. . LIBRARY . .
Connecticut
Agricultural College.
voL...-..._i)-.i.5y —
CLASS NO J.....-'....V.
COST "J.-vvt...
DATE iV^^r.^....?. — ISO-*-
Digitized by the Internet Archive
in 2009 with funding from
Boston Library Consortium IVIember Libraries
http://www.archive.org/details/vermilionironbeaOOclem
DEPARTMENT OF THE INTERIOR
MONOGRAPHS
OF THE
United States Geological Survey
VOLUME XLV
WASHINGTON
GOVERNMENT PRINTING OFFICE
19 0 3
' i '
H'J
UNITED STATES GEOLOGICAL SURVEY
CHAKLES P. WALCOTT, ])irector
THE
VERMILION IRON-BEARING DISTRICT OF MINNESOTA
\VITH AN ATLAS
J. M:oRG^]sr cle]mej^ts
CHAELES RICHARD VAN HISE, Gkologist in Charge
AYASHINGTON
GOVERNMENT PRINTING OFFICE
1 9 0 3
^15^
CONTENTS.
Page.
Letter of transmittal 1 '
Outline of monograph 19
Chapter I. — General description of the district 29
Introduction 29
Previous geologic work in the district 29
Scope of the paper 31
Geographic limits 32
Stratigraphy 33
Physiography - 34
Belief 34
Mesabi or Giants range. 35
Area north and northwest of Giants range 36
The gabbro plateau 37
Gunfiint Lake area 38
Drainage 39
Hydrographic basin 39
Streams 40
Lakes 41
Differences in water level - 42
Water power 43
. Origin of tlie lakes 43
Plankton 46
Exposures 46
Forests 47
Soil 50
Game and fish 50
Culture 52
Indian reservation 54
Chapter II. — Resume of literature 55
History of exploration and character of the routes 56
Geological literature , 64
Chapter III. — The Archean 129
Section I. Definition and subdivisions 129
Section II. Ely greenstone 130
Features of the greenstone 130
Occurrence and character 131
Distribution - 131
Exposures 134
Topography 134
Structure 135
Petrographic characters 136
The amygdaloidal structure 139
The spherulitic structure 141
The ellipsoidal structure 144
5
6 CONTENTS.
CiiAiTER III. — The Akchean" — Continued. Page.
Section II. Ely greenstone — Continued.
Oceurrence and iliaraeter — Continued.
^Microscopic characters 150
Texture I.5I
Schistose greenstones 153
General characters 153
Origin 1 54
Contact metaniorphisni 155
Contact effect of granite on the greenstone 155
Mineralogic composition of the metamorphosed rocks 157
Contact effect of gabbro on greenstone 161
Relation to adjacent formations 162
Economic value 163
Interesting localities 165
Possible tuffs associated with the greenstones 166
Evidences of volcanic character 166
Metamorphism of the greenstones 169
Section III. Soudan formation 172
Occurrence and character 172
Distribution 172
Exposures I73
Topography 175
Structure I75
Petrographic characters 179
Macroscopic characters of the fragmental portion of the Soudan formation 179
Microscopic characters of the fragmental portion of the Soudan formation 180
Macroscopic characters of the iron-bearing formation proper 181
The iron ores , 183
^Microscopic characters of the iron-bearing formation proper 185
Origin 188
Eelations to adjacent formations 191
Relations to the Ely greenstones 191
Resume of relations to Ely greenstones 198
Eelations to the Archean acid intrusives 199
Relations to the overlying sediments ■ 199
Relations to basic eruptives 200
Age 200
Thickness 200
Interesting localities 200
Intricate foldmg of the Soudan formation 210
elastics associated with the iron-bearing formation 212
The iron-ore deposits 213
Historical sketch 213
Ore horizons 215
The Ely iron-ore deposits 216
Deposits occurring at the bottom of the iron formation 216
The Tower and Soudan deposits 222
Deposits occurring at bottom of the iron formation 223
Deposits occurring within the iron formation 224
Origin of the ore deposits 227
Methods of mining in the Vermilion district 234
Prodncticin and shipments of iron ore from the Vermilion district 241
Prospect ing 243
CONTENTS. 7
Chapter III. — The Akchean — Continued. I'age.
Section IV. Granites 246
General statement ""**'
Age of the acid intrusives 2^"
Granite of Vermilion Lake '- 247
Distribution, exposures, and topography 247
DistriVjution 247
Exposures 247
Topography 248
Petrographic characters 248
^Macroscopic characters 248
Microscopic characters 250
Folding 250
Structural features and metamorphism 251
Sericite-schists 253
Chlorite-schists 253
Schistose granites and schists derived from granites 253
Relations to adjacent formations 254
Eelations to Lower Huronian series ; - - - 254
Interrelations of granites of Vermilion Lake '- 254
Interesting localities 255
Localities showing relation between granite of Vermilion Lake and
Ely greenstone 25o
Relations of the acid intrusives to the Soudan formation 256
Relations of the different varieties of the acid intrusives of Vermilion
Lake to one another 257
Granites of Trout, Burntside, and Basswood lakes 258
Distribution, exposures, and topography 258
Distribution 258
Exposures 258
Topography 2.59
Petrographic characters - 2o9
Macroscopic characters 259
Microscopic characters 260
Relations to adjacent formations r 260
Relations to Ely greenstone 260
Relations to other intrusive rocks 261
Age 261
Folding 262
Interesting localities 262
Granite between Moose Lake and Kawishiwi River 263
Distribution and exposures - 263
Distribution 263
Exposures 264
Petrographic characters 264
Relations to adjacent formations 264
Relations to Archean 264
Relations to Lower Huronian 265
Relations to Keweenawan 265
Granite of Saganaga Lake 265
Distribution, exposures, and topography 266
Exposures -""
Distribution 266
Topography - 266
8 CONTENTS.
Chapter III. — The Archean — Continued. Page.
Section IV. Granites — Continued.
Granite of Saganaga Lake — Continued.
Petrograpliic characters 266
Macroscopic characters 266
Microscopic characters 267
Eelations to adjacent formations 268
Relations to Ely greenstone 268
Relations to Lower Huronian sediments 268
Metamorphic effects of the granite of Saganaga Lake 273
Interesting localities 273
Chapter IV. — The Lower Huronian 275
Section I. Sedimentary rocks 275
Occurrence and subdivisions 275
Vermilion Lake area 277
Distribution, exposures, and topography 277
Distribution 277
Exposures 278
Topography 278
Structure 278
Relations of the Ogishke conglomerate and Knife Lake slate 281
Relations to adjacent formations 281
■Relations to Archean 281
Relations to Giants Range granite 283
Relations to basic dikes 283
Age 284
Ogishke conglomerate 28-1
Petrograpliic characters 28-1:
Macroscopic characters 284
Origin of the conglomerates 285
Thickness 286
Interesting localities 287
Knife Lake slates 293
Petrographic characters 293
Thickness 295
Interesting localities 295
Knife Lake area 297
Subdivisions 297
Distribution, exjiosures, and topography 297
Distribution 297
Exjiosures 298
Topography 299
Structure 299
Relations 303
Relations of the sedimentary members of the series to one another 303
Relations of Lower Huronian sediments to adjacent formations 303
Relations to the Archean 303
Relations to Ely greenstone 303
Relati<ins to the Soudan formation oO-l
Relations to the granite of Saganaga Lake 305
Relations to i^ower Huronian 305
Relations to the Giants Range, Snowbank, and Cacaquabic granites,
and various dikes of granites and granite-porphyry 305
Relations to certain basic and intermediate dikes of Lower Huronian
age 306
Relations to the Upper Huronian series 306
CONTENTS. 9
Chapter IV. — The Lower Hueonian — Continued. Page.
Section I. Sedimentary rocks — Continued.
Knife Lake area — Continued.
Relations — Continued.
Relations of Lower Huronian sediments to adjacent formations — Continued.
Relations to the Keweenawan 307
Relations to the Keweenawan gabl>ro 307
Relations to basic dikes 307
Age 308
Ogishke conglomerate 309
Petrographic characters 309
Macroscopic characters 309
Microscopic characters 312
Metamorphism 313
Thickness 317
Interesting localities 317
Agawa formation (iron bearing) 324
Distribution and exposures 324
Distribution 325
Exposures 326
Structure 326
Petrographic characters 327
Origin 329
Relations to other formations 329
Age 330
Thickness 330
Interesting localities 330
Knife Lake slates 335
Petrographic characters 336
Macroscopic character 336
Microscopic character , 337
Metamorphism 340
Contact effect of the granite 340
Contact effect of the gabbro ; 342
Petrographic characters of the metamorphosed slates 344
Microscopic characters 344
Thickness 346
Interesting localities 347
Structure and relations of the slates 347
Contact metamorphism of the slates 349
Section II. Acid and basic intrusives 352
Introduction 352
Giants Range granite 353
Distribution, exposures, and topography , 353
Distribution 353
Exposures : 354
Topography 354
Petrographic characters 354
Macroscopic characters 354
Relations to adjacent formations 356
Relations to Ely greenstone 356
Relations to Soudan formation 356
Relations to the Lower Huronian sediments 356
Relations to the gabbro 357
Age 358
1 0 CO^'TENTS.
Chapter IV. — The Lower HrROXiAX — Continued. Page.
Section 11. Acid and basic intrusives— Continued.
Giants Range granite — Continued.
Folding 35S
iletamorphic action 359
Interesting localities 359
Relations to the Ely greenstone 359
Relation to Soudan formation 359
iletamorphism caused by granite 359
Snowbank granite : 361
Distribution, exposiires, and topography • 361
Petrographic characters 361
Relations to adjacent formations 363
Relations to the Lower Huronian 363
Relations to the Keweenawan gabbro 363
Interesting localities - 364
Cacaquabic granite 364
Distribution, exposures, and topography 365
Petrograjihic character 365
Relations to adjacent formations 368
Relations to the Lower Huronian 368
Relations to- the gabbro 368
Age - 369
Interesting localities 369
Various acid dikes 369
Distribution 369
Petrographic characters 370
Macroscopic characters 370
Microscopic charactere 370
Relations to adjacent formations 370
Basic and intermediate intrusives 371
Intei'esting localities 373
Chapter V. — The Upper Hcroxiax (Asimikie) 374
Section I. Gunflint formation 374
Distribution, exposures, and topography 375
Distribution 375
Exposures 376
Topography 376
Structure - 376
Petrographic characters 377
Relation to other formations 387
Relations to the Lower Huronian series — Ogishkeconglomerateand Knife Lakeslates. 388
Relations to the Keweenawan (Duluth) gabbro 389
Relations to basic dikes 390
Thickness 390
Section II. Rove slate 390
Distribution, exposures, and topography 391
Distribution 391
Exposures 391
Topography 391
Structure 392
Petrographic characters 392
Microscopic characters 393
Contact metamorphism 393
CONTENTS. 11
Chapter V. — The Upper Hi'RONian (Aximikie)— Continued. Page.
Section II. Eove slate — Continued.
Eelation to adjacent formations 395
Relations to the Keweenawan dolerite 395
Relations to the Keweenawan (Duluth) gabbro 395
Age 395
. Thickness - 395
Chapter VI. — The Keweenawan 397
Section I. Duluth gabbro and Logan sills 397
Distribution, exposures, and topography 398
Distribution 398
Exposures 399
Topography 399
Petrographic characters of the gabbro 401
Macroscopic characters 401
Microscopic characters 404
Petrographic characters of the Logan sills 405
Macroscopic characters 405
Microscopic characters 406
Constituents 406
Relations of the gabbro to adjacent formations 406
Relations to the Ely greenstone 406
Relations to the Lower Huronian sediments 407
Relations to the Giants Range granite 407
Relations to the Snowbank and Cacaquabic granites ' 407
Relations to the Upper Huronian sediments 407
Relations to the Keweenawan 407
Relations of the Logan sills to adjacent formations 408
Relations to the Upper Huronian ( Animikie) 408
Relations to the Keweenawan 410
Relations of the gabbro to the Logan sills 410
Conclusions as to age and relation of the gabbro and sills -- 417
Metamorphic effect of gabbro and sills 418
Archean (Ely) greenstone 418
Lower Huronian • 419
Upper Huronian (Gunflint formation) 419
Rove slates 419
Endomorphic action 419
Iron-oxide bodies in the gabbro 420
Character, occurrence, and origin ot the iron-oxide bodies 420
Section II. Acid dikes younger than the Duluth gabbro 422
Section III. Basic intrusives younger than the Duluth gabbro 422
Petrographic characters 423
Macroscopic characters • 423
Microscopic characters 423
Relations to adjacent formations, and age 424
Metamorphic effects 424
Chapter VII. — The drift 425
Chapter VIII. — Topoc;raphy of the district in its relations to geologic structure 431
Chapter IX. — Geologic history of the Vermilion district 437
Index 449
ILLUSTRATIONS.
Paee.
Plate 1. Colored map showing disti-ibution of pre-Cambriaii and other rocks in the Lake
Snperior region, and the geographic relations of the Vermilion district of Minnesota
to the other iron-bearing regions of Lake Superior 32
II. General geologic map of the Vermilion district 34
III. .-1 and B, Spherulitic texture in the greenstones 142
IV. A, Ellipsoidal parting in greenstone; B, EUipsoidally parted greenstone, showing
spherulitic development 146
V. A, Amygdaloidal greenstone (metabasalt); B, Magnetitic chert, showing possible
lines of false bedding 168
VI. A, Folded jasper about one-half mile east of Jasper Peak; B, Contorted jasper and ore
in the NE. i sec. 2.5, T. 63 N., K. 12 AV.; C, A breccia of jasper and chert fragments
in greenstone matrix 176
A^II. Folded jasper and slate, showing slaty cleavage developed in slate bands 178
VIII. Panoramic view of the ore basin north of Ely, showing shafts of the Chandler and the
Pioneer mines 216
IX. A, View of main level timbering, Jlinnesota mine; B, Filling system, Minnesota
Iron Company, Minnesota, with chute for discharging refuse from upper levels 236
X. A, View showing method of loading cars; B, View of main drift which has begun to
cave. Chandler mine 238
XI. Scrammers in mine using caving system 240
XII. A, Photomicrograph, showdng granules in Gunflint formation; B, Photomicrograph,
showing details of granules 382
XIII. .4, View of sawtooth hills of Rove slate capped with dolerite sills, at the northeast
end of Rose Lake, international boundary; B, View on an island in Burntside Lake,
showing granite of Burntside Lake cutting amphibole-schists, metamorphosed Ely
greenstone 392
Fig. 1. Reproduction of sketch by A. Winchell showing the intricate relationship between the
granite of Burntside Lake and the amphibole-schists lo8
2. Reproduction of sketch by A. "Winchell showing the intricate relationship between the
granite of Burntside Lake and the amphibole-schists 159
3. Sketch showing Soudan formation infolded in Ely greenstone, both cut by Keweenawan
dolerite dike , , 205
4. Illustration showing distribution and relations of Ely greenstone, Soudan formation,
and Ogishke conglomerate south of Moose Lake 206
5. Sketch showing association and relations of Ely greenstone, Soudan formation, and
Ogishke conglomerate 207
6. Sketch showing relations between Soudan formation and Ely greenstone on Otter Track
Lake - 208
7. Sketch made in the field, showing relations of Soudan formation and Ely greenstone on
Jasper Lake 209
•8. Vertical section across Chandler ore body along line E-F of fig.' 9 216
13
14 ILLUSTRATIONS.
Page.
Fill. 9. Horizontal section througli fourth and sixth levels of the Chandler mine 217
1 0. A'ertical east-west section through the Chandler mine 218
11. Vertical .section through the Chandler ore basin along the line A-B of fig. 9 219
12. Reproduction of sketch showing replacement of jasper by iron ore 231
13. Cross section at Ko. 8 shaft, Soudan, Minn ' 235
14. Horizontal section through the fifth level of No. 8 shaft, Soudan, Minn 236
15. Longitudinal section through Soudan mine 237
16. Cross section of Soudan mine showing raise 238
17. Detail geologic map showing exposures in a small area on West Gull Lake 272
18. Detail map of east end of Ely Island 282
19. Sketch showing intricate relationship of granite-porphyry and overlying Ogishke con-
glomerate on Ely Island, Vermilion Lake 290
20. Sketch showing relationship of Ely greenstone and overlying Ogishke conglomerate on
island in Ogishke Muncie Lake ; 22
21. Diagrammatic section across the west end of Gunfiint Lake, illustrating the character-
istic topograph}' of the Rove slate area 392
22. Large-scale section through the Rove slates with intercalated Logan sills 400
23. Sketch map showing the distribution of the Vermilion moraine in the Vermilion dis
trict of Minnesota ..' -- 427
ATLAS SHEETS.
Sheet.
Title I
Contents II
Legend and key map Ill
Topographic map of part of tlie Tower quadrangle IV
Topographic map of part of the Soudan quadrangle Y
Topographic map of part of the El}' quadrangle VI
Topographic map of parts of the Basswood and Fall Lake quadrangles VII
Topographic map of parts of the Ensign and. Snowbank Lake quadrangles VIII
Topographic map of part of the Knife Lake quadrangle IX
Topographic map of part of the Gunflint Lake quadrangle X
Geologic map of part of the Tower quadrangle, with structure sections XI
Geologic map of part of the Soudan quadrangle, with structure sections XII
Geologic map of j)art of the Ely quadrangle, with structure sections XIII
Geologic map of part of the Basswood and Fall Lake quadrangles, with structure sections XIV
Geologic map of part of the Ensign and Snowbank Lake quadrangles, with structure sections. XV
Geologic map of part of the Knife Lake quadrangle, with structure sections XVI
Geologic map of part of the Gunflint Lake quadrangle, with structure sections XVII
Detail map of a portion of Tps. 61-62 N. , E. 15 W. , Minnesota XVIII
Detail map of a portion of Tps. 62-63 X., R. U W., Minnesota XIX
Detail map of a portion of Tps. 62-63 N., E. 13 W., Minnesota XX
Detail map of portions of Tps. 62-63 N., E. 12 W., Minnesota XXI
Detail map of a portion of T. 63 N., E. 11 W., Minnesota XXII
Detail map of Tower and Lee hills XXIII
Detail map of Soudan Hill XXIV
Detail map of laoint south of Mud Creek Bay, Vermilion Lake, ilinnesota XXV
Detail map showing actual exposures in NE. \ sec. 25, T. 63 X., E. 12 W., with structure sec-
tions XXVI
15
LETTER OF TRANSMITTAL.
Department of the Interior,
United States Geological Survey,
Washington, D. C, June 30, 1902.
Sir: I transmit herewith the manuscript, illustrations, and atlas of
a monograph on the Vermilion iron-bearing- district of Minnesota, b)^
J. Morgan Clements.
This monograph is the fifth one of a series of six which treat of the
iron-bearing districts of the Lake Superior region. The monographs on
the Penokee, Marquette, Crystal Falls, and Mesabi districts have already
been published. The last of the series is a monograph on the Menominee
district, by W. S. Bayley. There has also appeared a monograph on ihe
copper-bearing rocks, by R. D. Irving. It is planned to close the work of
the United States Geological Survey in the Lake Superior country by a
final monograph on the Lake Superior region as a whole.
This report contains the first series of detailed maps of the Vermilion
district. This region is one in which the rocks of Archean age contain
economic deposits. The geologic mapping of the intricately folded Archean
rocks has been a task of very great labor, requiring the full field seasons of
several men from 1897 to 1899, inclusive, and a part of that of 1900. The
area mapped in detail is about 1,000 square miles.
The topographic work for the report was done by Robert Muldrow
and E. C. Bebb, with various assistants. The geologic mapping has'
been done more largely by J. Morgan Clements than anyone else, but
W. S. Bayley and C. K. Leitli have also done a large amount of areal
mapping, and W. N. Merriaim has made a number of large-scale, detailed
plats of certain areas having exceptional economic importance. My own
MON XLV— 03 2 17
18 LETTER OF TRANSMITTAL.
part of the work lias been a general super-vision of the survey. This
has involved frequent trips into the district, made in order to assist in
solving the general structural jDroblems.
In our work on the Vermilion district we have had the willing help of
the officers in charge of the mines, and we are very greatly indebted to
them for their assistance. In this connection we would especially mention
Mr. D. H. Bacon, who formerly was president of the Minnesota Iron Com-
pany, and Mr. T. F. Cole, now president and general manager of the
Minnesota Iron Company.
Very respectfully, C. R. Van Hise,
Geolofjist in Charge.
Hon. Charles D. "Walcott,
Director of United States Geological Survey.
OUTLINE OF MONOGRAPH
Chapter I. The Vermilion iron-bearing district of Minnesota resembles the
other iron-bearing districts of the Lake Superior region in that the rocks are of
very great geologic age. Its economic importance has been known for a rela-
tively short time, the first published statement of the occurrence of iron ore in this
district having been made in 1850. A brief statement is made of the geologic work
previously done in this district, including the names of the geologists hj whom it
was done, and the scope of the paper is then outlined. The territory included in
the Vermilion iron-bearing district lies in the extreme northeastern portion of
Minnesota, including portions of St. Louis, Lake, and Cook counties. The district
has an area of approximately 1,000 square miles. It is a narrow belt trending east-
northeast, which ranges from 2 to IS miles in width, and has a length of somewhat
over 100 miles, extending from the west end of Vermilion Lake to Gunflint Lake,
on the international boundarj'. From a topographic standpoint the Vermilion
district is divisible into four areas, each of which is characterized b}' a fairly
distinct type of topographic development. The first of these areas described is
the one including the Giants range, the most prominent topographic feature of the
Vermilion district. The range reaches an extreme height of 2,120 feet above sea
level, but in general is not a very prominent feature throughout its extent. It
forms the backbone of the district, extending across it in a northeast direction and
dividing it into unequal areas. The second area described lies north of the Giants
range and includes all of the areas underlain by the most important iron-bearing
formation. This area is characterized by ridges trending N. 70°-80'^ E., with
intervening valleys, the larger ones usually occupied by streams or lakes. In this
area the topography is very rugged, but the range in altitude is not great. The
third area described is the high plateau country Ijnng southeast of the Giants range
and underlain by gabbro. The fourth is a small triangular area at the extreme east
end of the district, lying between the Giants range on the north and the gabbro
plateau on the south. In this area a rather peculiar topography- is developed. The
hills have abrupt north escarpments and gentle south slopes. These ridges lie the
one south of the other, and present in profile the appearance of a series of saw
teeth; hence they are commonly spoken of as "'sawtooth mountains." That the
drainage system is immature is shown by the abundance of lakes in the district, by
19
20 OUTLINE OF MONOGRAPH.
the absence of large streams, by the fact that the siiiull and short streams which
do exist serve mei-ely to connect the lakes into strings, and by the fact that these
streams are frecjuently interrupted in their courses by rapids and falls. Large
swamps still further emphasize this imperfect drainage. The lakes and streams
that feed and drain them belong to the large basins of the St. Lawrence Hirer
and Hudson Bay. The area belonging to the St. Lawrence drainage basin is very
small and is drained by only one small stream, representing the headwaters of the
Embarrass River, which flows south and linalh' empties into Lake Superior. By
far the greater part of the district belongs to the Hudson Bay drainage system.
All the waters of this sj'stem flow north and west, and are collected in Rainy Lake.
The streams are short, narrow, and shallow, and form merelj' the connections
between the numerous lakes. The lakes are far more abundant in the eastern than
in the western poition of the district. They lie in basins which in general trend
east-northeast and constitute the main routes of travel within the district. Most
of the lakes have had a mixed mode of origin, owing their existence to pre-
Glacial erosion, which scooped out deep valleys, and then to the drift, which left
dams across these valley's at intervals along their lengths, forming the strings of
lakes that we now find. Other lakes appear to owe their present location and
existence solelj' to glacial action. Thej- are depre.ssions in the drift which have
been filled hy water. -Rock exposures are numerous, especially in the immediate
vicinity of the lakes, and are particularly abundant in the eastern part of the
district. Only a small area in the district is wooded with old forests. A very
large part of the district, particularly the eastern portion, has been burned over
repeatedly, and here almost all growth is wanting, or there is but a meager second
growth of small timber present. The district is well supplied with fish and game.
There are only four towns in the district — Tower. Soudan, Ely, and Winten. Tower
is the oldest: it was settled in 1882, and has 1,366 inhabitants, according to the
Twelfth Census. Elv. the largest place, has 3,717 inhabitants. The first three
places named depend almost altogether upon the mining industry. • Winten is a
small village whose existence is dependent upon two sawmills which are rapidly
cutting the timber remaining in the district. There is one Indian reservation in the
district, that of the Bois Fort band of the Chippewa Indians, on Sucker Point,
where there are reported to be 808 Indians living. As a matter of fact, there are
rai'ely more than 7.5 or 100 Indians actually upon the reservation, at least during
the summer, the remainder being scattered through the surrounding country. They
are not progressive, and while apt in the acquirement of the vices of civilization,
do not appear to be willing to bear any of its burdens.
Chaiter II. In this chapter there is given a 1)rief statement of the main canoe
routes of the district. The. methods of travel are described by quotations from the
journals of the fur traders, as this shows the conditions existing when tht> country
was being opened up. The methods of tra\el are now essentially the same, by
canoe, although manv of the old customs have died out. The remainder of the
OUTLINE OF MONOGRAPH. 21
chapter is devoted to abstracts of articles dealing with the geology of the district.
In these abstracts the authors have been quoted very freely. From a perusal of this
chapter one can obtain an idea of the growth of knowledge of the geology of the
district, which is comparativelj^ difficult of access.
Chapter III. This chapter deals with the Arphean.
Section I gives the definition and subdivisions of the Archaen. As a result of
studies made largely in this district it was found necessary to modify the definition
of the Archean so as to include within it some small quantities of sediments. The
Ai'chean of the Vermilion district is divided into thi'ee formations, as follows, given
from the base up: The Ely greenstone, the iron-bearing Soudan formation, and the
granites of Vermilion, Trout, Burntside, Basswood, and Saganaga lakes.
In Section II the Elj' greenstone is described. This formation consists of basic
to intermediate igneous rocks, and is the lowest member of the geologic column.
These greenstones are very widely distributed and occur normally in anticlinal areas,
as is shown by the distribution of the overlying sedimentaries. A petrographic
study of the greenstones shows that they were originally rocks corresponding in
character to intermediate andesites and basic basalts. They have been extremely
altered, but retain in many cases in striking perfection the original structures, such
as ellipsoidal parting and spherulitic and amygdaloidal structures. A study of their
various textures and structures shows that these greenstones are unquestionaljl}^ of
igneous origin, and are largely of volcanic character. With the volcanics there are
associated, of course, some intrusives of essentially the same age. These have been
subjected not only to the ordinar}- processes of alteration that have metamorphosed
the greenstones, but have been strongly compressed and in many cases have become
schistose. Actual green schists, however, are very subordinate in quantity. The
greenstones have also been strongly affected by the contact metamorphism due to
the intrusion of great granite masses. As a result of this intrusion there have been
produced from the greenstones amphibole-schists, which form a marginal facies of
the greenstones, lying between them and the adjacent granites. The gi'eenstones
have also been metamorphosed by the Duluth gabbro of Keweenawan age, and
granular rocks have thus been produced which in most cases show the original
textures of the greenstones, but contain also a development of fresh biotite, hyper-
sthene, brown-green hornblende, and magnetite. These greenstones have very
slight value at present, although they make good road material.
In Section III the iron-bearing Soudan formation of the Archean is treated. The
iron formation is widel}^ distributed in the western part of the district, but is
practicall}' wanting in the eastern half. Where it occurs it is found mosth' in
narrow belts, which consist largelj^ of greenstone so intimately associated with the
iron formation that it has been impossiVjle to separate them on the map. In spite
of the resistant character of the rocks constituting the formation, exposures are not
verj' good, and it has been difficult to trace out continuous belts. The Soudan
being the oldest sedimentary formation in the district has been subjected to all the
22 OUTLINE OF MONOGKAPH.
oroo-eiiic movements that have occurred since its deposition. In consequence of thi?
its rocks hu\e been most intricately folded. Where it is exposed most prominenth"
it forms anticlines, although upon these are numerous minor rolls, giving- folds with
Steep pitches. The formation consists of (1) a very subordinate fragmental portion
made up of some conglomerate, clearly recognizable as having been derived from
the underlying greenstones, grading up into sediments of finer character; and (2)
lying above this fragmental portion, the iron-bearing formation proper, which
consists of siliceous rocks, largely white cherts— though varying in color from white,
green, yellow, and purplish to black — with red jasper and carbonate-bearing chert,
griinerite-magnetite-schist, hematite, magnetite, and small quantities of pyrite.
These various rocks occur in bands of varying thickness. Where banded they
rarely exceed a thickness of 5 or 6 inches. The hematite occurs in certain places
in masses of variable size, which constitute the ore deposits. These iron-bearing
rocks are clearly of sedimentary origin. They do not now present their original
characters, but are presumed to have been derived from rocks that were largely
carbonate-liearing, ferruginous cherts. The relation of the iron formation to the
adjacent greenstones is clearly that of a sedimentary overlying an igneous series.
The few basal conglomerates of the iron formation that have been found consist of
pebbles derived from the underlying greenstone, showing conclusively their relation-
ship. This relationship is obscured, however, in most places, by the absence of
the conglomerates, and by the fact that the iron formation has been very closely
infolded in the greenstone. In consequence of the extreme folding and of the
impossibility of determining different horizons in the iron formation, it has been
impracticable to ascertain its thickness.
The first published statement of the occurrence of iron ore in the Vermilion
district was made by J. G. Norwood in 1850. After a brief period of exploration
for gold in the sixties the attention of explorers was turned to the development of
the iron deposits. As the result of this development a railroad was built to Tower
in 1884, and shipments of the ore began. The ores are extremely hard, massive,
blue hematites. In the Chandler mine the ores have been brecciated, but the
fragments of the breccia are .still the hard blue hematite, averaging about 63. T per
cent iron, 0.05 per cent phosphorus, -±.78 per cent silica, and 5.5 per cent water.
The iron-ore deposits of the Vermilion district show a striking analogy wifh
those of the Marquette district. Like them, they may occur in two positions with
respect to the iron-bearing formation. They are found first at the bottom of this
formation, and second within it, the ores in both cases being the same in character.
The largest known deposits are at Ely. These are typical of the deposits occurring
at the base of the formation. They are found at the bottom of a closely compressed
.syncline of the iron formation which lies in the relatively impervious greenstone.
The source of the iron was. in the first instance, the Ely greenstone. From this
it was removed through the action of water and collected in the Archean sea to
form the sedimentary deposits of the .Soudun formation. After tlH> folding of the
formation this disseminated iron was carried l>y downward-percolating waters into
OUTLINE OF MONOGRAPH. 23
places favorable for its accumulation, such as the bottom of this synclinal trough,
where it was precipitated by oxygen-bearing waters coming more directly from the
surface. Pari passu with this precipitation silica was removed, affording space for
the accumulation of the iron to form the ore deposits as now known. The Tower
and Soudan deposits differ only in detail from the Ely deposit. They were accu-
mulated in favorable places both at the bottom of the formation, where it rests
against the greenstone in which it is infolded, and within the formation in basins
formed by the intrusion and subsequent folding of igneous rocks. The mode of
accumulation in these is the same as that briefly outlined for the Ely deposits.
The methods of mining in the Vermilion district are briefly described.
In Section IV are described certain acid intrusives varying from fine- to coarse-
grained granites, and from porphyries with very fine-grained groundmass to granite-
porphyries. The granites are known from the topographic features with which
they are associated, as the granites of Vermilion, Trout, Burntside, and Basswood
lakes, the granite between Moose Lake and the Kawishiwi River, and the granite of
Saganaga Lake. All of these rocks are younger than the Ely greenstone, for they
occur in it as dikes. A number of these dikes are found also in the iron-bearing
Soudan formation, which is of more recent origin than the greater part of the Ely
greenstone. That these intrusives are older than the Ogishke conglomerate (Lower
Huronian), which succeeds in age the Soudan formation, is shown conclusively by
the fact that pebbles derived from them occur in this conglomerate. The general
period of intrusion of all of these acid igneous rocks is placed between the time of
the deposition of the latest sediments of the Archean and that of the deposition of
the earliest sediments of the Lower Huronian series. Some were perhaps intruded
near the beginning of this interval, others probably near the end, but it is now
impossible to give their exact ages. In the portion devoted to the granites of the
different areas the various intrusives are described somewhat in detail. Their petro-
graphic characters are given as hornblende- and mica-granites, and the various
schistose rocks produced from them are described.
Chapter IV. This chapter is devoted to a description of the Lower Huronian
series. In Section I are discussed the sedimentary rocks of this series, which have
a very large surface extent in the Vermilion district. They are present in two
large detached areas, one of which, known as the Vermilion Lake area, extends from
the western limit of the acea mapped, in the vicinity of Tower, to within about 11
miles of Ely on the east. The second area begins about 7 miles west of Ely and
extends eastward to the eastern limit of the area mapped. This is known as the
Knife Lake area. The rocks of these two areas, although of slightly different
petrographic character, are of essentially the same age. At the base of the series
there lies a great conglomerate, known as the Ogishke conglomerate. The relation
of this conglomerate to the formations previously described is conclusively shown
by the fact that it consists of pebbles and finer detritus derived from the Ely green-
stone, the Soudan formation, and the various acid intrusives already mentioned.
Above this conglomerate in the eastern portion of the district there are found m a
24 OUTLINE OF MONOGRAPH.
few localities small masses of the iron-bearing Agawa formation. This formation
is petrographically the same as the Soudan fonnation. In it, however, there is in
places a development of the carbonate-bearing^ facies. No iron ores have been
found in it, and it is of so small a surficial extent, and so thin, that no large iron-ore
deposits will probablj' ever be found in it in the United States in the Vermilion
disti'ict proper. A reconnaissance made in the adjacent portion of Ontario indicates
that it is there better developed than on the United States side of the border, and it
may possibly contain iron deposits in this area, although this does not seem to be
very probable. This iron-bearing formation is wanting in the western portion of
the Vermilion district.
Overlying the Ogishke conglomera,te in the western portion of the district and
the intervening iron-bearing Agawa formation where present in the eastern portion
of the district, there occurs a thick series of slates of varying character, to which
the name Knife Lake slates has been given. These slates have been very closelj'
folded. Owing to the lack of well-defined horizons in the conglomerates and in the
slates it has been impossible to trace out the structure of this series by following
key rocks. The folding has, however, been proved in manj' localities by a study of
the distribution of these rocks. The relation of this series to the older rocks is
shown by the fact that it consists of detritus derived from these older rocks. In
three large areas granites which are younger than the sediments are associated
with them. These granites are known as the Giants Range granite, the Snowbank
granite, and the Cacacjuabic gi'anite. This relationship is proved by the fact that
these granites cut through, send offshoots into, and have metamorphosed the
sediments. As a result of this metamorphism, micaceous conglomerates in which
the conglomeratic structure is still recognizable have been produced from the
Ogishke conglomerate, and mica-schists have been produced from the Knife Lake
slates. These sediments are also metamorphosed by the Duluth gabbro, which has
changed them into mica-schists. Hence the gabbro is younger than the sediments.
In addition there are found in the rocks of the series certain basic dikes which are
similar to others which cut the Duluth gabbi'o, and which are considered to be of
Keweenawan age.
In Section II of this chapter various acid intrusives of the same general
character petrographically, and of the same geologic age, are discussed. They are
granites and granite-porphyries whicli occur in large masses and in dikes penetrating
the surrounding Lower Huronian sediments and other adjacent rocks. From their
occurrence in the vicinity of the Giants Range, Snowbank Lake, and CacaquaMc
Lake these names have been given to the gTanites occurring in these areas,
respectively. There is included also a description of some acid and intermediate
intrusives of the same age as the large masses of acid intrusives. The Giants Range
granite is a h()rnl)lende-mica-granite, and varies from very tine-grained rocks
through medium-grained to coarse-grained rocks. The Snowbank gi-anite also
varies from tine- to coarse-grained forms, with medium-grainod facies as the most
OUTLINE OF MONOGRAPH. 25
abundant type. Cei'tain porphyritic facies of the granite also occur. This granite
varies from a normal mica- and hornblende-granite to an augite-granite, and by
loss of quartz to a syenite. The Cacaquabic granite is somewhat more interesting
than the preceding ones, in that it is one of the rather exceptional augite-soda-granites.
The main mass of this granite is developed as a medium-grained gra}^ or pink to red
granite, whereas on the periphery of the granite area a finer-grained granite and
also a granite-porph^-rv facies of the rock are developed. In addition to this there
are various granite and granite-porphyry dikes whose immediate relationship to the
granite niassives already described could not be traced in the field. A section ia
devoted to a brief description of certain basic and intermediate intrusives of doleritic
and lamprophyric character, which bear the same relations to the various adjacent
formations as do the acid rocks previously described.
Chapter V. This chapter treats of the Upper Huronian (Animikie) series.
This series is found in the extreme eastern portion of the district, where it underlies
a relatively small area. It is known, however, to have enormous development
to the east, immediately beyond the limits of the Vermilion district, and also to the
south-southwest, in the adjacent Mesabi district. This Upper Huronian series may
be readily divided into two facies of I'ocks that are quite different petrographically.
At the bottom of the series occurs an iron-bearing formation known as the Gunllint
formation. Above this occurs a great slate-graywacke formation to which the
name Rove slate has been given. The Gunflint formation is correlated with the
Biwabik formation of the Mesabi district. It has a verj^ limited development in
the Vermilion district, and its most interesting phases are especially well developed
in the vicinity of Akeley Lake. In general the rocks of this formation have a
monoclinal dip to the south-southeast at a low angle, but variations in the strike
and dip indicate clearly that the structure is not so simple as it appears to be.
Minor folds have been traced. Petrographically the rocks of the Gunflint formation
are peculiar. Where least metamorphosed, they consist of thin bands of nearly
pure chert alternating with cherty and granular quartzose bands containing varying
percentages of iron carbonate, bands of jasper, magnetitic chert, and other bands
consisting of quartz as a basis with actinolite and griinerite crystals. With these
minerals are always associated more or less ferruginous carbonate, magnetite,
hematite, and limonite. In these rocks we find developed the peculiar oval, crescent-
shaped, and rounded granules which are so characteristic of the Biwabik formation
of the Mesabi range — granules made up in their freshest condition of a hjalrous
ferrous silicate of varying shades of green. These rocks have been extremely
metamorphosed by the Duluth gabbro. Where most metamorphosed the iron-bearing
Gunflint rocks are composed of coarsel}' crystalline bands of quartz, of varying
width, alternating with coarselv crystalline bands of magnetite ore reported to vary
from 1 inch up to 10 or 12 feet in thickness, and of bands of dark-green, brown, or
black rocks that consist of combinations of quartz, augite, hypersthene, hornblende,
olivine, and magnetite as the principal minerals, but associated occasional^ with
2G OUTLINE OF MONOGRAPH.
some ferruginous carbonate, actinolite, and griinerite. The rounded granules are
sometimes preserved in tliese rocks, sliowing their derivation from the least
metamorphosed forms previouslj' mentioned, although the granules consist of
minerals different from those in the least metamorphosed forms. The Rove slate
conformably overlies the Gunflint formation. The rocks of this series show nothing
of especial interest. They have been metamorphosed slightly as a result of the
contact action of the adjacent Duluth gabbro mass and the intrusive Logan sills.
Chapter VI. This chapter treats of the Keweenawan series. The only rocks
of this age in the Vermilion district are gabbros forming a part of the Duluth gabbro
mass of northeastern Minnesota, certain great basic sills to which the name Logan
sills has been given, and some few basic and acid dikes which cut all the rocks
of the district, including the aforementioned Duluth gabbro and the Logan sills.
The studies of the writer and his associates have been confined chiefly to the
northern edge of the Duluth gabbro, which appears in the Vermilion district.
Several reconnaissance trips have also been made into the area underlain by the
gabbro. As a result of these studies, the Duluth gabbro is found to vary in
texture from a coarse-grained granular rock to a relatively fine-grained rock. It
also has in places a gneissic structure. Under the microscope the texture is seen to
vary from granular to ophitic. The Logan sills are great masses of doleritic rocks
that occur for the most part as sills interbanded with the Upper Huroniau sediments,
and at times cut in dike form across them. Petrographically these dolerites range
from coarse-grained rocks, found in the centers of the sills, with an imperfect
granular texture and verj^ similar to the gabbro, through normal ophitic dolerites.
to intersertal textured basalts on the selvages of the sills. The gabbro is found to
metamorphose all of the sediments already enumerated, and is thus shown to be
one of the youngest rocks of the district. It is also found to be intrusive in the
Keweenawan volcanics. A number of facts are enumerated to show that the gabbro
and the Logan sills are of essentialh" the same petrographic character, although
they exhibit minor differences that are readily explicable when one considers the
relative amounts of the two rocks. After a consideration of these facts and of
the stratigraphic relationship of the rocks the conclusion is reached that the gabbro
and the sills are of essentially the same composition and age, having been derived
from the same pai-ent mass of magma. In certain localities in the Duluth gabbro
there are found masses of titaniferous magnetite of varying size, with some associated
minei'als. These masses grade into the surrounding gabbro, and were formed as the
result of processes of segregation. No published description has yet been given, so
far as the writer knows, of any large continuous masses of titaniferous magnetite in
these gabbros, and he knows of none from personal observation. If, however, large
masses do exist their content of titanium would prevent them from being of value at
the present time, when, according to the modern iron-smelting practice, titaniferous
ores can not be smelted economically. In a short section mention is made of the
acid dikes that are younger than the Duluth gabbro, and of certain basalt anddolerite
OUTLINE OF MONOGRAPH. 27
dikes that are younger than the gabbro and that cut the acid dikes, which themselves
cut the gabbro.
Chapter VII. In this chapter the drift is briefij^ described, the general distribu-
tion of the Vermilion moraine is outlined, and the locations of certain glacial lakes
are stated.
Chapter VIII. This chapter consists of a brief discussion of the topography of the
district in its relation to the geologic structure.
Chapter IX. This chapter is devoted to a discussion of the general geologic
history of the district as determined by the various facts set forth in previous
chapters.
THE VERMILION IRON-BEARING DISTRICT
OF MINNESOTA.
By J. Morgan Clements.
CHAPTER I.
GENERAL DESCRIPTION OF THE DISTRICT.
INTRODUCTIOIV.
The Vermilion irou-beariug district of Minnesota is like all of the other
iron-bearing districts of the Lake Superior region in that the rocks are of
verv great geologic age. Its economic importance has been known, how-
ever, for a comparatively short period. The first statement of the existence
of iron ore in this district is credited to J. Gr. Norwood, who observed it upon
his explorations in 1850 and refers to it in his report." It was not until the
early eighties that a determined effort was made to develop the iron resources
which some then knew were in this district. In 1884 the railroad from
Duluth was completed to Tower, and the first shipment of iron ore was made.
From this time on the development of the iron resources of the district was
rapid, as is shown by the annual increase in the shipments of ore. This
•increase, with minor fluctuations in 1893 and 1898, caused by financial
conditions, continued up to the season of 1902, when the maximum ship-
ment for the district, 2,083,784 tons, was reached.
PREVIOUS GEOLOGIC WORK IN THE DISTRICT.
Mr. Bailey Willis, special agent of the Census Office of the United
States, spent one month, October 10 to November 10, 1880, studying the
"Report of a geological survey of Wisconsin, Iowa, and "Minnesota, by D. D. Owen, 1852, report of
J. G. Norwood, p. 417.
29
30 THE VERMILION IRON-BEARING DISTRICT.
geology of the part of the Vermilion district in the immediate vicinity of
Tower. In 1883-1885 Prof R. D. Irving spent several months studying
the geology of this district. He was assisted in 1883 by Mr. AY. M.
Chauvenet and in 1884 by Mr. W. M. Chau-\-enet and Mr. W. N. ]\IeiTiam.
These studies were continued in 1885 and 1886 by Mr. W. N. Meniam,
assisted in 1886 by Mr. W. S. Bay ley during a portion of the season. In
1888 Prof. C. R. Van Hise visited the district, traversing it from end to end.
The general results of these trijDS, which were made for the United States
Geological Survey, were embodied in various papers which are refen-ed to
under the review of the literature (Chapter II of this monograph) and in
manuscript reports that are preserved in the office of the Survey. Various
members of the Minnesota Geological Surve}- have spent parts of or entire
field seasons in the district, and their results are published in the reports of
the State survey.
In pursuance of a plan to study each of the Lake Superior iron-bearing
districts and make detail reports on them the United States Geological
Survey resumed work in the Vermilion district in 1897. This work has
been under the general charge of Prof. C. R. Van Hise. The geologists in
the field were Messrs. W. S. Bayley, C. K. Leith, and J. Morgan Clements.
The field work continued through the field seasons of 1897, 1898, 1899,
and 1900. Professor Van Hise was in the district for short periods daring
the diiferent seasons, and in 1899 he spent a large part of the season in
active field work. Mr. Bayley spent the seasons of 1897 and 1898 "in the
field; Mr. Leith spent the seasons of 1897, 1898, and 1899; and Mr. Clements
(the writer) was present every year, remaining throughout the entire season.
In preparing this report the writer has, of course, made use of the
material obtained by the other members of the survey, and is very greatly
indebted to them for the assistance given by their carefully prepared notes. .
He is, however, chiefly under obligations to Professor Van Hise, who, in
the first place, gave him the opportunit}- to prepare the report, and wlio
has ever been ready to assist him both in the field and in the office.
The mining men of the Vermilion district have, almost without exception,
shown high appreciation of tlie work done in other districts by the United
States Geological Survey, and have rendered all legitimate assistance
witliin their power during the progress of the work. The Minnesota Iron
Coinpanv, under the presidencj- of 'Sh: D. H. Bacon, and later of ]\Ir. T. F.
INTRODUCTION. 31
Cole, gHYe invaluable aid. In 1899 the company began a careful geologic
survey of its lands, which was made in far greater detail than was
possible by the United States Geological Survey under existing conditions.
This private survey was carried out by Mr. W. N. Merriam, assisted in
1900 by Mr. Oscar Rohn All of the material resulting from this survey
has been placed at the disposal of the writer and his collaborators; a great
deal has been used in compiling the maps published herewith, and it has
added very materially to their comj^leteness. Moreover, Mr. Merriam has
taken pains to make drawings, some of which are reproduced in this report
(credited to him), and otherwise to render assistance. To the company
which he represents, and to him especially, the United States geologists are
deeply indebted. The writer wishes also to acknowledge here the great
assistance rendered by Mr. E. R. Maurer and Mr. C. F. Graff, who have
prepared the drawings from which the maps and plates are made, and by
Mr. F. B. Van Horn, his efficient stenographer.
SCOPE OF THE PAPER.
The attempt has been made to make this repovt a complete epitome of
our knowledge of the Vermilion district. At the same time many details
have necessarily been omitied, although in most cases these concern the
formations of the district that are not of economic value and are not likely
to become important. These details, without adding to the g'eheral results,
would have very much increased the bulk of the volume. Moreover, it
was feared that they would obscure important facts and thus defeat the
object of the monograph.
The report is intended jjrimarily to give to mining- men and to jjresent
and prospective owners of property in the district information concerning
the distribution of the important iron-bearing formations and their relations
to the other rocks associated with them. The text gives a full description
of these formations.
The atlas of maps and the plates in the volume are for the purpose of
aiding in an understanding of the textual descriptions. Actually observed
exposures of the rocks of the district could not be indicated in all cases on
the maps because their scale is too small. Large-scaled maps of certain
portions of the district that contain the important iron-bearing formation
in its best development, and in which areas any industrial developments
32 THE VERMILION IRON-BEARING DISTRICT.
of tlie future are likely to occur, have been prepared. On these plates
the exposures actually seen during the field AA-ork have been indicated,
drawn to very nearly correct scale. Man}- of the exposures are so small
that in order to represent them on the maps at all It has been necessary to
exaggerate them. On these maps are given the data which were tised as
the basis for drawing the formation lines in these economically important
areas. If anyone is unable to accept the conclusions reached, he may
draw his own inferences from the data given, which are essentially connect,
although, as will be understood by those who know the limitations under
which such geologic mapping is done, a number of minor errors may be
disclosed by very close work and very exact location of exposures with
instruments of precision. All geologic maps are but approximations to the
truth. The aim has been to make the present apjDroximation as close as
practicable.
A great deal of time has been devoted to examining all available literature
that refers in any way to this district. Fairly complete quotations are made
from the various works cited, so that a careful reading of the review of the
literature will enable one to familiarize himself with the changes of opinions
concerning the structure, character of rocks, and other details, and also with
the gradual increase in knowledge concerning the geology of the district.
GEOGRAPHIC LIMITS.
The territory designated by the name " Vermihou iron-bearing district"
lies in the extreme northeastern portion of Minnesota, including portions
of St. Louis, Lake, and Cook counties. The district has an area of
approximately 1,000 square miles. It is a narrow belt extending east-
northeast from near the west end of Vermilion Lake, in longitude 92° 30'
west from Greenwich, on the west, to the vicinity of Gunflint Lake, on the
international boundary, on the east, in about longitude 90'^ 45' west from
Greenwich. The district lies between 47° 15' and 48° 15' north latitude.
It attains its maximum width at the west, where it is about 18 miles wide,
and gradually narrows eastward, until at Gunflint Lake its minimum width
is 2 miles. The geographic relations of the Vermilion iron-bearing district
of Minnesota to the other iron-bearing districts of the Lake Superior region
can be seen on PI. I.
The western limit of the district as given on the map (92° 30') is purely
arbitrary. West of this the country is heavily drift covered and timbered, and
U, S- GEOLOGICAL SURVEY
MONOGRAPH XLV PL I
.VRCHK.^.N- ;^^^i^-''-^^'L
PO ST^fVLGON KIAN
nUROXIAN EEWEEKAWAK
\_/R__] HHl [I^KU I Pa I
Including considerable lncladui|), diiisidernhle
areas orAlgunkuui ^raiute areas of Arch e mi
I llie iron -hearing 3ai<'s '
GEOLO(ViC MAP OF VMi'V OF THH LAIvJ^ SUl^KRIOli ItKUlOJs
SlTOUTJ«IO PRK-rAMHlUAN' HOCKS
('aiiipilednMjuoffi[-iaJ jimpsur ITnitcd SlAtes.SMle.aiidC&iiaUuui survfyf.
SchJc
ULius ei£Naco urH.N.r
HI* RON IAN
*h7
4/
nligi
Ah*
J/>
AhH
lfr«o*(
srs^,
Aht7
FolO/tl
liivti i/(
i\ada
STRATIGRAPHY. 33
great miiskegs" extend over large areas. In that region outcrops are very
scarce, but exposures of banded jasper and iron ore have been found associ-
ated with greenstones, granite, and clastic sediments. A reconnaissance
trip was made through it in 1886 by Mr. W. N. Merriam for the United States
Geological Survey, and this work, as well as that done in that portion of the
district for private corporations, to whose results the Survey has had access,
shows the futility of attempting at present to trace out formation lines in that
region. Hence the Survey has done no detailed work west of the above line.
The same rocks that occur at the eastern end of the district are known to
continue for many miles eastward, both in the United States and in Canada,
Hence this eastern limit also is arbitrary, and includes, indeed, rocks that are
the direct eastward continuation of the topographic feature known as the Me-
sabi range, which in the western part of the district lies south of the Vermilion.
The southern and northern limits are sharply defined, and are well
marked geologically by granite and gabbro. The gabbro bounds only the
southern side of the district in the eastern part.
STRATIGRAPHY.
The stratigraphic succession in the Vermilion district is as follows, in
descending order :
Pleistocene , - . Drift.
Ke weenawan Duluth gabbro and Logan sills.
( Unconformity. )
Upper Huroiiian (Animikie series). ConijRove slate.
fined to east end of district iGunflint formation ( iron-bearing) .
(Unconformity.
Lower Huronian
( Unconformity.
Archean i ._
Intrasives. Granites, granite-porphyries, dolerites, and
lamprophyres.
Knife Lake slates.
Agawa formation ( iron-bearing ) .
Ogisbke conglomerate.
Intrusive gi'anites, granite-porphyries, and some green-
stones.
Soudan formation (the iron-bearing formation).
(Minor unconformity. )
Ely greenstone, an ellipsoidally parted basic igneous and
largely volcanic rock.
« These muskegs, as they are called by the Indians, are great open swamps that are comparable
in a way with the northern tundras. They have been formed in most cases by the drying up of large
bodies of water, and in many of them there is now an open area occupied by the remnant of a larger
lake. Over the area surrounding the water there is spread a growth made up largely of sphagnum
moss, wild cranberry bushes, and other water-loving plants, with occasional swamp-growth shrubs that
attain a height of 1 to 3 feet. Out of this thick undergrowth there rise isolated tamarack and spruce
trees, usually of small size. Where these muskeg swamps border the large lakes they are sometimes
flooded during high water.
MON XLV — 03 3
34 THE VERMILION IRON-BEARING DISTRICT.
The evidence upon which the above formations are grouped into series
and these correlated witli the formations in other districts of the Lake
■Superior region will appear in subsequent pages. In this place is given
merely a categorical statement of the problems to be treated.
The accompanying general map (PI. 11) shows the distribution of the
various formations enumerated above. The reader is able to get a better
idea of the relationship of the formations and their distribution throughout
the entire district from a study of this general map than he could from the
examination of the larger-scale maps in the atlas, which are of relatively
small areas. On the larger-scale sheets, in the accompanying atlas, details
of topography are shown which could not be shown on the general map.
These atlas sheets and the other more detailed sheets on a still larger
scale are more accurate than the general map and should be used in a
detailed geologic study of the district having in view the location of
possible productive properties.
PHYSIOGRAPHY.
RELIEF.
That portion of Minnesota included within the limits given above is
most commonly known in commercial reports and locally as the "Vermil-
ion iron range." The term "range" is in this case, however, a misnomer,
if one understands thereby an area with strongly marked topographic
features which cause it to stand out from the adjacent areas. The Ver-
milion district is one in which the relief is not very great. The maximum
elevation is attained by a hill in sec. 28, T. 65 N., R. 4 W., near the east
end of the district, which reaches a height of 2,120 feet above sea level, or
1,518 feet above the mean level of Lake Superior, which is 601.5(3 feet
above the sea. This is one of the highest points in the State, the highest
hill having a reported altitude of 2,230 feet." The lowest valley is that
occupied by Bass wood Lake, in which the water level is 1,300 feet above
the sea, or 698 feet above Lake Superior. There is, then, a difference of 820
feet between the lowest water level and the highest hill within tlie district.
The above extremes in height are found at opposite ends of the district,
and, as the general slope is to the northwest, the average relief is very
much less than 820 feet, approaching 400 or 500 feet. It is to be further
"Geol. ami Xat. Hist. Survey of Minnesota, Final Kept., Vol. IV, 1S99, p. 481.
I S GEOLOGrCAL SURVF
RELIEF. 35
noted that in general the changes in rehef are very rapid, and as a conse-
quence the district is extremely rugged in detail, diversified by hills and
many lakes, with an occasional muskeg. Over the greater portion of the
area we find east-northeast trending ridges alternating with valleys occupied
by long lakes, or chains of lakes, or streams. As a consequence, in
traversing the district from north to south one is continually ascending a
steep ridge to descend on the opposite side into a valley which is usually
occupied by a lake.
The Vermilion district, considered broadly, may be divided into four
areas, each of which is characterized by a fairly distinct kind of topographic
development. These are:
(1) The area including the Giants range, which is the most prominent
topographic feature of the Vermilion district.
(2) A broad area north and northwest of the Giants range, including
all the areas underlain by the iron-bearing formation. This is very rugged,
but the differences in altitude are not great.
(3) x\n area of high plateau country southeast of the Giants range,
underlain by gabbro.
(4) • A small triangular area at the extreme eastern end of the district.
The apex of the triangle is toward the west, and lies between the Giants
range on the north and the high plateau to the south.
Mesabi or Giants range. — This is a fairly well-marked east-northeast
trending range of hills," which runs obliquely across the district. It forms
the backbone of the Vermilion district, although it is unsymmetrical and
divides the district into unequal areas. It enters the district in T. 62 N.,
R. 12 W., and extends in an east-northeast direction along the Kawishiwi
River, south of Snowbank Lake and Cacaquabic Lake, and north of
Lake Gobbemichigamma to the east side of T. 65 N., R. 4 W., where it
leaves the district and enters Canadian territory.^ The maximum height
of this range is attained by a liill in sec. 28, T. 65 N., R. 4 W., already
« This range has been known as the Mesabi for an unknown length of time by the Indians
inhabiting this region, and has been so called in the reports of Western explorers or else translated by
them into Giants range. In late reports Prof. N. H. Winchell has applied the term Mesabi to a range
of hills lying south of that known as the Giants range proper, to which the above statements apply.
Winchell has his ilesabi and Giants range proper unite a short distance southwest of Birch Lake and
form the Giants range to the east. West of this point he discriminates the range into the Giants range
to the north and the ^lesabi to the south. Geol. and Nat. Hist. Survey of Minnesota, Thirteenth
• Ann. Kept., 1885, p. 22; Final Kept., Vol. IV, 1899, p. 232.
f'N. H. AVinchell, Geol. and Nat. Hist. Survey of Minnesota, Thirteenth Ann. Rept., 188-5, p. 38.
36 THE VERMILION IRON-BEARING DISTRICT.
referred to as the liig-liest hill in the district, which reaches 2,120 feet
above sea level. This is about 460 feet above the general level of the
surrounding- country. As a general thing the range does not stand out
very prominently from the rest of the district. Between Gobbemichi-
gamnia and Cacaquabic lakes, however, there is a subordinate range, with
Twin Peaks as the highest points, which forms a very prominent feature of
this part of the district. The Giants range is not continuous throughout.
It is made up of a great number of small hill ranges having in general
the trend of the main range to which the}" belong. Its contours are
commonly smooth and rounded, as the result of glaciation. On its slopes
are inany minor irregularities caused by glacial deposits. Among these
deposits we find now small glacial lakes almost upon the summit
of the range.
Area north and nortluvest of Giants range. — It has been said that the
Giants range divides the Vermilion district topographically. The area
with the largfest surface extent is that lying north and northwest of the
rauffe. This area merg-es to the south into the Giants range, and continues
to the north beyond the limits of the area mapped. Within this area the
topography is that which has been brieflj' described on p. 35 as fairly
typical for the entire district. It consists in ridges trending N. 60°-80° E.,
and separated by valleys which are usually occupied liy a long lake, a
string of small lakes, or a stream. The ridges are usually about 200 feet
above the lakes. The greatest height in this portion of the district is
reached by Chester" or Jasper Peak, in sec. 35, T. 62 N., R. 15 W., which
is 1,710 feet above sea level. The topography is less rugged in the
western part of the district, where the hills and ridges have been apparently
more affected by glaciation. They are there generally rounded and the
slopes are much gentler than in the eastern portion. In the east the area
is underlain by a great slate formation, and the jointing of the slates has
caused the development of minor drainage lines and ridges transverse to
"The name Chester was the first recorded name given to this pealv by \vliite men. It was so
called in honor of Prof. A. H. Chester, who did the first imjiortant work towawl exploiting the iron
deposits of this district. The peak is the most i)roniinent topographic feature of this part of the
district. It is an alnfost bare knob of ja.sper. This jasper is one of the important rocks of the
Lake Superior iron region, and as everyone is more or less familiar with it, the peak has naturally
been called after the rock of which it is formed. The writer thinks that it will be impossible to cause
the name Chester to be generally used, although by pricirity this name rightfully should be given to
the peak.
RELIEF. 37
the trend of the other topographic features. Moreover, the slates break
off, forming steep chffs that stirround the ridges and hills, instead of the
moderate slopes more common in the western part of the district. The
valleys are almost flat and are exceptionally wide. In short, the wide
U-shaped form is the common one here, rather than the flaring V-shaped
valley characteristic of rivers. The modification of the topography from
the V-shaped to the U-shaped forms is attributed to glacial erosion and
deposition.
One area in which rather interesting topography was observed is
that extending from about 1^- miles southeast of Ely, in sec. 2, T. 62 N.,
R. 12 W., southwestward to sec. 30, T. 62 N., R. 12 W., including about
14 square miles. This area is underlain by the Griants Range granite, and
throughout the relief is very slight, the greater portion of the land surface
being only a foot or so above the level of the lakes. As a result the major
portion of it is a marsh. The knolls are of granite, with low, rounded
surfaces rising only a few feet above the swamp area. A few of the knolls
are composed of glacial drift. Evidently pre-Grlacial drainage had been
especially vigorous here, and this is a small, nearlv base-leveled area, with
the lakes as the base-level.
On a reconnaissance trip along the international border a similar
area was noted surrounding the southeast side of Iron Lake, in sees.
11 and 12, T. 66 N., R. 13 W., and sec. 7, T. 66 N., R. 12 W., outside of
the Vermilion district. Here base-leveling has proceeded farther than in
the area previously mentioned; the islands in this portion of the lake rise
just above the water level, and the small streams entering the lake here
flow with meandering courses through wide marshes.
The gabbro plateau. — In northeastern Minnesota, southeast of the
Vermilion district pi'oper, there is a large area underlain by gabbro. Only
a small portion of this area comes within the region shown on the accom-
panying general map, and that portion is a strip on its southern and
southeastern edge. Knowledge of the plateau has been derived from a
study of this strip, where for the most part stratigraphic work ended; from
the results of a reconnaissance trip within the area underlain by the
gabbro; and chiefly from the description by Dr. U. S. Grant in the last
Minnesota report, " to which the reader is referred for greater detail. Dr.
"Geol. and Nat. Hist. Survey of Minnesota, Final Kept., Vol. IV, 1899, pp. 434-436.
38 THE VERMILION IRON-BEARING DISTRICT.
Grant describes tlie gabbro area as an elevated plateau, upon whose surface,
however, there are many minor irregularities, but few of which rise 100
feet above the surrounding country. Lakes are common, but very shallow.
"The general plain-like character of the gabbro-covered area can be
ascribed to weathering, erosion, and glaciation acting on a surface composed
of a single rock mass (the gabbro) uniform in constitution, grain, and
resistance to disintegrating agents.""
Gimflmt Lal-e area. — Near the east end of the district, as shown on
the map (PI. II), there is an area having approximately the shape of an
isosceles triangle, with the apex of the triangle pointing west. The north
side is bounded by the Griants range, the south side by the gabbro
plateau, and the base is the eastern limit of the region sho\\'n on the
general map. Within this area a very interesting kind of topography is
developed. Tlie following description is based upon a personal visit to
the greater portion of the area described, and upon the published reports of
Dr. U. S. Grant."
This area is underlain by a series of Upper Huronian slates which
have a low dip to the south-southeast. The continuity of the slate series
is interrupted by numerous sills of coarse dolerite of varying thickness,
which were intruded approximately parallel to the beds. Pre-Glacial
erosion developed here a system of east-west ridges and valleys. Each
rido-e is capped by a layer of dolerite and below, protected by this hard
upper layer, lie the slates. The ridges slope gently to the south. The
slope follows approximately the upper side of the dolerite sill and corre-
sponds closely to the dip of the sediments. To the north the ridges break
off abruptly, giving a steep or precipitous escarpment with a talus below.
The narrow valleys between the ridges are usually occupied by lakes, and
each lake is higher than the one in the valley next north of it. A cross
section would show on the north a talus surmounted by a clitf which forms
the brow of the ridge, the latter sloping gentl)- to the south to the valley,
which contains a lake. Then the talus, cliff, and slope are repeated. Such
a north-south section would have ranch the jagged appearance of the edge
of a saw. From this character these hills are frequently spoken of as the
"sawtooth hills."
"(Trant, u]). cit., p. 4o5. ''(.>ii. i-il., p|i. •'!17. 4S2.
RELIEF AND DRAINAGE. 39
The entire district has been overrun by glaciers, and in consequence
glacial drift is widely distributed. In places it is very thin, liut in other
places it has accumulated to a considerable depth over large areas. In
these areas the striking features of the topogi-aphy are due essentially
to the drift. However, the present relief can easily be seen to have been
superimposed upon a pre-Glacial topog'raphy of very different character.
A deep covering of drift occurs in all four of the greater areas above
outlined, and modifies the topography locally.
DRAINAGE.
One can not glance at the topographic maps of the Vermilion district
in the accompanying atlas without being impressed by the abundance of
lakes in it. These numerous lakes, with their connecting streams, make
the district comparatively easy of access. They have enabled us to study
the geology with a much smaller expenditure of time and money than
would be requii-ed if they did not exist. The presence of these lakes
is clearly indicative of an immature drainage S3'stem, which is further
shown by the absence of streams of large size, by the fact that the
small and short streams which do exist merely serve to connect the
lakes into strings, and, by the fact that these streams are frequently
interrupted in their courses by rapids and falls. The presence of large
muskegs in some portions of the district still further emphasizes this very
imperfect drainage.
Sydrograpliic basins. — These lakes and the streams which feed and drain
them belong to the large hydrographic basins of the St. Lawrence River
and Hudson Bay. The area belonging to the St. Lawrence drainage basin
is very small. It is drained by a small stream — the headwaters of the
Embarrass River, a tributary of the St. Louis — which rises in Putman
Lake, sec. 18, T. 61 N., R. 14 W., and flows south, finally emptying into
Lake Superior. It is interesting- to note that this small stream, flowing
south, runs for a considerable distance, in the area outside of the district
mapped, nearly parallel with Pike River, a stream 5 miles west of it, whose
waters flow north and belong to the Hudson Bay drainage.
By far the greater part of the district belongs to the drainage basin of
Hudson Bay. The waters in this district flow north and west, collecting-
fin ally in Rainy Lake and draining- through the river of the same name into
40 THE VEKMILION IRON-BEARING DISTRICT.
the Lake of the Woods and Lake Wmnepeg, and finally entering Hudson
Bay through the Nelson Eiver. It was in the country reached by means
of this string of connecting waters that the great battle of commercial
supremacy in the Northwest was fought in the early part of the century by
the Hudson Bay Company and its younger rival, the Northwest Fur Com-
pany. The Hudson Bay people came up the Nelson and carried out their
goods for the most part the same way. The Northwest Fur Company came
from Lake Superior and went to a great extent through the streams and
lakes bordering the Vermilion district down the Rainy River and returned
over the same route, although at times an all-Canadian route north of the
international boundary was used.
Streams. — The streams of the district are, with one exception, short,
narrow, and shallow, and form merely the connections between the
numerous lakes. The Kawishiwi" River, which runs through T 63 N.,
Rs. 9, 10, and 11 W., is the exception. This is a fairly long stream, which
for a portion of its course is within the southern border of the district.
In places it is both wide and deep. It is interrupted, however, by rapids,
and is full of widenings which really may be considered as lakes, so that
by a strict interpretation it could perhaps be classed with the other strings
of lakes. The course of this stream can be followed on the accompanying
maps, and an examination of the geologic map shows that it follows the
contacts of the various formations occurring in the part of the district in
which it runs. The Kawishiwi River is peculiar in certain portions of
its course and shows clearly that over a greater portion of its extent it is,
as has already been stated, merely a string of lakes. The main stream
flows through sees. 20 and 21, T. 63 N., R. 9 W., just below the margin of
the area mapped. At this place there is a large island, about 2 miles long,
extending northeast-southwest, but not caused, as one would naturally
think, by the stream di\dding and flowing around both sides of it. It is
due to the fact that to the north of the island, in sec. 16, T. 63 N., R. 9 W.,
there is a lake which has two outlets, and from which the water flows
to the southwest and to the northeast. The Avater running southwest
joins the Kawishiwi after flowing about one-fourth of a mile. The water
"The Indian name of this river is reported to l)e Mishiwishiwi, meaning "Big Beaver
House River." The Minnesota maps give it as Kawishiwi, and it is l:nown in local parlance as the
Cashawav.
DRAINAGE. ^ 41
flowing northeast follows this course for a short distance, then turns to the
southwest and joins the Kawishiwi about 2 miles from the outlet of the lake.
Somewhat farther west there is a striking case of the division of the
streain and the formation of an island in this way. The island here referred
-to is only partly included in the area shown on the present map, and
the reader is referred to plate 78, Vol. IV, Geological and Natural History
Survey of Minnesota, Final Report, where the course of the river
around it is shown. The area is briefly described by Grant, page 400 of
the same report. Extending through T. 62 N., Rs. 10, 11, and 12 W., is
a laro'e island, with a maximum lensfth of about 1 1 miles and a breadth
of 4 miles, which is completely surrounded by the north and south
branches of the Kawishiwi and the lakes which are developed in their
courses. The water of the main Kawishiwi divides in sec. 26, T. 63 N.,
R. 10 W. A part of it, forming the north branch proper, flows nearly
due west a distance of about 8 miles, in which distance it descends
about 70 feet. That portion forming the south branch flows south and
southwest, then north and northeast. The total length of the Soutii Kawish-
iwi is about 30 miles, and it d.escends 70 feet before it joins the first or the
nortli branch.
Lakes. — The lakes are the most characteristic drainage feature of
the Vermilion district. They are a source of great relief to the geologist,
who, wearied with a day's trainp through the brush on the hills, returns
to his birch-bark canoe and paddles back to his camp, situated at some
pleasant spot on the shore. They likewise aiford a constant source of
pleasure to the traveler through the district, whose interest is ai'oused.
by the rapid changes from a narrow lake with rocky clifi"s to others
showing broad reaches of open water studded with green islets. This
interest is sustained by the fact that each succeeding lake entered affords
something new to attract the attention. The scenery, although all of the
same general character, is constantly changing in its details. Occasionally
a moose or caribou may be seen swimming from shore to shore, and the
fishing- is generally excellent. The lakes vary greatly in size. Vermilion
Lake, the largest, covers about 70 square miles, excluding islands, more
than fifty of which are in the area mapped. Other large lakes are Bass-
wood (Bassimenan or Whitewood) Lake and Saganaga Lake, which border
the district. From these large ones the lakes grade in size down to mere
42 THE VERMILION IRON-BEARING DISTRICT.
ponds. A rough coimt gives 250 lakes within the region shown on the
topographic map.
The hikes are somewhat scattered in the western portion of the district,
but are far more numerous in the eastern part. There are many small
ones with no visible surface outlet, and these are usually completely
surrounded bv drift. The lakes lie in basins that trend northeast-south-
west, and with their short connecting streams they constitute the main
routes of travel through the district. Indeed, were it not for them the
district could be traversed only with extreme difficulty. A trip north
and south across the district is very arduous, as the trails, if any exist, are
lono- and over high, rough ridges, while the waterways are so naiTOW in this
direction as to make it seem useless to carry canoes for such long distances,
in \aew of the short distance they can be used, whereas without them such
a route would be thoroughly impracticable. On an east-west trip, however,
one can start from Tower, and go along the lines of lakes to CTunflint, on
the Canadian border, by canoes — a distance of 75 miles in a straight line,
and much greater hj the route traveled — and on the trip make only about
20 portages, not aggregating in all over 4 miles. A number of these
portages, moreover, are mere lift overs, from 10 to 50 yards long, and
others are the demies or petites decharges of the French voyageurs, where
it is only necessary to lighten the canoe in order to float with safety over
the bowlders.
Differences in water level— The differences in level of the bodies of
water in the district are very considerable; but, owing to the fact that these
differences are rarely shown by bodies of water near one another, there are no
high falls. The highest lake above the level of the sea is a small one very
near the top of one of the highest hills in sec. 20, T. 65 N., R. 4 W., in the
eastern part of the district. This is at an elevation of 1,880 feet. Bass-
wood Lake, the lowest body of water in the district, is 1,300 foet above sea
level. The difference of these extremes, separated by 26 miles, is only 580
feet.
Water power. — The streams connecting these lakes have for the most part
unimpoi-tant rapids in them. In a number of places, however, considerable
water power can be developed, as, for instance, at the Kawashachong Falls,
where the Kawishiwi empties into Fall Lake: at the Pipestone Falls on
Newton Lake; at several places on the upper Kawishiwi in sees. 30, 28,
DRAINAGE. 43
and 24, T. 63 N., R 10 W.; in sees. 19 and 20, T. 63 N., R. 9 W.; in sec. 1,
T. 64 N., R. 9 W., on Basswood Lake; at the falls between White Iron and
Birch lakes; and at the falls between White Iron and Stuntz lakes, just
below the south edge of the district. Of these the most accessible and the
best, and hence the ones most likely to be used, are the falls of the
Kawashachong, with a fall of about 32 feet, and Pipestone Falls, and the
falls between White Iron and Birch lakes. By a small dam at the outlet
of many of the lakes, large reservoirs could be formed and considerable
water power developed at little cost.
Origin of the lakes. — General statements are very frequently made con-
cerning the lake regions of the Northern States lying within the limits of
the glacial drift, especially of the lake regions of Wisconsin and Minnesota,
which would lead the casual reader to suppose that all of the lakes in these
regions owe their origin solely to the agency of the drift, i. e., that they are
mere depressions within the general drift mantle which have been filled with
water. This is certainh" true for a great number of the lakes in the drift-
covered portion of North America. In the case of the Vermilion district of
Minnesota, however, this simjjle mode of origin can be predicated of but
few of the lakes, and those ai-e all small. The greater number of the lakes
have had a mixed mode of origin ; they owe their existence to pre-Glacial
erosion, which scooped out deep valleys, and then to the drift, which left
dams across these valleys at intervals. It is very probable that glacial
erosion was also active in widening and deepening these pre-Glacial
valleys, changing V-shaped into U-shaped valleys. Many of the lakes
empty over rocky rims. They occupy basins formed by the damming of
pre-Glacial valleys by drift, and their present outlets are higher than the
original mouth of the valley. However, the writer nowhere observed
rock-basin lakes which he could interpret as due to glacial erosion.
The lakes in the western part of the district can be readily- divided into
those which owe their present location and existence solelv to glacial action,
and those which owe their existence to Pleistocene glaciation, but whose
present location and configuration are chiefly due to the geologic structure
of the pre-Glacial rocks and to pre-Glacial drainage. To the first kind
belong, among others, the oval or irregular lakes lying in the deep
morainal di'ift which stretches northeast-southwest through T. 61 N., R. 14
W^, and T. 62 N., R. 13 ^Y.
44 THE VERMILION IRON-BEARING DISTRICT.
Lake Vermilion is a good illustration of tlie second kind of" lake. Its
very irregular outline, with its islands and bays, is due chiefly to the geo-
logic structure and differential erosion of the various closelv folded rocks
touching its shores. The reader is referred to the statement on page 432,
wherein attention is called to the fact that in many cases the islands are the
crests of anticlines of harder rocks, the basins between being in the slate
synclines; and also to the statement that the large bays on the east end of
the lake are found invariably to be in the younger Lower Huronian rocks.
However, even in this western part of the district where the drift is rela-
tively heavy, the general trend of the long direction of the lakes corresponds
to the trend of the structural features; that is, it is about N. 60°-80° E.
These lakes in the western portion of the district have relatively large
drainage basins, and are usually bordered by low shores clothed with small
second-growth timber. Near these shores and back from them within their
drainage basins one very commonly finds swamps of considerable extent,
which are not very much above the lake level. The water of the lakes
is clear but is almost invariably tinged by the coloring matter brought in
from these tributary swamps. This coloring varies nuich in intensity in the
different lakes, and although the writer's observations extended over onl}' a
portion of the year — the months of July, August, September, and into
October — it was very noticeable that the intensity of the coloring varied
in the same body of water, being less in the late fall, when the water was
low, than in the early summer, just after the heavy rains, when the swamps
and streams were flooded. Sometimes the organic coloring matter is so
plentiful that a bucketful of the water shows a decided brown color. Such
waters, although clear, are not very transparent. It is almost impossible at
times to distinguish dark bodies 6 inches below the surface of such water.
Canoeing in smooth water of this nature is somewhat hazardous; the bow-
man, even when keeping a .sharp lookout, can scarcely see the reefs and
snags in the water until he is upon them, whereas in rough water their
presence is shown by the way in which the water breaks on them.
The lakes in the eastern part of the district oifer a striking contrast to
those in the western portion which have just been described. They are,
with the exception of some of the largest lakes, almost uniformly long and
narrow, are surrounded by high and bare rocky clitts, and lie in distinctly
structural basins. Later a number of instances will be cited (p. 432) to show
DRAINAGE. 45
the relationship of the distribution of the lakes to the various structural feat-
ures of the district; here only two will be mentioned. The lakes in sees. ^,
8, and 9, T. 65 N., R. 6 W., are separated by barriers of glacial drift. The
depression iu which they lie was evidently determined by the structure of
the slates, and is clearly of pre-Glacial origin. The existence of this string
of lakes is due to the low drift barriers in which the connecting streams are
now cutting. The most striking instance which the writer has observed of
this relationship of the lakes to the structure is seen in the string of lakes
just north of the international bovmdary known as That Mans, This Mans,
Agawa, and the Other Mans lakes. They lie in a great depression in a
syncline of slates, and, like those first mentioned, are separated by barriers
of drift.
The lakes in the eastern part of the district have, with a few exceptions,
very small drainage areas, and but few swamps of any size are tributary
to them. Consequently the organic matter which colors the water of the
lakes west is wanting, and as a rule the water is beautifully clear and
transparent.
A simple sounding apparatus was used for one season with a view to
o-etting the depths of the lake basins. This apparatus consisted of an oiled
silk fishing line, with knots 1 meter apart, wound on a large reel, with a
three-fourths pound lead plumb bob attached to the free end. The reel
was screwed to an arm of light wood. One end of this arm was fastened to
a crossbar in the bow of the canoe; the other end, on which the reel was
screwed, was free to swing. When the reel was not in use this arm lay
close to the left side of the canoe and was suspended from a hook, which
kept the reel in place and prevented it from unwinding. When the reel
w^as to be used the arm w^as swung in front and to the right of the man in
the bow, and rested on the gunwale of the canoe. The reel was thus sus-
pended over the water, and soundings could readily be taken and the
approximate deptlis read by counting the knots and estimating the fractions
of meters. By this means soundings were taken in a number of the lakes.
These showed that the lakes in the western part were shallow. For
instance, Lake Vermilion, the largest body of water in the district, gave iu
two places a depth of 10 meters. The average depth of the lake would be
much nearer 6 meters. In contrast to this the narrow, clear-water lakes in
the eastern portion of the district, those with high, rocky shores, were
-!6 THE VERMILION IRON-BEARING DISTRICT.
found to be deep. These are the lakes in which tlie trout and bass are
most abundant and shoAV best their fighting quahties. A maximum depth
of G0| meters (199 feet) was found near the center of Lake Gobbemichi-
ijamnia. The writer did not have a chance to sound in some of the other
lakes of the district, in which, if one may judge from the character of the
water and of the surrounding shores, even greater depths would be found.
The soundings taken are recorded on the accompanying maps. The points
at which the soundings were made were located approximately by a rough
system of triangulation, and are indicated on the maps by dots, adjacent to
which are placed the figures giving the depth in meters.
PlanUon. — During the season of 1899 a seining apparatus was carried
and collections of the plankton of thirty of the lakes of the Vermilion dis-
trict were made. The lakes from wliich the specimens were taken were
scattered from the western to the eastern part of the district, and, it would
seem, should give a fair idea of the general character of the plankton of
this district. These collections were given to Dr. E. A. Birge, of the
University- of Wisconsin, for study. He reports as follows: '-The col-
lections made by Mr. Clements have been examined. They contain very
few Crustacea and no species except those whose presence in these lakes
would be a matter of course, since they belong to genera and species very
widely distributed on this continent. In view of these facts, a more
detailed report does not seem advisable."
EXPOSURES.
In spite of the glacial drift, the rocks are very well exposed. This
is due to the fact that, as before stated, the drift was originalh' not very
thick, and tliat since its deposition it has been considerably removed. It
is also due to the presence of the great number of lakes, excellent exposures
()f the rocks a])pearing around their shores and upon the small, rocky islands
dotting tlieir surfaces. For example, within the immediate vicinity of
^'ermi]ion Lake, on the islands within this lake, and around its shores,
wh(-re drainage has been especially effective, the rocks rise up hi bare
liills. The eastern part of tlie district beyond Moose Lake contains also
vast areas in wliich clean rock surfaces are exposed nearly everywhere,
;ni(l uianv of these exposiu'es are of very large size. Between Tower and
xMoose Lake there are a number of square miles in which the drift deposits
FORESTS. 47
are so deep — this is especially true in the area overlain by the Vermilion
moraine — that but few exposures could be found. Moreover, within this
area of deep drift the forest growth is especially luxuriant, and this tends
to conceal those exposures that do exist. As a consequence, the difficulty of
determining the structure of these areas is greatly increased, and the results
are less reliable than for other portions of the district.
FORESTS.
With respect to the forests also the Vermilion district may be divided
into two contrasting areas, a western and an eastern. These areas are
separated approximately by a line drawn south from the international
boundary at the western end of Knife Lake, through the eastern end of
Ensign or Iron Mountain Lake, across the north side of Snowbank Lake to
Moose Lake, and then through the eastern end of North Twin (or North
Triangle) Lake to the Kawishiwi River.
The western area is to a considerable extent heavily .wooded with old
forests of mixed growth. On the whole, the hard wood,, especially birch,
seems to predominate ; but scattered through the hard wood there are large
areas of white pine (Pinus strohus) and Norway or red pine {Pinus resinosa).
The value of these forests at present is chiefly due to these conifers. With
the birch are found some jjoplars and scattering soft maples, jack pines
black pine (Pinus hanksiana), spruce, and balsam fir. Tamarack (hackma-
tack, or American larch) and white cedar (arbor vitae) are also present in
varying quantity. The undergrowth consists of smaller birch and poplar,
soft maple, mountain ash, black ash, willow, alder, hazel, pin and choke
cherry, jack pine, balsam fir, spruce (the last two in places forming
almost impenetrable thickets), ground hemlock, the high-bush cranberry,
a viburnum (Vihurnmn ojndus), June berry or service berry (Amelanchier
canadensis), and some other less important kinds. In some portions of this
western area, especially south of Eagle Nest Lakes, southeast of Fall Lake,
near the North and South Twin lakes, and south and west of Pine Lake,
the country takes on the aspect of a true pinery. If may be noted here
that the above-mentioned areas are the ones in which the drift is especially
heavy. Thus one may see the intimate relationship existing between the
geology of the district and its forest growth. In these areas red and Avhite
pine is the chief growth, with the former rather in the ascendency. There is
48 THE VERMILION IRON-BEARING DISTRICT.
very little uudergTOwth in these places, and this is chieflj' cherry, balsam,
spruce, and ground hemlock. Extensive lumbering operations are carried
on in these pineries, and in a verj^ few years the pine will have been cut
from most of the large tracts. It will then probably be l^ut a year or two at
the most before fire will get into the old pine slashings, and any isolated
uncut tracts will therebj' be destroyed, as will also adjacent hardwood areas.
Scattered through this western area are larg-e tracts which have been
biu'ued over one or more times within the last ten to twenty years. In
some places the fire was so severe as to destroy the humus as well as the
timber. As a consequence of the removal of these protections, the major
portion of the soil, and even in some cases the subsoil, has been washed
into the valleys, and the hills are now practically bare rock. Such an area
is that known as the "Burned Forties," in sees. 23 and 24, T. 62 N., R. 15 W.
Where the soilonly was removed it has required some time for the snbsoil
to reach a condition suitable for plant growth, and the hills in snch areas
are covered only with grass, weeds, and stunted poplars, birch, and jack
pine. In other areas the fire occurred so long ago that sufficient soil h<is
accnmulated to support a dense growth of poplar, birch, and jack pine,
which has reached fair size. In some places in such burned areas the
second growth is almost exclusively poplar; in other localities the jack
pine or birch may predominate. The usual history of such an area
after it has been burned is as follows: The year after the fire has run
through the forest there is always a heavy growth of fireweed (so called in
that region) — mare's tail — which springs np. This is soon succeeded by
pojjlar, cherry, bircli, jack pine, and rarely seedling white and Norwaj^
Ijine. As a result of the deadening of the original forest trees and their
consequent weakening, they very readily succumb to the strong winds.
They are blown down, and this fallen timber, with the dense second growtli
that springs up between the recumbent trunks, renders such areas extremely
difficult to tra\'erse. Not many years elapse before this second growth 1-=
swept by fire, and in its turn falls and is replaced by a third growth. The
repetition of such occurrences renders it increasingly diflicult to traverse
such burned country uidess the fire has been very recent and of suflicient
intensity to destroy completely both standing and fallen timber. An
occasional rotted and partly liurned log of large size in the midst of the
pines seems to indicate that long ago fires ran through even tliose areas in
FORESTS. 49
which the present forests are frequentl}' spoken of as the original growth^
and destroyed an earher forest then existing.
This northern country offers obstructions to the explorer such as can
probably be met -with elsewhere onl}- in tropical countries. It is compara-
tively easy to travel thi-ough the forests of standing Norway and white
pine, for here one finds but sparse undergrowth ; but only a very small part
of the district is covered by such open forest; the greater portion, especially
in this western part, is covered by exceedingly dense forests of birch,
balsam, and jack pine, with undergrowth that is almost impenetrable in
places. Between the areas of high ground covered with the above-
mentioned forest growth there lie some swampy areas of tamarack and
cedar and open muskegs. During wet years, many of these swamps are
flooded, so that in crossing them one wades in water 2 to 3 feet deep.
Windfalls have destroyed vast patches of timber and have left the trunks
piled upon one another in inextricable confusion, and a second growth in
places adds further to the entanglement and increases the difficulties of
the traveler. One inexperienced in a country of this character would feel
that the task were well-nigh hopeless were he called upon to leave the
canoe routes and beaten trails and explore this wilderness. It sometimes
requires two hours to advance a mile, and to run a line 5 miles in length
and explore the area for a few hundred yards on both sides is a good day's
work.
In this western jjortion of the district there are a number of very
extensive wild cranberry marshes and other marshes that would be suitable
for the cultivation of cranberries. There seems, indeed, to be no good
reason why these marshes should not be improved and cranberries grown
upon them for the market. In other States such marshes have proved a
good investment, and it would seem that a good opportunity for their
development is offered in this district.
The eastern half of the Vermilion district may be spoken of as the
burned area. In it there are but a few isolated and very small patches
of large timber. This portion of the district seems to have been frequently
swept by fires, and at present the growth covering it is, with few exceptions,
very small. It is probable that the character of the ground has been a promi-
nent factor in determining the size of the second growth. This portion of
the district as a whole is very rocky, and the drift and the soil are much
MON XLV — 03 4
50 THE VERMILION IRON-BEARING DISTRICT.
thinner than to the Avest. The timber is the same as that which occurs
farther west, with the ditference that, since it is nearly all second growth,
poplar, jack jiine, and birch predominate, in the order given. In some
places within this area the fire has been so intense that even the swamp
growth has been destroyed, and in place of the original cedar swamps we
now find grassy meadows. There is no way of determining accm-ately
just when this area was denuded of its forests. On the Government plats
there is a note to the efi^ect that the country near Gunflint Lake w^as burned
over in the sixties In some ijlaces the section corners are marked on
second-growth trees, showing that the burning took place a number of
years prior to the time that the region was surveyed. In other places the
second-growth birch is at least twenty- years old, and here no survey lines
of any kind were to be found. On the other hand, there is abundant evi-
dence that fires liaA^e run over portions of the area since it was surveyed,
for large trees with the marks of the corners and quarterposts, and the bear-
ing trees Avith their marks on them, haA^e been scorched since these marks
were made; and, indeed, in many cases, the marks themselves have been
nearh' obliterated.
SOIL.
The soil throughout tlie district is thin, but what there is, being of
glacial origin, is of A-ery good character and lends itself readily to cultiva-
tion. In the A'alleys the soil has accumulated in places to considerable
depth, and where some of the swamps have been drained and properly
treated the crops produced are excellent. However, farming is injured by
the climatic conditions, which are unfavorable to the groAvth and maturity
of all but a feAv crops. Hay can be successfully raised. Potatoes, cab-
bage, and rutabagas of excellent quality can also be grown, and all of these,
especially the hay, bring good prices. Suitable land for farming on a
large scale is found in but fcAV places. On some natural meadows in dried
lake basins and along the margins of the streams and lakes good crops of
hay are made.
GAME AND IISH.
This portion of Minnesota is fairly Avell stocked Avith game. Moose,
deer, and bear Avere seen repeatedly. In many places the swamps are trav-
ersed by deep cut, recent moose trails, and occasionally there were found
small areas so tramped and torn bA- these animals as to resemble a cattle
GAME AND FISH. 51
yard. Moose must be fairly abundant, therefore, although no great num-
ber were seen, this being- due chiefly to the fact that the party made no
attempt to go quietly through the woods. The animals were frequently
heard crashing through the underbrush. Only one caribou was seen,
though in portions of the district their tracks and runways were common.
Pickerel," wall-eyed pike,' bass, the namaycush or lake trout, and white-
fish are the kinds of fish most used for food, and occur in abundance, about
in the order given. Many of the lakes are teeming with fish, but they are
usually the least desirable kind, pickerel and perch. These can be obtained
in all of the lakes, and they are so common and relatively such a poor g-ame
and food fish that the fisherman ordinarily throws them back into the lake
with disdain. Usually, however, he first kills the pickerel, as they are
the recognized enemy of the game fish. Pike are not so abundant as
pickerel, but they are found in most of the lakes. Bass occur in only
a few, but where found they are in fairly large numbers. Trout
(Salvelhms namaycush) are confined almost exclusively to the deep lakes
in which the water is uncolored, although they by no means occur in all
such lakes. Since these conditions are most commonly fulfilled in
the eastern portion of the district and in the lakes along the international
boundary and just across the boundary on Hunters Island, the trout are
most common in the eastern portion of the Vermilion district. In excep-
tional cases lake trout were found in some of the lakes with colored
water — for example, Ogishke Muncie — and these were slightly diflferent
from the trout in the lakes with uncolored water. They are considerably
darker in color and appear to have proportionally heavier bodies and
smaller heads. They give the impression of being a heavier and slower
fish In the streams and lakes from Peter Lake east to Fay (Paulsons)
Lake trout were caught which seemed slightly different from the normal
lake trout They are called mountain trout by the woodsmen. They
<•' Pickerel is the name commonly applied to the true pike (Esox lucius) throughout this State, as
■well as in Wisconsin. It is easily discriminated from the wall-eyed pike by its shovel-shaped nose and
the light spots on the dark background of the body. It is a fish which lives in sluggish waters, among
the weeds, and very near the surface of the water generally. It is very slimy and has a disagreeable,
strong, fishy odoi. The flesh is soft in summer.
''The wail-eyed pike (Sihostedion vitreum) , or pickerel, as it is sometimes called in this region —
sometimes dory and jack-fish— is an excellent food fish, with firm, well-flavored, white fiesh. It has
a golden-yellow color on the sides of the belly, grading up into the darker color of the back, with dark
mottlings. Tnese mottlmgs also occur on the fins.
52 THE VERMILION IRON-BEARING DISTRICT.
were uot carefully studied, so uo g'ood detailed description can be giveu of
tliem. Ther seem from general appearance to be more nearly like the
ordinary speckled brook trout (Salvelinus fontinalis). Some of the markings
on the brownish back resemble those on the speckled trout, and they have
crimson spots on their sides, but they ai"e in other respects different from
these. This is probably one of the numerous ^-arieties of the lake trout.
According to the repeated experience of members of the party who, for
three diflPerent seasons and at different times during those seasons, had
fished in the same lakes, the fish caught in the same lake usually run about
the same size, showing very slight variations indeed.
White-fish {Coregonus clupeiformis) are abundant in a number of the
lakes, but since they are caught only in nets they are not to be considered
by the sportsman, although they are very important and very delicious as
food. They have been netted on Basswood for many years, and shipped
to southern ^Minnesota markets. They also occur in Vermilion, Saganaga,
Knife, Otter Track, Ogishke Muncie, and other lakes.
CTJIiTURB.
There are four towns in the Vermilion district — Tower, Saudau, Ely,
and Winten. Tower is the westernmost town of the district, and is situated
on Vermilion Lake. It was settled in 1882 (at that time there was one Lig
cabin there), and according to the Twelfth Census (1900) has 1,366 inhab-
itants. These depend almost exclusively for employment upon lumbering-
operations, a sawmill, and the mines of Soudan. All of the stores and
saloons of this portion of the district are located in Tower, and they supply
the people of Soudan as well as the people within the Tower limits.
Soudan, an unincorporated place, is 2 miles northeast of Tower, at the
foot of Soudan Hill. It has grown up around the Minnesota group of
mines, and has about 1,000 inhabitants. It is essentially a mining town,
and most of the people are recent immigrants with American-born children.
The population of the toAvn consists entirely of employees of the Minne-
sijta Iron Company and their families. The company allows no stores or
saloons here.
Ely is situated about midway the district, on the south shore of Long
Lake. It is the most prosperous town on the range. It has 3,717 inhab-
itants, who are for the most part employees of the Minnesota Iron Company
CULTURE. 53
and their families; iu addition there are, of course, a relatively few people
who are employed in the usual stores and small industries of various kinds
which are essential to the life of a town of this size.
Winteu is a small village at the west end of Fall Lake. It is the
eastern terminus of the Duluth and Iron Range Railroad. It owes its
existence to the presence of two thriving sawmills, which are rapidly cut-
ting all of the timber of this part of the district. There are about 500
people within a radius of a mile from the mills at Winten.
The name Silver City is not infrequently employed in the conversa-
tion of explorers and travelers ai'ound Ely, and may lead to confusion in
the mind of the stranger. The name is applied to the site of an old
exploration which, iu the sanguine owner's eyes, was the nucleus around
which there was to be developed a city of importance. Nothing exists
there now; in fact, the writer does not know that a single house was ever
built there. The location is at the White Iron Lake portage, in sec. 32,
T. 63 N., R. 11 W., and being on one of the canoe routes and frequently
used as a camping place, the name is still current among the woodsmen.
The towns just mentioned are connected by the Duluth and Iron
Range Railroad, which is the only one that at present gives service in the
Vermilion district. Consequently this road handles all of the lumber and
ore that is shipped. The eastern end of the district is touched by the
Duluth, Port Arthur and Western Railroad, which was projected to connect
Port Arthur and Duluth. This road was built from Port Arthur as far
west as Paulson's mine at Gruuflint, in sec. 28, T. 65 N., R. 4 W. There
are a few houses here, but since the abandonment of the mine, no inhab-
itants. The two termini, Paulson's mine on the east and Winten on the
west, have, however, never been connected except by the railroad survey.
At present the road within the United States for the 6 miles from the
boundary to Paulson's is impassable, the trestles and many ties having been
burned. It would require extensive rebuilding before it could be used,
and if rebuilt a new route for a part of the way should certainly be
selected, as it would be almost impossible to lay out any route that would
not be an improvement over some 23arts of its pi'esent location. Wagon
roads throug-hout the district are few in number. Those near the towns
are kept iu fair condition, but elsewhere they are very poor. Even the
county road between Soudan and Ely is poorly kept. Very few branches
54 THE VERMILION IRON-BEARING DISTRICT.
run off from this road, so that the country to the north and south of it
must be reached by means of the few homesteaders' trails that exist, or
else by tramping through the woods. East of AVinten the traveler can
proceed in summer only on foot and by means of canoes.
INDIAN RESERVATION.
Jn Sucker Point, a large point of land projecting northeast into
Vermilion Lake just across the bay from the mill at Tower, there is a
reservation which is occupied by the Bois Fort band of the Ojibwa or
Chippewa Indians. According to the last report of the Commissioner of
Indian Affairs, there were 808 Indians living on this reservation on June
30, 1900. Of these, however, a considerable number really live outside of
the reservation, many of them being located in the vicinity of Ely. The
Indians are found in large numbers near the reservation only about the
Fourth of July, and at the times when the regular Grovemment payments
are made. During the winter they are widely scattered over the country,
hunting and trapping, and in summer are encamped on the shores of
the lakes, where fish and berries are abundant. In 1898 the Government
selected a location on Sucker Point and erected thereon a number of
commodious buildings to be used for dwellings for the teachers and Indian
children and for school purposes, but these Indians are not progressive, and
do not take kindly to the advantages offered them by the Government to
become educated agriculturists, or otherwise good citizens. They are very
apt in acquiring the vices of ci^dlization; and instead of cultivating the
available land on their reservation, they prefer to gain a precarious liveli-
hood by hunting, fishing, and trapping. Only a very small portion of the
arable land on the point is cultivated.
CHAPTER II.
RESUME OF LITERATURE.
The Vermilion district lias been studied from a geologic point of view
only since the first quarter of the nineteenth century. A number of years
before this, howevei-, portions of it had been visited by fur traders. The
well-known international boundary canoe route passed part way along its
northeast and northern boundary, and has been used since time immemorial.
Some of the early fur traders and explorers kept journals of their travels,
and these make mention of this route. The first few pages of this
chapter are devoted to a very brief description of the main canoe routes
of the district, and of the methods of travel and customs of the fur traders
along the boundary route when this northwest country was first opened.
The remainder of the chapter consists chiefly of abstracts of articles dealing
with the geologic character of the Vermilion district. In this abstract the
author of each paper has been allowed, in most cases, to speak for himself.
Where, for various reasons, this was not considered best, the attempt has
been made in every case to give exactly the author's meaning, although
his precise words may not be used. While innumerable details have been
of necessity omitted, it is believed that the salient points of the papers
reviewed have been noted and that each author's views have been correctly
represented. Should it be found in any case that the author's views have
not been correctly stated, the fault is due to error in interpretation. In
statins' the views of the various writers, comments are for the most part
refrained from, as discussions of their statements will be found at the proper
places in the descriptive part of the monograph.
Throughout the abstracts the writer has followed the spelling of the
proper names given in the original article. The great variation in the
spelling of these names will be clearly seen if one follows the name through
a luimber of the reports.
In some cases a report or an article may have been preceded by one
or more papers discussing the bearing of some of the facts presented in
the final report. In such cases the final paper has been abstracted, and
references only have been given to the others.
56 THE VERMILION IRON-BEARING DISTRICT.
The literature of the Lake Superior region was very fully reviewed
by Prof. C. R Van Hise in Bulletin No. 86 of the United States Greoloffical
Survey, and that review has been continued by him and Mr. C. K. Leith
up to the present time. The writer has used this material freely, and wishes
to make acknowledgment here of the assistance afforded Ijy these reviews.
The abstracts are arranged chronologically, in the order of the publi-
cation of the articles abstracted. By following these abstracts critically
the reader can acquaint himself with all pulilished articles dealing with
the territory. He can follow the development of the views on the geology
of the district, which is comparatively difficult of access, and can see how
the knowledge concerning it was increased vear by year.
HISTORY OF EXPLORATION^ AXD CHARACTER OF THE ROUTES.
The international boundary trends a little south of east and north of
west, and forms the eastern, northeastern, and northern boundary of the
district for a total length of 75 miles. From Lake Superior to Rainy Lake
the international boundary follows a chain of rivers and lakes, crossing-
necks of land at two places. One of these, known as the Height of
Land, is between North and South lakes, and is the divide between the
headwaters of Pigeon River, flowing east to Lake Superior, and the
waters flowing west to Rainy Lake and finally to Hudson Bay. The
other is a narrow strip of land in sec. 24, T. 66 N., R. 6 W., about 600
paces in width at the portage that separates the watei's flowing- northeast
into Saganaga Lake and then northwest around the north side of Hunters
Island, so called, from the waters flowing northwest and around the south
side of Hunters Island, both of these finally uniting in Lac La Croix. This
naiTOw strip of land, about 600 paces wide at the portage, is all that
prevents the body of land, with an area of approximatelj^ 1,000 square
miles — to which the name Hunters Island was given erroneously, as we
now know — from being in reality an island instead of a peninsula.
As is well known, the rivers and chains of lakes marking the
international boundary have been for the Indians the main route of travel
from Lake Superior into the Northwest from time immemorial. It was this
same route that was followed by the fur traders and e.xplorers who first
earned civilization to the Indians of the extreme Northwest — a civilization
characterized, when first presented to them, by honesty and good morals in
RESUME OF LITERATURE. 57
minimiim amount, and dishonesty, lasciviousness, and rum in maximum
quantities. The Vermihou district is traversed for its entire length bv a
canoe route, which leaves the intei'national boundary route at Gunflint
Lake and continues westward. At Vermilion Lake this route joins a canoe
route that comes from Rainy Lake, by way of Vermilion River. It then
ascends Pike River, crosses the divide — the Giants range — south of Ver-
milion Lake, and thence continues on down St. Louis River to Duluth,
where Lake Superior is reached. The Vermilion district can also be
reached from Lake Superior by canoe routes from Grand Marais, Beaver
Bay, and other points on the lake shore.
The route along the boundary is, of course, well known to every
student of the history of the Northwest, and for one imbued with a love of
history as well as nature a pleasauter journey can scarcely be conceived
than that which can be so delightfully made in canoe from Grand Portage,
on Lake Superior, to Rainy Lake, or farther to the northwest if one chooses."
The easternmost part of the route is known by the name of Grand
Portage. This name was at first applied to the portage, 9 miles long, from
Lake Superior to Pigeon River, and has since been given to the settle-
ment at the Lake Superior end- of the portage. This part of the route is
mentioned in the accounts of nearly all of the early explorers, and if they
did not use the route they at least visited or heard of the port, as it was
one of the most important settlements on the chain of Great Lakes.
The literature dealing with the part of the international canoe route
that touches the Vermilion district has not been fully examined, but some
pains have been taken to get references to it, descriptions of it, and
mode of travel over it from the works of the eai'ly explorers. Jonathan
Carver* mentions the Grand Portage settlement, which he visited, but
describes Rainy Lake and the route to it only from hearsay. Alexander
Mackenzie,'' who must have ti-aversed the region a number of times, gives
0 "The route from Grand Portage to Rainy Lake traverses about 100 miles of diatance, the direct
line being not far from 70 miles. There are 29 portages. The first, or 'grand portage,' is 8} miles,
the third is IJ, the ninth is 1 j, and the residue vary from a few steps to a half mile. The total of
portages, 15 miles. The water communications are chiefly small lakes of 3 to 10 or 20 feet deep."
Hanchett and Clark: Report on Geology of Minnesota, pp. 47-48, 1865. See Vermilion literature
references, p. 66 of this monograph.
6 Travels Through the Interior Parts of North America, by Jonathan Carver, Edition 1778, pp.
106-115.
« Voyages from Montreal, on the River St. Laurence, Through the Continent of North America, to
the Frozen and Pacific Oceans, in the years 1789 and 1793, by Alexander Mackenzie, London, 1801.
58 THE VERMILION IRON-BEARING DISTRICT.
the best description of the route, as well as of the method of travel over it,
which the writer has thus far found. As essentially the same method is in
use at the present day, with the difference that, since the transport of furs
over this route is no longer of impoi'tance, the canoes used are not so large
and the number of men employed is A'ery much smaller, the desciiption of
that portion of the route leading from Lake Superior into the Vermilion
district seems to be of sufficient interest to warrant its insertion here in the
author's words, with the addition of a few footnotes, added chiefly for the
purpose of enabling the reader to identify the lakes by their present names
with the lakes as known to Mackenzie. His description of the route is
given in connection with his account of the rise, progress, and condition of
the fur trade, in which the author was ioterested as one of the partners
of the Northwest Fur Company.
Mackenzie's description of Grand Portage Bay and its surroundings,
at the eastern end of the canoe route, is very good. Let us refer to this
description and attempt to see the bay as he saw it, sm-rounded by hills
rising to a height of 730 feet, its bosom dotted with canoes and its shores
bearing the tents and wigwams of the fur traders and Lidians. Back from
the shore and on the slope of the hill was the fort, which was occupied bv
the traders and trusted employees, and in which the goods were stored.
The traders with their stores came fi'om ^Montreal, but we shall not attempt
to follow their journey in detail.
A quantitv of their goods are sent from Montreal in boats to Kingston, at the
entrance of Lake Ontario, and from thence in vessels to Niagara, then overland 10
miles to a water communication, by boats, to Lake Erie, where they are again
received into vessels, and carried over that lake up the river Detroit, through the
lake and river Sinclair to Lake Huron, and from thence to the Falls of St. Mark's,
when the}' are again landed and carried for a mile above the falls and shipped over
Lake Superior to the Grande Portage [p. xxxix]. ... At length they all arrive at
the Grande Portage, which is 160 leagues from St. Marys, and situated on a pleasant
bay on the north side of the lake. . . .
At the entrance of the bay is an island which screens the harbor from every wind
except the south. The shallowness of the water, however, renders it necessary for
the vessel to anchor near a mile from the shore, where there is not more than 14
feet water [p. xl].
********
The bottom of the ba}-, which forms an amphitheater, is cleared of wood and
incio-sed. and on the left corner of it, beneath a hill 3(i(> or 400 feet in height, and
RESUME OF LITERATURE. 59
crowned bj' others of a still greater altitude, is the fort, picketed in with cedar
pallisadoes and inclosing houses built with wood and covered with shingles. They
are calculated for everj^ convenience of trade, as well as to accommodate the i3ro-
prietors and clerks during their short residence there. The North men live under
tents; but the more frugal pork eater lodges beneath his canoe. The soil immediatel}'
bordering on the lake has not proved verj^ propitious. . . . There are meadows in
the vicinit}' that 3'ield abundance of haj' for the cattle; but, as to agriculture, it has
not hitherto been an object of serious consideration.
I shall now leave these geographical notices to give some further account of the
people from Montreal. When thej' are arrived at the Grande Portage, which is
near 9 miles over, each of them has to cany eight packages of such goods and
provisions as are necessarj' for the interior country. This is a labor which cattle
can not conveniently perform in summer, as both horses and oxen ^v•ere tried by
the compan}" without success. . . .
Having finished this toilsome part of their duty, if more goods are necessary to
be transported, they are allowed a Spanish dollar for each package; and so inured
are thej'- to this kind of labor, that I have known some of them set off with two
packages of 90 pounds each, and return with two others of the same weight, in
the course of six hours, being a distance of 18 miles over hills and mountains."
This necessary part of the business being over, if the season be early they have
some respite, but this depends upon the time the North men begin to arrive from
their winter quarters, which they commonly do early in July. At this period it is
necessary to select from the pork eaters a number of men, among whom are the
recruits, or winterers, suificient to man the North canoes necessary to carry to the
river of the rainy lake the goods and provisions requisite for the Athabasca country,
as the people of that country (owing to the shortness of the season and length of
the road can come no farther) are equipped there, and exchange ladings with the
people of whom we are speaking, and both return from whence they came.* This
voyage is performed in the course of a month, and they are allowed proportionable
wages for their services [pp. xliii-xlv]. . . .
* * * * * ** *
The trade from the Grande Portage is, in some particulars, carried on in a dif-
ferent manner with that from Montreal. The canoes used in the latter transport are
« Further on in his narrative, Mackenzie cites examples of men who have taken seven packages
each, and carried them without stopping across a portage one-half league in length [p. Ixi.]— J. M. C.
6 The system of living at Grande Portage was decidedly feudal, and is described by JNIackenzie
as follows: " The proprietors, clerks, guides, and interpreters mess together, to the number of some-
times a hundred, at several tables, in one large hall, the provision consisting of bread, salt pork,
beef, hams, fish, and venison, butter, pease, Indian corn, potatoes, tea, spirits, wine, etc., and plenty
of milk, for which purpose several milch cows are constantly kept. The mechanics have rations of
such provision, but the canoemen both from the North and Montreal have no other allowance here,
or in the voyage, than Indian corn and melted fat ' ' [p. xlvi]. The Indian corn mentioned is hominy
prepared at Detroit. — J. M. C.
60 THE VERMILION IRON-BEARING DISTRICT.
uow too large for the former, and some of about half the size are procured from the
natives, and are navigated by four, five, or six men, according to the distance which
they have to go. The}' carry a lading of about thirty -live packages, on an average;
of these, twenty-three are for the purpose of trade, and the rest are employed for pro-
visions, stores, and baggage. In each of these canoes are a foreman and steersman —
the one to be always on the lookout, and direct the passage of the vessel, and the
other to attend the helm. They also carry her whenever that office is necessary.
The foreman has the command, and the middle men obey both; the latter earn only
two-thirds of the wages which are paid the two former. Independent of these a
conductor or pilot is appointed to every four or six of these canoes, whom they are
all obliged to obey, and is, or at least is intended to be, a person of superior
experience, for which he is proportionabh' paid.
In these canoes, thus loaded, they embark at the north side of the portage, on
the river Au Tourt,"^* which is ver}' inconsiderable, and, after about 2 miles of a
westerlj' course, is obstructed by the Partridge Portage, 600 paces long. In the
spring this makes a considerable fall, when the water is high, over a perpendicular
rock of 120 feet. From thence the river continues to be shallow, and requires great
care to prevent the bottom of the canoe from being injured bv sharp rocks, for a
distance of oi miles to the prairie, or meadow, when half the lading is taken out, and
carried hy part of the crew, while two of them are conducting the canoe among the
rocks with the remainder to the Carreboeuf Portage, 3i miles more, when they
unload and come back 2 miles and embark what was left for the other hands to
carrj", which they also land with the former, all of which is carried 680 paces, and
the canoe led up against the rapid. From hence the water is better calculated to
carrj' canoes, and leads b}- a winding course to the north of west 3 miles to the
Outard Portage,* over which the canoe, and everything m her, is carried for 2,400
paces. At the farther end is a very high hill to descend, over which hangs a rock
upwaid of 700 feet high. Then succeeds the Outard Lake,'' about 6 miles long, lying
in a northwest course, and about 2 miles wide in the broadest part. After passing a
very small rivulet thej^ come to the Elk Portage,'' over which the canoe and lading
are again carried 1,120 paces, when they enter the lake of the same name, which is a
handsome piece of water, running northwest about -4 miles, and not more than 1^
miles wide." They then land at the Portage de Cerise,"^ ovei which, and in the face
of a considerable hill, the canoe and cargo are again transported for 1,050 paces.
This is only separated from the second Portage de Cerise by a mud pond'' (where
* Here is a most excellent fishery for whitefish, which are exquisite.
" According to Coues, tliis is the abbreviation for Tourtes. This was even then known as Pigeon
River.— J. M. C.
''Known now as Fowl Portage. — J. Jl.C.
« Called on Thompson's map, 1792, Goose Lake. Now called South Fowl Lake. — J. M. C.
''Called now and also on Thompson's map Moose Lake and Portage. — J. M. C.
'Big Cherry Portage now. — J. M. C.
./■Mud Portage.— J. M. C.
RESUME OF LITERATURE. 61
there is plentj^ of water lilies) of a quarter of a mile in leugth, and this is again
separated by a similar pond from the last Portage de Cerise/' which is 410 paces.
Here the same operation is to be performed for 380 paces. Thej" next enter on the
Mountain Lake, running northwest b}^ west, 6 miles long and about 2 miles in its
greatest breadth. In the center of this lake and to the right is the Old Road, by
which I never passed ; but an adequate notion maj' be formed of it from the road 1
am going to describe, and which is universallj^ preferred. This is, first, the small
new portage'' over which everj'thing is carried for 626 paces, over hills aud gullies.
The whole is then embarked on a narrow line of water '^ that meander's southwest
about 2i miles. It is necessary to unload here, for the length of the canoe, and then
proceed west half a mile to the new Grande Portage, which is 3,100 paces in length,
and over very rough ground, which requires the utmost exertions of the men, and
frequently lames them; from hence thej' approach the Rose Lake, the portage of
that name being opposite to the junction of the road from the Mountain Lake. They
then embark on the Rose Lake, about 1 mile from the east end of it, and steer west
by south in an oblique course across it, 2 miles; then west-northwest, passing the
Petite Perche to the Marten Portage, 3 miles. In this part of the lake the bottom is
mud and slime, with about 3 or 4 feet of water over it; and here I frequently stuck
a canoe pole of 12 feet long without meeting anj' other obstruction than if the whole
were water. It has, however, a peculiar suction or attractive power, so that it is
difficult to paddle a canoe over it.'' There is a small space along the south shore
where the water is deep, and this efl'ect is not felt. In proportion to the distance
from this part, the suction becomes more powerful. I have, indeed, been told that
loaded canoes have been in danger of being swallowed up, and have only owed their
preservation to other canoes, which were lighter. I have, myself, found it difficult
to get away from this attractive power, with 6 men and great exertion, though they
did not appear to be in any danger of sinking.
Over ao-ainst this is a verv high, rockv ridge, on the south side, called Marten
Portage, which is but 20 paces long, and separated from the Perche Portage, which is
480 paces, hj a mud pond covered with white lilies. From hence the course is on
the lake of the same name," west-southwest 3 miles to the height of land, where the
waters of the Dove or Pigeon River terminate, and which is one of the sources of
the great St. Laurence in this direction. Having carried the canoe and lading over it,
« Little Cherry Portage.— J. M. C.
6 Watab Portage on Minnesota geological survey maps. — J. M. C.
cThis is Eove Lake.— J. M. C.
''This phenomenon is very familiar to every one who has used a canoe. After paddling over
one of these muddy bottoms, apparently barely making the canoe move with the greatest exertion,
there is a remarkable sense of relief and an increase in the rapidity of the canoe's motion -when there ■
is a change to a sandy or gravelly bottom. An increase in depth of the water between the top of the
mud and the bottom of the canoe will have essentially the same effect. The cause of this is that the
canoe is actually floating in a thin mud, and the friction between this mud and the canoe is \'ery much
greater than between the clear water and the canoe. — J. M. C.
«Now called South Lake.— J. M. C.
62 THE VERMILIOX IRON-BEARING DISTRICT.
679 paces, they embark on the lake of Hauteur de Terre,* " which is in the shape of an
horseshoe. It is entered near the curve and left at the extremity- of the western
limb, through a very shallow channel, where the canoe passes half loaded for 30
paces with the current, which leads through the succeeding lakes and rivers and
disembogues itself by the River Nelson into Hudson's Bay. The tirst of these is Lac
de Pierres ii Fusil,* running west-southwest, 7 miles long and 2 wide, and making an
angle at northwest 1 mile more, becomes a river for half a mile, tumbling over a
rock and forming a fall and portage, called the Escalier,'' of 55 paces; but fi-om
hence it is neither lake or rivei", but possesses the chai-acter of both, and ends
between large i-ocks, which cause a current or rapid, falling into a lake pond for
about 2i miles, west-northwest, to the portage of the Cheval du Bois.-' Here the
canoe and contents are carried 380 paces between rocks; and within a quarter of a
mile is the Portage des Gros Pins,'° which is 640 paces over a high ridge. The
opposite side of it is washed by a small lake 3 miles round; and the course is through
the east end or side of it, three-quarters of a mile northeast, where there is a rapid.
An irregular, meandering channel, between rocky banks, then succeeds for 7^ miles
to the Maraboeuf Lake, which extends north i miles, and is three-quarters of a mile
wide, terminating by a rapid and decharge of 180 paces, the rock of Saginaga being
in sight, which causes a fall of about 7 feet and a portage of 55 paces.
Lake Saginaga takes its name from its numerous islands. Its greatest length
from east to west is about 14 miles, with verj' irregular inlets; is nowhere more
than 3 miles wide, and terminates at the small portage of La Roche,-'' of 43 paces.
From thence is a rocky, stony passage of 1 mile to Prairie Portage, which is very
improperly named, as there is no ground about it that answers to that description,
except a small spot at the embarking place at the west end. To the east is an entire
bog, and it is with great difficulty that the lading can be landed upon stages, formed
by driving piles into the mud, and spreading branches of trees over them. The
portage rises on a stony ridge, over which the canoe and cargo must be carried for 611
paces. This is succeeded bj- an embarkation on a small baj',^' where the bottom is
the same as has been described in the west end of Rose Lake, and it is with great
difficulty that a laden canoe is worked over it, but it does not comprehend more than
a distance of 200 yards. From hence the progress continues thi-ough irregular
channels, bounded bv rocks, in a westerlv course for about 5 miles to the little
* The route which we have been traveling hitherto leads along the high, rocky land or bank of
Lake Superior on the left. The face of the country offers a wild scene of huge hills and rocks,
separated by stony valleys, lakes, and ponds. Wherever there is the least soil it is well covered with
trees.
" Now called North Lake.— J. M. C.
''GunflintLake.— .J. M. C.
'Little Rock Portage.— J. M. C.
''Wood Horse Portage. — J. M. C.
<' Pine Portage. — J. jM. C.
./"This leads from Saganaga into Swamp Lake. — J. I\L C.
<J At east end of Otter Track Lake.— J. M. C.
RESUME OF LITERATURE. 63
Portage des Couteaux, of 165 paces, and the Lac des Couteaux, running about
southwest by west 12 miles, and from a quarter to 2 miles wide. A deep bay runs
east 3 miles from the west end, where it is discharged by a rapid river, and after
running 2 miles west it again becomes still water. In this river are two carrying
places, the one 15 and the other 190 paces. From this to the Portage des Carpes is
1 mile northwest, leaving a narrow lake on the east that runs parallel with the Lake
des Couteaux, half its length, where there is a carrj'ing place which is used when
the water in the xiver last liientioned is too low. The Portage des Carpes is 390
paces, from whence the water spreads irregularlj^ between rocks 5 miles northwest
and southeast to the portage of Lac Bois Blanc, which is ISO paces. Then follows
the lake of that name," but I think improperly so called, as the natives name it the
Lake Pascau Minac Sagaigan, or Drj' Berries.
Before the smallpox ravaged this country and completed what the Nodowasis
[Sioux] in their warfare had gone so far to accomplish, the destruction of its inhab-
itants, the population was very numerous; this was also a favourite part, where
they made their canoes, etc. , the lake abounding in fish, the countrjr round it being
plentifully supplied with various kinds of game, and the rockj^ ridges, that form the
boundaries of the water, covered with a variety of berries.
When the French were in possession of this country the}^ had several trading
establishments on the islands and banks of this lake. Since that period the few people
remaining, who were of the Algonquin Nation, could hardly find subsistence; game
having become so scarce that they depended princi]3allj' for food upon fish and wild
rice, which grows spontaneously in these parts.
This lake is irregular in its form, and its utmost extent from east to west is 15
miles; a point of land called Point au Pin, jutting into it, divides it in i^o parts;
it then makes a second angle at the west end to the lesser Portage de Bois Blanc, .200
paces in length.
This description will serve to give the reader unacquainted with the
area some knowledge of the character of the route and of the character of
the voyagevu's and Indians. Further interesting details of the international
canoe route and the methods of travel can be obtained from Alexander
Henry's and David Thompson's journals for the years 1799-1814, pp.
xlvii-liii.''
"Commonly called at present Basswood Lake. — J. M. C.
''New light on the early history of the Greater Northwest. The manuscript journals of Alexander
Henry, fur trader of the Northwest Company, and of David Thompson, official geographer and explorer
of the same company, 1799-1814. Edited by Elliott Coues. New York: Francis P. Harper, in three
volumes 1897.
64 THE VERMILION IRON-BEARING DISTRICT.
GEOLOGICAL, UTERATtiRE.
1825.
BiGSBT, John J. Notes on the geography and geology of Lake Superior:
Quarterl}^ Journal of Science, Literature, and Arts, Vol. XVIII, 1825, pp. 1-34, 222-
269, with map.
Bigsby, in 1825, describes the rocks at man}' points along the north
shore of Lake Superior. He also follows the old route from Lake Superior
to the Lake of the Woods for 430 miles, and observes an alternation of chlo-
ritic greenstone and amphibolitic granite. Near and on the Lake of the
Woods the greenstone passes into gneiss and mica-slate which is peneti'ated
bv graphic granite.
This statement concerning Bigsby's observations is obtained from Bul-
letin U. S. Geological Survey No. 86, p. 51. The article to which these
statements were credited not having been found, they can not be verified;
and whether any further observations were made by Bigsbj- in the area
included in the Vermilion district has not been learned.
1841.
Houghton, Douglass. [Fourth] Annual Report of the State Geologist, 1841.
State of Michigan. House of Representatives, No. 27. Reprint in "Memoirs of
Douglass Houghton, First State Geologist of Michigan," bj' Alvah Bradish, Detroit,
1889; 302 pages.
In this report Dr. Houghton refers incidentally to that poition of
Minnesota which extends along the international boundarj-, a part of which
is included in this monograph, in the following- words :
The hills rise in broad and somewhat knobby steppes or plateaus, to heights
varying from 400 to 1,200 feet above the lake, and the summits of these hills are
usually not farther inland than from 10 to 20 miles. The rocks of the hills are very
frequently- bare over considerable areas, and the vallej-s containing arable soil are
few and very narrow.
The route of the fur trade to the northwest, via Rainy lakes. Lake of the Woods,
and Lake Winnepic, was formerly wholly carried on by passing over these hills
from a point a few miles west from the mouth of Pigeon River. The trail or
portage path passes over a low jDortion of the range, and tinally falls upon Pigeon
River, which is ascended to its source, from which, l\v a series of portages, the
sources of the streams flowing northwesterly are reached. The hilly portion of the
country, though of exceeding interest in a geological point of view, is the most
desolate that could be conceived.
RESUME OF LITERATURE. 65
1852.
Owen, David Dale. Report of a geological surve}' of Wisconsin, Iowa, and
Minnesota; and incidentally of a portion of Nebraska Territory'. 1852. (Dr. J. G.
Norwood's report, pp. 213-ilS.)
Ill 1848 Dr. Norwood ascended St. Louis River, and crossing the
divide descended Vermilion River to Vermilion Lake. This portion of the
river is now known as Pike River. He then crossed the lake and went on
down Vermihon River proper to Rainy River. His observations were not
numerous. On the divide, the Missab^ Wachu (Big Man Hills) or Giants
range, he observed syenitic granite associated with gneiss. As Vermilion
Lake was approached, outcrops of slaty hornblende rock, micaceous clay
slate, and siliceous slate appeared in the banks and bed of the river,
forming riffles and falls. On the south side of Vermilion Lake talcose and
mica-slate and micaceous schists were exposed. On the north side of the
lake, at the outlet, and continuing on along tlie outlet, mica-slate and
granite were found. He notes that the general trend of the ridges is east-
northeast and west-southwest (p. 313). Structurally, he considers the
northeast part of Minnesota (p. 333) to consist of northeast-southwest
alternating anticlinal and synclinal folds, rivers sometimes occupying the
synclinal valleys. A range of greenstone, beginning at the great bend of
St. Louis River and running northeast (N. 30° E.), forms a true anticlinal
axis, the line of elevation crossing the boundary line between the sources
of Arrow Lake and Mountain Lake (p. 3361."
In 1849 Dr. Norwood followed the international boundary from Pigeon
River to Saganaga Lake. He mentions the occurrence of siliceous slate
and hornblende and ferruginous rocks on Gunflint Lake, and while the
statement is not perfectly clear, he seems to have the idea that these have
been disturbed by the intrusion of the granite, which he observed exposed
from Gunflint Lake to Saganaga Lake. This granite forms a range which,
if continued on the southwest, he states (p. 417) would pass in the line of
the Missabe Wachu (Giants range) and Pokegama Falls on the Mississippi.
He thus implies the con-elation of the granite of Saganaga Lake with that
of the Mesabi range, a position taken by some of the later geologists, as
will be seen below. He commends the northwest shore of Lake Superior as
a Norwood erred in making this statement, as this axis — the Giants range — crosses the interna-
tional boundary between Gunflint and Saganaga lalies.
MON XLV — 03 5
QQ THE VERMILION IRON-BEARING DISTRICT.
perhaps the best extinct volcanic region in the ^Yorld in wliich to study
igneous intrusion. It is to be regretted that Owen did not closely follow
Norwood's • notes in compiling his general map, as the map would then
show far more accurately than it does the distribution of the rocks of the
Vermillion district as disclosed by Norword's reconnaissance survey.
Sections 1 and 2 on PL 2 N of Owen's report have no legend other
than that giving a general location, and consequently one can not be abso-
luteh- sure of the rock represented. It is clear, however, that Norwood had
the idea that the metamorphic rocks in both the Vermillion and the Saganaga
area are cut by the granite of the Mesabi range and Saganaga Lake and
folded in between the granite I'idges.
1S61.
Anderson, C. L.. and Clark, Thomas. Report on geolog-y: Document No. 12,
Minnesota legislature. St. Paul, 1S61; 26 pages.
There was reprinted in 1 860, by order of the Minnesota senate, portions
of the publications of the geological surveys of Wisconsin for 1854 and 1868,
which, owing to the juxtaposition of the States, it was thought would equally
apply to Minnesota. Mr. Thomas Clark, of Lake City, was chairman of the
committee having in charge the publication of this first geologr>.al report of
the State of Minnesota. The same year that this report was published a
commission was appointed, consisting of C. L. Anderson and Thomas Clark,
above referred to, to report on the geology of the State and on a plan for a
geological survey of the State. This report was sent to the legislature by
the governor, who, however, declared himself not ready to advise the com-
mencement of a geological survey.
In the report the commissioners call attention to some of the general
geologic features of the State. The only points of interest in connection
with the present study of the literature of the region is the recognition
of the existence of granite and metamorphic rocks, forming northeast-
southwest trending ranges in the northeastern part of the State, and the
insistence on the investigation of this area, with the view to determining the
existence of metalliferous deposits in the rocks in this region.
1S65.
Hanchett, a. H.. and Clark, T. Report of the State geologist, Aug. H.
Hanohett, ^I. D., together with the physical geography, meteorology, and liotany
of the northeastern district of Minnesota, by Thomas Clark, assistant geologist.
St. Paul, 1865; 82 pages.
RESUME OF LITERATURE. 67
The agitation of the question of the organization of a geological survey
of Minnesota evidently had some effect, as is shown in the publication of
this report. Among other things, the State geologist reports that "speci-
mens of hematitic specular iron ore were obtained from a heavy deposit said
to lay between a lake forming the affluence of the Upper Embarrass River
and Vei'million Lake. The precise percentage of commercially pure iron
contained in this ore has not been ascertained" (p. 6).
1866.
Eames, Henry H. Report of the State geologist on the metalliferous region
bordering on Lake Superior. St. Paul, 1S66; 23 pages.
Eames gives merely a brief description of the then known metalliferous
rocks of northeastern Minnesota. He mentions the occurrence in the Ver-
million district of the siliceous and talcose slates (p. 10) of Vermillion Lake,
in the last of which are found auriferous and argentiferous quartz veins and
the hematite iron ore (p. 1 1) of Vermillion Lake, which is associated with
quartzose jasperoids and serpentine rocks.
Eames, Henrt H. Geological reconnoissance of the northern, middle, and
other counties of Minnesota. St. Paul, 1866 ; 58 pages.
This contains the results of a geologic reconnaissance of the State. It
was found that the granite uplifts have their greatest development in the
northeastern part of the State. They trend northeast, and reach their
greatest altitude at or near the Missabe heights. The most prevalent rocks
found in the northern part of the State are granite, porphyry, hornblendic
slates, siliceous slates, trap, greenstone, talcose slate, primitive schistose
rock, gneiss, and Potsdam sandstone. Li the region occupied by these
rocks are found immense bodies of magnetitic and hematitic iron ore. In
the talcose slates and primitive schistose rock are veins of quartz, carrying
auriferous and argentiferous sulphides of iron and copper
In a report by Richard M. Eames, assistant, a number of details of the
Vermilion Lake district, chiefly concerning veins, are given, and a geologic
map showing the outline of the formation surrounding Vermilion Lake is
said to have been prepared and handed in, but does not accompany the
published report.
68 THE VERMILION IRON-BEARING DISTRICT.
Whittlesei, Col. Charles. Geology and minerals. A report of explora-
tions in the mineral regions of Minnesota during- the years lS-i8, 1S59. and lS6i.
Cleveland, 1S66; 5-i pages.
According to Colonel Whittlesey the Minnesota shore of Lake Superior
structurally represents the northern edge of a synchue, in the basin of
which Lake Superior lies, and we consequently get essentially the same
succession of rocks that is found on the southern shore of Lake Superior
in j\Iichig-an and Wisconsin — a belt of alternating sandstone beds and trap
flows, succeeded inland by a belt of trap. These two constitute the series
which is known as the Keweenawan, Back of the trap he finds a belt of
hornblende rocks, or hornblende-slates, as they are called. This is the
Animikie series of the Minnesota geologists.
Behind the hornblende system is the imperfectly defined region of the granite,
syenite, mica slate, siliceous, and talcose rocks, extending to and across the national
bouudarj'. The Mesabi range occupies the watershed between the waters of Lake
Superior and those of Hudson's Baj'. In many cases the sj^enite and granite appears
to be more recent than the metamorphic slates, having all the appearance of
intrusive rocks [p. 7].
That part of the Vermilion Lake region, including the rocks just
mentioned — the mica-slate, siliceous and talcose slates — which lies to the
north of the Mesabi range is described in some detail. Leaving the syenitic
Mesabi range he proceeds over the portage trail to Vermilion River (now
known as Pike River) and passes in the order given over syenite, "gray,
compact quartz," mica-slate, and distinctly layered novaculitic quartzite.
Just before entering Vermilion Lake fine-grained micaceous rocks are
ex^aosed, and these continue along the western shore, becoming more
talcose and slaty as the explorer goes north. The slaty laminje strike
northeast by east, and dip 75°-80° northwest. These slates are also very
much jointed, causing the formation of rhombohedral blocks. On the north
shore of Vermilion Lake, Colonel Whittlesey saw and recognized clearly
the relations of the granite to the sedimentaries, mica and talcose slate, as
he calls the rocks. He says, "The granite appears as a protrusion in the
slates, and is therefore more recent" (p. 45).
RESUME OF LITERATURE. 69
1871.
Kloos, J. H. Article in The Minnesota Teacher, referred to by N. H.
Winchell.
Wiuchell " says that Kloos suggests the possibility of the hematitic
and magnetitic iron ore at Vermilion Lake being in the lowest member of
the Huronian.
Kloos, J. H. Geologische Notizen aus Minnesota: Zeitschr. deutsch. geol.
Ges., Vol. XXIII, 1871, pp. 417-W8.*
In this article (p. 199) Kloos states that he has become acquainted
with gneisses and finely crystalline clay slates from Vermilion Lake, which
appear to him to belong to the Laurentian. A small rush to this area was
caused by the discover}^ of gold-bearing pyi-itiferous quartz veins, cutting
the metamorphic schists, but did not result in the opening up of productive
mines. He farther on mentions having heard favorable reports concerning
the iron ores of Vermilion Lake.
1873.
Bell, Robert. Report on the country between Lake Superior and Lake
Winnipeg: Geol. Survey of Canada; Report of Progress for 1872 and 1873,
1873, pp. 87-111.
This contains a report (pp. 92-94) of a reconnaissance along a j^art of
the international boundary from Guuflint Lake to Whitewood " (Basswood)
Lake, which is included in the Vermilion district. He observed sedimenta-
ries, slates, and dolomite associated with trap on Gunflint Lake. These he
correlated with the Lake Superior coj)per-bearing rocks. They are suc-
ceeded to the northwest by the Laurentian granite of Seiganagah (Saganaga)
Lake. West of this Laurentian area the route crosses from Poplar Lake
(Swamp Lake of the boundary commission map) to Whitewood (Basswood)
Lake, a belt of Huronian black to green and gray schists, slates, and
quartzites, cut by dikes of trap. The sediments strike S. 15°-30° W., and
dip about 80° W. The only rocks observed around the shores of White-
wood Lake are fine-grained gi'ay to reddish-gray syenite.
«N. H. Winchell: Geol. and Nat. Hist. Survey of Minnesota, Final Kept., Vol. I, 1884, p. 103.
t> A translation of this article by X. H. Winchell is published in Geol. and Nat. Hist- Survey of
Minnesota, Tenth Ann. Rept., 1SS2, pp. 175-200.
<■ The usual name given to this lake is Basswood. No basswood was seen by Bell about the lake,
however, and my own observation showed me that it is certainly not present in great quantity there.
He states that " the lake is said to derive its name Lac de Bois Blanc, or AVhitewood Lake, from the
whitewood, or balm of Gilead, a kind of poplar" (p. 94).
7(» THE VERMILION IKON-BEARING DISTRICT.
WiNCHELL, N. H. First Ann. Kept. Geol. and Nat. Hist. Survey ^Nlinn.. pp.
129. Second edition, 1884.
The general distribution of the nonfo.ssil-bearing rocks of Minnesota,
referred to as "granitic and metamorphic rocks" (p. 64), is given in this,
tlie first annual report of the second Minnesota survey. It is stated that
they occup)- a great portion of the northern part of the State. These
rocks are regarded as Laurentian and Huronian. At the time of this
publication the great deposits of iron ore, which are now of" such national
as well as local importance, were but little known, as appears from the
indefinite statement that "iron ore in unlimited quantities is said to exist
in the dividing ridge between Lake Superior and Vermilion Lake" (p. 67).
1S76.
Whittlesey, Col. Charles. Physical geology of Lake Superior: Proc. Am.
Assoc. Adv. Sci., Twenty-fourth Meeting, 1875, Part 2, pp. 60-72, with map,
Whittlesey finds nowhere on the American side of the boundary, except
at Vermilion Lake, rocks which are like the Laurentian of Canada. The
great masses of granite and syenite, around which the Huronian is formed,
do not resemble the Laurentian of the Canadian geologists. Between the
Canadian and American Huronian there is a very close resemblance. The
conclusion of Foster and Whitney that the traps of Lake Superior are of
Potsdam age is adopted.
1877.
Streng, a., and Kloos, J. H. Ueber die krystallinischen Gesteine von Minne-
sota in Nord-Ame'rika: Neues Jahrbuch fiir Mineralogie, Geologic, und Paleonto-
logie, 1877, pp. 31-56, 113-138, 225-242; translation, by N. H. Winchell, in the
Eleventh Ann. Kept. Geol. and Nat. Hist. Survey Minn., for 1882, pp. 30-85.
■ The authors state that "the St. Louis River rises in northeastern Minne-
sota south of Vermilion Lake, in a region of granite, gneiss, and crystalline
slates, which form a branch from the Laurentian formation as it is displayed
in the region north of Lake Superior" (p. 36).
1879.
Winchell, N. H. Seventh Ann. Rept. Geol. and Nat. Hist. Survey JNIinn.. for
1S78, 1S7'J, pp. 9-25.
In this we find a report by C. W. Hall on a reconnaissance in the
northeastern part of Minnesota. In the summary of geologic results (p. 10)
EESUME OF LITERATURE. 71
the trend of the formations is stated to be more nearly east and west than
it had previously been supposed to be. Hence the formations —
cross the coast line at an acute ano-le, the later formations being- toward the south.
and the older along the international boundary line. . . . The formations that
compose the coast line" . . . seem to be something a^i follows, in descending order:
(1) Metamorphic shales, sandstones, and quartzite. These are cut by dikes, and are
interbedded with igneous rock. ... (2) Ferruginous and aluminous sandstones.
These seem to be metamorphosed into a iii'm basaltiform red rock, as seen in the
Palisades and at other points. ... (3) A quartzose conglomerate, seen at the Great
Palisades and Portage Ba}' Island — probably more properly a part of No. 2. (4) The
quartzites and slates of Grand Portage Bay. ... (5) The jasper, flint, and iron-
bearing belt of Gunflint Lake and Vermilion Lake, and of the INIesabi range. (6) The
slates and schists which the Canadian geologists particularly' designate Huronian.
(7) The syenites, granites, and other rocks that have been classed as Laurentian. (8)
The igneous rocks, known as the Cupriferous series [p. 10].
Of these, the onl}' ones with which we are much concerned in the
Vermilion district are Nos. 4, 5, 6, 7, and 8. It is therefore of interest to
learn the relations of these to one another, as given in the report.
Nos. i, 5, 6, and 7 are probabl}- conformably arranged in succession; at least
thej^ have been so seen at places. Nos. i and 5 are closely associated, and perhaps
the latter is but a local phase of the former, while Nos. 6 and 7 are as closeh^ related,
being conformably interbedded and stratified. No. 5 is conformable with No. 6 in
the iron district along the southeastern side of Vermilion Lake [p. 11].
1S81.
WiNCHELL, N. H. Ninth Ann. Rept. Geol. and Nat. Hist. Survey Minn., for
1880, 1881.
That portion of this report containing a mention of the northeastern
part of the State is apparently merely the published field notes of the State
geologist. No attempt is made to reduce these notes to an orderl}^ discus-
sion of the general geology, and the reader must glean such general facts
as he can find among the man}^ details given. Thus, apro^jos of a specimen
taken from one of the beds, we learn that a greenish, schistose, porpln-ritic
rock, cut by veins of milk}- quartz, is found in nearly a vertical attitude
on Gunflint Lake. This is supposed to be the Canadian Huronian, and
underlies the quartzite and Gunflint beds, apparently unconformably
« The northeast-southwest trending coast line of Lake Supei'ior is here meant.
72 THE VERMILION IRON-BEARING DISTRICT.
(p. 81). The Knife Lake chloritic or serpentinous quartzite is regarded as
Huroniau (p. 86).
On the south side of Vermihon Lake are beds of jasper and iron
hematite whi(^h are regarded as the equivalent of the Gunflint beds. These
are conformable with the magnesian schists and slates which are in a
vertical attitude. They pass down into the schists, and in places the
schists and schistose structure penetrate the jasper and iron (pp. 103-4).
WiNCHELL, N. H. The Cupriferous series in Minnesota: Proc. Am. Assoc.
Adv. Sci., Twenty-ninth Meeting, 1880, pp. •122-425; Ninth Ann. Rept. Geol. and
Nat. Hist. Survey Minn., for 1880, 1881, pp. 385-387.
In this description of the Copper-bearing series the impression is made
that the series gradually becomes more and more changed and crystalline
as one goes away from the shore of Lake Superior. The following
relations are mentioned:
The tilted red shales, conglomerates, and sandstones at Fond du Lac ....
are the same as those associated with the igneous rock all along the shore. They
lie there on a white quartz pebbly conglomerate of a few feet in thickness, which
lies unconformabl}^ on the roofing slates of the Huronian, the same foi-mation
which succeeds to the red-rock formation .... at Ogishke Muncie and Knife
lakes, northwest of Grand Marais [p. 387].
1SS2.
WiNCHELL, N. H. Preliminary list of rocks: Tenth Ann. Rept. Geol. and
Nat. Hist. Survey Minn., for 1881, 18S2, pp. 9-122.
In this rejjort the publication of Winchell's field notes is continued.
From them we learn that the iiint and jasper formations of Gunflint Lake
appear to be in apparent unconformit}^ with the underlying slates and
syenites (p. 88). On Ogishkie Muncie Lake is found a great conglomerate.
The conglomerate . . . contains large rounded pieces of the "Saganaga
granite," which proves the greater age of that granite and the unconformability
of this slaty conglomerate. ... The conglomerate also here contains red jasper
[p. ys].
The descending succession in northeastern Minnesota is given as
follows:
(1) The nearly horizontal quartzites and slate. ... (2) The coarse grit
or fine conglomerate. (3) The jaspery and calcareous . . . Gunflint bods. (4)
Gray marble. (5) The tilted, slat}' conglomerate, and the great conglomerate
RESUME OF LITERATURE. 73
about Ogisiikie Muncie Lake [bearing Saganaga granite bowlders]. (6) The auiphi-
bolite and chloritic slates. (7) Mica schists and alternations of mica schists and
syenite. (8) The syenites and granites of Saganaga and Gull lakes [p. 94].
Whether the great quartzite and slate formation (No. 1 above) is the
same as the highly tilted slate and quartzite formation which passes into
the great conglomerate (No. 5 above) of Ogishkie Muncie Lake is an open
question, although there are several things which indicate that they are
the same. They have been treated throughout, however, as different
terranes. The following' are given as the considerations which appear to
support their equivalency :
(a) Where the horizontal slates approach the syenites at the east end of Gunfiint
Lake there is nothing to be seen of any beds representing the tilted slates. The
syenites and their associated schists come on at once, {h) Where the tilted slates
and the conglomerates associated with them are traceable from the syenite upward
to the gabbro, as south of Ogishkie Muncie Lake, there is nothing to be seen of any
beds like the horizontal black slates of No. 1. (e) The " Gunfiint beds " appear to
belong to the horizontal slates of the international boundar}^ at Gunfiint Lake, but
their supposed equivalents at Ogishkie Muncie Lake belong to schistose and tilted
slates and conglomerate, {d) Although the horizontal slates and quartzites of the
international boundary strike west and southwest across the State, forming one of
the most important topographical features of the northern part of the State, and can
be followed for many miles as such, yet they are lost entireh^ in the region of the
upper St. Louis, and the tilted slates are the only ones seen where that river cuts
the rock at Knife Falls and below, (e) The great gabbro belt which surmounts the
horizontal slates along the international boundar3^ and prevails to the east and south
of their line of strike, is seen to pass to the west of Lake Superior at Duluth, and
to disapjjear from sight suddenl}' between Duluth and Fond du Lac as if its con-
tinuance depended on the maintenance of the horizontal formation with which it is
associated. ( f ) Where the Guuflint beds become a jaspery hematite, as south and
east of Vermilion Lake, the structure of the tilted slates passes into the iron ore as
if of the same formation [p. 95].
1883.
Irving, R. D. The copper-bearing rocks of Lake Superior. : Mon. U. S. Geol.
Survey Vol. V, 1883, 464 pp. , 15 1. , 29 pi. and maps. See also Third Ann. Rept. U. S.
Geol. Survey, 1883, pp. 89-188, 15 pi. and maps; Science, Vol. I, 1SS3, pp. 140, 359,
and 422; and Am. Jour. Sci., 3d ser., Vol. XXVIIl, 1884, p. 462, Vol. XXIX, 1885,
pp. 67-68, 258-259, 339-340.
In the above Irving gives a detailed account of the copper-bearing
rocks of Lake Superior, and also discusses the relations of this series to the
74 THE VERMILION IKON-BEARING DISTRICT.
rocks (if otlier ag-es with wliicli it is associated, and gives many details
conceniiiig- the associated rocks. The Copper-bearing series does uot
include tlie so-called Lower group of Logan, the Animikie group of Hunt,
and also the horizontal sandstones known as tlie Eastern and Western
sandstones; although it includes the dolomitic sandstones, -with accompany-
ing crystalline rocks occurring between Black and Thunder bays, in the
valleys of the Black Sturgeon and Nipigon rivers, and about Lake Nipigon.
The Keweenaw or Copper-bearing series then includes the succession of
interbedded traps, amygdaloids, felsitic porphyries, porphyry-conglomerates,
sandstones, and the conformable overlying sand.stone typicalh* developed
in the region of Keweenaw Point and Portage Lake. These rocks have
their most widespread extent about the western half of Lake Superior,
but also occur in the eastern part of the lake. Their entire geographic
extent in the immediate basin of Lake Superior is about 41,000 square
miles.
The Animikie series in the Thunder Bay district is of great thickness,
probably upward of 10,000 feet, comprising quartzites, quai-tz-slates, clay
slates, magnetitic quartzites, sandstones, thin limestone beds, and beds of
chert}' and jaspery material. With these are associated in great volume, in
both interbedded and intersecting masses, coarse gabbro and fine-grained
diabase, like those well known in the Keweenaw series. A broad examination
of the region shows that there is little ground for the belief in one crowning
overflow. The Animikie series is lithologically like the Penokee series in
Wisconsin; both series bear the same relations to the newer Keweenawan
rocks and the older gneisses, and the two groups are regarded as the same.
The iinimikie rocks have been traced by Bell and also by N. H.
Winchell as far west as Gunflint Lake, and are the equivalent of, if not
actually continuous with, the Mesabi iron range running to Pokegama
Falls and the slates of St. Louis Rivei', although the latter are aftected b}-
slaty cleavage.
The iron-bearing schists of Vermilion Lake are so like the Huronian
that they are regarded as a folded continuation of the Animikie beds,
and a generalized section showing the supposed original conneotiou of
the Animikie group and the Yermihon Lake iron-bearing schists ovi-r tlie
c'ranite of the Mesabi rany-e is introduced.
That the Animikie Muronian is beneath the Keweenawan rocks is
RfiSUMfi OF LITERATURE. 75
shown by the fact that the Keweenawan beds along the Minnesota coast
are passed in descending order until the Animikie slates are reached at
Grand Portage Bay, but there is not a direct downward continuation of the
Keweenawan into the Animikie, for between the two there has been an
intervennig period of erosion. This is shown by the fact that at Grrand
Portage Bay, where the two formations come together, the iinderlying slates
suddenly rise entirely across tlie horizon of 600 or 700 feet of the Kewee-
nawan sandstone. Also in northeastern Minnesota and in the Penokee
district the overlying Keweenawan is now in contact with one member of
the underlying series and now -with another. Further, in the Keweenawan
sandstones of Thunder Bay are found chert and jasper pebbles from the
Animikie, while in the Wisconsin Keweenawan are quartzite pebbles
ai^parently from the underlying Huronian.
18S4.
Chester, A. H. The iron region of northern Minnesota: Eleventh Ann. Rept.
Geo], and Nat. Hist. Survey Minn., for 1882, 1884, pp. 15-1-167.
This report gives in detail the result of two expeditions sent out,
the one in 1878, the other in 1880, by private parties for the purpose of
exploring the reported iron-ore deposits in the Mesabi iron range and on
Vermilion Lake. The earlier of these two expeditions paid little attention
to the Vermilion Lake deposits, and the following facts were obtained
chiefly as the result of the expedition in 1880.
The prevailing rocks in the Vermilion Lake iron district are the slates
and schists and mica-schists and quartzite found in other Hin-onian areas in
connection with iron-ore beds. The belt of iron ore is well defined. The
ore is found in connection with jasper and quartzite, and in many cases
with well-defined walls of slate. The ore deposits are intimately bedded
with the rocks of the country, slates, schists, and mica-schists and quartzite,
and stand nearly vertical, with perhaps a slight inclination to the south,
and trend generally east and west, though this varies from place to place.
The strata are much folded and contorted (p. 161).
Exploration developed what seemed to be two principal deposits of
ore, running nearly east and west, and about a mile apart. The more
northei-n one, nearest the lake, has a total length of nearlj^ a mile, lying in
sees. 28 and 27, T. 62 N., R 15 W. (p. 162).
76 THE VERMILION IRON-BEARING DISTRICT.
WixcHELL. N. H. Note on the age of the rocks of the Mesabi and Vermilion
iron district: Eleventh Ann. Rept. Geo!, and Nat. Hist. Survey Minn., for 1882, pp.
168-170.
In this report the general successiou < >t' rocks in uortheasteru Minnesota
is given in descending order, as follows: (1) Potsdam, including the Kewee-
nawan sandstones, shales, and cong;lomerates, changed by igneous gabbros
and dolerites locallj^ to red quartzites, felsites, quartz-porphyries, and red
granites; (2) Taconic g'l'oup, including' the Animikie series, the Gunflint
beds, the Mesabi iron rocks, the Ogishke Muncie conglomerate (?), the
Thompson slates and quartzites, and the Vermilion iron rocks; (3) Huronian
group (?), including magnesian soft schists, becoming syenitic and porphy-
ritic, found .on the north side of Gunflint Lake, along the international
boundary, at Bass\yood Lake, and at Vermilion Lake; (4) Montalban (?),
including mica-schists and micaceous granites at the outlet of Vermilion
Lake and on the ^Mississippi ; (5) Laurentian, including massive horn-
blende-gneiss and probably the Watab and 8t. Cloud granites.
1SS.5.
WiNCHELL, N. H. Notes of a trip across the Mesabi range to Vermilion Lake:
Thirteenth Ann. Rept. Geol. and Nat. Hist. Survej- Minn., for 18S1, 1SS5, pp. 20-3S.
As the result of a trip from Two Harbors to Vermilion Lake, Winchell
finds between these two points two rock ranges, the first being the ]\Iesabi
proper, and the second the Giants range. Resting unconformably upon the
syenites of the Giants range are the Huronian conglomerates and greenstones
of Vermilion Lake, while south of this range are the slates and quartzites
of the Animikie, overlain by the gabbro and red granite of the ^Mesabi
range, which is in turn overlain by the trap rocks of the Cupriferous
series. The Huronian is considered as resting conformably below the
Animikie, although not appearing at the surface. There are three iron-ore
horizons — the titanic iron of the gabbro belt, the iron ore of the Mesabi
range belonging in the Animikie, and the hematite of the Vermilion mines,
which seems to be the equivalent of the Marquette and Menominee iron ores.
Several pages (pp. 25-35) are devoted to a description of the deposits
exposed by the stripping operations of the various mining companies, and
to analyses of the ore.
A few details are also given concerning the distribution and subdivi-
sion iif the cr^•stnllin(' rocks of Minnesota (p]). 36-38), from which we
RESUME OF LITERATURE. 77
learn, in addition to the facts already presented above, that below No. 1
the slates and quartzites of the Animikie lie :
(2) Soft greenish slat_y schists, which hold lenticular masses of light-colored
protogine gneiss, and also beds of diorite. The horizon of the Vermilion iron mines
is thougiit to be near the bottom of this subdivision or at the top of the next, but en
the opposite side of a Laurentian axis, dipping north, and that of the Mesabi iron
range, in the foregoing subdivision, dipping south. (.3) Conglomeritic and quartz-
itic slates, which become tine, arenaceous quartzites, and also embrace beds of sili-
ceous marble.
Still further north [lie the gneiss and sj^enite], accepted ... as the Laurentian.
[pp. 37-38.]
WiNCHELL, N. H. The crystalline rocks of the Northwest: Proc. Am. Assoc.
Adv. Sci., Thirty -third Meeting, pp. 363-379. Reprinted in Thirteenth Ann. Rept.
Geol; and Nat. Hist. Survey Minn., for 188i. 1885, pp. 124-140.
In this paper Wincliell divides the rocks of the Northwest into six
groups. These groups, in descending order, are as follows, the rocks of
the Vermilion range being assigned to Nos. 4, 5, and 6: (1) Granite and
gneiss with gabbro; (2) mica-schist; (3) carbonaceous and arenaceous
black slates and black mica-schists; (4) hjdromica and magnesian schists,
the iron-bearing horizon at Vermilion Lake; (5) gray quartzite and marble,
which in Minnesota seems to run along the south side of Ogishkie Muncie
Lake, near the international boundary, including, perhaps, the great slate-
conglomerate which is there represented ; (6) granite and syenite with
hornblendic schists. This lowest recognized horizon has frequently been
styled Laurentian. In Minnesota it is found on the international boundary
at Saganaga Lake, and large bowlders from it are included in the overlying
conglomerate at Ogishkie Muncie Lake, showing an important break in the
stratigraphy.
Van Hise, C. R. Enlargements of hornblende fragments: Am. Jour. Sci., 3d
ser.,VoL XXX, 1885, pp. 231-235.
Van Hise describes some enlargements of hornblende fragments and
crystals seen in the Ogishke Muncie conglomerate as developed on Keke-
kabic Lake. A brief description of the macroscopic, appearance of the
conglomerate, taken from W. M. Chauvenet's notes, is also given.
Irving, R. D. Preliminary paper on an investigation of the Archean formations
of the Northwestern States: Fifth Ann. Rept. U. S. Geol. Survey, 1885, pp. 175-
242, 10 pis.
In this paper, published in 1885, Irving gives a preliminary account
of an investigation of the x\rchean formations of the Northwestern States.
78 THE VER:\IILI0N IRON-BEARING DISTRICT.
The problems to be solved in eacli i-egiou are briefly discussed. Here
reference will be made only to discussions of rocks occurring' in the disti'ict
described in the present paper.
It is maintained that the series of rocks first called Animikie by Hunt
belongs below the Keweenawan rocks. These rocks are presumed to con-
tinue with interruptions from Thunder Bay along the boundary to Grunflint
Lake, and thence southwest to Pokegama Falls on the Mississippi.
Throughout a considerable part of its extent the Animikie series of rocks
is bordered on the north by a belt of granite and gneiss, forming the Giants
range, to the north of which again come the strongly folded schists of the
Vermilion district proper. The main question to be determined here is
what the relations of the Animikie to the schists inay be. The hypothesis
is advanced that they were probably originally connected over the inter-
vening granite range, and are thus really a unit, which as the result of
erosion has been separated.
Most of the rocks occurring in the region are sedimentaries that have
been indurated by metasomatic action. With these are associated erup-
tives, some of which have been so modified as to become schists. The
cherty and jaspery rocks are supposed to be some sort of original chemical
sediment, certainly not, however, the result of metamorohism of ordinary
sedimentary material.
1SS6.
Irving, R. D. Origin of the ferruginous schists and iron ores of the Lake
Superior region: Am. Jour. Sci., 3d ser., Vol. XXXII, 1SS6, pp. 255-272.
In this paper Irving discusses the origin of the ferruginous schists and
iron ores of the Lake Superior region. An examination of the Animikie,
Penokee, Marquette, Menominee, and Vermilion districts reveals the fact
that in all of them is found abundant carbonate of iron, which oftentimes
grades into the othei' foi-ms of the iron-bearing formation. The silica
of the jasper, actinolite, magnetite-schists, and other forms of the
iron belt never .show any evidence of fragmental origin, so easily
discovered in the case of the ordinary quartzites and graywackes, and
is presumed to be of chemical origin. Associated witli the iron-bearing
beds is often a considerable quantity of carbonaceous or graphitic schists.
It is concluded, (1) that the original form of the iron-bearing beds of the
Lake Superior region was tliat of a series of thinly bedded carbonates,
RESUME OF LITERATURE. 79
interstratified with carbonaceous shaly layers in places, which were more
or less highly ferriferous; (2) that by a process of silicification these car-
bonate-bearing layers were transformed into the various kinds of ferruginous
rocks now met with; (3) that the iron thus removed from the rock at the
time of silicihcation passed into solution in the percolating waters, was
redeposited in various places, and thus formed the ore bodies and bands of
pure oxide of iron; (4) that in other places, instead of leaching out, the
iron has united with the silicifying" waters to form the silicates now found,
such as actinolite; (5) that the silicifying process went on partly before the
folding, partly afterward, and to the latter period belong probably the
larg'er bodies of crvstalline ore.
Willis, Bailey. Report of a trip on the Upper Mississippi and to Vermilion
Lake: Tenth Census Report, Vol. XV, 1SS6, pp. 4-.57-ti:67.
Willis, in 1886, describes the rocks and the structure of a small part
of the Vermilion district. The area surveyed lies along the south shore of
Vermilion Lake, in T. 62 N., R. 15 W., and comprises about 8 square miles.
One month was devoted to the study of this, and although for the greater
part of the time work was done on snowshoes, many details were carefully
noted. Traverses were made one- eighth of a mile apart, and observations
for magnetic variation and dip were taken.
The prominent topographic features are described as approximately
east and west trending anticlinal ridges of hard jasper, separated by syn-
clinal valleys, in which lie the younger and softer rocks. The north main
ridge or group of ridges is known as the Vermilion range. Southwest of
this, and separated from it by a valley three-fourths of a mile wide, extends
the Two Rivers range. Southeast of the Vermilion range lies Chester
Peak (this is now known as Jasper Peak), the west end of the thii'd ridge.
The iron-bearing series has a dip of between 85° and 90°, The succession
from the base upwards is as follows: (1) Light-green, thinly laminated,
chloritic schist. (2) V^hite, gray, brown, and bright-red jasper, inter-
stratified with layers of hard blue specular ore, which also occurs in ore
bodies of considerable superficial extent, running across the bedding;
thickness 200 to 600 feet or more. (3) Chloritic schist, similar to 1; original
thickness probablj^ about 150 feet. (4) Quartzite, dark gray, white, or
black, of saccharoidal texture, containing grains of magnetite which
make it a readily recognized magnetic formation; probable thickness 200
80 THE VERMILION IRON-BEARING DISTRICT.
feet. (5) Conglomerate, consisting of sandstone pebbles and traces of black
slate inclosed in siliceous chloritic schist. (6) Compact homogeneous rock,
composed of quartz grains, chlorite, hornblende, plagioclase feldspar, and
calcite. This rock may be an eruptive quartz-diorite, but is considered a
metamorphosed sedimentary transition bed between 5 and 7. (7) Black
clay slate, fissile and sonorous. It occupies a broad area north of Ver-
milion range. In section 28 huge masses of jasper form the crown of
the arch and are embedded in green schist, with which they agree in
strike and dip. The jasper blocks are rectangular and several hundred
feet long; the ends of the bands come out squarely to the contact with the
schist as to a fault
1887.
Ieving, R. D. Is there a Hm-onian group?: Am. Jour. Sci., 3d. ser.. Vol.
XXXIV, 1887, pp. 20Jr-216, 249-263. 365-37i.
In these papers Irving discusses the separability of a Huronian group
from an underlying series and demonstt-ates the possibility of such separation
in the original Huronian region, in the Marquette and Menominee districts
of Michigan, in the Penokee district of Wisconsin and i\Iicliigan, and finally
in the Vermilion Lake region of Minnesota and Canada (Ontario). In the
Vermilion region the gently tilted Animikie series of slates, gray wackes, and
iron-bearing rocks, with interstratified sheets of diabase and gabbro, resem-
bles very strongly in hthologic aspect the Penokee series of the south
shore and rests in palpable unconformity upon a folded series of schists,
granites, and gneisses. Above it is the Keweenaw series, which bears the
same unconformable relations to the underlying rocks as it does to the
Penokee series.
Thus the Animike series occupies very pLainl.y the stratigraphical position of the
original Huronian and of the various iron-bearing groups of the south shore of Lake
Superior. Since it is also intrinsicallj^ so extraordinarily like the Penokee series as
to leave no doubt of their identity, and since the Penokee is as evidently the equiva-
lent of the original Huronian, we seem to be left no choice as to calling the Animike
Huronian also [p. 263].
North of tlie Animikie beds are schistose iron-bearing rocks, which
extend from Vermilion Lake to the vicinity of Knife and Saganaga lakes.
These are flanked by gneisses and granites, and on account of their litho-
RESUME OF LITERATURE. 81
logic similarity to the Animikie rocks are taken to be their folded equivalent.
While there is not here the same palpable unconformities as in the other
regions discussed, it is believed that there are two gTOups of rocks, one of
crystalline schists, and another of newer detrital rocks, the apparent
unconformity between these being due to the intense folding.
WiNCHELL, A. Report of geological observations made in northeastern Minne-
sota during the season of 1SS6: Fifteenth Ann. Rept. Geol. and Nat. Hist. Survey
Minn., for 1886, 1887, pp. 5-307.
In this report are given the detailed observations made on an extensive
trip in northeastern Minnesota.
The region presents a series of schists flanked on the north and south
by massive crystalline rocks. In the western part of the district these
rocks are gneissic on both sides, but in the eastern part the schists extend
on the north beyond the limits of the map, while on the south the gneissic
rocks are replaced by gabbro and greenstone. The schists and bedded
crystallines stand in nearly vertical attitude.
It is said that the indications of a genetic connection between graywacke
and the mica-schists are very noteworthy, a gradation from one to the other
having been noted in numerous instances (p. 176). Likewise the schists
grade into the gneissic rocks, there being nowhere an abrupt passage from
one class to the other. In the passage from the schists to the gneisses there
is, first, an increase in frequency of ramifying veins, next, lumps of gneiss
or granite occur in the schists, and finally there is interstratification of the
schists and gneisses (p. 178). The author expresses himself (p. 179) as
uncertain whether or not the conglomerate at Ogishkie Muncie Lake, which
attains an enormous development and contains varieties of granitic and
quartzose bowlders, as well as flint, jasper, porphyry, and greenstone, exists
as far west as Vermilion Lake; however, there is apparently no doubt that
it lies in the strike of the schists occurring at the west end of the range on
Vermilion Lake. It thus seems to be a local development of the schists.
The beds of bowlders are interbedded with flinty argillites, wliich attain
their greatest development north of the conglomerates; the southern border
of the conglomerates is concealed by overlying greenstone and gabbro.
Some sericitic beds have been discovered within the conglomerate formation.
These facts lead the author to conclude that the conglomerate belongs in
stratigraphic position within the northern border of the sericitic schists,
MON XLV — 03 6
82 THE VERMILION IRON-BEARING DISTRICT. ,
and the southern border of the argilhtes as they appear farther west. The
entire SA^stem of g-neisses, schists, and slates is regarded as belonging to one
structural system, as they all possess a common dip and pass by gradations
into each other, both along the strike and across it (p. 181). The iron-
bearing rocks are interlaminated with the country schists, and while they
exhibit much persistence in the direction of the strike, they do not continue
without interruption; they appear in the midst of the schists sometimes as a
strictly local phenomenon (p. 182). In structure the region is a simple
synclinical fold, the strata of which have a thickness of 106,204 feet. The
succession, from the bottom upward, is granite, gneiss, micaceous and
hornblendic schists, graywacke, argillite-schist bearing conglomerates, and
sericitic and chloritic schists bearing iron ore (p. 191). As the plainly
fragmental rocks grade by imperceptible stages into the gneiss and the
gneiss into the granite, the whole is regarded as a sedimentar}'' series (p. 193).
While granite pebbles, are found in the conglomerates, these are not derived
from the underlying granite, as many of the fragments diflPer in character
from the inferior granite (p. 194).
The author places the conglomerates stratigraphically below the iron
ores and jaspers. He makes the following statement:
We find flints and jaspers, which, as far as we have explored, could not be
afforded by an}^ part of this system. We find nothing which indisputably could
have been derived from an}' member of the system — the Vermilion S3'stem — ranging
from the granites to the earth}' schists. Those older rocks whose destruction afi'orded
material for the building of the Vermilion system belonged to an earlier age, and
were parts of an older system [pp. 19-1-19.5].
They all belong to one system, for no grounds were discerned which
would justify the division of this series of rocks into several systems
(p. 195).
WiNCHELL, N. H. Geological report: Fifteenth Ann. Rept. Geol. and Nat.
Hist. Survey Minn., for 1886, 1887, pp. 209-399, with map.
In this report Winchell gives very numerous details as to the geology
of northeastern Minnesota. A preliminary geologic map accompanies the
report. At several places there are transitions between the granite-gneiss
and a fine-grained mica-schist. In the syenite are sometimes found
angular fragments of mica-schist. The Vermilion group is defined as
iiichiiling the lower portion of the complex series of schists designated as
RESUME OE LITERATURE. 83
Keewatin by Lawsou. It embraces the mica-schists and hornblende- schists
of Vermilion Lake and their equivalents, and lies between the graywackes
on the one side and the basal syenites and granites on the other.
Adjacent to Vermihon Lake are hematite ores associated with jasper,
which are inclosed in a greenish magnesian schist, the bedding of which
stands vertical. This schistose rock is probably of igneous origin, and in
its relations to the jasperoid rocks it fills all their cavities, overlying them
unconformably, and holding fragments of the jasper, all indicating its later
origin. This igneous rock passes into a chlorite- schist, and this into the
sericite-scliists and graywackes, which show unmistakable evidence of an
aqueous arrangement (pp. 219-221). The jasperoid is a sedimentary rock
(p. 245 et seq.) and not an eruptive, as has been supposed by Wadsworth.
The rock was not, however, deposited in its present condition. The beds
have been upturned, folded, crushed, and affected by intense chemical
action. The ore is regarded as a result of chemical or metasomatic change.
The ore is a hard hematite and of such good quality as to warrant a guar-
antee of 67 per cent or more of iron, and 0.06 j^er cent or less of phosphorus.
The general succession from above downward is as follows: (1) Gabbro.
(2) Diabasic dolerite. These rest unconformably upon the lower members.
(3) Reddish gneiss and syenite, which includes the Misquah Hills, White
Iron Lake and the Giants range (Mesabi heights). This is a ease of a
fusion of sedimentary beds in situ, although it is not generally complete.
(4) Graywacke, sericite-schist, argillite, quartzite, and jaspilite, which occur
about Vermilion Lake. (5) Mica-schist, hornblende-schist, and diorite — the
Vermilion group. (6) Mica-schist and granite, veined with syenite and
granulite. (7) Lower syenites and gneisses, generally regarded as Lauren-
tian. Nos. 3 to 7 are conformable, and Nos. 4 to 7 graduate into each other
(p. 355).
The author states it as his opinion (p. 356) that there is reason for
believing that the Animikie rocks overlie the greenstone (No. 2) and
underlie the gabbro (No. 1) of the above succession.
18SS.
WiNCHELL, N. H. Sixteenth Ann. Rept. Geol. and Nat. Hist. Survey Minn.,
for 1887, 1888. pp. 13-129.
Winchell, as customary, publishes in the annual report his field notes
for the preceding ye-a.v, giving an abundance of details about the rocks of
84 THE VERMILION IRON-BEARING DISTRICT.
the Vermilion district. As a result of a traverse made into Canadian ter-
ritory from the east end of Gunflint Lake, he comes to the conclusion that
the Animikie, on Gunflint Lake, while not found in exact superposition on
the Keewatin, bears such relation as to render it probable that the t^Y0
formations are discordant. A short distance north of the xinimikie the
Keewatin rocks are found with a dip of 80° south, and these, a little farther
to the north, grade conformably into micaceous and hornblendic schists.
He continues as follows:
In the report of 1881, p. 95, . . . some reasons are given for considering the
"quartzite-slate formation" seen at Gunflint Lake (the Animike) in horizontal posi-
tion, the equivalent of the great quartzite and slate forniatiou at Ogishke ISIuncie
Lake, which passes into the Ogishke conglomerate. . . . At that time the tilted schists
and graA'wackes of the Kewatin series, with their contained iron ore, were con-
sidered an integral part of the same tilted series as the slates and quartzites associated
conformably with the Ogishke conglomerate, and the iron ore of the jasper ridges
at Vermilion Lake were considered the equivalent of the iron ore seen in the
Animike. . . . But since the separation of the Animike from the Kewatin has been
established by mai'ked unconformities, and by constant differences in lithology
(including a constant difference in the kind of iron ore associated and their respective
mineral accompaniments), it remained still to answer the question. To which series,
the Animike or the Kewatin, does the quartzite-slate conglomerate of Ogishke
Muncie Lake belong^ [p. 79.]
An attempt was made to trace the Animikie westward and get its
relations to the rocks on Ogishke Muncie Lake. The Animikie rocks
rest unconformably on the gneiss west of Gunflint Lake. The Pewabic
quartzite is a magnetited rock apparently near the top of the Animikie.
The gabbro is observed overlying the Animikie (Pewabic quartzite) at
many places. The Animikie lies unconformably on the Keewatin north
of Gunflint Lake (p. 87). In passing from Gunflint Lake the Animikie
is found to have a dip varying from 12° to 65° SSE. At Gabemichigama
Lake a gradation is supposed to exist from the flat-lying Animikie into the
Ogishke Muncie conglomerate, with interstratified quartzite and slate beds
sti'iking northwest and dipping 88° northeast (p. 91).
Studies made around Ogishke Muncie Lake show that the Ogishke
conglomerate can be divided into, first, an old, eruptive-looking, massive
schistose and decayed conglomerate, which belongs to the Keewatin, and
extends from Stuntz Island, in Vermilion Lake, past Fdy (wliere it was
RESUME OF LITERATURE. 85
recently examined at the iron mines) to Twin Mountain and Frog Rock
Lake; second, the Ogishkie conglomerate proper of later date, fresher
aspect, and more siliceous, evidently derived largely from the disintegration
of the other, upon which it lies unconformably. With this second phase
the Animikie quartzite and slates are interstratified (p. 98).
Partly surrounding Cacaqiiabic Lake is found a green schist Avhich
belongs apparently to a date about the same as the Keewatin portion of
the Ogishke conglomerate or is its immediate successor and conformable
upon it. Nevertheless they are markedly different. The green schist is
apparently formed of basic erupted materials in a fragmental condition and
received its stratified arrangement through the agency of water. Volcanic
vents in the immediate neighborhood must have given origin to this vast
supply of basic materials (p. 108).
It is concluded that a great basic eruption separated the Animikie and
Keewatin, as shown by this volcanic fragmental material, as well as by the
existence of mountains of g-reenstone, which are to be regarded as the
probable sources of the fragmental rock (p. 108).
WiNCHELL, A. Sixteenth Ann. Rept. Geol. and Nat. Hist. Survey Minn., for
1887, 1SS8, pp. 133-391. Unconformability between the Animikie and the Vermilion
series: Am. Jour. Sci., 3d ser., Vol. XXXIV, 1887, p. Sltt. See also: The uncon-
formities of the Animikie in Minnesota: Am. Geologist, Vol. 1, 1888, pp. 14-24. Two
sj'stems confounded in the Huronian: Am. Geologist, Vol. Ill, 1889, pp. 212-214:,
389-340. Systematic results of a field study of the Archean rocks of the Northwest:
Proc. Am. Assoc. Adv. Sci., Thirty-seventh Meeting, 1888, pp. 205-206. The geo-
logical position of the Ogishkie conglomerate: Proc. Am. Assoc. Adv. Sci., Thirty-
eighth Meeting, 1889, pp. 234-235.
In the above papers Alexander Winchell reports that he finds upon
Wonder Island, in Lake Saganaga, a conglomerate which contains abundant
rounded pebbles in a groundmass of syenite (p. 219). This occurrence
gives rise to the following suggestion in the author's mind:
The inferences from the occurrence are important. A pudding stone like this
is universally regarded as of fragmental origin. Not onlj^ that, but of origin
through aqueous agency. . . . So, if this conglomerate is sedimentary in nature,
the S3^enite groundmass must, at the time of the deposition of the pebbles, have
been also in a state of semifluiditj' under the influence of water. It may have been
subjected simultaneously to energetic thermal action; but it was not in that state of
fluidity which accompanies and results from recent eruption as molten matter from
86 THE VERMILION IRON-BEARING DISTRICT.
some deep source. This view of the orig-jii of granitic rocks I have heretofore
maintained, and this remarkable observation is a gratifj'ing confirmation of the
correctness of the opinion [p. 219].
The lower limit of the conglomerate is very abrupt, and the conglom-
erate is figured as overlying the syenite. Nevertheless it is concluded
that —
The epoch of the paste [in which the pebbles lie] and that of the deposition were
the same. The conglomerate and the syenite were put in place simultaneously. The
syenite was not "erupted" after the conglomerate existed. The conglomerate was
not laid down on the solidified syenite [p. 321].
On. the north side of Gimflint Lake he finds argillites standing nearly
vertical.
The}' are not at all ambiguous. They are the Knife Lake slates preserving to
this point their steady yerticalitj', and here remaining uncovered by Animike.
This looks like a solution of a vexed problem. The Animike and the Vermilion
slates are not one . . . The dip [of these slates] is S. 89*^'. The strike of the sheet is
N. 72^ E. [p. 253].
The Animikie .slates are found resting unconformably upon A-ertical
schists, gneisses, and syenites at several points on Gunflint Lake [p. 323].
On the west side of West Sea Gull Lake the conglomerate and syenite are
reported as interbedded. This conglomerate is thought to be comparable
with that of Wonder Island (p. 293). On the north side of Sea Gull Lake
the syenite contains sharply limited rounded pebbles and irregular masses
of hornbleudic and diabasic material (p. 298). On Epsilon Lake the argil-
lite has schistic planes standing vertical, while the bedded structure has a
dip of only 23° toward S. 40° W. (p. 322).
From the summary of facts concerning the region we learn:
Within the region here considered the geographical distribution of the several
terranes is east-northeast in the Vermilion district and nearly east in ' the district
eastward from Knife Lake. Throughout the entire region the clastic rocks — not
excluding the so-called granites and sj'enites, present a liedded structure — sometimes
indeed obscure, but everywhere discernible over all considerable exposures. Among
the granites and gneisses the bedding may possibly lie regarded as the result of
foliation alone; Init I have been led to think that its direction was predetermined l)y
pianos of sedimentation. . . . For similar reasons, I regard the bedding of the
crystalline schists as primitively sedimentary. The two older systems of rocks ha\-e
their planes of bedding nearly vertical — inclining only a few degrees in one direction
RESUME OF LITERATURE. 87
or the other. The bedding of the newer system is nearly horizontal — inclining five
to fifteen degrees southward in the regions here reported on [p. 330].
811111101110' up, we get the following- succession (pp. 330-364):
1. At the base are the r/ranifoid and gneissoid rocks in three areas — the
Basswood, White Iron, and Saganaga lakes. The White Iron granite area
is made to include the area on and near Snowbank Lake that is underlain
by granite. These granitic masses have everywhere a bedded structure,
more or less distinct. They are traversed by quartzose and g-ranulitic
veins, as well as by dikes of diabase.
2. The gneisses and granites are flanked by vertical crystalline schists of
the Vermilion group. The transition from' the gneisses to the crystalline
schists is never abrupt, but is a structural gradation, near the line of
junction the beds of gneisses and schists occurring in many alternations.
3. Above the Vermilion group are the Keivatin semicrystaUine schists,
the two series being everywhere conformable; but there is a somewhat
abrupt change from one group to the other, and this indicates the possibility^
of an original unconformity. If such an unconformity existed, as is
thought improbable, it has been destroyed by laterial pressure. There has
been no actual connection traced between the Kewatin schists north of
Grunflint Lake and those of Knife Lake. The Kewatin schists are almost
everywhere vertically bedded. When the bedding is obscure this is some-
times due to the action of erupted masses, but more often the metamor-
phosed condition of the strata is not ascribable to any visible cause. The
Kewatin schists include graywacke, argillite, sericite-schist, chlorite-scliist,
porphyrellite-schist, and hematite. The Ogishke conglomerate is placed
as part of the Kewatin system, as it is traced liy actual gradations into
the adjoining argillites. These argillites and associated schists are in
continuity with the argillites and schists of Vermilion Lake, ^vliile in the
conglomerate itself are local developments of sericite-schist. The .bedding
of the conglomerate is nearly vertical; its pebbles are metamorphosed;
they include numerous varieties, among which are syenite resembling the
Saganaga syenite, greenstone, porphyry, red jasper, flint, quartz, petrosilex,
ordinary syenite, diorite (coarse and fine), porphyroid, siliceous schist, and
carbonaceous siliceous argillite. On structural as well as lithologic
grounds the Ogishke conglomerate seems to be part of the Kewatin,
although there are some reasons for suspecting it to belong to the Animikie.
88 THE VERMILION IRON-BEARING DISTRICT.
The Ogishke conglomerate is not to be confounded with the Stuntz
conglomerate, which occupies a lower stratigraphic position than does the
Ogishke. That the Kewatin schists are eruptive is regarded as improbable.
4. The Animikie series, resting uncouformably upon the Keewatiu,
stretches from Thunder Bay as far as Duluth and still beyond to the
Mississippi, and perhaps includes some of the slates as far west and north
as Knife Lake. The Animikie formation is generally in a nearly horizontal
position, the dip not being more than from 5° to 15° SSE. The formation
is essentially an argillite, which embraces jaspery magnetitic, hematitic,
and sideritic beds. At Gabimichigama Lake the Animikie, represented
by the "muscovado," is in its characteristic horizontal position, while the
vertically bedded terrane underlies it. The sedimentary series is cut by
dikes of igneous rock, determined macroscopically as diabase-norite and
porphyry.
For the system of semicrystalline schists subjacent to the Animikie, to
which the term Kewatin has been applied, Marquettian is pi'oposed. The
succession of terranes in northeastern Minnesota is, in descending order,
summarized as follows (pp. 366-367):
HuKONiAN SYSTEM (compai'c scc. 2 of this report"), over 4,082 feet.
Magnetitic group. 32 feet.
Dark, laminated, shaly argillite, sometimes magnetitic, 29 feet.
Magnetitic lieds, often uppermost, 8 feet. Place of sideritic bed?
Muscovado, uppermost when the two above are wanting, 4 feet.
Siliceous group. 50 feet.
Siliceous argillites and siliceous and jaspery schists, 50 feet.
Argillitic group. 4,000 feet.
Dark, laminated, shaly argillites, over 4,000 feet in Minnesota.
(Bottom of the .sj'.stem not reached at contacts seen with gneiss and
Marquettian.)
Marquettian system. 27,500 feet.
Ogishke group. 10,000 feet, but local. (Perhaps half this).
Ogishke conglomerate, slaty and diabasic. 4,500 feet each side of synclinal.
Ogishke dolomite, included in the conglomerate, 10 feet.
Conglomerate greenrock. 500 feet each side of synclinal.
Tower group. (Earthy schists.) 15,000 feet.
Sericitic and argillitic schists, with beds of hematite, 5,OU0 feet.
(These sometimes changed to chloritic .schists.)
"This refers to Sixteenth Ann. Kept. Minn. Geol. and Nat. Hist. Survey.
RESUME OF LITERATURE. 89
Marquettian system — Continued.
Tower group — Continued.
(The_v pass eastward into schists prevailingly porphyi'ellitic.)
Stuntz conglomerate, porodyte and porphyrel, 20 feet.
Graywaclve group. 2,500 feet.
Gra3'wacke and horufels.
Graywacke with indications of fine mica and hornblende. ("Nascent mica
schists.")
Laurentian system. 89,500 feet.
Vermilion group. Over 1,500 feet.
Crystalline schists — micaceous, hornblendic, dioritic, granulitic.
Gneissic group. Over 88,000 feet.
Chlorite-gneiss. (Not universally developed.)
Saganaga, White Iron, and Basswood gneisses.
Thus the crj-stalline schists and gneisses fall entireh' within the Laurentian
system. There are no Huronian gneisses in Minnesota. We find nothing of ' ' older "
and "newer" gneisses. We find no ''claj' slates" beneath the horizon of the
crystalline schists. But I can not deny the existence of a different state of things in
other regions. To me it seems probable, however, that a comparative^ undisturbed
region like northeastern Minnesota must approach near to a normal exhibit of the
real sviccession of the Archean rocks.
Irving, R. D. On the classification of the early Cambrian and pre-Cambrian for-
mations: Seventh Ann. Rept. U. S. Geol. Survey, 1888, pp. 365-45i, with 22 plates
and maps.
Irving, in 1888, discusses the classification of the early Cambrian and
pre-Cambrian formations, and particularly those of the Northwestern States.
The relations of the Animikie, Penokee, Marquette, Menominee, and Vermil-
ion Lake iron-bearing series to the underlying and overlying series are again
fully discussed. The Keweenawan is held to overlie the Huronian every-
where by a very considerable unconformity. At the base of the Keweenawan
is a great mass of gabbro, which extends from Duluth northeast to the inter-
national boundary, more than 100 miles, and at its maximum is more than 20
miles wide. This basal gabbro is now in contact with one member of the
Animikie, and now with another, while in other places it is in contact with the
lower crystalline schists or granite. In the Huronian are placed the original
Hiu-onian, the Iron-bearing series of Michigan and Wisconsin, the Black
River Falls iron-bearing series, the Animikie series, the St. Louis and
Mississippi slate series, the Vermilion Lake iron-bearing series, the Baraboo
90 THE VERMILION IRON-BEARING DISTRICT.
quartzite series, and the Sioux quartzite series. Under the Hurouian is the
Laurentian, separated from it by a great iincouformity. This is a series of
D-i-anites, gneisses, hornblende-schists, mica-schists, and other green scliists.
1889.
Hall. C. W. The di.stribution of the g-ranites of the Northwestern States, and
their general lithologic characters: Proc. Am. Assoc. Adv. Sci., Thirty-seventh
Meeting, 1889, pp. 225-226.
In the above paper Hall describes the distribution of the granites of
the Northwestern States, particularly those of Minnesota.
In Minnesota they occur (1) in several belts along the Canadian
boundary projecting southwesterly into the State; (2) as quite prominent
masses, wliether connected with those along the boundary or not, around
Vermilion, Snowbank, and other lakes; (3) forming the Mesabi or Giant
range; (4) at a number of places in the central part of the State. These
are found to be either intrusive or granitic vein-stones, the latter iDeing
insignificant in quantity. Tlie granites of Minnesota as to age are probably
later than the Laurentian floor of the continent, but earlier than those of the
Agnotozoic era.
They belong to one of tlie three or four grand periods of ei-uptive
activity determinable in the Northwestern States.
WiNCHELL, N. H. Seventeenth Ann. Rept. Geol. and Nat. Hist. Survey ^Nlinn.,
for 1888, 1889, pp. 5-74; see also the Animikie black slates and quartzites, and the
Ogishki conglomerate of Minnesota, the equivalent of the '^ Original Huronian:"
Am. Geologist, Vol. I, 1888, pp. 11-14; also Methods of stratigraphy in studying the
Huronian: Am. Geologist, Vol. IV. 1889. pp. 342-357.
In this, Wincliell gives a review of the work done upon the crystal-
line rocks of northeastern Minnesota by the State survey, showing the
progression in the ideas held concerning their characters and stratigraphy
(pp. 6-28). Then follows a summary of the results of the investigations
as they appear up to that time (pp. 28-74). In many points the conclusions
and facts are the same, of course, as in the previous reports. The Lauren-
tian age (pp. 28-31) is made to include the gneiss, granitic, and syenitic, Init
excludes the crystalline schists. It is the fundamental gneiss of iMinnesota.
"It resulted from the fusion and recrystallization of the earliest sedi-
ments" (p. 28).
RESUME OF LITERATURE. 91
The Laurentian g-neiss is represented in the Vermilion district by the
Basswood Lake and pei'haps the Saganaga Lake g-ranite. However:
There may be spots, or considerable areas, within this original gneissic belt,
where, bj' subsequent deeij-seated hj^drothermal fusion, these primitive Laurentian
sediments have been rendered plastic and then fluid, and have b}' pressure been
extended through fissures in the crust to the surface or have been uncovered as lacco-
lites b}' the destruction of the overlying strata; but wherever these exist the}' are
presumed to show their later origin by their nongneissic structure, or by their
overlying some later sedimentary strata. The distinction, howe\-er, between the
eruptive condition of the fused Laurentian sediments and the primitive sediments
that have been converted in situ into the fundamental gneiss is one that requires
more study before it can be defined. That both conditions exist there can be no
question; that they can always be distinguished is not to be afiirmed [p. 29].
Closely associated with the belts of fundamental gneiss are areas of
massive erruptive syenite which have resulted from such hydrothei-mal
fusion of the gneiss. Such syenite is found north of Gunflint Lake, on the
shores of Cacaquabic Lake.
The Laurentian gneisses are seen at places to be conformable with,
and to grade into the hornblendic and micaceous "crystalline schists" — the
Vermilion schists, which are the equivalents of Lawson's Coutchiching. At
other places the gneisses and schists are uncomfoi'mable and here they both
play the r61e of eruptive rocks interpenetrating, in the form of transverse
dikes, and inclosing fragments of each other. This relationship is con-
sidered to be eA'idence of volcanic action. "It is manifest, therefore, that
the supposition of the advent of a characteristically eruptive era, closing
the quiet Laurentian sedimentary age, will account for both an unconform-
able and a conformable transition, such as are seen, from the Laurentian to
the Vermilion" (p. 35).
The Vermilion group passes by conformable transition into the Keewa-
tin. The character of the Keewatin rocks indicates that there was active
volcanic action during the whole period, and that the ejectamenta were
received and distributed by the w^aters of the suiTOunding sea. This is
indicated by the alternation of the breccias and volcanic material with
truly sedimentar)- strata. The name Kawishiwin is proposed for the
massive greenstone stage of the Keewatin. The Keewatin is the iron-
bearing formation. The iron ore is associated with the jaspilite, which
is of a sedimentary orig-in. Above the Keewatin is a profound uncon-
92 THE VERMILION IRON-BEARING DISTRICT.
foriiiity (pp. 37-46). Above this lies the Aniinikie. This has the Ogishke
conglomerate as its base.
This coiii^loinerate is followed by an immense thickness of dark slat^' rocks,
often chert}', or flinty, frequently very dark-colored, generally siliceous, alternating
with thin quartzites and grayish feldspathic quartzites, all in conformable stratitica-
tion, as a whole. Variously interbedded with these slates and quartzites, from
bottom to top, are beds of basic eruptive rock, . . . [pp. 47—48].
The Animikie series of Minnesota, bearing iron at one horizon, is the
equivalent of the Marquette series, the iron-bearing group of Rominger, of
the Peuokee-Gogebic series of Michigan and Wisconsin, of the Mesabi
series in Minnesota, of the Black River iron-bearing- schists in Wisconsin,
and of the quartzites of the Black Hills. All are of Taconic age, for the
Lower Cambrian is equal to the Taconic, the Huronian is equal to the
Taconic, therefore the Lower Cambrian is equal to the Huronian (pp. 46-48).
La the Potsdam sandstone, which is uncomformably on the Taconic,
are included the upper quartzites of the original Huronian, certain of the
quartzites of Marquette, the Sioux quartzites of Dakota, and the quartzites
of Minnesota and Wisconsin. This is also the age of the copper-bearing
rocks, which are an alternation of basic and acid eruptions with interbedded
sandstones and conglomerates. The great gabbro eruption is later than the
beginning of the Potsdam age. Unconformably above the Potsdam is the
St. Croix sandstone (pp. 51-57).
The general succession in descending order, is as follows (p. 68):
Calciferous. Magnesian limestones and sandstones.. | Dikelocephalus horizon
bt. Croix, bandstones and shales ) '^
Overlap unconformity.
Potsdam. Quartzite, gabbro, red granite, and Keweenawan_ __Paradoxides horizon
Ovet^laj) unconformity.
Taconic. Black and gray slates and quartzites, iron ore (Huronian,
Animike) . _ - Olenellus horizon
Overlap unconforin Ity.
Kewatin. (Including the Kawishiwin or greenstone belt, with its jaspilite),
sericitic schists and gray wackes -
Vermilion. (Coutchiching) crystalline schists
Eruptire uncorfortn ity.
Laurentian. (xneiss —
Archean
EESUME OF LITERATURE. 93
WiNCHELL, H. V. Report of field observations made during the season of 1888
in the iron regions of Minnesota: Seventeenth Ann. Rept. Geol. and Nat. Hist.
Survey' of Minn., for 1888. 1889, pp. 77-14.5; see also The diabasic schists containing
the jaspilite beds of northeastern Minnesota: Am. Geologist, Vol. HI, 1889, pp. 18-22.
In the above H. V. Winchell gives further observations on the iron
regions of Minnesota. On the Giants range the Animikie is found to rest
upon the syenite. There is a semicrystaUine rock betvreen tlie two, which
gi-ades into the syenite. The character of the transition is not metamorphic,
but rather fragmental, there appearing to be a certain amount of loose
crystalline material which has resulted from the decay and erosion of the
syenite lying on top of this rock in the bed of the sea, upon and around
which the Animikie sediments were deposited. The coarse detritus grades
up into the fine detritus of the Animikie (p. 86). The Animikie beds are
found also to rest unconformably upon the upturned edges of the Keewatin
schists (p. 87). The same relations are found to prevail in the Birch Lake
region (p. 91). The gabbro containing ores in the vicinity of Kawishiwi
River are found to contain fragments of the Animikie slates and quartzites,
and is therefore of later origin (pp. 96-97). At Gunflint Lake the Animikie
rests unconformably upon the Keewatin (p. 104). The Keewatin schists
are largely of eruptive origin (p. 132). The contacts of the jaspilite with
the basic schists are abrupt and angular, and numerous fragments are found
contained in the schists. The jaspilite is regarded as a sedimentary forma-
tion, which was broken up and involved in the eruptions of Keewatin
time. The Huronian quartzite, associated with the magnetite, lying
unconformably upon the syenite, is believed to lie conformably upon the
Animikie slates (p. 133).
Grant, Ulysses S. Report of geological observations made in northeastern
Minnesota during the summer of 1888: Seventeenth Ann. Rept. Geol. and Nat. Hist.
Survey Minn., for 1888, 1889, pp. 147-215.
In this report Grant gives an account of the geologic observations
made by him in northeastern Minnesota.
North of Gunflint Lake the vertical Keewatin slates and Vermilion
crystalline schists, with an east and west strike, strike directly across a
range of immediately adjacent gneisses, the schists showing no evidence of
being twisted or bent within 200 feet of the gneiss. In the syenites of
Gunflint Lake are found fragments of schist, which indicate that the syenite
is eruptive later than the schists (p. 159).
94 THE VERMILION IRON-BEARING DISTRICT.
WiN'CHELL, Alex. Conglomerates enclosed in gneissic ten-anes: Am. Geologist,
Vol. III. 1889, pp. 153-1*35. 256-262.
Ill this jjaper Alexander Wiiicliell restates his conclusions concerning the
origin of the Sagaiiaga (Sixteenth Ann. Rept., p. 219) and Sea-Gull (Sixteenth
Ann. Kept., p. 298) syenite conglomerate. He maintains in this paper that
the conglomerate is produced from a fragmental rock by selective meta-
morphism, the completely crystalline gneissoid rocks retaining rounded
frao-ments which are residual clastic material. The conglomerate of Wonder
Island is not one consisting originally of a mass of pebbles, over which a
fluid magma has been poured, for the pebbles are not in contact; they
could not have lain where they are before the magma existed. The gneissic
magma was contemporaneous with the pebbles, and supported them and
prevented their contact. The magma must have been plastic, but it was
low-temperature igneo-aqueous plasticity.
^VI^'CHELL, N. H. Some thoughts on eruptive rocks, with special reference to
those of Minnesota: Proc. Am. Assoc. Adv. Sci., Thirty-seventh Meeting, 1889, pp.
212-221.
N. H. Winchell, in 1889, in a general discussion of the origin of the
eruptive rocks, maintains that there are four epochs of basic eruption in
Minnesota: First, that represented by the crystalline schists of the Vermilion
group ; second, an epoch succeeding the gray wackes of the Keewatin and
forming a part of the Keewatin; third, one succeeding the Animikie, during
which the Great gabbro or Mesabi overflow was outpoured.
1890.
Report of the Royal Commission on the Mineral Resources of Ontario and
Measures for their Development. Toronto, 1890, pp. 123-126.
In this report there is a brief description of observations made by three
members of the commission who visited the Vermilion district. No state-
ments of geologic character which are of any interest are given. State-
ments were gathered from miners and prosj^'^ctors which show that promising
iron-bearing formations exist in Ontario in tlie noi'theastern extension of the
Vermilion iron range.
WixciiELL, N. H. and H. V. On a possible chemical origin of the iron ores
of the Keewatin in Minnesota: Am. Geologist, Vol. IV, 1890, pp. 291-300, 382-386;
also Proc. Am. Assoc. Adv. Sci., Thirty-eighth Meeting, 1S90. pp. 235-242.
KESUME OF LITERATURE. 95
lu the above papers N. H. and H. V. Winchell maintain that the iron
ores of the Keewatin of Minnesota are not derived from a carbonate, but
are probably a direct chemical precipitate; for there is no evidence of the
existence of carbonate of iron at any time, and the nature of the country
rock is such as to imply that no carbonates in amounts required could he^ve
been deposited at the time the rocks were formed.
Winchell, Alexander. Some results of ArchEean studies: Bull. Geol. See.
Am.. Vol. I, 1890, pp. 357-394.
Alexander Winchell in 1890 repeats his general conclusions as to the
stratigra])hy in northeastern Minnesota already given in his reports of field
work in the Vermilion district for the yeai's 1886 and 1887, and published
in the fifteenth and sixteenth annual reports of the Minnesota survey, pages
5-207 and pages 133-391.
Summed up, these conclusions are briefly as follows: In northeastern
Minnesota there are large areas (in the Vermilion district four) of granitoid
and gneissoid rocks which have oval outlines trending- in general northeast-
southwest. Gneissoid rocks predominate, and the rocks approaching a
granitoid condition are foimd only at the centers of the areas. These
gneissoid areas are surrounded by cry stalline schists, mica-schists, hornblende-
schists, or mica-hornblende-schists, known as the Vermilion series. These
strike east-northeast. The dip increases away from the granitoid areas
until it becomes vertical. The gneissoid (granitoid) rocks and schists are
intimately connected, and the author, while declining to make a definite
statement, clearly intimates that the gneiss, g-ranites, and schists are all of
sedimentary origin. Referring to those rocks, he says:
They are so inseparable on an}' fundamental g-rounds, and are so blended together,
both structurally and mineralogically, that no reasons appear to exist for a reference
of one class to a mode of origin fundamentallj' different from the mode of origin of
the other class. On this question, however, I only jDropose at present to cite some
observed facts. The interpretation of them maj^ be subsequentlj- undertaken.
The crystalline schists are succeeded bv a system of semicrystalline schists [the
Keewatin]. Thej' range, however, from fragmentai crj^stalline to earthv. They,
succeed in perfect structural conformity with the older schists, with only slight indi-
cations of stratigraphic disturbance. Their attitude is generally vertical or steeply
inclined. Their position is between and surrounding the gneissoid areas [p. 377].
In the Vermilion district, as they lie between two elongated gneissoid
areas, they have a persistent east-northeast strike for 70 miles. In each of
96 THE VERMILION IRON-BEARING DISTRICT.
the intervals between such gneissoid areas the semicrystalline schists prevent
the structiire of a simple synclinal fold. Petrographically the semicrys-
talline schists (Keewatin) are sericitic schists inclosing beds of hematite,
argillite, including the Ogishke-Muucie conglomerate, porphyrellite and
chloritic schists, porodites, agglomerates and tuffs, and graywackes.
Wherever the crystaUiue and semicry.staUine schists are seen in juxtaposition
their stratification is strictly conformable. Whei'ever the crystalline schists are
wanting the semicrystalline schists are found in conformitA- with the gneisses.
Moreover, whether the semicrystalline schists occur in juxtaposition with the crys-
talline schists or the gneisses, there exist frequently those transitions by alternation
which characterize the passage from the crystalline schists to the gneisses. This
mode of transition, however, is much the most characteristic of the passage fj'om
the semicrystallines to the crystallines [p. 383].
Statement is also made of the petrographic gradation between the
semicrystalline and crystalline schists.
The uncrystalline schists (Animikie) are chiefly thin-bedded black
argillites grading into graywackes and even into conglomerates, with flint
and jasper schist and beds of magnetite. These rocks overlie the semi-
crystalline Keewatin schists with strong unconformity.
The enumeration made embraces all rocks up to the Keweenawan.
So far as these groups are concerned the order, in descending succession, is
as follows:
5. The uncrystalline schists (Animikie, Huronian).
4. The semicrystalline schists (Keewatin).
3. The crystalline schists (Vermilion).
2. The gneissoid rocks 1 ,_ , . ,
,„, . . , , ;- (Laurentian).
1. I he granitoid rocks J
WiNCHELL, N. H. and H. V. The Taconic iron ores of Minnesota and of
western New England: Am. Geologist, Vol. VI, 1890, pp. 263-274:.
In 1890 N. H. and H. V. Winchell state that the iron ores of Minne-
sota are, at five different geologic horizons, in descending order, as follows:
(1) The hematites and liinonites 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) olivinitic magnetites, just below the gabbro in the liasal portion of the
Mesabi rocks; (4) the hematites and magnetites of the Vermilion range
\
RESUME OF LITERATURE. 97
in the Keewatin formation; (5) the magnetites of the crystalline schists of
the Vermilion formation. It is maintained that the upper iron deposits
of the Mesabi and those of the Penokee-Gogebic are the equivalents of the
Taconic ores of western New England.
Irving, R. D., and Van Hise, C. R. The Penokee iron-bearing series of Michi-
gan and Wisconsin: Mon. U. S. Geol. Survey Vol. XIX, 1892, pp. 53i, with pis.
and maps. See also Tenth Ann. Rept. U. S. Geol. Survey, 1890, pp. 341-507, with
23 pis. and maps.
Irving and Van Hise in 1890 and 1892 give a detailed desci;iption of
the Penokee series of Michigan and Wisconsin, and of the complex of rocks
south of this series. They discuss the relations which the Penokee rocks bear
to the underlying and overlying series, as well as to tlie Eastern sandstone.
The Marquette and Felch Mountain series of Michigan, the Menominee
series of Michigan and Wisconsin, and the Animikie and Vermilion Lake
series of northeastern Minnesota, and Ontario are alluded to, since they con-
tain large developments of rocks which are almost exact reproductions of the
iron formation rocks in the Penokee series. The Animikie is considered in
more detail than the rest, since a comparison of its iron-bearing formation
shows that it consists of the same kinds of rocks which have been derived
from an iron carbonate in the same manner as those of the iron formation
of the Penokee series.
A further comparison of the Penokee series proper and the Animikie
series shows that they also occupy the same relative positions with refer-
ence to overlying and underlving rocks, one dipping northward under the
basin of Lake Superior and the other dipping southward under the same
body of water. They are therefore regarded as equivalent. The rocks in
various other areas in the Lake Supeiior basin refeiTcd to the Upper
Huronian are regarded as probably equivalent with the Penokee series
1891.
Van Hise, C. R. An attempt to harmonize some apparently conflicting views
of Lake Superior stratigraphy: Am. Jour. Sci., 3d series, Vol. XLI, 1891, pp.
117-137.
In the above paper Van Hise describes the physical break between a
Lower and an Upper Huronian series. That the two series are separated
MON XLV — 03 7
98 THE VERMILION IRON-BEARING DISTRICT.
by a great unconformity is shown by numerous contacts. At these contacts
the lower quartzite of the Upper series contains abundant fragments of the
Lower series which had reached their present condition before being
deposited in the former. That the Lower series has been greatly folded and
deeply truncated before the Upper series was deposited is further shown
by the much banded and contorted jasper abutting at all angles against the
beds of theriptilted but simply folded Upper series, and also by its more
crystalline character.
Since great belts of conglomerates containing abundant fragments of
ore and jasper are found in the Upper Vermilion, at Ogisliki Lake, and in
the Upper Kaministiquia series, it is argued that the source of this material
is the great belts of iron ore and jasper contained in the Lower Vermilion,
Hunters Island, and Lower Kaministiquia series. That the Vermilion Lake
conglomerate is unconformably above the schists in vertical attitude,
bearing ore and jasper, is further indicated by the fact, discovered by
Merriam, that on the islands of Vermilion Lake the conglomerate is found
to be in a series of gentle folds although having a vertical cleavage
developed. Merriam regards the conglomerate as a comparatively thin
formation overlying and overlapping the Lower series. Both the Animikie
and Upper Vermilion are unquestionably separated from the Lower Ver-
milion by an unconformity. Tlie Animikie is believed to be the equivalent
of the Upper Vermilion.
It is concluded that the confusion in correlation of the formations about
Lake Superior is due to the failure to recognize this general unconformity.
Once recognized, the structural conclusions to which the various writers
have most steadfastly held are found to be in general harmony. Above
the physical break, and constituting the Upper Huronian (equivalent to the
Original Huronian) are the Animikie and the Upper Kaministiquia, Upper
Vermilion, Upper Marquette, Western Menominee, Penokee-Gogebic proper,
the Dakota, Iowa, Minnesota, and Wisconsin quartzites surrounded by the
fossiliferous series. In the Lower Huronian is the Kewatin (in part at
least), the Lower Kaministiquia, Lower Vermilion, Lower Marquette, Felch
Mountain iron-bearing series, Menominee ]n-oper, and the Cherty limestone
at the base of the Penokee series, and the Black River Falls iron-beai-ing
schists.
RESUME OF LITERATURE. 99
Lawson, Andrew C. Lake Superior stratigraphy: Am. Geologist, Vol. VII,
1891, pp. 320-327.
Lawson discusses Lake Superior stratigraphy in an article which
owes its inception to the paper by Prof. C. R. Van Hise entitled "An
attempt to harmonize some apparently conflicting views of Lake Superior
stratigraphy,"" which is abstracted above, p. 97.
Lawson argues for the indivisibility of the Archean, meaning- by this
term all those rocks that existed prior to the denudation epoch, during
which time the floor was formed on which has since been deposited
with strong unconformity the Animikie series and its equivalents. The
Archean thus includes, in upward succession, the Laurentian gneiss and
granite, the crystalline schists of the Coutchiching, and the crystalline
schist and elastics of the Keewatin. The Coutchichincf and Keewatin are
so knit together by the Laurentian foliated granite as to warrant the union
of all of these under the term Archean.
Lawson then argues against Van Hise's correlation of the Upper Ver-
milion with the Animikie series. He states tliat the granite of Sag'anaga
Lake is found, with abundant and clearly observed evidences of eruption,
breaking through the Keewatin rocks, including' the Upper Vermilion (Van
Hise) fragmental rocks of Ogishki Lake with their associated slates and
grits. It is concluded that the break between the Upioer and the Lower
Vermilion described by Van Hise is within the Keewatin group, dividing
it into an upper and a lower series, and that this break is therefore below
the Animikie; that these series are united by the Saganaga graiiite intru-
sions and that they belong in the schist-granite-gneiss complex of the
Archean; hence this break is below the Animikie. It is further said that
the conglomerates of the Upper Kaministiquia series come out close to the
shores of Thunder Bay and form the basement upon which tlie undisturbed
Animikie rock rests with strongly marked unconformity. The following
succession foi- the region northwest of Lake Superior is presented:
[Keweenawaii, or Nipigou group.
Paleozoic — Algonkian system] Unconformity.
[Animikie group (possibl}' Huronian)
a Am. Jour. Sci., 3d series, Vol. XLI, 1891, pp. 117-1:^7.
100 THE VERMILION IRON-BEARING DISTRICT.
Unconformit)'. Greatest erosion interval in American geology.
Keewatin group (possibly Huronian)
Archean
Ontarian systemjuueonformity (?).
Coutchiching group.
Irruptive unconformity.
Laurentian s system.
Upper series.
Van Rise's break.
Lower series.
WiNCHELL, N. H. Record of field observations in 1888 and 1889: Eighteenth
Ann. Rept. Geol. and Nat. Hist. Survey of Minn., for 1889, 1891, pp. 7-47.
N. H. Wiuchell in 1891 gives numerous additional field observations.
The relations of tlie jaspilite, argillite, and green schist are considered, and
the argillite at least is regarded as a sedimentary rock (p. 9). In the Stuntz
conglomerate is found a large bowlder which contains pebbles of chalce-
donic quartz and quartzose felsite — these contain pebbles of vitreous
quartz — and pebbles resembling the porphyrel at Kekekebic [Cacaquabic]
Lake (p. 31). A study of the ore formation leads to the conclusion that —
All three of the known agencies for rock forming were intermittent!}^ at work
and concerned in the formation of the iron ore, viz: Eruption to afford the basic
eruptive material; sedimentation, to arrange it (in the main), and chemical precipita-
tion in the same water, to give the pure hematite and chalcedonic silica [p. 42].
The following facts are given as evidence that the Grreat gabbro of the
Cupriferous formation lies below the Animikie slates and that the Kewee-
nawan includes both the Animikie slate and the Huronian (Potsdam)
quartzite.
The most important and significant fact that bears on the stratigraphical position
of the gabbro, respecting its relation to the Animike black slates, is its occurrence
along a wide extent, reaching from Gunfiint Lake as far southwestward as to the
railroad crossing at Mallmann's (at least), next to and immediately south either of the
gneiss of the Giant's range or of the "greenstones" of the Kawishiwin, without the
appearance of any of the lilack slates between them. There is an appearance of
quartzite, with olivine grains and with magnitite, geographically between the gneiss
and the gabbro, the same being unquestionably the Pewabic quartzite seen near
Gunflint Lake. This quartzite is sometimes impure and linionitic, and seems to be
the chief iron horizon of the Mesabi range. This near conjunction (which is some-
times apparently an exact contact) of the gabbro with the gneiss, and the absence of
the Animikie proper between them, has been supposed to be due to a local overlap
of the gabbro beyond the strike of the Animike, covering it from sight, the idea
RESUME OF LITERATURE. 101
being that the gabbro flowed back noithward over older formations and came onto
the gneiss [pp. -itt-iS].
Bowlders of characteristic gabbro and red syenite, and of quartz porph}- ry, occur
abundantly in the later "traps" of the Cupriferous [p. i5].
WiNCHELL, H. V. Geological age of the Saganaga syenite: Am. Jour. Sci., 3d
series, Vol. XLI, 1891, 386-390.
H. V. Winchell in 1891 states that the syenite of Sagauaga Lake is
conglomeratic in places and contains pebbles which are similar to each
other, being mostly composed of lamellar angite, with or without grains of
feldspar, but there are no pebbles of sedimentary rocks or of syenite or
jasper such as occur in the Kewatin conglomerates.
In the Saganaga syenite at the end of the portage on Granite River
is a band of silica 1^ inches in diameter and 3 feet in length. This is
presumed to have been formed by chemical j^i'ecipitation from heated
oceanic waters."*
North of Saganaga Lake the syenite grades into greenish feldspathic
and sericitic schists and agglomerates without the usual intervening belt of
crystalline Vermilion schists. From these facts it is concluded that the
syenite is simply the result of the locally intense metamorphism of
Kewatin rocks, and is thus of Kewatin age.
Finally, as bearing upon the economic side of the question it is
suggested that —
If the Saganaga syenite be of the Keewatin age and contain chalcedonic silica in
an original, unchanged condition it is not unlikely to contain also Keewatin iron-ore
deposits free from titanium and of high grade in other respects. It can thus no
longer be laid down as a law for explorers in the Northwest that the gneisses contain
no iron-ore deposits [p. 390].
AViNCHELL, N. H. and H. V., The iron ores of Minnesota, Bull. No. 6, Minn.
Geol. and Nat. Hist. Survej', 1891, 430 pages, with geological map and section.
. N. H. and H. V. Winchell in 1891 give an extended treatment of the iron
ores of northeastern Minnesota and the, rocks in which they are contained.
Magnetic iron ore is not of great importance. Isolated deposits are reported
in the mica-hornblende-schists and in a massive hornblende-mica rock of
« Winchell, N. H. and H. V., On a possible chemical origin of the iron ores of the Keewatin in
Minnesota: Proc. Am. Assoc. Adv. Sci., Thirt3'-eighth Meeting, 1890, pp. 235-242. Am. Geologist,
Vol. IV, 1891, pp. 291-300; 382-386. Also The iron ores of Minn., Geol. iSfat. Hist. Survey ilinu.,
1889, Bull. No. 6, 430 pages.
102 THE VERMILION IKON-BEARING DISTRICT.
the Vennilion series. Deposits in the schists are presumed to be of sedi-
mentary origin, the magnetite having been produced by hydrothermal
action £i-om hematite, Hke that of the Keewatiu. Those magnetite.s in the
massive rocks are of igneous origin and are analogous to the titaniferous
magnetite found in the gabbro.
Hematite is the only ore actually rained on the Vermilion range up to
tliis time, and therefore the deposits of iron of this character are the most
important and the ones with which this report chiefly deals.
The ore is always found in schistose or massive greenstone of Keewatin
age and is -always associated with jaspilite. The ore bodies vary much in
size and are of lenticular shape, with long axis trending southwest-northeast,
parallel to the schistosity of the inclosing schists.
The jaspihte and schist of the Keewatin are found to occur sometimes
minutely interlaminated ; at other times the jasper is in irregular layers,
which never have any great extent, and finally always pincli out; at other
times it is in oval forms, the greater lengths being parallel with the schistose
structure. Again, the jaspihte is in great fragments within the green or
massive diabasic schists, the masses having sometimes such relations with
each other as to show that they are a broken continuous layer. The
branches from the large bodies of jaspilite are supposed to be Caused by
the crumpling, breaking, and squeezing of the entire rock structure, by
which the thinner sheets have been buckled out and thrust laterally among
the inclosing schists. The ore and jasper are regarded as a direct chemical
deep-sea precipitate from an ocean of hot alkaliuic water which was
continually disturbed by acid rains and flows of basic lava due to volcanic
activity. The iron for the ore was extracted from the basic lavas.
The rocks of the Animikie, equivalent to the Huronian and included in
the Taconic, consist chiefly of carbonaceous and argillaceous slates, with
siliceous slates, fine-grained quartzites, and gray limestones. At the bottom
of the series is a fragniental quartz sandstone, 300 feet in thickness, whicli is
named the Pewabie quartzite. The slates, conglomerates, and quartzites are
profoundly affected and intermingled with eruptive material similar to that
found so abundantly in the Kewatin. These beds have the appearance of
consolidated beds of basic lava or of porous tuff, but where this prevails
there is a sensible gradation from the dark trap-looking beds to the thin
beds of slate. At Ogishke Muncie Lake there is a slate conglomerate
o
RESUME OF LITERATURE. 103
similar to that on the north shore of Lake Huron. This conglomerate is
not the same as the ag'glomerates of the Kewatin, such as that on Stuntz
Island, at Vermilion Lake, and Ely. The Kewatin is always nearly vertical,
while the dip of the Taconic rarely exceeds 15°. The iron-ore beds of the
Taconic are: (1) The quartzose, hornblendic (or olivinitic), magnetitic group
of the Pewabic quartzite; (2) an impure jaspilite, hematite, and limonite
group; (3) a carbonated iron group; (4) a gabbro titanic iron group. The
jaspilitic hematite group has the same lithologic peculiarities as the
jaspilite beds of the Vermilion range. The gabbro in which the titanic
iron occurs constitutes the Mesabi range. This has been before regarded
as the base of the Keweenawan, into which it fades upwardly, but it has been
found that this great gabbro flow was outpoured at an earlier date, and it is
placed at or near the bottom of the Animikie.
Considered as to origin, the ores of the Taconic found in groups 1 and
2 above are supposed to be due to chemical oceanic precipitation. Those
of group 4 are of ig'ueous origin and are an integral part of the gabbro
The origin of those of group 3 is not definitely stated (pp. 144-145).
None of these Taconic ores are thus far mined in Minnesota.
1S93.
Batlet, W. S. Notes on the petrography' and geology of the Akeley Lake
region in northeastern Minnesota: Nineteenth Ann. Rept. Geol. and Nat. Hist.
Survey Minn., for 1890, 1893, pp. 193-210.
This is chiefly a petrographic description of rocks from Akeley Lake.
The three important results reached, largely by the microscopic study, are
summarized as follows :
(1) Most of the rocks designated as Pewabic quartzite in the neighborhood of
Akelej' Lake are not quartzites, but they are granulitic phases of gabbro. The
remainder are crystaUized aggregates of quartz. None of them are sedimentary
rocks, and consequently none can serve to determine the age of the ore associated
with them or of the gabbro in which they occur.
(i:) On the other hand, the granulitic gabbros may be traced into true granitic
gabbros and into quartzose phases of granulitic varieties. Hence, the granulitic
beds and their associated ores are of the same age as the gabbro, whose structural
relations to the younger and older formations must be appealed to in order to settle
the question of age.
(3) Since so much of the "Pewabic quartzite" is not quartzite in anj' sense of
the word, and since different beds that have been given this name are not all certainly
104 THE VERMILION IRON-BEARING DISTRICT.
of the same age, it is evident that great care must be taken in the use of the " Pewabic
quartzite" for correlation purposes. Several different rocks have been included
under this one title, hence the "Pewabic quartzite" as defined can not be relied
upon as marking a definite horizon in the succession of the geological formations in
northeastern Minnesota [pp. 208-209].
In an appendix Dr. Bayley states that the first two of the above
conchisions had been reached by W. M. Chauvenet a number of years
before, as the result of work done in the vicinity of Akeley Lake for the
Lake Superior division of the United States Geological Survey in 1883 and
1884. Dr. Bayley was not aware of Mr. Chauvenet's conclusion's until
after his own had been arrived at, Mr Chauvenet's being contained in an
unpublished report submitted to Prof R. D. Irving (p. 209).
WiNCHELL, N. H. The Kawishiwin agglomerate at Ely, Minn. : Am. Geologist,
Vol. IX, 1892, pp. 359-368.
This article contains a description of a greenstone which is found in
the Keewatin of the Vermilion district of Minnesota and which possesses a
peculiar structure.
On clean exposures the greenstone is seen to be not homogeneously
massive, but to be composed of irregularly rounded to oval bodies of mas-
sive greenstone ranging from 6 to 15 inches in diameter, surrounded by
and separated from each other by relatively narrow masses of fine-grained
chloritic schist, which winds about among the masses, its schistosity coin-
ciding with the surfaces with which it is in contact. The peripheries of these
massive bodies are all amygdaloidal, the long direction of the pores being
perpendicular to the surface of the round body.
The author explains the rock as an agglomerate, the rounded bodies
being bombs that were hurled into the air by volcanic forces and fell into a
hot ocean, in which was being deposited a fine volcanic mud which now
forms the fine schist between the bombs. In other words, "The source of
such rocks was igneous, but their structure is aqueous" (p. 367).
Van Hise, C. R. Correlation papers, Archean and Algonkian: Bull. U. S.
Geol. Survey No. 86, 1892, pp. 51-208, map, PI. Ill, op. p. 52, and pp. 440-529.
This bulletin, on the pages indicated, contains a thorough digest of the
various articles that had appeared concerning the Lake Superior pre-Cam-
briau geology prior to the time of its publication. The author's interpretation
of the structure and stratigraphy of the various regions is based upon his
RfiSUME OF LITERATURE.
105
very w'de personal kuowledg-e of the occurrences described in the articles,
and this lends additional value to his opinion.
The following are his conclusions about the Vermilion district, with
which we are especially interested. The succession, compared with that of
the Marquette district of the south shore of Lake Superior, which, having
been carefully worked out, we can use as a standard, is as follows (p. 195):
Northern Minnesota. Marquette (Michigan) district.
Keweenawan.
Unconformity.
Alffonkian jAnimikie and Upper Ver- Upper Marquette.
milion.
Unconformity.
Lower Vermilion.
Unconformitj' (?).
(Coutchiching?).
Eruptive unconformity
Unconformity.
Lower Marquette.
Unconformity.
Archean
Fundamental complex (not yet
separated in mapping).
Laurentian.
Grant, U. S. The stratigraphic position of the Ogishke conglomerate of north-
eastern Minnesota: Am. Geologist, Vol. X, 1S92, pp. 4-10.
Grant states that the Animikie rests unconformably upon the Saganaga
granite; that the Ogishke conglomerate is intruded by the Saganaga granite,
and therefore that the Ogishke conglomerate is earlier than and separated
by a great structural break from the Saganaga granite. As the Keewatin
has the same relations to the Saganaga gi'anite as the Ogishke conglomerate,
the same thing is true of the Animikie and Keewatin.
The Ogishke conglomerate is younger than the most of Keewatin, but
is considered as a part of it.
1S93.
Van Hise, C. R. An historical sketch of the Lake Superior region to Cambrian
time: Jour. Geol., Vol. L 1893, pp. 113-128.
In this historical sketch the five subdivisions given for this region are
the Basement complex or Archean, the Lower Huronian, Upper Huro-
nian, and Keweenawan, the last three together constituting the Algonkian
and the Lake Superior (Cambrian) sandstone. Each of these divisions is
separated from the others by unconformities. The only rocks of the Ver-
milion district treated of are the Lower Vei'milion and the Animikie series,
106 THE VERMILION IRON-BEARINCx DISTRICT.
tlie Lower Vermilion series being placed in the Lower Huronian and the
Animikie being placed in the Upper Hnronian.
The Lower Huronian is largely crystalline; the L'pper Huronian semi-
crystalline. Locally, along axes of intense plication, both the Lower
Huronian and Upper Huronian have been transformed into completely
crystalline schists.
WixcHELL, N. H. The crystalline rocks — some preliminary considerations as
to their .structure and origin: Twentieth Ann. Rept. Geol. and Nat. Hist. Survey
Minn., for 1891, 1893, pp. 1-28.
In this aiticle Professor Winchell g^ives the following' as the descend-
ing succession of strata in northeastern Minnesota. This detemiination of
the succession represents, according to him, the consensus of opinion of
several geologists who have given special attention to the field evidences.
From this statement must, however, be excepted the Great gabbro horizon,
No. 3, as by some it is presumed to have preceded, and not to have
followed, the Pewabic quartzite.
1. Keweenawan or Nipigon series, unconformablv beneath rocks
bearing' the "Dikellocephalus" fauna, and consisting of fragmental and
eruptive beds, the upper portions being almost entirely red sandstones.
2. Alternating beds of eruptive sheets and fragmental rocks. The
fragmentals are thin-bedded slates, actinolite-schists, magnetitic jaspers,
cherts, and quartzites. The sheets are ordinary eruptives or pyroclastics.
3 Immense quantities of true gabbro, often bearing titaniferous mag-
netite, are associated with contemporaneous felsites, quartz-porphvries,
and red granites. This gabbro includes several masses of the next older
strata, particularly the Pewabic quartzite.
4. The Animikie. This series is characterized by a great quartzite
associated with the iron ores and cherts. The quartzite (Pewabic) lies
iinconfomiably on all the older rocks. It is often conglomeratic, bearing
debris of the underlying formations. Within it is mingled volcanic tufts
from contemporaneous eruptions. The Pewabic quartzite includes that of
Pokegama Falls, on the Mississippi, and of Pipestone County. In the
vicinity of contemporaneous volcanic disturbances its grain is fine, like
jasper, and sometimes it has acquired a dense crystalline sti'ucture fi'om
contact with the gabbro.
5. The Keewatin. This is a volcanic series of great thickness, com-
posed mainly of volcanic tuffs, presenting evidence of aqueous sedimen-
EfiSUME OF LITERATURE. 107
tation, but conglomerates, graywackes, quartzitic schists, and glossy
serpentinous schists are present. The Kawishiwin formation, apparently
the upper member of the series, embraces the great bulk of the greenstones,
chloritic schists, jaspers, and hematites. The iron ores are in lenticular
lodes, and stand upright, conformable with the general position of the rocks.
6. The Keewatin series becomes more crystalline toward the bottom,
and passes conformably into completely crystalline mica-schists and horn-
blende-schists, which are named the Vermilion series. The rocks are usu-
ally stratiform, contain magnetic iron ore, and embrace some dark massive
greenstone belts, in which no stratification bands are visible.
7. The Laurentian. When not disturbed by upheaval the Vermilion
schists pass into Lain-entian gneiss, there being a gradual increase in the
feldspathic and siliceous ingredients. Even after the Laurentiaji characters
are apparently fully established, conformable bands of Vermilion schists
reappear, from which it is plain that the base of the Vermilion is an uncer-
tain plane, which can not be located exactly. This normal passage from
the Vermilion to the Laurentian is frequently disturbed by the intrusion of
numerous dikes of light-colored granitic and basic rocks. These were
both in a fluid state, the only nonfluid rocks being the schists which are
embraced within them in isolated pieces. In a similar manner small
areas of Laurentian granite are sometimes directly in contact with schists,
which have the imperfectly crystalline condition of the Keewatin.
Nos. 3 and 4 are separable from No. 2 by divergence in dip and strike,
as well as by a marked diiference of lithology. There is consequently some
evidence of unconformity between them. Below No. 4 is a great physical
break, which separates Nos. 1, 2, 3, and 4 from 5, 6, and 7 throughout the
Lake Siiperior region. This break is the greatest erosion interval which
has been discovered in Paleozoic geology. Nos. 1, 2, 3, and 4 together
constitute the Taconic. Nos. 5, 6, and 7 constitute the fundamental com-
plex or Archean, which is a unit in its grander featui'es.
Grant, U. S. Field observations on certain g-ranitic areas in northeastern Minne-
sota: Twentieth Ann. Rept. Geol. and Nat. Hist. Survey Minn., for 1891, 1893,
pp. 35-110. One map.
Grant, in 1893, publishes his notes made on a trip in northeastern
Minnesota. The areas visited were those of Kawishiwi River, Snowbank
Lake, Kekequabic [Cacaquabic] Lake, and Saganaga Lake.
108 THE VERMILION IRON-BEARING DISTRICT.
In the study of tliese areas there was uo evidence found of a transition
from semicrystalline and crystalline schists into granite. On the other
hand, abundant evidence was found of the irruptive nature of the granite
rocks into the surrounding sediments. The Kawishiwi River and Snow-
bank Lake massive rocks are hornblende-syenites. The Saganaga rock is
a coarse hornblene-granite. That around Kekequabic Lake is a pyroxene
granite, and associated with it is peculiar pyroxene-granite-porphyry
(pp. 37-38).
The intrusive character of the granite is particularly well shown where
the line between sees. 31 and 32, T. 63 N., R. 10 W., cuts the shore of
Clearwater Lake, and in the SE. i of the SW. \ sec. 26, T. 64 N., R. 9 ^Y.,
on the west shore of Snowbank Lake.
[Along-] the Kawishiwi River . . . iive distinct rock tj^pes [gabbro, syenite,
mica-schist, graywacke, etc., greenstone, and quartz-porphyrj-] are present. The
gabbro is the most recent; it covers part of the older rocks. . . . The .syenite is
older than the gabbro and Is younger than the greenstone and mica-schist, both
of which it cuts. . . . The mica-schists, graywackes, etc., stand vertical, and have
a general east-northeast stiike; thej' belong to what has been mapped as the Ver-
milion series, but there seems to be good reason for putting all of this type of rocks,
in the area of this map, into the Keewatin. The greenstone is presumably of
Keewatin age, and is .probably younger than the mica-schists, graywackes, etc.
Quartz-porphyrj' dikes are found cutting the greenstones in several places, but
they have not been seen in the other rocks in this immediate vicinity [p. 59].
The conclusions of this report differ from the general succession given
by Professor AVinchell in the fundamental point that there is no gradati&n
between the granitic rocks and the metamorphosed sedimentary rocks.
Also all of the metamorphosed sedimentary rocks are regarded as belonging
to the Keewatin (Lower Huronian '?), while the Vermilion schists are not
found. If there now exists in this area the original basement upon which
the sedimentary rocks were deposited, this has not been found. It is of
course possible that such a Basement complex does not exist in the Kawishiwi
River area, the one which was most closely studied.
Grant, U. S. The geology of Kekcqualnc Lake in northeastern IMinnesota,
with special reference to an augite soda-granite: Twenty-tirst Ann. Rept. Geol.
and Nat. Hist. Survey Minn., for 1892, 1893, pp. 5-58. With map, PI. II.
In tliis article Grant describes in great detail an area approximately
ilcs s([uai-e surnninding Kekequabic Lake. The distribution of the
RESUME OF LITERATURE. 109
various kinds of rocks present in the area is carefully determined and
represented upon the map (PL II), which accompanies the article. With
the exception of the Keweenawan gabbro and certain diabase dikes, whose
age is undetermined, all the rocks described are iiicluded in the Keewatin.
The points of chief interest in the paper are of a petrographic character,
and consist in a description of some anomalous green schists, of a
hornblende-porphyrite, and of an augite soda-granite. Evidence is also
presented to show that this is a true igneous granite, and is not due to
crystallization of sediments in situ, as had been previously maintained in
papers on the region hj other writers.
1894.
Grant, U. S. Preliminary report of tield work during 1893 in northeastern
Minnesota: Twenty -second Ann. Rept. Geol. and Nat. Hist. Survej"^ Minn., for 1893,
189i. pp. 67-78.
That part of the region studied by Mr. Grant, which is included in
the Vermilion district as described in this paper, lies in the Gunflint Lake
area, north of T. 63 N., and between Rs. 3 and 7 W. In Ts. 65 and 66
N., Rs. 4, 5, and 6 W., are Keewatin rocks, including the usual types —
volcanic tuff, greenstone-schists, greenstone, and the Ogishke conglomerate.
The Saganaga granite is intrusive in the Keewatin, the rocks of which it
metamorphoses. The author feels himself justified in stating:
(1) That the rock.s called Vermilion in the region of the writer's field work are
not necessarily lower in the geological scale than the Keewatin, but that thej" occur
at various horizons in the Keewatin; (2) that thej are only a more crystalline
condition of these same Keewatin rocks; and (3) that they probably owe their more
crj'stalline natui-e largely to their close proximity to areas of intrusive granite [p. 71].
Th*e Animikie iron-bearing rocks of Akeley Lake lie upon the
Keewatin greenstone to the aorth, and on the south are overlain b}" the
Great gabbro mass. The belt has a width of from 300 to 1,300 feet and
a dip varying from 20° to almost vertical, but averaging 45° to 50°.
Where widest it has an average dip of 30°, which would make a maximum
thickness of 650 feet. The iron ore is a nontitaniferous magnetite.
The Animikie rocks are little disturbed, except locally, having an
average dip of 8° or 10° a little east of south. The Animikie beds
are interleaved with diabase sills. These give parallel east-west ridges,
110 THE VERMILION IRON-BEARING DISTRICT.
wliich are gently sloping on the south sides, and steep on tlie north.
This topography has led Lawsou to the conclusion that the apparent
large number of sills is due to monoclinal faulting of fewer layers,
but of this there is no evidence. The Animikie strata are divided as
follows: An upper or graywacke-slate member, L,900 feet thick, com-
posed of slates and graywackes, with fine-grained quartzites and C|uartz-
slates; a middle or black slate member, 1,050 feet thick, composed mainly
of black slates, apparently carbonaceous, with a fine-grained, siliceous and
flintv layer at the base 60 feet thick; and a lower or iron-bearing member,
composed largely of jaspery, actinolitic, siliceous, and magnetitic slates,
usuallv thinly laminated, and some beds of cherty iron carbonate.
The Akelev Lake rocks, first called Pewabic quartzite, are similar to the
Gunflint iron-bearing rocks, and different from the Pewabic quartzite and
conglomerate found at the base of the Animikie farther west on the Mesabi
range. From the new data obtained, the Akeley Lake iron-bearing rocks,
which rest directly upon the Keewatin, are placed as the iron-bearing
member above the Pewabic quartzite.
Elftjiax. a. H. Preliminary report of field work during 1893 in northeastern
Minnesota: Twentv-second Ann. Rept. Geol. and Nat. Hist. Survey of Minn., for
1893. 1894, pp. 141-180.
This contains manv details concerning the structure and character of
the rocks north and west of Snowbank Lake. A section is given from
iloose Lake tt) Snowbank Lake, showing relations of rocks in the
intervening area as detei'mined b}' him.
Interest centers in the porphyry and granite. The porphyry is the
oldest eruptive. It is found sending long apophyses across the strike of
the Keewatin rocks, and contorting and metamorphosing them. On Snow-
bank Lake there are two granites, a red hornblende- and a gray augite-
granite, formerly known as red syenite and gray syenite, respectively,
which are considered as having been derived from parts of the same
magma. The gray augite-granite is not found in contact with the sedi-
ments. This augite-granite, the porphyry referred to above, and also the
sediments are cut by the hornblende-granite. Where the sediments are
cut bv the granite thev are metamorphosed to schists.
In foniiectioii with the precedino- it iniyht be of interest to note that the
hornlilende and mica schists of Snowbank and White Iron lakes grade into argilla-
EESUME OF LITERATUEE. Ill
ceous slates and conglomerates. The schistose character is most fully developed at
the contact with the granite. All evidence tends to show that the schists are due
to the intrusion of the granite, and suggests that the narrow belts of schist generally
found between the granite and the Keewatin rocks, and which have hitherto been
designated as a separate formation (Coutchiching or Vermilion) are only altered
portions of the Keewatin, which have been subjected to the heat and action of the
intrusive granite [p. 159].
The author also adduces further evidence to prove that, in accord with
Dr. Grant's statement to the same effect, the so-called Pewabic quartzite
between Birch and Gunflint lakes belongs in reality to the middle iron-
bearing member of the Animikie. (Cf abstract of Grant's report above.)
189.5.
Smyth, H. L., and Finlat, J. Ralph. The geological structure of the western
part of the Vermilion range, Minnesota: Trans. Am. Inst. Min. Engineers, Vol.
XXV, 1895, pp. 595-645.
Smyth and Finlay describe the western part of the Vermilion range.
The sedimentary rocks fall into two divisions. The older is a fragmental
slate formation, while the younger is an iron-bearing formation litholog-
ically identical with certain phases of the lower iron-bearing formation of
the Marquette district. To all apjaearances it is devoid of clastic material.
It is believed, from analogies with other iron-bearing districts of the Lake
Superior region, that the jaspe)' of the Vermilion district is derived from a
cherty iron carbonate or from a glauconite greensand, or both. However,
as the jasper is a final product of the alterations, it is not possible to show
this.
Intrusive igneous rocks are very abundant, cutting or being interleaved
with the sedimentary rocks in masses running from the thickness of a knife
blade to those 100 feet across. In qaantity the igneous rocks exceed,
perhaps, several times the sedimentary rocks. The oldest igneous rocks
are greenstones. These vary from massive to schistose, and in some
places are what is called conglomei'ate breccias. The acid rocks were
intruded later than the basic rocks. They were originally for the most
part quartz-porphyries, but these have been extensively changed to sericite-
schists and conglomerate breccias and to rocks intermediate between these
and the original form. Within the larger masses of the igneous rocks,
112 THE VERMILION IRON-BEARING DISTRICT.
botli basic and acid, are frequently included fragments from both the
slate and iron formations, from those of small size to masses more than
100 feet long.
The conglomerate breccias are of dynamic origin. The first step in
the development of the breccias was the formation of two intersecting sets
of planes of fracture, dividing the originally massive rocks into roughly
rhomboidal l^locks. Their further development depended on continued
movement between these blocks imder pressure, which resulted in enlarging
the shearing zones at the surfaces of contact, and rounding the angles.
The slate and jasper inclusions originall}^ plucked off from the rocks which
the porphyries and greenstones invaded shared, of course, the subsequent
history of their captors. The fact that the jasper inclusions are frequently
rounded, while those of slate are not, is explained by the difference in the
elasticity of the two rocks. The slate inclusions readily yielded and finally
took a permanent set under the deforming forces, while the harder and
more rigid jasper, in fragments of limited size and diverse orientation,
behaved like the inclosing porphyry. Tlie boundaries of the inclusions
were geuerall}" the surfaces along which rupture took place, although, as
has already been said, jasper in a few instances is found partly held in
porphvry inclusions.
As to structure, the main slate area is anticlinal ; both north and south
of this area the jasper succeeds the slates. The southern jasjjer continues in
a complex syncline, and south of this is found the northern limb of another
anticline of slates, the southern limb not being exposed. Still farther south
is the jasper of Lee and Tower hills, which appears to form the southern
and western edges of a complex syncline. All of these folds pitch toward
the east.
The ore deposits, a number of figures of which are given, as well as
many details concerning them, are found to conform in occurrence to the
laws worked out by Van Hise in reference to other districts of the Lake
Superior region; that is, (1) they occur for the most part in pitching
troughs having impervious basements, the basement being usually one or
more of the different varieties of the eruptive rocks; (2) they are secondary
concentrations produced by downward-percolating waters, the silica being-
leached out and the iron deposited.
RESUME OF LITERATURE. 113
1896.
Elftman, Arthur H. Ore deposits of Minnesota: Engineers' Year Book,
Univ. of Minn., 1S96, pp. 115-117.
This is a brief statement of the ores known to exist in the State. Of
a number mentioned, the iron-ore deposits are the only ones which have
been developed lo any extent. These deposits occur in the Vermilion and
Mesabi ranges. In the Vermilion the -ore is a hematite, with low content
of phosphorus and sulphur, rang-ing- from soft to hard ore. The deposits
are in the Keewatin of the Lower Huronian, and are mined only at Tower
and Ely. Eastward from Ely, extending- tlu'ough the eastern part of
Hunters Island, are very favoi'able indications of ore deposits.
1897.
Eby, J. H., and Berket, Chas. P. Co^jper minerals in hematite ore: Engi-
neers' Year Book, Univ. of Minn., 1897, pp. 108-117. (Rejarinted from Proc. Lake
Superior Min. Inst.)
Mr. Eby describes the occurrence of a number of copper minerals in
the hematite ore of the Montana mine, of Soudan, Minn. The minerals
associated with the native copper found in the hematite are cuprite,
malachite, azurite, and chalcopyrite. The native copper seems to have
been the source of the copper minerals, as in one case an octahedron was
found which consisted of metallic copper at center, surrounded by layers
of cuprite (CuO), and this surrounded by copper carbonate.
Mr. Berkey describes the minerals, excepting the chalcopyrite, which
was not found in the specimens he had for study.
WiNCHELL, N. H. Some new features in the geology of northeastern Minnesota:
Am. Geologist, Vol. XX, 1897, pp. 41-51.
Winchell presents some additional points on the geology of north-
eastern Minnesota.
The Laurentian includes, in Minnesota, an acid crystalline schist of
sedimentary origin and a massive igneous rock, although the igneous rock
is younger than the crystalline schist portion and should have a different
desig'nation. The conclusions reached are that (1) the sedimentary Lau-
rentian is a crystalline condition of sedimentary strata, which are con-
formably a portion of the sedimentary schists; (2) the igneous Laurentian
MON XLV — 03 8
114 THE VEKMILION IRON-BEARING DISTRICT.
is the result of a more intense metamorphisra, carried even to fusion of
some sti-ata. These conclusions result particularly from the study of a
section from Tower northward through Vermilion Lake, and of an area on
the west side of Outlet Bay, in the corners of sees. 13, 14, 21, and 32, T.
63 N., R. 1 7 W., and along the shore 'for one-half mile westward.
It is evident that the Stuntz conglomerate on the south shore of Ver-
milion Lake is a true water-deposited conglomerate of the same fonnation
as the slates and graywackes of the district, the conglomerate grading
into the quartzite and graywacke, and this into argillaceous slate. Further-
more, as supposed by Van Hise, the conglomerate lies uncoufonnably on
the iron-bearing' formation, and contains very numerous fragments of
jaspilite. The jjosition of this unconformity, whether at the base of the
Taconic or lower, is not ascertained.
1898.
Grant, U. S. Sketch of the geology of the eastern end of the Mesabi iron
i-ange in Minnesota: Engineers' Year Book, Univ. of Minn., 1898, pp. 49-62;
with sketch map.
Grant sketches the geology of the eastern end of the jMesabi iron range
in Minnesota, including T. 64 N., Rs. 3 and 4 W., and parts of Rs. 2 and
5 W., with some adjacent portions of Ontario. The rocks can be separated
into three divisions. The chief one of these is the Animikie series, contain-
ing the iron-bearing rocks of the Mesabi range. Older than the Animikie
is a senes of granites, greenstone both massive and schistose, conglomerates,
slates, and other clastic rocks, called the pre- Animikie. Younger than the
Animikie are some diabase sills and the Great gabbro mass of noi'theastern
Minnesota.
Of the pre-Animikie rocks, the greenstones and clastic rocks have been
called Keewatin. As the greenstones are usually associated with the
Mesabi iron-bearing rocks, these alone of the Keewatin rocks are described.
They lie to the north of the iron-bearing rocks in T. 65 N , R. 5 W., and
extend eastward to the center of T. 65 N., R. 4 W., where they disappear
under the Animikie strata. In general, the greenstones are at present
diorites; originally some were certainly diabases, others were of the nature
of andesites, and a large part were diorites or possibly gabbros. At places,
especially along the east side of sec. 27, T. 65 N., R. 5 AY., the greenstones
RESUME OF LITERATURE. 115
contain angular and subangular fragments of rock almost like themselves,
and some may be regarded as composed of fragmental volcanic rocks.
Associated with the greenstones, especially in sees. 22, 23, and 24, T. 65
N., R. 5 W., are small masses of more acid rocks, quartz-porphyries, and
quartzless porphyries, which are probably younger than the greenstones.
The pre-Animikie granite has its typical development on the shores of
Saganaga Lake. In a number of places it may be seen in intrusive
relations with the greenstone. A quarter of a mile south of the northeast
corner of sec. 23, T. 65 N., R. 5 W., many granite dikes cutting the green-
stone are seen, and on the south shore of West Sea Gull Lake granite dikes
of the same nature as the immediately adjacent main mass of granite of
Saganaga Lake are seen cutting the greenstone. Both granite and green-
stone are cut by another series of finer-grained, more acid granite dikes.
The Animikie rocks rest uncomformabl}^ upon the pre-Animikie rocks,
and are usually exposed on the south slope of the Giants range, which is
composed essentially of granite. The strike is approximately east-northeast,
and, the dip in general about 10° SE. The thickness varies from nothing
to 4,000 feet. The Animikie is separable into four conformable divisions —
(1) the lower or qnartzite member, called the Pewabic quartzite; (2) the
iron-bearing or taconite member; (3) the black-slate member; (4) the
graywacke-slate member.
(1) The quartzite member is well developed in Itasca County, but
disappears before reaching the eastern side of St. Louis County.
(2) The rocks of the iron-bearing member are similar to those in St.
Louis County on the western end of the range, described by Spun-." They
differ, however, in two features. They are more completely crystalline,
and the iron is magnetite instead of hematite. The rocks consist chiefly
of jaspers, amphibole- (griinerite) schists, greenish siliceous slates, cherts,
cherty carbonates, and magnetite slates. It is believed that these rocks
were originally glauconitic greensands; that the ore has been derived from
the iron in the glauconite, and that the ore bodies result from concentration
and replacement. In this part of the Mesabi range no ore bodies have
yet been found which are at the same time both rich enough and large
enough for profitable mining, although vast quantities of magnetite ore
«Geol. and Nat. Hist. Survey Minnesota, Bull. No. 10, 1894.
116 THE VERMILION IRON-BEARING DISTRICT.
occur at or near the surface. The dip of this formation varies from an
average of 45° to 50° on the west to less than 16° on the east, and the
thickness varies from 650 feet or less on the west to 900 feet on the east.
(3) The black slate is essentially a fine-grained, black, more or less
siliceous, apparentl}- carbonaceous slate.
(4) The graywacke-slate member is composed of black to gray slates
and fine graywackes, with some flinty slates; the upper part shows coarser
detrital material, and the highest beds seen are fine-grained quartzites and
quartz-slates. This member is well exposed on the south shore of Loon
Lake.
Associated with all of the strata of the Animikie are diabase sills, and
bounding the Animikie rocks on the south is the Grreat gabbro mass. These
are igneous rocks of later date than the Animikie. Near the contact with
the gabbro the Animikie rocks show marked metamorphism and usually
complete recrystallization. The gabbro varies from a nearly pure plagio-
clase rock to titaniferous magnetite.
The pre-Auimikie rocks here described, according to the nomenclature
used by the United States Geological Survey, belong to the Lower Huronian
series of the Algonkian system, and probably also in part to the older
Archean or Basement complex; the Animikie is regarded as the equivalent
of the Upper Huronian series of the Algonkian, and the gabbro as a part
of the Keweenawan series of the Algonkian.
1S99.
Sardeson, F. W. Report of secretary of the Geological Club of the
University of Minnesota: Science, Vol. IX, pp. -412-413.
Prof C. W. Hall discusses "The extent and distribution of the Archean
in Minnesota." The following quotations are from the secretary's report:
Accepting the Archean as that original "crust" or solidified portion of the
earth, ... he defined it as an era of igneous origins, whose rocks represent the
original crystallization of earth matter added to from below by successive solidifica-
tion and many subsequent intrusions. By this definition all overlying elastics or
irruptions into or through the elastics are excluded from the Archean. . . .
Between Rainy Lake and Lake Superior there are several belts of schists, with
alternating granites and other rocks having a general northeast-and-southwest trend.
Concerning one of these, Irving noted in ISSfi "that we have among the rocks . . .
two types, in one of which the crystalline structure is complete and in which there is
RESUME OF LITERATURE. 117
little or none of an original fragraental structure, while in the other the fragmental
texture is still distinct and the alteration has progressed to a smaller degree." He
then adds "that the supposed older one of the two groups of schists in the Vermilion
Lake belt is intricatelj' penetrated by the granites of the great areas north and south
of the belt."" Hence areas of Archean lie north and soutli of these older schists.
It is clear that Professor Hall believes in the presence of Arohean rocks
in the Vermilioji district, although the careful reader will see that Professor
Hall's conclusion as to their occurrence there does not follow from his defini-
tion of Archean as reported here and from the quotation from Professor
Irving's report.
WiNCHELL, N. H., Grant, U. S., and Elftman, A. H. Twenty-fourth Annual
Report Geol. and Nat. Hist. Survey of Minnesota for years 1895-1S9S, 1899.
In this annual report, which it is stated is the final one, there are pub-
lished the lists of rock specimens, with annotations, collected by N. H.
Winchell in 1896, 1897, 1898; a record of the field work of U. S. Grant,
1892-1898, and a list of specimens collected by him in 1898; also a list of
specimens collected by A. H. Elftman in 1895, 1896, 1897.
In these we find statements concerning the Vermilion district, but the
material is not digested, and no general conclusions are stated. Conse-
quently it is impracticable to review the report and show its actual contents.
Winchell, N. H., et al. The geology of Minnesota, by N. H. AVinchell, U. S.
Grant, James E. Todd, Warren Upham, and H. V. Winchell: Final Report Geol.
and Nat. Hist. Survey of Minn., Vol IV, 1899, pp. 630. With 31 geologic plates.
Structural geology of Minnesota, by N. H. Winchell: Final Report Geol. and
Nat. Hist. Survey of Minn., Vol. V, 1900, pp. 1-80, 972-1000.
The first of these volumes contains an account of detailed field work
in northeastern Minnesota, with incidental discussion of general problems.
The area is treated by counties and smaller arbitrary geographic
divisions, in the description of which several men have taken part. This
manner of treatment leads to repetition in the discussion of the general
geologic features, and in many cases it is extremely difficult to correlate
the facts recorded in the different sections.
Volume V contains an account of the general structural geology of'
the State, by Professor Winchell, based on the detailed work described in
a Seventh Ann. Rept. U. S. Geol. Survey, p. 437.
118 THE VERMILION IRON-BEAKING DISTRICT.
\'(il. IV. This general discussiou of Vol. V is reviewed, with such
reference to the facts recorded in Vol. IV as is necessary to make the
summary intelligible.
Dr. Grant's views, as indicated in the detailed descriptions of special
areas, in some cases differ somewhat widely from those of Professor
Winchell.
Winchell discusses the general structural geology of northeastern
Minnesota. The ancient rocks of this area he places in two main systems,
the Archean and the Taconic. Tlie former is further subdi-\dded into the
Upper and Lower K eewatiu, separated from each other b}' an unconformity.
The Pewabic quartzite also is placed with the Keewatin, but is not
assigned to either of the main divisions. Overlying the Archean with
strong unconformity is the Taconic, represented by Animikie and Kewee-
nawan rocks, these divisions being supposed to represent, I'espectively,
the Lower and Middle Cambi-ian of other parts of the country. The
Coutchiching and Laurentian rocks before mapped as separate formations
are now included within the Keewatin.
The Lower Keewatin comprises greenstone, with associated surface
volcanics which are both subaerial and subaqueous, argillitic slates, siliceous
schists, quartzites, arkoses, "greenwackes," 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
indefinitely westward b}^ way of Gobbemichigamma Lake, the Kawishiwi
River, White Iron Lake, and Tower. 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 appai'ently 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 impor-
tant 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
RESUMfi OF LITERATURE. 119
of Tower and Ely ou the Yermiliou iron range occi;r in tlie 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 erup-
tive. 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 Bowstring, 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 thev dip l^eueath the later formations in the southwestern
portion of the State, and probably occupy a wide patch in South Dakota.
South of the Giants range they occur also, but as they are covered by the
gabbro and Animikie toward the east and the di'ift 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 Ogishke Muncie Lake 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 soiith shore of Disappointment Lake; on the north
shore of Fraser Lake; on the south shore of Gobbemichigamma; at Akeley
Lake, forming the so-called Akeley 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.
The Upper Keewatin occurs in troughs in the Lower Keewatin, and
particularly in one main trough the axis of which is traceable from Vermil-
ion Lake to Saganaga Lake. The northern arm of this syncline, consist-
ing of granites, gneisses, associated mica-schists, and in some places earlier
120 THE VERMILION IRON-BEARING DISTRICT.
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, pene-
trated by the granite of the Giants range, extends from Pokegama 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
conglomerates, but also includes graywackes, argillites, quartzites, and
jaspilites, 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 parallelism
with the earlier folding, with the result that the Upper Keewatin lies in
narrow S5niclines in the Lower Keewatin 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
hio-her parts of the Giants range and the Snowbank Lake granite. The
earlier granite is represented by the granites at Kekequabic (Cacaquabic)
Lake, Saganaga Lake, Basswood Lake, Burntside Lake, Vei-milion Lake,
Lac la Croix, and Kabetogama Lake. The origin of the granite is dis-
cussed and the same conclusions reached as in a previous article."
The laconic. — This is unconformably above the Keewatin rocks. It
comprises the Animikie and Keweenawan divisions.
The Animikie rocks enter the State at Pigeon Point, run westward along
the international boundary to the eastern part of sees. 22 and 27, T. 65 N.,
R. 4 W. They reappear again south westward from Birch Lake on the north-
west side of the gabbro mass, and thence continue along the south side of
the Giants range, constituting the IMesabi iron series, to Pokegama Falls.
The hio-her parts of the Animikie are best developed toward the east while
the lower parts are best developed toward the west.
"The origin of the Archean igneous rocks, by N. H. Winchell: Proc. Am. Assoc. Adv. Sci., Vol.
XLVII, 1898, pp. .303, ,304 (Abstract). Also Am. Geologist, Vol. XXII, 1898, pp. 299-310. Summar-
izeil Jour. Geol., Vol. VII, 1899, p. 194.
RESUME OF LITERATURE. 121
The Animikie rocks comprise the Pokegama quartzite, Mesabi iron-
bearing formation, some hmestone and slate, all stricth' conformable with
one another. The thickness is several hundred feet, sometimes reaching
nearly 1,000 feet. The dip of the series is miiformly to the south, 8° to 12°.
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 some 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 Poke-
gama Falls 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, appears
the iron-bearing or taconite member of the Animikie, which contains the
iron-ore deposits of the Mesabi iron range. The ore is usually hematite in
the western part of the range and magnetite in the eastern part. It was
previously supposed to have been derived from the alteration of a greenish
glauconitic 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 Keweenawan (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 VV.,'and doubtfully on the shores of Gunflint Lake. This
limestone may be regarded as the basal horizon of the next overlying 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 members are intruded
bv diabase sills
122 THE VERMILION IRON-BEARING DISTRICT.
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 stratigraphic position have not been
satisfactorily established.
The top of the Animikie has not been identified. The first recogniz-
able datum jjlane 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 Cambrian for the
following reasons: It graduates upward into Upper Cambrian rocks as seen
on the south side of Lake Superior. The derivation of the iron ores from
a glauconitic greensand indicates that larg-e quantities of foraminiferal
orsranisms 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 Keiveenaivan. — The Puckwunge conglomerate is taken to be the
fragmental base of the Keweenawan, although certain igneous rocks which
antedate it, and which, perhaps, are contemporaneous with the upper por-
tions 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 conMomerate are cong-lomerates and sandstones of Ke-
weenawau 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 tlie
gorge of the Kettle River in Pine County. This in turn grades up
into typical Upjjer Cambrian sandstones of the St. Croix Valley. The
term Potsdam is restricted to the Puckwunge conglomei-ate 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,
RESUME OF LITERATURE. 123
the New Ulm quartzite iu Cottonwood County, and the quartzite in Pipes-
tone County.
The igneous rocks of the Keweenawau vary in age from the late
Animikie time to the top of the Keweeuawan series. They are divided
into two groups, the Cabotian or Lower Keweenawau, and the Mfur't a or
Upper Keweenawau.
The Cabotian division inchides gabbro and contemporaneous red rock
and their surface lavas, and all other dikes and sills which are associated
with, but younger than, the Animikie clastic rocks, and which are older
than the Puckwunge 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 W. On the north it is bounded by the
Animikie 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 N., 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 plainl}- intrusive on the older formations, Animikie and
Keewatin.
From the northern border of the gabbro many sills offshoot and pene-
trate 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
Keewatin series, there are various altered rocks which can be connected in
places with the gabbro and in places with the underlj-ing 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 ha^^e 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 probably have been the
result of the metamorphism or refusion of the Keewatin greenstones.
The anorthosite masses of the Beaver Bay diabase, supposed by Lawson
124 THE VERMILION IKON-BEARING DISTRICT.
to be Ai-ehean and to uuclerlie 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 beheved to represent the upper portion of
the Great gabbro flow, and to be due to the first and greatest movement of
the 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 River to near Grand
Marais, except where replaced at intervals by the Beaver Bay diabase or
some of the intersheeted fragmentals. They occur also in the neighborhood
of Grand Portage Bay, but their extent here is not definitely known.
General. — The most important petrologic conclusions reached 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
clastic rocks adjacent to the observed intrusions, rendered plastic by the
force of dynamic metamorphism accompanied by moisture.
2. The Keweenawan gabbro and its derivatives are derived from the
metamorphism and refusion of the Archean greenstones and their attendants.
Comment. — The two main petrologic conchasions announced by Pro-
fessor Winchell as the most important results of his final petrologic work,
summarized in the closing general paragraph, would be dissented from by
most of the other geologists who have worked in this area."
The Cambrian age of the Animikie strata has long been maintained by
Professor Winchell, and above are summarized his arguments in support of
this position. The first argument, that the Animikie grades into the Upper
Cambrian rocks, is not in accord with the observations of most of the
geologists above referred to. The second argument, based on the similarity
of the unaltered greensand in the Mesabi district to that in the Cambrian
of the eastern United States, loses weight when we consider the fact that
the similarity is not great, the differences being many and significant;
"Some of these geologists are: R. D. Irving, C. R. Van Hise, J. Morgan Clements, W. S. Bayley,
U. S. Grant, J. E. Spurr, A. H. Elftman, C. K. Leith.
RESUME OF LITERATURE. 125
and if the similarity were complete, the correlation would involve laying
too much stress on lithologic similai-it}^ of widely separated formations.
Professor Winchell's latest conclusion, that the Mesabi greensand is
volcanic and not organic, while entirely dissented from by others who have
studied this rock, in itself spoils his argument based on similarity. The
third argument in favor of the Cambrian age of the Animikie, based on the
extent of the unconformity beneath the Animikie, has little value when
unsupported by the other lines of evidence. Professor Winchell's conclusion
as to the Cambrian age of the Animikie strata is thus not adequately
sustained by the reasons given. The view that the Animikie is Upper
Huroniau (pre-Cambrian) is the commonly accepted one. The evidence
favoring this view is summarized by Van Hise."
Further comment on the above work would require reference to the
detailed observations made in northeastern Minnesota during four years by
the Lake Superior Division of the United States Geological Survey. The
results of this work are published in this monograph. In general it may
be stated that now, as in the past, there is a divergence in the conclusions
reached by the Minnesota survej^ and by the United States Greological
Survey concerning the position and importance of the unconformities, the
correlation of series, and the nomenclature.
1900.
Coleman, A. P. Copper and iron regions of Ontario; with Report of the Ontario
Bureau of Mines, 1900, pp. 143-191.
This paper deals incidentally with the Vermilion district (pp. 150-154).
Before the regular field work was begun in Ontario the author, accom-
panied by Prof. Arthur B. Willmott, visited the Lake Superior iron ranges
of the United States. The Vermilion range was visited, among others, and
a few desultory observations concerning the mines and rocks in the vicinity
of the mines are recorded.
Grant, U. S. Contact metamorphism of a basic igneous rock: Bull. Geol. Soc.
Am., Vol. II, 1900, pp. 503-510.
Along the northern edge of the Great gabbro mass of northeastern
Minnesota there occur certain peculiar crystalline rocks. These have been
produced by the contact action of the gabbro on rocks of varied lithologic
"Correlation Papers, Archean and Algonkian, Bull. U. S. Geol. Survey No. 86; Principles of pre-
Cambrian geology : Sixteenth Ann. Kept. V. S. Geol. Survey, Pt, I, 1896, pp. 571-874.
126 THE VERMILION IRON-BEARING DISTRICT.
character. It is the object of the paper to give in outline an account of the
phenomena seen on these rocks. An outline of the local geology is given.
The rocks range from Archean, through the Lower Huronian (or Keewatin)
and Upper Huronian (or Animikie), to the Keweenawan. The Animikie of
this area is subdivided from the base up into an iron-bearing member, a black-
slate member, and a graywacke-slate member. The Keweenawan is rep-
resented by the gabbro, which has metamorphosed the Gunflint iron-bearing
beds, and the graywacke and black slate members of Upper Huronian age.
The metamorphism of the black-slate and graywacke-slate members is
relatively insignificant, and consists of the more or less complete recrvstal-
lization of the rocks, which are now made up of granitic aggregates of
quartz, feldspar, biotite, muscovite, and occasionally cordierite.
The most complete recrystallization and the most interesting' phe-
nomena are shown in the metamorphosed iron-bearing beds.
The original rock is regarded as a glauconitic greensand in which there
is more or less iron carbonate. This has been altered to a quartz-magnetite-
amphibole-slate, the amphibole being in the form of actinolite, griinerite,
cummingtonite, and hornblende. This has been profoundly changed by
the gabbro, and is now a coarse-grained aggregate of quartz, magnetite,
olivine (which is frequently fayalite), hypersthene, augite, hornblende, and
occasionally griinerite and cummingtonite. These rocks, like the rocks
from which they are derived, are beautifully banded, the separate bands
being composed of quartz, or of magnetite, or of silicates, or of a mixture of
any two or more of the minerals.
Satisfactory reasons are given showing that these rocks are a part
of the Animikie (Upper Huronian) and not a border facies of the gabbro, as
has been thought to be the case by some.
The gabbro is in contact with very diverse strata of the Keewatin, and
the resulting metamorphic rocks differ greatly. Biotite and hypersthene
are prominent in these contact rocks.
The Archean consists of granites and greenstones. The granites have
not been affected by the gabbro in a noticeable way at least. The green-
stones have been affected in such a way as to reproduce the minerals
of the original rocks from which the greenstones were derived. Com-
monly a granular product is found which is quite similar in appearance to
a gabbro.
RESUME OF LITERATURE. • 127
Comments. — The members of the United States Geological Sm-vey who
have been studying this area place the dividing lines in some cases at places
somewhat different from those where Grant places them. They make the
same main divisions, however. The glauconitic greensand of Grant is now
known not to contain potassium, the green granules being composed of a
hydrous ferrous silicate. "
The United States geologists concur in the general conclusions reached
by Grant as to the character and cause of the metamorphism.
1901.
WiNCHELL, N. H. The geology of Minnesota: Final Report Geol. and Nat.
Hist. Survey of Minn.. Vol. VI, 1901. Geological atlas with synoptical descriptions,
88 plates.
This is a collection of maps of Minnesota. The earlier ones, from
Franquelins map of 1688 up to and including Nicollet's of 1842, are repro-
duced, and then the later maps published by the Geological and Natural
History Survey in the reports preceding- this. Brief explanatory notes
accompany each plate.
WiNCHELL, N. H. Glacial lakes of Minnesota: Bull. Geol. Soc. Am.. Vol. XII,
1901, pp. 109-128, pi. 12.
Winchell gives a brief description of a number of glacial lakes occur-
ring in Minnesota. Among these there are two, Lake Norwood and Lake
Onnamani, which lie partially or wholly in the Vermilion district.
Van Hise, C. R. The iron-ore deposits of the Lake Superior region: Twenty-
first Ann. Rept. U. S. Geol. Survey, Pt. Ill, 1901, pp. 305-434, 12 plates, 6 of
which are geological maps.
This paper contains a brief description and comparison of the various
iron-bearing districts of the Lake Superior region.
The first chapter contains a general discussion of jwhiciples. The
rocks of the region, disregarding the late formations, are stated to be divisible
into the following five series, enumerated from base up : Archean, Lower
Huronian, Upper Huronian, Keweenawan, and Cambrian.
The chief varieties of the iron-bearing rocks and their alterations are
described. They are shown to occur in the three series, the Archean, Lower
«The Mesabi iron-bearing district of Minnesota, by C. K. Leith: Mon. U. S. Geol. Survey Vol.
XLIII, 1903, pp. 110.
128 THE VERMILION IRON-BEARING DISTRICT.
Hurouian, and Upper Hurouiaii. The genesis of the ore deposits is then
discussed.
The genei'al process of ore formation for the Lake Superior region as
a whole is the same as that ah'eady described in the monographs on the
Penokee and Marquette districts. The iron is leached from an older forma-
tion— in case of the Archean from igneous rocks, the greenstones — and is
then deposited as a sedimentary formation, in the form of a cherty carbonate
or some iron compound, which, however, becomes this cherty carbonate.
These formations are folded and intruded by igneous rocks, and pitching
troughs with relatively impervious sides and bottom are formed. Meteoric
waters carry downward in solution iron derived from the iron-bearing for-
mation, and this is precipitated as an oxide in the structural basins in the
formation and in the spaces left by removal of the silica. As result of this
replacement, enrichment of the iron-bearing' formation occurs at favorable
places, and the ore deposits are formed.
Sections are devoted to a consideration of the influence of topography
and denudation on ore deposition, and to a discussion of the time and
depth of concentration of the ore deposits.
In Chapter II the individual districts are taken up. The section devoted
to the Vermilion district is written by Van Hise and Clements, and in this
is given the first detailed statement made by the United States Geological
Survey on this district. A preliminary map accompanies the description.
Since this section on the Vermilion district is in fact a very brief abstract
of the present monograph, it will not be reviewed in detail. It may be well
to mention the fact that the Archean is here made to include certain sedi-
mentary rocks In the last chapter, the third, there is a comparison and
summary of all the districts. The Vermilion district is again briefly con-
sidered, and some suggestions are offered in regard to exploration in it.
CHAPTER III.
THE ARCHEAN.
SECTION I— DEFINITION AND SUBDIVISIONS,
The Archean, as heretofore defined by Professor Van Hise, was made
to indude all pre-Algonkian rocks, and these were supposed to be igneous
rocks only." As a result of the work of the field season of 1900 on the
north shore of Lake Superior in Minnesota and Canada, it has been found
necessary to modify our ideas of the Archean and to change the definition
of the word accordingly. The term Archean, as used in the present paper,
comprises rocks older than the Algonkian, whicli are predominantly of
igneous origin, but with which may be included some subordinate amounts
of sediments.
From the study of the Vermilion district it has been found possible to
divide the Archean of that district into three stratigraphic divisions, which,
enumerated in order of age, beginning with the lowest, are: Ely greenstone,
Soudan formation, and a series of granitic rocks. The Ely greenstone
consists of basic to intermediate igneous rocks, and is the lowest member
of the geologic column in this district. Above this occtirs an iron-bearing
formation, the Soudan formation, of totally different lithologic character
and mode of origin, whose base is marked here and there by a conglom-
erate of small extent. The iron-bearing formation is followed by a series
of acid intrusives varying from rhyolite-porphyries to granites and granite-
porphyries, with normal granites as the predominant form. These rocks
show in many places their intrusive relationship to both of the earlier
formations. These three formations constitute the Archean and are sepa-
« Correlation papers — Archean and Algonkian: Bull. D". S. Geol. Survey No. 86, 1892, pp. 156-199.
Also Principles of North American pre-Cambrian geology, by C. R. Van Hise: Sixteenth Ann. Rept.
U. S. Geol. Survey, Pt. I, 1896, pp. 581-872.
MON XLV — 03 9 129
130 THE VERMILION IRON-BEARING DISTRICT.
rated from tlie next succeeding series by a great unconformity. The
fomiations will be taken up in order of age and described separately in the
following pages.
SECTION II— ELY GREENSTONE,
FEATURES OF THE GREEXSTO^TE.
In the Vermilion district there is a great complex of igneous rocks
whose members possess one general character in common. They are
almost universally colored some shade of green ; hence, for want of a better
one, the descriptive term "greenstone" is applied to the complex. Tojudge
from microscopic examination, no chemical analyses having been made,
the rocks constituting this complex vary in chemical character from inter-
mediate to basic rocks. They likewise possess varied physical characters.
The various parts of this complex are not of exactly the same age, as in a
number of places one member of the complex intrudes other members ;
nor are the rocks all of exactly the same mode of origin, as some are
effusive and others intrusive, although all are igneous. These differences
in age and mode of formation are, however, only such as we normally expect
to find in rocks belonging, as these do, to one great period of eruptive
acti-vdty which certainly extended over a great length of time.
Finally, a number of patches of fragmental rock are found in the midst
of the Ely greenstones. These patches are too small to be shown on the
accompan^-ing maps. Moreover, it is still doubtful whether these are tuffs
contemporaneous with and interbedded v>hh the flows, or nonnal conglom-
erates derived from the greenstones and infolded in them. It is probable
that both of these rocks are present, but owing to the imperfect exposures
it was impossible to distinguish them.
As a unit the Ely greenstones bear the following general relations to
the rest of the rocks of the district. The}' are in all cases older than the
other rocks. From them have been largely derived the later elastics and
upon them rest all of the sediments. Througli them have been intruded
all of the remaining igneous rocks. The Ely greenstones are all
Archean, or, if a more general term is desired, they form the basement
complex of the Vermilion district. This complex is very well developed m
the immediate vicinity of Ely, the largest city of the district, which is
literally built upon a firm foundation of tliese greenstones, and the complex
s
ELY GREENSTONE. 131
shows some of its most ty|3ical and interesting' characters right in the streets
and lots of the city. For this reason the formation has been appropriately
called the "Ely greenstone.""
OCCURRENCE AKD CHARACTER.
DISTRIBUTION.
In the westernmost part of the district, in the vicinity of Vermilion
Lake, the area underlain by the greenstones has the form of a nmnber of
large westward-projecting tong'ues. Beginning their enumeration from
south to north we find the first large tongue extending through the northern
portion of T. 61 N., R. 15 W. North of this there is another tongue, in
sees. 35 and 36, T. 62 N., R. 15 W*. This is followed to the north by a third
tongue, in sees. 24, 25, and 26, of the same township and range. A number
of very small tongues are to be found in the northern portion of sec. 21, T. 62
N., R. 14 W. A very narrow greenstone belt extends to the southern
portion of sees. 15, 16, 17, and 18, T. 62 N., R. 14 W. Still another
such tongue is in sec. 12, T. 62 N., R. 15 W., and extends, except in
one small area, eastward through sees. 7, 8, and 9, T. 62 N., R. 15 W.
Still farther north we find a tongue south of Bass Lake, in sees. 1, 2, 3, and
4, T. 62 N., R. 15 W., and one immediately north of this, just along the
line between T. 62 N., and T. 63 N. The northern side of Vermilion Lake
is bordered by this Archean greenstone, which has been followed out to the
west to the limit of the area mapped. The greenstone extends a long dis-
tance to the west of the Vermilion district, although it is discontinuous over
great areas. While it may be that this interruption of the continuity of the
greenstone in this portion of the State is due to its concealment in places by
overlying drift, it is also highly probable that even were this drift removed
we should find that the continuation of the greenstone is interrupted, as it
is in the vicinity of Vermilion Lake, by the overlapping of the younger
formations.
In all cases these tongues, when followed otit to the east, unite with
the main mass of the Archean which, along the line between Rs. 13 and
«The term "Kawishiwin" has been proposed by the Minnesota survey (Geol. and Nat. Hist.
Survey of Minnesota, Final Kept., Vol. IV, 1899, pp. 270-271 and 546) to comprise the two formations
which in this volume are treated under the terms the "Ely greenstone" and the "Soudan formation,"
as well as certain other rocks. Since this throws together two important formations that are here
treated separately, it has seemed necessary to introduce new names for each of these.
132 THE VERMILION IRON-BEARING DISTRICT.
14 W., covers almost the entire central portion of the district and has there
an approximate width of 10 miles. This great width of the Ely green-
stone continues to the east very nearly as far as Pine Lake, in R. 10 W.
Within this area its continuity is interrupted in a number of places by nar-
vow belts of sediments of later age trending approximately east to east-
northeast. These, altliough of minor importance so far as areal distribu-
tion is concerned, are of great economic importance, as within the area
just described they consist for the most part of the iron-bearing formation.
About 7 miles west of Ely, in sec. 4, T. 62 N., R. 13 W., begins a series
of sedimentary rocks which splits the Ely greenstone approximately in
the center. This belt of sedimentary rocks continues on to the east with
approximately uniform width beyond the end of Fall Lake, in sec. 31,
T. 64 N., R. 10 W., where it ends. Just about three-quarters of a mile
beyond this there begins another belt of sediments which continues
eastward, widening very rapidly, and corresponding to this widening
there is a rapid reduction in the width of the greenstone areas lying
north and south of it. The northern belt of greenstone narrows more
rapidly and disappears north of Moose Lake, in sec. 16, T. 64 N., R. 9 W.,
where it is covered by overlapping sediments. The continuation of this
belt is wanting for a distance of about a mile. It then begins again in sec.
10, T. 64 N., R. 9 W., and continues thence northeast to the international
boundary.
From here on to the northeast the extension of the Ely greenstone
has been traced into Canada as the result of a reconnaissance survey. The
boundaries of the formation in Canada, as given upon the accompanying-
maps, can not, therefore, be regarded as nearly so correct as those given
witliin the United States. They have been found in most places as a result
of the study of exposures along the lakes and of a few traverses inland.
This reconnaissance shows us that the width of the Ely greenstone, as it
continues to the northeast, varies greatly as a result of the folding to which
it has been subjected. As a result of this, also, its continuity is interrupted
by the infolding of the younger rocks whose are.cS have been delimited.
Upon the map it appears to cover a larger area than it does in reality, for
the reason that the important iron-bearing formation has not been delimited,
although its presence within the greenstone area is known from its having
been observed at a great number of j)lnces.
ELY GREENSTONE. 133
Returning to the sovitbern arm of the greenstone, we find it covering
the major portion of T. 63 N., R. 10 W., with a small portion of T. 64 N.
As we follow it eastward into T. 63 and T. 64 N., R. 9 W., we find that its
width is materially reduced. This results from the fact that in this area we
are upon a great anticline plunging to the east, around which wrap the
younger formations. These are likewise infolded in synclines within the
greenstone, as is to be expected, for instance, in sec. 17, T. 63 N., R. 9 W.
In the southern portion of T. 64 N., R. 9 W., this infolding is beautifully
shown. As a result of this the greenstone is divided into a number of
nan-ow belts having, in general, an east-west trend, each belt being-
separated from every other, and from the main mass of the greenstone to
the south, b}^ a trough containing later sedimentary deposits. As results
of cross folding the greenstone occurs in a number of places in anticlinal
boss-like areas plunging down under the sediments and completely
surrounded by them.
From the center of R. 9 W. eastwai-d this greenstone is totally wanting
until we reach the center of R. 8 W., where another anticlinal area of
greenstone is found. This is surrounded on three sides by overlapping
sedimentaries, the fourth, the eastern side, being cut off by the gabbro.
As we go farther east we find that the greenstone is not continuous over
any very broad areas. It occurs for the most part in rather narrow, long
belts; for instance, such a belt begins on the point projecting westward
into Knife Lake in sec. 21, T. 65 N., R. 7 W., and extends thence east-
ward into sec. 11, T. 66 N., R. 6 W., with a maximum width of about
one-half mile. Alongside this belt, however, there are small isolated
bosses surrounded by the younger rocks, as may be seen in the southwest
quarter of sec. 17 and the southeast quarter of sec. 18, T. 65 N., R. 6 W.
Similar greenstone areas occur south of Knife Lake in sees. 29, 30, and
31, T. 65 N., R. 6 W., and in sees. 25 and 36, T. 65 N., R. 7 W., and other
small areas occur also in sees. 25, 26, and 27, T. 65 N, R. 6 W. Consid-
erably larger is the greenstone massive forming the ridge upon which the
Twin Peaks are prominent points, extending along the line between Ts.
64 and 65 N. eastward to Gobbemichigamma Lake. Still larger is the
area that extends over sees. 18 and 19, T. 65 N., R. 6 W., eastward to
sec. 27, T. 65 N., R. 4 W., having an east-west length of approximately
10 miles. This last belt starts in at the west with two westward-plunging
134 THE VERMILION IRON-BEARING DISTRICT.
anticlinal tongues separated by sedimentary formations, and, after varying
materially in width as the result of cross folding, finally ends at its eastern
extremity also in two tongues — eastward-plunging anticlines partially
surrounded by the younger sediments.
EXPOSURES.
The exposures of greenstone in the areas outlined above are very good.
It is no uncommon thing to find almost absolutely bare surfaces several
hundred feet long and possibly one-fourth as wide. Such exposures are
most commonly rounded surfaces. Occasionally, however, cliffs of green-
stone are seen. In spite of the large size and the great number of the
exposures, it was very difficult — in fact, in most places almost impossible —
to determine the relations of the different kinds of greenstone to one another,
for the contacts have usually been concealed either by di'ift or by the
effects of erosion, so that where most needed, as is commonly the case, the
exposures are wanting. Mention will be made later of a few places where
some of the best exposures were found.
TOPOGRAPHY.
In the Avestern portion of the district, where the the greenstone under-
lies broad areas, the topography is ver}^ much broken. The minor idges
are numerovis and form the most prominent feature. In this portion of the
district there is a series of parallel ridges with narrow valleys between
them. Usually the sides have a steep slope, and there are sometimes abrupt
escarpments, but as a rule the hills and ridges are well rounded. It will
thus be seen that in detail the topography is very rugged. Especially is
this so north of Fall and Long lakes. The ridges throughout the green-
stone area lie in essentially parallel chains extending east-northeast, a
direction corresponding to the trend of the structure of the district. It has
already been stated that the Ely greenstone occurs very rarely in broad
areas in the eastern portion of the district, being there usually found in
comparatively small areas surrounded by younger sediments. There is a
very noticeable difference in the topography of areas underlain by the
greenstone and those underlain by the surrounding sediments, due to
differential erosion. As a rule, the greenstone forms the prominent hills
and main ridges. Usually, in traversing the country, one finds that after
leaving the sediments, which lie within a topographic depression, there
ELY GREENSTONE. 135
follow the low, rounded knobs or ridges of the greenstone. After passing
over these and ascending a gentle slope the top is reached, which is iisually
a broad, flat dome. The descent on the other side carries one over similar
topography, with the topographic break intervening in most cases between
the greenstone and the sediments. In these large ridges the minor details
of the topography are usually not very strongly accentuated, but in each
case blend in the main ridge which, while forming a very marked topographic
feature, is in general not separated into distinct peaks.
STRUCTURE.
In view of the essentially homogeneous, igneous character of thfe Ely
greenstone, it will be readil}^ seen that the geologic structure of the green-
stones areas could not have been determined without the aid of the younger
sedimentary formations. As the result of the study of the district, we
find that the greenstones have been intricately folded, the folds have in many
instances been carefully traced, and it has been found that in general the
greenstone has been folded into a great syuclinorium. The character of
this is better brought out in the western than in tlie eastern part of
the district. Within this synclinorium the synclines are occupied by the
younger rocks, whereas the anticlines are of greenstone projecting throug'h
sediments of younger age. Typical anticlines of the greenstone, partially sur-
rounded by the sedimentaries, occur in the vicinity of Vermilion Lake, in the
western part of the district and are enumerated on page 432. Attention is here
again called to the possibility that the greenstones reported to occur west of
that part of the district mapped are perhaps the crest of greenstone anticlines
projecting through the drift. The rocks of the Vermilion district have been
affected by a second system of folds lying approximately at right angles to
those that form the great east-west trending synclinorium. The effect of
this cross folding is best shown by the steeply plunging anticlines and
synclines in the sediments of later age, as well as by the distribution of the
formations in general. If we examine the map including the area near the
west end of Moose Lake, we find that as a result of the main folding the
gi'eenstone has been divided into a number of narrow belts separated from
one another north and south by still narrower belts of sediments lying in
synclines between the greenstones. It will be noted, also, that some of these
belts are completely isolated and that others have but slight connection.
136 THE VERMILION IRON-BEARING DISTRICT.
The isolation of the belts is due to the effect of the cross folding which has
produced anticlines of greenstone plunging down iinder sediments that
wrap around them. The most striking eases of these isolated anticlines are
those shown by the distribution of the greenstone in the vicinity of Knife
and Cacaquabic lakes and between Ogishke Muucie and Grobbemichigamma
lakes. Looking at the distribution of the greenstone proper, we see that
the presence of the synclinal structure is most marked in the western part
of the district where the younger formations are infolded into the greenstone
and where the greenstone predominates. In the eastern part of the district,
on the other hand, the anticlinal structure of the greenstone is most marked
for the reason that the minor synclines are very deeply buried by the
sedimentaries, which have a great surficial extent, as a result of which only
the crests of the anticlines are exposed where they project through the
sedimentaries.
PETROGRAPHIC CHARACTERS.
The rocks comprised in the Ely greenstone originally corresponded in
character to intermediate andesites and basic basalts and, like the recent
representatives of these families, must have been black or dark gray, and
presumably likewise corresponded to them in mineralogic character. These
Archean rocks have undergone for so long a period the vicissitudes to
which all rocks are exposed that it is not to be wondered at that they have
for the most part been exceedingly altered and never show all of their
original characters. Indeed, it is surprising that they retain any of their
original structures. Most of the changes which have affected them have
been in the character of the minerals, and these will be described in the
proper place. The changes that are most obvious macroscopically are
the chemical ones which have affected their color and the mechanical ones
which have affected their structure.
While the rocks, on the whole, are of a greenish color . (lience the
general name of greenstone), the various phases of them show all possible
variants of this, ranging from very light-colored greenish gray to very
dark greenish black, with the light grayish and brownish greens
predominating. The rocks appear lighter on the weathered surface, as a
rule, than upon fresh fracture surfaces. Some of the greenstones are very
nnxcli more feldspathic than usual, and in such cases they weather with a
light-pinkish crust, which causes them not uncommonly to be mistaken,
ELY GREENSTONE. 137
when viewed from a distance, for the more acid granites. The macroscopic
textures commonly seen are the ophitic, the poikilitic, and the porphyritic.
The rocks possessing these textures vary from fine-grained, almost aphanitic,
ones to those which are very coarse grained and, in exceptional cases,
have some constituents an inch and a half in length. The porphyi'itic
rocks have as phenocrysts feldspar or hornblende, or both, in a matrix
w'hich vai'ies from fine to coarse in grain. Some of the feldspar phenocrysts
are an inch and a half in length. Many of the finer grained forms of these
rocks are amygdaloidal and also frequently show beautiful cases of
spherulitic development.
Good columnar parting is totally wanting in the greenstones of the
Vermilion district, but, apparently taking its place, ellipsoidal parting" is
abundantly present. All combinations of the above structures and textures
may be found in this complex and all gradations between the rocks
possessing them. Thus we find gradations from fine to coarse forms
and from the nonporphyritic to the porphyritic. Those that are not
amygdaloidal at one place may become amygdaloidal elsewhere, and with
this change we may find the rocks becoming porphyritic, possibly showing
ellipsoidal parting. Fine-grained ellipsoidal and sjDherulitic basalts grade
into coarse-grained ellipsoidal spherulitic basalts, or into coarse-grained
basalts that are neither ellipsoidal nor spherulitic.
The greenstones are predominantly massive. Nevertheless they show
the effect of dynamic action and are in many cases finely jointed. The
dynamic action aifecting them has resulted in the production in several
places of very excellent friction breccias (reibungs-breccias) which can
with difficulty be separated from tuffaceous or conglomeratic deposits. On
the bare hills south of Moose Lake these basalts are in places brecciated,
producing rocks that strikingly simulate greenstone conglomerates. In
most cases the brecciated zone has a width of only a few feet. These
breccias might readily be mistaken for true conglomerates if the adjacent
massive rocks were covered and the breccias only were exposed. The
ellipsoids on great numbers of the exposures have very numerous and
prominent gashes w^hich traverse them at vai'ious angles, though usually
nearly at right angles to the direction of elongation. These are clearly
indicative of the mashing to which these ellipsoids have been subjected.
"Mou. U. S. Geol. Survey Vol. XXXVI, 1899, pp. 112-124.
138 THE VERMILION IRON-BEARING DISTRICT.
The masliing of the Ely greenstone has resulted in producing very com-
monly an imperfect schistose structure in tlie rocks composing it. In
exceptional cases and locally this development of schistosity has advanced
to such a point that the rocks have become completely schistose, and even
finely fissile. Gradations from the massive, coarse-grained greenstones to
green schists of dynamic origin have been observed in a nmnber of places.
This schistose structure has, however, certainly not reached such universal
development that one would be warranted in speaking of the Ely green-
stone as a green schist complex.
Finally, there occurs in a number of places with the rocks described a
conglomerate or tuff, whose structural relations to the greenstones are
somewhat doubtful. This facies is found commonly in small and isolated
exposures or under other conditions that preclude the determination of
its relationship to the nearest greenstones. These rocks consist of more
or less rounded but irregularly shaped fragments of greenstones of
various kinds, but corresponding to those that occur all around them
in the district. One can not say, however, that these deposits consist
chiefly of fragments of greenstones like those that are nearest them. No
definite indications of bedding have in any case been found, nor do the
rocks occur in connection with any sediments of which they can be the basal
conglomerates. In some places they lie between exposures of the massive
greenstone, and one is inclined to interpret such a field relationship as due
to alternation of flows and tuff deposits. On the other hand, however, one
m&j readily interpret this relation as due to infolding of the elastics in the
greenstones. Rocks very similar to these, but showing the transition to
finer-grained, clearly sedimentary deposits have been found in several
places, and are descril^ed Avith the Soudan formation, to which they
belong. The latter deposits clearly owe their field relationship to infolding
within the greenstones. It is highly probable that most, if not all,
of these conglomeratic rocks should be so classed. However, a few cases,
which will be mentioned under the heading " Interesting localities," have
been doubtfully referred as tuffs to the Ely greenstone. These elastics
have nothing to do with those belonging to the Ogishke conglomerate,
which will be subsequently coiasidered.
In numerous places the altered greenstones have been more or less
thoroughly discolored and impregnated with iron. This impregnation is, in
ELY GREENSTONE. 139
all of the cases observed, almost purely superficial, extending at most only a
few feet down into the rocks. Such occurrences have led to considerable
waste of money in the sinking of prospect holes. The joints and g-ashes in
numerous places throughout the district have been found filled more or less
completely with quartz, occurring- both as vein quartz and in a saccharoidal
condition, more or less intimately mixed with carbonates. In several places
large veins of quartz traverse the greenstones, but minute ones are more
common. The largest veins have been prospected for gold, and several
gold mines, so called, have been opened along them. Where the quartz
veins are mixed with carbonates the carbonate usually carries a consider-
able content of iron, so that on weathered surfaces such vein deposits are
quite ferruginous. This infiltrated carbonate-bearing material" is especially
common in the interstices and in the schistose matrix between the ellipsoids.
In this same position, and apparently but a further alteration product
of such secondary carbonate-bearing deposits, is a white, black, or purplish
chert, and less frequently a red jasper. Not uncommonly, also, the non-
ellipsoidal greenstone near the jasper belts contains irregular bunches and
lenticular areas, varj'ing in size, of rather coarse white and black chert,
with more rarely the true red jasper. Such deposits are certainly, in many
cases, composed of infiltration products brought from overlying formations
and deposited in the interstices of the basal greenstones. They are never
of very large size, and it is of course useless to prospect in such places for
paying ore bodies.
Certain of the macroscopic structures, namely, the amygdaloidal, the
spherulitic, and the ellipsoidal, mentioned above as being present in these
greenstones, are very well developed, and on account of this, but chiefly as
they offer a clew to the mode of formation of the greenstones, they are of
some interest and will be described m detail. '
THE AMYGDALOIDAL STRUCTURE.
Of the three structures mentioned above the most common is the
amygdaloidal. This- is rarely seen in the very coarse greenstones, but is
usually present in the finer-grained varieties. This structure is most notice-
able on weathered surfaces, where it may be recognized by the presence of
rounded or oval spots scattered over the rock. On examining the internal
« Mon. U. S. Geol. Survey Vol. XXXVI, 1899, pp. 130-135.
140 THE VERMILION IRON-BEARING DISTRICT.
structure of the greenstone, one sees that these spots, which are also scattered
through the body of the rock, are frequently cross sections of irregular tubes
lying perpendicular to the surface of the flow. They are sections through
filled gas pores, the filling being known as amygdules. The amygdules
consist of chlorite, calcite, quartz, actinolite, and epidote. Rarely is any
one of the minerals absolutely alone in the amygdule, though usually one or
the other will greatly predominate. The amygdules are filled with chlonte ;
chlorite and quartz; calcite; chlorite and calcite; chlorite and epidote;
chlorite and actinolite; chlorite, calcite, and quartz; calcite and quartz; and
quartz alone. The materials constituting these amygdules are arranged
above in approximately the order of abundance. According to whether
the light or the dark minerals predominate, we get light-colored to white
gi-anular areas on the one hand, grading to light-greenish to silky dark-
green areas on the other. The gas pores of these rocks owe their origin,
as has already been hinted at, to the pressure of the gas in an originally
molten magma. When this magma reached a position where the pressure
was markedly diminished, the gas separated, segregated, 'and expanded,
and the magma became more scoriaceous on the surface; and the pores are
found to diminish in number and size as those portions of the original
molten magma that were under greater and greater pressure are reached.
A concomitant of the formation of the gas pores is the relatively rapid
cooling of the magma, producing rocks of a glassy nature or of very fine
grain. Both of these conditions commonly confront us in A^olcanic rocks
or, in other words, in rocks that have overflowed upon the surface. It is
true that amygdules have been observed in rocks which occur clearly as
sills and dikes, and which therefore never actually reached the surface in a
molten condition. These cases are, however, relatively rare, and one can
readily see that the enormous reduction of pressure occasioned by the intru-
sion of the sills into their present places from much lower positions would
readily permit the expansion of the included gas. Moreover, in such cases
amygdules are far from numerous, showing that the pressure was dimin-
ished to such extent that relatively few pores were foiTued. In the case of
the amygdaloidal greenstones in the Vermilion district we observe the
following conditions: First, almost universally when amygdules are present
they occur in great quantity and are very commonly of large size ; second,
the amygdaloidal structure accompanies a fine-grained condition of the rock. ,
ELY GREENSTONE. 141
•The combination of these two characters and their g-eneral distribntion
among- the greenstones seem to indicate that these greenstones, in great
measnre at least, were poured forth at the surface.
THE SPHERTXLITIC STRUCTURE.
The Ely g-reenstones are very frequently marked by small, rounded,
raised areas, which differ in color from the matrix, being either lig-liter
or darker. They range in size from that of a pin's head up to 3 inches
in diameter. No structure is visible on the very small areas. They
merely stand out on the weathered surface of the rock as so many small
nodules, their relief being the result of differential weathering. On the
larger bodies, however, a distinctly radial arrangement can be seen, and
this is especially well shown on weathered siirfaces. The essential
characteristic of spherulites is tliat they are formed of radiating or
diverging groups of crystals which commenced to crystallize from one
point or a center. These are the characteristics of the objects mentioned.
They are similar in general characters to the spherulites of the acid lavas,
but differ from them in mineral ogic, and hence, of course, in chemical,
composition. One can see witli the naked eye that chlorite in radiating
fibers is the chief constituent of some of the spherulites. Most of them
are formed of a grayish material whose character can not be recognized
macroscopically. The characters of these spherulites as seen under the
microscope will be described below (see p. 152). From the pubhshed
descriptions of various occurrences of spherulites it appears that they
are generally found in rock masses that are believed to be flows, or, more
rarely, upon the selvage of small dikes. So true is this that spherulites,
like porous and slaggy sti'uctures, have come to be considered as fair
evidence of the original extrusive character of rocks in which they
occur. This spherulitic structure is not found, however, accompanying
the amygdaloidal rocks in the Vermilion district. On the contrary, the
conditions for the formation of gas pores in large quantity seem to have
precluded the formation of spherulites, although some few vesicles may
occur with the spherulites. On one good exposure a traverse showed
the fine-grained amj^gdaloidal rock grading downward into a rock growing
gradually coarser and coarser, in which the amygdules disap^^ear, and when
they had completely disappeared the spherulites were found to have
142 THE VERMILION IRON-BEARING DISTRICT.
developed in large quantity. These pre-Cambrian basic lavas exhibit con-
ditions almost exactly the same as those observed by Iddings in the acid
lavas of much more recent times in the Yellowstone Park and described
by him as follows:"
In recapitulation, then, this rhyolitic lava is a flow about 100 feet thick, except
where it has piled up in a small valley. It is glassy, except the lithoidal portion
in the valle}', and is fi'ee from phenocrysts. The obsidian is dense in a lower part
of the edge and carries numerous spherulites. Large vesicles occur in the upper
portion, and toward the surface of the flow the spherulites disappear and the glass
become.'? filled with gas cavities and passes up into pumice. * * * These
characteristics repeat themselves in the rhyolite in various parts of the park.
Up to the present time there have been, to m)^ knowledge, only two
occurrences of basic spherulites or spherulitic rocks (variolites) described
from North America. The one was by Ransome from California '' and the
other by myself from the northern peninsula of Michigan." In the case
of the Ely greenstone the spherulitic structure is, as has already been
intimated, one of the most common, most characteristic, and most striking
features of the rock. The spherulitic greenstones are distributed in dis-
continuous exposures over a great number of square miles. For instance,
they are very common and beautifully developed on the bare hills north
of Long Lake in sees. 10, 11, 14, 15, 16, 21, and 22, T. 63 N., R. 12 W.
They are especially common just southeast of Jasper Lake in sees. 1 and
12, T. 63 N., R. 10 W., and sec. 6, T. 63 N., R. 9 W. They are also
very common west of North Twin Lake in sees. 10, 11, 14, and 15,
T. 63 N., R. 10 W. Well-developed spherulites occur also about a mile
north of North Twin Lake at the northeast corner of sec. 12, T. 63 N.,
R. 10 W. In many of the places mentioned, especially in sec. 6, T. 63 N.,
R. 9 W., the exposures are almost solid masses of spherulites, which show
very beautifully on the weathered surface their radial structure. Figures
A and B of PL III are illustrations made from a polished and a weathered
specimen, respectively, of these spherulitic rocks, and give a correct idea of
their appearance.
" Geology of the Yellowstone National Park, Descriptive Geology, Petrograpliy, ami Paleoutology,
by Hague, Iddings, Weed, Waleott, Girty, Stanton, and Knowlton: 3Ion. U. S. Geo). Survey Vol.
XXXII, Part II, 1S99, p. 365.
'' The eruptive rocks of Point Bonita, by F. Leslie Eansome: Bull. Tniv. of Cal., Vol. I, 1S03, p. 99.
''The Crystal Falls iron-l^earing district of Michigan: Jlon. U. S. Geol. Survey Vol. XXXVI,
1899, p. 108.
U. S. GEOLOGICAL SURVEY
MONOGRAPH XLV PL. Ill
SPHERULITIC TEXTURE IN THE GREENSTONES.
A. Fragment taken from the periphery of a spherulitic ellipsoid. The outside of the ellipsoid is made of the finest-grained material, with an occasional
soherullte. Near the center the spherulites are more numerous and larger.
B. This illustrates the spherulitic character of many of the greenstones, especially those which show the ellipsoidal parting.
ELY GREENSTONE. 143
When the clearly recognizable characters of the spherulites, as shown
in the above illnstrations, and the extent of the distribution of these
spherulitic greenstones are taken into consideration, it is with very great
surprise that one finds that the only recognition which the spherulites
have received was by N. H. Winchell, who states, in his report," that the
surface of the rock is mottled by small areas of lighter color than the matrix
in which they lie, and refers to them as indicating an original amygdaloidal
or fragmental structure. Spherulites are now known to exist in the ancient
acid volcanics over various regions of the United States. They have also
been described from numerous localities where more recent acid lavas are
developed. Iddings well states the probable reason for the more frequent
occurrence of such crystallizations in acid than in basic lavas in the following
words -.^
The greater frequencj' of lamination and localized erystallization in acid lavas
as compared with basic ones is a consequence of the generally greater viscosity of acid
lavas at the time of their eruption. The basic rocks have a considerably lower melting
point and are much more liquid up to the temperature of solidification. Hence,
diffusion would take place more rapidh' and the magna would be more homogeneous,
other things being equal.
The spherulitic metabasalts or greenstones are extraordinarily abun-
dant in the Vermilion district. They have a very great development in the
adjacent portions of Ontario underlain by greenstones. The spherulitic
structure occurs in similar Huronian rocks in the Crystal Falls district
of Michigan, and is likewise developed in the rocks of the Menominee
district of Michigan. Mr. C K. Leith reports the occurrence of similar
rocks in the Mesabi district of Minnesota. Considering the extraordinarily
widespread development of this structure in the areas mentioned, one is
led to wonder at the fact that it is not present in similar rocks which have
a widespread occurrence in the Penokee-Grogebic district of Michigan and
Wisconsin, and in the Marquette district of Michigan. Furthermore, it
seems surprising that this structure should not exist in the somewhat
more recent metabasalts of the Keweenawan of the Lake Superior region,
and in the still more recent basalts of the Triassic of the Atlantic coast.
It seems highly probable that this spherulitic structure must exist, at least
«Geol. and Nat. Hist. Survey of Minnesota, Final Rent, Vol. IV 1S99, p. 253.
6 Geology of the Yellowstone National Park; De'crip ive Geology, Pttrology, anil Paleontology,
by Hague, Iddings, and othen-. : Mon. U. S. Gi;ol. Survey Vol. XXXIl, Part II, 1899, p. 4:'.5.
144 THE VERMILION IRON-BEARING DISTRICT.
iu a limited development, in the rocks of the areas mentioned, and that
its occurrence has simply been overlooked, or else that the spherulites
were not recognized as such, but were called amygdules, as they were in
the notes of some of the observers in the Vermilion district before attention
was called to the true character of these bodies.
THE ELLIPSOIDAL" STRUCTUEE.
The ellipsoidal structure iu pre-Cambrian greenstones (metabasalts)
was described by the author in 1899,* and the attempt was made to account
'for its occurrence. It was concluded that the greenstones correspond in
general characters and in mode of origin to aa lava, and that the ellipsoids
were due to the breaking up of this relatively viscous lava. It was
concluded -that the shajie of the ellipsoids was determined to a great extent
by the rolling over and over of these units and the pressure under which
they existed at this time as well as their cooling and consequent contraction,
witli possibly, as an additional and less nnportant factor, the pressure to
which they were subjected" subsequent to their complete cooling. In a
recent article Gregory describes the ellipsoidal structure in Maine andesites,
and writes of the brecciated rock (glassy and stony) which fills the
interstices between the ellipsoids." This is confirmatory of my statement
that the matrix between the ellipsoids in the Lake Superior region was
originally a breccia, in part at least, which is now, however, as the result
of pressure, almost always distinctly schistose. The conclusion as to this
orio-inal brecciated character of the matrix was reached chiefly as the
result of microscopic study of thin sections of the schistose matrix.'' A
clastic matrix has been observed filling the interstices of rocks with similar
elhpsoidal structure and is found described in the work of Geikie on the
« The advantage ot using the term ' ' ellipsoidal, ' ' applied to designate the peculiar parting found
in some of the basic igneous rocks, was emphasized in the description of the Crystal Falls iron-liearing
district of Michigan : Mon. U. S. Geol. Survey Vol. XXX VI, 1899, pp. 112-124. The writer has observed,
in conversation with various geologists, that in practically every case in which the term "spheroidal"
was applied to this parting, the person using it had the idea that it corresponded very closely to the
secondary spheroidal parting which is so well known in all igneous rocks. Very naturally confusion
is thus caused in the minds of the geologists by the use of the term spheroidal to designate the original
parting in the rocks on the one hand, and the structure produced as the result of weatliering processes
on the other. It seems to me, therefore, more than ever necessary to confine the term ellipsoidal to
this original parting and the term spheroidal to the structure produced by weathering.
f'Mon. U. S. Geol. Survey Vol. XXXVI, 1899, pp. 112-124.
c Am. Jour. Sci., 4th series. Vol. VIII, 1899, p. 367.
i' Loc. cit.
ELY GREENSTONE. 145
ancient vqlcanics of Great Britain." In some of the cases mentioned by
him, however, this clastic matrix is clearly of sedimentary origin, as lines
of sedimentation, with separation into the finer and coarser grained material,
are clearly recognized. In none of the ntimerons cases studied in the Lake
Superior region is there any indication that this clastic material is of
sedimentary origin, hence it has been concluded that it was due to
brecciation. The reservation must be made, however, that some of it inay
well be a tuff deposit in which, as the result of the small amount that can
be studied, no differentiation in grain, etc., is shown.
The mode of formation of these ellipsoids, as suggested, and the
presence in them of great quantities of amygdules, seem to point
conclusively to the fact that the lava in which they occur is a surface
flow, although the flows may have been of submarine formation.
Ellipsoidal basalts identical in every way with those from Michigan,
whose characters have been already described, occur in the Vermilion
district of Minnesota, and are very widespread. An occurrence at one
locality was described several years ago by WinchelL' More recently, in
the last volume of the Minnesota report," a number of other localities in
the Vermilion district have been enumerated, in which rocks having this
structui'e occur. The greenstones possessing this structure have, however,
a much wider distribution in the Vermilion district than would be inferred
from the description given by the State geologist. Corresponding to their
wide distribution in the Vermilion district proper they have also been found
by reconnaissance to cover large areas in the adjacent portion of Canada
forming the continuation of this district. This distribution is practically
the same as that of the Ely greenstones, for this structure can be seen in
more or less perfect development on nearly all of the large exposures of
that rock. The accompanying illustration (PI. IV, A) shows nothing
essentially different from the sketches reproduced in Monograph XXXVI,
but is taken from a photograph and is, therefore, of much greater value.
The photograph represents an exposure about 50 paces south of the county
road 1 mile east of Soudan, just northeast of Jasper Peak.
The various observations recorded in Monograph XXXVI concerning
a Ancient Volcanics of Great Britain, by Sir Archibald Geikie, Vol. I, 1898, pp. 18-1 and 193.
''The Kawishiwin agglomerate at Ely, r\linnesota: Am. Geologist, Vol. IX, 1892, pp. 359-368.
cGeol. and Nat. Hist. Survey of Minnesota, Final Kept., Vol. IV, 1899, pp. 255-267, 274, 276.
MON XLV — 03 10
146 THE VERMILION IRON-BEARING DISTRICT.
the peripheral and concentric an-ang-ement of the amj^gdules in the ellipsoids
were confirmed by repeated observations ou similar occurrences in the
Vermilion district. At one place on the hill just west of Ely and not very
far from the last house in the town, it was noticed that the arnvgdules were
concentric ou one side of the ellipsoids, although a few were scattered
through the ellipsoids. In this case the exposure showed a transition from
amygdaloidal ellipsoidal rock to amygdaloidal nonellipsoidal basalt, both of
essentially the same grain. The exposure is about 20 paces in width north
and south. It looks very much as though the ellipsoidal portion of this
ancient lava represents the surface of the flow, which was more viscous
than the inner portion and consequently more readily broken. This
being true, such a relationship readily explains the occm-rence of a transi-
tion from ellipsoidal to nonellipsoidal foi'ms of the same rock, as discussed
in detail in Monograph XXXVI, to which reference has repeatedly Ijeeu
made. Such gradations in these ancient lavas are shown by a number of
observations taken at different places. One passes in the field from a fine-
grained amygdaloidal ellipsoidal basalt to an ellipsoidal basalt in which, by
gradual transition, the grain has become considerably coarser; it is then an
ellipsoidal anamesite, if we use the terms basalt, anamesite, and dolerite to
express the degrees of coarseness of crystallization; it then grades into a
coarse-grained ellipsoidal dolerite, which in its turn grades into an even
coarser dolerite without marked ellipsoidal parting. Continuing, this same
sequence is g'one over in reverse order, from the coarse, massive dolerite to
the fine-grained ellipsoidal basalt. The best place to get this complete
sequence is on the bare hills south of Moose Lake, along the section line
between sees. 32 and 33, T. 64 N., R. 9 W. Another place in which the
transition from the fine amygdaloidal ellipsoidal basalt to the massive
dolerite can be excellently seen is north of Long Lake. It is about 500
paces, or one-fourth mile, north of the southeast corner of sec. 9, T. 63 N.,
R. 12 W.
Observations show that the amygdules are not the only features which
are common in the ellipsoids, as the ellipsoids are also commonly spherulitic."
In fact, the spherulites have been observed only on exposures which show
a more or less perfect ellipsoidal parting-. The best spherulites have been
seen Avliere the ellipsoids are typically developed and show the following
"Cole anil Gregory, Quarterly Jour. Geol. .Soc, Vol. XLVI, ISVlO, p. 311.
U. S. GEOLOGICAL SURVEY
MONOGRAPH XLV PL. IV
A. ELLIPSOIDAL PARTING IN GREENSTONE.
J?. ELLIPSOIDALLY PARTED GREENSTONE, SHOWING SPHERULITIC DEVELOPMENT.
ELY GREENSTONE. 147
relations to these ellipsoids. The smallest spherulites occupy the extreme
outside of the ellipsoids. From the outside they increase in size toward
the center, where, if the rock is there spherulitic at all, the largest
spherulites are developed. Sometimes the development of the magma
into a spherulitic rock did not reach entirely to the center, which is then
developed as a massive dolerite of normal character. It is very noticeable
that while the spherulites occur in the very fine-grained lavas they are
apparently equally common in some of the more coarsely crystalized forms
when these phases are ellipsoidal. The spherulitic and amygdaloidal
structures sometimes occur together, but most commonly they are not
developed in the same rock. Apparently the presence of one does not
altogether preclude the existence of the other, although it amounts very
nearly to this. Thus, in passing over the jjarticular section through the
ellipsoidal amygdaloidal lavas, which was noted above as occurring north
of Long Lake, we find that the amygdaloidal ellipsoidal fine-grained lava
is first nonspherulitic. Soon, however, the spherulites begin to appear and
gradually increase in importance, the amygdules decreasing correspondingly
in quantity and the grain of the matrix between the spherulites increasing
also in size. The spherulites, however, are wanting in the coarsest non-
ellipsoidal phases of the lava. The arrangement of the spherulites in
concentric circles made up of spherulites of larger and larger size as the
center is approached shows, as does the concentric arrangement of
amygdules in the ellipsoids, that each ellipsoid must be reckoned with as
a unit. Observations show that small spherulites occur on the outside of
a spherulitic mass, where crystallization continues for only a short time,
while the larger spherulites, requiring proportionally a longer period for
their formation, occur deeper down in the rock.
Magnificent exposures of these spherulitic ellipsoidal greenstones
occur on the hills north of Long- Lake for the greater portion of the distance
between this lake and Bass Lake, in sees. 9 and 10, T. 63 N., R. 12 W.
One of the finest exposures seen was that occurring- on the high hill about
500 paces south of the meander corner on the shore of Bass Lake. The
illustration presented in B of PI. IV is made from a photograph of this
exposure and shows the spherulitic character of the ellipsoids; the eight
spots in the photograph are the spherulites. The somewhat schistose,
brecciated matrix between the ellijDsoids can also be seen in the illustration.
148 THE VERMILION IRON-BEARING DISTRICT.
Other good exjDOsures of splierulitic, ellipsoidal greenstones, in wliicli four
or five rows of spherulites can be seen, occur in the northeast corner of
sec. 7, T. 63 N., R. 11 W. Perhaps even better ones can be seen south and
southeast of Jasper Lake, especially in sec. 6, T. 63 N., R. 9 W. Here
the hills are nearly bare and exposures extend almost continuously from
the south shore of Jasper Lake eastward along the section line to the south
quarter post of sec. 6, T. 63 N., R. 1 1 W.
These ellipsoidal or aa greenstones have been subjected to orogenic
movement, and when in the zone of fracture" they have been jointed, and
in places brecciation has also taken place. A difiFerent result followed
when the rocks were more deeply buried and subjected to great pressure,
which produced interesting structures that are in some places now
exposed at the surface. The ellipsoids then, instead of being fractured,
were mashed into disks, just as one could mash a Imnp of stiff dough into
a disk-shaped body. The cross section of such a mashed ellipsoidal
greenstone shows that the ellipsoids have enormously elongated axes,
approximately parallel to the direction of minimum pressure, and a
proportionally short one in the direction of greatest pressure. Various
stages of this deformation have been observed. In extreme cases these
rocks have a banded appearance, the material of the ellipsoids forming
bands of dense material alternating with other bands which consist of what
was the matrix between the ellipsoids. This rapid alternation of bands of
differing material — the bands derived from the matrix may be an inch and
a half in thickness, the bands from the ellipsoids being usually a little
thicker — very closely simulates true bedding, and miglit very readily be
construed as such on a hasty examination. Especially might it be so taken
in cases where, as freqifently happens, the exposures are onlj* a few square
feet or at most a few square yards in area. If one had large areas of these
massive ellipsoids to study, however, the bands, if examined in detail, would
be found to be relatively short and to be made up of enormously elongated
lenses. Such large exposures are, however, rare. One of the best exposures
of rock of this soi-t is on the high ridge south of the exjjloring camp on
the south side of Moose Lake. The rock here is the typical splierulitic
ellipsoidal greenstone, and shows very nearly clean exposures over an
"Principles of pre-Cambrian geology, by C. R. Van Hitie: Si.xteenth Ann. Kept. U. S. Geol. Survey,
Pt. I, 1896, p. 696.
ELY GREEN;<TONE. 149
area several hundred paces long. In the center of this large exposure the
ellipsoids are relatively little mashed, and show the aplierulitic structure
within them as well as the matrix between them. This was evidently a
part of the rock mass that acted as a buttress, and was not affected by the
mashing, as was the rock on each side of it. On the sides of the ridg-e the
ellipsoids are much flattened. The rock in places passes into a greenstone-
schist in which the ellipsoidal structure is totally obliterated. Between
these green schists on the one hand and the typical ellipsoidal greenstone
on the other there are various gradations. The intermediate phases show
a certain coarse banding which, by a careless observer, might be mistaken
for lines of sedimentation. This banding is produced in the way indicated
above, the bands being of two kinds, one kind produced from the matrix
and the other produced from the original ellipsoids.
Still another excellent example of this pseudobedded structure may
be seen in the Archean just north of the railroad near the east end of the
place where the Duluth, Port Arthur and Western Railroad first reaches
the shore of Gunflint Lake from the east. This is north of the railroad
track and distant from it from 75 to 150 paces. To the north the green-
stone is fairly massive, and in places is distinctly ellipsoidal. Toward the
south, nearer the overlying sedimentaries and consequently nearest the
plane along which movement must have taken place during the folding of
the rocks, it becomes decidedly schistose. The ellijasoidal masses are
flattened to such an extent as to give a rough banding to the rock.
This description of the ellipsoidal structure in these greenstones would
not be complete if attention were not called to the frequency of its occur-
rence in the various districts of the Lake Superior region. Thus, for
example, it has been described from the Marquette and the Crystal Falls
districts of Michigan, and one can state with a fair degree of assurance,
from the occurrence of large quantities of greenstones in the Penokee-
Gogebic of Michigan and Wisconsin, that it also occurs there, although it
has not been described from that district. It has also been observed by the
writer in a number of places in the Menominee district of Michigan and in
the Mesabi district of Minnesota. Lawson describes it in the rocks of
the Lake of the Woods region." The same structure has been described
"Geology of the Lake of the AVoods region, by A. C. Lawson: GeoL.and Nat. Hist. Survey of
Canada, 1885, pp. 51-53otf.
150 THE VERMILION IRON-BEARING DISTRICT.
from the Micbipicoten iron-bearing district on the east side of Lake
Superior b}^ A. B. Wilhnott," and Dr. S. Weidman, of the Wisconsin Greolog-
ical and Natural Histor}' Survey, states that it occurs in greenstones of
supposed Huronian age in the vicinit}- of Wausau, Wis. It has been
observed in the Archean greenstones on Lake Nipigon in Ontai'io, Canada.
As the result of field studies of the Keweenawan volcanics of the north
shore of Lake Superior in 1900, the writer knows that it occurs also in
them. Although so very common throughout the Lake Superior region in
the rocks of pre-Cambrian age, it appears to be relatively rare in the petro-
graphically similar rocks of later age found elsewhere in North America.
This structure has been found to be so common throughout the Lake
Superior region that it is now considered characteristic of the j^re-Cambrian
greenstones of the region. It is not, however, confined to any one of the
divisions of the pre-Cambrian rocks. The rocks in which it occurs range
from the Archean of the ^^ermilion district of Minnesota and the adjacent
Canadian districts and the Marquette district of Michigan, to the Keween-
awan,. It occurs within the greatest surficial areas of the Archean. This
same structure has been found by Ceikie in the lavas of Great Britain.*
In a letter to the writer Geikie says: "This remarkable structure appears to
be far more common in lavas of all ages than I supposed. It is admirably
developed in our Arenig lavas, and I have lately found it in those of the
Old Red sandstones and Carboniferous system."
MICROSCOPIC CHARACTERS.
The rocks composing the Ely greenstone have been divided according
to their macroscopic characters into the porphyritic and non-porphyritic
varieties, the normal diabasic or ophitic textured forms, the amygdaloidal,
spherulitic, and ellipsoidal forms. Stress has been laid upon some of the
principal macrosco])ic characters, and these divisions have been made
merely for the purpose of aiding in the study of the rocks and not because
the varieties were distinguished liy important diftereuces in microscopic
characters, except in a few cases. As the reader would infer from their age
and from the use of the name greenstone in connection with them, these
"The Michipicoten Huronian area, by A. B. Willniott: Am. Geologist, Vol. XXVIII, 1901, p. U.
The uomenclatnre of the Lake Superior I'orniation.s, by A. B. Wiluiott: Jour. Ueol., Vol. X, 1902,
p. 71.
''Am-ieiit N'oleauics of Great Britain, liy Sir Archibald Geikie, Vol 1, ISSKS, pp. 184 and 193.
ELY GREENSTONE. 151
rocks are very much altered. The original minerals that remain are
very few. The microscope discloses the following original constituents:
Hornblende, augite, feldspar, quartz, titaniferous magnetite, and apatite.
The original hornblende is the common brown variety. The augite varies
from yellow to yellowish green and possesses its normal characters. The
feldspar usually shows broad twinning lamellse, although in some cases
it was found in imperfect sheaves. In one case it was distinctly seen
to have been formed prior to the titaniferous magnetite, as the magnetite,
occurring in large plates, incloses lath-shaped feldspars. The feldspar is
generally very much decomposed, so much so that one can not determine
its exact characters. It is presumed to be a labradorite. There is very
little quartz, but some was found occurring in micropegmatitic intergrowth
with the feldspar, and is presumed to be a primarj^ constituent. Sometimes
it fills in-egulai- interstices between the other minei-als as primary quartz
representing the last product of the crystallization of the rock. Magnetite
and apatite show nothing uncommon.
The secondary constitiients are common green hornblende, actinolite,
biotite, chlorite, sericite, epidote, zoisite, sphene, rutile, feldspar, quartz,
pyrite, and hematite. The feldspar has iisually altered to a mass of sericite,
kaolin (?), feldspar, and quartz. In some cases it is completely saussuritized.
There were observed occasional irregular but in general rounded serpeu-
tinous areas, which are strongly suggestive of aggregates of olivine indi-
viduals in which the olivine possesses no definite crystallographic outline.
TEXTURE.
The great majority of the greenstones are massive rocks, varying from
fine to coarse in grain. The textures they originally -possessed have to a
certain extent been obscured by the various processes of alteration to which
they have been subjected. Both fine- and coarse-grained greenstones and
all of the intermediate phases show locally porphyritic texture, the pheno-
crysts being usually of feldspar, but occasionally of brown hornblende.
In the coarse-grained rocks the ophitic texture predominates; in the
even-textm-ed, fine-grained rocks the following are commonly developed:
Ophitic, microophitic, intersertal, pilotaxitic, hyalopilitic, flowage, and
spherulitic textures. The ophitic and microophitic textures are the most
common, and the mineralogic composition is generally that so characteristic
152 THE VERMILION IKON-BEARING DISTRICT.
of the nietadolerites (diabase) and metabasalts. The rocks possessing these
textures occur in very large quantity throughout the district. These
textures are common in the recent Ijasahs. The raineralogic composition
of the rocks is also the same as would be produced in recent basalts by
alteration. Hence, in the absence of chemical analyses, the wi-iter feels
warranted in asserting that the greater portion of these greenstones was
derived from the alteration of originally basaltic rocks.
Spherulitic texture is fairlv common in these altered basalts, and on
account of its somewhat greater interest deserves a little more detailed con-
sideration than has been given to the others. The spherulite occasionally
has at its center a very small crystal of plagioclase surrounded by fine
sheaves of feldsjaar, and these spherulites are ver}^ similar to those described
some years ago from Michigan." The feldspars are brownish when seen
under low power and grayish when examined by high power, as the result
of the innumerable minute crystals of epidote, a few hornblende individuals,
and reddish-brown to black spots of ferruginous material. Other spherulites
consist largely of feldspar, but between the feldspars occur needles of
actinolite, which seem to have been derived from some original ferromag-
nesiaii mineral which, with the feldspars, formed the spherulite. There were
found in one case in a much altered greenstone, instead of the usual feldspar
spherulites, radial masses of rich green chlorite with silky luster. The
mici'oscope showed a few crystals of magnetite and some epidote in these
spherulites in addition to the chlorite.
With the above kinds of rocks, which are unquestionably of basaltic
character, there are rocks that possess an intei'sertal, pilotaxitic, and hyalo-
pilitic texture, in some of which porphyritic feldspars, occurring in isolated
individuals or in groups, are very common. In these rocks there seems to
be a large proportion of brown hornblende, sometimes occurring as pheno-
crysts. The general appearance of the rocks is like that of the andesites.
It appears that, associated with the basalts and playing a subordinate role
in this district, there are rocks of intermediate composition which were
originally andesites — both hornblende- and pyroxene-andesites — and that
we are justified in stating that meta- andesites form a part of the Ely green-
stone.
"The Crystal Falls iron-bearing district of Michigan, by J. Morgan Clements: ]\ron. V. S. Geol.
Survey Vol. XXXVI, 1899, p. 111.
ELY GREENSTONE. 153
SCHISTOSE GREENSTONES.
The various greenstones thus far described Jiave been very much
changed by chemical action, as is shown by the number of secoiidary
minerals which now replace the original ones. In many cases dynamic
action subsequent to or accompanied by the above changes has produced
schistose forms of these greenstones. These schistose greenstones are not
nearly so common, however, in the Vermilion district as one would be led
to suppose from a perusal of the literature which has been published on this
district. In this literature these rocks have frequently been spoken of as
greenstone-schists. This term seems to the writer to convey a wrong idea
of the character of the rocks as a whole, though it is fitting in certain cases.
The greenstone-schists or green schists are in reality a very subordinate
phase of the Ely greenstone, and in the great majority of cases are of purely
local occurrence and very subordinate extent. They have been formed
along zones of excessive deformation and grade into massive granular rocks.
For this reason the term schistose greenstone has been 23referred to indicate
them. In these schistose rocks all of the original minerals have been
changed by metasomatic action, and as a result of movement in the rocks
produced b}^ shearing stresses, the original textures have also been almost
completely obliterated.
GENERAL CHARACTERS.
These rocks are schistose in character and appear in various shades of
gTcen. Only one macroscopic structure has been observed which would
lead to the determination of the original characters of the rocks. An imper-
fect, nearly obliterated, amygdaloidal structure was observed in one case.
A microscopic study of the rocks shows that the constituents are small and
the rocks very dense in texture. The various minerals to be enumerated
generally have their long directions approximately parallel, this arrangement
producing a schistose structure. In some cases almost complete recrystal-
lization seems to have taken place, and in these cases larger individuals have
been produced. Occasionally the hornblende and chlorite appear in large
porphyritic individuals inclosing other constituents of the rock. Constituents
of these rocks are biotite, muscovite, chloi'ite, sericite, calcite, epidote,
zoisite, pp'ite, and limonite. In some cases the secondarily produced
hornblende has undergone a tertiary change and has been chloritized.
154 THE VERMILION IRON-BEARING DISTRICT.
The above rocks, when examined in the laboratorj^, give no cine to the
character of the rocks from which they were derived, if we except the case
of the specimen containing the amygdviles. Classified according to their
mineralogic composition we would call them chlorite-, amphibole-, and
biotite-schists, and gneisses. When studied in the field, however, their
intimate relations with the greenstones and the gradations observed between
the schists and the massive greenstones prove conclusively that they have
been derived from rocks similar to those from which the massive greenstones
that now predominate throughout the district have been derived — in other
words, from dolerites, basalts, and andesites.
ORIGIK OF ELY GKEENSTOIs^E.
There can certainly be no reasonable doubt in the mind of the reader
as to the original character of the rocks described as constituting the Ely
greenstone of the Vermilion district of Minnesota. The various textures
and structures that the rocks possess are such as are present onh^ in igneous
rocks. However, even though it may be conceded that the greenstone
formation is of igneous origin, there remain still the further queries: Are
the rocks constituting it intrnsi^•e or effusive in their character, or are
rocks of both of these modes of formation present in tlie complex?
Furthermore, if both occur, which mode of origin is the 2:)i'edominant one?
These points may well be discussed here. To the first query the answer
must be given that the observations recorded show that both kinds of
rocks — both effusive and intrusive — are present. The answer to the second
query, as to which of these predominates, can be given only with some
doubt, as it is very difficult to make a quantitative estimate of the areal
distribution of rocks that are so similar in character. From personal
observations, however, the writer has been impressed by the Aery wide
distribution of the greenstones that possess characters indicative of
effusion. This has led him to place the greenstones with the surface flows;
but the reader must be cautioned to include under the term sin-face flows
those which maj' have been poured out under water — submarine flo%\s —
and winch were thus, perhaps, under relatively high pressure, as well as
those that reached the suiiace of the land. In either case the mode of
origin outlined for many of these greenstones postulates a surface upon which
they could rest. Hence, if interpreted in the strictest sense, they do not
actually represent the original crust of the earth, as Winchell considers them
ELY GREENSTONE. 155
to do." Nor, showing volcanic characters as they do, can they be considered
as a part of the earth's crust produced by downward crystaUization. They
must be, indeed, somewhat younger than the original crust. Nevertheless,
since the greenstones are the basement upon which rests a great series of
sediments that can be correlated with sediments in other areas which have
been regarded as of Lower Algonkian age, we have classed the igneous
greenstone basement in the Vermilion district as of Archean ag"e.
The intrusives that are considered to form a part of this complex
are those which are of essentially the same nature as the volcanics, but
which differ slightly in their mode of occurrence. They are those portions
of the magma that penetrated the contemporaneous flows as dikes, and in
some cases, perhaps, are the material filling the conduits which connected
some of the flows with the magma mass from which the}" came. In one
instance a frag-ment of a p-reenstone was found included in a somewhat
difi"erent greenstone. The fragment was identified as being similar to, and
presumably derived from, one of the greenstones of the complex. The
intrusives included in the Archean greenstone complex belong to the same
period of formation as the lava flows with which they are associated. These
intrusive rocks are of essentially the same mineralogic and chemical com-
position as the volcanics themselves.
It remains to be stated that we recognize the possibility, and, indeed,
the great probability, that there have been included in the areas mapped
as underlain by this Archean complex an occasional intrusive rock
considerably younger than the Ely greenstone proper. These greenstones
are cut by a number of dikes of relatively recent age and yet of essentially
the same character as the greenstones, except that they are less metamor-
phosed. No doubt many others were unrecognized, and, indeed, were
altogether unseen on account of poor exposures.
COIS^TACT METAMORPHISM OF ELY GREEIN^STOKE.
CONTACT EFFECT OF GRANITE ON THE ELY GREENSTONE.
In the preceding pages general statements have been made concerning
changes which the greenstones have undergone since they were formed
In addition to those mentioned, which were essentially changes brought
about as the result of ordinary mountain-making forces and of percolating
«The origin of the Archean greenstones of Minnesota: Geol. and Nat. Hist. Survey of Minnesota,
Twenty-third Ann. Eept., 189.5, pp. 4-24.
156 THE VERMILION IRON-BEARING DISTRICT.
waters, other changes of a far-reaching- character have taken place in them.
In all of the instances which will be cited the chief agent of metamor-
phism appears to have been the contact action of certain intrusive acid
rocks. It is not for a moment to be supposed, however, that the
metamorphism of the rocks should be ascribed solely to the action of these
intrusives; yet this is the most obvious cause, and probably the final
controlling cause.
In the course of the field work on the Vermilion district, it was noticed,
when the exposures of greenstone possessing the general characters ah-eady
outlined for that rock were studied, that a great number of them were cut
by dikes of acid rock, and that these dikes were of varying size. It was
further observed that near the central portion of the district these dikes
were relatively few, but that as the southern and northern limits were
approached they gradually increased in number until the greenstones were
in places literally permeated by dikes of acid rock. On continuing farther
from the central part of the district the main body of the granite was in
every case finally reached. When this body was reached, however, it was
found to contain occasional masses of Archean rocks of varying size, which
were practically surrounded by and thus included in the granite. The
relations are clearly those of intrusion, a younger acid rock being' intrvided
into and including fragments from the older Ely greenstone. In brief,
the relations are the same as those which exist between the batholiths of
granite and the contiguous greenstones of Rainy Lake" and Lake of the
Woods, and which have been so clearly described by Lawson. It was
further noted that this intrusion was accompanied by a marked change in
the character of the Archean complex. Where the granite dikes are few,
the characters of the greenstone formation remain essentially unchanged.
When the dikes have become numerous, however, the greenstones are
altered to amphibolitic and to a less extent to micaceous rocks, usually of
somewhat darker color than the normal greenstones. The main macro-
scopic characters are ])ractically unchanged. Thus, for example, in these
amphibolitic rocks one can still recognize the characteristic ellipsoidal and
amygdaloidal structure of the greenstones. A splendid exposure of these
hornblendic rocks can be seen in the southeast quarter of sec. 3, and
the northeast quarter of sec. 10, T. 61 N., R. 14 W. These rocks, while
"Report on the geology of the Rainy Lake region: Geol. Nat. Hist. Survey Canada, 1S89, F.
ELY GREENSTONE. 157
possessing on the whole a massive structure, nevertheless have, as it were, an
incipient fissility, or cleavage, which has been produced by the process of
recrystallization thi'ough which they have gone, as the result of which there
has been a production of amphibole needles and chlorite flakes, and also a
general tendency toward a parallel arrangement of the needles and flakes.
The secondary feldspar has also been affected in its crystallization, and
aids in emphasizing the parallel structure. This parallelism has developed
a fissility which is not sufficiently marked to warrant their designation as
schists. They merely split more readily in one direction than in another.
When the contact between the iiiain granite masses and the Archean
greenstone is approached, the Archean rocks are usually found to have lost
all of their characteristic features and to have been recrystallized into
amphibolitic schists and gneisses which very rarely retain any recognizable
greenstone character. The gradation is, however, so gradual and the steps
can be followed so clearly in the field that after a field inspection no doubt
as to the con-ectness of the above conclusions can remain in the mind of
any close and impartial observer.
MINERALOGIC COMPOSITION OF THE METAMORPHOSED ROCKS.
These amphibole-schists and mica-schists, derived from the green-
stones, consist of the following constituents in varying proportions: Com-
mon green hornblende, actinolite, biotite, muscovite, chlorite, epidote,
calcite, sphene, quartz, feldspar, pyrite, and magnetite. The mica is pres-
ent in very small quantity and is always associated with amphibole. It is
only occasionally that the mica occurs in such quantity that the rock can
be referred to as a mica schist. Banding is very commonly present,
as the concentration of some of the darker minerals was greater in certain
portions than in the areas immediately adjacent.
The origin of these metamorphosed greenstones, now schistose amphi-
bolitic rocks, is such as would be expected from their distribution and from
their relationship to the granites. They always lie between the normal
greenstones and the granites, occupying a belt of varying width, which
it is impossible in the field to delimit sharply. This zone of schists has
therefore been only approximately indicated on the maps.
The presence of these amphibolitic schists adjacent to the granite has
been noticed by nearly every observer who has been in this district. On
158
THE VERMILION IRON-BEARING DISTRICT.
a manuscript map by Irving- this belt of schists is duThued. Special
attention has been called to them by both A. and X. H. Winchell in
tlieir published reports. The earliest explanation oflPered was that of
A. Wincliell," who studied the good exposures of these schists upon Burnt-
side Lake and there found them, as has been described, permeated by the
granite. Figs. 1 and 2, from his report, illusti-ate the occurrence. His
conclusion was that they were derived from gray wackes by metaraoi-phism. '
Fig. 1. — Reijroduction of steteh by A. Winchell, shoivlng the intricate relationship between the granite of Burntside Lake
and the umphibole-schists.
N. H. Winchell " refers to this belt of schists, and concludes that they
have been "produced bv the granitic intrusions or by the force which
accompanied tliem," and that when acid clastic rocks were affected the
mica-schists were pr.oduced, and when the basic greenstone was involved
the amphibole-schists ^veve produced. T(i these rocks Winchell applied
the name Coutchiching, using- it in tlie sense proposed by Lawson.'' In his
"Geol. and Nat. Hist. Survey of Minnesota, Fifteenth Ann. Eept., 1887, pp. 40-41.
''Geol. and Nat. Hi.st. Survey of iMinnesota, Fifteenth Ann. Kept., 18S7, pp. 172-178. Ueol. and
Nat. Hist. Survey of Jlinnosota, Final Kept., Vol. IV, 1899, p. 246.
cQeol. and Nat. Hist. Survey of Minnesota, Final Kept., Vol. IV, 1899, pp. 272, 273, and 283.
'' Report on the geology of the Rainy Lake region: Geol. and Nat. Hist. Survey of Canada, 1889,
F. pp. 21-3.5 et seq. Geology of the Rainy Lake region: Am. Jour. Sci., .Sd series, Vol. XXXIII,
1887. p. 477.
ELY GREENSTONE.
159
general statement in the preface of the final volume of the Minnesota survey,
Winchell abandoned the use of the term Coutchiching, for his studies
showed that he could not in this district include any definite series there-
Fig. 2. — Reproduction of sketch by A. Winchell, showing the intricate relationship between the granite of Burntside
Lake and the amphibole-schists.
160 THE VERMILION IRON -BEARING DISTRICT.
under". The rocks that were first included under this term can be shown
to l^e to a large extent formed by metamorphism from the Ely gi-een-
stones as above described. The remaining portion has been formed by
metamorphism of sediments of Lower Huronian age, as described in
Chapter IV. Should Lawson's name Coutchiching be applied to the
amphibole- and mica-schists lying between the granites of the district and
the rocks that have been intruded by the granite, we should have included
under this term two series of rocks which, though possessing the same
schistose characters, are demonstrably of different age, both as regards
their initial period of formation and their period of metamorphism.
The use of- the name Coutchiching is not wan-anted in connection with the
rocks of the Vermilion district of Minnesota, and Lawson's insistence '' on the
presence of a series of rocks in this district comparable to his supposed
Coutchiching series. is explainable only as due to his unfamiliarity with the
district.
The character of the metamorphism involved in the change of the
greenstone of the Archean from a massive rock to a predominantly schistose
rock of a different mineralogic character might be made a matter of question
by some who wish to classify metamorphic rocks into those produced by
contact action and those produced by regional metamorphism. The ag'ents,
however, in both cases are the same. Thej- are heat, pressure, and water,
and whether these agents owe their activity to the intrusion of an igneous
mass of rock or to orogenic movement is merely a matter of detail. In
the present instance the field relations of the greenstones to the metamor-
phic rocks and the granite show that the metamorphism of the greenstones
accompanied the intrusion of the granite. Hence, as this was the prime
agent in their 'production, they hav6 been classed under contact metamor-
phic products. Yet while these schistose rocks may well have been pro-
duced by the intrusion of igneous masses that caused recrystallization of
their already partiall}- altered original minerals, nevertheless essentially the
same chemical constituents are present in them now as were present in
them formerly. The rocks have merely been recrystallized under pressure.
"Geol. and Nat. Hist. Survey of Minnesota, Final Rept., Vol. IV, 1S99, pp. 14 and 15.
''Geol. and Nat. Hi.-^t. i^arvey of >Iinne.«ota, V)y N. H. Winohell; Final Rept., Vol. IV. Review-
by A. C. Lawsiin, Am. .Toiir. Sci., 4th series, Vol. IX, 1900, p. 1.51.
ELY GREENSTONE. 161
This has resulted in the formation of minerals with higher specific gravity" —
hence such as occupy less space — and produced an arrangement of these
minerals which makes them conform to the law of the production of
secondary minerals in schists, as explained by Leith.'' Moreover, the great
additional space within this portion of the earth's crust which was required
by the intrusion of the granite has been partly supplied by this very change
of the preexisting greenstones into related rocks that occupy less volume.
In their formation, pressure has, of course, been a very important factor;
hence the more frequent occurrence among them of schistose forms of
rocks.
CONTACT EFFECT OF GABBRO ON ELY GREENSTONE.
In three areas the Archean EI3" greenstone lies in juxtaposition with
gabbro of Keweenawan age. • A contact of the greenstone with the gabbro
occurs east of Disappointment Lake, in sees. 26 and 35, T. 64 N., R. 8 W.
Here the anticline of Ely greenstone has been cut across on its east side by
the gabbro. Another contact occurs in sees. 1 and 2, T. 64 N., R. 6 W., at
the southwest side of Grobbemichigamma Lake. From sec. 25, T. 65 N.,
R. 5 W., eastward through sees. 30, 29, 28, and 27, T. 65 N., R. 4 W., we
find an Archean anticline which is not in contact with the gabbro, being
separated from it by a minimum distance of perhaps 20() paces and a
maximum distance of half a mile. The greenstone has been metamorphosed
by the gabbro, although it has not been affected nearl}^ so extensively as it
is where it is in contact with the granite. Macroscopically no great
difi"erence can be observed between the metamorphosed and the
unmetamorphosed greenstones. They are in all cases massive rocks,
and the metamorphosed portions appear to have essentially the same '
characters as the remaining unmetamorphosed portions, although the
former weather somewhat more readily than the latter and have a
rusty brown color.
The effect of the gabbro on the greenstone in producing
metamorphosed rocks can be best seen in exposures on the west side
of Gobbemichigamma Lake in the sections above mentioned, on
« Metamorphism of rocks and rock flovvage, by C. R. Van Hise: Bull. Geol. Soc. Am., Vol. IX,
1897, p. 291.
6 Manuscript.
MON XLV — 03 11
162 THE VERMILION IKON-BEARING DISTRICT.
the south flank of the Twin Peaks ridg-e. Here the texture of the
greenstone is characteristically ophitic, a secondary hornblende taking
the place of the original pyroxene. When metamorphosed by the galjbro
we find the ophitic texture perfectly preserved with, however, a large
quantity of biotite as a secondary pi'oduct. This biotite has accumulated
around the edges of the hornblende between the hornblende and the feld-
spar, and is especially concentrated along evident shearing planes where
normally — that is, in the greenstone unaft'eeted b}^ the gabbro — one would
find a large amount of chlorite derived from the hornblende. With rocks
like the above there is associated anotlier, showing the ophitic texture
poorly preserved and with brownish-green, massive hornblende constituting
most of the rock, and with hyperstheue occurring in more or less porphyritic
areas. This hypersthene is very fresh and seems to be a product of the
action of the gabbro on the greenstone. In other cases ophitic textured
greenstones seem to contain a very much larger amount of a brownish-
green hornblende and magnetite than these green.stones normally contain,
and in this instance the large quantity of magnetite, and possibly also the
brown hornblende, is assumed to be due to the action of the gabbro. In
general, there are produced from the greenstone, by metamorphism of the
gabbro, rocks which contain a large percentage of biotite and varying
quantities of hypersthene and magnetite. As a result of their mineralogic
character such rocks have a rusty-brown color, and the texture, although
distinctly ophitic, is inclined to become granulai' as the new minerals
increase in quantity. These rocks disintegrate much more readily than do
the greenstones.
RELATION OF EliT GREENST01S1E TO ADJACEXT FORISIATIONS,
The relations of the gi-eenstone complex to the adjacent formations
have already been briefly stated, but will be recapitulated. Wherever the
gi'eenstone complex is in contact with any sediments all the larger masses
lie above and are infolded in it. When these sedimentaries are normal
clastic deposits the}^ lie above and contain numerous fragments of the green-
stones, showing that the gi-eenstone complex is the older formation. When
the gi'eenstones lie next to other igneous rocks they are found to be
penetrated by them. Hence all of the relations of the greenstone complex
to the various adjacent formations prove its greater age. A detailed
ELY GREENSTONE. 163
description of some of the contacts of the various formations of the district
with the greenstones, in which their relations to one another will be given,
will be found under the discussion of these formations.
ECONOMIC VAIiUE OF THE ELY GREENSTONE.
The Ely greenstone rocks of the Vermilion district are suitable for
building stones, especially for foundations, for which the unworked stones
can be used. They are very tough, and there is hardly sufficient demand
for stonework to warrant their being cut and used for the superstructure
of buildings, even if their color were suitable. Their color is, however,
uniformly so dark that they would rarely be used for other portions of
buildings than foundations and trimmings. This stone is eminently adapted
for use as road material, as there is an inexhaustible supply, and it is gener-
ally so distributed that it can be obtained for use on existing roads at small
cost.
Occasionally the discovery of bodies of magnetite ore in the green-
stones is announced. The greenstone contains large quantities of magnetite
as an essential constituent. This occurs, however, disseminated through
the rock in very small particles, which make up an exceedingly small
percentage of the total mass. While the occurrence of the ore bodies
reported has in no case been verified, it would not be at all surprising should
iron-oxide bodies, of very limited extent, however, really be found. The
explanation of their occurrence would be similar to that of the occurrence of
almost identical iron-oxide masses in the gabbro ; that is, they are the result
of processes of segregation from the basic magma. It is not believed, how-
ever, that any such bodies that may be found would prove to be of commer-
cial value. The iron oxide occurring in minute quantities scattered through
the greenstone contains titanium — it is a titaniferous magnetite — and the
probability is that any ore bodies found in this greenstone would likewise
consist of titaniferous magnetite. They would then correspond in . their
chemical composition, as well as in their mode of origin, to the ore bodies in
the gabbro. Moreover, since the processes of liquation and fractional crys-
tallization, as the result of which such bodies are formed, would be most
fully carried out in those cases where the rocks remain under essentially
the same conditions of temperature and pressure for a great length of time,
we should naturally expect to find the largest bodies of oxide in the coarsest-
grained rocks. Hence, continuing the comparison of tlie bodies of oi'e
164 THE ^'ERMILION IRON-BEARING DISTRICT.
which one would be likely to find in the Archean greenstones with those in
the adjacent Keweenawan gabbro, we see that the}' would probably be
very much smaller than those in the gabbro, since the greenstones with
which they would occur are of much finer grain than the gabbro. More-
over, as the size of the mass of magma has an important bearing upon the
rate of cooling, we may say that the larger the mass of magma the larger
the ore body. For this reason also we should expect to find smaller bodies
of oxides in the greenstone than in the gabbro.
In many portions of the world very important ore bodies containing
other metals than iron are found associated with rocks of essentially the
same composition as those forming the greenstone complex. The question
may well be asked, What are the chances of finding silver, nickel, and
cobalt ores, to mention some of the most important, in association with
these greenstones? I would answer that there is practically no chance.
In other regions the ores mentioned occur as contact deposits which owe
their occurrence to the intrusion of rocks allied to these greenstones into
younger rocks, the deposits being found in fissures occurring within the
younger rock, within the older i-ock, partially in both, or along the contact
between the two. Although these greenstones cover a broad area, yet,
since they are themselves the oldest rocks, we can not expect to find such
deposits in them in very large quantity unless they occur within the
greenstones themselves as the product of processes of segregation — pro-
cesses which, as has been intimated, may have given origin to certain iron-
ore deposits reported to occur in them, but whose existence remains
unverified. Winchell refers to the occurrence of a gold-bearing quartz
vein in the following words:"
At the west end of Long Lake, SW. i SW. i sec. 30, T. 63 [N., R.] 13 [W.]. is a
conspicuous display of quartz and granite, the former carrying gold. An average
sample selected from the dump, assayed by F. F. Sharpless, gave §8.64. Some casual
working has been done on this vein, and numerous assays show, according to the
statement of Mr. Mcintosh, one of the owners, an average of over $10 per ton. The
vein is traceable about an eighth of a mile, a little north of east, with an irregular
width reaching a maximum of about SO feet. It accompanies a granite dike. The
ore is not abundant, but is in irregular streaks in the quartz.
Thus far no gold-bearing veins which have paid Tor the working of
them have Ijeen found in the Vermilion district.
«N. H. Winchell, Geol. and Nat Hist. Survey of Minnesota, Final Rept., Vol. IV, 1899, p. 258.
ELY GREENSTONE. 165
INTERESTING LOCALITIES.
Under this heading there will be found special descriptions of certain
localities where the rocks show especially well some of the characters
already described, or for other reasons are considered worthy of more
detailed mention than has been made of them in ^Jreceding pages. These
descriptions of localities may not perhaps be read by the general reader,
but it is hoped may be useful to future students of the geology of the
district who may wish to verify the statements herein made, and who would
therefore desire to visit some of these places.
Some of the best places at which the general characters of the Ely
greenstone may be studied, are on the hills near Ely. These hills are
very nearly bare, and numerous exposures of the greenstone may be found
on them. The ellipsoidal parting is well shown in exposures in the cut
on the south side of the railroad track west of the station, and can be
seen in numerous places on the bare hills between Ely and Long Lake.
Ajuygdaloidal structure is also very commonly present. No spherulites
were observed here, although these are abundant on the high hills due
north of Ely, on the north side of Long Lake. On the hill west of the
town and south of the water tank ellipsoidal parting, with peripherally
arranged amygdules, may be observed, and at one place not very far from
the road leading up to the cemetery the transition from the ellipsoidally
parted portion into the nonellipsoidal greenstone can be seen. The
greenstones on this hill are cut by dikes of granite-porphyry. Just
northeast of the Methodist church on the east side of Ely numerous basic
dikes are found cutting the greenstones. About a mile and a half south
of Ely is a bare ridge, bordered on the north and south by considerable
depressions, on which there are many exposures that show the amygdaloidal
and ellipsoidal characters of the greenstones. Irregular lines, which seem
to represent flowage lines, run through these rocks in many places. This
ridge offers a fairly good place for the study of the volcanic characters of
the greenstones. It must be noted, however, that the greenstones in this
ridge have been extremely altered and in many places are more or less
completely schistose, and would now be spoken of as amphibole-schists.
This alteration is due to the intrusion of the Giants Range granite, which is
^present in these rocks in numerous dikes, and which occurs in mass a
166 THE VERMILION IRON-BEARING DISTRICT.
short distance to the south. These exposures, therefore, besides offering
opportunity for a study of the general characters of the greenstone, are
very favorable for a study of the metamorphism of the greenstone into the
amphibole-mica-schists, which, as seen in isolated exposures, in many cases
offer little evidence of their derivation from the greenstones.
POSSIBLE TUFFS ASSOCIATED WITH THE GREENSTONES.
Reference has already been made to the fact that, associated with the
greenstone, we not uncommonly find masses of tuffaceous-looking rocks,
which, since they show no characters clearly indicative of sedimentation,
have been included with the greenstones as interbedded tuff deposits.
Included in this category are deposits occumng at the following places:
North 200 paces, west 1,950 paces from southeast corner sec. 17, T.
62 N., R 13 W.
North 600 paces, west 1,000 paces from southeast corner sec. 30, T.
63 N., R. 11 W.
North 1,930 paces, west 1,000 paces from southeast corner sec. 3, T.
63 N., R. 13 W.
North 2,000 paces from southeast corner sec. 3, T. 63 N., R. 13 "W.
On east shore of large lake in T. 62 N., R. 14 W., just south of the
east-west section line between sees. 25 and 36.
Another area is that occurring 1,650 paces north of southeast comer
sec. 20, T. 63 N., R. 10 W. Here the tuffaceous rock has been sheared
and where most schistose, with the schistosity striking N. 70° E., black
jasper has been infiltrated.
EVIDENCES OF VOLCANIC CHARACTER.
Beginning about 1,500 paces north of the southeast corner sec. 3, T.
62 N., R. 12 W., and extending south along the section line to the
quarter post, there are numerous exposures of dark-gray to green rocks
which have irregular lines running through them, and possess a more
or less perfectly preserved amygdaloidal and ellipsoidal structure. The
lines referred to are thought to be flowage lines. These, in connection
with the other structures mentioned, seem to be fair proof of the A'olcanic
character of the rocks. These volcanics are penetrated by dikes of granite
which vary in size from very small ones an inch or more in width to some
haviug a width that is measurable by yards. The rocks themselves are
ELY GREENSTONE. 167
impei-fectly schistose, and may well be called ampliibole- and mica- schists.
The rocks in this locality represent one of the passage phases between the
normal g-reenstones on the one hand and the ampliibole- and mica-schists
on the other, which, when close to the main mass of the granite, show
none of the volcanic structures that enable then- original character to be
easily determined here. This passage from the normal greenstones
through the amygdaloidal and ellipsoidal green schists to the normal schists
next to the granite can be seen still better just north of the quarter post
between sec. 19, T. 62 N., R. 12 W., and sec. 24, T. 62 N., R. 13 W.,
and also along the quarter line in the sou-theast quarter of sec. 24, T. 62
N., R. 13 W. At this last locality we pass from the granite into an area
in which the schists and granites are most intricately mixed. The schists
occasionally still possess an imperfect amygdaloidal structure. To the
north we soon pass from very schistose greenstones to those which are
onl}^ slightly schistose and in which amygdaloidal and ellipsoidal structures
are well developed.
The ellipsoidal structure and the presence of spherulites, found most
frequently in association with these ellipsoids, have been referred to as
common features of the greenstones. Greenstones possessing both of these
characters occur very commonly in large exposures throug-hout sec. 10
and the west half of sec. 11, T. 63 N., R. 10 W. They can be very clearly
seen at a number of the exposures here, especially on one about 200
paces north and 1,000 paces west of the southeast corner of sec. 10,
T. 63 N., R. 10 W. Here the greenstone is separated into large ellipsoids
and the spherulites are arranged in concentric circles within the ellip-
soids. The smallest spherulites occur near the periphery of the ellipsoids;
the largest, 3 inches in diameter, occur nearer the center. These large
spherulites show their radial structure in sections on the weathered
surfaces of the rock. Some of them now consist of a chloritic mineral
having a dark-green color and a silky luster. In most cases they are
lighter colored, and the mineral constituting them is feldspar. The inter-
ference of the spherulites with one another in the process of growth is
very prettily shown in many places. Very rarely are they perfectly
round. In most cases they have interfered with each other, and while in
some places nearly perfect spherulites may be observed, they are most
commonlv irregularly rounded and surrounded by others which have the
168 THE VERMILION IRON-BEARING DISTRICT.
form of segments of circles. It is evident that they did not all begin to
form at just the same time or else their rate of growth was not just the
same, for if their origin were simultaneous and their rate of growth equal
we would get in cross section through such masses a structure resembling
that of a honeycomb. These spherulitie greenstones are both fine and
coarse grained, the ellipsoidal and spherulitie portions being continuous
with the nonellipsoidal and nonspherulitic greenstones. It would seem
that the ellipsoidal and spherulitie portions of the greenstones represent the
surface of greenstone lavas which are to be considered as effusive sheets or
flows. These greenstones are cut by dikes of granite-porphja-y.
One mile north of North Twin Lake, at the northeast corner of sec. 12,
T. 63 N., R. 10 ^Y., these ellipsoidal spheruhtic greenstones continue from
south of Jasper Lake in almost continuous exposures eastward along the
south section line of sec. 6, T. 63 N., R. 9 W., almost as far east as the south
quarter post of that section. Here the ellipsoids reach a diameter of 3 to
4 feet and some of them are solid masses of spherulites, each spherulite
showing its radial structure very beautifully on the weathered surface.
About 400 paces north, 100 paces west from the southeast corner of
sec. 35, T. 62 N., R. 14 W., interbedded amygdaloidal and ellipsoidal basalts
occur. They are all somewhat schistose. As we continue southward
studying the exposures, we find that they show an increasing degree of
metamorphism. They finally become amphibole- and mica-schists and
gneisses, and but for the presence of the elongated ellipsoids and the
amvgdules, filled with chlorite and pinkish quartz, one could not be sure,
from the field study, of theii- igneous origin. The rocks in this locality
resemble in a striking degree the crystalline schists occurring in the
vicinity of Bone Lake", in the Crystal Falls district of Michigan.
It has been stated repeatedly that certain of the greenstones possess
an amygdaloidal structure, and that with these tuffs are associated. These
facts have been cited as evidence of the volcanic nature of the greenstones.
Such associated and presumably interbedded tuifs and amygdaloidal and
porphyritic greenstones are very well exposed upon the west and north-
west slopes of a high hill in the northwest quarter of sec. 19, T. 64 N.,
R. 10 W. Here tlie greenstones, which are both fine and coarse grained,
"The Crystal Falls ■iron-bearing district of Michigan: Mon. U. S. Geol. Survey Vol. XXX VI,
1899, pp. 148-152.
U. S. GEOLOGICAL SURVEY
MONOGRAPH XLV PL. V
m;
(^)
A. AMYGDALOIDAL GREENSTONE (M ETABASALT).
B. MAGNETITIC CHERT, SHOWING POSSIBLE LINES OF FALSE BEDDING.
The bands of brilliant red jasper commonly show lines similar to these, but without the irregularities indicative of false bedding.
ELY GREENSTONE. 169
are dotted with abundant amygdules. In some instances amygdules are
scattered over large areas. In other instances they are collected chiefly
along certain lines which run about northeast. The individual lava flows
could not be distinguished. The amygdules are now (ival, with the long
axes of the ovals parallel with the schistositv of the rocks, wliich trends
N. 60° E. The amygdules range from very small ones to others which
are 3 inches long and three-fourths of an inch across the shortest diameter.
The accompanying illustration, PI. Y, .4, is a reproduction of the polished
surface of one of these metamorphosed amygdaloidal basalts. The rock
is now an amphibole-schist, and but for the presence of the amygdules
and its associations in the field it would not be recognized as a basic lava.
These greenstones are cut by dikes of granite as well as by narrow dikes
of basic rock.
The ellipsoidally parted greenstones are well exposed in the west half
of sec. 17, east half of sec. 18, northeast quarter of sec. 19, and north-
west quarter of sec. 20, T. 63 N., R. 9 W. The ellipsoidal parting and
amygdaloidal characters of these rocks show clearly their identity with
the Ely greenstone in other parts of the district. The ellipsoids vary in
size considerably, and the matrix between them varies from one-half inch
to 2 inches in thickness. This matrix has been silicified to a consider-
able extent, and in places appears very much like a black chert. The
rocks have been extensively metamorphosed. This metamorphism is prob-
abl}' due largely to the effects of the Keweenawan gabbro, which at
present is separated fi'om the greenstone by the width of the Kawishiwi
River. Formerly, however, the gabbro unquestionably overlaid the green-
stone to the north of the river. The greenstone is cut by a number of
dikes of basic rocks varying in width from a few inches to 30 feet. The
dikes are dolerite and camptonite (f).
METAIVIORPHISM OF THE GREENSTONES.
The greenstones of the Vermilion district have been extremely
metamorphosed by the intrusion of younger rocks, as well as — perhaps
chiefly — by the intense folding to which they have been subjected. The
oldest rocks, the Ely gi-eeustones, have naturally been most metamor-
phosed, since they have been subjected not only to all of the folding
which affected the younger rocks, but to previous metamorphic action.
170 THE VERMILION IRON-BEARING DISTRICT.
The following description of an area on the portage between Wind and
Moose lakes makes clear the difficulty which one experiences in attempt-
ing" to discriminate between these o'reenstones and some of the rocks that
are associated with and derived from them. The rock on the north side
of the high ridge overlooking Wind Lake is undoubtedly a green schist
derived from the Ely greenstone. It is broken by diagonal joints running
in two directions, lines bisecting the acute angles of the rhomboids formed
by the intersection of these joints being parallel to the strike. The minute
diagonal fractures have been largely cemented by some material, as is
shown by ridges that run along the weathered surfaces. The rock is veined
with white quartz, but there is no definite banding except that produced
by this material which has filled the fractures. There is no appearance
whatever of pebbles in the rock. The schistosity is A-ery strongly marked.
Along the schistosity there are numerous fine quartz veins and some
fractures filled by other materials, so that the rock shows a fairly distinct
banding parallel ti;> the schistosity.
South of this green schist there is a peculiar rock ■\^'nicll shows ver}'
fine banding along the schistosity, as does the green schist abo^'e described.
This banding does not seem to be due to secondary cementation, but appar-
ently results from the mashing of originally heterogeneous material. On
weathered surfaces of this rock that lie transverse to the schistosity there are
obscure roundish or elliptical spots having their long directions parallel
with the trend of the schistosit)'. These roundish spots are presumably
mashed pebbles. They are very numerous, and increase in distinctness as
one passes southward from the contact, and within a distance of 30 t)r 40
feet the rock assumes a distinctly conglomeratic apj^earance. When the
rock is split the pebble-like masses are seen to be g-reatly extended in
the direction of the schistosity. These masses are several times longer
in the direction of the dip than along the strike, and from two to ten times
as broad (along the strike) as they are thick. In these respects tiie rock
is identical with the remarkable schist conglomerates of Vermont described
by Hitchcock" in 1860.
In the conglomeratic rock at Wind Lake the regular system of fractures
spoken of as occurring in the mass first described — the green schist adja-
cent to the conglomerate — are not present, at least with any such regularity,
"(ienlojry of Veniinnt, Vol. I, lS(il, iip. 28-42.
ELY GREENSTONE. 171
although there are irregular diagonal fractures wider apart. It seems as if
there had been distributire movement about each pebble-like area, so that
in this wa}^ readjustment occurred largely in the softer matrix-like material
rather than in the regular fashion shown in the homogeneous rock to the
north. These facts lead to the conclusion that the southern part of
the ledge is decomposed, much mashed, detrital material derived from and
resting upon homogeneous schistose material. Here it is rather difficult to
draw the dividing line between the Ely greenstone and the conglomerate
derived from it. At other places, where the mashing has not been so
extensive and where the pebbles are far more distinctive, and especially
where the rock contains pebbles of jasper and granite as well as of green-
stone, the line of separation can be much more readily drawn.
Indeed, a little farther southeast, along this same trail — that is, on the
Wind Lake-Moose Lake trail — a perfectly distinct conglomerate occurs
which consists predominantly of greenstone j^ebbles and bowlders, some as
large as 3 feet in diameter, but which contains many associated granite
pebbles. From the repetition of different zones of conglomerate occurring
on this portage it would seem that the rocks had been Yerj intricately
folded here and that the several different zones are reall}- a single zone
repeated as a result of the close folding.
In the southeast quarter of sec. 18, T. 62 N., R, 13 W., exposures are
pretty numerous, and one finds here good opportunities for studying the
transition from the greenstone, with ellipsoidal parting here and there, to
the amphibole-mica-schists. These schists occur in their best, most typical
development in close proximity to the Giants Range granite, which, as has
already been stated in detail elsewhere, is considered to be the cause of
their existence. In all such cases the farther one goes from the contact the
less altered are the greenstones found to be, whereas the nearer the contact
the more intricately are the greenstones cut by the granite dikes, and the
more nearly they assume the characters of typical schists. The same
observations, pointing toward the production of the schists from the green-
stones by the intrusion of the granite, may be made at many places along
the boundary between the granite and the greenstone, extending from
sec. 31 northeastward through sees. 32 and 29 into sec. 28, all in T. 62 N.,
R. 13 W. At numerous places along this line the hornblende-schists still
possess the ellipsoidal and less commonly the amygdaloidal characters
172 THE VERMILION IRON-BEARING DISTRICT.
of the unaltered greenstones. In some places the vesicles are half an
inch in diameter, and their character is undoubted. Most commonly these
are filled, and the amygxlules are very prominent, though in other cases
the amvgdules have been weathered out. Not infrequently faint lines
of somewhat lighter color than the main mass of the rock may be observed
crossing the exposures diagonally to the schistosity. These lines seem to
be most common where the amygdaloidal structure is most noticeable, and
they appear to be the traces of flow structm-e in the lavas.
In the southeast quarter of sec. 16, T. 64 N., R. 9 W., just north-
west of the southwestern end of Newfound Lake, there is a high hill,
bare over a great portion of its surface, consisting of the Ely greenstone
cut through and through by the granite of Basswood Lake. In some places
the two rocks are present in about equal amount, though in general the
greenstone predominates. The greenstone has been so much metamoi'phosed
by the granite that it would be more accurately classed as an amphibole-
schist. In many places the structural peculiarities of the Ely greenstone
may be observed. This hill affords a good opportunity for study of the
relations between the granite and the schist, and especially for obser-s-ing
the general characters of the amphibole-schists. Moreover, the fact, already
stated, that the greenstone does not consist of a single rock, liut of a com-
plex, is clearly shown by the presence of a coarse, much metamorphosed
dike of dolerite which cuts the schist and includes it and which in turn is
cut by the younger granite.
SECTION 111 —SOUDAN FORMATION.
The Soudan formation, a division of the Archean, lies above and is
mainly younger tlian the Ely greenstone. It contains the important iron-
ore deposits of the district and is well developed and exposed at the town
of Soudan, where are located also some of the most important mines of
the district.
OCCURRENCE AND CHARACTER.
DISTRIBUTION.
The iron-bearing formation begins 10 miles east of the western limit of
the district as outlined in this report, and caia be traced eastward for many
miles, the easternmost occurrence seen being a very limited exposure south
of Moose Lake, in sec. 4, T. 63 N., R 9 W. However, the same formation
SOUDAN FORMATION. 173
occurs just north of the boundary in Ontario, and is known to continue
northeastward for many miles within Canadian territory.
The Soudan formation has its greatest development in the western part
of the district, the most prominent areas extending from Tower, on Ver-
milion Lake, in T. 62 N., R. 15 W., on the west, to Fall and Garden lakes,
in T. 63 N., R. 11 W., just a few miles east of the well-known town of Ely,
on the east.
The formation is most notably exposed in areas lying about midway
between the north and south limits of the district. At Tower and Soudan
it underlies broad areas and forms the prominent topographic features
known as Tower, Lee, and Soudan hills, and Chester, or Jasper, Peak.
Other fairly large areas occur in a belt just north and east of Ely, in sec.
25, T. 63 N., R. 12 W., and in sec. 30, T. 63 N., R. 11 W. North and south
of and between the areas mentioned, the formation underlies rather narrow
belts trending east-northeast to west-southwest. Each of these belts is made
up of a series of narrow bands of the iron formation, interbedded in some
cases with small quantities of fragmental rocks and intimately associated
with the Ely greenstone and the late intrusives, which cut through both
the Ely greenstone and the Soudan formation. As shown on the maps
in the accompanying atlas, some of these belts, especially those near the
center of the western part of the district, can be followed for a number of
miles east and west; one was traced for 16 miles. Others are very much
shorter, having been traced for only a few miles, and these small areas
grade down to those which are mere patches, a few inches or feet across
and a few feet or paces in extent — that is, along the strike. However, it is
believed that all of these, from the largest to the smallest, with the excep-
tion possibly of certain small vein-like masses which will be mentioned
later, are parts of one general formation, now separated from one another
by folding and erosion.
A glance at the maps will show that the areal distribution of the
Soudan formation is closely connected with that of the Ely greenstone.
EXPOSURES.
One would be inclined to think, judging from the resistant nature of
the rocks constituting the iron formation, that it would be well exposed
throughout the district. Such is, however, very far from the case. Except
in a few places, notably at Tower, Lee, and Soudan hills and Jasper Peak;
174 THE VERMILION IRON-BEARING DISTRICT.
in sec. 25, T. 63 N., R 12 W.; sec. 30, T. 63 N., R. 11 W.; .sees. 3 and
4,T. 61 N., R. 15 W.; sees. 7 and 8, T. 62 N., R. 14 W.; sec. 6, T. 62 N;,
R. 14 W., and sec. 1, T. 62 N., R. 15 W.; the exposures are very poor. But
the fact that the exposures of the iron formation are scarce and small in some
localities does not necessarily mean that the formation at .such places is not
now or may not become in the future of very great economic importance.
For example, the immensely valuable iron deposits at Ely, extending fi-om
the Chandler mine on the west to the Savoy on the east, occur where jasper
exposures are remarkably few.
In the belts traced through the district the exposures are small and
discontinuous, both along and across the strike, and this would make it
impossible to trace out any horizons in the iron-bearing formation, even if
they could be determined, but, owing to the uniformity of the formation,
such horizons can not be fixed. In making use of the accompanying maps
it should be clearly understood that the colors or patterns indicate merely
that the iron formation has been found in the areas so colored. The limits
fixed do not necessarily imply that the area is underlain wholly by the
formation, for, as has ah'eady been intimated above, in many instances
exposures of greenstone, equally as numerous and as large, occur in these
belts in intimate association with the jaspers. With these occur also
younger intrusives, which cut through them. In fact, it is impracticable to
say which of these two kinds of rock preponderates in many of such areas.
The iron-bearing formation certainly does preponderate in a number of the
well-known areas which will at once occur to those acquainted with the
district — for example, on Tower and Lee and Soudan hills, Jasper Peak,
ridge in sec. 25, T. 63 N., R. 12 W., and ridge in sec. 30, T. 63 N., R. 11 W.
The above statement will hold true, however, on the whole, for the smaller
belts. The belts outlined represent the possible ore-bearing areas, and
such areas having once been outlined as closely as possible by the geolo-
gist, it then remains for the mining companies to make more detailed studies
of them than it was possible for the members of the Sm-vey to make in the
limited time at their disposal.
In the course of the field work the occurrence of the iron formation
has been reported from various localities, but search failed to reveal expo-
sures in these places. It is highly possible that in the future other areas
SOUDAN FORMATION. 175
than those which are outhned on the accompanying maps will be found,
but it can be confidently stated that they will in all cases be small and
presumably of very sliglit importance.
TOPOGRAPHY. ,
The amount of the iron formation in the district is relatively so small
that it can scarcely be said to have had any great effect upon the general
topography. In those areas where it is best developed it does influence
the topography very materially. As the result of the resistant character
of the jasper, which is the predominant rock in the formation, strongly
marked hills persist where it is present in large quantity. Of these, the
most striking are Lee and Tower hills, Jasper or Chester Peak, the hill
forming the prominent northeast point of Stuntz Bay, and the hills in sec.
7, T. 62 N., R 14 W., and in sec. 4, T. 61 N., R 14 W.; also the prominent
ridge extending through sec. 25, T. 63 N., R 12 W., and sec. 30, T. 63 N.,
R. 11 W. In the various belts containing the iron formation the jasper
very commonly occupies minor prominences, the low ground between being
occupied presumably by the associated greenstones and sediments.
STRUCTURE.
The iron-bearing Sondan formation being the oldest sedimentary
formation in the district has been subjected to all of the orogenic move-
ments which have occuri'ed in the district since its deposition. Since there
were several of these movements, and since the forces producing them
were verv intense, the formation has been most intricately folded. It is
indeed difficult to describe or represent the intricacy of the folding which
it exhibits npon nearly every exposure of any size.
On a large scale the formation has been folded into anticlines and
synclines, and its structure is now shown to a certain extent by the topog-
raphy. Thus, for example, the prominent hills — Lee, Tower, and Soudan —
are great anticlines with minor synclines and anticlines s;iperimposed upon
them, whereas Jasper Peak is situated at the end of a syncline, and on its
western and southern sides shows very prettily the jasper folded into a
series of rolls pitching a little to the east of north. It is also highly prob-
able that some of the east- west trending belt's of the iron formation are to
be considered as synclines of jasper infolded in the older greenstone.
Upon the more prominent anticlines and synclines numerous minor folds
176 THE VERMILION IRON-BEARING DISTRICT.
are superimposed, giving the intricac}" of structure already referred ti^. It
is of interest to note in connection A\-itli this remarkably close folding of
the jasper — some of the bands are actually folded upon themselves within
a radius scarcely greater than the width of the belt — that the jasper for the
most part has not been very much fractured. This is very clearly indica-
tive of the great depth at which this formation lay at the time the folding-
took place. This close folding' without fracture can be explained only by
assuming that the rocks were under such great pressure that they acted
practically as ^jlastic bodies. North of Fall Lake the close folding of the
jasper is shown in one place where bands 4 to 5 inches in width have been
turned so sharply that the two ends are now only 1 foot apai't, and here
the jasper, usually considered a very brittle substance, shows no indica-
tions of fractures, but has comported itself as a viscous material.
PI. VI, A and B, reproduced from sketches made by W. N. MeiTiam in
the field, from actual exposiu-es near Soudan, shows very well the extreme
intricacy of the folding.
Both the longitudinal and the cross folding of the iron formation is
composite; that is, superimposed upon the major folds in each direction are
folds of the second order, and upon these are folds of the third order, and so
on down to minute plications. The pressure has been so great as to give all
variety of minor folds, including isoclinal and fan shaped. Moreover, these
varieties of folds may be seen almost equally well on a ground plan or on
a vertical cross section. The}- are beautifully shown at various places
about Tower and Ely, but perhaps the most extraordinarily complex folding-
seen is that at the west end of the large island in the east part of Emerald
Lake. Figs. A and B of PL VI, which are upon the whole representative
of the district, show that the folding, notwithstanding the extremely brittle
character of the rock, was accomplished without major fracture. The
deformation, therefore, is deformation in the zone of rock flowage, and no
better instance is known to the writer of this kind of earth movement.
Frequently a solid belt of jasper is bent back upon itself within a radius
of its own width Avith no sign of fracture.
Though the folding is so complex as to give even fan-shaped folds, the
turns are ordinarily round rather than angled, thus differing from those
acute-angled folds frequently seen in the Menominee district. The round-
ness of the folds is well illusfrnted in the figures.
U. S GEOLOGICAL SURVEY
MONOGRAPH X LV PL VI
rA)
ilEN & CO.LlTM.N 1
FOLDED AND B RECCIATED JASPER OFTHE SOUDAN FORMATION.
SOUDAN FORMATION. 177
The Vermilion district, therefore, appears to be one of the best regions
in the world to illustrate complex folds, or folding in two directions at right
angles to each other, and the formation that best exhibits this folding- is
the Soudan. This is due to the very marked banding of that formation,
by means of which the position of bedding is readily determined, and to
the fact that for the most part it does not take on any secondary structure.
Furthermore, it frequently is found in contact with the Ely greenstone,
which also gives the pitch of the cross folds.
This remarkably complex folding partly explains the distribution of
the Soudan formation with reference to the Ely greenstone. Naturally,
where the formation is thick it is found along the border of the greenstone.
However, since upon the major folds are superimposed secondarj^ and
tertiarj^ folds, numerous patches of the jasper occur in the greenstone.
Moreover, because of the cross folding, these patches may be very narrow
at one place, widen out very rapidly so as to make a thick formation, and
again narrow. When the extraordinary complexity of this folding is
understood one has only to premise an erosion extending to different
depths in the Soudan formation before the Lower Huronian was deposited
in order to see how in the greenstone the jasper may range in size or extent
from patches a few feet in width and length, to the great continuous forma-
tion about Tower and Ely. Moreover, such premise fully explains the
extraordinary variation in width of the jasper belts at some places and their
persistency and uniformity at others.
Occasionally there is associated with the iron formation and inter-
banded with the jasper some bands of slaty material. In places the
amount of this slaty material is so great that where folding has • taken
place a slaty cleavage has developed in these layers. This cleavage,
however, does not pass through the bands of ii'on oxide or chert. These
bands with the slaty cleavage afford excellent oj^portunities for makino-
observations upon the relations of cleavage to the direction of pressure.
In these bands this development of slaty cleavage is seen to obey the laws
of slaty cleavage, as explained by Van Hise." PI. VII, a representation of
a specimen taken from the folded jaspers, shows this cleavage so clearly
that textual explanation is scarcely needed.
"Principles of North American pre-Cambrian geology, by C. R. Van Hise: Sixteenth Ann.
Kept. U. S. Geol. Survey, Pt. I, 1896, pp. 363-369.
MON XLV — 03 12
178 THE VERMILION IRON-BEARING DISTRICT.
Even wliere the bands of slaty material in the rocks are not more than
one-fourth of an inch across, the slaty cleavage is perfectly developed and
stops abniptly at the adjacent more biittle cherty material. Thus we have
in tills phase of the iron formation numerous layers showing good slaty
cleavage alternating with others in which it is absent. The slaty cleavage
is in such ])osition in reference to the plications as to show that it developed
normally to the pressm-e. The lack of parallelism of the cleavage upon
opposite sides of the folds beautifully illustrates the pnnciple that on
anticlines the cleavage on opposite sides of folds diverges downward and on
synclines converges downward. These alternating slate and jasper bands
are well shown in the so-called "Burnt Forties" adjacent to Vermilion Lake.
While usually deformation has taken place without fracture, the jasjjer
is sometimes brecciated. We sometimes find very pretty "reibungs" or
friction breccia formed of the jasper fragments cemented together by vein
quartz. Not uncommonly such a brecciated zone occurs near the base of
the iron formation, between it and the lower -lying greenstones, and is thus
clearly the result of movement along the plane separating the two kinds of
rocks. In such instances, the jasper, being the more brittle of the two
rocks, forms the angular to parti}' rounded fragments of the breccia, whereas
the greenstone, in some cases at least, is found to have been forced in
between the jasper fragments and to play the part of a matrix cementing
tlie l)reccia- together (PI. VI, C). The plane of brecciation being more open,
has been es})ecially favorable for the free movement of underground water.
Similar brecciated zones at the base of the jasper — that is, between it and
the greenstones — due to movements along this plane, occur near the west
end of Emerald Lake, just north of the international boundary, on the
point that projects eastward from the south shore of this lake. Consequent
upon this lirecciation there has been infiltration of various substances,
especially of quartz and iron oxide subsequent to the formation of the
breccia, which also tends to cement the fragments together and likewise to
tllseolor the rock.
On the east end of Lee Hill, on the south side of the old North Lee
mine, there is a brecciated zone in which the above-mentioned conditions
can be observed. It is further ver}' noticeable here — and the same thing
may be seen at otliei- places — that the fragments are frequently cemented
together by very pure hematite, and when there were favorable cavities of
sufficient size subordinate bodies of very high-grade ore were deposited.
U. 8. GEOLOGICAL SURVEY
MONOGRAPH XLV PL. VII
FOLDED JASPER AND SLATE, SHOWING SLATY CLEAVAGE DEVELOPED IN SLATE BANDS.
SOUDAN FORMATION. 179
In spite of the intricacy of the folding of the iron formation, it has been
possible to determine that in general the axes of the major folds strike east-
northeast to west-southwest, the clearest instance of such a large fold beina:
the syncline at Ely. The dip of the axial plane of these folds appears
almost without exception to be steep to the north, indicating that the close-
ness of the folding has been very great and overturns are common results
of this. It is interesting to find that this axial plane lias been subject to
torsional movement, as in the case of the plane of the Ely syncline. This
upon the west end dips to the north, but underground explorations at the
east end show that it has here a reversed dip to the south.
PETROGRAPHIC CHARACTERS.
MACROSCOPIC CHARACTERS OF THE FRAGMENTAL PORTION OF THE SOUDAN FORMATION.
The Soudan formation may be divided into a fragmental series of
sediments and into the iron formation jDroper, whose origin is not distinctly
clastic.
The clastic portion of the formation will be described first, for the
reason that it always underlies the iron formation j^roper. The very few
occurrences of these clastic sediments are so widely separated from each
other areally that it is impossible to say that they all belong to the same
beds, although they everywhere bear the same relation to the iron forma-
tion. And in this connection it must be borne in mind that there is a
possibility that the different bands of the iron formation are not of exactly
the same age, but represent stages of deposition of slightly different age,
although all of the same general period. The sediments of the clastic
division of the iron formation are grayish green and black in color, and
consist of a conglomerate at the base, grading upward into the finer-grained
deposits. The conglomerate lies next to the greenstone, and consists — both
matrix and pebbles — of the material derived therefrom and some pebbles
of vein quartz. The finer sediments are chiefly finer material of the same
character, derived from the same source. The exception to this statement
would be certain soft, black, graphitic slates which are found associated with
the jaspers. Such may be observed, for example, upon the westernmost
exposm-es of Lee Hill, just back — that is, north — of the houses of Tower.
A somewhat similar graphitic slate is found on the southern slope of Soudan
Hill, about 200 yards northeast of No. 12 shaft, which is northwest of the
180 THE VERMILION IRON-BEARING DISTRICT.
Minnesota Iron Company's warehouse. The presence of these graphitic
slates with the iron foi'mation probably accounts for the large masses of
gi-aphitic rock on the twelfth level in No. 8 shaft at Soudan. There is a
mass of this graphitic rock 10 feet long and from 6 to 8 feet thick
completely lying in the soap rock. It was cut in a third dimension for 11
feet, and how much farther it may extend in this direction is unknown.
This graphitic rock was tested, it is said, by Mr. John H. Eby, sometime
mining engineer of the Minnesota Iron Company, who reported it as
graphite.
These clastic sediments are interbedded with the jasper and other
materials constituting the iron formation proper. Toward the iron
formation the bands apparently become more frequent, and the clastic
sediments decrease in amoimt, and there is thus a gradual transition into
the iron formation proper.
The lower clastic portion of the fonnation is l^y no means characteristic.
It is rarely present, and when present is A-ery thin. In one place about
40 feet of sediments, chiefly conglomerate, were seen, but the exposures
were so poor that it was impossible to tell whether the beds were duplicated
bv folding or not. It is possible that some fairly wide areas separating
jasper exposm-es from greenstone exposures may be underlain by the
clastic formation, but this is not probable, for tlie practical absence of the
formation, winch is fairly resistant tlu'oughout the district, shows that it
must have been very subordinate. Yet, in spite of its subordinate position
quantitively, this clastic portion of the formation is of great stratigraphic
importance, as no matter at how few places it has been found or how thin
it may be, it nevertheless is clear proof of a very important change in
conditions, marking the transition from the period of volcanic acti^^ity in
which the greenstone had its origin to the period of sedimentary deposition '
in which the Soudan formation was laid down.
MICROSCOPIC CHARACTERS OF THE FRAGMENTAL PORTION OF THE SOUDAN
FORMATION.
The conglomerate and noiTnal fine-grained sediments, belonging for the
most part below the iron-bearing formation proper, show nothing under the
microscope which is worthy of detailed description. The conglomerates
are clearly recognizable in the field as elastics, and luider the microscope
SOUDAN FORMATION. 181
one can distinguisli the extremely altered greenstone fragments and the
matrix derived from the greenstones, both consisting now largely of
actinolite, chlorite, and epidote, with quartz. As the sediments get finer
the clastic characters disappear as the result of the extreme alteration, and
one can only surmise the mode of origin of these rocks by their intimate
association with and gradations from the coarse elastics, and, when no
gradation is visible, by the presence of banding and false-bedding lines.
The microscopic examination shows these sediments to be made up of
chlorite, actinolite, epidote, sericite, sphene, quartz, carbonaceous material
(graphite), and some iron oxides, in various proportions, so that these
metamorphosed slates vary from absolutely black, greasv-feeling, graphitic
slates to dark-green and fairly light greenish-gray rocks. The graphitic
slates consist essentially of graphite and quartz in exceedingly fine grains
and in some cases in very small quantity. In one specimen the place of
the quartz seemed to be taken by feldspar, which is altered to sericite, so
that the sediment consists of graphite and altered feldspar.
MACROSCOPIC CHAEACTERS OF THE IRON-BEARING FORMATION PROPER.
The iron-bearing formation proper (that is, that portion in which the
ore bodies occur) consists of cherts of various colors — green, white,
yellow, black, and purplish — red jasper, carbonate-bearing chert, slaty rock,
showing in some cases intimate association with the clastic formation proper,
griinerite-magnetite-schist, hematite, magnetite, and some pyrite. To the
formation as a whole the miners and prospectors apply the name "jasper,"
although only a portion of it falls strictly under this designation. These
various kinds of rock occur in bands of varying- thickness, rarely exceeding
5 or 6 inches, and commonly in extremely thin laminaj. Usually the
individual bands appear to be homogeneous. Occasionally there is a
banding within the bands, which is due to the arrangement of the mineral
constituents. In one place such a banding simulated the false bedding
of normal elastic sediments (PI. V, 5).
The alternate bands of material of different color combined with the
complicated folding make the formation a very striking object, which on
exposures almost always attracts the attention of the traveler, even if he is
not accustomed to closely noticing rocks.
182 THE VERMILION IRON-BEARING DISTRICT.
The hematite, besides being interbanded with the other materials, also
occurs very frequently in masses of variable size and shape. These consti-
tute the ore deposits of the district, and will be considered under a separate
head. The bands are not arranged in any definite order, but alternate with
one another, giving a very regular ribbon or banded structure. All the
colored cherts have the white as a base, the difference in color being due
chiefly to the presence of the iron, either in the form of magnetite, hematite,
or limonite, or as a combination of these. The black cherts, frequently
called black jasper or hungry jasper, always contain a large quantity of
magnetite, to which they owe their color. The brilliant-red jasper owes its
color, as is well known, to the thin transparent plates and minute specks
of blood-red hematite. The color of the brown cherts is due to the limonite.
The colors of the other varieties — gray, brown, and ocherous yellow — depend
on the mixtures of the above oxides or of their alteration products. Of
somewhat rarer occurrence is the slightly greenish and grayish chert, which,
although subordinate in quantity, is important in reference to the genesis
of the iron-bearing formation. This chert contains a considerable amount
of iron carbonate and griinerite, to which it owes its color. These chert
bands become yellow and brown on weathering, on account of the for-
mation of ocher by the decomposition of the carbonate and griinerite. The
hematite and magnetite bands associated with the cherts are very rarely
pure. On examining them it will be found in almost every case that the
hematite bands contain varying percentages of magnetite, and vice versa.
With these of course one is always sure to find a variable quantity of
quartz. Iron pyrite is mixed with these various rocks in small amounts,
but it is not known to occur in large quantity in this district.
The most intimate relationship exists between the various above-
mentioned members of the iron-bearing formation proper. Gradual transi-
tion from one into another may be traced. Near the west end of Tower
Hill, in following the strike of the rocks, one finds the jasper becoming less
brilliantly colored and grading with continuous exposures into the black
magnetitic jasper. The iron formation has been folded, and as a consequence
is traversed by more or less frequent fractures. These fractures have been
filled by veins of quartz which run transverse to the banding in the iron
formation. The smaller cracks have very commonly been filled by iron
oxide that is in all respects identical with that occurring interbanded with
SOUDAN FORMATION. 183
the jaspers. The secondary nature of the oxide that fills the cracks is
indisputably shown by this occurrence, and is strongly indicative of the
secondary origin of that in the bands, the two probably being of contem-
porary formation. In studying the formation it was noted that two series of
cracks had been formed in the jaspers, the older having been filled with
vein quartz and the younger with hematite.
THE IKOS ORBS.
The uj^per part of the Soudan formation is in a strict sense the ore-
bearing portion. Indeed, this is the iron-bearing formation which has given
to the Vermilion district its great economic importance, since from this have
been derived great quantities of the high-grade ore which has assisted
materially in making the Lake Superior region the greatest single factor in
the development of the iron industry of the United States and of the world.
The ores of the Vermilion district comprise several varieties — massive,
graniilar hematite, specular hematite, and insignificant amounts of mag-
netite and limonite. There are, of course, also all kinds of mixtures of
these, showing gradational phases from one variety to the other.
The predominant ore is an exceedingly hard, massive, granular, steel-
blue hematite. The specular ore occurs locall}'^ in small masses. The
magnetite is obtained only in small quantities, and is intimately associated
with the hematite. Occasionally small bodies of magnetite ore are found,
not large enough to be of special value, or to warrant an attempt to obtain
a grade of magnetite ore. Such occurrences are very exceptional. The
■ limonite is very subordinate, occurring only associated with the hematite.
There seems to be a general misapprehension as to the character of the ore
in the Chandler mine at Ely, the greatest producer of the district. It is
very commonly spoken of as a soft ore. This is, however, purely a relative
term, in this case depending upon the brecciated condition of the ore, which
enables it to be won with less drilling and with much less expenditure
of high explosives than is required, for instance, in the Minnesota Iron
Company's mine at Tower. The ore is found in an extremely brecciated
condition by the miners, and this brecciation is taken advantage of and
really increased by the method of mining employed. As a result of this
more or less finely brecciated condition the ore is obtained to a great
184 THE VERMILION IRON-BEARING DISTRICT.
extent by the use of picks. The fragments of tlie breccia are, however, the
same hard ore that occurs in the other mines of the district, but as a result
of the brecciation, a great deal of very finely comminuted ore is associated
with the larger fragments and occurs between them. The cause of the
brecciated condition of this ore will be discussed more at length under the
heading " Ore deposits."
The ores of the district are rendered impure by various mechanical
mixtures of quartz, calcite, chlorite, iron pyrites, native copper, the oxide
of copper (cuprite), and the carbonates (malachite and azurite). These
copper ores are present in very small quantity, however, and are of chief
interest on account of the fact that this occui-rence of these minerals in
association with the ores at Soudan is the first recorded from the Lake
Superior region." Mr. Pengilly informed the writer that native copper had
been found in the Chandler mine several years prior to its known occur-
rence at Soudan. Quartz, calcite, chlorite, and pyrite occur locally, but in
considerable quantity, and as a result large quantities of ore are thrown
away in the attempt to get rid of these impurities. Good hand specimens
showing these minerals can always be obtained from the dump piles and
even the stock piles of the Soudan mines. The minerals occur along the
walls of vugs of various sizes which exist in the ore bodies.
The iron content of the Vermilion iron ores, computed froin cargo
analyses made during 1899, varies from 60.47 to 67.37 per cent, and
averages about 63.7 per cent. The phosphorus content varies from 0.04 to
0.131 per cent, and averages about 0.057 per cent. The silica content
varies from 2.55 to 7.67 per cent, and averages 4.78 per cent. The water
content varies from 1.04 to 7.956 per cent, and averages about 5.50 per
cent. The ore bodies are of such importance that their origin, the
occurrence of the ore in them, etc., will be considered in detail under
separate headings.
The i)hysical character of the ores is such as to make them much desired
by the smelters for admixture with the softer, finer-grained ores. The ores
are hard and are obtained in large pieces and run tln-ough crushers and
"The occurrence of copper minerals in hematite ore, Montana mine, Soudan, Minnesota; descrip-
tion of the occurrence, tiy J. H. Kby; study of tlie minerals, by V. P. Berkey: Trans. Lake Superior
Mining Institute, Vol. IV, IS'JG, pji. G9-79.
SOUDAN FORMATION.
185
broken. The following table, made by Mr. R. B. Green, in 1898, at the
Minnesota Iron Company laboratory at Two Harbors, Minn., shows the
coarseness of the Chandler and Pioneer ores of Ely. Ores from Soudan
are, perhaps, even coarser.
Percentage of ore from Chandler and Pioneer mines, Ely, Minn., that passes through
screens of specified mesh per inch.
[Determined in natural state after taking from cars and drying.]
Kind of ore.
Chandlei' ore (28 cargoes)
Long Lake ore, Chandler mine (13 cargoes)
Pioneer ore (23 cargoes)
Pilot ore, Chandler mine (2 cargoes)
Does not pass
8 meshes.
8 meshes to
20 meshes.
20 meshes to
100 meshes.
82.05
9.14
5.86
76.99
10.94
8.08
77.50
11.00
7. 08 i
65.65
13. 93
11.44
100 meshea.
2.94
3.98
4.42
8.97
MICROSCOPIC CHARACTERS OF THE IROX-BEARING FORMATION PROPER.
The iron-bearing formation proper consists of the various colored
cherts, griinerite-magnetite-schists, hematite, magnetite, and limonite. With
the iron-bearing formation proper, but in quantity very subordinate to the
cherts, jaspers, and iron oxide, there occur some greenish to gray slates, and
also graphitic slates. Upon close examination under the microscojDe these
do not show evidence of their clastic character. False bedding would per-
haps indicate their clastic origin, but no other evidence of this has been
found other than their similarity to the slates above mentioned, which grade
into the recog'nized elastics.
The white chert, in combination with various minerals which color it,
forms the bases of the colored varieties of cherts already enumerated on
page 181. When pure the white chert consists of quartz varying in size
of grain from that which is minutely crystalline to that which is somewhat
more coarsely crystalline. The grains are polygonal and generally more
or less roundish. In one case, in a relatively coarse-grained chert, three
quartz grains were observed which showed a roundish core outlined by
a film of iron oxide, and beyond the iron oxide a zone of clear quartz.
This secondary enlargement might possibly be taken as evidence of the
clastic origin of the grains, but this structure is not sufficient evidence
of such origin, and even if it were considered sufiicient proof for the
grains it would not be sufficient evidence to prove this origin for the
186 THE VERMILION IRON-BEARING DISTRICT.
suiTOUiidiug cherts. A more or less perfect false bedding' observed in
some of these cherts (PI. V, B) might also be considered the result of
water motion. But this would not prove the cherts to be of mechanical
origin, as we find this structure in sediments of organic origin also. The
fine grains of quartz constituting the chert contain scattered through them
minute crystals and specks of magnetite and hematite and areas of limonite.
In some places these crystals are very small — mere dust as it were ; in others
they are of considerable size. At some places the quartz grains will con-
tain very few of these dust specks; at others they are nearly full of tliem
and appear almost opaque. The crystals of the iron oxides may occur at
any and all places in these quartz grains, from the center to the periphery,
and when the crystals are large they not infrequently extend from one
quartz grain to another, running across the junction of the grains. In some
cases these polygonal g-raius are outlined very distinctly by iron oxide,
occurring either as a mere film or as a layer of considerable thickness.
Instances were observed where the grains of quartz in the chert were
heavily impregnated with particles of iron ore on the periphery, leaving but
a small, fairly clear center. Other instances were observed where there was
less of the clear central quartz present, and, in fact, there seemed to be all
gradations from these cases up to those in which there was an opaque mass
of ore giving but an occasional indication of the presence of quartz.
These facts seem to show that the ore in these rocks is not primary, but
is a secondary product which has been accumulated either in bands or in
irregular masses as the result of the replacement of silica by iron oxide.
There is one variety of the chert which is interesting-, for it seems to
give a clue to the siliceous rock which has been replaced by the ore. This
variety is the gi-eenish carbonate-bearing chert to which reference has
already been made. Under the microscope such cherts show up as finely
granular aggregates of silica in normal rounded polygonal g-rains, but asso-
ciated with the silica grains is a carbonate which occurs in rounded rhombo-
hedra. The rounding of these grains is not the result of mechanical action.
A study of the slides shows the alteration of the carbonate to limonite. A
further change, resulting from dehydration, would produce a hematite-
bearing chert, and in cases where the oxidation of the carbonate took
place with access of insufficient amount of oxygen there would be produced
magnetitic chert. In rocks containing a large propoi'tion of carbonate and
SOUDAN FORMATION. 187
a relatively small proportion of chert we might thus get as a result of the
alteration of the carbonate a ferruginous rock, possibly with alternate bands
rich and poor in iron. These presumably served as a nucleus from which,
by replacement, were derived the more ferruginous bands and ore deposits.
Detailed descriptions of these processes have been given elsewhere by Van
Hise," and will not be discussed in tliis place. The presence of the limonite,
hematite, and magnetite in the cherts gives us the varieties of the feri'uginous
chert, as it is commonly called — the red jasper and the black or lean,
hungry jasper, respectively.
Associated with these cherts and ores are the griinerite-magnetite
rocks. These are not present in large quantity. In places we find a green
rock consisting essentially of chert with griinerite and but few crystals of
magnetite; at other places there are rocks in which magnetite is the essential
constituent with but little griinerite; and all gradations between exist.
This griinerite is very nearly of the composition of the hydrated ferrous
silicate which is so abundant in and forms such a conspicuous part of the
iron-bearing rocks of the Mesabi range. This material has been described
in detail in the monograph on this range by C. K. Leith.'' Altered forms
of this same material occur in the iron-bearing Gunflint formation at the
eastern end of the Vermilion district. The griinerite may very well have
been derived from this material by a simple process of dehydration, or it
may have been produced from an iron carbonate by silicification, as it was
in the Marquette district, as described by Van Hise." Indeed, since, as is
concluded later, on page 191, iron carbonate was the original rock of the
iron-bearing formation, it is presumed that the griinerite was for the most
part formed by the silicification of the carbonate.
In those rocks in which the griinerite occurs no traces have been found
of the peculiar oval and globxilar structures so characteristic of the Biwabik
and Gunflint rocks. The absence of these rounded bodies is not, however,
conclusive evidence that they did not originally exist. These rocks have
a The Penokee iron-bearing series of Michigan and AVisconsin, by C. R. Van Hise; Mon. U. S.
Geol. Survey Vol. XIX, 1892, p. 283 et seq. The Marquette iron-bearing series of Michigan, by C. R.
Van Hise; Mon. IT. S. Geol. Survey Vol. XXVIII, 1897, p. 402.
6 The Mesabi iron-bearing district of Minnesota, by Charles Kenneth Leith: Mon. U. S. Geol.
Survey Vol. XLIII, 1903, p. 101 et seq.
'The Marquette iron-bearing series of Michigan, by C. R. Van Hise: Mon. U. S. Geol. Survey
Vol. XXVIII, 1897, p. 367.
188 THE VERMILION IRON-BEARING DISTRICT.
been extremely altered, and with the recrystallization of the elements there
may have taken place complete destruction of the original structui'es, with
the exception of banding, which is still evident, and, in fact, this may have
been further emphasized by the rearrangement of the constituents.
ORIGIX.
The banded structure of the iron-bearing formation is exceedingly
regular throughout the district. It is difficult to say how persistent tlie
individual bands are, however, for the outcrops do not allow them to be
traced over very long distances. Most of the bands seen at a particular
exposure persist entirely across the exposed surface. A few, however,
run out to a feather edge even on small exposures, and thus disappear.
The 251'esumption is that all the bands feather out within a shorter or
longer distance. This banding is so well marked and so eminently
chai'acteristic that from its presence alone one is fully warranted in making
the statement that the structure of the formation is essentially that of a
sedimentary rock. Furthermore, the composition. and textxire of the rocks
making up the formation are such that we can assert with confidence that
none of the members are of igneous origin. It has alread}^ been stated
that in places the iron formation proper — that is, the interbanded iron
oxides, cherts, and jaspers — overlie conformably a series of clastic rocks,
beginning at the bottom Avitli a conglomerate and grading upward into
finer material, and also that we find similar fine clastic material interbanded
with the iron oxides, cherts, and jaspers. This clearly indicates that the
rocks of the iron formation are of sedimentary origin. The presumption
is that the elastics were first formed; that there was then a period with
changing conditions, during which tlie slates and iron-formation rocks were
interbedded, and that finally the conditions controlling the deposition of
rocks of the iron-bearing formation became more persistent when the orig-
inal rocks of the iron-bearing formation were deposited. This enables us
to explain certain characteristics of the contact between the green-
stone and the iron formation which, in view of the known relations
of these rocks, could not otherwise be explained. At a point 2,000 ^jaces
east, 700 j^aces south of the southeast corner of sec. 17, T. (i2 N., R. 13
W., there is an exposure of jasper on the south side of massive green-
stone. At this exposure there seeins to be a gradation of the greenstone
SOUDAN FORMATION. 189
into file jasper. On the nortli side of the exposm-e the greenstone is
massive, bnt as it nears the jasper it becomes more and more schistose, and
with this increasing- schistosity there is a more or less imperfect banding,
brought about b}' an alternation of bands of the greenstone discolored with
iron with those which are not so colored. Farther south the true jasper
appears. Here apparently is a fine-grained mechanical sediment derived
from and immediately overlying' the greenstone, a deposit which is
essentiallv indistinguishable from the greenstone so far as its composition is
concerned, its mode of origin being indicated by the imperfect banding' and
the presence of feiTuginous material. Here there is no well-marked clastic
deposit between the greenstone and the iron formation, and this is one of
the localities where the quiet conditions controlling the deposition of the
iron formation had already set in, while in other parts of the district
elastics were being formed. For instance, in sec. 10, T. 63 N., R. 10 W.,
west of North Twin Lakes, bands of the jasper lie parallel to the strike of
the edge of the greenstone in contact with it, as though the banded
rocks were a sedimentary deposit laid down upon it as a base. But no
clastic sediments were found in this case between the jasper and the green-
stone, their absence being due probably to the fact that here, likewise,
the conditions were those of quiet deposition, during which no elastics
could be formed. From the fact that the rocks of the iron formation are
conformable with the clastic sediments near their contact one might be
inclined to infer that the iron-formation rocks also are of mechanical
origin. But microscopic examination, however, shows that the cherts and
jaspers and ores do not possess the minute textvires indicative of mechanical
sediments, although one case has been noted in which three possibly clastic
quartz grains were associated with the chert, thus showing' that the condi-
tions fluctuated from those suitable for the formation of clastic sediments
to those giving rise to organic sediments, as shown also by the microscopic
occurrence of interbanded elastics with the cherts.
In recapitulation it may be said that the banding' possessed by the
iron formation and the slat}" bands associated with it, which show true
sedimentary characters, and which were evidently originally of detrital
mud, give the clearest proof of the sedimentary origin of the iron formation
itself. The question which may next be raised concerning its mode of
origin is whether it could not have been a chemical deposit. N. H. Winchell
190 THE VEKMILION IRON-BEARING DISTRICT.
and H. V. Wiuchell, who have had excellent oppoi-tunities for studying
the Minnesota deposits, have so construed them." A condition which
would admit of the precipitation from a sea of a rock as acid as a
chert, consisting esseutiall}^ of pure silica, followed immediately by the
precipitation of rock as basic as the bands of pure iron ore, is so
anomalous as not to be tenable. The only explanation which seems most
nearly to meet and to answer the requirements of texture, structure,
and composition is that the rocks are now in a very different condition,
both chemically and physically, from what they were when originally
deposited. Their present condition may be interpreted as due to secondary
changes acting upon rocks of banded character.
The exact character of these original rocks is a question of much
moment. As the source of the iron-bearing rocks of the Mesabi range
Spurr has suggested an original glauconitic greensand, partly of foraminif-
eral origin. The green material, called by Spurr glauconite, has been
carefully studied by C. K. Leith, and has been found to be not glauconite
but a hydrous ferrous silicate without any potassium, and it has been called
"greenalite.'"* Microscopic study of the rocks from the Archean iron-
bearing formation of the Vermilion district has shown no evidence of the
former existence of such foraminiferal rocks or glauconitic greensands; nor,
indeed, has any rock been found Avhich can be proved to be the original
rock from whicli the ores and associated rocks have been produced. How-
ever, that kind of rock which approaches nearest to the supposed original
rock is the cherty iron carbonate forming a part of the iron formation.
Thi« now appears to represent a stage in the process of metamorphism
between the cherts and jaspers and ores on the one hand, and the relatively
pure iron carbonate on the other.
Tlie presence of this cherty iron carbonate in the Vermilion district, in
association with the other members of the iron-bearing formation, oifers also
a striking analogy between this district and those on the south shore of Lake
Superior." In the various monographs u^son the iron ranges in the United
States portion of the Lake Superior region. Professor Van Hise has presented
"Geol. and Nat. Hist. Survey of Minnesota, Final Rept., Vol. IV, 1899, p. 547; Geol. and Nat.
Hist. Survey of Minnesota, Bull. No. 6, 1891, pp. 105-111.
' ''The Mesabi iron-bearing district of Minnesota, by C. K. Leith: Mon. U. S. Geol. Survey
Vol. XLIII, 1903, p. 115.
''Tenth Ann. Rept. II. Si Geol. Survey, pp. 896-.'W7; Monofrraphs U. S. Geol. Survey Vols. XIX,
XXVHI, and XXXVI.
SOUDAN FOEMATION. 191
the proof in favor of his view that the cherty iron carbonate is the original
rock of the iron-bearing formations. In the Verraihon district tlie iron
carbonate is present in very small quantity, but it is significant that we
find siderite in large quantity in the ranges east of this but in line of its
strike, for instance near Port Arthur and in the Michipicoten district.
No proof of the supposition that a cherty iron carbonate is the original
rock of the Soudan formation has been found in the Vermilion district.
From the analogy of this with the other iron-beariug districts of the
region it seems most probable that in this, as in the districts above referred
to, the cherty iron carbonate was the chief original rock of the iron-bearing
i'ormation. In the monographs cited above, details are given which will
enable the reader to follow the various changes which, by leaching and
deposition, transform the cherty iron carbonate into chert, jasper, and ore.
While stress has been laid upon the formation of the jasper as the
result of secondary processes acting upon an originally ferruginous rock, it
must be stated that in certain places in the district the jasper doubtless
owes its origin to processes of secondary infiltration. Such jasper occurs
in vein-like form and in irregular bunches. It may occur also as the
cement of the brecciated greenstones, and is found occasionally lying
between the greenstone ellipsoids. The possibility that some of this
jasper may be due to secondary action must of course be admitted.
This is admitted, however, only for very small masses. These masses
were jjrobably formed by infiltration in openings in the greenstones, from
above, during the time that the overlying clierty iron carbonates were
being changed to the present condition of the iron-bearing formation. That
the infiltration is of relatively recent date in one particular instance is
shown by the presence of chert veins in an acid porphyry which cuts the
jasper and the greenstone north of Mud Creek Bay. Had iron been present
in this siliceous solution jasper with small masses of ore instead of the
veins of chert might very well have been formed.
REIiATIOlSrS OF SOUDAK FORMATIO?^ TO ADJACENT FORMATIONS.
RELATIONS TO THE ELY GREENSTONES.
From a scientific point of view one of the most interesting problems
confronted in the study of the Vermilion district is that of the relation of the
iron formation to the Ely greenstones. From an economic standpoint this
is also one of the most important problems. It was likewise most puzzling
192 THE VER:\I1LI0N IRON-BEARING DISTRICT.
and most difficult. It is believed that the relations between the gi-eenstones
and iron-bearing rocks have now been determined.
In approaching" the question we must bear in mind the fact that this
iron- bearing formation was probably at one time spread over nearly the
entire district with more or less uniformity. Ever since that time, however,
it has been subjected to mountain-making- forces and active processes of
erosion. The few areas of this formation which we now find are t>nly those
remnants of it which, being infolded in the underlying formation and
-thereby to a certaiii extent protected from erosion, have foi-tuuately
remained.
It has already been stated that most commonly the contacts between
the ffreenstone and the iron formation are wanting-. Still a sufficient
number of good contacts were found to leave no doubt whatever as to the
usual relation. Where not wanting the contact is usually exposed only
over a very small area; where favorable exposures have been found the
usual relations are such as are described in the following jmragraphs.
The jasper of the iron formation occurs in the greenstone in lenses of
varying size, ranging from 6 inches in width upward, the smaller ones
being very common. The larger ones are rarely exposed over their entire
surfaces, 3^id such partial exposures make it very difficult to trace the
various bands through the full length of the lenses, and it is practically
impossible to recog-nize the same bands at different places unless one can
trace them over the intervening- areas. About a quarter of a mile north of
the north shore of Fall Lake, in the NE. i of sec. 13, T. 63 N., R. 12 W.,
the jasper is found in the greenstone in narrow bands, from 4 to 5 inches
wide, which are bent sharply upon themselves, showing the extreme fold-
ing to which they have been subjected. Similar bands varying in width
from a few inches to 2 feet are found in the NE. \ of sec. 7, T. 63 N., R.
11 W., infolded in the amygdaloidal ellipsoidal greenstones, and there form
small synclines pitching east. South of Moose Lake, in sec. 4, T. 63 N.,
R. 9 W., on the narrow neck of land separating the two small lakes, the
infolding of the jasper in the gi-eenstone is well shown. In many excellent
exposures it is clear that the jasper overlies the greenstone. One especially
good exposure showing this relation very clearly can be seen on the north
side of Jas2)er Lake. Here the iron-bearing formation is in Canadian ter-
ritory, but is in direct continuation of and really in couuiectiou with the
SOUDAN FORMATION. 19
Q
Vermilion range of Minnesota. At this place the jasper stands with a clip
of about 85° west, with the greenstone both to the east and to the west of
it. One might be inclined to say, and with good reason from this part of the
exposure, that the jasper is included in or interbauded with the greenstone.
However, as we go over the end of the exposure, coming south down the
hill along the strike, it can be seen that this relation is due to the exceed-
ingly close infolding. The jasper is closely compressed and lies in a
syncline of greenstone, and in the inclined section through the jasper and
greenstone displayed on the hill slope, the greenstone can be seen to pass
down under the jasper, from both the east and the west sides. Indeed,
at one place a subordinate anticline of greenstone was observed projecting
up through the jasper, the jasper wrapping around it and dipping away
from it. On Otter Track Lake essentially. ,th,e same relations can be seen to
exist. Actual contact between the irqufbearing formation and the green-
stone was observed, but no fragmental material occurred between them.
Clearly, however, the jasper overlies the g-reenstone, having been deposited
upon this as a basement, for the bands run parallel with the contours of the
greenstone mass. In this particular instance the exposures are riot quite so
good as those upon Jasper Lake, but are still good enoug-h to enable one to
determine the relations with certaint}^. On Emerald and Big' Rock lakes,
both of which lie in Canadian territory, north of Knife Lake, there are
several exposures of jasper in intimate association with the greenstone.
All the jaspers of Emerald Lake, especially, are beautifully and curiously
folded. This folding is particularly well seen on the western edge of the
large island about a mile and a half west of the end of the lake. As one
rows along the shore he may see most intricately folded jasper bands which
closely resemble the jaspers on Otter Track Lake figured by H. V. Winchell
in the Minnesota reports. There are fan-shaped folds and curious inter-
loekiugs which would be almost incredible if not seen. The jasper,
although so rigid, has evidently obeyed the law of flowage by filling up
every chink and corner throughout the mass.' It is still somewhat question-
able how far this formation may have been folded before it was jasperized.
At this ledge upon the island the broad jasper bands were peen to be bent
around in a curve having a radius of from 2 to 4 inches, sections across
them giving a roundish surface, making' them appear almost like a series of
closely laid pipes.
MON XLV-03 13
194 THE VERMILION IRON-BEARING DISTRICT.
In the cases mentioued the iron-bearing- formation was found in imme-
diate contact with the greenstone at a great number of places. In all cases
the contact is sharp. There is no gradation from the greenstone through a
distinctly clastic transitional rock into the iron-bearing formation. The
field relations described are evidently such as could be produced only by
the closest infolding of a superimposed rock in the rock below it. This
intricate infolding is furthermore shown by the contorted chai'acter of the
banded iron formation.
The iroii formation is not confined to the normal relatively unmetaraor-
phosed greenstones, but is also found in those which have been metamor-
phosed by contact with the granite into the amphibole- and mica-gneisses,
as in the occurrence in the SE. \ of sec. 32, T. 62 N., R. 13 W., south of
Burntside Lake. In passing, it may be mentioned that the presence
of the iron formation in these amphibole-schists and gneisses is further
proof of the correctness of the statement already made that these are but
much metamorphosed greenstones, and essentially the same, at least so
far as their original condition is concerned, as the greenstones which are
not very far distant.
Reference has been made to the fact that the jasper at the contact
with the greenstone is sometimes brecciated. One especially clear case of
this is the occurrence on the south side of the North Lee pit. Here the
jasper fragments derived from the iron-bearing formation are, especially
near the greenstone, more or less completely surrounded by a matrix
of the adjacent greenstone, which evidently, under the pressure exerted
upon it, became more readily plastic than did the more brittle jasper. Com-
monly brecciation does not occur at the contact. The greenstone is usually
schistose along such contacts. This is illustrated 220 paces north of the
southeast corner of sec. 15, T. 62 N., R. 13 W. Here there is a bare roche
moutonnee, which consists for the most pai't of the very dense massive
greenstone. At one place there was found a belt of schistose greenstone
about 3 feet wide lying immediately next to the massive form above
mentioned. In this schistose greenstone there are two naiTOw bands of
east-west striking jasper, each about 4 inches in width. These were
separated from each other and from the massive greenstone by tlie schistose
form of the greenstone, whicli to the south as well as to the north grades
into the massive variety. It is hardly possilile to interpret this oci'iirrence as
SOUDAN FORMATION. 195
anytliino' else than a case of infolding. As the result of the accompanying
movement, the gi'eenstone was rendered schistose immediately adjacent to
the jasper and nearest to the plane between the rocks of diverse character
along which the greatest movement naturally took place, this schistosity
diminishing in degree away from the plane of greatest movement.
In several places in the district careful search made expressly therefor
has disclosed the greenstone with a conglomerate lying on it and con-
sisting of material derived from the greenstone. The conglomerate is
followed by finer-grained clastic sediments of essentially the same character
as the conglomerate itself, and these sediments are in turn succeeded by
the iron formation proper, consisting of the normal cherts, jasper, and iron
oxide in alternate bands. Occasionally a clastic band occurs with the iron
formation jjroper. Such definite relationships were observed at several
places, as in sec. 10, north of Armstrong Lake. Here the greenstone is
overlain by a conglomerate derived from it, which is succeeded to the north
by the iron formation, which contains a large quantity of iron pyrites. The
greenstone also contains larg-e quantities of the iron pyrites scattered
through it in crystals which liave to a great extent been changed to
limonite. Another locality is north of Robinsons Lake, just south of the
north quarter post of sec 7, T. 62 N., R. 13 W. A number of other places
were found in which, however, the relations were not quite so clear, the
complete sequence being interrupted by lack of exposures ; they are there-
fore not referred to specifically. The above-mentioned relations show
clearly that the main part of the iron formation rests upon the greenstone
as a basement, and consequently is younger than the greenstone.
But is all of the iron formation younger than all of the greenstones of
the Vermilion area'? Apparentl}^ not, but there is occasionally an inter-
bedding of the iron fonnation with some of the greenstone. The evidence
for this is found in the distribution of the iron formation in long belts
separated from one another by areas of varying width underlain by the
greenstones. These belts have been traced for various distances, in one
case for a distance of 16 miles. In no case is the iron-bearing formation
continuously exposed over such extent, but the exposures are, nevertheless,
so numerous as to show conclusively that the intervening areas without
exposures are underlain by the iron-bearing rocks. Associated with the
iron-formation rocks of these belts there is more or less greenstone. This
196 THE VERMILION IKON-BEARING DISTRICT.
is frequently found in contact with the iron-bearing rocks and tends to
separate the formation into a number of small belts whose continuity can
not be traced on account of the rarity of exposures. The greenstones
which occur in this iron-foriuation belt are of different kinds, and it is
supposed that they inay re^^resent flows of greenstone geologically con-
temporaneous with rocks of the iron formation. The exposm-es are so
isolated, however, that it is not feasible to coiinect them and separate the
rocks into ■ individual flows or sills. Furthermore, it is in these belts of the
iron-beai'ing rocks that the above-mentioned clastic rocks derived from the
greenstones and underlying conformably the iron-bearing formation have
been found. In view of these facts, we are led to believe that some of the
rocks of the iron-bearing formation, while resting upon the basement of
greenstones, are likewise overlain by greenstones, or, in other words, that
some of the iron fonnation is interbedded with greenstones which are of
volcanic origin.
Thus the clastic sedimentary deposits derived from and overlying the
lava flows may grade up into nonclastic sediments. The conditions of
sedimentation var}' from place to place in the area, hence we get a gradual
change from mechanical sediments to organic sediments (cherty ferruginous
carbonates). Where the conditions were not favorable for the formation of
the clastic sediments the nonclastic sediments were deposited without the
conglomerates and graj^wackes intervening between them and their igneous
rock basement. Hence we no^^' find them resting upon the greenstones with
a sharp line of demarcation between them, or at most with a narrow zone
of schistose gTeenstone intervening. These sediments were in their turn in
some cases buried by lava flows, which again at a later date were covered
by succeeding- sedimentary deposits. These processes continued through-
out a shorter or longer period. It is due to this fact that such intimate
relationship exists between the greenstones and the associated — in the main
youngei- — iron-bearing formation. As a result of this intimate relationship
it has been impossible to logically separate the two in a more marked way
than has been done in the above pages. AVhile of a distinct method of
origin, their formation took place within essentially the same period of time.
In general, however, it is possible to recognize the greenstone as the true
basement rock of the district, correlative with the Archean rocks of the
other Lake Superior iron-bearing districts.
SOUDAN FOKMATION. 197
^ Within the greenstone are areas of jasper of very irregular shape and
size which do not possess the regular banding seen in the jasper of the large
areas of the iron formation. Such small areas of jasper are not uncommon
in the northern half of sec. 21, T. 62 N., R. 14 W., from 1,150 to 1,260
paces north, 950 paces west of the southeast corner; again, in sees. 1, 2,
and 3, T. 61 N., R. 15 W. These jasper areas are of irregular shape and
appear to owe their origin to a process of infiltration similar to that which
forms veins. This is clearly the mode of origin of these irregular masses of
jasper which occur in the midst of the ellipsoidal greenstones, filling the
angular interstices between the ellipsoids (see page 139). Some of these
irregular masses may be remnants of the iron-bearing formation deposited
in irregularities of the underl^dng rocks, but this mode of origin could not
be proved for any occurrence. There is a further possibility that some of
these masses may be inclusions of the iron formation in the eruptive green-
stones. This explanation has been offered by H. V. Winchell and others
for a large part of the iron formation of this district and has been cited as
proof that the iron formation was older than the greenstones. However, as
already shown, the presence of the conglomerates clearly disproves this age
relationship for the greater part at least of the iron formation. Infolding
may as readily explain the intimate character of the relationship between
the greenstone and the iron formation as the suggested intrusive relationship.
For instance, when we find small isolated lenses of the iron formation lying
in the greeiistone, with the surface only exposed, or when, as not uncom-
monly happens, narrow bands of the formation are bounded on two sides
by the greenstone, their lateral extension being concealed in the other two
directions, the exposures are too imperfect to enable one to determine the
exact relation of each mass of the iron-bearing formation to the greenstone.
Nevertheless, when considered in connection with other instances, such as
have been mentioned and described, where the relations of the large masses
of iron formation to the greenstone are clearly those due to infolding, it
will be readily admitted that the relations of the other doubtful cases are
also best explained as due to this same thing. Especially are we inclined
to this conclusion when the close folding to which the rocks of the district
have been subjected is fully recognized. Admittedly some of the greenstones
may be younger than some of the iron formation; for instance, the sills
and dikes forced into the iron-bearing belts toward the close of the period
198 THE VERMILION IRON-BEARING DISTRICT.
of volcanic activity, or some old flows which have poured out from the land
into the adjacent sea while the iron-bearing formation was being dej^osited.
In no case, however, has a jasper mass been found which could be conclu-
sively shown to be included in the greenstone as a result of igneous intrusion.
EESUMi: OF RELATIONS TO ELY GREENSTONES.
In view of the above-described modes of occurrence of the iron
formation in association with the greenstone, we reach the following
conclusions concerning the age relations of the two : A portion of the iron
formation — and this appears to form by far the predominant part of it — is
clearlv younger than the greenstone; for instance, those masses of the
formation that overlie the clastic sediments which have plainly been derived
from the ixnderljdng greenstone. Other very subordinate portions of the
formation are interbedded with the greenstone, and hence are partly
contemporaneous with it. This is shown in those cases where there is
greenstone overlying the iron formation. Moreover, it is not improbable,
although not susceptible of definite proof, that some of the smaller areas of
the iron formation are included in a greenstone which has been later
intruded through the iron formation. Lastl}', small areas of jasper, chert,
and ore, similar in general characters to the iron-bearing rocks, are second-
arv infiltration products.
In order to get a clear understanding of the conditions which would
permit such a variety of relationships between the greenstones and the iron
formation, it is necessary that we call to mind the conditions under which
these two formations originated. A study of the greenstones has led to
the conclusion that they were formed by volcanic outbursts. Just as at
the present day we have lavas and tuff masses outpoured upon the land
and partly occupying adjacent water areas, sedimentary^ deposits being
formed off"shore where the conditions are favorable for them, just so did
we liave similar conditions in the early history of the Vermilion district.
As a consequence of this volcanic outburst on the land and the simultaneous
formation of sedimentarj^ deposits in the sea, we now find the two
intermingled. In this way we can conceive that clastic sedimentarj-
deposits might be derived from aud overlie lava flows, and grade up into
nonclastic sediments. Where conditions were not favorable for the
SOUDAN FORMATION. 199
formation of clastic sediments, nonclastic sediments were formed without
the conglomerates, etc., intervening between them and their igneous-rock
basement; hence we now find them i-esting upon the greenstones with a
sharp line of demarcation between them. Conditions of sedimentation
varied; hence we get a gradual change from mechanical sediments to
organic sediments (iron-bearing rocks). The sediments in their turn were
buried by lava flows, which again at a later date were covered up by
succeeding sedimentary deposits, and so on. So far as we can ascertain,
volcanic activity continued only during the time when the lowest sediments
were being formed. We have no evidence that volcanic actiAdtv continued
during that later period in which the iron-bearing sediments were deposited.
Lastly, through this series of sediments and lavas intrusive masses Avould
be forced, which would, in some places at least, include portions of the
sedimentar}^ deposits as well as of the lava associated with them.
RELATIONS TO THE ARCHEAN ACID INTRUSIVES.
On Soudan Hill and elsewhere the iron-bearing formation is intruded
by acid intrusives belonging to the Archean eruptive series. At the
Eaton explorations at the SE. ^ of sec. 7 and SW. J of sec. 8, T. 62 X.,
R. 14 W., the jasper and ore are cut by granite-porphyry, which carries very
large quartz phenocrysts. Also north of Mud Creek Bay, in the SE. J of sec.
1, T. 62 N., R. 15 W., the jasper is both cut by and included in a granitic
eruptive. On the north shore of the lake, in sec. 18, T. 62 N., R. 12 W.,
the jasper is cut by granite-porphyry. Granite dikes cut the iron-formation
belt south of Ely in sees. 3 and 4, T. 62 N., R. 12 W. The jasper belt
extending through the south half of sec. 10, T. 63 N., R. 10 W., is also cut
by dikes of granite and granite-porphyry. Similar occurrences coiild be
multiplied, all showing the iron-bearing foi-mation cut by the younger acid
eruptives, but it is not necessary to further emphasize this relationship,
which is indisputably clear.
RELATIONS TO OVERLYING SEDIMENTS.
The iron formation is in places overlain b}^ a series of sediments of
clastic origin. Where contacts between these series were observed it
was found that the relationship existing was that of two unconformable
sedimentary deposits. The proof of this is in the fact that the upper.
200 THE VERMILION IRON-BEARING DISTRICT.
3^ounger sedimentary series contains in it fragments of the underlying,
older series, the iron formation. The details concerning the relations of
these two formations are given under the discussion of the later sediments.
RELATIONS TO BASIC EROPTIVES.
In several places the iron-bearing formation is found to have been
cut by basic eruptives. Thus, for examj^le, in sec. 27, T. 62 N., R. 15 W.,
the jasper is cut by dikes of greenstone which must be younger than the
jasper, and very probably belong with the Lower Hurouian basic intrusives.
Again, south of Ely, in sees. 3 and 4, T. 62 N., R. 12 W., basic intrusives
are found to cut across the iron formation.
AGE.
From the preceding paragraphs it will have been learned that the
Soudan formation is in general younger than the Ely greenstone, the oldest
rock of the district, but on the whole so intimately associated with it that
the two must be considered as belonging to the same great period of the
earth's history, the Archean.
THICKNESS.
It is impossible to make any reliable estimate of the thickness of the
iron formation, and this for many reasons. In the first place, the exposures
of the formation are so isolated and the formation itself throughout is of
such uniform character that it is impossible to recognize the same horizons
in it in different parts of the district. No definite basement has been found
from which to begin an estimate of the thickness. The ver}- close folding'
to which the rocks of the district have been subjected adds to the complica-
tions. The thickness of the Soudan formation, as inferred from its surficial
extent rather than from any definite measurements, is presumed to reach
several hundred feet
INTERESTING LOCALITIES.
On the bare hills just nortli of the northernmost houses of the town of
Tower there are a number of exposures that show the relations between
the iron formation and the associated rocks. For instance, the southern-
most exposures on these hills are conglomerates made up of pel^bles of
jasper, slate, and chert. These materials have been derived from the iron-
SOUDAN FOKMATION. 201
formation rocks which immediatel}' underhe them, aud in phices are seen
in juxtaposition with the conglomerate. Erosion has removed the con-
gk^merate in some cases, leaving small areas of the jasper only a few
yards in extent surrounded by the conglomerate. Associated -with the
jasper at this place are very narrow bands of black graphitic slate. It
is from some of these bands that the fragments of slate in the overlying
conglomerate have been derived. There is an area about 100 yards
wide on the slope of this hill in which the conglomerate and underlying-
jasper are intimately associated. Nox'th of these exposures occur the
iron-formation rocks, consisting of jasper, cherts, and iron ore interbanded
and closely infolded with the green schists. The iron formation has its
normal characters, which have already been described. The green
schist associated with it ^Dossesses an exceedingly well-developed fissility,
* which strikes N. 70° E. The schist is much crumpled in places and
show-s minor faulting, with bending of the lines of schistosity. The
faults cut across the schistosity at an angle of 45° aud extend about
northwest-southeast. This schist is impregnated in areas of irregular
outline with u'on, especially along the southern side of the exposures
nearest the jasper. A great deal of vein quartz has also been iniiltrated
into the schist and is found in thin sheets marking the planes of schistosity,
aud also in fine systems of rectangular veins which cut the schistosity.
The green schist and iron formation are most intimately infolded, forming
a series of anticlines and synclines having very steep pitches. The axes of
the folds have a strike approximately coinciding with the strike of the
schistosity in the green schists, N. 70° E., showing an exceedingly close
folding of the rocks. A great number of these small folds was observed,
and in some cases the bands of iron formation or of schist could be traced
through several folds. It is very clear that the close infolding here has
produced a kind of fluted structure which is best developed on the saddle
connecting Tower and Lee hills, which stand en echelon to each other from
northwest to southeast.
The same kinds of intricate plications of jasper and green schist can
be seen on the numerous exposures on Soudan Hill. An especially good
one occurs on the north flank of the hill about 1,040 paces north, 330 paces
west of the southeast corner of sec. 28, T. 62 N., R. 15 W. Numerous
other cases may be observed west of the Montana pit and on the north
202 THE VEKMII.ION IRON-BEARING DISTRICT.
flank of the hill north of No. 8 shaft. Several good exposures occur also
along- the road leading from the top of the hill to the compressor, and also
near the top of the hill on the right side of the tramway leading to the
compressor.
On the south shore of Vermilion Lake in the SE. J of sec. 20, T. 62
N., R. 15 W. on the point between Swede Bay and Vermilion Lake there
are a number of exposures of the iron formation in close association
with the later sedimentaries. Lidenting the northeast shore of this point
there is a small bay, on the west shore of which (almost due west of a
small reef of conglomerate) there is an exposure of jasper. This jasper
exposure is only about 75 paces across and is extremely plicated, showing
a number of distinct anticlines and synclines with axes striking east and
west and plunging east at the high angle of 80°. On this exposure there
are also some beautiful examples of friction breccias on a small scale.
Following this exposure inland we pass over several other jasper expo-
sures showing notliiug of especial interest, so far as the iron formation itself
is concerned, but lying above the iron formation here and there we find a
patch of conglomerate derived from it and showing distinctly its relation-
ship to the underlying jaspers. About halfwaj- across the point we reach
a place where a considerable area of the jasper is exposed. At this point
the jasper is extremely plicated, like that upon the shore. The axes of the
plications strike about N. 75° E. and form an arc of an oval whose long
axis trends N. 75° E. It seems very clear that we are at this place just
east of or near the apex of a small east-west anticline. A conglomerate
lies around the jasper, bordering it, with an occasional patch still remain-
ing on the top, and hence lying in the midst of the jasper area. The
conglomerate has its greatest development to the north, while to the south
the slates are most common.
Good exposures near the North Lee pit on Lee Hill, northeast of Tower,
offer excellent opportunities for a study of the relations between the green
schist and the jaspers. A green chloritic schist is exposed in a practically
solid mass for about 125 paces to the south, theii follows the jasper, and
again, north of the jasper, comes the solid greenstone for something like 75
paces, where an alternation of jasper and green schists begins, eventually
followed, still farther north, by a lai'ge mass of greenstone. The schistosity
in tlic scliists strikes east and west. Nearest the main mass of the jasper the
SOUDAN FOKMATION. 203
schist contains a few small lenses and stringers of jasper and chert, which
extend along the planes of schistosity and clearly have been introduced into
the rock since it was rendered schistose. The surface exposures of the schist
east of the jasper are disconnected, but it is reported by the mine captain
who had charge of the North Lee pit, now filled with water, that under-
ground the jasper is cut off by the greenstone which surrounds it to the
east. The jasper exposed, especially to the west of the North Lee pit, is
very badly fractured, and ore has been introduced since the fracturing,
healing the cracks and filling the cavities. Moreover, along the edge of
the pit from which the ore body has been removed there can be seen
remnants of banded lean ore. These bands are continuous with the bands
in the jasper lying' next to and continuous with the ore. There is here a
gradation from a . small mass of rich ore through a very hematitic jasper
into the normal jasper. The fracturing and brecciation of the jasper are
indicative of the extreme folding to which it has been subjected. This
is further shown by the presence of a fairly extensive breccia along the
contact between the southern wall of green schist and the jasper. Along-
this contact has occurred brecciation of the chert and jasper, and angular
to subangular fragments of these form the pebbles for the most part, with
the green schist occurring chiefly as the matrix. Occasionally a fragment
of the green schist occurs, also as a pebble, in. the mati'ix. Since the
formation of the breccia, iron has been infiltrated into it, and this has
tended in many places to cement it together, so that at some localities it
contains A^ery nearly enough hematite to be considered a lean ore. In
places along this contact a great deal of white quartz has been infiltrated,
showing that water has been verj^ active here.
Li the SW. 4 of the NW. i of sec. 3, T. 61 N., R. 15 W., there are
a number of exposures of jasper which show extreme plication and
infolding of jasper and green schist. Here also there is a band of breccia
consisting of fragments of jasper and chert in a green schist matrix.
Similar breccias have been observed also at other places throug-hout the
district. The jasper upon these exposures is for the most part that which
is usually called the black or hungry jasper. It is a magnetitic chert.
This in places shows a fine banding, probably sedimentary banding. In
the NE. 4 of the SE. 4 of sec. 1, T. 61 N., R. 15 W., there is an occin-rence
of jasper which has been explored by means of test pits and diamond di'ill.
The jasper belt here has a width of from 15 to 30 feet, strikes N. 70° W.,
204 THE VERMILION IRON-BEARING DISTRICT.
and dips 90°. It is exposed for a distance of about 100 paces in an east-
west direction. South of the jasper there is massive greenstone. On the
north side of the jasper there is a greenstone which has eUipsoidal parting
and i)i places appears tuifaceous. No good sedimentary banding is shown,
however. Here, it seems, there was an interbanding of the iron fomiation
with the greenstones, it having been covered up, perhaps, by a flow of
lava represented b)^ the ellipsoidal and tuffaceous rock. In the SE. ^ of
the SE. J of sec. 7, T. 62 N., R. 14 W., on what is known as the Eaton
property, there is a large exposure of much plicated iron formation which
in one place near the shaft has been cut tlu'ough by dikes of granite-
porphyry containing large phenocrysts of quartz. The intrusive relation-
ship of this porjjhyrv to the jasper can be well seen here The jasper
occupies prominent hills in the midst of an area containing heavy drift
deposits which conceal the greater portion of it, but numerous exposures
of greenstone north and northwest of the jasper areas indicate that the
greenstone at least partially surrounded the iron formation, and, as shown
by relations of these rocks elsewhere, underlies the jasper. Just west of
the shaft, about 300 paces distant, is a bare knob of greenstone cut through
by a dike of granite-porphyry which is believed to be a continuation of
that cutting the jasper near the shaft. On this bare knob there were
observed in some places structures which looked fragmental, making the
rock appear as though it were partially a greenstone tuif or a brecciated
greenstone. It could not be determined whether this fragmental portion
of the greenstone was a volcanic tuff' or a basal conglomerate lying upon
the greenstone and below the iron formation.
About one-fourth of a mile north of the southeast corner of sec. 1,
T. 62 N., R 15 W., there are a number of fairly good exposures of the iron
formation. This is here very intimately associated with greenstone, with
which it is clearly infolded, both the jasper and the greenstone there lieing-
cut by acid dikes. The belt in which this iron-formation material occurs
was traced to the north of east by means of a number of discontinuous
exposures for about 2J miles. There is a large exposure just north of the
little lake on the section line, between sees. 5 and 6, T. 62 N., R. 14 "W.
It is very much contorted, and represents a southwestward-plunging
anticline. Mining has been done at this ])oint to a slight extent, some
ore having been brought to the surface, although none has been shipped.
SOUDAN FORMATION.
205
A numbei* of explorations farther east along the same belt of rock have
likewise disclosed the presence of the iron formation, but thus far only
very small quantities of ore have been found.
Fio. 3. — Sketch showing Soudan formation infolded in Ely greenstone, both cut by Keweenawan dolerite dike.
South of Moose Lake small stringers of jasper have been observed at a
number of places, associated with the Ely greenstones, which are there well
exposed on the bare hills. On the narrow strip of land in the NW. 5 of
sec. 4, T. 63 N., R. 9 W., sepai-ating two small lakes, there are exposed
several narrow belts of iron formation intimately associated with the
206
THE VERMILION IRON-BEARING DISTRICT.
greenstone. Five distinct bands of iron-formation material were observed,
each only a few paces in width, and in the belts farthest south the
exposures were sufficiently good to show very well the intimate relations
of the srreenstone and the iron formation. The iron formation is very
Xf^
Ogishke conglomerate
I ron-beanng formation
Greenstone
Scale
40
sofeet
Fig. 4. — Ilhisiralion showing distribution aiwi relations of Ely greenstone, Soudan I'ormiition. und Ogishke eonylnm-
erate at a place south of Moose Lake.
clearly infolded in the greenstone, and subsequent truncation of these two
formations has resulted in ])roducing very intricate surface relationships.
At one place a broad dike of dolerite has cut across botli the preexisting-
formations, and, indeed, it was traced tor idxmt half a mile across the
country. Fig. 3 illustrates the relations observed at this place. About
SOUDAN FORMATION.
207
one-fourth of a mile due north of this locality, in sec. 33, T. 64 N.,
R. 9 W., there are some exposures showing very clearly the relations
which exist between the greenstones, the iron-bearing formation, and the
sediments that occur in such large quantities in this area south of Moose
Lake. At this place the greenstones occupy the opposite sides of a
considerable depression in which occur most commonly the exposures of
the fragmentals. Occasionally an irregular area of the old ellipsoidally
parted greenstone rises through these fragmental deposits (fig. 4). At
the place mentioned, a narrow belt of iron formation, about 20 feet in
length, was observed lying on the north side of such a small irregular
area of greenstone. South of this
there were small, irregular areas of
iron formation completely surrounded
by the greenstone. Immediately ad-
jacent to the iron formation and the
OgishUe conglomerate
j^i^^Soudan form at I o nTircMT^beanrrg)
N
A
greenstone
occurs the Ogishke con-
r
(oreensto tc
Sfeet
Fig. .5.— Sketch showing association and relations of Ely
greenstone, Soudan formation, and Ogishke conglom-
erate.
glomerate, consisting of fragments of
greenstone with, in the immediate
vicinity of the jasper, considerable
numbers of fragments of jasper, show-
ing very clearly that it is derived from
the jasper and the greenstone, which
are the underlvino- formations. Fig-.
5 is a sketch illustrating' the associa-
tion of the rocks at this locality. It
is very evident that the irregular distribution of the greenstone is due to
the intricate folding to which all of the rocks have been exposed, and the
subsequent truncation of the folds, which has left the outcrops of the forma-
tions in their present relations.
On the Canadian shore of Otter Track Lake, on the west side of the
strait leading to the north and south arms, and just beyond the main portion
of the lake, there is a considerable exposure of iron formation in contact
with the old greenstones. The formations are well exposed along the lake
shore, being present in cliffs which are about 75 feet high and very nearly
perpendicular. The contact between the jasper and the greenstone is
very irregular on a large scale, although when examined in detail the line
208
THE VERMILION IRON-BEARING DISTRICT.
is usually pretty sharp. The actual contact between the two was closely
studied to find whether or not a zone of clastic rocks intei'vened between
them, but such a zone was not found, the contact being very sharp. The
greenstone, it is true, is somewhat schistose along the contact, but this may
Scale
100 0 100 200 300 AOOfeet
Fig. 0.— Sketch sho'wiiig ri-latiniis Ijclwi-cn Sninlnn fiinniilion and Kly greenstone vn otter Track Lake.
be due to the moA'ement that has taken place between the tAvo formations
as a result of the folding. Fig. 6 is a sketch showing tlie occurrence of
these rocks at Otter Track Lake.
The regular canoe route was followed nortliward from Otter Track
Lake to Jasper Lake, and on the norfli shore of Jasper Lake, west of the
SOUDAN FORMATION.
209
bay from whicli the portage leads northward from the lake, exposures of
iron formation were found and carefully stxidied. Here, again, special
Greenstone ^.^ ■■
Scale
50 100 feet
/
/
Soudan formation /
(iron-bearing) /
Fig. 7.— Sketch showing relations of Soudan formation and Ely greenstone on Jasper Lake.
MON XLV — 03 li
210 THE VERMILION IRON-BEARING DISTRICT.
atteutiou was paid to the contact between the irou formation and the green-
stone, but very careful search showed no indication of anj clastic material
between them. Nearly everywhere this greenstone is perfectly massive; in
a few places it shows a slight schistosity parallel to the contact, probably
the result of the shearing caused by the close folding. The iron formation
is closely infolded in the syncline in the greenstone. The broadest part of
the syncline — that is, the place where the jasper has its greatest width — is
at the top of the hill. From this point down to the lake shore the descent
is at a rather sharp angle, and the fold has here been beveled off. As a
result of this beveling a subordinate anticline is first brought to the surface
as an oval area near the center of the main syncline, and as we come still
farther south the deeper truncation of the fold has separated the main
syncline into two subordinate synclines occurring in very narrow belts
separated by the intermediate greenstone area, as shown in fig. 7, from a
sketch made in the field.
On the portage between Big Rock Lake and Emerald Lake, north of
the international boundary, a mass of iron formation was seen which was
estimated to be about 100 feet across, north and south. Its contact with
the greenstone south of the trail was found, and is here knife-like in its
sharpness, there being absolutely no clastic material between the greenstone
and the jasper. The jasper is much broken, has very fine jointing-, and
the brilliant red variety makes up the greater portion of the formation.
Some of the bands of the bright-red jasper are 6 inches across. Bands of
white chert are infrequent, although present, and the iron ore in narrow
bands makes up the remainder of the formation.
INTRICATE FOLDIJiTG OF SOUDAlSr FORMATION.
The intricate folding of the iron formation can be well seen at a
number of places just north of the international boundary; for example,
on Emerald Lake, where it is most beautifully and curiously folded. On
the west side of the largest island, which is about in the center of tlie
lake, bands of the jasper are bent into fan-shaped folds and curious
interlockings, a description of which would be almost incredible. The
jasper, although so rigid, has evidently obeyed the law of flowage in filling-
up every chink and corner throughout the complexly deformed mass.
How far these rocks were folded before complete jasperization took place
is questionable. At this ledge tlie l)r()ad jasper bands turn around and
SOUDAN FORMATION. 211
form an elbow, having a radius of from 2 to 4 inches, giving a roundish
surface on top, looking like cross sections through a set of closely laid
pipes. The minor folds at this exposure pitch to the west at angles varying
greatly, but ranging, mostly between 50*^^ and 30°.
The intricate infolding of the green schists and jas,pers may be
observed at numerous places on Lee and Tower hills, just north of the
town of Tower. A study of these will impress one with the extraordinary
complexity of this folding. Not only are the dips substantially vertical,
but the pitches of the folds are vertical, or nearly so. The result of this is
that where the green schist and the jasper come together the contact is
most extraordinarily complex. It runs in and out, and in places the jasper
might be supposed to be over the green schist; in other places the reverse
seems to be the case; and in still other instances one is. strongly inclined
to believe that one of them cuts the other as an intrusive. In some
places there is brecciation along the contact, so that between the green schist
and jasper there is an intervening zone of pseudo-conglomerates. One of
these cono-lomerates has a srreen schist matrix in which are bedded numerous
fragments of banded jasper. Some fragments are well rounded, others
subangular, others have curious points, and a considerable number are in
roughly rhomboidal form (PL VI, C). The schistosity extends roughly
east and west. It cuts the schist, but usually stops abruptly at the jasper
bands. This adds still another feature to the complexity of the structure
at the contacts. However, although one may be confused by examining
the details of some of these exposures, if one follows the broad distribution,
he will find in many places that the schist and jasper occur in belts which
can be separated as such, the major folds being made out in many cases.
One of the largest exposures of jasper in the Vermilion district is that
which forms the prominent peak known very commonly as Jasper Peak,
although the name Chester Peak has prior claim. The jasper is exposed
over almost the entire area of the hill. The outcrops, while not solid, are
nevertheless so numerous as to enable one to determine very easily the
structural features. The south side of the hill is nearly vertical, and gives
very good sections through the intricately folded iron formation. A study
of the hill shows that the iron formation is folded into a great synclinorium
made up of a number of closely folded synclines and anticlines. The gen-
eral strike of the axis of the synclinorium is N. 60° E., and the axis plunges
to the east, though the exact angle is not known.
I
212 THE VERMILION IRON-BEARING DISTRICT.
There are some large exposures of jasper in the NE. \ of sec. 25,
T. 63 N., R. 12 W., and this locality has been prospected thoroughly by
means of diamond-drill borings. The way in which the iron-bearing
formation is closel)^ infolded in the Ely greenstone is well shown upon
Sheet XXVI of the accompanying atlas, the data for the construction of
which have been obtained from the Minnesota Iron Company.
CLASTICS ASSOCIATED WITH THE IROK-BEARIKG FORMATION.
There is only one place in the Vermilion district at which the clastic
■ocks are associated with the iron formation in such a way that no question
can be raised that they belong together and that there is a gradation from the
one into the other. This place is just south of the north quarter post
of sec. 7, T. 62 N., R. 13 W., on the south slope of a hill. At this place the
narrow bands of iron formation are interbanded with bauds of conglomerate
and slate. There are evidently several series of these bands, although it is
possible that there may be in places a reduplication due to the close folding.
The conglomerate is made up of fragments derived from the immediately
subjacent greenstones, and as these fragments grow smaller the sediments
grade upward through gray wackes into finely banded greenish slates, next
to which occurs the iron-bearing formation, consisting of chert, jasper, iron
ore, and an occasional band of material that may correspond to the finer-
grained slates adjacent.
Somewhat similar conditions were observed on the hill in the NW.
^ of the SE. -4 of sec. 10, T. 62 N., R. 14 W., Avhich overlooks the large
swamp to the northwest, although the proof of the relationship is, perhaps,
not quite so clear. Here there is a large exposure of jasper in which a shaft
has been sunk and through which a considerable quantity of pyritiferous ore
has been hoisted. The jasper is followed to the south by a fragmental rock
made up of fragments of greenstone. This is about 10 feet distant from
the jasper. The fragmental rock is thoroughl}' impregnated with partial
pseudomorphs of limonite after pyrite. South of this fragmental occurs an
amygdaloidal lava. The sediments and the greenstone are both slightly
schistose, the schistosity striking about east and west. South and west of
tliis locality a number of other exposures of greenstone, associated with a
fragmental rock made up of greenstone fragments, was observed. In none
of the sediments could well-marked sedimentary banding be found.
SOUDAN FORMATION. 213
THE IROlSr-ORE DEPOSITS.
HISTORICAL SKETCH.
The first mention of the occnrrence of iron ore in the Vermilion dis-
trict was made by J. Gr. Norwood, who observed it during his explorations
in 1850 and published a statement concerning it in the report accompanying
that of D. D. Owen." The iron that he observed is that which occurs near
Gunflint Lake, at the extreme east end of the district, and which geologic-
ally belongs with the ores of the Mesabi range. In this part of the Ver-
milion district the ores have never been exploited to any extent and are at
present of no commercial importance.
Interest in what is now known as the Vermilion iron-bearing district was
aroused in the sixties by the rejjorted occurrence of gold in the slates and
schists in the region of Vermilion Lake. There was considerable excite-
ment for several years and a small rush to the district. Shafts were sunk
and stamp mills were erected, the machinery having been packed in from
Duluth, partly on the backs of Indian packers, over the Vermilion trail. A
town site was laid out near Pike River, at the southwest extremity of Ver-
milion Lake, and some buildings were erected. In all a good deal of money
was fruitlessly expended, as no gold deposits of any importance were found.
A reference to the possible occurrence of hematitic iron ore in the Ver-
milion district, in the strict sense, was made in Hanchett and Clark's report
for 1866. The State geologist says:
Specimens of hematitic specular iron ore were obtained from a heav}^ deposit
said to lay between a lake forming the affluence of the upper Embarrass River and
Vermilion Lake. The precise percentage of commercially pure iron contained in
this ore has not been ascei'tained.*
A more detailed mention of the occurrence of the iron ore on the
iron range at Vermilion Lake was made by H. H. Eames, who investigated
this disti'ict in regard to the reported occurrences of gold and silver and
described the iron-ore deposits as follows : "
«Keport of the Geological Survej' of Wisconsin, Iowa, and Minnesota, by D. D. Owen, 1852;
Eeport of J. G. Norwood, p. 417.
f> Hanchett and Clark: Eeport of the State geologist, Aug. H. Hanchett, M. D. , together with the
physical geography, metallurgy, and botany of the northeastern district of Minnesota, by Thomas
Clark, assistant geologist, St. Paul, 1865, p. 6.
c H. H. Eames, Report of the State geologist on the metalliferous region bordering on Lake
Superior, St. Paul, 1866, p. 11.
214 THE VERMILION IRON-BEARING DISTRICT.
The iroa range of Lake Vermilion is on the east end [of the lakej. on the stream
known as Two River, which is about 60 feet wide. There are two parallel ridges
forming the boundary of this stream, and at the mouth on each side are extensive
tamarack swamps. This range is about one mile in length, it then ceases, and
after passing through a swamp, another uplift is reached, from 250 to 300 feet high.
The iron is exposed at two or thi'ee points between 50 and 60 feet in thickness: at
these points it presents quite a mural face, but below it is covered with detritus of
the overcapping rock. On this account its exact thickness could not be correctlj'
ascertained. The ore is of the variety known as hematite and white steely' iron, and
is associated with quartzose. jasperoids and serpentine rocks. It genei-ally has a cap
rock from 3 to 20 feet thick. A little to the north of this is an exposure of mag-
netic iron of very good quality, forming a hill parallel with the one described.
The hematitie iron has a reddish appearance from exposure to atmospheric influ-
ence; its fracture is massive and granular; color, a dark, steel gray. The magnetic
iron ore is strongly attracted by the magnet and has polarity: is granularly massive;
color, iron black.
The timber here is very abundant and good, of the same class as prevails else-
where in this region.
Some time after this, in 1875, the first exploratory work in this district
was taken up by Mr. George R. Stuntz, accompanied by Mr. John Mall*^
mann, who began to prospect the Vermilion ore deposits on Lee Hill,
southwest of tlie bay of Vermilion Lake, which is now known as Stuntz
Bay, named after Mr. Stuntz. In 1880 Prof A. H. Chester examined
the Vermilion Lake ore deposits for private parties, and Mr. Bailey Willis
studied them for the Census Office. Systematic and extensive eftbrts were
made in the late seventies and the earl}- eighties to develop the iron resources
which were known to be present in this district. By tins time the Minnesota
Iron Company had been organized and all of the properties which at that
time were known to contain ore and great stretciies of country which were in
the continuation of the ore range had been purchased, the company owning
over 20,000 acres of land on the Vermilion range proper and in the vicinity
of the good harbor on Lake Superior, known now as Two Harbors. On
August 1, 1884, the Duluth and Iron Range Railroad was completed from
Two Harbors to Tower. This road was 72 miles long. At a later date it
was connected with Duluth, 25 miles away. Daring the first year (1884)
62,122 tons of ore were shipped, some of this having come from the stock
piles which had been growing during the years of development jireceding
the opening of the railroad.
SOUDAN FORMATION. 215
Prospectors were busy in the years prior to the opening of the raih-oad
in prospecting the range to the east of Tower, and in 1883 outcrops of ore
were found by Mr. H. R. Harvey in sec. 27, T. 63 N., R. 12 W. The
body of iron ore indicated by these outcrops was further tested in 1885-86
and led to the opening up of the great deposits at Ely on which are now
working the Chandler, Pioneer, Zenith, Sibley, and Savoy mines. During
1888 there were shipped from the Chandler mine 54,612 tons of high-
grade ore.
From this time on the development of the range was rapid, as is shown
by the annual increase in the shipments of ore (pp. 242-243).
ORE HORIZONS.
The iron-ore deposits of the Vermilion district show a striking' analogy
with those of the Marquette district. Like them, they may occur in two
positions with respect to the iron-bearing formation. They are found, first,
at the bottom of this formation and, second, within it, the ores in both
cases being the same in character. The ores occurring at the bottom of
the iron formation rest immediately upon the Ely g-reenstone, which thus
forms the foot wall, and are overlain by and g-rade up into the jasper and
associated rocks of the iron-bearing formation, which usually forms the
hanging wall. Ores occurring within the formation either rest upon some
impervious part of the formation above its base or else lie in the midst of
the iron-bearing rocks, their position being determined by certain factors
which will be discussed below. Workable ore deposits are known at two
localities — Soudan and Ely. At Soudan there are a number of deposits
belonging together, so far as mode of occurrence is concerned, and they
are worked through a number of shafts, all belonging' to the Minnesota
Iron Company. With these ore bodies belongs that deposit of the old
North Lee mine on Lee Hill, near Tower, the ore of which has long since
been exhausted. At Ely there are two and possibly three ore bodies lying
in an approximately east-west line, and exploited by the Minnesota Iron
Company by means of a number of shafts. Future developments may
show that these ore bodies are actually continuous and form one immense
body of ore.
216
THE VERMILION IRON-BEARING DISTRICT.
THE ELY IRON-ORE DEPOSITS.
DEPOSITS OCCURRING AT THE BOTTOM OF THE IRON-BEARING FORMATION.
To this group belong the deposits occun-ing near Ely, now worked by
means of the shafts of the Chandler, Pioneer, Zenith, Sibley, and Savoy
mines. The jjarticular deposit upon which the Chandler and Pioneer mines
Seal
oofeet
No. I Shaft
^'g'ifc;«iil^^:::.;\:-;V-':-^v
•J -^ •, •' ','
Fig. 8.— Vertical section across Chandler ore body iilongr line E-F of fig. 9,
are Avorking will probably prove to be one of the largest continuous bodies
in tlie Lake Superior region (PI. VIII). This particular ore body affords so
perfect a conhrination of the law of the occui-rence of ore deposits as stated
by Van Hise that it is of very great scientific as well as economic
importance, and will therefore be described in .^ome detail.
SOUDAN FORMATION.
217
The workings at the Zenith, Sibley, and Savoy have not been extended
far enough to enable a final statement to be made as to the shape of these
ore bodies or the exact structural conditions under which they exist, or,
indeed, to warrant a positive statement that they may not eventually^ be
found to be connected with one another and possibly also with the Chandler-
Shaft NoA^
\Shaft No.2
55 joofeer
Fig. 9.— Horizontal section through fourth and sixth levels of the Chandler mine.
Pioneer ore body. The Chandler-Pioneer ore body, as well as the ore
bodies to the east, occurs at the bottom and comes part way up the sides of
a narrow canoe-shaped synclinorium whose axis trends about 80° E. The
rocks in this vicinity have been very closely folded, and, indeed, slightly
overturned, so that at the west end of the Ely trough both walls dijD at a
very high angle to the north. As the result of tinderground work, it is
218
THE VERMILION IRON-BEARING DISTRICT.
found that the south wall dips at an angle of about 70^ X. The north wall
ranges from a vertical position to a dip of 80° to the north. However, this
northerly dip does not continue tliroughout the trough, but, on the contrary,
it is found to be reversed at the east end of the tr()ugh, where the walls are
overturned in the opposite direction to the walls at the west end, and now
Scale
250
Fip. 10. — Vertical east-west section through the Chamller mine.
dip at a high angle — about 80° — to the south. It is not known exacth'
at what point in the trough this change takes place. It probablv occurs
about the center of the trough, near the east side of the Pioneer jiropertv.
This reverse of dip is possibly caused by the occurrence of a small subor-
dinate cross anticline, which has produced a warping in tlie strata, with an
SOUDAN FORMATION.
219
eastward pitch. These facts are shown by the accompanying iUustrations.
In fig. 8, a vertical section across the extreme western end of the basin, there
is shown a narrow synchne with both foot and hanging walls dipping to the
north. In fig. 9 there may be seen horizontal sections through the fourth
and sixth levels of the Chandler mine, with position of section shown in
fig. 1 1 , indicated by line A-B. From study of this figure in comparison
with figs. 8, 10, and 11 we see that farthest west and nearest the surface
the syncline is narrowest, that as we go down we are compelled to go
eastward to follow the base of the ore body (which therefore pitches
eastward) and that the ore body widens very considerably.
,_pr/gine/ surface I ne
Open p
Fig. 11.— Vertical section through the Chandler mine along the line A-B of fig. 9.
North of the southernmost narrow syncline there is an eastward-pro-
jecting tongue of greenstone which indicates a subordinate anticline with a
second syncline lying north of it and en dchelon with the southernmost syn-
cline. The workings of the mine where they extend to the bottom of the
ore deposit (fig. 11) show the conditions which exist there.
From this section we see that the bottom has not a simple basin shape,
but in section from north to south shows several subordinate rolls. These
are indicated also by the irregularities of the western foot wall, which, instead
220 THE VERMILION IRON-BEARING DISTRICT.
of showing' a smooth surface of paint rock, projects to the east in several
more or less prominent tongues. The horizontal plans of the various levels
show that these tongues project farther and farther eastward as the deeper
and deeper levels are reached. By the time the eighth level is reached the
tongue there shown is relatively naiTow. It is possible that it may die out
before it goes much farther east, and it is very probable that another small
roll will be found to begin north of and en echelon with it. This irregu-
larity in the western foot wall of the Chandler basin is well brought out by
the mining operations. The di-ifts which have been put in to open up the
west end of the ore body run in the paint rock (altered greenstone), and
have been maintained along a course which carries them approximately
parallel with the margin of the ore body. As a result they have a winding
course. One going through these and keeping his course may see that he
follows the tongue to the east, bends around this projection in the paint rock
corresponding to the anticline, and then follows back west around the suc-
ceeding syncline of ore. Occasionally when the syncline of ore is very
narrow the drift may cut directly across it, and in such instances a small
tongue of ore surrounded by the paint rock is shown in cross section.
The foot and hanging walls of this overturned syncline, as well as
the western wall and the bottom of the basin, so far as is known from the
mining work, are of paint rock or soap rock. This soap rock is identical in
character with the more or less schistose amygdaloidal and ellipsoidal
greenstones which occur in such abundance upon the sm-face in the vicinity
of Ely, and are found surrounding the north, east, and south sides of the
Chandler basin in the numerous exposures. This greenstone, as has been
already stated, is an altered basalt (see p. 152). Where the greenstone
lies in contact with the jasper and ore it is almost invariably very schistose
as the result of movements which have taken place between rocks of such
different physical characters, and which have l^een more effective along
this plane than elsewhere. Moreover, since such plane of contact rep-
resents a directioia of easy flowage for the percolating watei-s descending
from the surface, the rocks here have been subjected to leaching and have
undergone very great metasomatic changes. As a result of these changes
the ininerals of these rocks have in places been altered to chlorite, the
formation of which has produced a soft schistose rock. The soft, soapy
feel of the rock causes it to be spoken of by the miners as soap rock or
SOUDAN FORMATION. 221
soapstone. With the other changes there has ahnost invariably been an
infiltration of iron oxide to a greater or less extent. Consequentlv tlris
contact phase of the greenstone is usually impregnated with iron oxide and
thereby colored red. To this fact the rock owes its name of paint rock,
a term which is very generally used by mining men for such altered
and red rocks. The plane of contact between the ore formation and the
greenstone has also been a plane along which actual movement has taken
place, and as a result a zone of brecciation has been produced which
includes a certain thickness of both the iron formation on the one hand
and the soap rock or greenstone upon the other. The thickness of this
zone varies greatly. In some places practically no brecciation has taken
place, but in others there is a considerable thickness of brecciated rock
The production of slickensides along these cracks indicates movement even
in the greenstone at a considerable distance away from the immediate line
of contact with the iron formation.
Immediately adjacent to the greenstone, and showing with it the
above-described irregular contact surface due to folding-, lies the ore. Upon
this ore lies a capping of jasper. Both the ore and the jasper are very
much cracked, being- penetrated by innumerable fractui-es, as is also the
greenstone, though, as a result of its brittle character, the cracks in the iron
formation are far more numerous and less continuous than those in the
greenstone. This fractured condition of the iron formation is clearly, due
to compression resulting from the production of the close synclinal fold in
which the formation lies. Hence, since the ore as well as the jasper is
brecciated, there is no escape from the conclusion that the ore must have been
formed, in great part at least, prior to the time of the last folding- of the
district which caused the fracturing of the rocks. The ore has been very
much broken up, and this breaking has been a g-reat boon to the mining
company, as it makes it relatively easy to mine. For this reason, as has
already been stated, the ore is frequently designated a soft ore, although
this is, in a strict sense, a misnomer. In reality the various fragments of the
breccia are fragments of hard ore. This natural brecciation has been taken
advantage of by the efficient manager, and the system of mining which has
been developed here merely continues the natural process of brecciation,
and thereby" the cost of winning the ore is greatly reduced. The method
of mining employed in the Chandler is described and illustrated on page 240.
222 THE VERMILION IRON-BEARING DISTRICT.
The plane of contact between the ore and the jasper agrees in general
with the contour of the basin (fig. 11). One noticeable feature is that this
plane of contact in the upper part of the basin dips on the south side to the
north, and on the north side of the basin to the south. The conditions, in
other words, are those of a normal syncline. As the deeper workings of
the mine are reached, however, this dip is found to be overturned, and upon
the north side the dip of the plane is to the north. It will be noticed that
the greatest depth of ore lies, as it normally should, in the center of the
basin. The ore follows up both the north and the south limbs of the
syncline; it goes higher, however, and is very much broader on the south
side than on the north side. This condition of occurrence is in accord with
the view held concerning the origin of the ore deposits, and will be referred
to below. The plane of contact between the jasper and the ore is
irregular in the extreme, no sharp line of demarcation existing between
them. As has alreadj^ been intimated, the merchantable ore grades upward
into lean ore, which in its turn merges by imperceptible gradations into
the jasper, with very small quantities of interlaminated iron-ore bands.
Bodies of jasper varying from minute pieces up to large masses project
downward into the ore. Occasionally a horse of jasper is included in the
ore. Again, a body of ore will project upward across the line into the
jasper. Within this lean ore, or mixed ore and jasper, one can find pieces
of jasper showing pai'tial change into the ore, the banding of the jasper still
existing in a more or less perfect condition in the ore. There is in reality
no sharp line between the ore and the jasper, but there is a gradual
transition from one to the other. These facts will be referred to again
under the discussion of the orig'in of the ore deposits, on page 230.
t
THE TOWER AND SOUDAN DEPOSITS.
Let US now consider the ore deposits at Tower and Soudan. At these
places the conditions of occiuTence are not nearly so simple as at Ely. In
the first place, the iron formation in the western part of the district has been
very much crumpled, and there appear to be many alternations of jasper and
a schistose green rock. The relations of the jasper and this green rock are
not in all places clearly shown. At some localities it appears that the green
rock is a greenstone belonging with the Ely greenstone, and hence the
basement on which the jasper rests. It owes its alternation with the
SOUDAN FORMATION. 223
jasper to the intricate infolding and subsequent truncation of the two
formations. In other places the evidence is fairly conclusive that we have
to deal with dikes which were intruded both parallel to the banding of the
jasper and across this banding. In both cases folding subsequent to their
intrusion has brought about structural relations similar to those existing
between the jasper and the basement greenstone. The rock forming the
dikes which cut the iron formation is liow uniformly green, except where
discolored by the iron. Its chief component at present is chlorite, but
certain facts — for instance, the presence of quartz phenocrysts, which were
observed and which will be considered in detail at a more fitting place —
show very conclusively that at least some of these dikes were originally
acid intrusives. The alterations that these acid rocks have undergone have
produced schistose rocks, which now are strikingly similar in macroscopic
characters to some of the greenstones derived from rocks of an originally
basic character. These dike rocks have been classed ' as basic rocks —
greenstones. In one case — that of the scliist a,t the Lee mine — Smyth and
Finlay" state specifically that it has been derived from a quartz-porphyry.
DEPOSITS OCCtTREING AT BOTTOM OF THE lEON FORMATION.
We have no conclusive evidence that any of the deposits of iron ore
at Soudan or Tower rest upon what is the actual basement greenstone.
Such an occurrence, if recognizable, would be found to be similar in all
essential characters to the occurrence of the ore at Ely. We would find
the ore at the bottom of a synclinal trough of the iron formation, with the
greenstone forming the impervious bottom and sides of the trough. It is
impossible to recognize with certainty the true basement greenstone on
account of the intrusion of acid sills and dikes in the iron formation in the
mines in the vicinity of Soudan and Tower; on account of the intricate
and intimate relationship existing between these two kinds of rocks, due to
this intrusion and heig'htened by the subsequent infolding of the eruptive
and the iron-formation rocks; and also on account of the resemblance of
the altered acid rocks to the greenstone. The deposits at the east end of
Soudan Hill, which have been mined out, are supposed to have been laid
oThe geological structure of the western part of the Vermilion range, by Henry Lloyd Smyth
and J. Ralph Finlay: Trans. Am. Inst. Min. Eng., Vol. XXV, 1895, p. 639.
224 THE VERMILION IRON-BEARING DISTRICT.
down iu syncliues upon the basement greenstone, and hence to belong to
the same class of deposits as do those occvirring at Ely. The one mined
from the old pit known as the North Lee mine is presumed also to have
been a deposit in an analogous position.
DEPOSITS OCCURRING WITHIN THE IRON FORMATION.
The deposits within the iron formation may be of two modes of
occuiTence: (a) They may have jasper both as a foot and hanging wall,
and hence may lie within it and grade in all directions into it (these
deposits are of small size); or (&) the)' may have paint rock (soapstone, or
soap rock, as it is indifferently called by the miners) as foot wall, below
which is again jasper, with similar paint rock or jasper as the hanging wall.
Of this latter character, with an occasional pocket of ore lying wholly
within the jasper, are the deposits worked at Soudan. The ore deposits
showing the different modes of occuri-ence just mentioned are of very
in-egular shape and varj' greatly in size. Masses of greenstone project
from them into the jasjoer as a result of the irregularities of the foot wall,
due chiefly to folding; or the plane of separation between the ore and the
jasper is very irregular and projections of jasper extend down into the ore,
and the ore extends into the jasper, such irregularities rendering the mining
very uncertain and expensive. Occasionally great horses of jasper occur*
in the midst of an ore deposit.
The deposits occurring within the iron formation are far less likely to
be as large and as continuous as those which lie at the bottom of the fonna-
tion and rest upon an impervious basement, for the reason that the deposits
occurring within the iron formation- owe their existence to the introduction of
igneous rocks in the form of sills or dikes, or to the peculiar conditions of
fracture. Where the dikes are numerous there may be a number of rela-
tively small deposits separated from one another by intervening waUs
(subordinate dikes) of soap rock or paint rock of varying thickness. This
condition is well illustrated in the case of the ore deposits on Soudan Hill,
which are being mined by the Minnesota Iron Company.
In structure Soudan Hill is a large anticline trending a little north of
east and pitching steeply to the west. The summit of this anticline is
occupied by a syncline having the same strike and pitch as the anticline,
SOUDAN FORMATION. 225
aud it is within this syncliue that the deposits occur. Before the jaspers
were as intricately folded as at present they were intruded by dikes and
sheets of acid rocks similar in composition and general character to those
now outcropping on the islands and shores of Vermilion Lake. These
sheets of igneous rocks were intruded essentially parallel to the bedding of
the jasper, and were at varying horizons in the jasper, and hence are sepa-
rated by varying vertical distances. The intrusion of these sheets has thus
divided the iron formation, as it wei'e, into a number of bands of different
thickness. The dikes which cut through the iron formation are of varying
trend, and these, as well as the intercalated sheets, were intruded in the
iron formation at various angles with the horizon. When, aftei- their
intrusion, the rocks were folded, the intrusive sheets behaved essentially as
intercalated beds in the iron formation and were crumpled with it into close
synclines and anticlines. When the folding took place the brittle jaspers
accommodated themselves to the movement by fracturing, whereas the less
brittle eruptive rock accommodated itself by shearing.
Owing to the fractured character of the associated iron formation the
downward-percolating waters passed readily tlu'ough it, but were stopped
and led along the relatively impervious acid igneous rocks. These were
thus intensely affected by the circulation of the water, which, bringing iron
in large quantity in solution, deposited considerable quantities of it in the
igneous rocks during their alteration. As a result of the action of the
percolating waters these I'ocks were intensely altered chemically. The
addition of iron rendered possible the formation of the chlorite, which is
not as a rule characteristic of the alteration of acid rocks. Now these
intrusives are essentially the same in general appearance as the soapstone
and paint rock derived at various places from the basic greenstones. From
the point of view of their influence in the formation of the iron-ore deposits
they are also absolutely identical with the above-mentioned soapstones,
originally of basic character, and they will be designated as soap rock and
paint rock, in accordance with the custom of the miners.
Occurring in the way described, in the large central syncline of the
iron formation, these eruptive sheets have divided it into a number of small
synclines, each with an essentially impervious basement of soap rock. The
sheet of rock forming the impervious bottom of one trough forms the
impervious top to the next lower synclinal troug-h. Between these lie
MON XLV — 03 16
226 THE VERMILION IRON-BEARING DISTRICT.
the intensely fractured rocks of the iron formation. Through these frac-
tures downward-percolating water carried the ore and deposited it upon the
impervious bottom of the troughs formed by the intercalated igneous sheets.
Hence it comes about that the ore deposits, being derived from only the
small quantity of iron formation which occurs between these adjacent
sheets, are relatively small, and for the most part disconnected. They
occur, however, in a syncline, as does the enormous Chandler-Pioneer
deposit of Ely, the essential difference being that at Soudan the ore body
in the syncline has Ijeen separated into a number of irregular bodies, the
one above the other, instead of occurring in one continuous ore deposit.
One might infer from the above statement that these sheets were
introduced very regularly into the iron formation. This would be an incor-
rect inference. Anyone familiar with igneous phenomena knows that the
dikes and sheets divide more or less frequently at in-egular iutervahs. They
have done so in Soudan Hill, so that we may find deposits joining other
deposits as a result of the disappearance of the subdividing sheet as we go
away from the point where it leaves the main mass, or a large deposit will
divide into two or more small ones as a result of the introduction of siTch an
oftshoot from a sill. Hence there may be a very remarkable irregularity in
the occurrence of the ore deposits formed in such synclinal basins where the
impervious bottom is due to the presence of intrusive sills. A further
cause of irregularity is the introduction of the more or less vertical dikes
which were contemporaneous with the introduction of the sills. These,
cutting through both sills and the associated iron formation, have still
further tended to subdivide the rocks into masses of varying size. Fur-
thermore they, like the iron formation, were much folded, and now occupy
various positions and are of greater or less importance in determining the
size of the ore bodies. Thus, for example, if a dike should have cut across
a sheet at nearly right angles, and in such a way that when the two were
folded a pocket with nearly imper^'ious bottom and sides was formed, an
ideal condition would have been produced for the deposit of ore, according
to methods described by Van Hise in numerous articles to which reference
has already been made. The larger the pocket the larger, other things
being equal, would be the deposit of ore.
Belonging with the ore deposits occurring within the iron formation
are certain small deposits of relatively slight commercial importance, but of
SOUDAN FORMATION. 227
considerable scientific interest. These deposits are those which have the
fii'St mode of occurrence mentioned above. They are surrounded on all
sides by jasper. Hence their occurrence does not depend on the formation
of a synclinal trough with impervious bottom, as in the previously con-
sidered cases. These deposits, as a rule, form so-called chimneys of
ore, having in general, as the name indicates, a rectangular outline, and
their origin is evidently due to the presence of fracture planes or zones
in the jasper. Along these zones water has percolated and has produced
the ore bodies from the iron formation by the well-known process of
replacement, the ore diminishing in richness as the distance from these
fractures increases.
ORIGIN OF THE ORE DEPOSITS.
Having now described the manner of occurrence of the ore deposits
and shown their relation to the geologic structure, we are prepared in the
light of the facts given to consider their origin. The origin of ore bodies
completely surrounded by jasper obviously depends on the occurrence of
fractures, for the ore is confined to the vicinity of the fractures and
diminishes in richness as the distance therefrom increases. The importance
of these fractures as channels for downward descending waters is also
obvious. Hence the connection between the occurrence of the deposits and
the action of percolating water is shown. This interdependence is further
impressed upon one when a study is made of the ci'oss section of the narrow
ore deposits of Soudan, and also of the cross section of the Ely trough. It
will be readily recognized that the plane of contact between the two forma-
tions will, as a rule, perform the same function as any large, continuous
fracture in the formation itself, in that the plane of contact will permit more
readily the passage of water, since it has a continuous line of weakness,
than will the small, discontinuous fractures which may exist in the forma-
tion. Hence the ore deposits in this case will be confined more or less
closely in their occurrence to this plane of contact. Where the converging
streams of descending waters meet, at the bottom of the trough, the action
has been most intense, and consequently the largest bodies have been
accumulated there. Theoretical considerations show that if such a trough or
other favorable place is to contain a large body of high-grade ore it should
228 THE VERMILION IKON-BEARING DISTRICT.
be a structural feature which has at some point or points exits for the
inflowing descending currents of water. Since the accumulation of the ore
depends on the circulating- waters, of which some parts bring in and other
parts cause the deposition of the iron, and since any given amount of water
must carry an exceedingly small percentage of iron in solution, it is evident
that the free circulation of large quantities of water must be an extremely
im^jortant factor in the production of an ore deposit. The second g-reat
factor is time — a long period, in which complete replacement may take
place, being more favorable than a shorter one. Of course it is here pre-
supposed that there exist the other conditions necessary to the accumula-
tion of the ore, which are the presence of ii-on-bearing material as the
source of the iron and the structural features for its accumulation.
A full discussion of the chemical reactions which result in the deposi-
tion of the ore has already been given by Irving and Van Hise in the mono-
graph on the Penokee series," and the reader is referred to this for the details.
The following- is a summarized statement of Van Hise's views of the
general chemical process of concentration as the result of which this and
similar ore deposits have been produced:
The next question to be considered is the chemical process of concentration of
the ores. For places where waters from different sources are converged, this proc-
ess has been fully given in Monographs XIX and XXVIII of the United States'
Geological Survey. In this paper the discussion will be only summarized. A part
of the iron oxide of the ore was deposited in its present condition as an original
sediment containing silica and other imparities. However, the nature of the sediment
may have been changed — that is to say, it may have been deposited in part as iron
carbonate, or in small part as iron sulphide or iron silicate, and later transformed to
iron oxide in situ. The lean material originalh' deposited where the ore bodies now
are has been enriched by secondary deposition of iron oxide. Brietly, the process of
enrichment is believed to have been as follows:
The source of the iron for the enrichment of the ores is believed to have been
mainly iron carbonate. Meteoric waters are charged with oxygen. As thej" enter
the soil the}' would be dispersed through innumerable minute openings. The waters
which early in their journey come into contact with iron carbonate would have their
oxygen abstracted. Such waters would be likeh' to be those following circuitous
routes. The deoxidation of the waters bj- the iron carbonate would produce ferrugi-
nous slates and ferruginous cherts. In this alteration the carbon dioxide would be
"The Penokee iron-bearing series of Michigan and Wisconsin, by E. D. Irving and C. E. Van
Hise: Mon. V. S. (leol. Survey Vol. XIX, 1892, pp. 283-284.
SOUDAN FORMATION. 229
liberated, aud would join the descending waters. Thus carbonated waters free from
oxygen would be produced. Such waters are capable of taking a considerable
amount of iron carbonate and some iron silicate into solution. Large quantities of
these solutions would be converged upon the sides or at the bottom of the pitching
troughs, or in other places where there were trunk channels for water circulation.
After an iron-bearing formation was exposed to descending waters for a consid-
erable time, a large part of the iron carbonate adjacent to the surface would be
transformed to ferruginous slates and ferruginous cherts. This change would take
place most extensively where waters were abundant and a somewhat direct course led
to the trunk channels. After this process was completed at such places, the waters
now following this direct route would pass only through the ferruginous slates
and ferruginous cherts and would reach the trunk channels charged with oxygen.
There the solutions bearing iron carbonate and those bearing oxygen would be com-
mingled. Iron sesquioxide would be precipitated. Therefore the iron oxide of an
ore body consists in part of iron compounds originally deposited in situ and in part
of iron brought in by underground waters. The material deposited in situ may
have been originally detrital iron oxide or it may have been derived from iron car-
bonate, iron sulphide, or iron silicate, which was oxidized in place, or from two or
all of these sources. It has been assumed that the part brought in by underground
waters was mainl}^ transported as carbonate, although a portion may have been
transported in some other form. Of the two sources of iron ores, the original material
and that added by underground water, the latter is upon the average probably more
abundant. But in some exceptional cases, where there is a large amount of detrital
iron oxide, the material added by underground waters may be subordinate. However,
in all cases it ma}^ be said that were it not for the secondary enrichment by under-
ground waters, through the addition of ii'on oxide, the material would not be iron
ore. The evidence of this -lies in the fact that the ore bodies are universallj' confined
to the places where underground waters have been converged into trunk channels.
The ore deposits contain upon the average a less quantity of silica than does
the average of the iron-bearing formations. It follows therefore that silica must
ha/e been dissolved. This doubtless was largely the work of the great volume of
water converged into the trunk channels. It has been seen that the waters which
carried iron carbonate to the ore deposits were carbonated. The precipitation of
iron oxide from carbonate liberated more carbon dioxide, so that the waters were
very heavily charged with carbonic acid. In some of tlie districts basic igneous
rocks occur within the iron-ore deposits or as basements to them. In all such cases
these basic rocks are found to have lost a large part or all of their alkalies. These
must have passed into the solutions. Hence the waters moving along the trunk
channels would in some cases contain alkalies besides being rich in carbon dioxide.
It is well known that such solutions are capable of dissolving silica. Therefore the
conditions which result in the precipitation of iron oxide also furnish conditions
favorable to the solution of the silica. Silica is thus largelv dissolved from the ore
230 THE VERMILION IRON-BEARING DISTRICT.
bodies and transported elsewhere. The removal of the silica is ordinarih- onh' less
important in the development of the ores than the addition of the iron. In many
cases the abstraction of the silica proceeded further than the deposition of the iron
oxide, thus making' the rocks verA^ porous and further rendering the conditions
favorable for abundant circulation."
As the result of the detailed study of the ores in the various Lake
Superior iron-bearing districts the conclusion has been reached that they
are essentially replacement deposits, and this conclusion, pronounced early
in the study of the district, has been strengthened by the observation of
numerous facts in all the other districts which have been studied since then.
The following facts from the Vermilion district alone seem to offer incon-
testable proof that this is the character of the deposits in this district, and
also gives clear proof of the time of the formation of these deposits. Near
the west end of the ore body worked from shafts No. 7 and No. 8 on Soudan
Hill there is a large mass of jasper, already described by Smyth and Finlay,''
lying directly across the <5re with banding con-esponding- to and continuous
with the banding of the adjacent ore. Again, on Lee Hill, along the south
and west sides of the old North Lee mine, a breccia between the iron
formation and the underlying schist has been produced. Some of the
fragments of jasper in this breccia have been replaced by hematite with,
however, a j^fii'tial retention of the banded structure of the jasper. That
the iron ore at this particular place was deposited later than the movement
which formed this breccia is shown by the fact that the fragments of the
breccia are cemented in many places by hematite ore, and numerous similar
bodies may be seen which have been formed in cavities within the breccia.
A study of the ore remaining in place at the North Lee mine and the
adjacent banded iron formation also shows intimate connection between the
two. The iron formation has an approxiiuately east-west trend and seems
to rest in a westward-pitching trough of chloritic schist, the schist occurring
both on the north and south as well as at the east end of the iron formation.
The ore body corresponds in trend with the strike of tlie formation itself,
and on its south and west sides lies next to the iron formation, the banded
jaspers. As the ore body is followed westward, the ore is gradually more
and more mixed with jasper, becoming lean ore, and then the stringers of
«The iron-ore deposits of the Lake Superior region, by C. R. Van Hise: Twenty-tirst .'V.nn.
Kept. U. S. Geol. Survey, Pt. Ill, 1901, pp. 326-328.
iOp. cit., fig. 8, p. 42.
SOUDAN FORMATION.
231
ore continue into the jasper, gro"wer fewer and thinner, until finally the iron
formation consists almost exclusively of jasper and chert, with but isolated
narrow layers of ore in it. The banding of the jasper is seen to be con-
tinuous with that of the ore, which still possesses a banding, though an
imperfect one.
At one place on Soudan Hill, north of open pit No. 6, a contorted
banded iron formation is cut by a dike which runs nearly north and south,
cutting across the bands of the formation (fig. 12). On the east side of
Fig. 12. — Reproduction of sketch showing replacement oi jasper by iron ore. After Smyth and Finlay."
the dike and between the dike and the jaspers and cherts there has been
formed a small ore deposit. Here the banding in the adjacent jaspers and
cherts appears to run right on through the ore, and although interrupted by
the dike is found to be continuous beyond this.
At Ely the rock has been very much brecciated, but even there the
banding in the ores and their intimate mixture with the bands of jasper
seem to show very conclusively that their relation is essentially the same
as that of the ores and jaspers at Tower and Soudan which have been
a Reproduced from The geological structure of the western part, of the yermilion range, Minne-
sota, by Smyth and Finlay: Trans. Am. Inst. Min. Eng., Vol. XXV, October, 1895, p. 643.
232 THE VERMILION IRON-BEARING DISTRICT.
described above. The only explanation as to the origin of the ore which
ajjpears to conform at all to the facts is that the ore is the result of a
process of replacement, and that the original rock was a banded rock,
either essentially the same as the present banded jasper or, as seems
more likely, the same kind of rock as that from which the jasper itself
has been derived by replacement. The presumed original nature of this
rock has already been discussed (p. 191) and the conclusion reached that
it was a cherty iron carbonate essentially similar to that described from
the various iron-bearing districts of Michigan, and especially from the
Penokee-Gogebic district of Wisconsin and Michigan.
An explanation of the ore as a chemical deposit" contemporaneous with
the deposition of the remaining portions of the iron formation is, as Smj^th
and Finlay have already stated, altogether incompatible with the .occurrence
described above.
In the case of every known bod}- jasper forms at least one boundary in some
part of it, under such circumstances that the bands, if continued, would run into the
ore. This fact, taken in connection with the tortuous form of many of the bodies,
seems to us quite inexplicable on any theory of contemporaneous deposition of jasper
and rich ore. For such a theory would involve the extraordinary assumption that
the conditions of sedimentation or chemical precipitation were so radically different
on opposite sides of an imaginary vertical plane in ocean water as to permit the con-
temporaneous deposition or precipitation of nearly pure silica on one side and nearly
pure ferric oxide on the other, and that such differences in conditions persisted long
enough to permit the accumulation, in some cases, of 100 feet or more of material.*
The theory that the ore is primarily the result of replacement b}^ iron
oxide of various substances," notably iron, calcium and magnesium carbon-
ates, and siHca, and of accumulation of the replacement products in places
especially suitable, the location of these places being due to geologic
structure, is in direct accord with the facts whicli have been observed in the
district, and which have already been discussed in considerable detail.
The time of the accumulation of the ore can be fixed approximately.
It was subsequent to the folding which produced the synchnal troughs
in which the ores are now formed. This folding was of course also
«The iron ores of Minnesota, by N. H. and H. V. Winchell: Geol. and Nat. Hist. Survey of Min-
nesota, Bull. No. 6, 1891, pp. 103-112; N. H. Winchell, Geol. and Nat. Hist. Survey of Minnesota,
Final Rept., Vol. IV, 1899, p. 547.
''Smyth and Finlay, op. cit., pp. 643-644.
cMon. U. S. Geol. Survey Vol. XXVIII, 1897, pp. 400-40.5.
SOUDAN FORMATION. 233
partly instrumental in fracturing the brittle iron formation and rendering
it thereby more permeable to percolating water, which was the agent
which effected the accumulation. A consideration of the above fact
further strengthens the theory that the folding and fracturing preceded
the accumulation of the ore. That this accumulation clearly took place
subsequent to this fracturing is furthermore proved by the fact that the
fractures which traverse the iron formation have been frequently cemented
by infiltrated iron ore.
In the case of the Tower and Soudan deposits there appears no evi-
dence of folding subsequent to the accumulation of the ore deposits, for the
ore is uniformly fairly massive; although such folding occurred. Moreover,
we do not find in these deposits the micaceous hematites or schist ores which
ai-e found occasionally in the Marquette district of Michigan, and which owe
their origin to the shearing to which they were subjected while they were
so deeply bui'ied that they were essentially in the zone of flowage and did
not undergo fracturing, svich as is produced under ordinary conditions.
The case is somewhat different, however, at Ely. There, it is certain,
more or less extensive earth movements took place after the ore was depos-
ited, for the ore and the overlying jasper are fractured through and
through, so that they resemble in places a breccia ; and since these ores are
very thoroughly fractured, we conclude that the movement to which they
owe this fracturing took place while the ores were relatively near the
surface, or, in other words, were in the zone of fracture for the ore and the
associated jaspers Had they been more deeply buried, micaceous hematites
would have been produced, and the cost of exploitation would have been
very much greater than it is at present. As it is now the ore is almost a
rubble, and can be mined much more economically than can the massive
ore at Tower and Soudan. It is interesting to note that subsequent to the
formation of this rubble the ore, at least at the extreme east end of the Ely
trough, has been cemented together by infiltrated material — iron ore to a
certain extent, but also calcite and siderite to a still greater extent. Where
this cementation of the brecciated ore has taken place, as, for instance, in
the Savoy mine, the ore is almost as hard as that obtained from the Soudan
mines.
From the above statements the impossibility of fixing the time of the
formation of the ore deposits very definitely will be recognized. The
234 THE VERMILION IRON-BEARING DISTRICT.
process of folding was inaugurated between Archean and Lower Huronian
time; but since the present attitude of the troughs in which the main ore
deposits are located was mainly produced by the folding of the Lower
Huronian, the replacement certainly occurred, for the most part, after
Lower Huronian time. Since there are no pre-Cambrian deposits later
than the Lower Huronian in this part of the district, the determination of
the time of the replacement process can not be more accurately made. The
process begun shortly after Lower Huronian time doubtless has continued,
perhaps with interruptions, to the present time.
METHODS OF MINING IN THE VERMILION DISTRICT.
All of the ores of the Vermilion district are at present obtained by
means of underground workings. The underground work follows one of
two systems — either that known as the "overhand stoping" system, or
that known as the "caving" system. Both systems have been modified in
certain particulars, according to the peculiarities of the deposit or in
ao-reement with the ideas of the management as to the most economical
methods of exploitation. It would therefore be impossible to give in this
place detailed descriptions of all the methods in use, as this would involve
practically a description of the system followed in every mine in the
district; but a brief space will be devoted to a description of the methods
used at two of the typical deposits — the mines at Soudan, which use a
system of overhand stoping, and the Chandler mine, of Ely, which is
exploited by means of the caving system.
In the mines at Soudan the system of overhand stoping is best
developed, and therefore these mines, and especially the workings of No.
8 shaft, may be regarded as a type of this system. The description which
follows is in part an abstract of papers b}' Bacon" and by Denton'' and of
the statements of Mr. F. Ahbe, sometime mining engineer in charge of the
mine, and in part is the result of personal observation hj the author.
The ore at Soudan, which is a hard, blue hematite, occurs in great
irregular bodies of more or less lenticular shape, which dip steeply to the
"Development of Lake Superior iron ores, by D. H. Bacon: Trans. Am. Inst. Min. Eng., Vol.
XXI, 1892, pp. 299-304.
(< Elements of methods of metal mining based upon Lake Superior practice, by F. W. Denton:
Engineers' Year Hook, University of Minnesota, Vol. IV, 1896, pp. 49-67.
SOUDAN FORMATION.
235
north at angles ranging from 65° to 80°, and pitch to the west at an angle
of 22° to 45°. As the result of this pitch, the deepest levels are farthest
west. The deepest shaft,
No. 8, was down 926 feet
at the thirteenth level in
1 902. Fig. 1 3, taken from
Bacon's account referred
to above, is a cross sec-
tion through shaft No. 8,
showing" the condition of
the Minnesota Company's
mine, presumably about
1893, the time of the pub-
lication of the article. It
shows the general arrange-
ment of the workings. The
ore bodies lie one above
the other, and are nsuallv
separated from one another
by impervious basements
of "paint rock" or "soap
rock." Sometimes they
are partially surrounded
by material of the iron
formation proper, that is,
the jaspers, cherts, and
interbedded bands of hem-
atite. AVhen first opened
up, the ore' deposits were
exploited by means of
open pits. These were
carried down to a maxi-
mum depth of 150 feet,
when it was found advisable to begin underground mining. From a shaft
in the soap rock which forms the foot and hanging walls, crosscuts are rui
Fig. 13.— Cross section at No. 8 shaft, Soudan, Minn.
236
THE VERMILION IRON-BEAKING DISTRICT.
off at levels about 75 feet apart. From such a crosscut a slice of ore
extending from the foot to the hanging wall, and from 15 to 20 feet thick,
is removed or stoped out. When this has been cleared out, drift sets
consisting of legs 9 feet long, and averaging 15 inches in minimum
diameter, with caps 11 feet long and averaging 16 inches as the minimum
diameter, with heavy lagging, are set up, running from the crosscut
through the stope, usually near its center. PI. IX, A, shows these main-
level timbers being put in on the floor of the stope. After the timbering
is completed, filling is begun. These drift-set timbers are apparently fully
strong enough, as 80 feet of rock which is present over some of the drifts
has not broken them. Fig. 14, a horizontal section through the fifth
level of the Minnesota mine near its connection with the shaft, shows the
Fig. 14.— Horizontal section through the fifth level of No. 8 shaft, Soudan.
arrangement of the drifts and their connection with the crosscut in the
foot wall connecting them with main shaft. On the main level the ladder
ways and chutes or mills, 6 feet square, requiring timbers 7 feet long and
averaging 12 inches in diameter, with a minimum of 9 inches in diameter,
are timbered up a few feet above the drift sets. The space from which the
ore has been removed is then filled and the drift sets covered with several
feet of rock. Fig. 15 illustrates the way in which these fills are made, and
shows how connection is maintained, by means of chutes and ladder ways,
with the inain drifts at the bottom of the level.
PI. IX, B shows the top of one of the fills in the Minnesota
Company's mine. In the background is seen the cribbed jiortion of the
U. S. GEOLOGrCAL SURVEY
MONOGRAPH XLV PL. IX
.1. MAIN-LEVEL TIMBERING, MINNESOTA MINE.
'^:^^M
B. FILLING SYSTEM, MINNESOTA MINES, SOUDAN, MINN., WITH CHUTE FOR DISCHARGING REFUSE FROM UPPER
LEVELS.
From photographs belonging to the Minnesota School of Mines.
SOUDAN FORMATION.
237
raise, which passes tlu'ough the level and from which rock for the filling
is obtained, with a loaded tram car below its mouth. This raise extends
upward through the foot wall, with ore forming the front side of the raise,
to the level above, through which it passes in a cribbed way, and so on to
the surface. Below the point shown in the figure the raise is cribbed, and is
Oi.i'.'o.'^
? -^
BOTTOM or PIT „■■ ! *
-^^mmzm^^^^^^^'
\\4-TH L^VEL
I 5TH LEVEL
eTH LEVEL
80 feet-
Fig. 15.— Longitudinal section through Soudan mine.
carried by this cribbing tlii-ough the fills on the various levels below. To
the left is shown a diamond drill, used in drilling holes in the ore fonning
the roof of the stope, into which dynamite is then introduced and
discharged, bringing down enormous masses of the ore.
The rock -for this filhng is obtained from the raises which are cut in
the soap rock at the foot or hanging wall, and which communicate
238
THE VERMILION IRON-BEARING DISTRICT.
from level to level with the bottom of the open pit. Such a raise passes
through a level with its fi-ont face cribbed and an opening in the raise at
the height at which it is desired to discharge the filling into the trams
waiting for it. When filling is desired at any place the entire raise below
that point is filled, and the filling
then run through the opening in the
cribbing at the proper place; or tim-
bers and rails may be laid across
the raise, making a floor, which pre-
vents the filling descending- below
that point, and from this place it is
then run out into the trams.
Rock to fill the topmost level,
or any other level being worked at
the same time with it, is taken from
the accumulations of loose material
at the bottom of the open pit, de-
rived from the caving in of its sides
and from the drift at its surface, or
else from material which has been
blasted down from the walls of the
pit. After the topmost level is
worked out filling may be run down
into the level below that, and so on
down as the development requires it.
Fig. 16 is a cross section through the
Minnesota mine showing how the
raise in the foot wall connects with
the surface, and how connection is
maintained with the levels at various
heights as the filling proceeds.
Upon this filling the workmen
stand, and here the (frills are placed, mounted on braced cribbing, or in
any other way that will give them ;i sufiiciently firm foundation. This
keeps the workmen near the roof, which is one of the advantages of oA-er-
hand stoping, in that the roof can be easi'y examined and accidents from
Ifi. — Cross section of Soudan mine show-ins raise.
U. S. GEOLOGICAL SURVEY
MONOGRAPH XLV PL.
A. VIEW SHOWING METHOD OF LOADING CARS.
B. VIEW OF MAIN DRIFT WHICH HAS BEGUN TO CAVE. CHANDLER MINE
From photographs belonging to the Minnesota School of Mines.
SOUDAN FORMATION. 239
falls prevented by breaking- down tbe rock as soon as it is observed to
be loose. From the top of the fill the roof can readily be reached and a
slice of ore about 10 feet in thickness is blasted down from it, broken
up, and tlu'own into the chutes (mills). These chutes are 25 feet apart.
Formerly they were much more widely separated, but experience has
shown that the distance now maintained is the best. The men han-
dling the ore which falls between can work both ways from the chutes
and can throw the ore into the chutes without tramming or second
handling. These chutes lead down to the drift below, and from them
tlie ore is let out into the tram cars. PI. X, A shows the method of
loading these cars in the main drift at the bottom of one of the chutes.
When filled the cars are hauled by mules to the shaft and thence hoisted
to the surface. As the stope is extended filling is let in from the raises,
and the chutes and ladder ways are extended upward to keep pace with
the filling.
The ore is very hard and the cost of breaking it is high. Percussion
or power drills are used to some extent, but diamond drills are more com-
monly used for boring the holes which are used in blasting the ore down.
This is probably the only mine in the district in which the diamond drills are
used for boring preparatory to blasting. It has been found that diamond
drills are in the long run cheaper for this work than percussion drills.
After having been hoisted to the svirface the ore is run tlii'ough Blake
crushers, which reduces it to sizes suited for furnace use. The ore is then
run directly into cars d;iring the shipping season, or during the winter
season is piled in stock piles and loaded from these by steam shovels into
cars when the shipping season begins.
The ore deposits at Ely are the most important and most interesting
in the district. The oldest and most productive mine at Ely is the
Chandler, in whicli the ore is mined on the caving' system. The great
body of ore that is exposed by the Chandler lies in a trough of greenstone
which plunges to the east at an angle of about 45°. The continuation of
this same ore body is worked on the caving system by the Pioneer mine to
the east of the Chandler, which, since it is on the pitch of the ore body,
gets all of the water from the Chandler. A layer of sheared greenstone
discolored by iron (paint rock), 20 to 22 feet thick, lies between the ore and
the comparatively massive greenstone. The foot and hanging walls of this
240 THE VERMILION IRON-BEARING DISTRICT.
paint rock dip to the north at an angle of about 70^. Figs. 10 and 11 are
respectively vertical E.-W. longitudinal and vertical N.-S. cross sections of
the Chandler mine showing the features here described. The ore is capped
by fractured iron-formation material, jasper, chert, and ore bands, which is
overlain by glacial drift. The ore body has been reached through five
shafts sunk in the greenstone. The greatest depth is attained by No. 5 shaft,
which was down in 1902 to the eighteenth level, at a depth of 740 feet
vertically. All of the shafts were originally vertical, but shafts Nos. 2 and
4 were sunk so close to the ore body that the upper portion slid into the pit
as the result of the caving. Inclined shafts with openings farther back
from the pit were sunk to intercept the shafts at a point where the caving
has not injured them. Most of the work is now done from shafts Nos. 3
and 5. The method of mining is the caving system, slightly different in
the newer woi'kings — that is, in the lower levels — from what it is higher
up. The following concise description by Denton will give an idea of
the system:"
Down to the eighth level the method of mining is as follows: Main levels are
driven 75 feet apart and generally' there are two main drifts at the bottom of each
block, running approximately parallel on opposite sides of the block of ore. From
these main drifts raises are put up at intervals of about 50 feet, and from these raises
four series of subdrifts are run. The sets in the main drifts are made of 9-foot caps
and 7-foot legs, and those in the "subs" of 6-foot caps and 6-foot legs. This leaves
about 8 feet of ore between the sublevels. The sets are placed 3 to i feet apart.
As the raises are put up, sets are placed to start the first subdrifts: but these drifts
are not run at once, but are omitted to strengthen the main drifts until the fourth,
third, and second "subs" have been worked out. When the subdrifts are completed,
the block of ore between any two levels is honej'combed with drifts with vertical
intervals of 8 feet of ore. When mining above has been completed, the removal of the
ore pillars on the top "subs" begins. The pillars are sliced away, the back is caved,
and the caved ore is removed in wheelbarrows to the chutes leading to the main level
below. The chutes are 4 feet square and lined with 2-inch plank placed on edge.
When the sand or overlying timber appears, a new slice is taken off the pillar and
the back of ore is caved, as before, until finally all of the subdrifts have been worked
out, when the operation of caving is continued in the block below, which in the
meantime will have been honeycombed by the first or preparatory subdrifts.
Below the eighth level the method of mining has been modified. What are
called intermediate main drifts are driven through the ore at intervals of 20 feet
"Trans. Am. Inst. Min. Eng., Vol. XXI, 1892, p. 355.
SOUDAN FORMATION. 241
instead of 75 feet, and no subdrifts are used. The intermediate main drifts are of
the regular size, 9-foot caps and 7-foot legs, which leaves about 10 feet of ore to be
caved, instead of 7 to 8, as before. Stations are made at the shaft for each inter-
mediate main level. Under this modified system the removal of each 20-foot block
will be done as before, but the putting up of raises will be saved, and it is intended
to use cars and thus do away with wheelbarrows as far as possible.
PI. X, B, shows a main drift which has begun to cave under the
weight caused by the wrecking of the subdi-ifts just above. PL XI,
shows the miners removing the ore from a part of the mine where the
caving has taken place.
The ore mined \>j the Chandler is good hard hematite, practicall}- as
hard as the Soudan ore. Subsequent to its formation (p. 233) it was frac-
tured by the orogenic forces which folded the rocks, and it was thereafter
not completely cemented. A more complete healing of these fractures
seems to have taken place in the ore exposed by the workings of the Savoy
•mine (old Section 26 mine), at the eastern end of the Ely trough. The
caving system described above, as followed in the Chandler mine under
Manager John Pengilly, takes advantage of the fracturing which already
exists in the ore and can-ies it farther through the pressure of the super-
incumbent load. As a result, the mass of ore obtained in this way is very
much brecciated, so that it can readily be broken up by picks. In conse-
quence of the ease with whicli it can be obtained by picking, the ore is
frequently erroneously spoken of as a soft ore. The individual pieces of
the breccia, it must be borne in mind, are, however, the hard hematite,
nearly as hard as that of the Soudan mines. In driving some of the
headings, where the caving has not affected the ore, machine drills are
very frequently employed.
PRODUCTION AND SHIPMENTS OF IRON ORE FROM THE VERMILION DISTRICT.
The following tabulated statement gives the annual production and ship-
ments of iron ore for the Vermilion district and the totals for the district
since the date of the first shipment (1884) up to the present. The figures
for the annual production during the early years of the mines were not
obtainable, but have been given as a lump sum for those years. The figures
have been compiled by the Minnesota Iron Company, and are the most
accurate that can be obtained.
MON XLV — 03 16
242 THE VERMILION IRON-BEARING DISTRICT.
Statement showing production and shipments from all Vermilion Range mines since 1881),.
Minnesota or Soudan.
Chandler.
Production.
Shipments.
Production.
Shipments.
1884 1
Long tons.
■ 1, 182, 882
312, 088
553, 172
518, 600
513, 667
568, 471
429, 170
448, 943
412, 636
427, 797
502, 738
426, 240
441, 000
310, 000
257, 677
307, 166
Long tons.
62, 122
Long tons. .
Long tons.
1885
227,075
307,949
. 394,911
454,019
1886
1887 ..
1888
169. 457
54, 612
1889
479,240 1 317.827
306, 220
1890
540, 013
508, 842
498, 353
485, 778
391, 612
431, 647
448, 970
592, 244
428, 054
456, 225
325,025
208, 284
275,168
364, 659
469, 741
718, 014
427, 595
588,475
347, 449
551, 310
586, 353
628, 268
648, 296
644, 053
659, 820
593, 750
336, 002
1891
373, 403
1892
651, 799
1893
435, 379
562, 088
1894
1895
600, 987
471, 544
1896
1897
438, 366
716, 049
808, 324
644, 801
1898
1899
1900
1901
627, 379
1902 --
645, 575
Totals
7, 612, 247
7.515,531
7, 715, 067
7, 672, 528
Year.
Pioneer.
Zenith.
Savoy and Sibley.
Production.
Shipments.
Production.
Shipments.
Production.
Shipments.
1884
Long tons.
Long ions. Long tons.
Long tons.
Long tons.
Long tons.
1885
........ 1-
1886
1887
1888
1889
540, 299
3, 144
12,012
3,079
2,651
1890
1891
1892
I 97, 961
r 14, 991
14, 388
1893
1894
1895
40, 054
149, 073
207, 103
'
1896
18, 765
I 40, 817
1897
"29,408
n In stock.
SOUDAN FORMATION.
243
Statement showing production and shipments from all Vermilion Range mines since
18S4 — Continued.
Pioneer.
Zenith.
Savoy and Sibley.
Production.
Shipments.
Production.
Shipments.
Production.
Shipments.
1898
Long tons.
30, 740
381, 304
492, 393
620, 659
669, 745
Long tons.
123, 183
339, 897
450, 794
678, 301
673, 863
Long tons.
3,924
76, 303
54, 252
73, 512
162, 006
Long tons.
Long tons.
42
97, 081
175, 251
194, 329
324, 865
Long tons.
1899
79, 322
60, 089
60, 037
167, 206
86, 191
1900
175, 118
1901
211, 799
1902
321, 054
Totals
2, 735, 140
2, 683, 154
467, 9.58
455, 615
820, 976
794, 162
Long tons.
Total production 19,351,-388
' Total shipments 19, 120,990
PROSPECTING.
From the facts of occurrence given in the preceding pages we are
enabled to draw the following conclusions concerning the localities at which
prospecting for ore might be advantageously prosecuted in the Vermilion
district At the outset it may be said that as a result of the mode of forma-
tion and occurrence of the ores it is very probable that all of the large
ore deposits which exist will somewhere reach the top of the iron formation.
However, in consequence of the glacial drift which covers a large portion
of this region, the ore deposits rarely reach the present surface of the
ground, being buried by the drift. In the case of the deposits at Tower
and Soudan, the ore, on account of its exceptional hardness, outcropped upon
the tops of the highest hills, which, owing to their height, are, to a very
considerable extent, free from the drift. But usually the ore is softer than
the adjacent hard jasper and chert and greenstones, and is likely to have
suffered more from erosion than these. Hence the ores commonly occu2)y
more or less marked topographic depressions.
At Ely the exposures were first found at the west end of a broad basin,
illustrating what the writer considers the tj^pical occurrence, at least for the
east end of the district. Since igneous rocks form the impervious basements
upon whicli the ore deposits rest, it follows that the igneous rocks should be
examined with great care, especially where they are impregnated with iron
and are in the condition in which they are known as the paint rock. The ideal
244 THE VERMILION IRON-BEARING DISTRICT.
condition for the accumulation of ore is, as has been shown, a pitching ti'ough
of greenstone, with a great thickness of fractured jasper lying in it. The
most favorable conditions for the formation of an ore body or ore bodies of
some size are present where there is an amphitheater of greenstone within
which lies the jasper, in a much crumpled and fractured condition, with the
axes of the synclines plunging toward the opening of the amphitheater of
greenstone. Unfortunately, even when these favorable conditions ai-e
found, only exceptionally has the accumulation of the ore taken place.
The size of the ore bodies varies much as the result of a number of
factors, but one can state with confidence that the larger the amphitheater
and the larger and thicker the mass of iron formation, and the fewer the
dikes contained within this formation — whose effect would be to subdivide
the large pocket into a number of smaller ones — the more .likely will an
ore deposit found prove to be a large one.
The Tower and Soudan dejDOsits are in synclines which occur on the
top of anticlines, foi'ming hills. As the result of this known occurrence,
the prospectors have appeared very generally to neglect explorations in low
ground. But there are a number of areas of low ground that are with
great degree of probability underlain by the iron formation, and some of
them possibly by iron ore, which should be explored, for, as already noted,
where the ore is soft it is generall}^ found to occupy the lower areas. The
difficulties attending prospecting" in these low areas are great, on account of
the water and the deep drift frequently found in them; but they may
contain ore deposits which will pay in proportion to the difficulties
attendant upon their discovery. It may be well to call to mind the fact
that some of the large deposits occurring in Micliigan — for instance, the
Aragon mine of the Menominee range and the Lake Angeline of the
Marquette range — are found in such positions.
But it should be emphasized that, as a matter of experience, no large
ore dei)0sits have been found except where the iron-bearing formation has
considerable breadth. At many points the favorable conditions mentioned
above, except the presence of broad bands of the iron-bearing formation,
have been found; but in no known case where the iron formation is narrow
have such localities yielded workable ore deposits. Certainly experience
in the Vermilion district does not justify the expenditure of money in
exploring the narrow bands of jasper. Many thousands, probably hundreds
SOUDAN FORMATION. 245
of thousands, of dollars have beeu spent in exploring these narrow bands
without any returns.
A close study of the map shows some places whicli seem favorable for
prospecting. In a general survey it is impossible to study a district in such
detail as to warrant an expression of opinion as to individual localities. In
fact, such A^'ery detailed study with a view of determining the exact location
of ore deposits can hardly be considered a j^art of the functions of a
national survev.
- The exploration of the iron-bearing formation as an economic problem
should be handled by the mining companies. They would unquestionably
find it greatl}' to their advantage to have competent geologists make exceed-
ingly detailed surveys of properties in which there are large belts of the
iron formation. If such surveys show that certain areas have conditions
favorable for ore deposits it would be advisable to burn over such areas
so as to increase the number (if exposures, and thereby make the areas more
accessible. Finally the favorable locations should be narrowed down still
more by careful dip and horizontal magnetic-needle observations. Only
when all this preliminary work is done should a decision be made in refer-
ence to underground work.
The cost of such preliminary surveys as are advocated is insignificant
when compared with the money required for diamond drilling and other
underground work. Many such expenditures in the past would not have
been made had the results of the surveys advocated been available. Apropos
of the cost of the -diamond-drill work, it may be stated that, from informa-
tion derived from various sources, the estimate has been made that one of
the old com^Dauies operating in the Vermilion district expended at least
Si, 000, 000 in exploratory work without having uncovered thereby any
body of ore of size sufficient to warrant its exploitation. The preliminary
expenditure of a very small fraction of this amount of money for good sur-
veys in advance of underground work would have made unnecessary the
expenditure of much of it, and perhaps would have rendered the expenditure
of a part fruitful.
In conclusion, it may be recalled that the only productive mines are at
two localities, one on the belt of ir<m formation near Tower and Soudan, and
the other on the belt of iron formation running east from Ely. Notwith-
standing extensive but more or less haphazard exploration of other large
246 THE VERMILION IRON-BEARING DISTRICT.
belts for many years, no additional deposits have yet been developed.
However, it is by no means proved that some of these belts may not yield
runs of ore. Bnt the iron-bearing formation and the Ely greenstone are
so intimately mixed that some of the belts which seem to be largely iron
formation may be really to a very large extent composed of greenstone and
jasper. Under these conditions, the systematic exploration of even those
belts which appear to be the most promising- is a matter of extraordinary
difficulty. Certainly the Vermilion district is the most difficult to explore
of any of the iron-bearing districts within that part of the Lake Superior
region in the United States.
- . SECTION IV— GRANITES.
GENERAI., STATEMENT.
At a great number of places throughout the Vermilion district acid
rocks of various kinds have been found. Their macroscopic and microscopic
features demonstrate their igneous character without possibility of question,
and their relations to the adjacent rocks give further proof of this, as they
are found cutting through both the Ely greenstones and the iron-bearing
Soudan formation of the Archean. These rocks vary from fine- to coarse-
grained granites and from porphyries with very fine-grained groundmass to
granite-porphyries. The normal granites predominate. They are known
from the topographic featm-es with which they are associated as: (1) The
granites of Vermilion Lake; (2) the granites of Trout, Burntside, and
Basswood lakes; (3) the granite between Moose Lake and Ivawishiwi
River; (4) the granite of Saganaga Lake. These granites will be considered
in detail in this section.
THE AGE OF THE ACID INTRFSIVES.
All of these rocks are younger than the Ely greenstone, for they
occur in it as dikes. A number of the dikes are also found in the Soudan
formation, which is itself of more recent origin than the greater part of the
Ely greenstone. That these intrusives are older than the next sedimentary
formation of the district — the Ogishke conglomerate, of Lower Huronian
age, which succeeds the Soudan formation — is shown positively b}-
the fact that thej^ occur as pebbles in this conglomerate, and that their
detritus largely constitutes the rocks of this formation. Speaking broadly,
ARCHEAN GRANITES. 247
the g-enei-al period of intrusion of all of these acid igneous rocks may be
placed between the period of the deposition of the latest sediments of the
Archean and that of the deposition of the earliest sediments of the Lower
Huronian series. Some were intruded near the beginning' of this interval,
others probably near the end, but it is now impossible to give their exact
ages. In the description of the rock from each of the large areas after
which it is named an attempt will be made, where there are any facts which
warrant this, to determine more closely its period of intrusion relative to
the other igneous rocks as well as to the sediments.
GRAjSTITE of VERMILIOIS^ LAKE.
DISTRIBUTION, EXPOSURES, AND TOPOGRAPHY.
Distribution.— Granites and related acid rocks are found in great
quantity on the islands in Vermilion Lake and on the adjacent shores.
They are not, however, confined in their distribution to the immediate
vicinity of the lake, for scattered dikes of similar rocks are found cutting
tlu'ough the various older formations in places many miles distant from
Vermilion Lake. Direct surface connection of these distant dikes with
the main masses can not of course be shown, but from their similarity to
the larger masses in the area it is presumed that both are derived from the
same deep-seated source.
JExposures. — The exposures of these acid rocks are very numerous, and
many of them afford opportunity for a study of their different kinds, but
they rarely show more than a single contact when the contact is between
members of the same eruptive series, so that in most cases it is impossible
to tell the exact relations of these rocks to one another — that is, to determine
which is of younger age. Some of the best places at which to see these
Vermilion Lake intrusives are the east end of Ely Island, the point south
of Mud Creek Bay, Stuntz Island, the conical island east of Stuntz, the
prominent point of land farther east of this conical island, and the high,
bare hills on the north side of the "Burnt Forties." Dikes of these rocks
are also numerous in the Ely gTeenstone north of Mud Creek Bay, where
one of them about 30 feet wide, cutting the greenstone and trending east
and west, can readily be seen from the water's edge as a white streak along
the hillside.
On the flanks of knolls occupied by the igneous rocks we very commonly
248 THE VERMILION IRON-BEARING DISTRICT.
find sedimeiltary rocks — eouglomerates and graywackes — whicli have been
derived from tliem. In many cases, moreover, it is only after very careful
and patient examination that the igneous rocks can be separated from the
derived sedimentaries, for where the constituents of these derivative rocks
have been merely cemented together without having been much rolled
and rounded, and where the deposits are not well stratified, the resemblance
between the true igneous rocks and the rocks derived from them is very
great indeed.
The igneous rocks that occur at a considerable distance from the lake
are exposed over only very small areas. In some cases the exposure is suf-
ficient to show that the rocks are dikes in older rocks, and very commonly
this relation is infen-ed from the occurrence of these rocks in the midst of
numerous exposures of rocks belonging to formations which, from facts
observed in other localities, are known to be of greater age than the
eruptives.
Topography. — The eruptives usually occupy the crests of hills, or occur
in rounded or oval hills higher than those occupied by the surrounding-
rocks. They thus are seen to influence the topography to a considerable
extent. This influence is, of course, best shown in those areas where the
rocks occur in large quantity, as, for example, the islands in Vermilion
Lake and the lake shores, rather than at places some distance away, where
they occur as very small masses.
PETROGRAPHIC CHARACTERS.
The rocks considered in this section form a complex varying in both
macroscopic and microscopic characters, as well as in chemical composition,
yet in spite of these variations their close field relations and their char-
acters as determined by laboratory study show that they all belong to one
petrographic province and that they were formed at the same geologic
period.
All of the rocks belonging to this series of eruptives are very light
colored, at the most showing slatj^-gray to gTeenish-gray colors upon fresh
fracture. On weathered surfaces they are usually white or light gray,
varying to yellowish i>r pinkish.
Macroscopic characters. — The rocks under discussion vary from fine-
grained granites to those of coarse grain, and from porphyries with telsitic
groundmass and rare phenocrysts to coarse-grained granite-porphyries.
ARCHEAN GRANITES. 249
The porphyritic rocks are most common. Quartz is the usual phenocryst,
but it is sometimes accompanied by feldspar. In some of the porphyries
the phenocrysts are very abundant, but in others they are very scarce. The
quartz phenocrysts range in size from those that are scarcely separable from
the quartz of the groundmass up to crystals nearly an inch in diameter.
Two characteristic porphyries especially are of common occurrence in the
district. One contains a great number of small vitreous-looking quartz
phenocrysts; the other usually shows only a few very large phenocrysts.
The quartz phenocrysts differ greatly in number in different types of the
rock. In some of the rocks only a few are present; in others they occur
in great abundance. Moreover, the quartz shows considerable variation in
character. Usually it is clear and vitreous; less commonly it has a some-
what bluish tinge, but is still vitreous. Very commonly white, opaque,
porcelain-like phenocrysts appear, and some are found that are black and
opaque. Usually the quartz phenocrysts of the rock occurring at a single
exposure are all alike; that is, all are clear and colorless or all are black,
but sometimes these are found intermingled.
Feldspar in white crystals appears also as phenocrysts in the
porphyries and, like the quartz, varies much in abundance. The dark
2^henocrysts seen in these rocks are either mica or hornblende, or
occasionally the two together, but as a rule dark phenocrysts are scarce.
The groundmass of these porphyries gives to them some of their
distinctive characters. In some the groundmass is dense and aphanitic;
in others it is distinctly granular; in still others it is coarse grained;
moreover, there are phases of the groundmass that show all gradations
between these different kinds. In some of the porphyries, as has been
said, the phenocrysts are practically wanting, and as these become reduced
in quantity the rocks gradually change to those which we would call
granites; and we find not only changes from porphyries into granites,
but gradational phases among the granites, varying- from fine-grained
microgranites to normal coarse-grained granites.
The granites and porphyries of Vermilion Lake occasionally include
fragments of the iron-bearing Soudan formation, both large and small, as
well as fragments of greenstones, but they contain no inclusions of a
recognizably sedimentary rock other than those derived from the
iron-bearing formation.
250 THE YERjVHLION IRON-BEARING DISTRICT.
Microscopic characters. — The result of microscopic examination sustains
tlie determinations made by macroscopic studies. It shows that there are
present in the acid iutrusives of Yennilion Lake the following peti'Ographic
varieties: Rhyolite-poi-phyry, feldspathic porphyry, microgranite, granite,
microgranite-porphyry, and granite-poi-]^)h}T.y. The minerals occurring
in all of these are essentially the same. Under the microscope quartz is
the most prominent primary constituent, and ranges from minute particles
taking part in the construction of the groundmass up to phenocrysts an inch
in length. Both orthoclase and jjlagioclase feldspar occur in the groundmass
and as phenocrysts. Polysynthetically twinned plagioclase predominates
among the phenocrysts. These feldspars are much altered and an accurate
determination of their characters was not made. Brown mica occurs occa-
sionally as phenocrysts and is almost always altered to chlorite. A few
indi\dduals of common green hornblende were observed, which appear to
be primarv. Apatite, sphene, zircon, and a little iron oxide were also
observed. From these various minerals there have been produced by
alteration the following secondary minerals: Calcite, which is distinctly
fen-iferous, chlorite, epidote, zoisite, sericite, muscovite, and rutile. Pyrite
in cubes is also commonly found in some of the altered intrusives. The
texture of these rocks is normally granitic, although occasionally a tend-
ency to a trachytic texture was observed in some of the poi-phyries and
more commonly a niicropegmatitic texture was seen.
No analyses of these inti'usive rocks have been obtained. Indeed no
special effort has been made to obtain analyses, for the reason that their
field associations and general characters show clearly that the vai-ious kinds
of rock included under the above head belong genetically together, and
for the further reason that the rocks are without exception considerably
altered, so that analyses might be misleading rather than helpful. It is
highly probable that analyses of fresh rocks, could such be obtained, would
show that these acid intnisives range from the granites toward the diorites
by increase in soda-lime-feldspar and diminution in quartz and orthoclase,
as is indicated in some of the feldspathic porphyries.
FOLDING.
These intrusives have been subjected to dynamic action, for they are
very commonly jointed, and in some places even rendered schistose. They
have also taken part in the folding, but owing to their general homogeneous
ARCHEAN GRANITES. 251
nature it is impossible on exposures consisting of intrusive rocks alone to
trace out the extent and character of this folding-. Where the intrusives
are associated with younger sedimentary rocks the folding is clearly shown,
and is described on pages 288-291.
STRUCTURAL FEATURES AND METAMORPHISM.
As a rule the intrusives are broken up by a series of joints, which are
sometimes so close together that the rocks l)reak up into small more or less
regular rhombs. The very general distribution of the jointing seems to
point to the fact that the rocks were not buried very deep, for had they
been so buried it is probable that they would have acted more as viscous
materials, and that schistosity would have been developed, instead of the
stress being relieved hj the formation of joints, as is the case. As a rule,
however, the rocks composing the blocks between the fractures show no or
very slight indications of schistosity. On these jointed rocks it is not
uncommon to find a few shearing planes along which schistosity has been
developed. Moreover, the rock is sometimes schistose along the small
fracture planes themselves. Reference has casually been made to the
difficulty sometimes experienced, even under favorable circumstances, of
distinguishing between these igneous intrusives and some of the sedimentary
rocks derived directly from them. When schistosity has been developed
in the igneous rock, even though it be imperfectly developed, the difficulty
is much increased, whether the test applied be that of macroscopic or
microscopic examination, or, as in most cases, the two combined.
Some very interesting occuiTences of pseudo-conglomeratic rocks
derived from these acid rocks by orogenic movement may be seen at
Vermilioii Lake. One of the best cases is shown on the flat island lying
just south of Ely Island, in the NE. i of the NE. i of sec. 30, T. 62 N.,
R. 15 W. This island is composed chiefly of rhyolite-porphyry, with fine
felsitic groundmass, having large quartz phenocrysts scattered through it.
This porphyry is intersected by two sets of partings, which vary from big
joints down to very minute partitig planes. One of these sets of fractures
strikes N. 16° E. and the other N. 80° W. These planes of fracture
separate the rock into numberless more or less regular rhombs. The large
joints, a fo.ot or more distant from one another, break the rock into large
rhombs, which in their turn are subdivided into a great number of smaller
252 THE VERMILION IRON-BEARING DISTRICT.
ones by systems of still smaller joints and parting planes lia\'ing the same
trend as the larger fractures. After the formation of the joints movement
took place along the fracture planes, and as a result of this movement the
i-hombs have been rubbed against one another and their angles have been
more or less completely rounded, so that the rhombs have acquired now a
more or less perfect oval outline. The long axes of the ovals agree, of
course, since the ovals were all produced by the same processes. The
rock is now strikingly like a conglomerate, and forms a fine example
of a pseudo-conglomerate. This pseudo-conglomerate may, nevertheless,
readily be distinguished from true conglomerates, such as occur in abundant
typical development at Vermilion Lake, and especially on the shores of
Stuntz Bay, by the fact that all of the pebbles of the pseudo-conglomerate
are of exactly the same kind of porphyry, and that the matrix between
the pebbles is merely a sheared form of the same porphyry. Moreover,
no indication of bedding whatsoever is found in this pseudo-conglomerate.
The true conglomerates contain pebbles of various porphyries, as well as
of the older greenstone and the iron-bearing formation, and gradations can
be followed from these conglomerates into the graywackes, and through
these into the overlying slates. The pseudo-conglomerate is most typically
developed at the location given abpve. It is seen in less typical develop-
ment on the point south of Mud Creek Bay and at some other localities on
Vermilion Lake. It is not so common, however, as one might be led to
suppose fi'om previous descriptions of the Vermilion district." Far more
numerous are the exposures of the porphyi')' on which the fracturing is not
very distinct, and on which movement has not been sufficient to produce
the rounding of the rhombs and the pseudo-conglomeratic structure. These
pseudo-conglomerates were desci'ibed by Smyth and Finlay' under the
name "conglomerate breccias," and the manner in wliich tliey were formed
was correctly interpreted. However, these authors vinfortnnateh' classed
in their conglomerate breccias the enormously and typically developed
sedimentary conglomerates that occur on the islands and shores at the east
and southeast side of Vermilion Lake, and in particular on Stuntz Bay of
that lake, where they are interbedded with and grade into the normal
fiuer-gi'ained graywackes and slates.
"Thej.'i'olngiciilfitnK'tiiivof the wes^tern part of the Vermilion Range, ^linnesota, by 11. L. Smyth,
and .J. Ralph Finlay: Tran.s. Am. Inst. Min. Eng., Vol. XXV, 1895, pp. 610-613.
''Op. cit., pp. 629-6,33.
ARCHEAN GRANITES. 253
Sericite-schists. — When the crushing- and accompanying- alteration are
considerably advanced, sericite-schists are prodviced from these porphyries
and granites. At one stage we find a few eyes of quartz and feldspar left.
These lie in a finely granular groundmass in which the parallel structure of
the secondary minerals is very evident. This parallelism is most pro-
nounced when the rock contains a g-reat deal of sericite, for then the plates
of sericite are arranged parallel to one another and greatly emphasize the
structure. The parallel arrangement follows around the ^^henocrysts,
showing that it was produced after their foi'mation. It may be that in
some cases this parallelism represents partly an original flow structure which
has been emphasized by the production of the secondary minerals. The
extreme stage shows a very fine-g'rained schistose rock which is of a
yellowish-green color macroscopically, and which under the microscope is
seen to be a finely granular aggregate of quartz, presumably some feldspar,
and flakes of sericite, these being the predominant minerals.
CMorite-scMsts. — In a number of cases the porphyries show abnormal
alteration to a green chlorite-schist instead of to a sericite-schist. Such
alteration was found in immediate association with the iron-bearing forma-
tion— that is, where the acid rocks had been intruded and then infolded in
the iron formation. The production of the chloi'ite and of the green color in
g-eneral is due to the infiltration of iron from adjacent foi'mations. In some
instances the green coloration is found only along the contact of an acid
dike with the iron formation and extends only a few inches into the
porphyry. In other cases, where the acid rock is in the midst of the
iron formation, the rock is distinctly green, and might be, and in fact has
been taken for a product derived from the altered basic rocks of the area —
the greenstones. In some places the quartz phenocrysts have been granu-
lated, but in others they are still intact and show clearly the fact that
these schists were derived from acid rocks. The efi^ect of the iron in
causing the production of chlorite instead of sericite can be seen in many
places in the massive acid rocks. In these we very commonly find that
when iron pyrites occurs it is almost invariably surrounded by a zone of
limonite of variable thickness, and beyond this zone of limonite there is
a chloritic zone in the groundmass, whereas elsewhere sericite occurs and
not chlorite.
Schistose granites and schists derived from granites. — Locally these gran-
ites have been very much crushed, and as a result of this crushing there
254 THE VERMILION IRON-BEARING DISTRICT.
has been developed in a number of cases a parallelism of the feldspar and
quartz and, especially, of the mica and secondary chlorite. The qu^.rtz
shows clearly the effects of the crushing in the very common undulatory
extinction, in the fractures that pierce the phenocrysts, and in the granula-
tion of the phenocrysts, which represents the final stage. This crushing
has, of course, more or less completely obliterated the textures and has
usually greatly altered the minerals. Fractures passing through rocks have
been healed by infiltrated quartz, or by secondary feldspar in cases where
the fractures cross the feldspar phenocrysts. In such a case the secondary
feldspar corresponds in extinction with the adjacent feldspar bordering the
fracture, but is fresh and clear, and is readily distinguishable by these
characters from the altered original feldspar.
RELATIONS TO ADJACENT FORMATIONS.
The acid rocks described above are younger than the adjacent Ely
greenstones and Soudan formation. They occur in dikes in both of these
formations and include fragments of both. Detailed descriptions of the
occurrences of some of these rocks will be found under the heading "Inter-
esting localities " (p. 255).
Relations to Lower Huronian series. — The relations of the granite of
Vermilion Lake to the Lower Huronian sediments will be discussed in
detail later. It will suffice here to state that these sediments have been
derived partly from the acid rocks and hence are younger than they. The
detailed proof of their relations will be given in the chapter devoted to the
discussion of these sediments.
Interrelation of granites of Vermilion Lake. — The acid intrusives, while
of the same general age with respect to the older and younger sedimentary
formations, show certain age relations among themselves which are inter-
esting. The fine granite seems to be rather more extensively developed,
on the whole, than the rhyolite-porphyries and the granite-porphyries.
This granite is cut at several points by dikes of fine-grained granite-por-
phyr)^ containing small quartz phen<x',rysts — for instance, on the point
south of Mud Creek Bay, on Stuntz Island, on the island just west of
Stuntz Island, and at a locality just north of the prominent jasper outcrop
on the east side of Stuntz Bay. This granite-porphyry is in its turn found
to be intruded by the granite-porphyry containing the large quartz eyes.
ARCHEAN GRANITES. 255
Such an occurrence was observed in the Burnt Forties, for example. Thus
the succession, beginning with the oldest of the intrusives, is: Fine- to
medium-grained granite, fine-grained granite-porphyry with small eyes,
and coarse granite-porphyry with large quartz eyes. The relation of the
rhyolite-porphyrj- to the other acid eruptives is not definitely known, as no
occurrence has been found in which the relations between them are shown.
INTERESTING LOCALITIES.
Localities shoiving relation between granite of Vermilion Lake and the Ely
greenstone. — The relations between the acid intrusives and the Ely green-
stone are clearly shown at many places in the Tower area of the Vermilion
district. In the following paragraphs some of the most accessible of these
places will be mentioned.
On the north shore of Mud Creek Bay, in sec. 1, T. 62 N., R. 15 W.,
immediately north of the westernmost island in this bay, there is a broad
dike of nearly white medium-grained porphyry which trends east and west,
and can be distinctly seen from the water. This dike cuts directly across
the Ely greenstone, showing sharp contacts with it in many places.
Just northeast of this place, on the section line between sec. 1, T. 62 N.,
R. 16 W., and sec. 6, T. 62 N., R. 14 W., are dikes of granite cutting the green-
stone and the iron formation infolded in the greenstone. In fact, one can
hardly go a quarter of a mile in any direction over these nearly bare hills
without finding one or more of these acid dikes. The greenstone is in many
places schistose, and the dikes are also frequently found to be more or less
schistose along their margins, the schistosity striking a little north of west.
The presence of this schistosity is clear proof that the area has been folded
subsequent to the period of the intrusion of these igneous rocks.
About the center of sec. 6, T. 62 N., R. 14 W., there is a large boss
of porphyritic granite, which is completely surrounded by more or less
schistose gTcenstone. Numerous dikes of granite, ranging from a few
inches up to 15 feet in width, and perfectly massive, evidently ofi'shoots
from this central boss, penetrate the greenstones. Frequently they follow
the schistosity, but in some cases they cut across the schistosity, and in
places they include fragments of the schist. Since the position of these
intrusives has evidently been influenced by the preexisting schistosity, they
were evidently intruded somewhat after those that have already been
256 THE VERMILION IRON-BEARING DISTRICT.
mentioned. They are believed, however, to belong to the same general
period of intrusion.
On the hills south of Mud Creek Bay and south of Mud Creek itself
there are also numbers of dikes of granite and porphyry which cut the ellip-
soidal g-reenstone.
The following are other localities where the acid intrusives cutting the
greenstone may be studied:
North 1,000 paces, west 1,000 paces, from southeast comer of sec. 9,
T. 62 N., R. 14 W. Here a granite-porphyry containing large phenocrysts
of quartz cuts through a dense greenstone forming the bluff overlooking the
swamp to the south. A similar dike is to be found at north 1,650 paces,
west 950 paces, from southeast corner of sec. 21, T. 62 N., R. 14 W.
West 1,000 paces, from southeast corner of sec. 10, T. 62 N., R. 14 W.
Here the porphyry is fine grained, and has a dense groundmass.
North 200 paces, west 100 paces, from southeast corner of sec. 27, T. 62
N., R. 14 W. This is one of the feldspathic porphyries.
Belations of the acid intrusives to the Soudan formation. — On the bold, bare
hills of jasper, at a point north 270 paces, west 200 paces, from the south-
east corner of sec. 7, T. 62 N., R. 15 W., a dike of granite-porphyry 20 paces
wide, containing large phenocrysts of quartz, cuts through the jasper. It
cuts across the strike of the bands of jasper in places and runs out into the
jasper in small stringers, and also includes fragments of the jasper. The
grain of the intrusive rock is seen to be noticeably finer along the contact
of the small stringers than it is in the main mass of the granite. Reference
has already been made to the granite dikes found cutting the jasper in sec.
1, T. 62 N., R. 15 W. In both of these places the relations are perfectly
clear.
Contacts between these two kinds of rock were not found at many
places, but where they were observed the relationship was clearly shown.
At 1,125 paces north, 1,300 paces west, of the southeast corner of sec. 20,
T. 62 N., R. 14 W., a dike of feldspar porphyry cuts through the jasper and
the associated green schist. This porphyry includes large fragments of the
jasper and small ones of the green schist, showing conclusively its relations
to them. In places these fragments are so numerous that the rock distantly
resembles a conglomerate.
ARCHEAN GRANITES. 257
Relations of the different varieties of the acid intrusives of Vermilion Lake
to one another. — On the bare ridge south of Mud Creek Bay the granite-
porphyry is found cutting the feldspathic porphyry at several places. One
such dike may be found about north 700 paces, west 1,400 paces, from the
southeast corner of sec. 7, T. 62 N., R. 14 W. Again, on the high hill in
the Burnt Forties overlooking the lake the granite-porphyry is found in
contact with, and apparently cutting, the fine-grained porphyritic granite,
and includes fragments of greenstone and jasper. It is intei-esting to
note that immediately around these jasper inclusions the acid intrusive has
become g-reen as the result of the infiltration of iron and the production of
secondary chlorite instead of sericite. From this green background the
phenocrysts of quartz stand out very prominently. A similar alteration
occurs in the acid sills that were intruded through the iron formation on
Soudan Hill.
East of Stuntz Island there is a conical island which is made up chiefly
of the fine-grained feldspathic poi'phyry, and on which there is a dike of
porphyritic granite, about 25 paces in width, running from northwest to
southeast. At certain places in this dike, especially on the southeast slope
of the island, the granite grades into a rock corresponding vexy closely to
the coarse-grained granite-porphyry. On the other hand, at other locali-
ties, the same porphyritic granite was observed to pass into a form of rock
very similar to 'Some of the phases of the feldspathic porphyry. It would
appear from this that these different kinds of rock were all derived from
the same source, and that they merely represent different phases of
development of essentially the same magma. Upon this conical island
there were noted also several small dikes of green schistose basic rock, one
of them running nearly east and west and having a width of from 8 to 12
inches.
On Stuntz Island itself, especially upon the northwest arm of the
island, a mass of this porphyritic granite was found trending about east
and west, cutting through the feldspathic porphyry.
Nearly all of these porphyries contain more or less oval vellowish-
green fragments of rock which apparently were derived from the greenstone
through which they were intruded.
MOX XLV — 03 17
258 THE VERMILION IRON-BEARING DISTRICT.
GBAIS^ITES OF TROUT, BURIS^TSIDE, AND BASSWOOD XiAKES.
DISTRIBUTION, EXPOSURES, AND TOPOGRAPHY.
Distribution. — The rocks described under the above heading occur
along the northern edge of the Yermihon district, and extend from Verniiliou
Lake on the west to the east side of Basswood Lake on the east, where they
cross the international boundary. Observations made on Hunters Island,
Province of Ontario, Canada, show that similar rocks are present in that
province and that they possess the same geographic and geologic relation-
ship to the members of the Kaministiquia iron range, which is the continua-
tion of the Vermilion iron range of Minnesota to the northeast, as do their
Minnesota analogues to the various members of the Vermilion iron range of
Minnesota. They are for this reason presumed to belong geologically with
the granite of Basswood Lake. While it is known that the granites of
Trout, Burntside, and Basswood lakes extend at least as far northeast-
southwest as the limits of the area mapped, these granites — or granite
closely related to them — are presumed to have a very much greater areal
extent, as upon the Canadian maps granites are shown covering large areas
in portions of Ontario which are continuous with the Vermilion range.
The Minnesota and Canadian maps and a traverse luade by canoe show
that ffranite extends a considerable distance north of the international
boundary. Since the prime object of the survey whose results are here
described was to study the Vermilion di-strict from an economic point of
view, no attempt has been made to study the outlying granite more closely
than was requisite to determine its relations to the rocks of the district.
Moi'eover, observations have been confined almost altogether to a very
naiTow area bordering the main mass of granite. The traverses usually
ended as soon as we were sure that the limits of the granite had been
passed.
Exposures. — The exposures are, as a rule, very numerous, and the line
of contact between the main granite area and the area of the Vermilion
range proper is usually marked by a topographic break of some kind.
Either a valley and stream are present or else a lake or chain of lakes lies
along the contact. In either case as soon as the depression is crossed, if
one comes from the south, for instance, the granite exposures usually begin,
and they continue in great number as far north as we have been.
ARCHEAN GRANITES. 259
Topography. — The grauite does not seem to affect tlie topography very
materially. One point noted is that the lakes iu the area of the sediments
have, as a rule, a northeast-southwest trend, agreeing thus with the struc-
ture of the district, upon which they are largely dependent, whereas in the
granite area they ai'e more likely to be of very irregular or more or less
rounded outliiie, owing to the more homogeneous character of the granites
by which they are surrounded. The hills in the granite area are usually
rounded as a result of river erosion and subsequent glacial action. In
detail the topography is very rough, as is that of all this portion of the
country, but there are no very great differences in elevation. The district
underlain by the granite does not in general seem to have been much more
strongly affected by erosion than the adjacent portions of the Vermilion
district. In the course of a reconnaissance it was observed that on the
southeast side of Iron Lake, which is on the international boundary, just west
of Crooked Lake, there is an area of country that has been reduced almost
to a base-leveled plain, with Iron Lake as the plane of base-level. The
shores of the lake for a considerable distance back from the water's edge
possess all the features of such a base-leveled plain. The streams enter the
lake through broad marshes having wide estuaries and flow in meandering
courses through these marshes. An occasional hill (monadnock) of granite
projects above this level plain. Some of the islands in the lake and points
projecting into it are so low that in many places by rising in the canoe one
can see over them.
PETROGRAPHIC CHARACTERS.
Macroscopic characters. — The granite of Trout, Burntside, and Basswood
lakes shows a considerable variation in character, as one might be led to
expect from its great areal distribvition. In color it varies from A'ery light
gray through pink and reddish facies to very dark gray. An equal variation
in grain may be seen. It ranges from very tine-grained to coarse-grained
forms and also to granite-porphyries. The structure of the rock is in gen-
eral massive, but with these massive forms occur gneissoid rocks varying
in color from light gray to very dark. Some of these gneissoid rocks pre-
sumably owe their structure to pressure applied subsequent to their consoli-
dation. In these the minerals show to a large degree the effects of pressure.
Other facies may be due to differentiation processes and to movements in
260 THE VERMILION IRON-BEARING DISTRICT.
the unconsolidated magma. Between rocks formed in this way and those
formed as the result of pressure, but in which complete recrystallization has
taken place, no distinction can be made. In the areas examined the
massive granites predominate greatly over the schistose rocks. In the fol-
lowing brief description only the massive granites will be considei'ed.
Microscopic characters. — The mineral constituents are green hornblende,
biotite, orthoclase, quartz, and plag'ioclase, with accessory sphene, zircon,
and iron oxide. These minerals have been very much altered, so that their
places are taken largely by secondary minerals, of which chlorite is the
most prominent, and, after this, epidote and sericite and secondar)" feldspar.
There is a variation in the mineral character, hornblende being practically
wanting in some specimens and increasing very much in quantity in others.
No cases were found in which the quartz was wanting, but it was reduced
in quantity in some cases. The Trout, Basswood, and Burntside lakes acid
rocks seem to vary from hornblende- and mica-granites to syenites, with the
granites predominant.
RELATIONS TO ADJACENT FORMATIONS.
Relations to Ely greenstone. — In approaching that portion of the district
in which the granite of Trout, Burntside, and Basswood lakes is exposed we
cross over a broad ai'ea underlain by the Ely gi-eenstone in its typical
development, in which only rarely is a granite dike to be seen. The closer
we get to the contact between the two above-mentioned formations the more
numerous, however, become these granite dikes, until in places they are so
common that we may almost consider the greenstone as having- been thor-
oughly permeated by the granite magma. This intimate relationship is
beautifully shown on the numerous exposiires at the west end of Burntside
Lake. It should be stated in this connection that the granite dikes cutting
through the rocks exposed on the shores of this lake do not all belong to
exactly the same period of intrusion, but show some slight differences in
• age. These differences are not, however, thought to be great. In other
words, all of the granites are believed to belong to essentially the same
period of intrusion.
From the intrusiA^e relations above illustrated it is clear that tlie granite
is younger than the adjacent greenstone. This intrusive relation is further
emphasized by the progressive metamorphism shown by the Ely greenstone
ARCHEAN GRANITES. 261
as exposures closer and closer to the main granite mass are examined, as
described and explained on p. 156 et seq. It is not uncommonly found that
the greenstone is schistose, and the granite dikes are seen to follow the
schistosity, proving its development prior to the intrusion of the granite.
The granite also includes fragments of schistose greenstone.
Relations to other intrusive rocks. — As stated above, the granite areas
were not studied in great detail, but a sufficient number of observations
were made to show that the granite is cut by both acid and basic dikes.
The basic rocks are cut by acid dikes, as is shown in the photograph repro-
duced on PI. XIII, B. The normal white to gray granite has been cut by a
red-weathering granite which traverses it in dikes, but the period of intru-
sion of these later red granites has not been determined, even approximately.
AGE.
The granite of Trout, Burntside, and Basswood lakes is evidently
younger than the Ely greenstone, which it cuts, includes, and metamor-
phoses. Dikes, offshoots from it, are found following the schistosity of the
greenstone and including these schists. Hence it was certainly intruded
subsequent to the formation of the schistosity in the greenstone. This
schistosity was prodiiced primarily as a result of the folding which took
place subsequent to the deposition of the iron formation and which caused
the folding of this iron formation. Therefore it is concluded that this
granite is younger than the iron-bearing formation, although the jasper is
in no place cut by dikes which can be connected directly with the main
masses of the granite.
This g-ranite is not clearly recognizable in the pebbles in the overlying
Lower Huronian sedimentary series, but where this series comes closest to
the granite its relations are sufficiently clear. Thus, for example, dikes of
this granite, as in sec. 16, T. 64 N., R. 9 W., north of Moose Lake, are
found to ciTt the greenstones underlying the sediments, but never to pass
the contact and penetrate the sediments, although the dikes are numerous
near the contact. Hence the conclusion is reached that the granite of Trout,
Burntside, and Basswood lakes is older than the Lower Huronian sediments.
262 THE VERMILION IKON-BEARING DISTRICT.
FOLDING.
If the granite has been subjected to severe mountain-making processes,
as it presumably has, it does not now show any very marked effects of
these. No folding, of course, could be traced in such a homogeneous rock,
and it is only natural that one should find an occasional shearing plane along
which the granite is more or less schistose. In general the granite, as
already stated, possesses a very massive character. Unquestionably these
rocks must have taken part in the folding of the district, but the presump-
tion is that in general this great granite mass bordering the north side of
the Vermilion district acted as a relatively imyielding area against which
the rocks to the south have been forced. As a result partially of this, the
adjacent greenstones, consisting primarily to a large extent of easily
alterable pyroxene, have been metamorphosed into amphibolitic schists.
INTERESTING LOCALITIES.
In the following- paragraphs will be found brief descriptions of some
localities which show the relations of the granite of Trout, Basswood, and
Burntside lakes to the Ely greenstone.
The relations between these rocks are very clearly shown at many
places along the north and west shores of the northwest end of Pine Island
and on the adjacent shore of the mainland, where granite dikes intrude the
greenstone. These dikes are scattered over a wide zone along the contact
between these two igneous rocks, and as a result of their intrusion the
greenstone has been altered to amphibolitic schists. The dikes frequently
contain large masses and smaller fragments of schists similar to those
surrounding them. About 2 miles northeast of Mud Creek Baj- and about
three-fourths of a mile due north of the Sheridan mine, the schist is intri-
cately intruded by the granite. The schist has been so broken up as the
result of the intrusion that in places there has been formed almost an eruptive
breccia, with the granite as the cementing material. In some cases the
intrusive granite assumes roundish forms, and where the schist predominates
one might almost consider the rock a pseudo-conglomerate with granite
bowlders in a green schist matrix.
The relationship between the granite of Burntside Lake and the
ellipsoidal greenstones is well shown at a great number of places on the
shores of Bm-ntside Lake. Coasting along the southern shore to the west
ARCHEAN GRANITES. 263
of the portage one observes dikes of this granite in the ellipsoidal green-
stone, which has been metamorphosed to an amphibolitic schist on the steep
north-facing slopes in the sonthwest quarter of section 23. Similar exposures
occur again on the point in the southwest quarter of section 22 and in the
southeast quarter of section 21. The intrusive character of the granite and
the intricacy of this intrusion is best shown on the almost continuous expo-
sures that border the northwest shore of the lake in sees. 30 and 20, T. 63 IST.,
R. 13 W. One starting at almost any place on the southern shore of Burntside
Lake, where the granite dikes are numerous, will find that they diminish in
number southward, and with this diminution in number at a distance from
the main granite mass one will find that the schists gradually lose their
schistosit}' and grade into the normal greenstones.
Two hundred paces north of the shore of Long Lake, on a line 1,000
paces west of the east line of sec. 21, T. 63 N., R. 12 W., there is a well-
marked eruptive breccia, produced by the intrusion of granite of Burntside
Lake, which includes a vast number of fragments of the greenstone forming
the main country rock. A similar breccia is found about 70 paces farther
north of the above location. A dike of the granite developed as granite-
porphjay cuts the greenstone at 1,000 paces north, 1,000 paces west from
the southeast corner of sec. 19, T. 64 N., R 10 W.
The relationship between the granite of Basswood Lake and the Ely
greenstone is well shown on a high hill at about 500 paces north of the
southeast corner of sec. 17, T. 64 N., R. 9 W., and on the hill in the
southeast quarter of sec. 16, T. 64 N., R. 9 W. At both of these places the
greenstone is penetrated by numerous dikes, and it has been metamor-
phosed in most cases to amphibolitic and occasionally micaceous rock, in
which schistosity is very frequently more or less well developed.
GRANITE BETWEEN MOOSE LAKE AND KAWISHIWI EITER, IN SEC. 5,
T. 6.3 N., E. 9 W.
DISTRIBUTION AND EXPOSURES.
Distribution. — In the SE. { of sec. 5, T. 63 N., R. 9 W., there is an
oval mass of granite, ha^^ng diameters of about one-half mile northeast-
southwest by one-fourth mile east-west. In the vicinity of this mass and
in the greenstone area for several miles to the west — in general we may
say in the territory between Moose Lake and Kawisliiwi River — there are
264 THE VERMILION IRON-BEARING DISTRICT.
a number of granite and granite-porphyry dikes which, since they are of
practically the same petrographic character as the large mass and show the
same relationship to the adjacent rocks that this mass shows to similar rocks
adjacent to it, are presumed to be offshoots from this large mass, or at least
to have come from the same deep-seated mass of magma from which it
came.
Exposures. — The exposures are fairly numerous where the large mass
of granite occurs, but over a portion of the area underlain by this there
is a large amount of fallen timber, which helps to conceal the rocks and
renders the area exceedingly difficult of access.
PETROGRAPHIC CHARACTERS.
On fresh fracture this granite is dark gray in color, though at times
it has a reddish tinge. On weathered surfaces it usually becomes grayish.
It is of medium grain, and is sometimes developed as a granite-porphyry
in which the feldspar and quartz phenocrysts can be easily seen lying in
the dark-gra}^ fine-grained groundmass. The granite-porphyry facies bears
a very strong resemblance to the porphyries of Vermilion Lake, as well as
to the porphyritic facies of the granite of Saganaga Lake. These rocks
show only the ordinary characters of granites, and a brief description of
them will suffice. The constituents are the iisual ones — quartz, orthoclase,
plagioclase feldspar so altered that no individuals suitable for close deter-
minations of character could be foimd, a little brown mica, and magnetite.
In the porphyry the feldspar is the most prominent phenocryst, occurring
in both larger and more numerous individuals than the rounded quartz
phenocrysts associated with it. The feldspar shows fairly good crj^stal con-
tours, though sometimes the crystals are rounded. In the porphyries the
groundmass in which the phenocrysts lie is a fine-grained aggregate of feld-
spar, quartz, calcite, epidote, zoisite, rutile, chlorite, biotite, sericite, and
pyrite, all in small individuals. Most of these are of secondar}^ origin, yet
some of the quartz and feldspar, and possibly some of the biotite may be
primary, although not recognizable as such.
RELATIONS TO ADJACENT FORMATIONS.
In immediate proximity to the main mass of this granite are the Ely
greenstone of the Archean and sedimentaries of Lower Huronian age only.
Relation to Archean. — In the vicinity of the granite the Archean green-
ARCHEAN GRANITES. 265
stone is cut by acid dikes which are petrograpliically similar to this granite
and are beheved to be offshoots from it. Hence the fact that the Ely green-
stone is older than the granite is indisputably shown.
Although the Soudan formation does not occur near the main mass of
the granite, nevertheless dikes similar to the granite are found cutting
through this iron formation at places a number of miles distant from the
main massive, and if it is admitted that these dikes belong to the same
period of intrusion as the main mass of granite, then it is equally plain that
the iron formation is older than the granite.
Relation to Lower Huronian. — The Lower Hui'onian sediments and the
granite occur close to each other. For the most part these sediments are
fine slates with graywackes and very few narrow bands of cong-lomerate.
However, at a few places conglomerates have been found overlying- and
derived from the granite, and very good proof of the relations between
the granite and Lower Huronian sediments is thereby given. Negative
evidence is furthermore offered by the fact that no dikes which can be
identified with the granite are found penetrating these Lower Huroniai^
sediments, although they occur in the greenstones that immediately underlie
these sediments, and are in close proximity to them.
Relation to Keiveenawan. — The granite is itself cut by narrow dikes of
coarse black diabase, which are supposed to be of Keweenawan age, the
very youngest intrusives occurring in the district.
GRAlSriTE OF SAGAKAGA LAKE.
The granite of Saganaga Lake has probably one of the best-known
names of any geologic formation occurring in the Vermilion district of
Minnesota, for it has appeared repeatedly in the Minnesota reports and in
other publications in which its field and age relations to the adjacent rocks
have been discussed."
"Winchell, A., Geol. and Nat. Hist. Survej' of Minnesota, Sixteenth Ann. Rept., 1888, pp.
211-233 and 330-334. Grant, U. S., Geol. alid Nat. Hist. Survey of Minnesota, Twentieth Ann. Kept.,
1893, pp. 83-95; Final Rept., Vol. IV, 1899, pp. 321-323 and 467; also Am. Geologist, Vol. X, 1892,
p. 7. Lawson, A. C, Am. Geologist, Vol. VII, 1891, p. 324. Geological age of the Saganaga granite,
by H. V. Winchell: Am. Jour. Sci., 3d series. Vol. XLI, 1891, pp. 386-390.
266 THE VERMILION IRON-BEARING DISTRICT.
DISTRIBUTION, EXPOSURES, AND TOPOGRAPHY.
Distribution. — This gi-anite is confined in its oeciuTence to Saganaga
Lake and its vicinity. It covers about 100 square miles in Minnesota, but
only a portion of its area is shown on the accompanying map (PI. II).
Exposures. — Within this area its exposures are very numerous, for the
country is in mau}^ places comparatively bare of vegetation, the drift is as
a rule thin, and the presence of large bodies of water — Saganaga, AVest
Gull, and Red Rock lakes and their tributary streams — insure frequent
exposm'es on the shore of the mainland and on the islands.
Topography. — Tlie topography of tliis granite area offers a strong con-
trast to that of the surrounding country. In the smTOunding- territory,
which is underlain by Archean greenstones and Lower Huronian sediments,
the topography is roug'h, being- marked b)^ prominent hills, generally, to be
sure, having the round contours characteristic of glaciated areas, but often
presenting high and rugged cliffs. Within the granite area the topographic
features are, for the most part, not strongly emphasized. The hills are low
and rounded and the hilltops seem to approach very nearly the same level,
so that in looking over this area from some of the higher surrounding-
elevations one g'ets an idea that this particular portion of the district has
been reduced very nearly to a peneplain.
PETROGRAPHIC CHARACTERS.
Macroscopic characters. — This granite is very coarse g-rained over a large
portion of the area in which it occurs and is usually developed as a grajiite-
porphvry in which the phenocrysts are large quartzes. However, as one
traverses the region from West Grull Lake through Red Rock to Saganaga
Lake — that is, as one goes approximately from the periphery toward the
center of the area — it is very noticeable that the grain of the rock, which is
relatively fine upon the exposvires on West Gvill Lake, gi'ows coai'ser toward
the center. This is one of the evidences in favor of the intrusive character
of the granite. In color the granite varies from light gray to pink, and
even to brick red. This last strong tint is usually present where the
alteration is the most pronounced.
The granite massive is cut in various places by fiue-gTained red aplite
dikes.
ARCHEAN GRANITES. 267
Here and there in the granite may be found masses of dark gray to
green rock. Some of them have been in planes along which movement has
taken place, and are extremely altered, and have become schistose. Others
have been much altered, but no actual motion appears to have occurred in
them, so that they are still massive. These rocks are generally basic- —
some are even ultrabasic — and vary from basalts to peridotites. They
are intrusive in the granite, but how much younger than the granite they
may be is not known. The schistose basic rocks presumably belong' to
a period of eruption later than the Lower Huronian, and correspond to
the dikes described in Chapter IV. The freshest basalts are presumably
of Keweenawan age and are similar to those described in Chapter VI,
under the heading "Keweenawan."
Microscopic characters. — Under the microscope the granite is found to
be either a mica- (biotite-) granite or a hornblende-granite. This last is the
predominant rock. It varies by loss of quartz to a syenite. Glrant" has
described still a different facies of the granite of Saganaga Lake — a fluorite-
granite which he observed upon an island in Saganaga Lake. The essential
minerals are mica, hornblende, quartz, orthoclase, and plagioclase. With
these occur as accessory minerals, and in very small quantity, some apatite,
sphene, and magnetite. These possess their usual characters and show the
relations common to such minerals in the granites. All of the rocks are
considerably altered. The usual secondary minerals — calcite, sericite,
actinolite, epidote, and chlorite — have been produced and are present in the
sections examined. The granite varies somewhat in textural character from
the normal granite to a granite-porphyry.
In the granite-porphyries the phenocrysts are quartz, hornblende, and
plagioclase. Around some of the feldspar phenocrysts there is occasionally
micropegmatitic intergrowth of the feldspar with quartz. These minerals
are distinctly of the first generation, and lie in a moderately fine-grained
groundmass of hornblende, feldspar, and quartz of the second generation,
in striking contrast in size to those of the first generation.
"Geol. and Nat. Hist. Survey of Minnesota, Twentieth Ann. Rept, 1893, p. 89; Geol. and Nat.
Hist. Survey of Minnesota, Final Rept., Vol. IV, 1899, p. 323.
268 THE VERMILION IRON-BEARING DISTRICT.
RELATIONS TO ADJACENT FORMATIONS.
The granite of Saganaga Lake is found in contact with and showing
clearly its relations to the Archean Ely greenstone and the Lower Huronian
sediments — the Ogishke conglomerate, and the Knife Lake slates.
Relations to Ely greenstone. — In the southern portion of the area under-
lain by the granite of Saganaga Lake, on the south shores of West Gull and
Gull lakes, and along the contact between the granite and the Ely green-
stone to the east of these localities, the granite penetrates the Archean
greenstones in numerous dikes. Moreover, the intrusive nature of the
granite is further shown by the fact that in the contact zone the greenstone
is metamorphosed by the granite to an amphibolitic schist, whereas at some
distance away from the contact zone — that is, beyond the influence of the
granite — the greenstones show their normal characters.
On the northern. side of the granite area, on the north and east shores
of Cache Bay of Saganaga Lake (this is within Canadian territory), the
same relations are clearly shown on a great number of exposures around
the shores of the bay. Here, too, the metamorphism of the greenstones
diminishes as the distance from the main mass of granite increases. Further-
more, the granite contains inclusions of rock derived from this greenstone.
On Red Rock, West Gull, and Gull lakes there are in places in the
granite irregular fragments of hornblendic rocks that are believed to have
been derived from the ancient greenstones tlii'ough which the granites were
intruded. This intrusive relationship of the granite and greenstone has
been recognized by all geologists who have studied this area, except H. V.
Winchell," who maintains that the granite is derived from the greenstones,
or Keewatin green schists, as he calls them.
Relations to the Lower Huronian sediments. — No such general agreement
has been reached among the geologists who have studied the Vermilion
district and the adjacent district in Ontario as to the relationship which
exists between the granite of Saganaga Lake and the adjacent sedimentaries.
A. H. WinchelP has decided that the granite is younger, as a granite, than
the sedimentaries, and that it was derived from them by processes of
progressive metamoi-phism. Lawson" describes it as intrusive in the sedi-
« Geological age of the Saganaga syenite, by H. V. Winchell: Am. Jour. Sci., 3d Series, Vol. XLI,
1891, p. 389.
f> Winchell, A., Geol. and Nat. Hist. Survey of Minnesota, Sixteenth Ann. Kept, 1888, p. 211.
'Lake Superior stratigraphy, by A. C. Lawson: Am. Geologist, Vol. VII, 1891, p. 324.
ARCHEAN GRANITES. 269
mentaries, and having no other connection with them. Gi'ant," having first
considered the granite as intrusive in the sedimentaries, examined it a
second time, and since then has maintained that the sedimentaries are
younger than the granite, having been derived from it.
The sedimentaries lie on the western flank of the granite area extend-
ing from Cache Bay of Saganaga Lake on the north to West GruU Lake on
the south. Over a considerable portion of this area — for instance, where
the drainage is imperfect — exposures are very few, thick morainal deposits
covering that part of the area extending approximately from sec. 30, T. 6G
N., R. 5 W., southward into sec. 5, T. 65 N., R. 5 W. At the extreme north
and the extreme south, however, Saganaga and West Gull lakes, respec-
tively, lie along the contact and give fairly good opportunities for a
study of the existing relations. On the northern exposures especially the
relations are so absolutely clear and convincing that the phenomena there
observed will be first described.
Following the international boundary route from west to east, one
passes in order through the long, narrow lakes of Knife and Otter Track
(Cypress), then through Oak (Swamp) Lake into a bay of Saganaga Lake.
The rocks exposed on these lakes are chiefly slates and graywackes, with
occasionally a fine interstratified conglomerate. On Knife Lake the strike
of the slates is about N. 70^ to 80° E. As we go eastward we note a
change in this strike, and when the east end of Otter Track Lake is reached
the strike has become N. 45° E. to N. 20° E. and N. 10° E., and even in
places is shown as nearly north and sojath. From Otter Track Lake we
cross on the portage a ridge of the slates and then enter Oak Lake (Swamp
Lake), where there are exposed over the greater portion of the shores the
same dark slates and g-raywackes that are fotnid on the lakes farther west.
On this lake the strike of these sediments has tui'ned until it is west of
north. On the east side of the lake the sediments are noticeably difl^erent
in character from those we have been observing. They are no longer
dark, but are light in color — pink to reddish — and instead of being fine
slates are predominantly coarse-grained arkoses. They show distinct
bedding and dip, and one can trace gradations from the coarsest-grained
rocks into the finer-grained ones. This alternation was noted bv earlier
observers, but was misinterpreted. These coarse ai'koses so closely resemble
"Grant, U. S., Geol. and Nat. Hist. Survey of JMinnesota, Final Kept., Vol. IV, 1899, p. 322.
270 THE VERMILION IRON-BEARING DISTRICT.
the granite of Sagauaga Lake (they are derived directly from it) that they
were actuall}' taken for that granite, altered, however, and were supposed
to have been intruded iu thin sheets parallel to the fine-grained beds
lying in alternation with them. This was Grant's idea upon his fii-st
visit, when he decided that the granite of Saganaga Lake was for this
reason j'ounger than the adjacent sediments." A subsequent visit caused
him to change his view to the correct one, upon recognition of their true
relations. The sedimentary characters of these bedded arkoses were noted
by N. H. Winchell,* and likewise the resemblance of these arkoses to
the granite of Saganaga Lake, the main mass of which lies east of and
beneath these sediments, and the relations were interpreted by him as
evidence of progressive downward metamorphism, the arkoses having
been fused and transformed into the granite of Saganaga Lake. Examined
under the microscope, these bedded rocks are seen to be made up of
fragments of quartz and feldspar and flakes of mica. None of the minerals
are well rounded, but neither do they show the same relationships to each
other that the same minerals always exhibit in unquestionable granites
Moreover, the well-marked sedimentary banding iu them and the gradation
from coarser- to finer-grained rocks show that these are without question
fragmental rocks or arkoses derived from the granite and consisting of the
same constituents as the granite from which they were derived. These
fragments of minerals have riot been very much worn, and, since they
were deposited here through the action of water, have been cemented
together so as to form a rock that is strikingly like a granite, especially to
one making a superficial macroscopic examination. A microscopic study,
and indeed a close macroscopic field study, immediately discloses the
characters above mentioned, and shows that they are different from the
granite. Excellent exposures of the arkose occur on the portage from Oak
Lake to the west bay of Saganaga Lake, and good exposures of the frag-
mentals showing distinct bedding may be seen on tlie north shore of this
bay, on the point just east of the portage. After passing along these
outcrops of sediments one finally comes to the clearly recognizable typical
massive granite-porphyry of Saganaga Lake. These rocks have the
distribution shown upon Sheet XVI in the accompanying atlas.
"Geo), and Nat. Hist. Survey of Minnesota, Twentieth Ann. Rept., 189.S, pp. 90-95. ''Loo. cit.
ARCHEAN GRANITES. 271
Farther east and north on the shores of Cache Bay of Saganaga Lake
the relations between the granite and these sediments are shown still more
conclusively, if that be possible, than at the localities above referred to.
On the southwest side of Cache Bay, at a number of excellent exposures,
the granite of Saganaga Lake is overlain to the west by a beautiful typical
coarse basal conglomerate. This conglomerate is made up almost solely
of bowlders of all sizes, derived directly from the immediately adjacent
granite of Saganaga Lake. The matrix between these larger fragments is
the finer detrital material derived from the same source.
The unconformable relations of the sediments known as the Ogishke
conglomerate and the Knife Lake slates, here classed as Lower Huronian,
to the granite of Saganaga Lake and the Ely greenstones of this eastern por-
tion of the district are clearly shown by the above-stated field occurrences.
On West Gull Lake exposures are not nearly so good nor so extensive
as on Saganaga Lake. Nevertheless, from a study of this area correct con-
clusions concerning the relations of the rocks were reached, and the later
study of the Saganaga Lake area merely served to emphasize their accu-
racy. The areal distribution of the rocks within a small part of the
district on the west shore of West Gull Lake is shown on the accompany-
ing map, fig. 17. Starting in at the granite outcrop between the two
meander corners at a point about one-fourth of a mile north of the southeast
corner of sec. 7, T. 6.5 N., R. 5 W., we find that the granite along the shore
shows its normal characters and is cut by several dikes of basic rock. As
we follow the exposure inland, however, the granite is fotmd to become
more and more schistose, and finally we notice that this schistose rotten rock
is made up of small gi-anitic fragments, with the finer granitic debris for
cement. Its fragmental character is most clearly shown by an occasional
very small fragment of jasper. In examining this exposure one can say,
when the extremes are seen, "Here is a granite, and here is a clastic derived
from the granite ; " but no sharp line of demarcation between them can be
drawn, for, indeed, there is no such sharp line. On the contrary, there is
an imperceptible gradation from the one to the other through the interme-
diate schistose material which probably represents the disintegrated portion
of the granite which was not removed by erosion. In several other places,
where the granite and sedimentaries come nearl}^ together, they are se-pa-
rated by a narrow area usually marked by a small topographic depression
272
THE VERMILION IRON-BEARING DISTRICT.
in which uo rocks are exposed or in which there is only an occasional iso-
lated exposure of rotten schistose rock. In such exposures the characters
Corner
ALGONKIAN
UOWER I^URONIAN
Ogishke conglomerate
ARCHEAN
Granite Granite porphyry
(with observed (without observed
strike and dip) strikeand dip)
Scale
O 1/4.
'/a mile
Fio. 17.— DL'tiiil geologic map sliuwing exposures in u smiill iiruii on West linll Luke.
ARCHEAN GRANITES. 273
of this rock can not be definitely recognized in all cases, but since it is
analogous in all respects to the material described above as occurring
between the granite and the clearly recognizable sediments, it is assumed
to be the schistose arkose that lies between the granite and the overlying
sediments. These sediments consist of interbedded slates, gray wackes, and
conglomerates, and. in these conglomerates pebbles were observed which
could be identified with the granite of Saganaga Lake. After a study of
the exposures here there can be no reasonable doubt that the sediments are
younger than, and partially derived from, the adjacent granite of Saganaga
Lake.
Within the area shown on the map forming fig. 17 there is a knob of
greenstone, penetrated by numerous dikes of g-ranite, similar to those
occurring in the greenstone adjacent to the granite of Saganaga Lake at
other places in this district. These dikes are presumed to be offshoots from
the granite. Overlying this granite are sediments similar to those overljdng
the not far distant granites, consisting to a considerable extent of pebbles
of granite and greenstone, showing them to be younger than both the
greenstones and the granite. Within this small area, therefore, we find
the Archean greenstone, the granite of Saganaga Lake, and the Lower
Huronian sediments, with their relations to one another clearly shown.
The Lower Huronian sediments are now folded into synclines within the
granite and greenstone, and hence wrap around these rocks, as is shown
on the accompanying map (fig. 17).
METAMORPHIC EFFECTS OF THE GRANITE OF SAGANAGA LAKE.
The granite of Saganaga Lake having been found intrusive only in the
greenstones of Archean age, we are able to study its metamorphic effects
upon these rocks alone. These effects are in all respects the same as those
produced upon the similar greenstones by the intrusion of the granites of
Trout, Basswood, and Burntside lakes, as the result of which amphibolitic
schists were produced. The processes of metamorphism induced by these
intrusions and the products resulting therefrom have been described in
preceding portions of this monograph (p. 156 et seq.).
INTERESTING LOCALITIES.
Exposures of the granite of Saganaga Lake are so extensive in the
area in which it occurs that it is unnecessary to refer to any special locality
at which its characters may be studied. There are, however, several places
MON XLV — 03 18
274 THE VERMILION IKON-BEARING DISTRICT.
at which the relations between the granite and the adjacent rocks may be
noted with advantage, and although these have already been mentioned,
attention will be again called to them.
The relations of the granite of Saganaga Lake to the Ely ellipsoidal
greenstone may be seen at almost any place along the contact between the
two on the south shore of Gull Lake. For instance, just below the north
section lines of sees. 22 and 23, T. 65 N., R. 5 W., numerous dikes of the
granite cut the greenstone. The greenstone near the contact with the
granite, where it is full of granite dikes, has been extremely metamorphosed.
The farther we go southward from the contact the less altered is the green-
stone and the better preserved are the ellipsoidal, amygdaloidal, and other
structm-es. The southwest shore of West Gull Lake and the small lake on
the portage route between West Gull and Gull lakes are easily accessible,
and here in the cliffs many dikes of granite cut the greenstone. The same
relation is very clearly shown on the northeast shore of Cache Bay, which
is the large bay of Saganaga Lake that extends into Canada. Along this
shore innumerable dikes of the granite cut these schists.
The granite of Saganaga Lake is found in contact also with the Ogishke
conglomerate, and its relation to the Ogishke conglomerate is well shown at
certain places (mentioned above, pp. 269-273) on the west side of West Gull
Lake, on Saganaga Lake, and on Cache Bay of Saganaga Lake. At all of
these places the conglomerate consists largely of bowlders and finer detrital
material derived from the granite. The rocks along the contact have in
places been closely folded, and as a result of this folding the contact
between the two is somewhat irregular and the relations appeared to be
complicated, but careful studies of the exposures have shown the relations
above stated.
i
CHAPTER IV.
THE LOWER HURONIAN.
SECTION L— SEDIMENTARY ROCKS.
OCCURRENCE AIN^D SUBDIVISIOIS^S.
The Lower Huroniau sediments of the Vermilion district have a very
large surface extent. They are present in two large detached areas — one,
known as the Vermilion Lake area, extending eastward from the western
limit of the area mapped near Tower, on Vermilion Lake, to within about 1 1
miles of Ely; the other, known as the Knife Lake area, beginning about
7 miles west of Ely and extending eastward to the eastern limit of the area
mapped. These same rocks extend farther eastward for an unknown but
great distance, passing north of and around the granite of Saganaga Lake
into Canada. Where the Vermilion and Knife Lake areas approach
each other — that is, west of Ely — the rocks have their least surficial extent,
rapidly widening as we follow them from this point eastward or westward.
This distribution is due to the fact that the sediments occur in two great
synclinoria. The short distance of about 5 miles by which the continuity
of the rocks is interrupted represents the place where, as a result of a
cross anticline, the lower (Archean) rocks have been brought to the sui-face.
This gap is so narrow, and the structure points so clearly to the original
extension of the sediments across it, that this lack of continuity is not con-
sidered important. Clearly the rocks of the two areas were continuous
before erosion separated them. Considered in a broad way, the sediments
of the Lower Huronian are fragmental rocks consisting predominantly of
conglomerates and slates, although fragmentals intermediate between con-
glomerates and slates are, of course, present.
It is difficult to estimate the relative quantity of the several kinds of
elastics included in the sediments. Moreover, they differ in respective
quantity in different parts of the area. The conglomerates form by far the
more striking portion, and the casual visitor to the district will notice
beautiful exposiires studded with brilliant-red jasper pebbles, and draw,
276 THE VERMILION IRON-BEARING DISTRICT.
})erhaps, the conclusion that the conglomerate is the predominant clastic,
but it is believed the slates make u]} the greater part of the sediments.
Included within the Lower Huronian there is a horizon of iron-bearing
carbonates. These carrj^ a considerable quantity of iron as carbonate in
addition to the calcium-magnesium carbonates. There is also developed at
a few localities an iron-bearing formation consisting of banded jasper,
cherts, and iron ore. This iron formation is present in very small
quantity, and certainly will never be of any importance on the United
States side of the international boundary. These two kinds of rock are
presumed to correspond to each other — that is, they belong to the same
horizon. On the scale on which the map is published, it would be impossible
to represent all of the different bauds of conglomerates, grits, slates, etc.
Consequently no attempt has been made to discriminate between these kinds
of the fragmental rocks further than to show the ai'eal distribution of the
extremes.
We are enabled to divide the Lower Huronian sediments into thi-ee
parts — (1) a lower division, which is predominantly conglomeratic and
which is most typically developed near Ogislike Muncie Lake, and is called
the Ogishke conglomerate; (2) a division represented only in the eastei-n
portion of the district, consisting of iron-bearing rocks and known as the
Agawa formation; and (3) a division which is predominantly a slate
formation and which we shall denominate the Knife Lake slates, since
these slates are well developed and splendidly exposed on and near
Knife Lake. Mention has already been made of the fact that the Lower
Huronian sediments occur in two separate areas within this district. In
each area both the conglomerate and slate are well developed. There are,
however, certain local differences in the rocks that underlie the Lower
Huronian, and as a consequence the sediments in the two areas are
slightly different. For this reason, and also as it simplifies exposition, it
is considered best to describe the rocks of the two areas separately. It
must in each case be clearly understood that the conglomerates of the two
areas are geologically contemporaneous and that the same contem^joraneity
exists in the case of the slate formation. The Agawa formation is present,
however, only in the Knife Lake area and can not be correlated with any
definite formation in the western area. The areas will be described
separately in the following pages.
THE LOWER HURONIAN. 277
VEBMILIOIf LAKE AREA OF THE LOWER HUROISTIAN SEDIMENTS.
DISTRIBUTION, EXPOSURES, AND TOPOGRAPHY.
Listribution. — The Lower Huroniaii of this area has been found to
extend very much farther west than it is shown to do on the accompanying
map (PI. II). It has been carefully studied, however, only in the area out-
lined thereon, and in this description we must consider it as beginning' at the
western limit of the map, where the rocks of the series cover a very broad
area, corresponding in width 23i'actically with the width of the map. Its
greatest breadth is something like 11 miles in Ts. 61, 62, and 63 N., R. 16
W. As this area, as outlined upon the map, is followed to the east, we note
that it is subdivided into a number of smaller areas by the various fingers of
Archean rocks that project westward into it. Beginning at the south, the
area underlain by Lower Huronian rocks is found to extend eastwai'd very
nearly to Bear Head Lake, and on the north from the Archean near West Two
Rivers southward to the limit of the area mapped. Indeed, a reconnaissance
shows conclusively that the same sediments continue beyond the limits of the
map, and are practically bordered on the south by the Griants Range granite.
The next area north of this projects eastward only so far as the town of
Tower. It is but a short tongue, and is bounded on the south by the
Archean greenstone and on the north by this greenstone and the associated
iron-bearing formation which constitutes Lee and Tower hills. North of
this there is a third tongue, occupying the valley between the anticlines of
Tower and Soudan hills, projecting eastward at least as far as the village
of Soudan. North of Soudan Hill, and occupying in general the basin
in which Vermilion Lake lies, there occurs the main portion of the area
underlain by the Lower Huronian sediments. This is also that part of the
area in which the best exposures occur. The rocks of this area have been
followed eastward as far as sees. 2 and 16, T. 62 N., R. 14 W. The main
area of the Lower Huronian sediments around Vermilion Lake may be
subdivided into a number of smaller areas, due to the structural relations
of the rocks. On the east shore of Vermilion Lake, for example, there are
a number of smaller tongues into which the area can be divided. These
will not here be described in detail, but may be found outlined on the
maps in the accompanying atlas. The length of this Lower Huronian
belt from the western limit of the area mapped to the eastern end of the belt
in which the exposures occur is about 17 miles. The Lower Hui'onian
278 THE VERMILION IRON-BE AKING DISTRICT.
sediments within this area are subdivisible into conglomerates and slates, the
formations occiuTing intermediate between these having been classed with
one or the other, according to the predominance of the one or other kind of
rock in the outcrops. The conglomerate of this area has been called the
Stuntz conglomerate." However, the two subdivisions of the series in the
Vermilion Lake area are correlative with the Ogishke conglomerate and
Knife Lake slates of the typical areas in the eastern part of the Vermilion
district, and will be called by the same name in the description of the
western area.
Exposures. — On the islands and on the shores of Vermilion Lake the
exposures of the conglomerate are, on the whole, excellent, and are both
frequent and of large size. In the inland areas, however, the exposures
are not so numerous and are usually small.
The slates are not so well exposed on the islands and shore of
Vei'milion Lake as are the conglomerates, but exposures do occui', and
they are usually of considerable areal extent, well cleared off, and good.
There are likewise good exposures in the broad area underlain by the
slates to the south and southwest of Tower, which is faii'ly well dissected
by stream erosion. This statement is especially true of areas in the
immediate vicinity of Pike River and along part of the course of West
Two Rivers.
Topography. — Considered broadly, the Lower Huronian rocks of this
area occupy relatively low ground, the higher elevations being formed by
the Archean greenstones and the iron-bearing formation, this arrangement
separating the Lower Huronian sediments into the various troughs which
have already been described. The topography of the areas occupied by
the Lower Huronian sediments has already been referred to (p. 36). It is
fairly rugged, but there are no great elevations. The rocks have been
carved into a series of north-northeast to south-southwest trending, rounded
ridges separated by valleys occupied by swamps, streams, or lakes.
STRUCTURE.
Considering the western part of the Vermilion district broadly, it will
be seen that the Lower Huronian sediments occupy a great synclinorium,
trending N. 80° E., with Vermilion Lake lying in its broadest part, and
that the sediments swing around the anticline of greenstone south of
rtGeol. and Nat. Hist. Survey of Minnesota, Final Kept., Vol. IV, 1899, pp. 282, 525-538.
THE LOWER HURONIAN. 279
Tower and spread out southward into a broad area in which exposures
are so rare that no structural details can be determined. It is presumed,
however, that this is also a great synclinorium, the south limb of which
is bounded by the Giants Range granite, here beyond the limits of the
map.
Within the Vermilion Lake area and immediately adjacent to it ex-
posures are sufficient to enable us to determine some of the details of the
structural features of the sediments in them.
In this part of the Vermilion district it willbe noted that the conglom-
erate lies upon the flanks of the anticlinal hills formed by the older under-
lying rocks. Within the area covered by the Lower Huronian sediments
alone, the Ogislike cong-lomerate occupies the anticlines, for example, at the
Pike Bay oval, which is an anticlinal area. Ely Island is made up chiefly
of the Ogishke conglomerate, but enough of the adjacent rocks are exposed
to show plainly the structure. The conglomerate occupies the main central
portion of the island — in fact, nearly all of the western two-thirds of the
island — with but a small area of the Knife Lake slates flanking it on the
south. In the eastern part of the island the conglomerate is intermixed with
eruptive rocks, the granites of Vermilion Lake, from which it is derived and
with which it is intricately infolded. On the eastern as well as on the
western end of the island the conglomerates are coarser near the center, and
grow finer and finer toward the sides. This change is most noticeable on
the south side of the island, where at several places along the shore the Knife
Lake slates grade into the conglomerate through graywackes of intermediate
grain.
In general the slates occur in synclines lying between anticlines of
older and harder rocks, and ordinarily these synclines coincide with the
topographic depressions. In some places, however — as, for instance, north
of Tower, between the west end of Soudan Hill and the point between
Swede and Middle bays — the slates occupy a minor synclinorium and are
extremely plicated. The slates of this particular synclinorium occupy at
this place higher ground than the adjacent conglomerate on its flanks,
and within this synclinorium the anticlines of slates are the structural
features that occur at the greatest elevation. Structural details, such as
strike and dip, were observed almost exclusively on the slates, and it is
consequently by a study of the slate exposures chiefly, assisted hj observa-
280 THE VERMILION IRON-BEARING DISTRICT.
tions of the distribution of the other formations, that the structure of the
series has been determined. It may be noted here that in the absence of
any striking key rocks the folds in the slates were determined chiefly by
the distribution of the slates and their variation in strike and dip.
The general strike of the slate beds is N. 80° E. The slates have
been very closely compressed, and consequently many of the exposures
show the most intricate plications. On the large folds, as well as on the
plications, the strikes extend varyingly to nearly every point of the compass,
the direction depending on the position on the fold of the place where the
strike is taken. The dips are high and range from about 70° S. to 70° N.
The northern dip is the more coinmon and is generally not far from 80°.
The axes of the folds trend approximately N. 80° E., and, as shown by the
predominant northward dip of the beds, the axial plane of these folds is
generall}' overturned, dipping slightly north. In addition to a south-
north compression — to be exact, the pressure came from a direction slightly
west of north and east of south — producing the folds trending east and
west, there has been jwessure at right angles to this, which caused a
corresponding development of north-south folds. As a result of this minor
cross folding, the axes of the major folds — the folds trending east and
west — have a pitch varying from 90° to 65°. As a result of the
compression, sevefal sets of joints have been formed in the rocks. One
trends from N. 60° to 80° E., in close agreement with the strike of the
bedding' and with the trend of the axes of the folds. Another set is
nearly at right angles to the above, and varies from north and south to
N. 10° W. The joints, however, evidently bear definite relations to the
folds, having been produced by the same forces that caused the folding, for
as the strike of the beds varies upon the folds the joints vary also. Thus
on the j)oint southeast of Sucker Point, where the beds strike N. 50° W.,
the joints strike N. 80° W. and N. 30° E. The close compression of these
slates has produced a fissility which is verj- uniform throughout the region.
It is verv noticeable in the slates, and its general strike is N. 80° E.,
althougli a variation of a few degrees to south or north can be found. In
general, there is an agreement of the strike of tissilit}' and bedding, but, as
Van Hise has demonstrated," tlie fissility bears diftereut relations to the
n Principles of North .American pre-Cainbrian geology, by C. E. Van Hiae: Sixteenth Ann. Eept;
U. S. Geol. Survey, Pt. I, 1896, pp. 656-659.
THE LOWER HURONIAN. 281
beds in different parts of the folds. Thus on the Hmbs of the folds it is
essentially parallel with 'he bedding, whereas at the ends of the folds it
cuts across the bedding nearly at a right angle to it. No places were seen
which appeared to promise a supply of good roofing slate. The rocks are
very g-enerally much broken up by minor joints, but at considerable depth
possibly rocks might be found in such condition that roofing squares could
be obtained.
These slates are everywhere intersected by numerous quartz veins,
especially south of Pike River Bay. The presence of these quartz veins in
the slates gave rise to the rumor of the occurrence of gold, and the early
history of the Vermilion district is the history of attempts to obtain gold
from veins in these sediments — as, for instance, at Gold Island, near the
northern part of the lake.
RELATIONS OF OGISHKE CONGLOMERATE AND KNIFE LAKE SLATE.
The relations of the rocks to one another are so clearly shown at so
many places that it is scarcely worth while to discuss them. There is a
great conglomerate normally overlain by and grading up into a great
thickness of slate through the intermediate graywackes. In the con-
glomerates occur masses of slate, and in the slate likewise occur masses of
conglomerate. EAadently the series is a geologic unit which is divisible into
two parts, the conglomerates and slates, only by an arbitrary line below
which the conglomerate predominates on the whole, and above which the
slates predominate. In an article on the Vermilion area Smyth and
Finlay" described the slates as the oldest rocks of the area, instead of
nearly the yoimgest.
RELATIONS TO ADJACENT FORMATIONS.
Relations to Archean. — Where the series is in contact with the Ely
greenstone and the Soudan formation, the conglomerate normally lies next
to these rocks, and consists to a great extent of pebbles derived from them.
Hence, having been derived from them, it must overlie them stratigraphic-
ally and is therefore younger than they.
Occasionally the conglomerate occurs in very thin belts, too narrow to
«The geological structure of the western part of the Vermilion range, Minnesota, by H. L. Smyth
and J Ralph Finlay: Trans. Am. Inst. Min. Eng., Vol. XXV, 1895, p. 602.
282
THE VERMILION IRON-BEARING DISTRICT.
be shown on the map without great exaggeration. Sometimes the con-
glomerate is practically wanting, and then the slates abut against the
Archean, although, as a rule, actual contacts of the slates and adjacent
Knife LaUe slates Ogishke conglomerate Granite Granite-porphyry^
twi*h observed (without observed (w It h ob se r ved (without observed
strike and di p) strike and d ; p) stri kfe and d i p) strike and d I p)
Fig. 18.— Detail map of the east end of Ely Islatid, Vermilion Lake, Minnesota, showing actual roeli o.^posures, by J. Morgan
Clements and C. K. Leith, 1899.
formations are wanting, erosion along the contact having removed the
slates, which are softer than the other rocks at these places.
The relation of the sediments to the granites of Vermilion Lake is
exactly the same as their relation to the greenstone and the iron formation.
The basal conglomerate lies next to these eruptives normally and consists
chiefl}^ of pebbles, which can be identitied with the rocks immediately
THE LOWER HURONIAN. 283
adjacent. Moreover, in places, as we get farther from the eruptives, the
gradation from the conglomerate to finer-grained sediments can be dis-
tinctly traced, showing plainly that the conglomerates were derived from
the eruptives, and that consequently they are of later age. The conglom-
erates and eruptives especially have been very closely infolded and they
now show the most intricate surface relations. T'he large-scale detailed
maps of the east end of Ely Island (fig. 18), and of the point south of Mud
Creek Bay (Sheet XXV of Atlas), will give some idea of the intricacy
of their surface relations and- will indicate the difficulties of successfully
determining the geologic structure. This intricacy of relationship between
the sediments and eruptives is in some places most puzzling. A sketch,
fig. 19 (p. 290), made in the field, illustrates the relations between these
two rocks which were seen on an exposure on the north side of Ely Island,
near the east end, and which will be described in some detail further on.
The character of the conglomerate is usually well marked, especially
when jasper pebbles are abundant in it. Under such circumstances the
pebbles of the acid igneous rocks in the conglomerate can also be identi-
fied with the adjacent masses of granite and porphyry. But where both
the igneous rocks and the conglomerate derived from them have been
very much mashed, and especially where there is a compai-atively fine-
grained sediment — in other words, a grit — it is very difficult to discrim-
inate between them, for both the igneous rocks and the grits derived from
them have been sheared into white to gray fissile sericitic schists which
have practically the same appearance. It is not improbable that some
mistakes have been made on the detailed maps in this discrimination, but
extreme care has been exercised, so that the mistakes are unquestionably
few, and while they may aff"ect the determination of the areal distribution
of these rocks they are not of such character as to affect the interpretation
of their general relations.
Relations to the Giants Range granite. — South of Tower, in the vicinity
of milepost 92, on the Duluth and Iron Range Railroad, the Knife Lake
slates are intruded and metamorphosed by a number of granite dikes which
have been coiTclated with the Griants Range granite. The fact that this
granite is younger than the Lower Huronian sediments is thus clearly shown.
Relations to basic dikes. — The Lower Huronian sediments, both the
Ogishke conglomerate and the Knife Lake slates, are cut by occasional dikes
284 THE VERMILION IRON-BEARING DISTRICT.
of basic rocks, which are thus }"Ounger than the sediments. Splendid
examples of such dikes can be seen on Stuntz Island and elswhere. Some
of these dikes are older than, and some of them are essentially contem-
poraneous with, the Keweenawan rocks.
AGE.
The above facts concerning the relations of the Ogishke conglomerate
and the Knife Lake slate to the adjacent formations are so conclusive that
no doubt can remain as to the relative age of these sediments. They lie
immediately on the Archean Ely greenstone and the Soudan formation,
and are youngei- than these and than the granites of Vermilion Lake which
penetrate these two latter formations. They are older than the Giants
Range granite and than certain basic dikes that cut them. Since the sedi-
ments lie immediately upon the Archean and are overlain by another series
of clastic rocks, as has been found from the study of the contemporaneous
rocks in the Knife Lake area, they are here placed at the base of the
Alffonkian, and are correlated with tlie Lower Huronian series of the other
iron-bearing districts of Lake Superior.
OGISHKE CONGLOMERATE.
This conglomerate was first so called because it is well developed on
and near Ogishke Muncie Lake, and the use of the term has been continued
on account of its appropriateness and becaiise it was used in the early
literature of the Vermilion district.
Atteiition is again called to the statement already made, that the con-
glomerate in some places differs somewhat petrographically from that of
the typical area, and that this variant phase has occasionally been called by
the local name of Stuntz conglomerate. (See p. 278.)
PETROGRAPHIC CHARACTERS.
Macroscopic characters. — The conglomerates in this western area all
possess a strong family resemblance. On the weathered surface the difterent
beds are white or grayish in color. This light color is due to pebbles of
rhyolite-porphyry, microgranite, granite, and granite-porphyr}', which, as a
rule, have very light-colored weathered surfaces and are the main constit-
uents of the conglomerates. In a few places there is a good deal of jasper
pi'esent in angular fragments of various sizes. Greenstone fragments are
THE LOWER HURONIAN. 285
found occasional!}-, but they are much rarer than one would expect them to
be. In addition to the kinds of rock already enumerated, pebbles of black
and gray cliert and yellowish-green sericite-schists were observed.
The conglomerates differ locally in degree of coarseness, varying from
coarse-grained conglomerates, with bowlders reaching 2 feet in diameter,
to those in which the majority of pebbles are about 4 to 5 inches in
diameter. This latter facies is the commoner. Associated with these
conglomerates there are of course considerable quantities of much finer-
grained rocks, which would naturally be called consolidated grits or gray-
wackes, but which are here mapped with the conglomerates. With these
are likewise occasionally areas of slate. On the map an attempt has been
made to discriminate, by means of the colors, between the conglomerate
and the slate, but a close examination in the field would reveal the fact
that in some of the areas marked as conglomerate there are in j^laces
considerable quantities of graywackes and slates associated with and lying
in the midst of the conglomerate. The areas of these rocks are so small
in proportion to the area of the conglomerates that no attempt has been
made to show them on the small scale maps published herewith.
It is interesting to note the dependence of the petrographic character
of the conglomerate upon the adjacent rocks from which it was derived.
Where, for example, it lies next to a certain characteristic porphyry,
the major portion of the conglomerate is formed of pebbles and fine detritus
of the porphyry. On the other hand, where the conglomerate lies near
the iron-bearing' formation, fragments of this formation become numerous,
although ordinarily they are scarce. The pebbles and bowlders in the
conglomerate are crossed by fracture lines which divide the individual
pebbles in it into more or less rhomboidal fragments. This fracturing of
the fragments and the occurrence of the pieces essentially in place shows
that the dynamic action that produced the fracturing took place after the
formation of the conglomerate and that only slight displacements -occurred
as the result thereof.
OEIGIN OF THE CONGLOMERATES.
When we study these conglomerates in the field and find that they
are made up of pebbles of various kinds of rock lying in a fine-grained
clastic matrix, the pebbles of a certain kind of rock being most numerous
286 THE VERMILION IRON-BEARING DISTRICT.
near an underlying mass of the same kind, and when, moreover, we find
beds of grits and slate alternating with the conglomerate, all of these
showing every gradation into one another and possessing both true bedding
and false bedding, the only satisfactory conclusion we can form concerning
the origin of these rocks is that they are true clastic conglomerates of
sedimentary origin. This mode of origin seems so obvious that its
statement appears almost uncalled for, and it is made only for the reason
that a strong argument has been made by previous students of the rocks
of this area for the brecciated origin of these conglomerates." Reference
has already been made in previous pages to the formation of pseudo-con-
glomeratic rocks from the granites of Vermilion Lake by dynamic processes.
The first description of these pseudo-conglomerates (friction conglomerates)
and the correct explanation of their origin was given by Smyth and
Finlay in the article . above referred to. They made the error, however,
of attributing this method of formation to all of the conglomeratic-looking
rocks of that part of the lake and adjacent shores which they studied,
including great masses of true normal conglomerates occurring in great
abundance on Stuntz Island, Stuntz Bay, and elsewhere. These rocks, it
is true, are intimately associated with the pseudo-conglomerates, but in
most places may be readily separated from them. That the true conglom-
erates were unquestionably included with the pseudo-conglomerates is
shown by the fact that reference was made to the conglomerates occurring
on Stuntz Bay and Island as examples of pseudo-conglomerates,'' whereas
in reality they are typical sedimentary conglomerates in which may be
observed the characters referred to above as proving indisputably their
sedimentary origin.
THICKNESS.
The bedding in the coarse conglomerate is poor, but grows more
distinct as the grain gets finer until, as in the rocks here called graywackes,
it becomes very distinct. It is, however, generally so obscure that it has
not been possible to determine it with great accuracy and frequency.
Moreover, the rocks have been extensively folded, and considerable redupH-
cation — which it has not been practicable to determine — may have taken
"Geological structure of the Vermilion range, by H. L. Smyth and J. Ralph Finlay: Ti-ans. Am.
Inst. Min. Eng., Vol. XXV, 1895, pp. 610, 629.
''Op. cit., p. 612.
THE LOWER HURONIAN. 287
place, and would vitiate any estimates. For these reasons it has been
found impossible to determine, even approximately, the thickness of the
conglomerate. In places it is wanting or is represented merely by a thin
mass. In other places, as, for instance, on Vermilion Lake, it shows great
development and must be very thick.
INTERESTING LOCALITIES.
The islands in Pike Bay offer good exposures of the typical Ogishke
conglomerate of the western area. There are also splendid exposures on
the large island in sec. 14, T. 62 N., R. 15 W., and on both sides of Arm-
strong Bay. Smaller exposures occur nearer Tower, one southwest of
Tower in SW. ^ of sec. 6, T. 61 N., R. 15 W., another just on the outskirts
of Tower, on the south slo23e of Lee Hill, and another on the south slope of
Soudan Hill.
One of the best places in which to study this conglomerate in its typical
development is on the southwest side of Stuntz Island, which lies across the
mouth of Stuntz Bay of Vermilion Lake. On the bare exposures here the
conglomerate is made up of pebbles and bowlders of granite-porphyi-y, por-
phyritic microgranite, rhyolite-porphyry, a feldspathic porphyry, jasper, and
comparatively rare fragments of greenstone. The coarsest conglomerate
lies near the center of the island and is separated from the acid intrusives
to the north by a marked depression. Pebbles of the intrusives are present
in the conglomerate. As we go southward across the exposures the con-
glomerate grows finer and occasional beds of grit, striking east and west,
occur in it until finally on the extreme southwestern point of the island there
may be seen at low water a few feet of typical Knife Lake slates. The
evidence here is conclusive that the conglomerate has been derived from
the sediments to the north and that there is a gradation from it into the
Knife Lake slates to the south. On the highest knob at the west end of the
island the conglomerate is penetrated by a number of basic dikes varying
from IJ inches to 6 feet in width. At one place near the highest point nine
dikes were counted within a distance of 60 feet, lying essentially parallel
and trending a little south of east. These dikes cut across the schistosity
and the bedding of the conglomerate and also across the fragments in it,
showing sharp contacts. They do not seem to have produced any contact
effect on the conglomerate. The dikes themselves are only very slightly
schistose, and the schistosity is confined to the edges.
288 THE VERMILION IRON-BEARING DISTRICT.
On the hill just south of Mud Creek Bay the conglomerate is exposed
at a number of places. As a result of the close folding- it appears in very-
complex relationship with different rocks, namely, the Ely ellipsoidal
greenstone and the various granitic rocks of Vermilion Lake. After a
careful study of the exceedingly intricate contacts between the porphyries
and the conglomerates, which at first led to the belief that the porphyries
were intrusive in the conglomerates, it was found that this relationship
was due to the close infolding of the rocks, giving zigzag and most in-egular
contacts. This relationship, as has already been stated in pre"vious pages,
was proved by the identification of the porphyrj* pebbles in the conglom-
erate with the adjacent porphyries. The detailed map, Sheet XXY of the
atlas, shows the areal distribution of these rocks on this point and will give
some idea of the intricacy of the distribution.
Reference has already been made to the rocks occurring on Ely Island.
A good place at which to study the close relationship of these rocks is
the east end. Here there is a most intricately folded complex of moderately
fine-grained granite-porphyry, conglomerate, and graywacke. The distribu-
tion of these is shown on the detailed map forming fig. 18. The graywacke
and porphyry when looked at casually resemble each other very strongly,
but when examined closely they can readily be distinguished. The por-
phyry is studded with small phenocrysts of quartz and, as a result of
weathering, develops an exceedingly rough surface in detail, something like a
nutmeg grater. The graywacke contains grains of quartz which in many
cases, and probably in most cases, are phenocrj^sts derived from the
porphyry and in some instances are very slightly worn. This graywacke
weathers in general with a smooth surface, and this difference in the
weathering alone will usually enable one to distinguish the two kinds of
rocks. Where the gi-aywacke is in very massive exposures, and especially
where the graywacke and porphyry have both been sheared, it is at times
extremely difficult to separate them. As the result of the shearing and
subsequent weathering both the porphyry and the graywacke are likely to
develop a series of small parallel ridges or corrugations on the surface.
This corrugated way of weathering was not so noticeable, however, on the
por])h}Ty as on the graywacke. At this place the infolding of the porphyry
and the sediments is exceedingly complex. We find fingers of the one
interlocked with fingers of the other, so that the contact forms a zigzag
THE LOWER HURONIAN. 289
line, each finger pointing to a small fold — anticline or syncline. The plane
of contact between the porphyry and the sediments varies greatly. In
most cases the porphyry is below the sediments, but in some cases the
fold is clearly overtm-ned, so that now the conglomerate frequently
lies under the porphyry. Between these extremes any position of this
contact plane, from flat to vertical, may be seen. Tliis irregularity
in the position of the plane caused considerable confusion at first in
the determination of the relationship between the rocks. For some time
it was thought that the porphyry was intrusive in the sediments. How-
ever, further study showed that the conglomerate was clearly derived
from the porphj-ry and that the relationship mentioned was due to close
folding. A further factor which led to confusion was that the porphjT.y
itself simulated somewhat a conglomerate, for it is marked by two series
of fracture lines lying close together and crossing each other at such an
angle as to produce small rhomboidal blocks. Further shearing took place
along these planes of parting, and eventually the angles of the fragments
were more or less completely rounded, and the areas between the sub-
angular fragments were filled with schistose material. On exposed sur-
faces the massive unfractured parts of such rocks weather less readily
than do the schistose portions lying between them, and consequently
stand out as small rounded 2Drojecting areas verj^ similar to the pebbles
in a conglomerate, whicli project above the surrounding matrix. Closer
examination of such surfaces, however, shows that the fragments are all
of one kind of rock and that the apparent matrix lying between them is
but sheared material of essentially the same nature as the massive portion.
This is the most obvious fact noticed in a study of them and enables one
readily to separate such fractured and sheared porphyries from the true
conglomerates derived from them, which are made up invariably of frag-
ments of different kinds of porphyries, with more or less abundant jasper
fragments and an occasional fragment of greenstone. The strike of the
axes of the main folds at this locality is about N. 80° E., showing that
the force that produced the folding was exerted along a line extending
approximately north and south. As the result of this compression schistosity
has been developed in the sediments and in the underlying intrusives.
This schistosit}'- cuts directly across the minor folds shown in the zigzag
contacts above described and continues from the sediments into and
MON XLV — 03 19
290
THE VERMILION IRON-BEARING DISTRICT.
through the igneous rocks. The schistosity, which strikes about N. 70° E.,
cuts at a sharp angle the sedimentary bedding, which varies from N. 80° E.
•aiw?^
J5»
_ -/
?^
^■^
**v
>,
'■^y^t^S^'^^'^v^'^^'^^'
Fig. 19,— Sketch showing intricate relationship of granite-porphyry and overlying Ogishke conglomerate on Ely Island,
Vermilion Lake,
to east and west. The dip of the bedding varies shghtly from 75° to the
north to vertical. Indeed, it is occasionally found with a dij) of 85° and
THE LOWER HURONIAN. 291
even 80° to the south, although the steep northward dip is the one which
unquestionably predominates. The sketch reproduced in fig. 19, which was
di-awn to scale in the field, illustrates very well the intricacy of the contact
between the sediments and the igneous rocks and shows other features which
at first tended to create a belief that the porphyry was in igneous contact
with the sediments. The exposure reproduced occurs on the hill at the
west side of the first bay west of the northeast point of Ely Island and on
the north shore of the island. Going- along the contact between these rocks,
one finds the contact plane lying- at diff"erent angles, but on this particular
exposure the folding has not been so great as to overturn the rocks and place
the conglomerate under the igneous rock. Just north of the first main fold
at the south end of the ex^josures sketched are a number of very small
flutings, and one of these is represented as it occui-s in nature — as connected
with the main mass of sediments merely by a small neck. A little farther
north of this place, on the hill, there was observed a small mass of con-
glomerate, rei^resented in the sketch, which was completely separated from
the sediment and sm-rounded by the igneous rock. This evidently was
closely infolded in the igneous rock and afterwards separated by erosion
from the main mass. Here the process of separation has been completed,
whereas in the mass previously described erosion had gone only far enough
to leave merely a narrow neck connecting it with the main area. This
isolated area of conglomerate appeared much like an inclusion of con-
glomerate in the porphyi-}^, and was so construed at first, but later more
detailed studies showed its true character as an infolded mass.
On the east side of Stuntz Bay of Vermilion Lake the conglomerate
is well developed and is exposed over large areas having white weathered
surfaces. Here, as at the other places noted, the conglomeratic character
is plain, the fragments being well rounded and ranging from minute pebbles
to bowlders 2 feet or more in diameter. At one place there is a coarse
conglomerate made up of fairly irregular bowlders, such a conglomerate as
is often deposited near a shore line on which the wave action has not
greatly rounded the fragments. Immediately in contact with this coarse
conglomerate is a belt, about 4 feet thick, of beautiful, regular conglom-
erate, such as would be produced by the consolidation of a shingle beach.
The majority of the pebbles of this bed vary from 1 to 6 inches in
diameter. The conglomerates have not been very much metamorphosed.
292 THE VERMILION IRON-BEARING DISTRICT.
The pebbles have been frequently crossed by fractures which divide them
into rhomboidal pieces, but have not in general been greatly deformed.
The fragments usually retain their relative positions. At least 90 per cent
of the pebbles in the conglomerate are rhyolite- and feldspar-porph^T}',
granite- porphyry, and microgranite, the last of which occurs in numerous
exposures in the immediate vicinity. Intermingled with these in very
small quantity are fragments of jasper and greenstone. The fragments of
porphyry are well-rounded pebbles, and the conglomerates grade into the
finer-grained grits and slates and are occasionall}- traversed by bands of
this finer-grained material, so that there can be no question whatever that
they are normal water-deposited conglomerates.
South of Tower, near milepost 92 on the Duluth and Iron Range Rail-
road, there is a cut which passes through the Ogishke conglomerate and
the associated Knife. Lake slates. The conglomerate is very Avell exposed on
the east side of the road, where the weathered surfaces give one a better
oppoitunity to study the different kinds of pebbles in the rock than can be
had in the small fresh exposm-es in the cut. The conglomerate is of essen-
tially the same character here as at the exposiu-es near Vermilion Lake.
The Knife Lake slates lie north and south of it, and the conglomerate and
the overling slates have been intruded by both acid and basic dikes, the
acid dikes apparently corresponding to the Giants Range granite, which
forms the main portion of the Mesabi or Giants range bordering the northern
portion of the Mesabi district. This occurrence of the conglomerate at
this place is e^^dently due to a subordinate anticline which raised it, erosion
having then removed the superimposed slates and exposed the conglomerate
as we now find it. The sediments here have all been altered, and now the
matrix of the conglomerate and the finer-grained bands that occur occasion-
ally in it have been metamorphosed to amphibole and especially to mica-
schists, whose origin could not be determined but for their association
with and gradation into undoubted sediments.
In the SW. i of sec. 6, T. 61 N., R. 15 W., at the second falls above
the bridge on the county road, on the west bank of West Two Rivers,
there is a cliff consisting of Ogishke conglomerate. This conglomerate is
here only a short distance from ellipsoidal greenstone of the Ely formation,
which is exposed on the east side of the river, and it consists of numerous
pebbles of schistose greenstone, e\-idently derived from the underlying
THE LOWER HURONIAN. 293
Ely greenstone, as well as predominant pebbles of the granite rocks of
Vermilion Lake. This is the point nearest to the basal greenstone at which
the conglomerate has been found in this part of the district.
Another basal conglomerate somewhat similar to the one just desci'ibed
occurs on the south side of the county road leading from Tower to Pike
River, about 325 paces east of the bridge over West Two Rivers. This
conglomerate has been sheared until it is quite schistose. It consists
chiefly of fragments of greenstone, jasper, and feldspar-porphyry.
KNIFE LAKE SLATES.
The conglomerate described in the preceding pages is overlain by the
important Knife Lake formation, which is excellently developed upon the
shores of Vermilion Lake in the vicinity of Tower. The name is given
to the formation on account of its typical development near Knife Lake
(p. 297).
PETKOGEAPHIC CHAEACTEES.
It has been stated that the dividing- line di'awn in the Lower
Huronian sediments between the Ogishke conglomerate and the Knife
Lake slates is purely arbitrary. The .transition between them is not
sharp. Among the conglomerates there are a few interbedded fine-
grained sediments, and among the slates there are a few fragmentals that
are coarser than the normal slates, and show gradations between the slates
and the conglomerates. However, the slates are by far the j)redominant
kind of rock in the areas marked on the accompanying maps with the
slate color, the grits playing a very subordinate r6le.
Corresponding to differences in mineralogic character there is in tlie
slates considerable variation in color and texture. The normal slates are
on fresh fracture generally a slate gray to dark-greenish gray, and even
light greenish. Sometimes they range through purplish and bluish-black
rocks to a dense and almost black slate. They usually weather with a
light-gray to brown crust. The grain of the slates is so fine that one can
distinguish no individual mineral, unless it be quartz, except in the phase
that approaches the grits. The banding in the slates is caused by slight
variations in the quantity of the minerals of different color constituting the
slates, and by a slight difference in size of grain. These bands within the
294 THE VERMILION IRON-BEARING DISTRICT.
slates vary in thickness from a fraction of an inch to several feet. The
bands of slate themselves, where interlaminated with grits and near the
conglomerates, also vary in thickness from a few inches up to 30 paces.
These slate beds show a gradual increase in thickness as they occur at a
greater distance from the conglomerates. The slates are in places — as in
the embayment between Tower and Lee hills — very heavily impregnated
with pyrite, which is scattered through them in cubes, usually altered
more or less to limonite.
Microscopic examination of the Knife Lake slates and associated gray-
wackes shows that the primary constituents are feldspar, quartz, and horn-
blende in fragments. With these occui* secondaiy products — chlorite,
epidote, calcite, sericite, sphene, and pyrite. In the coarse-grained rocks
the various constituents can readily be distinguished. In the finer-grained
ones only the coarser particles can be clearly recognized, and these lie in a
very fine-grained dark matrix whose characters can not be positively
determined, but which probably consists of 'fine dust particles derived
fi-om the other constituents, with which may occasionally be associated
some carbonaceous material (although this was not recognized as such)
and feiTUginous matter, the last being the chief cause of the dark color.
These slates vary from the normal slates described, which prepon-
derate, to rocks found in certain portions of this area, which, although
showing all the macroscopic features of bedded elastics, nevertheless under
the microscope are seen to have been recrystallized, and now may properly
be called mica- and amphibole-schists and gneisses. These mica- and
amphibole-schists and gneisses vary from light-gray to neai-ly black rocks.
The schists have a light-brownish weathering crust. One can disting'uish
in all cases in them the mica flakes, the amphibole, and very frequently
the feldspar and quartz. Difierences in color and size of grain produce a
banding in these metamorpliosed rocks. Usually the banding stands out
much more plainly on their weathered surfaces than upon the fresh fracture
planes. This banding in the schists unquestionably corresponds to lines of
original bedding, for it can in places be traced uniuterruptedly from the
slates into the banded mica-schists, in both of which rocks it shows the same
strike. Moreover, at one place south of Tower, on exposui-es east of the
Duluth and Iron Range Railroad, near milepost 92, one niay see these
schists in various stages of formation, and on these schistose rocks there
THE LOWER HURONIAN. 295
are still present most perfect examples of false bedding in the normal
unmetamorphosed slates.
These metamorphosed slates consist of green hornblende, actinolite,
mica (biotite, muscovite, and sericite), feldspar, quartz, chlorite, rutile,
ejjidote, sphene, apatite, calcite, garnet, and iron oxide. In some of these
the garnet and muscovite appear as porphyritic constituents full of inclu-
sions of tlie other minerals of the rock, thus showing that their origin is
later than that of these constituents.
THICKNESS.
The folding- of the slates has resulted in excessive crumpling and a
slight overturning with an average dip of about 80° N. That this condition
exists is shown by a number of minor anticlines and synclines which have
been observed. It is very probable, therefore, that the thickness of the
slates has been several times repeated in the area. Bearing the above facts
in mind, one will readily appreciate the statement that an estimate based
on the width of the slates and the width of the area would probably give a
thickness many times too great. As such an estimate would only lead to
erroneous conclusions, and as we have no better means of making a more
accurate estimate, no attempt is made to give the thickness.
INTERESTING LOCALITIES.
The general characters of the Knife Lake slates occurring in the
western part of the Vermilion area can be seen at many places on the
islands in Vermilion Lake and on the shores of the lake. The slates are
well exposed on the east end of Sucker Point and on the adjacent shores
of the mainland, and here one has good opportunities to examine them at
localities that are readily accessible. The high hills east and southeast of
Swede Bay, in the SE. ^ of sec. 20, T. 62 N., R. 15 W., afford a number
of bare rounded surface exposures of these slates, and here, too, the results
of the intricate folding to which they have been subjected can be studied.
Similar slates may be observed at several places on the south shore of Ely
Island, on Canoe Island, and on the island south of its eastern end, and also
on the south shore of Pine Island, as well as at a great many places on the
lake. These slates are also exposed on the south slope of Soudan Hill, just
above the road, and on the road at the crest of the small hill in the town
296 THE VERMILION IRON-BEARING DISTRICT.
of Soudan. Being' near the base of the formation, the slates here contain
small amounts of conglomerate and graywacke.
More interesting than these common phases are the slates which occur
in the southern portion of the area, and which have been subjected to
nietamorphic action to such an extent that they have been transformed
into mica- and amphibole-schists. Excellent op^^ortunities for the study
of these metamorphosed sediments are afforded by exposures near Pike
River. The best places for such studv, however, are in the cuts along
the Duluth and Iron Range Railroad between Emban-ass and Tower,
and especially in those between East Two Rivers and milepost 92. On
these exposures the sedimentary character of the rocks is clearly shown by
sedimentary banding, false bedding, the presence of large bluish frag-mental
quartz eyes which stud some of the beds, and in the vicinity of milepost
92 by the fact that exactly similar sediments are there interbedded with the
Ogishke conglomerate, into which the slates grade. Some of the sediments
have been so extremely metamorphosed, however, that but for their field
relations it would be impossible to recognize them with absolute confidence
as derived from sediments. It should be noted that the sediments at these
exiDosnres are cut by granite dikes, and that the change in the sediments
from normal slates to mica- and amphibole-schists coincides with the
appearance of the dikes. The metamorphism of the rocks increases south-
ward alongr the railroad, in which direction the dikes become more numerous
as one approaches the large granite areas on the Giants range, from which
the dikes are presumed to be offshoots. Winchell," who has noted the
metamorphism of the graywacke and the slates to mica-schists south and
west of Tower, attributes this metamorphism to the Giants Range granite,
but classes these sediments in his Keewatin dixision. The sedimentary
banding, which still shows very plainly, even at places where the rocks
have been metamorphosed to mica-schists, is evidently indicative of a differ-
ence in original mineralogic, and hence chemical, composition. In spite of
the metamorphism these original differences have continued to exist, and
hence the sedimentary banding is retained.
oGeol. and Nat. Hist. Survey of Minnesota, Final Rept., Vol. IV, 1899, p. 254, and PI. LXVII
anJ LXXXVI.
THE LOWER HURONIAN. 297
KOT:FE liAKE AREA OF THE LOWER HUROJSriAN SEDIMENTS.
SUBDIVISIONS.
The Lower Huronian sediments are mucli better developed and more
extensively disti'ibuted in the eastern than in the western portion of" the
Vermilion district. Knife Lake, a prominent topographic feature of the
eastern part of the district, lies in the sediments, and therefore this portion
of the disti'ict in which the sediments occur will be called the Knife Lake
area.
The Lower Huronian sediments of the Knife Lake area may be con-
veniently subdivided into (1) the basal Ogishke conglomerate, (2) the Agawa
formation (iron bearing), lying conformably above the conglomerate, and (3)
the Knife Lake slates, which overlie conformably the preceding formations.
Thus it will be seen that in this eastern area there is a tripartite divi-
sion, whereas in the Vermilion Lake or western area the Lower Huronian
could be subdivided into only the Ogishke conglomerate, and the Knife
Lake slates, time equivalents of the Ogishke conglomerate, Agawa forma-
tion, and the Knife Lake slates of this eastern area. The intermediate iron-
bearing Agawa formation of the Knife Lake area has no known stratigraphic
equivalent in the western part of the Vermilion district.
DISTRIBUTION, EXPOSURES, AND TOPOGRAPHY.
Distribution. — The eastei'n area of the Lower Huronian sediments
begins a few miles west of Ely, in sec. 4, T. 62 N., R. 13 W., and extends
east for a long distance beyond the international boundary, which is the
eastern limit of the portion of the district included in the accompany-
ing map (PI. II). Where these sediments begin at the west the area
underlain by them is very narrow, and this tongue continues narrow for
several miles to the east, gradually, however, widening out. Finally, in
the vicinity of Moose Lake, the continuation of this narrow belt is found to
■join the main Lower Huronian sedimentary area. The distribution of these
rocks for this part of the area will be mentioned later. To the south of the
east-west trending area above mentioned there lies a narrow belt of sedi-
ments which begins on Farm Lake, extending about east and west. This
belt lies along both sides of the North Branch of the Kawishiwi Rivei-,
extending southward below this for some distance, where it abuts against
298 THE VERMILION IRON-BEARING DISTRICT.
the iuti-usive granite and continues eastward into sec. 29, T. 63 N., R. 10
W. The continuation of these sediments to the east is also inteiTupted
by the intrusive granite. The same series of sedimentary rocks is
again found east of the above-mentioned granite in sec. 20, T. 63 N.,
R. 9 W. From here they have been traced to the northeast, where they
are found to connect with and to form part of the same \arge area south
of Moose Lake into which the northerly tongue pre\'iously mentioned
merged. In this portion of the district, that is, in the vicinity of Moose Lake,
the Lower Huronian sediments are found to extend over the greater por-
tion of the area surveyed. These sediments are, however, subdivided into
several partly disconnected areas or tongues by intervening areas underlain
by Archean rocks as well as by intrusive masses of acid rocks somewhat
younger than the sediments. Continuing our observations on the dis-
tribution of the Lower Huronian sediments from the area west of Snow-
bank Lake, we note first that on the south this area is disconnected from
an area underlain by related rocks on the southeast side of Snowbank
Lake by the intervening Snowbank granite. This belt, however, extends
around the east side of the Snowbank Lake area and connects on the
northeast with the similar sediments which sweep around the northwest
side of the lake. Where these join, to the northeast and east of Snow-
bank Lake, they underlie an area which has very nearly the same
width as the Vermilion district. This main mass of the sediments
continues on east over Ensign and Knife lakes. To the south of Knife
Lake the main area underlain by the sediments is interrupted by small
areas of Archean rocks as well as by the Cacaquabic granite, which is
younger than the sediments. To the east of Ogishke Muncie Lake the
Lower Huronian sediments are divided into two main belts by a west-
ward-projecting massive of Archean greenstone which lies immediately
south of the granite of Saganaga Lake and in juxtaposition with it. These
belts, a southern and a northern one, can be traced around the interrupting
Archean greenstone and granite of Saganaga Lake for a great distance
beyond the Canadian border to the north of this separating area. On the
south this sedimentary series ends just east of Gobbemichigamma Lake, in
sec. 30, T. 65 N., R. 4 W.
Exposures. — The country underlain by the conglomerates and slates is
cut up by numerous lakes and is for the most part bare of timber of large
THE LOWER HURONIAN. 299
growth, partly by reason of extensive forest fires and partly for lack of
good and abundant soil; consequently the exposures of these sediments are
numerous and usuallj^ of exceptionally large size.
Topography. — The topography of the Knife Lake area of the Lower
Huronian sediments is very rough, although the features as a whole are on a
comparatively small scale. In general the topography in this area is much
more accentuated than it is in the area underlain by the same sediments in
the western portion of the district already described (p. 278). The maxi-
mum diiference in elevation is 400 feet, the difference between the level of
Ogishke Muncie Lake and the adjacent hills. Reference has already been
made (see p. 45) to the fact that the lakes in this part of the district are rela-
tively deep, as has been shown by the few soundings taken. The maximum
depth found in a lake in the sediments is nearly 200 feet. In reality, then,
the difference between lowest valleys and lake basins and highest hills is
about 600 feet. The hills and ridges have the usual east-northeast-west-
southwest trend, with narrow, deep valleys occupied by streams and lakes
lying between. The slates, on the other hand, generally form the lower
hills. These, while occasionally rounded, are generally more or less angu-
lar, more nearly coi-respoiiding to the appearance of the slate hills in non-
glaciated territory, although by no means so angular as these.
Normally the conglomerates occupy higher levels than do the slates
which lie next to them, and these hills of conglomerate have fairly well-
rounded contours. In portions of this area, however, the slates are very
siliceous, and as a result of their great hardness form some of the highest
hills. In the area underlain by slates sheer cliffs are common, some of
them reaching a height of 100 feet above the lakes.
The topography has been greatly influenced by the structure. This
will be referred to in the succeeding paragraphs.
STRUCTURE.
The Lower Huronian sediments, from the westernmost point where
they are found (see PI. II), just west of Ely, to their eastern extension, where
they abut against the Saganaga anticlinal area, occupy a sjmclinorium
which trends approximately K. 70° E. and continues around the northern
side of the Saganaga anticlinal area into Canada. This synclinorium is
narrow -in its western part and widens out toward the east. In that portion
300 THE VERMILION IRON-BEARING DISTRICT.
of the district where the sediments have a considerable width their conti-
nuity is intemipted by numerous anticlines of older rocks. Here and there
a boss of granite which has been intruded through these sediments is found,
the sediments wrapping around it. These areas also are anticlinal in
structure. For the most part the various anticlinal areas are commonly
outlined by conglomerates which lie on the flanks of anticlinal hills whose
centers are occupied by an older rock. Where the sediments alone occur the
conglomerates occiipy the centers of the anticlinal areas. The bedding is
so poorly preserved in the conglomerates as a rule that one can not get
many strike and dip determinations to assist in interpreting the structure.
Unquestionably these conglomerates must have been folded with the other
sediments, although such folding can not be traced in detail on their
exposm'es. It is shown, however, by the distribution of the slates, which
dip away from such anticlinal areas of conglomerate. The slates invariably
occur within the syncliues, forming depressions as a result of their initial
position at the bottom of the syncline and as a result of the relative ease
with which they are eroded. This is shown, for example, in the broad area
of slate surrounding Knife Lake. Exceedingly fine-grained, verA^ cherty
slates, breaking with conchoidal fracture, lie about in the axis of Knife Lake.
As we go farther south from this point the sediments get coarser, graywackes
gradually becoming associated with the slates, and finally the sediments
grade into conglomerates. This same condition exists north of the lake,
although there the conglomerates are not so greatly developed as to the south
of it. Within this and other broad slate areas small slate anticlines very
probably occur, for although no such anticlines have been clearly demon-
strated to exist, indications of them have been found.
As is shown on the map, this broad area, underlain by the Lower
Hm-onian sediments, is separated from several detached areas to the south
by intervening highlands, occupied by the Ely greenstone and the Snow-
bank and Cacaquabic granites, named in order of age. In the area south
of these highlands, formed of the older rocks, the structure of the sediments
is totally different from that seen in the large area to the north. South of
these anticlinal highlands the sediments occur in a southward-dipping mono-
cline which extends with few interruptions from the vicinity of Snowbank
Lake to the eastern end of the slate area on Paul Lake. There is a contin-
THE LOWER HURONIAN. 301
uous narrow monocline of slate extending from Cacaquabic Lake east to
Lake Gobbemichigamma, and lying between the high Twin Peaks anticline
on the north and the g-abbro on the south. This belt of rocks has been
much metamorphosed by the gabbro.
The slates of the Lower Huronian show the effect of the pressure much
better than do the conglomerates, and the remainder of the descri23tion of
the structure of the sediments applies specifically to the slates forming the
upper part of the series. In addition to the close folding, which is indicative
of great pressure, the Lower Huronian Knife Lake slates have been jointed
and faulted, and schistosity and cleavage have been produced. In general,
the major joints have an east-northeast ti-end, and the cross joints have a trend
not quite at right angles to the first. These joints make the slates break
into rhomboidal blocks and are the chief cause of the formation of the high
clifi"s. The strike of the joints varies with the direction from which the
pressure producing them came. Thus, in the western part of the area,
where the pressure was apparently N. 10° W. to S. 10° E. — that is, perpen-
dicularly to the axis of the folds and to the strike of the bedding — one set of
joints trends about N. 80° E., and another trends in a direction very nearly
at right angles to it, making an angle a little less than a right angle with
the first set of joints. In the eastern part of the district, however, where
the slates abut against the granite of Saganaga Lake and wrap around it, the
direction of the joints changes. Thus in sec. 35, T. 66 N., R. 6 W., three sets
of joints were noted. The first set strikes N. 25° E., and dips 30° to the
northwest, corresponding closely with the strike of the schistosity. The
second strikes N. 10° W. and dips 85° to the west. This agrees with the
bedding. The third set strikes N. 60° E. and dips 85° to the southeast.
The strike of these joints evidently influences very materially the shape of
the lakes in this part of the district. For instance, in the case of the lake
in sees. 34 and 35, T. 66 N., R. 6 W., these joints can be seen to determine
the long direction of the lake and the trend of the bays.
In a few places we find that the slates show minor faulting along the
joints. No cases were seen, however, where the throw was more than
about 1 foot. South of Fox Lake, at a place north 1,915 paces, west 600
paces from' the southeast corner of sec. 35, T. 65 N., R. 6 W., the interbedded
slates and graywackes are broken and slightly faulted. The fault plane
runs N. 10° W. The shearing accompanying the faulting has affected a
302 THE VERMILION IRON-BEARING DISTRICT.
zone about 3 feet wide. In the midst of this zone there is a horse of the
country rock. Around it the material is sheared and brecciated, and the
infiltration of silica has taken place subsequent to this shearing. The
beds in the sediments on both sides of the fault have been bent. The
amount of displacement could not be measured, but seems to have been
slight — a ver}^ few feet at the most.
The schistosity and cleavage which have been produced are well
marked on the slates. They show the variable relations to the bedding
planes which are shown by Van Hise " to be consequent upon their mode
of fonnatiou, and are clearly the result of the compressive forces which
caused the folding. Thus they may be essentially parallel to the bedding
on the flanks of the folds, and vary from this position to a position at right
angles to it, near the apices of the folds. This cleavage can be well seen
on the good exposures southwest and west of the portage from Moose into
Flask Lake. South of Ogishke Muncie Lake, where the beds strike N. 25°
to 30° W., the schistosity strikes N. 60° E. The diff"erence in the behavior
of the soft and hard beds — that is, the weak and the strong beds — under
the same condition of pressure are well brought out at one place upon
Ogishke Muncie Lake. At the southwest end of the long point projecting
southwest into sec. 27, T. 65 N., R. 6 W., at the southwest end of Ogishke
Muncie Lake, there are in the slate near the water's edge alternating bands
of harder and softer materials. In the softer bands, cleavage running
parallel to the bedding has been produced, while in the harder ones cross
joints have been formed, i-unning practically perpendicular to the cleavage
in the soft beds. This difference is evidently due to the difference in the
elastic strength of the two rocks. The one, the slate, practically flowed
under pressure, while the other was fractured. The deformation evidently
took place while these rocks were in the zone described as the combined
zone of flowage and fracture.'
Excessive crumpling is very noticeable in the cherty layers and in the
slates. This crumpling is especially well shown on the portage between
Fox and Agamok lakes, and is illustrated in fig. 1, PI. Y, Minnesota
Geological Survey, Vol. IV. The bands here are fractured along planes
"Principles of North American pre-Cambrian geology, bj'C. R. Van Hise: Sixteenth Ann. Rept.
U. S. Oeol. Survey, Pt. I, 1896, pp. .573-874.
''Il)icl.
THE- LOWER HURONIAN. 303
which, make angles somewhat less than a right angle with each other. The
schistosity in the softer bauds, joroduced as the result of shearing, is nicely
shown in places on these sediments.
RELATIONS.
KEIxATIONS OF THE SEDIMENTARY MEMBERS OF THE SERIES TO ONE ANOTHER.
The relations of the Ogishke conglomerate, the iron-bearing Agawa
formation, and the Knife Lake slates to one another is that of tlxree
conformable formations, with the Ogishke conglomerate at the base and
the Knife Lake slate at the top. Thej occur constantly in this position,
the iron-bearing formation being wanting at some places, but present at
others. There are gradations between the formations. The lines which
have been drawn are based upon the petrographic character of the rocks
and the preponderance of the various kinds.
RELATIONS OF THE LOWER HURONIAN SEDIMENTS TO THE ADJACENT FORMATIONS.
RELATIONS TO THE .\J!CHEAS.
Relations to JEly greenstone. — The relations of the Lower Huronian
sediments to the Archean greenstones are clearly shown at a number of
places where the Ogishke conglomerate has been found in association with
them. As a rule the conglomerate lies upon the flanks of the greenstone
anticlines and is made up chiefly or solely of pebbles and bowlders which
can be identified with the rocks constituting the Archean complex, so as to
show unmistakably their source. Thus, for example, at a great number of
places south of Moose Lake the conglomerate was observed in direct
contact with the greenstones, which occur in conspicuous ridges forming
the cores of the anticlines. In some places the conglomerate lies immedi-
ately adjacent to the fine-grained ellipsoidal greenstone, and at other
places, where the ellipsoidal portions have been removed by erosion from
the greenstone mass, the conglomerate lies against the coarse-grained
greenstone which normally is at some distance from the border of the
greenstone areas. Moreover, wherever finer-grained forms of the elastics
showing bedding occur, it is usually found that this bedding is essentially
parallel with the contact of the sediments and the underlying greenstones.
The contact of the Ogishke conglomerate with the greenstones was
also observed on the north side of Twin Peaks ridge and the occur-
304 THE VERMILION IRON-BEARING DISTRICT.
rence there was in general agreement with the description given by
N. H. Winchell." Furthermore, the contact was found between the con-
glomerate and the small ridge of greenstone which lies just along the south
shore of Ogishke Muncie Lake, and a number of additional contacts were
observed on the south side of the great anticline north and northeast of
Gobbemichigamma Lake. The fragmental character of some of these
deposits was recognized by the Minnesota survey, but it was not seen that
they were sedimentary deposits of later age than the greenstone. They
were, on the contrary, regarded as fragmental volcanic rocks, and were
included, with the greenstone, in the Archean.''
Relations to the Soudan formation. — The actual contact between the
conglomerate and the iron-bearing formation was observed at only one
place. Here, however, the evidence is indisputably' clear. The jasper of
the Soudan formation is overlain by a conglomerate containing fragments
of jasper derived from it as well as fragments of greenstone derived from
the greenstones, which in their turn underlie the iron-bearing formation.
In addition to this direct contact, where the evidence is perfectly clear,
there have also been found at a number of places scattered all over the
district quantities of jasper pebbles in the conglomerate. Their presence
is sufficient, of course, to prove that the Soudan formation is older than
these conglomerates.
Tlie fragments of slate and of the conglomerate or breccia which
occur in the Ogishke conglomerate south of Moose Lake are of especial
interest, since they indicate the existence of a series of clastic sediments
prior to the formation of the Ogishke conglomerate. The field e-vadence
for such a clastic deposit below the normal iron-bearing formation has
already been given. In this series there are slaty rocks associated with
conglomeratic elastics. The fragments of slate and conglomerate may very
well have been derived from these. In the conglomerate there were found
a number of slate pebbles. Their source has not been very satisfactorily
accounted for. If we accept the presence of certain sediments mentioned
as lying in a position between the iron-bearing formation and the green-
stone as indicative of the fragmental sedimentary horizon underljnng the
"Geol. and Nat. Hist. Survej' of Minnesota, Fifteenth Ann. Rept,, 1886, pp. 372-374. Final
Kept, Vol. IV, 1899, p. 451.
''Geol. and Nat. Hist. Survey of Minnesota, Final Rept., Vol. IV, 1899, p. 466.
THE LOWER HURONIAN. 305
jaspers, then tliese slates are accounted for, as they occur in conglomerates
younger than and containing fragments of the jasper. In a similar way
the conglomerate or breccia pebbles observed can be accounted for. It
should be noted, however, in connection with the explanation of the source
of these pebbles, that we do not know whether they are true conglomerates
or merely friction breccias, or pseudo-conglomerates. In the greenstone
south of Moose Lake there were numerous small zones which had been so
extremely fractured and then after the fracturing had been so sheared that
in ma;ny cases friction breccias have been produced which closely resemble
normal conglomerates and from which it would be perfectly possible to
derive the pebbles seen in the overlying conglomerates were the breccias
produced before the sediments were formed.
Belations to the granite of Saganaga Lake. — In the northeastern part of
the district the Ogishke conglomerate is very close to the granite of
Saganaga Lake and in several places contacts between these two rocks
have been found and their relationships thereby made perfectly clear. In
several places a great bowlder conglomerate has been found immediately
overlying the granite of Saganaga Lake and consisting essentially of frag-
ments of this granite. Detailed description has already been given of the
field relations of the granite of Saganaga Lake to this conglomerate under
the description of the granite (p. 271), and it was there shown that the idea
held b}" Lawson that the granite of Saganaga Lake was intrusive in the
conglomerate was untenable," hence it will not be necessary to repeat this
description here. From the evidence it is perfectly clear that the Ogishke
conglomerate is young-er than the granite of Saganaga Lake.
RELATIONS TO LOTVEK HUKONIAN.
Belations to the Giants Range, Snowbank, and Cacaquabic granites, and
various dikes of granite and granite-porphyry. — In the western portion of
the Vermilion district there is found a conglomerate — correlated with the
Ogishke conglomerate — which is in contact with the Giants Range granite,
and has been penetrated by dikes from this granite. In the vicinity of
Snowbank Lake a similar conglomerate practically surrounds the area
underlain by the Snowbank granite, and in a great number of cases it has
been found to have been penetrated by dikes from this granite. A portion
of the area underlain by the Cacaquabic granite is likewise surrounded by
a Lake Superior stratigraphy, by A. C. Lawson: Am. Geologist, Vol. Ill, 1889, pp. 320-327.
MON XLV— 03 20
306 THE VERMILION IRON-BEARING DISTRICT.
the Ogishke conglomerate and, as in the preceding cases, the conglomerate
is found to be penetrated by offshoots from this granite massive. At other
places in the district, where the conglomerate is distant from the granite
massive, it is cut by dikes of rhyolite-porphyries or granite-porphyries, or
possibly of both. In all cases, however, the relationship is clearly that of
intrusion, the conglomerates being intruded and metamorphosed by the
granites. Hence the conclusion that the conglomerates are of greater age
than the granites which penetrate them.
Relations to certain basic and intermediate dikes of Lower Huronian age. —
At numerous places dikes of slightly varying character — altered basalts
and lamprophyres — have been found cutting the Lower Huronian rocks.
None of these dike rocks are found as dikes in the overlying Upper
Huronian series. Hence they are believed to have been intruded in the
Lower Huronian rocks at about the time when they were being folded and
intruded by the aforementioned granites. Certainly some of the dikes are
later than some of the granites, as they are found cutting the granites.
REL.VTIONS TO THE UPPER HURONIAN SERIES.
It was found that the most difficult problem of relation to be solved'
was that of the relationship between those rocks which are here classed as
the Lower Huronian (consisting of the Ogishke conglomerate, the Agawa
formation, and the Knife Lake slates) and the Upper Huronian (Animikie)
rocks. These rocks come closest together in the vicinity of Gobbemichi-
gamma Lake, and here, if at all, their relations Avere to be determined.
A considerable time was therefore spent in the study of the rocks in this
vicinity. Unfortunately where the rocks of the series come closest together
the Lower Huronian is represented by conglomerates which give no good
strikes and dips, and the Animikie is represented by the metamorphosed
iron formation which lies at its bilse in this district.
In the area referred to the Lower Huronian rocks are extremely folded,
and where this series is in contact with the Animikie, the vertical or very
steejoly dipping rocks of the Lower Huronian were found to strike in such
a direction on the east side of Gobbemichigamma Lake as to bring them
very nearly at I'ight angles against the Animikie, which has a very low dip
to the south, with a strike slightly north of east. Only in two places
were the Ogishke conglomerate of the Lower Huronian and the iron-
bearing formation of the Upper Huronian series observed in contact, and
THE LOWER HURONIAN. . 307
in botli places there is a ver}^ striking difference in the Hthologic character
of the two rocks. The iron-bearing foraiation is made np essentially of
beds of magnetite alternating with very quartzose bands, whereas the
conglomerate is the normal greenstone conglomerate, although very much
altered. There is no transition between the two, and the relationships
appear to be those of two unconformable series of rocks. The evidence of
this unconformity is, however, not absolutely conclusive in the Vermilion
district. The opinion of the majority of those who were studying these
rocks for the Survey was in favor of this unconformable relationship, and
in this respect was in thorough agreement with the conclusions reached and
already published by the Minnesota survey. However, in view of the fact
that some of the rocks were intensely plicated, it was recognized that it was
possible for them to change both their strike and dip within a very short
distance, even within the distance which separated them from the Animikie
and in which there wei;e no exposures. Such a change might possibly
bring them into perfect conformity with the Animikie. In view of this
possibility we could not all agi-ee to accept the unconformable relationship
as proved. In 1900, shortly after work was begun in the Mesabi, Mr. C. K.
Leith had the good fortune to find a place where the relationship between
these two series is unmistakably shown." At this point, north of Biwabik,
the vertical Lower Huronian beds were found overlain by the low south-
ward-dipping rocks of the Upper Huronian series, with a thin basal
conglomerate at the bottom The correctness of the opinions pre^aously
held were thus demonstrated beyond all doubt.
RELATIONS TO THE KEWEENAW AN.
Belations to the Keweenawan gahhro. — In Ts. 63 and 64 N , Rs. 8 and 9
W., the Ogishke conglomerates and Knife Lake slates are found in manv
places almost in juxtaposition with the Duluth gabbro mass of Minnesota.
In no cases were actual contacts observed between them, as invariably a
topographic depression, occupied either jnerely by lower ground or, as in
most cases, by water, intervened. The Keweenawan gabbro has been long-
recognized as one of the youngest rocks occumng in Minnesota. Where
the conglomerates and gabbro are in contact the gabbro has been found to
metamorphose the conglomerates and slates very extensively, and hence
«Mon. U. S. Geol. Survey Vol. XLIII, 1903, p. 181.
308 THE VERMILION IKON-BEARING DISTRICT.
the conclusion is unavoidable that the gabbro is very much younger than
the Ogishke conglomerate and Knife Lake slates.
Relations to basic dikes. — Cutting through the Ogishke conglomerate and
Knife Lake slates at various places, basic dikes of the character of dolerites
have been observed. These dikes are of exactly the same nature as those
which are found cutting thi-ough the gabbro which represents the youngest
member of the series in the Vermilion district, excluding, of course, as was
done in the statement at the beginning of this monograph, the glacial
deposits. Since the dikes in the conglomerate are lithologically the same
as those cutting the gabbro, they are assumed to be of the same age,
although direct relationship between the dikes in the conglomerate and slates
and those in the gabbro have never been observed.
AGE OF THE LOWER HURONIAN SEDIMENTS.
The descriptions given in the above paragraphs of the relations
existing between the conglomerates and slates of the Lower Huronian and
the adjacent formations throughout the district enable us to make with
confidence the following summary statement concerning the relative strati-
gi'aphic position of these sediments in the Vermilion district. Since they
lie unconformably above the rocks of Archean age, they must of necessity
be younger than the Archean rocks. They are older than the Snowbank,
Cacaquabic, and Giants Range granites, which cut and metamorphose
them, and older than some basic dikes which are intrusive in them. The
chief interest, however, is in their relationship to the Animikie sediments,
which are very generally recognized as being of Upper Huronian age.
The sediments here classed as Lower Huronian are unmistakably of an
older period of formation than these Animikie sediments, since these
Animikie sediments rest unconformably upon them, as is shown by the
relations observed in the adjacent Mesabi district. Hence the tlii-ee
conformable formations, the Ogislike conglomerate, the Agawa formation,
and the Knife Lake slates form one series of rocks of Lower Hui'onian age.
In the eastern part of the Vermilion district these sediments bear the same
relations to the adjacent formations as in the western part. They lie at
the base of the Algonkian sediments, and rest miconforniabl)- upon the
Archean rocks, and are correlated with the Lower Huronian series of the
rest of the Lake Superior region.
THE LOWER HURONIAN. 309
OGISHKE CONGLOMERATE.
PETROGRAPHIC CHARACTERS.
Macroscopic characters. — The Ogishke cong-lomerate varies from a
coarse bowlder conglomerate with bowlders up to 20 iuches iu diameter, as
shown on the southwest side of Cache Bay of Saganaga Lake, down
through all intermediate gradations of coarseness into rocks which are
designated as grits, and thi-ough these into slates. The grits are, of course,
interbedded with the conglomerates, but no attempt has been made to
separate them from the conglomerates on the majjs where it was recognized
that they occurred in very subordinate quantity. The conglomerates contain
a great variety of pebbles. We find among these a great number of kinds
of altered basic eruptives, both massive and schistose, coarse and fine
grained, porphyritic and nonporjDhyritic, amygdaloidal and nonamygdaloi-
dal, some showing flowage lines produced by parallelism of the feldspars,
and others with sjjherulitic structure. Among the most striking of these
are the porphyritic rocks in which the feldspar and hornblende are the
])henocrysts and occur either alone or together. Upon one ledge seven
different kinds of greenstones were counted. The granites which occur in
pebbles and bowlders in the conglomerates show varieties ranging- from
coarse and fine evenly grained to porphyritic and nonporphyritic forms.
There are several kinds of fine-grained acid porphyries also. A few
slate fragments and two fragments of a conglomerate were likewise seen
in the coarse elastics. Black and gray chert, jasper, vein quartz, and a
number of fine-grained gray pebbles, whose characters were undetermined,
occur associated with those mentioned.
The brilliant red-jasper fragments lying in the green matrix give the
conglomerate a very handsome appearance. With this jasper-bearing
conglomerate, and a phase grading over into the Knife Lake slates, there
occiirs a dark-green medium-grained graywacke with a faint speckling, due
to the small, bright-red fragments of jasper scattered through it. The
amount of small jasper fragments varies in quantity, being rare in some
cases, and in others so numerous as to influence very markedly the color
of the graywacke.
Many of the jasper fragments which occtir in this conglomerate possess
a well-developed zonal structure. The centers of the fragments are red
310 THE VERMILION IRON-BEARING DISTRICT.
and the peripheries are bhxck, producing a xerj striking appearance. This
zonal structure, moreover, is parallel to the irregular contours of the
pebbles. The alteration is evidently due to the reduction of the iron oxide
(Fe203zrhematite) to magnetite (FegOa FeO). The zonal structure in these
fragments is verj^ good, and must have been formed after the jasper
had acquired its present fragmentary^ character, as the zones run parallel
with the irregular margins of these fragments. Some of the fragments are a
foot long, although the majority of them are only a few inches in diameter.
Where there is brilliant red jasper lying near the conglomerates it is
natural to look for some iron-bearing formation as the local source of the
23ebbles. However, a great deal of the conglomerate occurring upon and
in the vicinity of Ogishke Muncie Lake has as its most striking constituent
brilliant-red jasper pebbles, and yet there is no known typical iron-bearing
formation nearer than that which occurs on Otter Track Lake, 5 miles
away to the northwest. The intervening area is underlain by finer sedi-
ments— graywackes and slates for the most part. The brilliant jasper from
this vicinity is in its general character similar to that of Lower Huronian
age which occurs at Soudan, Ely, and other places in the district. It seems
scarcely reasonable to derive these pebbles from a source so far distant as
the exposure on Otter Track Lake, especially as the jasper occurs in such
large quantity in the sediments exposed in the area extending ajjproxi-
mately from West Gull Lake southwestward to Cacaquabic Lake. Win-
chell" reports the occurrence of a small quantity of jasper upon Townline
Lake, but search failed to reveal it. IVloreover, the Archean area to the
south and east of these sediments has been hunted over for the Soudan
formation, which it was supposed might be b'ing in troughs within it, but
it was not found. The Soudan formation, if it ever existed in this area,
and it is highly probable that it did exist, has been deeply buried under
the sediments or completely removed liy erosion. The probability of
such a removal will be seen to be great when we consider the enormous
thickness of sediments whicli lie west and northwest of the Archean and
which consequently indicate a long period of erosion and deposition.
The matrix of the Ogishke conglomerate is the finely triturated
material derived from the various rocks whicli have been mentioned as
occurring in pebbles in the conglomerate.
"Geol. and Nat. Hist. Survey of Minnesota, Sixteenth Ann. Rept., 1S85, p. 31.5.
THE LOWER HURONIAN. 311
The proportion of the different kinds of pebbles varies greatly, and
consequently we find conglomerates of very different chemical and physical
aspects. The dependence of the character of the conglomerate on the
petrographic nature of the adjacent rocks from which it has been derived is
well illustrated as we follow northward the line of contact between the
granite of Saganaga Lake and Ogishke conglomerate on Cache Bay of
Saganaga Lake, where we get the Ogishke conglomerate in contact with
the Ely greenstone, which has been cut by numerous dikes of the granite
of Saganaga Lake. As we go in this direction we find that the basal
conglomerate contains occasionally fragments of g'reeustone, and these
become more and more numerous, the granite appearing in proportionally
smaller quantity as the area in which the greenstone occurs is approached.
Finally, when we get well within the area in which the greenstone occurs,
the conglomerate is made up cliiefly of greenstone fragments, the matrix
likewise beino- detrital material from the srreenstone with onlv an occasional
granite pebble, derived probably from the gi-anite dikes which traverse
the greenstone or possibly transported from the main granite area. The
changeable cliaracter of the basal conglomerate within a very limited area,
its character depending on the petrographic nature of the surrounding
rocks, is also seen here, as above described, and shows the uncertainty of
correlations which depend on the similarity of the lithologic character of
sedimentary deposits occurring in widely separated areas. The differ-
ences in the Ogislike conglomerate at various localities is clearly due to
the varying character of the immediately adjacent rocks from which the
conglomerates have been derived. This is shown at a number of places.
Thus in some conglomerates all of the pebbles are greenstone, and the
matrix is likewise made up of the dust from these greenstones, so that the
resulting rock is green, with pebbles showing various textures, such as occur
in basic rocks. Jasper occurs in numerous fragments in these rocks, and
their brilliant color offers a very striking- contrast to the usual monotonous
green of the conglomerates. Here and there a pebble or bowlder of granite
will appear, and again there may be an approximately even mixture of
pebbles of granite and greenstone. In other places the frag-ments of granite
are present in such quantity that they are the predominant bowlders. In
such places the matrix likewise is found to have changed from the green of
the greenstone conglomerates to the gray or pinkish color characteristic of
312 THE VERMILION IRON-BEARING DISTRICT.
gi-anite debris. The most noticeable differeuces in the appearance of the
Ogishke cong'lomerate are between those varieties which are made up almost
exclusively of greenstone, which are therefore g-reeu in cokir, and those in
which the granite bowlders form the g-reater portion, such varieties having a
gray to reddish color on weathered sxirfaces. Between these there are, of
course, all gradations. In general the gray, bluish- to greenish-gray, slate-
colored, and green-colored rocks predominate. These different kinds of
conglomerates were noted before they were all correlated, and, for
convenience, the conglomerate made up essentially of greenstone pebbles,
which is so typically developed in the vicinity of Moose Lake, was spoken
of as the greenstone conglomerate. These greenstone conglomerates were
for a while rather puzzling, as it was not easy to determine whether they
were volcanic tuffs or true sedimentary rocks, in the one case contempo-
raneous with and in the other older than the greenstones with which they
were associated. Grant, after having studied the district, reached the
conclusion that the clastic rocks on the south flank of the Archeau tongue
lying south of Gull Lake and extending thence eastward were tuffs derived
from these greenstones." Field work having for its object the determination
of this particular point has been carried on, and as a result sufficient
indications of the sedimentary origin of the elastics have been found to
justify their classification as conglomerates. Moreover, in numerous other
places elsewhere in the district, interbedding of the finely bedded sediments
with these conglomerates and gi-adations between them have been observed,
so that there can be absolutely no doubt that they are normal sediments.
The conglomerate at Ogishke Muncie Lake contains pebbles of greenstone,
granite, jasper, and otlier varieties of rocks, find is the normal basal conglom-
ei'ate of the Lower Huronian for the eastern part of the Vermilion district.
Microscopic characters. — A certain amount of microscopic study was
made of the conglomerates. This consisted primarily in the determination
of the characters of the pebbles and of the matrix. Li addition to the
constitiients recognizable macroscopically, which have already been enum-
erated, the microscope discloses the presence of fragments of basaltic lavas
with various microscopically recognizable textures, spherulitic rhyolite,
rhyolite-porphyry, pieces of quartz and feldspar in pegmatitic intergrowth
"Geology of the eastern end of the Mesabi iron range in Minnescita, by U. 8. Grant: Engineers'
Yearbfwk, University of Minnesota, 1898, p. 54.
THE LOWER HURONIAN. 313
(presumably derived from pegmatitic granites), round to subangular grains
of cloudy feldspar, quartz, ragged pieces of biotite altering to chlorite,
green hornblende, augite, apatite, zircon, and a large amount of fine inde-
terminable interstitial material derived from the trituration of the various
materials already mentioned. From the decomposition of this interstitial
material, as well as from the associated larger fragments, there has been
produced the secondary minerals chlorite, ealcite, actinolite, sericite,
epidote, quartz, feldspar, and pyrite, which occur in large quantity in the
sediments. The prevailing green tones of the sediments is especially due
to the very large quantity of the green hornblende, epidote, chlorite,
augite, and sericite which is present. In a few cases the well-bedded gray-
wackes associated with the conglomerates, and especially those near the
conglomerates that are composed chiefly of greenstone fragments, are found
to consist very largely of fragments of crystals of hornblende, augite, and
feldspar, with an occasionally well-preserved entire crj^stal. Such sedi-
ments resemble the crystalline tuffs of volcanic origin. The beautiful
bedding and association with other sediments show clearly that these rocks
are water-deposited sediments.
METAMOEPHISM OF THE OGISHKE CONGLOMERATE.
It should be borne in mind that the conglomerates thus far descnbed
are by no means in their original condition, but have been extensively
metamorphosed. This metamorphism has been that produced chiefly by
cementation, due largely to infiltration of silica and some calcium carbonate
and chemical change of the constituents producing new minerals, and also
secondary enlargements of the old minerals. As the result of these changes
and additions the conglomerates have been thoroughly indurated. This is
the widespread metamorphism which is common throughout all of these
Lower Huronian sediments. The kinds of metamorphism now to be
described are exceptional; they are not those most common in these con-
glomerates:
The Ogishke conglomerate has been metamorphosed both as the result
of orogenic movements and in consequence of the intrusion through it of
various granites, as well as of its contact with the Duluth gabbro of Kewee-
nawan age. The result of crustal movements is well shown in the schis-
tose conglomerate which may be seen at the easternmost jDoint south of the
314 THE VERMILION IRON-BEARING DISTRICT.
channel between Birch Lake and Sucker Lake. Here the conglomerate
consists almost exclusively of fragments of the greenstone which lies on the
north side of the channel. An occasional fragment of vein quartz is present.
This rock has been so extremely mashed that the pebbles have been rolled
out and flattened, and it is now fairly difficult to recognize its true character.
Its fragmental character is best shown on the poi'tions of the exposure
where one gets sections transverse to the direction of greatest flattening in
the pebbles. The difficulty here is increased by the fact that, as stated
above, the conglomerate consists almost exclusively of the greenstone, the
matrix, of course, being derived from the same source and consisting of
the same material.
A mashed cong'lomerate similar to this in every respect was seen at a
number of places on the very irregular stream that connects Cache Bay of
Saganaga Lake with Saganagons Lake in Canada. The best place at which
to see this conglomerate is on the south shore of the stream where it turns
northeastward and flows into the southwest bay of Saganagons Lake. In
this vicinity the greenstone and the overlying sediments have been very
closely infolded, and as a result of the folding excessive shearing has taken
place along the limbs of the folds. Here, again, if the conglomerate is
viewed transversely to the bedding, its nature can be readily recognized.
On some of the clifi"s, liowever, which run parallel with the bedding of the
conglomerate, and essentially with that of the schistosity, the conglomeratic
nature is not so readily recognized. The pebbles occasionally stand out as
more or less rounded patches on the cliff" face, but very connnonly blend
with the matrix so nicely that the rock appears almost homogeneous.
Naturally where the conglomerate is made up of a great variety of
pebbles, its true character may be recognized with greater ease than in the
above-mentioned instances. Usually where the conglomerate has been
folded, the pebbles have not been much affected. The conglomerate may
in general have a schistose character, and tliis schistosity usualh^ agrees,
approximately, with the long direction of the pebbles. Closely exannned,
it will be found that the pebbles themselves are in most instances not even
fractured, but preserve their original shape perfectly. Pebbles of granite
have been obtained from this conglomerate which were as symmetrical in
shape and apparently, to the naked eye, as fresh in character as pebbles
obtained from a modern shingle beach of Lake Superior. The pressure
THE LOWER HURONIAN. 315
on the conglomerate has been reheved by movement in the matrix, and
this matrix has in most cases been rendered perfectly schistose. It will be
found that the schistosity, when it approaches a pebble, gradually bends
so as to run around the ends of the pebble, and then upon the flat sides
continues in its normal direction.
The contact metamorphism resulting from the intrusion of io-neous
rocks seems to have been more far reaching in its character — at least so far
as it has produced changes in the petrographic character of the cono-lom-
erate — than the metamorphi,sm due simply to orogenic movement. In all
probability the action of the intrusives is complicated by the fact that their
intrusion took place subsequent to some orogenic movements, so that thev
acted on rocks which had already been somewhat metamorphosed. The
best area in which to study this contact action is in the vicinity of Snow-
bank Lake. Snowbank Lake lies in a granite massive, which has received
its name, the Snowbank granite, from the lake. The Ogishke conglom-
erate surrounds this lake and is exposed with bare surfaces over large
areas. At a considerable distance away from the lake the conglomerate
possesses its normal characters, but as the lake is approached it will be
seen that gradually it changes. Tliis change is for the most part a petro-
gi'aphic one, and has consisted of the production of micaceous and horn-
blendic schists from the finer-grained sedim.^nts associated with the
conglomerates and from the fine matrix between the pebbles of the con-
glomerates. The pebbles in the conglomerates have also been altered,
certain kinds, of course, very much more than others. In general these
finer materials are now well-developed mica-schists in which the cono-lom-
eratic character can, however, be readily recognized by the presence of
comparatively unaltered granite pebbles. This alteration has reached its
extreme where the conglomerates are nearest to the granite.
Long after the intrusion of the granite and the metamorphism of the
conglomerates, the Keweenawan gabbro was intruded, and it has in turn
modified the conglomerates, which were already metamorphosed by orogenic
movements and by the granite. The effect of the contact action of the gab-
bro has extended for a considerable distance from the present exposm-es of
the gabbro. The exact distance can not be determined with certainty, as
its metamorphism blends with that produced by the granite. Possibly
a more detailed field and petrographic study than was warranted in the
316 THE VERMILION IRON-BEARING DISTRICT.
jjreseut case might enable a boundary line to be drawn between the con-
glomerates metamorphosed by the granite only and those metamorphosed
by both the granite and gabbro. The effect of the gabbro seems to
have extended at least as far north as the northwest shore of Disappoint-
ment Lake. Reference has already been made to the beautiful exposures
of conglomerate at this place. The matrix of this conglomerate contains
biotite and hornblende very abundantly, and the pebbles and bowlders of
the conglomerate are to a great extent of a hornblendic rock in which in
many places large porphyritic hornblendes have been produced. On the
weathered surface the pebbles generally decay faster than the matrix, and
hence are removed, leaving numerous roundish depressions. The rock has
not been very strongly mashed, for while the longer dimensions of the
pebbles are in the planes of schistosity, one could not say that the lesser
dimension in the other direction is not explicable as due to the original
shingling action of the pebbles. The rock is very irregularly veined by
quartz, and here and there by some granitic veins derived from the Snow-
bank granite. All these factors give the rock a very rough, knotty
appearance on the weathered surface. While in general the outlines of the
pebbles can be readily traced, nevertheless, when they are broken the
fractures extend cleanly through the pebbles and matrix alike. This shows
the close , union between the matrix and the pebbles which has taken place
as the result of metamorphism. This is further shown by the fact that m
some cases secondary porphyritic hornblendes, which are produced in
certain of the pebbles and in the matrix alike, will be found to extend from
the pebble across the contact into the matrix. As we go southward — in
other words, as we get closer to the gabbro — a study of the rocks on the
small i.slands in Disappointment Lake and on the south shore shows that
the clastic nature of the rocks is not so apjDarent here. A microscopic
study of these rocks shows that there have been produced in them in large
quantity minerals — hypersthene, green and brown hornblende, brown mica,
augite, magnetite — whose origin is clearly due to the action of the gabbi-o.
Similar rocks may be studied in the area extending from the north shore
of the Kawishiwi River in sees. 16, 17, and 20, T. 63 N., R. 9 W.,
near the shore. They are extremely metamorphosed and it is only by
rather close observation that one can recognize tlieir conglomeratic nature.
The pebbles and the matrix of the rocks consist to a great extent of the
THE LOWER HURONIAN. 317
same material, and as the result of the metamorphism essentially the same
new minerals have been produced in them, and this production of new
minerals has tended to render the characters of the rock more uniform. It
is only here and there, where a pebble occurs, whose mineral composition
was not so extremely changred by the metamorphism that the conglomeratic
chai'acter of the I'ock can be distinctly recognized. In such cases these
pebbles seem to withstand . the weathering better than does the adjacent
material in which have been produced, as above stated, the basic minerals
associated with the gabbro. This portion of the rock weathers more readily
than the less affected pebbles which stand out from the rest of the' rock
and show their true pebble characters.
THICKNESS OF OGISHKE COXGLOJIEEATE.
No data have been obtained that would enable us to make an accurate
determination of the thickness of the conglomerate. In places it is wanting
or is represented by a few feet of rock at most, and from this it runs up to
a thickness of possibly a thousand or more feet.
INTERESTING LOCALITIES.
The portage between Moose and Flask lakes is a good place at which
to study the Ogishke conglomerate as developed south of Moose Lake. A
short distance east of the portage landing on Moose Lake as the hill is
ascended we find slates similar to those occurring along the east shore,
which appear to grade into a fine conglomerate and then into a coarse
conglomerate which crowns the brow of the hill. The rocks along tliis
gradation zone are much mashed, so that one can not be certain that the
change is due to actual continuous sediments difi^ering only in coarseness.
The conglomerate is very coarse at this place, having bowlders u]j to 2^
feet in diameter. The fragments in the conglomerate are of many different
rocks — various kinds of porphyries, granites, many diflPerent varieties of
greenstone, jasper, an occasional slate fragment, and two fragments of
a conglomeratic or brecciated rock, the fragments and matrix in these two
pieces being very much alike, and apparently both derived from green-
stone. Still farther along the trail the same conglomerate occurs at
numerous places, and here and there are exposures of much contorted
slates associated with the conglomerates. Some dikes of granite- porphyry
with large quartz phenocrysts may also be observed cutting the older
318 THE VERMILION IRON-BEARING DISTRICT.
sediments. After leaving Flask Lake one finds, in traversing the portage
between Flask Lake and Snowbank Lake, a number of good exposures
showing a i-ather coarse greenstone conglomerate, which is well developed
in this vicinity. These conglomerates very rarely contain any pebbles of
granite or jasper, most of the fragments being of greenstone, and in this
respect the conglomerates are different from those occurring farther north
and west, nearer Moose Lake. They have also suffered mqre from meta-
morphism, since they are nearer to the main Snowbank granite massive
which has intruded them.
The northwest shores of Disappointment Lake afford the best places
in the Vermilion district for studying the Ogishke conglomerate. At the
time of the survey here reported the country had been recently bui'ned
over and the hills were practically bare. There was a little scanty
vegetation, but one could see bare rock exposed in great flat or slightly
rounded surfaces nearly everywhere. Great beds of coarse bowlder
conglomerates were exposed, grading into finer-grained conglomerates,
and these through gray wackes into slates, to be succeeded by a repetition
of these gradations. The strike of the bedding is very uniformly N. 20° W.,
the dip varies from 75° west to 80° east, but is very commonly nearly
vertical. The conglomerate at Disappointment Lake differs from the typical
Ogishke in respect to the absence from it of the jasper which is so common
in the typical Ogishke. No pebbles of this kind were found on Disappoint-
ment Lake. The pebbles consisted chiefly of varieties of granite and
porphyry, and especially of numerous varieties of greenstone. In fact,
many of the beds of conglomerate consisted exclusively of pebbles of
different varieties of greenstone, and the matrix between the pebbles
consisted of finer detrital matter derived from the same source. The
conglomerates had been intruded b}' a number of dikes, basic as Avell
as acid, and they had been metamorphosed by the Snowbank granite,
from which the granite dikes are offshoots, and subsequent to this
metamorphism had been further metamorphosed b}' the great Kewee-
nawan gabbro. Consequently the nearer one approaches the Snowbank
granite the greater the metamorphism, and the changed character of the
sediments is still further increased as one goes along the margin of the
granite mass and approaches nearer to the gabbro. In some places, even at
considerable distances from both of these intrusive rocks, the conglomerates
THE LOWER HURONIAN. 319
haTO been so much altered that their true characters could be detected
onh" on careful examination. The greater the variety of pebbles the more
difficult it becomes to conceal the true character of the conglomerate, as
even in the most exti'eme cases it is probable that some one or more of the
kinds of pebbles may retain very nearly their normal characters. But
when, as was frequently found to be true, th^e conglomerate beds are made
up essentially of one kind of rock, the greenstone, and when this has been
metamorphosed, it is found that it is sometimes difficult to recognize the
original chai-acter. In fact, in such a case as this large secondary
hornblende crystals were found to have been produced throughout the
conglomerate, and in the matrix as well as in the pebbles, and likewise
grew from the pebbles out into the matrix.
From Disappointment Lake north to the vicinity of Ensign Lake,
and from Ensign Lake east to Lake Cacaquabic, there are a number of
areas outlined on the map (PL II), in which the Ogishke conglomerate
is exposed. In all of these areas the conglomerate consists essentially
of greenstone pebbles, with gTanite pebbles secondary in abundance.
Jasper is practically wanting. In general appearance the conglom-
erate is green as the result of the preponderance of the greenstone, and
since the brilliant-red jasper pebbles are wanting it does not present the
appearance of the typical Ogishke conglomerate. On the north shore of
Cacaquabic Lake the Ogishke conglomerate is also exjjosed. Here the green-
stone is practically the only kind of pebble iu the rock, and the conglomerate
is very similar to some of the greenstone tufFs of other districts of Lake
Superior. It was while studying this rock that Van Hise" observed the
secondary enlargement of hornblende fragments. On the south shore of
the long east arm of Cacaquabic Lake the normal Ogishke conglomerate is
exposed here and there, and shows its typical characters especially well at
the foot of the high cliff about half a mile east of the main body of the lake.
Here, especially on the bowlders which lie just a few inches or feet below
the surface of the water, the jasper pebbles, with their brilliant red color,
stand out conspicuously. The granite pebbles increase in quantity and,
as above stated, the rock is the typical Ogishke conglomerate.
At the narrows of the lake at the center of sec. 28, T. 65 N., R. 6 W.,
is" a phase of the Ogishke conglomerate different from those heretofore
"Am. Jour. Sci., 3d series, Vol. XXX, 1885, pp. 231-2.35.
320 THE VERMILION IRON-BEARING DISTRICT.
described. This is made up exclusively of both pebbles and matrix of
granite debris. The matrix contains many subangular individuals of
feldspar, derived in all probabilit}' fi-om a porphyritic granite or feldspar-
porphyry. In some places the fine graywacke, with the jDorphyritic
feldspars which are associated with the conglomerates, simulates very
much a feldspathic porphjiy. East of this lake are here and there
exposures of Ogishke conglomerate, usually in rather thin beds, asso-
ciated with graywackes and slates. About a quarter of a mile south of
the small lake just west of Ogishke Muncie Lake, the Ogishke conglom-
erate is again exposed in large masses with jasper pebbles present in great
abundance. This cong'lomerate extends eastward to the lake and along- its
south shore, and eventually is connected with the great mass of conglom-
erate to the east of Ogishke Muncie Lake. As we go from the west end
of the lake eastward, the jasper pebbles rapidly diminish in quantity, aiid
eventually disappear, so that the conglomerate exposed on the south side of
Ogishke Muncie Lake is made up chiefl}- of pebbles of different varieties
of greenstone, with an occasional feldspathic porphyry and granite-
porj^hyry pebble. The typical Ogishke conglomerate also occurs north of
the west end of Ogishke Muncie Lake and is likewise exposed along the
greater portion of the north shore of the lake. Here it is in all cases the
typical jasper-bearing Ogishke, and this ma}' be followed over the areas
outlined on the maps and traced with almost continuous exposures through
to West Gull Lake. It will be noted that we have here the two phases of
Ogishke conglomerate — that known as the typical form, consisting of
striking red jasper pebbles with large quantities of granite and greenstone,
and that variety which consists essentially of greenstone pebbles with no
jasper pebbles and only a few granite pebbles — separated from each other
by the width of the lake — about half a mile. Between these lies a syncline
of the Knife Lake slates. As these phases of the conglomerate are followed
to the east the distance between them gradually increases, a headland con-
sisting of Ely greenstone and granite of Saganaga Lake coiuing in between.
These conglomerates are evidently the same. The difference in petro-
graphic character can be readily explained as due to a difference in the
underlying rocks from which they were derived. North of the headland
of Ely greenstone and granite of Saganaga Lake the conglomerate con-
sists, to a great extent, of jiebbles of granite derived from the gi-anite of
THE LOWER HUEONIAN. 321
Saganaga Lake and pebbles of greenstone and jasper. The jasper
evidently was derived from masses of the Soudan formation which were
presumably infolded in the Ely greenstone. This was not present in large
quantity and is now buried under the sediments, or, as the result of erosion,
it has all been removed and is now found only as pebbles in these sedi-
ments; at least no masses in situ have thus far been found. South of the
above-mentioned headland, where the conglomerate derived from the Elr
greenstone is penetrated by an occasional dike of the Saganaga granite, we
find that the pebbles are predominantly greenstone with only an occasional
granite pebble. Jasper is also wanting here. As we follow these two
belts westward they come closer and closer together, and petrographicalh^
they also approach more nearly to each other as the result of the inter-
mingling of the granite pebbles and jasper pebbles, until, b}^ the time we
reach the first-mentioned area at the west end of the lake, the two conglom-
erates are close together and are petrographically the same. Clearly this
was the place where the currents mingled the debris derived from the granite
on the north side and the greenstone on the south side of this great west-
ward-projecting headland,
The relations of the conglomerate to the undei'lying rocks are clearly
shown by the fact that they consist of pebbles from these underlying
rocks, and this relationship can be seen at a number of locations, to which
reference has already been made in preceding pages (p. 268 et seq). The
conglomerates have been found in actual contact with the greenstone and
with the granite, so that there can be absolutely no doubt as to their
actual relationship. Such a conglomerate, lying between the Knife Lake
slates and the great greenstone mass forming the Twin Peaks range south
of Ogishke Muncie Lake, was described by N. H. Winchell."
The location of this conglomerate could not be determined from
Winchell's statement, but the contact along this range was followed out
for a long distance. In many places the slates were found in contact with
the greenstone. Where these first contacts were found the greenstone was
schistose, arid no distinct conglomerate was observed. Eventually,
however, on the east slope of the prominent northward-trending hill of
this greenstone, at a point 840 paces south and 650 paces east of the
meander corner between sees. 27 and 26, T. 65 N., R. 6 W., the greenstone
«Geol. and Kat. Hist. Survey of Minnesota, Fifteenth Ann. Rept., 1886, pp. 372-374.
MON XLV — 03 21
322
THE VERMILION IRON-BEARING DISTRICT.
is found with about 9 feet of a conglomerate derived from it overlying it.
Above this folloAv well-banded slates and graywackes. At another point,
875 paces south and 700 east of the same location, is another contact
between the greenstone and the sediments. Here we find a band of con-
glomerate formed of poorly rounded greenstone pebbles which grades up
by rapid alternation of conglomerate beds with finer-grained sediments
into the normal Knife Lake slates. The bands of fine-grained conglomerate
vary in thickness from a few inches to
3 feet, the thinner ones alternating with
bands of the slates. The entire gradation
here takes place within a distance of about
10 paces from the greenstone on one side
to the normal slates, without marked con-
glomerate bands, on the other. The strike
of the beds here is N. 60° W., with a dip
of 80° to the north. As may be seen from
the map, the strike follows very closely
the outcrop of the greenstone.
(3n the northern slope of the green-
stone ridge which forms a subordinate
anticline at the southwest end of Ogishke
Muncie Lake, another contact was found
between the Ogishke conglomerate and
the greenstone. The contact occurs on
the hillside at a place 225 paces south and
20 paces west of a point on the shore
opposite and just south of the west end
of the westernmost island shown on the
map (PI. XVI, atlas). The conglon^erate
here is fine grained and consists of greenstone pebbles with occasional jasper
pebbles. Tlie conglomerate is coarsest near the eruptive greenstone and
grows progressively finer northward, evidenth' grading upward into the
slates that occupj^ the central portion of Ogishke Muncie Lake.
The large island northeast of the east end of this same greenstone
anticline well deserves examination, as it shows the relations of the green-
stone and the conglomerate. The large-scale sketch (fig. 20) shows the
200 feet
Fig. 20. — Sketch showing relationship of Ely green-
stone and overlying Ogishke conglomerate on
island in Ogishke Muncie Lake.
THE LOWER HUEONIAN. 323
distribution of the rocks. The conglomerate occurs both north and south
of the greenstone, but on the east side of the island near the top of the hill
there is a small area of slate and conglomerate which is clearly a part of
the Ogishke conglomerate that was infolded in the underlying greenstone
and has not been completely removed by erosion.
When this greenstone anticline which occurs at the southwest end
of Ogishke Muncie Lake and also on the island just described is followed
eastward along the south side of the lake, it is found to disappear for a
distance of about one-third of a mile, it having plunged down under the
conglomerate, which has not been eroded deep enough to show the
greenstone. It reappears again near the north-south section line between
sees. 27 and 26, T. 65 N., R. 6 W. Here the conglomerate is found
in contact with the greenstone on the north slope, and extends north of
the greenstone over a considerable area to the lake shore. South of the
greenstone, however, no conglomerate was found. Near the west end of
this anticline the slates seem to come in almost immediately. The actual
contact between the two was wanting, and a thin conglomerate may
occur in the depression between the two. This same greenstone anticline
continues eastwai'd and comes out again on the west side of the bay of
Ogishke Muncie, into which empties the stream which comes from Fox,
Agamok, and Gobbemichigamma lakes. Conglomerate is here again
exposed all along the north side of the anticline, whereas on the south
side the slates come up very nearly in contact with the greenstone. This
southern edge of the greenstone was here followed for a considerable
distance, in search of an actual contact between the greenstone and sedi-
ments, and at one place, just before the greenstone exposures cease and
where a swampy area begins, a small patch of conglomerate was found
hanging on the sotith face of the greenstone. The conglomerate here must
be very thin, as the slates begin again a few feet south of the face of
the greenstone ledge. If the map (atlas, PI. XVI) is referred to it will
be noted that south of these small greenstone anticlines bordering the
south shore of Ogishke Muncie Lake there occurs a broad area of the
Knife Lake slates which continue south to the great Twin Peaks green-
stone anticline. The presence of this great breadth of slates between these
anticlines, and the fact that where they are in contact with the greenstone
there is but a very thin mass of conglomerate, indicate that during Lower
324 THE VERMILION IRON-BEARING DISTRICT.
Huronian time this area was occupied by a comparatively protected sea,
in which wave and current action was not \erj strong', and in which the
slates were deposited with very subordinate masses of conglomerate.
On the south slope of the great greenstone anticline lying south of
West Grull Lake and Gull Lake contact between the greenstone and the
conglomerate derived from it can be found almost anywhere if one follows
the boundary line closely. The conglomerate is in most places, however,
so coarse that sedimentary banding is comparatively rare, and this may
account for Grant's error in considering it a volcanic tuff." The
conglomei-ate here is exposed over an area about If miles wide, and this
great width, as well as the coarseness of the conglomerate is evidence of its
great thickness at this place. Moreover, where bedding is shown, for
instance, on the hill 725 paces north of the northeast bay of Paul Lake,
the beds are found to dip very steeply to the north. Clearly this greenstone
was the shore of a great headland which was exposed to violent wave
action, for otherwise such a coarse and thick conglomerate would not have
been formed. The same statement is true for the great conglomerate which
occurs to the north of this headland, and which consists to a great extent
of granite derived from the granite of Saganaga Lake, although here the
cono'lomerate is not so thick as that south of the headland.
THE AGAWA FORMATION (IRON-BEARING).
DISTRIBUTION AND EXPOSURES.
At several places in the eastern part of the Vermilion district there is
found a carbonate-bearing and jaspery iron-bearing formation which is very
intimately associated with the Knife Lake slates and is reall)' but a phase of
these. This formation occurs in widely separated areas, and in each instance
it is exposed in comparatively small masses; consequently it is not possible
to assert with i)erfect confidence that all of these ferruginous rocks belong-
to exactly the same horizon, although we are sure that they are very nearly
contemporaneous. The areas in which it occurs in the United States are so
small that we can state confidently that it will never be of economic impor-
tance. The formation is, however, very much more extensively developed
in portions of Ontario adjacent to the Vermili(in district of Minnesota, and
"Geology of the eastern end of the Mesabi h'on range in Minnesota, iiy IT. S. Grant: Engineers'
Yearbook, University of Minnesota, 1898, p. 54.
THE LOWER HURONIAN. 325
it may be that ore deposits will eventually be found in this area, although
it is very doubtful whether the formation carries ore in paying- quantit}^ at
any of the places examined. The greater part of the data for the descrip-
tion of this formation was obtained from Canadian areas, in which a brief
reconnaissance was made and in which this formation is best developed. It
derives its name from Agawa Lake, where it is well exposed.
Bistributton. — The iron-bearing formation occurs only in narrow belts
which are not continuous for great distances. The westernmost occurrence
is on the portage between Wind and Moose lakes, "^ and the exposures here
consist of thin bands of iron oxide, chert, and jasper interbanded. The for-
mation is again found on Sucker Lake, and extends thence northeastward
into Birch and Carp lakes Here it is in character a ferruginous, carbonate-
bearing slate. It has been followed into Ontario for about 12 miles in a
direction a little north of east through the string of lakes which lie about
1^ miles north of the international boundary on Knife Lake, and are
known from west to east as That Mans, Agawa, This Mans, and The
Other Mans lakes. In this area it is present in very characteristic devel-
opment, and consists chiefly of bands of chert, jasper, and iron oxides, with
a carbonate-bearing chert and ferruginous slate in very subordinate
quantity. The next area in which it is known to occur is on the northeast
arm of Ogishke Muncie Lake on both the northwest and southeast shores.
Especially on the southeast shore is it well developed. Here it is the
carbonate-bearing phase, like that which occurs on Birch and Carp lakes.
In a traverse made southward from Knife Lake along- the section
line between sees. 29 and 30, T. 65 N., R. 6 W., at about 200 paces south
of the lake shore there is a high ridge of Knife Lake slates and gray wackes,
and the finer-grained slate grades directly down into banded slate and
jasper. Only a short distance south of this occurs the basal greenstone on
which rests the slate series. Here jasper bands are interlaminated with the
bottom part of the slate series which seems to correspond to the iron-bearing
formation observed on the Moose Lake- Wind Lake portage, and at the other
points noted above. It could not be followed to the east and west. A similar
occurrence of jasper bands in the Knife Lake slates has been noted upon
Pickle Lake.'' These occurrences are interesting as showing the existence
aGeol. and Nat. Hist. Survey cf Minnesota, Final Kept., Vol. IV, 1899, p. 278.
& Grant: Ibid., pp. 440 and 460.
326 THE VERMILION IRON-BEARING DISTRICT.
of this iron-bearing' formation at a number of places in the area, but in each
instance the occurrence is so small that no attempt has been made to show
them on the map. ]\Iention of these localities is made under the heading
"Interesting localities," page 330.
Exposures. — The exposures are not very numerous, but by means of
them the belts may be confidently traced out along the strike, and these are
shown on the accompanying map oidy as far as they have been followed.
STRUCTURE.
The iron formation occurs in narrow belts, having a very uniform
strike, but the beds of these belts show varying strike and dip throughout
their extent, indicating that the formation has been folded to a greater or
less extent. The greatest amount of folding was noticed in that portion of
the iron-bearing formation tliat occm-s on the portag-e between Wind and
Moose lakes. At this place the jasper is extremely crenulated and broken.
In some places this folding is so extreme that the bands have been fractured
and the fi-agments drawn out into pebble-like areas having no apparent
connection with the adjacent pieces; in others, however, the thin string-like
ends may be connected with other laminae of jasper whicli, when followed
out, thicken and grade into other pebble-like areas of jasper.
On the divide on the Wind Lake-Moose Lake portag'e the iron
formation is exposed in three different belts, all tln-ee essentially parallel
in trend. These are exposed only for a short distance along their strike.
The belts have very much the same appearance. They are intensely
plicated, and it seems from close study that they all belong to the same
horizon; that we have here, in other words, a single iron formation which
was intricately folded into the subjacent conglomerates, and that the folds
were then truncated, making the formation outcrop at three different places
in one horizontal section so as to look like three superim])Osed belts. It
is confidently believed that if the exposures were perfect they could l)e
followed along the strike and connected with one another.
On That Mans, Agawa, This Mans, and The Other Mans lakes, in
Canada, tlie iron formation is closely t\)lded into a syncline, the beds
standing practicalh' on edge. Slates occupy the center of the syncline,
and in going away from this center in both directions, north and south
across the strike of the l)eds, one goes from lower to lower beds. j\Iore-
THE LOWER HURONIAN. 327
over, a duplicate succession of the iron formation and other rocks could be
determined, showing the structure to be synclinal.
The iron-bearing formation on the Canadian lakes is locally consid-
erably folded, but in no cases does this plication reach the extreme that it
does in the Soudan formation. In a way we may consider the plication
of the rocks as a measure of the frequency and intensity of the folding.
Therefore this very noticeable difference in the folding of the two iron-
bearing formations, which are essentially of the same petrographic character,
is indicative of the lesser age of the one under consideration, which has
been assigned to the Lower Huronian.
The iron formation at the northeast end of Ogishke Muncie Lake is
also in a syncline, the formation appearing on both the southeast and north-
west shores of the lake The formation is here a carbonate-bearing slate
and is especially prominent on the southeast shore, where at several places it
occurs in high cliffs with a deep brown ocher-colored crust. When this is
removed it discloses a clean, white, almost pure carbonate rock forming the
main mass of the ledge. No crumpling could be detected in the formation
itself, although the folding of the formation in general is shown by the
synclinal structure of the lake basin.
PETROGEAPHIC CHAKACTEES.
The iron-bearing Agawa formation consists of two petrographic facies,
a carbonate-bearing slaty facies, and a chert, jasper, iron-oxide, and slate
facies. These do not occur together, but as they occupy the same relative
position, at the base of the Knife Lake slates, they are supposed to belong
to the same horizon. The presumption is that the carbonate-bearing facies
is of essentialh^ the same kind of material as that from which the chert,
jasper, and iron-oxide facies has been derived as the result of processes
of metamorphism similar to those which have taken place in the production
of the normal jaspers and iron oxides from the ferruginous cherts in the
other iron formations of the Lake Superior region (p. 192).
The first phase of the iron-bearing formation, the carbonate-bearing
slates, are best developed on the southeastern shore of Ogishke Muncie
Lake. There they lie at the base of the Knife Lake slates, resting imme-
diately upon the Ogishke conglomerate, which has been derived from the
subjacent Ely gi-eenstone. These carbonate-bearing slates have been
traced along the southeast shore of the northeast arm of this lake by means
328 THE VERMILION IRON-BEARING DISTRICT.
either of distinct outcrops or of marked topographic depressions indicating
their continuation. The lake here lies in a syncliue in the sediments, and
on the opj^osite shore of the lake — that is, upon the northwest shore — there
are carbonate-bearing rocks overlying conglomerates which seem to repre-
sent the northwest limb of this syncline and to be a repetition of the car-
bonates on the southeastern shore. In the rocks on this northwest shore the
carbonate-bearing character is not nearly so marked as in those on the
southeast limb of the syncline. The high cliffs of the carbonate on the
southeastern shore have been noted by previous observers, and have been
referred to by Winchell as a limestone. This rock is piil-e enough at some
of the exposures on the southeast shore of Ogishke Muncie Lake to be called
a limestone. Microscopic examination shows that it consists chiefly of a
carbonate with a small amount of fine-grained cherty silica and some cubes
of iron pyrites. The carbonate is decidedly ferruginous, as is shown by
the marked vellow, ocherous weathered crust. It passes down into a
carbonate-bearing slate and then into the Ogishke conglomerate. In the
other direction, upward, it passes into the Knife Lake slates.
This carbonate-bearing, horizon is assumed to be the representative of
the cherty iron carbonate rocks from which, it is presumed, the iron-bearing
rocks at the same horizon at other places were derived.
The second facies of the iron-bearing formation of the Lower Huro-
nian of the Vermilion district is better suited to bear this name than tlie
carbonate-bearino- rocks just described, wdiich contain but little iron. This
second facies consists of chert, jasper, iron oxide, and slate interbanded.
The iron oxide is chiefly magnetite with very little hematite. The chert
varies from white to gray and even darker when it has more magnetite mixed
with it, becomes red when it contains hematite, and thus passes over into
the brilliant-red jasper. This jasper is relatively rare in this iron formation.
The slates are the normal gray to bluish and greenish slate carrying more
or less ferruginous carbonate, like the above-mentioned cherts. The
slates are very much changed, and their clastic characters are not recog-
nizable. They are now ^-ery fine-grained, fissile, well-banded slates, con-
sisting of small chlorite flakes, grains of quartz, some ferruginous calcite in
rhombohedra, and crystals of magnetite. These rocks show nothing of
especial interest, being essentially the same as similar rocks forming the
iron-bearine- Soudan formation of the Archean and hence are not described
here in detail.
THE LOWER HUKONIAN. 329
ORIGIN.
The uormal iron-bearing Agawa formation — that which consists of the
chert, jasper, and iron bands, as, for instance, on the Wind Lake-Moose
Lake portage, and at the Canadian locality on That Mans, Agawa, This
Mans and The Other Mans lakes — is identical, so far as its petrographic
character is concerned, with that which has already been described as the
Archean Soudan formation. In the course of the description of this formation
the conclusion was reached that this had been derived as the result of
alteration — the nature of which was also discussed — from an original
cherty, iron-bearing carbonate. It is believed that this iron-bearing
Agawa formation was derived from the same kind of rock, and as the
result of processes analagous to those by which the Soudan formation was
produced. In the Soudan formation very little carbonate was found,
the reason being, very evidently, that the alteration had proceeded
so far that practically all of the carbonate had been changed. Some
carbonate-bearing bands were found associated with the jaspers and
cherts on That Mans Lake, and they bear a striking- resemblance to the
carbonates in the Soudan formation. The carbonate-bearing phases of the
ii'on-bearing Agawa formation, to which reference has been made, contain
a comparatively high percentage of iron, as is shown by the very rich
brown ocherous crust which is found wherever the rocks have been
weathered. It is believed that this unaltered carbonate-bearinor horizon
corresponds to the jasperized horizon, and that these unaltered rocks repre-
sent an earlier jjhase of the jasperized rocks, the alterations by which the
jaspers and cherts were produced not having taken place for some reason
as yet unexplained.
RELATIONS TO OTHER FORMATIONS.
In the j)receding pages statements have ah-ead}^ been made of the rela-
tions which these rocks bear to the adjacent formations, and the details will
be given under the heading "Interesting localities."
At this place the relations of the iron formation to the adjacent forma-
tions will be concisely stated. In the first place, it is clear that the iron
formation lies above the Ogislike conglomerate, with which it has been
found in contact at several places. In every instance it lies above the con-
330 THE VERMILION IRON-BEARING DISTRICT.
glomerate and between it and the overlying Knife Lake slates, into which
at other places the conglomerate grades. While this foi-mation is not found
everywhere between the conglomerates and slates, in those places where it
does occur it always occupies that position. It is clearly, then, an inter-
mediate horizon of comparatively local origin, and our studies have shown
that in the Vermilion district it is unimportant from an economic standpoint.
AGE.
In age it is therefore younger than the Ogishke conglomerate, and older
than the great mass of Knife Lake slates, and forms a part of the Lower
Huronian series.
THICKNESS.
The thickness of the formation varies considerably. On the Wind
Lake-Moose Lake portage it has a thickness of about 6 feet. At other
places on the United States side of the boundary the exposures are so poor
that nocon-ect determinations could be made of its thickness, but at all of
these places it appears to be considerably thicker than at the locality just ,
mentioned. The best opportunity for determining its thickness was aiforded
by the exposures in Canadian territory, where very accurate determination
could be made. The best exposures seen in this area are those on the range
of hills crossed by the portage from Agawa Lake into This Mans Lake.
The rocks are here exposed in a syncline, and on each side of the center
of this syncline, which is occupied by a belt of slates, there was found the
alternating series of belts of jasper and slates forming the iron-formation
complex and having a total thickness of about 50 feet.
INTERESTING LOCALITIES.
The best place at which to stud)- the characters of the Agawa forma-
tion is on the string of lakes known as That Mans, Agawa, This Mans,
and The Other Mans lakes, which lies at an average distance of about
1^ miles north of the international boundary on Knife Lake, and trends
about N. 45° E. Exposures of the formation may be seen at intervals
along the south shore of the string of lakes, on the islands near the
center, and on the necks of land which separate the lakes and across
which the portages run. On the point north of the north end of the
portage that comes from Emerald Lake on the south to That ]\Iaus Lake
THE LOWER HURONIAN. 381
there is a series of interbedded jaspers, cherts, iron ore, carbonate-bearing
slates, and normal slates and graywackes, having a width of about 50 feet.
The dips on this exposure are all to the south. No dips were taken which
were less than 55°, and most commonly they were 60°. On the southeast
side of the bay northeast of this point there is an excellent exposure which
gives a cross section of the iron formation. The best exposures in this
area, however, are those on the range of hills crossed by the portage from
Agawa Lake into This Mans Lake. On the hill immediately north of
the west end of the portage there is well exposed a series of narrow
interlaminated bands of jasper, iron oxide, chiefly magnetite and chert, with
some slaty bands. Between these, and interlaminated with them, there
occurs well-banded and thinly laminated gray slate. These interbanded
belts of slates and iron formation proper continue to outcrop along the
hill to the east. From this hill a north-south traverse was made, and here
it was found that the youngest rock, that which occupies the center of the
area, is a gray sericitic slate having a width of about 75 feet, and striking
about N. 45° E. On each side of this — that is, both north and south of it —
there occurs the iron formation, consisting of a complex of three belts of the
iron formation proper, and two intervening belts of slate. The approximate
width of the complex on each side of the central belt of slates is 50 feet.
North and south of the iron formation there is a considerable width of gray
slates, which are in their turn succeeded by intei'bedded graywackes and
slates, and, finallv, north and south of these sediments comes the Elv g-reen-
stone as basement. This greenstone was not seen at just this locality, but
its relation to the sediments was obtained on the strike of these beds to the
west, and but a short distance away from this point. From the repetition
of these various rocks it is clear that the structure at this place is
synclinal. The greenstones on the south and north represent the oldest
rocks, the younger sediments occurring above them toward the center
of the syncline. The structure of the rocks evidently determined the
topography in this region. The lakes lie in the center of and at the bottom
of this slate syncline. It is true that at this place the iron formation does
not lie absolutely at the base of the slates, since there are some slates
between it and the conglomerate. Nevertheless it occurs essentially at the
base of the formation. This was an area in which, as is shown by the
rocks, the conditions of deposition were rapidly changing. Slates were at
332 THE VERMILION IRON-BEARING DISTRICT.
first formed, and as the conditions changed they graded into the rocks
forming the iron formation, which in their tnrn, as the conditions again
changed, graded again upward into normal slates. This was repeated at
least three times. On good exposures the belts of iron formation proper
are made up predominantly of jasper, iron oxide, and chert, but here and
tliere slate bands are present, and on the sides of such a belt the slate
gradually increases in quantity, jasper and ore gradually diminishing, until
we pass into a belt of finel}^ laminated slate shiowing on the weathered
surfaces alternating pink, white, and greenish bands, with which there is
jDractically lao jasper.
The carbonate-bearing rocks occur at various places on Birch Lake.
They were always found along the contact between the greenstones and
the slates, and were a sure guide to the close proximity of the greenstones.
No jasper was found with these rocks. Essentially the same conditions
prevail on the north side of Carp Lake. At a number of places here the
belt of ferruginous carbonate-bearing slate was found between the green-
stones and the slates. At one place a series of bands of chert and jasper
was observed. These occur at the west end of the north arm of Carp
Lake, on the little point just opposite an exposure of the Ely greenstone.
They appear very similar to the slates and jasper that occur in the series
at This Mans Lake, northeast of this place. The greenstone shows its
typical ellipsoidal characters and is separated from the jasper by an interval
of 2 feet. This occurrence corresponds very closely to that on the north
flank of the small greenstone anticline south of Knife Lake, in sees. 29 and
30, T. 65 N., R. 6 W.
One can readily see how, in the course of the deposition of slates of
such great thicknesses as those that make up the Knife Lake slates of the
Lower Huronian in the Vermilion district, there could occiu- at different times
conditions favorable for the deposition of rocks from which might be derived
materials similar to the iron-bearing formation. We would thus expect to
find liere and there throughout these slates rocks essentially similar to those
of the iron-bearing formation. Li this way we may account for the occur-
rence in the slates west of Ely, near the east quarter post of sec. 4, T. 62 K.,
R. 13 W., of a series of alternating chert and cherty-slate layers bearing
some iron. The less cherty layers contain a considerable quantity of iron,
and weather Avitli fairly brilliant red color. In this way, .by the alternation
THE LOWER HURONIAN. 333
of the red bands and the gray chert bands, these rocks simulate the jaspers
and cherts of the iron formation pi'oper. A somewhat similar occurrence is
that noted at the south end of the portage coming into the north side of
Pickle Lake, where there are alternating slate and pui-plish chert bands,
striking N. 35° E. in the midst of the Knife Lake slates. These are
probably the continuation of similar chert bands which occui- upon the
southeast shore of Pickle Lake.
On the bare hills crossed by the Wind Lake-Moose Lake portage
tnere are good exposures of the iron-bearing Agawa formation, showing
its relationship to the adjacent rocks. To the north is a conglomerate
consisting' of greenstone pebbles with occasional pebbles of acid rocks, feld-
spathic porphyry, rhyolite-porphyry, and granite. Above the conglomerate
is about 3 feet of coarse graywacke and fairly coarse slate, followed by a
belt about a foot and a half in thickness, in which the interbanding is closer
with the slate predominating. Then comes 3 feet of the iron-formation
member, consisting chiefly of the black chert — black jasper as it is some-
times called — greenish chert, and some red jasper, although this is in rather
subordinate quantity. With these cherts there are found a few fine-grained
slaty layers ranging in thickness from a fraction of an inch to 4 inches.
This band of the iron-bearing formation is 6 feet wide. South of this iron-
bearing formation, and making up the remainder of the section of this hill,
and exposed for a hundi-ed feet or more, across the strike, is a conglomerate
similar to the conglomerate below the jasper band in its essential characters,
but different from it in several important respects: First, it contains several
narrow but minutely crenulated complex layers of interlaminated gray-
wacke, slate and jasper; second, it contains many roundish, lenticular, and
also angular areas of black chert, red jasper, and a black slaty-looking
jasper, and various combinations of these. When these areas are examined
closely, they appear to be true beds which have been broken by dynamic
action. The undoubted jasper formation itself is extremely crenulated and
broken. Generallj^ the pebble-like areas continue out into thin strings
which may be connected with laminae of jasper, but this is not invariably
the case. If the jasper is in true fragments the slate and graywacke also are
in true fragments, for they, too, occur in this material in similar roundish or
angular masses. It has been suggested that this conglomerate lying above
the jasper belt first described is younger than the jasper, and that these
334 THE VERMILION IRON-BEARING DISTRICT.
in-egular masses which have just been mentioned are pebbles derived from
it and lying in the conglomerate. While one can understand how this
interpretation could be made, the conglomerate above the jasper baud being
regarded as evidence of another structural break, nevertheless the undoubted
l^ands of jasper in the conglomerate, and the certainty that many of these
irreo'ular areas are derived from these bands and owe their character to
dynamic agencies, are strong evidence against this view. It may be sug-
gested that it is very probable that some of these irregular masses may be
due to a later infiltration of jasper material.
The interbanding of the jasper, slate, and conglomerate is particularly
well seen about 50 paces west of the trail near the top of the hill. Here
in a width of 6 feet one may count 6 distinct bands of jasper interbedded
with fine graywacke and slate. The bands which were counted as jasper
bands contain thin laminge of slate ranging from a fraction of an inch to an
inch across, and, vice versa, the bands counted as clastic sediments contain
minute bands of jasper. At this place the extreme mashing to which the
rocks have been subjected in this area is beautifully illustrated. The
jasper is plicated in an extremely complex manner. In some places it
bends without major fractures, and in others it has broken through and
through. In places narrow bands of the jasper are severed by diagonal
shearing planes into areas which are now more or less lenticular in shape,
and may be immediately in juxtaposition or somewhat removed from one
another. In the clastic sediments lying between the continuous jasper
bands there are some angular and roundish areas of jasper, but these
appear to have been derived by djmamic action from the continuous belts
of jasper, and not to be clastic fragments deposited b}' water in their
jDresent place. The whole is made more complex by secondary veining
and jasperization of the rocks since the folding of them took place. South
of the last-mentioned jasper belt, there occurs, measured across the strike,
about 50 feet of graywacke, slate, and conglomerate. On the south side of
the exposure this material changes into a fine-grained conglomerate which
in the space of 3 or 4 feet grades up into a slate which underlies the 150 to
200 paces intervening between the last jasper exposure on the ridge and
Moose Lake. A close examination of these jasper bands on the hills sliows
that the second one to the south, which lies between two ridges of con-
glomerate, is of just about the same width as the jasper band which appears
THE LOWER HURONIAN. 335
farthest north. This suggests that the double appeai-a.nce of this band
is due to infolding, the closeness of which is evidenced by the extremelj
plicated character of the iron formation itself East of this ridge, in the
low ground, the iron formation was found exposed, but- so poorly that it
was impossible to determine its width. It is followed to the south by
slates, whereas the conglomerates are the nearest rocks exposed to it on the
north. It seems almost unquestionable that we have here an iron-bearing
formation of very limited thickness, which occupies a horizon between
the conglomerates north of and below it, and the younger slates soiith of
and above it. This iron formation could not be traced to the east or
the west of the area mentioned. At one place, however, on the bare
hill in tlie SE. J of sec. 16, T. 64 N., R. 9 W., overlooking the swamp
which runs down to the southwest end of Newfound Lake, occur o-ood
exposures of a ver}^ feldspathic fragmental rock. This coarse fragmental is
interbanded with fine-grained slates. Here were found, in a few places,
interlaminated with the slates, narrow bands of black chert up to 3 inches
in width. This chert is conformable with the slates, and seems to be
contemporaneous with them, but may possibly consist of silicified lenses
of carbonate-bearing rock. These bands disappear after being followed
for only very short distances. The}^ occur fairly close to the greenstone
to the northwest, on which rest these sediments in unconformable relations
without intervening thick masses of conglomerate.
KNIFE LAKE SLATES.
One of the largest lakes on the canoe route along the international
boundary has been known since the time of the fur traders as Knife Lake
(Lac des Couteaux). The lake was so named because of the flinty con-
choidally fracturing slates which surround it and which, with their sharp
knife-like edges, cause a great deal of inconvenience to the moccasined
traveler, and even to him with thick boots. Probably this name is but a
translation of one given to it by the Indians long before the advent of
the French voyageurs. At any rate, the name aptly describes one of the
characteristics of the slates, and the lake bearing the name is so prominent
a featui-e of the hydrography of the district that the name has been given
also to the slates.
336 THE VERMILION IRON-BEARING DISTRICT.
PETKOGRAPHIC CHARACTERS.
Macroscopic characters — In mapping the Knife Lake slates it has been
found desirable to include with them numbers of beds of grit and fine-
gi'ained conglomerates which belong structurally with them. These, how-
ever, are so unimportant relative to the great mass of the slates that they
will not be considered in the further description unless they possess especial
characters that warrant reference to them. The slates vary from those
which are exceedingly fine grained and aphanitic to grits. As a rule, the
exceedingly fine-grained forms predominate. They are for the most part not
eartliv clay slates, but are flinty, break with a ringing sound, have con-
choidal fracture, and form fragments with sharp, cutting- edges. The normal
clay slates are in decidedly smaller quantity than the above-mentioned
flinty forms. The color of the slates is in general rather dark on fresh frac-
ture, varying from dark gray and olive green to bluish black. Associated
with these dark slates are light-grayish and greenish-colored slates and
graywackes. Occasional bands of white to gray and purplish black cherts
occur with the slates. On weathered surfaces the normal slates have a light
gray to light-brownish color. The flinty slates, however, weather with an
almost snow-white crust, showing maci'oscopically by this weathering that
they consist to a very considerable extent of silica in an exceedingly fine
state of division. This is in strong contrast to their invariable dark bluish-
black color on fresh fracture.
In general, there is a difference iii the slates in different portions of the
district, due primarily to the character of the rocks from which they are
derived. As a rule, where the Knife Lake slates and graywackes lie next to
the Archean greenstones, without large masses of Ogishke conglomerate
between them, they are green, and grade into the normal gra}- and blue
Knife Lake slates, made up in large proportion of granitic debris only at
considerable distance from the greenstones. Slates of this greenish color
are especially noticeable on Birch, Carp, and Ensign lakes, and in the area
southwest of Emerald Lake. From Moose Lake east to Ensign Lake the
slates range from the greenish ones, looking much like extremely fissile
green schists, to sericitic, gray, fissile slates, which predominate. IS ortheast
of Ensign Lake the slates again grade into the green ones, and in Bass
Lake the normal Knife Lake slates are at some places very flinty, and
THE LOWER HURONIAN. 337
at others have white weathered surfaces. There occur with the slates at
times graywackes which are i-ather puzzhng. They are more intimately
associated with the slates than with the conglomerates, and hence will be
described in this place. South of the east end of Moose Lake, and in fact
at a number of places on the hills south of this lake, especially in the
viciiaity of the portage from Moose Lake to Flask Lake, there occurs a
graywacke which has a greenish color, and from its green background
there stand out very prominent rounded porphyritic feldspars. The
appearance is very similar to that of a feldspathic porph3^ry which occurs
in the immediate Adcinity of these graywackes. Discrimination between
the two is difficult. In fact, there is evidently a gradation between them,
the graywacke representing the disintegrated jDortion of the porphyry, the
particles of which have been but slightly moved and worn. Bedding is not
very distinct in it. It is now very schistose, and one can trace the passage
from this schistose graywacke into the fairly massive poiphyry.
Somewhat similar feldspathic sediments with the feldspars appearing
almost like phenocrysts occur on the bare hill in the SE. ^ of sec. 16,
T. 64 N., R. 9 W., just west of the southwest end of Newfound Lake.
These occurrences correspond very closely to those graywackes which
occur at Vermilion Lake (p. 288) in immediate contact with the various
acid inti-usives, and which in many cases can not be distinguished from
them in the field.
Microscopic characters. — The microscopic examination reveals nothing
of especial interest. The essential primary constituents are feldspar, quartz,
brown mica, white to green and violet-brown pyroxene, and greenish-brown
hornblende, and then there is always an amount of the fine interstitial
material, the very fine product of the attrition of the grains of minerals and
fragments of rock forming the slates and graywackes. This material in all
cases studied has been recrystallized, and does not show up now as a dark
interstitial mass except by low power. By high power the individual con-
stituents can usually be recognized and will be referred to below. In the
graywackes which are associated with the slates there occur occasional
minute fragments of the various rocks which have been mentioned in
previous pages as forming a part of the conglomerates. The primary min-
eral grains and likewise the interstitial dust have very frequently been
extensively altered, and from these have been produced the following
MON XLV— 03 22
338 THE VERMILION IRON-BEARING DISTRICT.
secondary minerals, which in some cases, where the rocks are completely
recrystallized, are the sole constituents: Chlorite, epidote, sericite, actinolite,
massive dark-brown and gi'een hornblende, quartz, calcite, and pyrite. The
material between the grains in the coarser sediments is made up of sericite,
chlorite, epidote, quartz, and feldspar, and this is believed to have been
produced, as stated above, from the fine detrital material originally lying
between these grains. In the very fine-grained rocks, where the crystalline
character of this interstitial material is recognizable, but where the indivi-
duals of it could not be determined, they are presumed to be the same as
those just enumerated. These materials are present in varying proportion,
which produces, of course, the difiFerences in color and chemical composition
of the rocks; for instance, some of the lighter colored rocks — the light
cherts, for example — are composed essentially of quartz in very fine
crystalline particles. Other rocks are made up essentially of quartz and
feldspar in waterworu grains, with some of the fine interstitial material
between. Othei-s again contain, in addition to the quartz and feldspar, a
large quantity of pyroxene and of hornblende, and are very dark, usually
dark green, and apparently fairly basic in character. A few rocks contain
calcite in considerable amount, but never in sufficient amount to be called
limestone. Moreover, this calcite is believed to be of secondary origin,
derived from the alteration of the minerals forming the rocks or else
introduced from other sources by infiltration.
The general alteration which the minerals of these rocks have already
uuderg'one has been referred to. The addition of new minerals is cleai'ly
shown in the case of the hornblende of some specimens. In these we find
the fragments of dark-brownish hornblende surrounded hj recently added
massive light-greenish hornblende. The hornblende grains in some cases
have been increased to two and even three times their original length."
The changes which have taken place in some of the rocks show ver}^
well how, by somewhat further changes, banded crystalline schists might
be produced which would offer no clue but the banding to a determination
of the kinds of rocks from which they were derived. Thus some of the
fine-grained sediments, rich in hornblende, have nearly all of the pieces
of hornblende, very commonly cleavage pieces, arranged with their long
" Enlargements of hornblende fragments, by C. R. Van Hise: Am. Jour. Sci., 3d series, Vol. XXX,
1885, pp. 231-235.
THE LOWER HUEONIAN. 339
directions parallel. Mauy of these have been added to by secondary
growths of massive hornblende or of fibrous actinolite. This addition of
the secondary material only tends to emphasize the parallel arrangement
of the jDarticles, since the greatest addition has been made on the ends of
the fragments. This was noticed in some of the graj'-wackes made up
almost exclusively of feldspar and hornblende, with mica next in abun-
dance, and very little quartz. In such rocks the biotite alters to chlorite.
Alteration of the feldspar with the production of new minerals also takes
place, and a few quartz grains alone remain to give evidence of the clastic
character of the constituents. When such rocks take part in orogenio
movements the quartz may be crushed, and in general a better degree of
schistosity produced than previously existed, and as a result of such move-
ments hornblende-schists may eventually be produced. The banding of
these hornblendic sediments has been referred to. These bands of finer
and coarser materials have essentially the same composition. There also
appears to be a relation between the fineness of the hornblende grains of
the original sediments and the character of the new amphibole. Thus it
was noticed that in tlie alteration of the very fine-grained sediments the
new amphibole is added in the form of fine actinolite needles, whereas in
the coarser graywackes the new amphibole is in general a massive horn-
blende. The final pi'oduct of the metamorphism of these rocks would
probably retain this essential difi"erence, so that we would g-et fine- and
coarse-grained ampliibole-schists, or possibly banded actinolite and horn-
blende-schists.
There is also a banding due to the separation of the kinds of minerals.
Thus there will be some dark Ijands made up essentially of hornblende and
mica with but little feldspar, and alternating with these bands there will
be bands of feldspar with but little hornblende or biotite. When such
rocks are metamorphosed there will be no very great migration of material
from the one band to the other, and certainly the original differences in the
bands will be shown to a certain extent in the differences in the bands in
the metamorphic product. Presumably the final product would be an
amphibole-schist complex made up of alternating bands of amphibole-
schists rich and poor in feldspar or other secondary products — quartz and
feldspar, perhaps — derived from the original feldspar and of different color,
corresponding to the difference in mineral composition.
340 THE VERMILION IRON-BEARING DISTRICT.
METAMORPHISM OF THE KNIFE LAKE SLATES.
The rocks possessing' the characters briefly described above are those
which we may call the normal Knife Lake slates. Even these so-called
normal slates have been very much metamorphosed. The metamorphism
has been caused by the infiltration of material (chiefly silica and calcium
carbonate), by the cementation of the particles by these substances, and
hj the further cementation of the rock by chemical changes of the
fragments, which produced new minerals and also caused the secondary
enlargement of the old mineral fragments. These rocks show locally the
effect of orogenic movements in more or less well-developed schistose
structure and fracturing.
At several places in the district the slates are in contact with later
igneous rocks, both acid and basic, and have been metamorphosed by the
intrusion of these rocks. In all cases the rocks, as stated above, have
been affected by processes of cementation and to a certain extent by
orographic movements, and in some instances these sediments have been
metamorphosed by acid intrusives and then have been acted upon by the
great Keweenawan gabbro and still further changed. Hence it is exceed-
ingly diflicult to discriminate between the kinds of metamorphic products
wliich are due to each one of these agencies. In the following paragraphs
a brief description will be given of the macroscopic characters of those
Knife Lake slates which have been metamorphosed by the contact action
of the acid and basic intrusives, and which occur in the four most important
areas: (1) South of Tower, along the Duluth and Iron Range Railroad, near
milepost 92; (2) on the Kawishiwi River; (3) at Snowbank and Caca-
quabic lakes; and (4) in the vicinitj' of Gobbemichigamma and Paul lakes.
Contact effect of the granite. — The sediments metamorphosed by the
Griants Range granite are well exposed near milepost 92 on the Duluth
and Iron Range Railroad, south of Tower, find this place is readily
accessible. They are also well exposed on the Kawishiwi River, near the
mouth of that river where it empties into Farm Lake ; along the shore east
thereof; and on the portage leaving the ba}- of the river, in the SE. \ of sec.
30, T. 63 N., R. 10 W., leading southeast to Clearwater Lake. In places
some conglomerates occur, especially on the islands in tlie river in sec. 26,
T. 63 N., R. 11 AV., but the coarse sediments are very subordinate. The
THE LOWER HURONIAN. 341
sediments are now metamorphosed to mica- and amphibole-sehists. Occa-
sionally garnets have been produced as the result of contact action. The
metamorphosed rocks retain the sharp banding of the original sediments,
and there is also noticeable at many places a rapid altei-nation of bands
of different grain and composition. These bands are composed of feld-
spar and quartz, with epidote, amphibole, and mica. Variation in the
quantity and combination of these minerals causes a difference in their
appearance. A study of almost any of the exposures in the area south of
the Kawishiwi River in which these rocks occur as outlined on the maps
shows clearly the cause of the alteration. They are penetrated at numer-
ous places by dikes of granite which are offshoots from the great mass
of Giants Range granite which lies in contact with these schists on the
south. The granite dikes become more numerous as we go farther south
nearing this contact, and at the south end of the portage above referred to,
on the ridge just overlooking Clearwater Lake, the granite dikes are more
numerous and of larger size than at other places. Here, moreover, there is
an exceedingly good example of an eruptive breccia. The breccia consists
of dark-gray and black schist fragments derived from these metamorphosed .
sediments, which are cemented by a matrix of the grayish and pink Griants
Range granite. Farther south, within the main mass of the granite, as on
the islands in Clearwater Lake and elsewhere, it attains its normal grain
and characters. In the dikes its characters are likely to vary as well as its
grain, depending upon the size of the dikes and the position in the dikes
from which the specimen is taken. Mountain-making movements may have
affected and unquestionably did affect these sediments to some extent. It
probably aided in making them schistose. What other effects it may have
had have been concealed, however, by the contact effects of the intrusive
, granite.
In the vicinity of Snowbank Lake, especially on the bare hills south-
west of it, in sees. 10 and 17, T. 63 N., R. 9 W., the Knife Lake slates are
well developed and are splendidly exposed over large areas. The slates
have been intruded by the Snowbank granite and from them have been
produced mica-schists, which predominate, with subordinate amphibole-
sehists. The same kind of effect has been produced by the intrusion of the
Cacaquabic granite on the slates nearest it, although, since the slates
are not exposed very near the granite, the effect is not so marked.
342 THE VERMILION IRON-BEARING DISTRICT.
The microscopic study of these contact rocks of the granite shows
nothing' of especial interest. The rocks are, as said, mica- (biotite-) and
amphibole-schists. The minerals constituting them are biotite and some
musco\ate, hornblende, actinolite, quartz, feldspar, epidote, and garnet
occasionally.
Contact effect of the gahhro. — The effect of the intrusion of the Snow-
bank and Cacaquabic granites on the suiTOunding sediments has been, as
said, to produce mica-schists, and, in a subordinate degree, amphibole-
schists. Subsequent to this intrusion these schists must have been affected
by orographic movements, but the effect of these movements is not recog-
nizable, for at a still later date the great Keweenawan gabbro was intruded
into the rocks of this district and produced important contact effects upon
them. It is practically impossible now to discriminate between the products
of these different periods of metamorphism. In general the sediments in
the vicinity of the granites and gabbro have been transformed into mica-
schists and amphibole-schists, whose chief characters may have been pro-
duced by the intrusion of the Snowbank and Cacaquabic granites alone.
South and southwest of the Snowbank granite, however, in the vicinity of
the gabbro, fine-grained rocks are very commonly spotted and resemble the
so-called spilosite, the spotted contact rocks of the diabase. This pecuhar
phase of the metamorphism is probably due to the contact action of the
gabbro. Some of these spotted rocks are at present about thi-ee-fourths
of a mile away from the nearest exposures of the gabbro. It is, however,
not necessary to believe that the gabbro was able to affect the sediments at
this distance. It seems almost certain that at one time the gabbro extended
farther north than the line where its present northern boundary appears,
and that at that time it overlay these rocks, so that in reality it was sepa-
rated vertically from the rock at the level of the present exposures by
perhaps only a few hundred feet of intervening rock at most. Similar
spotted rocks occur in the slates upon the prominent hill north of the west
end of Paul Lake, in sec. 32, T. 65 N., R. 5 W. The total absence, near
these spotted sediments in this area, of any acid intrusives which could have
produced them, and their presence here in close proximity to the gabbro,
seems to show pretty conclusively that the spotted rocks in this place, as
Avell as the similar ones mentioned above as occurring south and south-
west of the Snowbank granite, are due to the gabbro contact action, and
not to that of the granite.
THE LOWER HURONIAN. 343
Some of the best places at which to study the slates that have been
extremely metamorphosed are on the east, southeast, and south shores of
Gobbemichigamma Lake. Here the metamorphism of the slates by the
gabbro has not been complicated by a previous metamorphism of the slates
by a granite, as is the case in the vicinity of Snowbank Lake. At various
places here the sediments are exposed, showing in places their characteristic
banding, but they have been so extremely altered that but for this banding
their derivation from the slates might not be recognized. The sediments
have acquired for the most part a granular character and brownish coloi",
and weather rather readily. As the result of their peculiar saccharoidal
appearance the name "muscovado," having reference to their resemblance to
brown sugar, was given to them b}^ Alexander Winchell." Their true char-
acter was not recognized by him. Since it is clear that they are but meta-
morphosed phases of the sediments, it seems totally unnecessarv to continue
the use of this term, which can not be applied to a rock of definite composi
tion, especially since, as pointed out by H. V. WinchelP and U. S. Grant,"
two different kinds of rocks, metamorphosed sediments, and certain phases
of the gabbros, are included under this term merely because they bear a
superficial resemblance to one another. This metamorphism has been pro-
duced by the Duluth gabbro, which at a number of places has been found
in direct contact with these rocks. One of the best places at which to study
the relationship is on the sn:iall island crossed by the town line on the
southeast side of Gobbemichigamma Lake, and also on the point north of the
portage from this lake east into Peter Lake. At these places the sediments
are overlain by the gabbro, and the contact line between them can be traced
ver}^ clearly. The vertical thickness of the contact rock as measured on
Gobbemichigamma Lake seems not to have exceeded 50 feet. At many
places along the shore there is a horizontal exposure of much more than
this in width. This represents, however, the beveled edge of the contact
zone, and since no data for the reconstruction of the removed material
showing the inclination of the surface overlain by the gabbro can be
obtained, no measurement can be made of the true width of the contact
zone.
On the island referred to above, and immediately next to the gabbro,
a Geol. and Nat. Hist. Survey of Minnesota, Fifteenth Ann. Rept., 1886, pp. 183 and 351.
6 Ibid., Seventeenth Ann. Eept., 1888, p. 130.
clbid., Final Eept., Vol. IV, 1899, p. 178.
344 THE VERMILION IRON-BEARING DISTRICT.
the sediments are, as above stated, granular and in character resemble much
more closely the gabbro than they do the sediments proper. At some
distance away from the gabbro contact rounded masses of dense greenish
and gray rock begin to appear, surrounded by the granular contact product.
This gives a conglomeratic appearance to the ex^josm-e. A little farther
away from the gabbro, on a vertical exposure at the lake shore, these
pebble-like areas were seen to be aligned, and eventually to pass into
parallel unbroken bands. The explanation of this occurrence is that the
gabbro in contact with the sediments caused them to be altered to the
peculiar granular contact product. The alteration naturally was most
eifective nearest the gabbro, and gradually spread, following along the
cracks in the rocks, the vertical as well as the horizontal cracks. Near the
gabbro all of the sediments were changed. Farther away the blocks or
fragments of sediments were changed on the exterior most completely.
Sometimes this change was so far reaching as to convert into the granular
contact product most of the sedimentarj^ block, and to leave only a small
core of the block, but even this was very much modified. Such a core
looks like a pebble in a matrix, and gives the rock a conglomeratic appear-
ance. Still farther away tlie alteration was less, following only along
certain of the most prominent parting planes, and leaving the sediments in
bands.
PETEOGKAPHIC CHARACTERS OF THE METAMORPHOSED SLATES.
Microscopic characters. — It would require a very much more detailed
study of these metamorphosed slates than has yet been made to enable one
to describe fully the various changes through which the original sediments
have passed in becoming the various kinds of rocks above described, and
numerous chemical analyses would be required in order to determine just
what, if any, changes in chemical composition had been produced by the
contact action of the gabbro.
The chief constituents of these contact rocks are: Plagioclase feldspar,
little quartz, biotite, muscovite, chlorite, green, bluish, and brownish
hornblende, light-green pyroxene, hypersthene, olivine (?), titanite, epidote,
garnet, and magnetite. The mica-, hornblende-, and pyroxene-schists and
gneisses derived from the slates by the conttict action of the gabbro almost
invariably contain very little (juartz, but are full of a rich-brown mica
THE LOWER HURONIAN. 345
and hornblende, with feldspar present in large quantity. The dark con-
stituents predominate, and the mica is usually more abundant than the
hornblende. With these constituents there occur varjang amounts of
hypersthene, light-green pyroxene, olivine (I), and magnetite. In some of
these gabbro contact rocks the hypersthene exceptionally is the predominant
constituent, usually associated with, considerable mica and magnetite. In
general we may say that the production of minerals rich in magnesium and
iron, in particular hypersthene, brown mica, and magnetite, is characteristic
of the gabbro contact. These are, of course, the kinds of minerals whi^ch, a
priori, would be expected in rocks greatly affected by a gabbro magma.
Analyses of these rocks and of the rocks from which they were derived
have not been obtained, and indeed a great many would be required to
prove the thesis that an actual transfer of material from the gabbro to the
surrounding sediments had taken place. However, in view of the produc-
tion in such large quantity of the magnesian and iron minerals in these
sediments it is believed that such a transfer of some magnesia and iron has
actually taken place from the gabbro magma to the sediments now con-
taining these minerals. The fact that these minerals occur more abun-
dantly in the rocks near the gabbro than in those farther away supports
this view.
The spotted contact rocks, the spilosites, are known to occur as the
result of the contact action of diabases and gabbros, and those occui-ring
in this district are believed to be without doubt the product of the gabbro
contact and to be characteristic of it. The spilosites from -the Vermilion
district are fairly common in the area southwest of Snowbank Lake. They
are very similar to the spilosite described from the Crystal Falls district of
Michigan," and, like them, the spots, wliich are in general of oval outline,
occur isolated or united along the long axis of the ovals in a series. These
spots are composed of aggregates of muscovite, epidote, little chlorite, and
sphene, in a tine groundmass (which predominates) of muscovite, chlorite,
epidote, sphene, feldspar, and quartz.
Somewhat different are the spotted rocks occurring north of Paul Lake,
for example. The white material forming the spots has essentially the same
single and double refraction as feldspar. The material includes biotite and
a Mon. U. S. Geol. Survey Vol. XXXVI, 1899, pp. 206-207. Also a contribution to the study of
contact metamorphism, by J. Morgan Clements: Am. Jour. Sci., 4th series, Vol. VII, 1899, pp. 81-91.
346 THE VERMILION IRON-BEARING DISTRICT.
chlorite flakes, and particles of iron ore. Grant" has described cordierite
occurring in similar rocks on Gobbemichigamma Lake, to the southwest
of the area above referred to. Very possibly the material forming- these
white spots is cordierite, but no conclusive proofs of this were obtained.
THICKNESS.
It has already been stated that in portions of tlie Vermilion district
the Knife Lake slates show a very great width. From this fact alone one
not taking into consideration the intensely plicated condition of these rocks
and hence the possibilit}^, or even the certainty, of more or less repetition,
niig:ht be led to infer that these slates are of enonnous thickness.
This folding, of course, points to a jirobable reduplication of the beds.
While the slate area can by no means be described as homogeneoiis,
nevertheless it is true that clearly recognizable key rocks are wanting.
Consequently there must be numbers of anticlines and S3"nclines which it
has not been possible under the existing conditions to ,recognize. These
facts render it exceedingly difficult to make any authoritative statement as
to the thickness of tliis series. At one place, however, the structm-e is
fairly simple. Between Ogishke Muncie Lake and Gobbemichigamma
Lake, two of the large lakes in the eastern part of the district, there are
two small lakes, Fox and Agamok lakes, which occupy the low ground.
North of these there is a high ridge occupied by Archean greenstone with
the Ogishke conglomerate on its southern flank. South of this string of
lakes there is a second ridge of Archean greenstone, forming a verv marked
topographic feature. Between these two ridges we find the Knife Lake
slates, showing a number of good exposures. Traverses across this area
from north to south gave at first a series of south dips, gradually becoming
flatter and flatter, until the bottom of the syncline just south of the lakes
mentioned as lying in the bottom of the depression was reached, where the
flattest dips were found. Continuing across as we ascend the south ridge,
this succession of dips is repeated in reverse order — i. e., the steepest dips are
nearest the ridge, and the dips are to the north. Moreover, it was possible
to observe a repetition of the rocks. The slates hei-e evidently occupied a
distinct syncline, and moreover this syncline seems to be a simple one.
"Geol. and Nat. Hist. Survey of Minnesota, Final Kept., Vol. IV, 1899, pp. 657-658.
THE LOWER HURONIAN. 347
From the data obtained here the total thickness of the slates across this syii-
cline is calculated to be 5,000 feet, which divided in half gives us a maxi-
mum thickness here of 2,500 feet. It is impossible to state positively
whether or not this represents the maximum thickness of the slates for this
area. The presumption is, however, that the maximum thickness here
obtained does not represent the maximum original thickness of the slates,
as they must have originally occupied the entire basin between the older
rocks, and possibly had a much greater vertical extent, the rocks above
those now remaining having' been of course long since removed by erosion.
INTERESTING LOCALITIES.
Structure and relations of the slates. — On Carp and Birch lakes the
relations of the topography to the geologic structure are well indicated.
The central headland of Carp Lake, as well as much of the western bay
and the headlands beside the narrows, are composed largely* of a coarse
graywacke which is characteristic of the lower part of the slate formation.
These headlands are directly along the strike of the plunging anticline at
the east end of the lake. Corresponding to them there is an eastward-
projecting headland in the west bay of the lake, and this also is composed
of the coarse graywacke. The southern part of the western bay is com-
posed of the normal slates. Thus, while Carp Lake is as a whole a part
of a monocline dipping to the south, it very plainly has a subordinate
anticline through the center which makes the graywacke at the base of the
slates "the predominant rock exposed.
Birch Lake, west of Carp Lake, was found to be surrounded by slates
for about a mile at its eastern end. Then, at a little bay occurring just
east of the long westward-projecting point, greenstone comes out upon the
north shore for a short distance. The slates appear again upon the point
to the southwest of the greenstone. The greenstone is again exposed in
the next bay to the north, and with a minor fold swings up into the
extreme north bay and thence inland, the shore being composed of slate.
The relations are such as to indicate a series of infolds, the slates occupying
the depressions, appearing as reentrants on the lakes, and the greenstones
occupying the headlands and highlands to the north.
Topographic depressions usually separate the Archean greenstones
from the slates which lie near them, hence the sequence of rocks from the
348 THE VERMILION IRON-BEARING DISTRICT.
greenstones to the slates is usually interrupted. A number of contacts
have been found, however, and in every instance where careful examination
was made at least a narrow belt of conglomerate derived from the green-
stone intervened between the greenstone and the slates. The localities at
which such conglomerates occur are described under the heading "Ogishke
conglomerate" (p. 317 et seq.). These conglomerates were found to be pres-
ent in practically every case in which careful search was made, and this fact
is the warrant for indicating on the maps in many places continuous belts of
conglomerate between the Knife Lake slates and the Ely greenstones. This
insertion has been made especially for the reason that thereby the structm-e
of the district can be made plainer. In almost every case, however, in
order to show this belt, it has been necessary to magnify many-fold the
true thickness of the conglomerate. In the NW. \ of sec. 17, T. 63 N., R. 9
W., contacts between the Knife Lake slates and the ellipsoidal Ely green-
stone can be found at many places. Here there is an eastward-plunging
trough of sediments infolded in the greenstones. Here and there a narrow
band of conglomerate occurs between the slates and the greenstone; else-
where, however, actual contacts have been found in which the slate lies
next to a zone of very schistose greenstone, which then grades down into
the normal greenstone. Evidently these deposits were formed in a fairly
protected area, as is indicated by the predominance of the slates over the
coarser gray wackes and the scarcity of conglomerates along the border of
the greenstone.
About 120 paces south of the Moose Lake end of the Moose Lake-
Flask Lake portage there occur numerous small outcrops of a porphyritic
greenstone which has a more or less schistose green matrix, with numerous
white porphyritic crystals of feldspar evenly distributed through it.
Toward Moose Lake this porphyry becomes more and more fissile until
finally it passes into a schist. This schist is so beautifully and finely
laminated and fissile that it can almost be spoken of as a slate. Upon tlie
surfaces the flattened phenocrysts are shown. This schist belongs with the
Knife Lake slates. South of it lies a graywacke made up of disintegrated
material derived from the porphyry, with which it lies directly in contact.
There are no pebbles in this material which can be clearly recognized as
such. If any exist they merge into the matrix, and the two rocks, the
schistose porphyry and the grit derived from it, resemble ench other so
THE LOWER HURONIAN. 349
strongly that it is onl}^ after very close study that they can be
discriminated.
At the southwest end of the lake, whose tip just extends into the NE.
J of sec. 11, T. 64 N., R. 8 W., there is a dike of granite-porphyry which
cuts across the bedding. Other granite dikes must occur throug-hout this
district cutting these slates, although few of them have been found. One,
for instance, occurs upon the south side of the large island in Knife Lake
north of sec. 31, T. 65 N., R. 8 W.
Contact metamorpJdsm of the slates. — An excellent place at which to see
the character of the metamorjjhosed Knife Lake slates is along the Duluth
and Iron Range Railroad, near milepost 92. This place is also very easily
reached.
In going south from Tower after passing the south side of the Ely
greenstone one observes the following series of sediments:
On the fii'st exposure south of the greenstone we get slates and gray-
wackes, which are, even in this first exposui'e, somewhat metamorphosed —
that is, they are very firm slates and quite thoroughly indurated gray wackes,
but still show verj clearl}^ their unmistakable sedimentary characters. The
first gi-anite dikes observed in these sediments occur about a mile south of
their most northern outcrop. Continuing- south from the first outcrops of
the sediments on the railroad, the ones to which we come thereafter take
on a more and more altered character as we go farther south. At milepost
92 the presence of good conglomerates with accompanying finer sediments
showing well-marked sedimentary structures such as normal and current bed-
ding, indicates with absolute clearness the mode of origin of the rocks. The
microscopic study of these rocks is not so satisfactory as the macroscopic;
the changes which the rocks have undergone have obliterated all features
which would enable one to determine with absolute accuracy by means of
the microscope the original characters of these rocks, although the coarse
macroscopic structures still remain. Farther south the outcrops become
scarcer as we approach the great muskeg area in the low ground north of
the Giants range. Nevertheless here the few outcrops which were observed
are exceedingly indurated banded rocks which can be more properly spoken
of as mica-schists than as graywackes. Still farther south the rocks are
mica-schists and mica-gneisses, with very much contorted banding, and
. are cut by granite dikes. The change in the character of the rocks as
350 THE VERMILION IRON-BEARING DISTRICT.
shown on the outcrops seems to point conclusively to the fact that these
changes are due to an increasing metamorphism of these sediments, con-e-
sponding to our approach to the main Giants Range granite mass lying far
to the south. In this western portion of the area the center of intrusion
lies in the Giants range, a number of miles south of the area descril^ed in
this monograph.
The metamorphism of these Knife Lake slates can be seen to advantage
in the vicinity of Snowbank Lake. On the north shore of Snowbank Lake,
at a number of places, mica-schists may be found wliich are cut through and
thi'ough by dikes derived from the Snowbank granite. These schists, when
followed inland for a considerable distance, are found to grade into the less
and less altered sediments, until eventually the normal Knife Lake sedi-
ments are reached. Somewhat similar metamorphosed mica-schists can be
observed upon the portage between Round Lake and Disappointment Lake.
These have been metamorphosed by the gabbro which lies only a short
distance to the south, although in all probability they were first affected by
the intrusion of the Snowbank granite. Here 'the cleavage is parallel to
the bedding. This mica-schist resembles in a remarkable degree the mica-
schist " which occurs as the upper part of the Upper Slate division of the
sediments at English Lake, near Penokee Gap. The most crystalline part
of this Upper Slate member runs from Penokee Gap westward, and here the
basal member of the Keweenawan, lyixig north of it, is a g'reat mass of
gabbro. The lower members of the Upper Slate at Penokee Gap, although
at lower horizons, and therefore presumably more deeply buried, and
moreover containing unquestionable intrusions of the diabase, are never-
theless composed of comparatively little metamorphosed black slate. It
seems conclusive that these mica-schists in the vicinity of Snowbank Lake
and those occurring in Penokee Gap both owe their present characters to
the alteration of an original slate by the gabbro.
At a point only a few miles west of ^Montreal River, and again at the
top of the Upper Slate member, near tlie gabbro, the rock is a gra}^, coarse,
strongly micaceous graywacke, the only recognizable clastic material in the
rock being the coarse quartz and feldspar. In Michigan, on the Penokee-
Gogebic range, east of ]\Iontreal River, the bottom members of the
Keweenawan are comparatively thin-bedded lava flows, dolerites, and
«Mon. U. S. Geol. Survey Vol. XIX, 1892, pp. 302-308.
THE LOWER HURONIAN. 351
amygdaloidal lavas, and here the top layers of the Upper Slate are the
ordinary or slightly metamorphosed black slates. From this it appears
clear that on the Penokee rang-e the metamorphism of the slates and mica-
schists is due to the gabbro, and the same conclusion seems to follow for
the similar schists in the Vermilion district.
The Lower Huronian slates appear at a number of places on the
north, south, and southeast shores of Gobbemichigamma Lake. On the
southeast and east shores these slates are in many places found in direct
contact with the Keweenawan gabbro, and where near it or in contact with
it they have been extremely metamorphosed. Much less metamorphism is
noticeable on the slates occurring on the north side of the lake. That part
of the south shore of the lake in the immediate vicinity of the section
line between sec. 1, T. 64 N., R. 6 W., and sec. 6, T. 64 N., R. 5 W., affords
a good opportunity for studying the relations between these two rocks.
There is a high cliff at the point where the section line above mentioned
comes to the shore. On this cliff one can readily detect bands of rock
which strike N. 10° E. magnetic and dip 20° to the east. This strike as
taken really represents the strike of the face of the cliff. Were it possible
to obtain it, the true strike of the beds would be found to be somewhat dif-
ferent. The bands in the cliff are made up of very dense-grained, hard,
siliceous rock. They are rarely more than 4 inches in thickness and fre-
quently very much thinner. Alternating with these bands there is a
rotten, brown, coarse-grained material which seems to weather very
readily — it certainly does so in comparison with the adjacent harder
bands — and appears much like gabbro. If we follow the shore around to
the northeast it is there possible to land and ascend the sloping hill leading
to the top of the above-mentioned cliff. In the ascent of this hill one
crosses the bands of rock, which are imperfectly shown in this section.
The harder bands are especially recognizable, as they are likely to form
shelves, the slopes between being formed of the softer gabbro-like material.
The relations here seem to indicate either that the banded sedimentary has
been included in the gabbro or else that the gabbro has been thoroughly
injected into the sedimentar}^, the injection following chiefly the bedding
planes as planes of least resistance. The normal coarse-grained gabbro
occurs on the shore just a short distance — about a quarter of a mile — back
of this headland. Following this shore from the cliff to the west we note
352 THE VERMILION IRON-BEARING DISTRICT.
ai^pearing at the water level a couglomeratic looking rock, the pebbles of
which seem to be a dense quartzitic graywacke or slate, whereas the
matrix is light colored, rather coarse grained, and appears like an exceed-
ingly feldspathic biotite-gabbro. This conglomeratic rock is cut by a sheet
of basalt 12 to 16 inches wide, which is very nearly horizontal, showing-
only very slight east-west rolls. The basalt sheet is separated into
very symmetrical hexagonal columns, and shows a distinct fine basalt
selvage, while at its center the sheet is coarser grained. Ascending the cliff,
above tliis conglomeratic rock, we pass from it into a coarse brown, more
or less friable rock, which calls to mind Winchell's muscovado. Still higher
up this rock is found to grade vertically into a coarse normal gabbro. This
conglomeratic-looking rock continues along the shore still farther to the
southwest.
Following the shore of Gobbemichigamma Lake to the east of our
starting point at the cliff, a pseudo-conglomeratic rock similar to that
described above begins on the shore just a little north of the meander
corner of the town line between Ts. 64 and 65 N., R. 5 W. The rock here
is rather fine-grained granular rock, weathering white to yellow and brown,
in which occur very frequent rounded areas all essentially alike and
seemingly of one kind of rock — a dense, green, fine-grained very quartzose
graywacke. A similar rock is well exposed on the little island just west
of the shore on which the meander corner stands, and here also its relations
to the gabbro and its true characters are better shown than elsewhere.
The occurrence observed here has already been described (p. 343). The
gabbro is evidently younger than the pseudo-conglomeratic rock, which
has, in fact, been produced from preexisting sedimentary rocks by the
intrusion into them and extensive metamorphism of them by the gabbro.
SSCTION 11— ACID AND BASIC INTRUSIVES OF THE LOWER HURONIAN.
I]SrTKODUCTIO>r.
In Section III of Chapter III various acid intrusives which are of the
same general petrographic character and geologic age are discussed. In
addition to these intrusives, there are found in the Vermilion district
thi-ee other large masses of granite and granite-porpln-ry, from which
iiumerous dikes have been given off. These large masses and accompanying
THE LOWER HURONIAN. 353
dikes penetrate the siuToundiug- Lower Huronian sediments and other adja-
cent rocks. The large masses especially have produced on the adjacent
rocks far-reaching metamorphism. In addition to these main areas and
the dikes which can be connected with them, there are throuo-hont the
district acid dikes which vary somewhat in petrographic characters and
which penetrate the various formations adjacent to them. The largest
masses of these acid rocks occur in the core of the Giants range, in the
vicinity of White Iron Lake, extending northeast and southwest of that
area, and in the vicinity of Snowbank and Cacaquabic lakes, and are
described below. In addition, a section will be devoted to a brief description
of the various dikes which can not properly be connected with these
masses, but are assumed to be of essentially the same age. Still another
section is given to a very brief description of certain basic and intermediate
dikes which have been found to bear the same relations to the adjacent
formations as the acid intrusives bear, and which are hence presumed to be
of the same age. ' '
GIAISTTS RAJiTGE GRAISTITE.
DISTRIBUTION, EXPOSURES, AND TOPOGRAPHY.
Distribution. — The Giants Range granite borders a portion of the south-
ern side of the Vermilion district, and is best developed in that part of the
area extending from the vicinity of Beaver River, in sees. 31 and 32, T. 62 N.,
R 13 W., eastward into sees. 24 and 25, T. 63 N., R. 10 W. Immediately in
the vicinity of White Iron Lake it reaches a very extensive development,
and to the southwest, in the portion of the Giants range in the Mesabi
district, it occupies the core of the range, and here shows its typical
characters, and has therefore been called the Giants Range granite. The
granite underlies a very miich greater area than is shown on the map
(PI. II). According to the Minnesota maps, and also as observed in a
reconnaissance trip, it extends a number of miles to the south of the area
here described, where it is bordered by the Duluth gabbro mass. This
gabbro mass likewise cuts across it in the northeast-southwest direction,
gradually nearing the area underlain by the granite as it is followed east-
ward, until in sec. 19, T. 63 N., R. 9 W., the granite is completely cut out
by the gabbro. The area underlain by this granite thus varies from a
very narrow strip on the east, where it actually feathers out, to an area 5
MON XLV — 03 23
354 THE VERMILION IRON-BEARING DISTRICT.
or 6 miles across, at a point south of White Iron Lake. The portion in
the area described in this report is rarely more than 2 miles wide and
usually less than that. Indeed, that part of the granite -s^-liich was actually
studied represents merely the border of the granite, for the reason that in
most cases our traverses were ended at the northern border, as the prime
object of the survey was the delimitation and study of the formations in
the iron-bearing district proper.
Exposures. — The exposures of granite are good, as the area in which
it lies is well dissected by streams and contains a number of lakes. The
exposures are especially good on White Iron Lake, in T. 62 N., Rs. 11
and 12 W.
Topography. — The topography is that usually seen in the granitic areas
of the Lake Superior region, consisting of low rounded to oval hills with
lakes here and there, in the intervening valleys. The range of hills formed
by the Giants Range granite is the topographic continuation to the north-
east of the Mesabi or Griants range of the Mesabi district. The hills are
nowhere high, however, and do not show very well the character of a hill
range.
About 2 miles southeast of Ely and extending from sec. 11, T. 62 N.,
R. 12 W., to sec. 25, T. 62 N., R. 13 W., there is an area underlain by this
granite which is \qyj similar to that occurring north of the district on Iron
Lake (see p. 259). This area is almost base-leveled, the lakes into which
it is drained representing the level to which the surrounding land has been
very nearly reduced. This area is very much larger than that occurring
near the international boundary before referred to. Here the swamps are
extensive and the elevations are very slight, being flat hillocks of granite
rising as a rule only a few feet above the adjacent low ground.
PETROGRAPHIC CHARACTERS.
Macroscopic characters. — The Giants Range granite includes a series of
granites ranging in color from light gray to very dark gray, to flesh color,
pinlv, and red. The grain varies also very materially, the rock passing from
very dense fine-grained granites through medium to coarse-grained ones.
While this rock is, as a rule, granitic in texture, there are also variations
to granite-porphyries and exceptionally to some that can be spoken of as
rhyolite-porphyries. These granite- and rhyolite-porphyry dikes are nor-
THE LOWER HURONIAN. 355
mally found cutting the greenstone which borders tlie Giants Range granite
on the north. They are described with the Giants Range granite, since they
are presumed to be offshoots from it, although their direct field connection
with it can not be. shown. Their petrographic similarity to the main
granite mass seems alone sufficient to warrant their description together
and to support the view that they were derived from the same deep-seated
source.
In places along the Kawishiwi River the granite is slightly schistose.
This schistosity is especially noticeable along the margin of the granite,
where it lies next to the Archean Ely greenstone. These schistose phases
can be well seen in the southern half of sec. 24, T. 63 N., R. 10 W., imme-
diately north of the Kawishiwi River.
The granite massive includes areas of dark hornblende and mica rocks
which are more or less schistose and consist of hornblende, mica, quartz,
and feldspar in about equal proportion. The relationship which the granite
is presumed to have to these is indicated by the use above of the word
"includes," for it surrounds these masses in some cases and sends offshoots
into them. In other cases the granite is found cementing an eruptive
breccia the fragments of which were derived from the above-mentioned
schists. Such a breccia, for example, is well shown just north of Clear-
water Lake, alongside the portage entering that lake from the Kawishiwi
River. These fragments in the granite are in some cases derived from the
Ely greenstone. In other cases the fragments represent a sedimentary series,
the Lower Huronian, which has been intruded and included by the granite.
It is difficult to determine the original characters of these included rock
fragments from a microscopic study after they have been metamorphosed.
In the field one has as a guide the proximity of the granite to larger masses
of metamorphosed sediments on the one hand or to the Ely greenstone
on the other. Naturall}^ the fragments in the granite are most likely to
have been derived from that rock to which the fragments are nearest.
Dikes of very fine-grained red aplite cut the Giants Range granite.
The constituents of these granitic rocks as disclosed by the microscope
are orthoclase . (microcline), plagioclase, quartz, hornblende, mica, zircon,
apatite, sphene, a little iron ore. These minerals possess their usual char-
acters. It is interesting to note that the microcline is especially abundant
in granites which show the pressure effects in the other minerals more evi-
356 THE VERMILION IRON- BEARING DISTRICT.
(k'litly tlian the rocks with a smaller amount of the microclme. This is
evidence iu favor of the microcline twinning having been produced by
pressure. The quartz and feldspar occasionally are in niicropegraatitic
interg'rowths. Some secondary minerals occur with these rocks, sucii as
chlorite, sericite, and epidote.
RELATIONS TO ADJACENT FORMATIONS.
The Giants Range granite is found at different places in juxtaposition
with the Ely greenstone, the Soudan formation, the Lower Huronian sedi-
ments, and the Keweenawan gabbro, enumerated in order from the base up.
Its relations to these formations are in each ciase quite clearly shown and
will be specifically described in the following paragraphs.
Relations to tlw Ely greenstone. — The granite cuts the greenstones con-
stituting this formation in innumerable dikes which individually seem to
have little effect upon the greenstone, judging from the lack of well-marked
contact zones adjacent to the dikes. As we get nearer the contact between
the greenstone and the granite massive, however, exactl}- the same kind of
metamorphic products are observed as are found associated with the contact
of the intrusive granite of Trout, Burntside, and Basswood lakes with the
Ely greenstone on the northern side of the district. As the dikes increase
in number the greenstones are altered to amphibolitic and micaceous schists,
frequently still retaining the unmistakable amygdules and ellipsoidal parting
of the original greenstones. Similar products of the granite intrusion have
been described under the description of the effect of the granite of Trout,
Burntside, and Basswood lakes on the Ely greenstone (p. 156 et seq.).
Relations to the Soudan formation. — The Soudan formation is, as has been
stated, of very limited extent, and consequently there are few opportunities
for observing granite dikes in it. However, such dikes have been observed
at a number of places (see p. 359), and their presence shows clearly that
the Giants Range granite cuts the Soudan formation and is hence younger
than it.
Relations to the Lower Huronian sediments. — The Duluth and Iron
Range Railroad, south of Tower, especially in the vicinity of milepost 92,
gives very good sections through the Lower Huronian sediments and shows
them to be cut by dikes of granite. These dikes ^■ary in size on different
ex])osures, ranging from 1 incli up to 4 feet in width. Tliey can not be
THE LOWER HURONIAN. 357
connected in the field directly with the granite dikes which cut the gray-
Embarrass granite," lying a number of miles to the south, but their general
characters are so similar that they are presumed to be of the same ag-e as
these dikes, and to have been derived from the same source. Moreover,
farther east jjractically the same granite is found in dikes which are clearly
offshoots from the Giants Range granite, and those at the extreme western
side of the district are likewise supposed to be offshoots connected under-
groiind with the Giants Range granite mass, although at the surface they
are a good many miles distant from it. The metamorphosing effect of the
Giants Range granite on these sediments has been described under the
description of the sediments themselves (pp. 340-341).
As we go northeast in the district beyond White Iron Lake, especially
along the Kawishiwi River, we find a comparatively small area of sediments,
conglomerates, graywackes, and associated slates extending from near the
mouth of the Kawishiwi River on Farm Lake, sec. 34, T. 63 N., R. 11 W.,
eastward into sec. 29, T. 63 N., R 10 W. The area underlain by these
sediments has a north-south extent from the Kawishiwi River to Clear-
water Lake of about 1^ miles. These rocks are identical petrographically
with the rocks which have been classed with the Lower Hurouian sediments
and are presumed to be of the same age. They are cut through and
through by dikes of the Giants Range granite, and as a result of this
intimate intrusion they have been metamorphosed to their present condition
of mica- and hornblende-schists and gneisses.
Relations to ilie gahbro. — Within the limits of the map (PI. II) the
Giants Range granite and the great Keweenawan gabbro are in proximity
to each other only along the Kawishiwi River, from sec. 34, T. 63 N., R.
10 W., to sec. 19, T. 63 N., R. 9 W. Although the area lying within the map
in which the granite and gabbro lie close together is small, nevertheless the
relations between the two rocks are sufficiently clear. The gabbro lies
obliquely across the northeastern continuation of the Giants Range granite
and even overlaps the Archean greenstones and the Lower Hurouian
sediments, which lie north of the granite. The way in which these rocks are
interrupted in their eastern continuation indicates an eruptive relationship
of the gabbro to them. Moreover that this is the true relationship between
"The Mesabi iron-bearing district of Minnesota, by C. K. Leith: Mon. U. S. Geol. Survey Vol.
XLIII, 1903, p. 186. ,
358 THE VERMILION IRON-BEARING DISTRICT.
the g-abbro and the Giants Range granite is couchisively shown b}- the
jDi-esence of a dike of gabbro which occurs in the Giants Range granite upon
the portage at the falls of the Kawishiwi, in sec. 19, T. 63 N., R. 9 W. This
dike was not directly connected with the gabbro, but is macroscopically the
same and is only about a quarter of a mile away from the contact line
between the gabbro and granite massives. Attention may be called to one
further fact indicative of the intrusion of the granite by the gabbro, and
that is that the Giants Range granite near its contact with the Duluth
gabbro appears a little more basic than elsewhere, and approaches the
gabbro somewhat in general appearance. Thus the granite is found to
contain some augite. There seems in many places to be a transition
between these two rocks, although ordinarily the contact is sharp. The
transition rock is in a few places broken into round masses and these are
cemented together by a schistose chloritic material. One might conceive
of such a transition phase being equally likely to result from the intrusion
of the gi'anite into the gabbro. However, the above-mentioned dike seems
to clinch the relationship existing between these two rocks.
AGE.
From the preceding statement of facts concerning the relation of
the Giants Range granite to the adjacent rocks, we are enabled to draw
the conclusion that the granite was intruded after the Lower Huronian
sediments were deposited and before the intnision of the Duluth gabbro
of Keweenawan age.
FOLDING.
As has been already intimated, the granite shows very slight effects of
crushing or of the action of mountain-building forces, but that it has been
exposed to a certain amount of pressure is clearly shown. Small granite
dikes cut through the adjacent greenstone areas and lie parallel to the
schistosity of the greenstones. They were probably intruded subsequent
to the production of this schistosit}^, and hence followed along the planes of
easiest parting, which were the planes of fissilitj^ in the rock. Subsequent
to their intrusion these granite dikes have, however, been squeezed, for we
find that occasionally they have been broken into pieces and these broken
portions have been separated, and, in fact, in some cases even the fragments
of the dikes have been rounded until the}' have acquired a more or less
THE LOWER HURONIAN. 359
lenticular shape. These effects are probably due to minor folding, which
has taken place subsequent to the period when the schistosity was produced
and the granite intruded, this second period of folding ha^dng, indeed,
emphasized the schistosity due to the first period of folding and having
slightly affected the Giants Range granite.
METAMORPHIC ACTION OF THE GIANTS RANGE GRANITE.
The metamorphic effect of this granite on the greenstones being essen-
tially similar to that produced by the intrusion of the granite of Trout,
Burntside, and Basswood lakes will not here be discussed, the reader being
referred to tlie previous discussion for the details and results of the process.
As a result of the intrusion of the Giants Range granite, the Lower
Huronian sediments have also been extensively metamorphosed. A dis-
cussion of this metamorphism may be found under the discussion of these
sediments (p. 340).
INTERESTING LOCALITIES.
Belations to the Archean Ely greenstone. — The relations of the Giants
Range granite to the Ely greenstones are well shown in the SE. \ of sec.
24, T. 62 N., R. 13 W., and in the NW. \ of sec. 19, T. 62 N., R. 12 W.
Relations to Soudan formation. — Dikes of the Giants Range granite
have been observed cutting the Soudan formation at several places; for
instance, at 200 paces west of the southeast corner of sec. 7, T. 62 N., R.
12 W., the iron formation is cut by injections of granite which run parallel
to the bands of the iron formation. Dikes of the same granite cut the
adjacent Ely greenstone just north and west of this locality. At 1,050
paces north, 550 prxes west of the southeast corner of sec. 28, T. 62 N.,
R. 13 W., the iron formation is likewise cut by granite dikes. Similar
dikes cut thts iron formation in sees. 3 and 4, T. 62 N., R. 12 W. These
dikes can be well observed at the places where they occur on the bare hills
south of Ely. In this particular instance the dikes are only a very short
distance away from the edge of the main mass of Giants Range granite,
and that they are offshoots from it can hardly be doubted.
Metamorphism caused hy granite. — Reference has already been made to
the fact that the Giants Range granite has had an important metamorphos-
ing effect on the rocks which it has intruded. Its effect upon the Lower
Huronian slates is well shown in the area south of the Kawishiwi River, in
360 THE VERMILION lEON-BEARING DISTRICT.
sec. 31, T. 63 N., R. 10 W., and in sec. 36, T. 63 N., R. 11 W. In fact, at a
o-reat number of places in this area a traverse almost anywhere will show it.
The granite dikes occasionally occur near the river, but become more and
more numerous as one proceeds southward and approaches the contact of
the main mass of granite. For the most part the sedimentar}- character of
the rocks can not be readily recognized, as they have already been
metamorphosed into hornblende- and mica-schists. At one place on the
portage leading southeastward to Clearwater Lake, proof of the sedi-
mentary character of these rocks may be seen. They are well-banded
amphibole- and mica-schists, but a few bands having a distinctly conglom-
eratic nature were observed, although even these fine-grained conglomer-
ates are now schistose and carry a good deal of mica, e^^dently of
secondary origin. Farther south the granite dikes become more numerous
and the metamorphism more extreme, so that practically the banding and
the connection with the rocks showing ^^nquestioned clastic characters are
the onlv indications of the sedimentary nature of these rocks.
The intrusive relations which the Giants Range granite bears to the
Ely greenstone are shown at a great number of places in that portion of
the Vermilion district adjacent to the area underlain by the Giants Range
granite. It is hardly necessary to enumerate the places at which offshoots
from this granite penetrate the adjacent greenstone, as they may be found
at almost any place in the portion of the area outlined in which large
exposures exist. Numerous dikes of the Giants Range granite have been
found in nearly every place where its contact has been followed. Manj' of
them are indicated upon the accompanjdng map (PI. II), but it has been
found impossible to show all of those which have been found. Numbers of
them were seen in sees. 24, 27, and 28, T. 62 N., R. 13 W. Many others
occur in sees. 7, 8, 17, 18, and 19, T. 62 N., R. 12 W., and they are especially
easy to find on the bare hills southeast of Ely, in sees. 1,2, and 3, T. 62
N., R. 12 W. A number of such dikes cut the hills south of Pickerel Lake,
along the line between sec. 2.5, T. 63 N., R. 11 W., and sec. 30, T. 63 N.,
R. 10 W. The bold hills on the north shore of the Kawishiwi, in sees.
20, 28, and 29, show a number of these dikes penetrating the greenstones. ,
Others, in considerable number, may be found on the hills north of Stone
Lake, in sees. 16, 17, 20, and 21, T. 63 N., R. 10 W. A number of others
likewise occur in sees. 10, 14, and 15, T. 63 N., R. 10 W., and at a number
of places which it is not necessary to enumerate. Throughout this part of
THE LOWER HURONIAN. 361
the district the relation of the granite to the greenstone, which is shown not
only by the presence of these dikes but also by the effect of the granite on the
greenstone, may be seen, if strict attention is paid to the changes which take
place, as the granite is approached from the north. In every instance where
the exposures are sufficiently numerous, as, for example, in the vicinity of
Ely, it will be seen that the greenstone changes its character, becoming more
and more schistose, and finally passing into marked amphibole- and mica-
schists in close proximity to the granite.
SKOWBASTK GRAHSriTE.
DISTRIBUTION, EXPOSURES, AND TOPOGRAPHY.
The Snowbank granite has received its name from the fact that it is
typically developed around Snowbank Lake, which covers several sections
and parts of sections of T. 64 N., Rs. 8 and 9 W., and T. 63 N., R. 9 W. The
granite is confined exclusively to the immediate vicinity of the lake, being
best developed on the southern shores of the lake and on the islands in that
portion of the lake.
The exposures are very numerous and excellent. Although the granite
occupies the center of a structural anticline, it nevertheless does not empha-
size this structure by forming an area of topographically higher ground
than that of the surrounding country. The topogi'aphy is not of marked
character, having the usual rounded gentle contours so common in the
glaciated granite areas of this district.
PETROGRAPHIC CHARACTERS.
The granite occm-s in good exposures in the vicinity of the shores of
the lake, where its characters may be easily studied. It is predominantly
pink to red in color, although on fresh fracture it is a gray or flesh-colored
rock. Some facies, however, are much darker colored, as the result of a
higher content of the dark minerals than is contained in the normal granite.
The granite varies from the fine-grained to the coarse-grained form, the
medium-grained facies being most abundant. Porjihyritic facies of the
granite also occur, but are not very abundant. The porphyries are developed
as granite-porphyries and microgranite-porjihyries with feldspar, augite, and
quartz phenocrysts in a fine-grained groundmass. These porphyries, as
well as the very fine-grained granites, occur chiefly as offshoots from the
362 THE VERMILION IRON-BEARING DISTRICT.
main medium-gi-ained mass, and penetrate the sediments wliicli smTound
the granite massive.
Miueralog-ically the Snowbank granite varies from a normal mica- and
hornblende-granite to an augite-granite, and, by loss of quartz, to a syenite.
The hornblende-granites are invariably much darker than the mica-granites.
These last tend to reddish colors, while the hornblende-granites are usually
dark gray or red if the orthoclase is very prominent and considerably
weathered. As the green augite takes the place of the hornblende, these
hornblende-granites pass over into the augite-granite. The augite-granite
is a grayish, flesh-colored to red medium-grained granite, and does not
differ materially from the normal Snowbank mica- and hornblende-granite in
macroscopic appearance. The red augite-granite has been observed to cut
the normal Snowbank granite, as for instance on the point projecting north-
eastward from the mainland and forming the NW. ^ of the SW. ^ of sec. 36,
T. 64 N., R. 9 W. Here the medium-gi-ained red augite-granite cuts a
hornblende-syenite phase of the Snowbank granite, and is in its turn cut by
a basalt dike. This red augite-granite is also reported to be cut by the
hornblende-granite " Both observations are correct, the ex^ilanation, as it
appears to me, being that they are of essentially the same age and are
differentiation products of the same magma. For this reason they are
included here together as constituting the Snowbank granite complex.
No attempt has been made to discriminate on the map between the
normal mica-granite, the hornblende-granite, and the augite-granite. They
show nothing of peculiar interest under the microscope. The normal
combination is mica, quartz, and feldspar, both orthoclase and plagioclase,
with some iron oxide in very small quantities. Hornblende is commonly
found with the mica, and as it increases in quantity the rock passes to a
hornblende-granite. Usually a great deal of sphene is present in the
hornblende- granite, and it is more prominent in the hornblende-syenites,
which are connected with the hornblende-granites and diifer from them
only in containing very much less or practically no free quartz. Augite
accompanies the hornblende in some of the hornblende-granites, and as it
increases in quantity and the hornblende diminishes there is the gxadation
to the augite-granite.
"Geol. and Nat. Hist. Survey of Minnesota, Final Rept., Vol. IV, 1899, pp. 427-428.
THE LOWER HURONIAN. 363
RELATIONS TO ADJACENT FORMATIONS.
Relations to the Loiver Hiironian. — The Snowbank granite is found in
contact with the adjacent Lower Huronian sediments, both the Ogishke con-
glomerates and the Knife Lake slates. In a number of places dikes, offshoots
from this mass, occur in the adjacent sediments. Moreover, the sediments
near the granite have been very much changed. They are full of mica in
relatively large crystals, and in general the rocks have been recrystallized
until they are now in places mica-schists. The crystalline character of
these rocks is most noticeable near their contact with the main granite mass
and at places where they are cut through by numerous dikes of the
granite and where the fragments of the sediments are inclosed in the dike
rocks. The farther away from this contact we go the less numierous the
dikes become and the less pronounced are the indications of metamorphism
until, at a distance varying in places from half a mile to a mile, the
sediments seem to show their normal character. The presence of the dikes
in the sediments and the contact effect of the granite on the sediments
clearly show the intrusive character of the granite. The facts referred to
briefly above were observed and recorded in their notebooks by the
members of the Minnesota survey, but the interpretation g-iven to these
facts by the State geologist" is very different from that given above.
According to him the Snowbank granite is a product of the metamorjjhism
of acid sediments, graywackes, and conglomerates, and the granite and
granite-porphyries are connecting links showing transitions to the sedi-
ments. The complete fusion of these graywackes produced the granite.
Incomplete fusion accounts for the metamorphosed sediments surrounding
the granite massive. The dikes in the sediments at some distance from the
border of the granite are portions of the molten sediments which penetrated
the unfused ones.
Relations to the Keweenawan gabhro. — The granite has not been found
in actual contact with the large mass of Duluth gabbro lying south of
and next to it. Along the contact there is a slight topographic break,
occupied by low ground, in which exposures are wanting. The granite
is of Lower Huronian age, and there can be no doubt that the Duluth
gabbro is younger than it is. However, if the gabbro exercised any
"Geol. and Nat. Hist. Survey of Minnesota, Final Rept., Vol. IV, 1899, pp. 287-294.
364 THE VERMILION IRON-KEARING DISTRICT.
metamorphic effects upon the granite, they have not been observed, nor
have any dikes that could be traced to the gabbro been found cutting
through it.
INTERESTING LOCALITIES.
The best portion of the area in the vicinity of Snowbank Lake in which
to study the relations of this granite to the adjacent sediments is east of the
lake, in general between its outlet and Disappointment Lake. In this area
the hills are bare, and good exposures are numerous. A number of good
exposures can be found on the shores of Snowbank Lake north and south
of its outlet, and here the Snowbank granite is found cutting the adjacent
conglomerates in numerous dikes. These same relationships ma}" be found
still farther inland, on the hills in the area to which reference has already
been made. Other exposures showing the same relations may be found at
almost any place along the north and northwest shores of the lakes, and
a traverse inland from these shores will almost invariably result in the
finding of dikes cutting the adjacent schist. There is nothing especially
peculiar in the relations of these dikes to the rocks which they cut or in
the occurrence and appearance of these dikes, consequently no detailed
enumeration of them will be given.
CACAQtTABIC GRAKITE.
This granite occurs typically on the islands in and the shores of
Cacaquabic Lake, from which it was named by the Minnesota survey. It
has been more or less extensively studied by the Minnesota State geologist
and his assistants, and mention and description of it occur in a number of
the State reports."
U. S. Grant has made an especially detailed study of this granite,'' and
as a result of his careful mineralogic and chemical analyses has deterinined
it as one of the comparatively rare augite-soda granites. Studies of the
Cacaquabic granite corroborate in the main the statements of Grant, l)ut
there has been no opportunity to make detailed mineralogic studies or
chemical analyses of the rock, and data resulting from these have been
obtained from Grant's articles, to which reference has been made.
«Geol. and Nat. Hist. Survey of Minnesota, Fifteenth Ann. Kept., 1886, pp. 361-369; Sixteenth
Ann. Kept., 1887, pp. 149-1.56; Twentieth Ann. Kept., 1891, pp. 70-79; Twenty-first Ann. Kept., 189L',
pp. 5-59, 2 plates. Grant, U. S., Am. Geologist, Vol. XI, 1893, i)p. 383-388; Geol. and Nat. Hist.
Survey of Minnesota, Final Kept., Vol. IV, 1899, pp. 294, 442, and 450; Final Kept., Vol. V, 1900,
descriptions of sections in various places.
''Loc. cit.
THE LOWER HURONIAN. 365
DISTRIBUTION, EXPOSURES, AND TOPOGRAPHY.
The granite is confined in its distribiition to the immediate vicinity of
Cacaqiiabic Lake, and is especially well developed on the southern side.
It extends back from the lake for a maximum distance of aboxit a mile,
reaching down into the SE. ^ of sees. 1 and 2, T. 64 N., R. 7 W. The
granite occupies on the whole higher ground than is occupied by the
adjacent rocks, forming, especially in sees. 1 and 2, fairlj- high hills. There
are excellent exposures on a number of islands in the lake and also on the
mainland near the shore.
PETROGRAPHIC CHARACTERS.
The Cacaquabic granite, like the other granite complexes of the Ver-
milion district, shows considerable variation both in texture and mineralogic
composition. The main mass of the granite is a medium-grained gray or
pink to red rock, whereas on the periphery of the gi'anite area the finer-
gi'ained and granite-porphyry facies are developed.
The granite-porphyry facies contains large phenocrysts of plagioclase
feldspar lying- in a fine-grained gray to red groundmass. The phenocrj'sts
themselves range from gray to red in color, depending on the degree and
the character of the alteration. When red in part or as a whole, the
phenocrysts give the gTanite-porphyry a very striking appearance. They
then stand out prominently from the lig'hter-colored groundmass. In general
there seemed to be no an-angement of the phenocrysts, but in one case the
phenocrysts of the granite-porphyry did show a distinct parallelism of their
major dimensions. There was thus produced a more or less perfect
macroscopic flowage structure.
The granite and granite-porphyry are massive, although in places
much jointed and separated into small blocks by the joint planes. In
places" the fractures in the granite are filled by veins of infiltrated quartz.
Mineralogically the granite varies considerably. While the main mass
is an augite-soda granite,'' an examination of the specimens collected shows
variations to a hornblende-gTanite and hornblende-mica-gTanite. Grant
reports an aug-ite-biotite-syenite facies." The minerals constituting the
«The Geology of Kekekabic Lake, by U. S. Grant: Geol. and Ivat. Hist. Survey of Minnesota,
Twenty-first Ann. Eept., 1892, pp. 34-36.
''Loc. cit., p. 33.
<^Loc. cit., p. 50.
3(36 THE VERxMlLlON IRON-JJEARING DISTRICT.
granite are quartz, feldspar (both orthoclase and plagioclase), augite, horn-
blende, mica, sphene, apatite, magnetite, and hematite. The phenocrysts are
of plagioclase. Most of the minerals have their normal characters and will
not be described. The feldspar and angite are somewhat exceptional, and
have been described in detail by Mr. Grant," from whose description the
following details concerning these are taken.
Gi-ant's study of the feldspar shows that the plagioclase has a specific
gravit}^ ranging from 2.58 to 2.62 and the chemical composition shown
below.''
Analysis of feldspar of Cacaquahic granite.
Per cent.
SiOj 67.99
AlA 19.27
FeA 82
CaO 75
MgO 02
K2O : 3.05
Na^O 6.23
H,0 90
Total 99.03
From this he concludes that the feldspar is an anorthoclase composed
of orthoclase, albite, and anorthosite molecules in the following propor-
tions: Org, Abi4, Aui.
The augite varies from a colorless one to a variety having a bottle-
green color. The colorless kind is not uncommonly intergrown with the
green variety. Sometimes this is in zonal intergrowth. The individuals
possess a very good crystallographic development. The specific gravity of
the augite as a whole is somewhat higher than 3. An analysis of the
augite given by Grant is here quoted:
Analysis of augite of Cacaqimbic granite.
Per cent.
SiO, 53.19
Al^O, 2. 38
FeA 9-5
FeO 315
CaO : 17.81
MgO 9- -13
K,0 38
Na,0 2.(«
H,0 01
Total 100.23
"Geol. and Nat. Hist. Survey of Minnesota, Twenty-tirst Ann. Kept., 1892, pp. 47-48. i-Ibid., p. 44.
THE LOWER HURONIAN.
367
Assuming- that this represents an isomorphous mixture of the diopside, hedden-
bergite, acmite, and fassaite molecules, and calculating their relative proportions,
we get approximateh^ the following result :
Per cent.
Diopside, Mg Ca SijOj 47
Heddenbergite, Ca Fe SijO^ 27
Acmite, Jsa Fe SijOg 21
Fassaite, Mg AUSiOs 5
In the considerable percentage of the acmite molecule this augite approaches in
composition the pyroxene of the more alkaline rocks, the eleolite syenites. This
analysis very probabh^ represents quite well the usual composition of the green
augite, as the proportion of zonal crystals, with colorless centers, and entire colorless
crystals, is small. The colorless augite is very similar to that of the well-known
augite granite from Laveline in the Vosges.
The analyses of the auorthoclase feldspar and of the augite which are
quoted above show such a proportion of sodium oxide that one would
expect the granite itself to exhibit a very high ratio of that substance. This
proportion is shown in the following analyses," I being that of the normal
Cacaquabic granite facies as given by Grant, and II being that of the
granite-porphyry facies.
Analyses of Cacaquabic granite and granite-porphyry.
SiO,..
TiO^ .
P2O5..
Al.Oa-
Fe^Os.
FeO..
MnO.
CaO..
MgO.
k;o..
Na^O.
H,0..
Per cenl.
66.84
II.
Per cent.
67.42
Tr.
18.22
2.27
0.20
3.31
0.81
2.80
5.14
0.46
Total.
100. 05
0.07
15.88
1.37
1.14
Tr.
3.49
1.43
2.65
6.42
0.05
99.92
«Loc cit., p. 41.
368 THE VEKMILION IRON-BEARING DISTRICT.
Secondary minerals, such as sericite, chloiite, and hornblende occui-
abundantly in some of the rocks. The rocks have locally been crushed,
but the crushing- has in no case reached such a degree as to produce schis-
tose rocks, although the microscope shows crushing effects reaching even to
the granulation of the quartz. The microscopic fractures are healed by
quartz and feldspar and also by hornblende when the fractures cross a horn-
blende individual. In such a case one can readily distinguish the second-
ary needles of nearly colorless hornblende which traverse the fractures and
unite the pieces of the old massive green hornblende individual.
RELATIONS TO ADJACENT FORMATIONS.
Relations to the Lower Huronian. — The Cacaquabic gi-auite lies adjacent
to the Lower Huronian sediments, which very nearly surround it. In the
area in which the sediments are wanting the gabbro Hes next to the granite.
The relations of the gi-anite to the adjacent formations are not nearly so
clear here as were the relations described for the Snowbank granite. The
granite contains dark-colored chloritic basic inclusions which may have
been brought up from the Archean greenstone that underlies the sediments,
an anticline of it lying a short distance southeast of the granite, and another
north of it. The Cacaquabic granite grows finer grained as the mantle of
sediments is approached, and it has been found, moreover, cutting the sedi-
ments in dikes which show clearly that it is intrusive in them and of
younger age. If it has metamorphosed them, as is probably the case, its
metamorphism is concealed by the later metamorphism caused by the gabbro.
Belations to the gabbro. — The statement that the gabbro is later than the
gi-anite is not warranted by any actual contacts that have been found
between them, or by the occurrence of any inclusions of the granite in the
gabbro, but is based chiefly on their field relations, shown on the nccom-
panying map, and on the comparatively youthful age of the gabbro. Thus
it will be seen that the granite everywhere, except on the southeastern
edge, is suiTOunded by the sediments. Presumably it was originally
completely surrounded by them: but for this narrow area, however, the
gabbro has cut out the sediments and overlaps on tlie area underlain by
granite.
THE LOWER HURONIAN. 369
AGE.
It will thus be seen that the Cacaquabic granite massive is an intrusive
of youup'er ao-e than the Knife Lake slates, but older than the Keweenawan
gabbro.
INTERESTING LOCALITIES.
At only two places has the granite been found showing its relations to
the adjacent sediments. One of these places is 700 paces north, 650 paces
west of the southeast corner of sec. 1, T. 64 N., R. 7 W., south of Caca-
quabic Lake, where the sediments are cut by dikes of the granite. ■ The
other point is just back of the southwest shore of Cacaquabic Lake south
of a large granite island. Here the porph3'ritic form of the granite cuts
the adjacent conglomeratic sediments.
Still other dikes occur in the southeast corner of sec. 1, T. 64 K.,
R. 7 W., on the hills north of the small lake in which the four sections
which meet at this place have their corner.
VARIOUS ACID DIKES.
Certain acid dikes, to which reference has already been made, are
found cutting through the various formations of the district, but can not
be directly connected with any of the large eruptive masses. They are
presumed to be of Lower Huronian age, and the evidence for this will
be given on another page, yet it is possible that some of them may
be of Keweenawan age, although if there are any Keweenawan dikes
included among them, they can not be recognized as such on account of
lack of evidence. They are not of sufficient interest or importance, how-
ever, to warrant a description of the individual dikes; consequently an
attempt will be made to give merely a general idea of their characters.
DISTRIBUTION.
The distribution of these dikes will be seen on the maps in the accom-
panying atlas, on which they are marked with the same symbol and the
same color as that used for the Griants Range, Snowbank, and Cacaquabic
granites. In most cases the exposures of these dikes are small, so that in
many instances it is practically impossible to state absolutely that they are
younger than the rocks near them. This presumption, however, is
MON XLV — 03 34
370 THE VERMILION IRON-BEARING DISTRICT.
AYarranted by tlie occuiTence of tiiese other rocks which suiTound them in
numerous exjDOSures. They are of such small area! distribution that they
do not materially influence the topography in general, although, since they
are generally harder than the rocks through which they cut, they do
exert a local influence and are found forming knolls or ridges.
PETROGRAPHIC CHARACTERS
Macroscopic. — These dikes vary in grain from very fine-gi'ained, almost
cryptocrystalline rocks, to those which may be classed as coarse-grained
ones. Moreover, a porphyritic structure is of very common occuiTence, the
quartz being sometimes the sole phenocryst though at other times both
quartz and feldspar occur as phenocrysts. In color there is likewise con-
siderable variation. For the most part the rocks are gray to pink on fresh
fracture. Some of the finer-grained ones, however, are dark purplish. The
weathered surfaces are nearly always pink to reddish. The dikes vary
greatly in width. They are usually narrow, but some dikes as much as 10
feet wide have been observed, and the grain usually varies with the width,
the finer-grained rocks occurring in the narrower dikes and as the selvage
of the wider ones.
3Iicroscopic. — Under the microscope one can recognize phenocrysts of
green hornblende in association with phenocrysts of feldspar and quartz-
The quartz is relatively scarce. These phenocr3^sts lie in an exceedingly
fine-grained groundmass with, in some cases, grains too small to permit
their characters to be recognized. Generally it can be seen that the ground-
mass is made up of quartz and feldspar, flakes of chlorite and sericite, and
some iron ore. Occasionally the grains and flakes are arranged in parallel
lines which so bend around the phenocrysts as to bring out a very well-
marked flowage texture. All of the minerals have their usual characters,
and hence no description will be given of them.
RELATIONS TO ADJACENT FORMATIONS.
It is believed that these acid dikes are offshoots from tlie various granites
described in this chapter. They show a general petrographic similarity to
them. Still they are not so much like them as to warrant one in making a
positive statement that they are the same. Moreover, they have not been
connected in the field, nor have any chemical analvses been made which
THE LOWER HURONIAN. 371
would enable one to determine their similarity in chemical composition to
the adjacent granites. The differences between them, which are textural,
can be explained by the fact that these offshoots occur in so much smaller
quantity that naturally they would not acquire the same textures as the
coarser-grained granites and granite-^oorphyries of the large massives.
It has already been stated that these acid rocks occur as dikes in the
adjacent formations. In some instances their dike character is not clearly
shown, actual contacts between them and the rocks occurring nearest to
them, and tlu'ough which they cut, being wanting. From the fact that
they are igneous rocks and more or less completely surrounded by other
eruptives or by sedimentaries, they are supposed to be intrusive in these.
They have been found cutting all of the rocks thus far described from the
Vermilion district, those of eruptive as well as those of sedimentary orig'in,
with the exception of the various Lower Huronian granites. They are con-
sequently known to be younger than all of the rocks which they cut. Their
relations to the Upper Huronian sediments and to the great Duluth gabbro
of Keweenawan age have not been determined conclusively. However,
as the result of a reconnaissance in the Keweenawan of the Lake Superior
region, it has been found that the Keweenawan is cut by acid rocks.
While these have not been connected petrographically with the dikes in the
Vermilion district, nevertheless it may be well to suggest the possibility
that at least some of the acid dikes in the Vermilion correspond to those
which cut through the Keweenawan rocks.
BASIC AND IlSrTEEMEDIATE IjVTRUSIVJES OF LOWER HURONIAN AGE.
At various places in the Vermilion district basic and intermediate
dikes have been observed cutting the country rock. These can be easily
divided into dikes of apparently different age by using as a criterion for
this the difference in alteration. This macroscopically determined difference
is substantiated by the condition of the rocks as shown by microscopic
examination. On the one hand there are certain dikes which are composed
of very fresh dolerite and basalt and which show distinct selvages. These
cut through all the other rocks of the Vermilion district, including the
gabbro. Just outside of the district are dikes identical in character with
these and cutting even the acid red rock of the Keweenawan, which itself is
known to cut the gabbro. These fresh dikes are clearly of Keweenawan
372 THE VERMILION IRON-BEARING DISTRICT.
or post-Keweenawan age, and will be described under the beading
"Keweenawan" in Chapter VI. On the other hand there are dike rocks
which cut all the rocks of Archean and Lower Hurouian age, but no
definite proof has been obtained that they i7itrude any of the rocks younger
than these. The much greater age of these dikes is shown in their more
extensive alteration, indicated macroscopically by their green color, and by
the occasional presence of an imperfectly developed schistosity. The orig-
inal characters of these dike rocks are rarely well enough preserved to enable
one to determine positively just the kind of rocks they are. It can be said
in general that they are of basic and intermediate character. Some have
unquestionably been derived from dolerites and basalts. Others, it is clear,
belong to the lamprophyric rocks.
The dolerites and basalts are invariably very much altered. Occasion-
ally a fairly well-preserved ophitic texture may be observed. Usually,
however, all textures have been destroyed as the result of orogenic
movements, and the rocks have become fairly schistose. They were
evidently intruded, however, in the Archean rocks after the latter had been
subjected to pressure, as they are found in some cases to have been injected
parallel to the schistosity of these rocks. The usual constituents are such
as are commonly found in these altered basalts: actinolite, chlorite, apatite,
calcite, muscovite, feldspar, a little quartz, sphene, and iron oxides.
The lamprophyric dikes above referred to occur usually in very narrow
dikes and while ordinarily extremely altered, nevertheless are generally not
so much altered as are the dolerites. These rocks consist of various
combinations of plagioclase and orthoclase feldspar, with biotite, hornblende,
augite, and iron oxide. The hornblende and augite are the predominant
dark silicates. A few serpentinous areas indicate the former presence of
olivine. The minerals are so much altered that a trustworthy separation
of these rocks into the various divisions of the lamprophyres to which they
belong could not be made. There seem to be represented among them,
however, chiefly biotite-kersantites, augite-kersantites, and the hornblende-
and augite- vogesites. With these there also seem to be some camptonites.
Certain other dikes in the district which were observed were so
extremely altered that one could only see that the original rock carried
hornblende, but the petrographic position of these dikes could not be
determined.
THE LOWER HURONIAN. 373
INTERESTING LOCALITIES.
No attempt will be made here to enumerate all the places where these
dikes occur. A number of them have been shown in exaggerated size on
the accompanying maps. On the west end of Stuntz Island there is a
plexus of these basic dikes cutting the Ogishke conglomerate. These dikes
run parallel to one another, diverge from one another in places, and at other
points cut one another. At one place on the top of the conglomerate ridge
just south of the small northwestward-pointing bay, nine dikes essentially
parallel and varying in width from IJ inches ujj to 6 feet were counted
within a distance of 60 feet. These dikes cut right across the fragments in
the conglomerate, giving sharp contacts. The jasper fragments in the con-
glomerate which have been cut by the dikes seem to have been apparently
unaltered by them. The dikes themselves are moderately schistose, espe-
cially on the edges. The rock of these dikes is extremely altered and
appears to have been a dolerite.
Similar dikes can be seen cutting across the granite island just east of
Stuntz Island, and still others were seen on the high hill along the shore
near the northeast corner of sec. 22, T. 62 N., R. 15 W.
A number of lamprophyric rocks were observed cutting the Ely green-
stone, the Ogishke conglomerate, and Knife Lake slates near the center of
sec. 17, T. 63 N., R. 9 W. Similar dikes occur on the hill on which Ely
is built, northeast of the Methodist church, and also on tlie greenstone
hills on the north side of Long Lake.
CHAPTER V.
THE UPPER HURONIAK (ANIMIKIE).
in the eastern portion of the Vermilion district there has lieen fonnd
overlying the Knife Lake slates and the Ogishke conglomerate where these
are present and, where they are wanting, lying immediately upon the Ely
greenstone or the granite of Saganaga Lake, a series of sedimeutarj- rocks,
of which a considerable thickness is exposed. These rocks belong to the
great sedimentary series which is best developed in a very wide area lying
along the northwest and north shores of Lake Superior and extending well
up into Canada, but which is also well developed in the Mesabi district on
the southern slope of the Giants range in .Minnesota. To this series the
name Auimikie, the Ojibway word for thunder, has been given by Hunt"
from the fact that these rocks are typically developed in the vicinity of the
two well-known topographic features of the north shore of Lake Superior,
namely Thunder Bay and Thunder Cape.
The Upper Huronian series of the Vermilion district may be readily
divided into two facies of rocks which are quite different petrographically.
At the bottom of the series occurs an iron-bearing formation, known
as the Gruuflint formation, consisting of bands of ferruginous carbonates,
quartz, magnetitic quartz, magnetitic ore, and augite, hypersthene, horn-
blende, olivine, griinerite, and magnetite rocks. All of these apparently
represent altered forms of some original ferruginous rocks. Above these
iron-bearing rocks there occurs a great slate-graywacke formation, known
as the Rove slate.
SECTION I.— GtTNFLiINT F0R»IAT10N.
The rocks of this formation are well developed on the north shore of
Gunflint Lake, from which their name has been derived. They extend in
a belt, shown on the maps in the accompanying atlas, for a number of
"The geognostical history of the metals, by T. Sterry Hunt: Trans. Am. Inst. Min. Eng., Vol. I,
pp. 3.31-395; Vol. II, pp. 58-59.
374
UPPER HURONIAN. 375
miles to the west of the lake, which is a well-known feature of the inter-
national boundary route. Rocks with which these are correlated occur in
the Mesabi district, and are there known as the Biwabik formation.
DISTRIBUTION, EXPOSURES, AND TOPOGRAPHY.
Distribution. — The iron formation has a wide distribution in the Mesabi
district, extending through it from end to end. The Gunflint formation
can be looked upon as the eastern continuation of the iron-bearing Biwabik
formation of the Mesabi district; In the Vermilion district this iron forma-
tion has a restricted distribution. The area in wliicli the rocks occur is in
places not more than about 200 paces wide, from this spreading out to a
width of nearly half a mile. Northeast of Paulson's mine, in sees. 21 and 22,
T. 66 N., R. 4 W., there is an east-west tongue of the Gunflint rocks project-
ing westward into Ely greenstone. About three-foui-ths of a mile east of
Paulson's mine, in sec. 27, T. 65 N., R. 4 W., where a great north-south valley
cuts directly across the Gunflint formation, the narrow belt of iron formation
joins a wider area of the same rocks, which extends over the greater portion
of sees. 23 and 26, T. 65 N., R. 4 W. The Gunflint formation is widest in
these sections, its great width being due chiefly to the fact that a fairly wide
synclinal fold has here been stripped, leaving exposed an unusuall}- large
area of the iron formation. East of these sections, toward the international
boundary, the formation thins down rapidly. The northern boundary of
the Gunflint formation in this area is marked by the Knife Lake slates and
Ogishke conglomerate of the Lower Huronian series, and the Ely green-
stone and granite of Saganaga Lake of the Archean. This is the order in
which the Gunflint formation comes in contact with these rocks from west
to east. This is also the stratigraphic order, passing from the youngest on
the west to the oldest on the east, with the single exception of the granite
of Saganaga Lake, which intrudes the greenstone.
The southern boundary of the iron-bearing formation over the greater
portion of the area in which it occurs is the northern edge of the Duluth
gabbro. From the SW. J of sec. 26, T. 65 N., R. 4 W., however, the direc-
tion of the southern boundary changes. From here it swings northeastward
and the Rove slates, which overlie it, begin to appear with a narrow edge
of a wedge widening to the east and separating the Gunflint formation from
the gabbro.
376 THE VERMILION IRON-BEARING DISTRICT.
Exposures. — The exposures in the areas outlined are very good. In
all instances they are sufficient to enable one to study the rocks in consid-
erable detail and ti-ace out their continuation without greater difficulty
than is offered by the very rough and thickly timbered character of the
country.
Topography. — Where the Gunflint formation occurs in sufficient quantity
to aifect the topography to a noticeable extent, the forms produced in it are
fairly characteristic. As the result of the monoclinal southward di]) and
the differential erosion of the harder and softer layers, a series of ridges are
produced which trend about east-west and have very steep northward-facing
escarpments with gentle southerly slopes corresponding approximately to
the dip of the beds. It may be well to mention here that sills of dolerite
lying approximately parallel to the bedding frequently form the top of the
larger ridges. This same kind of topography, but in a more marked form,
is also developed in the area underlain by the Rove slates, which is adjacent
to that in which the Gunflint formation occurs, and will be found described
in greater detail elsewhere (pp. 391-392).
About a mile east of Paulson's mine there is one very noticeable topo-
graphic feature which is not in agreement with the general topography —
a large cross valley, running about north and south, which appears to rep-
resent an old pre-Glacial valley formerly occupied, perhaps, by a fore-
ranner of the present Cross River, which flowed through it on its way north
into Boundary River and Saganaga Lake.
STRUCTURE.
The structure of the Gunflint formation in that ^^ortion which is
exposed in the Vermilion district is not very complicated. There is a
small northeast-southwest trending area of Gunflint formation rocks exposed
on the southeast shore of Disappointment Lake. Here the sediments have
a strike corresponding very closely to the trend of the area itself — that
is, northeast-southwest — and they dip to the south. In rocks of simihir
age on Gobbemichigamma Lake the structure is a little bit more compli-
cated. In this case the sediments have been folded, and as a result we
now find them forming in the main a syncline plunging toward the
northwest, but with a subordinate anticline near the center wliieli has
an axis plunging to the southeast. In the narrow belt extending from
UPPER HUKONIAN. 877
sec. 34, T. 65 N., R. 5 W., eastward to the gi-eat cross valley in sec. 27,
T. 65 N., R 4 W., the members of the series rest upon the older rocks
and unifoimly dip to the south. The regularity of this dip is, however,
interrupted by a number of minor flexures whose axes plunge south-
southeast. As a result, the amount of the dip varies considerably, ranging
from about 10° to 65° to the south, the higher dips occurring invariably
at the western end of the belt, the dips becoming flatter as the belt is
followed to the east. Moreover, these dips vary rapidly within short dis-
tances. Likewise the strike, although in general following the trend of
the belt, is found to vary gradually within short distances. The uniform
dip to the south shows the very simple structure which prevails in this belt,
but the changes in angle of dip and in strike clearly indicate the presence
of a number of subordinate rolls in these monoclinal southerly dipping
series of sediments. The gradual diminution in the angle of dip as the
sediments are followed to the east corresponds to the less folded condition
of these sediments in this part of the area. Attention has already been
called to the areal distribution of the sediments and the westward-trending
tongue of sediments occurring in sees. 21 and 22, T. 65 N., R. 4 W., which
is good evidence of an infolded syncline of these sediments at this place.
The dip of the sediments as observed upon the outcrops in this area gives
further proof of the occurrence of this syncline.
In general, then, the sediments have a uniform dip to the south, with
minor irregulaiities, these irregularities being most marked in the western
part of the area and in general wherever the sediments lie against the
older rocks. Some very considerable irregularities have been noted in a
few cases along the margins of certain enormous masses of dolerite which
occur in the midst of the sedimentary area. These dolerites, it may be
stated here, are intrusive in the sediments, and this fact sufficiently explains
the contorted character of the sediments immediately adjacent to them, for
this contorted chai'acter is confined only to their immediate vicinity, the
uniform low southerly dip appearing by the time one has gone some
distance from such contact lines.
PETROGRAPHIC CHARACTEPS.
The Gunflint iron-bearing rocks at the east end of the Vermilion district
correspond stratigraphically to, and are indeed the eastern continuation of,
the iron-bearing rocks of the Biwabik formation, which are so well developed
378 THE VERMILION IRON-BEARING DISTRICT.
and of sucli enormous economic value in the Mesabi range. Although
stratigraphically the same as the Biwabik, the rocks at the eastern end of
the Vermihon district, constituting the Gunflint formation, have been in
o-eneral much more metamorphosed than the Biwabik, and while showing
their derivation from rocks similar to those constituting the Biwabik, they
are now petrographically very different from them.
The rocks of the Biwabik formation have been described by the Min-
nesota Geological Survey, especially by N. H. and H. Y. Winchell " and
J. E. Spurr,'' and a later and more accurate description has been given by
Leith." To these articles the reader is referred for details.
. The following brief summary made by Leith, while preparing the
report on the Mesabi district, describes the petrographic character of the
rocks typically developed in that district and may aid in interpreting
the petrography of the Gunflint iron-bearing rocks:
The Mesabi iron formation rocks are mainly ferruginous chert, but contain
also iron ore, small quantities of iron and calcium carbonates, thin seam.s of slate and
paint rock, and, finall}-, certain peculiar green rocks containing minute dark-green
o-ranules resembling an indurated greensand. The ferruginous chert and the iron
ores have been shown to develop mainl}^ from the alteration of the last-named rock.
The original green granules under the microscope are seen to lie in a matrix of chert
with a variety of textures. They have round, oval, crescent shaped, gourd shaped,
or more irregular forms; their color varies from a bright green through a shade of
yellowish green to dark brown; under crossed nicols a tine aggregate polarization
appears, so fine that the substance appears practically isotropic. The green granules
were supposed by Spurr to be true glauconite, but later work by the United States
Geological Survey** shows the substance to be essentially a hydrated ferrous silicate
lacking potash, and quite different in composition from glauconite. Moreover, instead
of being entirely organic, as supposed by Spurr, the substance of the green granules
is supposed to be the result of chemical developement in a manner analogous to the
development of the iron carbonates described by Van Hise for the other districts of
the Lake Superior region.'' The shapes, however, may be due to filling, replacement,
or accretion about minute organic bodies, which are probably commensui'ate in
variety both with those depositing glauconite and with those giving the granule
aGeol. and Nat. Hist. Survey of Minnesota, Bull. No. 6, 1891, pp. 113-146.
i> Geol. and Nat. Hist. Survey of Minnesota, Bull. No. 10, 1894, p. 259.
t-The Mesabi iron-bearing district of Minnesota, by C. K. Leith: Mon. U. S. Geol. Survey
Vol. XLIII, 1903, pp. 101-159.
''Ibid., p. 108.
« Twenty-first Ann. Kept. U. S. Geol. Survey, Pt. Ill, 1901, jip. 32&-328.
UPPER HURONIAN. 379
shapes to much of tho CUnton ore. All stages of the alteration of this green ferrous
silicate rock to the ferruginous cherts and iron ores are to be observed. Scarcely a
slide of the cherts does not show some traces of the granules. The alterations have
been for the most part characteristic of surface conditions and have consisted in the
decomposition of the ferrous silicate, the oxidation of the ferrous iron to the hydrated
hematite form, and its segregation from the silica. Where metamorphosed by the
Keweenawan gabbro the alteration of the granules has consisted in the development
of a variety of amphiboles, including actinolite, grunerite, cummingtonite, and
perhaps others, of which grunerite is the most abundant, and the partial oxidation of
the feri'oiis iron to the magnetite form.
In the Grunflint formation the rocks very commonly have structural
characters indicating their development from ferrous silicate granules in
the manner characteristic of the metamorphism by the gabbro — that is,
traces of the granular structures still remain; but the characteristic min-
erals are magnetite and the amphiboles resting in a chert matrix. In
addition to the rocks that give a good indication of the kind of rock
from which they were produced, there are others that give no such clue.
They are without characteristic structural features. We know that fer-
ruginous carbonates form a part of the iron-bearing formation, and it is
presumed that some of these metamorphosed products have been derived
from such carbonates. It is impossible, however, to give any quantitative
estimate of the relative abundance of the ferruginous carbonate and ferrous
silicate rocks; so that we can not say which of these has been most impor-
tant in furnishing material for the rocks of the Grunflint formation.
In general, the least metamorphosed of the Gunflint rocks are thin
bedded and consist of bands of nearly pure chert alternating with cherty
and granular quartzose bands containing varying percentages of iron car-
bonate, bands of jasper and magnetitic chert, and others consisting of quartz
as a basis with actinolite and griinerite crystals, with which minerals are
always associated more or less ferruginous carbonate, magnetite, hematite,
and limouite. A description of the least altered Gunflint beds lias been
given by Irving and Van Hise in their monograph on the Penokee iron-
bearing series." The cherty ferruginous carbonates occur in better develop-
ment just outside of the Vermilion district in Canadian territory, on the
north shore of Gunflint Lake, than in the Vermilion district proper. The
«Moii. U. S. Geol. Survey Vol. XIX, 1892, pp. 260-268.
380
THE VERMILION IRON-BEARING DISTRICT.
following analyses, made by Mr. Thomas M. Chatard, of the United States
Geological Survey," give the composition of some of these carbonates.
Analyses of iron-hearing carbonates.
VIII.
IX.
!~ilK'a
Titanic oxide
Alumina
Iron sesquioxide . .
Iron protoxide
Manganese oxide. .
Calcium oxide
Magnesium oxide .
Carbon dioxide . . .
Phosphoric acid . . .
Iron sulphide
Water at 110°
Water at red heat .
Total
58.23
Trace.
.06
5.01
18.41
.25
.38
9.59
5.22
.03
.14
.07
2.01
46.46
Trace.
.24
.64
26.28
.21
1.87
3.10
19.96
.13
.11
.07
1.15
23.90
None.
.07
.44
10.65
.28
22.25
8.52
32.42
Trace.
.13
.99
99.40
100. 2:
99. 65
VII (specimen 10575) , iron carbonate from Gunflint beds on eastern side of outlet of Gunflint Lake
on international boundary; VIII (specimen 10598), from same beds, but from northern side of Gun-
flint Lake; IX (specimen 10588), ferriferous carbonate from another part of north side of Gunflint Lake.
Under the microscope most of the above-mentioned rocks show nothing
of especial interest. With these one finds chertv ferruginous rocks which,
when examined under the microscope, are of interest, since thev show the
relationship of these rocks to the less altered normal rocks of the iron for-
mation of the Mesabi range, concerning which a brief statement was made a
few pages back (p. 378). These rocks consist of rounded areas of fine-
o-rained crvstalline silica and limouite and rarelv hematite — corresijonding-
exactlv in shape to those granules which have been mentioned above —
which lie in a groundmass of crystalline silica (see PI. XII, A). These
areas are surrounded by a border df limouite, hematite, or these oxides —
most commonly limouite — are more or less uniformlv distributed through-
out the g'ranule or occasionallv concentrated at tlie center. Within tlie
V)()rder crystalline silica sometimes predominates, although scattered through
it tliere is more or less limouite, sometimes actinolite and griuierite and a
ferruginous carbonate. The iron oxide has frequently a definite arrange-
ment. It lias accumulated in aggregates at the centers of fibrous quartz
" Men. U. S. Geol. Survey Vol. XIX, 1892, pp. 191-192.
PLATE XIL
381
PLATE XII.
A, Photomicrograph showing the granules in tlie Giinflint formation. These granules, which
originally consisted of a green to brownish-green hydrated ferrous silicate, may, after alteration,
consist of limonite, hematite, magnetite, ferruginous carbonate, silica, actinolite, and griinerite, in
various combinations. Limonite and silica occur very commonly. In this slide the granules consist
of hematite and silica. The spherulitic character of the siliceous matrix is well shown by the black
crosses. (Slide 29446, 21 diameters, with analyzer, p. 380.)
B, Photomicrograpli showing the details in a limonite silica granule. The limonite occurs at
the centers and around the p-iripheries of small spherulitic or granular areas of silica. In the upper
right-hand quadrant an area with well-defined agate structure is distinctly shown. (Slide 7004, 80
diameters, without analyzer, p. 383. )
382
U. S. GEOLOGICAL SURVEY
MONOGRAPH XLV PL XII
A. PHOTOMICROGRAPH SHOWING GRANULES IN GUNFLINT FORMATION.
B. PHOTOMICROGRAPH SHOWING DETAILS OF GRANULES.
UPPER HURONIAN. 383
spherulites, and around each one of these spherulites there occurs also a
fihn or thicker layer of limonite, with, in a few cases, some small quantity of
hematite. This structure is interpreted to mean that the ferrous silicate
originally occupying- these areas has been altered into its constituents, iron
oxide and silica, the silica forming the radiating areas above mentioned, and
the limonite having been retained either at the centers of these areas or
forced outward during the processes of crystallization, so as to form a ring
now surrounding these areas (see PI. XII, S). That a large part of the
silica of the granules is a secondary deposit is shown b}^ the fact that an
imperfect agate structure is not uncommon (see PI. XII, B). A similar
agate structure also occurs between the large rounded granules referred to
Projecting from the sides toward the centers of the spaces between these
granules occur also segments of or complete quartz spherulites. This
spherulitic structure showing black cross is reproduced in PI. XII, A.
The rocks briefly described above are the least altered forms of the
rocks of the iron-bearing formation, and when weathered exhibit on the
surface a brown ferruginous crust. As we follow these rocks westward we
find that they change somewhat, passing into ferruginous cherts and cherts
which have been more or less completely recrystallized into relatively coarse-
grained rocks that might be spoken of almost as quartzites — although they
are not, as should be clearly understood, metamorphosed clastic sandstones —
actinolite, griinerite, and magnetite rocks, in which there is practically no
carbonate, or but very little. These rocks, of course, vary greatly in color,
ranging from white or gray to brownish, light green, dark green, and prac-
tically to black, the color depending on the quantity and kind of the minerals
mentioned which are present in them. This is especially true of those
rocks that occur in the narrow belt extending from a short distance east of
Paulson's mine west nearly to Gobbemichigamma Lake. Here the gabbro
is either in immediate contact with or but a short distance from these rocks.
The rocks in this area are made up of coarsely crystalline bands of quartz
of varying width in alternation with coarsely crystalline bands of magnetite
ore, reported to range from 1 inch up to 10 or 12 feet in thickness, and
bands of dark-green, brown, or black rocks, which consist of combinations
of quartz, augite, hypersthene, hornblende, olivine, and magnetite as the
principal minerals, associated occasionally with some feiTuginous carbonate,
actinolite, and griinerite. These bands, consisting largely of ferromagnesian
minerals, vary from medium grain to coarse grain. Occasionally they
384 THE VERMILION IRON-BEARING DISTRICT.
are characterized by large poikilitic plates of hyperstheue several iiiches in
length, which show bright reflecting faces. Most of these rocks when
examined under the microscope appear as granular aggi-egates of the
various minerals enumerated and give no clue to the original rock from
which they were derived. Some of them, however, still contain the
rounded areas to which reference has already been repeatedly made, and
show conclusively that they have undergone a more or less complete
recrystallization. In these the areas are always outlined by a zone of
magnetite, rarely with some hematite. In some cases this magnetite occurs
as a very fine dust; in others the magnetite is in relatively large masses.
Ordinarily the boundaries between these areas and the adjacent quartz of
the groundmass are sharply marked by the magnetite zone. "When the
areas are close together the magnetite border of tlie one unites with that of
the ones adjacent, and such union tends to destroy the regularit}' of the
areas. Indeed, when the areas are closely crowded they run together more
or less. When, in addition to being close together, the interior of the areas
is occupied by magnetite, as is commonly the case, the resulting rock is
composed of a mass of magnetite with little quartz and none of the rounded
granule areas are visible. In man}^ of the areas quartz is the chief con-
stituent, in relatively coarse grains. Within these grains occur dust-like
crystals of magnetite which are accumulated either at the centers of the
grains or just within their peripheries. Outside of the areas occurs the
matrix, which consists now of coarsely crystalline quartz. When viewed
between crossed nicols, however, it is seen that the large quartz individuals
of which the matrix is composed pass across the boundary and extend into
these areas.
We can readily see how such a rock as this might be produced from
the one already described (p. 380), in which essentially the same conditions
existed, with the difference that the rounded areas of silica and limonite
were bounded by limonite, and that the quartz was in fine fibrous spheru-
litic aggregates with limonite at the centers and bounding their perij)heries
(PI. XII, ^-1). By dehydration of the limonite there would be produced
hematite or, if insufficient oxygen were present, as appears to have been the
case throughout this region, magnetite. With the limonitic rocks there is
found associated ferruginous carbonate, which also contains undoubtedly
some lime and magnesia. Recrystallization of this material in combination
UPPER HURONIAN.
385
with the h'on and sihca of the adjacent rock might, where the elements were
present in proper proportion, very well produce the actinolite, griinerite,
and massive hornblende which in some places more or less completely fill
out these rounded areas.
The following- partial analyses of the iron ores of these Gunflint beds
are quoted from Wiuchell and Grant, and are of interest as showing the prac-
tical absence of titanium from them." In this respect they differ very
essentially from the titaniferovis magnetite ores which form a part- of the
Duluth gabbro of this vicinity.
Analyses of iron ores of Gunflint heds.
Metallic iron, Fe
Manganese, Mn .
Silica, SiO^
Alumina, AI2 O3
Phosphorus, P..
Titanium, Ti
I.
II.
.58. 40
54.01
4.92
5.02
8.22
9.37
0.52
.0.07
0.036
0.032
None.
None.
0.028
Trace.
62.05
7.14
0.113
These iron-bearing rocks were observed by Chauvenet in his recon-
noissance through this district in 1884.'' They were also studied by
Bayley ° and described by him in his articles upon the Duluth gabbro of
Minnesota. Chauvenet apparently considered these rocks as a part
of the gabbro. W. N. Merriam has also mapped these rocks as part
of the gabbro, and Bayley has also described them as a peculiar periph-
eral phase of the gabbro. H. V. Winchell, of the Minnesota survey,
first suggested that they were a phase of the Gunflint formation meta-
morphosed by the gabbro.'* This is beheved to be the correct expla-
nation of the orighi of these peculiar rocks. This explanation was
adopted by Spurr and also by Grant, who has described the rocks in
a Analyses Nos. I, II, III, and IV, are from Geol. and Nat. Hist. Survey of Minnesota, Bull.
No. 6, 1891, p. 138. No. V is from Geology of the Mesabi Iron Range, by U. S. Grant: Engineers'
Year Book, University of Minnesota, 1898, pp. 49-62. Geol. and Nat. Hist. Survey of Minnesota,
Final Eept., Vol. IV, 1899, p. 480.
("W. M. Chauvenet, U. S. Geol. Survey, MS. Notes.
cThe basic massive rocks of the Lake Superior region, by W. S. Baylev: Jour. Geol., Vol. I,
1893, pp. 433-716.
''Geol. and Nat. Hist. Survey of Minnesota, Twentieth Ann. Rapt., 1893, pp. 120-121, 134-136.
MON XLV — 03 25
386 THE VERMILION IRON-BEARING DISTRICT.
some detail and has brought forward evidence in favor of the hypoth-
esis that they originated by metamorphic action of the gabbro on the
Gunflint formation. Grant bases his conclusions as to their mode of
origin on the following facts: The magnetite in the Gunflint beds is, as
shown by the analyses quoted above, nontitaniferous, whereas that of
the g'abbro is titaniferous ; the large amount of quartz in these beds
could not possibly be derived in such quantity from the cr3"stallization
of the gabbro magma; feldspar is absent, whereas it is, of course, an
essential constituent of the gabbro itself A further fact, which should
be considered as evidence against the view that these iron-bearing beds
with the bands of ferromagnesian minerals are a contact or border
facies of the gabbro, and as favoring the hypothesis that they are an
exomorphic contact product of the gabbro — the explanation which is
believed to be the correct one — is the coarseness of the beds in comparison
with the recognizable border phases of the gabbro itself. These iron-
bearing rocks range from medium- to coarse-grained rocks. In general,
they are coarser than the border phase of the gabbro. Such a condi-
tion is anomalous. Ordinarily the contact is the finer grained the farther
it occurs from the main mass of the igneous rocks. If this were intei'preted
as a contact phase of the gabbro, here we would have next to the main
mass of gabbro a relatively fine- to medium-grained gabbro and then
this coarse-grained facies, which in places is made up of bands of
coarse-grained pure quartz and the other bands mentioned. The original
rocks from which the iron-bearing rocks, and eventually these rocks,
were derived, judging from analogy with the correlated iron-bearing
formation of the Mesabi district, are supposed to have consisted largely
of chert with a hydrous ferrous silicate, that which occurs in the green
granules, with which is associated more or less iron, calcium, and mag-
nesium carbonate. From rocks of this composition it is easy to see that
the coarse-grained rocks, consisting of quartz, magnetite, olivine, horn-
blende, augite, and hypersthene, might have been derived by simple
recrystallization, without presuming any transfer of material from the
gabbro. We know that fai'ther west in the district, where the gabbro lies
in contact with the Lower Huronian slates and conglomerates, it has
metamorphosed them extremely, producing in them secondar}- ferro-
magnesian silicates, hypersthene, hornblende, biotite, and augite, with
UPPER HURONIAN. 387
varying quantities of magnetite. It is not necessary, therefore, to assume
any abnormal conditions other than the contact action of the gabbro on
beds having the proper composition. Complete recrystallization of properly
constituted beds, the process taking place slowly and extending over a long-
time, would readily account for the existence of these abnormal Gunflint
beds. Since we consider this recrystallization of the rocks and production
of magnetite, etc., to have taken, place as the result of the metaiuorphism
produced by the Duluth gabbro, it is evident that the iron in the rocks
must have accumulated prior to Keweenawan time." As the result of the
metamorphism the rocks were so changed that no further concentration
of iron took place, and consequently we find these deposits in this part
of the district differing both in petrographic character and in size from the
great deposits of the western Mesabi or Mesabi proper, whose concentra-
tion was not seriously interfered with except locally during Keweenawan
times, but has continued right on up to the present.
RELATIONS TO OTHER FORMATIONS.
The peculiar Gunflint formation, found at the base of the Upper
Huronian, rests upon rocks of different character and of varying age.
These range from the granite of Saganaga Lake in sees. 23 and 24, T. 65 N.,
R. 4 W., through the Ely greenstone to the west of this area, and then up
to the Ogishke conglomerate and the Knife Lake slates still farther west.
The Duluth gabbro lies against and upon the southern edge of the Gunflint
formation.
The Gunflint formation of the Animikie series is found in relationship
with the Ely greenstone of Archean age, at one especially well-exposed
place in the north side of the cut of the Duluth, Port Arthur and Western
Railroad, where it cuts the east end of the high cliff of greenstone on the
north shore of Gunflint Lake. Here the iron formation is well banded,
and rests, with a very slight dip to the south, on the crinkled green schists
derived from the Ely greenstone. At one place about 1 foot of con-
glomerate was found at the base of this formation. This conglomerate con-
sists of green schist and quartz pebbles, and above this comes a layer of
banded white chert about a foot thick in places, and somewhat brecciated.
The iron formation proper does not actually appear at the particular
«Geol. and Nat. Hist. Survey of Minnesota, Bull. No. 10, 1894, pp. 199, 358.
388 THE VERMILION IRON-BEARING DISTRICT.
point where the conglomerate was seen, but it does appear a few paces to
the east. On the north side of the road, in sec. 30, T. 65 X., R. 4 W.,
about 500 paces east 6f Fay Lake, there were found several trenches
which cut through the iron formation and showed its contact with the EK-
greenstone. Here it seemed to rest upon this greenstone without any
intervening conglomerate. Still farther east, at the west end of the
Duluth, Port Arthur and Western Railroad, just west of Paulson's camp,
the cut has exposed the Ely greenstone with a film of the Gunflint forma-
tion lying above it. At this place no well-marked conglomerate exists.
The greenstone is more or less broken up and some of the iron formation
material has been infiltrated in these cracks, so that on the surface it looks
conglomeratic. A glance at the maps in the atlas will show that the area
just mentioned, in which the two exposures of greenstone in contact with
the Gvmflint formation occur, is at the place where the Ely greenstone
makes its greatest bend to the south.
Relations to the Lower Huronian series — Ogislike conglomerate and Knife
Lake slates. — The Ogishke conglomerate occurs east and west of the south-
ward projecting point of Ely greenstone in sec. 30, T. 65 N., R. 4 W., and
sec. 25, T. 65 N., R. 5 W. It is very thin to the east, and in fact its pres-
ence has been detected in only one place, as the result of an examination
of the dump heaps of the pits northeast of Paulson's mine, in the NW. \ of
sec. 27, T. 65 N., R. 4 W. These pits are just north of the ridge of Gun-
flint formation, and are in typical bedded rock. This bedded rock occurs in
the upper part of the pit, as one can readily see. The lower part of the pit
is now filled with water, but some rock on the dump and that forming the top
part of the dump, presumably material last taken from the pit, has a distinctly
conglomeratic appearance. The matrix is, liowever, very coarsely crystal-
line, and the supposed pebbles are well rounded. The kinds of rock which
constitute the pebbles could not be determined. This conglomerate certainh-
I'esembles very closely, if it is not identical with the Ogishke conglomerate,
wliiclL occurs farther to the west. There is a bare possibility that it repre-
sents a conglomerate at the base of tlie Gunflint formation belonging with
the Upper Huronian series, but, if so, it could not be discriminated from
the Ogishke.
West of the southward-projecting greenstone point above mentioned
the Ogishke conglomerate appears in typical development. ^X is first seen
UPPER HURONIAN. 389
along the north side of the road leading to Fay Lake, where it is very thin,
but to the west it increases greatly in thickness, until it is found covering
an area having a width north and south of nearly a mile along the line
between sees. 26 and 27, T 65 N., R. 5 W. The Gunflint formation has
been found in direct contact with this conglomerate at a number of places,
but in no place could a conglomerate be found which could be said to be at
the base of the Gunflint formation and separable from the Ogishke con-
glomerate. At no place in this district can positive e'V'idence be found of
the relation of the Gunflint formation to the underlying Lower Hui'onian
series. Where these rocks are in contact no strike or dip could be found in
the conglomerates below the iron-bearing formation. The strike and dip
can be determined where the iron formation is separated from the Ogishke
conglomerate and Knife Lake slates by an area within which there are no
exposures. In these cases the strikes of the rocks are nearl}^ at right
angles to each other, and there is a great discrepancy in dip. This evi-
dence weighed in favor of an unconformable relationship. The evidence
was not considered conclusive, for in view of the close folding in the
district the possibility was recognized thtit conformable rocks may have
been so folded that with lack of exposures showing the actual connection
and transition they may appear unconformable.
Lidubitable proof of the unconformable relationship of the Gunflint
iron-bearing beds to the Lower Huronian series was found by Leith in the
Mesabi district.". There the conformable series of rocks — Upper Huronian —
to which the Gunflint formation belongs, was found overlying the Lower
Huronian series with a basal conglomerate between. Thus the conclusion
reached from the study of the imperfectly exposed beds in the Vermilion
district was confirmed.
Belations to the Keweenawan (jDuluili) gabbro. — In all cases where the
Gunflint foi-mation is exposed in the Vermilion district it is found that the
Duluth gabbro is in contact with the series on its southern side. It is
very noticeable, also, that in all cases where the Gunflint beds are in con-
tact with this gabbro the rocks have been extremely metamorphosed. This
metamorphism is most noticeable immediately at the contact, diminishing
in extent as the distance away from this contact increases. These facts
f'The Mesabi iron-bearing district of ilinnesota, by C. K. Leith: Mon. U. S. Geol. Survey Vol.
XLIIl, 1903, p. 180.
390 THE VERMILION IRON-BEARING DISTRICT.
afford indisputable proof that the gabbro is younger than the Grunflint
formation.
Relations to basic dikes. — The Gunfliut beds have been cut bv dikes
and sills of basalt similar to dikes which cut the Duluth g-abbro.
THICKNESS.
The Grunflint formation is shown on the map in the atlas as feathering
out in sec. 33, T. 65 N., R. 5 W., near the end of Paiil Lake. East of this
point it widens very much and reaches its maximum width in sees. 23 and
26, T. 65 N., R. 4 W. East of these sections it again narrows down. Where
it is Avidest the beds have been somewhat crumpled locally by great intni-
sive sills. Grant" has estimated the maximum thickness to be about 825
feet, calculated on an average dip of 10° S., but states that this estimate is
possibly too great.
Irving and Van Hise estimated the thickness of the iron-bearing mem-
bers in the Penokee district of Wisconsin and Michigan to be from 800 to
1,000 feet. '' This seems to be, however, very close to the true thickness
for the Gunflint iron -bearing beds, as shown by comparison with correla-
tive beds in other districts. Thus the Biwabik formation of the Mesabi
district, with which this is correlative, has been estimated by Leith '' to
vary from 200 to 2,000 feet in thickness, and to have an averag-e thickness
of about 1,000 feet.
SECTION II.— RO^TE SLATE.
The sediments constituting this formation lie immediately above the
Gunflint formation and have been called the Rove slate from Rove Lake,
a lake sitviated just north of the international boundary and east and out-
side of the Vermilion district, and lying in a large area underlain by these
slates in typical development. Slates occupying the same stratigraphic
position and possessing the same general characters occur in the Mesabi
district and are there known as the Virginia formation.
"Oeol. and Nat. Hist. Survey of Minnesota, Final Rept., Vol. IV, 1S99, p. 471.
''Mon. U. S. Geol. Survey Vol. XIX, 1892, p. 189.
«The Mesabi iron-bearing district of Minnesota, by 0. K. Leith: Mon. U. S. Geol. Survey Vol.
XLIII, 1903, p. 166.
UPPER HURONIAN. 391
DISTRIBUTION, EXPOSURES, AND TOPOGRAPHY.
Distribution. — lu the Mesabi district south and west of the Vermilion
range these slates cover a large area. The Rove slates, which are found
in the Vermilion district, represent merely a portion of the slates that cut
across the east end of the district and that are not cut out by the Duluth
gabbro.
The westernmost exposures of these slates in the Vermilion district are
found in sec. 26, T. 65 N., R. 4 W. The formation underlies a very narrow
area in the south-central part of the. above section, but rapidly widens to
the east. The northern boundary of the slates extends northeastward, and
is limited by the Gunflint formation and a grea.t dolerite sill. The southern
boundary trends east-southeast and the Duluth gabbro everywhere marks
their southern extent. At the eastern limit of the map the extreme width
of the Rove slate area in the United States is only about 2 miles, and a
great deal of this width is taken up by intrusive sills of dolerite, which very
materially reduce the areal distribution of the sediments. Beyond the
limits of the map the slates have an enormous development in Minnesota
and in the adjacent portion of Canada. They may be seen especiallv well
along' the Canadian shore of Lake Superior and on the islands in the lake
from Pigeon River northeastward to Thunder Cape.
Exposures. — The exposures are usually very good along the lake and
wherever steep escarpments occur, which is usually immediately along the
lake shores. When the hills stand some distance back of the lakes it is not
uncommonly found that although the northern slope is fairly steep, a heavy
talus conceals the greater portion of the slates. The slates rarely show any
exposures at all, or but very poor ones, on. the gentle southern slope of the
hills.
Topography. — The topography is that which is usually developed in
areas of monoclinal dipping rocks. Ridges have been formed whose trend
correspoiids approximately to the strike of the slates, here about east and
west. These ridges have very steep escarpments on their north faces,
where the rocks have been cut directly across the dip, and very gentle
slopes to the south which agree in general with the dip. The depressions
between the ridges are occupied by lakes, or if not by lakes then by low
ground with a stream which eventually flows into a lake. Seen in profile
392
THE VERMILION IRON-BEARING DISTRICT.
the ridges and intervening- low g-roiind present an appearance very similar
. - to that of the teeth of a saw, and from this circumstance
•£ they are sometimes called sawtooth mountains. Starting
I at the uorth at a lake one ascends a steep ridge rising
0 200 or 300 feet above the lake in many instances, then
1 descends the gentle dip slope to the south, which leads
I down to a second depression occupied by a lake, then
Z ascends again a steep northward-facing- hill with gentle
M southerly slope, and so on. Dolerite sills occur interca-
I lated in these Rove slates and usually cap the hills.
I Their influence on the topography is referred to later on
I (p. 400). The topographic character of that portion of
a. Minnesota underlain by the Rove formation can be seen
i in PI. XIII, A, and fig. 21 in the text.
STRUCTURE.
^.2
The structure of the slates in this area is very
simple. Wherever they have been examined the)'' are
found to have a verj- uniform dip of from 5° to 25° to
the south-southeast. They evidently form a part of the
great mpnoclinal series of slates which are known as far
west as the Mississippi River in the Mesabi district, and
which continue directly east, cross the east end of the
Vermilion district, appear on the north shore of Lake
Superior north of Grand Portage, and continue thence
eastward around Tlumder Bay and northeastward along
the sliore of Lake Superior for some distance. As indi-
cated by the variation in dip — from 5° to 25° — the
monocline of slates is occasionally interrupted by minor
rolls, which, though of little importance, can be noted by
close examination of almost any of the great clift's that
give good exposures.
PETROGRAPHIC CHARACTERS.
The slates form the bulk of the Rove formation, but
with them are associated graywackes, some quite slaty,
others very massive, and also some fairly pure quartzites.
U. S. GEOLOGICAL SURVEY
MONOGRAPH XLV PL. XIII
SAWTOOTH HILLS OF ROVE SLATE CAPPED WITH DOLERITE SILLS. AT NORTHEAST END OF
ROSE LAKE, INTERNATIONAL BOUNDARY.
Ji. VIEW ON AN ISLAND IN BURNTSIDE LAKE, SHOWING GRANITE OF BURNTSIDE LAKE CUTTING
AMPHIBOLE-SCHISTS— METAMORPHOSED ELY GREENSTONE.
UPPER HURONIAN. 393
This series of sediments has been divided by Grant," of the Minnesota
survey into a "black slate member," with a "graywacke slate member"
above it. In our work no attempt has been made to discriminate between
these two petrographic facies of the Eove formation. They are not
separable by any time interval, but represent merely slight changes in the
conditions of deposition. Macroscopically they are very fine-grained black
carbonaceous slates, grading up into dark-gray graywackes of medium
grain, with occasional bauds of material almost sufficiently pure to be called
quartzite. In no case were any conglomerates, even fine-grained ones,
found associated with these. The slates are unquestionably the predomi-
nant kind of rock in the Vermilion district. They are commonly very
fissile, although in places these carbonaceous rocks are fairly massive.
Microscopic characters. — The Rove sediments are composed of angular
quartz and feldspar grains in a dark cement. In some cases the character
of this cement can be partly seen, and one can then recognize shreds of
biotite and chlorite. Between these is a very fine-grained dark material
which is presumed to consist of nnnute dust particles of quartz and feld-
spar and ferruginous and carbonaceous material. Many of these rocks
are so well crystallized that they may almost be called phyllites. In these
crystalline rocks the matei'ial between the grains, jjrobably formed from
the decomposition of the fine matrix above referred to, consists of flakes
of biotite and chlorite, with quartz and ferruginous matter.
CONTACT METAMORPHISM OF THE ROVE SLATE.
The Rove slate, as has already been stated, is in contact on its
southern border with the Dulutli gabbro. At numerous places within
the formation there are gi-eat intrusive sills which are considered to be
offshoots from the Duluth gabbro. The reasons for this -sdew will be given
in a later chapter. The gabbro and the sills have liad a slight contact
effect upon the slates adjacent to them. Actual contacts of the sills with
the slates in this district were not seen, but a number of contacts of similar
sills on similar slates were seen along the Lake Superior shore in the
Thunder Bay district of Canada, and in all such cases the slates merely
showed a slight induration. Outside of the district, as, for instance, on Pigeon
aGeol. and Nat. Hist. Survey of Minnesota, Twenty-second Ann. Kept., 1893, p. 74; Final Kept.,
Vol. IV, 1899, p. 470.
394 THE VERMILION IRON-BEARING DISTRICT.
Point, Minnesota, certain gabbroic intrusives are known to have had a very
far-reaching contact eflPect on these sediments." Along- the southern and
southeastern sliores of Loon Lake were collected several specimens of
sediments which were near, although not in actual contact with, the sills.
One of these specimens shows a spotted character and is a spilosite such
as is fairly common in sediments near the contact with the great mass of
gabbro and occurs also in other districts near great dolerite dikes. This
spilosite contains a large amount of chlorite in clumps embedded in a
matrix of quartz and presumably some feldspar, and forms the microscopic
spots. In the Mesabi range some of the slates near the gabbro contact
show clearly recognizable cordierite, forming the white spots, and the slates
have been metamorphosed to a cordierite-hornstone.* In general the slate
adjacent to these sills in the Vermilion district shows its normal chai-acters
with at most a little metamorphism due to cementation.
A contact of the gabbro with the Rove formation at a point at the
southwest end of Loon Lake was examined. This contact is of the gabbro
on the "graywacke slate member" of Grant. The sediments at the top
of the section were within about 3 feet of the gabbro. This is as near as
we found the sediments to the gabbro. Here the rocks are interbanded
slates and graywackes which were quite crystalline and hard. Microscopic
examination of them shows that the gabbro had effected a partial recrys-
tallization of the sediments and discloses in the sediments a large amount of
secondary biotite and muscovite. Both of these occur in relatively large
porph-sTitic plates inclosing grains of the other materials constituting- the
slate, recognizable quartz, and ferruginous material. As the rocks are
studied, as we go down the slope, they are seen to be less indurated, until
near the bottom of the section at the water's edge, about 50 feet below the
gabbro, the sediments do not appear essentially different from the ordinary
rocks of this character and of this age. It is clear from this that the effect
of the gabbro has not been felt at a very great distance from the actual
plane of contact with the sediments.
«0n some peculiarly spotted rocks from Pigeon Point, Minnesota, by W. S. Bayley: Am. Jour.
Sci., M series, Vol. XXXV, 1888, pp. 388-.393. Abstract, Nature, Vol. XXXVII, 1888. p. 91 (.5 lines).
Rocks on Pigeon Point, Minnesota, and their contact phenomena: Bull. V. S. Geol. Survey Xo. 109,
1893, pp. 121.
("The Mesabi iron-bearing district of iliunesuta, by C. K. Leith: Mon. U. S. Geol. Survey Vol.
XLIII, 190.3, pp. 171-172.
UPPER HURONIAN. 395
RELATIONS TO ADJACENT FORMATIONS.
The relations of the Rove slates to the other formations of the district
are easily deteriiiined. The oldest rock with which they are in contact is
that which has been described as the Gunflint formation. The slates are a
conformable series of sediments overlying- this formation, and consequently
younger than it. In previous pages the relations of the Gunflint formation
to the other older rocks of the district have been described, and it is not
necessary to add anything- to the statement concerning the age of the Rove
slates other than that they are young-er than all of the rocks below the
Gunflint formation.
Relations to Keiveenaivan dolerite. — In places the Rove slates are found
in contact with great masses of dolerite. Near the contact the sediments
are found to be harder than elsewhere, and in some places to have had
produced in them minerals which are evidently of secondary origin, corre-
sponding to the products of contact metamorphism, which have been
studied in other districts. This induration is undoubtedly due to the
metamorphic action of the dolerite. This alone is proof of the fact that the
dolerite is younger than the sediments. In addition to this proof, however,
we have the further evidence of the contortion of the slates, which has been
noticed in a number of places where the beds were in contact with the
dolerites, having been intruded by them. Moreover, the dolerites them-
selves are much finer grained at the edge than elsewhere. These three
facts — the fine-grained character of the edges of the dolerite masses, the
induration of the slates along this contact, and the contortion of the beds —
form indubitable evidence that the dolerites are younger than and intrusive
in the slates.
Relations to Keweenawan (Bulutli) gahhro. — The only place where a good
contact between the gabbro and the slates was observed is that mentioned
above, on the southwest side of Loon Lake. Here the gabbro overlies the
slates, and produced considerable changes as the result of its contact with
them. The superposition of the gabbro and the contact zone in the slates
afford conclusive proof of the relative ages of the two, the Duluth gabbro
being very clearly younger than the Rove slate.
396 THE VERMILION IRON-BEARING DISTRICT.
AGE.
Fi'om the foregoing statements we see that the Rove slates and
graywackes form the youngest member of the Animikie series in the
Vermihon district. The only rocks younger than it are the dolerite sills,
the Duluth gabbro, and the occasional basic and acid dikes which cut
through the gabbro.
THICKNESS.
Only a very small portion of the sediments which constitute the Rove
slates in the Lake Su])erior region are represented in the Vermilion disti-ict.
As has been shown by the distribution, only the apex of a V which rapidly
widens to the east is there present. The gabbro of Keweenawan age comes
in from the south and swings up northwestward, cutting across the east-
west striking slates, and ^^roducing the V above referred to. In the Ver-
milion district, then, the Rove slates vary from a minimum, at the point of
the V, up to a maximum for that district which attains a considerable thick-
ness. No attempt to measure the maximum thickness for this district has,
however, been made, as it would give merely the thickness of a portion of
the slate formation, and not that of the formation as a whole. The latest
estimate for that part of the slates present in the Vermilion district is
that made by the Geological and Natural History Survey of Minnesota.
According to this, the "Graywacke Slate Member" has a thickness of l,(i50
feet, the "Black Slate Member" a thickness of 950 feet, and the sills intruded
in these rocks a thickness of about 250 feet. This gives a total thickness for
the sediments of the Rove formation exposed in this district of 2,600 feet.
No statement is made as to the section on which the estimate of this thick-
ness was based, but it was presumably between Gunflint and North and
South lakes, just east of the limits oi the area shown on the accompanying
map of the Vermilion district, PL II. The formation has been studied at
various places by a number of geologists, and varying estimates have been
made of its total thickness. According to estimates made by Irving," the
Animikie series of slates corresponding to the Rove slates of the report has
a thickness of 10,000 feet. Ingall has estimated this thickness at 12,000 feet.*"
In 1892 Irving and Van Hise'' gave an estimate of 11,000 feet as the
maximum thickness of the Animikie slates in the Penokee disti'ict.
«Mon. U. S. (ieol. Survey Vol. V, 1883, p. 380.
''Geol. and Nut. Hint. Survey of Canadii, Ann. Kept, for ISSS, II, p. 2().
'Mon. U. S. Geol. Survey Vol. XIX, 1892, p. 299.
CHAPTER VI.
THE KEWEENAWAN.
IKTRODUCTION.
The only rocks of Keweenawan age in the Vermilion district are
gabbros which form a part of the Duluth gabbro mass of northeastern
Minnesota, certain great basic sills to which the name Logan sills has been
given, and some few basic and acid dikes which cut all of the rocks of the
district, including the aforementioned gabbros and Logan sills. As a
result of the studies reported in this monograph, it has been determined
that stratigraphically the Duluth gabbro and the Logan sills belong together,
cilthough they show slight differences in lithologic character. These
differences are due essentially to variations in the conditions of consoli-
dation. Since these two rocks belong together, they will be described
under the same section in the following pages. A second section will be
devoted to a brief mention of the basic and acid dikes, which are the
youngest rocks of the Vermilion district, excluding always the Pleistocene
glacial-drift deposits.
SBCTIOX I.— DULUTH OABBRO ANT> LOGAN SILLS.
Refei-ences to the OTeat o-abbro mass of northeastern Minnesota are
common in the geologic literature of the Lake Superior region. The name
Duluth is given to this gabbro since it is so well developed near tlie city of
that name. This rock is conspicuously developed on the north shore of
Lake Superior, where it forms a prominent part of the Keweenawan series
of northeastern Minnesota, underlying several hundreds of square miles. It
is also well known upon the south shore in the Keweenawan district of
Wisconsin.
North of the Duluth gabbro, and extending all around the north shore
of Lake Superior as far as the Slate Islands of the northeast shore of the
lake, it has been found by the various geologists who have worked in this
397
398 THE VERMILION IRON-BEARING DISTRICT.
territory, beginning with Logan," that the sedimentary rocks of this region,
slates, quartzites, and graywackes, have intercalated in them at various
horizons sheets of basic igneous rock ranging in thickness from 1 to 100
feet. These vary in character from distinctly gabbroic rocks in the
centers of the large masses through all gradations of finer-grained granular
and porphyritic rocks to the very fine-grained basaltic phases which form
the thin sheets and occur as well-formed selvages of many of the thicker
sheets. These are the intrusive sheets which have been called the Los'an
sills by Lawson,* in recognition of the geological work done by that
pioneer of investigation in this field.
DISTRIBUTION. EXPOSURES, AND TOPOGRAPHY.
Distribution. — The Duluth gabbro forms the southern boundaiy of the
pre-Keweenawan rocks throughout the greater portion of the Vermilion
district. The westernmost point at which the Duluth gabbro touches the
district is in sees. 26 and 3.5, T. 63 N., R. 10 W., and section 3, T. 62
N., R. 10 W. From these sections on along the Kawishiwi River the Duluth
gabbro swings off to the northeast with a broad sweep, extending just
within the area mapped as far east as the vicinity of Paulson's mine, in
sec. 28, T. 65 N., R. 4 W. From this place its edge trends to the southeast,
passing beyond the limits of the area mapped toward Lake Superior.
A couple of small isolated outliers have been found north of Grobbemichi-
gamma Lake. The southernmost one is only a quarter of a mile from the
northern edge of the main mass of the gabbro, northwest of Paulson
Lake, and the other is about three-fourths of a mile from the nearest point
on the edge of the gabbro and lies in the NW. 4 sec. 29, and NE. -^ sec.
30, T. 65 N., R. 5 W.
The sills lie well within the district to the north of the edge of the gabbro
mass, varying in distance from this edge. The first exposure of such a sill
was noticed on the southwest side of Gobbemichigamma Lake, but this can
not be traced far. The next one was seen near Bingoshick Lake. This sill
has been followed to the east for several miles to a point east of Paulson's
mine, having throughout this distance an almost continuous outcrop. Par-
allel to this sill sevei"al small and relatively unimportant sills have been
"Geological Survey of Canada, 1846-47, p. 13.
''The lacolitic .sills of the northwest coa.st of Lake Superior, r)y .-\. ('. LawHon: (ieol. ami Xat.
Hist. Survey of Minnesota, Bull. No. 8, pt. 2, 189:i, jip. 48.
THE KEWEENAWAN. 399
observed. Beyond Paulson's mine the Upper Huronian sediments begin to
widen, rapidly increasing in width as they are followed to the east, as
alread}^ described. Corresponding with this widening, we find an increas-
ing number of sills having in general a trend east and west and lying
approximately parallel to each other. During several trips to Gimfllnt
Lake and to the country to the south a number of these sills were followed
along their strike for short distances and were also crossed at right angles
to the strike. Their relations to the sediments were thus clearly seen.
No attempt was made to trace out the individual sills. This work has
been done in previous years by Chauvenet" and Merriam,'' of the United
States Geological Survey, and in more recent years by U. S. Grant, ° of the
Minnesota Survey.
The data for the distribution of the sills which are shown on the accom-
panying map have been taken chiefly from the reports of these men.
Exposures. — Throughout the area underlain by the gabbro, as well as
the sills, exposures are very numerous and usually of large size, affording
excellent opportunities for the study of the characters of these rocks, their
variations in grain, and also their relations to the adjacent sediments.
Topography. — The line of contact between the gabbro and the older
rocks adjacent to it is fairly well marked by a slight topographic break. The
gabbro normally has a steep north face sometimes showing an escarpment of
varying height. It is never very high, but is considerably higher than any
topographic features in the area north of it for some distance. The contact
is frequently marked by a lake or a stream. This difference between the
topography of the gabbro area and that to the north exists at the immediate
contact, but examining contrasting areas as a whole we find that in general
the gabbro area is lower than that underlain by the older formations to the
north. Locally the gabbro area has been reduced almost to base level. In
fact, this area may be described as very nearly a plain, but one with minor
but pronounced irregularities. The uniformity of the surface is due in g-reat
part to the homogeneous character of the gabbro mass, which has caused it
to be about equally affected by the various agents which have attacked it.
The minor pronounced irregularities are usually found to be due to erosion,
which has been controlled very frequently by the joints of the gabbro, and
« W. M. Chauvenet, XJ. S. Geol. Survey, manuscript notes.
i Mon. U. S. Geol. Survey Vol. XIX, 1892, PL XXXVII.
cGeol. and Nat. Hist. Survey of Minnesota, Final Eept,, Vol. IV, 1899, pp. 487^88.
400
THE VERMILION IRON- BEARING DISTRICT.
to differences in composition where such exist. For example, the anorthosite
masses usually stand out conspicuously from the surrounding more basic
and less resistant portions of the gabbro.
The lakes of tlie gabbro area are, as a rule, shallow, and they are also
very irregular, and can not be said to possess uniformit}- of long extension
in any one direction, as is so markedly the case in the lakes of the other
Fia. 22.— Set'tiun through the Eove shitcs. with intercaUited Logan sills south of Gunflint Lake.
portions of the Vermilion district. On the contrary, they spread out in all
directions, sending off luimerous bays, some of which a)-e very long and
narrow, and all very irregular in shape.
The Logan sills exercise a very material influence upon the topography
of that portion of the district north of the gabbro in which the}' occur. It
will be recalled that the Upper Huronian (Animikie) sediments in this vicin-
ity have a iiionoclinal dip to the south. The sills have been injected essen-
THE KEWEENAWAN. 401
tially parallel to the bedding- of the sediments, although occasionally they
are found cutting across the beds at low angles. Erosion has been most
active in this portion of the district in a direction parallel to the strike of
the beds, and consequently most of the large valleys trend in agreement
with these, approximately east and west. The resistant sills now form the
caps of the ridges, the slates having been removed down to the sills. The
massive rock forming the sills breaks off along the joint planes, and this
breaking results in forming perpendicular cliffs, below the foot of which
talus from the sills and from the easily weathering Rove slates give a
gentle slope. These sills are sometimes very nearly concealed by the
accumulated talus deiived from them.
The efiFects of erosion have produced a series of hills with ver}^ nearly
vertical north escarpments, and a gentle slope from the crest of the hills to
the south. This slope corresponds very closely to the dips of the Rove
slates and the upper surface of the dolerite sills. Fig. 22 shows a some-
what idealized section through the ridge on the south side of Gunflint
Lake, taken from W. M. Chauvenet's manuscript notes.
PETROGRAPHIC CHARACTERS OF THE GABBRO.
Macroscopic characters. — It is not the purpose of this report to consider
in detail other rocks than those of pre-Keweenawan age which make up
the Vermilion district in its strict sense. In order, however, to give a
complete description of the area shown on the maps in the accompanying
atlas, it is essential to consider at least briefly tlie Dulutli gabbro. Speci-
mens have been taken here and there along its margin, and several trips
have been made well down into the gabbro, during- which specimens were
collected of the varieties seen and observations made on their relations.
Tlie following brief description of the gabbro is the result chiefly of the study
of these specimens. No attempt has been made to obtain specimens from all
parts of the gabbro, and consequently numerous facies which would be seen
only after very detailed studies of the gabbro have, of course, not been
found. For more detailed descriptions of this gabbro the reader is referred
to the reports of the Minnesota survey, especially to the articles by
Elftman", Grant,'' and Winchell," and to the petrographic study of the gabbro
aAm. Geologist, Vol. XXII, 1898, pp. 131-149.
»Geol. and Nat. Hist. Survey of Minnesota, Final Rept., Vol. IV, 1899, p. 476 et seq.; Twenty-
third Ann. Kept., 1894, pp. 224-230.
cAm. Geologist, Vol. XXVI, 1900, pp. 151-188, 197-245, 261-306, 348-388.
MON XLV — 03 26
402 THE VERMILION IRON-BEARING DISTRICT.
by Bayle}'." The gabbro, as a rule, was found to be a medium- to coarse-
grained rock, with essentially the same granular texture throughout.
However, near the contact of the gabbro with the other rocks to the
north of it, it is found to grow much finer grained. This gradation
is rapid. Thus the gradation from a medium fine-grained to a normal
coarse-grained rock was completed within a distance of about 10 paces.
Occasionally large areas of the fine-grained phase of rock which has been
called granulitic gabbro occur in the midst of the main gabbro mass. This
fine-grained material is found to have very sharp contacts'* with the coarser-
grained gabbro, and small areas of this fine-grained material are also
included in rounded as well as irregular masses within a coarse-grained
gabbro, possibly indicating that there is a slight diff"erence in age between
the two. This fine-grained gabbro at the point referred to has a remarkable
horizontal jointing, as the result of which it looks at a short distance like a
bedded rock in layers of from 2 to 6 inches thick. It also has a sheeted
structure striking in a general way east and west and dipping to the south.
This structure is brought out by the differential weathering. In some
cases also the gabbro has a very distinctly banded structure, as has been
described by Grant." He describes an exposure near south end of Bald
Eagle Lake as having a gneissic structure which is practically vertical and
runs N. 15° W., making the rock break more readily in this direction than
in any other. "In some places the gabbro lies in horizontal beds from
2 to 4 inches thick. The rock seems to be almost entirely composed of a
feldspar (probably labradorite) and a mineral which is probably olivine;
this, when not decayed, is of a yellowish green color." Microscopic studies
show this mineral to be olivine. This occuiTence is very similar to the
banded faces of the gabbro which may be seen upon the St. Paul and
Duluth Railroad between Short Line Pai'k and Smithville, and also upon
the shore of Lake Superior between Split Rock Bay and Beaver Bay.
A number of varieties of the gabbro were seen upon Little Saganaga
Lake. The gabbro varies from the very coarse-grained varieties to forms
"The basic massive rocks of Lake Superior, by W. S. Bayley: Geol. and Nat. Hist. Survey of
Minnesota, Final Kept., Vol. I, 1884, pp. 688-716; Vol. II, 1888, pp. 819-825; Vol. Ill, 1895, pp. 1-20.
6 Grant, U. S. Geol. and Nat. Hist. Survey of Minnesota, Final Rept., Vol. IV, 1899, p. 447,
PI. MM, figs. 5 and 6, and p. 489, fig. 89.
«Geol. and Nat. Hist. Survey of MinnesoUi, Seventeenth Ann. Rept., 1888, p. 164.
THE KEWEENAWAN. 403
which have rather a fine grain, these two grading into each other. In
places the gabbro becomes so feldspathic that it can be correctly spoken of
as anorthosite. These anorthosite masses usualh- weather white, and being
more resistant than the more basic gabbro stand out as bare, white,
conspicuous knobs. In examining these anorthosite masses, which are
beautifully exposed in numerous places on the islands and west and
southwest shores of this lake, one finds scattered through them irregular and
roundish areas of what appears to be normal gabbro. This grades du'ectly
into the anorthosite. Furthermore there are also seen finer-grained facies
of the gabbro in small, rounded areas occurring in the midst of the
anorthosite and grading into the surrounding anorthosite. It thus appears
that the anorthosite grades both into the normal gabbro of coarse grain
and also into the normal gabbro of fine grain, thus showing both a
mineralogic and textural gradation. The more basic areas which are
scattered through the anorthosite range in size from IJ inches in diameter
to 4 or 5 inches in diameter. Between these basic masses lies the
anorthosite, which makes up the greater portion of the rock, covering
much larger areas than are occupied by the basic parts. The basic
portions weather more readily than the anorthosite, producing a pitted
surface upon the exposures. When disintegration proceeds much farther,
the anorthosite is apt to break down into rounded bowlder-like masses.
In many places, and especially near the northern contact, the gabbro
is found to be very friable, and this character seems to be due to a
considerable extent to some character of the rock dependent upon its
contact with the adjacent formations, for specimens taken farther within
the mass were uniformly fresher and harder.
The exposures normally show a rock of dark color, either dark-reddish
brown, or black, varying, as is stated above, to the anorthosite, wliich is of
rather rare occurrence, and has a gray to white color. The other extreme
in the variation from the anorthosite is represented by masses consisting
essentially of titaniferous magnetite, such as is well developed at Mayhew
Lake" and especially at Iron Lake.''
The chief components, plagioclase, feldspar, pyroxene, olivine, titan-
iferous magnetite, are clearly recognized in hand specimens. With these
«W. M. Chauvenet, TT. S. Geol. Survey, manuscript notes.
6Geol. and Nat. Hist. Survey of Minnesota, Final Kept., Vol. IV, 1899, p. 489.
404 THE VERMILION IRON-BEARIxNG DISTRICT.
are found occasionally as accessory minerals, liypersthene, biotite, and
hornblende.
3Iicroscopic characters. — The g-abbros were refen-ed to above as being
generally distiucth* granular rocks. In addition to the granular texture
one can recognize under the microscope the poikilitic and ophitic textures.
The poikilitic is the most common, and in rocks possessing this texture we
find plates of hornblende, augite less commonly, and biotite very rarely,
the poikilitic minerals including individuals of other minerals. The ophitic
texture occurs in the coarse gabbros, but is more common in the finer-
grained forms. The presence of this ophitic texture in these rocks is
interesting as showing the gradations in textures from the very coarse
gabbros to the finer-grained dolerites. The following are the original
constitutents which are still present in the gabbro: Augite, liypersthene,
olivine, a little brownish-green hornblende, biotite, apatite, and magnetite.
The plagioclase has been determined by Bay ley" to be near basic labradorite
in character. These minerals do not possess any very exceptional characters.
Moreover, they have been described in great detail by Bayley'' in his
articles on the Duluth gabbro mass of northeastern Minnesota. The only
secondary mineral that has been observed is the serpentine. The rocks are
uormall)^ very fresh indeed. They vary considerably in mineralogic
composition. In some cases the feldspar is practically the only mineral
present, associated with only occasional grains of magnetite, and rounded
individuals of augite, forming anorthosite. With the feldspar occurs
occasionally a large amount of olivine and some titaniferous magnetite.
From the anorthosite phase the rock grades through facies containing the
ferromagnesian minerals in increasing quantity to the nearly pure titan-
iferous magnetite ores as the extreme variation. Occasionally the rock
consists of nearly pure augite with very little feldspar. Sometimes the
biotite is present in considerable quantity, producing the biotite-gabbro.
These are the varieties which we have observed. Many details concerning
these gabbros are given by Bayley" in the papers alread}- referred to. The
chemical composition of the gabbro is also here given. These analyses
"The basic massive rocks of the Lake Superior region, bj' W. S. Bavlev: Jour. Geol., Vol. I,
1893, p. 700.
''The basic massive rocks of the Lake Superior region, by W. S. Bayley: Jour, (xeol.. Vol. I,
1893, pp. 433-716; Vol. II, 1894, pp. 814-825; Vol. Ill, 189.5, pp. i-20.
t'Loc. cit., p. 712.
THE KEWEENAW AN.
405
were made for Dr. Bayley in the Survey laboratory, and were reported by
him in the papers above referred to.
No. 8589 contains a large proportion of diallage and olivine, while
No. 8786 is more nearly of the average composition of the entire mass.
Analyses of Duluth gabbro.
Constituent.
8589.
8786.
SiO^ .
TiOj.
PA-
AI2O3
CrjO,
FeO .
Fe,03
NiO .
MnO
CaO .
MgO.
K,0.
Na^O
H2O at 105° - - . .
H„0 above 105°
Total ; 100.03
45.66
46.45
.92
1.19
.05
.02
16.44
21.30
Tr.
13.90
9.57
.66
.81
.16
.04
Tr.
Tr.
7.23
9.83
11.57
7.90
.41
.34
2.13
2.14
.07
.14
.83
1.02
100. 03
100. 75
PETROGRAPHIC CHARACTERS OF THE LOGAN SILLS.
Macroscopic characters. — The rocks forming the Logan sills are nor-
mally black, medium- to coarse-grained rocks, although varying to fine-
grained aphanitic facies upon the edges of the sills. Occasionally the rock is
a porphyry, with the feldspars as the phenocrysts. Some of the pheno-
crysts reach 4 inches in length, but they are normally smaller, ranging
from 1 inch to 2 inches. Very frequently we find these feldspars collected
into large masses which are made up almost entirely of these minerals.
Such areas usually possess as the result of weathering' a light gray or
almost white color. The relatively slightly altered masses resemble very
closely in appearance the auorthosite of the Duluth gabbro mass. As will
be seen from the description given below, the sills are formed of rocks which
have the composition and characters possessed by the modern dolerites, and
they are here called dolerites.
4UG THE VERMILION IRON-BEARING DISTRICT.
Microscopic characters. — In exceptional cases in the very coarse-grained
rocks the texture is almost granular. More commonly we find an ophitic
texture imperfectly developed, with the feldspar occurring in very irregu-
larl}- bounded but in general lath-shaped forms, and the augite in more
rounded gi-ain-like forms than is common. The normal ophitic textui-e
is very commonly developed in these rocks, and this grades over into
the intersertal texture which is especially well developed in the fine-
grained border facies. The fact should be emphasized that while the
ophitic texture is the one which is most commonly developed in these rocks,
there is occasionally observed an imperfectly ophitic texture grading into
a more or less gi'anular texture, and this will be referred to later as
evidence in favor of the close relationship of the rocks of these sills with
the gabbro.
Constituents. — The rocks are very fresh, and the constituents of them
are, in order of prominence, augite, feldspar, titaniferous magnetite, then
brownish-green hornblende, and some brown biotite. The biotite is occa-
sionally present in sufficient quantity to warrant our speaking of the rocks
as mica-dolerites. The only secondaiy mineral, observed is a light-green
amphibole derived from the uralitization of the augite. The rocks are
normal dolerites, as shown both by their mineralog'ic composition and
textures.
RELATIONS OF THE GABBRO TO ADJACENT FORMATIONS.
Within this district the gabbro lies adjacent to the following formations,
given in order of age from below up:
The Ely greenstone (Archean).
Lower Hiu'onian sediments: Ogishke conglomerate and Knife Lake
slates.
Giants Range, Snowbank, and Cacaquabic granites.
Upper Huronian sediments: Grunflint and Rove formations.
Relations to the Ely greenstone. — The gabbro cuts across the greenstone
anticline which occurs to the east of Disappointment Lake, and is also
in close proximity to but not in absolute contact with the greenstone
of the Twin Peaks anticline on the southwest side of Gobbemichigamma
Lake. Li both of these cases the greenstone has been metamorphosed by
the gabbro, showing conclusively the relative epoch of formation of the
two rocks.
THE KEWEENAWAN. 407
Relations to Lower Huronian sediments. — At a number of places which
may be seen by reference to the maps in the athas (Pis. XV, XVI) the
gabbro is in contact with the Lower Huronian sediments, and in all cases
where the contacts have been studied the sediments have been found to
have been extremely altered as the result of their proximity to the gabbro.
Minerals have been produced in these sediments which are in some cases
closely related to, in others identical with, the minerals occurring in the
gabbro itself. The quantity of these minerals increases as the gabbro
is neared, and all evidence points unquestiouably to the intrusion and
metamorphism of the sediments by the younger gabbro.
Relations to Giants Range granite. — The Giants Range granite and the
gabbro occur together in the vicinity of the Kawishiwi River, and their
relations are disclosed by a dike of gabbro, which is found cutting this
granite just to the south of the falls in sec. 19, T. 64 N., R. 9 W., showing
that the gabbro is younger than the granite.
Relations to Snowbank and Cacaquahic granites. — No contacts have been
found between the Snowbank granite and the gabbro, or between the Caca-
quabic granite and the gabbro. Since both of these granites are older than
the Upper Huronian sediments, which we know have been intruded by the
gabbro, the conclusion is evident that they must be older than the gabbro.
Relations to Upper Huronian sediments. — The gabbro is in contact, at a
number of places, with the Upper Huronian sediments, both the Gunflint
formation and the Rove slates. In all cases, metamorphism, which the
gabbro has produced upon these sediments as the result of its contact, offers
conclusive evidence that it is younger than they are.
Relations to the Keweenaivan. — Irving has placed this gabbro in the
Keweenawan chiefly as the result of his studies of it in Wisconsin," and
his later work sustained him in his views, as is shown by the fact that after
his studies in Minnesota* he still retained it in its same stratigraphic position.
This is not the place for a detailed historical review of the various
opinions which have been held as to the stratigraphic position of the
gabbro. Reference to the annual reports of the Minnesota survey will
show that the opinions entertained by the members of that survey as to its
o Geology of Wisconsin, Vol. IV, p. 171.
^The copper-bearing rocks of Lake Superior by E. D. Irving: Third Ann. Rept. U. S. Gaol. Sur-
vey, 1883, pp. 93-180. Mon. U. S. Geol. Survey Vol. V.
408 THE VEKMILION IRON-BEAKING DISTRICT.
relations to other rocks have varied greatly, although in the Final Report
(Vol. IV) it is regarded as of Keweenawan age, in general agreement
with the results obtained by others who have worked upon this problem.
Winchell" makes it the igneous base of the Keweenawan.
As the result of work done during the field season of 1900 fairly
conclusive proof has been obtained of the fact that the gabbro is in reality
intrusive in the volcanic Keweenawan, and consequently younger at least
than a portion of the Keweenawan. This intrusive relation was observed
just west of the west end of Brul^ Lake. Brule' Lake lies within a syncline
of Keweenawan lavas, bounded on the south and west by the Duluth gabbro
and on the north by the "red rock" of the Minnesota survey. From this great
gabbro mass at the west end of the lake an eastward-projecting tongue of
gabbro was traced into the volcanics. This tongue near the gabbro pos-
sessed the normal characters of the main gabbro mass, but to the east
it narrowed rapidly and its lithologic characters changed until where nar-
rowest, just before it disappeared in a topographic depression, it had graded
into a porphyritic rock of comparatively fine grain, with a selvage which
was very nearly basaltic. The lavas were upon both sides of this tongue.
The actual contact between the gabbro and the lava was not found, but
they were separated at one place by an interval of only about 1 foot,
and this was the place where the tongue showed its finest grain. The
connection of this tongue with the gabbro and the variation in grain in
the tongue seem to be very good evidence of the intrusive character of the
gabbro.
RELATIONS OF THE LOGAN SILLS TO ADJACENT FORMATIONS.
Relations to the Upper Huronian (^Animikie). — Up to the time of the
publication in 1893 of Lawson's* paper on the laccolitic sills of the
northwest coast of Lake Sujjerior, all of the earlier writers on the Lake
Superior region, with the exception of Irving" and Ingall'' had held the
sheets of basic rock which they observed intercalated between the Huronian
slates to be of volcanic origin — that is, surface flows interbedded with the
«Geol. and Nat. Hist. Survey of Minnesota, Final Kept., Vol. IV, 1899, p. 298.
''The laccolitic sills of the northwest coast of Lake Superior, by A. C. Lawson: Geol. and Nat.
Hist. Survey of Minnesota, Bull. No. 8, pt. 2, 1893, pp. 24-48 See this for references to writers giving
details of sills which can not be given in present paper.
«Mon. U. S. Geol. Survey Vol. V, 1883, p. 379.
''Geol. and Nat. Hift. Survey Canada, Ann. Kept., 1888, H, p. 25.
THE KEWEENAWAN. 409
sediments. lug-alls held that some of these masses were of intrusive origin.
Irving considered them all as intrusive.
Chauveuet, in his manuscript notes, refers to a dike which he places
with the Logan sills, cutting- the Animikie slates on Pig*eon River. This
rock is clearly an intrusive dike in the slates. The edges of the slates next
to the dike are much shattered and broken, as the result of this intrusion.
In the Minnesota" reports these sills are referred to as intrusions in the
Animikie slates, but are included in the description of the Animikie which
corresponds to our Upper Huronian Series.
Lawson* asserts that these sills are all intrusive, and fortifies his state-
ment by proof which seems to be unquestionable, as the following- quota-
tions of the summary of his argument from his paper will show:
I. The trap sheets associated ^vith the Animikie strata are not volcanic flows,
because of the combination of the following- facts:
1. They are simple geological units, not a series of overlapping- sheets.
2. They are flat with uniform thickness over areas more than 100 square miles
in extent, and, where inclined, the dip is due essentially to faulting and tilting.
3. There are no pja-oclastic rocks associated with them.
•i. They are never glassy.
5. They are never amygdaloidal.
6. They exhibit no flow structure.
7. They have no ropy or wrinkled surface.
8. They have no lava-breccia associated with them.
9. They fcame in contact with the slates after the latter were hard and brittle,
and had acquired their cleavage; yet they never repose upon a surface which has
been exposed to subaerial weathering.
II. They are intrusive sills, because of the combination of the following facts:
1. They are strictty analogous to the great dikes of the region: (a) In their
general relations to the adjacent rocks, and in their field aspect; (i) in that both the
upper and lower sides of the sheets have the facies of a dense aphanitic rock, which
grades toward the middle into a coarsely crystalline rock.
2. They have a practicallj^ uniform thickness over large areas.
3. The columnar structure extends from lower surface to upper surface, as it
does from wall to wall in the dikes.
rt. They intersected the strata above and below them after the latter had been
hard and brittle.
6. They may be obsei-ved in direct continuity with dikes.
6. They pass from one horizon to another.
"Geol. and Nat. Hist. Survey of Minnesota, Final Kept., Vol. IV, 1899, pp. 476; 487-488.
*Lawson, loc. cit., p. 29.
410 THE VERMILION IRON-BEARING DISTRICT.
8[7]. The bottom of the sedimentary strata above them, wherever it is observable,
is a freshly ruptured surface.
9[8]. Apophyses of the trap pass from the main sheet into the cracks of the slate
above and below.
10[9]. The trap sheets, particularh' at the upper contact, hold included fragments
of the overlying slates.
11[10]. They locally alter the slates above and below them."
The writer does not believe that the faulting- sug-gested in I, 2, above,
explains the structure of the rocks, as no evidence in favor of this faulting-
has been observed. Facts have been observed in the Lake Superior region
which are corroborative of Lawson's other statements in all their essentials,
and confirm his conclusions.
Belations of sills to Keweenawan. — Lawson'' goes still further, and draws
the important conclusion from evidence observed during- his field work that
these sills are not only later than the Animikie sediments, but are intrusive
in the Keweenawan, and states it as his opinion that they are identical in
age with many of the heavy sheets of dark diabase or gabbro which prevail
on the Minnesota coast, particularly in the eastern portion. He therefore
places the sills as post-Keweenawan, and possibly of Silurian age. The
writer dissents altogether from the idea that these sills and dikes can be
post-Keweenawan, since nowhere in the Lake Supei'ior region have the
Cambrian rocks been found to be cut by dolerite sills and dikes, or indeed
by any intrusives. While the sills may be younger than part of the
Keweenawan, there is no reason for supposing them younger than all of
the Keweenawan, but they may belong to the same period of igneous
activity and be merely one of the later expressions of this activity, when
the molten magma was too deeply buried to reach the surface freely as
flows, and was intruded between the sediments of the Animikie and the
lava sheets of the Keweenawan wherever conditions were favorable. Thus
considered they would be of Keweenawan age instead of post-Keweenawan,
as has been supposed by Lawson.
Belations of the gabbro to the Logan sills. — The question of the relation-
ship of the gabbro to the Logan sills has been discussed by Grant, who
states that he "is inclined to separate the sills from the gabbro, but admits
that this separation is not proven." The various facts which he considers
"Lawson, loi:. cit., pp. 44-45.
i Geol. and Nat. Hist. Survey of Minnesota, Bull. No. 8, 1893, p. 47.
THE KEWEENAW AN. 41 1
as evidence" against the relationship of the two kinds of rocks he summarizes
as follows:
1. The sills are considerably altered, i. e., the pyroxene has usually largely
been replaced by secondary nainerals, while the gabbro is usually fresh and the
olivine as well as the pyroxene is usually unaltered.
2. The sills are essentially nonolivinitic; at least traces of olivine, even when
altered, are not common. The gabbro is normallv olivinitic.
3. The sills are quite rich in ferromagnesian minerals, giving a dark-gra}' or
black color to the rock. The gabbro is usually rich in feldspar and rather poor in
ferromagnesian minei'als, and the rock is light gray in color. When a basic mineral
predominates, it is mostly iron ore, which is not the case with the sills.
i. The sills are in structure ophitic; the gabbro is granitic. This holds true
also of the coarsest-grained sills and of the finest-grained gabbro. In this connection,
it might be well to mention some sills in the Animikie at Akeley Lake in the Akeley
Lake plate; these are apparently of gabbro. They are fine grained at their edges,
but even here the structure is more nearly that of the gabbro, and not that of the
ordinary sills.
5. The sills are very fine grained, almost glassy at the lower and upper sides,
even in the thickest sills. The gabbro is not very fine grained at the contact with
the Animikie rocks, and even on the edges of the apparent gabbro sills mentioned
above, the fineness of the grain nowhere approaches that of the edges of the ordinary
sills.
6. The sills, even the largest ones, have macroscopically altered the Animikie
rocks for only a very few feet, or even inches, from the contacts, while the
metamorphism of the Animikie at the gabbro contact is profound, extending for a
distance of several rods.
7. The gabbro and sills Tiave not been traced together, neither have they been
found in contact. In the map the sills have not been shown in contact with the
gabbro; this is on account of lack of exposures. They may, of course, come into
actual contact with the gabbro.
8. Where the sills and the gabbro come nearest together the two rocks are easily
distinguished, even in the field. The few specimens about which there is question
have been, as far as examined microscopically, easily referred to one or the other.
In closing his summary Grant says that he is incliaed to the idea that
these sills are of earlier date than the gabbro.
Several 37'ears before the publication of the above statement Lawson
expressed his opinion * that the sills are identical with many of the heavy
sheets of dark diabase and gabbro which prevail on the eastern part of the
Minnesota coast, and which are associated with the Keweenawan. The
aGeol. and Nat. Hist. Survey of Minnesota, Final Kept., Vol. IV, 1899, p. 488.
i Geol. and Nat. Hist. Survey of Minnesota, Bull. No. 8, 1893, p. 47.
412 THE VERMILION IRON-BEARING DISTRICT.
writer is prepared to go much farther than Lawson, and to express the
opinion that these sills are the equivalent in age of the Duluth gabbro of
northeastern Minnesota, and even that they were derived from the same
magma from which it was derived. In the following paragraphs Grant's
statement, as quoted above, will be discussed and evidence will be presented
to support the above-mentioned view of the intimate relationship of the sills
and gabbro. This evidence was collected partly during work on the Ver-
milion district and during several reconnaissance trips into the Keweena-
wan gabbro from the Vermilion district, in the course of a portion of a
season's field work on the Keweenawan of Minnesota and Canada, made in
1900.
No. 1. The sills are considerably altered, i. e., the pyroxene has usually larg-ely
been replaced by secondary minerals, while the gabbro is usualh' fresh and the
olivine as well as the pyroxene is usually unaltered.
The facts are essentially coiTect, and no explanation can be ofi'ered for
this difference, unless it is that it results from the sills being of smaller
mass and intercalated in the slates and having been exposed in consequence
to a more energetic action of water than has the gabbro. The gabbro
disintegrates very easily, and even when the state of aggregation is such
that the rock can be easily crushed in the hand the constitutents are rela-
tively fresh. This gabbro, as a result of this readiness to disintegrate, has
had its outer disintegrated portion removed by glacial action and water
erosion. In general, erosion has kept pace with the disintegration of the
gabbro. Hence the rock which we now observe is very fresh. The sills,
on the other hand, are very much more resistant. All those examined
were fairly fresh and exceedingly hard and tough. If one could get a
specimen from the rocks of the sills deep down in the mass, doubtless the
rock would be found to be about as fresh as the ffabbro.
to"-
2. The sills are essentially nonolivinitic; at least traces of olivine, even when
altered, are not common. The gabbro is normally olivinitic.
This is also true. But one must not neglect the fact that locally the
gabbro is also practically free from olivine. The local absence of olivine
in the gabbro is due to conditions of crystallization of the gabbro magma
and possibly slight chemical differences also, and just in the same way can
one explain the absence of olivine from the sills. No analyses have been
THE KEWEENAWAN. 413
made of the various facies of the gabbro to determine the chemical differ-
ences which may exist between them, nor have analyses been obtained of
the rock of the sills to prove its identical chemical composition with the
gabbro as a whole or with any of its special facies.
3. The sills are quite rich in ferromagnesian minerals, giving a dark -gray or
black color to the rock. The gabbro is usually rich in felds2:)ar and rather poor iu
ferromagnesian minerals, and the rock is light gray in color. When a basic mineral
predominates, it is mostly iron ore. which is not the case with the sills.
The writer does not concur in this statement. The fact that the sills
are generally darker than the gabbro is due largely to tlie finer grain of the
sill rocks. When, however, the sills are very coarse grained one finds
patches that are made up almost exclusively of feldspar in large individuals
and such areas iu the sills are a very light gray and become, when the feldspar
is kaolinized, nearly snow white. On the other hand, some of the sections
from the sills which have been examined are made up largely of magnetite,
with pyroxene second in abundance, and last, the feldspar. This proportion
of minerals is not the rule in the sills, but neither is it the rule in the gabbro,
as witness the light-gray anorthosite with but little of the ferromagnesian
compounds.
4. The sills are in structure ophitic; the gabbro is granitic. This holds true also
of the coarsest-grained sills and of the linest-grained gabbro. In this connection it
might be well to mention some sills in the Animikie at Akeley Lake, in the Akeley
Lake plate; these are apparentl}' of gabbro. They are fine grained at their edges,
but even here the structure is more nearly that of the gabbro and not that of the
ordinary sills.
The writer must disagree with the above statement of the texture of the
rocks, as his own studies have shown that while the gabbro is predomi-
nantly granitic, nevertheless the ophitic texture is very frequent in the
finer-grained facies. On the other hand, the sills show, in places where the
rock is as coarse as is the finer-grained gabbro, a distinctly ophitic texture,
grading into an imperfectly granular one, and from this down into very
fine-grained intersertal textured basalts. A porphyritic texture which was
not observed in the g-abbros is common in the sills. These facts indicate a
textural gradation between the gabbro and sills, the differences in general
textural characters being due to the difference in the conditions between
the slow crystallization of an enormous mass of magma, as in the case of
414 THE VERMILION IRON-BEARING DISTRICT.
the gabbro, and of a relatively small mass and quick cooling and crystal-
lization, as in the case of the sills. This difference in the rapidity of cool-
ing is shown also by the porphyritic structure of the rocks of the sills and
the fine-grained rock occurring upon the edges of the sills. The relative
rapidity of the cooling is further indicated by the feldspar pheuocrysts
which occur in the sills. These very commonly reach a length of 2 inches,
with a maximum length of 4 inches. Had the magma cooled under the
conditions under which the phenocrysts were formed, obviously the rest of
the constituents would have also attained much larger size than they did,
and the resulting rock would have been coarse grained and doubtless as
granular as any of the gabbro.
But let us not neglect the evidence presented in the above-quoted state-
ment itself — that offered in the statement that there are sills at Akeley
Lake which are apparently of gabbro and not, like the other sills, different
from the gabbro. Let us attempt to conceive of the conditions under which
these sills were formed. They were injected into the sediments only a short
distance away from the edge of the main gabbro mass, and have been traced
parallel with this edge for a number of miles. Every observer of the rocks
in which these sills lie intercalated states that the rocks are those which show
the most extreme effects of the gabbro contact action. They are metamor-
phosed so that it is nearly impossible to determine their original character.
Evidently they were exposed to the high temperature of the adjacent gab-
bro for a long time, and the magma of the sills must have profited by this
high temperature of the parent mass of magma and the heated sediments
when it was intruded, and cooled more slowly than it otherwise would have
done, and much more slowly than the sills farther from the parent mass.
Hence the sill rock approaches in its texture much more closely that of
the parent mass of gabbro. No reason can be seen why these sills should
be connected with the gabbro as gabbro sills and separated from the other
sills in their vicinity, and even farther away, which occur under practi-
cally identical conditions, and show but relatively unimportant petrographic
differences from them.
5. The sills are very fine grained, almost glass_y at the lower and upper sides,
even in the thickest sills. The gabbro is not verj' fine grained at the contact with
the Animikie rocks, and even on the edges of the apparent gabbro sills mentioned
above, the fineness of grain nowhere approaches that of the edges of the ordinary sills.
THE KEWEENAW AN. 415
This difference in grain of the rock on the edge of the sill and of that
on the edge of the main mass of the gabbro is the difference which normally
occurs where there is such a great discrepancy in the size of the masses as
there is in the case under consideration. Witness, for example, the character
of the rock at the edge of any very large granite massive and that upon
the edge of the dikes radiating from it. Undoubtedly, however, the gabbro
is materially finer grained on the periphery of the mass than it is in the
interior, and there is an imperceptible gradation between the fine and
the coarse facies. Moreover, the border facies of the gabbro is about of
the same degree of coarseness or possibly even less coarse than the rock
occui'ring in the interior of the thickest sills. From the nature of the
occurrence one should not expect the border of the gabbro to be as fine as
the rock upon the edge of the sills.
6. The sills, even the largest ones, have macroscopically altered the Animikie
rocks for only a very few feet, or even inches, from the contacts, while the meta-
morphism of the Animikie at the gabbro contact is profound, extending for a distance
of several rods.
The relative intensity of the metamorphic action of the gabbro and sills
depends largely, as does the size of the grain of the rocks, upon the masses
of the magma, since this influences the rate of cooling. Of necessity a small
mass of rock like a sill would have less effect upon the sediments than
the gabbro.
One is better prepared to appreciate the difference in the effect of the
sills and main gabbro mass when one thinks that the thickest sills are only
about 400 feet thick, and that these are utterly insignificant when compared
with the Dviluth gabbro mass which covers about 2,400 square miles in
Minnesota."
7. The gabbro and sills have not been traced together; neither have thej^ been
found in contact. In the map the sills have not been shown in contact with the
gabbro; this is on account of lack of exposures. Thej' may, of course, come into
actual contact with the gabbro.
It is true that no actual contacts of gabbro and sill ha.ve been
observed, although they have been seen separated only by a short distance.
At one place on the Duluth, Port Arthur and Western Railroad, between
"The geology of the Keweenawan area in northeastern Minnesota, III, Pt. II, Geology of the
Keweenawan series, by A. H. Elftman: Am. Geologist, Vol. XXII, 1898, p. 132.
416 THE VERMILION IRON-BEARING DISTRICT.
the first and second trestles east of Paulson's mine, there is a small mass of
crumpled, much metamorphosed black slate, with the main mass of the
gabbro to the south, and a small mass of gabbro to the north. This slate
is considered by Grant to be an inclusion in the gabbro. The writer is
inclined to think this slate is caught in the fork between the main gabbro
mass and a sill which is an offshoot from it. Lack of exposures prevented
the tracing of the connection between them. It must be said in this
connection that in our work on the Vermilion iron district this relation
between the gabbro and the sills was considei'ed a problem of importance
secondary to that of mapping the iron-bearing formations, and no especial
attempt was made to trace the relation between these rocks in the field. It
is believed that this connection would be shown to exist if the edge of the
gabbro and the adjoining Auimikie area were mapped in detail.
In this connection also it seems that the fact that the srabbro and sills
have not thus far been found in contact cotild be used as evidence more
strongly against the sills being older than the gabbro, as Grant" suggests,
since the gabbro is found in contact with every other rock in the district
which has been proved to be older than it is. If the sills are older than
the gabbro, they would form the one exception.
8. AVhere the sills and the gabbro come nearest together the two rocks are
easily distinguished, even in the field. The few specimens about which there is
question have been, as far as examined microscopically, easity referred to one or the
other.
Exception is taken to this statement, and the reader is referred to
section 4 above, where certain sills are mentioned in these words:
In this connection it might be well to mention some sills in the Animikie at
Akeley Lake in the Akeley Lake plate; these are appai'eutl}' of gabbro. They are
fine-grained at their edges, but even here the structure is more nearly that of the
gabbro, and not that of the ordinary sills.
The rock of the sills can in general be readily distinguished from that
of the main gabbro mass, but so can the dike rocks of rhyol-ite, etc., from
the coarse granite of the^massives from which demonstrably the dikes are
offshoots. This difference is not evidence of importance against the rela-
tionship of the sills and of the gabbro.
«Geol. and Nat. Hist. Survey of Minnesota, Final Rept., Vol. IV. 1899, p. 4SS.
THE KEWEENAW AN. . 417
CONCLUSIONS AS TO AGE AND RELATION OF THE GABBRO AND SILLS.
In conclusion, there is presented the following summary of the facts
observed which seem to indicate the true relationship between the gabbro
and the Logan sills and their correct stratigraphic position. The gabbro
and the rock of the sills are petrographically the same and textural gTada-
tions have been observed which indicate their close relationship. The
gabbro, while predominantly coarse-grained and granular, is locally fine-
grained and poikilitic, and in one place was found as a dike in the Kewee-
nawan and there graded into a porphyritic facies and even into a fine-
grained ophitic dolerite. The rock of the sills is in places, in the midst
of the thick sills, a good granular gabbro in texture, and ranges from this
through ophitic and poikilitic-textjired dolerites into fine-grained aphanitic
intersertal-textured basalts upon the selvage. Mineralogically they are the
same, excepting that in the relatively few specimens from the sills which
have been studied no olivine nor hypersthene has been observed, nor do
the sills show such great miueralogic variation from titaniferous magnetite
rocks to enormous anorthosite masses, although there ai-e small anorthosite
masses in the sills. Such differences in variation are, however, easily
explicable as due to the enormous difference existing between the masses
of magma forming the gabbro and that forming the indi-\ddual sills.
It is admitted by all that both the gabbro and the sills are younger
than the Upper Huronian, since they both have been observed at numbers
of places to cut the rocks of this age. The point of difference is the rela-
tionship between the gabbro and sills and the Keweenawan. The writer
found the gabbro cutting a portion of the Keweenawan rocks, whose exact
stratigraphic position with relation to the remainder of the Keweenawan is,
however, not known, and hence considers it and the sills as younger than
some of the Keweenawan.
The gabbro is believed to be a great laccolitic mass which in general
follows approximately the contact plane between the Animikie series
(Upper Huronian) and the Keweenawan. In the Vermilion district there
are local departures from this which will be described in the following-
paragraph.
Over a great part of the southern edge of the Vermilion district the
gabbro followed essentially along the plane between the Upper Huronian
(Animikie) and the lower lying sediments, uplifting thereby the Upper
MON XLV — 03 27
418 THE VERMILION IRON-BEARING DISTRICT.
Huronian sediments, for at several places on the edge of the Vermilion dis-
trict and just south of it are found isolated patches of the lowest part of
the Guuflint formation (Animikie) included in the Keweenawan gabbro.
The gabbro in this part of the district may originally have been completely
covered by the Animikie series, but in this part of the district, also, the
rocks are much more closely folded than in the east near Gunflint and in
the west in the Mesabi district, and as a result of the erosion in this area,
where the rocks have been folded and fractm-ed, all of the Upper Huronian
(Animikie) but the few included patches has been removed.
In the eastern part of the Vermilion district the gabbro began to rise
and cut across the Upper Huronian, reaching higher and higher beds to the
east, and then spread out essentially along the plane between the Upper
Huronian (Animikie) and the base of the Keweenawan, sending sills and
dikes into the Rove slates of the Upper Huronian and also into the
Keweenawan rocks, as can be seen on Brule Lake.
The writer is inclined to believe that the gabbro is a great basic
igneous mass which represents a basic differentiation product of a magma
from which perhaps the major portion of the Keweenawan volcanics were
derived, and which basic magma has, perhaps, as its complementary acid
rocks, the great mass of intrusive "Red Rock" and the related rhyolite
flows of the Keweenawan.
METAMORPHIC EFFECT OF GABBRO AND SILLS.
The effect of the gabbro upon the various rocks with which it is in
contact has been considered under the description of those various rocks,
for example, under the Ely greenstone, the Rove slates, etc., but a brief
summary statement will be made here concerning the character of this
metamoi'phism. The most noticeable general effect of the gabbro has
been to produce a very large quantity of rich brown biotite in the rocks in
contact with it.
Archean {Ely) greenstones. — In the meta-dolerites and meta-basalts
(greenstones) the general effects are much the same as in the case of the
sediments. The ophitic texture of those greenstones which have been
studied is still retained with a fair degree of distinctness. There is, how-
ever, a tendency to gradually destroy the texture and produce granular
rocks therefrom by the production of large quantities of biotite, with
THE KEWEENAW AN. 419
liyperstheue, augite, brownish-green hornblende, and magnetite, and
possibly the recrystallization of the feldspar.
Lower Huronian. — In the Lower Huronian sediments this production
of biotite has been accompanied by a recrystaUization of the minerals of
the rock, whereby the sedimentary structures are mostly destroyed and
mica-schists and gneisses are produced. Immediately along the edge of
the contact with these schists in the metamorphosed sediments occur large
quantities of ferromagnesian minerals, such as augite, hypersthene, and
brown hornblende. In the case of the conglomerates we find, since there are
a number of different kinds of pebbles, and these differ from the matrix in
which they lie, that the pebbles when affected furnish a somewhat different
product from the matrix, and this product usually stands weathering better
than the matrix, so that the pebbly character of the conglomerate is retained.
Upper Huronian (Gunflint formation). — Where the gabbro has been in
contact with or very near the rocks of the iron-bearing Gunflint formation,
it has affected them in a very marked way, and has produced magnetite
ores interlaminated with bands of qiaartz and a rock composed of olivine,
hypersthene, augite, hornblende, and magnetite, with quartz in varying
proportions. The rocks thus produced are different from any original
igneous or sedimentary rocks with which the writer is acquainted.
Bove slates. — The metamorphism of the Rove slates by the gabbro
has been such as to produce, from rocks consisting of angular grains of
quartz, feldspar, and plates of chlorite, and a dark interstitial material,
more or less completely crystallized mica-schists, or, with larger percentage
of feldspar, mica-gneisses, if these terms can be applied to rocks that have
an essentially granitic texture, although the original banding of the sedi-
ments remains in the recrystallized rock and causes it to break more readily
along these bands than in any other dir«iCtion.
The action of the sills is very slight upon the slates, in most cases
producing rocks which are mica-schists, but not so completely recrystallized
as in the case of those affected by the gabbro.
Endomorphic action — There is, along the margin of the gabbro, a
somewhat finer grained facies than further in the mass. The sills also
show distinct selvages, but neither the gabbro nor the sills appear to have
had their textures or general characters otherwise modified as a result of
contact with the various rocks mentioned.
420
THE VERMILION IRON-BEARING DISTRICT.
IRON-OXIDE BODIES IN THE GABBRO.
It has been known for a long time that there were iron-oxide masses-
in northeastern Minnesota within the area which is underUxin by the great
Keweenawan gabbro mass. One of these, which occurs on Mayhew Lake,
was seen and described by Chauvenet in 1883 and 1884," and since that
time there have been a number of brief mentions made of these gabbro
iron-oxide masses in the various papers on the geology of Minnesota to
which reference has been made in this chapter. The iron oxide occurs in
bodies of varying size and the existence of these bodies has aroused a
considerable interest among investors.
CHARACTER, OCCURRENCE, AND ORIGIN OF THE IRON-OXIDE BODIES.
These bodies consist of titaniferous magnetites, and have, according to
H. V. Winchell,* the following composition:
Analyses of iron-oxide bodies.
Constituents.
Silica, SiO,
Alumina, AUOj ,
Titanium binoxide, TiO™
Magnetic oxide of iron
Protoxide of iron
Chromium sesquioxide, CrO
Magnesia, MgO
Lime, CaO
Phosphoric acid
2.02
2.68
12.09
80. 78
2.40
Metallic iron.
Tr.
.03
100. 00
58.48
II.
20.90
1.75
2.23
70.29
2.01
2.63
Tr.
None.
99.81
52. 46
In view of the fact that a great deal of money has been spent in this
district in exploring for nickel ore, which has been reported as occurring in
the titaniferous magnetites in the gabbro, the following- statement by Grant
is valuable:
Nickel does occur in similar situations,'' and is frequently found as an impurity
ill one of the gabbro minerals (olivine), but as yet no reliable reports of nickel ore
" W. M. Chauvenet, U. S. Geol. Survey, manuscript notes.
''Geol. and Nat. Hist. Survey of Minnesota, Bull. No. 6, 1891, p. 141.
'■This evidently has reference to its occurrence in other districts; not in Minnesota, however.
Clements.
THE KEWEENAWAN. 421
rich enough for mining- have been received. A determination of the niekel in iron
ore from the locality of the specimens above analyzed showed less than one-half of
one per cent/'
The iron-oxide bodies occur in the midst of massive granular normal
gabbros, and are not separated from the gabbro by sharp lines. The titan-
iferous magnetite body on the margin of any outcrop next the gabbro is
lean, and has a large projjortion of pyroxene, olivine, and feldspar, in
general of gabbro minerals, mixed with the titaniferous magnetite. These
minerals become fewer toward the main magnetite mass, but in the direction
of the gabbro they increase in quantity, first giving varieties of highly
mtignetitic gabbro, but gradually passing- into the crystalline gabbro, with
black and gray mottlings. There is then a transition from the gabbro into
the titaniferous magnetite bodies, which are but very magnetitic portions of
the gabbro.
The high percentage of titanium in the magnetite bodies, as shown by
the above analyses, is very important from both the economic and the
scientific standpoint. In the first place, the titanium renders the magnetite
at present valueless, since, in the present iron-smelting practice, titaniferous
ore can not be smelted economically. The magnetites can, therefore, not
compete now with the cheap nontitaniferous ores, nor can they in the
future, unless new discoveries give a higher value to titaniferous ores, or
cause changes in iron smelting which will place the titaniferous ores on an
even basis vsdth the other iron ores.
The injurious effect of the titanium in rendering the magnetite unmar-
ketable would, of course, apply to these titaniferous magnetite bodies, what-
ever their size. However, so far as we know, up to the present time no
published description has been given of any large continuous masses of the
practically pure titaniferous magnetite.
The content of titanium is of interest from a scientific standpoint, in that
it gives evidence (additional to that offered by the occurrence) of the intimate
connection between the gabbro and the ore, and enables us to determine its
source. The gabbro contains everywhere titaniferous magnetite in small
quantities, and the large amount collected in these magnetite bodies owes
its accumulation to those little understood processes generally spoken of as
processes of segregation. As the result of these processes the titaniferoiis
aLoc. cit., p. 62.
422 THE VERMILION IRON-BEARING DISTRICT.
magnetite elsewhere widely disseminated in small quantities through the
gabbro was locally concentrated, while the magma was in a more or less
fliiid state. We conclude that these titaniferous magnetite bodies belong to
those ii'on-ore deposits of igneous origin, which are at present of little value.
All of the other ore deposits of this distiict, even when now magnetitic, as
in tlie case of the Gunflint ores, are nontitaniferous, or contain only traces
of titanium, and were originally of sedimentary origin.
SECTION II.— ACID DIKES YOUNGER THAN THE DLTIilTTH GABBRO.
The great Keweenawan gabbro mass is cut through at various places
by small dikes of red granite. Several of these were seen beyond the
limits of the Vermilion district. One especially was noted upon the island
on the east side of Cross River, just opposite the bay into which the portage
from Snipe Lake enters. This dike is 3 feet in width and trends north and
south. It is a fi-esh biotite-granite. Grrant" reports a granite dike cutting
thi'ough the gabbro on an island in Gobbemichigamma Lake in the NW. ^
of the NE. ^ of sec. 6, T. 64 N., R. 5 W. These occun-ences are sufficient
to prove that there is a g-rauite later than the gabbro.
South of the Vermilion district there are large areas underlain by a
bright-red weathering acid rock, varying from syenite to granite, which sends
off shoots into the adjacent gabbro. No attempt has been made to trace
the connection between the granite dikes mentioned above and these lai'ger
masses of "red rock" occurring in the midst of the Keweenawan. The
possibility of their close relationship is suggested, however.
SECTION III.— BASIC INTRUSIVES YOUNGER THAN THE DULUTH
GABBRO.
At numerous places basic dikes have been noted in the gabbi'o. These
were found as the result of the limited studies made upon the gabbro.
These studies were confined chiefly to the margin of the gabbro and to a
few excursions made within the Duluth gabbro mass. Unquestionably great
numbers of other dikes of similar character would have been found had the
gabbro been more closely examined. A number of basic dikes were also
found in the Upper Huronian sediments and in the older rocks of the Ver-
milion district, both tlie eruptive and sedimentarv ones. These dikes corre-
spond in every detail with those found in the gabbro. All of these very
oGeol. and Nat. Hist. Survey of Minnesota, Final Kept., Vol. IV, 1899, p. 479.
THE KEWEENAW AN. 423
fresh rocks are basalts or dolerites, and are believed to belong to the same
general period of formation.
The distribution of these dikes can be seen on the maps in the accom-
panying atlas, where those of sufficient size to warrant it have been shown.
The majority of the dikes are small and have been omitted in most cases,
although a few have been inserted upon the map but are greatly exaggerated.
These dikes cross the strike of the slate and other sediments and are also
found to run parallel to their bedding. In some of the schistose rocks
the dikes very clearly followed the schistosity.
The dikes can not be said to follow a definite system in their occur-
rence, although presumably if they were studied in sufficient detail it would
be found that they followed in general the lines of fracture which prevail
in those portions of the district in which they occur.
PETROGRAPHIC CHARACTERS.
Macroscopic characters. — The rocks are invariably dark colored, black
or greenish or brownish black. The rocks in the smaller masses, the
narrower dikes, show a very fine grain and could properly be called basalt.
The rocks in the centers of the larger dikes show up as coarse-grained
ophitic textured rocks and are dolerites. However, gradations between the
fine- and coarse-grained forms are shown in that the finer-grained phases
appear upon the margins of the large dikes grading up by increasing size of
mineral constituents to the coarse-grained dolerites, occupying the centers
of the large dikes. The fine-grained basalts seem to predominate.
Microscopic characters. — Under the microscope, the rocks are found
to be very fresh, as one would infer fi-om the fact that they are usually
decidedly black. In some few of them slight alterations producing greenish
or brownish-green minerals tend to vary these to the brown or greenish
tones already referred to. The constituents of the rock are yellowish to
violet augite, green hornblende, a feldspar near labradorite in composi-
tion, olivine, apatite, ilmenite, and magnetite. These minerals show their
normal characters and but rarely give evidence of being altered. Where
altered there has been produced chlorite, epidote, calcite, and hornblende.
The ophitic texture predominates in these rocks, although the intersertal
texture is also common and merges at places into an imperfect fluidal
texture brought out by the parallel arrangement of the feldspars. Some of
the rocks are porphyritic.
424 THE VERMILION IRON-BEARING DISTRICT.
RELATIONS TO ADJACENT FORMATIONS AND AGE.
These basic rocks wherever found are clearly intnisive in the rocks
which surround them. Hence, in all cases the rocks are younger than those
in which they occur. These dikes cut all the rocks of the district from the
Archean up to and including the Keweenawan gabbro, but have not thus
far been found cutting the granite dikes which were found in the gabbro in
the Vermilion district. However, in the Keweenawan area south of the
Vermilion district similar basalt dikes have been seen cutting through the
syenite and granite massives referred to above, and it is strongly probable
that these dolerite dikes are of the same age. Since, then, these intrusives
cut all other rocks of the district they must be the youngest of the pre-
Cambrian rocks which are represented.
METAMORPHIC EFFECTS.
In several instances where the contacts between the Rove slates and
the dikes were exposed, observation showed that the slates were indurated
in the vicinity of the dikes, this induration diminishing as the distance
from the dikes increased. Special observations would be required to deter-
mine accurately the character of the metamorphism which produces the
induration. These examinations were not waiTanted by the amount of time
available for the study of the district, consequently the details of this change
must be left for future studies. In all probability this change consists in
the recrystallization of the quartz and feldspar and the production of biotite,
as is found to be the case in the contacts of the Logan sills and dikes upon
the same slates.
CHAPTER VII.
THE DRIFT.
The Vermilion district, like all the rest of the Lake Superior region, was
overridden by the great ice sheets of Glacial time. The tendency of the
glacial action during the advance of the ice sheet was, of course, to reduce
this district to a general level, to round the hills, and produce strise upon
the rock surfaces, thus marking the direction of its movement. In its retreat
this process of leveling was continued by the filling in of the pre-Glacial
valleys with morainal deposits. While it is known from the researches of
the glacialists that the Lake Superior region, and of course that part of it
here discussed, was covered several times by ice sheets, we can recognize
in the Vermilion district the effects of the ice only during the last or Wis-
consin stage of the Glacial epoch, to which consequently belong all of the
deposits which will be briefly described.
According to Prof. J. E. Todd, the glacial deposits of northeastern
Minnesota can be refen-ed "to two great lobes of the ancient ice sheet, a
shorter one moving southwest through the Lake Superior Basin, an,d a longer
one moving ai'ound this from the noi^theast to the west and southwest.""
In order that the reader may get a clear idea of the glacial history of
the Vermilion district, it may be well to make some general statements
concerning the glacial history of that portion of Minnesota which is adjacent
to but outside of the district considered in this paper. Extending- in an
approximately northeasterly direction through northeastern Minnesota there
is a height of land which forms the watershed between the hydrographic
basins of Lake Superior on the south, belonging to the continental St.
Lawrence basin, and between the basin of the Rainy River and Lake of the
Woods, wliich belongs to the great Hudson Bay basin on the north. This
a A revision of the moraines of Minnesota, by J. E. Todd: Am. Geol., Vol. XVIII, 1896, pp.
22.5-226, a paper read at the August meeting of the American Association for the Advancement of
Science, before Section E, Geology and Geography. Also, Am. Jour. Sci., 4th ser., Vol. VI, 1898,
p. 473.
425
426 THE VEKMILION IRON-BEARING DISTRICT.
heig-lit of land may be subdivided into several minor ridges following the
general trend indicated. Dui-ing its greatest advance the Lake Superior
lobe and the ice lobe to the north, called by Elftman" the Rainy lobe, were
confluent. As the ice receded these lobes became separated, their -separa-
tion being determined by the high land just mentioned as lying north of
Lake Superior. For a time they were close together, forming an inter-
lobular moraine. As they receded and became more widely separated, each
formed independent moraines.' Between these moraines there occurred
V-shaj)ed areas, with the apex of the V pointing to the northeast, the arms
becoming more widely" separated as they are followed to the west.
No deposits of the Lake Superior lobe are known in the Vermilion
district. There has been recognized, however, and described by Upham,"
a great moraine deposited by the Rainy lobe, which has been named by
him the Vermilion moraine. Moreover, the records of strise collected by
Upham "* and inserted on his map in the same article show the direction of
the ice flow to have been in the main to the south-southwest, varying from
S. 10° to S. 50° W., and seem to corroborate Todd's division of the ice sheet
in this region into the two lobes as mentioned above. Since Upham's work
this Vermilion moraine has been further described and more accurately
delimited by Elftman.' No special study of the glacial deposits of the
Vermilion district has been made by the United Spates geologists, as the
work in this district was primarily undertaken with the object of deter-
mining the pre-Glacial geology. Nevertheless, observations made in the
course of the survey enable us to add a little to the knowledge of the course
in detail of this moraine. These observations have been made use of, and
accordingly the moraine has been traced as is shown on the accompanying
map (fig. 23. At the time of the formation of the moraine the ice in the
northeastern portion of the district passed over this east end of the Giants
range, and the moraine was deposited to the south of it. As we follow
« The geology of the Keweenawan area in northeastern Minnesota, by A. H. Elftman: Am. Geol.,
Vol. XXI, Feb., 1898, p. 108.
ftTodd, op. cit. Am. Jour. Sci., 4th ser.. Vol. VI, 1898, p. 473; Elftman, op. cit., pp. 90-108.
'■ Preliminary report of field work during 1893 in northeastern Minnesota, chiefl>' relating to the
glacial drift, by Warren Upham: Geol. Nat. Hist. Survey of Minnesota, Twenty-second Ann. Rept.,
1893, pp. 18-66.
'' Upham, op. cit., pp. 38—40.
f Elftman, op. cit, p. 94.
THE DRIFT.
427
the moraine westward we find,
however, that it gradually ap-
proaches the Giants range,
which has a trend to the south
of west, and crosses it in T. 63
N., R. 9 W. West of this point
the ice tongue reached a point
considerably farther south, and
it had sufficient strength to ad-
vance well up the slopes of the
range, but was never able to
surmount it. From here on
the moraine is found lying first
high up upon the northern
slope of the range, with the
distance increasing between it
and the range as we go farther
west. In the western portion
of the district it is nearly 15
miles north of the range.
Exceptionally heavy deposits
of drift are known to occur
in the following areas: Sees.
21-28, T. 61 N., R. 15 W., and
extending' from there on up to
the northeast in a belt approx-
imately 2 miles wide through
sees. 19, 30, 18, 29, 16, 21, 10,
15, 22, 2, 11, 14, 1, 12, T. 61
N., R. 14 W.; sec. 6, T. 61 N.,
R. 13 W.; sees. 14, 15, 22, 23,
T. 62 N., R. 13 W.; sees. 25, 26,
34, 35, 36, T. 63 N., R. 12 W.;
sees. 30, 31, T. 63 N., R. 11 W.;
sees. 20, 21, 16, 15, 14, 11, 12,
T. 63 N., R. 11 W.; sees. 3, 4, 6,
428 THE A^ERMILION IRON-BEARINQ DISTRICT.
8, 9, 10, T. 63 N., R. 10 W. The main moraine has been traced by cou-
necting up these heavy diift dejiosits. Its distribution can be seen in
lig. 23. In the eastern portion of the area the course of the moraine is
essentially that as given in Elftman's paper, referred to above, with but a
few minor changes. This main moraine is not, however, the only moraine
which is present in this district. There are several other areas which have
been observed in which a clearly developed kettle moraine topography is
present. Such a series of areas have been connected extending from near
the south shore of Vermilion Lake to the north of east, running approxi-
mately along the town line between Ts. 61, 62 N., Rs. 14, 1,5 TT., and
then extending to the northeast across Eagle Nest lakes into the western
poi-tion of T. 62 N., R. 13 W. This small moraine is for the greater part of
its extent nearly parallel with the main Vermilion moraine. In the east,
however, it approaches the moraine, and finally coalesces with it. It is
evidently a moraine representing one of the stages in the recession of this
lobe during which this extreme southwestern portion of the lobe in the
lower land retreated relatively much more rapidly than did the southeast-
erly margin of the ice.- Still another stage in the recession is represented
by morainal deposits extending from north of Armstrong Bay of Vermilion
Lake northeast through the northern portion of T. 62 N., R. 14 W., and the
southern portion of T. 63 X., R. 13 W., south of Burntside Lake. Still
other deposits were observed on Birch Point and on Pine Island, in Ver-
milion Lake. These could probably be traced to the noii;heast of Vermilion
Lake, but no sjjecial work having been done in this area the continuation
of these deposits is not known. Other terminal moraine deposits north of
the main Vennilion moraine are known to occur in the southern poi-tion of
T. 64 N., Rs. 9, 10 W.; in the southwestern portion of T. 64 N., R. 9 W.,
north of Moose and Newfound lakes; and in the southern part of T. 66 N.,
Rs. 5 and 6 W., to the northwest of West Grull Lake.
Over the remainder of the district the di-ift is comparatively thin, and
in fact in places only a few bowlders on the tops of the hills with occasional
strife and the rounded outline of the hills indicate the former presence of the
ice sheet. The thinness of this didft is especiallv noticeable in the eastern
l)art of the district. In the western portion of the district the general di-ift
mantle is much thicker than it is in the eastern poi'tion. The general effect
of this drift is, (.)f course, to cover all of the rocks, and in agreement with
THE DRIFT. 429
the distribution given above we find that in the western portion of the
district exposures of rock in situ are far less numerous on the whole than in
the eastern part, where the drift was apparently mucli thinner to begin
with, and where since the original jDre-Glacial relief was more marked, it
has to a considerable extent been removed from the crests of the hills by
post-Glacial erosion, which is there correspondingly more vigorous.
The width of the Vermilion moraine proper can not be given with any
very great degree of accuracy. It varies considerably, ranging from a
half mile, and perhaps even less, up to between 2 and 3 miles in T. 61 N.,
R. 14 W.
The depth of the drift constituting the moraine is also variable.
Test pits and drill holes have cat through it in a number of places to a
depth of 75 feet. It is, of course, in many places much thinner than this.
Judging from the very considerable irregularities noticed — for instance, in
the area southwest of Eagle Nest lakes, along the township line between
Ts. 61 and 62 N., E. 14 W., and in the moraine extending northeast-
southwest across T. 61 N., E. 14 W., and also in the area southeast of Ely —
it must run up to at least 150 to 200 feet in depth, and may even reach a
greater thickness than this.
Between the south edge of the moraine and the Giants range, especi-
ally in Ts. 60, 61 N., Es. 14, 15, and 16 W., extensive deposits of drift,
modified by the waters from the edge of the melting glacier, are well devel-
oped. To the north of the moraines in various places similar modified
drift is found, which evidently owes its origin to the modification by water
from the Eainy lobe after it had passed to the north of the Vermilion
moraine.
It will be remembered that the drainage of this district, which is
approximately bounded on the south-southeast by the high laud of the
Giants range, is to the northwest. It is to be supposed that as the glacier
retreated the waters from its melting edge must have been dammed between
this range and the ice lobe to the north. It seems highly probable, there-
fore, that glacial lakes of considerable size must have been formed in the
Vermilion district. A study of the topography shows that there are many
areas which must have been favorable for the development of such lakes,
but no definite evidence, such as lake beaches and clay deposits, pointing
to the existence of large glacial lakes, has been found. It seems highly
430 THE VERMILION IRON-BEARING DISTRICT.
probable, however, that such lakes existed on the site of what is uow Ver-
milion Lake, Basswood Lake, and Sagauaga Lake, and probably in many
other places. Indeed, Winchell and Grant* have reported terraced gravels
around Long Lake and White Iron Lake, which give clear evidence of the
existence of lakes at these places during Glacial time, when the water was
considerably higher than the present water level and which consequentlv
covered larger areas than those of the present lakes. WinchelP has recenth-,
since the above was written, given a brief description of the glacial lakes
which occur partially or wholly in the Vermilion district.
According to him. Lake Annamani covered the present site of Vermil-
ion Lake, being 10 or 15 feet higher than it is. Lake Norwood was south
of the present Vermilion Lake, and covered the low flat area north of the
Giants range along the Embarrass River.
«Geol. and Nat. Hist. Survey of Minnesota, Final Rept., Vol. IV, 1899, p. 235.
''Glacial lakes of Minnesota, by N. H. Winchell: Bull. Geol. Soc. America, Vol. XII, 1901, pp.
125-126.
CHAPTER VIII.
TOPOGRAPHY OF THE DISTRICT IN ITS RELATION TO
GEOLOGIC STRUCTURE.
The velationsliip of topographic rehef to geologic structural features
is 251'ohably nowhere better brought out than in the Vermilion district.
From the preceding pages the reader will have learned of the gabbro
plateau (pp. 37, 399) with its simple topographic features due chiefly to the
homogeneous character of the gabbro, the country rock, and of the peculiar
topography of the sawtooth hills area (pp. 38, 391, 400) due to the presence
of the sills intercalated between the beds of slates with their monoclinal
dip to the south-southeast. The descriptions which follow apply especially
to the broad area north-northwest of the Giants range and to the Giants
range itself
In these portions of the Vermilion district the rocks are closely folded
into synclines and anticlines, frequently ai-ranged en echelon, which, in
general, have an east-northeast trend. Moreover, the oldest rocks are the
hardest, and these usually occupy the anticlines; whereas the troughs are
occupied by the younger, softer rocks. The minor structures, such as
cleavage and fissility, are in general parallel with the above structural
arrangement. As a consequence of the combination of these factors, which
are of the greatest importance in determining the topography, we find the
main ridges to be usually anticlines of older rock, with the intervening
valleys in synclines of younger and less resistant rocks. Almost withoiit
exception the main ridges and valleys, and to a very great extent the minor
ones also, agree with the trend of the geologic structure and run east-
northeast.
Beautiful examples of the relationship between the topography and
geologic structure are shown in the cases of Tower and Lee hills at Tower
and of Soudan Hill at Soudan. Here the anticlinal hills of resistant
431
432 THE VERMILION IKON-BEARING DISTRICT.
Archean jasper are surrounded by valleys in thfe softer rocks of Lower
Huronian age. The numerous anticlinal hills of Archean greenstone
between Moose Lake and the Kawishiwi River, in Ts. 63 and 64 K., R. 9 W.,
and those between Knife Lake and Gobbemichigamraa Lake, in Ts. 64
and 65 N., Rs. 6 and 7 W., show similar relationships, being surrounded
by sedimentaries of Lower Huronian age. Somewhat less characteristic
are the numerous small anticlines shown on the islands of Vermilion Lake
and the adjacent shores, with Ely Island, the jioint south of Mud Creek
Bay, and the point east of Stuntz Bay as the most striking cases. Just
across the international boundary in Ontario there is an area reconnoitered
by the United States geologists where a great Archean anticline is found
separating the Lower Huronian Knife Lake slates from the Archean iron-
bearing Soudan formation in Emerald and Big Rock lakes, and another
Archean anticline separates the iron formation of these last two lakes from
the Lower Huronian syncline in That Mans, This Mans, the Other Mans,
and Agawa lakes. These four narrow, aligned lakes are the most striking
cases of synclinal lakes found in this region.
The shapes of the lakes depend also to a very great extent upon the
structural features of the rocks surrounding the lakes. They lie, as has
already been intimated, in structural basins which are occupied by the
younger rocks. Examining the large lakes in detail one finds that the
prominent salients are formed by the anticlines of older rocks, while the
reentrants are synclines occupied by the younger ones. This condition is
very noticeable on the east end of Vermilion Lake. Beginning at the south
with Stuntz Bay we find the east side of this bay, which continues to the
east in a mai'ked depression, is in a syncline of Lower Huronian sediments
with an anticline of Archean greenstone and jasper forming- Soudan Hill to
the south, and a corresponding anticline of Archean jasper to the north,
which forms a salient. The western side of the bay shows clearly the
relation between the differential erosion of rocks of the same series. It lies
in a syncline of Lower Huronian slates with the underlying- conglomerate
of the same series forming- the north arm, and Archean jasper with occa-
sional patches of unremoved Lower Huronian conglomerate forming the
south arm. Armstrong Creek and Bay is another case of a syncline occu-
pied by the Lower Huronian slates with Lower Huronian conglomerates on
both flanks, and forming ridges Still farther south and north of the con-
RELATION OF TOPOGRAPHY TO STRUCTURE. 433
glomerate come anticlines of Arcliean i-ocks. Mud Creek, about a mile and
a half north of Armstrong Bay, flows also in a depression occupied by the
Lower Huronian sediments, which are flanked by Archean greenstone.
Similar cases are shown in the synclines of sediments extending from Ver-
milion eastward to Bass Lake, in sec. 2, T. 62 N., R. 15 W., and extending
eastward along Rice Bay of Vermihon Lake, in sees. 34 and 35, T. 63 N.,
R. 15 W., still farther north.
In T. 64 N., R. 9 W., Moose Lake occurs, lying in a syncline of Lower
Huronian slates. On the west side of the northern bay of Moose Lake,
that one out of which the portage to Wind Lake goes, the salient on the
west side is formed by an Archean greenstone anticline with reentrants on
both sides in the Lower Huronian slates. Knife Lake shows a beautiful
case in the 2-mile long point of Archean greenstone projecting west
through sees. 22 and 23, T. 66 N., R. 7 W. Both to the north and south of
this erosion has very nearly removed all of the Lower Huronian sedi-
ments from contact with this anticline, small patches only being found on
the north and south flanks, as shown on the geologic maps in the accom-
panying atlas. Lake Gobbemichigamma, in sees. 31-32, T. 65 N., R. 5 W.,
and sec. 6, T. 64 N., R. 5 W., and sec. 1, T. 64 N., R. 6 W., lies right at the
junction of three great formations and is influenced in its shape very
markedly by them. On the shores each formation shows the topographic
forms characteristic of it. The formations meeting here are the Archean
greenstone and the Lower Huronian Knife Lake slates, which overlap
upon the greenstone, and in contact with both of the above and over-
lapping them is the Keweenawan gabbro. The Twin Peaks Archean
anticline forms a bold east-west ridge, the end of which overlooks the
lake and projects into it from the west, forming two salients. The sligdit
embayment to the north of this greenstone is along the contact of the Archean
and the Lower Huronian slates, while the embaj'inent to the south follows
a little to the south of the contact of the Archean greenstone and of the
Keweenawan gabbro. The greenstone runs down in a very steep slope to
the water. The north side of the lake is bounded by the slates, and here
there is a gentle slope down to lake level. On the southeast and south sides
of the lake the gabbro appears in steep cliffs and prominent headlands, the
irregularities of the shore being influenced in their trend by the marked
jointing of the gabbro.
MON XLV — 03 28
434 THE VERMILION IRON-BEARING DISTRICT.
The shapes of many of the lakes, especially those which lie com-
pletely in rocks of more or less uniform character, have been determined
by the direction of the joint planes of the district. The major joints agree
very closely with the direction of the predominant schistosity and trend
N. 60-80° E. The next important system of joints is approximately at
right angles to that direction. The effect of this jointing can be seen best
in the lakes lying within the well-jointed Knife Lake slates. Knife Lake
itself shows its dependence upon the jointing of the rocks by its principal
direction, which agrees with the jointing, and by the occasional arms
nearly at right angles to it. The main southeast arm of Knife Lake owes
its direction, as can be readily seen by reference to the maps in the atlas,
to the influence of the Archean anticline which forms its north shore. The
lakes between the east end of Knife Lake and Saganaga Lake are within
the area wherein the influence of the anticline formed partly of the granite
of Saganaga Lake makes itself felt, and where as a result of this the bed-
ding of the slates, as \ve\l as the jointing, which agrees fairly well with the
direction of bedding, turns strongly to the east of north, getting more nearly
north, and finally turning a little to the west of north as the northern portion
of the granite mass is approached. This change in direction of the structure
in the slates and the relationship of the extension of the lakes thereto is
shown by the string of lakes in sees. 26, 34, and 35, T. 66 N., R. 6 ^Y., and
sees. 3 and 10, T. 65 N , R. 6 W. This string of lakes connecting the east end
of Otter Track Lake and the east end of the south arm of Knife Lake has
a trend in general of N. 20° E. Another specific instance of the influence
of the jointing in these slates can be seen in the lakes in sec. 30, T. 66 N.,
R. 5 W., and sees. 24-25, T. 66 N., R. 6 W., just west of the boundaiy
between the Knife I^ake slates and the granite of Saganaga Lake. Here
the jointing and the bedding have turned to about N. 10° W. These two
lakes have their major direction following this line of major jointing. A
second system runs N. 25° E. and controls the trend of the ends of some of
the points and the greatest width of the lakes.
Another factor which has in many cases determined the position of the
topographic features has been the plane of contact between the difterent
formations and the differential erosion of these formations. A notable case
is that of Otter Track Lake, on the international boundary. This lake lies
along the contact of the Archean greenstone and the Lower Huronian Knife
RELATION OF TOPOGRAPHY TO STRUCTURE. 435
Lake slates. The factor of differential erosion was aided here by the strike
of the slates and the jointing, which agrees closely with the strike of the line
of contact. Only in one place has any of the Lower Hnronian slates been
left upon the north side of the main arm of the lake. Before reaching the
east end of this lake there is a large north-trending bay which is in Cana-
dian territory. The direction of this bay has been controlled, like that of
the main body of the lake, by the contact of the Lower- Hnronian slates
and the Archean formations which here swings sharply to the north.
In the western part of the district, extending from Rice Bay of
Vermilion Lake, in T. 63 N., R 15 W., over sec. 36, T. 64 N., R 12 W.,
over nearly to Basswood Lake, there is an almost continuous line of lakes
and streams marking the boundary between the Archean greenstone and
those schists which have been produced from this greenstone by the
metamorphic action of the Trout, Burntside, and Basswood granites upon it.
This line begins near Rice Bay, as mentioned above, and can be followed
east through the creek flowing into this bay from the east nearly to the
vicinity of Mud Lake in sec. 3, T. 62 N., R. 14 W. For a few miles then
it runs south of Burntside Lake. From sec. 32, T. 63 N., R. 13 W., it is
followed approximately by Burntside Rivel- up to sec. 24 of above township
and range. From there on to the east it follows approximately a string of
lakes — Little Long Lake, Bass Lake, and Mud Lake. Another striking
instance of this influence of the contact of two rocks on the topography is
shown in the case of the Kawishiwi River. Beginning in sec. 15, T. 63 N.,
R. 9 W., where it enters the Vermilion disti-ict, it runs southwest verv
closely along the contact between the gabbro and the Lower Hnronian
sediments. Then when the sediments disappear it runs along the contact
between the Archean greenstone and Keweenawan gabbro, and where the
Giants Range granite commences it follows the contact between the gabbro
and the granite. Shortly after the granite is reached the Kawishiwi divides
in sec. 26, T. 63 N., R. 10 W., the south arm running southwest very closely
along the granite-gabbro contact. The north arm of the Kawishiwi continues
very nearly due west, following along the contact between the granite and
the schists produced by the contact action of the granite on the Archean
greenstone; then between the greenstone and the overlying Lower Hnronian
sediments, and finally bends northwest, cutting across the greenstone. In
the eastern part of the district, in sees. 34 and 35, T 65 N., R. 5 W., a string
436 THE VERMILION IRON-BEARING DISTRICT.
of lakes connected by a stream is fonnd along the contact between tlie iron
formation of the Lower Huronian and the Lower Hnronian Og-ishke con-
glomerates. A similar topographic depression can be followed between the
contact of the granite and the greenstones in sees. 26, 27, 28 and 29, T. 62
N., R. 13 W., and in sees 17, 9, 3, 11, and 1, T. 62 N., R. 12 W., to sec. 31,
T. 63 N., R. 11 W., south of Ely.
The above-mentioned instances are only some of the more important
and most noticeable of the many cases which occur in the Vermilion
district, and which migiit be cited as illustrating the dependence of topog-
raphy upon geologic structure. Some of the other cases have been
referred to in the more detailed descriptions of the vai'ious kinds of rocks
and of the special areas.
CHAPTER IX.
GEOLOGIC HISTORY OF THE VERMILION DISTRICT.
The earliest period o£ the history of this district, as of all others, is
entirely hidden from ns. We can go no farther back than the time of the
formation of the earliest rocks now exposed to view. Whether during this
early time the area of the Vermilion district remained continuously xnider
water after the formation of the primeval ocean, or after an ocean existed
there was a land area antedating the development of the oldest rocks now
found, can only be conjectured. However this may be, the evidence we
now have indicates that this region was for the most part, if not wholly,
under water at the time the Ely greenstones were formed.
The Ely greenstone, as has been shown, consists entirely of igneous
rocks. These are almost wholly lavas and largely of ellipsoidal greenstones
which are amygdaloidal and spherulitic, and are therefore presumed to be
surface igneous rocks. These greenstones within the district have a great
but unknown thickness, and the formation extends far beyond its confines
in a continuous belt nearly to Lake Nipigon, Ontario. Furthermore, similar
formations occupy equivalent positions in other parts of the Lake Superior
region; hence we infer that the time of the deposition of the Ely greenstone
was one of regional vulcanism paralleled in magnitude only by the more
important later volcanic periods, such as those of the Keweenawan and the
Tertiary.
The length of time involved in the formation of the Ely greenstone
can only be conjectured. It is well known that volcanic matei'ial accumu-
lates with great rapidity; hence, so far as the masses of material exposed
to view may determine our judgment, we would not be justified in con-
cluding- that this period was one of extraordinary length. However, if we
437
438 THE VERMILION IRON-BEAliING DISTRICT.
unite the events preceding- the time of the Ely greenstone with that of the
greenstone itself the time thus represented would be very long- indeed.
This time is only a part of the Archean. So far as the Vermilion district
is concerned, and, indeed, so far as the entire Lake Superior region is
concerned, we get no farther back than that period of the primeval ocean
in which the Ely greenstone was formed. If there were land areas in this
region or elsewhere in the world at an earlier time we have no evidence of it.
After regional volcanic activity continued for an unknown time, it
died out, as volcanic activit}^ has elsewhere. Probably this process was a
very slow one, although we have no direct evidence upon this point.
Following the Ely greenstone there were orogenic movements and
erosion. Correlative with these was the deposition of sediments in the
Vermilion district. The mechanical sediments formed at this time are
insignificant in amount. The depth of water over the Vermilion district was
sufficiently great to make the mechanical sediments entirely subordinate.
Here, under quiescent conditions, the iron-bearing carbonate of the Soudan
formation was laid down. For ranch of the district this formation rests
directly upon the Ely greenstone, with no intervening mechanical material.
The iron-bearing carbonates are chemical or organic sediments, or were
more probably formed by a combination of chemical and organic agents.
That life was present in the sea at the time of the deposition of the
Soudan formation and furnished organic material to reduce the limonite
and carbonate to protoxide is indicated by the graphitic material now
associated with these rocks. The iron for the carbonates may have been
partly absorbed from the Ely greenstone underlying the formation, but
probably was more largely abstracted from the areas of Ely greenstone
raised above the water outside the district at present considered. The
underground and surface streams would there dissolve the iron salts. Thev
were brought to the Vermilion sea, and there were probably precipitated
as limonite and transformed to iron carbonate by processes previously
explained.
Following the deposition of the Ely greenstone there was a second
great outbreak of igneous activity. At this time various igneous rocks
were intruded within the Ely greenstone and the Soudan ft)rmation. At
this time, or earlier, were the great intrusions of granite represente<l bv the
GEOLOGIC HISTORY. 439
Arclieau acid intrusives, the granites of Trout, Basswood, Burntside, and
Saganaga lakes and connecting- areas. The main masses of these granites
are vast batholiths, from which there are innumerable oifshoots. These
intrusions cut the Ely greenstone most intricatelv, l)ut all of the g-ranites —
for example, the granite of Saganaga Lake — have not been found in such
relations to the Soudan formation. It is believed, however, that this age
relationship exists and that it is only owing to absence of the Soudan
formation in good development near the granite of Saganaga Lake that
dikes of the granite have not been found in it. This may, therefore,
antedate the Soudan formation and reall}^ be of early Archeau age, although
if this were the case it is thought probable that the disturbances attending
its intrusion would have raised the Ely greenstone above the sea at various
places in the Vermilion district, and thus would have resulted in the
formation of thick mechanical sediments below the Soudan formation. It is
therefore thought to be more probable that the granite is contemporaneous
with the others, and is later than the Soudan formation.
The Ely greenstone, the meclianical sediments preceding the Soudan
jaspers, the Soudan jaspers themselves, and the intrusive rocks following
the Soudan constitute the Archean rocks.
Probabl}'^ contemporaneous with these great intrusions of igneous rock '
were the powerful orogenic movements following Archean time. These
movements probably raised the entire Vermilion district above the sea.
They certainly folded the rocks into mountain masses of enormous extent
and exceeding complexity, so that the Vermilion district, which up to the
present time had been a sea area, was now a land area. No sooner was the
district raised above the sea than the epigene forces attacked the land and
the process of degradation began. This was very long continued, and land
and sea erosion cut deeply into the previous formation. For larg-e parts of
the district it removed the entire Soudan formation; in places it cut away
the Ely greenstone, exposing the underlying intrusive granite.
Attending the granitic intrusions, the orogenic movements, and the
erosion were the metamorphic chang-es in the rocks. Dependent upon the
granite intn^sion was the deep-seated alteration of the YAj greenstone,
resulting in the production of amphibole-schists and amphibole-gneisses.
Adjacent to the granite masses the more superficial agents of metamorphism,
and especially those of weathering, greatly changed the carbonate of the
440 THE VERMILION IRON-BEARING DISTRICT.
Soudan fonnation to fen-ugiuous slates and ferruginous cherts, although
residual iron carbonate undoubtedly remained.
It therefore appears that the batholithic and dike intrusions of granite,
porphyry, etc., and doubtless extrusions, and the orogenic and profound
erosion and the metamorphism were contemporary events, or at least largely
overlapping. AVhile the intrusive masses are placed in the Archeau series,
these events chiefly mark the inter- Archean-Huronian time, the most con-
spicuous evidence of which is the great unconformity between the Archean
and the Lower Huronian series. It is clear, however, that Archean time is
not sharply separated from inter-Archean-Huronian time, but is connected
through the intrusive masses. If there were volcanics contemporaneous
with the batholiths of granite and the intrusive masses of porphyry, these
were wholly removed by the great period of erosion mentioned below, since
no such rocks are found in the district.
After erosion had long continued either the land was reduced to the
level of the sea by this process or else subsidence came, or perhaps erosion
and subsidence combined to reduce the outer portion of the Vermilion
district to the level of the sea. The sea then encroached upon the land.
However, the land was uneven, with highlands here and lowlands there, so
that the time of the advance of the sea over the district varied considerably.
The areas first encroached upon received a thick layer of gravel and
bowlders, the material being derived from highlands which had not yet
been covei-ed by the sea. Thus there was built up the great Ogishke con-
glomerate, the lowest formation of the Lower Huronian series. When at
last the sea had succeeded in entirely overi'iding the district, the conditions
were no longer favorable for the deposition of coarse mechanical sediments;
also it is probable that a certain amount of subsidence further favored
quiescent conditions at the bottom of the sea. At this time the Agawa
formation was laid down in certain favorable localities, but in small quan-
tity, in the eastern half of the district.
In the western part of the district no chemical or mechanical deposit
was found which may be correlated with the Agawa formation in the
eastern half of the district. It may be supposed that here the water was
not deep enough for such sediments to be produced.
Following the deposition of the rocks of the Agawa formation must
have been a long-continued subsidence, f^r upon the formation was laid
GEOLOGIC HISTORY. 441
down the great thickness of mechanical sediments marked by the Knife
Lake slates. These were deposited as muds, grits, and fine gravels. The
sea, therefore, must have been one of moderate depth; land areas must
have been adjacent, as the formation is so thick that a continual subsidence
must have occurred in order that the g'reat mass of material could be piled
up. A part of the material was clearly derived from the adjacent land
areas; some portion was contributed by contemporaneous volcanoes, for
within the Knife Lake slates at various localities are considerable propor-
tions of volcanic ash, showing that volcanic ash was spread far and wide
from volcanic centers, and was thus important for upbuilding the Knife
Lake slates. The source of this ash has not been discovered. The
Ogishke conglomerate, Agawa formation and the Knife Lake slates together
constitute the sediments of the Lower Huronian series.
Following the upbuilding of the sediments was another great period
of igneous intrusion marked by the Giants Range granite, the Snowbank
granite, and the Cacaquabic granite. While the acid intrusives mentioned
were the dominant ones, there are certain more or less schistose basic and
intermediate intrusives occurring in isolated dikes in the various rocks thus
far described which belong to this same general period of igneous activity.
The Snowbank granite and granites of equivalent age, etc., are placed with
the Lower Huronian series, although they more probably belong with the
great geologic revolution following the compai'atively quiescent conditions
of Lower Huronian sedimentation, and contemporaneous with lavas probably
formed at the surface. If such lavas were laid down they were apparently
wholly removed by the deep-seated erosion mentioned below.
This revolution was caused by the orogenic movements which now
followed. These were of the most severe kind, and continued long. As a
result the Lower Huronian series were thrown into a set of close east-west
folds and steeper cross folds. This folding must have produced great
mountain masses. No sooner did the orogenic movements raise the land
above the sea than the epigene forces again began their work of hewing it
down. The period of erosion was very long, so long in fact that in various
places the entire Lower Huronian series was cut through, laying bare the
Archean. Contemporaneous with and largely caused by the folding and
partly by the intrusion of the igneous rocks and by the erosion was a
second great period of metamorphism. The great mass of muds of the
442 THE \'ERMILION IRON-BEARING DISTRICT.
Lower Hnroniau series became transformed to slates. The sediments
near the Giants Range and Snowbank granites were transformed to
schists and gneisses. From the iron carbonates of the Agawa forma-
tion ferruginous slates and ferruginous cherts were formed, although
this process by no means iieared completion. It appears that following-
Lower Huronian time, as in inter -Archean-Huronian time, igneous inti-u-
sions, orogenie movements, erosion, and metamorphism were largely
contemporary, or at least overlapping, and that they were largely the result
of the same causes; that is to say, there was a time of the readjustment of
the earth's crust where before there had been a time of sedimentation.
These earth movements resulted in folding and in vulcanism, and conse-
quent upon this were erosion, and dependent upon all this was metamor-
phism, both deep seated and surface.
Following- the unconformity above Lower Huronian time the Vermilion
district was again depressed and the Upper Huronian (Animikie) series
was deposited above the upturned rocks of Archean and Lower Huronian
age. Only a small area in the eastern part of the Vermilion district is
covered by the Upper Huronian sediments, and we have little data in this
district for determining the history of this time. The basal formation of
the Upper Huronian in the Vermilion district is the Gunflint formation,
whose character is such as to indicate that it was deposited in relatively
deep water or a protected area of the sea. Southwest of the Vermilion
district in the Mesabi district a conglomerate occurs at the base of the
Upper Huronian below the Biwabik formation which is correlative with the
Gunflint formation of the Vermilion district. We thus conclude that while
the Upper Huronian sea advanced, conglomerates were formed to the west,
but that in the Vermilion district the conditions were not favorable for the
production of this kind of rock. Over the Giinflint formation was then
deposited possibly 10,000 and more feet of Rove slates and graywackes.
These rocks presumably covered the entire Vermilion district and probably
extended far northward beyond the district, but all evidence of such
extension has been removed by erosion.
Following the period of deposition of the Upper Huronian there came
an upheaval succeeded by a long- period of extensive erosion. This erosion
removed great thicknesses of the overlying Upper Huronian rocks and in
some cases the entire thickness, and having cut through the Upper
GEOLOGIC HISTORY. 443
Huronian even cut down into the Lower Hnrouian and Arcliean rocks.
From evidence in other districts of the Lake Superior region, but not from
evidence in the Vermihon district, the unconformity represented by this
period of erosion is known to have been important.
Following the Upper Huronian unconformity came the lava flows of
the Keweenawan. These probably accumulated to immense thickness
upon the beveled edges of the Upper Huronian series. There is no
evidence that these flows were not subaerial — that is, deposited upon the
Upper and Lower Huronian and Archean series while they were still land
areas. How far the process of upbuilding' of the Keweenawan had
continued before the next great event it is impossible to say, but it is
probable that thousands of feet of lavas were erupted, and that sand-
stones and conglomerates were deposited in the upper parts of the series
between these lavas. If this be the case, the land must have again sub-
sided below the sea. However this may be, after a very considerable
thickness of Keweenawan lava had accumulated, there came the great
laccolithic intrusion of Duluth gabbro, which now has a surface area of
nearly 200 square miles and extends from Duluth eastward beyond the
eastern end of the Vermilion district, and which bounds the Vermilion dis-
trict on the south from the Kawishiwi River eastward. The planes between
the various formations would be the natural planes of easiest resistance
which an intrusive would follow, and these planes the Keweenawan gabbro
seems to have followed for the most part. In the western and central
portion of the district the gabbro, to judge from the way in which it
overlaps on the Archean and Lower Huronian rocks, and from the fact that
it has included in it at various jjlaces along its northern edge masses of the
Gunflint formation of varying size, seems to have followed along the plane
between the Upper Huronian and the Archean and Lower Huronian. In
the east the gabbro laccolith began to rise and there beveled the edges of
the Upper Huronian, and at one place is even found intrusive in the
Keweenawan lava flows. It is believed that the numerous sills, named
Logan sills by Lawson, which are so abundantly found in the Animikie,
are but part of the same magma from which the gabbro came and that they
were introduced at the same time. Finally the great diabase dikes of
Beaver Bay and other places along the Minnesota coast, which intruded the
Keweenawan lavas, are probably connected with this great gabbro laccolith.
444 THE VERMILION IRON-BEARING DISTRICT.
It is not supposed that the intrusion of this enormous mass of lava,
probably the g-reatest known laccolith, occurred within a brief time. It
has been noted that this laccolith is 1.50 miles long and probably thousands
of feet in thickness — how thick is entirely unknown. The intrusion of so
vast a mass of material must have occupied a very great length of time.
The parts earlier intruded were doubtless solidified long before magma
ceased to enter. Thus these latter parts would be found as dikes in the
earlier solidified parts. There would be great variation in its coarseness
of crystallization. There would be ample time for the various processes of
differentiation by fractional crystallization and separation by gravity and
other 23rocesses, and thus is explained the structural complexity of the gab-
bro and its great variation in mineral and chemical character. Perhaps
contemporary with the intrusion of the gabbro, perhaps later than it, perhaps
in part both, was the gentle tilting of the Keweenawan lavas, the Duluth
gabbro, and the Animikie, all together, to the south, under Lake Superior,
and the much more pronounced northwest tilting toward the same lake
of the Penokee series south of Lake Superior. To this tilting in opposite
directions on opposite sides of the lake is due the Lake Superior Basin.
It is believed that at the time of the formation of the Lake Superior
syncline the Giants range anticlinal area was correspondingly upheaved,
and that thus the present Giants range was actually created by this
movement, although, as has been stated in the previous pages, the location
of the range was actually determined possibly as early as the folding
following the Archean when the protaxis of the range is thought to have
been first formed. Since this earliest time repeated movements of
elevation, particularly the one at the close of Lower Huronian time,
succeeded by subsidence and erosion, have followed along this old line of
weakness. The actual present condition of the range in its minor features
is of course due to the erosion and then glacial deposition which have
occured subsequent to Keweenawan time aiid which are briefly outlined
in the following jDages
Contemporaneous with and following the intrusion of the Keweenawan
gabbro is the 2:)eculiar metamorphism which marks the rocks of the Lower
Huronian, Upper Huronian, and Archean along its border. It has been
noted that the Gunflint formation adjacent to it was changed to a banded
granitic textured rock, consisting of iron silicates, magnetite, and quartz.
GEOLOGIC HISTORY. 445
These iron silicates comprise chrysolites, pyroxenes, and amphiboles.
They are very coarse grained and they stand as the extreme of deep-seated
static metamorphism of an iron-bearing cai'bonate. The rands and grits of
the Knife Lake formation and the conglomerates of the Ogishke formation
adjacent to the gabbro were completely crystallized largely into granitic-
textm-ed rocks described (pp. 315, 342). These are the best representatives
of the production of granitic textured rocks from heterogeneous mechanical
sediments known to the writer. They were produced under deep-seated
static conditions where high temperature prevailed.
All of the complex events thus far described preceded Cambrian time.
This history is Archeau and Algonkian. In another place it has been
shown by Walcott" that the Cambrian transgression over the North
American continent began at the southeast and extended to the northwest,
and that it continued through Lower and Middle Cambrian time before the
sediments were deposited in the Lake Superior region. This great erosion
period was probably partly contemporaneous with the tilting which pro-
duced the Lake Superior syncline. The erosion of this time laid bare the
great laccolith of gabbro, as well as tlie beds of the overlying- lavas, and
thus exposed to light of day all of the great series of rocks — within the
Vermilion district itself the Archean, the Lower Huronian, and the Upper
Huronian series; south of these the great batholith of gabbro; and south
of this the Keweenawan lavas. Finally, however, the sea overrode this
region, and the Cambrian sandstone was laid down in the Lake Superior
Basin and along its border. Remnants of it have been found far inland,
but none in the Vermilion district itself However, it can not be doubted
that over the Vermilion district were deposited Cambrian, or Silurian rocks,
and therefore the Paleozoic was there represented.
The next great step in the history of the region was the elevation of
the land above the sea and long- continued denudation. This period of
denudation has generally been known as the Cretaceous period of base-
leveling. Whether the sea actually overrode the Vermilion district and
there deposited the Cretaceous rocks is uncertain. However, it is certain
that the Cretaceous sea reached within a comparatively short distance of
the district, for outliers of the Cretaceous rocks are now known within 20
« Correlation Papers: Cambrian, hy C. D. Walcott: Bull. U. S. Geol. Survey No. 81, 1891, p. 36-1.
446 THE VERMILION IRON-BEARING DISTRICT.
miles southwest of ^'ermilion Lake, and it seems liighly probable that Cre-
taceous rocks were laid over the district. However, iu this case the long-
continued erosion of Cretaceous time had probably removed all of the
Paleozoic sediments, and had reduced the land to a rough peneplain before
the de^josition of the Upper Cretaceous rocks.
It can not be said that this period of base-leveling in the Vermilion
district was nearly so complete as in central Wisconsin. However, the
hills rise to approximately the same altitude. If one ascends to some high
point he finds an approximate horizon line above which onl}^ a few points
project, as, for instance, the Sawtooth range.
Following the period of Cretaceous base-leveling the land was raised
approximately to its ^^resent altitude, and a second cycle of erosion was
inaugurated. This cycle has continued to the present time. In the early
part of this long cycle river erosion was the important factor, and at this
time were scooped oiit the longitudinal valleys following the softer rocks
and the various structures of the rocks. The drainage was adjusted to the
character of the rocks. The slate areas were largely valleys; the more
resistant areas were lai'gely highlands. Finally, there came the vai'ious ice
advances, at which time the valleys were widened and deepened, the hills
were rounded, and glacial debris was dropped here and there, but especially
in the valleys. When the ice last receded the pi*esent topography was sub-
stantially shaped. The depressions filled with water until they overran
their rims at the lowest points, thus forming the lakes. The lakes were
thus connected by streams, and the present irregular drainage (discussed
on i)p. 39-46) was inaugurated. Thus we have explained the present
topography of the district.
In conclusion, we see that the Vermilion district presents one of the
longest geologic histories of any region in the world. Apparently the Ely
greenstone, the most ancient formation, was laid down in primeval time
and the Soudan formation was deposited above it. Since that time there
were five great periods of deposition: The Lower Huronian, the Upper
Huronian, the Keweenawan, the Paleozoic, and the Cretaceous.
There were four great periods of igneous activity: The Ely greenstone,
tlie great batholithic intrusions at the end of Archean time, the hardl}' less
important batholitliic intrusions at the end of Lower Huronian time, and the
great Keweenawan period of volcanic extrusion and intrusion. There was
GEOLOGIC HISTORY. 447
also possibly contemporary volcanic activity at the time of the Knife Lake
slates.
Finally, there were four great periods of orogenic movements,
denudation, and metamorphism : Following the Archean series; following
the Lower Huronian; following the Upper Huronian; and following the
Keweenawan.
Also there were three other great periods of denudation: The Cam-
brian and the Cretaceous periods of base-leveling, and finally the period
following the Cretaceous, extending to the present time.
INDEX
.A.. Page.
Agawa formation, age of -- 33,308,330
characters of — 24,327-328
distribution of .- 324r-326
, exposures of -- 326,330-335
origin of - — 329,440
petrographic characters of 327-328
relations of — 303,329-330
stratigraphic position of ._ ■_. 33,308,329-330
structure of 326-327
thickness of - 330
Agawa Lake, Agawa formation at 325, 329, 330, 331
Agriculture of the district, character of 50
Ahbe. F., cited on mining methods in Vermilion dis-
trict 234
Akeley Lake, Gunflint formation at -.- 25,109-110
iron-bearing rocks of - 109-110
Logan sills at _ „ 411
Pewabic quartzites at 103-104,110,119
Akeley Lake series, occurrence of 119
Algonkian period, geologic history of _ 437-445
Algonkian rocks, components of 105
map showing 32
Amphibole-schists derived from greenstones, com-
position of 157
Amygdaloidal greenstones, occurrence of 139-141,165
plate showing _ 168
Anderson, C. L., and Clark, Thomas, report made
by, on geology and geological survey of
Minnesota 66
Annamani Lake (a glacial lake), site of 430
Animikie series, age of 106,114,123,124-125,396
areas of -__ 25,88,375
character of -. 25-26,
74, 88, 96, 97, 102-103, 106, 120-122, 377-387, 392-398
components of 25, 33, 88, 102-103, 109, 115, 121, 374
deposition, of conditions of 442
dip of. 88,109
divisions of , 25, 33, 88, 102-103, 109, 115, 121, 374
equivalents of 80-81,92,118
. exposures of 120-122
formations composing 23, a3, 102-103, 121, 374
metamorphism of 25-26, 122. 383-387, 393-394
relations of , to adjacent formations 84,
86, 88, 93, 96, 97. 105, 109-110, 114-116, 387-390, 395
toHuronian quartzite 93
to Kewatin rocks.. 84,88,93,109
to Logan sUls 408-410
to Vermillion slates 86
stratigraphio position of 78, 80-81, 99, 106
thickness of 74,390
See also Upper Huronian.
MON XLV — 03-
-29
Page.
Archean history, sketch of 437-439
Archean rocks, character of 95-96, 99
components of 33,99,100,105,116-117,129-130,439
definition of... 129-130
divisions of . 139-laO
Ely gi-eenstone of 130-173
granites of :,.. 124,246-274
intrusive rocks of, relations of, to Soudan for-
mation 199
relations of 95,96,281-284
Soudan formation of 172-246
Archean history, sketch of 437-439
Area and boundaries of the district .32
Argentiferous quartz veins near Vermilion Lake,
occxirrence of 67
Arkose, occiirrence of 270
Armstrong Bay, conglomerate at 287
Armstrong Lake, conglomerate and greenstone at. 195
rock banding near 195
Augite of Cacaquabic granite, analysis of 366
Am-iferous quartz veins near Vermilion Lake, oc-
currence of 67
B.
fiacon, D. H., acknowledgments to 18,30
cited on mining methods in Vermilion district . 2.34
Banding in Ely greenstone 149
Bass Lake, slates at 336
Basswood gneiss, stratigi'aphic place of 89
Basswood Lake, elevation of 34
granite at, age of 261
distribution of ; 258
exposures of 258
folding in 262
petrographic characters of 259-260
relations of 260-261
topography of .' 259
granitoid and gneissoid rocksat 87
greenstones near 118
syenitenear 69
Upper Kewatin rocksat _ 120
Bayley, W. S., citation from, on Duluth gabbro.. 403,404
on iron-bearing rocks of Gunflint formation. 385
on metamorphism of slates by gabbro at
Pigeon Point, Minn 394
on petrogi-aphy and geology of Akeley Lake
region 103-104
Bayley, W. S., geologic work in Vermilion district
done by 17,.30
Bayley, W. S., Smyth, H. L., and Van Hise, C. R.,
cited on origin of iron ores 2.32
449
450
INDEX.
Page.
Beaver Bay diabase, occurrence of 123-124
Bebb. E. C, work done by 17
Bell, Eobert, cited on geology of the district 69
Berkey, C. P., cited on copper minerals of Minne-
sota - --- 113,184
Big Eock Lake, iron formation at, relations of 210
jasper interbanded with greenstone at 19i)
Bigsby. John J., geologic work in Lake Superior re-
gion done by 6't
Bingoshick Lake, Logan sills at -- 398
Birch Lake, Agawa formation at 325
Pewabic quartzite near --- 119
slates at. - 336,347
Birch Point, glacial deposits at - ''28
Birge, E. A., cited on plankton of lakes of the dis-
trict - -_ *6
Biwabik formation, character of.. - 377-379
equivalent of-.- 25,377-3i9
thickness of - - 390
See also Pewabic.
Black Eiver schists, equivalent of .- 92
Bois Blanc. See Basswood.
Bone Lake, Michigan, ci-ystalline schists at -- 198
Breociation, examples of 194-195
Brule Lake, Duluth gabbro at 408
Burned areas, locations of -■ - 48,49
Burnt Forties, banded rocks in 178
granite porphyi-y in 25i
location of - ^
Burntside Lake, dikes at 262-263
glacial deposits at 428
granite at, age of - 261
distribution of -- 258
foldingin - 262
petrographic characters of 259-280
relations of .- 260-261
granites and schist at, figures showing rela-
tions of . 158,159
metamoi-phosed gi'eenstones at. . 194
view at 392
C.
Cabotian division of Keweenawan rocks, character
and extent of 123
Cacaquabic granite, age of 369,406
analysisof ' 367
augite of, analysis of 366
character of --- 25,365-368
contact of Ogishke conglomerate with 305-306
distribution of 365
exposui'es of -- 369
feldspar of, analysisof.. 366
intrusion of. date of .- 441
metamorphism of slates by 341
occurrence of 24,2.5,364
petrogi-aphic characters of 365-368
relations of, to ad jacent formations 24,368
to Duluth gabbro 407
topography of 365
Cacaquabic granite-porphyry, analysis of 367
Cacaquabic Lake, geology of region near 108-109
granite at 364-369
green schist near.. 85
greenstone near, structure of :... 136
Lower Huronian slates at 301
Ogishke conglomerate at 319
syenite at 91
Page.
Cache Bay, Saganaga Lake, gi-anite cutting green-
stone at 274
greenstone in contact with contact at 311
Ogishke conglomerate at 274,309.311
Cambrian period, erosion in 445,447
orogenic movements in .' 445
Canada, Ely greenstone in _ 132
Canoe Island, Knife Lake slates at 295
Carbonates, iron-bearing, analyses of 3S0
Cai-p Lake, Agawa formation at 325.332
graywacke at 347
slates at 336
Cashaway River. See Kawishiwi.
Carver, Jonathan, cited on route from Grand Por-
tage to Eainy Lake.. .57
Chandler mine, drift of, view showing 238
mining methods at 2.34,239-241
ore of... 22, ia3-l&5, 217-222. 241
sections across.... 216.217,218.219
shaft of, view showing -. 216
Chatard, T. M., analyses of iron-beaidng carbon-
ates by 380
Chauvenet, W. M., citation from, on Duluth gabbro. 403
on iron-bearing rocks of gunflint formation. .3.'>5
on iron oxide bodies in Duluth gabbro 420
on Logan sills --- 399,409
on Pewabic quartzite 104
figures cited from --. 392,41X1,401
geologic work in Vermilion district done by 3D
Chert, gi-een, carbonate-bearing, charactei-s of . - . 186-187
magnetitic, plate showing 168
white, chai'acters of - 185-186
Chester, A. H., cited on iron region of northern
Minnesota 75
exploratory work done by 214
Chester Peak, height of 36
naming of 36
See also Jasper Peak.
Chippewa Indian Reservation, location of 20
Chlorite-schists, occurrence of.- 253
Clark. Thomas, and Anderson, C. L., report made
by. on geology and geological survey of
Minnesota 66
Clark, Thomas, and Hanchett, A. H., citation from,
on occurrence of hematite at Vermilion
Lake 67,213
on route from Grand Portage to Rainy Lake . 57
report made by, on geology of Minnesota 66-67
Clearwater Lake, breccia at 3.55
metamorphosed rocks near __ 350
Cleavage, slaty, occurrence of.. 177-178
Clements, J. M., citation from, on spherulites in
Michigan rocks 1-52
on spilosites .- 345
work done by.. 30
Clements, J. M., and Smyth. H. L., cited on crystal-
line schist at Bone Lake, Michigan 168
Cole, T. F., acknowledgments to 18
aid by -- 3n-;il
Cole, G. A. J., and Gregory, J. W., cited on spheru-
litic structure. 146
Coleman, A. P., report by, containing references to
geology of Vermilion district 125
Conglomerates, occurrence of 2-51-252
Conglomerates and pseudoconglomerates, associ-
ation of 286
distinctions between 252
INDEX.
451
Page.
Contact inetamorpliisiii, notes by U. S. Grant on. 125-127
Copper minerals, occurrence of _--. - 113,184
Copper-bearing rocks, age of - 93
cbai-acterof _ — 73-"5
relations of. .-- 73,73-75
CoutcMcliing, use of name... 159-160
Coutchiching group, equivalents of 91
geologic place of __ 100
Cretaceous history of the disti-ict, sketch of 445-446
Cretaceous period, erosion in 445-446,447
Cross River, dike at _....* 432
Cupriferous series. See Copper-bearing rocks.
X>.
Denton, F. W., cited on mining methods in Ver-
mUion district 234,340-241
Diabase dikes, intrusion of 443
Dikes, occurrence and character of, aplite 355
basalt 34,26,306,371-373,433-424
dolerite 300,205-306,308,400,40.5-406,433-434
gabbro 393-394,407,408^19
granite 33,34^35,1.56,165-166,
199, 22.3-226, 346-274, 2&3-284, 305-341, 353-371
gi-anite-porphyry 34-35,16.5-166
greenstone. 155,300,223-326
lamprophyre 308,371-373
(luartz-porphyry - 108
Disappointment Lake, greenstone in contact with
gabbroat. 161
metamorphosed conglomerate at 315-316
metamorphosed slates near 350
Ogishke conglomerate at.. -. 318-319
Pewabic quartzite at . 119
Dolerite, Keweenawan, relations of 395
Drainage system of the district, features of . . 19-20, 39-46
Drift, extent and character of 42.5-430
Duluth gabbro, age of 24,26,33
analyses of 405
character of- _ 36,401-405
dikes in 423-424
distribution of - 398-399
exposures of --.- 397,399
intrusion of , date and conditions of 443
intrusive rocks in 42^-424
iron oxide bodies in :.. _ 430-432
metamorphismby.. 342-344,393-394,418-419
nickel in _. 430-421
petrogi'aphic characters of 401-405
relations of, to adjacent formations. 34,33,408-408,417
to Cacaquabie granite 368.369,407
to Ely greenstone... _ 4(;i6
to Giants Eange granite 357-358,407
to Gunflint formation 389-390,407
to Keweenawan series 407-408
to Knife Lake slates 307-308
to Logan sills 410-418
to Lower Huronian sediments 407
to Rove slates... 393,394,395,407
to Snowbank granite ___ 363-364,407
to Upper Huronian sediments 407
stratigraphic place of 33
topography of 399-101
Duluth and Iron Eange Railroad, completion of 314
location of ._ 53
metamorphosed rocks along , 396, 340, 349
Duluth, Port Arthur and Western Railroad, expos-
ures of Gunflint formation along 337, 388
E.
Page.
Eagle Lake, Duluth gabbroat __ 402
Eagle Nest lakes, glacial deposits at 428^439
Eames,H.H.,citationfrom.ongeology of thedistrict 67
on occurrence of iron ore at Vermilion
Lake 313-314
Eames, E. M., cited on geology of the district 67
East Greenwood Lake, Cabotian rocks at 123
Eby, J. H., cited on occurrence of copper minerals
in hematite ore 184
graphite reported by 180
Eby, J. H., and Berkey, C. P., cited on copper min-
erals and hematite ore 113
Elftman, A. H., citation from, on Diduth gabbro. 401,415
on geology of northeastern Minnesota 110-111
on glacial hLstory of Minnesota 438
on ore deposits of Minnesota 113
Elftman, A. H., Winchell, K. H., and Grant, U. S.,
cited on geology of the Vermilion dis-
ti-ict 117
Elhpsoidal greenstones, deformation of 148
Ellipsoidal structure in Ely gre.?nstones, occur-
;renceof . 144-150
plate showing. 146
Ely, amygdaloidal structure in rocks near 146,165
dikes at and near 199,300,360-661,373
Giants Range granite near 354
greenstone exposures near 165
iron-ore deposits at 32-33
location of 52-53
Lower Huronian sediments at _. 399
mining methods at 234,339-341
ore bodies at 215
oi-e outcrops at 243-344
oresfrom, coarseness of 185
panoramic view near 316
population of 20,52-53
Soudan formation near .• 173
Ely greenstone, age of 33,197-198,406
amygdaloidal structure in 139-141
character of 21,136-150,437
colors of 136
conglomerate in 138
dikes in. 153,356
distribution of 131-134
economic value of 16-3-164
ellipsoidal structure in _. 137,144-150
exposures of 134
features of 31,130-131
formation of, date of 4.37^38
granite contacts with, effects of 155-157
granite dikes in... 356,360-361
jasper areas in 197
jointing in 137
magnetite ore in 163
mashing of. 137-138
metamorphism of 31 , 1.55-163, 169-173, 418.419
microscopic chai-acters of 150-152
naming of 130-131
origin of 154-1.55,437-4.38
original characters of 136
petrographic characters of 136-150
qiiartz veins in c 139
relations of, at Jasper Lake 308-309
at Moose Lake 308-207
at Otter Track Lake 307-308
to adjacent formations . 33, 161, 162-163, 191-199, 255-
356, 260, 268, 274-, 381-383. 303-304, 356, 360-361, 406
452
INDEX.
Page.
Ely grreenstone, relations of , to Duluth gabbru .. 161.400
to Giants Range granite 356,359,;i60-361
to iron-bearing formation - 198-199
to Lower Huronian sediments 281-283, 303-3(14
toSaganaga granite 268,274
to Soudan formation 191-199
to Trout, Bumtside. and Basswood lakes
gi-anites 260
to Vermilion Lake granite 255-256
scMstose forms of,. - 153-lo4
Soudan formation infolded in, figure show-
ing .. -1 - 205
sphemlitic and ellipsoidal structure in 141-144,
146-148, 167-168
stratigi'apliic position of. 33
structure of 135-136
textures of .- ---- 137,151-152
topography of 134-135
tuffs associated with -- 138,166
Tolcanic character of 166-169
Ely Island, conglomerate at - 279,288,291
geologic map of east end of -- 28:i
gi-aywacke at - -- - 288-291
Knife Lake slates at -.- 295
porphyry at - 251-252,288-291
Emerald Lake, brecciated rocks at 178
folding of Soudan formation at 210-211
iron-bearing formation at, relations of.. -. 210
jasper inter banded with greenstone at 193
slates at ._ --- 336
Soudan formation at- --- - --- 210-211
Ensign Lake, conglomerate at -. - 319
slates at .- - --- 336
Epsilon Lake, argillites at _.- 86
Exploration of the district, history of 56-63
F.
Fall Lake, greenstones at and near 118,132
jasper folded at- - 176
jasper interbanded with greenstone at.. 192
Soudan formation at .- 173
Farm Lake, metamorphosed rocks at 340
Fay Lake, Gunflint formation at 288
Feldspar of Gacaqiiabic granite, analysis of 366
Finlay, J. R.. and Smyth, H. L., citation from, on
age of slates of Vermilion district 281
on conglomerates of Vermilion Lake 252
on geology of the Vermilion range 111-112
on iron ores, origin of - 232
on jasper mass at Soudan Hill -- 230
on Ogishke conglomerates, origin of 2f^6
on schist at Lee mine, origin of 223
figure cited from - 231
Fish of the district, kinds of 50-52
Flask Lake, cleavage at - 303
graywackesat .- - - 337
greenstone, porphyritic, near - :il8-349
Ogishke conglomerate near 317-318
Forests in the district, area and character of 20, 47-50
Fox Lake, gray wackes and slates interbedded and
faulted at ..- :iiH
Frascr Lake. Pewabic quartzite at 119
Gr.
Gabbro, mc'tainon)hism by 161-162. 1^42-344
Gabbro plateau of northeastern Minnesota, topo-
graphic features of 37-:j.s
Page.
GabimichigamaLake. See GobbemichigammaLake.
Game of the district, kinds of ..- 50-52
Garden Lake, Soudan formation at .- 173
Geikie, Archibald, citation from, on ellipsoidal
structure in volcanic rocks of Great
Britain- - 144-145.150
Geographic location of the district - 32-33
Geologic history of the district, sketch of 437-447
Geologic structure, relations of. to topography... 431-436
Giants Range, Animikie rocks in 93
area north and northwest of, topography of 36-37
course and general features of.-- 35-36
glacial deposits near 426-428,429
origin of.. - 444
relations of topography and geology at . - 431^36
iSee o/.so Mesabi range.
Giants Range granite, age of 358,406
aplite dikes in 355
conglomerate in contact with 305-306
dikes in 355
distribution of _ 353-3.54
exposures of ^4
folding in 358-359
intrusion of, date of 441
in the Ely greenstones 165-166
Knife Lake slates in contact with 340-342
gi'anite, metamorphic action of 24,359
petrographic characters of 354-356
relation of, to adjacent formations 356-358
to Duluth gabbro _ - 407
to Ely greenstone.- 356,859,360-361
to Keweenawan gabbro - 357-358
to Lower Huronian sediments 356-357
to sedimentary rocks 283
to Soudan formation 356,359
schists in contact with 171
topography of - 354
Glacial drift, extent and character of 425-430
Glacial lakes, occurrence of 127.429-430
Glaciationiu the district 27,39
Gobbemichigamma Lake. Animikie series at 88
conglomerate-greenstone contact at 304
depth of .- -- --.■ 46
dike cutting gabbro at .- 423
Duluth gabbro at -.- ."" 39S
folding at --. 306-307
gabbro-slate contact at 351-352
greenstones at IS. 133. 136
greenstone -gabbro contact at -.- 161
Gunflint rocks near 383
iron-bearing formation near 383
Logan sills at j 398
Lower Huronian sediments at . . - 298, 301
metamorphosed slates at 343
Pewabic quartzitesat j.19
L"'i)ijer and Lower Huronian at, relations of 306
slatesat 343,351-352
topography and geology at 43:^
unconformity at 307
Gold, reported occurrence of 213
Gold mining, early attempts at, in Vermilion dis-
trict - 213
(Told-bearing rocks. Vermilion Lake, occui'rence of. 69
Graff. C. F.. acknowledgments to 31
Grand Portage, rocks at - 122
: Granite, age of 90.24(^24*'
chanicter of ^2. 107-ia^. 124. 24(>-250. 258-205. 266 -an*
dikes of 255-256
INDEX.
453
Page.
Gtranite, distribution of - --- 90
relations of 254-2.56, 2.59-261, 264-265, 268-27.3, 282-283
Kawishiwi Eivev area, distribution of _ 263-264
exposures of .- 264
petrograpliic cliaracters of.. 264
relations of-. 264-265
Knife Lake area, occurrence of J .-. 24
Moose Lake area, distribiition of 26.3-264
exposures of 264
petrographic characters of 264
relations of. 264-265
Saganaga Lake area, distribution of 266
exposiires of _ 266
metamorphic effects of. _ 273
petrograpbic characters of 266-267
relations of 268-273
topography of 266
Trout, Burntside, and Basswood lakes areas, age
of 261
distribution of-. 258
exposures of -. 2.58,262-263
folding in _ 262
petrographic characters of 259-260
relationsof 260-261
topography of 2.59
Vermilion Lake area, distribution of ._ 247
exposures of _ _ 247-248
folding in 250-251
petrographic characters of 248-2.50
relations of, to adjacent formations 254-2.57.
282-283
topogi'aphy of 248
sti'ucture and metamoi-phism of __. 251-254
Grant, U. S., citation from, on Cacaquabic granite. 364
365, 366, .367
on contact metamorphism in jVIinnesota . . 12.5-127
on cordierite at Gobbemichigamma Lake. . . 346
on Duluth gabbro 401,402
on dikes in Duluth gabbro 422
on gabbro plateau of northeastern Minne-
sota. 37-38
on glacial lakes .■ 430
on granitic areas in Minnesota 107-109
on Gull Lake rocks 312
on Gunflint formation. 390
on Gunflint Lake area. 38,9.3,109-110
on iron ores of Gimflint beds 385
on jasper in slates at Pickle Lake 325
on Kawishiwi River 41
on Logan sills ,399
on Mesabi iron range, geology of 114-116
on Logan sills and Duluth gabbro, relations
of..., 410-416
on metamorphism 12.5-1.26
on muscovado 343
on Minnesota geology 118-125
ov Ogishke conglomerate 324
onSpganagaLake granite 265, 267, 269
Grant, U. S., Winchell, N. H., and Blftman, A. H.,
cited on geology of the Vermilion dis-
trict 117
Graphitic rocks, ocurrence of 179-1 SO
Great gabbro, stratigraphic place of IM-ini
See also Duluth gabbro.
Green, R. B., table prepared by, showing coarseness
of iron ores \. 185
Greenalite, occurrence of I9fi
Page.
Greenstones, amy gdaloidal 168-169
belts of ■ 19,5-196
ellipsoidal 144-150
deformation of 148
inclusions of, in iron-bearing formation 197
relations of, to jasper 200-210
schistose, occurrence, and character of 1.53-154
spherulitic 141-144
Gregory, H. R., cited on ellipsoidal structure in
Maine andesites 144
Gregory, .J. "W., and Cole, G. A. J., cited on spheru-
litic striicture 146
Grtlnorite, origin of 187
Gull Lake, granite and greenstone at 274
greenstone and conglomerate in contact at 3.24
Gunflint formation, age of .33,406
character of 25,377-387
deposition of. time and conditions of. 443
distribution of , 375
equivalence of Biwabik formation to 25,377-379
exposures of 376
granules in, photonaicrograph showing 382
iron ores of, analyses of .. 385
iron-bearing rocks of... 377-387
metamorphism of 419,444^445
petrographic characters of 377-.387
relations of, to adjacent formations 387-.390
to basic dikes cutting. ,390
to Duluth gabbro 407
to Keweenawan gabbro 389-390
to Knife Lake slates
to Lower Huronian series
to Ogishke conglomerate. 388-389
stratigraphic equivalent of 25
stratigraphic position of .33
structure of 73,376-.377
thickness of . 390
topography of 376
Gunflint Lake, Animikie rocks at 74,84,86,93
argillitesat 86
banded rocks at 149
geology of region near 65,73,109 '
greenstones near ng
Gunflint formation at 387
hematite at 72
Huronian rocks at 71
iron-ore deposits at 213
jasper at 73
Keewatin slates at 93
Logan sills at 399
sections at ...^ 392,400
syenite at 91
Vermilion schist at 93
topography near ._, 38.39
H.
Hall, C. W., citation from, on Arehean rocks of Min-
nesota ..■ 116-117
on geology of the district 70-71
on granites of the Northwestern States. 90
Hanchett, A. H., and Clark, Thomas, citation from,
on occuiTence of hematite near Vermil-
ion Lake 213
on route from Grand Portage to Rainy Lake ... 57
report made by, on geology of Minnesota. 66-67
Harvey, H. E., iron.ore outcrops discovered by 215
454
INDEX.
i'age.
Hematite, features of 182
Hematitic specular iron ore, occurrence of 67
Henry, Alexander, citation from, on ti*avels in the
Northwest _ 63
BDighest point in district, location of 34, :i5-36
Hitchcock, Edward, cited on schist conglomerates
of Vermont - 170
Houghton, Douglas, geologic work in Lake Superior
region done by 64
Himt, T. Sterry, Animikie series named by 374
Huronian group, components of 76,80-81,88,89-90
Huronian quartzite , relations of 93
Iddings, J. P., citation from, on crystallizations in
acid lavas 143
on lavas of Yellowstone Park 143
Igneous intriisions, periods of 4iiT-147
Indian reservation in the district, location of 20, 54
Indians in the district, number and character of. . . 20, 54
Ingall,E.D., citation from, on Animikie series 396
on Logan sills 408
Intrusive rocks in Duluth gabbro, occurrence and
character of _ __ 422-424
Iron-bearing carbonates, analyses of 380
Iron-bearing formation, age of ' 195-196, 200
banding of 188-190
belts of 195-196
characters of 22
clastic rocks associated with 212
deposits at bottom of 223-224
deposits within 234r-227
greenstones associated with 196
inclusions of, in greenstones 197
interbedded volcanic greenstones with 196
macroscopic characters of 181-183
mici'oscopic characters of ^ 185-188
origin of 188-191
relations of, to adjacent formations. . . 197-198, 207-208
unconformity between Ogishke conglomerate
and -. 307
Iron Lake, Duluth gabbro at 403
Iron Mountain Lake. See Ensign Lake.
Iron-ore deposits, age of 232-234
character of 101-103, 113, 137-128, 183-185
distribution of 78-79
geologic horizons and relations of. . . 96-97, 127-128, 315
historical sketch of exploration of 2i:3-215
localities of 67,96-97
methods of mining 234-341
origin of - 78-79,95,100,101-103,227-234
prospecting for, mode of 24:j-246
Iron ores, analyses of 385
eharacterof 101-103,113,183-185
coarseness of _ 185
copper minerals in 184
iron content of 184
methods of mining 334-241
phosphorus content of... 184
production of 241-243
shipments of 2)5,241-243
silica content of 184
Iron oxide bodies in Duluth gabbro, analyses of 420
eharacterof 420-432
Irving, R. D., citation from, on Animikie series... 74,396
on Archean formations of Minnesota 77-78
on Cain))r jui iind iir<'-(.'anil)riiLn formations. 89-911
Page.
Irving, R. D.. citation from, on copper-bearing
rocks _ 73-75
on Huronian group _ 80-81
on iron-bearing schists 71
on ii'on ores of Lake Superior region, origin
of.. 78-79
on Logan sills 408
on relations of Duluth gabbro to Kewee-
nawan series 407
geologic work in Vermilion district done by 30
Irving, R. D., and Van Hise, C. R., citation from, on
Animikie slates 396
on Gunflint beds. 379
on iron ores, origin of 228
on mica-schist at Enghsh Lake 350
cited on Penokee iron-bearing series 97, 390
Jasper, brecciation of ._ 194-195
color of 182
folded and contorted, plates showing 176,178
relations of, to greenstones 200-210
unhanded, occurrence of _.. 197
Jasper Lake, spherulitic greenstones at and near 143,
148,168
iron-bearing formation at, relations of , 208-209
jasper inter banded with greenstone at 192-193
Jasper Peak, height of. ._ 1___ 36
jasper at _ 311
Soudan formation at 173,175
Kawishiwi River, course and character of 40-41
conglomerates, metamorphosed, near 316-317
dikes along 357
gabbro near 93,407
granite along and near 363-364, 355, a57,;)59-360, 407
greenstones near 118
ellipsoidal, near 169
metamorphosed rocks along . . , 316-317, 340-341, 359-360
rocks of various kinds near 108
Kawishiwin, name proposed 91
Kawisbiwin agglomerate, character of 104
Kawishiwin greenstone, age and geologic relations
of 91-118
Kawishiwin rocks, components of (note). 131
6'efo/so Ely conglomerate ojjrfSoudanformation.
Keewatin series, age of 11)6-107.114
Kewatin series, eharacterof.. 96,106-107,114-115
deposition of, conditions of . 91
ironores of, mode of origin of 95
origin of 88,93
relations of 87, K)5, 114-115
stratigi-aphic place of 91-92, 100, l(Hi-107
Kekekabic, Kekekebic. Kekequabic. Set- Cacaqua-
bic.
Keweenawau dolerite, relations of Rove slates to . . 395
Keweenawan gabbro, metamorphism of slates bj'. 342-344
relations of, to Cacaquabic granite 368,369
to Giants Range granite 357-3.58
to Gunflint formation 389-;i90
to Knife Lake slates 307-^08
to Ogishke conglomerate 307-^308
to Rove slates 395
to Snowbank granite 363-364
See o/.vo Duluth gabbro.
INDEX.
455
Page.
Keweenawan series, age of -. - 106
Cabotian division of --- -- 123-124
character of — - - 26-27,106
components of 100,122-124
dikes in. — - 422-434
distribution of
Duluth gabbro of..
equivalents of US
exposures of 297,299
formations composing - - ^
gabbroof- - -- 124,397-422
igneous rocks of --- 123
intrusive rocks in 422-424
iron oxide bodies in ._ 420-422
Logan sills of.. 397-422
Manitou division of -- -- 124
metamoi-phism by ..- 418-419,424
origin of -- 443-444
petrographic characters of 401^06,423
Potsdam rocks incliided in... 132-123
Puckwunge conglomerate of 122
relations of, to adjacent form,ations 73-75,408-416
to Logan sills _ 410
stratigraphic place of -. 89,99
topography 399-401
Kloos, J. H., cited on geology of the district 69
Kloos, J. H., and Streng, A., cited on geology of the
district - 70
Knife Lake, Agawa formation at 325
Ely greenstone at 133,136
jasper interbanded with greenstone at 193
Lower Huronian rocks at and near 23,297-303
topography and geologry at, relations of 433,434
slates at and near 300,301
Knife Lake slates, age of 33,73,384,308,406
characters of 393-296, 336-a39, 344-346
dikes in 308,341
dips and strikes of 280
exposures of _. 295-296,347-352
features of. __ _ 34
folding and faulting in 301
granite contacts with 340-342
metamorphism 301,340-344,349-351,445
naming of 335
origin of . _ 44X
petrographic characters of 293-296, 336-339, 344-346
stratigraphic position of 33
pyritein _ 394
relations of, to Agawa formation 303
to Gunflint formation 388-389
to Keweenawan rocks 307-308
to Ogishke conglomerate... 281,303
to Snowbank granite _-. 363
strikes and dips of. 280
thickness of 295,346-547
r..
Lac Bois Blanc. See Basswood Lake.
Lake Gobbemichigamma. See Gobbemichiganima
Lake.
Lake Superior Basin, origin of 444
Lake Superior region, pre-Cambrian history of . . 105-106
Lake Vermillion. See Vermilion Lake.
Lakes of the district, character of.. 41-43
depths of 45-46
origin of. _ 43-46
Page.
Lakes, glacial, occurrence of 429-430
Lauren tian rocks, characters and components of .. 76,
89, 90-91, 107, 113-114
Lawson. A. C, citation from, on Coutchiching 158-160
on ellipsoidal structure of rocks of Lake of
the "Woods region 149
on granite of Saganaga I^ake 265, 268-269, 345
on greenstones of Rainy Lake region 156
on Lake Superior stratigi'aphy 99-100
on Logan sills 398,408,409-410
Lee Hill, breccia at.. 178,330
conglomerate at 287
graphitic slates near 179
greenstones and jaspers at 303-303
Soudan formation at 173, 175, 201
topography and geology at, relations of 431
Leith, C. K., acknowledgments to 56
citation from, on Biwabik formation 378-379,390
on dikes in Embarrass granite 357
ongreenalite 127,190
on griinerite 187
on Gunflint and Lower Huronian rocks, re-
lations of 389
on metamorphism of slates by gabbro in
Mesabi range. 394
on Ogishke conglomerate and iron-bearing
formation, relations of 307
on secondary minerals in schists 161
on spherulitic structure in rocks of Mesabi
district 143
work done by 17,30
Literature of the district, resume of 56-128
Little Saganaga Lake, gabbro at.. 402-403
Logan, W. E , cited on Logan sills 398
Logan sills, age of , 33
characters of 26,405-406,411-416
distribution of 398-399
exposures of 397-398,399
intrusion of 443
metamorphism by ... 418-419
peti'ographic characters of 26,405-406,411-416
relations of. to adjacent formations 133, 408-418
section showing intercalation oJ, in Rove slates. 400
stratigraphic place of 33
topography of 399-401
Long Lake, amygdaloidal rocks at 146
breccia at 363
dikes at 373
gold-bearing quartz at 164
granitic intrusions at 363
greenstone exposures near. 165
spherulitic greenstones at _. 143
Loon Lake, gabbro-slate contact at 394,395
Lower Huronian intrusive rocks, acid dikes of. . . 369-371
character of 23-34
distribution of.... 24^25, a>3-354,;361, 365, 369-370, 371-373
exposures of 354, 359-361, 364, 365, 369, 373
relations of 370-371
topography of .^... 354,361,365
Lower Huronian sediments, age of 23,308
areas of 23,277-296
characters of 24-35,
384^295, 309-313, 327-328, a36-339, 344-346
dikes cutting 306,356-357
divisionscf 23-24,33,275-276
exposures of 278,298-399
formations composing 23-24, 33, 275-276
456
INDEX.
Lower Huroniau sediments, granite dikes in 3oQ-'So7
Knife Lake area of - --- 297-849
metanioi"phism of -- -- -- 419
relations of, to adjacent formations . . . 281-284, 303-308
to dikes - 283-284
to Duhitli gabbro - -.- 407
to Giants Range granite __- 254,2S3,35&-357
to granite of Vermilion Lake 254-256
to Gunflint formation _ 388-389
toSaganaga granite - 268-273
to Snowbank granite - 363
to Upper Hm-onian._ .- 97-98,306-307
petrograpbic cbaractei*s of -. - 284^295,
309-313, 327-328, 336-339, 344^-346
stratigrapbic place of ...,. 308
structure of --- 278-2SL 299-3 13
Vermilion Lake, area of 277-296
Lower Kewatin rocks, occurrence and character
of --- 118-119
Lowest point in district, location of 34
Mackenzie, Alexander, explorations made by 57-63
Magnetitic chert, plate showing - 168
Mallmann, John, exploratory work by. 214
Manitou rocks, character, thickness, and extent of. 124
Marquette district, iron-ore deposits in 22
Marqiiette series, character and thickness of 88-89
eciuivalents of --- - 92
Marquettian, name proposed - 88
Maurer,E. R., acknowledgments to 31
Mayhew Lake, Duluth gabbro at 403
Merriam, W. N., citation from, on iron-bearing
rocks of Gunflint formation • 385
on Logan sills 399
geologic work in Vermilion district done by 17,
30.31.33
sketches of folds in Soudan formation made by. 176
Mesabi range, course and general features of 35-36
geology of- - 114-116
origin of name of _ 35
metamoi'phic contact of gabbro and slate in 1^4
See also Giants range.
Mesabi series, equivalents of _ 92
occui'rence of 121
Metabasalt, plate showing 168
Mica-schist derived from greenstones, composition
of - 157
Mining, methods of - ^-- 234,241
Minnesota, copper minerals of _ 113
crystalline rocks of .- 106-107
ore deposits of 113
Minnesota Geological Survey, reports of, citation
from, on altitude of highest point in dis-
trict 34
citation from, on Mesabi and Giants ranges. 35
on island in Kawishiwi River 41
on Sttintz conglomerate 278
on Ogishke conglomerate 304
on Agawa formation 325
on Cacaqiiabic granite 364
on titaniferous magnetite at Iron Lake 403
on Logan sills 409
on augito-granite cut by hornblende granite . :^2
See also Winchell. Grant, and other Minnesota
geologists.
Page.
Minnesota Iron Company, view showing filling sys-
tem of 236
Minnesota mine, view of main-level timbering at . . 236
Mishiwishiwi. See Kawishiwi.
Montalban group of Vermilion district, rocks com-
posing 76
Moose Lake, Agawa formation near 325,
326,329,330.333-335
amygdaloidal rocks at : 146
breccias near 137, 305
conglomerate at _. 206,304,317
conglomerate and greenstones at, relations of . . 206,
303,304
dikes at 261
Ely greenstone near, relations of 132, 206
glacial deposits near 428
granite at and near. 2ol,264r-265
gi'eenstone at 132,1^5-136,206
greenstone conglomerate at 313
greenstones, ellipsoidal, at. 148-149
greenstones, porphyritic, near __ 348-349
iron-bearing formation near, relations of 205-207
jasper infolded in greenstone at 192
Lower Huronian rocks at _ 297,298
Ogishke conglomerate at and near 206, 303^ 304, 317
reibungs-breccias at 305
rocks at, figure showing 206
slates at 302.336,337
slate fragments in conglomerate at 304
Soudan formation near 172
structure at 135
topography and geology at 433
Mud Creek, granite dikes near _ 256
Mud Creek Bay, chert veins in acid porphyry at . .. 191
conglomerate at 288
granite dike in greenstone at 255
granite-porphyry dike at 257
granitic intrusions in schist at _ 262
intrusive rocks in iron-bearing formation at 199
Muldrow, Robert, work done by.- _ 17
Muscovado, occurrence of, at Gobbemichigamma •
Lake 88,343
Muskegs, occurrence and character of 33
nsr.
Ne wf oiind Lake, Agawa formation at .■- 335
greenstone and granite at 172
glacial deposits near 428
graywackes at. 337
metamorphism at 172
Nickel, occurrence of, in Duluth gabbro.. ._ 421
Nipigon series, age and character of 106
stratigraphic place of 99
See also Keweenawan.
North Twin Lake, jasper near _ 189
spherulitic greenstones near _ 142,168
Norwood, J. G., cited on iron ore in the district 29
geologic work in district done by 65-66
publication of occurrence of iron ore by 22,213
Norwood Lake, site of 430
O.
Oak Lake, arkose at 270
Ogishke conglomerate, age'of 33, 1(15, 284, 308, 406
anticlinal position of 279
character of 23-24,81,284-285,309-313
INDEX.
-457
Page.
Oglshke conglomerate, dikes, basic, in 308
dikes, granite, in 305-:*6
divisions of 84-85
exposures of - 287-293,317-324
origin of - --- 2&5-286,44n
metamorpliisni of 313-317
petrographic charaotei-s of 284-285 , 309-313
relations of, to Agawa formation 303
to Cacaquabic granite 305-306
to Ely greenstones 206-207,303-304
toDuluth grabbro 307-308
to Gunflint formation -. 388-389
to Keweenawan rocks 307-308
to Knife Lake slates- ■- 281
to Saganaga granite 305
to Snowbank granite .-- 353
to Soudan formation ._ 205-207
to Stuntz conglomerate 88
slate fragments in ._ 304
stratigraphic place of 33, 81, 87, 88, 105, 284, 308, 406
thickness of - - 286-287,81?
Ogjshke Munoie Lake, Agawa formation at . . . 323, 327, 328
conglomerate at and near ■ 72,
81, 84-85, 102-103, 276, 284-293, 310, 320-324
geology of region near 73
greenstones and conglomerates at, relations of. 304,
332-323
greenstone near, structure of 136
Lower Hvironian sediments at... 298
Lower Kewatin rocks at --. 119
schistosity of rocks at 302
Ontarian system, geologic place of 100
Orogenic moTements. historical sketch of 437-447
Otter Track Lake, Ely greenstone at 193,207-208
iron- bearing formation at. relations of 207-208
jasper at - - -- 193,310
Soudan formation at, relations of 207-208
topography and geology at, relations of 434
Owen, David Dale, geologic work in northwest done
by -, 65-66
iron-ore deposits first mentioned in report of. . . 313
Paul Lake, Lower Huronian sediments at 300
spotted rocks at _.- 345-346
Paulson Lake, Duluth gabbro at 398
Paulson's mine, Gunflint rocks at 375
Pengilly, cited on occurrence of native copper 184
Penokee series, character and relations of 97
equivalents of 92
Pewabic quartzite, chai-acters of 103,110
occurrence of 119
relations of__ 110
stratigraphic place of. _•- 111,U8
thickness o. _. 102
See aJso Biwabic.
Physiography, features of 34-46
Pickle Lake, Agawa formation at 325, 333
Pike Bay, conglomerate on islands in 287
Pike River, rocks exposed near. . -.- 65,68,296
Pine Island, Vermilion Lake, glacial deposits at 428
Knife Lake slates at — 395
Pine Lake, greenstones near 132
Pioneer mine, ore body at ---- 317-318
ores in, coarseness of 185
shaft of, view showing 216
Page.
Plankton of lakes in the district, collections of 46
Pokegama quartzite, character of 121
Potsdam rocks, equivalents of, in Vermilion dis-
trict... 76,92
Pttckwunge conglomerate, stratigraphic place of . . 122
Ti.
Rainy Lake, Lower Kewatin rocks at 119
Range (Vermilion iron), explanation of use of term. 34
Range 1 west, township 63 north ; 123
R. 2W.T. 63N 123
R. 2W., T. 64N... 133
R. 4W., T. 63N : 123
E. 4 W., T. 65 N., section 21 37.5,377
section 22 375,377
section 23 375,387,390
section 24 387
section26 375,390,391
section 27 119, 133, 161, 375, 377, 388
section 28 34,35,161,398
section 29 161
section 30 161,298,328
section 34 377
R. 5'W., T. 64N 352
section 6... 3.51,423,4.33
R. 5W., T. &5N 253
sections 269
section 7 .- 271
sections 22,23 274
section 25 ^ 388
sections 26, 27 389
sections 29, 30.... 398
sections 31, 32.-. 433
section33 390
section 34 119,377,435
section 35 435
R. 5 W., T. 66 N... 428
section 30 269,434
R. 6W,,T. 60 N -- 123
R. 6W., T. 64N.... 433
section 1.. 161,351,433
section 2 161
R. 6W., T. 65K 432
sections 3, 10 434
sections 11, 18, 19, 25 133
section 26 133,321,323
section 27 133,302,321,323
section28 319
section 29 325,332
section 30 325,333
section 31 133
section 35 301
R. 6W., T. 66 N 438
sections 24, 25, 26, 34 434
section 35 301,434
R. 7W., T.60N .'. 123
R. 7 W.,T. 64N 433
section 1 365,369
section 3 365
R. 7W., T. 65N-. .— 432
section 21 133
sections 22, 23 433
sections 25,36 133
R. 8W., T. 64N.. 361
section 11. 349
sections 36, 35 161
458
INDEX.
Page.
R. 9 W.. T. liSN 133,361,427,43-2
section! 172.192.2C6
section 5 263
section G 142,168
section 10 -.- 341
section 1.5 435
section 16 316
sectionl7 ia3, 316, .341, 34», 373
section 19 353,357,458
section 20 169,2as,316
R. 9 W., T. 64 N 133,361,428.4.32.433
section 10 __. 132
section 16 132,172,261,263,337
section 17 263
section 19 407
section 32 146
section .33 146,207
section 33 362
R.IOW., T. 58N 123
R. low.. T. 62N., sections 398
section 30 119
R. Ill W., T. 63N. 138
section 1 142
sections 3-5, 8, 9 427-12S
section 10 142, 167, 189, 199, 360, 427-428
section 11 142,167
section 12. ...' 142,168
sections 14, 15.. 142.360
sections 16, 17 360
section 20 166,360
section 21 360
sectioif24 .3.53,355
section25 353
section 26 398,435
section 29 298,359
sections!) 340.360
section 31 360
section 34 357
section a5 398
R. low., T. 64N 428
. sectionl9 168,263
section31 132
R. 11 W..T. .55]Sr 123
R.ll W., T. 62N.. 354
R. UW., T. 6:3N.. 173
sections 148
section 7 : 148,192
section 26 340
section 30. 166,173,174,175.427
section 31 427,436
section 34 357
section 36 360
R. 12 W., T. 62 N.. .55.354
section 1 360,436
section2 37,380
sections 166,199,200,359,360,436
section4 199,200,359
section 7 359,360
sections 360
section 9 436
section 11 354,436
section 17 •. 360, 4«)
section 18 199,360
section 19. 167,359,360
section .30 37
R. 13 W., T. 63 N., section 9 146,147
section 10 142,147
Page.
R. 12 W., T. &3N., section 11 : 142
section 13 192
sections 14-16 142
section 21 142,263
^ection22 142
section 25 173,174,175,212,427
section26 427
section 30 164
sections 34-36.. 427
R. 12 W., T. 64 N., section 36 435
R. 12 W.. T. 66 N., section 7 37
R. 13 W., T. 61 X., section 6 427
R. 13W.,T. 62N 428
sections 166
section! 132.297.332
section 7 359,360
section 14 427
section 15 194,427
section 17 166,188
section 18 171
sections 22,23 427
section 24 1 167,359,360
sections26,27 436
section 28 171,359,360,436
section 29 171,4:36
section 31 171,353
section 32 171,194,353
T. 63 N., sections 20,30... 263
section 32 435
T. 66 N., sections 11, 12 37
T. 60N 429
T. 61N 428,429
sectionsl,2. 427
section 4 175
section 10 1.56.427
sections 11, 12, 14, 1.5, 16.18,19,21,22,
29,30 247
T. 62N 166,428,429
section2 277
section 3 _ 435
sections 204
section6 174,204,255
section? 174,175,199,204.257
sections 174
section9 2-56
section 10 212,256
section 15 131,277
sections 16-18 131
section 20 256
section 21 1.31,197,256
section 27 256
section35 168
R. 15W., T. 60N 429
R. 15W.. T. 61N 131,428,429
section 1 197,20:3
section 2 197
sections 174,197,203
section 4 174
section 6 287,292
sections 21-28 427
R. 15 W.. T.02N'.. 173.428
section 1 1:31,174,199,204,255,256
section 2 1:31,43:3
sections 3, 4 -. 1:31
section 7 131.256
sections8,9, 12 131
section 14 287
R. 13 W,
R. 13 W.
R. 14 W.
B. 14W.
R. 14 W.
INDEX.
459
Page.
R. 15 W.,T.63N., section 20 202,295
section S2 3T3
section 27 200
section 28 -... 301
section 35 36,131
section 36 131
E. 15W., T. 63N-. - — 435
sections 34, 35 - 433
R. IIJ W., T. 60N 429
R. 16W., T. 61N 277,429
R. 16 "W"., T. G2 N .-..: 277
R. IGW., T. 63N.. 277
Ransome, F. L., cited on spherulitic rocks of Cali-
fornia 143
Reibungs-breccia, occurrence of 137,178
Relief, features of 34-35
Rivers of the district 40-41
Roa ds in tlie district, character of 53-54
Robinsons Lake, rock banding near 195
Bohn, Oscar, aid by 31
Rose Lake, view at 392
Round Lake, metamorphosed slates near 350
Routes of travel in the district, character of 56-63
Rove slate, age of 33,396,406
deposition of, time of 442
distribution of 391
exposures of 391
gabbro contact with.. 394
metamoi-phism of 393-394,419
naming of 390
occurrence of... 25
petrographic characters of 392-393
relation of, to Duluth gabbro 407
to Keweenawan dolerite and gabbro. 395
section through, showing intercalated Logan
sills.. 400
stratigi*aphic position of 26,33
structure of 392
thickness of 396
topography of _. 391-392
Royal Commission on Miuei*al Resources of Ontario,
report of, cited _ 94
S.
Saganaga Lake, arkose at 270
conglomerate at 85,274,309
Ely greenstone and Ogishke conglomerate at,
contact of sll
geology of region near 65
granitoid and gneisso'd rocks at 87
granite at, correlation of 65
dikes in. 115
distribution of 266
exposures of '. 266
metamorphic effects of 273
Ogishke conglomerate contact with 374
petrographic characters of 266-267
relations of. 268-273
topography of 266
Ogishke conglomerate at 85-86,374,309,311
relation of topography and geology at 434
syenite at 101
Upper Kewatin rocks at 119-120
Saganaga gneiss, stratigraphic place of... 89
Saganaga, granite, relation of, to Kewatin rocks .,^ 109
to Ogishke conglomerate 305
Page.
Saganaga syenite, age of . 101
Saganaga syenite conglomerate, origin of. 94
St. Cloud granite, age of _ 76
St. Louis River, geology of region near 73
St. Louis River, rocks near _ 70
St. Paul and Duluth Railroad, gabbro along... 402
Sardeson, F. W., report of. cited 116-117
Savoy iron mine, reference to 217
Sawtooth Hills, location of 38
Sawtooth Mountains, origin of name of 19
Schistose greenstone, occurrence and character
of. 153-154
Schists, occurrence of 253-254
Scrammers in mine iising caving system, view
showing 240
Seagull Lake, syenite at 86
Seagull syenite conglomerate, origin of 94
Sericite schists, occurrence of 252
Sibley iron mine, reference to 217
Silver City, location of. 53
Silver-bearing rocks, Vermilion Lake, occurrenceof . 67
Slate, folded, plate showing 178
Slaty cleavage, plate showing. 178
Smyth, H. L., and Finlay, J. R., citation from, on
iron ores, origin of 232
on Ogishke conglomerates, origin of 286
on Jasper at Soudan Hill 230
on pseudo-conglomerates _ 253
on schist at Lee mine. 223
on slates of Vermilion district 381
on Vermilion range, geology of 111-112
figure cited from '_ 231
Smyth, H. L., Bayley, W. S., and Van Hise, C. R.,
cited on iron ores, origin of 232
Snowbank granite, age of 406
distribution of 361
exposures of 315,361,364
intrusion of 441
metamorphism of slates by 341
occurrence of 24, 120
petrographic characters of 361-362
relations of, to Duluth gabbro 363-364, 407
to Keweenawan gabbro 363-364
to Lower Huronian sediments 363
topography of 361
Snowbank Lake, conglomerate at and near .. 305,315,318
geology of region near 110-111
granite at and near 87,110-111,315,364
Keweenawan gabbro at 315
Knife Lake slates at 341
Lower Huronian sediments at 398, 300
metamorphism at and near 315, 350
Soil of the district, character of 50
Sondan, graphitic slates at 180
iron-ore deposits at 23, 215, 234-335, 243, 244
location and population of 52
mining methods at ^.. 334-339
sections in mines at 235,236,237,238
Soudan formation, age of 33,195-196,200
character of 179-188
dikes in 356
distribution of- ..' 173-173
divisions of 179
exposures of 173-175
features of 31-33
folding in 175-177,310,312
figure showing 205
460
INDEX.
Page,
Soudan foiination. gi-anite dikes in 356
iron ores of - - 183-185
life content of - - 438
microscopic characters of fragmental part of. 180-181
origin of -_ 188-191,438-439
petrogi-aphic characters of 179-188
relations of . to adjacent formations --. 191-200
to Arcliean intrusives 199
to basic eruptives -. --- 200
to Ely greenstone.-- 205-207
to Giants Range gi-anite --- 356,359
to intrusive rocks --- 256
to Lower Huronian, Vermilion Lake area. 281-283
to Ogishke conglomerate.- - 206-209
stratigraphic position of - 33,195-196
sti-ucture of - 175-179
thickness of 200
topogi'aphy of- 175
Soudan Hill, conglomerate at --- 287
graphitic slate at - - 179
intrusive rocks in iron-beai*ing formation at 199
iron-bearing formation and associated rocks
at - 201-202
Knife Lake slates at 295-296
jasper, banded, at 230
jasper replaced by iron ore at - 231
Soudan formation at 173,175
structure of .-. -_ 224-225
topography and geology at, relation of 431
Specular iron ore, occuia-ence of 67
Spherulites, occurrence of 167-168
Spherulitic structure in Ely greenstone . . - 141-144, 146-148
Spherulitic texture in greenstones, plate showing.- 142
Spilosites, occurrence of - 345
Spurr, J. E., cited on Biwabik formation 378
citation of, on iron-bearing rocks 115,385
on iron-bearing rocks, origin of 190
Stratigi-aphy of the district, table showing 33
Streams of the district 40-41
Streng, A., and Kloos, J. H., cited on geology of the
district . - 70
Stuntz, Ct. R., exploratory work by 214
Stuntz Bay, Vermilion Lake, conglomerates at 252,
291-292
naming of - - 214
Stuntz conglomerate, relations of, to Ogishke con-
glomerat e 88
Stuntz Island, conglomerate at 287
dikes on - -- -,. 257,373
granite-porphyry at 257
Sucker Lake, Agawa formation at - 325
Sucker Point, Indian reservation at -.. 20,54
Knife Lake slates at -- -- 295
Swede Bay, Knife Lake slates at ; 295
Taconic system, components of 76,120
That Mans Lake, Agawa formation at 325,326,329,330
The Other Mans Lake. Agawa formation at 325,
:«6.:$».:^30
This Mans Lake, Agawa formation at 325,
326,:J2tt.:^^,;«l,332
Thompson, David, cited on travels in the North-
west 63
Thunder Bay, Animikie series at 74
Titaniferous magnetite, occurrence of 2U
Page.
Todd, J. E., cited on glacial deposits in Minne-
sota _._ 425.426
Topography of the district, features of . .- 34^46
relations of, to geologic structure - 4;:Jl-i:36
Tower, conglomerate at 387,292
greenstones and jasper folded near 211
gi-aphitic slatesat - 179
gi'eenstones near ,_ - 211
iron-bearing formation and associated rocks
at.. 2((U-201
iron-ore deposits at 23
location of 52
Lower Huronian rocks at 277
metamoi-phosed slates near 349
ore depositsat 243,244
population of _ 20.52
railroad built to 22,29
slatesat 292
Soudan formation at. 173,200-201
Tower group, stratigraphic place of - 8S
Tower Hill, jasper near 182
greenstones and jaspers at, relations of 202-203
iron-bearing formation and associated rocks at- 201
Soudan formation at 1 73, 175
topogrj,phy and geology at, relation of 431
Townline Lake, jasper reported to occur at 310
Towns in the disti-ict. location and population of. 20,52-54
Township 55 North, Range 11 "West 123
T.58N.,R.10W - m
T.60N..R.6W ...- - — . 123
T.60N.,R.7 W.-- 123
T.60N..R.14 W 429
T.60N..R.15W- - 429
T.60N.,R.16'W -, 429
T.61 N.,R. 13 W., section 6 -- 427
T.61 jSr.,R.14W ---- 428,429
sections 1,2 ■ 427
section 4 . .- --_ 175
section lU 156,427
sections 11, 12, 14. 15,16, 18. 19.21,22,29,
30 -- -.- 427
T.61N.,R.15W - 131,428,429
section 1-- 197,203
section 2 197
sections -.,_ 174,197.203
section 4 174
section6 287.292
sections 21-28 427
T.61N.,R.Ui W 277,429
T.62N.,R.2W..- - -.-- 123
T.62N..R.4W- 12:3
T. 62 N.,R. 10 "W.. sections -... 39S
section :30 - 119
T.62N.,R.ll W- - -- 354
T.62N.,R.12 W 35.354
section 1 -... 3(iO,4;i6
section 2 - - 37.;360
section 3 16»j. 11)9. mi a59. m). 4;J6
section 4 199,20i),359
section 7 a59.:360
stctionS 360
section 9 436
section 11 354.4^36
section 17 .' 360.436
sectitm IS.. 199.:*M)
section 19 167.359..M)
section :30 37
INDEX.
461
Paffe.
T.fi2N.,E.13W -- 428
section 3 -. - 166
section 4 132,297,332
section? -.- 195,212
section 14 427
section 15 194,427
section 17 166,188
section 18 ., — 171
sections 22,23 427
section 24 167,a59,360
sections 26,27 436
section28 ..-. 171,359,360,436
section 29 171,436
sectional.. 171, .353
section 32 171,194,353
T. 62 N., R. 14 W 166,428.429
section 2 277
sections 435
section5 204
section 0 174,204,2.55
section? 174,175,199,204,257
section 8 174
section9 2.56
section 10 212,256
section 15 131,277
sections 16-18.. 131
section 20' 256
section 21. 1.31,197,256
section27 2.56
section 35 168
T.li2N., E. 15W 173,428
section 1 1.31 , 174, 199, 204, 255, 256
section 2 1,31,433
sections .3, 4 131
section? 131,2.56
sections 8, 9, 12 131
section 14 287
section 20 202,295
section 22 373
section 27... 200
section 28 201
section .35 36,131
section 36 131
T. 62 N., R. WW 277
T.63N., R. IW 123
T. 63N., B. 9W 133,361,427,432
section 4. 172,192,205
sections 263
section 6 142,168
section 10.... 341
section 15 4.35
section 16 316
section 17 133, .316, 341, 348, 373
section 19 353,357,:i58
section 2u 169,398,316
T. 63N., R. low 133
section 1 142
sections 3-.5, 8,9 427-428
section 10 142, 167, 189, 199, 360, 427-428
section 11 142,167
section 12 142,168
sections 14, 15 142,.360
sections 16, 17 360
section 20 ._ 166,360
section 21 _. 360
section24 353,355
T.63N.
T.63N.
T. 63 N,
T.63N.
T.63N.
T. 63N.
T. 64 N.
T. 64 N.
T. 64 N.
T. 84N.
T. 64N,
T^ 64 N,
T. 64 N,
T. 64N
T. 65 N
Page.
R. low., section 25 353
section 26 398,435
section 29 298,359
section:*).. _. 340,:360
section31 360
section 34 357
section :i5 398
, R.U W 173
section 6 148
section 7 148,192
section 26 340
section 30 : 168; 173, 174, 175, 427
section 31 427,436
section Si _ 357
section 36 i .360
, R. 12 W., section 9 146,147
section 10- 142,147
section 11 142
section 13 192
sections 14-16 142
section 21 142,263
section 22 142
section 25 173,174,175,212,427
section 26... 427
section 30 164
sections 34-86 427
, R. 13 W., sections 20, 30. 263
section .32. 435
, R. 15 W 435
sections 34, 35 433
. R. 16 W 277
, R. 2 W.... 123
. R.5 W 3-52
section 6 351,422,433
, E. 6W 432
section 1 161,351,4.33
section 2 161
, R. 7W ^ 433
section 1 365,369
section 2 365
, R.8 W 361
section 11 349
sections 26, 35 161
, R. 9W 133,361,428,432,433
section 10 : 132
section 16.. 132,173,261,263,337
section 17 2a3
section 19 , 407
section 32 146
section 33 146,207
section 36 362
, R. low 428
section 19 168,263
section 31 132
,, R. 12 W., section 36.. 4.35
,,JR. 4 W., section 21 375,377
section 22 ._. 375,377
section 23 375,387,390
section24 387
section 28 375,390,391
section27 119, ia3, 161, 37.5, 377. 388
section 28 34, .35, 181, 398
section29 : 161
section 30 181,298,388
section 34 377
4(32
INDEX.
Page.
T. fwN., R. 5 W - 352
section ij 269
section 7 .- _ 271
sections 22, 23 274
sections.) .-- 388
sections 26, 27 389
sections 29, 30 ., 398
sections 31, 32 433
section Si — - 390
section 34.. 119,377,435
section 35 435
T.&-)N., R. OTV 432
sections 3,10 434
sections 11,18.19,25.... 133
section 26.. 133,321,323
section 27 133.302,321,323
section 28 319
section 29 325,332
section30 325,332
section 31.. 133
section 35 301
T. 6oN., R. 7W 432
section 21 133
sections 22. 23 433
sections 25, 36 133
T. 66 N., R. 5 W 428
section ;30 : 269,434
T. 66 N.. R. 6 W - 428
sections 24. 25, 26, 34 _ 434
section 35 301,434
T. 66 N.. R. 12 W.. section" 37
T. 66 N., R. 13 "W., sections 11, 12 37
Travel in the distrir-t. routes and metliods of .. 20-21,56-63
Trout, Burntside, and Basswood lakes, granites of,
age of 261
distribution of 258
exposures of 258,262-263
folding in 262
petrographic characters of 259-260
relations of 260-261
topography of 259
Tuffs, occurrence of. associated with greenstones.. 166
Twin Lakes, ellipsoidal spherulitic greenstones
near - 168
Twin Peaks, greenstones at 133
greenstone and gabhro in contact at 162
Twin Peaks Ridge, conglomerate and greenstone in
contact at 303-304
TJ.
Upham. Warren, cited on glacial deposits in Min-
nesota 426
Upper Huronian sediments, area of 25, 374
character of -. 25-26
deposition of. conditions of ,. 442-443
formations composing _.., 25,33
Gunflint formation of 374-390
metamoi*phism of 25.26,419
sediments, relation of. to Duluth gabbi-o. 407
to Logan sills 408^410
to Lower Huronian 97-98
Rove slate of 390-395
See also Animikie, Biwabic, Gunfiint. and
Rove.
Upper Kewatin rocks, occurrence and chai'acter
of 119-120
Page.
Van Hise, C. R.. acknowledgments to 56
citation from, on Animikie rocks, age of 125
on Archean and Algonkian rocks 1(^HJ6. 129
on zone of fracture in rocks 148
on fissility and bedding, relations of 280
on geology of Lake Superior region .. 97-9S,
105-106, 127-12S
on grunerite. origin of 1S7
on hornblende fragments, secondary en-
largement of 77,319.338
on iron-ore deposits of Lake Superior region,
origin of 127-128,190-191.228-230
on metamorphism 161
on schistosity and cleavage as related to
bedding ;3(J2
on slaty cleavage 177
geologic work in Vermilion district done by 30
Van Hise, C. R., and Irving. R. D., citation from, on
Animikie slates 396
on Gunflint beds.. - 379
on iron ores, origin of 228
on mica-schist at English Lake 350
on Penokee series 97-390
Van Hise, C. R.. Bayley, W. S., and Smyth, H. L..
citation from, on iron ores, origin of 232
Van Horn, F. B., acknowledgments to 31
Vermilion gi-oup, relations of rocks of 87
Vermilion Lake, area of 41
character of 114
conglomerate at 291-292
stratigraphlc relations of ." 98-114
conglomerates and pseudo-conglomerates at . 251-252
depth of 45
geology of region near 65,68.69
glacial deposits at 428
gold-bearing rocks near 67-69
gold mining at 213
gi*anite at, age of 68
distribution of 247
exposures of 247-248
folding in 250-251
petrographic characters of 248-250
relations of, to Ely greenstone 255-256
structure and metamorphism of 251-254
greenstones at and near 118,131.135
str uctiire of.. _ 135
hematite ores near 67,83
iron-bearing rocks at, relations of 74,2(.e
iron ores at and near 67,75,83,213.214
jasper and hematite near 72
Knife Lake slates at 295
Lower Huronian rocks at and near, age of.. — 284
distribution of 277-278
divisions of 275-276
exposures of 278
Knife Lake slates of 293-296
Ogishke conglomerate of 284-293
relations of 281-284
structure of 278-281
topography of - 278
micaceous rocks near 68
ores near ^
origin and general features of 44
pseu do -conglomerates at 251-25:J
schists at -- 253-254
sedimentary rocks at 23
INDEX.
463
PagL'.
Vermilion Lake, silver -bearing rocksnear _ 67
slate and jasper banding near _. 178
Soudan formation at _ 173
Stimtz conglomerate at 114
succession of rocks at 79-i5()
topography and geology at, relations of. . . 432-433, 435
Upper Keewatin rocks at ._ 119-120
Vermilion group, relations of - _ 87,91-92
str atigr apbic place and thickness of 89
Vermilion iron range, explanation of use of term. , 34
Vermilion moraine, extent and general features
of. 426-429
map showing 427
Vermilion range, geology of 111-112
Vermilion River. See Pike River.
Volcanic activity, periods of 437-447
A\"
Walcott, C. D., cited on Cambrian Sea transgression
in North America 445
Watab granite, age of 76
Water power in the district 42-43
Weidman, Samuel, cited on ellipsoidal structure in
Wisconsin rocks '. 150
West Gull Lake, glacial deposits near 428
granite and greenstone at 274
greenstone-conglomerate contact at 324
map shovring exposures at 272
rocks at „.. 271-273
West Seagull Lake, conglomerate and syenite at. .. 86
granite at 115
West Two Rivers, conglomerate at 292-293
White Iron gneiss, stratigraphic place o£ 89
White Iron Lake, granite at 353-354
granitoid and gneissoid rocks at 87
greenstones near 118
Whitewood Lake. See Basswood Lake.
Whittlesey, Charles, cited on geology of the dis-
trict 68,70
Willis, Bailey, cited on succession and structui-e in
part of Vermilion district 79-80
work in district done by 29-30, 214
Willmott. A. B., cited on ellipsoidal structure in
Lake Superior rocks 150
work in district done by 125
Winchell, Alexander, citation from, on Archean
rocks of Minnesota 95-96
on conglomerates 85-86,94
on geology of northeastern Minnesota. 81-82, 85-89
on granite of Saganaga Lake 258, 265
on jasper at Townline Lake 310
on muscovado 343
on schist and granite at Burntside Lake, re-
lations of 158
on succession of terranes in northeastern
Minnesota 88-89
on uncomformities in Vermilion district 85-89
Page.
Winchell, H. V., citation from, on granite of Sagan-
aga Lake. 265,268
on inclusions of iron-bearing formation in
greenstones 197
on iron-bearing rocks *of Gunflint forma-
tion ._ 385
on iron oxide bodies in Duluth gabbro 420
on iron region of Minnesota 93
on muscovado. _ _ , 343
on Saganaga syenite 101
Winchell, N , H . , citation from, on Archean
rocks 70
on arkoses of Saganaga Lake . 270
on basalt, ellipsoidal, in Minnesota 145
on conglomerate of Ogishke Muncie Lake., 72,
304,321
on copper -bearing series 72
on crystalline rocks of Minnesota 106-107
on Duluth gabbro 401,408
on Ely greenstones, origin of 154-155
on eruptive rocks of Minnesota 94
on geology of Minnesota 70,71-73,76-77,
82-a5, 90-92, 94. lOO-lOl. 104, 106-107, 113-114, 117-125
on Giants and Mesabi ranges 35
on glacial lakes 127,430
on gold-bearing quartz at Long Lake 164
on great gabbro 100-101
on iron ore at Vermilion Lake 69,76
on iron ores of Gunflint beds 385
on Kawishiwin agglomerate 104
on Mesabi and Giants ranges 35
on metamorphism of slates to schist 296
on Ogishke conglomerate and Ely green-
stone, contact of 304
on schists and granites at Burntside
Lake 158
on Snowbank granite, origin of 363
on spherulitic mottling of Ely greenstones . 142
on structural geology of Minnesota 117-125
maps, geologic, collected and published
by... 12T
Winchell, N. H. and H. V., citation from, on rocks
of the Biwabik formation 378
on iron ores of Minnesota 94-95,
96-97, 101-103. 189-190. 233
Winchell. N. H., Grant, U. S., and Elftman, A. H.,
report prepared by, on geology of the
Vermilion district 117
Wind Lake, Agawa formation near. 325, 326, 829, 330, 333-335
conglomeratic rock at 170-171
green schist near 170
Winten, industries of 20
location and population of 53
Wonder Island, conglomerate at 85-86
Zenith iron mine, reference to... 317
O
PUBLICATIONS OF UNITED STATES GEOLOGICAL SURVEY.
[Monograph XLV.]
The serial publications of the United States Geological Surrey consist of (1)
Annual Reports, (2) Monographs, (3) Professional Papers, (-4) Bulletins, (5) Mineral
Resources, (6) Water-Supply 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 ma}' be had
on application.
MONOGRAPHS.
I. Lake Bonneville, by G. K. Gilbert. 1890. 4°. xx, 4.38 pp. 51 pi. 1 map. Price $1.50.
II. Tertiary history of the Grand Canon district, with 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°.
XV, 422 pp. 7 pi. and atlas of 21 sheets folio. Price $11.
IV. Comstock mining and miners, by Eliot Lord. 1883. 4°. xiv, 451 pp. 3 pi. Price S1.50.
V. The copper-bearing rocks of I fke Superior, by E. D. Irving. 1883. 4°. xvi, 464 pp. 15 1. 29 pi.
and maps. Price $1.85.
VI. Contributions to the knowledge of the older Mesozoic flora of Virginia, by AV. M. Fontaine. 1883.
4°. xi, 144 pp. 54 1. 54 pi. Price $1.05.
VII. Silver-lead deposits of Eureka, Nevada, by J. S. Curtis. 1884. 4°. xiii, 200 pp. 16 pi. Price
$1.20.
VIII. Paleontology of the Eureka district, by C. D. Walcott. 1884. 4°. xiii, 298 pp. 24 1. 24 pi.
Price $1.10.
IX. Brachiopoda and Lamellibranchiata of the Earitan clays and greensand marls of New Jersey, bv
E. P. Whitfield. 1885. 4°. xx, 338 pp. 35 pi. 1 map. Price $1.15.
X. Dinocerata. A monograph of an extinct order of gigantic mammals, by 0. C. Marsh. 1886. 4°.
xviii, 243 pp. 56 1. 56 pi. Price $2.70.
XI. Geological historv of Lake Lahontan, a Quaternary lake of northwestei'n Nevada, bv I. C.
Eussell. 1885. 4°. xiv, 288 pp. 43 pL and maps. Price SI. 75.
XII. Geology and mining industry of Leadville, Colorado, with atlas, by S. F. Emmons. 1886. 4°.
xxix, 770 pp. 45 pi. and atlas of 35 sheets folio. Price $8.40.
XIII. Geology of the quicksilver deposits of the Pacific slojje, with atlas, by G. F. Becker. 1888.
4°. xix, 486 pp. 7 pi. and atlas of 14 sheets folio. Price $2.
XIV. Fossil fishes and fossil plants of the Triassic rocks of New Jersev and the Connecticut Valley,
by J. S. Newberry. 1888. 4°. xiv, 152 pp. 26 pi. Price $1.
XV. The Potomac or younger Mesozoic flora, by W. M. Fontaine. 1889. 4°. xiv, 377 pp. 180 pi.
Text and plates bound separately. Price S2.50.
XVI. The Paleozoic fishes of North America, by J. S. Newberrv. 1889. 4°. 340 pp. 53 pi. Price
$1.00.
XVII. The flora of the Dakota group, a posthumous work, bv Leo Lesquereux. Edited bv F. H.
Knowlton. 1891. 4°. 400 pp. 66 pi. Price $1.10.
XVIII. Gasteropoda and Cephalopoda of the Earitan clays and greensand marls of New Jersej', bv
R.P.Whitfield. 1891. 4°. 402 pp. 50 pi. Price $1.
XIX. The Penokee iron-bearing series of northern Wisconsin and i\Iichigan, bv E. D. Irving and
C. E. Van Hise. 1892. 4°. xix, 534 pp. Price $1.70.
XX. Geology of the Eureka district, Nevada, with an atlas, by Arnold Hague. 1892. 4°. xvii, 419
pp. 8 pi. Price $5.25.
XXI. The Tertiary rhynchophorous Coleoptera of the LTnited 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 $1.
XXIII. Geologv of the Green Mountains in Massachusetts, bv Raphael Pumpelly, T. N. Dale, and
J. E. Wol£ 1894. 4°. xiv, 206 pp. 23 pi. Price $1.30.'
MON XLV — 03 30 I
II PUBLICATIONS OF UNITED STATES GEOLOGICAL SURVEY.
XXIV. Mollusca and Crustacea of the Miocene formations «i Xew Jersey, bv R. P. Whitfield. 1894.
4°. 19.S pp. 24 pi. Price 90 cents.
XXV. The Glacial Lake Agassiz, by Warren Uphani. 1S9.5. 4°. xxiv, 658 pp. 38 pi. Price SI. 70.
XXVI. Flora of the Ambov clavs, by J. S. Xewberrv; a posthumous work, edited bv Arthur Hollick.
1S9.^. 4°. 260 pp. 58 pi. ■ Price 81.
XXVII. Geology of the Denver Basin in Colorado, bv S. F. Emmons, Whitman Cross, and G. H.
Eldridge. 1896. 4°. .556 pp. 31 pi. Price SI. .50.
XXVIII. The Marquette iron-bearing district of Michigan, with atlas, by C. R. Van Hise and W. S.
Baylev, including a chapter on the Repul;)lic trough, by H. L. Smyth. 1895. 4°. 608 pp. .35
pi. and atlas of 39 sheets folio. Price So. 75.
XXIX. Geology of old Hampshire County. Mas.?achusetts, comprising Franklin, Hampshire, and
Hampden counties, by B. K. Emerson. 1898. 4°. xxi, 790 pp. 35 pi. Price 81.90.
XXX. Fossil Medusce, by" C. D. Walcott. 1898. 4°. ix, 201 pp. 47 pi. Price 81.50.
XXXI. Geology of the Aspen minins district, Colorado, with atlas, by J. E. Spurr. 1898. 4°. xxsv,
260 pp. 43' pi. and atlas of 30 sheets folio. Price S3.60,
XXXII. Geology of the Yellowstone National Park, Part II, descriptive geology, petrography, and
paleontology, bv Arnold Hague, J. P. Iddings, W. H. Weed, C. D. Walcott," G. H. Girtv,"T. W.
Stanton, and F. H. Knowlton. 1899. 4°. xvii, 893 pp. 121 pi. Price 82.45.
XXXIII. Geology of the Xarragansett Basin, by N. S. Shaler, J. B. Woodworth, and A. F. Foerete.
1899. 4°. x"x, 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. .52 pi. Price 81. .30.
XXXV. The later extinct floras of Xorth 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, bv AV. S. Bavlev, and an introduction bv 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, witli
descriptions of a few other forms and a systematic list of the non-rhvncophorous Tertiary Coleoptera
of Xorth America, by 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 pp. 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 Mesabi iron-bearing district of Minnesota, bv C. K. Leith. 1903. 4°. 316 pp. 33 pis.
Price 81.50.
XLIV. Pseudoceratites of the Cretaceous, by Alpheus Hyatt, edited by T. \V. Stanton. 1903, 4°.
350 pp. 47 pis. Price .
XLV. The Vermilion iron-bearing district of Minnesota, with atlas, bv J. M. Clements. 1903. 4°.
463 pp. 13 pis. Price .
All remittances must be by MOXEr order, made payable to the Director of the
United States Geological Survey, 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.
LIBRARY CATALOGUE SLIPS.
[Mount each slip upon a separate card, placing the subject at the top
of the second slip. The name of the series should not be repeated
on the series card, but add the additional numliers, as received, to
the first entry.]
Clements, J[ulius] Morgan.
. . . The Vermilion iron-bearing district of Min-
nesota, with an atlas; by J. Morgan Clements.
Charles Richard Van Hise, geologist in charge.
Washington, Gov't print, off., 1903.
(U. S. Geological survey. Monographs v. xlv. ) 463 p. 13 pi.
30""'. and atlas (26 sheets) 53x46"=°'.
Clements, J[ulius] Morgan.
. . . The Vermilion iron-bearing district of Min-
nesota, with an atlas; by J. Morgan Clements.
Charles Richard Van Hise, geologist in charge.
Washington, Gov't print, off., 1903.
(U. S. Geological survey. Monographs v. xlv.) 463 p. 13 pi.
30""'. and atlas (26 sheets) .53x46™'.
U. S. Geological survey.
Monographs.
V. 45. Clements, J. M. The Vermilion iron-bearing
district of Minnesota, with an atlas. 1903.
U. S. Dept. of the Interior.
see also
U. S. Geological survey.
Live Music Archive
Librivox Free Audio
Metropolitan Museum
Cleveland Museum of Art
Internet Arcade
Console Living Room
Books to Borrow
Open Library
TV News
Understanding 9/11