 38. HO, 129. 349. relations to Iruai-beariug member, 185. relations to slates, 36. Garnet, analysis of, 214. of actinolitic slate, 195, 213, 214. ofniica-sebist, 119. of Upper slate, 214. Geiliie. definition of graywacke, 306. Geodes in ferrnginous ebert, 205. Germania mine, 162, 228. Geyserite, origin of. 251. Gneiss, 2. 6. 20, 36, 37, 47, 5'J. 80, 81. 103. 107-111. 116-122, 123, 139, 388, 428. 434, 435. 442. 443, 449, 452, 462, 465, 467, 471, 473. de.scribed, 107-111, 116-122. foliation of, 117. grades into granite, 107, 123-124, 125. origin of. 122. 125-126. relations to Clierty limestone. 446. relations to Eastern sandstone, 389. relations to graywacke. 118. relations to (Treeiislone-eonglomerate, 420, 421. relations to Iron -bearing member. 185. relations to mit a schist, 85, 308. relations to Qnartz-slate, 109, 143. 171. (See Biotite-gncins. Chlorite-gneiss, Epidote-gneiss, Gmnlfe. Bonthlenile-gnei's, f-'ckist, Sericite-gneiss.) Gneissoid granite. {See Gnei'^s, Granite.) Gogebic lake. 2, 3, 4. 6. 10, 11, 12,14,15,16-17.30,31,32,66.67, 105, 187, 421, 434. 461, 463, 464, 466. Gogogasbngnn river. i^eeMontreal river. West branch,) Goocli referred to, 256. Grand Portage bay, 262. Granite,2.6, 15,19, 20, 67, US. SI. 303, 118, 145,146.344,388,394, 395, 428, 434, 435. 442, 443, 444, 449, 450, 451, 452, 453, 454, 457, 462, 465. ^167. 469, 471, 473. crystallization of, 113. described. 106-107. 111-113. 122. grades into gneiss, 107. 123-124, 125. of Keweenaw series, 80. of Menominee district. 34-36. 81. recomposed, 388, 391, 394, 395. 403. 407, 435. 449, 451, 454. 462. relations to Cberty limestone, 127-128,445. relations to Eastern sandstone. 389. relaiiona to Greenstone- conglomerate, 420, 421. relations to Huronian. 34-36, 37-38. relations to Iron-bearing member, 71-72, 185, 187, 189. relations to Keweenaw series. 13-14, 104. relations to Penokee series, lii5. relations to Qnartz-slate, 128, 1 13. 171. 174. 179, 180. 445, 448. relations to schist. 104-105, 116-117, 124, 442, 445. (^ee GneihSy Microgranite.) Granite, nnconforniably below Animikio series. 261,262. Granite areas, relations of. 10.'). (iraiiitoid-gneiss. (See GnnvB, tirnnite.) (irapliitic niaturial of iron carbonate, I9U. Graywacke. 3. 261, 262, 301. 302. 303,304.344,301.433,465,472. described, 161, 167-168, 170. 306-3;t7, 3l.''.-316, 317-318,319, 321. 322, 323, 325,326-327.328. 329-330. 330-331, 332-;)38, 371, 396-397, 398, 401-402. 512, 514. origin of, 77-78, 334-335. relations to cluy -slate, 305, 333. relations to gneiss, 118. relations to iron ore. 285. relations to micit-schist, 307, 308, 309, 341. relations to mica-slate. 307. 308, 309. (See Graywacke-hlato.) Graywacke-slate. 3. 303. 304, 332, 389, 391, 465, 469. 572. described, 159-160, 161, 1G5-166, 306-307, 318-310, 321,322, 324, 325, 326, 329. 331, 397-399. 400. 401. 402-403, 404, 484. relations to iron ore, 285. (See Graywacke.) Great Britain, chert of. 252. Greenstone. (See Gabbro, Eruj)tives,t>r€enstone-conglomerate^ Diabase.) Greenstone-conglomerate, 429, 430, 431, 432,434,436. described, 374-377, 381-382, 383-387, 426, 518. grades into fragniental rock, 379-380. origin of, 377-381. relations to amygdaloids, 380, 418, 419. relations to day -slate, 374, 375. relations to diabase, 410. relations to gneiss and granite. 420. 421. relations to .jasper conglomerate. 369, 370. relations to Keweenawan. 419. 420, 435. relations to porphyrite, 378, 416. relations to schist, 420, 421. relations to slate. 419, 420. relations to Sontbern Complex, 419, 420, 435. thickness of, 423-425. Greenstone range, relations to Eastern area rocks, 422, Greenstone-schist, .52, 71. Griinerite. possible presence of, 215. Giimbel on schalstein, 374. Gnnflint lake, 192, 248, 261, 262, 268, 469, 470, 500, 506. Hague referred to, 251. Hall referred to, 7, 27. Hematite, 280,281,291. described, 368. from iron carbonate, 199, 204, 205. of concretions, 206. of actinolitic slate, 195, 196, 197. 198, 210. 211. 215, 262. of diabase, 356. of ferruginons chert, 202, 203, 209, 254, 265. of ferruginous slate, 193, 202, 204. of iron carbonate, 190, 201. of quartzite, 369. of slate, 392. pseudomorphous, 265. (See Iron oxide.) Hematite-schist, 391. Hillebrand. analyses by, 130-131. 191. Hinde, on origin of chert, 140, 251, 252. Hobbs referred to. 413, Hornblende, alteration of. 122. enlarpment of, 86, 353, 411, 412, 528 INDEX. Hornblende, from angite, 4^5. from dialUtge, 115. from feldspar, 118, 125, 353. from pyroxene, 115, 353. inclusions, 119. of diabase, 353, 359. of gabbro, 115. of gneiss, 108, 109, 110, 116, 118-119, 120, 121. of granite, 112, 123, 123. of greenatone-t'onglomerate, 377. of syenite, 114,115, 122. of syenite-schist, 125. pararaorphic. 353, 412. Hornblende-biotite-syenite, described, 478. Hornblende-gneiss, 108, 109-111, 116, 120-122, 476. origin of. 111. Hoinblende-grauito, described, 111-113, 478. Hornbleude-roclc, 3'i. Hornblende-schist, described, 476. origin of. 111, 121. Ilornltlende-ayenite, 114. llornstoue, cliaracter of, 200. Hudson river, 263. Hunt referred to, 44, 56, 79. Huronian, 1-2, 10, 37, 47, 48, 59, 62-65, 86-87, 468, 472, 473, 474. relations to Aniniikie. 2, 81. relations to basement complex, 81-82, 86. relations to granite, 34-36, 37-3S. relations to Keweenawan, 47, 65-66, 67, 81, 82, 8G. relations to L»irentian, 43-44, 47, 58, 61, 82. (See Peiiokec scries.) Hydro-miea-slate. position of, 80. Hyperathene of diabase, 354. Iowa, 473, 474. Ireland, chert of, 140, 251. Iron-bearing member, 3, 12,15,17,71-72, 146, 182-295,297.361, 362, 363, 364. 365, 369. 429, 430, 432, 433, 430, 438. 440, 441, 455-456, 457, 458, 459, 460, 464, 465. 468, 469, 471, 472, 473, 474, 490, 492. 494, 496, 498, 500. 502, 504. content of iron in. 182-184. diabase in, 348, 350. 359. dikes in, 12, 271-274, 347, 348, 355-358, 359, 465-466. flexures of 437. 438. jasper fragments from, 174. magnetic attractions of, 186. nonfragmental, 245, 246. quartzite fragments in, 175. relations to eruptivcs, 346, 361. relations to gabbro, 185. relations to gneiss, 185. relations to granite, 71-72, 185, 187, 189. relations to Ke-n-ceuawan, 187, 188, 362. relations to quartzito, 181, 185. relations to Quartz-slate, 144. 175, 176. 181, 184-185, 188, 189, 200, 294, 299, 363, 455. 456, 464. relations to Southern Complex. 189. relations to Upper slate, 200, 294, 296, 297, 298, 299, 300, 456, 464-465. thickness of, 187, 188, 189-190, 361, 362. topography of, 145, 188-189, 301. Iron Belt mine, 269. Iron carbonate, 280, 364, 379, 464, 466, 469, 471. alteration of, 199, 201, 204, 205, 254. 283, 284, 286-290, 291, 292, 294, 295, 393, 434. analysis of, 191, Iron carbonate, descrilied, 190-192, 200-203, 332, 233. 235, 236, 237, 238, 239, 262-264, 496, 498, 506. grades into actinolitic slate, 246. 257,258,360,362.266.267. 294, 362. ., grades into ferruginous chert, 199, 202, 246, 256, 259, 266. 294. grades Into ferruginous slate, 193, 201-203, 205, 253, 294. of actinolitic slate, 258, 259. of Animikie series. 259, 268. of concretions, 207, 208. of Eastern area, 365. of ferruginous chert, 257, 258. of feiTuginous slate, 193, 203, 204. of other series, 252, 260. origin of, 246-253, 263. position of, 198. pseudomorpbs of, 201. relations to iron ore, 260, 268, 282, 283. eiderite of, 256. silica of, 199, 202, 256. (See SideHte.) Iron King mine. (See Mount Bope mine.) Iron mines, Birkinhineon, 87-92, 92-96. Iron ore, 67. 09-71, 102, 198. 268, 26 l, 281, 283, 291, 471, 508, 510. analyses of, 381 . character of, 280-283. concentration of 254, 283-292, 295. depth of, 274-275, 292-293. foot-ivall of, 270-271, 274, 376, 277. from iron carbon.ate, 260, 283, 284, 286-292. 295. in Laurentian, 94. manganese in, 280-381, 392. of actinolitic slate, 258. of Eastern area, 365-366. of ferrnginons slate, 203, 204. of other districts, 293. origin of, 85-86, 281-290. position of, 184, 185, 198. 268-271, 294. relations to clay-slate, 285. 286. relations to dikes, 93, 274-275. 276, 277, 287, 289. 290, 291. 292, 295, 365. relations to ferruginous chert, 202, 203, 275, 276, 283. 287. 295. relations to gray wacke, 285. relations to greenstone, 12. relations to iron carbonate, 260. 268, 282, 383. relations to mang.anese. 91-93, 282. relations to quartzite, 94. 185, 199, 274, 276. 285, 286, 387. 391, 292, 294, 365. relations to Quartz-slate, 135, 199,269, 270, 286, 287, 294, 395. relations to sandstone, 285. relations to silica, 282, 284. relations to soapstone, 91, 94, 294. sulphnr in, 281. (See Iron oxide.) Iron oxide from iron carbonate, 253, 268. 393. 434, of actinolitic slate, 210, 212, 215, 257, 266. of chert, 133. of concretion, 205, 206, 208. of feiTuginous chert, 254, 257. of gneiss, 119, 120. '"• of graywacke, 333. of greenstone-conglomerate, 375. of quartzite. 153. of slate, 305, 370, 376, 434. (See Ferrite, Hematite, Iron ore, Magnetite.) Ironton mine, 273. INDEX. 529 Irving, referred t«. xiii. Xiv, xv. 3. 8. P. 33. 34, 3R. 40. 41-43. 44-r>2. 57, 5S. «t(-ti7. 77-78. 7»*. 70. 81 . 82. 82, 83. 85. 8((, 87. OR, 183, 250, 200, 2(11, ;J10, 377, 412, 438, 402, 405. .Tiuksnn, roforri'tl to, 5, 0. 13, 14, 15. Jasper, 145. 208, 363. 4«2, 469. 471, 504. described, 233, 234, 238. 239, 240. .Iiixtf.3fi9.3fln. I'ragiueuts iii Eastern area. 423. of couglonu-rate. 174. 180. 305, 448, 4,55. of irun-bearing member. 189. 19i), 193, 100, 204, 209. origiu uf, 249. relations to chert, 133. (See C'lieif, Flivt) rraspereongloinerate, described, 372. relations to Greenstone i'ouj;lomerate, 309, 370, relations to iiorpliyrite, 414. Julien. referred to, 33, 52. K. Kakabikka falls, 192. Kaministiquia river. 192, 202. Kaministiqiiia district, 473, 474. Kaolinfrom feldspar, 115, 118, 151, 155, 179, 333, 334. 353, 413, 414, 417,418. Kaoliuite of argillaceous slate, 155. of augite-porpliyrite, 415. of clay-slate, 305. of diabase, 359. of gneiss, 109. of gray wacke, 333, 334. of Quartz-slate, 148, 151. of slate, 370. of soapstone, 358. Keewatiu,408,473. Keweenawan, 2, 19, 20, 30, 36. 37, 38, 47, 48, 59, 00-67. 80, 81, 86, 261, 349, 358, 377, 424, 430, 431,434, 436, 441, 456, 465, 406, 407, 473. eruptives, relations to P«uokee eruptives, 349. 358. relations to Animikie series, 201, 409. 470. relations to Basemertti Cora,x)lex, 470. relations to dikes, 465. relations to Eastern sandstone, 47, 401-463, 406. relations to granite, and syenite, 13-14, 104. relations to greenstone conglomerate, 419. 420, 435. relations to Huronian, 47, 65-06, 07, 81, 82, 80. relations'to Penokee series. 80, 187, 188, 298, 299, 300, 301. 347. 348, 358, 302, 443, 450-461, 409-470. topograpby of, 301. unconformably below Lake Superior sandstone, 47. {See Copper -bearing acricii.) Kieselschiefer, 143. Kimball, referred to. 56. L. Labradorite, analysis of, 352. of augite-])orpliyrite, 415. of diabase, 352, 357, 413. of gabbro, 352. Lac des Anglais. {See English lake.) J.ake Superior, 470. Lake Superior region, N. H. Wincbell on, 78-81, Lake Superior sandstone, 47, 48. (See Eastsrn sandstoin'.) Lake Superior synclinal, 407, 470. MON xi:^ 34 Laphnm referml tn. 7-8, 22-27. Laureiilian. 47. 48. ."ilMil. 94. 139. 468, 473. ivlalion 1o IIuninian.43-44,47,58,0I,82. (Sue lifiNvmvnt vumpU'X, Sonthfni coinplfx.) Lava Hows, 360, 301,302. Lava of greensloiir conglomerate, 379. Lawscm referred to. 121. Lawton referred to, 92. Lead ores of LeiidviHo. 293. Leadville. (,'oln?iicbi. 293. Leucoxeue from menaccauite, 414, 415, 416. of augite-porphyrite, 415. of diabase, 351. of gre.enatoue-conglomeratc, 375. 378. of sehiat. 122. Life in iron carbonate, 250. Limestone, 52, 139, 205. . al)scnce of life remains in, 140, 141. analyses of. 130^131. brecciation of. 207. described, 130-132, 133, 139. concretions in, 205. origin of, 140-141, 142. relations to cbert, 127-128. 132, 139, 141. relations to actiuolitic slate, 259. (See Chert!/ litnesfone,- Dolomite.) Limonite of diabase, 3561 of iron carbonate, 190. of quartzite, 369. of slate, 392. Little Falls, Minnesota, 85. Little Presque Isle river, 128, 188, 199, 360, 361, 368, 369, 389, 414, 421 428, 433. Little Presque Isle river area, 71. Livingston Manor House, New York, 263. Logan referred to, 56. Longyear, J. M., indebtedness to, xiv, 12. M. Magnesian slate, position of, 80. Magnetic attraction. 180, 195, Magnetite, alteration ()f, 350, 358. from iron carbonate, 205, 302. in concretions, 206. in upper slate, 297. in aetinolitic slate, 195. 197, 198, 210, 211. 212. 213. 215. 257, 258, 206, 267, 279, 294, 302. of chert, 133, 134. of chlorite-rock. 111, of clay-.slatp., 305, 300. of diabase, 350, 351, .354, 350, 358. of ferruginous chert, 202, 203. 209, 254. 205, 266, of ferruginous slate. 193, 202, 204, 204, 260. of gabbro, 115,351. ofgnei.=is, 108, 109, 111, of iron carbonate, 190, 200, 201, 266. of quartzite, 309. of .schist, 121. of slate, 303, 393. origin of, 258. '^ pseudomorpha. 201, 258, 265, 200. relations to actinolite, 258. relations to biotite, 354. relatitms to siderite, 433. {See Iron oxide). Magnetite -schist, described, 24;3. 530 INDEX. Magnetite-slate, 52, 190, 194-198, 242, 294. (See Actiiwlittc date.) Malacolitb of mica-hornbleiKle-syenite, 115. Maiigauese, relatiou.s to iron ore. 91-92, 282. Marca,site or graywacke, mIJd, 337. Marengo river, 107, 128-129, 138, 175, 476. Marengo river iron range, 217. Mareuisran, 473. Marquette district, sill, 2, 10, 37, .56-58, 8.n, 26U, 261, 293. 356, 460. 468, 471-473. unconformities in, 471, 472. " Marquette series, relations It) Penokee series. 56. 470-472. Melaphyre of Keweenawan. 349. Mellon ,)iinction, 30.'), 318. Menaccanite. alteration of. 414, 415, 416. of angite-porpliyrite, 415. of dialiase, 351, 414. of gabbro, 351. of greenstone-conglomerate, 377. of horubloniie-gneiss, 120. of porphyrite, 416, 417, of schist, 122. Menominee district, 1, 2, 10, 34-36, 37, 260, 261, 293, 356. 472, 473. Menominee river, 81. Mesabi, Huronian, 2. Mesozoic, 87. Metamorphism, 467-468. ^ Mica from feldspar, 179, 335-340, 343. of biotite-sdiiat, 339, 340, 341. of biotite-slate, 338. of granite, 107. of gray w.acke, 304, 307, 334, 336, 337, 338. of mica-pyroxene-syenite. 115. of mica-schist, .'108, 309, 345. of mica-slate, 308, 309. of Quartz-Slate, 148, 149, 151, 179-180, 468. of Upper slate, 303. Mica-pyroxeue-syenite, 114-115. Mica-schist, 3, 4, 34, 52, 67. 84-85, 152, 302, 303, 332, 344. 345, 388, 465, 466, 409, 472. described, 307-309. foliation of 308. origin of 77-78, SO, 81. 107, 108, 335, 339-341. relations to gneiss, 85. 308. relations to gray wacke. 307. 308, 309. 341. (See jlffca-bia/f, BiitUc-datc, Biotite-sehist.) Mica-slate, 3, 52, 84-85, 302, 303, 332, 344, 465, 472. described, 307-309. origin of, 335, 341-343. relations to gray wacke, 307, 308, 309. (See Mica-tichisf, Biotite-slate, Biotite-schittt.} Microeline, alteration of 335. of conglomerate, 448. of granite, 106, 112, 123. of granitoid gneiss, 123. of gray wacke, 333, 335, 336. of Quartz-slate, 150. of schist, 118. of syenite-schist, 114. Microgranite. crystallization of 112-113. Milwaukee Iron Compau,v referred to, 8. Miner t Wells option, 191, 286. Minewawa mine, 51U. Mining, rules, for, 276-279. Minnesota, 36, 260, 261 , 411, 470, 474. Mississippi iron carbonate, 247. Montreal mine, 269, 270. Montreal river, 127, 299, 308, 456, 467, 482. 502, 508. west branch, 147, 161-162, 173, 173, 178, 227, 369, 418. Montreal river area 6, 7, 10, 11, 12, 15, 17-19, 31, 32. 43, 48, 67, 71, 84, lll-llo 115-116, 145, 30(. west brancii 84, lOi, 109-111, 121, 129, 144, 146,304, 328,329. Mount Hope mine, 16i, 231, 209, 270, 272-273, 281. Mount 'Whittle.se.v , 144, 145, 146, 176, 189, 454. Murray referreu to, 56. i Muscovite from feldspar, 305, 333, 334, 335, 336, 337, 338, 339. of biotite-sohist, 339, 340, 341 . of biotite-slate, 339. of granite, 106. of gray wacke, 307, 333, 334, 335, 336. of mica-schist, 307, 308- ofmica-slate, 307, 308. of Qmti'tz-slate, 148, 151- Nipigon, 473. Nonfragmental and fragmental rocks, gradations between, 246. Norrie mine, 84, 92, 273, 277, 281. North lake, 261. Novaculite, 147, 148, 178. described, 154-155, 164. Numakagon lake, 2. 186, 456, 460, 464, 468. Nnmakagon lake area. 9, 11. 66. Numakagon river, 104, 457, 469, 470. O. Ogishki conglomerate, 473. Ohio iron carbonates, 247, 263, 490. Oley, indebtedness to, xiv. Oligoclase. alteration of, 335. of gray wacke, 355. - of Quartz-slate, 150. Olivine of diabase, 350, 351, 352. Ontario, 260, 469, 470, 473. Ontonagon river area. 6. 8, 14, 30. Ophitic structure of diabase, 330, 351, 410. Ores. (See Iron ore.) Ore veins, number of, 94. Organic matter in iron carbonate. 250. in .slate, 250. Orthoclase, alteration of 335, 338. ofbiotite-schi.st,340. of biotite-slate, 338- ■ of gneiss, 109, 123. of granite, 106, 112, 123. of gray wacke, 333, 335, 336. of Quart z-.slate, 1.50. of schist, 118. of syenite-schist, 114. Owen referred to, 7, 17, 19, 27. Oxide of iron. (See Iron oxide.) P. Paint rock, 270. . (See Soapstone.) Paleozoic. 87, 252. Palms mine, 164-165. 174, 179, 191, 233, 448, 449, 451, 454, 471. Paramorphic amphibole, 435. hornblende, 353. Prtrker, referred to, 94. Peale. referred to, 256. Pegmatitic structure, 106, 123. Pence mine, 273»508. INDl'.X. 53i Peninsular Mining Onnipnny. Hifl. PtjnliHylviiniii Iran rnrlioimio, 217. Pt'UokiM'crnptivert, rchitiouH fo Kewfrnawaii (-'riiptivrs. :m;). Penokeu Unp, xm, 22-27. 30, 40, 77, 104-10."., 127, i:;o, i:i2. 13:), urn -139, 144, irM-l.W, 171. 170, 1S3. IH4, I.S.-., 1117,200.213, •.>l«-22;), 298, 300, 303, 344, 430, 410. 152, ir.l, 4C5. Penokeo Ciogubic, imiiio (•iiiiMi(UM'fiu feldspar, 118. 180.305.333-340.342,343.413.414. iueliided in hiM'nlih'ndi', 118. of aetiiuditie slate, 195, 210. 211,213, 215. 258, 2,50, 206, 3(j2, 3(i4, of biotitesehist, 339, 340, 341. of biotite-slate, 338, 339. of chert. 123. of chert-conglomerate, 451: of cherty limestone, 127. of chlorite rock. 111. of clay-slate, 305. of concretions, 207. of conglomerate, 174, 365, 395, 448. of diabase. 356. of ferruginous chert, 205. 254, 255, 257, 362. of ferruginous slate, 193, 204. of gneiss. 108, 109. 110, 116, 110, 120, 121, 123, 125. of granite, 106. 107, HI, 112, 113, 122, 123, 344. of graywacke, 306, 307, 332, 333, 334, 335, 336, 337. of greenstone-conglomerate. 375, 376, 377, 379. of iron-bearing member, 267, 268. of iron carbonate, 190. 200. of limestone. 132. of mica-schist, 345. of mica-slate, 342. of niierogranite, 112, Tl3. of porpliyrite, 417, 418. of quartzite, 153, 154, 369, 391. of quartz-rock, 133. of (Juartz-slate, 134, 143, 147. 148, 149,150,151,152.179-1811. 443, 408. of.sebist, 117,118, 121. 125. of slate, 370, 376, 380. of syenite, 113, 114, 122. . ol' Upper slate, 303, 343, 344, 345, 465. replaces piagioclase, 115. solubility of, 256. (See Chert, Silica.) Quartz amygdules, 377, 418. Quartz enlargements, 112, 132, 150, 152, 153, 180, 185, 209, 267, 288, 305, 334, 335, 339, 343, 345, 377, 395, 396, 444, 462, 468. Quartz veins, 418. Quartzito, 34, 30, 147, 148, 149, 176, 177, 178, 179, 180, 261, 301, 303, 304-305, 347, 363, 368, 369, 391, 303, 394. 395-'106. 438. 440, . 441 . 444, 440-451, 455, 457, 460, 464, 468, 471. 472, 473. 474. described, 153-154. 1.55-156, 157, 158, 159-160. 161. 162, 103, 164,105.166. 167, 160,170,171, 320.371,372.373,390,400, 404-105,408-409,498,518. dip of, 270. fragments of, in iron-bearing member, 84, 175. origin of, 77-78. relations to dikes, 272, 274. relations to Iron-bearing member, 181, 185. relations to Iron ore, 94, 185. 199, 274, 276, 285, 286, 287, 291, 292. 294. 365. relations to C|nartz-rock, 245. 632 INDEX. Quartzite relations to shafts, 277.,279. Qtiartz-porpliyry, 462. Qnartz-rock, 290. * aesurilied, 133, 136, 130. recomposed, in quartz-slate, 172. relations to quartzite, 245. Quartzsehist, de.scriljed, 216, 217, 21R-219, 220-223, 323-224, 239. Quartz,slate, 3, 4, 93, 143-181, 18C, 209, 38.5, 397, 363, 368, 369, 428- 430. 432, 433, 435, 436, 438-440, 443-444, 459-460, 4C7, 468, 409, 471, 472, 474, 482, 484, 4,S6, 488. basal conglomerate in, 139, 172, 173, 174, 180, 181. tle.'Cure3 0i;437,43S. of Eastern area, 366, 308-371 . recomposed quartz-rock in, 172. relations to clierty limestone, 130, 134, 139, 141, 142. 143, 144, 147, 171, 180, 1 81, 443, 454, 464. relations to gneiss, 143, 171. relations to granite, 128, 143, 171, 174, 179, 180, 445, 448. relations to iron-bearing member, 144, 175, 176, 181. 184-185, 188, 189, 300, 394. 299. 363, 455. 456, 464. relations to iron ore, 185, 19,1, 36S, 270. 286, 287. 294, 295. relations to schist, 129, 143, 171, 173, 178,445, 446-449. relations to Southern Complex, 143, 171, 172-174, 179-180, 181, 444-454. relations to Upper slate, 296, 297. tbiclaieas of, 143, 144, 146, 180, 444. • topography of, 145. 188-189, 301. Quaternary, 48. E. Randall, referred to, 7, 17. Eigga relerred to, 191, 192. Rohrbach referred to, 413. Rominger referred to, 5, ia,.67-73, 78. Roscoe referred to, 259, 289, Roth referred to. 355. S. St. Louis district, 1, 472. Sandstone, 147, 148, 149, 256, 456-457, 462, 463, 464. described, 154-155, 103, 165, 167, 486. relations to iron ore, 285. Schalstein. Giimbel on, 374. Schist, 2, 20, 80, 81. 94, 103, 104-105, 107-111, 116-122, 125, 438, 435, 438, 440, 442, 443, 450, 451 , 452-453, 454, 462, 469, 471, 473. of Iron-bearing niembei', described, 217, 244-245. of Qu.artz-slate. described, 160. of Southern complex, described, 107-111, 116-122. of Upper sl.ate, described, 316-317, 397, 404. foliation of, 103, 114, 117, 125. 308, 442. origin of, 124, 125-126. relations to cherty^ limestone, 137-128, 446. relations to granite, 104-105, 116-117, 123-124, 442, 445. relations to greenstfuie-conglmnerate, 420, 421. relatiou.s to quartz-slate, 129, 143, 171. 173, 178. 445, 440-449. rehations to syenite, 445. uuconforniably below Auiniikie series, 261, 262. {^ee J^ctlnolitic-schist, Sioiitr-schist^ Gneiss. Greenstone- schist, Hornblende schist, Mica-schist, Syenite-schist.) Schistose rocks, Whittlesey on, 20. Schorlcmnier referred to, 259, 289. ' Sericite from feldspar, 118, 333, 334. of chert, 133. of cla.v-slate, 305. of gneiss, 109, 111, 110, 118. of gray wacke, 307, 333, 334. of mica-.schist, 307, 308. of mica-slate, 307, 308. Sericite of Quartz-slate, 148, 151, 370. Sericite-gneiss, 110. Serieite-sohist, described, 324,400-407. foliation of, 428. Sericite-slate, 391. described, 404, 405^00. Serpentine of diabase, 356. Shafts in Peuokee-tTOgebic r.ange, relations of, 377, 279. Shale, 148, 177, 178. described, 167, 480. (See Clay shale, Clay slate, Slate.) Siderite, alteration of, 201, 303, 253, 283, 362. described, 337, 338-330, 234, 340-242. 367-368. grades into ferruginous slate, 201-202. of actinolitic .slate, 258, 266, 267, 362, 364. of clay-slate, 434. of ferruginous chert, 258, 265. of gray wacke, 333. of iron carbonate, 200, 201, 258, 263, 204, 268, 294. of slate, 392. • of veins, 202. position of, 199. relations to actinolite, 433. relations to iron ore, 282. relations to magnetite, 433. (See Iron carbonate.) Sideritic chert, described, 490, 498, 500. slate, described, 490. Silica, concentration of, 255. deposition of, 384, 28S-290, 292, 293. of actinolitic slate, 198, 211, 212, 257, 258, 259, 266, 267, 294. of chert breccia, 395-. of concretions, 207, 208. . of ferruginous chert, 202, 203, 205, 212, 254, 255, 256, 257. of ferruginous slate. 192, 193, 194, 202, 203, 204. 253. of geyserite, 251. of iron carbonate, 190, 1_99, 200, 202, 256, 263, 264, 294. of jasper conglomerate, 370. of quartzite, 396. of slate. 392. relations to iron ore, 282, 284. solubility of, 256, 288, 289, 290, 292, 293. Silurian, Irving on, 86. Silver creek area, 43. Sioux district, 2, 472. Slate, 7, 15, 52, 67, 80, 143, 145, 147, 148, 156, 158, 245, 250, 261, 262, 363, 364, 368, 309, 370. 376, 431, 438-440, 449. described, 154-155, 159, 161, 164, 165, 107, 168, 170, 171, 299, 313-315, 360. 373, 381, 382, 401-402, 402^04, 405, 406, 484. dip of, 390. relations to cherty limestone, 128, 134, 139,445. relations to gabbro, 36. relations to granite and scliist, 128-129. relations to greenstone-conglomerate, 419, 420. relations to ii-on oi-e, 282, 283. relations to shafts, 279. (See Actinolitie slate, Biotite-slate, Chloiitc-slate, Clay- slate, Clay-shale. Ferruginous flate, Grayuacke-slute,. Magnetite-slate, Mica-slate, Quartz-slate, Shale, Upper slate.) Bmaragdite from augite, 415. 417. from diallage, 353. ^ from feldspar, 353. 413, 414, 415, 417, 418. of diabase, 359, 413. of porphyrite, 415, 417. relations to augite. 413. 435. Soapstone from diabase, 294, 355, 356, 357, 359. INDEX. )3;5 Soapatoiio of Irnn-ltnarin^ nu'inln'r, 12. 271, X>t\ '.i^>7, rcliitioiis t»» iron (>ii\m,U4.2ii-*. SuUilH ri'lVlTi'il tn, 2">l, 25li. ^■llrhy rol'i'rrcd to. 427. Smitiii-ni (!iim|il>'S, 10:1-120. 115, 429,4:11. 1411, 141 4411, 1.18. 4.'.9, 41111, 4i;i , 4li7, 47:1, 474, 47li, 478. iliiiiia-io ui; 349, :ir)8. ilikrs or, :i49. mi'.'ili ol'. 124-120. rolatinii.s f(i bayal roiiLjlomprato. 422. rclalitma Ut Kastorn sainlstimo. 4Rl-4t]:t, rrlation.s ti) gr»jii8linu>-)-(Hi;iloiiusj'uUi, 419-420, 4:15. . rehitioiis to Froli-bo^^rini; nii'liiber. 180. velation.-i to Pfiiokeo aerius. lO.'i, 109. :!4:l. 444-l.'i4, 457. relations to Quartz-slate, 109, 14;i, 171, 172-174, 179-18Q, 181, 444-4.-)4. relatiinis to Upper slate, 34:i-344, :J45. uucoiitbriuably lielow chorty linie.stoue, 444-454. topography of, 145, 189, 301. (See Basenii'itt Complex, Laui-entian.)' Specular ore of aetinolitic .slate, 19G. Spitzliergeu, chert of, 140. 251, 253. Sponge spicules, chert formed from, 140. 251. Stanrolite of niica-scbist, 119. Sunday late, 129, 130-131, 139-140, 144. 146, 105-166, 179, 188, 187, 191, 25.5, 209, 271, 290, 291, 292, 302, 437, 438, 444, 449, 454, 456, 459, 400. ' Sunday lake area, 145, 154, 279. Sunday lake mine, 273. Sunday lake outlet, 283. Sweet referred to, 9, 39-40, 183. Swineford referred to, 59. S.yenite, 36, 80. 103, 105, 111, 110,120,122,123,344,443. described, 113-115. grades into syenite-schist, 125. relations to ehert^\' liniestoue, 445 relations to Kcweenawan, 13-14. relations to schist, 445. Syenite-schist, 103, 114, 116, 117, 120, 125. described, 125. (See Kornbhnde-syeuite. 2Uca-kornblende-sye7iite, Mica- pyroxene-syejiite.) Teall referred to, 413. Tertiary iron carbonate, 247, Thunder bay, 261, 470. Tilden mine, 164, 2:!3. Titanite of hornblende-gneisa, 120. of greenstone-conglomerate, 375, 378. Topography of Iron-bearing member, 145, 188-189, 301, of Keweenawan, 301. of Quartz-slate, 145, 188-189, 301. of Southern Complex, 145, 189, 301. of Upper slate, 301-303. Tourmaline of gneiss, 119. Town. 43 N., E. 7 W., Wisconsin, 104, 186, 451, 457. 43 N., ,R. 6 W., Wisconsin, 186. 44 N ., E. 6 W., Wisconsin, 186, 215, 216, 458. 44 N., E. 5 W., Wi-sconsin, 104,107,128,129, 130,131, 135, 138, 144, 175, 186, 188, 316, 217, 346, 443, 458, 476, 496. 44 N., E. 4 W., WisConsin,129, l;i5, 138, 144, 146, 155, 150, 175, 170, 185, 188. 217, 298, 351, 458. U N., E. 3 W., Wisconsin, 104, 129, 135. 136, 138, 139, 140, 156, 157, 158, 171, 176, 185, 197, 217, 218, 219, 220, 297, 29S. 303, 309, 310, 311, 312, 315, 459, 476, 482, 480, 494, 490, 504. 512, 514, 516. 4,'-, N . K. 1 W 45 N. K. -.1 \V. 45 N. li. 2 W. 45 N. li. 1 W Town. 44 N., 1{. 2 \V., Wisconsin, 10. 129, i:i0, 14 1, II.'., 140 1.19, 171, 170, 177, 197, 220, :ilK, 319, :121,:147, 510. 1 I N., 1;. 1 W., Wisi'onsiji, 108, 19S. Wisconsin, 180. Wisconsin. 42. Wisconsin, 10, 291). :i2l. , Wisconsin, 129. 149, 1.14. 159, 184, 185,220, :i,208, 209, :!U4,:i05, 321. 322, 32;i, :I24, :i25,;i20, :'.51, 459, 494, 504, 612,510. 45 N., K. 1 !•;., Wisconsin. 129, 144, 140, 159, 100, 178, 200, 223, 324, S25, 209, 302, 303, 305, 306, 325, 327, 440, 440, 462, 488. 45 N., R. 2 E , Wisconsin, 42, 161, 191, 220, 227, 297, 305, 306, 327, 490. 46 N., R. 1 K., Wisconsin, 328. 46 N., E. 2 E., Wisconsin, 104, 105, 139, 144, 140, 101, 102, 227, 228, 269, 274, 302, :i08, 309, 328, 329, 33U, 448, 476, 488,492. 46 N,, K. 44 W., Michigan, 105. 47 N.,E. 42 W., Michigan, :i87, 388, 389, 393, 394, 405,406, 407, 409, 422. 424, 425, 431, 450, 459, 461. 47N., E. 43 W., Michigan, 6, 105, 144.191,301,362.303,364, 365, 367, 308, 309, 370, 371, 372, 373, 374, 379, 380, 381,386, 387, 388, 389, 392, 393, 394, 396, 397, 398, 399, 400, 401, 402, 403, 404, 414, 422, 433, 424, 425, 426, 429, 430, 431, 449, 450, 459,461. 47 N., E. 44 W., Michigan, 4, 128, 139, 130-131, 137, 138, 139, 140, 170, 171, 179, 185, 187, 188, 199, 340, 241, 242, 343, 244^ ■ 245, 360, 361, 363, 366, 367, 368, 369, 374, 380, 381, 382, 383, 384, 387, 388, 389, 391, 392, 396, 410, 415, 416, 422, 423, 424, 420, 429, 430, 431, 433, 441 , 443, 459, 461, 480, 518. 47 N., R. 45 W., Michigan, 129, 136, 137, 139, 144, 146, 154, 165, 166, 167, 168. 169, 171, 179, 187, 191, 198, 199, 237, 238, 239, 240, 268, 269, 290, 298, 346, 355, 437, 449, 452, 454, 480, 482, 484, 486, 504. 47 N., E. 46 W., Michigan, 105, 144, 146, 164, 165, 174, 179, 187, 189, 191, 199, 231, 232, 233, 234, 235, 236, 237, 255, 269, 272, 280, 283, 290, 297, 305, 330, 331, 346, 351, 357, 448, 449, 456, 484, 490, 502. 47 N., E. 47 W., Michigan, 112, 144, 146, 154, 162, 163, 104, 179, 199, 228, 229, 230, 231 , 269, 305, 329, 357, 486. 48 N"., E. 46 W., Michigan, 6. 49N.,E. 41 W., Michigan, 6- 65 N., E. 2 W., Minnesota, 262. 65 N., R. 3 W., Minnesota, 262, 498. 65 N., E. 4 W., Minnesota, 506. Trap range, relations to Upper slate, 301, 302. Tremolite of limestone, 130, 131-133, 138, 141, 259, 260, 207. Trimble mine, 274, 610. Trimmingham chalks, 251, 252, 250. Tuff, volcanic, 360, 361, 379, 380. Tylers Fork, 154, 159, 177, 178, 184, 185, 188, 189, 198, 206, 220-223, 279, 298. 299. 300, 302, 303, 304. 307, 309, 321, 324, 369, 428, 456, 459, 460. Tylers Fork area, 129, 145. U. Unconformities. 470. in Marquette district, 471 , 472. Unconformity below Auiraikie series, 261, 362. below Eastern sandstone, 388, 461-463. between Animikie series and Keweenawan, 470. between Cherty limestone and Penokce sei'ies, 454-455. between Cherty limestone and Southern Complex, 444-454. between Huronian and Basement Complex, 81. between Huronian and Laurentiau, 47, 82. between Keweenawan and Fluronian, 47, 81, 82. between Lake Superior sandstone and Keweenawan, 47. between Penokee series and Keweenawan, 455-461. 534 INDEX. Unconformity between Penokee series and Southern Complex, 56, 77, 81, 82, 96, 107-108, 109, 129, 173, 343, 444-454, 469. between Quartz-slate and Southern Complex 1, 29, 173, 444, 454. Upper slate, 285, 296-345. 431, 434, 436, 438, 457, 459, 460, 468, 471, 472,474,512,514,516. bedding of, 296. cleavage of, 296. diabase of. 348, 359. dikes of, 358. greenstouea of, 285. oris'in of, 332-345. relations to uctiiiolitic slate. 214. relations to cruptives. 347, 348, 358. relations to greenstone-conglomerate. 420. relations to Iron-bearing member, 200, 294, 296, 297, 298, 299, 300, 456, 464^65. relations to Keweenawan. 297-299, 300, 301, 302, 347, 348,358. relations to Quartz-slate, 296, 297. relations to Southern Complex, 343-344, 345. thickness of 296, 298-299, 456, 459-461. topography of, 301, 302. (See Slate.) Van Hise referred to, 9. 10-11, 77-78, 84-85, 86, 102. Veins of chlorite, 418. of concretions, 206-207, 208. of epidote, 418. Veins of ferruginous cliert, 254. t)f iron carbonate. 202. of quartz, 418. Vermillion district, 2, 260, 201, 293, 356, 468, 472. 473. 474, 496. Vermillion lake, 496. Viridite of iron carbonate, 190. Volcanic tuft'. (See Tuf.) W. - Wadsworth referred to, 56-58, 71, 73-75, 75-77, 249, 412. Wales, chert of, 140, 251, 252. "White, ackuowledguieuts tn, xiv. WhitUeld referred to, 256. Whitney refcn-ed to, 6, 10, 13-14, 15. 16-17, 56, 71, 73-77. Whittlesey referred to, 7, 17-22, 27-30, 34, 36. Wight referred to, 9. Williams referred to, 412. Winchell, Alexander, referred to, 99-102, 473. Wiuchell, N. H., referred to. 78-81, 96-99, 101-102. Wisconsin, 1,41, 474. Wisconsin Central Kailroad, XIU, 347. Wisconsin geological survey referred to, 3, 5, 7, 8, 9, 10, 11, 20, 22, 27, 39, 40. 45, 58. 66. 84, 127, 128, 138, 143. 183-184, 186, 195, 196. Wright referred to', 3,9, 40-41 , 52-56, 58-59, 78, 83-84, 104, 186. T. Yellowstone National Park, 251, 256. Yorkshire, chert of, 140, 251.