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DEPARTMENT OE THE INTElilOR
MONOGRAPHS
OP THE
United States Geological Survey
VOLUME XXXVI
WASHINGTON
GOVERNMENT PRINTING OFFICE
1899
UNITED STATES GEOLOGICAL SURVEY
CHARLES I). WALCOTT, DIRECTOR
THE
CMSTAL FALLS IRON-IARING DISTRICT OF MICHIGAN
liY
J. MORGAN CLEMENTS and HENRY LLOYD SMYTH
WITH
A CHAPTER ON THE STURGEON RIVER TONGUE
WILLIAM SHIRLEY BAYLEY
AND
AN INTRODUCTION
BY
CHARLES RICHARD VAN HISE
WASHINGTON
GOVERNMENT PRINTING OFFICE
1899
"A^o»).
CONTENTS.
Page.
Letter of trans.mittal ,. xv
Introduction, iiY C'h.\rle8 Riciiaku Van Hise xvii
Outline of monograph xxix
Pakt I. — The western part tiF the Crystal Falls district, isy J. Morgan Clements. 11
Chapter I. — Introdiction 11
Previous work in the district 13
Mode of work 22
Magnetic observations 24
Chapter II.— Geographical limits, structure and stisatigraphy, and physiography 25
Geographical limits 25
Structure and stratigraphy 25
Physiography 29
Topography 29
Drainage 31
Timber and soil 36
Chapter III. — The Archean 38
Distribution, exposures, and topography - 38
ReLations to overlying formations 39
Petrographical characters 10
Biotite-granite (granitite) 40
Gneissoid biotite-granite, border facies of granite 43
Acid dikes in Archean 45
Basic dikes in Archean 46
Schistose dikes 46
Massive dikes 48
R^sum6 49
Chapter IV. — The Lower Hdronian series 50
Section 1.— The Randville dolomite 50
Distribution, exposures, and topography 50
Petrographical characters 51
Relations to underlying and overlying formations 53
Thickness 53
Section 2.— The Mansfield slate 54
Distribution, exposures, and topography 54
Possible continuation of the Mansfield slate 55
Petrographical characters 56
Gray wacke 56
Clay slate .and phyllite ■■ 57
Origin of clay slate .and phyllite 58
Present composition necessarily dift'erent from that of rock from which derived 58
Analysis of Mausfield slate 59
Comments on analysis 59
Comparison of analysis of Mansfield clay slate with analyses of clays 60
Comparison of analysis of Mansfield clay slate with analyses of other clay slates. .. 61
Siderite-slate, chert, ferruginous chert, and iron ores 62
Rjlations of siderite-slate, ferruginous chert, and ore bodies to clay slate 63
Relations of Mansfield slate to adjacent formations 63
Relations to intrusives 63
Relations to volcanics 64
V
VI CONTENTS.
Chapter IV. — The Lower Huroniax series — Continued.
Section 2. — The Mansfielil slate — Continued. Page.
Structure of the Mansfield area 64
Thickness 64
Ore deposits 65
General description of Mansfield mine deposit 66
Relations to surrounding beds 68
Comijosition of ore 68
Microscoiiical character of the ores and associated chert bands 69
Origin of the ore deposits 70
Conditions favorable for ore concentration 72
Exploration 73
Section 3. — The Hemlock formation 73
Distribution, exposures, and topography 73
Thickness 74
Eelatious to adjacent formations 75
Relations to intrusi ves 77
Volcanic origin 78
Classification 79
Acid Tolcanica 80
Acid lavas 80
Khyolite-porphy ry 81
Texture 83
Aporhy olite-porphy ry 87
Schistose acid lavas 87
Acid pyroclastics 94
Basic volcanics 95
Basic lavas 9.5
General characters 95
Nomenclature 93
Metabasalts 98
Nonporphyritic metabasalt 98
Petrographical characters 98
Chemical composition 103
Porphyritic metabasalt 103
Petrographical characters 104
Chemical composition 105
. ' Variolitic metabasalts 108
EUip.soidal structure 112
Origin of ellipsoidal structure 118
Amygdaloidal structure 124
Flattening of amygdaloidal cavities 12l)
Alterati on of the basalts 126
Description of some phases of alteration 127
Pyroclastics 135
Eruptive breccia 135
Volcanic sedimentary rocks 136
Coarse tuffs 137
Fine tuffs or ash (dust) beds 142
Relations of tuffs and ash (dust) beds 143
Volcanic conglomerates 143
Schistose pyroclastics 145
The Bone Lake crystalline schists , 148
Distribution 148
Field evidence of connection with the volcanics 149
Petrographical characters 150
CONTENTS. VII
CnAPTKR IV. — Thk I.owkk Hukonian skriks — Continuod.
Section 3. — The llciulock t'onnation — Continued. I'agn.
Normal scdiinontaiit^s (if the Hemlock formation 152
Economic products 1,")3
Building and ornamental stones 153
Road materials 154
CHAPTEU V. — TllK UlTKI! HURHNIAX SICHIISS 155
Distril)Ution, oxposurea, and topograpbj- 1.55
Magnetic lines 1.56
Thickness 1.57
Folding 158
Crystal Falls syncline 158
Time of folding of tbo Upper Huronian 161
Relations to other series 162
Relations to intrusives 16i
Correlation 164
Petrograpliical characters 165
Sedimentary rocks 165
Microscopical description of certain of the sedimentaries 169
Igneons rocks 174
Ore deposits 175
History of opening of the district 175
Distribution 175
Western half of sec. 34, T. 46 N., R. 33 W 176
Sec. 20, T. 45 N., R. 33 AV , 176
The Amasa area 177
The Crystal Falls area 178
Character of the ore 180
Relations to adjacent rocks 182
Origin 183
Size of the ore bodies 184
Methods of mining 184
Prospecting 185
Production of ore from the Crystal Falls area 186
Chapter VI. — The Intrusives 187
Order of treatment 188
Age of the iutrusives 188
Relations of folding and the distribution of the intrusives 189
Section 1. — Unrelated intrusives 190
Classification 190
Acid iutrusives 190
Geographical distribution and exposures of granites 190
Biotite-granite 191
Micropegmatites 192
Muscovite-biotite-granite 193
Relations of granites to other intrusives 194
Dynamic action iu granites 194
Contact of granites and sedimentaries 194
Evidence of intrusion 195
Basic intrusives 198
Metadolerite 199
Geograiihical distribution 199
Petrographical characters 199
Maeroscoiiical 199
Microscopical 200
VIII CONTENTS.
Chapter VI. — The Intrusives— Continued.
.Section I. — Unrelated intrusives — Continued.
Basic intrusives — Continued.
Metadolerite — Continual. Page.
Relations to adjacent rocks 20S
Relations to Lower Huronian Mansfield slates 203
Relations to Lower Huronian Hemlock volcanics 204
Relations to Upper Huronian 204
Relations to other intrusives 204
Contact metaniorpbism of Mansfield slates by the dolerite 204
Spilosites 206
Analyses of spilosites 207
Desiuosites 207
Adinoles 208
Analyses of adinoles 208
Comparison of analyses of rtormal Mansfield clay slates and the contact prod-
ucts 209
No endomoriiliic effects of dolerite intrusion 211
Metabasalt 211
Ultrabasic intrusives .' 212
Picrite-porphyry (porphyritic limburgite) 212
Geographical distribution and exposures 212
Petrographical characters 212
Gray tremolitized picrite-porphyry 213
Dark serpentinized picrite-porphyry 217
Classification 220
Section II. — A study of a rock series ranging from rocks of intermediate acidity through
those of basic composition to ultrabasic kinds 221
Diorite 222
Nomenclature 222
Distribution and exposures 223
Petrographical characters 223
Description of interesting variations 226
Sec. 15, T. 42 N., R.31 W 226
Across river from Crystal Falls 227
Southeiistof Crystal Falls 227
Analysis of diorite 231
Gabbro and norite 233
Petrographiciil characters 233
Description of interesting kinds of gabbro 240
Hornblende-gabbro in sec. 15, T. 42 N., R. 31 W 240
Sees. 15, 22, 28, and 29, T. 42 N., R. 31 W 241
Hornblende-gabbro dikes 243
Bronzite-norite dike 244
Sec.29,T.42N., R.31 W., 1200 N., 200 W 245
Dynamically altered gabbro 247
Relative ages of gabbros 249
Peridotites ^249
Distribution, exposures, and relations 249
Petrographical characters 249
Peridotite varieties 252
Wehrlite 2.53
Amphibole-peridotite 253
Gradations of amphibole-peridotite to wehrlite and olivine-gabbro 254
Process of crystallization 257
Analysis of peridotite 259
CONTENTS. IX
CiiAriKK \'I. — TiiK iNTUfsivES — Coutiiiued.
Si'ction 11. — A study of a rock series, etc. — Continued.
Peridotites — Continued.
I'eridotito varieties — Continued. Page.
Pcridotito from sec. 22, T. 42N., R.31 W., I'JSION., loOW 26()
Relations of peridotites to other rocks 261
Ago of peridotites 262
General observations ou the above series 262
Textural characters of the series 262
Chemical composition of the series 263
Relative ages of rocks of the series 265
Part II. — Thk kastern part of the Crystal Fall.s district, including the P'elch
Mountain range, by Henry Lloyd Smyth.
Chapter I. — CiEOORAPHicAL limits and physiography 32&
Introduction 329
Preliminary sketch of geology 331
Character of surface 331
Drainage - 334
Chapter II. — Magnetic observations in geological mapping 336
Section I. — Introduction 336
Section II. — Description of the magnetic rocks 338
Section III. — Distribution of magnetism in the magnetic rocks 339
Section IV. — Instruments and methods of work .' 341
Section V. — Facts of oliservatiou and general principles 344
(1) Observed deticctions when the strike is north and south and the dip vertical 344
(2) Detleetious of the horizontal needle 345
(3) Deflections of the dip needle. , 347
(4) Horizontal and vertical components when the magnetic rock dips vertically 349
(5) Horizontal and vertical components when the magnetic rock dips at any angle 350
(6) Determination of depth 854
(7) Summary 356
Section VI. — Applications to special cases 3.56
(1) The magnetic rock strikes east or west of north and dips vertically. 357
(2) .The magnetic rock strikes east and west 359
(3) Two parallel magnetic formations 361
Section VII. — The interpretation of more complex structures 366
(1) Pitching synclines 367
(2) Pitching anticlines 370
(3) Formations split by intrusives 371
(4) Summary 372 ■
Chapter III. — The Felch Mountain range 374
Section I. — Position, extent, and previous work 374
Section II. — General sketch of the geology 383
Section III. — The Archean 385
Topography 386
Petrographical characters 387
Section IV. — The Sturgeon quartzite 398
Distribution, exjiosures, and topography 398
Folding and thickness 399
Petrographical characters 401
Section V. — The Randville dolomite 406
Distribution, exposures, and topography 406
Petrographical characters 408
X CONTENTS.
Chapter III. — The Felcu Mountain range — Continued. Page.
Section VI. — The Manisfield scliists 411
Distribution, exposures, and topography 411
Petrographical characters 412
Section A'll. — The Groveland formation 415
Distribution, exposures, and topography 415
Petrographical characters 417
Section VIII. — The mica-schists and quartzites of the Upper Huronian series 423
Petrographical characters 425
Section IX. — The iutrusives 426
Chapter IV. — The Michigamme Mountain and Fence River areas 427
Section I. — The Archean 428
Section II. — The Sturgeon formation .' 430
Section III.— The Raudville dolomite 431
Distribution and exposures 431
Folding and thicliness 432
Petrograpliical characters 434
Section IV. — The Mansliold formation 437
Distribution, exposures, and topography 438
Folding and thickness 438
Petrographical characters 439
Section V. — The Hemlock formation 440
Distribution, exposures, and topography 440
Folding and thickness 441
Petrographical characters 442
Section VI. — The Groveland formation 446
Distribution, exposures, and topography 446
Folding and thicliness 448
Petrographical characters 448
Chapter V. — The northea.stkrn area and the relations between the Lower Mar-
(jtette and the Lower Menominee series 451
Chapter VI. — The Sturgeon River tongue, by William Shirley Bayley 458
Description and boundary of area 4.58
Literature 459
Relations between the sedimentary rocks and the granite-schist complex 461
The IJasement Complex 463
The gneissoid granites 463
The amphibole-sihists 465
Origin of the amphibole-schists 466
The biotitc-schists 467
The intrusive rocks 469
Comparison of the Sturgeon River and the Marquette crystalline series 470
The Algoukian trough 471
Relations between the conglomerate and the dolomite series 472
Relations between the dolomites and conglomerates and the overlying sandstones 473
The conglomerate formation 473
Important exposures 474
Petrographical descriptions 477
The dolomite formation 479
Important exposures 480
Petrographical descriptions 481
Slates and sandstones on the Sturgeon River 481
The igneous rocks 482
The intrusive greenstones 482
Petrographical description 482
The banded greenstones 485
Petrographical description 486
ILLUSTRATIONS.
Pago.
Plate I. Colored map ahowiug the distribution of pre-C';imbriau and other rocks in the Lake
Superior region, and tlie goographical relations of the Crystal Falls district of
Michigan to the adjoining Marquette and Menominee districts of Michigan 11
II. Topographical map of the Crj-stal Falls district of Michigan, including a portion of
the Marquette district of Michigan In pocket.
III. Geological map of the Crystal Falls district of Michigan, including a portion of the
Mar([uette district of Michigan In pocket.
IV. Portion of geological map of the Menominee iron region, by T. B. Brooks and C. E.
Wright 18
V. Generalized sections to illustrate the stratigraphy and structure of the northwestern
part of the Crystal Falls district of Michigan " 28
VI. Generalized sections to illustrate the stratigraphy and structure of the southern part
of the Crystal Falls di,striet of Jlichigan 28
VII. Generalized columnar section 30
VIII. Map of a portion of the Crystal Falls district, showing iu detail the glacial topog-
raphy and illustrating the development of the Deer Kiver 32
IX. Sketch of the Mansfield mine as it was before it caved iu, iu 1893 66
X. A, Reproduction of the weathered surface of a variolite; B, Reproduction of the
polished surface of a variolite 110
XI. Colored reproduction of an ellipsoid, with matrix, from <in ellipsoidal basalt 116
XII. Mount Giorgios, viewed from its west flank, in April, 1866, illustrating the charac-
teristic block lavas, from Fouque's Santorin et ses Eruptions, PI. VIII 120
XIII. Reproduction iu colors of a basalt tuflf 140
XIV. Idealized structural map and detail geological map, with sections, to show the dis-
tribution and structure of the Huronian rocks in the vicinity of Crystal Falls,
Michigan 160
XV. Portion of Brooks's PI. IX, Vol. Ill, Wisconsin Geological Survey 172
XVI. Detail geological map of the vicinity of Amasa, Michigan 176
XVII. Detail geological map of the vicinity of Crystal Falls and Mansfield, Sheet 1 178
XVIII. Detail geological map of the vicinity of Crystal Falls and Mansfield, Sheet II 178
XIX. J, Inclusions iu a fractured quartz pheuocryst; />', Quartz phenocryst with rhombo-
hedral parting 268
XX. yl, Micropoikilitic rhyolite-porphyry; /?, Micropoikilitic quartz-porphyry 270
XXI. J, Very fine-grained micropoikilitic rhyolite-porphyry viewed without analyzer;
B, Very fine-grained micropoikilitic rhyolite-porphyry viewed with analyzer 272
XXII. A, Perlitic parting in aporhyolite; B, Perlitic parting in aporhyolite 274
XXIII. A, Schistose rhyolite-porphyry; B, Aporhyolite breccia 276
XXIV. .4, Schistose rhyolite-porphyry ; /?, The same viewed between crossed nicols 278
XXV. ^,Amygdaloidal texture of basalt; i>, Amygdaloidal vitreous basalt 280
XXVI. A, Amygdaloidal vitreous basalt ; B, Amygdaloidal vitreous basalt showing sheaf-like
aggregates of feldspar 282
XXVII. ^, Reproduction in colors of amygdaloidal basalt; i>, Pseudo-amygdaloidal matrix
of ellipsoidal basalt ; C, Water-deposited pyroclastic 284
XXVIII. A, Fine-grained basalt with well-developed igneous texture; B, Illustration of the
obliteration of the igneous texture of a basalt by secondary products when viewed
between crossed nicols 286
XI
XII ILLUSTRATIONS.
XXIX. J, Basalt showing characteristic texture in ordinary light; B, Basalt showing
obliteration of texture between crossed nicols 288
XXX. A, Basalt showing iu ordinary light a distinctly amygdaloidal texture; B, The same
basalt with its amygdaloidal texture obliterated when viewed between crossed
nicols 290
XXXI. A, Basalt affected by calcification process; B, Basalt affected by calcification process
viewed between crossed nicols 292
XXXII. A, Illustration of perlitic parting in a fragment from a basaltic tuff; B, Sickle-
shaped bodies in volcanic tuff 294
XXXIII. A, Water-deposited sand; B, Gradation iu water- deposited volcanic sediment 296
XXXIV. A, Contact product of granite ; B, Brecciated matrix between ellipsoids 298
XXXV. .4, Contact between granite and a metamorphosed sedimentary ; /?, Contact between
granite and a metamorphosed sedimentary viewed between crossed nicols 300
XXXVI. A, A variety of spilosite with white spots ; B, A variety of spilosite with white spots
viewed between crossed nicols 302
XXXVII. -J, Normal spilosite or spotted contact product; .B, Normal spilosite of somewhat
different character 304
XXXVIII. J, Passage of spilosite into demosite; i?, Occurrence and alteration of bronzite in
bronzite-norite 306
XXXIX. A, Biotite-granite viewed between crossed nicols ; B, Mica-diorite viewed between
crossed nicols 308
XL. A, Quartz-mica-diorite-porphyry ; B, Quartz-mica-diorite-porphyry viewed between
crossed nicols 310
XLI. J, Porjihyritic poikilitic hornblende gabbro; i?, Poikilitic hornblende gabbro 312
XLII. ll, Moderately fine-grained hornblende gabbro showing parallel texture; £, Mod-
erately fine-grained hornblende gabbro showing parallel texture viewed between
crossed nicols 314
XLIII. A, Normal granular hornblende gabbro; B, Schistose hornblende gabbro viewed
between crossed nicols 316
XLIV. J, Moderately fine-grained hornblende gabbro; iJ, bronzite-norite 318
XLV. J, Bronzite-norite-porphyry ; i>, Feldspathic wehrlite 320
XLVI. J, Feldspathic wehrlite viewed between crossed nicols; Bj Feldspathic wehrlite 322
XLVII. Relations of magnetic beds to variation and dip 352
XLVIII. Relations of magnetic beds to variation and dip 362
XLIX. Geological map of the Felch Mountain Range 374
L. Geological map of a portion of the Crystal Falls district 450
LI. Geological map of the Sturgeon River tongue 458
LII. Map of exposures in sec. 7 and portions of sees. 8, 17, and 18, T. 42 N., R. 28 W 474
LIII. Schist conglomerate from dam of Sturgeon River 476
Fig. 1. Reproduction of a portion of the geological map of the Upper Peninsula of Michigan,
by William A. Burt. 1846 15
2. Enlarged reproduction of a portion of a map of the Lake Superior land district, by Foster
and Whitney 17
3. Enlarged reproduction of a portion of a geological map of the Upper Peninsula of
Michigan, by Rominger, Brooks, and Pumpelly, 1873 18
4. Granite-porphyry with inclusions of gneissoid granite 45
5. Illustration of the effect on the topography of the differential erosion of basic dikes
and granite 46
6. Concentric cracks formed by the caving in of the Mansfield mine 65
7. Sketch of the surface of the outcrop of an ellipsoidal basalt, showing the general char-
acter of the ellipsoids and matrix 112
8. Sketch showing the concentration of the amygdaloidal cavities on one side of an ellipsoid,
this side prob.ably representing the side nearest the surface of the flow 113
9. Ellipsoids with sets of parallel lines cutting each other at an angle 114
10. Reproduction of illustration of aa lava, after Dana 120
11. Profile section illustrating results of diamond-drill work 177
ILLUSTRATIONS. XIII
Page.
Fi(i.l2. Sketch illustrating CDiitortion of Uppor Huron I an strata 179
13. Skotili show i mi; chaiigo of strike of Upper lliironiaii beds, <lue to the folds 179
14. Sketch to illustrate the occnrrance of ore bodies 182
15. Magnetic cross section in T. 45 N., R. 31 W 345
16. Circles of attraction 346
17. The forces acting on the dip needle 347
18. Curves showing the relations between the horizontal components at the points of maxi-
mum deflection, for rocks dipping at various angles and buried to various depths 354
19. Truncated anticlinal fold with gently dipping. limbs 364
20. Truncated anticlinal fold with steeply dipping limbs 365
21. Plan and cross sections of a pitching syucline 367
22. Magnetic map of the Groveland Basin 370
23. Plan and cross sections of a pitching anticline '. 371
24. Magnetic map of a single formation split by au intruded sheet 372
LETTER OF TRANSMITTAL.
Department of the Interior,
United States Geological Survey,
Washington, D. C, April 23, 1898.
Sir: I transmit herewith the manuscript and iUustrations of a mono-
graph upon the Crystal Falls Iron-bearing District of Michigan, by
J. Morgan Clements and H. L. Smyth. The district is thus called from its
principal mining town. The area reported upon connects the Marquette
district on the north and the Menominee district on the south.
This monograph is one of the series which is to treat of the iron-bearing
districts of the Lake Superior region. The first of the series was that on
the Penokee district (Monograph XIX); the second of the series was that
on the Marquette district (Monograph XXVIII). The present report is the
third of the series, and the report upon the Menominee district, now being
prepared by W. S. Bayley, will be the fourth.
No previous detailed report has been issued upon the Crystal Falls
district, although the area has been touched upon by Messrs. T. B. Brooks
and Carl Rominger. The present report is the first which contains
geological maps and sections of the disti'ict.
The field work upon which the present report is based began about five
years ago. The work of the first season was a topographical survey and a
reconnaissance geological survey. The work of the second season was
detail geological work by H. L. Smyth, W. N. Merriam, and their assist-
ants. The work of these two seasons was done for private parties. These
parties turned over to me the original specimens, notes, and maps to assist
in the preparation of this report. The detail geological mapping of the
second year had covered only a part of the area included within the present
report, and in the remainder of the area in the following years detail
surveys were made by J. Morgan Clements and W. S. Bayley.
XVI LETTER OF TEANSMITTAL.
In the vicinity of the mines and the iron-bearing formations the exami-
nation of the district has been of the most detailed character, practically all
of the ledges having been visited and advantage having been taken of
underground workings and borings. Moreover, in order to determine the
succession, for large areas, it was necessary to make a close magnetic
survey with the dial compass and dip needle. In the areas more remote
from the mines the work was of a less detailed character.
The western half of the district is treated by J. Morgan Clements in
Part I of this monograph. The eastern half of the district, with the excep-
tion of the Sturgeon River tongue, is treated by H. L. Smyth in Part II.
The chapter upon the Sturgeon River tongue was prepared by W. S. Bayley.
My own part of the work has been a general supervision of the entire surve}',
with frequent trips into the region to assist in solving the general structural
problems.
To the gentlemen who furnished the complete results of their surveys
for the first two seasons, we are deeply indebted. The drawing for the
maps was done by E. C. Bebb. The colored plates were prepared b}-
J. L. Ridgway.
Very respectfully, your obedient servant,
C. R. Van Hise,
Geologist in Charge.
Hon. Charles D. Walcott,
Director United States Geological Survey.
INTRODUCTION.
By C. R. Van Hise
This report is a full account of the Crystal Falls iron-bearing- district
of Michigan.
The rocks of the district comprise two groups, separated by uncon-
formities. These are the Archean and the Algonkian. The Algoukian
includes both the Lower Huronian and the Upper Huronian series, and
these are also separated by unconformities. The terms Lower Huroniau
and Upper Huronian are applied to the series which occur in this district
because they are believed to belong to the same geological province as the
Huronian rocks of the north shore of Lake Huron, and to be equivalent to
the Lower Huronian and Upper Huronian series which there occur. The
reasons for this belief are fully given in Bulletin 86.'
The Archean is believed to be wholly an igneous group, and there-
fore no estimate of its thickness can be given. It covers a broad area in
tlie eastern part of the district, and from this several arms project west.
West of the main area there are two large oval areas of Archean.
The Lower Huronian series, from the base upward, comprises the Stur-
geon quartzite, from 100 feet to more than 1,000 feet thick; the Randville
dolomite, from 500 feet to 1,500 feet thick; the Mansfield slate, from 100
feet to 1,1(00 feet thick; the Hemlock volcanic formation, from 1,000 feet to
10,000 or more feet thick; and the Groveland fonnation, about 500 feet
thick. We tlms have a minimum thickness for the series of about 2,200
feet, and a possible maximum thickness of more than 16,000 feet. However,
^ Correlation papers, Archeau and Algonkian, by C. R. Van Hise : Bull. U. S. Geol. Survey No. 86,
1892, pp. 156-199.
MON XXXVI IX xvn
XVIII INTRODUCTION.
a large part of the latter is cjmpowed of volcanic material. It is not likely
tliat the sediments at any one place are as much as 5,000 feet thick.
The Upper Huronian is a great slate and schist series, which it is not
possible to separate on the maps into individual formations, and it is impos-
sible to give even an approximate estimate of the thickness of this series.
Various igneous i-ocks intrude in an intricate manner both the Upper
Huronian and the Lower Huronian series.
The aim of the following paragraphs is to sketch very briefly the
history of the district.
THE ARCHEAN.
The Archean consists mainly of massive and schistose granites and of
gneisses. Nowhere in the Archean have any rocks of sedimentarj^ origin
been discovered. The Archean has been cut by various igneous rocks,
both basic and acid, at different epochs. These occur in the form both of
bosses and of dikes, the latter sometimes cutting, but more ordinarily show-
ing a parallelism to, the foliation of the schistose granites. The granites
must have formed far below the surface, and therefore nmst have been
deeply denuded before the transgression of the Lower Huronian sea. The
Archean granites and gneisses and the earlier intrusives alike have been
profoundly metamorphosed, and at various places have been completely
recrystallized.
THE LOWER HUROXIAI^ SERIES.
The Sturgeon quartzite, the first deposit of the advancing sea, when
formed consisted mainly of sandstone, but in places at the base of coarse
conglomerate. The conglomerate is best seen in the Sturgeon River
tongue. Elsewhere e\ddence of conglomeratic character at the base of
the formation is seen, but the metamorphism has been so great as nearl}^ to
destroy the pebbles. However, in the Sturgeon River tongue is a great
schistose conglomerate, which, while profoundly metamorphosed, still gives
evidence of the derivation of its material from the older Archean rocks.
The sandstone has been changed to a vitreous, largely recrystallized
quartzite, which now shows only here and there vague evidence of its
clastic chai-acter.
The Sturgeon formation varies from probably more than 1,0()0 feet in
INTHODUCTION. XIX
tliickiu'ss in tlic Sturiieoii River tono'uo to less tliaii 100 fec^t in tliickness at
])lii(.T's in the Felcli Mountain ran<^'e, and is altogether absent in the north-
eastern j)art of the district.
In tlu' southeastern j)art of the district the Sturgeon quartzite is over-
lain l)\- the Handville dolomite. In the central part of the district the
([uartzite Ix^tween the Archeau and the Randville is so thin that it can not
be reijresented on the maps as a separate formation. In the northeastern
part of the <listrict a quartzite resting on the Archean, l)ut occupying a
higher position stratigraphically than the Randville dolomite, is overlain bv
an iron-bearing formation. It appears, therefore, that the Sturgeon sea
gradually overrode the district, and that at the time the Sturgeon quartzite
was deposited in the southeastern part of the area, the Archean was not yet
sulimerged in the central and northeastern parts of the district. However,
since the quartzite resting on the Archean in the latter area can not be sepa-
rated lithologically from the Sturgeon quartzite, both are given the same
formation color, but the later quartzite is given a separate letter symbol.
The quartzite color therefore represents a transgression deposit of the
same general lithological character, rather than a formation, all parts of
which have exactly the same age. While nowhere in the district is there
an}' marked discordance between the schistosity of the Archean and the
Sturgeon quartzite, the conglomerates at the base of the latter formation in
the Sturgeon River tongue are believed to indicate a great un<"onfoi-mity
between the Archean and the Lower Huronian series. The change from
the Sturgeon deposits to those of the Randville was a transition.
The Randville dolomite is a nonclastic sediment, and is believed to
mark a periftd of su.bsidence and transgression of the sea to the northeast,
resulting in deeper waters for much of the district. Since the Randville
dolomite has its full thickness on the Fence River just east of the western
Archean oval, and does not appear at all about the Archean oval a short
distance to the northeast, it is probable that the shore line, during Rand-
ville time, was between these two areas and that tlie land arose somewhat
abruptly toward the northeast. As the Randville formation has a thick-
ness of 1,500 feet, it probably represents a considerable part of Lower
Huronian time.
Following the de^^osition of the Randville dolomite, deposits of very
different character occur in different parts of the district. These deposits
XX INTRODUCTION.
are: (1) The Mansfield formation, (2) the Hemlock volcanic formation, and
(3) the Groveland formation.
The Mansfield formation was a mudstone, which has subsequently been
transformed into a slate or schist. The Hemlock formation is mainly a
great volcanic mass, including both basic and acid rocks, lavas, and tuff's,
but it contains also subordinate interbedded sedimentary rocks. This for-
mation occupies a larger area than any other of the Lower Huronian forma-
tions and is perhaps the most characteristic feature of the Crystal Falls
district. The Groveland is the iron-bearing formation. It includes sideritic
rocks, cherts, jaspilites, iron ores, and other varieties characteristic of the
iron-bearing formations of the Lake Superior region. In all important
respects these rocks are similar to those of the Negaunee formation of the
Marquette district, with the exception that in the southeastern part of the
Crystal Falls district, associated with the nonclastic material, there is a
considerable proportion of clastic deposits. The Groveland formation
contains iron carbonate and possibly glauconite, from which its other
characteristic rocks were derived.
The variability in the character of the deposits overlying the Randville
formation is probably caused by the great volcanic outbreaks in the western
part of the district. In the southern and southeastern parts of the area the
deposit overlying the Randville formation is the Mansfield slate and schist.
North of Michigamme Mountain and of the Mansfield area the Mansfield
formation is replaced along the strike by the Hemlock volcanic formation,
which directly overlies the limestone for most of the way about the western
Archean oval. The effect of the volcanic outbreak apparently did not
reach so far as the northeastern part of the district.
Overlying the Mansfield formation in the southeastern part of the dis-
trict and the Randville formation in the central part of the district is the
Groveland iron-bearing formation. In the Mansfield slate area the iron-
bearing rocks appear near the top of the Mansfield formation intercalated
with the slates. The Groveland formation can not be certainly traced
farther north than the northeastern portion of the western Archean oval.
It is apparently replaced along the strike by the Hemlock volcanics.
In the northeastern part of the district the Groveland formation,
equivalent to the Negaunee formation of the Marquette district of Michigan,
INTRODUCTION. ■ XXI
is found above the Ajibik formation. The occupation, in the western part
of tlie district, by the Hemlock volcanics of the same part of the geological
column as the Hemlock volcanics east of the western Archean oval, the
Mansfield slate, and the Groveland formation, is explained by the fact
that in the western part of the district the volcanoes first broke out and
there continued their activity longest. While north of Crystal Falls the
volcanic rocks were being laid down, the Mansfield formation was being
deposited in the southeastern part of the district. This activity continued
there through the time that the Groveland formation was being deposited
in other parts of the disti'ict.
From the foregoing it appears that the Hemlock formation in the
western part of the district is equivalent:
(1) East of the western Archean oval, to the Hemlock volcanics found
there and the overlying Groveland formation ;
(2) At Michigamme Mountain, to the Mansfield slates and the Groveland
formation ;
(3) In the Mansfield area, to the Mansfield slates and the Hemlock
volcanics occurring there; and
(4) In the southeastern part of the district, to the Mansfield and
Groveland formations.
The replacement of an iron-bearing formation by the great volcanic
fonnation just described is exactly paralleled in the Upper Huronian rocks
of the Penokee iron-bearing series, where the pure iron-bearing formation
is replaced at the east end of the district by a great volume of volcanic
rocks intercalated with slates and containing bunches of iron-formation
material.^
Following the deposition of the Lower Huronian series the region was
raised above the sea and eroded to different depths in difterent places. In
the Felch Mountain range the only formations above the Raudville dolomite
are a thiu bed of slate and the Groveland iron formation. In the north-
eastern part of the district only a thin belt of iron-formation rocks remains.
In the central and western parts of the district there is a great thickness of
volcanics. Tliis, however, does not imply a difference of erosion equal to
' The Penokee iron-beariug district of Michigan and Wisconsin, by R. D. Irving and C. R. Van
Hise: Men. U. S. Geol. Survey, Vol. XIX, 1892, pp. 428-433.
XXII INTRODUCTION.
the difference iu thickness of these rocks, for doubtless when the volcanics
were built up there was contemporaneous subsidence, so that at the end
of Lower Huronian time there may have been little variation in the
elevation of the upper surface of the series, but very great difference in its
thickness.
THE UPPER HURONIAIV.
After the Lower Huronian series was deposited the district was raised
above the sea, may have been gently folded, and was eroded to different
depths in different parts of the district.
Following the earth movements and erosion the waters for some reason
advanced over the district and the Upper Huronian series was deposited.
The basal horizon was a conglomerate, which has, however, very different
characters in different parts of the district.
In the eastern half were Archean rocks, the Sturgeon quartzite, the
Mansfield slate, and the Clroveland iron formation. Upon these was depos-
ited a sandstone which locally was very ferruginous. This has subsequently
been changed into a ferruginous quartzite. The typical occurrence of this
quartzite is at the east end of the Felch Mountain range. It also appears
between the Archean ovals in the northeastern part of the district. If
distinct conglomerates were formed at the bottom of this quartzite, they are
buried under glacial deposits or have disappeared as the resiilt of meta-
moi'phism.
In the western part of the district the rocks of the Lower Huronian at the
surface are the great Hemlock formation, and here the basal horizon of the
Upper Huronian is a slate or slate conglomerate, the fragments of which are
derived mainly from the underlying Hemlock formation. The sandstones
and conglomerates varied upward into shales and grits, which have been
subsequently altered into mica-slates and mica-schists. After a considerable
thickness of mudstone and grit was deposited, there followed a layer of
combined clastic and nonclastic sediments, the latter including iron-bearing
carbonates. These appear to be at a somewhat persistent horizon, and in
this belt are found the iron-formation rocks, and iron ores in the Upper
Huronian in the vicinity of Crystal Falls. Above these ferruginous rocks
there was deposited a great thickness of shales and grits, which have been
transformed into mica-slates and mica- schists.
INTRODUCTION, XXIII
JX>T.DI>r<; OF TIIK ARCHKAN AND IIURONIAN SERIES.
The Crystal F.iUs district liad now been an area of deposition for a
very lonj;- time, and a great tliickness of sediments had accumnlated. A
profound pliysical revohition next occurred, the greatest since Archean
time. The region was raised above the sea and was folded in a most com-
plex manner. As a consecpience, the more conspicuous folds vary from a
north-south to an east-west direction. The closer folds in the northeastern
piu-t of the area are nearly north-south. In the centi-al part of the area the
closer folds strike northwest-southeast. In the eastern and southeastern
parts of the district the closer folds are nearly east-west. All of these
folds, however, have steep pitches. It therefore follows that the region
was subjected to great compressive stresses in all directions tangential to
the surface of the earth, and that the yielding was mainly in one direction *
here and in another there, although on every fold there is evidence of yield-
ing in two directions at right angles to each other. Some of the folds are
very close, as in the case of the Huronian area between the two Archean
ovals in the northeastern part of the district, and in the Felcli Mountain
range. In other areas — as, for instance, in the Crystal Falls syncline — -
the major fold is somewhat open. However, upon the open folds are super-
imposed folds of a higher order, so that the detail structure is very compli-
cated. So far as known, the district has nowhere been faulted.
Subsequent to or during the late stage of this time of folding there
was a period of great igneous activity, probably contemporaneous with the
Keweenawan. At this tin:ie there were introduced into both the Lower and
the Upper Huronian rocks vast bosses and numerous dikes. The intrusives
vary from those of an ultrabasic character, such as peridotites, tlii'ough those
of a basic character, such as gabbros and dolerites, to those of an acid char-
acter, such as granites. These intrusives, while altered by metasomatic
changes, do not show marked evidence of dynamic metamorphism — there-
fore the conclusion that they were introduced later than the period of intense
folding, already described.
A few illustrations are mentioned. The Archean and other great
massifs are less profoundly altered than are the softer and weaker deposits
of the Huronian. In these more rigid formations, such as the granites and
quartzites, all phases of alteration by granulation and recrystallization are
XXIV INTRODUCTION.
beautifully exliibited. The Sturgeon River area affords one of the best-
known illustrations of a schistose conglomerate the matrix of which has com--
pletely recrystallized and, therefore, can not be discriminated from a gneiss
of igneous origin, but contains numerous pebbles and bowlders flattened in
the plane of schistosity.
The great Hemlock volcanic formation varies from rocks which ai"e
altered chiefly by metasomatic change to those which have become com-
plete crystalline schists containing no vestige, either macroscopically or
microscopically, of a texture or structure which may be interpreted as
Igneous.
SUBSEQUENT HISTORY.
After the introduction of the intrusives the region was subjected to vast
denudation, which reduced it approximately to its present configuration
This period of erosion continued until late Cambrian time, when the sea
again overrode the district and deposited upon the older rocks Upper Cam-
brian sediments. Long after the deposition of the Cambrian, and perhaps
later Paleozoic rocks, the district was again raised above the sea, and the
major part of the Cambrian deposits have been removed, although they are
found in patches throughout much of the district, and occur as a continu-
ous sheet just east of the area discussed.
The district niay have again been submerged in Cretaceous time ; but
if so, the deposits formed were removed after the area finall}'' emerged from
the sea. Since Cretaceous time the region seems to have been one of
erosion. During the Pleistocene period a thick mantle of glacial deposits
was spread over the entire district. Since Pleistocene time erosion has
advanced far enough to uncover the rocks here and there.
METAMORPHISM.
The folding varied in its closeness in different parts of the district.
Moreover, the formations are of very variable character, including a great
variety of sediments and of igneous rocks. The formations, therefore, xarj
greatly in their capacity to resist stresses. It thus follows that during the
folding process certain formations yielded to a much greater degree than
others. The amount of contained water and other conditions were also
variable. As a result of these many variable factors, it is one of the most
characteristic features of the district that there are to be found neai'ly all
INTRODUCTION.
XXV
varieties <^f niotamorphism in various stages of advanceineiit. The working
out of the details of the transformations of the different kinds of rocks
during their processes of inetamorphism is one of the chief scientific results
which has come from a study of the district.
CORRELATIOX.
In order to compare the succession in the Crystal Falls district with
that in the adjacent Marquette and Menominee districts, the descending pre-
Cambrian succession in each of the tlii'ee districts is here given in parallel
columns, the formations which are thought to be equivalent being placed
opposite one another :
Descending succession of formations in the Marquettef Crystal Falls, and Menominee districts.
MARQUETTE DISTRICT.
Upper Marquette.
(1) Micbigamme formation, bearing a sbort
distanre above its base an iron-bearing
horizon, and being replaced in much
of the districtbj the Clarksburg vol-
canic formation.
(2) Ishpeming formation, being composed of
the Goodrich quartzite in the eastern
part of the district and of the Goodrich
quartzite and the Bijiki schists in the
western part of the district.
ITjicon/ormity.
Lower Marquette.
<1) Negaunee iron formation. 1,000 to 1,500
feet.
(2) Siamo slate, in places including inter-
stratified amygdaloida, 200 to 625 feet
thick.
(3) Ajibik (luartzite, 7l)0 to 900 feet.
(4) "Wewe slate, 550 to 1,050 feet.
(5) Kona dolomite, 550 to 1,375 feet.
(6) Meanard quartzite, 100 to 670 feet.
CRYSTAL FALLS DISTRICT.
Upper Huronian.
(1) Michigamme formation, bearing^
a short distance above its base
an iron-bearing horizon.
(2) Quartzite in eastern part of dis-
trict.
Unconformity.
Lower Huronian.
(1) The Groveland formation, about
500 feet thick.
(2) Hemlock volcanic formation,
1,000 to 10,000 feet thick.
In western part of district also
occupies the place of (1) and
f3).
(3) Mansfield formation, 100 to
1,900 feet thick.
(4) Kandville dolomite, 500 to 1,500
feet thick.
(5) Sturgeon quartzite, 100 to l.UOO
feet thick.
MENOMINEE DISTRICT.
Upper Menominee.
(1) Great Slate formation
Unconformity .
Lower Menominee.
(1) Vulcan iron formation con-
taining slates.
(2) Antoine dolomite.
(3) Sturgeon quartzite.
Unconformity.
Archeaii.
Unconformity.
Archean.
Unconformity.
A rchean.
From the tliree columns it appears that the equivalents in the different
districts can be made out with a considerable degree of certainty. There
are, however, various differences, due to several causes.
For Upper Hui'onian time, omitting the Clarksburg formation, the sue-
XXVI INTEODUCTION.
cession in the Marquette, Crystal Falls, and Menominee districts is substan-
tially the same. The Clarsksburg formation in the Marquette district may
be omitted from consideration, because it is volcanic and replaces in jjart
the Michigamme and Ishpeming formations. The Upper Huroniau was a
great period of slate and grit deposition. The chief difference which appears
between the Menominee district and the other two districts is that in the
former no iron ores have been found in the Ujjper Huronian within the
district proper, although such rocks occur a short distance to the west, at
Commonwealth and Florence, in Wisconsin.
The succession for the Lower Huronian in the three districts can be
paralleled with a high degree of probability. The chief differences are
due to the disturbance of the great volcanic outburst in the western part
of the Crystal Falls district and to the uneven surface of the Archean land
at the beginning of Lower Huronian time. As a consequence of the latter,
the waters did not reach the western part of the Marquette district and the
northeastern part of the Crystal Falls district as early as the eastern part of
the Marquette district, the central part of the Crystal Falls district, and the
Menominee district. The transgression of the Lower Huronian sea for the
region covered in these three districts was therefore from the southeast
toward the northwest.
The Negaunee iron formation of the Marquette district is equivalent to
the Grdveland iron formation of the Crystal Falls district and the Vulcan
iron formation of the Menominee district.
The Siamo slate and the Ajibik quartzite of the Marquette district are
approximately equivalent to the Hemlock volcanic formation in much of
the Crystal Falls district, but in places where the latter formation displaces
the j\Iansfield formation they are equivalent to only a part of the Hemlock
volcanic formation. The Wewe slate of the Marquette district is equivalent
in the western part of the Crystal Falls distinct to a part of the Hemlock
volcanic formation, and in the southeastern part of the district is probably
equivalent to a part of the Randville dolomite. It appears that the Siamo
slate, Ajibik quartzite, and Wewe slate of the Marquette district, and the
Mansfield and Hemlock formation of the Crystal Falls district, are equiv-
alent to a part of the Antoine dolomite of the Menominee district.
The great dolomite formation occurring in all of the districts is sup-
posed to be equivalent, except that, as just explained, the de^iosition of
INTKODUGTION. XXVII
liiiu'stone contiimod longer in the southejistern part of the Crystal Falls
district and in the Menominee district than in the remainder of the region.
The absence of the limestone and lower formations in the* western two-
thirds of the Marquette district and the northeastern part of the Crystal
Falls district is explained by the fact that during early Algonkian time this
part of the region was not submerged. The Mesnard quartzite of the Mar-
quette district and the Sturgeon quartzite of the Crystal Falls and Menom-
inee districts stand opposite each other.
From the foregoing it is apparent that the tlu-ee districts together pre-
sent a most interesting and complex structural problem. While there is
sufficient similarity in the formations for one to feel considerable assurance
of their general equivalence in the different districts, it is certain that the
formations of similar kind did not begin and end at the same time. Slore-
over, there are remarkable lateral transitions in sedimentation, as a result of
the uneven surface of the Archean at the beginning of Algonkian time and
because of volcanic outbursts. As a resitlt of the first of these conditions, it
is necessary to equate fragmental formations which occur in the central and
western parts of the Marquette district and the northeastern part of the
Crystal Falls district, with nonfragmental limestones in the area to the east
and south. Consequent upon the Upper Huronian volcanic outbursts in the
Marquette district, the Michigamme and Ishpeming formations are largely
replaced by the Clarksburg volcanics. Similar outbursts in the western part
of the Crystal Falls district in Lower Huronian time placed volcanic rocks
for this part of the district opposite the Mansfield slate and the Groveland
iron formation.
The foregoing relations, combined with the great variety and complexity
of the sediments of the district, the presence of many forms of contempora-
neous volcanic deposits, the intrusion of the widest ^•ariety of igneous rocks
of various ages from Archean to later Algonkian time, and the complicated
folding and metamorphism to which the district has been subjected, will
readily convince one that the working c)ut of the detail structure of the
district by Messrs. Clements, Sn^yth, Bayle}', ]\Ierriam, and others has not
been accomplished without most painstaking and laborious work, especially
as the region is covered by timber or brush and is overspread by a mantle
of glacial deposits.
OUTLINE OF THIS MONOGRAPH,
Part I.
Chapter I. The Crystal Falls district is situated on the Upper Peninsula of Michigan, and
foniis a connecting link between the Marquette and Menominee districts of Michigan. A history of
the previous work in the district, accompanied by a summary of the literature, is given, and there is
reproduced a series of maps which indicate the development of knowledge concerning the distribution .
of the rocks and their structural relations. As explanatory of the locations given, the mode of work
is described and the object and method of taking magnetic observations is briefly outlined.
Chapter II treats of the geographical limits, structure, stratigraphy, and physiography. The
portion of the district here described includes approximately 540 square miles. Structurally it is
closely related to the Maniuette district; the essential features being a northwest-southeast set of
folds, with a superimposed series trending northeast-southwest. The oldest rocks belong to the
Archean. They cover an oval area which is surrounded by the Algonkian rocks represented by the
Lower and Upper Huroniau series. The Archean and Algonkian are overlain with strong uncon-
formity by rocks of the Cambrian division of the Paleozoic. The drift deposits of the Quaternary
are everywhere present. Only the pre-Paleozoic rocks, however, are discussed. The most noticeable
topography is that of the drift, which in places is seen to be superimposed upon pre-Pleistocene
topography. The maximum elevation is 1,900 feet above sea level, and the minimum 1,250 feet. The
conclusion is reached that this portion of Michigan before Glacial times had been reduced to the
condition of an approximate peneplain. The drainage is chiefly by a few large streams which flow
into Green Bay of Lake Michigan. A small part is drained by streams flowing into Lake Superior.
In portions of the area the drainage has reached an advanced stage, in other portions it is very
youthful. The development of the drainage is illustrated in the ease of the Deer River. The timber
and soil vary much in character.
Chapter III treats of the Archean. The rocks of this nge form an elliptical core, following the
axis of a northwest-southeast trending anticline. Exposures are few because of the superimposed
drift. The Archean is overlain uncouformably by Algonkian sediments derived from the granite, and
there is absence of contact action. These facts indicate that it was the floor upon which the over-
lying sediments were deposited. Petrographically it consists chiefly of biotite-granite. On the
periphery of the area a biotite gneissoid granite is very well developed. Some of this at least is
of dynamic origin. The Archean is cut by acid and basic dikes which are now both schistose and
massive.
Chapter IV treats of the Lower Huroniau series. This series is subdivided into the following
formations, from the base upward: The Raudville dolomite, the MausHeld slate, and the Hemlock
volcanics.
Section I. The Randville dolomite is poorly exposed. It consists of quartzose dolomite, grading
down into quartz-schist and recomposed granite. It has evidently been derived partly from the
granite, and is cousiderably younger than that. Its relations to the overlying formations were not
observed. The thickness could not here be determined, but, in the area studied by Smyth, its maxi-
mum is about 1,500 feet.
XXX OUTLmE OF THIS MONOGRAPH.
Section II. The Mansfield slate is best exposed in the vicinity of the town of the same name.
It here occupies a valley, through which flows the Michigamme River. Petrographically this
formation includes graywatkes, clay slate, phyllite, siderite-slate, chert, ferruginous chert, and iron
ores, with various metamorphic products derived from them. The slate predominates. The Maualield
slates are intruded by basic igneous rocks, which underlie them. They are overlain by volcanics,
which contain fragments of the slates, and are hence younger than they. The Mans6eld slates strike
north and south and dip on an average 80^ to the west. They represent the limb of a westward-
dipping monocline. The maximum thickness of the belt is 1,900 feet. Followed south away from
the point of maximum thickness it rapidly thins out and disappears. In the slate but a single ore
body of commercial importance has been found. This is exploited by the one Bessemer ore-producing
mine of the district. The ore body is presumed to have resulted from the alteration of a siderite,
and the concentration in a favorable position of the iron from the portions of the ferruginous beds
removed by erosion. A possible continuation of the Mansfield slate is suggested by the occurrence of
small outcrops of somewhat similar slates about 5 miles slightly to the west of north.
Section III. The Hemlock formation consists almost exclusively of volcanic rocks, both b.asic
and acid, with crystalline schists derived from them. Sedimentary rocks play a very unimportant
rule. Exposures are numerous west of the belts of previously described rocks, and where erosion
has removed the drift the formation has a marked influence ou the topography. The thickness is
estimated from the dip to reach 23,000 feet, but this is probably illusory because of reduplication due
to folding. In the northern portion of the district the formation overlies the Kona dolomite. In the
southern portion it overlies conformably the Mansfield slate. It is probable that volcanic activity
began in the north and moved south, and that some of the volcanics to the north are contemporaneous
with the Mansfield slates. The volcanics are cut by a few acid dikes. Basic dikes forming enormous
bosses of basic rock are of freiiuent occurrence. The volcanic origin of the major portion of this
formation is perfectly clear. Some of the volcanics are submarine. The greater proportion, however,
were derived from volcanic vents, which could not be located, but were probably situated near the
Huronian shore line. The Hemlock volcanics are divided into igneous and sedimentary rocks. Under
the igneous rocks there are described both acid and basic lavas and pyroclastics. Under the sedimen-
tary rocks there are described volcanic sediments, both of eolian and water-deposited character. By
extreme metamorphism crystalline schists have been produced from l)Oth igneous and sedimentary
rocks. The acid volcanics include rhyoliteporphyries and aporhyolite-porphyry. The rhyolite-
porphyry shows interesting micropoikilitic textural characters. Some of the porphyries have been
rendered schistose by pressure. Acid pyroclastics are scarce and were derived from the aporhyolite.
The basic lavas correspond to the modern basalts. They are much altered. To indicate these facts
and at the same time show their correspondence to the Tertiary aud recent basalts, they are called
" metabasalts." The basic lavas include noni^orphyritic, porphyritic, aud variolitie types. A columnar
structure was not observed, but an ellipsoidal structure is very common. This structure is described
ill detail and the couclusiou reached that basalts possessing this structure were originally very
viscous and correspond to the modern aa lavas. The amygdaloidal structure, which is almost univer-
sally jiresent in the volcanics, is described aud illustrated. The alteration of the basalts is discussed
aud speci.al cases described. As result of this alteration the textural as well as the mineralogical
characters may be completely changed, and the volcanic origin of the resulting rocks could not be
determined but for their association. In the zone of weathering, calcification is the controlling
alteration jirocess. In rocks more deeply buried, silicification is the process which predominates.
The pyroclastics comprise eruptive breccia, including thereunder friction breccias and flow breccias,
and volcanic sedimentary rocks. The eolian deposits, which arc described as tuft's, grade from fine
dust deposits up into tliose in which the fragments are bowlders. The water-deposited volcanic
fragmentals are known as volcanic conglomerates, and likewise grade from those of which the particles
are of minute size into those of which the fragments are of very large size. At various places
clastic rocks occur which are now schistose, and whose exact mode of origin — that is, whether eolian
or water deposited — could not be determined. Normal sediments consisting of slate with limestone
OUTLINE OF THIS MONOGRAPH. XXXI
liMises form u loiitiiulai' deposit in the volcuiiics uiiar tlie top of tin; formation. 'I'licy are inHijjiiiH-
cant in quantity.
Under the Uouo Lalvu crystalline schists there' arc included rocks of completely crystalline
character, but which by field and microscopical study have beeu connected with the volcanics and
are considered to have been derived from rocks similar in nature to them.
The rocks composiun' the Hemlock formation arc little likely, owing to their somber color, to
be much used for building or ornamental purposes. They offer, however, an inexhaustible supply of
the best cjuality of road-Unildiug material.
Chapter V treats of the I'pper Hurouian series. This series is connected in the northern part
of the area described with the Upper Maniuette series of the ad.joiniug Marquette district, and is
considered to correspond stratigraphically to the Upper Maniuette. Owing lo lack of exposures and
to the intricacy of folding, the series could not be subdivided. It covers a great area surrounding
the Hemlock formation, and extends beyond the limits of the map. The exposures are scanty. It
inlluences the topography only in a very general way, being for the most part heavily drift covered.
Its thickness could not be estimated. The rocks of this series wrap around the subjacent Lower
Huronian series. The line between them is uudulatory. The indentations in the Lower Huronian
represent minor cross eyuclines, and the jirotuberauees represent minor cross anticlines. The most
prominent fold of the series is known as the Crystal Falls syncline. The strike of the axis of this
syncline is in general to the south of west, and pitches in the same direction. The syncline is not
simple, but has minor rolls, as shown in various exposures. This folding has beeu productive
of extensive reibungsbreccias. The folding occurred immediately preceding the deposit of the
Keweenawau series iu other parts of the Lake Superior region. The Upper Huroniau is penetrated
by iutrusive rocks of acid, iutermediate, and basic composition. The rocks constituting the series
may be divided into those of sedimentary and those of igneous origin. The sedimentary rocks are
graywackes, ferruginous graywackes, micaceous, carbonaceous, and ferruginous clay slates and
their crystalline derivatives, and thinly laminated cherty sideriite-slate, ferruginous chert, aud iron
ores. In two places rocks of conglomeratic nature occur. The extensive folding which the series has
undergone, coupled with the intrusions of the igneous rocks, has produced crystalline schists from the
muds and grits. These are extensively developed iu the southern portion of the district in the vicinity
of the Paint and Jlichigamme rivers. The igneous rocks which have penetrated the Upper Hurouian
subsequent to the folding which affected it are not described under the series. Interlaminated with
the crystalline schists there are, however, certain rocks, now perfectly crystalline hornblende-
gneisses, which are presumed to have resulted from the metamorphism of either basic intrusive sheets
or interljedded flows.
The economic development of the district followed that of the adjoining Menominee district.
The exploited ore deposits occur near Amasa, and iu the vicinity of Crystal Falls. The ore is hematite
and limonite. The grade is non-Bessemer. The ore is associated with white and reddish chert, and
this formation lies between carbonaceous slates. The ore bodies in general pitch to the west at
varying angles, which correspond to the pitch of the axes of the syncliues iu which they occur. These
minor folds in turn correspond to the western pitch of the main Crystal Falls synclinorium. The ore
bodies are concentrates in synclinal troughs, as described by Van Hise for other Huronian districts.
The mining is now for the most part undergouud, and is carried on iu open stopes. The greatest
shipment of ore from the area, including the Lower Hurouian Mansfield mine, was .586,970 tons in
1892. The total shipment for 1898 reached 325,814 long tons.
Chapter VI treats of the intrusives. There is here included a varied assortment of rocks,
exhibiting iu common intrusive relations to the sedimentary and igneous rocks. The term " intrusive"
is not to be interpreted as synonymous with the "dike rocks" of some authors. These rocks are
never found to penetrate the Cambrian rocks, and have not been affected by the folding which meta-
morphosed the Upper Huroniau sediments. They are presumed to be of Keweenawau age. The intru-
sives have in some cases beeu injected along the axes of the folds, these representing the lines of
greatest shattering, and hence least resistance.
XXXII OUTLINE OF THIS MONOGRAPH.
In Section I there are described a number of intrusive roclis which can not be connected genet-
ically with one another. These comprise ordinary biotite-granites, with micropegmatitic varieties,
muscovite-biotite-granite, metadolerite, metabasalt, and picrite-porphyry. AVhere the granites have
intruded the Upper Huronian series they have contorted the strata, and include and metamorphose
the rocks, producing muscovite-biotite-gneiss, and staurolitiferous and garnetiferous mica-schists.
The metadolerites possess no special points of interest in themselves, but where they have intruded
the Mansfield slates they have caused interesting exomorphism. The slates are converted into
adinoles, spilosite8,and desraosites. Chemical analyses indicate the chief change which has taken
place in the production of these rocks from the clay slate to have been in the increase of silica and
soda as the contact is approached. There seems thus to have been a direct transference of sodiuip
and silicon from the igneous rock to the sedimentary. The metabasalt dikes are of little interest.
The ultrabasic picrite-porphyries are extremely altered. This alteration has produced in one case
chiefly tremolite and in another serpentine. One of these serpentine picrite-porphyries is polar-mag-
netic. The picrite-porphyries .ire presumed to have contained a vitreous base, and correspond to the
modern Tertiary limburgite.
In Section II there is given a study of a series of rocks varying from those of intermediate
acidity through those of basic composition to ultrabasic kinds. The exposures of these rocks are
found iu an area underlain by the Upper Huronian series, extending from Crystal Falls southeast to
and a short distance beyond the Michigamme River. The prevailing rocks are, on the one hand,
diorites of intermediate acidity, ranging to more acid rocks, tonalites, quartz-mica diorites, and
granite. On the other hand, we have hornblende-gabbros, gabbros, norites, and, lastly, peridotites
of varying luineralogical character. Only those kinds of rocks of which analyses have been obtained —
mica-diorite, hornblende-gabbro, norite, and wehrlite — are discussed. The diorites are holocry-
stalline rocks of medium to coarse grain. In texture they show some variation from those which are
hypidiomorphic granular to those in which the texture is imi)erfectly ophitic. As facies of the dioritic
magma there are described diorite, mica-diorite, quartz-niica-diorite, tonalite, and plagioclase-bearing
granite. A quartz-mica-diorite-porphyry occurs in narrow dikes cutting the mica-diorite. Analysis
of the mica-diorite shows it to stand upon the border between the lime-soda feldspar rocks and the
orthoclase rocks. Thegabbros and norites arehulocrystalline rocks of moderately iine to coarse grain.
They show considerable v.ariation in texture. The hypidiomorphic granular texture predominates,
but some few show a good parallel texture. Others are noticeably porphyritic, a few have poikilitic
textures, and less commonly there is an approach to the ophitic texture. Hornblende and labradorite
is the must common mineral association, giving typical hornblende-gabbro. A monoclinic pyroxene at
times becomes .abundant, giving a transition to the normal gabbro. Bronzite at times is the promi-
nent bisilicate constituent of these rocks, giving bronzite-norite. A bronzite-norite-porphyry also
occurs. Locally the hornblende-gabbro has been crushed, and there is produced therefrom a schistose
rock which represents a transition to a hornblendo-gueiss. Of these rocks the hornblende-gabbro was
first formed. It was intruded by normal gabbro, and both of these types were then cut by dikes of
bronzite-norite and bronzite-norite-porphyry. The rocks included under the peridotites show con-
siderable mineralogical variation. There is produced an amphibole-peridotite, which, when augite
becomes predominant, grades to wehrlite. This in its turn becomes feldspathic, and indicates a tran-
sition to olivine-gabbro. The amphibole-peridotite also becomes feldspathic and quartzitic, indicat-
ing a transition toward diorite. When the order of crystallization of the minerals composing the
granular rocks of the entire series described above is considered, it is seen to have been as follows:
Bronzite is apparently the oldest. The olivine and monoclinic pyroxene come next and are of essen-
tially the same age. Mica and hornblende follow, and are contemporaneous. Then comes plagio-
clase, orthoclase, and qu.artz. A consideration of the chemical analyses of the rocks above described
shows them to belong to a series ranging from a diorite, on the one hand, to hornblende-gabbro and
norite and to peridotite on the other. On the acid side of the series variations are shown microscopic-
ally, but of these rocks chemical analyses have not been obtained. It is not possible to state which
of these rocks most nearly resembles in its composition the original magma of which the di£ferent
OUTLINE OF THIS MONOGRAPH. XXXIII
types ropreseut tlm differentiation products. Tlie lioruMende-jjiilibro i« that typn whicli apparently
first loaclied its present geological position. It was followed in the acid part of the series liy the
diorite, which in its turn was succeeded by the diorite-porphyry. Along the basic scries hornbleude-
gabbro was succeeded by gabbro, followed by the bronzite-norite and the peridotite. In general the
forces of differentiation have been toward increasing acidity and increasing basicity.
Pakt II.
Chapter I treats of geographical limits and physiography. The geographical limits of the area
described arc given and a brief statement made concerning the eouditious under which the work was
done. In the preliminary sketch of the geology the rocks represented are stated to range in ago from
Archeau to early Paleozoic. In that part of the district north and west of the Michigamme River the
Archean is exposed in several regularly outlined oval areas from 10 to 12 miles long and from 2 to 6
miles wide. The intervjils Ijetweeu these ovals are occupied by highly tilted metamorphosed sedimen-
tary and igneous rocks of Algonkian age. In the southern and eastern portions of the district the edges
of the tilted rocks are covered by the gently dipping Cambrian sandstone. Xevertheless, field work
shows that the distribution of the Archeau in ovals, which is so characteristic for the areas north,
also holds here. The chief surface feature is a rolling plain, which slopes gently to the southeast,
and upon which is superimposed the glacial drift with its characteristic topograjihical features,
multitudinous in variety and detail, but insignificant in relief. While the details of the topography
are mainly glacial, the broader features have often clearly been determined by the presence of the
more resistant Archeau and Algonkian rocks. The drainage is to the southeast, mainly into Lake
Michigan, through the Michigamme aud the Sturgeon rivers. Details of the drainage have been
determined by the distribution of the rocks. It is interesting to note that the Michigamme flows
along the eastern edge of its drainage basin, having no eastern tributaries.
Chapter II treats of magnetic observations in geological mapping. Certain of the rocks
occurring in the Crystal Falls district contain magnetite in such quantity that they have a marked
influence on the magnetic needle. Advantage is taken of this fact in the mapping of the rocks where
exposures are w.auting. The instruments and methods of work used in making magnetic oI)Servatious
are described. Facts of observation are mentioned, aud general principles are laid down. Applica-
tion of these principles to special cases is then considered, and finally a description of the method used
in the interpretation of complex structures follows.
Chapter III. In Section I the position, extent, aud previous work done in the Feloh Mountain
range is described. An abstract of the literature covering the area is given.
Section II contains a general sketch of the geology of this range. The rocks range from Archean
to early Paleozoic. These last are not considered for the jiresent. The Archeau is distributed in areas
which represent the cores of large arches formed over the whole region by mountain building energy,
and subsequently truncated by deep Cambrian denudation. The rocks, chiefly of sedimentary origin
intermediate between the Archean and Paleozoic, to which the name Algonkian is applied, occupy a
nari-ow strip ranging from a mile aud a half to less than a mile in width, and extending east and west
for a distance of over 13 miles. This strip constitutes the Felch Mountain range. It is bordered on
the north and south by the Archean. The lowest part of the Algonkian occupies parallel zones nest
to the Archean lioth on the north aud on the south, aud is succeeded toward the interior of the strip
by the younger members. The general structure therefore is synclinal, Iiut is not simple. The strip
contains two or more synclines separated by anticlines. They have likewise been affected by cross
folds, which give a different pitch to the axes of the east and west folds. The structure is also compli-
cated by faulting. The Algonkian is divided into two series separated by an unconformity. In the
first occur, from the base upward, the .Sturgeon quartzite, the Eaudville dolomite, the Mansfield
schist, and the Groveland iron formations. Above these follows a youuger series which is undivided.
Section III treats of the Archean. This limits the Algonkian rocks on the north and south, and
is very well exposed. The topography is very rough. Usually, but not always, a topographical
MON XXXVI III
XXXIV OUTLINE OF THIS MONOGllAPH,
(lepressiou, occupied by a swamp or by a stream, exists along the coutact betweeu the Archeau and
the Algonkian. Petrographically the Archean consists of (1) granites or granitic gneisses, (2)
gneisses, (3) mica-schists, (4) hornblende-gneisses, or amphibolites. The granites possess the usual
characters of such rooks. The gneissoid members of this division are merely crushed granites,
and are connected with the massive rocks by indistinguishable gradations. The gneisses are banded
laminated rocks, the minerals of which have crystallized in parallel elongated forms. Subsequent
to crystallization they have been acted on by great stresses. The mica-schists are even, medium-
grained rocks, with generally well-developed schistosity. The original character of these schists
is wholly indeterminable. Their relationship with the granites and gneisses is perhaps a reason
for regarding them as derived from originally massive granites by dynamic metamorphism. The '
hornblende-gneisses, or amphibolites, are black or dark-green rocks, which are universally foliated.
They occur in narrow bands in the granites and gneisses. Their boundaries are sharp and frequently
cut the foliation of the amphibolites and of the gneisses. The field relation as well as the composi-
tion of the amphibolites leads to the conclusion that they are old dikes of basic rocks which have
been metamorphosed and recrystallized.
Section IV treats of the Sturgeon quartzite. The Sturgeon formation is the most widespread
member of the Algonkian series in the Felch Mountain range. It occurs in two parallel zones of
varying width, immediately adjoining the Archeau to the north and to the south, except when
displaced from thi.s position by faults. It is fairly well exposed. It frequently forms distinct lineal
ridges, which, with but few exceptions, seldom rise to the mean altitude of the adjoining Archean.
Owing to the comjileteness of recrystallizatiou, the original sedimentary features have almost been
obliterated, so that it is difiScnlt to find places suitable for dip observations. SufiScient dips have
been found to show that subordinate folds occur within the quartzite. The average thickness is
probably not less than 450 feet, and may bo more. Petrographically the formation includes massive
quartzites and mashed quartzites or micaceous quartz-schists, in some of which the relations of the
quartz present unusual features.
Section V. The Randville dolomite, consisting of crystalline dolomitio rocks, overlies the
Sturgeon quartzite. The Randville dolomite covers a larger share of the surface in the Felch Moun-
tain range than any other member of the Algonkian. Natural exposures are fairly numerous and
very evenly distributed. Moreover, test pits and diamond-drill borings have shown the pi-esence of
the formation in the covered areas. Relatively the dolomite is a weak rock, and occupies relatively
low ground. An average thickness of 700 feet is estimated for the Randville dolomite within the
Felch Mountain range. Petrographically the formation consists of a rather coarse-grained, thoroughly
crystalline dolomite, with more or less abundant crystals of tremolite and a number of other minerals
of minor importance.
Section VI. The Mansfield schist is only exposed in certain test pits. Its presence has also
been determined by diamond-drill borings. The thickness is so small — not more than 200 feet — that,
though it weathers readily, it produces no noticeable eft'ects on the general topography. Petro-
graphically it consi.sts of fine-grained mica-muscovite or mica-biotite-schists, probably derived from
the metamorphism of a clasfc. It shows nothing of especial interest.
Section VII. The Groveland formation is magnetic and has been traced by means of compass and
dip needle. Excellent natural as well as numerous artificial exposures render the data concerning the
distribution of the formation very satisfactory. The most prominent hills in the Algonkian belt owe
their relief to the fact that they are underlain by the Groveland formation. Petrographically we may
recognize two main kinds of rock. The usual kind consists of quartz and the anhydrous oxides of
iron, while the other and much rarer consists essentially of an iron amphibole with quartz and the
iron oxides as associates. Both of these kinds are clearly of detrital origin. The conclusion is
reached, based on certain microscopical structures, that iron and silica were originally present
largely in the form of glauconite.
Section VIII. Mica-schist aud ferruginous quartzites of the Upper Huronian series occur in
the eastern part of the Felch Mountain range. The rocks constituting the series are soft iron-
OUTLINE OF THIS MONOGRAPH. XXXV
stained mica-schists, with thin, interbandod beds of ferrugiuous aud miciiccous (juartziti!. Neither
kind shows traces of chistic origin. V'rom their structures and general relations they are believed to
have been derived from sedimentary rooks by metamorphism.
Section IX. The Algonkian rocks are cut by iutrusives, among which both acid and basic rocks
are reiiresented. The acid rocks are granites occurring in narrow dikes. No dikes of granite .ire
known to cut the Kaiidville or Manstield formations.
Chapter IV treats of the Miehigamme Mountain and Fence River areas. These areas occur in
the central part of the district. In the Fence River area the structure! is very simple. In the Miehi-
gamme Mountain area the structure is comple.x.
Section I treats of the Archean. The prev.alent rock is granite, cut by acid and basic dikes.
Section II treats of the Sturgeon form.-itiou. This is scarcely known as a distinct Algonkian
member in this area .apart from the Randville formation. In one section purely clastic sediments
were observed, for which it is convenient to retain the name. These exposures cousist of slates iind
gray wackes, with some layers of a coarser texture.
Section III treats of the Randville dolomite. In the Fence River area the dolomite lies on the
east side of the Archean and occupies a belt about one-half mile in width, aud extending from the
mouth of the Fence River about 10 miles to the north and west, where it leaves the portion of
the district studied. In the Miehigamme Mountain area the dolomite tops the low arcli in a broad
crumpled sheet. The form<atiou in the Fence River area occurs in an eastward-dipping monocline
with a number of minor plications. An average thickness of about 1,500 feet is estimated for the
formation in this area. In the scattered outcrops of the Miehigamme Mountain area the dolomite
strikes .and dips toward all points of the compass, caused by the gentle archiug from the general
northwest-southeast axis, combined with sharp local folds which run nearly east and west. Petro-
o-raphically the formation ranges from coarse saccharoidal marbles, sometimes very pure but usually
filled with secondary silicates, to fine-grained, little-altered limestones, which are occasionally so
impure as to be calcareous or dolomitic sandstones and shales. The prevalent colors are white, but
various shades of pink, light .and deep blue, aud pale green occur. Some of the varieties are oolitic.
This structure does not seem to have been noted previous to this in limestones of pre-C'ambri,an age.
Section IV treats of the Mansfield formation. The typical locality of this formation is in the
vicinity of the Mansfield mine, which lies to the west of the district studied. Where it occurs in the
Miehigamme Mountain area, the formation consists of phyllites or mica-slates of various colors. The
structure of this area is so complex and the outcrops so few as to forbid any but an approximate
outlining of the general boundaries of the formation. The geological position of the form.ation is
free from doubt. It overlies and passes downward into the Randville dolomite. The formation does
not seem to influence the topography. Like the preceding one, it has been extensively folded.
The average thickness is probably not less than 400 feet. The mica-slates or phyllites possess no
especial petrographieal interest.
Section V treats of the Hemlock formation. The Mansfield formation of the Miehigamme
Mountain area changes along the strike into rocks of a ditiferent character, to which the above name
is given. In the Fence River area it occupies a belt between 2,000 and 3,000 feet in width, between
the Randville dolomite on the west and the Groveland formation on the east. The l>est exposures
occur on the sections made by the Fence River. No folds have been observed within this formation.
The thickness probably varies from 0 to 2,300 feet as a maximum. The rocks of the formation are
chiefly chloritic and ophitic schists, with which are associated schists bearing biotite, ilmenite, and
ottrelite ; greenstone, conglomerates or agglomerates, .and amygdaloids. The general characters of the
schists are (1) a groundmass composed of chlorite, quartz, m.agnetite, epidote, and in some cases
plagioclase microlites, and (2) the presence in this groundmass of much larger porphyritic individuals
of several secondary minerals. As evidence of the origin of these schists, first, there is the absence
of rocks possessing any sedimentary characters; next, lavas and also greenstone-conglomerates or
agglomerates are undoubtedly present in the series ; furthermore, the minerals which compose the
schist are those which would result from the alteration in connection with dynamic metamorphism of
XXXVI OUTLINE OF THIS MONOGRAPH.
igneous rocks of basic or intermediate chemical composition ; and finally, the grain and character
of the groundmass, and in some slides tlie jireaence of plagioclase microlites disposed iu oval lines
point directly to an igneous origin and to consolidation at the surface. The conclusion is reached that
the Hemlock formation of the Fence River area is composed of a series of old lava flows varying in
composition from acid to basic.
Section VI treats of the Groveland formation. This is of wide extent throughout this part of
the Crystal Falls district, but its outcrops are limited to three localities. Its distribution has been
determined by means of its magnetic properties. It is not topographically prominent, except in the
Michiganime Mountain area, where it forms part of Michigamme Mountain. In the Fence River area'
it is probably not folded. It there dips to the east. At Michigamme Mountain it is found iu several
well-marked folds. The thickness of the formation is estimated to be approximately 500 feet. The
rocks are iuterbauded ierruginous quartzite and actiuolite and griinerite schists, which still contain
evidence of detrital origin.
Chapter V treats of the Northeastern area and the relations between the Lower Marquette and
the Lower Menominee. The territory included in the Northeastern area extends from the northern-
most outcrops of the Fence River area to the northern end of the Republic trough, a distance of about
11 miles. Outcrops are scarce throughout this area, and the main conclusions are drawn from the
magnetic work. Through the structural and lithological results of the magnetic work the gap
between the Marquette and the Crystal Falls district is bridged, and it is shown with a high degree
of probability that the Negaunee iron formation of the Marquette range is identical with the Groveland
iron formation of the Felch Mountain range.
Chapter VI treats of the Sturgeon River tongue. In the southeastern part of the Crystal
Falls district and just north of the Felch Mountain range a tongue of fragmental rocks extending
eastward has been studied. The extreme leugth is 12 or 13 miles. Its width at its eastern end is li
miles; to the west it widens rapidly. It is bounded both to the north and to the south by Archean
granites and schists ; to the east it is overlain by Paleozoic sandstones and limestones ; and at its west
end it is covered by glacial deposits. Within the tongue two small granite islands occur. The
Archean or Basement Complex rocks comprise gneissoid granites, hornldeude-schists, and biotite-
schists, which are cut by dikes of greenstone and granite, and veins of quartz. The sedimentary rocks
comprise conglomerates, arkoscs, quartzites, sericite-schists, clay slates, rocks that are probably
tufaceous, dolomitic limestones, and calcareous sandstones and slates. They may be divided into a
conglomerate series and a dolomite series. From the distribution of the exposures of the two series
it is concluded that the conglomerate series is the older, and that conformably above it follows the
dolomite series. The two form a westward-pitching syncline. The conglomerate series and the
dolomite series are correlated respectively with the Sturgeon quartzite and the Randville dolomite of
the adjoining Felch Mountain range.
THE CRYSTAL FALLS IRON-BEARING DISTRICT OF MICHIGAN
Part I.— THE ^A^ESTERN PART OF THE DISTRICT
MON XXXVI 1
CONTENTS.
Page.
ChAPTKR I. — INTKOIIUCTION ; ^^
Previous work in the district 13
Mode of work ^2
Maj^uetic observations 24
Chapteh II— Gkograpiucal limits, structure and stratigraphy, and physiography 25
Geograiihical limits 2o
Structure and stratigraphy 25
Physiography -"
Topography 29
Drainage 31
Timber and soil 36
Chapter III.— The Archean 38
Distribution, exposures, and topography 38
Relations to overlying formation,s - 39
Petrograpliical characters '10
Biotite-granite (grauitite) 10
Gneissoid biotite-granite, border facies of granite 13
Acid dikes iu Archuan • 15
Basic dikes in Archeau '1'^
Schistose dikes 1'^
Massive dikes ■1°
Re8umi5 .
49
Chapter IV. — The Lower Huronian series •'^O
Section 1.— The Rand ville dolomite 50
Distribution, exposures, and topography 50
Petrographical characters - - °1
Relatious to underlying and overlying formations 53
Thickness 53
Section 2. — The Mansfield slate 54
Distribution, exposures, and topography s^l
Possible continuation of the Mansfield slate ^>3
Petrographical characters 51)
Graywacke ^'^
Clay slate and phyllite 57
Origin of clay slate and phyllite 58
Present composition necessarily different from that of rock from which derived 58
Analysis of Mausfield slate 59
Comments on analysis ''"
Comparison of analysis of Mansfield clay slate with analyses of clays 60
Comparison of analysis of Mansfield clay slate Tvith analyses of other clay slates.. 61
Siderite-slate, chert, ferruginous chert, and iron ores 62
Relations of siderite-slate, ferruginous chert, and ore bodies to clay slate 63
4 CONTENTS,
Chapter IV.— The Lower Huronian series— Contiuned.
Section 2. — The Mansfield slate— Continued. P?ge.
Relations of Manstield slate to adjacent formations ., 63
Relations to intnisives 63
Relations to volcanics 6l
Structure of the Mansfield area 64
Thickness 64
Ore deposits - 65
General description of Mansfield mine deposit 68
Relations to surrounding beds 68 '
Composition of ore 68
Microscopical character of the ores and associated chert bauds 69
Origin of the ore deposits 70
Conditions favorable for ore concentration 72
Exploration - 73
Section 3.— The Hemlock formation .- 73
Distribution, exposures, and topography 73
Thickness 74
Relations to adjacent formations 75
Relations to intrusives 77
Volcanic origin 78
Classi fication 79
Acid volcanics • 80
Acid lavas 80
Rhyolite-porphyry 81
Texture 83
Aporliyolite-porphyry 87
Schistose acid lavas 87
Acid py roclastics - ^
Basic volcanics 95
Basic lavas "5
General characters 95
Nomenclature 95
Metabasalts 98
Nonporphy ritic metabasalts 98
Petrographical characters 98
Chemical composition 103
Porphyritic metabasalt 103
Petrographi cal characters 104
Chemical composition 105
Variolitic metabasalts 108
Ellipsoidal structure 112
Origin of ellipsoidal structure 118
Amygdaloidal structure 124
Flattening of amygdaloidal cavities 126
Alteration of the basalts 126
Description of some phases of alteration 127
Pyroclastics 135
Eruptive breccia 135
Volcanic sedimentary rocks 136
Coarse tuffs 137
Fine tufi's or ash (dust) beds 142
Relations of tuffs and ash (dust) beds 143
Volcanic conglomerates 143
Schistose pyroclastics WS
CONTENTS. 5
ClIArTKK IV. — TllK LoWKR HURONIA.N SKUiKS — C'outiiimjd.
Section 3. — The Hemlock formation— Continued. Page.
Tl)<« Bono Lake crystallini^ scliists 148
Uistriliution 148
Field evidence of connection witli tlie voleanics 149
I'etrogiiipliical iharacter.s 150
Normal 8edimentarie.s of the Hemlock formation 152
Economic products 153
Building and ornamental stones 153
Road materials 154
Chapter V. — The Upper Hukonian series 155
Distribution, e.xposures, and topography 155
M.agnetic lines 156
. Thickness 157
Folding w 1.58
Crystal Falls syncline 158
Time of folding of the Upper Huronian 161
Relations to other series 162
Relations to iutrusives 164
Correlation 164
Petrographical characters 165
Sedimentary rocks 165
Microscopical description of certain of the sedimeutaries 169
Igneous rocks I74
Ore deposits j75
History of opening of the district I75
Distribution I75
Western half of sec. 34, T. 46 N.,R. 33 W 176
Sec. 20, T. 45 N., R. 33 W 176
The Amasa area I77
The Crystal Falls area 178
Character of the ore 180
Relations to adj acent rocks 182
Origin I83
Size of the ore bodies 184
Methods of mining Igj.
Prospecting I85
Production of ore from the Crystal Falls area 186
Chapter VI. — The Intrusives 187
Order of treatment 188
Age of the intrusives 188
Relations of folding and the distribution of the intrusives 189
Section I. — Unrelated intrusives _ igo
Classification 190
Acid intrusives igO
Geographical distribution and exposures of granites 190
Biotite-granite 191
Mieropegmatites 192
Muscovite-biotite-granite I93
Relations of granites to other intrusives I94
Dynamic action in granites I94
Contact of granites and sedimeutaries I94
Evidence of intrusion I95
(3 CONTENTS.
Chapter VI.— The Intrusives — Continued.
Section I. — Unrelated intrusives — Continued. "P^ge.
Basic intrusives 198
Metadolerite 199
Geographical distribution 199
Petrographical characters 199
JIacroscopical 199
Microscopical 200
Relations to adjacent roclis 203
Relations to Lower Huroniau Mansfield slates 203
Relations to Lower Huroniau Hemlock rolcanics 204
Relations to Upper Huroniau 20+
Relations to other intrusives 204
Contact metamorphism of Mansfield slates by the dolerite 204
.Spilosites 206
Analyses of spilosites 207
Desmositcs 207
Adiuoles 208
Analyses of adinoles 208
Comparison of analyses of normal Mansfield clay slates and the contact prod-
ucts - 209
No endomorphic effects of dolerite intrusion 211
Metabasalt 211
Ultrabasic intriisives 212
Picrite-porphyry (porphyritic limburgite) 212
Geographical distribution and exposures 212
Petrographical characters 212
Gray treniolitized picrite-porphyry 213
Dark serpentinizod picrite-porphyry 217
Classification 220
Section 11. — A study of a rock series ranging from rocks of intermediate acidity through
those of basic composition to ultrabasic kinds 221
Diorite , 222
Nomenclature 222
Distribution and exposures 223
Petrographical characters 223
Description of interesting variations 226
Sec. l.^T. 42N.,R. 31W 226
Across river from Crystal Falls 227
Southeast of Crystal Falls 227
Analysis of diorite 231
Gabbro and norite 233
Petrographical characters 233
Description of interesting kinds of gabbro - 240
Hornblende-gabbro in sec. 1.5, T. 42 N., R. 31 W 240
Sees. 15,22,28, and 29, T. 42 N.,R. 31 W 241
Hornblende-gabbro dikes 243
Bronzite-norite dike 244
Sec. 29, T. 42 N., R.31 W., 1200 N., 200 W 245
Dynamically altered gabbro 247
Relative ages of gabbros 249
Peridotites 249
Distribution, exposures, and relations 249
Petrographical characters 249
CONTENTS. 7
CiiArTKK \I. — TiiK iNiiiUsivBS — Coiitinuefl.
Sootioii II. — A study ot':i rock series, etc. — Continued.
Peridotite — t'outiuucd. Paje.
Periilotite varieties 252
Wehrlito 253
Amiiliiliole-puridotite 233
Gr.adatious of ampbiboK'-peridotite to wehrlite and olivine-gabbro 254
Process of crystallization 257
Analysis of peridotite 259
Peridotite from sec. 22, T. 42N., R.31 W., 1990 N., 150 W 260
Relations of peridotites to other rocks ., 261
Age of peridotites 262
General observations on the above series 262
Textiiral characters of the series 262
Chemicil composition of the series 263
Relative ages of rocks of the series 265
ILLUSTRATIONS
Page.
Pl-ATE I. Colored insii) showing the distribution of pre-Cambriiin and otber rocks in the Lake
Superior region, and the geographical relations of the Crystal Falls district of
Michigan to the adjoining Marquette and Meuominee districts of Michigan 11
II. Topographical map of the Crystal Falls district of Michigan, includiug a portion of
the Marquette district of Michigan In pocket.
III. Geological map of the Crystal Falls district cf Michigan, including a portion of the
Marquette district of Michigan In pocket.
IV. Portion of a geological map of the Menominee iron region, by T. B. Brooks and C. E.
Wright 18
V. Generalized sections to illustrate the stratigraphy and structure of the northwestern
part of the Crystal Falls district of Michigan 28
VI. Generalized sections to illustrate the stratigraphy and structure of the southern part
of the Crystal Falls district of Michigan 28
VII. Generalized columnar section 30
VIII. Map of a portion of the Crystal Falls district, showing in detail the glacial topog-
raphy and illustrating the development of the Deer River 32
IX. Sketch of the Mansfield mine as it was before it caved in, in 1893 66
X. A, Reproduction of the weathered surface of a variolite; B, Reproduction of the
polished surface of a variolite 110
XI. Colored reproduction of an ellipsoid, with matrix, from an ellipsoidal basalt 116
XII. Mount Giorgios, viewed from its west flank, in April, 1866, illustrating the charac-
teristic block lavas, from Fouque's Santorin et ses Eruptions, PI. VIII 120
XIII. Reproduction in colors of a basalt tuft' 140
XIV. Idealized structural map and detail geological map, with sections, to show the dis-
tribution and structure of the Huronian rocks in the vicinity of Crystal Falls,
Michigan 160
XV. Portion of Brooks's PI. IX, Vol. Ill, Wisconsin Geological Survey 172
XVI. Detail geological map of the vicinity of Amasa, Michigan 176
XVII. Detail geological map of the vicinity of Crystal Falls and Mansfield, Sheet 1 178
XVIII. Detail geological map of the vicinity of Crystal Falls and Mansfield, Sheet II 178
XIX. ^, Inclusions in a fractured (juartz phenocryst; 7i, Quartz phenocryst with rhombo-
hedral parting 368
XX. ^, Micropoikilitic rhyolite-porphyry; 5, Micropoikilitic quartz-porphyry 270
XXI. ^, Very fine-grained micropoikilitic ryholite-porphyry viewed without analyzer; B,
Very fine-grained micropoikilitic rhyolite-porphyry viewed with analyzer 272
XXII. j4, Perlitic parting in aporhyolite; £, Perlitic iiarting iu aporhyolite 274
XXIII. J, Schistose rhyolite-porphyry ; B, Aporhyolite breccia 276
XXIV. J, Schistose rhyolite-porphyry; i?, The same viewed between crossed nicols 278
XXV. ^, Amygdaloidal texture of basalt; i}, Amygdaloidal vitreous basalt 280
XXVI. J, Amygdaloidal vitreous basalt; B, Amygdaloidal vitreous basalt showing sheaf-like
aggregates of feldspar 282
XXVII. A, Reproduction in colors of amygdaloidal basalt; B, Pseudo-amygdaloidal matrix of
ellipsoidal basalt ; C, Water-deposited pyroclastic 284
XXVIII. J, Fine-grained basalt with well-developed igneous texture: i?, Illustration of the
obliteration of the igneous texture of a basalt by secondary products when viewed
between crossed nicols 286
9
10 ILLUSTKATIONS.
Page.
Plate XXIX. ^, Basalt showiDg characteristic texture in ordinary light; B, Basalt showing
obliteration of texture between ci'ossed nicols 288
XXX. A, Basalt showing in ordinary light ii distinctly amygdaloidal texture; B, The
same basalt with its amygdaloidal texture obliterated when viewed between
crossed nicols 290
XXXI. A, Basalt aft'ected by calcification process; B, Basalt affected by calcification
process viewed between crossed nicols 292
XXXII. A, Illustration of perlitic parting in a fragment from a basaltic tuff; B, Sickle-
shaped bodies in volcanic tuft' 294
XXXIII. ./, Water-deposited sand ; B, Gradation in water-deposited volcanic sediment . .. 296
XXXIV. .1, Contact product of granite; /?, Brecciated matrix between ellipsoids 298
XXXV. A, Contact between granite and a metamorphosed sedimentary; B, Contact
between granite and a metamorphosed sedimentary viewed between crossed
nicols 300
XXXVI. A, A variety of spilosite with white spots; B, A variety of spilosite with white
spots viewed between crossed nicols 302
XXXVII. A, Normal spilosite or spotted contact product; i?, Normal spilosite of some-
what dift'erent character 304
XXXVTII. .1, Passage of spilosite into demosite; B, Occurrence and alteration of bronzite
in bronzite-norite 306
XXXIX. A, Biotite-granite viewed between crossed nicols ; B, Mica-diorite viewed between
crossed nicols 308
XL. A, Quartz-niioa-iliorite-porphyry ; B, Quartz-mica-diorite-porphyry viewed be-
tween crossed nicols 310
XLI. J, Porphyritic poikilitic hornblende gabbro; /?, Poikilitic hornblende gabbro.. . 312
XLII. .-(, Moderately fine-grained hornblende gabbro showing parallel texture; B, Mod-
erately fine-grained hornblende gabbro showing parallel texture viewed
between crossed nicols. ..'- 314
XLIII. J, Normal granular hornblende gabbro; B, Schistose hornblende gabbro viewed
between crossed nicols 316
XLIV. ,1, Moderately fine-grained hornblende gabbro; i?, bronzite-norite 318
XLV. J, Brouzite-norite-porpbyry ; /?, Feldspathic wehrlite 320
XLVI. J, Feldspathic wehrlite viewed between crossed nicols; /i, Feldspathic wehrlite. . 322
Fig. 1. Reproduction of a portion of the geological map of the Upper Peninsula of Michigan,
by William A. Burt, 1846 , 15
2. Enlarged reproduction of a portion of a map of the Lake Superior land district, by
Foster .nnd Whi tney 17
3. Enlarged reproduction of a portion of a geological map of the Upper Peninsjila of Michi-
gan, by K'ominger, Brooks, and Pumpelly, 1873 18
4. Grauite-porpbyry with inclusions of gneissoid granite 45
5. Illustration of the effect on the topograjjhy of the differential erosion of basic dikes and
granite 46
6. Concentric cracks formed by the caving in of the Mansfield mine 65
7. Sketch of the surface of the outcrop of an ellipsoidal basalt, showing the general char-
acter of the ellipsoids and matrix 112
8. Sketch showing tlie concentration of the amygdaloidal cavities on one side of an ellip-
soid, this side probablj' representing the side nearest the surface of the flow 113
9. Ellipsoids with sets of parallel lines cutting each other at an angle 114
10. Reproduction of illustration of aa lava, after Dana 120
11. Profile section illustrating results of diamond-drill work 177
12. Sketch illustrating contortion of Upper Huronian strata 179
13. Sketch showing change of strike of Upper Huronian beds, due to the folds 179
14. Sketch to illustrate the occurrence of ore bodies 182
us GEOLOGICAL SURVEY
MONOGRAPH XXXVI PL
GEOLOGIC MAP OF FART Ol'^ THl.; LAKE HUI'KRIOH REGKW
Compilftd from Official maps of U. S.MiniiPsoUi.aiid Caiiadian Sui^'eys
Scah.-
Ah* The Afrnamuirr IrotvBrarirtfiSent^
. Ah5 Thf n'usconj>ui f'niliry Stntes
Hi KONlA^ ■, Ah6 T/tf fmokeelran-Bttifinff Series
Ah 7 Hi/- St Louis Sfrifj
Ahe The Chippewa VitUey QuartMitei:
Ah 9 The Blaek River TrowBearin^ .Srfiwfj
Ah 10 The Aiumikie Series
Ah 1 1 TTie Fhlded SrJUata afOinadfU
urn STAT Ml
THE CRYSTAL FALLS IRON-BEARING DISTRICT
OF MICHIGAN.
PART 1. THE WESTERN PART OF THE DISTRICT.
By J. Morgan Clements.
CHAPTER I.
INTRODUCTION.
The present report is an account of a portion of the Crystal Falls dis-
trict of Michigan, so called from the most important town, Crystal Falls,
the comity seat of Iron County. The iron-bearing district along the Paint
River, near the site of the town of Crystal Falls, was first called in literature
the Paint River district by Brooks.^ As soon as the town was begun, about
1880, the name of the town was applied to the district.^ It is situated
on the Upper Peninsula of Michigan, adjoining the northeastern border of
Wisconsin, and serves as a link connecting the two well-known iron-ore-
producing districts of Michigan, the Marquette, and the Menominee. The
Crystal Falls district is of itself of considerable economic importance, as
will be seen, though not deserving to be ranked with either of the two
above-mentioned iron districts. Since the geological relations of the rocks
' The irou-bearing rocks (economic), by T. B. Brooks: Geol. Survey of Michigan, Vol.1, Part I,
1873, p. 182.
"Rept. Com. Miu. Statistics Mich, for 1881, p. 222.
11
12 THE CRYSTAL FALLS IRON- BEARING DISTRICT.
of the Marquette district have now been ascertained/ it is hoped, by means
of the determination of the succession in the intermediate Crystal Falls
district, that the Menominee rocks may be closely correlated with those of
the Marquette district.
The accurate delimitation of the iron-bearing or coal-bearing forma-
tions, or any other formations containing valuable mineral products, is of
inestimable value to miners and investors. In the iron districts of Michigan
alone innumerable test pits have been sunk in areas of solid granite, and at
great distances outside of the possible iron formations, thus wasting large
sums of money. Although the investigations carried on in the Crystal Falls
district, the results of which are here recorded, do not enable us to point out
definitely the places where the prospector will find iron deposits, they have
enabled us to delimit in a broad way the various formations, and warrant
the statement that iron deposits may occur in certain areas and that the
prospector will assuredly not find iron dejjosits in certain others.
The opportunity of studying the Crystal Falls district was given me
tlu-ough Prof. C. R. Van Hise. In the prosecution of the field studies and
in the preparation of the report I have availed myself of his advice and
suggestions, which have been generously ofi"ered and which have been
found of greatest value. To him I am most deeply indebted.
The report is based not only on my own field work, but also on the
field work done by a number of other geologists, whose notebooks have
been placed at my disposal. The names of these geologists may be found on
page 22. Among them, the notes of Mr. W. N. Merriam and Dr. W. S.
Bayley have been found especially valuable. Mr. Merriam, assisted by Dr.
Bayley, spent a season in doing very detailed work on the area shown on the
sketch map at the bottom of PI. Ill, between Crystal Falls and Mansfield,
and from this point northwest to some distance beyond Amasa. The mag-
netic lines represented in this part were traced by Mr. Merriam, and the
geology in general is the same that he outlined on his final field map
I wish to thank Mr. C. K. Leith, who has been of the greatest clerical
assistance, and Mr. E. C. Bebb, by whom the maps were drawn; also Mr.
• The Marquette iron-bearing district of Michigan (preliminary), by C. R. Van Hise and W. S.
Bayley; with a chapter on the Republic Trough, by H. L. Smyth: Fifteenth Ann. Rept. U. S. Geol.
Survey, 1895, pp. 477-650. Ibid, (final), Mon. U. S. Geol. Survey, Vol. XXVllI, 1897.
PKEVIOUS WORK. 13
J. L. Ividg-wiiy, by wlioin tlie colored plates of natural size specimens w(;re
j)repure(l.
PREVIOUS WORK IN THE DISTRICT.
( )u account of its comparatively slight economic importance, and also
on account of its isolation, very little work of whicli the results have been
published was done in this district prior to that on which this monograph is
based. As a rule, the earlier observers began the season's work either in
the Marquette or in the Menominee range, and working westward the
Crystal Falls district was reached only as the season neared its close, or as
the appropriation was nearly exhausted. The published work upon this
district is given below in chronological order.
X850.
Burt, Wm. A. Report of linear surveys with reference to mines and minerals,
in the Northern Peninsula of Michigan in the years 1845 and 184C. Dated March 20,
1847. Thirty-tirst Congress, first session, 1850; Senate documents, Vol. Ill, No. 1,
pp. 842-882, with map.
Dui'ing the year 1846 a linear survey was made of that part of the
Upper Peninsula of Michigan described as being bounded on the north by
the fifth correction line, on the south by the fourth correction line and the
Brule River, on the east by ranges 23 and 26 W., and on the west l)y
range 37 W. This includes in its limits the district under discussion. In
the course of the survey, geological observations were made by William A.
Burt, the deputy surveyor in charge of the work. The report and accom-
panying geological map embodying the results of these observations are
concealed among the Senate documents of the Thirty-first Congress. The
following quotations from this report give all the observations on the part
of the territory surveyed in which we are at jjresent interested:
Topography. — Wcst of range 31 west, and north of the Brule River to the fifth
correction line, is a tract of about 43 townships in which the rock is mostly greenstone
and hornblende slates. This part of the surveyed district is less broken than that
above described, and a large proportion of it may be denominated rolling lauds.
There are, however, many ridges and conical hills of various heights upon this part of
the survey, and also deep valleys of streams, many of which have ledges upon their
sides. These general characteristics are often changed for cedar, spruce, or tamarack
swamps, which are most numerous in townships 46, 47, and 48 N. [This includes the
part of the district supposed to be the continuation of the northern Wisconsin
peneplain (p. 31).]
14 THE CRYSTAL FALLS IRON-BEARING DISTRICT.
Granite (and syenite). — These Tocks occupy au area of about 22 townships on tlie
northeast part of the survey, between the fifth correction line and the south boundary
of township 45 N., and east of range 32 W., in a series of irregular uplifts, frequently
forming high cliffs and sloping ledges on the most elevated portion of this district.
[This covers a part of the Archeau granite oval of the Crystal Falls district, as well
as the large Archeau areas northeast of it.)
Argillaceous slates. — The argillaccous slates alluded to in townships 42 and 43 N. are
generally overlaid by deep drift; their boundaries, therefore, could not be satisfactorily
defined. West of the Peshaknmine River these slates appeared to have undergone
considerable chauge by igneous action, aud were often associated with an oxide of
iron; but east of the Peshakumine no change by igneous action in the slates was
observed, and on this part they have generally a reddish color
They dip variously at a high angle, and are supposed to conform to the greenstone
on the north and west, and to overlie or pass into the mica-slates on the south ; and
in their middle portion they di|) about 90°, with strike nearly east and west. (These
slates correspond to our least metamorphosed phases of the Upper Huronian.]
Greenstone and hornblende slate. — Thcsc rocks occupy a larger area lu the district
surveyed than any other class of rocks. They extend from the granitic and other
rocks east of them westward beyond the survey. [See their outline on map,
tig. 1.]
The greenstone and hornblende slates form a less broken surface than the
granitic range; and next to it is the most elevated range in this district, having an
estimated altitude, in many places, of from 1,001) to 1,100 feet above Lake Superior.
These rocks are frequently seen in the beds and bauks of streams aud in ridges
aud conical hills of various heights, often forming precipitous ledges upon their
sides
The greenstone of this region is generally more or less granular and syenitic,
with a dark green color when moist; its composition is hornblende, feldspar, and
quartz — the former mineral greatly predominating. In some places the feldspar and
quartz are nearly or quite wanting, leaving a granulated hornblende rock. Another
variety of this rock was freciuently seen which was composed of the same ingredients
but very tine grained and compact and having frequently a laminated or slaty
structure, the cleavages of which generally dip from the granitic rocks at a very high
angle
Some of these hornblende slates have in their seams and cleavages a silky luster,
from the presence of mica or talc in very fine grains
All of these rocks are traversed by many quartz veins, from a line to 4 feet or
more in width, and with still larger veins and dikes of more recent trap rock. This
range is supposed to have become blended with the trap range of Keweenaw point as
it passes under the red sandstone lying between them, and probably farther west the
two are united in one range. [These are the altered and more or less schistose basalts
and accompanying- fragmeiitals which are comprised in the Hemlock formation. [
Mica-siates. — Thcsc slatcs strctch along the southerly side of the argillaceous slates
on the south part of the survey. They extend from the Brule River on a course east-
uortheast for about 22 miles, in townshij)s 41 and 42 N., ranges 29, 31, and 32 VV.. aud
have an average breadth of about 4 miles
The mica-slates are supposed to dip northerly under the argillaceous slates at a
high angle, varying at the surface from 45° to 80°
PREVIOUS WORK.
15
This rock is conii)<)se(l of iniwi, ((uartz, and feldspar. Its lainiita' are uudiilatiiig
or waved, but its cleavages, on a large scale, are even and regular
Those luica-slates are best (levelo])ed on the south bouiidaiy of township 4L! N.,
ranges 31 and 32 W., in the beds and banks of the Peshakunime and Mesqua-cum-a-
XXX III
XXXII
XXXI
XXX
XXIX
x'Xvul
Scale of miles
5
Fig. 1. — Reproduction of a portion of tbe geological map of the Upper Peninsnla of Michigan hy Wnj. A. Burt, 1846.
cum-sepe, and at the falls near the junction of the latter stream with the Brula River.
[These, according to our observations, are the most altered phases of the Upper
Huronian.]
That part of the geological map accompanying the above report which
corresponds to the Crystal Falls district is reproduced iu fig. 1.
16 THE CRYSTAL FALLS IRON-BEARING DISTRICT.
1851.
Foster, J. W., and Whitney, J. D. Report on the geology of the Lake Supe-
rior land district. Part II. The iron region, togetlier with the general geology.
Thirty-second Congress, special session, 1851; Senate documents, Vol. Ill, No. 4,
pp. 406, with maps and section.
In 1851 there was published a report by Foster and Whitney on the
iron regions of the Lake Superior land district, together with the general
geology. This gives the first connected account of the results obtained by
the various surveyors who had been engaged on the Government survey of
the Upper Peninsula of Michigan. Accompanying this report there are
two colored maps and a section. The subdivisions of the rocks as made by
Burt in the Crystal Falls district are not retained in this report by Foster
and Whitney. The map is generalized, and the hornblende-slates, etc., of
Burt are included under the general term "crystalline schists," and are
placed by the authors in the Azoic system. There are represented here and
there throughout this Azoic area a trajjpean knob and bed of marble.
Tlie granite area shown on Burt's map is very much reduced in size,
and no longer connected with the large granite areas to the east. The
granite on the lower reaches of the Michigannne, in T, 42 N., R. 31 W., is
here indicated for the first time. In these respects only does this portion
of the map show a decided advance in knowledge of the distribution of the
rocks. A copy of the map, showing the distribution of the rocks by
symbols instead of colors, is reproditced as fig. 2.
1873.
Brooks, T. B. The iron-bearing rocks (economic). Geol. Survey of Michigan,
Vol. I, Part I, 1873, pp. 319. With Atlas Plate IV and general map, by Romiuger,
Brooks, and Pumpelly.
The next mention of the district that I have been able to find was
made in 1873 by Maj. T. B. Brooks, in his report on the iron-bearing rocks
of Michigan.
However, this report seems to show a decided decrease in knowledge
from that possessed by Burt concerning the geology of this district. It is
trae that indications of iron had been seen, but the observations made were
so meager that nothing could be done toward determining the relations of
the rocks or unraveling the structure of the area.
Upon the map accompanying the report (Geol. Survey of Micliigan,
PKEVIOUS WORK.
17
1S73), a {jortion of wliicli is reproduced iu ti<i-. 3, Brooks lias failed to
outline the granitic areas kuowu to the previous explorers. Except iu a
Scale ormiles
IS
=1
Fig. 3. — Enlarged reproduction of a portion of a map of the Lake Superior laud district, by Foster and Whitney.
G;=graDite; T^trappean rocks; cT^ii'on; ;^|P| = beds of marble.
Metamorphic formation.
Azoic system.
Igneo 18 formation.
(Associated with the Azoic.)
Trappean rocks. Granite.
Quartz. Crystalline schists.
few places, which have been left tincolored, the district is covered with the
■color representing the Huroniau.
MON XXXVI-
18 THE CRYSTAL FALLS IRON-BEARING DISTRICT.
Brooks refers the ore-beariiig rocks to the Huroiiian in the following
. wor
■ds:
Too little is known about the reoiote Paint River district, in townships 42 and
43, ranges 32 and 33, to enable me to give anything of interest regarding its geological
I —
Scale of miles
Fig. 3. — Enlarged reproductiou of portion of a geological map of the Upper Peninsula of Michigan, by C. Kominger,
T. B. Brooks, and R Pumpelly, 1873.
structure. The Iluronian rocks are extensively developed there, and contain deposits
of hard hematite ore. I had the opportunity to examine only two localities at the
Paint River Falls, sec. 20, T. 43, R. 32, and sec. 13, T. 42, R. 33, (p. 182).
N2-t'l
rO
NZ-t'X
o
t
"5 "
I;
I
a.
^ k ^
Q
* !i « t
5 o ~ 0:
O i Q I,
Uj Q w %
10 Q ■» o
k ° fe o
o t '^ o
m U] w i
S S 5 5
(t t ft a
K. K K uj
V) "O </l 10
rr'tc
\ o
o:
■^5"? I-
in
H
< I/)
Si
2 O
in
\ Z ^
o
- > O <-
t ">
E i
qo
-i in
PREVIOUS WORK. 19
II*' also <;-ives liis analysis (No. 68, p. 302 ot" Brooks's report) of au ore
sample t'roin the district, and calls attention to its abnormally high water
content, freedom from silica, and richness in iron as compared to those of
the more eastern mines in tlie Menominee region.
1880.
Brooks, T. B. Geology of the Menominee region. Geol. Survey of Wisconsin.
Vol. Ill, Part V, 1880, pp. 430-0.35. Atlas folio. Pis. XXVIII ami XXIX, and PI.
XXX, by C. E. Wright.
In an article on the geology of the Menominee region, which wa.s
written for the geological survey of Wisconsin, the same author brief!}'
touched on that part of the adjoining Michigan territory which is included
in the district under consideration. His observations were thus confined to
a few exposures in a limited portion of the area. He attempts to correlate
certain beds by means of their lithological character with those with which
he was familiar in the Marquette district and refers them uniformly to the
higher members of the Huronian.
They are also so referred on the map which accompanies the report,
thougli this is dated a year earlier than the date of publication of the report.
That portion of the map covering a small part of the Crystal Falls region
is reproduced on PL IV. The present survey enables us to add very little
Jo this, aud these additions are chiefly of a petrological character.
. 1881.
RoMiNGER, Carl. Geology of the Menominee iron region. Geol. Survey of
Michigan, Vol. IV, 1881.
In 1880 Dr. Carl Rominger, at that time State geologist of Michigan,
spent a season in the Menominee district, and in his report gives detailed
descriptions of a few occurrences in the Crystal Falls district, to which I
shall refer later on. He considers the rocks in general to belong to the
Huronian, and distributes the beds among his diorite group, iron-ore group,
and arenaceous-slate group, as given and defined in the previous report on
the Marquette district No attempt at more definite coiTelation was made.
isso.
Van Hise, G. R. An attempt to harmonize some apparently coutlicUng views
of Lake Superior stratigraphy. Am. Jour, of Sci., 3d series, vol. 41, 1891, p. 133.
On December 30, 1890, Prof C. R. Van Hise read a paper on Lake
Superior stratigraphy before the Wisconsin Academy of Sciences, Arts, and
20 THE CRYSTAL FALLS IROX-BEARING DISTRICT.
Letters, the same article being published the following year in the American
Journal. The iron-bearing series of this district was in this article referred
to the Upper Marquette (Upiser Huronian).
1893.
Wright, C. E. Report of State geologist from May 1, 1885, to June 1, 1888, hi
Rapt, of the State Board of Geol. Survey of Michigau, 1893, pp. 33-44.
State geologist, Charles E. Wright, in a report for the seasons from 1885
to 1888, inclusive, merely mentions the general strike of tlie rocks of the
district, and makes no attempt to determine their age nor to unravel the
structure.
Wadsworth, M. E. Sketch of the geology of the iron, gold, and copper dis-
tricts of Michigau. lu Rept. of State Board of Geol. Survey for years 1891-92, 1893,
pp. 75-186.
Dr. M. E. Wadsworth, who on Mr. Wright's dea,th succeeded him as State
sreoloffist of Michig-an, mentions the occurrence of carbonaceous slates, of
granite and melaphyre, and of conglomerate near Crystal Falls, but does not
enter into a discussion of the relations of anv of these rocks. Dr. Wads-
worth agrees with the correlation of Professor Van Hise, and places the
Crystal Falls ore deposits in the Upper Marquette series (Wadsworth's
Holyoke formation) (pp. 117, 132), but the evidence for so doing is not
given in the report. He also is the first to recognize the volcanic nature of
the rocks in the vicinity of Crystal Falls (p. 134).
lS9o.
RoMiNGER, C. Geol. rept. on the Upper Peniusula of Michigan. Geol. Survey
of .Michigan, Vol. V, Part. 1, 1895, pp. 1-164.
In liis report of work done on the Upper Peninsula of Michigan from
1881 to 1884, published in 1895, he follows the same plan, referring the
various rocks exposed h\ mining operations to his different groups.^
Clements, J. Morgan. The volcanics of the Michigamme district of Michigan.
Jour, of Geol., Vol. Ill, 1895, pp. 801-822. ^
In a preliminary article on this district, by the writer, published in
1895, the volcanic character of the rocks which cover a large area of the
Crystal Falls district was emphasized, and in a sketch maj) in the same
' Geol. Survey of Micbigan, Vol. IV, 1881, p. 8.
'After the publication of this article, the name Michigamme haviug been applied to a formation,
it was deemed advisable, in order to avoid confiisiou, to change the name of the district to the Crystal
Falls district.
PREVIOUS WORK. 21
article was oiveii an outline of the distribution of the various rocks for a
portion of the ilistrict, with their stratig-ra[)hical succession (p. 803), the dis-
cussion of the structure and correlation l)eino- left for the present report.
The above-mentioned sketch map, with the maps by Burt, Foster and
Whitney, Brooks, Brooks and Wright, the section by Foster and Whitney,
and the section by Brooks, along the Paint and Michigannne rivers, are the
oidy maps or sections which, so far as can be learned, have been published
of that part of the Crystal Falls district under discussion.
MISCELLANEOUS REFERENCES.
JuLiEN, Alexis A. Apiit-ndi.x A. Lithology. Geol. of Michigan, Vol. II, 187.3,
PI). 1-185.
WiCHMANN, Arthur. Microscopical observations on the iron-bearing rocks
from the region south of Lake Superior. Brooks's Geol. of the Menominee Iron
Region, 1880, Chap. V, pp. 600-65.").
Wright, Charles B. Geology of Menominee Iron Region. Geol. of Wis-
consin, Vol. Ill, Part 8, 1880, pp. 665-741.
Lane, A. O. In sketch of the geology of tlie iron, gold, and copper dejwsits of
Michigan. Rept. of State Board of Geol. Survey for 1891-92, 1893, p. 182.
Patton, H. S. Microscopic study of some Michigan rocks. Uept. of State
Board Geol. Survey for 1891-92, 189.3, p. 186.
During the progress of the Michigan and Wisconsin State surveys
specimens from outcrops were collected, and descriptions of these discon-
nected specimens are found in the State reports.
References to the pages on which the individual descriptions may be
found will be given under the petrographical discussion of similar rocks
here described.
UNPUBLISHED WORK.
In 1891 a survey was organized by a private corporation, and put in
charge of Prof C. R. Van Hise. He consented to take charge of this work
on the conditions that all maps and notes should be available for this report
and that no other compensation was to be made by the company. The
object of this survey, known as the Lake Superior survey, was to study that
part of Michigan of which Crystal Falls is the center, in order to determine
the feasibility and advisability of opening up the mines of that district.
This survey was vigorously prosecuted, and an excellent topographic map
made of an area 32 miles north and south and 42 miles east and west, cover-
ing a large part of four 1.5-minute atlas sheets of the United States Geological
22 THE CRYSTAL FALLS IKON-BEAKING DISTRICT.
Survey. At the same time, in connection witli the topographic work, a
reconnaissance geological survey was made.
The following is a list of those who took geological notes for this
survey: Andrews Allen, A. H. Brooks, W. S. Bayley, J. P. Channing, E. T.
Eriksen, J. R. Finlay, F. J. Harriman, F. T. Kelly, E. B. Matthews, E. R.
Maurer, J. A. McKim, F. W. McNair, W. N. Merriam, and H. F. Phillips. '
The following season was devoted to a detail study of the iron-bearing
belts which had been outlined by the reconnaissance. This detail work in
the western part of the district was prosecuted by parties in charge of W. N.
Merriam, and in the eastern part of the district by parties in charge of
H. L. Smyth. When they ceased work, the two areas mapped were sepa-
rated in the north b}' about 12 miles, and a narrow belt separated the
mapped areas to the south. During the season of 1894, under the direc-
tion of Profes.sor Van Hise and assisted by Gr. E. Culver, and during part
of the season by S. Weidman, I was engaged iu completing this unfinished
work for the United States Geological Survey, preparatory to connecting
this district with the Menominee iron-bearing district to the southeast. This
work was carried on in 1895 by Dr. W. S. Bayley, S. Weidman, and myself,
and the mapping of the district extended as far as the Menominee district.
Mr. H. L. Smyth has written Part II of the present report, covering
the portion of the district which was worked by his party. My description
of the part of the district worked by me is based largely on my own obser-
vations. Many of the facts of field occurrence, however, mentioned in the
following paper were observed and recorded by the several men mentioned
above, and were subsequent!)' verified by my own observations in portions
of the area surveyed by myself, and by visits to localities in other portions.
The topography of the greater portion of the district was taken by the
members of the Lake Superior Survey. The remainder we owe to the
topographical division of the United States Geological Survey. The areas
covered l^v tlie respective organizations are shown on the sketch map below
the topographical map (PI. 11).
MODE OF WORK.
As explanatory of the locations given in the paper, it is perhaps not
out of place to give a brief description of the plan of work followed by the
Lake Superior Division of the United States Geological Survey in this as
MODE OF VVOKK. 23
■well as in tlie otliiT Lako Superiiir iniu-buariuy (listricts wliicli lia\u been
previously surveyed.
The Upper Peninsula of Michigan affords an excellent example of the
■excellence wliicli can be obtained in the rectangular land survey, when
properly carried oiit l)^■ the Government. The section corner posts originally
established are in many cases still to be seen, and of course the bearing
trees are even more common. Since the original survey the timber value
has increased so much that in certain forested areas the section lines have
been resurveyed. Not uncommonly trails follow the section lines for long
distances. Moreover, the roads are frequently laid out along the section
lines, thus giving permanent land boundaries. The section corners con-
sequently offer the most reliable points from which to make locations.
Ti'averses are made across each section, either frcm east to west or from
north to south, and at varying intervals, according to the discretion of the
geologist and the exigencies of the case. Each geologist is accompanied by
a compassman, whose duty it is to determine the course of the ti-averses by
means of a dial compass, and the distance traveled by pacing at the rate of
2,000 steps to the mile. Corrections are made at the corner and quarter
posts. The compassmen employed are Michigan woodsmen, land lookers
or cruisers as they are frequently called, and it is reniarkable with what
accuracy they will pace mile after mile through swamp and over rough
hills, windfalls, etc.
The geologist explores the territory on both sides of the line followed
by the compassman. Ledges are located by the geologist pacing to the
compassman as he comes opposite him in a due east-west or north-south
direction. With two coordinates thus determined, the ledges are located
with reference to the starting point. For uniformity and to facilitate ref-
erence and cataloguing, it is customary to give the location with reference
to the southeast corner of the section. Thus, 1,000 N., 1,000 W., SE. cor.
sec. 5, T. 42 N., R. 33 W., gives the location of the outcrop at the center
of the section, and affords a means of finding that ledge which could not
be so accurately and concisely stated by the use of any ordinary land-
marks. Moreover, easily recognized landmarks, such as houses, quarries,
etc., are few, and exceedingly great changes may occur very rapidly, such,
for instance, as those caused by widespread forest fires, so that such a
method of location is practically valueless.
24 THE CRYSTAL FALLS IRON-BEARING DISTRICT.
MAGNETIC OBSERVATIONS.
It has long been known that many rocks are possessed of decidedly
magnetic properties, due to the presence in them of varying quantities of
magnetic iron ore. By the mining engineers and prospectors this property
has been turned to a practical use in aiding in the location of iron mines
where the ore is of a magnetic kind. It is only in the past three decades
that this property has been used to any extent by geologists as an aid in
the interpretation of the structure of a region. So far as I can learn, the
best published account of its use thus is in Brooks's report on the iron-
bearing regions of Michigan.' Conclusive proof of its geological value
was given in the mapping of the Penokee area, in 1876, by R. D. Irving
of the Wisconsin survey.^ That area extends for about 60 miles northeast-
southwest, and is on the average about 4 miles wide. For the eastern part
of the Wisconsin area the outcrops are few, and Irving located the iron
formation by magnetic work. Along that belt have been sunk shafts
belonging to various mines which have raised quantities of ore, and in no
case has a shaft sunk outside of the limit indicated by Irving come upon
paying ore.
By means of the dip needle and solar compass, observations were taken
which enabled us to trace a curving magnetic formation and connect the
outcrops, which were separated by about 16 miles. The same bed was
further delimited, and the direction partly checked, by the occurrence, at
varying distances along this course, of outcrops of rocks of the underlying
formation.
Since the second part of this report contains an exhaustive article on
the methods and use of the magnetic needle,' the subject is not further
treated here. The lines of maximum magnetic disturbance — or briefly, the
magnetic lines — are represented on the accompanying general map, PI. Ill,
by blue lines marked with letters I) and E.
• Magnetism of rocks .and the use of the magnetic needle in exploring for ore, by T. B. Brooks.
Geol. Survey of Michigiin, Vol. I, Part I, 1873, pp. 205-243.
■^Geol. of the eastern Lake .Superior district, by K. D. Irving. Geol, of Wisconsin, \o\. Ill,
1880, pp. 53-238. .\tlas sheets, XI-XXVI.
3 See Part 1 1, Chapter II, by H. L. Smyth, pp. 33(j-373.
CHAPTEE, II
GEOGRAPHICAL LIMITS, STRUCTURE AND STRATIGRAPHY,,
AND PHYSIOGRAPHY.
GEOGRAPHICAL lilMITS.
The portion of the district here described extends from the north line
of T. 47 N. to the south line of T. 42 N., and from the c.enter of R. 31 W.
to the west line of R. 33 W., and contains approximately 540 square miles.
Upon the small sketch majj at bottom of PI. Ill is outlined the por-
tions of the district which have been studied and described by the different
authors.
The detail character of the formations is imknown for parts of the
area under discussion. This is especially true of the north, west, and
southwest parts, where, owing to the readily decomposable nature of the
rocks, as determined by the few ledges observed, and to the drift mantle,
very few outcrops are to be found.
STRUCTURE AKD STRATIGRAPHY.
The Crystal Falls district is not sharply defined petrographically, but
is continuous with the Marquette district on the northeast and the Menomi-
nee district on the southeast (PI. I). It is, however, remarkable for the
vast accumulation of volcanic rocks, which, while by no means absent from
the adjoining districts, do not there play so conspicuous a role.
StriTCturally this district can hardly be better separated from the
Menominee and IMarquette districts than it can be petrographically. The
important sedimentary troughs of the two adjacent disti-icts are separated
by an average width of 40 miles. The area between the districts on a.
direct course is occupied chiefly by Archean rocks, with narrow infolded
troughs of Huronian rocks playing a very subordinate role. At the east
25
26 THE CRYSTAL FALLS IRON BEARING DISTRICT.
the Archean is overlain by the sedimentaries of the Paleozoic, the Cam-
brian, and the Silurian. The connecting Crystal Falls rocks are west of
this Archean dome.
In the Marquette district the essential structural features have been
shown ^ to be a g-reat east-west synclinorium, upon which more open north-
south folds are superimposed. At the western end^ of the district the ■
superimposed north-south folds become close, and the Republic trough is a
close fold with an axis in an intermediate position. In the adjoining Crystal
Falls district there are also two sets of folds with their axes approxi-
mately at right angles to each other. The closer folds are represented by
the great anticline in the central part of the district. This anticline has its
axial plane trending west of north and south of east, and the axis plunges
down both at the north and south ends.
The more open set of folds at right angles to the above set, is repre-
sented by the Crystal Falls syncline, with its axis striking to the south of
west, and plunging west. Farther south the axes of the folds become much
closer and more nearl)^ east and west, thus nearly according in direction
with the close folds of the Menominee district. Thus the structural features
of the Crystal Falls district merge into those of the Menominee district,
which joins the Crystal Falls district on the southeast, where the great
structural feature is a synclinorium similar to that of the Marquette, but
with its axis trending north of west and south of east.
A glance at PI. Ill will show the presence in the eastern part of the
northern half of the district of an oval-shaped mass of Archean, and, nearly
surrounding this, a number of rock belts.
The Archean ellipse is 11 miles long and 3 miles wide on the average.
The rocks are mainly granite and gneiss. They are cut by rather infre-
quent acid and basic dikes.
Immediatel}^ surrounding the Archean is a quartzose magnesian lime-
stone formation, to which the name Randville dolomite has been given.^ In
the eastern half of the district described by Sm3^th, where more numerous
exposures are found than occur in the western half, the formation has an
estimated thickness of about lj500 feet.* Not only are the exposures
' Mon. XXVIII, cit., p. 566 et seq.
2Loc. cit., J). 570.
2 See Part II, Chapter IV, by H. L. Smyth, p. 431.
<See Part II, Chapter IV, Sec. Ill, p. 433.
STUIIGTURE AND STUATIGKAPHY. 27
more numerous, but. owing to the tact that the strata stand on edge, due to
the chjser t'ohling of the rock series here, a more accurate estimate of their
thickness can be made.
According- to Smyth, this limestone formation, in the southeastern end
of the elHj)se, at its upper liorizon becomes mixed with slates, and these
increase in quantity until the formation passes above into a slate formation,
called the Mansfield slate.' This slate formation is found overlying the
limestone to the west of the central ellipse likewise, but as few outcrops
have been found, it is not positively known to exist as a continuous zone
encircling the northwestern end. In a direct line with its probable continua-
tion to the north, a graywacke was found at one place, sec. 19, T. 46, R. 32.
This single outcrop is insufficient evidence to warrant the introduction of a
graywacke formation as the northern equivalent of a part of the Mansfield
slates, and it is probably but a phase of that formation. The only mine of
this district producing Bessemer ore is in a deposit in the Mansfield slate.
The close of the Mansfield Slate time was marked by the extrusion of
a great series of volcanics, which constitute the next formation in the
succession. This volcanic formation has its best and most typical develop-
ment west of the western Archean ellipse. Because the Hemlock River
and its tributaries have exposed good sections in the volcanics, and becavise
this river drains a great portion of the volcanic area, the name " Hemlock
formation " is applied to the volcanics. The dip of the flows and of the tuff
beds wherever observed is about 75° west. The maximum breadth is about
5 miles. Deducting 15° for initial dip, this would give the enormous maxi-
mum thickness of 23,000 feet to the volcanics, upon the supposition that no
minor folds occur.
These volcanic rocks have associated with them rocks of unquestionably
sedimentary origin, as is shown by their well-bedded condition and the
rounding of the fragments. The subaqueous rocks are, however, composed
of little-altered volcanic materials, and evidently point to oscillations of the
crust during the time of volcanic activity — such oscillations as have long
been known to be common in volcanic regions.
Following the volcanics, and overlying them, probably unconformably,
comes a series of sedimentary rocks, believed to belong to the Upper
Huronian. These comprise chloritic, ferruginous, and carbonaceous slates,
'See Part ir, Chapter IV, Sec. IV.
28 THE CRYSTAL FALLS IRON-BEARING DISTRICT.
associated with quartzites, graywackes, and small amounts of carbonate-
beds. The general character of the series is wliat one would expect in
rocks the detritus of which was from the Hemlock volcanics. It is in this
slate series that, with the exception of the Manstield mine, the ore deposits
of the Crystal I'alls district are found. The sedimentaries extend west
from the Hemlock volcanics to the limits of the district, underlying thus a
very broad expanse of country. Where exposed, the}' show frequent
changes of character. This prevents the identification of individual beds
for any considerable distance. Owing to the imperfect exposures of the
beds and their close folding, it has been found impossible to subdivide this
series of rocks into distinct formations.
The series has in places been highly metamorphosed, resulting in the
production of gneisses and mica-schists, in places garnetiferous and stauro-
litic. The series corresponds in a broad way stratigraphically and litho-
logically to the Michigamme formation of the Marquette district.^ Since,
however, it has been found impossible to subdivide this series, and because
it may possibly include more than the Michigamme formation of the Mar-
quette district, it is considered advisable to speak of it simply as the Upper
Huronian series. The generalized sections through the western half of the
Cr}^stal Falls district, which are given on Pis. V and VI, will aid in the
com^jrehension of the structural and stratigraphical features thus briefly
outlined.
Here and there in the Crystal Falls district isolated patches of Upper
Cambrian Lake Superior (Potsdam) sandstone are found. This occurs in
beds which are either horizontal or only a few degrees inclined from the
horizontal. They overlie unconformably the steeply inclined Huronian
strata. The great lapse of time represent -d by this unconformity is indi-
cated by the deposits of the Keweenawan and Lower and Middle Cambrian
time, found elsewhere. The Lake Superior sandstone grades fi'om the very
coarse basal conglomerate below into a moderately coarse sandstone above.
The sandstone is of a reddish brown to gray color, and is not well indurated
as a rule, but is loosely cemented with ferruginous and in places calcareous
material. As a result of this imperfect induration, the sandstone is not very
resistent to the agents of disintegration. Hence it is that only remnants
have been found, but enough is present to indicate that the greatei- part,
' Fifteenth Annual, cit., p. 598, anil Moii. XXVIIT, cit.. p. 444.
us GEOLOGICAL SURVEY
MONOGRAPH XXXVI PL V
HcmZ,y.:k P
Alk
Alk /R^r
i<- --r; '-
Sketch sliowin^ locaiioii oT sections
" on
General Map
EfeS
\/;^^0yy/^-^/y'^^i^:y^^^
i IN', \ ^^', Vv , •'V, IV ' V
r-' -,'. s-/ /- 1 ^^ V - ' /- I ^
Alh
l\\i.
-^'J.:^-^^^— -^gr Ala
fiUiM' -. ~ v^v"/ V ''■'V/ 'V'V — '-r^ ;- ^ — V / •* im
MM/^ ^ /^ "_'-,nv ^V"-.nv /-')'-> \; n'"'-,M«
«W '-'/v -'n- '-/v,^''-'-'A n'i''-'/n v' ''- '- ,\
ty-/ ' - V- - ^- I '- V- ,• '- >1 \ - ,' /-'\^y S- ^ //- I '1 \
AlaAIn Au AIn Ala
/Rgr Au AInAla -^^r-
-i%.\-,/.'-~s-,/-/'l>-'/-''^-/,./-,/,i--\-' - i^'-s-'/-/- .^'a>^-:v:v,;jaaKK-/ /--s-O/ri'- v-/^-i'L\-. /-'-'- \-/-'-\-,-'-i'^-\-//-i'l\-.^/-
^gr
Ala AIn
V / V , ' / \ ,
1 - ~/"_,N v', -'
x-N-,-,)-/^- s-,/-l/_N-/ /-/-■- I-,'/-/ -,N-<~
N.E-
Au
AIn Als Ala
US BIENaCO LITH TJ v
GENERM.1ZED SECTIONS THROUGH NORTH WESTERN PART OE CRYSTAL FALLS DISTRICT
HORIZONTAL SCALE, 1 LMCH = 1 MILE. VERTICAL SCALE, I lNCH-1320 ?'EET.
ELEVATION OF BASE LINES 1000 FEET.
NOTE: Formations are brought to the surface only whel-e exposures have been obsenved
ALGONKIAN
ARCH E AN
a<ii< &Airj;
L0WERJ1L'R0NLAJ>J
UPPER HURONIAN
Granile Sturricon am! Ajibik Koao amlRaiidvillt
quartzile dolomite
Hemlock formaUoii Growlaitd ^Xe^iiiinee UndixTded
lonnation
6
us GEOLOGICAL SURVEY
MONOGRAPH XXXVI PL VI
sw
^'/i!/^</!<^<6M
yaX^:y.,;v'./^\,!^AA^
PaintR - ^1^^
-S^^^^^
Au Au ''^'^
" ^/v v'vWvi J
Sketch showing locaUon of sections
on
General Map
^Ih^"'^^' Ado^Au-
N F
Aim Aim
Mir AlsAIr Alg Air AU
^jr
, \ /^ <-;-
Air P Air
/Rgr
ULius B<e^aco lith wv
ARCH E AN
GENERALIZED SECTIONS TMROUCiH SOTTHERN PA FIT OF CRYSTVl. FALLS DISTRICT
HORIZONTAL SCALE, 1 1N'CH= 1 MILF:. A'ERTICAL SCALE, 1 INCH- 1320 FEET.
ELEVATION OF BASE LINES 1000 FEET.
NOTE: Formations are broir^hl to the surface only where exposures have been observed
ALGONKIAN
LOWER HURON L\N
LrpPER HURON IAN
Grajiite
Slurgpuii aii<l Ajibik KonoaudRaiidviUi:
quailzile dolomite
AirnSALs j
Mansfield and
Sianio slate
Hemlock formation I'roveland SNegaunee
formation
-JA-Ur---.-^
INTRUSIVE
■
PLEISTOCENE
UNDIVLDED
Undivided
Dolerite
Gabb ru
STRUCTURE AND STRATIGRAPHY. 29
iiiid |)r<»l)al>ly thu entire C'lystal Falls district, was covered by Caiiil)riaii
deposits. The thickness of tlie Cambrian deposits can not be determined.
The next hi<.>lier })ortion of the geological time scale represented in the
district is that part of the Pleistocene penod which in this part of the
United States is characterized l)y the past existence of great ice-sheets.
The evidences of the existence of the ice ai*e everywhere present, either in
the rounding and polishing and scoring on the surfaces of the rocks
exposed or in the character of the drift deposits. The direction of the ice
movement was clearly from the northeast to the southwest, as is shown l^v
the trend of the stria', which were observed upon the rounded rock out-
crops in N'arious places. The thickness of the drift deposit varies very
materially. In places it has been almost entirely removed by denudation,
if in such places it ever formed anything more than a thin veneer upon the
surface. In other places it reaches a very considerable thickness, as is
shown by the glacial topography characteristically developed in T. 45 N.,
R. 32 W.
As the present report is confined to the pre-Paleozoic rocks, no detail
description will be given of these Cambrian and Glacial de2:)0sits, nor are
they represented on the map, except in those places where it has been found
impossible to map the underlying rocks. The generalized columnar section
on PI. VII gives in condensed form our knowledge concerning the formations
mentioned.
PHYSIOGRAPHY.
TOPOGRAPHY.
The topography in its large features is pre-Glacial, and in some cases
this older topography is rather distinct. For instance, in the case of the
Deer River Valley, drift covers the gentle slopes and bottom, but is not
sufficiently deep to completely hide the pre-Glacial Deer River Valley.
In the southwestern part of the district west of Crystal Falls, or, more
generally, west of the Paint River, pre-Glacial topography is seen in places.
Here we find the drift as a veneer and only partly hiding the bed-rock
topography, which depends mainly on the strikes, dips, and varying charac-
ters of the rocks.
It is so well known that this part of the country was at one time
-covered by ice, that it is useless to cite such proof as the rounding and
30 THE CRYSTAL FALLS IRON-BEARING DISTRICT.
scoring of the rocks and the character of the di'ift material, a good portion
of which can be readily seen to have been brought from some other region,
no such rocks as those forming it existing where the bowlders now lie. The
ice-sheet left a deposit of di-ift, and we find the pre-Glacial topography essen-
tially modified by it. As a result of this, the prevailing and most noticea-
ble topography of the western half of the Crystal Falls district is that of the
drift, and is characterized by short ridges and broken chains of hills, usually
oval, though at times of very irregular outline, between which are lakes
and swamps. The swamps are even occasionally found on rather steep
slopes, where a thick spongy carpet of moss (sphagnum) retains sufficient
moisture for cedars and other trees and shrubs characteristic of the Michigan
swamps to grow. The swamps follow the carpet of moss up the hills to the
spring line.
The Glacial drift topography is especially marked where the drift was
of considerable depth. These conditions are well exhibited in parts of
T. 45 N., Rs. 31, 32 W., shown on the large-scale map, PI. VIII. Here,
even though the ground is very heavily timbered, one may easily trace out
the sinuous course of the eskers. When traversing the country, one is
constantly descending into pot-holes or is climbing ridges, some of them
75 to 100 feet high, often with a crest only a few feet, in some places not
more than 4 feet, wide.
Where the drift mantle has been removed, the rounded character of
the rock exposures is usually shown. This holds good especially for the
more resistant rocks, such as the granites and massive greenstones. Slates
and tuifs, weathering more readily, have in numerous cases had time since
the ice retreated to be weathered into rough broken ledges, some of which
show perpendicular cliffs.
The elevations range usually from 1,400 to 1,600 feet above sea-level.
The hills rarely rise more than 200 feet above the low ground at their bases.
The extremes of height noted in the district are from 1,250 to 1,900 feet
above sea-level, corresponding, respectively, to the valley of the Michigamme
on the south and the watershed between Lake Superior and Lake Michigan
on the north. Between these two extremes there is a strip of territory, 25
miles across from north to south, in which the variations in height are
within the limits of 200 feet.
A consideration of the slight difference of level which prevails over
U. S. OEOLOGICAL SURVEY
MONOGRAPH XXXVt PL. VII
I
Period. Formation name.
_IO
£LO
Potsdam sandstone.
2-i
l-O
< CD
o
U.
Colum-
nar SEC-
TION.
■Cp.
Mansfield siate.
LI_Li
^.^.^^^^^^
t^S
I'll . r
XJ,
Randvilla dolomite.
I ^gr.
II" I
I?
Character cf rocks.
Usual characters.
Thickness unknnwn Yellowish to reddish brown sandstone, not
thornughly cemented, therefore disintegrates readily, Found in patches
in many places, and always lying either in beds wh'ch are horizontal or
else possess slight dip to the south This may represent the initial dip
with Vkhiqh the beds weie deposited.
A series of very great bul unknown thickness. It consists of a'ternat-
ing beds of slates, graywackes. siderite, and chert With Ihese, espe-
cially associated wilh \he last two, are found hematite and limonite ore
bodies of variable size and of great economic importance. From this
series is derived nearly all the ore supplied by the Crystal ^a'ls distnct.
In the southern part of the district, espec-ally well exposed in the
vicinity of the Paint and Michigamme rivers, the slates and graywackes
have bet^n metamorphosed into schists and gneisses. This series is cut
by dikes of rock ranging from acid to ultrabasic, which have, in places,
metamorphosed the sediments.
The thickness of this vast pile o' volcanic ejectamenta can not be
estimated with any degree of accuracy. It consists chiefly of in'.erbed-
ded acid and basic lavas and associated tuff deposits, and the water-
deposited materr Is derived from them. Near the top of the volcanics
a lent'Cula: aiea of norma' sediments, slates witn lenses of limestone, is
found. This formation is cut by acid and basic dikes
1500
Estimated to be about 1 ,500 feet thick. It consists of interbedded f rag-
mentals, slates, and graywackes and. associated with these, fen uginous chert
and carbonate From these last has been derived the ore found associated
wiih them The Mansfield mine, by which is exploited the only ore body
in the Mansfield formation, supplies the only Bessemerore of the Crystal
Falls district These slates are cut and metamorphosed by basic dikes.
The thickness 's that estimated for this formation in the eastern part
of the district by Smyth, The prevailing rock is quartzose dolomite, of
a veiy friable character.
It shows the usual characters of granite. It is schistose on flanks of
m.assif, and is cut by acid and basic dikes, which are mafsve ai-.d
schistose.
GENERALIZED COLUMNAR SECTION.
PHYSIOGltAPHY. 31
the "Teatcr i)iirt of tlie Crystal Falls district has U-d Smyth to the conclusion
that this portion of Michigan liad before Glacial times been reduced to the
condition of an approximate peneplain. (See Part II, Chapter 1.) This
peneplain is a continuation of tlie peneplain of northern Wisconsin, and
lies between the northern Michigan base-level on the nortli and the central
Wisconsin baselevel on the south, to both of which attention has recently
been called by Van Hise.^
DRAINAGE.
The greater heights in the Michigamme district are in the northern
part, where some few of the hills rise to a height of 1,800 feet, and
one to a maximum of 1,900 feet above sea-level; but tlie majority do
not rise above 1,600 feet. The belt including these higher elevations
extends about NE-SW. This belt represents the crest of the watershed,
from which all streams on the northern side flow to Lake Superior, and on
the southeastern side all flow to Green Bay of Lake Michigan. A part of
this watershed is undivided, and it is not uncommon to find extensive swamps
in which streams flowing to opposite sides of the watershed take their
origin. The portion of the Crystal Falls district which is tributarj- to Lake
Superior is so small that it will be totally neglected in the further discussion
of the drainage. The topographical map, PI. II shows the general slope
and drainage of the district to be SSE. The eastern part of the district is
drained by the Michigamme^ River with its tributaries, the Fence (Mitchi-
gan), and the Deer, while the Paint (Mequacumecum) River, with its main
tributaries, the Hemlock and the Net, di-ains the west and northwestern por-
tions. The Brule (Wesacota) flows along the southern part of the district,
being for the most part just below the southern limits of the present map.
It forms throughout its course the boundary line between Michigan and
Wisconsin. The Paint flows into the Brule in sec. 12, T. 41 N., R. 32 W.,
and the Brule and the Michigamme unite in sec. 16, T. 41 N., R. 31 W., to
' A central Wisconsin base-level, by C. K. Van Hise : Science, new ser., Vol. IV, 1896, pp. 57-59, 219.
A northern Michigan base-level : ibid., pp. 217-220.
-The Indian names which the streams and lakes of this district formerly bore have either been
dropped or else, in a few cases, have been replaced by translations, though most commonly they have
been replaced by English names, which are altogether new. Those names which have been retained
receive various spellings at the hands of dift'ereut authors, and even at the bauds of the same writer.
The Michigamme Kiver, for example, is fre<iuently spelled by Burt iu the same article Feshakmnme and
Pesh-a-kem-e. The name Michigamme is also spelled on various maps ilachigamig and Michirjamig.
Whereas the Paint we find spelled Mequacumecum, as above most commonly, though Burt spells it
AIesi{uacumecum and also Mesijuacnm-a-cuui.
22 THE CEYSTAL FALLS IRON-BEAKING DISTRICT.
form the Meuomiuee River. This last flows southeast through the adjoin-
ing Menominee district, and is the boundary hue Ijetween Michigan and
Wisconsin from its source to its mouth.
A glance at the map, PI. II, will show the presence, especially in the
northern half of the district, of a great number of lakes of varying sizes.
These lakes of clear water, with bottoms of gravel, or most commonly of a
thick deposit of decayed vegetable matter, are a very characteristic feature
of the landscape. Many are in the midst of swamps, sun-ounded on all
sides by a quaking bog, which prevents one from approaching very closely ;
others are surrounded by steep but low di-ift hills. The lakes may or may
not have a visible inlet and outlet. In all cases the present water levels
are considerably below the original water levels. In many cases the lakes
are but remnants of much larger bodies of water. They are gradually
filling ujD with silt and vegetable growth. These lakes, covered with float-
ing lily pads and surrounded by more or less extensive hay marshes, are
favorite places for the deer, which in many parts of the district are still
fairly numerous. The numerous lakes indicate the youthful character of
the di-ainage. Many of the streams head in the lakes. In other cases they
flow tlii'ough them, connecting them in chains. This indicates the mode of
origin of the most of the streams of the area. The youthful character of
the drainage, is still further shown by the fact that with but few excep-
tions the rivers have not reached rock. They are still cutting in drift.
In the case of the Deer River this gradual development from the
original condition of a chain of lakes to the present condition of a river in
which the lakes play very subordinate parts is well shown. Moreover, its
development illustrates very well several of the stages passed through by
rivers in general, and for these reasons it may be well to describe it in detail.
The life history of the Deer River,' as it is to-day, began with the deposit
of the di'ift, which destroyed the former streams of the district and concealed
their records. It appears proljable from the topography that the river
occupies the same, or approximately the same, bed in which its pre-Glacial
forerunner moved. The noticeable valley occupied by the stream is at a
maximum about 3 miles broad, though its drainage area is a strip averaging
' The substance of the following was presented to the Wisconsin Academy of Sciences, Arts,
and Letters at the annual meeting, September 27, 1895, in a paper entitled "Some stages in tlie
(levelnpiiieut of rivers, as illustrated by the Deer River of Michigan." An abstract of the paper was
published iu Amer. Geol., Vol. XVII, lS9t>, p. li'6.
8
X ^ -
=^M
X
<H^Ps= 1
S =: ^ - ■*o
2:;
■i<-^ y-
:::
ii - H
"^
—
^- ' ^
< <i r.
PHYSIOGRAPHY. 33
f) or <! miles in l)rcailtli. 'I'lic lulls between wliicli the stream flows are not
very lii^^li aintxc flic river l)e(l, the niaxinuuu elevation bein<>- 175 feet,
'{'he tew rock outcro])s are in all cases foiiiifl on the tops ami flanks of these
hills, w here they have been exposed by denudation. At one jjoiut only
has rock been found in situ near the river bed, and that is toward the mouth
of the river. The conclusion is natural, since the river is 175 feet below
these exposed rocks and has not reached rock, that it must be flowing'
thr-Hiiih a i)reexisting depression or valle}- parti)- flUed by the drift of the
Glacial epoch.
The partial filHng of this valley at the time of the retreat of the ice
to the northeast was accompanied by the filling of the depressions in the
drift by the water flowing from the front of the melting glacier. After
the depressions were filled, the overflowing water naturally followed the
general southeastern slope, which exists throughout the area and is shown
by the topographical maps and by the flow of the rivers. The immediate
course of the water was determined by the former valley, which was not
completely obliterated by the drift deposit. Drift barriers across the valley
separating the ponded water, or lakes, from one another were cut through,
the material eroded being spread over the bottoms of the lakes below. Thus
was formed a chain of lakes, connected usually by narrow streams; the
processes by which the channels were cut out and the lakes drained and
filled up with the debris were going on at the same time. The result has
been to obliterate the lakes to a great extent and to accentuate the char-
acter of the stream.
The final eflect of the processes, briefly outlined, would be to destroy
the lakes entirely and produce a stream.
By following on PI. VIII the Deer River from its mouth to its source,
we may see the several stages in its development, which are also typical for
other streams of the glaciated portions of the world. The river is about 20
miles long and has a width near where it enters into the Micliigannne of
20 to 30 yards. Near its mouth it is a slow-flowing, sluggish stream, which
has nearly reached its base-level of erosion, and like many of the older
streams of the Coastal Plain region of the United States is gradually filling
portions of its channel with the silt and vegetable matter brought down
from above.
A short distance from its mouth it resembles such streams also in the
HON xxxvi 3
34 THE CRYSTAL FALLS IRON-BEARING DISTRICT.
meandering character of its channel. Tliis resemblance is still further
enhanced by the presence of a remnant of a crescent-shaped cut-off, so
characteristic of the old age of rivers. Just opposite this cut-off is a lake,
which is of interest on account of its possessing two outlets, both leading
into the river. Unfortunately this fact was observed on the topographical
sheet too late to permit of a return to the field for the purpose of determining
the cause of the presence of the two outlets.
Passing up the stream we soon reach the lakes, which farther on become
more numerous. The life history of these lakes is inseparably connected
with that of the river. They reached maturity dui-ing or at the close of the
Glacial epoch, and since that time their history is that of decline. This
part of the history of these lakes may be brieflj' stated as follows: As the
erosion continixes, the areas of water are reduced and the surrounding swanijD
areas are correspondingly increased. If a lake were lai-ge and considerable
inequalities existed in its bottom, two or more small lakes connected by the
stream flowing through them may be formed. The final stage is a swamp,
traversed by the slow-flowing river.
The various stages in the history of the lakes are well illustrated on
the accompanying map, P|. VIII, by the following series of lakes. In Nos.
1 and 2 the general character of such bodies of water, which may be con-
sidered essentially as mere expansions of streams, is seen. No. 3, and Deer
Lake, have long since reached maturity and are advancing rapidl}' to the
point where they will each be separated into two bodies of water. No. 4
has already reached this stage, and in the swamp marked A we have the
last stage, the swamp, with the stream flowing between peaty banks.
On Light and Liver lakes, in the lower part of the Deer Ri-ver, we may
see all but the last of these stages illustrated. The lakes are attached to
the main river by very short streams. The main river after leaving the
rapids above, where it accunmlates considerable detritus, enters a flat por-
tion of its course partly occu})ied by the two lakes in question. Here, its
rapidity being diminished, the stream deposits the detritus. Thus it has
gradually built a delta, now for the most part covered by swamp growth.
This tends to advance the shore line, and thus diminish the water area.
The rapid cutting down of the barrier immediately below the lakes by the
swiftly-flowing stream tends to lower the lakes and thus diminish their
surface area still more.
PHYSIOGRAPHY. 35
The couil)iiR'd effect of the draiiiiuy and tilHuf^ lias been to separate
what was tormcrK- a long- narrow lake trending- NE-SW. into three rounded
bodies of water, two of which are connected with each other, the larger of
these two and the third bdcc being- connected with the main stream by very
short necks. An artificial dam has been built across the narrow cliannel
below the lakes, and the effect has been to flood the delta and unite the
lakes into one large bod)' of water, occupying, approximately, the area
covered h\ the glacial hike, thus restoring the conditions which existed
before the natural barrier had been trenched.
In the remainder of the course of the Deer River the tendency of other
artificial dams to restore the river to its original condition, that of a series
of connecting lakes, is well shown. These dams were built by lumbermen
at the foot of the lakes or swamps when it was desired to retain a large
body of water at these places. When, on the other hand, the desire was to
enable the logs to pass rapids, a dam (marked B on the map) was built near
the head of the rapids. The l)ack water would bring the logs to the dam,
and on opening- the gates the flood would carry them over the rapids into
the deeper water beyond. The Deer River thus, after having reached a
somewhat advanced stage, has been rejuvenated by the Michigan lumbermen.
A study of the small tributaries shows the same condition of things,
although not on so large a scale nor so perfectly as in the main stream.
The source of the Deer River is in the copious sjjrings which rise out
of a spongy, marshy piece of ground less than 125 yards distant from
Bone Lake, and about 20 feet below the usual water level of Bone Lake,
and are really fed by the lake water percolating through the drift and
appearing at this point. From the springs there is a depression which leads
up to the lake. The highest point of this depression was about 3 feet above
the normal water level of the lake.
The outlet of Bone Lake is Ihe Fence River. The river leaves the
lake at a point three-quarters of a mile distant from the head of the Deer
River Valley. In order to obtain a supply of water for driving- the Fence
River, Bone Lake has been converted into a reservoir. A dam was built
at tbe outlet which raised the water about 4 feet, and the result was to turn
some of the water of the lake into the Deer River, necessitating also a dam
across this small valley near the lake shore. At present only a few strokes
of the shovel would be necessary in order to turn the water of the flooded
36 THE CRYSTAL FALLS IROIST-BEARING DISTRICT.
lake from tlie Fence into the Deer River, thus gaining for it a drainage area
extending 7 miles farther north and including three large lakes, the main
sources of tlie water suj)ply of the western branch of the Fence.
I ha^-e no data which would enable me to show that the valley at the
head of Deer River was ever a channel for the waters of Bone Lake. I am
incUned to believe that such was not the case. For had it existed with the
present slope, 20 feet in 375 feet, or even a much lower one, the water
would have had a marked erosive power, and it would have cut back its
channel mucli more rapidly than the Fence, which for a mile below the lake
is a comparatively sluggish stream, and would have eventually captured
Bone Lake and its feeders.
The Deer River is still continuing the process of lengthening its chan-
nel, and the springs which give it birth are gradually undermining the
barrier at its head, so that it is possible that it will, unless artificially
restrained, obtain much more water from Bone Lake than it does at present.
A change in atmospheric and other conditions, which would insure a state
of equilibrium Ijetween the incoming and outgoing waters, thus preserving
the waters of Bone Lake at their present level, would be favorable for the
final successful robbery of the upper Fence River system by the Deer
River. This favorable condition, as may be readily seen, would be greatly
increased in proportion as the increase of inflowing over outflowing water
raised the level of the lake.
TIMBER AND SOIL.
The district was at one time very heavily timbered, with hard wood
and ])ine, the former predominating on the whole. Along the flood j^lains
of the large streams one finds sandy j)ine barrens where once there were
heavy pine forests. On the headwaters the pine are found scattered
through the hard wood. Individually these trees are very much larger and
better tlian the thick and therefore smaller growth of the plains. Lumber-
ing, which had been confined for years to the main drainage channels of
the district, has of late been rapidly extended, following all the ramifica-
tions of the tributary streams, until at present there remains in this district
only a few years' cut of pine at the very headwaters of the rivers.
Following the lumbermen comes the forest fire, which finds its most nourish-
ing food in the dry resinous pine tops left by thein. The fires, once started.
riJYSIOGliAPUY. 37
are not confined, however, to the cut pine, but .sjjread to tlie adjacent
standing- ])ine and even into the hard-wood forests, carrying destruction
with tliein, and leaving but the gaunt, l)are, and blackened truidcs to mark
the sites of what were forinerh' thick forests.
The i)ine-covered areas have a thin soil and are jtoorly adapted to
agriculture. The areas covered with hard wood have, on the contrary, soil
well adajtteil to the crops of the latitude.
The advance of the lumberman has necessitated the damming and
clearing of streams and the blasting of channels to permit the floating of
the logs, and this has driven the fish> especially the sjjeckled trout, which
formerly crowded all the streams, into the smallest and most inaccessible
ones. Rutfed grouse, Bonasa umbellus, and deer are still rather plentiful in
certain portions of the area, although the pot-hunter with set guns, spring
nooses, and pitfalls is rapidly exterminating them. The deadh- character
of such appliances is brought vividly to mind, when, as happened in my
own case, one is suddenly arrested, while following a deer trail through the
underbrush, by a hay wire noose around his neck, and he may be thankful
if the bent sapling, having been bent so long- as to lose its elasticity, fails
to spring up and render the device effective.
CHAPTER III.
THE ARCHEAN.
DISTRIBUTIOlSr, EXPOSURES, AXD TOPOGRAPHY.
The granite described in this chapter belongs to the oldest system in
the district, and forms the western elliptical core designated on PL III as
Archean. It is surronnded by sedimentary strata, which have a quaqua-
versal dip away from the granite as a center. The portion of the Crystal
Falls district, in which the granite outcrops, is about 13 miles long by
3 miles wide, its longest axis extending in a NW. and SE. direction and
covering parts of Ts. 44, 45, and 46 N., Rs. 31 and 32 W.
The exjjosures of granite are especially numerous in the southeast part
of the o^•al area, where, owing to the proximity of large streams, the Fence
and Deer rivers, and the consequent increased erosion, the drift has to some
extent been removed. In the northwest part of the area, with rare excep-
tions, all the rocks are deeply covered with drift.
In general the topography of the area is that of the drift, but in the
southern part it is seen to have been considerably influenced by the char-
acter of the iinderlying rocks. The granite usually outcrops in small,
rounded, and isolated knobs, whose relations to one another can onh' he
conjectured. Where an occasional knob is composed of massive granite
and more or less gneissoid granite, the exposed surface is so small as
to prevent the observer from determining the relations between the two.
Cutting the massive and schistose granite are certain long narrow masses
of dark-colored rocks of rather fine gi-ain, and, with few exceptions, very
schistose. From their geological occurrence it was concluded, in »\nte of
their appearance, that they are dike rocks cutting the granite. The follow-
ilig paragrajjli, quoted from the inanuscript notes of G. O. Smith, describes
very cleai'ly their field occurrence:
The gaps in this grauite ridge seem to indicate greenstone dikes, as here the
granite usually has a facing of the greenstone more or less extensive, and often in
the center of the gap there are several small areas of greenstone. In all cases the
38
RELATIONS OF THE ARCIIEAN. 39
greenstone is markedly more affected by weathering than is the granite. A stndy of
the rehitions at the tew points of contact did not yield mncli more than negative
resnlts, but these pointed to the intrusive character of the greenstones.
UEIiATIONS TO OVERJOYING FORMATIONS.
The relations of tlie granite to the sedhnentary rocks might be explained
in two ways; the former may serve as the base of the latter rocks, or it may
penetrate them. The occurrence of the granite in an elliptical sliape, with
sediments surrounding it showing quacpiaversal dips, might be regarded as
evidence of its intrusion in the Huronian sediments, and on this theory it
would follow that the granite is of Huronian or post-Huronian age. If
intrusive, it should be found to penetrate and metamorphose those sedi-
ments. Against the intrusive character of the granite, and in favor of its
pre-Huronian age, are the following facts: (1) There is a total absence in
the surrounding sedimentary^ strata of any dikes which are lelated to the
granite. (2) There is a total alisence of any metamorphic action, so far as
observed, in the sedimentaries. (3) On the east flank of the granite core,
on the west bank of the west branch of the Fence River in the SW. corner
sec. 1, T. 45 N., R. 32 W., is a recomposed granite, which passes up into a
tine sericitic quartzite, with false bedding. These rocks evidently derived
their material from the granite, and hence mark the beginning of sedimenta-
tion in this area.
Thus the positive evidence confirms the negative, and since the granite
underlies the oldest sedimentary rocks, whose age has been determined to
be Huronian, the former is classified as Archean, that term being used here
to designate those rocks of undoubted igneous character which form the
foundation upon which rest the oldest determinable sedimentary rocks.
It is not the province of this paper to enter into a speculative discussion of
the origin of the Archean rocks of the district. For such a discussion the
reader is referred to Professor Van Hise's exhaustive disquisition on the
Principles of North American pre-Cambrian Geology,^ where the conclusion
is reached that "the Archean is igneous and represents a part of the original
criist of the earth, or its downward crystallization."'" The Archean has
gradually reached the surface by the removal by erosion of the superjacent
rocks.
I Sixteenth Ann. Rept. U. S. «eol. Survey, Part I, 1896, pp. 571-874.
- Loc. cit., p. 752.
40 THE CRYSTAL FALLS IRON-BEARING DISTRICT,
PETROGBAPHICAIi CHARACTERS.
The rocks of the Archean comprise biotite-granite, gueissoid biotite-
grauite, and acid and basic dikes.
BIOTITE-GRANITE (GRANITITE).
The rock occupying the main and central ])art of the Archean area is
a biotite-ffranite. This rock is also found to some extent on the border of
the area. The rocks of this kind vary in color from light-graj' rocks to
those having various tints of red, depending usually upon the degree of
alteration. They vary also from medium to coarse grain. Some varieties
show a decided porphyritic texture, and in some cases also an approach to
a laminated structure. The porphyritic character is due to the presence of
large crystals of feldspar, which stand out from the surrounding granitic
groundmass, thus producing a typical granite-porphyry. The feldspar phe-
nocrysts lie with their longer axes parallel, and thus help to produce an
imperfect laminated structure. This parallel structure in the granite-
porphyry is apparently analogous to the flow structure of the volcanic
rocks, and probably was produced by movements in the magma before it
had reached even a viscous state, as we find that the phenocrysts give no
evidence of having undergone excessive mashing or torsion. The different
textural varieties grade into one another in such a way as to indicate that
the}^ are merely modifications of the same magma. In addition to these
textural varieties, which are original, we find in certain places a passage
from massive to schistose rocks, in which the schistosity is of dynamic
origin, i. e., of secondary nature.
In the thin sections these rocks show the normal granitic texture and
the usual mineral constituents which characterize biotite-granites. The
chief minerals are orthoclase, microcline, plagioclase, quartz, and biotite.
Zircon and apatite are the accessory minerals present, and the secondary
minerals include epidote-zoisite, chlorite, muscovite, rutile, and iron pyrites.
Quartz occurs in grains forming the cement and molding around the
other minerals. In one of the granites it has a peculiar saccharoidal char-
acter macroscopically, and under the microscope such portions are resolved
into very fine aggregates of quartz grains.
The quartz is also frequently found in round blebs of varying size
included in the best crystallized feldspar crystals. Thus the crystallization
PETKOGKAPHIOAL CnAKACTEKS OF AKCHEAN.. 41
of the (|uartz, unless such qunrtz rej)reseuts the "([uartz tie corrosion" of
the French autliors, conthiued through the entire time occupied by the crys-
tallization of the feldspars, since it is included in the ohlest feldspar of the
rocks, and also forms the matrix in which lie the youngest feldspars. Undu-
lator\- extinction, so general in the quartzes, shows that the rocks have been
subjected to pressure, and in some cases it has been sufficient to produce
the extreme cataclastic structure of very greatly mashed rocks.
The quartz includes numerous gas and fluid inclusions, the latter
frequently with dancing bubbles and forming negative crystals, by means
of which it is easy to orient the irregular grains. The quartz of one of the
specimens was found to contain liquid inclusions, each of which, besides the
usual bubble, held a small rectangular crystal. These crystals are trans-
parent, with a light greenish tinge. A crystal similar in ajDpearance found
in the same quartz individual is partly inclosed by a large (j -shaped bub-
ble, and gave inclined extinction, though no further optical tests could be
made upon it.
Three kinds of feldspar are present : (1) A finely striated plagioclase ;
(2) a feldspar, unstriated, or at most showing Carlsbad twins, and presumed
to be orthoclase ; and (3) microcline, these last two being frequently inter-
grown after the manner of pertliite. The plagioclase was the first feldspar
to crystallize. It is invariably so altered that the twinning laminae are
nearly obliterated, thus preventing accurate measurements. It is probably
oligoclase; and if so, it is highly probable that much of the white mica
produced by its alteration is pai'agonite instead of muscovite, a fact not
determinable microscopically. The phenocrysts are orthoclase, usualh'
in Carlsbad twins, and thus at first sight appear to have been the first feld-
spar to crystallize; but I find that these ])henocrysts not uncommonly
inclose small rectangular, more or less automorphic,^ crystals of plagioclase,
which is in reality the oldest feldspar. Hence these orthoclases, notwith-
standing their porphyritic character, are later than a part of the plagioclases.
One phenocryst with Carlsbad twinning was observed in which one part of
' Automorph, Xenomorph; Uber die Eruptivgesteine iiu Gebiete tier Schlesisch-Maehrischen
Kreideformatiou, by Carl E. 11. Kohrbach : Tsch. Jliu. Pet. Mit., Vol. VII, 188(3, p. 18.
Idiomorph, AUotriomorph ; Rosenbusch : Mik. Phys., Vol. II, 1887, 2tl ed., p. 11.
L. V. Pirssou has recently proposed in a paper, read before the Geological Society of America,
on A Needed Terra in Petrology, the term anbedra for minerals which do not possess crystallographic
outlines and are xeuomorpbic, in contradistinction to those which we properly call crystals and which
are automorphio : Geol. Soc. Am. ,Vol. VII, 1896, p. 492, and Am. Jour. Sci., 4th series, Vol. II, 189G, p. 150.
42 THE CRYSTAL FALLS lEOX-BEARING DISTRICT.
the individual shows microclinic striations. The other part was untwinned,
and near the center of the phenocryst, bisected by the Carlsbad twinning-
plane, was found a rectano-ular plagioclase crystal.
The mierocline is usually the best crystaUized feldspar in the gTound-
niass, and also by far the freshest. In the few cases in which it was
observed in contact with plagioclase, the latter molded it, and is therefore
older than the mierocline, which in its turn is older than the orthoclase.
In one ease a mierocline individual showing the lattice structure over a
portion of its surface possesses no twinning lamellaj in another portion, the
twinning lamellpe fading until they totally disappear. Thus no sharp
delimitation is apparent between the twinned and untwinned portions of the
individual.
In most slides all the feldspars are much altered, but even in those in
which the mierocline is fresh the plagioclase and orthoclase alwaj's show
alterations, the plagioclase altering most easily and usually being so changed
that it is with difficulty that one can recognize the twinning lamellse. Hence
some of them may have been taken for the nonstriated orthoclase. In an
early stage of the alteration of the feldspars minute dark ferrite particles
which impregnate them are hydrated, and this gives the feldspars a more or
less distinctly red tinge. In a more advanced stage of alteration, muscovite
and a httle epidote-zoisite are produced. Another alteration of the feldspar
is alwavs associated with marked pressure phenomena, and hence is pre-
sumed to be the result, partially at least, of dynamic action. This is the
partial or complete granulation of the feldspar and the production from
that mineral, with the addition from other sources of the iron and magnesia
necessarv, of secondary white mica and quartz, and some biotite. It is
highly possible that some of the small limpid grains considered to be
secondary quartz are really an acid feldspar. Orogenic movements are
also indicated by the bending of twinning lamellae, and were probably the
partial cause of the twinning.
Biotite occurs in plates, and as a rule shows better-developed crystals
than does the feldspar, though it frequently occurs in decidedly ragged
flakes. It is strongly pleochroic, showing absorption in the following colors:
Pale straw yellow to yellowish brown, for rays vibrating perpendicular to
cleavage, to very dark chocolate brown and greenish brown for those par-
allel to cleavage. In the case of the biotite showing a greenish color this
P1«:TK()GKA1'111CAL CHAUACTKRS of AKCIIEAN. 43
seems to be the result of ine'ipient alteration", since the edges of the flakes
are ragu'ed, and in many cases almost the entire biotite of the section is
altered to a chlorite, which shows ordinary white to li<>-ht o-reenish pleoch-
roism, with the sinuiltaneous production of epidote and l)undles of needles
with high single and double refraction, having yellowish or l)rownish color.
These needles are taken for rutile. The biotite is found usually lying
between the feldspar and quartz grains almost as though it had l)een tlie last
product of crystallization. It has suffered crashing with the other minerals.
Apatite and zircon were observed in a few crystals. No onginal iron
ore was seen. As intimated above, by the use of the term "epidote-zoisite"
the exact character of this secondary material is not always determinable.
In some instances parts of an epidote crystal show^ the dee}) blue inter-
ference color of zoisite, apparently indicating a mixture of the zoisite and
epidote molecules, the latter predominating in the crystals.^ The remaining
secondary minerals mentioned as occurring in the granite show their usual
characters.
GNEISSOID BIOTITE-GRANITE, BORDER FACIES OF GRANITE.
About the central area of biotite-granite just described, and in part
formina- the border of the Archean area, are rocks having a gneissic
structure. With these are associated the biotite-granites. The gneissoid
rocks in general are markedly darker in color than the granites, showing
normally a rather dark gray. They vary little from one another in texture
and are much fmer grained than the granites. The fine-grained condition
of these schistose and banded rocks has perhaps a great deal to do with
their dark color, though this is primarily owing to the amount of biotite
present.
In some of the specimens the bands can be readily distinguished under
the microscope, and are seen to contain a white mica and a nxuch smaller
amount of biotite. These two minerals are present in fine films between
the crushed quartz and feldspar grains, gi\'ing to the rocks a very decided
schistose character. These mica folia are much more numerous in certain
areas than in others, thus producing a more or less perfect banding. The
mica plates are not all regularly parallel, although ordinarily having a
'On some granites from British Columbia and the adjacent parts of Alaslca and the Ynkon
district, by F. D. Adams : Canadian Record Sci., Sept., 1891, p. 3+6.
44 THE GEYSTAL FALLS lEON-BBAKING DISTKICT.
tendency to this arrangement, and are usually parallel to the banding.
The most perfect schistosity is thus developed parallel to the micaceous
bands. The banding and the schistose structure are plainly of secondary
oris'in, the result of dynamic action.
Others of the gnessoid granites, however, when examined under the
microscope, are decidedly massive, and it is only on a large scale that the
banding shows distinctly. In such cases the cause of the banding could not
be determined, and might by some be ascribed to differentiation, though,
from the association of these gneissoid granites with those just described, it
is assumed that the banded str icture is due to dynamic action. If this be
the case, however, a complete recrystallizatiou has taken i)lace, and slight
dynamic effects are now shown. The strike of the banding, wherever it
was taken, was uniform, varying from N.-S. to nearly N. 45'^ W., agreeing,
on the whole, with the trend of the Archean oval area.
The microscope shows that the constituent minerals of the gneissoid
granites are the same as those which compose the granites just described.
These show also the same relations to one another and the same general char-
acters as in the granites, except where mashing has completely obliterated
the original texture, and hence no further description of them is necessary.
The crushing to which the gneissoid granites have been subjected is
very clearly shown in the present cataclastic condition of the quartz and
feldspars.
As stated above, both the gneissoid granite and the granite proper are
found in the border area of the Archean. In those rocks in which the con-
tact shows a gradual transition from the banded rock to the unbanded, the
micaceous bands are clearly secondary, and are the result of the crushing
of the original granite, these lines representing macroscopic and microscopic
shearing planes along which the feldspar and quartz have been thoroughly
granulated, and sericite and some biotite produced, as was found to be the
case also in some of the granites. These rocks thus agree in their dynamic
origin with a similar but apparently more extensive and better developed
gneissoid border facies in the Morbihan (Brittany) granites, which have
been described, and whose origin has been so cleai-ly demonstrated by
BaiTois.^ Numerous other similar cases have been described recently from
the Canadian granite massifs and from Sweden and other districts.
' Anu. Soc. G6ol. du Nord., 1887, p. 40.
ACID DIKES IN ARCIIEAN. 45
ACID DIKES IN ARCHEAN.
Observations upon diki's of acid rocks {•uttini^- the Arcliean <»-r;uiite are
very i'ew, and we may suppose this to be partly due to tlieir occurrence in
isolated knobs, which prevented the determination of the relations of adjacent
exposures of rocks of sliji'litly different character. Some few dikes were,
nevertiieless, ol)serve<l, and are j^Tauites varying from medium to coarse
gi-ained, grauolitic' to })orphyritic rocks. The porphyritic facies is the
most conuuon. The dikes do not show differences from the main mass of
the Archean granite suflKcieiit to warrant detailed petrographical description.
The following description of one mass of granite-porphyry is given, as it offers
good proof of its relation to the schistose border facies of the granite. In
this case the gneissoid rock is found as inclusions in the granite-porphyry, as
is illustrated in the accompanying diagrammatical sketch, fig. 4, taken from
a ledge in the field. In this sketch the
sharply outlined angular and lenticular
areas represent the gneiss included in the
gi'anite-porph)Ty. The jjlienocrysts of
this granite-porphyry have a parallel
arrangement, the long direction of the
phenOCryStS agreeing also with the trend Fio 4— Granite imrpl.yry with mclusions of gneis-
soid granite.
of the longer axes of the inclusions. The
banding and foliation in the inclusions strike at a right angle to the flow-
age structure of the granite. The lines of separation between the areas of
gneiss and the granite, as shown in this outcrop, are sharp, and |)oint to
their nature as inclusions, and such is accepted as the true explanation
of their angular character and sharp outlines. As this por^jhyritic granite
was intruded throu"h the border facies of the Archean oranite, these fras"-
ments were taken up, and were so arranged as to agree with the direction
of movement in the intruding mass. This occurrence shows this granite-
porphyry to he younger than the great mass of Archean granite, whether
we consider the inclusions to be a border facies of the Archean granite,
derived from it by dynamic action, and thus of secondary origin, or to have
resulted from differentiation of the molten magma.
' This terra has been proposed by a committee on nomenclature for the geologic folios of the
United States Geological Survey, for use in place of "granitic."
46
THE CRYSTAL FALLS IRON-BEARING DISTRICT.
BASIC DIKES IN THE ARCHEAN.
The influence of the dikes on the character of the topography has
already been mentioned. They occur in long narrow bands of varying
widths, and with one exception are markedly schistose. Considering the
granite on a large scale as an approximately homogeneous mass, we would
expect to find lines of weakness, which might be indicated by the arrange-
ment of the dikes. No such definite arrangement can be seen, however, as
the dikes are found to extend in all directions. A good example of their
mode of occurrence may he seen in tig. 5, which also illustrates very clearly
their hifluence on the topography. ' A small valley, in sec. 1, T. 44, R. 32,
througli which a brook flows, is occupied by the main dike, from which
diverge the smaller ones,
SC!
iGnANlTE(CflTACLASTlC)
?
Scale ormiles
Fio. 5.— Illustration of the effect ou tile ttipograpiiy of the differential erosion
of basic dikes and granite.
penetrating the granite on
both sides. These, not
having been much more
deeply eroded than the
granite, do not form chan-
nels deep enough to be
shown on a map with a
1 0-foot contour interval.
It is without doubt owiny
to the fact that the dikes
weatlier so much more
readily than does the granite that we may partly explain the comparative
scarcity of exposures. The depressions separating the granite knobs are
believed to indicate in many cases the position of dikes, liut being now
filled with g-lacial deposits, the underlying dike rocks, if such are present,
are covered. Thus we find them exposed onl}' where erosion has cleared
this debris away, or where portions of the dike still border the steep sides
of the granite on the sides of dej^ressions.
The dikes may be classified as (1) earlier dikes, showing a schistose
structure, and with no trace of igneous textures, and (2) later massive dikes,
showing original igneous textures.
(1) SCHISTOSE DUCES.
The general character of these rocks occurring as dikes may be briefly
mentioned. The)' are schistose, tor the most part fine grained, and black
BASIC DIKES IN ARCnKAN. 47
ill color. The ci»iis>titueiits of tliu schistose eruptives, arranged according
to tlieir rehitive importance, arc liiotite, liornlilende, clilorite, (juartz, feld-
spar ('?), cah'ite, epidote, iron oxide, sphene, and ninscovite.
Tile clear limpid grains whicli form the gronndmass are nndoubtedly
for the most jiart (jnartz. No satisfactory results were obtained in the tests
for feldspar, but it is highh' probable that some is associated with the
quartz. Dark chocolate-brown to light-brown biotite is almost an invari-
able constituent. In some cases it is aQCompanied by a little chlorite, which
apj)ears not to have been derived from the biotite. In a few rare instances
biotite is absent altogether, chlorite taking its place. The biotite and chlo-
rite are usually found between the quartz grains. They have a parallel
arrangement, and this gives the rock its schistosity. Biotite and epidote
are found included in the grains of quartz of the groundmass. Muscovite
is rarely present, but when found is in medium-sized automorphic plates.
Ragged pieces of ore, either ilmenite or titaniferous magnetite, and sphene,
secondarv to these, are found in almost all specimens, and in a few instances
iron pyrites was observed. Calcite is invariably present in irregular, fairly
large grains, almost equaling the quartz in quantity. Epidote is found in
large qutmtity, both in crystals and in irregular grains, the crystals occurring
among the bunches of biotite and included in the grains of quartz. The
large amount of epidote in association with the calcite seems to point to the
very basic character of the feldspar of the original rock.
A bluish-green hornblende is rather frequently associated with the
mica. In rocks in which the hornblende predominates mica is always pres-
ent, but the reverse is not true, the most micaceous rocks being entirely free
from the hornblendic component.
The hornblende is found in lai'ge prismatic individuals without terminal
faces. This mineral contains some of the other constituents of the rock in
which it is found, such as quartz, epidote, and more rarely ircin oxides.
The interspaces between the hornblende crystals are filled with irregular
biotite flakes and with grains of quartz, epidote, and iron oxide. This
hornblende is apparently one of the last, if not the last, mineral to develop.
The hornblendic rocks are not nearly so schistose as the micaceous ones.
The secondarv origin of the hornblende is clearlv shown in one of the
sections which is traversed by a fissure; the hornblende can be seen extend-
ing into, and in places crossing, this fissure. The other minerals are
48 THE CRYSTAL PALLS mON-BEARING DISTRICT.
presumed to be secondary, but this can not be proved tor them. The
schistose character of the rocks is evidence of dynamic action. The pres-
ence of undulatory extinction was noticed in the quai'tz of some specimens,
but its absence is the rule. However, from the absence of great pressure
phenomena, and the remarkably fresh condition of the minerals composing
the basic rocks, which contrasts strongly with the generallv altered condition
of the minerals of the more refractory acid rocks including tliem, it would
appear that complete recrystallizatiqn has occurred.^
The schistose structure can undoubtedly be referred to the dynamic
action which resulted in the upturning of the sedimentaries and caused the
de^■elo[)ment of schistosity in certain portions of the border of the granite.
This dynamic action was in all probability also the chief force in the pro-
duction of the secondary minerals.
The schistosity of the dikes does not agree in direction with the gen-
eral strike of the schistosity throughout the entire district, but is always
nearly pai-allel to the long extension of the dikes. These dikes represent
belts of weakness, and it is therefore natural that the movements should
occur along these belts rather than across them.
This schistosity of the dikes also furnishes a slight clue as to their age.
Younger than the granites they cut, they must have occupied their present
position at the time the dynamic revolution took place which resulted in
the development of schistosity in the granite, as well as in the sedimentaries.
It is impossible to bring the date of their intrusion within narrow limits.
It seems very probable, however, that they were formed at the time of the
extrusion of the basic Hemlock volcanics, though it is impossible to proye
their connection with them.
(2) MASSIVE DIKES.
The only dike rock which retains to some extent its original texture is
a much-altered medium-grained dolerite (diabase). The alterations it has
undergone are those usual for such basic types of rock, and this one exhibits
nothing peculiar or of special interest. An ophitic texture, while still recog-
nizable, is more or less obscured by the uralite which has developed out of
the pyroxene. The remnants of the original plagioclase feldspar ])resent
show exceedingly slight pressure effects. The alteration processes would
'Principles of North American pre-Cambriau Geology, cit.,pp. 706-707.
RfiSUMfi OF AIJCHEAN. 49
tlierefore seem to have been due to the action of percolating water, without
special mechanical influence. Hence we may date the intrusion of tliia
particular dike after the orogenic movements which affected the granite
core, rendering portions of it schistose, and crushing all of it to a greater
or less extent. These movements are presumed to have taken place just
prior to or during Keweenawau time; and therefore the age of this dike is
Keweeuawan or post-Keweenawan.'
In the above- described granite massif we have a rock of pre-Huronian
ao-e, as shown by its relations to the overlying sedimentaries. It possesses
in general a coarse granular, and in places porphyritic texture. Along its
border it contains portions which are much finer grained, darker than the
rest of the mass, and very well banded. The boundaries between the
banded rock and the granite at times are sharp, but frequently are very
indefinite. This banded schistose portion is found to be due to pressure,
causing the gradual passage from the granular granite to the gneissoid,
schistose granite.
One instance of undoubted inclusion of gneissoid granite by a true
granite was observed. If the gneissoid granite was derived by pressure
from the Archean granite, then the particular granite dike which includes
the fragments must be of later age than the great mass of granite of the
Archean area.
The Ai'chean is cut by basic dikes of two ages. The earlier ones were
rendered schistose, and the production of this secondary structm-e was
accompanied by a total obliteration of the primary igneous texture and
the production of a large amount of mica and hornblende. All the dikes
were probably injected at the time of the volcanic activity when the vol-
canics of the higher series were ejected, but no proof of their connection
can be produced. They were, however, injected before the folding of the
.area took place, as shown by their having been rendered schistose by it.
A single dike belonging to the later series was studied. It is massive,
and therefore was irrupted after the folding which produced the schistosity
in the earlier series of dikes. It belongs probably to a Keweeuawan or
post-Keweenawan period of eruption.
' For a discuasiou of the orogenic movements which aft'ected the Crystal Falls district, the reader
is referred to p. 158 et seq.
3I0N XXXVI 4
CHAPTEE IV.
THE LOWER HURONIAN SERIES.
This series is represented in the Crystal Falls district by the following
formations, given in order from the Dase upward: The Randville dolomite,
the Mansfield slate, and the Hemlock formation. At the beginning of the
deposition of the Lower Huronian series the entire district was covered by
the pre-Cambrian sea, with the possible exception of a small island in the
Archean area.
SECTION I.— THE RANDVILLE DOLOMITE.
The best exposures of this dolomite are found near the center of the
district east of the western ellipse and in the extreme southeastei-n part of
the district in the Felch j\lountain range. Both areas are described by
Smyth, to whom we owe the name, and the reader is referred to his descrip-
tion on p. 406 and p. 431 for the detail characterization of the formation.
It will suffice for our purpose to state that it is a medium-grained crystallme
dolomite.
The few outcrops which I shall mention are important as showing the
relations of the formation to the underlying rock, but are, petrographically
considered, rather exceptional phases of the formation. Hence my descrip-
tion will be brief.
DISTRIBUTION, EXPOSURES, AND TOPOGRAPHY.
The area in which the Randville dolomite immediately underlies the
drift is a continuous zone adjacent to and surrounding the Archean core.
The belt varies slightly in width along the sides of the ellipse. At the
ends it is two or three times the width at the sides. This is due to the
lower dip of the beds at the ends. Exposures are found m the area studied
by me only on the northeast and southwest flanks of the granite core.
The west branch of the Fence River follows the limestone area for a
short distance in the northeastern part of the district, skirting the Archean
50
PETKOGHAPIIICAL CUAUACTERS OF KANDVILLE DOLOMITE. 51
• -•raiiito. On the whole, however, the Kiiudville (loh»inite has hud uo
uuirked effect on the topoyrapliy or the drainage.
PETROGRAPIIICAI. CHARACTERS.
Ill general the Randville dolomite consists petrographicall}' of a fine-
grained dolomite, with some quartz. This grades down through a calca-
reous quartzite by increase of quartz into a true quartzite. The nearer the
granite, the more quartzitic is the formation. At the southeast corner of
sec. 2, T. 45 N., R. 32 W., on the west bank of the west branch of the
Fence River, is a ver}- good exposure of the quartzite. Its derivation
from the underlying granite is here shown. The rock is a very fiue-grained,
almost novaculitic, quartzite. It shows current bedding in some places,
though no true bedding was observed. Immediately below this quartzite
is a very schistose rock, in which one can readily distinguish macroscopic-
ally rounded to lenticular quartz areas, with masses of sericite flakes
between them. The contact between tlie quartzite and the schistose rock
seems very sharp when viewed from a short distance, but is found to l)e
indefinite when closely examined. A close search was made along a con-
tact for pebbles from the granite, but such were not found. However,
small rounded pieces of vein quartz, most probably derived from the granite,
were observed. The schistose rock in its turn grades down into a graj-ish
granite, which is also more or less schistose. We have here evidently a
transition from the granite, through the intermediate schistose recomposed
granite, to the true sedimentary rock above. The meaning of this transi-
tion is considered below.
Under the microscope the cause of the schistosity of the rock inter-
mediate between the granite and the quartzite is plain. Quartz and sericite,
with some feldspar, are alone present in it. The quartz is grayish and
granulated, and mashed out into oval areas representing original quartz
grains. Various fragments constituting the areas are, however, angular
and more or less equidimensional, and when not so never have a definite
orientation of their longer axes. Between these large areas, but not between
the individual small fragments constituting the areas, sericite is abundant.
When the sericite is not predominant, the flakes lie in a fine mass of quartz
grains, each of which agrees in long direction with the mica plates and
large oval quartz areas. The sericite flakes are both included in this quartz.
52 THE CRYSTAL FALLS IRON-BEAfiING DISTRICT.
and also lie between the grains. In one instance fragments of the original
feldspar were found in the midst of such an area. These quartz-sericite
areas are unquestionably of secondary origin, and the minerals have devel-
oped in connection with pressure. They were probably produced from
feldspar which existed in the original gi-anite.
Whether this schistose rock was formed from a weathered but not
transported granite, from an arkose or feldspathic sandstone, or from the solid
granite, it is impossible to say. A similar sericite-schist which developed
from recomposed granites has been described by Van Hise as occurring at
several localities in the Marquette district.^ In these cases at places the
fragmental characters are still sufficiently clear to admit of the statement
that the rocks are sedimentary. In the Crystal Falls rock mashing has
destroyed all original characters. The rock occupies an intermediate posi-
tion between a metamorphosed sedimentary and a metamoiphosed eruptive,
and grades on the one liand into the sedimentary and on the other into
the eruptive. This makes it impossible to say whether it belongs exclusively
to the one or to the other, or in part to both. Similar relations . in other
parts of Michigan were explained by Rominger" as cases of progressive
metamorphism of sediments, the granite being supposed to be the extreme
stage of alteration of the sedimentary rock. Later the finding of basal
conglomerates at or near these localities has shown conclusively that this
explanation is incorrect, and it has been abandoned by Rominger.
The quartzite, which immediately overlies the rock of doubtful char-
acter, is composed of angular grains of quartz, between which are plates of
sericite which have an imperfect parallelism, thus giving a certain degree
of schistosity to the quartzite, possibly enough in places to warrant its
being called a quartz-schist. The rock shows no conclusive microscojjical
e\'idence of a sedimentary origin, but differs from the cherts, with which it
might be confused, in the size of the grains and in the presence of sericite.
This rock was originally probably chiefly composed of quartz sand, with
some feldspathic material from the disintegrated granite. Coincident with
the pressure which produced the striking schistosity in the underlying rock,
this sand was also mashed, resulting in the production of sericite and quartz
' Mou. U. S. Geol. Survey Vol. XXVIII, 1897, p. 226.
-The Marquette irou district, by Carl Rominger: Geol. of Michisau, Vol. IV, Parti, 1878-1880,
pp. 15-52.
lM:TROGKAPniCAL CHARACTERS OF RANDVILLE DOLOMITE. 53
trom the feldspiir and in the rrusliing- of the quartz grains, thus completely
destro}'ing- the rounded chistic grains and oljliteratiug all the sedimentary
character of the rock, except the macroscopic structure of cun-ent bedding.
On the west side of the granite ellipse, at N. 1750, W. 1550, sec. 12,
T. 44 N., R. 32 W., about 100 yards from the granite, to the nortli, and
lower down on the slope of the same hill on which the granite is found, is
found a carbonaceous quartzite or quartzose dolomite. The strike is N.
25°-35° W. The surfiice only is seen, so that the dip could not be taken.
Microscopical examination shows the rock at the eastern side of the exposure
to be made up of quartz grains held together by a fine-grained carbonate
cement. This grades up to the west by increase of calcite and correspond-
ing diminution of quartz to a quartzose dolomite.
At N. 500, W. 1550, sec. 1, T. 44 N., R. 32 W., one-fourth mile distant
from the granite, is seen another outcrop of a very dense quartzose dolo-
mite, appearing macroscopically almost like a vitreous quartzite, but really
with just enough quartz grains in it to enable the qualifying term "quartzose"
to be appropriate!}- used. The brown ferruginous crust on the weathered
surfaces point to a percentage of iron in the magnesium-calcium carbonate.
The pure limestones are to be sought slightly farther away from the
Archeau shore, where the conditions were more favorable for the production
of a pure nonclastic sediment.
REIiATIOlSrS TO UNDERLYING AND OVERLYING FORIMATIONS.
At only the one place cited above has a contact between the granite
and the Randville dolomite been found. It is probable that unconformable
relations exist, even though no basal conglomerate has been discovered as
evidence of wave action on the Archean coast.
Relations between the Randville dolomite and the overlying- forma-
tions have not been observed in the part of the district studied b}- me.
THICKNESS.
Reliable data for estimating the thickness of the Randville dolomite
have only been obtained in that area surveyed by Smyth. (See p. 433.)
According to his estimate, the formation possesses a maxinunn thickness of
1,500 feet.
54 THE CRYSTAL FALLS lEON-BEARmG DISTRICT.
SECTION II.— THE MANSFIELD SLATE.
The formation of the Lower Huronian, which is next higher than the
Randville dolomite, is composed of sedimentary beds, in which a' slate pre-
dominates.
This formation is fonnd in its most t^q^ical development in a narrow
valley through which the Michigamme River flows, and in which the village
of j\Iansfield and a mine of the same name are situated. The valley and the
slates are well known in the Crystal Falls district on account of their eco-
nomic importance. For this reason the name "Mansfield slate" is here
applied to this formation.
DISTRIBUTIONS, EXPOSURES, AXD TOPOGRAPHY.
The part of the valley occupied by the Mansfield slates begins at the
northern section line of sees. 17 and 18, T. 43 N., R. 31 W., and extends due
soiith for 3 miles to the southern section line of sec. 29 of the same township.
The slate belt is widest at the north, being over one-fourth mile wide on
the westren side of section 17. To the south it gradually dimini.shes in
width, until it finally disappears in sec. 29. The strike of the sedimentary
rocks is almost due north-south, except in a few places where the rocks
have been gently flexed and the strike varies a few degrees. The dip is
high to the west, ranging from 65° to 80°.
The influence of the Mansfield slate belt upon the topography is
strikingly shown by the depression in which the slates are found, and
which contains the Michigamme River. The slates are surrounded on
all sides by igneovis rocks which form fairly high hills, those to the west
being composed of rocks of volcanic origin, those to the north, east, and
south being intrusive, and later than either the sedimentaries or the vol-
canics. The Michigamme River flows south through sec. 1, T. 43 N., R.
32 W., and meets the east and west ridge of intrusives in the northeastern
part of sec. 12 of the same township and range. It cuts through this at an
oblique angle, changing its course to the southeast In sec. 7, T. 43 N.,
R. 31 W., it leaves the intrusives and penetrates a short distance into the
volcanic rocks, their contact not being able to cause a change in the course
of the river, owing to the slight diff'erence in resisting power between the
intrusives and the volcamcs. Still flowing to the southeast, it finds at
THE MANSFIELD SLATE. 55
the Michigamnu' (lain, on the section line between sees. 7 and 18, near the
southeastern and northeastern corners, respectively, the contact between
the three kinds of rock, the sedimentaries, the volcanics, and the intrusives.
Where the water leaves the eruptive and enters the sedimentary area the
more easily erodible nature of the rocks of the latter is well shown by the
■falls which have been formed, the volcanics constituting the barrier over
which the water plunges into a deep basin worn from the slates. Crossing
the slates in the same direction, i. e., southeast, the river strikes squarely
ao-ainst the intrusive dolerites and is deflected to the south, following the
contact between the two rocks for a short distance, then gradually working
to the west into the center of the sedimentary area, the river takes an
almost directly southerly course, with only minor bends. In the slates the
river has fairly low flat banks on both sides. In the southern portion of
the area the valley is narrower, owing to the progressive narrowing of the
sedimentary belt. As soon as the river leaves the Mansfield slate belt,
it resumes the sinuous course it had before the Mansfield belt is entered,
and flows between high banks through the intrusives, out through the sand
plains near Lake Mary.
POSSIBLE CONTINUATION OF THE MANSFIELD SLATE.
In sec. 10, T. 44 N., R. 32 W., about 7 miles northwest of the extreme
northern end of the Mansfield area of slate, there are one or two exposures
of much crumpled interbedded brown and black slates. Their strike is about
N. 16°-20° W., but owing to their plicated condition the dip varies from 55°
southwest over to 85° northeast. The average dip, however, is presumed
to be to the southwest, which is in accord with the general structure of the
area.
The slate exposures are surrounded by coarsergrained basic intrusives,
dolerites, which outcrop within short distances on all sides. The nearest
sedimentary beds are quartzose dolomite ledges which outcrop 1 J miles to
the east, in sees. 1 and 12, T. 44 N., R. 32 W., rather close to the Archean
granite. A section across the Lower Huronian rocks at this point shows
the Archean granite overlain by quartzose dolomite, which is in its turn
overlain b3' the slates. The relations which these rocks bear to one another
are those which similar ones bear to one another near Michigamme Moun-
56 THE CRYSTAL FALLS IKON-BE ARING DISTRICT.
tain/ and the slates of the two areas are consid red to he of the same age.
Since the slates correspond stratigrai^hically to the slates of the Michigamnie
Mountain and to those of the Mansfield area, they have been connected ou
the map with the slates of Michigannne Mountain by a narrow belt included
between dotted lines; but this belt is not based on any connecting exposures.
These two ledges of slate are taken as the northernmost outcrops of the
Mansiield slate formation, although a number of miles north and in direct
continuation of them along the strike there was found a single doubtful out-
crop of a graj'wacke, showing neither strike nor dip. Whether it represents
a shallower water deposit contemporaneous with the slates it is impossible
to say. However, on such slight evidence it was not deemed advisable to
continue the slate belt to this point.
PETROGRAPIIICAL CHARACTERS.
A petrographical description of the Mansfield slate belt must neces-
sarily be very brief, owing to the small area and to tlie scarcity of the
exposures.
The rocks of the Mansfield slate belt are graywackes, clay slates,
phyllites, siderite-slates, cherts, ferruginous cherts, and iron ores, with the
various rocks which have been derived from them l^y metamorphism.
They vary from coarse-grained rocks to very fine grained slaty ones. The
latter predominate, and for that reason this belt is called a "slate " belt. The
color of the rocks varies from an olive green and purplish lilack to bright
red for those which are very ferruginous and more or less altered.
The ordinary detrital rocks may be divided into the coarser and the
finer kinds. The first are the graywackes, and the second are the ordinary
clay slates and ^shyllite. There is, however, a gradation from the one to
the other.
GRAYWACKE.
The graywackes consist largely of grains of quartz and feldspar of
unquestionably detrital origin. Associated with these is a large amount
of mica, chlorite, and actinolite, with invariably more or less rutile. This
last is in minute grains as well as in crystals. Many of the crystals show
fine knee twins, triplets, and more rarely, heart-shaped twins. Tourmaline
' See Part II, Chapter IV, Sec. IV, by H. L. Smyth.
I'ETROGRArillCAL COAHACTEKS OF MANSFIELD SLATE. 57
is sometimes pirst'Ut. The feiTO-niagnesiau minerals dc-velop cliiefly from
the alteration of the feldspar, and from the finer detritns which is jjresmiied to
have existed between the grains. As a consequence, the secondary minerals
lie between the original grains. Many of the quartz grains are enlarged,
and here the secondary minerals are included in the new areas of the enlarged
grains. In numerous cases the new quartz occupies about as much space
as the original grains themselves. This shows very clearly the porous
character of the original sandstone. All original grains of the rocks show
signs of extensive mashing. Some specimens contain a large amount of
tourmaline in long slender crystals, which penetrate both the feldspar and
the quartz grains. The presence of tourmaline is especially interesting as
indicating that these sedimentaries may have been subjected to a certain
amount of fumarole action. According to the proportion in which the
vai-ious minerals have developed, we obtain sericite-, actinolite-, or chlorite-
schists produced from the graywackes.
CLAY SLATE AND PHYLLITE.
The clay slates are dull and lusterless and are black, olive green, or
red in color. They are usually impregnated with more or less iron pyrites
in large macroscopical crystals. One can distinguish in them quartz, Avhite
mica, a few needles of actinolite, rutile, hematite, with a small proportion
of a dark ferruginous and carbonaceous interstitial material.
The amount of iron which these clay slates contain varies considerably.
In some, hematite is present in such quantity as to cause the slates to be
appropriately called hematitic slates. Such, for instance, is the one forming
the foot wall of the Mansfield ore body. The iron oxide gives to the slates
a very bright red color where they are weathered. Tliese weathered
hematitic slates are very commonly known in the district as red slates, or
as "paint rock" or "soapstone," though rocks of very different character are
also at times designated by these names.
The phyllites have a silky luster and a bluish-black color. They are
composed essentially of white mica quartz, some feldspar, innumerable
minute crystals of rutile and dark ferruginous specks. These seem to differ
from the rocks called here clay slates only in that they are more completely
crystalline, the interstitial material of the slates having disappeared.
58 THE CRYSTAL FALLS IRON-BEARING DISTRICT.
ORIGIN OF CLAY SLATE AND PHYLLITE.
The origin of the clay slates of the Mansfield formation is probably to
be looked for in the disintegration and decay of the Archean granite, and
the subsequent metamorphisra of the resulting clay. For between the
srranites and the slates no other rock masses are known to have existed from
which the clay could have been derived. The phylUtes are presumed to
have resulted from the metamorphism of the clay slates.
PRESENT COMPOSITION NECESSARILY DIFFERENT FROM THAT OF ROCK FROM
WHICH DERIVED.
It is a well-recognized principle of rock weathering that in the altera-
tion of rocks near the surface of the earth there is a relatively rapid
diminution in the quantity of the more soluble constituents. Hence a clay
shows a lower percentage of alkalies and alkaline earths than is found in
the parent rock, with an increase in the percentage especially of alumina
and water. This relation is made clear by Adams in a statement of the
comparison of the composition of certain slates and granites:^ "On com-
paring the analyses of a series of granites with those of a series of slates,
as, for instance, those given in Roth's ' Gesteins Analyzen,' the latter are seen
to be on an average considerably higher in alumina and much lower in
alkalies, while at the same time they are lower in silica, which has been
separated both as sand and in combination with the alkalies which have
gone into solution, and in most cases contain more magnesia than lime
instead of more lime than magnesia, as is usual- in granites." Adams con-
cludes further, after a comparison of the alkalies in the slates and granites,
that "The slates thus contain on an average about two-thirds of the amount
of alkali present in the average granite."^ An examination of series of
analyses of granites shows that while the percentages of soda and potassa
vary considerably, now the one being predominant, now the other, on the
whole in the typical granites the potassa is higher than the soda.' This is
the relation which we would expect in the case of an ideally pure granite.
1 A further contribution to our knowledge of the Laurentiau, by F. D. Adams: Am. Jour. Soi., 3d
aer., Vol. L, 1895, p. 65.
"Loc. cit.jp. 65.
■^'Zirkel states that iu the weathering of granites the soda is much more readily removed than
is the potassa; Lehrbuch der Petrographie, Vol. II, 1894, p. 32.
PETROGRAPDICAL CnARACTERS OF MANSFIELD SLATE.
50
ill wliicli no aiiorthoclase replaces the orthoclase. As a consequence of
tlie easier solubility of the soda, this relation between the two alkalies,
soda and potassa, is maintained, and is often made more strildng in the
clay slates. An average of 31 analyses of clay slates taken from various
sources shows two and one-half times as much potassa as soda. In the case
of the Mansiield slate this difiPerence has been increased, so that there is
ten times as much potassa as soda present.
ANALYSIS OF MANSFIELD SLATE.
Mr. George Steiger, of the United States Geological Survey, has
prepared a complete analysis (No. 1 in the following table) of a typical
specimen of the Mansiield clay slate. Analyses Nos. 2 and 3 were pre-
pared by W. Maynard Hutchings,' and numbered by him Nos. 2 and 5,
respectively.
Analysis of the Mansfield clay slate.
Constituent.
1.
2.
3.
SiO-
60.28
.69
22.61
2.53
.45
Trace.
.13
.04
1.35
5.73
.54
.60
3.62
.03
None.
.97
59.28
53.57
TiO.
Al,03
21.85
1 5.80
24.53
6.51
FeaOa
FeO
MnO
CaO
.45
.76
BaO . .
MeO
1.24
4.13
1.18
I 6.25
1.81
4.34
.97
7.65
K,0
NajO
H.Q at 100
HiO above 100^'
PjO,
CO. . . .
c
Total
99.57
100. 18
100. 12
COMMENTS ON ANALYSIS.
That which is the most striking about the analysis is the relative pro-
portion of the alkaline earths, lime, and magnesia, the latter being present
'Notes on the composition of clays, slates, etc.. and nn sonip points in their contact metamor-
phism: (ieol. M.ag., Vol. I, 1894, p. 38.
60 THE CRYSTAL FALLS IRON-BEARING DISTRICT.
iu the greater quantity. As a rule, in all of the igneous rocks (and to
the igneous rocks all clay slates owe their ultimate origin), except in the
nonfeldspathic ultrabasic ones, the reverse condition exists, namely, the
magnesia subordinate in quantity to the lime. The difiference in amount
of soda and potassa is very striking and should be noticed, in view of cer-
tain points to which attention will be called in subsequent pages. The
percentage of alumina is higher than is usital in the clay slates. It will be
noticed that considerable water is present, but in consideration of the char-
acter of the rock this is to be expected. If anything, the value is rather
lower than would be expected, indicating a possible loss of water due to the
rock having already undergone some dynamic action. The carbon present
is considered as offering trustworthy evidence of the presence of organic life
at the time of the deposit of the slates, though no more satisfactory evidence
of the existence of life has been found.
COMPARISON OF ANALYSIS OF MANSFIELD CLAY SLATE WITH ANALYSES OF CLAYS.
During the last few j^ears there have appeared in the Geological
Magazine, from the pen of Mr. W. Maynard Hutchings, some very elaborate
and suggestive articles upon the composition of clays, shales, and slates,
and from one of these ^ I have taken two analyses of Carboniferous clays
for comparison with the Mansfield clay slate. These two analyses, Nos. 2
and 3, p. 59, are from the very fine grained clays, in which the quartz was
not distinguishable with the microscope, and are the analyses showing the
highest and lowest percentages of silica. Mr. Hutchings says of his
analyses that the samples were di-ied at 220° F., and that the titanic oxide
was not determined but is contained in the silica and alumina. Concerning
the clays, he writes:
From these analyses it will be seeu that these clays would be capable, chemically
considered, of transformatiou iuto very typical " clay-slates." Mineralogically they
are clay-slates, having already undergone all, or nearly all, the mineral changes
requisite to constitute the normal (unaltered) slates. Nothin.g more is needed but
physical changes, such as compacting, arrangement of mica iu a plane, increase of
size of mica, etc.^
The great similarity of these clays with the Mansfield clap slate is very
evident. The only material difference which exists between them is in the
'Notes on the compositiou of clays, ."ilates, etc., autl on some points iu tlieir contact metamor-
phism, by W. Maynard Hutchings : Geol, Mag., Vol. 1, 1894, p. 38.
•Loc. cit., p. 38.
rETllOGUAPUlOAL CHAKACTERS OF MANSFIELD SLATE.
61
liiyhor pLTcentiii^-e of water (•(tntaiued in tliu clay.s. This difference is
natural, clays iisuall}' containing- about twice as much water as do the
slates.
COMPARISON OF ANALYSIS OF MANSFIELD CLAY SLATE WITH ANALYSES OF OTHER
CLAY SLATES.
In tlie following talkie there are given, for purposes of comparison with
the Mansiield clay slate, analyses of typical clay slates, roofing slates from
the Cambrian of Vermont and New York.
Analyses of tyjncal clay slates.
Constituent.
SiO^
TiO,
AUG,
Fe.20,
FeO
MnO
CaO
BaO
MgO
KjO
NasO
H,0 at 100^ ...
H2O above 100^
P2O5
CO;
FeS,
C
Total
60.28
.69
22.61
2.53
.45
Trace.
.13
.04
1.35
5.73
.54
.60
3.62
.03
None.
.97
99.57
62.37
.74
15.43
1.31
5.34
.22
.77
.07
3.14
4.20
1.14
.34
3.71
.06
.87
.06
Trace.
3.
59.70
.79
16.98
.52
4.88
.16
1.27
.08
3.23
3.77
1.35
.30
3.82
.16
1.40
L18
.46
99.80
100. 05
67.61
.56
13.20
5.36
1.20
.10
.11
.04
3.20
4.45
.67
((.45
ft 2. 97
.05
None.
.03
100.00
67.89
.49
11.03
L47
3.81
.16
1.43
.04
4.57
2.82
.77
a. 36
6 3.21
.10
1.89
.04
100. 08
aHjO at 110-.
b U2O above 110^.
No. 1. Black slate, Bp. 32497, N. 450, W. 1620, sec. 17, T. 43 N., R. 31 W., Michigan. Analyzed by
George Steiger.
No. 2. Sea-green slate, Griffith & Nathaniel Quarry, South Poultney, Vermont. W. F. Hillebraud.
No. 3. Black slate, American Black Slate Company, Benson, Vermont. W. F. Hillebraud.
No. 4. Red slate, three-fourths mile south of Hampton Village, New York. W. F. HiUebrand.
No. 5. Cireen slate, three-fourths mile northwest of Janesville, Washington County, New York.
W. F. Hillebraud.
Nos. 2, 3, 4, and 5 taken from Analyses of rocks and analytical methods, 1880-1896, Clark and HiUe-
brand: Bull. U. S. Geol. Survey, No. 148. Nos. 2 and 3 are, respectively, C and F, p. 277, aud Nos. 4 and 5
are A aud D, p. 280.
62 THE CEYSTAL FALLS lEOK-BEAElNG DISTEICT.
The strong similarity between the composition of these clay slates is
at once apparent, and needs no further comment. The only marked differ-
ence between the Huronian clay slate and the Cambrian ones is the higher
percentage of alumina present in the former. .
SIDERITE-SLATE, CHERT, FERRUGINOUS CHERT, AND IRON ORES.
The two most interesting kinds of rock from the Mansfield slate belt
are those known as the siderite- or sideritic slates and the cherts or ferrugin-
ous cherts, according to the quantity of iron carbonate and iron oxide
present. These alternate with each other, and are found also interstratified
with the fragmental slates, and thus there can be no question as to their
sedimei^tary character. The siderite-slates are of a light to dark gray color.
They are well laminated, and in some places cleave rather readily along
the laminae, though at other places they break with an almost conchoidal
fracture. The weathered siderite slates are covered by a crust of reddish-
brown hydrated iron sesquioxide.
Microscopically the siderite slates are composed of siderite, or of sider-
ite and exceedingly fine grained cherty silica. Roundish rhombohedra of
siderite compose the purer sideritic portions. If one passes from the pure
to the less pure slates, the siderite gradually diminishes in quantity, the
silica grains increase correspondingly, and the rock grades into the chert
which, in bands, is commonly associated with iron carbonate in the Lake
Superior region. As the carbonate alters to the oxide or hydrated oxide
ferruginous cherts are produced. The cherts are white to red, depending
on the amount of iron oxide present. The manner in which the siderite
alters to limonite and hematite, and the various steps of the process have
been so well described and beautifully illustrated in Monograph XXVIII,
that the reader is referred to that volume for further information. None of
the brilliant red jasper or jaspihte, such as that found in the Marquette
district, is associated with the Mansfield slates. Iron ores of economic
importance, however, are found associated with these slates, and are
described in detail farther on. None of the sideritic slates, ferruginous
cherts, or ores, although interbedded with the fragmental slates, show any
evidence of fragmental origin so far as the indi\'idual grains of the minerals
composing them are concerned.
PETKOGUAPIIICAL CUAHAGTEliS OF xMANSFIELD SLATE. 63
RELATIONS OF SIDERITE-SLATE, FERRUGINOUS CHERT, AND ORE BODIES TO
CLAY SLATES.
Owing- to the scarcity of tlie outcrops of the sedinientaries in thu i\Iaus-
tic'ld Valley, it is practically impossible to decipher the relations of the
individual lieds. Neither the study of the surface exposures nor the expo-
sures in the mine workings liave given definite results. Tliat the heds repre-
sent interbedded strata is well understood, but the sequence of the strata is
indeterminable. It is of especial interest to determine, so far as possible,
the relations of the ferruginous rocks, in order that the possible iron-ore
deposits associated with theni may be found. A cross section through the
Mansfield mine from east to west shows the following relations: The foot-
wall of black heinatitic slate is overlain by 25 to 30 feet of fernaginous
chert and iron ore. This stratum is succeeded by "red slate," so called by
the miners, which is probably weathered greenstone impregnated with iron.
This is followed b}' a conglomerate, and this by amygdaloidal greenstone,
of the overlying volcanic formation. The ore body extends north and
sovith, agreeing thus with the strike of the slates. All drifts end on the
north in mixed ore, and on the south in mixed ore, with "quartz-rock" and
"lime-rock" of the miners in some places. From these facts we may justly
conclude that the ore-bearing ferruginous cherts exist in beds in the slates
or as lenticular masses which agree in dip and strike with the surrounding
slates. This conclusion is confirmed b}- test pits along the strike of the
exposed beds, which have disclosed similar ferruginous cherts at various
places for a distance of half a mile to the north.
RKLATIONS OF MANSFIEIiD SLATE TO ADJACENT FORMATIONS.
RELATIONS TO INTRUSIVES.
The Mansfield slates are surrounded on three sides— east, north, and
south — by coarse-grained basic eruptive rocks. The fact that the}' are so
surrounded by these rocks, which cut them off in the direction of their
strike, points to the later origin of these eruptives. Moreover, the quartzitic
character of some of the sedinientaries shows that they could not have been
derived from the eruptives which stratigraphically underlie them, for iu
these no quartz is found. The quartzitic character would thus seem also
to indicate that the slates are older than the intrusives. Wherever the
64 THE CEYSTAL FALLS lEON-BEAEING DISTEICT.
igneous rocks and slates are in contact or in close association, the latter
have been metainorpliosed, and adinoles, spilosites, and desmosites have been
formed which are similar to those described as occurring in other areas
along the contact zone of basic intrusives. Although no single instance of
a dike penetrating the slates has been found, it can hardly be doubted from
the relations which have been outlined that the slates are older than the
intrusive dolerites.
RELATIONS TO VOLCANICS.
The sedimentaries are overlain by volcanics, both lava flows and tufa-
ceous deposits. In these tuff's, at the northeast corner of sec. 7, T. 43 N.,
R. 31 W., angular black-slate fragments have been found similar in every
respect to the slates of the Mansfield belt. From this it is clear that at
least some of the volcanics are younger than part of the slate formation.
In section 29 similar relations obtain, the <»nly difference being that the
masses of slate and graywacke are inclosed in rather larger fragments in a
volcanic conglomerate, and still retain very closely their normal strike. In
the conglomerates near the Mansfield mine are found chert fragments and
in some places fragments of iron oxide. These latter were evidently not
included as oxide, but as fragments of cherty carbonate. Like the great
mass forming the ore body, the fragments have since their deposition been
altered, forming iron-oxide bodies of small size. Further discussion of the
relations between the volcanics and slates will be found under the heading
" Hemlock formation."
STRUCTURE OF THE ]MA]SrSFIELD AREA.
It has already been seen that the Mansfield rocks strike north and
south and have a high westerly dip. The two possible explanations of this
structure which are compatible with the facts in other portions of the area
are (1) that they form a westward dipping monocline, and (2) that they are
the western limb of an anticline.
THICKNESS.
As the sedimentaries forming the Mansfield belt now dip west at a very
high angle, and as there is no evidence of duplication of strata due to fold-
ing, I feel comparatively safe in giving an estimate of their thickness. The
belt is widest at the north end, and there has a breadth of about 1,950 feet.
THICKNESS OF MANSFIELD SLATF.
65
Till- i'.vi'i-aj^-o dip of tliu beds is 80^, and this gives a maxiumm thickuess
(if l,!lO() feet. Toward the south the belt rapidly narrows, initil it is cut
out l)y tile intruding (hilerites. A thickness of 1,500 feet is probably not
tar from the average.
To the east of the ^Mansfield slates is a belt, varying in width up to
about 1,200 feet, in which are found large masses of metamorphosed slates,
surrounded by intrusive dolerite. In this belt the slate masses still show
a general north-south strike, with slight variations to the east or west, and a
westward dip. One might, perhaps, consider this a slate area which has
been ciHn])letely saturated with intrusives. If it should be so considered,
this thickness should lie added to the estimated thickness of the slates as
above given, but as intrusives predominate in it, the slate being, as it were,
merely incidental, I have preferred not to include it in the belt with the slate.
t)RK DEPOSITS.
Although a great deal of exploring for iron ore has been done in the
Mansfield slates, only one lai-ge body of ore has thus far been discovered, in
which is the Mansfield mine. This mine
is situated on the west liank of the Michi-
gamme River, in sees. 17 and 20, T. 43
N., E. 31 W. The mine was apparently
prospering wlien, on the nigdit of Septeni-
ber 28, 1893, a cave-in occurred, letting
in the waters of the jMichigamme River
and drowning' 28 miners.
N
MAI,N
SHAFT
For two Lours after the caving occurred,
the bed of the river below the mine was bare,
the water tlowing into the mine worlviugs.
The accompanying tigure, fig. C, prepared by J.
Parke Channing, October 8, 1893, shows the
relative position of the shaft and the river, and
the couceutric- cracks caused by the caving of
the mine. (Plan copied from address of presi-
dent: Proc. Lake Superior Inst. Min. Eng.,
Vol. Ill, 1895, plate opposite p. 42. ) The timber shaft is near the center of tliese
cracks.
After the caving the mine remained idle until recently. At tlie present
writing the DeSoto Mining Company has obtained control of tlie mine and,
I understand, have freed it from water.
MON XXXVI 5
Fig. 6. - Concentric cracks formed by the caving in
of tlio 5I.iiiatield mine.
6(3 THE CRYSTAL FALLS IKOXBEARING DISTRICT.
Ill May tbey began the task of diverting the cbauuel of the river to a point several
hundred feet sonth of the old course. They have dredged out a cut 2,050 feet in
length by 100 feet wide and 18 feet deep. At the upper end of the new channel a
cofferdam containing 14,000 cubic yards of earth has been constructed, and where the
waters join the old outlet several hundred feet below the mine another embankment
has been constructed across the course of the old bed that has 8,000 cubic yards of
earth. This task was a very expensive one, and it has been well completed, the old
channel being perfectly dry.
The turning of the river's course brings out with startling distinctness the criminal
negligence or carelessness of those who were working the mine at the time of the
accident. The upper tier of timbers in the mine are plainly seen, as also the ground
that had been cut out to receive the set that was being gotten into place when the
waters broke through. This shows the miners had worked up to within 12 feet of
the water of the river. A great crack in the formation shows where the water first
gained entrance. The ore made up the bed of the stream — was a portion of the bed
in fact — and the walls of the mine were nearly vertical. The ore deposit had a width
of about 20 feet. The water pressure must have been considerable, and the blasting
of the ore (as it is hard, and explosives are needed to loosen it) shattered the thin
piotection over the miners, permitting the water to find ready and unimpeded
entrance Into the mine. An engineer could not have been employed and the wildest
sort of guessing must have been done by those who had the work in charge.
No sane man would have permitted the opening of the deposit so near the river's
bottom."
Owing to the long abandonment of the mine, the dii'ect sources of infor-
mation have been closed. For a description of the ore body I am com-
pelled to rely on such data as are available from existing notes and plats.
I am especially indebted to a manuscript description of the mine by
J. Parke Channing, and to Mr. C. T. Roberts, of Crystal Falls, for plats of
the mine. The sketch of the mine here introduced, PI. IX, is compiled
from an original drawing of J. Parke Channing, reproduced on the plate
cited, and from data obtained from other sources.
GENERAL DESCRIPTION OF MANSFIELD MINE DEPOSIT.
The Mansfield mine has au ore body varying from 16 to 32 feet in
width. It is in ahnost vertical position ; it has well-defined foot and hang-
ing walls composed of impervious rock; it has a somewhat indefinite longi-
tudinal extent. The ore is Bessemer and occurs in an iron-bearing formation,
which corresponds in every particular to those of the other iron-bearing
' Report of Commissioner of Mineral Statistics of Michigan, George A. Newett, for 1896, p. 84.
::^^^g«ftas»«g?g»- ■=»;«<: r-
feK^^>^.^,
5 il
ORE DEPOSITS IX MANSFIELD SLATE. 67
districts ot" the Lake Superioi- region. 'I'lie ore was iirst toiuid in a test pit
wliicli passed through 0 feet of drift. The main working' shaft was then
located about 100 feet west of this j)oiut. It was put down to a deptli of
4()0 feet before ore was struck. From this shaft crosscuts were driven east
at average intervals of 70 feet, and the ore body was met at a distance vary-
ingf from 74 feet at the first level to 10 feet at the sixth level. The cross-
cuts, in every case after leaving the greenstone, pass through so-called red
slate, at the maximum about 25 feet thick, before ore is reached, this rock
constituting the hanging wall. From these data the dip of the ore body
may be calculated to be about 80° W., agreeing well with the observed dip
of the slates, which outcrop over the area. The thickness of the ore, as
shown by the cross sections, averages about 25 feet. The extreme variation
in thickness ranges from two sets, or 16 feet, to four sets, or 32 feet. The
strike of the slates is north and south, and the trend of the ore body
agrees with this. This brings its southern end under the original course
of the Michigamme River as the stream bends slightly to the west, south
of the shaft. An examination of the longitudinal (north-south) section
through tlie ore body does not determine whether or not it has a pitch.
The southern boundary is nearly vertical from top to bottom, while the
northern boundary lengthens about 140 feet between the first and the fifth
levels.
In the northern end of the mine — that is, in line with the strike of the
sedimentaries — the ore body terminates, in a more or less irregular Avay, in
so-called mixed ore. This mixed ore continues to the north for over half a
mile, as shown by the numerous test pits which have been bottomed in it.
To the south of the mine shaft the ore body proper extends for 200 feet. It
then changes its character, becoming a lean non-Bessemer ore. A long drift
(335 feet) at the second level was run through this ore, and after leaving it
penetrated a mixed ore, the so-called lime rock (sideritel) and quartz rock
(chert?) of the miners. Three crosscuts along this drift show the ore body
to vary from 20 to 30 feet in thickness, with the same foot and hanging wall
as for the remainder of the mine. The same condition exists also lower
down, as shown by a drift from the fourth level, 260 feet south. The
figures on PL IX, giving longitudinal and cross sections of the mine, show
clearly the dimensions of the ore body.
68 THE CRYSTAL FALLS lEON-BEARING DISTRICT,
RELATIONS TO SURROUNDING BEDS.
The foot wall of the ore is a black slate, described as being rich in
hematite and liearing large crystals of iron pyrite. No crosscuts have
been driven for any distance into the foot wall, so that it is impossible to
say what thickness of the hematitic black slate there may be before the
greenish pvritiferous slate begins. In places a gray "soapstone" takes the
place of the black slate as the foot wall.
The dump obtained by sinking the shaft in the material overlying the
ore shows lai-ge masses of conglomerate, the pebbles of which are rounded
and predominantly of volcanic rocks, with pebbles of chert and slate from
the iron formation and slates below. These fragments are well rounded.
The microscope also shows quartz grains with secondary enlargements, so
that there can be no doubt that the rock is a true conglomerate. Similar
conglomerates, except that the sedimentary fragments are w^anting, have
been noticed farther north along the west side of the river. Just west of
the Ijridge at IMansfield, near the mine, there is also a small exposure of con-
glomerate, which shows an alternation of coarse and fine sediments, with a
strike nearly north and south, and a dip of 80^ W. To the west, above this
conglomerate, and not more than 15 to 20 feet distant, are found the lavas
of the Hendock volcanics. According to the mine captain, the succession
west from the ore body in the hanging wall is 20 to 25 feet of paint rock,
or, as it is usually called, red slate, then conglomerate, then greenstone.
It is difficult to diagnose the paint rock, as no specimens are to be had, but
it is highly probable that it is a ferruginous and extremely altered lava
sheet. Similar rocks are commonly found thus altered in association with
the ores in the Penokee-Gogebic and Marquette districts. Lending weight
to this conclusion is the fact that in some places an amygdaloidal green-
stone has been exposed in test pits immediately above the iron-bearing
formation.
COMPOSITION OF ORE.
The Mansfield mine up to the present time has raised only Bessemer
ore, and is the only mine in the Crystal Falls district which has supplied
any considerable quantity of ore of this character. An average of a num-
ber of analyses gives the following composition for the Bessemer ore:
OllE DEPOSITS IN MANSFIELD SLATE. 69
]\lft;illir iron, 04.80; pliospliorus, 0.037; .silica, 3.70.' According- to Dr.
N. P. Ilulst,- those ore deposits in tlie Menominee range which have ijoorlv
defined walls carry a uiininiuni of phosphorus. This body, however, .shows
that the same conditions do not exist at the Mansfield mine, since, while it
has both sharply defined foot and hanging walls, it contains ])ut a low
l)er cent of phosphorus. P'rom an e.xamination of the analyses from which
the aliove average was obtained I find that the percentage of phosphorus
shows a marked increase in the lower levels of the mines over that of the
higher, and there is also a slight corresponding decrease in the content of
metallic iron. Increase of phosphorus with depth is also found in the
adjoining Menominee range, as noted by Messrs. E. F. Brown, ^ of '^he
Pewabic mine, and Per Larsson,^ of the Aragon. It is impossible to state
whether or not this distribution is due to the action of percolatino- water, as
suggested by Hulst," Larsson,*' and other Michigan mining engineers. Onlv
a large number of good analyses from carefully selected ores and asso-
ciated rocks and a detailed study of conditions of occurrence could lead to
any accurate determination of the reason for such distribution, and a dis-
cussion of these reasons is by no means warranted by the few and imper-
fect analyses of the Mansfield ores, which I have been able to obtain. The
ore body changes in composition to the south of the shaft, as shown by the
drifts in this direction. The ore in this part of the mine contains more phos-
phorus, alumina, and calcium, and less iron. This low-grade lean ore then
passes over into the banded chert and ore nnxed with the lime and quartz
rock mentioned above.
MICROSCOPICAL CHARACTER OF THE ORES AND ASSOCIATED CHERT BANDS.
The ore varies from a soft limonitic hematite to a moderately hard
hematite. It is for the most part opaque under the microscope, but in
places shows bright-red to brownish-red color in transmitted li(i-ht In
incident light the ore for the most part shows a dull-brown or reddish color,
though in places it has a bright metallic reflection. In places in the ore
' An average of 62 per cant metallic irou and .030 per cent phosphorus is reported in Report of
Commissioner of Mineral Statistics of Michigan (G. A. Newett) for 1896, p. 85.
•The geology of that portion of the Menominee range east of the Menominee River, by X. P.
Hulst: Proc. Lake Superior lust. Min. Eng., Vol. I, 1893, p. 28.
'Distribution of phosphorus and system of sampling at the Pewabie mine. Irou .Mountain, by
E. F. Brown : Proc. Lake Superior Inst. Min. Eug., Vol. Ill, 1895, p. 49.
*0p.cit.,p.52. ^0p.cit.,p.28. '0p.cit.,p.53.
70 THE CRYSTAL FALLS IRON-BEAEIXG DISTRICT.
are spots, in wliicli is a large quantity of chert mixed with iron oxide. As
such ferruo-inous-chert areas increase in quantity the ore grades into the
ferruf>-inous chert and chert which is found associated with it in bands and
lenticuLir areas.
ORIGIN OF THE ORE DEPOSITS.
The mode of occurrence and general characters of the ore body hav-
ino- been described, we are now prepared to determine the cause of concen-
tration of the iron at this particular point and the source. From the
description it was seen that the appearance of the body of ore was that of
a bedded deposit. The microscopical examination shows, however, that
the ore presents no evidences of clastic origin. An examination of the
cherts and rocks of the area which are interbedded with the ore, and
also a studv of the southern contact of the ore body, shows that
the ore is a chemical deposit, or the result of a replacement process,
by which the original rock was largely removed, and its place taken
by the present ore. It has been shown (p. 62) that the siderite bands
pass into hematitic and limonitic chert bands. It has been seen that in
the southern end of the mine the lean ore merges into a mass of ore bedded
with chert and mixed with a rock called by the miners lime and quartz
rock. I interpret this rock to be banded siderite and chert, possibly with
some quartzite bands, all of which are found outcropping at the surface.
The siderite evidently has been changed into iron oxide and the silica
replaced by iron oxide, the banding of the original rock not having been
destroyed thereby. Irving^ considered siderite to be the source of, similar
ore and associated chert and jasper. Van Hise= has fully explained the
process of the concentration of the ores of the Penokee-Gogebic and
Marquette districts, and has applied the explanation to the other districts
' Origin of the ferruginous schists and iron ores of the Lake Superior region, by R. L). Irving :
Am. Jour. Sci., 3d series, Vol. XXXII, 1886, pp. 255-272.
-The iron ore of the Marquette district of Michigan, by C. R. Van Hise: Am. Jour. Sci., 3d series,
Vol. XLIII, 1892, pp. 116-132.
Iron ores of the Penokee-Gogebio series of Michigan and Wisconsin, by C. R. Van Hise: Am.
Jour. Sci., 3d series, Vol. XXXVII, 1889, pp. 32-48.
The Penokee iron-bearing series of Michigan and Wisconsin, by R. D. Irving and C R. Van Hise,
Tenth Ann. Rept. U. S. Geol. Survey, Part I, 1890, pp. 341-507.
The Penokee-Gogebic iron-bearing series of Michigan and Wisconsin, by R. D. Irving and C. R.
Van Hise: Mou. U. S. Geol. Survey, Vol. XIX, 1892, pp. 245-290.
The Marquette iron-bearing district of Michigan, by C. R. Van Hise and W. S. Bayley, with a
chapter ou the Republic trough, by H. L. Smyth : Mon. U. S. Geol. Survey, Vol. XXVIII, 1897, pp. 400-405.
ORK DEPOSITS IN MANSFIELD SLATE. 71
in tlio Liiko Suporior ivg-ion. I shall not do more, therefore, than to add
that the investij^ations in this area have sliown the probable correctness of
this explanation.
It is very interesting- from an historical standpoint to note that as far
back as 1868 Credner had made the suggestion, with reference especially to
the ^Marquette district, tliat the ores were derived from an original iron
carbonate. The following- quotation will show his idea of the processes of
development of the ore:^
Spliaerosiderit wurde aus kohlensjiurereichen Gewiisseu abgesetzt, duioh eine
theilweise Oxydatiou desselbeu eiitstaud Magneteisenstein, durch weitere Anfnalime
von Sanerstoff' das Gemenge vou Magneteisenstein und liotbeisensteiu uiid endlich
reiner Eotlieiseiisteiu; aus diesem sporadisch durch Zntritt von Wasser Brauneisen-
stein.
Credner's suggestion seems to have been lightly considered by other
workers in that area. In 1886 Irving-^ suggested the theory of replacement
of an original fen-uginous carbonate to explain the Penokee-Gogebic iron
ores. This theory has since then lieen elaborated by Van Hise, and shown
to have a wider application to the other Lake Superior ore districts. He
has also traced the iron to its source in the rocks removed by denudation,
and shows why it occurs in the positions in which the ore bodies are at
present found to occur. Moreover, Van Hise has also explained the process
of development in detail, and, what is perhaps far more important, the
reason certain ores develop and not others. In its essentials, however, the
process is the same as that suggested by Credner in the lines quoted above,
though in them no suggestion of the replacement to which is due the
em-ichment of the ore bodies is made.
Much of the iron of the Mansfield ore is presumed to have resulted
directly from the alteration of the feiTuginous carbonate in place, but a large
amount was brought in from above by infiltrating waters. The ferruginous
matter, which was taken into solution during the denudation of the area, has
been carried down by percolating waters and deposited at places favorable
for its accumulation. The beds are now on edge, off'ering the most favorable
condition to percolation. The conclusion is obvious that these deposits
' Die vorsilurischen Gebikle der " Oberen Halbiusel von Michigan " in Nord-Amerilca, by H.
Credner: Zeitschv. dentsch. geol. Gesell., Vol. XXI, 1869, p. 547.
-On the origin of the ferruginous schists and iron ores of the Lalse Superior region, by R. D.
Irving: Ani. Jour. Sci., Vol. XXXII, 1886, p. 263.
72 THE CRYSTAL FALLS IKON-BEAEING DISTRICT.
were formed after the beds were tilted, and the iron derived from the
upward extension of the rocks, which has been removed by erosion.
CONDITIONS FAVORABLE FOR ORE CONCENTRATION.
The conditions favorable for the accumulation of ore deposits have
been ascertained by Van Hise from studies in the other iron-bearing dis-
tricts of the Lake Superior region. He summarizes these results as follows:^
[1] The iron ore is contiued to certain definite horizons, known as tlie iron-bearing
formations. . . . frtj All ore bodies have been found to be distributed very irregu-
larly in these iron-bearing formations. Thi.s is due to the fact that they are secondary
concentrations produced by downward percolating waters, and the ore bodies therefore
occur at the places where water is concentrated, in accordance with the laws of the
underground circulation of waters, [b] These places are just above an impervious
formation, at the contact of the Upper Huroniau and Lower.Huroniau aud where the
rocks are .shattered. \c\ The impervious basement formation m.ay be a surface
volcanic, a subsequent intrusive, an argillaceous stratum, or any other impermeable
formation, [d] These impervious basements are most efl'ective when they are in the
form of pitching troughs, thus concentrating the waters from the sides along a well-
defined channel. These pitching troughs may be formed by a single one of the above
rocks or by a combination of two or more of them. The horizon marked by the uncon-
formity between the Upper and Lower Huronian is a great natural zone of percolating
waters. Here oftentimes the basement formation of the Upper Huronian is itself a
lean ore, having derived its material from the Lower Huronian, but in this case a
secondary concentration has occurred in order to produce the present ore bodies.
[e] Finally, as a result of folding, the iron-bearing formations have been shattered,
thus producing natural water-courses. More frequently than not, more than one of
these classes of phenomena are found together where the great ore bodies occur, and
in many cases all are combined. The original source of the iron ores has been ascer-
tained to be in many cases a lean carbonate of iron, often with a good deal of
carbonate of calcium and magnesium, formed as an ocean deposit.
Van Hise adds to the above statement that generally the ore bodies,
as a result of their methods of concentration, somewhere reach the rock
surface.
The Mansfield ore body has well-defined foot and hanging walls of
normally impervious rock. The iron-bearing formation is much fractured.
We thus have certain of the conditions favorable to the concentration of an
ore body. Wliether a trough is completed by a slight cross fold in the
formation, or possibly by an intersecting dolerite dike, has not been
determined.
'Foiirteeuth Ann. Kept. U. S. Geol. Survey, Part I, 1893, pp. 107-108.
OKIi DEPOSITS IN MANSFIELD SLATE. 73
EXPLORATION.
Exploration has developed no other deposits ulong- the Mansfield slate
belt.' It' other deposits exist, it is hig'hly jjrobable that they extend to the
rocK surface — that is, are covered by the drift mantle alone.
The intervals between possible ore bodies along the strike of the slates
ar'^ probably occupied by mixed chert and ore or ferruginous chert. Explora-
tions should extend from the impervious slate below the iron-bearing forma-
tion to the impervious rock above the iron-bearing formation. In order to
explore the belt thoroughly, rows of pits cross-sectioning the formation ought
to be made at intervals not greater than 100 feet, and even with such inter-
vals an important deposit might be missed, for it frequently happens that at
the surface of the rock an ore deposit is smaller than it is at a moderate
depth.
SECTION III.— THE HEMLOCK FORMATION.
This formation, the most interesting petrographically in the Crystal
Falls district, consists almost exclusively of typical volcanic rocks, both
basic and acid, with crystalline schists derived from them. Sedimentary
rocks play a very unimportant role. With one exception they have been
formed directly from the volcanics, and occur interbedded with them.
Cutting through the volcanics are intrusive rocks, which likewise include
both basic and acid kinds. Chemically the intrusive and extrusive rocks
show very close relationships. The name Hemlock has been given to this
volcanic formation because the river of that name flows through it for a
number of miles, and in places affords excellent exposures.
DISTRIBUTIOSr, EXPOSURES, AXD TOPOGRAPHY.
Beginning in sec. 36, T. 46 N., R. 32 W., the place where the Hemlock
formation enters the part of the district studied by ine, the formation has a
width of one-half of a mile. From this place the formation has a north-
western coui'se for about 5 miles, gradually widening. It then bends to the
west, and after a short distance to the south, which course it follows for
about 9 miles. In township 45 N., Rs. 32 and 33 W., the belt has a maxi-
mum width of 5 miles. At the end of the southern course the formation
' Since tbe above w.as Tvritten I hare been informed that Mr. George .T. Maas, of Negaunee, has,
with a diamond drill, located a body of bessemer ore 30 feet thick on lot 6, sec. 20, T. 43 N., E. 31 W.,
1 mile sonth of the Mansfield mine.
74 THE CRYSTAL FALLS IRON-BEARING DISTRICT.
bends to the southeast, and continues with this general trend for about 16
miles into T. 42 N., R. 31 W., where my field study of it ended. At the
north the belt runs into the eastern half of the district described by Smyth,
and swings south, which course is followed for some 15 miles. The
entire belt thus foiius an oval suiTOunding the sedimentaries, except in the
southeastern part of the district. Another area of Hemlock volcanics is
found in T. 43, Rs. 32 and 33, just north of Crystal Falls. This area is
about one-half a mile wide just north of the city of Crystal Falls, but rap-
idly widens as it is followed to the west until at the western limits of the
area it is about 3J miles wide. A third small isolated area is found in sees.
17, 18, 19, and 20, T. 42 N , R. 32 W, and sec. 24, T. 42 N., R. 33 W.,
about 4 miles south of Crystal Falls.
The topography of the Hemlock formation is exceedingly rough where-
ever erosion has succeeded in cutting through the drift mantle. This occurs
only adjacent to some of the streams. The rough topography at these
places is due to differential erosion working upon rocks approximately on
edo-e, and of varying hardness. The valleys usually indicate the location
of beds of tuff and the higher grounds are almost universally occupied
by dense rocks forming the lava flows, or of the coarse-grained massive
intrusive rocks. In a few places, however, the thoroughly consolidated and
indurated tuffs form high hills. In traversing the Hemlock formation one
makes an abrupt ascent, followed by a sharp descent into a nan-ow swamp,
then another ascent, and so on. Exposures appear for the most part in
small areas along the edges of the swamps and scattered over the faces of
the hills. These are fairly numerous, but so small and disconnected as to
prevent the tracing out of the individual flows, although this might be pos-
sible if the traverses were made at very short intervals and the area mapped
in ffreat detail.
° THICKNESS.
As has been seen, the belt of eruptives varies in width from one-
half of a mile to nearly five miles. The dip of the rocks is about 75° W.
The enormous thickness of 25,500 feet which these data would give is
probably illusory.
In the case of the assumption of the thickness of a series of lava flows
and tuffs, it is important that the initial dip, which these deposits must have,
be considered. This dip varies greatly, depending on the slope of the
THICKNESS OF HEMLOCK FORMATION. 75
cone, whic'li iu its tiivn, is dept'iideiit on the viscosity of the lava and the
j)resenco of varying' (jnantities of fragmental jirodncts. If we assume these
pre-Cambrian volcanic products to have had an initial dip of 15'^, I lielieve
we are within limits for products consisting, as these do, of what was i)rob-
ably moderately viscous basalt and vast masses of fragmental material.
This estimate is based on the assumption that the volcanics here represented
were deposited for the most part upon the westward slope of a volcano, or
a series of volcanoes. This initial dip of 15° is then to be deducted from
the present dip, 75°, of the flows. Taking this into consideration, we get a
thickness of 23,000 feet for the volcanics.
It is highly probable that the rocks have been subjected to close
folding, and for this reason also the apparent thickness would be much
greater than the true thickness. The schistose character of some of the
rocks shows clearly that they have been severely mashed, and this mashing
was probably produced in connection with folding. It is probable that this
possible maximum thickness shoiUd be very materially reduced, jDOssibly
to one-half or one-third of the amount. However, even the maximum
above calculated is probably paralleled by the vast masses of volcanic
material accumulated in certain volcanic areas, such as those of Hawaii or
Iceland. Geikie writes:^
The bottom of these Iceland Tertiary basalts is everywhere concealed under the
sea. Yet their visible portion shows them to be probably more than .3,000 meters in
thickness.
An especial interest belongs to this Icelandic plateau because volcanic action is
still vigorous upon it at the present day.
RELATIONS TO ADJACENT FORMATIONS.
In the northern part of the Crystal Falls district the volcanics overlie
the quartzose dolomite formation known as the Randville dolomite. In
the central part of the district, through which the Deer River runs, as shown
in section G-H, PI. VI, outcrops are so scarce that it has been found
impossible to trace the boundaries of the formations with any degree of
accuracy. Consequently this part of the district is mapped as Pleistocene.
From the few outcrops of slate, probably equivalent to the Mansfield
slate, which have been found in the Deer River area, it has been thought
'The Tertiary basalt-plateaux of northwestern Europe, by Sir A. Geikie; Quart. Jour. Geol.
Soo. London, Vol. LII, 1896, p. 395.
76 THE CRYSTAL FALLS IRON-BEARING DISTRICT.
higlilv probable that this slate iu an extremely plicated condition may
underlie the volcanics of this area, and it is so represented in section G-H,
Plate VI. As evidence of this, in T. 43 N., R. 31 W. the volcanics
overlie the Mansfield slate unconformably.
In places test pits have disclosed an amygdaloidal lava flow immedi-
ately overlying the Mansfield slates. At one place, at the northeast corner
of sec. 7, T. 43 N., R. 31 W., angular fragments of the underlying black
slate have been found in the tufaceous deposits of the Hemlock volcanics.
Farther south, along the contact just west of the Mansfield mine, a con-
glomerate is exposed, which contains fragments of slate, lava, and rounded
grains of quartz with secondary enlargements. The rock is evidently
water deposited. There is also obtained from the workings of the mine a
conglomerate, taken from just alcove the ore, which consists of lava frag-
ments and pieces of chert and ore, as mentioned on pp. 64, 68. From these
occurrences it is clear that some of the sedimentaries are unquestionably
older than some of the volcanics, and yet the conglomerates bearing the
fragments of ore and slate contain also fragments of lava, showing the
existence of some of the volcanics before the deposition of this conglom-
erate. The only explanation of all of the facts which has occurred to me
is as follows: After the ore-bearing Mansfield slate was deposited, an erosion
interval occurred. Then followed a volcanic outbreak. It is highly
probable that this outburst began far north of the Man.sfield mine, coincident
with the upheaval which resulted in the erosion of the Mansfield slate.
The volcanic ejectamenta were mixed with the sedimentary fragments and
all together were rounded and bedded, forming in places conglomerates.
In places along the shore lava flows descended, some reaching into the sea
and covering the sedimentaries along the shore where no conglomerate had
been formed. At other places deposits of scoriae, etc., including fragments
of slates from the sedimentaries through which the volcano burst, were
made, and thus deposits of tuff are found overlying the sedimentaries.
The various deposits, though really separated by a slight physical break,
are practically conformable with the series below, all having a north-south
strike and a high westward dip.
The formations which underlie the volcanics in the northern and
southern parts of the district are of different character. This difference
RELATIO^TS OF HEMLOCK FORMATION. 77
niav be oxplaiucd by supposing the voleaiiocs broke out in tlie nortliern
])art, while the Manstielil shite was still being deposited in the south.
Gradually, however, the volcanic activity spread toward the south, proba-
l)ly following- a fissure along the pre-Cambrian shore, and igneous materials
buried the Mansfield slate. Hence, while on the whole these volcanics are
younger than the Mansfield slates, some of the lower of them ai-e con-
temporaneous with some of the upper Mansfield beds. The volcanics
invariably overlie the Randville dolomite, and are unque.stionably of later
age than that formation.
The Hemlock volcanics are overlain throughout their extent b}- the
Upper Huronian series of graywackes and slates. Near the contact line
with the volcanics wherever the Huronian outcrops, or has been exposed
by exploration, it has been found to be characterized bv a line of magnetic
attraction. By means of magnetic observations the line of contact has been
traced, where owing to lack of exposures it would have been otherwise
impossible to comiect the isolated outcrops.
RELATIOlSrS TO USTTRUSIVES.
High ridges composed of dolerite are found extending in a general north-
west and southeast direction through the volcanics. That these masses
were forced up through the Hemlock formation is indicated by the folding
which they cause in certain places. Such rocks are unquestionably younger
than the volcanic series. There may be seen also on the map, in T. 44 N.,
R. 32 W., a number of isolated knobs. These are also doleritic, and are
presumed to be, like the larger ridges, intrusive in the volcanics.
The dolerites have in their turn been cut by acid dikes. These are
coarse micropegmatitic granites. Similar acid dikes have been found
cutting the surrounding volcanics. This set of acid dikes may be looked
upon as the youngest intrusive igneous rocks occurring in the Hemlock
volcanic formation.
Cutting the volcanics are also basic dikes varying from fine to moder-
ately coarse grain. It is well known that during a volcanic epoch the out-
poured lavas and clastic volcanic deposits are penetrated by dikes coming
from the same magma. Whether or not these dikes are of this origin, and
are hence contemporaneous with the later volcanics, or are of later age, and
78 THE CRYSTAL FALLS IRON-BEAEING DISTRICT.
correspond to the coarse dolerites, it is impossible to determine with
certainty. They are presumed, however, to form an integral part of the
Hemlock volcanics, as no connection between the dikes and the unques-
tionably intrusive dolerites could be traced in the field.
VOLCANIC ORIGIN.
In spite of numerous occuiTences of ancient volcanics which have
recently become known, the late Professor Dana makes the following
statement:^
It is not yet certain that a volcano ever existed on tlie continent of Ifortli
America before the Cretaceous period; for the published facts relating to supposed
or alleged rolcanic eruptions in the course of the Paleozoic ages are as well explained
on the suppo.sition of outflows from fissures and tufa ejections under submarine
conditions; and none of the accounts present evidence of the former existence of
a volcanic cone, that is, of an elevation pericentric in structure made of igneous
ejections. .
The presence in the Hemlock formation of a quantity of pyroclastics,
great in proportion to the solid lavas, and the absence of any great sheets
of lava, so important a product of great fissure eruptions, seem to point to
the derivation of the Hemlock rocks from a volcano or volcanoes situated
near the border of the contemporaneous Huronian sea, rather than from a
simple fissure. While some of the eruptives may have been submarine,
the occurrence of large quantities of clearly subaerial deposits shows that the
eniptives were largely on the land. Thus it appears that neither a fissure
flow nor a submarine volcano will wholly explain the Hemlock formation.
However, it is hig'hly probable that this volcanic outburst, which piled
masses of volcanic material upon the land, was accompanied, as have been
all or nearly all the great outbursts of recent times, by submarine lava
flows and tuff" ejections. No such clear evidence of the presence of a Pale-
ozoic or pre-Paleozoic volcano on the North American continent has been
adduced as that given by the English geologists for certain volcanoes
in the British Isles. But while the j^i'esence of a central cone with peri-
centric arrangement in the Hemlock district is not conclusively j^i'oven,
the presumption in favor of such a cone or cones having existed is certainly
strong.
'Manual of Geology, by J. D. Dana: 4th ed., 1895, p. 938.
VOLCANIC OUIGIN OF HEMLOCK FORMATION. 79
An attempt was made to locate the vent or vents from wliicli the
material was derived, l)iit no evidence could be found, unless we consider
tlie vents to have been where the accunudations were the greatest. The
coarse-grained rocks which were first supposed to represent the plugs of
ancient volcanoes, on careful and detailed examination appear to be later
intrusives, or else are indeterminate.
CXiASSIFICATIOlSr.
The general character and distribution of the Hemlock formation hav-
ing been given, we may now proceed to a petrographical consideration of
the rocks comprising it. This will be given in more detail than for the
other rocks of the Michigamme district because this great pre-Cambrian
volcanic formation possesses peculiar interest.
The rocks of the Hemlock formation are chiefly of direct igneous
origin. Some interleaved sedimentary rocks occur, which, however, with a
single exception are composed of fragments of the igneous rocks. For the
sake of easy reference, the usual classification into igneous and sedimentary
rocks will be used. The massive igneous rocks are subdivided according to
chemical composition into acid and basic rocks. The acid rocks include
rhyolite-porphyries,' aporhyolite-porphyries, and acid pyroclastics. The
basic rocks include altered uonporphyritic basalts, porphyritic basalts, and
variolite and basic pyroclastics. The sedimentary rocks are divided into
the volcanic sedimentaries and the nonvolcanic sedimentaries or normal
sedimentaries. The first include tuffs and ash beds — the aeolian deposits,
and volcanic conglomerates — subacpieous deposits. The normal sedimen-
taries are represented by slates and limestones. Various schists are locally
produced from these numerous kinds of rocks through metasomatic changes
and dynamo-metamorphic action. Many of these schists resemble one
another very closely, though, as will be seen later, they are derived from
both the massive rocks and from the elastics. These have been described
in connection with the rocks from which they have been derived.
'According to a late riiliug of the Director of the United States Geological Survey, based on the
recommendation of a committee on nomenclature for geologic folios, "porphyry" is to be used only
with a textural significance. Hence "quartz-porpbjry," according to this ruling should no longer be
used as a rock name. The rhyolite-porphyries here described are what have been kuown as normal
quartz-poriihyries.
80
THE CRYSTAL FALLS lEONBEAKING DISTRICT.
The following table will show the arrangement outlined above, which
will be followed in the descriptions:
Classification of the rocks of the Hemloch formation.
Igneous .
. . , Uavas * Rhyolite^poiphy ry .... J gchistose acid lavas .
fAcid .. <'^''''""' ( Aporhyolite-porphyry . S
( Pyroclastics
( Nonporphyritic
( Lavas Metabasalt <; Poipbyritic
Basic . .' . ( Vaiiolitic
Pyroclastics . . Eruptive breccia
Volcauic sediments
Flow breccia .
Sedimentary
Normal sediments ,
) Eohan deposits J Ash beds
I Subaqueous deposits.. .Conglomerates
S Slate
( Liiuestone ,
Crystalline
schists.
ACID VOLCAlSriCS.
The acid volcanics are comparatively unimportant in quantity. They
may be conveniently subdivided into the lavas and pyroclastics.
ACID LAVAS.
The acid lavas occur in such small quantity as to make it impossible
without very great exaggeration to place them upon the accompanying
small-scale general maps, though they have been introduced upon the
detail maps wherever the scale permitted. They usually form isolated
ridges, and their relations to the surrounding basic volcanics are obscured
by lack of exposures. The trend of the individual ridges agrees with the
general strike of the banding in the basic tuffs. Moreover, in nearl}^ all
cases the isolated exposures which are closest together lie in such relations
to one another that when connected the large sheets thus formed follow the
strike of the tuff banding, as do the individual ridges, and they are there-
fore confidently assumed to be the isolated portions of acid flows inter-
bedded with the basic volcanic rocks.
The rock types represented are the two closely related rocks — the
rhyolite-porphyry and the aporhyolite-porphyry. Under the rhyolite-
porphyries are included the porphyritic acid lavas, which have, so far as
can be determined, an original holocrystalline groundmass. Under the
aporhyolite-porphyry, following Miss Bascom's use of cqm,^ I include those
1 structures, origin, and nomenclature of the acid volcanic rocks of South Mountain, Penn-
sylvania, by Miss Florence Bascom : Jour. Geol., Vol. 1, 1893, p. 816.
ACID V'OLGANICS OF HEMLOCK FORMATION. 81
acid lavas which are now hkewise hoh)cr)stalhue, but wliich owe this
cliaracter to the devitriHcatiou of an orig-iual ghissy base, supposing- tliem
in tlieir original vitreous condition to have corresponded to tlie modern
liyalorhyolite-porphyries.
RHYOLITE-PORPHYRY.
The rhyolite-porphyries on fresh fracture are dark grayish-blue to lilack.
From this the)' grade with advancing alterations through chocolate brown
to purplish. The weathered surface A^aries from white to reddish. The
weathering has in one case brought out very well the fluxion banding of
the rock. Tlieir texture is very pronouncedly porphp-itic. The quartz and
feldspar crystals stand out ^ilainl}- from the groundmass, which is usually
dense with a somewhat resinous luster. The porphyritic quartzes average
perhaps the size of a small pea, and hence are macroscopically very plainly
visible. They frequently stand out on the weathered surface and show
their cry.>-tal forms, and in other cases we see the angular cavities out of
which they have fallen, like the kernel from the nut.
Under the microscope the rocks are seen to be typical rhyolite-por-
phyries. The phenocrysts are chiefly corroded dihexahedral crystals of
quartz. Crystals of plagioclase and orthoclase are less common. These
lie in a very fine-grained holocrystalline groundmass, composed largely of
feldspar and quartz, with some zircon in small crystals, and here and there
magnetite.
These are presumed to be the original constituents of the groundmass.
Associated with them are considerable quantities of secondary chlorite,
epidote, biotite, muscovite, calcite, and reddish to brown alteration products
of the magnetite. Included in the groundmass are here and there oval
areas of finely crystalline secondary quartz, probably filling former
amygdaloidal cavities.
In thin section the crystal contours of the quartz phenocrysts are
more or less rounded, with here and there embayments of the groundmass
projecting into them. The crystal form is, however, always cleai'ly
marked. In some cases the individuals have been broken before the cool-
ing of the magma, the fragments of an individual, though now separated,
being seen to conform to one another. That they have been subjected to
23ressure is shown by the undulatory extinction and also by the separation
MON xxxvi 6
82 THE CRYSTAL FALLS IRON-BEARING DISTRICT.
of the black cross of uniaxial minerals into hyperbolfe. Embayments of
groundmass, and liquid inclusions in which a dancing bubble may be seen,
are in places rather thickly distributed through the quartz. The liquid
inclusions have very commonly an hexagonal form, corresponding to the
contours of the inclosing quartz.
These liquid inclusions are certainly in some cases secondary. This
character is well shown in some of the crystals, which are broken across,
giving along the line of fracture a very wavy extinction. Along this line
of fracture the greatest quantity of inclusions are seen, both with and
without bubbles. As the distance from a fracture increases, both the undu-
latorA^ extinction and the number of inclusions diminish. (See fig. A,
PL XIX.) These fractures in the quartzes are but continuations of those
which extend in many cases all the way across the section. The fractures
have since been healed by secondary quartz. This secondary quartz has
also in some cases healed the fractured quartz phenocrysts, and then agrees
with them in orientation. The possession of an imperfect rhombohedral
parting is very noticeable in a number of quartzes, and especially those
wdiich, being on the edge of the section, are very thin. (See fig. B, PI.
XIX.) Similar parting in the quartz occurs in various rocks studied in this
district.
The phenocrysts of the porphyries are traversed by fractures, some of
which are more or less circular, and simulate very imperfectly perlitic cracks.
With the exception of those in porphyries in two localities, the quartz
phenocrysts are surrounded by zones, largely of quartz, of varying widths,
and considerably lighter than the remainder of the groundmass. Much of
the quartz of these zones has the same optical orientation as the phenocrysts.
In those sections in which the zones are observed they occur around every
section of quartz.
The feldspar phenocrysts are orthoclase and plagioclase, the latter
apparently predominating. They occur usually in rounded, badly corroded
crystals, with indentations filled with groundmass. They are always altered,
and have associated with them as secondary products calcite, epidote,
muscovite, biotite, and chlorite
No large original ferro-magnesian phenocrysts appear to have been
present in the porphyry. Their former presence is at least not indicated
by any aggregates of secondary products. Whatever ferro-magnesian min-
ACID VOLOANICS OF HEMLOCK FOliMATlON. 83
erals were preseut must have been scattered tlin)Ugli the <>Toiin(lniass, and
have been completely altered. The secondary minerals contained in the
groundniass are chlorite, calcite, epidote, niuscovite, and biotite.
TEXTCRE OF THE I'OUrilYpRIES.
The texture of the dense groundniass varies according to the mode of
association of the two chief minerals — quartz and feldspar. The commonest
variety is the rhyolite-porphyry with microgranitic groundmass (porphyre
granulitique of Michel Ldvy). A second variety is the rhyolite-porphyry
with micropoikilitic groundmass.' The microgranitic texture is too well
known to warrant a description of it here.
The micropoikilitic texture presents certain characters which render a
further description desirable. This peculiar phase of the micropoikilitic
textm-e was briefly described by the writer and illustrated by microphoto-
graphs in 1895.^ Shortly after the separates of this article were disti-ibuted,
I received from H. Hedstrom, of the Swedish Geological Survey, an article
published in 1894 containing a description of what appears to be very
nearly the same texture.^ If I have understood the description correctly,
however, there seems to be one essential difference. In order to explain
clearly this difference, I shall describe the textm-e in detail.
In certain of the rhyolite-porphyries, as already mentioned, the quartz
phenocrysts are surrounded by certain zones. These zones in the rocks
having a micropoikilitic texture possess exactly the same texture as does
the groundmass. The zones are composed of minerals which are of suffi-
cient size to permit readily their determination. Quartz and feldspar are
the essential components, with some chlorite, epidote, muscovite, and iron
oxide. The first two are the important minerals, and will alone be referred
to in the further description. The chief peculiarity of the zone is in the
arrangement of the two minerals, and this character is best shown on the
accompanying microphotographs. This textm-e can be seen even in
ordinary light. It is brought out better when the field is partly shaded, so
' Eruptive rocks of Electric Peak aud Sepulchre Mountain, by J. P. IiUlings : Twelfth Anu. Eeirt.
U. S. Geol. Survey, 1891, p. 589. On the use of the terms poikilitic aud micropoikilitic in petrography,
by G. H. Williams: Jour. Geol., Vol. 1, 1892, pp. 176-179.
- Volcanics of the Michigamme district of Michigan, by J. Morgan Clements: Jour. Geol., Vol
III, 1895, pp. 814-816, flgs. 1 and 2.
» Studier iifver Bergarter fran Moriin vid Visby, by H. Hedstrom : Geol. Foren i Stockholm
Forhandl, Bd. 16, H. i, 1894, pp. 5-9.
84 THE OKYSTAL FALLS IRONBEAKING DISTRICT.
as to exhibit the difference in reHef of the minerals, and, best of all, between'
crossed nicols. (See fig. A, PI. XX.) The zones are seen to be made up
of reticulating areas of clear quartz, in which lie embedded iiTegular pieces
of feldspar. Where two or more of the quartz stringers or needles unite,
one sees broad ai-eas of limpid quartz. The network of quartz is best seen
when it exhibits its highest polarization color, as then the feldspar is for the
most part dark. The pieces of feldspar in such a quartz area for the most
part have irregular orientation, as is shown by their varying extinction,
although a number extinguish simultaneously. This quartz net is connected
with the quartz phenocrysts, as shown by the continuation of the quartz of
the phenocrysts and that of the zone, and the consequent agreement in
orientation. The lack of a uniform optical orientation of the feldspar grains
is made especially apparent when the quartz is cut perpendicular to the c
axis, and consequently remains dark under crossed nicols. Under the
above circumstances we see certain feldspar grains polarizing in the zone
around the quartz, and as the stage revolves other particles lighten as those
which polarized in the previous position of the stage become dark. From
this description it is evident that the texture is not micropegmatitic according
to the generally accepted definition of the term, but corresponds to the
micropoikilitic, as described by Iddings and Williams.^
A gradation toward a spherulitic texture was noticed in one instance
where a number of long quartz stringers were arranged pei-jjendicular to
the periphery of the quartz phenocryst. (Fig. B, PI. XX.) The texture
of this micropoikilitic mass, it will be observed, is finer than that before
described.
The groundmass of the porphyries is formed of irregular roundish
areas having exactly the same micropoikilitic texture as the zones surround-
ing the quartz phenocrysts. An explanation of the origin of the zones
should therefore also explain the texture of the groundmass. Certainly in
many cases, probably in most cases, the groundmass areas result from
tangential sections through one of the micropoikilitic zones surrounding
the quartz phenocrysts.
The description given by Hedstrom^ of this same structure as observed
by him is essentially the same as the above, if I have understood him cor-
rectly. The following difference is, however, to be noted. In speaking of
1 Op. cit., pp. 589 and 179. ^Op. cit., p. 8.
ACID VOLCANICS OF UEMLOCK FORMATION. 85
t\w stnic'turt' where tliu (iiuutz i.s siuTuunded by this luicrojjoikilitic zone, he
calls it the graiiophyric structure. As I liave already emphasized alcove,
the feldspars in the network of (piartz have varying orientation, and tlie
structure is, strictly speaking, micropoikilitic, and in no sense granophyric
(micropegmatitic). Moreover, he describes in addition to tlie above type
one in which are found i)henocr}'sts of quartz lying in a micropoikilitic
groundmass with the above reticulating texture, but the phenocrysts abut
sharply against the groundmass, instead of being connected with it by
means of these zones.
The micropoikilitic texture has been held in some cases to be of sec-
ondary origin and the result of devitrification. While recognizing that
there may be certain unquestionable cases where a ixiicropoikilitic structure
results from the devitrification of a glassy groundmass, I can find no evi-
dence in the rocks here described that points to this origin for the micro-
poikilitic texture under discussion. On the other hand, there is an absence
of evidence that indicates its unquestionably primary character. Rather
than to regard the quartz as secondary and influenced in its orientation by
the phenocrysts, as in the enlargements of quartz grains, it seems natural
to suppose that when the lava was extruded after the crystallization of the
phenocrysts, there began, consequent upon the diminished pressure and
temperature and other factors, a rapid crystallization of the mineral elements
from the remaining magma. This resulted in the production of the feldspar
in very imperfect and small crystal individuals. At the same time the quartz
of the phenocrysts continued to grow, and in so doing inclosed these small
feldspars in its meshes.
In certain rhyolite-porphyries the micropoikilitic texture is somewhat
different from that above described. In these the quartz phenocrysts are
surrounded by zones which are illustrated in figs. A and B, PI. XXI.
These appear to correspond very closely to the ones described by Michel
Ldvy^ and VViUiams,- and since described by many other writers. The
zones have a much higher index of refraction than the quartz of the
phenocrysts, and hence contrast strongly with it. Examined closely,
they are seen to be composed of chlorite, epidote, and black or reddish
'Annales des Mines, Vol. VIII, 1875, pp. 378, 381.
=iDie Eruptivgesteiue der Gegeucl vou Tiiberg im SchwarzwaUl, by G. H. Williams: N. Jahrb.
fill- Jliu., Bil. II, 1883, p. 605.
86 THE CRYSTAL FALLS lORN-BEARING DISTRICT.
ferruginous grains, which He in a white matrix. This matrix shows the
following characters: The greater part of it extinguishes and lightens
simultaneouslv with the quartz phenocrysts which it surrounds, and is
consequently believed to be quartz. When the matrix and quartz pheno-
crysts are dark, one sees scattered through the matrix, making up a very
small proportion of the total zone, certain irregular areas which show
polarization effects. These are believed to be feldspar grains, though this
could not be determined. With the highest magnification no radial
arrangement of the quartz and feldspar could be observed which would
wai-rant the inclusion of these aureoles under Michel Levy's term " sjjhero-
lites a quarts (jlohidaire."^ Where two quartz crystals with different orienta-
tion are in juxtaposition, each possesses its own zone corresponding with it
in orientation. The way in which the zones about the quartzes are confined
to the quartz is clearly shown in one case in which a very much altered
feldspar phenocryst was found, one portion possessing a typical coarse
microjDCgmatitic texture. In this case where the quartz of the micropeg-
matitic intergrowth touches the groundmass, it grades into a micropoikilitic
area, whei'eas the feldspar does not do so.
The texture of the zones about the quartzes is apparently but a fine-
grained variety of the micropoikilitic texture, the coarser phases of which
are illusti'ated on PI. XX.
The groundmass of the rocks showing the texture is composed of
roundish areas of exactly the same composition as the zones around the
phenocrysts, with a feldspar of small dimensions here and there between
these areas. (See fig. B, PI. XXI.) The texture approaches very closely
if it does not correspond exactly to the quartz ^pongeuse phase of the
quartz-globulaire texture of the French." In one part of a section of rhy-
olite-porphjTy the quartz phenocrysts have aureoles and the groundmass
has the texture just described. In another portion of the section the quartz
phenocrysts have no aureoles and the groundmass possesses an imperfect
microgranitic texture (structure microgranulitique of Michel Ldvy). This
shows the passage of a micropoikilitic textured rock into one with a micro-
granitic texture. I explain the aureoles and the roundish areas in the
' structures et classification des rocbes 6rui)tives, by A. Michel L(?vy, Paris, 1889, p. 21.
= lt is found to show exactly the same texture as seen in a section obtained from Paris and
labeled " Porjihyre ;i quartz globulaire de la Sartbe.''
ACID VOLGANICS OF HEMLOCK FORMATION. 87
groundmass as original, in exactly the same way as has been suggested hy
Williams' tor those which he described. This is essentially the same expla-
nation which I have given on a previous page for the less common, coarse
micropoikilitic phase. The cause of the formation of the microgranitic
phase appears, however, rather difficult to discern. Its development seems
to depend upon peculiar local conditions.
APORHYOLITE-PORPHYKY.
Intimately associated with the rhyolite-porphyries are rocks very similar
to them in mineral constituents, both macroscopically and microscopically,
so that the description of the rhyolite-porphyries will largely answer for
the aporhyolite-porphyries. Flow texture, however, is well shown by the
aporhyolite-porphyries. A beautifully developed perlitic parting, fig. A, PI.
XXII, is taken to indicate the presence of an original glass; hence the rocks
are classed with the aporhyolites. The perlitic cracks are well brought out
in ordinary light by the chloritic flakes along them. Between crossed
nicols these disappear, and the groundmass resolves itself into a fine-grained
mosaic of quartz and feldspar. (Fig. B, PI. XXII.) This groundmass has
all the characters of that of a microgranite.
No evidence wdiich would point to the devitrification of a glass could
be seen other than the presence of a perlitic parting, as described. For
recent excellent descriptions of similar devitrified lavas in which various
structures characteristic of vitreous lavas have been identified, the reader is
referred to the articles already mentioned, and the one by Dr. Bascom,- in
which a moderately full bibliography is found.
SCHISTOSE ACID LAVAS.
The results of the ordinary alterations of the acid lavas, chiefly meta-
somatic in character, by which the phenocrysts and the matrix ha^•e been
changed, have been briefly described. The results produced by dynamic
action are more interesting and perhaps more striking. The mashing, result-
ing in chemical changes and schistose structure, has in many cases almost
obliterated the porphyritic texture, and in extreme cases destroyed all inter-
nal evidence of igneous origin. Even the fluxion banding, as is well known,
at times simulates very closel}" sedimentary bedding, and thus increases
'Op. cit., p. 607.
-Acid volcanic rocks of South Mouutain, by Dr. Florence B.ascom: Bull. U. S. Geol. Survey No.
136, 1896, p. 87.
88 THE CKrSTAL FALLS IRON-BEARING DISTRICT.
the difficulty of determiniug the igneous character of the rock. In the rocks
to be described the phenocrysts may still be observed, though more or less
deformed, and the fluxion banding has been in one case exceptionally well
preserved, so that no doubt is felt as to their igneous character. Dynam-
ically metamorphosed rhyolite-porphyry flows have been found in two areas
in the Hemlock formation. In the following each area will be described
separately, the one in which the original character of the porphyry is least
in doubt being considered first.
The Deer River schistose porphyries are found in the SE. J sec. 36, T.
44 N., R 32 W., beginning at 400 N., 250 W., and continuing to 600 N.,
350 W., of the southeast corner near the bridge on the Floodwood road.
They occur in several outcrops which are practically continuous, being
separated by very short distances, and are so much alike both macroscopic-
ally and microscopically that there is sufficient reason for the conclusion
that they belong together. Their field relations to other rocks have not
been observed. No data have been found which offer any clue as to the
time of eruption of these rocks other than the fact that they are surrounded
by the basic volcanics of the Hemlock formation and have undergone the
same dynamic action.
The porphyries are dense, bluish-black rocks, with porphyritic crystals
of red feldspar. A fluidal structure is not present in them. The schistose
structure is apparent to the eye, especially upon the weathered surface, and
the cleavage of the rock also indicates it. The cleavage face of the rock
has a silky luster, due to the sericite and biotite flakes parallel to it. The
rock breaks readily in various directions at angles to the cleavage, so that
it is impossible to obtain well-shaped hand specimens. The schistosity in
these porphyries is clearly brought out by weathering, the weathered rocks
showing perfect schistosity, while fresh specimens from the same exposure,
although splitting easiest in one direction, appear perfectly massive in hand
specimens when broken across the schistosity. That the dynamic action
was greatest along certain zones of the rock, other portions being more
or less exempt, is shown by the fact that of several specimens collected
with the view of obtaining different stages of alteration from different
portions of the same exposure some are markedly schistose, while the least
altered approach a fairly massive character.
I shall give a brief description of tliis least altered phase, and then
AOID VOLGANIGS OF HEMLOCK FORMATION. SB"
cou.sider the cliiuigx's wliicli liuve tukeii place and tlie character of the
rock which has resulted in the more altered phases.
The slightly schistose rock, like all the porphyries, is very fine gi-ained
and l)lack, with a more or less silky luster on fresh fractures parallel to
the schistosity. The porphyritic character is not very strongly marked.
Maotroscopically, comparatively few small feldspar phenocrysts are visible.
Under the microscope the rock is seen to be a micropegmatitic rhyolite-
porphyry in which the silica has not crystallized as quartz phenocrysts,
bi<it has remained in the groundmass. The feldspar phenocrysts are both
orthoclase and plagioclase. The latter shows its usual characters, but is
iK)t present in well-formed crystals. The orthoclase, on the contrary, is
well crystallized, occurring iii Carlsbad twins. While some of the feldspar
crystals are broken, they as a rule do not show many signs of pressure
The fine-grained micropegmatitic groundmass is made up of the quartz and
feldspar intergrowth and of secondary mica, both muscovite and biotite,
and remnants of iron oxide. Micropegmatitic intergrowths of quartz and
feldspar occur in irregularly shaped areas which frequently have a fairly
large quartz at the center. Very similar irregular areas which seem to be
composed altogether of unstriated feldspar also occur. These two kinds of
areas compose the greater part of the rock. The mineral particles fre-
quently show undulatory extinction. Between the micropegmatitic inter-
growths one finds here and there granular aggregates of quartz and striated
and unstriated feldspar. These feldspar grains, and likewise the feldspar
intergrown with the quartz, are considerably altered. Sericite and biotite
are present in considerable quantity. The former possesses the better crys-
tallographic outlines, the biotite being usually found in ragged fragments.
The two micas occiu- in the feldspars and lie between the quartz grains, but
not in them. They appear to be secondary products from the feldspar.
The micas lie with their long directions approximately parallel, and impart
to the rock its schistose character. A few automorphic crystals of apatite
were found. There occur also a few irregular grains of a dark reddish
brown mineral with high single refraction, but which is isotropic. This
mineral is presumed to be allanite, though conclusive tests could not be
made. Some crystals of zircon were also observed. The iron oxide is evi-
dently titaniferous, probably titaniferous magnetite. Secondary calcite is
scattered through the rock in considerable quantity.
90 THE CEYSTAL FALLS IROiSI -BEARING DISTRICl.
In a more altered phase of the porphp-}' exhibited in a number of
specimens, the schistose structure is much better marked both macroscopic-
ally and microscopically. The macroscopical appearance is otherwise
quite similar to the one just described. Under the microscope the pheno-
crysts show up well. These are rounded and shattered orthoclase and
plagioclase feldspars. They lie usually with their long direction in the
lines of marked schistosity of the rock. The larger crj'stals have been
much more generally fractured than have the smaller ones, and seem to
have obtained relief from sti-ain in that way, the individual fragments not
showing very strong dpiamic effects. The small crystals are more or less
rounded. The crushing to which the rock has been subjected has severed
the fragments in a number of cases. Triangular areas on two sides of the
broken or unljroken feldspars in the direction of schistosity are filled with
what appear to be secondary quartz and flakes of biotite. The feldspars
as a whole have undergone considerable chemical changes, the freshest
being red and very cloudy. Those more altered show secondary muscovite
and biotite scattered through them. The character of the triclinic feldspar
could not be determined. It appears, however, to be very rich in calcium,
as in some of the badly weathered sections the feldspar fragments may
almost be said to lie in a calcite matrix, resulting appai-ently from the
alteration of the feldspar and not from infiltration. No quartz phenocrysts
retaining their normal character are found. There occur here and there,
however, small rounded mosaics of quartz, the individual grains of which
show undulatory extinction. These are e^^dently the result of the granula-
tion of quartz grains, such as occur in the freshest specimens. It is well
known that the quartz is more easily affected by pressure than feldspar,
and Futterer^ has shown that they may be found in a completelj^ crushed
condition, in the same section with feldspars which still retain their regular
crystal contours.
The groundmass of the poqihp-y is made up of quartz and feldspar,
in and between which lie leaflets of biotite and sericite. The holocrystalline
granular mixture of quartz and feldspar is very fine grained, and the pres-
ence of the feldspar was only determined by difference in the refraction of
the two minerals. No striated feldspar grains were observed. The second-
1 Die "Ganggranite" von Grosssachsen, und die Quartzporpbyre von Thai im Thiiringerwald,
by Karl Fiitterer, Heidelberg, 1890, pp. 31, 126.
ACID VOLCANICS OF HEMLOCK FORMATION. 91
ary micas appear usiuiUy in ragged flakes, though the shglitly greenish-
yeUow sericite flakes approach crystal nntlines rather frequently. The
biotite is lm>\vnish-green and strongly i)leochroic. A few spots of brown
iron hydroxide and small heaps of grains of sphene probably indicate the
former presence of titaniferous iron ore. The few crystals of apatite present
are broken and separated, but otherwise retain the usual characters of this
mineral.
The groundmass has a very marked schistose structure, brought out
especially well by the i)arallel arrangement of the mica flakes. The way
in which these lines of schistosity flow around the mashed phenocrysts, one
line never coalescing with another, but remaining continuous, may be seen
with great distinctness where the lines abut sharply against the crystal at a
very obtuse angle. As the angle becomes less and less obtuse, the ends of
these lines bend up slightly in the direction which would enable them to
pass the crystal, and then end, so that along the face of the crystal one can
follow them, as it were, in a series of steps until those lines which strike the
crystal near enough the edge to flow around it bend slightly, and passing
around continue on the opposite side. The fact noted by Fiitterer^ that an
increased amount of sericite occurs on the two sides of the feldspar crystals
parallel to the schistosity is very patent in these porphyries. The diminu-
tion in grain of quartz and feldspar seems to accompany the increase in the
amount of the sericite. The slides are crossed by narrow fractures cutting
the planes of schistosity, which are filled with secondary quartz, showing
marked sti-ain efi"ects. Associated with the quartz were observed some
crystals of brown nitile. In one of the more altered slides these fissures
have been filled with calcite, whether or not as a replacement of the quartz
could not be told.
Schistose porphyries showing the extreme alteration phases are found
from N. 300, W. 300, to N. 400, W. 250, in the SE. i sec. 4, T. 44 N., R.
32 W. They form a rough escai-pment upon the southeast side of and near
the base of a large hill, and overlook McCutcheon's Lake. The exposure is
not continuous throughout, though practically so, but the unexposed parts
are sufiicient to prevent a perfect sequence being traced. The appearance
of the rock is strongly like that of sedimentary rocks. Difterent bands
nearly on edge may be seen, dipping 60°-90° SW. and striking N. 30° W.
I O]). cit., p. 40.
92 THE CEYSTAL FALLS IRON-BEARING DISTRICT.
At a point about 100 feet higher and three-fourths of a mile distant, oq
the very northwest flank of the same hill, at N. 1725, W. 775, from the south-
east corner of sec. 4, T. 44 N., R. 32 W., there is a small ledge of schistose
jDorphyry, macroscopically and microscopically similar to those to the south-
east of it, and with its schistosity striking N. 20° W. and dipping 80° SW.
The striking agreement in strike, dip, and general character of these two
separated outcrops points to their being merely isolated portions of the
same mass. There seems to be no discrepancy between the dip and strike
of the schistosity and that given above for the bands.
The most striking macroscopical characteristic of these mashed por-
phyry flows is the occuiTCUce of phenocrysts in a schistose and beautifully
banded rock. These phenocrysts stand ont clearly from the groundmass^
in all cases. The general appearance of the rocks is that of the well-known
very dense banded halleflintas of Elfdalen, Sweden. The bands vary in
color, ranging on the weathered surface from light creamy white, tln'ough
light greenish, to red and almost black. Tlie rocks which have very light
colored weathered surfaces are always bluish black on a fresh fracture, and
very dense, and those weathering red are usually cream colored on freshly
fractured faces; Many of the areas which appear macroscopically to be
single phenocrysts are resolved under the microscope into tangled groups
of individuals, though in rare cases the individuals show the imperfect
radial arrangement rather frequent in medium-grained micropegmatitic
rhyolite-porphyries.
The feldspar has iindergone considerable alteration. In the least-
changed grains there is a cloudiness caused by numerous indeterminable
specks, probably of iron oxide, which give a reddish tinge to the mineral.
Further changes result in the production of muscovite and epidote, with
biotite in rare cases, accompanied by the obliteration of the twinning
lamellse. The greater part of the phenocrysts seem to be orthoclase,
though associated with them are found pieces which show indistinct traces
of the polysynthetic twinning of plagioclase feldspar. The feldspars
exhibit marked strain eff'ects, especially in their flattening into long oval
and spindle-shaped areas. Some crystals have been broken and separated
perpendicular to the direction of the schistosity. The spaces between the
fragments are filled with secondarj- muscovite, quartz, and feldspar. Sur-
rounding the phenocrysts — that is, between the phenocrysts und the ground-
ACID VOLOANICS OF HEMLOGK FOKMATION. 93
mass proper — we find a mass of small angular, finely striated, limpid grains
of feldspar, associated with similar grains of quartz, tlie two lumng in
places between tliein sericitic fiakes. In one especiall}- clear case, this
secondary aggregate fills half the space formerly occupied by an individual
feldspar, the other half being still occupied In^ the remnant of the appar-
ently simply twinned feldspar from which it was derived. (Fig. A, PI.
XXIII.)
While no large quartz phenocrysts were observed, a mosaic of qviartz
is found in small round or oval areas in various sections. The individual
fragments exhibit the usual strain effects of crushed minerals. (Figs. A, B,
PI. XXIV.)
The groundmass consists of the same preponderant minerals as the
scliistose porphyries, which have been previously described. The accessory
minerals are apatite, which is present in very small quantity, and rutile,
which in one of the slides is present in verj' considerable quantity in the
form known as "thonshiefer-nadelchen." Calcite is found in all of the
slides, the amount varying very much. Those which contain a great deal
have a scoriaceous-looking surface, due to weathering out of the calcite.
The flow structure mentioned as having been observed in the schistose
porphyries of the Hemlock formation is perhaps of sufficient general interest
to warrant a few comments. This is well marked only on one hand speci-
men. In this there is an alternation of pink and dark grayish-blue bands
which are rarely more than a fraction of an inch thick. Some, especially
the thicker bands, are remai-kably persistent. Even macroscopically on the
weathered surface the pinkish bands can be distinctly seen to wrap around
the pink feldspar phenocrysts and oval areas of the grayish-blue part of the
rock. Under the microscope the bands which macroscopically are the
darkest are clear and transparent, while the pink bands are much less trans-
parent. The microscope shows the diff"ereuce in the color of the bands to
be due chiefly to tlie fineness of grain, and brings out the flow structure
even more clearly than the weathered surface. (Figs. A and B, PI. XXIV.)
Accompanying this variation of grain there is also a diff"erence in mineral-
ogical composition. The dark bands are composed essentiall}- of quartz
grains, with feldspar, sericite, some magnetite, considerable calcite, and
rare crystals of apatite and rutile, the quartz including- many black and
indeterminable specks. The pink bands are very fine grained, so much
94 THE CRYSTAL FALLS IRO:J^-BEARING DISTEICT.
so that tlie clear white mmeral grains composing it can uot be determined,
though probably both quartz and feldspar are present. These bands are
darkened by innumerable black indeterminable specks and long rutile
needles, with a small amount of biotite. It is possible that some of the
minute biotite flakes have been mistaken for rutile needles when viewed on
edge, but it is certain that( these bands contain a great deal more rutile
than do the others. Whether or not there is a still more essential chemical
difference between the bands than that indicated by the increased quantity
of rutile, was not determined.
It has become more or less common of late to attribute the banding
found in metamorphosed eruptives altogether to the pressure to which they
have been subjected. In the present instance I can not but consider the
banding as being an original fluxion structure, with the slight original
difterences between the bands emphasized, as it were, by subsequent
pressure. It appears highly probable that the rock was originally more
or less glassy and showed a flowage structure, and that the present miner-
aloo-ical character of the groundmass is due to the process of devitrification
wliicli did not destroy the banding of the original glass.
ACID PYROCLASTICS.
The only acid pyi-oclastic rock found was formed from the aporhyolite-
porphyry. This is a true eruptive breccia. The fragments are angular to
rounded in shape, weather to a pure white color, and have an exceedingly
rough surface. This roughness is due to a great extent to perhtic partings,
which are macroscopically visible, and give the rock an almost scoriaceous
appearance. Other inequalities on the surface adding to its rouglmess are
caused by the leacliing out of feldspars, and by the fact that many of the
quartz phenocrysts have fallen out of the inclosing matrix The fragments
are all aporhyolite-porphyry, containing a very large proportion of quartz
and feldspar phenocrysts. The cement of the breccia is aporhyolite. This
contains far less numerous phenocrysts, and is, therefore, on the whole much
finer grained than the fragments. The weathered surface of the cementing
aporhyolite appears a bluish gray, and is very smooth compared to the
scoriaceous appearing surface of the fragments ah-eady described. Tliis dif-
ference in weathering shows the brecciated character admirably, as the finer-
grained matrix stands out sharply and delimits the contours of the fragments.
BASIC VOLCANICS OF HEMLOCK FORMATION. 95
Movi'iiK'Hts of the inayina are shown by a flowage sti'ucture in the
matrix and by the fracturing of the quartz and feldspar phenocrysts and
separation of the pieces in ))oth the cement and fragments. (Fig. B, PI.
XXIII.)
This eruptive breccia can be seen in its best development in the NW.-
SK. trending ridge, just west of the small lake, crossed by the Chicago,
Milwaukee and St. Paul track in sec. 32, T. 44 N., R. 32 W.
BASIC VOLiCANICS.
The basic volcanics are considered under the main headings of lavas,
pyroclastics, and Bone Lake crystalline schists.
BASIC LAVAS.
GENERAL CHARACTERS.
The basic lavas are so very characteristically developed that no one
could for a moment doubt their trae natm-e, even upon the most superficial
examination. One of the nearly general characters is the presence of a
well-marked amygdaldidal texture. (Figs. A^ B, Pis. XXV and XXVI,
and fig. A, PL XXVII.) Some of the lavas are so full of amygdules that
they may be correctly said to have been scoriaceous. The amygdaloidal
portions of the rock masses — which may be considered the surface parts —
grade over into other portions, the interiors of the lava flows, which are,
macroscopically at least, nonamygdaloidal. Owing to the homogeneous
character of the basic magmas, a fluxion structure is rarely shown macro-
scopically, though microscopically it may be more or less well developed.
Columnar jointing was nowhere observed. An ellipsoidal parting, on the
other hand, is common.
NOMENCLATURE.
In a preliminary article on the Hemlock volcanics,^ I made a brief
mention of the occun-ence on the Upper Peninsula of Michigan of the
basic pre-Tertiary equivalents of the post-Tertiary and Recent family of
basalts. Following the Danas, Wadsworth, Williams, Iddiugs, Kemp,
Darton, and Diller, some of the most influential of the men who, in the
' The volcanics of thj Michigamme district of Michigan, by J. Morgan Clements: Jour. Geol.,
Vol. Ill, 1895, pp. 801-822.
96 THE CRYSTAL FALLS IRON-BEAEING DISTRICT.
United States, have advocated the simplification of petrogTaphical nomen-
chxture, I used the term basah, now ordinarily used for the Tertiary or post-
Tertiary basic rocks. This term was, however, modified by the prefix
"apo," as indicating their altered condition and the presumed presence of a
glassy base.^ This was a logical continuation of the use of the prefix as
proposed by Dr. Bascom' for devitrified acid lavas.
More detailed studies upon the Hemlock volcanics have shown the
presence of rocks which were apparently originally holocrystalline, and
therefore do uot belong with the altered vitreous basalts, the apobasalts, and
others in which some of the glass is apparently unaltered. Consequently,
since the apobasalts comprise only a portion of the Hemlock volcanics, the
replacement of that term as a general heading by the older, more general,
one of basalt was considered.
The use of this term is, however, not altogether satisfactory, for the
rocks, while clearly recognizable as basalt derivatives, do not possess the
mineralogical composition of the basalts. The term "apo" having been
restricted, as above indicated, can not be applied to them, for their altera-
tion is in many cases metasomatic and dpiamic, "and in most cases not
devitrification. If we adopt the prefix "meta" to indicate alteration of all
kinds, then these rocks could be called "metabasalts."
The terms "metadolerite," "metadiabase," etc., were proposed by Dana^
for metamorphic dolerites, diabases, etc., and first used by Hawes* in the
description of the altered rocks around New Haven. Recently these tenns
have been revived, but with a very diff"erent significance from that with
which they were first used. It is proposed to designate bj^ such terms
"rocks now similar in mineralogical composition and structm-e to certain
igneous rocks, but derived by metamorphism from something else."^ Fol-
lowing this suggestion, an uralitized dolerite (diabase) would be called a
metadiorite. Such a use of the term does not seem justified, and the
1 Loc. cit., p. 805.
'^The structures, origin, and nomenclature of the acid volcanic rocks of South Mountain, by
Florence Bascom. Jour. GeoL, Vol. 1, 1893, p. 82s.
3 Chloritic formation of New Haven, CoDuecticut, by J. D. Dana : Am. Jour. Sci., 3d ser., Vol. XI,
1876, pp. 119-122.
••The rocks of the "chloritic formation" on thevresteru border of the New Haven region, byG. W.
Hawes : Am. Jour. Sci., 3d ser., Vol. XI, 1876, pp. 122-126.
'■ On a series of peculiar schists near Salida, Colorado, by Whitman Cross : Proc. Colo. Sci. Soc,
g). 6, footnote. Paper read Jan. 2, 1893.
BASIC VOLOANIOS OF HEMLOCK FORMATION. 97
ol))i'C'ti(iii to it (•iiu not he g'wvn butter tliau by (juotiug the words wliicli
Zirki'l uses in the disoussion (tf the uietamorphisni of rocks:'
Bei solcheu nietaraorphiscli veriiiulerten Gesteinen ist es nicht zweckmilssig, sie
niit dem Nameu desjeiiigeu Typus zu belegen, dein sie durcb die Veriiriderung iilinlich,
oft bios scheinbar iihiilicli gewordeu siiid. Eiiie solclie Bezeichnuiig werde iiur za
inissverstiiiidliclien Autt'assuug der von dem (iestein gespielten geologisclieu Eolle
fiihren, welcbe niemals ausser Acht gelasseii werden darf. Und so ist es deiin ent-
scUicdenvorziizielien, der Beneiiiimig soldier Gesteineeiue Form zu gebeu, in wlielcber
zuviirderst auch zuin Ausdruck koiiimt, was sie friiber gewesen sind, and nicbt eiueu
Nauieu zu wiibleu, der sie iu erster Linie zu etwas stem pelt, mit welcbem sie genetiscb
keine Gemeiuscbaft baben.
Using these terms in the way suggested by Cross, attention is most
pointedly directed to that variety of rock which the secondary product now
resembles mineralogically, rather than to the type from which it was derived,
and which in all likelihood it still resembles most closely in its chemical
constitution. Whether or not a petrographer will use the term "metadio-
rite" or the term "metadolerite" (diabase) for a metamorphosed dolerite
will depend on whether or not he prefers to emphasize the present miner-
alogical composition of the rock, or its original characters, and thereby its
chemical constitution and genetical relations. In the present report the
terms "metabasalt" and "metadolerite" are used as including all those
altered rocks which demonstrably were originally basalts and dolerites.
These same strictures hold good in the case of Giimbel's term "epidior-
ite," when used, as it is very commonly, in the literature of the Lake
Superior region and elsewhere, for rocks avowedly derived from dolerites
(diabases), and characterized by the presence of fibrous secondary amplii-
bole. It is preferred, in accordance with the above statement, to use the
term "epidolerite" (epidiabase) instead of "epidiorite" for such altered
dolerites. None of these rocks, unless extremely changed, would resemble
chemically a diorite, and we have come of late years to rely more and more
upon the chemical composition, combined of course with the mineralogical
composition and textures of the rocks, to separate the vanous kinds from
one another. As stated above, the term "epi," associated with the rock
name, has come more and more to be restricted in its use solely to a rock,
the epidiorite, characterized by a specific alteration product, the amphibole.
In respect to this restriction to specific alteration, the term corresponds to
' LehrbucU der Petrographie, F. Zirkel: 2(1 ed., Vol. f, \>. 573.
MON xxxvr 7
98 THE CRYSTAL FALLS IRON-BEARING DISTRICT.
"apo," and it is iiufortuuate that these two terms should have been so nar-
rowly confined. As it is, the epi- and apo-basalts would be subordinate to
and therefore included under the metabasalts, as this term is used in this
report. In the first the production of secondary hornblende is character-
istic; in the second the process of devitrification, and hence the original
presence of a vitreous base, is the chief characteristic.^
METABASALTS.
All of the basalts belong to the plagioclase type. They may be most
conveniently divided into nonjjorjjhyritic and porphyritic kinds, according
to their most obvious macroscopical characters. There has also been found
a single occurrence of a spherulitic basalt, which will be described under the
head " variolite."
NONPORPHVUITIC METABASALT.
The nonporphyritic rocks possess a fine-grained or aphanitic structure
and are amygdaloidal or nonamygdaloidal. There are included under
this general name the microophitic-textured fine-grained pre-Cambrian
basalts (diabases in part), the very amygdaloidal forms of tlie basalts
(spilites),^ and the melaphyres in part. In these rocks the former presence
of a considerable amount of original glass is probable, and they show the
various textures known as navitic, intersertal, pilotaxitic, and hyalopilitic.
With the nonporphyritic basalts there have been included some rocks
which are to a considerable extent devitrified glasses, and others in which
only a few microlites have developed. These last two vitreous types occur
more especially in fragments in the tuff's, and are quantitatively unimportant.
petrographicai characters. — 111 color the uouporphyritic basalts on fresh fracture
show various uniform shades of green, dark olive green usually prevailing.
Much less common are purplish-black rocks, and these are much more vari-
able in color. In one of them is seen lighter-colored schlieren, which pass
over into the ordinary dark colors. The lighter-colored portions are seen
on microscopical examination to be due to a smaller quantity of the iron in
them and to a greater quantity of chlorite than occurs in the rest of the rock
' The above discussion was written and the determination to use the terms porphyry — without
textural significauce, as in rhyolite-porphyry — metabasalt, etc., was reached, in 1896, before the com-
mittee on petrographic nomenclature of geologic folios was appointed by the Director of the United
States Geological Survey.
'^ Microscopic characters of rocks and minerals of Michigan, by A. C. Lane: Kept. State Board of
Geol. Survey for 1891-92, 1893, p. 182.
BASIC VOLCANIOS OF HP:ML()CK FORMATION. 99
mass. Where weathered, the rock.s are usually covered by a thin crust, in
which ^rav, brown, and pinkish tints prevail.
The rocks vary in texture very much, from the dense aphanitic kinds
to medium fiue-grained varieties. The latter are usually less amygdaloidal
than are the aphanitic forms, and approach in ajjpearance both macroscopic-
ally and microscopically the coarser-grained basalts or dolerites represented
m the Michigamme district by the coarse-grained intrusives. 0\<'ing to the
basic nature of the rocks, they have generally suifered mucli alteration, and
as a result the original texture is in many cases poorly preserved. On the
whole, however, it is remarkable, considering their age and basic character,
how well preserved it is. Where it is preserved it varies from the micro-
ophitic to the various microlitic textures, such as intersertal, navitic, pilo-
taxitic, and hyalopilitic, and lastly glassy. In places a flowage structure is
beautifulh" brought out by the jjosition of the feldspar microlites, especially
around the amygdules.
The constituents present are plagioclase, light-green fibrous hornblende
epidote-zoisite, chlorite, calcite, muscovite, apatite, sphene, quartz, mao-net-
ite, and pyrite. Of these the feldspar, apatite, and iron oxide alone are
original. In some places the hornblende is wanting, the chlorite then
appearing in correspondingly greater quantity.
The feldspar ordinarily occurs in lath-shaped crystals showing twins of
the albite type, but where the texture is fine the feldspars are microlitic, and,
while showing their prominent long extension, the edges of the various crys-
tals interfere, and the outlines consequently are less sharp.
In some of the rocks which appear to have been vitreous the feldspar
forms feather and sheaf like aggregates (figs. A, B, PI. XXVI), apparently
quite similar to those described by Ransome in rocks from Point Bonita,
California.^ No reliable measurements could be made upon the microlites,
and consequently their character could not be determined. The feldsj)ar is
more or less completely altered to aggregates of epidote-zoisite which have
chlorite associated with them or are altered to sericite. In a number of
places minute limpid spots of secondary qviartz and albite are present. The
very small quantity of apatite present shows its usual characters. Titauif-
erous magnetite ore is apparently the only oxide present. It occurs in crys-
tals and in irregular grains, which in a few cases are not entirely altered,
' Eruptive rocks of Point Bonita, by F. Leslie Ransome : Bull. Univ. of Cal., ^'ol. 1, 1893, p. 84, fig. 6.
100 THE CRYSTAL FALLS IRON-BEARING DISTRICT.
thoug'h iu most cases they are replaced by spheue. In some cases the
alteration product is not well enough individualized for one to diag-nose it
as sphene, and it should perhaps be called leucoxene. In some of the fine-
grained rocks the matei'ial in the angles between the feldspars consists jjre-
dominately of grains of magnetite. This abundant magnetite renders the
rock very dark, giving the rare purplish-black lavas.
The most of the hornblende has a light-green color. A lesser portion
shows a decided bluish tinge, and gives fairly strong pleochroism. This
resembles the hornblende, which in the coarse dolerites is undoubtedly sec-
ondary after the augite and it is considered to be secondary after the origi-
nal augite in these rocks.
The original augite was presumably in most cases pi-esent in wedge-
shaped pieces filling the spaces between the feldspars, and consequently the
hornblende pseudomorphs never show augite outlines. No unaltered augite
was observed amongst the hornblende fibers. The fine fibers frequently
form a fringe beyond the original boundaries of the pieces and penetrate
the adjacent felds])ar. Quite frequently the secondary hornblende shows
partial alteration to chlorite and epidote.
Though careful search was made for olivine or indications of its pres-
ence, no traces of it were found, and I have concluded that these basic vol-
canics were essentially nonolivine bearing, though it would be rash to state
that such rocks did' not contain some olivine.
The calcite is usually found in irregular secondary granular aggregates
scattered tlirough the rock, and evidently replaces the other minerals. Less
commonly it is seen as an infiltration product along fissures.
A second form of the occurrence of calcite in the nonporphyritic meta-
basalts, and one not so conimon as the granular aggregate, is that of large
porphyritic atitomorphic rhombohedra and scalenohedra which lie embedded
in the eruptive groundmass. Such a rock, as, for instance, Sp. 32472, shows
macroscopically large rhombohedral phenocrysts in a green groundmass.
On the weathered surface are ferruginous rhombohedral cavities, once occu
]3ied by the carbonates. The groundmass consists of rather fresh plagioclase
microlites, between which are observed some quartz, fresh magnetite crys-
tals, and lastly chlorite flakes as alteration products of originally present
bisilicates or glass, or l)oth. The texture is undoubtedly that of an eruptive.
The carbonate is more or less ferruginous, brown iron hydroxide resulting
BASIC VOU'ANICS OF HEMLOCK FORMATION. 101
from its alteration, and as it ctiervesces quite readily with cold IICl, it is
supposed to l)t' ferruginous falcite.
Sericite is found in minute flakes replacing the feldspars, and it is also
found in large porphyritic plates occurring in the eruptive grounihiiass asso-
ciated witii the porphyritic carbonate al)ove descrilied. In some cases we
find epidote in these altered basalts, in others zoisite. In a great number
of instances the same individual exhibits the high interference colors of
epidote and the low l)lue interference color of zoisite in different parts.
These different portions, formed respectively of the epidote and zoisite mole-
cules, are most closely intergrown, and I have therefore used the compound
term "epidote-zoisite," indicating this fact. Associated with this, one finds
m many of the specimens small mineral aggregates which merit somewhat
further notice. These aggregates have a brownish-yellow color and possess
a verv high single and also a high double refraction. In these masses the
single and double refractions of the granules composing the aggregates
appear to be higher than that of epidote. In shape the aggregates vary
from perfectly round, zonally arranged spheres and irregular, elongated,
rounded aggregates to forms giving oblique quadratic sections. All of these
ao-o-reo-ates are found at times included in the epidote-zoisite crystals. In a
few cases the oblique quadratic sections were seen to occupy the centers of the
epidote-zoisite crystals, having exactly identical outlines. It is believed that
they are composed of an epidote much richer in iron than the common variety
with which they are associated. This increase in iron explains the darker
color and the increase in single and double refraction, as shown by Forbes.'
Why it should appear, especially in the aggregates, can not be explained.
The chlorite does not appear to be entirely an alteration product of the
secondary hornblende with which it is associated. There is usually rather
more chlorite than it would seem could pos.sibly have been formed from the
alteration of the hornblende alone. In some of the rocks the larger angles,
as well as the extremely fine areas between adjacent feldspars, are occupied
by a very fine felt-like chloritic mass. The chlorite which is not secondary
after hornblende is considered as the product of an altered glassy base.
No glass was observed in the nonporphyritic basalts occurring in large
masses, but in one of the fragments of basalt in a tuff a dark chocolate-
brown glass forms the matrix in which are lying well-developed plagioclase
'Epidote ami its optical proiierties, by E. H. Forbes : Am. Jonr. Sci., 4th ser., Vol. 1, 1896, p. 30.
102 THE CRYSTAL FALLS IRON-BEARING DISTRICT.
microlites. The glass where thick appears isotropic, but where thin appears
to be full of globulitic de^'itrification products, which show slight polariza-
tion effects between crossed nicols.
The original ])resence of glass in other basalts is considered to be indi-
cated bv the occurrence of amvgdaloidal cavities, with very sharply defined
walls marked by accumulations of magnetite.
The character of one basalt points strongly toward its glassy condition.
It is amygdaloidal, the amygdaloidal cavities being sharply defined. The
groundmass contains at present no indication of the existence of any orig-
inally crystalline elements whatever. It is now a dense mass of felty
chlorite and minute epidote grains. Through this mass and around the
amyg'daloidal cavities wind lines which are somewhat difterently colored
from the rest of the matrix, and seem to indicate the direction of flowage.
The amygdules are not all elongated, though some are, and these agree in
direction of elongatiou. It is really impossible to describe the groundmass
so as to do justice to its appearance and convince one who has not seen it
of its devitrified character. The general impression it makes is that of a
devitrified glass, and the photomicrograph (fig. B, PI. XXV) gives a
fairly good idea of its appearance under the microscojje, and will j^robably
prove more convincing than any description that might be given. Fig. A,
PI. XXVII, represents a polished face of the specimen in its natural size.
Another kind of glassy basalt is represented in this district. This rock
resembles the one just described, but differs from it in that it was not alto-
gether glassy. In it one sees long, slender, much-altered feldspar microlites
scattered through the matrix. These feldspars occur in needles, which
fringe out at the ends. They do not give the groundmass textures usually
found in the basalts, but occur in sheaves and imperfect spherulitic forms;
the rock thus approaches in texture the variolites. The base in which the
feldspars lie is brownish gray, and consists of recognizable chlorite, epidote,
some clear mineral in minute particles, probably quartz or feldspar, or both,
and aggregates of yellowish granules, which are apparently of a single
kind and are so minute as not to permit of determination. The grannies show
very slight polarization effects mider crossed nicols, and the groundmass in
many places where they occur in great quantity appears almost isotropic.
It seems highly probable that a large portion, if not all, of the ground-
mass was originally a glass. Further evidence of the originally glassy
BASIC VOLCANICS OF HEMLOCK FORMATION.
103
iiiitiire ot" the gTouudiuas.s is afforded by tlie grouudinass, wliicli, under the
microscope, shows variable li<»-hter and darker shades of brown, and these
poi-tions interpenetrate, forminj^- an imperfect eutaxitic structure. Such
structures are especially common in the glasses. The photomicrographs
(figs. A, Jl, PI. XXVI) show veiy well the general microscopical characters
of this rock.
Chemical composition. — Tile followiug couiplcte aiialvsis, for wliicli I am
indebted to Dr. Henry Stokes, of the United States Geological Survey,
shows the chemical composition of one of these pre-Cambrian nonporphy-
ritic metabasalts. The rock is very fine grained and microophitic, with a
marked amygdaloidal character. The amygdaloidal cavities are filled with
chlorite, into which project crystals of epidote-zoisite and calcite. The
altered condition of the basalt is very clearly shown by the high percent-
ages of water and carbon dioxide. The other oxides show nothing
remarkable except that the percentage of titanium oxide is quite high. On
the whole, however, the analysis is very similar to those of recent fresh
rocks of the same character.
Analysis of pre-Cambrian nonporphyritic metabasalt.
Constituent.
Per cent.
Constituent.
Per cent.
SiO,
46.47
1.28
16. 28
3.15
8.96
.09
6.56
7.90
3.64
K5O
0.21
.13
1.26
TiO>
P2O,
CO.
AliOi
FeOi
CI
FeO
SO,
MnO
H.O at 110°
.28
3.89
MgO
H;0 above IIC^
Total . ...
CaO
100. 11
Na,0
PORPHYKITIC METABASALT.
The porphyritic rocks are fine grained, and may be or may not be
amygdaloidal. They include diabase porphyrites and porphyritic forms of
the melaphyres. These last in the textures of their groundmass correspond
to the labradorite-porphyrites, the equivalents of the andesite-porphyries,
though more basic than they. The phenocrysts lie in a fine groundmass
which shows the same kinds of texture already mentioned as having been
104 THE CRYSTAL FALLS IRON-BEARING DISTRICT.
observed in the corresponding nonporphyritic basalts, the microophitic, inter-
sertal, navitic, pilotaxitic, and hyalopilitic. The various basalts are connected
by transition phases. The close connection between the different varieties
is well shown where one passes from the fine-grained amygdaloidal rock
through the fine-grained nonamygdaloidal over to the porphyritic macro-
scopicallv nonamygdaloidal type.
petrographicai characters. — As statcd lu tlic gcueral descriptlou, these rocks do
not differ essentially from the nonporphyritic basalts just described. The
most important difference is in the presence of the feldspar phenocrysts,
giving them a porphyritic texture.
Measurements upon the phenocrysts, made against the albite twinning
plane on zone _L to 010, according to the Michel Ltivy method, give an
average extinction angle of about 18°, which points toward its character
as labradorite. However, angles obtained lower than this indicate the
possibility of the association of andesine with the predominant labradorite.
The feldspars show the usual alteration products. One very infrequently
finds augite phenocrysts which have been completely uralitized associated
with the feldspars. Other phenocrysts are now represented by masses of
chlorite, with or without epidote, evidently pointing toward the basic and
maonesian nature of the original mineral. As uralite is the common sec-
ondary product of pyroxene in these volcanics, the altered phenocrysts
represented b^s' chlorite masses are not believed to have been pyroxene.
The original mineral was perhaps olivine. The very noticeable scarcity of
augite phenocrysts in the basalts stamps them as difterent from the great
majority of basaltic rocks and as being very similar to the basalts described
by Judd,^ from the Brito-Icelandic petrographicai province, in which por-
phyritic crystals of augite are seldom if ever seen and in which the pheno-
crysts are feldspar and sometimes olivine.
The groundmass in which the phenocrysts lie have generally the same
mineralogical composition and texture as the nonporphyritic rocks already
described, and the two kinds are supposed to have been originally similar.^
' On the gabbros, dolerites, and basalts of Tertiary age in Scotland and Ireljind, by J. W. Judd:
Quart. Jour. Geol. Soc, Vol. XLII, 1886, p. 79.
-The groundmass of one of these porphyritic forms dift'ers somewhat in one important respect.
In it were observed numerous round areas of small size occupied by a clear white aggregate, polariz-
ing in low gray colors. The centers of some of the areas were occupied by clumps of yellow grains,
with here and there a minute flake of chlorite. Others contain only the white material, which is
apparently secondary. The round areas are not sharply delimited, and hence are most probably not
microamygdules. Their general appearance is strikingly like that of leucite in those plagioelase
BASIC VOLCANIOS OF IlKMLOCK FOKMATION. 105
Measurements made on the feldspar microlites (»f the g-roundmass gave 17°
as tlie maxinmm extinetiou in zone perpendicular to 010. This angle points
toward tlie microlites being acid labradorite. The microlites thus seem to
agree essentially with the phenocrysts in composition. Flowage structure
around the phenocrysts is most distinctly shown by the arrangement of the
feldspar microlites. In one case in which the porphyritic texture and the
ilowage structure are very good, secondary actinolite crystals have devel-
oped parallel to one another and parallel to the flowaee direction, o-ivino-
the rock under the microscope a distinctly schistose appearance.
Chemical composition. — In the preliminary article upon these Hemlock vol-
cauics published in 1895/ the occurrence of andesites as well as basalts was
mentioned. This determination was based solely on the microscopical study
of the rocks, and the rocks which were presumed to be andesites were those
porphyritic forms which have just been described. Since the publication of
that article the following analyses (Nos. 1 and 2) have been obtained of
the porphyritic rocks. The rocks selected for analysis were those which
appeared to be especially rich in feldspar, and, having a rather lighter color
than the others, seem to be somewhat more acid than the average. These,
it was thought, might have the composition of andesite.
The comparison of series of rocks derived presumabl}- from the same
magma is more profitable than the study of single analyses. This line of
investigation, as followed by Rosenbusch," Iddings,^ Lang,^ Broegger,'
Becke,* and Michel Li^vy,' has been very fruitful.
basalts iu which it is present in very small quantity, filling irregular but iu general rounded areas
between the other constituents. It would, of course, be impossible to base the determination of the
former presence of leucite in these pre-Cambrian rocks upon such scant evidence as has been obtained.
Still it is worth while to notice even such a doubtful indication of its former presence as has been
mentioned above.
' Jour. Geol., cit. pp. 805-806.
'Ueber die chemischen Beziehungen der Eruptivgesteine, bv H. Rosenbusch: Tsch. Min u Pet
Mitt., Vol. II, 1889, pp. 144-178.
3 Origin of igneous rocks, by J. P. Iddings : Bull. Phil. Soc. Wash., Vol. XII, 1892, pp. 88-214. Tlie
eruptive rocks of Electric Peak and Sepulchre Mountain, by J. P. Iddings: Twelfth Ann. Kept U S
Geol. Survey, 1892, pp. .571-664.
< Ordnung der Eruptivgesteine uach ihrem chemischen Bestand, by H. Otto Lang: Tsch. Min. u.
Pet. Mitt., Vol. XII, pp. 199-252. Beitrage zur Systematic der Eruptivgesteine, bv H. Otto Lang- Tsch
Min. u. Pet. Mitt., Vol. XIII, 1892, pp. 115-169.
■• Die Eruptivgesteine des Kristianiagebietes, by W. C. Broegger. I. Die Gesteine der Grorudit-
Tinguait serie. II. Die Eruptiousfolge der triadischen Eruptivgesteine bei Predazzo in Siidtyrol.
Videnskabsselskabets Skrifter, I Matheuiatiek-naturv. klasse. No. 4, 1894; No. 7, 1895.
<i Gesteine der Columbretes, by F. Becke : Tsch. Min. u. Pet. Mitt., Vol. XVI, 1896, pp. 308-336.
'Note eur la Classification des Magmas des Roches Eruptives, by A. Michel L^vy; Bull.de la
Soc. G6ol. de France, .Sd ser., Vol. XX V, No. 4. .Inly. 1897, pp. 326-377.
106
THE CRYSTAL FALLS IRON-BEARING DISTRICT.
The A-alue of such an investigation largely depends on the freshness of
the rocks examined and the amount of variation. The Hemlock volcanics
are all more or less altered, and the variation in character is slight. I wish,
however, to call attention to the close relationship exhibited by the types of
which analyses were made, and to that end the analysis of the nonpor-
phyritic very basic appearing basalt (No. 3 of Table I) is repeated and is
placed by the side of the analyses of the porphyritic ones. The complete
analyses made by Dr. Henry M. Stokes, of the United States Geological
Survey, are found in the first table. In the second table there is given the
molecular proportion of the chief oxides, those which are not here repre-
sented having been first proportioned among them. From this table,
following Rosenbusch,' there were obtained the figures in the tliird table,
showing the atomic proportions of the metals present."
The analyses are arranged according to the increasing percentage of
calcium.
Analyses of porphyritic nietabasalt.
TABLE I.— COMPLETE ANALYSES.
Oonstitnont.
1.
2.
a 3.
SiO,
47.20
3.30
15. 36
3.06
8.87
.20
5.05
4.20
4.72
1.40
.36
52.59
1.36
15. 93
6.12
3.96
.25
5.55
5.04
.5.79
.67
.15
46.47
■ 1.28
16.28
3.15
8.96
.09
7.90
6.56
3.64
.21
.13
TiO.
AI,Ot
FeOi
FeO
MnO
CaO
MjrO
NaiO
K.;0
P2O,
CI
SO. - . '
COi
3.34
.16
3.04
None.
.16
2.16
1.26
.28
3.89
H,O:itll0>;
H2O above IIC^
Total
100.26
99. 73
100. 11
aNo. 3 is the analysis by Dr. Stokes of the nonporphyritic basalt, and is given for comparison.
I Uber (lie cliemischen Beziehungen der Eniptivgesteine, by H. Rosenbusch : Tsch. Min. n. Pet.
Mitt., Vol. XI, 1890, p. 144.
2 Tables Xos. II and III were calcuLated for me by Mr. Victor H. Bassett, assistant in cbeniistry
ill the University of Wisconsin.
BASIC VOLCANICS OF HEMLOCK FORMATION.
107
TAUI.E II.— MOLKCULAK rKdl'dRTloN Ol' THE CIIIKB- OXIDES.
SiO.
50.55
3.54
16. 45
3.28
9.72
5.41
4.50
5.05
1.50
54.07
1.40
16. 38
6.29
4.33
5.71
5.18
5.95
.69
49.15
1.35
17.22
3.33
9.57
8.36
6.94
3. 85
.22
TiO;
Al.Oi
Fe,0 1
FeO
CaO
MgO
Na^O
K 0
Total
100.00
100.00
100. 00
TAHLE III.— ATOMIC PROPOKTION OF METALS.
Si
46.97
2.48
18.07
9.87
5.42
6.25
9.16
1.78
49.49
.97
17.73
7.67
5.63
7.09
10.61
.81
45.41
.94
18.80
9.74
8.33
9.58
6.93
.27
Ti
Al
Fe
Ca
JI"
Na
K -
Total
lOO: 00
100. 00
100. 00
1
As tne calcium increases there is a corresponding increase of magne-
sium and a diminution in potassium. A decrease in sodium is also shown
if Nos. 1 and 3 alone are compared. The percentage of sodium present is
rather high and with the potassium indicates a magma family rich in alkalies.
The magnesium is notably high; such high percentages as we have here
usually accompanying much lower percentages of alkali. It may also be
noted here that the presence of the magnesium in such amounts indicates
the former presence of olivine or the presence still of its alteration products,
a point to which attention was directed in the microscopic description of the
rocks. No. 1 is remarkable for its percentage of titanium, which is very
high, even when compared with that contained in the others, which are
themselves considerably above the average. All of the rocks contain a
large amount of water of hydration. The percentage of CO2 contained in
Nos. 1 and 2 indicates also that they are much altered.
These analyses show that the rocks can not be classed with the typical
andesites. Should they be called andesites at all, they must be classed with
108 THE CRYSTAL FALLS IRON BEARING DISTRICT.
the augite-andesites, and placed on the border Hne between them and the
plagioclase basahs. It is preferred to hichide them under the basalts, though
it can not be doubted but that if analyses of perfectly fresh rocks could be
obtained, there would be found some which would incline more decidedly
toward andesites than do the above specimens.
VAKIOLITIC METAUASALTS.
Variolites are spherulitic basalts, usually very vitreous. Since the tend-
ency to crystallization is so much stronger in the basic than in the acid
rocks, it is not surprising that they should be far less common than the cor-
responding acid kind. Moreover, the basic glasses are very susceptible to
alteration, which naturally obscures the original characters of the rocks.
This probably partly accounts for the fact that they are very nifrequently
observed. This spherulitic phase of the basalts is well known in Europe,
but there has thus far been found only one reference to its occurrence in
the United States. Ransome^ has described a variolite from Point Bonita,
California. To this there may now be added a single occurrence in the
Crystal Falls district of Michigan. This variolite exposure occurs at N.375,
W. 900, sec. 4, T. 44 N., R. 33 W., in close proximity to the remnant of a
basalt sti'eam which shows well-marked flowage structure. The relations
of the two rocks are not determinable from the exposures.
The rock presents a very rough manimillated surface, due to diiferential
weathering. The varioles, being more resistant than the groundmass sur-
rounding them, form the protuberances. These protuberances vary in shape
from round to oval, and very rarely are irregular. The varioles vary also
in size from minute ones to those about one-half inch in diameter, and con-
stitute by far the greater part of the rock. These general characters may
be seen on the photogi'aph, fig. A, PI. X, taken from the hand specimen. The
color of the weathered surface of the rock is gray or light brown, while the
fresh surface is in general a dark green. Upon the polished surface of a
fresh rock the varioles have an olive-green color, with, in the majority of
cases, a distinctly darker center of purplish color. Less frequently this
center is lighter green than the remainder of the variole. The varioles are
usually separated from each other by narrow areas of groundmass, darker
than the varioles themselves, with a purplish or very dark olive-green
• The eruptive rocks of Point Bonita, California, by F. Leslie Ransome: Bull. Dept. Geol. Univ.
of Cal., Vol. 1, 1893, p. 90.
PLATE X.
109
platp: X.
Fig. .4.
(Sp. No. 32273. Natural size.)
Photographic reproductiou of the weathered surface of a variolite. This brings out very clearly
the uiaumiillated surface of the rock, which is due to the diftereutial weathering of the varioles and
of the groundmass between them. The rounded character of the varioles, and their gradation irom
those of very small to those of much larger size can readily be seen. (Desc., p. 108.)
Fig. B.
(Sp. No. 32273. Natural size.)
Reproduction of the polished surface of a variolite. This is designed to show the circular
character of the varioles, and the fact that each is separate and distinct from the one adjoining it.
It can be seen that some of the varioles have very dark and others much lighter centers. (Desc,
p. 108.)
110
U. S. GEOLOGICAL SURVEY
MONOGRAPH XXXVI PL. X
(-A) Weathered surface of variolite.
iS) Polished surface of variolite.
BASIC VOLCANIOS OF HEMLOCK FORMATION. 1 1 1
color. In places the varioles are in juxtapositiou. However, they do nut
coalesce, but each is separate and distinct (tig. B, PI. X).
The rock when examined under the microscope is seen to be extremely
altered. The only ori<;-iual minerals present are feldspar, apatite, and pos-
sibly some magnetite.
The groundmass consists of a finely crystalline secondary aggregate
of flakes of chlonte, associated with minute limpid grains, some of which
are probably qua/tz and others feldspar. Scattered tlu-ough this aggregate
are grains of epidote, calcite, a few crystals of original apatite, and mag-
netite, and numerous dark reddish brown and black fen-uginous specks.
The varioles are readily distinguishable from the matrix. From this,
as well as from each other, they are invariably separated by a crack, along
which reddish-brown ferruginous matter has been infiltrated. The varioles
are in o-eneral much finer grained than the groundmass, and at times
exhibit phenocrysts of feldspar. The composition of the varioles is the
same as that of the groundmass, except that apatite is more common in
them, and that in addition to the minerals mentioned as occurring in the
groundmass a small quantity of original feldspar may be recognized, both
as phenocrysts and as part of the groundmass of the varioles. Where
these feldspars occur, they are to a great extent replaced by a mass of epi-
dote, chlorite, sericite, quartz, and feldspar. The phenocrysts are found
near the center of the varioles, and the occasional light-colored centers
which were observed macroscopically are due to the presence of these
altered feldspar phenocrysts. The more frequent dark centers are due to
an acctunulation of the dark ferruginous specks in varioles in which the
phenocrysts are wanting.
No textures could be determined from the remnants of the original
minerals. In one variole aggregates of secondary epidote grains and fer-
ruginous specks lie in such a position as to produce a distinct radial
arrano-ement. With advancing alteration, spherulites in acid rocks are
frequently found to have between their radial fibers secondary deposits of
epidote and ferruginous matter, which mark very clearly their radial
arrangement. The similar radial arrangement in these varioles of epidote
and ferruginous matter seems to point to the varioles having possessed the
spherulitic character, though it is now impossible to determine the nature
of the fibers forming the spherulites.
112
THE CRYSTAL FALLS IKON BEARING DISTRICT.
TIIK ELLIPSOIDAL STRL'CTUUE IN THE iMETAIS ASALTS.
Upon examining the flat surfaces of many of the lavas one is immedi-
ately struck by their resemblance to a conglomerate formed of round
bowlders, all of the same kind of rock, lying in a matrix of very small
quantit}' and of very different color.
Fig. 7 is a sketch showing a portion of such a lava flow. I find that these
ellipsoidally parted rocks have been called "massive conglomerates," and the
blocks have been spoken of as "bombs" in the manuscript notes of some of
the men who have worked among them. The latter term was undoubtedly
due to the resemblance of the ellipsoids to the spindle-shaped pieces of lava
m
^ - H
■^fmm'^:rA
Fig. 7.— Skitch of ihe siirliice dl' Uic «Hitci'ui» of au ellip.suidal basalt, aliuwiufi the gtin^ral rliaiaiter of tlit- ellipsoids find
matrix-
which one finds around the modern volcanoes. Ellipsoidal basalt is very
common throughout the Hemlock volcanic area. It is found most frequently
in isolated ledges. However, it is also associated with and grades into non-
ellipsoidal varieties. In one good exposure it is overlain by a fragmental
scoriaceous mass which separates it from another mass of similar ellipsoidal
basalt. While the scoriaceous portion may represent the brecciated surface
of a lava flow, it is not so considered, biit is presumed to be a tuff" deposited
upon the flow represented by the ellipsoidal basalt. According to this view
the ellipsoidal basalt is on the surface. In another exposure an ellipsoidal
basalt overlies a bed of water-deposited clastic I'ock. There is no passage
between the two kinds of rock. The contact between the two is an undu-
lating one, and is marked by a mass of schistose material about 2 inches
thick and similar to that which is between the ellipsoids. This particular
BASIC VOLOANICS OI'" HEMLOCK FOKMATIOX.
113
biisiilt is \cr\- iK'iisi.', with ouh' otHMsioiiuUy small cliloritc-lilU'd \c.sicli's in
it, iiiul tlicic is no true flow structure observable. The facts cited seem to
show thai this elli[)soiihil [lortiou was tlic surface of a lava flow, whether
the top or the bottom is immaterial In c(n-taiu cases the (■lli|)soi(hil tiu-ies
ma\' constitute an entire How. Where tlie direction of How coidd with' anv
de"Tee of certuint\' be deterniinc^d, it w;is seen that the two hiu"er axe.s of
the ellipsoids are in the })laue of tlie flow.
Tlie ellipsoids vary in size from a few inches to G or 8 feet in diameter,
and are usually spoken of as spheroids. Attention has already l^eeu called
to tlie incorrect usaye of this term by F. Leslie Ransoiue, in lii.s iuterestinu-
paper on "The eruptive rocks
of Point Bonita, California."'
The outlines of the Ixtdies are
circular onlv in exceptional
cases. Un the other hand,
sections in all directions
through them give almost in-
variably ellipses, aiid there-
fore they are more })roperly
ellipsoids than spheroids. On
the surfaces exposed the long-
axes of the ellipses lie in the
same general direction.
The ellipsoids are formed
of a very fine-grained porphy-
ritic or nonporphvritic rock.
This is amygdaloidal or non-
amygdaloidal. Where amygdaloidal, the amygdules are as a rule dis-
tributed throughout the ellipsoids, though on the whole the masses are more
scoriaceous on the periphery than near the center. In exceptional cases,
the amygdules are much more numerous on the west side of the ellipsoids
than on the east side (fig. 8). In such cases the west sides are toward the
tops of the lava flows. The ellipsoids are very commonly split up by
cracks. Some of them have a roughly radiate arrangement. These may be
due to the effects of contraction in the early stages of the existence of the
Fig. 8. — Sketch showing the couceDtr.^tion of the ain.vgdak)idal cavities
ou one aide of au ellipsoid, thia side probably representing the side
nearest tlie surface of the How.
MON XXXVI-
' Op. cit., p. 75.
114 THE CRYSTAL FALLS IRON-BEARmG DISTRICT.
ellipsoids. Others, and by far the greater number, liave one set of lines par-
allel and another parallel set which in different cases cut the first set at dif-
ferent angles, very rareh' at a right angle (figs. 8, 9). One of these sets is
usually tranverse to the long axes of the ellipsoids (figs. 7, 8).
The blocks are separated from one another by a thin layer of a schistose
matrix, rarely more than 3 inches in thickness, though exceptionally nearly
8 inches thick. (Of figs. 7 and 8 of this pajjcr, and fig. 1 by Ransome.^)
Since the above description of the Crystal Falls ellipsoidal lavas was
written in 1896, there has appeared Sir Archibald Greikie's valuable work
on the Ancient Volcanoes of Great Britain,- in which several similar occur-
rences are mentioned. His illustration of this structure on page 184, as can
rcadih' be seen on comparison, would answer, but for the absence of a well-
defined schistose matrix between the ellipsoids, very well for
>^^^^C ^ sketch of a Michigan pre-Cambrian ellipsoidal lava.
ImI 4x1/1 'I ll '^riw schistose matrix between the ellijjsoids upon the
i|i''Tj+l/ll^/. weathered surface is seen to be made up of layers concentric
'/^^^f with the ellipsoids. It is possible that these layers are not
EiG. 9.-Eiiipsuids absolutelv concentric in the third dimension. However, no
■with sets of parallel • i r l l • • c ^ • • f^i
lines cutting each exposui'e ])ermitted 01 the determination ot this pomt. iTe-
quenth' certain layers seem to grade off into others of a some-
what different character. The matrix between any two ellipsoids usually
separates near the center; where apparently the greatest movement having
occurred the schistosity is most developed. One can often as easily knock
an ellijjsoid out of its encircling matrix as one can the kernel out of a nut.
In some cases there is no absolutely sharp line of demarcation l)etween
matrix and ellipsoid, but a gradation from one into the other. At the ^^laces
■where three blocks are in juxtaposition one frequently finds, instead of a
triangular space entirely filled by the matrix, in the center of the matrix a
triangular area of infiltrated vein quartz (figs. 7, 8).
In certain cases the minerals which compose the schistose matrix are
not thoroughlv cemented and give it a somewhat friable character, causing
it on weathered surface to appear granular.
In very rare cases a matrix with a distinctly brecciated character was
observed, but in this as well as in the cases above described a certain degree
' Op. cit., p. 76.
= Ancieut Volcanoes of Great Britain, by Sir Archibald'Geikie, Vol. I, 1897, pp. 26, 184, 193.
PLATE XI.
115
PLATE XI.
(Sp. Xii. 2367."). Xatiinil si.e.)
Thi.s colmed plate rej>re.seiits tlic pnlisUeil surface of iiu ellipsoiil with a portion of the matrix
which surrounds it and .separates it from the adjacent ellipsoids. The dense character of llie center
is nicely .shown. Aroiiud this oval area we get narrow concentric zones of alternatiug light-green
and dark-green material. The light-green material corresponds to that in the center, and represents
the least-altered basalt of the ellipsoids. The dark-green areas are the cliloritized basalt. Beyond
this, forming the outermost greeuish-gray zone, one linds the matrix, which possesses distinctly frag-
mental cdiaracters, though in spite of this with a marked schistose character. The schistosity of the
matrix conforms to the contours of the ellipsoid.
116
us GEOLOGICAL SURVEY
MONOGRAPH XXXVI PL XI
JULIUS eiENaco lith n v
BASALT ELLIPSOID WITH MATRIX.
BASIC VOLCANICS OK IlKMLOCK FORMATION. 117
(it' scliistositA' is iiotici'iiblc. Tlif iiiaTrix lictwccii llic ellipsoids viirics very
iniicli in (l('<>Tee of" srhistosity, color, aiul coiniiositioii.
Tlic most schistose, and 1)\' far tlic most comuKin variety, is tlie dark
tiTcen matrix, whicli consists essentially of chlorite, ei)i<lote, and zoisite.
This material is clearl\- the resnlt of the chloritization and the e[)idotization
of the original basalt constitnting the ellii)soids, for we see it alternating
with hands of and grading into the less altered basalt. (PI. XI.)
A second facies of the matrix is that which possesses only a moderate
deo-ree of schistosity, and appears at times almost massive. This matrix may
l)e lio-ht colored, almost white or greenish, or a dark bluish-blax-k. It is
cdimn o-rained or aphanitic. The light-coktred matrix consists essentially
lit (piartz and calcite. When a little chlorite or epidote is present, it has a
greenish tinge. The very dark variety consists of (piartz and siderite, col-
ored with minute particles of iron oxide. The quartz-calcite or qnartz-siderite
aggregates owe their origin to essentially the same processes, calcification, or
sideritizatiou, respectively, followed by silicification of the original basaltic
material. They are therefore briefly described together here. Their charac-
ters and origin will be found discussed in detail on page 130 et seq. Some
of the peculiar characters of this matrix are illustrated in fig. B, PI. XXVII.
The least common variety of matrix found between the ellipsoids is of a
light greenish-gray or brownish color, and possesses a noticeably brecciated
character (fig. B, PI. XXXIV, and PI. XI), but with at the same time a certain
degree of schistosity. Its characters are best seen under the microscope by
moderate magnification. Its brecciated character is then well shown.
The frag-ments of such a matrix are of all sizes and are angular. Thev
show quite commonly a separation into zones. The fragments now consist of
chlorite and epidote, and in the fragments with zonal ari-angement chlorite in
exceedingly fine flaky aggregates occupies the center and ejoidote the out-
side. Now^ and then there may be several alternating zones -of chlorite and
epidote. In all cases both epidote and chlorite are present in the zones, but
the one mentioned is in great quantity, while the other is very subordinate.
The epidote is very commonly the dark ferruginous kind mentioned on
l)age 101, and marks otf the outer limits of the fragments. Now and
tlien the limits are outlined by a zone of brownish ferruginous material,
whose exact character could not be determined. The fragments of the
breccia show now neither original minerals nor textures. To judge from
118 THE CRYSTAL FALLS IROX-BEARIXG DISTRICT.
the unitovni character of the zoues in the fragments, the original material
was very homogeneous, most probably a basalt glass.
The spaces between the fragments are occnpied by a iinely crystalline
aggregate of quartz and chlorite,. with a small amount ( )f epidote. This aggre-
gate outlining the original fragments owes its origin most probably to the
process of iniiltration. The long a.xes of the quartz grains and chlorite flakes
in this aggregate usually show a general parallel arrangement. Moreover, tlie
long directions of the fragments are in general parallel with each other, and
vvnth tlie (juartz-chlorite aggregate between them. This parallelism results in
giving an imperfect schistosity to the matrix. The schistosity of the matrix
is in general parallel to tlie contours of the ellipsoids which it surrounds.
Origin of the ellipsoidal structure. — ElHpsoidal structurcs Similar to those just
considered have been described by various authox's.^
' On colnmnnr, fi.ssile, nnil spheroidal stnictnre, by T. G. Boiiney : Quart. Jour. Geol. Soc, Vol.
XXXII. 1S76, pp. 140-1.">4.
Uelier niechiiuische Gcsteiusuniwandlungeu bet Hainii--heu in Snrbsen, by A. Rotbpletz : Zi:-itschr.
(lent. (ieol. Gesell., Vol. XXXI, 1879, pp. 374-397; Vol. XXXII, 18S0. p. 447.
Report (lu tlie jjeology of iiortbein New Brunswick, by R. W. Ells: .\un. Rept. Geol. and Xat.
Hist. Survey of Canada, 1879-80, D, p. 24.
E. Datlie: Jarb. K. pieiiss. geol. Laudesanstalt, 1883, p. 432.
K. Dalmer: C'f. Zirkel Pet., Vol. IT, p. lioO.
Report on the geology of the Lake of the Woods region, by A. C. Lawson : Geol. and Nat. Hist.
Survey of Canada, 1885, CC, pp. 51-.53.
The greenstone-schist areas of the Meiioiniuee and Marquette regions, by G. H. Williams: Bull.
U. S. Geol. Survey, No. 62, 1890, pp. 137, 166-168, 173, and 203.
Ou the variolitic rocks of Mont Genevre, by G. A. J. Cole and J. W. Gregory ; Quart. .Jour. Geol.
Soc, Vol. XLVI, 1890, pp. 29.5-332.
On a variolitic diabase of the Fichtelgebirge, by .J. W. Gregory: Quart. .Jour. Geid. Soc., Vol.
XLVII, 1891, i.p. 45-62.
The Kawishiwin agglomerate at Ely, Minnesota, by N. H. Winchell: Am. Geol., Vol. IX, 1892,
pp. 359-368.
The eruptive rocks of Point Honita, California, l)y F. L. R.ansome: Bull. Dept. of Gecd. I'uiv.
of Cal., Vol. I, 1893, pp. 71-114.
Editorial note on the above paper, by N. H. Wiuchell : Am. Geol., Vol. XIV, 1894, p. 321.
The geology of Angel Island, by F. L. Ransome: Bull. Dept. Geol. Univ. of Cal., No. 7, 1894,
p. 202.
Variolite of the Lleyn and associated volcanic rocks, by C. Raisin: Quart. .Jour. Geol. Soc, Vol.
XLIX, 1893, pp. 145-165.
Ou a radiolarian chert from Mullion Island, by H. Fox and .J. .J. H. Teall: Quart. .lour. Geol.
Soc, Vol. XLIX, 1893, p. 211.
Ou greenstone associated with radiolarian chert, by J. J. H. Teall: Trans. Roy. Geol. Soc. of
Cornwall, 1894: C'f. Rosenbusch, Mikroskopische Physiographie, 3d ed., p. 1064.
The volcanic rocks of the Michigamme district, by J. M. Clements: .Jour. Geol., Vol. Ill, 1895,
p. 808.
The geology of Point Sal, by H. W. Fairbanks: Bull. Uept. Geo!. Univ. of Cal., Vol. II, 1896,
p. 40.
Geology of the Fox Islands, Maine, by G. O. Smith, 1896, pp. 16-18.
The Ancient Volcanoes of Great Britain, by Sir Archibald Geikie, London and New York, 1897,
|.p. 26, 184, .and 193.
BASIC VOLOANIOS OF HEMLOCK FORMATION. 119
^'il^illus attcnijirs liavc liccn made to explain tliis peculiar structure
Honnev, hatlii-, au<l ivaisiu I'eg-ard contraction as tlie f(tr('(_' wliicli proilucctl
the rounded masses.
Datlu' and Dalmer show l)y the presence of the concentrically arrang-ed
amyydules that the ellipsoids were units, and were formed l)efore solidi-
tication of the rock. This arrangement of the amygdules, as well as the
arrangement illustrated in fig. H on page 113, precludes at once the idea, that
the structure owes its origin to the well-known weathering process which
by exfoliation produces spheroidal blocks.
Rothpletz and Williams look upon the ellipsoids as due to mechanical
forces which ground down the angles and edges of a fractured la^'a flow,
the idea of both authors apparently being that the fractures were h)ng sub-
sequent to the movement of the flow.
Ells ami Lawson mention the structure as concretionary.
Winchell considers the cases described by him as agglomeratic
accumulations.
Cole and Grregory see in the masses evidence of lavas rolling over
among themselves. In the later paper, published alone, Gregory detinitely
states that the lava first contracted into spheroids, which then rolled over
one another.
Ransome explained the Point Bonita occurrence as a basalt which
flowed "as a viscous pahoehoe, one sluggish outwelling of lava being piled
upon another to form the whole mass of the flow."^ In the description of
the basalts, he writes: "A certain amount of crushed and sheared material
fills the interstices between the spheroids and seems to be made up of com-
minuted fragments of the same rock. It is, however, too crumbling and
too full of secondary products for a satisfactory determination."^ In the
second occurrence, in a fourchite (augitite ?), the relations of the rocks are
such as to prove "conclusively that such structure can not be rigidly
restricted to surface flows, although it is still believed that lavas exhiljiting
it must have been erupted under very nearly surface conditions."^
Teall agrees with Ransome in comparing the ellipsoidally parted
masses of basalt to pahoehoe lava. Teall concludes that such ellipsoidally
parted basalts are submarine flows.
In a recent paper Smith has described from certain volcanics bodies
' Op. cit.,p. 112. "Op. cit., p 78. » Angellslaiu;, op. cit., p. 202.
120
THli CRYSTAL FALLS IROX-KEARING DISTRICT.
which ill cross section give elHptical iigures, l>ut whose iudetenninate down-
ward extension sliows them to be cohimns. The rounding of the cokimns,
which were presumably originally prismatic, he ascribes to dynamic action-
He also sug'srests that elliijsoidal masses could result from a similar dynamic
modification of a mass of lava parted into shorter prisms, or even
ellipsoids.
In the description of the eruption at >>antorin, Fouque' mentions a
viscous lava exuded in the form of a mass of blocks. These blocks, tum-
bling o^'er one another as the mass is pushed from l^ehind, have accumulated
in a rough pile, PI. XII. Fouque climbed these piles of block lava shortly
after their production, and noticed the breaking off of pieces from the sides,
due to the cooling and contraction of
the individual blocks.^
In general this character agrees
well v.'ith that of the aa lava of Hawaii,
as descriljed by tlie late Prof. J. D.
Dana.' He describes the formation of
the blocks as due to the slow for-
ward movement and contemporaneous
breaking up of the viscous lava. The surface contrasts with the ropy
surface of the more liquid pahoehoe. The aa is as a rule compact as
compared with the })alioehoe, though the exterior "is roughly cavernous,
horribly jagged, with projections often a foot or more long that are bristled
all over with points and angles." From the illustrations of this lava (see
fig. 10, taken fi-om Dana) the blocks may be seen to be, while irregular, still
in general distinctly rounded. This is the shape which viscous material
would naturally tend to take when sitbjected to the rolling action attendant
upon the onward motion of the stream of which they form an outer portion,
or in certain cases the entire thickness. This is clearly shown from the
following quotation from Dana's description of the constitution and concli-
' Santorin t-t ties truptions, Ijy F. Foiujue: Paris, 1879. Clia]). II. Compare especially Pis, VIII
and XIII.
2 Op. cit., p. 54.
'Characteristics of Volcanoes, by .I.D.Dana: New York, 1890, pp. 9, 241, and Am. Jour. Sci., 'ii\
ser.. Vol. XXXIV, 1887, p. 362.
"An aa or arate lava stream consists of detached masses of lava as far as is visible from the
outside. The masses are of very irregular shapes and confusedly piled up to nearly a comuion level,
altliough often coveriuj; areas mauy miles long aud half a mile to a mile or more wide. The size of
the masses in the coarser kind varies from a tew inches across to several vards."
Fig. 10. — Rt-produetion of illustr.4tion of aa lav.i, after
Daua (Characteristics of VoleaDoes).
S G
BASIC VOLCANICS OF HEMLOCK FOh'MATION. 121
tidii of the ;i;i strciini wlu-ii in motion:' "(1) A mass of rouyii blocks outside,
jii-fciscly like the cooled aa stream; (2) tlie motion extremelv slow, indi-
catiiifi- a semifluid condition heneath: . . . (.")) the Idocks of the upper part
of tlie front, as the stream creeps on, tund)lin»' down the liigh slope, owinj^-
to retardation at bottom from friction, and tlnis a rollinj^- action in tlie front
part."
Dana describes the gradation of pahoehoe into aa lava. He writes, "a
lava stream may chano'e from the smooth-flowing or pahoehoe condition to
the aa and back again to the smooth-flowing."-
Platania' describes from Aci-Trezza and Aci-Castello basalts with
globular structure. The interspaces between the globes are filled with silt,
or silt and tufF, and the exterior of some of these globes presents a thin vit-
reous cracked crust (cf. p. 117). These globular basalts are apparentlv but
a modification of the block or aa lavas described by Fouque' and Dana, in
which the separate portions of the lava have assumed a sufficiently rounded
character to be called globes. However, Platania's fui-ther descriptions show
this term to be clearly inapplicable unless the word "globe" is used with
considerable latitude.
The Santorin block lava, the Hawaiian aa lava, and the Aci-Castello
globular lava are all products of a slowly-flowing comparatively viscous
mass. They will in the further description be included under the general
term " aa lavas," as this is the most common form of occurrence of such
viscous lavas.
The elhpsoidal basalts of the Crystal Falls district appear to be com-
parable to the Hawaiian aa lava and block lavas of the kind described b}"
Fouqu^. The lavas have subsequent]}- been exposed to great pressure and
are considerably altered. The most obvious character of these masses, their
rounded outline, is believed to be due to considerable extent to the onw^ard
motion of the stream as desci'ibed by 1 )ana.
Contraction caused by cooling, accompanied by falling off of fragments
from the outside, as observed b}- Fouque'' in the Santorin block lava, would
also tend to round blocks which were originally angular. (PI. XI.) In
'Characteristics of Volcanoes, liy J. D. Dana, New York, 1890, p. 242; and Am. .lour. Sci., 3tl ser.,
Vol. XXVI, p. 100.
2Am. Jour. Sci., 3d ser., Vol. XXXIV, p. 363.
^Geological notes of Acireale, by Gaetano Platauia: The Southern Italian Volcanoes, H. J.
Johuston-Lavis, editor, Naples, 1S91, Chap. II., p. 41.
<0p. cit., p. 54.
122 THE CRYSTAL FALLS IRON BEARING DISTRICT.
some cases the separate portions of the lava may have been originally
nearly globular, similar t(i the ones described by Platania. The ellip-
soidal basalts, however, are so common in the Crystal Falls district and
such globular basalts are so rare that this peculiar form is not considered
worthy- of much consideration in the further discussion, the first two kinds
being chiefly the forms from which these were derived.
The lava blocks rolling over one another as the lava stream advanced,
would lie witli their axes in all positions, but pressure and the onward
movement of the flow would, in the lower portion of the stream at least,
be sure to produce from the blocks ellipsoidal bodies with their two longest
axes corresponding — the one to the direction of flow and the other to the
lateral extension of the stream. After the stream ceased to flow and the
lava solidified, there would be a gradation from the ellipsoidal into the non-
ellipsoidal portion of the flow.
An aa stream, such as described and shown in fig. 10, when subjected
to great pressure subsequent to burial beneath thick deposits, would be
compacted bv the breaking up of the jagged outer portions, which, falling
down, would fill the spaces between the blocks. This broken material
filling the spaces would be most exposed to movement and to the action of
percolating waters. It would consequently be very much altered, as in the
material described above (p. 119) by Ransome. Such alterations would
result in producing a matrix of exactly the same general composition as the
altered ellipsoids. It is the common case of inetamorphic action producing
from rock masses of essentially the same chemical composition, but of
difterent character, similar end products. This brecciated character of parts
of this matrix is well shown in parts of PL XI, and fig. B, PI. XXXIV. In
this case silica has been introduced, filling the spaces and marking out the
outlines of the fragments. Where mashing has been excessive, the outlines
of the fragments are obliterated and the matrix rendered schistose. There
may even be a gradation fi-oni the schistose matrix into the altered basalt of
the ellipsoid, which at the center is massive.
Let me recall the statement made on previous pages concerning the
distribution of the amygdaloidal cavities in the ellipsoids. This is one of
the characteristic features of the lavas. We have (1) amygdaloidal cavities
distributed about evenly throughout the ellipsoids, the cavities being some-
what smaller in the center than upon the periphery ; (2) the cavities are
BASIC VOLCANICS OF HEMLOCK FORMATION. 123
(■(luct'iitrittctl upon the |ifri})lu'ry witli tV-\v (ir only iiiicntscopical ciivitics in
the center; (3) tlle^' ;ire concentrated on one side of the ellipsoid, this side
rei)re.senting' apparentl\- that side of the ellipsoid tiirucd towanl tlie upper
surface of the lavii stream. 'I'he following- explanation is offered for this
difference in occurrence. The distribution of cavities is determined by
three factors: The viscositv of the lava; the difference in specific gravity
between the l)ubl)les tilling' the cavities and the lava; and the expansive
action of the gas. In the case of (1) the ellipsoids are considered to have
consisted of lava in a viscous condition through wliich the gas [)ores formed,
but in which, owing- to the high degree of viscosity, they remained nearly
or quite in the positions in which they were formed. Here viscosity was
the determining factor. In case (2) the gas pores, influenced chiefl}- by the
expansion of the gas, collected upon the periphery — ^^just as, for instance, in
the steel ingot while the center is compact the outer surface is porous.
The lava in this case was prolialily less viscous than in the former. In the
last described condition of distril)ution (3), where the gas cavities are on
one side, which is the upper surface, the lava was still less viscous than in
the preceding cases. Here specific gravity was the controlling factor, and,
as a result of the speciflc gravity and the less viscous nature of the lava,
the gas bubbles rose and collected upon the upper surface.
The explanation of the ellipsoidal basalts which has been offered — viz,
that thev are comparable with aa or block lava — seems to offer a ready
explanation for all of the observed characters. On the whole, the e\\i\)-
soids owe their origin and certain peculiarities to the viscous nature of the
lava. Thev possess also characters which are due to contraction, others
which are due to original flowage, and still others which are the result of
subsequent orogenic movements.
In certain places we may find the ellipsoids only half formed — that is,
attached by one side to the main unbroken part of tlie lava flow, the other
side showing a rounded outline. This probably represents a place where
the aa grades into a pahoehoe or smooth-flowing form. Such an instance
is possibly that illustrated by Ransome.'
Both Ransome and Teall compare the ellipsoidal basalts studied by
them with pahoehoe lava. The latter also suggests a submarine origin for
the basalts studied by him. It should be noted that pahoehoe lava in its
1 Point Boiiita, op. cit., fig. 2. p. 77.
124 THE CKYSTAL FALLS lEON-BEAKlNG DISTRICT.
typical occuiTeiK-e in Hawaii is fuuiid only in dry places, whereas tlie aa is
confined to those jjarts of the lava stream — which in other portions of its
course is perhaps developed as pahoehoe — where it crosses moist valleys or
other depressions presumed to have contained a considerable amount of
moisture.^
In the case of some of the block lava of Santorin described by Fouque,"
with which this may be compared, the conditions were such that the lava
practically welled up through the water.
From Dana's description it appears that lava in the pahoehoe form can
not exist in the presence of moisture, being changed to the aa form. It
would thus seem that Teall's statement of a submarhie origin for the
pahoehoe lava is untenable.
Wherever the ellipsoids have been studied in the Cr^ystal Falls district,
they have been found to exist as separate units, thus indicating the extremely
viscous character of the lava. It would seem that the analogy between
these basalts and the aa or block lava is much greater tlian that which
exists between them and the pahoehoe or smooth-flowing lava.
AMYGDALOIDAL STUUCTURK.
The amygdules in the basalts are composed of nearly the same min-
erals as those which occur secondarily in the rock mass itself Arranged
in order of frequence of occurrence, they are as follows: Chlorite, ei)idote-
zoi.site, quartz, calcite, feldspar, iron oxide, and biotite. An araygdule may
consist entirely of one of the above minerals, or, as is most commondy the
case, of two or more of them. In the latter case tlie minerals are usually
arranged in concentric layei's. The nonoccurrence of zeolites is very
noticeable. Their al)sence from these Huronian volcanics is especially
striking since they are so common in their altered modern equivalents,
and also occur in basalts as old as those of the Keweenawan of Lake
Superior' and of the South Mountain of Pennsylvania.^
' Cf. Characteristics of Volcanoes, by J. D. Dana : Ne-sv York, 1890, p. 243.
= 0p. cit., Chap. II.
'Faragenesis and derivation of copper and its associates on Lake Superior, by Raphael I'lim-
pcUy : Am. Jour. Sci., 3d ser., Vol. II, l.STl, ji. 1K8; also Cieol. Survey, Michigan, Vol. I, part 2, 1873, pp.
19-46; Gcol. of Wisconsin, Vol. Ill, l.-SO, p. 31.
The copper-bearing rocks of Lake Superior, by R. D. Irving: Mon. U.S. Geol Survey, A'ol. V.
1883, p. 89.
■iTbe volcanic rooks of South Mouutain m Pennsylvania and Jlaryland, by G. H. Williams: Aiu
Jour. Sci., 3d scr., \ol. XLl V. 1892, p. 491.
BASIC VOI.GANICS OF HEMLOCK FOIIMATIOX. 125
It is also ot' iiitcrcsr to notice tliat there is a total absence of indica-
tions ot" copper in these Huroiiian volcanics,as well as in tliosi; ot" the Peiiokee-
Gofebic, althoiiiih it is associated with similar rocks in the nix-ita above
referretl to as well as in many others.
The aniN-^'dn.les, with the exception of those of chlorite and of l)iotite,
are of nnich lighter color than the body of the rock, and from a short dis-
tance gWe the rock the appearance of a porphyry. AVeathering- gives tlie
rock a different appearance according to the materials filling the vesicles.
Where these weather readily they are removed and the rocks become
scoi-iaceous. Where, on the other hand, as frequently happens, the vesicles
are tilled with (juartz, the matrix weathers more rapidly and the rounded
quartz cores stand out on the face of the rock like the rjuartz })ebbles from
the softer matrix of a conglomerate.
In a few cases hematite is disseminated tlii'ovigh the quartz of the
amvgdules, giving it the bright-red color of jasper, and by some these
amvgdaloidal fillings have lieen taken for included jasper pebbles.
Careful study was made of the filling of the vesicles with the object of
determining the order of deposition of the minerals. However, it was found
that the aniA'gdules in a single slide contain very different fillings, one
chlorite, another calcite, a third epidote, and so on; and that even in the
same slide the relations are not always the same, a mineral which here
occupied the center of an amygdule being found there on the periphery.
]kIoreover, the same mineral species was found at times occupying tlie out-
side and the center of the same amygdule.
It is clear that the fillings are not the result of a solution connnon to
all the lavas, but that the same kinds of solutions were active in the various
lavas at different times and even in the same la\a. at different times. How-
ever, the conclusions reached were that the chlorite was generally the first
product deposited an<l the quartz usually the last. From the stud}' of the
related amygdaloids upon Keweenawan Point, Pumpelly^ long ago reached
the conclusion that chlorite was the earliest product of alteration — hence we
may conclude the first to be deposited in the amygdaloidal cavities; and
that the latest mineral deposited in the cavities, omitting copper from
' The paiageuesis and derivation of copper and its associates on Lake Superior, liy Raphael
Pumpelly: Am. .lour. Sci., 3d ser., Vol. II, 1871, ]>. 29.
Jletasiiuuitio development of the copper-beariuj; rocks of Lake Superior, liy Raphael Pumpelly :
Proc. Am. Acad. Arts and Sci,, Vol. XIII, 1878, p. 307.
126 THE CRYSTAL FALLS IRON-BEAKING DISTRKJT.
consideration, was quartz, the tendency naturally being to replace more
alterable witli less alterable minerals.
Flattening of amygdaloidal cavities. 111 SOlllC of the amVgdaloids (fig. B, PI.
XXV) the cavities retain their circular shape, as though the rock had not
flowed to any great extent. ]\Iore commonly the ca^'ities are di-awn out
into irregular (fig. A, PI. XXV) or lenticular shapes, the long axes agreeing
with the direction of flowage in case their deformation resulted from this, or
with the direction of schistosity in those cases where the rocks have been
extensively mashed. In some cases the cavities have been so extremel}'
flattened that the amygdules appear almost the shai)e of a melon seed,
showing a mere streak of chlorite in the sections cut perpendicular to the
schistosity, and in the planes of schistosity large lustrous oval areas.
In some few of the basalts the groundmass immediately surrounding
the amygdules is characterized by an accumulation of ferruginous matter.
In most cases, however, this part of the groundmass does not diff"er in any
respect from the rest of the groundmass of the basalts and points to a very _
gradual cooling.
Al.TEKATION OK THK BASALTS.
The descriptions given are of the freshest and most characteristic
basalts. As already explained, the mineral constituents in even these
freshest ones have undergone a veiy far-reaching alteration. The rocks
which show a more advanced stage of alteration exhibit merely a difference
in degree rather than in kind, and the minerals which result are in all cases
the same. They are uralite, actinolite, epidote-zoisite, chlorite, white and
brown mica, calcite, sphene, quartz, and feldspar.
The amount of these secondary minerals varies greatly, showing that
the alteration products resulting from the same kind of original rock may
differ very materially according to the process of metaniorphism.
In a general way the alteration of the basalts, as observed under the
microscope, has taken the following course: Even in the rocks nearest their
original condition the augite has largely changed to uralite. The vitreous
base, if any was present, has become devitrified. Rocks in this stage of
change still show the more important external characters of igneous rocks,
including in many cases those which are characteristic of glass. Some of
the rocks at this stage are light gray to green and exceedingly tough.
Many of these break with a ringing sound almost like phouolites. At a
' BASIC VOLCANICS OF HEMLOCK FiJUMATlON. 127
furtliLT stage of (.•luingL- thu t'cld-spars are partly altered to a graiuilar
■io(ire<'-ate of A'arious niinerals. In ordillar^' liylit the textures of iyueous
rocks are still preserved, Imt in polarized light none are seen, with the
exception of aniygdules which may be present. In some cases even these
are obliterated, and the original nature of the rock can only be determined
from its mode of ttccurrence and its association.
Further changes may produce rocks which consist practically of calcite,
and may be nearly white.
Ag-ahi, from these basic rocks there may be produced in extreme cases,
bv a process of silicification, a rock which consists practically of ])ure silica.
Description of some phases of alteration. As iUuStratiug SOme CaSBS iu wlucll the
same alteration products, but in different proportions and arrangement, give
rocks dift'ering very essentially, there are given the following Ijrief descrip-
tions of some of the rocks studied.
The flow structure was noted as being exceedingly well develoj^ed in
the microlitic rocks, and in some of them the production of amphibole
needles and cldorite flakes has taken place parallel with the long direction
of the feldspar microlites (the flowage direction), thus develo})ing, in com-
bination with the unaltered microlites, a well-marked schistosity. The
feldspars are still fairly well preserved.
In another case the feldspar microlites have become completely sericit-
ized, the interspaces between them being occujjied by epidote, chlorite,
and iron oxide. The preservation of the feldspar shapes, showing in ordi-
nary light the igneous texture of the rock, gives the only clue to its original
nature. (Figs. A and B, PI. XXVIII.) In some of the basalts the feldspar
is replaced chiefly by epidote-zoisite, and, as in the above case, such rocks
show their igneous character only when examined in ordinar}- light or by
uncrossed nicols. (Figs. A and B, PI. XXIX.)
In still other rocks calcite is very abundant. Its occun-ence in jjor-
phyritic rhombohedra and scalenohedra was mentioned in the description of
some of the rocks. These porjjhvritic calcites have thus far been found only
in the fine-grained microlitic types of groundmass, the coarser ophitic rocks
having it only in the usual granular aggregates. Muscovite, occurring in
large porphyritic ^^lates, conforms in occurrence to the calcite. When
muscovite is present, calcite is found associated with it in every case,
though the calcite may occur alone, and this latter is also b}* far the more
128 THE CEYSTAL FALLS IKON BEARING DISTRICT. '
common. These crystals give a secondary porpliyritic character to the havas,
and the microscopical appearance of the rocks varies somewhat according
to the occurrence of the calcite. Such rocks, for instance where the
rhombohedra occur, look on fresh surface l^y rapid examination like
por^jhyrites in which the feldspar sections are all c[uadratic. In the others
the scalenohedral sections resemble in general lath-shaped feldspar pheno-
crysts Iving scattered in all directions on the surface of the rock.
Another case of extreme alteration is shown in a light greenish-gray,
much-altered schistose rock from sec. 21, T. 46 N., R. 32 W. Upon the
weathered surface long grooves are noticed — one measuring 60 mm. long
bv 5 nmi. wide — which on the fresh sm-face are tilled with calcite. On
faces perpendicular t(^ the long extension of such grooves they appear as
narrow slits, with the long direction of the slit, that is, the width of the
groove, agreeing with the schistosity. These are clearly flattened amygda-
loiilal pores, and but for them the igneous nature of the original rock could
not have been determined. The extreme flattening of these amygdaloidal
cavities and the schistose nature of this rock produced from an original
volcanic, points toward mashing as one of the causes, if not the main cause,
of its present characters. It is now composed of fairly large automorphic
actinolite individuals, a very small amount of biotite and chlorite flakes, and
masses of grains of cpiartz, calcite, epidote-zoisite, magnetite, with ilmenite
and hematite in thick plates filling in the spaces between the actinolites.
If any feldspar was originally present, it is now entirely concealed hv tlie
calcite and epidote-zoisite.
The calcite phenocrysts are found in the ftiirlv fresh lavas. They are
beautifully automorphic and are certainly not replacement pseudomorphs of
some original phenocrysts, hnt replace the various minerals of the fine-
grained mass. Moreover, it is clear that they were formed subsequent to
all dynamic action, as their crystal Qutlines are perfect and the}' never
show any evidence of pressure. This is so even in those cases where the
amygdules which have been markedly elongated are filled with calcite
The process of replacement could not be followed, but it is evidently con-
nected with the development of chlorite, those rocks in which a great deal
of the calcite occurs having chlorite developed instead of actinolite.
In other sections in which the amount of porpliyritic calcite or calcite
and museovite is much greater than in the rocks just described, the amount
BASIC VOLCANIOS OF HEMLOCK FORMATION. 129
of i-lilorite, iron oxidt', rutik', and quartz is also greater. Tlie quartz is in
very fine graius. The jn-esence of the feklspar cau only be d(itermiued
witli (hffiouhy, and usually only on the edges of the sections, as the laro-e
amount of chlorite in the center conceals it. The textures caused by the
feldspar and the amygdules still indicate the original character of such
extremely altered stages. Figs. A and B, PI. XXX, illustrate such a rock,
showing the secondary porphyritic muscovite and calcite, and also the
original amygdaloidal character.
A still further stage of alteration gives a rock whose groundmass is
composed of the finest-gTained quartz and of grains and needles of brown
rutile (auatasef). In this lie rhombohedra of ferruginous calcite, plates of
muscovite, and irregular flakes of chlorite. The rock is macroscopically
gi-ay, hard, and quartzitic, has a ferruginous, brown, weathered crust,
effervesces with cold HCl, and yet shows its volcanic character by the
numerous beautiful amygdules. These stand out on the surface like
pebbles in a conglomerate. In some cases the weathering bnngs out the
concentric character of the filling very nicely. For example, some may
be seen in which the core is quartzitic, and is standing suiTounded by a
ring-like depression, showing by difference in the weathering the diff"erent
character of the mineral filling. Under the microscope the only amyg-
dules which happened to be cut by the section were found to be filled with
fine-grained quartz, with chlorite in automorphic flakes at the center of the
amygdules, and lying in the quartzitic mass. The macroscopical appear-
ance of some of the amygdules shows that just the reverse condition also
exists, that is, that quartz forms the centers and chlorite surrounds it.
The extreme stage of such an alteration is a rock which shows no
amygdules macroscopically or microscopically, but is otherwise like the
groundmass of the above last-described rock. It would be impossible to
determine the original character of such a rock except by its association.
The extremes of texture obtained in the alteration ^w'ocesses are, on
the one hand, a porphyry with eruptive groundmass and secondary pheno-
crysts; on the other, a porphyritic schist, in which all elements are secondary.
These extremes are connected by gradation varieties, in some of which the
calcite and muscovite approach more closely to the size of the elements
composing the groundmass, and which consequently approacli the ordinary
schists in structure.
>I0N xxxvi 9
130 THE CRYSTAL FALLS IRON-BEARING DISTRICT,
111 these rocks the porphyritic characters are unquestionably due to
the itroductioii of secondary pheuocrysts of mica (muscovite) and calcite,
not by contact metaniorphisni Ijut by dynamic action.^
It has not been found possible to determine definitely from a study of
the specimens, in many cases from widely separated exposures, on which
the above observations were made, whether the process which has taken
place in the production of such rocks has been a combination of calcification
and silicification, or a process by which carbonate is being replaced by
silica or the reverse. The replacement of carbonate by silica, as shown by
Irving and Van Hise,^ has taken place extensively in the case of the ferru-
ginous carbonates of the Penokee-Gogebic and Marquette iron ranges of
Wisconsin and Micliigan. The automorphic character of the carbonate
would seem to point toward calcification as the controlling process in the
Crystal Falls rocks.
Though the presence of quartz as the last filling of the amygdaloidal
cavities points toward silicification as being the process which would
eventually predominate, it is most prol^able that both processes of calcifi-
cation and silicification are active; but whether the one or the other is the
controlling one depends upon the depth of burial of the rocks which are
altering.
This statement appears to be supported by the facts to be described in
the following pages. The following oljservations, which were made upon
sections taken fi-oiu an ellipsoidally-parted basalt occurring on top of the
bills to the west of and overlooking Mansfield, illustrate the changes which
take place in the passage from the massive rock of the ellipsoids into the
schistose material of the mati-ix. • The change is one of increasing altera-
tion. This alteration is largely one of carbonation followed by silicifica-
' Metamorpbism of clastic feldspar in conglomerate schist, by J. E. Wolff: Bull. Mvis. Couip. Zool.,
Vol. XVI, 1891, pp. 173-183. Pis. I-XI. Cf. also Wolff on Green Mountains, Mon. U. S. Geol. Survey, Vol.
XXIII.
Principles of North American pre-Cambrlan geology, by C. R. Van Hiso: Sixteenth Ann. Rept.
U. S. Geol. Survey, Pt. 1, 1896, p. 692.
Phases in the metamorpbism of the schists of Southern Berkshire, by W. H. Hobbs: Bull. Geol.
Soc. Am., Vol. IV, 1894, pp. 169-177.
'^ Origin of the ferruginous schists and iron ores of the Lake Superior region, by R. D. Irving:
Am. Jour. Sci., 3il ser., Vol. XXXII, 1886, pp. 255-272.
The iron ores of the Penokee-Gogebic series of Michigan and Wisconsin, by C. R. Van Hise:
Am. Jour. Sci., 3a ser., Vol. XXXVII, 1889, pp. 32-48.
The Penokee iron-bearing series of Michigan and Wisconsin, by R. D. Irving and C. R. Van Hise :
Tenth Ann. Rept. U. S. Geol. Survey, 1889, pp. 341-507 ; Men., Vol. XIX, 1892, pp. 254-257.
BASIC VOLOANICS OF BEMLOGK FORMATION, 131
tioii. It may be characteristic nho of basalts with uo ellii)S()idal parting,
but it has l)eeu possible to follow the siiccessive changes only in the ellip-
soidal basalts. This is due to the fact that each ellipsoid shows all stages
from the comparatively fresh material of the center to the much altered
material on the periphery, and to the most altered basaltic material forming
the so-called matrix sun-ounding the ellipsoidal bodies (p. 114).
The freshest part of the interior of an ellipsoid from this occurrence is
a very fine-grained micro-amj-gdaloidal basalt, in which in ordinary light
lath-shaped feldspar microlites can be readily distinguished. Upon close
examination the feldspars are found to be nmch altered, and in many cases
their crystal outlines are almost completely filled out by grains of calcite
and flakes of sericite and chlorite in a quartz-albite (!) aggregate. The
spaces between the feldspar laths are now occupied by large crystals of
epidote-zoisite, grains of iron oxide, a few flakes of chlorite, and innumera-
able small round yellowish-brown and greenish indeterminable Ijodies. The
epidote-zoisite crystals also include large quantities of the brown and green
globular bodies, showing that they were produced previous to the epidote-.
zoisite. The substance in which this aggregate is embedded could not be
determined, as the aggregate is either so dense that nothing could be dis-
cerned or else underlain by feldspar. In the last case the substance is r.oen
to be clear white. The minerals mentioned, with the exception possibly of
the iron oxide, have evidently been produced secondarily from the sub-
stance or substances originally filling the spaces between the feldspars.
Nothing points toward the original substance or substances having been
crystallized, and I am inclined to believe that it was glass.
Toward the exterior of the ellipsoid the rock is more altered. The
zoisite and calcite are more abundant. The calcite occurs in the spaces
between the feldspars, as well as occupying parts of their outlines. (Figs.
A and B, PI. XXXI.) All of the other products drop into the back-
ground, owing to the fact of nonproduction, or concealment by the zoisite-
calcite aggregates.
Still nearer the exterior of the ellipsoid the calcite frequently fills the
spaces once occupied by the feldspars with long scalenohedi-al crystals,
which in a way maintain the original igneous structure. The calcite is,
however, not confined to these feldspar areas alone, but, as stated above,
also occurs between them.
132 THE CRYSTAL FALLS lEON-BEAEOG DISTRICT.
The matrix, representiug the most altered phase, is a granular aggre-
gate of calcite, in which one may here and there discern small clear limpid
grains of secondary quartz and feldspar (?) and flakes of chlorite. The
calcite includes in considerable quantity the globular bodies mentioned.
These are found also in the spaces between the calcite grains, as though
pushed away from the gi-ains as thej^ crystallized.
Some of the calcite in the first stages of the alteration of the rock may
have been derived from a basic feldspai; It is clear, however, that the
o-reat mass can not owe its origin to this process, but nmst be the residt of
infiltration. The calcite grains derived from, and lying in, the feldspar
acted as nuclei, around which the infiltrated calcite was gradually col-
lected, producing pseudomorphs after the feldspar laths. Quite recently
Dr. W. S. Bayley^ has noted in the Clarksburg submarine volcanic forma-
tion of the JMarquette district, Michigan, the occurrence of tuffs, in which
calcite has been introduced in such quantity that they may almost be called
limestones.
In another case in which the alteration of the ellipsoid (PI. XI) appar-
ently proceeded along the lines of shearing, and produced the kind of
aggregates of chlorite, including crystals and aggregates of epidote-
zoisite, which were described (p. 117) as the usual matrix of such elhpsoids,
one can see in thin section the calcite entering the chlorite aggregate along
minute fissure lines. The calcite literally eats its way into the chlorite,
and produces by an interchange of elements a mass of calcite (magnesian 1)
and epidote, besides including epidote which originally occurred scattered
through the chlorite aggregate.
The carbonation of the original basalt or of the secondary chlorite
mass results in producing a mass of carbonate which has associated with
it some secondary quartz, chlorite, and epidote. This carbonate mass may
be almost massive or it may be decidedly schistose. When schistose, the
grains of calcite and quartz have a uniform elongation, and the schistosity
is matei-ially enhanced by flakes of chlorite, which are not uncommonly
found in thin streamers or thick masses in the carbonate aggregate, at times
in sufficient quantity to give it macroscopically a decided green tinge.
I have used the term "carbonate," although having described in detail
above the calcification of the basalt, for the reason that at times, and for no
' Mod. U. S. Geol Survey, Vol. XXVIII, p. 473.
BASIC VOLCANICS OF HEMLOCK FORMATION. 133
discernible re.ason, the iron carlionate (siderito) may replace the ealcite, in
which case we get a dark bluish-black variety of" matrix (p. 117). The
siderite masse.s do not ditier essentially from the ealcite, tliough in some of
them a very small quantity of actinolite is found associated with the chlorite.
As illustrating the purely local development of these two carbonates, I
would mention having observed in one section a band of siderite separated
from a band of carbonate, which from its color appeared to be quite pure
ealcite. One may also see commonly in exposures areas of pure white
ealcite, almost in juxtaposition with areas of siderite.
It is a fact generally recognized that carbonation is a process confined
to the outer crust of the earth, so that we may perhaps best explain the
local occurrence of these carbonates replacing the basalt as products of
carlDonate-bearing waters. That such carljonation of the igneous rocks
through which these waters percolate is now taking place seems certain.
The carbonate grains in the rocks described are shattered and elongated,
or at least show undulatory extinction. They thus give evidence of having
been more or less mashed since their production, and this mashing probably
took place after they had been more or less deeply buried, and was, as a
matter of fact, to some extent due to the pressure of the superincumbent rocks.
The probability that these rocks have been thus deeply buried subse-
quent to their formation is to be borne in mind with special reference to the
next process to which they have been subjected, that of silicificatiou. This
process is most clearly shown in the siderites, and the phases of alteration
noted in their study will be briefly described.
The microscope shows the siderite matrix to be a coarsely granular
aggregate composed essentially of crushed siderite grains. Between these
grains in a few places are small grains of quartz, flakes of chlorite, and very
rarely needles of actinolite. Large quantities of black ferruginous specks
are included in and also lie between the quartz grains, and such specks are
also to be seen included in siderite areas, but close inspection shows that
they are also associated with blebs of quartz. The chlorite flakes and
quartz grains are generally elongated in the same direction, and the quartz
shows wavy extinction.
A more advanced stage in the process of silicificatiou was studied in
the case of a rock which is bluish-black in color, exceedingly fine grained,
and minutely schistose, the schistosity agreeing with the contours of the
134 THE CRYSTAL FALLS IKON-BEARING DISTRICT.
ellipsoid from around which it was broken. This is essentially an exceed-
ingly fine-grained quartz rock, with chlorite flakes and black ferruginous
specks scattered through it, and here and there an irregular oval siderite
o-rain remaining. Very few and unimportant grains of epidote were also
noticed. This rock represents nearly the last stage in the process of silici-
fication by which the siderite has been replaced, and a part, probably the
greater part, of its iron content oxidized. Some chlorite and epidote has
been produced, clearly from the lime and magnesian impurities in the
siderite. Essentially the same process of siliciflcation has been described
bv Van Hise in his various articles on the Penokee-Gogebic and Marquette
iron ranges, to which refei'ences have been so frequently made. I have
desired especially to call attention to it here, however, on account of the
fact that it shows the possibility of the production of an iron ore from an
original eruptive rock by the combined processes of earbonation and silici-
flcation. It is true that the end product in the case described does not
contain enough iron to be an ore deposit, but that is a mere detail. May
not this serve also to some extent to explain the numerous clearly marked
belts of magnetic attraction which occur throughout this area of altered
basalts, in Avliich little of the original magnetite remains i;naltered to exert
an influence npon the magnetic needle"? To explain this we must suppose
the influence to be exerted by secondarj^ magnetite accunudated along cer-
tain lines. The magnetic lines traced out agree in a very marked way
with what has been determined to be the trend of the lava flows and tuff
beds. The condition which would determine the presence of such a line
of earbonation, if we may so put it, may be the presence of a scoriaceous
lava flow or a bed of tuff, which offers exceptional facilities for the passage
of carbonate-bearing waters. It is thus intimated that there is possibility
of flnding purely local ore bodies of small size even in the midst of this
volcanic area.
The process of siliciflcation is generally considered as a deep-seated
one, occurring far below^ the outer weathering zone.
When the rocks exhibiting these various phases of siliciflcation are
exposed in the zone of weathering, certain interesting re.sults are obtained
which are worth noticing. Rocks are produced from these which upon the
surface strongly resemble amygdaloids, but in which the pseudo-amygda-
loidal cavities are of purely secondary origin. For instance, when the
siderite mass has become onlj' partially replaced by silica, weathering
BASIC VOLCANICS OF HEMLOCK FOKMATION. 135
afcncies It'iu-li out the reinaiuiug siderite areas and leave the tliiu lihns of
silica which lie between them standing up, thus giving the rock the appear-
ance of a ver}' dark pumice. As the silicificatiou progresses the siderite is
very much reduced in (puuitity, the intervening siliceous areas increasing
correspondingh'. The pressure exerted upon the rock has caused the
isolated siderite areas to take on an oval cliaracter, the longer axes in
general agreeing and being perpendicular to the ])ressure. When such
siderite areas are leached out, the silica bands remain, and pseudo-amyg-
daloidal cavities are produced, giving a very perfect jiseudo-amygdaloidal
structure to the hand specimen. This is the origin of that character of
matrix which some of the geologists have described in their field notes
as like rotten, worm-eaten wood (tig. B, PI. XXVII).
Although at present the material between the ellipsoids differs so
markedly from the rock forming the ellipsoids themselves, nevertheless
there is no reason for supposing the original composition of that part of
the rock mass to have been essentially different. The change in the
character of the basalt in passing from the ellipsoids toward the schistose
matrix is in mineralogical character much as has been described for other
basalts from this same district. The reason for the more complete degree
of the replacement process in passing away from the ellipsoids may be
readily understood from the discussion of the origin of the ellipsoidal
parting of the basalts, where the conclusion was reached that the matrix
between the ellipsoids resulted from the comminution of basaltic material
of the same general character as that of the ellipsoids. This matrix was ot
course more porous and probably more vitreous than the basalt, and hence
more liable to be altered.
PYROCLASTICS.
The majoi-ity of the clastic rocks have been derived from the basic
volcanic rocks already described. These elastics are veiy characteristic of
the Hemlock formation and constitute the greater part of it. They
comprise several classes, the more important of which are the eruptive
breccias, volcanic sedimentary rocks, and schistose pyroclastics.
ERUPTIVE BRECCIA.
The term "eruptive breccia" is here used to include those clastic rocks
in which angular fragments of an igneous rock are surrounded by a matrix
136 THE CRYSTAL FALLS lEON-BEARlNG DISTRICT.
also of igneous origin. In an eruptive breccia the fragments may be like
or unlike. Likewise the matrix may be hke or unlike the fragments.
Where the fragments have been rounded during the movement of the erup-
tive magma surrounding them, the resulting rock may be called an eruptive
pseudo-conglomerate.
Eruptive breccias are not very common in the Crystal Falls district.
"Where they do occur, the fragments, while predominantly angular, are to
some extent more or less rounded, and are similar in nature to the matrix
in which they lie. Since the rocks which form them preserve the main
characters of the massive lava flows which have just been described, they
will not be discussed in detail. The exact method of the formation of these
breccias could not be told.
In one case, in which both fragments and matrix are amygdaloidal,
it appears probable that the occurrence represents a true flow breccia in
which the broken surface of a lava flow had been recemented by a later
lava flow of the same kind of rock, or that it represents a very possible
case in wdiich the lava welled up through and floAved over portions of its
own crust, cementing the fragments. In such breccias a flow structure
around the fragments is quite plainly shown and the matrix possesses a
peculiar ropy appearance. In one instance, in which both the fragments
and matrix were niacroscopically nonamygdaloidal, it is probable that they
were formed under considerable pressure, and that this was a case in wdiich
lava was forced up through a previously consolidated mass of rock of like
character, and in its passage carried with it various fragments, forming
an eruptive "reibimffs-hreccia" or friction breccia.
VOLCANIC SEDIMENTARY ROCKS.
Under the term "tuffs" have been very generally included all kinds of
volcanic clastic rocks.^ This is probably due to the fact that there is fre-
quently considerable difficulty in discriminating between eolian deposits
and those which have been deposited in water. It seems desirable, wdierever
it is possible, to make this discrimination. To that end I shall in the fol-
lowing pages restrict the term "tuff" to eolian deposits. The term "volcanic
conglomerate," or, for the sake of brevity, simply "conglomerate," will be
used for those coarse deposits which have l^een sorted by and deposited
' Text-book of Geology, by Sir Archibald Cieikic : 3d ed., p. 135.
PYROCLASTICS OF HEMLOCK FORMATION. 137
in wutor, and wlio.se t'niynientsj show a rounded character. Slntuld the
fragiueut!:) be aiiguhir, the rocks may be called "volcanic breccias."
It has been found practicable to maintain this distinction in earlier
studies on Tertiary volcanics,^ and it is also maintained in the ^wesent study
of pre-Cambrian volcanics. I am confident the same distinction could he
made more generally tlian it is, and would in that case tend to a greater
precision in the separation of rocks of diiferent characters. However, it is
rather difficult to separate true eolian deposits of volcanic fragmentary
materials from those in which tlie fragments have been deposited rapidly
through water without liaAing eml^edded organic remains and. without
having undergone sufficient attrition to he much rounded. More or less
rounding, it is well iinderstood, results from the attrition of the volcanic
ejectamenta during their ascent and descent through the air, so that they
may in this respect resemble many of the sedimentaries. The exact mode
of origin of many of the volcanic fragmental deposits of the Michigamme
district is not clear. The greater portion appear to be of true eolian origin,
and where the origin of any is in doulit it has been put with those of eolian
origin.
COARSE TUFFS.
The coarse tuffs include rocks composed of fragments of all sizes, from
the large volcanic blocks to the fine-grained particles of sand and dust
which fill in the interstices. The ejectamenta may be uKire or less rounded
by attrition during their progress through the air, so tliat if a refinement of
the nomenclature should be needed one might very properlv be justified in
speaking of tuft' breccias and tuff conglomerates.
Tuffs are very common and characteristic for the district. The char-
acters of the beds is best shown on the weathered surfaces. Here the
scoriaceous and dense light-green fragments stand out well from the
brownish-red matnx of more altered, finer fragments and cement. On a
fresh surface the interstitial material usually has a darker green color than
the fragments. The fragments have a prevailing green color, but many,
especially in sections, are brown, much darker than anv of the rocks
forming the lava flows. The larger fragments are usually sharply angular,
but in many cases are more or less rounded because of attrition durino-
• Die Gesteine des Duppauer Gebirges in Nord-Bohmen, by J. Morgan Clements : Jahrbucli K.-k.
geol. Relchaanstalt, Vol. XL, 1890, p. 324.
138 THE CRYSTAL FALLS IRON-BE ARIXG DISTRICT.
their j^rogress tlu'ough the air. (PI. XIII.) They are for the most part
not .scoriaceous, though rather commonly amygdaloidal. The macroscopi-
cally dense fragments seem to predominate, though the amygdaloidal ones
do occur in some specimens in nearly equal quantity.
The fragments of the tuffs are derived from the various kinds of basalt
already described as forming the lava flows.
Among the fragments some of the most typical of these rocks liave
been found, and remarkable as it may seem, some of the thin sections from
them show the least-altered basalts.
In addition to the kinds mentioned under the basalts there are a number
which differ slightly from them, and ajjparently represent more glassy modi-
fications of the basalt magma. In one of these the amygdules are more
sharply outlined by the accumulation of iron oxide around the edges of the
amygdule than is the ease in the crystalline flow rocks. An especially
well-preserved fragment shows perfectly fresh plagioclase microlites exhib-
iting well-developed fluidal structure lying in a dark-brown apparently
isotropic glassy base. Where the section is thin, globulitic devitrification
products can be seen, and tliere also the base no longer appears isotropic,
but very feebly double refracting. There is very frequently found among
these tuffs amygdaloidal fragments which appear to have been derived from
what was originally a completely glassy rock, no indication of the presence
of any original crystals having been preserved. The background of these
fragments consists of a fine felt of a green chloritic mineral, dotted with
innumerable grains of epidote, in which one may distinctly discern concen-
tric circles and arcs of circles outlined liy aggi'egates of epidote grains.
These circles probably rejjresent perlitic partings. (Fig. A, PI. XXXII.)
The dark-ljrown fragments mentioned as occurring with the prevailing green
ones are very dense, appear to be very rich in iron, and may possibly
represent a very basic devitrified glass. Should accumulations composed
essentialh' of such glassy fragments be found, they could j^roperly be
called "palagonite tufts."
In addition to the rock fragments, a few rare ones of large plagioclase
crystals were found, and also in one case a fragment of a violet-brown
augite, the only specimen of fresh pyroxene thus far found in any of the
volcanics.
The tuffs show in places fairly well-developed banding, caused by the
PLATE XIII.
139
PLATE XIII.
(Sp. No. 23644. Natural size.)
This illustration is a faitliful representation of the appearance of the polished surface of a
pyroclastic from the Hemlock formation. It is somewhat doubtful whether or not the fragments
composing the rock have been deposited through the mediation of water or air alone. The larger
fragments are rather dense. Vesicular fragments are more common among the smaller particles.
Pyroclastics similar in appearance to this are of very common occurrence in the Crystal Falls district,
and huge clift's of it are readily accessible from the railroad.
140
U S GEOLOGICAL SURVEY
MONOGRAPH XXXVI PL XIII
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BASAI.T TtrFF.
I'YUOOLASTICS OF HEMLOCK FORMATION. 141
iiitoi-bcHl(liii<i' of layeris in which course and liiicr tVayinent.s prevail, illus-
trating well the varying intensity of the volcanic discharges.
Owing to the fragmental nature of the exposures, it is impossible to get
a correct idea of the maximum thickness of any of the tuflf deposits.
Exposures were seen in the north half of sec. 5, T. 43 N., R. 32 W., which
o-ave a thickness of over 500 feet for some of these deposits, but as their
fiirther continuation had l:)een cut otf by valleys, most probably eroded in
the tuffs, no means was afforded of determining their total thickness.
It is almost needless to state that the most of the tuffs have undergone
a great amount of alteration. The alterations were apparently due to an
interchange of the various elements without any essential variation in the
chemical nature of the rock as a whole. Since water is the chief agent
through which alterations occur, these always begin along the interstices.
In the case of the fragments the alteration accordingly proceeds from the
outside inward, and ordinarily at an equal rate all around the fragments,
following its contours. In this way zones of somewhat different mineralogical
composition are formed, surrounding the less altered part of the fragment.
This secondary zonal structure may be observed more or less imperfectly
in almost all of the sections made from the Ijreccias, but is much better
shown in the field, where the concentric zones are well brought out on the
larsre weathered surfaces of the bowlders.
In each case the outside, hghter-colored zone is chiefly made up of
chlorite, from which project light-green hornblende needles into the matrix
beyond. Less commonly we find it composed of epidote grains and chlo-
rite. Inside of this zone the mineral elements composing the fragments
sometimes can not be determined with any great degree of certainty.
Where determinable, the alteration products are found to be the same as are
produced from the corresponding rocks in the lava flows. As also in the
lava flows, some of the fragments of the denser rocks have become almost
opaque from the quantity of minute secondary epidote and sphene grains.
These have a lighter green color than the less altered fragments.
In examining many of the tuffs, one is repeatedly struck by the large
amount of space occupied by the cementing material. In some cases cavi-
ties of very considerable size were left between the fragments. It appears
that the fragments must have been lying very loosely. This fact tends to
confirm the eolian origin of the rocks, since ^vater deposition tends to bring
142 THE CEYSTAL FALLS IRON-BEARING DISTRICT.
the fragments in close contact, and also to fill the intervening spaces with
fine detritus. Those cases must of course be excepted where the material
fell upon water so deep that after sinking to the bottom the action of the
waves was not felt. Under such circumstances one can imagine the blocks,
being partly supported by the water, coming to rest in a more unstable
position than they would in the air.
The cement diff'ers in diff'erent specimens. The minerals constituting
it are quartz, feldspar, calcite, chlorite, epidote, and hornblende. The
minerals are found including one another in such a way as to make it prob-
able that they usually formed simultaneously. The calcite is an exception,
as it is usually pi-esent in greater quantity near the surface of the exposures,
and is therefore a Aveathering- product. It was noted above that the horn-
blende and chlorite frequently extend from the rock fragments into the
clearer elements composing the cement. Hornblende needles in many
cases constitute a large part of the cement. Where two fragments are very
close together, a perfect network of needles may extend from the one across
the intervening space and penetrate the other, and the fragments thus prac-
tically grow together. The cavities — especially the large ones mentioned
above — have quite frequently been filled with concentric growths of vari-
ous minerals. In general, chlorite seems to be the first mineral deposited,
and quartz the last, but in weathered specimens calcite is the last.
FINE TUFFS OR ASH (DUST) BEDS.
The fine tuffs or "ash" beds occur plentifully in the Crystal Falls district.
In many cases they ^^ossess a very well developed cleavage, and were very
puzzling in the field on account of their striking resemblance to true sedi-
mentary slates. They are of interest as emphasizing the resemblance
between pre-Cambrian ejectamenta and Tertiary and Recent ones. In one
respect they differ from the modern forms. The dust from Krakatao in
1885 and from other volcanic explosions consists mainly of fragments of
minerals and glass. The constituents of the Crystal Falls beds are usually
fine lava and glass fragments and less commonly minerals.
The rock fragments are angular, vesicular, and completely altered.
The glass fragments are likewise angular, and have the characteristic curved
shapes from which they are usually described as sickle-shaped bodies. (Fig.
B, PI. XXXII.) Such are formed when a pumice is broken up, and each
PYROCLASTICS OF HEMLOUlv FOHMATION. 143
represents a portion of tlic jilass Ixmudino- tlie vesicles. Here and there is
a fragment with a more or less perfect vesicle remaining. The few mineral
fragnu'Uts fouml — feldspar — were angular, Init (piite fresh. The rocks sliow
no intermixture of i-ounded tragments, and they are consequently regarded
as volcanic dust dei)osited through the air. These ash beds show a delicate
banding of iiner and coarser-grained fragments. In a single slide a grada-
tion can be traced from a moderately fine grained sand composed of distinct
volcanic fragments into a very fine grained mass composed of hornblende
needles, biotite, chlorite, epidote, and sphene, cemented by what is prob-
ably quartz, perhaps having associated with it some feldspar whose charac-
ters could not be determined.
Relations of tuffs and ash (dust) beds. — Tlic pyroclastics sceui to predominate in the
northwestern part of the district in the neighborhood of the small town of
Amasa. Special opportunities for observing the relations between the tuff
and the ash beds are offered l)y the third cut of the Chicago, Milwaukee
and St. Paul Railway west of Balsam, Michigan. Gradation can be traced
from coarse tuffs to delicately banded fine tuffs. The average thickness of a
single ash bed probably does not exceed 5 feet. In the same exposures the
tuft" beds are from 50 to 100 feet thick, and even more.
VOLCANIC CONGLOMERATES (TUFFOUENE SEDIMENTS, REVER).
That certain of the pyroclastics have been brought together and
rearranged by the agency of water is made clear by their characteristic
structure. Such rocks are the volcanic conglomerates. In very jnany
respects they are strikingly like the various eolian deposits, tuffs, etc.,
described above. They agree with them in color. The same varieties of
volcanic rocks are represented that are found in the tufts. They are true
basalt conglomei'ates.
The pebbles are ver}- commonly sharply outlined by accumulations
of epidote grains on the periphery. Some of the fragments have a reddish-
brown to purplish-black color, and stand out strongly from the green
matrix. Such pebbles are found to contain large quantities of magnetite,
the oxide being in beautiful sharp crystals and in absolutely fresh condi-
tion, forming a sharp contrast to the altered condition of the fragments in
which it occurs. In one case in which the main mass of the frag-ments
now consists of chloi'ite and epidote, magnetite occurs in large quantity,
144 THE CEYSTAL FALLS IltON-BEARING DISTRICT.
and in chains of crystals forming- dendritic growths. The oxide is clearly
secondary in these altered rocks. Since it also occurs secondarily in the
cement, it appears highly probable that it is an infiltration product formed
during or after the metasomatic process.
In these conglomerates feldspar fragments are far more common than
they are in the titffs, and they show a well-defined, round, waterworn charac-
ter. (Fig. A, PI. XXXIII.) Likewise masses of uralite are commonly
associated with the rounded feldspar. The uralite is taken to be altered
Ijyroxene fragments, though no proof of this beyond its association could be
offered, as the fragments show no characteristic pyroxene outlines. The
well-rounded nature of the volcanic pebbles makes it certain that they
have been deposited through the mediation of water and enables one easily
to distinguish the typical examples in the field.
In size the fragments diflPer froin one another just as they do in the
case of the eolian deposits (fig. B, PI. XXXIII). Many of the largest
are several feet in diameter, but more commonly they vary from a couple
of feet in diameter to small pebbles. Partly filling the interspaces and
aiding in cementing the larger fragments, witli which they are associated,
are very fine grained fragments derived from the trituration of the water-
worn lapilli and blocks. The coarse bowlder conglomerates grade through
finer conglomerates into very fine material. This fine material shows beau-
tifully marked false bedding. (Fig. C, PI. XXVII.)
In one of the finer-grained rocks, in addition to the usual sedimentary
banding, there are bands which appear to have been caused by a further
sorting of the materials, some of these bands being composed almost exclu-
sively of uralite. They consequently represent bauds which were originally
composed mainly of pyroxene fragments. In this case, when the fine ejecta-
menta settled through the water they were separated according to size of
gi'ain and specific gi'avity, as in ore-dressing processes.
Under the microscope other points of difference in addition to those
above mentioned are noted between the conglomerates and the tuffs or
eolian deposits. The fine-grained rocks corresponding nearly to the vol-
canic sand, do not consist of distinguishable rock fragments, but of clearly
rounded feldspar grains which have been enlarged by peripheral additions
of feldspar substance, bunches of uralite, some chlorite, and of sphene
secondary after titanic iron. The photomicrograph. Fig. A, PI. XXXIII,
PYKOCLASTICS OF HEMLOCK FORMATION. 145
illustrates the appearauee of the thin .section of such consolidated sand,
which possesses in no place the structure of an igneous rock, and which,
moreover, grades into a finely banded rock composed of minute needles of
hornblende, chlorite, and grains of epidote, lying" in a clear minutely crys-
talline groundmass of quartz or of feldspar, or possibly of both.
The tiner material cementing the recognizable fragments is the same as
it is in the tutfs. The large bowlders and pebbles lie in a matrix of smaller
pebbles, and these in turn lie close together in a paste which has been com-
pletely altered, and does not in all cases show clastic characters. The
cement is composed of hornblende, chlorite, sericite, epidote, feldspar, and
quartz, and in one case large porphyritic rhombs of a ferruginous carbonate
are scattered through the finer-grained material of the cement.
In the cement of the tuffs hornblende is present in large quantity and
feldspar is not so abundant. In the cement of the conglomerates feldspar
seems to be rather plentiful, hornblende is present in a comparatively small
quantity, and chlorite is more abundant, thus reversing the order of these
minerals in the conglomerates.
SCHISTOSE PYROCLASTICS.
At various places in the Hemlock formation there occur clastic rocks
w^hich have become schistose. Two isolated exposures of pyroclastics are
known whose characteristics have been so changed that, while recognizable
as elastics, it is impossible to say whether they belong to the eolian or
the water-deposited class. Upon weathered surface the rock is covered
with brownish ochre, and on fresh fracture it is dark green and very schis-
tose. Neither in exposui'es nor in hand specimens does it give any indication
of its origin.
In thin section, however, one may see macroscopically the fragmental
characters. The fragments are elongated and rounded. The amygdaloidal
texture is also seen, showing the volcanic nature of the fragments, though
the majority of the fragments are dense. Under the microscope the fi-ag-
mental nature of the rocks as a whole and the volcanic character of the
fragments forming it are still more clearly seen. In the centers the frag-
ments are seen to be composed of chlorite flakes in such great quantity as
partly to conceal the character of the clear white cement, which is supposed
to consist for the most part of quartz, though lath-shaped areas with poly-
MON XXXVI 10
146 THE CRYSTAL FALLS IRON-BEARIXG DISTRICT.
synthetic twinning, showing the presence of plagioclastic feldspar, were
seen in places on the edge of the section. In the chlorite and quartz occur
large grains of fresh titaniferous iron ore, altering on edge to spheue, and,
most striking of all, large porphyritic, beautifully automorphic calcite rhombs
and muscovite plates in isolated individuals as well as in heaps of crystals.
The carbonate, which predominates, effervesces readily with cold HCl, but
is evidently ferruginous as it is yellowish when altered, and from it results
some of the ochre which colors the weathered surface of the rock. In other
sections the calcite phenocrysts are scalenohedra, with very few rhombo-
liedra. The terminal faces are not sharply defined. The sections resulting
from the scalenohedra are long, lath-shaped, and have pointed or irregular
ends, ])arallel extinction, and oblique cleavage.
In passing from the centers toward the edges of the fragments, -we note
a marked diminution in the amount of carbonate, muscovite, chlorite, and
iron oxide, causing a consequent lightening in color of the periphery. This
gives the zonal structure noticeable ujjon the macroscopical examination of
the thin section. The schistose character of the fragments is caused by the
parallelism of the chlorite flakes.
The cement between the fragments is composed of quartz in rather
coarse grains, chlorite in larger flakes than in the fragments, and carbonate
in large porphyritic crystals, and also in minute rhombs included in the
quartz grains. Another phase contains considerable secondary plagioclastic
feldspar associated with the quartz in the coarser-grained portion of the
cement. . As in some of the conglomerates and tuffs, the fragments are
observed lying in juxtapo.sition, the only cement between them being the
secondary interpenetrating nainerals. In some cases the edges of the frag-
ments have been so welded that one may pass from one pebble to the
adjoining one across an intervening lighter zone without detecting the
transition, unless changes in the amount of chlorite, iron oxide, and carbonate
are noticed.
Nothing thus far mentioned would indicate the igneous origin of the
fragments, but that is indisputably proven by the amygdaloidal texture of
the specimens, than which I have seen none better, even in the freshest
yolcanics. The outline of the cavities is marked by an accumulation of
grains of iron oxide, and the cavities themselves are filled by fine-grained
quartz having a small amount of chlorite associated with it. (Figs. A and B,
PYKOCLASTICS OF HEMLOCK FORMATION. 147
1*1. XXX.) 'riic'se specinu'iis lia\c the characters of tin- porpiiyritic schis-
tose lavas described above, l)ut show clearly tliat they liave been derived
t'roiii igneous clastic rocks.
Other .schistose elastics occur in lar<,fe quantities in sec. 24, '\\ 4(1 X..
R. 33 W. They are penetrated by a boss of" coarse jxiikilitic dolerite
wiiich nvdv have aided in rendering" them schistose, though their schistosity
a"Tees with the general strike of that of the rest of tlie district, and is
probaljl}' chiefly due to the general folding.
Macroscopically their clastic structure niav be (dearly seen. The peb-
bles iire dense greenish gi'ay in color and oval in outline. The matrix is a
much darker green. The schistosit}' of the rock is marked. The ])ebbles
are tmifoi-mly elongated, and they have the appearance of having been
mashed. The schistosity agrees in direction with the elongation of the
pebl)les.
The microscope shows the pebbles to be basaltic, with a type of stiaic-
ture intermediate between the navitic and iiitersertal structures, and another
with approach to the trachytic structure. Considerable brown mica is
present in both kinds, and occurs in flakes which are probably secondary,
though the pebbles sho\\' few traces of alteration. The matrix consists
essentially of actinolite in rather coarse needles, large grains of fresh
magnetite, but ver}' little mica, and that such as is seen in the pebbles,,
all lying in a cement of quartz and calcite. The passage between the
cement and the pebbles is a more or less gradual one, there being a change
as we pass from the center of the pebble, where isolated actinolite needles
and epidote grains occur, toward the edge, where these minerals increase in
amount until between them here and there are twinned feldspars. In tlie
matrix projier the quartz is the predominant white silicate, though here and
there limpid feldspar is also seen. The pebbles are gradually being eaten
U}), so to speak, by the actinolite, and we can imagine the final result to be
an actinolite-schist showing no clastic structure, and giving aljsolutely no
indication as to the rock from which it originated.
There is no microscopical evidence of mashing in the minerals, and
since this is absent from the quartz in the cement, I conclude that no original
clastic cement is now present, and that the quartii is a secondary crystalli-
zation product derived from infiltrated material, and from material obtained
from the adjacent pebbles. Whether the rock is an eolian deposit or a
148 THE CRYSTAL FALLS IRON-BEARING DISTRICT.
watei'woru sediment can not now he definitely detei-mined, thougli that it
belongs to the first appears more probable.
Still another rock very similar in general character but differing in
detail, and showing a slightly different result, has been examined. The
pebbles are basalt, and in them the secondary nature of the biotite, which
lias chlorite ass(iciated with it, is clearly shown. Near the centers of the
pebbles very little is present, but it rapidly increases in amount toward the
l)eriphery, until at the edge only here and there the feldspars may be seen
between the mica and chlorite flakes. The cement between the pebbles
consists of angular fragments of altered orthoclase feldspar, quartz, and a
great quantity of chlorite, and some biotite. This cement is present in
large (juantity.
In both of these last the secondary minerals are parallel, and produce
rocks of most decided schistose character. These schistose pja-oclastics
may be compared with the rocks described by Williams^ andBayley' from
the related rocks in the adjoining Marquette district.
THE BONE liAKE CRYSTALUNE SCHISTS.
Under this name are included certain crystalline schists which are best
developed in the northern part of the Crystal Falls district, in the vicinity
of Bone Lake. If one examined isolated specimens of certain of these
rocks, it would be impossible to determine their origin. Studied in connec-
tion with the alteration of the altered and schistose lavas and pyroclastics
already described, the problem becomes greatly simplified. These
schists, as will be shown on the following pages, are but extremely meta-
morphosed members of the Hemlock volcanic formation. Since in the
limited area in which the rocks occur the secondary characters are domi-
nant, while the primary volcanic characters have nearly all disappeared, a
brief separate description of these rocks seems wan-anted, but they are not
represented by a separate symbol on the map.
DISTRIBUTION.
The crystalline schists predominate in T. 46 N., R. 32 W. Near the
western limit of this township the belt occupied by these rocks is about 2
miles wide. As it is followed to the east past Bone Lake, and then to the
'Bull. U. S. Geol. Survey, No. 62, cit., pp. 185-191.
•Mon. U. S. Geol. Survey, Vol. XXVIII, cit., pp. 160-169.
JJONE LAKE (JitVSTALLl>'E SCHISTS. 149
soiitlR-iist, it jiTiulually iijirniws, until iu sec. 3G, 'V. 4(i X., li. 32 W., the
eusteru limit of the Jirca studied hy lue, it is only about half a mile wide.
E.xt-ept iu the vicinity of Bone Lake, where erosion has uncovered some
of the knol)s, outcrojis ar(> ^^erN' scarce, since the drift is very lieaNA',
and the drainage is poorl\' develo})ed.
FIELD EVIDENCE OF CONNECTION WITH THE VOLCANICS.
If one examines attentive^' the Hendock formation in its typical
dcvidopnient, 1 )eginning, say, in sec. 27, T. 45 N., R. 33 Vs., and f(dlowiiig
its northw;n-d exteusion through sees. 22, 1(!, and 15 of the same township,
he will observe instances of banding in the tuffs and of schistositv in the
amygdaloidal lavas and pyroclastics. The strikes and dips of the jjrimarv
and secondary structures approximately coincide, both having a general
north-south strike and dipping high to the west. Tlii-oughout this area,
h()wever, the unmistakable massive volcanics are the predominant rocks.
Continuing the examination farther nc^rth into sec. 34, T. 46 X., R. 33 W.,
rocks are found which possess almost invariablv a strongly marked schis-
tositv, but with their volcanic origin clearly shown by the flattened amyg-
dules. This is also true for the exposures east of this place on the under
side of the Hemlock belt, in sec. 31, T. 46 N., R. 32 W. The strike of the
schistosity of the amygdaloids varies from N. 30° to 70'^ E., and the dip is
high to the northwest. Farther along this belt to the northeast, in sec. 24,
T. 46 N., R. 33 W., schistose pyroclastics were observed striking N. 80° E.
The original characters of these pyroclastics liave been almost entirely
obliterated. The exposures next to the east in sec. 16, T. 46 N., R. 32 W.,
possess all the characters of crystalline schists. Somewhat farther east,
however, associated with these schists are isolated outcrops in which traces
oi flow structure and remnants of amygdides were observed, and, in some,
traces of igneous textures were seen under the microscope. The schistosity
of these rocks strikes for the most part south of east, varying from N. 65°
to 80° W. and dipping to the nortJieast. Following the belt as it now turns
to the southeast, the crystalline schist characters prevail, the volcanic char-
acters being obliterated. The schistosity at the same time bends farther
around to the southeast, pointing toward the continuation of this area of
volcanics to the southeast, outside of the area studied.
From field observations the conclusion seems iiecessarv that these
150 THE CRYSTAL FALLS IROX-BEARmU DISTRICT.
.schists are metamorphosed volcanic rocks, and this conckision is strength-
eued l)y detailed petrog'raphical examination.
PETROGRAPHICAL CHARACTERS.
■ The crystalline schists are tine to medium grained schistose rocks
which vary in C(ilor from a moderately light green for the more chloritic
phases to a ^ery dark green and purplish-l^lack for those in which the
hornl)lende, mica, and iron ores are prominent. The minerals of which
the rocks are composed, arranged in order of importance, are hornblende,
biotite, feldspar, chlorite, epidote muscovite, quartz, magnetite, hematite,
ilmenite, and rutile. Under the microscojje the schistose structure is seen
to be produced by the general parallelism of the liisilicate constituents.
The por])hyritic texture is seen in a few specimens, and hornblende forms
the phenocrysts.
Hori:l)lende occurs in iine needles and also in coarse crystals which
are automorpliic in the prismatic zone, but on which no terminations have
been observed. It also occurs rather comm(inly in sheaf-like bundles of
ragged crystals. The marked orthopiuacoidal development so common for
actinolite is quite noticeable. The crystals show the usual strong pleoch-
roism: c = bluish-green, lj = olive green, it r= yellow, whereby i:>>lj>a.
The hornblende crystals frequently contain large fjuantities of the minerals
of the groundmass, many of them iu such quantity that there are really
only skeleton hornblende crystals present. The general character of the
hornblende in all these rocks is that of a secondary porphyntic constituent,
and seems to be analog'ous to such minerals as garnet, staurolite, etc., which
are produced in clearly metamorphic rocks.
Brown biotite is rather common in some of the rocks. Though usually
subordinate to the hornblende, it is at times the predominate bisilicate. It
is light brown and shows the usual characters of biotite. It is present in
small irregular flakes, and also in larger individuals which show poor
pinacoidal development. In one case such a mica individual in perfectly
fresh condition may be seen with its ragged edges interlocking with the
fringed periphery of an altering feldspar crystal. The liiotite appears to
have derived some of its necessary elements from the feldspar and to be
eating int(i it, and consequently to be a secondary product.
Feldspar is not found as an original mineral in any of the crystalline
BONE LAKE CRYSTALLINE SCHISTS. 151
schists. It occurs as a socond.ary constituent. It is found, however, as a
primary constituent in a few rocks which, as they still possess remnants of
orifi'inal igneous textures, strictly speaking, should not perhaps be included
with the crystalline schists. They represent more properly the transition
stao-es to the cr^'staHine schists, but the process of the alteration of the
feldspar is so well shown in these that it is considered expedient to mention
it at this place. The original feldspar occurs in this transition phase in
the large tangled intergrowths connnonly seen in andesitic and basaltic
rocks, as individual phenocrysts, and as microlitic lath-shaped individuals
in the groundmass. The greatest interest centers in the phenocrysts, as in
them the changes which take place are more clearly seen. The feldspar
phenocrysts ai-e always cloudy, due to numberless black ferruginous inclu-
sions. They also inclose the various secondary dark silicates composing
the gTOundmass, grains of epidote, ilakes of biotite, and crystals of horn-
blende. These are usually surrounded by very narrow clear zones, appar-
ently feldspar. Near the edges of such altered crystals, and especially in
the more altered individuals, these inclusions ai-e more numerous, and are
accompanied by grains of quartz and new feldspar (albite !). These last
two have certainly been derived from the alteration of the feldspar, but
that mineral may possibl}^ also have contributed something to the produc-
tion of the dark silicates.
The secondary feldspar, that of the schists proper, is found in grains
usually imstriated, though in a few cases striations were observed. This
feldspar was not determined, but is probably albite The chlorite is in flakes
scattered through the schists, showing the usual characters.
Epidote, muscovite, quartz, and rutile appear as usual.
Ilmenite is present in one case as micaceous titanic iron oxide, and is
then in extremely thin plates which show a beautiful hexagonal develop-
ment, though more frequently the plates are rounded. They are transparent
with the characteristic clove-brown color. The thicker plates are thin
enough to be transparent only along the edges.
The iron oxides, magnetite, and hematite occur in some of these rocks
in large quantity. In certain pai'ts of the area underlain by these schists
considerable excavations have been made in search of iron, the presence
of which was indicated by the magnetic needle, and moderately large
bodies of ore have been found, though in no case in sufficient quantity to
152 THE CRYSTAL FALLS IRON-BEARING DISTRICT.
admit of successful mining. Such ore bodies probably owe their presence
in great part to processes active subsequent to the formation of the schists.
(See p. 134.)
According to the quantity and association of the minerals described
above as occurring- in tlie schists, the following- rocks mav result from
the complete metamorphism of tlie basic volcanics : Amphibolites, chlorite-
schists, epidote-schists, mica-schists, mica-gneisses, and possibly siliceous
hematite and magnetite ore. The complete metamorphism of dense basic
lava flows into crystalline schists has been described by Williams^ for the
Menominee and Marquette districts, and also by Van Hise and Bay ley ^ for
the Marquette district. Williams^ has also described the production of
schists from the igneous elastics in the Menominee district and similar
products have been described from the Marquette di.strict both by Williams*
and by Bayley.^
The above-described schists cover a considerable area, with only iso-
lated exposures of rocks associated with them in which volcanic characters
are recognizable. They are confidently believed to represent extremely
metamorphosed volcanics of the same general original character as those
constituting the Hemlock formation and belonging to the same relative
period of extrusion.
The same conclusions have been reached by Smyth for similar schists
along the Fence River to the southeast of those described. It is noticeable
that the most intense metamorphism of the volcanics has taken place in the
northern and northeastern ^^fii't of the Crystal Falls district, that part in
which the crystalline schists have been produced, though the explanation
for this can not be offered.
NORMAL SEDIMENTARIES OF THE HEMLOCK FORMATION.
The normal sedimentaries are in small quantity. It has been seen
(p]i. 64, 78) that the Mansfield slate is overlain by a conglomerate in which
volcanic material predominates, but which contains partly rounded frag-
ments of chert and slate and round quartz grains derived from the under-
lying sedimentaries. But for the interming'liug of this normal clastic debris
I Bull. U. S. Geol. Survey No. 62, cit. ^ Op. cit., p. 158.
•Mod. U. S. Geol. Survey, Vol. XX VIII, cit., pp. 152-159. °0p. cit., pp. 160-169.
= 0p. cit., p. 133.
NOKMAL SEDIMENTAKIES OF HEMLOCK FORMATION. 153
with the l)vnl(•hl^sti^•s, the coni^loinerate shows iiothiu;^- ditfercut from the
volcaiiie con<>louierate already described. It is a transition rock between
the tuffs and tlie normal sedimentaries.
Similarly, in sec. 34, T. 4.') N., R. 33 W., a gradation occurs in the
upper horizon of the Hemlock formation from the ^■ok•anic conglomerates
to the true normal sediments. The sediments are slates about 17.5 feet
thick, containing- lenticular raas.ses of limestone. These beds dip 80° to the
west, generally strike north, but vary in places a few degrees to the west.
They are underlain by conglomerates containing well-rounded volcanic
pebbles. This volcanic conglomerate grades from the coarse conglomerate
up into Avhat might be termed a water-deposited volcanic sand. Tlie peb-
bles are all of volcanic material. Between the conglomerates and slates is
a small area without outcrop. Overlying the slates is a succession of tuffs
and lava flows.
The slates in color range from light gray and green to purplish red,
and the lenses of limestone vary from cream color to purplish red. In thin
section the slates are seen to be composed of a felt of sericite, chlorite, and
quartz, with associated innumeralile minute rutile crystals, and here and
there a large spot of limpid quartz. A ferruginous carbonate is present in
all of them in porphyritic rhombs. Where chlorite is abundant, the slates
are a light green. Where iron oxide is abundant and the chlorite less plenti-
ful, the slates are purplish.
The lenses of limestone are rather pure, consisting mainly of calcite,
with some few scattered areas of cherty silica. On the edges of the lenses
some of the slate material is found forming bands in the carbonate. These
intermediate phases grade on the one hand into the pure carbonate, and on
the other hand into the slate beds. From the crust of limonite, which may
be seen on the weathered surface of the rock, the calcite is evidently rather
ferruginous. The process of alteration is clearly seen under the micro-
scope, where many of the grains are surrounded by rims of hydrated oxide
of iron and hematite.
ECONOMIC PRODUCTS.
BUILDING AND ORNAMENTAL STONES.
The rocks of the Hemlock formation are not likely to be much used
for building purposes. The compact basalts possess in a high degree the
154 THE CRYSTAL FALLS lEONBEARING DISTRICT.
two essential features of strength and durability. For trimming in contrast
with lighter stones they might be found desirable, and it may be suggested
that they are especially suitable for mosaics in which rich greens are
desired. They are of too somber a color to l)e used in large quantity for
anything else than foundations. Moreover, the difficulty and consequent
expense of quarrying them, and their remoteness from cities of large size,
will operate strongly against their use.
The pyroclastics are natural mosaics, and some of them have a very
pleasing appearance (PI. XIII) and are suitable for table tops, wains-
coting, etc.
ROAD MATERIALS.
The importance of good roads in aiding in the material development
of a region can hardly be overestimated, and in the building of good roads,
especially in thinly inhabited regions, the proximity of good road material
is of prime importance.
Thus far the 15 miles of good road between Crystal Falls and tlie
adjacent mining villages have been covered with the ferruginous chert and
slates from the dumps of the mines, and unroll themselves to the traveler
like red ribbons laid through the green woods.
No rock is better suited for use in building macadamized roads than
the liasalt, and of this the Hendock formation offers an inexliaustible
supply. The fine-grained compact basalts are by far the best rocks
obtainable, and, other things being equal, should of course be chosen
rather than the scoriaceous and consequently weaker facies, but these
weaker kinds and also the pyroclastics are preferable to the cherts and
slates which have been used. The cherts are very hard and durable, but
the dust and sand froiu them possess but slight capacity for cementation.
Consequently the roadways upon which quartzite and chert have been used
are more likely to wash out than are the roads macadamized with basalt,
since the dust in this latter case serves as a cement which binds the larger
fragments more firmly together. The road commissioners have thus far
used very little basalt, chiefly for the reasons that no crusher was at their
disposal, and the chert and slates were at hand ready for use.
CHAPTER V.
THE UPPER HURONIAN SERIES.
The upper series of tliis district is connected in tlie northeastern part
of the area witli the Upper Marquette series of the Marquette district
ah-eady described in the Fifteenth Annual Report and Monograph XXVIII.
In these reports the Upper Marquette series is regarded as part of the Upper
Huronian. As has been stated, the Crystal Falls district is the southwestern
extension of the Marquette district, and consequently we should expect the
chief formations of the two districts to be continuous, as they are. Because of
the drift and becaiise of a change in the character of the rocks, in mapping
the western part of the Crystal Falls district it has not been practicable to
divide the Upper Huronian into several formations, corresponding to those
in the ^Marquette district. No independent name will be given to it, but
it will simply be called Upper Huronian, with the understanding that it
corresponds stratigraphically to the Upper Marquette series.
DISTRIBUTION, EXPOSURES, AND TOPOGRAPHY,
Beginning in the northeastern part of the area discussed by me (see
PI. Ill), this series covers the southern parts of T. 46 N., Rs. 31 and 32,
where it is only 4 miles in width. It is here a northwest-southeast syncline.
From this place it stretches beyond the northern limit of the map. With
slight interruptions where intrusives occur, it extends in a broad area to the
west and south about the Hemlock volcanics to a point lying beyond the
limit of the map. On the eastern side of the district it abuts against and is
folded in synelines in the Archean granite.
Exposures are scanty for the greater part of the area in the Crystal
Falls district underlain by the Upper Huronian series. This is due to two
conditions, first, to the soft character of the rocks constituting the series, and,
second, to the presence in places of the Cambnan sandstone, and more
especially to the deep covering of glacial drift which is found spread over
the entire district. The Upper Huronian is composed in great measure of
155
156 THE CRYSTAL FALLS lUON-BEARING DISTRICT.
slates, which are interbedded with much smaller quantities of graywackes
and chert. The slates are eroded much more readily than the associated
harder beds, and therefore, except along valleys, we rarely find the soft
slates exposed. The graywackes and cherty rocks are the ones ^^'llich form
the striking topographical features of the landscape, the slates forming
softly-rounded hills. The drift is also an important factor in the scarcity
of outcrops. In the northern and western parts of the districts especially
the drift is very heavy. In this portion the youthfulness of the topography
is emphasized by numerous swamps, lakes, and generally imperfect di-ain-
age. In the southern an<l southwestern parts of the district, owing to the
presence of larger streams, and consequently more advanced erosion, the
drift has been removed to a greater or less extent, so that the topographical
forms approach much nearer to those of an unglaciated region. For
instance, the general strike of the graywacke and cherty ferruginous slate
beds in the southern portion of the area, T. 42 N., Rs. 32 and 33 W., can
be closely followed by the north-south to northwest-southeast ridges which
they form, the intervening valleys being in all probability underlain by the
softer carbonaceous clay slates. Also in this vicinity, from the Chicago
and Northwestern Railway eastward to the Michigamme River, exposures
of intrusives with some sedimentaries stand out from the sand plains as
rounded knobs.
MAGNETIC LINES.
A considerable amount of detail magnetic work has been done in
the vicinity of the ore-bearing areas, in the hope that with the assistance
aftV)rded by the magnetic needle the iron belts might be better traced than
tliev could be by means of the very scanty outcrops. I shall here describe
those lines of magnetic disturbance which have been traced for considerable
distances. They are indicated on the map, PI. Ill, by solid blue lines.
Magnetic line D. — Tliis lluc of uiaximum uiagnetic disturbance was traced
northwest from near the southeast corner of T. 46 N., R. 32 W., around
Bone Lake, then southwest and south tlu-ough T. 46 N., R. 33 W., until
finally lost near the south side of sec. 34 of the same township. The
tracing of this line was begun where outcrojjs were wanting, and it was
not possible to connect it directly with any magnetic formation until sec.
34, T. 46 N., R. 33 W., was reached. Here it was connected with outcrops
of magnetitic slate and gravwacke which overlie the Hendock formation,
MAGNETIC LINES. 157
but with no contact exposed. Tlirouoliout its extent tlie lint; of disturbanco
is separated from the line of outcroj)s of the Hemlock volcanics by a short
interval. It is, however, always distinctly separated from them.
Magnetic line E — Tliis magnetic line passes directly tlu'ough the open pits
at the Hemlock. As the line is traced north from this point it passes just
west of an amyydaloidal lava one-half mile north of the mine. From this
point until it is lost in sec. 16, T. 45 N., R. 33 W., there is no evidence in
regard to the nature of the rock causing the attraction. Tracing the line
south from the Hemlock mine it is found to swing about 200 paces east of
the Michigan mine, near the north line of sec. 9, T. 45 N., R. 33 W.
A quarter of a mile farther south it swings back again, apparently in the
line of the continuation of the iron-bearing formation, which it follows for
one-half mile farther, where it is lo.st. The only place along this line where
it has been possible to determine the I'ock causing the attraction is at the
Hemlock mine. Here it was found that it is not the ore foi'mation proper
which is magnetic, but that it is the foot wall. This is a magnetitic slate,
about 42 feet m thickness, as shown by the diamond-drill borings.
The above are the only lines of maximum magnetic disturbance of
this part of the Crystal Falls district which it has been found possible to
connect in any way closely with the iron-bearing rocks. A large number
of lines of distui'bance, however, were traced within the limits of the
Hemlock formation, but on account of their slight economic importance
they are not inserted on the majj. In these cases the influence on the
needle is evidently exerted by the magnetite of the lavas and pyroclastics,
and in proof of this the lines can very commonly be connected with
exposures of the various volcanic rocks. It is of interest to note that the
trend of the lines in the volcanics invariably agrees with that of the tutf
beds, and with the general strike of the formations of the district, and the
reader is reminded of the suggestion already offered (p. 1 34) that they may
be caused by magnetite accumulated by secondary processes, especially
active in the tuff beds and scoriaceous portions of the lava flows.
THICKNESS.
Since the Upper Huronian sediments cover a broad area, their thick-
ness must be very considerable. Owing, however, to the scarcity of
exposures, it is impossible to give even an approximate estimate.
158 THE CEYSTAL FALLS IRON-BEAlilXG DISTlilCT,
FOLDING.
The extreme northwestern part of the area has not been stuclied in
such detail as to enable the minor folds to be determined. In general, the
series may be said to fold around the Lower Huronian, foUowino- the
general outline indicated by its color, as shown on PI. Ill, and having a
steep dip away from it. In sec. 20, T. 45 N., R. 33 W., large outcrops of
chert are folded in a most complicated fashion and are locally brecciated.
South from this point the evidence of subordinate cross folds is marked.
As a result, the line between the Lower Huronian and Upper Huronian is
undulatory. The indentations in the Lower Huronian represent minor
cross synclines, and the protuberances represent minor cross anticlines.
CRYSTAL FALLS SYNCLINE.
Near Crystal Falls is the most important of these synclines. This
town and a number of small outlying mining villages are situated on a
syncline. The character of this syncline is shown better by the distri-
bution of the Hemlock volcanics than by the sedimentaries, owing to the
scarcity of the outcrops of the latter (Pis. XVII and XVIII). Tlie broad
belt of northwest-southeast trending volcanics, situated 3 miles northeast of
Crystal Falls, bends in sees. 11, 12, and 13, T. 43 N., R. 32 W., to tlie
south, and gradually changes to a slight southwest trend. In the reentrant
ans'le of this volcanic formation is the CrA^stal Falls syncline, its course
Ijeing that of a southwestward-opening U. The axial line of this U probably
has a westward pitch, corresponding- with the general folding of this part of
the district.
Near the center of the U and just a little northwest of Crystal Falls,
in sees. 17 and 20, T. 43 N., R. 32 W., is an area underlain by volcanics,
which trends east and west, and can be followed westward into sec. 1, T.
43, R. 35, beyond the limits of the area represented on the map. It varies
in width from one-fourth mile to 4 miles, averaging about 2 miles. The
contacts of these volcanics with the overlying Upper Huronian sediments
are not exposed. Hence definite proofs of their interrelations can not be
given. The volcanics have been folded with the sediments, and subsequent
erosion has exposed them along the axis of an anticline.
The southei'n arm of tlie curved syncline bends around tlie extreme
southern projection of the Hemlock volcanics in sees. 1 and 2, T. 42 N., R.
PLATE XIV.
159
PLATE XIV.
IDEALIZED STRUCTURAL MAP AND DETAIL GEOLOGICAL MAT, WITH SECTIONS, TO SHOW THE DISTRIBU-
TION' AND STRUCTURK OF THE lURONIAN ROCKS IN' THE VICINITY OI' CUYSTAL KALLS. .MICHIGAN.
Idealized .structural map of the viciuity of Crystal Falls. An attempt has beeu made to
illustrate upon this map the distributiou of the Hurouian rocks, aud at the same time our conception
of the general features of the structure of this area. The drainage is merely introduced for the
purpose of orientation. The topography as here represented does not agree with the true topography
of the area. The bottom of the geological basin now occupies, as the result of erosion, the highest
places topographically.
Detail geological map, with sections, to show the distribution and structure of the Hurouian
rocks in the immediate vicinity of Crystal Falls. This serves as a key to the accompanying idealized
structural map.
160
FOLDlNd OF iri'l'KU HUUONIAN SEKIES. 161
31 W., and swing-s e;ist imrtli of Lakc^ Mary into st'c. 32, 'V. 43 N., R. 31 W.
Ht'ir tt'iTUfiinon.s slates aiv ex])ose(l, borderinfi' tlic ]\Iicliig-annne River
at the so-called Gliddeu exploration. The extension of these lowermost
U))j)er lluronian heds east from this }X)int soon passes nndcr the sand plains
and drift hills and is lost. The higher heds of the series are, however,
exj)osed in the lower course of the Michig-anmie, Paint, and Bi-ule rivers,
which give good sections across them. In this portion of the area discussed
the extension of even these higher parts of the formation can not, however,
be followed farther east than the Michigamme River.
That the Crystal Falls synclinal basin is not simple, but has minor
rolls, is shown by the way in which the Upper Huronian series indents the
Lower Huronian at the eastern end. Also the close and complicated folding
is shown by mining work, and can be nicely seen in the open pits of the
Columbia and Crystal Falls mines, in the exposures in the I'ailroad cut near
the Crystal Falls mine, and also along both banks of the Paint River near
the town of Crystal Falls. PI. XIV shows the general character of this
svncline. The folding has produced extensive "reibnngs-breccias." Near
Crystal Falls, along the river bank, about one-fourth mile south of the rail-
road bridge, may be seen such a breccia, which has been formed at the
junction of a chert with the slates.
TIME OF FOLDING OF THE UPPER HURONIAN.
The latest folding to which the rock of tlie Crystal Falls district has
been subjected is that which affected the Upper Huronian and likewise
involved the underlying Archean and Lower Huronian rocks. Therefore
the determination of this period of folding is of especial interest, as mark-
ing the close of orogenic movements in this district.
Overlying the Upj^er Huronian is the Potsdam Cambrian, or Lake
Superior sandstone. The beds of this formation are horizontal, or else
show a very slight tilting, following the general inclination of the district,
which perhaps to a great extent may be explained by the initial dips of tlie
beds. They overlie with strong unconformity the upturned and strongly
plicated beds of the Upper Huronian. This unconformity^ marks a. lapse of
time represented in other districts by the following events: (1) A period of
u^jheaval and denudation of the Upper Huronian; (2) the subsidence and
deposition upon the truncated Upper Huronian sediments of the hetero-
MON XXXVI 11
162 THE CRYSTAL FALLS lEONBBAKING DISTRICT.
geiieous volcanic and sedimeiitary Keweenawan series; (3) the ii])lieaval
and truncation of the Keweenawan, in which movement of course tlie Upper
Huronian in the Keweenawan areas was likewise involved. Subsidence of
the laud areas and the transgression of the Cambrian sea followed, with
deposition of the horizontal Lake Superior sandstone upon the inclined
Keweenawan and Upper Hui-onian rocks. The Upper Huronian of the
Crystal Falls district may have been involved in one or both of the foldings
which took place prior and subsequent to the Keweenawan; or, second,
since no Keweenawan deposits are known in the Crystal Falls district, it
may be that it suffered ah early period of powerful orogenic movement,
which raised the rocks above the sea, and was synchronous with the pre-
Keweenawan upheaval. A long period of erosion, accompanied perhaps
by other less important orogenic movements, may have followed contem-
poraneous with the activity of the Keweenawan volcanoes and the f)scilla-
tory movements of tlie Keweenawan region. The latter I conceive to be
the more jjrobable view. If this is correct, the intense folding of the Upper
Huronian sediments in the Cr3'stal Falls district took place immediately
preceding the deposition of the Keweenawan series in other parts of the
Lake Superior region.
KELATIONS TO OTHER SERIES.
It has been seen that in the western part of the district the Hemlock
volcanics are the highest member of the Lower Huronian. At the end of
the volcanic activity there must have taken place a very general transgres-
sion of the sea, as is eAadenced by the continuous belt of sedimentary rocks
which encircle the volcanics. The very marked change in the character of
the rocks from suliaerial volcanics to true sedimentaries partly marks the
division of the Upper Huronian and Lower Huronian series. The deter-
mining points in favor of this subdivision are found in the eastern part of the
district described by Smyth, and in the Marquette district still farther north-
east. In only one jjlace in the western part of the Crj^stal Falls district, in
sec. 26, T. 44 N., R. 33 W., has a contact between the two series been obtained.
A drill hole here passed through a mottled slate just before entering the
Lower Huronian volcanics. A similar slate was obtained at Amasa over-
lyino- conglomeratic volcanic material, which outcrops at the surface, but
no direct contact has been found. With most careful examination I have
been unable to determine whether the conglomeratic rock is a true volcanic
RELATIONS OF UPI'Elt UUKONIAN SEHIES. 103
tiirt" (li'|)osit(.'<l iipou tlif laiiil, or is water-deposited volcanic material, and
thus possibly a hasal conjilouierate of the U])])er Ilunmian.
Insec. 34/r. 4() X., II. ,')3 W., the Lower lliironianand I'pper Iluronian
are found in ver\-close proxiniitv. Here the Upper Huronian is a fei'i'uyinous
graxwacke and is separated li\' oidv aliout 10 yards from the schistose vol-
caiiics. In tliis case careful search failed to reveal the intermediate rock.
In onlv one case in addition to the Amasa instance has a distinct conglom-
erate been found which can be considered as a possible basal conglomerate-
Tliis is in sec. 9, T. 42 N., R. 31 W., along the Michigamine River. Here
there is a thick mass of conglomerate overlain bv soutliward-dijjping schists
The conglomerate contains pebbles of extremely altered basic amygdaloidal
rocks and of acid rocks which rest in a matrix of chlorite-schist. This
detrital rock is such as might be derived from the Hendock volcanics, but
between it and these volcanics is found a mass of ferruginous muscovitic and
chloritic schists at the Cllidden exploration (sec. 32, T. 45 N., R. 31 W.), wdiich
are very similar to those occurring immediately south of the conglomerate,
and like them have apparently a southern dip. The true relations between
this conglomerate and the schists at the (Hidden workings are not certain.
The conglomerate niay be below them. In that case the Gliddeu schists
would correspond to those south of the conglomerate, the beds having
received their present distribution from the close folding to which they have
been subjected.
If such be the case, the schists at the Glidden are the northern limb of
a syncline, and the conglomerate and the overlying schists, with an average
dip of 70° S., represent the southern limb of a steep anticline, whose crest
and northern limb have been cut oft' and covered up. The considerable
width of the conglomerate exposed may be partly due to the fact tliat it
has been doubled upon itself.
That the folding in this part of the district was probably fully sufhcient
to produce such structural relations and also the petrographical changes
of the conglomerate matrix to a chloritic schistose mass is shown by the
changes which the sedimentaries south of them in this part of the district
have undergone. If this interpretation is correct, the space between the
schists at the Glidden exploration and the Hemlock volcanics should be
occupied by the equivalents of the conglomerate. Section K-L, PL VI,
embodies this idea of the structure.
164 THE CRYSTAL FALLS IRON-BEARING DISTRICT.
RELATIONS TO INTRUSIVES.
The Upper Hurouian, as well as the Lower Huronlan, has been pene-
trated bj^ intrusive rocks. The diflference in the character of the intrusives
of the two series is, however, interesting-. As has been seen (p. 77), the
Lower Huronian is cut by vast masses of basic rocks and by rare dikes of
acid rocks. In the Upper Huronian of the southern part of the district the
acid rocks are more abundant but still subordinate to the basic intrusives.
North of Crystal Falls is a great east and wf st basic dike. Similar rocks are
known in a few small bosses near Crystal Falls. Moreover, on the Michi-
gamnie River, in sees. 31 and 32, T. 43 N., R. 31 W., and in a few places
to the southeast of this area, the Upper Huronian is cut by the southern
continuation of the basic masses whose principal occurrence is in tlie Lower
Huronian area. Finally basic and ultrabasic intrusives pierce the Upper
Huronian sediments in sees. 15, 28, and 29, T. 42 N., R. 31 W , and in a few
other places. The acid rocks occur in isolated knobs near Crystal Falls,
in sec. 28, T. 43 N., R. 32 W., and in sec. 4, T. 42 N., R. 32 W. They
increase in (piantity toward the southeast, and in the vicinity of Lake
Tobin, in sees. 21 and 28, 42 T. N., R. 32 W., tlie}' form a series of small
hills rising- Ixjldly out of the sand plains; and finally they occur in larg-e
quantity in sees. 19, 20, and 29, T. 42 N., R. 31 W., between the Paint and
the Michigamme rivers.
CORRELATION.
The Upper Huronian sedimentary rocks were first studied in the field
by Brooks, and the conclusion reached by him that they are late Huronian.^
Rominger correlates these same rocks very correctly with the schists
exposed aro^^nd Lake Michigamme, although he has the erroneous idea that
they follow down the ^lichigamme River, instead of making a wide curve
to the west, as subsequent study of the area has shown.^ He considers the
rocks as forming the middle portion of his Arenaceous Slate group.'
Recent work in the district has shown the Upper Huronian rocks to be
' Geolojiy nf the Menominee region, by T. B. Brooks: Geol. of Wisconsin, Vol. Ill, 1880, p. 44.
-"The mioa-schists seem to continue southward along the course of the Michigamme River, as
we find iu its lower course. 5 or li miles north from its entrance into the Brule River, and from there
down to the mouth, mica-schist to lie the iirevailing surface rock. Along the lower course of the
adjoining Paint River the mica-schists likewise are the only rocks seen in the exposures." (Geol. of
Michigan, Vol. V, 1895, p. 81.)
»0p. cit., p. 79.
CORRELATION OF UPPP^K HURONIAN SERIES. 165
iinqn(>stioniil)ly tliu wcsturu (•iiiitiiiuatioii of the .Michiginiinic- fonnatinn, In
wliicli tlu' rocks correspond |»etro<i;Taj)liically. The Micliifianime fonnalioii
has recentlv been carefully studied by Van Hise, and described l)y liiiii in
detail in Monograph XXVIII (Chaj). IV). A less detailed description is
g-iven in the Fifteenth Annual Report (pp. .'')98-604). Since such great
])etrograp]ucal silnilarit^' between the Upper Iluronian dej)osits in the west-
ern half of the Crystal Falls district and the above formation in the adjoining
districts exists, and since nothing of exceptional interest has been observed
in their stud}-, the reader is referred to the articles mentioned for details.
The following general description, while based ujjon the study of many
exposures, specimens, and 75 sections of these Crystal Falls rocks, may still
be considered to some extent as an abstract of the above articles in which
the few changes made necessary by the slightly different characters, have
been incorporated.
PETROGRAPHICAL, CHARACTERS.
From the above general statement it is seen that the Upper Huronian
comprises rocks both of sedimentary and of igneous origin.
The preponderant deposits of the western half of the Crystal Falls
district were muds and grits. With these were subordinate quantities of
carbonates. In a few places sheets of basic rocks were intruded between
the sedimentary beds and are now foiuid alternating with them. Widely
distributed basal conglomerates, coarse quartzitic conglomerates, and
quartzites, such as characterize the lowest horizon (the Goodrich quartzite)
of the Upper Huronian of the Marquette district, are absent. Work already
completed outside of the immediate area covered by this report shows the
presence of a small area of surface volcanics associated with the modified
Upper Huronian sediments. This evidence of contemporaneous A'olcanic
acti^^ty is closely paralleled by the Clarksburg volcanics of the Upper
Marquette of the adjoining district.'
SEDIMENTARY ROCKS.
The sedimentary rocks of the Ujjper Huronian series in the western
part of the Crystal Falls district are graywackes, feri-uginous graywackes ;
micaceous, carbonaceous, and ferruginous clay slates and their crystalline
'Fifteenth Ann. Rept. U. S. Geol. Survey, cit., pp. 604-607; Monograph U. S. Geo]. Survey, Voi.
XXVIII, cit.. pp. 460-486.
166 THE CRYSTAL FALLS IROlSr-BEARING DISTRICT.
derivatives; and thinly laminated cherty siderite-slate, ferruginous chert,
and iron ores. With these we find only in two places rocks of a well-
developed conglomeratic nature.
Some of the rocks have undergone great metamorphism, and we find
the graywackes and slates passing into chlorite-schists, mica-schists, and
mica-fneisses. The ore deposits of the distinct are associated with the least
altered sedinientaries.
The graywackes and slates are found chiefly in the northern and
western parts of the district, while the single conglomerate, the metamor-
phosed or micaceous graywackes and slates, the mica-schists, and the mica-
gneisses are confined to tlie southern portion.
Near Crystal Falls on both banks of the river, between the wagon and
railroad bridges, there is exposed a conglomeratic phase of graywacke.
Several bands of these coarse conglomeratic graywackes are interlamiuated
with bands of fine-grained graywacke and chert. A well-developed chert
reibungsbreccia is also associated with these. I do not consider this con-
glomeratic graywacke as representing anything more than a purely local
and very slight nnconformity. This is evidently the same occurrence of
conglomerate which has already been mentioned by Wadsworth.^
The well-developed conglomerate found in sec. 9, T. 42 N., R. 31 W.,
along the Michigamme River, contaios pebbles of both basic and acid
eruptive rocks in a chlorite-schist matrix. Toward the south the rock
grades up into chloritic graywackes and chlorite-schists, which possess the
ordinary characters of similar rocks in other portions of the area. The
graywackes and slates of the district in general differ from each other
chiefly in coarseness of grain. They are commonly interbedded in the
same exposures. The rocks vary in coarseness from medium-grained
graywacke to aphanitic slates, and in color from gray to green and black,
the aphanitic slates being usually the darkest. These fine-grained rocks
always show well-developed slaty cleavage. Throughout the area the
rocks are very thoroughly consolidated, and in places where they have
been most altered they are completely crystalline schists.
The crystalline rocks which are believed to have developed from the
elastics are found in the southern portion of the district, beginning in
sec. 16, T. 42 N., R. 31 W., and are well exposed in the river section at Nor-
' Sketch of the geology of the iron, gold, and copper districts of Michigan, by M. E. Wads-
worth: Ann. Rept. State Board of Geol. Survey for 1891-92, 1893, p. 128.
rETIJOGKAPIIICAL CHARACTERS OF UlTEli HURONIAN. 167
way farry (portai^v). Tlic rocks iit this point are an altcniatiuy succession
of inica-sdiists, some of wliicli are very ([uartzitic, and hornblende-gneisses
(altered igneous rocks) in thick beds. In these the Ix'ddini;' and schistosity
agree, or else ditter so slightly as not to be noticeable. These rocks out-
crop in bold knobs for some distance away from nnd on both sides of the
Michigannne River at the carry and also form the steep clitfs between which
the river flows. In some places they are more or less ferruginous, and
at one point near the head of the rapids exjjloring for iron has Ijeen done.
These crystalline schists .jre sei)arated from the undoubted sedimen-
taries at the north by an interval of about one-half mile, and are separated
to the south by a smaller interval from the next rocks, which are also of
sedimentary origin, though of highly metamorphosed cliaracter. These
latter consist of micaceous graywackes which grade over by increasing
metamorphism int<i rocks which are indistinguishable from mica-schists and
mica-gneisses. There can he no doubt as to the clastic character of these
rocks, as oue may see on the outcrops of the least altered phases the normal
as well as the false bedding. The bedding of these micaceous graywackes
agrees with that of the mica-schists at the Norway carrv, and in them the
schistosity is usually nearly parallel to the bedding, thougli at times cutting
it at varying angles. These rocks vary in grain from fine to very coarse.
The}^ are most all a light-gray color. In two cases the presence of large
porphyritic crystals of staurolite was observed. Garnet was found in but
a single specimen, and no andalusite was seen. The scarcity or absence of
such minerals is made noticeable by the fact that they are so abundant in
the adjoining Marquette district. The contrast is the more marked since the
Crystal Falls rocks are cut by and included in granite, and in the Mar-
quette district these intrusives are absent. Splendid sections through the
metamorphosed sediments are offered by the river sections in sees. 14, 24,
35, and 36, T. 42 N., R. 32 W., on Paint River, and at Peevie^ Falls, sec.
32, T. 42 N.,R. 31 W., on the Michigamme River.
It should be noted that thus far no gi-iinerite-schists which owe their
origin to the metamorphism of feiTuginous sediments have been observed
in the Crystal Falls district; nor does the brilliant red jasper — jaspilite —
which accompanies certain of the ores in the Marquette district, occur
associated in large quantity with them in the Crystal Falls district.
' This name has been given by the lumbermen to the falls as they lose so many peevies here in
breaking jams.
168 THE CRYSTAL FALLS lEON-BBAEING DISTRICT.
The iron-bearing- rocks of llie Upper Huronian comprise cherts,
siderite-slates, ferruginous cherts, iron ores, and subordinate quantities of
ferruginous graywackes and chiy slates.
The least altered of these is a siderite-slate. This is a fine-grained gray
rock composed almost entirely of siderite, usually in rounded rhombohedral
crystals, with very little minutely crystallized silica between them in
places. Wherever they have been exposed to the weather any length of
time, these rocks have a deep reddish-brown oxidation crust. Alteration
also follows along crevices, and thus the siderite is rapidly oxidized. The
main ])roducts derived from these siderites are like those of the more
important ore-producing parts of the Penokee and Marquette districts,
namely, hematite and limonite. Little magnetite has been found. These
siderites are interbanded with the black carbonaceous clay slates. In some
cases the dividing line is sharp. In others, as the siderite lessens in quan-
tity, fragmental material increases until only a few crystals of siderite are
found scattered through the elastics. Their association with the carbona-
ceous fragmentals would seem to indicate, as pointed out by Van Hise.Hhat
the siderite owes its formation to tlie presence of organic material.
The ferruginous cherts (the term is here used as defined by Van Hise)
are banded chert and hematite, with some magnetite, in which the iron
oxide is derived from a previously existing siderite, and the cherty bands
are not of fragmental origin. This alteration from the siderite to hematite
may be easily followed from the fresh siderite through that which is slightly
discolored, to the reddish-brown earthy mass, and then to the crystalline
hematite. Such alteration processes have been illustrated and clearly
described a number of times by Van Hise, so that no further mention will
be made of them.
The ferruginous graywackes may be described as rocks which are
partly of fragmental and partly of chemical origin. For instance, the tran-
sition may be traced from a rather micaceous magnetitic graywacke, in
which ordinary and false bedding may be seen, to a rather schistose rock,
in which magnetite is predominant, Init in which is considerable fragmental
quartz and secondary muscovite and chlorite. This rock represent.s an
original grit containing more or less siderite. Metamorphism has changed
I Fifteenth Ann. Eept. U. S. Geol. Survey, oit., p. 601; Mon. U. S. Geol. Survey, Vol. XXVIII, cit., p. 447.
PETHOGKAPHIOAL CHARACTERS OF UPPER HURONIAN. 169
the sideritc to nui^iuftite, and pnxliiccil tVoiii the tiue frag-iuental mud tlie
imiscovite and cldorire.
MICROSCOPICAL DESCRIPTION OF CERTAIN OF THE SEDIMENTARIES.
In tlie foUowin^j;- pa<^-es 1 sliall describe in a brief way the graywackes
anil shxtes, the most common rocks of tlie district, and the rocks which
have been ])rodnced from tliem by metamor])hism.
The g-raywackes and slates consist chiefly of readil}- distinguishable
fragmental quartz and feldspar grains, which are embedded in a matrix
consisting of fine-grained quartz, feldsj^ar (?), biotite, muscovite, chlorite,
some siderite, epidote, small quantities of magnetite, hematite, and iron
pyrites, and a dark clayey mass. This mass appears to contain a consider-
able amount of black carbonaceous material and reddish-brown ferruginous
matter in finely disseminated specks. The greater the quantity and finer
the character of this matrix the more difiicult it becomes to determine its
constituents with any degree of certainty. In the slates the matrix plays
the chief role, while in the graywackes the large fragmental grains form
the predominant material. By a diminution in quantity of the matrix and
fragmental feldspar grains, the coarser-grained elastics approach very closely
to true quartzites, but in no case was a pure quartzite found.
nie constituents which can be recognized without difficulty as original
ones are the larger grains of feldspar and quartz. These show pressure
phenomena of all grades, from slight, wavy extinction to complete granula-
tion. Many of the large fragmental quartzes are mashed into oval-shaped
areas or are broken into numbers of frag-ments. The large feldspars are
broken, and are altering to quartz and secondary clear feldspar with a
simultaneous production of epidote and mica. In their least altered condi-
tion the original feldspars are cloudy, and hence may be readily distin-
guished from the limpid secondary grains.
The small mineral particles of the matrix, including the mica, do not
show undulatory extinction like the large fragmental quartzes and feldspars.
These micaceous minerals are in automorphic plates, and wrap around the
quartz grains, and in some cases likewise project into them. These con-
stituents of the matrix are all believed to be secondary minerals derived
from the original clayey matrix, and from the alteration of the feldspar
fragments, with the possible addition of infiltrated material. At places all
170 THE CRYSTAL FALLS IRON-BEARING DISTRICT.
of thes^e minerals occur together, liut more commonly one finds various
combinations of certain of them. When muscovite is present in large
quantity, it is usually not accompanied by biotite or chlorite, the iron and
magnesium necessary for the production of liiotite and chlorite evidently
not having been present. These last two, however, are always associated.
As the mica increases, the schistosity of the rock increases in a, corre-
sjionding naanner, and the rocks liecome those which may be spoken of as
micaceous graywackes.
These micaceous graywackes represent a somewhat ni(ire advanced
stage of metamorphism of the rocks than the graywackes just described,
and the extremely altered varieties of these are very close to the mica-
schists and mica-gneisses, according to the respective amounts of secondary
feldspar jJi'esent. No distincti(in, however, can be made in the iield between
some of the less metamorphosed graywackes and these micaceous ones.
The chief difference appears to be in the fact that in the micaceous gray-
wackes the larger feldspars are almost completelj^ altered and the finer
matrix completely recrystallized into readily distinguishable mineral parti-
cles. In these more metamorphosed rocks the parallel intergrowth of
secondarA' muscovite and biotite is nicely shown, a thin leaf of biotite being-
included l^etween two lamella- of nmscovite. A considerable quantity of
epidote is scattered in large grains tlu-ough the micaceous graywackes,
besides occumng in aggregates of small grains. Some crystals of apatite
and tourmaline were observed. Rutile is found in some quantity, and with
it is also spheue, both of them possibly resulting from the alteration of
titanium-bearing iron ores in the original graywackes. The iron present
in the original graywackes as siderite and the minute specks of oxide have
been collected into large crystals of magnetite and also into aggregates of
smaller, well-defined magnetite crystals.
The alteration of the feldspar and the production from it of quartz,
secondary feldspar, ejiidote, and mica is well shown in one case. In this
the nucleus of original feldspar, in the center, contains minute grains of
epidote and flakes of muscovite, besides reddish, presumably ferruginous,
specks. These with a low power cause the feldspar to appear cloudy.
Surrounding this core is a mottled zone in which secondary felds^Dar and
quartz occur. In one place in this zone a flake of biotite is observed. Some
epidote also occurs in it, but no muscovite.
PKTUOGUAl'HU'AL OIIAKACTEIIS OF UPPER HUROISIAN. 171
Tlic iiltcM-iitidU of tlui t'eldispar usually l)c-;;ius at the periphery, and
jii-aduallv advances toward the center. It thus l)reaks tlie ori<i-inal oraiii up
into irreji-ular areas and strinji-ers of feldspar, many of which arc attaclicd to
the unaltered center. IVtween these n-sidual areas of feldsp;n- th(M-e are
irrcuuhu- yrains of .secondary linn)id felilspar and ((uartz. Tlie fartlier the
alteration is advanced the less of the irregular center may l)e seen, and in
the final sta<;-e the fehlspar core disappears.
A\'hile the alteration nearly always heifins at the periphery, one case
was noticed where it ap[)arently be<i'an at various places in the g-rain, the
result l)eing the production of a secondary micropoikilitic structure. This
original feldspar is cloudy, with the usual alteration products, ])ut scattered
through this are a number ()f more or less roundish s])Ots of quartz, the
majority of which extinguish simultaneoitsly, and have a different position
of extinction from the including feldspar. Considering these two elements
alone, the structure is near the micropegmatitic; but there are other areas
which extinguish in different positions from both the quartz and the original
feldspar. These are of decidedly more angular shape than the secondary
quartzes, and appear to be secondary acid feldspar. The small size of these
secondary minerals prevents the use of any })hysical tests other than the
differences in refraction. The nnmded quartz appears in many respects
very much like the corrosion quartz of the French petrograjijhers. The
majority of the secondary feldspars are imstriated, but a few show striations.
No satisfactory sections upon which to make measurements were found.
Biotite and nuiscovite flakes are included in the quartz, and smaller auto-
morphic plates of biotite niay be seen lying partly within the altered feld-
spar grain, as though growing partly at its expense.
These highly metamorphosed micaceous rocks included under the gen-
eral term "micaceous graywackes" have the interlocking groundmass struc-
ture of the schists, but some of the larger grains sliow clastic forms. No
sharp line can be drawn between these metamorphosed sediments on the
one hand and the mica-schists and mica-gneisses on the other.
In the mica-schists and mica-gneisses all of the original mineral grains
have been completely crushed and recrystallized, and we can find no micro-
scopical criteria which enable us to class them with the sedimentary rocks.
Dynamic action in the district had sufficient power and duration to comjjlete
locall}' the metamorphism of the original sedimentaries and produce per-
172
THE CRYSTAL FALLS IRON-BEARING DISTRICT.
fectly crystalline schists, as described by Van Hise in the Penokee and
Marquette districts.^ No rocks corresponding in content of carbon to the
carbonaceous slates which occur among the rocks around Crystal Falls and
south of that town have been found among these crystalline schists. These
crystalline schists are throughout moderateh' fine grained, and consist of
quartz, feldspar, and mica, with associated epidote, rutile, tourmaline, and
iron oxides, and in a few exceptional cases crystals of staurolite and garnet.
In some of the rocks quartz and mica are preponderant and feldspar is prac-
tically wanting, and we have mica-schists. In others all three essential min-
erals are present, and we have mica-gneisses. The presence of the feldspar,
and to some extent the proportion of the mica and other minerals, depend
on the character of the original sediments. Conclusive evidence of the
sedimentar)' origin of these schists is furnished by their occurrence in the
field, where are found all gradations between them and rocks of unques-
tionably sedimentary character.
In PI. XV there is reproduced the part of Brooks's PI. IX in Vol. Ill
of the Geological Survey of Wisconsin which comes within the Crystal
Falls district and includes a part of the area luiderlain by the Huroniau
sediments. There is here given his macroscopical description^ of the rocks
collected in his study of the area.
Brooks^s macroscopical description of rocks collected in the Crystal Falls district.
No. of
bed.
ThickDeas
in feet.
Mica-scMst, alternating with gneiss, generally flaggy or
schistose (2452) ' 1,250
About IJ miles west autl a little north is an outcrop of
mica.-sehist containing staurolite (2160). The same rock
crosses the Paint 1 mile farther west and north, where it
also contains staurolite. This is important and interest-
ing, since this miueral characterizes the same bed of mica-
schist (XIX) in the Marquette region, where it is associated
with andalusite. Neither mineral has been observed else-
where in the series.
I Mon. U. S. Geol. Survey, Vol. XIX, cit., pp. 332-343; Mon. U. S. Geol. Survey, Vol. XXVIII, cit.,
pp. 449-450.
^Geology of the Menominee iron region, by T. B. Brooks: Geol. of Wis., Vol. Ill, 1880, Pt. VII,
p. 496.
PETKOGRAPEIICAL CHAKAOTEKS OF UPPKK HUKONIAN.
Brooks's macroscopical description of rocks, etc. — Continued.
173
Xo. of
bed.
t.
Hornblende-Kchist or yneiss, black, slaty, liamled with Kneissic
layers (245:^ )
(iniias iind luica-nrhial
Covered
Stauiolilir iiiica-schisl ou south, overlaid with niica-arliial :inil
gneiss ( 2455)
Covered
V. \ (! iieiss
\v. ! Mint-schist
Micaceous ijuartz-schist or (jnciss (2381, 2457)
Covered
Gneiss
Thickness
ill fe»t.
100
mo
130
400
150
100
100
250
500
150
He then iiieutiniis the occurrence farther np the Miehigainnie Kiver of
exposures of "a heavy l)e(l of uneiss, dipping north at a high angle," and
also speaks of "outcrops along the river of hornhlendic and other rocks, often
granitic in appearance." He writes also of "chloritic and hornblende schists
exposed for a thickness of over 1,000 feet" at the Norway portage, farther
up the river. "They dip south at a high angle under tlie granite horn-
blende Ijelt just described, and are proljably the equivalents of some
portion of the Long Portage series on the opposite side of this synclinal,
although these differ from the prevailing rocks of that series in being
decidedly more chloritic and hornblendic'
It is seen from the above quotations that the general characters of the
rocks were recognized by Brooks. His schists are for the most part the
metamorphosed sediments, and the hornblendic and granitic rocks are the
various basic and acid rocks Avhich intrude them.
Specimens from these interesting beds were collected by the Michigan
and Wisconsin surveys, and were described by Julien, Wichmann, and
Wright in lithological reports appended to the reports of those survevs."
' Geology of Jleuouiinee iron region, by T. B. Brooks: Geol. of AVis., Vol. Ill, 1880, Pt. VII, i).497.
- Microscopical observations on the iron-bearing rocks from the region soutli of Lake Superior,
by Arthur Wichmann: Chapter V of Brooks's Geology ofthe Menominee iron region, Geol. Survey of
Wis., Vol. Ill, 1880, pp. 600-G5(3.
Lithology, by A. A. Julien: Geol. of Mich., Vol. II, 1873, pp. 1-197. Appendix A.
Geology of the Menominee iron region, by C. E. Wright: Geol. Survey of Wis., 1S80, part 8,
pp. (;90-717.
174 THE CRYSTAL FALLS IROD-BEAEING DISTRICT.
Specimens of these mica-scliists and chlorite-schists were regarded by
Wichniann as nonfragmental.' Tliis is little to be wondered at, since lie
had never studied their field relations. In the field these rocks can, however,
be traced into rocks of" unquestionably fragmental origin. The same speci-
mens were described as "micaceous quartz-schists" by AYright." Wright
also mentions a staurolitiferous mica-schist in the Michigannne River, and
also in the Paint River. The second occurrence is 2 J miles northwest from
the first and in the direction of its strike. Julien describes metamorphic
rocks from Long Portage as "fine-grained grayish black gneisses."' From
these descriptions it is seen tliat the least altered sedimentaries on the one
hand and the crystalline schists on the other were recognized by these
earliest students of the metamorphic rocks. However, the fact to which
I would especially call attention, that the crystalline schists are derived
from the clastic rocks b}' metamorphism, was evidently not understood.
The contact action produced by igneous intrusions into these series
of sedimentaries will be discussed in connection with the intrusive rocks
(p. 194 et seq.).
IGNEOUS ROCKS.
The igneous rocks which are found to have penetrated the Uppei-
Huronian after the important folding of tlie rocks took i)lace are not
included here, but may be found described under the heading "Intrusives"
(p. 187). In this place it is desired to call attention to certain hornblende-
gneisses wliicli occur near J^orwa)' carry, on the Michigannne River, and
also extend in large outcrops west of the river for about 2 miles and east
for about a mile. These are interlaminated in thick masses with the mica-
schists. Thev are perfectly crystalline hornblende-gneisses. They consist
of connnon hornblende, quartz, feldspar, and some iron oxide. The horn-
blende is present in large (juantity, the parallel })lates of that mineral
giving the rock its schistosity. None of the minerals are automorphic, but
all occur in interlocking grains. Without going into a detail description
of these rocks, it will suffice i)erhaps to state that they are similar in all
respects to hornblende-gneisses which in other parts of the Lake Superior
'Op. cit.,pp. 635,646.
-Op. cit., p. 693.
■ Op. cit., p. 130.
'Bull.TJ. S. Geol. Survey No. 62, by <i. II. Williiims, 1890; Mou. U. S. Gcol. Siirvoy, Vol. XXVIII,
jip. 152-1.59, 203, 208.
ORE DEP08ITrf OF UPPER IIUKONIAN. 175
reo'ion have Ix'Oii traced into ij^-iicous rocks/ ^''hcsi' jiiicisscs are believed to
be i<;neous rocks, either intrusives whicli were injected ])arallel to the ])ed-
ding- ot" the U])|)er Huronian sediments prior to tlic folding, or contempo-
raneous \(ilcanics. 'riie\' have l)een uietaniorpliosed and rendered schistose
bv the same forces which metamorjjhosed the sediments. This explains the
perfect agreement of their schistosity with that of the adjacent sediments.
ORE DEPOSITS.
HISTORY OF OPENING OF THE DISTRICT.
For a number of years after the opening of the mines of the Menomi-
nee range, prospectors worked in Aarions places, among others in the vicinity
of Crystal Falls, seeking to follow the iron range west of the Menominee
River. As a resnlt of this endeavor, the deposits at Florence, Wisconsin,
and then those farther north and west at CJrystal Falls, Michigan, were in
turn located. It was not until 1881 that sufficient exploratory work had
been done at Crystal Falls to warrant a belief in the future of this iron-
bearing area. In April, 1882, the Chicago and Northwestern Railway com-
pleted its branch to C'rystal Falls, and the shipment of ore began. The
Amasa deposits were not exploited to any great extent until the year 1888,
when the Chicago and Northwestern Railway built a branch from Crystal
Falls to Amasa. The Chicago, Milwaukee and St. Paul Railway, in 1893,
completed a line from Channing to Sidnaw, which runs through Amasa.
DISTRIBUTION.
The iron-bearing rocks trend nortlnvest and southeast from Crystal
Falls. East of Crystal Falls some of the ore deposits are found in prox-
imity to the Henilock volcanics, and follow along a line located a short
distance from them. Otlier deposits are those at Amasa, about 12 miles
northwest of Crystal Falls. These are near the contact between the Upper
Huronian and Lower Huronian, and above the Hemlock volcanics, like
the deposits east of Crvstal Falls. Four miles north of Amasa are the
explorations in sec. 20, T. 45 N., R. 33 W., in which the iron-bearing beds
are exposed. Another exposure of the iron-bearing formation is in sec. 34,
T. 46 N., R. 33 W., about 4 miles still farther north.
These are the northernmost known exposures of the iron-bearing rocks
of the Upper Huronian in the Crystal Falls district. However, dial-
compass and dip-needle work has located a line of magnetic attraction for
176 THE CRYSTAL FALLS IRON-BEARING DISTRICT.
about 12 miles to the north aud east. By meaus of this line of magnetic
attraction, with the assistance aflforded by occasional outcrops of Lower
Huronian, Hemlock volcanics, the possible continuation of the iron-beai'ing
belt was approximately located.
I shall take up the four localities mentioned in which an iron-bearing
formation has been found, and discuss them in some detail, beginning at the
northern and least important and passing to the southern aud most important
part of the district.
WESTEKN HALF OF SEC. 3-t, T. 40 N., E. 33 W.
In the western half of sec. 34, T. 46 N., R. 33 W., there are outcrops
of a magnetic graywacke which grades into a rock that might properly be
called a magnetite-schist but for the tact that its partial fragmental natui-e is
still apparent. The rock contains a varying quantity of magnetite, always
enough to exercise great influence over the magnetic needle. However, in
no case have true ore deposits been found in it, although the vicinity has
been extensively test pitted. The strike is in general north and south, Avith
a high dip to the west, thus agreeing in general character with the trend of
the Hemlock volcanics. The highest outcrop of the volcanics is a schistose
amygdaloid. After an interval of no exposure of about 30 feet, graywacke
appears, and this grades up intt) the magnetitic beds.
SEC. iiO, T. 45 N., R. 33 w.
To the south, in sec. 20, T. 45 N., R. 33 W., are outcrops of ferruginous
chert, which in places contains "bands and shots" of ore, the thicker bands
being an inch and a half across. These outcrops have tempted prospectors
to do considerable exploring by means of both test pits and diamond-drill
holes. The results have been negative. The general map, PI. HI, shows
that the Upper Huronian at this place indents the Lower Huronian series,
indicating, as has already been said, the presence of a westward-pitching
svncline. The presence of th"s syncline is further shown by the strike
obtained on the outcrops of chert found at this locality. For the most
part, a nearly north-south strike i)revails. The greater part of the northern
ledges give an east-west strike, with a variation of lint a few degrees to the
north of east. The southernmost outcrops show a strike which varies from
X. 27° E. to N. 34° W. The dip is in all cases high, ranging from 80° to
U.S GEOLOGICAL SURVEY
MONOGRAPH XXXVI PL, XVI
R 33 W
R 33 W
DETAIL GEOLOGICAL, MAP
VICINIT\^ OF AMASA, MICHIGAN
JULIUS aiENaCO LITM.NY
SCALE : 2 INCHKS - 1 MILE
CONTOUR INTERVAL 20 FEET
0 Outcrops wthoul obser\-ed strike or dip
T Outcrops vdth dptermined strike and dip
X Outcrops with slatinoas or Nrhisto.sity
. Tost pits bottomed in rook
ALGONKIAN
LOWER HIIRONIAN
HEMLOCK FORMATION
Yoltamcs Normal Sediments
'^'hl
UPPER HURONIAN
UNDIVIDED
I ^ I
ORE DEPOSITS OF UPPER HURON IAN.
177
DRILL HEMLOCK
87°. Tlu' si'vero dotoniiatiou is olcarly shown l)ytlu' plication of the beds,
and by faults whose extent can not be determined, but which are accom-
panied by rather extensive reiljuugsbreccias. The breccias are cemented
b\- iron oxide.
THE AMASA AREA.
The Aniasa deposits must of necessity be very briefly described, as I
have l)een unable to obtain much information concerning the relations of
the rocks as shown in the closed mine. In the early days of the mine it
was thought by the
mine captain that the
volcanics fonned the
foot wall of the ore,
and on liis authority
Van Hise says, "The
ore of the Hemlock
mine rests upon a
stratum consisting of
surface volcanic mate-
rial " ^ ^"^' ^^' — '^^***^^® section illustrating results of diamond-drill work.
Probably this is a mistake, for the section from west to east (fig. 11),
i. e., from the higher to the lower beds, obtained in two drill holes, is as
follows :
Feet.
Gray sericitic ulate, discolored liy iron 115
Chert and jasper 59
Pyritiferous black slate aud quartzito 180
Ore formation .* 3q^
Magnetitie slate 40
Mottled slates, red and green, containing iron. Drilling ceased after passing through 70
The thickness of the beds given is the true one and not the thickness
passed through by the drill, which cut through the beds at an angle. These
beds projected to the surface are found to be immediately underlain by
greenstone, in some places massive, in others tufaceous. Moreover, an
identical section is shown by a drill hole 4 miles southeast of Amasa, in sec.
26, T. 44 N., R. 33 W. It was carried deeper, however, and after passing
through the mottled slate was bottomed in greenstone. From these drill
LOWER
huroniaN
The iron ores of the Marquette district of Michigan, by C. R. Van Hise; Am. Jour. Sci., vol.
43, 1892, p. 130.
MON XXXVI 12
178 THE CRYSTAL FALLS IRON-BEARING DISTRICT.
holes it appears eertaiu that the ore formation is immediately underlain and
overlain by black slates. The foot and hanging slates are much alike, the
hanoino-, however, being verj^ pyritiferous, and the foot containing much
more iron than the hanging. This iron is in the form of hematite and mag-
netite. Below the black magnetic slate is the ferruginous mottled slate,
which apparently lies next to the Lower Huronian Hemlock volcanics.
The so-called ore formation consists of banded chert and jasper, with which
the hematite bodies are associated.
The results obtained from these holes show the lenticular character of
tne ore bodies and the difficulty in finding them. One of the holes passed
through the ore formation, l)ut missed the ore body, which subsequent
undero-round work showed it would have struck had it cut the formation 50
feet farther north.
The distribution of the Huronian rocks in the vicinity of Amasa is
shown on the map, PI. XVI.
THE CRYSTAL FALLS AREA.
The most of the observations upon the ore bodies and their relations to
surrounding beds have naturally been made in the vicinity of the town of
Crvstal Falls, where, owing to the extensive development of the mines, the
underground conditions could best be studied. The conclusions reached,
however, are confidently believed to hold good for the entire Upper Huro-
nian of the district.
In the description of the folding of the Upper Huronian it was stated
that the Crystal Falls area is in a syuclinorium forking as the result of a
subordinate central anticline so as to produce a U opening to the south of
west. It is in this basin that the important mines of the Crystal Falls dis-
trict are situated. One row of mines — the Hollister, Ai-meuia, Lee Peck,
and Hope— as shown by the map (PL XVII), lies to the west and north-
west of the main mass of Hemlock volcanics between Crystal Falls and
Mansfield. A second and more important set of mines follows an east-west
line south of the subordinate area of volcanics, which lie just north of Crys-
tal Falls in the midst of the Upper Huronian sediments. The second set of
mines, including the Crystal Falls, Great Western, Lincoln, Paint River,
Lamont, Youugstown, and Claire, lies near the axis of the syncline — that
is, along the line of major folding, and consequently greatest mashing. The
17
U.S. GEOLOGICAL SURVEY
DKTAII, GE0L0(;K;AI, M.U' OF THK MCIMT^OF (m'STAL I'AIJ.S AM) MAN'SFIKI.D. SHKF.T I
I-ONTUL-Ii 1.NIEUVAL.20 Hi 1. 1
ned wnlic Juitl <li|i -f Outcrvpe wiili alatuicss or sctiiSU)Bl(>-
LOTVER IjURONlAN"
MAN9FI&l.n SLATE HKMLOCK FORMATION
I Aim 1 rAJK:^^!
lliPP55_5S^oNiAN"
INTRUSIVE
DIOHITE
18
I
T^
US-GEOLOOrCAL SURVEY
MONOGPAPH XXXVI PL XVIIl
DKTAII. GI:01.0GR:AK MAP Ol-' THK MClXm OK CRVSTAL FM.LS AND NLVN'SMKLO. SlIEP:'!' IT
CONTOUW lNTKH\-.'U..ZO I'L^KT
cltijja "11" uotDiTiunBi] BinKi- unaiUp 'V OUUit-pn vvilli a
■ Tcsi T""* ''olioiiicil 111 rork
AlXJONKTAN
LOWKR irt-nONLW UPPER HLTtONlAW
HEMl^CK VOnUXnOtt L'KPn.TDCU
nOLERITE
DIORITK
OKE DEPOSITS OF UPPEK UURONIA>;.
179
CV'luinlnu, Duuu, Miistodou, juid others to the west (PL XVIII), sire prol)-
ablv the westeru conthiuatiou of tliis hue of luiiies, and follow the trend of
the main synclinal axis of the district. The position of these mines with
reference to the main structural features of the district can be seen on the
relief map and the sketch map corresponding to it, PI. XIV.
The section made through the closely folded Upper Huronian lieds by
w
Fill. 12.— Sketcli illustraliiig coiitortioD of Ujiper Huroiiiaii strata.
the Paint River affords the best opportunity in the district for studying the
rocks, but the rocks are so crumpled that even here the succession was not
made out with certainty.
The sketch fig. 12, by W. N. Merriam,^ shows the folding of the slate and
chert strata as seen in the railroad cut between the Paint River and the Lin-
coln mine. The strike of the rocks is about N. 80° E. The sketch is taken
looking almost along the strike
of the beds. In fig. 13 a sec- ,..-•''" \.
ond sketch is given, also hj
W. N. Merriam, which illus-
trates the rapid change in strike
in these beds, due to the con-
tortion of the strata. This
change is seen near the east
end of the wagon bridge just
across the Paint River from
OrVStal Falls At this nOint ^'^' ^^' — S^^tch showing change of strike of Upper Huronian beds
•^ ' i ilue to the folds.
the beds bend from a strike ot
S. 40° E. to W. 10° S. The change takes place by means of three very
sharp bends.
The following are the observations made by Rominger upon these expo-
sures near Crystal Falls:
Among the recently discovered productive fields for irou mining, the vicinity of
Crystal Falls has become famous for its wealth in ore. The formation enclosing the ore
' Manuscript notes.
180 THE CRYSTAL FALLS IRON-BEAKING DISTRICT.
deposits, has there a great thickness, but its determination by actual measurement is
, impossible, on account of the much folded condition of the strata, and for want of con-
nected exposures transverse to the stratification. Estimating its thickness to several
thousand feet is surely not far beyond the truth. This folded condition of the strata
is in many instances an obstacle in the decision, whether in a given locality we have
under observation a descending or an ascending succession of beds.
If we follow the railroad from Crystal Falls village upward along the bed of
Paint River, we find, in the first cut the road makes into rock beds, a series of hard,
black slates, transversely intersected in almost vertical positions, and, according to
their cleavage planes, dipping in southwest direction. This cross cut is 210 steps long;
thence, for the distance of 100 steps, no rock ledges are touched by the roadbed, but
on the left side of the road similar slate rocks are denuded, which apparently represent
a continuation of the former succession. From here for eighty steps a cut is made
through similar slate rocks, but interlaminated with numerous quartzite seams;
further on, the intersection of slates in alternation with quartz seams continues for
quite a while, but these slate rocks are more graphitic than the former and readily
disintegrate, on exposure, into splintery fragments, as they contain a large proportion
of iron pyrites and rusty ferruginous seams causing the decay. By this time we have
reached close to the river below its falls, and find, laid open in its embankments formed
by the bluffs thirty feet high, a further conformable series of graphite-schists, 300 feet
wide. Beneath the graphite-schists, close to the water level at the foot of the falls,
succeeds an ore belt six feet wide at the surface, but widening to fifteen feet, followed
into the hillside.'
Below the ore belt follows an immensely large succession of thinly laminated
banded ferruginous quartz-schists of dark, rusty color, which beds, in steeply erected
position crossing the river bed diagonally, give a cause to falls eight or ten feet in
height. The exposed succession of beds amounts at the falls to a thickness of over
800 feet. Intermixture of pyritous shaly seams with the quartzite beds, induces their
rapid disintegration on exposure, into shelly fragments covered with an iridescent
varnish like coating of oxide-hydrate. These beds are, in the embankments on the
opposite river side, remarkably corrugated, describing in their flections perfect coils.^
CHARACTER OF THE ORE.
The ore obtained from the Crystal Falls district is chiefly soft red
hematite, though iu places it is hydrated aud graded as brown hematite
(limonite). The ore is very porous and shows many crystal-lined cavities.
At places a hard steel hematite ore is found, which runs as high as 70 per
cent metallic iron. This ore occurs in very small quantities associated with
the soft ores, and appears for the most pai-t to have formed iu geodal
cavities. When the cavities are still partly open, the Ore has botryoidal and
stalactitic forms. The ores are very similar to the ores of the Michigamme
'Iron and copper regions of the Upper and Lower Peninsulas of Michigan, by C. Rominger:
Geol. of Mich., Vol. V, 1895, Pt. 1, p. 74.
-Ibid., p. 75.
ORE DEPOSITS OF UPPEK IIURONIAN.
181
slatos of the Upper ^I;ir(|iu'tte series, Ijut (litter very coiisidei'ablv from tliose
of the Lower ]\Iai-quette series, in whieh the hard hematites and magnetites
arc important ores, and from the ores of the Menominee district, wliieh pro-
duces Uirg-e quantitie(5 of soft Ijhie hematite, some inartite, and also some
specular ore.
The following figures show the average composition of the ores for the
district. They were taken from analyses furnished b}- the management of
the various mines and from the reports of the State commissioner of mineral
statistics of Michigan.
The metallic iron of the ores ranges from 54 to 63 per cent, the aver-
age being about 59 per cent. Phosphorus in exceptional cases is as low
as 0.05 per cent, though usually ranging from 0.1 to 0.7 per cent, most com-
monly approaching the higher figure. Silica averages about 3 per cent.
These analyses show the ore to be ratlier low grade.' It is due to this that
this district has been so sensitive to the prices of iron ores. A low market
price makes the cost of production exceed the selling value, and under
these conditions work necessarily stojis.
Some of the ores in the Crystal Falls district contain a ver}' high
percentage of AI2O3, CaO, and also of manganese. It is reported that
some very good deposits of manganese have been found, one unauthenti-
cated statement being- to the effect that an analysis of the ore runs as follows:
Metallic iron, 17.46; manganese, 29.81; phosphorus, 0.064; silica, 0.009(?).
' Brooks states that " the ores are unlike those in the more easterly part of the Menominee region
in being richer in iron, freer from silica, and in containing more water." (Analysis 68, p. 302.) Geol-
ogy of Michigan, Vol. I, part 1, p. 182.
Since the above was written the volume on Mineral Resources of the United States, 1896, Part
V, of the Eighteenth Annual Report of the United States Geological Survey, has appeared, and the
following analyses of ores from the Crystal Falls district are taken from Mr. John Birkiubiue's article
on iron ores in that report.
The analyses were prepared for the ore association at Cleveland. Ohio, and show the average
cargo analyses of iron ore as shippeil from the various mines. The analyses were made from ores
dried at 212^, the amount of natural moisture being added.
Iron.
Silica.
Phos-
phorus.
Man-
ganese.
Alu-
mina.
Lime.
Mag-
nesia.
Orgjinic
Sul- and
l)hur. ivolatile
' matter.
Mois-
ture.
Crystal Falla ..,.
Dunn ^.
58.55
58.61
60.20
60.86
61.00
4.25
3.88
4.71
3.86
4.50
.721
.573
.309
.240
.350
.20
.58
.39
.45
.30
1.16
1.88
2.81
1.67
2.75
2.64
1.80
1.87
2.14
.50
.77
.83
1.32
1.73
.30
. 0U8 t 2. 92
.033 1 5.44
.010
.010
.075
7.20
8.70
6.62
6.50
9.00
Lincolu
Mastodon
182
THE CRYSTAL FALLS IROK-BEARING DISTRICT.
One of the best results gives as high as 61.5 per cent metallic Mn.
The ores thus range from a manganiferous iron ore to a manganese ore.
The further statement is made that the bed lies very close to the surface,
and is from 6 inches to 3 feet in thickness. From 4 to 6 feet of bog iron
is found underlying tlie bed of manganese.
RELATIONS TO ADJACENT ROCKS.
The ore is associated with white or reddish chert, which in places is
jaspery. The cherty iron formation passes into ore by a decrease of the
silica. An intermediate phase is chert with "bands and shots" of ore. In
places the chert is more or less brecciated, and the ore often has a similar
character. Commonly the ore is completely surrounded by the chert beds,
or chert and ore, forming the so-called mixed and lean ore. In such cases
theA" form both the foot and hanging walls of the ore
bodA". But the ore-bearing chert formation is always
associated with black carbonaceous slates, which consti-
tute the base on which the ore-bearing formation rests.
In the Youngstown mine 3 feet of so-called "graphite"
was jjassed through before the usual carbonaceous slates
were reached.^ The hanging wall is also carbonaceous
slate. At places thin quartzitic beds which approach a true quartzite are
associated with the slate.
The ores occur in the cherts in pockets and lenticular masses, which
always agree in greater dimensions with the strike of the beds with which
thev are associated. The lenticular character is well shown in the Dunn,
Columbia, and Great Western mines. In the Dunn mine the bodies over-
lap. In the Great Western mine in 1887 seven different ore bodies in an
east-west line, separated by areas of barren rock, mostly slate, were being
mined. In following these isolated ore bodies to the east, at various places
thev are found to turn around a horse of rock. Their occurrence is illus-
trated by the horizontal section, tig. 14. Evidently the ore bodies accumu-
lated in westward-pitching synclinal troughs, in which the hanging wall
appears to the miners as a horse of rock.
The ore bodies in general pitch to the west at varying angles corre-
FiG. 14.— Sketch to illus-
trate the occurrence of ore
bodies.
'This information was furnished by Mr. C. T. Roberts, of Crystal Falls,
obtain a specimen of the graphite for examination.
It was not possible to
ORE DEPOSITS OF UPPER HURONIAN. 183
.spoiiding- ro the pitches of the axes of tlie syiielhies in which thev occur.
Tlie pitdu's of these folds in turn correspond to the westward pitch of tlie
Crvstal Falls syncliuoriuin, of which the secondary synclines containing
rlie ore l>odies are a part. A typical example of the occurrence is shown in
the Armenia mine ore body, which is found, according- to Van Hise, "at the
bottom and on the sides of a synclinal trough, pitching at an angle of about
^,-o"i rpi^^^ trend of the axis is to the south and west.
The dip of the ore bodies is always steep, and generally to the south,
but varies in places to a few degrees north.
ORIGIN.
The fact tliat tlie important mines in tlie district are located in a
synclinal basin and that they all possess an impervious footwall of black
slate gives very clearly the reason for their existence and indicates their
mode of origin. They are concentrates in synclinal troughs.
In the ]\Iarquette and Penokee-Gogebic districts the ore Ijodies are
frequently found associated with dikes of dolerite (diabase), which have
been altered to "diorite "-schists, and so-called soapstone or paint rock.^
Only one such association is known for the Crystal Falls district. Wads-
worth mentions having seen a dike in the Paint River mine.^
In the field notes of the Lake Superior survey for 1<S91, I find the
statement made that "the strata of the ore formation, which here strikes
nearly east and west, is cut by an eruptive dike which runs about north-
west and southeast. This dike hades to the west, and forms with the
hanging slates of the ore formation a trough pitching to the west at a very
steep angle. In this trough is situated the ore body upon which the Paint
River and the Monitor '^ mines are working." This ore body is stated to be
about 100 feet wide, 300 feet long, and of unknown depth. When I was
in the district, the mines were closed, or only shipping from stock piles, so
that I had no opportunity of verifying this observation. From this state-
ment it appears that in this particular case the ore is due to the presence of
Iron ores of the Marquette district of Michigan, l>y C. R. V.an Hise; Am. .Tonr. .Sci., 3d series,
Vol. XLIII, 18fl2, pp. 130.
-Mernaui also mentions in his notes a dolerite dike found cutting the ferruginous rocks at the
Glidden exploration. In this case it does not appear that an ore body was foruu-d.
" Sketch of the geology of the iron, gold, and copper districts of Michigan, by M. E. Wads-
worth : Kept. State Board of Geol. Survey for 1891-92, 1893, p. 108.
■•Now known as Lamont mine.
184 THE CRYSTAL FALLS IRON-BEARING DISTRICT.
this dike, as It occurs in a pitching trough, formed by its junction with the
impervious slate. These same relations are well known to be the cause of
similar occurrences in the Lake Superior districts above mentioned.
The original rock from which the ores were formed was cherty iron
carbonate, which in many places is found associated with the iron-bearing
formation. The cherty carljonate shows the various stages of alteration
from the compact cherty siderite to the banded ore and chert rocks which
form the nuclei for the addition of the iron obtained from the higher exten-
sions of the beds. Percolating waters have been the agents in this process
of replacement and concentration. Consequently where the rocks have been
most shattered, we find the water was especially active. Hence it is, also,
that we find the deposits in this closely folded part of the Upper Huronian.
As to the origin of the cherty carbonate itself, we know nothing
definite. Its association with the carbonaceous slates would indicate the
ao-ency of organic matter in its prodiiction, possibly in some such manner
as is rather generally accepted for the formation of the Carboniferous
carbonate ores. The Upper Huronian ores, as well as the Lower Huronian,
are supposed to have been formed in this same way, and from the same
kind of rock. Under the discussion of the Lower Huronian ores (p. 70)
these points were discussed more in detail, and references given to the
literature, and the reader is referred to that discussion for further details.
SIZE OF THE ORE BODIES.
No definite general statement can be made as to the size of the ore
bodies, as this varies considerably. None of the bodies which are being
worked, so far as I can learn, are less than 30 feet wide. In one of the
old mines crosscuts disclosed a width of nearly 200 feet. This same ore
body is reported to be at least one-foin-th of a mile long.
METHODS OF MINING.
The first develoinnent of the iron ores of this district was by the
stripping and open-cut method, very few resorting at once to under-ground
work. Nearly two-thirds of the product of certain of the mines has been
from open-pit work. When the open pits become too deep to be readily
worked as such, shafts are sunk and the exploiting of the ore body is
carried on under ground, at times both open-pit and under-ground work
being carried on simultaneously. The Mastodon presented the unusual
ORE DEPOSITS OF UPPER HURONIAN. 185
siflit of ;m o\)(.'U pit c'Xten(liii<4- down 200 feet, part of tlic wdrkinf^'s still
boiii"' roofed over by an enormous arch of ro(dc. All work in tlie district
is at present under gTound.' As a rule, the nndcr-yroun<l work has not, thus
far, been carried to very great depth, the two deepest mines beinf^^- the
Great Western and the Dunn, which are down, respectively, 700 and 720
feet. The cithers are down to depths varying- from 100 to 450 feet, the
lowest figures being, as a rule, for the youngest mines, the more important
ones having nearly reached the 400-foot level or passed beyond, it.
In the early mining days of the district an extensive system of
timbering was resorted to, but gradually, as the cost of timber increased
and this item became burdensome, careful attention was paid to this point,
and, where practicable, the system of caving or the sj'stem of tilling was
introduced (Mastodon). At the present time most, if not all, of the
important producers are mined with open stopes, pillaj's being left only
when necessary. From this it would appear that the rocks had not been
much broken, but it should be borne in mind that the ore deposits them-
selves are later than the folding, and in the process of their formation many
of the cracks in the surrounding beds could have been filled with ore or
other material and the rocks thus quite rigidl}' united again. That
cementation has taken place is evident from an examination of almost any
hand specimen or slide, where one may see veins of infiltrated quartz
traversing them at various angles. The extremely wet character of the
Great Western would seem to indicate that localh' the rocks may still be
very much fissured.
PROSPECTING.
Owing to the impossibility, with our present knowledge, of mapping
the various beds of the Upper Htironian, it is not possible to g'ive any
directions with reference to the exact lines which should be followed in
searching for ore. However, since the areas which are underlain by
igneous rocks have been delimited, there is no longer an}" excuse for wast-
ing time and money in prosjjecting in these unpromising portions of the
district. W^here indications point to considerable rock movements, and
where the sideritic rocks are found associated with impervious slates,
explorations are warranted.
' This waa written in 1896. .Since tlien a large part of tlie Mansfield ore body has been stripped,
and this \u:\y be worked at present by open-cut methods.
186 TOE CRYSTAL FALLS IRON-BEAKING DISTRICT.
PRODUCTION OF ORE FROM THE CRYSTAL FALLS AREA.
In the following table the iirst column contains the names of the mines
or combinations of mines of the Crystal Falls area. These are arranged
alphabetically for ease of reference, and not according to date of opening
or amount of ore produced, as is so usually the case. In one case, that of
the Claire mine, the mine was operated by the company operating the
Youngstown, and its output accredited to that mine until 1891, when the
two mines were separated. Following the name under which a mine is
known at present, there is given in parentheses in chronological order the
name or names by which the mines were formerly known. The second
column gives the location of the mine. Following these data there are
arranged in columns the yearly shipments from the time of the first open-
ing to the closing of the mines, or to January 1, 1899. In many cases the
ore body had been definitely located and considerable work done and ore
accumulated upon stock piles several years before the first shipments were
made, but it would be impossible to give the' exact date of the opening of
the mine unless we considered the year of first shipment as such. In a
column following the yearly shipment the total shipment for each mine is
given. This is followed by a column giving the year in which the maxi-
mum shipment was made, and by another giving the amount of this ship-
ment. In the horizontal column at the foot of the page may be found the
total shipments for each year and the total product of the district since its
first exploitation. The figures for the district have been obtained either by
correspondence with the mining companies or from the annual reports of
the commissioner of mineral statistics of Michigan. Acknowledgments are
due to certain of the companies which have, through their managers, been
ver-v obliging in furnishing infonnation concerning the mines they operated.
From a comparison of the total shipment of the area for 1898 with the
total shipment of the Menominee range 2,522,265 long tons and of the entire
Lake Superior region 14,024,673 long tons for the year, it will be seen that
the Crystal Falls area furnished 13 per cent of the total iron-ore shipment
of the range and 2^ per cent of the region.^
' Total shipments for 1898 were obtained through Mr. .John Birkinbiue from the Iron Trade Review.
IRON-ORE SHIPMENTS OF THE WESTERN HALF OF THE CRYSTAL FALLS DISTRICT, GIVEN IN TOSS OF 2,240 POUNDS.
Name of mine.
Location.
Operated by —
1S6-2
_
issa
1$S4
ISSo
18S6
1VS7
18)«»
1S89
1800
1S91
1S98
1893
1894
1805
1890
1897
I8D8
Total
shipmpnl of
indiviihial
miDes and uf
district 10
date.
T.ar
ot niitx-
ilniiiu
Bliip.
ment.
Ani„,iiit
orshi),-
nieiit.
-Van.e „r ,„ine.
E.J SE. J sec. 23. T. ja N., R. 33 W ,
47,775
26,649
2,045
76.460
121. 963
484. 190
1.341
266. 331
33.246
1,025,679
375,742
375, 023
4,098
17,818 .
140.871
2,844
45. 174
8,204
306, 845
425.290
1,702
222,371
158,899
1889
1692
1896
1882
1898
1886
1801
1892
1^97
1890
1892
1892
1892
1892
1889
1893
1890
1895
1890
1890
47,775
57,351
87,202
•1,341
128, 233
17,684
162,721
87,487
96,032
2,020
15,543
42,819
2,844
26,020
4,006
69,558
66, 559
1,071
62.654
41,460
ClairC
>'E.4SE.Jsec.l9.T.43N.,K.32^V
NW.|NW.^SfC.31.T.43N..E.32W
Lot3.scc.20.T.43K., B.32W
Claire Mining Co
55, 000
70,770
57,351
57.682
9,012
22, 426
Annenia<Siiiitii,.
15, 940
1,341
4.334
.77.
U,282
2.337
10,936
11.365
60, 133
10, 300
70,867
87,202
24.623
14.199
Crvstal Falls
ColtUBbia (1. Uuion ; 2- .Sbeldon i iScliaf.^r;
Crystal Fiills
SE.iNE.jBBC.21,T.43N.,E.32W
XE.jS'W.4 8ec.24,T.42N.,K.33'W
Corrigau.McKinnev & Co
3.975
14,387
44,526
95.210
123,233
Delphic
3,410
508
9.813
17,081
1,8)1
118.001
21,861
151,828
38,451
156, 653
71,719
162,721
62,464
35. 531
1,057
133, 945
87,487
6.5,459
1.021
15, S43
42, 819
2,844
26,020
58, 261
661
11,323
90,886
47.081
31,062
49,381
Great WesterD (ImD Star)
NE.iSW.Jaeo.21,T.43N.,B.32'n'
NW.JSW.i8ec.4,T.44N.,R.33-\V
Iron Star Co
5S7
22, 825
20,722
25,725
23,2(0
DuoD.
Hemlock
1,046
05.767
96,032
69,865
Holliater
NW.5ST;r.jBec.l3,T.43N..E.32W.'.
1 i
2,020
HoUister.
Hoped- Blaney; 2, AVauneta) |
NE.JSE.l8ec.27,T.43N.,R.32W
1
2,275
13,777
Laniont (Monitor) '
Lot(J,NW.iSE.jBec.20.T.43N..E,32W
W.jNE.iaec.2C.T.43N..R.32W...
2.090
21, 620
31.139
26, 226
2,600
Lament (Monitor).
Lee Peck.
Lincoln (Fairbanlcs)
Manhattan (Soutli Mastoclon).
Mans&tld (Caleilonial.t
Msalodon.
Lee Peck ..-.
■n'.iSW.Jsec.21,T.43N..R.32-W
Lincoln Mining Co
8,131
453
1 ■
1,813
8,767
i 1
Manbattan (Sonlh Mastodon)
KE.J SE. 1 sec. 13. T. 42 N., E. 33 W
2,722
4.000
1,476
18.303
66,559
. .. .
1
Mansfield (Caledonia)*
KW.iNW.i8eo.20.T.43N..E.31W
SE.JNE.Jsec.l3.T.42?r.,E.33-W
De Soto Mining Co
;:;.:;::::::::::
49,836
45.370
69,259
9,150
09,558
23, 485
505
39,612
60,877
Miistoilon '
3,477
18, 577
18, 020
11,73?
jl,»0
49, lis
51,293
63.086
23,781
1,071
Michigan
NE.jN\V.j8ec.9,T.44K.,E.33-\V
XE.iSE.i8ec.20,T.43N..E.32-n-
Michigan Exploring Co
210
Paint Ki ver
Paint River Iron Co
Illinois Steel Co
6.515
6.188
5,073
15,292
11,652
8, .143
2.373
13, 033
25,638
10.210
34, 418
12,506
12.700
32,700
7,471
02,654
44.460
45. 435
3,705
18,390
Paint River.
Youngstown,
Yoiingstown
J.nv.} Sn'.isec.20. T.43N.. E.32W
13
601
1
Total
42. 1S9
70, SCI
06, 019
22,953
138,902
140, 621
2J2, 799
378, 322
.^40, 040
559, 828
580,070
220, 040
12,000
134,096
274. 576
286. 610
392, 555
4,036,448
MON XXXVI — fact' ]). ISi;
■ Previous to IfiDl Clairo and Yonngstown products were quoted together and credited to Youngstown, though they were about eiiually divided,
t LowiT Iluionian. Only pruduclive EeSBi-mer mine in Crystal Falls area.
1
■
CHAPTER YI.
THE INTKUSIVES.
Under tliis general head there is here included an extremely varied
assortment of rocks exhibiting in common intrusive relations to sedimentary
and igneous rocks. This division is here used merely because it simplifies
the classification of the rocks of the district, and the term "intrusives" is not
to be interpreted as synonymous with the "dike rocks" (ganggesteine) of
some authors, a petrographical division which, in the opinion of the writer,
is not warranted.
These intrusive rocks differ very materially in field occurrence, petro-
gi-aphically, and in point of age from the igneous rocks thus far described.
In age much younger than the volcanics, they still bear a close resemblance
to some of them; indeed, some forms are identical in character. Massive
granular rocks are the common forms. Porphyritic varieties are very
subordinate.
The rocks are all considered as intnisives into either the Lower or the
Upper Huronian. In most cases the intrusive relations may be said to be
rather inferred than demonstrated, for the direct contacts have been observed
in very few cases. However, where isolated sets of knobs of eruptive rocks
are found in areas, the greater portion of which are underlain hj sedimen-
taries, the natural inference is that they penetrate these sedimentaries.
Where isolated sets of knobs are composed of the same kind of rock or
show variations of the same type, they may be presumed to be connected.
For the most part the dikes and bosses are too small to admit of indication
upon the accompanying map. Wherever their size has warranted it, they
have been represented, as in the case of the acid intrusives between the Paint
and Michigamme rivers, and of the basic intrusives north of Crystal Falls.
187
188 THE CRYSTAL FALLS IRON-BEARING DISTRICT.
ORDER OF TREATMENT.
For the sake of eon^•eniellce and to avoid repetition, the age of the
intrusives a.s a whole i.s first determined, and then follows a brief descrip-
tion of the effect of the folding of the district on the distribution of the
eruptives.
The rocks described ha^-e been di^aded into those of acid, basic, and
ultrabasic comjjosition, and the usual order of discussion from the acid to
the basic will be followed. In Section I rocks are described which are
geologically disconnected. In Section II is a description of a series of
rocks which constitute a geological unit, and are especially interesting from
a petrogenetic standpoint. Here also the order of discussion is from the
acid to the basic. The geographical distribution of the rock of each divi-
sion is given, only those outcrops being accui'ately located which are of
very large size or which for other reasons are of special interest from a
stratigraphical oi- a petrological standpoint.
After the petrographical description of each of the rocks a statement
will be made of its lield relation to the adjacent rocks, if its relations have
been discovered. Tliis will be followed by a brief account, in those rare
cases in which a contact has been observed, of the metainorphic action
which it has caused in the sediments through which it was forced.
AGE OF THE INTRUSIVES.
The intrusives have forced their way through the Lower and Upper
Huronian sedimentaries, but have never been found to penetrate the hori-
zontal Lake Siiperior (Cambrian) sandstone. These facts alone are con-
clusive proof that their period of intrusion falls in the time which elapsed
between the deposition of the Upper Huronian and that of the Cambrian.
Ill tlie discussion of the time of the folding of the Upper Huronian the
conclusion was reached that this folding preceded the deposition of the
Keweenawan series. If the intrusives to be described had existed at the time
of folding, they must certainly have suffered from the orogenic movements.
Examination of the exposures of the intrusives has not shown schistose
masses, nor has detailed microscopical study disclosed tlie eataclastic textures
which accompany powerful dynamic movements, except in one case, which
is described on p. 1 94, and is presumed to be due to purely local movements.
Such being the facts, the conclusion follows that the intrusives were iiitro-
AGE OF INTKUSIVES. 189
cluced subsequent to the folding of the Upper Huroniun, (.)r are of Kewee-
nawau or post-Keweenawan age.
A closer approximation to the age of tlic intrusives is not possible,
unless we rely upon petrographical similarity. The dolerites of the Crystal
Falls district are similar to those forming the flows and dikes of the Kewee-
nawan on Keweenaw Point. They are also similar to the basic intrusives
of the Marquette district, with which this is jjractically a geological unit,
and likewise they agree petrographically with the dolerite dikes of the
Penokee-Gogebic district. In both districts the late intrusives have been
considered to be of Keweenawan age.^ Rominger," in his report for 1881
and 1884, calls attention in a general statement to the possible connection
of the doleritic dikes penetrating the Michigan Huronian with the flows
and dikes of the "Copper-bearing formation" (Keweenawan).
While congelation by means of petrographical similarity would not
hold for widely separated areas, it seems to be well worth considering for
areas which are so closely connected as are the iron districts of the U})per
Peninsula of Michigan.
Judging from the evidence thus presented, the dolerites of the Crystal
Falls area are probably contemporaneous with the intrusions of the Penokee-
Gogebic and Marquette districts and with the volcanics of Keweenawan
time.
EBI.ATIONS OF FOXiDHSTG A3^D THE DISTRIBUTIOK OF THE
INTRUSIVES.
In the preceding- chapter, in the sections on folding of the Upper
Huronian, p. 158, it was shown that the main folds of the district follow an
approximately northwest-southeast course, and that upon these were super-
imposed minor folds approximately at right angles to these. The lines of
weakness parallel to the axes of the main folds have been taken advantage
of by certain of the intrusives, especially the dolerites.
A glance at the general map (PI. Ill) shows that the dolerite dikes
which have been traced for considerable distances — that is, ai'e more than
great knobs uncovered by erosion — have a northwest-southeast trend, in
agreement with the general direction of the major folding of the district
1 Mod. U. S. Geol. Survey, Vol. XIX, p. 349; Vol. XXVIII, p. 218.
= Geol. of Mich., Vol. V, cit.,p. 6.
190 THE CRYSTAL FALLS lEON-BEARIXG DISTRICT.
The only apparent exception is that part of the great mass in T. 43 N., R.
31 W., which extends north and south along the Michigamme River; but
this is reall}' not an exception, since the folds of the Mansfield slates here
run in the same direction.
SECTION I.— UNRELATED INTRUSIVES.
CLASSIFICATION.
Under the above heading are included intrusive rocks which occur in
such isolated outcro^js that no definite relations can be shown to exist
between them and other igneous rocks of similar or related characters.
The intru.sives described under this heading comprise rocks of acid,
basic, and ultrabasic composition, represented respectively by the granites,
the dolerites and basalts, and the ^jicrite-porphyries.
ACID INTRUSIVES.
The acid intrusive rocks may be divided into ordinary biotite-granite
(granitite) with a micropegmatitic variety, and muscovite-biotite-granite
and rhyolite-porphyry.
GEOGRAPHICAL DISTRIBUTION AND EXPOSURES OF GRANITES.
The biotite-granite proper does not occur in the district in large quan-
tity. It is found in dikes penetrating the Upper Huronian rocks in sees. 15
and 22, T. 42 N., R. 31 W., and in dikes, cutting diorite intrusives in the
Upper Huronian, in sec. 22, T. 42 N., R. 31 W.
The micropegmatitic variety of the biotite-granite is confined to an
area underlain by the Lower Huronian rocks. It occurs at N. 750, W. 740,
sec. 17, T. 43 N., R. 31 W., and N. 1750, W. 1580, sec. 29, T. 43 N., R. 31
W., in small quantities in isolated dikes, cutting the dolerites which penetrate
the Lower Huronian series.
Owing to their small size, none of the above-mentioned exposures are
represented on the maps.
The muscovite-biotite-granite forms large, bold, isolated knobs in sees.
19, 20, 29, and 30, T. 42 N., R. 31 W., between the Paint and Michigamme
rivers. These knobs are so closely related petrographically that they are
presumed to represent a large boss, and they are therefore represented as a
unit on the geological map of the district, PI. HI. Other smaller areas of
granite occur and are shown on the detail map, PI. XVIII.
ACID INTltUSlVES. 191
BIOTITE-GRANITE.
The biotite-granites vary iu colur tVoiu li<>'lit reddisli-browii to dark-
^av and greenisli rocks, ami iu j^-rain from tine to coarse. The structure
ordinai'ih' is tliat of a normal granite. In some of them the micropey-
niatitic intergrowth of quartz and feldspar may be observed in small quan-
tity. In others this forms the characteristic part of the rock, and these
rocks may be properly termed micropegmatitic granites. In most of the
sections the usual constituents in ordinary proportions occur.
In all the rocks the main mass of the quartz forms irregular grains,
molded on the other constituents. Sometimes round areas of quartz are
included in the feldspars. The quartz contains very commonly, and usually
in great quantities, liquid inclusions with dancing as well as stationary
bubbles.
The feldspar is nearly always of two kinds — orthoclase and plagio-
clase. Microcline was also observed, but in neglectable quantity. These
feldspars in the great majority of slides show fairly good rectangular out-
lines, and in some cases these are strikingly well developed. In some slides
the plagioclase is observed in rectangular crystals and the oithoclase is
found in large irregular plates which are jnolded on the plagioclase, show-
ing conclusively their relative age. The plagioclase is finely twinned
according to the albite law, and also in some slides exhibits pericline twin-
ning. A case was observed in which two crystals, one showing albite
twinning, the other both albite and pericline twinning, were grown together
so as to correspond to the Carlsbad twins of orthoclase. The jjlagioclase
gives low extinction angles, which show it to be rather acid. The amount
of plagioclase in some of the sections — for example, in those of the numer-
ous small dikes cutting the schists near Norway portage in sec. 15, T. 42 N.,
R. 31 W., and the one cutting the gabbro at the SE. corner of sec. 22,
T. 42 N., R. 31 W. — is very large, denoting an increase in soda and lime
and indicating a relationship to the diorites. The geological relations are
not such, however, as to enable this connection to be shown in default of
chemical analyses.
The orthoclase is for the most part untwinned, or else shows simple
Carlsbad twinning. In some sections the feldspars are quite fresh, but in
others they are seen to be opaque, porcelain-hke, and iu still others the
192 THE CKYSTAL FALLS IRON-BEARING DISTRICT.
original feldspar material is almost entirely replaced by a mass of musco-
vite, with some little epidote-zoisite and biotite. The muscovite in these
secondary aggregates gives excellent though small rectangular sections,
showing its fine cleavage very distinctly. Well-determinable kaolin flakes
were not found.
The mica is well represented Ijy both Ijiotite and muscovite. Both
occur in very well developed crystals, the muscovite showing the most per-
fect development. The biotite has j^artly altered to chlorite, with a simul-
taneous production of rutile, sagenite, and sphene. Between the chlorite
laminae one frequently sees lenticular areas of secondary calcite. Quite
commonly the sagenite is found included in these areas.
In onlv one specimen was hornblende oljserved. This was from a
granite dike whicli cut the dolerite. The hornblende is of a noncompact
variety, which upon the edges is finely fibrous. It corresponds exactly to
that which is found in the adjacent dolerite.
The contact between the granite and dolerite appears irregular, as
though the dolerite had been to some extent broken. As the contact is
approached from the granite side the hornblende increases in quantity. It
is thought probable that the hornblende in the granite along the contact is
secondary after pyroxene, and that this pyi'oxene was obtained by the
inclusion of fragments of the dolerite.
Accessory minerals are not present in large quantity.
Iron oxide is not present in great quantity, and when seen it is usually
titaniferous, as rutile is found as an alteration product. In one case the
hexagonal plates show the presence of ilmenite. Apatite is rare, as a rule,
though occurring in some sections in considerable quantity. Zircon is
scarce, as are also sphene and rutile. Epidote is rather common. In some
cases it is seen in biotite surrounded by a pleochroic halo, and in such cases
it is probably original. The secondary minerals, muscovite, biotite, chlorite,
epidote, sphene, rutile, and calcite, show their usual characters. Calcite is
abundant, and is more or less ferruginous. It is found in rhombohedra and
also in irregular masses. In all cases its secondary origin is clear.
MICROPEGMATITES.
The micropegmatitic varieties of the biotite-granite show the same
variations in color, from reddish to gray and greenish, and in grain from
ACID INTKUaiVES. 193
tine to mc'iliuiu, as do tlu' biotite-iirauitcs j)r()[)er. In o'euenil tlu;y niuy be
described as biotite-granites in wliicli the niicropegniatitie intergrowtii of
quartz; ami feldspar instead of being sul)ordinate preponderates. Many of
the well-crystallized feldspars are surrounded by a border of micropegraa-
tite, which varies from a narrow strip to a very wide border, usually in
inverse ratio to the size of the feldspar nucleus. The feldspar in the inter-
growth is continuous with that of the nucleus. A coarsely radial arrange-
ment of the niicropegniatitie intergrowtii was frequently observed. Where
the feldspars and quartz are predominantly poi-jihyritic, and micropegmatite
forms the groundmass, the rock grades over into the rhyolite-porphyries with
micropegmatitic groundmass — the inappropriately named granophyres of
Eoseiibusch.
The biotite of the micropegmatitic granites has partly altered to
chlorite and sageiiite. In some of these rocks the biotite is collected into
large aggregates of imperfect individuals, which surround larg-e pieces of
iron ore. In some instances it is included in the plagioclase. The biotite
flakes in the feldspar are sometimes so numerous as to conceal almost com-
])letely the feldspar substance. In one instance the feldspar of such a
micropegmatitic intergrowtii is completely replaced by biotite. These sec-
ondary biotite flakes surrounding the remaining more or less rounded quartz
areas of the micropegmatite produce a rock which is strikingly like a mica-
schist in places, although it is of -unquestionably eruptive character.
Tourmaline is a rare accessory in these granites, a small smoke-brown
crystal having' been observed in one section. Titaniferous iron ore alterinff
to leucoxene or sphene and rutile is found, as are also the common accessory
minerals — apatite, rutile, and zircon. They contain also the same secondary
minerals as the normal biotite-g'ranite.
MUSCOVITE-BIOTITE-GRANITE.
These are medium-grained rocks, and, owing to the fact that the mus-
covite is more abundant than the biotite, have a light-gray color.
The muscovite is noticeably automorphic with respect to the biotite,
though the biotite is also in well-developed automorphic plates. Plagio-
clase is present in these granites in very large quantity. It shows an excel-
lent zonal development, with diminishing angle of extinction — that is,
increasing acidity — from the center outward. The center of the individuals
MON XXXVI 13
194 THE CRYSTAL FALLS IRON-BEARING DISTRICT.
is nearly always extensively altered, while the outer zones are compara-
tively fresh. A maximum extinction angle of 15° against the twinning
planes in the zone perpendicular to 010 was observed on an unaltered zone
surrounding an altered core. This would indicate the feldspar to be per-
haps as basic as labradorite at the center. The other essential minerals,
quartz and orthoclase, occur in usual quantity and show nothing of espe-
cial interest. Sphene and apatite are the only accessory minerals present.
An apatite crystal was observed which was included in quartz, and con-
tained the brown apparently vitreous core so frequently seen in the apatites
of basic rocks. Where included in biotite, it is surrounded by a pleo-
chroic halo.
No analyses were obtained of these granites, but from the quantity
and character oi the feldspar as noted above, these rocks are thought to be
closely related to dioritic rocks. Indeed, it is a (question if they should
not be classed as quartz-diorites.
RELATIONS OF GRANITES TO OTHER INTRUSIVES.
In two cases alreadv mentioned granite dikes cut the diorites. Granite
also cuts the gabbro, and dikes of granite were observed penetrating the
dolerites, thus indicating that tlie granites are younger than tliese igneous
rocks.
DYNAMIC ACTION IN GRANITES.
An examination of the granites with particular reference to pressure
phenomena shows that the}' exhibit a great difference in this respect. Some
show scarcely any traces of pressure, while others quite closely associated
may have been affected thereby to such an extent that a more or less
strongly wavy extinction of their mineral constituents is general. However,
but a single instance of a supposed granite possessing an excellent cataclastic
structure and imperfect schistosity was observed, and this rock was so
extremely altered as to render dcnibtful a determination of its original
character.
CONTACTS OF GRANITES AND SEDIMENTARIES.
The largest intrusive granite mass is found l>etween the Paint and
Michigamme rivers, in sees. 19, 20, 29, and 30, in T. 42 N., R. 31 W.
The granite is a inusco\'ite-biotite-granite. The sedimentaries are mica-
ceous graywackes, which have been described on p. 170. Let it suffice
ACID INTliUSIVES. 195
here to repeat that thev li;ivc Ix-cii luiicli mashed and recrystalhzcd, Imt
that some of them still show their fraymeiital nriuiii. New hiotite, musco-
vite, feldspar, iiiid cjuartz have developed. Where most altered, they are
mica-schists and mica-gueisses. These chang-es are presumed to Ije due, for
the most part, to the orogeuic forces which were active prior to the iutrusiou
of the fi'ranite (p. 170). If su(di be the case, the g-ranite beg-an its metamor-
phic action upon a rock already greatly changed from its original couditiou.
EVIDENCE OF INTRUSION.
The intrusive character of this large mass of granite is indicated by
its stratigraphical position, and is further confirmed by the mechanical
effects ])roduced by the intrusion of the granite upon the sedimentaries, by
the contact effects produced in the sediments, and by the presence in the
sedimentaries of granite dikes forming offshoots from the main mass.
The mechanical effects are well shown by the inclusion of sedimentary
fragments and by the dislocation and folding of the beds. Inclusions are
rather common and are usually of considerable size.
The dislocation and folding are lieautifully shown at N. 1970, W. 570
paces, sec. 30, T. 42 N., R. 31 W. In general the layers in the graywacke,
which are alternately rich and poor in mica, strike N. 15° to 30° W., but
where the intrusives are, these layers are found to strike almost due north
and south. At the aliove location the beds are folded into small, closely-
compressed anticlines and synclhies, which pluiige to the east at an angle
of about 80°. At this place the micaceous graywacke is broken into small
pieces, which are thoroixghly injected and cemented by the granite, thus
forming a typical eruptive breccia. The granite cement is microgranitic,
with comparatively little quartz and a small amount of chloritized mica.
The fragments of micaceous graywacke in the breccia appear to be rather
more feldspathic than usual, but otherwise seem not to have been nuich
affected.
Owing to the altered condition of the sediments i)rior to the granite
intrusion, and to the alternation of sediments of somewhat varying char-
acter, we can not expect to find such clearly outlined concentric zones
suri'oundinp- the aranite as in cases where the sediments are uniform. In
one case a contact was observed between the granites and apparently the
main mass of the sediments. Along this line of contact biotite and white
196 THE CRYSTAL FALLS IKON-BEARING DISTRICT.
mica have developed in great abundance. Mica is well known as one of
the minerals produced in granite contacts, and it evidently here owes its
abundance to the presence of the granite.
At a considerable distance from the nearest intrusive outcrop (2 miles)
a mica-schist was observed which was characterized by numerous small
but prominent nodules that stood out upon its weathered surface. The
rock contains a considerable quantity of an apparently original chlorite
in large automorphic plates. The nodules were produced by large individ-
uals of staurolite. The staurolite has almost completely altered, remnants
only of the original indi^^duals remaining. These remaining grains show
a very poor cleavage, and extinguish parallel to it. These include blebs of
quartz and particles of iron oxide. Thev have the usual pleochroism for
staurolite, varying from golden yellow for c to yellowish white for a and 6.
The alteration products in which the grains lie are fine scaly aggregates
of minute leaves of muscovite, with here and there larger plates of the
same mineral. A few grains of quartz and a small amount of iron oxide,
possilily jiartly original, are found in the mass. This observation of the
alteration of the staurolite to muscovite confirms the observations of Thiir-
ach^ and Pichler.' A similar staurolitiferous mica-scliist, occurring in the
same locality, was described by C. E. Wright for the Wisconsin survey.^
On these particular specimens the characteristic twins of staurolite may be
observed macroscopically as well as in thin section. Wright has also
described a garnetiferous mica-schist from this area of metamorphic schists.*
Both of these schists contain prisms of bluish tourmaline in considerable
quantity.
It appears highly probable that these staurolitiferous and garnetiferous
schists owe their origin to the intnision of the igneous rocks, though no
well-marked contact zones could be outlined.
The exomorijhic contact effect of the granite is more noticeable where
a large body of the granite contains a sedimentary intrusion than elsewhere.
The determination of the sedimentary origin of the fragments included
in the granite is based primarily ujjon the probability that in its passage
I Thiirach, GrotU's Zeitschr., Vol. II, p. 423.
' A. Pichler, Beitriige zur Mineralogie Tirols, Nenes Jahrb., 1871, p. 54.
^Geology of the Menominee iron region, by C. E. Wright: Geol. of Wisconsin, Vol. Ill, 1878, Part
8, 11.695.
* Log. cit., p. 695.
ACID INTUUSIVES. 197
tlirou"-!! the seiliiii('ntiU-\' rocks whirli now surniuiKl it tliu granite iiichuU-d
fraj'-meiits of tliciu.
In addition to this a well-defined banding- is still ])resent in these frag-
ments. Thougli by no means conclusive evidence, this is considered as an
indication of their having been originally deposited through the mediation
of water. The sediments have been comi)letely recrystallized into fine-
grained mica-gneisses.
The sedimentary fragments included in the granite now sliow the
foliowiug characters. They are composed of layers of two kinds. The
one kind of layer is very fine grained, of gray color, and consists predomi-
nantly of biotite in fairly good automorjihic plates, muscovite in small
quantity — but in automorphic jilates — even with respect to the biotite,
feldspar, quartz, and iron oxide. The feldspar is in small equi dimensional
grains. Only one finely striated feldspar was observed, the greater part
possibly being orthoclase. It shows in places a well-developed zonal
structure, the zones conforming to the outlines of the grains. The zonal
structure probably depends upon varying quantities of the soda and potash
molecule. Quartz occurs in grains in very small quantity.
The second kind of layer which is seen in the fragments is nuicli coarser
grained than is the first, just described, and is very much darker. It is
composed mainly of muscovite and biotite, in about equal quantities, feld-
spar, quartz, magnetite, and ilmenite, with some tourmahne. Both feldspar
and quartz occur in grains, the former in small quantity. The biotite is in
small plates less well developed than the muscovite, though mostl}- auto-
morphic. It is partly bleached and has associated with it here and there
some secondar}^ epidote. The muscovite is in very large automorphic
plates, some of them twinned according to Tschermak's law, and includes
flakes of biotite and grains of quartz and feldspar. This gneiss contains
also a large number of crystals of tourmaline, showing strong dichroism
from light pinkish to dark grayish blue. Iron oxide is present, both as
magnetite and as ilmenite. The quadratic individuals of the one and the
hexagonal plates of the other at times are very well developed. No signs
of pressure whatsoever are seen.
The muscovites evidently represent the last product of crystallization,
as shown by their including all the minerals which had been previously
formed. (Fig. A, PI. XXXIV.) These muscovites are probably to be
198 THE CRYSTAL FALLS IRON-BEARING DISTRICT.
looked upon as the product of mineralizers, dependent upon the presence-
of the hot g-ranite injection, to whose action may also be referred the
presence of the tourmaline.
The line of contact between the granite and the sedimentary frag-
ments, though somewhat irregular, is macroscopically very well defined by
the difference in size of the mineral constituents of the two rocks. Here
and tliere in the fragments there are seen thin masses of the granite which
were injected along the planes of sedimentation, and also traverse cracks
penetrating the sediments. On the granite side of such a contact there is a
nan-KW zone very noticeably richer in large biotite flakes tliau the granite
is ordinarily. No diff'erence can be noticed on the sedimentary side of the
contact. The microscopical examination of the contact emphasized the
endogenous character of the metamorphic action. There is more biotite
present than in the normal granite.
The feldspar in the normal granite has a decided tendency toward
autoniorphic development. Where the feldspars of the granite touch the
metamorphosed sediments they are partly rounded, and the mica plates
developed in the sediments have in general a parallel structure around that
side of the feldspar which is turned toward the fragments.
At another point the quartz of the granite, where it comes in contact
with the sediments, has developed as an automorphic individual, and looks
as though it were pressed into the fragment. The quartz crystal contains
near its edge grains of feldspar and flakes of mica, thus making an imper-
fect narrow poikilitic zone. Beyond this zone there is the sedimentary rock
jjroper, and there the mica plates lie parallel to the contours of the quartz.
An illustration of such a contact is shown in figs. A and B, PL XXXV, as
seen in ordinary light and also between crossed nicols.
It appears that the formation of the quartz and feldspar noted above
caused the arrangement of the constituents of the gneiss parallel to their
contours. It would thus seem probable that in this particular case the
recrvstallizatiou of the original graywacke into the gneiss which we now
find in its place followed and was, chiefly the result of the intrusion of the
m-anite.
° BASIC INTRUSIVES.
The basic intrusives are represented by metadolerites and metabasalts.
The dolerites are the most important, and will be treated in detail. Rocks-
BASIC INTRUSIVES. 199
very siniihir to tlie l);iisalts have alread}' been described at length under the
Hendock volcanics, and since they are found in loniparatively few dikes
they will be passed over with very Ijrief mention.
METADOLERITE.i
GEOGRAPHICAL DISTRIBUTION.
The dolerites of the Crystal Falls district for tlie most part form liigh
ridges extending in a northwest-southeast direction. Their principal occur-
rence is in the area immediately north, northeast, and east of Crystal Falls.
Beginning in sees. 32 and 33, T. 43 N., R. 31 W., there extends a great
intrusive mass, varying from a mile to a mile and a half in width, due north
to sec. 6, T. 43 N., R. 31 W. There it bends to the northwest, and ends in
sec. 3,' T. 43 N., R. 3"2 W. The northwestern extension of this mass is much
narrower, never exceeding a half mile, and at many places it is only a few
hundi'ed yards in width. In the northern part of T. 42 N., R. 31 W., are a
number of knobs which are evidently connected below with these large
masses, although the exposures are discontinuous.
A large dike begins in sec. 1, T. 42 N., R. 32 W., and extends for about
5 miles to the northwest into sec. 19, T. 44 N., R. 32 W. This averages
about one-eighth of a mile in width, though in places it is three-fourths of
a mile Avide. Another dike begins in sec. 28, T. 44 N., R. 32 W., and runs
for 3^ miles to the northwest into sec. 18, T. 44 N., R. 32 W., and, like the
above, is narrow, being only about one-eighth of a mile in average width.
A narrow dike less than one-eighth of a mile in width extends in a high
ridge from in sec. 16, T. 43 N., R. 32 W., to the northwest for 3 miles and
ends in sec. 7 of the same township and range. Numerous isolated knobs
occur in T. 44 N., R. 32 W. A small boss is in sec. 24, T. 46 N., R. 33 W.,
and another at 1,600 paces N., 1,000 W., sec. 19, T. 47 N., R. 33 W.
PETROGRAPHICAL CHARAf^TERS.
iiacroscopicai. — Tlic doleritcs vary in color from greenish to dark olive-
green and almost black. The weathered surface is usually of a very light
color, rather a light gray, with frequently a reddish tinge. The texture is
'luse the name " ilolerite " here merely to iudic;ite the macrostiuctural dift'erence between the
rocks inclnded under it and the fine-grained and aphanitio rocks of the same composition included
under the basalts. It is also extended to include jja^fo as well as neo cruptives. As nearly all, if not
all, the paleodolerites have undergone great alteration, the prefix meta — indicating alteration without
reference to any specific kind — is very generally applicable.
200 THE CRYSTAL FALLS IRON-BEARING DISTRICT.
medium to coarse. Probably the most striking textural characteristic is the
pecuhar mottled appearance described as "luster-mottling" by Pumpelly,'
to which the name poikilitic" has of late years been more generally applied.
This texture is almost always brought out macroscopically on the weathered
surfaces by the difference in the weathering of the feldspar and the augite
or uralite, these lieing the prominent mineral constituents of the rock.
This poikilitic texture is most common in the coarsest of the metadolerites.
In such rocks the augite or uralite areas are of large size, quite commonly
2 centimeters in diameter, and show their mottled character very plainly to
the naked eye.
In the medium-grained dolerites the ordinary ophitic texture is pre-
dominant, though in these rocks poikilitic areas may be seen. In fact, these
poikilitic areas really possess an ophitic texture, according to the definition
of that texture by A. Michel Levy, for the feldspars are developed as laths
and the pyroxene is the mesostasis in which the feldspars lie.^ It is thus
clear that, restricting the statement to these dolerites, the ophitic texture is
at times included in the poikilitic, and that under such circumstances the two
can not be considered as totally different and independent textures, but are,
on the contrary, practically identical.
In one dike of dolerite the influence which the conditions of consoli-
dation exert upon the texture is well shown. This dike is only 8 feet wide,
but the center is developed as a dolerite, while along the edges where cool-
ing was more rapid, the rock is a porphyritic basalt. The porphyritic
texture is caused by the development of pyroxene and feldspar phenocrysts,
which lie in a dense basaltic groundmass.
Microscopical. — Tlic OHgiual miucrals of which the rocks were composed
were feldspar, quartz, pyroxene, olivine, biotite (?), apatite, and titano-
magnetite. The minerals which are now present in the rocks are for the
most part secondary. They are hornblende, muscovite, epidote-zoisite,
chlorite, biotite, sphene, leucoxene, calcite, albite, quartz, and iron pyrite.
Of these, hornblende is by far the most prominent constituent. A study of
the isolated specimens of these rocks might result in their determination as
' Metasomatic develoitmeiit of the coppei-beariug rocks of Lake Superior, by Raphael Pumpelly:
Proc. Am. Acad. Arts Set., Vol. XIII, 1878, p. 260.
'On the use of the terms "poikilitic" aud "micropoikilitic" in petrography, by G. H. Williams-
Jour. Geol., Vol. 1, 1892, pp. 176-179.
= Structures et classiljcatiou des roches eruptives, by A. Michel Levy, Paris, 1890, }>. 30.
BASIC 1NTKUS1VE8. 201
uralitic dolerites, cpidioriti^s, or even dioritus, as it is impossible, without a
stHiiieuce of cliaujj^es, to deteriniue wlietlier the hornhk'iide is original or
secondary. Rare rocks contain quartz in sutfieient (juantity to warrant
their designation as quartz-dolerites. However, they differ in no essential
respects from the other dolerites. The quartz is in micropegmatitic inter-
growth with feldspar, filling the angular interspaces of the rock. These
intero-rowths were evidently the last elements to crystallize.
The feldspar occurs in large automorphic lath-shaped crystals, which
in most cases show poly synthetic twinning. In a few cases unstriated crys-
tals were obseiwed. Owing to the alteration of the feldspar, which has in
most cases almost completely destroyed the striations, it has been impossible
to make many accurate measurements. Measurements ou the zone perpen-
dicular to 010 gave equal e.xtinetion angles against twinning planes of 37°
as maximum, showing the feldspar to be bytownite. The chief alteration
products of the feldspar are epidote and zoisite. With these are usually
associated more or less muscovite, some chlorite, and, more rarely, scales of
biotite. Accompanying these alteration products one very fi-equently finds
hmpid spots of secondary albite or quartz. Some few of the feldspars are
smoke-colored, and as the coloring appeared homogeneous even under the
highest powers, it Avould seem to be due to some pigment in the mineral
and not to minute inclusions.
Pyroxene is very rare, having been observed in only a few sections and
in the majority of these is present merely as small remnants surrounded by
its secondary product, uralite. The pyroxene possesses the usual char-
acters of common augite. The augite is quite free from inclusions. Along
the edge its alteration to the light-green hornblende, uralite, can be readily
followed, and in one case an octagonal basal section of augite was observed
which was completely occupied by uralite fibers.
The former presence of olivine is based upon verj^ slight proof, viz,
the existence in some of the pyroxene and uralite crystals of areas which
are oval or round in shape and are occupied by pilite. The presence of this
pilite in the altered augite might possibly be explained as an alteration
product of the augite itself, but it is difficult to explain why pilite should
develop in one part of the augite and secondary coarse hornblende in the
other part. Moreover, the general characters of the rocks are such as to
lead one to expect to find olivine present in some of them.
202 THE CRYSTAL FALLS IRON-BEARING DISTRICT.
Biotite occurs in large irregular masses whicli are considered to be
primary, as well as in the scales which occur within secondary products of
the rock and are considered to be secondary. It is scattered throughout the
rocks in irregular pieces, usually associated with iron oxide. Where fairly
fresh, it is brown and shows its ordinary character. By weathering it
becomes green, having still a high double refraction. By further weath-
ering it passes into a nearly colorless mass that has the faintest tinge of
green and scarcely polarizes light. Such masses are crossed by lines of
hair-like crystals, some of which intersect one another at angles of 60
degrees, extinguish parallel to their long directions, and show high single and
double refraction. These are taken to be rutile. Other crystals, somewhat
coarser, also lie irregularly in the biotite masses. They show the same inter-
sections as the rutile. They are very faintly greenish, have a high single and
double refraction, are positive in the long direction, and have a maximum
extinction angle of 46 degrees. These characters were not sufficient to
determine the mineral by, and no other characteristics could be observed.
Ilmenite and titano-magnetite are in irregular grains. These minerals
are more or less altered to leucoxene or to sphene. Very frequently the
alteration product incloses bands of the iron oxide, which intersect one
another at angles of 60 degrees, pointing toward the hexagonal character
of the ore. In one case a beautiful example of the alteration of such an
ilmenite to rutile was observed. It is exactly similar to that described by
Williams in the case of the greenstones of the Menominee district.^ By
low power the mass has a semimetallic luster, and, as it seems to. be almost
solid, has very much the general appearance of an ore, but by higher power
it is resolved into a mass of small golden-brown crystals. These frequently
intersect one another at angles approximating 60 degrees (120°), similar to
the fine needles in sagenite.
The hornblende is mainly in large xenomorphic plates inclosing the
automorphic feldspars. This is the variety of hornblende known as uralite
and is all presumed to be of a secondary nature. In no case does it pos-
sess the compact nature of original hornblende, but is always more or less
fibrous, its fibrous nature being best seen along the edges, and less clearly
shown, though still observable, where the sections are thicker. It varies
1 A letter to Neiies Jahrbiich,Vol. II, 1887, p. 263. The greenstone-schist areas of the Menominee'
and Marquette resious of Michigan. Bull. U. S. Geol. Survey, No. 62, 1890, p. 99.
BASIC INTRUSIVES. 205
iVoin scarc't'lv colored needles to those wliieli are stroujilv pleoeliroic. The
pleocliroism varies from yellowish for a to yellowish oi- dlixe •'•reen for ItJ,
and in many cases to a dark hluish-green for c. In a few cases mucli of
the hornblende has frequently a darker shade in the center than at the
border, althon<rh of the same color.
A somewhat different variety of hornblende is observed occupying'
round to oval areas in the dolentes. This is in tang-led aii-ffresrates of
needles, with which some chlorite is associated. This hornblende is auto-
niorphic in prismatic zone, ragg-ed at the ends. These aggregates seem to
be ver}' coarse pilitic pseudomor])lis after olivine. The areas occupied Ijy
these aggregates are similar in appearance to the pseudoamygdules described
bv Pumpelly as occurring in the Keweenawan lavas.
The hornblende is largely altered to masses of chlorite and epidote,
usitally with the production of some calcite, and to this is due the present
extremely cliloritic and epidotic characters of niany of the badl}" altered
specimens.
The secondary minerals, chlorite and epidote-zciisite, possess their usual
cliaracters. The chlorite is present in very large quantity, and next to it
the epidote-zoisite is most common. These two minerals make up a large
proportion of the rock. In one ease porph}-ritic scalenohedra of calcite
were found in a medium-grained dolerite, the occurrence in every way
being similar to that described in the volcanics of the same region.
None of the original minerals of these intrusive greenstones give evi-
dence of having been severely mashed; consecpiently we inay safely conclude
that they have not participated in the orogenic movements in pre-Keweena-
wan time which have universally aifected the older rocks of the Crystal Falls
district.
RELATIONS TO AD.JACENT ROCKS.
Relations to Lower Huronian Mansfield slates. Tile relatioU of tile dolcriteS tO the
Mansiiekl slate is quite clearlj' shown along the line of contact between
them. This extends from sec. 7 S. to sec. 32, T. 43 N., R. 31 W., on the
east side of the Michigamme River, near ilanstield. The presence of
numerous large inclusions of the slate in the dolerite and the occurrence of
contact rocks in the slate plainly show that the dolerites are younger than
the slate. Another piece of evidence pointing to this same relation was
found in sec. 28, T. 44 N., R. 32 W. Here was found an angular inclusion
204 THE CRYSTAL FALLS IRON-BEARING DISTRICT.
of sedimentary rock in a dolerite. The rock now possesses the characters
of a spilosite, and was evident!}- brought up from below by tliese iutru-
sives. No slate is exposed near this point, but it is presumed to underlie
this area, althoug-li below the exposed volcanics.
Relations to Lower Huronian Hemlock volcanics. The dolcote HdgeS wllicll OCCUr lu
the area underlain b}' the Hemlock volcanics are surrounded on all sides
by rocks of related petrographical character. The number of localities
at which the relations between the two may be observed are very few,
biit their relations where seen are clear. For instance, in sees. 18 and 30,
T. 44 N., R. 32 W., the coarse intrusives break through the volcanics. In
sefc. ?7, T. 46 N., R. 33 W., a boss of the dolerite occurs in the midst of schis-
tose volcanic tuffs. The volcanics surrounding the intrusives exhibit symp-
toms of more or less violent dynamometamorphic action, whereas the doler-
ites in no case show any evidence, microscopically or macroscopically, of hav-
ino- undergone the metamorphism from which the volcanics have suffered.
The dolerites are thus clearly younger than the effusives of the district.
Relations to Upper Huronian. — Ouly a fcw Isolated doleHtc outcrops liave been
found in the area imderlain by the Upper Huronian. The most conspicu-
ous outcrops are the large dike in sees. 7, 8, and 9, T. 43 N., R. 32 W., and
the knobs in Ts. 42 N. and 43 N., R. 31 W. These last are practically
continuous with the great dike which penetrates the Lower Huronian along
the Michigamme River immediately to the north. One isolated knob has
also been found in the extreme northwestern part of the district in sec. 19,
T. 47 N., R. 33 "W. Although in none of these areas have the dolerites been
found in contact with the Upper Huronian, as it has been shown by the
stratigraph}' that these areas are underlain I))- the Ujjper Huronian sediments,
the statement seems warranted that the dolerites are intrusive through them.
Relations to other intrusives. — In ouc placc a dolerite Is lutruded by a dolerite
of later age, and it is highly probable that there are many more simila,r
cases never observed. The dolerites are cut by small granite dikes at sev-
eral places east of Mansfield.
CONTACT METAMOKPHISM OF MANSFIELD SLATES BY THE DOLERITE.'
The contacts between the dolerites and the sedimentaries are very
rarely observable. For the most part where the sedimentaries are altered
' A contribution to tlie study of contact metamorphism, by J. Morgan Clements : Am. Jour. Sci,,
4tli series, Vol. VII, 1899, py. 81-91.
CONTACT AIETAMOKl'HISM liV INTIIUSIVKS. 205
1)V cinitiU't iictiou tllu^■ arc surrouiulcd on all sides i)V the dolerites, hciuii'
ill tact iiiclusious, but without tlic iiiiiiicdiate contacts ex])osed. Such
inclusions arc rather numerous on the east side of the Micliigannne River,
iVoni sec. 29 N. to sec. S, T. 43 N., R. 31 W., near the boundary line
')ct\vccn the Mansfield slates and the dolerites.
The Mansfield slates are uniformly rather fine grained, and the contact
products are also fine-grained rocks, which still show in some cases the fine
l)anding of the original slates. They are very dense "hornstone"-like
rocks, have a splintery and at times almost conchoidal fracture, and vary in
color from light to very dark gray and greenish The weathered surfaces
in almost all cases are covered by a thin white to light-yellowish crust.
This weathering brings out very clearly the banded and spotted, character
of some of the rocks.
The mineralogical components are quartz, feldspar, biotite, chk)rite,
white mica, actinolite, rutile, epidote, spliene, and iron oxide. Quartz is in
very minute grains. Much of the feldspar shows fine striations, but owing
to the minute size of the grains their exact characters are not determinable
with the microscope, although from the very high percentage of soda shown
by analysis to be present in the rocks the conclusion is drawn that they
are grains of albite.
Biotite is present in small quantity in some of the contact products.
This production of secondary biotite has been noted as rare for "diabase"
contacts.^ Plates of chlorite and white mica and needles of actinolite, the
latter of much larger size than the individuals of the other minerals men-
tioned, lie scattered through the fine-grained mass of feldspar and quartz.
Usually they are gathered together in bunches and sheaf-like or radial aggre-
gates, but they occur at places in isolated individuals. Scattered through
the slates is unmistakable rutile in coarse crystals with pyramidal ends. In
only one case are the crystals very fine, and in that case they approach
closely the appearance of needles in the clay slates (Thonschiefernadeln).
These needles are commonly aggregated into tangled and roughly radial
growths. The needles show very pretty knee- and heart-shaped twins.
Various combinations of the minerals occur, and the structures which
accompany the combinations likewise vary. As a result of these variations
there are found the different types of contact products described as spilosites,
I Mikroskopische Physiographie, by H. Rosenbusch, Stuttgart, 1896, Vol. ii, p. 244.
206 THE CRYSTAL FALLS IRON-BEARING DISTRICT.
desmiisites, and adinoles.^ The general characters of the minerals being- the
same, I shall describe the structure which characterizes the rocks.
SPII.OSITES.
The ordinary spilosites are distinctly mottled in hand sjjecimens and
show clearly to the naked eye in thin section tlie oval spots which char-
acterize them. These oval areas are commonly 4 millimeters long-, and in
rare cases even longer. They are ti-eqiiently connected, forming chains.
The spots are appreciably darker than the mass in which they lie, and are
composed of chlorite, quartz, feldspar, and rutile, with a small amount of
muscovite, the chlorite being the chief mineral. The sun-ounding mass
consists essentially of nuiscovite, quartz, and feldspar, with rutile crystals
and Hakes of hematite, and witii a very slight amount of chlorite. The
different proportions of chlorite and nmscovite seem to cause the difference
between the spots and the groundnia.ss (fig. A, PI. XXXVII). In some of the
spilosites Ave find a few flakes of biotite and needles of actinolite; however,
these are always very subordinate in quantity to the chlorite.
In the ordinary' form described these spots consist essentially of bisili-
cates. Others also have been noted in which these spots are white and lie
in the fine-grained dark mass composing the greater part of the slides.
Thus far it seems onlv one instance of the occurrence of sucli a variety of
the spilosites has been described. This is by Van Werveke, to whose
description reference is made by ZirkeP and Rosenbusch.^ These white
spots are composed essentially of albite feldspar, with onh" a minor amount
of chlorite and epidote. The feldspar grains are much larger than those
which take part in the constitution of the mass surrounding the spots.
This mass is made uj) of quartz and feldspar, chlorite, epid(ite, and some
sphene, with sheaves of actinolite scattered through it, and in one section
clumps of biotite were observed mixed with the chlorite, though in very
subordinate quantity (figs. A and B, PI. XXXVl).
' Uber lien spilosit aud desmosit Zincken's, by Losseu ; Zcitsclir. Dentsch. fJeol. GeselL, Vol.
XXIV, 1872, p. 701.
Durcli iliabaa veriimlertu Scliiel'er im Gcbiet iler Saai' auil Mosel, l),v Vmu Werveke: Leonard's
Jahrb., Vol. II, 1884, p.225.
An interesting contact rock, witb note on contaet luetaniorphism, by W. JI. Hntchiugs : Geol.
Mag., Vol.11, pp. 122, 163.
Numerous other references may be found in t'bemisolie Geologie, by Roth, Bd. Ill, and in petro-
graphical works of Zirkel and Eosenbusch.
-Zirkel, Pet., Vol. II, p. 719.
' Rosenbusch, Vol. II, 3d ed., p. 1177.
OOXTALT METAMOHPIIISM BY INTKUSIVES.
20'
Dirt'e-riuji' sliij;litly t'niiu the oiiliiiary spilositc i.s oiu' in wliicli the spots
<ire of iiiicrosi-opical size, and consist of r<i<i'<4e(l Ininches of chlorite and Jigyre-
_g'ates of sphene and epidote, witli sonic Hakes of l)iotite, which lie in a quartz-
feldspar mass. The photomicrograph, tig. 11 of 1^1. XXXVII, illustrates the
appearance of the rock. As these spots increase in number the)' approach each
other and unite, forming streamei's, which in their turn unite and form Ijands
(photomicrograph, tig. A, PI. XXXVIII,). The spilosites or spotted contact
products thus ])ass over into the desinosites or banded contact products.
Analyses of spilosites. — Aiialyscs of two of tlie spilosltcs (Nos. I and II) were
made by Dr. H. N. Stokes in the laboratory of the United States Geoloff-
ical Survey, and are here appended. With them there is given an analysis
(No. Ill) l)y E. Kayser of a spilosite from the Harz Mountains
Analyses of sjMositen.
J (32827).
11(32861).
III.
SiOi
57. 77
.92
19.35
None.
1.29
3.37
Trace.
1.71
None.
Trace.
4. 3.-)
8.22
. 22
None.
None.
.04
.18
2..S4
None.
None.
None.
52.51
1.70
19. 00
None.
3.31
7.19
Trace.
1. 55
Trace.
Trace.
3.29
6.72
.70
Trace.
None.
.15
.34
3.26
Noue.
None.
Trace.
55. 56
TiO ■
Al.,0)
18.15
Cr .O3
F&iOs
5.08
7.04
.51
1.40
FeO
MnO
CaO
BaO
SrO
MgO
3.17
4.20
2.25
NajO
KG
Ll-O
CO2
.10
P.O..
H.OatllO^
1 2. 79
H-O at 110^+
S and so ,
CI
F
Total
99.76
99.72
100. 25
DESMOSITES.
Under the desnK)sites are included contact products composed of the
;same mineral constituents as the spilosites, but which show a distinctly
banded structure. As shown in the discussion of the spilosites, the two
must be very closely related and grade into each other.
E. Kaj-ser, from Roth's Cheni. Geol., Vol. Ill, 1890, p. 143, No. IV.
208
THE CRYSTAL FALLS IRON-BEARIiSfG DISTRICT.
ADIXOI.ES.
Chlorite has been the chief dark luineral in the contact products thus
far mentioned, with actinolite as an accessory. In the adinoles actinolite is
the characteristic constituent. The mineral constituents in the adinoles are,
as a rule, more uniformly distributed than is the case with the spilosites;
however, the spots are composed essentially of actinolite. The actinolite is
in sheaf-like g-rowths. These actinolite sheaves lie in an exceedingly fine
grained mass of quartz and albite, with some flakes of ehloi-ite and grains
of epidote. The groundmass is formed of such minute mineral constituents
that no conclusive test could be obtained for the determination of the
limpid grains, and their nature has been concluded from the analyses. The
rock is rendered rather dark by minute black specks disseminated through
it. In places these are collected in irregular or lenticular heaps. They
seem to be carbonaceous matter.
Analyses of adinoles. — Thc followiug is au aualysls (No. I) by Mr. George
Steiger, of the United States Geological Survey, of one of the typical
adinoles from this district. With this there are given for comparison two
adinole analyses (Nos. II and III) by E. Kayser.'
Analyses of adinoles.
I (324li5).
III.
SiO.2
TiO.
AI2O3
Fe,03
FeO
MnO
CaO
BaO
MgO
K;0
Na;0
H.O at 100^ — .
H.,0 at 100'-' + .
P.O5
CO2
FeSj
C
74.16
.37
11.85
.82
1.66
.06
2.10
None.
2.10
.15
6.57
.05
.52
.08
.09
75.25
.18
11.80
Trace.
1.76
72.63
15.81
32
.74
1.02
1.57
.61
7.54
.81
1.21
.75
8.33
.61
.49
Total
100.76
100.15
101. 10
' E. Kayser, Zirkel, Vol. II, p. 721, Analyses III and VI.
CONTACT METAMORPniSM BY INTRUSIVES. 209
There is still a kind ot' contact rock in \\ liidi iictiuoiitc is the chief dark
iiiincral, and in which the actinolite, thougii in clnnips, is mainly collected
in l)ands. This convsjidnds to the desmosites in structvire, though ditiering-
from them in mineralogical composition.
These chlorite and actinolite contact rocks may be expected to grade
into each other, and such a gradation is sliown in one specimen, in which
actinolite and chlorite are present in about equal quantity. The actinolite
occurs in crystals and sheaves, forming the spots, whereas the main mass of
the slide surrounding the spots is formed of chlorite as the raetasilicate asso-
ciated with feldspar, quartz, and some epidote.
It w^ould be of great interest to determine which of the contact prod-
ucts, the desmosites, the spilosites, or the adinoles, represent the greatest
amount of metamorphism, as shown by the relations to the intruding mass.
Unfortunately, the records of the specimens do not enable me to determine
this, although for other contact zones in other regions it has already been
determined that the adinoles are next to the contact, while the spilosites
(and desmosites) are intermediate between them and the clay slates.
COMPARISON Ot" THIS ANALYSES OF THE NORMAL MANSFIELD CLAY SLATES AND THE CONTACT
PRODUCTS.
In a series of analyses designed to illustrate the chemical changes
which accompany the increasing metamorphism of a rock, it is of great
importance that the various phases, from the unmetamorphosed to the most
metamorph(5sed form of the rock, be represented. Moreover, the order of
succession from the unmetamorphosed to the most metamorphosed foi-m of
the rock should be definitely known. In the present case pertain pha es
of the metamorphosed rocks are represented, but it has been impossible,
owing to poor exposures, to determine in this locality the order of succes-
sion. This has, however, been done. so satisfactorily by Lessen and others,
and the characters of each fades in the progression have been so well
described, that I have no hesitation, after a microscopical study of the thin
sections of the specimens analyzed, in presenting the series of analyses in
the following table as illustrative of the changes which have taken place
in a clay slate, in the contact zone of dolerite, in its passage to spilosite
and adinole. The analyses are given in the order of approach to the
dolerite as determined by the character of tlie rocks. No. 1 is the uimieta-
MON xxxvi 14
210
THE CRYSTAL FALLS IRON-BEARING DISTRICT.
morphosed form of clay slate ; Nos. 2 and 3 are the intermediate phase, and
No. 4 is the jnost metamorphosed phase.
Comparison of analyses of clay slate, spilosites, and adinole.
1.
2.
3.
'4.
SiO,
60.28
.69
22.61
52.51
1.70
19.00
None.
3.31
7.19
Trace.
1.55
Trace.
Trace.
3.29
.70
6.72
Trace.
X.34
+3.26
.15
None.
57.77
.92
19.35
None.
1.29
3.37
Trace.
1.71
None.
Trace.
4.35
.22
8.22
None.
X.18
+2.34
.04
None.
74.16
.37
11.85
TiO,
Al.,03
Cr>0"
Fe O3
2.53
.45
Trace.
.13
.04
.82
1.66
.06
2.10
None.
FeO
MnO
CaO
BaO
SrO
M"-0
1.35
5.73
.54
2.10
.15
6.57
KG
Na.O
Li,0
H 0 at 100° — . .
.60
3.62
.03
None.
.97
.05
.52
.08
.09
.18
HoO at 100°+
PjO,
CO,
c .
SandSOi
None.
None.
Trace.
None.
None.
None.
CI
F . .
Total
99.57
99.72
99.76
100. 76
XHoO at 110°.
+HiO above 110°.
No. l = Clay slate (.Sp. 32497). Sec. 17, T. 43 N., R. 31 W., 450 N., 1620 W. ; George Steiger.
No. 2 = Spiiosite (Sp. 32861). Sec. 7, T. 43 N., E. 31 W., 750 N., 1380 W. ; H. N. Stokes.
No. 3=Spilosite (.Sp. 32827). Sec. 7, T. 43 N., R. 31 W., 250 N., 325 W. ; H. N. Stokes.
No. 4 = Aillnole (Sp. 32465). Sec. 8, T. 43 N., R. 31 W.,500 N., 475 W. ; George Steiger.
In these analyses the usual increase of silica as the dolerite is
approached is at once noticeable, and hand in hand with it goes the diminu-
tion in percentage of alumina and iron oxides. The content of water and
carbonaceous matter also suffers a diminution. The most iKiteworthy differ-
ence between the clay slate and the contact rocks is shown in the relations
of potassa and soda. This is well Ijrought out in an examination of analyses
Nos. 1 and 2. It will be seen that there is only about one-eighth as much
potassa in the contact rocks as in the normal clay slate; while, on the con-
CONTACT METAMOUPHISM BY INTKUSIVES. 211
triir\', aljout 12 times as much soda as there was in tlie shitt^ has hceii added
to the contact rock. This causes a reversal of the relations of the soda and
potassa, so that, whereas in the clay slate there is present 10 times as nuicli
potassa as soda, we find in the contact rock taken as a sample very nearly
1») times as nuich soda as potassa. The very considerable change in chemi-
cal t-omposition, especially in the amount of silica and soda, seems to lend
great weight to the supposition that in such contacts an actual transfer of
material (soda-silicate) takes place from the basic intrusive to the slate.
This idea is upheld by Roth,^ Zirkel," and others. W. Maynard Hutchings^
advocates this view, and has described some interesting products as a result
of the contact of the Whin Sill which still further support it.
XO KNDOMUUPIIIC EFFKCTS OF DOLKRITE IN'TUUSION.
Although the exoraorphic contact effects of the dolerite intrusion were
so obvious, no evidence is found that the dolerite itself suffered any change
consequent upon its intrusion.
METABASALT.
Basalt has been described at length under the volcanics, where it plays
an exceedingly important role. Basalt as a dike has been found in only
two places, and therefore very little remains to be added.
The two basalt dikes occur within a very short distance of each other,
in sees. 15 and 16, T. 42 N., R. 31 W., and are found penetrating the crystal-
line schists of the Upper Huronian. Their reUitions to the other intrusive
rocks of the same region are not known. The}' are probably of the same age
as the dolerites, of whicli they should most likely be considered offshoots.
These dikes are a porphyritic basalt. The phenocrysts were of augite,
olivine, and labradorite. They were in a very fine groundmass of feldspar,
augite, and iron oxide. However, the former existence of the augite and
olivine phenocrysts is determinable only by means of their outlines. They
are in very small quantity and are entirely altered to pilite. The feldspar
phenocrysts are in coarse, heavy crystals and are remarkably fresh. The
groundmass is very fine grained, and ranges from an exceedingly fine micro-
' Chem. Geol., by .1. Roth, Berlin, 1890, Vol. Ill, p. 145.
^ Lehrbuch tier PetrograpUie, by F. Zirkel, Vol. II, 1894, p. 722.
' Notes (III thu composition of clay slates, etc., and. on some points in their contact metamorpliism,
by W. Maynard Hutchiugs : Geol. ila','.,Vol. I, Dec. 4, 1894, p. 75. Cheni. Geol., Vol. Ill, p. 145. An
interesting contact rock, with notes on contact metamorphism, by W. Maynard Hutchlngs : Geol. Mag.,
Vol. II, 1895, pp. 122-131, 163-169.
212 THE CRYSTAL FALLS IRON-BEARING DISTRICT.
ophitic texture to the pilotaxitic texture. The feldspars in it are in small lath-
shaped individuals, and, like the phenocrysts, are fresh. The augite of the
groundmass is to a great extent altered to uralite, and the iron ores to spliene.
One of the dikes is about 5 feet wide. In the center it is a moderately
fine-grained rock; on the edges it is a dense aphanitic basalt. Even in thin
section the gradation from the rock with microophitic groundmass to the
one with a dense pilotaxitic groundmass is well shown. A dike of larger
size might readily have cooled sufficiently slowly to have crystallized at its
center as a dolerite.
UliTRA-BASIC INTRUSIVES.
'^ Under this head are the descriptions of the picrite-porphyries (porphy-
ritic limburgites).
PICRITE-PORPHYRY (PORPHYRITIC LIMBURGITE).
GEOGRAPHICAL DISTRIBUTION AND EXPOSURES.
The picrite-porphyries occur in isolated outcrops of comparatively
small size in sees. 9, 22, and 27, T. 44 N., R. 32 W., in the area supposed
to be underlain by the Lower Huronian Hemlock volcanics. They are
surrounded by outcrops of the altered poikilitic dolerites, but the exposures
are not such as to allow their relations to be determined. Their occurrence
points to an intrusive character. It is on account of their field occui--
rence alone that we feel justified in describing them here under the general
heading for this chapter, "Intrusives," instead of under the volcanics with
the basalts, their proper place from a strict petrographical standpoint.
PETROGBAPHICAL CHARACTERS.
The picrite-porphyries are medium-grained rocks, which varj^ in color
from gray to dark green and almost black. In general they have a por-
phyritic character. This is, however, not so well marked in the gray as in
the darker-colored rocks. The gray ones have a spotted appearance. The
sppts are- gray in color, fibrous, ver}" rarely larger than 3 or 4 millimeters
in length, and lie in a finely fibrous, dark-green matrix. In the dark rocks
the porphyritic crystals reach a length of 1 centimeter, and are bluish to
black, with silky luster. They lie in a fine-grained, more or less fibrous,
green groundmass. In one of the dark rocks the magnetite is very notice-
able. The crystals project from the weathered surface and the rock is
strongly polar-magnetic.
ULTHA BA81G INTIUTSIVBS. 213
Tlic rocks ()rii;iiiall\' cousisti.'il l;ii';ic'ly i>t (ilixiiic, i)\i'().\ciic, linruhlciiiU',
biotite, inaynetitt', and ilinenite. Tliey now contain alsso, in consideralile
quantity, a cliloriti<' product wliicli seems to lia\e been dei'ived from the
alteration of an original vitreous base. All of the specimens are exceed-
in<j'lv altered. The orig-inal mineral constituents have to a great extent been
determined from their form, which in some cases has been preserved by the
})rodncts of alteration, and by certain structures in the i)seudomorphs. The
minerals now com])Osing the rock are original hornblende, biotite, apatite,
magnetite, and ilmenite, with secondary amphibole, serpentine, chlorite,
calcite, sphene, and rutile.
The two kinds of rocks, the gray and the dark-colored ones, were
evidently derived from rocks of essentially the same composition. They
have undergone difterent processes of alteration, and upon this depends the
difference in color. As the study of these picrites is chiefly one of the
alteration products of the minerals which composed them, it seems best
to describe sej^arately the two rocks showing the different products of
alteration.
GRAY TUEMOLITIZED riCUITE-POUPHYRY.
In the gray rocks the spots which are macroscopicallj^ observed are
found under the microscope to consist of an aggregate of minerals. Exami-
nation of these aggregates shows them to be entirely secondary. A careful
study of these aggregates .shows them to consist of amphibole, magnetite,
ilmenite, and serpentine, the first being pi'edominant. No trace of the origi-
nal minerals remains. The aggregates are the same in- all of the crystals,
and the only clue to the original mineral is the form of the pseudomorphs
and certain structures in the aggregates. By means of the fomi the pheno-
crysts are readily divisible into three kinds. The first kind has a long
prismatic habit, with pyramidal faces meeting at rather an acute angle.
The iron oxide is arranged along certain lines, giving the characteristic
mesh structure of Serpentinized olivine. The second kind is a short, thick
prism, for the most ])art with rounded ends, in some cases the pyramidal
faces meeting in a rather obtuse angle. The iron oxide in some of these
cases marks an imperfect parting perpendicular to the long direction of the
prism. These are supposed to be pseudomorphs after a pyroxene. The
third kind consists of round and irregular grains or plates, some of which
may be referred to py^roxene, others to olivine.
214 THE CRYSTAL FALLS IKON-BEARING DISTRICT.
The amphibole is in small needles. It has a very faint greenish tinge.
In cross section it shows marked prismatic development. The character of
the needles is plus in the long direction. The maximum angle found
between c : c is 18 degrees. The needles appear to be tremolite containing
some iron, and thus approaching actinolite in composition. Usually the
needles have no regular arrangement, but in some of the pseudomorphs
with rectangular outlines there is a parallel arrangement of such a number
of the needles parallel to the long axis of the pseudomorphs as to give to
the pseudomorjih a distinctly uniform polai-ization effect.
Isolated crystals of magnetite and brownish transparent plates of
ilmenite are scattered among the actinolite needles. By far the greater part
of the iron oxide is collected in aggregates of small crystals and irregular
grains. The formation and arrangement of tliese aggregates has in some
cases taken jjlace along fracture and cleavage planes of the original minerals,
and thus in the pseudomorphs we see the mesh structure of olivine and
transverse parting of pyroxene clearly brought out. In other cases the iron
oxide is in irregular masses collected at the center or outlining the periphery
of the pseudomorphs or scattered in small masses through them.
Between the tremolite needles and the iron oxide is a small quantity
of minute fibers. They have a greenish tinge and low double refraction.
Their extinction is parallel to the long c axis, which is also the axis of least
elasticity. They are believed to be serpentine fibers. No definite arrange-
ment of these fibers could be discerned over the greater part of the pseudo-
morphs, but in one crystal, on the edge of the section, where it is esjjecially
thin, the arrangement of these needles perpendicular to the long direction
of the iron aggregates outlining the meshes is unmistakable. Calcite is in
considerable quantity in some of the pseudomorphs. It is highly probable
that it owes its origin to the alteration of the original mineral, though some
of the calcium went into the amphibole.
Besides the above-described pseudomorphs after olivine and pyroxene,
a few large prismatic and irregularly bounded areas were found among the
phenocrysts, which now consist chiefly of chlorite, with grains of calcite,
titanite, magnetite, and minute plates of ilmenite scattered tlu-ough them.
It is clear that the chlorite is derived from a hornblende, as shown by the
presence of ragged remnants of hornblende which possesses uniform orien-
tation throughout each area. This hornblende shows weak pleochroism in
ULTUAHASIG INTRUSIVES. 215
tlu^ light-VLillow to yreenisli tones of actinolite, ulthough its character is
more that of the compact hornblende. In one case its secondary natm-e
was shown l)y the presence of a small irreg'ular area of brown hornblende
lyinji' in a mass of the green. The two have the same orientation. In this
case the chlorite is apparently a tertiary product, the orig'inal mineral
1 )eing the brown hornblende, from which w.as formed the light-green variety,
which in its turn alters to the chlorite.
Between the various pseudomorphs are irregular plates of compact,
dark-brown hornblende, plates of biotite, large crystals of magnetite, and
roug'h branching aggregates of ilmenite. These, while molded on the
phenocrysts, themselves lie in the chloritic mass already mentioned, which
also often completely surrounds the j)henocrysts, and which is probabl}" an
altered vitreous base.
The pieces of brown hornblende which remain unaltered show moder-
ately strong pleochroisin, reddish brown for c and tt and light brownish
yellow for a. Czrb>>a. This hornblende contains inclusions of iron oxide
and has all the appearance of an original mineral. By alteration it passes
through a compact greenish amjjhibole to a much lighter colored, reedy,
actinolitic variety of amphibole. In the secondary amphibole occur certain
golden-brown grains with high single and double refraction, which are
supposed to be rutile formed from the hornblende, and also some brown
transparent plates of ilmenite. The orientation of the secondary horn-
blende is the same as the original. No further alteration of this amphibole
was observed, but it is believed that the prismatic crystals altered to
chlorite, calcite, and magnetite, as described above, are the extreme cases
of alteration of an automorphic form of a brown hornblende very similar
to the part described.
The biotite between the phenocrysts is in ragged areas either surround-
ing iron oxide or associated with it or with the hornblende. It is very
pleochi'oic, the absorption jjarallel to the basal cleavage being so strong as
to render the section opaque. Perpendicular thereto the color is a dai-k
chocolate brown. The mica does not show its usual bright polarization
colors in sections cut parallel to crystallographic c. This may be due in
some measure to the very strong absorption. In some cases the biotite is
seen to have a strong blue to violet metallic luster in incident light. The
biotite has partly altered to chlorite. The alteration proceeds along the basal
216 THE CRYSTAL FALLS IROJf -BEARING DISTRICT.
cleavage. As this alteration progresses there is a lighteuiiig of the color of
the biotite, and, as a consequence of this the whole cause of the metallic
luster and the partial cause of the color of the biotite is disclosed. In the
lighter biotite one by careful examination can see innumerable small plates
of a brown or smoky color. At first sight they remind one strongl}- of the
inclusions so common in man}' hj^perstlienes. Closer examination only
emphasizes this reseiiiblance, and they are believed to be micaceous ilmenite
plates. These inclusions were studied by means of an oil immersion
objective givmg a magnification of about 1,250, and were found to have
mainly a roundish or hexagonal outline. In addition to these, some plates
of long, irregular form were observed. These are all isotropic and lion-
pleochroic. These minute plates lie parallel to the biotite lamellse. The
consequence of this is that in sections parallel to c one sees, for the most
part, onl}^ short black streaks — the edges of the plates — whereas in the basal
sections of the biotite one can determine the irregular or rounded contours
of the plates. The plates ai-e too small to allow the metallic luster to be
seen on an isolated one. En masse they j^roduce a very decided blue
metallic shimmer, as seen in some of the biotite fragments.
Numerous apatite crystals occur. They are usually clear white, but
one crystal was seen showing a dichroism from faintest brownish for rays
perpendicular to crj-stallographic c to light smoky brown for rays parallel
thereto. This crystal contains a core of brown glass.
• Some of the iron oxide is in roughly rectangular masses, and appears
to be magnetite. This is associated with an iron oxide, which occurs in
opaque, ragged masses formed of long, irregular, and knotty stringers.
These at places are parallel to one another and at other places cut one
another at various angles, and at still other places meet at a common cen-
ter, forming an opaque mass of varying dimensions, but usually small.
Now and then one of the large magnetite masses constitutes a center from
which extend the knotty, irregular stringers. The general appearance of
these ragged masses is that described by Grerman petrographers as mrhackt.
When these stringers pierce the section at an oblique angle, the ends
are translucent, with a brown color, becoming more opaque as the section
gets thicker. Such masses have all the appearance of ilmenite, and are
believed to be that mineral. Similar ilmenite stringers are included in the
chlorite, wliich results from the alteration of the biotite
ULTRA BASIC INTUUSIVES. 217
The cliloritc in tlio iiiirainorplis after liiotite sliows extrenielv low blue
polarization color, and the characteristie pleoehroism — yellowish, tinged
with red, when the rays vibrate perpendicular to the cleavage, and green
when i)arallel thereto. Apatite needles included in the l)iotite are unaltered
in the secondar}' chlorite.
Some of the minute octahedral crystals in the amphibole pseudomorphs
after olivine appear to be slightly pellucid, with a brown color. If so, thev
might be referred to picotite, but there is doubt of the correctness of the
observation, in view of the high power used, the oil immersion lens, and
the fact that the search was for picotite. Close search was also made for
perovskite, but none could be found, unless the transparent crystals very
doubtfully referred to picotite are realh' iierovskite.
Forming the matrix in which the pseudomorphs after hornblende,
olivine, and pyroxene of these rocks lie, and frequently surrounding
isolated crystals, one sees an aggregate composed chiefly of a fine felt of
chlorite fibers. This alteration product contains a few apparently original
apatite needles, some secondary grains of magnetite, and crystals of amphi-
l)ole which are colorless or else show but the faintest tinge of green, and are
larger than the amphibole crystals in the pseudomorphs. It is a secondary
amphibole very poor in iron, probably highly calcareous, and approaching
tremolite in composition. This chlorite aggregate shows no indication
whatever of crystal forms. It seems to be the product of a homogeneous
mass, such as would result from the decomposition of a vitreous base. Such
a base the agg-regate is presumed to represent, althoug'h no trace whatever
of the glass has been observed in the rock, nor in view of the altered con-
dition of the rock could such a glass be reasonably expected to still remain.
DARK SERI'ENTINIZED PICRITE-rOIiPH YRY.
The second variet}' of the jncrite-porphyries is very dark greenish-
black, and represents the results of a slighth' different process of alteration
from that by which the gray forms just described were produced. These
dark picrite-porphyries show a very much better developed porphvritic
structure than do the gray ones. This is due to the fact that the oliAines in
these rocks were well developed and reached a length of a centimeter. The
olivines are completely altei-ed, serpentine, pilite, and magnetite, being the
products which form the pseudomorphs. The characteristic mesh structure
218 THE CRYSTAL FALLS IRON-BEARING DISTRICT.
of altered olivine is well broug-lit out l^y the serpentine and iron ore. In
the centers of the meshes there remain small masses of a felt of tremolite
needles (pilite). This alteration of the olivine corresponds to that first
described by Lewis/ and more recently by Professor Bonney and Miss
Raisin,^ from a rock — kimberlite — very similar to the picrite-porphyries
here described. He writes as follows: "It frequently happens that while
sei'pentinization begins at the outside of a crystal, iibrous tremolite begins
growing- within, finally forming a mass of asbestiform fibers surrounded by
a zone of green serpentine."
The minerals which composed these black picrite-porphyries were the
same as those constituting the gray ones. These minerals were olivine,
pyroxene, hornblende, biotite, magnetite, and ilmenite. They were cemented
by a glass matrix. The glass is completely altered. All of the minerals
are represented by pseudomorphs. Remnants of the original hornblende
and biotite alone are preserved.
The contours of the original pyroxene crystals are filled with pilite,
serpentine, and magnetite. The serpentine is present in greater quantity
in these pyroxene pseudomorphs than it was in tlie pyroxene pseudomorphs
in the o-ray picrite-porphyries. The alteration of the hornblende results
in the production of an aggregate of chlorite inclosing grains of caleite,
some sphene, and iron oxide, similar to that in the gray picrite-porphyries.
The biotite, magnetite, and ilmenite also show those characters which have
been described for the same minerals in the first-described picrite-porphyries.
Between all of the foregoing minerals we find a fine felty chlorite
mass eontaimng grains and dendritic masses of iron ore and a few needles
of tremolite. This corresponds to the material forming the cement for the
minerals in the gray porphyries, and, hke that, is believed to represent an
original vitreous matrix.
In one of the dark picrite-porphyries the magnetite is present in large
quantity and is very noticeable, crystals of it standing out upon the weath-
ered surface. This rock did not affect the magnetic needle very powerfully,
though it was expected that it would do so. However, another one of
1 Ou a dlamondiferous peridotite and the genesia of the diamond, hy H. C. Lewis: Geol. Mag.,
3d ser., Vol. IV, 1887, p. 22.
Papers and notes on the genesis and matrix of the diamond, hy the late Henry Carvill Lewis,
edited hy Prof T. G. Bonney, London, 1897, p. 14.
2 Notes on the diamond-hearing rock of Kimherly, South Africa, Part 11, hy Prof T. G. Bonney
and Miss C. A. Raisin : Geol. Mag., 4th series, Vol. II, 1895, p. 496.
ULTKA-BASIC INTRITSIVES.
219
tliosc porphyrios, in wliicli, by tlio way, tlic! iron content is relatively low,
is unitpie, in tiiat it is very stronj^ly \)o\av niag-netio, and in this, as well as
its probable original niineralogical composition, may be cdinpanMl with the
polar magnetic wehrlite from the Frankenstein, Hesse-Darmstadt, Germany.'
The German rock shows tremoliti? scattered through the serpentine result-
in"- from the olivine. It is a coarse, evenly granular rock, differing in this
respect from the Crystal Falls rocks which are porphyritic.
Au analysis (No. 1) of the polar magnetic serpentinized picrite-
porphyry, in which great abundance of olivine was originally present, is
here given, and there is placed with it for comparison an analysis (No. 2)
of a very similar rock described by Darton and Kemp," from New York.
Both analyses were made by Dr. H. N. Stokes, United States Geological
Survey. In No. 1 Ba, Sr, Li, CI, S, and SO' Avere not looked for.
Analyses of picrite-porphyry.
SiO- 37.36
TiO, : "t^
Al.O:, ITO
Cr,03 62
Fe,03 n.61
FeO 6.12
MnO Trace.
XiO ; .04 I
CaO [ 1.19
BaO
SrO
MgO I 31.11
K^O ;1 Trace, i
Na^O 1 I
P.O5 1 -06
CO. None.
SO3 !
S I
HiOatllO^ -65
HnO above 110° 10-37
36.80
1. 2G
4.16
.20
Less 0=S . .
Total
8.33
.13
.09
8.63
.12
Trace.
2.5. 98
2.48
.17
.47
2.95
.06
.95
.51
6.93
99.68
100.22
.47
99.75
1 Der raagnetstein von Frankenstein an der Bergstrasse, by Audreae and Kfinig: Abhaudl. der
Senkenberg. naturf. Gesell., Frankfort a Main, 1888, pii. 59-79.
Cf. Above article, p. 66, footnote, for references to other occurreuces of tremolite as.soclated
Tvith serpentine.
- Newly discovered dike at Dewitt, near Syracuse, New York, by N. H. Darton and J. F. Kemp:
Am. .Jonr. Sci., Vol. XLIX, 1895, p. 461.
220 THE CRYSTAL FALLS IRON-BEAKIiSG DlSTKiUT.
CLASSIFICATION.
These rocks just described, from their miiieralogical composition, if we
admit the presence of a vitreous base, would belong- with the picrite-
porphyrites of Rosenliusch.' This designation does not seem, however, to
be appropriate, as he states" that he uses the term "porphyrite" onlv for
certain textural phases of rocks containing lime-soda feldspar. He has
evidently extended that definition so as to be' able to iise it for these
picrites, considering that the glass possesses the necessary ingredients for
the formation of such lime-soda feldspar, provided the conditions under
which it cooled had been favorable for the feldspar development.
The porphyritic texture of these Crystal Falls picrites and the presence
of a vitreous base^ show them to be closely related to rocks of effusive char-
acter. Those which they most closely resemble among the younger basaltic
lavas are the porphyi-itic forms of the limburgites (magma basalts).
One of the best-known rocks with which this may be closely compared,
as far as association is concerned, is the rock first described by H. Carville
Lewis as a saxonite-porphyry,* later called kimberlite. This was described
by him as volcanic, and as associated with dolerites and melaphyres. He
described it as a basic lava.^ Other occurrences of very closely related basic
rocks havino- a vitreous base have been described from the United States bv
Diller, Williams, Merrill, Brainier and Brackett, Kemp, and Darton and Kemp."
' Microscopische Physiograpliie, by H. Roseiibusch: 3fl ed., Stuttgart, Vol. II, 1896, p. 1191.
-Op. cit., p. 436.
'Should the vitreous baso be considered as not having been pre.sent and the rocks be pnt among
the peridotites, then they would correspond very closely to the wehrlite described on p. 2."i4.
••Papers and notes, cit., p. .">0.
'■The genesis of the diamond, by H. C. Lewis : Science, Vol. VIII, 1886, p. 345.
On a diamon<liferous peridotite and the genesis of the diamond, by H. C. Lewis: Geol. Mag.,
3d series, A'ol. IV, 1887, p. 22.
"Dikes of jieridotite cutting the carboniferous rocks of Kentucky, by J. S. Diller: Science. l,S8.i,
p. 65; Notes on the peridotite of Klliot County, Kentucky, by ,J. S. Diller: Am. .lour. Sci., Vol. XXXIl,
1886, p. 188; Hull. U. S. Geol. Survey, No. 38, 1887.
The serpentine (peridotite) occurring in the Onondaga salt group, at Syracuse, New York, by
G.H. Williams: Am. .lour. Sci., Vol. XXXIV, 1887, p. 137; Proc. Geol. Soc. Am., A'ol. I, 1889, p. 533;
Perowskit in serpentin von Syracuse, New York, by G. H. Williams : Neues Jahrb. Vol. II, 1887, p. 263.
On a peridotite from Little Deer Isle, in Penobscot Bay, Maine, by G. P. Merrill : Proc. U. S. Nat.
Mus., 1888, p. 191.
The peridotite of Pike County, Arkansas, by J. C. Brauner and R.N Brackett: Am. Jour. Sci..
Vol. XXXVIII, 1889, p. 50.
Peridotite dikes in the Portage sandstone of Ithiica, New York, by .1. F. Kemp: Am. ,Iour. Sci..
Vol.XLII, 1891,p. 410.
A newly-discovered dike at Dewitt, near Syracuse, New York, by N. H. Darton and .1. F. Kinij)
Am. Jour. Sci., Vol. XLIX. 189.3, p. 456.
riie rock described liy F. L. Rausome as a fourchite should i>erhaps also be compared with these
ULTRA-BASIC INTRUSIVES. 221
Hatch' lias also described a very similar pre-Tertiary rock from Kug-
laiid as a limburgite. Kemp- emphasizes the resemblance of the Dewitt
dike to liiiil)urgite, and states that it should be called limburgite.^ If we
attempt to extend the use of the term " limburgite" to include the pre-Tertiary
vitreous basalts, we shall have to include under it the rocks heretofore desig-
nated as picrite and picrite-porphyrite. Rosenbusch has now put the jiicrites
and picrite-porphyrites with the effusive rocks, and if of these two sets of
terms there is one to be discarded, it should be the name "limburgite." It
seems preferable luider the rules of priority to retain the name "picrite." It
would then seem very suitable to apply to these pre-Tertiary porphyritic
limburgites Hussak's old term, "picrite-porphyry," using the term "por-
phyry" simply witli a textural significance.*
SECTION II.— A STUDY OF A ROCK SERIES RANGING FROM
ROCKS OF INTERMEDIATE ACIDITY THROUGH THOSE OF
BASIC COMPOSITION TO ULTRA-BASIC KINDS.
Beginning near the town of Crystal Falls, in isolated knobs, and
extending southeast toward the Micliigamine River, where the exposures are
larger and better connected, there is found a series of rocks wnose charac-
ters are of such interest petrogeiietically as to warrant a detail descrijjtion
of them.
These rocks are all intrusive in character, with few exceptions are
medivim to coarse grained, and, while the granitic texture is predominant,
there are certain facies in which the texture is porphyritic and even parallel.
They have been only slightly affected by dynamic action, and these cases
are purely local. Analyses show them to vary in chemical composition
from those of intermediate acidity to those of ultra-basic character.
The prevailing rocks are, on the one hand, diorites of intermediate
acidity ranging to more acid rocks, tonalites, quartz-mica-diorites, and
rocks, re[)re8enting as it probably does the oliviae-free form of the limburgite (augitite). Geology of
Angel Island, by F. L. Ransoine : Bull. Geol. Dept. Univ. of Califoruia, Vol. 1, 1894, p. 200.
' The Lower Carboniferous volcanic rocks of East Lowthian (Carlton Hills), by F, Hatch: Trans.
Royal Acad. Edinburgh, Vol. XXXVII, 1892, p. 115.
-Op, cit.,p.460.
"'Taking plutonic rocks as practically the granitoid, and volcanic as the porphyritic, the
Dewitt rock is a basaltic dike of the same composition and texture as limburgite, and should be called
limburgite, even if it is not a surface flow." (Loc. cit., p. 460. )
■•I believe E. Hussak was the first to use this term for a somewhat similar rock. Pikrit-pliorphyr
von Steierdorf im Banat, by E. Hussak : Verhandl. K.-k. geol. Reichsanstalt, 1881, pp. 258-262.
222 THE CRYSTAL FALLS IRON-BEARING DISTRICT.
granite (plagioclastic), and, on the other, hornblende gabbros, gabljros,
norites, and, kxstly, peridotites of varying mineralogical character. These
rocks thus resemble in their -sanations those Scottish plutonic rocks so well
described by Messrs. Dakyns and Teall.^
The rapid changes in mineralogical composition and texture in a single
rock, and the changes from one kind to another through intermediate facies,
show verv clearly the intimate relationship of these rocks to one another,
and warrant the assumption that they all belong to a geologic unit, a con-
clusion reached a luimber of years since by Williams for a somewhat sim-
ilar series, the Cortlandt series, from New York.
Granite is present as a local facies of the diorite. Howevei', it is very
subordinate in quantity and not altogether typical, and as no analysis has
been obtained, its jjosition is still more or less doubtful.
In the following pages only those kinds of rocks of Avhich analyses
have been obtained will be included in the final discussion. Others will be
described in detail or merely mentioned, as representing facies of the main
types, according to their petrological interest. The rock tyj^es of which
analyses have been made are as follows: Diorite, gabbro, norite, and
peridotite.
DIORITE.
NOMENCLATURE.
Diorite, according to the generally accepted definition, is a granular
rock consisting essentially of hornblende, which must be primary, and a
soda-lime feldspar.- The term has been used in a difi^erent sense by many
writers on the Lake Superior and other regions. It has been used to com-
2irise rocks which contain hornblende and plagioclase as preponderating
constituents, it is true, but in which the hornblende is secondary, therein
diff'ering from a true diorite. These so-called diorites have been regarded
as derived from an original dolerite (diabase) by uralitizatiou of the pyrox-
ene. By some writers these rocks have been classed ^yh]^ the epidiorites,
thus recognizing their secondary nature, but by this name, epidiorite,
unfortunately implying a false relationship.
In this paper, following Brogger, I restrict tlie name to the granitic
' On tlie jiltitonic rocks of Garabal Mil! and Meall Breac, by J. K. Dakyns, es(|., M. A., and J. J.
H. Teall, esq., M. A., F. R. S., F. C. S.: Quart, .hmr. Geol. Soc, Vol. XLVIII, 181W, pp. 104-121.
-Lehrbuch der Petrojjrapbic, by F. Zirkel, Leipzig-, Vol. II, 1891, p. 465.
DIOKITE INTUUSIVES. 223
rocks of iiitermediiite acidity, in which the feldspar is ])hiy'iochiso and the
bisilicate constituent is mica or primary hornblende. The feldspar is a lime-
soda plagioclase.'
DISTRIBUTION AND EXPOSURES.
The dist.ril)ation of the diorite is limited to a few localities, all of which
are in the area underlain by Upper Huronian sedimentaries. The most
typical occurrences, and those showing greatest variations, form knobs
beginning near Crystal Falls and continuing to the south and south-
east. Especially large outcrops form the hills in sees. 19 and 20, T. 43 N.,
R. 31 W. The smaller occurrences are not indicated on the map. These
diorite exposures are always good, so far as getting fairly fresh specimens
are concerned, but their contacts with other rocks are almost invariably
deeply covered with drift; hence their relations in many cases are doubtful.
PETROGRAPHICAL CHARACTERS.
The diorites are holocrystalline rocks of medium to coarse grain. In
texture they show some variation from those which are granitic to those in
which the texture is imperfectly ojjhitic. The color is, on the whole,
moderately light gray or reddish, but at times when the dark minerals
become more prominent in the basic facies, especially where we g"et basic
schlieren, the rock is very dark gray or greenish brown.
The important mineral constituents are feldspar, quartz, biotite, and
hornblende. The accessory minerals are epidote, apatite, zircon, sphene,
and iron oxides. The secondary niinerals, white and larown mica, chlorite,
biotite, epidote-zoisite, calcite, and rutile are also present.
Feldspar. — Plagioclasc feldspar, orthoclase, and microcline occur together.
The plagioclase is found in individuals which are fairly automorphic. In
the ophitic textured diorites, the plagioclase is the best developed of all the
essential constituents. In the granular rocks the degree of automorphism
is highest where orthoclase and quartz are present in the largest quantity,
and diminishes as these diminish, when the plagioclase individuals beghi to
interfere with one another's development. For the most part the plagioclase
' Die EruptivgeBteine des Kiiatiauiagebietes. I. Die Gesteine dor Grorudit-Tiiigiiait-Soiic,
by W. C. Brogger, 1894, No. i, p. 93. II. Die Eruptionsfolge der tiiadischen Eruptivgesteine bei
Predazzo in Siidtyrol, 1895, No. 7, p. 35. Videnskabsselslvabets Skrifter, I Mathematiskuaturv. Klasse.
224 THE CRYSTAL FALLS IRON-BEARING DISTRICT.
gives rather thin sections, thoug-li they can hardly be correctly called lath-
shaped. No other form of plagioclase, showing a uniformly better or
poorer development, or any other difference in character indicating the
presence of two kinds of lime-soda feldspar, was observed.
The plagioclase sections almost invariably show polysynthetic twinning
according to the albite law, with twinning lamelhie which vary fi'om very
thin to moderately thick plates, the thinner being the more connnon form.
Very common is the combination of the albite and Carlsbad twinning laws
in one individual. Less commonly we find individuals twinned according
to the pericline as well as the albite law, and sometimes a Carlsbad twin is
made up of individuals twinned according to the albite and pericline laws.
In determining the character of the feldspar, the Levy method was
followed.' A great number of measurements made on the zone perpen-
dicular to 010 gave equal extinction angles, varying chiefly around 15
degrees, but running as a maximum to 19 degrees. From this it appears
that the plagioclase is andesine, probably a somewhat basic kind. That
these andesines vary slightly in composition is shown by a very slight but
noticeable zonal structure, the moi-e basic character of the center of the
individuals being most admirably brought out by the more advanced con-
dition of alteration of the center as compared with the periphery.
The andesine is for the most part very much altered, to such an extent ~
that in many sections the boundaries of the twinning lamell:3e are so blurred
that measurements are rendered impossible. Muscovite, which appears in
minute rectangular sections showing good cleavage, is the chief secondary
product from the feldspar, with epidote-zoisite next in importance. Calcite
and biotite are present, but in comparatively small quantities. In some
cases muscovite almost replaces the feldspar; in others epidote-zoisite does
so. In such a case one sees in the center of- the feldspar only a mass of
secondary mineral. As the examination is carried from the center toward
the outside, the original feldspar material is distinguished as a thin film
between the secondary minerals. This increases in mass until we reach
the ontside narrow rim of practically unaltered feldspar.
orthociase. — This is prescut in large quantity in irregular plates which
form a part of the mesostasis for the plagioclase and bisilicates. Less com-
monly we find it in micropegmatitic intergrowth with the quartz. It is
' fitude sur la determination des feldspaths, by A. Michel Lf'vy, Paris, 1894.
DIORITE INTRFSIVES. 225
iiivariahly more or less (IcconiiKiscd, and shows innuiiicralile mimite dark
specks scattered through it. The (juantity (if orthocdase varies in tliese
dioritic rocks consideral)ly; at times it almost equals or even exceeds the
plagioclase, when the rocks apjjroacdi the granites, and at times it sinks to
a few large plates in each section, when the rocks are a normal diorite.
Microciine. — Thig mineral is not abundant. It is in individuals which
frecpiently, though not in all cases, are automorphic with re.spect to the
orthoclase and (piartz. It is remarkably fresh.
Quartz. — Quartz, at times, is an essential constituent, and again it dimin-
ishes in amount until it is present only in a few grains, or even disappears
altogether. Like the orthoclase, it is completely xenomorphic, and with the
orthoclase constitutes the mesostasis. Undulatory extinction in the quartz
gives indication of slight pressure.
Biotite. — The original bit)tite in the granular dioritic rocks is automorphic
with respect to all minerals but the hornblende. In the ophitic forms it
has a development about equal to that of the hornblende. It shows a dark
rich chocolate-browu or greenish-brown color for h and C, and a lio-ht
yellowish-brown for a. The biotite includes small ei)idote crystals with
})leochroic courts and some grains of sphene. Both of tliese are original.
The biotite is almost invariably more or less altered, lileaching in s(ime
cases to a very light colored mica with exceedingly high polarization colors.
This bleaching follows along the laminae of the biotite and results in givino-
sections parallel to the vertical axis a banded appearance resembling parallel
intergrowths of muscovite and biotite lamiiuie. More commonly it alters
to chlorite, rutile (often present as sagenite), sphene, epiclote-zoisite, and
calcite. There is also a distinct banding of the biotite and the chlorite in
places. In the alteration of the biotite we very commonly find lenses of
calcite produced between the lamiure. In some cases the epidote-zoisite is
clearly a product of the alteration of the biotite, for in many cases it is found
in the rectangular shape of the biotite section, and in other instances in spaces
between the feldspars in the ophitic rocks, which in fresher specimens are
found to be occupied by the biotite. Moreover, in the epidote-zoisite are
minute grains of sphene similar to those contained in the original biotite.
Where it is present as a secondary product, it occurs with the musco-
vite and is xenomorphic with respect to it. The green tone is absent from
the secondary biotite.
MON xxxvi 15
226 THE CRYSTAL FALLS IRON-BEARING DISTRICT.
Hornblende. — Tlic lioriibleude in the diorites shows a most excellent devel-
opment ill the prism zone; very nmch less well developed are the termi-
nating planes. The color varies from dirty green to a reddish-brown. The
brown hornblende occupies the center of the crystals, while the green occu-
pies the outside, the green agreeing perfectly, optically, with the brt)wn. A
zonal structure is indicated bv the difference in the character of the horn-
blende, though the zones are not sharply delimited, but grade into one
another. In a few cases the greenish hornbleude grades into one which is
almost colorless. The pleochroism is as follows: Brown hornblende : a, light
yellow or light reddish-yellow; b, light reddish-brown; C, darker reddish-
brown. Green hornblende: a, light yellow; Hj, bright green; c, dull or
olive green. This green hornblende is clearly original and not to be con-
sidered as a secondary product after the brown hornblende. Both kinds
are free from inclusions.
Accessory minerals, — Tlic cpidote Is observed very frequently inclosed in the
altered biotite, and is surrounded by pleochroic halos. In such cases it is
considered a primary constituent. The accessory minerals, apatite, sphene,
and zircon, show none other than their usual characters. Titaniferous
magnetite is present in the diorites in very small quantity.
According to the relative proportion of the important minerals just,
described — plagioclase, orthoclase, quartz, hornblende, and biotite — com-
posing the diorites, we get the following- varieties: Mica-diorite, quartz-
mica-diorite, quartz-diorite, and tonalite. These grade into one another, as
stated above; and, as will be shown later, certain of them grade into
o-ranites. On account of these variations these dioritic rocks are especially
interesting.
DESCRIPTION OF INTERESTING VARIATIONS.
SEC. 15, T. 42 N., R. 31 W.
A dike of rock 4 feet wide, occurring at 425 paces N., 1050 W., sec.
15, T. 42 N , R. 31 W., near Norway portage, shows the following min-
eraloo-ical variation. A specimen taken from the center of the dike shows the
rock to be there a typical fine-grained granitite with little or no plagioclase.
(Photomicrograph, fig. A, PI. XXXIX.) Along the sides the dike rock
is a mica-diorite consisting of mica and plagioclase without any (piartz.
Measm-ements on zone perpendicular to 010 gave equal extinction angles
DIORITE INTRUSIVES. 227
with ii luiixiiiuiiu (it" If) (legrties. Only oiw kind of plajiioclase i« distin-
•fuisliable by its mode of development, and this is rich in Ca( ), as shown by
its alteration products. The feldspar rang'es at most from all)ite to andesine.
No chemical anahsis has been obtained of either the "ranitite or flic mica-
diorite phase, but the mineralogical composition is sufficiently marked to
show conclusively that we have here a gradation from a granitic to a dioritic
rock rich in CaO. The idea has been suggested by Johnston-Lavis ^ that
in some cases the variation in chemical composition of intrusive rocks,
especiall}' where this variation is one between the center and the peripher^-
of an intrusive mass, may l)e due to resorption by the intrusive of parts of
the rock intruded. The sharp line of demarcation which exists between
the dike and the intruded hornblende-gabbro in the occurrence described
above seems to preclude the possil)ilitA' of a fusion and mingling of the two
rocks.
ACROSS RIVER FROM CRYSTAL FALLS.
Near Crystal Falls, just across the river from the town, are a number
of small knobs of granite grading into quartz-mica-diorite. They are
medium-grained rocks, reddish to gra}- in color. They take a very fine
polish and are well adapted to ornamental stonework, as is shown by the
columns made from them which are used in the court-house at Crystal
Falls. When examined under the microscope, the rocks are found to con-
sist of autoinorphic biotite and plagioclase, with xenomorphic orthoclase
and quartz, these last forming the cement. Some of the slides show
beautiful micropegmatitic intergrovvths of quartz and feldsjjar. The amount
of quartz, plagioclase, and orthoclase varies so that, depending upon the
specimen examined, one would call the rocks forming the knobs granite or
quartz-mica-diorite. Most coimnonly the rock is a plagioclase-bearing'
granite. No analysis has been obtained of the granite, but it is confidently
believed that the chemical composition would sustain the microscopical
diagnosis. Within the granite there are found lenticular schlieren of
considerably darker color than the main mass, in which the plagioclase is
the preponderant feldspathic constituent. The rock of these lenses is
essentially a quartz-mica-diorite.
' The basic eruptive rocks of Grau (Norway) and their interpretation ; a criticism by H. .1.
Johnston-Lavis: Geol. Mag., 4th ser., VoL I, 1894, p. 252.
The causes of variation in the composition of igneous roclis, by H. J. Johnston-Lavis : Natural
Science, Vol. IV, 1894, pp. 134-140.
228 THE CEYSTAL FALLS lEOI^-BEARma DISTRICT.
These knobs are cut bv a nuinl^er of small dikes from a fraction of au
incli to 3 inches in width. In all of these dikes the rock shows the same
characters. It is very light gray Xo pink in color, and aphanitic.
An examination under the microscope enables the separation of each
dike into a verv compact fine-grained saalband and a somewhat coarser-
grained porphyritic central portion. In the central part of the dike pheno-
crvsts of quartz, feldspar, and biotite lie in a very tine groundmass of
quartz and feldspar. Tlie texture of this groundmass is microgranitic.
The saalband is composed of the microgranitic groun(hnass without the
phenocrysts. The (juartz phenocrysts show the usual characters. The
feldspar phenocrysts are in most cases so completely replaced by a musco-
vite aggregate as to preclude any exact determination of their original
character. In some cases indistinct remains of poly synthetic twinning are
seen. Even when the main mass of the feldspar phenocrysts is entirely
altered, there is a narrow zone of very fresh feldspar material surrounding
it. Twinning in the center is also continuous through this zone. More-
over, this zone itself shows a very noticeable zonal structure by the change
in extinction angle observed in passing from the inner to the outer portion.
This less altered zone of feldspar contains numerous inclusions of quartz
from the groundmass. The character of the feldspar phenocrysts could not
be determined, but the presumption is that they are of the same character
as the feldspar in the coarser main mass — that is, andesine — with a more
acid feldspar, possibly oligoclase, surrounding them. The further presump-
tion is tlien wan-anted that the feldspar of the groundmass agrees with this
outer feldspar zone in character — that it is also oligoclase, or at least is
more acid than the phenocrysts. Automorphic biotite plates are now repre-
sented bv chlorite pseudomorphs, with here and there some secondary epidote.
The groundmass consists chiefly of quartz and feldspar, but contains
disseminated through it many minute plates of white mica and a few
crystals of zircon. The feldspar of the groundmass is too small to permit
of its accurate determination. A plagioclase feldspar in sections indicating
an approach to automorphism was observed. Its character as oligoclase (f),
( )r at least a feldspar of a more acid character than that of the centers of
the ithenocrysts, is sunnised for reasons given above. Microcline in sec-
tions showing characteristic twinning and in more or less rectangular out-
lines was obsei-ved in considerable quantity. An untwinned feldspar was
DIORITE INTKUSIVKS. 229
deterniined as ortlioclasc l)y tlic (lirtereiicci shown ))v its i-efractive index
and tliat of the twiuucid plajriorlase. Quartz was also recognized in tliis
way in the g-roundniass. The quartz and ortliorlase form tlie cement for
tlie other constituents. The muscovite in tlie groundmass is ])resumably
secondary, as is tliat in the phenocrysts. (Figs. A and />', I'l. XL.)
The rock is here inserted as showing an exceedingly line grained j)or-
l)hyritic form of the quartz-mica-diorite. It may compare to tliis mica-
diorite as does the tonalite-porphyrite of Becke' to the tonahte described
by Inm, and one miglit call it a quartz-mica-diorite-porphyry.
No analysis of this rock has thus far l)een obtainable. Possibly its
chemical composition may indicate it to be more closely allied to the true
granites than is believed to he the case, judging from its mineralogical
composition and its association with the rocks on the border line between
granites and diorites.
SOUTHEAST OF CRYSTAL FALLS.
Southeast of Crystal Falls, in sec. 16, northwest of Lake Tobiu, and
extending southwest into sec. 28, T. 42 N., R. 32 W., is a range of hills
upon which are numerous exposures of a uniformly medium-grained rock.
The main mass of the knobs is of tonalite, which shows several facies. A
miarolitic texture is very common in this massif. The cavities are now
filled with calcite, quartz, and epidote-zoisite alone or togetlier. This last
mineral occurs in single large individuals or in tufts of individuals, which
radiate from one side of the cavity. Li one case a cavity incompletely
filled by such a tuft has been completely filled by a later infiltration of
quartz. The color of the rock varies from light pink to very dark greenish
gray. The areas of the lighter-colored rocks may be seen extending in
finger-like projections into the darker-colored phases. There are, however,
no sharp lines between these varieties, but a gradual passage from the lio-hter
to the darker rock. These diff"erent phases evidently belong to a single
rock mass. Under the microscope, however, important variations in the
textural and mineralogical character of the rock masses are seen. The
main mass of the rock is granular tonalite. The essential constituents are
plagioclase, orthoclase, quartz, hornblende, and mica. The most common
association of minerals is hornblende and mica in automorphic crystals,
' Petrographische stndien am Tonalit der Rieserferner, by F. Hecke: Tschermak.s mineral
Mittheil., Vol. XIII, 1892, p. 435.
230 THE CRYSTAL FALLS IRON-BEARING DISTRICT.
witli plag-Ioclase somewhat less well developed. Between these there is
found the quartz, with some accessory orthoclase, and microcline as the
last products of crystallization. In some cases these two minerals are
present in micropegmatitic interg-rowths. A textural variation, which the
facies mentioned below also undergo, is from a granular to an im]:)ei'fectly
ophitic texture. In such cases the order of crystallization of the hornblende-
mica and plagioclase is reversed, the plagioclase being most automorphic in
the ophitic varieties.
The rock resembles the tonalite described by Becke from the Rieser-
ferner.^ It also closely resembles some slides of the typical Adamello tonalite
with which I have been able to compare it. The chief difference between
them is that the plagioclase and hornblende have a better crystallographic
development in the Crystal Falls rocks than in the Adamello tonalite, and
that the accessory allanite of the Adamello rock is wanting in the Crystal
Falls tonalite, though the normal epidote may represent it. The horn-
blende also differs slightly from that of the Adamello rock in that it is not
throughout reddish brown. Tlie central portion of some of the crystals
shows this color, but the outer portion is a dirty green, even grading into
an almost white hornblende.
The tonalite grades, by diminution of biotite, with corresponding
increase of hornblende, into a quartz-diorite, and by diminution or disap-
pearance of the hornblende and increase of the biotite into a quartz-mi ca-diorite.
Hornblende never occurs alone in the rocks, whereas biotite may
occur as the only l^isilicate C(instituent. It is a very common thing to find
in the diorites rounded basic segregations consisting chiefly of mica with
hornblende suljordinate and just a little accessory feldspar. When the
orthoclase and quartz diminish, we get the mica-diorites. Orthoclase is
always present in all of these dioritic rocks. In certain facies orthoclase
and quartz are very abundant and the plagioclase is correspondingly dimin-
ished. Such rocks are clearly plagioclase-bearing granites, and represent
gradations between the ordinary tonalite and granite, and point to close
relationship of this occurrence with the occurrences nearer Crystal Falls
alread}' described, in which the granitic facies predominates and the dioritic
facies is subordinate.
' Petrographlsche studien am Tonal it iler Rieserferner, by F. Becke: Tschermaks mineral. Mitt-
heil., Vol. XIII, 1892, pp. 364-379.
BIORITE INTKUSIVES.
231
Similar "Tiuliitions have l)eeu noted \)y lierke in tlic tonalito Iruni the
Riesei-t'einu'i-.' Tlie diorite massif (if the Crystal Falls district seems to cor-
respond very closely to the granodiorite masses of Becke, Turner, and Lind-
gren,- which on the one hand grade into the granitites and <ai the other into
the diorites.
ANALYSIS OF DIORITE.
It has not been found possible thus far to obtain analyses of all these
varieties. The more acid facies of the diorites seem in their mineralogical
composition to show very clearly their gradations toward the tonalites and
granites. This being the case, it was deemed of more importance to study
the relations of the more basic dioritic facies in order to determine tlie rela-
tionship of these rocks to those of tlie more basic gabbro and peridotite
families which are found iu association with them. To this end an analysis
of one of the mica-diorites was obtained.
This rock contains the dark constituents, biotite and hornblende, in
large quantity, and of these the mica predominates. Plagioclase predomi-
nates among the white silicates, with orthoclase and quartz very subordinate.
The mica is considerably altered, but on the whole the rock is fairly fresh.
Fio-. B, PI. XXXIX, is a photomicrograph of the rock and shows its general
characters. The following analysis was made by Dr. H. X. Stokes, in the
laboratory of the United States Geological Survey:
Analysis of diorite.
SiO:-.
TiO, .
ALO:,.
Fe,0,
FeO-.
MnO
CaO .
MgO.
Per cent.
58.51
.72
16.32
2.11
4.43
Trace.
3.92
3.73
KjO
Na,0
H;0 at 100°
H;0 above 100=
PjOfS
Co,
Total
4.08
3.11
.23
2.00
.30
None.
99.17
iPetrographische studieu aai Tonalit der Rieserferner, by F. Becke: Tschermaks mineral. Mitt-
heil.. Vol. XIII, 1892, p]). 379-464.
-The granodiorite of California appears from Lindgren's description (Granitic rocks of Cali-
fornia, by W. Lindgren : Am. Jour. Sci., 4th series, Vol. Ill, 1897, p. 308 ; where can be found references
to mention and descriptions of granodiorite) to correspond very closely to tonalitc, though Turner
nses the name as synonymous with quart/.-mica-diorite (Geology of the Sierra Nevada, by H. W. Turner:
Seventeenth Ann. Kept. U. S. Geol. Survey, Part I, 1896, pp. 636, 724).
232 THE CRYSTAL FALLS IRON-BEARING DISTRICT.
The altered character of the rock is readily seen in the large content
of water. It is, nevertheless, not so marked as to render the analysis use-
less for purposes of determination.
The character of the plagioclase feldspar is clearly indicated by the
relatively high percentage of lime. This high content in lime and the large
amount of alkalies present, 7.18 per cent, clearly show its relationship to the
diorite family. The content in potash feldspar and the possible derivation
of the rock from a granitic magma is shown by the high content in potash.
Possibly a considerable amount of the potash, with the greater part of the
mao-uesia, should be deducted for the biotite which is so abundant.
This rock is one which it is somewhat difficult to place definitely in
the existing division of rock families. The large amount of lime and the
relatively low percentage of alkalies prevent the placing of the rock with
the syenites. On the whole it approaches close to the monzonite group
according to the chemical composition of the group given by Brogger.
But it differs from this in that the lime (3.92 per cent) is too low to bring
the rock within his limits (4.52 to 10.12 per cent).' However, if we con-
sider the total of the alkaline earths (7.65 per cent) in the rock under dis-
cussion, we find that it comes well within Brogger's range (6.05 to 17.52
per cent) for a total of magnesia and lime. Moreover, the alkali total (7.19
per cent) is about right to warrant its classification in the monzonite grouj)
as a representative of the type of biotite-monzonite.
On comparing the analysis with that of trne normal diorites. we find
that the relative proportions of the alkalies are abnormal. The lime con-
tent is also too low for rocks of this character, and the magnesia is too higli.
The above considerations seem to make clear the relationship of the
rock to the monzonites and diorites. However, it is so intimately asso-
ciated with and so evidently but a facies of the tonalite which is the domi-
nant type where this rock occurs, that it is considered to be more closely
related to the lime-soda feldspar rocks, in which the orthoclase is but acces-
sory, than to the monzonite family of orthoclase-plagioclase rocks. It is
therefore considered to be a mica-diorite.
' Op. cit., Part II, p. 51.
GABBRO AND NOIilTE INTltUSlVES. 233
GAUnno AND NDRITE.
PETROGRAPHICAL CHARACTERS.
The <;iil)l)r()s and iiorites are liolocrystalline rocks of luoderatel}^ fine
to coarse grain. They show a considerable variation in texture. Some, the
finest-grained forms, possess a very good parallel texture (PI. XLIJ, figs.
A and B); others are noticeably porphyritic. A few have poikilitic tex-
tures (PI. XLI, figs. A and B); less common is an approach to the ophitic
texture of the dolerites. Most connnon of all the rocks are the hypidio-
morphic granular ones (PI. XLIV, fig. A, and PI XLIII, fig. A).
The rocks vary from a light grayish-green color for some of the coarse-
grained ones, through darker greenish colored rocks to those of a dark
brownish or greenish-black color for the finest-grained forms.
The important origiiial mineral constituents are feldspar, mica, horn-
blende, pyroxene, and olivine. Apatite, sphene, zircon, rutile, octahedrite
(anatase), brookite (?), and iron oxide occur as accessory minerals. White
and brown mica, chlorite, hornblende, talc, serpentine, sphene, rutile, and
calcite occur as secondary minerals.
Feldspar. — Both plagiocksc and orthoclase are present in the gabbros and
norites. Plagioclase is by far the most important. It occurs normally in
the coarse-grained kinds of rock as broad tabular individuals. In the finer-
grained, especially the })orphyritic and poikilitic facies, the feldspar sections
aiisume a broad, lath-shaped character. The sections show the character-
istic polysynthetic twinning. Twins, according to the albite, pericline, and
Carlsbad laws, are present, usually the albite and Carlsbad or the albite and
pericline being combined; in some cases all three occur together. Twin-
ning lamellse vary in breadth, but on the whole are moderately narrow.
Measurements made on the zone normal to 0 1 0 give equal extinction angles
against the twinning plane, which reach a maximum of 34 degrees. The
feldspar is evidently labradorite. A zonal structure is noticeable, and is
especially shown by the alteration being more advanced in the centers of
the individuals. It is possible that the labradorite is accompanied by a
small amount of more acid feldspar, andesine in zonal growth with it.
The alteration of the feldspar results in the production of the same sec-
ondary products formed from the slightly more acid feldspar of the diorites.
234 THE CRYSTAL FALLS IRON-BEARING DISTRICT.
mica (both muscovite and biotite), epidote-zoisite, and calcite. The plagio-
clase shows very beautifully tlie effects of dynamic action in local granu-
lation of the peripheries of the indi\T[dual. Such lines of granulated feldspar
can be followed through the sections, probably indicating shearing planes.
Inclusions are common. Some stout rutile crystals were observed in the
feldspar. In scTme cases minute hair-like needles, which in a few instances
were of a size sufficient to admit of their ready determination as rutile,
were also found penetrating the plagioelase. Crystals of apatite and iron
ores are also commonl)' included in it. There have also been found in a
few cases minute hexagonal plates, which are translucent, with brown color,
and are presumably micaceous ilmenite.
Mention of the presence of oi'thoclase in these rocks is made with con-
siderable doubt. Here and there a few plates of untwinned feldspar,
showing a character somewhat different from that of the plagioelase plates,
were observed. As will be seen, the possibility of its presence is indicated
by the potash shown in the analysis.
Biotite. — The biotite in the coarsely granular rock is in irregular plates.
They are frequently included in and attached to the outside of the horn-
blende. Its period of crystallization thus overlaps that of the hornblende,
though on the whole being contemporaneous with it. In the fine-grained
rocks biotite is better developed than the hornblende, and is apparently
for the most part older than it. In color it vai'ies from a rich reddish brown
for rays vibrating parallel to the cleavage to a light yellow for those per-
pendicular thereto. It includes crystals of zircon and apatite, which are
surrounded by pleochroic halos.
Hornblende. — In most of the sectlous hornblende is the most striking- com-
ponent. It is present in the gabbros in three different varieties. The most
prominent kind is a reddish-brown hornblende, which has a dirty green
hornblende commonly associated with it and frequentl}' intergrown with it
zonally. This liornblendo occurs without the green, but the green is
invariably associated with the brown. The two are optically continuous in
the intergrowths. It is possible, though not susceptible of proof, that the
green is the result of the incipient alteration of the brown. The second
kind is compact, strongly pleochroic, common green hornblende, and the
third is a noncompact, reedy variety of light-green hornblende. The first
GABBRO AND NORITE INTRUSIVES. 235
two kinds of lioniblondc^ ;ire presumed to l)e primary. The tliird variety is
secoiidarA', hut secondary after tlie orio-inal liornblende, thus not atfectiii"-
the diaracter of tlie rock.
The first variety, the reddisli-browii hornblende, occurs in the g-abbros
in anh('(ba. A zonal .structure is marked l)y l)rown liornblende occupying
the centers of the crystals and by dull green hornblende, which agrees
optically with the brown, occupying the outsides. The brown hornblende
is somewhat lighter colored than basaltic hornblende. The pleoehroism is
strong in the following colors: Brown hornblende: tl, light yellow or red,
with tinge of green; ft, red brown; c, same or darker red bi'own, excep-
tionall}' yellowish-brown; C^b>a. Green hornblende: a, greenish-
yellow; Jjs, yellowish- or brownish-green; c, dull olive green, frequently
with bluish tinge; C>I)>-a.
This hornblende, with respect to its rather exceptional pleochroi.sm
and its general characters, seems to agree very well with that described b}^
van Horn from ver}^ similar rocks from Italy, and, like that, is probably a
very basic hornblende.^ Twinning parallel to 100, co P oo, is very common.
An imperfect parting parallel to the orthopinacoid 100, oo P oc, was also
observed in some cases. It is also indicated by the jjlaty inclusions which
lie in this plane In some of the sections where the green hornblende is
not intergrown with the brown the green kind shows very commonly a
system of fine striations parallel to the positive orthodome 101, P oo. In rare
cases the brown hornblende is intergrown with almost colorless hornblende,
one end of a crystal being brown, the other faintly yellowish. Irregular
mottled intergrowths of the two were also found.
The normal brown-green hornblende is rendered poikilitic in some
specimens by a few rounded grains of perfectly fresh pyroxene, and also by
plagioclase crystals which it includes. This same kind of hornblende is
frequently rendered very dark by the number of exceedingly small inclu-
sions which it contains, and in this, and also in its reddish-brown color,
resembles so strongly many hypersthenes as to be readily mistaken for them
upon cursory examination. These inchisions are of several kinds, all dis-
tributed throughout the same individuals. It is impossible in studying them
' Petrographische Untersuchungcn fiber die noritischen Gesteine der Umgegend von Ivrea in
Oberitalien, by F. R. van Horn: Tschermaks mineral Mittheii., Vol. XVII, 1897, Paxt V, p. s.J9.
236 THE CRYSTAL FALLS IRON-BEARING DISTRICT.
to get any optical tests, except that of extinction, owing to the minute size
of the inchisions and to the fact that where large enough for exaniinatiou
the tests were vitiated b^s' the presence of the hornblende.
Of these inclusions some are readily distinguishable as rutile. Some
of the larger of the crystals reach a leng-th of 0.04.5 nun. and a thickness
of 0.0125 mm. Numbers of them show the characteristic heart-shaped
and geniculated twins of rutile, so that there is no doubt as to the determi-
nation. Associated with the rutile are other crystals 0.019 mm. long, which
show the typical 2)ointed pyramidal development of octahedrite (anatase).
Still others show a flat tabular development somewhat similar to that of
brookite, though these could not be jjositively determined as that mineral.
The hexagonal plates of clove-brown color so frequent in hornblende and
also in hypersthene occur also in this hornblende. They are believed to be
micaceous ilmenite. The thin ^ilates are translucent, thicker ones are less
so, and those which are still thicker are opaque and metallic. The thin
plates appear when on edge as fine, hairlike streaks. The thick ones
appear in the same position as more or less rectangular bars or rods. Often
these small plates are associated with masses of iron oxide, also included in
the hornblende. This iron ore occurs in the plates and bars characteristic
for ilmenite. These ilmenite masses are translucent only on the edges,
where the slide has cut the mass in such a manner as i o give an exceedingly
thin section of the ore. At such places the ore is translucent with the
same brown color as the thin plates. Another rare variet}' of the inclu-
sions occurs in round grains of rich green color, and may possibly be a
spinel.
In those sections in which both original brown and original green
hornblende occur the inclusions are confined to the brown kind. Where
the brown kind is surrounded by the green hornblende the inclusions grad-
ually diminish in quantity as we approach the green zone. With this goes
also, hand in hand, a lightening of the color of the including mineral
(brown hornblende), and there is thus an imperceptible change from the
brown to the green hornblende. Where the green hornblende occu.rs alone
it is frequently as full of inclusions as is the brown hornblende of other
sections. Individuals of the same sections difter from one another with
respect to the quantity of the inclusions, some being crowded with them,
while others are practically free from them.
GABBRO AND NOKITE INTRUSIVES. 237
Thi.s lirowii li(.nil)lcU(lLs on alturatiou, breaks up into aggregates of
ei)i(lote-zoisito and light-green chlorite.
The second kind of hornblende is the perfectly fresh, compact, coni-
nidu dark-green kind, with pleochroism varying from yellow for a, to
yelloAvish-green for Jji, and to bluish-green for C; C>b>a. This is found
in very few cases. It ap])ears in every instance to be a primary constituent.
The third kind of hornblende may ht-. primary, although the evidence
obtainable points to its secondary nature. It has a light-green color, and
when examined for pleoclu-oism exhibits a scarcely noticeable change.
This hornblende differs very nuich from the other two hornblendes
described, in that it is not compact, but occurs in aggregates of coarse reed-
like (schilfaehnliche) individuals. Such aggregates do not at all resemble
uralite. The individuals are far too coarse and wedge out at short distances
within the aggregates. The aggregates occupy irregularly shaped areas.
The aggregates consequently have a coarse patchy polarization. They are
frequently surrounded by ragged pieces of biotite, just as are the plates of
compact hornblende. Moreover, they occur in rocks which show pressure
effects, and are best developed in those in which such effects are most
marked. The aggregates rather frequently occur with irregular pieces of
the greenish or brown compact interposition-bearing hornblende bordering
the aggregates or in the midst of them. The light-green reedy hornblende
never contains such interpositions, but does have associated with it fairly
large grains of rutile, which may perhaps be considered as having been
derived from the various titanium-bearing microlites in the original brown
hornblende. The general appearance of these aggregates and their asso-
ciation with the original hornblende seem to point toward their secondary
origin from the latter through the effects of pressure.
Pyroxene. — Thc pyroxeue comprises both a monoclinic and an ortho-
rhombic kind. These are the first of the bisilicates to crystallize in these
o-abbros. The monochnic kind is of two varieties. The first is in colorless
to faint-pink grains included in large plates of original brown hornblende.
These grains have a well-developed prismatic cleavage. One basal section
shows very nicely the characteristic pyroxene cleavage. The extinction
measured against the prismatic cleavage reached as high as 50 degrees.
This pyroxene is presumed to be augite. It never shows diallagic parting.
In other sections the monoclinic pyroxene is a clear white to faintest
238 THE CRYSTAL FALLS IRON-BEARING DISTRICT.
green malacolite or diopside. This is in roundish grains included in the
original green hornblende, which it equals in quantity.
The orthorliombic pyroxene occurs in individuals which show fairly
good prismatic development, but with rounded terminal faces. The pris-
matic cleavage is very well developed. A transverse parting is also com
mon. The pyroxene is usually colorless, but in some cases a scarcely
noticeable pleochroism was observed, varying from a faint-greenish tinge
for rays ^'ibrating parallel to C, to a yellow for those parallel to a and
h. It contains small, dark, streak-like intrusions, some of which under
exceptionalh' favorable conditions are transparent,, with a faint-greenish
tinge The exit of the bisectrix in basal sections, as Avell as the parallel
extinction, renders it easily distinguishable from the monoclinic pyroxene.
The optical angle could not be measured, but was clearly very large, as the
hyperbolas passed completely out of the field of view. The orthorliombic
pyroxene is evidently enstatite or bronzite, and the pleochroism clearly
points to its position near bronzite. A few crystals of rutile and also some
of the ilmenite plates, so common in hypersthene, were found occurring in
the bronzite. The ilmenite plates are in rather rare irregular patches in the
crystals.
In many cases along the cleavage lines or around the edges of the
crystals or along the transverse parting planes are narrow zones of a sec-
ondary yellowish-green, finely fibrous, serpeutinous mineral. Beyond
these zones is a pure white aggregate of secondary talc scales (Fig. B, PI.
XXXVIII). Among these scales are a few minute rutile crystals, and also
a few Ijlack ferruginous specks, these products being probably derived from
the inclusions in the bronzite, and the ferruginous material possibly to some
extent from the mineral itself In some cases, instead of the intermediate
serpentine zone, the rather rare occurrence is observed of the passage of
the bronzite directly into the talc aggregate.
Olivine. — The determination of the original presence of olivine in the
gabbroic rocks is based upon very slight evidence. In some of the sections
containing augite almost every individual of this augite has near its center
a rounded, very rarely irregular area of yellowish-green fibrous serpentine.
These areas are sharply delimited from the surrounding pyroxene, and the
conclusion seems warranted that it resulted from the alteration of some
mineral included in the pyroxene. Tlie only bisilicate found in the rocks
GABBliO AND NOltlTE INTKUSIVES. 239
of the district, which ciystalhzud before the jjyroxene is ohviiie. In the
peridotite, to l)c described in the next section, this is usunllv surrounded
bv nionochnic or orthorlioinl)ic pyroxene. This idtered niinend is not
important in (punitity.
Iron oxide. — Jlinouite aud titaniferous magnetite occur in some of tlie rocks
in considerable quantity. Both alter to sphene and rutile.
Apatite. — Among- the accessory minerals apatite is jjerhaps the most
common, and, as usual, one of the very earliest minerals to crystallize. It
is contained in all the essential constituents, and in biotite is surrounded by
pleochroic hales. In some cases it has even crystallized before sphene. It
is noticeable in some sections that great numbers of apatite crystals are
arranged along lines representing sections of planes between the jjlagioclase
plates, thus practically outlining the feldspar individuals.
Sphene. — In many cases in these gabbros sphene is found contained in
some of the freshest rocks as an original accessory constituent. It is present
in largest quantity in the very finest-grained gabbros, which «how a parallel
texture. In these rocks sphene in some cases surrounds an iron ore, which,
to judge from the rod-like sections which are so common, is ilmenite. One
might be led to think that the sphene was secondary in such cases, but the
iron ore is perfectl}' fresh, and, considering that in the same thin section
crystals of apatite are also surrounded by sphene, it seems clear that we
may consider such sphene as original. It thus appears that a portion of the
titanium oxide combined with the iron oxide first, forming the titanic iron
ore. This was followed by the crystallization of the calcium-titanium
compound, thus giving the sphene. In these rocks sphene is not in crystals,
but in grains. These grains are arranged in long chains hing between the
other mineral constituents and with the long direction of the individual
grains, as well as of the lines of grains, parallel to the long directions of
the other constituents of the rock.
Zircon and rutile. — Zircou is iu vcry small quantity. Rutile shows its usual
characters. It is most commonly associated with the octahedrite (anatase)
and brookite (?) as inclusion in the hornblende. The iron oxide is chiefiy
present as ilmenite, with some titanic magnetite.
The secondary minerals have alread}^ been mentioned and their char-
acters described under tlie description of the minerals from which they are
derived.
240 THE CRYSTAL FALLS IRON-BEARING DISTRICT.
DESCRIPTION OF INTERESTING KINDS OF GABBRO.
The minerals described above as the leading essential constituents of
the rocks to be described may be combined in varying quantities. Accord-
ing to these combinations a number of different mineralogical types of rock
may be produced. The wide range in mineral composition of the gabbroic
rocks is equaled, if not surpassed, by similar variations noted by Fairbanks
in certain rocks from Point Morrito, California.' It may cause the further
description to be more readily understood if we preface it by the statement
that all of these types, however, are simple facias of a single magma. The
important phases which will be described are, in the order of their impor-
tance, hornblende-gabbro, consisting essentially of liornblende and lab-
radorite; gabbro, consisting of monoclinic pyroxene and labradorite, and
bronzite-norite, consisting essentially of bronzite and labradorite. The
various mineralogical types exhibit very interesting ranges in texture in
certain cases, t-o which attention will be called.
HORNBLENDB-GABBRO IN SEC. 1.5, T. i'2 N., R. 31 W.
A hornblende-gabbro forms a large knob in sec. 15, T. 42 N., R. 31 W.,
just at the foot of the Norway Rapids, on the west bank of the Michigamme
River. This exposure shows very prettily a change in texture. The change
in texture is also accompanied by a slight mineralogical change. The knob
is composed partlv of a fine-grained granular, but more largely of a coarse-
grained porphyritic, gabbro. The fine-grained portion is a pure gabbro
composed of plagiocla^e and brown hornblende, with very little brown
mica. No quartz was observed, nor was any orthoclase definitely deter-
mined. The plagioclase is in fairly well-de 'eloped automorphie plates.
The hornblende is the brown variety, with numerous minute inclusions,
which has already been described, and is not always so well developed as
is the plagioclase. In places it plays rather more the role of a cement.
This relation of the two minerals results in forming an imperfect ophitic
texture in places, though on the whole the two minerals are about equally
developed, and produce a granular structure. (Fig. ^, PI XLIV.)
' The geology of Point Sal, by H. W. Fairbanks : Bull. Dept. Geol., Univ. California, Vol. II,
1896, p. 56 et seq.
GABBRO AND NORITE INTRUSIVES. 241
The coarsse-iiTniiU'd })()r])liyriti('. gabbro foriiiing- tlie greater ])art of the
knol) consists of phigioclase, liornbleiide, biotite, and iron oxide, witli a very
small amount of pyroxene. The hornblende occurs in phenocrysts which
have irregular rounded shaj)es instead of being well crystallized. Some of
the largest phenocrysts have a diameter of slighth^ less than 1 centimeter.
They are poikilitic, rendered so by inclusions of lath-shaped plagioclase and
rounded grains of pyroxene. (Photomicrograj)h, fig. A, PI. XLI.) This por-
phyritic liornblende is a dark reddish-brown -variety containing such great
numbers of minute inclusions as to be opaque in many places, which grades
over into, and is in many places in optical continuity with, a dirty green
hornblende. This green hornblende is in anhedra and forms the cement
for the feldspar, and the two together the groundmass for the brown horn-
blende phenocrysts. Tlie plagioclase is most commonly in broad, well-
developed crystals, which frequently give quadratic sections. Some few
grains of a pink monoclinic pyroxene are included by the hornblende.
SECS. 15, 22, 28, AND 29, x. 42 n., b. 31 w.
Exposures of a hornblende-gabbro with interesting facies associated
with it occur in the southeastern corner of sec. 15, at tlie southeastern corner
of sec. 22, extending east and west through the northern part of sec. 28, at
the southeastern corner of sec. 28, and on the west bank of the Micliigamme
River in sec. 29, T. 42 N., R. 31 W., at the location N. 100, W. 1,250 paces.
This is medium to coarse grained and of a gray color from a short distance.
Examined at moderately close quarters, one distinguishes very readily a
milky white feldspar and a black or dark-green hornblende in about equal
quantities. The microscopical examination adds to these two minerals in a
very subordinate quantity biotite, pyroxene, and orthoclase. The labra-
dorite plagioclase is in medium-broad, irregular plates, though at times
approaching a very distinctly lath-shaped form. The orthoclase is present
in a few rare individuals in the form of irregular plates. The hornblende
constituent is in irregular plates and varies in character. It may be the
brown or the green variety ah-eady described, or the two together in sepa-
rate individuals, or even the brown grading into the green. This green is
original and not the alteration product of the brown. Biotite is the normal
reddish-brown kind in irregular plates. The pyroxene is usually absent
MON XXXTI 16
242
THE CRYSTAL FALLS IRON-BEARING DISTRICT.
from the sections of these rocks, but when present it is ver}- rare, and occurs
in small irregular grains not uncommonly intergrown with the hornblende
and evidently older than the hornblende. It is light green in color, 'with a
scarcely noticeable pleoclu-oism. Its monoclinic character was readily deter-
minable; but a more exact determination was not made. It does not, how-
ever, show diallagic parting, and is diagnosed as possibly diopside.
The feldspar shows the best development of the accessory minerals.
It can rarely, however, be said to be automorphic. The texture is, on the
whole, granular.
From a mineralogical study of the rocks alone, one would unhesitat-
ingly place them with the diorites, especially if those facies were seen in
which the pyroxenic constituent was wanting.
The following analysis (Sp. 23354), obtained from Mr. George Steiger,
of the United States Geological Survey, shows the chemical composition
of one of these rocks :
Analysis of hornhlendegahhro.
SiO, .
TiO, .
Al.Oa
FeiOa
FeO .
MnO.
CaO .
MgO.
Per cent.
49.80
.79
19.96
6.32
.49
11.33
7.05
K,0
NajO
H.O 100^—
H2O 100^-f,
P2O5
CO2
Total
Per cent.
.61
2.22
.13
1.71
.07
.15
100.63
An examination of the analysis shows that the microscopical determi-
nation of the rock as a diorite would be incorrect if we accept, as has been
done in the preceding pages, Brogger's characterization of the diorite and
gabbro families.^
The rock analyzed is hornblende-gabbro, as shown by the relatively
high content in the characteristic alkaline earths, esj^ecially magnesia,
which usually appears in inverse proportion to the silica, and in the low
percentage of alkalies.
' Op. cit., Part II, pp. 35, 39.
c
GABBRO AND NOIUTE INTKUSIVEB. 243
HOUNBLKNDK-CSABUKO DUCES.
One (tt" tlic exposures of the above-descTibod. liornlilende-g-abln-o — tlie
one on the west bank of the Michigamnie River in section 29— is very
interesting on aeeount of at least two different series of dikes which cut it.
The coarse-grained liornblende-gabbro forms the main mass of the knob.
The dike rocks may be divided into the fine-grained hornblende-gabbro
forming the earhest dikes and the coarse-grained bronzite-norite forming
the latest dike.
Some of the specimens of the fine-grained rock are massive, but the
o-reater portion possess a distinctly parallel texture. These are distinctly
micaceous. The rock of the dikes has in general very much the macro-
scopical appearance of a biotite-mica-schist. The dikes are very narrow,
never more than 18 inches wide. The larger ones send off branches, and
in places inclose angular pieces of the coarse diorite. Thus the relation of
this fine-grained rock to the main gabbro mass is perfectly clear, though in
places it so closely resembles, macroscopically, pieces of mica-schist that in
spite of the branching of these dikes, indicating their intrusive nature, they
were supposed by one observer to be curiously shaped stringers of the
mica-schists included in the main mass of the gabbro.
The notes do not indicate from just what portions of the dike the
specimens were taken; hence it is impossible to state positively that the
more schistose parts are nearest the edges and the more granular portions
nearer the center, as one would naturally expect. However, in all but one
of the specimens which show a contact between the dikes and gabbro
which they penetrate, the rock nearest the contact shows a parallel struc-
ture. Hence it may be stated that in some cases the edges of the dikes
are the more schistose portions. The one specimen referred to above is
granular and much finer grained along the contact than farther from it.
The microscope shows the rock of these dikes to be a gabbro differing
little in character from the main mass. The plagioclase is well developed
in rectangular, more or less lath-shaped crystals. Mica of a rich brown
color is rather more abundant than usual, and is in about equal quantity
with the hornblende. The hornblende is brown or of a dirty greenish color,
containing the inclusions mentioned in the detail descriptions of the minerals
of the gabVjros. Some irregular grains of a light-green augite (diopside)
were observed in the sections. Orthoclase (?) in grains is present simply
244 THE CEYSTAL FALLS lEON-BEAElNG DISTEICT.
as an accessory. Ilmeuite occurs in irregular masses in rod-shaped pieces:
Sphene is present in original grains and also as a secondary ])roduct after
ilnienite. Apatite occurs and is sometimes included in the sphene.
The most striking feature of the rock is its textural variation. Some
of the sections show very good granular texture; others have a fair ophitic
texture ; the most common is a striking parallel texture which macroscopic-
ally gives to the rock a schistose appearance. This may be seen even in
the same section with the ophitic texture, the two grading into each other.
The parallel texture is occasioned by tlie arrangement in a common direc-
tion of the long diameters of nearly all of the minerals (figs. A and B, PI.
XLII). The grains of sphene often lie in long trains agreeing with this
general parallelism. One's first idea would probably be that the texture was
due to the cause which has produced parallel structures in most other ancient
rocks — pressm'e. However, it can not be referred to this, as the minerals —
with some individual exceptions — show slight or no pressure efi"ects.
Apparently the only explanation borne out by the facts in this case is that
we have to do with a fluidal texture, produced by the movement in the
magma consequent upon its injection along the fissures in the gabbro, the
parallelism of the minerals agreeing with the bounding sides of the fissure.
BRONZITE-NORITE DIKE.
The main hornblende-gabbro and the fine-grained dikes just described
are cut by a dike, about 3 feet wide, of coarse rock which resembles that
forming the main mass of the knobs in every way except that it contains a
very much altered orthorhombic pyroxene (bronzite) in greater quantity
than the hornblende. The rock is a very pure kind of bronzite-norite (fig.
B, PI. XLIV). The following analysis (No. 1, Sp. 23755), by Mr. George
Steiger, shows the gabbro affinities of the rock. The high percentage of
magnesia gives a clear indication of the important role played by the bron-
zite in the composition of the rock.
With the analysis of the bronzite-norite there is placed for comparison
an analysis (No. 2) of a norite from Ivrea, Italy, which is essentially the
same as the above in mineralogical as well as chemical composition. In
the Italian rock hypersthene, instead of bronzite, is the chief pyroxenic
constituent.^
' Petrographische Uutersuchiingen iibcr die uoritischeu Gesteine der Umgegcnd von Ivrea in
Oberitalien, by F. E. van Horn: Tschermaks mineral, Mtttbeil., Vol. XVII, 1897, Part V, p. 404.
GABBKO AND NORITE INTRUSIVES.
Andh/scs of iiorites.
245
SiO,
TiOj
ALO,
Fe,0.,
FeO
MnO
CaO
MgO
KaO
Na^O
H;0 100°—.
HiO 100° + .
PjOa
CO,
1.
48.23
1.00
18.26
1.26
6.10
Total
9. .39
10.84
.73
1.34
.26
2.00
.07
.43
99.91
49.95
.69
19.17
4.72
6.71
Trace.
9.61
.5.03
.74
3.13
.09
Trace.
9.84
SEC. 29, T. 42 N., R. 31 W., 1,200 N., 200 W.
On the east bank of the Michigamme River at 1,200 paces N., 200 W.,
sec. 29, T. 42 N., R. 31 W., there is an outcrop which shows even better
than the occurrence just described the variation in mineralogical character
and the relative ages of these varieties. At tliis place there is a knob com-
posed of hornblende-gabbro essentially like the general type described as
typical for this district. This knol) is cut by a rock which is coarser in
grain and a trifle darker than the variety which it intrudes. Examined
under the microscope, it is seen to differ from the normal phase also in min-
eralogical composition, and resembles rather closely in this respect the
porphp-itic variety described on p 240. Like that, the hornblende is reddish-
brown, containing a large number of inclusions and grading over into the
green hornblende. This hornblende includes rounded grains of a white to
pinkish monoclinic pyroxene in sufficient quantity to be characteristic. The
pyroxene is never automorphic, as one would perhaps expect to find it,
though it was evidently one of the first minerals to crystallize. Contained in
the pyroxene individuals are rounded areas of yellovvish-greeu serpentine.
These areas are sharply outlined from the pyroxene and do not appear to be
the result of its alteration; cousequenth' the conclusion is reached that the
246 THE CRYSTAL FALLS IROX-BBARING DISTRICT.
serpentine results from the alteration of a mineral older than the pyroxene.
Most 'irobably this mineral was olivine, though no positive statement to
that effect can be made. This pyroxene I'ock also contains an exceedingly
large quantity of apatite. No analysis was obtained of this facies, liut the
microscopical characters enable us to place it as a gabbro (possibly olivine-
bearing) and to consider it, like the bronzite-norite, as a facies of the pre-
dominant hornblende-gabbro.
This same exposure of gabbro is cut by a coarse peridotite (wehrlite),
a description of which will be found on p. 253. In this peridotite there is
found a narrow strip of rock, about 2 inches wide, which is presumed to be
either an inclusion or a yerj narrow dike in the peridotite. The exposure
does not admit of its relations being determined more accurately. The
presumption is that it is a dike. Whether an inclusion or a dike, it is
younger than the massive hornblende-gabbro forming the main exposure.
In this respect it corresponds to the gabbro just described (p. 243) as cutting
the normal gabbro.
This dike rock is macroscopically a fine-grained, granular, dark-gray
rock. The microscope shows it to be very fresh, porphyritic in texture,
and composed of phenocrysts of bronzite lying in a finely granular ground-
mass of plagioclase, hornblende, and orthorhombic and monoclinic pyroxene.
No quartz whatever was found associated with these minerals. The pla-
gioclase is the usual kind, labradorite. Some unstriated feldspar, possibly
orthoclase, was also observed, though in very small quantity. The bronzite
is in narrow prisms which reach a length of 1.23 mm. Commonly they
have well-developed transverse partings. It is worthy of note that a few
of the crystals contain the brownish platy inclusions so common in hyper-
sthene. There is, however, no relation between the color and pleochroism
of the mineral substance and these brownish plates. The bronzite is very
clear, with weak color, showing a scarcely distinguishable greenish tinge
for the rays vibrating parallel to C, with yellowish for the rays parallel to a
and I). In one case the bronzite was seen altering to a greenish-yellow
fibrous aggregate of serpentine. In the groundmass this same orthorliombic
pyroxene is represented by irregular grains. The hornblende is the usual
reddish-brown kind, but differs frOm that seen in the other gabbros of this
district in that it contains ilmenite inclusions only, without any rutile oi*
anatase. It is in anhedra. A faint-greenish pyroxene occurs in irregular
p
GA15BKO AND NOHITR INTUUSIVES. 247
xenoniorpliic individiuils, ioi'iiiiiiy a soiiu'wliat smaller proportion of the
"Touiuliiiass than does the hornblende, l)ut i.s more abundant than the
bronzite of the groixndmass. Sphene is pi-esent in considerable (quantity,
and likewise ilmenite (fig'. A, PI. XLV).
This rock stands between the hornblonde-gabbros and the norites. In
texture it might be compared with the norite-porphyrite (enstatite-porphyrite
of Ivosenbusch) with holocrystalline groundmass. But it differs essentially
from this rock in the presence of a large quantity of hornblende. This
hornblende connects it with tlie hornblende-gabbros, from which its content
of pyroxene, both orthorhombic and monoclinic, tends to separate it. Owing
to its obvious relation to the bronzite-norites, which, like it, occur as differ-
entiation facies of and dikes in the hornblende-gabbro, I shall call it a
"bronzite-norite-porphyry," using the term "porphyry" purely in a textural
sense.
The rocks may be compared, in their variation, to those described by
G. H. Williams from Maryland,' by Chester from Delaware," and by Fair-
banks from California.' A series of basic rocks similar in many respects
to those of Crystal Falls has also recently been described in two interest-
ing papers by Van Horn * and Schaefer.^
DYNAMICALLY ALTERED GABBRO.
Near the junction of sees. 26, 27, 33, 34, T. 43 N., R. 31 W., there is
a large gabbro mass which shows raai-ked evidence of dynamic action, and
may well be cited as an example of a metamorphosed gabbro, or, perhaps
more clearly, as a rock intermediate between a hornblende-gabbro and a
hornblende-gneiss. None of tlie gabbros thus far described show any evi-
dence of having taken part in very extensive orogeuic movements. The
minerals of but few of them show more than the common phenomenon of
slight wavy extinction. Hence it is clear that their intrusion took place
'The gabbros and associated hornblende rocks occurring near Baltimore, by G. H. Williams:
Bull. U. S. Geol. Survey No. 28, 1886 ; Oiitliue of geoIo;,ry of Maryland, Baltimore, 1893, p. 39.
-The gabbros and associated rotks in Delaware, by F. D. Chester: Bull. U. S. Geol. Survey
No. 59, 1890.
^The geology of Point Sal, by H. W. Fairbanks: Bull. Dcp. Univ., California, Vol. XI, 1896,
p. 56 et seq.
^Petrographische Uutersuchungen iiber die noritischeu Gesteine der Umgegend von Ivrea im
Oberitalieu, by F. R. van Horn: Tscliermaks mineral., Mittheil., Vol. XVII, 1897, Part V, pp. 391-420.
^ Der basische Gesteinzug von Ivrea im Geliiet desMastalloue-Thals, by K. W. Schaefer: Tscher-
maks mineral., Mittheil., Vol. XVII, 1898, Part VI, pp. 495-517.
248 THE CRYSTAL FALLS IRON-BEARING DISTRICT.
after the occun-ence of the inountaiu-building inovemeuts to which the
intruded rocks have been subjected. It is not beheved that the mass
referred to in this description is an exception to the general rule, but that
its metamorphosed condition is referable to local causes, such as have pro-
duced the shearing planes to which allusion has been made (p. 234). It may
perl laps occur near or along some minor fault plane, though no indications
of a fault have been observed.
The outcrop is composed in part of massive gabbro and in part of the
metamorphosed kind. The gabbro in its typical massive form (fig. A,
PI. XLIII) is a medium to coarse grained granular rock, composed essentially
of plagioclase and dirty brown-green compact liornblende, the latter being
quite full of the inclusions mentioned above as commonly occurring in the
hornblende of the rocks in this region. In the metamorphosed rock the
grain is much finer, the rock possesses imperfect schistosity, and the color
is a lighter green than in the original. The component minerals are chiefly
hornblende and feldspar, with some quartz, chlorite, epidote, calcite, and
rutile. The hornblende has a light-green color. It occurs mainly in aggre-
gates of small irregular grains. In some cases these surround an angular
nucleus of dirty brown-green original hornblende containing the same
inclusions and in every way similar to that of the coarse-grained uncrushed
rock. The light-green hornblende contains none of these minute inclusions
and onlv here and there grains of rutile, which, if the diagnosis of the
interpositions as titanium minerals is correct, may possibly be considered as
havino- been derived from them. This hornblende is believed to be sec-
ondarv. It is produced by a process of mechanical Ijreaking down of the
original hoi'nblende, accompanied by recrystallization. The process is
somewhat similar to the crushing of ordinary plagioclase and the produc-
tion of a more acidic variety. As may be seen from the photomicrograph
(fig. B, PI. XLIII) of a section, the feldspar exhibits signs of intense crushing.
The twinning lamellse are strongly bent, and the pieces possess wavy
extinction. Many, in fact most, of the feldspars are fractured. Wherever
these fractures occur the feldspar along the edges of the fractured portions
has been altered, producing secondary epidote, muscovite, plagioclase feld-
spar, and quartz; the last, however, in small quantity. The biotite has
been crushed. This has partly altered to chlorite, and the latter contains
many grains of epidote. Some granular calcite and considerable iron
GABBttO AND NOUITE INTRUSIVES. 249
pyritt's are also toiind in the altered rocks. These secondary products are
easily distinguished from the original minerals by the total absence m them
of wavy extinction. The effect of the crushing upon the texture has been
to render it more or less schistose, the resulting rock resembling in its
mineralogical character, and in texture, an ordinary honiblende-gneiss, or,
when (piartz is present in but small quantity, a plagioclase amphibolite.
RELATIVE AGES OF GABBROS.
An attempt to determine the relations of the varieties described resulted
in the establishment of the following order of intrusion: The typical
coarse-grained hornlilende-gabbro was the first formed, and seems to be the
most widely distributed. This was then intruded by dikes of gabbro, which
contain both monoclinic pyroxene and hornblende in association and rep-
resent a gradation to the normal gabbro. These hornblende-gabbros and
normal gabbros were then cut by the dikes of bronzite-norite and bronzite-
norite-porphyry.
PERIDOTITES.
Until recently all of the ultrabasic rocks were included by most
petrographers under the family name of peridotite. ■ In the last edition of his
Mikroskopische Physiographie, Rosenbusch has separated these rocks into
the plutonic rocks and the volcanic rocks. The term " peridotite " is
restricted to the first, and the last are called the "picrites." The charac-
teristic differences between these two types are very clearly shown in the
specimens from the Crystal Falls district which I have had the oppor-
tunity of studying. The rocks here described agree with Rosenbusch's
narrower definition of the peridotites.^
DISTRIBUTION, EXPOSURES, AND RELATIONS.
The peridotite dikes were all found near the Michigamme River, in
sees. 29 and 22, T. 42 N., R. 31 W. Typical wehrlite with very little green
amphibole is found on the east bank of the Michigamme river, near the
center of sec. 22, T. 42 N., R. 31 W. It shows no relations to the other
rocks. The amphibole-peridotite, exhibiting marked variations to wehrlite
and olivine-gabbro, forms an outcrop on the east bank of the Michigamme
River in the NE. ^ sec. 29, T. 42 N., R. 31 W. This dike cuts the gabbro.
' Op. cit., p. 343.
250 THE CRYSTAL FALLS lEON-BBAKIXG DISTRICT.
A rock to be described last, connecting the diorites, gabbros, and peridotites,
was taken from near the northeast corner of sec. 22, T. 42, B. 31, where it
is found cutting the gabbro.
PETROGRAPHICAL CHARACTERS.
The peridotites are very dark green to black coarse-grained rocks,
showing in almost all cases a granular texture. In one case an excellent
poikilitic texture was observed, and in another the same texture in a very
imperfect condition was seen. The almost total absence of any pressure
phenomena in these rocks excludes the idea of their having been subjected
to any powerful dynamic action. The chief mineral constituents, arranged
in order of relative amounts, are pyroxene (monoclinic and orthorhombic),
olivine, hornblende, and biotite. The following minerals also occur associ-
ated with these: Feldspar, apatite, green and brown spinel, and iron oxide.
Pyroxene — Thcrc is prcscut in these rocks both orthorhombic and mono-
clinic pyroxene. The orthorhombic has a 3'ellowish tinge, and contains a
few tabular inclusions. Only a few grains of this pyroxene were found,
but upon one section a figure was obtained and the positive character of the
mineral was determined. It appears to be bronzite. The bronzite grains
are found included in the hornblende. Its relations to other minerals are
not shown in any of the thin sections.
The monoclinic p\TOxene, though one of the earliest minerals to crys-
tallize, is likewise in xenomorphic individuals, many of them twinned, some
polvsynthetically. It is usually of a light yellowish color, and is then
nonpleochroic; in some sections, however, it is sufficiently colored to show
pleochroism from light yellowish to brownish. It includes a number of the
clove-brown tabular interpositions like those in the hornblende, and these
at times give the pieces a decided violet tinge. Less commonly than the
tabular interpositions one observes green needles and laths, more rarely
rounded grains or plates of larger size and having a brownish-green color.
They are so minute as to defy positive determination, but are presumed to
be hornblende microlites. The orthopinacoidal parting is in some cases
well developed in the augite. This diallagic augite is in large quantity,
and though usually inchided in the hornblende, nevertheless includes both
hornblende and biotite in a few ragged plates. Such sections are pi'obably
those which have passed tlu-ough tlie outer edges of the augite crystals.
PERIDOTITK INTRTTSIVES. 251
Olivine. — 'riiis is present most commonly in round iuilit'dra, and is usually
almost entirely altered to yellowish-green serpentine tihers. When the olivine
is completely altered, the mesh structure affords a ready means of" recogniz-
ing the original mineral, especially when taken in conjunction with grains
of a rich-green isotropic mineral, and also octahedra and grains brown in
color, probably spinels, which are found included in the ])seudomorphs.
Hornblende. — Tlic luost of tlic hoHiblende in the peridotites is a brown
variety showing strong pleochi'oism. a is light cream-yellow, c is yellowish
brown, and h is reddish brown; tJ>>C>H. Patton ' has already called
attention to the pleochroism of the hornblende, which "is exceptional,
inasmuch as the brownest color is that of rays of light vibrating parallel to
the orthodiagonal axis." The brown hornblende is accompanied by a very
small quantity of green hornblende. Moreover, the brown hornblende
grades over into a light green, the two being in perfect crystalline conti-
nuity. In rare cases this brown hornblende is also intergrown with a light-
green pyroxene in such way as to give a mottled polarization effect. The
pinacoidal cleavage of the hornblende continues through the pyroxene, and
the extinction angle of the hornblende against this cleavage is 19 degrees,
while that of tlie pyroxene runs up to 34 degrees. The hornblende includes
numerous anhedra of pyroxene, somewhat fewer grains of olivine, and, less
commonly, ragged pieces of biotite, giving it a poikilitic character. It is
also very full of opaque metallic or brownish translucent plates of ilmenite.
In some individuals minute clear microlites, similar to those described
above in the hornblende of certain gabbros, are noticed in small quantity.
These are irregularly distributed in the hornblende, giving the crystals a
patchy appearance. In respect to its color and these inclusions, this horn-
blende, as noted by Patton, bears a rather striking resemblance to hyper-
sthene on superficial examination.^ Iron ore in large masses is fairly
frequent as an inclusion, and it is very commonly noticeable that where
such inclusions occur the zone of hornblende immediately surrounding
them is free from the platy inclusions mentioned above. Such a clear zone
is also observed at times surrounding the inclusions of biotite and olivine,
but never in case of pyroxene. Whei-e these clear zones surround the
' Microscopic study of some Michigan rocks, by H. V. Patton: Rept. State Board of Geol. Surv.
for 1891-92, 1893, p. 186.
' Loc. cit., p. 186.
252 THE CliYSTAL FALLS IRON-BEARING DISTRICT.
included biotite or olivine, these two minerals have associated with them
numerous small grains of ore, which probably represent the iron that
would have been incorporated in the sun-ounding hornblende but for some
selective influence exerted by the olivine and biotite.
Biotite. — This mineral is present in flakes of very irregular outline. The
pleochroism varies from cream color to yellowish red or brown. Although
one of the last minerals to crystallize, its crystallization began before that of
the pyroxene or hornblende had entirely finished. Hence we find flakes of it
included in these minerals, but near the edges of the crystals. The biotite
itself is almost free from inclusions, containing only a little hematite and mag-
netite. It alters to a brilliant green, strongly pleochroic, chloritic mineral.
Feldspar. — Tliis is prGscut lu specimcus from two outcrops, and in these
hardly reaches the rank of an essential constituent. It was the last mineral
to crystallize, and is consequently in anhedra, forming the mesostasis. All
of the feldspar sections were tested, but no determinative measurements
could be made. It is probably very basic.
Apatite. — Apatite is present in small quantity. It exhibits its usual
characters.
Spinel. — There is a spinel found in round grains which are included in
the olivine (serpentine). It is g-reen in color, and is possibly pleonaste. A
second spinel, probably picotite, occurs in small brown grains and octahedra
in the olivine.
caicite. — This mineral, derived partly, if not wholly, from the altering
minerals, is found in lenses between the biotite lamellse and in minute veins
which traverse the slide.
Iron ores. — Irou Ore IS rcprcseuted by hematite, magnetite, and pj^rite.
The hematite is in blood-red transparent flakes inclosed in the biotite.
Magnetite is included by all of the chief minerals, and is in irregular masses
without good crystal development. The iron pyrite is found in good crys-
tals, though not in large quantity, and is scattered here and there through
the slides.
PERIDOTITE VARIETIES.
The relative proportions of the minerals described above diff"er very
much, and we have different kinds of rocks corresponding to these min-
eralogical variations. These kinds are not sharply sepai'ated, but are seen
under the microscope to grade into one another.
rEKlDOTlTE INTKUSIVES. 253
Tlie purest form of peridotite is welirlito, which is composed essentially
of olivine and augite. When, besides these minerals, hornblende is present
in large quantities, the rocks Ixdong to the ampliil)ole-peridotite type. In
some specimens biotite is almost in sufficient abundance to warrant the
naniini;- of them biotite-peridotite. Ag-ain, in other specimens feldspar is
present in considerable quantity and the rock approaches an olivine-gabbro
or oliviue-hornblende-gabbro.
WEHRLITE.
This is represented by a coarse-grained rock which is mottled and has
a dark-green color. (Specimen 23763, from sec. 22, T. 42 N., R. 31 W.,
N. 1,500, W. 900 paces.) Under the microscope the mottling is seen to
be due to the association of very dark greenish-black serpentine pseudo-
morphs after olivine with a light-colored augite.
Olivine and augite were present in about equal quantities. They are
in anhedra, and therefore must have crystallized at about the same time.
The olivine is, with very few exceptions, completely altered to serpentine.
Augite has a very poor development. Between the olivine and augite are
small quantities of irregular plates of biotite. A few small irregular
pieces of a very light colored greenish hornblende were observed. Thev
are intergrown with the pyroxene and give it an imperfect poikilitic texture.
This wehrlite is unquestionably the same as specimen 1247 of the
Geological Survey of Wisconsin, described by Dr. A. Wichmann as serpen-
tine, consisting chiefly^ of serpentine with some unaltered olivine and
augite.
AMPHIBOLE-PERIDOTITE.
This variety of peridotite was obtained from the outcrop N. 1,260, W.
200 paces from the southeast corner of sec. 29, T. 42 N., R. 31 W., on the
east bank of the Michigamme River. The rock is very coarse grained,
and possesses poikilitic texture. It is composed of hornblende, pyroxene,
olivine, biotite, and iron oxide. The hornblende equals in quantity all of
the other constituents. Some of the hornblende individuals measure 3 cm.
in length, and include all of the other constituents except the biotite. The
pyroxene and olivine seem to have crystallized at about the same time, as
'Microscopical observations of the iron bearing (Huronian) rocks from tbe region south of Lake
Superior, by Dr. Arthur Wichmann, Leipzig, 1876: Gool. of Wisconsin, Vol. Ill, 1880, p. 619.
254 THE CRYSTAL FALLS IRON-BEAKmG DISTRICT.
they never include each other. They are both, however, included in the
hornblende, which with the biotite forms, as it were, the mesostasis. Biotite
is present in this specimen in very small quantity, and is essentially the
same kind as that above described (p. 252), except that it shows a trifle
higher absorption parallel to the cleavage and becomes a yellowish-red.
The rock is very fresh and shows scarcely any traces of alteration. This is
partly due to the erosive action of the Michigamme River having removed
the weathered crust, thus making fresh specimens obtainable.
This rock, from the description just given, would be classified as an
amphibole-peridotite, with accessory diallage, bronzite, and biotite. It
approaches Williams's cortlandtite. In some specimens the biotite is pres-
ent in very large quantity, though hardly in sufficient quantity to waiTaut
the designation of any of the rocks as biotite-peridotite.
GRADATIONS OF AMPHIBOLE-PERIDOTITE TO WEHRLITE AND OLIVINE-GABBEO.
There were taken from the same exposure whence the above-described
amphibole-peridotite came some specimens which macroscopically can not
be distinguished from those of the amphibole-peridotite except in that they
are a trifle finer grained. Examined under the microscope, however, we
find difl^erences. In some the hornblende is very much reduced in quantity,
and varies from the brown kind just described to a light-greenish color, the
two being in optical continuity, and the augite and olivine are increased in
quantity. These are good types of a wehrlite. In some of the wehrlites
there is a variable percentage of feldspar. In certain cases it reaches an
amount which would almost warrant the classing of the rock as an olivine -
gabbro. Patton described a rock from the same outcrop in which the horn-
blende still predominated, but in which there was also a certain amount of
plagioclase.^ He called it a hornblende-picrite.^ According to the ter-
minology here used, if the plagioclase is to be neglected, it would be au
amphibole-peridotite.
The thin sections of the feldspathic phase of this rock seem to show
that it approaches more closely to a gabbro — that is, to be more feld-
spathic than the one described by Patton. They certainly contain far less
hornblende than his, judging from his description, and more feldspar. The
' Mikroscopisctie Physiographie, by H. Roseiibiisch ; 3a ed., Stuttgart, Vol. II, 1896, p. 352.
-Op. tit., p. 186.
PERIDOTITE INTKUSIVES. 255
constituents ai'c tlir same in tlie two rocks, and with some few modifications
Ins description would answer.
Aiigite is the chief constituent, and following it, in order of im])ortance,
come olivine, hornblende, biotite, and feldspar. The diallagic augite is more
automorphic (see fig. B, PI. XLV) as the feldspar increases in quantity.
It is the only one of the minerals which shows any marked degree of
autt)morphisni. The augite present in the sections which I have studied has
a light-brownish color, differing from that described by Patton, which is
green to colorless. The augite contains the inclusions occurring in hyper-
sthene, as well as the green (hornblende?) ones already described. It is
invariably surrounded by a narrow rim of light-brown hornblende, and
includes in places on the edges irregular patches of the same brownish
hornblende.
J. Romberg describes the augite in Argentinian gabbros,^ both with
and without olivine, as being almost always surrounded by a rim of green
hornblende. In one case, however — that of the olivine gabbro from the
island of Martin Gai'cia, in the La Plata River^ — both brown and green
hornblende is present around the augite. The brown hornblende forms
part of the periphery of a crystal; the green the remaining portion. Of
the green hornblende some is fibrous, and is considered by Romberg to be
certainly secondary.
The olivine possesses its usual properties. It is in annedra, with the
exception of three or four individuals, which show a fair degree of auto-
morphism. The olivine includes rounded grains of a brown spinel, and is
traversed by anastomosing veins of the iron oxide. It shows the usual alter-
ation to serpentine, and the iron oxide is the result of this serpentinization.
The olivine is of exceptional interest on account of the fact that it is sur-
rounded by certain zones Avhere it is close to the feldspar (figs. A and B,
PI. XLVI). The characters of zones observed in sections from this same
locality, and which are almost, if not quite, identical with these which I
shall proceed to describe, have already been described by Patton.' There
are two of these zones. An inner one is composed of a mineral which is
probably an orthorhombic pyroxene. It was so determined by Patton in
' Untersuchungen an Diorit-Gabliro-und Amphibolitgesteiuen aus dem Gebiete der Argentini-
Bcaen Republik, by J. Romberg: Neues Jabrbuch fiir Miueral., BB. IX, 1894, pp. 320-321.
-Op. cit., p. 322. »0p. cit., p. 168.
256 THE CEYSTAL FALLS IRON-BEARING DISTRICT.
the specimens collected and studied by him. I can obtain no positive proof
for or against this statement. If it is an orthorhombic pyroxene, it agrees
with the inner zones of related occurrences wliich have been described by
Tomebohm, G. H. Williams, Adams,^ Romberg,^ and others. This zone is
at any rate composed of a colorless, compact mineral, with high single and
moderately high double refraction. Its single refraction is nearly equal
to that of olivine. The mineral, as a rule, extinguishes parallel to the lines
of cleavage. In a few instances the line of extinction made a scarcely
noticeable angle with the cleavage. It is separated from the olivine by a
shai'p line. At times this inner zone seems to disappear, and at others
becomes considerably broader than the average. The width is usually
about 0.02 mm., thoug-h it becomes at times 0.08 mm.
Outside of this pyroxene zone there is a very much broader zone of
light-green hornblende. This is compact, and is in optical continuity with
the ordinary brown hornblende, which is the dominant hornblende in the
rock. This, in its compact nature and in its relation to the compact brown
hornblende of the rest of the slide, differs from the short fibrous actinolite
zone ordinarily described as taking part in such " reaction rims." This
hornblende zone reaches an extreme width of 0.15 mm. The outer edge of
this zone is penetrated by tubular ramifying growths of a colorless mineral,
which usually extend inward, perpendicular to the periphery, and which
appear to be continuous with the feldspar. This portion of the hornblende
rim is about 0.05 mm. wide No such intergrowth of feldspar with the
brown hornblende was found, nor have I been able to find elsewhere any
description of such an outside zone.^ However, Romberg describes the
iutei'esting occurrence in an olivine-gabbro from the Argentine Republic
of zones around the hornblende which are very much like those above
described, except that the pseudopodia-like growths, as he describes them,
consist of a dark-green spinel instead of a clear white feldspar, as in the
Michigan rock.
In some cases, where the olivine and augite are in juxtaposition, the
inner orthorhombic pyroxene zone completely surrounds the olivine. The
outer hornblende zone, however, surrounds both the augite and the olivine
' Uber das Norian oder Ober-Laurenti<an vou Canada, by F. D. Adams: Neues, Jahvbuch fiir
Mineral, BB. VIII, 1893, p. 466, where refereuces to observations made previous to 1893 may be found.
- Op. cit., p. 322. ^ Op. cit., p. 323.
PEKIDOTIl H INTRUSIVES. 257
witli its ortliorlioinbic pyroxene zone. Where it. is in contact with tlie
an<j-ite it is the brown variety of hornblende, but is in optical continuity
with the green, which is the kind around the olivine and the orthorhonibic
pyroxene.
Of the remaining mineral constituents brown hornblende is the next
one in importance. It has in it patches of inclusions, previously described
as occurrino- in the hornblende of these ultrabasic rocks. It includes also
the augite and olivine. This brown hornblende is comparativel}- rarely
found in large plates, but usually as a rim of varying width around the
augite and olivine, as already described. Where it occurs in large plates it
is in that part of the section which is free from feldspar, and more closely
resembles the araphibole-peridotite phase.
The biotite has a cream to light yellowish-brown color, and occurs in
in-egular plates. The plagioclase feldspar is in irregular broad plates, and
forms the mesostasis. The feldspar contains, in uot verj- large quantity,
small microlites, which by very high power are translucent and show a
greenish tinge. They are supposed to be hornblende microlites.
PROCESS OF CRYSTALLIZATION.
From the relations described as existing between the various minerals
it seems that the following stages may be outlined in the progress of the
consolidation of this rock. From the coarse even-grained character, and
from the fact that neither a fine-grained groundmass nor glass is present,
the conclusion seems to be warranted that it crystallized under high pressure
and must have, of course, at some time been under very high temperature
also.
The augite and olivine were the first and chief silicate constituents to
form, and crystallized out of the magma at apjjroximately the same time.
The magma soon reached a condition unfavorable for further production of
olivine, probably on account of increasing acidity. Immediately around
the olivine there was formed, at this stage for a short while, the orthorhombic
pyroxene. The monoclinic p} roxene continued to grow during the forma-
tion of this orthorhombic variety. Finally, however, the condition was
reached when, in place of the monoclinic and orthorhombic pyi'oxenes, the
crystallization of hornblende began.
It is not known what the conditions were which caused the formation
MON xxxvi 17
258 THE CRYSTAL FALLS IRON-BEARING DISTRICT.
of the hornblende subsequent to and in such intimate association with the
pyroxene which it surrounds in zonal growth. An explanation of such
occurrences has been attempted by Becke in a recent ai-ticle, ^ in which the
conclusion is reached that the formation of the hornblende and pyroxene
depends upon changes in temperature and pressure. His explanation is
based upon the facts of occurrence of pyroxene and hornblende in plutonic
and effusive rocks, and also upon the well-known fact that under high
temperature and atmospheric pressure they can not exist, but when fused
recrystallize as pyroxene; and in addition to this, upon the experimeiits of
Von Chrustschoflf,^ who has obtained hornblende at a temperature of 550 C.
with the presence of water, under which conditions a high pressure must be
developed. However, attention should be called to the fact that his explana-
tion does not take into account other important factors which certainly
influence the crystallization of minerals — for example, the chemical composi-
tion of the magma and the fusing point and specific gravity of the minerals.
Whatever the factors are which determine its crystallization, the fact is
that hornblende began to crystfillize from this peridotite magma in the
place of pyroxene.
The biotite appears to have been formed at the same time with the
hornblende. The production of these two minerals, hornblende and biotite,
then continued until the remaining magma had reached the composition of
basic feldspar, which then crystallized and now forms the mesostasis.
A zone of orthorhombic pyroxene, succeeded by one of hornblende,
has been described as surrounding the olivine in this peridotite. The term
reaction rim has been applied to similar zones by various observers, but it
seems to me that this term is inapplicable to such zones. It is not probable
in such a case as this that there is a reaction between the magma and the
olivine. Moreover, the zones should not be compared to the resorption rims
found so commonly in certain effusive rocks, where from the fusion of the
hornblende crystals pyroxene has been produced.
Such a zonal growth aroimd the olivine seems to me comparable to
such a case as that described by Washington,^ where colorless diopside
' Gesteine der Colunibretes ; Anhang : Einiges iiber die Beziehung von Pyrosen und Amphibol
in Gesteinen, by F. Becke: Tschermaks mineral. Mittheil., Vol. XVI, 1896, pp. 327-336.
= Bull. Acad. imp. sci. St.-P^tersbourg, 1890, p. 13. Cf. Becke, Op. cit., p. 337.
'Italian petrological sketches; 4. The Eocca Monfina region, by H. S. Washington : Jour.Geol.,
Vol. V, 1897, p. 254.
PElilDOTlTE INTEUSIVES.
259
pluniocrysts are sun-ouiulcd 1)a' a narrow border of A-ellowish-o-reen auo-ite
wliicli corresjionds to tlie small au<>-ites in the g-roundmass, or to those cases
which are so roninion in plutonic rocks — even in this rock described — where
hornblende is found surrounding- the pyroxene.
A general explanation which would account for tlic successive crys-
tallization of hornblende and pyroxene in this rock should be applicable to
such a zonal g-rowth as occurs around tlie olivine, taking- into consideration,
of course, the jirobability that a factor of slight importance in the one case
may be the controlling factor in the other. Such occurrences seem clearly
to indicate a change in the chemical composition of the mag-ma as the chief
factor in the crystallization of the diflPerent minerals, in the pressure, in the
temperature, and also in other factors, either one alone or more of these
combined.
ANALYSI.S Ol' I'ERIDOTITE.
The peridotite just described was analyzed by Dr. H. N. Stokes of the
United States Geological Survey, and his results are here given (No. 1) :
Analysis of peridotite.
SiO;..
TiO, .
A1,0:,.
Cr:0,.
FejO,
FeC.
MnO .
NiC.
CaO
MgO
KcO
NajO...
HjO at 110^ . . .
HjO above HO'-^
PiOs
COi
Total
1 (23353).
44.99
.97
.5.91
.25
3.42
8.30
Trace.
8.79
21.02
.74
.91
.63
3.19
.05
Trace (f).
2 (22981).
37.36
.79
4.76
.62
6.61
6.12
Trace.
.04
1.19
31.11
Trace.
.65
10.37
.06
None.
99.17
99.68
260 THE CEYSTAL FALLS lEON-BEAEING DISTEICT.
It will be seen from the analvsis that the silica is somewhat too hig-h
for the typical peridotites. This same fact is also emphasized by the
teiidenc}' manifested in some facies of the peridotite for feldspar to develop,
and thus for transitions to norite and g-abbro to be produced.
With this analysis of the peridotite there is placed for comparison the
analysis (No. 2) by Dr. H. N. Stokes of the picrite-porphyry already
descril^ed. The close resemblance chemically liecomes at once manifest,
althouiih the latter is more nearly a typical peridotite in composition. It
can not be denied that possibly this picrite is but a further differentiation
product of the same magma to which the 2:)eridotites belong, although its
occurrence is so remote from these that it is impossible to connect them in
the field.
PERIDOTITE PROM SBC. 22, T. 42 N., E. 31 W., N. 1,990, W. 150.
Just west of the northeastern corner of sec. 22, T. 42 N., R. 31 W.,
there is a bold outcrop of hornblende gabbro, which is cut by a dike, about
10 feet wide, of a very massive, coarse, granular black peridotite. Macro-
scopically one can readily distinguish in the peridotite flakes of Ijiotite,
poikilitic plates of hornblende, and a smaller amount of white feldspar.
Under the microscope the constituents are, in order of imjjortance: Horn-
blende, augite, feldspar, biotite, bronzite, olivine, magnetite, and quartz.-'
Hornblende. — Tlils Is thc rlch browu kind, full of inclusions, g-rading into
the green variety which was described on p. 234 as occurring in the gabbros
of this district. It is jiresent in anhedra inclosing liiotite, pyroxene, and
olivine.
Pyroxene. — Tliis is represented by mouoclinic and orthorhombic varieties.
The mouoclinic pyroxene, augite, is most abundant, and is in light-yellow
to pink-colored anhedra, except where it touches the feldspar; there the
augite is automorphic, and is surrounded by a narrow border of light-
brown hornblende.
The orthorhombic pyroxene is present in a few anhedra, which are
colorless or have a faint cream tint. It is presvimed to be bronzite.
Feldspar. — This fills tlic intcrspaccs between the other constituents, and
occurs in grains which are polysyntheticalh' twinned after the albite law.
' Only one section has been prepared from this specimen, and it may not give a correct idea of
the true proportion of these minerals in thr rock mass. In tlie macroscopical examination of the
hand .specimen the biotite seemed to he subordinate only to the hornblende.
PERIDOTITK INTKUSIVES. 261
Mfiisurements ji-avo a, syninietrical t'xtiiiction of 32 eaeli side of the twiniiiii"-
plane on zoiie_L01(». I tlierefo.- ; eonclude tlie feldspar to be labradorite.
Biotite. — This is the ordinary yellow to brownish kind, and is in irre"'n-
lar plates. It shows its usnal characters and is inclnded in the hornblende.
Magnetite. — This mineral occnrs in (;rystals and gTains, inclnded in all
the other constitnents.
Quartz. — A few grains of (jnartz were fonnd associated with the feldspar.
The presence of dihexahedral liquid inclusions easily gave a clew to the
orientation of the grains.
The rock composed of the above-described nainerals offers a good illus-
tration of that gradation which is one of the fundamental laws of nature
and is nowhere better exemplified than in the rocks. On the one hand,
from its texture and from the presence of the dominant hornblende, with the
small quantity of quartz, this rock may perhaps be considered to be closeh'
related to the diorites. On the other hand, the presence of the pyroxene
and olivine seems to point toward its connection with a gabbro.
Its geological occurrence points most satisfactorily toward its corre-
spondence in age and its intimate relationship to the peridotites of the dis-
trict. The predominance of the bisilicates indicates it to be of very basic
character, and for these reasons I have called it " peridotite," althouo-h I
have not succeeded in getting an analysis to prove its ultrabasic nature.
RELATIONS OF PERIDOTITES TO OTHER ROCKS.
The peridotites occur in such small quantity that general conclusions
concerning their relations to other rocks occurring in their vicinity are
scarcely warranted." However, from the fact that they are so intimately
associated with the gabbro — cutting it in two cases where the contact ^\'as
observed — and from the fact that among the peridotites themselves certain
phases approach in mineralogical composition certain of the gabbros (see
p. 254), it seems advisable to conclude that they represent ultrabasic differ-
entiation products of the same magma from which the gabbro types were
derived. The inappreciable differences in grain between the portion of the
i-ock nearest the contact between these basic rocks and the gabbros and
those farther away can be explained by supposing their intrusion to have
taken place while the main mass of the gabbro retained considerable heat
and thus prevented their rapid cooling.
262 THE CRYSTAL PALLS lEON-BEARING DISTRICT.
AGE OF PERIDOTITES.
The only statement which can be made concerning the age of the peri-
dotite dikes is that they are younger than some of the gabbros, and that,
not having suffered the defonnation of the pre-Keweeuawan orogenic move-
ments, they are Keweenawan or post-Keweenawan.
GEKERAL OBSERVATIOXS ON THE ABOVE SERIES.
TEXTURAL CHARACTERS OF THE SERIES.
There are represented in the above series rocks with moderately fine
grain as well as those of very coarse grain. They vary from those with
parallel texture, through those vith porphyritic, poikilitic, and ophitic texture,
to those with granular texture. There is, howevei", tlu-oughout a clear
preponderance of the medium to coarse granular rocks. The rocks are
evidently not of effusive character, though some jDOSsess the textures prev-
alent in effusive rocks.
The order of crystallization of the minerals in the rocks of granular
.exture, excluding the iron ores and the accessory minerals, is as follows.
Tlie order in the imperfectly ophitic and porphyritic rocks is not considered,
as those are rather exceptional occurrences.
In the lists those minerals are hyphenated of which it has not been
possible to determine accurately the order of crystallization. It seems that
either they were formed at the same time or, in some cases, their formation
has overlapped. In such cases the one placed first is the one presumed to
have begun its crystallization first.
DioRiTE. Gabbroand horn- Bronzitb-noritb, Pkridotite.
BLKNDE-GABHRO.
Hornblende. Olivine. Bronzite. Olivine.
Biotite. Monoclinic pjToxene. Monoclinic pyroxene. Orthorhombic pyroxene.
Plagioclase. Biotite-hornblende. Blotite-bornblende. Monoclinic pyroxene.
Microcline. Plagioclase. Plagioclase. Biotite-hornblende.
Orthoclase-nuartz. Plagioclase.
For the entire series the order may be arranged as follows: Olivine,
bronzite, monoclinic pyroxene, mica-hornblende, plagioclase, orthoclase,
quartz. This is the same order that is exhibited by the most basic rock
represented in the series, the peridotite, so far as this rock contains the
minerals.
SERIES OF INTKUSIVES.
263
The order of" crystallization of the minerals throughout the series is due
to their relative solubility in the eruptive raagnui. Among- various factors
affecting solubility the fusion point of the chemical compounds constituting
the different minerals, the temperature of the magmas, and the pressure
under which the minerals crystallized, are important. The porphyritic and
the ophitic textured rock facies, having crystallized under different condi-
tions of pressure and of temperature from those under which the granular
rocks were formed, show, as is to be expected, a different order of crystal-
lization of minerals.
CHEMICAL COMPOSITION OF THE SERIES.
In the following tables there are reproduced the analyses which have
been obtained of the various types. They are arranged according to dimin-
ishing acidity. Nos. 1 and 4 were analyzed by Dr. H. N. Stokes, Nos. 2
and 3 by Mr. George Steiger, both of the United Slates Geological Survey :
Table I. — Analyses of Crystal Falls rocks.
SiOj
TiOj
Al,03
CrjOa
Fe^Os
FeO
MnO
NiO
CaO
MgO
K2O
NaaO
HjOatllOo ..
HjO above 110'
P2O5
CO,
Total...
1 (26023).
58.51
.72
16. .32
None.
2.11
4.43
Trace.
None.
3.92
3.73
4.08
3.11
.23
2.00
.30
None.
2 (23354).
3 (23755).
49.80
.79
19.96
6.32
.49
99.46
11.33
7.05
.61
2.22
100°— . 13
1000+1. 71
.07
.15
100. 63
48.23
1.00
18.26
1.26
6.10
9.39
10.84
.73
1.34
100°— .26
100° +2. 00
.07
.43
99.91
4 (23353).
44.99
.97
5.91
.25
3.42
8.30
Trace.
None.
8.79
21.02
.74
.91
110° . 63
110°-j-3. 19
.05
Trace. ( ?)
99.17
(1) Mica-diorite (quartzitic); (2) Hornblende-gabbro ; (3) Norite; (4) Peridotite (wehrlite).
264
THE CRYSTAL FALLS IRON-BEARING DISTRICT.
Table II. — I'ercentaijes of chief oxides reduced to 100.
1.
2.
3.
4.
SiO.
60.36
.75
16.83
2.17
4.57
4.04
3.85
4.21
3.21
50.52
.80
20. 25
6.41
.50
11.50
7.15
.62
2.25
49.64
1.03
18.79
1.30
6.28
9.67
11.16
.75
1.38
47.33
1.02
6.22
3.60
8.73
9.25
22.11
.78
.96
TiO-
Al,Oi
FeO,
FeO
CaO
UsO
K,0
Na.O
Table III. — Atomic 2}''oportions of metals.
Si..
Ti .
AI .
Fe.
Cii.
Mg.
K..
Na-
55.85
46.53
.53
.56
18.41
22.03
5.08
.834
4.04
11.42
5.32
9.85
4.99
.74
5.78
4.04
45.27
.71
29.26
5.70
9.51
15.22
.88
2.45
42.48
.70
6.60
9.02
8.98
29.67
.91
1.67
The analyses show that all of the rocks contain a moderately large
amount of water. Nevertheless,' they are sufficiently well preserved to
warrant a discussion of their analyses for classification purposes. This is
especially true of No. 4, which is remarkably fresh for so basic a rock.
The chief rock-makiiio- oxides in the above analyses appear in Table
II reduced to 100. The molecular proportion of these oxides was then
obtained. From these data the atomic proportions of the metals were
derived, and are given in Table III. These calculations were kindly made
for me by Mr. V. H. Bassett, assistant in the chemical laboratory of the
University of Wisconsin.
If we examine Table II we see that, in passing from the more acid to
the basic end of the series, in correspondence with tliis decrease in silica
the alumina increases rapidly, then decreases until it reaches the extreme
basic rock, when it cbops suddenly to 6.22 per cent. The analyses also
show an increase in iron, which is best brought out in Table III. The
alkalies decrease with diminishing silica, whereas the MgO, which for
rocks of this character is very characteristic, shows a decided increase.
Within the gabbro-norite-peridotite series (Nos. 2, 3, and 4) the lime shows
SERIES OF INTKUSIVES. 265
a constant diniinution f()iTes))()ndin<i' to tlie incTeasinj^' nia<inesi;ui cliaracter
of tla^ rocks. The potash increases as the soda sliows a decrease.
The rocks represented l)y tlie analyses are believed to belong to a
series ranging- troiu a diorite on the one liand, through horublende-gabbro
and norite, to peridotite on the other. It should be borne in mind that the
diorite is somewhat excei)tioual, representing a gradation toward the ortho-
clase rocks. On the acid side of the series the microscope also shows
variations to tonalitic and even granitic rocks very rich in quartz and ortho-
clase, consequently much more acid iu character than the diorite repre-
sented in the analysis.
It is a difficult matter to estimate quantitatively the amount of the one
or tlie other kind of rock present in the Crystal Falls district. We are thus
prevented from drawing from the predominance of the one kind or the
other the conclusion that those represented in the minority are the results
of the differentiation of a magma most nearly resembling in its original
constitution that which predominates. Moreover, since the analyzed rock
types were not selected as representatives of the extremes of the process of
differentiation, it would not be wise to endeavor to give the mean composi-
tion of the parent magma from the analyses of the differentiation products
Avhich have been presented. The main thesis, however, is estaljlished that
the separation of a magma into the various products described has taken
place, as is indicated by the relations in the field, and as has been shown by
the microscopical and chemical analyses.
RELATIVE AGES OF ROCKS OF THE SERIES.
Study of the relative periods of eruption of the various rocks results in the
determination of the hornblende-gabbro as the rock which first reached its
present position. It was followed in the acid part of the series by the diorite,
which in one place cuts it. The diorite is cut by the diorite-porphyry.
Along the basic series the order has been determined as hornblende-
gabbro, gabbro, bronzite-norite, peridotite.
In general the forces of differentiation seem to liave been active in two
directions, tending toward increasing acidity and increasing basicity of the
products of differentiation, thus agreeing with the law of succession of
igneous rocks as propounded by Iddings.^
■ The ongiu of igueous rocks, by J. P. Iddiugs: Bull. Philos. .Soc. Wash., Vol. XII, 1892, p. 195.
PLATE XIX.
267
PLATE XIX.
Fig. .i.
(Sp. No. 32756. Without analyzer, x90.)
Photomicrograph of fractured quartz phenocryst from a rhyolite-porphyry. It includes num-
berless liquid inclusions, which diminish in quantity as the distance from the plane of fracture is
increased, thus indicating their close connection with the fracturing of the cfuartz. The fracture in
the quartz phenocryst continued into the grouudmass, as may be seen on the left-hand side of the
figure. It has been healed with secondary quartz. (Described, p. 82.)
Fig. B.
(Sp. No. 32914. With analyzer, x47.)
Photomicrograph of a section of rhyolite-porphyry, designed to show the ihombohedral part-
ing, which is very common in many of the quartz phenocrysts. (Described, p. 82.)
268
U. S. GEOLOGICAL SURVEY
MONOGRAPH XXXVI PL. XIX
(A-) INCLUSIONS IN A FRACTURED QUARTZ PHENOCRYST.
(B-) RHOMBOHEDRAL PARTING IN A QUARTZ PHENOCRYST.
The MEftlDEM GRAVURE CO.
PLATE XX.
269
PLATE XX.
Fig. a.
(Sp. No. 32119. With aualyzer, x 90.)
Micropoikilitic rhyolite-porpbyiy, showing the peculiar texture of the zones which invariably
surround the quartz phenocrysts in sections in which the texture occurs. The same texture prevails
in the groundmass. The irregular white areas which are continuous with the quartz phenocrysts and
are connected with each other represent quartz. Disconnected dark and light areas between the
quartz stringers are feldspar grains. These do not possess uniform orientation; hence the texture is
not micropegmatitic. (Described, p. 84.)
Fig. R.
(Sp. No. 32137. With aualyzer, x 90.)
Photomicrograph of micropoikilitic rhyolite-porphyry. In this rhyolite-porphyry the micropo-
ikilitic texture is much finer than that represented in Fig. J, and the quartz in the zones shows a
tendency toward spherulitic development. Owing to the extreme fineness of grain it is difficult to
distinguish the (juartz and feldspar in many cases. The greater part of the light areas shown in the
photomicrograph are quartz. The dark areas between the quartz, and also some of the lighter areas,
represent irregular pieces of feldspar. (Described, p. 84.)
270
U. S. GEOLOGICAL SURVEY
MONOGRAPH XXXVI PL. XX
MICROPOIKILITIC RHYOLITE-PORPHYRY.
THE MERIDEN GRAVURE CO.
PLATE XXI.
271
PLATE XXI.
Fig. a.
(Sp. No. 32136. Without analyzer, x 90.)
Rhyolito-poriihyry with aureoled phenocrysts. The finest-grained type of micropoikilitic
testiuf is here represented. Tbe grouudmass of this porphyry consists of rounded areas of material
("quartz ^pongeuse"), corresponding to that forming the zones around the phenocrysts. Between
these areas there may be found in places small feldspars. These photomicrographs, represented in
figs. .-( and B, PI. XX, and in this figure, show every gradation iu the micropoikilitic texture, from
that which is with difficulty distinguishable as such to the coarser-graiued unmistakable variety.
(Described, p. 85.)
Fig. B.
(Sp. No. 32136. With analyzer, x 90.)
Rhyolite-porphyry with aureoled phenocrysts. This is the same section as is represented above
■when viewed Itetweeu crossed nichols. The texture of the groundmass is brought out somewhat
better. The feldspars especially become more noticeable. For instance, one Carlsbad twin may be
seen at the lower right-hand corner of the phenocryst partly indenting the aureole. Other feldspars
may be noticed through the groundmass. In other portions of the section from which this photo-
micrograph is taken the (luartz phenocrysts have no aureoles and the groundmass possesses an imper-
fect microgranitic texture. This figure brings out clearly the gradation toward that texture.
■(Described, p. 85.)
272
U.S. GEOLOGICAL SURVEY
MONOGRAPH XXXVI PL. XXI
{B)
MICROPOIKILITIC RHYOLITE-PORPHYRY.
■ME HERIDEN GHAVURE CO.
PLATE XXII
MON XXXVI 18 273
PLATE XXII.
Fig. a.
(Sp. No. 32732. Without analyzer, x 18.)
Aporbyolite showing beautifully developed peilitic parting. The perlitic cracks are brought
out clearly by the chlorite which has accumulated in them. (Described, p. 87.)
Fig. B.
(.Sp. No. 32732. With analyzer, x 18.)
Aporhyolite showing perlitic parting, when viewed between crossed nicols. The groundmaes
lesolves itself into a fine grained mosaic of quartz and feldspar, showing microgranitic characters.
The perlitic parting is thereby almost completely obscured. (Described, p. 87.)
274
U. S. GEOLOGICAL SURVEY
MONOGRAPH XXXVI PL. XXII
i^'srv-
{^)
(S)
(.A) PERLITIC PARTING IN APORHYOLITE.
(B-) PERLITIC PARTING IN APORHYOLITE BETWEEN CROSSED NICOLS.
The MERinEN GRAVURE CO.
PLATE XXIII.
275
PLATE XXIII.
Fi(j. A.
(Sp. No. 22953. Without analyzer, x 38.)
Rhyolite-porphyry rcntleied schistose liy crushing. Granulation of the feldspars ami the result-
ing production of schistose aggregate's of secondary quartz, feldspar, and sericite is here shown.
The two large areas shown near the center of the figure were formerly occupied entirely by fcUlspar.
The greater portion of this has now become altered, mere remnants of the original remaining. This
secondary aggregate has especially well-developed parallelism. (Described, p. 9.S.)
Pig. B.
(Sp. No. 327-26. With analyzer, s 18.)
Photomicrograph of aporbyolite-porphyry breccia showing the fractured character of the
quartz and feldspar. Certain portions of the section show the perlitic parting, with accumulations
in these areas of chlorite. (Described, p. 93.)
276
U. S. GEOLOGICAL SURVEY
MONOGRAPH XXXVI PL. XXIII
CA^ SCHISTOSE RHYOLITE PORPHYRY.
<B) APORHYOLITE BRECCIA.
THE MPRDE.-* GRAVUftE CO.
PLATE XXIT.
27V
platp: XXIV.
Fig. a.
(Sp. No. 22968. Without analyzer, x 18.)
Schistdse rbyolite-porphyry with well-developed ilowage structure. A feldspar phenocvyst
•which has been more or less rounded by crusliing, occupies the center of the figure. There is also
shown iu the upper left-hand quadrant of the figure a small crushed quartz phenocryst. (Described,
p. 93.)
Fig. B.
(Sp. No. 22968. With analyzer, x 18.)
When the section represented iu fig. A is viewed between crossed iiicols, the crushed character
of the feldspar phenocrysts is therel)y well brought out. The minutely granular character of the
grouudmass is also well shown. (Described, p. 93.)
278
U.S. GEOLOGICAL SURVEY
MONOGRAPH XXXVI PL. XXIV
::^is^
:**-'^
(B)
(/») SCHISTOSE RHYOLITE PORPHYRY.
(S) SCHISTOSE RHYOLITE PORPHYRY BETWEEN CROSSED NICOLS.
TH= MFRIDEN QHAVUtiE CO.
PLATE XXV.
279
PLATE XXV.
Fig. a.
(Sp. No. 32116.)
Photograph, with very slight enlargement, of the polished surface of a Tery fine grained but
very amygdaloidal basalt. The amygdules are of irregular shape, but iu general with .a rounded
or tubular character. The original cavities have been filled vfith chlorite and quartz. The chlo-
ritic amygdules are the most common. A few of the white quartz amygdules may be seen on the
left-hand side of the figure. It should be noted that owiug to the softness of the chlorite some of the
amygdules have become impregnated with the powder used iu pnli.shing the specimen. This could
not be removed, and in many cases may be seen filling as well as outlining the chlorite amygdules.
(Described, p. 95.)
Fig. B.
(Sp. No. 32903. Without analyzer, x 18.)
Photomicrograph of a section of the fine grained, possibly vitreous, amygdaloidal basalt repre-
sented in fig. A of PI. XXVII. The amygdules consist of chlorite, quartz, and feldspar. The greatest
interest centers in the groundmass. This consists of a fine felt of ehlorite with minute epidote grains.
Traversing this, one sees in places delicate flowage lines. This is believed to represent a once vitreous
basalt, t Described, p. 102.)
280
U, S. GEOLOGICAL SURVEY
MONOGRAPH XXXVI PL. XXV
(A) AMYCDALOIDAL BASALT.
(B) AMYCDALOIDAL BASALT.
THE MERIOEN GHAVURE CO.
PLATE XXVI.
281
PLATE XXVI.
FiCr. A.
(Sp. No. 32541. Without analyzer, x IS.)
Fine-grained amygdaloidul basalt. The only recognizable original constituent in the ground-
mass is the feldspar in microlites which luo.st commonly fringe out at the ends. They are not infre-
quently arranged in sheaf-lilie aggregates. These are best seen with high-power objectives. The
major portion of the groundmass consists of a fine felt of chlorite, with minute grains of epidote. It
is considered to have resulted from the alteration of a vitreous base. The amygdules consist of calcite.
(Described, p. 99.)
Fig. B.
(Sp. No. 32541. Without analyzer, x 35.)
Portion of section from which fig. A was taken viewed with a higli power. In this the sheaf-lilco
aggregates of feldspar can be seen. (Described, p. 99.)
282
U.S. QeOLOGICAL SURVEY
MONOGRAPH XXXVI PART I PL. XXVI
M)
(B)
(/» AMYCDALOIDAL BASALT.
(S) SHEAF-LIKE FELDSPAR AGGREGATES.
THE MERIDEN ORAVURE CO.
PLATE XXYII.
283
PLATE XXYII.
Fig, a.
(Sp. No. 32903. Natural size.)
Reproduction of a very fine grained, possibly vitreous, .imydaloidal basalt. The amygdaloidal
cavities show very little contortion. The amygdules cousist of white quartz, piuk feldspar, and
dark-green chlorite. Compare this figure with photomicrograph fig. B, PI. XXV. (Described, p. 102.)
Fig. 73.
(Sp. Xo. 32910. Natural size.)
Colored reproduction of the pseudoainygdaloidal phase of the siderite-quartz matrix which
occurs iu places between the ellipsoids. The original matrix was first replaced in the zone of weath-
erinf by siderite. Deep burial of the rock resulted iu the mashing of the siderite and the subsecjueut
replacement of a great portion of it by silica, leaving a few oval areas uusilicifled. Brought into the
zone of weathering again by erosion, these siderite areas are removed on the weathered surface
giving a scoriaceous appearance to it, as may be seen on the figure. (Described, p. 135.)
Fig. C.
(Sp. No. 33507. Natural size.)
Fine-grained volcanic clastic. The water-dejiosited character of this specimen is unquestion-
able. It can be traced in the field down into a coarse bowlder conglomerate. Tlie fragmental nature
can only be seen under the microscope in the coarser-grained portions. The finer-grained material
represents apparently the excessively fine-grained mud derived from the trituration of the coarser
fragments. The eolian-deposited sands and dust have essentially the same appearance as the specimen
here represented. The microscope, however, shows the very angular character of the fragments in
the coarser-grained portions. The very fine eolian-deposited dust can not be distinguished from that
which has been deposited through water. It is sometimes very difficult to determine the nature of
rocks of this fine-grained character. (Described, p. 144.)
284
U S GEOLOGICAL SURVEY
MONOGRAPH XXXVI PL XXVII
S BlENaCO L1TM N V
A. AilYGDALOIDAL BASATiT.
B. PSEUDO-AMYODiU,OmALi\L:VTRIX OF EUJPSOIDAI, UAHALT
C. VOLCANIC CLASTIC.
PLATE XXVIII.
285
PLATE XXVIII.
Fig. a.
(Sp. No. 32909c. Without analyzer, x 35.)
Photomicrograph of section of flne-grained basalt with well-developed igneous texture. The
feldspar outlines are now occupied chiefly by flakes of muscovite aud grains of zoisite. (Described,
p.m.)
Fig. B.
(Sp. No. 32909c. With analyzer, x 35.)
Photomicrograph of the same section when viewed between crossed nicols. showing the obliter-
ation of the igneous texture. The lath-shaped mineral is muscovite (Described, p. 127.)
286
U. S. GEOLOGICAL SURVEY
MONOGRAPH XXXVI PART I PL XXVIII
(/») BASALT SHOWING CHARACTERISTIC TEXTURE.
(S) BASALT SHOWING OBLITERATION OF TEXTURE BETWEEN CROSSED NICOLS.
HE MEOlnEN QDAVURE CO.
PLATE XXIX.
287
PLATE XXIX.
Fig. a.
(Sp. No. 32582. Without analyzer, x 38.)
Photomicrograph showing the normal igneous texture of a Ijasalt. The area formerly occuijied
by the feldspar substance is now occupied by a granular aggregate of various minerals. (Described,
p. 127.)
Fig. B.
(Sp. No. 32582. With analyzer, x 38.)
The same section of basal't when viewed l)etweeu crossed nicols. The only indication of au
igneous texture is shown by the amygdnles present. The igneous texture of the rock is completely
concealed as .soon as the aggregates occupying the feldspar areas break up into their constituent grains.
(Described, p. 127.)
288
U. S. GEOLOGICAL SURVEY
MONOGRAPH XXXVI PART I PL. XXIX
{B)
(/» BASALT SHOWING CHARACTERISTIC TEXTURE.
(SI BASALT SHOWING OBLITERATION OF TEXTURE BETWEEN CROSSED NICOLS.
HE MERIOEN GRAVURE CO.
PLATE XXX.
MON XXXVI 19
PLATE XXX.
Fig. a.
(Sp. No. 33313. Without analyzer, x 38.)
Photomicrograpli of a section of basalt from a pyroclastic showing in ordinary light a distinctly
amygdaloidal character. The alteration of the hasalt has, however, reached such a stage that the
groundmass materials have for the most part been completely altered, with the production of porphy-
ritic rhombobedra of calcite and plates of muscovite, which may be seen in abundance in the lower
right-haud side of the figure. (Described, pp. 129, 145.)
Fig. B.
(Sp. No. 33313. AVith analyzer, x 38.)
Photomicrograph of the same section of basalt, showing in ordinary light a distinctly amygda-
loidal character when viewed between crossed nicols. The igneous texture is almost completely oblit-
erated by secondary products. The secondary calcite and muscovite stand out very sharply from
the very tine grained groundmass. (Described, p. 129.)
290
U. S. GEOLOGICAL SURVCY
MONOGRAPH XXXVI PART I PL. XXX
C-A) BASALT FRAGMENT IN A PYROCLASTIC SHOWING AMYCDALOIDAL TEXTURE.
CB) BASALT FRAGMENT SHOWING OBLITERATION OF THIS TEXTURE BETWEEN CROSSED NICOLS.
THE MfRiflEf* QHAVURE CO.
PLATE XXXI.
291
PLATE XXX I.
Fir. a.
(Sp. No. 32909c. Without aualyzer, x 35.)
Photomicrograpli illustrating the igneous texture of the basalt from the center of an ellipsoiil.
This has already uutlergone advanced alteration, and the feldspars have been replaced by a granular
aggregate of calcite, serlcite, chlorite, epidote, quartz, and albite. With advancing alteration the
igneous texture is destroyed, and infiltrated calcite becomes more prominent. The photomicrograi)h
shows in the lower left-hand corner a portion of the section In which a large quantity of calcite is
present. (Described, p. 131.)
Fig. B.
(Sp. No. 32909c. "With analyzer, x 35.)
Photomicrograph illustrating the igneous texture of the basalt from the center of an ellipsoid,
when viewed between crossed nicols. The igneous texture is almost completely destroyed by the
breaking up of the secondary aggregates. (Described, p. 131.)
292
U.S. GEOLOGICAL SURVEY
MONOGRAPH XXXVI PART I PL. XXXI
w
M)
(B)
CA) BASALT
CS) BASALT SEEN BETWEEN CROSSED NICOLS.
HE MERIDEN QRAVURE CO
PLATE XXXIl.
293
PLATE XXXIl.
Fia. A.
(Sp. No. 23645. Without analyzer, x 35.)
Perlitic parting In a fragment from a basalt tuff. The perlitic cracks are marked by accumu-
lations of epidote grains. The remainder of the fragment consists of an exceedingly fine chlorite
felt, with here and there a small feldspar microllte embedded in it. This was probably a fragment of
basalt glass. (Described, p. 138.)
Fig. B.
(Sp. No. 23646. Without analyzer, x 35.)
Photomicrograph of a section of basalt tuff. The illustration shows the sickle-shaped bodies
which are characteristic for the fine eolian-deposited volcanic ejectamenta. (Described, p. 142.)
294
U. S. GEOLOGICAL SURVEY
MONOGRAPH XXXVI PART I PL. XXXIl
CA) PERLITIC PARTING IN BASALT (CLASS?).
(B) TUFF.
THE UEBIDEN GBAVURE CO.
PLATE XXXITI.
295
PLATE XXXIII.
Fig. a.
(Sp. No. 32713. Without analyzer, x 35.)
Water-deposited sand. This volcanic sand consists chiefly of feldspar and liornblende. The
rounded nature of the feldspar grains is readily seen. Under the microscope some of these show
secondary enlargements. The hornblende is in irregular areas, and is presumed to be secondary after
fragments of augite. (Described, p. 144.)
Fig. B.
(Sp. No. 32711. Without analyzer, x 17.)
Water-deposited volcanic sediment. This illustration shows very clearly the griidation from
the medium-grained volcanic sand in the lower portion of the section, through the finer-grained mate-
rial, to the very fine grained dust. The very fine grained material was deposited following the con-
tours of a large pebble, which is shown In the upper portion of the figure. The fragments of this
sand consist chiefly of basalt. There are some which were probably feldspar. (Described, p. 144.)
296
U. S. GEOLOGICAL SURVEY
MONOGRAPH XXXVI PART I PL. XXXIII
(^)
'm
■^^f
C/t) WATER DEPOSITED SAND.
(B1 GRADATION IN WATER DEPOSITED CLASTIC.
THE MERmEN QRAVURE CO.
PLATE XXXIV.
297
PLATE XXXIV.
Fig. a.
(Sp. No. 23746. Without .analyzer, x 17.)
Contact product of a granite. This muscovite-biotite-gueiss ( ?) is the result of contiict action
of a granite upon a graywacke. Complete recrystallization of the sedimentary rock has t.aken place,
with the production of a porphyritio schistose structure. The large porphyritic muscovite plates
seen as white areas in the photomicrograph evidently represent the last products of recrystallization,
as they include all of the minerals which have Iteen previously formed. They are jjossihly to be
looked upon as the product of mineralizers, to whose action may also be referred the presence of
tourmaline, which occurs in the section. The irregular white areas represent quartz and feldsp.ar.
Dark greenish-brown biotite is included in the muscovite, and with iron oxides occupies the areas
which in the figure are dark. (Described, p. 197.)
Fig. B.
(Sp. No. 23226. Without analyzer, xl7.)
Photomicrograph showing the brecciated character of the matrix which is at times found between
the ellipsoids iu the ellipsoidal bas.alts. In this specimen the schistose character is not so marked
as it is at times. 'Described, p. 117.)
298
U.S. GEOLOGICAL SURVEY
MONOGRAPH XXXVI PL. XXXIV
(/») CONTACT PRODUCT OF GRANITE.
(B) BRECCIATED MATRIX BETWEEN ELLIPSOIDS.
THE MER10EN QRAVURE CO.
PLATE XXXV.
299
PLATE XXXV.
Pig. a.
(Sp. No. 260.59. Without analyzer, x 18.)
Photomicrograph showing an eruptive contact between granite and metamorphosed sedimentary
rock. Ab a result of this contact the elements of the granite have become partly automorphic. The
center of the iigure is occupied by a quartz phenocryst which is partly surrounded by the schistose
metamorphic product. It contains, near the edge, grains of feldspar and flakes of mica, and thus an
imperfect poikilitic zone is produced. The mineral constituents of the metamorphosed sedimentary
are arranged parallel to the contours of the phenocrysts. (Described, p. 198.)
Fig. B.'
(Sp. No. 26059. With analyzer, x 18. )
The same section as the above, viewed between crossed nicols. (Described, p. 198.)
300
U.S. GEOLOGICAL SURVEY
MONOGHAPH XXXVI PART I PL. XXXV
A*,
c*i^
^
m
.i*"^
(fi)
(>») CONTACT OF GRANITE AND SEDIMENTARY.
(S) CONTACT OF GRANITE AND SEDIMENTARY SEEN BETWEEN CROSSED NICOLS.
THE MFR DEN GRAVURE CO-
PLATE XXXVI.
301
PLATE XXXVI.
Fig. a.
(Sp. No. 32827. Without .analyzer, x 38.)
Photomicrograph illustratiug a rather exceptional form of siiilositc with white spots lying iu
the fine-grained groundiiKiss. Albite, with a very small amount of chlorite and epidote, forms the
spots. (Described, p. 206.)
Fig. B.
(Sp. No. 32827. With analyzer, x 38.)
This is the same section as above, viewed between crossed nicols, showing the aggregate char-
acter of the spots of this spilosite. (Described, p. 206.)
302
U.S. GEOLOGICAL SURVEY
MONOGRAPH XXXVI PART I PL. XXXVI
'^l""' wa
r^^??^
■ »',
(^)
(-6)
(>») SPILOSITE.
(S) SPILOSITE SEEN BETWEEN CROSS NICOLS.
IFRIDEN GHAVOKE CO.
PLATE XXXVII.
303
PLATE XXXVII.
Fig. a.
(Sp. No. 32958. Without analyzer, x 18.)
Normal spilosite. Oval spots of xuacroscopical size, in which chlorite is the predominant con-
stituent, with some quartz, feldspar, rutile, and muscovite, are sharply defined from the surrouuding
fine-grained grouudmass, consisting of the same constituents, but with the muscovite in large quantity
and the chlorite very subordinate. (Described, p. 206.)
Fig. B.
(Sp. No. 32861. Without analyzer, x 38.)
Spilosite in which the spots are of microscopical size and consist predominantly of chlorite
aggregates lying in tbe fine-grained quartz-albite groundmass. (Described, p. 207.)
304
U. S. GEOLOGICAL SURVeV
MONOGRAPH XXXVI PART I PL XXXVII
■ .•^►>*
'•^te* ■ ^'■^'\'' ■'•if;'-'' ■
(^)
(/I) SPILOSITE.
(B) SPILOSITE.
nEN GSAVUHE CO.
PLATE XXXVIII.
MON XXXVI 20 305
PLATE XXXVIII.
Fig. a.
( Sp. No. 32826. Without analyzer, x 38. )
Photomicrograph illu8trating the passage of spilosite to a desmosite. lu the upper jiortiou,
especially iu the upper left-hand corner, of the figure, chlorite .aggreg.ates similar to those illustrated
in lig. i', PI. XXXVII, are seen. These hecome united, and thus there is a passage into the banded
product. This banded character is well shown in the lower half of the photomicrograph. (Described,
p. 207.)
Fig. B.
(Sp. No. 23755. Without analyzer, x 90.)
Occurrence and alteration of bronzite in bronzite-norite. This illustration shows the way in
which the bronzite occurs in the bronzite-norite. It is frequently included in the hornblende. The
bronzite alters around the edges and along the cracks to a yellowish-green fibrous serpentine mineral,
which is represented in the section by the dark fibrous material next to the unaltered bronzite. This
secondary mineral then .alters to a scaly aggregate of talc. These two secondary products can be
seen bordering tba bronzite, especially well where it is traversed by a crack. (Described, p. 238.)
306
U.S. GEOLOGICAL SURVEY
MONOGRAPH XXXVI PL. XXXVIII
:»^'
M)
(5)
(/») PASSAGE OF SPILOSITE INTO DEMSOSITE.
(S) ALTERATION OF BRONZITE IN BRONZITE NORITE.
THE MtRIDEN ORAVURE CO.
PLATE XXXIX.
307
PLATE XXXIX.
Fig. a.
(Sp. No. 23321. With analyzer, x 35. )
Pliotomicrograph nf a section of biotite-granite from the center of a dil^e 5 feet Tvido. On the
horders of the dike the magma has crvstallized as a normal mica-diorite -.vithout qnartz and orthoclase.
(Described, p. 226.)
Ficf. B.
(Sp.No. 26023. With analyzer, x 3.5.)
Mica-diorite showing tendency toward an opliitic texture. Plagioclase is the most automorphic
mineral- Biotite is next, but it is poorly developed. Orthoclase and quartz fill irregular areas
between the plagioclase. (Described, p. 231.)
308
U. S. GEOLOGICAL SURVEY
MONOGRAPH XXXVI PL. XXXIX
(^)
(B)
(») BIOTITE GRANITE.
(B) MICA DIORITE.
THE MERinEN GHAVURE CO.
PLATE XL.
309
PLATE XL.
Fig. a.
(Sp. No. 32643. Without analyzer, x 35.)
Qnartz-mioa-diorite-porphyry. The phenocrysts of feldspar aud quartz stand out clearly from
the fine miciogranitic grouudmass. Mica phenocrysts are not seen in this figure, which is intended
chiefly to illustrate the character of the feldspar phenocrysts. Muscovite has resulted from theii
alteration. The zone which surrounds the altered center is very fresh, and is rendered poikilitic hy
inclusions of minutt- grains ot fpiartz. (Described, p. 229.)
Fig. B.
(Sp. No. 32643. With analyzer, x 35.)
Qnartzmioa-diorite-porphyry. The same section, viewed lietweeu crossed nicols. The micro-
granitic texture of the groundmass is well shown. (Described, p. 229.)
310
U. S. GEOLOGICAL SURVEY
MONOGRAPH XXXVI PL. XL
-0^-^^.
'^
■k:
J" w,' "'
•y
V^/'^
•^^r^;
--*v- i.
M)
(B)
(A-) MICA DIORITE PORPHYRITE.
(S^ MICA DIORITE PORPHYRITE SEEN BETWEEN CROSSED NICOLS.
TMt MERlOEN QRAVUBE CO.
PLATE XLI.
311
PLATE XL I.
Pm. A.
(Sp. No. 23320. Without aualyzer, x 18.)
Pnrphyritic poikilitic hornblende-gabbro. Tho brown honibk-ndo occupying tho center of the
phenocryst grades over iuto a dull-green hornblende. Feldspar and pyroxene are included in the
hornblende. (Described, p. 241.)
Fig. B.
(Sp. No. 23344. With analyzer, x 18.)
Hornblende-gabbro showing a poikilitic texture. The tendency of the feldspars toward a lath-
shaped development is very evident. Were they lath-shaped tho texture would agree with the L^vy
definition of tho ophitio texture. (Described, p. 233.)
312
U.S. GEOLOGICAL SURVEY
MONOGRAPH XXXVI PL. XLI
(/» PORPHYRITIC POIKILITIC HORNBLENDE CABBRO.
CS) POIKILITIC HORNBLENDE.
HE MERIOEN QRAVURE CO.
PLATE XLII.
313-
PLATE XLII.
Fig. a.
(Sp. No. 23754. Without analyzer, x 38.)
A moderately fine grained hornbleude-gabbro showing parallel texture. This liornblende-
gabbro occurs in dikes cutting the coarse forms of hornblende-gabbro. The sppcimen shows very
clearly the parallel texture rather commonly found in sections from these dike rocks. This texture
is best developed nearest the contact, and i.s presumed to be a flow texture. The chief mineral
constituents — plagioclase, hornblende, and mica — can be readily distinguished in the section.
(Described, p. 244.)
Fig. B.
(Sp. No. 23754. With analyzer, x 38.)
The parallel texture in the hornblende-gabbro is brought out somewhat better when viewed
between crossed nicols. (Described, p. 244.)
314
U. S. GEOLOGICAL SURVEY
MONOGRAPH XXXVI PL. XLII
(^)
(B)
(/» HORNBLENDE CABBRO.
(S) HORNBLENDE CABBRO SEEN BETWEEN CROSSED NICOLS.
The meriden qravore co.
PLATE XLIII
315
PLATE XLIII.
Fig. a.
(Sp. No. 26070. Without analyzer, x 17.)
Normal granular hornblende-gabbro. This illustrates the normal medium-grained hornblende-
gabbro -which occurs in this district. The mineral constituents hornblende, feldspar, aud iron oxide
can be readily distinguished. (Described, p. 248.)
Fig. B.
(Sp. No. 26069. With analyzer, x 18.)
Schistose hornblende-gabbro. This illustrates a crushe<l specimen of a normal hornblende-
gabbro such as is represented above in fig. A. In this the feldspar has been partly granulated, and ne^
feldspar and quartz has resulted therefrom. The hornblende has also to a considerable extent been
recrystallized as light-green secondary hornblende. The mica has become chloritized. These are the
important secondary products. A very fiiir schistose structure has resulted from the crushing.
Carried to its full extent, metamorphism of this rock would result in producing an amphibolite or a,
hornblende-gneiss. (Described, p. 248.)
316
U. S. GEOLOGICAL SURVEY
MONOGRAPH XXXVI PL. XLIII
^.
[A)
(B)
(A) COARSE HORNBLENDE CABBRO.
(B) SCHISTOSE HORNBLENDE CABBRO.
THE MERinE)4 GHAVURE CO.
PLATE XLIV.
317
PLATE XLIV.
Pig. .-1.
(Sp. No. 23319. Without analyzer, x 35.)
Motlerately fine grained hornblende-gabbro. The constituents horublende, plagioclase, and iron
oxide can be readily recognized. Many of the hornblende individuals contain a great number of
minute inclusions. (Described, p. 240.)
Fig. B.
(Sp. No. 23755. Without analyzer, x 35.)
Bronzite-noiite. The rock consists of bronzite, hornblende, and plagioclase. The bronzite
individuals show a, tinge of green aud ijinlc, and are slightly fibrous, due to beginning alteration.
Cracks, filled with alteration products, cross them. The bronzite is frequently surrounded by a rim
of brown hornblende. In some parts of the rock it is partly automorphic. The hornblende can be
readily recognized by its strong yellowish and reddish-browu color. It is in auhedra. The plagioclase
is the white mineral including numerous minute dark specks. Jt, like the bronzite, is xenomorphic.
(Described, p. 244. )
318
USGEOLOGICAL SURVEY
MONOGRAPH XXXVI PL XLIV
.r:^r^^m^:'
/^ /T Denniston_ del
JULIUS BIEN a CO LITH N.V
A. HOBNBLKNDE-a\BBHO.
B. BRONZITE-NOIUTE.
PLATE XLV.
319
PLATE XLV.
Fig. a.
(Sp. No. 23356. Without analyzer, x 35.)
Brouzite-norite-porphyry. The pUenocrysts of bronzite lie in a grouudmass of bronzite, mouo-
■olinlc pyroxeue, hornblende, and feldspar. In the drawing the bronzite and monoclinic pyroxene of
the groundmass can not be distinguished. (Described, p. 247.)
Fig. B.
(Sp. No. 23353. Without analyzer, x 18.)
Fcldspathic wehrlite. One can readily distinguish the constituents — augite, hornblende, olivine
and feldspar — in the illustration. The augite is automorphic where it is near the feldspar. It is
surrounded by a brown hornblende zone. Between the olivine and the feldspar there are always two
zones. One, the inner one, best seen between crossed nicols, is orthorhombic pyroxene; the other,
the outer one, is compact green hornblende. This green hornblende is in optical continuity with the
browu hornblende which surrounds the augite. (Described, p. 255.)
320
US.GtOLOGICAL SURVEY
MONOGRAPH XXXVI PL. X LV
F K. Denniston, del
JULIUSBIEN6C0 LITH N ■-
A, BHONZITE- NORITE -PORPHYRY.
B. FELDSPATHIC IVEREJLITE.
PLATE XL VI.
MON XXXVI 21 32X
PLATE XLVI.
Fig. a.
(Sp. No. 23353. With analyzer, x 90.)
Wehrlito. There are shown iu this illustration the zones of orthorhombic pyroxene and horn-
bliintle which lie between the olivine and feldspar. (Described, p. 255.)
Fig. /}.
(Sp. No. 23353. Without analyzer, x 90.)
Wehrlite. The ramifying feldspar growths in the hornblende can be seen better when the
section is viewed with the analyzer in, as in iig. A. (Described, p. 255.)
322
U.S. GEOLOGICAL SURVEY
MONOGRAPH XXXVI PL. XLVI
M)
{H)
(A; WEHRLITE VIEWED BETWEEN CROSSED NICOLS.
fS) WEHRLITE.
THE WERIDEN QRAVURE CO-
THE CRYSTAL FALLS IRON-BEARING DISTRICT OF MICHIGAN
Part II.— THE EASTERN PART OF THE DISTRICT>
INCLUDING THE FELCH MOUNTAIN RANGE
By HENKY ULOYD SMY'm
A CHAPTER ON THE STURGEON RIVER TONGUE
By WILLIAM SHIKLET BAYLET
323
CONTENTS.
Page.
Chapter I.— Geographical omits and physiographv 329
Intvoduction 329
Preliminary sketch of geology 331
Character of surface 331
Drainage 334
Chapter II. — Magnetic observations in geological mapping 336
Section I. — Introduction 336
Section II. — Description of the magnetic rocks 338
Section III. — Distribution of magnetism in the magnetic rocks 339
Section IV. — Instruments and methods of work 341
Section V. — Facts of observation and general principles 344
(1) Observed deflections when the strike is north and south and the dip vertical 344
(2) Deflections of the horizontal needle 345
(3) Deflections of the dip needle 347
(4) Horizontal and vertical components when the magnetic rock dips vertically 349
(5) Horizontal and vertical components when the magnetic rock dips at any angle 350
(6) Determination of depth 354
(7) Summary 356
Section VI. — Applications to special cases 356
(1) The magnetic rock strikes east or west of north and dips vertically 357
(2) The magnetic rock strikes east and west 3.59
(3) Two parallel magnetic formations 361
Section VII. — The interpretation of more complex structures 366
(1) Pitching synclines 367
(2) Pitching anticlines 370
(3) Formations split by intrusives 371
(4) Summary 372
Chapter III. — The Felch Mountain range 374
Section I. — Position, extent, and previous work 374
Section II. — General sketch of the geology 383
Section III. — The Archean 385
Topography 386
Petrographical characters 387
Section IV. — The Sturgeon quartzite 398
Distribution, exposures, and topography 398
Folding and thickness 399
Petrographical characters 401
Section V. — The Randville dolomite 406
Distribution, exposures, and topography 406
Petrographical characters 408
Section VI. — The Mansfield schists 411
Distribution, exposures, and topography 411
Petrographical characters 412
325
32(i CONTENTS.
Chapter III. — The Felch Mountain Range — Continued. Page.
Section VII. — The Groveland formation 415
Distribution, exposures, and topography 415
Petrographical characters 417
Section VIII. — The mica schists and quartzites of the Upper Huronian series 423
Petrographical characters 425
Section IX. — The intrusives 426
Chapter IV. — The Michigamme Mountain and Fence River areas 427
Section I.— The .\rchean 428
Section II. — The Sturgeou formation 430
Section III.— The Randville dolomite 431
Distribution and exposures 431
Folding and thickness 432
Petrographical characters 434
Section IV. — The Mansfield formation 437
Distribution, exposures, and topography 438
Folding and thiclsness 438
Petrographical characters 439
Section V. — The Hemlock formation 440
Distribution, exposures, and topography 440
Folding and thickness 441
Petrographical characters 442
Section VI. — The Groveland formation 446
Distribution, exposures, and topography 446
Folding and thickness , 448
Petrographical characters 448
Chapter V. — The northea.stern area and the relations between the Lower Mar-
quette and the Lower Menominee series 451
Chapter VI.— The Sturgeon River tongue, by William Shirley Bayley 458
Description and boundary of area 458
Literature 459
Relations between the sedimentary rocks and the granite-schist complex 461
The Basement Complex 463
The gneissoid granites 463
The amphibole-schisls - 465
Origin of the amphibole-schists 466
The biotite-schists 467
The intrusive rocks 469
Comparison of the Sturgeon Eiver and the Marquette crystalline series 470
The Algonkian trough 471
Relations liet ween the conglomerate and the dolomite series 472
Relations between the dolomites and conglomerates and the overlying sandstones 473
The conglomerate formation 473
Important exposures 474
Petrographical descriptions 477
The dolomite formation 479
Important exposures 480
Petrographical description 481
Slates and sandstones on the Sturgeon River 481
The igneous rocks 482
The intrusive greenstones 482
Petrographical description 482
The banded greenstones 485
Petrographical description 486
ILLUSTRATIONS
Page.
Plate XLVII. Relaiious of maguctic beds to variation auil dip 352
XL VIII. Relations of magnetic beds to variation and dii> 362
XLIX. Geological map of the FeUb Mountain range 374
L. Geological map of a portion of the Crystal Falls district 450
LI. Geological map of the Sturgeon River tongue 458
LIT. Map of exposures in sec. 7 and portions of sees. 8, 17, and 18, T. 42 N., R. 28 W 474
LIII. Schist conglomerate from dam of Sturgeon River 476
Fig. 15. Magnetic cross section in T. 45 N., R. 31 W 345
16. Circles of attraction 346
17. The forces acting on the dip needle 347
18. Curves showing the relations between the horizontal components at the points of maxi-
mum deflection, for rocks dijiping at various angles and buried to various depths 354
19. Truncated anticlinal fold with gontly dipping limbs 364
20. Truncated anticlinal fold with steeply dipping limbs 365
21. Plan and cross sections of a lutchiug syncline 367
22. Magnetic map of the Groveland Basin 370
23. Plan and cross sections of a pitching anticline 371
24. Magnetic map of a single formation split by an intruded sheet 372
327
THE CRYSTAL FALLS IRON-BEARING DISTRICT
OF MICHIGAN.
part ii. the eastern part of the district,
i:n^cludixg the felch mountain range.
By Henry Lloyd Smyth.
With a Chaptek on the Sturgeon River Tongue, bv William Shirley Bayley.
CHAPTER!.
GEOGRAPHICAL LIMITS AND PHYSIOGRAPHY.
INTRODUCTION".
The teiTitory to be described in this and the four following chapters is
situated in the Upper Peninsula of Michigan, between the Marquette and
Menominee iron ranges, and is all embraced within T. 42 N., Rs. 28-30 W.,
and Ts. 42-47 N., Rs. 30-31 W. The area of about 300 square miles
included within these townships had for the most part been covered hastily
by previous reconnaissances of the Lake Superior Division of the United
States Geological Survey, the results of which were placed at my disposal.
Our task was to go over with especial care those portions in which outcrops
had been found by our predecessors, or which seemed likely to contain the
iron-bearing formations. At the same time much of the rest was examined
more hurriedly.
The tract surveyed in detail comprises a continuous belt about 30
miles in length, and of width varying from 2 to 5 miles, lying wholly
within the drainage basin of the Michigamme River and its principal upper
tributary, the Fence River, and extending southward from the northern end
of the Republic tongue, where it was connected with rocks of well-
determined Marquette types, as far as the south line of T. 43 N., R. 31 "W.
From this line we passed southeast (leaving a gap of 5 miles) across the low
329
3B0 THE CRYSTAL FALLS lEON-BEARING DISTRICT.
di^■ide between the Michigamme and the headwaters of the Sturgeon, to the
Felch Mountain range, which was then carefully studied for a distance
extending 13 miles to the east.
Until within the last few years the larger part of this area had been
very difficult of access, and much of it is dithcult still. The rock surface is
almost wholly concealed by a cover of glacial deposits of various kinds;
dense forest and great swamps also obscure the rocks, and make traveling
difficult and slow. It is therefore not a field to invite geological study.
While exploration for iron ore has here and there passed the frontiers of the
productive ranges on either side, the general ill-success which attended the
early enterprises has discouraged the active search that would at least have
resulted in important additions to geological knowledge. For these reasons
the area as a Avhole, with the exception of the Felch Mountain range, has
remained almost unknown geologically, until our work in 1892. The i-efer-
ences to it in geological literature are consequently but few In number, and
are for the most part merely the records of the unrelated observations of
casual visitors.
The district, nevertheless, deserves attention from both the economic
and the geological standpoint. The iron-bearing formations of the Mar-
quette range extend into it from the north, those of the Menominee range
from the south. On the west the ore deposits of the Crystal Falls area are
connected geographically at least with the western extension of the Menom-
inee range. Between these boundaries the area stands as the largest one
remaining in Michigan in which iron-bearing formations are known to occur,
but as yet not yielding important bodies of ore. Here, too, if anywhere,
the questions of the equivalence or nonequivalence of the individual for-
mations of the Marquette and Menominee iron-bearing series are to be
answered.
It is proper to state that the field study, in consequence of the condi-
tions under Avhich this work was done, was almost wholly directed to eco-
nomic questions, and that it was not originallj- anticipated that the results
were to be published as a monograph on the district. This will explain the
very brief space devoted to the Archean in the following pages. The field
work was begun and ended in 1892. Since that time there has been no
opportunity to revisit localities, and the conclusions now stand essentially
as they were reached in the field. Considering both the obscurity and com-
INTRODUCTION. 331
plexitv of the area, it is very probable* that further study of important local-
ities would clear away many of the difficulties, as well as uiodify certain of
the opinions now held.
The writer was efficiently aided in the field work by Messrs. Samuel
Sanford and Charles N. Fairchild for nearly the whole period, and E. B.
Mathews and H. F. Phillips for part of it, as assistant geologists, and by
Messrs. Lewis and Forbes as skilled woodsmen.
PRELiIMINARY SKETCH OF THE GEOL,OGY.
The rocks of the Michigamme and Felch Mountain areas range in age
from Archean to early Paleozoic. North and west of the Michigamme
River, where geological boundaries are most susceptible of determination,
the granites and gneisses of the Archean come to the surface in three oval
areas of great regularity of outline, from 10 to 12 miles long by 2 to 6
miles wide, while the intervals between the Archean ovals are occupied
by highly tilted sedimentary and igneous rocks of Algonkian age. The
lower member of the Algonkian has derived its materials from the wasting
of rocks lithologically similar to the underlying granites and gneisses. In
the southern and eastern portions of the district the edges of the tilted
older rocks are partially covered by a blanket of gently dipping sandstones
of Cambrian age, very soft and easil}" disintegrating. These rocks first
appear near the ]\Iichigamme River as detached outliers. In going south
and east from that river the separated patches become larger and more
a,bundaut, until finally a few miles beyond the eastern limit of our work in
the Felch Mountain range they unite and entirely cover the pre-Cambrian
formations.
CHARACTER OF THE SURFACE.
In its most general aspect the surface throughout this area is a plam,
somewhat rolling indeed, which slopes gently upward from the southeast
toward the northwest. The surface is formed partly by the soft and gently
inclined Upper Cambrian sandstones and partly by the much harder and
highly tilted pre-Cambrian rocks of diverse physical and mineralogical
characters, and yet over all it maintains a very uniform slope. On the
southeast, in the Felch Mountain range, the plain has an average elevation
above the sea of 1,200 to 1,300 feet. In the northwest, in the southern
332
THE CRYSTAL FALLS IRON-BEARING DISTRICT.
sections of T. 47 N., R. 31 W., the average elevation is 1,800 to 1,900 feet.
Since the intervening' distance is somewhat more than 30 miles, the gen-
eral slope is therefore less than 20 feet to the mile.
The minor topographical features based upon this plain are multitudi-
nous in variety and detail, but generally quite insignificant in relief. The
maxiinum difference of elevation between the top of the highest hill and
the bottom of the neighboring valley is less than 300 feet, and this is
reached in Ijut two cases. The country possesses no commanding emi-
nences, and in the widest panoramas now and then obtainable from the
summits of glaciated knobs the background is restricted to a radius of a
few miles. In these the general evenness of the sky line is usually broken
only by the remnants of the old forest, which have not yet succumbed to
fire and the lumberman.
These lesser features have been shaped mainly by the work of the conti-
nental ice-sheet, both through the materials which it brought in and through
those which it earned away. In the areas underlain by relatively massive
rocks, particularly the Archean crystallines, the surface has been left mam-
millated with rocky knobs,' which doulitless were the unattacked cores rising
into the pre-Grlacial zone of disintegration. These are separated by the
similar inverse forms, now for the most part occupied by swamps. In the
Archean borders of the Felch Mountain area, where the glacial cover was
originally thin, the periodical fires that have followed lumbering operations
have burned out the organic matter from the soil and so loosened it that,
on the steeper slopes, it has been entirely washed away and the rock
surface laid bare. The hummocks and bowls are generally elongated east
and west, which is the direction both of the gneissic foliation and of the ice
movement. The elevations rise, often with steep, smooth walls, for 5, 10,
20, or even in some cases 60 feet, above the intervening depressions. The
latter hold muskeg to the rims. In the wet season they fill with water,
which ovei-flows to the next bowl below, but permanent lines of minor
di-ainage, here as elsewhere in the Archean areas, are very infrequent.
Over most of the area, however, the ice has spread a sheet of till, and has
here and there deposited the materials swept along in the subglacial streams
in characteristic complexity of form and grouping. The more prominent
elevations are, in fact, deposits of modified drift, although occasionally
TOPOC.KAPHY. 333
small rock masses like Michig'amine Mountain, which is composed of mate-
rial that offers a most stubborn resistance to all degrading- agents, reach an
elevation of 100 to 200 feet above the general level of the surrounding
country. The fact that the name "mountain" has been applied to hillocks
ot this order by the surveyors and woodsmen, who have the widest knowl-
edge of the Upper Peninsula, conveys perhaps the clearest idea of the
generally level character of the surface.
While the details of the topography are thus mainly glacial in origin,
the broader features of the next order of importance have often clearly
been determined by the presence of the more resistant rocks. The large
structural domes of the Archean, which are such characteristic geoloo-ical
features, are also indicated by a general upward swell of the surface of the
areas which they occupy. The topographical transitions at the margins of
these swells are frequently abrupt, and sometimes for considerable distances
are marked by scarp-like slopes in the granites, caused by the almost ver-
tical contacts with the softer Algonkian formations. Considerable portions
of all three of the Archean ovals in the northern part of the district display
this slight topographical prominence. Marginal scarps are particulai-ly well
shown in the oval west of Republic, in sees. 19 and 30, T. 47 N., R. 30 W.,
and along the south side of the oval which lies between the Fence and
Deer rivers, near their junctions with the Michigamme. The more impor-
tant bodies of greenstone also are generally expressed by a noticeable degree
of elevation. Thus the great intruded sheets folded in with the Lower Mar-
quette series in sees. 24, 25, and 36, T. 47 N., R. 31 W., give rise to long
broad ridges that closely follow the changes in the strike. But in all these
cases the topographical emphasis is very slight, and the plain as a whole
may truly be said to maintain its general slope with practical indifference
to the weather-resisting differences in the underlying rocks.
These broader swells of the harder rocks are separated by broad,
slightly lower-lying plains, in many of which a valley character is still dis-
tinctly recognizable in spite of the fact that they especially have been
favored with deposits of modified drift. The present drainage, in its main
lines, largely follows these older valleys, although much confusion, which
is especially noticeable in the details, has of course resulted from their
partial choking by the drift.
334 THE CRYSTAL FALLS IRON-BEARING DISTRICT.
DRAINAGE.
Nearly all the surface water ot" this district finds its way to Lake
Michisran tlu'ouo'h the Michiijamme and the Sturcreon riA'ors, which are inde-
pendent bi-anches of the Menominee — the largest river flowing- into Lake
Michigan from the west. A few square miles along the eastern boundary,
however, are tributary to the Ford, which flows into Green Bay north of
the Menominee. Of these the Michigamme drains by far the largest j^art
of the district. This river heads in Lake Michigamme, which it leaves in
sec. 9, T. 47 N., R. 30 W., near the northeast corner of the area shown
on the general map (PI. II), at an elevation above the sea of 1,580 feet.
Thence it flows for 8 miles southeast to Republic, in a synclinal valley cut
out of the soft schists of the Michigamme formation. This valley, which
is nearly a mile wide at the northern end and less than .half as wide at the
southern, is bordered on both sides by the harder Archean granites, which
rise with rather steep slopes to the general level of the plain. Tlu-oughout
the length of the valley the river flows over glacial drift, but at Republic,
where the soft rocks come to an end, it breaks across rocky barriers in a
succession of rapids, and continues first nearly due south (leaving the dis-
trict covered b}' our map), and then flows southwest over glacial deposits,
which completely mask the bed rock for 10 miles. South of the Archean
oval, which occupies the western part of T. 44 N., R. 31 W., and the east-
«
ern part of T. 44 N., R. 32 W., the limestones and slates of the pre-
Cambrian are again exposed, and over these the Michigamme flows in close
conformity to the general strike as far as the range line.
In the southern sections of T. 44 N., R. 31 W., the Michigamme receives
two tributaries from the north — the Fence River, which comes from the
eastern side of the Archean mass just mentioned, and the Deer River, which
comes from its Avestern side. The headwaters of the Deer and of the west-
ern branch of the Fence flow through the same section (21) in T. 46 N.,
R. 32 W., north of the Archean oval, but farther south they diverge to an
extreme distance of 10 miles, and afterwards converge so that their points
of junction with the Michigamme are but 4 miles apart. The area thus
inclosed is broadly concentric with the Archean oval. In the case of the
Fence, at least, the river is placed within a wide depression coincident with
the softer stratified rocks of the Algoukian, and follows very faithfully their
DRAINAGE. 335
general .strike. Depo.sits of glacial sand and gravels are very abundant
witliin this valley, and for these the river often swings aside across the strike
for a mile or more. In sees. 21 and 29, T. 45 N., R. 31 W., and in sec. 10,
T. 44 N., R. 31 W., excellent rock sections are afforded by such digressions.
Tlie old valley between the two Archean ovals west of the Republic
tongue (see PI. Ill) is on the south entirely filled with glacial gravels to
the level of the old divides, and the large brook known as the east branch
of the Fence is diverted to the till-covered western of the two Archean
ovals. The valley is clearly indicated, however, by an interesting series of
lakes, of which Squaw, Trout, and Sundog, each about 1 mile in length,
are the most considerable.
The area drained by the Sturgeon lies in the extreme southeastern part
of the district, wholly within the marginal fringe of sandstone. The rela-
tion of its course to the geology is known in detail only within portions of
the Felch Mountain range. This it first enters in Ihe northern portions of
sees. 35 and 36, T. 42 N., R. 30 W., in a loop into the Algonkian, from the
northern Archean margin, to which it again returns. Five miles farther
east it crosses the trough from north to south, transverse to the strike of the
Algonkian formations, to the contact with the southern Archean mass. It
follows this contact eastward for 2 miles, and then strikes southward across
the Archean to the Menominee River, not again returning to the Felch
Mountain range. The river valley in the Felch Mountain range is very
distinct, and where bordered by Potsdam outliers is rather deep, with pre-
cipitous banks. It is but slightly affected by drift deposits. Its course
shows an almost complete disregard of the structure of the Algonkian and
Archean rocks, and so has the usual characters of a superimposed stream.
The Michigamme River, as was early noted by Pumpelly, has practically
no eastern branches within this district. The Escanaba and Ford rivers,
which reach Lake Michigan directly, and the Sturgeon, which joins the
Menominee below the mouth of the Michigamme, all head within 2 or 3
miles of the latter, the course of which is transverse to their general direc-
tion. The Michigamme thus flows along the eastern edge of its drainage
basin. This fact — the most striking in the general distribution of the streams
of the district — is the result of causes which, in part at least, go back to very
remote geological periods.
CHAPTER II.i
MAGNETIC OBSERVATIONS IN GEOLOGICAL MAPPING.
SECTION I. INTRODUCTION.
As has been said already, the area in which our work was done is
largely drift covered, to somewhat varying but usually considerable depths;
the mantle on the whole is so evenly spread that outcrops of any rocks
except those belonging to the Archean are in many sections few and scat-
tered and sometimes are almost entirely lacking over whole townships.
Under these circumstances, and since also the pre-Cambrian rock
structure is complex, even a general outlining of the old formations would
be impossible by the usual geological methods, and if we were restricted to
these there would be no alternative but to map most of the territory as
Pleistocene. It happens, however, that the Algonkian rocks of Michigan
contain a large amount of magnetite, which is known from observation in
the developed ranges to be characteristic of certain geological formations.
It undoubtedly occurs in more or less amount in all the sedimentary rocks
and is also present, sometimes in considerable quantities, in rocks that are
not sedimentary, as, for example, around the margins of the old intrusive
diorite bosses. But generally speaking, its occurrence in large quantities is
confined so closely to definite geological formations, in which it is found in
characteristic association with certain other minerals, or to horizons within
those formations, that it can be guardedly used in identifymg them, and in
tracing them from localities where they outcrop through areas in which they
are buried. This use is not only justified, from an empirical standpoint, by
the presumption in favor of analogies to which no exceptions are known, but
it has a rational basis, in the view of the late Professor Irving,^ which is
' This chapter is abridged from a paper of the same title presented at the Colorado meeting of
the American Institute of Mining Engineers in September, 1896.
^ Classificatior of early Cambrian and pre-Cambrian formations, byR. D. Irving: Seventh Ann.
Kept. U. S. Geol. Survey, 1888, pp. 451-452.
336
MAGNETIC OBSEUVATIONS. 337
steadily j^aiiiinji' j^rouiul, tlint at least luucli of the iron of this iiuvgnetite was
originally Iniricd in thesanu' t\tnnations in which it now occurs, through the
ageiic}' of organic life. From this point of view the magnetite is in a cer-
tain sense a fossil, but with the important practical advantage over other
organic remains, that it need not be dug up in order to prove its existence.
These magnetite-bearing rocks always produce disturbances in the
compass-needles held in their neighborhood. By a systematic location and
comparison of these disturbances the position of the rocks which produce
them can be determined with a considerable degree of precision, even when
they are deeply buried. Besides their position on the map, the magnetic
observations may, and often do, indicate certain other geologically impor-
tant tacts, such as whether the rocks are flat lying or highly tilted, the
direction of strike and dip, and, in some cases, the depth to which they are
buried. The methods employed in the field work were based on those
described by Maj. T. B. Brooks,^ who perfected the dial compass and pre-
dicted the importance of magnetic methods in geological mapping; but the
results reached in interpretation were gradually developed in the progress
of this work, as we were daily brought face to face with phenomena which
called for explanation.
It must be clearly understood at the outset that in the iron ranges of
the south shore of Lake Superior magnetite is i-arely concentrated in large
bodies, and that, in fact, its known occurrence as such is restricted to a
small part of the western Marquette district, where in one producing mine
it now forms practically the whole product and in another a variable but
usually important part of the whole. It is therefore well understood in the
Upper Peninsula that disturbances of the magnetic needle, however great,
do not mean workable deposits of magnetite. Whatever significance such
disturbances possess is stratigraphical, and properly interpreted may lead to
discoveries of rich ore, other than magnetite, in formations to the position
and attitude of which the attractions may furnish a clew. But it may be
asserted as a general proposition, the essential truth of which has been
established by the experience of many years, that in the region referred to
magnetic disturbances usually mean that magnetic iron ore in a workable
deposit does not exist in the area of disturbance.
' Geological Survey of Michigan, Vol. I, Part I, 1873, Chapter VII.
MON XXXVl 22
338 THE CRYSTAL FALLS IRON-BEARING DISTRICT.
SECTION II. DESCRIPTION" OF THE MAGNETIC ROCKS.
The magnetic rock of the Lower Huronian series of the western por-
tion of the Marquette area, which is of special importance to notice, since
it forms one of the chief horizons of reference to which our work is tied,
is the Negaunee iron formation. It is finely exposed at the south end of
the Republic trough; but farther north has been greatly reduced in thick-
ness, or locally cut out altogether,' by the Upper Marquette denudation,
and, where present at all, is usually drift covered.
This rock often possesses a very distinct banding, caused by the alter-
nation of layers, in which one of the constituent minerals predominates
over the others, sometimes, indeed, to their total exclusion. In the lower
part of the formation, quartz and griinerite constitute the bi;lk of the
rock, with magnetite scattered somewhat indiscriminately through them.
Higher up, the magnetite and quartz relatively increase, until near the top,
but below the jasper, the griinerite goes out almost entirely, and the rock
consists of quartz bands, heavily charged with magnetite, in alternation with
bands of nearly pure magnetite. In the Negaunee formation, as exposed
at Republic, the magnetite therefore occurs concentrated in some of the
parallel bands and disseminated through the others.
In the same district there is another much less prominent locus of mag-
netite at and near the base of the Upper Marquette or "hanging-wall"
quartzite. Along the strike of this zone, which is of small thickness, the
distribution of magnetite is very irregular; and for this and the additional
reason that when the magnetic portion of the Negaunee formation comes
up to it the disturbances which it produces can not be discriminated from
those produced by the latter, the position of the plane usually can not be
inferred.
In the Menominee district and its extensions there are two horizons in
the lower series, characterized by the presence of magnetite. The lower
of these is not known to outcrop, but it occurs somewhere near the juiac-
tion of the dolomite and the underlying quartzite. The magnetic disturb-
ances due to this formation ai'e feeble, but they are quite persistent in the
Felch Mountain area, and have thrown some light on the geological structure.
' The Marquette iron-bearing district of Micliigan, by C. R. Van Hise and W. S. Bayley, with a
chapter on the Republic trough, by H. L. Smyth : Mou. U. S. Geol. Survey, No. XXVIII, 1897, pp. 531, 537.
MAGNETIC OBSERVATIONS. 339
The other formation which produces disturbances is that wliicli I liave
correhited witli the Neg-aunee formation and named in a former paper^ the
Michiganmio jasper, but which is here renamed the Groveland formation.
This rock, while varying a great deal in character, is generally much like
that magnetic phase of the Negaunee formation in which the griinerite is
rare or absent. From the fact that it now survives for the most part only
in shallow and shattered synclines, it often lacks the regular banding; and
hematite is always present in greater or less amount. The relative propor-
tions of the two iron minerals vary along the strike also. The rock as a
whole, however, is very magnetic, but not so strongly so as the Negaunee
formation in the Republic trough.
In the Felch IMountain rang-e there is still a third magnetic formation,
which seems to overlie unconformably the lower series, and is therefore
provisionally assigned to the Upper Huronian. This formation consists of
ferruginous schists, interstratified with layers of ferruginous fragmental
quartzite. It is generally much less highly inclined than the magnetic
rocks of the lower series as well as less rich in iron, and the disturbances
produced by it are correspondingly small.
Besides these rocks of sedimentary origin, with which this paper pro})-
erly deals, it may be mentioned that along the Fence River there is a
considerable area of metamorphic eruptives, which are often exceedingly
magnetic. These are restricted to a definite geological horizon, within
which the magnetic disturbances are remarkable for their complexity and
irregularity, no doubt as the result of a very irregular distribution of
magnetite and of the formations which chiefly contain it. The rocks in
portions of this belt outcrop freely, and the disturbances can therefore
easily be assigned to the proper causes.
SECTION III. THE DISTKIBUTION OF INIAGNETISM IN THE IMAGNETIC
ROCKS.
Magnetite occurs, therefore, in these Algonkian rocks in different
ways. In some instances it is mainly concentrated in nearly pure parallel
layers ; in othei-s, it is more or less evenly disseminated through non-
magnetic material; and still again it is present in both forms. Moreover,
'Relations of the Lower Menominee and Lower Marquette Beries of Michigan (Preliminary),
by H. L. Smyth: Am. .Jour. Sci., Vol. XLVII, 1894, pp. 217, 218, 223.
340 THE CRYSTAL PALLS IRON-BEARING DISTRICT.
these rocks have all been folded, more or less strongly, at more than one
period ; and wherever they are exposed, they are seen to be inclined to the
horizon, often at high angles, and to be traversed by intersecting sets of
joint-planes and cleavage-planes, some of which always cut the bedding,
and often have been the seat of the development of secondary minerals.
By the crossing of these various surfaces, the rocks are divided into small
unit masses, at the boundaries of which there is either an actual physical
parting or a break in the continuity of the magnetite.
It is well known that when a bar magnet is broken and the severed
ends are again joined, the two pieces do not unite to form one magnet, but
remain as two. It may be conceived, therefore, from the manner of distribu-
tion of the magnetite, and the secondary partings existing in these rocks,
that their magnetism is seated in an enormous number of small separate
magnets, at least one for each of the physicallj^ distinct unit volumes.
It is a fact of observation, as will appear hereafter, that the upper
surfaces of these magnetic rocks invariably attract the north end of the
compass needles and, of course, repel the south end. From this it must be
inferred that the small magnets are generally similarly oriented, and have
their north ends, which would repel the north end of the compass needle,
pointing downward, and their south ends, which attract it, pointing upward.
As this is the arrangement that would result from induction from the earth's
magnetism, it can be (joncluded further (as, of course, might be assumed)
that these rocks are magnetic from the earth's induction.
It is also well known that when sevei'al bar magnets are joined in line
at opposite poles, the effect upon a compass needle within the range of influ-
ence is nearly the same as if the joined magnets were replaced by a single
magnet of the combined length. For each member of the pairs of inter-
mediate poles, one attracting and the other repelling, is about the same
distance away, and their effects so balance each other. The result, there-
fore, is to leave one pole unchanged in position and to remove the other to
the end of the last magnet added. If enough magnets are added, the iinal
result is to carry the moving pole so far away that it has no appreciable
influence upon the needle. This is a condition which, from the distribution
of the magnetite and the parting surfaces which run through the magnetic
rocks, must always be realized more or less completely. It is a necessary
consequence of such an ai-rangement of the small magnets that, in the case
MAGNETIC OBSERVATIONS. 341
of a tliiu sheet of mao-netic rock lying at a low angle itf dip, the l)urie(l
north poles would not be much farther removed than the upper south poles,
and consequently the compass needle should be relatively only slightly dis-
turbed. This is j)recisely what is found to be the ease.
Thus there is firm ground for the conception of the magnetic rocks as
made up of sheaves of small magnets, all similarly oriented in a general
way and all having their south poles upward at or near the rock surface,
while the effective north poles, by the continual addition of similarly ori-
ented sheaves below, are carried down, when the rocks are vertical or nearly
so, to such depths that their influence is greatly diminished or altogether
imperceptible. In equal small areas the individual magnets are no doubt
of -very unequal number or strength. This can be proved by holding a
swinging needle close to the surface of a magnetic rock, shifting its position
without moving it out of the parallel plane and observing the changes in
the pointing. These are almost always large and are undoubtedly due to
the variations in strength of small areas of the upper poles. In consequence
of the law of magnetism, by which the attraction (or repulsion) varies
inversely as the square of the distance, the areas immediately surroiuiding
tlie needle are very much more important factors in the resultant than those
farther removed. When the needle is held higher up, or, what is the same
thing, the magnetic rock is buried, the effects are much more regular, since
a larger number of the unit areas enter into the resultant with equal weight
due t<^ equal distance, and the extremes of indi^'idual ^•ariation are lost in
the general mean. Since successive magnetic cross sections over bui'ied
rocks showr on the whole a great degree of regularity, we can finally con-
clude that the magnetic force of these rocks is seated in an immense, prac-
tically an infinite, number of small magnets, which furnish free magnetism
at the upper and lower bounding surfaces of the magnetic formation, and
that on the average there is about the same number, of about the same
aggregate strength, or, in other words, equal intensity in equal areas of
these surfaces, if the areas are taken large enough.
SECTIOIf IV. THE INSTRUMENTS AND METHODS OF WORK.
The instruments used in this work are simple and well known. The
dial compass is an ordinary compass, carrying a 2^-inch needle swinging
inside a cii'cle graduated to degrees, which is further supplied with a grad-
342 THE CRYSTAL FALLS IRON-BEARING DISTRICT.
uated hour circle. It is therefore a portable sundial. Tlie gnomon is a
thread, which is attached at one end to the center of graduation of the hour
circle near the rear sight and at the other to a point in the forward sight so
taken that the angle made by the thread with the plane of the hour circle
is equal to the latitude of the place. When this instrument is leveled and
set up in the meridian on a sunny day, the thread will cast a shadow on the
hour circle at the correct apparent solar time, from which mean time may
be determined by applying the equation of time. Conversely, if it is so set
up that the shadow of the thread falls on the correct apparent time, the
sights of the instrument are in the true meridian. In this position the
declination of the horizontal needle maj" be read off from the graduated
circle. At work, this instrument is mounted on a light Jacob's staif, or it
may be held in the hand. The Jacob's staff, although often inconvenient
to carry, is preferable, as with it the readings are all taken at the same
height above the ground and the leveling is more exact and steady. In
a correctly constructed instrument, Avith good time, the readings may be
made to half a degree. Coirectness in the time, however, is indispensable
to good work, and this is best secured by keeping a standard watch in camp
and referring the working watches to it daily.
The dip needle needs no description. In geological work that form
known as the Norwegian, in which the needle is pivoted on a universal
joint, is not so useful as the type in which the needle is rigidly confined to
one plane. In taking the readings, this plane in which the needle is free
to swing is made to coincide with the vertical plane determined by the
pointing of the horizontal needle. The circle is graduated to single
degrees, and with skillful work the readings ai'e reliable to one or two
degrees. It ma}- be added that the south end is weighted, in order, either
partly or completely, to balance the vertical component of the earth's force.
It was found better not to balance it completely, but only to such an extent
that the north end of the needle would dip about 10° (the graduation zero
being horizontal) in an area removed from local disturbances. It is no
doubt desirable that all the dip needles used in the same work should be
brought to approximately the same index error, in order that the readings
may be more directly comparable. In practice, however, it was found quite
impossible to keep our three needles in unison, on account of the rough
usage to which they were unavoidably subjected. As, however, the form
MA(}NET1C 015SEKVATI0NS. 343
of the (lip curves is veiil.y the subjeet souyht, and since these, in the pres-
ence of consideraljle disturbances, are sensiljly indejjendent of .small differ-
ences in the index error, it is not indispensable that the needles should be
exactly together.
These instruments are simple, and, of course, do not give precise results.
But the observations are rapidl}' and chea])ly made, and to a sufficient
degree of acctn-acy for the end in view. It may be stated again that the
object is to detect and compare relative magnetic disturbances, and to find
out the bearing of these disturbances on the distribution and attitude of the
rocks which produce them. For this purpose the instrmnents are exceed-
ingly well adapted.
The field work was canned out by parties of two men each, one of
whom, a skilled woodsman, carried along the line and observed the hori-
zontal needle, while the other read the dip needle, kept the notes, and
atteiided to the geology. According to the general plan of the field work,
a series of parallel lines was run either north and south or east and west
across each section. The direction of the lines of travel was chosen so as
to cut the strike of the rocks at the largest angle. The jn'obable direction
of strike for each day's work could be inferred in advance from what had
ji'one before. If it were more nearly north and south than east and west,
the traverse lines were run east and west, and vice versa. These directions
were in many cases not the most desirable for the magnetic work alone, but
the choice was practically limited by the lines of the United States Land
Survey, which give for each square mile eight points of departure (at the
four corners and four quarter posts of each section), which are generally
identifiable on the ground. On these lines of travel the instruments were
read at various intervals, from 6 to 10 or 100 paces, depending upon the
local complications. The intervals Ijetween the lines varied from one-
sixteenth to one-fourth of a mile, and were determined not onl)' by the
magnetic complications, but by the character of the surface, it being
especially desirable that the ground should be so closely covered that no
outcrop could escape detection. The distances along and off the lines of
travel were measured by pacing. The general accuracy of the pacing is
remarkable, and is essentially within the platting error of the scale of the
maps. The aA^erage closing error for August, 1892, during which about
100 miles' of traverse lines were run, was 20 paces per mile, or 1 per cent.
344 THE CRYSTAL PALLS IRON-BEARING DISTRICT.
Two-thirds of tlie errors averaged 10 paces per mile, or 1 in 200, while the
maximum was 1 in 30. But this was better thau the average for the season.
The observations at each station consisted in a reading of the horizon-
tal and dip needles. When there was no local magnetic disturbance, the
horizontal needle would come to rest in the magnetic meridian, which in
this region is about N. 2° to 3° E., or almost coinciding with the true merid
ian. The dip needle, when held in the same meridian, would indicate the
index error. When, however, disturbing mateiial was present, the horizon-
tal needle would point to the east or west of the magnetic meridian, at an
angle determined by the direction of the resultant of the horizontal com-
ponents of the earth's and the local forces. The dip needle would come to
rest in the same vertical plane, at an angle with the horizon determined by
the amount and direction of the three forces, the whole pull of the earth's
force, the whole pull of the local forces, and the balancing weight, and in
general would show a downward deflection. After making and recording
the set of observations at a station, the party proceeded to the next, and so
on to the end of the day. At the end of each day, or as soon as possible
afterwards, the day's work was jjlatted on a large-scale map, on which the
readings of the horizontal needle were represented by short arrows drawn
through the stations, turned east or west of the true meridian, as the case
might be, and carrying the amount of declination written at the arrow
point. The dij) observations were laid off to scale immediately below the
stations, measuring all from the same horizontal line, and the })oints thus
established were connected by a free-hand curve.
8ECTIOX V. FACTS OF OBSERVATION AXD GET^ERAIj PRINCIPI.ES.
I. OBSERVED DEFLECTIONS WHEN THE STRIKE IS NORTH AND SOUTH AND
THE DIP VERTICAL.
If a magnetic rock, striking north and south and dipping vertically, is
crossed by an east-and-west traverse, it is found, as the disturbing belt is
approached, say from the western side, that the horizontal needle points
toward the east of north, and that this easterly pointing gradually
increases to a maximum. Continuing east from the maximum point the
eastward declination decreases, and soon a station is reached at which the
needle points due north. Still farther east the declination changes to west-
ward, and soon thereafter reaches a westward maximum, beyond which
MAGNETIC OHSERVATIOTSrS. 345
again tlio westward pointing in its turn gradually decreases, until finally
the needle reaches its normal eastward declination, after passing through a
second zero. The dip-needle readings at the same stations generally
mcrease slowly at first, and then rapidly, and soon reach a maximum at the
first zero point between the converging arrows ; beyond this to tlie east they
decrease corresj)ondingly, so that the dip curve is symmetrical east and west
of the maximum These statements will l)e made clear bv a reference to
fig. 15, which represents an actual traverse in T. 45 N., R. 31 W.
2. DEFLECTIONS OF THE HORIZONTAL NEEDLE.
It is evident that in crossing- a rock belt which stretches away indefi-
nitely in both directions, only a limited part of it will affect the readings on
» 10 10!^ II IS 15 14 II 8K 0 13 7 93< II 13% ISii 13 11>J ■0>i
_b:p Cui V
.-^
Fig. 15.— Magnetic cross .sectiiin in T. 45 N., R. :il W.
a given cross section. Since the })ull of the poles of a magnet on a com-
pass needle diminishes with the square of the distance of separation, it
follows that the limits to the material that would noticeabl}' disturb com-
paratively insensitive instruments would soon be reached. If we consider
for the moment only the horizontal components, and call the distance a
(fig. 16) at which the needle would respond to the attraction of material
possessing the magnetic force of that with which we are dealing, then at
any station, P, the material inside a circle di'awn with P as a center and
radius a (shaded in the figure) would exert force on P, the material outside
would not. If the circle drawn from a station, P', does not reach tii the
magnetic belt, the needles at P' will not be disturbed.^
For reasons of symmetry, it is seen that the attraction of the magnetic
' The actual distances at which disturl)<atues of the Deedles can be detected are exceedingly
variable, since they depend (as will be shown hereuftei') not only upon the lithological character of
the magnetic formation, but also upon its strike, dip, thickness, extent, and nearness to the .surface.
One formation in which all the conditions are exceptionally favorable distinctly deflects the dial-
compass needle at a distance of 3^ miles.
346
THE CRYSTAL FALLS IRON-BEARING DISTRICT.
belt would act along the line P N, drawn through P perpendicular to the
strike of the rock. Since there is as much material on one side of this
line as on the other, the components perpendicular to it will balance each
other, and the instruments at P will be affected exactly as if all the attract-
ing material were concentrated along this line. The horizontal needle will
take a position in the line of the resultant of the two forces which act upon
it, namely, the horizontal component of the earth's magnetism (which acts
in the line of the magnetic meridian) and the horizontal component of the
material within ths circle of attraction (which acts along the line P N).
The force which de-
flects the needle from
its general local direc-
tion is the component
along P N, and it is
evident that the great-
er this component the
greater will be the de-
flection of the needle,
since the direction in
which it acts always
remains the same at
all stations for any
given direction of
strike of the rock.
For if ft is the strike
of the rock measured
from the north, and H and H' are the horizontal components of the earth's
and the rock force respectively, it is readily shown that 5, the angle of deflec-
tion of the horizontal needle at any station, P, is given by the equation:
Fig. 10.— Circli-s of attraction.
tan d rr
COS /?
H
(1)
E
^ ± sin /?
From this equation it is easily seen that, no matter how great may be
the horizontal component of the force of the magnetic rock, the horizontal
needle can not be deflected past the normal.
MAGNETIC OHSKRVATIONS. 347
. As 1' moves towM-iI the iiiiiyiietic belt the liorizoiital coinponent ut first
increases, nnd with it the westward deflection of the needle. Finally, the
niaximuin westward deflection is reached, beyond which tlie needle begins to
return; it is evident, therefore, that at this point the horizontal component
has reached a maximum value.
3. DEFLECTIONS OF THE DIP NEEDLE.
The balanced dip needle (i. e., without index error), in an area of
no local disturbance, is in equilibrium under the action of two couples,
namely, the vertical component of the
earth's magnetism and the added weight.
When displaced from the position of __ »*-^ —
equilibrium, the horizontal couple re-
stores it.
In fig. 17 let PP be a balanced Fig. n.-The rcees acing o„ ti,e .n,, «ee<iie.
dip needle which has been displaced
through the angle a. At the two poles the attraction and repulsion of the
earth's magnetism may be resolved into horizontal and vertical components,
H and V.
Taking moments about C, we have, if «=zO, the needle in equilibrium
under the couples,
V. 2h — m(/. a = 0,
where 2&rrPP, mg^the added weight, and a its distance from the center.
If this needle, so balanced, is carried to a station within the influence
of a magnetic rock, its dip will be determined by the composition of the
new forces with the old. The vertical plane will be that in which the hori-
zontal needle points at the same station. The equations above give us a
ready means of determining- the angle of dip in terms of all the forces.
Suppose the needle finally comes to rest at the angle a with the hori-
zontal (the north pole being depressed). Then
V,.. 2h . cos a — mga cos a — H^ . 26 . sin az=0, . . . (2)
where H^ and V^ signify the resultants of the horizontal, and vertical com-
ponents of the earth's and the local force.
348 THE CRYSTAL FALLS IRON-BEAEING DISTRICT.
Equation (2) is easily reduced to
—'-^h!^'' ■■....- (3)
If, however, the dip needle is not balanced, but has, where there is no
local disturbance, a constant index error 0 (measured from the horizontal),
it is readily seen that
In an area of local disturbance the angle of dip a is given by equation
(3). Since V and V always act in the same line.
Substituting this and the value of mga fi-om equation (4), equation (3)
becomes
±V' + Htan0, .,.
tan a =.-^ U== '- {p)
If the index error is 0' , and the corresponding deflection «', we have
tan a _± V + H tan 0
tana'"~±V' + H tan &
Therefore at the same station, the greater the index error the greater
is the angle of dip in the same or two similar instruments. It is also evi-
dent that the greater the vertical component of the pull of the rock, the
less will be the difference between the deflections in the two cases.
From an inspection of equation (5) it is seen tiiat tan a — qo, or
a:zi90° only when H,.— 0. H^ is, in general, given by the equation
H,= VH^+H''±2 H H' sin ^,
where /? is the strike of the rock measured from tlie north. H,. can there-
fore equal zero only when /?=^, or the rock strikes east and west, and at
the same time H'. is numerically equal to H, and acting in the opposite
direction. Dips of 90° can not occur in other cases, no matter how strong
MAGNETIO OBSERVATIONS. 319
the magiu'tic force ot tlie rock iiiay ho. It is also evident that iu <>eiieral
Hr has its iniiiimum vahie when H':= — H sin /?. When the rock strikes
north and south or /?rr(), 11^ is a minimum when H':rrO.
4. HORIZONTAL AND VERTICAL COMPONENTS WHEN THE MAGNETIC ROCK
DIPS VERTICALLY.
If we assume that the magnetic rock has a uniform strike in any direc-
tion, a vertical dip and a surface width or thickness equal to 2a, it is easy to
show that the horizontal and vertical components of the rock force are given
by the following equations, where x is the horizontal distance of the station
of observation from the middle plane of the formation, h is the depth of
surface covering, assumed to be uniform, and a? is a constant.
GO -'^^li^J^(x-af ^^
^'-'> ^tan->^±^-tan-i'-^^' J (7)
GO
TT/
In equation (6 ) — rr 0 when ;r — 0 ; therefore a point of no deflec-
GD
tion of the horizontal needle is found vertically over the middle jjoiiit of the
magnetic rock. It is also evident that at corresponding stations on opposite
sides of the middle point, the horizontal components are equal, but act in
opposite directions.
To obtain the points of maximum or minimum values of the horizontal
component, we differentiate the right-hand side of equation (6) with respect
to X, and place the result equal to zero. This gives
irrr±V'F+^2 (8)
which determines two points, at equal distances from 0 on opposite sides of
the rock, at which the horizontal component has maximum values. Writing
for X the measurable distance d, and squaring, we have
d''=}i'+a^ (9)
The thickness of the magnetic formation is therefore always less than
the distance between the points of maximum horizontal deflection, except
when 7irzO, or the rock is uncovered, in which case the thickness and sepa-
ration of the maxima are the same.
350 THE CEYSTAL FALLS IRON BEARING DISTRICT.
By differentiating tlie right-hand side of (7), with respect to x, it is
easy to show that V has a maximum value when .czrO. When the rock
strikes north and south, this also corresponds to a minimum value of H„ as
has already been shown; and, therefore, by a reference to equation (5) it
is readily seen that a point of maximum dip coinciding with a point of no
horizontal deflection is in that case found over the middle plane of the
bui'ied magnetic rock.
Where the strike is inclined to the meridian, the points of maximum
dip and zero deflection will not coincide, since the maximum value of V
does not occur at the same station as the minimum value of H^. As has
already been shown, H^ is a minimum when H'=H sin fi (/? being the ang-le
of the strike), and this is in general on the side of the rock on which the
angle made with an east and west traverse is obtuse. The point of maximum
dip will be situated on the same side of the rock between this station and
the point of no horizontal deflection, and will approach the latter as the
strike approached the meridian, and also as V increases relatively to H'.
With strongly magnetic rocks the points of no deflection and maximum dip
practically coincide on maps platted to the scale of 4 inches to the mile,
except where the strike is nearly east and west.
5. HORIZONTAL AND VERTICAL COMPONENTS WHEN THE MAGNETIC ROCK
DIPS AT AN ANGLE.
Under the last heading it was assumed that the magnetic rock dips
vertically, and that it continues indefinitely downward at this angle; In
consequence of this assumption, and also of the conception of the manner
in which magnetism is distributed through magnetic rocks, it has been con-
cluded that the north poles of the rock, which repel the north end of the
compass needle, are situated so far below the surface that their effect may
be neglected. Therefore we have taken into account only the south poles,
which are situated at the rock surface.
In the case of rocks which do not dip at high angles this assumption
can not safely be made, and the influence of the bottom poles must be
taken into account. Since the force of these poles acts in opposite direc-
tions from that of the upper poles, and since they are more deeply buried,
it would seem that their influence in general must be to diminish the total
force which acts upon the needles at any station, and therefore that the
MAGNETIC OBSERVATIONS, 351
deflections botli of tlie horizontal and dip needles caused by the same rock
should be less in amount, ceteris paribus, where that rock dips at a low
ang-le than wliere it dips at a high angle.
In tlie course of the field work certain peculiar deflections of the
needles were encountered in traverses across rocks dipping at moderate or
low angles. These were not thoroughly understood at the time, but the
cause was believed to be connected with the angle of dip of the rock. For
example, it was found along traverses crossing certain north-and-south-
striking rocks, which were known to have a westward dip that may have
been either high or low, that the two points of maximum deflection of the
horizontal needle were not situated at equal distances from the point of no
deflection between them, but that the distance of the western maximum
was much the shorter. It happens in this region that no east-dipping rocks
occur which are so far removed from other magnetic formations as to be out
of range of their possible influence, but, so far as they go, traverses across
these showed that the nearer maximum was situated on the eastern side of the
point of no deflection. It therefore seemed probable that the cause of the
inequality in the distances from the zero point to the maxima was the dip
of the rock, and that the dip was in the direction of the nearer maximum.
If the magnetic formation has a surface width nz b, is uniformly buried
to the depth h, and dips at the angle ^, then, if A — tan z/, it may be
shown that the horizontal and vertical components at any point P, the hori-
zontal distance of which from the lower edge of the formation is x, are
given by the following equations:
H' _ \' , h'+x- 2 A
S 4. -1 x—h — Xh , ,x — Xh
< tan '- — — =- — tan^
h >
Xx—Xl) + h Xx+li] ^^^^
-1
— - =2 j tan-i^-tan-i^
w ( h h
V+x"" . 2
+ i-nr2lo&i
tan-Kli^tML±^)_tan- i+ii^ [. . . (11)
A-(l+u;) Ji—XJjj^x x)
352 THE CRYSTAL FALLS IRON -BEARING DISTRICT.
If A:iicc> , and the coordinates are referred to axes in the middle of the
rock, these equations reduce to equations (6) and (7).
By differentiating the right-hand side of equation (10), placing the
result equal to zero, and solving for x, the positions of the stations at which
H' is a maximum may be determined. This gives:
■^-"^^=^ 2l ^ ^^
Calling the difference of the roots, or the measurable distance between
2a
the maxima, 2d, and substituting for h its value -■ — -7- 2a being the true
thickness of the rock, we have:
d2_^jL+^' ■. . . . (13)
sm ~/l
For rocks of high dip, therefore, the distance between the maximum
points is but little greater than it would be were the dip vertical, and it
increases inversely as the angle of dip.
A general algebraic determination of the points at which H' is 0 and
V is a maximum is impossible, since it involves the solution of equations of
a degree higher tlian the fifth. However, hj assuming numerical values
XT' Y'
for A, h, and a (or li) curves expressing the relations between — and — and
GO GO
X can be plotted, from which the maximum and zero points can be deter-
mined in any desired number of special cases.
Let us first assume that A zr 3 (or that the rock dips at an angle of about
70° 34'), I1 — 2, and a — 6. The ordinates to the curves of fig. 1, PI. XLVII,
TI/ Y'
give the values of — and — corresponding to different values of x. The
GO GO
ordinates to — do not reijresent the deflections S of the horizontal needle
GO
from the meridian, but quantities that are connected with those deflections
by equation (1). The deflections, however, vary as H' varies, and will have
maximum and minimum values at the same points.
From this figure it appears, first, that the nearer maximum is situated
on the dip side of the rock; secondly, that the point of no deflection is not
over the middle plane, but is nearer the upper edge; thirdly, that the hori-
zontal force of the rock is numerically less at the nearer than at the more
U. 6. OeOLOGlCAL SUXVE^
MONOGRAPH XXXVI PL. XLVII
(^)
(^)
\,
RELATIONS OF MAGNETIC BEDS TO VARIATION AND DIP.
MAGNETIC OBSERVATIONS. 353
distant m;ixinium, and, fourthly, that tlie distance between the maximum
points is nearly the same for the inclined I'ock as for the vertical.
In I'l. XLVII, fi<>-. 2, the constants have the same numerical values as
before, e.\ce]it //, which now zr4 instead of 2. The rock is thus bm-ied to
twice the depth of the former case. The same conclusions are true for this
case as for the first. The zero point is still nearer the upper edge of the
rock, and the maxima are farther apart.
Let it next be assumed that A = 0.5 (or that the rock dips at an angle
of al)out 26° 34'), /(rr2, and (i-^6. These data lead to the curves of
PI. XLVII, fig. 3, in which, as in the case of the rock ot higher dip, the
maximum })oints are unsymmetrical to the }joint of no deflection, the nearer
lying on the dip side.
In PI. XLVII, fig. 4, we have a rock of the same thickness and dip at
a depth /;^4; and the same conclusions hold true.
From these four curves, which represent formations dipping at high
and moderately low angles, and buried to depths which are in the one case
small and in tlie other great, relative to the thickness, it is probably safe
to draw the following general conclusions:
(«) The direction of dip of a magnetic formation is toward the nearer
and (for north-and-south-striking rocks) the numerically smaller maximum.
(li) The point of no deflection between the converging maxima is not
situated over the middle plane of the formation, but is nearer the upper
edge. But with increasing depth and diminishing angles of dip, this point
may pass beyond the upper edge.
(c) With slightly inclined rocks, for moderate deptlis of siu-face cover-
ing, the disturbances are spread out over a much wider zone on each side,
and the maxima are less sharp, particularly the maximum on the dip side.
Under these 'circumstances irregular and anomalous deflections would be
expected in practice, as will be seen in the following sections.
(d) The curves of the vertical component show maximum values near
the zero value of the horizontal component only in the case of the rock of
high dip. In the case of the rock of lower dip, the vertical component has
a negative value, or is directed upward over a wide zone on the side of the
rock opposite to the dip side. Over this zone the readings of the dip needle
will be less than normal, or even negative if V > H tan 9. This is in
accordance with the facts of observation.
MON XXXVI 23
354
THE CRYSTAL FALLS IRON-BEARING DISTRICT.
It is also interesting- to determine the relative values of the horizontal
components at the two maximum points, for different angles of dip, and for
different ratios of /; to a — that is, for different depths of burial. Fig. 18
shows in graphical form these relations for all angles of dip between 90°
and 0° for seven different ratios between h ^ ^7. and h =z 4«. The ordinates
50
to the dotted curves (concave upward) give the relative values of the hori-
zontal component at the maximum point on the dip side (B maximum);
those to the full cur^'es (convex upward) the values at the maximum point
90^ 80" 70J 60° 50° 40° 30° 20o IQo 0°
Flo. 18.— Curves showing the relations between the horizontal components at the points of luaximum deliectioo, lor rocks
dipping at various angles and buried to various depths.
on the other side of the rock (A maximum). These curves show that the
horizontal component at the B maximum is always numericallv much less
than at the A maximum, and that for moderate depths of burial it diminishes
very rapidly at both points with small ang-les of di]).
6. DETERMINATION OF DEPTH.
The relations between the dip and thickness of a magnetic rock, the
distance between the horizontal maxima, and the depth of covering are
given in equation (13). By assuming numerical values for ^, and either
for a, the half thickness of the rock, or for r/, the half distance between the
maximum points, it is easy to plot the curve which expresses the relations
ma(ini<:tic observations. 355
between the other two quantities. It' d is taken as tlie constant, the equa-
tion re})reseuts a circle; it' (i, it represents a hyperbola.
This e(iuati(in wvax have a useful application in making it possible to
judge, in advance of actual test pitting, of the probable depth of surface
covering over a magnetic rock, for which the original assumptions are ful-
filled, and the numerical values of ^, «, and d are determinable. It will be
remembered that the assumptions upon which equation (13) rests are the
fundamental ones of Section III, and also that the rock has a uniform
strike. In practice, the uniformity of the strike can be established Ijy other
traverses on each side of the one in question, and J, the angle of dip, may
usually be determined by the outcrop of other formations in the same
series.
For any jjractical application it is also necessary that a (half the
thickness of the rock) and d (half the distance between the stations at
which the horizontal component is a maximum) should be known. From
an inspection of the equation it is evident that any close determination of
h, except for great depths of covering, depends upon very precise knowl-
edge of the ratio between a and d. The practical difficulties in the way of
the measurement of a and the ever-present probability that the rock may
vary from point to point, not only in actual but in effective magnetic thick-
ness (which is what a actually signifies), make it clear that for the most
part h can only be found approximately. Also, in the case of a rock
striking due east and Avest the methods fail, from the fact that 2d can not be
determined on the ground.
The determination of h is therefore hedged in with important limita-
tions; yet in many cases the information supplied by the equation may be
very useful. The difficulties in the way of measuring a are disposed of in
the event that along one traverse on the strike of the rock li is known, as it
may be, by the sinking of a test pit. This value of li at once gives a value
of a, which may be used on other traverses across the same rock with much
more accurate results, in the lack of disturbing factors, than if a were
known only by measurement It should be added that when the traverse
crosses the strike of the magnetic rock at the angle ,)', the distance d meas-
m.*ed on the line of the traverse must be multiplied by sin y in order to get
the value of d to be ixsed in the determination of /;, and also that h must be
. corrected for the height of the instrument.
356 THE CRYSTAL FALLS IRON-BEARING DISTRICT.
More general inforinatiou as ti) relative depths of Ijurial is also given
by the dip curves. It is easily seen that where the superficial covering is
small the vertical component of the rock force must remain small, except
immediatel}' over the rock. This condition is, therefore, indicated bv steep
slopes in the di^) curves. On the other hand, where the depth of covering
is considerable, the vertical component increases slowly and steadily, Ijegin-
ning at stations at a distance from the rock, and the resulting dip curve
approaches the maximum with gentle slopes.
7. SUMMARY.
(1 ) The strike of a magnetic rock is given by the line joining the points
on successive traverses, at which the horizontal needle is not deflected from
the local meridian between the converging aiTOws, or at which the dip
angles are a maximum. When the rock is vertical, this line lies in the middle
plane of the rock and fixes its position. It may be called a line of mag-
netic attraction.
(2) The dip of. a magnetic rock is toward the nearer horizontal
maxinuini.
(3) The thickness of the magnetic formation must, if buried, always
be less than the distance between the maximum points.
(4) Where the superficial cover is not very great, a change in the dip
of a magnetic rock from moderate or high angles to low angles is attended
with a rapid decrease in the values of the horizontal component, with a
corresponding decrease in the deflections of the horizontal needle.
SECTION VI. APPLICATIOT«rS TO SPECIAIL, CASES.
In the preceding section certain general conclusions have been estab-
lished with regard to the relative positions of the stations at which the
horizontal and vertical components of the force of a magnetic rock have
maximum and zero vakies. The deflections produced by these comjjonents
from the ^Jositions which the magnetic needles assume under the action of
the earth's force have maximum and zero values at the same stations at
which the components have maximum and zero values, and therefore the
conclusions as to the relative positions of these points are true for any
anffle of strike. But certain numerical relations between the deflections
depend upon the orientation or strike of the magnetic formation and ujion
the direction of dip, and these will now l)e considered.
MAGNETIC OBSERVATIONS. 357
1. THE MAGNETIC ROCK STRIKES EAST OR WEST OF NORTH AND DIPS
VERTICALLY.
Let US first take the case of a rock strikino' east of north. At the
stations within range of the h)cal influence on the east side of such a rock
belt the liorizontal needle is pulled west of the meridian, reaches a west-
ward niaxiiuum, tlieii points north, then on the west side of the belt, east
of the meridian, and reaches an eastward maximum. It is observed, how-
ever, that the westward deflections on the east side of the belt are generally
not so great as the corresponding eastward deflections on the west side of
the belt. The reason for this is easily seen. At each station east of the
belt the local })ull acts along the normal to the belt di-awn through the
station. This normal makes with the local magnetic meridian an acute
angle. The needle will come to rest within this acute ang-le alonar the line
of the resultant of the horizontal components of the two forces, the earth's
and the local force, which determine its position. However strong the
local pull may be, the horizontal needle can not be deflected past the
normal.
At the corresponding stations on the west side of the disturbing belt
the local pull also acts along the normal from the station to the belt, and
has the same numerical value. But in this case the normal makes an
obtuse angle with the magnetic meridian. For two points equall}' distant
from the magnetic belt, one on the east and the other on the west, the
resultant for the western point will, therefore, make a larger angle with the
meridian than that for the eastern.
On the other hand, when the rock strikes west of north, it is observed
that the horizontal deflections are greater on the east side than on the west,
and the explanation is entirely similar to that given above.
The dip-needle observations at the same stations show general })he-
nomena quite like those in the case in which the strike of the rock coincided
with the meridian. They gradually increase to a maximum near the sta-
tion, where the horizontal needle stands at zero between the converging
arrows, and gradually decrease from this maximum on the other side. It .
is noted, however, that the readings are not equal at corresponding stations
on opposite sides of the maximum. When the strike is east of north, the
western station shows a higher dip than the eastern; when the strike is
358 THE CRYSTAL FALLS IRON-BEARING DISTRICT.
west of uorth, the eastern station shows a higher dip than the corresponding'
station on the west. Generally stated, then, the stations on that side of the
magnetic rock on which the angle between the strike of the rock and the
line of traverse is obtuse show greater dip angles than the corresponding
stations on the side on which this angle is acute. As the angles of dip are
represented graphically by a continuous curve, this is the same thing as
saying that the dip curve is steeper on the side of the acute than on that of
the obtuse angle.
These facts are easily explained by the following considerations. The
vertical components tend to lower the needle, and would cany it to a ver-
tical position except for the action of the horizontal forces, which tend to
keep it horizontal. At any station on the acute-angle side of the magnetic
belt the resultant of the two horizontal components is larger than at the
corresponding station on the obtuse-angle side, the two being represented
by the longer and shorter diagonals of a parallelogram. Since the vertical
forces are the same at the two stations, it follows that on the obtuse-angle
side the angle of dip must be larger than on the acute-angle side. Or,
expressed algebraically, since H,. is the only variable on the right-hand side
of equation (5) it is evident that tan a, and therefore a, the angle of dip,
increases with a decrease in H,.
If the rock dips at an angle less than 90°, these results are either
intensified or gi-eatly modified, depending upon the direction of dip. It
4 was shown in the last section that the horizontal component of the rock
force is smaller on the dip side. If the strike and dip are both toward the
same side of the meridian (e. g., if the strike is northwest and the dip south-
west), it is evident that the numerical difference between the deflections of
the horizontal needle on the two sides of the rock will be still greater than
if the rock were vertical. On the other hand, if the strike and dip are
toward opposite sides of the meridian (e. g., if the strike is northeast and
the dip northwest), the diff"erence between the deflections on the two sides
is less than for a vertical dip, or may even be reversed.
The deflections of the dip needle in the case of rocks dipping at angles
less than 90° are also greatly influenced by the direction of dip. If strike
and di}) are toward the same side of the meridian, the diff'erence noted
above in angle of dip on the two sides of the rock is neutralized, and the
dip curve tends to become symmetrical; while if thev are toward opposite
MAGNETIC OBSERVATIONS. 359
sides of tlu' meridian, the difference is increased. It is to be noted that the
intiuencc on hotli instnnnents of tlio direction and angle of dip of the rock
becoini's weakened with an increase in surface covering.
2. THE MAGNETIC ROCK STRIKES EAST AND WEST.
When a vertically dipping magnetic rock strikes east and west, or
nearly so, the traverse lines must be run north and south so as to cross it
as nearly as possible at right angles. In approaching such a belt from the
south the instruments give little warning. The readings of the horizontal
needle .show either no deflections, or else very slight deflections, from the
mairnetic meridian. Past the middle of the foi'mation the horizontal needle
is strongly deflected, often through an angle of 180°, so that it may point
due south. But as the magnetic rocks having this strike which were
encountered in our work were not deeply buried, and had also quite irregular
upper surfaces, generally the needle pointed either east or west of south on
account of the weight which the nearness to the surface gave to the adja-
cent material, either from the irregular distribution of magnetite or from
the protrusion of small masses above the general level. Continuing noi-th,
the horizontal deflections gradually diminish and eventually disappear.
The behavior of the horizontal needle is explained in the same way
as in the preceding cases. The position of the needle at any station is deter-
mined by the resultant of the horizontal components of the two forces — the
eai-th's force and the rock force — that act upon it. South of the magnetic
rock these two components act in the same direction and essentially in the
same line, since the magnetic meridian practically coincides with the true
meridian. The resultant, therefore, is equal to their sum and coincides with
them in direction, and consequently there is no deflection. North of the
magnetic rock the two horizontal components act in opposite directions, and
when they are in the same line the needle takes up its position in the direc-
tion of the greater, which determines that of the resultant; when H' is
greater than H (which often happens near and north of the rock) this direc-
tion is due south. When the two components do not act in exactly the
same line, the needle will point east or west of south at an angle which
depends on the angle between the two forces and their ratio.
Still farther north the horizontal component of the rock force dimin-
ishes rapidly and we consequently first pass tln-ough a zone of large and
360 THE CRYSTAL FALLS lEOX-BEAEING DISTRICT.
diminishiug deflections to the east or Avest, depending on the side of the
meridian on which this component falls ; and finally, when it becomes
insensible, the needle rests again in the meridian.
In the case of a rock striking east and west, the points at which the
horizontal component of the magnetism of the rock has maximum values
become indeterminate by the methods hitherto described, from the fact that
throughout the traverse the two components act in or nearly in the same line,
and the deflections from the local magnetic meridian, therefore, do not
indicate the relative strengths at difi"erent stations of the horizontal com-
ponent of the rock force.
The dip-needle readings for an east-and-west-striking rock are as fol-
lows: At some distance south of the rock the angles are constant at the
index error. As the rock is approached, the angles of dip depend upon the
depth of burial. If the surface covering is considerable, an increase in the
dip angles begins at a considerable distance away, and progresses continu-
ously as the magnetic belt is approached. If the rock is near the surface,
the dip needle shows either the constant index error or else angles of dip
less than the index error for all stations south except those very near the
southern margin of the rock. The maximum reading is attained north of
the middle plane of the rock, at a distance from it which also depends upon
the depth of covering. Farther north the dip angles decrease slowly and
are in general greater than at the coiTesponding stations south. The form
of the dip-curve, therefore, shows a steeper slope south of the magnetic
rock than north of it. The reasons for these differences will be evident
from the following considerations.
Let it be supposed, for the sake of simjjlicity, that throughout the
north-and-south traverse the two horizontal components act in the same line
in the meridian. At any station south of the magnetic rock they act in the
same direction, and their resultant will be their numerical sum. At the
cori'esponding station north they act in opposite directions, and their result-
ant will be their numerical difl^erence. The angle of dip is giveii by
equation (5):
V'+ H tan &
tan a rz !— yt
For the two corresponding stations, V will be the same. The other
quantities are all constants except H^. For the south station H^rz H' -f H;
MAGNETIC OBSERVATIONS. 361
for t\\v north station, 11^ = 11 — 11', where 11 ;iufl H' are, respectively, the
horizontal components of the magnetism of tlie earth and of the rock, as
before. The numerator of the right-hand side of the etjuation will be the
same for both stations, while the numerical value of the denominator will
be less for the north station than for the south. Consequently tan a, and
therefore «, will l)e greater for the north station.
For great depths ' of superficial covering, however, these differences
become almost imperceptible, owing to the fact that H' is so small that
Hr is essentially tKe same at the two con-esponding stations. The tendency,
therefore, as h increases is for the dip curve to become symmetrical.
In the special case in which H'r= — H, H,.=: 0, and the dip needle stands
at 90°. This can only take place north of tile rock, and may, depending
on the strength of H', be found at two stations, one on either side of the
station at which H' is a maximum. At the same stations the horizontal
needle is not acted on by any unbalanced force, and rests indifferently in
any position.
The dips less than normal which are often observed at stations south
of a magnetic rock which lies A^er}^ near the suiface are also easily under-
stood by a reference to equation (5). At these stations the resultant pull
of the rock is so nearly horizontal that the vertical component V is very
small in comparison witli the horizontal component H'. If V is a negligible
quantity, equation (5) becomes
TT
tan a z= jj — Yn ■ *^^^ ^
rl-|-xl
In sucli cases the angle of dip is therefore less than the index error. With
north or south dipping rocks, where V is negative, tan a becomes negative
when V'>H tan 0.
3. TWO PARALLEL MAGNETIC FORMATIONS.
The cases so far considered have involved only one belt of magnetic
rock, which has been assumed to ha^•e a uniform dip in one direction, or, in
other words, to be a monocline. In practice, however, owing to complexi-
ties of structure and other causes, which will be considered hereafter, it
frequently happens that two or more approximately parallel belts are Ibund
within the range of one another's influence. Under these circumstances
362 THE CRYSTAL FALLS IRON-BEARIKG DISTRICT.
the effects produced upon the magnetic needles are corresponding!}^ com-
pHcated.
For the purposes of ilhistration it is sufficient to consider a few extreme
H' V
cases, and to represent the vahies of — and — for these graphically. Let
it tirst be assumed that the two parallel belts are vertical, that the distance
between them is 8, and that a^3, and /?zr2. This represents the conditions
when the distance of separation is large compared Avith /;. The ordinates to
XT'
the curve of PI. XL VIII, fig. 1, give the values of — which correspond to
the different stations of observation. Those parts of the curve which are
above the horizontal axis of •coordinates represent the portions of the trav-
erse in which H' is directed toward the west; the parts below, those in
which it is directed toward the east. It is seen that besides the middle point
there are two other points of no horizontal deflection, which do not exactly
correspond with the points A'ertically over the middle of the magnetic forma-
tions, but are somewhat nearer the adjoining edges; and also four points of
maximum deflection, one on each side of each rock. The maximum points
inside of the two formations have smaller deflections than those outside, as
shown by the relative lengths of the ordinates to the curve, and also
between the inside maximum points the horizontal components are directed
away from the middle point.
V
The curve of — , represented by the dotted line, has two maximum
valvies, which fall nearly over the two rocks.
If next we assume that the distance between the rocks is 8, and that
fflrz3, and h-=8, we obtain the curve of PI. XLVIII, fig. 2, which represents
TT/
the value ol — when h is large compared with the distance of separation.
This case shows but one point of no horizontal deflection between the two
rocks and but two points of maximum deflection, one on the outside of
each.
V
The cvirve of — , represented by the dotted line, shows the interesting
feature of three maximum points, one at the center and one over each
rock. If h were relatively a little greater, these would evidently coincide
at the center.
U. 8. GEOLOGICAL 8URVEV
MONOGRAPH XXXVt PL. XLVIII
RELATIONS OF MAGNETIC BEDS TO VARIATION AND DIP.
MAGNETIC OBSERVATIONS. 363
It" the two parallel toriuatious are not vertical, l)ut dip in tlie same
direction at the same angle, the resulting curves are somewhat ditlerent.
I'l. LXVIII, tigs. 3 and 4, show two cases in which the elements are the
same, excejit the depth of covering and the thickness of" the intervening non-
magnetic material. Here the rocks dip at an angle of 71 ^ 34'. and the width
at the rock surface is 6.3 for each.
In fig. 3, PL XLVIII, where Ji-=2 and the width of the nonmagnetic
bed is 10.7, and the covering is, therefore, relatively small, the presence of
two rocks is distinctly shown by the curves of both components, and the
chief result of their interaction is to introduce an additional point of no hori-
zontal deflection between them, on each side of which the horizontal arrows
diverg-e. The positions of the maximum and of the other zero points are
hardlv disturbed, and consequently the direction of dip is very clearly
indicated.
In fig. 4, PI. XLVIII, where /?rr4 and the formations are separated by
nonmagnetic material 4.7 wide, there is but one zero point, nearly over the
middle of the upper formation, toward which the pointings of the hori-
zontal needle converge. West of this are two points of maximum eastern
deflection, between which a faint minimum represents the backward pull of
the lower formation.
If the two magnetic formations are parallel in strike, but dip toward each
other at equal angles, the resulting curves of the two components are shown
in PI. XLVIII, figs. 5 and 6. Fig. 5 illustrates the eff'ects on a s^Tichne
with steeply dipping sides, the supei-ficial covering being relatively shallow.
These conditions result in a point of no horizontal deflection over the mid-
dle of the trough with diverging arrows on each side, and besides a point
of no horizontal deflection over each rock, toward which the aiTows con-
verge. The positions of the two maximum points for each rock, and of the
zero between them, is nearly the same as if the other rock were absent, and
conseqiiently the fact that the rocks dip toward each other is clearly
indicated by the unsymmetrical distances.
In fig. 6, PI. XLVIII, the depth of the rock surface is much greater
relatively to the inside distance between the legs of the syncline, and the dip
is flatter. In this case there are but two points of maximum deflection, one
on each side of the syncline, and but one point of no deflection, over the
middle of the trough. The maximum points represent the outside maxima
364
THE CEYSTAL FALLS IRON-BEARING DISTRICT.
of the former case, and the result of the interaction of the two legs is to
increase the numerical values of these, as" well as to bring them nearer
too-ether. It is evident that the deflections of the horizontal needle in this
case could hardly be distinguished from those that would be produced by a
single vertical fonnation buried to a considerable depth.
Let it next be supposed that the two rocks dip toward each other at
different angles, the strikes remaining parallel, and also that the rock of
lower dip is buried to the greater depth. This, then, is a case in which the
magnetic effect of one limb of the syncline is much stronger than that of
the other.
In PI. XLVIII, fig. 7, the curves of the two components are given for
the special case in which the right-hand limb of the synclinal dips at an
angle of 90°, and has a surface covering /i=r2, while the left-hand limb
dips at an angle of 26° 34', and has a surface covering h=i4:. It is interest-
Fig. 19 — Truucated anticlinal fold with gently dipping limbs.
ing to compare the theoretical results of this figure with the curves of PI.
XLVIII, fig. 8, which represent deflections actually observed, and not
components.
In the latter figure the strike of the two rocks is represented by the
heavy lines. The two rocks are the same formation, brought up by folding
on opposite sides of a synclinal trough. The synclinal is slightly pushed
over, so that the eastern rock dips nearly vertical, while the western has a
much lower dip toward the east, and is also more deeply buried. These
facts rest on independent evidence, yet they might all be inferred from the
observations recorded in this figure.
The dip curve in this case shows two distinct maxima, a smaller under
MAGNETIC! OKSERVATIOXS.
865
the zone of retardation and a lar«iCT over tlie ))oint of no liorizontal deflec-
tion, uliicii correspond i-espeotively to the two niaji'netic i-ocks.
If the two formations are parallel in strike, l)nt diii away from each
other, the cnrves of the horizontal and vertical comjjonents for different
anjrles of dij) and different relations of thickness and depth of coverino- are
shown in figs. 19 and 20. In fig-. 19 the formations are widely separated,
k is relatively small, and the angles of dip are equal and low; the inter-
action of the two rocks therefore extends over a narrow zone only, and the
curves of the components clearly indicate the presence of two formations
and the direction of dip of each. In fig. 20 the anticlinal is so truncated
that magnetic material occupies the whole space on the rock surface
between the outer boundaries
of the two formations. The
angles of dip are equal, and are
hig-her than in the preceding
case, while the depth of cover-
ing is relatively much greater.
The horizontal component is
zero in the axial plane of the
anticlinal, and has maximum
values at two points, one on
each side of the zero. The ver-
tical component is a maximum
at one point, also in the axial
plane. The deflections pro-
duced by these conditions could
not be distinguished in practice from those produced by a single vertically
dipping fomiation.
In general, therefore, when two magnetic formations lie within range
of each other's influence, the deflections are determined by the relative
magnetic strengths of the .two rocks, hj their distance apart, by their strike
and dip, and by their depth of burial. It is evident that for certain given
relations among these factors the special cases above described will occiu-,
and it is found that they really do occur in practice. For other relations it
is not possible to make a general statement either as to the number or the
position of the maximum and mininuini points.
rig, 20.— Truncated anticlinal fold with steepl.y dipping limbs.
366 THE CRYSTAL FALLS lEON-BEARING DISTRICT.
SECTION VII. THE INTERPRETATION OF MORE COMPIjEX STRUCTURES.
The existence of two parallel belts of magnetic rocks may be accounted
for geologically in more than one way. They may represent two distinct
formations occurring at different horizons in the same series, or they may
represent the same formation either duplicated by folding or faulting or
separated into two parts by the intrusion of a sheet of igneous rock parallel
to its bedding. Since, then, two magnetic lines, the existence of which has
been established b)' observation, ma}' have more than one interpretation,
the discrimination of these cases, when possible, is of special importance.
The question whether any given case belongs to the first of these cate-
gories can generally be settled only by following the lines of attraction into
a district which affords a geological section across the formations involved,
or by the occasional outcrop of the rocks which give rise to the disturl^ances,
in which case lithological resemblances or differences, the relations to other
formations, and the observed structure will decide the matter one way or
the other. In the special case in which either or both lines can be followed
completely round an anticlinal dome or a synclinal basin, which of course
can only rarely happen, the question would be settled affirmatively, e^^en if
outcrops were entirely lacking.
In the other instances the magnetic observations themselves often give
means of discrimination, even when the outcrops are so few or so obscure
as to be in themselves indecisive. It is characteristic of the folds in the pre-
Cambrian rocks of this region that the axes are not usually parallel with
the horizon for long distances, but are often inclined to it; in other words,
when followed for greater or less distances they pitch. The outcropping
edges of any formation involved in an anticlinal or synclinal fold which has
been cut by a plane of denudation will be parallel to each other wherever
the axis of the fold is horizontal, but will approach each other where the
axis is inclined. In an anticlinal fold they converge in the direction in
which the axis sinks, while in a synclinal the}" converge in the direction
in which the axis rises. If the formation is a magnetic one, conformably
placed between beds of nonmagnetic character, the magnetic lines to which
the outcropping edges give rise will therefore run parallel to each other
when the axis is horizontal and will converge or diverge when the axis
pitches. The convergence or divergence takes place gradually, since the
angles of pitch usually are not large.
MAGNETIC OBSERVATIONS.
367
111 the case also of a «iu<>-le tbrinatiou which stands on edge and has
been spUt b}- the intrusion of a sheet of eruptive rock parallel to the bed-
ding planes, the magnetic observations will often show two parallel lines,
which, at the extremities of the eruptive rock, where it wedges out, merge
into one.
In general, therefore, two parallel magnetic lines which represent two
distinct formations preserve their identity, and do not pass into each other;
when, however, they represent the same formation, they will often come
together if followed far enough. The principles which have already been
applied to the analysis of simpler cases are useful in discriminating among
the three cases of convergence.
I. PITCHING SYNCLINES.
Let us first consider a pitching synclinal fold, which is repi*esented in
plan and by successive cross sections in fig. 21. It is evident that on the
lines of traverse along Sections
I and II the deflections of the
needle will observe the usual
sequence for two parallel belts,
the details depending upon
separation and depth of cover-
ing, while on lines along Sec-
tions III, IV, and V the phe-
nomena will be those caused
by a single belt of magnetic
rock. Also, on account of the
rise in the axis, the south poles
of the rock are brought continually nearer the surface on these successive
cross sections, and therefore the two components of the rock force will be
smaller for each traverse than for the one preceding. Since the magnetic
material comes to an end at A, it is no longer true that there is as much
magnetic material on one side of these sections as on the other. Conse-
quently the horizontal component due to the pull of the rock does not
become zero at any point along these sections, but for every station has a
positive numerical value and acts in the general direction in which the syn-
clinal pitches. At the station in the plane of symmetry of the fold this
V^
CRCSS CCCTIONS
Fig. 21 . —Plan and cross sections of a pitching syncline.
368 THE CRYSTAL FALLS IRON-BEARIKG DISTRICT.
component acts parallel to the axis. The direction and amount of the deflec-
tion depend upon the direction of strike and pitch of the synclinal.
Let us suppose, first, that the axis of the synclinal strikes north and
pitches north. In this case Section I, in fig. 17, is the most northern,
Section V the most southern.
The traverses along Sections I and II display the usual phenomena
for two parallel belts. East of the eastern limb and west of the western
the horizontal needle will be deflected toward the syncline. Between the
two limbs there will be at least one point of no deflection, and "frequently,
depending upon the relations between the depth of burial and the thickness
of the intervening nonmagnetic material, either two other points of no
deflection or two zones of retardation, one on each side of this middle zero.
Along Sections III, IV, and V there will be but one point of no
deflection of the horizontal needle, which will correspond with the axial
line of the fold. Since this axis is north and south, and so coincides with the
magnetic meridian, the horizontal component of the rock force coincides in
direction with the horizontal component of the earth's pull, and consequently
there is no deflection of the horizontal needle. For other stations east and
west of the central station the deflections are toward the west and east,
with the usual maximum points.
The deflections on successive sections south grow smaller, since the
angle between the two horizontal components progressively diminishes.
The relative value of the horizontal component of the rock force also
progressively diminishes, since the thinning of the magnetic material due
to the rise in the axis of the fold brings the buried north poles into promi-
nence. Therefore the deflections of the horizontal needle after the mag-
netic rock has been left behind very soon become imperceptible.
The dip needle deflections for the northern sections, I and II, reach
their maximum values at the usual points, over the central zero and near
the outside zeros or points of retardation. For the southern sections the
dips grow less, since the horizontal restoring couple due to the rock has
always a positive immericai value, and also because the vertical component
of the rock force diminishes, owing to the nearness of the south poles. As
the section approaches the limits of the magnetic material the points of
maximum dip become less and less clearly defined, and the dip curve [)asses
into an irregular line, slightly depressed below the line of no deflection.
MAGNETIC OBSERVATIONS. 369
Tlie reasons for this iire, of course, obvious from what has been said
above.
In the case of a synchnal fohl pitcliing soutli, Section I (fig. 17)
becomes the most southern, Section X the most northern, hue of traverse.
Sections I and II present the same general phenomena as before for both
needles. In Sections III, IV, and V the horizontal component due to the
rock has a positive value for all stations as before, but in this case acts in a
generally opposite direction to that of the horizontal component of the
earth's force. Therefore, on these sections we should expect at first greater
deflections of the horizontal needle, which would diminish rapidly as the
sections approached and passed beyond the northern limit of the magnetic
material, but whicli, for corresponding sections, would be greater than for
the northerly pitching fold. The deflections of the dip needle would also '
be gi'eater for the same reasons.
For a synclinal pitching west, Section I is the most western, Section V
the most eastern, traverse. In this case, along I and II, the deflections of
the horizontal and dip needles are dependent for their details upon the ratio
of de^^th to distance of separation, but if far enough to the west will show
clearly two belts of magnetic material, aj^proximately parallel, and striking
approximately east and west. For Sections III, IV, and V, in which the
distance of separation is either nothing or relatively small, the phenomena
will indicate but one belt. On these sections, owing to the fact that the
horizontal component of the rock pull is nowhere zero, and has everywhere
a general westerly direction, the deflection of the horizontal needle will be
westerly throughout, and will reach a maximum north of the east-and-west
axial plane of the material, where the ang-le which it makes with the magnetic
meridian is more than 90°.
In accordance with the general principles stated in the discussion of a
single belt with the same strike, the angles of dip are in general smaller
south of the sjaicline than north, and the maximum dip is reached at a
point north of the axial plane. On sections farther to the east, near the
limits of the rock and beyond them, the dip-needle deflections, like those
of the horizontal needle, rapidly diminish and soon become imperceptible.
These facts are well shown in fig. 22, which represents a series of north-
and-south traverses across the Groveland basin, the limits of which are
defined by outcrops on the eastern side.
MON xxxvi 24
370
THE CRYSTAL FALLS IRON BEARING DISTRICT.
In this figure it is instructive to notice tlie small dip angles in the
sections east of the end of the syncline. In the first of these the dip
curve shows a hollow near the axis of the fold or angles of depression less
than the normal. This is easily understood upon considering that, since
the surface covering is here small, the vertical comjjonent of the I'ock
force becomes very small at these stations compared with the horizontal
component.
For an eastward-pitching syncline it is obvious that the facts will be
entirely similar to those stated above, except that tlie deflections of the
horizontal needle will be toward the east instead of toward the west. This
;
(
','
'I
,43 -^.(^
^■'--
■IS6
Its'
S6,
»71
y"
1/. ^ /_ „,. I_
lis.
\
24
>'.
--»V
I
I
I ■>,
I
1
I
'I
I
Y I
I
I
I
I
1
I
0 1
Fig. 22. — Magnetic map of the Grovelantl Basin.
is also well shown at the western end of the Groveland basin in fig. 22.
This basin does not show the jihenomena of two lines, however, from the
fact that it is so narrow and shallow that it does not include in its interior
any overlying nonmagnetic material, and there is accordingly no sej^aration
of its rims.
2. PITCHING ANTICLINES.
In the cases of pitching anticlines (fig. 23) the sequence of observa-
tions in the area of separation of the rims is very similar to that of pitching
synclines. In the zone of coincidence the structural diff'erence in the two
cases is that the material does not come to an end, but continues as one
band, which, as the axis sinks, is ^progressively buried to a gi'eater depth.
MAGNETIC OBSERVATIONS.
371
Thereftiro, in yoneral, tliL- l)uried north poles of the magnetic formation are
not brought nearer the surface; and this, together with the fact that the
material continues on in the line of the axis, jiroduces characteristic phe-
nomena in the magnetic sections.
These phenomena, "the details of which can be easily followed out for
any given direction of pitch, and need not here be described, show in gen-
eral two lines of attraction luerging into one, which continues in the same
direction as a strong line, showing, as it is followed, the peculiarities due to
an increasing deptli of burial. The points of maximum deflection of the
liorizontal needle continue to
separate from each other on suc-
cessive sections. The dip curve
shows a definite maximum
closely corresponding, except for
due east-and-west strikes, to the
point of no horizontal deflection.
Where the axis of tlie fold is so
oriented that these points can
be establi.shed, they indicate the
nature of the fold. If the strike
is east and west, in which case
they become indeterminate, the
continuity of the line and its
A'ery gradual decrease in power may give an excellent basis for inference
as to the nature of the fold.
r^7n\
A
Fig. 23.
CROSS SECTIONS
-Pliiii aud cruss sectioBs of a pitching anticline.
3. FORMATIONS SPLIT BY INTRUSIVES.
When a single formation has been split into two by the intrusion of a
nonmagnetic igneous rock, there are in the area in which the igneous rock
occm-s two pai-allel magnetic formations, which give rise on cross traverses
to phenomena the precise features of which depend upon the strike and dip
of the formation and upon the relation which the width of the intruded
mass bears to the depth of burial. To describe these would involve a mere
repetition of what has been said before. Such intruded masses always have
a definite limit in length, which is usually not very great. When the limits
are reached, the two parallel lines pass into a single line which continues on
372 THE CRYSTAL FALLS IRON-BEARING DISTRICT.
in undimiilislied vigor. Also, such intruded masses are seldom confined to
definite horizons for great distances and seldom split the formation into
symmetrical halves. Two nearly parallel lines of unequal strength, which,
as they, are followed, become equal, and then again become unequal, with
the stronger on the opj^osite side, are often, therefore, characteristic phenom-
ena of this case. A good illustration of the unequal division of a magnetic
rock at diff'erent points along its strike by an intruded sheet which wedges
SS 84 3S 39 40 40 40 CO 61 IS 59 IS 69 64 60 C3
f > ^ >■ f f r f f r \ \ \ \ \ 1-
.^i ,^
Jl 34 36 40 44 45 48 ST II. 46 VA'^i " 08 M 68
V. , f t f * f 'Y WX" ' ' *
31 81 34 31 41 47 49 69 16 -^i 3«4 66 86 M "
t r t t ' r f f > ^V \ * ' *
36 34 40 43 46 46 ST^S «* 68 « 19 63 6J
N '\vC>v
3? 35 4. .} 4; 48 6^66 ^9>;jVl » «T
39 41 43 41 •jjg eiSji > 84>«1 « •}
' » J. / » « y '^y^^ — s — i-
^
Fig. 24. — Magnetic map of a single formation split by an intruded sheet.
out at both ends is given in fig. 24. Between the second and third trav-
erses from the north end the existence of this sheet has been proved by
drilling.
4. SUMMARY.
The means of discrimination among these cases of convergence are
therefore founded on the deflections in the critical areas, where the separated
bands of magnetic material merge into one. Strong deflections toward the
point where they run together, with a rapid disappearance of all disturb-
ances within a short distance of this point, indicate a pitching synclinal
MAGNETIC OBSERVATIOXS. 373
fold. A louy (.•(iiitiiuiaiu-e of the disturbances, with the cliaracteristic phe-
nomena attending deeper burial, beyond the point of coincidence, indicates
an anticlinal fold. A coincidence in both directions and the continuation of
the disturbances without diminution indicate an intrusive sheet. To these
may be added the delicate criterion which the unsymmetrical distances of
the horizontal maxima from the central zero may afford. If in the area
of separation the two belts depart from each so far as to be out of range of
each other's influence, and it is found on successive cross sections that the
nearest maxima are inside the lines of no deflection which directly indicates
the position of the rock, it can be concluded that the rocks dip toward each
other, and, on the other hand, if the nearer maxima are outside these lines,
that they dip away from each other. In the one case a syncline and in the
other an anticline would be indicated, and, of course, in either case it would
be certain that tlie phenomena could not be due to an intruded mass.
OHAPTEE III.
THE FELCH MOUNTAIN RANGE.
SECTioisr I. posiTio:^, extent, and previous work.
Our map (PL XLIX) of the Felch Mountain range includes 12 sections
in the southern tier of T. 42 N., Rs. 28, 29, and 30 W., beginning- with sec.
33, T. 42 N., R. 28 W., on the east, and ending with sec. 34, T. 42 N.,
R. 30 W., on the west. The range is known to extend beyond these limits
both to the east and to the west. Rominger states^ that it has been traced
4 miles east of our eastern boundary, and also west of our western boundary
to the Menominee River north of Badwater Village. From a hasty recon-
naissance of the country to the east it seemed probable that but few addi-
tional facts could be determined, because of the swamps and the extensive
cover of the Paleozoic sandstone, and these sections were therefore not
studied in detail. We were not able to continue the work to the west, on
account of the lateness of the season, but it is desirable that this should l)e
done at some future time. The sections surveyed include, however, that
portion of the range in which outcrops are most abundant and which has
been the principal seat of exploration for iron ore.
The strong magnetic attractions in several of these sections and the
prominent outcrops of ferruginous jaspers at Felch Mountain in sec. 32,
T. 42 N., R. 28 W., and in sec. 31, T. 42 N., R. 29 W., Avere early noticed
by the United States land surveyors and indicated on the township plats.
With the rapid development of the Marquette range after the close of the
civil war the attention of miners was quickly drawn to these as to other
outlying prospects, with the result that vigorous exploration was begun on
this range even earlier than on the Menominee range proper.
'Geological report on the Tipper Peninsula of Michigan, by C Koniinger: Geol. Survey, Mich.,
Vol. V, 1895, p. 35.
374
49
us GEOLOGICAL SURVEY
MONOGRAPH XXXVIPLXLIX.
I R 28 W
/R^r
US BIEN SCO L
.\RCHEAN
Granite
GEOLOGICAI. MAP OF THE FELCH MOUNTAIN RANGE
TOPOGRAPiri'.\XU GEOLOGY
BY H.L.SMYTH
SCALE 2 INCHES - 1 MILE ^ "^'TOIIR INTERV'AL 20 FEET
HORIZONTAL SCALE OF SECTIONS :: INCHES -1 MILE \'ERTICAL SCALE 1 INCH - 1320 FEET
ELEVATION OF BASl. LINES 600 FEET
W Oulcrops wilhoi' t-bscH-ed Bli-jlu. or rtiu
; Trel pa« botlomed mrock
o Drill holes
ALGONKIAN
CAMBRIAN
LOWER HUHONIAN
StiirqoonQiinrtzilr
I Als I
Rand^■ille Dolomito Mansfield Schist
AlrH
Aim
Grovelaiul Formntion
UPPER HURONIAN
Ulldmded
INTRUSIVE
Diabase
Granite
Au
X
D
PREVIOUS WORK ON FELCH MOUNTAIN RANGE. 375
Tlio following abstracts of the published literature upon the Felch
jrouiitMin range are given as far as possible in the author's words:
1850.
Bi'BT, Wm. a. Geological report of the survey, "with reference to mines and
minerals," of a district of townsbip lines in the State of Michigan, in the year 184^0,
and tabular statement of specimens collected. Dated March 20, 18-1:7. Thirty-flrst
Congress, lirst session, 1849-50. Senate Documents, Vol. Ill, No. 1, pages 8J:2-875.
With maps.
The earliest mention of the part of the Upper Peninsula included
within the Felch Mountain range was made by Burt in describing the dis-
tribution of the talcose and argillaceous slates of the area covered in the
coui-se of his land survey in 1846. He states that the argillaceous slates
"are developed in ])arts of township 42, ranges 29 to 30 west" (p. 846).
The existence of the iron ore in this area was discovered l)y this
explorer.
The flrst bed discovered of this ore was found while traveling from the Pesha-
kumme Falls, near the Meuomonee River, east to Fort River, before it was surveyed
in May last, but was not discovered again during the suivey. It is believed, however,
that this bed of iron ore is not far distant from the corner of townships 41 and 43 N.,
between ranges 29 and 30 W. It was found in a low ridge about 3 chains wide,
course WNW. This ridge appeared to be nearly one mass of iron ore, stratified and
jointed ; consequently it may be quarried with ease. This ore has generally a granular
or micaceous structure, but specular varieties sometimes occur; color, iron black, pass
ing into a steel gray; luster when fresh broken, metallic, but soon oxidizes when
exposed to the atmosphere. This is supposed to be an extensive and rich bed of iron
ore. The variation of the needle was taken on the east side of the ridge at the cross-
ing of a hunter's trail, and its north end stood S. 82° E. Three or 4 miles west of
this, on the north side of a ridge, near a cedar swamp, the variation was N. 45° 30'
W. Probably iu this vicinity may be found another extensive bed of similar iron
ore (p. 849).
1851.
Foster, J. W., and Whitney, J. D. Report on the geology and topography of
the Lake Superior land district. Part II. The iron region, together with the general
geology. Dated November 12, 1851. Thirty-second Congress, special session, 1851.
Senate Documents, Vol. Ill, No. 4; 406 pages; with maps and plates.
Foster and Whitney, in sketching the distribution of the rocks of their
Azoic system, which comprises "for the most part gneiss, hornblende,
chlorite, talcose and argillaceous slates, interstratified with beds of quartz,
saccharoidal marble, and immense deposits of specular and magnetic oxide
of iron" (p. 8), after describing the main area of these rocks to the north-
west and north of the Felch Mountain range, say: "Another arm about
376 THE CRYSTAL FALLS IRON-BEARING DISTRICT.
18 miles in length and 10 in breadth extends easterly into T. 42 and 43,
R. 28" (p. 14). This east-and-west trending area corresponds in part with
the Felch Mountain range, but also includes two .adjacent troughs, one to
the north of it, and the other to the south.
Subsequent to the above reports, nothing is known to have been pub-
lished containing any matter concerning this iron-bearing area until 1869.
1869.
Ceednbr, Hermann. Die vorsiluiiachen Gebilde der "oberen Halbiiisel von
Michigan" in Nord-Auierika. Zeits. der deutschen geol. Gesell., Vol. XXI, 1869, pp.
516-568. With map and three plates of sections.
In Credner's article we find a considerable advance in knowledge
concerning the relations of the rocks of the Felch ]\Iountain area. This
advance is indicated by his general map (PL IX) and by his two profiles
(fig. 3, PI. IX, and fig. 1, PI. X).
In profile No. 3, going from south to north, the granite is represented
as overlain by quartzite, separated by a narrow interval from the marble,
which in its turn is overlain by a great thickness of the ore formation. The
dip is steep to the noi'th. Unconformably upon the last two formations — the
marble and the iron formation — there are caps of Potsdam sandstone, with
the beds dipping flat to the north.
In fig. 1, PI. X, the gneiss overlain by the quartzite, dipping steep to
the north, are the only pre-Paleozoic rocks shown. The same profile shows
the unconformable Potsdam, with Silurian dolomite resting conformably
upon it.
In the text there is mention, with an illustration (fig. 5, PI. IX), of the
granite dike cutting across' the iron formation in the upper course of the
Sturgeon River. Beyond this no reference to the area under discussion
occurs in the text.
1873.
Brooks, T. B. Iron-bearing rocks (economic). Geol. Surv. of Michigan, Vol.
1, 1869-1873, New York, 1873, Part I. 319 pages. With maps.
In the year 1873 Maj. T. B. Brooks's very important study of the
Michigan iron ranges appeared, and in it the Felch Mountain area for the
first time is distinctly separated as "the North Iron Range" from the "South
Iron Range" of the Menominee River iron region, both together, however,
constituting the Menominee.
The north iron range, about 12 miles from the other in the south i>art of T. 42,
Rs. 28, 29, and 30, is in places a prominent topographical feature. The capping of
PREVIOUS WORK OJf FELCH MOUNTAIN RANGE. 377
horizontal sandstones, wliicli -has already been mentioned as characterizing- the
Menominee liills, gives a somewhat more even character to the crest lines, and in
places produces a strikingly ditterent prottle (p. 72).
As stilted by Major Brooks in a footnote on p. 157, Hie facts contained
in the chapter on the Menominee iron reg'ion were derived largely from the
sm-veys and explorations of Prof. R Purapelly and his assistant, Dr. H.
Credner. The following passage is the most important statement conceni-
iug the Felch Mountain area, or the "North Range:"
The north iron belt or range has a coarse nearly due east and west, and is all
embraced, so far as known, in the south tier of sections of T. 42, Rs. 28, 29. and 30.
The most easterly discovered exposure of ore, known as the Felch Mountain, is in the
N. J of sees. 32 and 33, T. 42, E. 28. Traveling due west, fragments of iron ore are
found in NE. ^ of sec. 31, T. 42, R. 28; after which no absolute proof of the presence
of iron is found (although it is probably continuous) until we reach sec. 31, T. 42, R. 29,
where, in the center of the section, is an immense exposure of iron ore in an east- west
ridge, which can be traced westerly halfway across section 36 of the next township.
The natural exposure of ore on section 31 is larger than at any other point in the
Menominee region, and the quality is as good, if not better, so far as can be judged by
surface indications. Magnetic attractions and iron bowlders found farther west and
southwest on this range prove its extension in that direction. Whether the westerly
course continues, or whether it curves to the southwest, as seems probable from the
position of the lower quartzite and local magnetic attractions in the northwest part of
T. 41, R. 30, has not been determined. The latter hypothesis is most in accordance
with the known facts, although the southeast dip of the quartzite on sections 17 and 18,
observed by Dr. Credner, is not explained. If this hypothesis is true, the iron range
should cross the Menominee somewhere in sees. 24 or 25, T. 41, K. 31, into Wisconsin.
There can be little doubt but that the north and south belts belong to one geological
horizon, hence somewhere come together (pp. 159-160).
A geological section through the north range on the line between Rs.
29 and 30, T. 42, is given, and is also represented as section CC on Atlas
Plate IV. The succession from south to north is as follows:
Granite.
Quartzite.
Interval.
Marble.
Iron-ore formation.
Interval.
Marble.
Interval.
Granite-gneiss and hornblende and mica schist.
As represented in the section, the beds all dip toward the north at
378 THE CRYSTAL FALLS IRONBEAKIJ^G DISTRICT.
high angles. In attempting to correlate these vhrions beds with those of the
sonth iron belt, Brooks expei-iences difficulty with the uppermost formation
of granite-gneiss and schist. He says:
The gueiss and granite outcrop above described may be almost regarded as a
typical Laurentian rock in its appearance. If future investigations prove them to be
Laurentian, a very trouble.some structural problem would be presented here, as we
would have Laurentian roclss conformably overlying beds unmistakably Huronian
(p. 175).
As will be seen, this objection which Brooks had anticipated was raised
bv later workers in the area and was explained by Eominger.
The main points of Brooks's conclusions may briefly be summarized :
(1) The iron-bearing rocks of the . Menominee region occur in two
approximately parallel east-and-west belts (the north belt being the Felch
Mountain range and the south belt the Menominee range), separated by a
broad granite area which narrows toward the west by the convergence of
the iron belts. The north-and-south belts were not traced into each other,
but their probable connection was inferred from their bending toward each
other and from the occurrence of rocks of the iron-bearing series west of
the granite area. The equivalence in age of the two belts was inferred from
the lithological and stratigraphical similarity exhibited by the great quartz-
ite and marble formations, by the probable continuity above referred to,
and by the similar relations of these formations to the basement granites.
(2) The iron-bearing formations of the Felch Mountain range were
believed to occur at two horizons. That of Felch Mountain itself in sec.
32, T. 42 N., R. 28 W., was held to be a ferruginous phase of the lower
quartzite. On the other hand, the exposures of sec. 31, T. 42 N., R. 29 W.,
were rea'ai'ded as belong'ing' to a horizon above tlie lower marble, and as
the close equivalent of unimportant lean ores of the Menominee range.
(3) In geological structure the Felch Mountain area was held to be a
northward-dipping monocline.
(4) As a consequence of this conception of the structure, Major Brooks
supposed that there were two marble formations.
1880.
Brooks, T. B. The geology of the Menominee iron regioii, east of the center of
Range 17 E., Oconto County, Wisconsin. Geology of Wisconsin, 1873-1879, Vol. Ill,
published in 1880, Part VII, pages 429-599.
PKEVIOUS WORK OX FELOU MOUNTAIN RANGE. 379
In Vol. Ill of tlie Geoloy-y of Wisconsin, published in this year, Brooks
reiterates the views previously published in the Michigan reports. The
only new material atlded is a table (p. 447) giving the estimated minimum
thickness for the north belt as 5,200 feet.
1881.
RoMTNGBR, C. Geol. Survey of Michigan, Vol. IV, Part II, Menominee iron
region. New York, 1881, pp. 155-241. With map. .
This, the first of the reports of the geological survey of Michigan pub-
lished while Dr. Rominger was in charge, contains a great number of details
concerning explorations in the Felcli Mountain range, as it is thus called
(p. 194) for the first time in the chapter on the Menominee iron region.
The iron-bearing belt along the Menominee River is referred to as the
Quinnesee range.
Rominger criticises Brooks's position with reference to the age of the
granite and gneiss along the northern border of the Felch Mountain range
as follows:
Major Brooks declares the granites and the gneisses north of the Felch Moun-
tain ore range as younger than the ore formation, which like them dips northward;
but their superposition upon the ore formation is nowhere observable; on the con-
trary, the south side of the ore range exhibits in several places the direct superposi-
tion of the ore formation on the granite. This fact is known to Major Brooks, but
he solves the dilemma by identifying the granites on the south side of the ore forma-
tion with the Laurentian; those on the north side, he claims, represent the youngest
Hurouian rocks. How he can do so I can not conceive, as the concerned granitic and
gneissoid rocks north and south of the ore formation are so absolutely identical that
no one who ever sees them can doubt for a moment the quality and age of these rocks.
Moreover, this identification of the northern granite with the Upper Huronian, and
of the southern with the Laurentian, implies another abnormity; groups of rocks,
usually separated from each other by thousands of feet of intervening strata, are in
this case thought to be in immediate superposition, which does sometimes occur, but
not in coincidence with another improbability like the one stated in this instance
(p. 207).
^ 1887.
Irving, R. D. Is there a Huronian group? Am. Jour. Sci., Vol. XXXIV, 1887,
pp. 204-263, 365-374. Read before the National Academy of Sciences April 22, 1887.
In discussing this question. Professor Irving takes occasion to refer to
the structure of the Felch Mountain range:
In the case of the Felch Mountain belt, which does not exceed a mile in width,
all of the strata are described by Dr. Rominger as dipping at a high angle to the
380 THE CRYSTAL FALLS IRON-BEARING DISTRICT.
northward; and in crossing the belt from the south to the north, after passing the
middle, one traverses a repetition of the belts crossed farther south, but in an
inverted order. It would seem that we have to do here with a case of a synclinal,
whose sides are folded close together (p. 256).
The relations of the Felch Mountain range to the rocks of the Menom-
inee River and the Marquette district are shown on a profile. The facts are
mentioned as having been derived from an unpublished manuscript report
(1881 to 1884) of Rominger to the Michigan geological survey.^
1888.'
Irving, R. D. On the classification of the early Cambrian and pre-Cambrian
formations. Seventh Ann. Rept. U. S. Geol. Survey, for 1885-86, Washington, 1888,
pp. 365-45-1.
In this article the statements made above are repeated (p. 435).
1891.
Van Hise, 0. R. An attempt to harmonize some apparently conflicting views
of Lake Superior stratigraphy. Am. Jour. Sci., Vol. XLI, 1892, pp. 117-137.
The author of this paper, after discussing the significance of the various
unconformities observed and after dividing the rocks of the Marquette dis-
trict into a Fundamental Complex and a Lower and an Upper series, attempts
to correlate the rocks of the Menominee region with these divisions.
Passing now to the Menominee and Felch Mouutain districts, our information is
less exact. It is, however, clear that in both of these areas we have the Fundamental
Complex — that is, the granites and the gneisses associated with crystalline schists
having the usual "eruptive contacts" — the equivalence in every respect of Lawson's
combined Laurentian and Coutchichiug period. Above this complex. Professor Pum-
pelly, with whom this whole subject has been discussed, and who has great famil-
iarity with the entire Lake Superior region, suggests as exceedingly probable that in
the Felch Mountain iron-bearing series only the equivalent of the Lower Marquette
occurs, the Upper series, if it once existed, having been removed by erosion (p. 133).
1892.
Van Hise, C. R. Correlation papers — Archean and Algonkian. Bull. U. S.
Geol. Survey, No. 86, Washington, 1882, pp. 549.
In a summary of the literature on the Lake Superior region, the state-
ment quoted above is incorporated without change (p. 190).
I Published iu 1895, Vol. V.
PKEVIOUS WORK ON FELGH MOUNTAIN KANGE. 381
18»3.
Wadsworth, M. E. Report of tlie State geologist for 1801-92. State Board
of C;eol. Siirv. for the years 1891 and 1892, Lansing, 1893, pp. 01-73. Dated October
17, 189L'.
In this brief report Dr. Wadswortli calls attention to the granite which
is intrusive into the sedimentary series in the Felch Mountain area.
In tbe Menominee region, especially in the Felch Mountain district, these granite
dikes are well exposed. Here they are seen not only to cut the gneiss, but to pene-
trate the Republic or iron formation. Mr. Wright, in 1885, pointed out one of these
dikes cutting the iron series near the Metropolitan mine, on sec. 32, T. 42, R. 28 W,
(p. 101).
He includes the Felch Mountain dolerites (p. 104) in his Republic for-
mation, and correlates this formation with Van Hise's Lower Marquette series.
1S95.
RoMiNGER, C. Geological report on the Upper Peninsula of Michigan, exhibit-
ing the progress of work from 1881 to 1884. Iron and copper regions: Geol. Surv. of
Michigan, Vol. V, Lansing, 1895, pp. 1-94.
Chronologically, this is one of the latest reports upon the Upper Penin-
sula of Michigan, yet in justice to the author it should be considered as
having priority over any articles published since Irving's, m 1887, to which
reference has already been made, as in that article Irving made use of the
observations recorded in the manuscript of this report, giving Rominger
full credit for them. In Rominger's report, in the chapter on the Granitic
Group, the granite dike cutting the iron-bearing formation in the Felch
Mountain range is described (p. 7). He also describes a wedge-like intru-
sion into the highly contorted strata in sec. 33, T. 42 N., R. 28 W., as follows:
We have here evidently before us a series of strata plicated into a synclinal and
another anticlinal fold, the latter ruptured by an intruding granite mass, which rock
is^there the general surface rock and comes on the south end of the exposure in
contact with the uppermost ferruginous strata of the overtilted anticlinal fold (p. 8).
• The portion of the chapter on the iron-ore group which deals with the
Menominee region is devoted to the Felch Mountain range and to outlying
prospects as far north as Michigamme Mountain. The description of the
Felch Mountain area is in great part a condensation of earlier scattered
observations. The strata are given as dipping high to the north and
consisting of the following succession:
The underlying rock of the iron formation is always formed of crystalline rocks,
granite or diorite. The lowest strata are generally heavy light-colored quartzite
382 THE CRYSTAL FALLS IRON-BEARING DISTRICT.
beds, with interlaininated thinner ledges and schistose seams, amounting to consider-
able thickness.
Above this belt an equally large succession of well-laminated, even-bedded, often
fissile, slate like micaceous quartz-schists follow, which have a silvery luster. Next
above them comes a series of micaceoas argiUites, amounting to a belt even larger
than the former, which varies greatly in shades of color, firmness of grain, etc.; some
layers are whitish, others gray or bluish and greenish, but the greatest portion of
them is intensely red colored by hematitic pigment. A part is a fatty, impalpably
fine mass of silky or also pearly luster, according to the size of the mica shales
incorporated with them. Another part is rough and gritty from the prevalence of
arenaceous constituents.
At this horizon and rather in the lower part of it occur locally large bodies of
crystalline limestone ledges, some snowy white like Italian marble, but of coarser crys-
talline grain and intermingled with radiating clusters of asbestine fibers and larger
prismatic crystals of colorless tiemolite, which sometimes forms larger concretionary
seams in the lime rock, and are then intimately associated with crystal masses of
sahlite, one mineral penetrating the other in a manner which suggests either a process
of paramorphosis in progress, changing the sahlite into tremolite, or the original
conditions, when the calcareous material combined with the silica by a slight modifi-
cation, induced simultaneously the crystallization of the almost identical chemical
combinations iu one and the other form ; which latter suggestion is more sustained
by the actual condition of the mingled minerals than the first, as some of the crystals
of both minerals tightly grown together are so perfect in form peculiar to each and so
sharply defined that they must be considered as crystal individuals which formed side
by side and altogether independent of one another. In other localities where such
crystalline limestone belts occur, the tremolite is only sparingly intermingled, but iu
its place colorless mica scales of nacreous luster are plentifully disseminated.
In the place of marble-like limestone, sometimes also ordinary lime rock of dull
aspect with conchoidal fracture, and variously tinged, occurs; it is then usually full
of flinty siliceous seams, resembling the limestones of the Quinnesec range; the
quartzose seams, locally even, prevail over the calcareous. Incumbent on the above-
mentioned micaceous argellites succeeds a belt, about 800 feet in width, composed of
thinly laminated, banded, ferruginous, quartzite ledges of dark purplish tints or hav-
ing a metallic luster from intermixture of specular ore granules. The banded portions
are formed of an alternation of narrow seams of specular ore with siliceous seams not
so richly impregnated with the oxide. Other strata in the succession are porous
cherty rocks charged with ochreous yellow or brown oxide of iron and inclosing i)ockets
of the limonitic ore. Also blood-red argillitic seams occur in the succession, and with
them sometimes pockets of soft crumbly hematite ore.
Within the first-mentioned banded alternation of narrow ore seams with quartz
seams, larger deposits of specular ore in slaty or in compact granular, or also in the
soft friable condition of the so-called blue ore of the Quinnesec mines occur, which
constitute the principal storage of ore sought for by the miner, besides the hematitic
and limonitic deposits mentioned before. The first impression of every observer exam-
iiiing this above described rock series will induce him to consider it as an ascending
succession, as the layers follow one another in apparent conformity; but in some local-
ites, after having crossed this succession so far, if we proceed farther in the same
GEOLOGY OP FELGH MOUNTAIN RANGE. 383
direction, we intersect the same series again in an inverted order, but retaining the
same di|), until we have reached again a hirge beltof compact fjuartzite ledges in close
contiguity with granite or also diorite, as it may happen, which latter rocks then form
the surface rock of large areas on the luirth side of the Felch Mountain ore formation
(p. 33).
The only satisfactory explanation which I can give of this repetition of the rock
beds in an inverted order is the suggestion of a folding of the beds and the overturn
of tlie fold by a pressure acting principally from the north side. If this is the case,
we would have to consider the light-colored quartzite next to the granite as the most
recent deposits and the dark, ore-bearing, banded quartz beds as the oldest, which
would bring the structure of the Felch Mountain ore formation in perfect harmony
with that of the Quinnesec ore range (p. 34).
SECTIOX II. GENERAL SKETCH OP THE OEOL,OGY.
The rocks of tlie Felch Mountain range extend from the Archean to
the early Paleozoic. The Paleozoic is represented by the Lake Superior
sandstone, of supposed Upper Cambrian age, and the overlying Calciferous
limestone. These formations were originally laid down over the upturned
edges of the older rocks in flat sheets or with low initial dips, and have not
since suffered relative displacement to any notable degree. As has already
been stated, subsequent erosion has to a great extent removed this over-
lying blanket and laid bare the older rocks, except for the covering of
recent glacial deposits. The Cambrian sandstone, and to a less extent the
Calciferous limestone, still, however, occupy considerable outlying areas,
detached from one another throughout most of the district, but gradually
coalescing beyond the eastern end, where they completely cover the older
rocks and limit all further geological study of these in that direction.
The Paleozoic rocks will not be considered further at present. On the
detailed Felch Slountain map (PI. XLIX) their known outcrops are repre-
sented by appropriate symbols, but except in the larger areas, where they
so completely conceal the older rocks that the distribution of these can not
be determined, they are assunaed not to be continuous, and to be non-
existent, and in this respect stand upon the same footing as the Pleistocene
glacial covering.
The Archean, which is here made up of granites, granitic gneisses, and
various kinds of crystalline schists, is the basement group of the region.
The areas in which these rocks are now ex])osed at the surface represent
the cores of the larger arches which were constructed over the whole region
by the early manifestations of mountain-building activity, and subsequently
384 THE CRYSTAL FALLS IRON-BEAEING DISTRICT.
truncated by the deep Cambrian denudation. Our studies have dealt with
the Archean only in narrow marginal zones, and have included little more
than the location of its outer boundaries, except when it was necessary to
go deeper in order to complete the work over a full section. Consequently
no attempt at classification can be made upon the map.
The rocks, chiefly of sedimentary origin, which are intermediate in age
between the Archean below and the Paleozoic above, and therefore fallwithin
the system to Avhich the name Algonkian has been given b}' this Survey,
occupy a narrow strip nowhere more than a mile and a half and usually
less than a mile wide, which as a whole runs almost exactly east and west
for a distance of over 13 miles. This strip constitutes the Felch Mountain
range. On the north and south it is bordered by the older Archean. The
lowest member of the Algonkian occupies parallel zones next to the Archean
both on the north and south, and is succeeded toward the interior of the
strip by the younger members. While the general structure, therefore, is
synclinal, a single fold of simple type has nowhere been found to occupy
the whole cross section of the Algonkian formations, but usually two or
more synclines occur, separated by anticlines, which may have different
degrees and directions of pitch and different strikes, or may be sunk to
diff"erent depths, and complicated besides both by subordinate folds and by
faults.
Among the Algonkian rocks we distinguish two main divisions or series,
which are probably separated from each other by an unconformity. Owing
mainly to the peculiar lithological and weak physical character of the
younger of these two series, actual contacts between them have not been
found, and the evidence of unconformability consequently consists not so
much in observed discordance of structure as in an inferred discordance
based upon their relative surface distribution. This evidence will be fully
stated hereafter.
In the lower of these two series are included four formations which
clearly appear to be identical in lithological character and order of super-
position with the four formations that, so far as is known, make up the lower
iron-bearing series along the Menominee River. These are, reckoning from
the base upward, (1) the Sturgeon quartzite, (2) the Randville dolomite, (3)
the Mansfield schists, (4) the Groveland iron formation.
Above this series follows the younger series, which lithologically and in
AHCHEAN IN FELCH MOUNTAIN DISTRICT. 385
its art'iil relations is very incompletely known. It inclndes mica-schists,
ferruginous schists, iuul thin interbeddetl ferruginous quartzites. These
rocks, which from our imperfect knowledge must for the present be grouped
as a single formation, are believed to have been deposited contempora-
neously with the somewhat similar rocks that occur in the Menominee
area, at Iron Mountain, but are most extensively exjKised west of the
Menominee River, and especially in the Commonwealth and Florence
district in Wisconsin.
SECTIOX III. THE ARCHEAX.
The Archean occurs in the Felch Mountain district in two belts, which
limit the Algonkian rocks on the north and on the south. The north-
ern belt for the most part does not fall within the limits of the detail map
(PI. XLIX). It occupies a triangular corner in sees. 34 and 35, T. 42 N.,
R. 30 W., at the extreme western end of the area sm-veyed, and even in
these it has not been directly observed, but its presence is inferred from
outcrops in the adjoining sections west and north and from the observed
strikes in the overlying Algonkian formations. For the next 11 miles east
its southern boundary lies in the tier of sections next north of those majDped
in detail and probably always less than a mile away. This boiindary is
therefore not very accurately drawn, as only enough outcrops were visited
to permit its position to be fixed in a general way. Our work first touches
the southern area of the Archean, which is much better known on the west
in sees. 3 and 2, T. 41 N., R. 30 W., a short distance south of the township
line. Thence for 3 miles eastward the boundary follows the township line,
and in sec. 31, T. 42 N., R. 29 W., crosses it with a trend somewhat north of
east. From the west line of sec. 31 to the east line of sec. 36, T. 42 N.,
R. 29 W., the Archean occupies the southern third of the south tier of sec-
tions. Thence for a mile and a half it bends northeast, and in sec. 32,
T. 42 N., R. 28 W., reaches its farthest north in the center of the section.
From this point the boundary runs southeast, with a sinuous embayment to
the south, and passes outside the limits of the map a little north of the
southeast corner of section 33.
Throughout the Felch Mountain range the southern Archean is much
better exposed than any of the other terranes. In the western portion of
the range, where hardly more than the contact zone falls within our limits,
MON xxxvi 25
386 THE CRYSTAL FALLS IRON-BEARING DISTRICT.
outcrops are not especially numerous; but in the six eastern sections, which
include a belt from a quarter to half a mile wide, a very considerable por-
tion of the surface is bare rock. This exceptional degree of exposui-e has
been brought about by the forest fires, which, by loosening the thin soil
and destroying the protecting cover of vegetation, have facilitated its
removal from the steep-sided knobs that are such characteristic featm-es of
the Archean topography.
TOPOGRAPHY.
The Archean areas, particularly the southern, are distinguished by
a characteristic rough topography. The surface is exceedingly uneven
on rather a small scale, and has already been described as consisting of
hunnnocky elevations alternating with bowl-shaped depressions. Both
hummocks and bowls are elongated in an east-and-west direction, in accord-
ance with the prevalent gneissic foliation.
While the surface is thus so full <>f small details that an adequate
delineation of it is the despair of the topogra]>her, the actual relief is
insiii'nificant. To men bred on the flat ])lains of the lower lakes, as were
most of the early surveyoi's and explorei-s, it may naturally have appeared
mountainous, since roughness is a quality particularly noticeable in a wil-
derness like this, that can be traveled only on foot. But from a broader
point of view the irregularities are almost wholly lost. The higher summits
in the same neighljorhood rise to within a few feet of each other. The
distant sky line is even in all directions. There is, however, a gentle
ascent from east to west, quite imperceptible on the ground, and made
evident only by the general course of the streams or by leveling.
Along the contacts between the Archean and Algonkian systems there
usually but not always exists a topographical depression, occupied by
swamp or streams. North of the southern Archean mass this depression
is a well-marked linear valley, extending with some interruption from
sec. 33, T. 42 N., R. 28 W., on the east, for 6 miles west to sec. 33, T. 42 N.,
R. 29 W. For 2 miles in tlie middle of this stretch the valley is occupied
by the Sturgeon River; thence west for 2 miles by a small feeder of the
Sturgeon, while the eastern third holds swamp with ill-defined drainage.
On the south the Archean boundary of this valley generally rises with
AKCHEAN IN FELCH MOUNTAIN DISTRICT. 387
steep slopes, which are frequently escarpmeut-like iu cliaracter, and tor
short distauces present smooth faces to the valley. In sec. 33, T. 42 N.,
R. 28 W., the mural face which runs southeast across the eastern half
of the section with the regularity of a ruled line is a true fault scarp.
Toward the western end of this valley, in sec. 32, T. 4-2 N., R. 29 W.,
the Hoor gradually rises ;uid the swamp area broadens, penetrating the
Archean in a network of thicker and thicker mesh about the hiaher hum-
mocks, until these are finally oveitopped.
PETROGRAPHICAL CHARACTERS.
The rocks of the Archean areas niay be di^dded into four quite distinct
types, namely: (1) Granites or granitic gneisses, (2) gneisses with banding
or distinct lamination, (3) mica-schists, and (4) hornblende-gneisses or
amphibolites. Between the first two divisions there is an extremely close
mineralogical and chemical lUieness, while in these respects the fourth
division stands against all the others in strong contrast.
(1) The granites of the first division are, as seen in the field or in the
hand specimen, holocrystalline rocks of fine to medium grain, in which the
eye can readily distinguish the presence of quartz, pink feldspar, muscovite,
and biotite. In color they are prevailingly of pink or reddish tints of light
shades. Structurally, they frequently appear in small areas to be entirelv
massive, but even in the most massive occurrences the hammer can usuallv
part them along roughly parallel surfaces which glisten with spangles of
mica, indicating a certain degree of alignment in these constituents. Gen-
erally, however, a rude foliation is more or less distinctly visible, and is
sometimes exceedingly well developed, even to the point of fissilitv. It is
always apparently due to the parallel arrangement of the micas, which
are more abundant as the foliation becomes more distinct.
The field relations show that the massive and more or less Ibliated
varieties of this divi.sion are closely bound together by indistinguishable
gradations and, indeed, often constitute a visibly integral mass. The usual
arrangement of the micas is not parallel to a surface but parallel to a line
which is generally inclined to the horizon at angles varying between 10°
and 35°. A hand specimen when turned about the direction of foliation as
388 ■ THE CRYSTAL FALLS IRON-BEARING DISTRICT.
an axis, shows a parallel arrangement of the micas on all sides, and a con-
tinuous glisten follows the revolution; while on a surface at right angles to
this direction the micas are not parallel and wind about the other constitu-
ents indifferently. In the more fissile varieties the outcrops often have a
rough, channeled surface, suggestive of the surfaces familiar in closely
crenulated mica-schists, or on the corrugated walls of a fault. Similar cor-
rugated surfaces frequently j^art more massive from more fissile parts of the
same outcrop.
Under the microscope the essential constituents of the granites and
granitic gneiss are seen to be quartz, orthoclase, microcline, plagioclase,
biotite, and muscovite, with the iron ores, titanite, and occasionally apatite
and zircon as accessories. In the massive phases the general relations of
these minerals to one another, and their order of crystallization, in no respect
differ from those of igneous granite. The quartz, which is the last mineral
to form, contains numerous fluid and gas inclusions, the former often with
a mo^^ng bubble. (Jf the feldspars microcline is nmeh the most common,
then plagioclase, while orthoclase is generally comparatively rare, although
sometimes it is more abundant than the microcline. The plagioclase, from
its relief and extinction angles, is probably not lower in the scale than oligo-
clase. The orthocUise is usually clouded with alteration products, and some-
times the dull interior is surrounded with a narrow unattacked rim. Botli
micas are always present as original minerals, and on the whole biotite is
the more abundant. They occur in small stout crystals, often as inclusions
in the quartz and feldspars. Magnetite is rare, but occurs in idiomoi-phic
forms in the later constituents, as do also minute crystals of zircon and
apatite. Thin sections of even the most massive-looking specimens invari-
ably show the effects of pressure in the undulatory extinction of the quartz
and in the bending and occasional fracture of the feldspar.
In the foliated varieties with which these massive varieties are closely
associated the effects of mechanical stresses are the striking microscopic
phenomena. The constituent minerals are essentially the same as in the
massive phases, but the micas are relatively more abundant. The quartz
and feldspar individuals are fractured and strained, and occur in irregular
cores sepai-ated by anastomosing zones of a fine quartz-feldspar mosaic. In
these last, new micas, in long curving individuals and clusters, have been
developed in great numbers.
AROHEAN IN FELOH MOUNTAIN DISTRICT.
3H9
The following analyses give tlie clu'Diical fouijUKsititiu of these granites:
Analyses of granites.
[liyUr. H. N. Stok
es, U. S. Geol. Sun-ey.l
1.'
2.»
3.»
SiO.
76.10
.07
Noue.
.02
12.95
Noue.
.65
.09
Trace.
None.
.12
.14
6.50
2.36
.17
.48
72.17
.37
69.69
.29
TiO;
CO.
P.O.
AljO,
CfiOj
14. 44'
15. 64'
Fe 1O3
1.02
.99
.90
1.62
FeO
MnO
NiO
CaO
.69
.70
4.84
3.65
1.22
.66
5.30
3.34
MgO
K2O
NaO
H.OatllO'^
H^O above 110^
Total
99.65
I Ba, Sr, Li, CI, S, SO3 were not looked for. ' Water not determined. ' Includes PiOr,.
No. 1. Specimen 34677, Lake Superior Division, U. S. Geol. Surv., 1,935 N., 1,040 W., sec 2, T. 41 N.,
R. 30 W., Upper Penin.sula of Michigan.
No. 2. Specimen 34828, Lake Superior Division, V. S. Geol. Surv.. 300 N., 1,8.50 W., sec. 36, T. 42 N.,
R. 29 W., Upper Peninsula of Michigan.
No. 3. Specimen 36081, Lake Superior Divi-sion, U. S. Geol. Surv., 15 N., 1,025 W.. sec 31, T. 42 N.,
R. 28 W., Upper Peninsula of Michigan.
No. 1, which is rather low in alumina, iron, and lime, is a granitic gneiss
in which the abundant secondary mica, wdiich has grown in long curving
plates in nearly parallel zones of granulation, is wholly muscovite. Nos. 2
and '6 are fine- and coarse-grained pink granites, which show comparatively
little crushing and development of secondary minerals in thin section.
The rocks of this division therefore have the chemical composition as
well as the physical and petrograjjhical characters of igneous granites.
The positive proof of igneous origin, hoA^ever — actual injection into older
rocks — we have not found. Irruptive contacts may possibly exist, and
may have escaped us, since neither the Archean as a whole nor its internal
relations were the objects of especially rigid scrutiny. Igneous granites
of Algonkian or later age ought to be found within the Archean areas, for
several granite dikes are known to penetrate various members of this over-
lying series. Whether the known granites within the Archean are really
lower-lying and larger masses witli which such dikes are genetically con-
390 THE CRYSTAL FALLS IRON-BEARING DISTRICT.
nected is not known, but the possibility must be admitted. The banded
gneisses are often so faintly foliated and resemble the granites so closely in
color and grain that the distinction can be made with only the microsco])e,
and igneous contacts between them might easily be overlooked. It is
certain, however, that if the granites have been injected into the banded
o-neisses it has not been in the form of narrow dikes, and the fact remains
that no case of an igneous contact is recorded in our notes.
The gneissic members of this division are merely crushed granites, and
owe their foliation partly to the crusliing and partly to the growth of fresh
mica in the fractured zones. The}' differ from the banded gneisses in fur-
nishing l»oth field and microscopic proof of the way in which the foliation
was formed and of the rocks from which they were derived.
(2 ) The banded gneisses of the second group have essentially the same
mineral composition as the granitic gneisses of the first. They are distin-
guished by the eye mainly by the fact that the component minerals occur in
ra(^re or less distinct layers, from a fraction of an inch ujjward in thickness.
The lamination, which only rarely is very regular, seems to be caused in most
if not in all cases by the alternation of darker layers, which are relatively
rich in biotite, with lighter layers, which are comparatively and sometimes
wholly free from it. The light layers are almost always coarser in texture
than the darker, and frequently are coarsely pegmatitic. The individual bands
are not indefinitely persistent, but wedge out to knife-edges. The banding is
sometimes so indefinite as to be lost in the hand specimen, the large surface of
an outcrop being necessary to bring out the slight differences in shade. In
color these rocks are light gray, dull ^vhite, or pink. The banding shows great
variations in angle of dip, but the strike is usually fairly constant within a few
deo-rees of east and west. In a few localities distinct contortion was observed
in the gneis.sic banding and pitching folds. The lamination of these gneisses
is, so far as observed, of tlie plane-parallel type. The bands are thoroughly
welded together, and as a rule, the rock breaks indifferently across them.
Under the microscope the composition of these rocks does not differ
from that of the granitic rocks of the first division. The structural charac-
ters, however, are in strong contrast. Even in those specimens which
possess the most indistinct foliation all the minerals are elongated in a
common direction. While the individual grains in most cases show more
or less strain and are frequently fractured, their mutual boundaries are
usually sharp and clear, and it is evident that the forms are not the direct
AKGHBAN IN FELGU MOUNTAIN DISTURIT.
391
result of the pressure tluit has ati'eeted their optical properties. The
evidence is quite clear that tlic niinerals now present have crystallized in
parallel elongated forms, and it is to this they owe their prevalent lamina-
tion even when the color 1 landing- is indistinct or wanting'.
Subsequent to the time of crystallization they have been exposed to
the action of great stresses, which not onh^ have left a record in the strains
now frecpieutly perceptible in the minerals of the early crystallization, but
also in many cases have produced roughly parallel fractures and fracture
zones sometimes coinciding with and sometimes oblique to the early lami-
nation. In these zones coarse micas have grown, reenforcing the old
lamination when parallel to it, and when oblique producing a less regular
secondary foliation, which is entirely analogous and probably contempo-
raneous with the foliation of the cnished granites.
The following analyses of these gneisses are interesting as showing
their striking chemical relationship to the granites (analyses of which are
given on p. 389), with which they are intimately associated :
Analyses of gneiss.
[By Dr. H. N. Stokes, U. S. Geol. Survey.]
1.1
2.2
3.2
SiO,
74.37
.07
None.
.01
13.34
71.79
.35
74.63
.09
TiO,
CO
P,0,
AID,
14. 79'
13. 951
CrO,
Fe,0,
FeO
.92
.21
Trace.
1.10
1.09
.35
.32
MiiO
NiO
CaO
.50
.27
6.70
2.50
.12
.44
1.11
.71
3.79
4.29
1.08
.22
6.73
2.55
MgO
K,0
Na.O
H.2O at 110"
H.:0 above llO"^
Total
99.45
' Ba. Sr, Li, CI, S, SO3 were not looked for.
- Water not determined.
' Includes P»0.r,.
No. 1. Specimen 34826, Lake Superior Division, U. S. Geol. Surv., 240 N., 1,250 W., sec. 35, T. 42 N.,
R. 29 W., Upper Peninsula of Michigan.
No. 2. Specimen 36058, Lake Superior Division, U. S. Geol. Surv., 325 N., 1,225 W., sec. 36, T. 42 N.,
R. 29 W., Upper Peninsula of Michigan.
No. 3. Specimen 36080, Lake Superior Division. U. S. Geol. Surv., 15 N., 1,025 W., sec. 31, T. 42 N.,
R. 28 W., Upper Peninsula of Michigan.
392 THE CRYSTAL FALLS IRON-BEARING DISTRICT.
(3) The mica-scliists are not widel}' distributed in the portion of the
Arcliean areas included in the Felch Mountain map. They are well rep-
resented in the northern Archean area beyond the limit of the area mapjjed,
but within this limit they are known only in sees. 34 and 35, T. 42 N., R.
29 W., where an overthrust fault brings them into successive contact with
the Randville dolomite and Sturgeon quartzite for a distance of three-
fourths of a mile. An excellent section, which includes the faulted con-
tact with the dolomite, is exposed along the Sturgeon River below the
dam in the northern portion of section 35. Though so feebly represented,
they possess an unusual interest both in their field relations and in their
microscopic characters.
The mica-schists when fresh are dark gi'ay, rather soft rocks, of fine to
medium grain, with a generally well-developed schistose structure. The
HKist noticeable constituent, in spite of the dark color, is muscovite, which
occurs in pearl}' flakes of large size plentifully sprinkled along the cleav-
age surfaces, and is especially characteristic of thin seams, which are much
more fissile than the rest of the rock and part it into parallel bands of nmeh
regularity. Biotite, however, is the more abundant mica, although in
smaller and less conspicuous plates, and to it the dark color of the rock is
due. Quartz and sometimes feldspar may also be recognized.
These rocks ofter little resistance to the weather. The biotite gives up its
iron with great ease, staining the outcrop a dull red. The final product is a
slightly coherent ferruginous mixture in which the large muscovite plates
alone are recognizable. At a less advanced stage of weathering the alterna-
tion of layers more rich in biotite produces color banding in reds and grays.
The mica-schists contain many intruded dikes and sheets of flesh-colored
pegmatite and also of amphibolite, both of which are generally parallel to
the foliation. The pegmatites are typical " schrift-granits," the feldspar being
microcline. Both pegmatites and amphibolites show ragged and intrusive con-
tacts with the schists when these are examined in detail. Both also are foliated.
Under the microscope the mica-schists are thoroughly crystalline aggre-
gates of quartz, biotite, and muscovite, always with more or less microcline.
Magnetite is always present as a primary mineral, and hematiteorsomehydrous
oxide of iron between hematite and limonite is very abundant in the zone of
weathering. Besides these, tourmaline is an abundant accessory in some
slides, and apatite, zircon, titanite, pyrite, and chlorite also commonly occur.
AKCDEAN IN FELCH MOUNTAIN DISTRICT. 393
Quartz occurs in small and often partly rounded areas, some of which
have a very clastic appearance. Except as stated below, it is generally free
from inclusions of the micas, which suiTOund and terminate against it in
such a way as to indicate that it crystallized the earlier. It is often crowded
with fluid and gas inclusions, and an occasional grain bristles Avith radiating
clusters of rutile needles. Minute crystals of magnetite are also frequently
inclosed. The inclusions of all kinds are frequently grouped in roughly
oval areas near the centers of the grains, while between the nuclei and the
wandering perimeters the quartz is relatively free from inclusions.
Biotite, varying in color from dark brown to light yellowish green, is
the predominant mica. It occurs in irregular plates, generally much larger
than the quartz; the great abundance and uniform alignment of these plates
produce the schistose structure. As already stated, it includes and is there-
fore younger than the quartz generally, but it is also found, though rarely
and always in very minute plates, included in the small quartz grains which
are so abundant in the fresh microclines. The latter occurrences belong to
an earlier generation than that of the larger biotites: The chief interest
attaching to the biotite is in its alteration under the attack of the weather.
The iron separates out along the cleavages in little spheroidal drops and
flattened plates, which are red and translucent, but not quite of the deep
color of hematite. Doubtless they contain some water, and are possibly
close to gothite in composition. Between the red globules the biotite sub-
stance becomes paler, its pleochroism diminishes, and double refraction
increases, and finally, in a slide containing no basal sections, it can not be
distinguished from muscovite. The separated ferric oxide remains in the
mica, and while the rock remains firm does not travel and stain the other
constituents. In these stages the slide contains a very faintly colored
bleached biotite, which is sprinkled through and through with the little
dots of bright red iron ore.
Muscovite is not very abundant. It is sometimes intergrown with the
large biotites, and occurs under similar conditions, but it chiefly comes in
little ragged inclusions in the secondary microcline. In the form of aggre-
gates of sericite it composes the macroscopically conspicuous pearly micas,
and also is an abundant constituent, and sometimes the only representative,
of the partly absorbed and older feldspars included in the microcline.
Microcline is always a secondary mineral, and is present in variable
394
THE CRYSTAL FALLS IRON-BEARING DISTRICT.
amounts in different sections. It incloses quartz, the micas, niag-netite, and
an older feldspar. These inclosnres are usually small; they often lie in par-
allel alignment in the same and adjoining microclines, and the lines in which
they are disposed sometimes bend, apparently indicating that the original
i-ock was minutely puckered. The inclosed quartz sometimes incloses
.smaller flakes of biotite and muscovite, as well as magnetite and rutile
needles. The inclosnres in the little grains of quartz are frequenth^ con-
centrated in the centers, as in the case of some of the quartzes outside the
microclines, as described above. The microcline sometimes occurs in a few
scattered grains; sometimes with its inclusions it makes up almost the whole
rock. In its manner of occurrence, its inclusions, and the way in which
these are disposed within it, it is strikingly like the secondary albite of the
Hoosac schists of western Massachusetts, described by Prof. J. E. Wolff.^
The microclines are distinctly elongated in a direction parallel to the
foliation, to which they thus contribute. In a few cases the elongation is
parallel to a line, and does not appear in thin sections cut normal to this
direction. But in most cases the crystals are flattened parallel to a plane.
These forms are those of crystallization ; except along the secondary fracture
planes the microline is entirely free from breaking or granulation.
The following is a complete analysis of a representative specimen of
the mica-schist :
Analysis of mica-schist,
[By ])r. H. S. Stokes, U. S. Geol. Survey.]
SiO.
64.71
.72
Noue.
.02
16.43
1.83
3.84
Trace.
TiOj
CO,
PC-
Al,03
Fe-Oj
i FeO
MnO
1
CaO
MgO
K-O
Na.O
H.jOat 110° ...
H,0 above 110°
Total
0.08
2.97
.5.63
.11
.31
2.79
99.44
No. 1. Specimen 34822, Lake Superior Division, U. S. Oeol. Surv., 1900 N., 1310 W., sec. 35, T. 42N.,
R. 29 W., Upper Peninsula of Michigan.
' Mon. IT. S. Geol. Survey, Vol. XXIII, pp. 59-63.
ARCHEAN IN FELCH MOUNTAIN DISTKK T. 395
In its low silica and lime, ami high iron and magnesia, this rock differs
in important partienhirs from the granites, to which in its mineral com-
position it is allied. In tliese respects, as well as in the great excess of
potash over soda, it closely approximates the composition of certain clay
slates.'
The original character of the mica-schists is indeterminate. They may
be altered sediments, as the chemical analysis indicates, but if so they no
longer contain any material which can be proved to be in its original form,
and in view of the complete recrystallization, for which the evidence is clear
and striking, this could not be expected. Their mineralogical relationship
and close association with the granites and gneisses is perhaps a reason for
regarding them as autoclastic rocks, derived from originally massive granites
by dynamic metamorphism. If this be true, then the crust movements
which crushed the parent granite l)elong to pre-Algonkiau time, for the later
stresses which folded and l:)rought the schists into faulted contact with the
Randville and Sturgeon formations found them with a parallel foliation
which it bent and crumpled, and no period of great stress earlier than this
is known in Algonkian time. The complete recrystallization may be
referred with probability to the period of quiescence following the faulting
and folding, during which also occurred the recomposition of the older
Algonkian formations.
(4) The amphibolites or hornblende-gneisses are widely and abundantly
represented in the Archean. Macroscopically they are black or dark-
green rocks of medium to fairly coarse grain, the fresh fractures of which
glisten with the cleavage surfaces of hornblende, which is much the most
abundant and often the only recognizable constituent. They are universally
foliated parallel to the foliation of the associated gneisses, and exhibit,
but in a more marked degree, the same varieties of structure. The folia-
tion is easily recognized by the eye as due to the parallel arrangement
of tlie hornblende prisms. Depending mainlj^ upon the position of the
hornblendes relative to the other constituents, the structure is either "
of the plane-parallel or lineai'-parallel type, the latter often superbly
developed.
The essential constituents of these rocks are common ffreen horn-
blende, plagioclase, biotite, and quartz. The structure is thoroughly crys-
' See analyses quoted by Kemp, Handbook of Rocks, p. 107, noB. 4 and 5.
396 THE CRYSTAL FALLS IRON-BEAEmG DISTRICT.
talliue. The lioniblende occiu-.s iu long prisms 3 to 10 mm. in length, which
lie close together, and inclose, partially surround, and abut against smaller
angular grains of plagioclase. The plagioclase is quite unstrained and is
iisually fresh and clear, and entirely without crystal boundaries. Brown
biotite is iiniversalh' present in small amount, in long plates parallel with
the foliation. It does not seem to be an alteration product from the horn-
blende. Quartz is the least abundant constituent. It is crowded with fluid
cavities and needles of rutile, and often incloses minute crystals of horn-
blende. The plagioclase, from its high extinction angles and alteration
products, is evidently basic. A little magnetite is present, but titanite has
not been observed.
The structural features are well brought out in thin section In the
linear-parallel type the hornblendes all lie with their crystallographic axes
parallel to a line. A thin section parallel to the foliation cuts essentially all
in the zone of the prism or near it; one across the foliation gives only sec-
tions across the prism. The grains of plagioclase are general!}" elongated
without strain. Their outlines are most irregular and quite independent of
the twinning lamellae. Their g-eneral apjiearance is that which would be
presented if numerous crushed contiguous grains had united by some proc-
ess of annealing or absorption to form the new individuals. In the plane-
jjarallel type the only difference is that the hornblende prisms have grown
parallel to a plane, in which, however, they may have any orientation An
indistinct banding is also often observable in this tyjie, caused by a partial
grouping of the light and dark constituents in parallel layei's. The order
of crvstallization seems to have been plagioclase first, but nearly contempo-
raneous with the hornblende and biotite, and the quartz last.
The amphibolites occur in comparatively narrow bands of indefinite
length in the granites and gneisses. The width usually does not exceed 8
to 10 feet, and their dip is always at high angles. The boiindaries are
invariably sharp, and frequently cut the foliation of the ampliibolite within
and of the gneisses without somewhat obliquely. There is a general imi-
formity of grain throughout the width; the wider bands are not coarser
than the narrower.
ARCHEAN IN FELGH MOUNTAIN DISTRICT.
397
The IbllowiugcoiupU'tc iuialysi.s sliows tliu cliemical character of a rep-
resentative specimen ofainpliiboHte:
Analysis of amphibolite.
[Hy Dr. H. N. Stokes, U. S. Geol. Survey.
1.1
SiOj 50.36
TiO, 1.77
CO: None.
PsOj. .20
Al.O, 13.26
Cr,0;,
Fe .0:, 6. 30
FeO 9.34
MnO Trace.
1.'
NiO 1
CaO
7.85
5. 55
1.14
2.11
.16
1. 55
MgO
K.O
Na.O
HjO at 110-
H-,0 above 1 10^
Total
99.59 i
' Ba, 8i-, Li, CI, S, SO3 were not looked for.
No. 1. .Siiecimeii 36407, Lakr .Superior Division, V. S. Cieol. .Snivey, 1140 N., 1000 AV., sec. 32,
T. 42 N., R. 28 W., Upper Peninsula of Michigan.
From this analysis it appears that the rock has essentially the compo-
sition of diabase or basalt. The composition of the amphibolites, as shown
by the above analysis, and their field relations leave little room for doubt
that they are old dikes of basic rock.
Their present crystallization is of course not that due to original
cooling, since among other reasons it bears nO relation either to their
thickness or t<:) distance from the walls. The evidence of complete recrys-
tallization in place after consolidation which they thus afford, and the
unquestionable community of origin between their foliation and that of the
gneisses, are significant facts in the metamorphic history of the Archean of
this district.
It is for this reason tliat they are described with the Archean and not
with the intrusives. Whether they are really Archean intrusions and not of
Algonkian age can not, perhaps, l)e known with certainty. Basic rocks
having approximately the same composition are known to have penetrated
the Algonkian, but they have not undergone the same recrystallization.
These last besides have their known analogues, equally unmetamorphic in
the Archean itself. For tliese reasons it seems probable that the amphib-
olites were intruded into the Archean before the Algonkian rocks of this
district were deposited.
398 THE CRYSTAL FALLS IRON-BEAEING DISTKIOT.
SECTION IV. THE STURGEOK QITARTZITE.
The lowest member of the Algoiikiaii in the Felch Mountain range
is a formation consisting mainly, but not exclusively, of coarse vitreous
quartzite. Typical exposures of this formation, as well as one of the rare
contacts between it and the underlying Archean, occur along the Sturgeon
River, and it is therefore named the "Sturgeon Quartzite."
DISTRIBUTION, EXPOSURES, AND TOPOGRAPHY.
The Sturgeon formation, next to the Randville dolomite, is the most
widespread member of the Algonkian series in the Felch Mountain range.
Its general distribution throughout the area mapped is in two ])arallel zones,
of varying width, immediately adjoining the northern and southern Archean,
except wlien displaced from this position for relatively short distances by
faults. These zones extend east and west for the whole length of the range.
Their surface width varies with the complexity of the structure and the
depth of erosion. In part of sec. 35, T. 42 N., R. 29 W"., the higher
formations have been entireh' removed, and the two zones come together,
leaving the (][uartzite as the onh^ Algonkian rock at the present surface.
On the whole the Sturgeon formation is fairly well but A^ery unevenly
exposed. Beginning at the west the zone in contact with the southern
Archean furnishes frequent outcrops from the south quarter post of sec. 34,
T. 42 N., R. 30 W., to the south quarter post of sec. 36, T. 42 X., R. 30 W.,
a distance of 2 miles. Then follows a gap of a mile in which no outcrop .
have been found. Near the north and south quarter line of see. 31, T. 42 N.,
R. 29 W., they l)egin once more, and are supplemented by test pits as far
as the West sixteenth line of section 32, next east.
Then follows another gap without exposures, 2^ miles in length.
Near the north-and-south quarter line of sec. 34, T. 42 N., R. 29 W.,
outcrops begin again and continue for a mile to the east, with frequent inter-
ruption, as far as the north-and-south quarter line of section 35, where in
the valley of the Sturgeon the southern zone broadens and jt)ins the north-
ern, in consequence of the general westward pitch which has carried the
higher formations above the present surface of denudation. East of this
point the quartzite is known in only a few scattered localities. In the
southern part of sec. 36, T. 42 N., R. 29 W., it is in contact with the
Archean on the south l)ank of the Stur"-eon River. South of Felch Moun-
STURGEON QUAKTZITE IN FELCH MOUNTAIN RANGE. 39y
tiiiu, in src. .'52, T. 42 N., Ji. 2S W., it oiitcntps iiniiieili;itel\- soutli of the
abiUidoiied Northwcstci-ii mine, iiud lifis also Ix-eii t'ouiiil iu (lrilliii<>' (in the
west and in test pits on llie cast of the natnral exposnres throngh a distance
of lialf a inih-. In sec. 33, T. 42 W., R. 28 W., a small ledge, a few feet
square, occurs between the overlying- dolomite and the Archean, 200 feet
east of the road to the Calumet and Hecla (iron) mines. East of this the
contact between the Archean and the Algonkian is a faulted one, and the
quartzite is buried beneath the overlying- formations.
The northern zone of the Sturgeon formation is not nearly so well
exposed, nor for the most part does it fall within the artificial line that
bounds our detailed work on the north. Sees. 34 and 35, T. 42 N., R. 30 W.,
on the west contain a few scattered outcrops, one of which is of exceptional
})etrog-raphical interest and to be noticed later. The next exposures are
o miles east, along- and just north of the north line of sec. 35, T. 42 N.,
R. 29 W. The main northern zone of the Sturg-eon formation coming from
the west lies south of these exposures and is entirely covered. Between
the two the tongue of Archean scliists already described is faulted up. Two
miles farther east quartzite again appears in test })its, low-lying outcrops
and drill holes along the northern border of sec. 31, T. 42 N., R. 28 E.,
and in section 29, immediately north of section 32, is well exposed in a broad
belt that reaches north almost to the east-and-west quarter line.
The quartzite often forms distinct linear ridges, which in spite of the
chemical staljility and apparent homogeneity of the rock seldom rise to the
mean altitude of the neighboring Archean areas. An exception to this rule
is the succession of ridges formed by the southern zone in the 3-mile stretch
west of sec. 31, T. 42 N., R. 29 W.; these frequently overtop the adjacent
Archean plateau. Very frequently, also, the quartzite zones occupy lower
ground not only than the Archean but even than the immediately overly-
ing dolomite. The southern zone, for some unknown reason, is a distinctly
weak belt east of sec. 32, T. 42 N., R. 29 W., and for several miles forms
the bed rock of the Sturgeon and the connecting- valleys.
FOLDING AND THICKNESS.
It is extremel}' difficult in most cases to determine directly the attitude
of the Sturgeon formation, owing- to its generally massive and homogeneous
character. This is due, as will be shown hereafter, to the completeness of
400 THE CRYSTAL FALLS IRON-BEARING DISTRICT.
the recrystallization, in consequence of which the ordinary sedimentary
features that it originally possessed have been almost entirely obliterated.
Faint color banding, itself of secondary development, but no doubt pre-
serving a distinction in original composition, alone remains, and only here
and there, as a guide to the former stratification. By scattered indications
of this sort, and by the better evidence afforded by the overlying dolomite,
often very distinctly banded, it is known that the southern zone of
quartzite on the whole dips toward the north. Southward dips also occur in
this belt, by which it is known that subordinate folds occur within the quartz-
ite itself. From the considerable variations in the surface width of the forma-
tion we are led to suspect the existence of more of these little folds than
we are able to prove. However, the secondary syncline, which extends
from the offset already referred to in sec. 35, T. 42 N., R. 30 W., for 6 miles
to the east to sec. 35, T. 42 N., R. 29 W., and includes no formation higher
than the quartzite, is very definitely determined.
In the northern belt of the Sturgeon formation the indications of dij^
are generally northward at very high angles. These indications, not in
themselves conclusive, are reenforced by a corresponding attitude in the
overlying dolomite, and it is therefore probable that there is a general, or at
least widespread, overturn in the dip of the northern belt.
Since the contacts of the Sturgeon formation with the underlying
Archean and with the overlying dolomite are (except in one case) covered,
it is impossible to obtain the data for very accurate determination of its
thickness. The uncertainty in most outcrops as to the dip of the quartzite
introduces an additional difiiculty. However, in sec. 35, T. 42 N., R. 30 W.,
on the west end of the range, and in sec. 33, T. 42 N., R. 28 W., 11
miles farther east, the covered intervals to the limiting formations are not
great, and if the contacts are not faulted (which is far from certain), the
minimum thickness is determinable within a reasonable limit of error.
In the western locality the surface width of the zone probably under-
lain by quartzite is about 500 feet. The quartzite itself is structureless, but
the overlying dolomite dips northward at an average angle of about 70°.
If the same dip holds in the quartzite, its true thickness is about 470 feet.
In the eastern locality similar data lead to a thickness of nearly 430 feet.
In these two sections the quartzite zone is much narrower than it is else-
where, either because undetected faults have reduced it, or because it is
STURGEON QITARTZITE IN FELOH MOUNTAIN RANGE. 401
uncomplicated by subordinate folds. It is probably safe to conclude, in
view of the uncertainties, that the average thickness of the formation is not
less than 450 feet, and may be considerably more. In a preliminary paper
on the district,' written before the field notes were fully analyzed, I have
placed the thickness of the quartzite at about 700 feet; but this figure is
probably too large.
PETROGRAPHICAL CHARACTERS.
The Sturgeon formation includes a few very closely related rock
varieties, of whicli quartzite furnishes the great majority of the exposures.
The quartzites are usually light gray in color, and break with a coarsely
granular or glassy fracture. To the eye quartz is often the only recogniza-
ble constituent in the body of the rock, although the numerous joint and
shearing planes shimmer with little silvery plates of muscovite. Occasion-
ally a weathered surface is dotted with minute specks of an opaque pinkish
substance, which leads one to suspect the presence of feldspar. Chlorite
also is now and then visible in the darker varieties.
The quartzites are almost uniformly massive, except for the secondary
fractures above mentioned. At scattered localities, however, a faint color-
banding, due to the presence of layers of a pinkish hue, which are inde-
pendent of the secondary fractures, seems to indicate the original stratifica-
tion. The color bands are generally only vaguely defined ; occasionally,
however, they are numerous and shaq).
Closely associated with the massive quartzites are sheared quartzites, or
micaceous quartz-schists. These rocks are merely varieties of the quartzite
in which secondary shearing planes, with their attendant growths of new
muscovite, are more abundant than usual. The shearing surfaces almost
invariably intersect, with the result that the new structure tends toward the
linear-parallel type, and is often as similar in appearance as it is in origin
to the structure already described in connection with the sheared granites.
In a locality already referred to, on the south bank of the Sturgeon,
in sec. 36, T. 42 N., R. 29 W., where the Sturgeon formation is in visible
contact with the Archean, the quartzite is underlain by a considerable
thickness of very fissile musco^-ite-biotite-gneiss, which incloses rather
sparingly obscure pebbles of granite and quartz. This gneiss, which no
'Relations of the Lower Menominee and Lower Marquette series in Michigan (Preliminary)- Am
Jour. Sci., Vol. XLVII, 1894, p. 217. .> ^ •
MON XXXVI 26
402 THE CRYSTAL FALLS IRON-BEARING DISTRICT,
doubt was formerly an arkose rich in feldspar, has recrystallized and after-
wards been sheared; the coarse micas to which the fissility is due, together
with other new minerals, have grown between the fractured surfaces and
recemented the broken mass. It affords beautiful examples of foliation
parallel to a line.
The thin sections of the Sturgeon quartzite are of exceptional interest.
The principal constituent is, of course, always quartz. With the quartz are
associated, in nuich smaller amounts, and not necessarily all in the same
section, numerous accessories, including muscovite, biotite, chlorite, micro-
cline, orthoclase, plagioclase, titanite, rutile, zircon, apatite, and the ores.
The relations of the quartz to the other constituents present very unusual
features, and indicate that the metamorphic changes by which the present
completely crystalline rock has been made from an original granitic sand
have proceeded along lines not hitherto distinctly recognized in the forma-
tion of rocks of this character.
Among the large number of slides examined, a broad distinction can
at once be made between those which show the effects of stress in a pro-
nounced degree and those in which such effects are subordinate or hardly
noticeable. Connecting these two classes is a pei*fectly graded series; and
it is therefore certain that those of the first are merely the more or less
modified varieties of an earlier stage, represented more nearl)^ by the second.
In the slides in which the effects of pressure are least apparent the micro-
scopic characters are as follows: The background is composed of large
irregular grains of quartz, the edges of which interlock with the most minute
and sharp interpenetrations. The longest dimensions of these grains range
from 1.5 to 6 mm., avei'aging perhaps 2.5 or 3. The"\' often have a rather
vague parallel elongation, which corresponds to tlie alignment of the minerals
which diey inclose. Scattered very abundantly through these large quartz
grains are the accessoiy minerals, some predominating in one slide, others
in another, but the micas and chlorite occurring in all. Through each
slide the accessory minerals, with the exceptions noted below, lie with
their long axes in a common direction, and frequently cross the serrated
boundaries between adjacent quartzes. The inclusions in many cases have
the form and other characters of clastic minerals, and thus preserve the only
microscopic evidence of the original nature of the rock.
The included micaceous minerals are usually in small plates, ranging
STURGEON QUARTZITE IN FELOH MOUNTAIN RANGE. 403
from 0.05 to 0.75 mm. in longest dimensions, but few, however, exceeding 0.2.
Many of tliese are bent and split, the clejir mistrained quartz of the host jiene-
trating from the frayed edges into the interior between the partly separated
leaves. Biotite and muscovite, and sometimes chlorite, occur in the same
individual, indicating alteration before inclusion in the quartz host took
place. Besides its conuuon occurrence as an alteration j^roduct of the
biotite, a few rounded areas of chlorite, made up of little radiating tufts,
seem to be pseudomorphs of garnet. Inclusions of titanite and magnetite^
or a related ore, are not uncommon in the larger micas, and the biotite and
chlorite sometimes inclose beautiful sagenite webs. Many of the smaller
micas, however, have clear sharp edges and depart from the general paral-
lelism of the other inclusions. These are either contemporaneous crys-
tallizations or else, perhaps, were primary inclusions in former grains of
clastic quartz which has since disappeared. Some of the clastic jjlates of
biotite are bleached and include spheroidal blebs of red iron ore, similar to
those described in the case of the Archean mica-schists.
The microcline inclusions are usually elongated in form, and frequently,
jjarticularly in the cases of the larger, have well-rounded clastic outlines.
The long dimension, which usually coincides with one of the cleavages of
the mineral, rarely exceeds 0.5 mm. or falls below 0.08 mm. The periphery
is frequently partly sin-rounded by a thin film of biotite. Within the micro-
clines are sometimes contained little blebs of quartz, which are not oriented
optically with the host, and also, more rarely, small plates of biotite. The
microcline individuals are sometimes broken into two or three differently
oriented parts, which may be separated from each other, in which cases the
quartz of the host has completely filled the interspaces. Fracture in the
feldspar is often unattended with the slightest appearance of strain in the
inclosing and cementing quartz, which extinguishes as one individual, and is
therefore unmistakably to be attriljuted to stresses previous to the crystalli-
zation of the quartz.
Besides microcline, both orthoclase and plagioclase are sometimes
inclosed in the large quartzes, but much more sparingly. They are invari-
ably more or less decomposed, and are sometimes surrounded partially or
wholly by a film of ferruginous material. They show the same phenomena
of fracture, and occasionally of separation with penetration of the host, as
the microcline, and occur in grains having a similar range in size.
404 THE CRYSTAL FALLS IRON-BEARING DISTRICT.
Titanite is of frequent, zircon of rather rare, occuiTence. The titanite
is found not only inek)sed, as ah-eady stated, in biotite and chlorite, but also
in well-rounded clastic grains which are often bordered with an opaque ore.
Zircon occurs in broken grains, without doubt clastic, and also in small
crystals which show no signs of wear. These -last were probably entirely
embedded in original clastic grains of quartz.
Besides the above minerals of usual occurrence, small quartz grains of
different orientation from the matrix are ^'ery rarely found included in the
large quartzes of the general background. Only two or three such cases
have been oljserved, and in these the included grain is surrounded almost
wholly with thin plates of mica. It is believed that these are original
clastic grains which, perhaps because protected by a film of material now
represented by the micas, have escaped the general fate of their neighbors.
One or two composite inclusions, made up of microcline, the micas, and
quartz, have also been noticed. These seem to represent original pebbles
of granite or a crystalline schist.
The pressure effects begin with the appearance of optical strain and
decided elongation in the large quartzes of the groundmass. This is fol-
lowed by fracture, either along or quite independent of the original sutures,
the crack often halting in the interior of a grain. The fractures preserve
very roughly the same general direction, but frequently intersect at -very
acute angles, or come together in sweeping curves. The breaking is fol-
lowed by movement, and this results in the production of a fine-grained
quartz mosaic between the parted surfaces. In the final stages shown in
the series of slides in my collection, the rock is made up of long, narrow
lenses, each of which is an enormously strained qviartz individual, separated
by narrow anastomozing zones of very finely subdivided quartz. After the
fracturing took place there seems to have been no further distortion of the
lenses, for the edges of adjacent individuals follow similar curves, which are
often reversed, and in many cases could be brought together with an
accurate fit.
If the Sturgeon quartzite represents an original sandstone, it is evident
from the facts stated alcove that the old quartz grains have undergone com-
plete recrystallization. The usual conception, since the time of Sorby, of
the process by which quartzites are formed from original deposits of sands
is that new quartz is deposited around each original fragmental quartz grain,
STURGEON QUARTZITE IN FELCH MOUNTAIN RANGE. 405
ill similiu- crystallogTaphic orientation with it, and that neighboring grains
thus enlarged finally interlock bv mutual limitation of one another's growth.
Tliis explanation evidently can not account for the background of large
interlocking qnai-tz areas in these rocks, for if it were true it would be nec-
essary to assume that the quartz grains were less numerous in the original
deposit than those of almost any other mineral, in some slides even than the
titanite or chlorite. There seems to be but one escape from the conclusion
that the large quartz areas nuist each represent a nnmber of original frag-
mental quartz grains, which, as deposited, nuist have lain in the rock with
their crystallographic axes disposed entirely at haphazard; and that is the
hy])othesis that this quartzite was not originall}' a sandstone, but consisted
mainly of soluble and easily replaceable material, such as limestone, with
the fragmental particles scattered through it, and that the large quartzes of
the background have replaced this soluble substance. I have been able
to tind no positive evidence to support this hypothesis, and I am com-
pelled to believe that the rock was a sandstone in which, in some way
not easy to understand, considerable numbers of adjacent quartz grains
have united to form or have been absorbed into a new individual, leaving
absolutely no trace of their former separate existence. The introduction
of new silica, or the separation of silica from decomposing silicates in the
rock itself, may well have been essential factors in the recrystallization.
I shall make no attempt to explain the process further than to point
out its probable analog}' with the ^^rocess by which the new microclines
were formed in the Archean mica-schists.
The close alignment of the clastic minerals inclosed in the large quartz
areas, their frequent fracture, and their occasional separation, indicate that
the time of crystallization probably followed a period of stress; while the
very vague j^arallel elongation of the individuals of the background in the
unstrained sections would seem to show that they crystallized under static
conditions. Unquestionable proof of a period of stress later than the crys-
tallization is given by the numerous slides, in which these grains are seen
to have suffered fracture and distortion. The microscopical study of the
quartzites thus supplies important evidence, not afforded by the outcrops,
as to the erogenic history of the district.
406 THE CRYSTAL FALLS lEON-BEARIXG DISTRICT.
SECTION V. THE RANDVlLIiE DOLOMITE.
The Sturgeon quartzite is succeeded by a formation consisting, so far
as is known, almost wholly of crystalline dolomitic rocks. Excellent
exposures belonging to this formation are situated within a short distance
of Randville station, on the Milwaukee and Northern Railway, and it may
therefore convenienth' be named the Randville dolojnite.
DISTRIBUTION, EXPOSURES, AND TOPOGRAPHY.
Owing both to its great thickness and to its intermediate ^^osition in
the series, the Randville dolomite in the Felch Mountain range coveis a
larger share of the surface than any other member of the Algonkian suc-
cession. The overlying formations are frequently interrujjted, because of
the changes in direction of pitch of the secondary synclines in which they
occur. Ill these gaps the dolomite covers the whole interior of the syncli-
norium. Wliere the higher formations are present, they divide the dolomite
into two or more parallel east and west belts, one of which lies south of the
northern quartzite and the other north of the southern. Only in portions
of sees. 35 and 36, T. 42 N., R. 29 W., wliere the rise in the axis of the
main syncline has lifted it above the present surface of denudation, is the
dolomite entirely absent from the main trough.
Natural exposures of the dolomite are not so numerous as of the
quartzite, but they are more evenly distributed. Moreover, owing to its
proximity to the Groveland iron formation, the dolomite has been penetrated
by many test pits and diamond-drill borings put down in search of ore,
and these supply important information in the covered areas. From the
western end of the map to sec. 34, T. 42 N., R. 29 W., the dolomite is for
most of the way separated into two or more parallel belts. The southern
belt is es^jecially well exposed in sees. 35 and 36, T. 42 N., R. 30 W., and
in sec. 31, T. 42 N., R. 29 W., and for 2 miles to the northeast, beyond
which it has been found only in test pits and drill holes. In the middle of
sec. 35, T. 42 N., R. 29 W., the base of the formation is brought to the
surface by the westerly pitch of the main fold and is well exposed along
Sturgeon River.
North of the strike fault, which, as alread's' described, has brought the
RANDVILLE DOLOMITE IN FELGH MOUNTAIN RAISIGE. 407
Archean inica-sclnsts into contact with tlie dolomite and quartzite in the
northern part of the same section, the Randville formation runs east in a
single belt, which probably continuously widens as the throw of the fault
diniini-shes. It has been found 'in several places in the north half of sec.
31, T. 42 N., R. 28 W., and near the east line of this section the appearance
of the overlying mica-schists again divides it into two belts, which pass to
the north and south of the Felcli Mountain syncline. The northern belt
has been proved by test pits only, but the southern is well exposed natui'ally
in the neighl)orhood of the Northwestern mine. Other exposures also
occur south of the unconformable mica-schists and quartzite of the upper
series, in the central ])ortion of sec. 33, T. 42 N., R. 29 W.
The dolomite is relatively a weak rock, and generally occupies lower
ground than either the quartzite below or the iron formation above it. The
belt in contact with the southern belt of quartzite especially is valley
making throughout most of its extent. The outcrops usually form low,
steep-isided knolls elongated with the strike and of slight relief above the
basement; these occasionally unite into linear ridges, as in sec. 35, T. 42 N.,
R. 30 W. The northern belt is one of low general relief, from which, how-
ever, similar isolated knobs often protrude. The largest and most prominent
of these is the peak in the northeast quarter of NW. ^ sec. 36, T. 42 N.,
R. 30 W., which rises 80 feet above its base, covering 8 or 10 acres.
No actual contacts between the Sturgeon and Randville formations
have been found, but from their close association and continuity, as well as
from the structural characters, Avhen these are determinable, they seem
everywhere to be strictly conformable. Near the quartzite the dolomite
becomes distinctly more impure and contains a larger proportion of silicates
and quartz. It is altogether probable that between them come transition
beds, as indeed is shown by some of the drill records. In one of these
" talckv mica-schists, micaceous limestone, altered actinolite-schist, and
quartzite" are described as being interbedded near the junction.
The determination of the thickness of the Randville formation is beset
with the same difficulties as are encountered in the case of the quartzite,
namely, the uncertainty as to the exact position of the contacts and the
possibility of faults and subordinate folds within the formation itself The
best sections give a wide range of values from a mininuim of about 500
408 THE CRYSTAL FALLS IRON-BEARING DISTRICT.
feet near Felcli Mountain to a maximum of nearly 1,000 feet in the western
part of the district. While the discrepancies may be partly due to lack of
precision in the data, it is probable that the thickness of the formation is not
uniform, but really increases from east to west. On the Fence River, 18
miles northwest of Randville, the thickness is probably about 1,500 feet.
Accordingly, accepting- each of these determinations as approximately
correct, 700 feet may be taken as a fair estimate of the average thickness
of the Randville dolomite within the Felch Mountain range.
PETROGRAPHICAL CHARACTERS.
The outcrops of the Randville formation consist exclusively of dolo-
mite, more or less pure, and always thoroughly crystalline. A few
comparatively thin layers of schists, probably both micaceous and
amphibolitic, and als(i of quartzite, are mentioned in certain drill records,
to which I have had access as occumng interbedded with the dolomite;
and wliile the lithological determinations are perhaps not entitled to much
weight, thev at least prove the existence of i-ocks which are not dolomite
within the formation. In the field, however, such interbedded layers do
not outcro}), and they must constitute an extremely small part of the total
thickness. From the results of my w(irk the Randville formation appears
as a lithological unit.
Macroscopically the dolomites are rather coarse-grained marbles, of
various shades of color, of Avhicli jjinkish or bluish white are the most
common. They always inclose, more or less abundantly, large flakes and
aggregates of tremolite, which are particularly noticeable from their projec-
tion above the weathered surface. Occasionally tremolite and other silicates
are the most abundant, and sometimes, for small thicknesses, are essentially
the only constituents. Quartz and chlorite are also often present, but in
much smaller amounts. The weathered surface is usually dulled to a light
brown or creamy yellow in a thin superficial skin, but is not deeply iron-
stained, except when the silicates containing ferrous iron are present.
The following partial analyses of three specimens from different parts
of the range show that the carbonate is normal dolomite. The insoluble
portion consists chiefly of tremolite. These analyses were made for me
by Mr. G. B. Richardson, a graduate student in geology in Harvard
University.
KANDVILLE DOLOMITE IN PELCII MOUNTAIN RANGE.
Analyses of Randville dolomite.
409
I.
II.
III.
Insoluble in HCl
2.0
1.2
53.2
42.3
9.7
2.1
48.9
38. 0
29.1
2.2
39.3
27.7
FOiOi
CaCO ,
MgCO ,
Total
98.7
98.7
98.3
The outcrops, while often entirely massive, usually possess decided
structural features. These are indicated by color banding, by differences
in texture, and by the banded arrangement of the components. Slight
variations in the body color of the rock, proceeding from no distinguishable
variation in composition, often occur in alternate parallel layers, which are
persistent within the limits of observation. With the color banding often
go variations in texture, which, however, are neither so regular nor nearly
so persistent. The characteristic form taken by these is in thin layers,
which as they continue open out into nodule.s. Such layers consist of
closely packed crystalline grains, very much coarser than the body of the
rock, which have grown normal to the boundaries. Adjacent layers are
not strictly parallel and sometimes cross each other. They are believed to
represent ancient fracture and slipping surfaces, which followed very closely
the original bedding, in which the new carbonate individuals have had
room for larger growth. The arrangement of the accessory minerals,
especially the tremolite, also is usually a banded one. Layers rich in
tremolite alternate with layers poor in tremolite, while within the layers
the orientation of the tremolite individuals is usually at random. The
structure brought out in these various ways is, on the whole, a parallel
structure. If corresponds with the strike and dip in all the localities where
these can be independently confirmed by the attitude of the adjacent for-
mations, and it also has been tin-own into minor folds. 1 therefore regard
the structm-e as having originated partly in chemical differences in the
material originally deposited and partly in secondary growths in the open
spaces and rubbing zones determined by relative movements along the sur-
faces of easiest fracture at the time of the earliest folding, and for both
reasons preserving in the subsequent metamorphism the true stratification
of the formation.
410 THE CEYSTAL FALLS IRON-BE ARINO DISTRICT.
Under the microscope the dolomites show uo featui-es of special inter-
est. They are thoroughly crystalline rocks, chiefly composed of coarse
o-rains of dolomite with which is associated a considerable number of acces-
sory minerals. Of these the most important are tremolite, diopside, chlorite,
muscovite, phlogopite, quartz, and rutile, while apatite, tourmaline, pyrite,
and magnetite are rare.
The dolomite is by tar the most abundant constituent in most of the
slides, and furnishes the general background for the accessories. The shape
of the grains in many sections is decidedly oval, and the long axes lie in
the same direction, thus producing a foliation.
Tremolite is abundant in some of the sections, and is entirely absent
from none. It occurs in long-bladed individuals and aggregates, usually
bounded by the prism, but one or both pinacoids are also sometimes present.
It includes portions of the carbonate background. Diopside is rather rare;
it occurs usually in small single individuals, with sharp crystal outlines. It
is sometimes surrounded by tremolite, from which it is distinguished by its
high obliquity of extinction and its almost rectangular cleavage. Partings
parallel to both ])inacoids, as well as a transverse parting in prismatic sec-
tions, are also observable. Quartz occurs in irregular grains completely
interlocking with the dolomite, and in some cases with tremolite. In the
slides examined it is in all cases a secondary as well as a rare constituent.
In no case is there any indication that it is clastic. Chlorite is an abundant
constituent of some of the slides, while from others it is entirely absent.
Muscovite in little frayed plates is plentiful in some sections. Quite pos-
sibly some of these may be original clastic particles. The most interesting
mica, however, is phlogopite, which is very abundant in one locality near
the base of the formation. It occurs in large, cleanly bounded plates,
each of which is a multiple twin, and evidently a product of secondary
crystallization. Some of these plates have been strongly bent, thus showing
that the dolomite, like the quartzite, has been deformed since it crystallized.
The thin sections therefore show that the rocks of this formation have
experienced even more nearly complete reconstruction than is shown in
the case of the quartzites, for here none of the constituents, except jjossibly
some of the smaller micas, are present in their original form. Also the evi-
dence for disturbance after crystallization is of similar character and equally
MANSFIELD SCHISTS IN FELOH MOUNTAIN RANGE. 411
strong. Accordiiig-ly, a close agreement in the sequence and in the charac-
ter of the i)rincipal events thus indicated in the history- of the two rocks
may he recognized. These considerations make it quite certain that tlie
recrystalhzation of the two formations was • essentially contemporaneous.
From the character of the accessory minerals in the dolomite it is probable
that the crystallization was not accompanied by the introduction of foreign
material from outside, in notable quantities, but consisted in a mineralog-
ical rearrangement of the elements present in the rock from the. beginning.
SECTIOX VI. THE MANSFIELD SCHISTS.
Above the Randville dolomite comes a formation composed chiefly of
fine- to medium-gr;iined mica-schists. Owing to their exceedingly soft
character and small thickness, these rocks are exposed naturally in only a
few localities in the Felch Mountain area. A series of phyllites less meta-
morphic but otherwise similar, and occupying the same stratigraphical
position, immediately above the dolomite, outcrop characteristically at the
Mansfield mine, and especially north of it, near the Michigamme River, in
T. 43 N., R. 31 W. For these reasons it is convenient to name the forma-
tion for the Mansfield locality.
DISTRIBUTION, EXPOSURES, AND TOPOGRAPHY.
The existence of the Mansfield formation in the Felch Mountain area
is known mainly from test pits and the records of diamond-drill borings
and early explorations. Fortunately, these are so widely distributed that
the persistence of the formation is well proved. Many drill holes have
passed tlu-ough it into the dolomite. Immediately above it comes the mag-
netic Groveland formation, which even when covered betrays its presence to
the compass needle. With the upper and lower limits thus determined, and
with the large body of data supplied by the test pits and records, there is
no difficulty in indicating its approximate boundaries for the greater part of
the map.
On the west, mica-schists belonging to the Mansfield formation have
been proved b}" diamond drilling to occur between the dolomite and Grove-
land formations in the south half of sec. 34, T. 42 N., R. 30 W. Farther
east there is a line of outcrops in the eastern portion of section 35, and the
412 THE CRYSTAL FALLS IRON BEARING DISTRICT.
schists have also been found in test pits on both sides of the western exten-
sion of the Groveland syncline in sec. 36, T. 42 N., R. 30 W. In sec. 31,
T. 42 N., R. 29 W. (the Groveland section), they have been penetrated in
10 drill holes, besides numerous test pits, giving altogether a cross section
more than half a mile in length from north to south. In the nortliern half of
sections 32 and 33 numerous test pits have exposed the Mansfield formation,
proving that it borders on both sides the narrow syncline, the interior of
which for a mile and a half is occupied by the magnetic Groveland jasper.
Tlu'ough sees. 34, 35, and 36, T. 42 N., R. 29 W., and sec. 31, T. 42 N.,
R. 28 W., the mica-schists have not been discovered, probably both because
they are but feebly represented and because but few test pits have been
sunk tlu'ough the Cambrian blanket. In sees. 32 and 33, T. 42 N., R. 28
W., the mica-schists have been found in scattered test pits and borings on
both sides of the interior jasper of the Felch Mountain spicline, and also
on the south side of section 33.
The thickness of the Mansfield formation is so small — not more than
200 feet — that it produces no very noticeable efi'ects on the general topog-
ra]3hy, in spite of the ease with which it weathers. In the western portion
of the district, througli sees. 34 and 35, T. 42 N., R. 30 W., with the dolo-
mite it underlies a broad low-lying plain, which is bounded on the south
by a ridge of the Sturgeon quartzite backed by the. Archean plateau. On
the north, a broad ridge, through which diagonalljnfass the Archean gran-
ites and gneisses, the quartzite, and the dolomite, defines this valley as far
east as the middle of section 35 ; in the northern and central portions of this
section it spreads out into a swampy lowland, diversified by glacial sand
plains, expressive of the gradual widening of the trough and of the gen-
erally horizontal attitude of the soft rocks of the interior. The most defi-
nite topographical feature directly due to the Mansfield schists is the narrow
steep-sided valley which runs east from this lowland for nearly 2 miles,
on the south side of the Groveland syncline. The ancient stream valley
filled with the Cambrian sandstone, already mentioned, follows along this
narrow belt.
PETROGRAPHICAL CHARACTERS.
The hand specimens from the various test pits, the drill cores, and the
few small outcrops indicate that the Mansfield formation is quite uniform
in character throughout the Felch Mountain area. The great majority of
MANSFIELD SCHISTS IN PELOH MOUNTAIN KANGE. 413
the speciinous ure of fine-grained mica-schists, the color of wliicli varies
from light to dark, according as muscovite or biotite is the })redominant
mica. Garnets, in some localities, are very abundant, especially near the
contacts with intrusives. It appears from the records of explorations that
thin seams of jaspery iron ore interlaminated with the schists have been
encountered in occasional drill holes and test pits, but no specimens of such
occurrences have been obtained. Their existence is of interest, as showino-
the likeness in an imjiortant character of these more altered rocks with
the slates occupying the same relative position in the Iron Mountain and
Norway areas.
The outcrops and specimens are frequently well banded in lighter and
darker layers, the color banding in seme cases not coinciding with the
schistosity. Just south of the GroA^eland mine, in a test pit which was
sinking at the time of my visit, the color bands which mark the true strati-
fication, as shown by the contact with the underlying dolomite, are closely
crumpled and cut by the foliation of the rock, which is much the more dis-
tinct of the two structures.
Near the contact with the overlying Grroveland formation the mica-
schists become both more siliceous and more ferruginous, and there is
accordingly a distinct passage between the two formations. This does
not necessarily signify a transitional character in the original sediments,
but may be altogether due to the downward transportation of silica and
iron from the upper rock.
The mica-schists are generally very tender rocks, and the material
on the dumps of test pits sunk in them is usually far gone in decomposi-
tion after a few years' exposure to the weather. From even the freshest
specimens the little flakes of mica often rub off on the fingers. Where
penetrated by intrusions, however, as in sec. 36, T. 42 N., R. 30 W., and in
sec. 31, T. 42 N., R. 28 W., they become very much harder.
Under the microscope the rocks of this formation are seen to be in the
main thoroughly crystalline, though very fine-grained, aggregates of biotite,
muscovite, chlorite, quartz, and feldspar, with the iron ores, rutile, tourma-
line, and apatite as the accessories. Garnets are abundant in some of the
sections, and with these also occur actinolite, epidote, titanite, and an unde-
termined colorless amphibole in stout single prisms. In the eight thin sec-
tions which I have examined from this formation I have found no material
414 THE CRYSTAL FALLS IRON-BEARING DISTRICT.
which is certainly original and fragmental, although almost every slide
contains grains that may possibly be such. On the other hand, it is evident
that the large majority of the individual grains have formed in place.
The micas are in most cases the most abundant constituent; sometimes
muscovite, though iisually biotite, predominates. The two micas are often
intergrovvn. The biotite is usually very deeply colored, both brown and
green, and, except in the thinnest slides, is almost opaque even in cleavage
sections. The larger mica flakes do not exceed 0.5 mm. in length, and
average not more than 0.25 mm.
Quartz generally occurs in irregular grains, full of fluid inclusions, and
inclosing the various accessories. It frequently appears in little triangles
in the interspaces between adjacent flakes of mica. Rarely part of the peri-
meter is rounded and embedded in a mica, thus suggesting a clastic origin.
Feldspar is very abundant in some of the slides and entirely absent
from others. Both microcline and plagioclase occur, and in forms similar
to the quartz. Biotite sometimes penetrates in irregular shredded edges and
filaments into the interior of the feldspars, and in such cases may be a
metasomatic product, as described by Irving and Van Hise ^ in the mica-
schists of the Gogebic district. But much of the feldspar, as shown by its
form and freshness, has recrj^stallized. The alignment of these minerals is
with the schistosit)^ of the rock, which they thus determine. When the
schistosity cuts the lines of stratification, as it frequently does, the latter are
but faintly marked in the thin section by very slight mineralogical diff'erences.
Thus a dark baud, which may be very striking macroscopically, may be due
merely to the predominance of deeply colored biotite; a light band, to the
predominance of muscovite. Sometimes, however, in these bands a grain
of quartz, or a stout flake of muscovite, lies out of the general orientation
and with the direction of the band. Such grains are very possibly original.
The schistose structure, as has already been stated, is determined by the
general parallelism of the long axes of the constituent grains. Since the
greater part, if not demonstrably all, of these grains have formed in this
position, and have not been forced mechanically into it, the cases in which
the schistosity cuts the bedding support the inference as to the time of
the general recrystallization of the series grounded on the facts observed
' The Penokee-Gogebic iron-bearing district of Michigan and Wisconsin, by R. D. Irving and
C. R. Van Hise : Mon. U. S. Geol. Survey, No. XIX, 1«92.
GKOVELAND FORMATION IN FBLGH MOUNTAIN RANGE. 415
ill the lower t'onujition, luunely, that this time foHowed a period of great
stresses. Also a period of still later stress has affected the recrystallized con-
stituents of the schists, just as it has those of the quartzite and dolomite. It
is shown 1)>' lines of fracture crossing the slides along which ferric oxide has
infiltrated, and by occasional straining and bending of the quartz and mica.
G-arnetiferous varieties of the schists are found in close proximity to
basic igneous rocks, probably in every instance intrusives, and are evidently
the result of contact metamorphism. With the garnets occur actinolite in
felted mats and clusters, and abundant magnetite ami pyrite. A colorless
amphibole in large single crystals bounded by the prism and clinopinacoid,
and giving low extinctions, is often associated with the actinolite.
SECTION VII. THE GROVELAND FORMATION".
The ferruginous rocks which compose this formation are well exposed
in the central portion of sec. 31, T. 42 N., R. 29 W., in the vicinity of
the Groveland mine, and thus may properly be termed the Groveland
formation.
DISTRIBUTION, EXPOSURES, AND TOPOGRAPHY.
The magnetite, which is always an abundant constituent of these
rocks, has made it possible to trace them for long distances throughout the
trough, by means of the disturbances effected in the compass needles. The
same disturbances had led to the sinking of a great number of test pits on
the part of former explorers for iron ore, and the material thrown out of
these has served to check and substantiate the inferences from the magnetic
attractions. Finally, in several localities excellent natural exposures of the
iron-bearing rocks occur. So, altogether, the available data as to the surface
distribution of the Groveland formation are fairly satisfactory.
On the west the presence of the Groveland formation through sees. 34,
35, and 36, T. 42 N., R. 30 W., is shown by one principal and other minor
lines of attraction, as well as by test pits and outcrops. The principal line
of attraction begins in section 34, near the southwest corner, and runs to
the northeast, in conformity with the strike of the northern belt of dolomite,
finally ending in the northeastern portion of section 36. This line of attrac-
tion is very vigorous and strongly marked. Two other lines, parallel with
the principal line, but more feeble and much shorter, cross the boundary
between sections 35 and 36, and on the northern of these ferruginous rocks
416 THE CRYSTAL FALLS IRON-BE ARING DISTRICT.
outcrop in the western part of section 36. Near the center of section 36
another hne, marking the western end of the Groveland syncUne, begins
and continues for a mile and a half east to the eastern portion of sec. 31,
T. 42 N., R. 29 W. Along the western portion of this line are many test
pits, and in section 31 the fine exposures of the Groveland hill.
Four hundred paces north of the center of sec. 32, T. 42 N., R. 29 W.,
another line of attraction begins, and may be followed toward the east
without interruption nearly to the east line of section 33 of the same town-
ship. Along this line, which is comparatively feeble and crosses wet
ground, there are but few test pits. In the eastern part of section 33,
beyond the point at which the attractions cease, many pits have been sunk
to and into the Mansfield formation, which is there somewhat ferruginous.
From this point east for 4 miles the Groveland formation has not been
recognized.
In the northern part of sees. 32 and 33, T. 42 N., R. 28 W., the fer-
ruginous rocks are again well exposed on Felch Mountain for nearly a mile
along the strike, and may be identified for half a mile farther by the vigor-
ous disturbances produced in the magnetic needles. In the southeastern
quarter of section 33 the Groveland formation is again encountered in a
small and much-disturbed area, in faulted contact with the Archean.
The most conspicuous hills within the Algonkian belt owe their relief
to the fact that they are underlain by the Groveland formation, but else-
where this formation has left but little impress on the topography, perhaps
because the local base-levels are cut nearly to the bottoms of the synclines
in which it is preserved. The two lulls referred to — Felch Mountain, in
sees. 32 and 33, T. 42 N., R. 28 W., and the Groveland hill, in sec. 31,
T. 42 N., R. 29 W. — stand 100 feet or more above the average level of the
surrounding Algonkian territory, and in both instances the infolded second-
ary synclines are exceptionally deep and broad. The magnetic lines which
indicate the other synclines pass through low ground, and the Ijelts of dis-
turbance are much narrower than in the cases of the two principal hills.
There seems to be, so far as the collected material warrants a judgment, no
litholoarical difference between the rocks of the naiTOw and those of the
broad and deep synclines, and accordingly the relief of the latter is believed
to be caused by their depth below the adjacent base-levels and not by their
more resistant character.
GROVKLAND FORMATION IN FELCH MOUNTAIN RANGE. 417
PETROGRAPHICAL CHARACTERS.
The rocks of tlii' (irntveliind formation have a general family likeness,
which makes it very easy to distingnish them in tlie field from all the other
members of the Algonkian series. Among- them two main mineralogical
kinds may be recognized, the usnal one of which consists of quartz and the
anhydrous oxides of iron, while the other, which is mucli rarer, is made
up essentially of an iron amphibole, quite similar to the griinerite of the
Marquette range, with quartz and the iron oxides as associates.
As seen in the field, the rocks of the first kind are generally siliceous,
heavy, and dark colored, the weight and color, which has a tinge of blue,
being due to the presence of abundant crystalline ii-on oxides. A large part
of the silica is easily recognized as crystalline quartz, in some instances,
indeed, in the form of detrital grains. The visible iron oxides occur both
as little spangles of specular hematite and also in irregular dark-blue
masses and single grains, the latter often having the crystalline form of
magnetite. Many, if not most, of these last, however, seem to be really
martite, as they give a dark purple streak, and in fine powder are not
attracted by a hand magnet.
In the first kind there is much variety in external appearance, deter-
mined by the varial>le proportions in which the chief constituents occur
and by the different ways in which these constituents are arranged. Con-
siderable areas, for example, consist maiidy of granular quartz merely
darkened by the intimately mixed iron oxides, and in these, so far as the
eye can judge, the rock is a ferruginous quartzite. Closely connected with
such occurrences, or included most iiTegulaidy in them, are others in which
the ferruginous constituents are so abundant and the quartz so subordinate
that they would pass for lean iron ores. Between such rare extremes we
find all intermediate proportions of mixtures of the quartz and the iron
oxides.
One form of arrangement of the constituent minerals is in narrow
parallel bands, in which the quartz and the iron oxides alternately predom-
inate. Such alternations are sometimes so frequent and regular as per-
fectly to reproduce the lean "flag ores" of the Marquette range.^ Regular
banding, however, is not common. Usually the light or dark bands are
'Geol. Survey Michigan, Vol. I, Part I, by T. B. Brooks, pp. 93-94.
MON XXXVI 137
418 THE CRYSTAL FALLS IRON-BEARING DISTRICT.
suddenly cut oft', as if by faulting, or tajjer to thin edges, or occur in sepa-
rated pebble-like forms. Neighboring lenses and fragments of bands are
most frequently i-oughly parallel with one another, but often they are
jumbled together in the greatest confusion. They no doubt represent an
original more continuous banding, which has suffered brecciation. Masses
thus shattered are also traversed and cemented by numerous small A-eins
filled chiefly with quartz, chalcedony, and specular hematite. The posi-
tions in which the separated patches of the Groveland formation now
survive, namely, in and near the bottoms of synclines, and therefore at the
jioints where sharp turning and crowding together have taken place, suffi-
ciently explain the extensive brecciation observed in these brittle beds.
Very prevalent in all the varieties of the first kind of rock, in massive,
banded, and brecciated alike, is the occurrence of some of the constituents
in small roundish spots, which give to the whole formation a very detrital
aspect. In the quartzitic phases, as well as in the most ferruginous bands,
the eye recognizes, besides the little grains of clear quartz, which seem to
be unquestionably detrital, numerous small dots of blue hematite and bright
red dots of jasper. These are more abundant in some layers than in others,
but seem never to be entirely absent, and are exceedingly characteristic of
the formation wherever found.
In a few localities the iron constituent is almost entirely in the form of
little micaceous scales of specular hematite, which have a parallel arrange-
ment. Hematite-schists, however, are not very common. The best exam-
ples occur in the northern part of sec. 36, T. 42 N., R. 30 W., along the
northern syncline.
The second kind of rocks of the formation, the griinerite-schists, have
been found in small thickness and in one locality only, namely, in the
southern parts of sec. 33, T. 42 N., R. 28 W., where they underlie, in a
series of small anticlines and synclines, banded siliceous beds composed of
quartz and magnetite or martite.
Under the microscope the essential constituents of the first or prevalent
kind of rock of the Groveland formation are quartz, magnetite, martite, and
hematite. With these, much smaller quantities of clilorite, epidote, and
apatite are generally associated as accessories. Of rarer occurrence are
calcite and probably siderite, sericite, tremolite, griinerite, pyrite, limonite,
chalcedony, rutile, titanite, tourmaline, microcline, and plagioclase.
b
GKOVELAND FORMATION IN FELCH MOUNTAIN EANGE. 419
Quartz occurs in two ways — first as rouuded detrital particles, and
secondly as grains which have crystallized in place. The deti'ital grains,
which are easily recognized by their form, size, and freedom from inclusions
of the ores, consist of single individuals, often surrounded with rims of later
growth. They are also usually larger than the neighboring indigenous
grains. While detrital quartz is not abundant and, indeed, is often entirely
absent from the thin sections, its occurrence is of interest as conclusively
establishing- the sedimentary origin of the iron-bearing formation!
The secondary quartz grains are the most abundant constituents of the
thin sections, and form the genei'al background for the other minerals.
They always inclose separate crystals of the iron oxides, usually in gi'eat
abundance, and often also chlorite and little prisms of apatite. These
grains usuall}' have the shape of irregular polygons bounded by straight
lines, frequentl} with reentrant angles, and adjacent grains completely inter-
lock. In size the secondary quartz grains range from about 0.03 to 0.4 mm.
in diameter. Grains of approximately the same size occur together in bands
or in the rounded areas to be mentioned later.
The iron ores include both magnetite, or martite, and crystalline hem-
atite, the former being much the more abundant. The magnetite and martite
can not be distinguished in thin section, as their color in reflected light and
crystalline form are the same. They occur in irregvilar bands composed of
aggregates of crystals, the edges of which interlock with the adjoining and
inclosed areas of quartz, and show the triangular, rhombic, and square sec-
tions of magiietite individuals. Magnetite also occurs in isolated, irregular
aggregates interlocking with the secondary quartz grains, and of similar
dimensions to these, but is especially abundant as single minute crystals
interposed in the grains of secondary quartz, ranging in size from such as
are barely recognizable under a No. 9 objective to octahedra 0.03-0.05 mm.
in diameter. A single quartz grain ^ mm. in diameter may inclose a hun-
dred or more such minute individuals. Hematite is much rarer than mag-
netite, and seems to be found only in the secondary quartz grains or in
veins. In the former it occurs in separate crystalline plates, of deep red
color in transmitted light, under the same conditions as to number and size
as the magnetic crystals. Throughout some sections, and in certain bands
and rounded areas in other sections, it is more abundant as inclosures than
magnetite. Such rounded areas formed of several quartz individuals, each
420 THE CRYSTAL FALLS IRON-BEARING DISTRICT.
of which thus holds a great number of hematite plates, appear macroscopic-
all)^ as the little jasper dots already described. Chlorite and apatite are
also often embedded in the secondary quartz grains, the former in thin
plates and the latter in small hexagonal prisms. Epidote is quite common
in small irregular areas intercalated between the quartz gi-ains or in the
magnetite bands.
Many of the slides contain a small amount of rhombohedral carbonate,
much if not all of which is calcite. It occurs chiefly in the quarts bands,
in irreo-ular grains which interlock with the secondary quartz grains, and,
like them, inclose little crystals of magnetite and hematite. Specimens the
slides from which contain carbonates effervesce freely in scattered spots
with cold dilute acid. Most of the carbonates are clear white under the
microscope, and are evidently calcite. Sometimes, however, the carbonate
areas have a very light-brown tint, and are partially surrounded with a
limonite border and jjenetrated by brownish filaments along the cleavages.
In such cases it is difficult to decide whether they are calcite stained with
limonite, or siderite partially oxidized to limonite. However, if part of
these areas are siderite it is nevertheless certain that the small magnetite
and hematite crystals which they inclose have not been derived from them.
These little crystals are inclosed in the carbonates just as they are in the
adjoining grains of secondary quartz, while the alteration of the siderite, if
it is siderite, is to limonite. Carbonates also occur with tremolite, quartz,
chalcedony, epidote, and hematite in the numerous thread-like veins which
traverse some of the thin sections.
The feldspars have l^een found in onl}^ a few thin sections, as well-
scattered but minute angular grains of microcline and plagioclase. ]\Iany
slides, however, contain areas of matted sericite and quartz wliicli probably
represent original grains of feldspar.
Rutile and tourmaline are also occasionally inclosed with the iron ores
in the grains of secondary quartz. Small roundish areas of titanite, prob-
ably detrital, occur very sparingl}^ in a few of tlie thin sections.
The most interesting features of the thin sections are certain very
distinct structural arrangements of the quartz and iron ores. In almost
every slide, in ordinary polarized light (with the analyzer out), the minute
interpositions of the iron ores are seen not to he equally distributed through-
out the background, Init to be concentrated in round or oval areas, never
GROVELAND FORMATION" IN FELCII MOUNTAIN RANGE. 421
excuodiug ;i inilliiiK'tur in (liiunL-ter. Tliese oval forms are coiitiiUMl to the
more siliceous bands, and are much more distinct in some of the slides than
iu others. ( )ften the outlines are reenforced b}- rims of closel}' set mag-
netite individuals, somewhat coarser than the dust-like crystals within. The
long' diameters of adjacent ovals are 2)arallel to one another and to the band
in which they lie, and ai'e often closely packed like pebbles. ()cca.sionall3'
the little grains of iron ore within the ovals have a distinctly concentric
arrangement.
Between crossed nicols these areas are seen to have had in some
instances a distinct influence on the crystallization of the secondary quartz.
When they are large and closely packed, each oval includes a large number
of interlocking quartz grains, and occasionally in such cases there is some
difference in size between the quartz grains inside and those outside the
ovals. In the triangular and quadrangular areas lying between the larger
ovals, and bounded by curving segments of their perimeters, the secondary
quartzes are frequently larger than those within, and are placed normal to
the boundaries, precisely as if they had grown outward from the ovals into
free spaces. Often, however, a single individual of secondary quartz lies
partly within and partly without the oval. On the other hand, when the
ovals are small, one or more may be completely or partially inclosed within
a single quartz individual. The interlocking quartz grains within the large
ovals show no indications of having formed in open spaces, even when the
included iron ores have a tendency, as occasionally happens, to a concentric
arrangement. The faulting and brecciation so plainly seen in many of the
thin sections have also displaced and separated the oval areas. It seems
perfectly clear to me that these forms represent a structure originally
possessed by the rock from which the various phases of the iron formation
have been derived, and which has been preserved through the subsequent
metamorphism.
From the facts described above, it is evident that the Groveland for-
mation is made up of highly metamorphic rocks, which still, however, retain
some original clastic material as well as certain original structural characters.
With the exception of the rather rare clastic grains of quartz, titanite,
feldspar, etc., the minerals which now chiefly compose these rocks — namely,
quartz and the crystalline iron oxides — are not cla.stic, but have crystallized
in place. It is a matter of great interest, therefore, to determine, if possible.
422 THE CRYSTAL FALLS IRON-BEARING DISTRICT.
in what form these constituents were present in the original deposit. (.)n this
question the microscopic structure seems to me to have a distinct bearing.
Forms similar to the ovals in these rocks occur in the iron-bearing
formations of other districts in the Lake Superior region. In the Gogebic
district of Michigan and Wisconsin, R, D. Irving and C. R. Van Hise^ have
supposed that such forms have resulted from pi-ocesses of solution and
redeposition after the rock was formed, and are tlierefore concretionary.
They regard that portion of the formation — which they have named fer-
ruginous cherts — in which such forms occur, as an alteration product from
an original dejiosit of cherty carbonate of iron. On tlie other hand, J. E.
Spurr' lias shown that similar forms are exceedingly abundant throughout
the iron-bearing formation of the Mesabi range of Minnesota, and are there
original. In the least-altered stages Mr. Spurr has found that these oval
and roundish areas are filled with a green substance, which chemically is
a hydrous silicate of iron, in composition A^ery close to glauoonite, with
which it is also optically identical. The oval and rounded forms, moreover,
are those characteristic of glauconite in green sands of all geological ages.
Starting with this original substance, which is very unstable when exposed
to oxidizing and carbonated waters, Mr. Spurr has traced an interesting
series of clianges, the final result of which along one line is the complete
oxidation of the iron to hematite or magnetite and the separation of the
silica as chalcedony and quartz. Tlu'oughout these changes the original
form of the glauconite grains is jireserved in the new minerals. Without
going into the details of these changes, and without accepting Mr. Spurr's
conclusions in their entirety as to the steps involved, he has clearly shown,
as I have satisfied myself from the study of the large number of Mesabi
slides in my own collection, that the green glauconitic substance is the
source of the iron and silica of the ferruginous cherts of the Mesabi range,
and that the peculiar spotted structure of these cherts is inherited from the
original forms of the glauconite grains.
Between the ferruginous quartzites of the Groveland formation and the
ferruginous cherts of the Mesabi range there is a very close resemblance,
especially in structure. The essential difi"erence is that the former contain
' Loc. eit., pp. 2.54-257.
■ The iiou-ljcaring rocks nf the Mesabi rauge iu Jliuiiesota, by .1. E. Spurr: Bull. Geol. aud Nat.
Hist. Surv. of Minn., No. X, 1894, 259 pp., 12 pis.
UPPER nURONIAN OF FELCri MOUNTAIK RANGE. 423
little or IK. chalcedony, the silica being crystallized quartz, while the latter
have a great deal of chalcedonic silica. Also the former contain small
amounts of detrital material, which the latter generally lack, but the essen-
tial difference between them is one of degree of crystallization only.
If the silica of the Mesabi cherts had originally crystallized entirely as
quartz, or if after passing through the stage of mixed chalcedony and quartz
it had subsequently crystallized as quartz, there would be no essential
difference between the iron formations of the two districts.
There are, then, at least two possible forms in which the iron and silica
of the Groveland formation may have been deposited originally, as indicated
by the conclusions of observers who have studied the similar iron-bearing
formations in other districts of the Lake Superior region in which these
formations are less altered than here. Either of these forms — namely, a
cherty iron carbonate, as on the Gogebic range, or a glauconitic greensand, as
on the Mesabi range— could give i-ise, under the action of vigorously oxid-
izing waters, to rocks of the mineralogical composition of those in question,
and since no trace of either original form has been found in the Groveland
formation the choice between them may perhaps be regarded as still open.
My own opinion, based on the microscopic structure which, as I interpret it,
shows that the Groveland formation was in the beginning largely made up
of rounded particles having the same general form as the glauconite grains
of the Mesabi range, is that the iron and silica were originally present
largely in the form of glauconite.
SECTION VIII. THE MICA-SCHISTS AND QUARTZITES OF THE UPPER
IIURONIAN SERIES.
Through the eastern part of sec. 32, T. 42 N., R. 28 W., and entirely
across section 33, runs a belt of mica-schists and thin-bedded ferruginous
quartzites which seem to have unconformable relations with the formations
just described. These rocks are seen on the west in a cut in the North-
western Railway in the SE. i of the NE. ^ of sec. 32. At the western end
of this cut the strike is northwest and the dip northeast at an angle of about
35°. At the eastern end there is a decided bending in the strike to a more
nearly east-and-west direction, and the bedding surfaces carry striations
which dip east at an angle of 10°, all indicating that these outcrops prob-
ably lie on the south limb of a gently eastward-pitching synclinal fold, and
424 THE CEYSTAL FALLS IRON-BEAEING DISTEIOT.
near the axial plane. East from this point similar schists and quartzites
form a ridge, low and flat-topped, which extends immediately south of the
railway almost to the east line of section 33, and sinks gradually beneath the
great swamp of the eastern portion of that section. The formation notice-
ably disturbs the compass needles, and this fact, together with the rusty
appearance of the outcrops, has probably led to the sinking of the numer-
ous test pits by which the continuity is chiefly established. But low-lying
natural exposures are not lacking.
North of the center of the NE. ^ of sec. 33 similar schists have been
found in two test pits. Also, parallel with the outcropping southern belt
and a quarter of a mile or more farther north, a faintly marked zone of
magnetic disturbances runs east and west through the swampy ground
south of Felch Mountain and probaljly connects the last-mentioned occur-
rences with the exposures of the railway cut. It therefore seems likely
that the low ground through the middle of sections 32 and 33 is wholly
occupied by an open syncline of these soft and easily disintegrating
rocks.
Between the exposed ^southern limb of this syncline and the southern
Archean the lower Algonkian formations are found in the southeastern
portion of section 33. Actual contacts are not visible, but there are note-
worthy discordances in strike and dip, and especially clear proof of great
disturbances in the lower rocks in which the upper have not shared. In
the SE. i of the NE. 4 of the SW. i of sec. 33, about 200 feet thickness of
the Randville dolomite, striking east and west and dipping north at about
70°, is exposed between the Sturgeon quartzite below and the mica-schists
to the north. Between the dolomite and the schists is a covered interval
of some 40 feet. The latter also strike about east and west, but dip north
at 30° or less. Between a quarter and three-eighths of a mile east of this
locality (the interval being without outcrops) the Mansfield and Grroveland
formations lie against the Archean gneisses with a faulted contact. They
have been thrown into a series of southeastward-pitching minor folds, and
have been intruded by a mass of diabase and also by a pegmatite dike.
The true strike of these formations at this locality is toward the northeast,
and the dip, as shown both by the direction of pitch and the order of sujjer-
position, is toward the southeast. Five hundred feet north of this disturbed
area and directh' across the strike of the lower formations therein, the
UPPER HURONIAX OF FP:LCH MOUNTAIN RANGE. 425
upper schists and (niartzites coiitiiiuc their .southeastward strike without
deviation.
These general relations indicate tliat the ferruginous mica-schists and
quartzites are part of an upper series which overlies unconformably the
Groveland and all the lower formations. This series lias not Ijeen found
elsewhere in the Felch Mountain area.
PETROGRAPHICAL CHARACTERS.
The rocks of this formation, as seen in tlie outcrops, are principally
soft and deeply iron-stained mica-schists in which occur frequent tliin beds
of ferruginous and micaceous qnartzite.
Under the microscope tlie schists are composed mainly of biotite,
quartz, inuscovite, and magnetite. Chlorite, as an alteration product of the
biotite, is frequently abundant, and garnets also occur in .some sections.
These schists are much coarser in grain than those of the Mansfield forma-
tion, and are wholly crystalline. No clastic material has been recognized
in the thin sections.
The quartzites also are thoroughly recomposed rocks, without recog-
nizable clastic particles. Quartz is the most aljundant con.stituent, and with
it muscovite, biotite, and magnetite are constantly associated. The micas
and the magnetite are frequently inclosed in a background of large inter-
locking quartz grains, which is very similar to the background of the Stur-
geon quartzite. Such inclosures lie in general alignment throughout the
thin sections, but, unlike many of the inclusions of the Sturgeon quartzite,
they seem not to be clastic particles but to have crystallized in place. In
one slide among the inclusions in the large quartzes of the background is a
colorless isotropic substance, of low refraction, occurring in large polygonal
areas, but without definite crystal form. It is usually stained with limonite,
which has penetrated from the margins along straight lines, as if following
cleavages. This interesting mineral, which is certainly not garnet, and
probably not opal, deserves further investigation
The rocks of the upper series, like those of the lower series, are greatly
altered. From their mineralogical composition and structure it is evident
that as originally deposited they consisted of beds of mud separated by
thinner beds of sand. But as they now stand they have been as greatly
changed from their original condition as tlie bedded rocks below. Also,
426 THE CRYSTAL FALLS IRON-BEARING DISTRICT.
since the time of inetamorphi.sm they liave been subjected to stress, as is
clearly shown by the optically strained condition of the secondary quartz
grains and the bending- and twisting of the micas.
From these facts we may reasonabl}^ infer that the general metamor-
phism of both series was accomplished after the deposition of the upper
series and before the latter was folded. Reconstruction so complete as
that shown by the upper series is not believed to take place except at con-
siderable depths below the surface, and hence the part of the upper series
now visible must then have been deeply covered by overlying rocks, which
were afterwards entirely swept away before the deposition of the Cambrian.
In the earth movements which folded this mass of material and brought it
up within the reach of denuding agents, we may recognize the causes which
have strained and broken the secondary minerals of both series alike.
SECTIO]y IX. TlIK IXTRITSIVES.
The Alffonkian formations of the Felch Mountain area have been cut
by later intrusives, among which both acid and basic rocks are represented.
The latter have also been recognized in the Archean, in which, indeed, the
freshest and least-altei-ed occurrences have been found.
The acid rocks consist of fine- to medium-grained pink granites,
occurring in narrow dikes. A number of these dikes have been found in
the Stvu-geon formation, both in the area of tine exposure on the south side
of sec. 3.5, T. 42 N., R. 30 W., and also in sees. 34 and 35, T. 42 N., R. 29 W.
Two o-ranite dikes are also known in the highest member of the lower
series, but none have been detected in the Randville or Mansfield forma-
tions. One of these occurs on Felch Moinitain, the other, a very coarse
pegmatite, is found cutting tlie Groveland formation in the southern part of
sec. 33, T. 42 N., R. 28 W.
Basic dikes and intrusive. sheets are found in many localities. Some
are highly schistose and greatly altered, others are massive and but little
changed. They probably belong to many eras of eruption. The least
altered are diabases, in one occurrence of which, from the Archean, the
aus'Ites are almost intact.
CHAPTER IV.
THE MICHIGAMME MOUNTAIN AND FENCE RIVER AREAS.
By reference to the general map, PI. Ill, it will be seen that an oval-
shaped Archean area, about 11 miles long from northwest to southeast and
liaAing an extreme breadtli of nearly 4 miles, runs through portions of
Ts 44, 45, and 46 N., Rs. 31 and 32 W. The country to he described in
the present chapter includes that portion of this Archean mass (together
with the younger rocks on its eastern border) which lies east of the line
between Ranges 31 and 32 W., as well as the territory to the south in the
prolongation of the axial line, as far as the south line of T. 43 N., R. 31 W.
A gap about 6 miles broad not covered by oui' work intervenes between
this arbitrary southern boundary and the western termination of the Felch
Mountain work at Randville.
In the northern portion of the area now under consideration (which
lies along and is twice crossed by the Fence River) the geological structure
is exceedingly simple, while in the southern portion, especially in the neigh-
borhood of Michigamme Mountain, it is rather complex. The boundary
between these two divisions falls in the neighborhood of the mouth of the
Fence River in sec. 22, T. 44 N., R. 31 W. It is therefore convenient
in what follows to refer to the northern portion as the Fence River area,
and to the southern as the Micliigamme Mountain area.
By referring to PI. Ill, the broad geological structure of the whole
territory of which the above-mentioned Archean oval is the center is evi-
dent at a glance. It is an anticlinal dome, the core of which is Archean,
around which the younger Algoukian formations run in a series of concen-
tric rings, on all sides dipping outward from the inner nucleus. In the
Fence River area, on the eastern long side of the dome, the Algonkian
formations have a constant eastward dip, and are free from important
secondary folds. In the Michigamme Mountain area, how^ever, which lies
in the prolongation of the main axis of the dome, these encircling forma-
428 THE CRYSTAL FALLS lEON-BEAEING DISTRICT.
tious fall away gently to the south in a series of waves, produced by
several concentric minor folds transverse to the main axis. Of these minor
folds but one is at all distinct to the east of the general anticlinal axis,
while to the west of this axis at least three are well made out within the
Michigarame Mountain area. The much greater bi'eadth of the Algonkian
formations on the west side of the dome than on the east is probably due
to the persistence of these minor folds toward the northwest.
The general character and aspect of the formations of the two areas
and their succession is iii so many respects identical with the formations of
the Felch Mountain range that no doubt can be entertained that they are
really the same formations. Nevertheless certain differences mark these
rocks with a distinct individuality. These differences will be considered in
detail in the descriptions of the several formations. In general they may
be summarized as involving a great reduction in thickness of the Sturgeon
formation, with a corresponding increase in the Randville dolomite, the
appearance of surface igneous rocks at the Mansfield horizon in the Fence
River area, and a less uniform and complete metamorphism in the whole
Algonkian series.
SECTION I. THE ARCHEAN.
The rocks of the Archean core are well exposed through the west-
central sections of T. 44 N., R. 31 W., while farther north in T. 45 N., R. 31
W., outcrops are few and scattered. Much less attention was paid to this
area than to the Felch Mountain Archean; our work, as a rule, stopped
with the location of the boundary, and, therefore, the following brief state-
ments as to its character embody observations along the southern and east-
em margins only.
The prevalent rock in the Archean is granite, varying from medium to
coarse grain, and often carrying very large porphyritic Carlsbad twins of
flesh-colored microcline. Banded gneisses and mica-gneisses and mica-
schists, such as are so abundant in the Felch Mountain Archean, are rare but
not entirely absent. While in many localities the granites are much crushed
and even sheeted along adjacent parallel fractures, their original!}^ massive
character is sufficiently evident. They have the composition and structure
of typical igneous granites. The primary minerals are entirely without
definite arrangement.
AUCIIEAN OF MlCeiGAMME MOUNTAIN AREA. 429
111 tlR' Arcliean areas p-raiiites of two af;es Imv'e been found, the
younger in tlie form of narrow dikes. Basic igneous rocks, also in dike
form, are rather abundant. One of these under the microscope proves to
l)e a but little altered diabase, in which the augite is almost intact. These
acid and basic intrusions are i)rol)ably connected with tlie surface flows of
like character which are so abundant at the ]\Ian.siield horizon along the
Fence River.
Of mucli interest is the occurrence of a small mass of quartz-porphyry
in contact with the Archean, and below the lowest Algonkian sedimentary
formation. The locality is in sec. 21, T. 44 N., R. 31 W., in the southeast
quadrant of the Archean oval. The upper surface of contact of this sheet
with the lowest sediments is covered, and hence it is not entirely certain
whether it is intrusive or extrusive, and therefore whether it belong-s to
Archean or Algonkian time. The general relations, however, appear to
indicate that it is a surface flow wdiich suffered erosion before the deposition
of the ]:)asal Algonkian member, and is tlierefore to be classed with the
Archean. The exposure is 250 feet long by 100 broad. The rock consists
of a very finely granular matrix of a warm gray color, through which
are sprinkled quite uniformly little grains of blue quartz, and larger rounded
grains of pink feldspar. Flakes of biotite are scattered through the ground-
mass and coat the cleavage surfaces, which are developed in two distinct
systems, intersecting at an angle of about 10°. Immediately below the
porphyry is coarse porphyritic granite, sheeted in waving surfaces parallel
to the contact, wdiich dips eastward about 40°. The lower portion of the
porphyry contains a number of fragments of the underlying granite, one of
which is over 4 feet in leuafth.
Under the microscope the groundmass is a line-grained crystalline
aggregate of quartz, greenish biotite, and a little feldspar. The quartz
phenocrysts are beautifully corroded, and have the characteristic bipyrami-
dal iovm, while the feldspars are extensively altered to biotite, sericite, and
quartz.
Biotite-gneisses related to this porphyry in external appearance occur
among the Archean outcrops inclosed in the "B" line of magnetic attraction
in sec. 7, T. 45 N., R. 30 W., and may be described here for comparison.
They are dark-colored, fine-grained rocks, which weather to light pink.
They are eminently schistose, and the cleavage surfaces are coated with
430 THE CRYSTAL FALLS IRON-BE ARIXG DISTRICT.
biotite plates of medium size. Minute grains of blue quartz are occasionally
distingiiisliable by the eye.
Under the microscope these gneisses have a fine to medium grained
groundmass composed of quartz, microcline, orthoclase, plagioclase, green
biotite, and nuiscovite, and a little scattered epidote. Within it are large
roundish areas of quartz and feldspar, sometimes single individuals, but more
often consisting of several fragments. The gneissic foliation is pronounced
and is caused by a general elongation of the constituent minerals in a
common direction. The onl}" essential differences between these gneisses
and the porphyries described above are this strong foliation and the coarser
groundmass.
SECTION II. THE STURGEOK FORMATION.
The Sturgeon formation as a distinct member of the Algonkiaii series
is hardly known in this area apart from the Randville formation. Neverthe-
less, purely clastic sediments unmixed with the carbonates of calcium and
magnesium were deposited and are now visible along one section between
the Archean granites below and the dolomites above, and for these it is
convenient to retain the name, although their total thickness is so small
and their continuity so uncertain that they can not be shown on the geo-
logical map. The general conditions of sedimentation here were such,
perhaps in consequence of the low relief of the neighboring land, that lime-
stones began to form a relatively short time after the submergence of the
Archean surface, so that the two lower Algonkian formations probably by no
means represent equal periods of time with the same formations in the Felch
Mountain range. The time represented by both together is perhaps not
g-reatly different in the two areas, but since in the entire absence of fossil
evidence it is impossible to draw the line of equivalence, while at the same
time the lithological break is a shai'p one, it seems desirable to carry over
the Felch Mountain names, extending the Randville dolomite downward to
the lower limit of limestone deposition, and retaining the name Sturgeon
formation for the basal sediments which are free from carbonates.
These basal sediments are found only in sec. 15, T. 44 N., R. 31 W.,
where they are exposed in low-lying outcrops in the banks and bed of the
Fence River. Elsewhere throughout the 10 or 12 miles through which the
Archean extends in this area no outcrops have been found in the flat and
generally swampy belt which intervenes between it and the dolomite above.
KANDVILLE DOLOMITE IN FENCE RIVER AREA. 431
The expofsuix's ret'cnxMl to consist of soft, light-weathering slates and
gi'aywackes, with which are interbedded layers of coarser texture. They
are very evenly banded in pale shades of yellow, red, and green, and the
structure thus brought out dips eastward at an angle of 52°. Besides this
a secondary cleavage is quite prominent, especially in the finer-grained
beds, also dipping eastward, but at a considerably higher angle. At the
eastern side the slates are overlain by the lowest marble beds, here
extremely impure and highly charged with chlorite and quartz sand. The
thickness of slates exposed is about 100 feet, and between the Archean and
the most western outcrops there is room for about as much more. The
total thickness, then, can not exceed 200 feet.
A thin section of a specimen from one of the coarser layers shows it
to be a graywacke, the most prominent constituent of which is qi;artz in
small roundish and oval grains. These are embedded in a groundmass
composed of chlorite in minute irregular plates, ferric oxide, and kaolin.
The quartz grains while having generally clastic shapes are bounded by
minutely rough edges which interlock with the fibrous minerals of the
groundmass. Evidently much new quartz has been deposited round the
original grains.
SECTION III. THE BANDVILLE DOLOMITE.
DISTRIBUTION AND EXPOSURES.
In the Fence River area the dolomite, as already stated, lies on the
east side of the Archean, and occupies a belt over half a mile in width,
which extends from the mouth of the Fence River on the south for about
10 miles to the north and west, to our western boundary near the north-
west corner of T. 45 N., R. 31 W. lu this distance it is twice crossed by
the river, and on these natural sections and in their neighborhood the only
known outcrops of the dolomite have been found. The northern river sec-
tion passes through sees. 22 and 28, T. 45 N., R. 31 W., and discloses an
excellent series of closely connected exposures for a distance of about
2,900 feet, measured at right angles to the strike. The southern section is
5 miles farther south, and is much less continuous, laying bare the extreme
upper and lower portions only of the formation. Elsewhere through the
dolomite belt the rock surface is concealed by swamps or glacial drift^ to
which last it contributes but few scattered bowlders of noticeable size.
432 THE CEYSTAL FALLS IRON-BEARING DISTRICT.
South of the Archeaii dome in the Michigamme Mountain area the
dolomite tops the hiw arch in a broad crumpled sheet, in the minor syn-
clines of which the higher formations are more and more implicated as we
go south. This broad sheet, with its included tongues of phyllite, extends
to the south line of T. 44 N., R. 31 W., beyond which it disappears beneath
the hig-her formations, except in a single narrow belt which continues along
the main axis for about a mile farther south. Exposures sufficient in num-
ber to indicate several minor folds are found along the Michigamme River
and scattered through sees. 28, 32, and 33, T. 44 N., R. 31 W., and sec. 4,
T. 43 N., R. 31 W.
FOLDING AND THICKNESS.
In attitude the Randville formation in the Fence River division of the
district is an eastward-dipping monocline, the inclination of which is gen-
erally moderate. The rocks are usually heavily bedded and nearly always
show distinct alternations in coarseness and color, so that structural obser-
vations are made with much more cei'tainty than in the Felch Mountain
range. The more conspicuous minerals secondarily developed here — coarse
carbonates and tremolite — have formed chiefly in the old planes of bedding.
Oblique structures are generally absent except in the close vicinity of the
basic dikes which intersect the formation along the upper river section.
The surfaces of contact with the dikes stand at high angles, and nearly
parallel to these the neighboring dolomite has well-developed cleavages,
along which new minerals have formed, intersecting the true bedding. It
is evident that the stronger igneous rocks in these cases have lurnished
resistant surfaces against which the dolomite has been kneaded in the
general tilting of the series.
The eastward-dipping monocline is a simple one, 5^et the observed
angles of inclination are by no means uniform. Thus, along the upper river
section the dip ranges from 25° to 60°, with 40° as the mean of about a
dozen observations. The variable dips are so scattered through the cross
section as to indicate no widespread roll in the formation as a whole, but
rather a great number of minor undulation's pi'obably distributed through-
out its thickness. Such undulations are visible in favorable localities, as,
for example, on the north bank of the river in the NW. ^ of NW. 4» sec.
28, T. 45 N., R. 31 W., where fresh surfaces have been exposed in blasting
FOLDING AND THICKNESS OF KANDVILLE DOLOMITE. 433
for tlie dam. The light-blue and pearly-white layers of tlie beautiful mar-
ble here seen are thrown into a series of unsymmetrical folds. The western
sides of the little anticlinals are short and overturned, while the eastern
sides are long and gently inclined. Evidently, if the same system of sec-
ondary folding holds throughout the entire thickness of the formation, sur-
face observations would show everywhere eastward dips at A'ariable angles,
dependent upon the portion of the fold which hapi^ened to constitute the
particular outcrop, and gentle dips would be more abundant than steep dips.
This would completely explain the observed variations.
Similar variations and lack of regular sequence in the dips are found
in the southern river section. Five good observations range between 20°
and 58°, all eastward, but none of the exposures is sufficiently extensive to
show minor folds. The mean of these observations is about 40°.
The surface width of the dolomite zone on each section is a little less
than 3,000 feet, assuming that a fair proportion of the covered zones on
each side is underlain by the same formation. If the average observed dip
is taken to represent the average dip of the rock, the thickness in each
case would be a little over 1,900 feet. This is probably too great, and is
certainly too great if the same kind of internal crumpling visible in parts
of the upper river section is characteristic of the formation throughout.
The average dip evidently would more nearly be represented by the dips
of the long eastern limbs of the little anticlines. Assuming that these are
less than the mean, we find the average of the dips below 40° to be 30° for
each section. This gives a thickness of about 1,500 feet, which still is per-
haps beyond the truth, but is probaljly much nearer it than the first value.
It is interesting to compare this result with the thickness of 500-1,000
feet obtained on the two Felch Mountain sections. A part of the increase
is probably due, as already explained, to the earlier beginning of limestone
deposition in the Michigamme area. But an important part of it is pi'ob-
ably not depositional at all, but is the result of plications. The whole
series here is but gently tilted as compared with the walls of the Felch
Mountain trough, and hence the strong horizontal pressures have acted
in a direction but slightly inclined to the bedding. The result has been
the secondary crumpling within the formation which must contribute in an
important degree to its present apparent thickness.
In the . scattered outcrops of the Michigamme Mountain area the
MON XXXVI 28
434 THE CRYSTAL FALLS IRON-BEARING DISTRICT.
dolomite strikes and dips toward all points of the compass. This irregularity
is cansed by the gentle arching over the general northwest-southeast axis,
combined with much sharper local folding about a series of axes which run
more nearly east and west. The l)est-defined east-aud-west folds occur
west of the main axis in sec. 32, T. 44 N., R. 31 W., in which three syn-
clines and three anticlines are found along a north-and-south section 4,000
feet long. Tlie two southern syuelines are sufficiently deep to include the
overlying Mansfield phyllites. The secondary folds die out toward the
main nortli-and-south axis and bi'oadeu toward the west. East of the
main axis Ijut one secondary* fold has been recognized, namely, the syncline
which forms Michigamme Mountain. This is tlie deepest of the secondary
folds, and the only one containing the Groveland formation.
PETROGRAPHICAL CHARACTERS.
The Randville formation in this area is richer in litliological varieties
than in the Felch JMountain range. As originally deposited, a much larger
proportion of sand and mud was mingled with the carlionates, and tlie prog-
ress of subsequent metamorphism also has been less uniform. Depending
upon the interaction of these two factors, we find, as the extremes of variation,
on the one liand coarse saccharoidal marbles, sometimes very pure, but
most often filled with secondary- silicates, and on the other hand fine-grained
little-altered limestones, which occasionally are ho impure as to be rather
calcareous or dolomitic sandstones and shales. The more impure varieties
occur, as might be expected, near the contacts with the adjacent formations.
On the Fence River, in sec. 16, T. 44 N., R. 31 W., the base of the
dolomite rests on the Sturgeon formation. The rock is filled with grains of
quartz and feldspar and scales of chlorite, and is so soft that it ma}^ be
crushed between the fingers. In sec. 32, T. 44 N., R. 31 W., the toj) of the
formation is in contact with the Mansfield slates, and between them is a com-
plete sei'ies of transition beds. Near the junction the limestone becomes dark
colored and contains thin bands in which the clayej' material greatly exceeds
the carbonates. These are succeeded by alternating beds of slate and impure
limestone in nearly equal volume, and it is only high up in the slate member
that the calcareous bands completely disappear. Apart from these belts of
extreme impurity at the base and top of the formation, the presence of scat-
tered fragmeutal grains of quartz and feldspar is j-ather general tln-oughout.
The prevalent colors are white, various shades of pink, both light and
PBTKOGKAPUIOAL CUAUACTKKS OF KANDVILLE D(JLOMITE. 435
(U't-p l)hu', iuifl pale green. Where weathered, the u.sual colons are light
brown or buff. The lighter-colored rocks in general are chai-acteristic of
the Fence River area where metaniorphisni is more uniform and more
intense, and the (hirker colors of tlie Michigamme Mountain area to which
the less crystalline forms are wholly contined. Bands differently colored
are nearly always present in the same outcrop.
In the Michigamme jMountain area the torsional strains attendant upon
the formation of folds in two directions have developed two systems of frac-
ture in the dolomite. In these secondary quartz has formed, occasionally
in large amount. Of nuich interest is the occurrence in close connection
witli such vein quartz of occasional thin bauds of pegmatite, doubtless aris-
ing from the action of deeply derived ' waters. In similar spaces coarse
secondary carbonates, tremolite, and oxides of iron also have commonly
formed. Over the small anticlinal axes and domes of this area the orieinal
bands of the rock have often been shattered, and are now recognizable only in
displaced fragments cemented together b}' the new minerals. In the Fence
Ri^'er area the general secondary folding has been attended Avith differential
movements along the bedding, which left narrow open spaces where the
adjacent surfaces failed to fit in their final position of rest. These sjiaces are
now indicated by coarsely crystalline carbonates and silicates arranged nor-
mal to the original walls. Where the space was a wide one the outer walls
are usually lined with coarse calcite, while the interior is filled with quartz.
In the Michigamme Mountain area certain pink bands of the dolomite
have a beautiful oolitic texture, which is most clearly brougjit out in Aveath-
ering by the geometrical regularity of distribution of the harder shells or
cores of the little rounded grains. The forms are not different from and are
quite as distinct as those in the oolitic limestones of recent deposition.
The chemical composition of the dolomites is illu.strated by the follow-
ing partial analyses by Mr. R. J. Forsythe, of Harvard University:
' Analyses of dolomites frovi Michigamme Mountain area.
I.
II.
III.
14.25
11.15
47.18
18. 48
9.34
. 12. 57
45.98
•19.22
Al2(Fe..)03
5.38
36.60
16.38
CaCOj
MgCOs
436 THE CRYSTAL FALLS IRON-BEARING DISTRICT.
The ratio of CaCOg: MgCOg is too great for normal dolomite, but
approximates that for 2CaC03+MgC03.
Under the microscope the chief differences in the various thin sections
are in the degree of metamorphism and in the quantity and character of the
foreign fragments. The least altered varieties are those highest in the series
from the Michigamme Mountain area. These consist of a background of
extremely fine-grained calcite, with a few rounded fragmental quartz grains,
and scattered particles of chalcedony. Mixtures of small quartz particles,
chalcedony, and calcite slightly coarser than the background occur in short
vein-like gashes. The prevalent deep color of these rocks is due to the even
sprinkling through the background of a black opaque pigment, which may
be carbonaceous. Altogether the microscopic characters are those of a
little-altered, slightly cherty limestone.
The more crystalline varieties of the dolomite contain several secondary
minerals, namely, tremolite, diopside, chlorite, muscovite, phlogopite, pyrite,
and the oxides of iron. Of these, tremolite is very common and abundant,
especially in the Fence River area, where the rarer pyroxene, diopside, also
is found. Phlogopite comes in but two of the thin sections, while muscovite
occurs in nearly all. The general habit of these siHcates is precisely the
same as in the dolomite of the Felcli ]\Iountain range. They are developed
pari passii with the passage of the unaltered dolomite into marble.
The fragmental inclusions within the dolomite are of interest. These
are little pebbles of quartz, feldspar, mica, titanite, magnetite, and augite;
and are evidently derived mainly from preexisting granites or gneisses.
Titanite and augite are very i-are; the others are represented in almost
every slide. The quartz grains are seldom more than a millimeter in diam-
eter and commonly are much smaller. While the general shape is oval or
rounded in most cases, the perimeters are usually extremely irregular and
interlock with the carbonate grains of the Ijackground, which indicates that
they have been enlarged since deposition by the formation of new silica.
This is very evident in the few instances in which the original smooth out-
line, or part of it, is preserved by a film of difi"erent material inside the
present perimeter. The feldspar pebbles include orthoclase, microcline, and
plagioclase, nncrocline being the common species. They are usually much
decomposed and iron stained. The feldspars are especially abundant in the
slides from the Fence River area.
PETIUKJRAI'HICAL GHAKAGTKRS OF UANDVILLE DOLOMITE. 437
Tho clastic |)cl)l)les give us striking' proof of the general and severe
internal strains suffered by the dolomite, the etfects of which have healed
over without a scar in the carbonate matrix. The pebbles are always
Optically strained. Very often they are fractured and the parts separated,
and sometimes they have been reduced to small fragments. In tliese cases
the breaks have been completely healed by the flow or redeposition of the
Si'roundmass in the interstices. These effects are foitnd in g-reater or less
degree in every thin section.
The oolitic varieties are very interesting under the microscope. They
consist of little oval or round areas, averaging 2 mm. in diameter, packed
together as closely as possjble. Each oval consists of a single or compound
nucleus, surrounded by several thin and very even concentric layers. The
nucleus in a few cases is a single roundish quartz individual, evidently a
clastic grain. In most cases, however, it is composed of a great number of
minute quartz grains, or of several coarse calcite grains, with films of iron
oxide between. The arrangement of these separate quartz and calcite indi-
viduals is such as to indicate that they have filled interior cavities. The
surrounding thin layers are calcite in all cases. Sometimes two adjoining
nuclei, each within its own rim of several layers, are together included
within a common series of shells. In one such case the outside rim trav-
ersed the edges of the rings surrounding one of the nuclei with a decided
unconformity, as if the latter had been eroded before the deposition of the
former. The oolitic structure, I believe, has not hitherto been noted in
limestones of undoubted pre-Cambrian age.
SECTION IV. THE MAIVSFIELD FORMATION.
The typical locality of the Man.sfield formation is the Michigamme
River valley in the vicinity of the Mansfield mine, which lies a mile west
of the border of my field of work, and is described by Mr. Clements. The
same formation, however, is present in the Michigamme Mountain area,
where its relations to the adjacent formations are clearly defined. In the
Fence River area rocks of very different character and derivation occur at
the Mansfield horizon. These occur in typical development to the west, on
the Hemlock River, and are hence called the Hemlock formation.
438 THE CRYSTAL FALLS IKON-BEARING DISTRICT.
DISTRIBUTION, EXPOSURES, AND TOPOGRAPHY.
The Mansfield rocks of the Michigamme Mountain area consist of
phylHtes or mica-slates of various colors. They are found in the series of
east-west synclines, which have already been described in connection with
the Randville formation. The best exposures occur in sec. 32, T. 44 N.,
R. 31 W., between the center and the west quarter post, and still farther
north along- the south bank of the ^lichigamme in the northwest quarter of
the section. The}" are also found round the western edge of the Michi-
gamme Mountain syncline in sec. 33, T. 44 N., R. 31 W., and in sec. 5,
T. 43 N., R. 31 W., but here the exposvu-es are mainly in test pits. Test
pits have likewise penetrated them in sec. 10, T. 43 N., R. 31, where they
succeed the dolomites as the surface rock over the general arch. Their
extent iu the covered portions of this area is probably considerable, but
the structure is so complex and the outcrops so few as to forbid any but the
most approximate outlining of their general boundaries.
The geological position of the Mansfield rocks is free from doubt. In
the princii)al syncline of section 32 they are seen to overlie the dolomites
and to pass downward into them by a relatively slow gradation, while on
the Ijorders of the Michigamme ^Mountain syncline they are jJi'oved to
underlie the Groveland formation. The passage to the higher formation
likewise is graded, though more rapidly, and is marked in certain bands by
an increase in clastic quartz grains and by changes in the character of the
matrix in which these are set.
The portions of the surface underlain by the Mansfield formation are
without special features, and are indistinguishable topographically in the
gently rolling plain, the greater portion of which is formed in the dolomites.
In section 32 the outcrops are miniature ridges elongated with the strike,
the height of which, however, is less than the contour interval of the map.
FOLDING AND THICKNESS.
The folding of the Mansfield rocks, so far as it can be determined
in this area, has already been described in the account of the preceding
formation, which they overlie. The rocks are known only in the sec-
ondary synclines winch lie transverse to the general direction of the
main axis south of the Michigamme River. In the southern of these syn-
clines, in sec. 32, T. 44 N., 11. 31 W., between the limestone rims on the
FOLDING AND T[IICKNESS ()1< MANSFIELD FORMATION. 439
north ;in<l soutli, a .superfiriiil widtli of iiltout 1,S()() feet of pliyllitcs is
exjjoseil. The most southern exposures (lip northward at a low angle. On
the northern rim the true beddinj;- is nearly \-ertical. P]lsewhere the ver-
tical cleavage structure alone is distinguishable. The ixpper limit of tlie
formation is not found in this syncline. Making the iii()>t liberal estimate
for ])ossible minor crumples, it is improbable that a less thickness than 300
to 400 feet occurs here. On the eastern side of the main axis the phyllites
below the Groveland formation are very mvicli thinner than this, the thick-
ness at the Interrange exploration, for example, being only about 100 feet;
but there, as well as along the whole western edge of the Michigamme
Mouulaiu syncline, the lower contact with the dolomite is ])robably faulted.
It seems entirely safe, therefore, to place the average thickness of the
Mansfield formation in the Michigamme Mountain area ;it not less than
400 feet.
PETROGRAPHICAL CHARACTERS.
The ]\Iansfiield formation consists almost entirely of ^'ery tine grained
mica-slates or phyllites. The prevailing colors are dark green, black, and
light olive-green. These are often mottled irregularly with red, due to the
infiltration of iron oxides along the secondary cleavages. The cleavage
surfaces have a dull luster, caused by the ])arallelism of the micaceous
minerals, which are too minute, however, to be distinguished by the eye or
lens.
The phyllites are often finely banded in different colors and shades.
Near the base of the formation bands of limestone and near the toj) thin
bands of graywacke are interbedded, as has already been stated. Quartz
and calcite lenses are not unusual in the minutely puckered portions of the
formation.
The secondary cleavage is the jjrominent structure, and, indeed, the
only structure of the outcrops where the color and texture bandings do not
appear. Its general direction is transverse to the main arch, or nearly east
and west, and its dip is almost vertical. The north-south compression thus
appears to have been the stronger, or to have been active somewhat later
in point of time than the east-west compression.
Under the microscope the phyllites are seen to be composed principally
of fine leaves of muscovite and chlorite, often also with a little biotite, and
with a variable and usually small amount of quartz, feldspar, and sometimes
440 THE CRYSTAL FALLS lEON-BEAEING DISTRICT.
calcite. Magnetite, ilmeuite, and limonite are usually rather abundant.
Pyrite also occurs in a few grains in nearly every slide. The differences in
color depend mainly upon the relative proportions of the chlorite and nuis-
covite, the former being characteristic of the dai'k-colored, the latter of the
light-colored, rocks. The very dark-green or black varieties contain also
an opaque and jn'obabl}' organic pigment in very minute particles. The
quartz and feldspar grains are usually verj- small and irregularly shaped.
The larger, however, of which a few occur in the slides froin the less com-
pressed rocks, have well-rounded contours. In other cases extremely
flattened and strung-out lenses composed of many small particles represent
what were doubtless <:)riginally single clastic grains.
Two varieties of cleavage are well illustrated in the thin sections,
namelv, that caused by the parallelism of the component minerals, and
" ausweichungs-clivage." The former is characteristic of the coarser-
grained varieties, and the latter of the finer grained, where the direction of
pressure has made a large angle with the bedding. In some cases the little
leaves of muscovite outline parallel and equal folds, less than 0.2 mm. from
crest to crest, each of which is ruptured, sometimes with slight displace-
ments, sometimes with none, entirely across the slide. The structure is most
distinct in the red phyllites, in which the fractures and the arrangement of
the muscovite plates afe clearly outlined by the ferruginous stain. Each
kind of cleavage in a different way tells the storj- of extreme pressure.
SECTION V. THE HEMLOCK FOR3IATION.
The Mansfield formation of the Michigamme Mountain area changes
along the strike into rocks of an entirely different character, which, as
already said, have been named the Hemlock formation.
DISTRIBUTION, EXPOSURES, AND TOPOGRAPHY.
*
The Hemlock formation in the Fence River area consists of several
varieties of schists which occupy a belt between 2,000 and 3,000 feet in
width between the dolomite on the west and the Groveland formation on
the east. The best exposures occur on the two river sections already
referred to (p. 431), but outcrops are by no means lacking elsewhere.' At
the northwest corner of the area, in sec' 6, T. 45 N., R. 31 W., the schists
are found striking N. 60°-70° W., and dipping northeast about 40°. East
THE UEMLOOK FORMATION. 441
of the center of sec. Hi, '1\ 46 N., li. 31, a few exposures occur, tlie sti'uc-
ture of which strikes a few degrees west of north and dips eastward at
angles of 45° to 50°. In sections 21 and 28, a mih- and a lialf south, numer-
ous outcrops in siniihn- attitudes are found ah)ng the northern river .section.
In sees. 3 and 4, T. 44 N., H. 31 W., a few scattered outcrops only have
been found, but throufiliout section 10 they are very abundant. For the
next 2 miles south tln-ough sees. 15 and 22, T. 44 N., K. 31 W., onlv a
few small exposures have been discovered which have the same northerlj'
strike and eastward dip. Thus for a distance of 1 1 miles along the strike
exposures occur at comparatively short intervals, the longest gap being 3
miles.
In general this belt is one of slight elevation above both the dolomite
country on the west and the iron formation country on the east. The areas
of best exposure are characterized by very rough topographical details,
which are entirely lost in the generalized curves of the map. Abrupt strike
ridges, separated by narrow ravines, succeed one another at short intervals.
In the covered areas the surface, while retaining its general elevation, has
been leveled off by the deposition of till in the hollows, and has the smoothly
undulating contours characteristic of till-covered areas.
FOLDING AND THICKNESS.
No secondary folds have been detected within the Fence River area of
the Hemlock formation, and on account of the metamorphism and cleavage
structural observations are not possible from wdiich they might be inferred.
The only clear evidence as to the attitude of the rocks was afforded by the
contact at one locality between beds of amygdaloid and agglomerate.
There the dip is eastward at an angle of 50°. The surface width of the
formation varies between 2,000 and 3,000 feet. If 50° is taken as the dip,
the thickness would be from 1,500 to 2,300 feet. If the average dip is
assumed to be 40°, or the average of the observed dips of the underl^-ing
dolomite, the thickness would be from 1,300 to 1,900 feet. Or if 30° be
taken, the average of the lower dips of the dolomite, the thickness would
be 1,000 to 1,500 feet. We may say, therefore, that the thickness north
of the southern river section is probably not less than 1,000 nor jn-obably
more than 2,300 feet. South of the southern river section the thickness
diminishes rapidly.
442 THE CRYSTAL FALLS IRON-BEARII^G DISTRICT.
PETROGRAPHICAL CHARACTERS.
The exposures tlu-ougli section 10 and the northern ])art of sec. 15,
T. 44 N., R. 31 W. — the southern river section — give us a nearly complete
seqv;ence across the Hemlock formation, the principal gaps being on the
extreme east and west, thus leaving the details of the relations with the
dolomites below and the iron formation above undisclosed. In this section
of 3,000 feet in length, the rocks are chiefly chloritic and epidotic schists,
with which are associated schists bearing biotite, ilmenite, ottrelite, and
araphibole, greenstone conglomerates or agglomerates, and amygdaloids.
These rocks ai-e characterized by a generally fine and even grain, by a lack
of sedimentary characters, and by a double structure. In most of the
varieties minerals, which have formed quite independently of and later than
these structures, are macroscopically conspicuous. The prevailing color is
green, passing to dark purple and black in the varieties in which biotite,
hornblende, and magnetite abound.
The distinction made in the field between the several varieties of the
schists is a rough one, indicating the predominating minerals rather than
iniplving the absence of the others. In fact all the varieties are intimaxely
related. The chloi-ite-schists are very fine-grained green rocks, usually
from their color evidently very epidotic; they weather to greenish or pink-
ish white. The cleavage surfaces are often plentifully sprinkled with little
flakes of Ijiotite. Frequently also black needles of ihnenite, brilliant plates
of ottrelite, and large clusters of actinolite run irregularly through them,
quite independent of the cleavages. The biotite-schists are nuicli darker,
and lack the green coloring Through them also the same metamorphic
minerals are frequentl}' interlaced. By an increase in these minerals the
passage to the other varieties in limited exposures is a very eas)^ one.
Greenstone-conglomerates and amygdaloidal rocks occur in a few
exposures. In the former, light green or gray aphanitic inclusions, of
angular shapes, ranging from an inch to 2 or 3 feet in long diameter, are
inclosed in a matrix of chlorite-schist or biotite-schist. The chlorite-schists
often hold round or lens-shaped eyes of epidote, and epidote and quartz.
That these are filled cavities can in most cases be shown only by the micro-
scope, vet some of the larger amygdules have a banded structure evident
to the naked eye. These rocks are of structural interest since they are the
PETROGRAPHICAL CHARACTERS OF OEMLOC^K FORMATION. 443
only Hiciiihers of rlic urea wliicli ])ossess undoubted bedding-. The i)lane of
contact, between an aniyg-daloid and a layer of greenstone-conglomerate in
SE. \ sec, 10, T. 44 N., R. 31 W., dips eastwanl at an angle of SO^.
Two well-marked systems of cleavage traverse all the rocks of the
southern river se(;tion. The angle betv/een their strikes is always acute
toward the north, varying from F)° to as high as 34° in different exposures,
while the direction of the bisectrix is almost constant at N. 8°-10'^ W.
The dip of both systems is toward the east at about the same angle,
namely 50° to 60°. The two systems are usually both well developed, so
that the outcrop edges break down by weathering along zigzag lines.
The character of the cleavages varies from fine partings which divide the
surface into rhombs, sometimes extremeh' i-egular in the more aphanitic
rocks to a single perfect scliistosity capable of minute subdivision, along
which the com])f)nent minerals are visibly aligned, in the more crystalline.
Along the cleavages seams of quartz and calcite have frequently formed.
Along the upper river section the rocks of the area are distinctly more
crystalline, and are chiefly biotite-schists and biotite-hornblende-schists, the
latter often veiy coarse. They are sometimes banded, but very irregularly,
the lenticular character of tlie banding- sug-o-estinp- the rhondjic cleavag-es
of the southern section. Some of the finer-grained biotite-schists contain
round or elongated areas of quartz and epidote, which resemble amygdules.
With these are associated considerable thicknesses of sericite-schists, full of
little eyes of blue quartz; these are evidently metamorphic acid eruptives.
The width of the northern section is about 2,000 feet.
Under the microscope the Hemlock schists of the Fence River area
have a general porphyritic habit. Two main divisions only are clearly
distinguished. One of these is the fine-grained mica (sericite) schists,
which are characterized Ijy the presence of muscovite as well as biotite in
the microcrystalline groundmass, and true phenocrysts of feldspar and
bipyramidal quartz, while the other embraces all the other varieties, which,
diverse as they undoubtedly are, have yet certain important characters in
common and are connectf^d by gradations. The sericite-schists are obviously
metamorphosed acid lavas, and need not be described in detail here.
The origin of the second division, however, is far more obscure. The
least altered of these rocks possess an exceedingly fine grained microcrys-
talline groundmass, made up of very pale chlorite and a colorless aggregate
444 THE CRYSTAL FALLS IRON-BEARIXG DISTRICT.
with feeble double refraction, which seems to be quartz. Between crossed
nicols the groundniass is almost isotropic, and it is by no means improbable
that certain reddish patches here and there may really be glass. Little crys-
tals of magnetite are abundantly scattered through the groundmass, and are
often arranged in parallel curving lines, very suggestive of the flowage
lines brought out on the surface of weathered rhyolites by the ferruginous
stains. In man}- sections the groundmass includes minute lath-shaped
plagioclase feldspars, much altered and with indistinct boundaries, which
are often arranged in pa,i-allel lines. The groundmass also is generally
sprinkled with little irregular grains of epidote and calcite.
In this groundmass are included in variable combinations and propor-
tions much larger crystals and grains of common hornblende, actinolite,
biotite, ottrelite, calcite, ilmenite, epidote, and zoisite. Of these biotite,
calcite, ilmenite, epidote, and zoisite are the most constant and abundant.
Biotite is present in all or nearly all of the thin sections. It is always
brown, and is characteristically developed in stubby individuals, very thick
for their basal dimensions. These individuals are large and lie scattered
through the slides. They frequently inclose portions of the groundmass.
The mica cleavage most frequently stands across the cleavage of the rock.
In many of the darker-colored schists, however, biotite plates intermediate
in size between the large porphyritic individuals and the small chlorite
plates of groundmass are present in large numbers, constituting a sort of
secondary groundmass. These are generally aligned with the cleavage
of the rock and are sometimes gathered in bands, but in color and stubby
habit are similar to the phenocrysts.
Ilmenite in brownish-black prismatic sections is a common constituent.
It usually lies at random through the slide. It incloses the quartz and
epidote grains of the groundmass. Epidote and zoisite are exceedingly
abundant, often in well-formed crystals. Manj^ of the epidote and zoisite
individuals contain darker colored inner nuclei, the nature of which is
uncertain. In some cases the nuclei are irregular in shape and have
the characteristic pleochroism of epidote, but are more strongly colored
than the surrounding zones. In other cases they have sharp crystal
boundaries, isomorphous with e])idote, are brown in color, and inclose
grains of magnetite; these may be allanite. The nuclei are too small,
however, for determination. Generally tliey do not extinguish exactly witli
PETKOGRAPIIICAL CHAKACTERS OF HEMLOCK FORMATION. 445
the .suiToiindinii- zoiu's. It is prohahlo that luauy of these nuclei represent
an early g'eneration of e[)idote, like the small irregular grains of the ground-
mass, which were subsequently enlarged to porphyritic size. Inclosui'es of
zoisite are not uncoiumon in tlie large epidote individuals. Large lenticular
aggregates of epidote with calcite, chlorite, and biotite are found partially
replacing feldspar individuals, which were no doubt original phenocrysts.
Similar aggregates unmixed with the remains of feldspar are not infrequent,
and may reasonably be attributed to the same source. Epidote with quartz
is also the common tilling of the amygdaloidal cavities.
Common hornblende, actinolite, and ottrelite are very common as
porphyritic constituents of the schists. Hornblende occurs in very large
well-formed single crystals and clusters placed at random through the
gToundmass. It is characteristically associated with ottrelite and biotite,
and often has formed somewhat later than the latter. It is always crowded
with inclusions, which in the laminated varieties carry the structure through
without reference to the position of the host. Ottrelite is abundant in some
of the sections, and is distinguished by its characteristic pleochroism. It
occurs in large individuals and multiple twins, and like the large horn-
blendes and biotites is full of inclusions.
The general characteristics of these schists then are, lir.st, a groundmass
composed of chlorite, quartz, magnetite, epidote, and in some cases contain-
ing plagioclase microlites, and secondly the presence in this groundmass of
much larger porphyritic individuals of several secondary minerals. The
varieties are determined by the varying ratio of the porphyritic constituents
to the groundmass, by the nature of the predominant secondary minerals,
and also by the diiferences in grain of the groundmass. This, while gen-
erally extremely fine grained is much coarser, Ijut without minei'alogical
change, on the northern river section where the schists are more distinctly
crystalline. The cleavage of the schists is determined by the aiTangement
of the minute particles of the groundmass, and not by the parallelism of
the large secondary minerals. These last, further, are never faulted or
broken, and in general are unstrained optically. They must have formed
then after the compression and tilting of the series.
The origin of these schists, I think, is not doubtful. As important
points of evidence we have, first, the absence of rocks possessing any sedi-
mentary characters throughout the whole section. Next we have the
446 THE CRYSTAL FALLS IRON-BEAKmU DISTRIC3T,
undoubted presence of lavas in the series, shown ))y the sericite-schists,
amyg'daloids, and greenstone conglomerates or agglomerates. Furthermore,
the minerals which compose the schists are those which would result from
the alteration, in connection with dynamic metamorphism, of igneous rocks
of basic or intermediate chemical composition. Finally, the grain and
character of the groundmass, and in some slides the presence therein of
plagioclase microlites disposed in flow lines, point directly to an igneous
orio-in and to consolidation at the surface.
Mr. Clements has reached similar conclusions for the formation above
the Randville dolomite' on the western side of tlie Archean dome. There
metamorphism seems to have progressed less far than in the Fence River
area, and among the more basic rocks he has recognized andesites and
basalts.
I conceive, then, that the Hemlock rocks of the Fence River area are a
series of old lava flows, varying in composition from acid to basic, which
first underwent dynaiBic disturbance, which developed the secondary cleav-
ages, and afterwards, in a state of rest, the porphyritic minerals were
formed. It is an interesting fact, for which I can suggest no explanation,
that metamorphism is further advanced in tlie northern part of the area
than in the southern, and the schists more distinctly crystalline. This is
also true of the underlying dolomite.
SECTION VI. THE GROVELAND FORMATIOIf.^
DISTRIBUTION, EXPOSURES, AND TOPOGRAPHY.
The Groveland formation in this area, as in the Felch Mountain range,
consists mainly of siliceous iron-bearing rocks, which hold much fragmental
material, together with certain subordinate schists. While it is of wide
extent throughout the area, its known outcrops are limited to three local-
ities, namely: The vicinity of Michigamme Mountain, in sees. 33, T. 44 N.,
' Volcanics of the Michigamme district of Michigan, by J. Morgan Clements: .lour. Geol., Vol.
Ill, 1895, No. 7, p. 801.
-This formation was originally named by me the Michigamme Jasper (Am. Jour. Sci., March,
1894). The name Michigamme was subsequently used for one of the Upper Marquette formations, in
the Preliminary Report on the Marquette District, l.'ith Ann. Rept. U. S. Geol. Survey. I now aban-
don the old name, although it is entitled to stand by the rules of priority, in order to avoid the con-
fusion which would necessarily arise from its retention.
THE GitOVELAND EOKMATION, 447
Iv. 31 W., and 3, '1\ 43 X., \\. 31 W.; the exposures and test pits at the
Shohleis exploration in sec. 21, T. 45 N., R 31 W., and the test ])its at tlie
Doane exploration in sec. K;, T. 45 N., K. 31 W. The last two localities
are 1 mile apart, and tlie inoiv soutliern is 8 miles north of Michiffamme
Mountain.
In spite of the poverty of the formation in, outcrops, its distribution
throug-hout the area has been well determined throug-h its magnetic jjroper-
ties (following the methods described in Chapter II). Adjacent to the
Fence River area of the Hemlock formation it gives rise to a strong mag-
netic line which passes through the outcrops and test pits of the Sholdeis
and Doane explorations. To the north this line was followed to the south-
ern side of sec. 32, T. 46 N., R. 31 W., where it is said to connect with a
magnetic line followed by Mr. Clements from the western to the northern
side of the Archean dome. To the south it continues into the Micliii>-amme
Mountain area to within a mile of the outcrops of Michigamme Moun-
tain. There the magnetic line gives way to a broad zone of disturbances,
feeble and difficult to interpret, but consequent I believe mainly upon the
flattening of the foi'mation as it begins to pass over the general northwest-
southeast anticlinal axis. This zone connects directly with the exposures
of Michigamme Mountain which produce similar irregular distui-bances of
the needles and which visibly constitute a thin crumpled sheet, on the
whole but gently inclined.
For the stretch of 13 miles just described the Groveland formation
occupies a continuous belt on the east side of the main anticlinal axis. In
the Fence River area it lies east of and upon the Hemlock formation, while
in the Michigannne Mountain area it holds the same relations to tlie Mans-
field formation.
The eastern belt was not traced farther than a mile soixtheast of Michi-
gamme Mountain. . In the central and southeastern portions of T. 43 N.,
R. 31 W., however', in the direct prolongation of the anticlinal axis, we
found a broad belt of slight magnetic disturbance, along the western mar-
gin of which lie volcanic rocks, dipping west. In sec. 26, T. 43 N., R. 31 W.,
this magnetic belt splits into two branches, one of which runs directly east
for a mile and then southeast indefinitely, while the other maintains a general
southerly course to the south line of the township. In section 26 large
448 THE CRYSTAL FALLS IRON-BEARING DISTRICT.
angular bowlders evideiith' derived from tlie Grovelaiid formation are found
in the zone of magnetic disturl^ance, but no outcrops have been discovered.
There can be little doubt that these disturbances roughl}' outline the position
of the Groveland formation in the axial region.
Except in ^[ichigamme Mountain, the most elevated point of the dis-
trict, the Groveland formation is not topographicall}' ^^I'ominent. In the
Fence River area it produces a more subdued and somewhat lower-lying
surface than the underlying formation, but the difference is slight and is of
little moment in comparison with the confusing effects of glaciation.
FOLDING AND THICKNESS.
In the Fence River area there is no reason to suppose that the Grove-
land formation contains within itself minor folds of any importance. Our
knowledge of its attitude is supplied almost wholly by the magnetic obser-
vations, and these indicate that it has a general eastward dip like the under-
lying members of the succession. Here and there it may be divided into
two or more parts by sheets of intrusive material, and also may be slightly
criimpled, but on the whole it must be regarded as a single persistent sheet,
having a general eastward dip.
At Michigamme Mountain the Groveland formation caps tlie hill in a
well-marked syncline, the axis of which runs northwest and southeast.
The structure is distinctly shown by the attitude both of the ferruginous
rocks and of the underlying ])hyllites. At the Interrange exploration half
a mile south, a secondary embayment of the same syncline, but more open,
is found. Tliese are the only folds of the Michigamme Mountain area
sufficiently deep to include the iron-bearing rocks. The thickness of the
formation can onlv be guessed at, as no complete section is exposed, and
the data for determining its upper limit are decidedly shadowy. The mag-
netic observations indicate a breadth of from 400 to 600 feet, and as in the
Fence River area it is certainly nuich thinner than the two lower forma-
tions, its thickness may be approximately 500 feet.
PETROGRAPHICAL CHARACTERS.
In general aspect the iron-bearing formation in this area is strikingly
like that of the Felch Mountain range, and all the varieties there found are
represented here also. It is therefore unnecessary to repeat the detailed
descriptions already given. By way of broad comparison, however, it may
PETKOGRAPHIOAL CHAHACTEKS OF GROVEL AND FORMATION. 449
he .said that tliu toriuatiou coutaius less iron than in tlie Felch .Mountain
range, and consequent!}- the lighter-colored varieties are more abundant,
that it contains niort' detrital material, and that in tlie ]\Iichigamme Moun-
tain area the texture is generally closer and less granular. Moreover, in
passing north from the Michigamme Mountain area to the Fence River area
we find at the Sholdeis and Doane explorations that the lower portion of
the formation is com])osed of ferruginous quartzite, which is succeeded
higher up by actinolite-schists and grihierite-schists similar in all respects
to the characteristic rocks of the Negaunee iron formation in the western
Marquette district. In this change in character as the Marquette district is
approached is found the lithological support for the view, first suggested by
the distribution of the lines of magnetic attraction, that the Groveland
formation is the southern equivalent of both the Ajibik quartzite and the
Negaunee formation of the Marquette district. The passage to a more
crystalline condition in going from south to north is also in accord with the
like changes already noted in the lower formations.
Under the microscope the close texture of the Groveland rocks of
Michigamme Mountain is seen to be due to the minuteness of the quartz
grains of the groundmass, and to the abundance therein of chalcedony.
The coarse quartz grains are all detrital and are often beautifully enlarged.
In many slides feldspar pebbles occur, and in many also sericite and chlorite
are prominent in the groundmass. The iron oxides, including both mag-
netite and hematite, in single crystals and also in agg-regates, are well dis-
tributed, as in the Felch Mountain sections. A similar grouping of these
in round j^ebble-like areas as in the Felch Mountain range is also beauti-
fully shown. In one slide tlie matrix is a rhomljohedral carbonate, prob-
ably calcite, in which are embedded quartz grains and the iron ores in
single crystals and irregular aggregates.
The most interesting features of the thin sections from Michigamme
Mountain are the pressure eff'ects. In many slides the detrital quartz
grains are strained to an extraordinary degree. In one case the stage was
rotated 45° before the black wave of extinction completely traversed a
little pebble 0.3 mm. in diameter. Almost every section is crossed in sev-
eral directions by fractures healed by the deposition of coarse quartz
and the iron oxides.
MON XXXVI 29
450 THE CRYSTAL FALLS IRON-BEARING DISTRICT.
In the Fence River area the lower portion of the formation consists of
quartzite, more or less ferruginous and micaceous. It contains beautifully
rounded and enlarged grains of quartz, and also less abundantly rolled
grains of microcline. Muscovite, biotite, and epidote occur, with the gen-
eral background of interlocking later quartz. The more ferruginous laj^ers
have a groundmass almost exclusively of hematite, in which the clastic
particles are set. The hematite is in parallel micaceous scales, which com-
pletely cover the cleavage surfaces. Above these layers come crystalline
actinolite-schists and griinerite-schists, the former with garnets and Ijoth
carrying particles of the iron oxides. These rocks are not distinguishaljle
in the field or in thin section from certain varieties of schists of common
occurrence in the Negaunee formation of the Marqiiette district.
50
n
I
m
!l
CHAPTER V.
THE NORTHEASTERN AREA AND THE RELATIONS BETWEEN
THE LOWER MARQUETTE AND LOWER MENOMINEE
SERIES.
From the northerninost outcrops of the Fence River area to the north-
ern end of the Repubhc trough the air-lhie distance is about 1 1 miles. This
intervening territory, on one side of which we find the typical formations
of the Menominee district and on the other the typical formations of the
Marquette district, remains to be described in this chapter. It may con-
veniently be referred to as the Northeastern area.
As was shown in the report on the Marquette district, in the pro-
ductive portion of the Marquette range between Negaunee on the east and
Repulilic on the west, the lower Marquette series consists of two or three
clearly marked formations, which, perhaps, may further be subdivided
according to individual taste. ^ The lowest of these, the Ajibik- quartzite,
which rests on the Archean complex, is fragmental in origin, and is prevail-
ingly a white vitreous quartzite, which in one or two localities is conglom-
eratic near the base. Often it is represented by a muscovite-schist as the
result of the dynamic metamorphism of the original arkose. In the eastern
part of the productive area of the Marquette district and along the northern
side of the main fold, in the western part of the district, this formation is
overlain by the Siamo slates.^ Elsewhere the slates are not present, or are
not known.
The next formation is the Negaunee iron formation,* which has already
been referred to in Chapter II. This rock, which has many phases, as there
noted, is clearly marked oif from the lower quartzite by its great richness
in iron and by the fact that over the whole Mai-quette district it nowhere
appears to contain fragmental material, except in the transitional zone
between it and the lower formations.
'The Marquette iron-bearing series of Michigan, by C. E. Van Hise and W. S. Bayley, with a
chapter on the Republic trough, by H. L. Smyth: Mou. U. S. Geol. Survey, Vol. XXVIII, 1897, p. 221.
2 Op. eit., pp. 528-529. ' Op. cit., pp. 313-315. ' Op. cit., pp. 328-407.
451
452 THE CRYSTAL PALLS IRON-BEAEING DISTRICT.
Above these conformable formations comes the unconformably placed
Upper Marquette series, tlie base of which rests now on one member, now
on the other, or on the Archean.
East and south of Negaunee, and extending thence to the shore of
Lake Superior at Marquette, is a series of rocks which resemble lithologic-
ally neither the Upper nor the Lower Marquette series in the productive
area. It consists, in ascending order, of quartzite with basal conglomer-
ates, dolomite, and slates, and thus bears a close resemblance lithologically
and stratigraphically to the three lower members of the Menominee series.
This series, named by Wadsworth the Mesnard series, has been regarded
b»y him as belonging with the Upper Marquette series, or at least as over-
lying the Lower Marquette formations just described. Maj. T. B. Brooks
had earlier correlated the dolomite with the Lower Marquette quartzite,
and had supposed that there was a gradual passage from one into the other
along the strike. Mr. C. R. Van Hise has recently stated that its position is
below the Ajibik quartzite.
This series is found only in the eastern part , of the Marquette area,
between Goose Lake and Lake Superior, a distance of about 6 miles.
Elsewhere, over by far the greater part of the Marquette district, no member
of it has been recognized.
The geological structure of the Marquette railge presents the general
features of an east-west striking complex syncline or synclinorium. The
pre-Cambrian sedimentary rocks, with their associated intrusive and extru-
sive igneous rocks, occupy the trough, in which there is much local com-
plexity of structure. The trough is flanked on the north and south by the
•older Archean crystallines.
At the western end of the district the peculiar Republic^ trough
branches from the main synclinorium, and runs southeast into the Archean
rocks for 6 or 7 miles, having a nearly constant width of about one-half to
three-quarters of a mile. In this trough the Algonkian rocks have been so
closely compressed that they stand essentially on edge. The interior is
occupied by the younger Upper Marquette quartzites dud schists, betAveen
which and the underlying Archean walls the older Lower Marquette iron
formation and quartzite here and there occur.
The northwestern end of the Republic trough is about the western
' Op. cit., p. 525.
THE NOETHEASTEEN AEEA. 453
rmiit of iniuing' develop lueiit, tliough not of exploration, on the wouth side
of the Marquette synclinorium. Up to this point outcrops, producing
mines, and old explorations are sufficiently abundant to permit the separate
formations to be traced and mapj^ed with comparative ease, and to indicate
at least the larger structural features.
At this northwestern end of the Republic trough the Lower Marquette
sei'ies makes an abrupt turn to the south, and may be followed for a mile
or more by occasional outcrops and test pits. The Negaunee iron formation
is persistently present beneath the U]3per Marquette .quartzite, and gives
rise to a very strong and persistent line of magnetic atl action, which was
followed in our work for about 12 miles to the south and southeast into the
Northeastern area. For about 4 miles from the sharp turn at the mouth of
the Republic trough it runs nearly due south; afterwards it turns somewhat
to the east of south, and follows that course for about 6 miles, after which
it turns more and more toward the east, and finally, where we left it, its
course w^as only slightly south of east. That this magnetic line is caused
by and marks the position of the Negaunee iron formation there can not be
the slightest doiibt, for that rock outcrops in a few scattered localities, occurs
abundantly in the i:lrift, aiul has been found in occasional test pits and drill
holes throughout this distance. The underlying quartzite outcrops beneath
the iron-bearing formation near the northern end of the line, but farther
south it is entirely covered by the drift, so far as the territory has been
examined. The overlying Upper Marquette rocks are also known to
be present just west of the Negaunee formation as far south as sec. 19,
T. 46 N., R. 30 W.
The magnetic line which accompanies the Negaunee formation may be
called the A line. Taking into account the connected Republic trough and
its exposures of the Lower Marquette rocks, it is seen that the A line par-
tially surrounds a dome of the Archean crystallines, ai^J that in going
from the interior of this dome outw^ard across the A line we pass from older
to younger I'ocks. The dip along the A line is, therefore, on the whole,
toward the west, although the observed dips at the few localities where
determinations have been made are either vertical or slightly inclined from
the vertical toward the east. The southern part of the A line, as far as it
has been traced, passes through sees. 5, 8, 9, 15, and 16 of T. 45 N., R. 30
W. In section 5 it is just 5 miles east of the Grroveland formation, which.
454 THE CRYSTAL FALLS lEON-BEAElNG DISTRICT.
as was shown in earlier chapters, is a magnetic rock occupying a definite
place in the Menominee succession, and is underlain by other typical
Menominee formations, and finally by the Archean.
Between the A line and the magnetic line caused by the Groveland
formation, which may be called the C line, is a third magnetic line, which
may be called the B line. This was traced parallel to the A line and less
than half a mile away, from near the south end of the latter to the north
end, and finally entirely round an elliptical area, closing again upon itself
at the starting point, the perimeter of the ellipse being 25 miles in length.
Throughout this entire distance not a single outcrop could be discovered
along the B line. Within the inclosed area, however, in sees. 6 and 7,
T. 45 N., R. 30 W., and in sec. 19, T. 46 N., R. 30 W., several exposures
of granites and crystalline schists were found, which left no doubt that
the greater part of the area inclosed by the B line is occupied by Archean
rocks of the same general character as those partially inclosed by the A
line on the east and entirely by the C line on the west. The area between
the A and B lines as far south as sec. 19, T. 46 N, R. 30 W., has been
proved to contain the basal member of the Upper Marquette series. The
southwestern quadrant of the B-line ellipse is nearly parallel to the C line
and only IJ miles away.
The known facts with reference to the B line, then, are these: (1) It
represents a magnetic rock; (2) this magnetic rock completely encircles an
Archean core. It may further be inferred with practical certainty that this
formation, which carries such constant magnetic properties for 25 miles,
must be sedimentary. With regard to' its structure, the foregoing con-
siderations would necessarily involve the conclusion that it dips away from
the Archean core on all sides, and this conclusion is fortified by the
unsymmetrical separation of the horizontal maxima on the magnetic cross
sections. It follows, therefore, that on the eastern side of the oval, where
the formation is parallel to the A line, it dips toward the east, and on the
western side, where it is parallel to the C line, it dips toward the west.
This conclusion is further supported by the dips within the ellipse in the
outcropping Archean rocks that show structure. These all happen to lie
east of the major axis, and all dip toward the east.
East of the B line, and between it and the A line, is found the basal
member of the Upper Marquette series. The rock which is manifest in the
THE NORTHEASTERN AREA. 455
B line must, therefore, be older than anj- member of the Upper Marquette
series. The Negaunee iron formation, represented in the A line, dips west,
while the rock of the B line dips east. They are both older than the basal
member of the Upper Marquette series, and are both j^'ounger than the
Archean. Thej' are both strongly and persistently magnetic. For 8 or 1 0
miles they run parallel to each other less than half a mile apart. Their
broad structural relations to the Archean basement of the region are pre-
cisely similar. Therefore, although the rock that gives rise to the B line
has never j-et been seen, it may be concluded with the utmost confidence
that it is the Negaunee iron formation, and that the A and B lines represent
this rock brought up in the two limbs of a narrow and probably deep
synclinal fold.
This conclusion carries the Negaunee iron formation 3 J miles farther
to the west, and in the northeast part of T. 45 N., R. 31 W., leaves a gap of
but 1^ miles between the Lower Marquette and the Menominee sei'ies.
Here, between the B and C lines, is precisely the same situation as
between the A and B. One magnetic rock, represented by the B line, dips
west; the other, the Grroveland formation, represented by the C line, dips
east. Between them no magnetic disturbances can be found. The area
between them must have a synclinal structure, and if they are not one and
the same formation each must undergo an extremely rapid and precisely
similar change in lithological character (namely, the loss of magnetite) in
a very short distance and be represented on the opposite side of the syn-
clinal fold by a nonmagnetic formation. Each of these rocks is ^Dersistently
magnetic in the direction of the strike for great distances. That each should
independently lose its magnetite in the direction of the dip in this particular
locality is very improbable. And, therefore, the grounds for the conclusion
that the B and C lines represent one and the same formation are quite as
firm as those upon which rests the conclusion that the A and B lines repre-
sent the same formation.
The greater portion of the Northeastern area is without outcrops, yet
through the structural and lithological residts of the magnetic work we are
able to bridge over the gap and to show with a high degree of j^robability
that the Negaunee iron formation of the Marquette range is identical with
the Groveland iron formation of the Felch Mountain range. Further,
when we recall the differentiation of the Groveland formation in the
456 THE CRYSTAL FALLS lEON-BEARING DISTRICT.
northern part of the Fence River area into ferruginous quartzite at the
base and griinerite-schist in the upper portion, it would seem probable that
the Groveland formation represents the underlying Ajibik quartzite as well
as the Negaunee formation of the western part of the Marquette range.
This conclusion has an important bearing on the interpretation of the
early geological history of what is now the Uj)per Peninsula of Michigan.
If the formations which constitute the whole of the Lower Marquette series
over the 25 miles or more of the productive and best-known portion of the
range are represented in the Menominee district and the intervening area
by a single formation, and that the highest in the Felcji Mountain succes-
sion— namely, the Groveland formation — the formations below the Grove-
land formation are all older than the Marquette rocks and do not occur at
all within the productive portion of the Marquette range. Why are these
lower formations absent?
To this question there seem to be two answers which are a priori
possible. It is conceivable that the quartzite, dolomite, and slates of the
south, or some of them, may have been deposited in a succession of
unbroken sheets over the whole Marquette area, in continuity with the simi-
lar Mesnard formations on the east, and that afterwards the main Marqtiette
area was elevated above the sea and entirely stripped of these formations
by long-continued denudation. Finally, when the time of deposition of
the Groveland formation came round, this elevated area had again been
reduced to sea level, and subsided below it, so that the Ajibik quartzite and
the Negaunee iron formation, and their southern equivalent, the Groveland
formation, were deposited in an unbroken sheet over the whole. If this
hypothesis is correct, two consequences should follow from it: First, we
ought to find some discordance between the Groveland formation or the
Lower Marquette quartzite and the lower formations in the marginal areas
between the Menominee and Marquette, and the Mesnard and Marquette
areas, respectively, or at least a gradual cutting out of these lower forma-
tions by the iron-bearing members and the lower qviartzite, and, secondly,
we ought to find, in the lack of discordance, rocks present in the areas of
continuous deposition which represent the time of denudation.
With regard to the first of these consequences no verification is possible,
at least in the territory between the Marquette and Fence River districts,
from lack of outcrops. Throughout the Northeastern area, from the north-
THE NORTHEASTERN AREA. 457
western end of the Republic trough in T. 47 N., R. 30 W., to the C hue in
T. 45 N., R. 31 W., there are no exposures whatever of the Algonkian rocks
which underhe the Groveland formation. Somewhere in this distance of
about 11 miles the lower formations disappear, but whether by unconformity
or overlap is an unanswerable question; nor (for the same reason) can it
be definitely settled whether elsewhere farther to the south there is any
discordance. That there is general parallelism between the Groveland
formation and the lower rocks, and strict conformity in some places, is true.
But this is not at all inconsistent with a period of erosion between them, if
that erosion antedated the later and more severe orogenic disturbance.
In the Mesnard area the observed relations have been interpreted by
Van Hise to mean that the lower formations disappear by overlap. The
facts at present known on the Felch Mountain side are capable of the same
interpretation, but they are not sufficiently definite to exclude the possibility
of a period of erosion below the iron-bearing formation.
With regard to the second consequence — the deposition in the sub-
merged areas of formations which would represent the erosion period in the
elevated area — the evidence at hand is decidedly against the existence of
such formations.
The alternative hypothesis is that the lower quartzite, dolomite, and
slate formations of the Menominee area were not deposited over the
western Marquette area at all, but disappear toward the north and east by
overlap, and this hypothesis is much more likely to be the true one. We
can suppose, as I have already pointed out,^ that this part of the Upper
Peninsula was a slowly subsiding area, the central portion of which, now
occupied by the Marquette rocks, stood initially at a greater elevation
above the encroaching sea than the rest. While the quartzite-dolomite-slate
triad was going down in the Mesnard area on the east and the Menominee
area on the south and west, the central Marquette area still remained
above the sea. At last, when the Groveland foi'mation began to be deposited,
the Marquette high land was finally submerged and covered, as the sea
marched over it, first, with a sheet of arkose made up of its own disinte-
grated ddbris, and, finally, with the same nonclastic sediments as chiefly
compose the Groveland and Negaunee formations.
' Relations of the Lower Menominee and Lower Marquette series in Michigan, by H. L. Smyth :
Am. Jour. Soi., Vol. XLVII, 1894, p. 222.
CHAPTER VI.
THE STURGEON RIVER TONGUE.
By William Shirley Bayley.
DESCRIPTION AND BOUNDARY OF AREA.
The Sturgeon River area of Algonkian sediments, like the Felch
Mountain area, is an east-west tongue of conglomerates, slates, and dolo-
mites, very narrow at its eastern extremity and widening out toward the
west until it finally plunges under drift deposits that separate it from the
large Huronian area of the Crystal Falls district. The tongue occupies
the western portion of T. 42 N., R. 27 W., the central and northern portions
of T. 42 N., R. 28 W., T. 42 N., R. 29 W., and T. 42 N., R. 30 W., and
the southern parts of T. 43 N., Rs. 28 W., 29 W., and 30 W. The best
exposures of the rocks constituting the tongue are found in sees. 7, 8, 17,
and 18, T. 42 N., R. 28 W., and in sees. 1 and 3, T. 42 N., R. 29 W., on or
near the northwest branch of the east branch of the Sturgeon River; hence
the name Sturgeon River tongue (PI. LI).
On the south the sedimentary rocks are bounded by an area of granites,
gneisses, hornblende-schists, and mica-schists, that are cut by granite and
quartz veins, by dikes of diabases, and by other greenstones. This area
separates the Sturgeon River tongue of sediments fi-om the Felch Mountain
tongue lying from 2 to 3 miles farther south. The exact line of demarca-
tion between the granite-schist complex and the sedimentary rocks is difficult
to draw, because for the eastern 7 miles the latter are bordered by green-
stones whose position in the granite-schist complex or in the sedimentary
series can not be determined at present. The line as drawn on the map
places the greenstones in the Archean. It begins near the south side of sec.
7, T. 42 N., R. 27 W., and runs a little south of west to the quarter post
between sections 17 and 18, in the next town west, then northwest to near
458
51
U.S GEOLOGICAL SURVEY
MONOGRAPH XXXVI PL LI,
GEOLOOK^AL MAP OF STIHGEOX HIVJ-JU TONOUE
SCALE i INCH - i MILE COT^TOUR INTKRVAL 2() FEET
'^ Outci'ops williout observed strike or dip =r Outoroijs vvilli deternuned strike and dip
TOutcrope with slaltnesH or 3(.'hiBtosity. ^ ■ Teel pilH bottomed in rock,
\^nTI('AL SCAIiE or SE(-/riON 1 INCH -i:i2<J FEET,
ARCHKAN ,AI^G0^11IAN__ PLEISTOCENE
Granite Stut;^eonquarLzite Raixdville dolamilft
A-Arhos*
SI'SIate
^-(iuart7ite
THE STUKGEON RIVER TONGUE. 459
the N. quarter post of sec. 13, T. 42 N., R. 29 W., and westward to near the
southwest corner of sec. 12, T. 42 N., R. 30 W. From this pomt the Hne
leaves the Sturgeon River tongue, curves southward, and returns east on
the north side of the Felch Mountain tongue. The eastern boundary of tlie
Sturgeon River Algonkian area is even less definitely determinable than its
southern boundary, because of the thick drift covering the rocks. This
boundary is placed at about the east lines of sees. 6 and 7, T. 42 N., R. 27 W.
because just east of this line, in the NW. ^ sec. 5, ledges of' Paleozoic
limestone occur. The northern boundary is the most indefinite of all.
The southern portions of T. 43 N., Rs. 28, 29, and 30 W., are so deeply
drift covered that but few ledges can be found in them, and these are widely
separated. In sec. 6, T. 42 N., R. 27 W., and in sees. 13 and 24, T. 43 N.,
R. 29 W., are exposures of granite. These, so far as is known, mark the
southern limit of an Archean area which stretches some miles northward
and separates the Sturgeon River fragmentals from those of the Marquette
district. The line marking the northern boundary of the Sturgeon River
tongue begins at the southeast corner of sec. 6, T. 42 N., R. 27 W., and is
assumed to run a few degrees north of west from this point until it reaches
the west line of R. 29 W., where it turns north.
Between the northern and the southern boundaries of the sedimentary
area as defined, and in the midst of the sediments, are two areas of granite,
the rock of one of which is unquestionably, and that of the other presum-
ably, older than the conglomerates within the tongue. The best defined of
these two areas lies in the northern portions of sees. 7 and 8, T. 42 N.,
R. 28 W., and sec. 12, T. 42 N., R. 29 W. It measures about 2J miles in
length and less than one-half mile in width. The extent of the second
area can not be so accurately outlined. It occupies about three-fourths of
a square mile and is entirely within sec. 3, T. 42 N., R. 29 W.
lilTBKATURE.
But few references to the existence of fragmental rocks in this portion
of the Upper Peninsula of Michigan can be found in the literature of the
region.
The early United States surveyors^ reported tlie occurrence of talcose
T. ■R^ *^^°'"''^^ observations upon the geology aad topography of the district south of Lake Superior,
by Bela Hubbard: Thirty-first Congress, first session, Executive Documents, 1849-50, Vol. Ill No 1
pp. 846, 847, 848, 855. ' ■ >
460 THE CEYSTAL FALLS IRON-BEARING DISTRICT.
and argillaceous slates in Ts. 42 and 43, Rs. 29, 30, etc., of mica-slates in
Ts. 41 and 42, Rs. 29, 30, etc., and of "calciferous sandrock" near the south
boundary of T. 42 N., Rs. 27 and 28 W.
In a list of specimens gathered from these townships Burt^ mentions
sienitic greenstone, trap, granite, granulite, and talco-micaceous slate. On
the land plats made by these surveyors conglomerate is noted on the west
line of sec. 8, T. 42 N., R. 28 W., and marble at the south corner between
sections 3 and 4 in the same township.
In 1851 Messrs. Foster and Whitney reported^ the existence of an arm
of Azoic rocks about 18 miles in length and 10 in breadth, extending east-
erly into Ts. 42 and 43 N., R. 28 W., and located its position on their map
of the Upper Peninsula.
Brooks,^ in his description of the northern iron belt of the Menominee
district, refers to the existence of outcrops of hornblendic rocks, mica-
schists, and gneisses, cut by trap dikes, which he regarded as equivalents
of the various greenstone-schists exposed along the Menominee River.
"Near the center of this hornblendic belt, in the north part of sees. 22, 23,
and 24, T. 42 N., R. 29 W., a line of weak magnetic attraction was observed.
This is regarded as an indication here of the existence of an iron-ore belt."
The gneiss, granite, etc., north of the north quarter post of sec. 31,
T. 42 N., R. 29 W., he declares to have the appearance of typical Laurentian
rocks. "If future investigations prove them to be Laurentian, a very
troublesome structural problem would be presented here, as we would have
Laurentian rocks conformably overlying beds unmistakably Huronian."*
The only distinct reference made by Brooks to the sedimentary beds
of the district is in the following paragraph:^
A range of marble associated with quartzite, chloritic and talcose rock, and
overlaid by a chloritic gneiss, with beds of chloritic schist and gneissoid conglom-
erate, the whole dipping at a high angle to the south, passes about 5 miles north of
' Geological report of the survey of a district of township lines in the State of Michigan, in the
year 1846, by Wm. A. Burt: Thirty-first Congress, first session, 1849-50, Senate Documents, Vol. Ill,
No. 1, p. 84.
^ Report on the geology and topography of the Lake Superior land district, hy J. W. Foster
and J. D. Whitney, Part II, The Iron Region: Thirty-second Congress, special session, 1851, Senate
Documents Vol. Ill, No. 4, p. 14.
3 Iron-Bearing Rocks (economic), by T. B. Brooks: Geol. Survey of Michigan, Vol.1, 1869-1873,
N.Y., 1873, p. 161.
* Op. cit., p. 175. ' Op. cit., p. 176.
LITBRATLTRB ON STUEGEON RIVER TONGUE. 461
the North belt (i. e., the Pelcb Mountain tongue). These may represent the north
side of the trough or basin, of which this iron belt is the south outcrop. No iron
has, however, been found, so far as I know, on this range.
In Romiuger's first I'eport on the Menominee district only a single
reference is made to this area. He declares that a series of test pits put
down in the W. ^ sec. 26, T. 42 N., R. 29 W., and in the SW. i sec. 14,
T. 42 N., R. 30 W., are in decomposed granite.^
A specimen of the conglomerate referred to by Brooks as overlying
the marble in the belt 5 miles north of Felch Mountain is described and
pictured by Van Hise^ in his paper on the Principles of North American
pre-Cambrian Geology (see also PI. LIII). It is stated to be from the
Felch Mountain district. The more exact location of the ledge from which
it was obtained is near the northwest corner of sec. 17, T. 42 N., R. 28 W.,
in the Sturgeon River tongue.
Thus the only distinct reference to a tongue of sediments north of the
Felch Mountain range is that of Brooks, although the existence of sedi-
mentary rocks in this portion of the Menominee district was reported by
Hubbard and Burt. Brooks believed that the Sturgeon River rocks repre-
sented the northern rim of a syncline whose southern i-im constitutes the
I'dch Mountain range, although both he and Rominger discovered a
granite-schist complex underlying the country between the two areas of
fragmental rocks.
RELATIONS BETWEEN" THE SEDIMENTARY ROCKS AND THE GRANITE-
SCHIST COMPLEX.
As has already been stated, the country between the Sturgeon River
tongue -of sediments and the Felch Mountain tongue is underlain by a com-
plex of granites and various schists, traversed by fresh and altered diabases
and by granite and quartz veins. Brooks recognized these rocks as pre-
senting a Laurentian aspect, although he felt constrained to call them
Huronian, because of the supposed structural difficulties involved in any
other view of their age.
No contacts of this granite-schist complex with the bedded rocks of
the Sturgeon River tongue have been discovered. Nevertheless, there can
be little question as to the relative ages of the two series. As has been
1 Geol. Survey of Michigan, by C. Rominger, Vol. IV, 1881, pp. 198-199.
2 Sixteenth Ann. Kept. U. S. Geol. Survey, 1896, p. 801 and PI. CXV.
462 THE CRYSTAL FALLS IROISr-BEAEING DISTEIOT.
stated, the granites and schists extend southward to the Felch Mountain
fragmentals, and here they are unconformably beneath the latter. Moreover,
since the Sturgeon River rocks and the lower members of the Felch Moun-
tain series are identical in character, it is probable that they are of the
same age, in which case the granites and schists that are older than the
Felch Mountain rocks are older also than those of the Sturgeon River
tongue.
The relations of the sedimentary series to the granites on the north
have not been determined, because no contacts are exposed. The granites,
however, can be traced northward until they are found unconformably
beneath the rocks of the Lower Marquette series at Republic, and these, so
far as is known, are the oldest sediments in Upper Michigan. There can
be little doubt, therefore, that the relations of the sediments to the northern
granites are the same as those with the southern schist complex.
The granites of the two areas surrounded by the sediments are prob-
ably of the same age as the northeni and southern granites. The rocks of
the area in sees. 7 and 8, T. 42 N., R. 28 W., and sec. 12, T. 42 N., R. 29 W.,
are demonstrably beneath the conglomerates, though their relations with the
dolomites have not been determined. A well-marked contact between the
granites and the conglomerates is exposed at the south base of a small hill
of granite in the NE. | sec. 7, T. 42 N., R. 28 W} The conglomerate here
is well bedded. Its strike is N. 60° W., and its dip almost vertical. It
consists largely of pebbles and bowlders of granite identical with the granite
composing the hill, and a matrix constituted entirely of granitic debris.
The contact, though exposed for only a short distance, seems to be an
erosive one. It is certainly not an igneous one.
From a consideration of the facts as given above, there can be little
doubt that the rocks of the granitic areas within the Sturgeon River tongue
and of those bounding it on the northern and the southern sides are older
than the sediments within the tongue, though this has not been proved for
the granites with respect to the limestones.
From the lithological similarity of the Sturgeon River fragmentals
with those of the Felch Mountain district, and from the structural relations
exisiting between the rocks of the two districts, it is practically certain that
the Sturgeon River sediments are of the same age as the Felch Mountain
1 The exact location of the coutaot is 400 paces N., 280 W., of the southeast corner of section 7.
BASEMENT COMPLEX OP STURGEON RIVER TONGUE. 463
ones — i. e., Meuomiuee (Huroiiiau) — while the granites and schists belong
to the Basement Complex on which the Lower Algonkian beds throughout
Michigan have been laid down.
THE BASEMENT COMPLEX.
• The Basement Complex rocks in the area studied comprise gneissoid
granites, biotite-schists, and hornblende-schists, cut by dikes of greenstone
and by veins of quartz and granite. The granites are best exposed in the
NE. 4 sec. 7 and the NW. ^ sec. 8 and the NE. i sec. 7, T. 42 N., R. 28 W.,
where they occur as bare knolls of a fairly coarse pink rock, separated
from one another by stretches of sand. The best exhibition of rocks with
the typical aspect of the Basement Complex is along the west half of the
east-west quarter line of sec. 19, T. 42 N., R. 28 W., and south of the center
of this section. Here we find hornblende-schists and hornblende-gneisses
cut by veins and dikes of red granite and by greenstones that are usually
schistose. Near the west quarter post of the section is a high hill bare of
vegetation. On this hill the rocks are especially well exposed. In addi-
tion to the types already mentioned, there is present here a coarse white
pegmatitic-looking granite that apparently cuts the hornblende-gneiss.
All the members of the Basement Complex in this area are so similar
to the corresponding members of this complex elsewhere in the Lake Supe-
rior region that they demand but little description. They are described
here only in sufficient detail to establish their character.
THE GNEISSOID GRANITES.
The gneissoid granites north of the fragmental tongue, and those of
the two areas surrounded by the sedimentary rocks, are mediumly coarse
aggregates of a dark-red feldspar, white quartz, and a dirty green chloritic
substance. The red feldspar is in excess, sometimes to the exclusion of the
other components, when the hand specimen resembles a dense red felsite.
Almost all specimens are gneissoid. The constituents are usually lenticu-
lar, but in a few specimens, particularly those taken from near the contacts
with the sedimentary rocks, they are drawn out into long slender string-like
masses, giving the specimens a streaked appearance.
The microscopical features of all the granites are those common to these
rocks elsewhere in the Basement Complex. They consist of clouded ortho-
464 THE CRYSTAL FALLS lEON-BEAEING DISTEICT.
clase, some plagioclase and a little microcline, quartz in varying quantity,
and more or less green chlorite that seems to have been derived from
biotite. All the constituents present abundant evidence of the efFects of
pressure. In the least-crushed rocks the quartz shows undulatory extinc-
tions, and the feldspar grains granulation around their edges. As the
criTshing action increased, the granulation increased, so that the most
crushed granites now consist of large grains of feldspar and of quartz in an
aggregate of broken fragments of orthoclase, quartz, plagioclase and micro-
cline, and a few wisps of green chlorite. Movement in the crushed rock
mass has drawn out the granulated aggregate between the large grains of
feldspar into bands and lines, thus producing the schistose structure noted
in the hand specimens and in the ledges. In the more highly schistose
granites a considerable quantity of new microcline and a small quantity of
new plagioclase have developed within the granulated aggregate, and in a
few instances muscovite has been found in fairly large plates of pale-yellow
color. This muscovite occurs on the contact between the larger quartz and
orthoclase grains, but more particularly in the granulated matrix.
The granites in the area between the Sturgeon River and the Felch
Mountain tongues are not so abundant as those in the northern area of Base-
ment Complex rocks, or in the areas surrounded by the sediments, but in
their essential features they are identical with these. Occasionally the sur-
face of a fresh fracture through these southern granites shows the outlines
of porphyritic orthoclase crystals, but these crystals are riot sufficiently
numerous to impart a porphyritic aspect to the rock.
Some of the granite specimens examined from this district are so com-
pletely granulated that they can with difficulty be distinguished in the hand
specimens from the schistose arkoses near the base of the fragmental series.
In thin section they differ from the latter in containing no rounded quartz
grains and in the possession of very little niica. The feldspathic constitu-
ents are nearly all decomposed, and very much of the quartz present in the
granites is of secondary origin.
Thus in all essential respects the gneissoid granites of this district are
like those in the Marquette district elsewhere described.^
'The Marquette iron-bearing district of Michigan, hy C. E. Van Hise and W. S. Bayley, with a
■chapter on the Republic trough, by H. L. Smyth : Mon. U. S. Geol. Survey, Vol. XXVIII, 1897, pp.
171-176.
BASEMENT COMPLEX OP STURGEON EIVEE TONGUE. 465
THE AMPHIBOLE-SCHISTS.
In additiou to gneissoid granites, the southern area of the Basement
Complex contams a number of ledges of dai-k-colored schistose rocks.
These, in some instances, are cut by dikes of granite similar to the granite
already described.
These dark schists may be classed as greenstone-schists and as horn-
blende-schists. The former are heavy rocks, with dull greenish-gray luster
and distinct schistose structure. They resemble closely in their micro-
scopic as well as in their macroscopic features the schistose dike greenstones
to be referred to later. They are doubtless altered or squeezed diabases or
gabbros.
The hornblende-schists are usually line-grained, bluish-black rocks,
with a very even schistosity, closely resembling slaty cleavage. On the
surfaces of cross fractures may be seen long slender prisms of glistening
black hornblende aiTanged in as distinct lines as the lines of particles in an
evenly bedded sedimentary rock. Often the cleavage surfaces are coated
with thin layers of golden-yellow mica scales. In most specimens there
may also be noticed a fine banding parallel to the foliation.
In thin section these hornblende-scliists differ from the schistose dike
diabases and from the greenstone-schists, referred to above, in the presence
of large quantities of quartz, and of some biotite, and to some extent in
structure. The greenstones owe their schistosity to the flattening of their
components, while in the hornblende-schists this structure appears to be due
largely to the crystallization of the hornblende in elongated prisms with
their major axes parallel. The parallel arrangement of the amphibole in
the latter rocks is thus much more pronounced than in the schistose
greenstones.
The hornblende-schists are comjDosed of hornblende, quartz, biotite,
plagioclase, magnetite, and sphene.
The hornblende is in long prisms of the usual yellowish green color.
The mineral is compact, but it is full of inclusions of quartz grains similar to
those constituting a large part of the matrix lying between the amphiboles.
It was evidently formed in situ as an original crystallization, and not, like
much of the hornblende of the schistose greenstones, by the alteration of
augite or of some other component of a basic crystalline rock. The biotite
MON XXXVI 30
466 THE CRYSTAL FALLS IRON BEARING DISTRICT.
is in small dark greenisli-brown flakes interspersed between quartz and
feldspar grains, wliich together constitute a matrix surrounding the other
components. The quartz is unusually free from inclusions. It contains a
few liquid inclosures and occasionally a few flakes of biotite and needles of
hornblende. The plagioclase, where present, is in irregular grains with
ragged outlines, as though a newly formed mineral. It apjjears to act the
part of a cement surrounding the other minerals with which it is in contact.
Small round grains of sphene and magnetite occur very abundantly scat-
tered through the matrix. Often the magnetites are surrounded by borders
of sphene ; hence it is probable that this mineral is a titaniferous variety and
that the round grains of sphene are pseudomorphs after mag'netite grains
that have been completely altered.
In a few specimens large colorless areas with the outlines of porphy-
ritic crystals are observed in the midst of the finer-grained groundmass of
schist. Between crossed nicols these break up into a coarse-grained aggre-
gate composed of the same minerals that constitute the rest of the rock,
except that in it altered plagioclase is common and amphibole is rare.
These probably represent phenocrysts of plagioclase which have suffered
alteration into quartz and new plagioclase that may differ somewhat from
the feldspar of the original crystal.
The banding of some of the hornblende-schists has already been
referred to. Under the microscope the only differences noted in the bands
are the quantity of hornblende present in them and a variation in the
coarseness of grain. The coarsest of the bands have the composition and
structure of the schistose greenstones. They contain large quantities of
plagioclase, both fresh and altered, and large grains of hornblende that are
not in the definite prismatic form characteristic of this mineral in the main
mass of the rocks.
OEIGIN OF THE AMPHIBOLE-SCHISTS.
From the gradations often observed between the hornblende-schists
and the greenstone-schists, it is plain that the two rocks are genetically
related. The latter, from their similarity to schistose dike greenstones in
composition and structure, are believed to have been derived from massive
diabases or gabbros. The hornblende-schists are in all probability derived
.from similar basic rocks, though the presence in them of what appears to
BASEMENT COMPLEX OF STURGEON EIVEK TONGUE. 467
have once been plagioclase pheiiociysts may indicate that the original rocks
were in the form of hxvas.
The principal difference between the hornblende-schists and tlie
greenstone-schists seems to be in the nature of the amphibole in the two
rocks and in the presence of quartz and newly formed plagioclase in the
first named. The materials of the greenstone-schist were derived from the
alteration of those of the original rock, as were also those of the hornblende-
schist, but the former now consist mainly of the direct jjroducts of this
alteration, whereas in the latter the substances now existing liaA^e been
worked over and entirely recrystallized.
THE BIOTITE-SCHISTS.
Mica-schists are not common in the Sturgeon River tongue. They
constitute by no means so large a part of the Basement Comj)lex in this dis-
trict as they do in the other portions of the Lake Superior region that have
been studied. Indeed, only a few ledges of this rock have been observed
in the country between the Sturgeon River and the Felch Mountain sedi-
mentary tongues, and most of these are along the southern edge of a g'reen-
stone knob 300 to 400 paces north of the southeast corner of sec. 17,
T. 42 N., R. 28 W.
The mass of this knob is a dark hornblende-schist. On the south side
of the top of the knob this rock is in contact with a A^ery evenly banded or
streaked rock of a general dark-gray color. In the hand specimen it resem-
bles very closely a fine-grained banded aug-en-gneiss. Near its contact
with the hornblende-schist the rock is apparently porphyritic, with pheno-
crysts of feldspar from 1 to 15 mm. in length, and an occasional one of
quartz scattered through a matrix composed of narrow altei-nating bands of
almost black and light-gray material. On cross fractures of the rock
the phenocrysts are seen to • be drawn out in the direction of the bands.
Cleavage takes place verj'- readily along the j^lanes of the banding, yielding-
siTrfaces covered with tiny scales of black biotite. A little farther from the
contact the light-colored bands are thicker and more distinct. At first
glance they appear to be uniformly thick for long distances, but a more
careful inspection shows that they wedge out rapidly and are replaced by
other bands of the same character. The dark bands are not thicker than
468 THE CRYSTAL FALLS IRON-BEARmG DISTRICT.
sheets of paper. They are the cross sections of the mica coatings on the
cleavage planes.
The inspection of this rock in the hand specimen and in the ledge
leads to the same conclusion — that it is an intermediate or an acid lava, a
porphyrite, or a porphyry that was squeezed until it became schistose and
sheared until it became fissile.
Under the microscope the feldspar phenocrysts, though much decom-
posed and filled with inclusions of quartz, muscovite, and other decomposi-
tion products, are well enough preserved to exhibit in some places twinning
striations. The greater portion of the phenocrysts are untwinned. The
twinned material borders the grains, fills in cracks between Carlsbad twins,
and is irregidarly distributed through the untwinned material, occurring
more particularly in those places where the decomposition of the original
feldspar is most complete. The twinned feldspar is fresher than the
untwinned variety. This fact and the manner of its distribution indicate a
secondary origin for it. The quartz phenocrysts are rare. They present
their usual characteristics.
The groundmass in which the phenocrysts lie is a fine-grained aggTe-
gate of biotite, quartz, and plagioclase. The biotite is a greenish-brown
variety. It occurs in large plates arranged in parallel position and in
small flakes occupying the same parallel position and lying between the
quartz and the plagioclase grains. The banding noticed in the hand speci-
men is due to the arrangement of the large biotite flakes in bands. These
are separated from each other by bands of quartz and plagioclase that are
free from the large biotites, though they contain innumerable small flakes
of this mineral. Only when a porphyritic crystal lies in the way of tlie
bands do these depart from their uniform directions. Here they bend
around the phenocrysts, leaving on both sides of them little triangular areas
in which the components are much finer grained than elsewhere in the rock.
The light-colored components are quartz and plagioclase. These min-
erals are in small grains that appear to be intercrystallized in the manner
of the secondary aggregate that constitutes the fine-grained matrix of many
greenstones, of the aporhyolites, and of other rocks that have sufi"ered
intense metamorphism. The quartzes are nearly always crossed by strain
shadows and the fresh clear plagioclase by interrupted and bent twinning
bars.
BASEMENT COMPLEX OF STURGEON RIVEE TONGUE. 469
Here and there in the midst of this fine-grained groundmass are noticed
lenticuhxr and k)iig' narrow aggregates composed of grains of plagiochise
that are much larger than the grains of this mineral occurring- in the sur-
rounding matrix. They look as though they might be the crushed remains
of what were originally plagioclase phenocrysts.
Thus the microscopic study of these rocks tends to confirm the results
of their field study. They were probably porphyritic lavas or intercalated
flows that have sufl'ered alteration as the result of intense pressure and
movement. Their present composition suggests that they were oi'Iginally
quartz-porphyries or perhaps andesitic porphyrites. Whatever their original
nature, their origin is different from that of the biotite-schists of the Mar-
quette district.-'
THE INTRUSIVE ROCKS.
The intrusives in the schists and gneissoid granites of the Basement
Complex are granites, identical with the gneissoid granites above described,
and greenstones. The former cut only the schists. They are probably
apophyses from the larger granite masses. The greenstones cut the schists
and the granites. They are similar in all respects to the greenstones in the
sedimentary series, and thus are the youngest rocks in the district, with
the exception of the horizontal Paleozoic sandstones and limestones that
cap some of the higher hills.
The greenstones are all more or less altered diabases. In some the
ophitic structure may be detected, but in most of them no traces of their
original constituents nor of their structure remain. Nearly all are more or
less schistose. The only evidence that the most schistose phases were once
massive igneous rocks is in their composition and their occurrence in dike-
like fissures. As the schistosity of these greenstones increases, the amount
of their altei'ation also increases; there is a greater abundance of horn-
blende present in them and a greater quantity of quartz, until in the most
schistose phases the rocks are now typical hornblende-schists.
One of the best examples of these greenstones occurs in the series of
ledges extending in nearly a straight line for 6 miles from the southern
portion of sec. 13, T. 42 N., R. 29 W., to the northeast corner of sec. 14,
T. 42 N., R. 28 W. Except in its eastern ledges the rock constitutes bold,
' Mou. U. S. Geol. Survey, Vol. XXVIII, pp. 200-203.
470 THE CRYSTAL FALLS lEON-BEARING DISTRICT.
rounded, bare knobs with ahnost perpendicular sides, usually situated in
the midst of swamps. The main mass of the knobs is a rather fine-grained,
slightly schistose, gray rock exhibiting the diabasic structure on weathered
surfaces. On the south sides of the knobs the rock is much denser, and in
most cases is much more highly schistose than the main rock mass.
Under the microscope these rock's present the usual features of schistose
dike greenstones. They consist almost exclusively of hornblende, plagio-
clase, and quartz. The hornblende, which is the common yellowish-green
variety, occurs in long plates and in columnar crystals, some of which are
idiomorphic in cross section, and also in slender needles penetrating the
quartz and feldspar. These two minerals form an aggregate between the
larger hornblendes. The feldspar is mainly a calcium-soda plagioclase,
though a small quantity of albite may also be present. It occurs as irreg-
ular grains embedded in a mosaic composed of rounded grains of the same
feldspar and of quartz, and appears to be a new crystallization subsequent
to that of the greater portion of the plagioclase. At any rate, a single
large grain often fills the interstices between numbers of the mosaic grains
and extinguishes uniformly over large areas. The magnetite in the rock is
titaniferous. It occurs in little crystals and in small irregular grains that
are often surrounded by a granular zone of leucoxene.
This rock may serve as a type of nearly all the other dike greenstones
in the district under discussion. Some may be more schistose than this one,
while a few ma}^ be more massive, but in general characteristics they are all
similar. The more schistose rocks diff'er from the less schistose varieties
simply in the possession of a greater amount of quartz and a greater quantity
of what appears to be newly formed feldspar. Their greater schistosity is
due to the more uniform elongation of their components.
The fine-grained greenstones found on the edges of the coarser-grained
ones, and occasionally as independent dikes, are weathered diabases of the
normal type.
COMPARISON OF THE STURGEON RIVER AND THE MARQUETTE CRYSTALLINE
SERIES.
The Basement Complex in this area is essentially like that in the Mar-
quette district, except that the altered tuffs so abundant in the northern area
are absent from that now under discussion. The biotite-schists of the two
ALGONKIAN KOCKS OF STURGEON EIVBR TONGUE. 471
areas seem also to be different in origin, althoug-h this can not be stated
with certainty, since the origin of the Marquette schists is not so clear as is
that of the Sturgeon River schists. There is enough similarity between the
crystalline series in the two areas to leave no doubt as to their practical
identity. If the Marquette Basement Complex is Archean, the crystalline
series underlying the conglomerates in the Sturgeon River tongue is also
Archean.
THE ALGONKIAX TROUGH.
The sedimentary rocks comprised within the Sturgeon River Algonkian
tongue may be separated into a conglomerate series and a dolomite series.
The conglomerate series consists of schistose conglomerates, arkoses, qnartz-
ites, slates, and certain sericitic schists that are squeezed arkoses. The
dolomite series embraces crystalline dolomites, a few thin beds of quartz-
ite, a few breccias and conglomerates, and some slates.
It is possible that a third series, composed essentially of slates, also
exists in the district, but if so it is not advisable to separate it from the
dolomite series, since its exposures are very few in number, and the slates
which comprise its main mass are so nearly like the slates belonging in the
dolomite series that they can with difficulty be distinguished from these.
Associated with the sedimentary rocks are great inasses of basic
igneous ones. Some of these are unquestionably intrusive masses, as
shown by their relations to the conglomerates, while others appear to be
interleaved sheets. A very few, apparently bedded greenstones, on close
examination seem to be composed of intermingled sedimentary and igneous
material. These may be altered tuffs.
Nearly all the sedimentary as well as the igneous rocks embraced in
the trough are schistose, and thus are sharply distinguished from the brown
Potsdam sandstones and the Silurian limestones that here and there lie
approximately horizontal on their upturned edges. The squeezing of the
pre-Potsdam formations has been so intense that both conglomerates and
dolomites have been forced into closely appressed folds, which in the con-
glomerates are for the most part apparently isoclinal. The strike of the
latter rocks is nearly east and west, and their dip nearly perpendicular,
except in one or two cases. The dolomites are less closely folded than the
conglomerates. Their dips are mucli less steep, and their strike varies
472 THE CRYSTAL FALLS IROJS[-BEAEING DISTRICT.
considerably, except in the narrow eastern portion of the tongue, where
it is approximately parallel to that of the conglomerate, i. e., a few
degrees north of east.
RELATIONS BETWEEN THE CONGLOMERATE AND THE DOLOMITE SERIES
AND CORRELATION WITH THE FELCH MOUNTAIN FRAGMENTALS.
The relations of the conglomerates to the dolomites are best shown
by the distribution of their respective outcrops, as members of the two
series are nowhere in contact. In the central portion of the tongue the
conglomerate outcrops are limited to the district between the central granites
and the southern area of the Basement Complex. The dolomites, on the
other hand, are limited to the country north of the central granite. Its
outcrops are found scattered over the northern tier of sections in T. 42 N.,
Rs. 28 W. and 29 W., and the southern tier of sections in T. 43 N., Rs. 28 W.
and 29 W. Between them and the granite to the north is a belt of country
devoid of exposures. It is heavily drift covered, consisting of sand plains
and sand hills, from beneath which no ledges of any kind protrude. This
barren belt measures about a half mile in width, in sec. 2, T. 42 N., R. 28 W.,
gradually increasing in width till it reaches the center of sec. 1 in T. -tS N.,
R. 29 W., where it opens out into the large Pleistocene area whose southeast
edge is shown on the map (PI. LI). In the eastern portion of the district
the northern granites and the conglomei'ates approach each other, and the
dolomite belt becomes very narrow, finally disappearing toward the east
side of T. 42 N., R. 28 W.
The relative distribution of the conglomerate and dolomite ledges,
when considered with reference to the triangular outline of the area
embraced between the northern and the southern granite-schist complexes,
suggests that the two formations constitute a western-pitching syncline with
the dolomite in the center and the conglomerates with their associated beds
on the two flanks. The conglomei'ates comprising the southern flank are
well exposed, but those of the northern flank are not seen. They are
believed to underlie the glacial deposits in the barren strip of country
bordering the northern granites. The conglomerates, according to this
view, are older than the dolomites.
Toward the center of the dolomite area, in the north half of sec. 6,
T. 42 N., R. 28 W., and at a few places farther west, there are ferruginous
ALGONKIAN EOGKS OF STURGEON ElVBR TONGUE. 473
beds in the dolomite series. If these represent the upper portion of the
dolomite formation, as is the case with similar rocks in the Felch Mountain
range, it is clear that as we approach the center of the Sturgeon River
tongue the rock beds met with are younger than those on its borders. This
is in line with the supposition that the Sturgeon River tongue is a westward-
pitching syncline.
The belief that the conglomerates are beneath the dolomites in the
Sturgeon River area is further strengthened by the fact that the jirincipal
conglomerate in the Felch Mountain range is benea.th a dolomite which is
identical in character with the Sturgeon River dolomite. This conglomerate
is regarded as the base of the Lower Menominee series in this district, with
the dolomite above it, known as the Randville dolomite, immediately suc-
ceeding it. If the conglomerates and dolomites in the two districts are the
same, the Sturgeon River rocks are Lower Menominee.
RELATIONS BETWEEN THE DOLOMITES AND CONGLOMERATES AND THE
OVERLYING SANDSTONES.
At several places the conglomerates- and dolomites are overlain hy
well-defined Lake Superior sandstone. The sandstone usually caps hills,
on the lower slopes of which ledges of the underlying rocks appear. The
contacts between the overlying sandstone and the underlying rocks are
rarely seen, but the fact that the former are always horizontal, while the
latter are always very steeply inclined, leaves no doubt that there is a
strong unconformity between them.
THE CONGLOMERATE FORMATION.
The conglomerate formation comprises very much squeezed granitic
conglomerates, arkoses, sericite-schists, quartzites, a few beds of banded
rocks believed to consist largely of tuffaceous material (see pp. 486-487),
and occasional beds of slates. Nearly all the members of the series are
schistose, the arkoses in some cases passing into very well characterized
sericite-schists. Occasionally the arkoses show obscure traces of ripple
marking, and more frequently very well defined cross bedding.
All the rocks of this formation strike in a nearly uniform direction, N.
76°-84° E., and dip almost vertically. In one or two instances observed
the dip is as low as 65°, but in most cases it varies between 85° N. and
85° S. The strike of the schistosity is approximately parallel to the strike
474 THE CEYSTAL PALLS IRON-BE ARIKG DISTRICT.
of the bedding, as is also the direction of the elongation of the pebbles so
abundant in the conglomeratic layers.
From the slight changes iii dip observed in the beds, as well as the
great width of the formation in some places, it is evident that folding must
exist. It is probable that in the wider portions of the area occupied by
these rocks there are present two or more folds, so closely appressed that
the beds on the opposite limbs can not be correlated. Hence they appear
as members of a consecutive series of conformable members with a nearly
uniform dip throughout. In the narrower portions of the area it can not
be told whether more than one fold is loresent or not. In any event, the
folding is practically isoclinal.
The ledges of the conglomerates and their associated beds occur in the
southern portion of the Sturgeon River tongue throughout its entire extent.
No exposures have been found north of the granite areas in the central and
western portions of the tongue.
IMPOKTANT EXPOSURES.
The arkoses, the sericite-schists, and the conglomeratic phases of the
series can be best studied at the dam of the Sturgeon River near the north-
west corner of sec. 17, T. 42 N., R. 28 W. Here they form a continuous
ledge of well-bedded layers striking N. 83° E., and dipping 85° S., which
measures at least 250 yards in width and 400 yards in length. (See PI. LII.)
The conglomerates are pink in color. They contain immense numbers of
white quartz pebbles and bowlders, fewer and smaller ones of pink granite,
and many fragments of red feldspar in a matrix composed of moderately
coarse granite debris. All the fragments and pebbles in these rocks, as well
as then- matrix, show plainly the eflPects of pressure (PL LIII.) The matrix of
all specimens is more or less schistose, and the coarse sand grains embedded
in it are in many cases elongated in the direction of the schistosity. Most of
the pebbles and bowlders in the conglomerate are also flat and parallel to
the schistose plane. How far these phenomena are due to mashing, to rota-
tion into parallel positions during flattening, and to original sedimentation,
respectively, can not be determined in most cases, since the schistosity of
the rock and the elongation of the pebbles are both approximately parallel
to the bedding — i e., the pebbles are nearlj^ in the positions assumed by
unequidimensional pebbles in a well-bedded conglomerate. In a few
us GEOLOGICAL SURVEY
MONOGRAPH XXXVl PL III
n \
^
mW^ ^ '^
Mm>^^'f
|;f %^^. vf f
^l\%^
%
,^
JULIUS BiEN aco.N.r
MAP OF EXPOSURES
IN SEC. 7 AND IN PORTIONS OF SECS. 8, 17, AND 18, T. 42 N.,P. 28 W., M ICH I GAN
VICINITY OF DAM ON EAST BRANCH OF STURGEON RIVER
I A. Arkose
SI. Slate
Cj. Quartzite INTRUSIVE
C. Conglomerate
G.T. Greenstonetiiffaceous
,,j G. Greenstone
G . S . Greenstone Schist
ALGONKIAN EOCKS OP STURGEON EIVER TONGUE. 475
instances the scliistosity may be seen to meet the bedding at a very acute
angle. In this case the pebbles are usually arranged with their longer axes
parallel to the schistosity, though there are always present a large number
that lie parallel to the bedding planes.
In the least schistose phases of the rocks the pebbles are nearly round
and the matrix possesses a well-defined fragmental texture, but in those
beds in which the schistosity is more pronounced the matrix is seiicitic and
the pebbles are lenticular. The most completely schistose jihases resemble
augen-gneisses. In these the matrix is an almost typical sericite-schist.
The quartz pebbles have been crushed and flattened into long narrow
stringers or plates of quartz, some of which are continuous for long distances
(6 or 7 inches), while others are broken into separate parts, which when
rounded on their edges yield quartz lerses like the "augen" of so many
augen-gneisses.-'
The nonconglomeratic beds interstratified with the conglomerates are
usually more completely schistose than the latter. The least schistose beds
are arkoses. These often show ripple marking and current bedding. As
the schistosity increases, the quantity of sericite present also increases, until
in the most highly schistose phases sericite-schists result.
Some of the arkoses, as well as some of the finer-grained conglomerates,
in addition to being schistose, are also foliated — i. e., they are built up of
plates or leaves, along the planes between which they split very easily.
When this is the case, the cleavage surfaces are covered by small scales of
silvery mica. The foliation is so pronounced in many cases that the rocks
are almost fissile.
Besides these rocks there are present near the dam great ledges of
coarse and fine grained greenstone (see PL LII), whose relations to the
sedimentary beds at first glance appear to be those of interleaved flows.
Upon close inspection some of these masses disclose intrusive features.
Although they almost invariably follow the bedding of the fragmental
rocks, some of the greenstones can be seen to cut across the layers in such
a manner as to leave no doubt of their intrusive character.
' The best examples of these extremely schistose conglomerates are not found in the exposures
referred to above, but they are well developed along the line between sees. 11 and 12, T. 42 N., K. 29 W.
Here the width of the series is but one-fourth mile, whereas the total width of these rocks and tbeir
associated greenstones near the dam measures a full mile.
476 THE CEYSTAL FALLS IRON-BEARING DISTRICT.
On the old road leading to the dam the conglomerates and arkoses are
intruded by an altered diabase in a most complex way. To the north of
the road is the great mass of the greenstone, within which are considerable
areas of the conglomerate. Within the belt of conglomerate, on the other
hand, are several bands of the eruptive rock which roughly follow the bed-
ding of the sedimentary one, but which cut across it in a minor way.
At the contact of the main mass of greenstone and the conglomerate
are numerous interlaminations of the two rocks, the greenstone having
intruded the conglomerate along- its bedding planes. At one place a dozen'
alternations of the two were noted within a foot. Moreover, for some
distance from the greenstone the conglomerate appears to be impregnated
with material from the intrusive, so that it has taken on a greenish tinge.
This impregnation in one instance has gone on so far as to produce what is
apparently a greenstone matrix containing separate pebbles from the con-
glomerate, the groundmass of the conglomerate having apparently been
absorbed. The greenstone adjacent to the conglomerate is traversed by
narrow pegmatite veins in various directions, some of the lai'gest being not
more than 2 inches in width. There is no evidence of a granitic intrusion,
the pegmatites appearing clearly to be the result of an interaction between
the basic igneous rock and the more acid fragmental one. At one place
along the contact there is a belt of very coarse hornblendic material that is
cut through and tlirough by the pegmatite veins.
East and west of the dam for some distance are other ledges of con-
glomerate. They, however, as a rule, present no features different from
those exhibited by the great ledge described above. In all, especially in
those occurring in sees. 9 and 10, T. 42 N., R. 28 W., the interbanding of
conglomeratic and nonconglomeratic layers is beautifully shown.
Near the north quarter post of sec. 11, T. 42 N., R. 28 W., the arkoses
have a purple rather than a pink tinge. On cross fractures they are seen to
be spangled with glistening black needles and plates of hornblende, which
lie with their long axes in all azimuths. The little crystals appear to be
more abundant in some layers than in others.
The best exposures of quartzite are found near the north quarter post
of sec. 11, T. 42 N., R. 28 W., and at 1,300 paces W., 150 N., of the south-
east corner of sec. 7, T. 42 N., R. 29 W. The rocks are black. They occur
in beds varying in thickness from a few inches to several feet.
ALGONKIAI^ EOCKS OF STURGEON EIVER TONGUE. 477
PETKOGRAPHICAL DBSCEIPTIONS.
As might naturiilly be expected, the least schistose of the arkoses and
conglomerates exhil^it the fewest evidences of alteration in the thin section.
In addition to the pebbles in the conglomerates, these rocks consist of
ronnded and angular grains of quartz, microcline, orthoclase, and of various
plagioclases, and a few of microperthite, embedded in a finer-grained
aggregate of the same minerals, tiny flakes of gi-een biotite and of color-
less mnscovite or sericite, a few plates of chlorite, particles and crystals of
magnetite, and little nests and isolated grains of epidote, with occasionally
some calcite. Many of the feldspar grains are altered into sericitic products,
colored red by small particles of various iron oxides and by red earthy
substances.
The composition and microstructure of the schistose arkoses and of
the schistose matrices of the conglomerates vary greatly in different speci-
mens, being determined largely by the original composition of the different
beds and the amount of squeezing to which they have been subjected. No
attempt will be made here to describe in detail all the changes suffered
by these rocks; a simple statement of the tendency of these changes will
be given.
The quartz pebbles in the moderately schistose conglomerates show
plainly that they have been under great stresses. The smaller ones all
exhibit undulatory extinction. The larger ones are sometimes peripherally
granulated, and sometimes etched or con-oded on their edges, as though
they had suffered partial solution. By this process small portions of the
original particles have been separated from them, and the dissolved silica
has been redeposited among the grains of the surrounding matrix as sec-
ondary quartz. In their interiors many of the larger pebbles have been
changed to a mosaic of differently oriented parts, which interlock so per-
fectly that they appear to have crystallized together.
The groundmass in which the pebbles lie is, in a few cases, a frag-
mental aggregate of quartz and several feldspars, with the addition of seri-
cite and other crystallized components. In most cases, and in all in which
schistosity is marked, no fragmental structure is noticeable. The ground-
mass is an interlocking mosaic of fairly large quartz grains that appear to
have crystallized in situ, between which are smaller grains of the same
478 THE CRYSTAL FALLS lEON-BBARING DISTRICT.
character, large and small spicules and plates of sericite, crystals of magne-
tite, and a few needles of chlorite and other secondary substances. Between
these, again, is often a cement of what seems to be secondary quartz. The
schistosity of the specimens is due to the arrangement of the sericite in
approximately parallel positions, and to the elongation of the quartz grains
in the same direction. The pink color of the rocks is produced by red
earthy substances in the feldspars and in their decomposition products.
In the most schistose phases of the conglomerates the quartz pebbles
have been mashed into plates, several of which join, end to end, forming
sheets, which in the thin section appear as long narrow lines of variously
oriented quartz grains, each of which is crossed by strain shadows.
The larger quartz grains in their matrices are broken into parts, and
these parts are differently oriented with respect to one another. Other
grains seem to have entirely recrystallized, for they are now made up exclu-
sively of the same kind of interlocking quartzes as are present in the fine
portions of the groundmass in which the coarse quartz grains are embedded.
In the groundmass of these rocks sericite is very abundant, and feldspar is
rare. From the proportions of these minerals present it would appear that the
former has been derived largely from the latter. Biotite is also present in
many specimens as small green flakes, but this mineral is not widely spread.
The conclusion from the study of the thin sections of the schistose
conglomerates is that there has been a crystallization of new substances,
principally quartz, sericite, biotite, and magnetite, from the materials of the
original granitic sediments. Perhaps a portion of the crystaUization was
the result of alteration of the original components before squeezing took
place. The larger portion, however, was accomplished under the influence
of pressure. The result of the mashing and recrystallization is a schist,
which between crossed nicols has the aspect of a typical crystalline schist,
but which in natural light exhibits its conglomeratic nature in the presence
of the large quartz lenses, with the outlines of flattened pebbles, in a fine-
grained groundmass.
The pink arkoses differ from the conglomerates simply in the absence
from them of the pebbles. The schistose varieties are similar in every
respect to the schistose groundmass of the squeezed conglomerates. Both
in the hand specimen and in the thin section the schistose arkoses exhibit
striking resemblances to jnuscovitic gneisses.
ALGONKIAN EOCKS OF STUKGEON ElVEK TONGUE. 479
The purple arkoses differ from the pink ones just described in contain-
ing chlorite and hornblende, and in addition some apparently newly formed
feldspar, notably a feldspar with the microcline twinning. As a rule, these
rocks are more feldspathic than the matrices of the conglomerates, and they
contain much less quartz. The larger grains of both quartz and feldspar
are corroded as if partially dissolved. They have lost their smooth, rounded
contours of sand grains, and now possess irregular jagged ones, which,
however, are not due to secondary enlargements.
The characteristic components of these rocks are the chlorite and the
hornblende. The former mineral is present in plates intermingled with
grains of epidote, while the hornblende is in dark-green or light-green
plates, and in acicular or columnar crystals that are idiomorphic in cross
section. The crystals are distributed indiscriminately through the rocks,
with their longer axes lying in all azimuths. They were evidently formed
after the squeezing that made the rock schistose. The plates, moreover,
include within themselves such great numbers of the other components of
the rock that their parts often appear to be independent. Under crossed
nicols, however, many of these apparently independent plates are discov-
ered to polarize together. No evidence is present in any of the sections as
to the source of the material that gave rise to the hornblende. The fact,
however, that all of the horublendic rocks are banded, that some layers are
rich in amphibole while others are completely devoid of this mineral, sug-
gests the notion that the hornblendic schistose arkoses consist partly of
sedimentary and partly of tuffaceous materials. As we shall see later, this
origin is ascribed with more confidence to some very peculiar rocks to be
discussed later.
Crushing effects are noticed in some of the hornblendic arkoses, but
their present condition appears to be due more to chemical changes pro-
duced in them than by mechanical action. The chemical changes were no
doubt superinduced by the mashing, but this can only be inferred from the
fact that they are more pronounced in the schistose phases of the rocks than
in those phases in which the schistosity is poorly developed.
THE DOLOMITE FORMATION.
The dolomite formation comprises, as has been stated, both dolomitic
limestones and calcareous slates, and occasionally quartzites, sandstones,
480 THE CEYSTAL FALLS IRON-BEAEING DISTRICT.
and conglomeratic and brecciated beds. As a rule, exposures are small and
scattered. Their distiibution lias already been described. All ledges
observed may be seen by reference to the map (PI. LI).
IMPORTANT EXPOSURES.
Good exposures of the dolomites occur in the NW. J sec. 6, T. 42 N.,
R. 28 W. The ledge nearest the northwest corner of the section is a hard
flesh-colored dolomitic marble, containing here and there little quartz
grains. This is cut by joints', and is traversed by small chert bands. The
bedding is more or less contorted, but its general strike is N. 45° E., and
its dip is 45° NW. About one-fourth mile east of this ledge is a small, bare
knoll, composed of interlaminated pink marbles, conglomerates, red sand-
stones, and red slates, varying in thickness from a few inches to a foot or
more. The conglomerate consists of marble pebbles and slate and chert
fragments in a calcareous quartzitic matrix. The strikes and dips are uni-
foi-m throughout the ledge, the former being nearly east and west and the
latter 45° S. The difference in dip of the beds of these two exposures
indicates plainly the presence in this place of a little westward-pitching
anticline.
Other prominent exposures of the dolomite series are in the NW. |
sec. 1, T. 42 N., R. 29 W., and in the SE. 4 sec. 35, T. 43 N., R. 29 W. In
the first-named locality is a high, bare knob, and a cluster of small ledges,
in which dolomites, conglomerates, and slates are all well exposed. The
dolomites, for the greater part, are massive pink marbles crossed by joint
planes. In places the rocks take on a greenish-yellow tinge, and become
schistose. At 1,500 paces N., 1,930 W., of the southeast corner of sec.
1, T. 42 N., R. 29 W., the dolomite forms a well-defined bed, striking
N. 45° E. and dipping 70° SE. Above this, to the southeast, is a bed of
coarse-grained granitic sandstone or quartzite, which in turn is over-lain
by beds of gray quartzite alternating with thin slates and fine-grained
conglomerates. Farther south is a ridge of well-bedded, fine-grained
quartzite and bluish-gray slate, the individual layers being usually less
than one-half inch in thickness. This rock grades into a gray schistose
dolomite, and the whole quartzite-slate series strikes N. 75° E. and dips
63° S. The exposures in section 35 are almost pure marbles, in which no
traces of bedding have been detected.
ALGONKIAN BOOKS OF STURGEON EIVER TONGCTB. 481
I'ETliOGUAPHlCAL DESCUIPTION.
Ill thill section tlie iniirl:)les appear as very close-gTaiued aggregates
of calcite aud dolomite, usually untwinned, but occasionally twinned in
the ordinary manner of these minerals. Here and there among the car-
bonates are rounded quartz grains, but the greater portion of this min-
eral appears to have crystallized in situ between the calcite and dolomite
individuals.
All the marbles are of the same general character. They diifer only
in the quantity of silica present and in the presence or absence of the tiny
dust grains producing the color. The schistose varieties owe their schistos-
ity to the elongation of their components.
The quartzites and slates interbedded with the marbles possess no
unusual characters. They are similar to the corresjjonding rocks inter-
stratified with the Marquette dolomites. The conglomerates interstratified
with the dolomites, slates, and quartzites are of two kinds. One is com-
posed of marble and slate fragments cemented by quartzite, and the other
of small granite pebbles embedded in granite sand. The latter are evi-
dently composed of the detritus of the granites underlying the dolomite
series, while the marble-bearing conglomerates, or perhaps more ^jroperly
breccias, are interformational beds conformable with the beds below them,
and also with those above. They are similar in every respect to the inter-
bedded breccias in the Kona dolomites on the Marquette range.
SLATES AND SANDSTONES ON THE STURGEON RIVER.
The rocks in the SW. ^ sec. 34, where the road to Sagola crosses the
Sturgeon River, are placed in the dolomite formation, although they differ
somewhat from that portion of the series described. These rocks are white
calcareous sandstones, that look very much like the Potsdam sandstone
where it overlies limestones, and a light-green slate, which near joint
planes and other cracks has a light purple color. According to Dr. J. M.
Clements, who visited the spot, the slate overlies the sandstone. "The
river," he writes in his notebook, "gives a section through these rocks, and
makes the strike seem to be N. 35° W., dip 50° N. It appears to me, how-
ever, that the true strike is about N. 85° E., and dip 40° S." If these rocks
belong to the marble series, they constitute its upper part. The slate
closely resembles some of the slates in tlie Kona dolomite formation of the
MON XXX.VI 31
482 THE CRYSTAL FALLS lEON-BBAEING DISTRICT.
Marquette range. It is a very fine grained rock composed of very small
splinters of quartz, flakes of sericite, and a few of chlorite.
THE IGNEOUS ROCKS.
The igneous rocks associated with the sedimentary beds in the Sturgeon
River tongue are all greenstones in composition. Many of them are unques-
tionably intrusive; a few ma}' be tuffaceous.
The intrusive greenstones do not differ essentially from those cutting
the Basement Complex. Some of them are in the form of small bosses.
Others are clearly dikes, though for the most part these dikes follow the
bedding of the sedimentary rocks. Still others may be intrusive sheets.
The rocks regarded as possibly tufPaceous are distinctly banded. Some are
made up of alternate bands of dark and light shades. The darker bands
consist principally of a schistose greenstone, and the lighter ones principally
of arkose or granitic sandstone. These rocks are well bedded, apparently
constituting a definite portion of the conglomerate series near its lower
horizon.^
THE INTRUSIVE GREENSTONES.
The intrusive greenstones are usually fairly massive rocks, with a dark
bluish-green color and a moderately tine grained texture. On their edges
they often pass into schistose phases, presenting the structure and appear-
ance of chlorite-schists. A very typical schist of this character occurs on
the southern edge of the great greenstone mass 1,525 to 1,600 paces north
and 300 to 400 west of the southeast corner of sec. 18, T. 42 N., R. 28 W.
In the hand specimen the rock appears to be a well-characterized chlorite-
schist, spangled with plates of a light-colored muscovite measuring 1.5 to
2 mm. in diameter.
The intrusive character of some of the greenstones is clearly shown by
the fact that they occur immediately on the strike of the conglomerate bands,
and often cutting across them, as is the case at 300 paces east of the north-
west corner of sec. 17, T. 42 N., R. 28 W. (see PL LII), and at 400 paces
south, 100 west, of this same corner.
PETROGRAPHICAL DESCRIPTION.
The greenstones intrusive in the Algonkian sediments are not essen-
tially different from those cutting the members of the Basement Complex.
' See Van Rise's Notebook 184, pp. 21-23.
IGNEOUS BOOKS OF STUKGEON EIVER TONGUE. 483
They differ from the latter in containing, as a rule, less quartz and a very
nmch greater abundance of epidote. The epidote is all secondary, as is also
the quartz, so that the only noticeable difference between the two sets of
greenstones is dependent upon differences in the nature of their alteration,
which in turn are probably the results of differences in environment. Both
sets of greenstones have been squeezed, but those in the Basement Complex
are associated with crystalline schists, while those in the Algonkian series
are associated with fragmental beds.
In addition to hornblende, plagioclase, epidote, and a little quartz,
almost all the later greenstones contain biotite, small crystals of magnetite,
and irregular grains of ilmenite or of a titaniferous magnetite. Their
structure is schistose through the arrangement of the larger hornblendes
and biotites and the elongation of the feldspar grains in approximately
parallel directions. As a rule, their thin sections present no unusual features.
They all show dirty green hornblende plates, greenish-brown biotite flakes,
magnetite crystals, etc., embedded in a mass of irregular grains of decom-
posed plagioclase, the principal decomposition product of the feldspar being
in almost all cases epidote.
Often the proportion of epidote present is very great. It occurs as
colorless crystals and grains scattered through the hornblende, and as light-
yellow plates and grains embedded in the mass of altered plagioclase. In
the rock at 500 paces east, 125 north, of the southwest corner of sec. 8, T. 42
N., R. 28 W. (PI. LII), the replacement of the plagioclase by epidote has pro-
ceeded so far that no trace of the feldspar can be discovered. In the hand
specimen the rock is seen to be a massive mixture of black ghstening horn-
blende crystals in a yellowish-green groundmass possessing a sugary texture.
In the thin section the hornblende is present as bluish-green plates that are
often idiomorphic in cross section. The groundmass in which they lie is
composed of epidote and quartz. The epidote is in large yellowish-green
irregularly-outlined plates, including particles of magnetite and small
rounded quartz grains. Most of the quartz is in isolated grains between the
epidote plates and in little nests of interlocking grains. Small magnetite
granules are scattered everywhere throughout the section, through all of the
components indiscriminately
The coarser greenstones show plainly in the hand specimen the ophitic
structure, even where the rocks are schistose. In the section this structure
484 THE GEYSTAL FALLS lEON-BEARING DISTEICT.
is often obscured by the abundance of decomposition products. Under low
powers of the microscope, however, it can nearly always be detected. In
a few of the finer-grained varieties, phenocrysts of plagioclase are occasion-
ally met with. They are clouded by inclusions of biotite flakes and shreds
of hornblende and by tiny ^^articles of a kaolinitic or sericitic mineral.
From their composition and structure, it is clearly evident that the intrusive
greenstones, whether massive or schistose, are altered phases of diabase or
of diabase-porphyrite.
The dark-green chlorite-schist referred to as occurring in the edge of
one of the greenstone masses is a chloritic biotite-schist spangled with large
flakes of a light-colored mica. The rock consists of biotite, chlorite, musco-
vite, quartz, and rutile. The liiotite is in broad thin plates, arranged
approximately parallel, and embedded in a mass of chlorite, the greater
portion of which is a greenish-brown variety that looks as though it may
have been derived from hornblende. A smaller ^sortion of the chlorite is in
light-green plates, like the chlorite so frequently found in chloidte-schist.
The quartz is in small rounded grains exhibiting strain shadows, scattered
here and there through the chlorite and between the biotite plates. It is
much more abundant in soiiie portions of the rock than in others, forming
bands rich in quartz, between others in which very little of this mineral is
present. The rutile is in large quantity. It constitutes large greenish-
yellow grains. Some of these are rounded forms, others are prismatic
crystals measuring 0.08 mm. to 0.12 mm. in length, while still others are
clearly defined elbow twins. They occur everywhere throughout the slide,
but are rare in the quartz. They are most abundant in the chlorite and in
the large plates of light-colored mica that have been mentioned as character-
istic features of the hand specimens. These have all the properties of mus-
covite. They lie indiscriminately among the other com2Donents, irrespective
of the schistosity of the rock, and contain very few inclusions, with the
exception of the rutile grains. The lines of biotite, to the arrangement of
which the rock owes its schistosity, do not bend around the muscovite as
they do around the eyes in an augen-gneiss, but they continue their courses
up to the edge of the muscovite grain, and there abruptly stop. From these
facts it is clear that the muscovites have originated since the rock containing
them was rendered schistose. As in the case of many other secondary
minerals, it appears that these were produced from the components of the
IGNEOUS ROCKa OF STURGEON RIVER TONGUE. 485
schist by a process which resuhed in the absorption of all of them except
rutile. The process may have been connected with contact action, but no
evidence in favor of this supposition has been obtained.
There are a few other types of greenstone occasionally met with
among the dike and other intrusive forms of the district, but they do not
diifer in any marked degree from those described, except that some are
quite schistose. One or two of these contain oval aggregates of epidote,
plagioclase, and quartz, that may represent inclusions of foreign rocks.
They are now, however, so much altered that it is difficult to determine
their character with any degree of certainty.
The rock of one or two other exposures in the area underlain mainly
by the conglomerates deserves mention before the banded greenstones are
discussed. The rock referred to is a heavy, lustrous, black schist that
resembles in many respects a hornblende-schist. On fresh fractures across
the schistosity parallel lines, darker than the main mass of the rock, may
be easily detected. These are the edges of cleavage planes, whose surfaces
are coated with brassy yellow mica plates. In thin section these rocks
differ very little from the schistose greenstones referred to above. They
consist of a heterogeneous schistose mass of green hornblende, cloudy
plagioclase, quartz, epidote, chlorite, and magnetite. Biotite flakes are met
with occasionally, but they are by no means common, except on the cleav-
age surfaces. Rocks of this class have not only been made schistose by
squeezing, but they have also suffered shearing along what are now the
cleavage planes. They are almost identical in microscopic and macroscopic
features with the hornblende-schists in the Basement Complex.
THE BANDED GREENSTONES.
Distincth' banded rocks, composed partly of basic material with the
composition of greenstone, form a well-defined hillock in sec. 17, T. 42 N.,
R. 28 W., about 250 paces north of the west quarter post of this section,
and a group of outcrops on the east bank of the Sturgeon River, imme-
diately west of this point.
The rocks in question are banded in mediumly coarse-grained dark
bands, containing large quantities of green hornblende, and in fine-grained
lighter ones, that resemble in the hand specimen bluish-black quartzites or
cherts. In some bands there are large lenticules of white quartz, that show
486 THE CRYSTAL FALLS IRON-BEARING DISTRICT.
plainly on weathered surfaces, like the flattened pebbles in a squeezed con-
glomerate or the drawn-out parts of quai'tzose layers in a mashed bedded
rock. These bands, though not very well defined, run continuously for
long distances, and strike and dip conformably with the conglomerate beds
exposed 200 paces to the north.
PETROGRAPIIICAL DESCRIPTION.
In the thin section the lighter-colored layers of these rocks are seen
to be composed of very irregularly outlined and rounded quartz grains,
cemented by a mass of finer quartzes and small grains of zoisite, little
clumps of chlorite, some decomposed feldspar, and particles of magnetite.
Occasionally a plate of yellowish epidote occurs in the midst of this aggre-
gate, and scattered here and there through it are large plates of green horn-
blende with the cellular structure so common to secondary minerals. These
hornblendes lie irregularly in the slide, and include grains of all the other
components. The quartz grains are small and are independently oriented,
but frequently little groups of them, with the outlines of sand grains, are
met with. There is Httle evidence of schistosity in these layers, but they
exhibit a banding produced by the alternation of coarser and finer constit-
uents. In the darker layers the proportion of hornblende is much greater
than it is in the hghter ones. Indeed, some bands consist almost exclu-
sively of large cellular plates and radial aggregates of plates of this mineral,
only the small interstitial spaces between the large amphiboles being filled
with an aggregate of quartz-zoisite, small hornblende needles, and magnet-
ite. In some sections biotite is also present. It occurs most abundantly
in the quartz-zoisite aggregate, filling the interstitial spaces between the
amphiboles, but is present also as inclusions in this latter mineral. Some
of the biotite in the hornblende appears to grade into its host, and certain
portions of the amphibole possesses the brown color of the mica, with the
optical properties of the hornblende. The large amphiboles are evidently
the youngest components in the rocks, though they were plainly produced
before the schistosity. In those layers in which the schistosity is strongly
marked this structure is produced mainly by the parallel arrangement of
the biotite and the small amphibole needles and plates in the quartzose
aggregate. The larger cellular hornblendes lie across the schistose planes,
and when they do so, the lines of biotite and of small amphiboles pass
IGNEOUS ROCKS OF STUEGEON EIVER TONGUE. 487
around tlieiu exactly as tliey would do were the large hornblendes present
before the rock was squeezed. Sometimes the amphibole masses that form.
so large a proportion of the schistose bands are single crystals, sometimes
they are fragments of crystals, and at other times they are groups of
radiating crystals. The magnetite is very much more abundant in the
hornblendes than in the surrounding quartz aggregate, sometimes being
confined exclusively to this mineral, as though it were one of the products
(the hornblende being the other) resulting from the decomposition of some
original constituent, probably augite. Little particles of hematite, on the
otlier hand, are abundantly disseminated through the quartzose aggregate,
and are practically absent from the hornblende. Much of it appears to
have been derived from magnetite.
The evidence derived from the microscopic study of sections of these
banded rocks, so far as it relates to their crigin, is disappointing. The
quartzose layers are, in all pi'obability, sedimentary. The hornblendic
layers, however, differ from these so much in composition that their material
must have had a different source. It is possible that the quartzose layers
represent sediments derived from the granitic portions of the Basement
Comjjlex, while the hornblendic layers represent sediments derived from
the basic portions of the Basement Complex; or, it may be that the acid
layers have the origin ascribed to them, while the basic ones are mixed
sediments and basic tuffs. The sections of the dark layers of these rocks
resemble so strongly the sections of the basic laj^ers in the Clarksburg, series
of mixed tuffs and sediments in the Marquette district that the writer is
inclined to regard the rocks as composed partly of tuffaceous material. On
the other hand, the banded rocks occur so close to the boundary between
the sedimentary area and the Basement Complex, which near this boundary
is composed mainly of basic schists, that it would seem but natural that
they should contain large qiiantities of basic material derived from these
schists. The original structure of the layers has been so completely
destroyed by mashing that it can not give any evidence as to the nature
of the beds. We are therefore compelled to rely entirely upon their com-
position to aid us in discovering their origin. This indicates simply that
much of their material was derived either from volcanic ashes or from the
debris washed from the basic portions of the Basement Complex.
INDEX
A. Page.
Aa structure, sketch of 120, 121, 122, 123. 124
(See Ellipsoidal structure.)
Aci Castello, basalt from 121
Aci Trezza, basalt from 121
Acid intrusives, described 45-49, 190-198, 426
age of 49
distribution of 190
in A rchean 45-49
in Felch Mountain Kange 426
in Hemlock formation 77
in "Upper Huronian 164
Acid lavas, of Hemlock formation, described 80-94
banding of 91,92,93,94
micropegmatitic texture in 89
pressure effects in 87, 88
schistose, described 87-94
Acid py roclastics, described 94, 95
Acid volcanics of Hemlock formation, described 80-95
Actiuolite from hornblende 215
of adinole .' 209
of cblorite-schist 442
of metabasalt 105, 133
of picrite-porphyry 214, 215
of py roclastics = 147
of slate 205,209
of spilosite 206
orientation of 105
(See Amx^hibole.)
Actinolite- schist from gray wacke 57
of pyroclastics 147
Adaraello tonalite 230
Adams, F. D., on analysis of slates and granites 58
on pyroxene zone about olivine 256
Adinole, analyses of 208
compared with analyses of clay slate and spilo-
site 210
in Mansfield formation 64
described 208-209
Ajibik quartzite 451
correlation of xxv, xxvi
relations to Groveland formation xxi,449,456
Albitefrom feldspar 151,201
of metabasalt 99
of spilosite, plate of 302
Algonkian, contact "veith Archean, effect on topog-
raphy 386
deposition of 456, 457
distribution of 331,427
folding of 427,428
of Felch Mountain Range, distribution of 384
succession in 384-385
structure of 384-385
intrusives in, described 426
Page.
Algonkian of Marquette district, distribution of 452-453
of Sturgeon River tongue, described 458-437
comparison with Algonkian of Felch Moun-
tain tongue 462
folding of 471-t72
igneous rocks of 482-487
pressure effects iu 471
relations to Archean 461-463
relations to Lower Marquette 462
relations to Archean xvil, 331, 399, 427, 458
relations to drainage 334, 335
relations to Paleozoic formations 331, 383
relations to quartz porphyry 439
(See Huronian, Upper Huronian, Lower Hu-
ronian.)
Allanite in acid lavas 89
included in epidote-zoisite 444
Allen, Andrews, referred to 22
Alteration of audesine ,. 224
of basic volcanics 152
of biotite 43, 393
of bronzite, plate of 306
of calcite 146
of diabase 469
of ellipsoidal basalt 292
of feldspar 42, 170, 171, 192, 201. 224, 228, 478
plate of 288
of glauconite 422
of grit 168-169
of hornblende. 234,235,237
of metabasalt 1 17
described 126-135
of picrite-porphyry 213
of slate 14
of tutfs 141
Amasa (town) 12,143,162
Amasa area, ore deposits of 177
succession in 177-178
(See Hemlock mine.)
American Black Slate Co., analysis of slate from 61
Araphibole of greenstone 486, 487
of metabasalt 127
of mica-schist - 415
of picrite-porphyry 214
orientation of 127,214,405,486
(See Actinolite, Hornblende, TremoUte.)
Amphibole-peridotite, described 253-254
crystallization of 257
gradation to olivine-gabbro 254-260
gradation to wehrlite 254-260
Amphibole-schist of Sturgeon River Archean, de-
scribed 465-467
Amphiboliie, analysis of 397
489
490
INDEX.
Page.
Arapliibolite of Arcliean, described 395-397
from basic volcaiiics 152
intruding tnica-scbist 392
Aniygdaloidal texture in metabasalt. . . . 102, 113, 122, 123. 442
described 124^126
plate of 280,282,284,290
cause of distribution of, in basic lavas 95
in eruptive breccia 136
in Hemlock scbist - ■^^S
in pyroelastics 138, 14C
Amygdules of calcite, plate of 282
of clilorite, plate of 284
of feldspar, plate of 284
of quartz, plate of 284
order of deposition of 125
pressure eftects in 126, 128
Analyses of adinoles 208
of iimpbibolite ^ 397
of clay slate 59,61
of clay slate, spilosite, and adinole, comparison of 21 0
of dolomite 409,435
of ;:;neiss 391
of granite 389
of bornblende 242
of hornblendegabbro 263-264
of iron ore, by Brooks, referred to 19
of iron ore 181
of Mansfield mine 69
of Mansfield slate 59,61
comparison witb contact products 209-211
of metabasalt 103,106.107
of mica-diorite 231,263-264
of niica-scbist ^9^
ofnorite 245,263,264
ofperidotite 259,263,264
of picrite-porphyry 219
of spilosite 207
Auatase included in bornblende 236
of gabbro and norite - 236
of metabasalt 129
{See Octabedrite.)
■ Andesine, alteration of 224
altering tomuscovite 224
altering to epidote-zoisite 224
of diorite 224
of gabbro and norite 233
of metabasalt 104
twinnina; of 104
Andesite, of Hemlock formation 107
Anticline determined by magnetic observations- 366, 372, 373
described - 370-371
Antoine dolomite of Menominee district corre-
lated XXV, XXVI
Apatite included in biotite 217,234,239
in feld spar 234
in quartz 194,419,420
in spbene 244
of acid lavas 91
of biotite -granite 192
of gabbro and uorite 239
of metabasalt 99
of periodite 252
of picrite-porphyry 216
of outlining feld spra- crystals 239
Apbanitic texture in metabasalt 98, 99, 212
Apobasalt, use of term 96,98
Page.
274
87
276
276
276
Aporbyolite "witb perlitic parting, plate of
Aporhyolite-porpby ry described
breccia, plate of
perlitic parting of. plate of
pressure effects in, plate of
use of term 80
Aragon mine 69
Arcbean, described xviri, 38-49, 385-397, 428-430, 463^71
acid dikes in, described 45-49
age of 39
basic dikes in, described 46-49
biotite-granite of, described 40-43
iUkes in, described 45-49
distribution of 38, 39, 331, 427
ellipse 26, 38, 333, 427
erosion of xxiv, 39
folding of xxiii
intrusives in xviu, 38, 45-49
metamorpbism of x viii, xxiii
of Crystal Falls district correlated witb Arcbean
of Marquette district xxv, xxvi
of Felcb Mountain range, described 385-397
distribution of 385
topography of 386, 387
of Marquette district correlated T,7itb Archean
of Menominee district xxv, xxvi
correlated witb Archean of Sturgeon River
tongue 470-471
distribution of 452-453
of Micbigamme Mountain and Fence River areas,
described 428-430
of Sturgeon River tongue, described 463-471
comparison with Archean of Marquette dis-
trict 470-471
relations to Algonkian 459, 460, 461-463
origin of 39
relations to Algonkian xvii,
331, 378, 379, 386, 399, 427, 458, 460. 461-463
relations to Cambrian 26, 331
relations to drainage 334,335
relations to Groveland formation 416,424
relations to Huronian. (See Relations to Algon-
kian.)
relations to Lo^er Huronian xix, 55
relations to Mansfield formation 424
relation.s to overlying formations 39
relations to Paleozoic rocks 26, 331
relations to quartz-porphyry 429
relations to Randville dolomite xix, 51. 53, 55, 407
relations to Silurian rocks 26
relations to Sturgeon quartzite xis, 398, 401
relations to Upper Huronian xxii
topography of 38,39,333
Arenaceous slate group of Rominger 164
Argentine Republic, gabbros of 255, 256
Arkose of Sturgeon River conglomerate formation
described 478-479
pressure etfects in 479
Armenia mine, description of ore body at 183
location of, 178; op 186
table of shipments from, op 186
Ash beds described 142-143
Azoic system 16, 17, 375
of Sturgeon Rivertongne 460
Augite altering to hornblende 100
altering to pilite 211
INDEX.
491
Augito altering to uralito , lOi, 2U1, 212
cU'avajroof 237
I'ryatallizatiou of 257
of amphiboU'-peridotito 255
of gabhroautl norito 237
of horubleiide-gabbro 243
of inetabaaalt 100,104,211
of iiietiidolerite 200,201
of peridotite 250
of tuffs 138
iu dolomite 436
included iu hornblende 237, 250, 255, 257
iuc'lnding biotito 250
including hornblende 250,255
including hyperathene 255
zonal intergrowth with hornblende, plate of 320
zonal intergrowth with olivine and hornblende. 255-260
Augite-andesite of Heinloct formation 108
Ausweichungs-clivage. (See Pyroxene) 440
B.
Bad Water village 374
Balsam village 143
Banding of acid lavas 87,88,01,92
described 93,94
of acid pyroclastics 95
of Archean granite 49
of ash bed 143
of biotite and chlorite 225
of biotite-granite 43-44
of biotite-schist 467,468
of Bone Lake crystalline schist 149
of chert of Mansfield mine 69
of gneiss 390
of granite 45,49
of greenstone 485-487
of Groveland formation 417-416
of hornblende-schist 465, 40G
of Mansfield schist 413
of mica-gneiss 197
of quartzite 401
of Handville dolomite 409
of rhyolite-porphyry ' 81
of spilosite-desraosite, plate of ,. 306
of Sturgeon formation 431
of tuff 138
of volcanic conglomerate 144
Barrois, Charles, referred to 44
Basalt. (See Metabasalt.)
Bascora, F., on devjtritied lavas 80, 87, 96
Base level. (See Peneplain.)
Basement Complex of Stnrgeon Kiver tongue de-
scribed (see Archean) 463-471
Basic intrusives described 198-212
age of 48, 49
correlation of 189
effect on topography, sketch of 46
in A rchean 49
in Felch Mountain range 446
in Hemlock formation 77
described 204
in Mansfield slate described 203-204
in other intrusives described 204
in Upper Huronian 164,211
described 204
metamorphism of Mansfield slate described 204-211
Page.
Basic intrusives, schistosity of 47-48
transfer of material to slate 211
Basic lavas described 95-135
Basic volcanics, alteration of 152
described 95-148
Ba83ett,V.H., indebtedness to 106,264
Bayley, "W. S., indebtedness to 12
on Clarksburg formation 132
on metamorphism of basic volcanics 152
on schistose pyroclastics 148
referred to xv. 22
Eebb, E. C., indebtedness to xvi. 12
Becke, F., on granodiorite 231
on relations of pyroxene and amphibole 258
on tonalite 230-231
on tonalite-porphyrite 229
referred to 1 05
Bedding of Upper Huronian 167
Benson, Vt., analysis slate from 61
Bessemer ore of Mansfield mine 68
of Mansfield slate 27
Biotite, alteration of 43, 393
altering to calcite 192,225
to chlorite- 43, 47, 192, 193, 215, 216, 225, 228, 248, 403, 425
to epidote 43
to epidote-zoisite 225
to rutile 43,192,202,225
to sagcnite 192,193
to sphene 192,225
banding with chlorite 225
crystallization of 258
from feldspar 42,82,89,90,92,150,170,192,201
included in augito 250
included iu feldspar 151, 171
included in hornblende 251, 260, 261
included in muscovite 298
included in quartz 47,171,394,403,466
included in plagioclase 193,484
including apatite 217,234,239
including epidote 225,226
including ilmenite 216
including iron oxide 252
including sagenite 403
including sphene 225, 404
including zircon 234
iutergrowu wifh muscovite 393, 414
of acid lavas 89,91
of amphibolite 296
of amphibole-peridotite 254, 258
of amphibole-schist 466
of araygdules 124
of basic dikes 47
of biotite-granite 42, 43, 192, 193
of biotite-schist 468,484
of Bone Lake crystalline schist 150
of diorite
of gabbro and norite
of granite ,
of graywacke
of greenstone
of Hemlock schist
of metadolerite
of metamorphosed Mansfield slate
of mica-diorite, plate of
of mica-schist
of muscovite-biotite-gneiss, plate of
225
234
198
170
4S6
444
202
205
308
392. 393
298
492
INDEX.
Biotite of mnscovite-'biotite- granite 193
of peridoiite 252,257,261
of picrite-porphyry 215
of pyroclastica 147
of sedimentaries developed by intrusion 195
of sedimentary inclusions in granite 197
of spilosite 206 ;
orientation of 393,425,468,486 ,
paralle] growth, -witb, muscovite 170 |
penetrating feldspar 4=14 |
pressure effects in 43, 248 j
relations of orientation to hornblende 486 .
relations of orientation to nmscovite 484 ^
replacing feldspar - 193 '
Biotite-gneiss of Arcliean 429
Biotite-granite described 40-43, 191-193
banding of 43,44
plate of 308
micropegmatitic structure in, described 192-193
schistosity of 44
Biotite-scbist of Algonkian of Sturgeon Kiver tongue 484
of Arcbean of Sturgeon Kiver tongue 467-469
of Hemlock formation 442
Birkinbine, John, on ore shipments 186
Blaney mine. {See Hope mine.)
Blocklava. (SeeEllipsoidalstructure, Aastructure )
Bog iron ore of Upper Huronian 182
Bone Lake described 35
referred to 15^
Bone Lake crystalline schists described 148-152
Bonney, T. G-.,on alteration of olivine 218
on ellipsoidal structure 118-119
Botryoidal ore 180
Brackett, 11. N., on ultrabasic intrusives 220
Brauuer, J. C , on ultrabasic intrusives 220
Breccia, eruptive, of Hemlock formation, described. 135-136
Tolcanic, use of terra 137
Breeciation of Groveland formation 418
of metabasalt 11''^
Brittany granite compared with Crystal Falls granite 44
Brogger, W. C, on diorite and gabbro families 242
on monzonito group 232
on rock analyses 105
on use of term diorite 222
Bronzite altering to serpentine 238
plate of... 306
altering to talc 238
plate of 306
included in hornblende 250
plate of 306
including ilmenite 238
including rutile 238
in zonal intergrowth with hornblende, plate of. . 318
of broDzite-norite 244
plate of 318
alteration of, plate of 306
of bronzite-norite-porphy ry 246
of gabbro and norite 238
of peridotite 250
Bronzite-norite described 244-247
plate of 318
analysis of 245
crystallization of minerals 2(32
intruding hornblende-gabbro 243, 249, 265
Bronzite -norite -porphyry 246
plate of 320
Paga
Bronzite-norite-porpbyry altering to serpentine 246
intruding hornblende-gabbro 249
Brooks, A. H., referred to 22
Brooks, T. B., on composition of iron ore 181
on correlation of Menominee rocks 19
on correlation of Upper Huronian 164
on Felch Mountain range 376, 377. 378, 379
on iron-bearing rocks of Michigan 16-17
on magnetic observations 24, 337
on Menominee district 19
on Mesuard series 452
on Paint Kiver district 11
on Sturgeon Kiver tongue 460-461
on Upper Huronian 172, 173
referred to xv, 21
Brooks, Kominger, and Pumpelly, map of Upper Pen-
insula of Michigan 18
Brown, E F., on analysis of iron ore 69
Brule Kiver described 31
referred to 13, 15, 161
Building stones of Hemlock formation described. . . 153, 154
Burt, William A., map of part of Upper Peninsula . 15
on Crystal Falls rocks 13
on Felch Mountain range 375
on Sturgeon Kiver tongue 460-161
referred to - 16,21
C.
Calciferous limestone, relations to Potsdam 383
Calcification of chlorite 132
of feldspar 131
of metabasalt 117,130,132,133,134
Calcite, alteration of 146
altering to limonite - 153
developed by dynamic action 432
from biotite 192,225
from feldspar 82,90,131,132
from hornblende 203, 21 5
of acid lavas 89,93
of amygdules 124
plate of 282
of basic dikes 47
of biotite-granite 192
of ellipsoidal metabasalt, plate of 292
of Groveland formation 420
j of marble 481
of metabasalt 100,101,117,127.128,129,132
j plate of 290
I of metadolerite 203
j of peridotite 252
; of py roclastics 146, 147
of tuff 142
orientation of 132
pseudomorphs after feldspar 132
replaced by iron carbonate 133
replacing chlorite 132
replacing feldspar 131
Caledonia mine. (See Mansfield mine.)
Calumet and Heckla mine 399
Cambrian sandstone 29
deposition of xxiv
erosion of xxiv
relations to Algonkian 331,473
relations to Arcbean xxiv, 26, 331
relations to Ke weenawan 162
relations to intrusives 188
INDEX.
493
Page.
Camliriaii sniulstonc, relatioiiH to Upper Hurouiaii xxiv,
155.161,162
Carbon of Mansflold slate 60
Carbonate ilevcloped by dynamic action 432
Carbonation of metabasalt 117, 130, 132, 133, 134
Carboniferous clays, analyses of 59
CataL-lastic structure in feldspar 44
in granite 194
in quartz 41, 44
Cellular texture in hornblende 486
Channing, J. Part, figure by 65
on Mansfield mine 66
referred to 22
Cbanning (town) : 175
Chert fragments in conglomerate 64
of MansfieUl formation described 62
of Upper Huronian 166
pressure efl"ect3 in 177
relations to ore deposits 182
road material 154
Cherty carbonate from organic matter 184
in conglomerate 64
Chester, F.B., (?) on gabbros 247
Chicago, Milwaukee, and St. Paul Eailway 95, 143, 175
Chicago and Northwestern Railway 156, 175, 423
Chlorite altering to epidote 132
banding with biotite 225
calcification of 132
from biotite 43, 47, 192, 193, 215, 216, 225, 248, 403, 425
from feldspar 82,99,111,131,201
from hornblende 100,203,214,215,237
included in quartz 403, 419, 420
including epidote 248
including ilmenite 146, 216
including iron ore 218
including sagenite 403
including titanite 404
of adinole 208 !
of altered slate 209 I
of amygdules 124,125 i
plateof 280,284 \
of arkose 479
of basic dikes 47 ,
of biotite-granite 192, 193 .
of biotite-schist 484 ;
of Bone Late schist 151
of dolomite 410 i
of graywacke 170
of metabasalt 98, 99,
101, 117, 118, 127, 128, 129, 131, 132, 133, 134
of mica-schist 196
of phy Uite 440
of picrite-porphyry 213, 217, 218
of pyroclastics 145
of spilosite 206
plate of 302, 304
of spilosite-desmosite, plate of 306
of slate 205, 209
of tuff 141^ 142
of volcanic conglomerate 143,144, 145
orientation of us, 127, 133, 146
pseudomorphs after biotite 217, 228
after garnet 403
replaced by calcite 132
Chlorite-schiat from basic volcanica 152
from gray wacte 57
Page.
Cblorite-scbist intrusive in .^Mgonkian of Sturgeon
River 432
of Hemlock formation 442
of Upper Huronian ig6, 174
Cbloritization of metabasalt 117
Chrustscbofr, C. von, on relations of pyroxene and
amjihibole 258
Claire mine, location of 178, op. 186
table of shipments from op. 186
Claire Mining Company. {See Claire mine.)
Clarke, F. "W., analysis by 61
Clarksburg volcanics 132,165
comparison with banded greenstones of Sturgeon
Kiver tongue 487
replacing Michigamme and Ishpeming forma-
tions XXVI
Clastic volcanic, plate of 284
Clay slates, analyses of 59, 61
analyses of, compared with altered clay slates .. 209,211
composition of 58,60
from granite 58
of Mansfield formation described 57-62
origin of 5g
relations to ferruginous chert 63
relations to ore bodies 63
relations to siderite-slate 63
Cleavage of .7cid lavas 88-89
of augite 250
of biotite-schist 467
of Hemlock schist , 443,445
of hornblende ? ^51
of phyllite 439, 440
of pyroxene 237
Clements, J. Morgan, on ellipsoidal structure,
referred to US
on Hemlock volcanics 446
on Sturgeon River sandstone 481
on volcanics of Crystal Falls district 20
referred to xx vi, 437, 447
Cole, G. A. J., on ellipsoidal structure 118, 119
Columbia mine, description of ore bodies 182
location of 179, op. 186
table of shipments from op. 1 86
referred to 161
Commonwealth 385
iron ores at xxvi
Compass, dial, use of 24, 341 , 342, 344
Concentration of ore in synclinal troughs (see Iron
ore deposits, origin of) 183, 184
Conglomerate altering to sericite-schist 475
basalt of Upper Huronian 163
of Hemlock formation 76, 152,153
* intruded by diabase 476
of Mansfield ore deposit 63-64, 68
intruded by greenstone 475-470
of Sturgeon River xviir, xxtv, 461,462, 481
described 472-479
of Upper Huronian xxn, 166
pressure effects in 474, xviii
volcanic, described 143-145
plate of 284
use of term 136
Copper, absence of, in Huronian volcanics 125
Corrigan, McKinney Sc Co. (*S'ee Crystal Falls mine.)
Cortlandt series, comparison with Crystal Falls in-
trusives 222
494
INDEX.
Page.
CoutcMchiiig 380
Credner, H., on Felch Mountain range 376
on Menominee district 377
on oritrin of iron ore 71
Cretaceous subaideDce xxiv
Cross, Whitman, on metadolerite 97
Crystal Falls area, ore deposits of 178
comparison with output of Menominee mines... 186
comparison with output of region 186
discovery of ore deposits 175
Crystal Falls district, drainage of 31-36
elevations in 30, 31, 332
folding of 26
geographical limits of 25
physiography 13,29-37,329-335
relations to Marquette district 11,25,329
relations to Menominee district 11,25,329
structure and stratigraphy of 25-29
Crystal Falls mine, analysis of ore from 181
location of 178, op. 186
table of shipments of op. 186
referred to 161
Crystal Falls series, correlation with Marquette
series xxv, xxvi
correlation with Menominee series xxv, xxvi
metamorphisra of xxiv, xxv, sxvi
Crystal Falls syncline xxni, 26, 178
' described 158-161
Cry stall ine schist of Bone Lake described 148-152
of Upper Huronian* 166, 167, 171, 172
Crystallization of minerals of basic rock described . . 257-259
of minerals of intrusive series 262
Culver, G.E., referred to 22
Current bedding in quartzite 53
D.
Dakyns. J. R.,on plutonic rocks 222
Dalmer, K.. on ellipsoidal structure 118, 119
Dana. J. D., ou ellipsoidal structure... 120,121,124
sketch by 120
on metadolerite 96
on origin of volcanics 78
referred to 95
Darton, N. H., ou ultrabasic intrusive^ 219-220
referred to 95
Dathe, E., on ellipsoidal structure 118, 119
Deer River 38, 75,88, 333
described 31,32-35,334
development of 32-35
topography of valley 29
Delphic mine, location of op. 186
table of shipments from op. 186
Desmosite gradation to spilosite, plate of 306
of Mansfield foi-mation 64
described 207
De Soto Mining Co. {se(? Mansfield mine) --- 65
Devitrification of aporhyolite 87
of metabasalt 102,103,126
of tuffs 138
Dewitt, N. Y.,picrite-porphyry at 219
Diabase, alteration of 469
altering to greenstone 466-484
altering to borublende-schist 466
intruding conglomerate 476
intrudiu^ granite 429
intruding Felch Mountain series 426
Page.
Diabase intruding Sturgeon River series 469
(^SeeMetadiabase.)
Dial compass, use of 24,344
described 341-342
Diamond-drill work at Hemlock mine, figure of 177
in8ec.20, T.45N.,R.33 W 176
Differentiation of magma 265
Dike, associated with ore deposits at Paint River
mine 183
in Monitor mine 183
in Paint River mine 183
acid, in Archean, described 46-49
basic, in Archean, described 46-49
effect on topography, sketch of 46
(See Basic dikes, Acid dikes.)
Diller, J. S.,on ultrabasic intrusives 220
referred to 95
Diopside of gabbro and norite 238, 212
Diorite, comparison with granodiorite 231
crystallization of minerals of 262
intruded by diorite-porphyry 265
intruded by granite 194
intruded by hornblende-gabbro 265
of intrusive series described 222-232
use of term 22.223
Diorite-porphyry intruding diorite 265
intruding hornblende-gabbro 265
Diorite-schist associated with ore deposits 183
Diorite. See Motadiorite.
Dip needle, use of 24. 344
described 342-343
Doane exploration 447, 449
Dolerite, contact with granite 192
dikes associated with ore deposits 18.1
grading into ba.salt 200
including sedimentary rocks 20:J
intruded by granite 194, 204
intruding Archean 48
intruding Hemlock formation 77
intruding pyroclastics 147
intrusive, endomorphic effects of 211
metamorpbism of Mansfield slate, described 204, 211
relations to intrusives of other districts 189
relations to picrite' porphyry 212
use of term 96
{See Metadolerite, Basic intrusives.)
Dolomite described 408-411, 431-437, 479-482
analyses of 409, 435
containing foreign minerals 436
metamorphisra of 432
of Michigamme Mountain and Fence River areas,
described 431-437
of Randville formation, described 408-411
of Sturgeon River tongue, described 479-482
distribution of 1 - 472
relations to conglomerate -* 471
relations to Felch Mountain fragmentals . . . 472-473
relations to Lake Superior sandstone 473
orientation of 410
(See Randville dolomite.)
Drainage of Crystal Falls district 31-36,334-335
Drift. (See Pleistocene.)
Dunn Iron Mining Company. {See Dunn Mine.)
Dunn mine, analysis of ore from 181
depth of 185
description of ore bodies 182
INDEX.
495
I'a-r.
Dunn luiiif, liK-aliuii of 179, op. 180
table of shipment H from op. 186
Dynamic iictiou. {Sec Pressure effects.)
Economic ])iodiict8 of Hemlock formation de-
scribed 153-154
Elevations of Crystal FiiUs district 30,31,332
Elldaleii, S-i\-eaeu LUlletlinta of 92
Ellipsoidal structure m metabasalt described 112-124
figure of 112,113,114
plate o( 116,292,298
Ells, R. "\Y., on ellipsoidal structure 118. 119
Enlargement of feldspar 144
of quartz 57,85,404-405
Enstatite of gabbro and uorite 238
Epibasalt, use of term 98
Epidiorite, use of term...^ 97,222
Epidolerite. use of term 97
Epidoto from biotite 225
/rom cblorite 132
from feldspar 82,92,111.169,170,201,248,483
from hornblende 100,203
included in biotite 225,220
248
151
47
444
47
445
47
- 443
151
442
226
included ni chlorite
included in feldspar
included in bornblende
' included in ilraenite
included in quartz
including zoisite
of basic dikes
of biotite-scbist
of Bone Lake schist
of chlorite-schist
of diorite
of greenstone 483, 485, 486
of metabasalt 101, 102, 117, 118, 127, 134
of py roclastics 147
of spilosite 206
plate of 302
of tuff 141
of variolite Ill
of volcanic conglomerate 143
Epidote-scbist from basic volcanic 152
of Hemlock formation 442
Epidote-zoisite —
from feldspar 42,99,127,192,224
from bornblende 237
including allanite 444
including spbene 225
of metabasalt 99, 131, 132
of araygdules 124
zonal structure in 101
Epjdotizatiou of metabasalt 117
Eriksen, E. T ., referred to 22
Erosion of Archean xxiv, 39
of Cambrian u xxiv
of Huronian sxi, xsii, xxiv
of Pleistocene xsiv, 29
Eruptive breccia formed by intrusion 195
of Hemlock formation described 3 35, 136
Escanaba Piver described 335
Eutaxitic structure in metabasalt 103
Exploration for iron ore 12, 73, 330
in Mansfield slate 73
of Mansfield ore deposit 67
Page.
Fairchild, C.N. .indebtedness to 331
Fairbanks, H. "W., on ellipsoidal structure 118
on gabbros 240
on ultrabasic rocks 247
Fairbanks mine. (See Lincoln mine.)
False bedding in TTjipcr Huronian 168
Felch Mountain range described 374-426
elevation of 1 331
geographical position of 374
literature on 375-383
relations to Sturgeon River rocks 462, 472, 473
Feldspar, alteration of 42, 127, 170, 1 92, 201, 224, 478
plate of 288
altering to albite 151, 201
altering to biotite 42, 82, 89, 90, 92, 150, 170, 192, 201
altering to calcite 82,90, 131, 132
altering to cblorite 82, 99, 111, 131, 201
altering to epidote 92, 111, 161, 170, 201, 248. 483
altering to epidote-zoisite 42, 99, 127, 192, 224
altering to feldspar Ill, 131, 151, 169, 170, 171, 201, 248
altering to iron oxide 42, 92
altering to mica 169, 170
altering to muscovite . . . 42, 82, 90, 92, 192, 201, 224, 228, 248
plate of 286
altering to paragonite 41, 42
altering to quartz 42,
99, 52, 111, 131, 151, 169, 170, 171, 201, 248
altering to sericite 52, 89, 99, 101, 111, 127, 131, 477
altering to zoisite 201
plate of 286
cataclastic structure in 44
enlargement of 144
included in dolomite _ 436
included in bornblende 202
plate of 312
included in microcline 294
included in uralite 202
including apatite 234
including biotite 151, 171
including epidote 151
including bornbleiide 151
including iron oxide _ 151, 234
including muscovite 468
including quartz 191.468
including rutile 234
of acid lavas 90
of ampbibole-peridotite 255
of amygdulea 124
plate of . 284
of basalt, plate of 282
of biotite-granite 41,42, 191, 193
of biotite-scbist 467, 468
of Bone Lake schist 150, 151
of bronzite-norite-ijorpbyry 246
of diorite 223-225
of dolomite 436
of gabbro 234, 241, 242, 248
of granite 198, 388. 464
of graywacke 56, 169
of greenstone 470
of Groveland formation 420
of metabasalt 99, 102, 104, 127, 129, 211
of metadolerite 200, 201
of micadiorite 227
of mica-schist 414
496
INDEX.
Page.
Feldspar of muacovite-biotite-gneiss, plate of 298
of peridotite - 252,257,260,261
of phyllite *«
of pjroclastics 146, 147
of quartz-niica-diorite-porphyry, plate of 310
cf rhyolite-porphyry 81, 82, 84, 85
pressureeffects, plate of
of sedimentary inclusions in granite
of slate, altered ' *•■
of spilosite -• 206
145
255
Ill
278
197
205
414
57
of tuff
of ultrabaaic roots
of variolite
of TOlcanic conglomerate 144,145
of volcanic sand, plate of 296
orientation of - 84,90,171
outlined by apatite '39
penetrated by biotite - - - •
by tourmaline
penetrating hornblende, plate of 322
perthitic 41,42
pressure eflects in 42, 90, 92, 169, 254, 388, 464
plate of 276
replaced by biotite 1^3
replaced by calcite 131
twinning of 41,42,89,191,201,224,233,468
zonal jntergrowtb witb borublende 256
plate of 322
zonal intergrowtb witb pyroxene, plate of 322
zonal structure in 197,233
Fence River described 31,35,334,335
referred to XIX, 38, 39, 50, 51, 152, 329, 333, 408, 427, 430
Fence Eiver area described 427-450
168
62
63
22
417
Page.
Folding of Upper Huronian, relations to intrusivea. 189-190
time of 161,188
Foliation of araphibole 395
of gneiss 430
of granite 387
of Sturgeon formation 402
Forbes, indebtedness to 331
Ford River, described 334, 335
Forsythe, E. J., analyses by 435
Foster, J. "W., and Whitney, J. D., on Felcb Mountain
range 375
on Crystal Falls district - 16
map of Lake Superior land district 17
referred to 21
Fouque, F., on ellipsoidal basalt 120, 121, 124
Fox, H., on ellipsoidal structure 118
Frankenstein, wekrlite from 219
Friction breccia of Hemlock formation 136
Fumar(dc action on sedimentaries 57
Fundamental Complex. (,Sce Arcbean.)
Futterer, Karl, on pressure eflects in quartz 90
on pressure effects in feldspar 91
Ferruginous cbert
of Mansfield formation described.
relations to clay slate
Finlay, J. E., referred to
Flag ore of Groveland formation
Floodwood road
Florence, iron ores at xxVi
175
87
278
45
244
discovery of ore deposits at
Plow structure in aporhyolite-porphyry
plate of
in granite
in bornblende-gabbro
inmetabasalt 99,102,105
plate of 280
in tufl's 138
f^&m Banding.)
Folding determined by magnetic observations, de-
scribed 306-372
of Algonkian xxill, 163,427,428
of Sturgeon Paver 471,472,474
of Arcbean - ^-^'"
of Crystal Falls district 26
of graywacke, caused by intrusion 195
of Groveland formation 448
of Hemlock formation 441
of Mansfield formation 438, 439
of Marquette district 26, 452
of Menominee district •. 26
of Eandville dolomite 432-434
of Sturgeon quartzite - 399-400
of Sturgeon Eiver series 471,472,474
of Upper Huronian described 158-162
sketch of
179
Gabbro described
altering to greenstone-schist
crystallization of minerals of
intruded by granite
intruding bornblende-gabbro
pressure effects in
relations to peridotite
Garnet of actinolite-scbist
of graywacke
of Mansfield schist
of mica-schist
pseudomorphs after chlorite
Geikie, A., on ellipsoidal structure
on Tertiary basalt
Geographical limits of Crystal Falls district
of Felcb Mountain range
Glass in metabasalt
in tuff
{See Devitrification.)
Glauconite, alteration of
of Groveland formation
of Mesabi iron-bearing formation
Glidden exploration
Globular basalt. (&m Ellipsoidal structure in meta-
basalt.)
Gneiss, analyses of
developed by intrusion
of Arcbean xviii
described
recrystallizatiou of
Gneissoid biotite-granite described
Gneissoid granite, included in granite-porphyry,
sketch of
of Sturgeon Eiver tongue described
pressure effects in ■
Granite altering to clay slate
altering to pbyllite
analyses of
gradation into quartzite
banding of
composition of
contact with dolerite
233-249
466 .
262
194
265
247, 248
261
450
167
413
415
403
114, 118
75
25
274
99
138
422
XX
422
161, 163
391
198
43
390,391
390, 391
43,44
45
463, 464
44
58
58
389
51,52
45
58
192
INDEX.
497
Page.
Granite, contact -with sodimeiUaries 194-198,300
pliito of 298
ioUationof 287
iiicliuUug sedimentary fragments 195
iutnuied by diabase 429
intrudi;ig iron-bearing formation 370,381,426
intruding dolerite 204
intruding Folcli Mountain range 426
intruding gneiss 381
intruding granite 429
intruding Sturgeon formation 426
intruding Sturgeon Kiver series 459
iloribiban (Brittany) compared witli Crystal
Falls granite - 44
of Arcbean xviii,38,45,49,428,429
described 40-13,387-389
of Micbigarame River 16
of Sturgeon River 459,464
of Sweden 44
porpbyritic 40,45
jjressure effects in 44, 194,464
relations to quartzite 376
relations to otber intrusives 194
(See Biotite-granite, Muscovite-biotite-granite,
G-rauitite, Gneissoid granite.)
Granite-porphyry, sketch of 45
Granitite 226
described 40-43
Granitic texture in diorite 223
Granodiorite, comparison with diorite of Crystal
Falls 231
Granopbyric texture 85
Granular texture in hornblende-gabbro 240, 244
plate of - 316
in perlodotite 250
Granulation of feldspar 42,169
plate of 276,316
of quartz 51,90,169
Graywacke altering to actinolite-scbist 57
altering to chlorite-schist 57, 166
altering to miea-scbist 57, 166
brecciated by intrusion 195
intruded by granite 194
plate of 298
magnetitic of Upper Huronian 176
of Mansfield slate 56,57
of Sturgeon formation 431
of Upper Huronian 166, 167, 168, 169-174, 176
of sec.l9,T.46X.,R.32 W 27
pressure etfects in 170
recrystallizatiou of 195, 198
plate of 298
Great slate formation of Menominee district sxv, xxvi
Great "Western mine, description of ore bodies at . . . 182
depth of 185
fispuring of rocks 185
location of 178, Op. 186
table of shipments from Op. 186
Green Bay, streams tributary to 31. 334
Greenstone 14
from diabase 484
from diabase-porpbj'rite 484
in conglomerate of Hemlock formation 442
intrusive in Algonkian of Sturgeon River tongue 482-485
intrusive in Arcbean 38
intrusive in conglomerate .' 475, 476
MON XXXVI — 32
Greenstone of Mansfield mine...
topograjihy of
Greenstone-schist from diabase .
from gabbro
Pago.
63
333
466
466
of Arcbean of Sturgeon River 465, 467
Gregory, G. W.,on ellipsoidal structure 118, 119
Grifdth Sc Nathaniel quarries, analysis of slate from. 61
Grit altering to chlorite ]68, 169
altering to muscovite 108, 169
Groveland formation described sx, 415-423, 446-450
correlation of (see Relations) xxv, xxvi
distribution of 415,416,446-448
folding of 448
intruded by granite 426
magnetic observations in 447, 448
origin of 423
pressure effects in 449
quartzites, resemblance to Mesabi chert 422-423
relations to Arebean 416, 424
relations to Ajibik quartzite xxr, 449, 456
relations to Hemlock formation xx, xxi, xxvi, 447
relations to Mansfield formation 411,413,438,447
relations to Negaunee formation xx, 449, 455
relations to Eandville formation xx
relations to Upper Huronian xxii, 425
thickness of xvii, 448
topogranby of 415, 416, 446-448
Groveland mine 413, 415
Grijnerite- schist of Groveland formation 418
Giimbel, , on epidiorite 97
H.
Halleflinta, of Elfdalen, Sweden
Hampton Village, N. T., analysis of slate from .
Harriman.F. J., referred to
Hartz Mountain, analysis of spilosite from
Hatch, F., on limburgite .
92
61
22
207
221
Hawaii, volcanics of 75, 120
Ha-wes. G. "W., on metadolerite 96
Hedstrtim, H., on micropoikilitic texture 83-84
Hematite deposits of Upper Huronian described... 180,182
of Groveland formation 417, 418, 419, 420, 450
of Mansfield ore deposits described 69-70
of Paint River district 18
Hematite from basic volcanics 152
from siderite 168
included in biotite 252
included in quartz 419
of am y gdules 1 25
of Bone Lake schists 151
of clay slate 57
of quartzite 450
orientation of 418
Hemlock formation described xx, 73-154, 440-446
acid volcanics of, described 80-95
classification of 79, 80
correlation of xxv, xxvi
distribution of 27,73,74,440-441
economic products from 153, 154
folding of 441
metamorphism of xxiv, 446
of Micbigamme Mountain and Fence River
areas described 440-446
origin of 27,78
pressure effects in xxiv, 75
pyroclastics of, described 135-148
498
INDEX.
Page.
Hemlock formation, pryoclastics of, plate of 140
relations to lutmsiyes 77,204
relations to Mansfield formation xs, xsi, xxvi, 64, 76
relations to Groveland formation xx, xxi, xxvi, 447
ralations to Randville dolomite xx, 75
relations to Upper Hurouian 77
rtiyolite-porphyry of, described 81-87
sedimentaries of, described 152, 153
thickness of ' XVII , 27, 74, 75, 441
topograpby of 73, 74, 440, 441
Hemlock mine 157, 1 77
analysis of ore from 181
diamond drill work at, figure of 177
exploitation of ore deposits at - 175
location of Op. 186
table of shipments from Op. 186
Hemlock Kiver described 31
Hesse-Darmstadt, wehrlite from 219
Hillebrand, "W. F., analvsis by 61
Hobbs, Wm. H., on luetamorpbism of schists 130
HoUister mine, location of 178, Op. 186
Holy oke formation 20
Hoosau schists of western Massachusetts 394
Hope mine, location of 178, Op. 186
table of shipments from Op. 186 |
Horizontal needle, deflection of, figure of 345 j
Hornblende, alteration of 234-235
altering to actinolite 215
altering to calcite 203,215
altering to chlorite 100, 203, 214, 215, 237
altering to epidote 100,203,237
altering to magnetite 215
crystallization of 257,258,259
from augite 100,201
grading into hornblende 241,245,251
plate of 312
included in augite 255
included in feldspar 151, 484
included in quartz 466
including anatase 236
including augite 250,255,257
including biotite 251,260,261
including brouzite 250
plate of 306
including epidote 47
including feldspar 235,241
plate of , 312
including ilmenite 236,246
including iron oxide 47,215, 251,487
including malacolite 238
including' olivine 251, 257, 260
including pyroxene 237, 241, 245, 251, 260
plate of - 3 12
including quartz 47
including rutile 236,239,248
including spinel 236
inclusions in 240, 245, 248, 260
plate of 318
intergrown with augite, plate of 320
intergro wn with augite and olivine ' 255-260
intergrown with bronzite, plate of 318
intergrown with feldspar 256
plate of 322
intergrown with pyroxene 251, 260
plate.of 320
intergrown with spinel 256
Page.
Hornblende of amphibolite 396
of amphibole-peridotite 253, 258
of arkose 479
of basic dike 47
of biotite-granite 192
of Bone Lake schists 150
of bronzite-norite-porphyry 246
of diorite 226
ofgabbro 234,237,248
of greenstone 470,483,486
of hornblende-gabbro 240, 241, 243, 245
plate of 312
of hornblende-gneiss 174, 465
of Hemlock schist 445
of metabasalt 100
of metadolerite 200,202,203
ofperidotile 251,252,257,260
of picrite-porphyry 215
of tuff 141,142,145
of volcanic conglomerate 145
of volcanic sand, plate of 296
orientation of 174,214,215,395,396,465
penetrated by feldspar, plate of 322
pbenocrysts in Bone Lake schists 150
pressure effects in 237
relations of orientation to biotite 486
zonal structure in 2-26, 234, 235, 236, 256
{See Hornblende intergrowth.)
Hornblende-gabbro described 240-244
plate of 312,314,316,318
analyses of 242,263-264
crystallization of minerals of 262
intruded by bronzite-norite 243, 249,265
intruded by diorite 265
intruded by gabbro 265
intruded by peridotite 246, 260, 265
Hornblende-gneiss of Arcbean described 395-397
intrusive in Upper Huronian 167, 173, 174, 175
pressure effects in 175
Hornblende-schists from diabase 466
from gabbro 466
intrusive in Sturgeon River Algonkian 485
of Arcbean of Sturgeon River 465-467
of Upper Huronian 173
Hornblende-slate described 14
distribution of 13-14
Hubbard, Bela, on Sturgeon River tongue 459,461
Hulst, N. P., on composition of ore deposits 69
Huron Mining Co. {See Columbia mine.)
Huronian rocks, correlation of xsv, xxvi
distribution of, plate of 160
erosion of xxiv
folding of XXII, 25, 163
metaraorphism of xxiii
relations to Arcbean 39, 378, 379, 380, 410
relations to Cambrian xxi v, 28
succession xxv, xxvi
unconformity within xxvii
(See Algonkian, Upper Huronian, Lower Huronian.)
Hutcbings.W.Maynard, analyses by 59
on composition of clay 60
on spilosites 206
on transfer of material from basic intrusive 211
Hyalopilitic texture in metabasalt 98, 99, 104
Hyperstbene included in augite 255
Hypidiomorphic texture in gabbro 233
INDEX.
499
Page.
75
105
84
265
95
Iceland, volcanica of
Iddiug-s. J. P., on niagmatic differentiation
ou micropoikilitic texture
on origin of igneous rocks
referred to ,
lUinoia Steel Co. {See Tonngstown mine.)
Ilmenito altering to rutile * 202, 239
altering to sphene 239
included in biotite 1 216
included in bronzite 238
included in chlorite 216
included in horubleude 236, 246
including epidote ' 444
ini-luding quartz 444
of basic dikes 47
of Bone Lake schist 151
of chlorite schist ' 442
of gabbro and uorite 236,238,239
of Hemlock schist 444
of metadolerite 202
of picrite-porphy ry 216
of sedimentary inclusions in granite 197
Instruments, magnetic, use of, described 341-344
Interrange exploration 439, 448
Intersertal texture in tuetabasalt 98, 99, 104
Intruaives described 187-265, 469-470, 482-485
age of 188,189
correlation of 189
effect on magnetic observations 371,372
in Archean xviii, 38,45-49
in Felch Moantain range 426
in Hemlock formation 77
in Huronian xxm, 190
in Mansfield slate 63,64,204^211,413
in Sturgeon Kiver tongue, described 469-470, 482-485
in Upper Huronian 164, 174, 175, 190
influence on topography 54, 333
metamorphism of Mansfield slate, described- .204^211, 413
relations to Cambrian 188
use of term 187
IntrusiTe series, related, described 221-265
composition of 263-265
crystallization of minerals of 262
relative age of rocks of 265
textures of 262
Intrusives, unrelated, described 190-221
Iron-bearing formation described sx, 415-423, 446-450
correlation of 20, 330
intruded by granite 381
magnetic observations in 338, 339
near Crystal Falls, succession in 179,180
of Felcb Mountain range 378
described 415-423 '
distribution 377
Eominger on 381-383
of Penokee district, located by magnetic work . - 24
of Upper Huronian 168
relations to marble 376
relations to volcanics of Penokee series , sxi
(See Groveland formation, Iron-ore deposits.)
Iron carbonate. {See Siderite.)
Iron County H
Iron Mountain 385
slates of, correlation witb Mansfield schist 413
Iron ore, analyses of : 69
by Brooks, re I'erred to 19
Page.
Iron ore, composition of. isi
from siderite 70. 71, 184
from eruptive rock 334
origin of 70, 71, 184
Credner on 71
Irving on 70,71,13.0
Spurr on 422
Van Hise on 39, 70, 71, 130, 168
Iron ore deposits associated with intruaives 183
comparison -with ore deposits of adjacent dis-
tricts 180-181
concentration of, at Mansfield mine 72
conditions favorable for, "Van Hise on 72
discovered at Crystal JFalls 175
discovered at Florence 175
explorations for 12,330
of Amasa area 175, 177
of Armenia mine, description of 183
of Crystal Falls area 178,186
production of ore from, table of op. 186
of Crystal Falls district, compared with Me-
nominee iron ores 19
table of production from op. 186
of Columbia mine described 182
of Dunn mine described 182
of Great Western mine described 182
of Felch Mountain range, Burt on 375
of Hemlock mine 177
of Mansfield formation 27,62
described 65-73
of Mansfield mine 63
described 67-70
of Paint Kiver district. Brooks on 18
of Paint Hiver mine ]83
of Pesbakumme Falls 375
of sec. 20, T. 45 K., K. 33 "W 176
of S6c.34,T.46N.,R.33'W 176
of Ux^per Huronian 28, 166
described 175-186
distribution of, described 176-180
history of 175
methods of mining in, described 184-185
origin of, described 183-184
prospecting in 185
relations to adjacent rocks, described 182-183
size of, described 184
sketch of occurrence of 182
table of shipments from op. 186
relations to claj' slate 63
search in Bone Lake schists 151
shipments from Crystal Falls area, table of op. 186
Iron oxide altering to sphene 212
from feldspar 42, 92
included in chlorite 218
included in feldspar 151, 234
included in hornblende 47, 215, 251
included in quartz 419
included in siderite 133
of acid lavas 89, 91
of amygdules 124
of biotite-granite 192
of Bone Lake schists 151
of conglomerate 64
of dolomite 53
of gabbro and norite 239
of Groveland formation 417, 419, 449
500
INDEX.
Page.
1,99,117,127,131
202
Iron oxide of metabaaalt
of raetadolerite
of musco^ite-biotite-gneisB, plate of
of peridotite
of picrite-porphyry 314,216,
of variolites
veins in Groveland formation
Iron pyrites of basic dikes
of peridotite "^^-^
Iron Star Co. {See Great "Western mine.)
Iron Star mine. {See Great "Western mine.)
Ishpeming formation, correlation of xxv, XXVI
Irving, K, D., in concretionary structure in ferrugi-
nous cliert *22
on Felcli Mountain range 379-380
on magnetic mapping ■ - 24, 336
on mica-schist ^^^
on origin of iron ore 70,71, 130
Ivrea, Italy, norite from, analysis of 244, 245
Italy, noritea from 235
Jacob's statf, use of, described 342
Janesville.N.Y., analysis of slate from 261
Johnston-Lavis, H. J., on resorption by intrusives. . 227
Judd.J.AV., on basalts 104
Julien, A. A., on lithology of Crystal Falls district . 21
on Upper Huronian 173-174
K:ayser,E., analyses by 207-208
22
394
221
220
219
95
162
Page.
Lake Superior Survey, reconnaissance by 329
topography by 22
Lamont mine, location of 178, Op. 186
table of shipments from Op. 186
[See Monitor mine.)
Lamont Mining Company. {See Lamont Mine.)
Land Survey, United States 23, 343
Lane, A. C, referred to 21
Lang, H. Otto, referred to 105
Larsson, Per, on composition of iron ore 69
Laurentian. {See Arcbean.)
Lavas, basic, described 95-135
Lawson, A. C, on ellipsoidal structure 118-119
on Laurentian and Coutcbiching 380
Lean ore in Upper Huronian 182
Lee Peck mine, location of 178, Op. 186
table of shipments from Op. 186
Kelley, F. T., referred to
Kemp, F. J., analysis of mica-schist
ou limburgite
on ultrabasic intrusives
on picrite-porphyry
referred to
Keweenawan series relations to Cambrian sandstone .
Kona dolomite, correlation of xxv, xxvi
Krakatao, tuff from 142
L.
La Platte River, olivine gabbro from 255
Labradorite of gabbro find norite 233
of bornblende-gabbro 241
of metabasalt 104,105
twinning of 104
Lakes of Crystal Falls district, origin of 32
Bone, described 35
referred to 156
Huron, Huronian rocks of north shore of xvii
Michigan, streams tributary to 31
Superior, streams tributary to 31
Light, described 34
Liver, described - 34
Mary 161
Squaw 335
SuuDog 335
Tobin 164
Trout 335
Lake Superior land diatrict, map of, by Foster and
Whitney 17
Lake Superior sandstone described 28
unconformity with Huronian 28
{See Potsdam sandstone.)
Lake .Superior Survey 183
Leith, C. K. , indebtedness to
Leucoxene of greenstone
of metabasalt
of metadolerite
surrounding magnetite
Lewis, H. C, on saxonite-porpbyry
ou alteration of olivine
Lewis, indebtedness to
Life in Mansfield slate, presence of carbon.
Light Lake described
12
470
100
202
470
220
218
331
60
34
Limburgite, porpby ritic intrusive, described 212-221
Lime rock of Mansfield mine 63, 67, 69, 70
Limestone of Felch Mountain range, Rominger on.. 383
of hemlock formation 153
{See Dolomite, Randville dolomite.)
Limonite deposits of Upper Huronian 180 .
from calcite 153
from siderite 1 68
of Groveland formation 420
Lincoln mine 179
analysis of ore from 181
location of 178, Op. 186
table of shipments from Op. 186
Lincoln Mining Co. {See Lincoln mine.)
Lindgren, "Waldemar, on granodiorite 231
Liver Lake described 34
Long Portage 174
Long Portage series 173
Lessen, K., on altered clay slates 206,209
Lower Huronian series described xvii,
xviil, 50-154, 398-423, 430-450, 471-481
erosion of XXI, xxii
magnetic rocks of, described 338-341
relations to Arcbean xix, 55
relations to intrusives 187, 190
relations to Upper Huronian xvii,
XXir, 158, 160, 161, 162, 163, 176, 424
auccessiou in xxv, xx vi, 50
thickness of xvii
Lower Marquette aeries, distribution of 453-455
relations to Crystal Falls series xxv, xxvi
relations to Menominee series xxv, xxvi, 451-457
relations to Sturgeon River series 462
ore deposits, comparison with Crystal Falls ore
deposits
Lower Menominee, relations to conglomerate of
Sturgeon River tongue
relations to Lower Marquette 451-457
Luster mottling in raetadolerite 100
181
473
INDEX.
501
M.
Page.
Magnotio instruuieuts, «3e of, described 341-344
Ma.i;netic ubaervations 24,77
described 336-372
efiect of iutniaivos on, described 371-372
iu Grnveland format ion 377, 415, 416, 447, 448
in Negaunee forniiition 453,455
in melabasalt 134
in Sturgeon Hi ver tongue 460
in Upper Hurouian 175, 176
described 156-157
enfolding 366-373
Magnetic rocks described 338-341
depth of, determination of 354-356
Magnetism iu magnetic rocks, distribution of 339-341
466
152
215
217
168
394
252
436
487
394
466
151
Magnetite altering to sphene
from basic volcanics
from hornblende
from olivine
from siderite
included in microcliue
included in biotite
included in dolomite
included in hornblende
included in quartz
of amphibole-schist
of Bone Lake schist
of graywacke' 170,176
of greenstone 470, 487
of iron-bearing formation 338, 417, 419, 421
of metabasalt 100
of metadolerite 202
of peridotite 261
of picrite-porphyry 218
of pyroclastics 147
of sedimentary inclusions in granite 197
of volcanic conglomerate 143
surrounded by leucosene 466, 470
surrounding amygdaloidal cavities 102
titaniferous 47
Malacolite included in hornblende 238
of gabbro and norite 238
Manhattan mine, location of op. 186
table of shipments from op. 186
Manganese ore of Upx>er Huronian 181, 182
Mansfield (to-mi) 12,54,130,178,203,204
Mansfield formation described xx, 54^73, 411-415, 437-440
adinole of, described 208-209
Bessemer ore in 27
clay slate, described 57-62
analysis of 59, 61
compared with analyses of contact product. 209-211
compared with clay 60
compared with New York slate 61
compared with Vermont slate 61
chert of described 62
correlation of xxv, xx\a, 413
desmosite of 207
distribution of 27, 54, 438
exploration in 73
folding of 438, 439
gray wacke of, described 56-57
iron ores of 27, 62
described 65-73
metamorphosed by intrusives 204^211, 413
of Felch Mountain range described 411-415
Page.
Mansfield formation of Michigamme Mountain and
Fence lii ver areas described 437-440
phyllite of, described 57-G2
possible continuation of 55
relations to adjacent formations gg 64
relations to Archean 424
relations to Groveland formation 411, 413, 438, 447
relations to Hemlock formation xx, xxi, xxvi, 64, 76
relations to intrusives 63, 64, 203, 204
relations to Randville formation xx, 55, 4 U , 434, 438
relations to slate of Michigamme Mountain 56
relations to Upper Huronian xxii
siderite-slate of, described 62
spilosites of, descri bed 206-207
structure of g^
thickness of xvn, 64, 65, 412, 438, 439
toijography of 54^433
Mansfield mine 27 54 411
described 65-70
analyses of ore from 59
cross section of g3
concentration of ore at 72
figure of cracks in Q5
location of op. 186
table of shipments from op. 186
Mansfield ore deposit, exploration of 67
concentration of 70
origin of 70
relations to surrounding beds 68
Marble of Randville formation 434, 435
described 408-311
of Sturgeon River dolomite 480, 481
relations to iron-ore formation 376
relations to quartzite 376
{See Dolomite, Randville dolomite.)
Marquette district, correlation of formations of XXA',
XXVI, 189, 330, 451-457, 470-471
folding of 26, 452
Clarksburg formation of iy2
iron-beai'ing formation of, connection with Crys-
tal Falls iron-bearing formation.. 330
Martin-Garcia, olivine-gabbro from 255
Martite of Groveland formation 417-419
Mary Lake jei
Mastodon Iron Co. {See Mastodon mine.)
Mastodon mine, analysis of ore from 181
caving system at 185
location of 179, op. 186
method of mining at 184,185
table of shipments from op. 186
Matrix of ellipsoidal metabasalt 122, 132,133,135
described 114-118
plate of 116, 284, 298
Matthews, E. B., indebtedness to 331
referred to 22
Maurer, E. R., referred to 22
McCutcheon'a Lake 91
McKim, J. A., referred to 22
McNair, F. W., referred to 22
Melaphyres 98
Menominee district, correlation of formations of xxv,
XXVI, 330
folding of 26
relations to Crystal Falls district 11, 25
structure of 378
iron ore of, compared with Crystal Falls iron
ore '. 19,181,186
502
INDEX.
Page.
Menominee Eiver 175, 335, 374, 375, 460
described 32, 334
ilenomiDce series of Sturgeon Pdver 462
Merriam, "W". N., indebtedness to 12
on dike at Glidden exxjloration 183
magnetic lines traced by 12
referred to xv,xxvi,22
sketcli of folding of Upper Huronian 179
Merrill, G.P., on ultrabasic intrusives 220
Mesnard quartzite of Marquette district 452
correlation of xx v, xxvi
Mesabi cbert, resemblance to Groveland quartzite. - 422, 423
Mesabi iron-bearing formation , origin of 422
Mesqua-cum-a-cum-sepe (riTer) 15
Metabasalt described 98-135
alteration of 126-135
aniygdaloidal, plate of 280,282,284,290
analyses of 103,106,107
carbonation of 132, 133; 134
conglomerate of Upper Huronian 163
devitrification of 102,103,126
ellipsoidal, described 112-124
figure of 112,113,114
plate of 116,292
matrix of, described 114-118
plate of 284,.298
eut-axitic texture in 103
flo'w structure in, plate of 280
fragments in volcanic sand, plate of 296
grading into dolerite 2O0
intrusives described 211-212
perlitic parting in, plate of 294
road material IS'i
eilicification of 133, 134
spberulitic 98, 108
texture of, plate of 280, 286, 288, 290, 294
use of term 96,97,98
variolitic, described 108-111
plate of 110
wea^tbering of 134-135
Metadiabase, use of term 96, 97
Metadolerite, described 199-204
analysis of 203
distribution of ■ 199
relations to Hemlock volcanics 204
relations to Mansfield slates 203, 204
relations to other intrusives 204
relations to Upper Huronian 204
use of term 96, 97
Metamorpbism of Arcbean x viii
of Crystal Falls series xxni, xxiv, xxv, xxvi
of Hemlock formation xxiv,44G
of quartzite 425-426
of Upper Huronian 28,425-426
{See Alteration.)
Method of location of ledges 23
Methods of mining 184, 185
Methods of work of Lake Superior Division 22, 23, 24
Metropolitan mine, granite dike at 381
Mica, from feldspar 169,170
included in microcline 394
of acid lavas 89, 91
of biotite-granite 43-192
of granite 198,388,464
of Hemlock schist 444
of bornblende-gabbro 243
Page.
Mica of bornblende-schist 465, 485
of Mansfield slate 205
oi" mica-schist 414
of quartzite 425
orientation of 43, 44, 89, 91, 198, 387, 388
Mica-diorite 226
analysis of 231,263,264
comparison with monzonite 232
opbitio texture in, plate of 308
Mica-gneiss, developed by intrusion 195, 197
from basic volcanics 152
of graywacke 170
of Upper Huronian 166,167,171,172,173
pressure efi'ects in 171
Miea-scbist described 392-395, 423-426
analyses of 394
developed by intrusion 195, 196
from basic volcanics 152
intruded by amphibolite 392
intruded by pegmatite 392
of Arcbean, described 392-395
of Pelch Mountain range, described 423, 426
of graywacke 170
of Mansfield schist 413
of Upper Huronian 166. 167, 171, 172, 173, 174, 425
pressure efi'ects in 171
relations to Kaudville dolomite 392
relations to Sturgeon quartzite 392
Mica-slate described 15
distribution of 14, 15
of Mansfield formation 439
Mica-titanite incl uded in d olomite 436
Michel-Levy on method of feldspar measurement. . . 104, 224
on ophitic texture 200
on texture of rhyolite-porphyry 81, 85, 105
Michigamme dam 55
formation, correlation of xxv, xxvi, 28, 165
replaced by Clarksburg volcanics xxvi
Michigamme jasper, magnetic observations in 339
Michigamme Mountain xx, 56, 446, 447, 448
analyses of dolomites from 435
elevation of 333
slate of - 56
Michigamme Mountain area described 427-450
Michigamme Piver 16,
21, 54, 65, 156, 161, 163, 164, 166. 167,173,
174, 187, 190, 194, 203, 204, 205, 240, 241, 243,
245, 249, 253, 329, 330, 331, 333, 411, 432, 438
described 31,334,335
course of 54, 55
changing channel of 56
Michigan Exploring Co. (See Michigan mine.)
Michigan mine 157
location of op. 186
table of shipments from — op. 186
Microcline, including feldspar 394
including magnetite 394
including mica ^ 394
including quartz 393, 403
of biotite-granite 41,42
of diorite 225
of mica-schist 394,414
of quartz-mica-diorite 228
orientation of 294
Microgranitic texture 83
described 86,87
INDEX.
503
Page.
Microgranitic toxturo in quartz-niica-diorite 228
in quartz-mioa-diorite-porphyry, plate ol' 310
Microlitic feldspar 99
Micro-ophitic texture in metabasalt .'-. 104,212
Micropegiuatitic texture in acid lavas 89
in biotite-granite 191,192, 193
indiorite 224
in gneisses 390
in metadolerite 201
in quartz-niica-diorite 227
in tonalite 230
{See Pegmatitic texture.)
Micropoikilitic texture in rhyolite-porpbyry de-
scribed 83-86
plate of 270,272
in feldspar 171
origin of 85
(See Poikilitic texture.)
Milwaukee and Nortliern Ewy 406
Mines, Aragon 09
Armenia, description of ore body at 183
location of 178, op. 186
table of shipments from op. 186
Blaney. {See Hope mine.)
Caledonia, {See Mansfield mine.)
Calumet and Hecla 399
Claire, location of 178, op. 180
table of shipments from op. 186
Columbia, descrijjtion of ore bodies 182
location of 179, op. 186
table of shipments from op. 1S6
referred to 161
Crystal Falls, analysis of ore from 181
location of 178, op. 186
table of shipments from op. 186
referred to 161
Delphic, location of op. 186
table of shipments from op. 186
Dunn, analysis of ore from 181
depth of 185
description of ore bodies 182
location of 179, op. 186
table of shipments from op. 186
Fairbanks. {See Lincoln mine.)
Great Western, dessription of ore bodies at 182
deptli of 185
fissuring of rocks 185
location of 178, op. 186
table of shipments from op. 186
Groveland 413,415
Hemlock 157, 177
analysis of ore from ■... 181
diamond drill woik at, figure of 177
exploitation of ore deposits at 175
location of op. 186
table of shipments from op. 186
Hollister, location of 178, op. 186
Hope, location of 178, op. 186
table of shipments from j op. 186
Iron Star. {See Great "Western mine.)
Lament, location of 178, op. 186
table of shipments from - op. 186
{See Monitor mine,)
Lee Peck, location of 178, op. 186
table of shipments from op. 186
Lincoln 179
Page.
Mines, Lincoln, analysis of ore, from 181
location of 178,op.l86
table of shipments from op. 186
Manhattan, location of op. 186
table of shipments from op. 186
Mansfield 27, 54, 411
described 65-70
analyses of ore from 69
Cross section of G3
concentration of ore at 72
figure of cracks in 65
location of op. 186
table of shipments from op. 186
Mastodon, analysis of ore from 181
caving system at 185
location of 179, op. 186
method of mining at 184, 185
table of shipments from op. 186
Metropolitan, granite dike at 381
Michigan 157
location of op. 186
table of shipments from op. 186
Monitor, dike in {see Lamont mine) 183
!North western 399, 407
Paint Kiver 183
dike in 183
location of 178, op. 186
ore deposits of 183
table of shipments from op. 186
Pewabic 69
Sheldon &. Schafer. {See Columbia mine.)
Smith. (See Armenia mine.)
ITnion. {See Columbia mine.)
"Wauneta. {See Hope mine.)
Toungstown 182, 186
location of 178, op. 186
table of shipments from op. 186
Mining, depth of 185
Mining methods 184-185
Miarolitic texture in tonalite 229
Mixed ore in Upper Hurouian 182
Monitor mine, dike in {see Lamont mine) 183
Morbihan, Brittany, granite, compared witb Crystal
Falls granite 44
Muscovite, development of 484
from feldspar 42, 82, 90, 92, 192, 201, 224, 228, 248
plate of 286
from schistose x>y roclastic 146
from staurolite 196
included in feldspar 468
included in quartz 171, 394, 403
including biotite 298
iutergrown with biotite 393, 414
of acid lavas 89
of basalt, plate of 290
of basic dikes 47 >
of biotite-granite 192
of biotite-schist 484
of Bone Lake schist 151
of granite 464
of gray wacke 170
of metabasalt 127,129
of mica-schist : 392, 393
of muscovite-biotite-granite 193
plate of 298
of pbyllito 440
504
INDEX.
Page.
Mascovite of sedimentarieBmetamorpliosed by intru-
sion 195,197
of spilosite, ijlate of 30^
parallel growth with biotite 170
relations of orientation to biotite - 484
Muscovite-biotite-gneiss, plate of 298
of Sturgeon forn?ation 401
Muscovite-biotite-granite ^^^
described 193,194
Navitic texture in raetabasalt 98, 99, 104
Needle, dip, use of 344
described 342-343
Keedle, borizontal, deflection of, figure of 345
Kegative crystals in quartz 41
Negaunee iron-bearing formation 451
correlation of XX, xxv, xxvi, 449, 455
distribution of ■ 453-453
magnetic observations in 338, 339
relations to Groveland formation xx, 449, 455
Ket Kiver described 31
New Haven ^^
Korite, analyses of 244,245,263-264
described 233-249
Nortb iron range. (See Felcli Mountain range.)
Northeastern area described 451-457
Northwestern mine --• 399,407
Norway Portage 167,173,174,191,226
Norway Rapids -40
Norway slates, correlation with Mansfield schist 413
0.
Octahedrite, included in hornblende 236
of gabbro and norite 236
(See Anatase.)
Oligoclase in biotite-granite 41 , 42
Olivine, altering to magnetite 217
altering to pilite 211,217
altering to serpentin > 213, 217, 251, 253, 255
altering to tremolite 218
crystallization of 257
included in hornblende 251, 257, 260
included in pyroxene 255,256
including spinel 252, 255
of amphibole-peridotite 255
of gabbro and norite 238,239
of metabasalt 104,211
of meladolerite 201
of peridotite 251
of picrite-porphyry 213, 217
pseudomorphs of 203, 213, 214
zonal intergrowth -with augite and hornblende.. 255-260
plate of 320,322
zonal structure in 258, 259
01i\'ine- gabbro, gradation to amphibole-peridotite . . 254-260
Oolitic testurein dolomite 435,437
Ophitic texture in diorite 223
in gabbro 233
in greenstone - 483
in hornblende-gabbro 240, 244
in metabasalt 212
in metadolerite 48, 200
in tonalite 230
Organic matter, origin of cherty carbonate 184
Page.
Orientation of actinolite 105
of amphibole 127,214,486
of biotite 393,425,468,486
of calcite 132
of chlorite 118,127,133,146
of dolomite 410
of feldspar 84,90,171,394,396
of hematite 418
of hornblende 174, 214, 215, 395, 396, 465
of mica 43,44,51,89,91,198,387-388,478
of phenocrysts of granite-porphyry 45
of quartz 51,84,118,132,133,171,402,404,475
Ornamental stones of Hemlock formation described. 153, 154
Orthoclase, alteration of 224
of acid lavas 89
of biotite-granite 191
of diorite 224,225
of granite 464
of m asco vite-biotite-granite 3 94
of rhyolite-porphyry 182
Ottrelite of chlorite-schist 442
of Hemlock schist 445
Oval structure in Groveland formation described... 420-423
Pahoehoe structure in basalt (see Ellipsoidal struc-
ture) 119,120,121,123,124
Paint Eiver 11,21,161,164,167,174,179,187,190,194
described 31
Paint River district 18
iron ore of 18
named by Erooks 11
Paint River Falls 18
Paint River Iron Company. {See Paint River mine.)
Paint River mine 183
dike in 183
location of 178, op. 186
ore deposits of 183
table of shipments from op. 186
Paint rock associated with ore deposits 183
of Mansfield ore deposit 57, 68
Palagonite tuffs, use of term 138
Paleozoic rocks, deposition of xsiv
relations to lower formations 26, 28, 376, 383
(See Potsdam sandstone.)
Paragonite from feldspar 41, 42
Parallel texture of hornblende-gabbro 244
plate of 314
Patton, H. S., on hornblende 251
on hornblende-picrite 254,255
referred to 21
Peevie Falls 167
Pegmatite in Randville formation 435
intruding mica-schist ., 392
veins in greenstone 476
Pegmatitic texture in gneisses ■ 390
Peneplain of Crystal Falls district, relations to
Michigan and "Wisconsin peneplain ] 3, 31
Penokee district, correlation of series xxi, 189
iron formation of, located by magnetic work 24
ferruginous carbonates of 130
Peridotite described 249-262
age of 262
analyses of 259,263,264
crystallization of minerals 262
distribution of 249,250
INDEX.
505
Page.
, Poridotite, ruliitious of 249,250
relations to g.ibbro 246,200,261,265
(See Welirlite.)
Peril tie purtiug iu aporljyoiito 87
I'l^itoof 274,276
iu Ij.tsalt tuH'. plate of 294
Pertliite in biotite-granlte 41,42
Pesbakiiiiiine Falls, iron ore at 375
Pesliakauinio JRirer 14, 15
Pewabie mine 69
Phenoorysts of granite-porpbyry, orientation of 45
Phillips, H.F, indebtedness to 331
referred to 22
Phyllite, of Mansfield formation 411,439
descri bed 57-62
Pblogopite of dolomite 410
Pbysiograpby of district 13,29-37,329-335
Pichler, A., on alteration of staurolite 196
Pickings, Matber & Co. {See Hemlock mine.)
Picotite of picrite-porpbyry 217
of olivine 252
Picrite-porpbyry described 212-221
Pilite from augite 211
from olivine 211, 217
of metabasalt 211
of metadolcrite 201
of picrite-porpbyry 218
pseudpmorpbs after olivine 202
pseudomorpbs after pyroxene 218
Pilotasitic texture in metabasalt 98, 99, 104, 212
Pine. {See Timber.)
Pinnite, including rutile 410
of dolomite 41Q
PlagiocJase included in hornblende 235, 241
including biotite I93 434
including hornblende 484
of acid lavas 89
of ampbibolite 296
of aiupbibole-schist 4gg
of biotite-granite 41 42191
of biotite-scbist 4q8 459
of diorite 223 ''24
of gabbro and norito 233 234
of greenstone 483' 484
of bcrnblende-gabbro 240 241
of metabasalt ' jq-^
of mica-diorite 227 23''
plate of gQg
of mica-schist ^^^
of muscovite-biotite-granite _ 193
of pyroclastics -^^g
of rhyolite-porphyry g2
of tuffs jgg
orientation of gog
pressure effects in 234
zonal structure in 193-194
Plagioclase-basalt of Hemlock formation 108
Poikilitio texture in amphibole-peridotite 253
in contact of granite and sedimentary, plate of. . 300
in granite jgg
in hornblende 235 '>5i
in hornblendegabbro '241
plate of 019
in metadolcrite 200
in pyroxene '_' 253
iu quartz-mioa-diorite-porphyry, plate of 310
inperidotite .' 250
{See Micropoikilitic texture.)
Point Bonita, California.
40,45
45
Page.
^ . ^,, . 99,108,113
Point Jlornto, California, gabbro from 240
Polar magnetic picrite-porpbyry 212,219
Porphyritic granite
sketch of
I'orphyritic limburgite described 212-221
Porphyritic texture in gabbro 233 241
plate of
in graywacke
iu granite
in metabasalt
in picrite-porpbyry - . .
in quartz-mica-diorite
in schistose pyroclastics
312
167
45
127,129
217
228
146
383
376
87,1
479
48
43, 248
177
{See Ehyolite-porphyry, ' Granite-porphyry,
Aporhyolite-porpbyry.)
Potsdam sandstone, relations to Calciferous lime-
stone
relations to pre-Paleozoic rocks, xxiv, 26, 28, 331, 376, 473
relations to Silurian
Pressure effects in acid lavas
in Algoukian of Sturgeon Paver tongue 471
in amygdules 120,128
iu aporhyolite-porpbyry, plate of 276
iu arkose
iu basic dikes
in biotite
in chert
iu conglomerate ^-^
iu feldspar 42, 90, 92, 169, 234, 248, 388, 464
plate of 276^278
in gabbro 217' "48
'°g™"e ''.'.'44, 194! 464
iu graywacke ■, ^a
iu Groveland formation 440
iu Hemlock formation ^c
in hornblende ^gy
i n hornblende-gabbro, plate of gig
in hornblende-gneiss i^c
iu mica-gneiss -.t^-,
in mica-schist ny,
iu quartz 41,51,53,57,81,82,91,93,
133, 169, 225, 388, 402, 404, 426, 464, 468, 477, 478, 48-(
Pl'i*e«f 268, 276^278
in quartzite jo
in Eandville formation 435-437
in rhyolite-porphyry, plate' of 276 278
in sedimentaries, caused by intrusion 195
Pleistocene deposits, XXIV 29,33 75 332 333
erosion of XXIV ' ' 29
relations to Huronian 155
Prospecting for iron ore jg=
Platauia, Gaetano, on ellipsoidal structure 121 122
Pseudo-conglomerate, eruptive igg
Pseudomorpbs after olivine 213 214 251 253
after pyroxene 214 218
of calcite, after feldspar 130
of chlorite, after biotite 217 228
of pilite, serpentine, and magnetite, after pyrox-
ene 218
of serpentine, after olivine 251 253
Pumpelly , Raphael, on amygdules 125
on Felch Mountain series 380
on luster mottling 200
on Menominee iron region 377
on Michigamme Eiver 335
506
INDEX.
Page.
Purapelly, RapliEel, ou pseudo-amygd ules 203
with Broolis and Rominger, map of Upper Pe-
ninsula of Michigan .'. 18
Pyroclastics, acid, described 94, 95, 135-148
of Hemlock formation described 135-148
plate of 140
intruded by dolerite 147
road material 154
Pyroxene altering touralite 48
crystallization of 258,259
included in hornblende 237, 241, 245, 251, 260
plate of 313
including olivine 255,256
including serpentine 245
lutergrown with hornblende 251
of amphibole-peridotite 258
of bronzite-norite-porphy ry 246
of gabbro 237,238,241,245
of metadolerite 201
of peridotite 250,260
of picrite-porphyry 213,218
of tuffs 138
pseudomorphs 213,214.218
zonal iutergi owth with feldspar, plate of 322
zonal intergrowth with hornblende 260
plate of * 320
zonal intergrowth with olivine, plate of 320, 322
Quartz and magnetite, oval structure in Groveland
formation 420-423
augen of conglomerate 475
cataclastic structure in 41 , 44
enlargement of 57,85, 404, 405
from feldspar . . 42, 52, 99, 111, 131, 151, 169, 170, 171, 201, 248
from granite 52
globular texture in 86
granulation of 51,90
included in feldspar 40,191,394,403,468
included in hornblende 47
included in ilmenite 444
including apatite 194,419,420
including biotite 47,171,394,403,466
including chlorite 403, 419, 420
including epidote 47
including hornblende 466
including iron oxide 394, 419
including muscovite 171, 394, 403
including rutile 394, 420
including sericite 51
including tourmaline 420
inclusions in. . 41, 82, 191, 261, 388, 393, 396, 403, 404, 425, 466
lenticules in greenstone 485, 486
micropoikilitic zones about, plate of 270
of acid lavas 89,91,93
of amphibolite 296,466
of amygdules 125,126
plate of 280,284
of basic dikes 47
of biotite-schist 443,468,484
of Bone Lake schists 151
of conglomerate 477, 478
of diorite 225
of dolomite 410,436
of granite 191,193,194,198,388,464
plate of 298
Page.
Quartz and magnetite of gray wacke 56, 57, 169, 431
of greenstone 470, 483, 486
of Groveland formation 419, 421, 449, 450
of Hemlock schist 444
of marble 481
of metabasalt 117, 118, 127, 129, 132, 133
of metadolerite 201
of mica-schiat 393,414
of peridotite 261
of phyllite '. 440
of pyroclastics 147
of q uartz-mica-diorite-porphy ry , plate of 310
of quartzite 52, 402, 404, 425, 450
of rhyolite-porphyry 81, 83, 84, 85, 86
pressure effects in, plate of 278
of rock intermediate between granite and quartz-
ite 51
of sedimentary inclusion in granite 197
of sericite-schist 443
of tuff 142
orientation of 51, 84, 118, 132, 133, 171, 402, 404, 475
penetrated by tourmaline 57
pbenocrysts, aureoled, of rhyolite -porphyry,
plate of 272
pressure effects in 41,
51, 53, 57,81, 82, 90, 91, 93, 133, 159, 225,
278,388, 402, 404, 426, 464, 468, 477, 484
plate of 268, 276
rhomboehdral parting in
veins lu greenstone
Groveland formation
in ore deposits
in Eandville formation
Quartz-biotite-granite
Quartz-diorite
Quartz-mica-diorite
Quartz-mica-diorite-poTphyry, plate of .
Quartz-porphyry
Quartz rock .
82
14
449
185
435
40
230
227,230
310
429
134
of Mansfield ore deposit 63, 67, 69, 70
Quartz-schist of Eandville formation 52
of Sturgeon formation 401
of Upper Huronian 173
Quartzite, apparent gradation into granite 39, 51, 52
correlation of xxv, xxvi
current bedding in 53
metamorphismof 425,426
of Pelch Mountain Eange described 398-405, 423^26
of Kaudville formation 51-53
of Upper Huronian xxii, 425
pressure effects in 52
relations to dolomite 51, 376
relations to granite 376
relations to ore, deposits 182
{See Sturgeon quartzite.)
E.
Eailways ; Chicago, Milwaukee and St. Paul 95, 143, 175
Chicago and Northwestern 156,175,423
Mil waukee and Northern 406
Eaisin, C, on alteration of olivine 218
on ellipsoidal structure 118
Eandville dolomite described . . xix, 27, 50-53, 406-411, 431-437
analyses of 409, 435
correlation of xxv, sxvi
distribution of 26, 50. 406-408, 431, 432
INDEX.
507
Page.
Kanavillodolmiti'. I'ohliugof 432-431
iifFcluh llouutaiu tongue described 406-411
ofMichigammo Mouutainaud Fence River areas
described 431-437
jiressuro eflfecta in 435, 437
quartzite of 51-53
relations to Archeau xix, 51, 53, 55, 407
relations to Groveland formation xx
relations to Hemlock formation xx, 75
relations to intrusivea 434
relations to Mansfield foi'mation xx, 55, 411, 438
relations to mica-schists 392
relations to Sturgeon quartzite.. xix, 51,407, 430,4:^1,434
relations to Upper Huronian 424
relations to underlying formations 53
thickness of xvii, xix. 26, 53, 407, 408, 433
topography of 50, 406-408
Kandville station 406
Range. {See Town.)
Ransome, F. L., on ellipsoidal basalt . 113, 114.118, 119, 122, 123
on feldspar sheaves 99
on ultrabasic iDtrusives 220
on variolites 108
Recomposed granite grading into quartzite 39
of Marquette district 52
Recryatallization of gneisses 390, 391
ofgraywacke 195,198
plate of .• 298
of hornblende, plate of 316
of mica-gneiss 171
of quartz 404
of Sturgeon river conglomerate xxiv
Reibungsbreccia of Hemlock formation 136
of Upper Huronian , 161,166,177
Republic (town) .■ 333, 334
Republic trough 26, 452, 453
Resorption rim about olivine 258
Reyar, E., referred to 142
Rhombohfedral parting in quartz 82
Rhyolite-porphyry 80, 190
described 81-87
banding of 81
plate of 278
microgranitic texture in described 86, 87
micropoikilitic texture in described 83-86
plate of 270,272
of Hemlock formation 80
described 81-87
phenocrysts of, plate of 268
pressure effects in, plate of 276, 278
Richardson, G. B., analysis by 408, 409
Ridgway, J. L., indebtedness to 13. xvi
Rieserferner, tonalite from 230, 231
Ripple marks in arkose 473
Road material of Hemlock formation 154
Roberts, C. T.. indebtedness to 66, 182
Romberg, J., on augite of gabbro 255
on olivine-gabbro j 256
on pyroxene zone about olivine 256
Rominger, C-, on correlation of Menominee rocks ... 19
on correlation of dolerite dikes 189
on correlation of Upper Huronian 164
on Felch Mountain range 374,379,381
on Menominee district 19
on iron-bearing formation near Crystal Falls 179, 180
' on progressive metamorphism 52
Page.
Komiuger, C.on Sturgeon River tongue 461
on Upper Peninsula of Michigan 20
referred to xv
with Brooks and Pumpelly, map of Upper Penin-
sula of Michigan 18
Kosenbusch, H., analysis by 166
on enstatite-porphyrite 247
on peridoti to 249
on picrite-porphyry 220,221
on spilosite 206
referred to 193, 105
Roth, J., on transfer of material from basic intru-
sives to slate 211
on weathering of granite 58
Rothpletz, A.,on ellipsoidal structure 118,119
Rutile from biotite 43, 192, 202, 225
from ilraenite 202, 239
from titanic iron 170, 192, 193, 230
included in bronzite 238
included in feldspar 234
included in hornblende 236, 239,248
included in pinnite 410
included in quartz 394, 420
of acid lavas 93
of biotite-granite 192,193
of biotite-schist 484
of Bone Lake schist 151
of gabbro 236, 239
of graywacke 56, 170
of nietabasalt 129
of metadolerite 202
of metamorphosed Mansfield slate 205
twinning of 205, 236
S.
Saalband in quartz-mica diorite * 228
Sagenite from biotite 192, 193
included in biotite 403
included in chlorite 403
of biotite-granite 192, 193
Sand, volcanic, plate of 296
Sandstone and slates of Sturgeon River dolomite
formation described 481,482
Sandstone, Lake Superior. (5'ee Potsdam sandstone.)
Sanford, S., indebtedness to 331
Santorin, basalt from 1200
Schafer, R. "W"., on basic rocks 247
Schistose acid lavas described 87-94
dikes in Archean 46-48
pyroclastics described 145-148
Schistosity of acid lavas 91
of basic dikes 47,48
of gabbro 2i8
plate of 316
of granite 44, 194
of greenstone 470
of hornblende-gneiss 175
of hornblende-schist 465
of metabasalt 127
of mica-schist 114
of pyroclastics 147
of Sturgeon River conglomerate 474, 475
Schists. (See Crystalline schists.)
Schlieren in metabasalt 98
Scoriae of Hemlock formation 70
Section. (See Town.)
508
INDEX.
Page.
Sedimentariea, contact with, granite, described 194-198
plate of 300
included in dolerite 203
included in granite 195
of Hemlock formation described 152. 153
of Upper Huronian described 165-174
volcanic, described ^ 136-145
plate of '. 296
Sericite from feldspar 52, 89, 99, lOi, 111, 127. 131, 477
included in quartz 51
of acid lavas 91
of nietabasalt 99,101,127
of Kandville quartzite 52
of rock intermediate between quartzite and
granite 51
orientatiou of 51, 478
Sericite-scbist from conglomerate 475
from graywacke 57
of Hemlock formation 443
Serpentine from bronzite 238
plate of 306
from bronzite-norite-porpbyry 246
from olivine 213,217,251,255
includ/sd in pyroxene 245
intergro wn witb augite 253
of picrite-porpbyry 213,214,218
pseudomorphs 214
pseudomorpba after olivine 253
pseudomorphs after pyroxene 218
Sbeldon &. Scbafer mine. (See Columbia mine.)
Sboldeis exploration 447,449
Siamo slate 451
correlation of sxv, xxvi
Siderite altering to hematite 168
altering to limonite 168
altering to magnetite 168
including iron oxide 133
interbanded witb clay slate 168
of gray wacke 170
of Groveland formation xx, 420
of metabasalt 117, 133
of siderite slate 62
origin of 168
replaced by silica 134
replacing calcite 133
source of iron ore 70, 71 , 184
Siderite-slate of Mansfield formation described 62
of Upper Huronian 108
relations to claj^ slate 63
Sideritization of metabasalt 117
Sidnaw (town) 1'5
Silicification of metabasalt 130, 133, 134
of siderite 134
Silurian rocks, unconformity with, underlying rocks . 26, 376
Slates described 1-1, 153, 169-174, 481, 482
alteration of 14, 166
of Mansfield formation described 204-211
altering to chlorite-schist 166
altering to mica-gneiss 166
argillaceous 14
fragments in Hemlock tufls 76
from Benson, Yt., analysis of 61
from Hampton village, N. Y., analysis of 61
from Janesville, N. Y., analysis of 61
from SoutbPoultney,Yt., analysis of 61
hornblende, described 13, 14
Page.
Slates of Hemlock formation 153
of Mansfield formation, described 57-02
of SturgeonRiverdolomiteformation, described. 481,482
of Upper Huronian 166
described 169-174
relations to ore deposits 68, 182
transfer of material from basic intrusives 211
(See Mansfield formation.)
Smyth, H. L., on peneplain 31
on Randville dolomite 27, 53
referred to xv. xvl 22, 26, 50, 74, 152
Smith, G. 0., on Archean granite 38
on ellipsoidal structure 118, 119
Smith mine. {See Armenia mine.)
Soapstone associated \\ ith ore deposits 68, 183
of Mansfield formation 57
Soil of Crystal Falls district 36,37
Solar compass, use of 24
Sorby, H. C, on enlargement of quartz grains 404
South iron range. (See Menominee range.)
South Mastodon mine. (See Manhattan mine.)
South Mountain, Pa 124
South Poultney, Yt., analysis of slate from 161
Sphenefrom biotite 192,225
from ilraenite — 239
from titanic iron 144, 146, 170, 193, 212, 239
included in biotite 225
incl uded in epidote-zoiaite 225
including apatite 244
of acid lavas 91
of basic dikes 47
of biotite-granite 192, 193
of gabbro 239
of gray wacke 170
of metabasalt " 100
of metadolerite 202
of tuff 141
of volcanic conglomerate 144
pseudomorphs after magnetite 466
surrounding magnetite 466
Spherulitic texture in metabasalt (<eeYariolite.). 98, 102, 108
Spilite 98
Spilosite of Mansfield formation 64
described 206,207
plateof... 302,304
analyses of 207
analysis of compared with analyses of clay slate
and adinole 210
gradation to desmosite, plate of 300
included in dolerite 204
Spinel included in hornblende 236
included in olivine 251, 252, 255
intergrown with hornblende 256
of gabbro and norite 236
of peridotite 252
Spurr, J. E,, on oval areas in iron formation 422
Squaw Lake 335
Stalactitic ore 180
Staurolite altering to muscovite 196
of gray wacke 167
of mica-schist 195
Steiger, George, analyses by 59,
61, 203, 208, 210, 242, 244, 245, 263, 264
Stokes, H. K., analyses by 103^
106, 207, 210, 219, 231, 259, 263, 264, 389, 391, 394, 397
referred to 260
INDEX.
509
Page.
Stratigraphy of Crystal Falls ilistrict 25-29
Structuro of Crystal Falls district 25-29
I'ln'oof 160
of Mausfiehl formation 64
Sturgeon quartzite dcscribod xviii, 398-405, 430-431
correlation of XXV xxvi
distribution of 398-399
folding of 399,400
intrtided by granite.. 428
of Fclcli Mountain range described 398-405
of ilenomiuee district, correlated xxv,xxvi
of ilicbigamme Monntain and Fence River
areas described 130, 431
relations to Archean xix, 398-401
relations to mica-schist 392
relations to Randville formation xix, 407, 430, 431, 434
relations to Upper Huronian xxii
thickness of xvii, x viii, 399-401, 431
topography of 398-399
Sturgeon Eiver 330, 376, 386, 401, 458, 485
described 334,335
Sturgeon Eiver dam 474
Sturgeon Eiver tongue described 458-487
Algonkian of, described 471-482
Basement Complex of, described 463-471
conglomerate of xviii, xxiv
described 473-479
literature on 459-461
relations to Felcb Mountain rocks 462
relations to Lower Marquette , 462
Sun Dog Lake 335
Sweden, granite of 44
Syenite, distribution of I3
Synclinal troughs, concentration of ore in 183, 184
Syncline, Crystal Falls, described 158-161
determined by magnetic observations. . . 366-370, 372, 373
of Groveland formation 416
Syracuse, N. Y., plcrite-porpbyry at 219
T.
Table of succession in Marquette, Crystal Falls, and
Menominee districts xxv xxvi
Table of iron ore shipments of Crystal Falls area... Op. 186
Talc from bronzite 238
Plateof 3Q5
Teall, J. J. H., onellipsoid.il structure 118,123,124
on plutonic rocks 000
Test pits in Mansfield ore deposits 67
Timbei' of Crystal Falls district 13, 36 37
Timbering of mine jg5
Titanic iron altering to sphene 144, 146, 170 193, 239
altering to rutile 170,193,239
of basic dikes 47
of biotite.;
included in biotite.
■J<i^ite 192,193
404
included in chlorite 146 404
included in quartz 146
of quartzite , 404
Thonschiefernadeln 93 oqs
Thiirach, on alteration of staurolite
Tobin Lake
Tonalite
comparison with Adamello tonalite
Topography by Lake Superior Survey
by U. S. Geological Survey
influenced by intrusive rocks
256
420
196
298
Page.
Topography, sketch of 45
of Algonkian 333
of Archean 38, 39, ,133, 386, 387
of Crystal Falls district 13,29-31,331-333
of Deer Eiver Valley 99
of greenstone 333
of Groveland formation 415,416,446-438
of Hemlock formation 73,74,440-441
of Huronian rooks, plateof 150
of intrnsives 46,54,333
of Mansfield formation 54 433 439
of Eandville dolomite 50 406^08
of Sturgeon quartzite 398-399
of Upper Huronian 153-156
Tornebohm, , referred to
Tourmaline included in quartz
of mica-schist
of muscovite-biotite-gneiss, plate ct
of sedimentary inclusions in granite 197
penetrating feldspar 57
penetrating quartz 57
Town.41N.,E.29W !.';;"l4,375,460
41N.,E.30W 375,377,460
41 U"., E. 30 W., section 2 355
section 3 30-
4iN.,E.3iw ^y^..^[.....[\[] u
41M".,E. 31 W., section 16 31
section 25 3^^
41 JJ.,E.32\7 14
41 N., E. 32 W,, section 12 31
section 33 igQ
42N.,E.27W ;-"-'^'!;!": 458,460
42 N., E. 37 "W., section 6 459
^ectiom ^58^^59
section 17 450
section 18 453
42N.,E.28 W 374,376,377,(158,460,472
42 N., E. 28 "W., section 2 472
section 3 4gQ
section 4 4^0
section 6 ^qq
''™tion7 459,462,463
section8 459,460,462,463
section 9 ^-jq
section 10 47^
section II .~q
section 14 4gQ
sectioun 458,461,467,474,482,485
section 18 458,482
section 19 ^q^
section 21 377
section 29 399
section 31 399,407,412,413
section 32... 374,377,378,381,385,399,412,416,423,424
section 33 374
377, 381, 386, 387, 399, 412, 416, 418, 423, 424, 426
42 N., E. 29 -n" 14, 329, 374, 375, 376, 377, 458, 460, 472
43 N., E. 29 W., section 1 458,480
section 3 458,459
section 7 475
section 12 459,463
section 13 459,469
section 22 459
section 23 459
section 24 460
section 26 454
510
INDEX.
Page.
Town. 42 N"., E. 29 W., section 31 377,
374, 378, 385, 398, 399, 406, 412, 415, 416, 460
section 32 387,399,412,416
section 33 386,407,412,416
section 34 392,398,406,412,426
section 35 392, 398, 399, 400, 406, 412, 426
section 36 385,398,401,406,412
42N.,R.30W 329 374,375,376,377,458,460
42 N., E. 30 "W., section 12 459
section 14 461
section 25 411
section 30 385
section 34 374, 385, 398, 399, 411, 412, 41 5
section 35 335, 399, 400, 406, 407, 412, 413, 415, 426
section 36 335, 398, 407, 412, 415, 416, 418
42 N., R. 31 W 14, 15, 16, 73, 199, 204, 329
42N., R. 31 W., section 1 158
section 2 158
section 9 163, 1G6
section 15 164, 190, 191, 211, 226, 240, 241
section 16 167,211
section 19 161,190,194
section 20 164,190,191
section 22 190, 191, 241, 249, 250, 253, 260
section 28 164,241
. section 29 164, 190, 194, 241, 243, 245, 249, 253
section 30 190,194
section 32 167
42K.,E.32W 14,15,18,156
42 N., E. 32 W., section 1 .
section 4
section 12
section 14
section 17
section 18
section 19
section 20
section 21
section 24
199
191
158
167
74
74
74
74
164
167
section 28 164,229
section 35 ." 167
section 36 167
42N"., E.33W 18,156
42N.,E.33 W.,sectionl3 18
section 24 74
42N.,E.28'W 329,460
43N,,E.28'W" 375,459,460,472,458
section 33 400
43N.,E.29"W 458,459,460,472
43 N., E. 29 "ST., section 1 472
section 13 459
section 24 459
section 35 480
43N.,E.30 \T 458,459,460
43N.,E.31 W 76,203,204,329,427,447
431f.,E.31"W.,section2 432
section 3' 446
section 5 438
sectione 199
section7 54,64,76,203,210
sections 205,210
section 10 438
section 17 64,61,65,190,210
section 18 54, 55
section 19 223
section 20 ; 65,223
Town. 43 K., R. 31 "W., section 26 .
Page.
447
247
section 29
54 64 190 205
section 31
164
section 32
161,163,164,199,203
199,247
section 34
. . 247
43 N., R.32W
18 74
43 K., R. 32 W., section 1
54
199
section 5
141
section 7
199 204
204
section 9
204
section 11
. 158
158
199
section 20
18 158
164
43 X., R. 33 ■S\''
18,74
43N.,R. 35 W., section ■
158
44N.,R.31 W
38 334 427 428 432
44 N.,R. 31 "W"., section 3
441
section 4
441
section 10
335
section 15
430,441,442
434
section 21
429
section 22
427 441
section 28
432
section 32
section 33 ,..-
432,434,438
432 438 446
44N.,R.32 W
44 N., R. 32 "W., section 1
38,199,334,427
46, 53
91 92
section 9
212
55
53
section 18
199 204
212
section 27
212
section 28
.. . 199 203
95
88
441T.,R. 33 "W.,section 4 ... .
. X08
44N.,R.42 W
77
45 !N"., R. 30 W., section 5
453
429
453
section 9
453
453
453
45N.,R. 31"W.
. 30, 38, 345, 427, 428, 431, 457
45 N. R. 31 "W". section 6 .
440
441 447
section 21
335,441,447
431
section 28
431,432,441
45N.,E.32"W
29,30,38,73,427
45 IN". R. 32 W. section 1
39
51
45N.,R.33 W
73
INDEX.
511
149
149
153
453
406
1,427
447
Pago.
Town. 45 N„K. 33 W.,aoction 9 157
seotiou 15 149
section IC 149_ 157
3ection20 158,175,176
section 22
section 27
section 34
46 N., K. 30 'W., section 19
section 40
46N., K.31 W 13,
46 N., R. 31 W., section 32
46N..K.32W 38,148,155,156,427
40N.,E. 32 W., section 16 149
section 21 128, 334
section 31 I49
8ection36 73, 149
46N., E.33 W 'l56
46 N.,R. saw., section 24 147,149,199
section 27 204
section 34 149,156,163,175,176
47]S-.,E.30W 329,457
47N'.,E.30 W., section 9 '. 334
section 19 333
section 30 333
47N.,R.31W 13,329,332
47N.,E.3i-\Y ;,g(,tion 24 333
section 25 333
section 36 333
47 N., E. 33 'NY., section 19 199,204
48N.,E.31 W
Tracliy tic texture in pyroclastics
Tremolite developed by dynamic action- . .
from olivine
included in clilorite
of dolomite
13
147
432
217
218
.. 408,410,436
of picrite-porpliyry 214,218
(iSee Amphibole.)
Trout Lalie 335
Tuff, basalt, perlitio parting in, plate of 294
breccia, use of term I37
conglomerate, use of term 137
of Hemloclc formation 64 75
described 137-143
thicl£ness of 141
origin of 141-142
palagonite, use of term 133
relations to asli beds 143
resemblance to Tertiary and Eecent tuffs 142
use of term 135
Tuffogene sediments described 143-145
Turner, H. TV., on granodiorite 231
Twinning of feldspar. ... 42, 104, 191, 201, 224, 228, 233, 261, 468
of muscovite 197
of pyroxene 250
of rutile 56,205,336
Tlltrabasic intrusives in Crystal Falls district
described 212-221
in Upper Huronian I64
Unconformity. (See Eolations under particular
formations and series.)
Undnlatory extinction in quartz (see Pressure
effects) 41, 81, 82, 90, 133, 169, 225, 388, 464, 477
Union mine. (See Columbia mine.)
United States Land Survey 343
Page.
United States surveyors on Sturgeon River tongue. 459-460
United States Geological Survey, topography by 22
Upper Cambrian. {See Potsdam, Lake Superior
sandstone.)
Upper Huronian described xviii, x.\i, 27, 155-186, 423-420
correlation of 155,164,165
distribution of 155 155
folding of 'I58-I62il88
sketch of - _ 179
relations to intrusives 189-190
intrusives in ; 174,175,187
magnetic observations in 156,157 339
metamorphism of 28,425 420
of Felch Mountain range described 423-426
of Penokee series ^xr
ore deposits of 28
described ^j.igg
table of shipments of Op. 180
relations to Archeau .
relations to basic intrusives .' 204,211,223
relations to Cambrian sandstone 155,161,162
relations to grit 155
relations to Groveland formation xxi, 425
relations to Hemlock formation 77
relations to intrusives ,. ., 164,190,204.211 223
relations to Mansfield formation xxir
relations to Michigamme formation 28, 105
relations to Lovrer Huronian xvii
XXII, 158, 160, 161, 162, 163, 176, 424
relations to Eandville dolomite 424
relations to Sturgeon quartzite xxii
relations to Upper Marquette 155
succession in xxv xxvi
thickness of 157
topography of ^ 155,136
Upper Marquette, correlation of xxv, xxvi
iron-bearing series referred to 20
iron ore deposits, comparison with Crystal Falls
iron ore deposits 180,181
relations to Upper Huronian 155
Uralite from augite 104 201 212
from pyroxene 43
including feldspar 202
of metadolerite 200
of volcanic conglomerate 144
V.
Van Hise, C. E., connection with Lake Superior Sur-
rey
indebtedness to
on alteration of siderite
on concretionary structure in ferruginous chert.
on correlation of Crystal Falls ore deposits
on Felch Mountain range
on formation of crystalline schists
on Lake Superior stratigraphy
on Mesnard area
on metamorphism of basic rocks
on mica-schist
on Michigamme formation
on ore concentration
on ore deposits of Armenia mine
on ore deposits of Hemlock mine
on origin of siderite
on origin of iron ore 39,70,71,
on sericite-schist from recomposed granite
21,22
12
168
422
20
380
172
19
452, 457
152
414
165
72
183
177
168
130, 168
52
512
i:n^dex.
Page.
Van Hise, C. E., on eilicification of ore formation on 13-1
Sturgeon River conglomerate 461
Van Horn, F. R.,onnorites 235,247
Van "Werveke, on spilosites 206
Variolite in metabasalt described 108-111
plate of 110
of Point Bonita , - 108
Veins of iron oxide in Groveland formation 449
of pegmatite in greenstone 476
of quartz 14
in Groveland formation 185, 449
in Rundville foi mation 435
Volcanic ashes altering to greenstone 487
Volcanic breccias, use of term 137
Volcanic elastics, plate of 284
Volcanic cones iu Hemlock formation 78
Volcanic conglomerate described ^ 143-145
use of term 136
Volcanic rocks of Crystal Falls district xx, 95-148
of Hawaii 75
of Iceland 75
of Penokce district xxi
{Sec Hemlock formation.)
Volcanic band, plate of 290
Volcanic sedimentary rocks of Hemlock formation
described 136-145
Vulcan iron formation, correlated xxv, xxvi
"W.
Wadsworlh, M. E., ou the iron, gold, and copper dis-
tricts of Michigan 20
on dike in Paint River mine 183
on Felch I\tountain range 381
on Mesnard series 452
on Upper Huronian conglomerate 166
referred to 95
Washington, H. S., on zonal structure in olivine 258
"Wauueta mine. (See Hope mine.)
"Weathering of amygdules 125
of biotite 202
of Mansfield slate 205
of metabasalt 134, 135
of metadolerite 199-200
"Weidman, S., referred to 22
"Wehrlite described 253
plate of 320, 322
analysis of 259,263-264
gradation to amphibole-peridotite described 254-260
(See Peridotite.)
We we slate, correlation of xxv, xx vi
Page.
Whin Sill 211
Whitney, J. D. (See Foster, J". W.)
Wichmann, Arthur, on iron-bearing rocks south of
Lake Superior 21
ou serpentine 253
on Upper Huronian rocks 173, 174
Williams, G. H., on alteration of ilmenite 203
on Cortlandt series 222
on Cortlandite 254
on ellipsoidal structure ]18, 119
on gabbros 247
on micropoikilitic texture 84,85,87
on production of schists from igneous elastics.. 152
on pyroclastics of Marquette district 148
on pyroxene zone about olivine 256
on ultrabasic iutrusives 220
referred to 95
Wincbell, N, H., on ellipsoidal structure 118-119
Wolff, J. E., on Hoosac schists 394
on development of mica 130
Wright, C.E., on Crystal Falls district 20
on garnetiferous mica-schist 196
on granite dikes in iron-bearing formation 381
on Menominee iron region 21
on staurolitiferous mica-schists 196
on Upper Huronian 173-174
referred to 21
Y.
Toungstown mine 182, 186
location of 178, op. 186
table of shipments from op, 1S6
Z.
Zircon included in biotite 234
of gabbro and norite 239
of rhyolite-porphyry 81
Zirkel, F., on rock nomenclature 97
on spilosites 206
on transfer of material from basic intrusives 211
Zoisite from feldspar 201
plate of 2R6
included in epidote 445
of metabasalt 101, 117
Zonal structure in amygdules 124
in epidote-zoisite 101
in feldspar 193,194,197,233
in hornblende 226, 234, 235, 236, 256
in metabasalt 117
in olivine 259
in pyroclastics 146
in tuff 141,142
[Monograph XXXVI.]
The statute approved March 3, 1879, establishing the United States Geological Survey, contains
the following provisions :
' ' The publications of the Geological Survey shall consist of the annual report of operations, geo-
logical and economic maps illustrating the resources and classiUcation of the lands, and reports upon
general and economic geology and paleontology. The annual report of operations of the Geological
Survey shall accompany the annual report of the Secretary of the Interior. All special memoirs^and
reports of said Survey shall be issued in uniform quarto series if deemed necessary by the Director, but
otherwise in ordinary octavos. Three thousand copies of each shall be published for scientific exchanges
and for sale at the price of publication ; and all literary and cartographic materials received in exchange
shall be the property of the United States and form a part of the library of the organization : And the
money resulting from the sale of such publications shall be covered into the Treasury of the United
States."
Except in those cases in which an extra number of any special memoir or report has been sup-
plied to the Survey by special resolution of Congress or has been ordered by the Secretary of the
Interior, this office has no copies for gratuitous distribution.
ANNUAL REPORTS.
I. First Annual Report of the United States Geological Survey, by Clarence King. 1880. 8'^. 79
pp. 1 map.— A preliminary report describing jjlau of organization and publications.
II. Second Annual Report of the United States Geological Survey, 1880-81, by J. W. Powell.
1882. 8°. lv,588pp. 62 pi. ,1 map.
III. Third Annual Report of the United States Geological Survey, 1881-'82, by J. W. Powell
1883. 8°. xviii, 564 pp. 67 pi. and maps.
IV. Fourth Annual Report of the United States Geological Survey, 1882-'83, by J. W. Powell
1884. 8°. xxxii,473pp. 85 pi. and maps.
V. Fifth Annual Report of the United States Geological Survey, 1883-'84, by J. W. Powell.
1885. 8*^. xxxvi, 469 pp. 58 pi. and maps.
VI. Sixth Annual Report of the United States Geological Survey, 1884-'85, by J. W. Powell
1885. 8°. xxix, 570 pp. 65 pi. and maps.
VII. Seventh Annual Report of the United States Geological Survey, 1885-'86, by J. AV. Powell.
1888. 8^. XX, 656 pp. 71 pi. and maps.
VIII. Eighth Annual Report of the United States Geological Survey, 1886-87, by J. AV. Powell.
1889. 8°. 2 pt. xix, 474, xii pp., 53 pi. and maps ; 1 prel. leaf, 475-1063 pp., 54-76 pi. and maps.
IX. Ninth Annual Report of the United States Geological Survey, 1887-'88, by J. W. Powell
1889. 8°. xiii,717pp. 88 pi. and maps.
X. Tenth Annual Report of the United States Geological Survey, 1888-89, by J. W. Powell.
1890. 8°. 2 pt. XV, 774 pp., 98 pi. and maps ; viii. 123 pp.
XI. Eleventh Annual Report of the United States Geological Survey, 1889-90, by J. "\V. Powell.
1891. 8^. 2 pt. XV, 7.57 pp., 66 pi. and maps; ix, 351 pp., 30 pi. and maps.
XII. Twelfth Annual Report of the United States Geological Survey, 1890-'91, by J. W. Powell.
1891. 8°. 2 pt., xiii, 675 pp., 53 pi. and maps ; xviii, 576 pp., 146 pi. and maps.
XIII. Thirteenth Annual Report of the United States Geological Survey, 1891-'92, by J. AV.
Powell. 1893. 8°. 3 pt. vii, 240 pp., 2 maps; x, 372 pp., 105 pi. and maps; x'i, 486 pp., 77 "pi. and
maps.
XIV. Fourteenth Annual Report of the United States Geological Survey, 1892-'93, by J. W.
Powell. 1893. 8°. 2 pt. vi, 321 pp., 1 pi. ; xx, 597 pp., 74 pi. and maps.
XV. Fifteenth Annual Report of the United States Geological Survey, 1893-'94, T)y J. W. Powell.
1895. 8°. xiv, 755 pp., 48 pi. and maps.
XVI. Sixteenth Annual Report of the United States Geological Survey, 1894-'95, Charles D.
Walcott, Director. 1895. (Part I, 1896.) S'^. 4 pt. xxii, 910 pp., 117 pi. and maps; xix, 598 pp.. 43
pi. and maps; xv, 646 pp., 23 pi. ; xix, 735 pp., 6 pi.
XVII. Seventeenth Annual ]?eport of the United States Geological Survey, 1895-'96, Charles
D. Walcott, Director. 1896. S-^. 3 pt. in 4 vol. xxii, 1076 pp., 67 pi. and maps; xxv, 864 pp., 113 pi.
and maps; xxiii, 542 pp., 8 pi. and maps; iii, 543-1058 pp., 9-13 pi.
XVIII. Eighteenth Annual Report of the United States Geological Survey, 1896-'97, Charles D.
Walcott, Director. 1897. (Parts II and III, 1898. ) 8^. 5 pt. in 6 vol. 1-440 pp., 4 pi. and maps ; i-v,
MON XXXVI 33 1
II ADVERTISEMENT.
1-653 pp., 105 pi. and maps; i-v, 1-861 pp., 118 pi. aud maps; i-x, 1-756 pp., 102 pi. and maps; i-xii,
1-642 pp., 1 pi. ; 643-1400 pp.
XIX. Nineteenth Annual Report of the United States Geological Survey, 1897-'98, Charles D.
Walcott, Director. 1898. 8". 6 pt. in 7 vol.
MONOGRAPHS.
I. Lake Bonneville, by Grove Karl Gilbert. 1890. 4°. xx, 438 pp. 51 pi. 1 map. Price $1.50.
II. TertiaryHistoryofthe Grand Canon District, with Atlas, by Clarence E. Dutton, Capt., U. S. A.
1882. 4'=. xiv, 264 pp. 42 pi. aud atlas of 24 sheets folio. Price $10.00.
III. Geology of the Comstock Lode and the Washoe District, with Atlas, by George F. Becker.
1882. 4°. XV, 422 pp. 7 pi. and atlas of 21 sheets folio. Price $11.00.
IV. Comstock Mining and Miners, by Eliot Lord. 1883. 4°. xiv, 451 pp. 3 pi. Price $1.50.
V. The Copper-Bearing Rocks of Lake Superior, by Roland Duer Irving. 1883. 4°. xvi, 464
pp. 15 1. 29 pi. and maps. Price $1.85.
VI. Contributions to the Knowledge of the Older Mesozoic Flora of Virginia, by William Morris
Fontaine. 1883. 4°. xi, 144 pp. 54 1. 54 pi. Price $1.05.
VII. Silver-Lead Deposits of Eureka, Nevada, by Joseph Story Curtis. 1884. 4°. xiii, 200 pp.
16 pi. Price $1.20.
VIII. P.ileontology of the Eureka District, by Charles Doolittle Walcott. 1884. 4°. xiii, 298
pp. 241. 24 pi. Price $1.10,
IX. Brachiopoda and Lamellibranchiata of the Raritan Clays aud Greensand Marls of New
Jersey, by Robert P. Whitfield. 1885. 4^. xx,338pp. 35 pi. 1 map. Price $1.15.
X. Dinooerata. A Monograph of an Extinct Order of Gigantic Mammals, by Othniel Charles
Marsh. 1886. 4°. xviii, 243 pp. 56 1. 56 pi. Price $2.70.
XI. Geological History of Lake Lahontan, a Quaternary Lake of Northwestern Nevada, by
Israel Cook Russell. 1885. 4^. xiv, 288 pp. 46 pi. and maps. Price $1.75.
XII. Geology and Mining Industry of Leadville, Colorado, with Atlas, by Samuel Franklin
Emmons. 1886. 4°. sxix, 770 pp. 45 pi. aud atlas of 35 sheets folio. Price $8.40.
XIII. Geology of the Quicksilver Deposits of the Pacific Slope, with Atlas, by George F. Becker.
1888. 4°. xix, 4fe"6 pp. 7 pi. and atlas of 14 sheets folio. Price $2.00.
XIV. Fossil Fishes and Fossil Plants of the Triassic Rocks of New Jersey and the Connecticut
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XV. The Potomac or Younger Mesozoic Flora, by William Morris Fontaine. 1889. 4°, xiv,
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XVI. The Paleozoic Fishes of North America, by John Strong Newberry. 1889. 4^^. 340 pp.
53 pi. Price $1.00.
XVII. The Flora of the Dakota Group, a Posthumous Work, by Leo Lesquereux. Edited by
F. H. Knowltou. 1891. 4^. 400 pp. 66 pi. Price $1.10.
XVIII. Gasteropoda aud Cephalopoda of the Raritan Clays and Greensand Marls of New Jersey,
by Robert P. Whitfield. 1891. 4^^. 402 pp. 50 pi. Price $1.00.
XIX. The Penokee Iron-Bearing Series of Northern Wisconsin and Michigan, by Roland D.
Irving and C. R. 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 Ehynchophorous Coleoptera of the United States, by Samuel Hubbard Scud-
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XXII. A Manual of Topographic Methods, by Henry Gannett, Chief Topographer. 1893. 4°.
xiv, 300 pp. 18 pi. Price $1.00.
XXIII. Geology of the Green Mountains in Massachusetts, by Raphael Pumpelly, T. Nelson Dale,
and J. E. Wolif. 1894. 4°. xiv, 206 pp. 23 pi. Price $1.30.
XXIV. Mollusca and Crustacea of the Miocene Formations of New Jersey, by Robert Parr Whit-
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XXV. TheGlacialLakeAgassiz, by Warren Upham. 1895. 4°. xxiv, 658 pp. 38 pi. Price $1.70.
XXVI. Flora of the Amboy Clavs, by John Strong Newberry; a Posthumous Work, edited by
Arthur Hollick. 1895. 4°. 260 pp. 58 pi. Price $1.00.
XXVII. Geology of the Denver Basin in Colorado, by Samuel Franklin Emmons, Whitman Cross,
and George Homans Eldridge. 1896. 4*^. 556 pp. 31 pi. Price $1.50.
XXVIII. The Marquette Iron-Bearing District of Michigan, with Atlas, by C. R. Van Hise and
W. S. Bayley, including a Chapter on the Republic Trough, by H. L. Smyth. 1895. 4'=. 608 pp. 35
pi. and atlas of 39 sheets folio. Price $5.75. .
XXIX. Geology of Old Hampshire County, Massachusetts, comprising Franklin, Hampshire, and
Hampden Counties, by Benjamin Kendall Emerson. 1898. 4°. xxi, 790 pp. 35 pi. Price $1.90.
XXX. Fossil Medusa?, by Charles Doolittle Walcott. 1898. 4'^. ix,201pp. 47 pi. Price $L 50.
XXXI. Geology of the Aspen Mining District, Colorado, with Atlas, by Josiah Edward Spurr.
1898. 4°. XXXV, 260 pp. 43 pi. and atlas of 30 sheets folio. Price $3.60.
XXXII. Geology of the Yellowstone National Park, Part II, Descriptive Geology, Petrography,
and Paleontology, by Arnold Hague, J. P. Iddings, W. Harvey Weed, Charles D. Walcott, G. H. Girty,
T. W. Stanton, and F. H. Knowltou. 1899. 4°. xvii, 893 pp. 121 pi. Price .
XXXIII. Geology of the Narragansett Basin, by N. S. Shaler, J. B. Woodworth, aud August F.
Foerste. 1899. 4°. xx, 402 pp. 31 pi. Price .
ADVERTISEMENT. Ill
XXXIV. Tho Glacial Gravels of Maine and their Associated Deposits, hj George H. Stone. 1899.
4^. xiii, 499 pp. 52 pi. Price .
XXXV. The Later Extinct Floras of North America, by John Strong Newberry; edited by
Arthur HoUiek. 1898. 4-. xviii, 295 pp. 68 pi. Price $1.25.
XXX\'I. The Crystal Falls Iron-Hearing District of Michigan, by J. Morgan Clements and
Henry Lloyd Smyth ; with a Chapter on the Sturgeon River Tongue, by William Shirley Bayley, and an
introduction by Charles Richard Van Hise. 1899.4'^. xxxvi, 51i! pp. 53 pi. Price .
In preparation:
XXXVII. Flora of the Lower Coal Measures of Missouri, by David White.
XXXVIII. The Illinois Glacial Lobe, by Frank Loverett.
— Flora of the Laramie and Allied Formations, by Frank Hall Knowlton.
BULLETINS.
1. On Hypersthene-Audesite and on Triclinic Pyroxene in Augitio Eocks, by Whitman Cross,
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2. Gold and Silver Conversion Tables, giving the Coining Values of Troy Ounces of Fine Metal,
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4. On Mesozoic Fossils, by Charles A. White. 1884. 8°. 36 pp. 9 pi. Price 5 cents.
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9. A Report of Work done in the Washington Laboratory during the Fiscal Year 1883-84. F. W.
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10. On the Cambrian Faunas of North America. Preliminary Studies, by Charles Doolittle
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11. On the Quaternary and Recent MoUusca of the Great Basin; with Description of New
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12. A Crystallograijhic Study of the Thinolite of Lake Lahontan, by Edward S. Dana. 1884. 8°.
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13. Boundaries of the United States and of the Several States and Territories, with a Historical
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14. The Electrical and Magnetic Properties of the Iron-Carburets, by Carl Barns and Vincent
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15. On the Mesozoic and Cenozoic Paleontology of California, by Charles A. White. 1885. 8°.
33 pp. Price 5 cents.
16. On theHigherDevonianFaunasof Ontario County, New York, by John M.Clarke. 1885. 8°.
86 pp. 3 pi. Price 5 cents.
17. On the Development of Crystallization in the Igneous Rooks of Washoe, Nevada, with Notes
on the Geology of the District, by Arnold Hague and Josejjh P. Iddings. 1885. 8°. 44 pp. Price 5
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18. On Marine Eocene, Fresh-Water Miocene, and other Fossil Mollusca of Western North
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19. Notes on the Stratigraphy of California, by George F.Becker. 1885. 8°. 28 pp. Price 5 cents.
20. Contributions to the Mineralogy of the Rocky Mountains, by Whitman Cross and W. F. Hille-
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21. The Lignites of the Great Sioux Reservation; a Report on the Region between the Grand
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22. On New Cretaceous Fossils from California, by Charles A. White. 1885. 8°. 25 np. 5 pi.
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23. Observations on the Junction between the Eastern Sandstone and the Keweenaw Series on
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24. List of Marine Mollusca, comprising the Quaternary Fossils and Recent Forms from American
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25. The Present Technical Condition of the Steel Industry of the United States, by Phineas
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26. Copper Smelting, by Henry M. Howe. 1885. 8°. 107 pp. Price 10 cents.
27. Report of Work done in the Division of Chemistry and Physics, mainly during the Fiscal Year
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28. The Gabbros and Associated Hornblende Rooks occurring in the Neighborhood of Baltimore,
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IV ADVERTISEMENT.
29. On the Fresli-Water Invertebrates of the North American Jurassic, by Charles A. White. 1886.
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30. Second Contribution to the Studies on the Cambrian Faunas of North America, by Charles
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31. Systematic Review of our Present Knowledge of Fossil Insects, including Myriapods and
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32. Lists and Analyses of the Mineral Springs of the United States; a Preliminary Study, by
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33 NotesontheGeologyofNorthemCalifornia, by J. S. Diller. 1886. 8°. 23 pp. Price 5 cents.
34 On the Relation of the Laramie Molluscan Fauna to that of the Succeeding Fresh-Water Eocene
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35. Physical Properties of the Iron-Carburets, by Carl Barus and Vincent Strouhal. 1886. 8°.
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37. Types of the Laramie Flora, liy Lester F. Ward. 1887. 8°. 354 pp. 57 pi. Price 25 cents.
38. PeridotiteofElliottCounty,Keutucky,byJ.S.Diller. 1887. 8='. 31pp. Ipl. PriceScents.
39. The Upper Beaches and Deltas of the Glacial Lake Agassiz, by Warren Uphani. 1887. 8"^.
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41. On the Fossil Faunas of the Upper Devonian — the Genesee Section, New York, bv Henry S.
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54. On the Thermo-Electric Measurement of High Temperatures, by Carl Barus. 1889. 8°.
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56. Fossil Wood and Lignite of the Potomac Formation, by Frank Hall Knowlton. 1889. 8^.
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. 57. A Geological Reconnoissance in Southwestern Kansas, by Robert Hay. 1890. 8°. 49 pp.
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63. A Bibliography of Paleozoic Crustacea from 1698 to 1889, including a List of North Amer-
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64. A Report of Work done in the Division of Chemistry aud Physics, mainly during the Fiscal
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ADVERTISEMENT. y
T , ',"?■ »-V'''^'*'''''V'.!!;^' "i^^^' l:'it"n"i"'"« C(.al FioUl of Pennsylvania, Ohio, and West Virginia, bv
Israel C. White. IWIl. y^. 212 p].. U jil. Prico 20 cents. ' i^mia, oy
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99. Record of North American Geology for 1891, by Nelson Horatio Darton. 1892. 8°. 73 pp
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101. Insect Fauna of the Rhode Island Coal Field, by Samuel Hubbard Scudder. 1893 8°
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VI ADVERTISEMENT.
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104. Glaciation of the Yellowstone Valley north of the Park, hy Walter Harvey Weed. 1893. 8°.
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105. The Laramie and the Overlying Livingstone Formation in Montana, by Walter Harvey
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106. The Colorado Formation and its Invertebrate Fauna, by T. W. Stanton. 1893. 8==. 288
pp. 45 pi. Price 20 cents.
107. The Trap Dikes of the Lake Champlain Eegion, by .Tames Fnrman Kemp and Vernon
Freeman Marsters. 1893. S'^. 62 pp. 4 pi. Price 10 cents.
108. A Geological Reconnoissauce in Central Washington, by Israel Cook Russell. 1893. 8".
108 pp. 12 pi. Price 15 cents.
109. The Eruptive and Sedimentarv Rocks on Pigeon Point, Minnesota, and their Contact Phe-
nomena, by William Shirley Bay ley. 1893. 8^'. 121 pp. 16 pi. Price 15 cents.
110. The Paleozoic Section in the Vicinity of Three Forks, Montana, by Albert Charles Peale.
893. 8°. 56 pp. 6 pi. Price 10 cents.
111. Geology of the Big Stone Gap Coal Fields of Virginia and Kentucky, by Marius E. Camp-
bell. 1893. 8^.''l06pp. 6pL Price 15 cents.
112. Earthquakes in California in 1892, by Charles D. Perrine. 1893. 8°. 57 pp. Price 10 cents.
113. A Report of Work done in the Division of Chemistrv diiring the Fiscal Years 1891-'92 and
1892-'93. F. W. Clarke, Chief Chemist. 1893. 8". 115 pp. Price 15 cents.
114. Earthquakes in California in 1893, by Charles D. Perrine. 1894. 8^. 23 pp. Price 5 cents.
115. A Geographic Dietioiiary of Ehode Island, by Henry Gannett. 1894. 8°. 31 pp. Price
5 cents.
- 116, A Geographic Dictionary of Massachusetts, by Henry Gannett. 1894. 8^. 126 pp. Price
15 cents.
117. A Geographic Dictionary of Connecticut, by Henry Gannett. 1894. 8°. 67 pp. Price 10
cents.
118. A Geographic Dictionary of New .lersey, by Henry Gannett. 1894. 8^. 131 pp. Price 15
cents.
119. A Geological Reconnoissauce in Northwest Wyoming, by George Homans Eldridge. 1894.
8°. 72 pp. Price 10 cents.
120. The Devonian System of Eastern Penuyslvania and New York, by Charles S. Prosser. 1894.
8°. 81 pp. 2 pi. Price 10 cents.
121. A Bibliography of North American Paleontology, by Charles Eolliu Keyes. 1894. 8°. 251
pp. Price 20 cents.
122. Results of Primary Triangulatiou, by Henry Gannett. 1894. 8^. 412 pp. 17 pi. Price
25 cents.
123. A Dictionary of Geographic Positions, by Henry Gannett. 1895. 8°. 183 pp. 1 pi. Price
15 cents.
124. Revision of North American Fossil Cockroaches, by Samuel Hubbard Scndder. 1895. 8^.
176 pp. 12 pi. Price 15 cents.
125. The Constitution of the Silicates, by Frank Wigglesworth Clarke. 1895. 8'^. 109 pp.
Price 15 cents.
126. A Mineralogical Lexicon of Franklin, Hampshire, and Hampden counties, Massachusetts,
by Benjamin Kendall Emerson. 1895. 8'-. 180 pp. 1 pi. Price 15 cents.
127. Catalogue and Index of Contributions to North American Geology, 1732-1891, by Nelson
Horatio Darton. 1896. 8-. 1045 pp. Price 60 cents.
128. The Bear Eiver Formation and its Characteristic Fauna, by Charles A. White. 1895. S°.
108 pp. 11 pi. Price 15 cents.
129. Earthquakes in California in 1894, by Charles D. Perrine. 1895. 8°. 25 pp. Price 5 cents.
130. Bibliography and Index of North American Geology, Paleontology, Petrology, and Miner-
alogy for 1892 and 1893, by Fred Boughton Weeks. 1896. 8°. 210 pp. Price 20 cents.
131. Report of Progress of the'Division of Hydrography for the Calendar Y^ears 1893 and 1894,
by Frederick Haynes Newell, Topographer in Charge. 1895. 8°. 126 pp. Price 15 cents.
132. The Disseminated Lead Ores of Southeastern Missouri, by Arthur Winslow. 1896. 8°.
31 pp. Price 5 cents.
133. Contributions to the Cretaceous Paleontology of the Pacific Coast: The Fauna of the
Knoxville Beds, bvT.W. Stanton. 1895. 8^. 132 pp. 20 ph Price 15 cents.
134. The Cambrian Eocks of Pennsylvania, by Charles Doolittle Walcott. 1896. 8°. 43 pp.
15 pi. Price 5 cents.
135. Bibliography and Index of North American Geology, Paleontology, Petrology, and Miner-
alogy for the Year 1894, by F. B. Weeks. 1896. 8°. 141 pp. Price 15 cents.
136. Volcanic Eocks of South Mountain, Pennsylvania, by Florence Bascom. 1896. 8°. 124 pp.
28 pi. Price 15 cents.
137. The Geology of the Fort Eiley Military Eeservation and Vicinity, Kansas, by Robert Hay.
1896. 8°. 35 pp. 8 p'l. Price 5 cents.
138. Artesian-Well Prospects in the A-tlantic Coastal Plain Eegion, by N. H. Darton. 1896. 8°.
228 pp. 19 pi. Price 20 cents.
139. Geology of the Castle Mountain Mining District, Montana, by W. H. Weed and L. V. Pirs-
sou. 1896. 8-^. 164 pp. 17 pi. Price 15 cents.
140. Report of Progress of the Division of Hydrography for the Calendar Year 1895, by Frederick
Haynes Newell, Hydrographer in Charge. 1896. 8*^. 356 pp. Price 25 cents.
ADVERTISEMENT. yil
141. The Eocene Peiiosits of the Middle Atlantic Slope iu Deluwaro, Maryland and Virsriui-i
by William Bullock Clark. 1896. 8'^. 167 pp. 40 pi. Price 15 cents.
112. A Brief Contrilnition to the Geolony and Paleontology of Northwestern Louisiana bv
T. Wayhind Vanghan. 1896. 8'^. 6.5 jip. 4 pi. Price 10 cents. '
113. A Bibliography of Clays and the Ceramic Arts, by John C. Branner. 1896. 8°. 114 pp
Price 15 cents. '■ ^ '
144. The Moraines of the Missouri Coteau and their Attendant Deposits, by James Edward Todd
1896. 8"^. 71 pp. 21 pi. Price 10 cents.
145. The Potomac Formation in A^irginia, by W. M. Fontaine. 1896. 8^. 149 pp. 2 pi Price
15 cents. '
146. Bibliography and Index of North American Geology, Paleontology, Petrology, and Miner-
alogy for the Year 1895, by F. B. Weeks. 1896. 8°. 130 pp. Price 15 cents.
147. Earthquakes iu California iu 1895, by Charles D. Perrine, Assistant Astronomer iu Charo-g
of Earthquake Obseryations at the Lick Observatory. 1896. 8"-'. 23 pp. Price 5 cents. °
148. Analyses of Rocks, with a Chapter on Analytical Methods, Laljoratory of the Uuited States
Geological Suryey, 1880 to 1896, by F. W. Clarke and W. F. Hillebraud. 1897. 8'-\ 306 pp Price
20 cents.
149. Bibliography and Index of North American Geology, Paleontology, Petrology, and Miner-
alogy for the Year 1896, by Fred Boughton Weeks. 1897. 8°. 152 pp. Price 15 cents
150. The Educational Series of Rock Specimens collected and distrilmted by the United States
Geological Survey, by Joseph Silas Diller. 1898. 8°. 398 pp. 47 pi. Price 25 cents.
151. The Lower Cretaceous Grypha^as of the Texas Region, by R. T. Hill and T. Wayland
Vaiighan. 1898. 8°. 139 pp. 25 pi. Price 15 cents.
152. A Catalogue of the Cretaceous and Tertiary Plants of North America, by F H Knowlton
1898. 8°. 247 pp. Price 20 cents.
^o^o ^P' ■*■ Bibliographic Index of North American Carboniferous Invertebrates, by Stuart Weller
1898. 8°. 653 pp. Price 35 cents.
154. A Gazetteer of Kansas, by Henry Gannett. 1898. 8°. 246 pp. 6 pi. Price 20 cents
1.55. Earthquakes in California iu 1896 and 1897, by Charles D. Perrine, Assistant Astronomer
m Charge ol Earthquake Obseryations at the Lick Observatory. 1898. 8^. 47 pp. Price 5 cents
156. Bibliography and Index of North American Geology, Paleontology, Petrology, and Miner-
alogy for the Year 1897, by Fred Boughton Weeks. 1898. 8". 130 pp. Price 15 cents'!
160. A Dictionary of Altitudes in the United States (Third Edition), compiled bv Henry
Gannett. 1899. 8^. 775 pp. Price 40 cents. ''
^T, x?-^' Earthquakes iu California in 1898, by Charles D. Perrine, Assistant Astronomer in Charo-e
ot Earthquake Observations at the Lick Observatory. 1899. 8°. 31 pp. 1 pi. Price 5 cents
In preparation:
157. The Gneisses, Gabbro-Schists, and Associated Rocks of Southeastern Minnesota, by C. W.
158. The Moraines of southeastern South Dakota and their Attendant Deposits, by J. E Todd
159. The Geology of Eastern Berkshire County, Massachusetts, by B. K. Emerson.
WATER-SUPPLY AND IRRIGATION PAPERS.
By act of Congress approved June 11, 1896, the following provision was made •
"Provided, That hereafter the reports of the Geological Survey in relation to the o-auo-ino- of
streams and to the methods of utilizing the water resources may be prin'ed in octavo form^'no'tto
exceed one hundred pages in length and live thousand copies in number ; one thousand copies of which
shall be lor the official use of the Geological Survey, oue thousand five hundred copies shall be deliv-
ered to the Senate, and two thousand five hundred copies shall be delivered to the House of Renre-
sentatives, for distribution." '
Under this law the following papers have been issued:
1. Pumping Water for Irrigation, by Herbert M. V'ilson. 1896. 8°. 57 pp. 9 pi
2. Irrigation near Phoenix, Arizona, by Arthur P. Davis. 1897. 8°. 97 pp 31 pi
3. Sewage Irrigation, by George W. Rafter. 1897. 8^. 100 pp. 4 pi.
4. AReconnoissanceinSoutheasternWashington, by Israel Cook Russeil. 1897. 8^^ 96 pp 7 pi
5. Irrigation Practice on the Great Plains, by Elias Branson Cowgill. 1897. 8° 39 pp 1'' pi'
6. Underground Waters of Southwestern Kansas, by Erasmus Haworth. 1897. 8^ 65 pp 12 pi'
7. Seepage Waters of Northern Utah, by Samuel Fortier. 1897. 8°. 50 pp 3 pi
8. Windmills for Irrigation, by Edward Charles Murphy. 1897. 8°. 49 pp. 8 pi
9. Irrigation near Greeley, Colorado, by David Boyd. 1897. 8°. 90 pp 21 pi
10. Irrigation in Mesilla Valley, New Mexico, by F. C. Barker. 1898. 8°. 51 pp. 11 pi
11. River Heights for 1896, by Arthur P. Davis. 1897. 8°. 100 pp.
12. Water Resources of Southeastern Nebraska, by Nelson H. Darton. 1898. 8° 55 pp 21 pi
1?" J'^"S'^*i°ii Systems iu Texas, by William Ferguson Hutson. 1898. 8''. 67 pp. 10 pi.
14. New Tests of Certain Pumps and Water-Lifts used in Irrigation, by Ozni P. Hood. 1889. 8^
al pp. 1 pi.
15. Operations at River Stations, 1897, Part I. 1898. 8'^. 100 pp.
16. Operations at River Stations, 1897, Part II. 1898. 8°. 101-200 pp
17. Irrigation near Bakersfield, California, by C. E. Grunsky. 1898. 8'=. 96 pp. 16 pi
18. Irrigation near Fresuo, California, by C. E. Grunsky. 1898. 8°. 94 pp. 14 pi
19. Irrigation near Merced, California, by C. E. Grunsky. 1899. 8°. 59 pp. 11 pL
20. Experiments with Windmills, by T. O. Perry. 1899. 8°. 97 pp 12 pi
Hall.
VlII
ADVERTISEMENT.
2 pi.
7 pi.
21. Wells of Northern Indiana, by Frank Leverett. 1899. 8°. 82 pp.
22. Sewage Irrigation, Part II, by George AV. Eaiter. 1899. 8°. 100 pp
23. Water-Right Problems of Bighorn Mountains, by Elwood Mead. 1899.
24. Water Resources of the State of New York, Part I, bv George W.
99 pp. 13 pi. > > J ^
25. Water Resources of the State of New York, Part II, by George W. Rafter.
101-200 pp. 12 pi.
26. Wells of Southern Indiana (Continuation of No. 21), by Frank Leverett. 1899.
27. Operations at River Stations, 1898, Part I. 1899. 8°. 100 pp.
28. Operations at River Stations, 1898, Part II. 1899. 8°. 101-200 pp.
In preparation:
29. Wells and Windmills in Nebraska, by Edwin H. Barbour.
30. Water Resources of the Lower Peninsula of Michigan, by Alfred C. Lane.
62 pp. 7 pi.
Rafter. 1899. S
1899. 8°.
8°. 64 pp.
TOPOGRAPHIC MAP OF THE UNITED STATES.
When, in 1882, the Geological Survey was directed by law to make a geologic map of the United
States there was in existence no suitable topographic map to serve as a base for the geologic map.
The prepnration of such a topographic map was therefore immediately begun. About one-fifth of the
area of the country, excluding Alaska, has now been thus mapped. The map is published in atlas
sheets, each sheet representing a small quadrangular district, as explained under the next head-
ing. The separate sheets are sold at 5 cents each when fewer than 100 copies are purchased, but when
they are ordered in lots of 100 or more copies, whether of the same sheet or of different sheets, the
price is 2 cents each. The mapped areas are widely scattered, nearly every State being represented.
About 900 sheets have been engraved and printed; they are tabulated by States in the Survey's
"List of Publications," a pamphlet which may be had on application.
The map sheets represent a great variety of topographic features, and with the aid of descriptive
text they can be used to illustrate topographic forms. Tliis has led to the projection of au educational
series of topographic folios, for use wherever geography is taught in high schools, academies, and
colleges. Of this series the first folio has been issued, viz :
1. Physiographic types, by Henry Gannett, 1898, folio, consisting of the following sheets and 4
pages of descriptive text: Fargo (N. Dak. -Minn.), a region in youth; Charleston (W.Va.),a region in
maturity; Caldwell (Ivans.), aregion in old age; Palmyra (Va!), a rejuvenated region; Mount Shasta,
(Gal.), a young volcanic mountain; Eagle (Wis.), moraines; Sun Prairie (Wis.), drumlins; Donald-
souville (La.), river flood plains; Boothbay (Me.), a fiord coast; Atlantic City (N. J.), a barrier-beach
coast.
GEOLOGIC ATLAS OF THE UNITED STATES.
The Geologic Atlas of the United States is the final form of publication of the topographic and
geologic maps. The atlas is issued in parts, progressively as the surveys are extended, and is designed
ultimately to cover the entire country.
Under the plan adopted the entire area of the country is divided into small rectangular districts
(designated quadrangles), bounded by certain meridians and parallels. The unit of survey is also the
unit of publication, and the maps and descriptions of each rectangular district are issued as a folio of
the Geologic Atlas.
Each folio contains topographic, geologic, economic, and structural maps, together with textual
descriptions and explanations, and is designated by the name of a principal town or of a prominent
natural feature within the district.
Two forms of issue have been adopted, a "library edition" and a "field edition." In both the
sheets are bound between heavy paper covers, but the library copies are permanently bound, while
the sheets and covers of the field copies are only temporarily wired together.
Under the law a copy of each folio is sent to certain public libraries and educational institu-
tions. The remainder are sold at 25 cents each, except such as contain au unusual amount of matter,
which are priced accordingly. Prepayment is obligatory. The folios ready for distribution are listed
below.
No.
Name of slieet.
State.
Limiting meridians.
Limiting parallels.
Area, in
square
miles.
Price,
in
cents.
^
Montana
/Georgia
110°-lllo
j. 85°-85o 30'
120° 30'-121o
840 30'-85o
1210-121° 30'
850-85° 30'
1050-105° 30'
85° 30'-80°
106° 45'-107o 15'
I 77° 30'-78°
450-46°
34° 30'-35o
38° 30'-39°
35° 30'-36°
38° 30'-59o
350-350 30'
380 30'-39o
350-35° 30-
380 45'-39o
390-390 30'
3,354
980
932
969
932
975
932
975
465
925
\Tennessee
California
Tennessee
California
Tennessee
Colorado
Tennessee
Colorado
Virginia
mest Virginia..
Maryland
3
25
4
25
•i
P,
7
R
Pikes Peak (oat of stock)
25
25
9
in
ADthracite-Crested Butte
50
ADVERTISEMENT.
IX
No.
11
12
13
U
15
16
17
18
30
Name of sheet.
State.
Limiting meridians.
Limiting parallels.
Area, in Price,
square in
miles.
Jackson ...
Estillville .
Fredericksburg
Staunton
Laason Peak...
Knoxville
Marys ville. .
Smartsville .
Stevenson .
Cleveland
Pikeville
ilcMinn ville.
Noiiiini
Three Forks.
Loudon
Pocahontas ..
MoiTistown..
Piedmont.
Nevada Citv.
fXevada City.
<^ Grass Valley.
iBanuer Hill .
rGallatin..]
/Tellow.stoue .Na. JCauyon...[
California
Virginia
Kentucky
.Tennessee
fMaryland
\Virgiuia
/Virginia
\*U^est Virginia.
California
Teunessue
North Carolina
California
California
{Alabama
Georgia
Tennessee
Teuue.-^see
Tennessee
Tennessee
fMaryland
\ Virginia
Montana
Tennessee
/Viiginia
OVest Virginia .
Tennessee
Virginia
Maryland
["West Virginia.
California
121° 00'
121° 01'
120° 57'
t tional Park.
Pyramid Peak -
Franklin
Brice ville
Euckbannon...
Gadsden
Pueblo
Do\\Tiieville ...
Butte Special..
Truckee
"Wartburg
Sonera
Nueces
Bid^y6llBar ...
Tazewell
I Shoshone.
[Lake
Wyoming
Boise
Kichmond
London
Teumile District Special.
Roseburg
Hoi yoke
California
/Virginia
\West. Virginia .
Tennessee
West Virginia .
Alabama
Colorado ....
California
Montana
California
Tennessee
California
'i'exas
California
/Virginia
\West Virginia.
Idaho
Kentucky
Kentucky
Cohii'ado
( )regon
/Hassacbuaetta
\Connecticut ...
112° 29'
120° 30'-121o
82° 30'-83°
770-77° 30'
79°-79° 30'
1210-122°
830 30'-81°
121° 30'-122o
1210-121° 30'
85° 30'-86°
84° 30'-85°
8o°-850 30'
85° 30'-8S°
76° 30'-77°
lll°-112o
84°-Sl° 30'
81°-S1° 30'
83°-S3o 30'
79°-79° 30'
25"-121o 03' 45"
35"-121o 05' 04"
05"-121° 00' 25"
120O-120O 30'
79°-79° 30'
84°-84° 30'
800-80° 30'
86°-860 30'
104° 30'-105°
120° 30'-121°
30"-112° 30' 42"
120°-120° 30'
840 30'-S5o
1200-1200 30'
100°-100o 30'
121°-121° 30'
81° 30'-82o
1160-1160 30'
840-840 30'
840-84° 30'
106° 8'-106° 16'
1230-1230 30'
72° 30'-73o
38°-38o 30'
360 30'-37o
38°-38° 30'
360-38° 30'
400-41°
350 30'-36°
390-39° 30'
390-390 30'
350-350 30'
350 30'-36o
350 30'-36o
380-380 30'
450-460
350 30'-36°
370-370 30'
360-360 30'
390-390 30'
39° 13' 50"-39° 17' 16"
39° 10' 22"-390 13' 50"
39° 13' 50"-39o 17' 16"
380 30'-39°
36°-36o 30'
38° 30'-39°
340-340 30'
380-38° 30'
39° 30'-40°
450 59' 28"-46° 02' 54"
390-390 30'
360-360 30'
37° 30'-38°
290 30'-30°
390 30'-40o
370-370 30'
430 30'-44°
370 30'-38°
370-370 30'
390 22' 30"-39° 30' 30"
43°-43° 30'
420-12° 30'
957
938
938
3,634
925
925
925
980
975
969
969
3,354
969
11.65
12.09
11. C5
3,412
963
932
986
938
919
22.80
925
903
944
1,035
918
950
864
944
950
55
871
25
25
25
25
25
25
25
25
50
25
25
25
25
25
25
STATISTICAL PAPERS.
Mineral Resources of tlie United States [1882], by Albert Williams, jr. 1883. 8^. xvii, 813 pp.
Price 50 cents.
Mineral Resources of tbe United States, 1883 and 1884, by Albert Williams, jr. 1885. 8°. xiv,
1016 pp. Price 60 cents.
Mineral Resources of tlie United States, 1885. Division of Mining Statistics and Technology.
1886. 8^^. vii, 576 pp. Price 40 cents.
Mineral Resources of the United States, 1886, by David T. Day. 1887. 8°. viii, 813 pp. Price
(50 cents.
Mineral Resources of the United States, 1887, by David T. Day. 1888. 8°. vii, 832 pp. Price
50 cents.
Mineral Resources of the United States, 1888, by David T. Day. 1890. 8°. vii, 652 pp. Price
Mineral Resources of the United States, 1889 and 1890, by David T. Day. 1892. 8^. viii, 671pp.
Mineral Resources of the United States, 1891, by David T. Day. 1893. 8°. vii, 630 pp. Price
50 cents.
X ADVERTISEMENT.
Mineral Resources of the United States, 1892, by David T. Day. 1893. 8'^. vii, 850 pp. Price
50 cents.
Mineral Resources of the United States, 1893, by David T. Day. 1894. 8^. viii, 810 pp. Price
50 cents.
On March 2, 1895, the following provision was included in an act of Congress :
"Provided, That hereafter the report of the mineral resources of the United States shall be
issued as a part of the report of the Director of the Geological Survey."
In compliance with this legislation the following reports have been published:
Mineral Resources of the United States, 1894, David T. Day, Chief of Division. 1895. 8'^. xv,
646 pp., 23 pi. ; xis, 735 pp., 6 pi. Being Parts III and IV of the Sixteenth Annual Report.
Mineral Resources of the United States, 1895, David T. Day, Chief of Division. 1896. 9P.
xxiii, 542 pp., 8 pi. and maps ; iii, 543-1058 pp., 9-13 pi. Being Part III (in 2 vols.) of the Seventeenth
Annual Report.
Mineral Resources of the United States, 1896, David T. Day, Chief of Division. 1897. 8°.
xii, 642 pp., 1 pi. ; 643-1400 pp. Being Part V (in 2 vols. ) of the Nineteenth Annual Eeport.
Mineral Resources of the United States, 1897, David T. Day, Chief of Division. 1898. 8°.
viii, 651 pp., 11 pi. ; viii, 706 pp. Being Part VI (in 2 vols.) of the Nineteenth Annual Eeport.
The money received from the sale of the Survey publications is deposited in the Treasury, and
the Secretary of that Department declines to receive bank checks, drafts, or postage stamps ; all remit-
tances, therefore, must be by money order, made payable to the Director of the United States
Geological Survey, or in currency' — the exact amount. Correspondence relating to the publications
of the Survey should be addressed to
The Director,
United States Geological Survey,
Washington, D. C, June, 1S99. Washington, D. C.
[Take this leaf out and paste the separated titles upon three of your cata-
logue cards. The hrst and second titles need no addition ; over the third write
that subject under "which you would place tho book in your library.]
LIBRARY CATALOGUE SLIPS.
United States. Department of the interior. {TJ. S. geological survey.)
Department of the interior | — | Monographs | of the | United
States geological survey | Volume XXXVI | [Seal of the depart-
ment] I
Washington | government prilitiug office | 1899
Second title: United States geological survey | Charles D.
Walcott, director | — | The | Crystal Falls iron-bearing district
of Michigan | by | .J. Morgan Clements and Henry Lloyd Smyth |
■with I a chapter on the Sturgeon river tongue | by | William
Shirley Bay ley | and | an introduction | by | Charles Richard Van
Hise I [Vignette] |
Washington | government printing office | 1899
A°. sssvi, 512 pp. 53 i3l.
Clements (J. M.), Smyth (H. L.), Bayley (W. S.), and Van Hise
(C. R.)
United States geological survey | Charles D. Walcott, di-
rector I — I The I Crystal Falls iron-bearing district of Michigan |
by I J. Morgan Clements and Henry Lloyd Smith | with | a chap-
ter on the Sturgeon river tongue | by | William Shirley Bayley |
and I an introduction | by | Charles Richard Van Hise | [Vig-
nette] I
Washington | government piinting office | 1899
4^. xsxvi. 512 Jip. 53 pi.
[U^'iTED States. Department of the interior. {U. S. geological survey.)
Monograph XXXVl.]
United States geological survey | Charles D. Walcott, di-
rector I — I The I Crystal Falls iron-bearing district of Michigan |
by I J. Morgan Clements and Henry Lloyd Smyth | with | a chap-
ter on the Sturgeon river tongue | by | William Shirley Bayley |
and I an introduction | by | Charles Richard Van Hise | [Vig-
nette] I
Washington | government printing office | 1899
4°. xxxvi, 512 pp. 53 pi.
[TJxiTED States. Department of the interior. {U. S. geological survey.)
Monograph XXSVI.]
U S GEOUODICAL SURVEV
Topograprir b, G E h,Di
H L Smiih.ana from «u
Su'xyeilm 1090-90
H
TOI'OC.liAPIIK'AI. MAP
( ItYSTAl. r.VI.I.S liisTUUT, MM iii(;ax
MAHQUHTTK lUHTHR'T. MICIUCAN
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