. . LIBRARY . . Connecticut Agricultural College. VOL 2i...4 /.-.e^ — CLASS NO .^....rr^.M COST .^.>*jt DATE (jilAJi....LS.». 19.0..^.. Digitized by the Internet Archive in 2009 with funding from Boston Library Consortium IVIember Libraries http://www.archive.org/details/mesabiironbearinOOIeit f.1 vol 4^ DEPARTMENT OF THE INTERIOR MONOGEAPHS OF THE United States Geological Survey VOLUME XLIII WASHINGTON GOVERNMENT PRINTING OFFICE 19 0 3 UNITED STATES GEOLOGICAL SURVEY CHARLES D. WALCOTT, PIEECTOE, THE MESABI IRON-BEARING DISTRICT OF MINNESOTA BY CHARLES KENNETH LEITH CHARLES RICHARD VAN HISE, Geologist in Charge WASHINGTON GOVERNMENT PRINTINC4 OFFICE 1 9 0 3 9^ 1 X.. CONTEI^TS. Page. Letter of transmittal Outline op monograph - Chapter I. — Introduction - ^^ Geography and topography - ^'^ General geology - Chapter II. — Brief history of district and summary of literature concerning it 25 History and literature of the district prior to its opening 25 Opening and development of the district - - 27 Literature on the district subsequent to its opening 29 Summaries of literature, arranged chronologically - 31 Chapter III. — The Basement Complex, or Archean - 63 Distribution -. - - - - - - - ^^ Kinds of rocks - ^^ Dolerites and metadolerites -. ^4 Basalts and metabasalts - 64 Diorites - ^^ Peridotite - ^^ Hornblendic schists 66 Micaceous schists and chloritic schists 68 Granite and porphyritic rhyolite , 68 Sedimentary rocks - 69 Structure ''^ Relations to other series' '^ Chapter IV. — The Lower Huronian series - - 72 Distribution - - '^ Kinds of rocks - - '"^ Gray wackes and slates '* Conglomerates - ' ^ Granites and porphyries - --- - -- '^ Inclusions in granite °0 Vein cfuartz °'' Metamorphism of Lower Huronian rocks by granite - 83 Relations of Lower Huronian granite to sediments and relations of both to other series 84 Structure ' - ' ^^ Thickness - - - ^'^ Chapter V.— The Upper Huronian series - - 88 Section I. Pokegama quartzite _ - - - 90 Distribution - - ^0 Kinds of rocks ^^ Quartzite - 90 Micaceous quartz slate - "^ Conglomerates - "* Structure 98 Thickness - 99 Relations to other formations - - 99 5 6 • CONTENTS. Chapter A'. — The Upper Hi-roxiax series — Continued.- Page. Section II. The Biwabik formation (iron bearing) 100 Distribution 100 Kinds of rocks lOl Greenalite rocks 101 Ferruginous, amphibolitic, sideritic, and calcareous cherts 116 Siliceous, ferruginous, and amphibolitic slates 143 Paint rock 149 Sideritic and calcareous rocks 150 Conglomerates and quartzites 154 The alteration of the iron formation by the intrusion of Keweenawan granite and gabbro. 159 Comparison of the metamorijhic effects of the granite and gabbro 164 Magnetic attraction 164 Structure 165 Thickness - 166 Relations to other formations ■ 167 Section III. Virginia slate 168 Distribution 168 Kinds of rocks 169 Slate 169 Limestone 1~1 Cordierite-hornstones resulting from alteration of the Virginia slate by the gabbro . . 171 Relations of the Virginia slate to the Biwabik formation 172 Comparison of slate of Virginia and Biwabik formations 176 Structure - 177 Thickness 177 Section IV. Structure of the Upper Huronian series 1 78 Section V. Thickness of the Upper Huronian series ISO Section VI. Relations of the Upper Huronian series to other series 180 Chapter VI. — Keweenaw an, Cretaceous, and Pleistocene rocks 182 Section I. Keweenawan rocks -- - 182 Duluth gabbro 182 Contact phases of gabbro 183 Diabase - 185 Embarrass granite 186 Proof of intrusion of Embarrass granite into the Upper Huronian series 187 Section II. Cretaceous rocks 189 Fossils 190 Section III. Pleistocene or glacial deposits 191 Chapter VII. — Reslme of geologic development and correlation 195 Section I. Resum6 of the geologic development 195 Section II. Correlation 200 Chaiteu VIII. — The iron-ore deposits 206 Distribution 206 Shape 207 Size 208 Kinds of ore 209 Minerals and rocks contained in tlie ore 210 Chemistry 212 Texture and structure 223 The rocks forming the bottoms and sides of the ore deposits 227 Structural relations of tlie ores to the adjacent rocks 227 Petrographic relations of the ores to the adjacent rocks 233 Drainage 234 CONTENTS. ^ Page. Chapter IX. — Oeigin of the iron ores 23^ General statement "''' Origin of the greenalite granules 239 Greenalite a sedimentary deposit 289 Similarity of greenalite to glauoonite 239 Composition of glauoonite and greenalite, by F. W. Clarke 243 Explanation of the occurrence of greenalite in granules - 247 Manner of deposition of greenalite - 2o3 I. Develoi^ment similar to glauconite r - - - 25.3 II. Direct precipitation from solution by organisms 2.55 III. Development similar to that of iron carbonate 255 Iron derived from the weathering of preexisting rocks and carried to the ocean as carbonate -'^^ The iron first precipitated in the ocean as hydrated peroxide 256 The iron first precipitated in areas of vegetation 256 The hydrated peroxide reduced by vegetable matter and the protoxiile of iron combined with carbon dioxide or silica 257 Conclusion with reference to the origin of the greenalite granules 259 Burial of the iron-bearing formation beneath the Virginia slate 260 Emergence of the iron-bearing formation from the ocean 260 Alteration of the iron-bearing formation by weathering and the secondary concentration of the ores '- 260 Localization of ores by circulation of water 265 Explanation of the apparent absence of ore deposits at the east end of the range 272 Cause of distribution of phosphorus 274 Points of similarity and difference between Mesabi ores and those of other Lake Superior ranges 276 Previous explanations of the origin of the ore and their relations to the explanation above given 27 ( Chapter X. — Mining, transpoetation, production, reserve, ownership, prices of ores, fur- nace USE op ores 280 Methods of mining 280 Mining by steam shovels in open cuts 280 MiUmg 282 LTnderground caving and slicing systems 282 Comparison of methods of mining - 283 Transportation 285 Production - 287 Reserve tonnage 290 Ownership and control 291 Price of Mesabi ore in comparison with old range ores - 293 Furnace use of Mesabi ores .., 294 Chapter XL — Exploration --' 295 Index "^"3 ILLUSTRATIONS Page. Plvte I. Sketch map of Lake Superior region, showing iron districts, shipping ports, and transportation lines -^^ II. General geologic map of the Mesabi district In pocket III. First train load of Mesabi ore, 1892, on Duluth, Missabe and Northern docks 28 lY. Detail map showing distribution of Upper Huronian, Lower Huronian, and Archean rocks northwest of Hibbing ^° V. Detail map showing distribution of Upper Huronian, Lower Huronian, and Archean rocks north of jNIountain Iron '" VI. Detail map showing distribution of Keweenawan, Upper Huronian, and Lower Huronian rocks in the vicinity of the Mailman camps 80 VII. Photomicrographs of normal and metamorphosed Lower Huronian graywacke 82 VIII. Greenalite rock ^^^ IX. Photomicrographs of greenalite granules 106 X. Ferruginous chert of iron-bearing formation 122 XI. Ferruginous chert of iron-bearing formation - - 124 XII. Ferruginous chert, "jaspery" phase, and ferruginous chert in contact with quartzite of iron-bearing formadon 1"° XIII. Photomicrographs of fresh and altered greenalite granules and ferrugmous chert concretion XIV. Photomicrographs of ferruginous chert granules, showing mottling 130 XV. Photomicrographs of ferruginous chert showing ]ater stages of the alteration of green- alite granules -- XVI. Photomicrographs of ferruginous chert of Penokee-Gogebic district - - 134 XVII. Photomicrographs of ferruginous and amphibolitic chert of iron-bearing formation near contact with Duluth gabbro --_ l^^ XVIII. Slate, ferruginous slate, and paint rock in iron-bearing formation and contact of iron- bearing formation with intrusive granite 152 XIX. Photomicrographs showing metamorphi-^m of Virginia slate into cordierite-hornstone in approaching Duluth gabbro 1 ' * XX. Views of contact of the ore with wall rock in the Biwabik and Mountain Iron mines. 232 XXI. Photomicrographs of granules and concretionary structures in Clinton iron ores 250 XXII. Plan of tracks in the Mahoning, Mountain Iron, Fayal, and Biwabik mines - . - 280 XXIII. Map of Adams mine with bottom of the ore deposit indicated by contours 282 XXIV. A, Railway cut in approach to Oliver mine at Virginia, showing close jointing and brittle nature of the iron-bearing formation; B, Preliminary stripping at Oliver mine, Virginia "*'■* XXV. Oliver mine at Virginia in 1900 - ----_- 284 XXVI. A, Mountain Iron mine, looking north through mine; B, Steam shovel "bucking" bank of ore, Mountain Iron mine --- 286 XXVII. A, Stripping operations, Sharon mine; B, View of Auburn mine, open pit and shaft. . 286 XXVIII. A, Panoramic view of Adams mine; B, Panoramic view of Fayal mine 292 XXIX. Adams mine, showing horses of rock in open pit 294 XXX. A , Mills at Fayal mine; B, ililling with steam shovel at Fayal mine 294 10 ILLCSTRATIOXS. Page. Plate XXXI. .1, Saunti\\ mine, showing we-stward monocliual tilting of ore strata: B, Faval mine, showing steam shovel "bucking" l)ank of ore 294 XXXII. .1 and B, Panoramic view of ilahoning mine: C and D, Panoramic view of Bi wabik mine 296 XXXIII. A, View of Hale mine, showing monocliual dip of strata of ore and rock: B, Duluth, Missabe and Xortheru docks at Duluth 298 Fig. 1. Detail map showing distrilsution of I'pper Huronian, Lower Hurrmian. and - Archean rocks northeast of E veleth 73 2. Sketch of contact of Lower Huronian granite and graywacke siate. showing intri- cate nature of granite intrusion 85 3. Detail map of part of sec. 3, T. 58 X., R. 17 W., showing separation of quartzite at the base of the Bi wabik formation from the Pokegama quartzite 89 4. Sketch showing distribution of the Biwabik and Pokegama formations at the ■ lower falls of Prairie River 91 5. Sketch showing relations of Embarrass granite to the Biwabik formation in the abandoned glacial gorge in the XW. -} of XAV. \ of sec. 17, T. 60 AV., R. 12 W. . 160 6. Details of contact of Embarrass granite and Biwabik formation in the gorge in theXW. iof XW. i-of sec. 17, T. 60 X., R. 12 W 161 7. Detail map showing distribution of Lower Huronian granite, Biwabik formation, Keweenawan gabbro and contact phase of granite with gabbro on the north shore of Birch Lake 184 8. Sketch showing jiossible connection of the Mesabi with Penokee-Gogebic iron- bearing series 203 9. Ideal cross section of a Mesabi ore deposit, showing relations to ferruginous chert and impervious slate layer 228 10. Ideal section parallel to the pitch of a Mesabi ore deposit, showing relations to ferruginous chert and to impervious slate layer 229 11. Section through Biwabik formation transverse to the range, showing nature of circulation of water and its relations to confining strata 266 12. Sketch showing three stages in the downward and lateral migration of an ore deposit, due t-.j the truncation of the iron formation by erosion 270 LETTER OF TRANSMITTAL. Department of the Interior, United States Geological Survey, Dr~-si0N of Pre-Cambrian and Metamorphic GtEOLOGY, Madison, Wis., June 23, 1902. Sir: I have the honor to transmit herewith the manuscript of a mon- ograph on The Mesabi Iron-bearing District of Minnesota, by Charles Kenneth Leith. Discovered only about ten years ago, in the early nineties, the Mesabi district has to-day no rival in its production or reserve of iron ore. The geological succession in the district, the unusual size, shape, and structure of the ore bodies, their manner of development, and tlie peculiar and rajjid methods of exploitation of the ore, all present features of unusual scientific and economic interest. This monograph is one of a series planned to treat the six iron-bearing districts of the Lake Superior region. Monographs on the Penokee- Gogebic, Marquette, and Crystal Falls districts have been published, in the order named. This, on the Mesabi, is therefore the fourth. The last two of the series treat of the Vermilion and Menominee districts. The former, by J. Morgan Clements, is ready for the printer, and the latter, by W. S. Bayley, is nearing completion. The execution of the full plan of the old Lake Superior Division of the United States Geological Survey will be marked by the ])ublication of a closing monograph entitled The Geology of the Lake Superior Region, which will contain a general discussion of the geology of that region, including a summary treatment of each of the iron-bearing districts, and a general discussion of the ore deposits. This monograph is in preparation. Very respectfully, your obedient servant, C. R. Van Hise, Geologist in Charge. Hon. Charles D. Walcott, Director of United States Geological Survey. 11 OUTLWE OF MONOGRAPH. Chapter I contains a general account of the geography, topography, and geology of the Mesabi district. The district lies northwest of Lake Supeior and extends from near Grand Rapids, on the Mississippi River, a little north of east to Birch Lake, a distance of approximately 100 miles. Its width varies from 2 to 10 miles, and the total area is about 400 square miles. The main topographic feature is a ridge, known as the Giants (or Mesabi) range, which extends the length of the district. The geologic formations represented in the district belong, in ascending- succession, to the Archean, Lower Huronian, Upper Huronian, Keweeuawan, Cretaceous, and Pleistocene. Thej^ are all separated by unconformities. The core of the Giants range is formed by Archean and Lower Huronian rocks, except for the portion in ranges 12 and 13, where Keweenawan gi-anite forms the core. On the south flank rest the Upper Huronian rocks, containing the iron-bearing formation, with gentle southerly dips. The Keweenawan gabbro lies diagonallj- across the east end of the district. The Cretaceous rocks are found in small isolated patches in the western portion of the district. Pleistocene drift forms a more or less heavy mantle over"all the underlying rocks. In Chapter II is given the historj^ of early explorations, discover}^ of ore, and the marvelous economic development of the district, together with summaries of literature on the geology of the area. Chapter III treats of the Archean rocks. They consist principally of green rocks of gi'eat variety, including dolerites, metadolerites, basalts, metabasalts, dioi'ites, and hornblendic, micaceous, and chloritic schists. The more massive rocks frequentlj^ have an, ellipsoidal structure which is characteristic of the green igneous rocks of other parts of the Lake Superior region. In addition to the green basic rocks there are present sniiall areas of granite and porphyritic rhyolite. In Chapter IV the Lower Huronian series is described. The series consists of sediments and granite. The sediments are graywackes, slates, and conglomerates, all metamorphosed, with bedding and schistosity practicall}^ vertical. Thej^ may be as thick as 10,000 feet, but it is thought more probable that the thickness does not exceed 5,000 feet. The Lower Huronian sediments rest unconformably u^jon the Archean rocks, as shown by basal conglomerates containing fragments of all the varieties of rocks found in the Archean. Previous to. the work done in con- nection with the preparation of this monograph, the presence of the Lower Huronian 13 14 OUTLINE OF MONOGRAPH. series of sediments had not been determined, but everything beneath the Upper Huronian or Animikie had been mapped as Keewatin or Archean. The Lower Huronian granite forms the main mass of the Giants range westward from a point near the east line of R. 14 W. It is intrusive into both the Archean rocks and the Lower Huronian sediments and has produced strong exomorphic effects in both. Chapter V contains an account of the Upper Huronian series, of importance because it includes the iron-bearing formation. The series consists of three forma- tions— the Pokegama quartzite at the base, above this the Biwabik formation (iron- bearing), and above this the Virginia slate. The Pokegama quartzite (Section I) comprises vitreous quartzite, micaceous quaitz-slate, and conglomerate. The thickness ranges from 0 to 500 feet, averaging about 200 feet. The conglomerate at the base indicates unconformable relations of the Pokegama formation to the Archean and Lower Huronian rocks. The Biwabik formation (Section II), the iron-bearing formation, comprises ferruginous, amphibolitic, Sideritic, and calcareous cherts, siliceous, ferruginous, and amphibolitic slates, paint rocks, " greenalite" rocks, sideritic and calcareous rocks, conglomerates and quartzites, and iron ores. All but the last are described in this chapter. The iron ores are reserved for description in Chapter VIII. Cherts make up the bulk of the formation. The original rock of the formation is shown to consist largely of minute granules of green ferrous silicate, thus conlirming Spurr's conclusion. The material was called glauconite by Spurr. but is here determined to be a hydrous ferrous silicate entirely lacking potash, and thus not glauconite. It is named "greenalite" for convenience in discussion. The cherts and ii'on ores are shown to develop mainly from the alteration of the greenalite granules. The slates are in thin laj'ers interbedded with the other phases of the iron formation. The paint rocks result from the alteration of the slates. The conglomerates and quartzites form a thin layer from a few inches to perhaps 15 feet or more in thickness at the base of the formation. They pass upward into ferruginous cherts of the iron formation rather abruptly, though usually at the contact the chert and quartzite are interleaved for a few feet. The conglomerate of the iron formation rests upon Pokegama quartzite, indicating a slight erosion interval between the Biwabik and Pokegama formations, although the interval is not shown by discoi'dance in bed- ding, which is parallel in both. Heretofore the quartzite and conglomerate in the iron formation have not been discriminated from the rocks of the Pokegama formation. In the eastern portion of the range the iron formation is in contact with the Keweenawan gabbro and granite, and near this contact has suffered pro- found metamorphism. The characteristic rocks of this area are amphihole-mag- nctitecherts. The thickness of the formation may vary from 20(i to 2,000 feet. The average may bo 1,000 feet. OUTLINE OF MONOGRAPH. 15 The Virginia slate (Section III) is essentially a soft slate or shale formation, but it contains graywacke phases, near its base a little limestone, and near its contact with the gabbro is metamorphosed into a cordierite-hornfels. The noi'mal slate phases of the formation may be distinguished with difficulty in isolated occurrences from the slate layers in the Biwabik formation. The separation of the two is of importance to the explorer, and hence an attempt is made to determine criteria for their discrimination. The thickness of the Virginia formation can not be measured within the district, but from analogy with the Penokee-Gogebic district and the extent of the low, flat-lying area south of the Mesabi range supposed to be occupied by the slate, the formation is believed to have a verj^ considerable thickness. The slate grades, both vertically and laterally, into the Biwabik formation. The structure of the Upper Huronian series is described in Section IV. The entire series is well bedded, conformable in structure (although having a thin conglomerate between the Biwabik and Pokegama formations), and dips in southerly directions at angles varying from 5 to 20 degrees and exceptionally at higher or lower angles. The series is gently cross folded and the axes of the cross folds pitch in southerly directions. Accompanying the folding is considerable jointing, especially in the brittle Pokegama and Biwabik formations. Indeed, in these two formations the folding is l^rought about mainly through relatively minute displace- ments along joints, while in the Virginia formation the folding has taken place mainly bj' the actual bending of the strata. The thickness of the Upper Huronian series (Section V) within the limits of the district mapped may average about 1,500 feet; but if the total thickness of the slate formation outside the limits of the district be taken into account, the total thickness of the Upper Huronian series is probably several times this figure. The relations of the Upper Huronian series (Section VI) to the subjacent for- mations are those of unconformity, as evidenced by basal conglomerates, discor- dance in dip, difl'erence in amovint of deformation and metamorphism, distribution of the series, and I'elations to intrusives. In Chapter VI the Keweenawan, Cretaceous, and Pleistocene rocks are described. The Keweenawan rocks (Section I) consist of gabbro, diabase, and granite, all of which are intrusive into the rocks with which they come into contact. The north edge of the gabbro runs diagonally across the east end of the district from south- west to northeast, resting upon the edges of each of the members of the Upper Huronian series, and at Birch Lake against the Lower Huronian granite. North of the gabbro margin in range 12 are isolated exjjosures of diabase which may represent sills associated with gabbro intrusion. The granite forms the crest of the Giants range through ranges 12 and 13. This granite has not heretofore 1(3 OUTLINE OF MONOGRAPH. been discriminated from tlie Lower Huronian granite. The exomorphic effect of the g-abbro and the granite upon the Upper Huronian series has been profound. Cretaceous rocks (Section II) are found in a few isolated remnants in the western portion of the range. They consist of conglomerate and shale, and contain fossils showing them to belong to the Upper Cretaceous — not older than the Benton and probablj' not younger than the Pierre horizon. The Pleistocene deposits (Section III) form a heavj" covering over the district. On the upper slopes of the range many rock exposures project through, but on the lower slopes the rock series are commonly buried to depths ranging from 20 to 150 feet. The glacial deposits consist of stratified and unstratified drift, belonging principally to the Itasca and Mesabi moraines of the latest ice incursion. The movement of the ice was mainly from northeast to southwest, as shown by glacial strife. Several remarkable, steep-walled gorges through the crest of the Giants range at high elevations are believed to be the work of glacial streams escaping from a great lake ponded between the Giants range and the ice front when the glacier had drawn back north of this area. Chapter VII contains a resume of the geologic development and a correlation of the formations. The resume of geologic development is itself a summary and will not be here repeated. The essential feature of the correlation is the equivalence of the Archean, Lower Huronian, Upper Huronian, and Keweenawan series with similar sei'ies in other parts of the Lake Superior countrj-. Prior to the work in the Mesabi district done in connection with the preparation of this monograph, everything below the Upper Huronian (or Animikie) had been mapped as Archean or Keewatin. The work of the Survej^ has shown the supposed Archean or Keewatin series to consist of two series, an igneous one below and a sedimentary one above, separated by a profound uncomformity, to be correlated respectively with the Archean and Lower Huronian series of other parts of the Lake Superior region. The remarkable similarity of the Upper Huronian series to that of the Penokee- Gogebic district is again emphasized and the probability of their area! connection is discussed. Chapter VIII is a description of the iron-ore deposits. The features treated are distri))ution, shape, size, kinds, minerals and rocks contained in the ore, chemistry', texture and structure, rocks forming the bottoms and sides, structural relations of the ores to the adjacent rocks, pctrographic relations of the ores to the adjacent rocks, and drainage. The ores are in basin-shaped deposits, with great variet}' and irregu- larity of shape, but the horizontal dimensions ai-e usually great as compared with the vertical dimensions. At the edges the layers of ore grade directly into the layers of wall rock, i)rincipally ferruginous chert. The deposits lie, for the most part, near tlie axes of gentle troughs formed by the folding of the iron -formation strata, but m OUTLINE GF MONOGRAPH. 17 many cases the strata in the ore and adjacent rocks have essential^ monoclinal dips, indicating the deposit to be independent of a synclinal structure in the iron-forma- tion layers. Chapter IX contains a discussion of the origin of the ores. The ores are shown to develop mainly from the alteration, under surface conditions, of green ferrous silicate granules, as first pointed out by Spurr. The green granules, however, instead of being glauconite, as maintained bj^ Spurr, are believed, from their lack of potash, to be of different nature, and have been given the name greenalite. Their development is believed to be analogous to that of the iron carbonates of other parts of the Lake Superior region. That is, the iron was carried to the Upper Huronian ocean in solution, probably as carbonate, was precipitated as ferric hydrate, was buried with the vegetable material and reduced to the protoxide form, and was then combined with silica to form ferrous silicate. In the Gogebic district, where silica was not present in so great abundance, the protoxide combined for the most part with carbon dioxide to form iron carbonate. The shapes of the granules maj- be due to replacement of minute shells, such as those depositing glauconite or those giving shape to the granules of much of the Clinton ore. The secondar}' concentration of ore into deposits has resulted from the surface alteration of the ferrous silicate (greenalite), under essentially surface conditions, since the iron formation was first exposed to weathering. The process has consisted essentially in the decomposition of the ferrous silicate, the oxidation of the protoxide of iron to hematite or ferric hydrate, and the segregation of the iron and silica. Where this has occurred on a small scale, banded ferruginous cherts have resulted; where on a large scale, the iron-ore deposits have been formed. During the change both iron and silica have been carried in solution. At the present time waters flowing through the altered portions of the formation are concentrating ore by the solution and abstraction of silica, but little iron being carried in solution, as shown b}' analysis. The localization of the ores by circulation of underground waters is described in detail. The ores are shown to develop both above and below ground water, to have been concentrated on impervious basements consisting of slaty layers in the iron formation, which have limited the circulation below, and to have been in part confined to troughs formed by the folding of such impervious strata and in part independent of them. They have, in short, developed in the irregular and ramifying channels of water circulation in gentlj'- dipping, much jointed strata. Finally, the apparent lack of ore deposits in the iron formation far under the edge of the Virginia slate is shown to be due to sluggish circulation under the slate because of the ponding of water under that impervious formation. In the eastern portion of the range the iron oxide is mainly magnetite, associated with amphibolitic chert, and has not yet been found in large enough deposits to MON XLIII — 03 2 18 OUTLINE OF MONOGRAPH. warrant mining. The explanation of the nature of the oxide and of the absence of ore deposits in this portion of the district is found in the presence of the Kewee- nawan intrusives. Prior to the Keweenawan intrusions the iron formation in the eastern end of the district had been exposed to erosion by the removal of the over- lying Virginia slate. In the central and western portions the slate had not been removed. When the Keweenawan rocks were intruded the iron-formation rocks of the eastern end of the district were brought under deep-seated conditions, during which the changes in the original greenalite rock were those of partial oxidation and silicitication, resulting in the production of amphibole-magnetite rocks. These rocks are stable and have not been considerably altered since their exposure to surface alterations by the erosion of the Keweenawan rocks. The rocks of the iron forma- tion in the central and western portion of the district were not exposed to weathering agencies until after Keweenawan time, when the Virginia slate had been removed by erosion, and thus were never metamorphosed by the gabbro. Their alteration has been throughout under surface conditions, where abundance of oxygen and carbon dioxide makes possible the complete oxidation of the iron and the removal of silica on a large enough scale to cause the concentration of the ore bodies. During the development of the iron-ore deposits erosion has continuously cut down the iron formation, and, because of the gentle dip of the strata, this truncation has been accompanied by the downward and lateral migration of the ore deposits. The deposits in their present position may be supposed to represent simply a stage in the process of concentration and migration. Glacial erosion has also cut down the ore deposits to a considerable extent. The phosphorus in the ores is shown to be a concentration and not a residual product. The original greenalite rocks contain little or no phosphorus, while their altered equivalents, the ores and cherts, uniformly show small contents of phos- phorus. The unaltered iron-formation slates also contain a lower percentage of phosphorus than their altered equivalents, the paint rocks. The points of similarity and difference between the Mesabi ores and those of other Lake Superior ranges are briefly summarized. Previous explanations of the origin of the ore are outlined and their relations to the present explanation shown. Chapter X is devoted to economic features of interest, such as mining, trans- portation, production, reserve, ownership, prices, etc. The open-pit steam-shovel method of mining, characteristic of the district, is illustrated. In Chapter XI exploration for ore is discussed and an attempt is made to give the criteria for locating explorations which the geologic structure and manner of development of the ore would seem to warrant. THE MESABI IRON-BEARING DISTRICT, MINNESOTA. By Chakles Kenneth Leitij. CHAPTER I. INTRODUCTION. The following mouograph tells of the geologic and economic features and of the sudden and gigantic development of the Mesabi iron-bearing district of Minnesota, the sixth, last, and greatest of the iton ranges to be discovered in the United States portion of the Lake Superior region. The writer's work upon the district has been done under the super- vision and advice of Prof C. R. Van Hise, geologist in charge. To him is due in large measure any credit this report may deserve. Indeed, the work has been largely the application in this district of jjrinciples and methods of work developed by him. Field work in the Mesabi district was begun in the spring of 1900 and continued during the summers of 1900 and 1901. In the year 1901 M. H. Newman served as field assistant. Near the close of the field season of 1900 Professor Van Hise spent some time in the district Immediately thereafter he and the writer prepared a brief preliminary report on the district, accompanied by a geologic map of the central portion, which was published in the Twenty-first Annual Report of the United States Geo- logical Sui'vey, as a part of a general paper on Lake Superior iron-ore deposits, by Professor Van Hise. At the close of the field season of 1901 the preparation of the final report and maps presented herewith was taken up. The topography of the Mesabi district shown on the accompanying maps was sketched during the summers of 1899 and 1900 by a party of the United States Geological Survey consisting of E. C. Bebb, D. L. Fairchild, 19 20 THE MESABI IRON-BEARING DISTRICT. Louis B. Weed, aud assistants, in charge of Mr. Bebb. Much information concerning section, town, and range lines and the subdivisions of sections has subsequently been furnished by Mr. D. L. Fairchild, who has had charge of parties engaged in locating the boundaries of properties of the Minnesota Iron Company. This information has applied particularly to the area between Eveleth and Mesaba station. Since the work was beg'un in the Mesabi district the Survey has had access to an elaborate set of maps of the iron-beai'iug series, containing records of practically all of the exploration work done on the range, pre- pared by J. U. Sebenius, under the direction of W. J. Olcott, for the Lake Superior Consolidated Iron Mines, now a part of the United States Steel Corporation. These have been kept up to date by Mr. Sebenius for the United States Steel Corporation. Exploration of the iron formation in the Mesabi district is done largely by test pitting and drilling through the glacial drift which deeply covers the district, and if the records of test pits and drill holes ai'e not collected and systematized at the time of the exploration the information is largely lost for purposes of mapping. It is apparent, therefore, that accurate mapping of the iron formation would have been quite impossible without access to such maps as those referred to. Many other mining men and explorers, indeed practically all interested in the ^lesabi district, have given the Survey information concerning their properties and have placed facilities for study at its disposal. The wi'iter finds himself quite unable to present an adequate list of names or to make a satisfactory selection of a few names for special mention. The mine photographs reproduced were furnished by W. J. Olcott, E. E. Sperry, Greorge Dormer, E. R. Buckley, and local professional photog- raphers. To all whose cooperation has aided in the preparation of this report the Sui'vey tenders thanks. GEOGRAPHY AND TOPOGRAPHY. The Mesabi iron district lies in the part of Minnesota which is north- west of Lake Superior. In shape and trend it is similar to the other iron districts of the Lake Superior region (see PL I). It extends from Grand Rapids, on the Mississippi River, in a direction ENE. to Birch Lake, a distance of approximately KM) miles, with a width varying from 2 GEOGRAPHY AND TOPOGRAPHY. 21 to 10 miles. Its area is about 400 square miles. Eastward from Birch Lake to G-unflint Lake and beyond are small patches of iron-formation material, and these areas have often been included in the Mesabi district, particularly by the early explorers. The main topogTaphic feature of the district is a ridge or "range" parallel to the longer direction of the district, known as the "Giants" or "Mesabi" range. Mesabi" (spelled also Mesaba and Missabe) is the Chippewa Indian name for "giant." In the west end of the district the Mesabi range merges insensibly into the level of the surrounding country, about 1,400 feet above sea level, or 800 feet above Lake Superior. Toward the east the elevation, with reference both to Lake Superior and to the surrounding country, increases; from range 18 to range 12 elevations of 1,800 and 1,900 feet above sea level, or 400 and 500 feet above the level of the surrounding country, are reached. For many miles both north and south of the range there is a comparatively low, flat area, and the Giants range, particularly its eastern portion, is a very conspicuous feature in the landscape. While the general trend of the range is ENE., there are many gentle bends in the crest line, and in range 17 a spur, known locally as the "Horn," projects in a southwesterly direction for 6 miles. The crest of the range is in places broad and flat, in others comparatively narrow and sharp. The southern slope is very gentle; the northern slope is somewhat less so. At frequent intervals both crest and slopes are notched by drainage channels. The Mesabi range, for the most part, forms a drainage divide, although it is crossed by drainage channels at several places. The drainage of the district is apportioned among three of the great river systems of the country — the Mississippi, St. Lawrence, and Nelson. In the western portion of the district, from Grand Rapids to within 3 miles of Hibbing, the southern slope is drained by the Mississippi River and its tributaries, the Prairie and the Swan. The Mississippi and the Swan cross the range. From 3 miles west of Hibbing to east of Iron Lake, near the east line of range 13, the district is drained to the south by the St. Louis River and its tributaries, the Swan, EmbaiTass, and Partridge The Embarrass River "The United States Board on Geographic Names has adopted this spelling. The term was origi- nally applied to the elevation made by the gabbro eastward from Allen Junction to Gunflint Lake and beyond, but has since been applied as above. 22 THE MESABI IRON-BEARING DISTRICT. crosses the range. The St. I^ouis empties into Lake Superior, and thence the waters of the system are contributed to the St. Lawrence. From the east hue of range 13 to east of Birch Lake the district is di-ained bv the Dunka River, which crosses the range and is tributary northward to a hike system which discharges through Nelson River into Hudson Bav. The northern slope of the Mesabi range is drained in pait bv tlie Mississippi, Prairie, and Embarrass rivers, flowing south, but aside from these the drainage of the north slope all goes northward into the lake system tributarv to Hudson Bay. One of the feeders of this system, the Pike River, reaches well down into the southward-projecting spur of the range between the towns of Eveleth, Vu'ginia, and McKinley, thus over- lapping the headwaters of the Embarrass. To anyone familiar with the Lake Superior region it is sufficient to say that the timber and soil of the Mesabi district are characteristic of the region. The forest includes the white pine, the yellow or Norway pine, tamarack, spruce, cedar, and balsam or balsam fir (jack pine). Scattered among them are hardwood trees, mainly poplar, birch, and maple. For the most of the district the forest is essentially coniferous, Init over small areas the hardwood trees predominate. Tamarack, cedar, and spruce swamps occupy considerable areas, particularly along the lower slopes of the rang-e. The exceedingly thick underbrush consists largely of hazel, maple, alder, ash, willow, cherry, and ground hemlock. Most of the pine has been cut. Old choppings, windfalls, fires, underbrush, and swamps have combined to make the scene a desolate one for much of the district, and to make the ti-aveling away from trails or roads most arduous. In the limited portions of the district where the original ])ines still stand, all the beauties of the Northern })ine forest at its best are to be observed. The district is heavily covered with glacial drift, consisting of sand, clay, and bowlders, the latter in some places so ^•ery numerous as to discourage attempts to clear the land for agricultural purposes. L^p to the present time practically no land has been cleared outside of town sites and mining locations. There are, however, considerable areas in which the soil would yield abundantly on cultivation. Along the Mesabi range are a number c>f mining towns, most of tliem marking mining centers; one oulj', Grand Rapids, on the west end of the GENERAL GEOLOGY. 23 range, owes its existence to lumber interests, and even this town is benefited by the exploration for ore in the western portion of the range. The towns are largely confined to the central portion of the district. Beginning at the Stevenson mine, in R. 21 W., there are towns at frequent intervals to Mesaba station, in range 14, and these intervals are likely to be further subdivided as the exploitation of the range proceeds. From Mesaba station to the east end of the range, and from the new town of Nashwauk to Grrand Rapids, near the west end of the range, a distance in each case of a little over 20 miles, there are no settlements. Three railways, all with terminals on Lake Siiperior, touch the range. The Duluth and Iron Range Railway crosses the district in R. 14 W., and sends out a branch to Biwabik, Stephenson, McKinley, Sparta, Eveleth, and Virginia. The Duluth, Missabe and Northern Railway approaches the range from the south through R. 18 W., and just before reaching the range sends out branches to Biwabik, Eveleth, Sparta, Virginia, Mountain Iron, and Hibbing. The Eastern Railway of Minnesota (the Great Northern) has three approaches to the range, one through R. 18 W., another through Hibbing, and a third through Grrand Rapids. Branches connect with Stevenson, Chisholm, Buhl, Mountain Iron, and Virginia. The only large parts of the range not immediately accessible by railway are those between the Hawkins mine and Grand Rapids, and between Mesaba station and Birch Lake. GENERAL GEOLOGY. The succession of formations in the Mesabi district appears in the following table: Succession of formations in 1 (esabi district. Pleistocene. (Unconformity.) Cretaceous. (Unconformity.) Keweenawan Great basal gabbro and granite, intrusive in all lower formations. (Unconformity.) Upper Huronian (Mesabi series) Virginia slate (upper slate formation). (Unconformity.) Biwabik formation (iron-bearing formation). Pokegama formation (quartzite and quartz-slate formation). Lower Huronian Granite, intrusive in lower formations. (Unconformity.) Slate-gray wacke-conglomerate formation (equivalent to the Ogishke and Knife Lake formations of the Vermilion district). Basement complex, or Archean Greenstones, including basalts, diorites, diabases, etc., hornblende- schists, and porphyritic granites and rhyolites. 24 THE MESABI IRON-BEARING DISTRICT. The core of the Giants range is made up principally of granite of Lower Hurouian and Keweenawan age, and subordinately of Archean igneous rocks. To the south of the igneous core, for a part of the district, are Lower Huronian sedimentary rocks, with bedding- approxi- mately vertical. Against the southern boundary of the Lower Huronian, or where the Lower Huronian is lacking, against the igneous core, lie the Upper Huronian sedimentary rocks. They dip gently to the south and underlie the greater portion of the southerly slopes of the range. On the southeast the Huronian rocks are limited by the Keweenawan gabbro, the north edge of wliich cuts across the Huronian formations diagonally from southwest to northeast. The Archean, Lower Huronian, and Upper Huronian series are separated from one another by unconformities. Glacial drift covers the district so thickly that rock exposures are rare on the lower slopes of the range, and only fairly numerous near the crest. CHAPTER II. BRIEF HISTORY OF THE DISTRICT AND SUMMARY OF LITERATURE CONCERNING IT. HISTORY A3>f D LITERATURE OF THE DISTRICT PRIOR TO ITS OPE]sri]srG. In penetrating- the vast wilderness north and west of the Great Lakes country, the early explorers were compelled for the most part to stick close to the waterways, for the nature of the country made travel for long distances exceedingly arduous by any other method than canoeing. Three of the canoe routes to the country northwest of Lake Superior cross the Mesabi range and its eastward continuation The Mississippi River and its tributaries, the Prairie and the Swan, touch the western portion of the district. Embarrass Lake, tributary to the St. Louis River and thence to Lake Superior and the St. Lawrence, crosses the Mesabi range near its east-central portion. Gunflint Lake, one of a chain of lakes tributary to Rainy River and Nelson River and thence to Hudson Bay, lies far to the east, on a continuation of what is now known as the Mesabi range. Thus it is that the first references to the Mesabi district found in literature concern the parts of the district immediately adjacent to these canoe routes. Brief descriptions of Pokegama Falls on the Mississippi River and adjacent areas were made by Maj. Z. M. Pike in 1810, by Lieut. James Allen and Henry R. Schoolcraft in 1832, and by J. N. Nicollet in 1841. In 1841 also Nicollet published his map of the hydrographic basin of the Upper Mississippi, on which the Mesabi range, called "Missabay Heights," was for the first time delineated, by hachures, although very imperfectly. In 1852 J. Gr. Norwood reported the occurrence of iron-formation material at Gunflint Lake and mentioned granite and gneiss seen in crossing the range at Embarrass Lake. In 1866 Col. Charles Whittlesey reported on explorations made in northern Minnesota during the years 1848, 1859, and 1864. He mentioned Pokegama Falls and made vague reference to the granitic rocks of the range. "Mesabi range" was used in an indefinite way 26 THE MESABI IRON-BEARING DISTRICT. to cover what are now known as the Mesabi and VermiHon ranges. In 1866, also, Hemy H. Eames, the first State geologist of Minnesota, reported granite and gneiss seen on a trip across the range at Embai-rass Lake. In describing the ranges of the northern part of the State, including tlie " Missabi Wasju," he stated that they appear to be traversed by metal-bearing veins. Presumably, liowever, this statement refei's mainly to the Vermilion range. In a second report, published the same year, Mr. Eames is more explicit, and, referring to the general elevated area of the northern part of the State, including the Mesabi range, states: "In this region are found also immense bodies of the ores of iron, both magnetic and hematitic, occurring in dikes and . associated with the rock in which it is found; in some of these formations iron enters so largely into its composition as to affect the magnetic needle." Pokegama Falls and Prairie River Falls were visited, and at the latter place the presence of " iron ore" was noted. These reports of Eames contain the first references to iron ore in the Mesabi district proper, although iron formation had been noted by Norwood in 1862 at Gunflint Lake From this time on desultory exploration work was done in certain portions of the district. It was confined for the most part to the area west of Bii-ch Lake, in Rs. 12, 13, and 14 W., and to the vicinity of the Prairie River. No published accounts of the earlier portion of this explo- ratory work are to be found. The fij'st examination of the Mesabi range by a mining expert with particular reference to the occurrence of iron ore in workable deposits, noted in print, was made in 1875 by Prof. A. H. Chester, of Hamilton College, New York. Striking the Mesabi range at Embarrass Lake, he worked eastward toward Birch Lake. In his report (published in 1884) he called attention to the magnetic character of the iron in this area, and to the fact that the alternating iron layers are not thick or continuous. The percentage 44.G8 was given as a fair average of iron in the rocks of this part of the district. In general, one gathers the impression that he was not favorably impressed with the economic prospects of this area. Between the time of Professor Chester's examination of the range, in 1875, and the i)ublication of his report, in 1884, Prof N. H. Winchell, State geologist of Minnesota, bi-iefly noticed the Mesabi range in two of liis reports. In 1879 he told of the occurrence of iron ore in R. 14 W., and pul)lished analyses. In 1881 he told of a trip from OPENING AND DEVELOPMENT OF THE DISTRICT. 27 Embarrass Lake east to I'ange 14, and noted the magnetic character of the iron formation in range 14, as well as it^ similarity to the formation at Gmiflint Lake. Indeed, the iron formation in range 14 is called the "Gnn- flint beds." In 1883 Irving called the Mesabi iron-bearing rocks series "Animikie," a term which had been applied to similar rocks at Thunder Bay and westward to Gunflint Lake, and correlated the Animikie rocks with the "Original Huronian" rocks of the noi-th shore of Lake Huron and with the iron-bearing series of the Penokee-Gogebic iron range of Michigan and Wisconsin. From this time on the term "Animikie" is much used in the literature on the Mesabi range to designate the iron-bearing series. In 1 884, in the same report in which Chester's report was published, N. H. Winchell discussed the age of the Mesabi series of rocks, assigning them to the "Taconic," or Lower Cambrian, and, following Irving, correlated them with the iron-bearing rocks of the Penokee-Gogebic district. In the late eighties a numbei* of other reports on the district were issued by the Minnesota survey, but they contain no important points not noted in reports above cited. This brings us to the opening of the district for mining. OPENIJS^G AKD DEVEIjOPMEIS^T OF THE DISTRICT. Since the late sixties there had been more or less exploration, partic- ularly along the eastern portion of the district, from Embai'rass Lake to Birch Lake, and the presence of iron-formation material had been recognized and discussed in the reports above mentioned. However, not a single deposit of ii'on ore of such size and character as to warrant mining had been shown up. In fact, the range had been "turned down" by many mining men who had examined it. This was largely because of the fact that they confined their attention principally to the eastern, magnetic end of the range, where exposures of the iron formation are numerous. Even up to the present time no ore has been found there in quantity. Yet the impression was gradually developing that iron ore in large quantity was to be found in this district, and a few prospectors were working diligently. Among the more persistent of the Mesabi range explorers were the Merritts — Lon Merritt, Alfred Merritt, L. J. Merritt, C. C. Merritt, T. B. Merritt, A. R. Merritt, J. E. Merritt, and W. J. Merritt— of Duluth, Minn. Their faith in the range was the first to be rewarded. On November 16, 1890, one of their test pit crews, in charge of Capt. J. A. Nichols, of Duluth, 28 THE MESABI IRON-BEARING DISTRICT. struck iron ore in the NW. ^ sec. 3, T. 58 N., R. 18 W., just north of what is now known as the Mountain Iron mine. This was followed in 1891 by the discovery of ore in the area now covered by the Biwabik and Cincin- nati mines. John McCaskill, an explorer, observed iron ore clinging to the roots of an upturned tree on what is now the Biwabik property. This led to test pitting, and test pitting by the Merritts on the area of the Biwabik mine, under charge of "W. J. Merritt, led to the discovery of this mine in August, 1891. The Cincinnati mine was opened the same fall. The Hale, Kanawha, and Canton mines were shown up in the spring of 1892. The discovery of ore near what are now known as the towns of Virginia, Eveleth, McKinley, and Hibbiug followed in rapid succession. The excitement following the first discovery of ore at Mountain Iron was greatly augmented by each succeeding find, and in 1891 and 1892 there was the inevitable rush of explorers. Up to October, 1892, there were two railways touching the range, the Duluth and Iron Range, crossing the range at Mesaba station on its way to the Vermilion range, and the old Duluth and Winnipeg (now the Great Northern), reaching the range at Gi'and Rapids. Both of these places were far removed from the exploring centers. Most of the explorers went through Mesaba station. Reaching this place by rail, they were compelled to travel 12 to 50 miles to the west along "tote roads," which were all but impassable. The time, money, and energy needed to conduct even modest explorations at this time can be appreciated only by those who have experienced the difficulties of inland travel in the Lake Superior region away from railways. The stories of this "toting" period contain the usual records of misfortunes, lucky strikes, and enterprise incidental to a mining boom. The railways were not long in getting into the field. In October, 1892, two lines were put in operation. The Duluth, Missabe and Northern Railway was built to connect Mountain Iron mine with the old Duluth and Winnipeg Railway (now the Eastern Railway of Minnesota, a part of the Great Northern system) at Stony Brook Junction, and later was extended to Duluth. Almost immediately after the connection with Mountain Iron a branch was sent out to Biwabik. About the same time the Duluth and Iron Range Railway sent out a branch from its main line to the group of mines at Biwabik. Very soon thereafter both railways got into Virginia. 0) 13 "e-s o Q Z < LU CO o Q < o ^ CO DEVELOPMENT OF THE DISTRICT. 29 Hibbing was reached by the Duluth, Missabe and Northern m 1893. Eveleth was reached by the Duluth and Iron Range in 1894, and by the Duluth, Missabe and Northern ver}:^ soon thereafter. The Mississippi and Northern (Eastern Railway of Minnesota) about the same time projected a spur from Swan River to the Hibbing district. With the advent of railways the development of the range went on by leaps and bounds. This marvelous development has continued to the present time. The only considerable check occurred during the period of general financial depression which the country underwent in 1894, 1895, and 1896. Almost an untouched wilderness in 1890, the district is to-day the greatest producer of iron ore in the world. The rapidity of the development of the mining industry' of the district, carrying with it all the prosperity of the range, can not be better told than by the following table of shipments from the district: Shipments from the Mesabi district." Gross tons. 1892 : . . . 4, 245 1893 613,620 1894 1,793,052 1895 2,781,587 1896 2, 882, 079 1897 4,275,809 1898 4,613,766 1899 : 6,626,384 1900 7,809,535 1901 9,004,890 1902 13,329,953 Total 53,734,920 LITEEATUKE ON THE DISTRICT SUBSEQUENT TO ITS OPENING. With such a phenomenal development, it is but natural that the litera- ture on the range, published since its opening, should be voluminous. The most important reports have been issued by the Minnesota State survey. In 1891 Messrs. N. H. and H. V. Winchell published jointly a general discussion of the iron ores of Minnesota. This was written for the most part prior to, the actual discovery of ore in quantity in the Mesabi district, but it contained also a brief notice of the Merritts' discovery of iron ore near Mountain Iron in 1890. A general account of the structural relations oOn pp. 287-289 may be found a full table of shipments from individual mines. All figures are taken from the Iron Trade Beview, except for 1902. 30 THE MESABI IRON-BEARING DISTRICT. of the ores of the Mesabi range was given, and a comparison with Vermilion ores. The iron formation was described and mapped as extending from Pigeon River to the Mississippi River on tlie west. Detailed descriptions were given of the principal explorations made up to this time, and the report was accompanied by a map showing location of iron indications in the district. A pi-ediction was made that the Mesabi range would be found to contain vast quantities of iron ore. The first report on the Mesabi district which was written after the discovery of ore — written largely because of the discovery of ore — was published in 1893 by H. V. Wiuchell. This contained a map of the part of the range then pi-oductive, and a general account of the history and geology of the range. The mining developments up to that time were fully described. In 1892 Van Hise correlated the Auimikie series with the Upper Huronian division of the Algonkian. In 1894 J. E. Spurr issued a" bulletin containing a map of the range and a full discussion of the geology, and maintained that the ores were developed from green granules which he called glauconite. This was the first serious attempt to determine the origin of the ores. His conclusion that the ores are mainly derived from green granules is confirmed by the work reported in the following monograph, but his determination of the green granules as glauconite is proved to be erroneous. In 1899 was issued Volume IV of the Final Report of the Minnesota survey, containing, besides a general map of the Mesabi range and its eastern extension to Gunflint Lake, a number of detailed maps, each of them accompanied by a description of the geology, by N. H. Wiuchell or U. S. Grant. This volume was followed in 1900 by a volume (V) containing a general discussion of the stratigraphic relations of the rocks of northern Minnesota, including the Mesabi rocks, by N. H. Winchell. In 1901 there appeared Volume VI of the same survey, containing practically all the maps published in previous volumes of the survey, and one new one, a general geological map of the State, accompanied by synoptical descriptions. The United States Geological Survey began field work in preparation for this monograph on July 1, 1900. While rather voluminous reports on the district above referred to had already appeared, these were difterentiu scope and execution from the United States Geological Survey series of reports on the iron ranges, and, moreover, they contained interpretation.s of tlie geology of the area with which the United States Geological Survev SUMMARIES OF LITERATURE. 31 was not in accord. It was tlierefore decided to map and report on the district. In 1901, in a paper on the iron-ore deposits of the Lake Superior reg'ion, by C. R. Van Hise, there was included a preliminary report on the geology of the Mesabi district, by C. R. Van Hise and C. K. Leith, accom- panied by a map of part of the district by C. K. Leith. This preliminary report contains many of the essential features of the following monograph. In addition to the above reports there have appeared a considerable number of articles concerning the economic features of the district, some of which are listed on pages 61 and 62. STJMMARrES OF LITERATUKE, ARRANGED CHROKOLOGICAliLT. In the following summaries of literature on the Mesabi range no refer- ence is made to a number of geological reports containing general discus- sions or incidental references to the general geology of northern Minnesota. Neither is any attempt made to summarize all the articles on the economic features of the Mesabi district which have appeared in the publications of engineering a,nd mining societies and trade journals the world over. The titles of some of these articles appear on pages 61 and 62. Only such reports are summarized as mark the historical development of knowledge of the range. The reader is likely to have difficulty in understanding some of the geological names used in reports summarized. A variety of terms have been used for the same formations, and the same term has occasionally been used with different meaning by different writers. The meaning of the names is discussed in the section on Correlation (pp. 200-205). isio. Pike, Maj. Z. M. An account of expeditions to the sources of the Mississippi and through the western part of Louisiana, performed by order of the Government of the United States during the years ISOo, 1806, and 1807. Philadelphia, 1810. A brief mention of Pokegaraa Falls is here made. 1833. Allen, Lieut. James. Report of Schoolcraft's expedition of 1832 to the sources of the Mississippi River. American State Papers, Vol. V, Military Affairs, page 330. 1833. 32 THE MESABI IRON-BEARING DISTRICT. Lieutenant Allen briefly describes Pokegama Falls, and states tliat the river breaks through a low ridge which trends northeast-southwest. This is the first mention of the ridge which was later known as the Mesabi. 1841. Nicollet, J. N. Report intended to illustrate a map of the hydrographical basin of the Upper Mississippi River, made while in the emploj' of the Bureau of the Corps of Topographical Engineers. Twenty-sixth Congress, 2d session, 1841, Senate Document No. 237, series No. 380, p. 63. The rocks at Pokegama Falls are briefly described and Missabay Heights are indicated on the map for the first time by Hachures. issa. V Norwood, J. G. Geology of the northwestern and western portion of the valley of Lake Superior. Published in the report of a geological suiwey of Wisconsin, Iowa, and Minnesota, made under instructions from the United States Treasurj' Department, by David Dale Owen, Philadelphia, 1852, pages 213-418. Dr. Norwood crossed the Mesabi range in two places — at Grunflint Lake and at Embarrass Lake. He reports the occuiTonce of iron-formation material on Grunflint Lake. Groing north across the Saganaga granite, he makes the statement that this granite range, if continued in a southwesterly direction, would pass in the direction of the Missapi Wachu and Pokegama Falls on the Mississippi (p. 417). In going up the north shore of Embarrass Lake he crossed the Mesabi range and observed syenitic granite associated with gneiss. 1855. ScHOOLCKAFT, Henry R. Summary narrative of an expedition to the sources of the Mississippi in 1820, resumed and completed upon the discovery of its origin in Itasca Lake in 1832, with appendices. This contains a brief mention of Pokegama Falls. 1866. Eames, Henry H. The metalliferous region bordering on Lake Superior. First Report of the State Geologist of Minnesota, St. Paul, 1866, pages 8 and 9. In 1865 Eames crossed the "Missabi Wasju" at Embarrass Lake, and in his report mentions the granite there seen. Eames, Henry H. Geological reconnoissance of the northern, middle, and other counties of Minnesota. Second Report of the State Geologist of Minnesota, St. Paul, 1866. pages 1-58. SUMMARIES OF LITERATURE. 33 Eames in 1866, referring to the "gigantic uplifts" in the northern part of the State, which reach their greatest altitude " at or near Missabe Heights," states: "In this region are found also immense bodies of the ores of iron, both magnetic and hematitic, occurring in dikes and asso- ciated with the rock in which it is foiind. In some of these formations iron enters so largely into its composition as to affect the magnetic needle, both in its horizontal deflection and vertical dip" (p. 18). In a geological reconnaissance Pokegama Falls and the falls of Prairie River were visited. The rock at Pokegaina Falls is referred to the Pots- dam. The lower or first falls of Prairie River were referred to as " an uplift of igneous and metamorphosed rocks, consisting of granite, coarse and fine, quartzite or Potsdam sandstone, and iron ore," which occurred in the following order in ascending the river : 1. Fine-grained quartzose granite. 2. Iron ore. 3. Quartzite (Potsdam sandstone). 4: Fine granite. 6. Primitive schistose rock. 6. Argillaceous slate. Passing the first falls near the upper end of the lake on the south side, an uplift is seen 50 feet high, showing the same succession of strata as above given, beginning with No. 4 and ending with No. 1. Nos. 5 and 6 are not seen here. The upper falls of the Prairie River cut through an uplift of granite rocks. Whittlesey, Col. Charles. A report of explorations in the mineral region of Minnesota during the years ISiS, 1859, and 1864. Cleveland, 1866. Colonel Whittlesey gives a vague account of the northern portion of Minnesota, using the term "Mesabi" range to cover the general elevated area in the northern part of the State, including the Vermilion range. The region is referred to as " an imperfectly defined region of granite, syenite, mica slate, siliceous, and talcose rocks extending to and across the national boundary." The quartzite of Pokegama Falls is referred to the Potsdam sandstone. MON XLIII — 0.3 -3 34 THE MESABI IRON-BEARING DISTRICT. 1879. AViNCHELL, N. H. Sketch of the work of the season of 1878. Geological and Natural Histor}' Survey of Minnesota, Seventh Annual Report, 1879, pages 9-25. Professor Wiuchell here tells of the occuiTence of iron ore near Grun- flint Lake and in Ts. 59 and 60 N., R. 14 W., and publishes two analyses of ore from range 14 (pp. 22, 23). The " Mesabi Heights" are referred to as due to drift and to hard quartzite. 1881. WiNCHELL, N. H. Geological and Natural History Survey of Minnesota, Ninth Annual Report, 1881, pages 106-109. Professor "Winchell writes of a trip down the Pike River, across the portage to Embarrass River, down the Embarrass to the dam, thence east to range 14, where pits in iron-formation material (called Gunflint beds) are reported in sections 14, 15, and 28, and other places. The magnetic character of the formation is noted. 1883. Irving, R. D. The copper-bearing rocks of Lake Superior. Mon. U. S. Geol. Survey Vol. V, 1883, pages 382-384. In his famous monograph on the copper-bearing rocks of Lake Superior, Irving correlates the Mesabi iron-bearing rocks as far west as Pokegama Falls with the slates and associated iron-bearing rocks at Gunflint Lake and thence eastward to Thunder Bay, and accordingly calls them Animikie. In 1873 Hunt had suggested the name Animikie (Indian for Thunder Bay) for the series at Thunder Bay, and the continuity of this series westward to Gunflint Lake had been established by Bell" and Winchell." At the same time Irving correlates the Animikie with the original Huronian of the north shore of Lake Huron and with the iron-bearing series of the Penokee-Gogebic district: This correlation has since been accepted by Van Hise and other United States geologists, and it is the one which is used in the following report. From this time on the Mesabi iron-bearing series (the Upper Huronian of the following repoi-t) is frequently referred to in literature as the Animikie «Geol Surv. of Canada for 1872-7.S, by Robert Bell, pp. 92-94. 6 Ninth Ann. Kept. Geol. Nut. Hist. Survey Minnesota, by N. H. Winchell, 1881, p. 76. SUMMARIES OF LITERATURE. 35 series, altlioug'li this term is given different age significance by different writers. The petrography of the gabbro of northeastern Minnesota is fully described. 1884. AViNCHELL, N. H. The geology of Minnesota. Geological and Natural His- tory Sui'vev of Minnesota, Final Report, Vol. I, I88i. The first volume of the Final Report of the Minnesota survey contains a very interesting account of the early explorations in Minnesota. The development of knowledge of the geography and geology of the State is sketched. Slight reference is made to the Mesabi district. Following the historical sketch is a description of the general physical features of the State. The discussion of the drainage systems and divides involves the discussion of the topogi'aphy of the Mesabi district. Timber, soils, glacial diift, and lakes are also discussed. Chester, A. H. The iron region of northern Minnesota. Geological and Natural History Survey of Minnesota, Eleventh Annual Report, ISS-i, pages loi-167. Professor Chester, of Hamilton College, in 1894, published an account of an examination of the eastern end of the Mesabi range, made in 1875 for private parties. This was the first examination of the range by a mining expert noted in literature. Coming from Duluth on the south, he struck the Mesabi range at the portage of the Embarrass, and worked northeast along the range to T. 60 N., R. 12 W. A large number of outcrops and pits were examined. The siliceous bands associated with the iron bands in the iron formation are called quartzite. Attention is called to the magnetic character of the iron ore and to the fact that the alternating iron layers are not thick or continuous. From the samples of iron ore brought back from his trip a large number of analyses were made; 44.68 is given as a fair average of the percentage of iron in the part of the district covered. This, of course, applies to iron above the level of ground water, as this is the depth to which most of the pits were sunk. Professor Chester expressed the opinion, that the iron-bearing rocks of the Mesabi district bear the same relation to the Huronian rocks as do the rocks of the Penokee iron range in Wisconsin. To the north of the magnetic schists constituting the iron formation is 36 THE MESABI IRON-BEARING DISTRICT. a red or pink granite forming the backbone of the Mesabi range. To the north of this ridge the rock strata are much more inclined, and consist of similar slates and quartzite, but without magnetite. The general trend is east and west, following the trend of the ridge. The second belt, the red granite, identical in appearance with that of the Mesabi backbone, is exposed on the long portage between Embarrass and Pike rivers in the southwestern jiart of T. 60 N., R. 15 W. In general the reader gathers the impression from Professor Chester's report that he is not favorablj' impressed with the value of the iron ores from the part of the district ^"isited by him. WiNCHELL, N. H. Geological and Natural History Survey of Minnesota, Elev- enth Annual Report, 1884, pages 168-170. Professor Winchell discusses the age of the Mesabi rocks. He classes them, with the Vermilion iron-bearing rocks, as Taconic, or Lower Cam- brian, equivalent in part to the Huronian of Michigan and Wisconsin. 1885. Winchell, N. H. Geological and Natural History Survey of Minnesota, Thir- teenth Annual Rei^ort, 1885, pages 10-24. Professor Winchell here describes the rocks of the Mesabi range seen in a trip across the range along the Dnluth and Iron Range Railway. Gabbro is crossed at Okwanim (Allen Junction). Two miles north of here is a cut in soft reddish iron-formation material. Four miles north of Okwanim is gray granite or syenite, forming the Giants rang-e. This is about a mile wide. Two miles to the north is red granite (Emban-ass). A cross section shows Huronian, Animikie, and Gabbro rocks lying with structural conformity with one another on the syenite of the Giants range, to the north of which is shown Huronian conglomerate and greenstone, with a dip to the north of about the same degree as the dip of the strata south of the range. In a general account of the crystalline rocks in the same report Professor Winchell describes the rock succession in the Mesabi and Vermilion ranges as follows: (1) At the top are slates and quartzites, with beds of diorite (the Ani- mikie group). These contain the jMesabi iron ores. (2) Soft greenish slaty schists, with lenticular masses of gneiss and diorite. These contain Vermilion iron ores near the bottom. SUMMARIES OF LITERATURE. 37 (3) Conglomeratic and quartzitic slates, qnartzites, and marble; per- haps contain the Vermilion iron ores. The Mesabi ores are probably the equivalent of the Commonwealth ores of Wisconsin, with no known equivalent in Michigan. 18S6. Willis, Bailey. Report of a trip on the Upper Mississippi and to Vermilion Lake, Minnesota. Tenth Census of the United States, Vol. XV, 1886, pages 457-i61. Mr. Willis briefly describes the rocks at Pokegama Falls and at the two falls of the Prairie River, and accompanies the description with a sketch map of both places. Irving, Roland D. Origin of the ferruginous schists and iron ores of the Lake Superior region. Am. Jour. Sci., 3d series, Vol. XXXII, 1886, pages 255-272. Irving concludes that the original form of the iron-bearing rocks of the Lake Superior region was iron carbonate, and that the iron ores and associated rocks of the iron formation have resulted from the alteration of this rock by percolating waters. With reference to the Gunflint district of the Animikie ores, the statement is made that a study of slides shows "com- plete gradations from the unaltered carbonates to cherty and jaspery mate- rials and even to actinolitic and magnetite schists " (p. 262). No direct reference is made to the ores of the Mesabi district. 18S7. WiNCHELL, N. H. Geological and Natural History Survey of Minnesota, Fifteenth Annual Report, 1887, pages 209-399. In this report Winchell mentions the occurrence of nontitaniferous magnetite in T. 63, R. 11 W., and Ts. 59 and 60 N., R. 14 W., and states that it is comparable to the "iron ore found at Black River Falls, in Wiscon- sin, and at the western end of the Penokee-Grogebic iron range on the south side of Lake Superior" (p. 216). 1S8S. Winchell, H. V. Geological and Natural History Survey of Minnesota, Sixteenth Annual Report, 1888, pages 438^-10. Here is a brief description of Pokegama and Prairie falls. It is said that the Pokegama were formerly much higher and have been worn down in the last twenty years. The Indians call them "Kakabikag" (rocky falls). They sometimes add a diminutive, and call them the "Little Rocky Falls." 38 THE MESABl IKON-BEARING DISTRICT. 1S89. WiNCHELL, H. V. Report of field observations made during^ the season of 1888 in the iron reg'ion of Minnesota. Geological and Natural History Survey of Minne- sota, Seventeenth Annual Report, 18S9, pages 77-145. H. V. Winchell here reports on a trip from Birch Lake southwest along the Dunka River, alouo- the Mesabi range to the Duluth and Iron Range Railway, and back again to Birch Lake. Descriptions of uumei'ous outcrops of the iron formation, Giants range syenite, and the gabbro are given. The magnetic character of the iron is emphasized. The contact of iron formation and syenite north of Iron Lake in the SE. J of NE. ^ sec. 35, T. 61 N., R. 12 W., is described. The iron formation rests with normal erosion unconformity upon the syenite. At the end of the season a visit was made to Pokegama Falls and Prairie River Falls, at the west end of the Mesabi range, but no new points were noted. Winchell, N. H. Geological and Natural History" Survey of Minnesota, Seventeenth Annual Report, 1889, pages 5-74. See also the Aniinikie black slates and quartzites and the Ogishki conglomerate of Minnesota, the equivalent of the "Original Hurouian." Am. Geol., Vol. I, pages ll-ll. Methods of stratigraphy in studying the Huronian. Ibid.. Vol. IV.. pages 343-357. The nontitaniferous iron ores of the Mesabi range are classed with the Taconic and placed as the equivalents of the Hui'onian of the Marquette district, the Penokee-Gogebic district, the Black River Falls schists, and the Black Hills quartzites. X890. Winchell, N. H., and Winchell, H. V. The Taconic iron ores of Minnesota and of western New England. Am. Geol., Vol. VI, 1890. pages 263-274. The iron ores of Minnesota are at five different geolog'ical horizons, in descending order, as follows: (1) The hematites and limonites of the Mesabi range, the equivalents of the hematites of the Penokee-Gogebic range in Wisconsin; (2) the gabbro titaniferous magnetites near the bottom of the rocks of the Mesabi range; (3) 01i^'initic magnetites, just below the gabbro in the basal portion of the Mesabi rocks; (4) the hematites and magnetites of the Vermilion range in the Keewatin formation; (5) the magnetites of the crystalline schists of the Vei'milion formation. It is maintained that SUMMARIES OF LITERATURE. 39 the upper iron deposits of the Mesabi and those of the Penokee-Gogebic are the equivalents of the Taconic ores of western New England. In the fall of this year occurred the first discovery of merchantable ore in the Mesabi district. The first shipment was made in 1892. 1891. WiNCHELL, N. H. Record of field observations in 1S88. Geological and Natural History Surve}^ of Minnesota, Eighteenth Annual Report, 1891, pages 7-59. This report was written before the discovery of ore in the district. Professor Winchell briefly describes the explorations of John Mailman 2 miles south of Hinsdale, near the Duluth and Iron Range track, calls attention to the similarity of the iron foraiation here with that at Grunflint Lake, and suggests that there is little likelihood of ore being found here similar to that at Tower. There is given also a brief account of a study of Pokegama Falls and of the country to the eastward for 1 6 miles to Grriffin's camp (the Diamond mine), accompanied by a sketch map of this area. The similarity of the iron formation to that at the Mailman camp, at Gunflint Lake, and on the Penokee range, and the difi'erence between this iron formation a".d that on the Vermilion range, are emphasized. The position of the Pewabic quartzite is left uncertain. It is consid- ered, however, to overlie the Animikie black slate, unless there are two great quartzites. This quartzite has heretofore been made the parallel of the great quartzite that overlies the Animikie unconformably, but it is possible that it runs below it conformably. The great gabbro of the Cupriferous formation is regarded as lying below the Animikie, among other reasons, because it lies next to and immediately south of the gneiss of the Giant range without the appearance of any black slate between them, and because bowlders of characteristic gabbro, red syenite, and quartz-porphyry occur abundantly in the later traps of the Cupriferous. Winchell, N. H., and Winchell, H. V. Iron ores of Minnesota. Geological and Natural History Survey of Minnesota, Bulletin No. 6, 1891, pages 430. With a geological map, 26 figures, and il: plates. This report was in part written before the discovery of ore in quantity in the Mesabi district, but was not published until afterwards. A notice of the discovery of ore is included. 40 THE MESABI IRON-BEARING DISTRICT. The report contains a general account of the structural relations of the iron ores of the Mesabi and Vermilion ranges. The Mesabi iron ores occur in the Taconic or Huronian, consisting chiefly of carbonaceous and argillaceous slates, but often there are siliceous slates, fine-grained quartzites, and gray limestones. Near the bottom of the formation is a fragmental quartz sandstone having an apparent thickness of 300 feet, which has been called the Pewabic quartzite. All of these fragmentals are intermingled with eruptive material. Near Gunflint Lake carbonates about 20 feet in thickness occur near the bottom of the Taconic. The authors say: The Taconic formation embraces a variety of ores — nontitanic magnetites at the bottom, jaspilitic hematites next above, soft hematites and titanic magnetites. These are found to constitute a well-marked belt extending from Pigeon River westward to the Mississippi River, although the titanic magnetites seem to diverge from this course and to run below the St. Louis River a few miles west from Duluth. Except the titanic magnetite of the gabbro, which is a primary constituent of the rock and is of eruptive origin, all the ores of the Taconic seem to be of chemical origin, and all, except those referable to concentration from oxidized carbonates, are due to chemical precipitation, as hydrated sesquioxides in the Taconic ocean under circumstances identical with those of the precipitation of the Keewatin hematites. On the accompanying geological map the Laurentian, Keewatin, Pewabic quartzite, and Animikie strata are differentiated and h'on indica- tions are marked in red. Comparison of this map with that published in connection with Volume IV of the Minnesota survey (see pp. 50-52) shows it to be very crude, but it was far in advance of any previous map. The Mesabi succession is also indicated in a general cross section of northeastern Minnesota. Descriptions are given of explorations on the Mesabi range, including the Stone mine at Mesaba (Mailman's original workings), the Mailman mine proper, in sec. 11, T. 59 N., R. 14 W., and the Diamond mine, in sec. 15, T. 56 N., R. 24 W.,and the discovery of ore at the Mountain Iron mine by the Merritt brothers is chronicled. One of the most interesting features of this report is a prediction as to the future of the Mesabi range. "The Mesabi ores are destined to play a very important part in the future development of the iron industry of the State" (p. 112). Later in the same report, just after intimation had been received of the first discovery of ore on the range, it is said, "There can be no reasonable doubt * * * there will yet be mined in the Mesabi SUMMARIES OF LITERATURE. 41 range even greater quantities of hematite than have been taken from that marvel of mining districts, the Penokee-Gogebic range * * *" (p. 160). 1892. Van Hise, C. R. Correlation Pajjers — Archean and Algonkian. Bull. U. S. Geol. Survey No. 86, 1892. In his correlation bulletin Van Hise refers the Animikie series of the Canadian boundary (the eastward continuation of the Upper Mesabi series) to the Upper Huronion, a part of. the Algonkian system, and correlates it with the Animikie and the Keewatin series of western Ontario, the Upper Vermilion series of the Vermilion district of Minnesota, the Upper Mar- quette series of Michigan, the western Menominee series, the upper series of the Penokee-Gogebic district, the Chippewa, and Baraboo, Minnesota, and Dakota quartzites. 1893. WiNCHELL, H. V. The Mesabi iron range. Geological and Natural History Survey of Minnesota, Twentieth Annual Report, 1893, pages 111-180. See also Trans. Am. Inst. Min. Eng., Vol. XXI, 1893, pages 611-686. Mr. H. V. Winchell here gives the most comprehensive discussion of the Mesabi iron range yet published, particularly in its western portion. It is also the first report written subsequent to the discovery of ore on the range. . The succession of the Mesabi in descending order is : 1. Gabbro unconf ormably on all the following Taconic 2. Black slates, Animikie _ Taconic 3. Greenish siliceous slates and cherts Taconic 4. Iron ore and taconyte horizon Taconic 5. Quartzite unconformable on 6 and 7 Taconic 6. Green schists of the Keewatin Archean 7. Granite or syenite of the Giants range : . .Archean The granite of the Giants range is bounded on the north by a belt of crystalline mica-schists and hornblende-schists, and on the south seems to have a direct transition into the green schists of the Keewatin. The green schist has a nearly vertical cleavage. The schists do not always follow the course of the granite range. They are unconformably covered in many places by the quartzite. The quartzite never has a high dip. Near the 42 THE MESABI IRON-BEARING DISTRICT. base It contains pebbles of quartz and granite, as well as jasper and gi-een- stone. This quartzite is correlated with the Pewabic quartzite of Gun- flint Lake, the Pokegama quartzite of the Mississippi River, that of Sioux Falls, S. Dak., and that of Baraboo, Wis. Conformable with the quartzite is the iron ore and taconyte horizon, the strata of which are siliceous and calcareous, and are banded with oxide of iron in beds of variable length and thickness. The ore is sometimes magnetite and sometimes hematite. To the banded jaspery chert associated with the ore the term taconyte is applied. The greenish siliceous slates or cherts constitute a transition stage between the rocks of the iron horizon and the black slates. There is also a considerable mixture of greenish material, apparently of eruptive origin. The greater part of the rock is a red, yellow, black, white, or green chert, sometimes having a thickness of 200 or 300 feet. It often has a peculiar brecciated appearance, having been shattered into angular fragments, and recemented by the same amorphous silica. The same fracturing is also visible in the iron ore. The siliceous slates and cherts pass upward into a carbonaceous argillite of great thickness, having a dip varying from the horizontal to 20 degrees to the south or southwest. Locally the dip is as high as 45 degrees, in which case the ore deposits lie close to the green schists. The gabbro flow is over all of the previous strata. The effect of the heat on the molten gabbro was to make the iron ore which ah-eady existed in the rocks hard and magnetic, although the magnetite in the rocks westward from Mountain Iron mine was probably too far from the gabbro to have developed in this way. There is good reason to believe that the iron ore deposits in their present condition have been principally formed since the gabbro overflow. The ore deposits occur as regular beds, which lie in almost their original positions, usually ha^dng a dip of less than 30 degrees and passing into the jaspery quartzite or taconyte in three directions, and occasionally on all sides. The theory of Irving as to the origin of the Grogebic ores is partially adopted. The quartzite is impervious to surface infiltration. The ore is regarded as produced by chemical replacement of some mineral, chiefly silica, by oxide of iron. As evidence of this, all stages of the process may be seen. Iron carbonate is found in the Mesabi rocks, but it does not appear in sufficient quautit}- to permit th-e assumption that the source of the ore was originally a carbonate. The solvent for the silica was ])robably carbon dioxide, and its source may have been the atmosphere, the l)lack slates, recently decaying vegetation, or the ore deposits higher SUMMARIES OF LITERATURE. 43 up the slojie. The sihca removed from the location of the- iron ores has been added to the grains of quartz in the quartzite, has been deposited as chalcedonic and flinty sihca, and has been deposited in cracks and fissures in the slate, which lies at a lower elevation, but stratigraphically above the ore. The source of the iron is believed to have been chemical and mechanical oceanic deposits, which have simply concentrated in the present situation, perhaps from rocks now completely removed by erosion. The water which brought in the iron ore to supply the place of the silica taken away in solution followed the natural drainage 'courses, surface and under- ground. The Giants range is regarded as having been uplifted at the time of the gabbro outflows, and to have been caused by them. Brief descriptions are given of the following mines : Biwabik, Cincinnati, Canton, Kanawha and Hale, Missabe Mountain, Ohio, Lake Superior, New England, Virginia, Paddock's, Lone Jack, Wyoming-, Security, Great Western, Rouchleau, McKinley, and others. The general economic features of Mesabi iron mining are discussed, such as method and cost of mining, quantity of ore, transportation, value to the State, etc. WiNCHELL, N. H. The crystalline rocks. Geological and Natural History Survej^ of Minnesota. Twentieth Annual Report, 1893, pages 1-28. Professor Winchell discusses tlie general age of the crystalline rocks of northeastern Minnesota. The Animikie series lie beneath the Keweenawan and above the Keewatin rocks. The Animikie and the Keweenawan together constitute the Taconic or Lower Cambrian. This series is characterized by a great quartzite associated with the iron ores and cherts. The quartzite (Pewabic) lies unconformably on all the older rocks. It often is conglomeratic, bearing ddbris of the underlying formations. Within it are mingled volcanic tuff's from contemporaneous eruptions. The Pewabic quartzite includes that of Pokegama Falls, on the Mississippi River, and of Pipestone County. In the vicinity of contemporaneous volcanic disturbances its grain is fine, like jaspilite, and sometimes it has acquired a dense crystalline structure from contact with the gabbro. Bayley, W. S. Actinolite-magnetite schists from the Mesabi iron range in northeastern Minnesota. Am. Jour. Sci., 3d sei'ies, Vol. XLVI, 1893, pages 176-180. 44 THE MESABI IRON-BEARING DISTRICT. The actinolite-magiietite-schists from the iron-bearing formation in the vicinity of Birch Lake are here described, and attention is called to their similarity to the actinolite-magnetite-schists of the Penokee-Gogebic district, described by Van Hise and Irving. WiNCHELL, N. H. Some problenas of the Mesabi iron ore. Geological and Natural History Sui'vey of Minnesota, Twenty -first Annual Report, 1893, pages 134-143. See also Am. Geol., Vol. X, 1892, pages 169-179. There is here given a general account of the rock succession and occuri'ence of iron ore in the Mesabi range and a discussion of the origin of the iron ore. It is concluded: First. The Mesabi ore is not satisfactorilj' explained by any theory that has ■yet been proposed for it, or for its equivalent (Gogebic) ore on the south side of the great lake. There are some facts that favor all of the theories that have been pro- posed, but they meet with opposing facts of greater import. Second. There is but one known cause acting with sufficient force, and on a geographic area sufficiently wide, to which we can appeal for the geographic and stratigraphic distribution of this ore — and that is oceanic sedimentation. That there has been a profound change in the sediments since their origination is quite evident; but whether this change took place, in whole or in part, prior to consolidation or after it is as yet unknown; and if after consolidation it is equally unknown whether it was accomplished in Taconic or in Recent time. There seems to have been some- thing peculiar either in the natui'e of the sediments of this horizon or in the intiu- ences to which they have been subjected, and this peculiarity is expressed on both sides of the Lake Superior basin. WiNCHELL, N. H. Field observations in 1892. Geological and Natural History Survey of Minnesota, Twenty-first Annual Report, 1893, pages 79-134. Here is a brief description of the following- mines: Hale, Cincinnati, Biwabik, McKinley, Missabe Mountain, Security, Virginia; the exploration of the Mesaba Syndicate Company in sec. 27, T. 60 N., R. 13 W. Mention is made of a green dike in the iron formation in the NW. \ of NE. \ sec. 32, T. 60 N., R. 13 W. The magnetic and siliceous nature of the ore in the eastern part of the range is again emphasized, and attention is called to the fact that tliis part of the range is not likely to be productive. 189-i. Elftman, A. H. Preliminary report of field work during 1893 in northeastern Minnesota. Geological and Natural History Survey of Minnesota, Twenty-second Annual Report, 1894, pages 159-169. SUMMARIES OF LITERATURE. 45 Mr. Elftman gives a detailed petrographical description of the actiuolite-magnetite-schists of the iron formation on Birch Lake and south- westward through T. 60 N., R. 12 W. The description is accompanied by a geological sketch map of the area. In approaching the gabbro contact, augite and olivine appear intimately associated with the actinolite and magnetite of the Animikie schists. The black slates, also, in the proximity of the gabbro, are changed to quartz-biotite-schists. The Pewabic quartzite at the bottom of the Animikie is not found east of Iron Lake. The black slates are not found east of the Dunka River. Outcrops of the Animikie below and inclosed by the gabbro between Birch Lake and Akeley Lake have the same lithological characters and composition as the actinolite-schists at Birch Lake, and are therefore included in the Animikie. The actinolite-magnetite -schists correspond very closely in petro- graphical character and origin to those in the Penokee-Gogebic series described by Van Hise and Ir^-ing. Spurr, J. E. Preliminar}' report of field work done in 1893. Geological and Natural History Survey of Minnesota, Twenty-second Annual Report, ISOi, images 115-124. Mr. SpuiT describes the general features of the Mesabi range. The Giants range granite (Laurentian) is found to be intrusive in the Keewatin schists. The Keewatin schists vary in origin, some being undoubtedly igneous and some detrital. Near the contact with the granite they have been metamorphosed into hornblende and mica-schists, which have hereto- fore been called Coutchiching or Vermilion, and assigned to a lower horizon than the Keewatin. The Animikie rocks lie unconformably ujjon the Keewatin schists. The succession and sti'ucture are represented in a north-south section across the Mesabi range, passing tlirough the town of Mountain Iron. Spttrr, J. E. The iron-bearing rocks of the Mesabi range in Minnesota. Geo- logical and Natural History Surve}' of Minnesota. Bulletin No. 10. 1894, pages 1-268. With 3 colored maps and 8 plates. See also Am. Geol., Vol. XIII, 1894, pp. 335-346. Mr. Spun- was the first to make a systematic study of the origin of the ores, and his conclusions, published in this report, are of much interest. 46 THE MESABl IRON-BEARING DISTRICT. The oldest formation of the district is the Keewatin, the most common rock of which is green schist, but associated with this, especially near the granites, are hornblende-schists and mica-schists. ■ The schists have a regional cleavage, which is nearly uniform in trend, about north 70 degrees east and nearly vertical. Next in age to the Keewatin schists is the hornblende-granite of the Giants range. This range has an average width of about 10 miles, and its direction is the same as that of the schistosity of the Keewatin rocks. The granite is intrusive in the schists, ^ as shown by numerous fragments embedded in it, by stringers of the granite in the schists, and by the metamorphism of the schists adjacent to the granite. Unconformably upon the Keewatin and granitic rocks is the Animikie series. The Animikie series has no marked folding, slaty cleavage, or schis- tose structure. The rocks of the series are in a gentle southern monocline, dipping perhaps 10 or 15 degrees east of south. This monocline has gentle vmdulations, with axes parallel to its dip, and in the Virginia area has been faulted. The amount of disturbance is greater adjacent to the central part of the district, where are found the Keweenawan rocks. It is probable that the weight of the Keweenawan rocks has produced a sinking in the area south of the Animikie, and that this has produced the tilting. The Anim- ikie series may be divided into three chief members — the Pewabic quartz- ite, the iron-bearing member, and the upper slates. The Pewabic quartzite is a fragmental rock, indurated by the enlargement of quartz grains. It occasionally passes into a fine-grained conglomerate. The iron-bearing member is composed of peculiar rocks, presenting no resemblance to the Pewabic quartzite or to the upper slate. The upper slates are of great thickness, and have at their base an impure limestone, often dolomitized or sideritized. The part of the iron-bearing member from Pokegama Falls to Embarass Lake is called the western Mesabi range, that from Embarass Lake to Gunflint Lake, the eastern Mesabi range, and from Gunflint Lake east, the international boundary area. The description of the iron-bearing member beloAv applies to the western part of the district. The ii'on formation has a thickness varying from 500 to 1,000 feet, with an average of about 800 feet. The dip varies from less than 10 to as much as 30 degrees, the aver- age being 10 degrees. The width of the formation varies correspondingly SUMMARIES OF LITEKATURE. 47 from 2 or 3 miles to less than half a mile, the average width being 1 mile. Resting upon the iron-bearing member is a great thickness of fine-grained slates, at the base of which is locally an impm-e dolomitic limestone. When this limestone is present the contact between the iron-bearing member and the upper sla.te can not be distinctly located. The least altered phase of the iron-bearing- member is a rock called taconyte, which consists of a background of cryptocrystalline, phenocrys- talline, and chalcedonic silica, in which are numerous granules. These are composed of glauconite, siderite, hematite, magnetite, limonite, and crypto- crystalline silica, in the very freshest phase, the two former being predomi- nant. One of these fresher phases showed, by analyses, about 35 per cent of siderite and 65 per cent of glauconite. In terms of percentage of the entire rock the glauconite contains the following bases: Per cent. Alumina (AlA) 1-35 Sesquioxide of iron (FejOj) 1. 96 Protoxide of iron (FeO) 6. 49 Lime (CaO) 63 Magnesia ( MgO ) 92 Water (H,0) 62 Soda (Na,b) 11 Potash (K,0) 10 Total 12.18 Since silica can not be separated from the free silica in the rock, its percentage is not known, but assigning the percentage of 50, which is the usual content of silica in glauconite, the composition of the green granules of the Mesabi iron formation, supposedly glauconite, is as follows: Per cent. Silica (SiO,) 50.00 Alumina ( AI2O3) 5. o-l Sesquioxide of iron (FejOs) S. 05 Protoxide of iron (FeO) - 26. 06 Lime (CaO) 2. .59 Magnesia (MgO) 3.78 Soda (Na.,0).... 45 Potash (K2O) .41 Water (H^O) 2.54 Total 99.92 In the freshest phase were seen, in thin section, probably detrital original grains of carbonate, recognized by their cleavage as calcite or 48 THE MESABI IRON-BEARING DISTRICT. dolomite. From the taconyte, by a complicated series of metasomatic changes, there have developed cherts and jaspers, which are sideritic, hematitic, magnetitic, or actiuolitic, or two or more of these combined. During the process the chert and iron oxides were largely concentrated in alternating bands. The cherts and jaspers are frequently concretionary and brecciated. They have often a prismatic jointing and horizontal parting. These transformations were caused hj downward-percolating waters, carrying as the chief agents oxygen and carbonic acid, and as subordinate agents sulphuric acid and alkalies. In the changes from glauconite and siderite to the oxides, there was an important shrinkage of the mass, and this has resulted in the brecciation, prismatic jointing, horizontal parting, and banding. The prismatic jointing is analogous in its formation to the shrinkage of basaltic columns of lava. The horizontal parting is caused by a later shrinkage along the least diameters of the columns formed hj the prismatic jointing. The banding is due to the removal of silica and the entrance of iron along the parting. The ore deposits rest upon the Pewabic quartzite, or upon the hard and little altered iron-bearing rock, in areas of especial weakness or dis- turbance, as (1) actual fault lines, (2) incipient fault lines, (3) apices of anticlinal folds and the troughs of synclines. These are places of fracture, and Avhere abundant waters were converged often form wide areas, and therefore places where large quantities of iron were supplied. The down- ward-percolating water, taking glauconite or iron carbonate in solution, pre- cipitated the iron as oxide in those places where there was an abundance of oxygen, and at the same time took the silica, in solution, thus forming the ore bodies. Between those of the largest size and the small local concen- trations there are all gradations. The larger deposits of ore occur where they are protected from glacial erosion on the north by a hard ridge of the Keewatin rocks, especially when the hard rocks give slight elevations on either side, so as to present a basin-like depression. The fflauconite in origin is believed to be the same as modern glauco- nites; that is, it has developed within foraminifera and other minute shells, as a result of a reaction between the organic matter within the shells and fine ferriferous clay As the formation contains only a small quantity of i)i-dinary fragmental quartz grains, it formed in water at a depth beyond SUMMARIES OF LITERATURE. 49 which much of these materials was deposited. As its upper horizon g-rades into Hmestone, this indicates a further subsidence of the area, so that the distance from the shore Hne became so great that very httle mechanical detritus was furnished, and the deposit was made up of calcareous matter. In the eastern Mesabi district the Animikie strata are pierced and intermingled with the northern border of the Keweenawan rocks, so that their normal attitude is often much disturbed. With this change the iron of the iron-bearing member becomes largely magnetic and the silica hard and crystalline. It is concluded that the iron before Keweenawan time was here in the state of sesquioxide, and that the heat of the igneous Keweenawan rocks and the disturbances of the Animikie series produced by them are the causes of the change of the sesquioxide of iron to its mag- netic form. Thus the normal process of decomposition and concentration was brought to a close, and this probably explains the poverty of this part of the district in large ore deposits. At the base of the Cretaceous are ferriferous detrital deposits derived from the Animikie. A study of these indicates that the metasomatic processes had gone far before Cretaceous time, although they have since continued to the present time. Upham, Waeren. Preliminarj' report of field work during^ 1893 in northeastern Minnesota, chiefly relating- to the glacial drift. Geological and Natural History Survey of Minnesota, Twenty-second Annnal Report, 1894, pages 18-66. Mr. Upham here gives a general discussion of the general geology of northern Minnesota. The area of the Mesabi range between Hibbing and the west end of the range is shown to be occupied by the Tenth or Itasca moraine; between Hibbing and Embarrass River, by moraiual material representing the merging of the Itasca moraine and the Mesabi or Eleventh moraine; eastward from Embarrass River to Birch Lake, by the Mesabi moraine. 1893-1895. Baylet, W. S. The basic massive rocks of the Lake Superior region. Jour. GeoL, Vol. 1, 1893, pp. 133-456, .587-596, 688-716; Vol. II, 1891, pp. 811-825; Vol. Ill, 1896, pp. 1-20. A detailed petrographic description of the gabbro and related rocks of northeastern Minnesota. MON XLIII — 03 4 50 THE MESABI IRON-BEARING DISTRICT. 1896. Van Hise, C. R. Principles of North American pre-Cambrian geology. Sixteenth Ann. Rept. U. S. Geol. Survey, Ft. I, 1896, page 790. The upper series of Mesabi and its eastern equivalent, the Animikie, are con-elated with the Upper Huronian division of the Algonkian (as in Bull. 86), and is placed as equivalent to the Animikie and the Keewatin series of western Ontario, the Upper Vermilion series of the Vermilion district of Minnesota, the Upper Marquette series of Michigan, the western Menominee series, the Upper series of the Penokee-Gogebic district, the Chippewa, the Baraboo, Minnesota, and Dakota quartzites, the Wisconsin Valley series, the Upper Felch Mountain series, and the St. Louis slates of Minnesota. X898. Elftman, a. H. Geology of the Keweenawan area in northeastern Minnesota. Am. Geol., Vol. XXI, 1898, pp. 90-109, 175-188; Vol. XXII, 1898, pp. 131-149. This report includes a detailed description of the gabbro and a brief account of the contact effect of the gabbro on the underlying rocks. The glacial history of the northern part of the State is reviewed, but no featui-es are given in addition to those previously published by Upham. 1899. WiNCHELL, N. H., and Grant, U. S. The geology of Minnesota. Geological and Natural History Survey of Minnesota, Final Report, Vol. IV, 1899. Illus- trated by 31 colored geological plates, 48 plates of photographic views, and 114 figures. This volume is accompanied by detail maps of the Mesabi district, a general map of the district, and maps of the counties in which the Mesabi district occurs. The county maps and the detail maps of the Mesabi are made subjects of special chapters. The maps are far in advance of any- thing thus far published, and are specially accurate with reference to the u-on-bearing formation. While the main features of the geology are essen- tially the same as those given in previous reports of the Minnesota sui-vey, there are a number of minor modifications in interpretations of the geology. The general succession for the area west of Birch Lake is, from the top dowi(i=: Animikie . A rchean . SUMMARIES OF LITERATURE. 51 Succession of formations west of Birch Lake. „ . / 1 • 1 J ■c^\ /Flat or nndiilatina till. Quaternary (glacial drift) i_ . , . '=' . ITermmal moraines. Cretaceous Shales, clays, and conglomerates. Cabotian (lower division of Keweenawan) Gabbro. 'Upper slates. Black slates. Taconyte (iron ore) . P jkegama quartzite. Granite (post-Keewatin). Greenstones. Mica-schists (in part in Keewatin) Pewabic quartzite and iron ore. Lower Keewatin. Spurr's theory of the origin of the ores from the alteration of glauconite grains is accepted. In some places the bottom of the Animikie is distinctly taconitic and ferruginous, while in others it is distinctly quartzitic and conglomeratic. The Pokegama qiiartzite is thus not a continuous formation and is believed to blend into the iron-formation strata in places. The "quartzitic rocks" extending eastward from Iron Lake near the east side of range 12, and geographically continuous with the Pokegama quartzite and iron formation, are mapped and described as Pewabic quartzite." The Pewabic quartzite is a rock in which giiinerite, magnetite, enstatite, diallage, hypersthene, olivine, and other minerals characteristic of the gabbro contact have been developed and is supposed to have resulted from the alteration of a jaspilitic phase of the Keewatin. The altered Pokegama and Pewabic quartzites are difficult to distinguish. The Pokegama quartzite usually dips less than 25 degrees, becoming horizontal, and the Pewabic usually more than 75 degrees, becoming vertical. The Pokegama quartzite is associated with taconitic iron ore and the Pewabic with jaspilitic. The former is not known to be titaniferous ; the latter is usually distinctly titaniferous. The Pokegama quartzite is never associated with the peculiar muscovadyte, but the Pewabic is never without it. The Pokegama quartzite, with its taconitic companion, is known to be overlain by the black slates of the Animikie, and occurs only westward from Iron Lake. The Pewabic qiiartzite is overlain and underlain invariably by muscovadyte, or by "gabbro," where the altera- tion was intense, and occurs only eastward I'rom the vicinity of Iron Lake. o The conclusion that the Pokegama quartzite blends into the iron formation and is replaced to the east by the Pewabic quartzite is one reached by Professor Winchell. Professor Grant, in his por- tions of the volume, regards the quartzite as a persistent horizon at the base of the Animikie series in the western portion of the district. 52 THE MESABI IRON-BEARING DISTRICT. The Keewatiu rocks in the part of the district west of Bircli Lake are not subdivided in the mapping, but in rock cuts along the railway in sees. 15 and 22, T. 58 N., R. 17 W., Dr. U. S. Grrant observed certain coarse fragmental rocks on the basis of which he suggests a division of the Keewatin into a lower igneous portion and an upper fragmental portion (pp. 372-374). The evidence for faulting in the Virginia area, given by Spurr, is considered inadequate. Eastward from Birch Lake, in the gabbro, are isolated areas of banded • ferruginous quartzites and olivinitic iron ores which are regarded as parts of the Animikie caught in the gabbro flow. These are found in the following locations: Just north of Muskrat Lake, in sec. 30, T. 62 N., R. 10 "W.; south of Disappointment Lake, in sec. 4, T. 63 N., R. 8 W.; northwest corner of Thomas Lake, in sec. 29, T. 64 N., R. 7 W.; north side of Fraser Lake, in sec. 23, T. 64 N., R. 7 W.; in sec. 20, T. 64 N., R. 6 W.; south side of Grabbemichigamak Lake, in sec. 1, T. 64 N., R. 6 W. Similar rocks are found to the east in a belt running through Chubb, Akeley, and Gunflint lakes, but as these lakes are in the area covered by the monograph on the Vermilion district their geolog'y is not summarized. The petrography of the gabbro itself is briefly described. 1900. WiNCHELL, N. H. Structural geology of Minuesota. Geological and Natural History Survey of Minnesota, Final Report, Vol. V, 1900, pages 1-80, 972-1000. This volume contains Professor Winchell's final conclusions on the general geolog}^ of nortliern Minnesota and the origin of the Mesabi iron ores. While much of it does not concern the Mesabi district, it is here fully summarized in order to show Professor Winchell's latest correlation of the Mesabi geology with that of adjacent areas. (See discussion of coirela- tion, pp. 200-205.) The ancient rocks of northern Minnesota are placed in two main systems, the Archean and the Taconic. The former is further subdivided into the Upper and Lower Keewatin, separated from each other by an unconformity. The Pewabic quartzite (see above summary of Vol. IV) also is placed with the Keewatin, but is not assigned to either of the main divi- sions. (Overlying the Archean with strong unconformit}- is the Taconic, represented ]^y Animikie and Keweenn\\'nn rocks, these divisions being SUMMARIES OF LITERATURE. 53 supposed to represent respectively the Lower and Middle Cambrian of other parts of the country. The Coutchiching and Laurentian rocks before mapped as separate formations are now included within the Keewatin. Lower Keewatin. — Tlie Lower Keewatiu comprises greenstone, with asso- ciated surface volcanics which are both subaerial and subaqueous, argyllitic slates, siliceous schists, quartzites, arkoses, " green wackes," iron ores, and marble. The greenstone, designated the Kawishiwin, is the oldest known rock in the State, and is supposed to represent a portion of the original crust of the earth. With its associated volcanic rocks it occurs in two main belts. The southern belt begins in the vicinity of Gunflint Lake and extends west- ward by way of Gobbemichigamma Lake, the Kawishiwi River, and White Iron Lake, to Tower, and indefinitely westward. The northern belt of greenstone enters the State from Hunters Island, appearing conspicuously at the south side of Basswood Lake. At Pipestone Rapids and Fall Lake it widens southward and apparently unites at the surface with the southern belt, the overlying Upper Keewatin being absent for a distance of a few miles. But farther west it is again divided by the Stuntz conglomerate, the northern arm running to the north of Vermilion Lake, west of which its extension, is unknown, and the southern one running south of the lake. The fragmental stratified rocks of the Lower Keewatin are most important toward the western part of the area of exposure of crystalline rocks. They occupy a wide area, south, west, and north of Tower. The iron ores of Tower and Ely on the Vermilion iron range occur in the upper part of the Lower Keewatin. It is probable that the immediately inclosing rock is a sedimentary one, although composed of the elements of a basic eruptive. The sediments extend south to the Giants range of granite, where they are metamorphosed to mica-schists by the granite. Toward the west they extend as far as the Mississippi River and its northern tributaries and across the Bowsti'ing, although the drift prevents the delimitation of the belt. To the northwest they extend toward Rainy Lake, in this direction being converted into mica-schists and gneisses by the intrusion of granite; in unmodified form they are found at one point only on Rainy Lake. These fragmental rocks of the Lower Keewatin doubtless also underlie most of the central and southwestern part of the State as far as the Minne- sota River. Here they dip beneath the later formations in the southwestern portion of the State, and probably occupy a wide patch in South Dakota. 54 THE MESABI IRON-BEARING DISTRICT. South of the Giants range they occur also, but as they are covered by the gabbro and Animikie toward the east and the drift deposits of the St. Louis Valley toward the west their geographic boundaries are mostly unknown. They appear in the central and western portions of Carlton County, where their line of separation from the Upper Keewatin is quite obscure, and in the central and western portions of Morrison County. The Lower Keewatin marble is seen at Lake Ogishke-Muncie and at Pike Rapids, on the Mississippi. The Lower Keewatin was terminated by a period of extensive folding and intrusions of granite and basic rocks. The Pewabic quartzite belongs with the Keewatin, but whether to the Lower or Upper Keewatin is not known. This formation includes altered quartzites and iron ores between the granite and gabbro in the immediate vicinity of Birch Lake and small patches of similar rocks in sec. 30, T. 62 N., R. 10 W.; on the south shore of Disappointment Lake; on the north shore of Eraser Lake; on the south shore of Gabbemichigamma ; at Akley Lake, forming the so-called Akley Lake series extending from the west side of sec. 34, T. 65 N., R. 5 W., to the eastern part of sec. 27, T. 65 N., R. 4 W. Upper Keewatin. — Tlic Uppcr Kccwatin occurs in troughs in the Lower Keewatin, particularly in one main trough the axis of which is traceable from Vermilion Lake to Saganaga Lake. The northern arm of this sjuicline, consisting of granites, gneisses, associated mica-schists, and in some places earlier greenstones, extends from the northern part of Vermilion Lake through Basswood Lake to the northern side of Hunters Island. The southern arm, consisting of Lower Keewatin green schists and other schists, penetrated by the granite of the Giants range, extends from Pokegania Falls on the southwest toward the northeast until cut out by the encroachment of the gabbro from the south. The Upper Keewatin consists very largely of con- glomerates, but also includes graywackes, argyllites, quartzites, and jaspi- lites, in general coarser than those of the Lower Keewatin. Volcanic rocks are less important than in the Lower Keewatin, although still present. There is no general order of succession in the Upper Keewatin excepting that it can be said that it is in general conglomeratic at the bottom. After Upper Keewatin time both the Lower and Upper Keewatin were subjected to another folding, the axis of which had a general paral- lelism with the earlier folding, with the result that the Ui)per Keewatin lies SUMMARIES OF LITERATURE. 55 in naiTow synclines in the Lower Keewatiu and in places is nearly or quite vertical. Associated with the Keewatin rocks are granites of at least two periods of intrusion, one later than the Lower Keewatin and one later than the Upper Keewatin. The later granite is believed to be represented by the higher parts of the Giants range and the Snowbank Lake g-ranite. The earlier granite is represented by the granites at Kekequabic Lake, Sag- anaga Lake, Basswood Lake, Burntside Lake, Vermilion Lake, Lac la Croix, and Kabetogoma Lake. The origin of the granite is discussed and the same conclusions are reached as in a, previous article." The Taconic. — This is uuconformably above the Keewatin rocks. It comprises the Animikie and Keweenawan divisions. The Animikie rocks enter the State at Pigeon Point and run westward along the international boundary to the eastern part of sees. 22 and 27, T. 65 N., R. 4 W. They reappear again southwestward from Birch Lake on the northwest side of the gabbro mass, and thence continue along the south side of the Giants range, constituting the Mesabi iron series, to Pokegama Falls. The higher parts of the Animikie are best developed toward the east, while the lower parts are best developed toward the west. The Animikie rocks comprise the Pokegama quartzite, Mesabi iron- bearing formation, and some limestone and slate, all strictly conformable with one another. The thickness is several hundred feet, sometimes reach- ing nearly 1,000 feet. The dip of the series is uniformly to the south, 8 to 12 degrees. The iron-bearing formation and the Pokegama quartzite constitute the base of the formation. The quartzite in places is beneath the iron forma- tion; in other places it is in the same horizon, and in still others is above the iron formation. Commonly the base of the Animikie is marked by a conglomerate containing- debris from the underlying Keewatin rocks. This is a narrow horizon which soon graduates upward into a quartzite known as the Pokegama quartzite from its typical development near Pokegama i'alls, on the Mississippi River. The thickness of the quartzite is not known to exceed 50 feet and is sometimes less than 25 feet. Above the quartzite, or in alternating beds with it, or below it, «The origin of the Arehean igneous rocks, bj' N. H. Winchell: Proc. Am. Assoc. Adv. Sci., Vol. XLVII, 1898, pp. 303, 304 (abstract); also Am Geol., Vol. XXII, 1898, pp. 299-310; sum- marized in .Jour. Geol., Vol. VII, 1899, p. 194. 56 THE MESABI IRON-BEARING DISTRICT. appears the iron-bearing or taconyte member of the Animikie, which contains the iron-ore deposits of tlie ^Mesabi iron range. The ore is nsuallv hematite in tlie western part of the range and magnetite in the eastern part. Origin of ore. — Thc Ore WPS previouslj snpposed to have been derived from the alteration of a greenish glanconitic sand rock, but later work has seemed to show that the green sand is a volcanic sand, and that the so-called taconitic rock itself has resulted from igneous forces. This is accounted for by supposing a chain of active volcanoes to have existed where the Mesabi iron range is now found. These volcanoes yielded flows and ejectamenta to the adjacent waters, which have been modified into the various phases of the iron formation now seen. This volcanic epoch may have a deep-seated connection with the Cabotian or lower division of the Kewee- nawan (described later). Above the iron- bearing member is an- impure, dark-colored limestone a few feet in thickness, not exceeding 20. It extends apparently the whole length of the Mesabi range, but has been identified in two places only, sec. 7, T. 58 N., R. 17 W., and doubtfully on the shores of Gunflint Lake. This limestone may be regarded as the basal horizon of the next over- lying rock. The black slate is probably several thousand feet in thickness and constitutes the bulk of the Animikie. In the neighborhood of Gunflint Lake it has been divided by Dr. Grant into a lower black slate division and an upper graywacke-slate division, both of which membei's are interleaved with diabase sills. In the Indian reservation at Grand Portage and at various places along the Grand Portage trail is a graywacke, which is supposed to overlie the black slate member, but its extent and stratigraphical position have not been satisfactorily established. The top of the Animikie has not been identified. The first recognizable datum plane after the close of the Animikie is the Puckwunge conglomerate, supposed to be the fragmental base of the Keweenawan. At one or two places southwestward from Birch Lake, and at Little Falls, on the Mississippi River, and in Morrison County, the Animikie has been converted into a mica-schist. The age of the Animikie is believed to be Lower Cauibrian for the SUMMARIES OF LITERATURE. 57 following reasons: It grades upward into Upper Cambrian rocks, as seen on the south side of Lake Superior. The derivation of the iron ores from a glauconitic green sand indicates that large quantities of foraminiferal organisms once lived in the Animikie Ocean, and Matthew has shown the existence of foraminiferal organisms associated with the iron ore in the St. Johns group of New Brunswick. Further, the Animikie has a uniformly low dip, while the lower strata are all highly tilted. There must therefore have been a great lapse of time between the deposition of the two series. The Keweenawan. — The Puckwuuge couglomcrate is taken to be the frag- mental base of the Keweenawan, although certain igneous rocks which antedate it, and which, perhaps, are contemporaneous with the upper portions of the Animikie, are also called Keweenawan. The conglomerate is found at Grand Portage Island, at Isle Royale, on the Baptism River, at Little Marais, on Manitou River, at the deep well at Short Line Park near Duluth, and at New Ulm. Above this conglomerate are conglomerates and sandstones of Keweenawan age which are stratified with lavas of diabasic nature. Still higher up the eruptive rocks become less in quantity and the fragmental rock is a sandstone, known as the Hinckley sandstone, quarried in the gorge of the Kettle River in Pine County. This in turn grades up into typical Upper Cambrian sandstones of the St. Croix Valley. The term Potsdam is restricted to the Puckwuuge conglomerate and the hardened quartzites immediately overlying it, represented by the Sioux quartzite, the Baraboo and Barron County quartzites of Wisconsin, the quartzite at Grand Portage Island and west of Grand Portage village, the New LTm quartzite in Cottonwood County, and the quartzite in Pipestone County. The igneous rocks of the Keweenawan vary in age from late Animikie time to the top of the Keweenawan series. They are divided into two groups, the Cabotian or Lower Keweenawan and the Manitou or Upper Keweenawan. The Cabotian division includes gabbro and contemporaneous red rock and their surface lavas, and all other dikes and sills which are associated with but are younger than the Animikie clastic rocks and which are older than the Puckwuuge conglomerate. The lower member of the Cabotian is the gabbro, which covers an enormous area. It extends on the east to East Greenwood Lake, in T. 64 N., R. 2 E. On the north it is bounded by 58 THE MESABI IRON-BEARING DISTRICT. the Auimikie strata of the Mesabi iron range. Its westernmost exposure is in the vicinity of Short Line Park, Duluth. The southern limit is irregular, swinging from East Greenwood Lake in a zigzag manner through T. 63 N., R 1 W.; T. 62 N., R. 2 W.; T. 62 N., R 4 W.; T. 60 K., R 6 W.; T. 60 N., R 7 W.; T. 58 N., R 10 W., and T. 55 N., R 11 W., to Duluth. Along the northern and northwestern side of the great gabbro mass the gabbro is plainly intrusive on the older formations, Animikie and Keewatin. From the northern border of the gabbro many sills offshoot and penetrate the Animikie strata parallel to the bedding. These are known as the Logan sills. . Near its contact with the underlying rocks, both the Animikie and the Keewatin series, there are various altered rocks which can be connected in places with the gabbro and in places with the underlying rocks. To these altered rocks the term "muscovadyte" has been applied. It includes the various so-called peripheral phases of the gabbro. On the southern and eastern border the gabbro is penetrated by and penetrates in a confused manner the red rock, with which it alternates both structurally and areally. It is believed to have resulted from the meta- morphism by the gabbro of the Animikie, and perhaps earlier fragmentals. As the granites of the Archean are believed to have resulted from the softening of acid fragmentals, so the gabbro may have been the result of the metamorphism or re-fusion of the Keewatin greenstones. The anorthosite masses of the Beaver Bay diabase, supposed by Law- son to be of Archean age and to underlie unconformably the Beaver Bay diabase, are believed to represent segregation phases in the main gabbro flow, and to be the same as anorthosite masses in the gabbro proper to the west. The Beaver Bay diabase is believed to represent the upper portion of the great gabbro flow, and to be due to the first and greatest movement of tlie gabbro toward Lake Superior. The Logan sills belong to this part of the gabbro flow. The Manitou division of the Keweenawan includes the surface flows, sills, and dikes which accompanied and followed the Puckwunge conglom- erate. These eruptives, with the elastics associated with them, do not have a thickness in Minnesota of more than 1,000 feet. These lava sheets extend along the shore of Lake Superior from near Baptism Rixcr to near Grand SUMMARIES OF LITERATURE. 59 Marais, except where replaced at intervals b)^ the Beaver Bay diabase or some of the intersheeted fragmentals. They occur also in the neighbor- hood of Grand Portage Bay, but their extent here is not definitely known. General. — Tho most important petrological conclusions determined from the examination of the Minnesota crystalline rocks are three in number: 1. All the granites of the Archean can be explained on the assumption that they are intrusives representing the metamorphosed conditions of clas- tic rocks adjacent to the observed intrusions, rendered plastic by the force of dynamic metamorphism accompanied by moisture. 2. The basic Keweenawan gabbro and its derivatives are derived from the metamorphism and complete re-fusion of the Archean greenstones and their attendants. 3. The green sand of the Mesabi iron-bearing formation appears to have resulted from a volcanic sand, and the taconite itself, from igneous forces. Grant, U. S. Contact metamorphism of a basic igneous rock. Bull. Geol. Soc. America, Vol. XI, 1900, pages 503-510. Dr. Grant describes the contact metamorphism caused by the great gabbro of northeastern Minnesota on the rocks with which it comes in con- tact. These are of particular interest as explaining the character of the iron-bearing rocks of the eastern portion of the Mesabi range. He says: That the great mass of gabbro at the base of the Keweenawan in Minnesota has features which indicate its intrusive rather than its extrusive nature; that one of the most important of these features is the marked contact zone along the lower or north- ern side of this mass; that in this zone a complete recr3'stallization of the strata has been effected, at times for a distance of a few hundred feet from the igneous rock, with less pronounced effects extending for a quarter of a mile or more; that the rocks resulting from the contact metamorphism of the iron-bearing member of the Animikie are peculiar I3' rich in minerals of the basic rocks — that is, in augite, hypersthene, and olivine; that the materials for these minerals were present in the quartz-magnetite- amphibole slates of the Animikie, and consequenth' that it is not necessar}' to con- sider these minerals as derived from the gabbro, and that the contact effects on some altered basic igneous rocks have been to reproduce the original mineral character of these rocks and to produce textures partially similar to true igneous rocks. The petrography of the gabbro itself is summarized. The above discussion is based primarily on facts observed eastward from Birch Lake. 60 THE MESABI IRON-BEARIMG DISTRICT. 1901. WiNCHELL, N. H. Geological atlas, with synoptical descriptions. Geological and Natural Histoiy Survej" of Minnesota, Final Report, Vol. VI. 1901. This atlas contains the maps published with Volume V of the Minnesota survey and in addition a geological map of the State. The synoptical descriptions of the plates contain no features not given in Volumes IV and V. The mapping of the iron formation between Dunka River and Birch Lake as Pewabic quartzite is abandoned. Van Hise, C. R. , and Leith, C. K. The iron-ore deposits of the Lake Superior region [the part on the Mesabi district]. Twenty -first Ann. Rept. U. S. Geol. Survey, Pt. Ill, 1901, pages 351-370. Accompanied by a map of central portion of the district, by C. K. Leith. This is a preliminary report on the district, containing a brief account of the essential features, more fully described in the present monograph. The first announcement is here made of the presence and distribution of the Lower Huronian and Archean series, as these terms are used by the United States Geological Survey. Spurr's conclusion that the ores have developed from a hydrous feiTous silicate is confirmed, but the original gi'een fen-ous silicate granules are thought not to be glaucouite, as named by Spurr. 1902. Sptjer, J. E. The original source of the Lake Superior iron ores. Am. Geol.. Vol. XXIX, 1902, pages 335-349. After the appearance of the above report by Van Hise and Leith, Spurr restated his position, concluding — 1. That the iron ores of the Mesabi range and the varied and peculiar rock types of the iron-bearing formation are derived from the alteration and rearrangement of a sedimentary rock containing large quantities of a green hydrous ferrous silicate, in generally rounded, small, separate grains. 2. That the rocks containing iron carbonate, including the phases called cherty siderites and sideritic cherts, are one of the results of alter- ation of this original rock, the iron carbonate, and also a large proportion of the silica, being derived from the green silicate. 3. That the green silicate was formed large!}' through the agency of organic matter. SUMMARIES OF LITERATURE. 61 4. That its habit, form, optical and chemical qualities mark it as belonging to the class of glauconites, and mark the original rock as a green sand. 5. That in accordance with what is known of the formation of green sand, the iron, silica, etc., of which the glauconite is comjDosed were probably derived largely from fine land silt; in pai't, also, from solution in sea v-^ater. 6. That the above conclusions probably apply to most of the other Lake Superior iron ores. For a full discussion of the literature covering the eastward continua- tion of the Mesabi range — that is, the area in the neighborhood of Akeley and Grunflint lakes and eastward — the reader is referred to Monograph XLV, on the Vermilion district. ECONOMIC REPORTS. In addition to the above reports, dealing mainly with the geology of the district, there have appeared a large number of articles on ihe economic features of the district, including descriptions of mines, mine methods, cost, production, transportation, etc. Below is given a list of such of these articles as are signed that have come to our notice. They are so numerous and so widely scattered in trade journals that it is certain that some have been overlooked: Bacon, D. S. Methods of working on the Mesabi iron range. Engineering and Min. Jour. , Vol. LXIV, 1897, pp. 306-307. Bailey, C. E. Mining methods on the Mesabi range. Trans. Am. Inst. Min. Engi- neers, Vol. XXVII, 1897, pp. 529-536. Brackenbuet, Ctril. Methods of mining on the Mesabi range. Mines and Min- erals, Vol. XXI, 1900, pp. 150-152. Channing, J. Parke. Lake Superior iron ore. The Mineral Industrj'; its Statistics, Technology, and Trade to the End of 1894, Vol. Ill, 1895, pp. 375-102. Chester, A. H. The iron region of northern Minnesota. Geol. and Nat. Hist. Surve.y of Minn.. Eleventh Ann. Rept., 1881, pp. 151-167. Denton, F. W. Methods of iron mining in northern Minnesota. Am. Inst. Min. Engineers, Vol. XXVII, 1897, pp. 344-390. Open pit mining, with special reference to the Mesabi. Proc. Lake Sup. Min. Inst., Vol. Ill, 1895, pp. 84-92. Elftman, a. H. Ore deposits in Minnesota. Yearbook Soc. of Engineers, Univ. Minn., Vol. IV, 1896, pp. 115-117. 62 THE MESABl IRON-BEARING DISTRICT. Head. jEREinAH. and Head. A. P. The Lake Superior iron-ore mines and their influence vipon the production of iron and steel. Proc. Inst. Civ. Engineers (Enoland). Vol. CXXXVH, 1899. pp. 72-102; discussion of sand, pp. 103-130, with Plate.s III and IV. HuLST, N. p. Lake Superior methods of mining. Proc. Engineers Soc. West. Penn., Vol. XV, 1899, pp. 62-104. Lamnees, T. L. Engineering and Min. Jour., Vol. LIV, 1892, p. 579. Lo^GTEAR, E. J. Explorations on the Mesabi range. Trans. Am. Inst. Min. Engineers, Vol. XXVII, 1897, pp. 537-541. Warken, O. B. The Mahoning iron mine. Iron Age, Vol. LXIV, 1899, pp. 1-3. Wedding, H. Stahl und Eisen, Vol. XVI, pp. 7-13, with 5 illustrations taken from the Iron Age and 1 from Hai'per's Weekty. Wilkinson, C. D. Systems of mining in Minnesota iron mines. Yearbook Soc. of Engineers, Univ. Minn., Vol. Ill, 1895, pp. 47-51. AViNCHELL, H. V. Methods of mining. Iron Trade Review, July 21, 1892. The Mesabi iron range. Geol. and Nat. Hist. Survey of Minn., Twentieth Ann. Rept., 1891, Minneapolis, 1893, pp. 111-180. Trans. Am. Inst. Min. Engineers, Vol. XXI, 1893, pp. 644-686. The iron ranges of Minnesota. Proc. Lake Sup. Min. Inst., Vol. IH, 1895, pp. 1.5-32. The Lake Superior iron-ore region, U. S. A. Trans. Fed. Inst. Min. Engi- neers, Vol. XIII, 1896-97, pp. 493-562. Accompanied by map of the Great Lakes, and 6 sections of the Mesabi. Minnesota iron mining economically and statistically considered. Geol. and Nat. Hist. Survey of Minn., Final Report, Vol. IV, 1899, pp. 581-616. WixcHELL, H. v., and Jones, J. T. The Biwabik mine. Trans. Am. Inst. Min. Engineers, Vol. XXI, 1893, pp. 951-961. WrscHELL, N. H., and Winchell, H. V. Iron ores of Minnesota. Geol. and Nat. Hist. Survey of Minn., Bull. No. 6, 1891. Winchell, N. H. Geology of the iron oi-es of Minnesota. Geol. Soc. Australasia, • Vol. I, 1892, pp. 171-181. The discovery and development of the iron ores of Minnesota. Minn. Hist. Soc. Collections, Vol. VIII, Pt. I, 1895, pp. 25-40, with geological map. WooDBRiDGE, D. E. Ii'on-ore mining on the Mesabi range. Engineering and Min. Jour.. Vol. LVI, 1893. p. 163. See also unsigned article in Iron Age, Vol. LVI, 1895, pp. 216, 277, and 386. CHAPTER III. THE BASEMENT COMPLEX, OR ARCHEAN. DISTRIBUTIOlSr. The Archean rocks of tlie Mesabi district are confined to its central portion. They are found north and northwest of Nashwauk, northwest of Hibbing; north and northeast of Mountain Iron; in the southerly projection of the Mesabi range known as the "Horn," bounded by the cities of Virginia, Eveleth, Sparta, and. McKinley; north of Biwabik, and eastward to near the east line of R. 16 W. With the exception of the portion of the Archean area east of Embarrass Lake, exposures are sufficiently common to allow of a fairly close determination of the boundaries. East of Embarrass Lake the mapping is based on the presence of abundant Archean fragments in the di'ift. Included in the areas mapped as Archean north of Mountain Iron are several small patches of Lower Huronian rocks. Exposures are so few, they are so mixed in the same exposure with Archean rocks, and they are metamorphosed to such difficultly recognizable forms that their accurate delimitation on the general map is not possible. Their distribution, so far as worked out, is shown on a special large-scale plat (PI. V). KINDS OF ROCKS. The Archean is represented by dolerites (and their altered equivalents, metadolerites or diabases), basalts (and their altered equivalents, metaba- salts), diorites, peridotites(?), micaceous, chloritic, and hornblendic schists, granites, and poi'phyritic rhyolites. In abundance the rocks stand in about the following- order: the micaceous, chloritic, and hornblendic schists, basalts, dolerites, porphyritic rhyolites, granites, and diorites. The basic rocks have commonly a green color and are usually referred to locally as greenstones or green schists. Tliey are so intricately intermingled that they are given one color on the general map, but in the area northwest 64 THE MESABI IRON-BEARING DISTRICT. of Hibbing, to illustrate their complexity, they have been separately indicated on a special large-scale map (PL IV). The acid igneous rocks, consisting of the porphyritic rhyolites and the granites, are mapped undei- another color. All of these rocks have their counterparts in other iron districts of the Lake Superior region. In the Vermilion and Crystal Falls districts, where especially well developed, Clements has described each phase in great detail. On this account the following description of the rocks of the Archean of the Mesabi district is A-ery brief. The names used by Clements in the Crystal Falls and Vei'milion districts are applied throughout. In case the reader desires to know moi'e of the details of the petrogra]:)hy, he is referred to the description of the Archean rocks in Monographs XXXVI and XLV. DOLERITES AND METADOLERITES. The dolerites are best developed in the gi-eat area of Archean mapped as extending from Virginia eastward to beyond Biwabik, although occur- ring also in the other Archean areas. On the weathered surface they show varied shades of green and brown, these colors grading into dirt}' white or light yellow. On fresh fracture the color is characteristiciall}' some shade of green, commonl}^ a rich dark green. The texture is typically ophitic, and varies from coarse to fine. Occasionall)'' a luster mottling- or poikilitic texture is present. Under the microscope the jDlagioclase feldspar is obscured by alteration products consisting largely of epidote, mica, quartz, and kaolin. The feldspar laths interlock to give the ophitic arrangement. The interstices are occupied b)^ secondary hornblende, fine-grained feld- spar, and their alteration products, mica, chlorite, and zoisite. In addition there are present minute quantities of ilmenite, sphene, and magnetite. The rock was originally a typical dolerite, but the alterations make the term metadolerite, or altered dolerite, appropriate for the greater mass of it. BASALTS AND METABASALTS. The basalts are most closely associated witli the dolerites — in fact, grade into them — and, like them, occur in the greatest quantity in the great eastern area of the Archean. The conspicuous features by which the basalts are distinguished from the dolerites in the field are tlieir fine apha- nitic and porphj'ritic textures. Their coloi- on fresh fracture also is THE BASEMENT COMPLEX, OR ARCHEAN. 65 frequently a somewhat lighter green than that possessed by the dolerites. Microscopically the basalts show much altered plagioclase-feldspar pheno- crysts and occasional quartz phenocrysts in a very fine-grained, although holocrystalline, grouudmass. The alterations of the feldspar are the usual ones to sericite, epidote-zoisite, and kaolin. The groundmass consists mainly of feldspar deeply discolored by an abundance of secondarv minerals, including chlorite, zoisite, iron oxide, and i>,alcite. No original augite is present and little or no secondary hornblende. The texture is sometimes of the fine, even grade known as "cryptocrystalline" or "micro- crystalline," and at others the irregular mottled kind known as "micro- poikilitic." With these textures may occasionally be seen a slight arrange- ment of the finer constituents, particularly the iron oxide or plag'ioclase laths (pilotaxitic texture), in such a manner as to suggest flowage lines of a lava. Rai'ely, also, the groundmass shows a spherulitic texture (625 paces north of the southeast corner of sec. 36, T. 59 N., R. 16 W.), and, in this case there seeins to be a considerable amount of quartz in phenocrysts and in the groundmass. The amygdaloidal texture is less rare. It is best seen northeast of Biwabik. The amygdules are filled with quartz and with finely fibrous minerals which are probably zeolites, the latter not infrequently so altered as to be visible only under crossed nicols. Other common structures are the tuflfaceous and ellipsoidal structures, which appear to best advantage on the weathered surface. The ellipsoidal struc- ture is typically developed north of Sparta. The ellipsoids themselves consist of basalt, and vary in diameter from a few inches to one or two feet. They are separated by narrow bands of somewhat lighter or darker basalt. The ellipsoidal structure is one supposed to have been induced in the rock when it first cooled from an extensive magma, perhaps subaqueous. A similar structure has been observed at many places in the basic igneous rocks of the Lake Superior country and has been fully described by Clements for the Crystal Falls and Vermilion districts." The figures in the Vermilion monograph representing the structure in the Vermilion dis- trict would represent the structure equall}^ well for the Mesabi district. As in the case of the diabase, the alteration of the basalts has been of such a nature as to make the name "metabasalt" appropriate for most of the phases. «Mon. V. S. Geol. Survey Vols. XXXVI and XLV. MON XLIII — 03 5 66 THE MESABI IRON-BEARING DISTRICT. DIORITES. The diorites are dark-gray or green rocks, which on weathered surfaces resemble hornblende-granite. Their principal constituents are plag-ioclase feldspar and hornblende, but there are alteration products, including mica, chlorite, epidote-zoisite, quartz, kaolin, etc. The texture varies fi-oin coarse to fine and from granitic to porphyritic. In the latter case the hornblendes, frequently showing enlargement, are the porphyi'itic constituents. By diminution in the amount of feldspar and increase in the amount and coarseness of the hornblende present the diorites grade into coarse hornblende rocks, which consist almost entirely of coarse, stumpy, dark- green hornblende crystals with random arrang-ement. The interstitial material is plagioclase feldspar and is exceedingly sparse. Ilmenite, showing alteration to deeply colored sphene, is present both in the horn- blende and in the matrix. The hornblende rocks diifer from ihe diorites onlv in the relatively greater amount of hornblende present, and really constitute but a special phase of the diorites. They might perhaps be called "hornblendites," but these rocks usually contain a small amount of augite. Tliey would fall under the general group of "perknites," a name recently suggested by Turner" for rocks consisting largely or entirely of monoclinic amphibole or pyroxene, or l^oth. PERIDOTITE. Peridotite has been found in exploration work in sec. 33, T. 59 N., R. 15 W. It is not certain that the rock found in this exploration is in place and not a float from the north; hence it is but mentioned. HORNBLENDIC SCHISTS. The hornblendic schists are best developed in the area north of Mountain Iron, but they are found throughout the areas mapped as Archean. In typical form they are rich dark-green rocks, sometimes almost black, and show many brilliant reflections of hornblende cleavage faces. Many variations of the type are to be seen. The schist may have a rather light gi'ayish-green color, or it may take on a yellowish color, due to the presence of a considerable amount of feldspar in the rock. Tlie texture varies from coarse to flue. Tlie hornblende crystals have a tendency to lie with their "Jour. (;eol., Vdl. IX, l!)i)l. ]<\k 507-.M1. T. 58 N. ■N 85 -1 THE BASEMENT COMPLEX, OR ARCHEAN. 67 columnar directions almost parallel, giving tlie rock its schistosity or cleavage. When broken with a hammer the parting of the rock parallel to the schistosity is observed not to follow a plane, but to be everywhere parallel to the columnar crystals. The pieces of the rock broken off roughly resemble in shape and dimensions the individual hornblende crystals making- up the rock. Each of the pieces of rock broken off exhibits the same glistening faces of hornblende, showing that in the breaking the elongated crystals have parted along their mineral cleavage planes. Under the microscope the hornblende appears in fresh green columnar crystals, almost certainly secondary, with a tendency to parallelism of their long axes, although many crystals are not so arranged. Rarely the horn- blende appears in two forms in the same slide — in large stumpy crystals almost as wide as long and with no parallel arrangement, and in slender columnar forms with parallel arrangement. An interesting feature here is that the parallel columnar hornblendes result occasionally from the parting or slicing of the large stumpy hornblende crystals along their cleavage planes; that is, under pressure the large hornblendes have parted along their cleavage planes, yielding a large number of slices which have much greater length than breadth or thickness and have been arranged parallel. The hornblende crystals lie in a fine-grained and much discolored matrix of feldspar, chlorite, and epidote-zoislte, these minerals showing a variety of proportions in different rocks. The usual accessory minerals, including magnetite and calcite, are to be observed. With increase in the amount of feldspar the hornblendic schist grades into amphibolite. With increase in the amount of quartz the hornblendic schists may become almost indistinguishable from the hornblendic g'ray- wackes of Lower Huronian age which are found associated with horn- blendic schists north of Mountain Iron and Hibbing (see pp. 69-70), and, indeed, it is not unlikely that some of the rocks containing- considerable quartz and feldspar, here described as hornblendic schists, may themselves have been derived from the alteration of sediments. Most of the horn- blendic schists, however, have unquestionably been derived from the alteration of basic igneous rocks of the Archean described above, and usually, moreover, have received their most characteristic feature through the metamorphic effect of the Lower Huronian granite. In the Mesabi 68 THE MESABI IRON-BEARING DISTRICT. distiict the actual transition from the massive basic rocks to the horu- blendic schists may be observed in but few places, but along the contact between the gi-anite and the basic igneous rocks of the Archean hom- blendic schists are everywhere abundant, and in the Vermilion district to the north hornblendic schists of the same character, age, and associations may be observed in all stages of alteration from basic igneous rocks, brought about mainly and finally through the intrusion of granite. MICACEOUS SCHISTS AND CHLORITIC SCHISTS. These rocks may be seen throughout the Archean, but to special advantage in the great Archean area eastward from Virginia and north of Biwabik. When typically developed they consist largely of chlorite and mica, principally biotite but partly muscovite, with locally more or less talc, l}"iug in a groundmass composed mainly of feldspar with subordinate amounts of quartz, hornblende, magnetite, ilmenite, and zoisite. A small part of the micaceous schists has resulted from the alteration of the acid igneous rocks of the Archean, but the greater part of the mica- ceous schists and the chloi-itic schists, like the hornblendic schists, has developed from the basic igneous rocks of the Archean. Why hornblendic schists should develop in some places and chloritic and micaceous schists in others is not always known, but in general it seems to be true that the hornblendic schists are characteristic of the contact of the granite with the basic igneous rocks of the Archean, while the chloritic and micaceous schists are characteristic developments from the folding and mashing of the Archean rocks away from the granite. GRANITE AND PORPHYRITIC RHYOLITE. The Archean acid igneous rocks include porphyritic rhyolite, porphy- ritic granite, and granite. The two former may be conveniently referred to as "porphyries." Between Virginia, Sparta, and Eveleth are three small areas of Archean porphyries. The one mapped as lying mainly in sec. 22, T. 58 N., R. 17 W., is a porphyritic granite. The rock weathers white, light green, and dirty yellow. The phenocrysts of quartz and plagioclase feldspar stand in a fairly fine-grained groundmass of quartz and feldspar in which appears a considerable quantity of secondary sericite, chlorite, and quartz, indicating considerable alteration. The large, clear phenocrysts of quartz stand out like eyes. An almost identical rock has been found in the THE BASEMENT COMPLEX, OR ARCHEAM. 69 Vermilion iron district, and in both the Vermihon and Mesabi districts the rock has been designated in the field the "white-eyed porphyry." The mashing to which the rock has been subjected, together with the develop- ment of secondary mica and chlorite, has in places altered the porphyries to chloritic schists and micaceous schists, either kind being more or less talcose. The porphyry mapped in sees. 16 and 21, T. 58 N., R 17 W., and along the quarter line of sec. 29, T. 68 N., R. 17 W., is almost the same in texture and mineral content as the one just described, except that the phe- nocrysts, instead of being both quartz and feldspar, are plagioclase feldspar alone. Moreover, a considerable amount of secondary calcite and pyrite are to be observed in the groundmass. The rock is called a feldspar- porphyry. Like the porphyr}^ above described, it shows much mashing, and by its alteration has yielded chloritic, muscovitic, and talcose schists. Near the west line of sec. 25, T. 59 N., E,. 18 W., is a dense dark-gray porphyry associated with hornblendic schist. Under the microscope the acid feldspar phenocrysts show zonal cloudy alterations to muscovite and kaolin. The feldspars lie in a fine but imeven grained matrix of feldspar and quartz, with a subordinate amount of muscovite, chlorite, kaolin, and zoisite. Seventy paces north of the southeast corner of sec. 6, T. 58 N., R. 16 W., there is an exposure of biotite-granite. The surrounding rock is Lower Huronian slate and graywacke, and we have no evidence that the granite itself is Archean. However, it is a considerable distance from the Lower Huronian granite, and, on the other hand, not far from the granites and porphyries whose age is known to be Archean, and it has been mapped as Archean. Orthoclase feldspar and rather abundant quartz form the mass of the rock. With this is a liberal sprinkling of biotite and less greenish hornblende. The texture is typically granitic, although rather fine. SEDIMENTARY ROCKS. North of jVIountain Iron and Hibbing, and westward, frag'mental rocks are intricately mingled with the Ai'chean igneous rocks (see Pis. IV and V). There is no positive evidence to show whether these rocks are Upper Huronian, Lower Huronian, or Archean. As they are closely folded with the Archean, it is probable that they are not Upper Huronian. Litholog- ically they resemble the Lower Huronian rocks, and hence they are 70 THE MESABI IRON-BEARING DISTRICT. described in couuection with the Lower Huronian. Nowhere in the district have sediments been found which are demonstrably of Archean age. However, certain facts seem to show that sedimentary rocks of Archeau age are actually present in the district. In the basal conglomerate of the Lower Huronian were found a few somewhat doubtful slate fragments and a single pebble of what is taken to be a fine-grained grit containing grains of quartz, feldspar, and iron oxide. While careful search has failed to reveal the counterparts of these rocks in the true Arcliean, it is possible that in the future they will be found. Indeed, it is not impossible that certain of the altered sediments included in the Archean and mapped as Lower Huronian may be truly of Archean age. On the other hand, the sedimentary fragments in the conglomerate of the Lower Huronian may have been brought in from distant areas and the Archean of the Mesabi di.strict in itself lack them. It makes little difference which is the case, for it is known that beneath the Lower Huronian rocks in the Lake Superior region are other subordinate sedimentary rocks associated with what has been mapped in the past as Archean. The pebble in the conglomerate here described offers additional evidence of this fact. Whether or not small areas of these Archean sedimentary rocks be found in place in the narrow confines of the Mesabi district is a matter of small importance. STRUCTUEE. The Archean rocks of the district, being igneous throughout, have only such structures as are characteristic of massive and schistose igneous rocks The original igneous structures have been mentioned above. While most of the Archean rocks show some cleavage, perhaps about half have enough cleavage to warrant calling- them schists. In general the plane of cleavage is nearly vertical and strikes parallel to the range, aliout N. 60° E. The hornblendic schists north of Mountain Iron have a cleavag-e of a linear parallel type, and the lines of the cleavage dip steeply to the northeast. In addition to cleavage there are many joints and faults with dis))lacoments of a few inches or feet, but no regular systems have been determined. RELATIONS TO OTHER SERIES. The Archean rocks, both basic and acid, form a basement u])on wjiich the sedimentary rocks of the region wei'e deposited, and hence between the Arclicaii and tlir nvo'K'iiiL;' rocks is a stnictural luiconfonuitv. IK' U. S. GEOLOGICAL SURVEY MONOGRAPH XLIII PL.V. LiUS BIEM aCO.LlTH.K."' THE BASEMENT COMPLEX, OR ARCHEAN. 71 The sediraentary rocks now lying next to the Archean are Lower Huronian for a part of the district and Upper Hiironian for another part. This results from the fact that the Upper Huronian is unconformably above the Lower Huronian and laps over the Lower Huronian onto the Archean. The Lower Huronian, near its contact with the Archean, is a coarse conglomerate, containing large pebbles and bowlders of ^he kinds of rocks found in the Archean, with the exception of some of the schists, which were formed by the mashing of Archean rocks subsequent to the deposition of the conglomerate. The actual contact of the Upper Huronian and Archean is drift covered, but from the known fact that the Upper Huronian is unconformably above the Lower Huronian and the Lower Huronian is unconformably above the Archean, it is certain that the Upper Huronian rests unconformably iipon the Archean. The Archean along its entire northeastern edge is in contact with granite which is intrusive into the Archean rocks. Actual contacts of the two are to be observed in a number of places, and at such places the Archean greenstones become micaceous or hornblendic. While some of the schists are clearly the altered equivalents of the Archean igneous rocks which have yielded pebbles to the conglomerates at the base of the Lower Huronian, others of the schists have not been proved to have resulted from the alteration of Archean rocks, but are supposed to be Archean from their lithological similarity to such rocks. In spite of such similarity, some of the schists may be of Lower Huronian age. To this doubtful group belong a part of those north of Mountain Iron, north- west of Hibbing, and in Rs. 22 and 23 W. CHAPTER IV. THE LOWER HURONIAN SERIES. DISTRIBUTION. Sedimentary rocks of Lower Huronian age appear in two considerable areas in the Mesabi district. One witli an average width of perhaps a niile extends from Eveleth northeast to Biwabik ; the other, somewhat less than a mile in width, extends from near the Duluth and Iron Range Railroad northeast to near the center of sec. 11, T. 59 N., R. 14 W. In the former belt there are areas of green schist forming the cores of the hills. One of them has been mapped, but others, while their presence is known by isolated exposm'es, are not sufficiently exposed to warrant their separation on the map. A number of small patches of Lower Huronian sediments are known also in other parts of the district, as follows: East of Biwabik, in the northern portion of sec. 1, T. 58 N., R. 16 W.; north of Biwabik, in sec. 34, T. 59 N., R 16 W. ; bordering the Archean north of the Genoa mine at Sparta; northwest of Virginia, along the line between sees. 31 and 32, T. 59 N., R. 17 W. ; northeast of Virginia, near the east side of sec. 34, T. 59 N., R. 17 W. ; bounding the Archean north of Mountain Iron, in sec. 34, T. 59 N., R. 18 W. ; intricately mixed with hornblendic schists and acid intrusives in a belt running through sees. 28, 27, 22, and 23, T. 59 N., R. 18 W. (see PI. V); northwest of Hibbing, in a narrow belt bounding the Archean in sees. 26, 34, and 35, T. 58 N., R. 21 W., also in deep drill hole beneath quartzite 1,025 paces north, 665 paces west, sec. 35, T. 58 N., R. 21 W.; in the area mapped as Archean in sees. 19, 30, and 20, T. 57 N., R. 22 W.; and near the contact of the hornblendic schist with the granite near the north line of sec. 2, T. 56 N., R. 23 W. Granite of Lower Huronian age forms the core of the Giants range and is exposed on its upper slopes from Grand Rapids eastward to near the east line of R. 14 W., with only one break, nortli of Mountain Iron, where it is interrupted for a short distance by Arcliean hornblendic schists. The granite tlms bounds on the north the other formations for most of the district. Our detailed work has not gone farther north than the granite boundary. 72 THE LOWER HURONIAN SERIES. 73 A dike of Lower Huronian porphyiy appears northwest of Biwabik, in the northern part of sec. 3, T. 58 N., R. 16 W. The porphyry in sec. 25, R.17W. R.i7W. Scale LEGEND ALGONKIAN UPPER HURONIAN Ab Biwabik (iron- bearing) formatioa Apq Pokegama quartzite LOWER HURONIAN Ahg Graywacke slate and congrlomerate ARCHEAN Basalt and porphyry m Outcrop ^ Outcrop (witJi bedding) » Out<;rop (amglomerote) a 0 K ImUe Contour interval 20 fee'^ Fig. 1.— Detail map showing distribution of upper Huronian, Lower Huronian, and Archean rocks northeast of Eveleth. T. 59 N., R. 18 W., described on page 69 with the Archean, may be Lower Huronian, but there is no evidence one way or the other. 74 THE MESABI IKON-BEARING DISTRICT. KIXDS OF ROCKS. The Lower Huronian rocks are both sediinentarj' and igneous. The sedimentary rocks include interbedded slate, graywacke, and conglomerate, and the igneous rocks include granite and porphyry. GRAYWACKES AND SLATES. The following description applies to the normal phase of graywacke and slate making up the bulk of the sedimentary portion of the series. The highly metamorphosed phases caused by the metamorphism of the granite are described in a separate section. The interbedded gray wackes and slates form the great bulk of the sed- iments. They are dull, dark-gray and dark-green rocks which usually weather to a somewhat lighter green or gray or to a dirty light yellow. The grain is usually fine, although it varies considerably. The bedding, shown by both color and texture, is conspicuous. Parallel to the bedding a secondary cleavage has been developed. As a result of variation in tex- ture, bedding, and secondary cleavage, there appear all gradations between metamorphosed coarse graywackes, banded graywackes, and finely fissile slates. Alf)ng the parting plane of some of the graywackes and slates may be seen glistening plates of mica or chlorite, conspicuous because of the fact that they appear in separate spangles on the dark background rather than in continuous layers, although, indeed, some of the more fissile slates show mica and chloi'ite in the continuous layers characteristic of slates. Under the microscope the graywackes and slates show little uniformity in texture and mineralogical composition. A composite slide from the less altered graywackes would show angular to subaugular grains of quartz and feldspar in about equal quantity and of rather unifoi-m, small size, cemented by a sparse, ill-defined matrix of the same material, in which there is nuich chlorite and micaceous material and cloudy alteration products of the feld- spar. While the particles are not well rounded, their general aspect leaves no doubt as to their clastic character. Certain slides show a predominance of quartz grains and others a predominance of feldspar grains. Certain slides have almost no cementing material ; in others it is so abundant as to make the clastic grains look almost like phenocrysts. In certain slides, again, tlio matrix is almost entirely an ill-defined greenish I'hloritic or THE LOWER HURONIAN SERIES. 75 micaceous siibstance ; in others, a fiiie-grained cloudy alteration of feldspar witli little of this material. The chlorite and mica in the matrix are in large, distinct plates parallel to the bedding. These are the ones which appear so conspicuously on the parting planes of the graywackes above referred to. The slates under the microscope show an exceedingly fine felty mass of quartz and feldspar almost obscured by an aggregate of micaceous and chloritic substances. In other words, they show the ordinary features of typical slates. A great variety of rocks intermediate between the gray- wackes and slates show microscopical features intermediate between those above described. In certain areas iron pyrites is fairly abundant in both the slates and the graywackes. This, while occasionally fresh, is for the most part altered to iron oxide, which retains the cubic form of the pyrites, or, if altered to iron ore, is weathered out altogether, being represented only by iron-stained cavities which frequently retain the cubic form. Iron pyrites may be especially well observed in the SW. | sec. 22, T. 58 N., R. 17 W. The graywackes and slates abovp described have resulted from the alteration of fine mud and feldspathic sand deposits. The metamorphism has consisted in their cementation into hard rocks, which has been brought about by the recrystallization of the finer materials in the background and perhaps the infiltration of quartz from without, and by the abundant development of chloritic and micaceous materials. Some of the mica, especially that in separate clear-cut plates, may have been originally deposited in its present position, but most of it, and especially that in continuous sheets on the parting surfaces, is undoubtedly a secondary development due to dynamic movement in the rock. In general, the mashing of the rock has not been sufficient to develop any secondary structure inclined to bedding, and its main effect has been in developing micaceous minerals parallel to bedding. CONGLOMERATES. The conglomerates are perhaps the most interesting of the Lower Huronian sediments. They are most abundantly and typically exposed in a belt running from the cut along the Duluth and Iron Range Rail- road in sec. 22, T. 58 N., R. 17 W., southwest through sees. 22 and 21 76 THE MESABI IRON-BEARING DISTRICT. into sees 20 and 29, T. 58 N., R. 17 W. Similar conglomerates are known in small patches bordering the greenstones north of the Grenoa mine, at Sparta. The conglomerates are massive rocks for the most part, with various shades of green on fresh surface and a lig'hter green on the weathered surface. The pebbles vary in diameter from 6 inches to a small fraction of an inch, in kind they are, for the most part, identical, both macro- scopically and microscopically, with the rocks in the Archean above described, including diabases, basalts, and granite-porphyries. The more basic pebbles are in greater quantity than the acid ones. One of the most characteristic pebbles is a peculiar, purplish, dark-green porphyritic basalt in which the phenocrysts, originally of feldspar, are now spots of greenish chloritic material. The conglomerates have a fine-grained green matrix, which was probably originally lai'gely of feldspar and quartz, but which is now almost obscured by chlorite and sericite alterations. The common green fragments and the green matrix of the conglomerates make the name "greenstone-conglomerate" very appropriate, and this, indeed, is what the rock has been called during the field work. In walking through the railroad cut above referred to, unless one looks very closely he is likely to suppose the rock to be an original basic igneous one. In other places many of the pebbles of the basic igneous rocks weather to a salmon pink, giving the impression that the rock is made up largely of porphyry pebbles. An examination shows that the apparently acid fragments are really basic, while the true porphyries weather grayish green and look basic. In addition to the common pebbles above named, there ^jpear a few pebbles of white and greenish-gray chert, which may represent altered slate (45494). Close examination of these fails to determine whether or not they are sedimentary slates, but one or two fragments are seen to have a very fine banding, which may indicate sedimentary origin. In the Duluth and Iron Range Railroad cut, also, one pebble was found which may be a fine gi'it or graywacke. It is a greenish-gray, fine-grained rock made up of roundish and subangular grains of quartz and much-altered feldspar in an abundant fine-grained matrix of similar materials, obscured by greenish alteration products. Throughout the rock are little specks of iron oxide. One of these appears on the weathered sm-face like a little fragmental THE LOWER HURONIAN SERIES. 77 grain of jasper, and was, indeed, the feature which first attracted attention to the pebble. The presence of this possible sediment in pebbles in the conglomerate at the base of the Lower Huronian series may indicate the presence of still older sedimentar}^ rocks somewhere in this area. Such an older sedimen- tary formation has been found in other districts of the Lake Superior region, the Vermilion and Marquette, but, with the possible exception of certain doubtful sediments north of Mountain Iron, no such rocks have been found in the Archean within the limits of this district. Further search may reveal them in small patches, but certainlj^ tliey occupy no considerable areas. In the NW. i sec. 34, T. 58 N., R. 17 W., just north of the Genoa mine, patches of conglomerate may be observed in the southerly exposures of the massive Archean greenstones. On weathered surface the light-gray, green, or pink angular to subangular fragments stand out consjDicuously from a dark-green matrix. On fresh fracture fragments and matrix have a dark-green color and can not be separated. They both resemble the underlying Archean basalt. The conglomerate is associated with a small quantity of banded rock of the same general character, which is probably an altered graywacke associated with the conglomerate. The rock next to the south is Pokegama quartzite, but the conglomerate has not been actually connected with the Pokegama quartzite, and because of its meta- morphosed character and similarity to the Lower Huronian conglomerates of other areas it is here described. The conglomerates, in common with the rest of the Lower Huronian rocks, have suffered metamorphism, but the extent of the alteration varies greatly from place to place. East of Mariska, in the railway cut referred to, the rocks show only re crystallization of the mineral particles, without marked development of schistosity. The alteration of the minerals is the same as that described above for the various rocks of the Archean. To the southwest of this cut the conglomerates have been much squeezed and are now very schistose. The recrystallization accompanying the squeezing, has made the rocks very chloritic and micaceous, and, in many cases at least, lias completely obliterated the clastic texture in the finer-grained portions. The pebbles have been elongated in the plane of schistosity (vertical and striking N. 60° E.), and on the weathered surface stand out in lenticular and 78 THE MESABI IRON-BEARIiNG DISTRICT. oval forms from the finer, more schistose, and more easily eroded mati'ix. Rocks of this character may be traced into schistose rocks in which, in peb- bles and matrix alike, nearly every vestige of sedimentary texture has been lost. GRANITES AND PORPHYRIES (Porphyritic Granites and Porphyritic Rhyolites). Lower Huronian granites form a continuous belt along the higher parts of the Giants range from near the east line of R. 14 W. to the west end of the district, except for a short distance north of Mountain Iron, where they are cut out by the Archean hornblende-schists. They also make up part of the shores of Birch Lake. Over this great area the granites show considerable lithological complexity. At Birch Lake the Lower Huronian granites are coarse gray and pink hornblende-granites. From the east line of R. 14 W. to the neighborhood of Mountain Iron the granites are similar to those on Birch Lake. It is noticeable that the coarser phases appear in the eastern end of this area. The hornblende varies in abundance, but is usually conspicuous. Rarely, as near the Mailman camps, the dark constituent iis augite (45435) instead of hornblende, or again it may be partly biotite. The feldspar is partly orthoclase with Carlsbad twinning, partly microcline, and in small part plagioclase, and all of it shows cloudy alteration and a zonal structure indicating two stages of growth. Occasion- ally also it appears in porphyritic form. The hornblende is a fresh green variety. Quartz is present, but very sparsely; indeed, certain phases of the rock have so little quartz that they might perhaps be called syenites. A characteristic accessory is brown sphene, showing in places stages of altera- tion from ilmenite. In places the rock becomes very slightly gneissic, and immediately next to its contact with the Lower Huronian sediment it becomes very fine grained. Next to the contact of the granite with the Keweenawan gabbro on Birch Lake is a metamorphic rock resembling granite, which is described in connection with the gabbro. From the neigborhood of Mountain Iron westward to the west end of the district the preponderating granite is somewhat finer grained than the granite to the east, possibly somewhat more gneissic, and usually of a pink color. Certain phases of this finer granite are similar to the hornblende- granite to the east, but by far the larger portion shows a considerably greater content of (juartz and a smaller content of the basic minerals. THE LOWER HURONIAN SERIES. 79 Instead of hornblendes we have in these rocks green or brown biotite and muscovite. The feldspar is partly microline and partly orthoclase, as in the hornbleude-granites, and the alteration of the feldspar is about the same. As in the hornblende-granites, also, the feldspar crystals occasion- ally stand out in porphyritic fashion. Associated with these two prevailing types are dikes of exceedingly fine-grained pink granite showing very little biotite. They may be well observed in the cuts along the main line of the Duluth and Iron Range Railroad. Other dikes are pegmatitic granite consisting of a pink feldspar with verjT- abundant quartz, and with the ferromagnesian minerals almost totally lacking. They may be seen to advantage at the upper falls of the Prairie River. In connection with the Lower Huronian granites should be mentioned a dike of feldspar-porphyry intruding the Lower Huronian sediments just northwest of Biwabik, in the NW. J of NW. ^ sec. 3, T. 58 N., R. 16 W. It is a very fine-grained, grayish, acid rock which under the microscope shows orthoclase-feldspar phenocrysts, now much altered, lying in the usual altered matrix of quartz, feldspar, and chloritic material. North of Mountain Iron, in sec. 34, T. 59 N., R. 18 W., near the contact of the granite, hornblende-schists, and Huronian sediments, is an exposure of a porphyritic rh3^olite in which quartz and feldspar phenocrysts of about equal abimdance stand in a fine-grained but holocrystalline matrix of quartz and feldspar with a spherulitic texture. There is no direct evidence to show whether the rock is Lower Huronian or Archean. At 1,400 steps north of the southeast corner of sec. 16, T. 59 N., R. 14 W., in the Lower Huronian sediments, is a dike, about 25 paces wide, of a dark-gray, fine-grained, schistose, chloritic and hornblendic granite. The rock under the microscope is seen to consist of orthoclase feldspar showing considerable alteration to sericite, kaolin, and zoisite, which is in about equal abundance with hornblende and chlorite. The hornblende is a green variety showing little alteration and usually having crystal foi-m. The chlorite is secondary. In sec. 11, T. 59 N., R. 14 W., and northwestward for a mile and perhaps more is a fine-grained porphyritic rhyolite or granite which apparently is intnided by the Lower Huronian granite. As to its age, all that can be said is that it is older than the Lower Huronian granite, but 80 THE MESABl IRON-BEARING DISTRICT. whether Archean or Lower Hurouian there is no dh-ect evidence. Because of its close association with the granite in its distribution and its dissimihxrity to the Archean porphyries, it is described in this connection rather than with the Archean. The distribution of this rock in sec. 11 is shown by the detailed sketch map (PI. VI), The rock is gray or pink, fine grained, and contains minute red and gray streaks. Under the microscope the texture is seen to varj^ from porphyritic to granitic. In certain slides large feldspar phenocrysts stand in a fine-grained granular matrix of quartz with a sub- ordinate amount of feldspar. The feldspar shows in places strain shadows, fracturing, peripheral granulation or even total granulation. In other slides the quartz and feldspar particles occur in about equal size. It is possible that the porphyritic texture may be in part the result of the granulation of a part of the constituents, lea'sdng- the remainder as pheno- crysts in a granulated background. In addition to the quartz and feldspar there are present greatl}^ varying but usually small quantities of horn- blende, biotite, and chlorite. Under the microscope the similarity of this rock to the altered phases of the Lower Huronian graywacke is striking. In hand specimens, however, they may be discriminated. INCLUSIONS IN GRANITE. Through a considerable portion of the district, and particularly north of Mountain Iron and Hibbing and from there westward, there are found intricately mixed up with the granite, and not separable on a map of ordi- nary scale, small quantities of hornblendic schist, chloritic schist, micaceous schist, diorite, basalt, or diabase, or their metamorphosed equivalents. Some of tlie micaceous schists are the altered equivalents of the granite, and some of the diorites are also apparently genetically connected with the granite; such rocks are of Lower Huronian age. Other rocks, par- ticularly the hornblendic schists, chloritic schists, diabases, and diorites, are intruded by the granite, and these might be either Lower Huronian or Archean. Still others, and perhaps the larger projiortion, are so intricately mixed with the granite that no determination of their age can be made. Many of the phases can be duplicated in the area mapped as Archean, and Indeed the line between the granite and the Archean in man}- places is determined by the relative abundance of these rocks. It is certain therefore that a considerable proportion of the rocks included in the granite are of Archean age. U.S. GEOLOGICAL SURVEY ■MTONDGTiAPH XLIII PL. VI LEGEND ALGONKIAN KEWEENAWAN |~AKgr [ Granite UPPER HURONIAN Cordierite hornstone of Virginia formatioa Interstratified slate of Biwabik (iron-bearing) formation Ferruginous chert of Biwabik (iron-bearing) formation Apq Pokegama quartzite LOWER HURONIAN Ags Graywacke and slate Afp Feldspar porphyry Outer op X DETAIL MAP SHOAVIKG DISTRIBUTION OF KEWEENAWAN, UPPER HURONIAN AND LOWER HURONIAN ROCKS IN THE VICINITY OF THE MALLMAN CAMPS Scale q M ii 1 mile Contour interral 20 feet 1 i PLATE VII. MON XLIII — 03 6 81 PLATE VII. PHOTOMICROGRAPHS OF NORMAL AND METAMORPHOSED LOWER HUROXIAN GRAYWACKE. Fig. a. — Lower Huronian graywacke. Specimen 45412, slide 15697. From 1,300 paces north of west of the southeast corner of sec. 16, T. 59 N., R. 14 W. With analyzer, x 50. This is the normal phase of Lower Huronian graywacke, consisting of quartz and feldspar grains, mainly the former, very imperfectly rounded, and a considerable amount of secondary biotite and muscovite or sericite. All the constituents have a dimensional parallelism, and the micas have also a crystallographic parallelism. Described pp. 74-75. Fig. B. — Lower Huronian graywacke. Specimen 45414, slide 15700. From 1,680 paces north of west of the southeast corner of sec. 16, T. 59 N., R. 14 W. With analyzer, x 50. This is nearer the intrusive granite contact than the specimen shown in A, and shows a more abundant development of the secondary minerals and a coarsening of the grain. The grain is coarser and more irregular, due to the recrystallization of the quartz and feldspar. Abundant biotite and green hornblende have developed; muscovite is nearly lacking. Abundant accessories are magnetite, ilmenite, rutile, sphene, and garnet. The rutile may be seen surrounded by and altering into sphene (titanomorphite). Described pp. 83-84. Fig. C. — Lower Huronian graywacke. Specimen 45415, slide 15701. From near the southeast corner of sec. 9, T. 59 N., R. 14 AV. AVith analyzer, x 50. This is still nearer the granite contact than the specimens figured as A and B, and shows correspondingly coarser crystallization and more abundant development of secondary minerals. The hornblende is the dominant secondary constituent, and biotite is almost lacking. The same accessory minerals are present as in fig. B. Described pp. 83-84. Fig. B. — Lower Huronian graywacke. Specimen 45416, slide 15703. From near the east quarter post of sec. 9, T. 59 N., R. 14 AV. AA'ith analyzer, x 50. The slide is cut from within an inch of the granite contact and shows the coarse recrystallization and abundant development of secondarj' min- erals in the Lower Huronian graywacke. The feldspar shows cloudy alterations, and the quartz shows undulatory extinction. The dark mineral is almost entirely fresh, green hornblende. The accessories are sphene, rutile, ilmenite, and epidote. If this rock were found by itself and not connected by gradations with normal graywacke and slate it could not be recognized as a derivative of a sedimentary rock. Described pp. 83-84. 82 U. S. GEOLOGICAL SURVEY MONOGRAPH XLIII PL. VII PHOTOMICROGRAPHS SHOWING PROGRESSIVE METAMORPHISM OF LOWER HURONIAN GRAYWACKE IN APPROACHING INTRUSIVE GRANITE. THE MERIDEN SRAVURE CO. THE LOWER HURONIAN SERIES. 83 VEIN QUARTZ. In both the Lower Huronian sedimeutar}' and granitic rocks, particu- larly the former, there are abundant veins of quartz, resulting from infiltra- tion along joints and brecciated zones. This vein quartz has yielded numerous and conspicuous pebbles to the conglomerates at the base of the overlying Upper Huronian series. At the contact of the granite and the Lower Huronian series, also, there has been a segregation of quartz in irregular veins and stringers, and in this case it is believed that such quartz was in part deposited from hot solutions accompanving the intrusion of the granite. META3IORPHISM OF LOWER HURONIAIV ROCKS BY GRANITE. The intrusion of the granite above described has further greatly meta- morphosed the graywackes and slates, which are themselves the altered equivalents of muds and sands. In approaching the granite they become more chloritic, hornblendic, and micaceous, and a marked, and usually much contorted, schistosity obliterates the bedding. They become, in short, chloritic, hornblendic, and micaceous schists. The planes of parting have Colors characteristic of chlorite, hornblende, and mica, and when weathered not infrequently exhibit silvery and bronzy lusters. Under the microscope the rock may be seen to have undergone extensive alteration. There has been abundant development of secondary chlorite and horn- blende and a lesser development of secondary biotite and muscovite. Abundant accessories characteristic of metamorphic rocks of this nature are present. They include tourmaline, staiirolite, g-arnet, rutile, ilmenite, ' magnetite, and apatite. The alteration of the ilmenite and rutile to sphene (titanomorphite) is well exhibited. (Specimen 45414.) It is noticeable that the development of secondary minerals is greater in rocks showing more feldspar and less in rocks consisting mainly of quartz, as would be expected. Accompanying this development of new minerals there has been a recrystallization of the original quartz and feldspar, which has resulted in increasing the size of the grains and in obliterating all evi- dence of their clastic character as well as of bedding. Rarely, also, the quartz particles have been made to lie with their principal axes parallel to the schistosity, thus showing crystallographic as well as dimensional 84 THE MESABI IRON-BEARING DISTRICT. parallelism. (Specimen 45492.) In the most altered phases feldspar crystals, which are several times the size of any found in the unaltered graywackes or slates, and which have somewhat irregular outlines, stand out among smaller quartz grains, and are larger than any of the feldspars in the unaltered graywackes. Following the recrystallization there has been a considerable cloudy alteration of the feldspar. Where the original rock was mainly quartz the grain of the metamorphic equiA-alent is uniformly finer than the grain of the rock originally strongly feldspathic. Between and around the quartz and the feldspar are abundant fresh secondary chlorite and hornblende and less abundant mica, in general roughly parallel, but in detail following the peripheries of the quartz and feldspar grains. While the highly developed schistose structure shows that the rocks have undergone great compression, none of the mineral constituents show any strain effects whatever because of the complete recrystallization. Starting at some little distance from the granite contact, the graywackes and slates are of the kind above described as normal for the formation. In approaching the contact the metamorphic features just described become more and more evident, until we find their typical development imme- diately at the contact. Were it not for the complete gradation it would not be possible from the character of the rocks to show that the highly altered schists near the granite are really of sedimeiitary origin and the metamorphosed equivalents of the graywackes and slates. The series of photomicrographs (PI. VII) show how the microscopic aspect of the gray- wackes and slates changes in approaching the granite. The hornblendic graywackes associated with the Archean hornblendic schists north of Mountain Iron and Hibbing correspond in all essential features with the hornblendic graywackes formed liy the contact of the granite. THE RET^ATIONS OF LOWER HURONIAN GRAIS^ITE TO SEDIMENTS, AND RELATIONS OF BOTH TO OTHER SERIES. The granites are throug-hout intrusive into the Lower Huronian sedi- ments. Actual intrusive contacts are to be observed in a number of places. The Lower Huronian shows the metamorphic effects of the intrusion, and near the contacts no conglomerates are to be observed. The contact of the "■ranite and Lower Huronian sediments is well exposed northwest of THE LOWER HCRONIAN SERIES. 85 Mesaba station in the SE. J of SE. i sec. 18, T. 59 N., R. 14 W., and northeast of Mesaba station 50 steps north of the east quarter post of sec. 9, T. 59 N., R. 14 W. Near the contact at the former place the graywacke is shot through and through with stringers of granite. The ahernating layers of graywacke and granite in some instances vary from a fraction ol an inch up to several feet. In general, the injection with the granite has been parallel to the planes of schistosity in the altered graywacke, but in a number of places the granites may be seen cutting across the schistosity. The sketch (fig. 2) shows the intricacy of the contact at this place. The contact effect of the granite on the sediments has already been described. The granite itself close to the contact becomes very fine grained, but otherwise does not differ essentially from tlie granite of the main mass. Fig. 2.— Sketch of contact of Lower Huronian granite and graywacke-slate, showing intricate nature of granite Intrusion. Another even more complex contact may be observed north of Mountain Iron, northwest of the center of sec. 34, near the Archean- Upper Huronian boundary. While the evidence is conclusive that the great mass of the granite is intrusive into the Lower Huronian, it is not at all certain that, for limited areas, the granites here mapj^ed and described as Lower Huronian may not contain granite of later date. The granites show great lithologic com- plexity, and where the points affording evidence of relation are as widely separated as they are, particularly in the western portion of the range, parts of the granite may be intrusive into the main granite mass and thus per- haps be of post-Lower Huronian date, and still no evidence of this appear. Because of the lack of exposures, it is not unlikely that even the most 86 THE MESABI IRON-BEARiNG DISTRICT. detailed field work, with this point alone in mind, would fail to delimit the later g-ranites. The conglomerate forming the great part of Lower Hiironian sedi- ments afiPords conclusive proof that the Lower Huronian sediments rest unconformably upon the Archean rocks. Every kind of pebble found in this conglomerate, with the possible exception of a few cherty slate pebbles, can be matched among the Archean rocks. The conglomerate can best be studied in the cut in the Duluth and Iron Range track east of Mariska, in the NE. i of NE. i sec. 22, T. 58 N., R. 17 W., and northwest of Biwabik, in the SW. i of SW. i sec. 34, T. 59 N., R. 16 W. At the latter place the actual contact of the two formations can be observed and the conglomerate at the base of the Lower Huronian contains pebbles identical with the adjacent Archean igneous rocks. Both the Lower Huronian sediments and granites are unconformably underneath the Upper Huronian series, as shown both by structure and by conglomerates at the base of the Upper Huronian sediments. This uncon- formity is described in connection with the Upper Huronian series. STKUCTLrRE. The Lower Huronian sedimentary series shows a conspicuous sedi- mentary bedding. The area has been so folded that the beds now stand on edge, the dip seldom varying more than 5° or 10° from vertical. Superposed upon the original bedding structure is an excellent secondary cleavage. The cleavage planes, for the most part, are approximately parallel to the bedding planes. The strike of both bedding and cleavage is uniform, about N. 60° E., although locally var}-ing 10° to 20° from this direction. Both the Lower Huronian sediments and the granites are jointed, the sediments particularly so. The sediments, moreover, show conspicuous faulting- and brecciation. These features may be well observed 425 paces west, 275 paces south of the northeast corner of sec. 32, T. 58 X., R. 17 W., and just south of the northwest corner of sec. 3, T. 58 N., R. 16 W. The breccias at these places might be mistaken for conglomerate, especially as at the latter place there is also present a small amount of true conglomerate (see p. 96), but they are believed to be breccias, for the following reasons: (1) The fragments are identical with the material of the THE LOWER HURONIAN SERIES. , 87 strata adjacent. (2) The fragments are angular; certain quartz fragments are rounded, but the rounding is due to the pinching out of quartz veins; intermediate steps of the process are to be observed. (3) The interstitial material is largely vein quartz. (4) Finally, the so-called breccias occur in definite vertical zones, striking almost north and south; that is, almost directly across the sedimentary bedding. The supposed breccia grades into the unbroken strata, which have a normal strike on each side. More- over, in attempting to match the beds on different sides of the brecciated zones it is found that there has been faulting, oftentimes as much as several feet. THICKIfESS. As the bedding stands directly on edge, the width of the formation across the strike may measure the thickness of the series. On this basis the thickness may amount to 7,000 feet. However, the beds may repre- sent limbs of a closely compressed fold, or j)erhaps several folds, which have been truncated, and in such a case the apparent thickness of the series is much greater than the true thickness. Large areas of the formation are not exposed, and while there is no positive evidence of duplication of beds the probability is that they are duplicated. In view of this probability, 3,000 to 5,000 feet is probably as great a thickness as can safely be assigned to the Lower Huronian sediments of the Mesabi district. CHAPTER V. THE UPPER HURONIAN SERIES. The sedimentary rocks of Upper Huroniau age occup}^ practically all the southern slopes of the range from one end of the district to the other, and extend also an unknown distance south beneath the g'lacial diift. The surface width of the series in the ai'ea included in the district described varies from less than 1 mile to 5 miles or more. The beds of the series have a flat dip to the south. Their upper edges being truncated, they appear in belts winding along parallel to the range, the northerly belts representing the lower beds and the southerly belts the higher beds of the series. The exposures of the Upper Huronian, particularly on the lower slopes, are so widely separated that the mapping of the series would have been an impossibility had it not been for numerous test ^jits sunk in search for ore, whicli were bottomed in the Upper Huronian series. These are particularly numerous along the central portion of the range, and have enabled the distribution of the Upper Huronian rocks to be indicated within rather ("lose limits for this part of the range. The U]:)per Huronian series comprises from the base up (1) the Pokegama formation, consisting mainly of quartzite, but containing also conglomerate at its base; (2) the Biwabik formation, consisting of ferruginous cherts, iron ores, slates, greenalite rocks, and carbonate rocks, with a small amount of coarse detrital material at its base; and (3) the Virginia slate. Between the Pokegama quartzite and the Biwabik forma- tion there is a slight erosion interval. Tlie Biwabik forniati(M\ grades conformably into the Virginia slate both vertically and laterally. In all previous geologic work on the district the detrital rocks forming the base of the iron formation (quartzite and conglomerate) have been con- sidered a part of the Pokegama formation, the presence of the slight break between such detrital rocks and the underlying Pokegama formation having been overlooked. On tlie iiccoiupanx iug geologic map, also, the 88 THE UPPER HURONIAN SERIES. 89 basal detritals of the iron formation have been included in the Pokegama qnartzite. This is done for the reasons that: (1) The layer of basal iron- formation fragmentals for the most part is so thin that it can not be indicated on the general map without exaggeration; (2) in much of the district exploration has not been sufficient near the boundary of the iron formation to allow of the discrimination of the quartzite and conglomerate R. 17 W R. 17 W. Scale LEGEND ALGONKIAN Ferruginous chert of BiwabikCiron- bearingOformation Abq Quartzite of Eiwabit (iron-bearing) formation Apq Pokegama quartzite Outcrop Fig. 0 X JimUe Contour interval 20 feet -Detail map of sec. 3, T. 58 N., E. 17 W., showing separation of quartzite at the base of the Biwabik formation from the Pokegama quartzite. f A correction of this map is shown on PI. II.) of the iron formation from that belonging with the Pokegama formation; (3) for economic purposes it is more desirable to indicate the boundary between the possible iron-bearing area and quartzite, regardless of the formation to which the latter belongs, than between the two structural geologic units. In order to show that discrimination between the Poke- gama quartzite aud the detrital material at the base of the iron forma- tion is possible where exploration has been sufficient, a detailed map has 90 THE MESABI IRON-BEARING DISTRICT. been made of a part of sec. 3, T. 58 N., R. 17 W. (fig. 3), where the relations of the Pokegama quartzite and the iron formation were first satisfactorily worked out. "While, for the reasons stated above, all the detrital material at the base of the iron formation has been included in the Pokegama formation on the general map (PI. II), the description of the Biwabik formation on a subsequent page includes the detrital material belonging with it. SECTIO^r I. THE POKEGAMA QUARTZITE. DISTRIBUTION. The Pokegama quartzite is the basal formation of the Upper Hurouian series. Because of the southerly dip and truncation of the series, the quartzite appears as a belt immediately south of and contiguous to the Lower Huroniau and Archean formations. The belt, varying from a few steps to a half mile or more in width, extends from the west end of the Mesabi district continuously to north of Mountain Iron. From here on to the east end of the range data are insufficient for mapping the quartzite as a continuous belt, and it is accordingly mapped as a number of discontinuous areas of varying width and length. It is possible that future exploration w ill result in extending and connecting some of these areas, but it is also certain that some of them are really cut ofP from one another because of the overlapping of the iron formation. The typical Pokegama quartzite is exhibited in exposures at Pokegama Falls, on the Mississippi River; at Prairie River Falls (fig. 4), north of the Arcturus mine in sec. 13, T. 56 N., R. 24 W.; north and northwest of Hibbing; near the south quarter post of sec. 20, T. 53 N., R. 17 W.; at the east end of the range in sees. 29 and 32, T. 60 N., R. 13 W.; and test pits and drill holes have been bottomed in the quartzite at many intermediate points. KINDS OF ROCKS. The Pokegama formation comprises vitreous quartzites of various colors and textm'es, micaceous quartz slates, and conglomerates. QUARTZITE. The bulk of the Pokegama formation is a vitreous quartzite. Bedding is well marked by alternating light and dark bauds, and rarely ripple marks may be oljserved. The quartz grains are well rounded, of medium size, THE POKEGAMA QUARTZITE. 91 and in g'eneral are bettei- rounded and coarser than in the Lower Huronian graywackes. The colors are dark green, grayish green, light yellow, or Quartzite 25 o .^75 feet Fig. 4. — Sketch showing distribution of Biwabik and Pokegama formations at the lower falls of Prairie Eiver. 92 THE MESABI IRON-BEARING DISTRICT. various shades of red and brown. In some cases at least the original colors have been yellowish and grayish green, and the red and brown colors have resulted from the infiltration of ferric iron and from the oxidation of ferrous compounds in the matrix of the quartzite. At Pokegaraa Falls the quartzite, for some feet from the surface, has a reddish color, but where blasted out near the dam the red colors are seen to give wav a few feet from the surface to the grayish or yellowish ones. In cracks and cre^aces the iron staining has penetrated much deeper. In other rocks the yellow or red colors are due to numerous grains of iron oxide, mainly limonite, associ- ated with the quartz grains. Weathering has not only discolored the quartzite, but has caused it to disintegrate to a certain extent by softening the cementing material. This phenomenon may also be observed at Poke- gama Falls. Under the microscope the quartzite is seen to be made up of well- rounded, sometimes subangular, grains of quartz and an occasional grain of microcline feldspar. In the proportion of quartz and feldspar the quartzite differs from the Lower Huronian graywackes, in which the feldspar and quartz are in al^out equal abundance. The fragmental grains show at places consideral^le effects of pressure by their undulatory extinction and cracking, but seldom are these effects conspicuous. More frequently are the grains cloudy, due to the inclusion of minute specks of other minerals. It is a noticeable fact that the clastic grains in the finer quartzites are less well rounded than in the coarser ones. Among the quartz grains there appears here and there a granule of iron oxide, mainly limonite, which in some cases seems to have partially or wholly replaced quartz grains and in others is mixed with a greenish or yellowish chloritic substance in such a way as to suggest that it may have replaced a granule of some mineral containing ferrous silicate. The appearance and distribution of some of the iron oxide grains strongly sug- gest their development tin-ough the alteration of iron jiyrites in an originally pyritiferous quartzite, but while such development is probable, no direct evidence of it has been observed. The cementing material may be (1) quartz which has grown out in optical continuity with the original grains, containing abundant iron oxide and chloritic discolorations, or (2) a confused aggregate of greenish material wliifli, with a high power, is found to be mainly chlorite, with subordinate amounts of actinolite or griinerite, (juartz, and feldspar. The green matrix THE POKEGAMA QUARTZITE. 93 may be so abundant as to make the rounded clastic quartz grains stand out from it like phenocrysts, or it may be sparse and give way to the discol- ored quartz cement formed by the enlargement of the quartzite grains. The oxidation of the ferrous iron in the chloritic cement in certain of the rocks, particularly the weathered ones, has yielded red and brown hematite, which has imparted to the cement a red or brown color. In other rocks iron oxide has been infiltrated from above, discoloring the cement in the same way. Comparing the slides with the hand specimens, it is seen that the color of the quartzite, as would be expected, is due to the nature of the cementing material. Where the cement is mainly quartz formed by the enlargement of the clastic grains of quartzite the rock is one of the lighter colored gray or yellow ones. Where the cement is an abundant chloritic substance the quartzite is dark gray or dark green. Where considerable hematite has been infiltrated or developed by alteration the quartzite has distinctly reddish or brownish colors. The dark-green and dark-gray quartzites, with an abundant chloritic matrix, are similar in general aspect to graywackes, but the clastic feldspar grains are too few to wai-rant the application of this name. The chloritic cement in the quartzite has been particulai-ly studied because of its close resemblance to the green ferrous silicate occurring in granules in the over- lying iron formation. It is indeed possible that a very small part of the material here called chlorite may in reality be a ferrous silicate corres|)ond- ing to that described in the iron forinatiou on a subsequent page, but no positive evidence of this has been found, and, on the contrary, positive evidence of the chloritic nature of most of it is at hand. Where the Pokegama quartzite is in direct contact with the basic igneous rocks of the Archean it takes on a character different from that noi-mal to the formation. In sec. 33, T. 59 N., R. 15 W., for instance, the particles, instead of being well-rounded quartz grains, are complex grains derived from the breaking down of the fine-grained underlying basalt, and both fragments and matrix are much discolored b}' green chloritic and hornblendic alteration. MICACEOUS QUAETZ-SLATE. Closely associated with and interbedded with the massive quartzites above described are thin-bedded slaty quartzites, or quartzitic slates, with a considerable amount of mica and excellent parting along bedding planes. 94 THE MESABI IRON-BEARING DISTRICT. They are very dififerent in appearance from many of the quartzites, and were they not actnally observed to grade into and to be conformably bedded with the quartzites at a number of places they would scarcely be referred to the same formation on lithologic grounds. They usually, though not always, overlie the massive quartzite. This may be well observed just northeast (jf Prairie River Falls (see fig. 4) and just southwest of the center of sec 18, T. 59 N., R. 1 4 W. The color varies from dark greenish gray to a light yellow or pink or red. The red and pink colors are frequently due to the infiltra- tion of iron oxide from above along bedding planes. The texture varies from that of a medium- grained quartzite to that of a rather coarse shale. The conspicuous features of the rocks are their excellent parting parallel to bed- ding planes and the mica plates on the parting plane. For the most part the parting planes are smooth, with slight ridges roughly resembling fine ripple marks, bnt not uncommonly they are somewhat rough and contorted. On these surfaces the mica does not form a continuous layer, but appears in separated plates, each with its own twinkling reflection. Under the microscope the quartz-slates are seen to be finer grained than the quartzites described above. The grains are more angular; the interstitial chloritic aggregate is more uniformly present; evidence of enlargement, while present, is not so conspicuous as in the quartzite; and, finally, the quartz-slates possess mica, while the quartzites do not. The frao-mental grains in the quartz-slate are mainly quartz and rarely microcline, as in the quartzite, and the green chloritic material is of the same nature. Only rarely are the effects of pressure to be observed. The mica is in separate flakes, with their greater diameters parallel to the bedding, as are also the longer diameters of the quartz grains. It resembles in its occur- rence the clastic mica plates sometimes seen in a sedimentary rock demon- strably unaltered. In a few cases where the quartz-slate gives evidence of having undergone much squeezing and alteration the mica is much more abundant and clearly secondary, and in such cases it has a tendency to form continuous layers along the parting planes rather than to occur in isolated flakes with definite outlines. CONGLOMERATES. From a structural standpoint the conglomerates are the most interesting rocks of the Pokegama formation. They form a very irregular layer but a few feet thick at the base of the quartzite. THE POKEGAMA QUARTZITE. 95 In the SE. J of SE. J sec. 18, T. 59 N., R 14 W., the conglomerate is in several small patches overlj'ing both the Lower Huronian graywacke and slate and the Lower Huronian g-ranite. The next rocks to the south are of the iron formation, and although there is no evidence of Pokegama quartzite occurring in this immediate vicinity, there is plenty of room for it, and it is known less than a half mile to the northwest. There is thus reason to believe that future exploration may show it here, but whether or not the quartzite or the iron formation immediately overHes the conglom- erate at this point, the conglomerate is basal to the Upper Huronian, and thus should be described at this place. A few steps west and north of the southeast corner of sec. 18 the conglomerate can be observed in a thin layer mantling over the Lower Huronian slate and graywacke. Neither the bed- ding of the conglomerate nor that of the underlying graywacke or slate is clear, but so far as any structure is present in either it is a horizontal one in the conglomei-ate and a vertical one in the lower series. The pebbles of the conglomerate are fairly small, sometimes reaching a size of 2 or 3 inches, but commonly being an inch or less. The most con- spicuous fragments are white, gray, or black vein quartz, but these are scarcely more abundant than pebbles of graywacke and slate identical with those of the underlying Lower Huronian series. With these abxindant pebbles are a few scattering and doubtful pebbles of granite. At 425 steps north and 460 west of the southeast corner of sec. 18 is again a thin layer of conglomerate mantling over the Lower Huronian granite. Only a short distance awa}^ the granite is found intrusive in the Lower Huronian sedi- ments, and hence the Upper Huronian age of the conglomerate is unques- tionable. Here the conglomerate contains pebbles and bowlders up to 2 feet or more in diameter, consisting of graywacke and slate, white, gray, or black chert, granite identical with that immediately underlying, and in addition pebbles of a fine-grained granite similar to that sometimes seen in dikes in the Lower Huronian granite. Certain pebbles of doubtful character may represent basic ig-neous rocks. The matrix is a quartzite in which the quartz grains are well rounded and set in a dark greenish- or purplish- black matrix, discolored by chloritic substances. In general the conglom- erates described in this area contain a typical assemblage of pebbles and bowlders representing the various phases of rock found in the immediately underlying Lower Huronian. 96 THE MESABI IRON-BEARING DISTRICT. A little southwest of the center of the XW. i of SW. ^ sec 33, T. 59 N., R. 15 W., is a pit which has passed through quartzite and conglomerate into Archean basalt. The conglomerate is a much metamorphosed one, containing basalt pebbles identical with the basalt below. The conglomerate is thus unconformably upon the Archean, and is basal to the quartzite. Near the powder house, in the NW. ^ of NW. J sec. 3, T. 58 N., R. 16 W., is a thin film of conglomerate on the upper svirface of the southernmost exposure of Lower Huronian slate and graywacke. The next rock to the south is the Pokegama quartzite. The conglomerate is composed mainly of vein quartz and slate, with a few feldspar porphyry pebbles. The vein quartz and porphyry pebbles are well rounded, while the slate fragments are angular. It is believed that this may be a part of the conglomerate at the base of the Pokegama quartzite. However, just to the north and north- west, in the Lower Huronian area, are certain breccias (see pp. 86-87) which are almost identical with the conglomerate except that the fragments are not so well rounded. It is not impossible that this supposed conglom- erate may be, in part, a breccia, although much of it certainly is not. On the road a little north and a little east of the northeast corner of sec. 3, T. 58 N., R. 16 W., in the drainage ditch close to the Biwabik mine, is an obscure conglomerate in contact with the Archean green schists. The rock is reddish, and the fragments, as nearly as can be made out, are of green schistose rocks like those of the Archean subjacent, but very niucli altered. The rock immediately to the south is the Pokegama quartzite. Just north of Roberts mine at McKinley are pits which have passed through Pokegama quartzite and conglomerate into Lower Huronian gray- wacke and slate. The conglomerate at the base of the Pokegama contains pebbles of the graywacke and slate below. Noi'th of Mountain Iron, just northwest of the center of sec. 34, T. 59 K., R. 18 W., are small patches of cong-lomerate intricately mixed up with the Archean and Lower Huronian rocks which come together at this ]ioint. (See PI. V.) The Upper Huronian rocks occur just to tlie southeast across a little valley. The older rocks show much intricacy of structure, erosion lias consequent!)' cut down into them unequally, and finally some of the contacts are drift covered, so that it has been exceedingh* difficult to determine the ti'ue relations of the conglomerate and the adjacent rocks. I I i THE POKEGAMA QUARTZITE. 97 However, there appear to be here Archean hornblende-schists and Lower Hiirouian graywackes and slates, and both of these are intruded by Lower Huronian granite. On top of this complex are patches of con- glomerate containiiag well-rounded pebbles of white weathering slate and graywacke, I'eaching a size of several inches, and resembling the Lower Huronian sediments near at hand. A few doubtful pebbles may be Archean igneous rocks. No granite pebbles were observed. The matrix is a well- bedded graywacke, much contorted, of dark-green or dark-gray color, with mica plates on bedding planes. It seems likely that the conglomerate is basal to the Upper Huronian. But there is a distinct possibility that the conglomerate after all is basal to the Lower Huronian sediments, and that the pebbles of sediments observed are from the Basement complex. This possibility is suggested by the similarity in appearance of the matrix of the conglomerate to the typical Lower Huronian graywackes close at hand and by the absence of Lower Huronian granite pebbles. In sees. 25 and 26, T. 58 N., R. 21 W., north of Hibbing, are large bowlders of conglomerate in the drift. They are foiind just south of the northern boundary of the quartzite and were undoubtedly carried there by the glaciers from this boundary. They are similar to the conglomei-ates north of Mesaba station, their principal pebbles being graywacke and slate, vein quartz, and granite, and in addition there are seen fragments of chlorite-schists and mica-schists similar to these rocks in the Lower Huron- ian and Archean areas adjacent. On the west line of sec. i^4, T. 58 N., R. 21 W., on the south escarp- ment of the southernmost exposure of Lower Huronian granite, is a con- glomerate with pebbles up to 4 or 5 inches in diameter of vein quartz, white, gray, brown, and reddish, and of granite identical with that of the solid ledge underneath. As usual the fine quartz pebbles ai-e the most conspicuous. At 1,200 steps south and 400 west of the northwest corner of sec. 3, T. 56 N., R. 23 W., is a conglomerate lying on the eastward -facing escarp- ment of the granite, that is, between the granite and the Pokegama quartzite. The pebbles are several inches in diameter. They consist of granite identical with that beneath, of vein quartz and of jasper. The vein quartz is very abundant and forms the most conspicuous pebbles. Certain of the pebbles are somewhat discolored by iron. One ellipsoidal pebble, with a greater diameter of 3 inches, seems to be a true, bright-red, MON XLIII — 03 7 98 THE MESABl IRON-BEARING DISTRICT. slightly bauded jasper. It is not impossible that this represents a very highly iron-stained phase of the vein quartz so conspicuous in the pebbles, but on this basis it is difficult to account for the banding. A careful exam- ination in the laboratory, both microscopic and macroscopic, leaves little doubt that the rock is a true jasper, that is, one of the phases of rock com- monly associated with iron ore. Jasper has not been found in the lower Huronian or Archean of this district, but it occurs abundantly in this series in other districts, and this particular joebble may have come from a very considerable distance. On the other hand, it is not impossible that future exploration may show small areas of jasper in the Lower Huronian of this district. In general, throughout the range the conglomerates which can safely be assigned to the base of the Pokegama quartzite or Upper Huronian vary only in relative abundance of the different kmds of pebbles. At every locality the pebbles are predominantly like the immediately subjacent rocks. The striking feature of the conglomerates throughout is their large content of vein-quartz pebbles. This feature was at first trouble- some, but search in the underlying rocks of the Lower Huronian has shown abundant vein quartz from which the pebbles could be derived. The vein quartz being hard, massive, and homogeneous, is not likely to be so much broken up as the granites and sediments during- the time it is worked over by water, and, for the same reason, fragments become better rounded. The few chert fragments and the one true jasper fragment found in the conglomerate have not yet been duplicated in the underlying rocks. STRUCTURE. The Pokegama quartzite in its lower and middle horizons shows but a faint bedding. In higher horizons the bedding is well shown by alternation of light and dark and coarse and fine bands, parallel to which is an excel- lent parting. The parting planes are made conspicuous by spangles of clastic mica. As a part of the Upper Huronian, the Pokegama quartzite has participated in the general tilting and cross folding to which the series as a whole has been subjected, and thus lies with a gentle flat dip to the south, with gentle cross folds whose axes are transverse to the range. (See structure of the Ujjper Huronian, pp. 178-180.) Aside from the tilting and general cross folding, the Pokegama quartzite has suffered little deforma- THE POKEGAMA QUARTZITE. 99 tion. Joints, while present, are inconspicnous, and little or no secondary cleavage has been developed in the formation. It is possible that there may have been a slight amount of differential movement between the beds of the formation due to the gentle folding, and this may account for a small part of the mica seen in bedding planes, but, as already noted, it is believed that a greater part of this mica is clastic. If there has been much movement it is probable that there would have been a greater development of secondary mica in continuous layers. THICKNESS. Because of the few exposm-es and the difficulty of ascertaining the variability of the dip, it is difficult to give an estimate of the thickness of the formation. Assuming an average southward dip of 8 degrees, and the average width of the quartzite belt to be 1,500 feet, the thickness of the formation may be little over 200 feet. But in some places the surface width is nearly 3,000 feet, and in other places the formation is cut out entirely. Moreover, the dip varies from 5 to 15 degrees. The thickness, therefore, while perhaps averaging about 200 feet, may vary between 500 feet and 0. E. J. Longyear in one place (1,025 paces north and 665 paces west of the southeast corner of sec. 35, T. 58 N., R. 21 W.) has drilled from the iron formation completely through the quartzite and found it to have a thickness of 69 feet. RELATIONS TO OTHER FORMATIONS. The Pokegama quartzite, forming as it does the base of the Upper Huronian series, rests unconformably upon the Lower Huronian and Archean series. This unconformity is considered under the heading "Relations of the Upper Huronian series to other series" (see pp. 180-181). The Pokegama quartzite is overlain by the iron formation, and while the two formations are essentially conformable in bedding and structure, there is between the two a thin but persistent layer of conglomerate, indicating a minor erosion interval. This is described in connection with the iron- bearing formation (see p. 154). 100 THE MESABI IRON-BEARING DISTRICT. SECTION II. THE BIWABIK FORIMATIOIV (IROI^-BEARIIS^G). DISTRIBUTION. The Biwabik formation extends along the slopes of the range for its entire length, from west of Grrand Rapids to Birch Lake, a distance of nearly 100 miles. The width of the formation, averages perhaps 1\ miles, bnt is in places as great as 3 miles and in others as small as a quarter of a mile. The total area is approximately 127 square miles. The bounding formation on the north is, for the most part, the Pokegama quartzite, but where this is lacking the Biwabik formation comes in contact with the Lower Huronian and Archean rocks. To the south the iron-bearing formation is bounded by the Virginia slate, except in range 12 and a part of range 13, at the east end of the range, where the Duluth gabbro laps np over the formation. On account of the covering of glacial drift, exposures of the iron- bearing formation, except in the eastern end of the district, are few. But the formation has been reached and pierced in thousands of places by drills and mining excavations, and it is therefore possible, particularly along the part of the range at present productive, to delimit the iron forma- tion with a fair degree of accuracy. In parts of the district where explo- rations and mining have not been so extensive, especially in the west end of the district, future explorations are likel}" to show that the boundaries, particularly the southern boundary, ai-e in some localities not correct. The iron formation in general occupies the middle slopes of the Giants range, and its north and soutli boundaries have fairly uniform altitudes for considerable distances. By an examination of the map, however, it may be seen that the elevation of the iron formation increases from the west end of the district to the east, the total difference amounting to as raiicli as 500 feet. This corresponds with the increased elevation of the range as a whole in this direction, although the higher elevation of the southern limit of the iron formation at the east end of the range is in part due to the fact that the lower parts of the formation are overlapped by gabbro. It may be further seen that the elevations of the north and sontli boundaries show local fluctuations as great as 200 feet, due to the folding of the formation ;iiid to differences in depth of ei'osiou. THE BIWABIK FORMATION. 101 KINDS OF ROCKS. The great bulk of the Biwabik formation is ferruginous chert" more or less amphibolitic, calcareous, or sideritic and ^ray, red, yellow, brown, or green, with bands and shots of iron ore. It is analogous to the jaspers of the other iron ranges but difPers in certain particulars, as will be seen on a subsequent page. Associated with the chert, mainly in the middle horizon, are the iron ores. Their surface area is only about 5 per cent of the total area of the iron-bearing formation, and the pi'oportion of their bulk to that of the iron- bearing formation is much less. Near the bottom of the Biwabik forma- tion is a small amount of conglomerate and quartzite — that is, coarsely clas- tic sediments. A minute conglomeratic layer has also been observed in the Mahoning mine, in about a central horizon of the formation. In thin lay- ers and zones throughout the iron-bearing formation, and particularly in its upper horizons, are layers of slate and of paint rock, the paint rock usually resulting from the alteration of the slate. Between the slate and the paint rock and the ferruginous chert are numerous gradational varieties, most of which come under the head of ferruginous slate. Associated with the slaty layers in the iron formation, or closely adjacent to the overlying Virginia slate, are green rocks made up of small green graniiles of ferrous silicate which are here called greenalite. It will be shown later that these are the original rocks from which most of the other phases of the iron formation, including the ores, have resulted by alteration. Finally, certain calcareous and sideritic rocks are present in small quantity, particularly near the upper horizons, associated with the greenalite rocks. The rocks of the iron formation are described below, beginning with the original type, the green- alite rock. The ores are reserved for a separate chapter. GREENALITE ROCKS. Ill limited quantity either just below the Virginia slate, or associated with some slate layer in the iron formation, are dull, dark-green rocks of rather uniform fine grain and with conchoidal fracture. Layers of slate, iron oThis rock has been called taconite by the geologists of the Minnesota survey, and the name has been much used locally. The term is not here retained for the reason that the rock ia not different from ferruginous cherts of other parts of the Lake Superior region, as described in the monographs of the United States Geological Survey, and there is no reason to complicate rock nomenclature by add- ing a new name. There is no objection, however, to its local use. 102 THE MESABl IRON-BEARING DISTRICT. ore, aud other phases of the iron formation usually mark their bedding. (See fig. B, PI. VIIl.) On close examination, and particularly when the sur- face is wet, there may be observed numerous ellipsoidal granules of a green substance of a very slightly lighter green than the matrix in which thev lie. They are so small and of a color so nearly like that of the matrix that they are likely to be overlooked unless especiallj- searched for. (See fig. A' , PI. VTII.) An occasional one is of much greater size tlian the average and looks like a conglomerate pebble in the rock. Under the microscope the granules are conspicuous. Their cross sections are round, oval, in some cases with much elongation, crescent shaped, lense shaped, gourd shaped, or even sharply angular (Pis. IX, XIII, XIV, and XV). Here and there a curved "tail" seems to connect one granule with its neighbor (PI. IX). Where in contact with a layer of iron carbonate or calcium carbonate, as they frequently are, the granules become more irregular in shape and project into or are included in the carbonate layers as irregular filaments and fragments. The carbonate is largely secondary and clearly replaces the granules, but some of it is perhaps original, and in this case the variation in shape of the granules where associated with the carbonate layers has a bearing on the origin of the ores, which is discussed on another page. One hundred and twenty measure- ments of the granules show an average greater diameter of 0.45 mm. and average least diameter of 0.21 mm., with average ratio of greatest to least of 100 to 47. The diameters rarely reach 1 mm. and seldom drop below 0.1 mm. Occasionally certain of the granules may be seen to be aggre- gated into larger granules^ with well-rounded outlines, making the con- glomerate-like fragments above mentioned. The greater diameters of the granules, for the most part, are parallel to the bedding, aud in fact this arrangement largely detei-mines the bedding. In ordinary light the granules are green, greenish yellow, brown, or black. The green and vellow ones are trans}3arent, while the brown and black are nearl}" or (piite opacjue Under crossed nicols the granules are either entirely dark or show a very faint lightening, hardly sufiicient to disclose a color. Here and there incipient alterations to chert, griinerite, cummingtonite, ov actinolite, scarcely discernible in ordinary light, give low ])olarization colors in minute s|)ots and make the term aggregate ])olarization applicalde. In ret)ecte, ;,, :.'■'- y (^■) ' L ; FERRljniNJniJ?; CHFRT of IRONRFARIMn FORMATiriM. PLATE XII. 12.5 PLATE XII. FERRUGINOUS CHERT, "jASPERT" PHASE, AND FERRUGrNOUS CHERT IN CONTACT WITH QUARTZITE OF IRON-BEARING FORMATION. Fig. a. — Ferruginous chert with gnarled and contorted banding. Specimen 45420. From drift fragments just east of Mesaba station in sec. 21, T. 59 N., R. 14 W. Natural size. This phase of ferruginous chert is characteristic of the basal horizon of the Biwabik formation. The red bands are iron-stained chert; the lighter ones are chert and vein quartz. Under the microscope the shapes of the granules can be seen to have been retained by chert and iron oxide. Described p. 120. Fig. B. — Ferruginous chert in contact with quartzite of iron formation. Specimen 40952. From the Cincinnati mine. Natural size. The chert is the gnarled and contorted phase characteristic of basic horizons. The sharpness of its contact with the ferruginous quartzite is to be noted. In some places ferruginous quartzite and chert of this kind are minutely interbanded at this horizon. Described, pp. 120 and 156. 126 U. S. GEOLOGICAL SURVEY MONOGRAPH XLIII PL. XII FERRUGINOUS CHERT, "JASPERY" PHASE, AND FERRUGINOUS CHERT IN CONTACT WITH QUARTZITE OF IRON-BEARING FORMATION, PLATE XIII. 127 PLATE XIII. PHOTOMICROGRAPHS OF FRESH AND ALTERED GREEXALITE GRANULES AXD FERRUGINOUS CHERT CONCRETION. Fig. a. — Greenalite rock with bands of carbonate. Specimen 45178, slide 15652. From 100 paces north, 500 paces west of southeast corner of sec. 22, T. 59 X., R. 15 W. Without analj'zer, x 50. Greenalite granules slightly altered to griinerite, iron oxide, and chert, stand in a matrix of chert. On the left side of the figure is a band of carbonate, probably largely calcium carbonate, but perhaps in part iron carbonate. Attention is called to the irregular nature of the greenalite granules near the contact with the carbonate band. The irregular dark fragments in the carbonate band are also greenalite and their alteration product griinerite. Described pp. 101-115. Fig. B. — Chert with granules and banding. Specimen 45419, slide 15706. From hill just east of town of ilesaba. Without analyzer, x 50. This is a phase of chert which is typical of basal horizons of the iron formation. The rock consists essentially of chert. Iron oxide occurs outlining the altered granules, and also occurs in contorted lines and bands representing flow lines in a lava. Described p. 120. Fig. C. — Greenalite granules. Specimen 45765, slide 16395. From Cincinnati mine. With- out analyzer, x 40. The granules are for the most part unaltered, and are dark green, light green, or yellow. Some of them show alterations to iron oxide and to dark-green chloritic material. Where altered they become dark brown, black, or dark green. The matrix is entirely chert. Evidence of crusliing is to Ije observed in minute cracks ramifying through the sUde. Note the remarkable similarity in shapes of these granules to those of the green granules in Clinton ores, illustrated PI. XXI. Fig. D. — Concretionary chert. Specimen 40767, slide 15415. From the NE. J of the SE. \ of sec. 35, T. 59 X., R. 17 W. With analyzer, x 100. This is one of the rare normal concretions in the iron formation of the Mesabi district. The interior is a single grain of quartz. This is surrounded by concentric layers of quartz and iron oxide, the latter somewhat hydrated. The matrix is chert. This structure resembles the concretions figured by Van Hise from the Gogebic district (see PI. XVI), and is believed to be quite different from the greenalite granules figured in the jireceiling plates. Described p. 118. 128 U. S. GEOLOGICAL SURVEY MONOGRAPH XLIII PL. XIII PHOTOMICROGRAPHS OF FRESH AND ALTERED GREENALITE GRANULES AND OF FERRUGINOUS CHERT CONCRETION, THE MERIDEN GRAVURE CO. PLATE XIV. MON XLIII — 03 9 129 PLATE XIV. PHOTOMICROGRAPHS OF FERRUGINOUS CHERT GRANULES, SHOWING MOTTLING. Fig. a. — Ferruginous chert with mottled granule. Specimen 4.5628, slide 15978. From near the center of the SW. J of sec. 10, T. 58 N., E. 19 W. Without analyzer, x 125. The granule here shown is composed of chert and reddish iron oxide. The chert occurs in small polygonal blocks separated by the oxide. Each of the chert individuals contains in its interior a more or less noticeable nucleus of iron oxide. The matrix is chert. A similar structure has been noted in the iron ores of the Ver- milion district and in the Clinton ores. In the latter cases the mottled structure is clearly due to the replacement of a shell with regular structure. Described p. 117. Fig. B. — Another granule in the same slide, showing a different aspect of the same feature. Described p. 117. 130 U. S. GEOLOGICAL SURVEY MONOGRAPH XLIII PL, XIV PHOTOMICROGRAPHS OF FERRUGINOUS CHERT GRANULES SHOWING MOTTLING. THE MERIDEN GRAVURE CO. PLATE XV. 13.1 PLATE XV. PHOTOMICROGRAPHS OF FERRUGINOUS CHERT SHOWING LATER STAGES OF THE ALTERATION OF GREENALITE GRANULES. Fig. ^1. — Ferruginous chert with granules. Specimen 45063, slide 15563. From near center of sec. 22, T. 60 N., R. 13 W. Without analyzer, x 50. The granules are outlined and in part replaced by iron oxide. The matrix is chert. The complex nature of one of the granules is to be noted,. Apparently one complete small granule is entirely inclosed in another large one. Desci -ibed pp. 116-120. Fig. B. — Griineritic ferruginous chert. Specimen 45603, slide 15974. From Clark mine. With analyzer, x 50. The rock consists of chert and iron oxide and griinerite. The iron oxide is a yellowish-brown hydrated variety, which is with difficulty distinguished from the griinerite. The granules have been entirely obliterated. Described pp. 116-120. Fig. C. — Ferruginous chert with granules. Specimen 45183, slide 15657. From 400 paces north, 35 paces west, sec. 28, T. 59 N., E. 15 W. Without analyzer, x 50. The rock consists almost entirely of chert with a small amount of iron oxide. The granules are marked by a light pigment which on a hurried examination would be scarcely noticed. Described pp. 116-120. Fig. D. — The same under crossed nicols. The cherty nature of the rock is here shown, and the granules are quite obscured by the double refraction of the chert. Described pp. 116-120. 132 U. S. GEOLOGICAL SURVEY MONOGRAPH XLIII PL. XV m^ k 'i' ,^i±i^ (D) PHOTOMICROGRAPHS OF FERRUGINOUS CHERT SHOWING LATER STAGES OF THE ALTERATION OF GREENALITE GRANULES. THE MERIDEN GRAVURE CO, PLATE XVI 133 PLATE XVI. PHOTOMICROGRAPHS OF FERRUGINOUS CHERT OF PENOKEE-GOGEBIC DISTRICT. Fig. a. — Concretionary chert. Specimen 9048, slide 2886. From Penokee-Gogebic district. Without analyzer. These are the normal concretions from the Penokee-Gogebic district supposed by Van Hise to be secondary developments during the alteration of an iron carbonate. (PI. XXII, fig. 1, Mon. U. S. Geol. Survey Vol. XIX. Described on pp. 227 and 228 of Mon. XIX.) Fig. B. — Ferruginous chert. Specimen 9625, slide .3150. From Penokee-Gogebic district. Without analyzer, x 60. The granules here shown are identical in aspect with granules in the IMesabi iron formation which can be shown to have developed from greenalite. (Reproduced from PI. XXVII, flg. 2, Mon. U. S. Geol. Survey Vol. XIX.) 134 U. S. GEOLOGICAL SURVEY MONOGRAPH XLIII PL. XVI =;.< ■>! •!. I-*:! (B) PHOTOMICROGRAPHS OF FERRUGINOUS CHERT OF PENOKEE-GOGEBIC DISTRICT. -HE MEfilOEM GRAVURE CO. PLATE XYII. 135 PLATE XVII. PHOTOMICROGKAPHS OF FERRUGINOUS AND AMPHIBOLITIC CHERT OF IRON-BEARING FORMATION NEAR CONTACT WITH DULUTH GABBRO. Fig. a. — Actinolitic, griineritic, and magnetitic chert. Specimen 4.5141, slide 15621. From south- east of center of sec. 17, T. 60 N., R. 12 W. Without analyzer, x 50. This rock is close to the contact with the Duluth gabbro, and shows the typical alterations characteristic of the contact. The chert is in much larger particles than in the western portion of the range away from the contact. (Com- pare with PI. XV. ) The particles fit in somewhat regular polygonal blocks. The iron oxide is mag- netite instead of hydrated hematite, and there is present actinolite and griinerite. The amphiboles are in small quantity in the slide shown, but the short actinolite needles may be seen inclosed in the quartz. Described pp. 159-161. Fig. B. — Actinolitic, griineritic, and magnetitic chert. Specimen 45147, slide 15626. From east of the north quarter post of sec. 3, T. 60 N., R. 12 W. With analyzer, x 50. This is still nearer the gabbro contact than the rock figured in A above, and shows correspondingly coarser chert. The radial fibers are griinerite, and perhaps in part cummingtonite, which project into the quartz. The differ- ence in the shade of the right and left portions of the photograph show but two large particles of chert to be represented. Described pp. 159-161. 136 U. S. GEOLOGICAL SURVEY MONOGRAPH XLIII PL. XVII if *-::i , -rf- ■'•TV WT A-*"^ m; ri>'; PHOTOMICROGRAPHS OF FERRUGINOUS AND AMPHIBOLITIC CHERT OF IRON BEARING FORMATION NEAR CONTACT WITH KEWEENAWAN GABBRO. THE MERIOEN UKAVURE CO. THE BIWABIK FORMATION. 137 cherts as due to the sti'aiii incident to the change of vohime during the alterations which the rock has undei'gone. Certain of the brecciated phases in higher horizons of the formation might be thus explained, but the persistent belt at the base is evidence of more concentrated movement along one horizon than could be attributed to chemical strains. Another phase closely associated with the above-described jaspery phase and characteristic of the lower horizon of the iron-bearing formation is a dense purplish-red chert in which the characteristic granules are very slightly differentiated by a somewhat lighter red or purple color. Under the microscope the rock appears as a fine-grained chert with granules marked by iron oxide, largely hematite and magnetite, sometimes ai-rauged peripherally. The rock differs from the jaspery rock above described only in lacking the gnarled and contoi'ted minute bandings and in containing less clear quartz. A rare rock associated with the ferruginous chert is composed of inter- bauded chert, iron, and greenish-yellow material which is partly chlorite, in the form of delessite, and partly serpentine. This may be observed in the Fayal mine. The ferruginous chert in several places exhibits ellipsoidal nodules with their greater diameters in the plane of bedding. They may be well observed in the railway approach of the Oliver mine. They reach a diam- eter of 6 inches, although commonly they are smaller than this. The nodules consist of ferruginous chert similar to that in the layers adjacent, differing only in being more massive and perhaps somewhat finer grained. The layers apparently do not continue through the nodules. Some of them abut against the nodules and others bend slightly in passing by. The nodules are similar to those in the overlying Virginia slate and to those found in slates and cherts in general. Still another type which should be specially mentioned is one which frequently occurs in the neighborhood of iron-ore deposits, a dense yellow chert owing its color to limonite. (Fig. A of PL XL) Under the micro- scope there appear granules with unusual characters. In ordinary light they are practically colorless ; under cross nicols they are almost isotropic, but show yellow polarization colors in minute spots, indicating incipient crystallization of quartz and perhaps other minerals. The matrix, at first glance, is apparently composed entirely of yellow limonite, but a high 138 THE MESABI IRON-BEARINCt DISTRICT. power reveals in addition the presence of abundant gi'iinerite or actinolite in typical sheaf-like and radial forms. The ferruginous cherts rarely contain a considerable amount of iron pyrites. The "gold mine," 600 steps west of the southeast corner of sec. 29, T. 60 N., R. 13 W., is a good example. The ferruginous chert is here a dark-gray and black, fine-grained, siliceous rock, in which the shapes of the greenalite granules can be easily distinguished, althougli the granules have been comjjletely altered to chert. The iron pp'ites occurs in large crvstals, replacing all the other constituents of the rock, and also as a filling in the interstices between the gi'anules, and marking the outlines or even partially replacing the granules. The iron pyrites in this case has crystallized during or subsequent to the alteration of the granules. An occasional rock may be seen to consist of a dense felted mass of dark-green and brown amphibole, which is probably griinerite or cumming- tonite, interbanded with carbonate of iron or calcium, or containing carbonate in rhombs, which frequently show a beautiful zonal alteration in their interiors. Chemically the cherts show wide variation. Comprising, as above described, rocks consisting almost entirely of silica, rocks consisting- very largely of iron oxide, and rocks of intermediate kinds, with greath" varying quantities of associated minerals, the variety of results in the analyses listed below is to be expected. The analyses in the following table are of cherts lacking any large amount of amphibole. The analyses of the chei'ts rich in amphibole are given in a separate table. A part of the analyses are incomplete. Dotted lines indicate that the substances have not been looked for, not their absence. THE BIWABIK FORMATION. Analyses of ferruginous chei'ts. 139 1. 2. 3. 4. 5. 6. 7. s. 9. 10. SiO^ 63. 92 None. 31.13 24.22 3.13 .49 None. JTrace. .48 1.12 Trace. None. .05- .021 None. .10 32.56 None. 66.02 46.45 .30 None. .18 J None. .32 .90 .16 None. .12 .052 None. .14 . 77.60 80.93 AI2O3 Fe,0, Fe 14.78 9.69 25.25 3 43 FeO MgO CaO .......J 1 NajO } ■- 1 K2O... i HjO- H2O+ TiOj CO., PA P None. .006 .021 .020 .023 .026 .062 009 SO3 MnO Loss on ignition... 11. 12. 13. !*• 15. 16. 17. 18. 19. 20. SiO 45.49 95.73 63.68 85.97 .67 11.40 57.00 1.43 27.05 61.57 16.83 5.S7 86 35 AI2O3 78 Fe,0, 7 41 Fe 33.61 53.53 1.72 19.53 51.41 FeO .90 .02 .01 .01 .01 .30 11.08 2.02 .40 .397 .113 6.41 3.44 .01 .12 1.59 4.70 3 46 MgO 05 CaO 01 Na^O 12 KjO . None. 01 HjO— . 1 01 H2O+ / TiOj CO2 1.22 P,0, .03 .0129 5.70 2.49 P .013 .025 Trace. .019 SO3 MnO Loss on ignition .91 140 THE MESABI IRON-BEARING DISTRICT. 1. Ferruginous chert below horizon of ore deposit (specimen 40751); from sec. 28, T. 58 N., R. 17 W., north of Virginia. Analysis by Geo. Steiger. 2. Ferruginous chert in horizon of ore deposits (specimen 40744); from Oliver mine. Analysis by Geo. Steiger. 3. Ferruginous chert showing red granules (specimen 45662) ; from pit in southwest of see. 3, T. 58 N., R. 17 W. Analysis by E. T. Ailen. 4. Ferruginous chert, basal phase with red granules (specimen 45688) ; from outcrop on Sparta road just east of Fayal mine. Analysis by R. B. Green. 5. Ferruginous chert (average of cores in several holes from beneath ore deposit); from Donora mine. Analysis by Lerch Bros. 6. Ferruginous chert (specimen 45590); from beneath ore deposit in NE. } of NE. J sec. 11, T. 57 N., R. 21 W. Analysis by Lerch Bros. 7. Ferruginous chert (specimen 45596); from just above slate in pump shaft of Penobscot. Analysis by Lerch Bros. 8. Ferruginous chert (specimen 45589); from north wall of Mahoning mine. Analysis by Lerch Bros. 9. Ferruginous chert (specimen 45543); from northwest of Mesabi Chief mine. Analysis by Lerch Bros. 10. Ferruginous chert (specimen 45672 B); from Donora mine. Analysis by Lerch Bros. 11. Ferruginous chert, hard bluish gray, seen gi-ading into blue and black ore (specimen 45694); from Biwabik mine, east end. Analysis by R. B. Green. 12. Ferruginous chert within ore deposit; from Adams mine. 13. Ferruginous chert, green, banded, siliceous (specimen 45653); from south of Virginia along Duluth, Missabe, and Northern tracks. Analj'sis by E. T. Allen. 14. Ferruginous chert, hard yellow, seen gradipg into limonite (specimen 45692); from Biwabik mine, east side. Analysis by R. B. Green. 15. Ferruginous chert, yellow (specimen 45603); from Clark mine. Analysis by H. N. Stokes. 16. Ferruginous chert, in middle horizon; from NE. J sec. 21, T. 59 N., R. 14 W. Analysis by Lerch Bros. 17. Ferruginous chert (specimen 65, chem. series No. 237). Analysis by C. F. Sidener, for J. E. Spurr. (See Geol. Nat. Hist. Survey Minnesota, Bull. No. 10, p. 116.) 18. Ferruginous chert (specimen 107, chem. series No. 243); from SW. ^ of SW. } sec. 2, T. 58 N., R. 19 W. Analj'sis by A. J. Hammond, for J. E. Spurr. (See Geol. Nat. Hist. Survey Minnesota, Bull. No. 10, p. 134.) ~ 19. Ferruginous chert, top of Biwabik formation (specimen 101, ch'em. series No. 239) : from SE. J- of NW. i sec. 18, T. 58 N., R. 18 W. Analysis by Alonzo D. Meeds, for J. E. Spurr. (See Geol. Nat. Hist. Survey Minnesota, Bull. No. 10, p. 148. ) 20. Ferruginous chert, Biwabik formation (specimen 27 A, variety 1); from sec. 33, T. 58 N., R. 17 W. Analysis by C. F. Sidener, for J. E. Spurr. (See Geol. Nat. Hist. Survey Minnesota, Bull. No. 10, p. 54. ) It is apparent froiu the above that the ferruginous cherts grade from almost pure cherts to iron ore. The percentage of phosphorus is uniformly lower than that of the ores (see pp. 214-217). The ferric iron is greatly in excess of the ferrous iron; calcium and magnesium oxides are rare; carbon dioxide is almost entirelj^ absent. Another characteristic feature which is not emphasized by the analyses is the presence of organic matter. The loss on ignition consists partly of the oxidation of organic matter, according to chemists who have handled the Jlesabi ores (see p. 218). The content of organic matter is not so gi-eat in the ferruginous cherts as in the slates. THE BIWABIK FORMATION. 141 The cherts in which the amphibole constituent is abundant show composition different from tliat of the cherts above analyzed. Analyses of amphihoUtic clierts. SiOj (total) SiO^ (soluble in HCl) AlA FeA FeO MgO CaO..^ NejO KjO... HjO- - H2O+ TiOj CO2 PA SO, Specimen 45028. Specimen 45648. MnO BaO Carbon in organic matter . 50.36 18.42 .64 6.46 I 32. 91 3.94 .23 None. None. .27 4.64 None. None. None. None. None. None. None. .18 83.82 3.52 .39 4.46 8.77 None. .60 None. None. .13 1.37 None. .72 .02 None. Specimen 45649. 23.94 12.76 6.00 .5.81 34.29 5.05 1.51 .28 None. .41 18.03 .11 Specimen 45689. 44.10 15.93 1.05 10.80 28.73 2.43 .33 None. None. .51 2.47 None. 9.71 .04 None. a Owing to presence of organic matter the determination of ferrous iron is probably high. Specimen 45028; from old Chicago mine in NE. \ of SE. \ of sec. 4, T. 58 N., E. 16 AV. Analysis by George Steiger. Specimens 45648 and 45649; from pit near Duluth, Missabe and Northern track, about one-half mile south of Virginia. Analysis \>y George Steiger. Specimen 45689; from Donora mine, near the east side of sec. 28, T. 59 N., E. 15 W. Analysis by George Steiger. In these four samples the amphibole is mainly dark brown or dark green, with inclined extinction, which might, from its microscopic char- acter, be common hornblende, cummingtonite, or gi-iinerite, but it is in part also colorless or nearly colorless actinolite. The lack of alumina shows that it is not hornblende. In all there is a small amount of iron oxide to be seen in the slide. In specimens 45649 and 45689 siderite is abundant. In strong hydrochloric acid a considerable amount of the dark material was dissolved out, leaving a white residue in specimens 45028 142 THE MESABI IRON-BEARING DISTRICT. and 45648, and a dark residue in specimens 45649 and 45689. It seems probable that the dark amphiboles and the iron oxide are the substances which are mostly dissolved by the hydrochloric acid. The analyses give results which accord with the microscopic observations. The ferrous iron present, except that combined with the carbon dioxide to form siderite, may be supposed to be mainly combined with silica to form griinerite, the magnesium and calcium oxides to be combined with the silica to form actinolite, or the ferrous iron and the magnesium and calcium oxides to be combined with silica to form cummingtonite. In three of the analyses magnesia is present in much higher percentage than calcium oxide." In the analyses of the greenalite rock on page 108 and of the amphibolitic slates on pages 144-145 the same fact may be noted. The principal amphiboles containing such a ratio of magnesium oxide to calcium oxide are antho- phyllite and cummingtonite, both of which are essentially silicates of magnesium and ferrous iron. The amphibole in the Mesabi rocks does not have the optical properties of anthophyllite, but has properties ranging from those characteristic of actinolite to those characteristic of griinerite or cummingtonite. The high proportion of magnesium oxide to calcium oxide would indicate, therefore, that the dark-colored amphibole is at least in part cummingtonite. The above aualy.ses show a lower percentage of comljined water than obtains in the analyses of unaltered greenalite gran- ules discussed on preceding pages, and it is apparent that the development of the amphiboles has involved dehydration of the original greenalite. The direct development of dark amphibole from the greenalite may be well observed in specimen 45689, where the change from, the greenalite apparently has been merely a, matter of the fine recrystallization of the original substance. The change is scarcely noticeable in ordinary light, and under crossed nicols is shown only by the low double refraction. In the slide no actinolite is to be observed. The matrix is chert. The carbon dioxide is supposed to be combined with the lime, magnesia, and a part of the ferrous iron; the carbonate can be observed in the slide. The remainder of the ferrous iron, together with the small amount of magnesium oxide "In this connection it in of intereat to note that an analysis of a •j;runorite-niaij;netite-sfliist from the Marquette district, given by Van Hise, shows a similar high proportion of magnesium to calcium. (Mon. V. S. Geol. Survey Vol. XXVIII, p. .338, Analysis No. 3.) Also an analysis of griiuerite by Lane and Sharpless shows a high percentage of magnesium and a lack of calcium. (Am. Jour. 8ci., 3d series, Vol. XLII, 1891, p. 506.) The analyses suggest that the amphibole may be more closely allied with I'liinniingtonite tlum trriinerite. THE BIWABIK FORMATION. 143 present, may be supposed to be combined with the soluble silica shown in the analysis to form the dark amphibole. The ferric iron and water are combined to form limonite, which may be observed in the slide. This would leave 17.60 per cent of FeO, and 15.93 per cent of soluble Si02. On the basis of 100 the proportions are FeO 52.5 per cent, SiOs 47.5 percent, which shows the green substance to be closely allied to griinerite. These figures show a proportion of ferrous iron and silica similar to that shown in the analyses of the unaltered green granules. They differ in showing less combined water. This similarity is in accord with the microscopic obser- vation that the amphibole has developed by simple recrystallization of the original green granules. SILICEOUS, FEKKUGINOUS, AND AJIPHIBOLITIC SLATES. Under this head are grouped a variety of slaty rocks which are interstratified with the other phases of the iron formation. They include dense, black, dark-gray, green, or reddish rocks with a tendency toward conchoidal fracture, and the slaty parting poorly developed if at all ; rocks showing banding of dark-green, black, gray, red, or brown layers parallel to the bedding, and a well-developed cleavage parallel to the same structure ; gradational varieties between these two, between them and the feiTUginous cherts, and between them and the iron ores (PI. XVIII, figs. B and C). Any of them may be hard or soft, carbonaceous or noncarbonaceous, fine grained or medium grained. Under the microscope the slates are seen to contain principally cherty quartz, iron oxide, either hematite or magnetite, usually in octahedra, or some hj^drated oxide, monoclinic amphibole which may be griinerite, cummingtonite, or actinolite, and possibly even common hornblende, a small amount of carbonate of calcium or iron, a little zoisite, and possibly, also, a little chlorite. From the optical properties, and from the analysis of the rock, it is thought that the amphibole is mainly griinerite and cummingtonite. There is much variation in the relative proportion of the principal constituents. Some of the slates consist almost entirely of fine cherty quartz with subordinate quantities of dark amphibole in radial aggregates or in irregular masses and of the iron oxides. Others are composed mainly of iron oxide, showing but small quantities of the quartz and dark amphibole. Others are composed of a tangled mass of yellowish, brownish. 144 THE MESABl IRON-BEARING DISTRICT. and greenish amphibole fibers containing minute particles of iron oxide, silica, and other subordinate constituents. The griinerite is far more abundant than the actinolite. The banding frequently shown in a specimen is due to the segregation of the above-named elements into layers. • While it may be convenient in description to refer to this or that slaty rock as a ferruginous slate, a siliceous slate, an amphibolitic slate, or an actinolite slate, depending upon the relative abundance of the constituents, usually all three constituents are present in one rock, and the rocks are really amphibolitic, siliceous, and ferruginous slates. Perhaps the most characteristic feature of the slates as a gi'oup is the abundance of the dark amphibole. Con-esponding to the mineralogic vai-iation in the slates there is considerable chemical variation, as shown by the following partial analyses of most of the phases of the slaty rocks. Dotted lines indicate that the substances have not been looked for, not their absence. Aiialyses o^ siliceous, amphibolitic, and ferruginous slates within the Bnoahik fannation. 1. 2. 3. 4. 5. (i. 7. SiO., 37.11 2.41 44.22 6.39 48. 16 53.86 9.14 35. 12 37. 01 AI2O3 1. 72 1 2. 19 Fe,0, 17. 51 Fe 22.20 24.53 29.90 15.90 42.18 41.00 FeO 26.13 3.70 MgO CaO Na^O ' - 09 K.,0 .62 .95 2.57 .22 6.16 .09 H,0- H.,0-1- TiO COj 12.60 .05 .14 .04 P,0, p .004 .098 .036 . 022 S03 None. 1.21 MnO . .11 .17 Mn c TO Vol 1.30 1.15 THE BIWABIK FORMATION. 145 Analyses of siliceous^ amp?dholitic^ and fey'ruginous slates within the JBiwabik formation — Continued. 8. 9. 10. 11. '] VI. 13. SiO^ 12.93 1 23.80 AI2O3 ! 7.95 Fe,0, 5.97 Fe 54.81 3.88 3.40 FeO .'. 32.21 MeO 5.89 CaO 4.67 NajO I .29 K2O .18 H.0--, 4.28 TiO CO, 11.84 P„0, - - - .012 .14 .06 p :. .009 .010 None. SO3 MnO Mn 1 Trace. C 11.34 Vol Loss on ignition . . 3. 35 1 1. Specimen 45461, from Moss mine. Analysis b}' Geo. Steiger. 2. Specimen 45591, from Penobscot mine, 278 feet beIoT\- taconite. Analysis by H. N. Stokes. 3. Specimen 45670, from sec. 7, T. 58 N., R. 18 W. Analysis by R. B. Green. 4. Specimen 45672 A, from Douora mine. Analysis b}' Lerch Bros. 5. Specimen 45600, from near southeast corner of NE. \ of SW. \ sec. 21, T. 58 N.,E. 20 W. Analysis by H. N. Stokes. 6. Specimen 45645, from cut south end of ilountain Iron mine. Analysis by A. T. Gordon. 7. From cut south end of Mountain Iron mine. Analysis by A. T. Gordon. 8. Specimen 45677, from Adams mine. Analysis )5y R. B. Green. 9. From north of Little Mesabi exploration. Analysis b)- Lerch Bros. 10. Specimen 45672, from Donora mine. Analysis by Lerch Bros. . 11. Specimen 45625, from northeast of Buhl, Analysis by H. N. Stokes. 12. Specimen 45678, from south of Spruce mine. Analysis by E. T. Allen. 13. Specimen 112 ( Chem. Series No. 240) NE. \ of SE. J of sec. 17, T. 58, R. 19 W. Analysis by A. D. Meeds, for J. E. Spurr. (See Geol. Nat. Hist. Survey Minnesota, Bull. No. 10, p. 10) . The essential features above shown are the variation in metallic iron, the considerable content of ferrous iron as compared with the ferric iron, low alumina as compared with true slate, but hig-h as compared with the other rocks of the iron formation, and the greatly varying- quantities of MON XLIII — OS 10 146 THE MESABI IR0N-BEARI>;G DISTRICT. carbon dioxide and carbon, both of them, however, fairly abundant. The lai'ge proportion of mag-nesium oxide and calcium oxide accords well with the microscopic determination of some of the dark amphibole as cummingtonite. In texture and in mineralogic and chemical composition the slaty rocks of the iron formation differ from true black roofing slates, such, for instance, as those from Vermont. The cleavage is not as good as in roofing slates, and what there is of it is parallel to the bedding and largely conditioned bv the bedding', and not by the deformation of the rocks. Griinerite is abundant, while in typical roofing slates micaceous and chloi'itic constituents are important. Finally, the percentage of iron, and in some case^, the percentage of silica, is higher, and the percentage of alumina is much lower than in typical slates. The slates in the iron forma- tion also differ from the overlying Virginia slates in a manner described in connection with the latter (see p. 176). Some of the typical occurrences of slate within tlie iron formation from which specimens have been collected are specified below : The pump shaft of the Penobscot mine in sec. 1, T. 57 N., R. 21 W"., passes through 278 feet of ferruginous chert and bottoms in slate (speci- mens 45591 to 45593). Northeast of the center of sec. 27, T. 58 N., R. 20 W., E. J. Longear drilled through 5(i feet of iron-formation material, mainly ferruginous chert, to slate (specimen 45600). Test pit in the NE. i of SE. i sec. 17, T. 5S N., R. 19 W. (specimen 45630). Drill hole hi the NE. i of NW. \ sec. 20, T. 58 N., R. 19 W. (specimen 45541). Test pit in NE. 4 of SW. i sec. 10, T. 58 N., R. 19 W. (specimen 45625). Test pit just southeast of northwest corner of sec. 8, T. 58 X., R. 18 W. (specimens 45639 and 45640). Test pit and drill hole in the NW. \ of NW. ^ sec. 7, T. 58 N., R. 18 W. (specimen 45670). Test pit south of A^irginia, west of the Duluth, Missabe and Northeni track in the SE. \ of SW. \ sec. H, T. 58 N., R. 17 W., near contact with the overlying Virginia slate (specimen 45652). Test ])it just north of the old Norman open pit in the SE. \ of NW. \ sec. 9, T. 58 N., R. 17 AV. (specimens 40741 and 40742). THE BIWABIK FORMATION. 147 Test pits south of the Spruce mine in NW. \ of NE. { sec. 6, T. 57 N., R. 17 W. (specimen 45678). Drill hole in Fayal mine. Slate reached under 200 feet of ferruginous chert (specimen 45734). Test pit just south of the Elba mine near the north line of the NE. ^ of NE. i sec. 24, T. 58 N., R. 17 W. (specimen 40863). Test pits and shafts of the old Chicago mine in the NE. ^ of SE. ^ sec. 4, T. 58 N., R. 16 W., near contact with overlying Virginia slate. Test pits in the Cincinnati mine in the SW. J of NE. ^ sec. 2, T. 58 N., R. 16 W., and the SE. 4 of NW. i sec. 2, T. 58 N., R. 16 W., near contact with overlying slate (specimen 45039). Test pit and drill hole in SE. i of NE. i sec. 28, T. 59 N., R. 15 W., the little Mesabi exploration (specimen 45672). Test pits in the SE. i of SE. i sec. 22, T. 59 N., R. 15 W. (specimen 45176). Test pit in SW. i of SW. \ sec. 26, T. 59 N., R. 15 W. (specimen 45737). Test pit in SE. i of NW. i sec. 28, T. 59 N., R. 15 W. (specimen 45191 calcareous). Drill hole in the SE. i of NE. i sec. 19, T. 59 N., R. 14 W. (speci- men 45003). Drill hole in the SW. i of SW. i sec. 16, T. 59 N., R. 14 W. (speci- men 45006). Test pit in the NW. i of SW. ^ sec. 21, T. 59 N., R. 14 W. (specimens 45009 and 45010). Test pits in SW. i of SW. i sec. 15, and NW. i of NW. J sec. 22, T. 59 N., R. 14 W. (specimens 45224 and 45228). Exposure (?) NE. i of SE. i sec. 21, T. 59 N., R. 14 W. (specimen 45699). Exposure ("?) east of the west quarter post of sec. 22, T. 59 N., R. 14 W. (specimen 45700.) From near the east side of range 14 eastward through ranges 13 and 12 the slates interbedded with iron formation are in great abundance, and, because of the good exposure of the rocks in this area, can be well observed. One may mention in particular the belt extending north and east of Mall- man camp (see map, PI. VI), and the slate along the road in the NE. \ of sec. 1, T. 59 N., R. 14 W. 148 THE MESABl lEON-BEARING DISTRICT. It has been maiutaiiied by certain of the mining engineers on the Mesabi range that the slates found within the area of the iron formation represent, in large part, patches of the Virginia slate, which, in these particular places, have been left as islands during the erosion which removed most of the slate from the area. The area is heavily drift covered, and the slates are, for the most part, found in isolated explorations and show no relations to surrounding rocks. It is possible that the occurrence of some small part of the slate may be explained in this way, but it is believed that practically all of it is interstratified with the iron formation, for the following reasons: (1) In a number of places explorations have gone through the iron formation into the slate, and then into iron formation again, xilso, in the east end of the district actual interbedding can be observed in exposures. (2) As the iron formation and Virginia slate are conformable and have the same dip, patches of the Virginia slate left on top of the iron formation, especially where well to the north, must necessarily have a considerably higher elevation than the surrounding rock surface. So far as can be ascer- tained under the thick covering of drift, the slates in the iron formation are not elevated above the siirrounding rock surface. (3) Commonly, wdiere exploration has gone far enough, the slate in the iron formation is found to come to the rock surface in a narrow band, with strike parallel to the strike of the iron formation layers, and not in an in-egular patch witli outlines determined by erosion. (4) In mineralogic and chemical composition the slate layers within the iron formation are somewhat different from the Virg'inia slate. (See p. 176.) If, is not improbable, indeed it is likely, that certain of the interstrati- fied th te layers in the iron formation have been mapped as Virginia slate; that is, where full data have been absent the Virginia slate boundary has been extended to the north to cover slate shown in some exploration which may really !. 3 a layer of slate interstratified in the iron formation. If this is true, future wo.]: will resvilt in carrying the Virginia slate boundary farther south. As ore deposits have not been found to occur under the solid Virginia slate, and as the slate layers in the iron formation have an influence on the concentration of the ore deposits hiter discussed, it is apparent that the discrimination of tlie various slates is a niattcr of importance to mining men. THE BIWABIK FORMATION. 149 PAINT KOCK. Throughout the iron formation, and particularly adjacent to the ore deposits, are thin seams of paint rock, which have resulted from the altera- tion of the slates above described. The paint rocks are essentially soft red or yellow or white clay. They retain the original bedding of the rocks from which they were derived, the structure being marked by alternation of bands of different color. In situ the paint rocks are moist and soft. When taken out and dried they become harder, but retain a soft, greasy feel. The alteration of the paint rocks from slates is proved by the numer- ous intermediate phases to be observed. (See PL XVIII.) At several mines also the paint rocks associated with the ore deposits are in the same horizon as slates immediately adjacent. At the Penobscot mine paint rock forms a persistent horizon near the bottom of the mine, while the pump shaft sunk in the adjacent rock struck slate at the same horizon. At the Biwabik mine there is a capping of paint rock over the ore, which, according to the superintendent, has been found by test pitting to grade southward into the true black slate mapped as Virginia slate. South of the exploration of the Medow Mining Company in the NW. ^ of sec. 3, T. 58 N., R. 15 W., a considerable quantity of paint rock is encountered along the north mar- gin of the Virginia slate. Other instances might be cited, but the proof is so conclusive at these places that further evidence is hardly necessary. Chemically, the paint rocks have the characteristics shown in the following partial analyses: ATwlyses of paint rock. 1. 2. 3. 4. 5. 6. 7. 8. SiOa 9.54 7.00 77.30 .45 55.57 3.28 36.57 32.16 31.39 20 94 A1,0, 6.58 7.75 7.62 7.23 19 01 FsjOa FeO Fe 25.60 54.20 47.49 47.48 56.72 17.56 30 88 TiOj .61 \ HjO - 9.55 13 40 H2O + / None. Trace. CO2 PA - .20 p .055 150 THE MESABI IRON-BEARING DISTRICT. 1. Paint rock (specimen 40661) from Maiioning mine. Analysis b)' George Steiger. 2. Paint rock (specimen 4.5.594) from Penobscot mine, beneath ore. Analysis by H. N. Stokes. 3. Paint rock from Franklin mine. Analysis by E. F. Johnson. . 4. Paint rock from Wacoutah property near Mountain Iron. Analysis by Lerch Bros. 5. Paint rock from Wacoutah property near Mountain Iron. Analysis by Lerch Bros. 6. Paint rock from Wacoutah property near Mountain Iron. Analysis by Lerch Bros. 7. Light portion of banded red and white paint rock (specimen 45646) from Mountain Iron mine. Analysis by A. T. Gordon. 8. Dark portion of banded red and white paint rock (specimen 45646) from Mountain Iron mine. Analysis by A. T. Gordon. SIDERITIC AND CALCAREOUS ROCKS. Associated with the slaty layers iu the iron formation, and particularly with the greenalite rocks, are carbonates of iron and calcium in small quantity Most of the carbonate reacts readily with cold dilute hydro- chloric acid and is certainly limestone, which, from the aualvsis of rocks containing it, is doubtless magnesian. Some of the carbonate, how- ever, is certainly siderite, as shown by the analysis on page 141. The carbonates occur in minute clear-cut layers interbedded with the iron formation (see PI. XIII, fig. A), in veins cutting across the bedding, iu irregular aggregates in iron formation layers, and in well-defined rhombo- hedral crystals in the same. In the carbonate bands are small quantities of iron oxide, ferrous silicate, and chert, and in the bauds of these minerals are small quantities of the carbonate. In some cases the carbonates are coarsely crystalline and fresh and clearl}- have resulted from the replace- ment of the other constitutents in the rock, particularly the ferrous silicate, or from infiltration along cracks and crevices. In other cases, especiall}' where in distinct layers interbedded with unaltered ferrous silicate phases of the formation, the carbonate layers seem certainly to be original. At the top of the iron formation and closely associated with the basal horizons of the Virginia slate are several feet of clear calcium carbonate, which is described in connection with the Virginia slate. North of Birch Lake is a siliceous and sideritic slate exposed in one pit. (See fig. 7, p. 184.) The carbonate is a peculiar grayish banded slate, which near the surface and adjacent to cracks is weathered to a rusty-brown color. The weathering penetrates several inches from the surface. Under the microscope the rock is seen to be largely made up of carbonate in bands and in isolated rhombs associated with magnetite and chert. The rusty weathei'ing of the carbonate and the fact that it effervesces but slightly, if at all, with cold hydrochloric acid indicate it to PLATE XVIII. 151 PLATE XVIII. SLATE, FERRUGINOUS SLATE, AND PAINT ROCK IN IRON FORMATION AND CONTACT OF IRON-BEARING FORJIATION AVITH INTRUSIVE GRANITE. Fig. .1. — Contact of Biwabik formation and Embarrass granite. Specimen 45138. From the NW. } sec. 17, T. 60 N., K. 12 W. Natural size. The granite is in intrusive contact with the iron formation. Note the purple quartz phenocrysts in both. Described pp. 186-188. Fig. B. — Banded slate. Specimen 45592. From Penobscot mine, 298 feet below ferruginous chert. Natural size. Described pp. 14.3-148. Fig. C. — Banded ferruginous slate. Specimen 45594. From Penobscot mine, 298 feet below ferru- ginous chert. Natural size. Described pp. 143-148. Fig. D. — Paint rock. Specimen 45587. From north wall of the Mahoning mine. Natural size. The derivation of the paint rock from the alteration of slate is evident. ^lany specimens have been collected showing complete gradation between the two. Described pp. 149-150. 152 U. S, GEOLOGICAL SURVEY MONOGRAPH XLIII PL. XVIII SLATE, FERRUGINOUS SLATE, AND PAINT ROCK OF IRON-BEARING FORMATION, AND CONTACT OF IRON-BEARING FORMATION WITH INTRUSIVE GRANITE. THE EIWABIK FORMATION. 153 be au iron carbonate or siderite. The siliceous phase of the slate is clearly a replacement of the carbonate. Indeed in hand specimens it is almost identical in appearance It is a minutely banded gray and black and brown rock with a poor parting parallel to the banding. Under the microscope it is seen to have a medium-grained, quartzose background, in which are octahedra of magnetite. The concentration of the iron oxide and the quartz in alternate bands gives the minute banding observed in the hand specimen. An analysis of the rock (specimen 45161) by George Steiger is as follows: Analysis of siliceous slate of iron fmvnation at contact of gahhro. Per cent. SiOj 78. 95 AljOg None. FeA 13.89 FeO ] . 23 MgO 18 CaO 81 NajO None. KjO None. H,0- 73 H^O-h 2.21 TiOj None. COa 1.59 P2O5 04 SO3 None. MnO 11 Total 99. 74 The rock analyzed is from the same mass as the carbonate, and before analysis was supposed itself to be partly carbonate. The total absence of any traces of greenalite granules and the occur- rence of the carbonate in well-bedded layers, evidently little altered, make it probable that the sideritic slate at Birch Lake is original. It is the nearest approach in nature and abundance to the original sideritic slates characteristic of the Penokee-Gogebic range to be anywhere seen on the Mesabi range. Eastward, in the neighborhood of Gunflint Lake, structures characteristic of the alteration of siderite are to be observed Its occur- rence at the east end of the district, together with certain structures in the Gunflint Lake region to the east characteristic of altered sideritic rocks, is suggestive of a gradation toward the east to phases characteristic of the Penokee-Gogebic iron formation. 154 THE MESABI IROX-BEARING DISTRICT. CONGLOMERATES AND QUARTZITES. At the base of the iron formation is a thin laver of fairly coarse fragmeutal Jiiaterial consisting in places of conglomerate alone and in other places of conglomerate and quartzite. At several localities the fnll succession from the iron formation above through the quartzite or conglomerate, or both, into the Pokegama quartzite below may be studied. In the SW. 1 of SW. i of sec. 3, T. 58 N., R. 17 ^Y. (see fig, 3, p. 89) are a considerable number of test pits which have either been bottomed in the quartzite belonging to the iron formation or have gone through this and have been bottomed in the Pokegama quartzite. At the Fayal and Adams mines green quartzites are found beneath the ferruginous chert, and so closely resemble it that they were classed as "taconite" by the drillers. Drill holes on the NE. \ of the SE. \ of sec. 4, T. 58 N., R. 16 W., put down by E. A. Sperry, penetrated the iron-formation conglomerate (45753), Pokegama quartzite, and Lower Huronian rocks, in the order named. One of the northward drifts of the Cincinnati mine penetrates the quartzite at the base of the iron formation. A little southwest of tlie center of sec. 18, T. 59 N., R. 14 W., is a trench in which may be observed the actual contact of the iron formation and the Pokegama quartzite. Here only the conglomerate appears at the base of the iron formation. In the S. i of SE. \ of sec. 29, T. 60 N., R. 13 W., the same thing may be observed in natural exposure. Here also the conglomerate alone is present at the base of the iron formation. A di-ill hole put down by E. J. Longear in the SW. 4 of SE. \ oi sec. 32, T. 59 N., R. 17 W. penetrated iron formation, quartzite, conglomerate, and Pokegama quartzite in the order given. (Specimens 40851 to 40855.) Thin films of conglomerate lie on the upper surface of the Pokegama quartzite a little north of the east quarter post of sec. 13, T. 58 N., R. 17 W.; just north of the Arcturus mine near the center of sec. 13, T. 56 N., R. 24 W.; at the falls of the Prairie River in range 25; and at Pokegama Falls in range 26. On the dumps of a number of the pits adjacent to the northern boundary of the iron formation are found conglomerate and quartzite with such n'lati4 Fig. 5. — Sketch showing relations of Embarrass granite to the Biwabik formation in the abandoned glacial gorge in NW. J of NW. J of sec. 17, T. f.O N., E. 12 \V. entirely disappeared, being obscured by magnetite, amphibole, and chert In the phases not showing the maximum alteration the}' are marked by magnetite, either as a rim about the granule, as a solid mass filling it, or in evenly disseminated particles through it. Not infrequently the granules maj^ be observed only in ordinary light and then by distribution of the magnetitic particles; in parallel polarized light they are obscured by the polarization of the amphibolitic and cherty constituents. Finally, in the eastern portion of the district certain minerals have developed which have not been found in the less altered rocks of the central and western portions THE BIWABIK FORMATION. 161 of the Mesabi district. lu the latter areas the ampliiboles are entirely gi-iinerite and actinolite, with little or no hornblende. In the eastern por- tion of the district the amphiboles include griinerite and actinolite, and in addition green and brown hornblende in considerable quantity. Associated with these minerals are small quantities of biotite, glaucophane (45057), andalusite (45124), zoisite (45119), and g'arnet. Still farther to the east, in the neighborhood of Gunflint Lake, the continuation of the Biwabik forma- tion has suffered metamorphism by the gabbro, and, according to Grant," there has been an extensive development of hypersthene, augite, and olivine, in addition to, and sometimes replacing, the minerals above mentioned. Fig. 6. — Details of contact of Embarrass granite and Biwabilj: formation in the gorge in N\V. i of NW. ^ of sec. 17, T. 60 N., R. 12 W. While hypersthene, augite, and olivine are abundant and characteristic in the true gabbro of range 12 and westward, these minerals are nearly, if not quite, lacking in the Biwabik formation. Although eastward toward Gunflint Lake the gabbro alone has been able to produce even greater metamorphic effects on the iron-bearing rocks, it is certain that the metamorphism of the iron-bearing rocks in the region under description has been produced jointly by the gabbro and granite. « See Chapter II. MON XLIII — 03- -11 162 THE MESABI IRON-BEARING DISTRICT. At the contacts of the iron formation with each of tliese rocks, detailed evidence of their metamorphic effects may be observed. For the gabbro contact the general description of the metamorphism above given applies almost in toto, the only exception being the statements concerning the augite, biotite, and andalusite, which are not found near the gabbro contact. The granites, before this work was taken up, had not been known to be intrusive in the iron formation, and hence a brief detailed description of its contact effects may be of interest. The contact effects of the granite may be well studied in the gorge in the NW. i of NW- i of sec. 17, T. 60 N., R. 12 W., where on both sides of the gorge are actual contacts of the rocks. (See figs. 5 and 6, and description of structural relations, pp. 187-1 88.) The iron formation is a very dark-gray and grayish-yellow ferruginous chert, highly siliceous, containing near its base a few undoubted well-rounded, fragmental quartz grains. The chert does not contain iron oxide in bands but in evenly disseminated particles and aggregates, in some layers in so small a quantity as to make the chert resemble a quartzite in general aspect. The only structure resem- bling a banding is a rough parting or jointing into thick, nearly horizontal, layers. The massive character of the rock here distinguishes it from the well-banded phases away from the contact. At the to]) of the gorge, and down to within a few feet of the contact, the iron-formation rocks look siliceous and have disintegrated, brown, sugary surfaces where weathered. Under the microscope the quartz appears in medium-sized grains which have a tendency to be of uniform size for certain bands, although varying somewhat in different bands. The quartz grains are of polygonal shape and are fitted together in a fairly uniform mosaic- No trace of clastic structure is to be observed, unless aggregation into indefinite bands be taken as evidence of sedimentary ori- gin. Evenly disseminated through the quartzose background are particles and irregular ao-OT-egfates of iron oxide, mainlv niao:netite, of o-reeu and brown hornblende, of actinolite or griiuerite, of zoisite, and of biotite. In abundance the minerals stand in about the order named. The actinolite and griiuerite are in niinute, isolated, columnar forms and in radial and sheaf-like aggregates. THE BIWABIK FORMATION. 163 Within a few feet of the contact the ferrug-inous chert shows a black, heavih^ feiTUginous background, in which beautifully rounded quartz eyes are highly conspicuous because of their translucent character and their dark reflections. Under the microscope the grains appear well rounded and in some cases even show incipient enlargement. The matrix consists of greenish, transparent zoisite plates and dark, greenish-black aggregates of magnetite, zoisite, hornblende, griinerite, and actinolite (45119). The forms of the ellipsoidal granules are still to be observed in ordinary light by distribution of the iron oxide. Under crossed nicols the other minerals named completely obscure them. It is clear in connection with the altera- tions traced in the central part of the district that the rock was originally composed of ferrous silicate granules and of fragmental quartz grains in about equal abundance, and that the alteration of the ferrous silicate has given the dense, dark-green and black background in which the quartz grains now stand so conspicuously. Immediately next to the contact with the granite, the quartz in the matrix of the ferruginous chert has been thoroughly recrystallized, consid- erably' increased in size, and the round shape lost. The granite is rich in quartz which occurs as purple phenocrysts. Within a few inches of the contact almost identical quartz phenocrysts have been developed in the iron formation (see fig. A, PL XVIII), and in addition numerous irregular stringers and veins of quartz ramify through the iron formation. It looks as if there had been a minute injection of the quartz into the iron formation close to the contact through the agency of hot silica-bearing- solutions accompanying the intrusion of the granite. Several specimens were collected, which consist of about one-half quartz in stiingers and phenocrysts and about one-half iron-formation material. The contact of the iron formation and granite is ordinarily exceedingly sharp (see fig. A, PI. XVIII) though in places an irregular gradation zone from an eighth of an inch to an inch thick separates the two. In this zone may be also obsei'ved jjink feldspars and purplish quartz characteristic of the granite, lying in a black, ferruginous chert matrix, which, toward the granite side, fades out and toward the iron-formation side becomes more abundant until it excludes the feldspar of the granite. In this zone andalusite (45124) and biotite (45124), commonly associated, and glaucophane have been developed in small quantity, and the quantity of green hornblende, actinolite, and griinerite has been increased. 164 THE MESABI IRON-BEARING DISTRICT. COMPARISON OF THE METAMORPHIC EFFECTS OF THE GRANITE AND GABBRO. The contact effects produced by the granite differ from those produced b}' the gabbro in the following ways : (1) While there has been thorough recrystallization next to the granite contact, the size of the grains has not increased nearly so much as next to the gabbro contact. (2) Accompanying the recrystallization next to the gabbro there has been a tendency toward the segregation of the magnetite into irregular masses and layers, it is possible to take out good-sized hand specimens of almost clear magnetite. Next to the granite contact it would be difficult to find magnetite well enough segregated to allow of this. (3) Next to the granite contact there have been developed andalusite, zoisite, biotite, and glaucophane which are not characteristic of the gabbro contact in the area west of Birch Lake. On the other hand, green and brown hornblende are much less abundant than near the gabbro contact, and olivine and h^'persthene, which in the Gunflint Lake area are charac- teristic of the gabbro contact (although not observed west of Birch Lake), are altogether lacking near the granite. (4) Tlie intrusion of the granite has caused the introduction of a considerable amount of quartz in stringers and phenocrysts into the iron formation adjacent to the contact. The gabbro has contributed little or no material to the iron formation in tlie contacts observed in the Mesabi district. To the east, near Akeley and Gunflint lakes, such transfer from the gabbro probably occurred." MAGNETIC ATTRACTION. The normal magnetic variation for northeastern ]\Iinnesota where there is no local disturbance is 7° east of north. Throughout the Biwabik formation the needle shows deflections from this direction. In limited areas the deflections of the needle are most capricious, yet by putting together observations taken throughout the range it becomes apparent that there is some regularity in the magnetic attraction. In the eastern portion of the district a high and variable deflection can be counted upon, for here, as we have seen, there is much magnetite, due to the alteration of the Keweenawan «See disi.'Ussion of (labbro contact by Grant, Bayley, ami utluTs sniiiiiiarizod in Cliajiter 11. THE BIWABIK FORMATION. 165 intrusives, and the drift covering is thin. Variations of 40° or 50° or even 90° are common. Throng-h the central and western portions of the district the magnetic needle, on an average, varies but a few degrees from the normal, but so far as there is any variation it is greater in certain zones than in others. One of these is just within the iron formation near its contact with the Pokegama quartzite. It has a width varying from a few steps to a few hundred steps. In this zone variations as high as 30° to 40° are common. From this zone tongues of liigh attraction project to the south. Also in the neighborhood of some of the ore deposits a slightly higher attraction has been noted near the contact of the ore with the wall rock than in the rock or in the ore. STRUCTURE. The most conspicuous structure in the Biwabik formation is the bedding, which may be observed in all pliases of the formation. The bedding layers may be several feet thick or as thin as those of shales. The more massive and irregular layers and the poorest parting are shown by the ferruginous chert, particularly the feiTUginous chert in lower horizons of the formation. The finer bedding, accompanied by a better parting, is shown by the iron ores, by the slaty rocks within the iron formation, and by some of the ferruginous cherts in middle and upper horizons. The iron formation is a sedimentary formation interbedded with typical quartzite and slate, and the bedding as a whole is an original structure, but the formation has undergone such great metamorphism that the bedding- has suffered many modifications. The iron formation originally consisted mainly of greenalite rocks with thin interbanded layers of slate. The alteration of the greenalite rocks to ferruginous chert has been accom- panied by the segregation of the iron and silica into irregular bands and lenses which serve to mark the position of the original bedding, but at the same time obliterate much of it and make the parting parallel to the bedding very poor. The alteration of the greenalite rocks to the ore deposits has not obliterated any of the original bedding, but on the contrary has made it even more conspicuous and has made the parting parallel to it an excellent one. The iron formation has been tilted gently away from the high land of the Giants range adjacent at an angle varying from 5° to 20°, but 166 THE MESABI IRON-BEARING DISTRICT. averaging' perhaps 8° or 10°, and has, in addition, been gently flexed both parallel and transverse to the range. All of these features are indicated by the variation in direction and degree of dip in different parts of the formation, and may be actually observed on a small scale in the open cuts of the mines. Accompanying the folding is fracturing and even brecciation. The formation is cut by numerous joints, which in the massive cherty portions are even and continuous, but which in the fine bedded portions, and particularly in the ore deposits, are extremely irregular and for the most part coterminous with the individual layers. In some places, as at the Biwabik mine, and at the Hale and Kanawha mines, there is evidence that the faulting- and brecciation of the iron formation has been greater near the contact of the iron formation with the underlving rocks than farther away from it. This is due to readjustment along the contact during the folding of the district. Water flows easily through the formation parallel either to the bedding or to the joints. In the massive portions it probably is more nearly parallel to the joints than to the bedding, because of the poorer parting parallel to the latter. But in the more finely bedded portions the water could move in almost any direction. Most of these structural features the iron formation shares with the other members of the Upper Hurouian series, and are discussed more in detail in connection with the structure of the Upper Huronian series (pp. 178-1 8U). THICKNESS. The thickness of the Biwabik formation has been directly measured in but one place, in sec. 34, T. 59 N., R. 14 W., where Mr. E. J. Longyear drilled from the Virginia slate through the iron formation into greenstone below, and found the iron formation to have a thickness of 576 feet. On the basis of average dips and width of exposure of the iron formation at any one place the thickness might be more than 2,000 feet or as little as 200 feet. Sucli great variation is in part real and in part apparent. The iron formation grades laterally into the Virginia slate (see pp. 172-176) and the fi)rmation was dejiosited against an irregular shore line and an irregular bottom of the Lower Huronian and Archean rocks, both causing variation in the real thickness of the formation. On the other Imnd the gentle foldings of tlic formation in two directions, cou])led witli the scarcity of dip THE BIWABIK FORMATION. 167 observations, make the selection of a fair average dip across the formation at any given place an exceedingly difficult matter, and some of the apparent variation in thickness is unquestionably due to failure to take into account the minor folding. The average thickness for the district is perhaps in the neighborhood of 1,000 feet. The eastward continuation of the iron forma- tion, near Gunflint Lake, has been estimated by Grant" to have a thickness of 825 feet. RELATIONS TO OTHER FORMATIONS. The Pokegama quartzite underlies the Biwabik formation for most of the district. The relations are those of a minor erosion interval. Structurallv the two formations conform to each other; so far as can be ascertained, the dips of the Pokegama quartzite are identical with those of the overlying iron formation. The Pokegama quartzite had not been folded prior to the deposition of the iron formation, and, as members of the Upper Huronian series, both formations have been affected by the gentle Upper Huronian folding. However, at a considerable number of places in the district, and, it is believed, throughout the district, a layer of conglomerate a few inches to several feet in thickness separates the iron formation from the Pokegama quartzite. The pebbles in this conglomei'ate are in small part of quartzitic rocks like those underlying and show some variety, and hence the con- glomerate must indicate .the lapse of some time interval prior to its deposition. Yet, taken in connection with the thinness of the conglom- erate, the characteristic scarcity of indurated quartzite fragments (see pp. 157-158), the comparatively small size of its fragments, and the corre- spondence in dip of the quartzite to the iron forination, it seems likely that the erosion interval represented by the conglomerate was not a g-reat one. The conglomerate is closely associated with a brecciated phase of the ferruginous chert, and in some places it is hard to draw a line between the two. The breccia indicates that in addition to the normal erosion unconformity there has been a slig'ht amount of readjustment along the jjlane of contact between the quartzite and the iron formation, .which probably occurred during the tilting- and folding of the Upper Huronian strata. The Pokegama quartzite and the iron formation pr,.,bably acted essentially as units during the deformation. « Geol. Nat. Hist. Survey Minnesota, Final Eeport, Vol. IV, 1899, p. 486. 168 THE MESABI IRON-BEARING DISTRICT. For short intervals in the district the Pokegama qnartzite has not been found between the Biwabik formation and the underlying rocks, and in such areas the iron formation rests unconformably on the Archean and Lower H'uronian rocks. Through much of ranges 12 and 13 the Biwabik formation lies directly upon intrusive granite, probably of Keweenawan age. The Biwabik formation is o^•erlain for the most of the district by the Virginia slate. In the eastern portion of the district evidence is not at hand to show the distribution of the Virginia slate, but jJi'esumably it is the rock immediately south and overlapping the iron formation until cut out by the overlapping of the Keweenawan gabbro. The Virginia slate and the Biwabik formation are perfectly conformable. The Biwabik formation throughout contains layers of slate which, near the upper horizons, become more numerous and finally form the Virginia slate. Moreover, the upper part of the iron formation grades laterally into slate, which has been mapped as the Virginia formation. The relations of the Virginia slate to the Biwabik formation are more fully discussed in connection with the Virginia slate. Throuffh much of rang-es 12 and 13 the iron formation is overlain directly by the gabbro, and on the north side of Birch Lake the north- easternmost tongue of the iron formation is bounded both east and west by the gabbro. SECTIOjS^ III. VIRGIIVIA SLATE. DISTRIBUTION. The Virginia slate bounds the iron formation on the south from the west end of the district to near the east side of sees. 5 and 8, R. 13 W., where the slate is overlapped by the gabbro. Still farther east in the SW. J sec. 25, T. 60 N., R. 13 W., drilling has shown altered slate to lie between Keweenawan gabbro on the south and Keweenawan diabase on the north, but whether it is an isolated mass at this point in the Keweenawan area, or is continuous with the slate to the west, explorations or exposures do not yet tell. The slate underlies the lower slopes of the Mesabi range and con- tinues soutli under the low-lying swampy area south of the Mesabi range for an unknown distance. The area overlain by slate is so thickly covered with drift tlint exposures of the slate are almost entirely lacking; its pres- ence and distribution have been determined bj' drilling and test pitting THE VIRGINIA SLATE. 169 done in the search for iron. Through the central portion of the district enough of such work has been done to show the position of the slate bound- ary with a fair degree of accuracy, although even here there are consider- able stretches where records of the occui'rence of slate are wanting. In the western and eastern portions of the district the distribution of the slate is largely hypothetical, particularly so in the western end of the district. In drawing the slate line on the map of this portion of the area, all that can be done is to connect the widely separated explorations which reveal slate. Wherever exploration has been detailed it is found that the slate boundary is not straight, but in gentle curves, and it is reasonable to expect, therefore, that future work will show numerous additional undulations in the slate boundary for the area at present not completely explored. KINDS OF ROCKS. SLATE. The normal Virginia slate is usually a gray rock, though in part black, reddish, or brown, with bedding shown by alternating bands of varying color and texture. Some of the beds are almost coarse enough to be called graywackes. Indeed, in the field the rock has been called a banded slate and gray wacke. Some of the slate is hard and siliceous, while other phases, especially the nonsiliceous and carbonaceous ones, are soft and can be whit- tled with a knife. Near the contact of the slate with the iron deposit in the underlying ii'on formation, as at Biwabik and in sec. 3, T. 58 N., R. 15 W., the slate becomes iron stained and soft and grades into paint rock. The slate in general has a very poor parting parallel to bedding planes, and there is little or no development of secondary cleavage. What there is of secondary cleavage has been developed parallel to the bedding planes and is marked by minute particles of mica there found. While the rock in general aspect and minei'alogic and chemical composition looks like slate, it differs from a true slate in lacking a true cleavage, and as this is one of the essential characteristics of slate, it may be doubtful whether the term slate ought to be applied to the rock. Yet the rock is not a shale, for it is too much metamorphosed and has too poor a parting parallel to the bedding-. In the eastward continuation of the Mesabi district, east of Gunflint Lake, the same formation shows the characteristics of a true slate, and the formation both here and in the Mesabi district proper has been known 1 70 THE MESABI IRON-BEARING DISTRICT. locally and in geologic literature as slate. Hence the term is here retained. Under the microscope the rock is seen to be a chloritic slate containing minute, more or less angular, pieces of quartz, and perhaps some of feldspar, associated with chlorite in confused aggregates. The chlorite makes up about half the rock. An occasional minute plate of mica is present with its greater diameter parallel to the bedding, possibly as a result of the incipient development of secondary cleavage. In general the grain, while exceedingly tine for some layers, is fairly coarse for a slate. The carbo- naceous slates differ from the ordinary type only iu containing minute specks of carbon tending to obscure the other constituents. A partial analysis of the slate (sjDecimen 4.o735), from south of the "Meadow" explorations in sec. 3, T. 58 N., R. 15 W., by H. N. Stokes, ot the United States Geological Survey, is as follows: Analysis of Yirginia slate. SiOj , 46. 61 AljOa 8. 04 FejOs 25. 77 Fe . - 18. 04 MgO 2. 82 CaO 13 PA 06 CO, None. The following analysis by Mr. George Steiger, of the United States Geological Survey, is of a composite sample of the Virginia slate made up b}' assembling seyeral specimens from two localities (specimen 45767 from excavation for watei* tank of Eastern Railwav of Minnesota at Virofinia; specimen 45463 from south of the Biwabik mine): SiOj 62. 26 AlA ; 16. 89 FeA - 1-76 FeO 4. 55 MgO 2. 95 CaO 42 Na,0 2. 29 K,6 :^.02 HjO- 0. 70 HjO-l- 3. 88 TiO, 60 CO, None. P,(i, 20 Organic inuk'tcrniintMl 99. 52 THE VIRGINIA SLATE. 171 I LIMESTONE. In the lower horizons of the Virginia slate, as well as in the upper horizons of the iron formation, is a considerable amount of limestone, interbedded with both slate and the iron" formation. The rock effervesces readily with cold dilute hydrochloric acid, and hence probably does not contain much magnesia. The layers vary in thickness from several feet to a fraction of an inch. Near the horizon of the rapid transition of the slate into the iron formation there is a fairly persistent layer of the limestone several feet thick, although at many places it is lacking at this horizon. This may be well studied in the excavation at the water tank of the Eastern Railway of Minnesota at Virginia. The limestones are gray rocks, not to be discriminated by their color or texture from some of the coarser phases of the banded slates. Indeed, in some cases it was not until a test had been made with hydrochloric acid that the presence of carbonate was suspected. (Specimen 45464.) Under the microscope the calcite is observed in irreg- ular grains and aggregates making up the bulk of the rock. Associated with the calcite are small quantities of quartz and chlorite in particles of about the same shape and size as those seen in the slate above described. COKDIEEITE-HORNSTONE RESULTING FROM THE ALTERATION OF THE VIRGINIA SLATE BY THE GABBEO. In approaching the gabbro, which overlaps the Virginia slate in ranges 14 and 13, the slate becomes more crystalline, harder, and characteristically breaks with a conchoidal fracture, and the color becomes darker and frequently is a bluish black. The rock, indeed, becomes a hornstone. Moreover, there appear minute, light-colored specks which on the weathered surface are likely to have disappeared and to be represented by pits. Under the microscope the white specks are found to be cordierite in typical development, standing as numerous phenocrysts in a fine-grained matrix of biotite, feldspar, magnetite, and certain doubtful microlites which may be actinohte or sillimauite, or both." (Figs. B and C of PI. XIX.) The cordierite ciystals appear in short, columnar forms or with hexagonal outlines, depending upon their position with reference to the plane of the slide. Their average diameter is between 0.10 and 0.20 millimeter, and " Cordierite in this formation was first noted and described by N. H. Winchell, Geol. Nat. Hist. Survey Minnesota, Final Kept., Vol. V, 1900. 172 THE MESABI IRON-BEARING DISTRICT. their length varies from 0.40 to 0.80 millimeter. The single and double refractions are low and striking-ly similar to those of quartz. Their dis- tinguishing feature is the twinning, which in basal or hexagonal sections causes the crystals to lighten and darken in alternating sextants in revolv- ing under crossed nicols. The cordierite crystals are more or less obscured by minute blades of biotite, and perhaps other micas, and by nmnerous columnar raicrolites of some greenish-white mineral, without pleoclii'oism and with high single and low double refraction, whose character because of their small size is doubtful. They may be sillimanite, one of the chai'acter- istic associates of cordierite. Both macroscopically and microscopically the metamorphosed slate is cordierite-Jiornstone, as this term is used by petrog- raphers such as Rosenbusch, Zirkel, and Harker. A partial analysis of the cordierite hornstone (specimen 45699), from near the southeast corner of the NE. J of SE. J sec. 21, T. 59 N., R. 14 W., by H. N. Stokes, of the United States Geological Survey, is as follows: Analysis of cordierite-h/)rnsto7ie. SiOj 70. 48 AljOj 12. 46 FejOj 4. 94 MgO 2. 04 CaC) 71 V.,0; 11 Cbj None. From the Crystal Falls district, south of Lake Superior, Clements" has described and figured a spilosite which shows a most remarkable similarity to the cordierite- hornstone here described The j^orphyritic crystals and the background have the same general aspect, but when examined closely the phenocrysts in the rock described hj Clements are. found to be albite instead of cordierite. RELATIONS OF THE VIRGINIA SLATE TO THE BIWABIK FORMATION. Reference has already been made to the fact that the relations of the Virginia slate to the underlying Biwabik formation are those of gradation, both lateral and vertical. It remains to discuss this gradation somewhat fully. The iron formation contains slate layers throughout. In upper and middle horizons they are perhaps more numerous than in lower hori- «The Crystal Falls iron-bearing district of Michigan, by J. Morgan Clements: Men. U. S. Geol. Survey Vol. XXXVI, IsiW, pp. 206-207, PI. XXXVI. PLATE XIX 173 PLATEXIX. PHOTOMICROGRAPHS SHOWING METAMORPHISM OF VIRGINIA SLATE INTO CORDIEKITE- HORNSTONE IN APPROACHING DULUTH (gREAT) GABBRO. Fig. a. — Slate. Specimen 45463, slide 15736. From water tank of Eastern Railway of Minnesota, Virginia. With analyzer, x 70. This is the normal flne-grained Virginia slate, consisting of quartz and feldspar, chlorite, muscovite, biotite, and iron oxide, all of the constituents arranged with their longer diameters roughly parallel, though the chlorite and iron oxide are arranged to a less extent than the other constituents. Described, pp. 169-170. Fig. B. — Same, without analyzer. Fig. C. — Cordierite-hornstone. Specimen 45285, slide 15679. From near east quarter post of sec. 27, T. 59 N., R. 14 W. Without analyzer, x 70. This rock results from the alteration of the normal Virginia slate by the contact of the Duluth gabljro. The light round and oval spots are cordierite, which in ordinary light can scarcely be distinguished from quartz or feldspar. The matrix is the flne-grained one normal to the slate. Described, pp. 171-172. Fig. D. — The same, with analyzer, x 70. The cordierite crystals may be distinguished by the trillings faintly shown on the round basal sections. As in ordinary light the cordierite resembles albite and quartz. In general aspect the cordierite-hornstone here described corresponds almost exactly to the spilosite of the Crystal Falls district described by Clements and to the feldspathic graywacke of the Penokee-Gogebic district, described by Van Hise. In both of these cases, however, the phenocrysts are albite instead of cordierite. Described, pp. 171-172. 174 U. S. GEOLOGICAL SURVEY MONOGRAPH XLIll PL. XIX PHOTOMICROGRAPHS SHOWING METAMORPHISM OF VIRGINIA SLATE INTO CORDIERITE HORNFELS IN APPROACHING GREAT GABBRO FORMATION. HE MERIDEN GRAVURE CO. THE VIRGINIA SLATE. 175 zons. Just below the solid, black Virgiuia slate there is a zone in which there are many interlaminations of iron formation and slate, the layers varying in thickness from several feet to a fraction of an inch. This zone is of varying and uncertain thickness. In many places at least the zone of minute interbanding is thin, not more than 15 or 20 feet, but, as already noted, layers of slate are found well down in the iron formation, and layers of iron formation are found well up in the slate, so that in a broad way the gradation zone may be several hundred feet. An examii^ation of the map will show the Virginia slate to encroach on the south margin of the iron formation to greatly varying distances, with a result that the surface outcrop of the iron formation ranges in width from 2 or more miles to less than a quarter of a mile. This might be explained by steeper dips at the narrow places than at the wide places in the iron formation, erosion having thus vmcovered less of the iron formation where the dips were steep. This is probably a par- tial explanation of the narrowness of the iron formation belt in the neigh- borhood of Biwabik. The dips are on an average somewhat greater at Biwabik than at Mountain Iron, for instance. However, the difference in dip is not sufficient to account for the extreme narrowness of the formation near Biwabik, nor in the area as a ^vhole is there uniform variation of the dip sufficient to account for variation in width of the formation. The distribution might be explained by the greater dip of the plane of surface erosion, either atmospheric or glacial, in places where the forma- tion is wide than where narrow, the greater dip of the surface bringing' it more nearly parallel with the dip of the iron formation and thus uncovering more of it. While this may help to explain the distribution, the dip of the the plane of erosion does not show the requisite variation in inclination to fully account for the observed distribution. Finally the distribution may be explained by the lateral gradation of the iron formation into the Virginia slate. This is believed to be the principal cause of the present distribution. In the Biwabik area, where the slates encroach on the soiithern boundary of the iron formation, the lack of sufficient steepness in dip of strata or flatness of plane of erosion leaves the lateral gradation of the iron formation into slate as the only alternative explanation of the distribution at this point. Moreover, in following the iron formation eastward trom the Biwabik mine through the Cincinnati 176 THE MESABI IRON-BEARING DISTRICT. property the iron formation and slate are actually found interbedded direct!}" along the strike of the iron formation. At numerous points in the district, as alreadv noted, the iron formation becomes interstratified with slate toward its upper portion. This being the case it would be strange indeed if the conditions favorable to the deposition of slate had been reached at exactly the same time all over the range. Indeed, it would be more reasonable to suppose that while at some places the iron fonnation alone was being deposited, at other places iron formation and slaty layers were being alternately deposited, and at still other places • slate alone was being deposited, thus giving a lateral gradation from iron formation into slate. COMPARISON OF SLATE OF VIRGINIA AND BIWABIK FORMATIONS. It is difficult to discriminate the Virginia slate from slate layers in the iron formation on lithologic, chemical, or textural grounds, for each of these slates has a variety of phases and some of them are common to both. But while in an individual case it may not be possible satisfactorily to dis- criminate between the two, in general, the differences are believed to be somewhat as follows: (1) The Virginia slate has a predominance of grayish tones, while the slates interstratified with the iron formation have red, brown, or black tones. (2) The slate in the iron formation is probabl}' more frequently broken into small parallelopiped blocks, or is more likely so to part, than the Virginia slate. Insufficient observations have been made to warrant posi- tive statements, but in this opinion the writer is in agreement with some of the leading mining engineers on the range. (3) The slates in the iron formation contain abundant griinerite, while the Virginia slate contains almost none of it. (4) The slate in the iron formation contains on an average a lower percentage of alumina, a higher percentage of iron, and in some cases a. higher percentage of silica than the Virginia slate. In the mapping, where pits or drill holes have shown slate of consid- erable thickness with few or no iron-formation layers in it, it has been mapped as Virginia slate. Where the slate is mixed with the iron-forma- tion layers in considerable quantity, or the iron-formation material has been found to the south of it, the slate has been included in the iron forma- tion. (See p. 14S.) THE VIRGINIA SLATE. 177 STRUCTURE. , 0}Dportunities for studying the structure of the Virginia slate in situ are so few that if the observer were dependent upon such observations alone he would be unable to make any statements concerning the structure of the formation beyond the fact that it dips at low angles away from the high land adjacent. However, the slate is a conformable part of the Upper Huronian series, the other members of which show clear evidence of folding and fracturing. The Virginia slate must have partaken in this deformation. The statements applied to the structure of the Upper Huronian series on pp. 178-180 will therefore apply to the Virginia slate, but with one modification. Drill holes going through slate into iron formation sometimes reach water under pressure, indicating that the water has been ponded in the iron formation under the slate. This indicates the relatively pervious character of the iron formation, and it seems likely that in the gentle folding of the Upper Huronian series the brittle iron formation was more broken than the soft slate and thus affords freer passage to water than the slate. THICKNESS. The thickness of the Virginia slate can not be determined in the Mesabi district. The drift covering is thick, mining exploration stops to the south where the slates are encountered, and the souther! 3^ extent of the slate belt is thus unknown. To the south of the range, however, there is a low, swampy area, west of the gabbro, extending southward for about 35 miles at its widest, which is presumably underlain by slate. If the slate occupies all of this area, it must have a vast thickness. In the neighborhood of Grunflint Lake and eastward the equivalents of the Virginia slate and their upward extension have been estimated by Grant" to have a thickness of 2,600 feet. This does not represent the total thickness of the formation, but simplj^ the part not covered by the gabbro. In the Penokee-Gogebic district the Upper Huronian slate lying above the iron formation has a present maximum thickness of 13,000 feet.** Thus the Virginia slate, which has an inconsid- erable thickness in the district covered by the general map of the Mesabi range, is continuous with a formation extending to the south which probably has a great thickness. oGeol. Nat. Hist. Survey Minnesota, Final Rept., Vol. IV, p. 48fi. 6Mon. U. S. Geol. Survey Vol. XIX, p. -!56. MON XLIII — 03 12 178 THE MESABI IRON-BEARING DISTRICT. SECTION IV. STKUCTURE OF THE UPPER HUEOXIAlSr SERIES. As a whole the Upper Huronian is a well-bedded series of sediments. The bedding is most pronounced in the middle and upper horizons. The beds have gentle dips, averaging between 5° and 20°, though locally greater or less, in southerly and southeasterly directions away from the older rocks forming the core of the Giants range, but locally the dips show much variation both in degree and direction. About the southerly projecting tongue of the Giants range, in the vicinity of Virginia, Eveleth, and McKinley, the dips are westerly on the west side of the tongue, southerly at the end of the tongue, and southeasterly on the southeast side; that is, throughout approximately normal to its periphery. Even more conspicuous than the change of dip at such a place are the minor variations between exposures. Seldom is it possible to get two identical readings in dip at exposures of rock separated by even short intervals, although the direction and amount of the dip come within the above limits. These facts indicate that the beds of the Upper Huronian series are tilted away from the core of the Giants range in directions normal to its trend, and that the gentlv tilted beds are not plane surfaces, but are gently flexed. By tabulating and comparing the dips it becomes further apparent that the greater flexures are not random ones, but generally have their axes normal to the trend of the range. Examining the attitudes of the beds still more in detail, it appears that the great flexures themselves are not simple, but have many subordinate flexures, some of them transverse to the major ones. The complexity of the structure may be likened to that of water waves. On the great swells and troughs there are smaller waves, on the smaller waves there are still smaller ones, and so on down to the tiniest disturbance of the surface. While perhaps the majority of the minor flexures in the Upper Huronian rocks have attitudes similar to the larger ones, many of them vary greatly in direction. They may be observed at almost any single exposure of the Upper Huronian series. "While the great flexures are very gentle, involving very small changes in degree and direction of dip, the minor flexures superimposed ujwn the greater ones are frequently sharp and conspicuous. The local dips may varv as much as 50° within a few hundred feet and change their direction considerably. Dips as high as 45° or even 60° may be seen in the iron- formation layers in some of the open pits of the mines, as at the Stevenson, STRUCTURE OF THE UPPER HURONIAN. 179 the Sauntry-Alpena, the Kanawha, and the Sparta. A series of dips taken at intervals through the Mountain Iron, OHver, and Biwabik open cuts are tabulated in connection with the description of the ore deposits (pp. 225-226). The iron formation shows more minor contortion than the rest of the series, because of the great chemical changes which it has undergone, but it is not probable that there is any great difFerence in the major folding. Accompanying the tilting and minor folding ofi the Upper Huronian series there has been a very considerable amount of fracturing, especially in the comparatively brittle Pokegama and Biwabik formations. Indeed, it seems likely that the folds of the two lower members of the Upper Huronian series are mainly the result of relatively small displacement along fractures, and only to a small degree the result of the actual bending of the strata with- out breaking. The ponding of water beneath the Virginia slate would seem to indicate that this formation has been less fractured than the iron formation because of its less brittle character, and had thus yielded to deformation by actual bending rather than by breaking. On almost every exposure of Pokegama and Biwabik formation rocks joints and minute faults are to be observed cutting almost perpendicularly across the bedding. In each case the joints seem to make up two or more systems crossing each other at various angles, but such sets have little constancy of direction in widely separated exposures, unless we except a set of joints which at a number of places have an average direction of somewhere between N. 60° and 70° E.— that is, approximately parallel to the trend of the range. In the massive rocks the joints are clear cut and continuous for considerable distances. In the well-bedded rocks, as, for instance, in the thin-bedded portions of the iron formation, the joints are usually more irregular, less continuous, and less conspicuous. In such places each individual bed may be more or less jointed without reference to the layers above or below. The displacement or faulting along joints has been, in general, small. The displacement is rarely 3 or 4 feet, and commonly it is measured by a few inches. In the neighborhood of some of the iron deposits of the Biwabik formation certain facts doubtfully indicate a greater displacement, but this is discussed in connection with the ore deposits. Certain of the joints and faults have been tilled with vein quartz and others not. It is rather surprising that so little vein quartz is to be observed. Where present in the harder rocks, where the joints are clear cut and 180 THE MESABl IRON-BEARING DISTRICT. continuous, the quartz veins appear likewise. In the well-bedded portions of the iron-bearing formation, where the joints are irregular and discon- tinuous, the distribution of the vein quartz is also irregular and discontinuous, being rather in a confused zone than a well-defined plane. After the Upper Huronian series were tilted and folded, the upper edges of the series were eroded away, with the result that the rock surface is now irregular, with dips corresponding roughl^y in direction, but not in degree, with those of the underlying rock strata, being, in general, less steep. SECTIOX V. THICK?rESS OF THE UPPER HUEOISTEAX SERIES. From what has been said concerning the thickness of the constituent members of the Upper Huronian series, it is apparent that accurate statements of the thickness of the Upper Huronian series as a whole are not possible. The Pokegama quartzite varies in thickness from 0 to 500 feet and averages perhaps 200 feet. The Biwabik iron formation ranges in thickness from 200 to 2,000 feet and averages perhaps 1,000 feet. The Virginia slate extends indefinitely southward beyond the limits of the area under investigation. Within the area studied an actual thickness of nearlv 400 feet has been observed. Assembling these figures, it appears that the maximum figure for the thickness of the Upper Huronian series within the limits of the district mapped may be 3,000 feet, while the average may be 1,500 feet. But the total thickness of the Virginia formation, including its southward extension, is probably several times as great as the thick- ness of the two lower members of the series combined, for the thickness of the formation in this area may be commensurate with its thickness in the Penokee-Gogebic area, as the extent of the flat area south of the range would seem to indicate. SECTION XI. RELATIONS OF THE UPPER HURONIAX SERIES TO OTHER SERIES. The Upper Huronian series lies iinconformabh' upon the Archean and Lower Huronian rocks. The proof of unconformity is as follows: (1) The conglomerates at the base of the series (listed imder the discussidii of the Pokegama quartzite) contain fragments derived from the underlying rocks. RELATIONS OF THE UPPER HURONIAN. 181 (2) There is discordance in dip. The underlying formations, where they have any parallel structure at all, are almost vertical. The Upper Huronian is well bedded, with a low dip. Moreover, in approaching the contact no change of dip is to be observed, either in the Upper Huronian or in the underlying rocks. (3) There is a difference in the amount of minor folding, fracturing, secondary cleavage, and further consequent metamorphism of the two series, the Upper Huronian being much less afiPected than the older series. (4) The Upper Huronian belt overlies Archean and Lower Huronian rocks indiscriminately. Near Biwabik, for instance, the northern edge of the Upper Huronian series lies diagonally across the contact of the Archean and Lower Huronian rocks. (5) The Lower Huronian is intruded by the granite making up much of the core of the Giants range. The Upper Huronian series is not so intruded, and, moreover, in the conglomerate at its base it bears fragments of this granite. The Upper Huronian series, in ranges 12 and 13, is in eruptive contact with the Keweenawan granite and gabbro, as fully shown in the section devoted to the Keweenawan. CHAPTER VI. KEWEENAWAN, CRETACEOUS, AND PLEISTOCENE ROCKS. SECTIOX I. THE KEWEENAW AX ROCKS. DULUTH GABBRO. A portion of the great mass of Keweeuawau gabbro of northern Minnesota comes within the limits of the Mesabi district. The northern edg'e of the mass hes diagonally across the eastern end of the district, extending from neai- the Duluth and Iron Range track, in range 14, northeasterly through ranges 13 and 12 to Birch Lake. Tkrough range 14 the gabbro is in contact with Virginia slate: in ranges 13 and 12 it is in contact with the Biwabik iron formation, and north of Birch Lake it is in contact with Lower Huronian granite. The northern edge of the gabbro forms a conspicuous northward-facing escarpment overlooking the low- lying area of the Virginia slate and of iron formation immediately to the north. To this the name Mesabi range was first applied. In the neigh- borhood of Birch Lake the gabbro comes well up on the crest of the Giants range, and here it does not stand above the adjacent rocks. The gabbro exposures show a parting into massi^'e bands usually 10 to 20 feet thick, but sometimes ranging down to a few inches. As certain layers have somewhat different textures from those above and below, it is certain that the stinicture is at least in part one induced in the gabbro when it first cooled, but also a considerable amount of the parting may be a secondary phenomenon. The banding of the gabbro in the IMesabi district may be well observed near Allen Junction, where it dips to the northeast at an angle of about 40°. In addition to the parting into bands the gabbro is cut b)' vertical joints. Occasionally a combination of the joints and banding structures causes the rock to weather into spheroidal l)locks, which at first glance look surprisingly like conglomeratic bowlders. This feature 182 THE KEWEENAW AN ROCKS. 183 also may be observed in the gabbro about three-quarters of a mile north of Allen Junction. The petrography of the gabbro of northeastern Minnesota has been exhaustively described by Irving," Bayley/' Winchell/ Grant/ Elftman/ Clements/ and others, and the part which comes witliin the Mesabi district shows no features not covered in these descriptions. In characterizing the gabbro in the Mesabi district one can not do better than to quote a brief summary of the petrographic character of the gabbro as a whole by Dr. U. S. Grant: Tlie gabbro is a coai-se-grained aggregate of plagioclase, which is near labrado- rite; augite, which is often diallagic; olivine and magnetite, with occasionallv h\'persthene, biotite, hornblende, and minor accessor}' minerals. In general, the mass is of fairly miiform composition. Variations, however, take place mainlv in three directions: First, by increase of feldspar the rock becomes an anorthosite; second, by increase of feldspar and olivine a forellenstein is formed; third, by increase of magnetite masses of titaniferous magnetic iron ore originate. Along its northern limit the gabbro, while at times assuming a liner grain, usually preserves its coarse grain and granular texture to its contact with the underlying rocks, s" The gabbro mass has been generally regarded as an extrusion forming the base of the Keweenawan, but recent work on the relations of the gabbro to adjacent formations, especially near Gunflint and Akeley lakes, has led Van Hise, Clements, Grant, and the writer to the behef that the gabb-o is a laccolitic intrusion. CONTACT PHASES OF GABBRO. On the north side of Birch Lake the gabbro, where in contact with the iron formation, shows a segregation of coarse magnetite octahedra in layers. Moreover, at this point biotite and the orthorhombic pyroxenes, particularly enstatite and hypersthene, become more abundant, while the monoclinic pyroxenes are corresponding!}' less abundant. These features are common to the gabbro contact in other parts of northern Minnesota. «Mon. U. S. Geol. Survey Vol. Y, 1S83. ''Jour. Geol., Vol. I, 1893, pp. 433-i56, 587-596, 688-716; Vol. II, 1894, pp. 814-825; Vol. Ill, 1895, pp. 1-20. <= Reports of Geol. Nat. Hist. Survey Minnesota, latest conclusion in Final Kept. , Vols. IV and V. ''Engineers Yearbook, Univ. of Minn., 1898, pp. 49-62. Bull. Geol. Soc. Am., Vol. XI, 1900, pp. 503-510; also in Reports of Minn. Surv. (see Chapter II). «Am. Geol., Vol. XXI, 1898, pp. 90-109, 175-188, contains full bibliography; Vol. XXII, 1898, pp. 131-149. /Mon. U. S. Geol. Survey Vol. XLV. !7Contact metamorphism of a basic igneous rock, by U. S. Grant: Bull. Geol. Soc. Am., Vol. XI, 1900, pp. 504-505. 184 THE MESABI IRON-BEARING DISTRICT. North of Bircli Lake also (see fig. 7) is a curious contact rock between the gabbro and Lower Hm-onian granite. Between the coarse gabbro and the normal coarse granite is a narrow zone, perhaps 150 feet wide, occupied by a fine-grained micaceous rock, varying from brown to pink in color, which looks in places like a fine-grained gabbro and in others like a fine-grained g-ranite. In the field it was not determined whether the rock was an altered gabbro, an altered granite, or an intermixture of the two, and microscopic study does not help us out much. The rock is R.12 W. LEGEND R.12 W. Scale 0 K K mile Fig. 7.— Detail map showing distribution of Lower Huronian granite, Biwabik formation, Duluth gabbro. and contact phase of granite with gabbro on the north shore of Birch Lake. composed mainly of feldspar, largely orthoclase, although so obscured by cloudv alteration as to make accurate determination difficult, biotite, and quartz. The texture is granitic. These are features characteristic of granite, and, whatever the rock once was, it should probably now be called a granite. However, it is still jiossible that the rock represents a fine- grained contact phase of the gabbro for the reasons that (1) the character of the feldspar is doubtful, (2) biotite is one of the characteristic minerals developed in the gabl)r(> near its contact, (3) in tlic area eastward toward THE KEWEENAW AN ROCKS. 185 Grunflint Lake free quartz has been fonnd in the gabbro itself, and (4) in this eastern area also the exomorphic effects of the gabbro contact on the granite are very slight. DIABASE. There are in the Mesabi district certain rocks associated with gabbro which are not covered in the above general account. In range 13 expo- sures of fine-grained diabase appear in the SW. J sec. 25, T. 60 N., R. 13 W., and in the central and northern portions of sec. 35, T. 60 N., R. 13 W. Bowlders of the same material indicate its extension for several miles east and west, and, taken together with the exposures, indicate a belt with a possible width of something less than a mile, a length of at least 3 miles, and jjrobabl)' much more, and a trend northeast and southwest — that is, parallel to the general strike of the formation boundaries in this part of the district. The diabase is a fine-grained dark-gray rock which, under the microscope, shows a well-developed ophitic arrangement of plagioclase feldspar crystals and the presence of abimdant hornblende and less abun- dant ilmenite and magnetite. The diabase corresponds lithologically to the diabase sills intruded in the iron formation in the neighborhood of Gunflint Lake, and there supposed to be either offshoots of the gabbro or intrusives bn thickness are being mined. The maximum depths of the workings of some of the mines are given in the following table: Maximum depth of mines in Mesabi district. [June. 1902.] Feet. Adams 20.5 Burt 1 20 Duluth 97 Biwabik 110 Mahoning 7.5 Sparta 12.5 Malta •. 110 Pillsbury 73 Spruce 211 Stevenson 40 Hull 176 Rust 192 Sellers 13S Auliurn 218 (ienoa 174 Fayal UiS Mountain Iron 150 THE IRON-ORE DEPOSITS. 209 KINDS OF ORE. The Mesabi iron ores are for the most part shghtly hydrated hematites with an average of 3 per cent of combined water shown by cargo analyses. It is likely that the true percentage of combined water may be somewhat larger than this, for before it is measured the ore is dried at 212° F., a temperature sufficient to drive off some of the combined water. Asso- ciated with the slightly hydrated ores are abundant layers of ores of varying thickness differing from hematites only in their higher percentage of combined water. They include turg-ite, 2 Fe203 (94.7 per cent), HjO (5.3 per cent) ; goethite, Fe203 (89.9 per cent), HjO (10.1 per cent) ; limonite, 2 FeaOs (85.5 per cent), 3 HjO (14.5 per cent); and possibly even xantho- siderite Fe203 (81.6 per cent), 2 H2O (18.4 per cent). Magnetite is present, but ver}^ sparsely, in the ore deposits. The slightly hydrated hematites are ustially dull blue-black or brown earthy varieties. The hard blue crystalline hematites are rare and the bril- liant specular hematites altogether lacking. The more hydrous ores are for the most part soft, earthy varieties. Most of them are locally called yellow ocher, or brown ore, but they include also hard crystalline varieties. Rarely both the hematite and hydrous ores appear in stalactitic or botry- oidal forms which are likely to be observed bordering cavities in the ore. Under the microscope the hematites appear oidy as dull-red and black opaque aggregates, and the hydrous ores as dull-yellow opaque aggregates, but when examined in hand specimens they are seen frequently to be made up of small ellipsoidal granules identical in size and shape to the greeualite granules of the greenalite rocks described on pages 101-115. The magnetite, when present, is black and crystalline, occurring typically in octahedra. East of the Iron Range track, through most of range 14, and all of ranges 13 and 12, the oxide is largely magnetite, which has not been segre- gated into deposits of sufficient extent to warrant exploitation. Westward from Mesabi station the magnetite gives way rapidly to more or less hydrous hematite, and through the central and western portions of the range is found in but small quantity. Slightly hydrous hematites and the more hydrous ores are almost everywhere interbanded in thin or thick layers, yet in most deposits con- MON XLIII — 03 14 210 THE MESABI IRON-BEARING DISTRICT. siderable zones are composed predominantly of one or the other class of ores. The hydi-ous ores are more abundant near the tops of the deposits, as in the Mahoning, Sharon, Clark, Oliver, Grenoa, Sparta, Elba, Commo- dore, Biwabik, Duluth, and Hale mines. To less extent they appear in middle and lower horizons in the deposits. JNniSrERAIiS AND ROCKS CONTAINED IN THE ORE. The principal mineral constituent associated with the ore is chert or quartz. Average cargo analyses of the ores show about 4 per cent of silica, but locally the percentage of silica in the ores runs much higher. There may be found all stages in the gradation, from ore with a low percentage of chert to ferruginous chert with only a low percentage of iron. Ferru- o'inous chert forms the wall rocks of the deposit, occurs as pillars, horses, or shelves projecting from the bottoms or sides, and occurs as small or large masses entirely included in the ore. Even where the percentage of chert is as low as 4 per cent the substance may be observed in minute grains with the microscope. Rarely a layer of the feiTuginous chert in a deposit may be so disin- tegrated that it is a soft, light-yellow powder resembling fine sand or tripoli powder. The particles are entirely angular. Analyses published by SpuiT give the following results: Analyses of silica j>owde7\'^ Constituent. Per cent. Fer cent. SiO, 77.89 13.55 1.83 .36 Trace. .58 .84 1 4.45 98.17 ALO, .50 Fe,0, 1.03 MgO Trace. CaO . . Trace. NajO .25 K,0 Trace. H2O- H.,0+ - Loss on itniition .19 99.50 100. 14 "Geol. Nat. Hist. Survey Minnesota, Bull. No. 10, pp. si nnd 211. THE IRON-ORE DEPOSITS. 211 Other similar layers, somewhat coarser, are found on examination to consist of waterworn sand (to be discriminated from the "sandy" ores described in the subsequent paragTaph). Some of the sand layers are cer- tainly the result of washing in of glacial sand along cracks from above, as the layers have been connected with the surface and consist of fragments of all the minerals found in the drift, but others may represent a disinte- gration of original sandy layers in the iron formation. In the western end of the rang-e some of the iron ores contain so much disseminated "sand" that up to 1902 they were considered unfit for use. The "sand" is uniformly disseminated tln-ough the ore, or occurs as more or less iron-stained layers. Under the microscope the "sand" is seen to consist not of water-rolled particles, but of subangular fragments of chert derived from the disintegration of the ferruginous chert. All stages of the disintegration may be observed from a typical ferruginous chert, in which the former existence of greenalite granules is indicated by 'the distribution of chert and iron oxide, to the loose particles of chert which to the naked eye look like sand. In the less disintegrated phases the polygonal and angular particles of chert may be seen to be separated by thin films of iron oxide, showing the cementation of the particles to be very weak. The disintegrated chert may be separated from the ore by washing, but whether or not this can be done successfully on a commercial basis is a question not yet satisfactorily decided. Experiments thus far conducted, while not decisive, indicate that it can be done. A small quantity of griinerite and actinolite in columnar forms or in radial sheaves may occasionally be seen associated with the chert and ore, especially in the ore containing considerable magnetite. Crystals of calcite, siderite, dolomite, quartz, adularia, pyrite, mica, pyrolusite, and many other minerals are common in vuggs. It is not known in what mineral form the phosphorus occurs in the ores, although it will be shown on a subsequent page that it occurs probably in combination with alumina. In the Michigan ranges Prof A. E. Seaman, of the Michigan School of Mines, has determined the phosphorus to occur largely in the form of apatite. Apatite has been searched for in the Mesabi ores, but has not been found in clearly identifiable crystals. Other phosphorus minerals, 212 THE MESABI IRON-BEARING DISTRICT. such as viviauite, wavellite, waguerite, aud collophanite, have been looked for, but without success. Layers of paint rock, ranging in thickness from a fraction of an inch to several feet, occur in almost every deposit. Kaolin, or clay layers, either white or yellow, and not sufficiently discolored by iron to warrant the name "paint rock," are also occasionally to be seen. Ferruginous slate, differing from the normal ores only in containing a higher percentage of alumina, forms layers in the ore deposits. Occasionally a small mass of bluish-black ore is encountered which nms high in manganese in the form of pyrolusite. Vein quartz, usually much brecciated, is a common feature in the ore deposits. It usually follows irreg'ular joints and fault zones, and occasionally follows the bedding for a considerable distance. The breccia- tion is direct evidence of considerable movement in the deposit subsequent to the inti'oduction of quartz. Iron pyrites is rarely to be observed in quantity; it is known to be sufficiently abundant to lower the value of the ore only along the edge of two deposits on the range. Still other rare rocks or minerals in the ore deposits could be mentioned. In the Fayal mine is a peculiar bluish rock with a greasy feeling, showing slickensides. It is a rock rich in magnesium and colored by ferrous iron. Its oi'igin is not known, nor is it important. CHEMISTRY. The following statements concerning the composition of the Mesabi ores are based on official cargo analyses, as published by the American Iron and Steel Association, on a great number of detailed figures furnished by mining superintendents, engineers, and chemists on the Mesabi range, and finally on general statements made by those best qualified to make them. Among those who have given especially full information on this subject should be mentioned the Lerch brothers, chemists, of Hibbing, Virginia, and Biwabik; Mr. R. B. Green, chemist ofthe Minnesota Iron Company; Mr. E.T. Griese, chemist ofthe Duluth, Missabe and Northern docks at Duluth; Mr. A. P. Silliman, mining engineer aud chemist, Hibbing; Mr. A. T. Gordon, chemist. Mountain Iron; Mr. E. J. Johnson, chemist ofthe Republic group. To mention superintendents and mining eiigineers who have given infor- THE IRON-OEE DEPOSITS. 213 mation would be to give a list of those connected with Mesabi mining. It must not be understood that each of the above-named gentlemen would agree with all of the following statements. Indeed, there is much minor diversity of opinion. Average cargo analyses of Mesabi ores shipped in 1901, according to figui'es compiled by the Iron and Steel Association, are as follows: 214 THE MESABI IRON-BEARING DISTRICT. <0 S -H PCs u ^__, _ _ = 01 -f lO (M 'M ' ?::? • ri ' 2 ' CO X -r CO 1-- ■ CO ta ■ -f' 'S o B >»c ^^ o o o ao c CO t^ CD O CO t^ CO t^ o: CT- t^ -* t^ CT CO cc 05 Iff ■^ CO 1-t CO ^ O 00 cc c^ o'c lO X Tf b- t^ 02 Ir^ C^l lO o- lO tH CO O CO ^ o iC Zi 3.M tIh' -*■ CO CO cc lO N T-^ Tf Th -* CO co cc '^ CO IC -^ CO CO -t co' t^* 1* Tt< o CD t^ X CO -+ CO t^ N ot (M CO lO cc 1— ( (M ■01 w: CO CO IC IC -f" -^ CO r^ -+■ JZ xnj a -* r- CO c o l-H C o o o o o o o o a. o o c o c o c= o s O C o c= o o c o o o o o o o o o cc o CO tP X CO (M o- cc -r Q IC CD CO t- -^ cc c= O -f t^ t> r^ t^ cc CO c t^ Ol o o cc iC O 1^ X cc CD iC' o o c o o O (>] I— t c^ (M (M — -— ' o o — 1—1 S "^ o t^ IC IC Cq d SS S2 o- t^ c o o 1 /— , LO o t-- a Th CO o cr. CO I— < c t^ X OJ X ^ CT t-- t^ X' CO r: o ■n T— * .—I I— i-H »-H C<1 oq OJ ^H CO CO ^ CO (M 3<1 C\ c-1 Z- _ *"■ O t^ \ CO lO CO o T— ( (N X o CO ^ r— 1 00 t^ 9 l^ S"' 2 - (N (M (M ,-t CD IC -f I>- o lO Tj^ CO CM -t -^ iC -t^ w o O oi iT -i- o I- - -: - ?^ TT CO- CO o t^ ■ S ^H CD on - I>- CO Tt^ 02 03 cc cc lO Od CO CO X C<1 IC CD iC X 0- -^ oc Cq IC (N l-H ^ co =2 t-: 03 o CO 00 c TfJ ^ CO CO t-^ cv: CO (>; CO Tj- I>1 I l> OS CO lO CO IC lO vo CO lO CO lO CO LO •CO lO iC CO iC cc iC CD IC CO ic iC -^ si q "oj o ? B CQ 1 c < < 0^ p T a) '5 G b a; B ^ a. 1 ? CK '/Z — r". M !32 _H ::; = CD X V. i: L. S 5 T. £ '— _^ _;£ c 3 o: 5 J _m _x u •c < .^ -^ ^ -t; £3 ^ -C r= :5 i3 THE IKON-ORE DEPOSITS. 215 13 .3 a o I "■SI r-C) CO =0 u I>- o rt< o O I— ( CO lO -M I— 1 o CO o lO 0 3 o uo -t^ LO CO -* cq C<1 -t- t^ CO cq cq .£S ,_, cr^ lO c^i ai crj b' o d d ^ ci CO d o , T— 1 I— 1 s >.« yD CO ^ o -t* :o 3= t^ CO a> o CO CO t^ X) CO •f o o o; ^ ^ DO o w ■o ■J3 m ■-0 o lO t- la CO CO o 5i t^ o CO -^ o m lO 5^1 a: C^l t^ LO CO rH o 00 J_bp i6 -r^ :o lO lO -v CO CO CO CO CO CO o4 c^i i-O l-O cq' ci co' co' cq ^ ^• r^ o o ■M -M O] -t^ lO CO 3 CO CO o 00' o c^ -V CO r-- o :o t^ r^ --C ■CO LO a-.' 95 -5 Tt^ CO o o o o o o o o o o o 0 _ft o o o o o o o o o o ^ o o o o o o o o o 0 "3 CO o i -t^ t^ ^ CO o r^ c^ LO ^1-1 o 1^ OO LO CO -t^ cq o CO lO 10 Si CO uO o Ol 1^ lA lO CO - o -? X Sc l^ CO r- w (M '— ' CO r-t 3: GO o cc ^ t-^ rH 0 d -: - " -^ ^ •-^ --< cq cq rH r^ , -t- ^ rH 'M cc T~i Cl CO co !M tM I— < br rt< •* o LO o cq ^ G:i CO t^ CO lO CO lO o o UO CO cn CO CO QO Tf in 0 1-- lO Xi b- o: CO ■r> lO ^ 5^ lO lO lO rf lO ^ cq cq cq cq ^ ^ ^ " — - — — o tie IT"- „ (M LO t^ CO -* CO o en o T(< o 1—1 uO CO oc Tt^ I>- iC t^ CM •* IC rH r* CO cq o lO rt^ CO 00 rH fM LO CO -f CI CO CO 03 CO OO VC Tf o cq CO CO CO t^ 00 ZfJ O O LO -* CO ct" ■M oi -r -f- CO ct Ol o~i CO co' -+ CO LO -J' CO t^ 6 cC -Tt- ?1 LO ^ ^ I^ _ lO C3 cc cn -a . ^ 'T cc CM CO o O: LO t^ CO a i> C-l X !N 00 c^" cc CO '^s. "S CO ^ ,_^ M cn CO c£ ir C£ IT ; ,^ ■5 £ W ___ ;- .^ a C c C3 s 'g ^ 1 c c « s !« 3 c ^ •^ C ^ ^ W a Q^ ^ a. ^ - ^7 s 'l ^ 'a 13 ^ » o; g "^ ^ 'i i -ti 1 X 1 = ,t c c ■ ■ a > a E c a C c 6 JO c c -1 3 5 1 c s c: c 03 cS i i4 216 THE MESABI IRON-BEARDs^G DISTRICT. o lill tP cc C-1 '^ ^1 C^l C3 (M ifD iO -^ iC CO O O O tN (M CC tM CO O 1-i i-H en « ro -h -f X i-~ CO CO c^ o c; CO tj- i-( o o o o o o o t^ c^ o t^ -* '^ (M lO CO t^ (M iM T-H 1— ( 1-1 ,— I O o o o o o o o CO Qocor^ooocoi— I OOOOOOi— lOi— ii-( Ot-.0000000 lO W O . t^ I>. <1D CO o o o o J>- O 00 -^ CM O C^ rH 1— I r-( CD lO O lO CD lO 1— < CTi CD lO --H C-H I-H CM cq 03 a o s ^ 00 I— ( cs CD o CO r- (>J O -t CO CD »0 Ci O CI Oi I-H rH C<1 CM a:^ 'Tt^ a:, t- CD I-H (M (M O:^ CO O .-I CM W t^ CO O CO LO o COQOOCOiOCOfMCO COt^lOTt^t^CDCOiM 1— I lO OS Ol -rt< T-H iC (M* (N T-H i>- CO lO CM CO ic Tt< t^ lO tM o i-H X cq O Hi CO (■(-, GO CM -^ (M r-1 (M a> C5 CO (>i CO 05 i-H iClCCCCO^-*CCC^t^CDTpcO^ O U Pi CO t^ c^ CO -f -* o o o O t^ C<1 03 lO O TJH —i CO in CO Tt^ -Tf CO CO -* oooooooo -* CO Tl< IC O t^ iO 00 CO Tji CO CO C-1 lO -:r CO lO o o o o o o o o o t^ o -* a> i>- O t^ O i-l CO c M c St c o pq I o 5 3 3 z -^ -^ o JS ,5 a 2 "a o >3 3 3 ■J 3 s o n p-l Ph 3 o 3 ^H M ^ d 3 3 O 55 f5 O o 3 3 _2 Oi Eh S3 - M THE IRON-ORE DEPOSITS. 217 a o O *> 5. .§5 ^. 03 0 0 U lO lO CO 0 ' 0 (M 0 CD cq CO 1— ( 0 (M S 00 CO CO 0 CD CO t-- 0 -* I>- -t t ■X 00 ^ cc '• CJ J>^ 06 ^ CJ CO OD ai T-H o rH ^H I— ( .-H rH >.«" JO I r-l J2 O l-^ 00 0 TlH CO t^ ' 0 00 DC LO Th r^ c: C^ OS CO >— 1 CO 00 00. rH , i~ a; !M o'S o ^ CO i-i CO -* l:~ T). CD rH ^ 03 rH CD < S CO I-- 1-H '-^.& CO lO (M' c^ ' '^* -t^' c-i c4 -*■ -4 -* CO -!t< CO ■ (>i c^ LO LO t-i ^ LO ^ ' 00 C<1 00 ^ p y? iC 2 cc — 1 o* t^ I> CD in ■*! t^ CO 00 CO CC CO ■ oc b- CD T*^ 1 o o o 0 c 0 (N oa w & c o c o £ o c 0 c: 0 (3 0 c 0 0 0 S 0 ! § 8 0 0 1 9 02 c: CO ' t^ 0 CD o (M t^ CO 0 ic CO m CD cq CO " 0 0= Csl o o b- CO 1:0 CO ' 10 -^ 00 CD ^H Oi Tl^ 1— f CD 10 ■ >— I CO -±< I— i 1 ^ I— o o C" |^^ 0 0 1 — 1 .— 1 0 C<1 C<1 0 CO ' (M 1— 1 01 01 1 s c ■ 1 r~t I M 0 CO o rH -# iC r^ 0 00 ^ Cft 10 00 LO ' 0 'M CO a 3 03 CD c^ cr^ 10 CO c^ ^ CO CO CO 10 10 CO CD ^ . CO CO t-* r^ 1 S -H G^ (M (N 1—1 I—f • C^l tM I— 1 rH I— 1 T-^ T-< -rt< -^ 1 c OS t^ 00 a CO O !M 0 t^ 0 0 ot •* ■* I 0 c<\ r-- oc .— 1 r^ CO cc tH a m Cs (M Cs CD C> (M 10 10 t^ c^l • 0 »-( c^ ^ 1 a (X l>- c 00 c Oi CO CO ; Cs T-H r- CO 00 t^ Oi 00 10 (N i ai CO 0 c~. 1 I— c< T-- T-H oi oi 1^ CO • CD CO rr lO o iO 00 0 Tt< t^ 10 00 rH ■^ 0 C: LO o (M <:<■ t^ 0 CO • t^ CO 00 0 CO 00 a. CO rH « 1-- CO TtH If: -^ cc £^ IC -* -i- CO 0 02 m c^ CD 10 00 !>. CO CO ■ CO CO CO LO 1 c \ -< 10 cn c3 Ttl CD CO IC 0 a> t- 00 T-^ . 0 CO t- .=^ -Tf I— 1 CO Oi oc t^ 10 00 10 t^ 00 OS (M CO Tt ^ 0 CO 10 iC ' 0 0; LO (M o- -* c Tt< 1> -Tt^ I> 0 t~ CO Tt (N c t^ cr 02 CO I:- Tt 00 CO CO Ci s w; iC <:C lO !M . "^ t- t-^ CO CO c^ cj CO c4 CO oi IC ^ ^ CO • OJ 1-A CO ci 10 OT 6 CO o 00 t^ CO Oi CD ]>. X3 . cc 00 d C-) 05 lO m IM 0 1> -* cc co cc en rH t^ iC a:> LO CO 1 &S IC -* t^ r^ CO LO - 1 >- '5 1 _c C "s^ ^ i 0 £ 0) s w a V ft "£ c c 'a 02 u 0 "s 0 02 1 oc i 13 0! > -2 S OQ 02 £ £ b CJ a; a) c 0 '^ ^ 6 s c« g S 0 C > > >- >- bX K p — ■ ?^ 1 03 C3 D. 02 s 03 1 1 02 0 ft 0 Eh 3 H 218 THE MESABI IRON-BEARING DISTRICT. The aboA^e fig-ures show that the Mesabi ores at present mined contain a hig-h percentag-e of iron — indeed, a higher percentage than is shown by the average of all the ores mined in the other ranges of the Lake Superior region. At the present time ore containing less than 58 per cent is not mined on the ilesabi, except in small quantity for mixing with higher grades, thus making the cargo g-rade above 58 per cent. Tlie slightly hydrated hematites make up the bulk of the ore shipped from the ]\Iesabi range, and therefore their average composition is approxi- mately that given above for the entire Mesabi shipment. The percentage of iron in the yellow or brown ores averages less than the tigures given for the hematites. Fifty-six to 60 per cent are character- istic figures. The loss on ignition, above shown, is presumabh' largelv combined water. However, this may not represent all the combined water, for the ores are dried at 212^ F., and it is extremely likel}' that at this temperature some of the combined water is driven off. Where Mesabi ore has been finel}" powdered and dried for a longer time than usual a half to 1 per cent more of water has been lost by this drying." Again, there mav be a really greater loss of combined water on ignition than is shown by the weight, because there is an actual gain of weight due to the oxidation of a ferrous oxide to a ferric oxide. On the other liand, the loss on ignition may be partly due to the burning- of organic matter, or to the conversion of a carbonate to an oxide, or to the decomposition of a sulphide whereliy the sulphur is eliminated. But while the average of 3 per cent is perhaps a trifle low, the figure probably represents nearly the average conditions. From these average conditions there are wide variations, for it is known that some of the yellow ores are highly liydrated, while others are slightly so. For instance, some of the "j'ellow ocher" in the Biwabik group of mines showed an average content of water, accordins' to H. V. Winchell, of 10.1 per cent, thus being goethite. The wide variation in moisture driven off at 212° — that is, hygroscopic water — is due to the character of the or and to local conditions. A porous ore is likely to contain more free water than a dense ore. An ore which has been standing in water is likely to contain more free water than ores which have been in drier places. In the Mountain Ii-ou deposit the content '■'As iH-r letter (if R. B. Green, chemist, Minnesota Iron Co.. dateil 'March 4. 1902. THE IRON-ORE DEPOSITS. 219 of free water varies fi'om 6 per cent at the top of tlie open cut to 15 per cent at the bottom, and all is above ground water. When mines are first opened up the content of water is usually larger than later when the mine has been partially drained. A heavy rainstorm also will make a difference of 2 or 3 per cent in the total. Only two deposits on the range are known to contain sulphur in injurious amounts. The sulphur is present in the form of iron pyrites and is usually confined to the edges of the deposits. In mining these parts of the deposits are simply passed by. The hydrous ores perhaps run a little higher in sulphur than the nonhydrous ores. The great vai'iation in silica is due to the fact that the iron ore grades into ferruginous chert. Rocks can be obtained showing all percentages of iron and silica, but those containing a sufficient amount of iron to be ranked as ores seldom contain over 8 per cent of silica. In the western portions of the range the silica content is on an average higher than in the central portion, and in some deposits is so high as to run the percentage of iron down below the salable limit . The variation in alumina is due to the varying content of clayey and slaty material. In general the alumina is a trifle higher in the yellow ores than in the blue or black ores, although there are exceptions. This appears in comparing the higher and lower grades of ore in the preceding table. Throughout the district the lower grades of ore are the ones which are likeh'' to contain more of the yellow ore than the remainder of the ores. For instance, in comparing the Mountain, Oliver, Juniata, and Preble grades of ore from the Mountain Iron mine it appears that as the iron runs down the alumina runs up. The paint rocks uniformly contain a much higher percentage of alumina than any other rock in the iron formation. As phosphorus in considerable quantity prevents use of ores for the acid Bessemer process, the phosphorus content in an ore is of the greatest importance. The Bessemer limit is vague, but is commonly placed at 0.045 to 0.050 in phosphorus. From 60 to 70 per cent of Mesabi ores would be ranked as Bessemer. However, ores containing a much higher percentage of phosphorus are used in quantity in basic open-hearth furnaces, and with the rapid growth of the open-hearth method of steel making such ores will be in even greater demand. In the little hydrated hematites the phosphorus is below 0.05 per cent, 220 THE MESABI IRON-BEAKING DISTRICT. although occasionally running a little higher. In the hydrous oi'es the average of the phosphorus is higher. The common figures are above 0.05 per cent. It is not necessary to give detailed figures for particular ores. Chemists, mining engineers, and mine managers all agree to this. As the hydrous ores are, on the whole, more abundant in the upper portion of the deposits than elsewhere, the phosphorus is correspondingly more abundant at this horizon. It will be seen below that the Mesabi ore is partly in the form of soft dirt and partly in hard lumps. To ascertain whether or not there is any difi'erence in the percentage of phosphorus in the hard and soft lumps, in order to regulate the sampling, Mr. R. B. Grreen, of the Minnesota Iron Company, made a considerable number of analyses from cargoes of Canton, Norman, and Fayal ores, with results as follows: Percentage of pho«2}horus in hard and soft Iwinps of ores. Lump. " Fine. 0. 046 0. 033 .043 032 .048 .053 .045 048 .059 067 .072 059 .064 050 .041 037 .047 036 . 0517 (Average) 046 These figures indicate in general a slightly higher percentage of phosphorus in the hard lumps than in the soft ones. A similar result was obtained by Mr. E. T. Griese in comparing the hard and soft lumps of the Biwabik mine. The soft material ran below 0.035, while the harder lumps ran up to 0.040 to 0.050. Ores containing over IJ jjer cent of manganese are shipped only to a small extent. However, there are present in the Mesabi district ores containing a considerable higher percentage of manganese. In the Mountain Iron, the Moose, and the Oliver deposits bunches of ore have been found to run locally from 15 to 61 per cent in mang-anese. The Oliver property is the only one on the range containing sufticient amoimt of manganese to prevent the shipment of any considerable proportion of its ore. Attempts have been made to utilize such ores as a manganese ore. THE IRON-ORE DEPOSITS. 221 but thus far without success. In the samples of hard and soft material analyzed by Mr. Green it was found that the manganese was slightly higher in the hard lumps, than in the soft ones. A similar result was obtained by Mr. Grordon of the Mountain Iron mine, as a result of analyses of Mountain Iron and Oliver ores. In the Biwabik mine the hard ore there found runs distinctly higher in manganese than the soft ore. Finally, certain of the yellow ores run as high as 1 per cent of manganese, while the blue and black ores seldom go over 0.50 per cent, except when close to the wall rock. Analyses of magnetite from the eastward continuation of the Mesabi, in the neighborhood of Akeley and Gunflint lakes, are as follows: Analyses of magnetite from neighborhood of Akeley and Gunflint lakes. Constituent. 1. 2, 3. 4. Fe - 58.40 54.01 63.98 61.95 Fe,Oi 85.55 SiOj 8.22 .52 9.37 .07 8.90 11.39 ALO, - Trace. CaO .22 MeO 3.44 P .36 4.92 None. .32 5.02 None. .28 None. Trace. .02 Mn Ti None. s Trace. 1, 2, and 3. (Average of 6 samples). From NE. \ of NE. \ sec. 29, T. 65 N., E. 4 W. (west end of Gunflint Lake). Analysis by Rattle and Nye, Cleveland. Published by N. H. Winchell, Geol. Nat. Hist. Survey Minnesota, Bull. No. 6. 4. From SE. \ sec. 30, T. 62 N., R. 10 W. (transported masses of gabbro). Analysis by C. F. Sidener. Published by N. H. Winchell, Geol. Nat. Hist. Survey Minnesota, Bull. No. 6. 222 THE MESABI IRON-BEARING DISTRICT. The magiietites at the east end of the range have not yet been found in pajang quantities. Analyses of magnetite from ranges 12 and 13 are as follows : Analyses of magnetite from ranges 12 and 13. Constituent. 1. 2. SiO„ 1.16 1.81 69.08 11.89 ALO, .34 Fe,0, - Fe.Oi 87.00 FeO 27.10 69.43 .25 0.53 None. .06 Fe 63.07 MgO .80 CaO . 0.20 TiOj None. p,0- p .056 MnO .33 S Trace. 1. From the east end of Mesabi, SE. i of SE. J sec. 34, T. 61 N., E. 12 W. Analysis by W. H. Melville, for W. S. Bayley. 2. From the NE. \ of sec. 23, T. 60 N., R. 13 W. (near Iron Lake). Analysis by C. F. Sidener. Published by N. H. Winchell, Geol. Nat. Hist. Survey Minnesota, Bull. No. 6. THE IRON-ORE DEPOSITS. 223 It will be noted that the magnetites contain little or no titanium. In this they differ from the magnetites occurring- in the gabbro. Analysis of the latter are as follows: Analyses of magnetites in gahhro. Constituent. 1. 2. 3. SiOj 20.90 1.75 Trace. 2.63 2.23 None. 2.02 2.68 Trace. 11.37 A1,0, 1.32 CaO .10 MgO 2.73 TiO, 16.03 P .01 P,0, .03 FeO 2.01 70.29 52.46 14. 42 Fe,0^ . . 80.78 58. 48 12.09 2.40 53.33 Fe . . . 49.40 Ti . . CrjO.. S .. Trace. 1. From the neighborhood of Iron Lake in T. 65 N., R. 3 W. Analysis by Prof. J. A. Dodge" Published by N. H. Winchell, Geol. Nat. Hist. Survey Minnesota, Bull. No. 6. 2. From SE. I sec. 36, T. 65 N., E. 3 W. (Iron Lake). Analysis by E. S. Eobertson, Pittsburg. Published by N. H. Winchell, Geol. Nat. Hist. Survey Minnesota, Bull. No. 6. 3. From sec. 36, T. 63 N., E. 10 W. Analysis by C. F. Sidener. Published by N. H. Winchell, Geol. Nat. Hist. Survey Minnesota, Bull. No. 6. The paint rock frequently associated with ore deposits has a imich lower average content of iron than either the hematite or hydrous ores. It averages all the way from 12 to 45 per cent or even to 55 per cent. Figures between 40 and 50 per cent are the most common ones. Phos- phorus is also usually but not invariably high. The usual range is from 0.070 to 0.150. Alumina is also much higher in the paint rock than in the hematite or hydrous ores. The figures run as high as 7 per cent (Lerch): 3 to 4 per cent (Griese) may be the average. The water content is char- acteristically large. TEXTURE AND STEUCTUEE. The Mesabi ores range in texture from large crystalline masses requiring the use of a crusher, as at Biwabik, to fine, soft dirt, which runs like dust between the fingers. In general the ores at Virginia and Eveleth and eastward are somewhat harder and coarser than those of Mountain 224 THE MESABI IRON -BEARING DISTRICT. Iron, Hibbing, and vicinity, although there are exceptions both ways. The deposits are thin, bedded in layers varying from a fraction of an inch to several inches, or even feet. Usually some of the layers are soft and pulverulent while others are more or less hard and broken up into small parallelopiped blocks by cross fractures. The best physical texture for furnace purposes, a medium hard ore in small lumps, is frequently found along the edges of the deposits. A stroke of the pick at almost any point in a deposit will loosen a mass of soft ore mixed with small blocks of hard ore which seldom exceed a few inches in length and breadth. The average texture of some of the ores is shown by the following screening figures of representative ores kindly furnished by Mr. R. B. Green, chemist of the Minuesota Iron Company: Average texture of ore. Ore not pass- ing through a screen with 8 meshes to the inch, a Ore passing through screen with S meshes to the inch. Ore passing through screen with 20 meshes to the inch. Ore passing through screen with 100 meshes to the inch. 1898. Audrey ore, Auburn mine (41 cargoes) . Auburn ore, Auburn mine (13 cargoes). Fayal ore (43 cargoes) Genoa ore (34 cargoes) Sparta ore (44 cargoes) Elba ore (6 cargoes) 1901. Norman ore (23 cargoes) Top Brown, Sparta mine (8 cargoes) . . . Vulcan ore, Spruce mine (18 cargoes) . . 6.79 6.92 5.85 Mountain Iron ore ; 75 to 80 ' 75 to 80 Per cent. 60.88 51.73 72.17 71.47 65.71 64.13 59.38 64.92 65.92 Per cent. 9.52 7.96 8.42 7.09 5.58 6.96 Per cent. Per cent. 18.72 10.88 20. 38 19.92 15.93 3.48 18.42 3.01 15. 64 13.09 20.05 8.80 18.16 16.42 15.35 75 to 80 15. 66 11.74 12.88 20 to 25 «' Determined in natural state after taking from cars and drying. The looseness of the ore as it lies in the deposit is shown by the fact that in computing the tonnage of Mesabi ores 11 J to 14, or rarely even 17 to 18, cubic feet are allowed per long ton (2,240 pounds), whereas in the hard-ore deposits of other ranges the common figures are 8 or 9 cubic feet. If tlie ore were all limonite with a specific gravity of 3.6, and if a ton be supposed to occupy 14 cubic feet, the calculated pore space would be 29 per cent. If the ore were all hematite with a specific gravity approaching 5, tlie pore space would be 48 per cent. If a ton of ore be supi)osed to THE IRON-ORE DEPOSITS. 225 occiii)j 11 J cubic feet, there would be 13 per cent poi'e space if it were limonite and 38 per cent if it Avere hematite. As the ore is an intermediate variety between hematite and limonite, it is probable that the average pore space is somewhere in the neighborhood of 35 per cent, though locally showing wide variations. In contrast to this the pore space in the old range ores is commonly less than 20 per cent. The bedded structure of tlie Mesabi ore deposits is a striking feature. A single bed commonly shows much persistence in color and texture where followed out laterally, but differs in these particulars from beds above and below, with the result that the bedding structure is made most conspicuous. (See Pis. XXIV to XXXIII.) White and colored efflorescence along the bedding still further emphasizes it. The bedding planes in general pitch gently, perhaps 8° to 20°, toward the lower end of the basin in which they lie. The beds may also dip gently from the side of the channel in toward its axis, as in the Oliver mine, or may have a monoclinal tilt, as in the Hale and Kanawha mines. (See PI. XXXIII.) In exceptional instances dips are as high as 50° to 60°, as at the Stephenson, Sauntry-Alpena, Sparta, and Kanawha mines. Close to the wall rocks of the deposits the dip of the beds of ore usually becomes suddenly stee^jer and may jump up 45° or more at the immediate contact. Besides having the above general attitudes the iron-formation layers are much contorted in a minor way. In walking through any of the open pits of the mines the gentle minor undulations of the layers are everywhere apparent, and here and there folds of unusual sharpness stand out conspicuously. Some of the dips observed in the Mountain Iron mine are as follows: Dips observed in Mountain Iron mine. AT LOWEST LEVEL ON EAST SIDE OF PIT GOING SOUTH. 8°S. 30° SSW. 8° S. 20° N. 1°S. 6° S. 5° NW. 2° E. 5°S. 4°N. 10° NNE. 0°. 10° ssw. 4°N. 6°S. 11° SE. 7°SE. 9° N. • 22° S. 3°S. 8° S. 35° N. AT SECOND LEVEL ON EAST SIDE OF PIT GOING NORTH. 19° SE. 40° SE. 6° NW. 20° SW. 3° NW. 6° E. 0°. 8° SE. 5° NNW. 39° SSW. 6°NW. 15° W. 11° S. 44° SSW. 5° NW. 20° W. 20° SE. 4°E. MON XLIII — 03 -15 7°SW. 226 THE MESABI IRON-BEARING DISTRICT. ^ Dips in tiie Oliver mine at Virginia are as follows: Dij>s in Oliver mine. GOING NOETH TO MIDDLE OF PIT. 9° NW. 7° NW. ACROSS SY^'CLINE AT UPPER END OF PIT FROM SOUTH TO NORTH. 21° NE. 9° NE. 19° SW. GOING WEST ALONG NORTH WALL. 12° SSW. 30° SSW. 15° W. 40° SSW. 35° SSW. 20° SSW. Dips in the Biwabik mine, begimiing at east end of mine and running along the north wall on the upper level, are as follows: Dijjs in Bvwabih mine. 28° S. 15° W. 12° S. 14° S. 18° SSW. 13° SSW. 16° S. 12° S. 19° WSW. 14° S. 17° S. 19° SW. 20° S. 20° SSW. The bedding, wliile in general roughly parallel to the surface, in some deposits shows discordance with the surface. In the Sauntiy mine, for instance, the surface of the deposit may be observed to cut diagonally across the layers of the ore deposit. Nodules of iron ore are frequently found with their gi-eater diameters parallel to the bedding. The nodules vary fi'om a fraction of an inch up to 6 or 8 inches or more in size. They are frequently hollow and some- times show concentric arrangement of hydrated and nonhydrated ores. In addition to the bedded and folded structures, the ore deposits show many fractures, especially along the contact with the wall rock. The individual layers, where hard enough, are broken into small blocks by fractures which in the main are independent of those in the layers above and below. Also numerous joints and faults cut across the ore deposit. Indeed, in some deposits there is scarcely a cubic yard which is not crossed by one or more such fractures. For the most part there is little displacement along fractures, and where displacement does occur it is measured by inches rather than feet. Rarely a fault of several feet may l)c ()l)sci-vc(l, and this is likely to be near the contact of tlie ore witli tlie wall rock. THE IRON-ORE DEPOSITS. 227 The numerous joints and faults suggest that the apparent gentle folds in the iron-bearing strata are not diie entirely to actual bending of the strata^ but are due in part to minute displacement along the closely spaced fractures. THE ROCKS FOKMIKG THE BOTTOMS ATsTD SIDES OF THE ORE DEPOSITS. The rocks adjacent to the Mesabi iron-bearing deposits are ferruginous cherts, more particularly the altered varieties, ferruginous slates, paint rocks, and even unaltered slates. In abundance the rocks stand in the oi'der named. In one mine, the Hale, the north wall of the ore deposit is a micaceous phase of the Pokegama quartzite associated with a schistose Archean basalt. The north wall of the Biwabik mine also may be quartzite, although exploitation has not yet shown this. The conditions in these two mines are exceptional. STRITCTURAIL, RELATIOlSrS OF THE ORES TO THE ADJACENT ROCKS. The shape of the ore deposits and the attitude of then- layers can be easily made out from the mining and exploration work done on them. But it is difficult to make positive statements as to the structural relations of the Mesabi iron ores to the adjacent rocks for the reason that a very small proportion of the deposits of the range have been exploited to a sufficient extent to show such relations. Many deposits have not been opened up at all, and the preliminary exploration does not reveal information of this sort. In the underground mining the di'ifts reach the wall rock in too few places to warrant general statements as to the structure. Most of the open cuts also have as yet either not reached the wall rock at all or have reached it only on one side. Assembling all the data available concerning the relations of the ore to the wall rock, the following general statements seem to be warranted: (1) In a few mines, as for instance the Oliver, the layers of rock adjacent to the ore have such attitudes as to show the ore to lie near the axis of a gently pitching trough formed by the folding of the rock layers. The ore deposits of this class have relations, which are expressed in fig. 9. On each side the wall rocks dip in toward the ore deposit. The upper layers abut against the ore deposit. The lower layers form a gentle trough beneath, which usually has many minor undulations. The iron-ore 228 THE MESABI IRON-BEARING DISTRICT. beds themselves are bowed iuto a gentle trough more or less complex, with slopes similar to those of" the rock layers below and at the sides. In other words, the layers of the ore deposit and the layers of the rock together make up a g-entle syncline, with even slopes. This does not mean that the slopes of the bottom of the deposit are gentle and even. Indeed, the sm-face of the contact of the rock and ore is exceedingly irregular, as ah-eady noted, and usually terraced, and the dip of the contact sui-face is usually steeper than the dip of the strata. In other words, the bottom of the deposit forms a trough with much more in-egular bottom and much steeper slopes at the sides than the trough formed by the bowing of the sti'ata of the iron formation and ore deposit together. a^^Slat^ Iron ofe Fig. 9. — Ideal cross section of a Mesabi ore deposit showing relations to ferruginous chert and impervious slate layers. The longer directions of the ore deposit in such cases are usually parallel to the axis of the trough. (2) The rock layers about other ore deposits form jjitching troughs, but the folding is so slight that the pitch is far more conspicuous than the dip of the limbs, and in this case the rock layers on all sides of the deposit have essentially a monoclinal attitude. The longer direction of the deposit may be either parallel or transverse to the monoclinal pitch, but it is usuallv transverse. The Canton, Biwabik (PI. XXXII), Cincinnati, and Saniitry (PI. XXXI) mines are instances of this. (3) About still other deposits the rock layers show no traces of synclinal flexure, and the dip is essentially monoclinal with minor variations in degree. It is not unlikely in some cases that the layers, while essentially monoclinal, are even slightly bowed into anticlines. The deposit ill this case lies ou tlie linib of one of tlic gentle folds into wliicli THE IRON-ORE DEPOSITS. 229 the iron formation is flexed, either the great major fold or one of the minor cross folds giving the gentle tilting of the iron formation to the south. Instances of such relations appear at Hale fPl. XXXIII), Kanawha, and Sparta mines. The longer direction of the deposit is nearly always transverse to the pitch. A generalized cross section of a Mesabi ore deposit parallel to the pitch of the fold or parallel to the dip of the monoclinal rock strata is shown in fig. 10. In general it is likely that more of the ore is to be found in structural synclines in the iron formation than elsewhere, but this may not be in all cases apparent in the attitude of the layers immediately adjacent to the ore deposit, for the ore deposit may fill only a very small portion of a broad and gentle syncline, or be one of several deposits in such a syncline, and Fig. 10.— Ideal section parallel to the pitch oi a Mesabi ore deposit, showing relations to ferruginous chert and to impervious slate layer. the minor structure of the iron-formation layers immediately adjacent to the ores may give no evidence of the existence of the major syncline in the iron-formation layei's. These structural relations are explained in connec- tion with the discussion of the origin of the ores, where it is shown that the ores have been concentrated through the agency of underground waters working their way through a gently folded and much fractured formation, in which the major flow of water has been directed by the broad gentle synclines in the iron formation, but in which the cross fractures have locally concentrated the circulation and given it most capricious turns. The structural relations of the ore to the wall rock at the immediate contact in the above cases may be any of the following: (1) The layers of the wall rock may grade into the layers of the ore without change of dip. (PI. XX, fig. B.) This is likely to be observed at the contact of the ore with horses of rock or islands of rock within the ore. 230 THE MESABI IKON-BEARING DISTRICT. (2) The layers of tlie wall rock may grade into the la^'ers of the ore, and at the contact there is a sharp downward deflection of the ore layers. (PI. XX, fig. A.) The layers of the ore may be almost perpendicular, and dips of 45° are common. Within a few feet, or at least a few yards, the layers of the ore deposit take on their usual gentle dip. (3) Accompanying the downward flexure of the ore layers there may be jointing, faulting, or brecciation. Where the flexures of the layers pass into fractures, the displacement is usually small, a few inches or at most a few feet. In certain mines the displacement between the ore and the wall rock may have been greater, although decisive evidence is lacking The steep walls to be seen in some deposits, as, for instance, the Oliver, Elba, and Canton, have sometimes been taken as evidence of extensive faulting. While the walls are steep as compared with the average of the wall-rock slopes on the range, in detail they are terraced, some of the terraces running- out 20 feet or more. It is apparent that there has not been any great displacement parallel to the wall, for the ter- races would have required a tremendous local disturbance of the ore close to the contact during the movement, and this does not appear. North of the Biwabik mine in the NW. ^ of NW. i sec. 2, T. 58 N., R. 16 W., a pit has been sunk 70 feet in ore just 60 feet south of a pit showing Pokegama quartzite. If the quartzite passes under the ore, it must do so with a dip of over 45°, a dip much greater than the average in the Upper Huronian, although such dips are indeed to be observed. The quartzite is much broken up and disintegrated here, and it is not impossible that we have here a fault with a considerable displacement. At the Hale and Kanawha mines the open cuts show the north wall to be the green rock of the Archean, with thin films of micaceous Pokegama quartzite and iron formation adhering to it. The vertical and lateral dimen- sions of the deposit parallel to the wall are greater than that normal t<^ it, and the dip of the iron formation and ore strata is steep, being in some places as high as 60° to the south. (See PI. XXXIII.) The attitude and steep dip of the deposit are in accord with the idea that it has been developed in a plane of weakness along tlie contact at the Archean and Upper Huronian, where there has been considerable movement, but there is no direct evidence of this. PLATE XX. 231 PLATE XX. VIEWS OF THE CONTACT OF ORE WITH WALL ROCK IN THE BIWABIK AND MOUNTAIN IRON jVHNES. Fig. -1. — This view is taken from the west end of the Biwablk mine. The strata on the left side of the picture are ferruginous chert, and those on the right side are ore. The fairly sharp downward bend of the strata from left to right near the center of the field is at the contact of the ore and ferruginous chert. The layers of the ferruginous chert are, for the most part, directly continuous with, and grade into, the layers of ore, the gradation being accomplished within a few feet. The flexure at the contact is due to the slumping down of the ore layers, which is caused by abstraction of silica in solution, as explained on page 262. This is a characteristic feature at the contact of the ore and wall rock in most deposits on the range. Fig. B. — This view shows a contact of the ore and the ferruginous chert forming the wall rock in the south railway approach of the Mountain Iron mine. The upper strata, appearing lighter colored and more coarsely bedded than the lower, are ferruginous chert; the lower flne-bedded strata are iron ore. The rock forms a thin shelf projecting over and into the ore deposit at this point. The layers of ore at the south end of the mine grade directlj' without any disturbance into layers of ferruginous chert forming the wall rock; the layers of ore and rock are strictly parallel. Near the contact they become interleaved, layers of ore extending well into the wall rock, and, vice versa, layers of wall rock extending wel into the ore. 232 U. S. GEOLOGICAL SURVEY MONOGnAPH XLin PL. XX A. CONTACT OF ORE WITH WALL ROCK IN BIWABIK MINE. j: CONTACT OF ORE WITH WALL ROCK IN MOUNTAIN IRON MINE. THE IRON-ORE DEPOSITS. 233 (4) The jointing, faulting, or brecciation may appear at the contact without any marked change in dip of the layers on either side. A single deposit may show part or all of these relations at the contact of the rock and ore. Indeed, it is rare that a deposit does not show most of them at different places along the contact. However, the relations described in (2) and (3) are by far the most common. In any case the ore does not run into the rock along an even plane, but in a series of terraces, as ah-eady described. Moreover, the contact is a zigzag one in plan. The contact of the ore and wall rock has usually followed vertical joints which intersect one another at many angles. These angularities may be either great or small; a large jDrojecting' corner may carry on it many minor corners, and these in turn small ones but a few inches in size. PETKOGRAPHIC RELATIONS OF THE ORES TO THE ADJACENT ROCKS. Where jointing, faulting, or brecciation does not prevent, the lateral transition of the layers of the wall rock into the layers of the ore can be clearly seen. The transition is sudden, usually being completely accom- plished within a fraction of an inch, but requiring in jjlaces several inches or even several feet. The bulk of the wall rock is ferruginous chert containing usually less than 30 per cent of iron oxide and 60 to 70 per cent of silica. Transition into the ore is represented almost entirely by the change in the relative proportions of these minerals. The discussion of the origin of the ore on a subsequent page will include an account of the genetic relations of the cherts to the iron ores. An attempt was made to ascertain whether or not any particular kind of ferruginous chert is uniformly represented by any particular kind of iron ore, but without any considerable success. In the Biwabik mine a hard yellow ferruginous chert (PL XI, fig. A) may be observed to grade into the yellow ocher of the deposit. The brown, red, and black hematites seem to grade impartially into any of the gray or reddish ferruginous cherts. The ferruginous cherts in the wall rock of the Mountain Iron mine may be seen to grade directly into purplish slaty ores with white specks. Finally, the slate in the wall rock may be observed to grade into the paint-rock layers within, below, and above the ore deposits. Evidence of the correspondence of slate and paint rock has been noted by several of the mining engineers of the range, although some of them object to using the term slate, preferang to keep this term for the overlying 234 THE MESABI IRON-BEARING DISTRICT. Virginia slate. The equivalence of the paint rock and slate is indicated by the actual transition to be observed at a number of cases, and also by their structural relations. The sump of the pump shaft of the Penobscot mine is bottomed in slate under 298 feet of ferruginous chert. At the bottom, of the adjacent ore deposit at the same horizon is a zone of paint rock. In the Biwabik mine there is a capping of paint rock. East and west along the strike of the paint rock there is found black slate. Also a well driven in the paint rock and ore 600 feet south of the southernmost pit pierces typical black slate at a depth which ought to show paint rock if the pauit rock continues southward with approximately the dip it shows in the mine. At the bottom of the deposit the ore is usually in sharp contact with feiTuginous chert, with practically no gradation, but paint rock is also fre- quently found immediately underlying the ore. This may be in single thin seams a fraction of an inch or several inches in thickness, or may be in several thin layers interleaved with the ore in a zone several feet thick. Beneath the paint rock, and to a certain extent mixed in with it, is some variety of fen-uginous chert or ferruginous slate, principally the former. Toward the sides of the channels the paint rock between the ore and the ferruginous chert becomes less abundant or altogether disappears, and the ore rests directly upon the ferruginous chert. Attention has been called to the fact that the sides of the trough are in a series of steps. Oil these steps there is seldom any paint rock separating the ferruginous chert from the ore. DRAINAGE. Because of the bedded and jointed structure of the ore deposits water is able to pass through them freely. Water probably flows along the beds more freely than across them, for the bedding partings present more continuous openings than the joints, which in the ore deposits are irregular and discontinuous, and frequently cut off by soft impervious layers, which have yielded to deformation by bending rather than by jointing, and, moreover, certain layers — for instance, limonite layers — are themselves very porous. Locally, however, the flowage along joints is dominant. The ore deposits are at present in part above water level and iii part l)clo\v. On the upper slopes the deposits are largely above water level, on tlic lower slopes below water level, although there are many exceptions to THE IRON-ORE DEPOSITS 235 both. Thus it is that certain mines are comparatively dry throughout the year while others are permanently wet. A good instance of this appears at Virginia, where the Columbia mine on low ground receives a vast quantity of water while the mines on high gTOund adjacent are comparatively dry. The pumping of vast quantities of water from mines below the level of ground water has materially reduced the general level of ground water in this and adjacent areas. The Penobscot mine, for instance, discharges in the neighborhood of 5,000 gallons per minute, and thereby drains the mines to the north and west. The cessation of pumping at the Penobscot mine would immediately raise the level of ground water in the adjacent mines were pumping not begun in them. At the Biwabik mine the level has been lowered from 75 feet below the surface to 150 feet below the surface. Below the level of ground water the amount of water to be handled in the deposits varies with the depth below the level because of the increased head and increased contributing area in the shafts. Many mines show increased flow toward the bottom. Before any artificial openings were driven into the ore deposits the flowage along the bottom may not have been any greater than, if indeed so great, as in upper portions, as shown by fig. 11, described in Chapter IX. The outlet for the water was then at a higher level, near the surface of the formation. Above the level of ground water the ore contains water, but is not saturated. In this zone the amount of water in the ore increases from the top down. In the Mountain Iron mine the amount of hygroscopic moisture in the ore is said by A. T. Gordon, chemist in the mine, to vary from 6 per cent in the upper part of the deposit to 15. per cent in the lower part. During periods of great precipitation the amount of water in the deposits above the level of ground water is increased and at such times also the level of ground water is raised. Where mine openings have been made above the level of ground water the amount of water contributed to the openings during such periods is of course increased. Many deposits in the range, though not all by any means, lie under surface depressions or surface drainage channels, due to development in original rock synclines, as shown on a subsequent page, or to gouging out by erosion to a greater extent than the adjacent harder rocks. Glacial drift has tended to obscure these rock depressions, and does so completely in many places. Where open cuts have been made in deposits under- 236 THE MESABI IKON-BEARING DISTRICT. Ij'ing sucli di-ainage basins great precautious have to be takeu to guard agaius flooding from this surface drainage. In 1900 the Mountain Iron open cut, which by pumping is ordinarily kept above the level of ground water, received 70,000,000 gallons of water in a few hours. At the same time the Fayal mine fared nearly as badly. The extensive drainage ditches to be seen about the open pits testify to the tendency for increased flow at such times. CHAPTER IX. ORIGIN OF THE IRON ORES. GElSnERAi STATEMENT. The iron ores have come chiefly from the alteration of rocks made up of minute granules of green hyclrated ferrous silicate, which we have called greenalite (see pp. 101-115 and Pis. VIII, IX, and XIII). A small part of the ore has resulted from the alteration of siderite. The proof of the development of the ores from greenalite is, briefly, as follows : 1. All stages of the alteration from the fresh greenalite granules to the other phases of the iron formation, including the ores, are to be observed. 2. Much of the ore and associated rocks show traces of the greenalite granules. The granules may be represented b}^ iron oxide, by chert, by actinolite or griinerite, by unaltered greenalite, by any combination of these substances, or by any gradation phase between them. 3. Greenalite is one of the less stable of the iron-formation materials, and its alterations to the other phases of the iron formation would be chem- ically characteristic of conditions under which the iron formation has existed during the concentration of the ore. The reverse change would not be probable under such conditions. Greenalite is found in parts of the iron formation which have suffered the least alteration; i. e., in association with slate layers in the iron formation or just below the Virginia slate. The proof of the development of a small portion of the ore from sid- erite is, briefly, as follows: Associated with the greenalite and slaty layers in the least-altered portions of the iron formation are thin layers of iron carbonate, some of which, from interlamination with the easily soluble greenalite, probably is original and not the product of alteration of green- alite.. Near Birch Lake undoubted fine-banded original carbonate is found. If iron carbonate is now found in the unaltered portions of the iron formation, it must have originally occurred in the more altered portions. Its alteration to the other phases of the iron formation now 237 238 THE MESABI IRON-BEARING DISTRICT. observed would be chemically characteristic of the conditions under which the iron formation has existed. Certain of the finely banded ores — as, for instance, some of those at the Mountain Iron mine — may have thus resulted, although there is no evidence in the ores themselves, aside from their fine banding, suggestive of development from banded carbonate. The fact that the ores of the Penokee-Grogebic range have been proved mainly to develop from u'on carbonate further suggests the probability of such a change having occurred. The development of the iron ores from greenalite has consisted in the breaking up of the greenalite into its constituents — mainly protoxide of iron (FeO), silica (SiOj), water (H2O) — the partial or complete oxidation and hydration of the protoxide of iron, and the abstraction of the silica, probably much of it as colloidal silicic acid (H^SiO^), in solution. Where the original rock was siderite the development of the ores occurred in a similar way. It consisted in the breaking up of the siderite into ferrous iron and carbon dioxide, the partial or complete oxidation and hydration of the protoxide of u-on, and the abstraction of carbon dioxide as carbonic acid in solution. Where the oxidation has been complete, the sesquioxides have devel- oped; these are usually somewhat hydrated. Where the oxidation has been partial, magnetite has developed. The derivation of the iron ores from greenalite and siderite has been brought about through the agency of surface weathering and of waters percolating thi-ough the formation both above and below the level of ground water. This is shown by the nature of the chemical changes which the ore and associated rocks of the iron formation give evidence of having undergone, by the fact that the silica and iron have been transported and rean;anged and the silica largely abstracted from what are now the ore bodies, by the frequent occurrence of the ore in nodules and in stalactitic forms in cavities, by the occurrence of the ore in underground drainage channels in the iron formation, and finally b}^ the fact that water may now be observed coursing through the formation bearing mineral constituents indicative of such changes. The greenalite rock itself is a sedimentary oceanic deposit, as shown liy its bedded character and its interstratification and conformity with ordinary slate and quartzite. Its development is believed to be both by chemical and oi'ganic processes. ORIGIN OF THE IRON ORES. 239 The above general outline of the genesis of the iron-ore deposits is filled in below. The development will be described in the order of sequence of events, and the first subject to be considered in detail is naturally the origin of the greenalite granules. ORIGIN OF THE GBEEKALITE GRANULES. GREENALITE A SEDIMENTARY DEPOSIT. The rocks containing greenalite grade into quartzite below, into slate above, into slate laterally, and contain interstratified slate layers. They show bedding which is strictly conformable to. that of the associated sedi- mentary rocks. There is therefore no question that the greenalite of the iron formation is a sedimentary deposit. The occurrence of greenalite or its altered equivalents between quartzite and slate would indicate the con- ditions of its deposition to be somewhat intermediate between those favor- able to sand deposition and those favorable to mud deposition. SIMILARITY OF GREENALITE TO GLAUCONITE. Spurr, in his work on the Mesabi range in 1894," noted the similarity of the greenalite granules here described with glauconite, a green silicate of iron and potassium, found in formations of the most various ages and brought up in dredging operations from the sea bottom near the edge of the continental shelf, where it occurs in all stages of development by depo- sition in the interiors of foraminifera and other organisms, and possibly in small part developed as entooliths and concretions independently of organ- isms. In color, size, shape, and optical properties the two substances are almost identical, but in other respects they differ. Murray and Renard, who have investigated modern glauconite depos- its tlxrough the dredgings of the Challenger expedition, emphasize the char- acteristic accompaniments of glauconite. "Glauconite is almost always accompanied by quartz, orthoclase (often kaolinized), white mica, plagio- clase, hornblende, magnetite, garnet, epidote, tourmaline, zircon, and frag- ments of ancient rocks, such a,s gneiss, mica-schists, chlorite rocks, granite, diabase, etc. In addition to these minerals, there seems always to be associated with glauconite, in modern deposits, a considerable quantity of organic matter, often apparently of a vegetable nature. The glauconitic a Geol. Nat. Hist. Survey Minnesota, Bull. No. 10. 240 THE MESABI IRON-BEARING DISTRICT. grains frequently contain traces of phosphate of lime and make up a con- siderable part of some phosphatic nodules, so that phosphate of lime may be said to be one of its constant accompaniments."" No trace of these sub- stances remains in the greenalite deposits under discussion. In thickness of the deposits the glauconite and greenalite deposits differ. The former are nowhere found unmixed with foreign material with a thickness exceeding 35 feet, while the gi'eenalite granules in the Mesabi district have made up a deposit with a thickness of 1,000 feet or more, with only thin layers af mud. In the fundamental property of composition they are dissimilar. Spurr himself noted this dissimilarity in composition, but in view of the wide variety in composition of glauconite, and the similarity in ])hysical prop- erties, concluded that the material was glauconite. A number of anah^^ses of so-called glauconite or greensand are given below. Some of them are not reliable, but it is scarcely possible to make a satisfactory separation of the good and bad without details as to methods. Analyses of glauconite. I II Ill IV v VI VII.... VIII... IX X XI XII.... XIII... XIV... XV.... XVI ... XVII .... XVIII . . . XIX XX SiOo. 58.17 53.46 49.10 50.80 48.99 49.40 50.20 47.60 49.50 43.60 46.91 50.62 54.18 50.42 49.09 50.70 46.58 56.70 48.45 .51.50 AljOa. 10.09 5.00 7.05 6.70 6.40 7.10 1.50 4.20 3.20 5.10 7.04 3.80 7.15 4.79 15.21 19.80 11.45 13.32 6.30 6.40 Fe.Os. 23.60 21.80 25.80 20.07 28.10 21.60 22.20 32.80 23.06 21.03 19.90 10.56 PeO. 18.75 21. 78 3.25 3.10 4.80 3.80 4.20 3.00 6.80 3.00 2.64 6.03 20.16 5.96 3.06 8.60 20.61 20.10 24.31 24.30 MgO. 3.37 6.21 4.20 Spur. 1.40 1.50 4.40 4.08 2.28 2.65 3.70 1.27 1.18 Trace, CaO. 0.78 2.40 2.95 NaoO. 0.91 3.21 ; .21 .55 2.49 1.62 1.21 .50 .98 KoO. 3.37 8.79 5.75 3.10 5.18 5.75 5.90 4.60 8.00 5.60 7.31 7.14 7.97 7.87 6.05 8.20 6.96 HoO. 12.01 9.96 6.25 4.76 10.10 9.80 8.98 12.75 8.60 14.70 9.50 7.70 4.71 9.14 5.74 5.28 11.64 8.50 9.66 MnO. MgCOa. CaCO: Total. .! 0.57 8.40 7.70 0.54 100. 00 100. 00 98.85 99.50 100. 93 98.87 98.50 99. 50 99. 20 99 30 93 99 99.86 99.28 99.92 100. 02 100. 00 100. 00 99.47 99.86 "Report of the voyage of H. M. S. ChaUcngcr, 1873-1876, Deep Sea Deposits, p. 383. ORIGIN OF THE IRON ORES. 241 Analyses of glauconite — Continued. SiO.,. AI2O3. FeoOs. FeO. MgO. CaO. Na-O. K.O. H,0. MnO. MgCOs. CaCOa. Total. XXI 50.75 53.26 57.56 58.74 49.42 51.24 49.76 46.90 52.86 56.62 50.85 51. 80 55. 17 27.74 48.95 4:}. 75 49.53 51.00 47.88 42.32 41.02 42.35 47.59 52.96 6.50 3.85 6.56 4.71 10.23 12.22 8.18 4.06 7.08 12.54 8.92 8.67 8.12 13.02 7.66 7.82 5.84 9.93 14.94 16.51 22.19 21. 43 17.99 12.76 16.01 13.44 16.00 27.09 7.20 15.63 24.40 24.21 21.59 39.93 23.43 22.26 20.06 18.69 17.13 18.91 18.49 8.17 13.95 13.56 22.14 24.15 20.13 21.06 3.00 3.06 3.77 3.60 19.48 1.18 1.66 1.54 1.95 1.76 1.32 2.36 5.95 1.98 2.68 2.80 2.06 5.78 3.70 2.34 12.96 5.36 4.88 3.26 7.91 7.50 7.57 6.16 2.23 2.52 4.21 3.86 3.36 .95 9.54 9.01 9.31 7.66 8.04 7.49 5.74 7.16 7.21 8.69 7.50 10.12 8.17 9.79 8.08 8.20 9.82 9.25 8.43 6.84 5,55 5.68 5.76 10. 85 4.93 5.16 4.91 5.83 5.91 7.48 7.88 4.45 5.27 4.91 99 85 XXII . . . . 1.10 1.70 1.48 3.78 3.93 3.97 .70 2.90 2.49 3.13 3.04 2.83 4.62 2.97 3.25 2.92 3.85 2.45 1.74 .69 1.93 1.80 4.11 1.73 1.04 .92 .31 .10 .41 .20 Trace. 1.69 1.26 1.27 1.34 1.19 .57 .56 .87 .56 2.20 1.96 8.37 1.22 1.60 .26 .31 .52 1.28 Trace. .90 .25 .25 .27 .62 .98 .30 .46 .35 .43 .42 .38 .43 .42 .47 101 17 XXIII . . . 100. 04 XXIV ... XXV .... 99.96 99.00 XXVI ... 100. 00 XXVII .. 100. 00 XXVIII . 99.24 XXIX ... 100. IS XXX.... Trace. 100. 41 XXXI... Trace. 100. 23 XXXII . . Trace. 100 32 XXXIII . Trace. 100 39 XXXIV . Trace. 100. 68 XXXV .. XXXVI . 100. 35 99.91 XXXVII. 99. 54 XXXVIII 100. 16 XXXIX . 100 02 XL 99 87 XLI 100 41 XLII .... 100 07 XLIII ... 99 15 XLIV.... 100. 10 I. Greensand, from Biiderich, between Unna and Werl. Dechen, Xat.-hist. Ver. Bonn, 1855, Vol. XII, p. 176. II. Greensand, from Dortmund in the direction of Witten. Marck, ibid., p. 266. III. Bindlacher Mountain, near Bayreuth. Haushofer, Journ. pr. Chem., 1866, Vol. XCVII, p. 353. IV. Sorg, near Kronach in Oberfranken. Ibid. V. Ortenburg, near Passau. Ibid. VI. Roding at Cham, in the Oberpfalz.' Ibid. VII. Ebendaher. Ibid. VIII. Benedictbeuern. Ibid. IX-X. Kressenberg. Ibid. XI. Insel Gozzo. Bamberger, Tscherm. Mitth., 1887, p. 271. XII. Havre. Haushofer, Journ. pr. Chem., 1867, Vol. CII, p. 36. XIII. Villers sur Mer, Dep. Calvados. Pisani, Cloizeaux Min., 1862, p. .542. XIV. Antwerp. Dewalque, Soc. geol. Belgique, 1874, Vol. II, p. 3. XV. Ashgrove, near Elgin, in Scotland. F. Heddle, Trans. Roy. Soc. Edinb., 1879, Vol. XXIX, p. 79. XVI. Island of Orleans, Quebec. Hunt, Geol. of Can., 1863, p. 487. XVII. Red Bird, Mo. Ibid. XVIII. (iay Head, Massachusetts. S. L. Dana, Hitchcock's G. R. Mass., 1841, p. 93. XIX. Cauley's pits, at Woodstown, X. J. Rogers, Geol. Rept. N. J., p. 201-204. XX. ScuUtown, N. J. Ibid. " MON XLIII — 03 16 242 THE MESABI IRON-BEARING DISTRICT. XXI. Polk Hill, Burlington County, N. Y. Ibid. XXII. New Jersey, southeast of Philadelphia. Fisher, Am. Journ. Sci., 1850, Vol. IX, p. 83. XXIII. Coal Bluffj Alabama. Mallet, Am. Journ. Sci., 1857, Vol. XXIII, p. 181. XXIV. Gainesville, Ala. Ibid. XXV. Svir R, Russia. A. Kupffer, J. B. Ch., 1307, 1870. XXVI. Ontika, Russia. Ibid. XXVII. Grodno Valley, Russia. Ibid. XXVIII. Agulhas Bank. Giimbel, Ber. Ak. Miinchen, 1886. XXIX. French Creek, Pennsylvania. Knerr and Schoenfeld, Am. Ch. J., Vol. VI, 1884. p. 412. XXX-XXXIII. Challenger dredging, lat. 34° 13' S., long. 151° 38' E., 410 fathoms. XXXIV. Challenger dredging, lat. 11° 38' 15" S., long. 143° 59' 38" E., 155 fathoms. XXXV. Padi, Gouv. Saratow. K. Glinka, Zeitschr. fiir Kryst., Vol. XXX, 1899, p. 391. XXXVI. Xasonovo, Giouv. Smolensk. Ibid. XXXVII. Ural. Ibid. XXXVIII. Traktemiroff, Gouv. Kiew. Ibid. XXXIX. Tschernofskoje, Gouv. Nischni-Novgorod. Ibid. XL. Karowo, Gouv. Kaluga. Ibid. XLI. The same, another portion. Ibid. XLII. Kosolapowo, Gouv. Nischni-Novgorod. Ibid. XLIII. The same, another portion. Ibid. XLIV. Udriass in Estland. Ibid. It is apparent from the above analyses that so-called "glanconite" has a very indefinite and vai'iable composition. In mineralog-ic text-books and in the work of Mnrray and Renard on modern glauconite deposits the substance is described as ^'■probably a mixture." Indeed, the variation in composition is so marked as to lead one to suspect that substances of dif- ferent origin have been included under this term. The main variation is in the iron. In most of the samples the iron is largely in the ferric form, and in the analyses of typical glauconite grains collected by the Challenger expedition this is true in each case. Indeed, Sir John Murray, after sum- marizing the results of their work, makes the statement that "the glauconite now foi'ming on the bottom of the sea is, like the glauconite of geological formations, a hydrous silicate of potash and of ferric oxide, containing always variable quantities of alumina, ferrous oxide, magnesia, and often lime." Yet in analyses of so-called glauconites from various geologic formations the ferrous iron is greatly in excess of the ferric iron. Comparing the analyses of glauconite above listed with those of the green granules of the Mesabi iron formation, the following differences appear: The amount of alumina in glauconite averages 9 per cent in the above ■ talile, wliile less than 1 per cent is found in the greenalite rocks, and tliis doubtfully Ijelongs with the greenalite. ORIGIN OF THE IRON ORES. 243 The ferric iron in all of the best analyses of glanconite is in greater percentage than the ferrous iron. In the green granules from the Mesabi district feme iron is nearly if not quite absent. The percentage of metallic iron in glauconite is on an average lower than that of the green granules of the Mesabi district. Glauconite contains a small percentage of soda. In the Mesabi granules soda is entirely lacking. Glauconite contains from 3 to 13 per cent of potassa, and, indeed, in descriptions of glauconite in standard text-books the content of potassa is noted" as a characteristic of it. In the green granules from the Mesabi district potassa is entirely lacking, and the granules are so fresh and unaltered as to preclude the idea that this substance was originally present and has been removed. Prof. F. W. Clarke, chief chemist of the United States Geological Survey, under whose direction the chemical work on the Mesabi green granules was done, kindly makes for publication here the following state- ment concerning the chemical differences between glauconite and greenalite THE COMPOSITION OF GLAUCONITE AND GREENALITE. B}' F. W. Clakke, Chief Chemist. "A scrutiny of the table of analyses of glauconite compiled by Mr. Leith reveals at tirst sight a hopeless discordance of data. This is due to the fact that glauconite, as actually observed and analyzed, is never piire and defi- nite, but contains a variety of imdetermined substances commingled with the true glauconite silicate. Some of the analyses, moreover, are old and obsolete, and evidently made without the precautions which are now recog- nized to be essential. Iron, for instance, is often all recorded as ferrous, no determination of the fact having been made. The figures for "water" often represent only loss on ignition, a method of estimation of notoriously falla- cious chai-acter. In order to discuss the composition of glauconite the analyses must be carefully sifted and criticised, for much of the pubhshed material is entirely worthless. " The recent series of glauconite analyses by Glinka," made upon sam- ples from various Russian localities, is of great value, for the reason that the glauconite grains were carefully separated" from admixed impurities by aZeitschr. Kryst. u. Min.; Vol. XXX, p. 390. 244 THE MESABI IRON-BEARING DISTRICT. means of Thoulet's solution, and the fig'ures indicate that the work was done with care. The analyses are ten in number, and the five best, XXXV- XXXVIII, XLIV, in the table on page 241, made upon material dried at 100°, are as follows: Analyses of glauconite. Constituent. XXXV. XXXVI. XXXVII. XXXVIII. XLIV. SiOa 48. 95 7.66 23.43 1.32 .57 2.97 9.54 .98 4.93 49.75 7.82 22.26 2.36 49.53 5.84 20.06 5.95 .56 2.92 9.31 .46 4.91 51.00 9.93 '• 18.69 1.98 .87 . 3.85 7.66 .35 5.83 1 52.96 AljO, 12.76 Fe,0, 13. 56 FeO 2.34 CaO MeO 3.25 9.01 .30 5.16 4.11 KjO 8.69 Na,0 .47 HjO 4.91 Total 100. 35 99.91 99.54 100. 16 99.80 " From these figures the following- molecular ratios can be derived: Constituent. XXXV. XXXVI. XXXVII. XXXVIII. XLIV. SiOii 0.816 .216 .103 .117 .276 0.846 .216 .114 .101 .287 0.825 .182 .165 .106 .273 0.850 .214 .138 .087 .324 0.883 R..0, .210 RO .135 E„0 .099 HjO .273 "Since RO and RoO vary reciprocally, replacing each other, these factors may all be reduced to the general equivalent of R', giving tlie subjoined empirical formulae: XXXV R'«oR'%2Si8,„0,5„„. 276 aq. XXXVI R'«oR'"«.SwO,555. 287 aq. XXXVII R'.,.R'"3o,Si„,50„„T. 273 aq. XXXVIII R',5oR'%8Si„5„0,5„;. 324 aq. XXXIX RVR'"«„Si8sA™. 273 aq. "If we neglect the water as extraneous — that is, as ' zeolitic ' or crystalline — and therefore not a part of the silicate molecule, these figures give quite closel}- the general formula R'R"'Si.A)„,+aq., in which 1!' is mainly K, K'" is mainl)' Fe, and witli the usual replacements ORIGIN OF THE IRON ORES. 245 of these radicals by others, as shown in the analyses. That is, glauconite, in its purest forms, must be regarded as a metasilicate, approximating more or less closely to the typical compound KFe'"Si206+aq., which, however, like many other silicate molecules, has not yet been found in a state of purity. Such a compound would easily lose alkali and take up water, jnelding, as Glinka observes, a ferruginous clay as its final product of alteration. Many of the observed variations in the composition of glauconite are due to partial alteration of this very obvious kind. The other variations represent the replacement of the iron salt by its aluminic equivalent and of the potassium salt by corresponding compounds of sodium, magnesium, and ferrous iron. "In the case of the mineral described by Mr. Leith, to which he has given the name of ' greenalite,' the evidence is less complete. The sub- stance is so intimately commingled with chert that it can not be isolated by ordinaiy physical means, and its composition, therefore, is only to be determined from that of the soluble portion of the mixture. This portion, according to the three latest analyses made by Mr. Steiger, representing the most carefully chosen material, has the composition given below." The summation gives the total amount of mineral decomposed by hydrocldoric acid in 100 per cent of the rock. Hygroscopic water is rejected. Constituent. 45758. 45765. 45766. SiO, 13.45 .37 15.00 10.28 2.33 .28 4.17 2.04 19.30 .61 13.83 17. 57 3.22 None. 5.74 33.11 AI2O3 .56 Fe,0< 6.44 FeO. 30.93 McO 5.35 CaO None. H2O - 6.13 CO, -- Total 47.92 60.27 82.52 "Professor Clarke's discussion was written without an opportunity to examine slides of the rocks analj'zed, and hence no account is taken of the rock alterations. However, the alterations are slight and in no way invalidate Professor Clarke's main conclusion that the substance of the Mesabi green granules is different from glauconite. Indeed, were the alterations taken into account, and especially the alteration to ferric oxide, the basis for his conclusion would be strengthened. On pp. 108-115 the writer has discussed the analyses with reference to the alterations. — C. K. L. 246 THE MESABI IRON-BEARING DISTRICT. "To compare these data they must l3e reduced to corresponding terms. According-ly, akimina lias been recalculated into its equivalent of ferric iron, maffuesia and lime into ferrous iron, and in the first example the carbon dioxide has been deducted, with its corresponding amount of monoxide bases. Then, recalculating to 100 per cent, we have the following figures to represent the composition of the soluble mineral : 45758. 45765. 45766. Constituent. Composi- tion. Ratio. Composi- tion. Ratio. Composi- tion. Ratio. SiO : ,.. 30.08 34. 8.5 25. 72 9.35 0.501 .218 .3.57 .519 30.49 23.52 36.92 9.07 0.508 .147 .513 .503 38.00 8.40 46.56 7.04 0.633 FejOj .052 FeO H^O .648 .391 Total 100. 00 100. 00 100. 00 "Although these ratios are not concordant, they still show something radically different from glauconite. If we restate them in the shape of empirical formulae, we have — No. 4.5758 Fe"35,Fe'%6Si5oAoi3- 519 H,0 No. 4.5765 Fe"5,3Fe'",s,Si5„80ic„o. 503 H^O No. 4.5766 Fe"e«Fe'"5.. Si63sO,„™. 391 H,0 "The first two of these expressions give quite sharply the orthosilicate ratio. The third represents a lower stage of oxidation, but something which is still far too high for a metasilicate. In the second case the com- position approximates to the simple formula, Fe"'oFe"3(SiO,)3. 3 H^O, in which the ratios are those of a hydrated ferroso-ferric garnet. In the last of the three analyses the ferric oxide is very low and the ferrous oxide very high, which suggests the possibility that the original mineral may have been wholly ferrous, and that it has undergone partial oxidation in the other samples. The ratios FeOiSiOg is here 1:1, indicating a possible hydrated FeSiOg as the normal substance. "The composition of greenalite, then, is uncertain. It may be a ferrous metasilicate, or it may be a ferroso-ferric orthosilicate. In either case tin- miiicrnl difters fundnmontnllv from ghuiconito, a ])otnssium ferric nietasili- ORIGIN OF THE IRON ORES. 247 cate, and can not be united with the latter species. Similarity, or even identity, of appearance under the microscope can not, in substances of this kind, offset the evidence of the ratios." F. W. Clakke. Spun-'s argument that the green granules of the Mesabi district are glauconite is based mainly on their similarity to glauconite in color and in shape, and on the fact that substances of a great variet}^ of composition have been called glauconite by mineralogists, making it allowable for him to use the term for a somewhat exceptional phase in the Mesabi district. It is shown above that the green graniiles of the Mesabi differ frOm glauconite deposits in their association, in their thickness, and in several features of their composition. It is further shown that while no decisive proof of a definite chemical composition of the substance of the green granules in the Mesabi district has been brought forward, the facts are such as to indicate a distinct possibilit)^ that the green granules of the Mesabi have a definite and uniform chemical composition. While all these differences are thought to be more or less significant, the most sig- nificant difference between the green granules of the Mesabi district and glauconite granules is taken to be the entire absence of potash in the former. There seems to be the fullest authority for the statement that potash is an essential constituent of glauconite. If potash is an essential constituent of glauconite, then its absence is sufficient warrant for the conclusion that the substance of the green granules of the Mesabi district, whatever its origin, is not glauconite. As the substance corresponds to no other known mineral species and since a name is necessary to facilitate discussion of the substance, the name greenalite has been coined, as noted on a preced- ing page. If mineralogists and chemists, in view of the differences between the two substances above described, still think it desirable to stretch the term "glauconite" to cover the substance under discussion, it is suggested that the name "greenalite" may be retained as a varietal name under "glauconite." EXPLANATION OF THE OCCURRENCE OF GREENALITE IN GRANULES. The green granules of the iron-bearing rocks have been shown not to be glauconite, but to be a substance with a different composition which has been called greenalite; but the occurrence of greenalite in granules has 248 THE MESABI IRON-BEARING DISTRICT. not been explained. Are the granules normal coucretious with radial and concentric structures about foreign nuclei, or are they concretionary or amorphous growths about, within, or replacing minute organisms, and thus directly analogous to glauconite? 1. None of the fresh green greenalite granules show any traces what- ever of radial or concentric structure or of any foreign nucleus which is characteristic of concretionary or oolitic structures. Where altered in a few cases the iron oxide and chert or the different kinds of iron oxide have a rough concentric arrangement which may indicate that the original material possessed a concretionary structure, but which more jDrobably has been developed secondarily and quite independent of the original structure of the granule. A few undoubted concretions consisting of concentric layers of iron oxide and chert about a nucleus of quartz are to be observed (PI. XIII, fig. D). A concretion of this kind may be distinguished at a glance from the greenalite granules or their derivatives, even when the latter have rough concentric arrangement, as may be seen by comparison of the figures of PI. XIII. These concretions are identical in shape with those described and figured by Van Hise as characteristic of the Peuokee- Grogebic iron-bearing formation, in which they have developed from the alteration of iron carbonate." (See PL XVI, fig. A) There is carbonate associated with greenalite in the Mesabi district, and it is not impossible that the Mesabi concretions may have developed from carbonate, although no direct evidence of this has been observed. ,The true concretions of the Mesabi district also find their counterpart in concretions in the widely distributed Clinton iron ores, in which they probabl}^ developed at the time of the deposition of the ore. They are similar in form also, though not in substance, to the bavalite, chamosite, and berthierine of European ores.'' Whatever the origin of the concretions in the Mesabi district, it is clear that they are quite independent of and subordinate to the greenalite granules, lacking evidence of radial or concentric structures or nuclei characteristic of true concretions. 2. The similarity of the granules of the Mesabi iron formation in general aspect to glauconite grains and to certain organic granules of the Clinton ores brings to mind very strongly the possibility that the shapes of the Mesabi STanules mav be determined bv similar conditions. While «Mon. U. S. Geol. Survey Vol. XIX, 1892. ''See Lacroix, Minerals of Fiaiue, |>. 401. PLATE XXI. 249 PLATE XXI. PHOTOMICKOGRAPHS OF GRANULES AND CONCKETIONAKT STRUCTURES IN CLINTON IRON OpES. Fig. vl.— Granules in Clinton iron ore. From lower bed Sand Mountain, New England City, Ga. Loaned tay C. H. Smyth, jr. Without analyzer, x 40. Granules of black and dark-brown hydrated hematite stand in a matrix of calcite. The latter areas within the granules are also calcite. Traces of organic shells in these slides are abundant. The granule a little to the right of the center shows this especially well. There can be no doubt as to the fact that the granules are for the most part replace- ments and accretions about shells and particles of shells. It is apparent also that there is a marked tendency for the granules to take on rounded and aval forms regardless of the shape of the original particles of shell. Note the remarkable similarity of these granules in shape to the greenalite granules illustrated in PL XIII. Fig. 5.— Green oolites in Clinton ore. From Clinton, N. Y. Loaned by C. H. Smyth, jr. With analyzer, x 40. Concentric layers of chloritic and siliceous substance, of various shades of green and yellow, surround angular, subangular, and rounded grains of quartz. The concentric greenish and yellowish bands under crossed nicols show black crosses characteristic of concretionary structures. The matrix is mainly calcite, but there are present also small particles of quartz. 250 U. S. GEOLOGICAL SURVEY MONOGRAPH XLIII PL. XXI PHOTOMICROGRAPHS OF GRANULES AND CONCRETIONARY STRUCTURES IN CLINTON IRON ORES. THE MERIDEN GHAVURE CO. ORIGIN OF THE IRON ORES. 251 the greenalite is different in composition from glauconite, it might still be developed in much the same way. The close resemblance to g-lauconite grains in shape and phvsical properties other than specific gravity has alreadj- been noted. Noting the remarkable similarity in external shape between the Mesabi granules and granules of the Clinton ores of Wisconsin, the writer asked Dr. C H. Smyth, jr., of Hamilton College, Clinton, N. Y., for slides of the Clinton ores, which occurr so widely in the eastern portion of the United States, and which he had described." Professcn- Sm^i:h kindly furnished the slides requested, and thus enabled the following' comparison to be made:* In the Clinton ores two kinds of gran- iiles are numero is: (a) Normal concretions of silica and iron oxide or of silica and some greenish siibstance with a ferrous iron base, the further composition of which is unknown, about a nucleus of quartz. (Fig. B, PI. XXI.) These are analagous to the few true concretions observed in the Mesabi district and to the concretions of the Penokee-Gogebic district, (b) Accretions of iron oxide about calcium carbonate shells and partial or complete replacements of the shells, in either case without or nearly- without radial or concentric structures. (Fig. A, PI. XXI.) The size is somewhat greater than that of the Mesabi granules. It is noticeable that while traces of shells are abundant in the Clinton granules the shapes of the granules are not closely dependent upon the shape of the shell or fragment. On the contrary there seems to be a uniform tendency for the granules to develop with rounded and oval outlines no matter what the form of the shells which they replace. The shapes are almost identical with those of the normal greenalite granules of the Mesabi, as a comparison of Pis. IX, XIII, XV, and XXI will show. The similarity in shape is as close as between greenalite and glauconite. ■ The crescent shapes, the gourd shapes, the much elongated ovals, and rods, which are seen associated with the round and oval forms in the Mesabi rocks are all to be seen in the Clinton ores. The calciiTui carbonate of the shells in the interior of some of the granules of some of the Clinton ores also has a mottled appearance very similar to that observed in some of the Mesabi slides figured in PI. XIV, although « Am. Jour. Sci., 3d series, Vol. XLIII, 1892, pp. 487^96. 6 In this connection a remarkable coincidence may be noted. In a letter of the same date Professor Smyth asked for information concerning the ilesabi granules. He had seen a brief preliminary description of them, by the writer, in Science, and had lieen struck with the similarity of certain of their features with those of the Clinton ores, with which he was familiar. 252 THE MESABI IRON-BEARING DISTRICT. beyond this there is no evidence that the two are the same.. In both the Clinton ores and the Mesabi rocks a not ■uncommon feature is the accretion of a considerable number of granules into somewhat irregular pebble-like ao-2-res-ates which have been waterworn as a whole and deposited parallel to the bedding. The Chnton granules differ from the unaltered Mesabi granules in that they are either iron oxide entirely or partly iron oxide and partly calcite, while the' Mesabi granules consist of ferrous silicate when fresh and largely of iron oxide and chert when altered. The Clinton ores, in their present form, may not be concretions or replacements subsequent to their deposition, for they have uniform composition in thin beds over great areas, which could not be the case were they subsequently concen- trated through underground water or other agencies. They may well be compared with the fresh greenalite granules of the Mesabi which also have undergone no concentration, rather than with the altered granules. If dm-ing the deposition of the Clinton ores the numerous minute shells had been surrounded and replaced by iron sihcate instead of iron oxide, greenalite granules identical with those in the Mesabi district may have resulted. It is concluded that the greenalite granules of the Mesabi district, while associated with a few typical concretions, are not for the most pait normal concretions with radial or concentric structures about an inorganic nucleus; that from their remarkable similarity in shape to glauconite grains which are mainly developed as replacements, secretions, or accretions about minute organisms (although perhaps partly in other ways), and their similarity to the accretions about and replacements of shells in the Clinton ores, they may owe their shape mainly to similar development either witliin or about or replacing minute organisms of the variety commensurate with that now observed both in modern glauconite deposits and in the Clinton ores; that the development of greenahte instead of glauconite or iron oxide was largely a matter of substances present which were available for accretion, secretion, or replacement. However, the absolute absence of organic structures, aside from the suggestive similarity in shape to granules of known organic origin, must still be kept in mind, and the conclusion here given must be regarded as a tentative one, lacking suthcient basis of dii-('ct observation to render it final. ORIGIN OF THE IRON ORES. 253 MANNER OF DEPOSITION OF GREENALITE. It now becomes necessary to determine how a compound like g-reenalite can develop under conditions such as those supposed to have existed in the Mesabi area. Two explanations suggest themselves. I. DEVELOPMENT SIMILAR TO GLAUCONITE. The material may have developed in a manner analogous to the develop- ment of glauconite. The manner of the development of glauconite is not by any means clear. Perhaps the most instructive work on the subject is that by Murray and Renard." Quoting from their report concerning con- ditions for the development of glauconite: Where the detrital matters from rivers are exceedingly abundant and where there is apparently a rapid accumulation, glauconite, though present, is relatively rare; on the other hand, along high and bold coasts where no rivers enter the sea, and where accumulation is apparently less rapid, glauconite appears in its most typical form and greatest abundance. * * * * * * * * * * With reference to its bathymetrical distribution, it appears to be most abundant about the lower limits of wave, tidal, and current action, or, in other words, in the neighborhood of what we have termed the mud-line surrounding continental shores. In the shallower depths beyond this line, that is to say, in depths of about 200 and 300 fathoms, the typical glauconitio grains are more abundant than in deeper water, but glauconitic casts may be met with in deposits in depths of over 2,000 fathoms. No typical glauconitic sands have, so far as we know, been recorded in process of formation in the littoral or sub-littoral zones.* Concerning its manner of development they state, tentatively : We are therefore inclined to regard glauconite as having its initial formation in the cavities of calcareous organisms, although we have admitted above that some grains which might be regarded as glauconite appear to be highlj^ altered fragments of ancient rocks or coatings of this mineral on these rock fragments. It appears that the shells are broken by the swelling out or the growth of the glauconite, and that subsequenfl}' the isolated cast becomes the center upon which new additions of the same substance take place, the grain enlarging and becoming rounded in a more or less irregular manner, as in the case of concretionary substances like silica, for example, which forms molds of fossils. * * * * * * After the death of the organisms their shells are slowly filled with the fine mud in which thej" are deposited. The existence of this organic matter in these cavities, and the absence of all other causes which might there ii^duce the deposition of the silicates, in fact, the constant association of these phenomena appear to demon- strate the existence of a relation of cause and effect. * * * If we admit that «Eeport of the voyage of H. M. S. Challenger, 1S73-1878, Deep-sea Deposits. ^iLoc. cit., pp. 382,383. 254 THE MESABI IRON-BEARING DISTRICT. the organic matter inclosed in the shell, and in the mud itself, transforms the iron in the mud into sulphide, which may be oxidized into hj^drate, sulphur being at the same time liberated, this sulphur would become oxidized into sulphuric acid, which would decompose the fine clay, setting free colloid silica, alumina being removed in solution: thus we have colloid silica and hydrated oxide of iron in a condition most suitable for their combination. To explain the presence of potash in this mineral we must remember that, as we have shown when speaking of the formation of palago- nite under the action of sea-water, there is always a tendency for potash to accumulate in the hydrated silicate formed in this way. and, as we have stated before, this potash must have been derived from the sea water." It is difficult to see how so high a percentage of iron as is found either in glauconite or in greenalite^ can be derived from the decomposition of mud filtered into the interior of the shell. If the mud were derived entirelj' from the disintegration of basic rocks, the percentage of metallic iron would not be far above 10 per cent, and the actual percentage found in the granules is far higher than this. The derivation of sufficient iron from the decomposition of mud would require a larger amount of foreign material than is contained in the casts. In the typical glauconite deposits foreign material is present outside of the shells, as shown Idv the above quotation from Murray and Renard concerning the constant accompaniments of glau- conite, and there seems to be no reason why all of this material should not be di-awn upon for the supply of iron. If the substance of the Mesabi green granules be supposed to have been derived from the decomposition of orig- inal detritus, this must have been present in enormous quantity, for the content of metallic iron shown in the analyses of greenalite rocks is 25 per cent, while the detritus available in the Mesabi area could scarcely have averaged as much as 7 per cent in metallic iron. As the greenalite rocks of the Mesabi iron formation accumulated to a thickness of perhaps 1,000 feet, it would be necessar}^ to assume that a thickness of detritus several times this figure originally was present to yield the necessary amount of iron to the granules. There is ample evidence that no such amount of detritus (in fact little or none beyond that now to be observed) was ever present in the Mesabi iron formation. This consideration calls for an additional source for the metallic iron of the Mesabi greenalite granules; other possible sources are discussed under II and III lielow. The great thickness of the Mesabi iron-bearing formation as corajiared ^^■ith known glauconite deposits is further presumptive evidence tliat processes other tliau those forming n Loc. cit., pp. 388,389. ORIGIN OF THE IRON ORES. 255 glauconite were active in the development of the Mesabi greenalite gran- ules, for otherwise it would be necessary to assume abnormal intensity and duration of glauconite-forming processes. While the explanation of Murray and Renard as it stands above scarcely seems applicable to the explanation of the origin of the Mesabi granules, it does not at all follow that the conditions and processes favorable to the development of g'lauconite when these are fully known will not be applicable, at least in part, to the development of the Mesabi greenalite granules. II. DIRECT PKECIPITATIOX FROM SOLUTION BT ORGANISMS. Where iron is being- contributed to ocean waters in considerable abundance, it is possible to conceive of minute organisms abstracting the same and depositing it directly in some such form as glauconite or greenalite. This subject would reauire elaborate treatment and the expla- nation is here but ixientioned. HI. DEVELOPMENT SIMILAR TO THAT OF IRON CARBONATE. The association of the green granules in the Mesabi iron formation with original iron carbonates and their analogous composition suggest an explanation of their origin similar to that applied to the development of iron carbonates from other portions of the Lake Superior region by Irving and Van Hise." This is outlined below. IRON DERIVED FROM THE WEATHERING OP PREEXISTING KOCKS AND CARRIED TO THE OCEAN AS CARBONATE. The iron and silica of the greenalite were brought into the ocean largely in solution. The rocks which at that time formed the shore (the Archean and Lower Huronian rocks) contained disseminated iron. The Archean "greenstone," which formed a very large proportion of tlie land area at that time, still contains from 7 to nearly 10 per cent of metallic iron, largely in the ferrous condition, and the Lower Huronian rocks less. By the ordinary processes of weathering the rocks were decomposed and the iron taken into solution by the surface waters, largely as carbonate. Most meteoric waters contain carbon dioxide (C0„), and it is more than proba- ble that sulphuric acid was also present, but was very subordinate in quantity to carbonic acid, and hence the sulphates were not important. a See Men. U. S. Geol. Survey Vols. XIX and XXVIII, and Twenty-first Ann. Kept. U. S. Geol. Survey, Pt. III. 256 THE MESABI IRON BEARING DISTRICT. Particles of the preexisting rocks were also carried into tlie ocean in suspension, and these, by subsequent decomposition, yielded iron to the developing iron-formation materials, but it is not likely that this was a great factor in the development of the iron formation, for the average amount of metallic iron in such particles was less than 5 per cent, while the amount of metallic iron in tlie original iron-formation rocks is commonly 25 per cent. THE IRON FIRST PRKCIPITi,TED IN THE OCEAN AS A HYDRATED PEROXIDE. When the waters bearing iron in solution reached the ocean most of the iron carbonate was broken up, the carbon dioxide given off, and the ferrous iron oxidized to a hydrated peroxide state and precipitated. The precipitate was probably first in the form of Fe„(OH)^, although the degree of hydi'ation may have speedily altered, as it is known to do in the laboratory" when it is allowed to stand or is subjected to various conditions. The reaction was 2FeC03+0+3H,0 = Fe,,(OH),+2CO,. Precipitation may have occurred either through the ordinary oxidation or with the aid of "iron bacteria."" THE IRON FIRST PRECIPITATED IN ARE.AS OP VEGETATION. It is probable that the ferric hydrate was thrown down in an area of abundant vegetation. Van Hise ^ has shown that the process of carbonation on a large scale, bringing the iron in form of carbonate to the ocean, is favored by the presence of abundant vegetation, the decomposition of which }delds carbon dioxide; that where vegetation exists in land areas it is also likely to be abundant in adjacent waters where the conditions allow it ; and that in such, places carbonates are formed. The very fact that a formation high in iron content was deposited in the Mesabi area shows that' some process was bringing in iron on a large scale, and no process would be more likely to be normal under the conditions under which the iron formation developed than carbonation. If the process of cai'bonation was occuriing on a large scale this implies the presence both on land and in water of abundant vegetation. Moreover, the iron formation contains original carbon and carbonates of iron and calcium, affording direct proof of the presence of vegetation in the water in which the iron ^^•as first pre- " For action of iron bacteria, see any standard text-book of bacteriology. ''Mon. U. S. Geol. Survey on ]Metamor|)bisin, in jireparation. ORIGIN OF THE IRON ORES. 257 cipitated. Still further, the iron formation, from its position between qnartzite and slate, is known to have developed under conditions inter- mediate between those of clear water and muddy water, which would be likely to be favorable to vegetation. It has already been pointed out that instead of the open ocean, there may have been over this area a semi- inclosed arm of the sea. Such a condition Avould have been favorable to the extensive occurrence of this process. THE HYDRATED PEROXIDE REDUCED BY VEGETABLE MATTER, AXD THE PROTOXIDE OF IRON COMBINED WITH CARBON DIOXIDE OR SILICA. As the hydrated peroxide fell to the ocean bottom and became ming-led with vegetable material, and buried with it, it was reduced, at least in part. Says Van Hise," "The reducing agent may be regarded as carbon monoxide, or some of the carburreted hydrogens, such as methane." The iron was tlien in the protoxide state and could easily combine with the carbon dioxide simultaneously developed by the oxidation, of the carbon of the organic material to reproduce iron carbonate. But in addition to the carbon dioxide developed under these conditions silica was present. As shown by Van Hise, it is often frequently associated with carbonates, and it was associated with carbonates at the time of the developinent of the Biwabik iron formation, as shown by its present occurrence both in the altered and unaltered portions of the formation. The best investigation on the subject indicates that the chert is in most cases formed through the agency of pelagic organisms which secrete the silica found in solution in the sea water or derived from the decomposition of the silicate minerals in the associated detrital material.'' The silica, especially where in a colloidal form, could combine with the ferrous iron present to foi-m ferrous silicate. Thus in the development of the Biwabik formation both carbon dioxide and silica were present, with either of which the iron protoxide could combine according to the following simple reactions: FeO-t- CO3 + nH,0 = FeCO, + nH,0 FeO+ SiO, + nH,0 -= FeSi03nH,0 The last-named formula essentially represents greenalite, though the subordinate constituents are not taken into account. Whether the iron combined with the silica or with the carbon dioxide was probably a function of the relative masses of the chert and carbon dioxide «Loc. cit. ^See C. D. Walcott, Mon. U. S. Geol. Survey Vol. XXX, 1898. MON XLiir — 03 17 258 THE MESABl IRON-BEAKING DISTRICT. available for combiuation. During the deposition of the Mesabi iron forma- tion, silica was unquestionably in much greater mass than the carbon dioxide, as shown by the present composition of both the altered and unaltered portions of the formation. Doubtless there are other factors con- cerned in the combiuation of silica or carbon dioxide with hon, but these are not vet known. Whatever the cause, ferrous silicate was formed in great abundance and the iron carbonates in small quantity. Calcium and magnesium oxides, which may have been present, as shown by the analyses, mav have reduced the active mass of the carbon dioxide available to unite with, and thus increased the proportional mass of silica. Both the magne- sium and calcium oxides are stronger bases than the iron, and would take precedence over it on going into combination with carbonic acid. In the iron formation of the Gogebic district of Micliigan, a forma- tion of the same age, general character, and associations, and supposedly of a similar origin, silica was less abundant in the original rock of the forma- tion (siderite) than in the original rock of the Mesabi iron formation (green- alite rock), and thus the dominant combination was protoxide of iron and carbon dioxide, producing iron carbonate, and the combination of protoxide of iron with silica was very subordinate, although it did occur, as shown by the small admixture of ferrous silicate rocks in the carbonates of this region, already noted. (See p. 118.) The preponderance of iron silicates in the Mesabi district and the preponderance of iron carbonates in the Penokee-Gogebic district suggests an analogy to the occurrence of zinc ores in Missouri and Wisconsin, described by Van Hise. In the former district zinc silicate is abundantly present with zinc carbonate. In the latter district zinc silicate is sparinglv present with the zinc carbonate. Van Hise concludes that "the almost complete absence of zinc silicate in Wisconsin and the presence of zinc silicate in southwestern Missouri are in accordance with the well-known law of mass action. Where silica is abundant, so that zinc silicate can form, it is jiresent plentifully; where silica, is absent or subordinate, it does not develop in any considerable quantity."" In a later work he states: "Where, under the conditions described, silica is abundant in proper form. " Preliminary report on tlie lead and zinc deposits of the Ozurk rftiion, liy 11. K. Hain. with an introihu'tion hy C. li. Van Uise; Twenty-second Ann. Kt'pt. l'. S. Gfol. .-^nrvey, Part 11, 1\H)2, p. 51. ORIGIN OF THE IRON ORES. 259 the law of mass action requires its union with the protoxide of iron. This principle is illustrated by the condition in which the oxidized compounds of zinc occur in the Wisconsin and southwestern Missouri districts. In the Wisconsin district silica is not especially abundant, and where zinc sulphide is oxidized the zinc oxide unites with carbon dioxide and forms smithsonite (ZnCOg) But in Missouri, silica, partly in the amorphous form, is very abundant; and there, when the zinc sulphide is oxidized, the oxide of zinc largely unites with the silica, forming calamine [(ZnOH) 28103]. Both smithsonite and calamine occur in both districts, but calamine occurs abundantly only where silica is alsundant. Similarly, where in lagoons the iron is reduced to the protoxide form, it would unite with the silica on a large scale, j^ro^dded that compound were abundantly present in a form suitable for union." " CONCLUSION WITH REFERENCE TO ORIGIN OF GREENALITE GRANULES. It is concluded that the explanation offered by Murray and Renard for the development of the modern glauconite deposits does not apply without much modification to the greenalite deposits under discussion; that the greenalite granules may possibly have developed directly from the abstrac- tion, through the agency of organisms, of iron from solution in sea water, whence it was contributed from adjacent land areas; finally that a reason- able explanation seems to be that the green granules, from their analogy in composition to iron carbonate, from their association with iron carbonate, from their great thickness, their uniform character, and analogy with other carbonate and silicate compounds, probably developed in a way similar to the development of the iron-carbonate deposits of other portions of the Lake Superior region; and that their shape, and not composition, is determined by organisms. It may be noted in closing that the several explanations of the origin of the Mesabi green granules above outlined are not mutually exclusive, and each of them may be found to be partly true when the complete explanation is known. «Mon. tr. S. Geol. Survey on Metamorphism, in preparation. 260 THE MESABl IRON-BEARING DISTRICT. BUEIAIi OF THE IRON-BEAEIXG FORIMATIOX BENEATH THE VIRGrNTA SLATE. The iron-formatiou materials, consisting mainly of gi'eenalite in the form of granules, abundant chert as cementing material and in layers, thin layers of u-on carbonate, and layers of mud, were deeply buried under a great accumulation of mud, which, by its metamorphism, has given the "\^irginia slate formation. Judging from the present maximum thictness of the slate in the Penokee-Gogebic district, the depth of burial of the Biwabik fonnation beneath mud was thousands of feet and m.ay have been as great as 13,000 feet. EarEEGElSrCE of the IEOX-BEARKS^G FORIVIATIOX FROM THE OCEA^'. The iron-bearing rocks came above sea level by the uplifting of the area. While the rocks were deeply buried under j-ounger sediments long- before the movement began, induration of the formation set in and con- tinued during the nplift and, for parts of the formation, long afterwards. ALTERATION OF THE mON-BEARIXG FOR3IATION BIT T\TEATHEEIXG AND THE SECONDARY CONCENTRATION OF THE ORES. As soon as the land appeared above the water, erosion began to wear down the rocks. The great thickness of slate had first to be removed, and it is probable that the iron formation was not exposed to weathering- agen- cies until a very long period had elapsed. While part of the iron formation in the eastern end of the district was exposed by erosion before Keweena- wan time, as shown Vjv the fact that the Keweenawan gablDro there lies on the eroded edges of botli the Virginia slate and the Biwabik formation, the central and western portions of the district in which the iron-oi-e depos- its are now found were jirobably not exp<:)sed until post-Keweenawan time. Throughout the Lake Superior region the Upper Huronian and Keweena- wan rocks were folded together, and tliere is reason to believe that this folding developed the Giants range and the southward tilting of the Upper Huronian strata over what is now known as the Mesabi district. This fold- lug also fractured the brittle iron formation nnd made it very pervious to water. As soon as the folding liad taken ])lace erosion set in, which, after a long j)criod, truncated the edges of the Upper Huionian series, giving them the distribution in belts parallel to the Giants range now to be observed. ORIGIN OF THE IRON ORES. 261 As the alteration of the iron formation and the concentration of the ore has been brought about by processes characteristic of. surface conditions, as will be seen on a subsequent page, the secondary concentration of the Mesabi iron ores did not begin until after the truncation following the post- Keweenawan folding. The alteration which then occurred was as follows: In its unaltered state the iron of the iron formation was disseminated through the rocks of the formation. Tlie average amount of metallic iron was about 26 per cent. The alteration was rapid at the surface; but alteration was also carried on below the surface, both above and below the level of ground water, by the downward-percolating siu-face waters. The bedded structure, the attitiide of the formation, and the cross jointing were favorable to the vigorous circulation of waters through the formation. Coming down the south slope of the Giants range they entered the trun- cated edges of the iron-bearing strata, dipping gently away from the range, and flowed southward along the layers. The chief alteration was that of the greenalite which, with the chert, made up the bulk of the formation. The waters from the surface carried carbon dioxide and oxygen derived from the atmosphere and the carbon dioxide, perhaps in part from tlie erosion of the overlying slate and carbonates. They carried also small quantities of sulphuric acid and the alkalies. (See analysis on p. 264.) Hydrous sili- cates are readily soluble in underground waters, and especially waters bear- ing carbon dioxide. In the long period during which the ferrous silicate has been in contact with circulating waters bearing carbon dioxide it is apparent that it must have gone much into solution. The iron was thus brought into solution mainly as carbonate, though perhaps partly as sul- phate or even as silicate. To illustrate the essential nature of the altera- tion of the greenalite, its composition will be assumed to correspond to the simple formula FeSiOguHaO, thus neglecting the small amount of other substances shown by the analyses. The simple ratio of ferrous iron and silica shown in the formula has not been proved in the analyses, but what evidence there is points strongly to such a proportion. With this assump- tion the reaction was FeSi03.H,0+H,C03+Aq=FeC03+H,SiO,+Aq The iron carbonate was either immediately oxidized and hydrated and thrown down as ferric hydrate through the agency of the oxygen carried 262 THE MESABI IRON-BEARING DISTRICT. by the solution which effected the carbonation, or the iron carbonate may have been in part .carried a greater or less distance, until it met waters carrying abundant oxygen and was then thrown down. That transporta- tion of iron in solution actually occurred is shown by the stalactitic and botryoidal ores in vuggs. Where ferrous compounds were abundant there was probably not oxj^gen enough to throw all of the ferrous compounds out of solution at once. The oxidation of the iron carbonate took place accord- ing to the following equation : 4 FeCOj+S H30-t-2 0=2 FeA-3 H,,0-1-4 COa The degree of hydration may have varied at the time of the precipita- tion or thereafter through varying temperature or other causes." The solution of the iron silicate (greenalite) meant the simultaneous production of soluble colloidal silicic acid (see reaction, p. 261, and analy- sis, p 264), which carried the silica for greater or less distances and by dehydration de^DOsited it as chert. The abstraction of the silica in this manner caused the iron oxides to slump and thus to be further concentrated. This process, combined with the actual transportation of the iron in solution, where carried out on a large scale, resulted in the development of iron-ore deposits; where on a small scale the alteration resulted merely in the local segregation of the iron oxide and chert, giving ferruginous cherts. The abstraction of the silica explains the slump near the contact of the iron-ore layers with the layers of wall rock described on pages 230-233. Indeed, one would expect the observed slump to be even greater than it is, for the silica taken out is over half of the volume of the orig'inal rock, as may be noted by comparing the analyses of the unaltered rocks and of the iron ores. But the ores after the concentration have a very large amount of pore space compared with the rocks from which they are derived, indi- cating that the iron ore has not fallen to an extent commensurate with the volume of the silica removed. « The effect of temperature ou the kind of hydrate precipitated is shown by the following analyses, by Haiijpe (Ch. Central-Blatt, 1889, II, 906): 0°. 20°. 25°. 30°. ■to°. 6O0. 80°. 100°. FeA 54.6 45.4 51.4 50.4 46.1 53.9 43. 9-53. 2 66.2 33.8 70.1 29.9 72. 3-92. 7 Loss on ignition (HjO ) 48.6 49.6 ORIGIN OF THE IRON ORES. 263 So far as the original iron-formation rock was an iron carbonate, which we know to have been present but to have been very subordinate in quantity to the greenahte, a similar set of changes occurred. The iron was oxidized and hydrated and the carbon dioxide removed in the manner described by Van Hise." At the same time the slaty layers within or adjacent to the iron formation which came into contact with actively circulating waters were altered to paint rock. The process consisted in the abstraction of substances other than the silica and alumina to a greater extent than these compounds, and staining red with iron oxide, due in part to the oxidation of the ferrous compounds contained in the slate and in part to introduction of ferric oxide. Since the concentration was started in post-Keweenawan times the development of the ore bodies has had interruptions, but the interruptions probably have been subordinate as compared with the long periods during which the concentration has been occurring. In Cambrian times the district may have been covered by Cambrian sediments, but this is uncertain. The nearest Cambrian sediments are now 80 miles distant. Again, in Cretaceous times, the western part of the district, and perhaps the eastern part, were buried to an unknown depth b}^ Cretaceous rocks, but when these were stripped ofP by erosion the concentration of the ores was resumed. In Glacial times the Mesabi district was oven-idden by the ice and the surface rocks planed off. Large quantities of iron ore were removed at this time, as shown by the fact that fragments of ore are found in the drift, some of them of considerable size. The so-called Moose Track mine, south of the Fayal mine, was a large mass of iron ore of about 30,000 tons inclosed in the drift. Vastly more ore was carried away than now appears in large pieces in the drift, for the ore is of soft character and would be likely to be finely disseminated through the drift, or even carried away in solution. The glacial cutting of the ore is shown further b}^ the fact that the rocks in the neighborhood of the iron-ore deposits, which are much harder than the ores, have been extensively cut down, as shown by the abundance of their de'bris in the drift; and as the surface of the ores is now usually below the harder ■wall rock, the softer ore was probabl}" gouged out to a greater extent than the adjacent rocks. If in the past the upper poi'tions of the deposits contained on an average a lai-ger amount of hydi-ated ores than tlie lower a Twenty-Erst Ann. Rept. U. S. Geol. Survey, Pt. Ill, pp. 326-328. 264 THE MESABI IRON-BEARING DISTRICT. portions, as tliev do now, then the glacial erosion removed more of the highly hydrated ore than of the less hydrated ore, and thus the present preponderance of hydrated ores in upper horizons is not so great as it might have been had glacial erosion not occurred. In general, the glacial cutting has probably not been so deep as in the Canadian iron-bearing districts, or perhaps the Vermilion or Marquette districts, because of the protection of the high land t() the north and also because of the interlobate position of the district, and this may help to explain the apparently greater abundance of ores in the Mesabi disti'ict than in the Canadian districts, as suggested by Van Hise." Since Glacial time the ore deposits have for the most part remained buried beneath glacial drift, but still the iron formation has been under- going essentially surface alterations. That the concentration of the ores is continuing at the present time is shown by the water analysis by Mr. E. T. Allen, of the United States Geological Sm'vey, given below. The water is from a drift between the Hull and Rust mines west of Hibbing: Parts per lidUion. CO, .71 SiO, 22. 35 SOj 2. 2 PO4 Trace? Fe Not a trace. Ca 10. 1 K 97 In all cases except in the determination of CO.., ] liter was used. The water contained no suspended or precipitated matter. The results are stated in terms of ions. Whether or not the ionization theory be accepted, the radicals given are the ones which have been actually determined in the analysis, whatever their state or combinatioa. The relatively high content of silica in the water indicates that this substance is now being taken out of the ore deposits and thus that concen- tration of the ore is actually occun-ing at the present time. The content of carbon dioxide, sulphuric acid, and the alkalies show the agents to be present now which presumably have been present in the past. The absence of any iron in the water is evidence that the concentration at the present time witliin the ore deposits already formed consists almost entirely, if not quite, in the abstraction of silica. It does not show, liowever, that in the past " Twenty-first Ann. Kept. U. S. Geol. Survey, I't. Ill, 1901, .pp. 411^12. ORIGIN OF THE IRON ORES. 265 the iron itself may not have been carried in solution to foi-m these ore deposits, nor that iron is not now being- carried in those portions of the iron formation! north of the Virginia slate and under the edge of the Virginia slate where greenalite or iron carbonate are available. At the present time ferrous silicate and ferrous carbonate are in exceed- ingly small quantity in the iron formation north of the Virginia slate, and their oxidation takes but little oxygen from the percolating- waters which traverse the ore bodies. The waters, then, are likely to have a high percentage of oxygen throughout the part of the formation explored and the part from which the water analyzed was derived, and any ferrous compound which may get into solution is quickly oxidized and precipitated. In the past, when the ferrous comjDOunds were abundant where the ore bodies now are, it may be supposed that the water was depleted in oxygen through the oxidation of the ferrous compounds before it had traveled any considerable distance into the formation, and that when the oxygen had been abstracted the ferrous compounds in solution may have been carried some distance before water bearing sufficient oxygen to oxidize and precipitate them was encountered. LOCALIZATIOlSr OF ORES BT CIKCULATIOlSr OF WATER. The segregation of the iron-ore bodies having occurred through the alteration of the greenalite by the agency of percolating underground waters, one would expect to find that the maximum alteration has occurred and the iron-ore bodies have developed at places where the circulation of the oxygen-bearing and carbonated waters was most vigorous. A consid- eration of the present and past circulation through the iron formation shows this to be the case. Meteoric water falling on the iron formation, or contributed to it from the slope of the Giants range from the north, enters the eroded edges of the iron-formation beds dipping gently to the south Water probably flows along the bedding openings more freely than along the joints, for the bedding openings present more continuous openings than the joints, which are irregular and discontinuous, and are frequently cut oif hj soft, imper- vious layers which have yielded to deformation by bending rather than by breaking. Locally, however, the flowage along joints is dominant. Certain of the iron-formation layers also are porous, while others, particu- 266 THE MESABl IRON-BEARING DISTRICT. larly slate and paint rock, are not, and thus the water flows more easily through certain beds than through others. The flowage of water above the level of ground water and the flowage below this level have distinctive features and may be considered separately. Above the level of ground water the water entering the iron formation tends to move vertically downward under the stress of gravity. If the substance wei'e homogeneous this tendency would prevail and the move- ment would be practically vertical. Vertical joints in the iron formation tend to allow movement in this du-ection, but the more important bedding openings and the combination of pervious and inij^ervious beds tend to make the water deviate from verticality and take up a course more nearly Iroii--beai'ijld iornia-tion Fig. 11.— Section through Bhvabik formation transverse to the range, showing nature of circulation of water and its relations to confining strata. parallel to the beds. The major flow seems to follow broad gentle syn- clines in the iron-formation layers, but locally the flow follows cross fractures. When the water has passed into the sea of ground water its flowage depends not only upon the structure and texture of the formation but upon the position of the outlet. It is certain that the water flowing through the Biwabik formation does not circulate vigorously far under the Virginia slate. Where the slate is drilled, in exploration, water is occasionally found under pressure, which shows that, in spite of the slight jointing of the slate, the formation is essentially impervious, as would be expected from an}- slate formation, and has ponded the water coming under it from the north. Tlius water flowino- tlu-ouoh the iron formation must therefore necessarilv overflow at the north edge of the slate Recent pumping ojjerations have lowered the ground water to such an extent that in places it stands below the slate margin, and in these places there is little or no ovei-flow at the edges of the slate. In the past, however, overflow at this level must have "■enerally occurred. Slate layers within the iron formation and the O m.' »■' Pokefi'ama formation at its base nmst also limit the water circulations, ORIGIN OF THE IRON ORES. 267 several of which may have contemporaneously existed. Finally the circulation is modified by joints. The general nature of the flow of water parallel to the pitch of the formation between a slate layer within the iron formation and the overlying Virginia slate, or between an}^ two "slate layers within the iron formation, or between the Pokegama quartzite below and a slate layer within the iron formation, is shown in fig. 11, based on a drawing kindly furnished by Prof C. S. Slichter. The lines of flow are drawn on the assumption that they enter the ground at equal distances along the upper slopes and emerg-e in a limited area in the drift near the margin of the slate. Probably more water enters the iron foi-mation on the upper slopes than on the lower slopes, being contributed from the Griants range to the north. Were it not for this fact it would be necessary to leave wider spaces between the points oi" entrance of the lines of flow on the upper slopes than on the lower slopes. The diagram shows that the flow is more vigorous near the area of escape than elsewhere, just as, when water is drawn off the edge of a basin, the flow is more rapid at the point of escape than in a distant portion of the basin. The actual flow through the iron formation diff'ers from that shown in the diagram in a number of particulars. The iron formation between the impervious strata is not homogeneous, but the attitude of its layers and openings is such as to cause little vertical modification of the flow shown in the above diagram, for the beds and bedd'ng openings would tend to carry the water in the same general direction. The effect of heterogeneity is only to make the vertical distribution more uneven than shown in the diagram. It must be supposed that while somewhat more water is coming to the surface near the edge of the slate than at any other horizon a considerable quantity of water is also coming to the surface in the iron formation a considerable distance away from this horizon, for the level of ground water is frequently in the drift above the rock sui-face. The total head, as shown by the configuration and attitude of the water table, varies from place to place and is seldom evenly distributed across the surface width of the iron-formation belt. An examination of the map will show much irreg'ularity in topography, and numerous swamps and lakes, indicating ground-water level unevenly distributed through the formation. Perhaps in general the head is greater in upper portions of the belt, nearest the underlining rocks, than near the slate. Finally, in the past the water level and head have been changed from time to time, as will be seen below. 268 THE MESABI lEON-BEARING DISTRICT. Yet, notwithstanding these variations from the conditions represented in the diag-ram, it is thought that the diagram will serve as well as any other to show the general average conditions of flow below water level in the past before artificial openings had modified them. It is qualitatively, if not quantitatively, correct. The lateral distribution of the flow depends upon the channels avail- able and the level of the points of escape. Other conditions being equal, the flow is concentrated along joints or pervious portions of the formation. The watej- escapes at the lowest point, and hence near such a point the flowage is likely to be concentrated. Fracturing of the overlying Virginia slate or the slate layers within the iron formation may determine the lowest point of escape for a given area; or diff'erential subaerial or glacial erosion may accomplish this result; or, finally, the original folding of the Upper Huronian series, followed by the truncation of the series by erosion, may result in making the low points of escape along synclines. It is probable in many cases that stream and ice erosion has followed the original struc- tural synclines in the formation. In other cases it is ceilain that the anticlines have been cut oif to as large or even a larger extent than the synclines. While the flow is limited below by impervious strata, if, because of the position of the outlet, the flow below the level of ground water is greater near the point of escape than deeper down, it follows that there is no concentration of the flow along the impervious basement, and thus, in so far as the iron formation is below water level, there is no reason why water circulation should be more vigorous along the troug'hs than along the arches of the gentle folds, provided the openings in each case are equal and the points of escape at equal elevations. If, in the case of drawing- off water from the edge of a basin, above cited, we assume the bottom of the basin to be gently flexed instead of flat, it is apparent that the circula- tion will be more vigorous near the top of the basin than below, and at the l^ottom there will be practically no difference in the circulation over the arches and the circulation over the troughs so far as both are below water level. Thus in the iron formation below the level of ground water, while the impervious layers limit the circulation below, their shape has no effect in concentrating the flow. The distriljutiou a,nd shapes of tlie iron deposits and their relations to ORIGIN OF THE IRON ORES. 269 the adjacent rock strata are fully in accord with the idea that they have been localized by the circulation of water just described. The history of their localization, it is believed, was as follows: When the formation was first exposed to surface alteration meteoric watei's began to enter the formation. This was probably in earh^ Cambrian time, long before the Glacial epoch, and the level of the ground water probably nowhere came to the rock surface. Indeed, there must have been over the exposed area of the iron formation a belt of weathering above the level of ground water, comparable in thickness with that now observed on the upper slopes of the Giants i-ange where the drift covering is not thick. It is not uncommon to find here a belt of weathering as much as 100 feet thick, sufficient to include the greater bulk of the iron-ore deposits of the range at the present time. In the belt of weathering the concentration of the ores, following the circiilation of the water, was controlled in its major distribution by the broad, gentle folding of the formation, but locally was controlled by cross fractures. The ores were developed along irregular and ramifying fractures in the broad and gentle synclines, and not uncommonly several more or less independent deposits were developed in the same syncline. As noted on a prior page, under these circumstances the occurrence of the ores in synclines is frequently not directly shown by the attitude of the iron- formation layers immediately adjacent to the deposits, for the fractures along which the ores have developed may cross any part of the syncline and thus intersect layers with almost any attitude. Alteration once begun in any area, the abstraction of the silica made the rocks porous and tended to confine further circulation and consequent alteration to the same area. That the concentration of the ore deposits has occurred along cross fractures is shown not only by their distribution and shape, but by the fact that the contacts of the ore with the wall rock or with horses of rock within the deposit are plane surfaces intersecting at various angles, and when the ore has been removed the rocks stand out in castellated forms. The verj' existence of horses of rock in the ore deposit is evidence of the concentration along cross fractures which leave intermediate rock masses. Still further showing the control of circulation by fractures is the fact that many hand specimens may be collected showing plane surfaces with alteration extending a little way in from the surface, indicating' the agent 270 THE MESABl IRON-BEARING DISTRICT. of the alteration — water — to have followed this surface. In general, the expression sometimes used locally on the range that the ore results from "rotting" along fractures may be close to the truth, although requiring certain modifications, as above indicated. When the water passed below the level of ground water it continued its work of alteration, though perhaps not so vigorously because of its depletion in oxygen and carbon dioxide. But while the concentration was limited below by the slate layers of the iron foriiiation, the concentration of the ores was not necessarily confined to openings in the broad synclines. In this part of its course the water concentrated ores along the largest and most continuous openings and near the lowest points of escape, regardless of the shape of the impervious basement. But in so far as the lowest point of escape was determined primarily by the folding, as it doubtless was to a great extent, the concentration occurred along synclines in the impervious Boundaries of /ron formation Erosion surface ■ ore deposit ° ?520 10eO_ Fig. 12.— Sketch shoiving three stages in the downward and lateral migration of an ore deposit due to the truncation of the iron formation by erosioji. basement. This is illustrated by the Biwabik mine. The ore deposit is in general parallel to the strike of the iron-formation strata, liut its thickest portion may be seen to lie near the lower point in the paint rock (the altered equivalent of the Virginia slate at this point) which may be seen to overlie the ore body. Also exploration to the south shows the slate to be cut back just opposite the "belly" of the deposit. As the process of erosion continued the surface of the iron formation was cut down, and this was accompanied by a migration downward of the le^•el of ground water, with the consequent emergence above this level of the part of the iron formation which had heretofore been below ground water. Any concentration which had occurred before this part of the formation came above water level was probably continued in the same areas, for such areas were rendered more jiorous by the alteration already undergone, but any new concentration was along the impervious basements of pitching troughs. The erosion of the surface of the iron formation resulted in the lateral ORIGIN OF THE IRON ORES. 271 downward migration of the ore deposits. As the ore was cut off above, the development of new ore continued downward along the dip, its lowest limit always marked by an impervious stratum. Tliree hypothetical stages of the development of the ore dejDOsit and its lateral migration are shown by fig. 12 (p. 270). It is apparent from this that the present state of affairs is merely a stage in a contii:iuous process of concentration and migration, which is continuing to-day as it has in the past. There is evidence that the area, before Grlacial time, may have been near the sea level in two periods, the Cambrian and Cretaceous. A.t these periods the level of ground water must have been near the rock surface; the circulation was feeble and the concentration of the ore was slow and possibly ceased. At intermediate and subsequent times the level of ground water may be supposed to have been some distance beneath the surface. The circulation was then vigorous and the development going on. In Grlacial times a considerable portion of the upper part of the forma- tion was scraped off. This included a large part of the formation which had been above the level of ground water, and thus many of the ore deposits and parts of ore deposits which had been concentrated in this belt. Associated with the glacial cutting was the deposition of a thick mantle of glacial drift, into which the ground water worked up. Since glacial time a large part of the iron formation has been below the level of ground water. In summary, the localization of the ores through the circulation of underground water has been controlled primarily by the broad, shallow synclines" into which the iron-formation layers have been folded, but other factors have greatly modified this control and locally have been dominant. Of the modifying factors the principal one has been the cross fracturing of the iron formation, yielding openings through which the waters have flowed in devious paths and causing the concentration of the ores in limited and irregular areas within the synclines. Of scarcely less importance have been the little fractured and relatively impervious slate layers within the iron formation and the Virginia slate above the iron formation, limiting the cir- culation above and below, ponding the underground water, causing lateral « Such synclines are not necessarily surface troughs, aa sometimes assumed by explorers. They are evidenced by the attitudes of the layers of the iron formation, and may not be apparent in the unequally eroded rock surface or at the surface of the irregular covering of glacial drift. 272 THE MESABI IRON-BEARING DISTRICT. movements toward the lowest points of overflow, and finally causing the flow to be not necessarily confined to synclines but in some cases to be vigorous over anticlines, all combining- to explain the concentration of iron- ore deposits with greater dimensions parallel to the strike of the iron- formation layers (parallel to the trend of the range), and deposits apparently independent of any synclinal structure in the iron formation. EXPLAKATIOIS" OF THE APPARENT ABSEKCE OF ORE DEPOSITS AT THE EAST EXD OF THE RANGE. Eastward from Mesaba station in the proximity of the Duluth gabbro the iron formation has no known ore deposits. The iron oxide present is mainly magnetite instead of hydrated hematite, and the asso- ciated minerals, aside from the chert, are characteristically monoclinic amphibole. These conditions indicate a history for this part of tlie district somewhat different from that above described for the part bearing the ore deposits. The iron formation in the east end of the district was originally similar in character to that in the western and central portions of the district, though perhaps containing a little more intercalated slate. While the iron forma- tion of the central and western portions of the district probably was not exposed to surface alteration until post-Keweeuawan time, in the' eastern end of the district the iron-formation rocks were exposed by the removal of the overlying .slate before Keweenawan time. In Keweenawan time the eastern end of the district, and perhaps the central portion, were buried beneath Keweenawan rocks, principal among which was the Duluth gabbro mass of northeastern iMinnesota." In the east end of the district the gabbro came to rest on the eroded edges of the iron formation and slate. In the central and western ends of the district the gabbro was separated from the iron formation by a vast thickness of slate which up to that time had not ^:■euce might be taken as evidence that the iron formation had not been exposed at the east end of the district prior to the intrusion of the gabbro. This evidence, however, it is believed will not stand against the evidence of the structural relations of the gabbro to the iron-formation rocks described on pp. 197-198. The lack of jaspers may rather indicate that the alteration under surface conditions prior to Keweenawan time haJD Qiiaiior-eecflon and sixtoenth linua 0 L O Q "U E T M I N B T.SeN R. 17W. SPRUCE 31 I N I N O ECONOMIC FEATURES. 283 is planked. As soon as one level or slice has been removed the surface or glacial drift is allowed to fall in or is blasted in, being kept from mixing with the ore by the planking at the bottom of the first level. Then another slice is taken out below on a sublevel, the planldng- and the surface allowed to fall in, as before, and so on, always working down. In the rooming or square-set method, rooms, consisting of three or more sets from 20 to 40 feet wide, are run up from the main drift to the top of the deposit, the sides being lagged. Pillars of ore of about the same size as the rooms are left. The glacial drift is allowed to fall in from above, filling the rooms. The intervening pillars are then taken out by slicing either froxn the top or bottom. Tramming is done mainly by hand or with mules, although of late electric haulas'e has bearun to be introduced. COMPARISON OF METHODS OF MINING. A discussion of costs is out of place in a report the subject-matter of which is mainly geologic; but even from the above cursoiy description of mine methods it must be apparent that in general the open-pit method of mining is far the cheapest of the methods employed; that the underground methods are the most expensive; and that the milling- methods are interme- diate in cost. The cheapness of the open-pit shovel method as compared with the underground mining is due to the large production possible, to the fact that timbering is not necessary, fewer men are required, lighting expense is less, all the ore can be moved (while in underground methods perhaps 10 per cent is lost), the ore can be better sorted, it has to be handled but once, tramming cost is saved, and the hoisting is by locomotive on a grade rather than through a shaft. If open-pit steam-shovel mining is so much cheaper than the under- ground methods, the question is often asked why any of the Mesabi ores are mined by any other method. In order that a deposit may be worked satisfactorily with the open-pit steam-shovel method it must have considerable horizontal extent as compared with its length in order to afford proper grades to the tracks. The deposit must lie in a position to allow of an easy grade to the approach; this condition is met where a deposit is on a side slope. The thickness of the drift to be removed must 284 THE MESABI IRON-BEARING DISTRICT. not be too great, for otherwise the cost of stripping will run up the total cost of mining. There must be available gTound with easy grades on which to deposit the dirt stripped off from the ore body. If there are capping layers of poor or unsalable ores these must be removed before the good ore can be mined. Or if it is desired to remove interstratified layers of good and poor ore independently it is not practicable by this method. Finally, the annual production must be large. Without a large annual production the interest on the preliminary investment for stripping, rolling- stock, etc., necessary before a pound of ore can be moved runs the price of ore per ton up to a high figure. This preliminary investment is in most cases large. On the other hand, when a deposit is opened up by an underground method there is little preliminary investment; no g'reat mass of stripping has to be removed and disposed of; no layers of poor ore have to be removed before the good ore can be reached; the accessibility of all parts of the deposit does not depend on grade; the mine can be worked all the year round; and finally, the ore taken out while the mine is being opened goes to defra}^ current expenses and to pay interest on the investment. Thus it is that while, where conditions allow it, open-cut mining costs less than half that of underground mining, in many cases it is still advisable to use underground methods. The milling method is a combination of the open-cut and undergi-ound methods, and combines some of the advantages and disadvantages of Ixith. It costs less than the underground methods of slicing and caving, liecause the timbering is less and all of the ore is saved, but it usually costs more than the open- cut steam-shovel method because of the shafts, drifts, uprises, and the tramming and hoisting. The recent tendency has been greatly to increase the use of the open- pit steam-shovel method of mining. More of the new mines are opened in this way than formerly, and several of the mines whicli liave in the past used underground methods will produce their ore by steam shovel in the future. There is also appearing a marked change in the policy of conserv- ing ores. In the past it has often been the practice, because of market conditions or because of desire for immediate large profits, to take out high- grade ores finding ready sale wherever they were found, without regard to the grades that were left. Tlie better, and fm- tlie most part tlie later, U. S. GEOLOGICAL SURVEV MONOGRAPH XLIII PL. XXlV ,1. RAILWAY CUT IN APPROACH TO OLIVER MINE, VIRGINIA, Shows close jointing and brittle nature of the iron-bearing formation, The rock is a slaty phase of the ferruginous chert. B. PRELIMINARY STRIPPING AT OLIVER MINE, VIRGINIA. U. S. GEOLOGICAL SURVEY MONOGRAPH XLIII PL. XX\/ hkMiMM^^^^ VIEWS OF OLIVER MINE, VIRGINIA, IN 1900. A, Looking west; S, Looking eist. ECONOMIC FEATURES. 285 practice has been to determine well in advance, in some cases even before anv mining has been done, the grades of ore and their distribution for the entire deposit, and in the mining to make such selection and combination of these grades as to leave the lo\Yest surplus of undesirable ores. The mining of the ore is planned to the end, as in building a structure, and is not influ- enced so largely as formei'ly by temporary conditions of market or man- agement. The change is possible lai-gely because of the new conditions of ownership whereby the control of the mines is in the hands of a few large steel interests. TRAlS^SPOETATIOiy. The Mesabi iron ores are transported to Lake Superior by rail, and thence by lake to terminal lake ports. The average rail haul is about 75 miles. Three i-ailways can-y the ore, the Duluth, Missabe and Northern, the Duluth and Iron Range, and the Eastern Railway of Minnesota. The Duluth and Iron Range Railway carried the ore from the follow- ing mines shipping ore in 1902: Auburn, Fayal, Elba, Genoa, Sparta, Roberts, Hale, Malta, Kanawha, Spruce, Union, Franklin, Corsica, Minorca, Bessemer, Pettit, La Belle, Section 33, Victoria, and Wills. The ore is delivered to Lake Superior docks at Two Harbors, Minn. The docks are five in number, with 776 pockets and storage capacity of 162,040 tons. The same railway also carries the ore for the Vermilion iron range, and the docks serve for both ranges. The Duluth, Missabe and Northern Railway carried the ore from the following mines: Adams, Duluth, Pillsbury, Sellers, Burt, Oliver, Day, Hull, Rust, Biwabik, Mountain Iron, Glenn, Lincoln, and Spruce. The ore is delivered to Lake Superior docks at West Duluth, Minn. These docks are three in number, with 960 pockets and storage capacity of 167,040 tons (PL XXXIII, B). The Eastern Railway of Minnesota hauled ore from the following mines: Chisholm, Clark, Commodore, Mahoning, Penobscot, Sauntry, Stevenson, Alpena, Sharon, Grant, Pearce, Jordan, Longyear, Agnew, Morrow, Croxton, Utica, Laura, Hawkins, and Leetonia. The ore is delivered to Lake Superior docks at Superior, Wis., two in number, with 500 pockets and a storage capacity of 103,000 tons. A third dock is in process of construction. 28Q THE MESABI IRON-BEARING DISTRICT. " The tonnage of ore from the Mesabi range carried by the three railways in 1902 is as follows: Ore carried from Mesabi district hy different railways. Duluth and Iron Kange Duluth, Missabe and Northern. Eastern Minnesota Total 1902. 3, 538, 978 5, 610, 407 4, 180, 568 13, 329, 953 In density of traffic and freight earnings per mile of track there are but half a dozen roads in the United States which compare with these three railways, a fact brought out in the famous ore-rate case in which the State of Minnesota has made an unsuccessful attempt to have the rate on ore reduced. The rate from the Mesabi range to Lake Superior is uniformly 80 cents per ton. The Duluth and Iron Range and the Duluth, Missabe and Northern railways and the docks at their termini are owned by the United States Steel Corporation. About 79 per cent of the ore which these roads; hauled in 1902 was owned by the same corporation. The Eastern Railway of Minnesota, as already noted, is a part of the Great Northern system. None of the ore earned by this road was mined by the Great Northern, iDut by traffic contracts and fees it controlled 70 per cent of the ore it carried. The ore has in the past been largely carried in wooden cars with a capacity of 25 to 35 tons. Within the last two years, however, steel cars of 50 tons capacity have been extensively introduced. Revenue loads for single trains in 1902 averaged 1,400 to 1,730 long tons for the three railways, and the loads for double headers on the Duluth and Iron Range Railway averaged about 2,100 tons. From Lake Superior docks the ore i > carried in vessels to Lake Erie jjorts, or to Chicago and Milwaukee. The rate per ton has, since the opening of the Mesabi range, varied between 57 cents and $1.29.1. The rate for 1902 was 75 cents. The larger interests in the Mesabi district control their own lake steamers. The largest fleet, of course, is that of the United States Steel Corporation, which numbered 112 vessels in 1902, enabling this comjianyto carry aljout 63 i^er cent of its own oi'e for that year. U. S. GEOLOGICAL SURVEY MONOGRAPH XLIII PL. XXVI VIEWS OF MOUNTAIN IRON MINE. A, Looking north through mine; B, Sieam shovel " bucking " bank of ore. U. S. GEOLOGICAL SUftVEY A. SHARON MINE, SHOWING STRIPPING OPERATIONS. «" - 4^^ '*<^*, * '*-- M, a. AUBURN MINE OPEN PIT AND SHAFT. ECONOMIC FEATURES. 287 PRODUCTION. The following table, taken (except for 1902) from the Iron Trade Review, shows the shipment, in gross tons, of iron ore in the Mesabi district since its opening in 1892: Prochiction in Mesabi district. Name of mine. 1892. 1893. 1894. 1895. 1896. 1897. Adams 59, 141 234, 562 17, 723 131,478 242, 565 , 16,261 57, 324 96, 280 22, 063 170 738 Aetna (Lowmore) ' 1,645 Auburn 108, 210 90, 048 213. 853 376, 970 247, 069 359, 020 17, 187 47, 700 175 263 Biwabik 151,500 24,416 26, 372 427 464 Canton Cincinnati 32, 912 12, 215 Cloquet ( Vega) 5,628 7,213 65, 137 37, 626 60, 798 Duluth Fayal 136, 601 286, 423 248, 645 231, 086 17, 136 70, 006 67, 659 167, 245 642, 939 30, 128 Franklin 46,617 223, 399 Genoa 309, 514 13 728 Hale 3,616 24, 167 31, 004 58, 123 117, 884 Lake Superior Group . 259 912 Mahoning .... 519 892 Minnewas 13, 858 119, 818 2,140 573, 440 38, 999 Mountain Iron ( and Kath ) 4,245 371, 274 93, 392 28, 943 500, 377 142, 021 77, 523 69, 925 808, 291 773,538 101, 077 Ohio 47, 350 OUver (1) 123, 015 505, 955 601, 072 Penobscot 11, 933 Roberts 18, 614 Sellers 47, 433 153, 037 Sparta .- 66, 722 Williams (North Cincinnati) . 3,046 11, 249 Total 4,245 613, 620 1,793,052 2, 781, 587 2, 882, 079 4, 275, 809 Name of mine. 189S. 1899. 1900. 1901. 1902. Totals. Adams 390, 860 720, 474 777, 346 829, 118 1, 242, 923 4, 425, 162 19, 368 Aetna (Lowmore ) Agnew 45, 582 38, 283 623, 128 45 582 Auburn 235, 630 385, 992 553, 836 99, 498 263. 69^ 427,510 410, 074 2, 143, 028 4, 053. 732 713,048 Biwabik Canton 383, 180 924, 868 :^88' THE MESABI IRON-BEARING DISTRICT. Production in Mesabi district — -Continued. Name of mine. isas. 1899. 1900. 1901. 1902. Totals. 34,573 200, 629 235. 202 Pincinnati 246 134 613 163 15 768 86 18 127 744 570 7,505 1,370 1,868 23 51 279 5 147 145 70 2,320 3 28 87 22 4,783 413 15 35 35 7,462 421 714 3,143 54 704 17 666 190 041 Clark 63, 071 199, 566 350, 799 436 1,621 444 Columbia .... 15, 627 35, 546 26, 838 697 Commodore . 80, 494 152, 947 278, 416 65, 833 59, 294 18, 594 106, 516 150, 220 214, 447 1,919,173 84, 534 399, 719 23, 875 51, 946 54, 289 5,892 147, 931 24, 830 70, 753 766, 312 3,575 28, 785 87, 779 22, 735 1, 030, 143 192, 874 447 Corsica 13? Croxton 594 Dav 18,651 112, 155 564 575, 933 200, 400 279, 677 1,975 165, 435 9; 547 1, 072, 257 60, 000 276, 559 14'> Duluth 128, 587 121, 707 1, 252, 504 168, 524 253, 651 150, 024 224, 630 1, 656, 973 39, 299 332, 022 047 Elba 895 Fayal 0''5 410 Genoa ''78 Glenn 875 Grant 946 Hale 18, 807 32, 901 30, 929 447 Hawkins S9'> Jordan 931 14, 963 64,218 41, 300 sn La Belle 753 Lake Superior Group 135, 404 154, 326 284, 023 594, 761 .5''0 Laura 575 Leetonia 785 Lincoln 779 Longvear 735 .Mahoninc; 520, 751 750, 341 28, 615 911,021 65, 346 765, 872 126, 299 149 Malta 134 Miniiewas 998 Minorca 35, 500 35, 572 1, 629, 576 500 Morrow . ; 57-> Mountain Iron (and Rath)... Norman 650, 955 110, 141 101,607 349, 100 1,137,970 1, 001, 324 1,058,160 321 13'' Ohio 293, 651 172,597 244, 876 073 Oliver (1) 5,420 5,131 54, 885 209, 431 17, 278 238, 122 28, 972 249, 837 4, 213 ■'37 Piarce 885 rcnohncot 29, 652 85, 619 146, 641 221,080 356 I'cttit •'7S I'illsl)urv 99, 691 106, 487 57, S47 53, 004 101,032 41,965 (iS, 560 120, 723 42, 756 328, 739 0^15 KiLcrtH 1R4 Samitrv-Alpena 700, 140 S •(•. -.VA 4,213 ECONOMIC FEATURES. Production in Mesahi district — Continued. 289 Name oi mine. 1898. 1899. 1900. 1901. 1902. Totals. Sellers Sharon 112, 765 174, 867 56, 280 34, 918 56, 810 156, 426 279, 515 666, 273 93, 109 193, 428 224, 526 252, 674 543, 397 1, 424, 864 103, 521 9,009 26, 465 772, 728 281 336 Sparta Spruce 226, 156 237, 143 202, 144 101, 675 56, 031 8,297 1, 141, 265 924, 587 Stevenson 2, 147, 168 Union . 204, 927 9,009 26, 465 44, 890 12, 159 Utica Victoria ! Williams ( North Cincinnati ) . 12, 357 18, 238 Wills 12, 159 Total 4, 613, 766 6, 626, 384 7, 809, 535 9, 004, 890 13, 329, 953 53, 734, 920 In 1891 the district had uot been opened up. In 1895 the Mesabi dis- trict became the largest producer in the Lake Superior region, that year passing the Marquette district which, since 1854, had held first place. In 1901 the total production of the Mesabi district was 44 per cent of that of the Lake Superior region, and two and one-half times as much as its near- est competitor, the Menominee, which in 1901 for the first time surpassed the Marquette district in production. The combined production of four of the largest producers in the Mesabi district for 1901, the Mountain Iron, Fayal, Adams, and Mahoning- mines,, was greater than the total for the entire Menominee district for the same year. Beginning in 1899 the Fayal and Mountain Iron mines have shipped over a million tons each year, a rec- ord which has been made by no other mine in the world. The Fayal ship- ment in 1901 was 1,656,973 tons, an amount almost as great as the shipment for the entire Vermilion range for the same year. Largely on account of the Mesabi production the State of Minnesota in 1901 passed the State of Michigan as the larg-est iron producer in the United States. In total shipment the Mesabi district is still behind the Marquette district. At the close of 1901 the Mesabi district had shipped a total of 40,404,967 tons, while the Marquette district, open since 1854, had shipped 62,847,473 tons. Comparing the Mesabi shipment for 1901 with that of the United States, it appears that the Mesabi district shipped 33 per cent of the total. MON XLIII — 03 19 290 THE MESABI IRON-BEARING DISTRICT. In 1902 the Mesabi sliipments showed an increase of 41 per cent over its own sliipment for tlie preceding year and constituted 49 per cent of the total Lake Superior shipments for that year. An examination of the general map of the Mesabi district (PI. II) shows that there are many iron-ore deposits on the range which have not yet shipped ore. This is because of their grade or because of their late discovery or because they ai-e controlled by companies wliicli have enough ore for present shipments in properties alread}' opened up. By leaving tlie ore untouched, taxes and the interest on the large investment necessary to open up a mine are saved. Other mines liave been opened up and but small shipments made because more desirable ores or more cheaply mined ores were controlled by the same company in other mines. With the mines already opened up, including the steam-shovel mines, the annual production of the district could be enormously increased without opening any more deposits. RESERVE TONNAGE. The tonnage of individual projjerties ranges from a few thousand tons up to a possible maximum of 70,000,000 tons. Several deposits are known to have between 20,000,000 and 40,000,000 tons of ore. The total amount of ore of present marketable grade — that is, containing above, per- haps, 58 or 59 per cent of metallic iron — at present in sight on the Mesabi range has been estimated at 500,000,000 to 700,000,000 tons. Six hundred million tons is a commonly accepted figure. Of this, with proper mixing, perhaps 60 to 70 per cent is Bessemer ore. These figures are necessarily based on incomplete data, but they are commonly accepted by those best qualified to judge. Ore running below 58 or 59 per cent in metallic iron is known to be present in enormous quantity, but the amount lias not been estimated nor are the data for an estimate likely to be available for some time to come. Within recent years steps have been taken to reserve the best of it. Ores running even as low. as 50 per cent, while not exploited, are being put aside by the large companies for future use. At the mines where it has been found necessary to move low-grade ore in order to get at lilgher-grade ores, the low-grade ores are in some cases being stock piled where this can be done at a small cost. The aggregate amount of high-grade ore in sight up to 1902 on all the "old ranges" of the Lake Superior region li;is Ix'cn thought not greatl}' to ECONOMIC FEATURES. 291 exceed 350,000,000 tons. Even if the estimates for the Mesabi range and the old ranges are considerably away from the truth, it is certain that the Mesabi holds a commanding position in the region in its reserve tonnage. When it is remembered that even before the discoveiy of the Mesabi ores the Lake Superior region was regarded as the richest iron-ore-bearing region in the world, it is apparent that the Mesabi district has no peer. OWNERSHIP AND COKTROL. On the general map of the range are indicated the principal mining properties known up to the time the map was submitted to the printer. This information is based partly on a list and description of properties prepared by Mr. J. H. Gruber, land agent of the Eastern Railway of Minnesota, for property owners in the Mesabi district, to aid in the appor- tionment of taxes, but shows many subsequent additions and changes. The large amount of exploration and the rapidity with which discoveries of ore have followed one anotlier make it certain that before the map comes from the printers iron-ore properties other than those indicated will be known. Moreover, properties are rapidly changing hands and the names of the mines are being changed, with the resiilt that some of the names shown on the map will be superseded. The name of the mine or lease rather than the name of the owners is g-iven whenever a name has been assigned. It has been the aim to include only such parcels of land as actually contained ore, but it has not been possible consistently to follow this procedure. It seems best in some cases to include the entire block of land covered by a well-known lease; for instance, the Kanawha mine is shown on the map to cover four forties, while only the southerly forties contain the ore. For the sake of convenient reference by mining men, the sections have been divided into forties. Where quarter posts have been found, subdivision has been based on their location. Where not found, the sections have been proportionately divided. In this connection it may be noted that the subdivisions of the sections in the vicinity of Virginia and Eveleth and thence eastward to Mesaba station have been made by Mr. D. L. Fairchild for the Minnesota Iron Company and kindly furnished us for use on the accompanying map. 292 THE MESABI IRON-BEARING DISTRICT. To give a full and accurate list of the owners of the iron-ore proper- ties would require an amount of labor which is scarcely warranted by the scope of this report. Moreover, the list would be out of date at a number of points before the books come from the printer, so I'apidly are the newer properties changing hands But in order to show the extent to which the control of the ore is concentrated into a few hands, the names of the com- panies or individuals controlling the shipments for 1902, and the mines operated by them, are given below : United States Steel Corporation: Adams, Glenn, Auburn, Chisholm, Clark, Day, Duluth, Faval, Genoa, Lake Superior group. Mountain Iron (and Rathbone), Oliver, Pillsbury, Sauntry -Alpena, Sellers, Spruce. In 1902 the United States Steel Corporation mined about 60 per cent of the total Mesabi production. Republic Iron and Steel Company: Franklin, Union, Victoria, Pettit, Wills. Pickauds Mather Company: Elba, Corsica, Minorca, Utica, Malta (with St. Clair). Interstate Mining Company (Jones & Laughlin) : Lincoln, Grant. Donora Mining Company (Union Steel Company) : Penobscot, Sharon. Joseph Sellwood: Hale, Kanawha, Croxton, Leetonia, Longyear, Morrow, Pearce. G. A. St. Clair: Malta (with P. M. Co.), Sparta, Sec. 33. Corrigan and McKinney: Stevenson, Commodore, Jordan. Todd, Stambaugh and Co.: Mahoning. Biwabik Mining Company: Biwabik. Fay Exploration Company: Laura. Drake, Bartwell and Co.: Roberts. Deering Harvester Company: Agnew, Hawkins. In most of the properties other parties are concerned, either in the fee or lease, l)ut not directly in the operation of the mine. #i I eT* :if ECONOMIC FEATURES. 293 PRICE OF MESABI ORBS IN C0MPARIS0:N^ AVITH OLD RA^STGE ORES. Below are listed the prices of Lake Superior ores at terminal lake ports from 1891 to 1901, quoted from figures given by A. I. Findley, editor of the Iron Trade Review: Prices of Lcike Siqjerior ores. Grade. 1891. i 1892. 1 1893. 1894. Mesabi Bessemer 12. 25 to %2. 65 Mesabi non-Bessemer 1.85 Marquette specular No. 1 Bessemer Marquette specular No. 1 non-Bessemer. Chapin S5.00 4.25 3.75 4.75 5.50 S5.00 4.25 3.75 4.50 5.65 S4.00 3.65 3.25 4.00 4.50 2.90 2.50 Soft hematites, No. 1 non-Bessemer Gogebic, Marquette, and Menominee No. 1 Bessemer hematites 2.25 2 75 Minnesota No. 1 hard Bessemer Vermilion No. 1 hard Bessemer 3.35 Minnesota No. 1 hard non-Bessemer . 4.85 4.85 4. 00 3.90 4.65 4.50 3 00 Chandler No. 1 Bessemer 2 95 Marquette extra low-phosphorus Besse- 6.00 5.50 3 50 Republic and Champion No. 1 5. .50 3 25 Grade. 1895. 1896. 1897. 1898. Mesabi Bessemer S2 25 to S2 70 §3. 25 to §3. 75 2.40 4.50 3.00 3.65 2.60 4.00 4. 55 §2. 10 to 12. 30 1.80 to 2.00 2. 80 to 3.10 2.45 2.40 2.25 2. 65 to 2.85 3.11 ?2. 15 to §2. 25 1 70 to 1 85 1.90 Marquette specular No. 1 Bessemer 3 10 to 3 35 Marquette specular No. 1 non-Bessemer. Chapin 2.75 2.55 2.25 2.90 3.40 2. 35 to 2.45 9 5g. Soft hematites, No. 1 non-Bessemer Gogebic, Marquette, and Menominee 2.20 2 75 to 2 95> Minnesota No. I hard Bessemer Vermilion No. 1 hard Bessemer 3.36- Minnesota No. 1 hard non-Bessemer Chandler No. 1 Bessemer 3.00 3.05 3.55 3.30 3.25 - 4.25 4.90 4.50 2.65 2.92J 3. 42 to 3.46 2.55' 3 13; ^larquette extra low-phosphorus Besse- mer 3 65' Republic and Champion No. 1 294 THE MESABI IRON-BEARING DISTRICT. Prices of Lake Superior ores — Continued. Grade. Metabi Bessemer Mesabi non-Bessemer Marquette specular No. 1 Bessemer Marquette specular No. 1 non-Bessemer. Chapin Soft hematites, No. 1 non-Bessemer Gogebic, Marquette, and Menominee No. 1 Bessemer hematites Minnesota No. 1 hard Bessemer Vermilion No. 1 hard Bessemer Minnesota No. 1 hard non-Bessemer Chandler No. 1 Bessemer Marquette extra low-phosphorus Besse- mer • Republic and Champion ,No. 1 1899. S2. 2.5 to ] . 90 to 3. 21 to S2.40 2.10 3. .50 2. .50 2. 73i 2. 00 to 2.15 2. 80 to 3. 25 2.65 3.35 3. 85 to 3. 90 1900. $4. 40 to 84. 90 4. 00 to 4.25 5. 93 to 6.48 5.00 4.96 4. 15 to 4. 25 5. 50 to 5. 75 1901. 5.10 6.00 6. 80 to 6.90 82. 75 to S3. 00 2. 35 to 2. 65 4. 66 to 4.92 3. 65 to 3.85 3.78 2. 85 to 3. 15 4. 25 to 4. 65 4.08 4.62 5. 65 to 5. 75 FURNACE USE OF MESABI ORES. When Mesabi iron ores first came on the market it was not found practicable to use them to a greater extent than 33 per cent of the furnace charges, for the reason that, because of their soft character, they packed in the furnace. The term "flue dust" was commonly applied to them. Since that time the percentage used in the furnaces has steadily increased until in 1902 the average furnace mixture contained 49 per cent of Mesabi iron ore and 51 per cent of old range ores. In other words, they are mixed on an average in about the proportion of their production. Individual ores are used in proportions ranging from 35 to 100 per cent of the ore burden. It is not unreasonable to expect that, considering the relative abundance of the Mesabi and the old range ores available, the percentage of i\lesal)i ores used in furnace charges will slowly increase in the future as it has in the past. Tliis change results in part from the present tendency to decrease tlie heiffilt of furnaces from tlie maximuni sizes reached in late A^ears, thus giving the ore less opportunity t( > pack. U. S. GEOLOGICAL SURVEY MONOGRAPH XLIII PL. XXX VIEWS OF FAYAL MINE. A. Mills; B, Milling with steam shovel. U. S. GEOLOGICAL SURVEY MONOGRAPH XLtll PL. XXXI A. SAUNTRY MINE, LOOKING NORTH. The layers of ore in both banks can be seen dipping to the west. At the north end of the cut the ore has been cut back to the rock fferru- ginous chert) and it there appears that the rock layers dip westward at the same angle. While the layers of ore and rock may be gently flexed into a great syncline, pitching to the west, this flexure would be likely to be overlooked because of the great extent of the oie deposit along the strike of the monoclinally tilted strata. B. FAYAL MINE, SHOWING STEAM SHOVEL "BUCKING" BANK OF ORE. CHAPTER XL EXPLORATION. Exploration for iron ore in the Mesabi district is partly a matter of chance, as it lunst be in any ore-bearing district and especially in a district heavily covered with glacial drift. The iron ore thns far found in the Mesabi district, however, has been within certain limits. Ore is confined to the Biwabik formation of the Upper Huronian series. The accompanying g-eologic map shows the distiibution of the iron formation as indicated by the facts available n]3 to the time of its pub- lication. Where exploration has been insufficient, as ill the western portion of the district, further work will show chang-es in the iron-formation boundaries. Examination of the drift may help to determine the boundaries m doubtful areas, for it is an observed fact that fragments of the iron formation have not been earned far in great abundance. The drift frag- ments have been carried in one direction, from northeast to southwest, and the discovery of fragments of iron -formation rocks indicates that the iron formation must be either beneath or to the northeast. Magnetic work may also be of assistance in locating the iron formation. In the productive portion of the district the iron formation is essentially nonmagnetic, yet over the area of the formation the magnetic attraction everywhere shows minor disturbance and this disturbance is particularly marked near the northern boundary of the iron formation. The workable deposits thus far discovered are confined to the portion of the iron formation west of Mailman camp, in range 14. East of Mesaba station the oxide is largely magnetite and the associated rocks are actinolitic, griineritic, slaty, and crystalline. Magnetite with such associates has not been found in workable deposits eithei- in the Mesabi district or in the Penokee-Grogebic district, and if the explanation of the origin of magnetites given on preceding pages is correct, there is reason to believe that no large deposits of ore will be found in this area. 295 296 THE MESABI IRON-BEARING DISTRICT. The westernmost ore deposit thus far discovered is in R. 25 W. The ores near this western limit contain abundant particles of silica resulting' from the disintegration of the chert associated with the ore, giving the ores' what is known locally as a "sandy" character. Such ores can be used, however, bv washing. The western limit of the area in which ores may be found in the future is as yet quite unknown. The map of the district accompanying this report covers an area extending only a little way west of Grand Rapids. It is certain that the iron formation extends well westward although deeply covered by glacial drift. Magnetic work and examination of glacial fragments has already shown the extension of the iron-bearing formation for several miles west of the limits of the map. Fig. 8, p. 203, shows the possible westward continuation of the belt and its connection with the Penokee-Gogebic series. If the concentration of the ore deposits is dependent upon vigoroiis circulation of the underground M'ater, it ma}' be suggested that the apparent flattening out of the Giants range toward the west may not give the waters of this area a sufficient head to circulate vigorously through the iron formation and concentrate the ores. The low-grade and sand}' nature of the ores also may be in some way connected with this feature. However, it may be that the rock surface still has a considerable slope which has been covered and obscured by the drift, and if this is the case there is no apparent reason wh}- ore should not be there developed. Cej'tainly exploration for ore is warranted well to the west of Grand Rapids. Unexplored land in the vicinity of known ore deposits is more likely to contain ore than is unexjylored land in the vicinity of areas which have been explored and found barren, for in the former case it is certain that the conditions in the area as a whole are favorable to ore concentration, while in the latter they may or may not be. Applying this principle to the district in general, unexplored land in the central portion of the district is likely to yield larger returns in exploration than unexplored lands in the western and eastern ends of the district. While ore has been found in the western portion of the district it is not nearly so abundant as that found with an equivalent amount of exploration in the central .portion of the district. It has been estimated that about 5 per cent of the area of the iron for]nation in the entire district is underlain b}?- iron ore, while for the central area, between Mesaba station nml tlie Hawkins mint', in R. "22 W., iron ore underlies more thnn 8 per cent ot'tlie surface. U, S. GEOLOGICAL SUHVEV PANORAMIC VIEW OF MAHONING MINE. 0<-C"w'*:- - -*»*■« PANORAMIC VIEW OF BIWABIK MINE. EXPLORATION. 297 The ore deposits have not been found in the portion of the iron forma- tion which runs under the Virginia slate, nor are they Hkely to be found there, for the reasons that the circulation accomplishing the secondarv concentra,tion probably is not vigorous undei- the edge of the slate (see pp. 266-267); that the iron-formation rock there f^und has usuall}^ a fresh unaltered green character; and finally, while the slate has been pierced in a few places by drill holes, no ore has been found any distance under the true black slate, although the same number of holes similarly distributed almost anywhere else in the iron formation has scarcely failed to reveal ore. Attention is called to the fact, however, that the absence of ore under the black slate has not yet been demonstrated by actual drilling. Considering the magnitude of the new fiield opened further drilling seems wai-ranted. Being confined to the part of the iron formation not covered by the Virginia slate, the ore bodies for the most part lie between elevations 1,450 and 1,750 feet above sea level. Most of the iron formation north of the slate is between these elevations, so that this is simply another manner of stating- that the iron ore is in the part of the iron formation not covered by the slate. The iron ore is more abundant about midway between the north and south areal boundaries of the iron formation than elsewhere, as shown by the location of the existing deposits; yet many deposits are known to crowd both the north and south boundaries of the iron foi-mation. The ore deposits ft'equently underlie surface depressions. Their sec- ondary concentration has been largely though not entirely along structural synclines; the leaching out of silica during concentration has caused a decided slump in the deposits; subaerial and glacial erosion has cut down the deposits perhaj^s to a larger extent than the surrounding harder rocks. The heavy and irregular mantle of drift deposited over the area by the glaciers has tended to obscure depressions, but in a large number of cases the underlying rock trough receives some expression in the overlying drift. Surface-drainage lines therefore are excellent areas to prospect. This does not mean that all surface-drainage lines mark the course of ore deposits, for the ores have not developed in all rock synclines, and, moreover, the depression in the glacial drift containing the surface drainage may be quite inde^jendent of the underlying i-ock topogTaphy. Neither is it true that every ore deposit occurs in a depression in the rock surface, for, as shown 298 THE MESABI IRON-BEARING DISTRICT. on pages 269-274, the ores iu developiug along planes of weakness below water level have not necessarily been confined to rock troughs, and often where so confined subsequent erosion may have cut down adjacent areas so irreo-ularly as to leave the ore in higher areas than the surrounding rocks. Northward swings of the northern margin of the iron formation or of the Pokegama quartzite may mark synclinal basins formed by tlie folding of the Upper Huronian series (though they may also mark areas in which erosion has not cut down so deep as in adjacent areas). As the ores commonly develop in synclines, the location of the synclines in this manner may be of assistance. As a matter of fact, however, the position of the northern margin of the iron formation is for the most part known after, and as a result of, exploration in the iron formation. The ore deposits being localized by underground circulation and the underground circulation being limited by slat}' layers, the location of such layers may give some information as to the direction in which the explora- tion should be carried. Because of the dip such slaty rocks may come to the rock surface. It is a fact that there is scarcely a large deposit in the district which does not show layers of paint rock, the altered equivalent of slate, below or above, dividing the deposit, or at any or all horizons. But in a given locality there is difficulty in determining whether the slate is really an interstratified layer in the iron formation, which may be associ- ated with ore, or is the Virginia slate, which is probabl}- not associated with ore. In general the slates within the iron formation are perhaps more jointed and broken up into small parallelopiped blocks than is the Virginia slate; thej^ have a predominance of red and brown tones, due to their large content of iron, as contrasted to gray and black tones in the Virginia slate; they are more siliceous and brittle; and they contain a lower percent- age of alumina. Because discrimination by these criteria is so frequently doubtful, the position of the slate with reference to the supposed iron- formation boundary, or with reference to surrounding explorations showing- iron formation or slate, is likely to be the guiding criterion in determining whether the slate belongs to the iron formation. or to the Virginia slate. Tlie ore deposits have not yet been found to be covered by any con- siderable thickness of barren rock for an}- large proportion of their area, although shelves and irregular masses of rock project from the walls out Ij. S. GEOLOGICAL SURVEY Monograph xliii pl. xxxili A. VIEW OF HALE MINE, SHOWING MONOCLINAL DIP OF STRATA OF ORE AND ROCK. The steeply dipping strata on the south are ferruginous chert. The longer dimensions of the ore deposit are parallel to the ferruginous chert wall. The deposit extends for a considerable distance to the west; its continuation is mined at the Kanawha shaft, to be seen in the distance. The layers of ore are continuous with those of the wall rock to the south, although showing minor disturbances at the contact. It is apparent from this view that the ore is not in a pitching trough formed by the folding of the iron-formation layers, but is really in a long, narrow basin, upon the upper edges of monoclinal tilted strata. B. DULUTH, MISSABE AND NORTHERN ORE DOCKS AT DULUTH. EXPLORATION. 299 over the deposit or into the deposit, or islands of rock may be surrounded above, below, and on the sides by ore. Wherever drills have reached ore in quantity after penetrating any considerable thickness of rock it has been found that the ore appears at the surface a short distance away. Up to the present time thei'e has not been enough deep exploration in rock to pi'ove the nonexistence of ore in bodies entirely covered b}^ rock and nowhere reaching to the surface. In view of the fact that all ore thus far known is only very locally and very partially covered by rock, shallow exploration over wide areas is likely to show the greatest percentage of finds, but deep drilling is also warranted. In the future many areas in which there has been shallow exploration will need to be explored deeply to prove whether or not ore actually occurs beneath them. The ore deposits are associated with characteristic altered varieties of the iron formation which are familiar to all who have worked long with the ores. Nearly every mining man or explorer has in mind certain phases of the ferruginous chert which he is accustomed to associate with ore deposits. Pis. X and XI show several of the phases frequently associated with ore. On the other hand, other phases of the iron formation are seldom found in association with ore, and thus are avoided in exploration. Fig's. A and B, Pis. VIII and XII, represent certain of these phases. Further dis- criminations are made by Mesabi explorers, but they are not described because of doubt as to their general application. The question has been asked, "What are the chances of finding ore in the Lower Huronian and Archean rocks!" Iron ore is known in both series in other districts of the Lake Superior region, and there is no a priori reason why ore should not be found in either or both in the Mesabi district. How- ever, both series are fairly well exposed, have been thoroughly examined, and not only has no iron ore been foiind, beyond a few ferruginous dis- colorations in the hornblende-schists, but no iron-formation rocks have been discovered. Stray fragments in the conglomerate at the base of the Upper Huronian show that iron-formation rocks were present in the Lower Huronian and Archean areas from which the fragments of the conglomerate were derived, but they may have come from a considerable distance outside the Mesabi district, perhaps from the Vermilion. Exploration should be governed by the above conditions so far as they are known, but it is never possible to know all of these conditions in 300 THE MESABI IRON-BEARING DISTRICT. advance of exploration. When the district was first explored none of them were known. To-day but a part of them are available for locating explora- tion, but all of them niay be of assistance in interpreting the facts brought to light after exploration has once begun. As a matter of practice but little exploration has been done in Avhich all these criteria have been taken advantage of, particularly the criteria developed from the flowage of ground water. In most of the cases a part of them have been used, but they have been subordinate to other conditions — the lands available for explora- tion at the time, favorable terms, etc. The unit of land transfers is one- sixteenth of a section, or 40 acres, and it has been a very common practice to put the holes down systematically over the "forty" regardless of any criteria for closer location of the ore which might have been present. Exploration is done in the Mesabi district b^s' test pitting with pick and shovel, by churn drilling-, and b}" diamond drilling-. In the early days the work was done almost entirely by test pitting-. In later years drilling has come commonly into use. In the year 1902, 200 drills, diamond drills and churn drills, were continuously in use in the Mesabi district. A hole is put down entirely by drilling or by test pitting until water or rock is struck and then by drilling below. Because of the fairly soft character of the formations churn drilling, in which the cutting is done by percussion, is more common than diamond di'illing, in which the cutting is done by rotation of a steel bit with diamonds set in its periphery. The cost of test pitting since the opening of the range has varied from Si. 25 to $3 per foot; $1.25 is the present price. The cost of drilling has ranged from S2 to $3.50 per foot for ore and from $4 to $7 for rock, depending upon the nature of the ore or rock. The higher prices are the later ones. E. J. Long-year has called attention to errors in determining the true composition of an ore by drilling. The choppings of the drill are brought to the surface by water forced through casing pipes, are allowed to settle, are di-ied, and then analyzed. If the ore is not allowed to settle for a considerable length of time the lighter materials associated with the ore are likely to be retained in suspension, and the analyses of the ore therefore show a higher percentage of ore and phos])horus than they ought to. A mixture of blue ore, brown ore, yellow ore, and paint rock was analyzed, and found to contain 59.23 percent of iron and .087 per cent of phosphorus. When treated with water and allowed to settle for twenty minutes, dried, EXPLORATION. 301 aud analyzed, the content of iron was 60.80 per cent and the content of phosphorus .094 per cent. When allowed to settle for sixteen hours, dried, and analyzed, the content of iron was found to be 59.67 per cent and that of phosphorus .088 per cent." In no other district of the Lake Superior region, or for that matter of any other region, have the rewards for exploration been so great as in the Mesabi. Since the opening of the range practically every explorer who has gone at the -^ork systematically and on a large enough scale has suc- ceeded in finding ore. a Trans. Am. Inst. Min. Eng., Vol. XXVII, 1898, p. 540. INDEX Actinolite-magnetite-schists from Birch Lake, 43-15. from Penokee-Gogebic district, 43, 44. Actinolite. {See Ampbiboles. ) Adams mine, analyses of iron ores from, 214. contour map of, 282. depth of, 208. ferruginous chert, analyses of, 139-140. milling in, 282. quartzite in Biwabik formation, 154. shipment from, 287, 289. slate in Biwabik formation, analyses of, 144-145. transportation of iron ores, 285. underground mining in, 282. view of, 292, 294. Adularia in iron-ore deposits, 211. .(Etna mine, shipment from, 287. transportation of iron ores, 285. Agnew mine, shipment from, 287. transportation of iron ores, 285. underground mining in, 282. Agriculture of Mesabi district, 22. Aitken, Minn., quartzite near, 203 Akeley Lake, analysis of magnetite from. 221. Animikie of, 45. 52, 54, 159, 201. literature on, 61. relations of gabbro to adjacent formations, 183. Albite in graywacke of Penokee-Gogebic district, 174. in spilosite in Crystal Falls district, 172, 174. Algonkian system of Lake Superior region, 200. Allen, E. T., analyses by, 139-140, 144-145, 156, 264. Allen. James, referred to, 25. , summary of literature, 31. Allen Junction, gabbro near, 36, 182-183. Alpena mine, transportation of iron ores, 285. Alumina in glauconite, 242. in greenalite rocks, 242. h;i iron-ore deposits, 219. in paint rock. 223. American Iron and Steel Association, referred to, 212, 213. Ampbiboles, alteration from greenalite, 102, 107, 117, 237; plate of. 106. in ferruginous chert, 119, 138,141-143, 159. 273; photomi- crograph of, 136. in iron-ore deposits, 211. in quartzite, 92. in slates. 143, 144. Amphibolite of Archean, described, 67. Amphibolitic chert, analyses of, 141. Amygdaloidal texture in basalt, 65. Analyses of amphibolitic cherts, 141. of cordierite hornstone, 172. of ferruginous cherts, 140. Analyses of ferric hydrate, 262. of glauconite, 47, 240. of greenalite rocks. 108, 245-246. of iron ores, 214-217, 221-223, 262. of paint rock, 149-150. of phosphorus in hard and soft iron ores, 220. of quartzites of Biwabik formation, 156. of samples in drilling, 300-301. of silica powder, 210. of siliceous slate of Birch Lake, 153. of slate in Biwabik formation, 144. of Virginia slate. 170. of water from iron formation, 264, Andalusite in Biwabik formation near contact with Embar- rass granite, 160, 163. of Embarrass granite, 187. Animikie, use of term, 27, 202. Animikie series, 41-i2, 51, 55. correlation of. 30, 34, 36, 41, 50, 52, 53, 56-57, 201. distribution of, 40, 45, 52, 55, 56. iron ores, origin of. 37. relations to other series, 36, 39, 43, 45, 46, 49, 56, 57, 58. (See Upper Huroniau.) Anorthosite of Beaver Bay and Duluth gabbro, 58, 183. Apatite in altered Lower Huronian rocks, 83. in iron-ore deposits, 211. in Michigan iron ranges, 275. Aplite dikes in Lower Huronian granite, 79. Archean, use of term, 202. Archean area, Lower Huronian in, 63, 67, 69-70. Archean series, described, 63-71; summary, 13. adjacent to iron-ore deposits, 227, 230. distribution of, 24, 60, 63, 66, 70. erosion of, 195. granite of, origin of, 58, 59. greenstone altering to gabbro, 59. greenstone as a source of iron for iron formation, 255. greenstone near Kimberly. Minn., 203. inclusions in Lower Huronian granite, 80. of Lake Superior region, 200. of Mississippi River, 204. of northern Minnesota, 52 pebbles in Lower Huronian conglomerate, 70, 76. possibility of iron ores in, 299. relations to other series 23, 52, 70, 71, 86, 195; described 70-71. sediments in Marquette and Vermilion districts, 77. similarity to Crystal Ealls and Vermilion rocks, 64. Arcturusmine, conglomerate of Biwabik formation, 154. Cretaceous near, 190. quartzite at, 90. Attraction, magnetic. (See Magnetic attraction). 303 304 INDEX. Auburn mine, analyses of iron ores from, 214. depth of. 20S milling in, 282. shipment from. 287. texture of iron ore in. 224. transportation of iron ores. 285. view of open pit and shaft, 286. ■ Augrite in contact rooks of gabbro, 45. 59, 160. of Dululh gabbro, 183. Augite-granite near Mallmnn camp, 78 (specimen 45435). Bacon, D. S., referred to, 61. Bacteria, iron. {SeelTon bacteria.) Bailey, C. E., referred to, 61. Bain, H. F.. referred to, 258. Bamberger, analysis by, 241. Baptism River, Manitou rocks of, 58. Puckwunge conglomerate on, 57. Baraboo quartzite, correlation of. 41, 50, 57. Barron County quartzite. assignment to Potsdam, 57. Basalts of Archean. 63; described, 64. adjacent to iron ore deposits, 227. include A in granite, SO. Basement complex of Lake Superior region, 200. {.Sfc Archean.) Bas-swood Lake, granite of, intrusion in Lower Keewatin, 55 Lower Keewatin greenstone of, 53. Bavalite compared with greenalite granules, 248. Bayley. W. S.. referred to, 11, 159. 183, 222. summary of literature, 43, 49. Beaver Bay anorthosite, 58. Bebb. E. C, indebtedness to, 19, 20. Bel), Robert, referred to. 34. Berthierine compared with greenalite granules, 248. Bessemer mine, transportation of iron ores, 285. Bessemer ore, proportion in Mesabi district, 219,' 290. Biotite in ferruginous chert near contact with Embarrass granite, 163. of Duluth gabbro, 183. Birch Lake, Animikie series east of, 45, 52. rocks of, 38, 45, 52, 54. 56. 59, 78. 150-153, 182, 183, 184, 18S. Biwabik formation, 46, 55; described, 100-167; summary, 14. alteration by granite and gabbro, 49, 273; described, 159-164: plate of, 152 (specimen 45138): summary, 17-18. altitude of, 100. area of, 100. burial under Virginia state, 260. circulation of water in, 265-268, conglomerates and quartzites of, described, 154-159. connection with Penokee-Gogebic series, 296. contact with Embarrass granite. 161; figures of, 162; plate of, 152. correlation of, 34, 37, 38, 40, 61-62: described. 201-205, 276, 296. deposition of, 196. distribution of, 100. erosion of. 260, 272, 273. ferruginous cherts of. (See Ferruginous cherts.) iron ores of. {See Iron ore deposits.) magnetic attraction in, 1G4-165. of eastern Mesabi, 21. 52, 136. 141, 160-163. 184, 207. 272-276. of Kimburly. 204. paint rock of. described, 149-150. proportion altered to iron ore, 20G, 296. quartzite of, 227; described, 154-159 (specimens 40851, 40855, 45G55, 45665, 45668, 45687, 45753, 46020. 46021, 46026, 46031, 46034). analyses of, 156. in contact with ferruginous chert of. 156; plate of, 126, Biwabik formation, quartzite of. in contact with Pokegama quartzite, 156-157. relations of. 35, 39. 46, 47. 51. 167-168. 182. 196. 230. 260. sideritic and calcareous rocks, described, 150-153. slates, described, 145-148. comparisons and relations with Virginia slate, 172-176. structure of, described, 165-166. thickness of, 46, 166-167. ISO. Biwabik mine, 43. 44, 62. alteration of slate to paint rock. 149, 169. 234. analyses of iron ores from, 214, 217. depth of. 208. dips in, 179. 226. discoverj' of iron ore in, 28. faulting in, 166. 230. drainage of. 235. ferruginous chert, analyses of, 139-140. goethitein. 218. limonite in, 210. manganese in hard and soft iron ores. 221. phosphorus in hard and soft iron ores of, 220. plan of tracks, 280, 282. relations of iron formation and slate, 175. shape determined by circulation of water, 270. shipment from, 2S7. solution of phosphorus in, 275. steam shovel mining in. 280. structural relations of iron ore. 228. transportation of iron ores, 285. view of, 296. views of contact of iron ore with wall rock. 232. wall rocks of, 227. Biwabik (town) . Archean and Lower Huronian rocks near, 63. 65, 68, 72. dips at, 175. railway connections, 23. 28. Black River Falls rocks, correlation of. 37, S8. Black Hills quartzites, assignment to Taconic, 38. Bowlder clay of Pleistocene. 192. Bowstring River. Keewatin sediments of, 53. Brackenbury. Cyril, referred to. 61. Breccia in Biwabik formation. 48, 116, 120, 137, 167, 230, 233. in Lower Huronian, S6, 87, 96. Brecciation of iron ore deposits. 230, 233. Brooks, referred to, 277. Buckley, E. R.. indebtedness to. 20. Buhl (town), railway connections, 23. slate in Biwabik formation, analyses of. 144-145. Burnside Lake, intrusion of grauite in Lower Keewatin, 55. Burt mine, depth of, 208. transportation of iron ores, 285. underground mining in. 282. Cabotian age of iron ore, 56. of Keweenawan described, 57. Calamine in Mi.';souri. 259. Calcareous rocks of Biwabik formation, 101; described 150-153. Calcite in Clinton iron ores, 250, 251. (.SfC Carbonate.) Cambrian, absence of in Mesabi district, 198. iufluence on concentration of iron ore. 263. 271 in Menominee district of Michigan. 198. relations to Keweenawan, 198. Canadian iron districts, glacial erosion of. 264. (.•^■fc Ontario.) Canton mine, 43. discovery of iron ore in. 28. faulting in. 230. manganese in hard and soft ores of. 221, INDEX. 305 Canton mine, phosphorus in hard and soft ores of, '220. shipment from, 287. structural relations of iron ore, 228. transportation of iron ores, 2S5. Carbon of Virginia slate, 169-170. Carbonate of Biwabik formation, 42, 101, 119; described, 47 48, 150-153. alteration of, 48, 237, 238, 273. of amphibolitic chert, 138. of greenalite rock, 102, 107, 117, 119-120; photomiero- g-raphs of, 128. of Gunflint Lake, 40, 153. of iron ore deposits, 211. of Penokee-Gogebic district, 153. Carbonation, processes of. 256. Carbonic acid, influence in concentration of iron ore, 261- 262. influence in formation of greenalite. 255-257. Carlton, rocks of, 54, 203-204. Caving. { See Mining methods. ) Chamosite compared with greenalite granules, 248. Challenger expedition referred to. 289, 253. Channing, J. P., referred to, 61. Chert pebbles in Lower Huronian conglomerate, 76. in Pokegama conglomerate, 98. {See Ferruginous chert.) Chester, A. H., referred to, 26,27,61. summary of literature, 35. Chicago mine, amphibolitic chert, analysis of, 141, ferruginous chert of Biwabik formation, plate of, 122. slate in Biwabik formation, 146-147. Chippewa quartzite, correlation of, 41,50. Chisholm mine, analyses of iron ores from, 214. shipment from, 288. transportation of iron ores, 2S5. underground mining in. 282. Chisholm (town), railway connections, 23. Chlorite of Virginia slate, 170. Chloritic schists, development of, 68, 69, 77, 83. included in granite, 80. of Archean, 63,68. Chubb Lake, Biwabik formation near, 52. Churn drills, 300. Cincinnati mine, 43, 44. analyses of greenalite rock from, 108-109. discovery of iron ore in, 28. ferruginous chert in contact with quartzite, plate of, 126. photomicrographs of greenalite granules, 128. quartzite of Biwabik formation, 154. shipment from, 287-288. slate in Biwabik formation, 146-147, 175-176. structural relations of iron ore, 228. transportation of iron ores, 285. Circulation of water in Biwabik formation, 166. localization of iron ores by, described, 265-272. (See Iron ore deposits, Biwabik formation, drainage.) Clarke, F. W., on composition of glauconite and greenalite, 243, 247. Clark mine, analyses of iron ores from, 214. ferruginous chert, analysis of, 139-140; plate of, 124; pho- tomicrograph of, 132, limonite in, 210. shipment from, 288. transportation of iron ores, 285. underground mining in, 282. Clay in iron ore deposits, 219. Cleavage in Archean rdcks, 67, 70. of Lower Huronian, 74, 77, 86. MON XLIII — 03 20 Clements, J. M., referred to, 11, 64. 65. 159, 172, 174, 183. Clinton iron ores, origin of, 252; photomicrographs of, 250. similarity of granules to granules of Mesabi and Goge- bic districts. 128. 130, 248-252. 279. Colquet (town), rocks near, 203, 204. Colquet mine, shipment from, 287, 288. Colorado formation of Upper Cretaceous, 190. Columbia mine, analyses of iron ores from, 214. drainage of, 235. shipment from, 288. Commodore mine, analyses of iron ores from, 214. limonite in, 210. shipment from, 287. transportation of iron ores, 285. underground mining in, 282. Commonwealth iron ores, equivalence to Mesabi iron ores, 37. Concretions in ferruginous cherts of Biwabik formation, 117-118, 24S; photomicrographs of. 128 (specimen 40767), 134. in Clinton iron ores, 248; photomicrographs of, 250. in Penokee-Gogebic iron-bearing formation, 117-118, 248; photomicrographs of, 134 (specimens 9048 and 9625). Conglomerate comparison with Lower Huronian breccia, 86-87. of Biwabik formation, 101, 167; described 154-159 (spec- mens 40851-^0855, 46020, 46021, 46026, 46031, 46032, 46034). of Biwabik formation, Mahoning mine, 159. of Cretaceous formation, 189. of Lower Huronian, described, 75-78. of Pokegama formation, described, 94-98. Copper-bearing rocks. (See Keweenawan.) Cordierite hornstones in Virginiaslate formation, described, 171-172 (specimen 45699), 185. photomicrographs of, 174 (specimen 45235). Correlation of Mesabi series, 200-205; summary of, 16. Corsica mine, analyses of iron ores from, 215. shipment from, 288. transportation of iron ores, 285. underground mining in, 282. Cottonwood county, quartzite in, assignment to Potsdam, 57 Coutchiching series, assignment to Keewatih, 53. origin of, 45. relations to Keewatin, 45. Credner, referred to, 277. Cretaceous series, 198; described, 189-191; summary, 16. effect on concentration of iron ore, 263, 271. fossils in 190-191 (specimens 45573, 45576, 45610, 45733). lignite in. 191. relations to other formations, 23, 49, 198. Croxton mine, analyses of iron ores from, 215. shipment from, 288. transportation of iron ores. 285. underground mining in, 282. Crystal Falls district, ellipsoidal greenstone in, 64^65. monograph on, 11. origin of iron ores in, 277. spilosite from, 172, 174, Crystallographic arrangement of quartz particles in schists, 83-84 (specimen 45492). Cummingtonite, alteration from greenalite, 102, 117. of Biwabik formation, 141-143. (See Amphiboles. ) Cupriferous formation. [See Keweenawan.) Dakota, Cretaceous fossils of. 190. quartzite, equivalence to Upper Huronian, 41,50. Dana, S. L., analysis by, 241. Day mine, shipment from, 2SS. 306 INDEX. Diiy mine, tmnsporation of iron ores, 2So. underground mining in, 282. Dectien, anaylsis by, 241. Deles-iiite of Biwabik formation, 119, 137. Denton, F. W., referred to, 61. Dewalque, analysis by, 241. Diabase, included in Lower Huronian granite, SO. , of Keweennwan, 58; described, 185-186. sills, 57, 58, 185-186. Diallage, of Duluth gabbro, 183. of Pewabic quartzite, 51. Diamond drills, 300. Diamond mine, 39, 40. ferruginous chert of Biwabik formation, plate of, 124. Dikes of granite and porpliyry, 79. Diorite included in Lower Huronian granite, 80. of Archean, 03; described, 66. Dip, initial, of Upper Huronian series, 197 Dip of iron ore deposits, 42, 225, 226. of Lower Huronian series, 24,86. of Upper Huronian of Gogebic district, 202. of Upper Huronian series, 24, 42, 46, 51, 55, 88, 98, 165, 166, 175, 178, 179, 202, 230, 260: Disappointment Lake, rocks of, 52, 54. Docks on Lake Superior, 285. Duluth, Missabe and Northern, view of, 28, 298. Dodge, J. A., analyses by, 223. Doleritesof Archean, 63; described, 64. Dolomite in iron ore deposits, 211. Donora mine, ferruginous chert, analyses of, 139-140, 141. slate in Biwabik formation, analyses of, 144-145. Dormer, George, indebtedness to, 20. Drainage of Mesabi district, 21-22. as a guide for exploration, 297-298. of iron ore deposits. {See Iron ore deposits and Circula- tion.) Drilling, analyses of sample, 300-301. cost of, 300. in exploration, 300. Duluth and Iron Range Railway, building of Jlesabi branch, 28, 29. iron ores hauled, 28.5-2.S6. ownership of, 2.S6. towns reached by, 23. Duluth and Winnipeg Railway, connection with Mesabi range, 28, Duluth, docks at, 285. Duluth gabbro, 57, 59; described, 182-183. Birch Lake, figure of, 184. distribution of, 57-58, 182. influence on concentration of iron ore, 272-274. intrusive nature of, 59. metamorphism, cau.sed by, 50, 59, 159-164, 183-185, 272. petrography of, 49, 52, 68, 59, 183. relations to Animikic scries, 36. relations to Logan sills, 58. relations to red rock, 58. relations to underlying rocks, 78, isi, 182, 184. {See ICeweenawan. ) Duluth mine, analyses of iron ores from, 215. depth of, 208. limonite in, 210. milling in, 2.^2. shipment from, 287, 2.S8. transportation of iron ores, 285. Duluth, Missabe, and Northern Railway, building of, 28,29. iron orfcs hauled, 285. ownership of, 2.SG. Duluth, Missabe, and Northern Railway, towns reached by, 23. Duluth, Jtissabe, and Northern docks, view of, 28, 298. Dunka River, 22, 193. rocks of, 38, 60. Eames, H. H., referred to, 26; summary of literature. 32. East Greenwood Lake gabbro, 57. Eastern Railway of Minnesota, 170, 171,174. building of Hibbing branch, 29. connection with Mesabi range, 28. iron ores hauled 2S.5-286. towns reached by, 23. Elba mine, analyses of iron ores from, 215. faulting in, 230 limonite in, 210. shipment from, 287. texture of iron ore in. 224. transportation of iron ores, 285. underground mining in, 282. Elevation of Lake Superior. 21. Elftman, \. H., referred t". 61, 1.59, 183, 192; sunimary of literature, 44-45, 50. Ellipsoidal structure in Archean basalt of Mesabi district, 65. in Biwabik formation, 137. in Crystal Falls district, 65. in Vermilion district, 65. Embarrass granite, described, 186-188; (specimens 4-5139 and 45075). age of, 198. alteration of Biwabik formition, 159-164. contact with Biwabik formation, 1.59-164; plate of, 152. dikes in Keweenawan, 186. relations to Lower Huronian granite of Birch Lake, 188. relations to Upper Huronian series, 181, 187-188, 198. use of name, 186. {Sec Keweenawan.) Embarrass Lake and River, early explorers on, 21. 22, 25. 26, 32, 36, 193, 194. Embarrass station, granite near, 186. Enlargement of quartz grains, 92, 94. Enstatite of Dulutli gabbro, 183. of Pewabic quartzite, 51. Epidote spherulites, 118. Epidote-zoisite, alteration from greeualite, 117. in matrix of ferruginous chert, 119. Eveleth, 178. discovery of iron ore near, 28. railway connections, 23. rocks near, 63, 72, 73. textures of iron ore at, 223. Exploration for iron pre described, 29.V301. by drilling, 300. by test pitting, 300. early, of Mesabi district, 25-31. Fayal mine, analyses of iron ores from, 215, 217. delessite in, 137. depth of, 208. drainage of, 236. ferruginotis chert, analyses of, 139-140. glacial erosion of, 263. milling in, 282. phosphorus in hard and soft iron ores of, 220. quartzite of Biwabik formation, 154; analysis of, 1.56. rocks in iron-ore bo '.ies, 212. shipment from, 2''", 2.S8, 289. slate in Biwabik formation, 147. steam-shovel mining in, 280. system of tracks in, 282. texture of iron ore in. 224. INDEX. 307 Fayal mine, transportation of iron ores, 285. underground mining in, 282. view of, 292. 294. Fall Lake, Lower Keewatin greenstone of, 53. Faircbild, D. L.. Indebtedness to. 19, 20. referred to, 291 . Faults in Archean rocks, 70. in Biwabik formation, 52, 166, 179. in Fokegama formation, 179. In Virginia area, 46. relations to iron ore deposits, 48, 230, 233, 278. Felch Mountain area, origin of iron ores in, 119, 277. Feldspar porphyry dikes of Lower Huronian granite, 79. Feldspathic graywacke of Gogebic district, 174. Ferric iron in glauconite, 243. in greenalite, 243. Ferrous iron in glauconite, 243. Ferrous silicate in quartzite, 92, 93. (See Greenalite.) Ferruginous chert of iron formation, described, 101, 116-143: plate of, 122 (specimens 43027, 45588); plate of, 124 (specimens 45035, 45309, 45603); plate of, 136 (speci- mens 45141, 4.5147). altered, front east end of range, 160 (specimens 45057, 45119, 45124), 163, 273 (specimens 4.5119, 45124). amphibolitie, photomicrograph of, 132 (specimen 45603), 136 (specimens 45141, 4.5147); analysis of, 141 (speci- mens 45028, 4564S, 4.5649, 45689). analyses of, 139-140 (specimens 40744, 40751, 45543, 45589, 45590, 45596, 45603, 45653, 45662, 45672 B, 4568S, 45692, 45694), 141 (specimens 45028, 45648, 4.5649, 45689). exploration for iron ore beneath, 298-299. granules in, described, 116-120; photomicrographs of, 128 (specimen, 4.5419), 130 (specimen 45628), 132 (speci- mens 45063, 45183, 45603). granules Penokee-Gogebic district, photomicrograph of, 134 (specimen 9625). in contact with quartzite of Biwabik formation, 156, plate of, 126 (specimen 409.52). in iron-ore deposits, 210, 211, 219, 233. jaspery phase, Biwabik formation, plate of, 126. phases of, as a guide to exploration. 299. relations to irou-ore deposits, 227-229, 233; figure of, 232. relations to paint rock, 234. Ferruginous slate of Biwabik formation, 101; plate of, 152 (specimen 45594) , adjacent to iron-ore deposits, 227. Fisher, analysis by, 242. Forellenstein of Duluth gabbro, 183. Fort Benton horizon of Cretaceous, 190. Fort Pierre horizon of Cretaceous, 190. Fossils of Cretaceous rocks, 190-191. Franklin mine, analyses of iron ores from, 215. paint rock of Biwabik formation, analyses of, 149-1.50. shipment from, 287-2SS. transportation of iron ores, .285. underground mining in, 282. Fraser Lake, rocks near, 52, 54. Furnace use of iron ores, 294. Gabbemichigamak Lake, rocks of, 52, 53, 54. Garnet of altered Lower Huronian rocks, S3. of Biwabik formation, 160. of Embarrass granite, 187. Gabbro. (See Duluth gabbro. ) Genoa mine, analyses of iron ores from, 215. depth of, 208. Limonite in, 210. Lower Huronian conglomerate near, 72, 76, 77 shipment from, 287, 288. Genoa mine, texture of iron ore in, 224. transportation of iron ores, 285. underground mining in, 282. Geographic J^ames, United States Board on, referred to, 21. Geography of Mesabi district, 20-21; summary of, 13. Geology, general, of Lake Superior region, 200. of Mesabi district, table of formations, 23-24: summary of, 13-14. Georgia, Clinton iron ore from, photomicrographs of, 250. Giants range, 178. constitution of, 24, 178. gorges in, 162, 193, 199. granite. (See Lower Huronian granite.) history of development of, 43, 199-200. use of term, 21. Glacial deposits, described, 22, 23, 24, 49, 191-194, 199; sum- mary, 16. effect on water circulation, 269, 271. thickness of, 269. Glacial erosion of Canadian iron district, 264. of Marquette district, 264. of Mesabi iron ore, 263, 271. Glacial gorges in Giants range, 162, 193, 199. Glacial lakes north of Mesabi range, 193, 194, 199. (See Lake Norw^ood, Lake Dunka.) Glacial strise in Mesabi district, 194. Glauconite, accompaniments of, 239, 240. alteration of, 30, 48, 51, 60, 278. analyses of, 47, 240, 241. as evidence of Cambrian age, 57. compared with greenalite, 115, 239-259. composition of. 61, 242, 243, 245. of cretaceous, 189. origin of, 48, 59, 60, 61, 189, 239, 253, 2.54. thickness of deposits, 240. use of term, 60, 247. volcanic nature of, 56. Glaucophane of Biwabik formation, 160. Glenn mine, shipment from, 288. transportation of ores, 285. underground mining in, 282. Glinka, K., analyses by, 242-244. Goethite in iron-ore deposits, 209, 21S. Gogebic district. Monograph on, 11. Gogebic series, concretions in, 117-118, 128, 248, 251; photo- micrograph of, 134 (specimen 9048). connection with Upper Huronian of Mesabi district, figpre of, 203. correlation of, 27, 35, 37, 38, 39, 41, 50, 202, 203-205, 296. feldspathic grayw_acke in, 174. greenalite in, 118^ 258; photomicrograph of. 134 (speci- men 9625). iron ores, comparison with Mesabi iron ores, 276; price of, 293; origin of, 238, 277. siderite in, 153. slate of, 177, 180. Gordon, A. T., analyses by, 144, 145, 150. referred to, 189, 212, 221, 235. " Grand Marais, Manitou rocks near, 58-59. Grand Portage, rocks of, 56, 57. Grand Rapids, exploration west of, 204, 29( location of, 22-23. railway connections, 23, 28. thickness of drift at, 192. Granite of Mesabi district. (See Archean granite. Lower Huronian granite, and Embarrass granite). Granite of northern Minnesota, 55. Grant mine, shipment from, 288. transportation of iron ores, 285. 308 INDEX. Grant mine, undersrround mining in, 282. Grant, U. S.. referred to, 30. .51, 52, .56, 159. 160, 107,177. 183,202. ■summary of literature. 59. Grant. U. S., and Winehell.N.H., summary of literature, .50. Graywaeke, feldspathic of Gogebic district, 174. of Carlton and Cloquet, 203. of Grand Portage. 56,57. of lower Huronian, described, 74-75. of Virginia slate, 109. Great Northern . (See Eastern Eailway of Minnesota. ) Great Western mine, 43. Green, R. B., analyses by, 139-140, 1«-145, 156, 220. referred to. 212, 221. screening figures by, 224. Greeualite. described, 14,17,102-116. alteration of, 116-117.132.137-138,237,238; photomicro- graphs of, 128, 130. analyses of specimens, 451S0, 45758, 45765, 46766, 108-109, 245-246. association mth iron carbonate, 258. comparison with bavalite, chamosite, and berthierine, 248. comparison with Clinton iron ores, 248, 279. comparison with glauconite, 114-115, 239-247, 2.53-255. comparison with iron carbonate. 255-259, 279. composition of, 108, 242, 243-247, 256, 261; conclusion, 113- 115. explanation of occurrence in granules, 247-252. in Penokee-Gogebic series, 118. origin of, 59, 239-259, 278. photomicrographs of, 106 (specimen 45178); 128 (speci- mens 45178, 4.5765) . rocks grading to quartzite and slate, 239. rocks, plate of, 104 (specimens 45176, 45647). specific gravity of. 107. use of term. 247. Greensand. (See Glauconite, Greenalite.) Greenstone. (.See Archean.) Griese, E. T., referred to, 212, 220, 223. Grifnn's camp, 39. Groveland formation, granules of. 119. Gruber, J. H.. referred to, 291. Griinerite, altered from greenalite granules, 102; photomi- crographs of, 128. of ferruginous chert, 119; photomicrographsof, 132 (spec- imen 4.5603). of iron-ore deposits, 211. of quartzite, 51, 92. (Sec Amphibole.) Giimbel, analyses by, 242. Gunflint iron formation, correlation of, 27, 34. origin of iron ores, 37, 277. Gunflint Lake, analyses of magnetite from, 221, carbonates at, 40, 56, 1.53. early e.xrilorers on, 25, 26, 32. iron formation, alteration of, 159, 160. litcnitvire on, 61. location of, 25. rocks near, 27, 40, 62, 63, 1.59, 169, 177, 183. 185, 201. Hale mine, 43, 44. analyses of iron ores from, 215. dip of iron-ore deposit layers, 225. discovery of iron ore in, 28. faulting at, 166, 230. limouite in, 210. rocks adjacent to iron-ore deposits in. 227. shipments from, 2.S7, 288. steam-shovel mining in. 2.S0. strtictunil reliitituiw of iron ore of. 229. Hale mine, transportation of iron ores. 285. view of, 294, Hammond, A. J., analysis by, 139-140. Hampe, analyses by, 262. Harker, referred to, 172. Hanshof er, analyses by, 241 . Hawkins mine, railway connections, 23. shipment from, 288. transportation of iron ores, 285. underground mining in, 282. Head, A. P., referred to, 62. Head, Jeremiah, referred to, 62. Heddle, F., anaysis by, 241. Hematite in iron-ore deposits, 209, 218. in quartzite, 93. pore space in, 224. (.See Iron ore.) Hibbing, analysis of water west of, 264. Archean, Lower Huronian, Upper Huronian rocks north- west of, 63, 64, 67, 69, 71, 72, 80, 84. discovery of iron ore near, 28. iron-ore deposits near, 206. railway connections, 23. texture of ore at, 224. Hinckley, sandstone, 57. Hinsdale, explorations near, 39. Hornblende. (See Amphibole.) Hornblendic schists of Archean, 63; described, 66-68. included in Lower Huronian granite, SO. Hornblende granite of Archean. (See Archean.) Horses of rock in iron-ore deposits explained by circulation of water, 269. view of, 294. History of district, 2-5-31. Houghton, referred to, 277. Hubbard, referred to, 277. Hudson Bay, 22, 25. Hull mine, analyses of iron ores from. 215. analysis of water from, 264. depth of, 208. transportation of iron ores. 285. underground mining in, 282. Hulst. N. P., referred to. 62. Hunt, analyses by, 241. referred to. 277. ^ Hunters Island, Lower Keewatin greenstone of. 53. Huronian, use of term, 202. (Sec Upper Huronian, Lower Huronian. ) Hyperstheno of contact rocks of gabbro. 51. .59. 160. of Duluth gabbro, 183. Ilmenite, of altered Lower Huronian rocks, S3, of Embarrass granite, 187. of Keweenawan diabase, 185. Iron bacteria procijiitating hydrated iron. 256. Iron-bearing formation. (See Biwabik formation.) Iron carbonate alteration from glauconite, 60. alteration from greenalite, 279. altering to iron ores, 37, 263, 277, 278. association with greenalite rocks, 258. comparison of development with greenalite. 255. 279. concretions in, 248. (.See Carbonate.) Iron Lake, analyses of magnetite from, 222, 223. contact of iron formation and syenite. :3S. Pewabic quartzite of, 51. Iron ore. absence in oast end of Mesabi range. 207, 272- 274, absence under N'irginia slate, 297. abstraction of silica from. 2li2. 264. INDEX. 309 Iron ore, cTiemistry of, 26, 212-223. comparison with otlier Lake Superior ores, 296. correlation of. (See Biwabik formation.) covered by ferruginous chert, 298-299. cubic feet per ton, 224. deposits 40, 48: described, 206-236; summary, 16-17. discovery of in Mesabi district, 39, 62. early explorers on, 33. elevation of, 297. exploration for, 295-300. furnace, use of, 294. glacial erosion of, 263, 271. locahzation by water circulation, 279; described, 265- 272. magnetic. {See Magnetic attraction. Magnetite.") migration of, figure 270. mixture with old range iron ores, 294. not mined, 290. of Lake Superior region, origin of, 61; paper on, 19; price of, 293-294. of Minnesota, horizons of, 38. of Vermilion district, 38, 40, 53. of west end of range, 211. origin of, 37, 40, 42, 43, 44, 48, 51, 56, 60. 237, 238, 260-272, 277-279; described, 56, 237-279; summary of, 16-17. 42, 263, 277. o^vnership of, 291-292. phosphorus in, 219, 274. pitch of deposits, 276. pore space in, 262. possibility of finding in Lower Huronian and Archean of Mesabi, 299. jjroperties, mapping of, 291. (.S'ee Mines.) proportion of area to that of Biwablk formation, 101, 20L-, 296. proportion of Bessemer, 219, 290. relations to adjacent rocks, 40, 48, 227, 234, 278, 299. reserve tonnage in Mesabi district, 290. reserve tonnage in old ranges, 290,291. sand in, 211,296. shipment by railways, 285. shipment from Mesabi, 29, 287-289. shipment from Michigan, 289. shipment from Minnesota, 289. shipment from United States, 289. Iron pyrites altering from greenalite grantiles, 138. in ferruginous chert of Biwabik formation, 138. in iron ore deposits, 211,219. in Lower Huronian sediments, 75. in quartzite, 92. Iron Trade Review, referred to, 29, 287. Irving, R . D. , referred to, 27, 42, 44, 45, 183, 200, 201, 2.55, 277, 278: summary of literature, 34. Isle Royal, Puckwunge conglomerate at, 57. Itasca or 10th moraine, 49, 192. (See Moraine.) Jasper in Biwabik conglomerate. 157-158. in Biwabik formation, 120, 157, 158; plate of, 126 (speci- men 45420). in Gogebic district, origin of, 272. in Lower Huronian conglomerate, 77. in Pokegama conglomerate, 97, 98. Johnson, E. J., analysis by, 150. referred to, 212. Joints in Archean rocks, 70. in Biwabik formation, 48, 166, 179. in gabbro, 182. in iron ore deposits, 226-227, 230, 233; filled -with vein quartz, 212. Joints in Lower HurOnian, 86. in Oliver mine, view of, 284. in Pokegama formation, 99, 179. influence on water circulation and concentration of iron ore, 265. 267, 269, 271, 279. Jordan mine, analyses of iron ores from, 215. mining in, 282. shipment from, 288. , transportation of.iron ores, 2S5. Kabetogoma Lake, granite of, intrnsion in Lower Keewatin, bb. Kakabikag Falls, use of term, 37. I Kame gravels of Pleistocene, 193. Kanawha mine. 43. analysis of iron ores from, 215. boundaries of, 291. • dip at, 179. dip of iron ore deposit layers, 225. discovery of iron ore in, 28. faulting in, 166, 230. shipment from, 288. structural relations of iron ore of, 229. transportation of iron ores, 285. underground mining in, 282. I Kaolin in iron ore deposits, 212. Kawishiwin greenstone of Lower Keewatin described. 53. I Keewatin, use of term, 202. , Keewatin series. 40, 41, 46, 52, 53-54, 201. correlation of, 41, 201-202. greenstone, alteration to gabbro, 58. mapping of, 52. relations to other series, 45, 46, 53, 54, 55, 58. schists, origin of, 45. Kekequabic Lake, granite of, 55. Kettle River, Hinckley sandstone of. 57. Keweenawan series, described, 57-59, 182-188; summary of, 15-16. correlation with Lower Cambrian, 52-53. folded with Upper Huronian, 260. relations to other series, 23, 39, 42, 43. 45, 46, 49, 56, 57-58, 197-198, 200, 260, 272. {See Embarrass granite, Duluth gabbro.) Kimball, referred to, 277. Kimberly, Minn., greenstone near, 203. magnetic attraction near. 204. quartzite near, 203, 204. Knerr, analysis by, 242. Knife Lake formation, 23. Kupffer, A., analyses by, 242. Labradorite of Duluth gabbro, 183. La Belle mine, shipment from, 288. transportation of iron ores, 285. underground mining in, 282. Lac la Croix, granite of, 55. Lacroix, referred to, 248. Lake. (See Akeley, Basswood, Birch, Burntside, Chubb, Em- barrass, Rail, Fraser. Gabbemiehlgamak, Gunfiint, Iron, Kabetogoma, Kekequabic, Muskrat, Rainy, Saganaga, Thomas, White Iron.) Lake Dunka, 194. Lake Huron, original Huronian rocks of, correlation with Upper Huronian, 27. Lake Norwood, 194. Lake Ogishke-Muncie. Lower Keewatin of, 54. Lake Superior, docks at, 285. elevation of, 21. Lake Superior Consolidated Iron Mines Company, indebted- ness to, 20. Lake Superior lobe of ice sheet, 194. 310 INDEX. Lake Superior mines, 43. (inalyses of iron ores from. 215, 217. shi[>ment from. 2S7. 2.SS. Lake Superior region, general geology of, 200. monograph on geology of. 11. sketch map of, 19. Lane and Sharple.ss, referred to. H2. Lanners. T. L., referred to. 02. Laura mine, shipment from. 2ss. transportation of iron ores. 28.'.. underground mining in. 2S2. Laurentian series, assignment to Keewatin, 53. distribution of. -10. Lawson. referred to, .58. Leetonia mine, shipment from. 2S.S. steam shovel mining in. 280. transportation of iron ores, 285. Leith, C. K.. referred to, 31, 243, 245. Leith. C. K.. and Van Hise. C. R.. summary of literature, 60. Lerch Brothers, analyses hy, 139-140. 114-145, 1.50. referred to, 212, 223. Lignite in Cretaceous, 191. Limestone in Biwabik formation described, 150-153. of Virginia slate. 171 (specimen 45464), 46, 56. Limonite, alteration from greenalite, 117. in iron-ore deposits, 209-210, 218-219. pebbles in Biwabik conglomerate, 157-1.58. pore space in, 224. Limonltic chert in Biwabik formation, 137; plate of, 124 (specimen 45035). Lincoln mine, shipment from, 288. transportation of iron ores, 285. underground mining in, 282. Literature of Mesabi district, 25-31, 61-62. Little Falls, mica-schist at, 56. Little Marais, Puckwunge conglomerate at, 57. Little Kocky Tails, use of term, 37. Locke, referred to, 237. Logan sills. 58. Lone Jack mine, 43. Lon,i;year, E. J., referred to, 62, 99, 154, 166, 186, 300. Longyear mine, analyses of iron ores from, 216. shipment from, 288. transportation of iron ores, 285. underground mining in, 282. Lower Cambrian, correlation with Animikie and Keweena- wan, 36, 43, 52-53. use of term, 27. Lower Huronian granite, 36,41,45,72,186,188,196; described. 7.S-.S0. contact with gabbro of Birch Lake, 78; figure of, 184, inclusions in, 80. relations to other rocks, 41, 45, M. 55, 70, 71, ,84. l.ss. variety of, 85. vein quartz at contact, S3, Lower Huronian porpliyries. (.sVi Porj.liyritie rliyolite of Lower Huronian.) Lower Huronian series described. 72-.S7 (specimen 4.5494): summary of, 13-14. alteration by granite, 67; photomicrograph of, 82 (speci- mens 45112, 45414. 4,5415, 4.5410), 83-84 (specimen 45114). as a source of iron for iron f(prmation, 255. deposition of, 195. distribution of, 24, 60, 66, 70, 72, 73, 80, equivalence to Ogishke and Knife Lake fornialions, 2:!, erosion of, 195, in Arcliean area, o:i-6", near Carlton and Cloquet, 203, Lower Huronian series near Mississippi River, 204, ol Lake Superior region, 200. possibility of iron ores in. 299. relations to other series, 23, 70, 71, 84-86, 195, 196. vein quartz in, 83. Lower Keewatin. (.S'ee Keewatin. i Magnetic attraction in exploration. 164-165, 295, 296. near Kimberly, 204. west of Grand Rapids, 204. Magnetite, alteration from greenalite. 117. 238. analysis of, 221, 222. of altered Lower Huronian rocks, S3, of Biwabik formation near gabbro, 26, 27, 35, 42, 44, 49. 1.59, 272, 273: photomicrograph of, 136: summary of, 17-18. of Duluth gabbro, 183: analysis of, 223. of iron ore deposits, 209, 211. of Keweenawan diabase, 185, of Pewabic quartzite, 51, of sideritic slate of Birch Lake, 153. origin of, 49, 117, 238. Manganese in iron ore deposits. 212, 220, 221. Manitou division of Keweenawan, 57, 58. Manitou River, Puckwunge conglomerate on, 57. Mallet, analyses by, 242. Mailman, referred to, 40. Mailman Camp, augite granite near, 78. rocks near, 39, 40, 147, 295; sketch, SO. Malta mine, analyses of iron ores from, 214, 216. depth of, 208. shipment from, 2SS, steam-shovel mining in, 280, transportation of iron ores, 285. Mahoning mine, analyses of iron ores from. 214. 216. analysis of ferruginous chert, 139, 140. articles on, referred to, 61, 62, conglomerate in, 101, 159. depth of, 208. ferruginous chert of Biwabik formation, p'ate of, 122. limonite in, 210. paint rock of Biwabik formation, analysis of, 149-150: plate of, 152. plan of tracks, 280, 281. shipment from, 287-289. steam shovel mining in, 2S0, 281. transportation of iron ores, 285. view of, 296. Map of Jlesabi district by U. S, Geological Survey, in pocket, by Jlinnesota Survey, referred to, 40, -50. 60. Marck, referred to, 241. Mariska, Lower Huronian conglomerate near, 7.5-77, 86. Marquette district, Arehean sediments in. 77. erosion of, 2t>4, griincrite-schist from, 142, monograph on, 11. Marquette iron ores, apatite in, 275. comparison witli Mesabi ores, 276. origin of, 277. price of, 293-294. sliipmelit t)f, 289. Maltliow. referred to, 57, MoKaskill. ,lohn, referred to, 28. McKinley, 178. .•\rchean rocks north of, 63. • discover}' of iron ore near, 2S. mine, 43, 44. railway connections. 23. Meadow exiilonition. analysis of slaie from, 170. Meeds, A. 1>.. analysis by. 139, 140, 144, 145. INDEX. 311 MelviUe, W. H.. analysis by. 222. Menominee district, Cambrian in. 198. monograph on. 11. origin of iron ores in, 277. shipment from. 289. Menominee series, equivalence to Animikie series. 41. Merritts. referred to 27, 29, 40. Mesaba station, contract of Lower Huronian sediments and granites north of, 85. exploration near, 40. ferruginous chert with granules, photomicrograph of, 128 {specimen 45419) . railway connections of, 28. Mesabi Syndicate Company. 44. Mesabi Chief mine, ferruginous chert, analysis of 139, 140 (specimen 45543). IMesabi. or Eleventh moraine, 49, 192. Mesabi range, divisions of, 46. elevation of, 21. shipments from, 29. use of term, 21, 25-26, 182. {See Giants range.) Mesozoic, absence of in Mesabi district. 198. Metabasalts. {Sec Basalts.) Metadolerites. {See Dolerites. ) Mica in iron ore deposits, 211. (S'eeBiotite.) Micaceous quartz-slate of Pokegama formation, described, 93. 94. {Ste Pokegama quartzite.) Mica-schists of Archean, 63; described, 68. development of, 68, 69, 77, 83. included in granite, 80. of Animikie, 56. Michigan Iron ores, apatite in. 275. comparison "with Mesabi iron ores, 276. price of, 293, 294. shipment of, 289. reserve tonnage of, 290, 291. {Sec Penokee-Gogebic, Marquette, Crystal Falls, Felch Mountain, Menominee.) Michipicoten district, origin of iron ores in, 277. Milling at Fayal mine, described. 282. view of, 294. Mines. {See Adams, iEtna, Agnew, Arcturus, Auburn, Biwabik, Burt, Canton, Chicago, Chisholm, Cincin- nati. Clark, Columbia, Commodore, Corsica, Croxton, Day, Donora, Duluth, Elba, Fayal, Franklin. Genoa. Glenn, Grant, Great Western, Hale. Hawkins, Hull. Jordan, Kanawha, Kimberly, Lake Superior mines, Laura, Leetonia, Lincoln, Longyear. Mahoning, Malta, Minnewas, Minorca, Moose, Morrow. Moss. Mountain Iron, Norman, Ohio, Oliver, Paddock's, Pearce, Penobscot. Pettit. Pillsbury, Roberts, Rouch- leau, Sauntry-Alpena, Security, Sellers, Sharon, Sparta, Spruce, Stephens, Stevenson. Union, Utica, Victoria, Virginia, Williams, Wills, Wyoming.) Mines, not shipping, 290. Mining methods, 43, 61-62; described, 280-285. Minnesota iron ore, price of, 293, 294. reserve tonnage of, 290, 291. shipments of, 289. Minnesota river, rocks of. 41, 50, 53. Minnesota Survey, Final Report of, 30. maps referred to, 30, 40, 50, 60. {See Winchell. Grant, Upbam, Elftman, Spurr.) Minnewas mine, shipment from, 287, 288. Minorca mine, analyses of iron ores from, 216. milling in, 2S2. Minorca mine, shipment from, 288. transportation of iron ores, 285. underground mining in, 282. Missabe Mountain mine, 43, 44. Mississippi and Xorthern Railway, 29. Mississippi River, 21, 22, 25. Keewatin sediments of, 53, 54. mica-schists on, 56. quartzite on, 90, 92. Missouri zinc ores, comparison with development of green- alite and carbonate, 258, 259. Moose mine, manganese in, 220. Moose Track mine, glacial origin of, 263. Moraine, Itasca or 10th, 49, 192. Mesabi or 11th, 49, 192. Morrison County. Lower Keewatin of, 54. mica-schists in, 56. Morrow mine, analyses of iron ores from, 216. milling in, 282. shipment from, 288. transportation of iron ores, 285. Moss mine, slate in Biwabik formation, analysis of, 144, 145 (specimen 45561). Mountain Iron mine, analyses of iron ores from, 215, 216. analyses of ferruginous slate from, 144, 145 (specimen 45645). analyses of paint rock from, 149. 150. comparison of grades in, 219. depth of, 208. development of iron ores from carbonate, 238. dips at, 175, 179, 225. discovery of iron ore in, 28. 29, 40. drainage of, 235, 236. ferruginous chert of Biwabik formation, plate of, 124 (specimen 45035). gradation of iron ore to ferruginous chert, 233. manganese in, 220, 221. magnetite near, 42. plan of tracks, 280, 281. railway connections, 23. rocks north of. 63, 66, 67, 69, 70, 71. 72. 78, 79, SO, 84, 85, 96: sketch, 70. shipment from, 287, 288, 289. slate in Biwabik formation, analysis of, 144, 145 (speci- men 45645). steam-shovel mining in, 280, 281. texture of iron ore in, 224. transportation of iron ores, 285. view of, 286. views of contact of iron ore with wall rock, 232. water in, 218, 219, 235, 236. Murray referred to, 239, 242. 253. 254, 255, 259. Muscovadite, association with Pewabie quartzite, 51. use of term, 58. Muskrat Lake, Biwabik formation near, 52. Nashwauk town. Archean rocks north of, 63. Nelson River, 21, 22, 25. New Brunswick, St. Johns group of, equivalence to iron ore, 57. New England City, Clinton iron ore from, photomicrographs of, 250. New England mine, 43. Newman, indebtedness to. 19. New Ulm, Puckwunge conglomerate at, 57. quartzite at, assignment to Potsdam, 57. New York, photomicrographs of Clinton iron ore from, 250. Nichols, J. A., referred to, 27. Nicollet, J, N., referred to, 25. summary of literature, 32. 312 INDEX. Sodules in Biwabik formation, 137. in iron-ore deposits, 226. in Virginia slate. 137. Norman mine, milling in. 2.S2. phosphorus in hard and soft iron ores of. 220. shipment from, 287-288. texture of iron ore in, 224. Norwood. J. G., referred to, 25. 26. summary of literature, 32. ^ Oeher. yellow, (.S'n Limonite,) Ogishke formation, equivalence to Lower Huronian, 23. (See Lake Ogishke.) Ohio mine, 43. shipment from, 287-288. Okwanin, gahbro near, 36. (.Sff Allen .Junction). Olcott, W. .!., indebtedness to, 20. Oliver mine, analyses of iron ores from, 215, 216. Cretaceous rocks in, 189. dip at, 179, 225, 226. faulting in, 230. ferruginous chert, analysis of, 139-140 (specimen 40744) limonite in, 210. manganese in, 220, 221. nodules in Biwabik formation, 137. phosphorus in limonite, 275. shipment from, 267, 288. structural relations of iron-ore deposits, 227, 228. steam-shovel mining in, 280. transportation of iron ores, 285. view of wall rock showing jointing, 284. views of, 284. Olivine in contact rocks of gabbro, 45, 51, .59. 160. in Duluth gabbro, 183. Ontario, western, correlation of Keewatin, 41, 50. Oolitic structure in Clinton iron ores, photomicrographs of, 250. in Mesabi iron ores, 248. Ophitic texture in dolerites, 64. Original Huronian beds, correlation ivith Animikie and Upper Huronian, 27, 34. Organic matter in ferruginous cherts, 140. in iron ore, 218, Paddock's mine, 43. Paintrock.lOl; described, 149; plate of, 152 (specimen 4.5.587). alteration from slate, 169, 234. analyses of, 149-150 (specimens 40661, 45594, 46646), composition of, 149-150 (specimens 40661, 45594, 45646), 223, effect of flowage of water, 265-266. relations to iron-ore deposits, 212, 223, 227, 234, 263, Paleozoic, absence of, in Mesabi district, 198. Partridge River, 21. Pearce mine, analyses of iron ores from, 216. shipment from, 288. transportation of iron ores, 285. underground mining in, 282. Pegmatitic dikes in Lower Huronian granite, 79, Penobscot mine, alteration of slate to paint rock, 149, 234. analyses of iron ores from, 210. drainage of, 236, ferraginous chert, analysis of, 139-140 (specimen 45696). ferruginous slate of Biwabik formation, plate of, 152 (specimens .15592, 45.594). paint rock of Biwabik formation, analysis of, 149-160 (specimen 45.59'l). shipment from, 287-2.88. slate in Biwabik formation, 146; analysis of, 144-145 (specimen 4,5.591); plate of, 1,")2 (specimen 45594), 146, Penobscot mine, transportation of iron ores, 285. underground mining in, 282. Penokee-Gogebie. (See Gogebic.) Peridotite of Archean described, 66. Perknite of Archean, 66. Pettit mine, shipment from, 288. transportation of iron ores, 285. underground mining in, 282. Pewabic quartzlte, 39, 40, 42, 43, 45, 46, 51, 52, .51, 60. {See Pokegama quartzite.) Phosphorus, comparison in hard and soft iron ores, 220. determination in drill samples, 300-301. in ferruginous chert, 140. in hematite, 219. in iron-ore deposits, 211, 219, 220, 274.' in limonite, 220. in paint rock, 223. precipitated by alumina, 275. solubility of, 275. source of, 274. summary of, 18. Pike Rapids, Lower Keewatin of, 54. Pike, Z. JI., referred to, 25. summary of report by. 31. Pike liiver, 22. granite of, 36. Pillsbury mine, analyses of iron ores from, 216. depth of, 208. shipment from, 288. transportation of iron ores, 285. underground mining in, 282. Pine County, Hinckley sandstone of, 57. Pine of Mesabi district, 22. Pipestone County, quartzite in, assignment to Potsdam, 67. Pipestone Rapids, Lower Keewatin of, 53. Pisani, analysis by, 241. Pitch of iron-ore deposits, 207-208. Pleistocene. (.8cf Glacial deposits.) Pokegama Falls, 37, 38, 39, 92, 154. early explorers on, 25, 26. 31-33. stria; at, 194. Pokegama quartzite described, 55, 90-99; summary of, 14, as a guide to exploration, 298. at Pokegama Falls, 92, correlation of, 42. deposition of, 196. influence on water circulation, 266, 267. metamorphism of, 43, near Aitkcn, Minn., 203. near Kimberly, Minn., 203-204, relations to iron-ore deposits, 227, 230, 278. relations to other formations, 42, 51, 55, 99, 196, 230. thickness of, 55, 99, ISO, {Sec Pewabic quartzite.) Porphyries of Vermilion district, 6.8-69. Porphyritic rhyolite of Archean described, 68-69. of Lower Huronian described, 78-80. Potash in glauconite, 17,243,247,2.54. ingrcenalitc,243,247. Potsdam formation, reference of Pokegama quartzite to, 33. Potsdam, use of term, 57. Prairie River, 21, 22, 25, 193. Prairie River Falls, early explorers on, 26, 33, 37, 38. rocks at, 79, 90, 94, 1,54, sketch of, 91. stri:e at, 194. Production, (.svr Shipments.) I'uckwungc conglomerate, 56, 57. Pyrites. (See Iron i>yrites.) INDEX. 313 Pyrolusite in iron-ore deposits, 211, 212. Quartz-biotite-schist of Animikie, 45. Quartz, enlargement of, 92, 94. in iron-ore deposits, 211. Quartz-slate of Pokegama formation, relations to massive quartzite, 94. Quartzite, Biwabik, described, 154-159 (specimens 40S51- 40855, 45119, 45665, 45668, 45687, 45753, 46020, 46021, 46026, 46031, 46032, 46034, 46070). Pokegama, described, 90-99. (See Biwabik quartzite, Pokegama quartzite,) Quartzite. (.See Pokegama, Pewabic, Baraboo, Cliippewa Falls, Barron County, Sioux, South Dakota.) Railways of Mesabi district, 23, 28. (See Great Northern Railway, Eastern Railway of Min- nesota, Duluth and Iron Range Railway, Duluth, Missabe and Northern Railway, Duluth and Win- nipeg Railway, Mississippi and Northern Railway.) Rainy Lake, 25. Keewatin rocks of, 53. Range 1 W.,T. 63 N., 58. 2E., T. 64 N., ,57. 2W.,T. 62N.,5S. 3 W., T. 65 N., section 36, 223, 4 W., T, 62 N., 58. iW., T. 65 N., section 27, 54. .section 29, 221. 5 W., T. 65 N., section 34, 64. 6W., T. 60 N., 58. 7 W., T. 60N.,6S. 10 W., T. 58 N., 58. 10 W., T. 62 N., section 30, 54, 221, 10 W., T. 63 N., section 36, 223. 11 W.,T. 65N., 58. 11 W., T. 63 N., 37. 12 W., 51, 147, 1.59, 163. (See Embarrass granite, Duluth gabbro, Biwabik formation.) 12 W., T. 60 N., 44-^5. section 3, 136. section 17, 136, 152, 160. 162, 187, 193. 12 W., T. 61 N., section 34, 222. section 35, 38. • , 147, 1.59-163, 182, 186. 13 W., 13 \V., T. 59 N., sections, 168. section 8, 168. section 34, 186. 13 W., T. 60 N., section 5, 185. section 8, 185. section 22, 132. section 23, 222. section 25, 168, 185. section 29, 90, 138, 154. section 32, 44, 90. section 35, 185. 14W., 26-27, 147, 182. 14 W., T. 59 N., 34, 37. section 1, 147. section 2, 186, 188. section 8, 193. section 9, 82, 85, 193, section 11, 40, 72, 79-80. section 14, 34. section 15, 34, 147. section 16, 79, 82, 147, 193, section 17, 193. section 18, 85, 94, 95, 154. section 19, 147. section 21, 126, 139-140, 147, 172. Range 14 W., T. 59 N, 14W.,T. 60 N, 15 W., T. 58 N. 15 W., T. 59 N. 16 W., T. 58 N. 16 W., T. 59 N.^ 17 W,, T. 57 N. 17 W., T. 58 N. 17 W., T. 59 N"., 18W.,T. 58N., 18W.,T. 59N., 19 W., T. 58 N. 20 W., T. 58 N, 21 W., T. 57 N. 21 W., T. 58 N. 22 W., 71. 22 W.,T. 57N. 23 W., 71. 23 W.. T. 56 N. , section 22, 147. section 27, 174. section 28, 34. section 34, 166, 186. , 34, 37. , section 3, 119, 169, 170. , section 22, 104, 106, 108-109, 128, 147. section 26, 147. section 28, 132, 141, 147. section 33, 66, 93, 96. section 35, 108. section 2, 230. section 3, 73, 79, 86, 96. section 4, 122, 141, 147, 154, 155. section 5, 1.55. section 6, 69. section 7, 193. section 34, 86. section 36, 65. , section 6, 147. section 3, 89, 139-140, 154, 155, 156. section 7, 66. section 8, 146. section 9, 146. section 13, 154, 155. section 15, 52. section 16, 69. section 20, 7.5-76, 90. section 21, 69, 75-76. section 22, 52, 68, 75-76, 86. section 24, 147. section 28, 139-140. section 29, 7-5-76. section 32, 87. section 33, 139-140, 155. section 34, 77. section 32, 154. section 35, 128. section, 3, 28. section 7, 144-145, 146. section 8, 146. section 18, 139-140. section 22, 72. section 23, 72. section 25, 69, 72. section 27, 72. section 28, 72. section 30, 193. section 31, 193. section 34, 79, 85, 96. section 2, 139-140. section 10, 130, 146. section 17, 144, 145, 146. section 20, 146. section 21, 144-145. section 27, 146. section 11, 139-140. section 25, 97. section 26, 97. section 34, 97. section 35, 72, 99. 2. section 19, section 20, 72. section 30, 72. section 31, 189. section 2, 72. section 3, 97. 314 INDEX. Rauge 24 W., T- 5i5 N.. section 13, 90. 154. 1«9. section 15, 40, 12i. section 24, 189, 190. 25 W\, 154. 295. 26\V., 154. 26 W., T. 55 N.. section 22, 189. secftion 23, 189. Rattle and Nye, analyses by, 221. Recrystallization of chert. 159, 273. of Lower Huronian graywacke. 75, S3-84. Red River, lobe of ice sheet. 194. Red rock, of Keweeuawan, 57, 58. Renard, referred to. 239, 242. 253, 254. 255. 259. Reserve tonnage of iron ore in Mesabi district, 290. Rhyolite, porphyritic. of Archean, 63: described, 68-69. of Lower Huronian, described, 7&-S0. Rivers of Mesabi range, 21-22. (See Mississippi, Prairie. Swan, Embarrass. Pike. Par- tridge. Dunka.) Rivot, referred to, 277. Roberts mine, analyses of iron ores from. 216 Pokegama conglomerate, 96. shipment from. 287-288. transportation of iron ores. 285. underground mining in, 282. Robertson, R. S., analysis by, 223. Rogers, analyses by, 241. Rosenbusch, referred to, 172. Rouchleau mine. 43. Rust mine, anjilysis of water from, 264. depth of, 208. shipment from. 288. transportation of iron ores, 285. underground mining in, '282. Rutile of altered Lower Huronian rocks, S3. Saganaga Lake, granite of, intrusion in Lower Keewatln,55. St. Croix Valley, Hinckley sandstone of. relations to Kewee- uawan, 57. St. Johns group of New Brunswick, equivalence to iron ore, 57. St. Lawrence River, 21, 22. St. Louis River. 21, 22, 25. graywacke and slate on, 50. 54, 203, 204. Sand in iron ores, 211, 296. Sand Mountain, Clinton iron ores from, photomicrographs of. 250. Sauntry- Alpena mine, analysis of iron ores from. 217. dip at. 179, 225. shipment from, 288. steam-shovel mining in, 280. structural relations of iron ore, 22b. surface of iron-ore deposits, 226. transportation of iron ores. 285. view of, 294. Schists. (.SeeHornblendic schists. Chloritic schists. Micace- ous schists.) Schoenfuld, analysis by. 242. Schoolcraft. H. R., referred to, 25. summary of literature, 32. Slichter. C. S., referred to. 267. Seaman, A. E., referred to, 211, 275. Sebenius, J. U,, indebtedness to, 20. Section. {Sec Range. ) Sections, subdivision of. 291. Section 33, mine, shipment from, 288. irunsportalion of iron ores, 285. Security miiR-. 43, 41. Sellers mine, depth of. 20n. (Shipment frv>m. 2.s7. 289. Sellers mine, transportation of iron ores, 285. underground mining in, 282. Serpentine of Biwabik formation, 137. Shale of Cretaceous formation, 189. Sharon mine, limonite in, 210. milling in, 282. shipment from, 289. transportation of iron ores, 285. ■\iew of stripping operations. 286. Sharpless and Lane, referred to. 142. Shipments, from Marquette district, 289. from Menominee district, 289. from Mesabi district, 29. 287-239. * from Vermilion district, '289. Shipping points, sketch map of. 19. Short Line Park, rocks of. 57. 58. Sidener, C. F., analysis by, 139-140, 221. 222, 223. Siderite. (See Carbonate.) Sideritic slate. Birch Lake, described, 150-153 (specimen 45161). Silica, abstraction from iron ore, 262, 264. in iron-ore deposits, 210, 219. powder, analyses of, 210. Silliman. A. P.. referred to, 212. Sills, diabase in Upper Huronian, 58, 185, 186. Sioux quartzite. assignment to Potsdam, 57. Slate of Biwabik formation, 101; described 143-148; plate of 152 (specimens 40741, 40742, 40863, 45003, 45006. 45009, 45039. 45175, 45191, 45224, 45228, 45391, 45461, 4-5541, 45591. 45592. 45593, 45594, 45600, 45625, 45630, 45639, 45645, 45652, 45670, 45672, 45672A, 45677, 45678, 45699, 45734, 45737, 45780). alteration to paint rock, 149; plate of, 152 (specimen 45587), 234. as a guide to exploration. 297-298. of Birch Lake, analysis of. 153 (specimen 45161). relation and comparison with Virginia slate. 172-176, 297-298. relations to iron ore deposits, 212, 219. 227, 229. 263. 266, 267, 271, 272, 279. {See Biwabik formation.) Slate of Carlton and Cloquet, 203. of Gunflint Lake, 169, 177. of Lower Huronian. described. 74. 75, 76 (specimen 45494). (Sec Lower Huronian.) of Penokee-Gogebic district. 177. ISO. pebbles in Lower Huronian conglomerate, 76 Slate. Virginia, described, 168-177. (.See Virginia slate. ) Slicing system of mining. 282-283. {See Mining methods.) Smyth; C. H.. referred to, 250, 251. Smyth, H. L.. referred to, 118. 277. Soda, in glauconite, 243. in greenalite, 243, Soil of Mesabi district, 22. South Dakota, Keewatin sediments of, 63. Sparta mine, analyses of iron ores from, 217 depth of, 208. dip of iron ore deposit layers, 225. limonite in, 210, shipment from. 287, 289, steam-shovel mining in, 280. structural relation of iron ore, 229. texture of iron ore. 224. transportation of iron ores, 285. Spai:ta mine, underground mining in, 282. Sparta (town), Archean rocks near, 63, 65, 76, railway connections. 23. INDEX. 315 Sperry, E. A., inrlebtedness to. 20. referred to, 1.54. Sphene, of Embarrass granite, 187. Spherulites of epidote, 118. Spherulitic texture in basalt. 65. Spilosite from Crystal Falls district, 174. Spruce mine, analyses of iron ores from, 215, 217. depth of, 208. slate in Biwabik formation, analyses of, 144-145 (speci- men 45678). shipment from, 289. texture of iron ore in, 224. transportation of iron ores, 285. underground mining in. 282. Spun, J. E., referred to, 14, 17, 30, 51, 52, 60, 114-115, 116, 120, 139-140, 144-145, 1-59, 210, 239, 241, 247. 278, 279- summary of literature, 45, 60. Stanton, T. W., on Cretaceous fossils, 190. Staurolite of altered Lower Huronian rocks, 83. Steam shovel mining, 280-282, 294. {See Mining methods.) Steiger, Geo., analyses by, 139-140, 141, 144-145. 150, 153, 245. Stephens mine, analyses of iron ores from, 217, steam shovel mining in, 280. Stevenson mine, analyses of iron ores from, 217. depth of, 208. dip at, 178. dip of iron ore deposit layers, 225, shipment from, 289. steam shovel mining in, 280, transportation of iron tres, 285. Stevenson (town), railway connections, 23. Stokes, H. N., analyses by, 139, 140, 144-145, 150, 171-172. Stone mine, exploration near, 40. Stony Brook Junction, railway connections, 28. Strike of Lower Huronian, Sd. Succession in Mesabi district, 36, 51. Sulphur in iron ore deposits, 21S, 219. (.See Iron pyrites.) Superior, Wis., docks at, 285. Swan Eiver, 21, 26. railway connections, 29. Syndicate Company, Mesabi, 44. Taconic series, 36, 38, 39, 40, 43, .52, 55-56. carbonate In, 40. iron ores of, 40. relations to Archean, 52. use of term, 27, 202. Taconite, 42, 47, 248. i use of term, 42, 101. volcanic nature of, 59. ('See Ferruginous chert.) Tertiary sediments, absence of in Mesabi district, 199. Test pitting, 300. Thunder Bay, Animikie of, 27, 201. Thomas Lake, Biwabik formation near, 52. Till, of Pleistocene, 192. Titaniferous magnetic iron of Duluth gabbro, 183, 223. Todd, J. E., referred to, 194. Tonnage, reserve of Mesabi iron ores, 290. Topography of Mesabi district, 20-21; summary of, 13. Tourmaline of altered Lower Huronian rocks, S3. of Embarrass granite, 187. Tower, Lower Keewatin greenstone near, 53. Townships. (.See Ranges.) Towns in Mesabi district, 23. (See Biwabik, Buhl, Chisholm, Eveleth, Grand Rapids, Hibbing, McKinley, Mesaba, Mountain Iron, Nash- wauk, Sparta, Virginia.) Transportation lines, sketch of, 19. Transportation of Mesabi iron ores described, 43, 285, 286. {.See Duluth and Iron Range Railway, Duluth, Missabe and Northern Railway, Eastern Railway of Minne- sota.) Tripoli powder in iron ore deposits, 210. Tuffaceous texture in basalt, 65. Turgite of iron ore deposits, 209. Turner, referred to, 66. Two Harbors, docks at, 285. Unconforniity between Animikie and Keewatin, 55. between Archean anC overlj'ing rocks, 23, 52, 70-71, 86, 195. between Cretaeeous and Keweenawan, 23. between Keweenawan and underlying series, 197, 198. between Pleistocene and Cretaceous, 23. between Upper and Lower Keewatin, 52. between Upper Huronian and the underlying series, 23, 86, 94-98, 180-181, 196. Underground mining. (See Mining methods. ) Union mine, analyses of iron ores from, 217. shipments from, 289. transportation of iron ores, 285. underground mining in, 282. United States Board on Geographic Names referred to, 21. United States Steel Corporation, indebtedness to, 20. iron ores controlled by, 285, 286. United States, iron ore production in, 289. Upham, W. N., referred to, 50, 192, 194. summary of literature, 49. Upper Cambrian of St. Croix Valley, relations to Keweena- wan, 57. Upper Felch Mountain series, equivalence to Upper Huron- ian, .50. Upper Huronian, 201; described, 88-205. absence of, north of Mesabi range. 196-197. correlation of, 30, 36, 41, 50, 201, 202-205. deposition of, 196. distribution of, 24, 66, 70, 73, 80, 88, 89, 91, 184, 203. folding of, 197, 198, 204, 260. initial dip of, 197. of the Lake Superior region, 200. relations to other series, 23, 35, 42, 70-71, 180-181, 187, 188, 196, 197-198, 260, 272. shore line of, 196. structure of, 15, 178-180. thickness of, 15, 55, 180. sketch of, near Mailman camps, 80. sketch of, near Hibbing, 66, 70. summary of, 14-15. Upper Keewatiu. (.See Keewatin. ) Upper Marquette, correlation of, 41, 50. Upper Vermilion, correlation of, 41, 50. Utica mine, shipment from, 289. tr-^nsportation of iron ores, 235. Van Hise, C. R., indebtedness to, 19. letter of transmittal, 11. referred to, 30, 31, 34, 44, 45, 115, 118, 128, 134, 142, 159, 174, 200, 201, 248, 255, 256, 257, 258, 263, 264, 277, 279. summary of literature, 41, 50. Van Hise, C. R., and Leith, C. K., summary of literature. 60. Vein quartz in iron-ore deposits, 212. in Lower Huronian, 83. in Pokegama conglomerate, 95, 97,* 98. in Upper Hiu'onian series, 179-180. near contact with graiiite, 83. Vermilion district, Archean sediments in, 77. correlation of iron-bearing formation, 36, 39, 40, 64. ellipsoidal greenstone in, 53, 65. 316 INDEX. Vermilion district, erosion of. 264. iron ores of. 36-37. 130. 277. 293-294. 299. Monogrrnph on, 11. 61. porphyries of. 68-69. shipment from, 289. Vermilion series, origin of. 45. Victoria mine, shipment from, 289. Iransporiation of iron ores, 285. underground mining in, 282. Virginia itown). 178. analyses of chert and slate from. 139-141. 170-171. discovery of iron ore near, 28. drainage near, 234, 235. faulting near, 46, 52. railway connections, 23. 28. rocks near. 63, 6S; photomicrograph of, 174; plates of, 72, 104. 139-140. 141, 171. texture of iron ore at, 223. Virginia mine. 43, 44. Virginia slate, described. 16S-170: summary of. 15; photo- micrographs of, 174 (Specimen 45463). analyses of. 170 (specimens 45463, 45735, 45767). cordierite hornstones in. described, 171-172. deposition of. 196. diabase sills in, 56. erosion of. 197, 199, 260. . influence on circulation of water and concentration of iron ore. 177, 179, 266, 267, 271, 279, 297. iron-ore deposits under, 206-207, 297. limestone of, 56; described, 171. nodules in, 137. relations and comparison toBiwabik formation, 47, 172- 176, 196, 297-298. relations to other formations, 42. 55, 182. thickness of, 46, 56, 177. 180, 260. Volcanic sand altering to iron ore, 56. W'acoutah property, paint rock of Biwabik "formation analysis of, 149-150. Walcott, C. D.. referred to, 257. Warren, O. B., referred to. 62. Water in iron ores. 218, 219, 235. in paint rock, 223. Weathering of gabbro. 182. Wedding, H., referred to. 62. Weed, L. B., indebtedness to, 20. West Duluth, docks at. 285. Western Jlenominee series, equivalence to Upper Huro- nian, .50. Western New England iron ores, assignment of Taconic to, 39. Wilkinson, C. D., referred to. 62. Williams mine, shipment from. 287. 289. Willis. Bailey, summary of literature. 37. Wills mine, shipment from, 289. transportation of iron ores, 285. underground mining in. 282. Winchell, H. V., referred to, 29, 30, 62. 159, 190, 191. 218. 278. summary of literature, 37, 38, 39, 41. 1 Winchell, N. H., referred to, 26, 27, 29. 30. 34, 36. 55, 62, 159. 171, 183, 221. 222, 223. 278. summary uf literature. 34. 35. 36, 37. 38, 39, 43, 44, 52, 60. Winchell, N. H., and Grant, f. S.. summary of literature. 50. Wisconsin epoch of Pleistocene, 191. Wisconsin Valley series, equivalence to Upper Huronian. 50. Wisconsin zinc ores, comparison with development of green- alite and carbonate. 258-259. Wisconsin. (See Commonwealth, Black Kiver Falls, Bara- boo.) White, C. A., on Cretaceous fossils, 190-191. White Iron Lake, granite, similarity '^ Lower Huronian granite, 188. Lower Keewatin greenstone of, 53. Whittlesey. Chas., referred to, 25. summary of literature. 33. Woodbridge, D. E., referred to, 62. Wright, referred to. 277. Wyoming mine, 43. Xanthosiderite of iron-ore deposits, 209. Yellow ocher. {See Limonite.^ Zinc ores of Wisconsin, comparison with development of greenalite and carbonate, 258-259. Zirkel, referred to, 172. Zoisite of Biwabik formation, 160, 163. O PUBLICATIONS OF UNITED STATES GEOLOGICAL SURVEY. [Monograph XLIII.] The serial publications of the United States Geological Survey consist of (1) Annual Reports, (2) Monograiahs, (3) Professional Papers, (tt) Bulletins, (5) Mineral Resources, (6) Water-Suppl}' and Irrigation Papers, (7) Topographic Atlas of the United States — folios and separate sheets thereof. (8) Geologic Atlas of the United States — folios thereof. The classes numbered 2, 7, and 8 are sold at cost of publica- tion; the others are distributed free. A circular giving complete lists may be had on apjjlication. MONOGRAPHS. I. Lake Bonneville, by G. K. Gilbert. 1890. 4°. xx, 438 pp. .51 pi. 1 map. Price 81.50. II. Tertiary history of the Grand Canon district, -svith atlas, by C. E. Button, Capt., U. S. A. 1882. 4°. xiv, 264 pp. 42 pi. and atlas of 24 sheets folio. Price |10. III. Geology of the Comstock lode and the Washoe district, with atlas, by G. F. Becker. 1882. 4°. sv, 422 pp. 7 pi. and atlas of 21 sheets folio. Price 811- IV. Comstock mining and miners, by Eliot Lord. 1883. 4°. xiv, 451 pp. 3 pi. Price SI. 50. V. The copper-bearing rocks of Lake Superior, by K. T>. Irving. 1883. 4°. svi, 464 pp. 15 1. 29 pi. and maps. Price SI. 85. VI. Contributions to the knowledge of the older Mesozoic flora of Virginia, by W. M. Fontaine. 1883. 4°. xi, 144 pp. 54 1. 54 pi. Price SI. 05. VII. Silver-lead deposits of Eureka, Nevada, bv J. S. Curtis. 1884. 4°. xiii, 200 pp. 16 pi. Price 11.20. VIII. Paleontology of the Eureka district, by C. D. Walcott. 1884. 4°. xiii, 298 pp. 24 1. 24 pi. Price 81.10. IX. Brachiopoda and Lamellibranchiata of the Raritan clays and greensand marls of New Jersey, by R.P.Whitfield. 1885. 4°. xx, 338 pp. 85 p). 1 map. Price $1.15. X. Dinocerata. A monograph of an extinct order of gigantic mammals, by O. C. Marsh. 1886. 4°. xviii, 243 pp. 56 1. "56 pi. Price 82.70. XI. Geological history of Lake Lahontan, a Quaternary lake of northwestern Nevada, by I. C. RusseU. 1885. 4°. xiv, 288 pp. 46 pi. and maps. Price 81.75. XII. Geology and mining industry of Leaclville, Colorado, with atlas, by S. F. Emmons. 1886. 4°. xxix, 770 pp. 45 pi. and atlas of 35 sheets folio. Price 88.40. XIII. Geology of the quicksilver deposits of the Pacific slope, with atlas, by G. F. Becker. 1888. 4°. xix,' 486 pp. 7 pi. and atlas of 14 sheets folio. Price 82. XIV. Fossil fishes and fossil plants of the Triassic rocks of New Jersey and the Connecticut Valley, ■ by J. S. Newberry. 1888. 4°. xiv, 1.52 pp. 26 pi. Price 81. XV. The Potomac or younger Mesozoic flora, by W. M. Fontaine. 1889. 4°. xiv, 377 pp. ISO pi. Text and plates bound separately. Price 82.50. XVI. The Paleozoic fishes of North America, by J. S. Newberry. 1889. 4°. 340 pp. 53 pi. Price 81.00. XVII. The flora of the Dakota group, a posthumous work, by Leo Lesquereux. Edited by F. H. Knowlton, 1891. 4°. 400 pp. 66 pi. Price SI. 10. XVIII. Gasteropoda and Cephalopoda of the Raritan clavs and greensand marls of New Jersey, b}^ R. P. Whitfield. 1891. 4°. 402 pp. .50 pi. Price 81. XIX. The Penokee iron-bearing series of northern Wisconsin and Michigan, by R. D. Irving and C. R. Van Hise. 1892. 4°. xix, 534 pp. Price 81.70. XX. Geology of the Eureka district, Nevada, with an atlas, by Arnold Hague. 1892. 4°. xvii, 419 pp. 8 pi. Price 85.25. XXI. The Tertiary rhynchophorous Coleoptera of the United States, by S. H. Scudder. 1893. 4°. xi, 206 pp. 12 pi. ' Price 90 cents. XXII. A manual of topographic methods, bv Henry Gannett, chief topographer. 1893. 4°. xiv. 300 pp. 18 pi. Price 81. XXIII. Geology of the Green Mountains in ilassachusetts, bv Raphael Punipellv, T. N. Dale, and J. E. Wolff. 1894. 4°. xiv, 206 pp. 23 pi. Price 81.30.' II PUBLICATIONS OF UNITED STATES GEOLOGICAL SURVEY. XXIV. Mollnsca and Crustacea of the Miocene formations of New Jersey, l)y R. P. Whitfield. 1894. 4°. 193 pp. 24 pi. Price 90 cents. XXV. The Glacial Lake Agassiz, by Warren Upham. 1895. 4°. xxiv, 658 pp. 38 pi. Price 81.70. XXVI. Flora of the Amboy clays, by J. S. Newberry; a posthumous work, edited by Arthur HoUick. 1895. 4°. 260 pp. 58 pi. Price SI. XXVII. Geology of the Denver Basin in Colorado, by S. F. Emmons, Whitman Cross, and G. H. Eldridge. 1896. 4°. 556 pp. 31 pi. Price §1.50. XXVIII. The IMarquette iron-bearing district of Michigan, with atlas, by C. R. Van Hise and W. S. Baylev, including a chapter on the Republic trough, by H. L. Smyth. 1895. 4°. 608 pp. 35 pi. 'and atlas of .39 sheets folio. Price §5.75. XXIX. Geology of old Hampshire County, Massachusetts, comprising Franklin, Hampshire, and Hampden counties, bv B. K. Emerson. 1898. 4°. xxi, 790 pp. 35 pi. Price 81.90. XXX. Fossil Medusfe, bv C. D. Walcott. 1898. 4°. ix, 201 pp. 47 pi. Price 81.50. XXXI. Geology of the Aspen mining district, Colorado, with atlas, by J. E.Spurr. 1898. 4°. xsxv, 260 pp. 4.3 pi. and atlas of 30 sheets folio. Price S3.60. XXXII. Geologv of the Yellowstone National Park, Part II, descriptive geology, petrography, and paleontology, by Arnold Hague, J. P. Iddings, W. H. Weed, C. D. Walcott, G. H. Girty, T. W. Stanton, and F.' H. Knowlton. 1899. 4°. xvii, 893 pp. 121 pi. Price .82.4.5. XXXIII. Geologv of the Narragansett Basin, bv X. S. Shaler, J. B. Woodworth, and A, F. Foerste. 1899. 4°. XX, 402 pp. 31 pi. Price 81. XXXIV. The glacial gravels of Maine and their associated deposits, by G. H.Stone. 1899. 4°. xiii, 499 pp. .52151. Price 81.30. XXXV. The later extinct- floras of North America, by J. S. Newberry; edited by Arthur Hollick: 1898. 4°. xviii, 295 pp. 68 pi. Price 81.25. XXXVI. The Crystal Falls iron-bearing district of Michigan, by J. M. Clements and H. L. Smyth: with a chapter on the Sturgeon River tongue, by W. S. Bayley, and an introduction by C R. Van Hise. 1899. 4°. xxxvi, 512 pp. 53 pi. Price 82. XXXVII. Fossil flora of the Lower Coal Measures of Missouri, by David White. 1899. 4° xi, 467 pp. 73 pi. Price 81.25. XXXVIII. The Illinois glacial lobe, by Frank Leverett. 1899. 4°. xxi, 817 pp. 24 pi Price 81.60. XXXIX. The Eocene and Lower Oligocene coral faunas of the United States, with descriptions of a few doubtfully Cretaceous species, by T. W. Vaughan. 1900. 4°. 263 pp. 24 pi. Price 81.10 XL. Adephagous'and clavicorn Coleoptera from the Tertiary deposits at Florissant, Colorado, with descriptions of a few other forms and a systematic list of the non-rhyncophorous Tertiary Coleoptera of North America, bv S. H. Scudder. ' 1900. 4°. 148 pp. 11 pis. Price 80 cents. XLI. Glacial formations'and drainage features of the Erie and Ohio basins, by Frank Leverett. 1902. 4°. 802 p]i. 26 pis. Price 81.75. XLII. Carboniferous ammonoids of America, by J. P. Smith. 1903. 4°. 211 pp. 29 pis. Price 85 cents. XLIII. The iMesabi iron-bearing district of Minnesota, by C. K. Leith. 1903. 4°. 316 pp. 33 pis. Price 81.50. In press. XLIV. Pseudoceratites of the Cretaceous, by Alpheus Hyatt, edited by T. W. Stanton. XLV. The Vermilion iron-bearing district of Blinnesota, with atlas, by J. M. Clements. All remittances must be bj' money order, made paj'able to the Director of the United States Geological Surve3^ or in currency — the exact amount. Checks, drafts, and postage stamps can not be accepted. Correspondence should be addressed to The Director, United States Geological Survey, Washington, D. C. [Take this leaf out and paste the separated titles upon three of your catalogue cards. The first and second titles need no addition; over the third write that subject under which you would place the book In your library.] LIBRARY CATALOGUE SLIPS. ■United States. Department of the interior. ( U. S. geological survey. ) Department of the interior | — | Monographs | of the | United States geological survey | .Volume XLIII | [Seal of the depart- ■ ment] | Washington | government printing office | 1903 Second title: United States geological survey | Charles D. Wal- cott, director | — | The | Mesabi iron-bearing district of Min- nesota I by I Charles Kenneth Leith | — | Charles Richard Van Hise, geologist in charge | [Vignette] | Washington | government printing office | 1903 4°. 316 pp.. 33 pis. Leith (Charles Kenneth). United States geological survey | Charles D. Walcott, di- rector I — I The I Mesabi iron-bearing district of Minnesota | by | Charles Kenneth Leith | — | Charles Richard Van Hise, geologist in charge | [Vignette] | AVashington | government printing office | 1903 4°. 316 pp., 33 pis. [United States. Department of tlie interior. ( L'. S. ijeotogical survey.) Monograph XLIII.) United States geological survey | Charles D. Walcott, di- rector I — I The I Mesabi iron-bearing district of Minnesota | by | Charlas Kenneth Leith | — | Charles Richard Van Hise, geologist in charge | [Vignette] | Washington | government printing office | 1903 4°. 316 pp., 33 pis. [United St.\tes. Department oj ilie interior. {V. .S. geological survey.) Monograph XLIII.] i. 19031 Scak' ^ ■ f , ,, ■? t Oonidur JtiliTviil 201c>«t ItlcvoiionorUiko siijiriioi- ih ci02 rrt-i LEGEND Al.UONKIAN urPEniiynoM.vs ~' i,o\m-:hhl^ cHAyrrc MiuuswswTK imiClMA?ttillAYV(MV;».NO fiJiTt. > MONOGRAPH N043 PL I