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HXT^''^ 



HARVAHD UNIVERSITY 



LIBRART OF THE 

MINERALOGICAL 
LABORATORY 

UNITEBSITY HOSEUU 



Transferred to 

CABOT SCIENCE LIBRARY 

June 2005 




folio 

Qe 




OEPillXGFE OF TBE IHTESIOB-D. S. aGOLOaiOlL BUEVEI 

CHAKLES D. WALCOTT, DIBECIOB 



GEOLOGY 



LITTLE BELT MOUNTAINS, MONTANA 



WITH NffTBe OS THE MINERAL DEPOSm OP THE KEIHAKT 
BABKEE, VOGO, AND OTHER IHSTRICIB 



WALTER HARVEY WEED 



A REPORT OS TRB PETROURAPHY OF THE ICSEOCS ROCKS 
OF THE DISTRICT 



E. V. RIRSSON 



WASHINGTOIT 

aOTERNMENT PKINTtNG OPPIOB 

1900 



H 



Z.'Ta 



S . t "^ 



GEOLOGY OF THE LIHLE BELT MOUNTAINS, MONTANA 

WITH NOTES ON THE MINERAL DEPOSITS OF THE NEIHART, 
BARKER, YOGO, AND OTHER DISTRICTS 

BY 

WAIiTEB IIAKTET WEED 

ACCOMPANIED BY 

A REPORT ON THE PETROGRAPHY OF THE IGNEOUS ROCKS OP THE DISTRICT 

BY 

L.. V. PIRSSON 



20 GEOL, PT 3 17 257 






-t ' 



.'■'] 



r 



CONTENTS. 



OEOLOOY OF THE LITTLE BELT MOUNTAINS, MONTANA, BY W. H. WEED. 

Paire. 

Chapter I. — Introdiiction 271 

Qeographic position 271 

SettlemeDts 272 

PrevioQs exploration ^ 273 

Topography . . T 273 

Drainage 275 

Vegetation 276 

Stractare 276 

Chapter II. — The rock formations 278 

Metamorphic rocks — gneisses and schists 278 

Algonkiau rocks 279 

Belt terrane *. 279 

Topographic aspect of Belt nreas 280 

Neihart quartzite 281 

Chamberlain shale 282 

Newland limestone 282 

Greyson shale 282 

Spokane shale 282 

General section 283 

Cambrian rocks 284 

Distribution and subdivisions 284 

Flathead sandstone 285 

Wolsey shale 2H5 

Meagher limestone 285 

Park shale 286 

Pilgrim limestone 286 

Dry Creek shale 286 

Yogo limestone 286 

Fossils 286 

Siluro-Devonian rocks 287 

Jefferson limestone 287 

Threeforks shales 289 

Carboniferous rocks 289 

Madison group 290 

Paine shales 290 

Woodhurst limestone 291 

Castle limestone 293 

Quadrant group '. 294 

Kibbey sandstone 295 

Otter shale 295 

Fossils 295 

Section on Judith River near Utica 296 

2.")9 



260 CONTENTS. 

Page. 

Chapter III. — Descriptive geology of the soathem and Judith areas 299 

Introductory 299 

Southern part of range 299 

East end of range 300 

Foothills 300 

Structare 300 

Dirty Creek section 301 

Bluff Monutain 302 

Newland Creek Hills 302 

Igneous intrusioDM 302 

Sheep Creek Valley and vicinity 304 

Smoky Moantain 304 

Volcano Valley fault 305 

Coxcomb Bntte 305 

Sheep Creek copper mines 306 

Canyon of Lower Sheep Creek 306 

Head waters of Sheep Creek 307 

Minette intrusions 308 

Mountains north of Sheep Creek 309 

Williams Mountain 309 

Wolsey Mountain 310 

Porphyry Peak 310 

Judith region 310 

Plateaus of the Judith 310 

South Fork 311 

Middle Fork 312 

Igneous rocks 313 

Chapter IV.— Descriptive geology of the Yogo district, Big Baldy Mountain, 

Wolf Butte, and Taylor Peak 317 

Yogo district 317 

Yogo Peak 318 

Location 318 

Character of rocks 318 

Western knob 318 

Shonkinite 318 

Dikes cutting shonkinite 319 

Middle knob 319 

Eastern knob 320 

Eastern shoulder 320 

Elk Ridge summit 321 

Dikes connecting Yogo and Woodhurst stocks 321 

Structural relations 322 

Contact metamorphism 322 

Group of encircling sheets about the Yogo stock 323 

Fringing dikes 325 

General relations 325 

Bandbox Mountain 326 

Eureka divide 326 

Woodhurst stock 327 

Structural relations 327 

Yogo Creek Valley 328 

Section north of Yogo 328 

Section at mouth of Bear Creek 329 

Sheep Mountain 330 

Ricard Mountain 331 

Running Wolf Ridge 332 



CONTENTS. 261 

Chapter IV. — DeBcriptive geology of the Yogo district, etc. — Continiiecl. Page. 
Yogo district — Continued. 

Rnnning Wolf Ridge — Continued. 

Minette intrusions 332 

Sage Creek Mountain 333 

Steamboat Mountain 333 

Big Baldy Mountain 335 

Description of the peak 335 

The igneous rock 337 

Dikes In porphyry mass 337 

Intrusions iu surrounding strata 337 

Belt Creek divide 338 

South and east flanks 339 

BigPark 339 

Head waters of Dry Wolf Creek '. 340 

Ore deposits 341 

Butcherknife Mountain and Creek 341 

Wolf Butte and Taylor Peak 341 

The laccolith rock 342 

Intrusive sheets near Wolf Butte 342 

Ore deposits 343 

Dry Wolf Creek dome 343 

Chapter V. — Descriptive geology of the Barker and Monarch districts 344 

Barker district 344 

Discovery and development 344 

Extent and topography 344 

Sedimentary rocks of the district 346 

Vicinity of Barker 348 

Igneous rocks of the district 349 

Intrusive sheets and dikes 349 

Chocolate porphyry 349 

Blenkinsop Creek sheet 351 

Trachyte or bostoni te sheet 351 

Sheet of Wol f porphyry 351 

Miuctte sheets 351 

Intrusive sheets of vogesite betweeu Barker and Mouarcli 352 

Dikes of the district 352 

Hughesville syenite stock 353 

Barker Mountain laccolith 354 

Barker porphyry 356 

Otter Mountain laccolith and intrusive sberts of Clcuduuuiu Mt. . 356 

Mixes Baldy intrusive mass 358 

Gold Run Basin 359 

Galena Creek dike 359 

Contact relations 360 

Monarch district 360 

General features 360 

Monarch cliffs 361 

Dry Belt section, Orange Cliffs of Monarch 362 

Section 8 miles south of Monarch 363 

Thunder Mountain ^ 364 

Tenderfoot Mountain 367 

Valleys of Pilgrim and Tenderfoot creeks 367 

Pilgrim Creek section 368 

Slnicebox Canyon 369 

Tiger Butte ... 369 



262 CONTENTS. 

Pago. 

Chapter VI. — Descriptive geology of the Neihart district 371 

Location and general featnrea 371 

Archean gneisses and schists 371 

Older igneous rocks 373 

Pinto diorite 373 

Younger igneous rocks 375 

Rhyolite-porphyry of Kock Creek 375 

Neihart porphyi-y 375 

Carpenter Creek porphyry 376 

Trap dikes (minettes) 377 

Sedimentary areas 377 

Quartzite rim rock 377 

Belt Creek parks 378 

Head- water valley of Belt Creek 379 

Intruded sheets at head of Belt Creek 379 

Outline of geologic history of Neihart district 380 

Chapter VII. — General geology 382 

History of region as interpreted from sedimentary rocks j 382 

Structural features of the range 384 

Nature of the uplift 384 

Relation of igneous intrusion to folding 385 

Dynamic geology 385 

Intrusive sheets 385 

Laccolithic intrusions 387 

Position and number 387 

General features 387 

Size 388 

Contact metamorphism 389 

Character of the rocks 389 

Jointing 389 

Horizon intruded 389 

Depth of intrusion 389 

Extent of denudation 390 

Accompanying dikes or sheets 390 

Form of intmsion 391 

Theory of origin and peculiarities of form of laccoliths 394 

Summary 395 

Gradation between laccoliths and stocks. 396 

Intrusive stocks 396 

Yogo stock 396 

General geology of Yogo stock 397 

Relation of stocks and laccoliths 400 

Theory of dike formation about igneous centers 400 

Chapter VIII. — Noces on the ore deposits of the Little Belt Mouatainn 401 

History and development of mining in the region 401 

Acknowledgments 402 

Neihart district 402 

Discovery and development 402 

The ores 405 

General nature 405 

Mineralogic character 406 

Ore minerals 406 

Gangne minerals 408 

Paragenesis — 410 

Value of the ores 413 



CONTENTS. 263 

Chapter VIII. — ^Notes on ore deposits of Little Belt Moantains — Cont'd Page. 
Neihart district — Continued. 

The veins 413 

Occurrence 413 

Fissure system 414 

Course 415 

Origin 415 

Relation to dikes 4 15 

Splits 415 

Effect of country rock on vein Hssurcs 415 

Vein structure 416 

Vein matter 416 

Vein filling 416 

Banding 417 

Ribbon structure 417 

Rock alteration 418 

Distribution of ore in vein 418 

Pay shoots 418 

Influence of splits 420 

Permanence in depth 420 

Crosscutting 420 

Suggestions for development ■». 420 

Ore deposition 420 

Superficial alteration of veins 421 

Secondary enrichment of veins 421 * 

Summary 423 

Kotes on mines of the district 423 

Neihnrt district proper 423 

. Queen 423 

O'Brien 424 

Mountain Chief 424 

Florence : 425 

Concentrated and Monarch 426 

Gait 426 

Moulton 428 

Cumberland 429 

IngersoU 429 

Queen of the Mountains 430 

IngersoU No. 2 430 

Rock Creek 431 

Lizzie 431 

Lizzie No. 2 432 

Dakota 432 

Broadwater 433 

Snow Creek mines 436 

Benton group 436 

IXL and Eureka 437 

Big Seven 437 

Maokey Creek 439 

Harley Creek 440 

Hoover Creek 440 

Barker district 441 

Discovery and development 441 

Occurrence of the ore deposits 442 

Notes on the mines - 442 

Barker 442 



264 CONTEXTS. 

Chapter VIII, — Xoies on ore deposits of Little Belt Moontniiis—CoDtM. Pag^. 
Barker distriet— CoDtinaed. 

Notes oa the minea — Continoed. 

Wright aud Edwards 443 

Paragon, Mar and Edna, and C'arier -144 

MoolU>ii, Ti^er, and T. W 444 

Liberty aud (/aeen Eather 445 

Other claims 446 

Middle Fork of. I adith River 446 

Viigo mines 447 

History 447 

General wcarrence of ores 448 

Notes on individaal pro|>ertie8 448 

(/QAker City and Delia 448 

CalifomU 449 

Christopher Colombo 449 

Weatberwax 449 

T.C.Power 449 

Little Emma...: .' 450 

Bine Dick 450 

Climax 450 

- Bill Cammins 450 

Banning Wolf district 450 

Ore defiosits 450 

Notes on the mines 451 

W^oodhnrst-Mortson 451 

Sir Walter Scott 451 

Mountainside 452 

Yankee Girl 452 

EnrekA 453 

Dry Wolf or Lion Creek district 453 

Mount Taylor mines 454 

Yog<i sapphire mines 454 

Iron ores of the Little Belt MoiintaiiiH ^ 459 

I'BTKOORAPIiy OF THE IGXKOUS KOCKS OF THE LITTLE BKLT MOl XTAIXS, 

MONTANA, II Y L. V. PIRSSON. 

Chapter 1. — Ifitrodiiction 463 

- Prefatory remarks 463 

Classification 463 

Chapter II. — The granular rocks 4G5 

Barker syeni t«; 465 

Angite-syenite of Belt Creek 4G8 

Annlcite- (nephelite- ) syenite 469 

The rocks of Yogo Peak 471 

8yeuite of Yogo Peak 471 

Local varieties of the syenite 474 

8torr P<'ak 474 

Monzonite of Yogo Peak 475 

Shonkinite of Yogo Peak 479 

Olivine and its resorption bands 481 

Feldspars 483 

Chemical composition 483 

Structure and classification 487 

Shonkinite at head of Running Wulf Creek 488 

Shonkinite of Otter Creek 488 



CONTENTS. 265 

Chapter II. — The granulur rocks — Continued. Pafre. 

Pinto diorite of Neibart 488 

MicroHCopic characters 489 

Chemical composition 490 

Structure and classification 492 

Contact facies 492 

Aplites 493 

Banatite-aplite variety 494 

Sheared aplite of Neibart 495 

Origin of microcline 495 

Granite-syenite-aplite of Sheep Creek 496 

Chapter III. — Acidic feldspathic porphyries of the laccoliths, dikes, and 

sheets 498 

Introdnctiou 498 

Granite-porphyry 498 

Wolf Butte type 498 

Contact 500 

Carpenter Creek type . 501 

Structure of quartz phenocrysts 501 

Yogo Peak type 502 

Microscopic characters 502 

Structure and classification 503 

Trausition forms 504 

Barker type 504 

Thunder Mountain 506 

Tiger Butte 510 

Big Baldy Mountain 510 

Granite-syenite-porphy ry 512 

Tillinghast laccolith 512 

Sage Creek Mountain 512 

Intrusive sheets 512 

Syenite-porphyry 513 

King Creek intrusives 615 

Diorite-porphyry - 515 

Steamboat Mountain type 515 

Quartz-dioritf-porphyry 518 

Syenite-diorite-porphyry 518 

Rhyolite-porphyry 520 

Megascopic characters ^0 

Microscopic characters 521 

Type No. 1 521 

Type No. 2 (Neibart porphyry) 522 

Type No. 3 522 

Chemical composition 522 

Localities 523 

Trachyte (bostonite) 524 

Chapter IV. — The laniprophyres and the effusive rocks 526 

The lamprophyres 526 

Minette 526 

Microscopic petrography of the minette 526 

Iron ore and apatite 529 

Biotite 529 

Augite 529 

Feldspar 530 

Secondary minerals 530 

Structure 580 



266 CONTENTS. 

Chapter IV. — The lamprophyres and the efiusive rocks — ContiDaed. Page. 
The lamprophyres — Continued. 
Minette — Cootinued. 

Microscopic petrogpraphy of the minette — Continued. 

Chemical composition 631 

Variolitic facies 532 

Microscopic characters of variolite 533 

Exomorphic contact phenomena 535 

Included masses ,^36 

Alteration of minettes 537 

Minette-like rocks 538 

Transition into kersantite '. 538 

Nephelite-minette 539 

Contact phenomena 540 

Vogesite 541 

Ana Icite- basalts 543 

Bandbox Mountain type 543 

Eureka divide type 546 

Barker type 547 

Big Baldy Mountain type i... 547 

Related types with polarizing base 550 

Transition from analcite-basalt to minette 551 

Sapphire rock of Yogo Gulch 552 

Microscopic characters 552 

Origin of the sapphires 553 

Character of the sapphires 5r4 

The effusive rocks 556 

Basalt rc>in 

Chapter V. — General petrology of the Little Belt Mountains .^58 

Introduction 5o8 

Rocks of the laccoliths 559 

Mineral composition 560 

Structure and classification 561 

Differentiation in the laccoliths 5(52 

Relation of rock structure to depth 562 

Rocks of the stocks and massi ves 563 

Formation of the aplitic dikelets : . . . 566 

Differentiation 566 

Variation in mineral composition 567 

Chapter VI. — DijBCussion of magmas by graphic methods, and absorption of 

sediments by magmas 569 

Discussion of magmas by graphic methods 569 

Relation to other magmas 571 

Extension of diagram 573 

Deduction of results 575 

Absorption of sediments by magmns 577 

Chapter VII. — Analyses of rocks 578 



ILLUSTRATIONS. 



Pftga 

Plate XXXVI. Topograpbic map of Little Belt Moantain region.. 271 

XXXVII. Af Arcbean Klopes at Neibart, MoDtana; £, Forest gro^tb on 

porpbyry benohes, stage road up Sawmill Creek, near 

Neibart, Montana 276 

XXXVIII. Af Valley of Belt Creek, cnt in gneiss ; £, Canyon of Belt ( 'reck, 

cut in Keibart quartzite, 3 miles soutbeast of Neibart. . . 278 
XXXIX. A, Conglomerate of limestone pebbles from Park sbile of tbe 
Cambrian; B, Intraformational oongionierate of lime- 
stone lenses, Pilgrim limestone of Cambrian; (% Cbert 
layers iu Cambrian limestone, sbowing relief weather- 
ing 282 

XL. Comparative columnar sections of Middle Cambrian forma- 
tions in central Montana 284 

XLI. (leologicmap 286 

XLII. Af Cbaract<eristic outcrop of Jefferson limestones in Little Belt 
region, summit of plateau, bead of King Creek ; &, Corals 

in Jefferson limestone 288 

XLIII. J. Limestone cliffs at Monarcb; /?, Outcrop of Paine shale 

alongside wagon road 4 miles east of Barker 290 

X LI V. Giant Rook, Belt Creek Canyon below Monarcb 292 

X LV. Slnicebox Canyon, Belt Creek 294 

XL VI. Columnar sections of tbe formations of tbe Quadrant group, of 

Lower Carboniferous age, in central Montana 296 

XLVII. Af Summit of Big Baldy Monntain, seen from tbe alpine 
meadow on top of Yogo Peak (shonkinite bowlders in the 
foreground) ; Ji, Yogo Peak, seen from tbe Belt Creek 

divide 318 

XLVIIl. Af Big Baldy Mountain, from bead of Carpenter (.'reek; 7^, 

Cliffs of Barker porpbyry,- wall of western ampbitbeater 

of Big Baldy Mountain 336 

XLIX. Af Ampbitbeater cut iu eastern side of Big Baldy Mountain; 

Bf Pinto diorite exposure, bead of Rock Creek, Neibart.. 338 
L. Af Clifis of limestone near Logging Creek station, Belt Creek ; 
B, Florence mine, Neibart, sbowing massive gneiss out- 
crops and gully cut in outcrop of veiu 368 

LI. Neibart, Montana, seen from tbe nortb, looking up Belt Creek. 370 
LII. Af Neibart quartzites overlain by sbale formations of Belt ter- 
rane ; /i, Intrusiveporphyrysbeet and characteristic talus 

slope, Sawmill Creek road southeast of Neibart 378 

LIII. Af O'Brien Creek reservoir, with Neibart Mountain in dis- 
tance; Bf Outcrops of gneiss near old silver smelter of 
Hudson Mining Company, Belt Creek, below Carpenter 

Creek, Neibart 380 

LIV. Af Crusted ore. Big Seven mine; Bf Crusted ore, Florence 

mine 410 

LV. Af Surface cut showing apex of vein. Empire claim, Neibart; 

Bf Sapphire Coul<$e, Yogo district 420 

LVI. Claim map of Neibart district, showing olaims surveyed np 

to December, 1898 422 

LVII. A, Queen of tbe Hills mine from tbe north, looking np Belt 

Creek; B, Queen of the Hills shaft-bouse and ore bins.. 421 

267 



268 ILLUSTRATIONS. 

Page. 

Plate LVIII. Plau of undergroQDd workings of Broadwater mine 426 

LIX. Shaft-house and ore bins of Gait mine in 1895, looking sooth 

across Rock Creek Gulch 428 

LX. Longitudinal section of Broadwater mine 430 

LXL Moulton mine, 1895 432 

LXII. Broadwater mine in 1895 '. 434 

LXIII. Summit of Neihart Monntain, showing character of qnartzite 

talus and bedding plane of the rock 438 

LXIV, A, Mackay mines, Dawn and Foster claims, head of Mackay 

Creek ; Jij Florence mine 440 

LXV. ^, Whippoorwill mine, Carpenter Creek, Neihart; /^, Mouth of 

Carpenter Creek, Neihart 442 

LXVI, Map showing location of sapphire mines, Yogo district 454 

LXVII. Sapphire mines of Yogo district: J, Shaft in 1897 (Ricard Peak 

in distance) ; if. Face of dike in open cuttings, 1897 456 

LXVIII Woodhurst-Mortson mine, Running Wolf Creek 458 

LXIX. Mountain rim northeast of Barker Basin, Barker district, Mon- 
tana V 460 

LXX. Igneous rocks of Yogo Peak: J, Syenite; B, Monzonite; C, 

Shonkinite 470 

LXXI. J, Intergrowth of two alkali feldspars in monzonite of Yogo 
Peak; 7?, Skeleton magnetite in olivine of shonkinite of 

Yogo Peak 476 

LXXII. Af Thin section of shonkinite, natural light; /i, Thin section 

of shonkinite, polarized light 486 

LXXIII. .:(, Diorite of Neihart (Pinto diorite); /f, Variolitic minette 

of Sheep Creek 488 

LXXIV. J, Thin section of resorbed olivine in shonkinite of Yogo Peak; 

/^, Thin section of granite-porphyry of Thunder Mountain. 508 
LXXV. A, Micro-drawing of rhyolite-porphyry; B, Micro-drawing of 

diorite-porphyry of Steamboat Mountain 516 

LXXVI. J, Thin section of minette; ^, Thin section of analcite-basalt. . 528 

LXXVIL Sapphires of Yogo../. 554 

Fig. 36. Index map showing location of range 272 

37. Minette sheets intruded in limestones, head waters of Sheep Creek.. 307 

38. Cross section of Judith Plateau region 311 

39. North-south section through Yogo Peak 318 

40. Strata Hexed by Steamboat Mountain laccolith, Eureka divide 335 

41. North west- southeast section through Big Baldy Mountain and Storr 

Peak 336 

42. Mapof Barkt^r district 345 

43. North-south cross section through Barker Mountain 355 

44. Contact between igneous rock and limestones on south side of Thun- 

der Mountain 365 

45. Upturned beds at north end of Thunder Mountain 365 

46. North-south section across Thunder Motmtain 366 

47. Diagram showing distribution of laccolithic masses and related intru- 

sions in Little Belt Mountains 388 

48. Profile section through Mount Marcellina, with ideal restoration of 

laccolith and cover (Cross) 392 

49. Ideal cross section of Mount Hillers laccolith, Henry Mountains (Gil- 

bert) 392 

50. Ideal cross section of Mount Holmes bysmalith, Yellowstone Park 

Iddings) 393 



ILLUSTRATIONS. 269 

Page. 
Fig. 51. Ideal orose section of northern flank of Little Belt Range, to show 

relation of asymmetric laccolith to fold or fuiiltiDg of uplift 393 

52. Map of Neihart district 4aS 

53. North west^soiitheast section across Neihart district 405 

54. Crystalline form of polybasite 407 

55. Crystal of barite ". '. 409 

56. Crystal of barite 409 

57. Diagram to show occurrence of niiueruls in qnartz streak of Big Seven 

vein 411 

58. Crystal of zinc blende coated with polybasite 411 

59. Diagram to show relative position of the Neihart veins and the rela- 

tion of pay ore to nature of country rock 419 

60. Fanit where vein crosses amphibolite, upper level Florence mine, 

August 13, 1897 426 

61. Face of vein Augast 14, 1897, npper tunnel Florence mine 427 

62. Diagram showing relation of Gait vein t-o porphyry dike, north end 

of Queen tunnel, August, 1897 427 

63. Workings on IngersoU vein, 1897 431 

64. Cross section of Broadwater vein 432 

65. Face of Broadwater vein, on stope below No. 3 adit level, under 

bhicksmith shop 435 

66. Big Seven vein in quartzite, face of upper drift, August, 1897 438 

67. Plan and section of Carter mine, Barker district 443 

68. Workings of May and Edna mine, Barker district 444 

69. Ideal transverse section of mine. Barker district 4-15 

70. Vanadinite crystal. Sir Walter Scott mine 452 

71. Vanadinite crystal, Sir Walter Scott mine 452 

72. Section of npper limit of sapphire- bearing dike, wall of Vogo 

Canyon 456 

73. Yogo Peak dikelets 494 

74. Manebach twin 504 

75. Zoned plagiocla8e(andesine-labradorite-andesiue) in diorite-porphyry 

of Steamboat Mountain 516 

76. Natural etched figures on sapphires from Yogo Gulch 556 

77. Diagram to show differoDtiation at Yogo Peak 564 

78. VariatiouH of minerals in rocks of Yogo Peak 568 

79. Ditferentiatiou diagram of Little Belt Mountain rocks 571 



U. S.GEOLOGiCAL SURVEY 




TOPOGRAPHIC MAP OF LITl 



Sc. 



o 



Contour Intrr 



TWENTIETH ANNUAL REPORT PART III PL. XXXVI 




JULIUS BIEN a CO LITH N Y 



,E BELT MOUNTAIN REGION 



12 

s3 



a! 200 feet. 



GEOLOGY OF THE LIHLE BELT MOUNTAINS, MONTANA. 



By W. H. Weed. 



CHAPTEE I. 

rN'TRODUCTIOX. 

The field work upon which the present report is based was done in 
Se]>tember, 1893, and Augast, 1894, while the writer wavS engaged in 
an areal geologic survey of the region. During both these years he 
was accompanied by Prof. L. V. Pirsson, of the Sheffield Scientific 
School of Yale University, who shared with him the vicissitudes of 
camp life and visited nearly all the localities seen. The writer is under 
obligations to him for many keen observations in the field as well as 
for the careful petrographic study which he has made of the igneous 
rocks of the range (see accompanying paper). 

Owing to the necessity of revising the topographic map, especially 
of the area about Neihart and Barker, the two mining settlements of 
the region, the preparation of this report was delayed until this 
revision was made, and in September, 1896, apart of the range was 
jfesurveyed by Mr. R. H. Chapman, but the heavy autumnal showers 
prevented further geologic field work in that year. In 1897 three 
weeks were spent at Neihart revising the geologic boundaries of the 
new map and visiting the ore deposits of the district. The author was 
in that year assisted by Mr. L. S. Griswold, formerly of the geologic 
staff of Harvard University, now of Helena, Montana. To him was 
intrusted the delineation of the contact boundaries of the igneous 
masses about Barker and a study of the ore deposits. 

All of the range does not appear on the folios issued by the Survey, 
though both the Little Belt Mountain folio, named from the range, and 
the Fort Benton folio include parts of the chain. 

It must be remembered that the study of the region was incidental 
to this areal mapping and that it was not a detailed investigation. 
For this reason, and because of the small scale of the map, this report 
must be regarded as a general description only. 

GEOGRAPHIC POSITION. 

The Little Belt Mountains are situated in the central part of the 
State of Montana, as shown on the index map (fig. 36). They form 
part of the Rocky Mountain region, being one of the eastern of the 

271 



272 GEOLOGY OF THE LITTLE BELT M0UNTA1M8, MOHTAMA. 

bordering or Iroiit ranges, which project from tlie geoeral monntain 
area into the open country of the Great Plains. They lie to the Koath 
of the plains of the Missouri River and midway hetween that stream 
and the Yellowstone. Their drainage is, however, all tributary to the 
Missouri. The range has a general north west- southeast trend. It 
is clearly de&ued between the plains country and the broad intermon- 
taue valley of Smith River, but its western extension is indefinite, 
though currently accepted as ending where Smith River has cut its 
valley northward through a relatively lower though somewhat moun- 
tainous country. The mountains were seen by Lewis and Clarke iu 
their historic trip up the Missouri River in 1804-1805. They undoubt- 
edly derive their name indirectly from Belt {or the Belted) Butte, a con- 
spicuous emiuence rising above the open plains country north of the 



mountains. Belt River, named from the butte, has its source in the 
range, and no doubt suggested the name of the latter, while the rela- 
tively low elevation and plateau character of the mountains, as con- 
trasted with the range south of it, enggested the adjective Little, in 
contradistinction to Big Belt. 

SETTLEMENTS. 

The gold discoveries which brought a host of energetic prospectors 
into the State in the sixties resulted in a general searching of the 
mountain region of .Montana for mineral deposits. It was not, however, 
until 1876 that the discovery of the silver-lead ores of Barker directed 
especial attention to this tract. Neihart is now the principal town, 
with a population varying from ttW to 2,000, according to the activity 
in -the mines. Barken, though nearly deserted for many years, has now 



wJtED.] EXPLOBATION AND TOPOGBAPHY. 273 

a few handred inhabitants. Monarch, a distribnting point for the 
Kibbey Basin, is the only other town within the mountains, though 
I>08t-offices are maintained at a few points for the benefit of the scattered 
settlers of the region. A branch line of the Great Northern Railway 
extends f^om Oreat Falls into the heart of the range, lines running to 
both Neihart and Barker. There are very few wagon roads, and the 
greater part of the range is accessible only on horseback, though the 
load to White Sulphur Springs runs across its center. 

The climate is rigorous, as would be expected from the elevation. 
Outside of the few small bottom-land areas along the creeks, and the 
meadows of Belt Park, no crop except hay can be raised* The arable 
land is limited and the region has no agricultural possibilities. 

The general elevation being over 6,000 feet, the snowfall is heavy, 
and the roads across the range are blocked by the deep drifts as late 
as June. June and October are commonly stormy months, but the 
intervening summer period is characterized by an almost ideal climata 

PREVIOUS EXPLORATION. 

In 1883 Prof. W. M. Davis crossed the range, visiting Neihart, in an 
economic exploration conducted for the Northern Transcontinental Sur- 
vey of the Northern Pacific Railroad Company. He published a brief 
account of his observations in the Report of the Tenth Census on Mine 
Industries.' He gives the main facts of geologic structure and strati- 
grapW '^ sequence, and noted especially the fine geologic sections in the 
northern part of the range. He was accompanied and assisted in this 
work by Mr. Waldemar Lindgren, who made a special study of the 
igneous rocks, described their occurrence, and gave a description of the 
principal rocks. His work will be alluded to later. In 1884 Prof. J. S. 
Newberry visited Neihart and looked over the ore deposits there, and 
in the same year he published a short account of his observations on 
the geology of the range.^ 

TOPOGRAPHY. 

The Little Belt Mountains are commonly spoken of as a range, but 
are more correctly designated as an elevated and eroded plateau region, 
as is plainly shown by the topographic map (PI. XXXVI). Individual 
peaks along the northeastern flank rise above the general level, and 
give the serrated crest line seen from the open plains. Compared with 
the compact, well-defined mountain ranges common in the Cordilleras^ 
the Little Belt tract is relatively low, wide, and composed of many spurs 
radiating from a central point. The mountains constitute, however, a 
tract sharply delimited from the adjacent plains country on the north- 
east and southeast, and separated by the broad and flat Smith Biver 
Valley from the Big Belt Range. Westward the deep canyon of Smith 

iRelationa of the ooal of Montana to the older roolu; with api>endixea on fossils by R. P. Whitfield 
and on eruptive rocks by W. Lindgren: Beport of Tenth Census, Vol. XV, pp. 696-737, Wasli^ 
ington, 1886. 

* Annals New York Acad. Soi., Vol. UI, No. 8, 1884. 

20 aEOL, PT 3 18 



274 GEOLOGY OP THE LITTLE BELT MOUNTAINS, MONTANA. 

Biver oatlines the border of the mountainB, the re'gion beyond being of 
gentler relief, in strong contrast to the western slopes of the Little 
Belt. The region thus delimited is 60 miles across from east to west, 
40 miles wide on its western border, and narrows eastward to a sharp 
point terminating at Judith Gap. The letter V represents the plan of 
the mountains, the angle pointing east. 

The water parting of the monntains is the approximate axis of the 
nplifb, though not the highest part of the mountains. It runs northwest 
and southeast, and this is commonly spoken of as the trend of the 
range. 

Throughout the greater part of the region the mountains are plateau- 
like. There are no narrow crest lines marked by aretes or sharp 
peaks, but broad, flat tops prevail. The average elevation is 7,600 feet, 
though in the central tract from which the spurs radiate the summit 
level is 8,000 feet. While this plateau character prevails over^the 
greater part of the tract, the higher summits found along the northeast 
border rise above this general level, are different in form, and more or 
less isolated. The highest summit of the mountains, called Big Baldy, 
reaches an altitude of 9,000 feet. The other peaks are much less in 
height. These mountain masses show rounded or dome-shaped sum* 
mits, which rise above and are distinct from the rest of the range. 
These individual mountain masses owe their prominence to geologic 
structure, as will be shown later in this report. The Little Belt Moun- 
tains are bounded by soft and easily eroded rocks, and owe their 
prominence as well as their uplifted position to this fact. Within the 
mountain tract rock characters and geologic structure determine topo- 
graphic form. So intimate is this relation that it is difficult to discuss 
one without discussing the other. 

There are no big and broad valleys within the mountains. The 
streams flow in deeply tren'ched courses, open and wide in the softer 
shaly rocks, narrow canyons in the harder limestone strata. The 
region is sufQciently rugged to be characterized as mountainous, though 
onlj* alpine about the highest peaks. Summit plateaus are bordered 
by high escarpments, and along the streams the towering limestone 
cliffs, walling in deep gorges, present difiiculties to travel and lend 
picturesqueness to the scenery. 

Secondary plateaus are common in the central area, where the beds 
are horizontal or gently inclined. Resistant rocks determine broad 
levels separated by deep gorges, and where such levels are emphasized 
by differences in vegetation, as is the case in Belt Park and the other 
quartzite parks near Neihart, the contrast with the wooded slopes 
above and below is very marked. Smaller benches, due to igneous 
sheets, are also common. The smaller parks of the monntains owe 
their existence to soft rocks. Bear Park and several other park val- 
leys are formed of synclinal folds of Oarboniferous shale. 

Along the mountain flanks the streams cut across the upturned beds 



vm>.] DBAINAGE. 275 

in narrow gulches. Here the abrupt transition from soft to harder 
rocks is strikingly shown in the resulting scenery. The streams pass 
through narrow clefts in the massive limestones — gateways from the 
open, arid plains to the verdure-clad mountain valleys. Along the 
larger streams such gorges are wider, though generally impassable, and 
the limestones present a bewildering array of pinnacles and towers, 
castle-like masses, natural archways, and ribboned walls. 

DRAINAGE. 

The region is well watered, the rainfall and snowfall being relatively 
abundant. The range is a center from which streams radiate in all 
directions, but the Judith, Belt, and Smith rivers are the trunk streams, 
draining the east, north, and west slopes, respectively. So far as it 
has been studied, the drainage is consequent. Smith Biver is, how- 
ever, a reversed stream, and a northward tilting in recent geologic 
time has accelerated northward-flowing streams and retarded south- 
ward drainage ways, and its effects are seen in one or two beheaded 
and reversed streams. 

The character of the streams is dependent upon the nature and struc- 
ture of the rocks, and accordingly the streams are perennial, inter- 
mittent, and interrupted.^ The fofmer occur only in impervious rocks, 
shales or the igneous rocks. The intermittent streams, flowing only 
in wet weather, or the time of melting snows, are common where the 
catchment area is small, but ar^ especially characteristic of limestone 
areas, where the waters sink and are lost in the rockd. Where the 
structure brings an underlying impervious stratum or an igneous rock 
to the surface, such waters often reappear as springs. About the 
mountain flanks the dry drainage ways which cross the surrounding 
plain often have springs a few miles from the mountains. The ^^ inter- 
rupted " streams have flowing water only where their channel is cut 
in impervious rocks. In the limestone areas, even a large and rapidly 
flowing stream will sink beneath the surface and disappear, leaving the 
stream channel bare and dry until, at some lower level, the water comes 
to the surface and forms a flowing stream for a short distance, and 
then disappears again. Dry Fork of Belt Creek, and Belt Greek below 
Monarch, and the forks of the Judith exhibit this character during the 
summer months. There are no waterfiftlls in the mountains. The 
streams have, however, a steep gradient, and water power is readily 
available. Belt Creek has a fall of 1,677 feet in 27 miles, from iN'eihart 
to the point where it leaves the mountains at Biceville. The grade is 
82 feet per mile between I^eihart and Monarch, 47 feet per mile between 
Monarch and Logging Creek, and 40 feet per mile through Sluioebox 
Canyon. 

> Terms proposed by R. T. Hill, Geolofio AUm U. S., IoUo 43, Nueces, Texas i Washington, 1898. 



276 GEOUX2T OF THE LITTLE BELT MOUFTAnrS, MONTANA. 

VEGETATION. 

The moantoiiis are yerj generally forest clad, their dark slopes \mng 
in somber eontrast to the sorroondiog arid plains. The timber is, 
however, mostly smalL Yellow or lodgepole pine {PinuM murrayana) 
is the prevailing species. In some tracts it forms forests whose trees 
are 10 to 14 inches in diameter; bnt more commonly it is smaUer, and 
bamed-over tracts hare a dense thicket of pole pine. The nsoal char- 
acter of the forest growth of this species of pine is shown on PL 
XXXVU, B. Spmce and fir are also found along wet stream bottoms 
and on moist and cold northern exposures. The white pine {P.flexUUi) 
is also found on the plateau summits, and south of Neihart has been 
extensively cut for lumber. 

The character of the timber growth varies with the exposure. On 
sonthward'focing slopes the growth is sparse and open, with grassy 
interspaces. On northerly exx)osnres thick and ditf k forests prevaiL 
The plateau summits are beautiful in their alternation of glade and 
grove. The grouping of pine and spruce is ideal in form, and the opens 
are bright with innumerable flowers. The growth also varies somewhat 
with the character of the rock, or rather with the physical nature of 
the soil formed. The barren Belt shMes produce but little soil and sup- 
port scanty vegetation. The sandstones and qnartzites and the fine 
debris of igneous rocks are generally densely wooded. The Cambrian 
shales, on the contrary, usually underlie an open, park country. 

STRUCTURE. 

The general structure of the Little Belt Range is that of a low flat- 
topped arch, which is 20 miles wide on the west and narrows to a point 
on the east. On the summit of the arch the rocks are gently inclined 
or horizontal. On the flanks or shoulders of the arch the rocks dip 
steeply away from the uplift. This is shown in the sections, drawn to 
natural scale, across the range, contained in folios Nos. 55 and 56, of 
the Oeologic Atlas of the United States. 

This simplicity in stmctare is modified by a great fissure extending 
from Yogo Peak northeastward for 13 miles. This fracture, filled by 
igneous rocks forming a stock,^ was the center from which numerous 
sheets and dikes were sent out as intrusions in the softer shaly strata. 
An even greater modification of the general arch of the range is seen 
along its northern flanks, where great bodies of igneous rocks are 
found intruded between the crystalline schists and gneisses and the 
overlying sedimentary rocks. These intrusions are laccoiithic in char- 
acter and arch up the beds above them, thus locally elevating the lat- 
ter far above the general summit of the range fold. These local arches 
are not confined to the mountain areas, but also occur on the outlying 
low plains country north of the range. As a result of the powerful 



1 An irregniar intmsiTe body of igneoas rock of anknown depth, breaking acroes and up throngh 
earlier rooks ; often a trunk mass for radial dikes and sheets. 



WBED.] STBUOTITRE. 277 

movements by which the range itself was uplifted and arched, and of 
those lesser movements by which the stratified rocks were subjected to 
further uplift and folding, there are many dislocations and minor fold- 
ings. As shown on the map, there are no great faults throughout this 
area, the dislocations being small and purely local in character. 

Ore deposits occur in many parts of the range, but up to the present 
time those of the Neihart and Barker districts are the only ones that 
have yielded productive mines. Their association with the igneous 
rocks appears to be very close, if not directly genetic. The mineral 
deposition is found to be relatively recent, being later than the youngest 
eruptive rocks. 

Since the uplift of the range, with the accompanying intrusion of 
igneous rocks, the region thus raised above the general level has been 
extensively denuded. This period of erosion has bared the larger 
laccoliths and worn down the general level to the plateau-like region 
now seen. In this process the relative hardness of the dififerent rocks, 
or their resistance to erosion, has resulted in a differential degradation, 
in which the harder igneous rocks are left in relief, and now form the 
higher summits by virtue of their physical character as well as their 
uplifted position. 

This region, unlike that of Castle Mountain to the south, has not 
held local ice sheets, nor has it been covered by the general glaciation 
from the north. A careful search was made for traces of glaciation, 
but none were found whose origin was certain, although it was supposed 
by Newberry that the range was once ice clad. The igneous rocks of 
the mountain are peculiar and easily recognized, but do not occur in 
glacial drift farther north. 

Stream erosion has been active, and the range presents evidence of 
stream adjustment and vigorous cutting as a result of later tilting of the 
region to the northward. 



CHAPTER II. 
THE BOCK FORMATIONS. 

The rocks of the Little Belt MoantaiDS are of maDy kinds and of 
diverse origin. The nacleus or core of the range, which is exposed 
about Keihart and on lower Sheep Creek, consists of gneisses and 
schists, a group which has at many localities been designated as a 
Basement Complex. Sedimentary rocks rest upon this central core, 
and cover the greater part of the range. They cover a wide range in 
lithologic character, and include formations of all the geologic ages. 
The sedimentary strata are intruded by igneous rocks, which break up 
through them in great stocks or in dikes, or are intruded between the 
beds as sheets or laccoliths. These igneous rocks also present a con- 
siderable variety in texture and composition. The areal distribution 
of the difierent rocks is shown on the geologic map, PI. XLI. 

METAMORPHIC ROCKS— GNEISSES AND SCHISTS. 

The crystalline schists which occur in the Little Belt Range have the 
characters common to the Archean or Basement Complex elsewhere. 
They are the oldest rocks and form the nucleal core of the range. The 
rocks are well banded, the layers preserving a uniform direction for 
many miles. nx)on the truncated edges of these bands the stratified 
rocks rest in an unconformity that is as marked as is the difference in 
texture between the metamorphic and the unaltered or but slightly 
altered' strata. North of Neihart, Cambrian rooks rest directly upon 
the gneisses. South of Neihart a thickness of 4,000 to 5,000^ feet of 
Algonkian rocks intt^rvenes, separated from the Cambrian by an 
unconformity. 

In no part of the series examined do the rocks possess the recogniz- 
able characters of altered sediments, and if subdivisions of the Archean 
are made it must be on purely lithologic grounds. A part of the 
series is demonstrably eruptive, and the rocks are cut by later and but 
partially metamorphosed igneous rocks. The banding and foliation 
that are so striking in general view do not afford a satisfactory basis for 
a subdivision of the series, since the rocks rapidly change in character. 
The general character of the Archean slopes is shown in PI. XXXYII, 
A, and PI. XXXVIII, A, made from photographs of the slopes near 
Neihart. 

The crystalline schists present considerable variety in color, texture, 
mineral composition, and in those physical characteristics which show 

278 



yllLES BELOW NEIH 



N or BELT CRE6K. ! 



wwa>.] ALGONKIAN B00K8. 279 

in weathering, and canse the rock to form bold outcrops or soil-covered 
slopes, massive talus debris blocks or slopes of fine, gravelly debris. 
These characters affect to a very marked degree the nature of the vein 
fissures, and also the character of the ore of the veins. ^Nevertheless, 
it was not found possible to map these distinctions, partly on account 
of the very small scale of the topographic map and partly because of 
the nature of the rocks themselves. Though presenting apparently 
so wide a variety of rock types, all the rocks can be classed as crys- 
talline schists, and are either gneisses or schists of various kinds. 
Gollectively they form a remarkably well-defined series of rocks, and 
they are mapped as a unit. 

The ^^crystalline schists" are rocks distinctly crystalline in texture, 
showing a streaked or foliated structure, due to the aggregation of dif- 
ferent constituents into parallel layers, which are generally distinct in 
texture and composition from adjacent layers. These layers or folia are 
not persistent, but are thin lenticular bands, which thicken and thin out 
rapidly, the ends of diff'ercnt folia interlocking. The rocks split along 
these folia more or less readily (schistosity). This arrangement is 
seen on a large scale^as banding; in a hand specimen it is generally 
recognizable, and always in microscopic sections. 

While part of the Neihart rocks are true schists, none of them are the 
wrinkled, puckered schists common in so many Archean areas. Most of 
the rocks are gneisses, using the word to denote or designate any crystal- 
line rock possessing a gneissic structure, and not confining it to a 
quartz- mica-feldspar rock simply. They are foliated rocks, not suffi- 
ciently fissile to be called a schist In none of the Neihart gneisses or 
schists has any evidence of a sedimentary origin been observed. On 
the other hand, many of them are still recognizable as metamorphosed 
igneous' rocks, mostly porphyries, which can be distinguished without 
difficulty from the later and unchanged intrusions of porphyry. Where 
the original character of the rock is not determinable it is distin- 
guished by the name of its predominant or characteristic mineral. 

ALGONKIAN ROCKS. 
BELT TEBBANE. 

Throughout the southern portions of the mountain tract there is a 
great thickness of generally barren slaty rocks underl3ring the 0am- 
brian formations. This series, which consists of several distinct but 
allied formations, collectively known as the Belt terrane, is shown upon 
the geologic map under the names of Neihart quartzite and Belt 
shale. This formation is wanting in the northern part of the range, 
where the fossiliferous Middle Oambrian rocks rest directly upon the 
crystalline schists. Throughout the entire southern part of the moun- 
tain area it is exposed where the denudation has cut deeply enough 
into the uplift, and these rocks here form the surface over extensive 
districts. Volcano Valley, between the Little Belt Eange and Oastle 



280 GEOLOGY OP THE LITTLE BELT MOUNTAINS, MONTANA. 

Mountain, is eroded in an anticline of these rocks, and the Big Belt 
Eange is a long anticlinal uplift formed of them. 

The Belt terrane consists of seven distinct fonnation8,only five of which 
are found in the Little Belt region. The formation was first recognized 
by Davis in 1882,^ and described as probably Lower Cambrian. It was 
also noted by Newberry in a trip across the Belt Mountains in 1884. 
The southern extension was mapped and described in folios 1 and 24, 
and in bulletins 110 and 139, of the United States Geological Survey, 
The best exposures of the Little Belt and Big Belt ranges were care- 
fully examined at different times in each successive field season for 
traces of fossil remains, and in 1895 a reconnaissance of the region was 
made by Mr. Charles D. Walcott, the director of the Survey, accom- 
panied by the writer, in search of fossil remains. Despite a prolonged 
search at that time, it was not until the summer of 1898, when a second 
visit to the region was made by Mr. Walcott, that fossil remains were 
found. Tlie character of these remains does not contradict the distinct 
stratigraphic evidence of a great unconformity and of an interval of 
erosion between these and the overlying beds, and the terrane is 
assigned to the Algonkian period. 

Topographic aspect of Belt area^. — Between the areas covered by the 
rocks of the Belt terrane and those of later sedimentary strata there is 
a marked contrast in topography, and usually in vegetation also. This 
is due to the prevailing shaly character of the Belt rocks, the thin and 
barren soil formed by them, and their usually rapid degradation. In 
general, the country covered by the Belt terrane is a hilly one, with 
smooth and rounded slopes, deeply trenched by flowing streams. The 
larger valleys are bordered by bluffs of these rocks. 

The formation as a whole is well indurated and somewhat metamor- 
phosed. In this respect it presents a strong contract to the overlying 
Cambrian strata, whose unaltered shales and limestones bear no resem- 
blance to the slaty rocks of the Belt. There is no true slaty cleavage, 
however, but there are bedding laminations, and the rocks are more 
properly called phyllites or argillites, though slate appears to be a 
more popular or commonly used descriptive name. 

No general review of the literature of the terrane will be given here. 
Van Hise has presented an admirable summary of it,^ as did also Peale^ 
in 1893. Neither Peale nor Davis recognized definite subdivisions of 
the terrane, though both allude to the variety of rocks composing it. 
The first attempt to subdivide the group was made by the writer in the 
Castle Mountain region.^ When the vicinity of Neihart was studied 
the base of the formation was named the Neihart quartzite, and the 
different subdivisions overlying this were recognized, though for pur- 

> Tenth CeiuBas, Vol. XV, p. 788. 

* Bull. U. S. Geol. Sarvey No. 86, pp. 286, 504. Principles of pre-Cambrian ffeology : Sixteenth Ann. 
Bopt. U. S. Geol. Snrvey, Part I, p. 818. 
» Ball. U. S. Geol. Survey No. 110. pp. 16-20. 
«Biill. U. S. Geol. Survey No. 139, p. 32. 



WEED.] BELT TEBBANE. 281 

poses of areal mapping they were grouped as Belt shale. In a paper by 
Mr. Walcott these formations were for the first time given individual 
names and defined as separate formations.^ The subdivision is, how- 
ever, based entirely upon lithologieal grounds, though the terrane pre- 
sents an ideal example of a cycle of deposition,'^ and the subdivisions 
grade into one another. The formations composing the terrane, corre- 
si)onding in part to those made by the writer in 1896,^ have been named 
by Mr. Walcott as follows, the beds being given in descending order: 

7. Marsh Creek sLale. 
6. Helena limestone. 
5. Spokane shale. 
4. Greyaon shale. 
3. New land limestone. 
2. Chamberlain shale. 
1. Neihart quart zite. 

The two upper formations are not found in the Little Belt Range, but 
occur on thiB flanks of the Big Belt Eange in a continuation of the ter- 
race to the west and north. 

Neihart quartzite, — Thc^ oldest recognizable sedimentary rocks of the 
Little Belt Mountains are the quartzite beds found in the vicinity of 
Keihart. On Neihart and Long Baldy mountains the quartzite is seen 
resting directly upon the crystalline schists, and the picturesque canyon 
about Neihart is cut in it. The rocks are in part true quartzites, grad- 
ing into well-indurated sandstones. They sometimes show well-devel- 
oped bedding, though they are often quite massive in general view. In 
color they vary from creamy white to gray or pink. Pebbles are occa- 
sionally found, masses of which sometimes form thin lenses, but their 
occurrence is local and no well-defined conglomerate beds occur. They 
consist for the most part of milky white, or pink, rarely gray, quartz. Red 
and white gneiss is occasionally seen, but no pebbles of the Pinto diorite 
or other igneous rocks intrusive in the crystalline schists were found. 

The lower 350 feet of the quartzite forms a very compact body, uni- 
form in character, which makes the escarpments so conspicuous from 
Neihart. The rock shows a prismatic structure, especially conspicuous 
in Neihart Canyon, and shown on PI. XXXVIII, B. About 300 feet 
above the base the character of the formation changes. The pink and 
white pure quartzites are replaced by more thinly bedded rocks, no 
longer of pure arenaceous material, but containing an admixture of 
greenish mica, which higher in the group forms the layers of mica 
shales interbedded with the quartzite. The higher strata are still 
more impure and the quartzite beds are but 6 to 12 inches thick, 
blackened by carbonaceous material that now forms a prominent feature 
of the intervening shales, becoming increasingly abundant until the 
latter rocks are true black shales in which the green mica no longer 

> FoBsiliferoas pre-Cambrian terranes : Bull. Geol. Soo. America, Vol. X, pp. 109-244. 
'Kewberry, oydea of deposition of Amerloan aedlmentary rocks: Proo. Am. Assoc. Adv. ScL 
Angnst, 1873, p. 186. 

sBuU. U. S. Geol. Survey No. 130, p. 82. 



282 GEOLOGY OP THE LITTLE BELT MOUNTAINS, MONTANA, 

shows. At the same time the quartzite beds decrease in thickness 
and purity, while the interbedded shale increases in thickness and 
purity, so that an arbitrary line must be drawn separating the two 
formations. 

Chamberlain shales. — These consist of a series of dark-gray, almost 
black, shales, frequently arenaceous, showing occasional ripple marks. 
The rocks are essentially slaty in fracture, but the beds are jointed and 
form clifis along the stream courses. The formation is characterized 
by these black shales, which form its middle part. At the base the 
admixture of arenaceous and micaceous material indicates transition 
into the underlying quartzite, while in the upper part of the formation 
calcareous beds appear alternating with the black shale, the latter 
becoming, less and less prominent and the calcareous shale becoming 
true limestone. There is thus a very gradual transition into the 
Newlaud limestone, and no sharp dividing line can be drawn. The 
estimated thickness of the Chamberlain is 2,000 feet, and it is typically 
developed along Chamberlain and Sawmill creeks south of l^eihart. 
The thickness on Sawmill Creek is estimated to be 2,078 feet. 

Newland limestone. — The base of the formation consists of beds of 
limestone but 12 to 30 inches thick, at first separated by 20 to 50 inches 
in thickness of shale. Higher up in the series the limestone beds 
become more frequent and thicker and the shale layers thinner, until 
in a few hundred feet the limestone largely predominates. The.se lower 
limestones are somewhat fissile and shaly. In the center of the forma- 
tion they are massive, very dense and compact, dark-blue in color, and 
show crystalline streaks and markings (calcite) and carbonaceous stain- 
ings. The rocks weather with a light-yellow or buff color, on which 
these crystalline markings are prominent, and are generally cracked by 
fine joints, frequently filled with calcite, which causes the rock to break 
into small cubical masses on weathering. In the middle of the forma- 
tion there is little or no shale. The limestone occurs in beds 3 to 6 feet 
thick, sometimes thicker, and is jointed, so that the exposures present 
a masonry-like effect when seen in cliffs. The formation is typically 
exposed in the bluffs on the north side of Newland Creek, from which 
it takes its name. The thickness along Sawmill Creek is estimated at 
567 feet; on Kewland Creek it is much greater. 

Oreyson shale. — This formation consists of dark-gray or black, fine 
and coarse grained siliceous shales. The lower part of the formation 
consists of pearly gray shales containing mica (sericite), which gives the 
rock a glistening, satiny sheen. These pass upward into more siliceous 
beds containing beds of intercalated sandstone a foot thick. The 
formation is exposed in the bluffs at Sawmill and Belt creeks, and covers 
large areas in the southern part of the mountains. The thickness on 
Sawmill Creek is 955 feet. 

Spokane shale. — ^The highest beds of the terrane seen in the Belt 
Mountains are the red shales given this name. The rocks vary from 



WMi>.] BELT TEREAITE. 283 

brick-red to pink in color, and are therefore nsually easily recognizable, 
thongh seldom forming good exposures. South of Neihart they are 
seen in Sawmill Creek, just below where the road makes a sharp bend 
to ascend to the park. The formation has not been recognized on 
Chamberlain, Belt, or O'Brien creeks. The unconformity between 
these shales and the overlying Flathead (Cambrian) quartzite is seen 
on Sawmill Creek. Southward these red shales are very prominent on 
lower Kewland Creek, where maroon shale is overlain by light-red and 
these by white shales. 

General section. — The following section was measured along Belt and 
Sawmill creeks south of Neihart: 

Section exposed on Belt Creek south of Neihart, Montana. 

Flathead BandBtone : ^eet. 

Cambrian sandfitone; dark red, containing white pebbles, with quite 
ferraginous matrix. But 40 feet exposed, the bed occurring in the 
creek channel one-fourth mile below the forks of the stream 100 

Plane of unconformity. == 

Spokane shale : 

Red shales 200 

Bed shales, laminated, brittle, .and quite hard 10 

Greyson shale : 

Shales; generally gray, rarely exposed, and forming densely wooded 

slopes • 700 

Gray sericitio shale, locally disturbed by a horizontal sheet of 

minette 170 

Shales; exposed in wall on west side of canyon. Thinly bedded 

slates carrying limestone 60 

No exposure 25 

Kewland limestone: 

Massive, block-Jointed limestones, blue on Aresh fracture and 

weathering earthy brown. Dip 20° S 15 

Calcareous shales or slates, not exposed 200 

Limestones, with massive outcrop, blue on fresh fracture, weather- 
ing on surface to an earthy buff color 60 

ISlates. (Prospect hole shows a minette intrusion) 125 

Limest-one; fissile and slaty 18 

Massively bedded slate 15 

Impure limestone 5 

Black or gray slate with glistening surface. Dip 30<^ 15 

Trachyte intrusion. Rock at contact badly twisted, probably not a 
sheet. 

Limestone and slate, not exposed 30 

Minette sheet. 6 

Limestones and shale 4.... 20 

Shales ; slaty, in part indurated. Dip 20^ S. ; strike S. 60° E 15 

Slaty shale; gray in color, well indurated 30 

Limestone; in 3-foot beds of dark-gray color with crystalline mark- 
ings S 

— 562 
Chamberlain shales : 

Gray shale, seen in debris only, no ledges being seen 1, 095 

Black shale ; Exposure being one-fourth mile below a western branch 

of the creek 363 



284 GEOLOGY OF THE LITTLE BELT MOUNTAINS, MONTANA. 

Cbamberlain shalea — Continued. Feet. 

Massively bedded, black sbale. Dip 20^ upstream 40 

No exposure 190 

Black shale 40 

Black crumbly shale. Dip 14^ 54 

Black shales 229 

Thinly bedded and fissile quartzite 5 

Black shale, somewhat slaty, carrying beds of green quartzite 40 

Shale, not exposed 22 

2,078 

Neibart quartzite: 

Quartzite 2 

Micaceous shale with fucoidal markings, resembling Cambrian 1 

Quartzite; greenish in color, occurring in beds 6 to 18 inches thick, 

with iDt>eryening black, carbonaceous shale 7 

No exposure ^ 95 

Shales ; micaceous, dark colored, generally green, and carrying 

thinly bedded quartzites, so that the entire series might be classed 

as quartzite 15 

No exposure ^ 170 

Green, micaceous shale and micaceous quartzite occurring in beds 

4 to 12 inches thick, in alternate layers 8 

Micaceous shales, resting upon basal quartzites 104 

Quartzite series, forming base of formation 300 

702 

Total 4,607 

Unconformity. 
Metamorphic gneiss. 

CAMBRIAN ROCKS. 

Distribution and subdivisions. ~T\xQ strata of this age are an impor- 
tant element in the geology of the region. They cover a wide extent 
of country and determine its topographic character, and to a certain 
extent the nature of the vegetation. The formations composing it have 
influenced the character of the mountain folding and determined the 
site and nature of the numerous and great intrusions of igneous rock 
which compose the most prominent mountain masses, and upon whose 
existence some, at least, of the ore deposits are dependent. 

The fossils show that the Cambrian rocks of this range are of Middle 
Cambrian age. The area covered by them is indicated by a single 
color on the geologic map (PL XLl), and the rocks are grouped under 
the name of the Barker formation. The beds thus grouped show well- 
: defined lithological subdivisions, and in the ranges to the south and 
in the Yellowstone Park have been subdivided into two formations. 
This subdivision is, however, unsatisfactory, and since a careful study 
of the fossils found by Mr. Walcott shows that the forms are all Middle 
Cambrian species, the writer has divided the rocks of this age into the 
following formations: 

7. Yogo limestone. 
6. Dry Creek shale. 
5. Pilgrim limestone. 
4. Park shale. 

8. Meagher limestone. 
2. Wolsey shale. 

1. Flathead sandstone. 






(DOLE CAMSRIik 



WBKD.] AMEBIAN BOCKS. 285 

The character and thickness of each of these formations are repre- 
sented graphically oq PI, XL, where colamnar sections measured at 
various localities in the range are shown. 

Flathead sandstone. — ^The base of the Oambrian is formed by a quartz- 
ite, which in the Little Belt Mountains is somewhat fissile, impure, and 
shaly in the middle, so that the formation consists of three members. 
The lowest bed is a granular but generally well-indurated sandstone, 
of a white, pink, or dark -red color, often cross bedded, and occurring 
in strata 3 to 10 feet thick. The base is often a conglomerate. The 
following section, made in the bluff of Belt Greek 8 miles south of 
Monarch, shows the composition of the formation. 

Section on Belt Creek 8 miles south of Monarchy Montana* 

Feet. 

Wolsey shale : 

FerruginonB sandstone carrying fossil remains 10 

Flathead sandstone : * 

Qnartzite; white \\ 

Sandstone; mst colored and rotten 5 

Sandstone; dirty white to bnff; flaggy 15 

Sandstone ; fissile, impure, purple, and rast colored 90 

Sandstone; massive 1 

Qnartzite; flaggy « 6 

Qnartzite; vitreons, hard, massive, knotty, neither well bedded nor fissile.. 60 

Augite-syenite sheet; 70 feet thick. 

Sandstones; dark red and ferruginous 25-^50 

Intrusions are frequently found elsewhere in the horizon here 
occupied by the syenite. The upper sandstone, a white granular 
rock, weathering with a pitted, pockety surface, covers Belt Park, and 
is exposed in the little gullies that indent its surface. On Sawmill 
Greek and along the O'Brien Greek road the basal bed was not measured, 
as no good exx)osures were observed, but its thickness was estimated to 
be 50 feet. A sheet of porphyry 50 to 60 feet thick is intruded between 
this lower bed and the higher qnartzite. The latter differs in appear- 
ance. The rock is not so distinctly bedded, and shows rounded, sphe- 
roidal weathering. It is well exposed near the Sawmill and generally 
over O'Brien Park. 

Wolsey shale. — The qnartzite is overlain by shale, which is well exposed 
in Keegan Butte and the hill south of it which rises above the open 
and nearly level surface of Belt Park. The shale is dark gray or 
greenish, often micaceous, and carries oval concretions of limestone a 
few inches thick and seldom over 6 inches long. These concretions 
contain fossils which Mr. Walcott has identified as Middle Gambrian 
forms. These shales average 150 feet in thickness, and are well exposed 
at. the old dam on Sheep Greek near Wolsey. 

Meagher limestone. — The summits of Belt Park buttes are capped by 
thinly and irregularly bedded limestones. The rocks consist of pure 
gray limestone mottled with patches of buff-colored, arenaceous, clayey 
matter. The exposed edge of the beds shows wavy — almost crinkled — 
bedding planes. Over 60 feet of these beds are exposed on Keegan 



286 GEOLOGY OP THE LITTLE BELT MOUNTAINS, MONTANA. 

Batte. The lower strata carry no distinguishable fossils, and weather 
into very small, irregular, gravelly debris. The upper beds are spotted 
with green glanconite grains and contain numerous fossil fragments. 

Park shale. — The greater part of the Cambrian rocks seen in the 
mountain area probably beloug to this formation. The lower strata 
are gray or greenish micaceous shales. Higher in the section these 
contain intercalated thin layers of limestones, which are impure and 
often consist of flat limestone pebbles — a true intraformational con- 
glomerate (PI. XXXIX, A). These beds are well exposed in the road 
cuttings at the head of Sheep Greek, in the valleys of Dry Wolf, Pil- 
grim, and Tenderfoot creeks, and near Barker. Their thickness is 
estimated at 800 feet 

Pilgrim limestone. — Above the Tenderfoot shale, whose ready weath- 
ering gives gentle slopes and generally open but deeply stream-trenched 
valleys, limestone beds form low cliffs or cap mesas on the broad sum- 
mit levels of the range. These limestones are somewhat massively 
bedded. They are gray, carry fossil remains, and in this region do not 
show the mottled appearance common to rocks of this horizon in the 
southern part of the State. The*basal beds are limestone conglomer- 
ates separated by thin layers of shale (see PI. XXXIX, B). 

Dry Creek shale. — Between the. beds of massive Pilgrim limestone 
and the dark chocolate-colored beds of the Jefferson formation there 
is a thickness of 40 to 50 feet of brick-red and bright-yellow sandy 
beds, whose fissile nature determined their designation as shales. The 
formation is a very constant one throughout central Montana, but 
owing to its ready weathering is seldom well exx)08ed. Good sections 
were observed at the head of King Greek, near Togo, in Big Park, 
on Belt Greek above Monarch, and on Pilgrim Greek, the average thick- 
ness being 40 feet. 

Togo limestone. — ^This formation generally consists of thin-bedded 
limestone flags, alternating with crumbly gray or greeuish shale, but 
grades into rather pure thick-bedded limestones. The entire section 
is well shown at the head of Sheep Greek and south of Monarch. A 
measured section made 8 miles south of that place is given in the 
description of that district (page 363). 

Fossils. — These Gambrian formations have all been grouped as the 
Barker formation for the purpose of areal mapping. In former publi- 
cations the series has been divided into two groups, the upper, or Gal- 
latin, and the lower, or Flathead. This distinction was originally based 
upon the occurrence of the Pilgrim limestone, whose resistant nature 
and peculiar mottled character made it a very convenient horizon to 
use in areal mapping. The fossil remains seemed to indicate the Upper 
Gambrian age of the beds above this, but larger collections recently 
made by Mr. Walcott prove that the Upper Gambrian fauna is wanting 
in all the collections thus far made, the forms studied being all of 
Middle Gambrian types. The Lower Gambrian (Georgian Olenellus) 



wiBD.] CAMBRIAN BOOKS. 287 

and XJpx>er Oambrian (Potsdam) are both wanting. The sections shown 
on PI. XL represent the strata of this age in the Little Belt Moan- 
tains, the Castle Mountain or Livingston sections being given for 
comparison. 

Fossils are abundant in the limestone concretions of the Logan shale 
of Keegan Butte and in the thin-bedded limestone of the Wolsey 
shale at the head of Sheep Greek. The Pilgrim limestone at the head 
of King Greek also contains fossils, as do the Yogo limestones near 
Togo settlement. The brachiopod species are few in number, and are 
associfited with Ptychoparia^ ConocorpphcBj and fragments of other 
trilobites, and an abundance of Eyolithes. Mr. Walcott, who has 
examined the collections, but not yet determined all the species, does 
not find any considerable differences of fauna from top to bottom. The 
following species have been identified by him : 

Dicellomas nanus M. and H..Pil^im limestone, Yogo; Wolsey shale. Sheep Creek; 

Flathead qaartzite, near Monarch and on Sheep Creek. 

Obolos (Lingnlella) ella Wolsey shale, Sheep Creek. 

Orthis remnioha Yogo limestone. 

Syntrophia primordialis Yogo limestone. 

Billingsella coloradoensis. .. Yogo limestone. 

Hyolithes primordialis Wolsey shale and Meagher limestone; Sheep Creek and 

Keegan Batte. 
Ptychopariagallatinensis... Pilgrim limestone, Pilgrim Creek. 

Ptychoparia llanoensis Yogo limestone. 

Ptychopariabipunotata Yogo limestone. 

Ptychoparia affinis Yogo limestone. 

Ptychoparia roemeri Meagher limestone, Fonrmile Creek. 

Agnostas sp. 
Obolas (Lingnlepis). 

Olenoides serratns Wolsey shale. Sheep Creek. 

Camarella Wolsey shale, Sheep Creek. 

Bathynriscns Wheeler Wolsey shale, Sheep Creek. 

These forms are all regarded as Middle Oambrian by Mr. Walcott. 
They show local grouping, and very often individual beds are made up 
of an aggregate of one species. This is especially true of Hyolithes 
in the limestones of the Park shale and Wolsey shale, and of the 
Ptychoparia gallatinensis of the Pilgrim limestones of Pilgrim Greek. 

SILURO-DEVONIAN ROCKS. 

Jefferson limestone. — Upon the geologic map the Jefferson formation, 
under the name of Monarch formation, is grouped with the Threeforks 
shale, which carries Devonian fossils. .Overlying the soft shales and 
associated limestones of the Cambrian there is a series of generally 
dark-colored, well-bedded limestones, whose lowest bed frequently 
forms a bold bluff or escarpment that rises abruptly above the shale 
slopes. The distinction is therefore usually a marked one in the topo- 
graphy, and is readily followed in mapping. This limestone bed is 
the basal member of the Jefferson limestone series. 



288 GEOLOGY OF THE LITTLE BELT MOUNTAINS, MONTANA, 

As a whole the Jefferson formation is characterized by chocolate-brown 
or steel-gray crystalline limestones, generally haviug a distinctly grann. 
lar or saccharoidal texture, which is especially noticeable on weathered 
surfaces. The rocks occur in beds 2 to 6 feet thick, ^hich are jointed, 
and weather like regular courses of masonry (see PI. XLII, A), 
Slight differences in hardness result in a pitting of the surface with 
sac-shaped cavities, due to weathering, and the edges of the beds often 
show ribbing and filagree work. The dark color is due to organic 
(nitrogenous) material, and the rocks emit a strongly fetid odor when 
struck with a hammer. The chocolate-colored beds are often mottled 
with light cream-colored patches, which in some instances are clearly 
recognizable as coral remains. The ledges near Barker show this 
especially well, as illustrated on PI. XLII, B. 

The only fossils collected from these rocks are corals. Mr. Charles 
Schuchert has identified Diphyphyllum ccespitosum Hall. In a report 
furnished the writer he says: << If this identification is correct the rocks 
are of Silurian ( Upper Silurian) age. This genus, however, like Atrypa, 
is not always of great value as a horizon marker." Later collections 
f^om this horizon contained specimens of Stromatopora^ Pachyphyllum 
(near woodmani H. and W.), and Aeervularia^ identified by Dr. Girty, 
and regarded by him as determining the age as Devonian. Up to the 
present no positive identification of Silurian rocks has been made in 
Montana. Frequent references to Silurian strata are made in the 
Hayden Survey reports, but the rocks therein called by this name are now 
known to be Cambrian. The uppermost beds of the Cambrian, the 
pebbly beds of the Togo limestone, contain fossils which were formerly 
regarded as possible Silurian, but the only paleoutological collections 
as yet thoroughly studied, those of the Yellowstone Park, prove to be 
Middle Cambrian. The nearest strata of undoubted Silurian age are 
those of the Bighorn Bange of Wyoming.^ 

The Jefferson limestones contain much arenaceous matter, and at 
several localities elsewhere in the State grade into quartzite and sand- 
stone. This is the case at Whitehall and Phillipsburg, Montana. At 
the latter place a collection of fossils showed the following species: 

Camarot(Bchia sappho. 
CamarotoBohia near C. oongregata. 
Glyptodesma rectum? 
Avicnlopecten sp. 
Cyatbopbyllum sp. 

In a letter to the writer. Dr. Girty makes the following statement 
about the fossils determined by him from this locality: 

Although comparatively little has been ascertalDed in the way of certain and 
exact Bpecific identification, yet I believe the horizon here represented can be 
referred with some certainty to the Middle or Upper Devonian. Avicnlopecten has 
not been recognized in this country below the Devonian, but is abundant in deposits 
of Devonian and Carboniferous age. Camarotachia sappho first appears in the 

1 C. E. Beecher, on the ocourrence of Silnrian strata in the Big Horn Mountaina, Wyoming, and in 
the Black HiUa, South DakoU: Am. GeoL, VoL XVIII, 1896, pp. 31-33. 



WEED.] CARBONIFEROUS ROCKS. 289 

Hamilton, but is known to extend into the Waverly. C oongregaia is likewise a 
Hamilton form, as is also Glyptadesma rectum. This faana can scarcely be earlier than 
Devonian, and I believe it represents middle or late Devonian time. 

ThreefarJcs shales. — ^The lighter-colored shaly limestones which immedi- 
ately overlie the dark-colored Jefiersou beds are known by this name. 
In the Little Belt Kange the formation has not been distingaished from 
the strata which immediately overlie it, as the latter rocks are identical 
in lithological character, and were in the field mistaken for this for- 
mation. The beds cover the surface of the bench so often made by the 
massive limestone bed forming the top of the Jefferson. In this region 
the formation, if present, grades into and can not be distinguished from 
the upper beds of the Jefferson. On King Creek Mountain the shaly 
beds immediately above the typical dark-brown Jefferson limestone 
hold Carboniferous fossils, and this is also the case in the exposures a 
few miles east of Barker. The beds usually weather readily and the 
actual contact of the two formations is rarely seen. At Monarch the 
debris of the cliff's hides the lower beds, the interval being estimated 
to be about 60 to 75 feet. 

It can not be asserted that the Threeforks shale is wanting over 
this region, but the collections from the shaly beds immediately over- 
Ijring the typical Jefferson limestone contain Carboniferous fossils at 
several localities. In the vicinity of Livingston and near Threeforks 
the formation contains an abundant and typical Devonian fauna. 
In the description of Castle Mountain,^ immediately south of the Little 
Belt Mountains, the limy shales at the base of the Madison limestone 
group was called Devonian, though no fossils were obtained from the 
beds; it now seems more probable that they represent the Paine shale 
of the Carboniferous. 

CARBONIFEROUS ROCKS. 

The Carboniferous rocks are the mountain-forming strata of the 
range. Their occurrence sharply defines the mountain area from the 
surrounding plains, and determines the rough and craggy character of 
the scenery of its border. Fossil remains, which are abundant through- 
out the series, show the rocks all to be of Lower Carboniferous age. 

The collections afford some evidence upon which to subdivide the 
series by faunal groupings. There is, also, a very marked grouping of 
the beds shown by lithological characters. The lower series, embracing 
a thickness of 1,000 feet of beds, is composed entirely of limestones of 
varying character. This is the Madison limestone of the geological 
map, a formation that is very persistent in occurrence and very uniform 
in character throughout central Montana. This limestone terrane is 
overlain by a series of shales, sandstones, and limestones, collectively 
known as the Quadrant formation, and the equivalent of this formation 
is developed in the southern part of the State, though presenting a 

very different development in this area. 

  

1 Ball. U. S. GeoU Survey Na 138, p. 38. 
20 GBOL, PT 3 19 



290 GEOLOGY OP THE LITTLE BELT MOUNTAINS, MONTANA. 

HADIBON GBOUP. 

This embraces three lithologically distinct horizons, viz : 

3. Castle limestones. 
2. Woodhnrst limestone. 
1. Paine shales. 

Paine shales, — These are dark-bine, qaite impnre, argillaceous lime- 
stones, which might be classed as calcareoas shales. The fresh rock is 
generally dark bluish-gray, but lighter-colored beds alternate with this* 
Upon weathering, the rock breaks up into flue shaly debris of a pink, 
buff, or straw color. The exposures of the lower beds form typically 
reddish or pink debris, and the weathered surfaces show an abundance 
of fossils, usually weathered in relief. The beds higher in the series 
and near the top contain fewer fossils, but the forms are silicifled, beau- 
tifully preserved, and project above the smooth weathered surface in a 
very striking manner. This silicification appears to be a very constant 
feature of this particular bed throughout the mountain region, and 
when carefully looked for has always been found by the writer. These 
strata show 6 to 30 inch beds of rather pure limestone, containing chert 
lenses one-half inch by 3 inches, separated by 3 to 10 foot beds of light- 
buff, varying to pink and dark bluish-gray limestone shale. The total 
thickness at Monarch is 175 feet. The ready weathering of these shaly 
beds gives rise to topographic depressions and vegetation bands, so the 
horizon is easily determinable (see PI. XLIII; Aj Monarch exposures; 
Bj Barker exposures.) 

This limestone is especially distinguished by an abundance of Bry- 
ozoan remains. The following table gives the list of species identified 
by Mr. Charles Schuchert, of the National Museum. The numbers 
refer to localities from which collections were made by the writer. This 
formation is undoubtedly the same as that called by Dr. A. O. Peale 
the Laminated limestones of the Threeforks section.^ 

FosaUsfram the Paine shales. 





MS. 


801. 


294. 


003.' 604. 

1 


660. 


668.|680. 


646. 


608. 


AnloDora sd. .•■.••••••••••.••• 


















Snlrorbis 2 sd..... 






















Fenfwtellii (mverAlnwo468>T ,,,.,.^ r.. 


X 
X 


X 
X 


• • • • 

• ••• 


X 
X 


w... 


X 


X 
X 








Cvstodictva 8D .••.•.......•••.............. 








Streb lotrv Dft sd 












Pinna toDora sd.. 


X 
X 


X 








X 










OrAnla ((ftrfftt<4 rd^m^Im) t T-n- ..-.,.,-,^-,, 
















Bblpldomella mlchelinl l*fir . .....*............r... 


X 


X 




X 






X 






Orthotbeteslnflatns White and Wbttf. 


X 










Cbonetes bd..... 










X 


' 








Prodootus bnrioofite White 


X 




B M M * 



































iBoIL U. 8. GeoL Snirey ITo. 110, 1803, p. 38. 



I" ll 



: j 



292 GEOLOGY OF THE LITTLE BELT MOUNTAINS, MONTANA. 



The following table shows the fossils collected irom the Woodhurst 
limestone in this region. The identifications have been made by Mr. 
Schuchert. 

Fossils from the Woodhurst limestone. 





1 

545. 


1 

547. 


553. 


554.1 


1 
558. 


561. 


1 

577. 


585. 


593. 


597. 


609. 


650. 


649. 


648. 


401. 


1 
PLftlvorinnB an . .............! 


\ 




1 
1.. ..!.... 


1 


1 

1 


Tflnnioflirt vA raninlofla T71 .......... 








t 


.' 










. ... 


1 






l^homlio'Dora dicbotomft Ul ...... .... 




[ 1 












.... 




X 




Khipidomella (small micbelini 
r^v 


1 





1 

1 


X 


.... 






t 









1 

RpliiKonlioriR A^rallovi Ha.ll. -. 


1 












X 
X 








- 


Orthothetesinflf.tus? White and 
Whitf 




X X 


X 


"**'p 1 

1 ! 
X 1 X : 


X 


X 


« » • « 
. _ . _ 


1 

y\ 1 . . . . 

I 


X 


.... 


T)Arliva an. undt ............... - 


.... 




1 

1 








dionntAfi iIlinoia(>nfiiA W^orthAm 






i 








1 


1 


X 


X 

X 
X 

« • «  


Chonotes loganeDsis H. and W . . 
Productella ahnmardiana Hall . . 


X ' — ! X 
, i 


1 . 

X ' X — X 

...J 


• •  • 


>' 


« a • - - - 





X 


T^roductQS cor& crrouD. «.*..... • 


.... 


. 1 

1 


. 






1 


Productufl IflBvicosta White 

Prodactua of. setigems or Roa- 
l>ridiliiB. ...................... 


X 




1 
....1- -1 -- -1 


. ... 








^ f • • « • 

1 


X 






X 


— —  X 










Productus cf. gallatineDsis n. sp. 
SDirlfer cf. biolicatus Hall 














! 


X 


X 
X 

X 








I 


1 










1 




Snirifer centronatQS WiDchell. . . 




X 


1 


X 


X 


X 


X 


X 


— 


— 


X ' X 

1 


X 


SDirifer triironalis Martin 




Reticularia ii. bd ................ 
























X 
X 
X 


Keticularia seticera Hall 




























Martiuia rostzlita n. sp 








" 




















SyrinsothyriB of. extenuatus 














X 














SDiriferina cristata Schlot 




























X 


Spiriferina transversa McChes- 
-ney ............................ 




X 


X 


X 


— 


— 


X 
X 


— 


X 


— 


X 




X 


— 


Sniriferina solidrostris White. . . 




Seminnla trinnclens Hall 

Cleiothyris orassicardinalis 
White '. 


— 




— 


^■* 


X 


— 


— 


— 


X 


.... 


— 


— 


X 
X 


X 

• • • * 


£ametrla marcyl Shnm 








X 
X 


X 
X 






f 










Camarotcechia cf. metallica 
White 


— 


— 


— 


X 






. 




X 




Plat vceras sp 
















' Biiomphalus luxus White 






X 




X 






X 














Proetns peroocidens Hall and 
Whitf 






1 








X 



































The nambers at the hoads of columns in the above table refer to localities at which 
collections were made. The specimens are now in the National Maseum and may 
be identified as to locality by the numbers. The dashes in the table mean that 
species are probably present at the points indicated, though they are not represented 
in the collection brought in. 

A comparison of the two tables shows that a number of species 
occurring in the Paine shale have not been found in the overlying lime- 
stone, viz : 

Fenestella, several species. 
Crania, striated species. 
Spirifer striatus Martin. 



T cnEEK CANYON BELOW MONAR 



WBED.] MADISON GBOUP. 293 

Spirifer desideratns Walcott. 

Cyrtina. 

Cleiothyris. 

Camarophoria cf. C. explanata McCheaney. 

Pagnaz mutata Hall. 

CamarotcBchia cf. C. metallica White 

GasteropodB, 5 species. 

Eiiomphalas spergenensis Hall. 

Bacanopsis textilis Hall. 

Conularia sp. 

Of the^e, GUiothyris hirsuta seems to be especially characteristic. 
Reticularia setigera extends into the basal beds of the Woodhurst lime- 
stone. The collections from this basal bed of the Woodhurst embrace 
seven species not found at any other locality. The species common to 
both formations are the most abundant types and comprise the bulk of 
the collections. The faunal differences may be due entirely to the 
character of the rocks. The Paine shale was a calcareous mud depos- 
ited in shallow waters, while the overlying limestones are very pure 
and indicate clear waters. 

The collections, as a whole, are referred by both Schuchert and Girty 
to the Kinderhook or Chouteau. This is entirely in accord with the 
results obtained by them from a study of the fossils from the vicinity of 
Threeforks, described by Peale,^ and from a study of those of the 
Carboniferous areas near Livingston collected by the writer. 

Castle liinestone. — The uppermost formation of the limestone series is 
a limestone that is very massive and shows no bedding. The rock is 
generally dense and rarely crystalline, always light colored, and in expo- 
sure shows inconspicuous division planes. It weathers with a rough 
cavernous surface, forms castellated craggy masses, and is easily dis- 
tinguished from the well-bedded rocks beneath it. The type of erosion 
is illustrated on Pis. XLIV and XLY. Its massive homogeneous 
character causes it to form very narrow canyons, and it is the '^ gate- 
way " rock of the mountain streams. It is frequently crushed and brec- 
ciated when folded, and in such cases often stained red by iron oxides, 
which are sometimes so abundant as to cause prosx>ectors to locate 
claims. The rock contains dark-brown chert scattered through it in 
round and lenticular masses, sometimes a foot in diameter, but the 
chert is light colored and not abundant enough to be conspicuous in 
most exposures. The beds cap the great clifi's of Belt Creek and are 
everywhere prominent upon the mountain flanks. The formation 
takes its name from the town of Castle, where it is especially well 
developed. The thickness measured near Monarch is 375 feet, and at 
several localities throaghout the mountains it is 300 to 400 feet. 

The following section represents the typical development of the Madi- 
son group in this region. It does not embrace the base of the Paine 
shale, and it is possible that a few feet may be wanting at the top. 

> Bull. r. S. Geol. Survey No. 110. 



294 GEOLOGY OP THE LITTLE BELT MOUNTAINS, MONTANA. 

This section should be compared with that, given with the description 
of the Monarch districti of the clifi's near Monarch. 

SeeUan ofCarhanfferouB Umeai&nes exposed in cliffe north of Dry Wolf Cretk, 1 vdU above 

Spring CouUe, 
Castle limestone: Feet. 

Limestone ; brownish gray in color, often conglomeratic 5 

Limestone; cream colored, blotched with pink 2 

Limestone; massive, bat brecciated in places 115 

Limestone; gray, somewhat massive, and forming bench on summit of 

buttress spars 30 

Well- bedded limestone, dark bine in color, weathering to baff gray; carry- 
ing chert bands of one-half inch to 2 inches in thickness at 40 feet above 
base ; generally heavy bedded with vertical Jointing and breaking into 

small prismatic pieces 170 

Limestone ; gray, massive, belonging to same series as that above 10 

Woodarst limestone: 

Limestone ; quite massive, heavily bedded or without recognizable bedding, 
weathering into rude prismatic blocks and forming buttresses projecting 
from steep slopes north of creek. The upper 75 feet shows rude bedding, 
with chert bands and drusy cavities scattered through the rock. Crinoid 

stems are abundant and other fossils are occasionally seen 250 

Limestone; gray, quite massive 15 

Limestone; massive, with indistinct bedding planes and rude prismatic 

fracture and irregular surface, and carrying scanty crinoid remains 60 

Limestone; thick and thinly bedded, quit« massive in exposure, breaking 

into splintery fragments and showing occasional fossils 65 

Paine shale : 

Thin bedded limestone, brownish or buff color on weathered exposure, with 
argillaceous layers of fissile limestone 2 to 6 inches in thickness between 
the purer and more massive beds. The rocks form buttresses with square 

Jointing, vertical faces, and a masonry-like appearance 85 

Thin-bedded, shaly limestone 10 

Limestone ; gray, weathering with rough surface and forming ledge above 

talus slope 8 

Limestone; thin bedded (4 inches to 2 feet), alternating with shaly lime- 
stone beds a few inches in thickness 27 

Debris concealing base of exposure and running down to creek 80 

Total 932 

QUADBA17T GBOUP. 

Above the massively bedded white limestones, which nearly every- 
where form the flanks of the range, there is a series of sandstones and 
shales with intercalated limestones that weather readily and form the 
narrow foothill hollows between the mountain slox>es and flanking hills 
of sandstone. This series is collectively named the Quadrant forma- 
tion or group, as it corresponds in stratigraphic position and in the 
changed conditions of origin which it represents to the formation 
given that name in the southern part of the State. In these moun- 
tains the shales and limestones of the formation contain an abundance 
of fossils which belong to the Lower Carboniferous, and these forms 
extend through the formation up to its very top. There is, however, 
evidence of an unconformity between this and the succeeding Jurassic 
formation. As noted above, the beds occur chiefly as an encircling 



I. BELT CREEK 



w«K».] QUADRANT GROUP. 295 

belt aboat the monntains, bat inliers formed by synclines also occar 
within the moantain tract at Bear Park and between projecting ridges 
of the range at Lone Tree Park and the divide west of Taylor Peak. 
The Quadrant group consists within the Little Belt region of two sets 
of beds. 

Qaadraptgroap: "" Feet. 

Otter Bhale' 300-600 

Kibbey formation 115-150 

The total thickness is 313 feet on Dirty Greek, 303 feet in Musselshell 
Oanyouy 453 feet at Biceville, and 1,400 feet near Utica. 

Kibbey sandstone. — The basal member of the Quadrant group is a 
sandy formation which is commonly brick-red in color, is seldom well 
bedded, and weathers quite readily. Where best exi)osed it is seen to 
consist of slightly indurated sandy clays, prevailingly red in color, 
with lumps of yellowish clay scattered through it. This grades 
upward into a well-bedded, somewhat fissile, red and purple sandstone. 
These beds are best developed near Kibbey and Biceville, where they 
contain intercalated gypsum beds. The thickness here is 150 feet. 

West of Utica, at the forks of the Judith, the lower clays are 75 feet 
thick and the sandstones 40, giving a total of 115 feet for the forma- 
tion. At the east end of the range, at Oka, the beds form an encir- 
cling hollow between the main mountain mass and an outlying limestone 
hill. In Musselshell Oanyon the formation is thin, the basal bed being 
but 15 feet thick. These beds immediately overlie the massive Castle 
limestones, and are therefore readily located, though seldom exposed. 
Near Woodhurst and north of Dry Wolf the beds are gray, var3dng 
to buff, in color. No fossils have been found in these beds. 

Otter shale. — ^This formation consists of green and gray shale, with 
intercalated limestones. The green shales form its most conspicuous 
feature. They are bright coppery green in color, and where well exposed 
are very striking features of the canyon walls. The interbedded lime- 
stones are thinly bedded, white or light gray in color, but are often 
markedly oolitic. They carry Carboniferous (Mississippianf) fossils. 
The variable nature of the formation is shown in the detailed section near 
Utica, given herewith, but the details differ greatly in different expo- 
sures, though the general character is constant. The thickness at Bice- 
ville is 300 feet; at Musselshell Canyon and Dirty Creek it is much less. 

Fossils. — The fossils from the Quadrant group are abundant at sev- 
eral localities, but only small collections have as yet been made. The 
collections firom Biceville are different in form from those of any other 
locality, the species showing a Spergen Hill facies. The following 
species fh>m this place have been determined by Mr. C. D. Walcott: 

Seminnla sabtilits HaU. 
Enmetria marcyi Shum. 
Pagnaz motata Hall. (Common.) 
Eapbemns carbonarins Cox. 



iHTamed in 1801. See Two MontaDa ooal fields : Boll. G«oL Soo. America, Vol. HE, 1892, pp. 801-880. 



296 GEOLOGY OP THE LITTLE BELT MOUNTAINS, MONTANA. 

Dielasma cf. formosa HaU. (Commou.) 
Semioala cf. trinuclea Hall. 
Eumetria yemeuiliana Hall. 
Spiriferina of. cristata Schl. 
Allorisma sp. nndet. (Like marionesis.) 
SchizoduB curtiformiB Walcott. 
Euphemas sp. andet. 
Lophophylluin sp. f 
Stenopora sp. f 

The following species, identified by Mr. Charles Schnchert, were col- 
lected from the Forks of the Jadith, west of Utica: 

Cy therella sp. f 

Anomphalas sp. f 

Orbiculoides sp. f 

Derbya sp. undet.' 

Seminula cf. trinuclens. 

Prodaotus semireticalatus Martin. 

Spirifer keokuk Hall. 

Nucula like parva bat larger. ^ 

On the southern flank of the range from Dry Fork of Belt Greek fos- 
sils obtained from the fissile limestones but a few inches below Jurassic 
fossils show a fauna differing from any found elsewhere. Mr. Schu- 
chert says: ^'The evidence, however, is such that no definite age can be 
assigned to this fauna closer than Carboniferous, and apparently Lower 
Carboniferous.^' 

Productus semireticalatus Martin, 
Seminula sp. undet. 
Dielasma sp. undet. 

SECTION ON JUDITH RIVER NEAR UTICA, 

The following section, which was measured in the foothills west of 
Utica on the Judith Basin, near the Sapphire mines, represents the 
lithological characters of the formation. Owing to several landslips, 
there is a little doubt about the thicknesses given for the upper part of 
the section, which may be excessive. 

Section of Carboniferous shales, Jurassic and Kootanie heds, eaq^ossd on north side of 

Judith Eiver near Utioa, 
Kootanie : Feet. 

Black shales f 

Red earths, alternating with 3-foot beds of sandstone 100 

Sandstone, massive and everywhere prominent 40 

Red earths f 

Buff sandstone 3 

Red earths 50 

Sandstone 6 

Sands 50-75 

> Very abundant. A similar form oooorrlng in the St. Louis formation is often identified with the 
Upper CarboniferoiiB 2>. crasaa. There are differences, however, between the two forms. 




COLUMNAR SECTIONS OF THE FORMATIONS OF THE QUADRANT GROUP. OF LOWER 
CARBONIFEROUS AGE. IN CENTRAL MONTANA. 



WEED.] QUADBANT GROUP. 297 

Jurassic : Feet. 

Limestone ; white, earthy 5 

Red earths and shales 55 

Limestoue changing to sandstone; sometimes a conglomerate; carries 

fossil gryphseas 20 

Sandstone ; fissile, breaking into plates one-half to 1 inch thick 3 

Sandstone ; brown ; contains typical Jurassic (marine) fossils 15 

Carboniferous-Triassic : 

Sand and red earths ? 

Limestone ; white 3-5 

Red earths 

Sandstone; white, with local lenses of earthy, light-bnfi;' limestone and 
gypsum, qnartz crystals, and locally with conglomerate and 6-inch balls 

of banded chert 25 

Red sandstone 30 

Limestone; thickly and thinly bedded, hard, dense, gray to dove-colored 

rock ; not crystalline 30 

Limestone ; massive bed of rough weathering, gray limestone 3 

Limestoue ; debris shows minute gasteropods 65 

Limestone; thin bedded * 5 

No exposure 25 

Sandstone ; red, earthy, fissile 2 

No exposure 8 

Sandstone; granular, friable, soft rock with massive, round-weathering 

outcrop 10 

Sandstone ; red 6 

No exposure 200 

Limestone ; marble-like and weathering into reddish-colored bits 200 

Limestone ; yellow, weathering with rough, prickly surface 2 

No exposure. Soil holds small masses of a purple-oolored limestone, break- 
ing into fine debris 140 

Dark-red sandstone ; locally a conglomerate, generally red 55 

Limestone ; white to dove colored, shaly, breaking in plates a foot square 
and one-fourth inch to 2 inches thick, weathering pink or lavender 

colored, and into red earths. Carries minute gasteropods f 125 

Sandstone; granular saccharoidal, buff to yellow, varying to white and 

pink, or red at top. Dip 10^' ; forms bench 45 

Shales; thin-bedded gray shale and impure limestone; drift of massive 
white limestone, breaking into Jt>locks 3 feet square, covers surface, but 
was not seen in place. These beds, of which 100 feet are exjKised in 

gully, resemble the Jurassic 200 

Limestone 7 

Shale ; dark gray, calcareous 25 

Shale; light green 10 

Shale ; purple 4 

Shale; greenish white 1| 

Green shale series : 

Minette sheet, with 1 foot of altered shale at contact. 

Green shale 10 

Oolitic limestone 1 

Green shale and purple shale 15 

Hornstone, or dark-colored altered limestone, weathering light 2 

Minette sheet 10 

Shale ; green j 15 

Limestone ; white and massive 5 



298 GEOLOGY OF THE LITTLE BELT MOUMl^AINB, MONTANA. 

Green shale series — Continued. Feet. 

Shale; hlaok 1 20 

Limestone; pearl gray, dense and compact 3 

Shale; greenish gray, soft and ommbly 20 

Limestone; dense, Jasper-like in texture, massive 6 

Limestone; buff, with earthy yellow staining 4 

Shale; crumbly and breaking into fine bits 22 

Limestone; white and massive 3 

Shale; not exposed 25 

Limestone; white, massive, and dense, very hard and rarely crystalline.. 12 

Shale; soft, gray, and earthy 15 

Limestone 5 

Shale; fissile or in plates one-half to 1 inch thick; gray-colored shale, 

alternating with soft, more earthy shale, with harder scalloped layers. . 75 

Sandstones; light-purple, gray sandstones and sandy earths, with shaly belts 

of 6 inches to 2 feet 40 

Bed (magnesian) earths, mottled and spotted one-half inch with green. Bep- 
resents knotted or lumpy beds seen along base of Little Belt Mountains and 

at Oka 75 

Massive white limestones. 



4 



OHAPTEE III. 

DESCRIPTIVE GEOIiOGY OF THE SOUTHERN AND JUDITH 

AREAS. 

INTRODUCTORY. 

In the following pages a detailed description of a large part of the Lit- 
tle Belt Range is given. A very brief summary of these facts has 
already been presented in the general account of the range and of its 
rock formations, but it is believed that the detailed descriptions will be 
useful to those who may desire to know more of the geology of particu- 
lar parts of the range, especially of those districts where mineral 
resources are from time to time reported. For this reason the descrip- 
tions, though summarized from notes taken in the field, are yet some- 
what full, particularly as an effort has been made to make the account 
of each district complete in itself^ necessitating an occasional repetition 
of facts previously mentioned. The observations on which the descrip- 
tions are based were made in the summers of 1893 and 1894, and during 
brief visits to Neihart in 1895 and 1897, while the mapping of the areal 
geology of the region was in progress. 

For the purposes of description, the region has been somewhat arbitra- 
rily divided into five districts, of which four have been named from the 
prominent settlements of the area. The first embraces the entire south- 
em part of the mountains, in which there are few prospects and no pro- 
ductive mines. It includes the country drained by the Judith River and 
by Sheep Creek. The four other general divisions made are designated 
the Yogo, Neihart, Monarch, and Barker districts. The subheadings 
given in the description and in the table of contents will inform the 
reader as to the district in which a peak or valley has been placed, if 
in doubt. 

SOUTHERN PART OF RANQB. 

The relatively simple structure and broad topographic features of the 
southern part of the mountains have led to its treatment under one gen- 
eral heading. In this part of the range the relation between topographic 
form and geologic structure is most strikingly illustrated. The moun- 
tains are parts of the great broad and flat fold of the range, cut . by 
streams and erosion into individual summits. The gentle slopes of the 
plateaus of the Judith and those of the higher mountain ridge forming 
the east end of the range follow the inclination of the easterly dipping 

299 



I 



300 GEOLOGY OF THE LITTLE BELT MOUNTAINS, MONTANA. 

beds of Paleozoic rocks which form their surface. The broad sheets of 
these limestones are seen covering the summits or lying in overlapping 
plates upon the outer flanks like the shingles on a roof. The region 
shows no evidences of glaciation, not even by those small and local ice 
pockets so common in the Kocky Mountain region during Pleistocene 
time. Therefore the topographic features and scenery are due to erosion 
alone, and there is a marked contrast between the areas where sofb 
rocks cover the surface and those covered by a harder, more resistant 
material. 

EAST END OF BANGE. 

The eastern end of the range loses the broad plateau character which 
prevails farther west, and narrows to an anticlinal ridge that termi- 
nates abruptly at Judith Gap, a few miles east of the limits of the area 
shown on the accompanying map, PI. XXXY I. This secondary range is 
alow mountain mass, whose dark forest-clad slopes and white limestone 
cliffs are in strong contrast to the surrounding plains. The highest 
summit is a flat-topped elevation whose westward-facing cliffs make 
the name Bluff Mountain an appropriate one. From this point east- 
ward the range declines very graduaUy. There are no sharp peaks, 
and the only individual summits are the two known as the Twin Peaks. 

Foothills. — To the south the mountain tract is flanked by a very nar- 
row belt of foothill ridges and an outer plain whose gently sloping, 
uniformly level surface extends southward to the meadows of the 
Musselshell River. North of the mountains the foothill tract is wider, 
the ridges forming pronounced topographic features and inclosing a 
basin of considerable extent. 

The inclined plain about the mountains is a feature common to most 
ranges of the State. It is underlain by soft and easily eroded shales 
and sandstones of Cretaceous age, leveled by the streams from the 
mountains to broad plantation terraces. Seen in profile this tract 
appears to consist of a number of terraces of different levels whose 
front is deeply trenched by lateral drainage ways and crossed by the 
larger mountain streams. 

Structure. — The structure is simple, this end of the range being an 
anticlinal arch whose axis pitches eastward, so that the range declines 
in height in that direction, and ends where the massive limestone beds 
pass beneath the plains. This structure may becompared to an inverted 
canoe whose prow forms the hills east of Twin Peaks. The rocks thus 
uplifted and folded are all of sedimentary origin, and in the mountains 
are all of Paleozoic age. Upon approaching the mountain area the 
slopes are seen to be sheathed with great curving plates of limestone, 
cut into scallops by the streams that trench the slopes. Traveling 
up any of the larger drainage ways, the strata are seen to dip away 
from the mountains. The inclination is slight, not over a few degrees 
upon the outlying terrace plain, but gradually increases as the moun- 
tains are approached, until it becomes 20^ to 30^ in the foothill tract. 



WEED.] SOUTHERN AREA. 301 

Continuing np the stream way, this dip increases until, in the narrow 
gorges ont in the limestones, it is generally 30^ to 40^. If one continues 
up the gorge, he will find that ibhe dip gradually lessens until the 
beds are nearly flat upon the summit, or dip at a low angle to the east- 
ward — that is, with the range and not away from its axis. The same 
structure will be found on the opposite side of the range, so that a cross 
section would show the structure to be that of a flat-topped arch with 
steeply sloping sides. 

The creek whose broad valley cut into the open plain is known as 
Hopleys Hole, shows in the foothill tract excellent sections of the beds 
which overlie the great limestone series. A section at this locality 
showed the usual development of the Quadrant formation and overly- 
ing Jurassic beds, but the overlying Cretaceous rocks which form the 
outer part of the foothills were not well enough exposed to warrant its 
subdivision into the formations recognized farther north. 

JHrty Creek section. — The stream known as Dirty Creek afibrds good 
exposures of the Quadrant formation, and the following section was 
measured: 

Section on Dirty Creek, 

Cretaceoaa conglomerate. Feet. 

Softer ahaly beds. 
Jurassic : 

Sandstone ; coarser, less firmly cemented, generally reddish ; forms surface of 

slopes and crest of hill 

Sandstone; yellow weathering, well bedded and cross bedded, carrying 

large gryph^eas 

Conglomerate (1 to 1^ feet), formed of chert nodules and limestone pebbles, 

arranged flat 

Carboniferous: 

Limestone; light brownish-gray, carrying black flints; dense, not crystal 

line, massive, 3 to 6 foot beds ; no fossils in lower 20 feet, but abundant 

in more fissile and shaly, yellow- weathering limestone forming upper part . . 35 
Limestone ; dark-blue, in 2 to 3 foot beds, alternating with 3 to 12 inch beds of 

shaly limestone, carrying fossils 18 

Limestone ; light-gray, heavily bedded, carrying dense black chert 10 

Shales ; light earthy colored and green, in part saudy 200 

Limestone ; black and shaly, changing to dense blue limestone above 60 

Shales ; with iuterbedded impure sandstones and red clays at the base 100 

Limestone; dense, massive, nearly white, not crystalline, carrying white 

chert in layers and nodules 15 

Massive Carboniferous beds. 

As the uplift of the range increases westward, erosion exposes the 
older nucleal rocks of the fold on the very summits west of Blufif 
Mountain. The thickness and relative positions of the different forma- 
tions are best seen in the stream gorges, but these are generally too 
narrow and precipitous for travel, especially where cut in the limestone 
series, and it is easier to follow the summit of the ridges. Upon cross- 
ing the rough outcrops of the Carboniferous, on which an abundant 
growth of pine flourishes, an open grassy sag or depression is found to 



36 



302 GEOLOGY OP THE LITTLE BELT MOUNTAINS, liONTANA. 

mark the presence of the shaly beds. From the base of the Madison 
formation to the summit of the range there are mountain meadows and 
groves of pine, and as one ascends ttie slope the low step-like outcrops 
of the dark-colored Jefferson limestone give place to the smooth slopes 
and rich herbage of the Cambrian shales. 

The strata on the summit have a gentle easterly dip of about 2^ to 3^, 
and this may be assumed to represent the pitch of the axis of the range 
at this point. The exposures of Oambrian shales capped by limestones 
in Bluff Mountain show, however, that the average pitch of the axis is 
1^ 30^ calculated on the basis of the known thickness of the strata and 
their elevation at the east end of the range. 

Bluff Mountain. — ^Bluff Mountain presents the most striking section 
of the general anticline to be seen in the range. Its steep western face 
shows several hundred feet of greenish micaceous Cambrian shales, 
with interbedded limestones, capped by the heavy bed of limestone which 
forms the cliff at the top and whose debris litters the slopes. Seen from 
the west these beds form an arch in Twin Peaks. The outlook from 
the summit is extensive and affords a comprehensive view of the entire 
southern and eastern portion of the range. To the west the broad plateau 
summit is notched by a depression cut through to the gray shales of the 
Belt formation, the nucleal core of the range. Beyond this the plateau 
character is more marked and continues westward for many miles. To 
the north the anticline is seen to be deeply cut by the South Fork of 
the Judith. The main stream penetrates to the Belt formation, but the 
resistant sandstone that forms the base of the Cambrian covers exten- 
sive tracts, forming a densely wooded plateau leveL Beyond the South 
Fork of the Judith the slopes show the exposed edge of the beds that 
underlie the Lost Fork Plateau, the beds dipping at an angle coin- 
ciding very closely with the slope of the plateau surfaioe. 

NEWLAND CBEEK HILLS. 

The low submountainous tract drained by Newland Greek is the 
southern extension of the Little Belt Bange. The topography is in 
strong contrast to that of the range proper, where the later Paleozoic 
rocks prevail. It is an ojien country, with well-rounded hills, gently 
modeled slopes, and broad summit levels. The streams cut the rocks 
readily and erosion is rapid. Over large areas the country is treeless, 
shows little or no soil and a scanty growth of grass. 

The Belt formation includes several horizons, and these are well 
developed in this region. The Newland limestones form cliffy border- 
ing the creek meadows, and the brick-red shales which locally form the 
top of the Belt terrane are well developed, weathering in gullies that 
separate the hills of drab shale from the wooded ridges formed by the 
outcrops of the Flathead quartzite. 

Igneous intrusions. — ^The shaly beds of the Belt formation are easily 
penetrated by igneous intrusions, but within this part of the range 



w»ED.] SOUTHERN AREA. 808 

only a few animportant masses of igDeons rock are found. Several of 
these are seenin the slopes bordering l^ewland Greek, their occurrence 
being shown on the geologic map. Kear the fork of the road on the 
north side of the creek the shales, which dip downstream at an angle 
of 18<^ to 20^, are penetrated by a small mass of hornblende-mica- 
porphyry. The rock is platy, occurs as an intrusive sheet conformable 
with the shales, and is apparently sheared by its upheaval with them. 
The porphyry is greatly altered. 

A mile below the point just indicated a white or straw-colored rhy- 
olite-porphyry is encountered, the debris of which forms a slide along 
the slope. The rock is much decomposed and shows little quartz crys- 
tals scattered through a fine-grained groundmass, and several small 
exposures occur on the southern side of the creek. The intrusive rocks 
do not disturb the general structure of the sedimentary beds nor 
produce any appreciable metamorphism of the inclosing shales. Farther 
down, the valley has been cut across the upturned Cambrian rocks. 
The basal quartzites are separated by a bed of shale into which a 
variety of granite-porphyry of rather fine grain has been intruded. 
The weathered surface is somewhat rough and pitted and shows only 
round quartz grains, but the fresh fracture shows glassy crystals of 
quartz an eighth of an inch across, small yellowish feldspars, and 
larger, tabular crystals of clear orthoclase a half inch or more across. 

^orth of the creek the Cambrian sandstones change their course, and 
the ledges extend along the slopes a few hundred feet above the creek. 
The overlying Cambrian limestones are cut through by the creek, the 
ledges dipping downstream. These rocks are intruded by a mass of 
granite-porphyry that does not disturb this general dip. The rock 
is fine grained, platy near the contact, and weathers in massive out- 
crops. Below this intrusion the limestones appear, with the same gen- 
eral dip of 250y the strike being N. 55^ W., or nearly at right angles 
to the creek valley. A small lateral drainage cutting into the west 
end of the ridge marks the approximate location of a fault which 
extends from the mouth of Fourmile Creek, north of Castle Mountain, 
along the limestone hills and ridges north of the Smith Biver Valley 
to this place. This fault, thou^ not so profound as the one that 
defines the northern border of Smith Biver Valley north of White 
Sulphur Springs, is yet an important element in both the geology 
and the topography of the legion, though the area afiected lies outside 
of the region under immediate discussion. Beyond the fault the lime- 
stones exposed on the slopes bordering Newland Creek are also of 
Cambrian age, but strike N. 20^ E., and dip at 10^ to 15^ upstream. 

North of Newland Creek Valley a broad, undulating summit extends 
to the borders of Sheep Creek Valley. This tract shows the gray 
shales and limestones of the Belt formations, which are intruded by 
sheets of porphyry a few miles south of the sharp, forested eminence 
called Coxcomb Butte (Black Butte). At the south base of this butte 



304 GEOLOGY OF THE LITTLE BELT MOUNTAINS, MONTANA. 

the rocks show local induration and are puckered aud wrinkled, but 
the general strike is east and west, and the dip 60^ to the south. The 
stratigraphic throw of the fault is at least 3,000 feet, as that thickness 
of beds is wanting between the quartzite and the Newland limestones. 
The fault must be as great as this, even if the quartzite is Cambrian 
and not Algonkian in age. 

SHEEP CREEK VALLEY AND VICINITY. 

Sheep Greek is the largest of the streams draining the western slope 
of the range and emptying into Smith liiver. The wagon road from 
White Sulphur Springs crosses the broad meadows of the low divide 
between Newland aud Sheep creeks, aud follows the latter stream 
through a broad valley to the pass across the range at the head of the 
stream. For several miles below this pass Sheep Greek flows over 
the bedded rocks in the direction of their dip, then, turning westward, 
has a course nearly coincident with the strike of the rocks. Below this 
broad strike valley the stream enters a canyon cut across upturned 
quartzite beds and descends with rapid grade through a narrow gorge 
cut in gneisses and schist.^ 

The geologic structure is as simple as that of the plateau country of 
the Judith. The stream follows a valley cut into a block of slightly 
tilted sedimentary beds, dipping southward from the parks of Belt 
Greek to the profound fault which bounds the mountains on the south, 
and brings up the lower formations of the Algonkian against Gambrian 
rocks. The strata have been eroded into connected but individual 
blocks aud the valley cut in soft shales of the Gambrian and Belt ter- 
ranes, whose underlying quartzites form the resistant beds that have 
determined the presence of the broad, park-like valley. There are sev- 
eral hay ranches in the bottom lands, at one of which the post-office of 
Wolsey has been established for many years. 

8M0KY MOUNTAIN. 

South of Sheep Greek dark wooded slopes rise steeply to the ridge 
whose highest summit is Smoky Mountain. This mountain block is 
formed of beds inclined southward at gentle angles, varying between 
30 and 50. The Gambrian quartzites exposed in the valley floor near 
Wolsey pass beneath the slopes, the overlying shales being concealed 
by the soil and vegetation. At 950 feet above Sheep Creek the low 
ledges of the brown and black Jeflerson limestones form low reefs 
extending along the slopes, and above them the more massive light-gray 
limestones of the Carboniferous are well exposed. The beds dip west- 
ward in the butte back of Kinneys, conforming to the gradual slope 
downstream of the quartzite beds of the valley floor. The summit of 
Smoky Mountain is capped by a flow of basalt, which extends down 

1 See Davis, Kelation of coal of Montana to older rocks: Tenth Census, Vol. XV, p. 708, Washington, 
1886. 



WEED.] SOUTHERN AREA. 305 

an easteru spur of the moantains to an elevation 400 feet or more 
below the Bummit, clearly proving the uuevenness of the surface on 
which the lava flowed. The rock is very dense, of a dark-gray color, 
and seems to be liberally sprinkled with minute dots of yellowish oli- 
vine and the spots of ocher left by the decomposition of that mineral. 
It is a normal basalt, very dense, and free from the vesicular cavities so 
often seen in basaltic lavas. Near the limestone contact the rock is 
lighter in color, banded, and shows considerable hornblende in acicular 
crystals and nests. 

The southern slopes of the mountain are more sparsely wooded and 
show better exposures than are seen on the Sheep Creek side. An 
intrusive sheet of porphyry occurs in the Gambrian shales 300 feet 
below the brown limestone horizon. This sheet of porphyry, which is 
estimated to be 100 feet thick, forms a prominent bench, and its outcrop 
is traceable along the smooth slopes for a mile or more. It is largely 
weathered to a rusty iron-stained mass, which has been prospected at 
several places along the contact for mineral deposits, but no ore was 
seen. The rock is a rather coarse-grained rhyolite porpbyry, in which 
white feldspars and black biotite scales, with occasional small grains 
of glassy quartz, are seen by the unaided eye. A second intrusive sheet 
of porphyry is cut by Newland Greek at the base of the slopes, and a 
dike of darker rock cuts through it and the shales, the contacts of both 
rocks having been unsuccessfully explored for ores. 

VOLCANO VALLEY FAULT. 

The head- water valleys of Newland Greek show the Gambrian shales 
faulted against those of the Belt terrane. The fault is traceable east 
and west, and is believed to be the extension of the long Volcano Val- 
ley fault, by which the Garboniferous and earlier Paleozoic limestones 
are thrown against the Belt shales. The amount of throw diminishes 
westward, however. This fault is recognized on the divide between 
Newland and Sheep creeks, where the Gambrian quartzite forms a 
small detached wooded ridge rising above the meadows, and the rocks 
show brecciation and polished slickensided surfaces. The course of the 
fault was not definitely located west of this, but it is believed to be 
continued in the faulted beds of Goxoomb Butte. The quartzite expo- 
sures just mentioned continue westward, and appear in low reefs 
dipping at 5^ downstream, and flanking the creek meadows a couple of 
miles below Kinneys. 

COXCOMB BUTTE. 

The Sheep Greek Valley ends at the base of the dark sharp- crested 
knob called Goxcomb Butte, sometimes designated as Black Butte. 
Quartzite is again seen at the southern border of the meadows at the 
base of this butte, but its age is uncertain, and it may be part of the 
Belt terrane, though mapped as Gambrian. The bed dips at 5^ toward 
the valley, and the rock is much fractured, iron stained, and jaspery, 

20 GEOL, PT 3 20 



306 GEOLOGY OF THE LITTLE BELT MOUNTAINS, MONTANA. 

with green copper stains, which latter led to the digging of prospect 
pits and the location of a mineral claim. Coxcomb Butte is formed of 
dense compact quartzite. The beds are on edge and dip at 80^ to the 
east, and are in contact with gray Belt shale whose ready weathering 
causes the quartzite to stand out in relief. The quartzite is probably 
faulted, for the upper red shale belt of the Algonkian is wanting, and, 
moreover, the quartzites show crushing and brecciation. The beds 
continue northward across the canyon, in whose walls they are seen 
dipping at 80° to 85° downstream. 

SHEEP CKEKK COPPRR MINES. 

Two prospectors who own a hay ranch at the lower end of the valley, 
Messrs. Weir and Tyler, have located claims on copper-stained seams 
in the quartzite forming the flanks of the butte. Their claims are 
located on a secondary ridge alongside of a small drainage cut into the 
east side of the butte several hundred feet above the meadow. 

The Virginia mine is located upon what appears to be a brecciated 
zone of iron-stained quartzite. The shaft was 70 feet deep in 1804 and 
a 30-foot drift had been run. Both the surface indications and showing 
made by workings were extremely poor. There is no evidence of a 
well-defined vein, and the ore is neither abundant nor rich. The loca- 
tion is probably on the fault line previously mentioned. Unaltered 
quartzite is exposed on a little ridge to the northwest of the claim. 
The summit north of the butte shows nearly horizontal beds of pink 
and gray quartzite, 200 feet thick, dip 1^ S. 

CANYON OF LOWER SHEEP CREEK. 

At the 'base of Coxcomb Butte the creek enters a canyon cut across 
ledges of steeply upturned beds of quartzite and indurated sandstone. 
The beds dip downstream at 80<^, and an estimated thickness of 200 
feet is seen. In following down the canyon the west side shows quartz- 
ite debris, but no exposures appear in the slopes above the wagon road 
until the red gneisses and schists of the Archean are seen at Moose 
Greek. The position of the quartzite between Archean gneiss and the 
Belt shales indicates that it is the Neihart quartzite. The steep dip of 
the beds and the fact that the flat summit west of the butte shows 
nearly horizontal beds of pink and gray quartzite capping the gneiss, 
as well as the finding of similar flat-lying beds of quartzite at the east 
base of the butte, would indicate a cross fault here at right angles to 
the main Volcano Valley fault. No direct evidence of this, however, 
was obtained. Below the mouth of Moose Creek, where an old dam 
still extends across the main stream, Sheep Creek flows in a narrow and 
rapidly deepening gorge cut in the Archean gneiss. ' The slopes of the 
Archean rocks are rough and covered with fallen timber, but the 
bedded rocks may be seen high up on the slopes, and several miles 
down the creek the limestones may be seen dipping at angles of 1^ to 
3° to the west. 



WMD.] SOUTHERN ABEA. 807 

The antiqaity of this valley is proved by the occarrence of a flow of 
denee black basalt. It ib first seen a balf mile below Moose Greek, 
formiugaclifl'lO tol5 feet in height facing tlie narrow allavial flat and 
making a narrow bench on the hillside. This extends at about the same 
level to the first creek below Moose, where it is 1:^5 feet thick. It is 
evideottbat itfiUs the ancient valley bottom, but no extensive accumn- 
lations of gravel were noted below it, and the cieek now flows in a 
channel cat between the basalt and the slopes to the south, leaving the 
basalt as a bench with gently sloping top, on the northern slopes far 
above the present stream. As the under surface of the basalt sheet 
slopes gently to the east it is clear that the direction of the drainage 
has been reversed since it occurred. 



The valley of Sheep Greek above Wolsey narrows as the thinly 
bedded UatnbriaD limestones and conglomerates appear. As already 
Doted in describiug Smoky Mountain, the beds dip at a low angle south 



Fu). a; M Incite sbeBM iuLruiled la liutealauea, faeaU wVUmol Sheep Creek, 

and the valley is cut along the strike of the beds, so that the higher 
beds are only encountered as the grade of the valley gradually carries 
one to higher elevations. Above the mouth of Lamb Greek the direc* 
tion of the main valley corresponds to the direction of the dip, and as one 
ascends the stream the descent of the creek is very nearly coincident 
with the dip of the beds. Near the low pass by which the stage road 
reaches the bead waters of Belt Greek the slope of the valley is, how- 
ever, a little greater than the dip of the beds, so that the strata seen in 



308 GEOLOGY OF THE LITTLE BELT MOUNTAINS, MONTANA. 

the steep slopes at the head of the creek are cat through and lower 
horizons appear lower down near the main valley. This is well shown in 
the intrusive sheets seen in this locality, which are shown upon the map. 

The shale exposures along this part of the creek contain abundant 
remains of Cambrian invertebrates, from which large collections have 
been made. The lower shales abound in the wedge-shaped Syolithes^ 
of which some of the beds are almost made up. Kearer the divide the 
shales contain nodules and thin layers of limestone. 
. Minette intrtisions. — Near the head of Sheep Creek the Cambrian 
series is intruded by a number of sheets of igneous rock of a totally 
different type from any of the intrusions observed farther south. A 
mile or more above an old dam (erected when timbermen were at work 
in the valley) the soft shales are seen to be cut by a sheet of dark basic- 
looking rock« The road has been cut across the sloi)e and shows a good 
exposure of both the intrusive rock and the Logan shales, while near by 
the creek banks also show excellent exposures. There are several 
sheets, at least three being noted near the creek. They are from 2 to 
4 feet thick, the two lowest separated by an equal thickness of shaly 
limestone and well exposed in the creek bank (see fig. 37). The latter 
forms a little cliff 6 to 10 feet high alongside the creek, resting on shale 
and capped by thinly bedded limestone. Seven ty -five feet above, another 
5-foot sheet of minette outcrops on the sIoi)e, separated by 15 feet of 
limestone and shale from a still higher 6 to 8 foot sheet. Several still 
higher sheets probably occur here, as they do in the same strata on the 
steep slopes east of the Neihart Pass, but they were not located. Though 
apparently regularly intruded, at least one sheet showed local thicken- 
ing, and it is believed that the intrusions vary somewhat in horizon, as 
the unaltered shales are soft and offer little resistance to the breaking 
through of the magma from one bed to another. In some exposures 
round bowlder-like masses surrounded by concentric shells of altered 
material are seen protruding from the face of the exposure and recall 
the bowlders of granite regions. These bowlders usually show cores of 
solid, less altered rock, surrounded by shells of material showing more 
and more alteration. 

These rocks are dark colored, and form low, generally rounded out- 
crops that stand above the shale slopes, the rock being exposed in fresh 
cuts made in building the wagon road and in low walls along the banks 
of the creek. Thoy occur as sheets intruded in the shale or, more rarely, 
as dikes. These intrusive sheets vary from a few inches to 20 feet or 
more in thickness, and have all i)roduced more or less contact metamor- 
phism of the shales. In the thinner sheets this amounts to merely an 
induration or hardening of the shale, which becomes slaty, and offers 
greater resistance to weathering than the unaltered rock. The thicker 
sheets produce a greater effect upon the rocks in which they are 
intruded. This is especially noticeable in the thinly bedded, impure 
limestones, which are indurated to hornstones or to adinole like rocks. 



WEBD.] SOUTHERN AREA. 809 

The latter are finely crystalline, banded rocks, of lavender, pale-green, 
and buii' tints, witb a conchoidal fracture, which show little nests of 
pyroxene and cavities lined with brown garnet. 

The intrusive rocks themselves are minettes, and are regarded as 
offshoots of the great stock intrusion of Yogo Peak. They are prevail- 
ingly of an olive color, and weather so easily that the rock crumbles to 
sand very readily. The rocks vary greatly in grain. In the thin sheets 
and dikes the rock is very dense or fine grained, resembles basalt, 
resists erosion well, and shows browish weathering extending only an 
inch or less from the surface into the rock. The thicker sheets consist 
of a type that is easily recognizable as granular, and that weathers 
readily to an olive-colored micaceous sand. The finer-grained rocks 
contain no visible minerals. The coarser forms glitter with the reflec- 
tions frdm innumerable little particles of mica, and sometimes show bio- 
tite scales or masses a quarter of an inch across, and more rarely as round 
blebs of olivine. With the lens the specimen is seen to consist of an 
intimate mixture of biotite and a white component (feldspar) in nearly 
equal proportions. Calcite seams and round amygdules are also seen 
in the decomi)osed rock. The exposures generally show the minette 
cut by narrow dikelets, 6 inches to 12 feet wide; these are of a light- 
colored rock which resembles a dense porphyry or aplite, but which 
petrographic study shows to be a granite- syenite aplite. These little 
dikes are irregular or crooked in course, and show massive jointing. 
The most interesting peculiarity of the sheets was observed in the con- 
tact portions of one of the intrusions, where the weathered surface was 
a mass of little round spheres and the fractured surface showed the 
rocks to be composed of light-gray spheres, smaller than peas, held in 
an olive matrix. It is a variolitic phase of the rocks above described, 
which is illustrated in PI. LXXIII, B. 

One hundred feet below the divide, on the Sheep Greek side, an intru- 
sive sheet of dense green rock 5 feet thick is seen, forming a low, 
rounded outcrop near the road. This sheet is intruded in shales and 
is itself cut by a 3-foot dike of a denser form of the same rock, the dike 
trending N. 55^ E. The petrographic description of these rocks, and of 
others of the same character from various neighboring localities, is 
given in the appended paper by Professor Pirsson. 

MOUNTAINS NORTH OF 8HREP CRKEK. 

Xorth of Sheep Greek the main divide of the range is a low line of 
rounded, heavily timbered summits that rise above the broader parks 
of Belt Creek and the fiat summit levels west of the divide. This 
mountain tract is formed of beds dipping gently southward or westward 
near Tenderfoot Creek. It consists of bedded rocks with conformable 
intrusive sheets of prophyry. Quartzite is the prevailing and most 
prominent rock here, as it is in the Belt Creek parks. 

Williams Mountain. — West of Moose Creek the quartzite is seen 
resting on Archean gneisses, tlie present gently southward-sloping sur- 



310 GEOLOGY OF THE LITTLE BELT MOUNTAINS, MONTANA. 

face of the sammit of Williams Mountain conforming very closely to 
the beddiug planes of the rocks. North of Wolsey the same structure 
is found, but tbe quartzite is here unmistakably Cambrian, as it con- 
tains fossil remains and overlies tbe Belt shales. It covers the summit 
of the mountain, whose slopes conform closely to the foldings of the 
rock, but, as the slope is a little steeper than the dip of the beds, the 
quartzite is cut through, occurring again in the valley at the foot of the 
mountain, as already mentioned. 

Wolsey Mountain. — This name is applied to the mountain north of 
Sheep Creek. North of Wolsey the lower slopes which rise above the 
hay meadows are formed of ridges running parallel with the valley. 
These ridges greatly resemble moraines, but upon close examination are 
seen to be the result of landslides, due to the slipping of the Flathead 
quartzite upon the soft Belt shales. The slopes above are thickly 
wooded, with occasional open parks, up to a level X,500 feet above the 
creek. Here the Cambrian quartzite is seen dipping south at 5^. 
Where the quartzite forms the surface the woods become more open, 
and spruces and firs occur, with frequent upland meadows which extend 
up to an altitude of 2,350 feet above the creek. Here an abrupt cliii* 
marks the front of a basalt cap that forms the summit of the mountain. 
The total thickness of basalt is about 75 feet. The rock varies some- 
what in character, ranging from a dense, compact, black basalt to a 
slaggy or brecciated, vesicular form, but presents no petrographic fea- 
tures of special interest. The main divide is covered by the usual heavy 
forest of lodgepole pine, and ^hows long talus heapings of prophyry, 
the debris from a sheet of rhyolite-porphyry which extends from Por 
phyry Peak to the head of Tillinghast Creek. 

Porphyry Peak. — ^This is the summit west of the pass at the head of 
Sheep Creek. It consists of shales with several intruded sheets of 
porphyry, one of which is now exposed and forms its summit. These 
sheets are seldom well exposed, for the rock breaks into small debris, 
which is usually much altered. In the eastern spur of the peak, run- 
ning down to the pass, two sheets were identified, but so widespread 
is the debris that others, if present, would scarcely be recognized in 
the dense woods that cover the slopes. A minette sheet was found 250 
feet below the summit, and another sheet is intruded in the shales 
beneath the porphyry cap of the summit, whose lower surface is cut 
by the minette. The rock is similar to that exx)osed along Sheep Creek 
at the east base of the peak. 

JUDITH REGION. 
PLATEAUS OF THE JUDITH. 

South of Yogo Creek the main summits and eastern slope of the 
Little Belt Eangeare a plateau country that is deeply trenched by the 
three forks of the Judith River. The summit has an eastward inclina- 
tion, which is slight on the main watershed but pronounced on the 



wiiD.] JUDITH REGION. 311 

eztensiou of tbe plateau eastward Into the interstream areas. TLtsue 

aummits are tniirifiDed by escarpiiitinta tbiit rise above tbe 

steep slopes of 8b urply V-sbaped valleys. The mouutain 

top is a park regiou, showing extensive areas of open 

^asB land with clumps of spruce and fir and groves of 

pine. Tbe streams bead in deep amphitbeatera cut in 

the Dearly tiorizoutal rocka of the aamin'ib, and are fed by 

perennial springs and by tbe melting of the great snow 

banks which gather in these amphitheaters during the 

storms of tbe long winter season. The three streams — 

Yogo Creek, Middle Fork, and South Fork, which uniting 

form the Judith River, just east of the range — all have 

sharply out head-water cournea leading into an open 

mountain valley. Below these valleys tbe atreama flow 

through deep and narrow canyoos whose pinnacled walls 

of white limeatone are extremely picturesque. 

The structure of this part of the range is simple. It is 
formed of the stratified rocks, whose heds are nearly hori- 
zontal on the main divide, the dip being but 5° eastward, 
while on Trospect Ridge and Lost Fork Plateau and in 
the walls of the lower canyons the dip is from 10° to 15°. 
As a consequence of this easterly dip the aumniit escarp- 
menta and creek canyons show these beds iu orderly 
sequeuce, and tbediU'ereut formations exhibit their pecul- 
iarities of weathering and of color iu remarkably well- 
exposed sections. The8ummitareaa,outhecontrai'y,sbow 
few natural aectiona, for the surface slopes eastward and 
very uearly coincides with the dip of the strata, su that 
large areas are often covered by a aiugle bed. As a con- 
sequence of the gradient of the stream channels, steep uear 
the head and low farther east, the three main valleys cut 
deepest iuto the sedimentary series 6 to 10 miles east of 
tbe divide, and the uplift of the range being greater to 
the aouth, the South Fork exposes tbe oldeat beds. 

In this aouthern part of the range there are do igneous 
rocks, but they appear in the form of sheets and dikes 
about tbe head wat«r8 of the Middle Fork, and become 
the most prominent and important elements of both struc- 
ture and scenery north of Yogo Creek. 



Tbe Sooth Fork exposes the only area of the Belt rocks 
found on theeastsideof tbe range. The rocksare the usual 
gray shales, which are deeply trenched by the streams. 
The beda do not dip directly east, for the strata dip away 
from the arch forming the eastern end of tbe range. The Belt shales 



312 GEOLOGY OP THE LITTLE BELT MOUNTAINS, MONTANA. 

are overlaiu by the reddish sandstones and quartzites of Cambrian 
age — the Flathead qnartzite, which is a very durable rock and forms 
extensive areas of densely wooded plateaus sloping northwestward. 
North of the stream these quartzites pass beneath the surface, and the 
parks of the region are underlain by the soft micaceous shales and 
thinly bedded limestones. The absence of igneous rocks coincides 
with the reported lack of mineral discoveries in this regiou. The cli- 
mate here is far too rigorous for agriculture, though the parks afford 
summer pasturage for large herds of cattle. The lower canyon is trav- 
ersed by a wagon road from Hoover to the forks of the river, the can- 
yon being wider and the bottom less rugged than that of the northern 
forks. 

MIDDLE FORK. 

The Middle Fork is the largest of the three branches of the Judith 
heading in the range. It drains a large part of the maiu summit region 
south of Yogo Peak, and one of its tributaries drains the plateau 
country to the south, generally known as Lost Fork Plateau. The lower 
canyon, below Lost Fork, is a narrow and tortuous gorge 6 miles long, 
in which the trail has to follow the creek channel for much of the way. 
Above Lost Fork the valley widens, and the creek is bordered by 
bench-land parks, on which hay has been cut and cattle have been 
pastured for several years past. The small head- water streams have 
received the name of prospectors who have explored them or found 
mineral deposits upon their borders. Many claims have been located 
and rich samples of gold, silver, and copper ores brought to Neihart at 
various times, but the inaccessibility of the region has deterred develop- 
ment, and no mining has been done nor any ore shipped during the ten 
or more years in which the various discoveries have been known. 

A.rude wagon road was built many years ago from the old Neihart 
stage road on O'Brien Creek to the summit of the range and down King 
Creek to an arrastre erected to treat oxidized ores found near by. The 
easiest outlet for the locality is down the stream to Utica, a longer but 
an easier route for a wagon road. 

The summit of the range at the head of Weather wax Creek is formed 
by a bed of massive limestone containing Cambrian fossils. This bed 
overlies the shaly beds in which the head-water valleys are all cut, and 
which are also exposed on the slopes west of the summit at the head 
of Sheep Creek. At the head of Weatherwax Creek the main divide 
is cut down through this bed, but it appears southward at the base of 
the limestone ledges which form the flat 8,000-foot summit rising above 
the divide at the head of Collins Creek. These higher rocks are also 
seen in a knob on the ridge between King and Harrison creeks, and 
in the maiu summit northward, where the Cambrian limestone (Gallatin 
group) is seen overlain by a few feet of greenish shale and then by red 
earthy shales, capped in turn by pinkish or buff'-colored shaly lime- 
stones that overlie the low masonry-like courses of black or dark-brown 



WEED.] JUDITH REGION. 313 

4 

limestone of the Monarch formation shown in PI. XLII, A. The beds 
are from 3 to 5 feet thick, and consist of a distinctly granular limestone 
with a peculiarly etched surface, and often pitted by cavities where cal- 
cite geodes or coral masses have weathered out. The section from the 
quartzite of Sheep Greek to the base of the brown limestone series 
shows a thickness of 1,000 feet of Cambrian strata. In the ridge north 
of King Creek the brown Monarch limestones are estimated to be 140 
feet thick. The beds dip eastward at a low angle and are covered 
by dark-blue shaly limestones which contain an abundance of fossil 
remains that show in relief upon the buff-colored weathered surfaces of 
the rock. These shaly limestones break readily into shaly debris, and 
are seldom seen in massive exposures. 

Igneous rocks. — The upper valleys of Middle Fork show many intru- 
sions of igneous rock. They occur chiefly in the soft, easily penetrated 
Cambrian shales, more rarely in the shaly beds of higher horizons. 
Both dikes and sheets occur, but the former are less common and do 
not form so important a feature of scenery and structure as the sheets. 
These intruded sheets are in some respects the most conspicuous rocks 
of the region, their resistance to erosion leaving them in relief as ledges 
banding the mountain sides. Their debris piles are conspicuous, and 
they are especially noticeable as crescent-shaped benches on the spurs 
or lateral ridges seen along the valley sides. The rocks are of two 
types — the rhyolite- or trachyte- porphyries, the light-colored rocks, form- 
ing the thicker, more conspicuous intrusions; and the dark-colored 
rocks, which are mostly minettes or closely allied rocks, and decom- 
pose readily, and though quite as common in occurrence as the former, 
are not recognizable at a distance, as they seldom form conspicuous 
exposures. 

The light-colored rocks are feldspathic porphyries of a pinkish tone 
on weathered surfaces, or straw colored on fresh fracture. The tints 
are due to the hydrated iron oxides disseminated through the rocks 
and resulting from the decay of some iron-bearing mineral. All of the 
rocks have been greatly altered, and in part mineralized, so that the 
contacts have been more or less prospected. The basic rocks have 
baked and altered the adjacent shales; the feldspathic porphyries have 
produced little if any alteration of the rocks in contact with them. 

The ridge previously mentioned, lying between King and Harrison 
creeks, is crowned by a mass of feldspar- porphyry, forming a sharp 
knob or summit. This porphyry is part of an intrusive sheet recogniz- 
able also in the summit to the north. It is estimated to be 150 feet 
thick, and as it lies upon the strata its lower surface, at least, con- 
forms to them in dip. It extends a mile or more eastward down the 
ridge. The rock is believed to be a remnant of an extensive intruded 
sheet uncovered by erosion, which extends far down the ridge to the 
east. It has produced almost no metamorphism of the underlying 
shaly limestones, though a prospect pit at the contact shows a little 



814 GEOLOGY OF, THE LITTLE BELT MOUNTAINS, MONTANA. 

ore, and the under contact is more or less mineralized. In the outcrop 
the rock is shattered and breaks into platy debris. It is light colored, 
with a reddish tinge on weathered surfaces, but of a light-gray color 
when unaltered. The dense, compact groundmass is dotted with occa- 
sional opaque white spots of feldspar of varying size up to one-half 
inch in diameter, with a sprinkling of small flakes of rather dull biotite- 
mica. The rock is altered, though less so than most of the igneous rocks 
of the neighborhood, and is probably to be classed as a quartz-syenite- 
porphyry (quartz-mica-syenite), as noted in the petrographic chapters. 

From the summit of the ridge a comprehensive view is obtained of 
the neighboring country. It is at once noticed that the benches and 
dark-colored d^.bris piles seen on the spurs or ridges between the vari- 
ous streams near by, all occur at about the same level. This suggested 
what subsequent visits proved to be true — that a single sheet of igneous 
rock forms all the exposures seen at the same horizon ; and as the prin- 
cipal exposures are lower, both in elevation and in stratigraphic se- 
quence, than the Jefferson limestones, the sheet forming them is intruded 
near the top of the Cambrian shales. This sheet is probably the one 
encountered in the bottom of Harrison Creek north of the King Creek 
summit, at 1,250 feet below the top, and 100 feet above the forks of King 
and Weatherwax creeks. At the latter locality the sheet is estimated to 
be but 15 feet thick, but it forms a ledge traceable around the slope and 
up Harrison Creek. One hundred feet below this porphyry sheet an 
intrusive sheet of minette cuts the shales, and at a still lower elevation 
another porphyry sheet 30 feet thick forms low cliffs alongside of 
WeatherWax Creek, and as the valley deepens is seen in bold cliffs and 
benches on the spurs. The minette sheet at the mouth of King Creek 
is seen for 3 miles as a prominent ledge on the grassy open slopes north- 
east of the creek. It forms a little level at the mouth of King Creek, 
where it is well exposed, and is seen to have produced considerable 
alteration of the shales and thinly bedded limestones in which it is 
intruded. The exposure is nearly horizontal, as the course of the val- 
ley is along the strike of the strata in which the sheet is intruded. 
This minette sheet was not followed beyond Harrison Creek, but it 
extends up that creek, and probably occurs on the spur between it and 
Cleveland Creek. 

A dike was found cutting the shaly limestones at the head of King 
Creek, which probably also cuts the porphyry forming the summit 
north of that stream. It is seen cutting the shales on the steep slopes 
descending the channel of Harrison Creek, where it is 20 feet wide, has 
a dip to the west, and has baked and altered the shale for several feet 
from the contact. The trend is northeast, but as the rock weathers 
readily it is not recognizable northward, where the ridges are capped 
with the Carboniferous limestones. The rock is altered to an olive or 
brown, sandy material, but is readily identified as a minette. It shows 
the peculiar warty surface and variolitic structure on contact surfaces 



WEED.] JUDITH BEGION. 315 

and also on joint (shrinkage?) planes noted in the Sheep Greek rocks* 
A second dike of minette, showing at the contact the variolitic faces 
noted in the sheets of Sheep Creek, is seen near the mouth of a tribu- 
tary of Harrison Greek below the dike just noted. ' The trend is N. 20o 
E. The rock is an augite-minette. It does not at all resemble the other 
minettes, as it is a dense, very dark-gray, almost black, rock, whose 
grouiidmass glitters with reflections from minute mica scales. The only 
mineral recognizable by the eye is brown biotite. 

On the north side of Harrison Greek the wash of the soft shale covers 
the slope, so that the underlying rock in place is seldom seen. A sheet 
of minette forms a bench 600 feet above the creek, and 200 feet above this 
rock a 25-foot sheet of hornblende-porphyry is found whose position 
and petrographic character correspond to those of the sheet found on 
King Greek. It is probably the northern extension of that sheet, though 
the interval between it and the minette sheet is 100 feet greater than 
at the mouth of King Greek. If this be true, it shows that the sheets 
vary somewhal! in the horizon of intrusion at different localities, as 
would be expected in these soft shales. The higher slopes north of 
Harrison show the same intrusive sheet that is found capping the sum- 
mit between King and Harrison creeks. 

The valley of the Middle Fork below Harrison Greek broadens out, 
and for several miles shows a beautiful open, meadow benchland at the 
base of the densely wooded southern slopes. These slopes show no 
exposures, though the porphyry sheet already mentioned forms a recog- 
nizable bench and black talus slope on all the spurs. The slopes north 
of the creek are scantily wooded or open, thin soiled, and show good 
exposures. At the mouth of Harrison Greek the stream cuts a sheet 
of greenish basic rock, too altered for absolute determination, but 
probably minette. At Cleveland Greek, where a cattle ranch has been 
established, there is a mass of basic rock intimately associated with an 
intrusion of light-colored porphyry that forms a broad bench north of 
the creek and is exposed in columnar cliff's extending down the stream 
for several miles. ^Tbe porphyry is probably laccolithic in form, for 
the Cambrian beds north of the creek dip away from it at an angle of 
25^, though the summit escarpments on either side show the usual 
easterly dipping beds. The general easterly dip of the Devonian and 
Carboniferous limestones is not altered, as the ledges may be seen far 
above the valley dipping gently eastward. 

The rocks exposed at the mouth of Cleveland Greek are the lowest 
beds seen, as below here the prevailing easterly dip just noted brings 
successively higher beds to the valley bottom. Two intrusive sheets 
were noted in the shales east of here, the rocks belonging to the minette 
group. About 2 miles above the mouth of Lost Fork the massive 
limestones, which dip downstream at a low angle, reach the creek bot- 
tom, and the valley narrows and soon becomes a deep canyon. The 
cliffs and creek banks show unusually good exposures of the softer beds 



316 GEOLOGY OF THE LITTLE BELT MOUNTAINS, MONTANA. 

of the sedimentary section. The brown Jefferson limestones are here 
seen to be overlain by shaly red limestones, often spotted with green. 
These are overlain by the thinly bedded limestones carrying Carbon if 
eroas fossils and forming the base of a series of bedded limestones whose 
cliffs are often orange colored. From these exposures eastward the 
creek canyon is cut in Carboniferous beds, and no igneous rocks are 
seen until the broad basin of the Judith is reached, in which the 
sapphire mines occur. 

Prospect Ridge is the broad block of limestone between Yogo Creek 
and Middle Fork. While the block shows the general easterly dip 
already noted, there is some local warping or buckling. The summit 
shows a gentle anticlinal arch, the beds dipping at 3^ to 5^ toward the 
valleys on either side. This inclination is more marked opposite Yogo, 
where it amounts to 10^. 



OHAPTEB lY. 

DESCRIPTIVE GEOIiOGY OF THE YOGO DISTRICT, BIG 
BAIjDY MOUNTAIK, WOIiF BUTTE, AND TAYIjOR PEAK. 

YOGO DISTRICT. 

The moantainons tract included between Yogo Creek on the south 
and Dry Wolf Creek Valley (Big Park) on the northwest is conven- 
iently described as the Yogo district. It includes several areas where 
prospecting and mining operations have been more or less vigorously 
carried on at various periods in the past, the best known being those 
of Yogo proper, the Yogo sapphire field, Lion Gulch, and Running 
Wolf Creek. The last named is the only one irom which ore has been 
shipped. The region thus delimited presents a far more irregular sur- 
face and a more complex geologic structure than that so far described. 
The highest point, Yogo Peak, is the westernmost summit of a some- 
what irregular ridge extending northeastward and forming the main 
water parting of the district. A northern spur of this ridge ends in 
Steamboat Mountain,«as the summit between Dry Wolf and Running 
Wolf creeks is locally called. The main divide continued eastward 
ends in two separate elevations, the northern of which is Woodhurst 
Mountain; the southern is, for lack of a name, here called Sage Creek 
Mountain. A fourth detached mountain, lying between Sage and Yogo 
creeks, is here called Ricard Peak. 

Yogo Peak and the ridge east of it are formed of a mass of igneous 
rock breaking through folded sedimentary rocks. The outlying moun- 
tain masses mentioned above are laccoliths — that is, great blister-like 
elevations of the strata lifted up by intrusions of igneous rocks. The 
igneous rocks determine, therefore, the main features of the topography 
of the region, as they do the geologic structure. 

The ridge of which Yogo Peak is the western end is formed of a 
continuous body of massive igneous rock extending from Yogo Peak to 
the eastern foothills of Woodhurst Mountain, a distance of 13 miles. 
This long and relatively narrow body of massive rock is an intrusion 
which fills an irregular fracture in upturned sedimentary rocks. The 
western end of this fracture, at Yogo Peak, appears to be a chimney 
or core, the center of disturbance at the time of igneous activity, from 
which the dikes seen for many miles about seem to radiate, and which 
for a distance of 10 miles or more is encircled by intrusive sheets in 
the softer, more easily penetrated formations of the sedimentary series. 

317 




318 GEOLOGY OF THE LITTLE BELT MOUNTAINS, MONTANA. 

The mass as a whole shows an apparently continuoas single body of 
rock that presents a remarkable gradation of types at the western 
end of the fractare. Owiug to the importance of the mass and the 
dedactions derived from the study of its rocks, the field observations 
are given in considerable detail. 

YOGO PEAK. 

Location. — ^Yogo Peak is from many i)oints of view the most conspic- 
uous elevation of the Little Belt Bange. Although not snow capped, 
it projects above the timber line, and its somber crown of crags, formed 
of massive igneous rock, is in sharp contrast to the ronuded summits 
and level plateaus adjacent. ( PI. XLVIl, B,) It is situated 8 miles east 
of Keihart and on the high divide between the waters of Belt Greek 

and the Judith Kiver, and 
YO0OF£A/( reaches an elevation of 8,625 

feet above tide water. The 
adjacent streams have cut 
deeply through the horizon- 
tal sedimentary rocks, and 
>J the broad valley bottom of 

Fio. 39.— North-sonth section through Yogo Peak. Dry Wolf CrCCk On the UOr th 

and the placer bars of Yogo 
on the south are 3,000 feet below the crags of the mountain summit. 
It is readily accessible, as a well-traveled trail, which is the route from 
Keibart to Judith Basin, crosses its summit. 

Character of rocks. — ^The peak is composed of coarse- grained massive 
rocks, showing a progressive gradation from very dark-colored augitic 
forms at the western contact to lighter-colored, more feldspathic, finer- 
grained rocks forming the eastern part of the peak. These rocks are 
fully described in the petrographic chapters, so that only the more 
general facts will be given here. 

WESTERN KNOB. 

A grassy depression or sag, which affords an easy descent for the 
trail to the head of Yogo Greek, divides the summit of the peak into 
eastern and w.estern parts. The western summit has an irregular sur- 
face, on which ruin-like masses and crags, with curiously shaped mono- 
liths and bowlders, rise above the small grassy plats lying between 
them. (PI. XLVII, A.) The rock forming these crags has a very 
massive jointing, is exceedingly tough, and breaks with great difQculty. 
On the western slopes of the peak the rock disintegrates readily on 
weathering and forms a coarse sand, the rock being seen only in little 
gullies washed in the slope. 

Shonkinite. — ^The rock appears at first sight to consist principally of 
coarse biotite. It is very coarse grained, and in the exposure is so 



A SUMMITOFBIG B*LDV MOUNTAIN. SEEN FROM THE ALPINE MEADOW OM TOP OF VOGO P 



B VOGO PEAK, SEEN FROM THE BELT CREEK 



WEED.] YOGO DI8TKICT. 819 

loosely textured that good specimens are obtained with difficalty. This 
type is the most basic one foand, and lies nearest the west contact. It 
consists of augite and biotite, which far exceed the feldspar in amoant. 
The glistening bronzy plates of biotite are the most conspicuous min- 
erals, though the augite is present in greater quantities. The rock also 
contains small masses of glassy olivine. The feldspar is almost wholly 
orthoclase, and the rock is a shonkinite — that is, a coarsegrained rock 
composed of augite and biotite with a smaller amount of orthoclase. 

Upon the summit the rocks composing the crags and monoliths are 
denser in grain than the type just noted. The exx)osures show, how- 
ever, a considerable variation in grain, the coarse and fine grained forms 
being mixed as if stirred together while still pasty. The denser rocks 
form the exposed masses; the coarser varieties weather down and form 
the grassy interspaces. All gradations may be found between very fine 
and very coarse grained types. The prevailing rocjiL seen in the massive 
exposures, though less coarse than that of the Western slope of the 
peak, is yet a coarse-grained rock of a very dark* gray color. The dark- 
colored minerals are in excess, and the rock glistens with the light 
reflected from the numerous plates of biotite, and is dotted with round, 
rusty-brown spots. With the lens an abundance of small augites are 
also seen in the feldspars. The feldspar is chiefly orthoclase. The 
microscopic study of this rock shows that it is also a typical shonkinite, 
but contains less of the dark-colored minerals than the coarser-grained 
form nearer the contact. 

BiJces cutting shonkinite. — The rocks of the western part of the sum- 
mit are cut by a number of tiny dikes which vary from a fraction of an 
inch to several inches in width (fig. 73). The dike rock is of uniform char- 
acter, very light gray or pink in color, and is recognizable as granular to 
the eye. The finer grain of the rock causes it to weather in relief upon 
exposed surfaces. The widest dike cutting the shonkinite is a rather 
coarse-grained syenite, containing inclusions of the shonkinite. The 
smaller ores are but little finer in grain. In one instance a 2-inch dike 
showed a well-marked darker colored, much denser contact band 1 mm. 
in width, but usually no such contact band is recognizable in the hand 
specimen. Dikes of a syenite-porphyry and of a dark-green augite- 
minette probably cut this rock, for their debris was found near the 
saddle separating the two summits, but the rocks could not be found 
in place. 

MIDDLE KNOB. 

In crossing the summit eastward to the saddle the rocks show a 
gradual change in character, becoming lighter colored and more feld- 
spathic, and have a general mottled appearance. The weathering is less 
massive, and there is a tendency to break into thick slabs and rectan- 
gular blocks, which is well illustrated in the sharp little knob on the south 
side just east of the trail. The rock is very clearly finer grained than 
the shonkinite, but is composed of the same minerals, though feldspathic 



320 GEOLOGY OF THE LITTLE BELT MOUNTAINS, MONTANA. 

constitaents are more abundant, giving it a decided gray color, of a 
greenish tint, on fresh fracture. As is shown in the i>etrograpbic 
description, the feldspar consists of nearly equal amounts of orthoclase 
and plagioclase (oligoclase andesine). It is a typical monzonite, and is 
quite clearly seen to grade into the shonkinite. 

EASTERN KNOB. 

The eastern summit of Togo Peak has a rounded form and is covered 
by the platy debris into which the rock weathers. The rock mass com- 
posing it and the high eastern shoulder of the mountain possesses a 
platy parting which causes it to split readily and to formj^iles of debris 
and talus slopes, above which project the low and much-jointed exi)o- 
sures of the rock in place. The joint blocks are short, stout rhomboids 
or heavy plates a foot or so long. They are very hard and tough, ring 
sonorously under the hammer, and are broken with difficulty, the rock 
being unaltered and fresh. These characters prevail for the entire 
Yogo Peak mass. On a freshly fractured surface the rock appears 
evenly granular, of moderately Hue grain, and is compact in character 
and with few miarolitic cavities. The color is a medium gray with a 
strong pinkish tone. The rock is clearly a feldspathic one, and of sye- 
nite asi)ect. Examined with the lens, it is seen to be chiefly composed 
of light-colored feldspar, dotted with small, dark, formless spots of green 
pyroxene or hornblende. The rock is very unlike that of the types 
previously described, but the monzonite forms a transition phase 
between this rock and the shonkinite. "So actual gradation of the mon- 
zonite into this syenite was observed in the field, a gentle grassy slope, 
scattered with black bowlders extending up to the base of the platy 
talus slide. The plates seen on the summit show partly rounded or 
subaTigular inclusions of a coarser-grained augitic rock, like the monzo- 
nite and shonkinite of the western summit of the peak. The rock is 
also cut by little dikelets, from an inch to several inches in width, of a 
lighter-colored, flner-grained, more feldspathic syenite, regarded as an 
aplitic form of the rock. This syenite continues eastward without 
change in character, to the steep slopes that form the eastern shoulder 
of the peak, there being an abrupt descent of GOO feet to the broad and 
open-parked summitof theridge. That the syenite extends to the south- 
ern contact is shown in the walls of the little amphitheater cut in its 
southeast side, the contact with the sedimentary rocks being seen on 
the shores of the lakelets at the bottom of this amphitheater. 

EASTERN SHOULDEK. 

The platy debris covering the eastern shoulder of Yogo Peak and 
the steep slopes that define it from the broad and wooded summit of 
the ridge to the east, shows a very gradual change in the appearance 
of the syenite, from the evenly granular rock of the peak to a decidedly 
porphyritic form constituting the talus of the eastern slope. The latter 



WEED] YOGO DISTRICT. 821 

maybe regarded as a transition form to the lighter-colored, more feld- 
spathic syenite-porphyry which is seen immediately east of the peak. 
The rock still has a granalar character, and contains some hornblende 
and aagite, but its distinctive feature is the presence of large square 
feldspar crystals. These resist weathering better than the ground- 
mass and stand out in relief upon the rock surface, giving it a slight 
resemblance to a conglomerate. Where the rock has weathered down, 
these feldspars form a sandy gravel. This rock contains microscopic 
quartz grains and may be classed as an amphibole-granite-x)orphyry or 
a quartz-syenite. 

ELK RIDGE SUMMIT. 

The slightly accidented surface of the ridge, designated Elk Eidge, 
which extends eastward from Yogo Peak to the clififs at the head of 
Lion and Elk gulches shows few exposures. A sharp point rises above 
the general level at the north edge of the summit. The slopes of this 
eminence consist of platy brown ddbris, while its top shows massive 
exposures of the rock. Several varieties occur here, presenting grada- 
tions similar to those noted at Yogo Peak. The prevailing form is a 
monzonite associated with a lighter-colored granite- syenite. Both these 
rocks are cut by a dike of dark basic rock, running toward Yogo Peak, 
but having a dip of 50^ NW. This dike rock shows porphyritic crystals 
of augite and olivine in a dense black groundmass, and is found to be 
an augite-minette. In the transverse ridge or summit, crossing the 
ridge near the head of Lion Creek, the rocks are again well exposed. 
They are no longer porphyritic, but are coarse and even grained sye- 
nite, rather dark in color, which grades into shonkinite in the escarp- 
ments on either side, the rocks being contact forms of the mass. The 
shonkinite also forms a narrow, dike-like mass which extends eastward 
and makes the divide between Lion Gulch and the waters of Yogo 
Creek. A contact form of this rock is an augite-minette. 

DIKES COXNECTINO YOGO AND WOODHUR8T STOCKS. 

At the locality last noted, which lies nearly north of the Yogo settle- 
ment, the intrusion is no longer a wide one, but splits up into three 
narrow, dike-like masses, only one of which continues eastward and 
expands into the broad masses seen at Sheep Mountain and in the 
chain of hills north of Bear Park and Sage Creek. This intrusion is 
but a few yards wide where it crosses the saddle back of Schoppe 
Mountain, whence it runs diagonally across the basin of Skunk Creek, 
where it is not over 200 yards wide, and is seen cutting the limestones 
in the walls of Sheep Mountain. In the saddle between Sheep and 
Bandbox mountains a similar dike-like mass runs transversely across 
the ridge, and shows a mass of shonkinite, flanked by porphyry on the 
north, the contact being abrupt and without gradation. The relations 
of this mass to the main intrusion were not determined. 

20 GEOL, PT 3 21 



322 GEOLOGY OF THE LITTLE BELT MOUNTAINS, MONTANA. 

STBUOTUBAL RELATIONS. 

The intrusion is a stock or intrusive mass that occupies an irregular 
fracture in the sedimentary rocks and has an abrupt, nearly vertical, 
contact. This is clearly shown at a number of localities where the 
contact is well exposed, though in general the actual plane of contact 
is hidden by debris or soil and is not seen. It is well exposed on the 
borders of the ponds seen in the floor of the amphitheater cut into the 
northeast side of Yogo Peak and also in similar amphitheaters south- 
west of the peak, where the plane of contact is vertical. Upon the west 
and southern sides of Yogo Peak the strata are locally undisturbed by 
the intrusion, but elsewhere the sedimentary rocks maintain the same 
general attitude as those of the surrounding undisturbed parts of the 
range. Thus, upon the northern spurs of Yogo Peak the limestones are 
tilted, the dip being 10° S. toward Yogo Peak, as seen in the photo- 
graphs reproduced on PL XL VII, B, The beds make benched slopes 
with steep cliffs and level benches sloping gently inward toward the 
peak. The beds here exposed can be traced continuously eastward in 
persistent cliffs and ledges, gradually running down the slopes below 
as far as Lion Greek, where the beds which form the creek bottom are 
the same as those seen high up on the northern shoulder of Yogo Peak. 
West of Yogo Peak the ridge connecting the peak with the western 
summits shows slightly tilted limestones, but at the base of the peak, at 
the immediate contact with the massive rocks, the strata are steeply 
upturned. These beds and those seen south of the peak appear to be 
broken and uptilted blocks produced by a punching up of the massive 
rock in opening its conduit. The rocks are so highly altered that it was 
impossible to determine their age. These upturned sedimentary rocks 
are well exposed on the south side of the peak, and on the spur which 
runs down into Yogo Cre^k that is traversed by the trail. The beds dip 
away fi'om the intrusion, the angle of dip being 60° to 65° south of the 
eastern summit of Yogo Peak. The lateral spurs extending Qouth from 
the peak toward Yogo Creek show crumpled and warped beds, which, 
however, pass gradually into less-disturbed strata in a very short 
distance from the contact. These disturbed beds appear to be abruptly 
uptilted by the intrusion. It is not a laccolithic uplift, and for the 
most part the uptilted strata are fractured and uplifted blocks — the 
broken-off ends of the beds through which the intrusion has occurred. 

CONTACT METAMOBPHISM. 

The sedimentary rocks are highly altered at and near the contact 
with the igneous rocks. This alteration is greatest, of course, near the 
contact, and becomes less and less marked as the distance from it 
increases. In no case is it intense, and the zone of noticeable meta- 
morphism is rarely over a few hundred yards wide and generally only 
a few yards across. Where the metamorphism is most marked the 



WEED.) YOGO DISTRICT. 323 

purer limestones form coarsely granular marbles. The less-pure lime- 
stones are aggregates of calcite, garnet, epldote, and other character- 
istic contact minerals. Farther away the heat of the intrusion has 
produced an induration of the beds and developed joints and strains in 
the rock, which cause it to break into fine angular debris on weather- 
ing. This contact metamorphism is most marked about Yogo Peak, 
and eastward as far as Lion Creek and Sheep Mountain. It is much 
less east of the places where the intrusion is a porphyry and not a 
coarsely granular rock. In general, it has been found that the more 
basic the rock the greater is the efifect of cont-act metamorphism. 

The altered rocks are mostly somewhat loosely textured and crystal- 
line, so that they weather more readily than either the massive igneous 
rock or the unaltered sedimentary ones. The prominence of the igneous 
rock, forming as it does sq high a peak and mountain ridge, is prob- 
ably largely due to this fact. The slopes of Yogo Peak rise steeply 
on all sides above the surrounding area of sedimentary rocks. From 
Yogo Peak eastward to Lion Greek and a little beyond, the igneous 
area has a relatively flat summit, bordered by cliffs of massive rock 
that rise as an escarpment above the contact zone of altered sediments. 
This steep slope of Yogo Peak and this escarpment are no doubt due 
in part to the resistant characters of the syenite, but more directly, 
perhaps, to the nature of the altered sediments, whose coarsely crys- 
talline texture yields readily to the processes of weathering, crumbling, 
and disintegrating. This material is removed rapidly by wind and 
water, so as to leave the igneous rock in relief, and the cliffs seen are 
probably the denude<l and exposed contact planes of the intrusion. 
This contact zone is often marked by seams and fissures of iron-stained 
or clayey material and by ore deposits. These ore deposits have been 
prospected at frequent intervals, and for many miles the contact can be 
traced by them. Eich specimens of ore have been found, but thus far 
no productive ore bodies have been discovered. At most of the locali- 
ties about the contact the altered rocks were originally limestones, but 
in the branches of Wolf Creek, west of Lion Gulch, there is a large 
amount of debris derived from the eruptive contact. This debris con- 
sists of hornstone and adinole — the dense rocks produced by the baking 
and alteration of the Cambrian shales. 

GEOUP OF ENCIRCLINa SHEETS ABOUT THE YOOO STOCK. 

Yogo Peak is the center from which a great number of sheets of 
igneous rock were intruded into the sedimentary rocks of the range. 
These sheets, which by denudation are now exposed, encircle the peak 
for a radius of at least 10 miles, and form conspicuous elements of the 
scenery and mountain structure, while if the laccoliths be regarded 
as enormously thickened sheets, the highest and largest mountains of 
the range are formed by them. The sheets occur chiefly in the soft 
and easily intruded Cambrian shales, where they are very abundant, 



324 GEOLOGY OF THE LITTLE BELT MOUNTAINS, MONTANA. 

but they are also fouud at higher shaly horizons which are easily 
intruded. 

These sheets are formed of varioas rocks which exhibit a consider- 
able diversity of texture and of mineral composition, but may be 
grouped as trachytic or rhyolitic rocks, and as lamprophyres or basic 
rocks, chiefly minettes. The former are the common porphyries of the 
region. They are hard rocks, which resist weathering better than the 
soft shaly rocks in which they are intruded. They form escarpments 
or mountain slopes and spurs, very often fronting benches determined 
by the occurrence of the sheet, and having broad talus slopes at the 
base. These sheets form the most prominent feature of the country 
about the head waters of Belt Greek, the Middle Fork of the Judith, 
and Sheep Creek, and their occurrence is described in the detailed 
description given of these localities. The basic sheets are probably 
quite as numerous as those of the acidic rock ; but they weather readily, 
generally forming a slightly coherent brown sand, and are therefore 
seldom recognizable on the mountain spurs and slopes; thus tliose 
which have been mapped and visited probably represent but a small 
part of the total number. These sheets generally produce considerable 
metamorphism of the rocks near their contact. The two classes of 
rocks forming the intrusive sheets exhibit various gradations, but may 
be classed as porphyries and as minette and analcite-basalt. They rep- 
resent in another form the same rock magma which at Yogo Peak has 
consolidated as shonkinite and syenite. In the sheets the rapid cooling 
of the thinner mass has resulted in different texture and somewhat 
different mineral cojnposition. The petrographic features of these rocks 
are given in detail in the accompanying paper by Professor Pirsson, but 
attention may here be called to their leading characteristics. 

While the connection of the various intrusive sheets observed in the 
range for many miles about Yogo Peak can not be directly established, 
there is strong evidence of it. Such sheets are abundant in the spurs 
of Yogo Peak and vicinity, and on the north side of the ridge they 
can be traced to within short distances of the massive rock. Though 
the actual physical connection was in no case established, there seems 
no reason to doubt their origin from that center. 

The lateral spurs of Yogo Peak contain at least three intrusive sheets 
in the upper part of the mountain slopes, showing as columnar cliffs 
where deeply trenched by streams, and forming pronounced benches 
indenting the crests of the spurs, with dark talus slides. The west 
wall of the second gulch southwest of Lion presents cliffs of well-bedded 
white limestones dipping 10^ NE., while the opposite eastern walls 
of the gulch show the brown Monarch limestones at the base of 
the bluff, capped by an intrusive sheet which forms the top of lesser 
ridges, but on the liigher ones is overlain by the white Carboniferous 
limestones. This same sheet is continued eastward, and may be seen in 
Lion Gulch, near the Gold Dust mine. It is a much altered syenite- 



WEED.1 YOGO DISTRICT. 325 

porphjrry. In the lower slopes several sheets are seen cutting the Cam- 
brian shales, and as these occar at the same horizons and show the 
same intervals between them, and consist of the same rocks, as those 
found in the spur of Big Baldy Mountain north of Big Park, it is evi- 
dent they once extended across the valley. 

West of Yogo Peak the ridge connecting it with the main divide of 
the range is largely covered by the debris of an intrusive sheet of por- 
phyry. The underlying rocks have a gentle dip. The Cambrian shales 
and shaly limestones are here, as elsewhere about Yogo Peak, intruded 
by sheets of porphyry, showing as ledges banding the slopes and as 
caps to the buttes which rise above the general summits. In the 
upturned strata seen at the contact with the main stock, three sheets, 
each from 5 to 10 feet thick, are intruded in the limestones. The rocks 
are very greatly altered, but are recognizable as fine-grained syenites or 
syenite-porphyries. A sheet 100 feet thick, forming the amphitheater 
walls southwest of the peak, appears to be continuous with the sheet 
seen on the ridge to the'west running across the ridge to the head of 
Wolf Creek, The rock is a syenite precisely like that noted at the 
shaft house on the west contact of Yogo Peak. 

South of the peak the open limestone summit at the head of Yogo 
Creek shows the usual prospect pits found in the contact zone. Two 
intrusive sheets that occur here in the upturned limestone are slightly 
faulted. The spur leading down into Yogo Creek from the last-named 
locality shows upturned strata in which two intrusive sheets were seen. 
To the east the lateral spurs of Yogo Peak show intrusive sheets 
whose darker weathering is in strong contrast to the white limestones. 
In the spur west of the Yogo settlement an isolated remnant of a sheet 
forms a turret, capping white limestones, and capped in turn by a smaller 
mass of white marble. The thickly bedded limestones of the Carbon- 
iferous do not split apart readily along their bedding planes, and it is 
rare to find intrusive sheets in these rocks. This probably accounts 
for the general observance of intrusive sheets in the rocks adjacent to 
the eastern extension of the Yogo Peak core. The sheets of Wolf Creek 
Park (Big Park) are described under that heading (p. 341). 

FRINGING DIKES. 

General relations. — The dikes which cut the sedimentary rocks for a 
radius of 12 miles about Yogo Peak are believed to have their origin in 
that locus of igneous activity, as in general they radiate from Yogo Peak 
as a center. They vary greatly in appearance and also in mineral com- 
position, but are nearly all basic rocks, trachytic or rhyolitic rocks being 
very rare in this group of dikes. They are generally dark colored, and 
when intruded in limestones or light feldspathic syenitic rocks are quite 
conspicuous. More commonly, however, they yield readily to weather- 
ing and are barely recognizable even close at hand. The minette 
dikes of Sheep Creek and of King and Harrison creeks have already 



326 GEOLOGY OF THE LITTLE BELT MOUNTAINS, MONTANA. 

been described. The granite-porphyry of Big Baldy Mountain is cut 
by analcite-basalts, which are seen on the summit and are prominent in 
the amphitheater walls. The shales and intruded sheets at the base of 
this mountain are cut by two dikes of minette. Another group is seen 
exposed on the saddle at the head of Eunning Wolf Creek, in the valley 
of that creek, on spurs of Steamboat Mountain, and on the ridge south 
of the creek. It is probable that but a small part of the total num- 
ber have been seen in mapping the geology. These dikes cut sheets 
of feldspathic porphyry at a number of places, and hence are certainly 
later. They are found much farther from the center than the acidic 
sheets, but no farther than sheets of similar basic rocks. The most 
interesting of these dikes are probably those of Bandbox Mountain and 
the dike from which the Yogo sapphires are obtained. 

Bandbox Mountain dikes. — Bandbox Mountain is the name given to 
a flat- topped elevation whose encircling cliffs and girdle of limestones 
have doubtless suggested the name. The mountain consists of gently 
tilted Carboniferous limestone, of which good exposures occur in the 
summit cliffs. The rocks are shaly, thin-bedded or flaggy, dark-gray 
limestones, and carry abundant fossils of Carboniferous types. The 
summit of the mountain is smooth, open, and grassy, and conforms in 
shape to the bedding planes of the limestone, dipping N. 13^, mag- 
netic. The massive bed of. limestone covering the summit is cracked 
by reticulated fissures into rhomboidal areas, several hundred feet on 
a side. These fissures are very prominent, as their course is marked 
by a rich and luxuriant growth of grass and fiowers, in strong con- 
trast to the scanty covering of the limestones. The fissures are filled 
by a soft, olive-colored or greenish material that is a decomposed dike 
rock. 

The dikes are commonly 1 to 3 feet wide, but the largest, which 
trends directly toward Yogo Peak, is between 9 and 10 feet wide. 
The decomposed dike rock commonly shows numerous mica plates, 
which lie in planes parallel to the sides of the dikes, giving the rock a 
flaky, fissile structure that is made prominent by weathering. The 
fresh material of one dike shows large, red olivines. These rocks 
are augite-minettes (or analcite-basalts), whose characters are fully 
described in the petrographic chapters. They weather peculiarly, as 
the micas alter to soft, green chloritic masses, which dot the rock with 
prominent spots. 

Dikes of Eureka divide. — The low saddle between Bandbox Mountain 
and Steamboat Mountain also shows an interesting group of dikes. 
The sedimentary rocks seen on the divide above the Eureka mine are 
Carboniferous, belonging near the base of the Madison formation and 
containing an abundance of silicified fossil shells which weather in 
relief. These rocks are cut by a number of dikes — ^twelve were counted — 
nearly parallel and trending directly toward Yogo Peak. The dike 
rock is seen in all stages of alteration to a soft, olive-colored clay, and 
has been very generally prospected for ore deposits. The dikes vary 



WEED.] TOGO DISTRICT. 327 

firom a few feet to a few yards in width. They are generally minettes, 
but include also a dike of analcite- basalt, similar in character to that 
cutting the summit of Bandbox Mountain. 

WOODHUEST STOCK. 

In Sheep Mountain and eastward to Woodhurst Mountain the intru- 
sion consists of a syenitic porphyry, and is no longer a coarsely granular 
rock. The general characteristics are the same, and the rock is to the 
eye the same in texture and composition throughout, though varying 
slightly in color. Under the microscope it is seen to present slight 
variations, but is throughout a syenite-diorite-porphyry, which passes 
into a rhyolite-porphyry at the extreme northeast contact. The rock 
is a general type common to the laccolit.hic masses of the range, and 
presents none of the remarkable variations in texture and composition 
seen farther east. It forms in rather massive exposures, but upon 
weathering the blocks break into platy debris, which frequently covers 
the exposures. On Sheep Mountain the igneous rock extends from a 
low saddle to a knob to the north, and also forms an eastern spur 
whose wooded slopes extend down to Bear Park. The intrusion con- 
tinues in the low range of wooded summits north of Bear Park, extend- 
ing as far as Woodhurst Mountain. 

Structural relatioris of Woodhurst stock. — In the vicinity of the settle- 
ment of Yogo, as in Lion Gulch, the general structure of the sedi- 
mentary strata, due to the uplift of the range, as a whole is not 
disturbed; but east of here, in Bear Park and on Eunning Wolf Creek, 
local foldings due to laccolithic intrusions are noticed. It is, how- 
ever, evident that the intrusive stock is not of this nature. East of 
Sheep Mountain successively higher horizons are cut, and in Bear 
Park, and again on the east footslopes of Woodhurst Mountain the 
Carboniferous shale (Quadrant formation) is cut by the igneous rock. 
At Woodhurst Mountain the channel of Willow Creek is cut along the 
eastern contact until the open country is reached. The contact at 
Woodhurst is chiefly with the massively bedded Carboniferous, but on 
the west face of the mountain a* sharp upturning of the beds, as if a 
block of strata were hinged and lifted up, exposed what appear to be 
Cambrian limestones, though too altered for a positive identification. 
The beds dip away from the porphyry at 40° to 50°, and on the south 
side the massive Carboniferous limestones dip away from the mountain 
at 30°. These dips are opposed to those of the adjacent regions and 
the general mountain folding, and can be explained only by supposing 
a laccolithic uplifting at the place. The northern or northwestern 
edge of the porphyry mass shows gently dipping beds of Carboniferous 
limestone, the inclination being about 2°. It is evident that there 
must be a fault along the course of the creek. The contact is not 
regular, but breaks through difl'erent beds. The rock found in the 
eastern hill is platy, the edge being parallel to the slopes of the hill. 



328 GEOLOGY OF THE LITTLE BELT MOUNTAINS, MONTANA. 

TOGO CREEK VALLEY. 

• 

Yogo Creek has cut a deep and narrow valley, whose southern slopes 
are so heavily timbered that few exposures are seen, while the slopes to 
the north show good exposures of the bedded rocks. To the south of the 
Yogo settlement the steep walls of Prospect Eidge rise nearly 2,0()0 
feet above the creek. Near the top the Carboniferous limestones are 
seen to dip gently toward Yogo, though the ledges gradually descend 
the mountain slopes eastward. In Schoppe Mountain and Sheep Moun- 
tain, the two knobs that cap the lateral spurs of the main ridge north 
of the settlement, the slopes also show northward dipping beds, and at 
the mouth of Bear Creek this is the direction of the dip. A mile north 
of Yogo, however, this dip is reversed and the strata are inclined south- 
ward. The creek has cut through the limestones and exposes Cambrian 
shales irom the mouth of Elk Gulch to Bear Creek. Above them the 
rocks are well exj)osed in the slopes north of the creek. Two sections 
were measured. The first, made during a snowstorm, may give slightly 
inaccurate thicknesses, as the aneroid could not be depended upon and 
the x)ocket level was useless. This section shows the succession of the 
beds observed in the slopes of Sheep Mountain immediately north of 
the town. The second section given was made on the spur between 
Bear and Yogo creeks. The Sheep Mountain section is given below. 
The beds dip at 10<=^ up Skunk Creek — ^that is, into Sheep Mountain. 

Section of beds exposed north of Yogo, 
Porphyry sheet: Feet. 

Thinly bedded, massively weathering, gray limestone 250 

Limestone, forming cliffs 50 

Impure, buff limestone, forming three bands, separated by 20 to 25 feet of 

thinly bedded, dark-gray limestone 115 

Massively bedded white limestone, weathering with prismatic jointing, 

breaking into splintery fragments, and carrying small cherts 35 

Limestones, in beds 5 to 8 feet thick, dark gray, and containing fossils which 

weather in relief on faces of shaly rocks 400 

Limestones, thinly bedded, impure and shaly; graj colored on fresh frac- 
ture, with reddish buff and lavender tones on weathered fragments. 

Numerous fossils 100 

No exposure ; porphyry sheet at 6,900 feet above sea level 100 

Limestone; gray and light gray, dense, not crystalline, beds of light brown 
weathering buff, alternating with gray- weathering beds. The uppermost 

75 feet is probably Carboniferous 1 190 

No exposure 15 

Black limestone containing calcite geodes ^ 20 

Black limestones. Forms base of Jefferson limestone series 30 

No exposure. 

Yogo limestone 10 

Dry Creek shale : 

Shales: micaceous, green and olive colored; of Cambrian aspect. 



WEBD.] TOGO DISTRICT. 329 

Section at mouth of Bear Creek j Togo Gulch. 
Castle limestone : Feet. 

Limestones, in heavy beds of white, almost structureless rock, in which the 
sharp limestone canyons and gateways of the creek are out. It is this bed 
that forms the canyon at Bear Park. 

Massive white limestone, underlain by a breociated rock carrying dark- 
brown, cherty limestones in fragments 6 to 10 feet across. The rock shows 
glistening grains resembling sand, and passes downward into a dark- 
gray rock 200 

Woodhnrst limestone: 

Buttress limestone, weathering yellow, generally gray on fresh fracture, 
carrying scanty fossil remains, including crinoids, corals, and spirifers. 
The rock forms liold buttresses, but has shattered on weathering into small 
fragments 2 to 3 inches across 100 

Limestone; dark gray, with crystalline fragments of crinoids and other 
fossils 200 

Limestone, in massive beds at base, capped by less dense layers of limestone 
above. The rock is a gray, massive limestone, but its character varies at 
different parts of the same horizon, and is often yellowish and decomposed. 
The upper part forms sharp pinnacles rising above the slopes ^ 125 

Limestone, thinly bedded, very fissile 100 

Limestone, gray and cherty, occurring in 2 and 3 foot beds 8 

Thinly bedded limestones, striped with wavy bedding lines and carrying 
chert lenses parallel to the bedding. The rock is dark ish gray and changes 
to a lighter-colored, more flaggy limestone 25 feet above the base. Dips 
at 30^ into the hillside 75 

Thinly bedded limestones, darkish-gray color, rather platy, and show abun- 
dant Carboniferons fossils 85 

Cliff limestone, often broken into buttresses ; heavily bedded with rough gut- 
tered surfaces ; dark gray in color and in quite prominent layers 25 

Darkish-gray, thinly bedded limestone, cherty, and carrying abundant 
fossils. This bed and the one above are seen in castellated masses at the 
month of a small easterly branch of Bear Creek, the stream draining the 

amphitheater of Ricard Mountain 45 

Paine shale : 

Limestone, gray, red, and purple, rather massive, and breaking into angu- 
lar d<«bris 60 

Fossiliferous gray limestone, probably argillaceous 1 

Shaly limestone, like that below, carrying abundant fossils 75 

Limestone, light brown, earthy colored, granular ; without fossils 1 

Threeforks shale : 

Shaly limestone, with reddish color, carrying fragments of small crinoids 
and resembling the Devonian horizon 15 

No exposure, but debris of light-brown limestone. 
Jefferson limestone : 

Limestone, light brownish-gray in color, weathering pink; probably part of 
the bed below. The rock is a granular crystalline one that occurs in 
flaggy beds 20 

Limestone, dark colored, granular, showing light markings a half inch 
long that look like scratches on the rock and appear to bo characteristic of 
the formation 50 

Limestone, dove colored, not crystalline, breaking into small fragments. 
No fossils seen 2 

Black limestone, forming cliff ledge at edge of bench 25 

Light, earthy-colored, fissile limestone, more argillaceous than that below. . 2 

Earthy-colored limestone, broken down into slope 10 



330 GEOLOGY OF THE LITTLE BELT MOUNTAINS, MONTANA. 

Jefferson limestone — Continued. FeeL 

Ledge of massive limestone, showing qnite fine statification lines. The rock 
does not break readily along these lines of stratification, bnt has a Joint- 
ing which is at angles to this 35 

Limestone, light earthy color, blotched with pink; clearly a part of the 
ledge below 3 

Black ledge, everywhere prominent. The limestone is crystalline, grannlar, 
and in the exposure is a glistening rock of very dark brown or black color, 
forming a persistent ledge that is readily recognizable. The rock has a 
fetid odor npon concussion, and the lowest 5 feet of the ledge is not black 

but an earthy-brown, rotten limestone 18 

Yogo limestone: 

Light earthy-brown limestone, occurring in beds that break readily into 
splintery fragments 2 to 3 inches across 60 

Shaly limestone of light earthy-brown color ; really part of bed above 6 

Dry Creek shale : 

Limestone, quite shaly, reddish in color, and weathering readily into red 

earths. It is really a laminated calcareous shale 25 

Pilgrim limestone : 

Limestone, thinly bedded, but not shaly 20 

Limestone, quite fissile, breaking into plates 1 to 2 inches thick, and forming 
ledge near creek 7 

Limestone, conglomeratic, forms lowest ledge exposed 10 

Laminated calcareous shales, occurring at the mouth of Bear Creek, where 
they are broken through by an intrusive sheet of minette 5 

Sheep Mountain. — Sheep Moantain is the sammit of a spur of the 
main ridge lying between Bear Park and Skunk Creek. The mountain 
is a synclinal fold, cut by a dike-like extension of the Yogo stock. The 
strata at Yogo dip north, while at the northern sammit of the moantain 
the dip is south. A section of the sedimentary rocks has just been 
given. In the dikes seen cutting the west face of Sheep Mountain the 
contact is vertical and in nearly undisturbed limestone, while the east- 
ern face of the same moantain shows that the contact is inclined and 
the igneous rock extends under the sedimentaries. The spur extending 
east to Bear Park shows porphyry flanked on the north by limestones 
dipping down the fork of Eunning Wolf Creek, The southern summit 
shows grassy slopes on top, with wooden spurs below, on which the 
porphyry debris and talus are seen extending down to Bear Creek. 

This spur is surmounted by a knob formed of the platy debris of the 
porphyry, the contact being seen in the little saddle west of the knob, 
where the contact line is, as usual, prospected, and the pits show 2^ to 
3 feet of specular iron ore between the crystalline limestones and the 
porphyry. The beds beyond this contact are dark-gray and bluish, thinly 
bedded limestones, showing the effects of the heat firom the igneous 
injections in a markedly crystalline texture. These rocks are succeeded 
by very fissile, dark-blue limestones, dipping down the spur. The top 
of the mountain, which is the northern summit of Sheep Mountain, is of 
rather massively bedded white limestone, ^orth of the summit the 
darker limestones appear, and 200 feet below it these beds are cut by a 
dike of basic rock 15 to 25 feet wide, the dike running toward Yogo 



\ 



WEED.] YOGO DI8TBICT. 331 

Peak. A second dike of porphyry was observed 400 feet below the top. 
The crest of the ridge is notched by a saddle or gap 600 feet wide 
and 600 feet or so below the top, which is cat in a sofb and readily 
weathering basic rock (shonkinite). This lies alongside of or forms a 
part of the porphyry extending up the slope to a point 200 feet above 
this saddle, the contact between the two rocks being marked by a min- 
ing shaft showing 2^ feet of iron ore (hematite). The rocks are rather 
poorly exposed, and the soil and vegetation prevented a determination 
of the exact relation of these rocks to one another. Beyond the por- 
phyry the sedimentary rocks form the slope up to the summit of Band- 
box Mountain, but are cut by a dike of shonkinite, on which a shaft 
has been sunk. The saddle between Eunning Wolf and Skunk creeks 
shows the usual contact marbles cut by two basic dikes. Two prospect 
shafts have been sunk in the contact, 150 or 200 feet below the summit 
on the Wolf Creek side. 

The stream west of Sheep Mountain, which is known as Skunk Creek, 
has a wide basin at its head, which is well wooded and thickly soiled, 
so that the rocks are generally concealed. A wagon road has been cut 
in the slope from the prospects west of Bandbox Mountain down to the 
Yogo settlement, but the cuts show only red shaly debris until near 
the Weatherwax mine, where porphyry is seen. 

BIOABD MOUNTAIN. 

Eicard Mountain is a detached elevation lying east of the mountain 
ridge formed by the Yogo stock. It is a laccolithic uplift in which the 
intrusive has broken irregularly through the various Paleozoic hori- 
zon^. The mountain is a dome shaped anticline, the limestones of the 
Carboniferous dipping steeply away from it in every direction. This dip 
being somewhat greater than the angle of the slopes, successively older 
and older beds are seen in ascending the slopes, and the beds exposed 
on the summit are still older than those on the f anks of the mountain. 
The dip flattens on the northern spur, the beds being nearly level 1,000 
feet above Bear Park, so that this may be taken as the center of the 
arch. In the saddle north of the highest summit the beds dip north at 
23°, and on the eastern spurtheeastward dip is20o. On the western face 
the small stream which is cutting its way into the anticline exposes the 
underlying black limestones with a central area of red shales supposed 
to belong to the Cambrian rocks. It is evident that the main body of 
the laccolith forming the core of the mountain is either in the Cambrian 
or beneath it, and that the igneous rocks seen on the summit are merely 
offshoots of this concealed core. 

The summit of the mountain is formed of several knobs separated by 
depressions several hundred feet below the higher point. The rocks are 
Carboniferous limestones containing an abundance of fossils, and vary- 
ing from a thinly bedded and flaggy, dark-brown limestone to the blue, 
compact limestones which lie beneath the massive white limestones that 



332 GEOLOGY OF THE LITTLE BELT MOUNTAINS, MONTANA. 

are the most conspicuous feature of tbe Carboniferous terrane. The 
southern summit is cut by a pipe or dike of quartz-porphyry that is 
several hundred yards wide and is marked by an abrupt contact, in 
which 25 feet of limestones, altered to coarsely crystalline marbles, 
separate two sheets of rhyolite-porphyry, one 15 feet and the other 20 
feet thick, from the surrounding limestones. The porphyry mass form- 
ing the summit weathers in small angular chips, and is altered so that 
quartz and an occasional large phenocryst of feldspar are the only 
minerals seen. This rock extends from a point just east of the saddle 
separating the main peak of the knob to the west, up to the summit, 
and along the summit for a quarter of a mile eastward. The northeast 
spur of the mountain shows a sheet of rhyolite-porphyry, intrusive in 
the massive limestones, which has been denuded and now caps the 
ridge. 

RUNNING WOLF BIDGE. 

The ridge extending east from Bandbox Mountain, and lying between 
Bunning Wolf and Galena creeks, is a block of tilted massive limestone 
strata, dii)ping eastward. The dip, which is 13^ on Bandbox Moun- 
tain, becomes gradually fatter toward the east. An intruded sheet of 
porphyry caps the ridge near its center, and several dikes and sheets 
are seen both in Eunning Wolf Valley and on the summit of the ridge, 
especially near the collection of log cabins belonging to the Woodhurst- 
Mortson mine. (See PI. LXVIII.) 

While the general dip of the beds is down the ridge, yet the north 
side shows an inward or southerly dip. This is due to the Steamboat 
Mountain uplift, the axis of the anticline thus formed running nearly 
parallel with Eunning Wolf Creek, along the edge of the ridge south 
of it. Several mining prospects have been found just north of this 
line, and the Woodhjirst-Mortson mine i& similarly situated. A por. 
phyry sheet is seen outcropping east of this mine, where it forms a 
talus slide, but was not seen west of the mine. The strata at this 
locality dip at 25^ into the ridge, and strike very nearly with it. On 
the summit the beds are more gently inclined. The surface, which is 
covered by open woods, and rises gradually to the west, terminates 
southward in precipitous clifl's. 

Minette iyitrtmons. — A half mile or so from the Woodhurst mine the 
bluish-black limestone bed covering the summit is cut by two dikes 
running transversely across the ridge. Farther west a third dike of 
basic rock is seen on the summit. The rocks are augite-miuettes, which 
weather to soft greenish or rust-colored materials, from which fresh 
unaltered material can seldom be obtained. The freshest specimens 
are green and contain large augite crystals. 

In ascending the ridge lower and lower beds are successively exposed 
to the knob or summit of the ridge. In the saddle between this summit 
and the steep slope of Bandbox Mountain thinly bedded blue limestones 
have a strike of N. 50^ W., and dip 20° NB., apparently with the ridge. 



WEED.] YOQO DISTRICT. 333 

The sag is due to an intrusive mass of basic rock, whose exact nature 
could not be determined, though it is probably a sheet. The rock is 
quite altered and rotten, but is recognizable as a minette by its abun- 
dant mica. It holds included masses of the shaly limestones in the cen- 
ter, which are altered to hornstone and cut by stringers of the igneous 
material. The outcrop is 60 feet across. The rocks at the borders show 
considerable induration and metamorphism. 

Similar intrusions of minette were observed at the mining cabins in 
the creek bottoms below the Woodhurst mine, the rock being a much 
altered augite-minette and forming an intrusive sheet dipping south at 
20°, conformably with the limestones, and showing on both sides of the 
creek. This is cut by a 6-foot dike of similar rock which appears 100 
feet above the creek on the slope to the south, and which trends north- 
east and southwest, the direction of Yogo Peak. 

SAGE GREEK MOUNTAIN. 

Like Eicard Peak, this mountain mass is a partly stripped laccolith. 
The .mountain shows a central mass of igneous rock, surrounded by 
massively bedded white limestones, which dip away from the mountain 
at steep angles on all sides. The drainage ways which score its sides 
carry water only where cut in the porphyry, being dry in the limestone 
areas. The porphyry slide covers the eastern knob of the mountain. 
The mountain is precisely similar to Kicard Peak in structure, but the 
massive limestones are not yet cut through, and so far as seen are 
the only rocks exposed. As this mass lies so near Ricard Peak, there 
is a sharp and sudden upturning of the limestones between the two 
mountains, and Sage Creek flows along a synclinal fold. 

STEAMBOAT MOUNTAIN. 

This mountain shows a laccolith just emerging from its cover of sedi- 
ments. The mountain flanks show the massive beds of Carboniferous 
limestone dipping away on all sides from the central peak. The dip is 
steepest near the summit, where it approximates 30°, and lessens rap- 
idly as the distance .from the summit increases. The southern slopes 
show the Cambrian and Devonian rocks lying beneath the laccolith 
and thb Carboniferous. The northern slopes were not visited, but the 
laccolith appears to lack symmetry and to be in contact with the Car- 
boniferous on that side. The saddle between Steamboat and Bandbox 
mountains, at the head of Running Wolf Creek, shows a sharp synclinal 
folding of the limestones where the northerly dipping beds of Bandbox 
are uptilted and the dip is reversed by the Steamboat laccolith. To 
the northwest of Steamboat the broader uplift, caused by a concealed 
intrusion east of Big Baldy, limits the upturning on this side. 

The laccolith appears to be intruded mainly in the soft shales of the 
Cambrian. These rocks are considerably metamorphosed at or very 
near the contact with the igneous rock, forming dense and glistening 



334 GEOLOGY OF THE LITTLE BELT MOUNTAINS, MONTANA. 

black schists, dense hornstones, and those garuet-calcite aggregates 
and other common forms produced by contact metamorphism. The 
Monarch limestones (Devonian) and the Carboniferous, being farther 
from the igneous rock, are less profoundly altered, though they show 
in their texture and color the influence of the metamorphic processes. 
The highest part of the summit is formed of these altered rocks. The 
laccolith rock is a diorite-porphyry. It is dark gray in color, and to 
the eye closely resembles the porphyries of Sheep, Woodhurst, and 
Sage Creek mountains, though darker in color. Under the microscope 
it has the same general habit, but the plagioclase feldspars predomi- 
inate. It forms a platy d6bris, covering the summit, which, even 
where wooded, shows little soil. 

The mountain slopes, when viewed from a distance, show the lacco- 
lithic nature of the uplift very plainly, the limestone beds forming curved 
sheets wrapped about the flanks, as shown in the accompanying dia- 
grammatic sketch (fig. 40). The succession of strata is best seen in the 
saddle to the south, the narrow ridge showing good exposures. The 
rocks are here cut by twelve or more basic dikes, radiating from 
Yogo Peak, as already noted. The debris of a similar rock was found 
on the summit of the mountain, and is probably derived from a similar 
dike. An oiishootof the laccolith breaks across and through the tilted 
rocks near the south contact, the rock being quite similar to that of the 
main laccolith. 

The southern spur shows a small fault when seen from the summit of 
Bandbox Mountain, the fault plane dipping very steeply to the north, 
the downthrow on that side being estimated at less than 100 feet. This 
was not observed when the ridge was visited, and is probably recog- 
nizable only in the cliffs forming the west face of the ridge. It is the 
only instance noted in the laccoliths of the range in which the cover 
shows faulting, other than that due to general asymmetry of the 
intrusion. 

The largest southerly spur of the mountain is that on which the Sir 
Walter Scott mine is situated (see p. 451). The altered Cambrian 
rocks exposed on this ridge show three intrusive sheets of porphyry 
between the summit and the saddle back of the knob on which the mine 
is situated. South of this saddle a lesser knob, formed by a poj-phyry 
sheet and another sheet estimated to be 300 feet thick, lies between the 
Cambrian and Devonian rocks. The rocks of this si)ur are cut by sev- 
eral dikes of the dark trap rocks, similar in character to those of the 
Eureka divide at the head of Running Wolf Creek, and believed to be 
extensions of the same dikes. The same minette rock was observed in 
the dump heap of the mine just mentioned. 

The northern base of Steamboat Mountain ends in the limestone 
cliffs of Wolf Creek. The dip is gentle, and exposes successively higher 
and higher beds as one travels from Lion Creek northward. Porphyry 



WEKD.1 BIG BALDT MOUNTAIN. 335 

is Been but 200 to 300 feet above the limestones wbich cross Dry Wolf 
Greek and lau up the walls east of Butcherknife. This porphyry is 
seen on the western spur of the mountain but a few hundred feet above 
the creek, while on the uortbero apar of the mountain limestone beds 
dipping at 30^ away from the peak are seen at least 500 feet higher. 
These exposures were not visited, but the relations are plainly visible 
from the opposite side of the valley, and the elevations were determined 
by hand-level measurement. It is assumed that the porphyry seen is 
the edge of the laccolith, which on the north side of the mountain has 
therefore broken u[) through the Carboniferous beds. 

BIG BAX.DY MOUNTAIN. 

DESCBIPriOK OF THE PEAK, 

The highest mountaiu of the range is Big Baldy. Its bare, smooth, 
dome-like summit rises to a height of 9,000 feet above tide water, and 
forms a couspicious feature of the region. Like all the more prominent 



Fio. 10.— Stistt a««d b; SUainbont UoudUId lurfolllh. Eoreh* dldds. 

mountain masses of the range, it is formed of igneous rock, a variety of 
granite- porphyry designated the Barker porphyry. This rotk here 
forms a great mass 3 miles wide and 4 miles long, with a vertical thick- 
ness exposed of 2,000 feet. Thenuiform character of the rock indicates 
that it constitutes a single body and is not the result of several intru- 
sions. 

Ou three sides this igneous mass is surrounded by stratified rocks, 
whose general attitude is that due to the uplift of the range, the beds 
being only locally disturbed by the intrusion. The broad arching of 
the strata seen in the laccolithic uplifts already described is not se«D 
here. The contact plane is not generally well exposed, but the rela- 
tions of the stratified rocks, as shown in the cross section (fig. 41), indi- 
cate that the contact is nearly vertical, and that the instrusion is of the 



336 GEOLOGY OP THE LITTLE BELT MOUNTAINS, MONTANA. 

type of laccolitli first named and described by Professor Iddin^s as a 
bysmalith.^ On the west the igneoas rock is in contact with the crys- 
talline schists, but there is no evidence of a fanlt or equivalent fold in 
the continuation of this contact line into the areas of stratified rocks. 
The source of the intrusion is not known, but the nearness of the 
Togo Peak center of igneous activity and the peculiarities of the Big 
Baldy rock show a close relation with that stock, and indicate that the 
intrusion is but one of the many masses associated with and forming 
part of the Yogo epoch of igneous activity. The observed relations of 
the mass to the adjacent rocks indicate that it is the result of the 
intrusion of a magma rising through a conduit in the gneiss and 
spreading out in the shaly beds forming the base of the Cambrian. 
The overlying rocks were lifted and arched; but, fracturing about the 
borders of the intrusion, were pushed upward as a dislocated block 
and raised 2,000 feet above the original position. The grain of the rock, 
which is similar to that of the laccolithic rocks of the range, is very 
uniform, and the crystalline texture indicates that the magma solidified 
beneath a covering of rock, now denuded. 



BifeBaldyMt 



Storr Pk. 




-0000 



. eooo 



- 7000 



Horizontal Scale- 
2 d « 



:lMilo4 



Fio. 41— North weAt-AonthAaBt section through Big Baldy Mountain and Storr Peak. Scale on right 

shows height above sea level. 

The dome-like outline of the mass is but slightly altered by erosion, 
and is seen in the profile of the mountain from most points of view, as 
shown in PI. XLVII, A, and PI. XLVIII, A, The northern slopes are 
smooth and rounded, but are slightly scored by gullies, and may repre- 
sent the denuded and as yet slightly altered surface of the intrusion.  
To the south the summit is indented by two deeply cut amphitheaters, 
but the intervening spurs still preserve a gentle slope and rounded sur- 
face, like that of the northern side of the peak. The diagram above (fig. 
41) shows the profile as it appears from Yogo Peak, the accompanying 
plate (PI. XLVII, A) being made from a photograph taken at that 
place. 

The summit slopes are everywhere smooth, rounded, and covered by 
platy debris, no massive exposures being seen until the actual summit 
is reached. The two immense amphitheaters cut in the southern side 
of the summit show magnificent exposures of the rock in their precipi- 
tous walls. The accompanying illustration (PI. XLVIII, B) shows the 
massive weathering of the rock. The great blocks seen in this view 



* Journal of Geology, October-November, 1898. 



F CARPENTER CREEK 



WMRD.) BIO BALDY MOUNTAIN. 387 

break readily, on falling from the cliffs, into platy masses, often several 
feet across and bat a few inches thick, and this debris accnmnlates in 
great tains slides at the base of the clifi's and fills the bottoms of the 
amphitheaters. The walls are not uniform surfaces, but show sharp 
projecting buttresses and intervening debris slides. A lakelet fills a 
hollow in the bottom of the amphitheater surrounded by talus heap- 
ings, overgrown with stunted alpine pines. If these heapings are 
morainal they constitute the only evidence of glaciation observed in 
the range. 

THE IGNEOUS BOOK. 

The rock has a light-gray or purplish-gray color, which weathers with 
a reddish tinge. It is clearly feldspathic, and shows very prominent 
rectangalar cross sections of a fresh, glassy-looking orthoclase feldspar, 
embedded in a dense groundmass dotted with small opaque white feld- 
spars and a peppering of minute black grains of hornblende and biotite. 
The rock is quartz-syenite-porphyry, but is related to and mapped as 
a phase of the Barker granite-porphyry. It is fresh and unaltered, 
and contains occasional small inclusions of gneiss, shale, limestone, and 
minette, which are apparently but little altered. The rock is very 
similar to those forming the eastern part of the Togo stock; i. e., at 
Bear Park, Woodhurst Mountain', etc. Its character and*affinities to 
these rocks are fully discussed in the accompanying paper by Professor 
Pirsson. 

BIKES IN POBPHYBY MASS. 

This rock is cut by dikes, but owing to the platy debris covering the 
summit their outcrops are obscure on the mountain top. The walls of 
the eastern amphitheater of the peak show several black dikes cutting 
the light-colored porphyry; and the debris, of a dense, very dark rock, 
which proves to be an analcite-basalt, occurs strewn over the south- 
western slope, mixed with the porphyry. Two dikes of lighter color 
than the porphyry of the amphitheater walls ate recognizable to the 
west of the black dikes seen on the west side of the eastern amphi- 
theater. 

INTRUSIONS IN SUBBOUNDING STBATA. 

Numerous dikes and intrusive sheets occur in the stratified rocks 
about the mountain, especially in the easily invaded Cambrian shales. 
Where these sheets have been actually located they have been shown 
on the map, but there are probably other sheets not exposed or exten- 
sions of the sheets mapped at localities not visited. They belong to 
the two groups of feldspathic and basic (trap) rocks, and are in the 
main regarded as offshoots of the Yogo Peak center, except in the case 
of the thick sheets exposed by Wolf Greek at the southwest base of 
the mountain, where their connection with the Big Baldy mass is proba- 
ble. The occurrence of these intrusions is noted in the succeeding 
pages, together with that of the stratified rocks in which they are 
intruded. 

20 GEOL, PT 3 ^22 



838 GEOLOGY OF THE LITTLE BELT MOUNTAINS, MONTANA. 

The relation of the Big Baldy mass to the sarroanding rocks has 
akeady been summarized. To the north the drainage channel of the 
head of Dry Fork of Belt Greek is cat along the contact and separates 
the porphyry from the Arehean gneiss, which forms a roagh, hilly dis- 
trict. To the west the ridge ranning to Neihart Moantain (Long 
Baldy) shows the same red and gray gneiss. 

BELT OBBEK DIVIDE. 

Soath of the moantain a narrow ridge dividing Wolf and Belt creeks 
shows thinly bedded Cambrian shales, with nameroas conformable 
intrasive sheets of igneous rock. The strata at the immediate contact 
with the porphyry are somewhat, but not profoundly, altered, forming 
a hard, dense, blue, shaly debris that obscures all massive exposures. 
An offshoot of the main intrusion cuts these rocks. The first good 
exposures seen near the contact are hardened shales and conglomerates 
that dip at an angle of 3^ toward the peak. South of the little sag that 
defines the mountain slopes ^m the ridge the same beds dip south at 
a low angle. The knobs or summits along the ridge are capped by por- 
phyry, and the sheets form crescentic benches on the slopes. The ridge 
has a general descent to the south, so that a single sheet may appear 
on the top of one of these knobs and reappear on the slopes to the south. 

The western spurs of this divide ridge are generally grassy or but 
sparsely wooded slopes, on which the intruded sheets form benches and 
little cliffs. The intervening shales weather down to a slope showing 
no exposures, except where locally hardened by contact metamorphism 
alongside of the intrusive masses. The spur visited showed eight 
intrusive sheets in a total thickness of 700 feet of shale. Two of these 
sheets are minettes; the others are feldspathic rocks varying somewhat 
in character as in thickness. The lower four acidic sheets are char- 
acterized by hornblende. The rocks are much altered and break readily 
into a platy debris. The map shows these sheets only upon the spur 
visited, but it is probable that they occur in the adjacent ridge, as the 
I>ersistency of the sheets intruded in the shale is a very striking feature 
of the region. The thicker sheets have produced considerable meta- 
morphism of the shales near the contact. A sheet 75 feet thick, occur- 
ring 400 feet below the top of the ridge, has baked and hardened the 
shale for 10 feet from the contact. These sheets, together with the 
inclosing strata, dip at a low angle to the west on the upper slopes, but 
this is reversed on the lower slopes, the dip being east, or toward the 
main divide, at 3^ to 5^, conforming to the general dip that prevails 
down Belt Greek. That this is the general dip of the beds forming the 
divide is proved by the attitude of the same rocks exposed east of the 
ridge, about the head waters of Wolf Greek, where the dip is also east. 

SOUTH AND EAST FLANKS. 

The eastern spur or shoulder of Big Baldy, which ends at the Big 
Park of Wolf Greek, shows good exposures of the bedded rocks, dip- 



wm>.] BIG BALDY MOUNTAIN. 339 

ping at a low angle away from the peak. The contact between them 
and the igneous mass is defined by a shallow notch or sag at the head 
of a small drainage way tributary to the stream from the eastern am- 
phitheater of the peak. The highest beds are the massive Carbon- 
iferous limestones, whose bold ledges are seen dipping at 6^ away 
from the peak in the western face of the spur. 

m 

BIG PARK. 

Big Park, as the meadows of Dry Wolf Greek together with the 
thickly timbered upper valley of that stream are called, is so closely 
connected with the Yogo Peak mass that it may properly be treated 
here. The lower parks of the creek are bordered by steep walls of 
limestone. The beds dip at a low angle (8^) down the creek, and as 
one ascends the stream successively lower horizons are passed, until 
near Lion Greek the white limestones of the Oarboniferous are seen 
underlain by the dark -brown beds of the Jefferson limestone. As usual 
about Yogo Peak, the shales are intruded by sheets and dikes of vari- 
ous rocks, offshoots of the Yogo Peak center of Igneous activity. In 
Lion Gulch the shales are seen overlain by limestones, and the mineral 
deposits of the gulch are found at the contact between the brown 
Jefferson limestone and an intrusive sheet of porphyry, about 150 feet 
below the base of the white limestone series. Big Park is due to the 
widening of the valley of Wolf Greek, owing to the presence of the 
Cambrian shales. . A measured section was made of the beds exposed 
on the eastern end of the spur, which forms the steep slopes west of 
Big Park, opposite Lion Greek. 

stratified rocka exposed in north toall of Big Park, opposite Lion Creek. 

Feet. 
Carboniferons limestoneB, 1,400 feet above creek ; fossils, FenesteUa, crinoids, 

brachiopods ; all of typical Carboniferous aspect 150 

Paine shale : 

Limestone, bluish and hard f 

Limestone, shaly , dark colored i 

Limestone, shaly, light colored j ^^ 

Jefferson limestone : 

Limestones, black; top of bed forms bench ^ 

Limestones, granular, crystalline, black, Devonian aspect / 

Limestones, massively bedded, light colors, pitted and rotten looking, 

often pinkish and gray 35 

Limestones, bluish, but not black 100 

Felsite- porphyry sheet 24 

Limestone, buff weathering; light brown on f^esh fracture; Devonian 

aspect 15 

Limestone, dark blue 25 

Intrusive sheet 15 

Limestone, Jefferson fades 10 

Yogo limestone : 

Limestone, thin-bedded, very dense, blae-gray rook, in beds of 2 to 4 
inches 65 



' 165 



340 GEOLOGY OP THE LITTLE BELT MOUNTAINS, MONTANA. 

Togo limestone— Oontinned. Feet. 

LimeatoDe bed, very peraiBtent, and forms ribbon-like line along slopes; 
always weathering out li 

Limestone, rotten, bn£f-colored rock, breaking with irregular fracture ... 10 
Dry Creek shales : 

Shales, red and purple, with impure, yellow, thinly bedded limestones. .. 40 
Pilgrim limestone: 

Limestone, purple colored and shaly .* 45 

Limestone, massive, blue, weathering yellow and rough ; irregularly flaggy . 25 
Park shale : 

Red shaly beds 115 

Meagher limestone : 

Limestone an4 conglomerate 300 

Intrusive sheet of igneous rock (5 feet). 
Wolsey shale : 

Shales, micaceous, forming wooded slopes without exposure 

Intrusive sheet of igneous rock (15 feet). 

Shales, soft, micaceous 

Intrusive sheet 

Shales, micftceous. 

The intrasive sheets and dikes noted in this section were the only 
ones examined on the eastern flanks of Big Baldy Mountain. The 
lowest sheet is traceable along the base of the slope for a mile or more 
to the soathwest, but is carried by its dip beneath the meadows to the 
north. The rocks are like those already described as forming the 
encircling sheets of Yogo Peak, and the different sheets exposed in 
this section also occur intruded at the same horizons, across Big Park, 
2 miles southwest of the mouth of Lion Creek. A still lower mass of 
nearly white intrusive rock is seen on the banks of the creek at the 
upper end of the park, where it forms white talus slopes. Going south- 
ward this sheet is seen to be over 100 feet thick, and west of the creek 
forms a ridge coming from the eastern amphitheater of the mountain. 
Its extent is not definitely known, but must be considerable. The rock 
is a dense, white rhyolite- porphyry. That the rock is a sheet is clearly 
seen at the point at which the stream from the Big Baldy amphitheater 
joins Dry Wolf Creek, where Cambrian shales are exposed underlying 
the rhyoliteporphyry. The igneous rock only is seen above, where it 
forms hillocks and debris piles extending northward. 

HEAD WATERS OF DRY WOLF GREEK. 

West of the stream from the western amphitheater of the mountain 
the trail up Dry Wolf Creek traverses a densely timbered bench or 
slope, on which there is much quartzite and porphyry drift, but no rock 
is found in place until the trail leaves the creek and ascends the spur 
that extends east from the Belt Creek divide. Here the porphyry, 
through which the little gorge of Dry Wolf has been cut, is overlain 
by red Cambrian quartzite and sandstones 200 feet thick, capped in 
turn by a sheet of porphyry 200 feet thick. Above this the Cambrian 
shales, with several intruded porphyry sheets, extend to the top of the 
divide, a total thickness of 1,400 feet. The beds dip east at a low 



WEKD] WOLF BUTTE AND TAYLOR PEAK. 341 

angle, so that the qaartzite must pass beneath Wolf Greek not far 
above the month of the amphitheater drainage. The horizon of the 
Big Baldy intrusion is therefore lower than the basal Cambrian 
quartzite at this point. 

ORE DEPOSITS. 

There are no mines on Big Baldy Mountain, but the flanks hare been 
prospected at many places near the contact. The rocks of the summit 
are generally weathered, but otherwise unaltered, and it is with sur- 
prise that one observes the boundary stakes and shallow open pits of 
mineral claims. Thin seams of iron- stained material, said to contain 
traces of gold, were observed, but nothing to* indicate the presence of 
valuable mineral deposits. 

BUTCHEBKNIFE MOUNTAIN AND CBEEK. 

The mountain (7,821 feet in height) north of Big Baldy is formed of 
sedimentary rocks, which arch over and conceal a laccolithic center of 
porphyry. The eastern side of the mass is deeply cut by the drainage 
of Butcherknife Creek. Butcherknife Gulch was not ascended. The 
stream gravels show an abundance of syenite- and rhyoliteporphyries, 
the former being the common type found in the intrusive sheets of the 
region. A dark-green, almost black, rock, which microscopic study 
proves to be an orthoclase basalt, forms peculiarly pitted bowlders. 
The most noticeable feature of the stream drifb is, however, the abun- 
dance of dark-bluish hornstones, very hard and dense forms. This is 
the more noticeable because it is the only place about Big Baldy where 
such products of contact metamorphism of the Cambrian shales is 
noticed. A t the mouth of Butcherknife Creek the limestones are nearly 
level, but the walls east of that creek show these beds dipping away 
from Big Baldy Mountain, the angle increasing northward and the 
beds rising higher and higher ap the slopes, the dip being fully 30^ on 
top of the ridge. 

WOLF BUTTE AND TAYLOR PEAK. 

From the open country of the Judith Basin, Wolf Butte appears as a 
sharply defined conical mass, situated in front of and a'little distance 
from the wooded slopes that mark the general front of the range. This 
very prominent peak is formed of granite porphyry that is part of an 
intrusive mass extending southward for 4 miles and having a width of 
2 miles. This intrusion has arched up the beds about it on the south, 
and, in fact, the limestones which surround it dip away from it on all 
sides, so that the mass probably constitutes a laccolith, though an 
asymmetric one — like all the other intrusions of this character seen in 
the range. To the south the Barker wagon road follows the synclinal 
trough produced by the meeting of the limestones dipping toward it 
from the mountain to the south and from Taylor Peak to the north. 
This trough preserves inliers of the Carboniferous shales which form 
the meadows at the south base of Taylor Peak and the foothills east of 



342 GEOLOGY OF THE LITTLE BELT MOUNTAINS, MONTANA. 

the peak. Taylor ^eak itself is formed of massively bedded, Garbonif- 
eroas limestones, dipping westward at 15^. 

The Wolf Bntte mass of igneous rock is seen in contact with the 
Garboniferons limestones except on its soathern side, where, on the 
saddle north of Taylor Peak, the older rocks are seen sharply aptamed, 
slipped, and intruded by sheets of porphyry, but clearly recognizable 
despite the alterations dae to contact metamorphism. In this saddle 
the brown Monarch limestones and 400 feet of the jasper-ribbed Gam- 
brian rocks are very easily recognized, thongh the dips increase from 
15^ in Taylor Peak to an almost vertical attitude at the contact, and 
the slipping of the upturned beds along shale horizons has resulted in 
the absence of such beds in the section seen at the contact. The 
Gambrian rocks in particular are toughened, baked, and hardened, and 
show contact minerals, especially in the joint and bedding planes. The 
uplift is progressively less and less northward, and at the base of Wolf 
Butte, at 5,600 feet, or 1,400 feet lower in elevation than tbe exposures 
just noted, the igneous rock is in contact with the shales and sandstone 
beds of the Quadrant group. 

The laccolith rock. — The intrusion consists of granite-porphjrry of a 
normal character, which has been given the name of Wolf porphyry to 
distinguish it from the very different-looking rocks of Barker Mountain 
and the other laccoliths of the region, rocks which are also granite- 
jwrphyries, though of a very different type. 

The Wolf Butte rock is a coarse-grained porphyry, weathering with 
a massive jointing^ splitting into immense slabs, and making crags and 
slopes and castellated forms that resemble those of a typical granite. 
The rock disintegrates readily to a coarse sand, so that it is not trans- 
ported far. It shows large crystals of glassy quartz and x>inkish 
orthoclase feldspars, with minute scales of dark mica. The promi- 
nence of Wolf Butte is due to the variation in the grain and jointing 
of the rocks and its consequent resistance to weathering, the rock 
being somewhat denser and more massively jointed than the more 
granular, easily disintegrated rock forming the amphitheater between 
the butte and the mountain south of it. The Wolf Butte contact with 
the limestones is abrupt and shows but little uplifting of the sedimen- 
tary rocks. The peak between Wolf Butte and Mount Taylor is 
formed by part of the contact rim of the intrusive mass, in which the 
rock is so dense as to constitute a rhyolite- porphyry. The rock is pink- 
ish or brown and has very pronounced laminations parallel to the con- 
tact plane. It breaks with a platy fracture into thin and rather 
small fragments, and this platy parting breaks across the laminations. 
The upturned strata at this south contact of the laccolith contain 
several intrusive sheets of dark micaceous rock, resembling minette. 

Intrusive sheets near Wolf Butte. — There are several intruded sheets 
of porphyry in the areas of Garboniferons shale south of Taylor Peak, 
but the character of the rock and manner of occurrence do not con- 
nect them with the Wolf Bntte mass, but rather with the intrusions of 



WEED] WOLF BUTTE AND TAYLOR PEAK. 343 

the Barker type of porphyry. These sheets occur along the Barker 
wagon road, at the head of Arrow Creek, and in the hills north of 
Geer. At the first locality the beds are nearly horizontal and the 
porphyry is too much altered for study. In the foothills lying south- 
east of the laccolith the shales are intruded by two sheets of quartz- 
diorite-porphyry, resembling a variety of the Yogo stock rock. The 
lower sheet is, perhaps, 200 feet thick, and can be traced for several 
miles, forming a distinct bench on the smooth shale slopes. The 
upper sheet is thinner and caps the knobs or summits of the hills. 
The rocks resemble those of the east end of the Yogo stock, showing 
large feldspars and hornblende prisms in a gray grouudmass. The 
rock weathers with a reddish surface and breaks in platy masses, leav- 
ing rounded exposures, it is cut by two minette dikes, which are 
therefore younger. 

Ore deposits. — But one mineral prospect was examined. This is situ- 
ated on the knob southeast of the saddle separating Taylor Peak from 
the mountain north of it. The ]>rospect is on an east-west fissure, and 
shows galena and cerusite, together with malachite, azurite, chalcedony, 
calcite, etc. 

Dry Wolf Creek dome, — Dry Wolf Creek cu^s a canyon through a low 
dome of Carboniferous limestone that rises above the open grass land 
formed by the shales of the Quadrant formation. The rocks dip away 
symmetrically on all sides from the dome, whose structural relations 
are such that it seems very certain that it is formed by a concealed 
laccolith of igneous rock. 



CHAPTER V. 

DESCRIPTIVE GBOIiOGY OF THE BARKER AND MONARCH 

DISTRICTS. 

BARKER DISTRICT. 
DISOOVEBY AND DEVELOPMENT. 

The Barker district is situated in the northern part of the mountains, 
on the headwaters of the Dry Fork of Belt Creek. The discovery of 
ore deposits, in the years 1875 to 1880, led to the rapid development 
of the district, and the towns of Barker and Hughesville were built 
Several mines yielded large amounts of silver-lead ores from 1880 to 
1883, but the inaccessibility of the region and the high cost of transpor- 
tation to smelting centers constituted a serious detriment to the devel- 
opment of the camp. A smelter was erected at Barker in 1881 and ran 
for a short time, but the ore bodies first discovered proved to be limited 
in extent, and when the Neihart deposits were developed iu 1884 the 
camp had seen its best days. The completion of the branch railroad 
to the camp in 1888 and the building of the silver smelter at Great 
Falls gave a temporary impetus to mining development, but as the ores 
are valuable only for their silver contents the place was practically 
abandoned when the drop in the price of silver occurred in 1893. In 
1894 the Tiger and Moulton mines were the only ones being worked, 
and these only on a small scale under lease. From that time to 1897 
the mines were worked for short periods at various intervals and by 
different leasers until the demand for lead ores at the smelters and the 
granting of cheaper rates for smelting and railroad charges led to an 
active prospecting of old and new properties. At the present time the 
future oi the camp looks more promising than at any previous period 
of its history. 

EXTENT AND TOPOGRAPHY. 

The Barker district proper embraces the basin like area lying north 
of Dry Fork of Belt Creek, as shown on the map (fig. 42), and 
inclosed between Barker Mountain on the west, Clendennin on the 
north, and Mixes Baldy and the adjacent peaks on the east. The basin 
is drained by Galena Creek, on whose banks the settlements of Barker 
and Hughesville are situated. The mountain slopes were formerly 
densely timbered, but about Barker the trees were long ago cut for 
burning into charcoal, and to-day the stumps and young pines cover 
considerable tracts, and the slopes north of the basin show a forest of 

344 



wuD.] BAREEB DISTBICT. 346 

bare, gray poles, the resnlt of exteDSive forest firee. Bock expOBures 
are uowhere proniiuent, and tbe couutry is not rongti or eapecially 
ragged. The limeBtones show on Barker Mountain and are promiaent 
There thewagOQ road croBses the divide to Otter Greek and theKibbey 
Basin. The igneous rocks are more often exposed, but are seldom seen 
in couspicuons exposures except on the bare mountain tops. They 
form extensive debris Blopes north of the basin, and intrusive sheets 
form low cliftB along Dry Fork of Belt Creek. The best exi>osures of 
the great limestone seriea are seen ou the outer slojiea of the moan- 
tains that inclose the Barker Basin. 

The town of Barker lies at the lower end of this basin and only a 
short diatance above Belt Creek, The old smelter and the charcoal 



FlO. iS.—'Utp of Barker dlntrtot. 

kilns were bnilt alongside Galena Creek, below the fork of this stream 
known as Gold Itun. These, together with the Carter mine opposite 
the mouth of Gold Run, determined the site of the town. The princi- 
pal mines were, however, farther up the basin — at its northern head, in 
fact — and another settlement w.ib established there, which was given 
the name of Clendennin on the maps, but was commonly called Hnghes- 
ville by the niiners, and is still known by that name. 

The railroad was not extended to the mines, but terminates at the 
month of Galena Creek. The line has not been operated regularly for 
Bome years, although trains are run from the junction with the Neihart 
branch at Monarch whenever there are a few carloads of ore ready for 
shipment. Well-graded wagon roads nm down Dry Fork of Belt Creek 



346 GEOLOGY OF THE LITTLE BELT MOUNIAINS, MONTANA. 

to Monarch, northward over a low pass to Otter Creek and the Kibbey 
Basin, and eastward ap a branch of Belt Greek and over the divide to 
Arrow Creek and Dry Wolf Creek and the Judith Basin. The region is 
nearly as high above the sea as Neihart, but receives less snowfall and 
has a somewhat milder climate. The soft natures of the Cambrian 
shales and the crumbly weathering of the igneous rocks that prevail in 
the center of the basin give smooth and rounded contours, and open, 
rather broad, and retreating slopes. The eastern branch of Galena 
Creek, which joins that stream at the settlement of Barker, is known as 
Gold Run. A fork from the west, entering above Barker, is called Green 
Creek, while its two head-water branches unite at Hughesville, the 
settlement 2 miles above Barker, one fork coming from Kibbey Gap, 
the other forming the gap north of Mixes Baldy. This will be made 
clear by reference to the map, tig. 42. 

The principal ore de[>osits discovered thus far lie at the northern 
border of this basin-like area, on the head waters of Galena Creek. 
There are also prospects along Gold Run and south of Dry Fork of 
Belt Creek, and one mine on the eastern bank of Galena Creek oppo- 
site Barker yielded a large amount of ore in the early history of the 
camp. The rocks show no very extensive areas of decomposition. 
Though changed by weathering and the ordinary processes of rock alter- 
ation, profound alteration of the rocks accompanying the ore deposits 
is confined to small areas immediately adjacent to the veins. The ore 
deposits all occur in connection with the igneous rocks which break 
through and have folded the sedimentary rocks. 

The geology of the district is more varied than that of any other area 
of equal extent in the range. The district is situated on the northern 
border of the Archean core of the range, where the uplift of the moun- 
tains has upturned the sedimentary rocks and tilted them northward. 
This normal tilting is, however, almost destroyed by the igneous intru- 
sions of the district. These are of various rocks, and occur intrusive in 
various ways. Barker Mountain is a great mass that is laccolithic in 
character, and has lifted up the sediments about it in a dome. Otter 
Mountain to the north is another laccolith which is as yet but partially 
revealed by erosion. Its sheets form Clendennin Mountain, whose slopes 
make the north wall of the Barker Basin. Mixes Baldy, the mountain 
to the east, and the peaks adjacent to it, are carved out of an intrusive 
mass punched through the strata — a great bysmalith. The center of 
the basin is occupied by a small stock of granular rock that may be the 
center of the igneous activity of the region. 

SEDIMENTABT ROOKS OF BABKEB DISTBIOT. 

The stratified rocks of the Barker district and vicinity present no 
features of especial interest. The different formations from Cambrian 
to Mesozoic are well developed, and exhibit the general sequence 
already described as common for the northern part of the range. The 



.] BAHEEB DISTRICT, 347 

lowest beds, the basal qaartzites, which rest apon the crystalline schists, 
are seen oii the slopes south of the Dry Fork of Belt Creek, and where 
a bend of this creek, below the railroad terminus, cuts through the 
schists. In general the basal beds are indurated sandstones, some- 
times true quartzites in nature, which consist of feldspar and quartz. 
The colors are, as usual, pink or reddish, weathering rusty brown. The 
rock sometimes grades into a conglomerate, but the latter form is 
neither common nor of more than local development. Gross bedding 
is often prominent. The rock is jointed and breaks into angular debris, 
but this is never abundant enough to be of more than local interest. 
The thickness on upper Dry Fork of Belt Creek is but 60 feet, and it 
is about the same back of the railway station. The quartzite base 
differs markedly in this respect Irom the section seen north of Neihart, 
where lower and upper sandstone layers are separated by shale. As 
noted below, this may possibly be concealed either by overlap or by 
overthrust. 

The structure of the sedimentary rocks is that of a monoclinal fold, 
being the northern side of the broad anticline forming the range. Near 
the gneiss contact the dip is always steep, but the inclination lessens 
away from the gneiss and schist areas. On the slopes south of the Dry 
Fork of Belt Creek, near Barker, the schist surface slopes steeply 
northward, and where overlapped by the sedimentary rocks the dip is 
40^ northward, away from the schist contact. Near the railway station 
(at the mouth of Galena Creek) the ridges to the northwest show a reef 
of quartzite 20 feet thick whose dip is 45^ to the north, and this angle 
of dip continues for some distance northward, in the second spur west 
of Galena Creek. Along the trail from Barker to Neihart the basal 
sandstones and quartzite are seen forming a ridge running up the slope, 
but are wanting on the divide and are not exposed along the contact 
on the southern side of this divide. On upper Dry Fork of Belt Creek 
the basal members of the Cambrian run up to and are cut off by the 
Big Baldy intrusion. These observations seem to point to a sharp uplift, 
and indicate that the floor of crystalline schists was a slipping plane on 
which the Cambrian shales were shoved up and over the gneisses. 
Definite proof of this hypothesis seems to be afforded by the exposures 
along the Neihart trail. If this be true it explains the absence of the 
lower shales and the basal members of the quartzite and sandstone 
series in the exposures noted. 

The Cambrian shales and interbedded limestones are seldom well 
exposed throughout the Barker district except over small areas, and the 
field work was not thorough enough to show any variations from the 
conditions observed at Belt Park. These rocks are seen generally along 
Belt Creek and about the town of Barker, and also on the summit of 
Otter Mountain north of the mines. Three miles east of Barker the 
slopes north of the Dry Fork of Belt Creek show Cambrian strata 
overlain by a succession of limestone beds forming cliff ledges and flat 



348 GEOLOGY OP THE LITTLB BELT MOUNTAINS, MONTANA. 

benches. Here the DevoDian limestones are especially well exposed, 
and contain an abundant fossil fauna, from which only a small collection 
was brought in. To the east of this place the continuity of the strata is 
interrupted by the great mass of Wolf porphyry, which cuts off all the 
rocks earlier than the Carboniferous. A good section of the Madison 
limestones is, however, exposed along the wagon road up Blenkinsop 
Greek (the Wolf Creek road), where the characteristics of the different 
horizons of its formations may be studied to advantage. Between 
Barker Mountain and the crystalline schist area the sedimentary rocks 
are sharply folded in a synclinal trough, as noted in the description of 
that mountain. 

Vicinity of Barker. — The geology immediately about the settlement 
of Barker is revealed by numerous exposures, for although the rocks 
are very generally covered by soil and vegetation, Galena Creek and 
Gold Eun both show the rocks along their banks. Back of the railroad 
station the ridges west of Galena Creek show the Archean gneisses. 
The lower quartzite bed forming the base of the sedimentary series was 
not recognized here, but the succeeding reddish earths are overlain by 
a bed of quartzite 20 feet thick, dipping 45^ E. into the hill and stand- 
ing up as a bold reef or wall above the shales on either side. The soft, 
micaceous shales above this show an intruded sheet of chocolate-colored 
porphyry, also upturned and forming a broken down ledge, succeeded 
by grassy slopes covered with the buff debris of shales, which extend 
up the ridge for a half mile, until a limestone bed is seen, also upturned 
and weathering as a wall. The ridge above shows nearly horizontal 
beds of white limestones. From Belt Creek to the settlement the 
benches on both sides of the creek show the shales and interbedded 
limestones of the Barker formation, which are conformable with those 
seen on the ridge to the west, though the dip is less, being but 15^. 
These are seen to be intruded by a dark basic sheet of igneous rock 
(minette), 4 feet thick, near the mouth of Galena Creek and one-eighth 
of a mile above, or north of the railroad. A 30-foot dike of granite- 
porphyry, an offshoot of the Mixes Baldy mass, is seen on the west 
(right) side of the creek. 

On the slopes west of Barker the Cambrian rocks are cut by an 
intrusion of porphyry. The Carter mine is situated on its contact. 
The shales extend several hundred feet up the slope, and are overlain 
conformably by the Monarch limestones. At the mouth of Gold Bun 
the shaly limestones of the Barker formation are seen in the bluff and 
knoll to the north, back of the post-office. On the east side of the gulch 
they are seen in contact with the granite-porphyry mass (Mixes Baldy 
intrusion), about 600 feet from the forks of the creek. Above the set- 
tlement the wagon road to the mines follows a bench on the east side 
of the creek, on which no exposures are seen, but the debris is a rusty 
weathered, rotted porphyry, which extends to the forks of the creek. 
At this point the road ascends the slope and runs around the ridge 



.] BABKER DISTRICT. 349 

separating Oalena Greek from its branch, Green Greek; the ground 
shows occasional exposures of syenite, and careful examination shows 
that Galena Greek defines the boundary between a mass of coarse* 
grained syenite and a porphyry mass east of it, the exact contact not 
being determinable owing to the amount of drift and debris. 

laNEOUS BOOKS OF BABKEB DISTRICT. 

Igneous rocks are exposed over a large part of the Barker district, as 
shown by the geologic map, PI. XLI. They constitute the most lmi>or- 
tant element in the geologic structure and history of the region, and 
merit, therefore, a somewhat detailed account of their occurrence. 
They are all intrusive rocks, but vary considerably in character as well 
as in manner of occurrence, for which reason they will be described 
under the following titles: Intrusive sheets and dikes, Mixes Baldy 
bysmalith. Barker Mountain laccolith, Hughesville syenite stock, and 
Olendennin Mountain intrusives. 

INTRUSIVE SHEETS AND DIKES. 

The igneous rocks occurring as intrusive sheets in the sedimentary 
strata are conspicuous features of many parts of the district, as they 
resist weathering better than their inclosing strata and often form cMs 
and reefis that are important elements of the topography. 

Chocolate porphyry. — The most important single intrusive sheet con- 
sists of a rock whose dark-brownish weathering suggests the designa- 
tion Ohocolate porphyry and gives its name to a small stream cutting 
through it. It occurs as a sheet of varying thickness, intruded in 
Cambrian shales and found at nearly the same horizon in many parts 
of the district. It is well exposed along Dry Fork of Belt Greek east 
of Barker, where it forms a low cliff alongside of the stream. At the 
junction of Dry Fork of Belt and Galena creeks it is seen near the 
water level and can be traced in almost continuous exposure eastward 
for 6 miles up Dry Fork of Belt Greek. The thickness varies at differ- 
ent points, but is probably about 50 feet most of the way. In the cliff 
beside the wagon road the rock shows two phases. In the prevailing 
type it is dark-brownish colored and breaks with a square and sharply 
defined jointing, while its associated form is pinkish and has a sphe- 
roidal weathering. Under the microscope the two rocks are seen to be 
so similar that these differences are merely superficial. About a mile 
below Blenkinsop Greek (the stream followed by the wagon road to the 
Judith Basin) the Ghocolate porphyry intrusive splits into three sheets. 
The lowest was not measure<l, as its base was not seen. It is separated 
by 60 feet of shale from a middle sheet 15 feet thick, and this is in turn 
separated by 12 feet of baked 'and indurated shale from the uppermost 
sheet, which is 6 feet thick. The intrusive has a dip of 15^ N., 
conformable to that of the inclosing shales. 

Farther south in the exposures revealed by Dry Fork of Belt Greek, 



350 GEOLOGY OF THE LITTLE BELT MOUNTAINS, MONTANA. 

above the point where the Judith wagon road leaves that stream, the 
Chocolate porphyry sheet is more irregular in its occurrence, and forms 
high cliffs and extensive talus heapings. Its structural relations in 
this locality were not determined, but the exposures actually seen seem 
to indicate that the pipe or conduit is located near the mouth of Gray 
Gulch. It forms cliffs 150 feet high, the rock resting upon baked Cam- 
brian shale seen in a 5 foot exposure alongside the creek. The western 
bank of the creek shows 7 to 8 feet of baked shale overlain by 100 feet 
of unaltered shale extending to the summit of the bench, so that either 
there is a fault or the porphyry has broken through the shale here and 
forms an irregular intrusion, whose sharp boundary wall has been 
removed by the down cutting of the creek. The rocks here strike with 
the creek and dip to the east. The igneous rock weathers in craggy 
masses, is well jointed, and forms rough debris piles. The rock shows 
a variation in grain, one form breaking into the sharp-edged blocks 
typical of the Chocolate porphyry; the other form, which occurs mixed 
through the first in stringers and masses, is much more altered and 
weathers in fissile, rounded, crumbly masses. Up the stream the under- 
lying shales pass underground and the creek cuts the Chocolate por- 
phyry. In the park above (at Crandali Creek), as well as on the grassy 
ridge lying between the Big Baldy Mountain fork and the main stream, 
the overlying shales are seen, the remnants of a higher sheet of por- 
phyry forming knobs on both sides of the western tributary stream and 
a ridge on the east. The closeness of these exposures to the margin 
of the Big Baldy intrusion suggests its probable connection with that 
mass. 

The extent of the Chocolate porphyry intrusion is illustrated by its 
occurrence at nearly the same stratigraphic position in the slopes south 
of Dry Fork of Belt Creek. The limestones of the Cambrian form little 
hillocks, separated from the quartzite and the underlying gneiss by 
depressions or saddles worn in the soft shales. The porphyry is seen 
250 feet above these saddles, with 200 feet of shale between the intru- 
sive sheet and the basal quartzite. To the northwest the quartzite 
forms a dark, rusty, black ledge, which is readily followed along the west 
side of the basin cat by Ontario Creek, the dip being 30° to 40^. The 
Chocolate porphyry is 40 to 50 feet thick and forms the crest of a steep 
ridge or is seen running down the slopes on both sides of the ridge in 
ledges. The actual connection of this exposure with that seen on Dry 
Fork of Belt Creek was not traced out. 

West of the junction of Galena Creek with Dry Fork of Belt Creek 
the Chocolate porphyry sheet is seen forming a broken-down ledge on 
the oi)en and grassy shale slopes a short distance east of the railway 
buildings. The sheet is tilted, dipping 45^ £., conformably with the 
basal quartzite ledges seen near by. The intrusion can be traced west- 
ward at this horizon 2 or 3 miles, but has not been recognized in this 
locality south of Dry Fork of Belt Creek. The Chocolate porphyry 



.] . BARKER DISTRICT. 351 

shows almost no variations in character throughout the entire extent of 
the exposures just noted. It is a distinctly porphyritic rock, of a gray 
or pale pinkish-brown color on fresh fracture, but generally covered 
by a brownish crust of altered rock. 

Opaque white feldspars are the most conspicuous phenocrysts, though 
fine needle-like prisms of hornblende are far more abundant, and glis- 
tening tablets of biotite-mica are also seen. In most of the exposures 
these dark minerals are green, being more or less altered to chlorite, 
and the feldspars are decomposed and pinkish in color. Microscopic 
study shows the rock to be a rhyolite-porphyry. 

BlenkinBop Greek intrusive sheet — The Carboniferous limestones 
exposed in the ravine cut by this stream dip gently up the creek, the 
angle being less thau 10^. They are intruded by a sheet of porphyry 
estimated to be 50 feet thick, whose characters are similar to those 
already described. The rock is gray and shows a stippling of small 
white feldspar phenocrysts and altered hornblende-micas in a dense 
pinkish-gray groundmass. The rock is a variety of rhyolite-porphyry 
related to the Chocolate porphyry sheet below. 

Trachyte or bostonite sheet, — A sheet of light-colored rock cut by Dry 
Fork of Belt Creek above the Sheep Creek parks should also be noted. 
The rock forms a low bench, with an apron of debris in front of it. The 
sheet occurs in Cambrian shale but a few feet above the Flathead 
quartzite and is chiefly interesting because of its petrographic char- 
acter. It is described in the appended paper by Professor Pirsson. 

Sheet of Wolf porphyry. — The upper sheet exposed on the north side 
of Dry Fork of Belt Creek, east of Barker, consists of granite-por- 
phyry of the Wolf Butte type. The sheet is regarded as an ofifshoot 
of the bysmalith mass of Mixes Baldy. It is exi>0Bed by the road cut- 
ting east of the railway terminus, and forms the grassy bench on which 
the cemetery is situated. By its debris and an occasional exposure it 
is traceable eastward as far as Blenkinsop Creek. It gradually thins 
out and is but 50 feet thick at the latter locality, where it appears to 
suddenly wedge out. Its upper surface forms a very marked sloping 
bench, which is 400 feet above the creek 2 miles east of Barker, but 
which descends westward and is traceable along the slope to Galena 
Creek. The interval between the base of this porphyry and the top of 
the Chocolate i>orphyry is not definitely known. It must be less than 
100 feet at the eastern end of the sheet and but a few feet at the mouth 
of Galena Creek. The dikes observed in the shales opposite Barker 
are believed to be an extension of this sheet, but a mile west of Barker 
no trace of it was found. The rock is a somewhat altered, denser 
variety of the Wolf porphyry type of granite-porphyry. 

Minette sheets. — Besides the Chocolate porphyry just described the 
shales of the district are intruded by the rocks occurring both as dikes 
and sheets. The most common of these is a sheet of dark trap-like 
rock, which has been found in almost every ^art of the district as an 



352 . 6EOLOGT OF THE LITTLE BELT MOUNTAINS, MONTANA* 

intnusiye sheet This, though only 4 to 5 feet thick, is of widespread 
extent, being found on Otter Mountain, at the mouth of Galena Greek, 
on Upper Dry Fork of Belt Greek, and filling the outcrop of the Gam- 
brian shales for several miles up and down the course of that stream. 
Like the trap-dike rocks of the district, it is too much altered for posi- 
tive identification, but may be classed as a minette. 

Intrusive sheets of vogesite between Barker and Monarch, — Four miles 
west of Barker the wagon road down Dry Fork of Belt Greek is cut 
across the outcrop of dark basic rocks intrusive in the shales. The 
sheet beside the road is perhaps 35 feet thick. The rock of this expo- 
sure is clearly a lamprophyre. It is soft and altered, of a dull-gray 
color, with a glistening luster, and shows no phenocrysts. The exx>o- 
sure shows the usual concentric bowlder weathering common to such 
rocks. The second and upper sheet is exposed to the little drainage 
from Barker Mountain at this locality, where it causes a waterfall, owing 
to the hardening of the shales at its contact. The lower sheet is 
exposed alongside of the road for a mile or more eastward to another 
stream coming down from Barker Mountain and opposite the point 
where the Neihart trail crosses Dry Fork of Belt Greek. On the oppo- 
site side of Dry Fork of Belt Greek the cliff alongside of the creek 
shows a wall of columnar rock that is undoubtedly part of the sheets 
that are exposed on the north side of Dry Fork of Belt Greek. There 
are really two sheets, the lower 14 feet thick and the upper 25 feet 
thick, separated by 8 feet of Gambrian shale. The latter rocks are 
baked and hardened for some distance above and below each sheet and 
between them, the contact action being noticeable for at least 10 to 15 
feet from the contact. The rock shows the same concentric weather- 
ing, spherical sheets peeling off rounded, bowlder-like masses. The 
rock includes fragments of the shale and also of the underlying gneiss, 
so it is probable that the rock came up here and spread out as a sheet 
in the adjacent strata. The beds dip at 20^ to 25^ to the south, or 
directly opposite to the prevailing dip of this vicinity. The shales 
above the .upper sheet are much contracted and puckered. They show 
an unusual amount of alteration, secondary minerals being developed. 

IHkes of Barker district. — The dikes of this district are comparatively 
few in number and play but a minor part in the structure of the region. 
They are mainly dark basic rocks, which in most exposures are too 
highly altered for petrographic study. Light-colored dikes also occur, 
but they form tongues of the Mixes Baldy mass and are therefore noted 
in the account of that plutonic plug. The dikes observed cut the sedi- 
mentary rocks, and in one instance the massive granular syenite near 
Hughesville. Their occurrence is shown upon the geologic map, but, 
owing to the ready decomposition and weathering of the basic rocks, it 
is probable that further study would add to their number. 

On the eastern spur of Barker Mountain above the Kibbey divide a 
12-foot dike of dark-greefiish minette cuts through Garboniferous lime- 



WBED.] BARKER DISTRICT. 353 

stones, which are marinorized near its contact. Dikes of similar rock 
occur on the slopes north of Dry Fork of Belt Creek a mile east of 
Barker, and another one a half mile beyond. Similar dikes were 
observed 4 miles north of Barker alongside of the Monarch wagon road 
and near the intrusive sheets of vogesite noted in the preceding pages. 
A dike cutting the Hughesville syenite stock is exposed at the Wright 
and Edwards mine and in the mine workings. The dike is about 20 
feet wide — a dark basic rock that forms one wall of the lode. The 
rock is dull olive-gray in color, is hard and dense, and rings under the 
hammer. It shows large phenocrysts of crackled, glassy, pale-brown 
quartz and of white or faintly brownish decomposed augite in a very 
dense groundmass. The rock is regarded as a kersantite. 

HUGHES VI LLK 8YEN1TK STOCK. 

The center of the Barker Basin is occupied by a mass of coarsely 
crystalline granular rock, which proves upon microscopic study to be 
a syenite. The area covered by it is nearly circular in outline, about 
a mile across, and is eroded into low ridges and hills forming the basal 
slopes of Barker Mountain and lying west of Galena Greek. This tract 
is generally smooth and rounded, showing only d6bris and soil, and at 
the present time is open, the timber having been burned or cut off. 
Galena Creek defines very nearly the eastern boundary of this syenite, 
while other portions of its coutact are in part also defined by small 
drainage ways. The rock is granular and weathers down, so that no 
good natural exposures occur, and good specimens of unaltered rock 
can be obtained only from the various mine openings made In it. The 
syenite forms a "stock" — an intrusive mass breaking abruptly through 
all other rocks. It is nearly surrounded by Carboniferous limestones, 
the older rocks showing on its southern border, while to the east it 
adjoins a mass of porphyry. The sedimentary strata along its west 
coutact are on edge or dip at 80^ toward the syenite, but this attitude 
may not be the result of the intrusion. The strata are altered by con- 
tact metamorphism. The contact is generally marked by decomposed 
rock and the presence of many shallow prospect pits and refuse dumps. 
The rock is exposed on the creek banks above Hughesville. It is much 
jointed and altered, even the fresher material from the underground 
workings being cracked and seamed with pyrite films. The rock is of 
a grayish or rarely purplish-gray color, and shows light reflected from 
the flat surfaces and narrow cross sections of tabular feldspars lying 
in a coarsely crystalline mass dotted with the small formless masses of 
dark ferromagnesian minerals, mica, and hornblende. 

The rock at the Barker mine is slightly finer in grain than that at the 
Wright and Edwards mine. The latter mine is on a shallow drain cut 
in the center of the ridge between Galena Creek and Green Creek, the 
contact being farther west. At this mine very fresh material has been 
extracted in driving a crosscut tunnel, but the greater part of the 
20 OEOL, PT 3 23 



354 GEOLOGY OF THE LITTLE BELT MOUNTAINS, MONTANA. 

material on the mine dump consists of a much altered rock of a lighter 
greenish or white color and holding much pyrite. This alteration con- 
sists in a sericitization of the feldspars and leaching out of the dark 
minerals. At a shaft west of this mine the rock has been altered to a 
white porous material resembling loaf sugar, and consisting of quartz 
and sericite, the latter mineral giving it a pearly luster. 

The Wright and Edwards tunnel cuts the syenite for about 200 feet 
before encountering a vein. The walls show the syenite to be fractured 
by eight or more fracture or sheeting planes, running nearly northeast 
and southwest, with slight reticulation by lesser cross fractures. These 
sheeting planes are marked by rusty iron-stained lines and a few inches 
of leached and altered rock. 

BARKER MOUNTAIN LACCOLITH. 

Barker Mountain is a broad and rounded mountain mass whose 
summit reaches an altitude of 8,152 feet — 2,000 feet above the limestone 
plateau north of Monarch and 3,000 feet above Dry Fork of Belt 
Greek or the Kibbey Basin. The mountain slopes are well timbered, 
largely with pole pine. A small part of the summit is bare, as the 
platy debris of porphyry affords no soil or foothold for tree growth. 
The lower slopes north of the mountain show good exposures, and 
isolated ledges are seen above Barker, but the forest effectually con- 
ceals the rocks in a general view. The mountain is a dome-shaped 
uplift, produced by a central body of porphyry. The igneous mass has 
been bared by erosion, but is as yet slightly scored by gulches, and 
over considerable areas on the west and south slopes presents what is 
probably the upper surface of the laccolith. About the central core 
the stratified rocks may be seen dipping away on every side. The 
lower or under side of the laccolith is nowhere exposed, but from the 
observed structural relations it is probably not the flat floor of the 
ideal laccolith, but a curved or warped one. The horizon of intrusion 
is probably the Cambrian shales immediately above the solid resistant 
floor of Archean rocks, and the base of the laccolith probably conforms 
to the arching of this surface due to the uplift of the range. On the 
north the laccolith has broken up through the older formations and 
the igneous rock is seen in contact with the Madison limestones. 
This peculiarity is also noticed in all the other laccoliths of the range 
front. It is believed to be the result of a common cause, and shows 
that the intrusion and doming accompanied the folding of the range. 
The rocks generally show no recognizable evidence of movement or 
shearing since consolidation. The uplifting and arching of the lacco- 
lith so close to the borders of the Archean rocks result in a very sharp 
folding of the stratified rocks. On the south side of Dry Fork of Belt 
Greek the strata are seen dipping steeply away from the gneissic area, 
and this dip extends northward across the creek on the lower slopes of 
the mountain. At higher elevations the dip suddenly flattens, and 



WBKD] BARKER DISTRICT. 355 

still higher is reversed, so that on the southern side of the mountain 
the Cambrian shales which form the benches and lower slopes along 
Dry Fork of Belt Creek pass under the Monarch and Madison lime- 
stones, but are*ezx)osed again above these rocks^ between them and the 
porphyry area. Near the railroad terminus the beds dip northward, or 
toward Barker Mountain, at 45^, and the second spur west of Galena 
Creek shows the Monarch limestones thus tilted. In the mountain 
slopes opposite the town limestone cliffs are seen, appearing nearly 
horizontal whcm seen from the town, though really dipping gently 
toward Barker Mountain and forming the edge of a basin fold whose 
central mass of Carboniferous limestones forms the eastern spurs of 
the mountain. 

The curving or warping of the stratified rocks about the porphyry 
mass is especially well shown along the course of a small creek, whose 
channel is parallel to and follows the western contact of the x>orphyry. 
The drainage is cut in the soft shales, but on the western bank the 
limestone beds are seen rising with the creek and curving with it around 

S. N. 

BarkerMt. O 




'ZOOO'above Sealevel 



i 1 t lVliles 



Fio. 43.— Kortb-touth cross, section tbroagh Barker Mountain. 

the mountain flank. Here and there little patches of porphyry — rem- 
nants of an intruded sheet — are seen capping the limestones. The dip 
is about 30° away from the mountain. The porphyry surface is readily 
distinguished from the sedimentary areas, as it is covered by a dense 
thicket of lodgepole pine and down timber, in marked contrast to the 
open or sparsely timbered sedimentary areas. The western slopes of the 
mountain are smooth and rounded, and form a great couchoidal surface, 
like part of a huge sphere. The surface is slightly indented by shal- 
low drains, but the general rounding is very marked. It is evidently 
the surface of the laccolith, from which the shale cover has been and is 
being stripped, and shows nearly the original face of the intruded mass. 
At the lower borders of the porphyry area the thinly bedded limestones 
which occur in the shales of the Barker formation are seen in imbri- 
cated outcrops sheathing the porphyry, like the scales of the cup of an 
acorn. The porphyry contact is not regular along the southern slopes 
and does not run uniformly about the slopes. On the middle south 
spur the contact extends down to 5,700 feet, the Cambrian shales, which 



356 GEOLOGY OF THE LITTLE BELT MOUNTAINS, MONTANA. 

are mach baked, dipping steeply away from the porphyry surface, but 
flattening out to 20^ a hundred yards from it. 

The eastern spur of the mountain running down to the Kibbey road 
shows a sharp synclinal folding where the beds upturned by the 'Bar- 
ker and Otter Mountain laccoliths meet. For several bundred feet 
above the divide the Carboniferous limestones dip westward, or into the 
mountains. At 400 feet above the road there is an abrupt and sharp 
change, the inclination being outward, or away from Barker Mountain, 
the dip being 45o. The rocks are cut by a 12-foot dike of greenish rock 
(minette), trending to the syenite area at Hughesville. The rocks adja- 
cent to the dike are marmorized, this eflfect being the more noticeable 
since there is no appreciable alteration or baking of the strata near the 
laccolith contact. The little creek emptying into Dry Fork of Belt 
Greek below the railroad terminus, heads in the saddle, 1,100 feet 
above the Kibbey divide, marking the contact between the laccolith 
porphyry and the blue Carboniferous limestones, the gulch being 
eroded along the contact. The beds dip 50^ E. and strike nearly 
north and south, the dip being up the lower thinly bedded limestones 
of the baaal portion of the Madison limestone series. The porphyry 
slide rock extends down 400 feet below the summit on the eastern spur. 
A detached bare knob, 100 feet lower than the main summit of the 
mountain, is not shown on the map. The porphyry near the contact is 
dense, whitish gray, and shows small feldspar phenocrysts. 

Barker porphyry. — The main mass of the mountain is formed of a 
finely crystalline or granular rock of gray color, dotted with large white 
feldspars, and peppered with green hornblende and biotite. It is a 
rock recalling many dacites, and from its appearance alone might be 
called ^ mica-hornblende-dacite-porphyry. Its chemical composition 
and detailed microscopic study show it to be a variety of granite-por- 
phyry, and it is designated the Barker porphyry. As already noted, 
the other laccolithic rocks are like it or near it in character. Very close 
to the contact with the sedimentary rocks the Barker porphyry is 
dense and slate-like, splitting into thin, irregular plates parallel to the 
plane of contact. These plates, upon weathering, break into small 
angular or sherdy fragments, owing to a network of minute joints. In 
places it is a dense felsitic rock, carrying scattered quartz phenocrysts 
and showing no visible hornblende or mica, and is thus a rhyolite 
porphyry. 

OTTER MOUNTAIN LACCOLITH AND INTRUSIVE SHEETS OF CLENDENNIN MOUNTAIN. 

The mountain slopes inclosing the Barker Basin on the north are 
part of a mountain mass jutting into the open country of the Judith 
Basin, and owing its relief to a laccolithic uplift and doming of the 
strata. The laccolith of igneous rock is revealed by the sharply incised 
drainage on the western side of Otter Mountain, and although the 
dome has not been dissected far enough to expose it elsewhere, the 



WEED. J BARKER DISTRICT. 357 

overlying cover of sedimeuts shows by its structure the nature of 
the uplift. Tliis is the more noticeable since the synclinal basin between 
this mountain mass and Taylor Peak shows a trough of quadrant shales, 
while the Cambrian rocks which are seen on its summit are 2.500 feet 
above the Cretaceous rocks on its western and northern sides, showing a 
lifting of at least 5,000 feet produced by the intrusion. On the summit 
Xhe beds are nearly flat or dip gently to the northeast. On the south- 
east flanks the dip is 20^ to 30° away from the center of the mountain. 
To the west the dip changes, and there is a local crumpling of the 
shales. The summit shows Cambrian rocks, the alternation of shale 
and limestone producing tables, clifts, and sloi)es, which are parked or 
open grassy slopes with patches of timber. A minette dike, 4 feet 
wide, and trending N. 2()0 E., crosses the summit. The laccolith is 
intruded in the Cambrian rocks as usual, and between it and the gneiss. 
The mass must be large and thick, as may be judged by the size of the 
arch and the vertical extent of the uplift. 

The little creek draining an amphitheater cut in the north side of 
Otter Mountain shows an excellent section of the strata overlying the 
Madison limestones. The beds dip away from the mountain at r>0^. 
The contact was not visited, but the stream drift shows an abundance 
of quartzite of light flesh color, pink, and gray, running into a fine 
conglomerate with pebbles one fourth of an inch to 4 inches across. 
Extensive talus slopes of porphyry are seen on the higher slopes, and 
the stream drift contains an abundance of the denser contact forms of 
this porphyry, and of the dense hornstones produced by the meta- 
morphism of the Cambrian shales. The structure is that of a breached 
anticline, and is very evident when the mountain is seen from the open 
country west of it, as the strata are seen wrapped about the jwrphyry 
core. The common form of the porphyry is a dense lavender-colored 
rock, dotted with occasional phenocrysts of white orthoclase, and 
sprinkled with minute black needles and scales of biotite and horn- 
blende. It is a variety of rhyolite-porphyry. A very dense felsitic 
rock also occurs, which probably represents a contact form of the rock. 
It is a pale, faintly gi eenish-gray rock, dotted with very dark purple 
spots. Small feldspars are the only phenocrysts seen. 

Like most laccolithic intrusions the Otter Mountain mass is accom- 
panied by, or bordered by, sheets intrusive in the sedimentary cover. 
These sheets are seen near the contact on the west side, but the exam- 
ination was not thorongh enough to prove their presence on other sides. 
The remnants of sheets which cap Crown Butte and vicinity may come 
from this or from the Barker Mountain mass. 

The Clendennin Mountain mass is a separate elevation, whose south- 
ern slopes are covered by heapings of porphyry slide roclt, which con- 
ceals all rock in place. From what is seen in the mines and on the gaps 
at the head of the basin, it is evident that the mountain is carved out 
of the southward-dipping beds forming the south side of the laccolith 



358 GEOLOGY OF THE LITTLE BELT MOUNTAINS, MONTANA. 

anticline. The beds are penetrated by two or more thick sheets, locally 
thickened and perhaps forming small laccoliths. T]^e sheets have been 
cut across in the erosion of the basin, and the rock thus forms the talus 
heapings seen here. These slopes shut in the basin above Hughesville 
and extend westward nearly to the Kibbey divide and eastward to the 
gap north of Mixes Baldy. The creek heading in the latter gap defines 
the boundary between it and the mass of Wolf porphyry to the south, ^ 
a road to the Moulton and Tiger mines being built along the base of 
the talus. The porphyry debris contains some limestone and shale. 
The mines are located on a vein in the porphyry near the limestone 
contact. 

The rock is mapped as a syenite-porphyry. It is a fine-grained, dense 
rock of a light-pink and gray color. It shows no porphyritic quartz, 
and has an andesitic look, but proves upon microscopic examination to 
be a variety of rhyolite-porphyry. It might perhaps be classed as a 
quartz-mica-porphyry, as it shows phenocrysts of orthoclase, plagioclasei 
and biotite in a groundmass of quartz and feldspar, so that although 
it was grouped with the syenite-porphyries in mapping, it is more closely 
related to the Barker i)orphyry. The border facies of the Otter Moun- 
tain laccolith is a rhyolite-porphyry. 

The rocks near the Tiger mine are sheared, showing that they have 
suffered movement since consolidation. This is believed to be the 
result of local thrust produced by the intrusion of the Mixes Baldy 
mass, which is, on this and other evidence, believed to be later and to 
cut off this rock abruptly. 

MIXES BALDT INTRUSIVE MASS. 

The eastern part of the Barker Basin, together with Mixes Baldy and 
the peaks south of it, is cut in a mass of Wolf porphyry. This great 
body, which is 1^ miles wide and 2^ long, and forms several mountain 
peaks, consists of a rock which is very uniform in appearance through- 
out the whole extent of the body. It is a dull rusty gray, with very 
prominent phenocrysts of smoky-colored quartz and larger ones of 
white feldspar. It is distinctly porphyritic, the quartz grains giving 
it a general resemblance to a conglomerate. The variations in texture 
occur near the margin of the mass and bear a definite relation to the 
contact plane, or the thickness of the offshoots from the parent body. 
This uniformity of character indicates what the field observations 
prove, that the mass is a single one, formed of a single body of magma 
injected at one time and by a single act. The rock is crumbly when 
weathered and does not form conspicuous exposures, and the slopes are 
generally smooth and rounded. On the mountain summits the rock is 
exposed, but is more or less altered, and the rocks (^.rumble beneath the 
hammer. The surface is covered with the coarse pearly sand into 
which the rock disintegrates, and in which the large feldspars, often 
an inch or so long, form conspicuous features. The rock is a typical 
granite-porphyry, like that of Wolf Butte. The quartz phenocrysts 



WEBD.] BARKER DISTRICT. 359 

are large, but cracked; the groundmass appears granalar and like a 
fine granite. 

Gold Run basin, — ^The mountains south of Mixes Baldy foi^m the rim 
of the amphitheaters cut in the mass by Gold Run. The summit is flat 
and gently round, open, and parked with groves of wind-swept pines at 
the edge of the slopes. A few craggy exposures which occur on the 
edges of the summit are particularly noticeable, as the big feldspar 
phenocrysts give the rock a bizarre appearance. More commonly the 
summit shows only the sand resulting from the weathering of the rock. 
The amphitheater or basin of Gold Bun is also cut in this rock. It is an 
extensive basin, with very steep slopes on the east, and separated by a 
big densely wooded ridge from the Tiger mine branch of Galena Creek. 
The basin is generally wooded, save in the center, where a sloping 
grassy bench suggests a change of rock, though it proves to be also cut 
in the porphyry. In the basin the creek cuts rather deeply into the 
rock, and about a mile above Barker crosses a massive exposure of the 
granite-porphyry, 200 to 300 feet high, cutting a deep trench partly 
through it and completing the descent in a very fine little waterfall. 
The outcrops here are especially good and very picturesque, as the 
Wolf porphyry weathers into massive crags, which on the right-hand 
side run toward the hill in smooth slopes dotted with castle-like and 
hoodoo forms of erosion. Some prospecting has been done on leads in 
this rock just above the falls, as well as farther up the creek, the ores 
being galena. Similar prospects were observed in the rock a mile east 
of Hughesville. 

The granite-porphyry mass is an intrusion that has broken through 
the previously tilted limestones and other sedimentary strata, subse- 
quent to the formation of the Otter Mountain and Barker Mountain 
laccoliths. The Mixes Baldy mass is not a laccolith. It is to be classed 
as either a bysmalith or a stock. The sedimentary rocks about its 
borders are, for a short distance from the contact, very steeply upturned 
on the eastern and northeastern flanks, but on the three other sides 
of the mass show little if any disturbance by the intrusion. There is 
very little contact metamorphism, and this occurs only at the immediate 
contact. The relation of the mass to the surrounding rocks is shown 
on the geologic map (PI. XLI). 

Galena Creek dike. — ^Au offshoot of the mass, or its border, is exposed 
along the north slopes of Dry Fork of Belt Creek, east of Galena 
Creek, where it occurs above Cambrian shales, and is covered in turn 
by other strata belonging to the same age. The wagon road is cut 
across it a short distance east of the railway terminus. From here 
eastward the sheet thins out, and is but 50 feet thick 1^ miles east of 
Barker, where it ends as a thin wedge in the Cambrian shale. This 
sheet is separated from the main mass of Wolf prophyry by a round- 
topped ridge thinly capped by shale and limestone. This mass is 
shown on the map as connecting with the main mass around the 



360 GEOLOGY OF THE LITTLE BELT MOUNTAINS, MONTANA. 

west end of this ridge. Owing to the drift and soil, it is impossible to 
establish this beyond all donbt. A 20-foot dike of this rock is seen 
catting the Cambrian shales exposed on the west side of Galena Creek, 
one-eighth of a mile above the railroad, and this is believed to be a 
western e^ttension of the intrusion just noted. Back of Barker fi. e., 
northeast) a similar dike of jjorphyry, which is of the Neihart type, is 
seen cutting the limestones and shale and trending southwanl to the 
town. The rock contains in it various fragments of shale limestone 
and a basic igneous rock. The contact of the main mass is seen east 
of the town of Barker, on the south side of Gold Run, 300 feet from 
the creek. 

Contact relations. — At the borders of the intrusion it is in contact 
with strata of various ages, lying at various altitudes. Near Barker 
the porphyry cuts across the strata of Cambrian age. Followed east- 
ward the contact is with successively higher and younger strata, until 
2 miles east of Barker the upper strata of the Madison limestones are 
cut by it. On the flanks of Mixes Baldy the contact is with Cambrian 
again. On the north it adjoins the intruded sheet rocks of Clendennin 
Mountain, and the syenite mass of Hughesville farther down. 

The strata at the border of the intrusion are apparently unaffected 
by it on the south. Near Barker the beds dip toward the intrusive mass 
at 150, and 1^ miles east of Barker the mountain slopes also show lime- 
stone benches dipping at 10^ toward the igneous contact. About the 
head of Arrow Creek (or Lonetree Park) the limestone strata are steeply 
npturned, dipping at 70° away from the contact (strike N. 10^ W.) 1 J 
miles southeast of Mixes Baldy, this altitude prevailing to the gap 
north of that peak, where the dip is 80^ to the southeast, into the moun- 
tain. The last locality is the only place where marked alteration of the 
contact rocks was observed, the limestones being marmorized and the 
shale indurated, but no contact minerals were seen. 

The divide above the Tiger mine is cut in limestone, the contact with 
the Wolf porphyry being 100 feet above the saddle on the south side. 
The beds strike N. 45o E. and dip at 70^ to 90° W., into the mountain, 
and form a wedge-vshaped block, extending down the gulch toward the 
Tiger mine to a point 500 feet below the saddle. The trail above this 
mine is, however, cut across slopes of Wolf porphyry to a point 100 feet 
below the saddle, and from there to the summit it crosses the mica- 
ceous porphyry of Clendennin Mountain. 

MONARCH DISTRICT. 

The region adjacent to the town of Monarch, together with Thunder 
Mountain, Pilgrim Creek Valley, and Tiger Butte, is conveniently 
described under this heading. 

GENERAL FEATURES. 

The general geologic structure of this district is simple. The strati- 
fied rocks have a general northward dip. away from the Archean center 



^KED] MONARCH DISTRICT. 361 

of the range. This stracture is disturbed by the dome-shaped uplifts 
of Tiger Butte and Thunder Mountain, due to laccolithic intrusions of 
igneous rock. 

The topography is varied. The most noticeable features are the deep 
canyons, presenting precipitous cliffs with almost ideal exposures of 
the sedimentary rocksJ Where the canyons are cut across the strike 
of the beds both sides of the gorge show good exiwsures, but the larg- 
est canyon, that of Belt Greek, shows receding slopes on the south and 
steep cliffs on the north, owing to the prevailing northward dip of 
the strata. The two mountain masses of Tiger Butte and Thunder 
Mountain dominate the topography, and are landmarks visible for 
great distances across the open country north of the mountains. The 
valleys of Logging, Pilgrim, and Tenderfoot creeks show the usual 
narrow gorges cut in the massive limestone series, with broader basin- 
like valleys where the softer Cambrian shales prevail. The region is 
well wooded south of Belt Creek, but the character and relative abun- 
dance of the timber varies with the exposure and also with the nature 
of the soil and rock; it is most abundant on northern slopes and on 
porphyry areas. Up to the present time no productive mines have 
been found in the district, and Monarch is the only settlement. Pros- 
pecting has shown the presence of ore deposits on the flank of Thun- 
der Mountain and Tiger Butte, and also in the valley of Pilgrim Creek. 
The iron ores of Thunder Mountain are described in the account of the 
ore deposits of the range. The other deposits are silver and gold ores, 
of which small quantities have been packed out over the horseback 
trails and shipped to the smelter at Great Falls. Limestone is quar- 
ried near the mouth of Logging Creek, and there is a sawmill near the 
head of the stream. With the exception of a few acres of arable land 
near Monarch, the district is not susceptible of agricultural develop- 
ment. The high limestone plateau north of Belt Creek is, however, a 
very fertile and productive wheat area. 

MONARCH CLIFFS. 

For several miles from Monarch up and down Belt Creek and the 
Dry Fork the slopes to the south are more or less wooded and show no 
prominent exposures, while to the north a line of cliffs rises in grand 
exposure several hundred feet high. These cliffs of white limestone 
are largely stained by the orange color and reddish material from less 
pure layers, and might be fittingly called the Orange Cliffs. Nowhere 
in the region are better and more imposing exposures of the Carbonifer- 
ous rocks. At Monarch the brown limestones, which take their name 
from the town, form the valley floor, and the cliffs northwest of the rail- 
road station show only the Carboniferous, the contact with the Monarch 
limestones being hidden by debris. The cliffs seen in Pi. XLIII, A, 
show, however, unusually good exposures of the impure argillaceous 



> See Davis, Tenth Censun, Vol. XIV, p. 708. 



362 GEOLOGY OP THE LITTLE BELT MOUNTAINS, MONTANA. 

limestones forming the lowest member of the Garboniferoas, which are 
BO generally characterized by the presence of silicified fossils. The 
rocks are dark-blue and dull straw-colored, earthy limestones when 
fresh and unaltered, but the weathered surfaces and shaly fragments 
into which the rocks break are of a buff or rosy-pink color, and fre- 
quently show the fossils weathered out in relief on exposed surfaces. 
These rocks are capped by the massive block-jointed beds of limestone 
which are so prominent in the Orange Cliffs and give them the banded 
effect, as the more massive beds alternate with shaly layers. The fol- 
lowing section was measured at the base of the cliffs immediately north 
of the station. (See PI. XLIII, A.) These cliffs are not everywhere 
persisteut to the top of the plateau, but show a general escarpment of 
300 to 400 feet, with branches and cliffs above, broken occasionally by 
coulees or deep and narrow gulches tributary to Belt Greek. 

Dry Belt teotion, Orange Cliff* of M<march, 

Castle limestone: Feet. 

Massive Carboniferons limestones; heavy bedded, rough snrfaced;, forms 

crags; holds round lenticular masses of chert up to 12 inches in diameter. 400 
Limestone ; platy and fissile. (Fossils 30 feet above the base of this bed) . . 100 

Limestone ; massive. (Fossils Just beneath this bed) 10 

Limestone, shaly .* 1 2nA 

Limestone, less shaly than above j 

(The three beds last described with underlying fissile limestont^ form bold 
red bluffs and walls.) 
Woodhurst limestone: 

White limestone bed, weathering as ribbon ledge above second line of but-' 

tresses 

Second buttress line. Thin bedded limestone, carrying fossils 

Limestone; dark gray, weathering buff ; shows argillaoeons lines; weathers 

down frequently 

Limestone, massive, 6 feet 

Limestone; very dark gray, alternating with light-gray rocks; forms first 

buttress line 

Limestone; gray, compact, pure, in beds 5 to 10 feet thick; forms base of 

massive cliffs 

Limestone ; impure and shaly ; carries silicified fossils 30 

Shaly limestone; light buff-colored, with one-half foot layers of harder lime- 
stone carrying chert lenses 10 inches across and 3 inches thick 35 

Paine shale : 

Fissile limestones, in6 to 10 inch layers; dense in texture, showing cross-bed- 
ding structure 5 

Banded limestones forming cliff face. Light-buff and gray limestones in 6 
to 12 inch layers, of rather hard, compact texture, carrying chert lenHcs of 3 
inches by one-half inch, alternating with earihy shaly liuieHtone in 3 to 6 
inch bands, usually of a light-buff to dark hlue-gray color, but varying to 
pink when weathered. The layers are inconstant and grade into one 

another horizontally * 45 

Monarch limestone: 

No exposure. 

Hrown limestones. 

South of Monarch the eastern side of Belt Creek shows a line of 
clitt's whose limestone beds dip at a gentle angle downstream. Near 



135 



wiED] MONARCH DISTRICT. 363 

Ubls Station, about 8 miles above Monarch, the Archean gneisses are 
seen overlain by sandstones and shales which are intruded by a great 
sheet of augite- syenite-porphyry, from 70 to 100 or more feet thick. 
This rock forms the steep walls of the canyon, and passes underground 
at Uhls, where it forms the rocky bed of the creek. At this place the 
cliffs to the east show an excellent section of the lower part of the sedi- 
mentary series. Owing to local landslips, the shale formation above 
the basal sandstone series is not well exposed, but it can not be 
very different from that of Keegan Butte, only a short distance to the 
southeast, where it was measured in company with Mr. 0. D. Walcott. 
The following section represents the complete series fi*om the Archean 
to the middle part of the Oarboniferous. 

Section of beds expoeed north of Belt Creek, 8 milee south of Monarch, 

Feet. 

Castle limestone : 

Massive bed of limestone, heavily bedded, in places reddish colored. 
Woodharst limestone : 

Fissile limestones, or calcareous shale, generally weathering down 50 f 

Limestone ; forms persistent blnff or reef on slopes, gray 25 f 

Limestone, not shaly ; upper 100 feet carrying silicified fossils 150 

Shales and shaly, dark-gruy limestones. Fossils 648 of table, p. 292 175 

Paine shale : 

Limestone ; cream colored, splintery fragments 5| 

Conglomerate ; gray brown Si 20 

Limestone; cream colored 2; 

Threeforks shale : 

Limestone; flssile, pink to light-buff colored shaly beds, believed to rep- 
resent shales of Devonian 36 

Jefferson limestone: 

Limestone ; rough, granular, brown (coffee colored) 15 

Limestone ; white, massive 5 

Limestone; granular, brown 5 

Limestone; light brown, splintery 6 

Limestone; light brown, sandy and granular, pitted, with large cavities 
(45 inches), in part due to limestone forms. Emits a fetid odor when 

crushed with a hammer; weathers in steep cliffs 15 

Limestone, yellow, forming ledge with blaok-streaked face, but elsewhere 

weathering down to a steep slope '. 50 

Limestone; forms biggest and most prominent ledge of mountain side, 
irregularly bedded, lilac-gray, roundish weathering, and resembles 
mottled limestone of Gallatin times. Ledge undercut, and forms cavern- 
ous recesses 55 

Yogo limestone : 

Limestones, thinly bedded (2 to 6 inches), alternating with 5-inch strata 
showing no lamination lines. Cherty, very dense, dove-gray, not crys- 
talline, breaking with block Jointing and into prisms ; weathers as cliffs. dO 

Limestones, alternately thick and thin bedded 15 

Limestones, gray, weathering buff, thinly bedded (6 to 24 inches) ; forms 

pediment of cliff 25* 

Limestone, thinly bedded, gray. Beneath big cliffs 25 

Lhnestone, thinly bedded. Beneath big cliffs *. S 

Dry Creek shale : 

Shale or shaly red limestone 5 



864 GEOLOGY OF THE LITTLE BELT MOUNTAINS, MONTANA. 

Dry Creek shale — Continued. Feet. 

Shaly limestone, light blue-green and thinly bedded white limestones. .. 15 
Shales, red, earthy, and not laminated like micaceous shale of lower part 

of Cambrian. Shales are crumbly and of bright dark-red or purple 

color 40 

Limestone, granular or saccharoidal, yellow and red limestone, with 

earthy portions irregularly distributed through bed 12 

Alternating thinly bedded limestone and conglomerate, and greeu fissile 

laminated shales and shaly limestones 15 

Pilgrim limestone: 

Alternating beds of thinly bedded limestone or conglomerate and shaly 

limestone 20 

Limestone, fissile or shaly, argillaceous 20 

Limestone, conglomerate 2 

Limestone, thin bedded, carrying a little conglomerate. (Bed is baked 

by intrusive mass. ) 1 35 

Volcanic sheet 40 

Limestones, thin bedded (one-fourth to one-half inch), alternating with 

micaceous shaly limestones and conglomerate. Exposure looks like a 

pile of boards 20 

Park shale : 

Shales, micaceous, leafy, black. Obscured by landslips; thickness given 

from estimate based on aneroid readings 600 

Meagher limestone-s : 

Limestones, flaggy, not well exposed. Thickness on Keegan Butte is 80 

feet 110 

Wolsey shale : 

Shales, leafy, grayish green, micaceous 125 

Flathead sandstone : 

Quart/ite, wh ite 1^ 

Sandstone, rusty, rotten 5 

Sandstone, tlaj^gy, white or buff-colored 15 

Sandstone, fissile, rusty colored, varying to purplish shale 30 

Sandstone, massive 1 

Quartzite, flaggy 6 

Quartzite, vitreous, and massive; neither well bedded nor fissile 60 

Augite-syenite, intrusive 70 

Sandstones, black, and at times ferruginous. Qnarzite, white and pink.. 25-50 

THUNDER MOUNTAIN. 

Thunder Moantain is the name given to the high mountain northwest 
of Belt Park. It is composed of a great mass of igneous rock, some 
4 miles broad. The outline of the mouutain is rather flat, but lacks 
the smooth dome shape of Barker and Baldy mountains. The tlanks 
are wooded, but the summit is generally bare and covered by platy 
debris. The igneous mass is not a typical laccolith, for, as described 
by Lindgren,* *Mt does not disturb the sedimentary rocks to a very 
great extent; its only action has generally been to turn up, perhaps 
even reverse, the edge of the nearly horizontal surrounding strata for a 
distance of 1,000 or 2,000 feet." In general, the rocks about the borders 
of the intrusion dip sharply away from it. Fig. 44, copied from Lind- 

» Tenth Census, Vol. XV, p. 720. 



iTEKD.] MONARCH DI8TBICT. 365 

gren, shows tbe general relations of the sediaieotary rocka to tbe iutra- 
Bion, as seen along the contacts 590 feet above the creek. 

This place was also visited by the writer. The limestones belong to 
the Jefferson formation ami show but little alteration, although so near 
tbe igneous rook. The latter, near the contact, shows a pronounced 
platy parting, which causes it to break into small bits. Tbe usual 
deposits of iron ore occur at tbe contact. Tbe attitude of tbe rocka 
shown by the figure is true only near the contact, for if this divide ridge 
be followed southward, toward the low wooded summit at the head nf 
Tillingbast aud Tenderfoot creeks, it is found that the rocks, instead 




of dipping away from Thnnder Moontain at from 3° to 6°, flatten out in 
a short distance and change to a dip of 20° to the north, or toward 
Thunder Mountain. Tbe section thus exposed in following the ridge 
or divide southward embraces the normal aaccession of formations seeu 
througbout tbe region. 

Ill these upturned beds south of Thunder Mountain eight sheets of 
poi^ihyry occur, conformably intruded in the limestones and shales. 
These intrusions consist mainly of hombleudic porphyries allied to that 
of Thunder Mountain, but minettesand augite-minettesalso occur, the 
rocks being generally too altered for definite recognition. At tbe north 
end of the mountain the 
trail from Monarch to 
Pilgrim Crt-ek follows 
along the contact, wbicb 
is well exposed at sever- 
al places. 

Fig. 45, by Lindgren, 

shows the prevailing t'-a- «--tpt..rpea l«d« -t norib «d «f Tb,m.l,r Sln„ 

structural relations of (DevonUn). 
the igneous rock and tbe 

surrounding siilimentson the northern aide of the mountain. On this 
side tile drainages cut deeply into tbe limestones and head in shallow 
gullies scored in tbe igneous rock. The spurs show a sag or depression 
between the igneous rock and the braestones, due to the presence of 
tbe Cambrian shales between the limestone and porphyry. This sag 
aft'ords easy traveling and is the route followed by the trail. Tbe shales 
show little, if any, appreciable metamorphism, aud it is only at Iron 
Creek, where bodies of iron ore occur along the igueous contact, that 
tbere is any metamorpbism. Here tbe lower shales are exposed, and 
tbey are indurated to dense homstoues. It may be noted, however. 



366 GEOLOGY OP THE LITTLE BELT MOUNTAINS, MONTANA. 

that the emptive debris is so extensive and slides down the slopes 
so freely that it conceals the actual contact plane, and it is not im- 
probable that similar contacts might prevail on other spars. The 
rocks at the Iron mine dip away from the mountain at 40^, bat flatten 
rapidly in a few hundred yards, and dip aboat 20^ on limestone knobs 
above the shale sags, this lessening to 6^ at Belt Creek. It is apparent 
that these beds are uplifted by the igneonsrock, since they are seen in Til- 
linghast Greek, the flat stream bottom being cut in them. The Jefferson 
limestones show 200 feet above the creek at its forks, where the beds 
dip at 30 down the creek. 

At the point where the trail descends from the shoulder of Thunder 
Mountain to Pilgrim Creek a mining prospect was observed, 1,000 feet 
above Pilgrim Creek. The brown Jefferson limestones appear 350 feet 
above the valley shale. These beds probably form a sharp trough face, 
for the Cambrian shales are found above and below them. The lime- 
stones are underlain by an intrusive sheet of porphyry along the red 
shale horizon. Along the trail the dip is 20^ down the mountain side, 
at a place perhaps 600 feet above Pilgrim Creek. 




5cal« 



& 



J. 



iMiies 



Fio. 40.— North-iu>iith section acrons Thunder Moantaln. 



The west slope of the mountain shows the same sharp infold as 
observed alongside the Pilgrim Creek trail. The igneous rock has 
pushed up the sediments so sharply as to form a trough, which forms 
cliffs, seen on an escarpment line above the landslide slopes of shale. 
The exposures, however, were not visited. 

The eastern side of the mountain was not visited. Seen from the 
south, it shows talus slopes, which are undoubtedly of porphyry, with 
limestone beds below, dipping away from the mountain at 30^. Lind- 
gren^ noted "black, somewhat metamorphosed Silurian (Jefferson!) 
limestone, dipping at 15° toward the eruptive." The valley of Tilling- 
hast Creek is cut in Cambrian shale, and forms fertile benches occupied 
by several ranches and watered by numerous springs. 

The Thunder Mountain intrusion is not a typical laccolith and does 
not arch up the strata about it, as plainly shown by flg. 46, as well as those 
already given. Lindgren says,^ '* The whole can be compared to noth- 
ing but an enormous plug driven up through the sediments." It seems 



» Tenth Census, Vol. XV. p. 721. 



"Ibid., p. 720. 



wiBD.) MONARCH DISTRICT. 367 

to be an irregalar laccolith, correspondiBg perhaps to the type of intru- 
sion to which the name of bysmalith has been applied by Iddings, and 
its structaral relations are noted later in the chapter on dynamic geol- 
ogy of the mountains. 

The igneous rock is a variety of the Barker porphyry type of granite- 
porphyry. Its resemblance to the rock of Barker Mountain is men- 
tioned by Lindgren, who gives a description of it under the name of 
hornblende-dacite. The rock is described in the report appended to this 
paper, and needs no further comment here. 

TENDERFOOT MOUNTAIN. 

Between the head waters of Tenderfoot and Tillinghast creeks, and 
west of Belt Park, there is a low, rounded, densely timbered mountain 
mass to which this name is given. It is a broad mass of igneous rock, 
from which the strata incline away at gentle angles in each direction. 
It is so densely wooded that good exposures are rather rare, and as only 
the northern side was visited the outline given on the map is merely 
approximate. The rock is similar to that of Thunder Mountain, and 
undoubtedly occurs as a laccolith intruded between the gneiss complex 
and the Cambrian formations. 

VALLEYS OP PILGRIM AND TENDERFOOT CHEEKS. 

Pilgrim Greek and Tenderfoot Creek valleys are broad mountain 
depressions cut in the soft Cambrian shales. They present similar 
geologic conditions, and at both places there are mining prospects from 
which small amounts of ore have been extracted and packed out. The 
valleys are formed by streams cutting into rocks whose general dip is 
in the direction of tiie stream. In the lower course both creeks cut 
deep and narrow canyons through the heavily bedded limestones, but 
as the dip of the beds is downstream the upper course is sunk through 
these rocks, and broader valleys are eroded in the soft micaceous shales 
of the Barker formation. The lower benches and slopes are parked by 
open groves of the stately yellow pine (Pinus ponderosa). The upper 
slopes were once densely timbered with the lodgepole pine, but are 
now burned, and only a forest of poles remains. The harder rocks are 
well exposed in the lower canyons and the escarpments which overlook 
the valleys, but the soft shales are rarely seen except aloug the narrow 
and deeply cut creek channel. 

In both Pilgrim and Tenderfoot creeks the shale valley is bordered 
by cliffs of limestones, the ledges running along the slope until the dip 
brings them down to the creek level, and they close in the valley. The 
valley slopes are often disturbed by landslides. The conditions are 
especially favorable for this, as the massively bedded limestones rest 
on soft shales, whose downward dip facilitates slippmg when the 
shales become soft and slippery by saturation. This is especially 
noticeable about the iianks of Thunder Mountain. 



368 GEOLOGY OP THE LITTLE BELT MOUNTAINS, MONTANA. 

About the bead waters of the streams the limestones exteud dowD the 
spurs leading into the valleys, as the dip is downstream. 

The only section measured in this part of the range was a partial 
section of the cliffs west of Pilgrim Creek, opposite the x>oint where the 
trail comes down from Thunder Mountain. The creek banks at this 
point show good exi^osures of the Park shales of the Cambrian, in which 
there are abundant fossil remains, identified for me by Mr. Walcott, as 
mentioned in the first part of this paper. The cliffs to the west of these 
exposures show a perfect section, but only the following partial section 
was made : 

Section on Pilgrim Creek, northwest of Thunder Mountain. 

Orange cliffs: Feet. 

Limestones, thin bedded (beds 3 to 6 feet). 

Shaly beds 60 

Limestones, block jointed, red, brownish weathering, in beds 3 to 4 feet thick. 

Limestone in broken ledges, chocolate colored. 

Poorly exposed beds of light chocolate-colored limestones. 

Limestone, forming prominent, heavy, black-jointed ledge, showing no 
striping. 
Chocolate beds : 

Alternating beds of thin chocolate-brown limestones and bnff-colored, argilla- 
ceous beds, the latter not of persistent thickness. 
Jefferson formation : 

Chocolate-colored limestone, with lighter cream-colored parallel lines strip- 
ing face of exposure. 
Yogo limestone: 

Persistent bidge of heavy, massive limestone, brown gray, and lighter than 
rocks above. 

Massively bedded, brownish-gray limestone. 

Base of big cliffs and bluff. 
Dry Creek shale : 

Impure, very argillacreous, and sandy limestone, weathering light buff and 
showing fncoid markings, sun cracks, :ind argillaceous partings 10 

Olive-gray shale 8 

Limestone ; straw colored, shaly, thinly bedded, impure 4 

Red and green shale 20 

Limestone, gray green to olive brown, weathering light pink or buff; are- 
naceous, with granular texture 3 

Gray-green shale 5 

Red shales, and straw-colored earthy or magnesian limestone 3()-35 

Pilgrim limestone: 

Sandstone 5 

Poor exi>08ure ; limestone varying to a conglomerate and shale 20 

Park shale : 

Conglomerate of limestone pebbles 

Soft micaceous shales 

Thinly bedded, impure limestone, with argillaceous mattc^r, mottled, and 
with characteristic weathering 

Soft micaceous shales J.355 

Ribbon beds or banded limestone, carrying fossils 

Thinly, irregularly bedded limestone in layers, 2 inches to 4 inches thick, 
separated by layers of soft, micaceous, argillaceous shale one- fourth inch 
to one-half inch thick 



ME5T0NE NE«R LOGGm 



WEBD.1 MONARCH DISTRICT. 369 

In Pilgrim Greek Valley, as is coramonly the case in the northern 
part of the Little Belt Range, the shales of the Cambrian formation are 
intruded by sheets of igneous rock. Intrusive sheets were noted in 
the Cambrian limestones at the head of Pilgrim and Tenderfoot creeks, 
and in most instances prospectors have sank shallow pits along the 
contacts looking for ore, but there are no extensive explorations at 
either x>lace. Both localities are at present accessible by trail only, 
but wagon roads of easy grade could be cheaply built if developments 
warranted it. In Pilgrim Creek, immediately above the main forks of 
the stream, two such sheets are seen intruded in shales, the silvery 
gray intrusives forming a narrow bench that defines the outcrop of the 
sheet for a mile or so along the slope. The mineral prospects seen in 
this vicinity are in connection with the upper sheet. The creek exposes 
a thickness of about 200 feet of shale here, but the shale is indurated 
and hardened by contact metamorphism, and as the intrusive sheets 
have not produced this alteration there is reason to believe that the 
main mass of the Thunder Mountain core lies not far below, though its 
nearest edge is seen a half mile distant and 600 feet above on the slopes 
to the east. 

SLUIOEBOX CANYON. 

Between Tiger Butte and the high limestone plateau that extends 
eastward to Barker Mountain there is a broad and well-defined valley 
cut in the Carboniferous limestone. The rocks possess a general north- 
erly dip of about 6^, but there are many small local flexures and faults. 
In this broad, rather shallow valley Belt Creek has cut a very narrow 
trench, from 50 to 150 feet deep, with vertical and overhanging walls. 
The creek flows in a succession of deep and rapid reaches of great 
beauty. The narrow gorge has been christened the Sluice-boxes, or 
Sluicebox Canyon, and from the railroad, which follows the old valley 
level above the canyon, the view is superb. The general appearance is 
well shown in the illustration (PL XLY). 

TIGER BUTTE. 

This rather sharp and prominent butte is caused by an asymmetric 
or irregular laccolith. The summit and sides are formed of limestones, 
dipping away from the mountain on all sides, and arching over its top. 
The general horizon of intrusion is not definitely known, but it is prob- 
ably the Cambrian.' On the north the igneous rock has broken through 
to the higher beds and is in contact with the Jurassic, and at this place 
it has been bared by erosion and is exposed to view. The rock is similar 
in character to that of the other laccoliths of the region, and belongs to 
what has been designated the Barker type of granite-jwrphyry. It 
weathers with the usual massively platy parting, is much shattered by 
joints and weathering, and forms extensive talus heaps. There are the 
usual silver prospects about the contact, but though rich samples of ore 
are said to come from the deposits, no workable bodies of ore have as 

20 GEOL, PT 3 24 



370 GEOLOGY OF THE LITTLE BELT MOUNTAINS, MONTANA. 

yet been brought to light. The batte lies some little distance in front 
and away from the general margin of the range, so that the laccolithic 
doming of the strata is very prominent. Only a part of the bntte comes 
within the limits of the accompanying map (PI. XXXYI). Logging 
Greek has cut its valley between the range and the batte, and a small 
stream defines its western borders. The latter shows no igneoas rocks 
in it<s gravels, so that althoagh not visited, it is believed that no igneoas 
rooks occar on the western side. 



CHAPTER VI. 

1>E8CRIPTIVE GEOLrOGY OF THE NEIHART DISTRICT. 

LOCATION AND GENERAL FEATURES. 

The principal settlement of the Little Belt Monntains lies in the center 
of the range, not far distant from the geographic center of the State. 
It is situated on the head waters of Belt Creek, in the bottom of the 
deep mountain valley of that stream, with the sharp x>oint of Keihaiii 
Baldy overlooking the town, and the high plateaus of Belt Park and 
its neighbors surrounding it on the west. (See PI. LI.) The general 
situation of the town is shown in the map of the district (fig. 52, p. 4(^). 
The location is accessible by wagon road from White Sulphur Springs and 
the Judith Basin, and is the terminus of a branch line of the Great 
Northern Railway running from Great Falls, from which place it is 65 
miles distant. The town owes its origin and existence to the ore depos- 
, its of the surrounding district. 

The Neihart district, embracing the drainage tributary to Belt Creek 
in the immediate vicinity of the town of Neihart, presents scenery and 
geologic structure quite different from those of the parts of the range 
already described. The town is situated near the southern end of an 
area of metamorphic rocks, which constitute the central core of the 
range and are believed to be of Archean age. These are overlain by 
stratified rocks which dip away from the Archean contact at steep 
angles to the south and at relatively low angles to the north. Th^ 
Archean rocks themselves are cut by eruptives of several different 
ages. 

ARCHEAN GNEISSES AND SCHISTS. 

The Archean rocks are well banded, have a prevailing dip or plane 
of schistosity of 45^ tx) the south, and show no traces of sedimentary 
origin. 

The ridge at Johnson Gulch shows a rough crest, formed of harder, 
dark-red, feldspathic schists, but no massive weathering gneisses are 
seen on the east side of the creek until the gully defining the shoulder 
of Neihart Mountain (or Neihart Baldy) is reached, where the massive 
rocks near the Broadwater mine are seen, with strike and dip of schis- 
tosity planes conformable with those found farther north. Viewed in a 
general way, the gneisses show very distinct and well-marked divisions 
of color and lithologic habit. Bed feldspar-gneiss, white or gray 
feldspar- gneiss, black mica-schist or amphibolite, and the more schistose 
rocks appear, to be persistent divisions. To a certain extent such dis- 

371 



372 GEOLOGY OF THE LITTLE BELT MOUNTAINS, MONTANA. 

tinctions can be made in a rough way, and they prove to be very impor- 
tant distinctions in the economy of the ore deposits, but when studied 
in the attempt to map the boundaries of such rocks they are found to 
grade into one another without order or system. Moreover, the very 
intricate and indented intrusion of the peculiar Pinto diorite has modi- 
fied the schists near its contact, and though somewhat gneissoid yet 
uniform in character, its surface is less varied than the gneiss. 

At only a few localities was the igneous character of the schists rec- 
ognized. Some 300 feet above the town, on the road from the Broad- 
water mine, a foliated rhyolite-porphyry was found. A similar rock, 
but leached and altered, occurs in the Queen mine. At the upper end 
of the Broadwater road a very fissile rhyolite-porphyry-schist was 
obtained. It is a pale cream-colored rock, weathering a buff or straw 
color. It is very fissile, splitting into slaty debris, which conceals its 
outcrop. This is due to an abundance of sericite fiakes developed 
along the folia. The edges of the fragments show a very decided flow 
structure, with quartz phenoerysts drawn out into lenticules. The 
rock is a sheared rhyolite-porphyry. Another form is much less 
schistose, and is a distinctly porphyritic rock. Large oval masses, 
one-fourth to three-fourths of an inch long, of sheared and drawn-out 
granules of red quartz, and smaller dull- white feldspars, appear in a 
dark-gray or greenish-black groundmass; the rock is a sheared rhyo- 
lite-porphyry. With these exceptions the rocks do not show their 
derivation to the naked eye. 

The rocks south of the town, and between it and the Broadwater 
workings, are schistose and fissile, breaking readily along the folia and 
seldom showing massive out<5rops (see PL XXXVII, A). North of the 
town, on both sides of Belt Creek, the rocks are gneisses, in which 
along Eock Greek a diorite is irregularly intruded. There is, however, 
no transition between these schistose and gneissic forms, and no evi- 
dence showing their character to be due to metamorphism produced by 
the diorite. The most notable varieties can be distinguished as gray 
feldspar-gneisses, red feldspar-gneisses, and am phibolite- gneisses. 

The gray gneisses show wide variations in texture, color, and folia- 
tion. In general, they are coarsely crystalline, with mica folia sepa- 
rating a coarsely granular mixture of quartz and feldspar. The red 
gneiss forms fairly well-defined bands, which, like the folia, are len- 
ticular in shape. The rock occurs as patches and streaks in mica- 
gneisses along Kock Creek and north of the town. Typical specimens 
from the Mouiton mine are evenly granular medium-grained rocks, 
showing a streaking of dark-green chlorite. The rock consists of a 
mixture of pinkish feldspar and quartz. A red gneiss from the mine 
workings — the 400-foot level west — is a very dense and finely granular 
rock, broken by small joints into diamond- shaped rbombs. The red 
gneiss encountered in the Qneen workings is coarsely granular, and 
forms patches separated by black mica-schist, and is associated with a 



WMD.) NEIHART DISTRICT. 373 

gray mica-gneiss. The altered qaartz-porpbyry-gneiss from this mine 
is like that of the Broadwater, but can not be mapped as separate from 
the inclosing gneisses. A black micBrgneiss consists of a biotite and 
quartz, with white lenses of qaartz and feldspar. 

The amphibolites also vary from coarse to fine-grained textures. The 
coarser rocks are usually evenly granular and show little foliation, 
except on dark reddish-gray weathered surfaces, where the feldspar 
shows as pinkish material and the rock is distinctly streaked. The 
rock breaks with a sharply angular fracture into thin flakes. The com- 
moner variety seen in the mine workings is a very tough rock with 
rounded fracture. It is a finely and evenly granular amphibolite, bat 
slightly schistose to the eye, and consists of a felty mass of interlocked 
hornblende needles. This rock is seen along liock Greek above the 
Moulton, and in the Florence and other mine workings. 

The gneisses of Carpenter Greek form uniform and rather smooth but 
steep slopes, having a few rock outcrops of reddish gneiss pillars, but 
covered by fine di^bris supporting a dense growth of small pine, so that 
in a general view the country appears smooth, and shows the topography 
typical of simple erosion of a homogeneous material. The grassy tops 
of the hills are smooth and bare of outcrops; the canyon walls rough, 
with picturesque masses. Their aspect at the mouth of Garpenter 
Greek is shown in PL LXV, B. 

On Snow Greek the porphyry of Poverty Mountain ends a short dis- 
tance above where the wagon road to the Gornucopia and Benton mines 
leaves the main creek. The prophyry is in contact with a hard and 
dense, blocky, gray gneiss banded with amphibolite. This gneiss pre- 
vails for several miles up Snow Greek, changing to a red feldspathic 
gneiss where the trail crosses the slopes west of the creek. The slopes 
between Snow Greek and the Gomncopia show alternate belts of black 
amphibolite and red feldspathoid gneiss dipping north. The Gornuco- 
pia is on a bench cut in the ridge between the forks of Snow Greek, 
in black micaceous gneiss, bat the Pinto diorite forms a bench to 
the north, below the shaft house. Follow ingthe road from the Gornu- 
copia to Keihart, porphyry is found at 8,000 feet, forming a spur down 
to 7,700 feet, where the black gneiss outcrops. 

OLDER IGNEOUS ROCKS. 
PINTO DIORITE. 

The gneiss series is intruded by a large and very irregular body of 
diorite which has been designated the Pinto diorite, from its spotted 
appearance. This rock occurs in a continnoas body on the ridges 
between Rock Pinto and Garpenter creeks, is cut across by the canyon 
of Belt Greek below Garpenter Greek, and forms the gateway of Harley 
Greek. It was traced up the last-named creek one-half mile, beyond 
which the exposure was not followed, owing to the dense timber and 



374 GEOLOGY OF THE LrFTLE BELT MOUNTAINS, MONTANA. 

debris covering the slopes. Isolated exposures of this rock were also 
foand at the extreme head of Carpenter Greek west of Long Baldy 
Mountain, at the Broadwater mine, and hear the Cornucopia and 
Barker mines of Snow Greek. At the last localities it is x>os8ible that 
there is a surface connection with the main body, but it was not deter- 
mined. In the very narrow and steep gulch near the head of Carpenter 
Greek, Pinto diorite occurs intrusive in black amphibolitic gneiss, at 
8,000 feet, or 300 feet below the top of the black amphibolite just below 
the summit. The exposure at this place is small and the rock contains 
many included fragments of gneiss, a common feature of the borders of 
the mass, though at this locality the denser contact facies of the rock 
was not recognized. It is cut by a minette dike, trending to the east. 
There is reason to believe that all these exposures are part of a single 
great body — a batholith, and that the several masses now exposed con- 
nect at an unknown depth. 

The rock has a striking appearance. Photographs of a hand specimen 
are shown in PI. LXXIII, A. The rock consists of roundish, pale-green, 
white, or pink feldspar masses or aggregates, one-half to 1 inch across, 
closely packed, with the interspaces iilled with schistose hornblende. 
In the freshest material obtained from underground workings the feld* 
spar is pale green in color. The porphyritic appearance produced by 
these feldspar groups is most striking on weathered surfaces, where 
they are reddish in color and stand out in relief. The rock is very uni- 
form in habit and character throughout the district. In most cases 
there is no change in grain at the contact, but the southern borders of 
the main body exposed along Eock Greek above the Moulton mine show 
a very marked contact facies, in which the feldspars are thinly tabular, 
and are arranged in a fluidal structure, which is very striking on a 
weathered surface. At this locality, also, both the denser contact form 
and the coarse-grained rock are cut by aplitic dikes, which are regarded 
as connected with the intrusion, the aplite being a fine-grained granite, 
much mashed. These dikes are often only an inch or two across and 
are accompanied by dark seams. They are seen only near the contact. 

The contact between the Pinto diorite and the schists is an extremely 
irregular one. Not only is it indented by projecting wedges and 
tongues of diorite in the schist and of sharp and irregular wedges of 
the schist in the diorite, but many masses of schist are included in the 
diorite. The relation of the two rocks is complicated by late dynamic 
forces, causing the development of a gneissoid structure in the diorite. 

The Pinto diorite has a more massive weathering than the schist. 
This is especially noticeable on the sharp ridges dividing the drainage 
of Rock and Snow creeks. At this place the diorite forms rough, 
craggy masses, shown in PI. XLIX, B. The rock breaks into great 
blocks, 2 to 10 feet across. It shows a distinctly spotted character on 
freshly fractured surfaces, but where long exposed to the weather is 
reddish and largely overgrown with lichens. These exposures, like 



WMD.] NEIHART DISTRICT. 375 

those nearer the town (above the Monlton mine), hold stringy masses 
of schist and are streaked with occasional seams of aplite. The 
included fragments of schist and gneiss do not show any melting on 
their edges, nor the development of contact minerals. 

YOUNGER IGNEOUS ROCKS. 
BHYOLITB-PORPHYRY OF ROOK OREEK. 

On the narrow divide between Snow Greek and Eock Greek (the 
gnlch jast back of the town) the Pinto diorite and gneisses are cut by 
a mass of porphyry, whose debris is scattered over the surface and 
is seen in several shallow prospect pits, though its boundaries could 
not be determined. The rock is not schistose, but shows a fracturing 
approaching flowage. It is a pale-green rock, with abundant pheno- 
crysts of dark-gray quartz 2 mm. in diameter. This quartz shows 
doubly terminated pyramidal crystals, and the grains commonly project 
above the ft'acture planes of the rock, though others fracture with the 
rock. The groundmass is extremely dense and felsitic, and is of a 
grayish-green color. The rock is much altered and the former feldspar 
phenocrysts are now spots of yellowish earthy matter, probably seri- 
cite and kaolin. The rock is a rhyolite-porphyry (quartz-porphyry 
of older nomenclature), but it differs in both texture and appearance 
from the Garx)enter Greek type of that rock. 

NEIHART PORPHYRY. 

The rhyolite-porphyry distinguished by this name occurs as dikes and 
large intrusive bodies cutting; the Pinto diorite and gneisses. It forms 
the dikes seen in the workings of the Oalt, IngersoU, Benton, and other 
mines, and is exposed on the surface north of Belt Greek. It also 
occurs in irregularly intrusive bodies in the slopes adjacent to Snow, 
Garpenter, and Mackay creeks, where various ore bodies have been 
found in it and mines located. Its outcrops are seldom massive, as the 
rock is crackled by a network of fine joints, and breaks into small angu- 
lar debris, which covers the slopes, and under favorable conditions 
forms extensive debris slides, such as those seen on Poverty Mountain, 
as the hill between Snow and Garpenter creeks is called. Throughout 
the different exposures the rock is fairly constant in habit, though it 
varies somewhat in appearance, owing mainly to the alteration that has 
occurred near ore bodies, or the changes produced by weatbering. The 
rock is anormal rhyolite- (quartz-) porphyry showing rather small pheno- 
crysts of quartz 1 to 2 mm. across, scattered through a very dense 
felsitic-looking groundmass, which is white in some cases, but in the 
vicinity of the ore deposits it is pale green. Irregular-shaped masses 
of rusty earthy material occurring in some specimens represent pheno- 
crysts wholly decayed — probably biotite. The altered rock of the ore 
deposits is impregnated with pyrite grains, and is often brecciated. 



I 



376 GEOLOGY OF THE LITTLE BELT MOUNTAINS, MONTANA. 

The fine rhomboidal or diamond-sliaped fractures which crack the rock 
cause it to break readily into angular debris on weathering. This rock 
is seen at the Gait, IXL, and Eureka mines, and at these localities the 
fragments often show a black surface, due to oxidation of pyrite. 
The Mackay Greek mass, in which the ores of that area have been 
found, is intrusive in the gray and red schists, and is found on both 
sides of this creek from the head to the mouth. It extends across to 
Carpenter Greek and forms the sharp ridge between that and Mackay 
Greek. On the opposite side the extent of the porphyry is not accu- 
rately known. A short distance (estimated at half a mile) below the 
mouth of Mackay Greek it is not seen, and gneisses are exposed. 

The ridge between Snow and Garpenter creeks shows granite-por- 
phyry at its very end, but the Neihart porphyry of Poverty Mountain 
appears one-fourth mile above the junction of the streams, as noted. 
The Neihart porphyry occurs on the spur below the Gornucopia mine 
on the ridge east of the Benton mine, being exposed alongside of the 
Gornucopia road at 6,900 feet, with black gneiss below it. The por- 
phyry extends across the slopes to the Benton workings, showing at 
the point where the road swings around to the south, where it descends 
to Benton Oulch, and the rock is cut by a crosscut level from the Ben- 
ton to the Flora vein. !N'o surface connection between this and the ^ 
mass at the IXL mine was found. \ 

West of the Snow Greek Basin porphyry outcrops 350 feet below the 
Bock Greek divide, just below the IXL mine, and is 200 feet wide on 
top of the ridge, and abuts against gneisses. To the east the schists » 

extend to Snow Greek, but the Neihart porphjrry forms the crest of the 
spur and connects with the Poverty Mountain mass near the junction 
of Snow and Garpenter creeks. 

GABPENTEB OBEEK PORPHYRY. 

The canyon wall nort>h of Garpenter Greek shows a ledge of light- 
colored rock running along the wall for several miles. It forms a verti- 
cal cliff 50 to 60 feet in height, whose base is about 300 feet above the 
creek. The ledge does not continue west along Garpenter Greek to Belt 
Greek, but as the crest of the ridge becomes lower it crosses the spur 
and appears on Belt Greek a half mile or more below Harley Greek ; and 
here it is very plainly seen to be an inclined dike or sheet dipping north 
at 45<=^. It is here 30 to 35 feet thick, and cuts through the Pinto diorite. 
It shows a contact band 3 to 5 feet wide at top and bottom, and the 
rock holds many fragments of schist and gneiss torn from the fissure 
walls. On the cliff seen north of Garpenter Greek this contact is very 
apparent, owing to its darker color and different weathering, due to a 
denser texture. 

The reef can be traced as far as the mouth of Snow Greek, where it is 
lost on the smooth slopes to the north. An exposure of it was found 
on the doab^ ridge between Snow and Gar])enter creeks, at the extreme 



> Triangnlar space between two Btreams at their Janotion; aee Century Dictionary. 



WMD.] NEIHART DISTEICT. 377 

end of the spar. The rock is light gray in color, and thinly spotted 
with crystals of pale-pink ortboclase an inch or less across, with smaller 
light-colored feldspars between. These lie in a gray groundmass 
sprinkled with black dots of biotite. The rock is not schistose, and is 
apparently of later age than the diorite. 

TRAP DIKES (MINETTBS). 

Besides the rocks already mentioned as intrasions in the gneiss com- 
plex of the Neihart district, dikes of minette also occur at the Gait 
and Ingersoll mines and at the head of Snow Creek. They are regarded 
as due to the igneous activity of the Yogo Peak center, since they 
possess the composition and characters peculiar to the rocks of that 
locality. They are soft and easily weathered rocks, which are seldom 
exposed except under the most favorable conditions. It is possible that 
they are more common about iN'eihart than at present supposed, since 
they would be concealed by gneissic debris on the surface, and in the 
altered condition prevailing about the veins in most mine workings 
would not be readily recognizable. They are an important bit of evi- 
dence, however, since their occurrence shows that the ore dex>osits 
which cut them are of later age, and therefore certainly post-Jurassic, 
and probably Tertiary. 

SEDIMENTARY AREAS. 
QUARTZITE RIM ROOK, 

The mountain slopes above Neihart show massively bedded quartzites 
capping Neihart and Long Baldy mountains (PI. LII, A), and the same 
beds are seen as a rim rock running along the edge of the plateau 
west of the town. The latter outcrops are nearly horizontal, but fol- 
lowed southward the beds are seen to dip in that direction, as they 
do on Long Baldy Mountain, and to be overlain by a thickness of 4,000 
feet of gray slaty rocks. The exposed edges of the quartzite beds are 
seen running down the slopes east of Belt Greek, and 2 miles above 
the town the creek has cut a very strikingly picturesque canyon through 
these rocks (see PI. XXXVIII, B). Above this canyon the slaty series 
composing the various formations of the Belt terrane are seen for several 
miles up Belt Greek and its several branches. The detailed section of 
these beds has already been given in the description of the terrane. 
The thickness varies greatly, however, at different localities. On the 
O'Brien Greek road it is not over 1,000 feet, probably less, while on 
Sawmill Greek it is 4,500 feet, and on Ghamberlaiu and Belt creeks a 
little less. 

The slaty rock beds of the formation are cut by sheets and dikes of 
intrusive rocks, which alter the strata, and are frequently prospected 
for ores. The intrusions are relatively small and part of the fringe of 
sheets and dikes surrounding the Yogo center. 



378 6E0L06T OF THS UTTLE BELT MOUNTAINS, MONTAHA. 



Akmg the stage road ap Sawmill Greek, as well as on theoM CFBrien 
Creek road, the slaty Belt roeks are seen OYerlain by reddish and yellow 
(Stws-bedded sandstones, the basal beds of Cambrian formations. 
The:4e rocks are well indurated, and as they are OYerlain by soft shales 
that weather readily, the sandstones form erteiisive grass-cuYered 
plateaa areas lying between the creek Yalleys. This is rendered more 
effectiYe by the intrusion of a thick sheet of prophyry in the fissile beds 
just above the base of the formation, a sheet whose characteristic cliffs 
and talus slopes are shown in PI. LII, B. These sheets outline exten- 
siTe plateau areas between Belt and Sawmfll creeks. The areas of 
porphyry and quartzite debris are very generally densely timbered with 
lodgepole pine, as shown in PL XXXYII, B, They are thus in strong 
contrast to the open grassy or sparsely timbered areas where shales cover 
the surflfice. This is noticed not only on the plateau but along Belt 
Greek on the Togo trail. These plateau levels are the most characteristic 
topographic features of the Neihart district. They are all due to the 
resistant nature of the quartzite or sandstone beds. Near Neihart it is 
the Neihart quartzite that determines the level area; south of Xeihart 
it is the Flathead quartzite. In going south from Xeihart along Belt 
Greek or any of its branches the stratified rocks are seen to dip to the 
south. On the summit of Long Baldy the dip is 20^ to 22^. Along 
Sawmill Creek it is at first 14^, but increases to 20^, which prevails 
until the sharp twist in the road is reached^ where the Cambrian Flat- 
head sandstones have a much gentler dip, changing to 2^ to 3^ on the 
grassy plateau levels. An angular unconformity exists between the 
two series, but it is much less marked than these dips indicate. The 
Cambrian shales are soft, easily intruded rocks, and in them the igne- 
ous intrusions are abundant, and the intruded sheets thickest. The 
shale formations attain a thickness of about 1,200 feet near the divide 
between Belt Greek and the neighboring drainage basins, and, as else- 
where, are separated by several formations of limestone. Nowhere 
within the actual limits of theXeihart district, as defined by the drain- 
age basin of Belt Creek, are more recent stratified rocks exposed. 

North of Neihart the crystalline schists form a hilly country, deeply 
trenched, yet with smooth and generally rounded slopes. West of 
Belt Creek the plateau area known as Belt Park is, like the smaller 
areas to the south, formed by nearly level beds of indurated sandstones 
resting upon the schists. The fissile impure sandstones found inter- 
bedded with the harder shales of this locality hold fossils of Cambrian 
age, and the two buttes which rise above the park are formed of shales 
and limestone whose fossils prove to be Cambrian types. The southern 
limit of the park, where it is cat off by Harley Greek, shows the Cam- 
brian resting directly upon the crystalline schists with no intervening 
beds. Similar conditions prevail all about the western and northern 



T QUART2ITES OVERLAIN BV SHALE F 



B. INTRUSIVE PORPI 



wi«D.] NEIHART DISTRICT. 379 

margins of the Archean area, no Belt rocks occurring north of an east- 
west line through Long Baldy Mountain at Neihart. 

HEAD- WATER VALLEY OF BELT CREEK. 

Belt Greek above Neihart Oanyon flows through a mountain valley, 
whose width and character vary with the nature of the rocks. The 
lower park, cut in Belt shales, extends for a mile above Chamberlain 
Greek. To the north the slopes have been densely wooded, the forest 
being now largely cut or burued over. The slox)es are steep up to the 
cliff fronts of the terraces formed by porphyry sheets. The normal 
sequence of the Belt rocks is seen in occasional exposures along the 
creek. The shales prevail to Ghamberlain Greek, where they are 
broken by an intrusive sheet of porphyry. Above Ghamberlain Greek 
the blue Newland limestones appear, in beds 3 to 5 feet thick, with 
interbedded shales. The limestones are either shaly or, in more mas- 
sive beds, the rock breaks into splintery fragments. They weather 
buff, with square jointiug, break into cubical blocks, and contain car- 
bonaceous markings and calcite streakings. They are succeeded by 
black shales, extending up the creek until the low ledges of dark- 
colored Gambrian quartzite appear. The beds dip upstream at low 
angles until the divide at the head of Running Wolf is reached, where 
the dip is westward, or down Belt Greek. 

The sedimentary rocks encountered on Ghamberlain Greek show the 
same beds. The channel is cut in shale for 2 miles above its mouth, 
and above hei^ in the Neihart qaartzite. A sheet of porphyry intruded 
in the sandstones of this formation covers with its drift a large part of 
the mountain side to the north. The intrusion is a sheet perhaps 300 
feet thick, tapering to the west, and regarded as an offshoot of Big 
Baldy. Gambrian beds here conformably dip upstream. 

Belt Greek above the quartzite exposures shows a wider valley, 
where it is cut in the overlying soft shales, and a beautiful park 
occupies its center. Up the Yogo trail few exposures are seen until 
the crest of the divide is reached, for dense but open woods prevail 
until the steep and open or parked higher slopes are reached, where 
the soil is thin and occasional low exposures of green shale and knotty 
surfaced, thinly bedded limestones are 6een, with one intrusive sheet 
exposed 725 feet below the top. The ridge to the north of the trail, a 
lateral spur of the divide, shows better exposures along its crest, and 
the soft shales are seen to be intruded by a dozen or more sheets of 
igneous rock. These are of two kinds. The first is the light-colored 
rhyolite-syenite-porphyry, which, on account of its hardness, forms 
benches. 

INTRUDED SHEETS AT HEAD OF BELT CREEK. 

These intruded sheets are syenite-porphyries, which vary somewhat 
in appearance according as the sheet is thick or thin. They are choc- 
olate or purplish-gray rocks, characterized by abundant phenocrysts of 



380 GEOLOGY OF THE LITTLE BELT MOUNTAINS, MONTANA. 

dull-white feldspars and black or greenish hornblende needles, and 
sometimes mica scales. These dark-colored minerals are altered to 
chlorite and rasty limonite spots in the weathered forms. The sheets 
break with a platy fracture, and their debris covers the slopes and 
obscures the lower contact. The rocks of the thicker sheets possess a 
strong resemblance to the syenite porphyry of the laccolith end of the 
east end of the Yogo stock. The sheet forming the crest where the 
Yogo trail descends to Belt Creek differs from the common type of 
porphyry forming the intrusive sheets. It occurs on the east crest, 
intruded in shaly beds that dip 5^ W., and swings across the ridge to 
the ridge to the west as lower beds appear in going north from the 
trail. The rock is a pale-greenish, very dense, hard, platy-looking 
rhyolite-porphyry, showing infrequent rectangular sections of pinkish 
orthoclasc up to one-fourth inch across, and a few scattered grains of 
a clear, glassy quartz. The weathered surface is buff colored and shows 
a minute pitting, revealing a banded or flow structure. 

The west slopes of this divide ridge contain numerous intruded 
sheets in the Oambrian shales, eight being counted on the spur visited, 
running down between two hi?ad-water branches of Belt Creek. The 
sheets vary from a few feet to 75 feet in thickness, and weather in 
step-like benches that terminate in rocky cliffs. The rocks are syenite- 
porphyries, but include two sheets of minette. The two thickest sheets, 
one 75 feet thick 400 feet below the top, and the other nearly 100 feet 
thick at the base of the slope, have baked and altered the rocks for 10 
feet from the contact. The lower 600 feet shows six intruded sheets. 
Although there is little doubt that these sheets, like those so generally 
observed in the district, are continuous for considerable distances, only 
two of them were seen on the slopes crossed by the Yogo trail, owing 
probably to soil and debris, but there is no reason to doubt that they 
would be found if carefully looked for in essentially the same horizons 
on all the spurs leading down into Belt and Wolf creeks. 

OUTLINE OF GEOLOGIC HISTORY OF NEIHART DISTRICT. 

The crystalline schists are the oldest rocks of the mountains, and 
constitute the central core or nucleus, about which the later stratified 
rocks have been folded. The bedded rocks show that the iN'eihart region 
has been the site of dynamic disturbance from the very earliest geo- 
logic time. The cliffs of Long Baldy Mountain and the rim rock of 
the plateau to tbe west are formed by sedimentary strata deposited 
along the shore of an Algonkian sea, in whose waters the very oldest 
forms of life known to the world existed, and remains of which were 
recently discovered near Neihart. The region of Belt Park and Hoover 
Creek, north of Keihart, was a land area, the shore being near the 
present site of the town. This location was along the "hinge line" 
between the northern continental land and the gradually sinking sea 
to the south, in which 4,500 feet of strata were laid down. The lines 



1 CR£E« fieSEBVOIR. WITH NEIHART MOUNTAIN IN DISTANCE. 



F GNEISS NEAR 



WEED.] NEIHABT DISTRICT. 381 

of weakness developed at this time may have been intensified by those 
later oscillations of level to which the entire region was subjected 
thronghoat later geologic epochs, but no definite data as to their effect 
upon this region are obtainable. 

It is probably to later igneous activity that the Assuring of the rocks 
is due. These represent at least two, perhaps three, periods of dy- 
namic disturbance, but their age is not known, for they do not cut the 
sedimentary strata. The most lmx>ortant intrusion was that of the 
magma forming the Pinto diorite. This rock is found penetrating the 
schists in every direction and over an extensive area. Its contacts 
appear to be without order or arrangement, and over the district dis- 
tinguished by mineral veins the observed facts indicate that it under- 
lies even a larger area than is indicated by its surface exposures. That 
its intrusion shattered the schists is shown by the numerous included 
fragments in it. In places it shows endomorphic contact phenomena, 
but more generally the observed facts indicate a thorough heating of 
the adjacent rocks and a slow cooling of its mass. That it is later than 
the schists is shown by the structure of the included fragments, and 
that it is (Barlier than the last dynamic movements producing schis- 
tosity is shown by its gneissoid structure. 

This Pinto diorite is of unknown age. It may be pre-Algonkian, but 
its occurrence near the base of the quartzite beds of Long Baldy Moun- 
tain and the absence of any fragments of it in the conglomerates of 
those strata are opposed to this hypothesis; its gneissoid structure 
would accord with this idea, since the Algonkian rocks are but slightly 
metamorphosed. Still later igneous forces ruptured the rocks, in whose 
crevices dikes of porphyry were injected, or formed irregularly intru- 
sive masses. These rocks cut the diorite as well as the schists, and 
although sheared, they are not gneissoid or schistose. The period of 
their intrusion is not known. 

A third period of energetic igneous activity accompanied the uplift 
of the range, or directly followed it, and the igneous rocks, which were 
sent out as sheets and dikes from various centers of disturbance, occur 
in the stratified rocks surrounding the Archean area of Neihart, and 
are sometimes found in it The period of vein formation and ore depo- 
sition either accompanied or followed these igneous intrusions. The 
precise geologic age of the ore dex>osits is unknown, because they are 
found only in the older rocks of the region. On the summit of Long 
Baldy the ores occur in the quartzite overlying the gneiss, but nowhere 
else do they cut stratified rocks. Inferentially the deposits are believed 
to be post-Oretaceous. 



CHAPTER VII. 

GENERAIi GEOIiOGT. 

HISTORY OP REGION AS INTERPRETED FROM SEDIMENTARY 

ROCKS. 

The sedimentary rocks of the regioD show that the same general 
conditions prevailed in the Little Belt region as in the rest of the east- 
ern part of the Bocky Mountain area of the State. The stratigraphic 
sections are very similar and show the same formations recognized 
throughout the geologic province* There is, however, one feature of 
especial importance shown in this section, namely, the overlap of the 
Cambrian beds fn>m the Algonkian rocks of the southern part of 
the mountains to the Archean gneisses of the northern. 

The Archean rocks are, so far as recognized, wholly of igneous origin, 
and may be supposed here as elsewhere to represent the downward 
crystallization of the original crust of the earth. These rocks were 
already metamorphosed to gneisses and schists of the same character 
seen today when the earliest known 'sedimentary rocks of the region, 
the !Neihart quartzites, were deposited, for occasional pebbles of red 
and gray gneiss are found in the latter. 

The Neihart quartzites were laid down ux>on what appears to be a 
very uniform surface. The lower beds consist of clean quartz sand, 
but silty material appears in the higher beds and in all the rest of the 
Belt terrane, which attains a thickness of at least several thousand 
feet in the southern part of the range, and of 10,000 to 12,000 in the 
Big Belt Bange. These beds are shallow- water deposits. During the 
period in which the rocks of the Belt terrane were deposited the north- 
em part of the Little Belt area was probably above water, for no Belt 
rocks are found there, and the beds immediately overlying the gneiss 
contain no material indicating the former existence of such rocks in 
the northern part of the range. Of the conditions prevailing through 
this part of Algonkian time there is little knowledge other than that 
which may be deduced from the character of the beds themselves. 
These show a cycle of deposition, with slowly deepening sea, followed 
by gradual emergence. The only forms of life yet discovered are those 
described by Walcott.^ 

After the formation of the Belt beds the whole region was uplifted. 
There is not only an entire absence of Lower Cambrian throughout this 
and the neighboring ranges of central Montana, but the succeeding 



1 Fouiliferoiu pre-Combrian temuiM : Ball. GeoL Soc. America, VoL X, 1899, p. 190. 
382 



WEED.) GEOLOGIC HISTORY OF THE REGION. 383 

beds consist of assorted sands whose material varies in character from 
place to place with the natore of the underlying rock. The belt forma- 
tions were extensively eroded in this interval, and as a conseqaence the 
Flathead sandstones, which constitate the basal beds of the Middle 
Cambrian, rest upon different formations at different places. Angular 
unconformity between the Belt and Cambrian rocks has been seen, but 
it is seldom very noticeable in exposures. The overlap of the beds is 
itself convincing proof of unconformity. The plane of contact on which 
tfae Cambrian rests is a very uniform one, and is especially so when it 
is the Archean. This is seen in the canyon wall along Belt Creek and 
Belt Park, as well as in lower Sheep Creek. This plane surface pre- 
sents the characters of a plane of marine erosion, or shore-line cutting. 
Crosby' has recently called attention to the widespread extent of this 
feature. Pebbles of Belt rocks in the basal beds of the Flathead, where 
the latter formation rests upon the Algonkian terrane, and of gneisses 
and schists over Archean areas, show the erosion caused by a spreading 
sea margin. These Flathead sandstones represent a general subsi- 
dence pf this region in common with the rest of the eastern mountain 
region of the State. The level plane of contact was apparently formed 
as the sea cut back and wore down the land during a gradual sub- 
mergence. The overlying beds show, in the shales, impure limestones, 
and intraformational conglomerates, as well as by the fossil remains, 
that the conditions prevailing were those that are known to have pre- 
vailed commonly in Cambrian time over large parts of the continent.' 

So far as at present known, there are no beds of either Upper Cam- 
brian or Silurian age in this region, the fossil remains from adjoining 
beds holding Middle Cambrian and Devonian forms, respectively. It 
is assumed, therefore, that the region was a land area during the 
Upper Cambrian and Silurian periods. 

The Devonian period is represented by arenaceous limestones whose 
fossil remains are mostly corals. No unconformity is recognizable 
between these beds and the succeeding layers of the Carboniferous, 
though a bed of red sands was observed in one exposure and may 
indicate estuarine conditions. 

The Carboniferous period was one of slowly deepening, followed by 
shallowing, seas. The rocks contain abundant fossil remains, which are 
wholly of Lower Carboniferous types, and the well-defined subdivisions 
recognized in the Mississippi Valley and the States east of it have not 
been distinguished. The lowest beds are impure, argillaceous, shaly 
limestones, and the fossils contained in thQ^l are shallow- water forms. 
The middle and greater part of the rocks of this age are, however, 
quite pure limestones, more rarely dolomites, and the life is such as to 
indicate relatively deep water. The close of the period was, however, 
one of elevation, and there is an abrupt change to very shallow- water 



iW.C. Crosby, Boll. Geol. Soc. America, Vol. X, 1890, pp. 141-164. 

*Walcott, Cambrian rooks of Pennsjlvania : Bull. U. S. Geol. Surrey No. lS4j and Correiatio& 
Papers--Gambrlan: Bull. U. S. Geol Snrvey No. 81. 



384 GEOLOGY OF THE LITTLE BELT MOUNTAINS, MONTANA. 

and estaarine areas in which red sands and gypsum deposits were 
formed. These conditions were followed by a gentle depression, with the 
formation of green shales alternating with limestones, whose fanna 
also indicates shallow and muddy waters. 

^o definite break or unconformity is in general recognizable between 
the topmost beds of the Carboniferous and those of the succeeding 
marine Jurassic. On the southern flanks of the range the highest 
beds of the Carboniferous show a fauna somewhat different from that 
prevailing elsewhere, and their fossils are found within 2 feet of Jurassic 
fossils. There is, moreover, some evidence of an erosion interval in the 
varying thickness and absence of certain beds of the Quadrant group 
of the Upper Carboniferous. The succeeding Jurassic period was at 
first one of quiet waters covering the entire area, but the limestones 
formed in this time are succeeded by beach sands holding shell remains 
evidently broken by wave action, showing a gradual elevation of the 
land, that resulted in the rapid deposition of sands which are barren 
of life. 

The succeeding Mesozoic rocks do not form any part of the mountain 
area, but it is necessary for a comprehension of the conditions determin- 
ing the uplift of the range to understand the fact that the sediments 
of Cretaceous age all show a thickening, with a great increase of sands 
and clays near the mountain front. The conditions indicate quite 
clearly that there was a general and widespread uplift in early 
Cretaceous time, accompanied by the formation of marsh lands and 
fresh- water lakes, while volcanic eruptions breaking forth at some 
distant point scattered ashes over this area. Succeeding this epoch, 
the plains area was depressed beneath the Cretaceous sea, while the 
Little Belt region remained a land area. This was not, so far as now 
recognized, a time of mountain folding. The infolds of Cretaceous 
rocks found at Castle Mountain, just south of the Little Belt Kange, 
and the general upturning of these rocks about the moantain flanks, 
indicate that this occurred at some later period, which the evidence in 
neighboring ranges indicates to have been post-Cretaceous.^ 

STRUCTURAL FEATURES OF THE RANGE. 
NATURE OF THE UPLIFT. 

The Little Belt Mountains form a part of the eastern front of the 
Rocky Mountain region. They are structurally connected with Castle 
Mountain on the south and through it with the end of the Big Belt 
Bange. They are formed by a broadly V-shaped uplift, pointing east- 
ward, which disappears beneath the plains at Judith Gap, but is never- 
theless continued eastward and reappears in the dome uplift of Big 
Snowy Mountain. Although connected with the general mountain 
folding, the Little Belt fold is practically a unit. It differs from the 
commoner and sharper folds of the region in being a broad, relatively 



>Wee<l, Laramie and overlyiDg LivlngBton formations in Montana: Ball. U. S. Geol. Sarvey No. 105. 



WEED.] STRUCTURAL FEATURES. 385 

low anticline, having a flat or slightly undulating top and steep sides. 
The southern border is formed by a secondary fold, pitching eastward 
and passing into a fault that outlines the margin of the uplift farther 
west in Volcano Valley and continues westward to Sheep Greek, near 
Kinney. With this exception no faults of any magnitude were found 
in the range, except those directly due to laccolithic uplift. The warp- 
ing is all broad in its features, and there are none of the narrow foldings 
and overturns seen farther south and west in the mountains near Boze- 
man and Livingston. On the north and south are troughs of dex>osition 
in which great masses of sediments were laid down. The Highwood 
Mountains on the north show several thousand feet of Oretaceous rock. 
The Crazy Mountains to the south are composed of 10,000 feet of latest 
Oretaceous and Eocene ( !) rocks, cut by igneous intrusions. The initial 
dips of these strata away from the Little Belt area probably determined 
the uplift. There is every reason to suppose that the range uplift is 
due to lateral compression. 

Relation of igneous intruaums to folding. — It is evident, however, that 
the minor doming and faulting which are observed at all the larger 
mountain masses of the range are due to igneous intrusions. These 
mountains are in most cases isolated and individual masses. It will be 
shown that they accompanied and were a corollary of the large uplift, 
and locally modified it greatly. To do this it will be necessary first to 
review the general character, structure, and relation of these igneous 
masses, a detailed account of which is given in the chapters on 
the descriptive geology. 

DYNAMIC GEOLOGY. 

The present structure and altitude of the Little Belt Mountains are 
plainly seen to be due, first, to a general uplift and folding of the range 
as a whole, and, secondly, to the presence of large intrusive bodies of 
igneous rock, of which the largest mountains — ^in fact, all the individual 
mountain masses — are formed and to which they owe their origin and 
present relief. As already stated, these igneous intrusions occur in a 
variety of ways. The smaller masses form dikes and sheets, the larger 
ones stocks and laccoliths or allied forms. 

Though presenting these different forms and structural relations, it is 
believed that the intrusions are all the result of the same general cause; 
that the rocks come from the same general source of supply, and were 
intruded at the same time or period of igneous activity. This hypothesis 
finds striking confirmation in the chemical and mineralogical relations 
of the rocks, as is forcibly shown by Professor Pirsson in the report 
on the petrography of the igneous rocks which follows this paper. 

INTRUSIVE SHEETS. 

The areal distribution of the intrusive sheets of the region as shown 
upon the geologic map does not convey an adequate idea of the abun- 
dance and the extent of such intrusions or of the part they play in 
20 GBOL, PT 3 26 



1 



386 GEOLOGY OF THE LITTLE BELT MOUNTAINS, MONTANA. 

determiniog the topographic developmeut of the region. This is dae 
to the fact that the sheets mapped are in most cases shown only at the 
places actually visited. A better idea of their probable occurrence 
would perhaps be given if the map had been made more diagrammatic, 
for many of the sheets are very persistent over many miles of country, 
though only prominent where the slopes show cliffs or debris heapings 
of the rocks. For this reason the valleys cut through those areas in 
which such sheets occur show their outcrops on both sides, conforming 
to the structure of the rocks in which they are intruded. 

The horizons invaded by these intrusions embrace parts of all the 
Paleozoic formations. As would naturally be expected, the softer and 
more easily intruded shales are selected, and as the Cambrian rocks 
present the most favorable conditions for intrusions the greatest num- 
ber of sheets are found in these rocks. In the Belt formations the 
rocks yield irregularly to intruding forces, and the ruptures are filled 
by dikes and irregular intrusions, which are rarely persistent, sheets 
following along definite bedding planes. 

In the Cambrian, on the other hand, the alternation of shale and of 
beds of sandstone or of limestone offer peculiarly favorable condi- 
tions for such intrusions, and they are here most effectively developed. 
In the higher formations the fissile beds lying between heavy-bedded 
limestones are the horizons of easy intrusion. The basic sheets are 
much thinner than the acidic ones, seldom exceeding 15 feet in thick- 
ness, and commonly being but 5 to 10 feet through. The acidic sheets, 
on the contrary, vary from a few feet up to 100 feet in thickness, and 
are commonly 20 or more feet through. The light-colored sheet rocks 
consist of rhyolitic and syenitic (or trachytic) porphyrites, and are 
usually much altered by the normal processes of weathering, but do not 
crumble down, as do the basic rocks. The basic sheet rocks are mostly 
minettes, showing considerable difference in grain, being very dense in 
the thin sheets and crystalline in the thicker; and the latter rocks alter 
and disintegrate rapidly when exposed, forming rounded exposures or 
sandy slopes, so that they are seldom conspicuous elements of the scen- 
ery. The two types are also contrasted in the effects which they have 
produced upon the sedimentary rocks in which they are intruded. The 
siliceous rocks have produced little if any contact metamorphism; the 
basic rocks, a good deal. This is usually apparent in a baking or hard- 
ening of the shales to hornstone and the development of a fine jointing 
that is independent of the bedding. This metamorphism is, of course, 
most marked about the largest sheets, and is directly proportional to 
the thickness of the intrusion. 

In the impure limestones secondary minerals are often developed and 
the rocks marmorized. In those observed instances where the two 
rocks cut one another the minette is the latest in age. This rock itself 
shows light-colored dikelets cutting it, but they appear to be aplitic in 
character and to merely represent parts of the same magma and not 
independent eruptions 



WEBD.] LACCOLITH IC INTRUSIONS. 387 

The siliceous rocks are relatively much more abundant, as the sheets 
are more numerous, and are always much thicker. It is difficult to 
form an estimate of the relative volume of the two types of rocks. 
Probably 1:500 would not overestimate the proportion of the lighter- 
colored rock. 

It seems certain that the igneous sheets, some of which can be traced 
for miles along a certain bedding plane, and which are found upturned 
and folded with the sedimentary rocks, could not have been intruded 
after the folding of the strata. No force, however powerful, could 
inject sheets of so uniform a thickness and so wide an extent into rocks 
folded as these are today. On the other hand, if the sheets were 
intruded before the folding of the sedimentary rocks they should show 
some evidence of the movement. It is incredible that the sheets should 
have been upturned with the inclosing shales, when they were tilted, 
without showing evidence of the dynamic forces in strain phenomena 
or cracking of the phenocrysts of the rock. Yet such evidences are 
lacking in the field exposures and are not revealed by careful exami- 
nation of the section under the microscope. The only alternative is 
the hypothesis that the intrusions were injected contemporaneously 
with the uplift and folding of the strata, their present position being 
in consequence of such folding and of the ^Hatent" cavities or planes 
of easy intrusion then developed. This hypothesis seems in perfect 
accord with the facts observed in the field. 

LACOOLITHIO INTRUSIONS. 

Position and number. — With the single exception of Yogo Peak, all the 
prominent mountain massies of the Little Belt Bange are formed of 
masses of igneous rock, whose structural features show that they con- 
stitute a group of closely related forms, grading from a typical laccolith 
to those which may represent plutonic plugs* or bysmaliths.^ The 
geologic map (PI. XLI) shows the distribution of these intrusions 
throughout the range by the exposed areas of igneous rock, or, where 
this is not yet revealed by erosion, by the outlying domes of older rocks, 
which are believed to cover such intrusions. 

Fig. 47 is a ground plan of these intrusions. This diagram shows the 
relation of the laccoliths to one another and their distribution in the 
range. The circular areas marked by letters represent the different 
mountain masses, the curved line representing areas of uplifting or 
doming of the strata where this has occurred, or of the boundary of the 
igneous rock where the intrusion has not disturbed the adjacent strata. 
The uplift of the whole range is defined by a line which marks very 
nearly the outer boundary of the Carboniferous limestones. 

General features, — It is evident from fig. 47, which is based upon the 
geologic map, that the intrusions occur along the northerly border of 

> I. C. Rnftsell. Jonr. Geol., Vol. IV, 1896, pp. 23-48. 

> J. P. Iddioffs, Juur. GeoL, Vol. VI, 1808, p. 70S. 



388 GEOLOGY OP THE LITTLE BELT MOUNTAINS, MONTANA. 

the uplift, most of them being situated on its northern limit or mono- 
cline. By reference to the geologic map it will be seen that they occui* 
only where, the Belt terrane is wanting. It will also be seen that the 
igneous rocks are in contact with strata of various ages, and that only 
in the uplifts outside of the range, as at Skull Butte, is the symmetrical 
doming of a typical laccolith shown by concentric outcrops of the differ- 
ent formations. The map shows plainly that the intrusions associated 
with the main range uplift the strata about a part of their periphery 
in some cases, and break through the beds in another part, while in 
other cases they show only a narrow zone of abrupt uplifting or none 
at all. These relations are also seen in the vertical sections across the 




Fig. 47.— Diagram showing difttribation of lacoolithic masaes and related introsions in Little Belt 
Moantaina. The outer line shows the limit of pronounced displacement or uplift (the springing line 
of the range), the rudely circular areas that of the laccoliths and related intrusions. A, Tiger Butte 
laccolith; B, Thunder Mountain dome; C, Tiliinghaat Mountain dome; D, Kibbey Dome; E, Barker 
Mountain; F, Clendenin and Otter Mountain dome; G, Mixes Baldy dome; H, Taylor Peak and Wolf 
Butte; I, Foothill Dome; J, Steamboat Mountain; K, Big Baldy dome and Steamboat Mountain; L, 
Dry Wolf dome ; M, Sage Creek Mountain ; K, Ricard Mountain; O, Skull Butte. 

range shown in folios Nos. 55 and 56 of the Geologic Atlas of the United 
States. 

It is apparent that the asymmetrip^ laccolith is the prevailing type. 
These sections and the geologic map also show that the asymmetric or 
faulted side of the laccolith is on the outer side — that is, next to the 
plains country, and away from the mountains, and that it coincides 
with the steep dip marking the flank of the fold. In other words, the 
asymmetric sides of the laccoliths coincide with the monoclinal fold 
along the flanks of the range anticline. 

Size, — The size of these intrusive bodies varies. Big Baldy, the 
largest, shows a surface exposure of 3 by 5 miles, and a vertical extent 
which is believed to be very nearly its original thickness of 3,000 feet 
at the center. The Barker Mountain mass has a diameter of about 4 
miles, as shown by the doming of the strata, and the mass is probably 



w«D.] LACCOLITHIC INTRUSIONS. 389 

about 3,000 feet thick in the center. The Steamboat, Sage, and Eicard 
jnonntain masses are smaller, the folding indicating a base abont 2 
miles in diameter. 

Contact metamorphism, — ^These intrusions have produced but little 
contact metamorphism of the sedimentary rocks. At the immediate 
contact there is a hardening and alteration of the adjacent rocks to 
homstones or marbles, but this extends at most only a few yards from 
the igneous rock. So far as observed, this alteration affects a greater 
thickness of rock over the top of the intrusion, as at Steamboat Moun- 
tain, than it does along the sides. 

Character of the rockn. — ^The rocks constituting these intrusive masses 
are uniform in mineral composition throughout the whole extent of 
each body, but exhibit a limited variability of texture, ranging fromdense 
aphanitio forms at the contact to finely crystalline textured porphyries 
in general. Those of the same type differ somewhat in the different 
'intrusions, showing variations in texture and in the relative proportions 
#f the component minerals, but the rock nevertheless preserves its 
general character and habit. The rocks are classed, first, as Barker 
porphyry, the prevailing type, which grades into diorite-porphyry at 
Steamboat Mountain; and, second, as Wolf porphyry. Both types are 
granite-porphyries, and are fully described in the report following this. 
Their field relations are given in the descriptive geology of the range. 

Jointing, — The rocks of each type show characteristic jointing, but 
the jointing is more dependent upon texture than upon mineral com- 
position. The Wolf porphyry at Wolf Butte shows typical granitic 
weathering, with well-marked jointing, forming great castellated crags 
and rounded bowlder masses, the so-called ^^ Woolsack'' structure. In 
general, however, the rock is crumbly and seldom forms good exposures. 
The Barker porphyry breaks with a conchoidal platy fracture, rarely 
showing massive forms. The best examples are those seen in the Big 
Baldy amphitheaters, illustrated in Pis. XL VIII, JB, and XLlX, A. 
The contact forms of both types show a fissile or platy structure, with 
the planes parallel to the contact. These platy masses break up into 
rather small, angular blocks, due to minute joints — the common weather- 
ing of dense rhyolitic porphyries. 

Horizon intruded. — In no case has the bottom contact of these intru- 
sions been seen. The correlative evidence afforded by the stratified 
rocks about the intrusion is, however, quite conclusive that the intru- 
sions rest on a fioor of Archean gneiss or upon the basal sandstones of 
the sedimentary series. The horizon invaded is therefore the shale 
formations of the Cambrian, and the lat^r formations of this age are 
seen uplifted about and encircling many of the intruded masses. 

Depth of intrusion. — ^The Archean rocks are covered in the vicinity of 
these intrusions by a thickness of 2,200 feet of strata. The Quadrant 
formation, 400 to 600 feet, and possibly the Ellis, 200 feet, were also 
probably present, but have been removed in the denudation of the 



890 GEOLOGY OF THE LITTLE BELT MOUNTAINS, MONTANA. 

range. How mucli of the Cretaceous, if any, formerly covered the range 
is not known. The shore-line features observed about the flanks of 
the range and their absence from the infolds of the region seem to indi- 
cate that these rocks never covered the Little Belt area. Skull Butte, 
which is believed to be a concealed laccolith, is still covered by Jurassic 
rocks and shows the Cascade formation (Lower Cretaceous) sharply 
upturned by it, so that it undoubtedly once covered the dome. This 
would mean a total cover of about 4,000 feet. 

None of these intrusions of this character occur where the Algonkian 
rocks are found; that is, they are absent where the sedimentary section 
is greater than 4,000 feet and were not formed under a cover of great 
thickness. 

Uxtmt of denudation, — The intrusions are seen to-day in various stages 
of denudation and erosion. At Thunder Mountain the original surface 
has been carved into abroad peak; at Big Baldy Mountain the smooth, 
rounded contour is preserved on one side, but the other is deeply cut* 
by amphitheaters. Barker Mountain is a bared but as yet almost 
uneroded laccolith, showing the original smooth, curved upper surface 
of the intrusion. Steamboat Mountain shows the cover of sediments 
just cut through, exposing a small area of the igneous core. The Kib- 
bey dome is believed to conceal a laccolith; the softer beds have been 
removed from the core, but the dome of Carboniferous rocks are only 
slightly attacked by erosion. At Skull Butte the Mesozoic rocks are 
not yet pared off, though the center of the dome is deeply trenched by 
a small drainage originating in this butte. Thus every stage is seen, 
from intrusions still covered by the domed strata to those almost com- 
pletely denuded and deeply scored and carved into peaks. 

The Kibbey and Dry Wolf Creek domes are crossed by streams flow- 
ing in deeply trenched channels cut in the arch. This peculiarity is 
due, as Gilbert has shown for similar examples in the Henry Moun- 
tains,^ to the formation of planation terraces — the work of streams radial 
from the mountains cutting across the soft Mesozoic rocks. As degra- 
dation progressed the Carboniferous limestones of the domes were 
uncovered, and, lateral corrosion by the stream being checked by the 
harder rock, the channel became fixed and a canyon was carved, while 
the dome of limestone was, at the same period, left in relief by the 
general degradation of the region. 

Accompanying dikes or sheets, — There is a general absence of dikes 
connected with these intrusions. Only one case was found of a dike 
that owes its origin to a laccolith or related form of intrusion. This is 
on the summit of Bicard Mountain. This is in accord with the facts 
observed in the Judith Mountains, but quite different from the Henry 
Mountain laccoliths. Intrusive sheets are, however, often found in the 
upturned beds about the intrusions. They are usually of little thick- 
ness, the larger sheets seen in the range owing their origin to the Yogo 
Peak center. 



1 Geology of the Henry Mountains, p. 126. 



wBml LACCOLITHIC INTRUSIONS. 391 

Form of intrusion, — The intruded masses whose occurrence and 
characteristics have been summarized in the hist few pages have been 
referred to as laccoliths. Their cross sections are shown in iigs. 43, 44, / 
45, and 46. In none of the denuded laccoliths is the simple, symmetrical, 
dome-shaped uplift realized. All the masses show at least a short arc 
on one side, along which the strata are broken, and the igneous rock 
has risen to a higher horizon across the fractured edges of the beds. 
This is the typical asymmetric laccolith. 

Barker Mountain, a cross section of which is given in fig. 43, shows 
this structure. On three sides the rocks are flexed and arch over the 
intrusion; on the remaining side they are broken and faulted. A sec- 
tion made in a different direction would show this better than that in 
the figure, which is intended to show, not the normal doming of the 
beds, but the asymmetric character of the intrusion. 

The cross section of Thunder Mountain (fig. 46) represents a differ- 
ent form of intrusion. It is easy to see that there may be every grada- 
tion, from the asymmetric laccolith, whose lack of symmetry is confined 
to a single side or a short arc of its circumference, as seen in ground plan, 
to one in which half the circumference is a line of fracture and fold, and 
from this to one that is fractured for three-fourths of the periphery. 
If we assume the contact plane is all fracture and no fold it becomes 
a bysmalith — that is, a laccolith in which the arched part equals zero. 
The laccoliths of the Little Belt Mountains are all asymmetric ones, and 
show various gradations into the bysmalith. It is noticed that the lacco- 
liths are smaller in area than the bysmaliths when the rock is the same 
in both. There are three bysmaliths, viz: Thunder Mountain, Mixes 
Baldy, and Big Baldy Mountain. The Wolf Butte and Steamboat 
Mountain intrusions are intermediate in type; the remaining six are 
asymmetric laccoliths of the Barker Mountain type, while the three 
outlying domes may belong to any one of these groups, since in each 
the intrusion is covered by an arch of sediments. 

The common form found in the Little Belt Mountains is, however, 
clearly entitled to the name laccolith, being symmetric for the greater 
part of the circumference. Moreover, as stated by Gilbert, " if the 
stratahad experienced anterior displacements, soas to be inclined, folded, 
and faulted, a symmetrical growth of laccoliths would have been imi)os- 
sible." ^ Cross,^ in discussing the laccolith, says : 

In regions where the beds are nnder orographic stress almost, or qnlte to the point 
of folding, a magma would find intrnsion on certain planes a comparatively easy 
matter. Such conditions are illustrated by the sheets in the spring of the arch of 
strata over the laccolith, fig. [48], and it seems probable that such occurrences as 
those of the Mnsqnito Range and Ten Mile district, where many of the sheets are 
intruded very regularly, represent localities where lateral pressure has already over- 
come the gravity of the strata to a great extent, and the intrusion planes were 
planes of easy parting. 

' Geology of Henry Mountains, p. 98. 

'Laccolithic mountain groups: Fourteenth Ann. Kept. U. S. Geol. Survey, Part II, 1893, p. 236. 



892 GEOLOGY OF THB LITTLE BELT M0DNTAIN8, MONTANA. 

He explains tbe asymmetric form of the Moant Marcellina laccolith 
by anpposiiig "a line of weakness • « • from axial teDsion, pre- 
esisting fracture or resistaDce offered by an earlier intrusion — the 
iutruding magma would develop this into dislocatiou." 

These statements apply with peculiar force to tbe conditions found 



Fio.U.— Pro91«snctiontbrougbU(iuiitHuvelllDa.witbIdMlragtoni[loiiofliwcollthKiidcoT«r{Cniu). 

in the Little Belt Mountains, The common type is like that of Mount 
Marcellina, given by Gross. Fig. 48, copied fh>m Cross, shows an ideal 
restoration of laccoliths and strata above it, based upou a profile sec- 
tion. If this be compared with the section of Mount Hillers, in the 
Henry Mountains, as given by Gilbert (fig. 49), it will be seen that the 



Fia. tB.-IdsalcroHnKUDnof MoDDt HlllerBUccollth, Henry UonDUlua (Ollbert). 

conditions observed there and those supposed by Gross are similar, but 
the interpretation is very difTerent. Tbe Thunder Mountain laccolith 
shows a crosB section (fig. 46) strikingly like that of Hillers. It is 
believed that the facts observed in the Montana region are in accord 
with the interpretation given by Cross. If the right-hand half of the 



*■■!>.] LACCOLITHIC INTRU8I0NB. 893 

diagram (fig. 48) be revolred aboat a yertical axis, the resulting form 
woald be the byamalitb of Iddings, as shown in bis diagram of Mount - 
Holmes, of the Gallatin Bange of the Yellowstone Kational Park (fig. 
60). It is eaaj to conceive of all gradations between laccoliths asym- 



Via. M.— Ideal orou UDtloa of Uoant Holnwa bjanuliUi, Telloiritone HatlonKl Park clddlogi). 

mdtrio for short arcs of the ground plan, throngh those that are 
asymmetric for half the circumference, to the typical bysmalith. It is 
believed that the Little Belt masses present actaal examples of all these 
forms. 
The occurrence of dikea filling flssares formed by extensile strains, 



FiQ. Gl — IdealoTOUMcMonarnortheniflaakofLiHlBBctt Kange, to ihoir relattoD of ujrnunctrla 
laccolith to foia or faulilng of uplift. 

dae to the stretching of the strata over the laccoliths, Is a yery nncom- 
men feature in the Little Belt Range, owing, it is believed, largely to 
the shaly nature of the overlying strata. The pipe of rhyolite- 
porphyry cutting the limestone that crowns Bicard Peak is bdiered 



394 GEOLOGY OF THE LITTLE BELT MOUNTAINS, MONTANA. 

to' be the filling of a fissure due to such extensile strain. Gilbert's 
hypothesis of stretching is, however, based upon hydrostatic pressure 
acting on horizontal beds. If the beds were flexed or being flexed by 
orographic forces, there would not be a stretching of the strata next to 
the laccoliths, but rather a compression, while the uppermost layers 
of the arch would be stretched, though the well-bedded stratified rocks, 
being coherent, would tend to fold rather than rupture, like the beds of 
the Colorado locality described by Cross. 

Theory of origin and peculiarities of form of laccoliths, — The origin of 
laccoliths, originally treated by Gilbert, has since been discussed by 
Gross, and recently by Pirsson. The former writer shows that relative 
^^ densities of intruding lavas and invaded strata" are not the cause, 
and finds in forces of different intensity, if not in kind, the factors 
determining whether the intrusion shall be a laccolith or stock.^ Pirs- 
son,^ in a very lucid and practical treatment of the subject, calls atten- 
tion to the physics of the problem and the analogy between sheets and 
laccoliths. Particular emphasis is laid upon the viscosity of the intrud- 
ing magma, and in this and the rate of injection he finds the main 
factors determining the character of the intrusion as a sheet or lac- 
colith, with upward force of magma, gravity, load of sediments, and 
their resistance to splitting as other factors. Owing to viscosity, the 
force is not transmitted as in a perfect fluid, and in a supposed instance, 
where the magma is intruded between strata when the sheet-like mass 
becomes of a certain radius, the force is greater over the supply pipe 
than elsewhere, and the strata arch up. The convexity of a laccolith 
is a function of the viscosity of the lava. 

In the same way, it is believed that if the force be great, fractures 
will occur and a bysmalith be formed instead of a laccolith. It is only 
carrying the process a step further, and the bysmalith bears the same 
relation to the laecolith that the latter does to an intrusive sheet. 
Pirsson ^ finds in rapidity of supply of the magma the cause why stocks 
and not laccoliths are formed, the magma <^ spreading out laterally as 
the lateral resistance diminishes, rupturing and tearing up the beds 
* *  or under lighter loads expanding laterally and uplifting beds 
so as to simulate laccoliths when seen from above, but having no proper 
floor." In the writer's opinion, such fan-like expansions are most apt 
to form in clay shales and similar easily ruptured beds. 

Summarizing the conditions determining the nature of the Little 
Belt Mountain intrusions, we find that— 

1. Sheets occur in easily separable, well-bedded strata; the intrusive 
rocks are of various composition. 

2. Laccoliths occur along planesof easy intrusion (usually between a 
solid resistant floor and overlying massive beds competent to form 



iLoc.oit.,p.240. 

'L. V. Pirsson, in Geolugy of Judith MountainA, by Weed and Pirsson: Eighteenth Ann. Rept. 
IT. S. Geol. Survej^ Pt. Ill, 1896, p. 586. 
*Loc.cit., p>587. 



WHBD.1 LACCOLITHIC INTRUSIONS. 395 

arches) when the invading magma is relatively siliceous, and hence 
yiscousJ Theoretically they should have a larger horizontal area than 
the sheets, other conditions being equal. Asymmetiy of form results 
from local weakness of the cover, due to one or more of several causes. 

3. Still more irregular laccoliths, or those forms called bysmaliths, 
will form instead of true laccoliths when the diameter or the ascensive 
force is greater, and hence causes more rapid nplifting. 

These three forms of intrusion all grade into one another, and are 
similar in that they all are covered by an arch of strata domed by the 
intrusion. Gradations from the laccolith to the stock will occur when 
there is a well-developed line of weakness or fracture along one side of 
the laccolith. It is merely a question of the ratio of resistance of 
lateral expansion and uplift to Assuring and ascension. 

In regions where easily separable strata of very unlike character, 
such as massive limestone alternating with micaceous shale, are folded 
by orogenic forces, it is evident that if molten magma has access to the 
folds intrusions will be easy in the span of the arch or along the slip 
planes on the limbs of the fold. The theoretical sha[>e of intrude<l 
masses in the span will give a crescent-shaped cross section, like the 
^^ saddle reefs" of the Australian ore deposits, which are strictly analo- 
gous deposits of quartz. This form of intrusion is, as Iddings has 
pointed out, the one to which Suess applied the term batholith — a term 
now generally used in a broader sense. Such meniscus-shaped intru- 
sions have been found by the writer on the summit of anticlines in the 
Gastle Mountain region south of the Little Belt Mountains. 

Unlike the laccoliths of the various localities hitherto described, the 
Little Belt Mountain examples are in a region where orographic folding 
has occurred. The sedimentary rocks consist of well- bedded strata 
resting on Archean gneisses. The lower member of the series consists 
of 900 feet of shale alternating with thinly bedded limestone, the mid- 
dle member of 1,000 or more feet of massive limestone, the upper mem- 
ber of 1,000 or more feet of shale and sandstone. It is the same 
series which under tectonic forces always formed folds in this region. 

Summary, — In the Little Belt Kange the observed facts show that 
the intrusions did not occur before the uplift nor after the uplift of 
the range. The hypothesis is advanced that the igneous intrusions 
accompanied the uplift, and that they invaded those places where 
the orographic stresses furnished planes of easy entrance, so that the 
force causing the range uplift and the upward pressure of the ascend- 
ing magma worked together. The laccoliths occur mainly on the flanks 
of the mountain uplift because it furnished faults or lines of weakness 
for intrusion, especially along the outer sides. The presence of the 
laccoliths on the northern part of the range only is in accord with the 
hypothesis of a relatively light load. Where the many thousand 



^Laoooliths of baBio rook ooonr In the High wood Mountaina of Montana and elsewhere, but are not 
oonaidered here, the contrasted type«i being those of the Little Belt region only. 



396 GEOLOGY OF THE LITTLE BELT MOUNTAINS, MONTANA. 

feet of strata of the Belt terraue lie between the gneisses and the 
Cambrian beds no laccoliths occur. Where the Belt beds are absent 
the intrusions had less than one-fourth of the load to lift. 

The asymmetric character of the intrusions, and the fact that the 
asymmetric side is always toward the o|)en plains, are quite in accord 
with the theory just given, as the line of monoclinal flexure on the 
flank of the range anticline would also be a line of weakness, which 
the ascensive force of the magma, added to the compression and fold- 
ing stresses of orographic movement, would tend to develop into a dis- 
location. This might readily pass into a fold upward and be revealed 
only by erosion. 

GBADATION BETWEEN LACCOLITHS AND STOCKS. 

Intrusive stocks may show fanlike sections and dome up the strata. 
In giving the conditions which he believes favor the formation of a 
stock rather than a laccolith, Pirsson mentions the conditions which 
the present writer believes determine the formation of bysmaliths — 
viz, too rapid forcing of magma upward for adjustment of load and 
arching of strata. It is, how'ever, a question how far the facts carry 
out Pirsson's hypothesis that ^' as the magma ascends from one horizon 
to another it continually spreads out laterally as the lateral resistance 
diminishes, rupturing and tearing up the beds and carrying their mate- 
rial with it;" or ^'diminishing lateral resistance, the lighter load may 
permit the magoias to seek relief by lateral expansion, and in so doing 
they may lift the lighter beds above them and solidify beneath them; 
then from above they would simulate a laccolith, but differ in having 
no proper floor." 

The soft Mesozoic beds which prevail in the Judith Mountain exam- 
pies are readily ruptured and do not flex well, but these formations are 
not present in the Little Belt examples, and the observed facts do not 
accord with this hypothesis. 

INTRUSIVE STOCKS. 

There are three intrusive masses in the range which differ in occur- 
rence firom those already described, and are best classed as stocks or 
intrusions breaking abruptly through all other rocks with a more or 
less vertical contact. The most northern of these masses occurs in the 
Barker district, and is a mass of rather evenly coarse-grained syenite 
about li miles across. But little is known of its structural relation 
further than the fact that it is later than the intrusive sheets of the 
vicinity. No connection with the laccoliths is known. 

YOGO STOCK. 

In the central part of the mountains there is a chain of peaks extend- 
ing in a general northeasterly direction 13 miles from Yogo Peak, the 
second highest summit of the region, to Woodhurst Mountain. This 



1 LACCOLITHIC INTRUSIONS. 397 

chain, to which the name of range might perhaps be applied, is geolog- 
ically of very different origin from the mountain types of the rest of the 
district, and finds its nearest known counterpart in the Buby Eange of 
Colorado, described by Gross.' 

At the southwest end of this chain is a stock whose culminating 
I>oint is Yogo Peak. This is connected by numerous dikes with another 
stock to the northeast, which is for convenience designated the Wood- 
hurst stock, as its extreme end forms Woodhursc Mountain. These 
two stocks and the connecting dikes constitute the filling of a great 
break or fissure extending across the eastern side of the Little Belt 
Bange. The Yogo Peak stock appears to have been a conduit or center 
for. the igneous activity of the region; the Woodhurst stock is in places 
a broad dike-like intrusion, breaking up through the rocks without 
regard to their structure, but at Woodhurst Mountain is laccolithic in 
character. It constitutes, in fact, a transition form between a stock 
and the laccolith-bysmallth types. Moreover, the rocks of this stock 
show an essential identity in composition, and even in details of struc- 
ture, with those of the laccoliths. This forms the bulbous eastern 
extension of the remarkable fissure line noted above. It consists of a 
mass of coarsely granular rocks, whose structural relations show that 
it was a center of igneous activity* 

The sheets which encircle the center, and the dikes, which show a well- 
marked radial arrangement about it, indicate that this stock formed a 
center firom which intrusions were sent out in various directions and in 
various forms for many miles into the rocks surrounding it. There is, 
however, no positive evidence to show that it had any direct connection 
with the various laccoliths of the range. The rocks composing the 
stock are also of unusual interest, since they show a systematic grada- 
tion of types Arom basic to syenitic rocks, and this arrangement has a 
definite structural significance. 

General geology of Yogo stock. — The rocks of the Yogo stock, together 
with those of the encircling sheets and radial dikes, form a natural 
group showing great variability of composition and structure. The 
stock rocks grade into one another, but are sharply contrasted in some 
small closely adjoining portions. The granular nonporphyritic types, 
the syenite, monzonite, and shonkinite, occur only in the stock and its 
larger offshoots, while the finer- grained porphyritic rocks occur in the 
stock and in dikes and thinner sheets intruded in the sedimentary 
rocks for many miles about it. The syenitic types vary in structure 
and composition in the different occurrences, but the changes in a single 
area are not rapid or marked except at Yogo Peak. At this locality the 
different types occur in such a manner as to suggest differentiation in 
place. It is possible that successive ii\jections occurred, but the general 
uniform gradation of the types is against this view. There is no reason 



> Laooollthio moantain gronpe: I'oarteenth Ann. Sept. U. B. Geol. Snrvey, Part H, 1898, pp. 

iM-aoi, 



398 GEOLOGY OP THE LITTLE BELT MOUNTAINS, MONTANA. 

to believe that the material ever found an outlet at the surface, but the 
stock appears to be the site from which not only the great mass of the 
stock itself but also the lesser masses of the encircling dikes and sheets 
were sent out. The variability of grain, the banding of some of the 
smaller portions of the Togo rock, the little included masses of por- 
phyry, which seem to fade into the coarser-grained rock, and the other 
variations observed in the field could have resulted from a differentia- 
tion of the mass. This might possibly Lave differentiated before the 
final act of intrusion and have been slightly mixed during the final 
movement, as explained in the petrographic report following this paper. 
If this is true, the differeutiatiou observed is a result of cooling after 
injection. Since cooling must have been about the same throughout this 
large body, it must have been one cause of the variation in character, 
since rock structure depends upon both temjierature and chemical com- 
position. The magma of the main stock would remain molten much 
longer than in the smaller fissures, and it is therefore in general much 
coarser grained. There is no evidence at Yogo Peak of very fine-grained 
or dense rock at the contact, and at the time of the final consolidation 
the surrounding rocks, as the metamorphism shows, were undoubtedly 
thoroughly heated, so that very fine-grained varieties were not formed. 

The porphyries forming part of the stock and the intrusive sheets 
and dikes may be divided into light-colored feldspathic and basaltic 
types. The light-colored rocks contain a little hornblende and biotite, 
and only the basaltic or trap like forms contain augite. The light- 
colored acidic rocks are cut in places by the dark-colored basic ones, 
and vice versa, and it is therefore certain that there were successive 
periods of fracturing and of dike filling. The evidence also shows 
that the intrusion of the great stock itself was practically simultane- 
ous, though there may have been several acts of dynamic action closely 
following one another, all forming one great period. When the first 
dynamic action fissured the sedimentary strata the magma at hand 
would fill the larger fissures and penetrate all the smaller cracks and 
solidify as sheets and dikes. There is no evidence that the stock broke 
out along a general syncline, which seems to be the most favorable con- 
dition for the injection of such large masses. That the force was an 
uplifting one is, however, shown by the abundance of intruded sheets. 

The intrusiorf of the sheets is generally parallel to the bedding of the 
strata, except in places where for short distances they may break across 
the beds. The association of light-colored acidic and of dark basic types 
is oftentimes very close, and sometimes they occur as parts of a coutinu- 
oas rock mass, but in no case observed is there distinct evidence of differ- 
entiation in the sheets, such as that described by Merrill ^ in the Gal- 
latin Valley, and as seen by the authors of this memoir in the neighboring 
Highwood Mountains. The thicker acidic sheets — from 75 to 100 feet 



> Proo. U. S. Nat. Mus., Vol. XVII, p. 637. 



WE1D.1 LACCOLITHIC INTRUSIONS. 399 

thick — are light-colored rocks, resembling types occurring in the Yogo 
stock. These sheets show plainly that in large part they were intruded 
at a time of uplift, when the stresses produced by dynamic forces aided 
in the injection of the igneous rock. The intruded sheets vary greatly, 
both in thickness and in horizontal extent, and present corresponding 
variations in texture and in mineral composition. In the thinner sheets, 
which are from 1 to 25 feet thick, the acidic light-colored rocks exhibit 
a very uniform habit. They vary in texturjB from dense aphanitic to 
fine crystalline forms, and, except in the very thinnest sheets, they 
exhibit an abundance of white feldspar phenocrysts or embedded crys- 
tals, which spot the rock and give it a typical porphyritic appearance, 
and smaller quantities of black ferromagnesian silicates, hornbleude and 
biotite, the relative proportions of which vary in different forms. Con- 
siderable variations in color are shown, and are due to variations in 
the amount or character of the groundmass and of the phenocrysts. 
As a whole, the rocks of the stock and its connected intrusions form a 
very complex group, intimately connected in geologic occurrence, and 
they are the result of a single period of igneous activity, forming a series 
exhibiting both transitions and contrasts in composition and in 
structure. 

The character of the intrusion is such that it shows that the force 
was great enough to force the magma upward and disru[)t the strata 
without greatly uptilting the beds, as previously mentioned. The mass 
thus injected is some 14 or 15 miles long, by about a mile wide, forming 
a continuous rock body, of which Yogo Peak is the southwestern end. 
Although accompanied, as stated, by numbers of sheets and some dikes, 
there has been no such vast amount of radial fissuring of the surround- 
ing strata as occurs in the Crazy and High wood mountains of this same 
general region, where the intrusions of streaks are in the soft Creta- 
ceous strata, with great surrounding zones or dikes. This is probably 
due to the harder, more resistant character of the Paleozoic beds. The 
greatest amount of fissuring of the strata appears to be at the north- 
east end, where the intrusion is connected by dike-like masses through 
the sediments with the large intrusive masses of Schoppe and Sheep 
mountains, which carry the whole line of intrusions to the northeast 
along a great general plane of fracturing. 

These intrusions appear to lack the depth and massiveness of the 
great Yogo stock, since they terminate toward the northeast with a 
distinct laccolithic jihase of intrusion, and the rock has also the por- 
phyritic structure of the laccoliths. And as in the laccoliths, there is 
no perceptible amount of differentiation in place. 

How far above the present surface the Yogo stock once extended is 
of course uncertain, but it is clear that a great load of sediments above 
it, and probably a considerable portion of igneous rock itself, have been 
removed. 



400 GEOLOGY OP THE LITTLE BELT MOUNTAINS, MONTANA. 

RELATION OF STOCKS AND LAOGOLITHS, 

The basic dikes and all the dikes found radial from the Yogo center 
are basic in character, are the latest rocks, and cat igneons rocks as 
well as the sedimentary ones. The minette dikes are believed to be of 
the same phase as the minette sheets, and in some instances at least 
to be directly connected with and to form a part of such sheets. 

On the other hand, the dikes of analcite-basalt and associated types 
cut through are later than the laccoliths, acidic sheets, minettes,or 
the granular rocks of the core. They can not, therefore, be offshoots 
from the massive rock of Yogo Peak, but must come from some deeper- 
seated source or the still liquid lower part of this Yogo stock. Both 
the basic dikes and sheets are found at far greater distances from the 
Yogo stock than the acidic rocks. 

The fact that the dikes are the very latest intrusions of the Yogo 
series is in accord with similar observations made by the author else- 
where in Montana. It indicates that the final forces produced a As- 
suring of the rocks adjacent to the stock. The analogy with the hot 
springs of Yellowstone National Park is worthy of notice in this connec- 
tion. While the hot waters have a free outlet they build up a mound 
about the spring, line theconduit with acalcareousdeposit, and, penetrat- 
ing every fissure adjacent to the conduit, tend to fill it up with a fresh 
deposit. There is thus gradually formed a definite conduit with solid 
walls. When deposition arches over and contracts the outlet, as is 
often the case, the water level is raised to keep pace with the building 
up of the deposit, until finally a level is reached where overflow all but 
ceases, and the rapidly forming travertine covers over and sometimes 
completely closes up the spring. The gases of the spring waters thus 
confined, together with an increased head resulting from the accumu- 
lation of water due to lack of outlet, make a gradually accumulating 
force, which, if it can not break a new outlet from some point in the 
conduit walls, will split the formation about the spring in radiating 
cracks, which generally extend far enough outward to give a much 
lower outlet for the water and thus renew the life of the spring. 

THEOBT OP DIKE FORMATION ABOUT lONEOUS GENTEBS. 

If we suppose that an igneous magma, under pressure, penetrates 
between sedimentary beds to form intrusive sheets, or raises them to 
make laccoliths, then chills in the pipe or central stock, but remains still 
liquid below, the result of such intrusions is to strengthen the sur- 
rounding structure and to make further intrusions as sheets more diffi- 
cult until the accumulated pressure brings about a final rupturing of 
the area adjacent to the center of activity, with accompanying intrusion 
of the dififerentiated magma, the regulus of the fluid, into these fissures, 
forming dikes. This, in fact, appears to have taken place at Yogo. 



CHAPTER Vm. 

NOTES ON THE ORE DEPOSITS OF THE lilTTIjE BEIiT 

MOUNTAINS. 

HISTORY AND DEVELOPMENT OP MINING IN THE REGION. 

The first recorded discovery of valuable deposits of the precious 
metals in the Little Belt region was made on October 21, 1879, at 
Barker. Several extensive bodies of rich silver-bearing lead carbonate 
were found in that district, and it became a producer in 1881. The 
same year that the Barker deposits were discovered the discovery of 
gold in the alluvial gravels of Yogo Gulch resulted in an inrush of 
fortune seekers to that locality. The attention thus directed upon the 
Little Belt Range resulted in the general exploration of the mountains 
in this and succeeding years. The carbonate and silver-lead deposits of 
the Wolf Creek district, north of Yogo, were developed, an* many hun- 
dreds of lode claims were staked out in various sections of the range. 
The Neihart deposits, discovered in 1881, attracted relatively little 
attention at first, but have proved the most valuable and permanent 
source of wealth of the entire region. The total production of thedif. 
ferent camps can not be given, nor is it possible to give that of any of 
the separate districts with any degree of accuracy. The entire product 
of the Barker and* Neihart districts for 1882 was only $65,000 to $70,000. 
That of Neihart up to 1898 is estimated at $2,140,000. In the year 
1883 mining development reached high water mark in all but the Nei- 
hart district, where it had just begun. The exhaustion of the bodies 
of rich carbonate ores resulted in stagnation and in a period of wait- 
ing until railroad communication was established. But little devel- 
opment work was done, though it was very generally believed that 
a rapid revival would follow the advent of the railroad. In 1891 a 
branch line of the Montana Central Railroad (now part of the Great 
Northern line) was built from Great Falls to Neihart and Barker, and 
another ^^ boom" period began. The well-defined veins and rich ore 
bodies of the Neihart region naturally directed especial attention to 
that district, and extensive development of mines and the usual mush- 
room growth of a mining town followed. Since that time the history 
of the mining development of the region is practically that of Neihart, 
the Yogo and Eunning Wolf districts having been but small producers. 
Since the ore bodies first discovered proved limited in extent, sufficient 
development work has not been done to prove either the value or the 
worthlessness of the properties, and they have been idle the larger part of 

20 GEOL, PT 3 26 401 



402 GEOLOGY OF THE LITTLE BELT MOUNTAINS, MONTANA. 

the time. Local excitement over discoveries on King Creek, Tender- 
foot and Pilgrim creeks. Tiger Batte, Sage Greek, and other localities 
proved short lived, and though small amoants of ore have been packed 
oat from various claims in these districts no producing mines have been 
developed. Up to the present time the geologic work reveals no such 
well-defined veins as those of Neihart, the deposits being mainly along 
contacts or associated with igneous intrusions. 

The prevailing inactivity, outside of the Neihart district, has made it 
difficult to obtain valuable data about the ore deposits. Wherever 
the workings were accessible they have been visited, but in many 
instances the surface geology and such facts as could be gathered from 
an examination of the ores and dump heaps, together with the infor- 
mation given by persons familiar with the region, are all the data avail- 
able for this memoir. Moreover, it should be remembered that the 
gathering of information upon the ore deposits was merely incidental 
to the survey of the areal geology of the mountains on a 4-miletothe- 
inch scale. Though the work was not in any sense a detailed study of 
the ore deposits of the region, it is confidently believed that the facts 
presented will be of general interest. As an account of the general 
geology of the region has already been given, the following pages are 
devoted entirely to a description of the ore deposits. The Neihart dis- 
trict, the principal producer of the region, is first described. This is 
followed by notes on the Barker district, the Yogo district, the Yogo 
sapphire mines, and the iron-ore deposits of the region. 

ACKNOWLEDGMENTS. 

The writer acknowledges with much pleasure the courtesy and assist- 
ance extended him by the mining men of Neibart and the other 
districts visited. To Mr. William Monroe thanks are especially due 
for copies of his claim map of the district and of individual mine sur- 
veys, as well as for the use of his office and many acts of kindness. 
Acknowledgments are due to Mr. D. 0. E. Barker, not only for many 
courtesies, but for the gift of particularly fine specimens of the rich 
ores of the Big Seven mine. Messrs. Jonathan McAssey, Daniel Lenny, 
and James Henley also were untiring in their kindness. The history 
of Neihart has been largely derived from the account by Mr. E. N. 
Abbot in the Neihart Herald. Thanks are also due to Messrs S. S. 
Hobson and Matthew Dunn, of the Yogo sapphire mines, for courteous 
attention and for specimens from the workings. 

NEIHART DISTRICT.' 
DISCOVERY AND DEVELOPMENT. 

The ore deposits of Neihart were discovered in July, 1881, by a party 
of prospectors from the neighboring town of Barker, and the first claim 
that was located, the Queen of the Hills, is still worked. The news of 



Montana district, unorganized, of United States Land Oftice. 



W11D.1 OKE DEPoarrs op nbihaet district. 403 

the discovery soon reached Barker, when Beveral parties started for 
the locality, and a large namber of claims were staked out io the first 
few weeks. The camp was named after J. L. Neihart, but the district 
was called the Montana district, and has never been officially organized. 
In 1882 a small settlement had sprang up, a wagon road was cat 
through to White Sulphur Springs, and siiiall amounts of rich ore were 
packed on horseback to the Barker smnlter. The first mine to be 



Flo. 5S.— Hap of Neihart dlatrlot. 

developed by oatside capital was the Gait, bonded by Governor Hauser 
in 1883. In that year the district was visited by Prof. W, M. Davis, 
of the Northern Transcontinental Survey, and in April, 1884, Prof. 
J. S. Newberry visited the camp on behalf of capitalists, who bonded 
the Monntain Ohief. The year 1881 was a lively one for the new camp. 
The Queen, Gait, Ball, and Mountain Chief mines were being actively 
developed and began shipping ore to the Omaha smelter. These ship- 



404 GEOLOGY OF THE LITTLE BELT MOUNTAINS, MONTANA. 

ments netted tbe owners $200 a ton, after deducting $100 per ton for 
freight and treatment. The Mountain Ghief was purchased for $18,000 
by the Hudson Mining Company, which spent over $10,000 in develop- 
ing the property, and acquired a group of six claims. The character 
of the ore uncovered by these workings led to the building of a con- 
centrator and smelter by this company in 1885-86. About 1,000 tons 
of concentrates and $50,000 to $60,000 worth of bullion were made. 
The works closed down in 1887, owing to the exhaustion of the rich 
surface ores and to the encountering of ores at slight depths carrying 
but 15 to 40 ounces of silver. It was found difficult to concentrate this 
ore, the tailings being, I am told, as high as 9 to 15 ounces of silver; 
and as large sums had meanwhile been expended in unproductive 
development, the enterprise was abandoned. 

In April, 1885, a group of claims acquired by the late Colonel Broad- 
water were consolidated to form the Broadwater mine. This property 
was vigorously exploited for a few months, but the ore bodies opened 
proved disappointing to the promoters, and work was soon suspended. 
These two failures gave the new camp a decided setback. Transporta- 
tion was costly, low-grade ores could not be successfully mined; so from 
1887 to 1890 the camp was deserted by all but its most sanguine men. 
In 1890 the building of a railroad line to Great Falls was assured, and 
a new era at once began. People flocked in from all over the State, 
buildings sprang up like magic, a large number of mines were bonded 
and sold, and mining development was actively carried on. The rail- 
road was completed on November 15, 1891, giving cheap transportation 
to the new smelter at Great Falls and to similar establishments else- 
where. The result was that development work had progressed so far 
in 1893 that a number of properties were in a condition where a steady 
output was assured, when the price of silver took its final drop in that 
year. It came like a thunderclap to the little camp. Work was sus- 
pended on all but a few mines until something of the future was known. 
From this blow the camp has not fully recovered, though the rich ores 
of the Florence, Benton, and Big Seven mines were profitably worked 
despite the low price of silver, more especially as those of the two last- 
named properties carried high values in gold. Before the drop in sil- 
ver, in this same year, exploratory work upon the Broadwater mine by 
its new owners had developed large bodies of rich galena ores, and this 
mine has continued to produce and ship ore up to the present time. 
From the figures given for various mines the total production of the 
district up to 1898 is estimated to aggregate 4,008,000 ounces of silver, 
$800,000 in gold, and 10,000,000 pounds of lead. As the ores have been 
very largely shipped outside the State, and the properties have changed 
hands several times, absolutely reliable returns could not be secured. 

While a large part of the known area of the district has been pat- 
ented, there is reason to believe that development of the camp has just 
begun. Very few of the veins known to exist have been explored. 



WBED.] 



ORE DEPOSITS OF NEIHABT DISTRICT. 



405 



WTiere tnnnels have been driven few, if any, upraises or crosscuts have 
been made. Discoveries in 1897 of rich ores on virgin ground on 
Mackey Greek, of the extension of the veins into the qnartzite hitherto 
believed to be barren, and of the existence of rich ores south of Belt 
Greek show that these places have been overlooked in the past. Tbe 
steep slopes are admirably adapted for the development of many of the 
veins by adit levels, and this system is generally followed. Cooperative 
working of several veins by a common level will undoubtedly cheapen 
development. Water power is abundant and close at hand, while the 
coal mines of Belt, only 40 miles away, insure a supply of cheap fuel.. 
Timber is abundant and near at hand, though large mine timbers are 
relatively scarce. 

The leasing system of workings has been adopted in a small way 
and is likely to become a i>owerful factor in the development of the 
camp. At present the rates are based upon an assurance of $3 for a 
day's work, a royalty of 10 to 15 per cent being exacted. The equitable 
system in vogue in Golorado of a definite and signed contract, with 
sliding scale of royalty according to the value of the ore produced, 
seems a sure way to develop many properties now idle into paying 
mines. Ooncentration of the low-grade ores now thrown aside must of 




Fia. 63.--Northwe6t-aoTitheAst seetion aoross Neihart distriot. Cb, Cambrian, Barker formation : 
jAgn, Archean gneiss ; pd, Pinto diorite; An, Algonkian, Neihart qnartzite; Ab, Algonkian, Belt 
sbale: Cm, Carboniferous, Madison formation ; Dm, Devonian, Je£fersou formation. 

necessity come soon. Several attempts have been made of late years 
to build works for this purpose, but so far without success, yet even at 
the present price of silver the future of the district is thought to be 
brighter than is generally believed. 



THE OBES. 

General nature, — ^The ores of the Neihart district are all silver bear- 
ing, and a large proportion of them are silver-lead ores. In the product 
of one or two mines gold forms an important constituent. In most of 
the ores it occurs in quite insignificant amounts. The amount of lead 
varies in the product of the different mines. It averaged between 7 
and 15 per cent in the shipments made from the principal mines for 
several years. In those mines producing the richest ores tbe amount 
of lead is too small to appear in the smelter returns. Zinc is present 
in all the ores, especially in those high in lead, and in the latter it often 
exceeds the 10 per cent limit allowed by the smelters. Gopper is often 
present in small amount, and the richer ores also contain antimony and 
a smaller amount of arsenic. The veins show but a relatively small 
amount of superficial alteration and have yielded but little oxidized 
ores. The silver-lead ores are usually "basic;" that is, they are not 



406 GEOLOGY OF THE LITTLE BELT MOUNTAINS, MONTANA 

siliceous, bat have a gangue of carbonates. The rich, '^dry" silver ores 
generally have a siliceous gangue. 

Mineralogic character of the ore, — The common type of ore consti- 
tuting the ore shoots of the Broadwater, Gait, !Nrevada, Moulton, and 
Queen mines and making up the greater part of the ore shipped from 
the district consists of an intimate mixture of galena and ankerite 
^^spar" — a mixture of lime, magnesia, iron, and manganese carbonates 
with silica. There is also some heavy spar (barite), pyrite, and blende. 
The dry or rich silver ores consist of polybasite (locally called brittle 
silver), pyragyrite, and quartz, with small amounts of galena, pyrite, 
chalcopyrite, sphalerite, ankerite <^ spar," and heavy spar (barite). 

ORE MINERALS. 

Galena (lead sulphide, PbS). — ^This is the most common ore mineral 
of the vein, and it forms the bulk of the great ore bodies thus far mined. 
It occurs massive in the ore shoots, in disseminated grains and small 
masses through the lean ore. The massive ore varies in grain from 
fine to coarse, but it is seldom that the cleavage surfaces exceed a half 
inch across. Barely the ore breaks into cubes an inch or less across. 
Galena also occurs in well-developed crystals with the quartz and 
other minerals of vugs. In some cases well- developed crystals one- 
fourth inch across appear coated with a soft metallic substance, and 
the mineral beneath is not so brittle as ordinary galena. Crystals lin- 
ing recent water courses show a dull surface coated with an undeter- 
mined material. It varies greatly in silver contents, from less than 1 
ounce up to 50 or more ounces per ton, and there seems to be no rela- 
tion between the coarseness or fineness of grain and the amount of 
silver in it. 

Pyrite (iron sulphide, FeS2). — This mineral occurs in nearly every 
vein in the district, both in the quartz and in the altered country rock. 
It is as common as galena in most of the ores, but is usually most abun- 
dant in the poorer varieties. It occurs massive in the vein filling 
as well- developed crystals on the surface of vugs and disseminat4'.d 
through the altered country rock of the vein matter. It is commonly 
pale in color, and in the massive form is intesgrown with blende and 
sometimes with galena. On the surface of vugs the massive form has 
a spongy, crystalline surface, though solid well-developed crystals also 
occur. In the altered rock of the vein filling and walls the crystals are 
minute, but perfect and of normal cubical forms. 

Blende (zinc blende, sphalerite, ZnS). — Sphalerite is an extremely 
common mineral in all the veins. The common variety is dark brown 
in color and has a resinous luster. It occurs in all the veins and is 
closely associated with galena. It is always crystalline, but usually 
massive in the primary ore. Where it is of later formation it occurs in 
well-formed crystals lining the open spaces or vugs of the veins. Some 
of these crystals are honey-yellow or greenish yellow, transparent, and 
show lustrous faces. The massive variety is dark brown, sometimes 




wwa>.] ORE DEPOSITS OP NEIHABT DISTRICT. 407 

almost black, and thoagh it occasionally forms a considerable proportion 
of the ore constitating the big ore shoots, and increases the charges for 
smelter treatment, yet the miners regard it with favor, as it is currently 
believed to accompany rich ores. No tests have been made to deter- 
mine whether it is silver bearing or not. 

Polyhasite (sulphantimonide of silver, 9AgS, As^Ss, with copper and 
zinc replacing silver). — This contains 64 to 72 per cent of silver. The 
brittle silver of the Neibart miners is not stephanite, but polybasite, 
and forms a very important constituent of the ores, being the most 
common of the rich silver minerals. It occurs both as a doubtful pri- 
mary ore mineral and as a product of secondary enrichment of the sul- 
phide zone. As a possible original mineral it occurs in crystalline 
masses, showing a minutely rough fracture, and is without crystal out- 
line. In color it is a steely gray and resembles gray copper (tetrahe- 
drite). It occurs intimately associated with galena and blende in ore 
from the Big Seven, and with copper pyrite in the product of the Flor- 
ence mine. It also occurs disseminated 
through the altered country rock of the 
vein matter in some of the other mines, 
usually in cobweb-like films on minute frac- 
ture planes. Its most important occurrence 

is, however, as a product of secondary Fio. 54.-Cry8talHne form of polyba- 

enrichment of the primary ores. It forms 

well-developed crystals in open cavities, but the crystal faces are usu- 
ally dull. Prof. S. L. Penfield, who has kindly examined for me the 
best specimen obtainable, has furnished me the following note : 

The crystals, owing to the uneven nature of the faces, were not well adapted for 
orystallographic measurement. They appear as 6 sided tablets, with triangular 
markings upon the basal planes, a development which is very characteristic for this 
species. This form is very close to that shown in fig. [54].' The material gave a 
Blight reaction for arsenic and a decided test for antimony and silver. 

In rare instances the mineral forms an open network of crystals whose 
surfaces are covered by a granular coating of a purplish and green 
iridescent substance of undetermined composition. In the smaller 
cavities it occurs coating quartz crystals, and as specks or minute 
crystalline aggregates on barite tablets or other minerals.' In most 
instances, however, it occurs as crusts upon the original galena-pyrite 
ore in small ox)en fissures, or filling minute fractures in this or the 
altered country rock. In such cases it is usually massive, rarely show- 
ing triangular terracing, more often coated by a sooty substance which 
is an extremely finely divided form of the mineral mixed with earthy 
oxide of manganese. It is the chief silver mineral of the Big Seven, 
Benton, and Florence ores. 

Argentite (silver sulphide, Ag2S). — This has been identified in min- 
eral collections of the Keihart ores, but it is not a common constituent 
of them. 



1 Copied from Am. Jour. Sci., 4th 8erie«, Vol. II, p. 24, July. 1896. 



408 GEOLOGY OF THE LITTLE BELT MOUNTAINS, MONTANA. 

Stepha/nite (brittle silver, salphantimoDide of silver, 5Ag2S, SbjSs). — 
This contains 68.5 per cent of silver when pare. This mineral, which 
was supposed to be of very common occurrence, proves to be quite rare, 
the material mistaken for it being polybasite. So far as known it does 
not occur in well-formed crystals. 

Pyrargyrite (dark ruby silver, antimouial silver sulphide, SAgsS, 
SlhSa). — This contains 60 per cent silver when pure. The dark-colored 
ruby silver is a common mineral in the rich silver ores. It is usually 
associated with polybasite, and in these ores is generally of secondary 
origin. It is found as grains scattered through vein quartz, and also 
in crystalline clusters, which rarely show well-formed crystals upon the 
surfaces of the vugs in the vein. 

ITative silver. — This is of common occurrence in the oxidized zone of 
the veins, and where the pyrargyrite has been subject to alteration. 
It occurs in the usual hair-like form, and as solid masses having a 
fibrous structure. 

Qold. — Free gold is reliably reported from the Big Seven ores. The 
quartzite ores of this property contain oxidized molybdenum, but no 
mineral is visible to the eye. 

Chalcopyrite (copper pyrite, GuFeSa). — Some of the veins contain this 
mineral in rich ores, where it occurs mixed with polybasite. It is, 
however, of rather uncommon occurrence, and has not been noted in 
the ore extracted from most of the mines. 

Pyromorphite (lead phosphate). — This has been observed in only one 
locality, a mine in Narrow Gauge Gulch, where it occurs in films and 
clusters of little crystals, coating a rusty iron-stained quartz. It is a 
bright yellowish green in color. 

GAN6UE MINERALS. 

Quartz is an abundant mineral of the veins. It is generally coarse 
grained, slightly turbid or white, though not the typical milky- white 
quartz common in some districts. This quartz consists of interlocking 
grains, and quite commonly shows transition into a well-defined comb 
structure. In the cavities and lines of vugs, which are often found in 
the vein, it forms typical comb quartz. The ores also show small cavi- 
ties lined with quartz crystals. All the other minerals occur as inclu- 
sions in quartz. In some cases the quartz coats a cryptocrystalline 
form of silica, which is best classed as chalcedony or opal, whose 
minute concentric banding and the frequent presence of a central 
nucleus of altered rock show its formation by metasomatic replacement 
of the country rock. This replacement of quartz and chalcedony is 
not prominent, however, though it is believed that the banding of the 
veins is in part due to such replacement. 

Sericite (white mica). — ^This is extremely common in the altered coun- 
try rock of the vein and its walls, but occurs in such minute scales as 
to be scarcely visible to the eye. It is this mineral which imparts the 
greasy feel to the altered rocks and which constitutes the chief con- 
stituent of the <' clays" encountered in the mine workings. 



WBED.] 



ORE DEPOSITS OF NEIHART DISTRICT. 



409 



Barite (heavy spar, BaS04). — This is a common gangue mineral, 
though not so abundant as the ankerite ''spar" noted later. It is 
commonly a pure white, lustrous mineral occurring in tabular crys- 
tals in the richest ores or in massive form in poorer ones. The platy 
crystals sometimes form septate masses, with the interspaces filled by 
ore sulphides. In the drusy lining of vugs it occurs in pale-yellowish, 
well-developed crystals. A specimen from the Florence mine has been 
studied and figured for me by Mr. H. H. Kobinson, under the direction 
of Prof. S. Jj. Penfield, of Dhe Sheffield Scientific School of Yale Uni- 
versity. .Mr. Bobinson furnishes the following notes: 

Id the barite ftom the Florence mine at Neihart the habit of the crystals is tab- 
ular, which is common ^rith this species^ the basal plane o (001), the prism m (HO), 
and the pyramid z (111) being the prominent faces. Faces of the brachypinacoid 
h (010), the brachydome o (Oil), and the macrodome I (104), althongh small, are 





Fio. 55.— Crystal of barit«. 



Fig. 66.— Crystal of barite. 



almost always present, as shown by fig. [55]. On a few of the crystals the macrodome 
d (102) and the prism n (320) were observed, as shown by fig. [56], which represents 
the comer of a crystal showing these forms in combination with the faces c, m, and 
Zf previously mentioned. 

Ankerite spar. — The most abundant gangue mineral of the ores is a 
white or very pale brownish or pinkish, coarsely crystalline substance 
which upon chemical analysis proves to be a mixture of lime, iron, mag- 
nesia, and manganese carbonates. In the small cavities of the galena 
ores, and rarely in the larger vugs, it is seen in rosette-like aggregates 
of small rhombohedral crystals, but no well-formed crystals were found 
available for measurement. Analyses of this gangue mineral have been 
made by Dr. H. N. Stokes in the laboratory of the United States 
Geological Survey. Analysis I is that of the pale-brownish or cream- 
colored material. Analysis II is of the white mineral. 

Analyses of carbonate gangue mineral {ankerite spar). 



CoDstituent. 



Iron carbonate, FeCOa 

Manganese carbonate, MnCOg. 

Lime carbonate, CaCOs 

Magnesia carbonate, MgCOa. . . 
Insoluble in HCl 



Total 



I. 



23.16 
35.99 
13.16 
14.93 
12.76 



100.00 



la. 



26.55 
41.26 
15.08 
17.11 



II. 



13.14 
17.66 
36.84 
18.02 
14.34 



Ila. 



15.34 
20.62 
43.00 
21.04 



100. 00 100. 00 I 100. 00 



The insoluble material consists of quartz and metallic sulphides. 
Eecalculating the analysis and leaving out this impurity, we get the 
proportions given in Columns la and Ila. 



410 GEOLOGY OF THE LITTLE BELT MOUNTAINS, MONTANA. 

The compositions given do not correspond to those of any one min- 
eral species, bat as siderite, dolomite, and rhodochrosite grade into 
one another, it is probable that the two minerals belong in this group. 
As the material is probably variable in composition, and may be a mix- 
ture and not a definite mineral species, the miners' name of '^ spar"* is 
used for it. In the lean ores it is the matrix in which the metallic sul- 
phides occur. Both this spar and the mineral grains in it are cut by 
thin plates of heavy spar (barite) and by strings and films of quartz. 

PARA6ENESIS. 

The association of the minerals with one another and tlieir order 
and mode of formation constitute paragenesis. Each of the minerals 
found in the ore occur of different generations, and most of them not 
only as constituents of the original or primary vein-filling, but also 
of later formation, filling crevices and fractures. This is clearly due 
to recent circulating waters altering the original vein constituents. 

The relative order of crystallization of the minerals is studied to the 
best advantage where the ores show successive "crusts" or bands of 
mineral, or where cr^'^stals of one mineral occur upon the crystal faces 
of another. The first arrangement is not usually well developed in the 
Neihart ores, but may be seen in several veins where the vein matter 
consists of fragments of country rock, cemented together by the vein 
filling. Similar crusts are also observable along vug lines, and even 
where the cavity is completely filled the crusted arrangement may be 
apparent. It is, of course, only in the open cavities that the crystals 
of one mineral are found upon the crystal faces of another. Where the 
vein minerals occur massive or show only poorly defined streaking it 
is often impossible to distinguish the order of formation by a study of 
hand specimens alone, and a careful microscopic examination has been 
made of thin sections of the ore. 

In the primary vein-filling pyrite appears to be the earliest-formed 
mineral, followed by blende and galena. Specimens of crusted ores 
from the Big Seven mine are illustrated in PI. LIV, A. While the ore 
is quartzose and somewhat ditierent from the more common "spar" ores 
of the district, the order of mineral deposition appears to have been 
nearly the same in the different veins. The specimens figured show 
the following sequence of deposition, beginning with the oldest: 

1. Quartz, radially fibrous, crystalline, with a little scattered pyrite. 

2. Blende, with some pyrite; forming a massive layer. 

3. Mixture of galena, pyrite, and quartz, with scattered grains of blende; showing 

rude banding ; a little barite also occurs. Crystals of galena project into layer 
above. 

4. Frosting of opaque white quartz in stout crystals. The pyrite crystals of No. 3 

project up through this crust. 

5. Polybasite, sometimes mixed with pyrargyrite. Filling of interstices between 

quartz crystals, specking on crystals (rarely good crusts). 



* Ankerite is a term used by some writers for such a mixture of carbonates, and this name is therefore 
adopted here because of its usefulness, although it is not the mineral ankerite. 




(A) CRUSTKDOnK.FlXJRENCK MIMK 
(Bl CRUSTKnORE,niGSRVEN MINK 



w«ED.] ORB DEPOSITS OP NBIHAHT DI8TBICT. 411 

In other specimens the layer No. i is well developed aod coDBists 
of quartz with masseB of pyrlte. la some cases the minerals form 
individual layers. 

Fig. 57, which is drawn from a specimeu of ore from the Big Seven 
mine, represents a cit)8S section of the little quartz " vein" which oon- 
stitutes the high-grade ore streak of the lode. The figure shows the 
relative abundance and association of the minerals, but does cot repre- 
seut the spongy tex- 
ture of the polybar ^^ 
site and its intimate ^| ^py^t%^^ 
admixture with both ^^ 
galeua aud pyrite |^| Q>ieus. 
(chaloopyritel), as j^m j,^^ 
this growth is too ^^ 
mossy to be repre- ^==j Bjenj, 
seuted well, and the '== 
mineral ia therefore EjFJ Qn«rti. abroa*. 
indicated as poly- ^_^ 
basite alone. The I | «"■'■"■ 
apecinten seen in 
thin section shows 
ruby silver and poly- 
basite intimately associated together and forming irregular, shreddy, 
and ragged patches. No positive identification of galpna as the 
nucleus of such masses was made, but the association with galena is 
such as to indicate a possible cliange to polybasite. The pyrite is 
broken and fractured, but the grains are always sharply deflued and 
no genetic relation to the silver sulphides is recognizable. A blende 
crystal seen isolated in the central quartz filling shows in thin aection 
a crust of polybasite, the latter holding minute inclu- 
sions of pyrite, as shown in fig. 58. The blende seen 
in another sectiou of such rich ore is invariably sur- 
rounded by a dark crust which is not iron oxide, nor 
does it appear to be an iron-rich blende. It is not defi- 
nitely determinable, but resembles galena or a silver 
Fio M -Crjstsi sulphide. 
°* ild"^ h**"!*' Another little quartz vein an inch across, also- in the 
^iie. *" Big Seven mine, shows walls of quartz radially arranged, 
with the center iilled by irregular grains of blende and 
pyrite held in a quartz filling. The sulphides are seen to be parts of 
shattered and broken grains. The fragments, though separated by 
quartz, show &ce8 fitting together, but more often are too much 
broken to show similar indentations. The blende was evidently 
formed before the pyrite, but continued to grow during the foima- 
tiou of that mineral, which is now seen in 8ac-shai>ed embayments in 
the blende and superimposed upon its faces. In general, the pyrite 



412 GEOLOGY OF THE LITTLE BELT MOUNTAINS, MONTANA. 

does not show crystal oatliiies, but the irregular form due to fracture 
and cleavage. The blende also shows fracture, but the borders have 
been softened by corrosion during the period of quartz filling. 

An examination of numerous specimens from the Florence and Big 
Seven mines shows that while polybasite is possibly a constituent of 
the primary ore, its more common occurrence, like that of pyrargyrite, 
is as a secondary mineral, filling cavities and cracks in the original ore. 
The material gathered from the lowest level of the Florence mine shows 
polybasite in the form of crystalline tablets upon barite and other min- 
erals, and also as a moss like mass of open skeleton texture which 
seems to represent arrested deix)sition. The latter form is believed to 
come from a place in the vein where mineral-bearing water is now 
depositing this mineral, together with spar, quartz, and probably galena. 
Studied under the microscope the polybasite appears to be an altera- 
tion product of galena, and itself to be mixed with and to grade into 
pyrargyrite, which is in some cases its undoubted alteration product. 
It is certain that polybasite, as the important constituent of many of 
the ores, is of secondary origin. It occurs on all other minerals, and 
is itself not coated or dotted by them. 

Thin sections show that the barite seen in the ore occurs in fractures 
of the original spar. Such fractures, cutting across spar crystals and 
shattering pyrite grains, sometimes show one part of the fracture filled 
by barite, with quartz filling beyond on the same line. In the little spar 
veins of the Ingersoll mine the metallic sulphides occur mainly in con- 
nection with quartz-filled fractures of the spar. Usually the barite is 
easily distinguished from the spar by the unaided eye, by its whiteness 
and its luster. 

Sphalerite also occurs in well-formed crystals in some of the vugs, 
and is one of the most recently deposited minerals. 

The products of superficial alteration — that is, oxidizing action — are 
those common to silver mines. The pyrargyrite alters to native silver, 
the galena to pyromorphite, cerusite, etc. These changes present no 
features of especial interest. 

Summarizing the mineral composition and mineral associations of 
the ores, we find : 

1. The common type of ore of the veins conBists of galena and spar; 

2. The rich ores of a few veins contain their chief valaes in polybasite and, more 

rarely, pyrargyrite. 

3. The primary vein minerals are pyrite, blende, galena, and a mixture of carbonates 

called spar ; chalcopyrite and in some instances quartz are of doubtful primary 
origin. 

4. The above minerals all occur of secondary formation; that is, are later than the 

primary ore. The polybasite and pyrargyrite are of secondary origin. Barite 
is especially common in these secondary ores. Quartz occurs of several genera- 
tions and is of latest formation. 



WB«D.] ORE DEPOSITS OP NEIHART DISTRICT. 413 

VALUE OF THK ORBS. 

The values vary greatly in different veins and in the same vein. The 
galena ores, which form the bnlk of the ore shipped, sometimes carry 
60 to 70 ounces of silver and sometimes as low as 10 to 12 ounces. The 
average shipments of the Broadwater when the great galena ore 
bodies were being extracted was 40 to 50 ounces. Where the rich sil- 
ver ores occur, as in the Florence, Benton, and Big Seven, the values 
run up very high — into the hundreds of ounces. The so-called high- 
grade ore of the Broadwater mine carries from 40 to 60 ounces of silver 
per ton ; the low grade 20 tp 30 ounces of silver and only traces of gold. 
Gold ratio of Big Seven, 5 ounces silver: $1 gold. The ores shipped 
by the Gait in 1807 ran 8 to 15 per cent of lead. Those of the Broad- 
water averaged 7 to 8 per cent lead while the large ore bodies were 
being stoped, but in 1897, owing to admixture of the altered country 
rock carrying films of silver sulphides, the amount of lead dropped 
to 2 to 3 per cent. In the early history of the district the ores from 
the upper part of the veins were exceedingly rich and values of $500 
to $1,000 a ton were not uncommon. The ores now being mined vary 
greatly in silver contents. The galena ores carry from a few ounces 
up to 100 ounces or more per ton. In the lead ores the smelter returns 
show that the shipments rarely average over 35 to 40 ounces of silver 
per ton. The purest galena is sometimes very poor in silver. The 
richer ores also vary greatly in values, and no general figures can be 
given* The high-grade ores shipped from the Florence, Big Seven, and 
Benton mines gave returns varying from $3,000 to $4,000 a carload, 
which is approximately $200 a ton. 

THE VEINS. 



> 



Occurrence. — The veins occur in a narrowly circumscribed area adja- 
cent to Kelhart, and traverse the gneiss and the various igneous rocks 
which penetrate it. So far as known they are confined to the gneisses 
and schists of supposed Archean age, penetrating in a few instances 
the quartzite which overlies those rocks. The geology of the area has 
already been described, so that here it will be sufficient to repeat only a 
brief summary of the leading facts. The crystalline schists of the Xei- 
hart district are composed of gneisses and schists of varying color and 
texture, in which various igneous rocks have been intruded as irregular 
branching stocks, as more regularly bounded intrusive bodies, and as 
dikes and sheets. The gneiss is roughly divisible by its mineralogic 
composition into white or gray gneiss, red gneiss, and black gneiss or 
amphibolite. The red and gray gneisses vary considerably in compo- 
sition and texture, and it is often difficult to determine whether a particu- 
lar modification should be classed as one form or another, as they occur 
intermingled. There is, however, a distinct banding. The intrusions are 
of several kinds and are all older than the veins. The Pinto diorite is 



414 GEOLOGY OP THE LITTLE BELT MOLTH'AINS, MONTAXA. 

the most common and characteristic rock of the district. It penetrates 
the gneiss without apparent order or system and is itself somewhat 
gneiHSoid in structure. The later eruptives are rhyolite- and granite- 
porphyries. The rhyoliteporphyry occurs in dikes and as bosses. The 
veins are independent of tliese eruptions also. There are no contact 
deposits. 

Heavily bedded qnartzites, inclined at an angle of 2(y^j dip southward 
from the borders of the district and form the base of a great series of 
stratified rocks. A few veins have been found in this quartzite on the 
summit of Long, Neihart, and Baldy mountains. 

The ores occur in normal fissure veins, whose character varies in the 
different rocks traversed by the fissures, but which fill well-defined fis- 
sures in all rocks except the porphyry. In this the rock is shattered 
for a considerable width, and this belt of fracturing might be classed 
as a ^^ crush ^ zone. The veins are primarily replacement deposits, with 
some filling of open fissures. 

FISSURE SYSTEM. 

As ore veins are fissures filled with ore, a knowledge of the struc- 
tural relations of the fissures is of the utmost importance in mining, 
since the success or failure of the property as an investment depends 
upon it. 

The vein fissures form part of a well-marked system of fractures trav- 
ersing the rock complex. As described elsewhere, the gneisses are 
banded, the bands dipping to the south at angles of 40<=^ to 65P. As this 
banding corresponds to differences in the character of the rock, it deter- 
mines the nature of the outcrop. These rocks show a well-defined sheet- 
ing by a system of fractures crossing them at nearly right angles to 
the banding. This sheeting is not uniform throughout the district, as 
the fractures occur in groups or zones of sheeting. The fissures are not 
open cracks, but mere fracture planes recognizable in jointed surfaces of 
the outcrops, especially in the more massive rocks. Underground they 
are most often marked by thin films of ocherous or clayey material or 
by a distinct but almost imperceptible fracture in slightly altered rock. 
The fissures occur at different intervals; that is, the spacing between 
them is not uniform. Like those described in other mining districts, the 
sheeted zones are separated by rock showing little or no fissuring. 

The vein thus has a sheeted structure, and consists of highly altered 
country rock inclosed by walls that practically limit the intense rock 
alteration. In some places such veins show areas where instead of 
sheete<l country rock the vein matter is brecciated, the fragments being 
held in the finer breccia or cemented by quartz and other minerals. The 
width of the vein a« thus defined varies greatly. It may be as great 
as that of the Gait vein, which is 60 feet or more between walls. 
Practically, however, the vein as worked seldom exceeds 8 feet in 
width. This sheeting is recognizable even where the original rock is 



wmtD.] ORE DEPOSITS OP NEIHART DISTRICT. 415 

entirely replaced, for the sheeting planes are often marked by comb 
quartz or vug lines, filling the original small open fissure. The hang- 
ing wall of the Ingersoll lode is sheeted, the fissure being 8 to 10 inches 
apart. In the Bock Greek vein the hanging wall shows five such 
planes in 5 feet. In both cases this hanging-wall rock is practically 
unaltered. 

Course. — The fissures have a general northeasterly and southwest- 
erly course. Owing to the difficulty of tracing them upon the surface, 
and the lack of an adequate base map, no attempt was made to map 
their outcrops. The maps of underground workings, so courteously 
placed at my disposal, show a course varying from true north to N. 45° B. 
The direction of individual veins is not constant, but the variation 
is not wide. The average trend is about N. 15° B. The vein fissures 
with but few exceptions dip west, the average dip varying between 
GQo and 65^, though there is considerable variation in the amount in 
different mines, and even in the same vein. The most extensive work- 
ings are those of the Broadwater mine. The map of the underground 
workings of* this property (PI. LVIII) shows the variation in the dip 
and strike of the veins in the different levels, and this may be consid- 
ered as typical for the district. 

Origin of fissures, — The fissures show the common features of com- 
pression fractures. Their occurrence in especial abundance at this 
locality is possibly due to a line of weakness first developed here in early 
geologic time, when this place was a shore line of a gradually deepening 
sea, or the hinge line of a strong fiexure, which has at many times since 
been subject to fracturing, as shown by the numerous igneous intru- 
sions. The fissure system to which the veins belong is, however, of 
later origin and probably coincident with the late dynamic disturbances 
of the region. When the range uplift occurred, the ore deposition fol- 
lowed a period of igneous activity. 

Relation to dikes. — The fissures intersect dikes of rhyolite-porphyry 
in several mines. In the Gait workings the vein follows the general 
course of the dike which forms the immediate wall rock of the vein in 
some places, as shown in fig. 62, on page 427. In other workings the 
fissures cross the dikes at considerable angles. 

Splits. — The fissures are approximately parallel, as already stated, so 
that several veins are workable from one crosscut tunnel. The vein 
fissures show, however, numerous *< splits," or smaller fractures running 
off in the country rock of either wall, and these bear a definite relation 
to the course of the vein, forming the second set of fractures common 
where the fissures are compression faults. The vein walls are com- 
monly quite well defined, and the crosscntting necessary in so many 
districts of sheeted rocks is not so essential here. (See note on Broad- 
water mine, p. 433.) 

Effect of country rock on vein fissures. — The character of the fissure 
varies with the nature of the inclosing rock. In the red and gray gneiss 



416 GEOLOGY OF THE LITTLE BELT MOUNTAINS, MONTANA, 

it is a well-defined, cleaa-cut fractare; in the more schistose rocks the 
vein is wider and less sharply defined. In passing from gneiss into the 
Pinto diorite there is a pronounced narrowing of the fissure, the veins 
that are 5 to 8 feet wide in the gneiss contracting to 3 or 4 feet in the 
diorite, usually with a loss of ore. 

In the rhyolite-porphyry masses of Snow Creek the vein splits up 
into a network of fractures, shattering the rock, so that a definite vein 
is not always recognizable. This is believed to be due to the brittle 
character of the rock, which is weakened perhaps by the strains devel- 
oi)ed in it in cooling from a state of fusion at the time of its intrusion. 
In ordinary weathering this rock breaks up into angular fragments 
which seldom exceed 2 or 3 inches across. 

Where the vein encounters black gneiss or amphibolite it is commonly 
spoken of as faulted. In the instances observed no true faulting was 
recognized, but the vein was deflected as shown in fig. 60, preserving 
its normal course beyond. In this case the tough nature of the rock 
resists sharp fracture and becomes a curving break, narrower than in the 
feldspar gneiss and with less regular boundary planes. Observations 
were not sufficient to determine whether the vein was or was not slightly 
thrown by this band. The former superintendent of the Monlton, Mr. 
Frank Eaymond, assured me that the vein of that mine, together with 
those adjacent to it, was cut through by a fault traceable southward 
to the Broadwater workings, and his observations upon the barrenness 
of the ores in the black gneiss, given in the account of the occurrence of 
the ore bodies, are pertinent in this place. 

VEIN STRUCTURE. 

Vein matter is the material which lies between the walls and forms 
the vein. It therefore includes various materials, the included frag- 
ments or horses of altered country rock, as well as the quartz and other 
ore and gangue minerals brought into the vein from the outside. 

Vein filling consists of the material brought into and filling the open 
spaces of the fissure. It consists of ore and gangue minerals, those 
found at Neihart having already been noted. The difference between 
true vein filling and the altered country rock is an important one to 
bear in mind, since it is the former alone that constitutes the ore. In 
the Neihart veins the bulk of the vein material is frequently the altered 
country rock or the clays resulting from it. This altered country rock 
is often too much decayed to determine its original composition, but 
where nucleal masses are found the rock is seen to be the same as the 
vein walls. The intense alteration results in the formation of a material 
composed of white mica, carbonates, pyrite, etc. This material varies 
slightly in appearance according to the rock from which it has been 
derived, but is all essentially similar in composition. All traces of the 
original minerals have disappeared, and the only mineral recognizable 
to the naked eye is pyrite in small, well-developed crystals. Where the 



WKBD] ORE DEPOSITS OF NEIHART DISTRICT. 417 

altered rock contains silver, the valnes are in secondary quartz and ore 
veinlets. The altered rock never constitutes workable ore unless such 
films of mineral are present. 

Metasomatic replacement of the rocks has been, however, a very com- 
mon phenomenon, and in the dolomitic gangue the altered rock is seen 
in various stages of replacement. The chalcedonic quartz resulting 
from so-called silicification of the country rock of the vein has also been 
noted. 

Banding. — The Neihart veins show a prevailing banded structure. 
Very often this is plainly seen to be merely sheeted rock with the par- 
allel cracks occupied by vein filling. More generally the vein near the 
productive ore bodies is more or less distinctly banded, but the appear- 
ance is due to a streaking of the ore minerals in the gangue. In the 
nearly solid galena there is often a decided banding or striping due to 
alternations of finer and coarser grained galena. In the usual ore com- 
posing the '^shoots " there is a marked banding, due to alternate layers 
of galena and blende. The banding is also due to lines of spar, which 
are not persistent, and in the center of which there is often a line of 
cavities or vugs. Moreover, the spar itself is commonly sjiotted with 
grains of galena, blende, and pyrite, which show a general banded 
arrangement. Such banding is in fact a common and almost constant 
feature of the ore bodies, even the ore itself being banded when seen 
in cross section in the ore shoots. As a result of later fracturing, the 
ore shows lines of vugs or open fissures, but such places are more often 
partially filled, only the broader parts remaining open. It is about 
such cavities that true crustification is observed. Commonly the vein 
does not show a filling of successive crusts of mineral. Barely the 
vein matter is a breccia of country-rock fragments cemented by filling, 
a structure that does not prevail throughout any one of the veins so 
far observed, but occurs in parts of several veins. The minerals about 
these fragments of country rock are arranged in concentric bands exhib- 
iting true crustification, and open cavities between adjacent fragments 
are often seen. The stringers often show little veins of solid quartz, 
and such veinlets also occur in the altered country rock of the vein 
matter. This quartz generally exhibits a radial or comb structure. 

Ribbon structure, — Secondary banding, duQ to a sheeting of the vein 
filling, as defined by Lindgren,' was rarely observed, but the ores 
exhibit a very plain secondary fracturing. The fractures cross the 
vein afa considerable angle, though they do not fault or very greatly 
disturb it, but merely crack it and leave minute crevices in part filled 
by later minerals. Where such fissuring of the ore is in parallel planes 
a banded structure is produced by reopening. These openings are 
usually small, are a common feature of the ore, and afford the oppor- 
tunity for processes of secondary concentration of the silver sulphides. 



' Gold-qnartz veins of California: Seventeenth Ann. Rejit. U. S. Geol. Survey, Fart II, p. 129. 

20 GEOL, PT 3 27 



418 GEOLOGY OF THE LITTLE BELT MOUNTAINS, MONTANA. 

ROCK ALTERATION. 

The rock walls of the vein ofteii show more or less sheeting and 
more or less chemical alteration. The amount of this chemical altera- 
tion is, however, quite variable along different veins, the zone of 
pronounced change varjdng from an inch or less to several feet in 
width. It is most pronounced in the rock material found within the 
vein walls and constituting the bulk of the vein matter. All stages of 
alteration are found in the vein walls, from the slightly altered rock to 
the light-gray or yellowish, soft, greasy, or clay-like material of the vein 
matter. All the rocks are subject to this alteration, even the Keihart 
quartzite. In the Pinto diorite it is less marked than in the gneisses, 
and apparently took place more slowly. The final product of altera- 
tion is very similar whatever the original nature of the rock, but in 
the less altered materials the former character is very often clearly 
recognizable. v 

The changes iu the rocks are those common to the quartz veins of 
California,^ being those due to the formation of carbonates, sulphides, 
and sericite. The changes are clearly those produced by metasomatie 
replacement. In studying the veins it is easy to distinguish between 
the quartz filling or '^veins'' deposited in open spaces and the altered 
country rock. The distinction is, however, of little use in most of the 
mines, since the ore occurs with carbonates. Although the varying 
composition of the Neihart rocks produces many local peculiaritit*B, yet 
the general process of metasomatie alteration is that described by 
Lindgren. 

The replacement of the altered rock by carbonates Is recognizable 
under the microscope in slides cut from some of the vein rocks, and it 
is thought that the banded structure of the veins may be largely due to 
the replacement of the layers of sheeted rock by carbonates and ore. 
Tests have not been made to determine whether the pyrite of the 
altered rock carries gold or silver, but the general absence of other 
sulphides — except as fracture fillings — shows that the ores were intro- 
duced from outside when the carbonates were deposited. 

DISTRIBUTION OF ORE IN VEIN. 

Pay shoots, — In general it may be said that the ore streaks are very 
constant, but the values "bunchy." The ore minerals occur in nearly 
all parts of the vein, but are found concentrated in workable ore^bodies 
in only a relatively smaller part of the vein, such masses forming the 
pay shoots. Throughout the more barren parts of the vein the sul- 
phides occur in streaks in the gangue minerals. Generally the ore 
bodies are composed essentially of galena with associated minerals. 
They occur in long, narrow, lenticular bodies, which as they wedge out 
pass into streaks; or they may end abruptly and another lens occur 

1 Lindgren, op. oit., p. 140. 



wax,.] ORE DEPOSITS OF NEIBAET DISTRICT. 419 

aloDgside ill the vein, tli« end overlappiug:; bat this, of coarse, varies 
' in tlie different veins of the re^OQ. 

The occurrence of the ore bodies thus far developed in the Broad- 
water mine i» shown on PL LX, The shoots have a general tendency 
to pitch northward. Mr. Baymond, the former superintendent of the 
Monlton mine, who made a careful study of the relationship of the 
productive ore bodies to the wall rock, found that, as a rale, the veins 
are prodnctive where they traverse the feldspathic gneiaaes and are bar- 
ren in the antphibolites and dark-gray gneisses. The Monlton work- 
ings, which tap three veins, show this relationship very plainly, and 
so far ae I was able to extend my observations it is true for the camp, 
but the number of productive mines appears to me to be too small to 
afford data for a positive statement. The accompanying diagram (dg. 
59) is intended as a plan of the area near the Monlton mine, on which 
the successive bands of gneiss are indicated, and the veins are shown 



GraygnolMi |w}' or 



lUttnreor country iwk. The vein* are, Tor t-aDTenieiice, npreMntad slmpl; w atnilKbt ItuH. 

in the order in which they occur, though for convenience their relative 
distances apart have not been preserved, the dit^^ram not being 
drawn to scale, 

Careful observations made in all the accessible workings show that 
thus far no workable ore bodies have been nncovered iu the Pinto 
diorite. This ia alao true of the dense and very tough ampbibolites, of 
relatively rare occurrence, which deflect the veins. As already noted 
in describing the vein fissures, these rocks narrow the vein, and the 
vein matter itself does not present the favorable physical conditions 
for ore deposition fonnd in the other rocks. 

Khyolite-porphyry in dikes does not a[>pear to show any association 
with the occurrence of pay ore. In the larger bodies the deposits so 
far worked proved very rich near the surface, but gave out in depth. 
The reason for this ia diacussed elsewhere. It is believed to be due to 
the spreading out, or, as it were, the diffasion, of the fissure in this easily 
shattered rock, so that the ores occur iu numerons veinlets and films 



420 GEOLOGY OF THE LITTLE BELT MOUNTAINS, MONTANA. 

scattered through a considerable extent of rock, rather than in com- 
pact, well-defined ore bodies. The same shattered condition is, how- 
ever, favorable to the penetration of later circulated waters and results 
in great surface enrichment, so that such properties show large amounts 
of such ores, as observed at the IXL, Eureka, and on Mackey Greek. 

Injiuence of splits. — Stringers or splits generally enrich and often 
enlarge the ore bodies of the main vein, and near the vein the ores of 
such splits are themselves generally richer than those of the ore bodies 
of the main vein. 

Permanence in depth. — The study of the region indicates that the fis- 
sures are deep. There is no reason to doubt that ore shoots occur in 
them at far greater depths than any yet attained. The rich silver sul- 
phide ores peculiarly characterize the upper parts of some of the veins, 
and it is open to question whether such very rich ores will be found at 
greater depths. That they are of secondary or later origin in most 
cases in this district is beyond doubt, but these minerals are of primary 
origin in the California veins,^ and may also be found so here. 

CR08SCUTTING. 

But few of the veins have as yet proved productive, though an un- 
usual amount of prospecting has been done in the district. Gross- 
cutting of the walls may be of value here, and one should always be 
certain that the wall is not a sheeting plane in the vein matter. 

SUGGESTION 8 FOR DBVELOPMENT. 

A careful regard to the nature of the country rock, a close observ- 
ance of splits or branches of the veins, and, above all, an exploration of 
the vein above those levels in which it is seemingly barren, are recom- 
mended. Past experience in the working of the veins shows that ore 
bodies have been overlooked because the shoots wedged out downward 
and did not show in the levels. The soft, altered rock of the vein is 
sometimes richer than the galena ore shoots, owing to minute films of 
ore, so that this rock should be carefully watched and assayed. 

ORE DEPOSITION. 

The observed facts of occurrence are believed to prove that the ores 
were deposited by ascending mineralizing waters, mainly in open 
cavities. Alteration of the country rock with metasomatic replace- 
ment took place at first along minute fractures in the rock and 
along cleavages in the minerals of the rock, the process being one of 
molecular substitution. The sheeted country rock between the vein 
walls was thus altered or partially replaced, as is well shown in speci- 
mens from the Florence mine. The metallic sulphides, if brought into 
the vein in sulphide waters, might perhaps be deposited by reaction 
between feldspar and sulphides,' which may account for the occurrence 

> Oral oommanication of W. Lindgren. 

* See F:emp, Ore Depoeita of the United States, New York, 1896, p. 34. 






1 1 

If 



WMD.] ORE DEPOSITS OF NEIHAUT DISTRICT. 421 

of the ore mainly in the feldspathic rocks. The original source of the 
metals is not known, but the presence of large intrusive bodies of 
ingeons rock (and the indications point to the existence of a general 
stock of such rocks underlying the district) aiibrds a possible source of 
the metals at no very great depth as well as at no great distance later- 
ally. The paucity of ore in the veins in the Pinto diorite itself indi- 
cates that the ore is not derived from this rock bv lateral secretion. 

ft* 

It is believed that the primary vein filling is due to heated ascending 
waters, but no direct evidence showing that the waters were hot is at 
hand. 

SUPERFICIAL ALTERATION OF VEINS. 

Superficial alteration of the vein mineral by circulating oxidizing 
waters has produced small amounts of oxidized or carbonate ores, and 
these ores are found mainly along what appear to be even now the pipes 
or channels of descent for surface waters. Surface waters altered in 
character and robbed of their oxygen in their descent by the changes 
they induce in the metallic sulphides of the upper parts of the veins 
are believed to be the agents producing the rich sulphide ores forming 
secondary enrichments. 

The outcrops of the veins do not show the great gossans or << iron 
caps " found at some localities. Surface alteration breaks up the vein 
matter in places and the quartz is rusty with iron oxide. The disinte- 
gration is, however, such as to make the outcrop rather inconspicuous 
on the debris-covered slopes. A view of a surface cut exposing the 
outcrop and upper part of the Empire vein is shown in PL LV, A. 
The planking seen on the right is an air box connecting with the 
underground workings. The vein, across which a hammer is seen, has 
sharply defined walls, and the contrast between the massive, unaltered 
gneiss on either side and the shattered quartz of the vein is very well 
shown. 

SECONDARY ENRICHMENT OF VEINS. 

Secondary enrichment has played an unusually important part in 
the development of the ore deposits of Keihart. The ores extracted 
in the earlier workings and those found to-day where new veins are 
opened all show silver sulphides deposited by secondary enrichment 
as crusts or crystals lining cavities, or as films or thin coatings along 
fractures of the primary ore, or in the oxidized zone as the so-called 
'^ sooty sulphide" ores that occur with manganese oxides. It is from 
this zone of enrichment that the high-grade ores, running from 200 to 
1,000 ounces of silver to the ton, or even higher, were obtained in the 
early history of the camp. Although such ores played out in depth 
and caused many disappointments and failures, their occurrence played 
a most beneficial part in causing the development of the veins. 

While the secondary enrichment of copper veins along a level 
between an upper zone of oxidation and the unaltered vein material 
below is a well-recognized fact, a similar enrichment of silver veins 



422 GEOLOGY OF THE LITTLE BELT MOUNTAINS, MONTANA. 

I 

appears to have escaped general recoguition. The secondary minerals 
recognized are chiefly polybasite and ruby silver, the former being 
more abundant. There are also bright metallic coatings, presumably 
argentite, on crystals and along fracture planes, and rarely in minutely 
crystalline masses. The chemical changes by which the primary sul- 
phides split up and yield these minerals have not been investigated. 
The change is presumably the result of superficial alteration in which 
the primary vein minerals are broken up (chemically) and their silver 
contents are partially leached out and carried downward and deposited 
in the upper parts of the sulphide zone. Briefly stated, the process is 
believed to be a partial leaching out of the silver contents from the out- 
crop of the vein by surface waters and the precipitation of the silver at 
somewhat lower levels. The superficial alteration of the Neihart veins 
is not a marked one, as there are no great zones of carbonates and oxi- 
dized ore. Such ores occur only in limited amounts, being most abun- 
dant in the Broadwater vein, where the partially oxidized ores extend 
down 170 feet below the outcrop, and in pipes and along drainage fis- 
sures reach even greater dei)ths. Generally, however, there is another 
zone of alteration below the level of these altered or highly altered ores — 
the zone of enrichment. These secondary enrichments are believed to 
be due to the alteration of argentiferous galena into the usual carbon- 
ates and sulphates, in which the silver and to some extent the other 
metals are taken into solution during the reactions produced by this 
alteration, the surface waters, changed in character by the formation 
of the lead carbonates, carrying the silver down and depositing it in 
crevices, open spaces, and minute fractures of the vein filling, especially 
of the metalliferous portions. The ore also occurs in the cracks of the 
shattered country rock, forming the veiu matter where it is associated 
with secondary quartz. Very commonly the x>olybasite occurs in crys- 
talline masses showing no definite crystal outlines. In the open spaces- 
and vugs of the vein, crystallized specimens have been found associated 
with barite. It appears that this mineral is in process of formation at 
the Florence mine, 200 feet below the creek level, which is about the 
water line. It is possible, of course, that it may be due to the meeting 
of surface and of deep-seated waters. The zones of impoverishment, 
of enrichment, and of unaltered primary sulphides recognized in the 
case of copper veins are clearly present here, though the uppermost 
is of limited extent; and the zones are not so sharply or definitely sep- 
arated from one another as they are in copper deposits, owing to the 
later fissuring of the vein filling allowing the secondary enrichment to 
be mixed with the unaltered sulphides. Polybasite is said by Dana to 
alter to stephanite and pyrite. In the Neihart ores this mineral seems 
to show an alteration to pyrargyrite and pyrite, and the former, in turn, 
changes to native silver in the upper zone. 



WEED. J ORE DEPOSITS OP NEIHART DISTRICT. 423 

SUMMARY. 

The Neihart ore deposits occar io metamorphic gneisses of supposed 
igneous origin and Archean age, and extend upward into the basal beds 
of the Belt series of Algonkian age. They are sheeted fissures that 
cut both ancient and later igneous rocks and are believed to be of post- 
Cretaceous age. 

The veins contain silver-lead ores ; more rarely rich silver sulphides, 
and a value in gold of $1 to 5 ounces of silver. The common ores 
consist of galena, blende, and pyrite, in a gangue consisting of lime, 
magnesia, iron, and manganese carbouates. The rich silver ores con- 
sist of polybasite with a lesser amount of pyrargyrite and native silver 
in the oxidation zone; chalcopyrite also occurs. Barite is a common 
gangue mineral, but occurs in much smaller quantity than the carbonate 
^^spar." The primary ore minerals are those mentioned above, except- 
ing perhaps pyrargyrite. Polybasite more commonly occurs, however, 
as a secondary mineral. 

The silver- lead ores vary from $20 to $60 per ton ; the richer ores, from 
$100 to $200 or more per ton. The most valuable carload shipped by 
the Benton mine returned $36,000. 

The fissures all belong to a single system, running about north and 
south magnetic, and dipping 60^ to 80^ W. The vein fissures are part 
of a general fissure system. The width of the fissure varies in the dif- 
ferent rocks. It is widest in the softer schistose rocks, narrow but 
sharp cut in the massive diorite, irregular and narrow in tough and 
knotty amphibolite, and becomes lost in a multitude of little fissures in 
rhyolite- porphyry. 

The veins are in part replacement zones of closely sheeted rock and 
in part filling of narrow open fissures. The rock between the fissures 
is intensely altered and decomposed, and the vein walls practically 
limit this alteration. The ores occur in more or less persistent streaks 
of spar, and rarely of quartz, in this altered rock or vein matter. The 
payable ore bodies occur in shoots. Secondary filling of open spaces 
by quartz has occurred in some of the veins, usually accompanied' by 
rich silver sulphide ores. 

Ore deposition was by ascending carbonated waters, producing 
metasomatic replacement along fissure lines. The veins have sufi'ered 
later fracturing and secondary enrichment of the zone at or below the 
water level. There is now but a small amount of superficially altered 
or oxidized ore. 

NOTES ON MINES OP THE DISTRICT. 
NRIHART DISTRICT PROPER. 

Queen. — ^This was the first vein discovered in the district. It was 
opened by a tunnel driven a short distance on the vein, which showed 
such favorable conditions that the Queen claim, together with its exten- 



424 GEOLOGY OF THE LITTLE BELT MOUNTAINS, MONTANA. 

sioDs, the Homestake and O'Brien, was bonded for $45,000 in 1884. 
Further development work exposing less ore than was expected, the 
property was abandoned to the vendors the following year. Later 
development work by the owners resulted in the discovery of ore bodies 
from which large shipments have been made. The present develop- 
ment (1897) consists of a three-compartment shaft 300 feet deep, with 
100-foot and 300-foot levels, and a tunnel driven 1,010 feet along the 
vein and extended several hundred feet farther by the owners of the 
Gait, who now work their property through this adit. The property 
is well equipx)ed with machinery and buildings. A general view is 
given in PI. LVII, B, showing the shaft house, ore bins, and waste dump. 

The Queen vein varies from 1 foot to 5 feet in width. The underground 
surveys show that the vein has an easterly dip, which is contrary to the 
prevailing dip of the other veins. The old shaft, 530 feet north of the 
entrance of the main tunnel, shows the vein to be vertical between this 
tunnel and the level 100 feet below, but to dip easterly at 80° below 
this. The levels driven from the new shaft show a less steep inclina- 
tion, the angle' being 80° above the IdO-foot level and 75° below it. 
The pay ore of the early workings is said to have occurred in kidney- 
shaped masses scattered through the vein fillings, and not in regular 
ore shoots. As the mine was shut down and the shaft flooded at the 
time of the writer's visit, no personal observations were possible. The 
map of the workings shows that the vein at the shaft house has a 
general course of N. 20° to 30° E. The long tunnel which connects with 
the Gait workings has a course of about N. 40° E. There is reason to 
believe that it is run on a stringer or cross vein, for the O'Brien and 
Queen veins appear to be either the same or two closely adjacent parallel 
veins, 

CPBrien. — This vein has been worked by a tunnel, which waft over 
420 feet long in 1896, and by a 324-foot crosscut from the main Queen 
tunnel to the vein. This surface tunnel and the drift from the cross- 
out show the vein to have a course of N. 10° E. and to dip west at the 
steep inclination usual in the district. 

Mountain Chief, — This vein, on which the mines of that name are 
located, crosses the mountain spur lying between Belt and Carpenter 
creeks, from the London over the slopes to the Eighty-eight mine. The 
Mountain Chief mine was one of the first properties of the district to be 
developed. It was purchased in 1884 by the Hudson Mining Company 
for $18,000, and extensive development work at once begun, a concentra- 
tor and smelter being erected to treat the product of the mine. The 
ores first extracted were very high grade, it being reliably reported 
that over $10,000 worth of ore was extracted in sinking the first 20 feet 
on the southern shaft. This rich ore did not continue in depth, and as 
the low-grade ore did not concentrate well and a large amount of money 
was expended in driving the Eighty-eight tunnel without any returns 
work was stopped upon the exhaustion of the stopes of the upper tunnels 



A QUEEN OF T 



WBBD.] MINES OF NEIHART DISTRICT. 425 

in 1890 and has not been resumed since. It is evident from an exami- 
nation of the workings that the lower or Eighty-eight tnnnel, thoagh 
1,700 feet long, is still several hundred feet from the vein, and that its 
course to the lead is not a direct one. The vein has a course of nearly 
true north and south, as shown in the upper workings, curving slightly 
toward the west in its southern extension. If, as is supposed, it is the 
same as the London vein, this change of course continues until, at the 
London workings, above Belt Greek, the course is nearly northeast. 
The dip is to the west, varying from 62° to 65o. 

The property was visited in 1894, when the two upper tunnels were 
examined. The lower of the two tunnels is 950 feet above Carpenter 
Creek and is about 1,000 feet long. The vein traverses the Pinto diorite 
for the first several hundred feet, and in this rock is but a foot or so 
wide, the filling being a rusty oxidized material, showing no values at 
first, but carrying a streak of ore farther in. About 600 feet from the 
mouth of the tunnel the country rook changes and the vein widens to 
7 feet across and carries an ore body stoped out to the surface. Two 
short crosscuts driven into the walls show the diorite to be quite solid, 
no sheeting being seen on either side of the vein. In the upper tun- 
nel, which is about 700 feet long, the vein traverses '' gashes" or included 
masses of the feldspar-gneiss in the diorite, and in part the vein 
appeared to have a Pinto • diorite hanging wall, with gneiss on the 
foot, so that the gneiss seen on the hanging wall may be projecting 
tongues or wedges cut by the veins. A porphyry dike is cut at about 
500 feet from the entrance. The vein, which is 30 to 40 inches wide, 
shows the ore shoot cut in the lower tunnel, which has been worked out. 
A crosscut from the upper tunnel into the east or foot wall of the Moun- 
tain Chief vein cuts a second vein 33 feet from it and another vein 10 
feet beyond. The first vein cut is 3 feet across, but is barren of ore 
material ; the second is only 2 feet wide, but shows a streak of good- 
looking ore. 

Florence, — This has been one of the most successfiil mines of the 
camp, having been a steady producer since 1893, and thus far has 
shown pay ore in every part of the vein worked. The ore consists of 
a mixture of galena, pyrite, and blende, together with chalcopyrite, 
polybasite, and pyrargyrite. The vein runs northeast and is nearly 
vertical. Its width varies from 4 to 6 feet, and the fissure is always 
well defined. It traverses a gray feldspar- gneiss, generally much altered 
along the walls, and giving place to a black amphibolite for a short 
distance., The fissure varies in width in the different rocks, and in the 
amphibolite is deflected slightly, and narrowed, as shown in fig. 60, but 
continues its normal direction and width beyond this rock. The vein 
shows occasional splits or stringers, and where these run off they 
usually carry for short distances an ore richer than that of the main 
vein. The ore shows a well-marked banding, but it is more commonly 
a streaking parallel to the fissure walls, formed by a stringing out of 



426 OKOLOGY OF THE LITTLE BELT MOUNTAINS, MONTANA. 

the minerals in the gangae, rather than a definite cmstification or 
banding <lae to alternating layers of did'erent minerals. 

The vein matter consists of leached and whitened gneiss, the richest 
ores being found where this alteration is most intense. This vein matter 
often has a fine sheeting or banding. The pay streak varies in width 
and in its position in the vein. In some places it fills the entire width of 
thevein,di£feringinthisreBpectiromanyotherTein worked in thedistrict. 
The ore carries barite very commonly. The face showed in Augast, 
1897, a 2-foot ore body averaging 60 ouDces of silver to the ton. A 
section of the vein seen in the &ce of the upper tunnel on Aagust 
14, 1897, is shown in fig. 61. At this point a spur comes in from the 
west and enlarges the ore streak to 2 feet. The figure iUastrates the 
banded strnctare of the vein due to the stringing out of the grains of 
ore minerals in the light-colored gangne. A line of yuga marks the 
center. The face figured in the diagram, fig. 61, shows an ore body 
decidedly above the average for the vein, both in width and in 
quality. The ore is partly brecciated, has vugs, and consists of white 
spar with some quartz, together with rich silver sulphides and galena. 

The property is developed by two adit levels and a two-compartment 
shaft, 135 feet from the entrance of the lower level. This shaft is 300 

/I'clay selvage 



Flo. eo.— Fnalt where Teln creasn Bmphibollle. upper level Florence mine, A trguet 13, 1887 ^k 

feet deep, the bottom level being 230 feet below Belt Creek. Id 1897 
the lower tunnel was 465 feet long. PI. LXIV, B, shows the ore and 
hoist houses and the entrance to the tnnnels. The view also illustrates 
the characteristic weathering of the gneisses near by. The shipments 
for 1S97 averaged 5 carloads per mouth. The claim has four well- 
defined veins in a width of 300 feet, only two of which have as yet 
been opened on the surface. 

The Florence ore, as shipped, will not average 10 i>er cent lead, but 
the higher the silver the higher the lead contents. The jKmrer ores 
are carefully sorted before shipment. Very little stoping has been \\ 

done on the upper part of the vein. 

ConceairaUA and Monarch. — These properties now belong to the 
Florence Company. The former owners ran a 1,500-foot tnnnel at a 
slight angle toward the Florence on a vein averaging Si feet wide. A 
crosscut at the end cuts a new lead, but no ore has been shipiwd from 
either vein. 




iF.STKlN6£ 




c^DWATEFI 



MINES OP NEIHART DISTBICT. 



427 



Gait. — Thia vein has beeo one of the large ore prodncers of the dis- 
trict since 1893. The Gait claim shows, it is said, four parallel veins, 
the Gait vein being the only ooe developed. It is now commonly 
regarded as the same as the Queen vein, though formerly the Equator, 
Gait, and Nevada claims were supposed to be on the same ledge. The 
vein has a course N. 20° to 30° E., and where the main ore body 
occurs is vertical, the dip being eastward to the north of the shaft 
and westward to the south yaauiie 

of it. A Hght-colored feld- 
spathic gneiss is the common 
wall rock, but in part the 
vein follows a dike of rhyo- 
lite-porphyry, the fissure 
cutting the dike and not 
being a truecontact. Pinto 
diorite is seen m only one 
place in the workings, and 
black gneiss (amphiboliteT) 
only in the fault at face of "^^ *' ~''";„^Z/™oiZ;a!r.'''''''''''''°'" 
the level. The vein is 10 to 

20 feet in width, but the vein matter is wider, and in the only places where 
the wall is cut through 60 feet of altered gneiss is encountered between 
the ore body and the unaltered country rock on the hanging-wall side. 
Crosscuts from the working level 160 feet above the Queen tnuuel, and 
from the winze 150 feet below the Gait tunnel (160 feet above the other 
crosaent), show 40 to 50 feet of vein matter. It is certain, therefore, that 
the vein is a wide one. The rhyolite- porphyry is a hard rock, showing 
qnart?. grains in a dense, felsitic groundmass, and quite like the N'ei- 



A.  

V 







Fio. «2.— Olagnin sbonrlng 



.rthendoTQuA 



hart porphyry described elsewhere (pp. 375-376). The dike is 1 to 5 feet 
wide, and its relation to the vein is represented in the accompanying 
diagram (Qg. 62), which shows its occurrence in the end of the Queen 
tunnel. The vein cuts the porphyry for a distance of 300 feet or more, 
and is seen forming one or the other of the vein walls, though not on both 
sides of the vein at one place; it also often forms horses in the vein. 
The same relation of vein and dike Is seen in the Gait tunnel, 450 feet 



428 GEOLOGY OF THE LITTLE BELT MOUNTAINS, MONTANA. 

vertically above the Qaeeu tunnel. Amphibolite or black gneiss was 
seen only at the extreme end of the lowest tannel, where it faults or 
deflects the vein as it commonly does other veins of the district, but 
there is apparently also a true fault oi^ slip cutting the vein at the face 
of the tunnel, as the ore and vein material are crushed and thrown to 
the northwest. 

The ore is galena with the usual accompaniments of blende and 
pyrite, and occurs in a shoot that has been developed some 600 feet in 
vertical extent. The pay streak varies from 1 foot to 4 or even 5 feet 
in width throughout: the workings. In the stopes visited the vein 
showed 2 to 24 inches of ore, separated by a seam of clayey matter from 
a porphyry foot wall. The ore has a gangue of spar. In the workings 
visited the ore showed a more or less distinct banding. The vein mat- 
ter is largely an altered country rock, which is sometimes sheeted or 
shattered, and these minute fissures are filled with rich ore. The vein 
shows a clayey or talc parting, sometimes on both sides of the vein, but 
more commonly only on one wall. It is currently reported that the 
vein is richest where the gouge is most abundant. The vein has been 
developed by three tunnels. The middle or main Gait tunnel runs for 
1,015 feet on the vein. A 150* foot shaft was sunk from this tunnel at 
450 feet from the entrance. PI. LIX shows the engine house and ore 
bins at the mouth of this tunnel as they appeared in 1895. Upon the 
extension of the Queen tunnel to the side line of the Oalt claim, arrange- 
ments were made by which the Gait vein could be worked from this 
tunnel, 450 feet below the level of the Gait tunnel, the level being 
extended beneath the earlier workings. In 1897 two levels, at 75 and 
150 feet above the Queen tunnel, opened up stoping ground, but the ore 
body had wedged out and the ore in sight was too low grade to warrant 
shipping. The mine was therefore shut down until early in 1899, when 
new development work disclosed good shipping ore. 

Moulton, — This mine, owned by the Diamond B. Mining Company, 
was prior to 1893 the largest producer of the district, and it is credited 
with a production of 450,000 ounces of silver. The ore is galena, to- 
gether with pyrite and blend in a barite and quartz gangue. The vein 
runs nearly north and south, and cuts bands of vari-colored gneiss. 
The vein dips 80^ to 90^ W., while the gneiss bands run east and west 
and dip 70^ S. The vein matter consists largely of crushed and decom- 
posed country rock, which is usually soft. No definite sheeting of this 
altered rock was observed, though it is commonly cracked and the 
joints are filled by ore. The vein is 3 to 7 feet wide, with walls of hard, 
unaltered rock. The pay streak is from a few inches to several feet in 
width, varying with the inclosing walls. Zinc blende occurs abundantly 
with the galena, and is looked upon favorably, for, contrary to the rule, 
it is here a sign of good paying ore, as the galena with blende carries 
more silver than the galena alone. 

The workings show very striking examples of the influence of the 



WEED] MINES OF NEIHART DISTRICT. 429 

country rock upon the vein, and the generalization made by the former 
superintendent, Mr. Frank Raymond, that the productive ore bodies 
occur only in the bands of feldspathic gneiss appears to be borne out 
by the workings of other mines. A fault is reported to run from the 
Queen of the Hills workings through the Moulton and across to the 
Broadwater. The development consists of a tunnel driven on the Inger- 
soll vein, which here runs through Pinto diorite and is barren of pay ore, 
with crosscuts to the Moulton and South Carolina veins. PI. LXI 
shows the shaft house and mine buildings as they appeared in 1895. A 
three-compartment shaft 5o0 feet deep, with levels at 100-foot intervals, 
develops the lower part of the vein. The 300-foot level was the only 
one visited. South of the shaft it runs through black mica-schists, and 
is ore bearing in the pink gneiss, beyond which it passes into black 
gneiss again. To the north the level does not extend to the feldspar- 
gneiss. The mine is the only one where careful ore assorting has been 
done. The average smelting ore is reported to run 50l to 60 ounces of 
silver per ton. 

Cumberland. — This vein is said to be an exception to the rule that 
the veins are ore bearing in the pink gneiss. 

Ingersoll, — Over $45,000 has been expended upon this vein in driving 
exploratory tunnels in the unsuccessful search for ore. This expendi- 
ture was without doubt incurred because the ore carries a considerable 
proportion of its value in gold. Up to 1897 the total amount of ore 
shipped amounted to only 6 carloads, the last of which, though sorted^ 
netted only (200 to the shippers. 

The vein is a well-defined one, but in the five claims owned by the 
Ingersoll Company has been worked only on the IngersoU claim. On 
this property it cuts both gneisses and Pinto diorite, as it crosses and in 
part follows the contact between these rocks. The gneiss is of both the 
red and the black varieties, and, as usual, in neither the latter rock nor 
the diorite does the vein contain any workable ore. Owing to the 
indented nature of the contact with the diorite, the vein cuts successive 
projecting tongues of the latter in the gneiss as well as the main body 
of the intrusion. 

• The vein has a course N. 10° to 20° E. The dip is to the west in 
the southern part of the workings, but at the end of the long tunnel 
is to the east, the angle of dip being from 60^ to 80^. In the ore 
body from which shipments have been made the dip is nearly 80o, the 
vein showing an ofifset of 25 feet in the 111 feet between the two levels. 
The width varies with the nature of the inclosing rock. In the red 
gneiss it is over 2 feet; in the diorite it is commonly but a few inches, 
and nowhere does it exceed 1^ feet. 

The property has been explored by two tunnels driven on the vein. 
The upper is about 150 feet long and exposes a small ore shoot; the 
face shows walls of hard and blocky rock, inclosing 30 inches of soft 
and whitened, much-altered gneiss that is sheeted by planes 6 to 10 



480 GEOLOGY OF THE LITTLE BELT MOUNTAINS, MONTANA. 

inches apart. The ore occurs as a streak of sparry galena lying on the 
foot wall, and as the vein rock is soft and clayey it slips easily in mining 
and caves down, leaving the ore attached to the foot. The wall rock 
in this tunnel is minette, but its nature was not recognized in the field 
and its relations are not known. 

The lower tunnel follows the vein for 1,000 feet and then changes to 
a crosscut driven west to the Queen of the Mountains vein. The ore 
seen in the upper tunnel extends down to this }evel, and the shipments 
made from the property came firom this shoot. In general, the tunnel 
shows the vein to carry a narrow streak of spar dotted with blende, 
galena, and pyrite, but averaging, it is stated, less than 6 ounces of 
silver to the ton. The diorite walls, which prevail beyond a point 700 
feet from the entrance of the tunnel, show alteration for a distance of 
3 or 4 inches. 

About 1,000 feet from the entrance the adit level leaves the Ingersoll 
lead and for a little over 100 feet follows a cross fracture in the diorite, 
which has smooth and polished walls and occasionally films of rich ore. 
The diorite is very solid, showing no shearing until within 25 feet of a 
vein 2 feet wide and running parallel to and about 150 feet west of the 
Ingersoll. This vein carries bunches of 5 to 6 ounce galena. 

One hundred feet beyond this vein the level leaves the fracture plane 
in the diorite and the course is almost due west till the Queen of the 
Mountains vein is encountered. These relations will be understood by 
reference to fig. 63. 

Queen of the Mountains. — This vein has been explored for several hun- 
dred feet by levels run north and south from the end of the Ingersoll 
crosscut. The crosscut itself is driven entirely in diorite until within 
40 feet of the vein, where a dike of rhyolite-porphyry is cut. This dike 
dips west toward the vein at 45o. The north level of the Queen of the 
Mountains vein shows a narrow streak of spar dotted with blende, 
pyrite, and galena, but no workable ore bodies. The level driven south 
from the crosscut tunnel was filled by a cave-in at the time the mine 
was visited, but it is said to be cut in black gneiss and to show no 
workable ore. 

Ingersoll No* 2. — The property is developed by tunnels run north on - 
the Ingersoll vein. About 1,000 feet from its mouth the lower tunnel 
is deflected and is cut across the country rock for 600 feet in order 
to crosscut the veins parallel and adjacent to the Ingersoll on the 
west (see map of workings, fig. 63). Throughout this tunnel the Inger- 
soll vein shows only 2 to 3 inches of lean ore, and by some of the miners 
is believed to be a ^' stringer," meaning an offshoot that runs parallel 
to the vein. 

In the crosscut at the end of the long tunnel the rock is a very hard, 
solid, unaltered Pinto diorite, breaking with blocky fracture, and is 
very hard to drill. It shows no sheeting or fracturing until within 
25 feet of the 300-foot east crosscut, where a stringer is seen having 




_IL 




iv£LS. 



^* ^^ ^.^ ^^ ^^ 

CevElsout. 



WEED.] 



MINES OP NEIHART DISTRICT. 



431 



smooth polished walls, the fracture running nearly west and dipping 
north. This crosscut shows a 2-foot vein carrying bunches of solid 
galena that holds but 5 ounces of silver per ton. This cross lead is 
followed 100 feet or more, showing occasional films of rich mineral, but 
no bunches of ore, though bunches of quartz occur in the hard rock. 

Rock Creek. — ^The three veius supposed to cross this property have 
not been suflSciently developed to prove their value. The workings 
comprise a tunnel with several branches. The Bock Greek vein pitches 
west at 60^, is 2 J to 12 inches wide, and runs through blackish or dark- 
gray gneiss holding much reddish gneiss and shot with tongues, 
stringers, and bunches of Pinto diorite, similar to that seen on the , 
surface workings. The ore thus far extracted yields, when sorted, 16 to 



QUCEN OF 
(CLAIM 




Fia. 03.— Workings on TngersoU vebi, 1897. 



18 per cent of lead, with 80 ounces or more of silver per ton. It shows 
galena with silver sulphide and some wire silver, and carries no gold. 
The workings throw little light on the vein structure. The west branch 
has a spur or offshoot to a stope from which the rich ore was being 
taken out and runs through Pinto diorite mostly. The east branch 
follows a narrow fissure showing a film of spar without any pay ore, 
running through solid gneiss, the face showing leached and whitened 
gneiss. A little offshoot from the vein shows the hanging wall to be a 
solid, blocky, black gneiss that is distinctly sheeted with five sheeting 
planes in a thickness of 4 feet, but as there is no talc or decomposed 
rock along the sheeting planes they are not prominent. These planes 
adjoin 6 to 8 feet of shattered gneiss. 
Lizzie. — This property is developed by numerous surface cuts and a 



432 GEOLOGY OF THE LITTLE BELT MOUNTAINS, MONTANA. 



N0.3 AOIT LEVEL 



short tannel. There are sapposed tx) be fonr veins in the claim, two of 
which are cut by this tnnnel, the easternmost being the Lizzie vein and 
Lizzie !No. 2, the vein parallel to it. The surface cats show a vein of 

brown,ocheron8,oxidized 
ore, containing residual 
masses of galena. This 
ore carries no gold. The 
tunnel, which starts near 
the end line of the claim, 
is 200 feet long and shows 
at its face a vein of white 
spar and quartz but a 
few inches wide. The 
ore occurs like a string 
of lenses edge to edge. 
The wall rock is solid 
and blocky in fracture 
and is largely amphibo- 
lite, in which shearing is 
not prominent. Higher 
up the hillside is an 80- 
foot tunnel. The discov- 
ery shaft is 73 feet deep. 
Lizzie No. 2. — This 
vein is developed by two 
tunnels, 110 and 120 feet 
long. The first carload 
of ore shipped was from 
the discovery shaft, and 
netted $786 for about 15 
tons. Up to 1897 the 
claim is estimated to 
have produced about 
$5,000 worth of ore. 

Dakota. — This p r o p • 
erty is said to have a 
good showing of low- 
grade ore in a well-de- 
fined fissure. In 1897 
the workings were not 
accessible, the tunnel be- 
ing filled by a cave-in 
about 300 feet from the 
entrance. This lower tunnel starts in the Pinto diorite, but the wall 
rock changes to black amphibolite-gneiss, and in a short distance this 
is replaced by the diorite again. No vein was observed in the 300 feet 




AfO. a AO/T LEVEL 



Fio. 64. — Cross section of Broadwater vein. 



^v*:kd.j mines of neihabt district. 433 

accessible in this tunuel. The lead is said to be 4 feet wide. The damp 
heap shows a leached white gneiss impreguated with pyrite. 

Broadwater. — This mine has been the chief producer of the district 
since 1893. The property embraces several adjoining claims situated 
on the api>er slopes of Neihart Mountain, southeast of the town, and 
from 700 to 1,000 feet above Belt Creek (see PI. LXII). The property 
was actively prospected in 1885, employing as many as 75 men at that 
time, but the ore bodies then found were not considered to warrant 
further development, and work was abandoned and the mine shut 
down. In 1892 the property was sold for $165,000, and the new owners 
at once began extensive development. Large bodies of argentiferous 
galena were at once encountered, and the mine yielded over 1,000,000 
ounces of nilver in the succeeding two years, the net profits being stated 
to have been (4()5,000 up to 1895. In December, 1896, the ore in sight 
was largely exhausted and a long adit tunnel seemed to limit the down- 
ward extension of the ore shoots, but further development uncovered 
new extensions, and the mine has since then continued to yield a steady 
output of ore. The shipments averaged 3 cars per week from January 
to July, 1897, and were then increased to 15 cars per week (300 tons). 

The vein has a general course of N. 26^ E. and dips west at angles 
varying between 60° and 8()o (see fig. 64). The vein has been, owing 
to its productiveness, more extensively prospected than any other in 
the district. PL LVIII, which shows the workings in 1896-97, gives a 
good idea of the regularity of the vein in lateral extent. 

The vein is strong and well defined and traverses light-colored schists 
and reddish or streaked gray feldspathic gneisses, which it crosses 
at nearly right angles throughout the greater part of its extent as 
developed. At the extreme northern end the level penetrates Pinto 
diorite, and no paying ore has been found in this part of the vein. 
Near the entrance to the lower tunnel sheeted gneissoid porphyry is 
found. In the Pinto diorite the vein is well defined and continues with 
unaltered course, showing a banding of rusty and bluish clay, and 
occasionally small bunches of lead ore, though in the diorite it has 
nowhere yielded any paying ore bodies. The vein varies in width in 
different parts of the workings, averaging between 3 and 6 feet. It 
has been explored for over 1,000 feet vertically and 2,400 feet in lateral 
extent. Offshoots or splits are numerous, mostly from the hanging 
wall, but are not large or important. In the upper workings the vein 
splits about a horse of country rock 50 feet wide and 100 feet long. 
Small horses 50 feet in length and half this in width are sometimes 
encountered. The workings indicate a splitting of the vein southward. 

The foot wall is usually well defined. The ore sometimes occurs 
" frozen " to it, but is more often separated by a band of clay a few 
inches thick, which sometimes is a rich ore, owing to films of silver sul- 
phides. No streaks or slickensides were observed on either wall. The 
hanging wall is usually a hard though little-altered rock, but crosscuts 
are few and its character is known only at such places. 
20 GBOL, PT 3 28 



434 GEOLOGY OP THE LITTLE BELT MOUNTAINS, MONTANA. 

The vein shows nsaally a very distinctly banded stmcture. This is 
not doe to snccessive layers or crasts of gangae and ore materials, 
bat chiefly to a sheeting of the altered country rock which lies between 
the vein walls and oonstitntes the greater part of the vein matter. 
This material, of which the waste-damp heaps are formed, is a greatly 
altered leached gneiss or schist. In some parts of the mine the vein 
matter is brecciated, fragments of country rock being cemented by 
barite and quartz with ore minerals, and to a lesser extent by clay. 
No evidence showing the amount of faulting was obtained. In some 
parts of the workings the vein is about 4 feet wide and shows 3 to 8 
inch ore streaks near both walls, with intervening altered country rock 
sheeted in plates one-half to 2 inches thick. As the ore itself is sheeted 
in this case and the minute fractures are coated with secondary pyrite, 
it is evident that post-mineral fracture has occurred here, as it has in 
several other mines of the region. 

Two faults are observed. The first is noted only on the lower level, 
and throws the vein to the east in its southern extension. The second 
is near the north end of the workings, and is a well-defined slip with dip 
of 50^ N., and is marked by 6 inches to 3 feet of soft clay or slickenside 
materials between hard country rock. The fault cuts off the pay ore, 
but the vein continues beyond it with unaltered course into the Pinto. 

The ore consists chiefly of galena, together with a little pyrite in a 
gangue of spar, with lesser amounts of barite and blende. The ship- 
ments often averaged 20 per cent zinc, 7 to 8 per cent lead, 40 to 60 
ounces of silver for the high-grade and 20 to 30 ounces of silver for 
the low-grade ores. In 1897 the ores contained only 2 to 3 per cent of 
lead. 

The vein generally shows first a streak of soft, muddy material, from a 
lew inches to a foot thicK, resting upon a well-defined foot wall. Ux>on 
this is the sulphide streak, consisting chiefly of galena, then a low- 
grade ore, which is a mixture of spar, heavy spar, and galena. The 
hanging- wall rock is commonly hard and unaltered, but in some parts 
of the vein the reverse is true. 

While the galena ore bodies do not commonly show a sheeted or 
banded structure, yet in some parts of the vein this structure is very 
marked, and even in hand specimens the ore shows a decided banding, 
due to minute layers of spar running through the galena. There is 
also a coarser banding, due to alternations of sparry ore with nearly 
pure galena. The ore is soft and easily broken and handled, one man 
averaging a carload (20 tons) every two days. The decomposed rock 
between the vein walls carries large amounts of carbonates, and the 
wall rocks also contain carbonate minerals. 

Fig. 65 shows a cross section of the vein observed on stopes below 
the third level near the south end, and illustrates a common appearance 
of the vein. It is impossible, however, to give a general section, as the 
vein varies &om point to point. While the ore minerals occur rather 



BROADWATER MINE, 1 



MINES OF NEIHABT DISTRICT. 



435 



a^ 






5 6' 



^- 



6°7 



generall; diBBeminated thronghont tbe ctparry parts of the vein, tbe 
pay ore occurs iQ shoots. PI. LX shows the space stoped ap to 
December, 1896, and illnstrates the size and dip of the shoota. 

These shoots are often persistent for maoy feet laterally, but vary 
somewhat io position in the vein. They vary from narrow streaks to 
lenticular bodies which are sometimes as much as 4 floet across. In tbe 
southerly workiugs, near the entrance, the ore occurs in isolated and 
well-defined lenses whose ends overlap. The best ore bodies consist 
of a nearly solid mass of galeua with a little barite, and will average 10 
ounces of silver. The ore above the So. i level (6,416 feet elevation) 
was largely osidiiced, show, 
ing residual bunches of 
galena. This surface oxi- 
dation extends several hun- 
dred feet or more below the 
outcrop, and is deeper along 
pipesorwiitercoarses. The 
richest ores are not the ga- 
lena ores, but those carry- 
ing silver sulphides, which 
occnr in the upper work- 
ings. In later development 
work at lower levels the 
sheeted vein material, which 
is a bleached and altered 
gneiss, is reticulated with 
minute films of silver sul 
phide, so that this rock, re- 
sembling waste, ismorc valuable than thegalena-orestreak. In one place 
the vein for 18 inches nearest the foot wall is sheeted, and shows a streak 
of poor galena ore bnt 6 inches across, that carries mnch spar and holds 
but 4 to 5 ounces of silver. The rock above this for a width of 5 feet is 
netted with cobweb-like films of blue sulphides, so that the rock as a 
vhole carries overSOOounces of silver. This rock seems to fade in to the 
country, and no good hanging wall was observed. Kear this same place 
the vein jtasses into a greenish gneias, and is filled by a coarse 
breccia of this rock held together by spar, the foot wall being a solid 
and blocky red gneiss and tbe ores low in zinc. Zinc-lined water 
courses were ob.served in a number of different parts of the mine. The 
workings are wet, owing to tbe surface openings, bnt are well drained 
by the lower adit level. 

In many places where the vein is not workable it shows a mass of 
spar peppered with grains of galena and pyrite- As shown in the map 
of the workings, the property has been developed mainly by adit levels. 
The relative elevation of these levels and the appearance of the surface 
works of the mine are well shown in PI. LXII. The property is admi- 



a. Ki — Fue cf Broadwater vein, on stflpe below No, 8 aAW, 

level, under blackimllh abop. a, wall rock; 1. 1" black 
:1a]-; t. while apar and atlpplemlnarati 3, oUf gaaKeand 
Fault cla;; t. rusty ahattered apar and noiue mineral^ t. 
V-V blaik tale with b»«t valuea; fr, ore strsak: «', aolt 
aIda rock and good ore; 7, wall; a, btockj rock, abowing 
iron -stained raff. 



436 GEOLOGY OF THE LITTLE BELT MOUNTAINS, MONTANA. 

rably Bitaated for the use of a wire-rope bucket line, but the ore has 
been hauled down a very steep wagon road — the lowest level being 400 
feet above the railroad — despite the fact that the teaming bill for six 
months more than eouals the amount necessary to have built a bucket 
line. 

SNOW CKEBK MINES. 

The veins found upon the north slope of Long and Neihart mountains 
cross the slopes drained by Snow Creek. Though not over 2 or 3 miles 
from Neihart, they are accessible only by wagon road up Carpenter 
Creek, and the long haul and rough roads make the mining of low- 
grade ore impossible under present conditions. Thus far no large 
bodies of silver-lead ores have been found, but the high grade character 
of the ores mined has led to extensive development work. The veins 
cut the various rocks of the district, and the nature of the country 
^ plays an important part in the economy of the mining development. A 
large number of claims have been prospected, but the only productive 
mines are the Benton group. Big Seven, and formerly the IXL and 
Eureka. The Cornucopia has, I am told, been extensively prospected, 
but has not been a large shipper. 

Benton group. — This was for many years the largest producer of high- 
grade ores of the camp, and the gold contents were so considerable that 
the mine was profitably worked from 1892 to 1896 despite the gen- 
eral depression in silver properties. The property consists of twelve 
claims, situated at the head of a southerly fork of Snow Creek under 
the western point of Long Baldy (locally known as Neihart Baldy). 
The only workings visited in 1893 were those of the new tunnel 
or uppermost adit of the mine. These workings nowhere cut entirely 
through the vein, exposing the walls. TJie vein matter is a bluish decom- 
posed gneiss carrying pyrite. The ore, though but a few inches in width, 
was very rich, consisting of loosely compacted sulphides with native 
silver. The hanging wall of the tunnel, which is driven on the lead, 
shows Pinto diorite, and the foot wall a quartzose gneiss, but the vein 
crosses both rocks and is not a contact lode. 

In the summer of 1897 the high-grade ore bodies of the vein were 
reported exhausted and active development work was suspended, though 
a couple of leasers were extracting some galena ore from the stopes near 
the face of the lower tunnel. The workings were visited, but were very 
wet, and the ore bodies were obscured by mud in most places. The main 
adit level is 2,400feetlong and is driven on the vein. In thislowor tunnel 
the vein is from 3 to 6 feet wide, and shows walls of both gneiss and 
Pinto diorite,*the former prevailing where the ore shoots occur and the 
vein pinching to a few inches in width in the latter rock. The vein 
is a breccia or gneiss, which is in places checked and sandy, but 
more generally is altered to a soft clay-like material, so that the mine 
workings are wet and muddy. The Benton ore has been unusually high 
grade, the values being chiefly in gold, with some silver; but the bodies 



WEBD.] MINES OF NEIHART DISTRICT. 437 

at the end of the new tunnel consist of lead ore, generally zincky and 
low grade. In the third tunnel the vein carries ore in bunches and not 
big shoots. In the main lead the values were largely in gold. The 
tunnel is said to cut two leads. The ore produced in the past has been 
much like that of the Big Seven. One carload netted $26,000, accord- 
ing to Mr. D.C. E. Barker, and the total product of the mine had exceeded 
$400,000 in 1898. 

IXL and Eureka. — This mine is developed by a 250-foot shaft and 
levels, but has been idle for several years. The ore obtained in the 
upper workings was very rich, but gave out at 90 feet below the sur- 
face, the vein consisting of rock checked by minute fissures and not 
showing a single well-defined fissure. This is the general experience in 
mining the ore deposits of the district which occur in porphyry. In the 
writer's opinion this is due here, as it has been found to be elsewhere in 
the State, to the physical nature of the country rock. The porphyry 
is fractured by a close jointing, fissuring the rock so that it was freely 
penetrated by the mineralizing waters, and the ore, instead of being 
confined to a well-defined vein, is disseminated in minute films through- 
out the fine joint fissures of a wide zone. Secondary enrichment of such 
deposits generally results in the concentration of the minerals near the 
surface as rich ores, which are probably silver sulphides — "sooty" sul- 
phides they are called in some cam])S. They contain much manganese 
oxide, and are quite unlike those sulphides found at greater depths in 
the neighboring mines. 

Big Seven. — This mine has been for several years past a large pro- 
ducer of the rich silver sulphide ores, carrying high values in gold. 
The company owns a group of seven claims ui>on the high mountain 
slopes north of the summit of Long Baldy. The ore being extracted in 
August, 1897, when the mine was visited, carried from 100 to 500 ounces 
of silver and $50 per ton in gold. The mine was producing from 2 to 3 
carloads a month of 300-ounce ore, derived from development work 
alone, no stoping being done. The veins are exposed by numerous sur- 
face cuts and by three adit levels. The veins have the general north- 
east course common in the district, and are well-defined fissures, cutting 
gneiss and a dike-like mass of Pinto diorite. Their upward extension 
is in the overlying Neihart quartzite. The vein dips west at a steep 
angle, and is from 5 to 6 feet wide (except in passing through the Pinto 
diorite, where it is 3 inches to 2^ feet in width). It carries a 1- to 2-inch 
streak of white quartz spotted with sulphides, which sometimes forms 
good ore; the rest of the vein matter is altered country rock cased in 
hard walls. There are often two ore streaks, but they 'are not per- 
sistent, though one is always present. The hanging- wall streak is 1 to 
6 inches wide, and occurs "frozen" to the wall. The vein widens 700 
feet from the entrance to the tunnel, and has a correspondingly wider 
streak of quartz and ore. At the face it was 12 feet wide, and a lens 
of ore on the foot wall was 1.8 feet wide. The hanging- wall ore streak 



438 GEOLOQT OP THE LITTLE BELT MOtJItTAniS, MONTANA. 

VM not exposed, as the banging wall was not cat. The v^eiu matter U 
a decomposed ^Deiss, which appears crashed, bnt shows do recogniz- 
able sheeting, and it is impregnated with qaartz spotted with ore. 

The Big Seven ore shows polybasite with ruby silver and (he usual 
baser eolpbides. It carries very little galena, and is generally regarded 
as a dry ore. This ore presente a wider variety of structure than the 
ore of any of the other veins. In the smaller quartz streaks the ore con- 
sists of quartz, with galena, blende, and pyrit« as primary filling, show- 
ing banding aud comb stmotare. The larger masses show much barite 
in a crisscross stmcture, the interspaces filled by pyrite, galena, aud 
blende, which along rags is capped by massive galena, with cbalco- 
pyrite at top, covered by quartz crystals. Replacement of country rock 
is seen in some specimens, bat more commonly there is a sharp demar- 
^ cation between altered 

country rock and filling, 
which is best seen in the 
' J breccia ore. 

V^ -^*  An unusaul typeofore 

f ,\' found here consists of 

"^ "^ '■ zinc blende with a little 

^ ' ; ^ . pyrite and galena, coated 

\ / ,' ' by plumose quartz whose 

, *^ . - surface is dotted with 

masses of pyrite, the ore 
being evidently formed 
v.^ iu I,. 1. , , . , r ,  , ir. . along a vug line. A car- 

rlQ.V. — blj[ Scvvn v«'1d Id quarlillQ, Tmc# of upper <lrlft, All' ^ ^ 

gan.iivj. a, quutiio, xnneohBt fi»Dr«i iiDdcfaeckeiii ». 3 load of this Ore ran, ziuc 
iBirh™ »( wiiiM. cifl) ; (, cr«k.d ud fi«u»d, ni.t HiiiiK^ 28 per cent, silver 800 to 

ol»7!/. brown-anJ; p. qunrtiitBore: ft, unalWrwlqn.rtnlle. 9W) onnceS per tOn, gold 

C150 to t200 per ton. 
The owners say that the smelter retarns do not give any lead £rom the 
Big Seven shipments, though the average ore always contains more 
or less galena visible to the eye and determined as such by the usual 
tests. Tbe ratio of the gold and silver contents of the Big Seven ores 
appears to be about $1 in gold to 5 ounces of silver. 

The most interesting feature of the mine is seen in the apper work- 
ings, where the vein passes into quartzite. In the only face seen the vein 
rnns X. 20° E., is nearly vertical and is 7 feet wide, and showed the 
cross section given in fig. 66. The upper workings were inaccessible, 
owing to a snowslide a few mouths previous to our visit. From infor- 
mation furnished by the owners and from tbe character of the veins 
in quartzite on the neighboring claims, it appears that the vein scatters 
in the qnartzite and is very irregular. There is reason to believe that 
rich ore deposits will be found along tbe contact plane between the 
gneiss and qnartzite, where tbe veins pass into the latter rock. Sam- 
ples of the oxidized rock assayed by Prof. 0. E. Monroe for the 



WBBD.] MINES OF NEIHABT DISTRICT. 439 

laboratory of the United States Geological Survey fifave 49.75 ouuces 
of silver and $145 in gold per ton. The qaartzite ore is in part a quartz 
filling and in part impregnated quartzite. It shows no free gold, and 
uo mineral other than a rastiness of the rock due to iron oxide is observ- 
able even in ore that assayed $1,600 per ton, but the ore is shown by 
chemical tests to contain a large amount of molybdenum. The average 
ores irom veins in quartzite yield 20 to 50 ounces of silver and $5 to 
$10 in gold per ton. A sample collected by the writer from a ledge 
(not located) on the head of Narrow Gauge Gulch, on the opposite side of 
the mountain, gave, upon assay by 0. E. Monroe, $4 in gold and 12.20 
ounces of silver per ton. 

In addition to the Big Seven mine there are a number of properties 
located upon working veins in the quartzite. The rock is very dense 
and hard to work and forms large talus blocks, as seen in PI. LXIIl, 
showing the summit of Neihart Mountain. The rock dips southward 
at about 30^, the bedding plane being seen in the illustration. The 
summit of this mountain shows a narrow but well-defined vein of rusty 
quartz, whose course is N. 80^ B., dip 85° N". It is uncovered by a 
trench 50 feet long and 5 to 6 feet deep, from which a low grade of ore 
has been taken. Other claims on the saddle between Neihart and 
Long Baldy Mountain, or east of it and a little higher than the Big 
Seven workings, show short tunnels that expose rusty ores of cemented 
quartzite fragments carrying $5 to $10 in gold and 20 to 50 ounces of 
silver per ton. Kich quartzite ore like that of the Big Seven has not 
been found in quantity in any of these claims. 

MACKEY CREEK. 

The discovery in 1897 of veins carrying rich surface ores along the 
course of this stream brought into prominence a part of the district 
that. had been generally regarded as barren. Several claims were 
located on the two veins now known as the Gold Rock and the Phillippi. 
The first named has a nearly uorth-and-south course, like those of the 
district generally, and is nearly parallel to the creek, whose waters 
in some places flowed over the lead. The veins all occur in the Neihart 
porphyry. This rock forms the ridge between Mackey Creek and 
Carpenter Creek, but shows few exposures, though the debris covers 
the ground. The exact areal extent of the rock was not determined, 
owing to this debris and to the dense growth of lodgepole pine that 
everywhere covers it. The workings seen in 1897 showed only bunches 
of ore in a checked and shattered porphyry. 

A fissure showed in the face of the Golden Dream, the lead having a 
course N". 20° E. and dipping 60° to the west. The surface cut seen 
showed a shattered and altered porphyry streaked with brown and 
black manganiferous ore. A short tunnel on this property showed no 
distinctly defined vein at its face, but an altered rhyolite-porphyry 
netted by fractures marked by reddish films carrying pyrite and some 
quartz. 



440 GEOLOGY OF THE LITTLE BELT MOUNTAINS, MONTANA. 

The Phillippi lode, from which some rich ore was being extracted, was 
at that time opened for only 100 feet, and the ore occurred on the con- 
tact between a hanging wall of dark-gray micaceous schist and a foot 
wall of altered porphyry. The Mackey Creek ore bodies illustrate the 
branching or diffusion of the vein fissures in the i>orphyry. Every- 
where that prospect cuts or drifts had been run some ore was seen. 
The ores are all secondary sulphide enrichments. Some nucleal masses 
of galena were seen, and a very little oxidized ore, but the greater part 
consisted of sooty-looking sulphides mixed at the grass roots with 
manganese oxides. As was stated in discussing the influence of wall 
rock on the character of the vein fissures, the porphyry is favorable for 
such surface ores, but the veins are not to be relied on in depth. The 
ores will undoubtedly change in depth to galena, with or without silver 
sulphides. Until development work shall prove them permanent the 
deposits found in porphyry will be looked upon as likely to decrease in 
both value and quantity in depth. 

At the head of the creek the Dawn and Foster lodes have been devel- 
oped by drifts several hundred feet in^length. The ores thus disclosed 
were low in grade and zincky, though there is a well-defined vein with 
quartz filling along a porphyry-gneiss contact. PI. LXIY, A, shows 
this property. Specimens of nearly pure galena from this claim have 
been assayed for the laboratory of the United States Geological Survey 
by C. E. Monroe, and prove very low in grade, carrying but 0.15 ounce 
of silver per ton. 

The Whippoorwill mine, shown in PI. LXV, A, was one of the 
earliest discoveries of the district, but has never been a profitable 
producer. It is easily accessible by the Carpenter Oreek wagon road, 
and the recent discovery (1897) of ore bodies on the claim by leasers has 
revived interest in this part of the district. 

HARLEY CRERK. 

. Mention should be made of the Harley Creek prospects, although 
they were not visited. The writer is indebted to E. K. Abbott, of 
Neihart, for the following notes: The ore samples from these prospects 
carry as high as 12 to 20 per cent of copper, with a few dollars per ton 
in gold. The Imperial group, comprising eight claims, was in 1897 
developed by a 450foot tunnel, the Eoyal by a 200-foot tunnel, and the 
Granite Mountain by a 225-foot tunnel on the vein. 

HOOVER CREEK. 

Claims located about the head waters of Hoover Creek jdeld low- 
grade auriferous silver-lead ores, but the prospects are not sufficiently 
developed to show their character. 



MACKAY MINES iDAWN AND FOSTER CLAIMS), HEAD OF MACKf 



II FLORENCE W 



WBED.1 ORE DEPOSITS. 441 



BARKER DISTRICT. 
DISCOVERY AND DEVELOPMENT 

The first discovery of the ore deposits of this district was made in 
October, 1879, when the Barker and Gray Eagle mines were located 
by P. H. Hughes and D. 0. E. Barker. It was after the latter that the 
district was named. The Wright and Edwards mine was discovered 
shortly afterwards, and both this and the properties named above were 
exploited, so that in 1881 the ore bodies in sight were sufficient to 
warrant the erection of a smelter at the camp. In that year a smelter 
with a daily capacity of 4 tons was built by the Glendennin Mining 
and Smelting Company near the mouth of Gold Bun, below the present 
town of Barker. The plant consisted of two Blake crushers, a set of 
Cornish rolls, a Baker blower, and a 40-horsepower engine. The ores 
were treated in two reverberatory furnaces with 10 by 45 feet hearths.^ 
This smelter, whose dismantled remains are still standing, began opera- 
tions in December, 1881, but financial reverses of the company caused 
the closing of the works until the summer of 1882, when it was started 
up again and continued with success the remainder of the year. In 
1882 the smelter treated 934 tons of ore, which jielded 240,542 pounds 
of base bullion carrying 20,527 ounces of silver and 41 ounces of gold. 
This bullion, cast into 97-pound bars, was hauled to Fort Benton and 
shipped by steamer down the Missouri Biver. 

In 1883 the smelter was idle part of the season for lack of fuel, and 
some difficulty was experienced in getting a constant supply of ore. 
Beehive kilns were erected, insuring a sufficeut supply of charcoal, and 
the Silver Belle mine was purchased by the Smelter Company. The 
output when running is said to have been 25,000 to 30,000 pounds of 
bullion per week in 1883. In those years the May and. Edna mine was 
the chief source of ore supply, together with the Wright and Edwards, 
Barker, and Gray Eagle, the last two furnishing but little ore in 1883, 
while the Silver Belle yielded about 25 tons per day. 

A second and smaller smelter, costing about $1,200, was erected in 
1884, but when the principal ore bodies of the mine noted above gave 
ojut, both smelters were closed, in 1884. This was, perhaps, also brought 
about by the lower grade of the ore encountered, whose decreased 
values, though too low to pay the costly smelting charges of the local 
plant, could be profitably treated if railroad communication were 
afforded to large reduction works. The railroad was completed in 1891, 
since which time the output of the mine has been shipped to various 
points. 

At present the increased demand for silver-lead ores for the Great 
Falls smelter has led to the leasing or purchase of several of the mines 



1 Report of the Director of the Mint for 1881, Washington. 1882. 



442 6EOLOGT OP THE UTTLE BELT MOUNTAnYS, MONTANA. 

by the Helena Smelter Company, and if energ^etic work is piosecated, 
8o that development work keeps ahead of ore extraction, the fntore of 
the mines seems bright. 

OGCUBBENCB OF THE ORB DEPOSITS. 

The ore deposits of Barker may be divided into two classes: First, 
those occurring in veins in the Haghesville syenite; second, those foond 
in limestones along contact planes between limestone and porphyry 
(fig. 69). The Barker and the Wright and Edwards mines belong to the 
first class. The second class includes a number of very productive 
ore bodies — the Carter, May and Edna, Tiger, Moulton, and other 
mines. Experience with several of these ore bodies having shown 
that the galena changes in depth into low-grade pyrite, it is a question 
whether this is a common phenomenon with deposits of this type. A 
plan and section of the Carter mine are shown in fig. 67. The May and 
Edna, Silver Belle, and Carter mines show the same phenomena. 

In this connection, also, may be noted the fact that the Cumberland, 
the greatest ore body of the Castle Mountain district in this State, is 
a pipe of galena in limestone. Here also the ore changes in depth into 
pyrite, at a i)oint where the fissure is occupied by a iK>rphyry dike 
that does not reach into the upper working. The workings at the 
Tiger and Moulton mines may not be deep enough to show whether the 
galena will be replaced by pyrite in depth, though the ore body of the 
mine is cut at 326 feet below the outcrop and is still a good grade of 
galena. 

In the Tiger the ore bodies near the surface were relatively flat and 
shallow, indicating a spreading out along joints and bedding planes, 
but as th^ rocks are much fissured and the structure is complicated at 
this point, no definite evidence showing their exact occurrence was 
obtainable i^ the short visit made there. The mines located upon ore 
bodies in the syenite itself appear, however, to be u])ou well-defined 
fissures in the syenite and include the oldest mines and largest pro- 
ducers of the district. 

NOTES ON THE MINES. 

Barker. — This and the Gray Eagle are the oldest mines of the dis- 
trict and yielded large amounts of ore in 1881, 1882, and 1883, and 
again in 1891. The mine is situated on Galena Creek, a short distance 
above the town of Hughesville. The vein is in the Hughesville syenite, 
the mine being near the contact with granite-i)orphyry. The ore is an 
argentiferous galena mixed with pyrite and a little chalcopyrite, in a 
gangue carrying calc spar and some barite. The syenite close to the 
vein is much decomposed and the granite x>orphyry encountered in the 
workings is leached and altered to a soft, white, crumbly mass. The 
main body of syenite is fresh and very hard, though traversed by films 
of pyrite. The workings were not accessible in the years when the 
district was visited. The mine was leased in 1898. The property 



INE, ORPENTER CREEK, r 



WEED.) MINES OP BABKEB DrSTRICT. 443 

ioclndes the Barker, Gray Eagle, aod Equator claimB, and is developed 
by a two-onmpartmeBt shaft, with levels and a tannel. 

Wrigkt and Edwards. — Tliia mine is also one of the oldest of the 
district, and one of the best producers. It is developed by a shaft 
said to be 250 feet deep, and Is therefore the deepest mine of the camp. 
The vein is a well-dedued flsBare in the Hnghesville syenite, which in 
part follows the contact with a trap dike. At the time visited the 
minu was closed down and only the tnnnel was accdBslble. This is ctit 
tbroQgb the solid syenite, muniDg in an easterly direction for some 300 
feet before reaching the lode. The syenite shows in this distance eight 
or ten well-defined parallel fractnre or sheeting planes that rtin north- 




east and southwest, or parallel to the vein, and also very slight reticula- 
tion by cross ftitctures. These planes are marked by a few inches of 
leached and whitened rock, but so far as known have not been pros- 
pected. The trap dike is 20 feet wide, and where seen seems to form 
the west wall of the fissure. This rock, as described in the previous 
chapters, is a kersantite. The syenite is so decomposed and leached 
near the ore bodies that it slacks in part to clay when exposed on the 
dump. It contains much pyrite in stringers and disseminated grains. 
The vein is said to average nearly H feet in width. The ore is like 
that of all the mines — an argentiferous galena — and occurs mixed with 
pyrite and zinc blende. Samples of the best ore seen showed 40 to 50 



444 GEOLOGY OF THE LITTLE BELT MOUNTAINS, MONTANA. 

per ceut of lead and 30 oances of silver per ton. It shows banding and 
crnstification and evidences of formations in open cavities, and occurs 
in well-deflned sheets separated by ore streakings or a low-grade ore. 
The property includes the Power, Neptnue, and Joe Hill claims, all pat- 
ented, and in 1898 was bonded by T. C. Powers et al. to the United 
States Smelting and Befining Company as a source of supply for the 
Great Falls Smelter. In 1897 the shaft was 180 feet deep, with the usual 
levels, and a tunnel was expected to tap the shaft at 300 feet. The 
mine was shipping in 1897. 




SCAL£ t 



Fig. 68v— WorklngB of Hay and Edna mine, Barker district. 

Paragon^ May and Udna'y and Oarter.-^These mines are on or near 
the Kibbey divide, where the wagon road from Barker and Hughesville 
to Kibbey and Belt crosses the mountains. The workings are in the 
Carboniferous limestones on the flanks of Olendennin Mountain, where 
the strata are upturned and intruded by sheets of rhyolite-porphyry. 
The workings were inaccessible when visited in 1894 and 1897. ThiB 
Paragon is developed by a 150- foot shaft, and is said to show 2 feet of 
ore. Like similar deposits in limestone, the ore occurs in chambers or 
bunches. The May and Edna was the principal producer in the early 
history of the district. 

Moultonj Tiger J and T. W. — These mines form a group in the guloh 



WSED.] 



MINES OF BARKER DISTRICT. 



445 



ZurfACe of ground 

r 



between Mixes Baldy and Glendenuin Mountain. The first two are said 
to be on the same vein, which is in the broken and folded limestones 
near the contact between the porphyry of Glendennin Mountain and 
that of Mixes Baldy. In the workings visited in 1894 the vein appeared 
in one place to have rhyolite-porphyry or Wolf porphyry on one wall, 
and to be in Barker porphyry. 

The Tiger has been a shipping mine for many years past, and was 
worked even when the other mines were closed down. The ore is 
argentiferous galena, which occurs in lenticular bodies, those seen in 
the workings in 1894 being lenses 3 to 6 feet wide, pinching out rapidly 
in depth. The workings at that 
time comprised a 335-foot tunnel 
and shaft. According to informa- 
tion since received, the tunnel has 
been extended to 550 feet, the shaft 
has been deepened to 120feet (1898), 
and an ore shoot 3 feet wide and 
carrying 30 to 40 per cent of lead, 
with 18 to 20 ounces of silver per 
ton , h as been developed . The prop- 
erty has been worked under lease 
by various parties since 1893, dur- 
ing which time theshipments of ore 
aggregate about a thousand tons, 
in shipments of 10 to 20 carloads a 
summer. 

The Moulton mine, comprising a 
group of four adjacent claims, viz, 
Harrison, Bellfont, Pioneer, and 
Moulton, has also been a shipping 
mine for several years past. The 
ore is a galena, carrying from 20 to 
40 ounces of silver* per ton. It is 
developed by a 100-foot shaft and a 
tunnel, completed in 1898, which 
tops the lode when 1,232 feet long, 
at a depth of 356 feet. This level shows a vein Ijring, it is said, on 
the contact between a porphyry hanging wall and limestone. The ore 
body opened by this adit shows a galena that is 7 feet wide in places. 
Three thousand tons of ore were shipped in 1898. The last ore body 
discovered is said to be 14 feet wide. 

Liberty and Qvsen Esther. — These mines are situated in the syenite- 
porphyry, near its contact with the granite-porphyry mass, whose 
highest point is the peak known as Mixes Baldy. 

The Liberty lode has a course N. 60o to 65° E. and dips at 50° to 
60^ to the south, into the mountain. The ore is the usual argentiferous 




Fig. 6Q.— Ideal transyene ■eotion of type of ooii> 
taot ore deposits, Barker district. 



446 OEOLOGr of the little belt mountains, MONTANA. 

galena, and occurs in a banded mixture with quartz. The pay ore occurs 
in a shoot that is 3 to 4 feet wide and 130 feet long, where it is cut hj 
the upper tunnel. The mine is developed by two tunnels driven along 
the vein, 110 feet apart, and an inclined shaft 100 feet deep following 
the vein from the surface to the lower tunneL The ore shoot was not 
crossed by the lower tunnel when the mine was visited in 1897, but 
was cut for 00 feet. Its vertical extent was still unknown. At that 
time there were 30 men employed and from 2 to 3 carloads of ore a 
week were shipped. This mine was also reported leased to the United 
States Beflning and Smelting Company in 1897. 

The Queen Esther mine shows a quartzose vein carrying a pay streak 
of galena up to 6 inches in width in a well-defined shoot. The vein, like 
the Liberty, dips into the mountain, and at nearly the same angle. The 
vein follows in part along the contact between syenite and a granite- 
porphyry dike which forms the east face in the lower tunnel. In 1897 
the vein was developed by two tunnels, one of 95 feet and one of but 
75 feet in length. Four carloads of ore were shipped in the summer 
of 1897. 

Other claims. — The McEinley was worked in 1898, the development 
work consisting of a 95-foot shaft, which shows a 3-foot ore body of 
galena and spar, and from which samples were reported to assay 40 to 
60 per cent lead and 90 to 120 ounces of silver per ton. This property 
was not visited. The St Louis was not visited, but is said to show a 
vein carrying an ore running 44 -per cent lead, 20 per cent zinc, and 
carrying 37 ounces of silver per ton. It is developed by a 200-foot 
tunnel. The Blackhawk, Ontario, Defiance, and Sunlight claims are 
reported to be promising, but no information concerning them was 
obtainable. 

MIDDLE FORK OF JUDITH RIVER. 

Several discoveries of copper, silver, and gold ores have been made 
in the region drained by the head- water branches of the Middle Fork 
of Judith River. On King Greek one such property-shows an oxidized 
ore carrying gold, which occurs in the limestones beneath an intrusive 
sheet of trachytic porphyry. A small amount of the ore was treated 
in an arrastre run by water power, but no work has been done for ten 
years past. 

The Fairview claim also shows a body of low-grade silver-lead ore, 
along a contact between a porphyry sheet and limestone. 

The Orendal claim has yielded ore carrying copper as well as lead, 
picked samples assaying 10 to 12 per cent of copper and $5 in gold, 
according to published statements in the local press. The development 
consists of a 75-foot adit tunnel and a 50-foot shaft. 



WKBD.] ORE DEPOSITS. 447 

YOGO MINES. 
mSTOBY. 

The discovery of gold in the allavial gravels of Togo Creek brought 
the castomary stampede in 1879. The town of Togo was started, and 
during its brief period of life is said to have contained 1,200 or 1,500 
people, with wellbnilt log houses and the usual accompaniments of a 
mining town. An active season's work, during which several miles of 
ditches were built and considerable amounts of gravel passed through 
the flumes, was followed by a clean-up so meager as to discourage further 
operations, and the boom collapsed. The town was all but deserted in 
1883, and in the many years that have since elapsed the log cabins have 
been carried off by settlers to the treeless country of the Judith Basin, 
until to-day a half dozen or fe^o are all that remain to mark the spot. 
The locality was visited in the summer of 1889 by G, E. Swallow,^ at 
that time State inspector of mines. In his report he states that at 
the Weatherwax mine, on Skunk Creek, he found a small mill having 
a crusher, Hunter oscillator, and Frue vanner, working 6 to 15 tons of 
ore a day, the ore coming from the Gold Belt mine and yielding tl5 
per ton. Four men were employed at the mine and mill together. An 
arrastre, run by an overshot wheel, was also running at Yogo on ore 
from the T. C. Power mine. In 1893 a dozen or more men were living 
there. In 1897 only three or four men were in the neighborhood. 

The alluvial gravels, though too poor to pay a return in the early 
days, have been worked at intervals ever since, though never by more 
than half a dozen men at a time. In 1897 but two men were thus 
employed, and they informed me that they made fair wages from the 
little strip of hillside gravel they were then washing. Panning a little 
of the gravel, the gold was found to be bright and clean, without 
quartz, rather rough, and in flattish grains ; no scale gold being noticed. 

During the single season of its existence Yogo was the center from 
which prospectors streamed out over the neighboring region in every 
direction. Many discoveries of minerals were made at this time and 
in subsequent years, when both Great Falls and Neihart contributed 
men who diligently sought for mineral deposits in this part of the 
Little Belt. Much work was done — seldom wisely — in this vicinity, but, 
so far as the writer has been able to ascertain, the total production of 
the mines at Yogo proper does not exceed one or two thousand dollars. 
The Weatherwax mine is reported to have yielded as much as this, but 
no definite information on this point could be obtained. On Running 
Wolf Creek two mines, the Woodhurst-Mortson and the Sir Walter 
Scott, have produced considerable ore, which was freighted to Great 

^ Beporto of the inspector and deputy inspector of mines for six months ending Noyember 30, 1889, 
by G. E. Swallow, inspector, and J. B. Trevarthen, deputy inspector: Joomal Pabllshing Company, 
Helena, Montana, 1890. 



448 GEOLOGY OP THE LITTLE BELT MOUNTAINS, MONTANA, 

Falls, bat definite returns conld not be obtained. Several pro8x>ects 
have also yielded small amounts of silver-lead ore from Banning Wolf 
Valley and Lion Greek. 

GENERAL OCCUBBENCE OF ORES. 

The ores of the .mines at Yogo proper are generally very low grade 
or occur in deposits too small to work. Galena, pyrite, cbalcopyrite, 
and their oxidation products are the chief minerals. The deposits 
occur in the altered limestones at or near the contact with the granular 
rocks of the Yogo stock or the sheets and dikes connected with it. 
The *' stock ^' contact can be very easily traced by the prospect pits, 
whose dump heaps of white marble are common features of the contact 
zone. The minette dikes cutting the limestones have also been quite 
often prospected, as their contact is frequently marked by bands of 
pyrite. The under contact of intrusive sheets of porphyry is also very 
often mineralized. There is a widespread miueralization, but, so far as 
developments show, the ore is too low grade to be workable under 
present conditions. The ores at Yogo which have been worked occur 
beneath intrusive sheets of porphyry, the ore being a replacement of 
the limestone and in part an impregnation of the leached and altered 
porphyry itself. The cursory examination made of these properties 
does not enable me to say what their future may be. Geologically, 
their occurrence is a favorable one. It is perhaps safe to predict that 
the oxidized ores found in several localities may yield a profit by the 
cyanide or some equally cheap process of extraction. 

The south contact of the Yogo stock was located for a distance of 10 
miles, being known as the " blue lode." So far as known, no ore has 
been shipped from any of the workings. At the head of Yogo Creek 
a prospect shaft near the trail shows decomposed lead ores and a rusty 
gossan, said to be slightly auriferous. West of Yogo Peak an old log 
shaft house stands on the contact between the main shonkinite mass 
and the altered limestones. No ore was seen here, but the dump heap 
shows shonkinite carrying stringers and films of pyrite. The northern 
contact of the stock has also been more or less prospected, but with- 
out success. The Lion Creek prospects (Judith mining district, unor- 
ganized) are on porphyry contacts some distance from the syenite 
mass. 

NOTES ON INDIVIDUAL PROPERTIES. 

The following notes were obtained during brief visits to the district 
in 1893-94, though few workings were accessible, and the few facts pre- 
sented are given for the reason that no other information whatever is 
obtainable. 

Quaker City and Bella, — These claims are situated on the open slopes 
of tbe basin at the extreme head of Elk Gulch. At this place the Car- 
boniferous limestones are fractured and show numerous sheets of 



wisBD.] TOGO MINES. 449 

syenite-porphyry, which are similar to, and probably offshoots from, 
the main mass of the mountain top. Dikes of dark miuette are also 
seen. The claims have been worked intermittently since 1892, several 
shallow prospect shafts, one 40 feet deep, being sank, and a tunnel sev- 
eral hundred feet long driven into the mountain side. Ore, said to 
contain free gold and to assay well, is reported to have been found in 
bunches, but I have been nnable to find that any of it has been shipped, 
and the samples gathered by myself and assayed for this oflQce carry 
too little value to be classed as ore. When last visited — 1897 — the writer 
was informed by the placer miners that the claims were no longer worked. 
When visited in 1893 the owners, Charles Ferris and M. E. Dornblut, 
were driving a tunnel on a dike of minette. The shaft showed 5 to 6 
feet of mineralized material said to be low-grade ore, but the ore body 
was cut off by porphyry. The tunnel was at this time about 300 feet 
long. The minette being soft and easily mined, it was followed, though 
the so-called ore was but a band 2 to 3 inches in width that was found 
at the contact of the dike and encasing limestones. The course of the 
dike is northeast and the dip 70^ W.; the trend is not constant, but 
somewhat sinuous. The limestones (Cambrian?) occur in layers 3 to 6 
inches thick, with shaly x>ortions, and show contact metamorphism 
alongside the dike, where they are altered to marble and a coarse 
aggregate of garnet, pyroxene, and calcite. The ore streak occurs 
with a thin clay selvage, which is not always present along the contact 
plane. The so-called ore, found at both the tunnel and the shaft, is 
pyritous, and of too low grade to work, picked samples gathered by 
the writer giving no gold and 0.05 ounce of silver as a result of careful 
assays made by C. E. Monroe. 

California, — This claim is situated on top of the ridge at the head of 
Skunk Creek, near the contact between the syenite and limestones. 
The vein matter is said to be 10 J feet wide, to have been developed for 
50 feet in depth, and to carry $7 per ton in gold. It is owned by the 
Judith Valley Mining and Milling Company and is worked by Louis 
Pepin, R, Giroux, and Joseph Sutler. 

Christopher Colombo. — This is the name given a prospect on the 
Bandbox Mountain side of the divide, at the head of a fork of Wolf 
Creek. Tlie workings comprise a 500-foot tunnel, a 75-foot shaft, and 
two crosscut levels. The lead shows only silver-lead ore, occurring in 
nearly flat beds of limestone. 

Weathertcax. — In the early eighties, when the Togo district looked 
most promising, this property was extensively prospected and a 5-stamp 
mill erected. The workings embrace tunnels driven into the slopes 
west of the gulch and tapping a lode said to be a contact deposit 
between limestone and a porphyry sheet. It was not accessible when 
the district was visited. 

r. C. Poicer, — This claim is on a sheet of porphyry showing on both 
sides of Skunk Creek and on Elk Creek. Considerable prospecting 
20 GEOL, PT 3 29 



450 GEOLOGY OF THE LITTLE BELT MOUNTAINS, MONTANA. 

has been done at varioas periods. The ore is a rotted, oxidized quartz, 
showing no minerals, bat said to carry low values in gold. A minette 
dike cuts across the claim. The property has yielded Kcveral hundred 
tons of oxidized ore, treated in the Yogo arrastre. In 1889 the ore is 
said to have assayed $15 per ton. The property was worked in 1893, 
but is now shut down. 

Little Emma. — This claim, situated west of the Blue Dick, shows a 
pyritous gold ore. Two 28-foot shafts and a 300-foot tunnel prospect 
the lead. The tunnel is 12 feet wide, and shows ore on contact of lime- 
stone and syenite that is said to assay $17 per ton but is not free 
milling. 

Blue IHclc. — This claim, on the slopes west of Skunk Creek, shows 
mineralized contact dipping at 45^ into the mountain. The ore is mainly 
pyrite carrying free gold with copper staining. The ore occurs at under 
contact of syenite- porphyry and limestone, and shows some brecciation. 
This property was worked by P. H. Hughes in 1893, the ore being 
crushed and amalgamated in the arrastre at the Yogo settlement. 
The ore carried a small amount of silver and gold. A sample of the 
oxidized **ore'' assayed for the Survey laboratory by 0. E, Monroe 
yielded only a trace of gold and 2.2 ounces of silver. One of the sul- 
phide ores gave $1 in gold and less than an ounce of silver to the ton. 

Climax, — This claim, like the Christopher Colombo, is on the Wolf 
Creek side. The ore is a mixture of manganese, pyrite, and galena, 
and carries considerable lead and some gold. 

Bill Cummins, — This prospect yields high-grade lead ores (128 ounces), 
but in small pockets. 

In addition to the prospects mentioned, there are many more in the 
vicinity of Yogo, some of which were visited, although the workings 
were generally so shallow that little could be said of the claim. 

RUNNING WOLF DISTRICT. 
OEB DBPOSITS. 

The ore dei)osits of the Eunning Wolf Creek district occur in lime- 
stone, and, as is so commonly the case with the ore deposits in this rock, 
they provied irregular in form and limited extent. That they occur on 
fracture planes either on or near eruptive contacts is apparent from 
their surface relations, and the ores and waste rock of the mines show 
that they are replacement deposits. So far as observed they occur on 
lines of faulting or disturbance too slight to show on the geologic map. 
The ores consist of galena and its alteration products. At one place, 
the Walter Scott, the ores are " dry" — i. e., without lead. The common 
ore is galena mixed with a jaspery gangue. The replacement is shown 
by a gradual passage from ore into jasper, and this into a silicified lime- 
stone showing knots and bunches of silica, and this into unaltered lime- 
stone. The jasper is really a mixture of chalcedonic or cryptocrystalliue 
silica, with a little quartz in small bunches and lining cavities. It is 



WKED.] MINF8 OP RUNNING WOLF DISTRICT. 451 

asually brown or reddish from iron oxide, and quite plainly occars as a 
replacement of limestone nsaally encasing the galena ore bodies. The 
caves and water coarses seen about such deposits are plainly of recent 
origin. 

The Sir Walter Scott, Mountainside, and Woodhurst are the only 
mines that have been producers. The region is easily accessible by 
wagon road, but is so remote from a railroad that low-grade ore bodies 
can not be worked at a profit The limited size of the ore bodies and the 
lack of well-defined veins render the future of the region uncertain. 
None of the workings were accessible when the region was visited in 
1894, and the properties are, it is understood, still idle. 

NOTES ON THB MINES. 

Woodhurst- Mortson. — This property shows the most extensive devel- 
opment work of any mine of this district. The mine is situated on the 
sonth side of Running Wolf Greek, in a recess or niche cut in the steep 
limestone slopes, some 600 or more feet above the creek. The ore now 
seen is chiefly galena in a Jaspery gangue showing much calcite. The 
mine buildings are shown on PL LXVIII, which also shows the tower- 
ing walls of massively bedded white limestone that rise behind it. 
These rocks dip southward, the synclinal folding being due, it is 
supposed, to the Steamboat uplift. The limestones are intruded by 
a sheet of syenitic porphyry, whose talus slide is seen alongside of 
the mine on the east. The material of the dump heap indicates that 
this porphyry is encountered in the underground workings of the mine. 
The ore occurs in a contact vein between the porphyry and the limestone. 
The vein is 2 to 7 feet in width. The workings comprise a shaft said tx) 
be 250 feet deep, together with levels aggregating nearly 4,000 feet in 
length. In 1880 it had yielded 500 tons of carbonate lead ore, carrying 
65 per cent lead and 30 ounces of silver per ton, with no zinc' A small 
smelter was built on the main creek, near the mine. It was run only 
a short time, and was not a financial success. Over $20,000 worth of 
work is said to have been done on the property. 

Alongside of the Woodharst-Mortson is a low-grade body of oxidized 
iron-copi)er ore, carrying a few ounces of silver. When visited in 1894, 
this had been prospected by a shaft 50 feet deep. 

8ir Walter Scott — This mine is situated on the fiat, elevated summit 
of a spur of Steamboat Mountain, north of the Woodhurst mine. The 
vein is said to be 2 to 5 feet wide, and is a contact deposit between 
limestone and porphyry.^ The deposit was worked for a short period, 
about 100 tons being shipped, on which a few thousand dollars was 
realized. The character of the ore is unlike that of the Woodhurst- 
Mortson, being a free milling silver ore, carrying 60 to 70 ounces of 
silver per ton and containing bunches of very rich ore. The geologic 
structure of the locality has already been noted. The limestones are 



> Report State Inspector of Mines, 1889, p. 23. 'Ibid., p. 24. 



452 GEOLOGY OF THE LITTLE BELT MOUNTAINS, MONTANA. 

thinly bedded and belong to tbe base of the Garboniferone series. The 
beds dip away front Steamboat Mountain, and are iutraded by several 
sheets of jiorpliyry and cut by trap (minette) dikes trending to Yogo 
Peak. The mine is accessible by a fairly good wagon road. The prop- 
erty in equipped with comfortable log bnildings for boardiug house, 
blacksmith shop, shaft house, etc The andergroand workings were not 
fWM»ssible wheu the property was visited by the writer. Miners familiar 
with the workings state that the shaft is 300 feet 
deep, with the usual levels. It is also reported that 
the ore occurred near the surface, and that the ore 
I body is woi^ed out. The levels tup a large cave, 

whose bottom is filled by a great pool of water. The 
I malarial seen on the ore pile shows galena, mixed 

with copper pyrite and duorite. Vanadiuite col- 
lected from this mice by Mr. R. H. Ohapman is well 
crystallized; the specimen has been kindly studied 
pra. TO.— v»n«diiiite for me by Mr, H. H, BohiusoD, and the crystals have 
oryatfti. ftwn Sir beeu drawD uuder the direction of Prof. S, L. Pen- 
w.iter soo.tn.in.. ^^j^^ ^^ ^^^ Sheffield Scientific Sciiool, Yale Univer- 
sity. Mr. Kobinson furnishes the following notes: 

The brownish crystals have the form of a hesagonal prism, nsually 
with slight truncations of the edges and a tendency to taper toward the 
extremities (fig. 70), thus giving the crystals n barrel like habit which 
is not uncommon with this species. The surfaces of the prisms are 
Qoeven, while the basal planes are rough and usually show a slight six- 
sided depression. On a few of the very small crystals the faces were 
sufficiently perfect to admit of measurement with the rejecting goni- 
ometer. The habit of these crystals is shown by fig. 71, the forms 
being the prism m (1010), the pyramid of the 
first order x(lOH), the pyramid of the second 
order v (1122), and the base e(OOOl). 

Mountainside. — This mine, near the Sir Walter 
Scott, was not visited. The property was being 
worked in 1804, and ia said to have yielded some 
830,000 to 840,000 worth of ore,, paying "from 
the grass roots down." 

Yankee Qirl. — This claim, discovered and (.,,, 7i._VMi»diDit« irvKtai. 
worked in 1894, is a deposit of argentiferous fromsirWsitersootimine. 
galena and its oxidation products found in lime- 
stone. The claim is streaked on the steep slopes south of Running Wolf 
Creek, about a mile above its forks. The ore body is clearly a replace- 
ment. No definite flssnre or fault plane was recognized, but the ore 
occurs in a zone of shattered limestone parallel to the bedding. The 
central part of the ore body ia a nearly pure galena, which grades into 
a mixture of brown jasper and galena, forming the outer part, and this 
is incased by an irregular crust of jaspery quartz which grades into tlie 



WKBD] DEY WOLF OB LION CEEEK DISTRICT. 453 

limestone. Several tons were shipped in 1894, the ore being sorted, 
sacked, hauled on ^< stone boats" down the steep trail, and thence carried 
by team to Oreat Falls. The ore body was G feet thick, 200 feet long, and 
was exposed for a width of 20 feet. The lens dips soath at 30^, being 
nearly conformable with the inclosing limestones. Several other claims 
in the vicinity — the Ada, Gaficade, and Keystone, north of the creek, 
and the Lookoat, one- fourth mile east of the Yankee Girl — show similar 
occurrences of lead ores in limestone. 

Eureka. — Tliis property is situated at the head of the creek, directly 
under the divide to Lion and Dry Wolf creeks. Small pocket>8 of silver- 
lead ore were found, but the mine is very generally regarded by the 
miners of the region as a ^< wild cat " proposition. The limestones are 
nearly horizontal, being the center of a synclinal fold or basin. They 
are cut by a number of dikes of basaltic rock running across the ridge, 
and the ore deposit is supposed to be connected with these or similar 
fissures in this zone of fractured limestones. The mine is equipped 
with a GOhorsepower engine and hoist. 

DRY WOLF OR LION CREEK DISTRICT. . 

The ore deposits thus far prospected in this district occur on the 
northern side of the Yogo divide, and properly belong to that district. 

The only claims examined are on Lion Creek, on the slopes south ot 
Big Park. The alluvial deposits at the mouth of Lion Creek contain 
gold and have been placered with more or less success at various times. 

Lion Creek is a small stream draining the mountain side directly 
north of the settlement of Yogo and cutting a deep trench in the sedi- 
mentary rocks. The strata dip eastward at a gentle angle and are 
intruded by sheets of porphyry. The ore deposits thus far discovered 
appear to be contact deposits, and they occur beneath these porphyry 
sheets. 

At the head of the creek small ore bodies are found along the con- 
tact between the limestones and the big syenitic stock, and in some 
cases on dike contacts, but no producing mines have yet been devel- 
oped. Iron ores also occur at the contacts between limestone and por- 
phyry in Iron Gulch. Large bowlders of this material seen near the 
Lion Creek settlement consist of very pure limonite carrying 40 to 60 
per cent of iron. 

The only property as yet developed is the Pierce and Higbee mine, 
^ embracing the Dry Wolf, Gold Dust, and Anything claims, which have 
been surveyed and patented. The property was deserted and the 
tunnels were not accessible when the place was visited. The ore heap 
showed galena and chalcopyrite, with their decomposition products. 
The deposit appears to be on a contact with a sheet of porphyry 
intruded in the dark-brown Jefferson limestones. 



454 GEOLOGY OF THE LITTLE BELT MOUNTAINS, MONTANA. 



MOUNT TAYLOR MINES. 

A lead discovered on a high shoulder a short distance below the 
summit of Mount Taylor has been developed by a shallow shaft, over 
which a log shaft house has been built. The vein is a fissure in lime- 
stone, running east and west, and carries galena and its decomposition 
products with jasper and oxidized copper ores. A road which has 
been cut through the forest on the south slopes of the mountain affords 
easy access to the mine, but sufficient development work has not yet 
been done to prove the property. , 

YOGO SAPPHIRE MINES. 

The Yogo sapphire mines, which are to-day the most valuable gem 
mines of the country, are situated in Fergus County, Montana, 13 miles 
west of the town of Utica. The locality is not accessible by railroad, 
but can be reached by wagon road from Utica, from which town a stage 
line runs daily to the railroad at Great Falls. There is also a short cut 
over the mountains by horseback trail to Neihart, the termination of 
the Belt Mountain branch of the Oreat Northern Railway. 

These mines are an illustration of the good luck which sometimes 
occurs in mining. In 1895 a placer-mining company was organized to 
work the gold-bearing gravels found in pockets upon the limestone 
bench land lying east of the Yogo fork of the Judith Biver. A ditch 
costing $38,000 was built and the waters of Yogo Creek were carried 
upon the bench land, with a head of 300 or 400 feet. The first season's 
work demonstrated that the gravels would not pay as gold placers, as 
a clean-up of but $700 was made as the result of the eutire season's 
work. The sluice boxes, however, contained a large number of blue 
stones, which were identified in November, 1885, as sapphires. 

A cigar box full of the gems collected at this time is said to have 
been sold to Tiffany & Co. for $3,750. Preparations were immediately 
made to work the gravels for the sapphires, which seemingly occurred 
in great abundance in certain parts of the field. It was believed at 
first that the gems, together with the gold, came from Yogo Gulch, 
and that the gravels represented an old and high channel of that 
creek. Their local derivation was discovered by John Ettien, a settler 
in the neighboring valley of the Judith Kiver, in February, 1896. 
While prospecting the ground above the placer he noticed a fissure in 
the limestone, whose soft filling resembled the outcrop of a vein. Two 
claims were located on it and some of the dirt was taken to the nearest 
stream and washed. The blue sapphires in the earth were noticed, but 
it was not until they were shown to the placer workers that their value 
was known. The importance of this discovery was recognized by Mr. 
Hoover, one of the owners of the placers, and he and his partners at 
once located the sapphire lead. It is now known that the gems occur 
in a dike of trap rock cutting white or gray limestones. This dike has 







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WBBD] YOGO SAPPHIRE MINES. 455 

now been traced for a distance of 5 miles from the meadows of the 
Jadith Biver westward to the canyon of Yogo Greek. The entire known 
extent of the dike has been located as a lode and a large number of 
claims have already been patented. 

The mines are situated in the center of a broad and open basin 
inclosed on three sides by the Little Belt Mountains, whose wooded 
slopes show white limestone outcrops that look like banks of snow. To 
the east high foothill ridges shut ii) the basin firom the open plains coun- 
try beyond. The Judith Biver flows through the center of the basin, 
its three forks uniting at the base of the mountain slopes to the west 
The most northerly fork is Togo Creek, and f^om it the mines take 
their name. The claims are located on the bare bench land lying north 
of the main stream and east of Yogo Greek. The surface has a gen- 
eral easterly slope, having a descent of 800 feet from the brink of 
Yogo Ganyon to the meadow land of the Judith. 

Several dry drainage ways traverse the sapphire basin, cutting gulches 
before they are lost on the alluvial bottom lands. The general aspect of 
the region is shown in PI. LY, B, n&ade from a photograph taken at the 
mine settlement, looking westward up the largest of these gulches. In 
a general view the sapphire locality shows rolling hills whose summits 
and slopes are formed by the bare and white surface of limestone. The 
intervening gullies are well grassed, and the gentle slopes show mat- 
like growths of ground cedar. Occasional small groves of stunted pine 
are seen in a few places, but the general lack of vegetation is in marked 
contrast to the wooded mountains near by. 

The geologic structure of the basin consists of a broad, basin-like 
fold, opening eastward. It is a synclinal basin, lying between the sharp 
uplifts on the north and south. While the general structure is thus 
quite simple, the massive limestones show many minor undulations, 
and it is largely to them that the present relief of the surface is due, 
the soft, red earths that overlay the limestone having been carried off 
from the greater part of the district. As already stated, the gems are 
found in a dike of igneous rock cutting the limestones. This dike is 
recognizable upon the surface only by a slight depression, a foot or so 
deep, emphasized by grass and herbage where it crosses the slopes of 
bare limestone. In the hollows and gulches the outcrop is recogniz- 
able only by the line of gopher and badger holes which mark its extent. 
No solid outcrops of the dike rock occur, as it alters on weathering to 
a soft clayey material. This is why its course is marked by gopher 
heapings, since the adjacent limestone is too hard and undecom posed 
for these animals to burrow into. These holes, indeed, proved the 
meafis of locating the dike when the claims were staked, and many of 
the finest stones yet obtained were picked up from the heapings made 
by these animals. 

The dike has a general trend S. 56^ W. It is from 3 to 6 feet 
wide, and, so far as shown by the workings, is vertical. It has been 
traced east and west from the meadow lands of the Judith to the walls 



456 GEOLOGY OP THE LITTLE BELT MOUNTAINS, MONTANA. 



of Togo Canyon, where it apparently ends, as it is not seen in the 
limestone walls of that canyon, but a few yards west of the crest. It 
has been found west of the creek bottom, however, at a lower eleva- 
tion, and, as shown later, it is not seen in the exposure because it did 
not break quite so far through the limestones at this point. The dike 
walls are rough, but not especially irregular; they show the bedded 
limestones slightly indurated by the intrusion. The dike material is 
somewhat variable in appearance. Near the surface it consists of a 
coarse breccia of limestone and shale fragments cemented by the igne- 
ous rocks. Where the upward termination of the dike is seen, at the 
westernmost workings, the top part of the dike is a blunt wedge and 
the material consists chiefly of these rock fragments, as shown in 

fig. 72. In the main work- 
ings, 2 miles farther east 
and several hundred feet 
lower in elevation, the ex- 
cavations show a similar 
breccia (PI. LXVII, B) at 
the surface, but the size and 
number of the fragments 
decrease with the depth. 
This is believed to be the 
fragmeutal material from 
thefissure walls, which has 
been floated upward as the 
molten rock rose in the fis- 




T~ 



s^v$?r^j 



I 



^flilSl4 



L\ 



] I 



I 



I 



( : . 

Fio. 72.— SectioD of upper limit of Bappbi re- bearing dike, 

wall of Yogo Canyon. 



sure, like chips on the surface of a stream of water. 

The workings were in 1897 entirely in altered rock, the shaft being 
at that time only 60 feet deep. (PI. LXVII, A,) At this depth it had 
passed out of the zone of surface alteration, and the ocherous, yellow 
clay was replaced by a blue clay which reminds one very much of the 
description given of the Kimberley diamond matrix. Throughout this 
clay there are bowlders of the unaltered rock, with kernels of solid 
material, which have been broken open and furnish specimens for 
petrographic description. In the shaft the entire width is 11.4 feet 
from wall to wall. Of this, 3.7 feet on the south wall was of solid 
minette, checked with calcite seams, but otherwise comparatively 
fresh and unaltered. The clay does not always bear the same relation 
to the walls, but jumps across from one side to the other. In the work- 
ings it is at once seen that the upper part of the dike is largely a brec- 
cia — that is, it consists of a mixture of dike material and the limestone 
fragments. It appears to be somewhat near the apex of the dike. 
The questions naturally arise, how far the relative abundance of these 
limestone fragments have influenced the formation of the sapphires, 
and whether the sapphires will continue in depth or not. For this rea- 
son a very careful examination was made of the shaft and of the blue 
clay which was seen there. 



WKKD.] TOGO SAPPHIRE MINES. 457 

The writer was unable to find any sapphires in the blue clay itself, 
bat was assured that when washed the blue clay yielded a fair pro- 
portion of the gems. There seems reason to believe that this blue clay 
will require a special treatment, inasmuch as it is very tenacious and 
does not yield readily to ordinary washing. In all probability it will 
have to be handled as the blue clay of Kimberley is handled for the 
extraction of diamonds. The gems in this blue clay are said td be much 
finer than those in the more altered material. This one can readily 
believe, because the sapphires in the altered rock are so checked and 
fissured that even when found in place they split up into chips of no 
value for cutting. In the blue clay the gems would not have been sub- 
jected to the strains and processes incident to the decomposition aiid 
expansion of the rock, and therefore should yield a very much larger 
proportion of cuttable stones. 

At present the greatest amount of material is derived from an open 
cut, some 400 or 500 feet in length, upon the highest part of the claims. 
Three windlasses are employed, and men are at work with pick and 
shovel, digging the soft, yellow earth and throwing it into shallow 
tubs, which are hauled to thesurface, where the earth is thrown into ordi- 
nary dump carts. It is then hauled about a quarter of a mile to the 
ditch and shoveled directly into 'sluice boxes. In the sluices the harder 
bowlders and the balls of blue clay are carried through the rifles and 
accumulate upon the tailing dumps. Probably not over 33 per cent 
of the gems are recovered in this first washing. By exposure to the 
atmosphere and by the ft*equent freezing and thawing which takes 
place in this fi'osty climate the blue clay slacks and disintegrates, so 
that the material can be washed over, with a further extraction of gems. 

At the time of the writer's visit some 20 loads, each approximating a 
square yard of earth, gave between 1,200 and 1,500 carats of cuttable 
stones. The value of the stones in London market is $6 a carat for the 
first quality, $1.25 a carat for the second quality, and 25 cents a carat 
for the gleanings. The larger stones found weigh, when cut, 4 to 5 
carats, and are then valued at $75 a carat. 

In sluicing the earth the process followed is similar to that of washing 
gold-bearing gravels, but no mercury is used. The gems drop between 
the riffles and are obtained at the close of each day's work by turning off 
the water and lifting the racks. This material from the riffle is then sifted 
and panned by hand, to get rid of valueless materials. The result is a 
concentration of the sapphires, together with grains of pyrite,from which 
the gems must be picked by hand. This pyrite is the only other min- 
eral found with the gems. It has been assayed for the writer and found 
to contain a little silver, copper, and nickel, but no gold. The pyrite is 
in moss-like aggregates and not in well-shaped crystals. The clay con- 
tained a few hexagonal crystals, which had the form of corundum, but 
consisted of some decomposition product and showed no traceof the orig- 
inal mineral. The shaft has been sunk to a depth of 300 feet, and con- 
siderable masses of corundum are said to have been found at that depth* 



458 GEOLOGY OF THE LITTLE BELT MOmrTAINS, MONTANA. 

As already stated, in the apper part of the dike the rock is largely 
altered to a yellowish clay in which only the fragments of sedimentary 
rock are recognizable. At depths of 20 to 40 feet below the surface 
bowlders of the igneous rock are found. They are clearly nucleal 
masses not yet decomposed by surface waters. In some places con- 
siderable masses of the solid dike rock are also found, and every 
gradation may be observed, from the tough and resistant dark-gray 
dike rock to the soft yellow clay into which it finally decomposes. 

The freshest material has been carefully studied and proves to be a 
lamprophyre rock. In the hand specimen it is dark gray, has an 
uneven, rough fracture, and is evidently a tough and heavy trap rock. 
The rock shows numerous angular inclusions of white or pale-green 
color, which vary in size frt>m those of microscopic dimensions to 
masses a foot or more across. The large inclusions consist chiefly either 
of quartz or of crystalline calcite surrounded by a rim of pale-green 
pyroxene of small but variable width. Some of the smaller inclusions 
consist entirely of this green pyroxene, and it is also recognizable in 
the calcite center of the larger pieces. 

The dike rock itself is very dense, dark colored, and glistens with 
the light reflected by innumerable flakes of biotite, of which the rock 
is seemingly comi)osed. Pyroxene is recognizable to the eye. A few 
scattered tablets of brown mica — ^the largest seen a quarter of an inch 
across — are the only phenocrysts. The rock has been described by 
Professor Pirsson,^ and the results of his microscopic study are given 
in the following paper. Under the microscope the rock is seen to 
consist of biotite and pyroxene in closely crowded masses. There is no 
feldspar present, but a small amount of interstitial kaolin-like material 
occurs. The rock is most like a mica-pyroxene-analcite-basalt. 

The sapphires occur embedded in this rock in well-formed crystals 
and in rounded masses -^ inch to f inch across. They were found in 
the freshest unaltered rock obtained, as well as in the altered decom- 
I>osed material. They show no connection with the included fragments 
and are always distinct and sharply defined. Their crystalline form is 
fully discussed by Pratt,^ a summary of his work being given in Pro- 
fessor Pirsson's report. 

The occurrence of the sapphires shows quite conclusively that they 
were formed in the dike rock itself. Their origin is believed to be the 
result of the action of the molten igneous rock upon fragments of clay 
shale or impure limestone, taken up by the former in its ascent, as 
suggested by Pirsson. This implies the complete assimilation and 
digestion of such material in the igneous rock. It is apparent from a 
study of the sapphires themselves that they crystallized out of the 
rock, but it is also evident that partial resorption took place before 
final consolidation, since many of the sapphires show deeply corroded 
surfaces; others are rounded masses whose crystalline outline is 

1 Am. Joar. Sd., 4th series, Vol. IV, 1897, p. 421. * Ibid., p. 424. 



IURST-M0RT50N MINE, RUNNING WOLF CREEK. 
.( cl.m wsn t.ck of build. ng. .iH ib.uBtiy to sumrnil o) ridgi 



WETO] IRON ORES. 459 

nearly effaced, while many of them are sarrounded by a blackish crast. 
If the molten rock could dissolve the sapphires at this stage, it is cer- 
tain it could dissolve clay shale as well. The dike undoubtedly extends 
a considerable distance in depth. The limestones are 1,000 feet thick 
in this vicinity, and rest upon nearly a thousand feet of Cambrian 
shale. The Belt formation is believed to be absent, but the Cambrian 
beds contain almost every possible variety of calcareous, siliceous, and 
argillaceous rocks. It is remarkable, however, that, though sapphires 
are found throughout the entire extent of this dike, they do not occur in 
the parallel dike of nearly similar rock that cuts the limestone 600 feet 
north of the sapphire claims, nor have gems been found in the augite 
minettes that occur as dikes and sheets in the shales of the Quadrant 
formation southeast of the mines, along the border of the Judith Biver 
bottom land. 

A parallel dike about 600 feet north of the sapphire dike weathers 
to a sandy, micaceous material that probably represents the outcrop of 
a minette dike cutting through the limestone. A few kernels of partly 
altered rock were found where prospecting had been done on the dike. 
The rock and its debris show no sapphires, although many cart loads 
of the dirt have been washed from different x)oints along the outcrop. 
It seems probable, however, that this dike is the source of the gold 
found in the placers, as the colors can be traced up the gulches to the 
outcrop of the dike and never occur beyond it. 

IRON ORES OF THE LITTLE BELT MOUNTAINS. 

Deposits of limonite and hematite are found at a number of localities 
in the Little Belt Mountains, and tbe small amount of development 
work which has been done thus far shows that they are of sufficient 
purity and extent to be workable and will some day be utilized in 
Montana furnaces. The material is very dense and hard, resisting ero- 
sion better than any of the rocks of the region, so that the float from 
such deposits is often very noticeable in the drift and gravels of the 
region. The occurrence of the ores is noted by Eldridge, who says:^ 

In the Judith Basin, at the southwestern edge of the map, there occurs, in addition 
to the stratified rocks already mentioned, a narrow belt of granite, width undeter- 
mined, accompanied by a band of magnetic iron ore, the relations of which to the strati- 
fied rocks were, for various good reasons, left undetermined. The ore is steel gray, 
strongly magnetic, unless, as in few instances, exposure to atmospheric influences 
has altered it to limonite. It outcrops in heavy masses from one-half ton to 20 tons 
weight, the outcrop varying in width from 2 feet to what would seem from the float 
to be at least 15 feet, though perhaps this may be too great a width. The question 
of width could only be solved by systematic prospecting. The trend of the ore body 
is, as nearly as could be determined, about north 70^ west. lu the vicinity of the 
head of Wolf Creek it was traced by float for 2 miles, but with an intervening space 
of a mile where it could not be found, owing perhaps to a cover of soil and debris. 
At one point on its course it was traced continuously for 3,000 feet. The guide who 
was with the party, and in whom every confidence as to truth in this matter can be 



1 Tenth CeDSos of the UDited States, VoL XV, p. 751. 



460 GEOLOGY OP THE LITTLE BELT MOUNTAINS, MONTANA. 

placed, states that he has traced it from 3 to 6 miles farther to the westward, or mid- 
way between the head of Wolf Creek and the Barker mining district, seen on the 
general maps of the Survey. Magnetic iron ore is also reported on good authority 
in the Barker mining district itself. 

The only other points at which iron ore was observed on the border of the Judith 
Basin were in the Judith Mountains, north of the Maiden mining camp, where is 
also found a limited amount of magnetic iron ore, together with a mass of bog ore 
(limonite), resulting from the breaking down of the magnetite, which occurs higher 
up on the mountains. 

The locality meDtioned by Eldridge is undoabtedly Woodhurst Moim- 
taiD, and the contact referred to is that of the Woodhurst stock. 

From the general study made of the region it is evident that no gen- 
eral lead extends across the mountains, but that lenticular bodies of 
ore occur at the contacts of many of the masses, of igneous rock. They 
were observed on Woodhurst Mountain, on Iron Creek, on a branch of 
Lion Oulch, at a point north of Yogo, on the mountain top above that 
place, and on Thunder Mountain. Iron-ore float was observed at 
other localities, and there seems little reason to doubt that it is of 
common occurrence about most of the larger igneous intrusions. 

Woodhurst iron mine, — The Woodhurst Mountain deposits have, so far 
as known, been prospected at only one place. These workings are on 
the southeastern flank of the mountain, at the head of a small drainage 
tributary to Galena Fork of Eunning Wolf Creek. The locality, though 
remote from a railroad, could be made readily accessible by wagon road. 
The deposit has been opened by surface cuts, exposing a continuous 
mass of hematite in a trench 40 feet long, cut at right angles to the 
contact and hence across the lens. The deposit lies at the base of a 
projecting tongue or offshoot of the porphyry body of Woodhurst 
Mountain, and is clearly a contact deposit. The porphyry is some- 
what altered and rotted, but the Carboniferous limestones show only 
slight alteration. The claims are 600 feet above the forks of Bunniug 
Wolf Creek. An analysis of the ore, made by Dr. W. F. Hillebrand 
in the laboratory of the United States Geological Survey, gave FejOs, 
83.7 per cent; FeO, 6.4 per cent; Mn, none; Ti02, none; P2O5, trace. 
The remainder is mostly silica. The ore is strongly magnetic, and is 
evidently a mixture of magnetite, hematite, and the hydrated oxides 
of iron. 

Iron Creek or Lion Oulch. — ^These deposits were not examined, but 
the character and amount of ore seen in the gulch warrants an exami- 
nation of the locality. 

Yogo deposits. — A lens of quite pure hematite 2J feet thick was 
observed on the mountain ridge east of the head of Skunk Creek, near 
Yogo. The deposit occurs at the contact between limestones and a 
dark-colored, coarsely granular rock (shonkinite). 

Thunder Mountain iron mines. — On the north side of Thunder Moun- 
tain, al the head of Iron Creek and at an altitude of 6,000 feet, the con- 
tact between the porphyry and sedimentary rocks is marked by contact 



WKBD.] IRON ORES. 461 

lenses of iron ore. The sedimentary rocks are locally baked and meta- 
morphosed, the soft micaceous Cambrian shales being changed to hard, 
flinty hornstones. The iron ore is, in part at least, a replacement of 
these rocks and occurs between them and the granite-porphyry. The ore 
is in lenses varying from a few feet to 20 feet in thickness, whose lateral 
extent is not exposed by outcrops or by the artificial openings thus far 
made. The ore is at present exposed in an open cut, and the quantity 
appears to warrant mining if there should arise a demand for such ores, 
since the ore could be easily transported, by some gravity system, to tbe 
railroad. Analyses of the ore, made for the owners and published in 
the i^eihart Herald, show Pe^^Oa, 76.90 i)er cent; FeO, 0.07 per cent; 
Mn, 0.03 per cent; SiOz, 8.80 per cent; AI2O3, 0.74 per cent; S, 0.03 per 
cent; H2O, 13.36 per cent. 

From the analyses it would appear that the ore is a fairly pure limon- 
ite mixed with a little quartz; but the ore is magnetic, and hence must 
be a mixture of magnetite with limonite derived from it. Openings 
along the contact on the southern side of the mountain also showed 
iron ores, but their existence was not determined. 



PETROGRAPHY OF THE IGNEOUS ROCKS OF THE 
LITTLE BELT MOUNTAINS, MONTANA. 



By L. V. PiBSSON. 



CHAPTER r. 

INTRODUCTION. 

PREFATORY REMARKS. 

The present work embraces a somewhat detailed description of the 
petrography of the region of the Little Belt Mountains. Although a 
considerable portion of the rocks are classified under simple and well- 
known types, or present but slight divergences from them, it has been 
deemed hesj: to give a rather full account of them and accompany it 
with analyses, since the area is a mining region which is growing in 
importance, and a detailed description of the igneous rocks, with which 
most of the mining industries stand in close relationship, will be of local 
service. 

It is also thought that, since even these well-known types of rocks, 
such as granite-porphyry, syenite-porphjnry, etc., are not devoid of 
certain regional characteristics and individual peculiarities, their 
description will be of interest to the petrographer. 

The greater part of the occurrences here described have been visited 
and collected from by the writer while in company with Mr. W. H. 
Weed, through whose kindness and cooperation he was enabled to 
enter the field during the progress of the area! mapping of the region, 
by Mr. Weed, for the United States Geological Survey. 

In this portion of the work only such details of descriptive geology 
will be given as will enable the reader to locate the types described 
and refer them to their proper places in the descriptive geology by Mr. 
Weed in the foregoing pages. 

CLASSIFICATION. 

In regard to this vexed subject, it has seemed best to divide the types 
to be considered primarily into four groups: a, the granular nonporphy- 
ritie rocks^ which are here of plutonic origin and which mainly form 

463 



464 IGNEOUS ROCKS OF LITTLE BELT MOUNTAINS, MONTANA. 

iDtruded stocks, but in a few cases are fouud as dikes; &, the acid /eld- 
spathic porphyrieSj which are usaally lij^ht-colored rocks composing the 
laccoliths of the region and a considerable portion of the dikes and 
sheets, and with which are included several dense types which lack 
phenocrysts and may be considered as imperfectly developed porphy- 
ries; c, the lamprophyres, or dark-colored basic rocks, composed mainly 
of ferromagnesian minerals, aud found only in dikes and sheets, usaally 
of rather small dimensions ; aud d, effusive rocks^ or lava flows, which in 
this area are restricted to two occurrences of basalt. 

In the nomenclature no regard has been paid to the fast-dying and 
nearly obsolete qualification of geologic age; tfaus, rkyoUte has been 
used irrespective of the consideration whether the rock so called is of 
Tertiary or of pre-Cambrian age. The term porphyry has been used 
simply as one of structure, not as the name of a kind of rock, and 
hence is used as a suffix to any of the names of the rock families, as, 
for instance, diorite-porphyry in place of dioriteporphyrite. This is 
merely restoring the word to its earlier, more logical, and correct usage, 
and is in full accord with the best and i)revalent American practice.^ 

In the first group of granular rocks are found representatives of the 
syenites, monzonites^ diorites, and shonkinites ^ in the second group are 
various representatives of the granite^ syenite, and diorite families in 
the form of porphyries and densely textured rocks ; in the third group, 
that of the laraprophyres, we find minettes, vogesites, and analcite- 
basalts of various types, with transitional forms as well; while in the 
last group there is only common feldspar-basalt. 

* See J. D. Dana's Manual of Mineralogjr and Petrography, 1887, p. 441. 



CHAPTER II. 
THE GRAIOJIjAR ROCKS. 

From the standpoint of general geology the granular rocks are by no 
means so important in this district as are the porphyritic. The most 
notable occurrence is that of the Pinto diorite of Neihart. In addi- 
tion, the list of occurrences includes the syenites of Barker and Belt 
Creek, the analcite-syenite of Otter Creek, and the stock of Yogo Peak, 
which is differentiated into various rock varieties, such as syenite, 
monzonite, and shonkinite. 

BARKER SYENITE. 

The syenite which forms the intruded mass north of Barker has sev- 
eral mines located upon its contact, like the Wright and Edwards, 
Barker, etc., from the dump heaps of which most excellent fresh mate- 
rial can be obtained. The rock is of a gray color, of moderately fine 
grain, with occasional large feldspars, half an inch or so long, which are 
phenocrystic in character. It is thickly dotted with small anhedrons 
of a black ferromagnesian mineral, and is locally termed ^^ granite." 

Under the microscope the minerals of a typical syenite are disclosed, 
viz, iron ore, hornblende, pyroxene, apatite, alkali feldspars, a little 
oligoclase, and a little quartz. The iron ore and apatite are of the char- 
acter usual in rocks of this class. The hornblende is mostly of a stringy, 
fibrous character, and plainly paramorphic after the pyroxene with 
which it is connected ; but in some specimens, such as those from the 
Wright and Edwards mine, the hornblende is idiomorphic, compact, 
and of the usual olive-green pleochroic character seen in many syenites 
and diorites, and must herel^e regarded as primary. The pyroxene, 
which has been mostly changed to hornblende, is a pale-greenish diop- 
side of a wide extinction angle. The feldspars are of greater interest; 
they consist mainly of alkali kinds which have no good outlines but 
always have a tendency to a broad tabular habit. They are soda- 
bearing orthoclases, and in them the section perpendicular to a exhibits 
2E varying from 40° to 60o, with extinction parallel to the very good 
cleavage lines which show the trace of 001 on the section. In sections 
perpendicular to jc — that is, nearly parallel to 010 — the extinction is 9^ 
pluSy as measured from the cleavage of 001 , the direction of the vertical 
axis and the orientation of the angle ft being shown by inclusions and 
a parting parallel to m (110). These soda orthoclases contain inter- 
laminated perthite bands of atbite; they also occasionally contain small 
cores of oligoclase; the included plagioclase may, indeed, be an acid 
andesine, as shown by Becke's method. Albite may be present in inde- 

20 GEOL, PT 3 30 465 



466 IGNEOUS ROCKS OF LITTLE BELT MOUNTAINS, MONTANA. 



pendent crystals and also oligoclase, as shown in some cases by the 
simultaneous illumination of sections in the zone perpendicular to 010 
of excellent Oarlsbad twins with their albite lamellsB, while other crys- 
tals are those of oligoclase. The amount of plagioclase present is 
always small in comparison with the alkali feldspar. Of the quartz, a 
small quantity is seen, mostly interstitial, the last product of crystal- 
lization, though sometimes in more or less small rounded anhedrons 
inclosed in the outer boundary of the feldspar. 

The structure of the rock is purely granitoid, though the occasional 
larger orthoclases give it sometimes a slight tendency to a porphyritic 
structure. The chemical composition is shown in the following table of 
analyses : 

AnalyseB of syenites. 



OonsUtaent. 


I. 


II. 


ni. 


IV. 


V. 


VI. 


SIO9 


64.64 

16.27 

2.42 

1.68 

1.27 

2.65 

4.39 

4.98 

.09 

.27 

.61 

.37 


65.43 

16.11 

1.16 

2.85 

.40 

1.49 

5.00 

6.97 

.19 

.39 

.50 

.13 

.11 


66.54 

17.83 

.74 

1.15 

.98 

1.92 

5.55 

5.58 

I .54 

.11 
Trace. 


68.34 

15.32 

1.90 

.84 

.54 

.92 

6.45 

5.62 

r .15 

I .30 

.21 

.13 


59.78 
16.86 
3.08 
3.72 
.69 
2.96 
5.39 
5.01 

1 1.58 

f 


1.077 
.158 
.015 
.022 
.031 
.047 
.070 
.053 


AlaOa 


FeaOa 


FeO 

MgO 

CaO 

NaaO 


KgO 


H«OatllOo 


HoO above 110° 






TlOo 


PjOs 




ZrOfl 






SO3 


Trace. 




Trace. 


1 


FeSa 


.07 
Trace. 
.08 
.05 
.23 
.03 




! 


C0« 

Fl 


.37 






.75 










CI 

MnO 


Trace. 
.18 
.08 


Trace. 
Undet. 
Undet. 
Trace. 


.04 
.07 
.08 
.04 
None. 






.14 

f 

f 




BaO 




SrO 

LiiO 






Total 










100.12 
.02 


100.18 
.04 


99.92 


99.95 


99.96 




Tiesa oxygen (0) 

Kemainder 


• 










• 100. 10 


100.14 





















I. Syenite (No. 633), Wright and Edwards mine, Barker, Montana. W. F. Hille- 
brand, analyst. 
II. Syenite, Moant Ascatney, Vermont. Jaggar and Daly. See Bull. U. S. G^l. 
Survey No. 148, by Clarke and Hillebrand, p. 68. W. F. Hillebrand, analyst. 



•] BARKER SYENITE. 467 

III. Syenite, Highwood Peak, Highwood MoantainH, Montana. L. V. Pirsaon and 

W. F. Mitchell, analysts. 

IV. Syenite, Head of Beaver Creek, Bearpaw Mountains, Montana. Weed and 

Pirsson : Am.- Jonr. Sci., 4th series, Vol. 1, 1896, p. 354. H. N. Stokes, analyst. 
V. Syenite, Bine Monntains, Colorado. W. Cross: Proo. Colorado Soi. Soo., 1887, 

p. 240. L. G. Eakins, analyst. 
VI. Moleonlar ratios of No. I. 

This analysis is quite typical for a syenite, in respect to the high silica, 
alumina, alkalies and low lime, magnesia, and iron. The silica is 
toward che upper limit of the syenite group, verging on the granites, and 
this explains the amount of quartz which is present. Since the quartz, 
however, is small and entirely microscopic, it appears best to classify 
the rock as a syenite rather than a granite. For sake of comparison, 
the analyses of some other quartzose syenites, mostly of western occur- 
rences, are given. The silicia in No. lY is very high, running into the 
granite group, but this is because the rock is almost pure feldspar; the 
actual amount of quartz present is very small. 

From the molecular ratios given in No. VI we may calculate the 
percentage of minerals present. In the specimen analyzed the only 
dark minerals present are iron ore and hornblende. We may assume 
that all of the alumina is in the feldspar, all the ferric oxide in mag- 
netite; then the excess of alumina over alkalies demands an equivalent 
of lime to form anorthite. Thus the excess of lime over the anorthite, 
the excess of ferrous oxide over the magnetite, and the magnesia are the 
elements forming the hornblende, and they are present in the ratio 

Ca : MgO, FeO :: 0.012 : 0.038 = 1: 3.1, 

while the theory for hornblende, Oa (MgFe)3 (^103)49 demands 1 : 3. 

This result is rem^kably close and is another proof of the marvelous 
accuracy of Hillebrand's analyses. These results show that the rock 
has the following mineral composition, in parts by weight : 

Per cent. 

Magnetite 3. 6 

Hornblende. 5. 5 

Anorthite 10.0 

Albite 37.4 

Orthoclase .mi 

Quartz 13.4 

Total 100.0 

If all the albite and anorthite were combined to form plagioclase, the 
resulting feldspar would be an oligoclase, Abuo Anas, or exactly Ab^ Aui. 
The microscope shows, however, that the average plagioclase present is 
not so rich in soda, but approximates to Ab2 Aui, and this permits of 
the presence of the albite molecule in the soda orthoclase and in the 
microi>erthite intergrowths.^ 

* The albite molecules are thus about equally divided, and the ratio of plagioolaee to alkali feldspar 
\b about 7 : 12, which shows a syenite verging toward the monsonite /rronp of Brogger. ^ 



468 IGNEOUS ROCKS OF LITTLE BELT MOUNTAINS, MONTANA. 

From the foregoing results we can calculate approximately the chem- 
ical composition of the hornblende present, and find it to be as follows: 

Per cent. 

SiOa 55.1 

FeO 9.3 

MgO 23.3 

CaO 12.3 

Total 100.0 

This is, in fact, the ordinary composition of a common hornblende, 
though perhaps a little richer in iron than usual. 

In making these calculations the minute amount of lime necessary 
to turn phosphoric acid into apatite, of ferrous iron belonging in ilmeuite, 
and of the barium and strontium in the feldspar, have not been taken into 
account; possibly if they had been the results would be a trifle more 
accurate. 

AUGITE-SYENITE OF BELT CREEK. 

This occurs as a thick intrusive sheet, reaching 100 feet in thickness 
in places in the Cambrian beds on Belt Creek, and extending from 3 to 
6 miles above its junction with the Dry Fork. It has been previously 
mentioned by Lindgren^ in his two publications on this region, and a 
brief description of it given. The collection of new and more varied 
material has added much of interest to the original study, and the whole 
is herewith given in full. 

The rock is of a medium fine grain, and in color of a grayish tone 
with light-pink spots. The general gray tone is due to the feldspar 
granules being thickly sprinkled with exceedingly minute dots of a ferro- 
magnesian mineral, to be seen only with a powerful lens. In these 
grayish feldspars are scattered many tablets of biotite, while occasional 
quite large formless inclusions or phenocrysts of a piuk alkali feldspar 
are seen. A few included quartz grains with greenish mantle were 
noted. The rock also contains some inclusions, as large as one's finger, 
of another rock, which appears to be a minette, with micaceous 
grouudmass and phenocrysts of biotite. The rock surface exhibits 
many small miarolitic cavities studded with projecting feldspars of 
poorly developed crystal form. The microscope discloses in the section 
the following minerals: Iron ore, titanite, apatite, diopside, biotite, 
alkali feldspar, and quartz. 

The iron ore and apatite are of the usual character ; the titanite is not 
very common and is in small, well-formed crystals. The pyroxene, 
which is of a clear, very pale-yellow green, has an extinction angle of 
about 450, and is of diopside character. It occurs rather sparingly in 
good-sized crystals 1 or 2 mm. across, but also in great quantities of 
very small, well-formed, slender prisms which are thickly scattered 
through the feldspars, the latter inclosing them in a i)oikilitic manner. 

1 Tenth Census of the United States, Vol. XV, p. 723; Proc. California Acad. Nat. Sci., 2d aeriea, 
Vol. Ill, p. 45. 



PIR880N.] ANALCITE- (nEPHELITE- ) SYENITE. 469 

The biotite also occars in the same maniier, and is in crystals of two 
sizes, the smaller embedded in the feldspar, but it is much less in 
amount than the diopside. The feldspar, which is the chief constituent, 
has a short, thick, tabular habit, giving square or rectangular cross 
sections; in a section parallel to b (010) and perpendicular to c the 
extinction is 10^ 30' from the trace of 001, and hence the feldspar is a 
soda orthoclase; and this is also shown by its marbled, patchy, moir6 
appearance between crossed nicols with high powers. Moreover, there 
being but one variety present, it would naturally be rich in soda. The 
average size of feldspar grain is between 1 and 2 mm. In some of the 
angular interspaces between the feldspars is a little quartz, or quartz 
and feldspar in micrographic intergrowths. 

The striking feature of this rock is the small augite prisms embedded 
in the feldspar. The large occasional pink feldspars are quite free 
from them, and must therefore be an earlier product. The rock is thus 
an augite-mica-syenite verging on the syenite-porphyries. 

A specimen collected at another outcrop of the sheet shows a slightly 
different character. There is more mica present, titanite is wanting, 
and the feldspars are smaller and tend more to a lath-like form. This 
variety agrees more closely with the description of Lindgren, and is 
undoubtedly the variety examined by him. He refers it to the mica- 
syenites or minettes, stating that it has a tendency to the augite- 
syenites. In his second article it is referred to the augite-syenites, 
where it clearly belongs. 

ANALCITE- (NEPHELITE-) SYENITE. 

Bocks of the nephelite-syenite group, or those composed chiefly of 
alkali feldspars in combination with a feldspathoid mineral of which 
nephelite may be taken as the type, are rare in the Little Belt Moun- 
tains, only one occurrence being known, which may be referred to under 
this heading. 

The rock forms an intrusive mass on the west side of upper Otter 
Greek, about 6 miles north of Barker. It is first mentioned and 
described by Lindgren,^ who gives some brief i)etrographic data con- 
cerning it. Some additional details are given in a later publication.* 
The following account supplements that given by Lindgren : 

It is rather fine grained and of a light-gray color, due to a mingling 
of the white feldspathic material with innumerable small, slender foils 
of biotite, which lie pointed in every direction and produce a markedly 
speckled appearance. In the maximum these foils of biotite attain the 
length of half an inch, but their thickness is not over one-twentieth of 
this, and commonly they appear not over an eighth to a quarter of an 
inch long. They are not the edges of thin tables of biotite that are seen 
on end, but actually have this curious rod-like development, and apx>ear 
at first sight much like slender hornblende prisms. Among these glit- 



' Tent h Census of the United States, Vol. XV, p. 723. 

* Proo. California Acad. "Nat. Sci., 2d series, Vol. III. p. 45. 



470 IGNEOUS ROCKS OF LITTLE BELT MOUNTAINS, MONTANA. 

tering biotites are roandish flecks of dull greeuish black, which are dae 
to small^ stoat prisms of pyroxene. The average size of grain and 
texture is similar to that of an ordinary granitic aplite. To these details 
the lens adds little beyond showing that the feldspars have a j)ro- 
nounced thin tabular habit. 

In thin section the microscope discloses the following minerals: Iron 
ore, apatite, pyroxene, biotite, muscovite, alkali feldspars, nephelite ( Y), 
and analcite, while a little calcite and serpentine are clearly secondary 
products. 

Iron ore occasionally occurs in rather large scattered anhedrons. The 
apatite in its usual prisms is closely associated with the iron ore and 
pyroxene. The pyroxene is moderately abundant in thick, short pris- 
molds. It has adear, pale gray-green color, and exhibits in places a fain tly 
perceptible pleochroism in tones of the same color. Its extinction angle 
on 010 is about 45^. It is clearly a member of the diopside family. In 
places along the cleavages and cracks it is altering into a yellowish, 
fibrous, serpentine-like product. 

The biotite has a strong pleochroism varying between a dark red- 
brown color and a very pale yellow, almost colorless. Its most marked 
feature is the development into long, slender foils. It is strikingly 
idiomorphic and free from inclusions. 

The feldspars appear to be wholly made up of alkali varieties. In 
some of them interior cores remain which are clear, homogeneous, and 
unaltered, and which seem to be of soda orthoclase. Oenerally, how- 
ever, the feldspars are more or less flecked and muddy. They contain 
occasional strips of muscovite, and more especially small bays and 
areas of a colorless isotropic mineral of low relief, which, as will be 
presently shown, is undoubtedly analcite. Generally between crossed 
nicols the feldspars show the mottled look characteristic of soda ortho- 
clases which are being altered. Small areas and occasional laths show 
the albite twinning, and are due to albite. The feldspars have a pro- 
nounced lath shape, giving the section a broadly trachytoid appearance. 

In the interstices between the feldspars is a colorless isotropic mineral 
of very low relief, which is analcite. There is considerable of it present. 
That it is analcite is shown by the. fact that if the powdered rock is 
boiled with extremely dilute acid the resulting solution when filtered 
and evaporated yields gelatinous silica abundantly. This shows the 
mineral is not leucite. The filtrate when tested with silver nitrate 
yields but the faintest perceptible trace of chlorine and none of sul- 
phates, which excludes sodalite and nosean. On application of heat 
in a closed tube the rock x)owder yields water readily and abundantly 
far below redness. These tests show a considerable amount of analcite 
present in the rock. In his earlier publication Lindgren was inclined 
to believe that this analcite was glass, and believing the rock to be of 
younger age he placed it among the trachytes, but in his later work 
he recognized that it might be a mineral of isometric character, perhaps 
sodalite, and classified the rock as an augite-syenite. 



PIB8SOK.] SYENITE OP YOGO PEAK. 471 

A little caJcite and inascovite are present as secondary alteration 
products. It seems probable that the analcite is also wholly, or at 
least in great part, secondary, probably after nephelite or sodalite in 
the angular interspaces, and partly after the albite molecule in the 
feldspars. The rock is thus somewhat changed along the line of zeolitic 
alteration. 

Whether we assume the analcite filling the angular interspaces to be 
secondary or original, the rock in any case belongs in the nephelite- 
syenite group* If we assume that it is secondary after nephelite, as 
seems most probable, the rock is to be classed as a miascite. The pres- 
ence of some nephelite in the rock could not be directly proved, but it 
is suspected from the section. 

THE ROCKS OF YOGO PEAK. 

The rock mass of Togo Peak and the different rock varieties into 
which it is differentiated have already been described by Mr. Weed 
and the writer.^ In that article only brief petrographic details were 
given, sufficient to make clear the discussion of the analyses and the 
facts bearing on theoretic petrography, which comprised its essential 
features. It is here proposed to treat these types in more detail, 
especially those points which are of interest to petrographers. The 
discussion of the facts from a standpoint of theoretic petrography is 
deferred until the latter part of this work. For details of descriptive 
geology the reader is referred to the description by Mr. Weed in the 
first portion of this work. 

SYENITE OF YOGO PEAX. 

That portion of the Yogo Peak stock which may be most properly 
classified as a syenite comprises the eastern shoulder of the elevated 
mass. The rock has a platy parting which causes it to split readily, 
making the joint blocks a foot or so long. They are very hard and 
tough, and specimens are broken off with difficulty. The rock is un- 
altered, fresh, and very suitable for study and analysis. 

On a freshly fractured surface the rock appears evenly granular, of 
moderately fine grain, and is compact in character and with few miaro- 
litic cavities. The color is a medium gray, with a pinkish tone. An 
illustration of it is shown in PI. LXX, A. Examined with the lens, it is 
seen to be chiefly composed of light-colored feldspar, dotted with small, 
dark, formless spots of green pyroxene or hornblende. 

The microscope shows the following minerals to be present: Apatite, 
titanite, iron ore, pyroxene, hornblende, biotite, orthoclase, oligoclase, 
and quartz. The apatite and titanite are of the usual characters 
common to such rocks. The iron ore is not abundant and occurs in 
small grains of about 1 mm. in diameter. The pyroxene is a very 
pale green diopside, and is much cracked and broken up. Frequently 

> Am. Joar. Soi., 3d series, VoL L, 1886, p. 467. 



472 IGNEOUS BOCKS OF LITTLE BELT MOUNTAINS, MONTANA. 

it appears like a bundle of rods. It is rarely alone, generally oocor- 
ring in common with a brownish-green hornblende. The two min- 
erals are very frequently found together in stout, ill-shaped crystals 
from 1 to 2 mm. long, the pyroxene forming a core surrounded by the 
hornblende. In such cases the amount of pyroxene is inversely pro- 
portional to that of the hornblende. The appearance and association 
of these two minerals indicate that the hornblende is paramorphic after 
the pyroxene. The latter rarely occurs alone, while the hornblende 
frequently does. Biotite is rare, and occurs only as occasional brown 
pleochroic shreds. 

Orthoclase is the predominant feldspar, occurring in irregular masses. 
A smaller quantity of plagioclase is also present, the optical characters 
of which prove it to be oligoclase. It is more idiomorphic than the 
orthoclase, frequently or even commonly occurring in rather rectan- 
gular elongated lath?, and is often surrounded by a mantle of ortho- 
clase. A small amount of interstitial quartz completes the list of 
minerals. 

In structure the rock is hypidiomorphic, but only partly so, as the 
pyroxene and hornblende are themselves rather ill formed and irregu- 
lar, and the tendency is toward an allotriomorphic structure. The aver- 
age size of grain is about 1 mm. 

The analysis given in Ko. I of the following table shows the chemical 
comx)osition, which is that of a syenite with rather high lime, iron, and 
magnesia for a rock of that group. The mineral and chemical charac- 
ter show it to have a somewhat dioritic tendency, and in fact it is closely 
related to the monzonite group, in which the feldspars are equal; that 
is, approximately the plagioclase equals the orthoclase. It is very 
closely related to certain of the syenites which have been called akerites, 
as the analysis of one of them tends to show. Moreover, the descrip- 
tion of these akerites as given by Brogger,^ with their rectangular 
zonal feldspars, applies closely to this rock. On the other hand, its 
relation to certain rocks which have been variously placed, sometimes 
among the syenites, sometimes among the diorites, is shown by the 
close agreement with the analysis of the rock from the Hodritsch Vale 
near Schemnitz. All these types clearly belong in a group by them- 
selves, and, following the proi)osal of Brogger,^ they may well be con- 
sidered an intermediate group between the normal syenites and diorites 
and be called banatitesy after the old name used by Yon Ootta. Thus the 
rock of Yogo Peak, although here called a syenite, as under a broad 
grouping according to present ideas of rock classification it would 
undoubtedly be called, would for petrographic puri)oses be better 
designated a banatite. Its connection with the monzonite of Yogo 
Peak, as part of a single geologic mass, is extremely interesting, as it 
shows, as exhibited by nature itself, that grouping and connection which 
Brogger has suggested on theoretic grounds. 



> SyenitpeismatJtgange der sndnorwegischen : Groth's Zeit. f. Kryst., VoL XVI, p. 51. 
*Triadi8cben Eruptlonsfolgo bei Predazzo, p. 60. 



PIR880N.] 



SYENITE OF YOGO PEAK. 



473 



Analyses of syenites. 



CoDBtituent. 



Silica (SiOi) 

Alumina (Al>0.i) 

Ferric iron (Fe-iOa) 

Ferrous iron (FeO) 

Magnesia (MgO) 

Lime (CaO) 

SodaCNa^O) 

Potash (K3O) 

Water (H2O) at nOQ.... 
Water (H.O) above 110^ 
Titanic oxide (TiOi) ... 
Chromic oxide (Cr^Oa) . 
Manganese oxide (MnO) 

Baryta (BaO) 

Strontia (SrO) 

Chlorine (CI) 

Phosphoric acid (PaO*) - 
Sulphuric acid (SO3) ... 

Carbonic acid (CO2) 

Lithia(LijO) 



I. 



61.65 

15.07 

2.03 

2.25 

3.67 

4.61 

4.35 

4.50 

.26 

.41 

.56 

Trace. 

.09 

.27 

.10 



II. 



III. 



IV. 



.33 



59.56 
17.60 
2.90 
3.38 
1.87 
3.67 
4.88 
4.40 

1.37 

1.22 

X=.44 

.03 

(t) 



61.73 
17. 45 

5.94 

2.29 
4.52 
3.12 
3.88 

1.16 

(t) 



1.027 
.145 
.013 
.031 
.092 
.082 
.070 
.048 



(f) 
(t) 



(f) 



(t) 



Trace. 



Total ' 100.15 



101. 32 



100.09 



L 



I. Syenite, Yogo Peak, Little Belt Mountains, Montana. W. F. Hillebrand, 

analyst. 
II. Syenite, akerite type, YettakoUen, South Norway. H. O. Lang : Nyt Mag. for 
Nat., vol. 30, p. 40. P. Jannasch, analyst. 

III. Syenite, diorite, banatite, Hodritsch Vale near Schemnitz. K. R. von Hauer: 

Yerhandl. K. k. Reich sans talt, Wien, 1867, p. 82. 

IV. Molecular proportions of No. I. 

By assaming that all the alumina is in feldspar^ and taking the equiv- 
alent of soda, i)otash, and lime for it, and then assigning sufficient 
ferrous iron to convert the ferric iron into magnetite, we may calculate 
the mineral composition with rather close approximation to truth; for 
the remaining lime, iron, and magnesia are to be divided between 
pyroxene and hornblende, which is readily done, while the excess of 
silica represents the quartz. This gives: 

Per cent. 

Magnetite 3. 1 

Pyroxene 5. 4 

Hornblende 12.9 

Anorthite 7. 5 

Albite 37.5 



474 IGNEOUS EOCKS OF LITTLE BELT MOUNTAINS, MONTANA, 

Per cent. 

OrthoclAse 27.5 

Quartz 6:i 

Total 100.0 

Components : 

Dark 21.4 

Light 78.6 

100.0 

The average plagioclase combining the above would be AbsAni, bat 
as a large part of the albite molecnle is present in the orthoclase, the 
oligoclase present does not average nearly so mach soda as this. 

Local varieties of the syenite, — ^Toward the high east shoulder of Yogo 
Peak, which descends to a saddle on the ridge, the talus forming this 
slope shows a variety of the rock in which the plagioclase diminishes 
almost to the vanishing point, and the rock therefore assumes the 
character of a normal and typical syenite; in other respects its charac- 
ter is that of the type just described, and it can not, indeed, in the 
hand specimen, be distinguished from it. The variation, like all types 
at Yogo Peak, is probably local, but it has a certain x)etrologic signifi- 
cance, which will be treated of in another place. 

At the prospect mining shaft which has been sunk not far from the 
contact on the south side of Yogo Peak, in the igneous rock, there 
occurs a light-colored rock which is another variation of the banatite in 
that it represents a more dioritic phase. The lath-like plagioclases 
clearly predominate over the alkali feldspar and form the main rock 
constituent. It is interesting to note in this variety that the horn- 
blendes, although quite compact and appearing, on the whole, origi- 
nal, yet occasionally carry interior cores or fragments of pale-green 
diopside. What the exact relation of this diorite-like facies is to the 
shonkinite and monzonite, which are the main rock types of the vicinity, 
could not be learned, as it is not apparent at the surface, but it must 
certainly be quite limited in amount when compared with them. 

Storr Peak, — ^The granite-porphyry of the great intruded mass of 
which Yogo Peak forms the western extremity passes again into more 
basic phases to the northeast. Thus, at the high i)oint at the head of 
one of the forks of Dry Wolf Creek, near where the llO^ 15' meridian 
crosses the boundary of the igneous rocks, here called Storr Peak, the 
rock passes into a phase that is between a i)orphyry and a granular 
rock, and also a transition to syenite. 

The hand specimen shows a rock that appears like a rather flue- 
grained syenite, gray in color, dotted with minute black specks of 
ferromagnesian minerals. In fact, megascopically, it exactly resem- 
bles the Yogo Peak syenite just described. 

In the thin section the feldspar phenocrysts, which are almost entirely 
soda orthoclase with a very little oligoclase, are of uniform size and 



Piiwaoii.] MONZONITE OP TOGO PEAK. 475 

packed so thickly together that the groundmasB is redaced to a very 
small amount; filling the little interspaces between them. It is a micro- 
grauite mixture of quartz and feldspars, and is quite fine grained. Some 
green hornblende, iron ore, and apatite complete the minerals. Such a 
rock is difficult to classify, as it is a connecting link between the granu- 
lar and porphyritic structures, and also between the granite and syenite 
families. It appears best to place it here with the syenites, as its por- 
phyritic character is seen only under the microscope, while the total 
amount of quartz is very small and is confined to the groundmass. 

MONZONITE OF TOGO PEAK. 

This name has been applied to a massive igneous rock occurring at 
Monzoni, in the Tyrol, and usually classified under the syenites, of which 
it has been considered a variety rich in plagioclase and in the darker 
ferromagnesian minerals, especially pyroxene. It has been shown in 
recent years that this tyx)e of rock is not confined to the vicinity of 
Monzoni, but occurs elsewhere in sufficient abundance to warrant the 
proposition that the name shall no longer be considered that of a misre 
variety of syenite, but of an independent rock group of the same order 
of significance as that of syenite and diorite, to be applied to those rocks 
in which the alkali and lime-soda feldspars are about equally balanced, 
thus avoiding the difficulties of classifying such rocks either with the 
syenites or the diorites.^ In the former article on Yogo Peak, by Mr. 
Weed and the writer,' it was shown in the petrographic description that 
the type of rock forming the middle knob of the peak was of unusual 
character, in which alkali feldspars were of about equal amount with 
plagioclase, and the name *<yogoite" was proposed for it. Later,^ 
however, recognizing that ^^yogoite" is essentially the same rock as that 
firom Monzoni and Predazzo, both chemically and in its mineral com- 
position, the name ^^yogoite" was withdrawn for the older and better- 
known term. Bocks of this character have been found in several 
localities in Montana, and the number of occurrences in this portion of 
the Bocky Mountain area will no doubt be increased in the future. It 
can scarcely be doubted that many types of rocks hitherto placed under 
diorites or syenites by difierent petrographers, really belong in this gen- 
eral group, and that the future will show the type to be a not uncommon 
one. In the localities so far described — at Monzoni and Predazzo in Tyrol, 
at the Bearpaw Mountains, here at Yogo Peak, and also in the High wood 
Mountains in Montana — the rock does not appear geologically alone 
and independent, but is accompanied by more feldspathic types on the 
one hand and by darker-colored, more basic, augitic varieties on the 
other. It is thus part of a differentiated complex, and considering the 
very medium chemical character it possesses, as a sort of petrographic 
mean, this should be exx)ected. 

' Brdggier, Eruptivgesteine des Kristianiagebietea, II, Predasso. 

* Am. Jour. Sci., 3d series. Vol. L, 1895, p. 467. 

* Weed andlPirsson. Bearpaw Moiintaiosof Montana: Am. Jour. Sci., 4th series, Vol. 1, 1890, p. 857. 



476 IGNEOUS BOCKS OF LITTLE BELT MOUNTAINS, MONTANA. 

At Yogo Peak the rock occurs most typically and best exposed at 
the central one of the three prominent knobs forming tbe x>eak. On 
the one hand it grades into the banatite variety of syenite, previously 
described, which forms the eastern shoulder, and on the other hand 
into the shonkinite of the western outcrops and exposures. 

The rock is divided by joints into short blocks, and is very firm and 
tough. On a freshly fractured surface it is rather dark gray, with 
a greenish tone, and appears of medium granularity. It is clearly seen 
to be somewhat mottled, by the contrast between the light-colored feld- 
spathic portion and the darker-colored ferromagneaian minerals, and 
recalls in its appearance many diorites. The dark minerals appear to 
make up half the bulk of the rock. The reflection of light from numer- 
ous biotite cleavages of small size is also noticeable. The appearance 
of the rock is shown in PI, LXX, B. 

Under the microscope the minerals seen are iron ore, apatite, biotite, 
pyroxene, hornblende, plagiodase, alkali feldspars, and quartz. 

The iron ore is not present in large amount, but is seen in scattered 
grains usually attached to pyroxene and biotite. The apatite is not 
abundant and shows nothing of special interest. 

The pyroxene is a clear pale-green diopside of wide extinction angle 
and rather idiomorphic in form. It is comparatively free from inclu- 
sions, save those of iron ore and apatite ] in a few cases some inclusions of 
a brownish substance, which may be glass, were seen. It is very fresh 
and unaltered, except for its connection with hornblende. It is the 
most abundant ferromagnesian mineral. 

The hornblende is of the olive-green color usually seen in common 
hornblende, is strongly pleochroic, and is generally seen surrounding or 
attached to the diopside. It occurs in places penetrating the latter 
in small flakes or rods, and sometimes the diopside is quite spotted 
with these bits of hornblende. When in larger pieces it does not have 
any distinct idiomorphic form. All these facts go to show very 
clearly its secondary paramorphic character. ]!^owhere does it show 
those evidences of primary character which Iddings has so well 
described and figured in the intergrowths of hornblende and pyroxene 
in the diorite of Electric Peak.' An estimate based on the sections 
places the amount of hornblende at one-tenth that of the diopside. 

The biotite is pleochroic, in tones of pale yellow and olive brown; 
basal sections are a deep umber brown. It is quite idiomorphic and 
has the usual apatite and iron-ore inclusions. 

The plagiodase is rather variable; studies of it according to recent 
methods show that it is mostly andesine, in small part oligoclase, and 
even a little albite is present. It occurs in rather broad tabular forms, 
giving in general idiomorphic sections; sometimes it is seen in rather 
slender laths, which are always smaller than the tables mentioned 
above, and while they are generally Carlsbad twins they often show no 
albite twinning, or at best but one or two strips. They are invariably 



* Twelfth Ann. Kept. U. S. Geol. Survey, Part I, p. 606. 



PiMflON.] MONZONITE OP YOGO PEAK. 477 

of aDdeslne. The larger tables, on the contrary, always show albite 
twinning, usually in very fine lamellsB, and sometimes are not Carlsbad 
twins; they are more irregular in their composition, are sometimes 
zonally built with basic cores, and sometimes consist of varying irregular 
masses without any regular crystallographio or zonal arrangemenjb, but 
with the albite twinning passing through as if the crystal were entirely 
homogeneous. Thus, in these crystals, while andesine is the most com- 
mon proportion of the albite and anorthite molecules, they vary through 
oligoclase to albite. 

The alkali feldspar is mostly a soda orthoclase, but this contains a 
microperthite-like intergrowth of another feldspar that is believed to 
be albite, but it is present in so narrow lamellae that this could not 
be proved; moreover, it does not show the albite twinning. All that 
can be safely said of it is that it is another feldspar and not quartz. 
The intergrowths are not exactly like the usual microperthitic lamellae 
of albite, but more nearly resemble micrographic intergrowths of 
quartz and orthoclase; they are shown in PI. LXXI, A, in which they 
are seen very greatly magnified; it does not require a very high power 
to see them clearly. 

The calculation of the chemical analysis shows that the total average 
alkali feldspar has the composition OriAb], but the microscope shows 
that although this may be the sum total, there is considerable vari- 
ability in the manner in which the albite and orthoclase molecules are 
arranged. 

The structure of the rock is a purely hypidiomorphic granular one. 
There is a strong tendency for the ferromagnesian elements to be 
together, and also for little areas to occur in which plagioclase is very 
abundant, others in which it is nearly absent, unstriated alkali feldspar 
predominating. Thus, while taken in mass the composition of the rock 
is very homogenous, on a microscopic scale it is variable, and it is dif- 
ficult to bring into the field of the microscope, except with extremely 
low powers, an area that would be typical of the rock as a whole. The 
alkali feldspar shows always a tendency to a broad, poikilitic character, 
tending to surround the other minerals. An extremely minute amount 
of interstitial quartz needs no further mention ; its role as rock compo- 
nent is here without importance. 

An analysis of the rock by Dr. Hillebrand is shown in the following 
table, and with it are given published analyses of four other monzonites 
from different localities. The older analyses are full of analytical 
errors and are not to be trusted. It will be noticed how nearly all 
these agree, and how little any one of them departs from the mean of 
the whole five given in No. YI. This mean may be taken then as the 
typical composition of a monzonite, especially as expressed in the near- 
est whole numbers and given in No. Via. The feature of this chemical 
composition is the very medium character expressed throughout. In 
all respects the monzonites stand as a mean between the different rock 
groups. 



478 IGNEOUS BOCKS OP LITTLE BELT MOUNTAINS, MONTANA. 



Analjfset of m<mzonites. 



Constituent. 

SiOa 

Al/Os 

FeaO, 

FeO 

MgO 

CaO 

Na^O 

KsO 

H2O — llQo. 
H,O + 110o. 

TiOa 

Fl 

CI 

P*05 

SO3 

CrjOs 

MnO 

BaO 

SrO 

LiaO 



I. 



II. 



III. 



IV. 



54.42 

14.28 

3.32 

4.13 

6.12 

7.72 

3.44 

4.22 

.38 

.22 

.80 



.59 



.10 I 
.32 ' 
.13 
Trace. , 



52.81 

15.66 

3.06 

4.76 

4.99 

7.57 

3.60 

4.84 

.93 

.16 

.71 

Trace. 

.07 

.75 

Trace. 

Trace. 

Trace. 

.24 

.09 

Trace. 



52.05 
15.02 
2.65 
5.52 
5.39 
8.14 
3.17 
6.10 



} 



.35 
.47 



51.00 
17.21 
2.41 
4.23 
6.19 
9.15 
2.88 
4.93 



V. VL 

54.20 I 52.89 



.63 
.13 



15.73 
3.67 
5.40 
3.40 
8.50 
3.07 
4.42 

.50 
.40 



.24 ! 

.21 

.02 



Trace. 
.33 
.03 



15.58 
3.03 
4.81 
5.22 
8.21 
3.23 
4.90 

.51 
.56 



50 



.11 
.47 



Trace. 
.42 
.28 



Total . . 100. 19 100. 24 100. 03 



Trace. 
.34, 

.14: 



99.60 



.70 



.33 

.15 



100.49 ' 100.00 



Via. 


VII. 


53.0 


0.907 


16.0 


.139 


3.0 


.021 


5.0 


.057 


5.0 


.152 


8.0 


.139 


3.0 


.055 


5.0 


.045 


.5 





.5 



.0 . 



.3 
.2 



100.00 



J 



I. Monzonite of Yogo Peak. W. F. Hillebrand, analyst. 

II. Monzonite of Beaver Creek, Bearpaw Mountains. Weed and Pirsson : Am. 
Jour. Sci.y 1st series, vol. 50, 1895, p. 357. H. N. Stokes, analyst. 

III. Monzonite of Highwood Peak, Highwood Mountains. Boll. U. S. Oeol. Sor- 

Tey No. 148, p. 154. E. B. Hurlbnrt, analyst. 

IV. Monzonite of Middle Peak, Highwood Mountains. Loc. cit. supra. E. B. 

Hurlbnrt, analyst, 
y. Monzonite of Monzoni. Brogger: Ernptivegesteine, Predazzo, 1895, p. 24. 

y. Schmelck, analyst. 
yi and yia. Average of above analyses reduced to 100. 
yil. Molecnlar proportions of No. 1. 

If we make two or three assamptions, as follows: That the biotite is 
nearly or practically free from ferric iron, and agrees with the biotite of 
Monzoni (which has been analyzed) in this respect; that the replace- 
ment of magnesia by ferroas iron is similar in the minerals into which 
these enter, and that the amoant of hornblende is one-tenth that of 
diopside, as shown by estimates made from the sections, we may calca- 
late from the analysis and the table of molecnlar proportions given in 
Ko. YII the mineral composition of the rock. Kone of these assump- 
tions is absolutely correct, but all of them must be approximately so; 



PIM80H.] SHONKINTTE OP TOGO PEAK. 479 

hence the following table, while not absolutely accurate, mnst represent 
the composition rather closely: 

Per oeot. 

Magnetite 5. 1 

Biotite 12.1 

Diopside 20.7 

Hornblende 4.5 

Anorthite 11. 3 

Albite 30.1 

Orthoclase 16.2 

Total 100.0 

Andeeine (AngAbs) 27.2 

Soda ortboolase (Or,Abi) 30.4 

Total feldspars 57.6 

Total feiTomagnesian minerals 42. 4 

Total 100.0 

The amount of the albite molecule present is just sufficient to turn 
the anorthite into the andesine demanded by the microscopic study, 
and have enough left to convert the orthoclase into a soda orthoclase 
where the relations are as 1 : 1 — a very common ratio for soda orthoclase, 
as, indeed, on chemical grounds we should exx>ect. The calculation 
shows also that the plagioclase and alkali feldspar present are equal, 
and again shows the impossibility of logically classifying these rocks 
either as syenites or diorites. The large proportion of ferromagnesian 
minerals present, forming two-fifths of the whole, also shows the middle 
position occupied by this type. 

SHONKINITB OP TOGO PEAK. 

This name has been given to dark-colored basic granitoid rocks con- 
sisting chiefly of orthoclase (or alkali feldspar) and augite, but in 
which, unlike the syenites, which are feldspathic rocks, the augite pre- 
dominates, producing an augitic or, as one might say, a gabbroid rock. 
Besides these chief components, olivine, biotite, and iron ore among the 
dark-colored minerals, and plagioclase among the light-colored ones, 
may be present, as accessory components, in considerable amount, but 
the orthoclase and augite are in all cases the determinant minerals. 
This type of rock is closely related to theralite, in that both are dark- 
colored basic augitic types and both are likely to occur associated with 
other types of rocks rich in alkalies, but theralite, the granular plu- 
tonic equivalent of the tephrites, has plagioclase and nephelite as its 
determinant white minerals. 

The first shonkinite described was that from Square Butte, in the 
Highwood Mountains,^ and later the occurrence at Yogo Peak was 
briefly mentioned.' This account it is now proposed to supplement 
with further details, and to mention another occurrence in this district. 

< Weed and Pinson, BalL GeoL Boo. America, VoL VI, 1894, p. 380. 
* Am. Jonr. Sci., 3d series, Vol. L, 1885, p. 487. 



480 IGNEOUS ROCKS OF LITTLE BELT MOUNTAINS, MONTANA. 

Besides these occurrences in the Little Belt and Highwood mountains, 
shonkinite has been described from localities in the Bearpaw Moun- 
tains,^ and it appears, as will be shown later, to occur at Monzoni, in 
the Tyrol, and doubtless other localities will be found as knowledge of 
the type becomes better known and petrographic research progresses. 

At Yogo Peak the shonkinite forms the rock masses of the western 
end, abutting against the sediments, and it also occurs about 4 miles 
northeast, on the ridge running out in that direction from Yogo Peak. 
Here it is found in contact with the limestones at the head of one of 
the main branches of Bunning Wolf Greek. 

The shonkinite rock does not possess the thick platy parting of the 
monzonite and syenite, but has an exceedingly massive character. The 
rock is very tough and breaks under the hammer with difficulty. On a 
fresh fracture it is of a very dark stone color, and at first glance recalls 
many coarse, dark gabbros. On inspection it appears that the quantity 
of ferromagnesian minerals is very large, and the eye is caught by the 
reflection of numerous plates of a dark-brownish biotite, which average 
several millimeters in diameter. With the lens a great abundance of 
small augites are also seen in the feldspathic constituent. 

At places, especially toward the contact, there is considerable varia- 
tion in the grain of this type; it sometimes occurs very much finer 
than the normal type mentioned above, and, on the other hand, at the 
extreme west end of the peak a variety is found that forms large, 
irregular masses, the rock being noticeable for the very large, spongy, 
biotite crystals which it carries. These biotites are at times 1 cm. 
across a cleavage face. They are made up of a number of smaller, 
nearly similarly oriented iudivldnals mixed with other constituents. 
Although the mica is really subordinate in amount to the other miner- 
als, it has the appearance of being predominant, and the rock seems at 
first glance to be almost wholly made up of these coarse biotite crystals, 
and has a very coarse-grained, curious appearance. Examination with 
the lens shows that although the biotite thus appears so important, it is 
merely because the crystals reflect the light from their cleavage surfaces, 
and thus seem more prominent than the other minerals; moreover they 
are very x>oikilitic, and are filled with augite grains. Thus the actual 
amount of biotite is less than that of either augite or orthoclase. The 
rock is shown in PL LXX, C. 

Under the microscope the minerals seen are iron ore, apatite, augite, 
hornblende, biotite, olivine, plagioclase, and soda orthoclase. 

Iron ore as an actual component of the rock is almost entirely want- 
ing. In one phase a few scattered grains surrounded by coats of bio- 
tite were observed, but in the other sections, representing different 
phases and areas of the shonkinite mass, it may be said to be entirely 
wanting. This is a very striking feature for so dark and basic a type, 
which, as the analysis shows, possesses considerable of the oxides of 

1 Weed and Firsson, Bearpaw Mountains of Montana : Am. Jour. Sci., 4th series, Vol. 1, 1896, p. 351 . 



PrassoM.] SHONKINITE OF YOGO PEAK. 481 

iron. It is therefore clear that it has gone into the ferromagnesian 
minerals present, and the green color and character of much of the 
biotite indicates that it mast approach lepidomelane in composition. 
It should also be stated that a very small amount of iron ore from 
the olivine resorptions, to be presently described, is also present, but 
this is in a way secondary and confined to these occasional minute 
areas. 

The apatite present in short stout crystals shows nothing of special 
interest. The amount of phosphoric anhydride in the analysis proves 
that 2.3 per cent of it is present, while the fluorine, in the absence of 
chlorine, shows it to be a flnor-apatite. The augite is a pale-greenish 
diopside like pyroxene of a very wide extinction angle. The prismatic 
cleavage is well developed ; it has no other, and there is no trace of any 
diallage-like character. It is quite idiomorphic, especially in the pris- 
matic zone, being bounded by the faces a(lOO), m(llO), and 2^(010), 
which have generally about an equal development. The ends of the 
prisms are less well developed and are likely to be rounded off. The 
habit is short, thick, and columnar. It contains inclusions of biotite, 
and, less rarely, of glass or iron ore. These inclusions are infrequent. 
In size the crystals vary from one-tenth to 1 mm. in diameter. 

Hornblende is not common, and its character and association are 
such as to lead to the belief that it is secondary, as described under the 
monzonite; its color, lack of definite form, and its association with 
pyroxene, are similar, but it is rather less in amount. 

Olivine and its resorption hands, — The olivine, in the most basic type — 
that is, the one containing the coarse poikilitic biotite — is mostly very 
fresh and clear, but in a few places is altered to a yellowish- red micaceous 
substance — one of the well-known alterations of olivine which need not 
be further mentioned. The olivine has no good crystal outline, but is in 
irregular masses. It has, as inclusions, shreds of mica, sometimes an 
ore grain, and occasionally little darker shadow-like spots which, when 
examined with very high powers, are seen to be skeleton magnetites 
presenting wonderful patterns of intricate grating structures. They 
resemble somewhat similar growths which have been previously 
described by other i)etrographers. They are illustrated in PL LXXI, B. 

The most interesting thing in regard to the olivines is the resorption 
phenomena they show. In the more basic and coarse-grained phase 
they are quite unaltered, except that they seem somewhat rounded, 
and where they come against alkali feldspar there is generally a band 
of green mica separating the two. From this character they pass, in 
other phases of the sbonkinite, into types which are surrounded by 
zones, as is often the case in gabbros. The zones, however, are of some- 
what diiierent character from those seen in the gabbros. Here the 
olivine is surrounded by, first, a band of granules of a biaxial mineral 
of high refraction and rather low birefringence, whose general characters 
indicate it to be enstatite; the granules are rather small in size and 
20 GEOL, PT 3 31 



482 IGNEOUS ROCKS OF LITTLE BELT MOUNTAINS, MONTANA. 

too confused for absolutely positive identification, but this also seems 
most probable, considering the composition of olivine. Next to this 
comes a band of green biotite and then the alkali feldspar. The iron 
in the olivine separates out as iron ore in black grains. This process 
goes on until no olivine is left, and only a yellowish mica-like substance 
dotted full of ore grains shows the place of the core of the original 
crystal. Such a resorption phantom is shown in PI. LXXIV, A. From 
this stage they may be traced gradually, by unaltered pieces of olivine, 
into the unchanged crystals. 

But the most interesting point in regard to this change is that it is 
directly proportional to the amount of feldspar which the rock con- 
tains. In the most basic, least feldspathic type of shonkinite the 
olivines, as noted above, are unaltered or surrounded only by a band of 
biotite where they touch the feldspar^ in the more feldspathic types 
they begin to be surrounded by the resorption bands, but there is gen- 
erally, though not always, some olivine substance leit. In the monzonite, 
a much more feldspathic phase of the Yogo Peak mass, these resorp- 
tions of olivine occur, but they are always resorptions (no olivine sub- 
stance is seen) and they are, moreover, not nearly so common. In the 
syenite (banatite) certain groupings of iron ore and biotite suggest the 
same thing, but are not conclusive. It is indeed interesting as a specu- 
lation whether these olivines formed before differentiation took place 
in the mass or afterward. 

The resorption zones, or '^ reaction rims," as they have been called, 
which occur around olivines in the plagioclase rocks have been so well 
described and their origin so fully discussed ' that they need no further 
mention here; but it may be said that the idea that they could have been 
formed in the shonkinite under discussion by any dynamic metamor- 
phic processes is not tenable for a moment; it does not even need to be 
discussed; we are dealing here with fresh rocks of a recent geologic 
period, breaking up through almost unaltered sedimentary beds. 

When we consider the chemical composition of the minerals involved, 
the cause and character of these resorption or <^ reaction" phenomena 
in the shonkinite become quite clear. If we consider that olivine was 
one of the first minerals to separate out of the original magma, it was 
because a mineral of that composition was capable of forming and was 
insoluble in the resulting and residual magma or capable of existing 
in it. As the process of crystallization proceeded, however, and the 
pyroxene, biotite, etc., crystallized out, the residual magma became 
richer in alkalies and alumina, until it eventually solidified as alkali 
feldspar. When this stage was reached the olivine was no longer 
insoluble in the molten feldspathic magma, and, redissolving and the 
magma crystallizing, the following reaction took place: 

OliTine. Orthocla«e. Hypenthene. Biotite. 

5(MgFe)«8i04 + K«Al«Si60,e =8(MgFe)Si03 + K8(MgFe)«Al«(Si04)3 

1 Rosenbasch, Mass. Gest., 1885-96, p. 814. 



PIB880K.] SHONKINITE OF TOGO PEAK. 483 

That is, the olivine and orthoclaBe give rise to hypersthene and biotite, 
and very naturally the hyx>erBtheue, the mineral richest in magnesia, 
lies next to the olivine; while the biotite, rich in alkali and alumina, 
lies next to the feldspar. Thus it is very easy to see why, on purely 
chemical grounds, the formation of such zones and their composition 
may be both expected and explained. 

It is to be noted that lime, which plays so imx)ortant a part in the 
zones around the olivines in the gabbros, is entirely absent in the 
above. In one or two cases slender needles were seen in the outer 
zone, and it may be that lime has been present and a little hornblende 
formed, as in Ihe gabbros. This is the exception, and not the rule, in 
the shonkinite. 

Feldspars, — The feldspars in the shonkinite are somewhat variable, 
especially the plagioclase. This is sometimes present and sometimes 
wholly absent, and t]iis within small areas, even within that of an 
ordinary thin section. It is usually in the form of laths, some very 
small and narrow, others broader and more columnar. It varies from 
interior cores as basic as a labradorite AbsAu^ to outer rims of ande- 
sine AbsAus; both albite and Carlsbad twins are generally present. 
The noticeable feature of this i)lagioclase is its strong idiomorphic char- 
acter, and this is especially noticeable when it lies embedded in the 
soda orthoclase. In some places, within a very minute area a con- 
siderable quantity of these plagioclase prisms will be heaped together, 
surrounded by broad regions quite destitute of them. Its total amount 
is small, and considered altogether it plays only the role of an acces- 
sory constituent. It seems to depend to some extent on the relation 
between pyroxene and biotite; thus, in the more basic phases, where au- 
gite is very abundant and its prisms thickly crowded, the plagioclase is 
almost wholly wanting, because the lime has all united with the magne- 
sia and iron in the production of pyroxene, while in those areas where 
it is not so common the magnesia and iron combined with alumina and 
{K)tash to form biotite, thus permitting the lime to enter into plagio- 
clase with the soda. 

The alkali feldspar ranks with the augite as the most important rock 
constituent. In sections perpendicular to the obtuse positive bisectrix — 
that is, approximately parallel to b (010) — ^the basal cleavage is easily 
seen and is usually good; at times a cleavage crosses this at 64^, which 
is probably parallel to the prism, a not unusual phenomenon in alkali 
feldspars. This gives the direction of the vertical axis and enables the 
section to be oriented, and it is then found that the extinction lies 10^ 
in the obtuse angle — that is, is positive — and therefore the feldspar is a 
soda orthoclase. This is shown also by its watery, moir^ appearance 
and by other phenomena which show that it is not a simple comx>ound. 

Chemical composition. — To show the chemical comi)osition of the shon- 
kinite there is given the analysis which has been made by Dr. 
Hillebrand; also some analyses of these rocks from other localities, 



484 IGNEOUS ROCKS OF LITTLE BELT MOUNTAINS, MONTANA. 



all of which show the characteristics of the type — rather low silica, 
low alumina, high iron, lime, and magnesia, with moderate alkalies 
and the potash predominating over *8oda. In No. Y is given the 
average of the first three analyses, and this may be taken as represent- 
ing the composition of a typical shoukinite; the analyses vary from it 
but little. 

Analyses of skonkiniits. 



Constitnent. 


 

I. 

48.98 


II. 




IV. 


V. 


i ^'• 


Vll. 


vni. 


IX. 

1 


SiOa 


; 50. 00 


46.73 


50.43 


48.90 


47.85 


50.82 


48.36 


0.813 


AliOn ! 12.29 


9.87 


10.05 


10.21 


11.07 


13.24 


n.44 


12.42 


.119 


Fe^O.! 

FeO 


2.88 
5.77 


3.46 
5.01 


3.53 
8.20 


11.57 


f 3.32 
I 6.33 


2.74 
2.65 


.25 
8.94 


5.25 
2.48 


.018 
.080 


MgO 9.19 


8.31 


9.27 


5.58 


9.06 


5.68 


14.01 


9.36 


.229 


CaO . 


9.65 


11.92 


13.22 


14.82 


11.59 


14.36 


8.14 


8.65 


.173 


Na^O. ..:... 


2.22 


2.41 


1.81 


1.48 


2.15 


3.72 


1.79 


1.46 


.036 


KaO 4.96 


5.02 


3.76 


3.70 


4.55 


5.25 


3.45 


3.97 


.052 


HaO— 110<^. .26 
HoO+llOo. .56 


.171 
1.16) 


1.24 


.87 


L13 


2.74 


.58 

* 


5.54 


.031 


TiOa 1.44 


.73 


.78 


Undet. 


.98 


• ■•«*•• 


.59 


1.18 


.017 


PsO* 


.98 


.81 


1.51 


.70 


1.10 


2.42 


.20 


.84 




SO3 




.02 
.31 
.08 
.16 
.11 
.07 
Trace. 
.32 
.07 
















CO, 




.18 


.52 












CI 












Fl 


.22 
Trace. 




"1 , 








CrOi 










.03* 


- 




KiO 










- 






MnO 

BaO 

SrO 


.08 
.43 

.08 


.28 

(t) 
(?) 




I 


.19 
.06 


.13 
.29 
















Li;.0 


Trace. 


Trace. 


Trace. 






f 






1 

Total .. 99.99 

1 


1 








100.01 


100.56 


99.88 


100.18 


100. a5 


100.49 


99.93 




0-Cl,Fl.. .08 


.08 

1 


.04 






- _ 












1 ' ' 


1 


1 


99.91 


99.93 


100.52 


1 






1 


!''-"i 











I. Shonkinlte, Yogo Peak, Montana. W. F. Hillebrand, analyst. 
II. Shonkinite, Bearpaw Mountains, Montana. Weed and Pirsson: Am. Joar. 
Soi.y 4th series. Vol. I, 1896, p. 360. H. N. Stokes, analyst. 

III. Shonkinite, Square Butte, High wood Mountains, Montana. Weed and 

Pirsson: Bull. Geol. Soc. Am., Vol. VI, 189.5, p. 414. L. V. Pirsson, analyst. 
As originally published MgO =^ 9.68, but a recalculation of the analytical 
data showed that a slight error in calculation had been made, and that the 
MgO should be 9.27. 

IV. Shonkinite, Monzoni. Lemberg: Zeitschr. d. Deutsch geol. (Resell., 1872, p. 

201. Lemberg, analyst. 
V. Average of I, II, and III. 

VI. Malignite, Poohbah Lake, Ontario. Lawson : Bull. Geol. Dept. University of 
California, Vol. I, No. 12, p. 350. F. L. Ransome, analyst. 



PLATE LXXII. 



485 



PLATE LXXII. 

Thin Section of Shonkinite. 

A, Shonkinite of Yogo Peak. Olivine (pale yellow), biutite (brown and oliye), 
pyroxene (green), and soda orthodase (white). Apatite and iron ore are also pres- 
ent, as well as a little plagioclase. Actual size of field 4 mm. multiplied by 19; 
section seen in natural light. 

By the same section seen in polarized light between crossed nicols. It shows the 
field occupied by a single large plate of orthoclase which incloses the other minerals, 
a common characteristic of this rock. The section here selected to show this does 
not contain quite the average amount of the dark ferromagnesian minerals. 

486 




UN SKCTIONS OKSHONKINITK OF VOGO PKAK 



FIR8BOH.] SHONKINITE OP YOGO PEAK. 487 

VII. Lamprophyre, between South Boulder and Antelope creeks, Montana. 
Merrill: Proc. U. S. Nat. Mas., Vol. XVII, 1895, p. 670. L. G. Eakins, 
analyst. 
VIII. Absarokite, dike south of Clark Fork, Wyoming. Iddings: Jour. Geol., 
Vol. Ill, 1895, p. 988. L. G. Eakins, analyst. 
IX. Molecular proportions of No. L 

The shonkinite magma is that which is characteristic of the class of 
rocks which bave been called lamprophyres. That this magma exists 
in other localities iu a different mineralogic, stractaral, and geologic 
form is shown by the comparison of the analyses given iu VII and VIII, 
the former a thick iutrasive sheet, the latter a dike. The relation 
between shonkinite and absarokite has been already noted by Iddings.^ 
In Ko. YI is given, for comparison, the analysis of a rock described 
by Lawson^ under the name malignite. Miueralogically it is closely 
related to shonkinite, iu that pyroxene and orthoclase are the prominent 
constituents; it differs in the presence of nephelite and in the character 
of the pyroxene, which is aegirite-augite, and these differences are caused 
by the larger amount of alkalies, especially of soda. Eosenbusch^ 
places it under the shonkinites, including both in the theralite family. 

Structure and classification, — The structure of the Yogo Peak rock is 
purely hypidiomorphic granular, and it has all the characteristics of a 
plutonic rock. The most l^triking and dominant microscopic feature is 
the poikilitic character of the orthoclase, which occurs in broad masses 
enveloping the other minerals and evidently the latest product of crys- 
tallization. This is shown in the figures given on PI. LXXII. Law- 
sou ^ mentions it as being also a characteristic of malignite. 

From a consideration of the molecular proportions given in No. IX of 
the table of analyses and the results of the study of the thin sections, 
it is estimated that the rock contains, on the average, by weight — 

Per cent. 

Pyroxene 35 

Biotite 18 

Olivine 7 

Hornblende, apatite, etc 5 

Audesine 10 

Soda orthoolase 25 

Total 100 

Dark constituents G5 

Light constituents 35 

Total 100 

This, of course, is not accurate, but the control is sufficient to make 
certain that the variation in the more doubtful constituents can not be 
more than a few per cent either way. 

A mere inspection of the above table shows that this rock can not be 
classed with existing rock groups, and that its erection into a new 
group is justified. But its occurrence in other localities and the accept- 

I Jonr. Geol., Vol. Ill, 1895, p. 053. ^Maas. Gesteine, 3d ed., 1895-96, p. 1308. 

>Biill. Dept. Oeol. Univ. Cal.. VoL I, March, 1896, pp. 887-302. « Loc cit. 



488 IGNEOUS ROCKS OF LITTLE BELT MOUNTAINS, MONTANA. 

ance of the group by other petrologists are already matters of history 
and render any further comment on this point superfluousJ It must be 
stated, however, that the persistent appearance of quantities of biotite 
in all these cases, due doubtless to the large amount of MgO and K^O 
in the magma, renders this mineral a much more constant feature of the 
rock type than was supposed would be the case when the original 
specimen from Square Butte was described. 

Shonkinite at head of Running Wolf Greek. — This occurrence, which 
has already been mentioned in the description of the Yogo Peak mass, 
has also been studied in thin section, and excepting the fact that none 
of it has been seen to carry any plagioclase and that the soda orthoclase 
is a little more abundant, it so exactly resembles the type already 
described that no further mention is necessary. 

Shonkinite of Otter Creek. — Besides the occurrence of shonkinite at 
Yogo Peak, there is another in the region of the Little Belt Mountains 
which deserves brief mention. It forms the large heavy mass intruded 
in the Upper Carboniferous beds on Little Otter Greek, about 2 miles or 
so above its junction with the main Otter stream. The mass is exposed 
at least 300 feet above the creek, and the outcrops, which are very 
columnar, extend in a long line, suggesting a sheet which must be 
extremely thick. The road quarry at one point has exposed quite good 
fresh material. 

In the hand specimen the rock is very dark gray and moderately 
fine grained, the components running from 1 to 3 mm. in diameter. In 
the section it shows the same minerals mentioned above for the Yogo 
Peak shonkinite, but the amount of olivine, which is very fresh and 
has no reaction rims, is considerably greater, while biotite is much less. 
The amount of andesiue is also less, only an occasional minute prism 
being present. The orthoclase as usual cements the other minerals. The 
rock in other respects so closely conforms to the description already 
given that it needs no further mention. 

PINTO DIORITE OF NEIHART. 

This rock forms the great massif intruded in the crystalline schists 
at IN'eihart, and hence it is there one of the most prominent rocks; the 
slopes north and east of the town are covered with bowlders of it, and 
it is equally prominent in the Oarpeuter Creek Gulch. Many mines are 
connected with it, and its striking appearance makes it well known 
among the mining population in the vicinity. 

The rock is noteworthy for its bizarre appearance, which is best 
seen on a large scale on some cliff wall or smooth surface of a large 

I In hia review of the original paper on Yogo Peak by Mr. Weed and the writer (Neues Jahrbuch, 
1896, Vol. II, p. 443), H. Bebrens quotes none of the analyseis omits all mention of the presence of 
orthoclase in the shonkinite, mentions especially the kind of plagioclase, states with emphasis that it 
resembles gabbro, and thus produces a totally false impression that only an ordinary gabbro had been 
described and decorated with a new name, an idea which anyone may see is patently wrong by reading 
the original description and observing the analysis. 



P1E880N.] PINTO DIORITE OF NEIHART. 489 

outcrop, where it has a mottled appearance, dne to ovoid masses of 
feldspar which vary from half an inch to an inch or more in 
diameter, and which lie so thickly scattered that they constitute the 
main portion of the rock, the interspaces between them being much less 
in bulk amount ; indeed, they run into one another in many places, leaving 
only rudely cusp- or lune-shaped spaces between them. Neither are 
these feldspar bodies always ovoid ; sometimes they are curved, bent, 
drawn out, elongated, and otherwise distorted, but the average shape 
inclines to the ovoid character. The interspaces between them are 
filled with a greenish-black ferromagnesiah mineral, and it is this strong 
contrast of these white spots on a black background that gives the 
rock the peculiar mottled appearance that is so striking, especially 
when seen on a large scale. When seen in this way the rock has also 
a gneissoid appearance, as the ovoid feldspar spots have their longer 
axes in one common direction, as a rule, and they are likely to succeed 
one another in lines, thus suggesting gneissoid structure. In the hand 
specimen it can be scarcely observed — the scale is too great — but an 
illustration is given in PL LXXIIl, A. 

This mottling produces also the effect of a huge coarse porphyry, the 
white spots appearing like phenocrysts. The appearance is, however, 
deceptive, for the rock is not a true porphyry, as will be shown later. 
The structure might be termed pseudoporphyritic. When the rock is 
examined closely, and especially with the lens, it is seen that the 
apparent phenocrysts — the white feldspar masses — are not made up 
of a single feldspar crystal, though they are likely to contain one feld- 
spar mass that is larger than all the rest. They are of a pale-green or 
gray color. The dark material filling the interspaces is in greater part 
a black lustrous hornblende, which has a somewhat schistose arrange- 
ment. Occasionally a shining leaf of biotite is seen among the horn- 
blendes. 

Microscopic cJiaracters ofdiorite. — In thin section under the microscope 
the minerals »een are apatite, iron ore, biotite, hornblende, plagioclase, 
and orthoclase; a very little sericite and calcite from alteration prod- 
ucts of the plagioclase are also present. The apatite in small prisms 
lies mostly embedded in the hornblende. The iron ore is in small 
anhedra and is limited in amount. Biotite is of the usual dark-brown 
pleochroic variety; it is variable in amount; sometimes there is consid- 
erable to be seen in the section, at other times it is almost wanting. 
This is the case in the sample analyzed. 

The hornblende is in small formless masses which tend to produce 
prismoid bodies which are more or less extended in one direction, thus 
aiding in the effect of schistosity. These prismoids are heaped together 
in clusters, without any other included mineral except apatite, and 
define the white feldspar areas. It is of the type of common horn- 
blende, with its pleochroism of deep grass-green, olive green, and 
yellow. 



490 IGNEOUS BOCKS OF LITTLE BELT MOUNTAINS, MONTANA. 



The plagioclase is in formless interlockiDg grains, always twinned 
according to the albite law, almost never accordiag to the pericline 
law. No Carlsbad twins were seen in spite of careful search. There 
is considerable variation in the size of the grains. In sections perpen- 
dicular to 010 the maximum extinction found was 24<^, which would 
indicate a basic andesine, according to the statistic method. There are 
no zonal growths to be seen among these feldspars; they are apparently 
all homogeneous and similar throughout. The calculation of the chem- 
ical analysis shows that they have the compo.Hition Abio An?, which con- 
firms the measurement given above; the plagioclase is basic andesine. 

Orthoclase is also present in rather small amount, and is generally 
interstitial in character between the plagioclases. Both of the feld- 
spars are more or less turbid from leaves of sericite, which as c<iual 
develops mostly along the cleavage cracks. The analysis shows that 
a little trace of a carbonate is present, and this may be due to some 
calcite deposited along cracks; none was recognized in the sections. 
A few minute grains of quartz were detected along with the orthoclase. 

Chemical composition. — The composition of this diorite is shown in 
No. 1 of the adjoining table, as made by Dr. Hillebrand. 









Analyses of diorites. 










Constituent. 


I. 


II. 


111. 


IV. 

52.05 
17.96 
4.09 
6.33 
5.03 
8.64 
2.99 
1.61 

.97 
.31 


V. 


VI. 


VII. 


vni. 


IX. 

.919 
.197 
.010 
.060 
.045 
.126 
.069 
.030 

.060 

.009 
.003 


X. 


8iO^ 


55.13 

20.27 

1.52 

4.29 

1.80 

7.05 

4.31 

2.84 

.14 

.95 

.74 

.40 

.26 

.13 

.06 

.11 

trace. 


53.48 

19.35 

2.37 

4.90 

3.67 

7.55 

4.07 

1.41 

.16 

.80 

1.07 


58.05 
18.00 
2.49 
4.56 
3.55 
6.17 
3.64 
2.18 

1 .86 


55.53 

16.80 

4.06 

3.35 

3.00 

6.96 

4.31 

3.57 

r .09 

I .55 

.95 

.47 

.09 

.16 

.13 

.11 

trace. 

100.17 


58.05 

15.46 

1.69 

5.09 

4.84 

6.94 

2.86 

2.14 

.10 

2.02 

.72 

.16 

none. 

.14 

.07 

traee. 

trace. 


58.63 

16.23 

1.91 

4.20 

4.28 

6.59 

3.51 

2.09 

.15 

1.17 

.74 

.20 

none. 

.11 

.06 
1 
trace. 

trace. 


55.54 
15.64 
1.19 
7.13 
4.84 
5.67 
3.17 
2.28 

I 2. 93 

1.24 
andet 
andet. 

(») 

undet. 
andet. 


55.80 
17.44 
2.59 
4.67 
3.59 
7.13 
3.67 
2.26 

1.13 

.88 
.33 


Al^Oi 


FeOa 


FeO 

MeO 


CaO 


Na.O 


KjO 


H.jO — 110°.... 
HaO + llO^.... 
TiOi 


P3O5 


.62 .17 


C0« 


.08 
.06 




MnO 


TionA. 


.43 
nndet. 
nndet. 




.17 
.11 
.09 


BaO 


. 19 andet 

. 11 mill fit. 


SrO 


LiaO 


traee. 


none. 




Total.... 






99.88 


100.00 


99.89 


100.79 


100.41 


100.28 


99.93 


99.63 





I. Diorite, Carpenter Creek, near Neihart, Montana. W. F. Hillebrand, analyst. 
II. Qaartz-diorite, Sweet Grass Creek, Crazy Moantains, Montana. Ball. U. 8. 

Geol. Survey No. 148, p. 143. W. F. Hillebrand, analyst. 
III. Diorite, Electric Peak, Yellowstone Park. Iddings: Twelfth Ann. Rept. U. S. 

Geol. Survey, Part I, p. 577. W. H. Melville, analyst. Includes 0.07 S0> 



piBMON.] PINTO DIORITE OP NEIHART. 491 

IV. Angite-diorite, StoDy Monntain, Ouray Connty, Colorado. Ball. U. S. Geol. 

Snrvey No. 148, p. 180. L. G. Eakins, analyst, 
v. Diorite, La Plata Monntains, Colorado. Bull. U. S. Geol. Sarvey No. 148, 

p. 181. W. F. Hillebrand, analyst. Includes 0.04 FeS^. 
YI. Diorite, near Sonora, California. Ball. U. S. Geol. Sarvey No. 148, p. 218. 

W. F. Hillebrand, analyst. 
YII. Diorite, Captains Bay, Unalaska Island. Ball. U. S. Geol. Sarvey No. 148, 

p. 232, with 0.04 Fe&j and 0.02 NiO. W. F. Hillebrand, analyst. 
YIII. Diorite, Lampersdorf in Silesia. Cf. Zirkel, Lehr. Petrog., Yol. II, p. 485. 
Hampe, analyst. 
IX. Molecular proportions of No. 1. 
X. Average of analyses I to YII, inclnsive. 

For the sake of compariaoD, a number of other aDalynes of diorites are 
quoted, mostly from Bulletin 148 of the United States Geological Sur- 
vey. It is interesting to observe how nearly alike they are and how 
clearly they show the characteristic magma composition for a typical 
diorite — ^that is, medium silica, high alumina, medium iron and magnesia, 
high lime, moderate alkalies, with soda predominating strongly over 
potash. The alumina, lime, and soda condition the appearance of the 
plogioclase; with an increase of magnesia and iron the ferromagnesian 
minerals would become more prominent and the rock would pass into 
the gabbros; with potash predomiuating over soda, biotite and ortho- 
clase would become prominent and it would pass into the monzonites. 
The analyses here given are the best that have been made of diorites; 
most analyses are very crude and untrustworthy, especially as showing 
the relation between alumina and magnesia, oxidation of the iron, etc. 
One of the best, taken Arom Zirkel, is given under YIII. The average 
of these seven analyses, given in X, may be taken as representing a 
typical diorite magma; as such it may be of value for comparison. 
Undoubtedly a portion of the water in the analyses is not secondary, 
but original, and comes from the biotite and hornblende.^ 

From the molecular proportions given under IX of the table we may 
approximately calculate the mineral composition of the rock by reckon- 
ing the feldspars from the alumina, the iron ore from the ferric iron, 
and the hornblende from the magnesia, a little correction being applied 
for the small amount of biotite, apatite, etc., but as the others are the 
chief minerals the error is small and can not materially aiiect the resulU 
From this calculation we find that the rock contains — 

Per cent. 

Iron ore 2.5 

Biotite, apatite, etc 3.5 

Hornblende 13.0 

Andesiue 63.5 

Orthoclase 17.5 

Total 100.0 

> Cf. Onwitit von Darbach, Saner: Mitt. d. Grossh. Baden. Geol. Landesaust., Bd. II, pp. 261-256. 



i 



492 IGNEOUS ROCKS OP LITfLE BELT MOUNTAINS, MONTANA. 

The feldspars present are, Ab=36.2, An=27.5, Or=17.6. The dark 
minerals are 19 per cent; the light minerals 81 per cent. 

Structure and classifi-cation. — The remarkable feature of this rock 
is its stractore, which is ocellar or psendoporphyritic. It is remark- 
able to find all the feldspars collected into snch lumps or eyes, and 
the interspaces filled with hornblende. It recalls the ophitic structure 
of diabases in which the feldspars are idiomorphic and the augite fills 
the interstices between them. The rock, it is true, has. been subjected 
to severe dynamic pressure, the geologic evidence in the field and its 
appearance in the hand si^ecimen and under the microscope agreeing 
on this point. The hornblende, however, shows no evidence of this, 
but the andesine is cracked and granulated and the albite twinning 
lamellae are curved and broken. But it is difficult to see how pressure 
and shearing could have produced this structure if there had been an 
ordinary diorite to work upon. It is very much what would be pro- 
duced if a corsite,^ with its orbicular masses, were crushed and sheared. 
But here there is so marked a separation of the light and dark min- 
erals that one feels compelled to assume that this must be an original 
character — produced in the magma, perhaps, as an ophitic structure on 
a huge scale which was afterwards crushed and broken down by the 
dynamic forces. With the facts at present at hand regarding the rock, 
it seems useless to speculate further upon the origin of this structure, 
but so far as the author knows it has not been elsewhere described, and 
he desires to call the attention of petrographers to it in the hope that 
some light may be thrown upon it. 

Contact fa^nes of diorite, — Where the diorite comes in contact with 
the crystalline schists, and especially where it penetrates them in 
apophyses, it shows a marked endomorphic contact modification. The 
rock becomes of quite fine grain, compact, very dark in color, and in 
some places has scattered through it numerous flat tabular feldspar 
phenocrysts; it is here a true porphyry. On a freshly broken surface 
these feldspar phenocrysts are dark and not especially noticeable, but 
on a weathered surface they are white and strongly contrasted against 
the background, and it is here seen that they have a strongly pro- 
nounced flow structure, the flattened tables being arranged in lines, 
curves, etc. The groundmass, or the rock devoid of iihenocrysts, is 
very dark and glitters with innumerable mica scales, so that it has a 
somewhat minette or kersantitelike appearance. 

Under the microscope the minerals seen are iron ore, a[)atite, titanite, 
biotite, hornblende, andesine, orthoclase, quartz, and sericite. 

The iron ore, the apatite, and the occasional titanite in grains offer 
nothing unusual. The biotite is not abundant, and in considerable areas 
is entirely wanting, as in the main diorite. Hornblende is present in 
considerable amount, much greater than in the diorite, and is in rounded 
grains or small masses. The feldspar phenocrysts are almost entirely 



* Orbicular diorite or Napoleonite of Corsica. Of. Zirkel, Lehrbach der Petrog., Vol. II, p. 471. 



PIM80N.] APLITE8. 493 

converted into masses of sericite, or at least into a flne, scaly, micia* 
ceous, colorless mineral of high birefringence, and it is difficult to say 
more about them, except to determine, from unaltered scraps, that in all 
cases they are a plagioclase, and a rather basic one. They are in strong 
contrast to the rest of the rock', which is very fresh, and the feldspars 
of the gronndmass are andesine, with a subordinate amount of ortho- 
clase and a very little interstitial quartz. The groundmass feldspar 
is dotted through with small grains of iron ore. 

The most interesting thing about this contact rock is its structure 
and petrologic position. The structure is the sugar granular one 
characteristic of aplites — that is, it is panidiomorphic in the sense of 
Eosenbusch; all the grains, felds[)ar and hornblende, have an isometric 
development. The rock has, however, compared with the main diorite, 
a very considerable enrichment of the dark minerals; in the hand spec- 
imen, aside from 'the type which has phenocrysts, it looks like a kersan- 
tite — that is, it appears like a lamprophyre or is melanocratic in the 
sense of Brogger.' This enrichment in the dark minerals is of very 
common occurrence in intruded igneous masses, and need not be further 
dwelt upon beyond the influence it has upon the character of the rock 
produced. Aside from the type with rather abundant thin, flat phe- 
nocrysts of feMspar, one could call this variety of diorite a type of the 
spesaartite of Rosenbusch,^ with the description of which it very closely 
agrees. Bosenbusch speaks of these rocks as sometimes having a hol- 
ocrystalline porphyritic structure, and if we accept the view that a 
lamprophyre can contain feldspar phenocrysts, this variety of the 
rock would be a spessartite-porphyry. The occurrence of a lamprophyric 
border facies of an intruded diorite is of great interest. The type, 
which is purely lamprophyric, without phenocrysts, is in places quite 
gueissoid from a parallel arrangement of the components. What the 
quantitative relation of the facies which is porphyritic bears to that 
which is not, could not, from the nature of the outcrops, be ascertained. 

APLITES. 

All portions of the rock masses composing Yogo Peak — the nuge 
heavy black knobs, monoliths, and masses of shonkiiiite forming the 
western end, the masses and rock heaps of monzonite forming the mid- 
dle portions, and the platy debris and slide-rock areas of the banatite- 
like syenite of the eastern shoulder — are everywhere cut by narrow veins, 
or rather dikelets, if they may be so termed, of a light-colored a[)litic 
rock. In places these veins are very numerous, and as they resist 
weathering much better than the basic rock which they cut, they are 
likely to project as ribs or slight ridges on the rock faces. The cut 
shown in flg. 73 is a drawing of a rock slab about 3 feet square, approx- 
imately drawn on the scale of 2V, and this shows how numerous these 



lEmptivgestelneKristiaDiagebieteH; III, Ganggefolge des Laardallta, p. 262. 
•Maats. Gesteise, 3d ed., 1895-96, p. 532. 



494 IGNEOUS EOCKS OF LITTLE BELT MOUNTAINS, MONTAMA. 

little dikes are. They vary iu widtb from a fraction of an inch to perhaps 
2 feet or more, and their length can not be told. They clearly repre- 
sent a later intrusion of more acidic feldspathic magma after the main 
masses had cooled, crystallized, and broken through contraction into 
innumerable fragments. 

These dikes are composed of a fine-grained nonporphyritic rock of a 
pale-red color. Under the microscope it is seen to be composed of the 
same minerals as the syenite and monzontte ^bich it cuts; that ia to 
say, pale-green diopside surrounded by mantles of common green horn- 
blende which appears paramorphiu in character, shreds of biotite, a very 
little iron ore and apatite, a very small amount of oligoclase, a soda 
orthoclase, which is the chief mineral, and a little interstitial quartz. 
The stmctore is granular allotriomorphic or panidiomorphic, as one 
chooses to regard it, characteristic 
of aplitic rocks, tehere the grains 
are of general isometric character, 
without distinct crystal form and 
nearly of a size, perhaps 0.5 mm, 
in diameter. 

The iron ore is a mere trace, the 
iron appearing almost wholly in the 
ferromagnesian minerals. These 
latter arein proportion to the white 
components about as in the bana- 
tite-like syenite of Yogo Peak, ia 
which the dark are to the light 

no. TS^Kock mrraoe »t Togo Pert, ahowing components a* 1 : 5 J that IS, 20 pCT 

cent of the former to 80 per cent 
of the latter, which in this case is almost entirely soda orthoclase, as 
the amount of plagioclase is very small, not nearly so great as in the 
banatite. 

Banatite-aplite variety. — In one of the dikes the rock contains dark- 
colored angular inclnsions which, so far as can be determined, seem to 
consist of fragments of the more basic shoukinitelike rocks through 
which the magma has pressed upward. This dike is also of interest in 
that, like the baiiatit«, it contains a large pro[>ortion of plagioclase, 
and the feldspar minerals have the same tendency to a sqnare, tabular, 
idiomori>hic form. Between these the minerals, orthoclase, and quartz 
filling the interspaces are rather finely granular, and there is thutr a 
tendency to a porphyritic strncture. The diopside has almost wholly 
given place to hornblende, and it is indeed an interesting fact that the 
diopside in the Togo Peak rocks changes into hornblende in direct 
ratio with the increase of silica. 

The Togo Peak massif is also cat by dikes of granite-porphyry, bat 
these are mentioned in connection with that rock (p. 502). 



PIB880N.] APUTES. 495 

SHBABED APLITE OF NEIHABT. 

The crystalline schists aronnd Neihart below the Belt qnartzite are 
in many places filled with granitic injections which appeared as dikes, 
etc. In the severe dynamic pressures and shearing to which these 
rocks have been subjected, these igneous rocks also have suffered. 
This is well illustrated by an aplite from near the Moulton mine which 
has taken on a gneissoid structure. When the thin section of one of 
these is studied under the microscope, it is seen that the minerals no 
longer have the forms characteristic of them in unaltered igneous rocks. 

The quartz is strewn out in angular fragments, which extend in long 
lines, and the larger pieces have an undulatory extinction, showing opti- 
cal strains due to pressure. The alkali feldspar also shows certain 
peculiarities. It is mostly a microcline, but the grains, instead of being 
either orthoclase or microcline in clear, well-defined areas, such as, for 
instance, one finds in many rocks, like some of the elseolite-syenite of 
Litchfield, Maine, are indefinite and indeterminate between orthoclase 
and microcline. Some larger grains of feldspar extinguish uniformly, 
are untwinned, and appear to be homogeneous orthoclase and remnants 
of larger crystals, perhaps phenocrysts. In others, however, there is an 
undulatory rolling extinction which is much like that in the quartzes. 
Unlike that in the quartzes, however, it often starts irom certain spots 
and rolls in several directions, and one may trace such areas into those 
of weakly defined microcline, and these into more perfect material. Yet 
the microcline itself is never sharp, clean, and well defined, as one finds 
it in pegmatite dikes; it, too, has a rolling extinction. These transitions 
occur not only in different grains but in the same grain, and every gra- 
dation may be traced in such a grain, from homogeneous orthoclase into 
areas of undulatory extinction, and from these into the microcline.^ 

Origin of the microcline. — The whole appearance of these two miner- 
als is such that one feels forced to believe that the feldspar can not be 
in its original condition, but has suffered a change of some sort — that 
either the orthoclase or the microcline is not the original mineral. More- 
over, the fact that these appearances occur in a rock that has been 
much sheared, has, of course, a great tendency to strengthen this opin- 
ion, though it is not the cause of it. At all events, it may be set down 
as almost certain that one or the other is not the original mineral, but 
whether the microcline has been changed into orthoclase, or vice versa, 
is not easy to say. Microcline occurs in aplites, but it is far less com- 
mon than orthoclase; the largest feldspars in the rock are orthoclase; 
the smallest fragments, or those most crushed, are invariably micro- 
cline. The microcline itself is ill defined in character, and these appear- 
ances lead to the belief that it is the secondary mineral. More- 
over, it is a common phenomenon that in a variety of minerals 
twinning is produced by pressure, and not the reverse; and it would 

' See also Zirkel, Lehrbuch der Petrog., 2d ed., Vol. II, p. 4, 18M. 



496 IGNEOUS BOCKS OF LITTLE BELT HOL^TAJNS, HONTAKA. 

• 

also be natural to expect that in such a crushing process, if a sub- 
stance were dimorphous, it would have its symmetry reduced from a 
higher to a lower state. Even if one believes that orthoclase is only 
snbmicroscopic microcline, it would be easy to understand that an 
extremely complicated system might be reduced to a simpler one. The 
general prevalence of microcline in the crystalline schists and its com< 
mou association with gneissoid structure are well known. In the Xew 
Haven area granites occur, which in those places where they have suf- 
fered dynamic metamorphism contain microcline of the same character 
as that described above. There seems, therefore, very clearly to be, 
in certain cases, some casual relatiou between the occurrence of micro- 
cline and dynamic metamorphism ; whether primarily induced by the 
pressure or other causes which the dynamic metamorphism has set at 
work, is not easily told. Its occurrence in pegmatites and in certain 
unaltered igneous rocks rich in alkalits, is of a quite different char- 
acter from this, and is not here considered. 

GBANITE-SYENITE-APLITE OF SHEEP CREEK. 

The sheets of minette intruded in the Cambrian shales at the head 
of Sheep Greek, and whose petrology is described later in this work, 
are cut in a number of places by narrow dikes of a white feldspathic 
rock from 6 to 12 inches in width, which has partly the aspect of a 
porphyry, partly of an aplite, and whose mineral character shows it to 
lie between the granite and syenite groups. The cutting made for the 
roadway along Sheep Creek has well exposed these narrow dikes in one 
or two places, and they are clearly seen cutting the dark-colored shales 
and intruded sheets of minette. The latter has a soft, decayed, smooth 
character in its exposure, across which the dikes break irregularly, with 
crooked courses and with very massive jointing for so small dikes. 

The rock in the hand specimen is gray, and appears like a very fine 
aplite of syenite aspect, rather thickly spotted with very small formless 
phenocrysts of feldspar which are not much larger than the ground- 
mass in which they lie. 

In thin section this groundmass is seen to be made up of alkali feld- 
spar and quartz in a rather coarse inicrogranitic structure, and is brown 
and turbid with kaolin. In it lie scattered small phenocrysts of oligo- 
clase, orthoclase, and green hornblende, with fewer of biotite. The 
oligoclase has an average character of AbsAui, and this makes the 
orthoclase a soda orthoclase of the composition Ori Abi. An analysis of 
the rock by Dr. W. F. Hillebrand resulted as follows : 



PIB880V.] 



APUTE8. 



497 



Analysis of granitC'syenite-aplite from Sheep Creek, Little Belt Mountains, Montana. 



Constitiient. 



SiO, 

A1,0:, 

Fe.O:, 

FeO 

MgO 

CaO 

Na,0 

KiO 

HaO-l-llOo ' 

H,0— 110° 

TiOa 

P.O5 

COa 

MnO 

BaO 

SpO 

U2O 

Total 



I. 


II. 


66.29 


1.105 

1 


15.09 


.146 


1.37 


.008 


1.17 


.016 


2.39 


.060 


2.38 


.042 


3.96 


.064 


4.91 


.062 


.60 


.033 


.39 




.27 




.15 




.45 


.010 


.06 




.30 




.07 




Trace. 








99.85 









I. Percentages. 
II. Molecular proportions. 

This composition recalls closely that of some of the granite-porphyries 
of the laccoliths of this region, as is shown later. From this analysis 
and the study of the section we may approximately calculate the min- 
eral composition as follows : 

Per oont. 

Magnetite 2. 

Biotite 3.3 

Hornblelide 7.4 

Oligoclase 16.2 

Soda orthoclase 47. 6 

Quartz 18.4 

Kaolin " 4.1 

Calcite 1.0 

Total 100.0 

The occurrence of this aplitic dike, although in texture it might 
equally well be classed as a porphyry, is from a genetic point of view 
of great interest, since we find it cutting the minettes which form so 
prominent a feature of this part of the district, and itself having the 
composition of the great masses of acid rock, the granite- porphyries of 
the laccoliths of the region, while the minette has very nearly the com- 
X>osition of the shonkinite of Yogo Peak. 

20 GEOL, PT 3 32 



OHAPTBE III. 

ACrDIC FEIiDSPATHIC PORPHYRIES OF THE liACCOIilTHS, 

DIKES, A1^I> SHEETS. 

INTRODUCTION. 

The classification and consequently the arrangement and description 
of these rocks offer some difficulties. Not only do they differ in their 
mode of occurrence geologically, but there are many transitional types 
among them, the transitions being in several cases of much more impor- 
tance locally than the more commonly known types of such rocks, and, 
further, these transitions occur not only in different masses, bat often 
in the same mass. On the other hand, the description, according to 
geologic occurrence, would involve a vast amount of repetition. It has 
seemed best, therefore, to describe the rocks under the headings of well- 
recognized types, and discuss the transitional forms under them. 

The types of acid porphyries occurring in the district are granite- 
porphyry, granite-syenite-porphyry, syenite-porphyry, syenite-diorite- 
porphyry, diorite-porphyry, rhyolite-porphyry (quartz-porphyry), and 
trachyte (bostonite). 

GRANITE-PORPHYRY. 

From a geologic point of view this rock is one of the most important 
in the Little Belt Mountains, as it forms many of the largest laccoliths 
and intrusive masses and comprises many of the sheets and dikes. 
The large amount of material these afford for study is grouped and dis- 
cussed under several types of this rock which are of local importance, 
and to which, therefore, local names have been given, such as the Wolf 
Butte, the Barker, the Yogo Peak, and the Carpenter Creek types. 

WOLF BUTTB TYPE. 

One of the most important occurrences of granite-porphyry is the 
large area of Wolf Butte and the peak southward from it. This great 
laccolith is intrusive in the shaly Cambrian beds below the Lower 
Carboniferous, whose heavy limestones it has upraised and displaced. 
The mass has been much eroded and cut into, so that the prevailing 
type of rock is clearly seen. It is of a pale-gray color, of a rather dense 
groundmass, thickly sprinkled with white feldspar phenocrysts from 
3 to 5 mm. in diameter, with occasional ones of about 20 mm. in length 
and of tabular form ; among them one sees, less frequently, dark-gray 
phenocrysts of quartz from 2 to 8 mm., averaging 3 to 5 mm., in 
diameter. The appearance of the rock is rather dull and luster less, 
especially the feldspar phenocrysts, as if considerably altered; some 
of the larger odcs, however, are clear, transparent, and of sanidine- 
like character; these are unstriated and are of orthodase. A very 

498 



PIB880ir.] 



GEANITE-POEPHYRY. 



499 



few small black tablets of biotite complete the list of megascopic 
minerals. The rock carries inclusions of a mica-syenite of rather fine 
grain which may be several centimeters in diameter. 

Under the microscope the quartz phenocrysts show the common 
phenomenon of rounding and of absorption embayments, and they are 
frequently surrounded by coronsB of the groundmass. The large 
phenocrysts and most of the small ones are of orthoclase, many of the 
smaller are of oligoclase, and small penocrysts of the latter are inter- 
grown in the large crystals. The biotite is of the usual pleochroic 
brown variety; a small amount of iron ore is present. 

The groundmass is a very fine grained mixture of quartz and alkali 
feldspar of distinct microgranitic structure, and the whole rock is thus 
seen to be a common and well-known type; it is, in fact, a typical 
granite-porphyry in all respects, and it is to be regretted that better 
material could not be procured for the analysis. The dull appearance 
of the rock is seen under the microscope to be due to the presence of 
considerable kaolin, which occurs chiefly in the groundmass, rendering 
it turbid and brown in color. A little calcite, and probably muscovite 
also, is present. 

An analysis of this type has been made by Dr. W. F. Hillebrand and 
is herewith given. 

AncUyais of granite-porphyry from Wolf Butte, Little Belt Mountains, Montana, 



CoiiBtitaaiit. 



SiO, 

Al^Oj 

FejOa 

FeO 

CaO 

MgO 

NaaO 

KaO 

H2O— 11(P 
H2O + IIOO 

TiOa 

P8O5 

COs 

80a 

CI 

MnO 

SrO 

BsO 

Total 



I. 



69.68 

14.97 

.79 

.34 

2.10 

• 66 

3.38 

4.40 

1.09 

.92 

.28 

.17 

.88 

Trace. 

Traoe. 

Trace. 

.06 

.14 



99)86 



II. 



1.161 
.145 
.005 
.005 
.016 
.036 
.054 
.047 



.050 



.020 



I. Percentages. 
II. Molecalar proportions. 



600 IGNEOUS ROCKS OF LITTLE BELT MOUNTAINS, MONTANA. 

Theoomposition is that of a typical magma of the granite family, and 
it needs no farther comment. The amount of water and carbonic acid 
shows that the rock is not very fresh ; the amoant of calcite seen in the 
section would not account for the amoant of carbonic acid, but as the 
rock is greatly cracked, making it difficult to obtain a fair-sized hand 
specimen, and as the laccolith has solidified under a heavy cover of lime- 
stones, since eroded away, it seems clear that these joints have become 
filled with secondary films of calcite, which might not show in the 
sections and yet be sufficient to account for the carbon dioxide. 

From the molecular proportions given in the second column, we may 
readily calculate the mineral composition to be: 

Per cent. 

Iron ore 1. 13 

Biotite 3.01 

Oligoclase 17.06 

Orthoclase 37.48 

Qaartz 30.24 

Kaolin 7.25 

Calcite 2.00 

Besidue 1.71 

Total 99.86 

The residue contains the exti a water below 110^, traces of barium, 
strontium, manganese, etc., of which no account has been taken and 
which can have no important bearing on the above result. The oligo- 
clase has been reckoned as Abs Aui, as the optical determinations, accord- 
ing to Michel Levy's method, on Carlsbad and albite twins have shown 
it to have that ratio of soda to lime. This proves the orthoclase to be 
a soda orthoclase of the composition Ab30r4, and the determination of 
the extinction angle on the face (010) of the phenocrysts made on cleav- 
age material shows the angle of extinction to be 7^ 30' -f, an extinction 
greater than that of pure orthoclase but not so great as many soda 
orthoclases show, which generally contain, however, more soda. 

Contact — At the contact with the shaly Cambrian beds, as is well 
seen on the saddle between Taylor Peak and the next peak north 
of it, the granite-porphyry magma has solidified as a dense, pinkish, 
felsitic looking rock without phenocrysts. It is, in fact, a typical fel- 
site of a well-known type, and as such deserves no farther mention. It 
gradually, as one proceeds inward toward the laccolith, changes to the 
granite-porphyry mentioned. Its lack of the larger phenocrysts of the 
granite-porphyry is very striking and is its most interesting feature, as 
it shows that these were not present in the magma at the time of it« 
intrusion, but were formed later. 

The exomorphic contact action is comparatively slight; the limy, 
8haly beds of the Cambrian are indurated for a few yards, and their 
cavities are coated with small crystals of grossular garnet and 
pyroxene; the metamorphism is similar to that described on page 540. 



piBasoN.] GRANITE-PORPHYRY. 501 

Mixes Baldy is composed of exactly the same type of rock, but the 
material shown in its outcrops and surface exposures is much more 
altered and decayed by weathering. 

OABPENTEB OBEEK TYPE. 

This is a variation of the foregoing type of granite-porphyry. It is 
called here the Carpenter Creek type, as it occurs locally well devel- 
oped as an intrusion in the crystalline schists above Carpenter Creek. 
It is a rather light-gray rock, thickly spotted with pale-pink orthoclase 
phenocrysts 15 to 20 mm. long, which are generally Carlsbad twins. 
Between them are many small light-colored feldspar phenocrysts, which 
the microscope shows are oligoclase', and rather infrequent gray 
rounded phenocrysts of quartz from 6 to 8 mm. in diameter. The gray 
groundmass is speckled with minute black dots of a ferromagnesian 
mineral which under the microscope proves to be biotite. The study 
of the section adds nothing of special interest (excepting a character 
of the quartz phenocrysts, to be described later) beyond what has 
been mentioned in the foregoing type. There is a recurrence, how- 
ever, of the biotite in the groundmass in little strips, shreds, etc. It 
differs from the preceding type chiefly in the hand specimen; that is, 
megascopically. The Wolf Butte type is remarkable for the abundant 
and large quartz phenocrysts. The feldspar phenocrysts are impor- 
tant, but not so important as in the Carpenter Creek type, while in the 
latter the quartz phenocrysts are not abundant. Both types have the 
microgranitic groundmass. 

To this type belongs the rock of the dike which cuts the limestone 
forming the bench north of Gold Kuu and near where it enters the 
main stream at Barker. The rock is of a brownish-gray color, and the 
feldspars and quartz phenocrysts are much smaller than in the preced- 
ing type. It contains hornblende, both as phenocrysts and in small 
prisms in the groundmass. Under this type come also the heavy 
intrusive sheets in the Cambrian beds, and just at their base, on the 
Sawmill Creek branch of Belt River. The road from ^eihart to White 
Sulphur Springs passes up this creek, and its cuttings have exposed 
the igneous material. The rocks are light-colored porphyries with abun- 
dant phenocrysts of feldspar and less abundant ones of quartz. The 
same may be said of the dikes cutting Yogo Peak, and of the dike on the 
divide between Yogo Peak and Big Baldy Mountain, which occurs a 
little distance south of where the heavy talus slopes of Big Baldy 
Mountain reach the divide. The intruded sheet at the base of the 
Cambrian, which is exposed at the head of Dry Fork Belt Creek, on the 
spur running northward from Big Baldy Mountain, although somewhat 
deficient in quartz andinclining toward the syenite-porphyries, seems 
best placed here. 

Structure of quartz phenocrysUt. — In the granite-porphyries which . 
have been previously described, especially in those from Wolf Butte 



502 IGNEOUS BOCKS OF LITTLE BELT MOUNTAINS, MONTANA. 

and Snow Oeeek, which have large to good-sized phenocrysts of quartz, 
a corions structure has been observed. It is a multiple intricate twin- 
ning, seen with crossed nicols, by which the quartz of the phenocrysts 
looks almost exactly like a microcline, but the structure is much finer 
than in the generality of microclines; it has the same basket-work 
appearance. The difference in birefringence between the parts is less, 
however, than in microcline, and the phenomenon of twinning is faint, 
as in leuoites. Not every phenocryst in a section shows this-^nly 
the larger ones. That the material is really quartz is shown by its 
form — that of the dihexagonal pyramid — the embayed character so well 
known in quartz phenocrysts, and its uniaxial positive character, all of 
which have been observed on crystals showing this twinned structure. 
I have been able to find no reference to anything similar in the litera- 
ture, and can suggest nothing plausible to account for this curious 
structure. 

TOGO PEAK TYPE. 

This rock forms a great mass on the divide running east from Yogo 
Peak. It begins in the saddle just east of Yogo Peak proper, the 
syenite of the eastern part appearing to merge gradually into this 
tyi)e. It is feldspathic, light colored, dotted with small amphibole 
prisms; its chief characteristic, however, is that it is very porphy- 
ritic, with large eqnidimensional feldspar phenocrysts. These resist 
weathering better than the groundmass and project, giving the rock a 
rude resemblance to a conglomerate. In the saddle mentioned many 
of them have fallen out and form a rough gravel. 

Microscopic characters, — In thin section the microscope discloses the 
following minerals : Apatite, zircon, titanite, hornblende, iron ore, bio- 
tite, orthoclase, oligoclase, and quartz. 

The apatite and zircon are found in occasional small grains, usually 
embedded in biotite; titanite sometimes occurs in well-formed crystals, 
but is more likely to be in irregular masses, and it commonly accompa- 
nies the hornblende. 

The hornblende occurs in irregular, shredded, broken crystals, which 
are often anhedral. Its angle of extinction is small, its pleochroism 
good, but not strong, with jc=blui8h green, ft=olive green, a=greenish 
yellow, absorption jc > |r > a. It contains titanite, iron ore, and bio- 
tite as inclusions, and some crystals are hollow and contain pieces of 
quartz and feldspar of the groundmass. 

The biotite is the usual dark-brown, strongly pleochroic variety found 
in granitic rocks. It is usually in good crystals. Its period of forma- 
tion appears to follow the hornblende. The actual amount of both 
hornblende and biotite is comparatively small, and they are strictly 
accessory minerals, as biotite usually is in granitic rocks. 

Following the biotite and hornblende in the order of crystallization 
comes the oligoclase. It is found in short, broad tablets inclining to 
columnar forms on the a axis, and sections across these are perfect 



PIB880N.] GRANITB-POBPHTRY. 503 

squares; it is, therefore, qaite idiomorphic. It ranges from 1 to 2 mm. 
in length, and is a rather abundant phenocryst. It is always twinned 
according to the albite law, often following that of the Carlsbad; a 
few pericline twins were seen. It varies in composition from Ab4 Ani 
to Abs Ani ; thns, a section oriented in the zone perpendicolar to 010, 
and showing a negative bisectrix centered in the field, gives extinction 
angles of 0^ for both sets of albite lamella; the section is zonal and 
the onter zone extinguishes at 50 ; the equal illumination between the 
two is produced at a little over 40<^, and we have here, following Michel 
Levy's tables, an oligoclase with from 15 to 20 per cent An. The par- 
allel extinction of numerous other sections in this zone confirms this 
determination. 

By far the most abundant mineral is the orthoclase, which occurs in 
pheuocrysts, from examples 2 or 3 mm. long, stout columnar on the d, 
axis, and showing the forms c (001), b (010), m (110), and x (lOl) ory (201), 
down to individuals 1 mm. long and less perfect in form. The largest 
pheuocrysts, while not abundant, are rather regularly sprinkled through 
the rock. 

The determination of the mineral as orthoclase rests on the fact that 
in sections parallel to h (010) an obtuse positive bisectrix emerges and 
the plane of the optic axes lies at 8^ from the trace of c (001) in the 
obtuse angle /?. Sections perpendicular to the negative bisectrix show 
a rather large angle in air (and therefore not sanidine), and the extinc- 
tion is rigidly parallel to the trace of o(OOl). The mineral is quite 
fresh and clear. 

Quartz occurs only very rarely in such a manner that it may be said 
to rank as a phenocryst; it then occurs in irregular areas with projecting 
tongues and small fringes of attendant quartz with similar orientation. 

The groundmass, compared with the total bulk of the pheuocrysts, 
is rather small in amount; it occupies mostly the angular interspaces 
between the phenocrysts, is rather coarse, and has a microgranitoid 
structure; it is chiefly composed of quartz and alkali feldspar, with a 
very subordinate amount of oligoclase. The quartz grains are very 
likely to be disposed like beads around the largest feldspars. 

Structure and classification, — The dominant character is given to this 
type by the great abundance of the phenocrysts, the small amount of 
groundmass, and the gradual passage of the phenocrysts by graduation 
in size into the groundmass. The structure is therefore really a transi- 
tion between the hypidiomorphic structure of granular abyssal rocks 
and the typical porphyritic structures. A strikiugfeature is the rounded 
granular, idiomorphic character of the quartz grains in the ground- 
mass, which latter, indeed, has the aspect of a rather fine aplite. 

In normal position this rock must stand between a typical granite- 
porph3rry with characteristic quartz phenocrysts and abundant quartz 
in its groundmass and a syenite- porphyry devoid of quartz phenocrysts 
and with little to no quartz in the groundmasa 



504 IGNEOUS ROCKS OF LITTLE BELT MOUNTAINS, MONTANA. 

With respect to its feldspar coastitaents, the alkali one so plainly rules 
that the rock clearly belongs in the granite-syenite series; it does not, 
however, belong to the alkaline family, but to the one approaching 
the diorites. If Brogger's group of monzonites, that is orthoclase- 
plagioclase rocks standing between the alkali feldspar series and the 
soda-lime feldspar series, be taken into account, then the type just 
described stands midway between the alkaline series and the monzonites 
(adamellite). 

Transition forms. — This type, by simple decrease of quartz and 
increase of other constituents, goes over locally into subvarieties which 
may be properly termed quartz-syenite-porphyries. One notices this 
first by the loss of quartz as a phenocryst and its entire restriction to 
the groundmass, by which it becomes very subordinate in amount. 
All the other components remain the same, and the gradational type 
needs no further mention. Though somewhat coarser grained, it is 
extremely similar to the type composing the great mass of Big Baldy 
Mountain, elsewhere described. For this reason no analysis of it has 
been made. It is the type which geologically borders the syenite of 
Togo Peak proper. 

BABKEB TYPE. 

The rock composing the great laccolith of Mount Barker is a granite- 
porphyry, but of diflFerent character from the typical one of the Wolf 
Butte and Mount Mix laccolithic masses. It is a grayish porphyry which 

appears rather granular, 

. ..■,.. -.,..... -..-^ 1 dotted thickly with feld- 

liif/ Jf I - ^ f-j /If /~f -f- rTy 'A spar phenocrysts from 5 

m/i l< ^4'-^^ "TJ-I- / / / "tL!-\»^ to 1^ J^ni. long and of tab- 

^irrr -- - Y"? ^rS^^^^ ^= --^^^^^^ - "^^ j?^ ^^*^ habit, with occasional 

\\J:^^^^^r^^^\z\^^^ \ ^ ''-\ --\ll scattered ones three to five 

\\:\X^^^ ^ times as large. These are 

V-  - "- ""^1 unstriated and of ortho- 

FiQ. 74.«Manebach twin. ^^^^^ rpi,^ g^^faCC of the 

rock is well sprinkled with small, dull, black spots which are an altered 
ferromagnesian mineral. Quartz as a phenocryst is almost entirely 
wanting, and this is the most marked feature in contrast with the typi- 
cal granite-porphyry of Wolf Butte. 

Under the microscrope the sections show phenocrysts of orthoclase 
and oligoclaseof the composition approximately Abs An i lying in a quartz 
and alkali feldspar groundmass. There is a little iron ore, some fresh 
brown pleochroic biotite, and pseudomorphs, in part of chlorite and mus- 
covite after biotite, in part of serpentine, iron ore, etc., after hornblende. 
In some sections the amphibole is fresh and of the usual columnar 
form and olive-green color of common hornblende. When it increases 
in amount, biotite diminishes, and vice versa. When most abundant, it 
is accompanied by occasional small, well-formed crystals of titanite. 

The groundmass is an interesting feature. It is somewhat variable 



OOf 



PIB8SON.] 



GB ANITE-PORPH Y R Y. 



505 



in its degree of granularity, bat is always micropoikilitic, and in tbe 
type analyzed from the sammit of the mountain it is rather coarse. 
When examined with a high power the small feldspar grains are seen 
lying embedded in fields of quartz, each of which forms one quartz 
crystal, while the feldspar has no orientation. There are a few clear 
areas of quartz which might almost be termed phenocrysts. 

One of the large orthoclase phenocrysts in the sections is of interest 
as it chances to be a Manebach twin cut almost exactly parallel to the 
face 010, and it shows the relations exhibited in fig. 74. There are two 
lines of growth clearly seen revealed by a sharp line of tiny inclusions 
and by a slight but x>erceptible difference in optical properties, the 
outer shells extinguishing at perhaps half a degree to a degree greater 
angle than the main inner portion, indicating probably an increase in 
the albite molecule. That the albite molecule is present is indicated 
by the extinction angle of 8^, and this is confirmed by the analysis of 
the rock. It is interesting to notice that the mineral shows a good 
parting parallel to the prism, as indicated in the figure. 

An analysis of the rock has been made by Dr. Hillebrand with the 
results here given : 

Analysis of granite-porphyry from Mount Barker , Montana, 





ConatltueDt. 


I. 


II. 




8iO« 


68.60 

16.13 

2.22 

.44 

.72 

1.36 

4.37 

4.89 

.20 

.58 

.32 

.18 

Trace. 

Trace. 

Trace. 

.27 

.09 


1.143 
.156 
.013 
.006 
.018 
.024 
.070 
.052 
.011 




AIjOs 




FejOa 




FeO 




MffO 




CaO 




NajO 




K.,0 




H,0 110^ 




HaO + llO^ 




TiOi 


.004 




P-^Os 




SO3 


t 




CI 


1 




MnO 






BaO 






SrO 






Total 






100.37 










I. 
II. 


Percentages. 
Moleonlar proportions. 







506 IGNEOUS HOCKS OF LITTLE BELT MOUNTAINS, MONTANA. 

From this analysis we may reckon, using the molecular ratios given 
in the second column and taking into account the relations and kinds 
of minerals seen in the section, that the rock has approximately the 
following composition : 

Peroent. 

Iron ore 1.4 

Biotite 4.6 

OUgoclaae 25.6 . 

Orthoolaae 36.2 

Quartz 26.2 

Hornblende and alteration prodacte 7.0 

Total 100.0 

This alteration material comprises the chlorite, serpentine, etc., and 
may be regarded as the residue; the figure given is not very accurate, 
but can not be far from the right amount. The oligoclase is reckoned 
as AbsAui, as shown by the microscope; the orthoclase by this reckon- 
ing becomes, as an average, exactly OriAbi, and must therefore be 
mainly a soda orthoclase. 

The peculiarity of this type of granite-porphyry lies in the fact that 
with so abundant quartz there is none appearing in definite pheno- 
crysts; it is all in the groundmass. Here, however, it is so abundant 
as to condition the poikilitic structure, and in fact, at first sight, the 
hand specimen appears granular, much like an aplite. Study with the 
microscope shows us, however, that the grains are really quartz sponges 
filled with unoriented grains of orthoclase. 

THUNDER MOUNTAIN. 

The porphyry composing the great laccolith of Thunder Mountain 
also comes under the head of this Barker type. It is a light-colored 
rock, thickly dotted with numerous phenocrysts of feldspar, which 
occasionally attain large size, and with small black spots of hornblende 
or shining biotite. The rock appears quite fresh and unchanged; in 
the enormous exposures on the mountain side it occurs in large plates 
and rhomboidal blocks. 

Under the microscope this type appears like the preceding; it con- 
sists of iron ore, apatite, biotite, hornblende, plagioclase, orthoclase, 
and quartz. 

The biotite, of the usual brown pleochroic type, is quite idiomorphio 
and moderately abundant; the hornblende is the olive-green variety of 
common hornblende. It is in columnar crystals defined by the prism 
faces. A few resorptions and alterations to augite (f ), iron ore, and 
biotite were seen; in these cases the biotite is on the outer boundary, 
where the magnesia of the hornblende has come in contact with the 
potash and alumina of the feldspar. The hombleude is more plentiful 
than the biotite. The plagioclase is variable; some of the phenocrysts 
are of audesine, some of basic oligoclase, some are zonally built; there 
is very little in the groundmass, and that appears to be oligoclase. 
It is not exactly easy, with these varying data, to determine what the 



PLATE LXXIV, 



507 



PLATE LXXIV. 

Thin Sections of Olivine and Granite-Porphyry. 

A, Resorbed olivine in shonkinite of Yogo Peak. The central core of yellow 
and black is the remnant of the olivine (now altered) ; aronnd this is a band of light- 
brown enstatite grannies, and ontside of this another band of green biotite; the 
whole surrounded by white orthoclase, with one or two grains of green angite and 
brown biotite. Actual size of field, 2 mm. multiplied by 38, section seen in polarized 
light, uncrossed nicols. 

Bf Granite-porphyry of Thunder Mountain. Ferromagnesian minerals, green 
hornblende and brown biotite, seen with uncrossed nicols; magnetite dark blue. 
Orthoclase, plagioclase, and quartz seen with crossed nicols. The groundmass has a 
micropoikilitlc structure. Actual size of field, 2 mm. multiplied by 38. 

508 



:0L0G1CAL SURVEY TWEI 




THINSKCTIONS 01-' OLIVINK AND GR.\N[TR -PORPHYRY 



PIB880N.] 



GRANITE-PORPHYRY. 



509 



average plagioclase is, for the pari>ose of oalcalating how much of the 
albite molecules shall be assigned to the anorthite and how much to 
the soda orthoclase. After carefhl study of the sections and estima- 
tion of all the data at hand, it is determined that it must be very close 
to an oligoclase Ab2Ani; it certainly runs in some cases into andesine, 
and it may thus be that the weight of plagioclase as reckoned is too 
great, but probably not very much. 

The orthoclase presents the same features as those described for the 
rock from Mount Barker; its extinction angle on 010 is 8^ to 9<^, which 
indicates the presence of soda, and the calculation of the analysis shows 
that it is Or3Ab2. Besides the usual Carlsbad twins, Manebach twins 
also occur, as shown in fig. 74. The orthoclase forms the largest pheno- 
crysts, the plagioclase is likely to be relatively small. 

The groundmass consists of alkali feldspar and quartz, with a little 
oligoclase. The structure is pronouncedly micropoikilitic, with quartz 
as the oriented cementing mineral; it is considerably coarser in grain 
than the Mount Barker variety, and the quartz sponges are much 
larger, but the effect is the same. An endeavor to show this ground- 
mass with its poikilitic groups has been made in PI. LXXIY, B. 

As this rock appeared much fresher than that from Mount Barker, an 
analysis of it has been made by Dr. H. N. Stokes, of the United States 
Geological Survey, with the following results: 

Analysis of granite-porphyry from Thunder Mountain, Montana. 



Conatituent. 



SiOa 

Al,Oj 

Fe.O:, 

FeO 

MgO 

CaO 

NaaO 

K,0 

H^O — li(P 
H.2O 4- 1100 

TiOi 

PiOft 




67.44 

15.78 

1.58 

.85 

1.43 

2.38 

4.11 

4.87 

.32 

.70 

.32 

.21 



SO3 Trace. 

CI ' Trace. 

MnO Trace. 

BaO 2^1 

SrO 09 



1.124 
.153 
.010 
.012 
.035 
.0-43 
.066 
.052 
.018 
.039 



Total 100.32 



I. PercentageB. 
II. Molecular proportions. 



510 IGNEOUS BOCKS OF LITTLE BELT HOUNTAINSy MONTANA. 

The correspondence in comiKmition between this and the Barker rock 
is very close — ^they are rather low in silica, and in this respect approach 
the syenite-porphyries. 

From the table of molecular ratios given in the second colamn, and 
aided by the study of the thin section, we may calculate the approximate 
mineral composition to be — 

Per cent. 

Magnetite 2.4 

Hornblende 3.5 

Biotite 2.0 

Oligoclaee 28.9 

Orthoclaae 42.8 

Qaartz 20.4 

Total 100.0 

The above composition places this rock in the granite-porphyries, 
though the lack of quartz phenocrysts gives it a tendency toward the 
syenites. It also has a tendency toward the diontes, and clearly does 
not belong in the alkaline subdivision of the granite group, but at the 
head of the granito-dioritic family. Lindgren,^ who has previously 
given a description, but no analysis, of this rock, called it a dacite, as 
its age was supposed to be post-Cretaceous and the prevaling method 
of classification at that time being based on the character of the feld- 
spar phenocrysts, and as these are more numerous than those of ortho- 
clase, it was naturally classified as a dacite. When, however, one has 
an analysis to calculate from, and the rock including thegroundmass is 
considered as a whole, then it is seen that orthoclase predominates, as 
shown above. 

If the lack of quartz phenocrysts be taken into account in the classi- 
fication, such rocks as these might well be caUed granite-syenite- 
porphyries. 

TIGER BUTTE. 

The rock composing the laccolith of Tiger Butte is of this type; in 
the hand specimen it is scarcely to be distinguished from the Thunder 
Mountain rock^ in thin section it is exactly like the Barker type, and 
needs no further mention. 

BIO BALDT MOUNTAIN. 

The rock composing the great laccolith of Big Baldy Mountain 
appears to belong in this group. Like those preceding, it is light col- 
ored, feldspathic, with numerous small phenocrysts of oligoclase, also 
some of orthoclase, with occasional large, well-formed idiomorphic crys- 
tals of orthoclase up to an inch or more long (15 to 25 mm.) which show 
the common forms c (001), b (010), m (110) and y (201); they are often 
Carlsbad twins. The groundmass is dotted with specks of ferromag- 
nesian minerals which never attain large size. 

The microscope shows in addition iron ore, titanite, apatite, horn- 
blende, biotite, and quartz. 

1 Tenth Censas, Vol. XV, p. 722 ; Proc. California Acad. Nat Sol., 2d series, VoL HI, p. 42. 



PIBSbON.] 



GRANITE-PORPHYRY. 



611 



Tbe hornblende is a rather carious pale leather-brown variety, very 
slightly pleochroic, of very low birefriugence and small extinction angle. 
The study of the plagioclase shows it to be a rather basic oligoclase of 
the average composition Ab^ Ani. The orthoclase is not only a soda 
orthoclase, bat is fall of microperthite growths of albite, and its patchy, 
flamed, anhomogeneous character exhibits the considerable amount of 
soda present. The calculation of the analysis in fact shows the aver- 
age of the alkali feldspars to be Ove Abs. The groundmass is of alkali 
feldspar and quartz, which is rather coarse and in a microgranitic 
structure, at times approaching to a micropoikilitic one. The analysis 
of the rock by Dr. W. F. Hillebrand gave the following results: 

Analy8i9 of porphyry from Big Bdldy Mountain j Montana, 



CoDttltaent. 



SiOz 

AI2O3 

FeaOs 

FeO 

MgO 

CaO 

NajO 

K^O 

HaO -I- 110° 
HaO— 110^ 

TiOg 

P«05 

MnO 

BaO 

SrO 

Total 



I. 



67.04 

16.25 

1.69 

1.13 

1.75 

2.17 

4.09 

5.10 

.51 

.56 

.20 

.21 

.05 

.33 

.08 



100.11 



II. 



1.117 
.148 
.011 
.016 
.043 
.039 
.066 
.054 
.028 



I. Percentages. 
II. Molecalar proportions. 

As in the former cases, by use of the molecular ratios in connection 
with the facts discovered in the thin section, we may calculate the 
approximate composition of the rock to be— 

Percent. 

Apatite, titanite, etc 1.0 

Magnetite 2.4 

Biotite , 3.2 

Hornblende 4. 8 

Oligoclase 22.2 

Orthoclase 47.0 

Qaartz. 19.4 

Total 100.0 



612 IGNEOUS ROCKS OF LITTLE BELT MOUNTAINS, MONTANA. 

This is very similar in composition to the rock of Thunder Mountain. 
As previously remarked, none of these rocks are to be considered 
typical granite-porphyrieSf since they lack quartz phenocrysts and the 
quartz present is only discovered under the microscope. They approach 
closely to the syenite-porphyries, as previously remarked, and might 
be termed granite-syenite-porphyries. 

GRANITE-SYENITE-PORPHYRY. 

Tillinghast laccolith. — What has been previously stated under the 
description of the granite-porphyry of Big Baldy Mountain, in regard 
to its transitional character, becomes still more evident in the case of 
several other occurrences. This is well illustrated in the porphyry 
composing the large laccolith just south of Thunder Mountain and 
lying between and drained by the heads of Tillinghast and Tenderfoot 
creeks. 

It is a gray feldspathic porphyry, thickly spotted with small feldspar 
phenocrysts and small black dots of mica and hornblende. It lacks the 
large orthoclase phenocrysts which distinguish the previous types, and 
has also none of quartz. 

In the section it appears that orthoclase and oligoclase are about 
equally divided in the number of phenocrysts. Hornblende is quite 
common, in well-formed prisms of the usual olive- green color; biotite 
is less abundant. These phenocrysts lie in a microgranitic ground- 
mass of quartz and alkali feldspar of rather fine grain. It is notice- 
able that the quartz is quite idiomorphic, its sections furnishing little 
squares; in amount there is far less of it than in the preceding tyi)e8, 
and the rock is therefore classed under the transition types. 

Sage Creek Mountain. — Under the type previously described should 
be included the igneous rock of the laccolith at the head of Sage Greek. 
The road passing over the divide at the head of Bear Park and down 
Sage Creek through the limestone canyon, passes the edge of this lacco- 
lith and affords excellent fresh material. It is so similar to the rock 
just described that it needs no further mention. 

Intrnsive sheets. — Intrusive sheets of porphyry at the following locali- 
ties may be placed under this type: In the Cambrian beds at the head 
of Sawmill Creek branch of Belt Creek, crossed by the road to White 
Sulphur Springs; in the Cambrian beds on the spur between Harrison 
and King creeks, where its exposure forms a mass on the ridge; in the 
Cambrian shales and Devonian beds on the northwest side of upper 
Dry Wolf Creek, where the successive sheets form a series of benches 
on the wooded hillsides. 

There is some variation among these types. In some there is a tend- 
ency to a micropoikilitic structure in the groundmass, the amount of 
quartz being too small for a full expression of the structure; in others 
the feldspars of the groundmass tend to assume lath-shaped forms and 
produce a transition toward the orthophyres mentioued later. Nearly 



PIR880H.] 8YENITE-P0EPHYBY. 513 

all of these sheets are greatly altered and decayed, the former ferro- 
magnesian mineral being indicated by pseudomorphs or rusty spots. 
Some have biotite alone, others biotite and hornblende. The feldspar 
phenocrysts are usually small, considerable oligoclase is present, and 
the rocks show tendencies toward the diorite group. They are none of 
them of the alkaline series. 

SYENITE-PORPHYRY. 

In several localities in the Little Belt Mountains there are intruded 
sheets of a rock which is best classified as a syenite-porphyry. Although 
there are small differences in texture and appearance among the rocks 
from the different localities, such are of minor importance, apd on the 
whole the different specimens resemble one another in a quite remarka- 
ble degree, so that one description will do for them all. In the hand 
specimen they are purplish or chocolate-colored rocks, dotted with 
numerous very small white feldspar phenocrysts, which are formless in 
shape, and with many phenocrysts of slender, dark, blackish-green 
columns of hornblende. The feldspars average about 2 mm. in diame- 
ter, the hornblende prisms 5 mm. in length. 

The groundmass becomes granular under the lens, and its purplish 
tone suggests the keratophyres of several foreign regions. All the 
different localities furnish material more or less altered, and in all the 
rocks are rather dull and lusterless. 

In thin section they are seen to be composed of phenocrysts of prtho- 
clase and oligoclase, which are of the common habit seen in such por- 
phyries, embedded in a groundmass of alkali feldspar with some quartz. 
Hornblende also occurs, of the usual olive-green pleochroic variety 
found in the syenitic rocks of the group rich in lime and magnesia. 
The structure of the groundmass is microgranitoid. The feldspars, 
which are often twinned as Garlsbads, are in short, broad forms and show 
sometimes a tendency toward a trachytoid structure, tending to throw 
the rocks into the orthophyre group. At all times this groundmass is 
rather fine in its granularity. The quartz in minute grains fills the 
interspaces in the groundmass, and it sometimes appears partly second- 
ary in character. The feldspars are more or less turbid from the pres- 
ence of kaolin, especially in the groundmass, and the hornblendes are 
in some places altered to calcite, chlorite, and perhaps serpentine, and 
there are some fine calcite particles scattered through the groundmass. 
A little biotite, often changed to chlorite, with iron ore, and apatite 
completes the list of minerals. 

20 OEOL, PT 3 ^3 



514 IGNEOUS ROCKS OF LITTLE BELT MOUNTAINS, MONTANA. 



An analysis of the freshest of these rocks has beeq made by Dr. 
Hillebrand, with these resalts : 

Analifsin of syenite-porphyry from Little Belt Mountains, Montana. 



Coniitituent. 



I. 



II. 



SiOi 

Al«03 

FeiOi 

FeO 

MgO 

CaO 

Na,0 

K2O 

H,0 + HOC 
H:0 — 110^ 

TiOa 

P«05 

CO, 

MnO 

BaO 

SrO 

Li,0 



Total 



1 

1 62.58 


1 

1.043 


' 16. 42 


.159 


' 2.46 


.015 


1.96 


.027 


1.84 


.046 


2.47 


.044 


4.57 


.073 


3.91 


.041 


1.40 


.077 


.38 




.40 




.33 





.77 




.017 


.08 




.41 




.10 




Trace. 
100.08 




•*.« ..«••• 







I. Analysis of syenite-porphyry of rock from ridge between Big Baldy Mountain 
and Yogo Peak. 
II. Molecular proportions. 

The locality is on the ridge between Yogo Peak and Big Baldy Moun- 
tain. The rock is from an intrusive sheet whose exposure forms a cap 
on the ridge. The 1.78 per cent of water shows that the rock has suf- 
fered some alteration, and this is confirmed by a calculation of its 
mineral character, which gives approximately the following relations: 

Per cent. 

Iron ore 3. 5 

Hornblende 8. 4 

Oligoclase 10.8 

Orthoclase 52. 7 

Quartz 11.5 

Kaolin 9.6 

Calcite 1.7 

Biotite, chlorite, apatite, etc 1. 8 

Total 100.0 

The oligoclase is reckoned from the study of the section as Ab4Ani, 
and from this the alkali feldspar or orthoclase averages a soda ortho- 
clase OrsAbs. 



PiMsoN.l DIORITE-PORPHYBY. 515 

Other localities for these rocks besides the one just mentioned are 
the sheet, perhaps 100 feet in thickness, intruded in the Oambrian beds 
at the head of Belt Greek, aboat 6 miles or so above ^Neihart, forming 
some heavy outcrops and talus slides close by the stream, and also in 
the Gamf^rian beds on the Sawmill Greek branch of Belt Greek below 
the heavy sheet of granite-porphyry at the winding of the road. This 
sheet is about 10 feet in thickness. Several sheets of this rock are also 
exposed on the crests of the spurs leading down from the ridge between 
Togo Peak and Big Baldy Mountain into the extreme head of Belt 
Greek; the rocks are sometimes quite fine-grained and dense, with platy 
parting, and in that case of gray color. Although somewhat different, 
on account of the finer grain, they are best placed under this type. 

King Creek intritsives, — At the head of Weatherwax and King creeks 
there occur, intruded as sheets in the Gambrian beds, a number of 
igneous rocks of porphyritic habit, which are best included under the 
syenite-porphyries. They are light-colored, feldspathic rocks, which 
generally have a pinkish tone from the iron hydroxide scattered through 
them and resulting from the decay of some former iron-bearing com- 
ponent. All of these rocks are greatly altered and kaolinized, and in 
part mineralized, so that they have been more or less actively pros- 
pected. They usually carry altered phenocrysts of orthoclase and pla- 
gioclase in a feldspathic groundmass. They are tf>o greatly altered to 
afibrd any satisfactory material for petrographic study. 

DIORITE-PORPHYRY. 

Between the granite-porphyries without phenocrysts of quartz but 
with abundant quartz in the groundmass, which pass into syenite- 
porphyries whose quartz is merely accessory, and the series of the 
diorite-porphyries, there are in this district many transitional rocks. 
Some of these transitions have been alreMy describe4. They occur 
chiefly in the sheets and laccoliths. The diorite series of this group 
of porphyries will now be described. The most typical is the rock 
composing the laccolith of Steamboat Mountain. 

STEAMBOAT MOUNTAIN TYPE. 

The great laccolith of Steamboat Mountain which has been injected 
into the Gambrian shales has domed them up, along with the heavy 
beds of the Garboniferous limestones, and now lies in considerable part 
exposed by erosion. The igneous rock has a thin platy parting, and 
the plates lie piled conformable to the laccolithic surface. This is 
unusual in laccoliths of acid feldspathic rock possessing a platy parting 
of the material near the laccolithic boundary, since generally the plates 
have a radial deposition and stand on end, perpendicular to the domed 
surface. The rock is of a dark-gray color, quite fine grained, thickly 
spotted with small, white, formless feldspar phenocrysts. In addition, 
the groundmass glistens with the reflection of light from numerous 



516 IGNEOUS HOCKS OF LITTLE BELT MOUNTAINS, MONTANA. 



cleavages of tiny black biotites which minately speckle its gpray sar- 
face. The rock appears granalar to the eye, bnt its grains are too fine 
to recognize. 

Under the microscope the minerals seen are phenocrysts of horn- 
blende, biotite, and plagioclase in a groandmass of plagioclase, alkali 
feldspar, and qnartz. Scattered grains of iron ore and a trace of titanite 
are present. 

The hornblende is quite idiomorphic in columnar crystals, having the 

color and pleochroism of ordinary olive-green hornblende. In addition 

there are smaller shreds and grains; a few lighter-colored grains may 

be of augite. 

The biotite also is somewhat variable in size, the larger crystals being 

well formed and mixed with smaller pieces. It is 
a light-brown pleochroic tyi>e, and is older than 
the hornblende, for the latter often incloses it. 

All of the feldspar phenocrysts appear to be of 
plagioclase. Their composition is somewhat vari- 
able, running through the oligoclases into the 
andesines, and even more basic varieties occur. 
Thus, as shown in fig. 75, a crystal oriented in the 
zone perpendicular to 010 shows a zonal banding. 
The albit« twins of the core extinguish at 12^, the 
zonal band at 20^, and the outer rim at 12^. Equal 
illumination takes place at + 36, and the core shows 
a nearly centered in the field. At 44^ the albite 
twinning also disappears. Hence the core is cut 
at 65^+ from zero, and contains 30 per cent of 
anorthite and is an andesine; the zonal band con- 
tains 38 per cent of anorthite and is an acid labradorite; the rim is 
also an andesine. 

The comx)osition of the small plagioclases in the groundmass is more 
acid than this, and while it is difficult to decide what the average pla- 
gioclase is, it is believed to be approximately about AbjAui ; that is, a 
basic oligoclase. 

In company with this plagioclase a considerable proportion of ortho- 
clase and quartz is also present, entirely, so far as can be told, in the 
groundmass. 

The structure of the groundmass is microgranitoid, and the plagio- 
clase has a tendency to a short lath form, which gives this groundmass a 
I)erceptible microlitic character. In it lie, irregularly sprinkled, the 
numerous small phenocrysts. The character of the rock and the struc- 
ture of the groundmass are illustrated in the microdrawing shown in 
PI. LXXV, B. 

The chemical composition is seen from the following analysis by Dr. 
W. F. Hillebrand: 





















. . «• . 


. m mm 


••• 






1 


.*'' 








*% 
% 




« 














/ 




••-• ' 


m^mm^ 


« 




1 




< 






r 








1 














{ 














1 














i 














1 














f 














1 
i 


1^ 










t 


1 


\ 










1 


L 


x 










t 


r 


\ 












fir 


/2* 


^ 


' 






^ 






^ 










^««* « » m ««' 












#n 








% 




# l\ 






/ 


* 


'*.. 




-- 


4 






Fio. 75.— Zoned plagioclase 
(andesine- labradorite-an- 
desine) in diorite- por- 
phyry of Steamboat 
Mountain. 



518 IGNEOUS BOOKS OP LITTLE BELT MOUNTAINS, MONTANA. 

QUARTZ-DIORITE-PORPHYRY. 

This occnrs at several localities, each of which is marked by some 
small local peculiarity. The phenocrysts of feldspar vary from oligo- 
clase to andesine, and they are generally rather small and well formed; 
there are sometimes present a few rare, scattered, large orthoclase phe- 
nocrysts. The dark-colored minerals are hornblende and biotite, both 
of which are present, sometimes fresh, sometimes altered, and generally 
of small size and inconspicuous, except under the microscope. The 
groundmass is made up of quartz and feldspar in about equal proi)or- 
tion, sometimes in a microgranitic structure, sometimes in a micropoiki- 
litic one. The proportion of orthoclase to plagioclase in this groundmass 
is generally about one to one. The quartz phenocrysts are not abund- 
ant, nor are they large or well formed. 

Examples of these rocks are found in the intrusive sheets in the 
Carboniferous beds at the head of Dry Wolf Creek; in the spurs run- 
ning southeast from Mount Taylor; in the dike cutting the Cambrian 
rocks exposed in the stream at Barker and opposite the railroad station; 
and in a sheet, about 15 feet in thickness, which is exx)Osed in the 
Cambrian shales on upper Dry Wolf Creek, in one of the spurs leading 
down from Big Baldy Mountain. At the latter locality the porphyry 
sheet was cut by a much-altered minette dike, with trend southward 
toward Togo Peak. 

A variation of this type is found in the porphyry comx)Osing the large 
flat area on the ridge east of Yogor Peak and east of the type described 
on page 502 as the Yogo granite- porphyry; it is between the head of 
Elk Gulch and the fork of Dry Wolf Creek, and is south of Storr 
Peak. The rock in the hand specimen is similar to the Yogo granite- 
porphyry, and has the same minerals and structure under the micro- 
scox>e, and the same beading of the feldspars with little quartz grains. 
The orthoclase of the granite-porphyry has given way, however, to 
oligoclase, which is the predominant feldspar, and the type has become^ 
in fact, a granite- diorite-porphyry, as there is still much orthoclase 
in the groundmass. 

SYENITE-DIORITE-PORPHYRY. 

Just as transition forms occur between the granite- and syenite- 
porphyries and the granite- and diorite-porphyries, there are also those 
between the syenite- and diorite-porphyries. Excellent examples of 
this type compose that part of the great Intrusive mass which runs 
eastward from Yogo Peak and which composes the slopes of Sheep 
Mountain and the masses east of it. An example from the talus slopes 
which border Bear Park at the head of Bear Creek above Yogo is 
selected for description. 

In the hand specimen the type closely resembles the other porphyries 
described, especially those forming the laccoliths in great part. In a 



PIR8SON.] 



DIOKITE-SYENITE-PORPHYRY. 



519 



very fine granular groundmass of very pale chocolate color lie, thickly 
crowded, small white phenocrysts of feldspar, which are only mod- 
erately well- formed crystals; the groandmass is farther spotted by 
numerous small black specks of shining biotite and small slender 
hornblendes. There are numerous tiny cavities of miarolitic character. 

In the section there are seen phenocrysts of soda orthoclase of 
homogeneous aspect, of plagioclase varying from andesine to oligoclase 
and about equal in size and number with the orthoclase, idiomorphic 
biotite and green hornblende in a microgranitoid groundmass of quartz 
and alkali feldspar, with some oligoclase, which is not so common, 
apparently, as the orthoclase. The rock is so similar to others which 
have been previously described that no further details are necessary. 

An analysis of this rock has been made by Dr. H. N. Stokes, with 
the following results: 

Analysin of diorite-ayenite'porphyry from Bear Park, Montana, 



Constituent. 


1 

1 I. 


II. 

1 

1.082 
.150 
.013 
.022 
. 066 
.055 
.068 
.041 
.047 


SiO, 


64.95 

15. 44 

2.02 

1.60 

2.65 

3.07 

4.25 

3.87 

.85 

.26 

.39 

.25 

.02 

.04 

Trace. 

.35 

.10 

100.11 


ALO3 


FeaOa 


FeO 


MgO '. 


CaO 


Na^O 


K2O 


' HaO-f-llO^ 


H3O 110^ 

TiOa 




PoOft 


__ 


SO3 


1 


CI 




MnO 


BaO 


' 1 

( 

t 


8rO 


1 


Total 







I. Analysis of diorite-syenite- (monzonite-) porphyry, talus slope west side of 

Bear Park. 
II. Molecalar proportions. 



520 IGNEOUS ROCKS OF LITTLE BELT MOUNTAINS, MONTANA. 

From this aualysis may be derived the molecular ratios given in the 
second column, and, recalling the study of the section, the rock may be 
calculated to have the following mineral composition : 

Per cent. 

Iron ore 3. 

Hornblende 6.2 

Biotite 7.7 

Anorthite 11. 7 

Albite 36.6 

Orthoclase 13.9 

Quartz 20.9 

Total 100.0 

From the study of the sections it may be estimated that the average 
plagioclase would be about Ab2An], and distributing the albite prop- 
erly we would have Ab2Ani 33.2 per cent, and of alkali feldspar or 
soda orthoclase, OriAbi(Or24Ab^), 29 per cent, which would place the 
rock between the two groups. The quartz, though abundant, is all in 
the groundmass, and the rock can hardly be considered a type between^ 
. typical granite-porphyries and typical quartzdiorite-porphyries. It 
has then, on the whole, seemed best to place it between the syenite and 
diorite groups, although in reality it stands between the rather abnor- 
mal granite-porphyry of the Barker type and the diorite-porphyry of 
Steamboat Mountain. It is a good example of a monzonite-porphyry, 
if one accepts the new monzonite group in its widest extent. 

RHYGLITE-PGRPHYRY (QUARTZ-PORPHYRY) . 

The group of rocks which are described under this heading are those 
which corresx)ond in all essential particulars to the types which have 
generally, especially by European petrographers, been called " quartz- 
porphyries." Since this term, however, carries with it the idea of age, 
and is also logically objectionable, it is discarded and the rocks are 
termed rhyolite-porphyries. 

Megascopic characters, — ^The rhyolite-porphyi'ies of the Little Belt 
Mountains are in general very dense, hard, compact, flinty or felsitic- 
looking rocks. The groundmass is so dense that it has generally a 
more or less horny texture, and its component grains can not be dis- 
tinguished by the eye, and rarely by the lens. It has a conchoidal or 
shell-like fracture, and the rock mass itself is generally much jointed, 
giving rise to small plates or chippy fragments. In color the rocks 
are always of light tones, sometimes pure white (head of Dry Wolf 
Creek), very commonly a pale yellow or buff turning to a light brown 
(divide west of Yogo Peak, Ricard Peak), or of a light pink or pale 
red (south edge of Mount Lupus, Dry Fork Belt Creek above Barker), 
more rarely a pale gray (Neihart Mountain). They usually contain 
very few or almost no phenocrysts, and these are generally small. 
The feldspar phenocrysts are commonly not of good crystal form ; the 
quartzes, which are dark gray in color, have the bipyramidal form 



piBMON] RHYOLITE-PORPHYRY. 521 

common in these rocks, and frequently break oat of the matrix, leaving 
a clear imprint of their crystal form. Some occurrences, like the south 
border facies of Mount Lupus, have no phenocrysts at all, and are there- 
fore not really porphyries but are placed here for convenience. The 
dominant megascopic character is the flinty aspect, the pale color, and 
the scarcity or lack of phenocrysts. 

Microscopic characters. — According to the structure which these rocks 
present when studied in thin section, they may be divided into several 
tyx>es or subsections as follows : 

Type N^o, 1. — This class is almost devoid of phenocrysts, and under 
the microscope shows only a microgranitic mixture of fine grains of 
quartz and alkali feldspar. The size of grain is somewhat variable, 
but always very fine. There are practically no dark components 
whatever, but the rocks contain small traces here and there of limonite 
and chlorite, which seems to indicate that originally a little biotite 
was present. They contain more or less sericite in the feldspar, and 
sometimes muscovite in good-sized crystals, which may be altered 
biotite phenocrysts. In addition, in two cases (Dry Wolf Creek and 
Mount Lupus) the groundmass is liberally sprinkled with minute 
shreds of light olive-colored pleochroic tourmaline (uniaxial, negative) 
and small irregularly shaped granules of colorless topaz. The mineral 
is biaxial, has one good cleavage, the plane of the optic axes is per- 
pendicular to the cleavage, and assuming the cleavage to be basal, then 
e:=jc, a = a, and b = b'j the refraction is about that of apatite, the 
birefringence that of quartz; the mineral contains many fluid cavities 
with bubbles. These properties prove the mineral to be topaz. 

The occurrence of tourmaline in rocks of this class is not uncommon, 
and has been previously described from this region (Castle Mountain)^ 
but topaz is much rarer. According to Bosenbusch, ' it is mentioned by 
Schalch and Schroeder as occurring in ^^microgranite^ dikes in one or 
two localities in Saxony, and its occurrence in lithophysae of rhyolites 
has been described by Cross ^ from localities in Colorado and Utah, in 
which it is found in very perfect crystals; but as a granular constituent 
of the groundmass of these rocks it has either been overlooked or does 
not occur, since this appears to be the first case noticed. It is a matter 
of considerable interest, as it links the rhyolites and x)orphyries in the 
pneumatolytic stages with those of the granites, already so well known. 
The rock whose analysis is given later is of this No. 1 type, and forms 
an intrusive sheet in the Cambrian beds on the divide leading west 
from Yogo Peak and not far from it. Other localities are the intru- 
sive sheet at the head of Dry Wolf Greek in the spurs coming down 
from Big Baldy Mountain (the rock is white and has a few quartz 
phenocrysts); and the contact facies of the granite-porphyry of the 



' Weed and Pirsaon, 6«ology of the Castle Mountain mining district : Ball. U. 8. Geol. Survey No. 139, 
p. 99. 
'Mass. Gesteine, 3d ed.. 1805-90, p. 655. 
"Am. Jour. Sci., 3d seriew, Vol. XXXI, 1886, p. 432. 



522 IGNEOUS ROCKS OF LITTLE BELT MOUNTAINS, MONTANA. 

Wolf Butte laccolith exposed on the saddle between Taylor Mountain 
and the next peak north. 

Type No, 2. — In this the gronudmass is of the same microgranitoid 
character described in type No. 1, but there are numerous phenocrystiS of 
feldspar and quartz. The feldspar phenocrysts are generally orthoclase, 
with smaller ones of oligoclase or andesine. Usually they are more 
numerous than those of quartz, but in one case, Neihart Mountain, the 
reverse is true. These are the types of rhyolite-porphyry (quartz- por- 
phyry) which tend to transitions into the granite-porphyries. 

The size of grain of the groundmass is variable; in one case (Kei- 
hart Mountain) it is exceedingly fine and suggests a possible devitrifled 
glass; others are much coarser. The dark components are rare, and 
are mostly chloritized biotites. None of the rocks are very fresh ; all 
are somewhat filled with calcite, sericite, or kaolin. In one phase (Ric- 
ard Peak) most of the feldspar and quartz phenocrysts are quite small, 
only to be seen with the lens, and there are many slender columnar 
hornblendes of the common green variety, also brown biotite foils, which 
megascopically spot the rock surface with slender black lines. Another 
phase of this tyi)e (intrusive sheets on Dry Fork Belt Creek above Bar- 
ker) is a transition into the granite-syenite-x)orphyries, described under 
the laccolith rocks. The phenocrysts of feldspar are more numerous, 
those of quartz less common ; the feldspar of the groundmass has a very 
slight but distinct tendency toward idiomorphism, giving a suggestion 
of the trachytoid structure of syenite-porphyries; the quartz dimin- 
ishes in amount and tends to fill angular interspaces; the dark min- 
erals are biotite and hornblende. The rocks are somewhat altered. 
Megascopically they exhibit many small phenocrysts of feldspar and 
hornblende and very few of quartz. They appear very much like the 
chocolate-colored syenite-porphyry previously described, and are, in 
fact, a transition between it and a typical rhyolite-(quartz-)i)orphyry. 

Type No. 3. — In the third and last type the groundmass is micropoi- 
kilitic, a mixture of orthoclase and quartz, with the latter oriented into 
spongy masses inclosing the feldspar. At times there are distinct 
transitions from these impure quartz sx)onges into regular clear quartz 
phenocrysts; the quartz, in crystallizing, has excluded more of the 
feldspar material and tended to become more homogenous. A draw- 
ing of this phase of the micropoikilitic groundmass is shown on PI. 
LXXV, A J from the contact facies of Mount Glendenniu, above the Tiger 
mine. The phenocrysts in this tyx>e are usually rare, and are of the 
character described above for the other types. Slender columns of 
hornblende are numerous in the Clendennin rock just mentioned. 

Chemical composition, — The chemical composition of rocks of tbis 
class is so simple and well known that it usually offers little of general 
interest. In the present series, moreover, the types are more or less 
altered, in many cases mineralized, and the material not well suited 
for analysis. As a control, however, upon the petrographic work, and 



PIB880N.] 



RHYOLITE-PORPHYBY. 



523 



to ascertain that these types present nothing unusual from a chemical 
point of view, an analysis was made of the rock forming an intrusive 
sheet on the divide just west of Yogo Peak, whose felsitic character has 
been previously mentioned. The analysis is by Dr. W. F. Hillebrand. 

Analysis of rkyoliie' (quartz-) porphyry from near Yogo Peaky Montana, 



CoDstUaent. 



I. 



IT. 



SiOa 

AI.2O3 

FeaOa 

FeO 

MgO 

CaO 

NaaO 

KaO 

H2O — 110° 

TiO.2 

P«05 

CO2 

MnO 

BaO 

SrO 

LiaO 



I 

L 



Total 



73.12 

14.27 

.51 

.26 

.24 

1.10 

3.43 

4.90 

.73 

.68 

.08 

.03 

.77 

.06 

Trace. 

Trace. 

Trace. 



1.218 
.138 
.003 
.003 
.006 
.020 
.055 
.052 
.040 



.017 



100.18 



I. Analysis of rook from divide west of Yogo Peak. 
II. Molecular propoTtione. 

The water and carbonic acid prove what the microscope has shown, 
that considerable alteration has taken place. The analysis is easily 
calculated into the mineral composition — 

Per cent. 

Iron ore 8 

Muscovite , 11.3 

Anorthite 8 

Albite 29.4] 

Orthoclase 21. 6J 

Quartz 33.6 

Calcite 1.7 

Chlorite 8 



51. 8 OrsftAbfis 



Total 100.0 

Localities. — The following list gives the more important localities in 
which the porphyries just described are known to occur : 

Eidge between Yogo Peak and Big Baldy Mountain; analyzed. 

Head of Dry Wolf Creek; intrusive sheets at base and top of Big 
Baldy Mountain spurs. 



524 IGNEOUS ROCKS OP LITTLE BELT MOUNTAINS, MONTANA. 

Contact facies of Moant Lupus, Mount Taylor saddle. 
Contact facies of Mount Clendennin, above Tiger mine. 
Intrusive in limestones, extreme bead of Dry Fork Belt Creek. 
Intrusive sbeets, Dry Fork Belt Creek a mile or two above Barker, 
chocolate colored. 
Intrusive (laccolith), head of Otter Creek. 

Intrusive forming hill on Eunning Wolf Creek below Woodhurst. 
Masses of Eicard Peak. 

Intrusive mass forming lowest north spur of Eicard Peak. 
Localities on Neihart Mountain and on slopes of Snow Creek Valley. 

TRACHYTE (BOSTONITE). 

On the upper main head of Dry Fork Belt Creek there occurs, intruded 
in the Cambrian, a mass of igneous rock which, so far as the exx)OSure8 
warrant supposition, api>ears to be an intrusive sheet. It is near the 
boundary of the Cambrian, and above the locality, along the creek, 
there are oi>en parks cut in Cambrian shale; the mass forms a wooded 
bench which yields a considerable talus along the stream. 

The rock is of a light-brownish color, rather x)orous in texture (from 
numerous small miarolitic or steam cavities), coated with limonite, has a 
rough, trachytic feel, and is rather dense. The phenocrysts are entirely 
inconspicuous and consist of numerous very small, thin, tabular, sani- 
dine-like feldspars, aud others which are soft, lusterless, and kaolinized. 
Ferromagnesian minerals are almost entirely wanting; occasional rusty 
specks may be the remains of former ones. 

In thin section the rock is found to consist almost entirely of orthoclase, 
with a very little quartz. The phenocrysts are homogeneous in struc- 
ture, untwinned, and show none of the microperthite, soda-microoline, 
or moir^ structures so common in this class of rocks. In sections per- 
pendicular to a the optic angle is small, the bars barely passing out of 
the field of the No. 9 objective of Fuess, i. e., 2E is in the neighborhood 
of 50O. 

The small feldspars of the groundmass are in thin plates tabular on 
010; they are extremely apt to be grouped together parallel to this 
face, so that at times they either actually form repeated Carlsbad 
twins or appear to do so, or are slightly divergent from one another. 
As these groups are cut at different angles by the section, the feldspars 
api)ear in rectangular laths in parallel positions or in broader formless 
plates. There is no sign of any albite twinning to be seen among them, 
and in all cases where they form rectangular sections with sharp clean 
edges without neighboring overlaps, which shows that the plate of 
feldspar (and therefore 010) is perpendicular to the section, they extin- 
guish rigidly parallel, as a monoclinic feldspar should. 

Practically the entire mass of the rock is made up of these plates of 
alkali feldspar, packed as closely together as is possible. The angular 
interspaces, which, so closely are they joined, are always minute, are 



PIB880K.] TRACHYTE. 525 

usually filled with a small amount of quartz ; sometimes they are empty, 
giving rise to minute miarolitic cavities. The structure of the rock is 
thus really the panidiomorphic of Eosenbusch, in the sense that the 
constituents have to a greater or less degree their own form; it is not 
of course the sugar granular structure of the aplites. Eosenbusch^ 
includes all of the aplitic rocks in one group of his dike rocks, mention- 
ing the panidiomorphic structure as being characteristic of the group. 
Brogger^ has later suggested their division into two groups, the aplitic, 
with sugar granular structure, and the bostonitic, with trachytoid 
structure. This latter is the structure, in a very high degree, of the 
rock just described, and in rock classifications, where the mode of 
geologic^ occurrence is strongly taken into account, the rock is atypical 
bostonite; a more general term for it is trachyte. 

The rock does not appear fresh enough to warrant a careful analysis, 
but since pure potash rocks are almost unknown, it seems clear that the 
alkali feldspar, its almost sole constituent, must be a soda orthoclase. 



1 Mass. Gesbeine, 8d ed., 1896-96, p. 388. 

> EruptiTgesteine Kristianiagebietes ; III, Ganggefolge des Laordaiits, 1898, p. 211. 



.1 



CHAPTER lY. 

THE liAMPROPHYRES AND THE EFFUSIVE ROCKS. 

THE LAMPBOPHYRES. 
MINETTE. 

Minette, which has been considered a rather rare rock in America, 
is freqaently found in the Little Belt Mountains, and usually in the 
form of sheets intruded into the thinly bedded horizons of the Cam- 
brian and Carboniferous. Its occurrence in dikes is much more uncom- 
mon, though a series of dikes occur at the head of Dry Wolf Creek 
which are closely related to these minettes. In a number of cases, how- 
ever, dikes of minette are found in connection with the sheets, being, 
in fact, the feeding canals to them. 

According as the minette is found in the very thin or in the thicker 
sheets it has a markedly different appearance in the hand specimen. In 
the thicker sheets it has, when fresh, a dark-gray color, and it is easily 
seen by the eye alone to be distinctly crystalline, and in certain cases 
it approaches the condition of a finegrained granular rock. Occasional 
large biotites, quite idiomorphic and about 2 or 3 mm. across, are found 
playing the r61e of phenocrysts. In these coarser varieties the ground- 
mass is easily differentiated into a white feldspathic component, in 
which lie innumerable small leaves of biotite, while smaller grains of 
a greenish pyroxene can be seen with the lens. This type of rock has a 
dull, hackly fracture. It occurs more abundantly among the sheets 
intruded in the Cambrian shales and limestones at the head of Sheep 
Creek. 

The work done on the White Sulphur Springs and Feihart road, 
which follows Sheep Creek to the divide and crosses the strike of 
the beds and the intruded sheets, has exposed in a number of places 
material which is quite fresh, especially in one locality. The rock as 
here seen was the least altered of any obtained, and for a rock of this 
family was remarkably fresh, as disclosed not only by the study of the- 
section but also by the analysis given later. 

In the thinner sheets and in the few dikes in which this rock occurs 
it has a quite different appearance. It is very dark stone gray, almost 
black, of a very fine, dense grain, with a distinct conchoidal fracture. 
It has, in fact, a prouounced basaltic appearance and merits well the 
field term ^< mica trap," applied by the older geologists to this class 
of rocks. Occasional phenocrysts of biotite are seen in the rocks, 
and they are often speckled by small spots of white, which are mostly 
due to altered and calcitized augite crystals and included fragments 
of calcite. 

MICROSCOPIC PETBOGBAPHY OF THE MINETTE. 

While the greater number of the occurrences of this rock are so greatly 
altered by weathering that no satisfactory results could be gained by 

526 



PLATE LXXVI. 



527 



PLATE LXXVI. 

Thin Sections of Lamprophyres. 

J. Minette from the sheets and dikes at the head of Sheep Creek. Brown biotite, 
green aiigite, and black iron ore, in a mixture of white orthoclase and plagioclase. 
Seen in natural light; actual size of field 2 mm. multiplied by 38. 

B. Analcit'e-basait from dike cutting Bandbox Mountain. Olivine (yellow), bio- 
tite (brown), and pyroxene (green), lying in an isotropic pale-brown groundmass of 
analcite. Natural light ; actual size of field 2 mm. multiplied by 38. 

528 



i GEOLOGICAL SURVEY TWENTI 




THiN SKC'f'IONS ()!■■ ItASK' IJiEvI 



.IT'n.K IIKI.T MOI'N'l'AlNS 



PiBflsoN.] MINETTE. 529 

an examination of them, about two dozen were in a sufficiently good 
state of preservation to furnish fair material for study in thin section, 
and of these about one-third were practically unaltered. These show 
that the ordinary minerals of a typical minette are present — iron ore, 
apatite, biotite, augite, orthoclase, plagioclase — and in the altered varie- 
ties several decomposition products, which are, indeed, not wholly want- 
ing in the very freshest examples. 

Iron ore and apatite. — These present their usual characters. The ore 
grains are, as a rule, rather small, averaging about 0.03 mm. They are 
at times clustered bead-like along the edges of the augites in such a 
manner as distinctly to suggest a pushing and exclusion of the already 
formed magnetite grains by the growing and expanding augite. In a 
few instances quite large crystals of apatite were noted rising almots 
to the dignity of phenocrysts. 

Biotite. — This has the typical micro character found in minettes. In 
the interior of the crystal the color is a brownish ocher-yellow, bordered 
by a mantle of deep brown, best seen in basal plates (see PI. LXX VI, A ), 
while in sections perpendicular to the cleavage the pleochroism is very 
strong, varying between pale yellow and deep brown. The larger 
phenocrysts are sometimes embayed, and the edges of the embay men t 
are bordered by hexagonal boundaries. Sometimes the larger crystals 
are made up of smaller ones in parallel position. The great majority 
of the sections do not give an opening of the axial cross sufficient to 
tell whether the mica is a meroxene or an anomite, but in one case a dis- 
tinct opening showed the trace of the axial plane perpendicular to one 
side of the hexagon; in this case the biotite is an anomite. In some 
cases the larger crystals exhibit a bending and deformation through 
stress, showing that they had crystallized out in the magna before it 
had attained its final resting place. In a few cases it is seen that the 
biotites have suffered magmatic resorption and are partially converted 
into opacite. 

Augite, — This is a pale-greenish' diopside. It rarely occurs in clear, 
well-formed, and distinct crystals. It is far more likely to be present 
in irregular masses and in collections of rounded grains with similar 
orientation or in small anhedrons. Compared with the biotite, it also 
varies considerably in amount and in relative quantity. In some cases 
it is scarcely i)resent, in others it increases until it equals the biotite, 
and in a few dikes it preponderates, giving transition forms into lampro- 
phyric rocks, to be described later. In a few of the coarser-grained 
minettes, where the augite is more idiomorphic, it frequently appears 
with secondary tufts of a finely fibrous hornblende attached to the 
basal plane. Both minerals have the vertical axis in common. This 
hornblende is a rich green in color and strongly pleochroic. Its angle 
of extinction is extremely small. Since its extension is optically posi- 
tive and its j)leochroism is in tones of green, it can not belong to the 
20 GEOL, PT 3 34 



630 IGNEOUS BOCKS OF LITTLE BELT MOUNTAINS, MONTANA. 

soda-iron group. In composition it differs from the very similar tafted 
groups described by Cross ^ as secondary on augite. 

From its method of occurrence it is inferred that the augite occurs 
at times in two genera tions, though by far the greater part, like the 
biotite, belongs to the second. 

Feldspar. — This is, of course, chiefly orthoclase, or at least an unstri- 
ated alkali feldspar. In the finer-grained varieties in the dikes and 
thinner sheets it has a lath-like form, giving a trachytoid appearance 
to the groundmass. In the coarser-grained types found in the thick 
sheets the orthoclase occurs in shapeless masses, which at times have a 
tendency to run into broad plates, inclosing the micas and other ferro- 
magnesian minerals in a poikilitic manner. It is usually more or less 
clouded by incipient kaolinization, even in the freshest types, and in 
those in which decay is far advanced it is greatly altered. 

Associated with this orthoclase there is a variable quantity of pla- 
gioclase. It has a strongly zonal formation and runs from interior cores 
of plagioclase as basic as labradorite down to albite on the outer bor- 
ders. At times such individuals have an exterior mantle of orthoclase 
surrounding them, and it is to be suspected that as the inner shells 
pass into albite on the exterior, this in turn is succeeded by anortho- 
clase or soda-rich orthoclases, the orientation of the whole being pre- 
served by this succession. In some of the smaller sheets the plagio- 
clase does not become so basic as labradorite, and in these cases there 
is also less of it. The relative proportion, indeed, of orthoclase and 
plagioclase is quite variable, not only in the different occurrences com- 
pared with one another, but even in the same rock mass. While in all 
cases orthoclase is strongly the predominant feldspar, yet in certain 
cases local enrichments of plagioclase are found to such an extent that 
the rock assumes a kersantite facies. Here it is that the plagioclase 
assumes its most basic form. In general the plagioclase is fresher and 
much more limpid than the orthoclase. 

Secondary minerals. — These are present in direct proportion to the 
amount of alteration the rock has sufiered. The biotite changes into 
chlorite, the augite into limonite and masses of carbonates, while, as 
previously mentioned, the feldspars are changed into kaolin and allied 
products. 

Structure. — ^The type of structure varies somewhat according to the 
mode of occurrence. In the dikes and thin sheets the lath-like form of 
the feldspar gives a somewhat trachytoid type of structure, very similar 
to that of the minettes so common in eastern Germany, but this type 
changes in the thicker sheets to one much more granitoid or hypidio- 
morphic in its character. This is caused by the gradual thickening 
and loss of definite form of the feldspar, which assumes the character 
of that seen in granites. It should, indeed, be stated that in some of 
the very thickest of the sheets the growing coarseness of the grain of 
these minettes, the absence of phenocrysts, and the structure described 

' Am. JTonr. Sci., 3d serieii, Vol. XXXIX, 1890, p. 369. 



PiBSfioir.] 



MINETTE. 



531 



above, cause them to assame facies which are clearly transition forms 
into the mica-syenites, or, since rocks so basic as these and with so 
great an abandanceof ferromagnesian minerals can scarcely be termed 
syenites with propriety, into rock types of which perhaps the durbach- 
ite of Saner' is at present the only type which has been distinctly rec- 
oguized and differentiated from the syenite group. 

A figure of one of these minettes is given on PI. LXXVI, A. 

Chemical composition, — The chemical composition of these minettes is 
shown by an analysis of the very fresh and rather coarse-grained type 
(No. 274) taken from material brought out in the road-cut on upper 
Sheep Greek. The analysis is by Dr. W. F. Hillebrand and is given 
in Column I in the table below. This shows the composition to be 
that of a normal lamprophyre with low silica, moderate alumina and alka- 
lies, with high iron, lime, and magnesia. The amount of carbonic acid 
is low for a rock of this class, and proves it to be fairly fresh. The 
water is no more than must be expected in a rock of this group, it being 
nearly impossible to obtain absolutely fresh material. Part of it prob- 
ably goes with the biotite. For the sake of comparison, analyses of 
two other augite minettes firom well-known and typical occurrences are 
given, and it will be seen that on the whole the agreement is very satis- 
factory. It agrees also fairly well with the durbachite of Saner except 
in the potash and ferric iron, and we may conclude that the durbachite 
contains a larger amount of biotite. 

Analysts of minettes. 



Constitnent. 



SiOa 

TiO« 

AlaOs 

CraOs 

FegOa 

FeO 

MnO 

MgO 

BaO 

SrO 

CaO 

Na,0 

K,0 

LisO 

H„0— llOo 
HsO + llQo 

P«Oft 

C0« 

Total . 



I. 



52.26 

.58 

13.96 

Trace. 

2.76 

4.45 

.14 

8.21 

.28 

.05 

7.06 

2.80 

3.87 

Trace. 

1.53 

1.34 

.52 

.49 



II. 



52.70 

1.71 

15.07 



m. 



51.15 

(t) 
15.91 



IV. 



51.05 

1.76 

14.49 



8.41 

(0 



4.63 
3.72 



7.23 



4.14 



4.16 
4.37 



8.16 



100.25 



5.33 
3.12 
4.81 



7.68 
1.92 
5.97 



5.11 
1.85 
7.25 



2.38 

Trace. 
Trace. 



2.75 
2.12 



100.76 



99.99 



1.05 
.70 



99.94 



' Miitoilungen d. Bad. Geol. Landesanstalt, II, 1892, p. 247. 



532  IGNEOUS ROCKS OF LITTLE BELT MOUNTAINS, MONTANA. 

I. Augitc-minette (No. 274); Sheep Creek, Little Belt Mountains, Montana. 
W. F. Hillebrand, analyst. 
II. Augite-minette ; Weiler bei Weissenburg, Alsace. Linck : Abh. z. Geol. Special- 
karte v. Elsass Loth., Bd. Ill, 1884, p. 55. G. Linck, analyst. 

III. Augite-minette; Leonhardskopf hie Flockenbach. Roth, Tab., 1879, XXVI. 

IV. Durbachite; Durbach, Schwartzwald. Sauer: Mitt. Baden Geol. Landesanst., 

Vol. II, p. 258. 

Yariolitic fades of minette, — In the compaoter varieties, of this rock 
occarring in the thiuner intrusive sheets and in the dikes a variolitic 
facies at the saalband is iiot uncommon. Megascopically, in such cases 
the freshly fractured surface is seen to be thickly spotted by circular 
masses averaging from 2 to 5 mm. in diameter. They are of a pale- 
gray color, while the matrix in which they lie is a dull brown with 
greenish tinge. The difference in color is so pronounced that the rock 
has a strongly mottled character that at once arrests attention. Their 
number is very great; they are rarely separated by more than their 
own diameter, and in places are so thickly crowded that t^hey touch 
one another, or even intersect in groups. They are more compact and 
coherent than the matrix, which on a fracture tends to break away 
from them, giving a gnarly, knotted appearance to the break. On a 
weathered surface the rock has a pale-brown color, and the matrix 
weathering faster than the spheres, leaves them x)rojecting. The rock 
surface has thus a pronounced warty appearance, and it rudely resem- 
bles the perlitic development of acid glasses. On the other hand, the 
fractured surface recalls certain rhyolites with a spherulitic develop- 
ment. Its appearance is shown in PL LXXIII, B. 

Examined with a lens, the varioles, as we shall call the globular 
bodies, are seen to be composed of a gray feldspathic mass flecked by 
tiny dark spots from which one catches the occasional reflection of a 
mica cleavage. As we approach the outer border of the variole the 
gray color changes within a narrow zone to pure white. Thus the 
variole is encircled by a mantle, which, on a fresh fracture, is easily seen 
by the naked eye as a white ring surrounding it. On close inspection 
with the lens, however, it may be seen that the border zone is not 
wholly composed of feldspar, but that it is traversed by many very 
narrow hair-like lines of black, which sometimes have a radial direction 
and sometimes not. These are the edges of very thin, small tables of 
biotite, commonly arranged so that the tabular face is either radially or 
tangentially disposed with respect to the spherical mass. 

Under the lens the matrix in which the varioles lie is found to be 
extremely rich in biotite, compared with the varioles. While the grain 
is dense, the mica tablets can be easily seen. The boundary of the 
variole against the matrix is not a perfectly geometric onC; but is rather 
wavy and irregular, though the boundary between the two colors is 
perfectly sharp. It is because of the greater richness of the matrix in 
biotite, which cleaves so readily, that the varioles may be separated out 
from it, and from its easy crumbling and decay that they are left pro- 
jecting like w^rts from a weathered surface. 



PIR880N.] MINETTE. 633 

The characters described above become less and less uoticeable as a 
greater distance from the saalband is reached, until eventually the rock 
attains its normal character. In the most developed case we have 
seen, the breadth of the variolitic band is about G inches, so that hand 
specimens can be made showing the varioles on all faces. 

Microscopic characters of variolite, — Under the microscope the usual 
minerals of the minette are seen -iron ore, augite, biotite, and feldspar. 
In plain light the appearance of the section at first glance is similar to 
that of an ordinary line- grained minette; the iron ore is in fairly good 
crystal form ; the small stout prisms of augite are quite idiomorphic, and 
these, with the biotite, appear to be scattered irregularly through the 
section. It is noticeable that the biotite is in very thin leaves, which, 
when standing perpendicular to the section, show as very slender 
pleoehroic rods. In certain places it is seen to be locally abundant. 
It is quite surprising how the strongly marked individuality of the 
varioles, which is so characteristic a feature megascopically, practically 
disappears in thin section, giving way to an appearance of uniformity. 
The thinner the section the more pronounced this becomes. 

Between crossed nicols the study of the feldspar areas, which in white 
light are of a very pale ocher color and appear slightly kaolinized, 
shows that they are not made up of a single feldspar individual, but 
that they are composed of bundles of fibers arranged in plumose or 
fan like forms, across which the brush of an interference cross waves as 
the section is revolved. They are formed, in fact, of spherulitic growths 
of orthoclase, and the sensitive tint, showing that the fibers are extende<l 
in an optically negative direction, indicates that the development of the 
feldspar is, as usual, parallel to the clino-axis. The study of the devel- 
opment and arrangement of these growths shows that they start from 
some common center, such as a group of augites or of iron ore, and 
spread radially in all directions until interrupted by the interposition 
of the mass of some of the ferromagnes an minerals. Then from these 
they again start, preserving approximately the general radial direction, 
until finally the outer boundary of the variole is reached, where they meet 
spherulitic growths coming in the opposite direction, and the varioles 
intersect or terminate at the interspaces. The whole arrangement is 
similar to a great quantity of brushes placed radially in rudely con- 
centric circles around some common point and mixed without arrange- 
ment with the ferromagnesian minerals. The mica, however, shows a 
tendency to arrange itself so that the basal plane is also radially 
extended. 

The cusp-shaped and annular interspaces between the varioles, which 
appear so plainly in the hand specimen, are seen by study of the section 
to be local enrichments of biotite tablets, which are generally arranged 
tangentially to the spherulitic growths and cemented together by deep- 
brown glass. The study of these interspaces has not been as satis- 
factory as could be desired, owing to the difficulty which has been found 
in preparing tliin sections. The varioles are much harder and resis- 



534 IGNEOUS EOCKS OP LITTLE BELT MOUNTAINS, MONTANA. 

tant, and in grinding the cmmbly brown material disappears long 
before the varioles are safficieutly thin. We have had to stady it, 
therefore, in rather thick section, from which the above characters have 
been made ont. 

From what has been stated above, it seems evident that the varioles 
are spherulites of feldspar qoite comparable to the spherulitic growths 
f)o characteristic of acid glasses. They differ, however, from the 
majority of these in that the sphernlitic growth has been interrapted 
by the presence of augite and iron ore and has then repeated itself, 
thns making the variole a compound sphernlite, while in acid glasses 
the growths asaally take place before other minerals have crystallized 
oat, and are hence not interrupted; they are the first products of crys- 
tallization, while in the variolites described they are the last. 

Spherical structure in the minette-kersantite rocks has been observed 
by various writers, but this, as described, is usually a contraction phe- 
nomenon occurring on a large scale and brought out by weathering, 
not a microscopic structure produced by a special process of crystalli- 
zation. A spherical structure on a minute scale is, however, mentioned 
by several authors,^ but, so far as we can learn from the literatuiei it 
appears to have been formed by an amygdaloidal filling of vesicular 
cavities. 

Pohlmann,* indeed, speaks of a kersantite in which are '* concre- 
tions" the size of peas, whose center consists of feldspar laths ^^ whose 
radial structure is not to be mistaken, " and between which lie aggre- 
gates of chlorite from decomposed augite and biotite. This descrip- 
tion seems to agree with the one we have given for the Sheep Greek 
minette. Pohlmann does not seem to offer a direct explanation for 
these structures, except to connect them with others contaiuiDg calcite. 
Bosenbusch^ sees in these structures the filling of amygdaloidal cav- 
ities; he does not apparently believe that they are of spherulitic nature, 
since he says "an evident divergent radial structure of the little 
spheroids is nowhere mentioned" ("auch wird eine evident excentrisch 
strahlige Structur der Ktigelchen nirgends angegeben"), a statement 
which does not seem to agree with Pohlmann's mentioned above. 

As we have not seen the material which these writers have investi- 
gated, we can not, of course, presume to offer any opinion as to the 
origin of the spherical masses they have described, but it is clearly 
evident that the varioles of the Sheep Greek minettes are not due to 
the filling of amygdaloidal cavities. They are, on the contrary, an 



1 E. Cohen, Eeraantit -von Laveline : N. Jabrb. Min., 1879, p. 858. 

R Coben, Ueber einige VogeBcngesteine : N. Jabrb. Min., 1888, p. 199. 

Liebo und Ziramennanu, JiiDgere Eruptivgebllde in sUdweaten OsUhiiringens: Jabrb. k. prenss 
1^1. Landesanst. fur 1885, p. 178, Berlin, 1886. 

Pohlmann, Untpersuobungen liber Glimmerdiorite und Eeraantit Sudtharingens und des Franken- 
waldes : N. Jabrb. Min., Bel. Bd. Ill, 1885, p. 78. 

Linck, Geog. petrog. Besohreibung des Grauwackengebietes von Weiler bei Weisaenburg; Inang. 
Diss. Strassburg 1. £. 1 884. 

*Loc. cit. pp.78, 96. 

> Massige Geateine, 3d ed., 1895, p. 519. 



PIB8SON.] MINETTE. 535 

original rock stmctare, and are quite similar to the variolitic facies of 
certain diabases, and to be compared to the sphemlites found in acid 
glasses. 

We have conceived their mode of formation to be as follows: In the 
magma, as it was gradually being forced into its final resting place, 
certain products had begun to crystallize, and it was filled with small 
crystals of augite, iron ore, and some biotite. On coming in contact 
with the cold limestones, between whose bedding planes the sheets now 
lie, a very much more rapid cooling began in the portion adjacent to the 
contact plane. This forced the remaining portion of the magma into a 
rapid process of crystallization, and from the minerals already present, 
which served as centers, rapid growths of feldspar branched out, form- 
ing sphemlites. These would generally include whatever minerals they 
found in their way, but it is quite conceivable that while a mica tablet, 
with its basal plane lying in the direction of growth, would be included, 
one with the same plane perpendicular to this direction might be pushed 
along some distance and excluded by the mass of growing fibers. At 
the same time the mass of feldspathic material growing in this manner 
would tend to take up those elements necessary for its formation, and 
to exclude those that were not; hence the residual material would be 
becoming richer in iron and magnesia, and therefore better fitted chem- 
ically for the development of the biotite. The biotite may thus have 
been excluded from the varioles and concentrated in the interspaces, 
partly as an already formed mineral and partly as chemical material, 
some of which formed biotite later. This explanation we believe 
enables us to understand why the outer zone of the sphemlites or 
varioles is of a lighter color, and why the cement is richer in biotite. 
Finally, the rate of cooling was such that the residual material now 
forming the cement did not have time to entirely crystallize, but partly 
solidified as glass. The whole phenomenon in these minettes, as we 
understand it from our studies, is clearly that of an endomorphic con- 
tact modification of the rock structure, due to quick crystallization 
induced by rapid cooling. 

Some of the observers^ who have studied the formation of feldspar 
spherulites in acid glassy rocks have attributed to absorbed aqueous 
vapors a preponderating rdle in the process. While by no means indi- 
cating that such is not the case, the minettes under discussion afford 
no evidence of their action. That they are not always necessary as a 
factor in spherulitic crystallization is clearly shown by the occurrence 
of such bodies, often of great beauty in their development, as an acci- 
dental product in artificial glasses. 

Uxomorphic contact phenomena, — Where the thinner sheets have come 
in contact with the limestones very little effect has been produced. 

— ,_— ■- 

I Cross, Constitution and origin of sphemlites In acid ernptive rocks: Boll. PhiIos.Soc. Washing- 
ton, Vol. XI., 1891, p. 411. Iddlngs, Obsidian cliff: Seventh Ann. Kept. XT. S. GeoL Surrey, 1888, p. 256. 
Iddings, Spherulitic crystallization : Bull. Philos. Soc. Washington, VoL X, 1891, p. 445. 



536 IGNEOUS ROCKS OF LITTLE BELT MOUNTAINS, MONTANA. 

Immediately next to the contact wall a thin layer of hardened, tough- 
ened material has been produced, and the limestone appears to have 
undergone some recrystallization. As the sheets become thicker this 
effect increases, and where the beds were shaly they are filled with 
cavities lined with minute crystals, or are much cracked and the walls 
of the crevices also filled with crystals. The minerals which are pro- 
duced by these processes are garnet and pyroxene. The former is of 
the grossular variety and shows the form of the dodecahedron. The 
pyroxene comprises the greater part of the altered material and is 
rarely well crystallized. The contact produced is similar in character 
to that described on page 540, produced by a closely related igneous 
rock, and that description includes all that has been observed in the 
case of the minettes. 

The shales on Sheep Creek have been greatly toughened and hard- 
ened, and their color is deepened. 

Included masses in imnette, — Occasionally in the minettes are to be 
found masses of a plutonic rock which have been brought up from 
depths below by the ascending magma. One of these is well exposed 
in the road-cutting previously alluded to. It is a large mass, some 
2 or 3 feet in diameter, and is a striking object to find in an intruded 
sheet at what must be at least a considerable distance from the point 
of ascent. In the hand specimen it is a grayish rock, rather coarse 
grained, and of a syenitic aspect. With the lens one sees tbat it is com- 
posed of feldspar and mica, with an occasional grain of pyrite. The 
feldspar has a waxy appearance, recalling paraffin; it does not possess 
lustrous cleavages, but is clearly altered. The biotite also has an 
altered, nonlustrous appearance; the rock, however, does not impress 
one as having suffered from weathering or atmospheric agencies, but 
rather from some other cause. Its line of contact with the minette is 
sharply defined, and the two may even be broken apart at the contact 
plane with comparatively smooth surfaces. Under the microscope the 
inclusion is seen to be a mica-syenite of rather coarse grain and of gra- 
nitic structure, w*ith a gneissoid tendency, as the micas are strung out 
along certain planes. This does not api)ear to be due to shearing forces, 
as the minerals are not at all granulated or broken, but is due rather 
to an original fluid movement before the rock was wholly crystallized. 

The minerals seen under the microscope are biotite, plagioclase, ortho- 
clase, iron ore, and apatite. The biotite has suffered from processes 
which in the younger extrusive rocks would be called resorption; it 
appears precisely like many of the resorbed micas which often occur in 
trachytes; it is partly or wholly converted into opacite or bordered by 
opacite rims. The feldspars are converted into masses of sericite; 
occasional unaltered fragments or cores in the crystals permit of the 
identification of the species, and from these it is seen that orthoclase 
very greatly predominates. The condition of the plagioclase, together 
with its small amount, does not permit of accurate determination of the 



pmsBON.] MINETTE. 637 

variety, but from the fact that in one example the twin striations which 
were approximately of eqaal angle of extinction on either side of the 
twinning line blended in the position of equal illumination with a 
Carlsbad twin, so that the whole appeared homogeneous, it must be 
inferred that it is a very acid one, in the albite-oligoclase group. The 
occurrence of this syenite as an inclosure in the minette, from the point 
of view of the genetic relationships of igneous rocks, is very interest- 
ing and significant; it shows that this rock exists in the depths, and 
that the minette is of later origin and connected with it. 

The minette itself does not seem to have suffered the least amount 
of endomorphic modification from its presence; it retains its normal 
minerals and structure directly to the contact line, and from this we 
may infer that the mass was taken up while the magma was extremely 
hot, and that it had acquired very nearly the temperature of the fluid 
mass before the latter began crystallizing. With regard to the altered 
condition of the minerals of the syenite, it seems certain that the change 
in the mica was occasioned by the action of the minette magma. In his 
valuable work on the inclosures of the volcanic rocks ^ Lacroix speaks 
of the alteration produced in the biotite of granitoid rocks by the 
action of an inclosing trachytie magma, and states that the mineral is 
converted into magnetite, green spinel, new biotite, and often hyper- 
sthene. It seems certain to us that heat alone has not produced these 
changes in tbe mica, as suggested by Lacroix^ (at least in the pres- 
ent case), because we should then expect the mica to be equally affected 
throughout, since it must all have attained the same temperature; 
whereas in reality some mica tablets are more affected than others but 
a few millimeters distant from them. If we attribute the alteration to 
mineralizing vapors in the magma acting with the heat, it is easy to 
see that some individual micas will be more affected than others, 
according as the rock varies in its permeability to the vapors from 
place to place. The new minerals formed are iron ore, new biotite, and 
a pyroxene in granules. 

The conversion of the feldspars into sericite or fine-leaved, fibrous, 
white mica is probably an effect due in part to the same cause, and pos- 
sibly also in part to weathering, since the feldspar of the minette 
appears to be similarly affected, though in less degree. 

Alteration of the minettts, — These rocks undergo the normal proc- 
esses of weathering, the biotite changes to a greenish chlorite, the 
pyroxene to masses of carbonates and iron ore, while the feldspars 
change to white mica in part, but mostly to kaolin. After a certain 
period the feldspathic portion seems to decay more rapidly than the 
biotites, and as the rock becomes soft and earthy the biotite appears as 
green scales of chlorite. Eventually the exposure crumbles down into 
soil, filled with these greenish altered scales, which serve to identify it 



> Enclaves den roches volcaniques, 1893, pp. 172, 175. 
'Log. oit., p. 355. 



538 IGNEOUS ROCKS OF LITTLE BELT MOUNTAINS, MONTANA. 

and show its former character. Many of the intruded sheets have had 
their outcrops reduced to this condition, making it impossible to obtain 
good material for investigation. 

Minette-like rocks. — Above the series of minette sheets occurring 
on Sheep Creek, which have been mentioned as exposed in the road-cut- 
ting, and below the sheets of the same rock which are seen on the 
divide, there occur intrusive masses, probably thick intrusive sheets, 
of a rock which is most closely connected with the minettes and yet 
differs from them in some particulars. The large talus masses which 
are found in the woods not far below the divide show a weathered rock 
of a brownish color, fine, dense, and with decayed ferromagnesian 
phenocrysts. Under the microscope this rock is seen to consist chiefly 
of orthoclase, with considerable amounts of biotite and augite, and 
some hornblende and iron ore. The orthoclase is in short, extended 
laths, as in the minettes, and gives a trachytoid structure; it is much 
altered and kaolin ized. The augite and biotite are similar in character 
to those in the minette; the amount of them is much less. They are 
greatly altered, and converted into masses of chlorite, limonite, carbon- 
ates, etc. The hornblende, which is comparatively rare, is generally 
fresh and of a dirty brownish-green color, with strong pleochroism ; some 
iron ore, apatite, and a few exotic grains of quartz, with mantles of 
decayed pyroxene and some plagioclase, complete the list of minerals. 
The material was not suitable for analysis. 

The occurrence of this rock is interesting in spite of its altered char- 
acter, because it furnishes a transition form from the minettes — ortho- 
clase rocks rich in ferromagnesian minerals — into the syenite-porphyries 
described on page 513, which are orthoclase-porphyries with the ferro- 
magnesian elements greatly diminished. In mode of occurrence in 
intruded sheets, in their moderate grain, and in the gradation of their 
mineral and consequently chemical character, these rocks form a closely 
connected series which are, genetically, intimately related to each other. 

Transition from minette into Jcersantite. — Another transition type of 
the minette is found in the dark basic dike cutting the syenite which 
forms the projecting knob of Storr Peak, about 3 miles northeast of 
Yogo Peak and at the head of one of the forks of Dry Wolf Creek. On 
a fresh fracture the rock has a very dark-gray color and appears fine 
grained and exactly like the finer-grained minettes previously de- 
scribed ; it glistens firom the fine cleavage surfaces of innumerable small 
biotites. Under the microscope it is found to contain large phenocrysts 
of augite, with a large amount of greenish fibrous hornblende, plainly 
secondary after the augite. The augite contains specks of iron ore 
zonally arranged. The groundmass is composed of biotite of a greenish 
color and lath-shaped feldspar mixed with considerable of the green 
hornblende. The greenish color of the biotite would indicate a variety 
rich in iron, and this seems to to be confirmed by the practically total 
absence of iron ore in the groundmass. The feldspar is a mixture of 



piBMOH.l NEPHELITE-MINETTE. 539 

the alkaline species with andesine twinned according to the albite and 
Carlsbad laws. Another of these transition tyx>e8 is found in a' black, 
dense, basic dike catting the syenite at the Wright and Edward's mine, 
above Haghesville. The rock contains large glassy, much-crackled 
inclusions of quartz and of feldspar, which appear to be taken up from 
rock masses through which the magma has passed on its way upward, 
in a manner precisely similar to the well-known lamprophyre at Aschaf- 
fenburg in Germany. 

Under the microscope this dense groundmass resolves into a felt 
composed of slender microlites of a feldspar varying from labradorite 
to oligoclase mixed with many of alkali feldspar and granules and 
formless masses of a completely decayed and altered ferromagnesian 
component. 

NEPHELITE-MINETTE. 

The normal minettes of Sheep Creek type, by increase in feldspathio 
components and consequent diminution of the amount of the ferro- 
magnesian elements, pass into types intermediate to the syenite- 
porphyries, and in another direction, by increase of plagioclase, they go 
over into kersantite-like facies. On the other hand, by their assump- 
tion of olivine and nephelite there are produced types which, although 
megascopically retaining the same characteristic minette-like habit, are 
by the microscope found to be lamprophyric rocks which do not corre- 
spond exactly to any hitherto-described rock types. These in their turn 
grade into rocks in which the minette-like chAxaotor is lost; they 
are black or very dark, dense lamprophyres, occurring in dikes, and 
although they can not be assigned to any definite type, in the present 
systems of classification, they clearly belong in the monchiquite-alnoite 
series of Bosenbusch, and have perhaps their closest analogies in some of 
the rocks described as <^ monchiquites." The most characteristic type of 
nephelite-minette occurs in a broad dike or intrusive mass cutting the 
limestones in the saddle or low point in the spur running eastwardly 
from Bandbox Mountain. The rock is of a clear dark-gray color, thickly 
mottled with small glittering tablets of black biotite, which occasionally 
reach 5 mm. in diameter, and with light-brownish siK)ts which are 
altered olivines. 

In thin section the following additional minerals are found to be 
present : augite, iron ore, apatite, alkali feldspar, nephelite, and sodalite. 

The augite is a clear, pale-greenish brown in the section, has a good 
cleavage, and shows no inclusions. It is often fringed by granules of 
iron ore. The phenocrysts are 1 or 2 mm. in length, the habit broad 
and stout, and the development of the crystal faces renders them 
idiomorphic. The phenocrysts are moderately common. Augite of a 
similar character is freely and abundantly scattered through the 
groundmass in small short prisms and rounded anhedral grains. 

The biotite which occurs so abundantly through the groundmass is 
in rather slender foils, seldom in broad tablets. It is intensely 



540 IGNEOUS ROCKS OF LITTLE BELT MOUNTAINS, MONTANA. 

j)leocliroic, between a very deep olive-brown and a pale yellow. In addi- 
lion to tbis normal variety there occur spots in the rock which contain 
local enrichments of biotite in larger leaves and tablets of peculiar 
<olor and appearance. Within it is of a light olive-green color, mottled 
and clouded with areas of brown, and fringed by a zone of brown on the 
outer edge; its pleochroism varies between the olive green and a pale- 
brownish color. It is everywhere thickly spotted by inclusions of 
iron ore. 

The feldspar is entirely an untwinned alkali variety, found in small 
grains and laths and mixed with irregular areas of nephelite and small, 
l)ale-brownish, rudely circular masses of sodalite, which are, of course, 
isotropic between crossed nicols. The presence of the nephelite is 
proved by the ready and abundant gelatiuization of the powdered rock 
in extremely dilute nitric acid, and by the faint negative uniaxial cross 
of basal sections between crossed nicols; the sodalite is shown by the 
strong reaction for chlorine given by the solution on qualitative testing. 

Excei)t for the alteration of the olivine, which appears to be quite 
thoroughly. altered to a micaceous substance which may be iddiugsite, 
the rock appears quite fresh and unchanged. In structure the rock is 
porphyritic, but holocrystalline, and in ordinary light it appeais of a 
strongly miuette-like character, due to the abundant foils of biotite, 
though rather richer in augite than a truly typical miuette. It is 
clearly and pronouncedly a lamprophyre, and the name of nephelite- 
minette seems to best define it. It appears to be a transition form from 
the regular minettes into the mouchiquite alnoite-lamprophyre series of 
Kosenbusch. 

CONTACT PHENOMENA. 

The most marked case of the metamorphism induced by the lampro- 
phyres wliich has come under our notice is that produced by the rock 
described above. Here a mass of the limestone into which the igneous 
rock has been intruded has been split off and immerged in the fluid 
mass. As a result it has been subjected to a more intense degree of 
couta<!t metamorphism than we have seen exhibited in any other local- 
ity, and as it shows the character of the phenomenon on a large scale 
and witli a more perfect development of new minerals, we have been 
led to study it in some detail, since its description serves to cover all 
of the less striking cases jireviously mentioned. 

The lime rock has become hard, dense, tough, and of a somewhat 
greenish color. It has been much cracked, and these cracks are often 
filled by small dikelets or apophyses from the igneous rock. Often 
these little tongues or ** flames" are but a few millimeters in width. In 
other places the rock is hollow and rather cavernous and the cavities are 
completely studded by the brilliant facets of very small, often minute, 
bright-green diopside crystals. The i)yroxene is characterized by the 
frequent occurrence of the forms m (110), A^ (331), and o (221), and is 
similar to the pyroxene occurring in the limestone altered by contact 



PIR880N.] VOGESITE. 541 

metatnorpbism of the diorite at Blackhawk on Castle Mountain, whicli 
has been described and figured by the aathorJ Associated with the 
pyroxene are small, often brilliant crystals of a wine-colored grossular 
garnet which shows only the planes of the dodecahedron and is at times 
two or three millimeters across the diameter of the crystal. 

Thin sections cut across the altered rock and the narrow dikelets, 
thus showing the contact plane of the two, exhibit several interesting 
features under the microscope. The minette rock whose normal char- 
acter is somewhat variable, as previously mentioned, seems to maintain 
its full size of grain to the contact wall. This is no doubt due to the 
mass of limestone having been an inclusion and not a limiting contact 
wall. It would therefore readily become so greatly heated as not to chill 
the magma and interfere with the process of crystallization. As the 
rock approaches the contact it becomes surcharged with lime, which 
marks itself by the production of great quantities of augite. The 
augite is present in large crystals and in great quantities of minute 
rounded anhedra. It finally becomes enormous in amount, and the large 
biotites inclose great quantities of these small grains poikilitically. A 
considerable number of sodalites close to the contact edge are also 
noticed. Although this contact edge, as seen by the eye alone, is a very 
sharp line, the microscope shows that there has been considerable pene- 
tration of the lime rock by the igneous magma. It penetrates in narrow 
threads and tongues, and although at the edge there is a great predom- 
inance of augite in the lime rock, this is mixed with patches and masses 
of alkali feldspar and biotite. These gradually fade out, and the rock 
becomes an almost solid mass of small pyroxenes in round grains, \^ith 
an occasional biotite flake; the minute interstices between the crowded 
grains are filled with a colorless isotropic substance of low refraction 
whose exact nature can not be deterniined. This appears to be the 
dominant tyx>e of the included mass. 

VOGESITE. 

Several types of minettes have been found in which the augite has 
apparently been converted into hornblende. Since in all the minettes 
from this district the proportion of augite is large, they should be 
termed augite-minettes. The hornblende, therefore, plays a conspicuous 
role, and whether such rocks should be called vogesite or not is an open 
question ; it being understood, of course, that the secondary hornblende 
predominates over biotite. They might, perhaps, be called pseudo- 
minettes or meta-minettes, indicating that an alteration of a prominent 
ingredient has taken place. 

In one or two instances, however, types occur in which it car. not be 
said, from any evidence afforded by the microscope, that the hornblende 
is secondary; and since the rocks are of typical lamprophyre habit and 



> Wee<l and Priseon, G^logy of the Castlo Mountain mining district: Bull. U. S. Geol. Survey No. 
130, p. 161. 



542 IGNEOUS ROCKS OP LITTLE BELT MOUNTAINS, MONTANA. 

method of occarrence, aud are composed of predominant hornblende and 
alkali feldspar, they may well be termed vogesites. The best instance 
of this type is fonnd in intraded sheets in the Cambrian on Dry Belt 
Creek several miles below Barker. T]ie road-cutting on the side hill 
has afiorded fairly fresh material. The rock is of a gray color, with 
an olive tone, and weathers with a brownish crust. It is cracked by 
prismatic jointing^ and on breaking one of the pieces it can be seen 
that a zone of alteration extends from each joint face inward, leaving 
only the central portion as an unaltered core. The grain is quite fine 
and the rock tough and hard to break. With the lens it appears dis- 
tinctly crystalline, but contains no phenocrysts of any kind. An 
occasional exotic fi*agment of included quartz or feldspar brought up 
from the gneiss below was noted. 

Under the microscope the rock is found to consist of a mixture of 
greenish-brown hornblende and alkali feldspar, with accessory plagio- 
clase, apatite, and iron ore, and with calcite, chlorite, and quartz as 
secondary alteration products. 

The hornblende is moderately fresh. It occurs in slender prisms, and 
its color varies from place to place from green to brown. It has a small 
angle of extinction and moderately high birefraction. It does not 
appear to be an alkali-bearing variety. It is present in very large 
amount; a rough estimate would place the proportion of hornblende to 
feldspar as two to three. ^ 

Of the feldspar an unstriated alkali variety decidedly rules; it is 
considerably altered, and filled with sericite leaves. It has a long, lath- 
shaped form, much like the hornblende, and the two are interwoven in a 
somewhat trachytic structure, with granules of iron ore, apatite, and 
decomposition products, such as chlorite, filling the interspaces. 

The plagioclase is also lath-shkped, like the alkali feldspar, but, if any- 
thing, more altered, so that its determination is not so satisfactory as 
could be wished. Nevertheless, since all sections of it seen extinguish 
nearly parallel with the nicol, no matter what twinning is present, we 
may safely conclude that it is an oligoclase. 

The quartz appears in the triangular interspaces between the feld- 
spars much as in many trachytes. It appears in part secondary, but 
much of it is clearly of primary origin and the last mineral which 
crystallized. 

The structure of the rock is dominated by the lath-shaped horn- 
blendes and feldspars; it is the structure of many well-known types of 
lamprophyres, and is somewhat trachytic. A rock of quite similar type 
occurs intruded in Cambrian shales on Belt Creek about 3 miles above 
Monarch; the hand specimen is lighter in color and the rock more 
coarsely crystallized ; it forms a transition to a fine-grained syenite very 
rich in brownish hornblende. 



PIB880V.] LAMPROPHYRES. 543 

ANALCITE-BASALTS. 

As previoasly stated, the nephelite-minette forms a transition type 
from trne minette to a series of lamprophyres which are foand most 
abandantly catting the limestones forming the top of Bandbox Moun- 
tain and the divide running north and connecting it with Steamboat 
Mountain. These rocks are analcite-basalts, the type of which was iirst 
described by Lindgren^ from the Highwood Mountains, Montana. 

BANDBOX MOUNTAIN TYPE. 

The rock forming the narrow dikes on Bandbox Mountain is generally 
more or less decayed, but in one instance good fresh material could be 
obtained. It strongly resembles the nephelite-minette in appearance, 
is a dark basic-looking rock, with great numbers of large, fresh olivines, 
small mica plates, and occasional augites as phenocrysts. 

Under the microscope the same minerals are to be seen lying in a 
colorless base. The large olivines are very fresh, clear, and limpid. 
The occasional large augites are also clear and nearly colorless; they 
appear to be of the diopside variety. The biotite is very peculiar. It 
has the striking red-brown color often seen in theralitic and leucitic 
rocks, but of so pale a tone that it appears a pale orange-brown; the 
pleochroism, while marked, is therefore unusual. 

Bays parallel to c = light yellow-brown. 
Bays perpendicular to c = colorless. 

The larger crystals of this mica, which may properly be termed pheno- 
crysts, are at times broken and bent and have embayed portions. The 
olivines and augites are also often broken and the pieces slightly sepa- 
rated from one another, the irregular contours of one piece exactly cor- 
responding to those of the one adjoining. This would seem to point 
to an earlier period of formation for these crystals, which have become 
cracked and separated in the upward movement of the inclosing viscid 
mass. It is noticeable that these interspaces in the case of the olivines 
are filled with the peculiar mica already mentioned, while a narrow fringe 
or mantle of it surrounds all of them on the outside. We are inclined 
to believe, from its color and from the chemical relations shown in the 
analysis, and a consideration of the minerals present, that it is a biotite 
rich in alumina and poor in iron. 

The minerals which have been described are lying in what may be 
termed a groundmass, consisting chiefly of pyroxene, with a consider- 
able amount of the mica already described in flakes and scattered shreds, 
cemented by a pale-brownish isotropic base. The second generation of 
pyroxene occurs in small, slender prisms, having a colorless diopside 
core, surrounded by a deep- green mantle of SBgirite. The brownish 
base examined with very high powers is really colorless, but dotted 
with innumerable small brownish specks, which give it, under low pow- 

< Tenth Census United SUtes, Vol. XV. 1886, p. 727. Kniptive rooks firom Montana : Proo. California 
▲cad. Nat. Sol., ser. 2, Vol. III. 1890. pp. 89-57. 



544 IGNEOUS ROCKS OF LITTLE BELT MOUNTAINS, MONTANA. 



ers, a general brownish tone. In ordinary light it appears much like 
a somewhat kaolinized feldspar. With a strong illamiuation between 
crossed nicols, it is seen not to be wholly isotropic everywhere but to 
have in places a feeble aggregate polarization; and it often contains 
minute flecks of brightly x>olarizing substances, perhaps due to calcite 
or muscovite. As will be shown later, this base consists of analcite, in 
agreement with what has been demonstrated for similar rocks from 
other regions.^ 

The essential character of the rock is given to it by the large olivines 
lying in the groundmass of interwoven small, slender pyroxenes with 
their green rims and the dusty brownish base of analcite cementing the 
whole, touched up here and there with the pale-orange biotites. The 
rock, from the presence of this mica and the green-rimmed pyroxenes, 
has an appearance which recalls that of some of the fine-grained thera- 
lites from the Crazy Mountains. (See PI. LXXVI, B.) 

An analysis of this interesting type (No. 576) by Dr. W. F. Hille- 
brand gave the results shown below in No. I : 

Analyses of analcite-hasalts from various localities. 



Constituent. 



SiOi 

AI2O3 

Fe-iOa 

FeO 

MgO 

CaO 

NaiO 

K.2O 

H2O + 110° 
H2O — 110'= 

TiO-t 

COj 

P2O5 

CI 

SO3 

CraOa 

MnO 

NiO 

BaO 

SrO 

Li,0 



Total 



I. 



48.39 

11.64 

4.09 

3.57 

12.55 

7.64 

4.14 

3.24 

2.56 

.28 

.73 

None. 

.45 

Trace. 

.08 

.07 

Trace. 

None. 

.32 

.15 

Trace. 



II. 


III. 


IV. 
43.50 


V. 


VI. 


VII. 


46.48 


42.46 


45.59 


45.58 


0.8065 


16.16 


12.04 


18.06 


12.98 


15.87 


.113 


6.17 


3.19 


7.64 


4.97 


4.65 


.026 


6.09 


5.34 


7.52 


4.70 


6.37 


.050 


4.02 


12.40 


3.47 


8.36 


8.32 


.314 


7.35 


12.14 


13.39 


11.09 


9.91 


.136 


5.85 


1.21 


2.00 


4.53 


3.42 


.067 


3.08 


2.68 


1.30 


1.04 


1.61 


.034 


4.27 


4.03 


1.22 


3.40 

.51 

1.32 


a 3. 14 


.142 


.99 


2.47 
.55 

.84 


2.10 


Undet. 




.45 
(f) 







.91 


(f) 




None. 
None. 


.05 
J. 03 








«*«• •*•••• 










.16 

1 


.14 


Trace. 




1 





(n 
(f) 


TJndet. 




.13 


(f) 




Undet. 


.12 




Trace. 


Trace. 


 



I 



99.90 



100.91 



99.51 



100. 20 99. 87 



98.86 



a. Ignition includes CDs, etc. 



6Zr0, 



1 Pirason, On the monchiqnito or analcite group of rocks : Jour. Geol., Vol. IV, 1896, p. 679. Cross, 
Analcitobasalt from Colorado: ibid., Vol. V, 1897, p. 684. 



PIB880N.] ANALCITE-BASALTS. 545 

I. Dike of analoite-basalt from Bandbox Mountain. W. F. HiUebrand, analyst. 
II. Analoite-basalt (moncbiqnite), Santa Cniz, Brazil. Hunter and Rosenbuscb, 
Tscber. Mitt., XI, 1890, p. 445 (see also Jour. Geol., Vol. IV, 1896, p. 679). 
M. Hunter, analyst. 

III. Analcite-basalt (moncbiquite). Castle Mountain district, Montana. Weed 

and Pirsson, Bull. U. 8. Geol. Survey No. 139. L. V. Pirsson, analyst. 

IV. Analcite-basalt (moncbiquite), Magnet Cove, Arkansas. Williams, Igneous 

Rocks of Arkansas, p. 295. W. A. Noyes, analyst. 
V. Analcite-basalt, The Basin, Cripple Creek district, Colorado. Cross, Jour. 

Geol., Vol. V, 1897, p. 689. W. F. Hillebrand, analyst. 
VI. Analcite-basalt (moncbiquite), Sbelbume Point, Vermont. Kemp, Bull. U. S. 
Geol. Survey No. 139, by Weed and Pirsson, Geology Castle Mountain mining 
district. H. T. Vulte, analyst. 
VII. Molecular ratio of oxides in No. I. 

It will be noticed that the rock has the chemical characters of the 
lamprophyre groap — low silica and modern alumina, with rather high 
alkali for so basic a type, and, at the same time, high lime and mag- 
nesia. In connection with the analysis, we have given all the analyses 
of rocks of this type which have been classed as monchiquites that 
we have been able to find iu ttie literature. It will be seen that it does 
not agree in its chemical relations very closely with any of (hem, nor, 
indeed, for that matter, do they agree very closely with one another. 
This is dne to the fact that under the heading of ferromagnesian min- 
erals in a colorless isotropic base quite diiferent varieties and propor- 
tions of minerals may be assembled ; the composition of the base may be 
variable, consisting of different isotropic minerals, analcite, leucite, 
sodalite, etc., and its proportion to the dark minerals which are present 
may also vary. The ratios given under YII furnish a means of deter- 
mining the character of the isotropic base. Neglecting small quanti- 
ties of nonessential minerals, we may assume the rock made up of diop- 
side, olivine, biotite, and the base. All of the lime is in the pyroxene, 
which gives the measure of its amount. The biotite produces some 
uncertainty, but we may consider it made up of an olivine molecule 
(MgFe)2Si04 and^a feldspathoid in which K2O: AUOs::!:!. The fer- 
rous iron molecules may then be added to the magnesia, a correspond- 
ent for the lime (as augite) deducted from the sum, and the remainder 
of the (MgFe)O molecules considered olivine. This leaves (NajO + 
K20):Al203:Si02:: .101: .113:420, which is — 1.1:4.1, or in round num- 
bers, 1:1:4, and this shows that the base has the general formula EAl 
(8103)2. When we consider the ratio of the soda to the water we find 
that it is 0.067 : 0.142 = 1 :2.1 or 1 : 2, and consequently it is clear, since 
the biotite contains a considerable part of the potash; and that the iso- 
tropic base has the composition NaAl(Si03)U20, or is made up of anal- 
cite. When we reflect that this is only an approximation, that the 
water in the biotite and the ferric iron present as SBgirite (though these 
two errors tend to counterbalance each other, since segirite requires 
NasO : Fe203 = 1 :1, and biotite requires H2O : AI2O3 : : 1 : 1) have not been 
considered, the agreement of these ratios is very remarkable, and it is 
20 GBOL, PT 3 36 



546 IGNEOUS ROCKS OP LITTLE BELT MOUNTAINS, MONTANA. 

dear that they are not accidental; and the feict of the isotropic base 
being made up of analcite may be considered demonstrated. Thns, 
all things considered, the rock shows that it is an analcite-basalt and 
should be classified as such. Its petrologic affinities will be pointed 
out later when some other related types have been considered. 

EUBESA DIVIDE TYPE. 

It has been mentioned that on the divide between Bandbox and 
Steamboat mountains a series of dikes occur. Like those on Bandbox 
Mountain, they are narrow and of dark basic rocks of basaltic charac- 
ter. They do not show any large phenocrysts, but are quite dense, of 
very dark-gray color, and one sees only the light reflected from numer- 
ous cleavage surfaces of minute biotites; they thus have a strong 
minette-like habit. 

Under the microscope they are found to consist of biotite and pyrox- 
ene in small crystals thickly crowded in a colorless base; the large 
olivines of the previous type are entirely wanting. The biotite is of 
the same character as in the former variety, and the pyroxene has the 
same form and mantle of segirite. The rock thus appears very similar, 
but without the large phenocrysts; there appears to be more varia- 
bility in size of the second generation of pyroxene and biotite, and 
relatively more of them in proportion to the amount of groundm$kSs; 
the amount of segirite is also less. The colorless groundmass is also 
thickly spotted with minute pale-brown specks that appear like kaolin; 
in places it shows a feeble aggregate polarization between crossed nicols. 

One peculiarity that distinguishes this from the former type is that 
its surface shows here and there minute round spots of a white min- 
eral. Under the microscope these spots are without crystal form and 
appear composed of an isotropic mineral; the other components fre- 
quently project into them ; and they frequently contain bright polarizing 
specks, which in convergent light give the negative, uniaxial, ringed 
cross of calcite. The rock powder is found to gelatinize with very 
dilute nitric acid, and qualitative tests show the absence of chlorine 
and the presence of considerable sulphuric anhydride in some sul- 
phate. The colorless mineral is probably nosean, and that it is rich in 
soda is shown by the fact that wherever the pyroxenes project into 
it they are invariably tipped with deep-green jBgirite, although the 
outer portion may be almost entirely a colorless diopside. Similar facts 
regarding a local enrichment of soda have been described by Oross.^ 

In this connection an interesting fact in relation to these rocks, as 
well as the Bandbox Mountain type, should not be passed by without 
mention, and this is the total absence of iron ore in them in spite of 
the very considerable amount of both ferrous and ferric oxides shown 
by the analysis previously given. The ferrous oxide has gone into 
olivine and pyroxene, the ferric into asgirite and biotite. The base 
consists of various isotropic minerals, partly altered; analcite is 



i6«ology Cripple Creek district: SixteeDth Ann. Kept. U. S. Geol. Sarvey, Part II, 1895, p. 85. 



PiBBflOH.] ANALCITE-BASALT8. 547 

ondonbtedly present, and nothing more definite can be stated con- 
cerning it. 

The only occurrence of a lamprophyre which we have been able to 
find in the literatare, similar in character to this just described, is one 
from Umptek which has been studied by Hackman.^ 

This consists also of olivine, biotite, and pyroxene in an isotropic base 
(probably of analcite), and the pyroxene has also the aegirite rims. Hack- 
man, adverting to the character of the pyroxene, which differs so much 
from the basaltic variety found in the types described as monchiquites, is 
inclined to believe that these rocks should not be placed in the same class 
with them. With more or less uncertainty regarding the character of 
the base existing, it appears to the writer that the formation of a new 
class based on the distinction of a variable pyroxene seems to be hardly 
advisable, and he has followed Hackman's conservative course. 

BABKEB TYPE. 

A rock which is closely related to the forms just described occurs as 
a dike on the north side of Dry Fork of Belt Greek, above the town of 
Barker. It is a dark, dense, basic-appearing variety, and the section 
shows a considerable number of rather small olivines lying in a felt 
composed of slender, colorless prisms of augite cemented by an isotropic 
base. The olivines are fresh and present no particulars worthy of 
mention^ they are accompanied by some iron ore and apatite. The 
augite, with low powers, shows the mossy appearance so frequently pre- 
sented by the slender microlites of SBgirite in tingoaites; it is a sort of 
felt, composed of very small, long, slender microlites densely interwoven, 
of a colorless pyroxene which only in a few cases was seen to pass into 
green segirite. When this mesh is studied with high powers occasional 
minute scraps of a brownish mineral, which is thought to be biotite, 
are seen ; the particles are so small that the determination must be con- 
sidered doubtfril. The base which cements the minerals is colorless, 
and though generally isotropic it shows in places a feeble polarization; 
it is probably of analcite. 

BIG BAXDY MOUNTAIN TYPB. 

This rock was not actually found in place, as the whole surface of the 
exposed laccolith on the south side, where the specimens were obtained, 
presents a smooth surface formed of large, flat-joint plates, the outcrops 
having broken down into slide rock. The course of the dikes, which are 
seen as black lines crossing the white walls of the enormous amphithe- 
aters on the east side, is here readily perceived by the dark fragmenf» 
mixed with the light-colored porphyry of the main rock. On a fresh 
frJEUsture the rock is a very dark stone color, or grayish black, and very 
dense in grain; only occasional small si)ots of a dark-brown minen^l, 
an altered olivine, are to be seen. The rock has, indeed, a pronounced 
basaltic habit. 



■Nephelinsyenitgebiet anf der Halbinsel Kola, Ramsey nnd Hackman: Fennia, II, No. 2, 1894, p. 178. 



548 IGNEOUS ROCKS OF LITTLE BELT MOUNTAINS, MONTANA. 



In the section the olivines are found to be quite rare ; they range from 
1 to one-half millimeter in diameter and are nearly always serpentin- 
ized; with this exception the rock is very fresh. The most common 
mineral and the one which forms by far the greater bulk of the rock is 
a pale brownish-green pyroxene, so light in color in the section as to be 
nearly colorless. It is in rather slender, square prisms, and ranges in 
size from 1 to one-tenth millimeter in length ; it contains numerous pale- 
brownish inclusions of glass. A moderate amount of iron ore is scat- 
tered around among the' pyroxenes. These minerals are lying thickly 
crowded in a groundmass which is filled with very great numbers of 
flat tabular microlites of a brownish-olive augite; these latter vary in 
size up to the dimensions of the smallest of the colorless pyroxenes 
already mentioned. The base which cements all of the minerals 
together is a colorless isotropic substance filled with minute dusty 
specks of a brownish or blackish color, which are no doubt of a ferrugi- 
nous nature or separated particles of ore. The general effect of the 
section with a moderately low power is precisely that of an augitite, 
great quantities of augite in idiomorphic crystals lying in a brownish 
base, which is isotropic. 

An analysis of this type by Dr. W. F. Hillebrand gave the results 
given in Column I: 

Analyses of lamprophyres. 



CoDBtitnent. 


I. 


n. 


III. 


IV. 


V. 


SiOa 


48.36 

13.27 

4.38 

3.23 

8.36 

9.94 

3.35 

3.01 

2.89 

.90 

.52 

.40 

.30 

.25 

Trace. 

.04 

.19- 

.54 

.09 

Trace. 


46.04 

16.04 

7.10 

8.23 

4.46 

10.19 

6.11 

2.85 

.33 


42.03 

13.60 

7.55 

6.65 

6.41 

14.15 

1.83 

.97 

1.08 


48.43 

11.41 

12.32 

.64 

8.23 

9.97 

3.59 

3.21 

1.33 


0.806 
.129 
.027 
.045 
.209 
.179 
.057 
.032 


ALOa 


■**■*» '-'O" "• ■••• •«-« •••• «••• 

FftiOa 


FeO 


Mir 


CaO 


NaaO 


KiO 


HaO-l- 11(F 


H3O 110° 




TiOj 


(f) 


3.70 
.57 

8O3,.08 
NaC1..05 


(f) 

(») 
COytrace. 




PgOli 




CO., 




Fl 






Cr^Oi 








NiO 










MnO 




Trace. 

(♦) 
(t) 


.34 

(f) 
(f) 




BaO 


(f) 
(f) 




SrO 




LiiO 




Total 










100.01 
.10 


100.35 


99.23 


99.47 




= F1 














99.91 



















I. Analcite-basalt dike rock of Big Baldy Mountain, Little Belt Mountains. W. F. 
Hillebrand, analyst. 



PiBssojf.] ANALCITE-BASALTS. 549 

II. Augitite, Upper Pioos vale, Vulcane der Capyerden. Doelter, 1882, p. 143. C. 
Doelter, analyst. 

III. Fourcbite, Fourche Mountain, Arkansas. Igneous Rocks, Arkansas. J. F. 

Williams, 1890, p. 111. Brackett and Koyes, analysts. 

IV. Vogesite, ForsthansWelschbruch, Alsace. Hosenbusch, SteigerSchiefer, 1877, 

p. 301. H. Rosenbuschf analyst, 
y. Molecular proportions of Na. I. 

The aDalysis shows strongly the characters of the lamprophyre 
groap — high lime, iron and magnesia, low silica and alumina, and mod- 
erate alkalies. 

Considering that the rock is composed essentially of angite and an 
isotropic base which has the chemical composition given above, we 
have not been able to find in the literature any type which precisely 
corresiK>nds with it. Two varieties of rocks of this class are com- 
pared, as to their chemical composition, under II and III, for the sake of 
example. The rock described by Doelter consists mainly of augite in 
a ''glass" base, with minute amounts of other minerals; that by Wil- 
liams consists of augite in an '' altered glass base." The former is an 
extrusive. With both, the type under discussion shows certain analo- 
gies, but at the same time important variations. When one compares 
it with the analyses given on page 544, the same is found 4o be the 
case; yet according to our existing systems of classification (and 
especially when one takes into account its generic relations) it appears 
to belong best under the camptonite alnoite series of Bosenbusch. 
From the chemical point of view the hornblende- vogesite whose analy- 
sis is given under IV in the above table agrees with the Big Baldy 
Mountain rock better than any we have been able to find. The agree- 
ment, except as to the iron, is quite remarkable indeed, and it must be 
confessed that the relations of the two iron oxides for a rock of this 
class, as given in Analysis IV, are not above suspicion. The ferrous 
oxide must certainly be too low; the ferric, too high. Taking this into 
consideration, the agreement would be even closer. It shows how 
extremely similar magmas may crystallize into quite different minerals. 

If we consider the molecular ratios given in the last column of the 
preceding table, they may be arranged as follows with respect to the 
minerals shown to be present: 

""^^^ «-^7=>agnetite. 

FeO 027=1J ^ 

FeO 018 > 2| 

MgO 030 i [olivine. 

SiOi 024=l) 

CaO 180=1) 

MgO 180 = 1 [Oiopside. 

SiOa 360=2) 

From the above there remains over — 

SiO.2 =0.424. 

AlaOs^: 129. 

(Na2O-f-K«O) = .089. 



550 IGNEOUS BOCKS OF LITTLE BELT MOUNTAINS, MONTANA. 

Of coarse such a compatatiou mast be considered as only radely 

approximate; there is an excess of alamiua over the alkali, and it is 

therefore probable, as asaally happens in snch cases, that some of it 

has found its way into the augite. The calculation is sufficient to show, 

however, that if an alkali- alumina silicate had formed as a residual 

product of crystallization, it must have been mainly one of the feldspa- 

thoid group, low in silica and rich in alumina and alkalies. This 

group, it will be remembered, is composed mainly of isotropic minerals, 

as leucite, analcite, sodalite, etc. The ratios, indeed, approximate to 

the relation 

NazO+KjOiAljOarSrOa:: 1:1:4, 

which would furnish a silicate of the formula (!NaK) Al (8103)2, which is 
that of leucite, or, with the addition of water, of analcite. 

Of the very considerable amount of water shown by the analysis it 
may be said that there is no apparent alteration product which could 
account for it except the serpentinized olivines, and they are entirely 
too minute in amount to furnish more than a very small fraction of it. 
It must therefore belong to the isotropic base, and could be furnished 
by analcite. That it is the latter is shown by the molecular ratios of 
the water to the other components — 

(NazO + K2O) : AI2O3 : SiOj : HjO : : .089 : .129 : .424 : .210, 

or approximately 1:1:4:2, which is the ratio required by analcite. 

The rock powder when treated with very dilute nitric acid is found 
to gelatinize readily and abundantly. This could not be due to leucite, 
which does not gelatinize, though decomposed with acid, but is un- 
doubtedly due to analcite. In this connection it is interesting to note 
that the molecular ratio of the soda to the water in the fourchite of 
Williams is .030 : .060=1 : 2, which is the ratio required by analcite, and 
undoubtedly this mineral is present in the base of his rock. 

From the molecular ratios furnished by the analysis it may be easily 
calculated that our rock contains — 

Per cent. 

Iron ore 6.7 

Pyroxene 39.6 

Olivine 4.2 

Base (mostly analcite) 49.5 

Total 100.0 

BELATED TYPES WITH POLARIZING BASE. 

It will be noticed by reference to the geologic map that the great 
intruded stock or mass of which Yogo Peak forms the western extremity 
is narrowed down at the head of Yogo Gulch to a dike-like character. 
As previously described, the western limb of this mass practically ends 
at this point in a mass of shonkinite, and then thins out into the dike 
which runs along the crest of the divide until it widens into the Sheep 



PIB880H.] ANALCITE-BA8ALTS. 551 

MoantaiD portion. The exposares at this point hardly warrant us in 
stating positively that the dike forms an actual apophysis of the mass 
and is not a separate intrusion , but the general relation of the intruded 
bodies inclines one strongly to the belief that it is an apophysis — ^a con- 
necting link between the masses, and not a separate intrusion. 

The rock forming this dike is dense black and similar to the lampro- 
phyric types previously described. In the section are found olivine and 
augite in a colorless base, thickly peppered with minute ore grains. In 
places the ore grains are wanting, and their place is filled by scattered 
leaves or skeleton crystals of biotite. The base when examined between 
crossed niools presents a mosaic of low polarizing grains of great fine- 
ness mixed with isotropic ones. The effect with a low power is that of 
an aggregate x>olarization. This may be due in part to zeolitization, 
but may equally well be a mixture of analcite with nepheline or ortho- 
clase, or both. The rock also contains a number of olivines and augites 
of large size, which are deeply corroded and bordered by heavy black 
opacite rims. They are probably of intratelluric origin and have suffered, 
in later movements of the magma, a partial resorption. An exact ana- 
logue of this type, except for the corroded olivines and augites, occurs 
in the drift brought down from Big Baldy Mountain in Butcherknife 
Creek. The rock was not found in place, but undoubtedly comes from 
some basic lamprophyre dike. 

These types appear to be closely related to the absarokites of Iddings/ 
which seem to belong in the monchiquite-alnoite series of lampro- 
phyres of Eosenbusch,^ with a base composed of alkali feldspars more 
or less mixed with feldspathoid minerals. In the majority of the types 
described as monchiquites the base is an analcite, as we have shown 
elsewhere;^ in the camptonites it is a soda-lime feldspar, and in the 
absarokites it is essentially orthoclase. If the base were nephelite the 
rock would have the essential character of many nephelite-basalts, and 
it is evident that these rocks with varying developments of the feld- 
spathoid minerals, but which are composed chiefly of ferromagnesian 
silicates, will show transitions into the various types of alkaline basalts, 
whose effusions are a common feature of many regions. 

TRANSITION PROM ANALCITB-BASALT TO MINBTTB (INCLUDING 
THE SAPPHIRE-BEARING ROCK OP YOGO GULCH). 

On the long ridge which extends eastwardly from Bandbox Moun- 
tain, and which forms a spur between the head branches of Eunning 
Wolf Greek, and in a deep saddle of which occurs the nephelite- 
minette previously described, are found several dikes cutting the lime- 
stone. One of these dikes is found to be composed of pyroxene and 

biotite, with large pseudomorphs of serpentine after olivine, lying in a 

' ' — J 

iJour. Oeol., Vol. III. 1895, p. 935. 
>Mr8h. Gesteiue, 34l ed., 1895, p. 536. 
•Jour. Geol., Vol. IV. 189«, p.«79. 



552 IGNEOUS ROCKS OP LITTLE BELT MOUNTAINS, MONTANA, 

colorless isotropic base. The pyroxene is of a pale-brownish variety, 
the biotite the usual deep-brown pleochroic kind found in igneous 
rocks. The base has in places a feeble aggregate polarization. A con- 
siderable amount of iron ore is present. Disregarding the olivine, the 
character, relations, and amount of the minerals are such that, seen in 
ordinary light, the section strongly recalls many of the minettes which 
have been previously described; the use of the analyzer shows, of 
course, that the base is not composed of feldspar. From the actual 
character of its minerals the rock is most nearly related to the analcite- 
basalt which has been previously described. ' 

The same type occurs also in the form of sheets intruded in the Cam- 
brian beds at the very head of Belt Creek, the structure and relation 
of the minerals being precisely similar ; the groundmass giving a feeble 
Aggi'^gate polarization, but presenting in many places minute spots 
of a doubly refracting colorless mineral, which may be orthoclase or 
nephelite. 

SAPPHIBE ROOK OF YOOO GULCH. 

By far the most important occurrence, from an economic standpoint, 
of the^e basic lamprophyric types is that of the corundum or sapphire- 
bearing rock near the mouth of Yogo Gulch. A preliminary account 
of this rock and its sapphires has been already published elsewhere,' 
but on account of its economic bearings and in the hope that its de- 
scription may aid in the search for similar occurrences in the region it 
is now repeated with additional details. 

The rock is of a dark-gray color and has an uneven fracture. It con-, 
tains small, light-green or white included masses, which form its most 
conspicuous feature, and these angular inclusions are pieces of lime- 
stone broken off and carried upward by the fluid rock in its ascent. 
They vary in size from those of microscopic dimensions to some that 
are half an inch or about a centimeter across. Many of them consist 
entirely of calcite, while others appear to be made up of a pale-green 
mineral, which is pyroxene. The largest inclusions, especially those of 
quartz, show a reaction rim of the same green pyroxene, the rim being 
about 1 mm. thick, while the entire center is of calcite with scattered 
prisms of the pyroxene. The rock shows only scattered tablets of mica 
as phenocrysts 2 or 3 mm. in diameter, while the groundmass glitters 
with minute flecks of biotite, and considerable pyroxene is seen. 

Microscopic characters. — In thin section the rock at once exhibits its 
character as a dark, basic lamprophyre, consisting mainly of biotite 
and pyroxene. There is a little iron ore present, but its amount is small 
and much less than is usually seen in rocks of this class. The biotite 
is strongly pleochroic, varying between an almost colorless and a strong, 
clear, brown tint. It occurs in ragged masses, rarely showing crystal 
outline, and it contains a large amount of small apatite crystals. The 
pyroxene is a pale-green diopside filled with many inclusions, now 

» Am. Jour. Sci., 4th Rerie«. Vol. IV, 1897. p. 421. 



pmsBON.] 8A.PPHIBE BOCK OF TOGO GULCH. 553 

altered, bat probably originally of glass; in some crystals these inclu- 
sions are so abundant as to render the mineral quite si>ougy. The 
grains sometimes show crystal form, but are mostly anhedral and vary 
in size, though the evidence is not sufficient to show two distinct gen- 
erations. 

These two minerals lie closely crowded together, and no feldspars are 
seen in the rock. The small interstices between them are filled with a 
clouded, brownish, kaolin-like aggregate, which appears to represent 
some former feldspathic component, possibly leucite, perhaps analcite. 
The rock appears to have its closest affinities in the analcite-basalt 
group, of which it may be considered a basic, somewhat altered, type. 
The abundance of biotite shows its relation to the minettes, but the 
rock is mach richer in the ferromagnesian components and lacks the 
feldspar of the minettes. It has evidently a close affinity with the 
minettes and shonkinite of the region, and is clearly of the same 
magma. It has the same richness in biotite and pyroxene as these, but 
differs in the feldspathic comp<ment. Togo Peak, with its shonkinite, 
is but a small number of miles distant from the locality. 

Borne calcite in agglomerated granules is also seen in the section, and 
this, as is so often the case in lamprophyres, does not appear as if 
secondary in origin, and is probably due to limestone fragments picked 
up, as previously mentioned. 

Origin of the sapphires. — The occurrence of such well-crystallized 
corundum in a basic igneous rock is of great interest. It seems clear, 
from the many different ways in which this mineral occurs, that there 
must be several methods in nature for its formation. The association 
with metamorphic rocks such as gneisses, schists, etc., is well known, 
and its occurrence with granites is also not uncommon. In all these 
cases, however, the association is with older metamorphic or granular 
crystalline rocks, and we know of its occurrence in more recent, 
undoubted basic igneous rocks in but few cases. Lagorio,^ in an arti- 
cle to be mentioned presently, gives a list of the known occurrences of 
corundum in igneous rocks, their tuffs, ejected fragments, and contact 
zones. 

By a series of important and interesting experiments Morozewicz^ 
showed that molten glass of a basic character dissolved alumina readily 
and in large quantity, and from this, on cooling, corundum and spinel 
crystals separated out. Lagorio,^ in commenting on these results and 
adding details of some experiments of his own, showed that the former 
idea which had been held concerning the origin of corundum in igneous 
rocks should now no longer be urged . This idea was that such corunduins 
had been torn loose from some place below where they had previously 
existed, and, being infusible, had spread the^nselves through themagma. 
Others again recognized in these corundums infusible but recrystal- 
lized portions of rock fragments inclosed in the magma, other portions 



1 Zeitoohr. fnr Kryst., VoL XXIV, 1895, p. 285. > Ibid., Vol. XXIV, 1895, p. 281. * Op. cit. 



554 IGNE0U8 BOOKS OF LITTLE BELT MOUNTAINS, MONTANA. 

being converted into spinel, cordierite, etc. Lagorio points out, how- 
ever, that this could not be the case, as corundum dissolves in molten 
glasses; and he calls attention to the confusion which has existed 
between fusibility of compounds in molten masses and their solubility 
in the same, the two being quite distinct. The characteristic form of 
corundum occurring with igneous rocks is the thin, flat, hexagonal 
table with low rhombohedron, described in a subsequent extract. 

This occurrence at Yogo Creek is an imi)ortant addition to the list 
of py rogenetic corundum. The clear-cut form of the crystals and their 
general distribution show that they have crystallized out of the magma 
with as much certainty as the well-formed phenocrysts of feldspar in 
a porphyry betray their origin. 

The general character of the rock, however, and its close relation- 
ship to the minettes and shonkinite of the region, show that it could 
not originally have been sufBciently rich in alumina to have allowed a 
general separation out of corundum. The condition of it, as men- 
tioned above, shows that the magma took up great quantities of inclu- 
sions from the sediments through which it passed. Among these 
sediments must have been a considerable thickness of clay shales. The 
liability of such beds to be shattered by igneous rocks ascending 
through them and included as fragments has already been shown else- 
where.' 

Such included fragments of shale, if the magma maintained its heat 
sufficiently, as it naturally would if confined in the form of an intruded 
mass, would eventually be dissolved, as the experiments described 
show. There would thus be formed local areas in the magma very 
rich in alumina, which on cooling would allow crystals of corundum to 
separate out. This explanation seems to us most in accord both vrith 
the facts observed in the field and with those obtained by experiment in 
the laboratory. The form of the crystals is also in accord with that of 
the pyrogenetic corundums. This occurrence, then, agrees well with 
the experiments and views of Lagorio, and is indeed an important con- 
firmation of them.^ 

Character of the saj^kires. — The sapphires occur in good-sized and 
generally well-formed crystals embedded in the rock and sometimes 
showing a slight blackish crust. At my request a complete crystallo- 
graphic study of these crystals has been made by Dr. J. H. Pratt, on 
materials kindly furnished for the purpose by Mr. G. F. Kunz, of the 
firm of Tiffany & Co., of New York. From the result of Dr. Pratt's 
study, which has been published elsewhere,^ the following is extracted: 

■Geology of Castle Monntaiii : Bull. U. S. Qeol. Survey No. 139, p. 72. 

* Since the above was written a large number of investigations have been pablished on the ocoar> 
rence of comndnm as a primary constituent of igneous rocks, and it is, indeed, now well recognized 
that this is the chief way in which it occurs. Cf. Morozewicz, Tschermaks Mitt., Vol. Xviil, 
U88; Pratt, Am. Jour. Sci., 4th series. Vol. VI, 1898, p. 49, and Vol. VIII, 1890, p. 227; Miller, Rept. 
Bureau of Mines, Ontario, 1899, p. 205; Coleman, ibid., p. 250. 

•Am. Jour. Soi., 4th series, Vol. IV, 1897, p. 424. 




SAPPHIRES OF 



v^ 



P1B840II.] SAPPHIRES OF TOGO GULCH. 555 

The sapphire orystalt are etched and striated to snoh a degree that no crystallo- 
graphio measnrements were possible on the reflecting goniometer ; bnt sufficiently 
accurate angles could be obtained with the contact goniometer to allow of the iden- 
tification of the faces. 

The prism of the second order a (11^), which is so common on corundum, was not 
observed on any of the crystals from this locality. The only two faces that could 
be identified were the base o (0001) and the rhombohedron x (3032), which is a new 
&oe for corundum. On one crystal two yery small faces were observed, which 
were too small to be measured with the contact goniometer, bnt were probably the 
faces of a pyramid of the second order. 

In determining the rhombohedron, ten or more independent measurements were 
made ofo^x. These varied from 66^ to 68^, but approximated closely to 67^, 
which agrees very well with the calculated value, 67^ 3', for 0001 A 30^2. 

The crystals are developed as shown in figs. [5, 6, and 7, PI. LXXYII], the prevailing 
type being like fig. [7]. The crystals vary from those where the base is very largely 
developed, having a diameter of 8 mm., while the rhombohedron is only 1 mm., to 
those that have the base and rhombohedron equally developed [fig. 5]. Where the 
fiftoes are more equally developed, the rhombohedral faces are generally rounded. 

The basal plane often shows characl^ristio striations which are parallel to the 
three intersections of the base c and the rhombohedron x, as shown in fig. [8] . These 
lines are sharp and distinct and on the very flat crystals can easily be measured, 
when examined under the microscope. The rhombohedral faces are very roughly 
striated without showing any distinct parallel lines. 

One very common development of these crystals is a repeated growth on the basal 
plane, of the rombohedron x (3032; and the base o (0001), as represented in fig. [6]. 
These growths are very varied, as is shown in figs. [1-4], where they are drawn in 
basal projection . In fig. [1] there is but one secondary rhombohedron and base, which 
has one of its rhombohedron faces a continuation of one of the rhombohedron faces 
of the crystal. Fig. [2] represents a repeated growth, each face of which is entirely 
distinct from the faces of the main crystaL In Ag, [3] there are represented two and 
in fig. [4] a series of such growths, where a number of the rhombohedral faces coin- 
cide. These growths occur most frequently on the flat crystals. The thickness of 
the rhombohedron rarely reaches 1 mm., and often they are so thin that they appear 
like striations. Figs. [la-4a] , representing the same crystals as figs. [1-4] , have been 
drawn as they appear under the lens, which brings out the relation of the base and 
rhombohedron to better advantage. 

Bauer,! i^ ^ recent article entitled " Ueber das Vorkommen der Rubine in Birma,'' 
has described this same style of development as occurring on the Burma rubies, but 
it is not so general as on the Montana corundums. 

Etching figures. — The etching figures, which were observed on nearly all the crys- 
tals examined, were on the basal plane. The figures are very perfect, and, although 
showing many different forms, they all have a rhombohedral symmetry. Fig. [76, a, 
p. 556], represents the common etching figure, which is a rhombohedral depression 
terminating in a point. The edges of the depression are sharp and well defined, as 
are also the intersections of the rhombohedral faces of the depression. These rhom- 
bohedral faces were smooth and gave fair reflections of the signal on the reflecting 
goniometer. In measuring them all the crystal but the depression to be measured 
was covered with a thin coating of wax. Two different crystals were measured 
which gave for rhombohedron on rhombohedron 22^ 30 ' ; this corresponds to the 
rhombohedron 1017, for which the calculated value is 21^ 50'. The same style of 
figures were observed whose edges were parallel to those of the negative rhombohe- 
dron; these, however, are not common in isolated figures. 

Another common form is represented in [o and e, fig. 76], where the depression is 
bounded by the basal plane, which at times is so large that the rhombohedral plane 

^■^1^ -■- — -  -  I  .^^^^^ 

1 Keaee Jahrbuch mr Min., Geol., nnd PaL, U, 1890, p. 209. 



556 IGNEQUS EOCKS OP LITTLE BELT MOUNTAINS, MONTANA. 

ia hardly visible. Fig. [76, h] Tepresents etching figares, where, on the basal plane of 
a shallow depression, there is another and sometimes two other etching figures. These 
second etching figures are like the common ones shown in [a] . The outer rhom- 
bohedral contour of these figures is generally rounded; this is also usually the case 
with the deeper depressions. 

Often the etching figures are intergrown [d], and when many of these occur 
together they have the appearance of raised figures rather than of depressions. 
This raised appearance is very striking when there is a combination of the plus and 
minus rhombohedron in parallel position and without overlapping each other [/]. 

The figures vary considerably in size, but most of them are near 1 mm. in diame- 
ter. A few were observed that were nearly 2 nmi. in diameter. 








Fig. 76.~Natural etched figares on sapphires from Togo Ooloh. After J. H. Pratt. 

THE EFFUSIVE BOCKS. 
BASALT. 

So far as is known, there are only two occurrences of effasive rocks 
or lavas in the Little Belt Mountains, and both of these are basalt. 
One is the mass resting on Cambrian beds at the head of Kinney Creek 
and shown on the map; the other is on the summit of Smoky Moun- 
tain, in the southern part of the area shown on the map and crossed by 
the line of 110° 45'. Such lava flows grow more abundant in the valley 
between the Little Belt Mountains and Castle Mountain, and their 
geological relation and petrographic character have been described.^ 

From both localities the rocks are very dark stone gray in color. The 
Eanney Creek occurrence is somewhat vesicular with small steam 
pores; both are very dense and compact, and as phenocrysts exhibit 
only a dull orange-colored mineral that is altered olivine. 



1 Weed and Pirsson, G^logy of Castle Mountain mining district : Boll. U. S. OeoL Survey Na 180, pp. 
71-73, 129-131. 



piEsacju.] BASALT. 557 

Under the microscope they are seen to be of a common tyx>e of fine- 
grained basalts; the groandmass, an intermingled mass of grains of 
iron ore and of pyroxene mingled with plagioclase feldspars, having a 
more or less pronounced lath-shaped development. In this lie rare 
phenocrysts of angite and great nnmbers of olivines, some of which 
are altered entirely to a fibrons orange-brown mineral; others contain 
still unaltered cores of olivine. This alteration mineral is very probably 
the same as that described by Iddings ^ and more lately discassed by 
Lawson,^ and named by him iddingsite; at least the completely altered 
mineral appears to have the characters ascribed to that mineral by 
Lawson. 

These lava flows appear to be the extrusions from dikes reaching to 
the surface, and it is probable that the dikes are of lamprophyric char- 
acter and that these flows and their feeding dikes are of the same age 
and character as those occurring at Oastle Mountain, whose origin has 
already been discussed.^ 

1 Geology'of the Eureka DiBtrlot : Men. U. S. GeoL Survey, Vol. XX, Appendix B, p. 888. 
*BuU. Bept GeoL XSnir. California, VoL I, p. 30. 
•BulL U. S. Geol. Surrey Ko. 189, 18M, pp. 131 and 142. 



OHAPTEE V. 
GENERAIi PETROIiOGY OF THE lilTTIiE BEIiT MOUNTArNS- 

INTRODUCTION. 

The igneous rocks of the Little Belt Mountains, taken as a whole, are 
of quite acid types. If the volumes of the various laccoliths, sheets, 
and dikes, as revealed by the study of their field relations, be taken 
together — that is, if they were melted down into one mass — it is clearly 
evident that the total amount of basic rocks, the sheets .and dikes of 
minette, and other lamprophyric rocks, would exercise almost no appre- 
ciable influence on the composition of the whole; and even if the 
diorite of Neihart and the shonkinite and monzonite of Togo Peak 
were added, the composition of the mass would still be an acid one. 

The same has also been shown to be true of the neighboring eruptive 
district to the southward, that of Castle Mountain.' 

It is the belief of the writer that in this case the average composition 
of the magma would be about that of a moderately acid syenite rather 
rich in lime and magnesia, and thus approaching an acid monzonite in 
character. Unlike the Castle Mountain area, however, the Little Belt 
area, as may be seen from the geologic map, does not represent a single 
important center of eruption, but a considerable number of separate 
centers. In these the magma has appeared chiefly in the form of 
laccoliths, attended by numerous sheets and dikes. 

It is possible that there was once a considerable amount of extrusive 
material, flows, and breccias in the district, but if so, they have been 
entirely removed by erosion, which has progressed so far that some of 
the deeper-lying laccoliths are now almost bared. 

Yogo Peak, however, is the only eruptive mass which, by it« character 
and relation to the sedimentary beds in which it is placed, suggests 
that it may have been the outlet of igneous material to a former upper 
surface. It thus seems on the whole more probable that extrusive 
material in this district was of comparatively limited occurrence, if 
indeed any existed. On the whole, therefore, in discussing the char- 
acter of the magmas it does not seem unreasonable to regard the 
character and amounts of the material now existent as representing 
rather closely the sum total of the products of igneous activity in this 

1 CMtle Mountain mining district: Ball. U. S. Geol. Survey No. 139, 1896, p. 189. 
658 



PIBMON.] 



ROCKS OF THE LACCOLITHS. 



559 



district withont aerioas loss by erosion. If this is admitted, the gen* 
eral magma of the district mast have been, as stated above, of a 
rather acid type. 

ROCKS OP THE LACCOLITHS. 

There is a somewhat striking similarity in the general character of the 
rocks of the larger laccoliths. This is dae not alone to their chemical 
composition, but also very largely to their texture and porphyritic 
nature. Comparing the analyses of the laccoliths whose rocks have 
been analyzed, we see that while there is a general similarity of type in 
the composition and that they may be so arranged as to show a grada- 
tion, in which, with decrease of silica, the lime, iron, and magnesia 
steadily increase, there is considerable difi'erence between the first and 
last terms of the series. 

CompoHHon of laoooliihs. 



Constituent. 


I. 


11. 


III. 


IV. 


V. 


VI. 


SiOi 


69.7 

15.0 

.8 


68.6 

16.1 

2.2 


67.4 

15.8 

1.6 


67.0 

15.3 

1.7 


65.0 

15.4 

2.0 


62.1 

15.8 

1.8 


ALO3 


FeaOa 


FeO 


.3 


.4 


.8 


1.1 


L6 


2.4 


MgO 


.7 


.7 


1.4 


L8 


2.6 


3.6 


CaO 


2.1 


1.4 


2.4 


2.2 


3.1 


4.1 


Ni^iO 


3.4 


4.4 


4.1 


4.1 


4.3 


3.9 


K>0 


4.4 


4.9 


4.9 


5.1 


3.9 


3.9 


1 



I. Granite-porphyry, Wolf Bntte laccolith. 
II. Granite-porphyry, Barker Mountain laccolith. 

III. Granite- porphyry, Thnnder Monntain laccolith. 

IV. Granite syenit^porphyry, Big Baldy Mountain laccolith, 
y. Diorite-syenite-porphyry, Sheep Monntain laccolith. 

VI. Diorite-porphyry, Steamboat Mountain laccolith. 

From the standpoint of the chemical classification of magmas the 
variation is sufficient to throw them into different rock groups, as has 
been done in this work. This difference in the chemical composition of 
the magmas i^hows itself most clearly in the mineral components, where, 
beginning with the most acid typQ, the alkali feldspars diminish, pla- 
gioclase increases, and with its Increase the amount of quartz finally 
falls off. 



660 IGNBOU8 BOCKS OF LITTLE BELT MOUNTAINS, MONTANA. 



Constitnent. 



I. 



II. 



III. 



SiO, i 69.7 

Al^Os ' 16.0 

F^iOa 8 

FeO .3 

MgO 7 

CaO 2.1 

Na,0 3.4 

KaO 4.4 



62.1 


66.8 


15.8 


15.5 


1.8 


1.7 


2.4 


1.4 


3.6 


1.8 


4.1 


2.5 


3.9 


• 4.0 


3.9 


4.5 



I. Mo«t aoid of laccoliths. 
II. Most basic of laccoliths. 
III. Average of the six analyzed. 

The average composition of the magmas taken together shows, as 
stated above, that it is of an acid syenite natare, standing at the extreme 
upper limit of this groap and overlapping the granites. It is also not 
of an alkaline type, but tends toward the granito-diorite series, and 
has thus a banatite or monzonitelike character. 



MINERAL OOMPOSITION OF LACOOLITHS. 

It is of interest to compare the laccoliths according to their mineral 
composition, as has been done in the annexed table: 

Table of mineral components of Uieoolithe, 



Component. 


I. 


n. 


III. 


IV. 


V. 


VI. 


Mamietite 


1.1 
3.0 


1.4 

4.6 

3.0 

25.6 

36.2 

25.2 


2.4 

2.0 

3.5 

28.9 

42.8 

20.4 


2.4 

3.2 

• 

4.8 
22.2 
47.0 
19.4 


3.0 

7.7 

6.2 

33.2 

29.0 

20.9 


2.5 
8.7 
11.5 
39.5 
21.0 
16.0 


Biotite 


Hornblende 


Plaffioclase 


17.1 
37.5 
30.2 


Orthoclase 


Quartz 





I. Granite-porphyry, Wolf Batte. 
II. Qranite-porphyry, Barker Mountain. 

III. Granite-porphyry, Thunder Mountain. 

IV. Granite-syenite-porphyry, Big Baldy Monntain. 
V. Syenite-diorite-porphyry, Sheep Mountain. 

VI. Diorite-porphyry, Steamboat Mountain. 

It must be confessed that on the basis of this comparative table alone 
the classification of No. Ill as a granite-porphyry and of No. IV as a 
granite-syenite-porphyry seems hardly justified ; that whatever one may 
be, the other is the same; but in this case it is not absolutely the mineral 
composition alone which has been taken into account, but to some extent 
the structure, associations, etc., and these appear to justify the division 
thus made. 



PIB880N.] EOCKS OP THE LACCOLITHS. 561 

STBUCTURB AND CLASSIFICATION OF THE LACCOLITHIO ROCKS. 

It is very interesting to observe how pertinaciously the well-defined 
porphyritic structure clings to these occurrences of acid laccolithic 
rocks, and this is true not alone here, but nearly everywhere in the Eocky 
Mountain region. Cross has shown its occurrence in the various moun- 
tain groups of the Colorado, Arizona, and Utah region,^ while its occur- 
rence elsewhere in the Montana region has been shown in the earlier 
petrographic descriptions of Lindgren,^ and more recently by Mr. Weed 
and the writer.^ Thus, in the Little Belt, the Castle, the Moccasin, the 
Judith, the Little Eocky, and the Bearpaw mountains and the Sweet 
Grass Hills, this type is constantly found as the characteristic rock struc- 
ture of the laccoliths and laccolithic masses of acid rocks occurring in 
these mountain groups. The rocks range from acid alkaline types with 
little free silica through increasing silica to very acid ones, and from 
these again into rocks low in free silica but containing cons'iderable 
lime and magnesia. The intermediate position occupied by many of the 
rock types in these mountain groups is shown in the discussion of the 
rocks of the Judith Mountains.^ 

That this porphyritic type of structure is due to magmas of a certain 
chemical type — that is, of acid feldspathic nature — being intruded under 
laccolithic conditions, and is not generally true of all magmas intruded 
as laccoliths, is well shown in the High wood Mountains,^ where Square 
Butte and other laccoliths composed of rocks whose magmas are of 
medium (56 per cent of Si02) to basic character have solidified with 
typical granular, nqnporphyritic structure. 

The rocks of the laccoliths which have been described are typical 
granite-, syenite-, and diorite- porphyries, with many connecting types; 
under the system of classification urged by Eosenbusch they are typical 
examples of granitic porphyritic dike rocks. It is to be noted, how- 
ever, that in this Eocky Mountain region dikes of this type, from both 
a geologic and a petrographic standpoint, play but an insignificant role 
when compared with the vast masses and importance of the great lac- 
coliths. Certainly one who from his experience in this region would 
propose a classification based on method of geologic occurrence would 
never dream of referring these rocks to a "dike rock^' subdivision, but 
would be far more likely to refer the acidic porphyries occurring in 
dikes and sheets to a division of "laccolithic rocks," the attendant, 
satellite-like attitude of the dikes and sheets toward these great lacco- 
liths, from whose parent isupply their material has been so often drawn, 

1 Laccolitliic mountain groups : Fourteenth Ann. Kept. TJ. S. Geol. Surrey, Part II, 1805, p. 166. 

'Eruptive rocks of Montana: Tenth Census U. S., Vol. XV, p. 719; Proc. California Acad. Nat Sci., 
Vol. Ill, 1890, p. 29; Am. Jour. Sci., 3d series, Vol. XLV, 1893, p. 286. 

»Am. Jour. Sol., 3d series, Vol. L, 1895, p. 309; 4th aeries. Vol. I, 1896, p. 283; Jour. Geol., Vol. IV, 
1896, p. 399. 

^Geology of the Judith Mountains : Eighteenth Ann. Kept. U. S. Geol. Survey, iVrt III, 1898, p. 663. 

sWeed and Pirsson, Highwood Mountains of Montana: Bull. Geol. Soc. America, Vol. VI, 1895, 
p. 4'JO. 

20 GEOL, FT 3 36 



562 IGNEOUS ROCKS OF LITTLE BELT MOUNTAINS, MONTANA. 

favoring this idea all the more strongly. Bat the very facts mentioned 
above show that geologic position and structure can not be taken as 
analogoas, and that in considering them chemical composition must also 
be taken into account. 

The fact, observed by Gross ' in the Plateau region and by the writer* 
in many Montana areas, that many of the phenocrysts of these porphy- 
ritic laccolithic rocks were not brought up from greater depths, but 
were formed where they now are, finds full confirmation also in the 
Little Belt occurrences. The distribution of the phenocrysts is often a 
local phenomenon. They may be very abundant in some parts of tlie 
mass and very sparse or wanting or of different character in other 
parts. They may be abundaut in the central portion and perhaps 
wholly wanting in the contact zone. They may be abundant in the 
main laccolith and wanting in the contemporary satellite dikes and 
sheets. And microscopically some of them may include all the other 
rock constituents, or they may, while growing, have excluded and 
arranged them. All such facts point clearly to their formation in the 
place where they now are, and tend to confirm the view held by ZirkeP 
in his discussion of this subject.^ 

DIFFERENTIATION IN THE LACCOLITHS. 

So differentiation of any perceptible kind has been noted in these 
laccoliths ; they appear to be, so far as can be told from the exposures — 
and in some cases they have been quite deeply dissected — entirely homo- 
geneous both in mineral composition and in structure. They thus 
differ most markedly from the laccoliths of the Highwood^ Mountains, 
which have such a different composition both in minerals and in struc- 
ture, and also from those of the neighboring Judith Mountains,^ where 
a certain amount of laccolithic differentiation is clearly indicated. 

There is no evident reason which at once suggests itself why differ- 
entiation has not taken place in these Little Belt laccoliths, but since 
they are all of quite acid and of rather simple composition, this, com- 
bined with a certain viscosity at the time of their entrance into the sedi- 
ments, has probably been the means of preventing such differentiation. 

RELATION OF ROCK STRUCTURE TO DEPTH. 

Here in the Little Belt, as in the other mountain groups of laccolithic 
character in the Hocky Mountain region, the depths at which the mag- 
mas are intruded appear to have exerted no preceptible influence on 
their granularity. These great masses intruded in the shaly beds of 
the Cambrian and bearing above them the enormous load of all the 



> Luocolithic mountain m^roupB : Fourteenth Ann. Rept. U. S. Geol. SurN'ey, Part II, 1895. p. 231. 
*PhenocryRt8 of intrusive igneous rocks : Am. Jour. Soi., 4th series, Vol. VII, 1899, p. 271. 

* Lehrbuch der Petrographie. 3d ed.. Vol. 1, 1893. p. 737. 

* See Am. Jour. Sci., 4th series, Vol. VII, 1899, p. 271. 

' Highwood Mountains: Bull. Greol. Soc. America, Vol. VI, 1895, p. 400. 

* Geology of the Judith Mountains : Eighteenth Ann. Kept. U. S. Geol. Survey, Part III, 1898, p. 572. 



PIR880N.] ROCKS OP THE STOCKS AND MA8SIVES. 563 

later Bedinients of the region are flue-grained porphyritic rocks, while 
their intrusive sheets at much higher horizons have the same structure 
and may even be quite typical granular rocks, as in the case of the 
nephelite- (analcite) syenite of Otter Greek intruded in Cretaceous sedi- 
ments. This shows how important a factor chemical composition is in 
rock structures.^ 

ROCKS OF THE STOCKS AND MASSIVES. 

From the standpoint of theoretic i)etrography the granular rocks of 
the Little Belt Mountains occurring in intrusive stocks and masses, 
aside from that of Yogo Peak, do not offer any individual features 
which are of especial importance. The syenite of Barker has a certain 
interest from the possibility that it may represent the source or center 
from which various masses around it may have come, which idea is 
favored by its position and granular structure. The Pinto diorite of 
Neihart is interesting chiefly for the evidences of dynamic shearing 
which it shows, its geologic age, and its connection with the mining 
industries. The augite-syenite of Belt (Jreek and the nephelite- (anal- 
cite-) syenite of Otter Creek are important from their petrographic char- 
acter but not from any direct relationship with other igneous rocks, as 
they are isolated and not a portion of a comx)lex, and therefore there 
is no direct connection between them and other masses. The chief 
interest of this kind centers at Yogo Peak, where, as was shown in the 
former paper on that area, there is a progressive change in rock types, 
so that one passes from an acid syenite over continuous rock masses 
through the mouzonite stage into shonkinite — a very basic rock. 

If one, however, as may be seen by referring to the geologic map 
and to the ftill description of the peak by Mr. Weed, takes the Yogo 
Peak mass as simply the westward extension of the greater intrusive 
stock which stretches some 4 miles more to the eastward, it will be seen 
that in this direction the syenite (banatite) of Yogo Peak passes into a 
still more acid stage, that of the syeuitic granite-porphyry of Yogo 
Peak type previously described. As one passes on to the eastward 
this gives way to syenite- porphyry and then to another occurrence of 
shonkinite, which forms the boundary against the sediments at the head 
of Eunning Wolf Creek. Thus the geologic position of the syeuitic 
rocks and the shonkinite masses is a peripheral one with respect to the 
more acid granite-porphyry and to each other, precisely the position 
demanded by that view of theoretic petrology which regards differen- 
tiation in rock masses as produced by the accumulation of basic material 
at the outer walls of the inclosing chamber, with the more acid material 
within. In strict accord with the theory one would expect the whole 
outer margin of this mass to be composed of syeuitic rocks encircled by 



' Cross. Lacc-olithic mountain groups: Fourteenth Ann. Kept. U. S. Geol. Survey. Part II, 1895, p. 
230. See nlso Geology Judith Mountains : Eighteenth Ann. Kept. U. S. Geol. Survey, Part III. 1898, 
p. 574. 



564 IGNEOUS ROCKS OF LITTLE BELT MOUNTAINS, MONTANA. 




Bhonkinite; it is indeed possible that this is so, bat it could not be defi- 
nitely determined in tbe field, partly on account of the nature of the 
ground, from which the exposures were either inaccessible or covered 
by talus slides and debris, and partly from lack of time to devote to a 
long and minute research. 

There seems, however, to be sufficient facts at hand, not only to 
show that the Yogo Peak mass and its eastward extension are an 
example of differentiation in place, but to add certain facts concerning 
such occurrences. This is shown in fig. 77, B, which is a little sketch 
map of the area. In A is shown, in ground plan — that is, iii horizontal 
section — the theoretical disposition that an originally homogeneous 
magma would assume on cooling, the more basic portions differentiat- 
ing toward the margins. It is very clear that, the rate of cooling being 

most rapid proportionately along 
the sides, the greatest amount of 
differentiated material would be 
found at the ends of the cavity; 
it would be very thin along the 
sides, and might even be wanting. 
Even if one considers difi'erentia- 
tiou to be a process of fractional 
crystallization, as has been sug- 
gested by Barker^ and elaborated 
by Becker^, the result would be 
the same. A familiar example 
would be the filling up of the cor- 
ners of a vessel containing a crys- 
tallizing saline solution and leav- 
ing a rounded cavity in the center, 
which contains the mother liquor. 
The figure given shows the 
ground plan of the cavity. The 
vertical extension is not here con- 
sidered. It is of the nature of a very broad, thick dike. The Yogo 
Peak mass, as shown in the sketch map, has approximately this gen- 
eral form and arrangement of material, and is from half a mile to a 
mile in width and several miles in length, and hence could hardly be 
called a dike, Yogo Peak proper is a somewhat bulbed extension, and 
it is here that the difi'erentiatiou of the magma is seen to best advan- 
tage, and has been already briefly described.^ The various types, pass- 
ing from the granite-porphyry southwest along the peak, gradually 
merge into one another along the exposures, and one passes from the 
granite-porphyry into syenite (banatite), then into monzonite, and 
finally into shonkinite. This is shown very strikingly by the comparison 

> Qaart. Jour. Geol. Soc., VoL L, 1894, p. 327. 
« Am. Jour. Sci., 4th series, Vol. IV, 1807, p. 257. 

'Weed and Plrsson, Igneous rocks of Yogo Peak: Am. Jour. Sci., 3d series. Vol. L, 1895, p. 467. 
Seo also the full account given by Mr. Weed In the preceding portion of this work. 




Fio. 77.— Diagram to show differentiation at 
Yogo Peak. 

A, theoretical arrangement of differentiated 
mass, acid center, basic periphery (horisontal 
plan) ; B, sketch map of Yogo Peak mass aud ex- 
tension; ff, shonkinite; m, monzonite; 6', banatite 
(syenit«); p, granite-porphyry; 6', syenite- 
porphyry; «',shoukinite; »mb, Yogo Peak mass; 
j9, plateau extension ; b', Storr Peak ; 8*, peak head 
of Kunning Wolf Creek. 



PIB880N.] 



COMPOSITION OF ROCKS OF TOGO PEAK. 



565 



of the chemical analyses of the tyi)es involved. Fine fresh material of the 
granite-porphyry suitable for analysis and equal in quality to the other 
rocks was not obtained, and therefore an analysis of the granite- 
porphyry of Thunder Mountain is selected in its place, as the two rocks 
must be very close together in their chemical composition, as shown by 
the study of thin sections. The Thunder Mountain rock has possibly 
not quite so much free quartz, and hence a little lower content of silica, 
but for our purpose such small differences as must exist may be practi- 
cally disregarded, since they could have no effect upon the general result. 

CompoaitUm of the rocks of Yoga Peak, Montana. 



Constituent. 



I. 



II. 



III. 



IV. 



la. 



SiO, . 
Al.Oa 

Fft,0:, 

FeO.. 
MgO. 
CaO . 
Na,0 . 
K,0.. 



67.4 
15.8 
1.6 
.8 
1.4 
2.4 
4.1 
4.9 




61.7 


54.4 


49.0 


1.124 


1.027 


( 


15.1 


14.3 


12.3 


.153 


.145 ' 


2.0 


3.3 

• 


2.9 


.010 


.013 1 


2.3 


4.1 


5.8 


.012 


1 

.031 


3.7 


6.1 


9.2 


.035 


.092  


4.6 


7.7 


9.7 


.043 


.082 


4.4 


3.4 


2.2 


.066 


.070 




4.5 


4.2 


4.9 


.052 


.048 





0.907 
.139 
.021 
.057 
.152 
.139 
. 055 
.045 



0.813 
.119 
.018 
.080 
.229 
.173 
.036 
.052 



I. Grauite-porphyry. 
II. SycDito (banatite). 

III. Monzonite. 

IV. Shonkinit-e. 

la — IVa. — Molecular oxides of preceding. 

At the east end there are no such excellent outcrops to furnish mate- 
rial for investigation and analysis as at Yogo Peak proper. The change 
is from the granite-porphyry to a more basic syenitic phase, with very 
little quartz and predomiuant orthoclase. This is characterized by a 
corresponding change in microstructure, the sections of the rock at 
Storr Peak, where good material is seen in the outcrops, showing, as 
previously described, a rock that is already nearly out of the por- 
phyritic stage and almost a fully granular one. In the hand specimen 
it appears wholly rather fine granular; the minute amount of ground- 
mass is seen only under the microscope. 

Still farther to the eastward, where good fresh material is found 
again, it is the coarser-grained shonkinite, with typical granitoid struc- 
ture. Thus, as so commonly is th^ case, chemical composition and 
structure are directly related, and this is shown at both ends of the 
mass. The tendency of the very acid magmas to form porphyritic 
rocks under the same conditions where the basic ones form granular 
types, is here well exemplified; and although one expects the center of 
a mass to x)ossess conditions more favorable for a higher degree of 
granularity than the periphery, this is manifestly offset in the case 
under consideration by the elongated form of the mass, which in a 



566 IGNEOUS ROCKS OP LITTLE BELT MOUNTAINS, MONTANA. 

great degree exposes its whole content to more nearly equal conditions, 
and thus we And granite- porphyry (with, however, very coarse ground- 
mass) and the coarse- grained shonkinite united in the same mass. 

As the theoretical diagram of fig. 77 would indicate, one would 
expect more basic material in slight amount to occur along the sides of 
the mass. It may Indeed do so, but this point the author is unable to 
decide from personal observations, for the reasons given above, but it 
can at least be said that if it does occur it is small and of no impor- 
tance, except from the theoretical point of view. 

A modification of the foregoing theory for the arrangement of the 
several parts of the Yogo stock might be suggested as follows: If a 
body of magma had been pressed upward into a cavity opened for it 
in the crust, and had then remained for a period at rest, it might have 
difi'erentiated, as required by theory, into a more basic outer envelope, 
passing into more and more acid material within. If at this time the pres- 
ent opening which the Yogo stock now occupies had formed, with a forc- 
ing upward of the differentiated material at the same time, the acid inner 
portion might have been driven through the more basic envelope, crowd- 
ing it back into the ends of the great fracture and itself occupying the 
inner part. It is thought, however, that the greater part of the differ- 
entiation has occurred after the material came to rest, or at least very 
nearly so, and that movements after the main injection have been slight, 
otherwise the orderly arrangement of the parts, taken as a whole, would 
have been disturbed, with a consequent large amount of mixing. 

FORMATION OF THE APLITIO DIKELETS. 

The numerous little syenite-aplite dikelets or veins which cut the 
Y'ogo rocks in all directions, previously described, are readily explained 
by the following hypothesis: As the upper and outer masses of the 
stock crystallized into rock and cooled they contracted and were 
broken into innumerable blocks, like all igneous rocks, the heavy 
masses, resting on the still molten, unconsolidated, acid, inner, lower 
portion, by their weight gradually forced it up into these fractures, 
where it solidified as the dikelets. That the material was forced into 
the fracture planes of the rocks while they were still very hot is showH 
by the granular character from wall to wall without sign of contact 
influence, and by the fact that such very narrow dikelets are really 
granular crystalline and not glassy or microcrystalline. The fracture 
planes would also serve as channel ways for escaping vai>ors from 
below and from, the walls, and these would tend to make the magma of 
the little dikes extremely fluid and enable it to penetrate the narrowest 
cracks. 

DIFFERENTIATION. 

Throughout this report this term is used to denote simply the cleav- 
ing of an igneous magma into two or more of different composition ; it 
does not denote any theory as to the process by which it is accomplished. 
That differentiation occurs the author regards as long since demon- 



PIBSiSON.] 



MINERAL COMPOSITION OP TOGO PEAK BOCKS. 



567 



strated. The process that causes it, whether molecular flow, convec- 
tion currents, fractional crystallization, or other of the various theories 
suggested, remains to be discovered. 

VARIATION IN MINERAL COMPOSITION. 

The character of the progressive variation in the rocks of Yogo Peak 
is shown very clearly by the study of the table of analyses, with their 
oxides, which has just been presented. These exhibit many features 
of interest when studied in comparison with the mineral composition 
and position of occurrence. Thus with respect to the mutual relations 
of lime and magnesia, the table shows that in molecular proportions, 
from granite-porphyry to shonkinite, they run as follows: 

Molecular proporiiont of lime and vMgnesia in rocks of Togo Peak, Montana, 



CniiHtituent. 



Granite- 
porphyry. 



(banTlU^). Monzonlte. Shonkinite. 



1 



MgO 

CaO 



35 
43 



92 

82 



152 

139 



229 
173 



J 



Thus in the granite-porphyry the lime is greater than the magnesia; 
in the shonkinite, the reverse. This shows itself clearly in the mineral 
products formed from the magmas. In the granite-porphyry there are 
hornblende and plagioclase. The surplus of lime controls and shows 
itself in the feldspar. Then, as the relative amountof magnesia increases, 
the latter begins to control ; more of the lime is taken up by it, and 
augite begins to apx)ear and the plagioclase to diminish. Finally, in the 
shonkinite, where the magnesia is considerably in the lead, hi)rublende 
' in its ratio to pyroxene almost disappears, and the magnesia takes the 
lime into augite before the plagioclase commences to form. Therefore, 
in the most basic type it is entirely wanting and we see the surplus of 
magnesia forming olivine, or in combination with potash and alumina 
producing biotite. These relations are also shown in the following 
table of mineral components, in which, however, the most basic type of 
the shonkinite is not represented, on account of the lack of its analysis : 

Table showing mineral components of rocks of Yogo Peak, Montana, 



Component. 



Iron ore.... 

Biotit« 

Hornblende 
Pyroxene .. 

Olivine 

Plagioclase 
Orthoclase . 
Qnartz 



Granite- 
porphyry. 

2.4 
2.0 
3.5 



Banatite. 

3.1 

.9 

12.0 

5.4 



Monzonite. 


Shonkinite. 


5.1 


1.0 


12.1 


18.0 


4.5 


4.0 



20.7 



28.9 
42.8 
20.4 



29.5 

42. 5 

6.0 



27.2 

30.4 



35.0 

7.0 

10.0 

25.0 



568 IGNEOUS ROCKS OF LITTLE BELT MOUNTAINS, MONTANA. 



They can be even more strikingly represented by a series of diagram- 
matic curves, as shown in fig. 78. In this equal distances have been 
taken as abscissas, and on the perpendiculars erected the amounts of 
each mineral have been set ofi* as ordinates, and through these points 
the curves have been drawn. The diagram shows several features of 
interest. Thus the pyroxene, determined by three points, is absolutely 
a straight line. The quartz, determined by two points only, is also 
drawn as a straight line. The olivine, for which one point only exists, 
is of course purely diagrammatic. The curves between points can not, 
of course, be very exact, but they are sufficiently so for this purpose. 
There is an interesting mutual relation between hornblende and bio- 
tite; they appear to complement each other. The preponderance of 
orthoclase over plagioclase in the more acid rocks — the granite-porphy- 
ries, passing into the syenites (banatites) — is practically lost in the 

monzonite and then suddenly acquired 
again in the basic types. In the more 
basic shonkinite, lying to the right of 
the diagram, it is clear that hornblende, 
iron ore, and plagioclase would be 
wanting, which is, indeed, the actual 
fact. 

It is clear, also, not only that the dia- 
gram thus constructed represents the 
mineral variation at Yogo Peak, but 
that by means of it we may obtain the 
mineral composition of any of the inter- 
mediate products of the mass by erect- 
ing at a suitable point a perpendicular 
parallel to those already drawn. Then the points at which it is inter- 
sected by the curves, measured from the foot with a millimeter scale, 
give directly the prox)ortion8 by weight of the minerals for the interme- 
diate rock type. 

The proper method to have constructed this diagram, with correct 
abscissas, would have been to have measured along a line from the 
granite-porphyry to the shonkinite boundary in the field, and, noting 
the distances at which the types taken for analysis occurred, they 
would have furnished correct data for the abscissas. Then to ascertain 
the mineral composition of the rock mass at any point it would only 
be necessary to measure from there to the nearest iK)int where the 
composition is known from the analyses, and, erecting a perpendicular 
at a corresponding point on the diagram, the curves would give the 
mineral composition. 




Fio. 78.— Variations of minerals In rocks 
of Toko Peak. 



OHAPTBE YT. 

DISCUSSION OF MAGMAS BY GRAPHIC METHODS, AKD 
ABSORPTION OF SEDIMENTS BY I^IAGMAS. 

DISCUSSION OF MAGMAS BY GRAPHIC METHODS. 

The introduction of graphic methods for the study and comparison of 
groups of related rock analyses we owe to Iddings,* and more recently^ 
to Becke,^ Michel L4vy,* and Brogger.' These methods have had the 
advantage of presenting more directly to the eye the facts ftirnished by 
the analyses, and thus they permit a more direct comparison of the com- 
positions of the rocks with each other than can be made from the table 
of figures giving the several amounts of the rock-making oxides in 
molecular proportions. This is more especially true of the diagrams of 
Michel L^vy and of Brogger. The diagrams of Iddings, on the other 
hand, do not serve so well in this direction, but are more useful in show- 
ing the mutual relations produced by processes of differentiation and 
the direction in which the oxides tend during such processes. These 
methods, then, may be said to represent pictorially the analysis and to 
make its results more easily grasped, especially by those who are not 
chemists and to whom the numerical results of analytical work do not 
readily appeal. 

It has always seemed to the writer, however, that the results so far 
obtained by these methods have been inadequate. They permit of com- 
parison, it is true, but what has resulted from the comparisons of the 
diagrams would mainly have resulted from comparison of the analyses 
themselves. It would seem that the analyses are mathematical data, 
which, if we knew how to use them rightly, should enable us to place 
the subject of differentiation or the derivation of magmas from one 
another on a more mathematical basis, and thus to gain a better state- 
ment of the laws governing these processes, and by analogy to suggest 
new ideas. 

Efforts tending in this direction by use of the graphic method have 
been made by Iddings^ and some very interesting data obtained. On 
the mathematical side Brogger^ also has given some very remarkable 

I Origin of igneous rocks: Bull. Pbilon. Soc. Washington, Vol. XII, 1892, pp. 89-214. 

*In revising the proof the author takes the opportunity to mention also the elaborate and beautiful 
diagrams of Loewinson-Lessing. which have Just been received, in his work Studien iiber Emp- 
tivgesteine: Compt. Keud. 7th Cong. Geol. Internat., 1899, p. 193. 

» Tscher. Mitt., Vol. XVI, 1896, p. 308. 

<Bull. Soc. G^ol. France, 3d series. Vol. XXV, 1897, p. 326. 

'Qauggefolge desLaurdalits, p. 255, Videuskab. Skrift I, Math.-nat. Kl. 1897, No. 6. 

•Jour. Geol., Vol. Ill, 1895. p. 057. 

' Ganggefolge des LaurdaliU, 1898, p. 276. 

569 



570 IGNEOUS ROCKS OP LITTLE BELT MOUNTAINS, MONTANA. 

data showing that the varied magmas of the soath Norway region can 
be derived by the admixture of parent magmas in proper quantities, 
and these latter, which represent types actually occnrrent in the dis- 
trict, may themselves be derived from simpler forms. When one con- 
siders the large number of variables in the magmas, these agreements 
furnish one of the most powerful and logical arguments against the 
contention of those who see no connection between the distribution and 
the chemical composition of igneous rocks and relegate the whole affair 
to chance and chaos. In the original diagrams of Iddings' and in suc- 
ceeding ones by Washington,^ Dakyns and Teall,** and Harker ^ the molec- 
ular ratios of the silica are made to serve as abscissas, while those of the 
metals are taken as ordinates. Supposedly the reason for this selec- 
tion is that the silica is the only acid present, the metals being the bases. 

It has appeared to the writer, however, that the silica, being a variant 
and closely connected with the other oxides, should also be used as an 
ordinate in any scheme which should graphically represent the varia- 
tions in the magmas of a given unit district. The difficulty arises that 
there seems to be no definite basis on which to select abscissas. We 
might, for example, select the relative volumes of the various rocks, 
but these are rarely even approximately known. An attempt in this 
direction has been made, however, with the Yogo Peak rocks, with the 
effect of furnishing some results which are not only very interesting, 
but even surprising. 

For this purpose equal distances of 2 centimeters have been arbitra- 
rily selected as abscissas, and at the points a, &, c, and d of the accom- 
panying diagram (fig. 79) perpendiculars have been erected. The 
reason for thus arbitrarily selecting these equal distances is the same 
as that previously given in the explanation of the diagram of mineral 
variation at Yogo Peak (fig. 78), because they represent in a gen- 
eral way those relations seen in the field. On the perpendiculars 
thus erected distances in millimeters have been measured off corre- 
sponding to the molecular relation of the oxides as shown by the 
analyses of the Yogo Peak rocks, the molecular ratios having been 
multiplied by 100 throughout for convenience. For greater accuracy, 
however, a scale of twice the size at which the figure is reproduced 
has been actually used in plotting and obtaining the results given in 
the following work.'* 

At d the molecular ratios of the shonkinite have been measured off 
and plotted, the silica being given twice the scale of the other oxides 
to condense the diagram, and this relation of the silica has been fol- 
lowed throughout the series of analyses, since its line does not intersect 



* Loc. cit. 

' ^gina and Methana : Jour. Geol., Vol. Ill, 1805, p. 160. 

« Garabal HiU and Meall Breac : Qaart Jour. Geol. Soc, Vol. XLVITI, 1892, p. 104. 

<Carrock Fell : Qaart Jour. Geol. Soc. Vol. LI, 1896, p. 125. 
- 'In reproducing the diagram, which was plotted and drawn with great care, the cut ia found to be 
not absolutely accurate in a vertical direction, though correct horizontally. This has occurred in the 
procos.H of reproduction, and should be kept in mind in case the measurements described in the follow- 
ing pages are repeated. 



PIB880N.] DISCUSSION OF MAGMAS BY GRAPHIC METHODS. 



571 



those of the other oxides. At c the monzonite has been similarly laid 
off, and at b the syenite (banatite). Since good material for the analysis 
of the granite-porphyry was not collected, the analysis of the fresh and 
precisely similar tj'pe of Thunder Mountain has been chosen in its 
place, as was done in the diagram of mineral variation, audits molecular 
relations have been plotted at a. Through the similar poiuts thus 
obtained on the four perpendiculars lines have been drawn, and the 
resulting diagram shows the directions of molecular variation at Yogo 
Peak. 

It is of interest to study this diagram and observe the mutual rela- 
tions and character of the lines or differentiation paths of the oxides. 
The silica, of course, does not intersect any of the others. It will be 



-==Tn 




Fio. 79.— Difierentiation diagram of Little Belt Hoantain rocks, a, b, e, d, aDalyses of rockn of Yogo 
Peak; a, granite-porphyry (of Thunder Moan tain) ; b, syenite (banatite) ; e, monzonite; d, shonkiiiite; 
y, minetto, Sheep Creek; x, diorite- porphyry, Steamboat Mountain; v, ayenitc-diorite-porphjry, Bear 
Park; t, R3'euite-granite>aplite, Sheep Creek; #, granite-porphyry, Barker Mountain; p, granito-por- 
phyry, Wolf Batt«; r, rhyolite dike, Yogo Ridge; z, missouritc. 

noticed at once how very nearly straight lines the alumina, silica, and 
ferrous iron are; the magnesia, the soda, aTid the potash seem to indi- 
cate very flat curves; and probably they should all be drawn as very 
flat curves. The approach to symmetry which the figure possesses 
seems to indicate that the abscissas have been taken with a fair degree 
of correctness, and that this is so will be shown in other ways. 

EELATION TO OTHER MAGMAS. 

On a previous page it was suggested that the mineral composition of 
any of the gradational types of Yogo Peak could be found from the 
diagram of variation there given ; and so, in respect to chemical compo- 
sition, the diagram of variation just described should give the chemical 
composition of any 6f the intermediate varieties. Unfortunately, hav- 
ing used all of the analyses to construct the figure, there are none left 



572 IGNEOUS ROCKS OF LITTLE BELT MOUNTAINS, MONTANA. 

to directly test this, but a study of the otber analyses of rocks of this 
region shows the striking fact that this is the diagram of variation for 
the igneous rocks of the Little Belt Mountains. 

Thus, if at the point y^ distance one-half centimeter from c and one 
and one-half from d^ we erect a perpendicular to cd^ the point where it 
intersects the lines of the various oxides, measured by a millimeter 
scale to its foot, will give the molecular ratios of a magma intermediate 
in type between c and d. This is shown in the following table, where 
the first column gives the intercepts in millimeters — that is, decimal 
parts of a meter, which are here considered as molecular ratios — which 
multiplied by the corresponding molecular weights give the percentages 
seen in the second column. This is the theoretical composition of a 
magma between o and d in composition. 

Actual and theoretical oompoHtion ofmineite. 



Constituent. 


I- 


II. 


III. 


IV. 


SiC 


.880 
.135 
.020 
.065 
. 175 
.115 
. 050 
.045 


52.8 
13.9 
3.2 
4.7 
7.0 
8.1 
3.1 
4.2 
3.0 


52.26 
13.96 
2.76 
4.45 
8.21 
7.06 
2.80 
3.87 
4.88 


53.0 
13.8 
3.2 
4.5 
6.9 
8.2 
3.1 
4.4 
2.9 


AliO; 

Fe.O, 


FeO 


MgO 


CaO 


NajO 


K,0 


X 


Total 





100.0 


100.25 


100.0 







I. Measured oxides 5 mm. left of c in fig. 79 equals molecular ratios. 
II. Above converted into percentages; X = TiOi, H^O, P^Og, etc., supplied by dif- 
ference. 

III. Actual analysis of minette of upper Sheep Creek. 

IV. Three parts of monzonite and one part of shonkinite mixed. 

In the third column is given the actual analysis of the minette of 
Sheep Creek, and it will be seen at once how close is the agreement 
between the two, only -a few tenths of 1 per cent, except iu the lime 
and magnesia. 

Tt is clear from the foregoing that the minette magma ^, considering 
its position between c and d, where 3yc = yd^ can be expressed in terms 
of c and d as follows : 



The solution of the equation is given in the fourth column of the 
adjoining table, and agrees very closely with both the theory derived 
by measurement of the intercepts and the actual analysis. 



piBSfiON.] DISCUSSION OP MAGMAS BY GRAPHIC METHODS. 



5.73 



The Sheep Greek minette is not, however, the only magma which can 
be thus derived from the differentiation diagram of Yogo Peak. If at 
the point a?, between a and 6, a perpendicular be erected we can obtain 
in the same way the composition of the magma of the Steamboat 
Mountain laccolith; similarly the perpendicular v gives the rock of 
Bear Park, and t the narrow dike of aplite (granite-syenite-aplite) 
cutting the minette of Sheep Creek, which is thus so well shown to be 
complementary to it. The result of these comparisons is shown in the 
following table. The symbol X stands, as before, for the titanic and 
phosphoric acids, barium, manganese, water, etc. 

It is also evident that the rock composing the laccolith of Big Baldy 
Mountain should be included in the series, since its analysis is nearly 
identical with that of Thunder Mountain. For this reason its calcula- 
tion from the diagram has not been attempted. 

Compariton of theoretical and actual magmas. 



Constituent. 


A'. 1 


A«. 


A». 


l.OSO 


B'. 


B'. 
64.95 


C. 
1.110 


66.6 


CK 


SiO,.... 


1.050 


63.0 


62. 18 


64.8 


66.29 


Al,03 . . . 


.146 


15.0 


15.77 


.150 


15.5 


15.44 


.150 


15.4 


15. 09 


Fe.O:,... 


.012 


1.9 


1. 83 


.013 


2.1 


2.02 


.010 


1.6 


1.37 


FeO.... 


.029 


2.1 


2.44 


.020 


1.5 


1.60 


.016 


1.1 


1.17 


MgO ... 


.080 


3.2 


3.55 


.060 


2.4 


2.65 


.043 


1.7 


2.39 


CaO.... 


.075 


4.2 


4.13 


.060 


3.3 


3.07 


.049 


2.7 


2.38 


Na^O ... 


.070 


4.3 


3.92 


.065 


4.0 


4. 25 


.066 


4.1 


3.96 


K.O .... 


.050 


4.7 


3.91 


.050 


4.7 


3.87 


.053 


4.9 


4.91 


X 




1.6 


2.50 




1.7 
100.0 


2.26 




1.9 


2.29 
99.85 






Total . 


100.0 


100.23 


!------ 

1 


100.11 


1 


100.0 



A'. Molecular proportions of oxides measured at x in fig. 79. 

A^ Above converted into percentages, X = TiO>, P^O.-,, etc., supplied by difference. 

A*^. Actual analysis of diorite-porphyry of Steamboat Mountain laccolith. 

B'. Molecular proportions at v in diagram. 

B'. AV>ove converted into percentages. 

B"'. Actual analysis of syenite-diorite-porpbyry of Bear Park. 

C. Molecular proportions at t in diagram. 

C-. Above converted into percentages. 

C^. Analysis of syenite-gpranite-aplite cutting minette at head of Sheep Creek. 

It will be seen from the table that the agreement between the cal 
culated values and those fonnd is very close. It may be said of these 
rocks that, given the percentage of one element, the chemical composi- 
tion of any intermediate type can be deduced from the diagram. 



EXTENSION OF THE DIAGRAM. 



The quite regular and symmetrical character of the diagram suggests 
that it might be carried out by prolongation of the lines, and that 
perhaps other magmas, on the one hand more acid, on the other more 



574 IGNEOUS ROCKS OF LITTLE BELT MOUNTAINS, MONTANA. 



basic, might be derived from it. The relations already shown to exist 
may also be true of other magmas of the district. That this is so is 
easily shown. Thus to the left of a, in fig. 79, we may extend the lines 
of the molecular oxides as shown by the dotted lines. They are carried 
out as far as r, which marks the limit of the diagram. The magnesia 
and ferrous iron have disappeared before this point is reached. If we 
now erect a perpendicular at 8 and measure and reduce to percentages 
as before, we obtain the theoretical magma shown in the column A^ of 
the annexed table, while A^ gives the actual analysis of the Barker 
Mountain rock. The agreement is very striking. In the same manner, 
perpendiculars erected at p and at r give other magmas whose relations 
to actual types are shown in the annexed table under B and C. The 
agreement is quite close on the whole, but it is to be noted that the 
alumina line, instead of being a straight prolongation of the ah section, 
really curves slightly downward, if we take the percentages found in 
the analyses as ordinates, and thus completes in its whole course a 
very flat, gentle curve. 

*Compariaon of derived and actual magmas. 



 


A}. 


69.0 


A". 


B». 


B» 


B«. 


C>. 
1.200 


C». 


Q\ 


D'. 


D». 


D3. 


SiOa .... 


1.150 


68.60 


1 
L165 


69.9 


69.68 


72.0 


73.12 


.750' 


45. 46. 06 


Al.Os-... 


.155 


16.0 


16.13 


! .158 


16. 2 14. 97 


.160 


16.4 


14.27 


.105 


10. 8 10. 01 


Fe,0:, ... 


.012 


1.9 


2.22 


, .008 


L2 


.79 


.005 


0.8 


.51 


.020 


3.2 3.17 


FeO .... 
MgO.... 


.010 
.020 


.7 
.8 


.44 

.72 


1 .005 
.013 


.4 .34 
.5 .66 






.26 
.24 


.105 
.300 


7.5 
12.0 


5.61 
14.74 


i 




CaO .... 


.035 


1.9 


1.36 


.030 


1.7 2.10 


.011 


0.6 


1.10 


.210 


11. 7 10. 55 


NaiO.... 


.065 


4.0 


4.37 


.064 


3.9 


3.38 


.062 


3.8 


3.43 


.020 


1.2 


1.31 


K.O 


.055 


5.1 


4.89 .055 


5.1 4.40 


.055 


5.1 


4.90 


.062 


5.8 


5.14 


X 

Total.. 




.6 
100.0 


1.64 




1.1 3.54 




1.3 

• 

100.0 


2.35 





2.8 
100. 


2.98 


1 


100.37 


1 


100.0 


99.86 




100.18 


99.57 



A'. Molecular proportion of oxides measured at « iu fig. 79. 

A«. Above converted into percenta^ifes. X = TiO:, P.O^, H..0., etc., by difference. 
A^. Actual analysis of granite-porphyry of Barker Mountain. 
B>. Molecular proportion of oxides fkt p in diagram. 
B^. Above converted into percentages. 
B"*. Analysis of granite-porphyry of Wolf Butte. 
C. Molecular proportion of oxides at r in diagram. 
C Above converted into percentages. 
C^. Analysis of rhyolite-porphyry dike on Yogo Ridge. 
D'. Molecular proportion of oxides at z in diagram. 
D*. Above converted into percentages. 

D^. Analysis of missourite of Highwood Mountains. Am. Jonr. Soi., 4tb series, 
Vol. II, 1896, p. 315. 

If now, on the other hand, we extend the lines to the right — that is, 
in the basic direction — we may derive basic magmas and examine what 
the products of the differentiation in this direction might have been. 



P1R880N.] DISCUSSION OF MAGMAS BY GRAPHIC METHODS. 575 

If at z^ in which the distance zd equals that of od^ we erect a perpeii- 
dicalar, the intercepts on the prolonged lines are seen in I)' of the 
annexed table, which yields the theoretical magma of D^ An inspec- 
tion of this magma shows at once that it is typical for a basic leucitic 
rock, and that this is so is shown by its general close agreement with 
the analysis of missourite of the Highwoo^ Mountains,^ a granular, 
intrusive augite-olivineleucite rock, the granular plutonic represent- 
ative of the leucite-basalts. 

The relation thus brought out is a most interesting one. It shows 
that if differentiation had gone on as far beyond the shonkinite as the 
latter is from the monzonite there would have been formed, not a 
pyroxenite, but a missourite, and it shows the intimate relation between 
the latter and shonkinite. This relation is confirmed in fact, for the 
stock which furnished the original missourite has shonkinite phases. 

We may thus deduce that in regions where monzonite occurs as a 
main stock type both shonkinite and missourite facies and dependen- 
cies are to be expected and should be carefully looked for. 

It is also quite true that this deduced magma might express its^f 
as a pyroxenebiotite rock (biotite instead of olivine and leucite), and 
especially if the magma crystallized under such conditions that water 
vax)or and fluorine, necessary for the formation of biotite, could not 
readily escape. 

The more basic type of shonkinite, described in the original paper and 
mentioned in this work under shonkinite, with its large and abundant 
poikilitic biotites, would thus in part be accounted for. 

Such a rock, though of pyroxenic habit, is very different from the 
pyroxenites which represent the differentiation end products of the 
gabbro-peridotite group, as shown by its abundant potash, and is not 
to be confounded with them. 

DEDUCTION OF BBSULTS. 

From the results which have been obtained in the foregoing it 
seems not too much to say of rocks of the Yogo series that, given the 
percentage of one elemept, the chemical composition of any rock of 
the series to within a fraction of I per cent can be deduced from the 
diagram. It is approximately true in practice and appears absolutely 
so in theory. There are a few analyses of rocks in the Little Belt Moun- 
tains given in these pages which do not fit into the diagram. One of 
these is that of the '^ Pinto " diorite of Neihart. This rock, how- 
ever, was injected and formed before the Paleozoic, and the whole of 
that vast period elapsed before the Yogo series was produced. The 
magmas then can have nothing in common; all sorts of changes 
and alterations in magmatic conditions may have occurred during the 
enormous lax)se of time involved in that great era. Another is the 
syenite of Barker, which comes very near the syenite-diorite ijorphyry 



> Am. Joar. Sci., 4th Beries, VoL II, 1896, p. 321. 



576 IGNEOUS ROCKS OF LITTLE BELT MOUNTAINS, MONTANA. 

of Bear Park. The differeuce lies chiefly in the relations of magnesia 
and alumina; these are not great, but still distinct. It is to be noted, 
however, that this syenite is a granular type and an independent 
abyssal rock. Those rocks, however, which connect geologically with 
the Yogo series, evidently of the same age and belonging to the some 
general center, and depart most clearly from the diagram, are the 
syenite-porphyry of Yogo Kidgeand the twolamprophyres, the analcite- 
basalt dikes of Big Baldy and Bandbox mountains. 

Why these depart from the diagram series can not be definitely 
explained, but the writer believes that these complementary types are 
rocks of secondary differentiation, since, in contrast to the view of 
Brogger,^ who holds the complementary rocks in his region to have 
been of deep-seated origin, he believes that in many areas they are 
produced by laccolithic differentiation, as in the Judith Mountains.' In 
this case they belong to a later order of differentiated products than 
those of the Yogo Peak series, and might well differ from them. 

In conclusion a few deductions can certainly be drawn from the 
remarkable relations which have been shown to exist in the Yogo series. 
The masses occur over a large area and their magmas have certain defi- 
nite relations to one another, so that any one magma may be definitely 
and mathematically stated in terms of the others or be derived from 
them. It is conceivable that the differentiation lines or paths of the 
molecular oxides of the diagrams might be drawn as curves and these 
curves mathematically discussed and their equations found. 

All this points clearly and unequivocally to the fact that these 
magmas have had a common, deep-seated origin — that they have been 
derived from a common source according to some definite law. It does 
not point toward these magmas having been produced by chance, from 
heterogeneous substances, or by the absorption of sediments or other 
material; on the contrary, it would be impossible to concede this and 
at the same time believe that harmonious relationships are due to the 
operations of natural law. 

Whether the relationships and manner of discussing them which have 
been shown for the Yogo series can be also shown at the present time 
for other regions is not easy to say. The great advantage in having a 
series of analyses from a unit rock mass whose differentiation repre- 
sents that of the region is evident, and it would seem as if this should 
be the point of departure in the study of other series. Some prelimi- 
nary work on other regions of which groups of analyses exist seems to 
indicate that generally the relations are much more complex than in 
the Little Belt area and more difficult to unravel. This may also be 
due in part to the unfamiliarity of the writer with these regions and 
their rock types. 



■Emptlvgesteinedes Chrlstianiagebietea, Groradit-Tingaait Serie, 1894, p. 152. 

* Geology of the Judith MountainB : Eighteenth AnxL Kept. U. S. GreoL Survey, Part III, 1898, p. 672. 



PIB880N.] IGNEOUS BOCKS OF LITTLE BELT MOUNTAINS. 577 

ABSORPTION OF SEDIMENTS BY MAGMAS. 

In regard to the question which has been brought up at various 
times, and which is being actively debated in Europe at present by 
Brogger, Michel L6vy, Lacfoix, and others, as to whether igneous 
masses are capable of dissolving and absorbing large amounts of the 
stratified rocks with which they come in contact, becoming changed in 
composition thereby, it can be said that the Little Belt area presents no 
facts in favor of such a view. Indeed, the only occurrence in the 
district which could lend any color to such a supposition is that at 
Yogo Peak, the change in chemical composition from center to sides 
being alluded to. But here again the facts speak against it. The con- 
tact metamorphism is too inconsiderable; the change in basicity at the 
ends of the mass and not on the sides would by this be incomprehensi- 

• 

ble, although easily explained, as previously noted, by the cooling and 
crystallizing of a differentiated mass; but it is especially in the com- 
parison of the chemical composition of the different phases that such a 
8up[)Osition receives a final blow. A reference to the comparative table 
of Yogo analyses shows that the greatest increase has been in magnesia, 
and that a rock containing an -enormous percentage of magnesia in com- 
])arison to the other elements should have been absorbed; but the 
difficulty is that no such rock occurs in the district. The section, as de- 
scribed in the foregoing pages by Mr. Weed, is known from the gneisses 
of the Archean upward, and consists of shales, sandstones (i. e., 
quartzites), and limestones, the latter containing but little iron and little 
magnesia compared with the lime. It is impossible on any chemical 
basis to account for the shonkinite as produced by the granite- 
porphyry having absorbed local limestones or shales. Moreover, the 
method of occurrence shows, as previously described, that the arrange- 
ment is local and must have been produced by local causes after the 
magma reached its present position. Other facts in this direction 
might be cited, but the above is decisive and therefore sufficient. A 
somewhat similar instance is found at Castle Mountain and has been 
described.^ It is not iiitended in this statement to generalize upon 
other regions. It should be clearly recognized that in different areas 
unlike conditions have often prevailed and given rise to quite differ- 
ing phenomena; and without regard to the general aspect of this 
question of absorx)tion, it is only intended to show that there is no 
evidence in favor of it in this region. 



1 Weed and PirsBOQ, Geology of Castle Mouutaiii mi Ding district: Bull. U. S. Geol. Survey Ho. 139, 
1896, p. 133. 

20 GEOL, PT 3 37 



OHAPTEE VII. 
ANAIiYSES. OF ROCKS. 

For conyenient reference tbe various analyses of rocks of the Little 
Belt Monntains which have appeared in the foregoing pages are here 
grouped in one table. Of these analyses, !N umbers I, V, VI, X, XI, 
XII, XIII, XIV, XV, XVII have been made by Dr. W. F. Hillebrand, 
and Numbers II, III, IV, VII, VIII, IX, XVI by Dr. H. N. Stokes, in 
the laboratory of the United States Geological Survey. The ])ainstak- 
ing accuracy and skill of these chemists are well known, and the conten- 
tion of Hillebrand ^ for more exact and careful work in rock analysis is 
being more and more justified as such data are given a mathematical 
usage, and the graphic discussion in the foregoing pages is a direct 
evidence of this. Xo problem meets the analytical chemist requir- 
ing more skill and special training than the complete and accurate 
analysis of an igneous rock; and without this training and careful atten- 
tion to the best modern methods even a very good analyst is liable to 
make serious errors. The too common method of turning rock analyses 
over to beginners and students is an absurdity on the face of it; more 
useful work on the whole would be achieved if the investigator, pro- 
vided he is a chemist, made the analyses himself and turned the petro- 
graphic work over to the student. Of the vast mass of rock analyses 
which confront the petrographer it must be confessed that very few 
are of any value from a modern standpoint; the most are mere approxi- 
mations, and many not even that. 

Even in the handbooks prepared by masters of the science many 
analyses are quoted which are obviously wrong, especially in the basic 
rocks. A type is described as being composed chiefly of augite and 
often olivine, but the analysis gives but a slight amount of magnesia 
and a large percentage of alumina; or the iron is given as ferric acid and 
titanic and phosphoric acids are not determined, while loss by ignition 
maybe largely carbon dioxide. The writer desires to emphasize these 



» Jour. Am. Chem. 800., Vol. XVI, 1894, p. 90; Bull. U. S. Geol. Survey No. 148, 1897, p. 13. 
578 



PIHS80N.1 ANALYSES OF ROCKS. 579 

points, because in bis opinion .this is the greatest weakness in petro- 
graphic work. Thus, for example, in spite of the great number of 
occurrences which have been described, there are very few reliable 
detailed and accurate analyses of European nephelite and leucite basic 
rocks. 



580 IGNEOUS ROCKS OF LITTLE BELT MOUNTAINS, MONTANA. 



Table of analyses of rocks 




I. Rhyolite-porphyry, intrusive sheet Yogo-Baldy Ridge. 
II. Granite-porphyry, Wolf Batte. 

III. Granite-porphyry, top of Barker Mountain laccolith. 

IV. Granite-porphyry, north end of Thunder Mountain laccolith, 
y. Granite-porphyry, top of Big Baldy Mountain laccolith. 

VI. Granite-syenite-aplite, dike head of Sheep Creek. 
YII. Syenite, Wright and PM wards mine, Hughesville, near Barker. 
VIII. Syenite-diorite-porphyry, talus, head of Bear Park. 



PIB8BON.] 



ANALYSES OF ROCKS. 



581 



of the Little Belt Mountain, Montana, 



IX. 
62.18 


X. 

61.65 


XI. 

62.58 


XII. 
54.42 


XIII. 


XIV. 

48.98 


XV. 

48.35 


XVI. 


XVII. 
55.13 


52.26 


48.39 


15.77 


15.07 


16.42 


14.28 


13.96 


12.29 


13.27 


11.64 


20.27 


1.83 


2.03 


2.46 


3.32 


2.76 


2.88 


4.38 


4.09 


1.52 


2.44 


2.25 


1.96 


4.13 


4.45 


5.77 


3.23 


3.57 


4.29 


3.55 


3.67 


1.84 


6.12 


8.21 


9.19 


8.36 


12.55 


1.80 


4.13 


4.61 


2.47 


7.72 


7.06 


9.65 


9.94 


7.64 


7.05 


3.92 


4.35 


4.57 


3.44 


2.80 


2.22 


3.35 


4.14 


4.31 


3.91 


4.50 


3.91 


4.22 


3.87 


4.96 


8.01 


3.24 


2.84 


.70 


.41 


1.40 


.38 


1.34 


.56 


2.89 


2.56 


.95 


.30 


.26 


.38 


.22 


1.53 


.26 


.90 


.28 


.14 


.55 


.56 


.40 


.80 


.58 


1.44 


.52 


.73 


.74 


Trace. 


Trace. 
.09 


.08 


Trace. 
.10 


Trace. 
.14 


Trace. 

.08 


Trace. 
.19 


.07 
Trace. 




.13 


.16 


.10 


.10 


.13 


.05 


.08 


.09 


.15 


.06 


.43 


.27 


.41 


.32 


.23 


.43 


.54 


.32 


.11 


Trace. 


Trace. 


Trace. 


Trace. 


Trace. 


Trace. 


Trace. 


Trace. 


Trace. 


.32 


.33 


.33 


.59 


.52 


.98 


.40 


.45 


.40 






.77 




.49 




.30 


.08 
Trace. 


.26 


Trace. 
.04 










1 












.'J2 


.25 
















100.23 


100.15 


100.08 


100.19 


100.25 


99.99 


100.01 


99. 90 


100.00 



IX. Diorite-porphyry^ Steamboat Mountain laccolith. 
X. Syenite (banatite), Yogo Peak, east end. 
XI. Syenite-porphyry, Yogo-Big Haldy Ridge, intrnBive sheet. 
XII. Mon/onite, Yogo Peak, middle knob. 

XIII. Miuette, intrusive sheet, head of Sheep Creek. 

XIV. Shonkinite, Yogo Peak, west end. 

XV. Analcite-basalt dike, Big Baldy Mountain. 
XVI. Analcite-basalt dike, Bandbox Mountain. 
XVII. Diorite, Carpenter Creek, near Neibart. 

Analyses I, V, VI, X, XI, XII, XIII, XIV, XV, XVH by W. F. HiUebrand. 
Analyses H, UI, IV, VII, VIII, IX, XVI by H. N. Stokes. 



INDEX. 



[Italic page nam bora denote illiiBtratloiiii — i. e., plates or f^gim^s.] 



A. 



Page. 



Abbott^ E. K., ackno wledgmentB to 402, 440 

Acer beadirei Lx., Arom Cascade Range 49, C4 

Add and baaic rocks, Togo Peak, Mon- 
tana, diagram showing differentia- 
tion of '564 

Acidic feldspathic porphyries of Little Belt 
Mountains, petrogniphic charactem 

of 4«8-525 

Acrostichum alinnlatum, n. ap., from Cas- 
cade Range 87-38,54 

Acturaa claim, Bohemia region, Oregon, 

featarea of 19 

Adularia from Idaho, form and composition 

of crystals of - 187-168 

Algonkian rocks, Little Belt Mountains, 

Montana, description of 279-284 

Alnus carpinoides Lx., from Cascade 

Range, notes on 40 

Alteration. Se4 Metamorphism. 
Amalgamation and concentration, gold and 
silver mines of western central 

Idaho 114-115 

Analoite-baaalt, Little Belt Mountains. 

Montana, analyses of 544 

I>etrographic descriptions of 543-551 

thin section of 6t8 

transition rocks from roinett« to 551-556 

Analcite- (nephelite-) syenite, Montana, pet- 

rographic description of 469-471 

Analyaes, adularia 167 

ankerite spar 400 

basalt 176 

analoite-basalta 544 

diabase 176 

diorite 251,490 

diorite-porphyry 517 

diorite-syenite-porphyry 519 

dloritio rocks from Idaho 82-83, 219-231 

granite-porphyry fhmi Montana . 499, 506, 511 

granitic rocks flrom Idaho 81-83, 210-231 

minettes 531 

monzonites 478 

qnarte-monzonite 219-231 

quartz-porphyry 523 

rhyolites, altered 179-184 

rocks of Little Belt Mountains, Mon- 
tana 678-681 

rhyolite-porphyry 528 

shonkinite 484 

spar (ankerite) 409 

syenite firom Montana 466, 467, 468, 473 



Pago. 
Analyses, syenite-porphyry from Montana. 514 
wall rocks of veins of Idaho silver-lead 

mines 219-231 

Andesites of Bohemia mining region, Oregon 12, 13 

Ankerite spar, Montana, analyses of 409 

occurrence and character of 409-4 1 

Aplite, sheared, Montana, petrographio de- 
scription of. 495-406 

Aplite dikes in granite, western central 

Idaho, occurrence of 85 

Aplites, Montana, petrographic descrip- 
tions of 4i)3-497 

Aplitic dikelets of Togo roc ks, character of. 494 

figure showing 494 

mode of formation of 566 

Argent vein, Wo<*d River mining district, 

Idaho, description of 204-205 

Argentitts Neibart district. Montana, oc- 

onrrence and character of 407 

Asplenium tenenim? Lx., from Cascade 

Range, noteii on 38 

Aiigite-syenite of Belt Creek, Montana, i>et- 

rographio description of 468-469 



B. 



Banatite-aplite, Montana, petrographio de- 
scription of 494 

Bandbox Mountain, Montana, analcite-ba- 

salts of 543-546 

dikes at 326 

Banded Mtructure, veins of Neihart district, 

Montana, description of 417 

Banner vein, Florence gold-mining district, 

Idaho, features of 236 

Barite, Nelhart district, Montana, charac- 
ters of 409 

forms of crystals of 409 

Barker, D. C. £., acknowledgments io 402 

Barker, Montana, geology of region near. 348-349 

outcrop of Paine shale near S90 

petrography of analcite-basalt from 547 

Barker  Basin, Montana, mountain lim 

northeast of 460 

Barker formation of Montana, featnreffof . 284-287 
Barker mine, Neihart district, Montana, de- 
scription of 442 

Barker mining district, Montana,dikes of. 852-353 
l^tory, geography, and geology of . . 344-360 

igneous rocks of 349-860 

map of S45 

ore deposits and mines of 441-446 

583 



684 



INDEX. 



Barker Mountain, Montana, Heotion through 5,m 

lact!oliihof 354-356 

barker porphyry, Moutuiia, character of. . . 356 

cllflsof... : SS6 

Barker syenite, Montana, aualypes of. 466, 467, 468 

petrographir. description of 465-468 

Basalt, Bohemia mining region, Oregon 13-14 

Idaho mining districts. 99-100,118-120. 174-176 

Little Belt Mountains, Montana .'>56-557 

Basic and ncid rocks, Togo Peak, Mon- 
tana, diagram showing differentia- 
tion of ''04 

Bear Park, Montana, anaiyaes of diorite- 

syenite-porphyry from 519 

Becker, G. F., cited on geology of wejitern- 

central Idaho 7r>-76 

Belle Peck vein, De Lamar mining district, 

Idaho, character of loi 

Belt Creek, Montana, augite-syenite of. . 468-469 

geologic features of 37R-:t80 

geologic features of regioii near 338 

sections on 283-284,285 

limestone cliffs on S68 

outcrops of gueisH on 380 

canyon of S78 

valley of 378 

Belt shales overlying Keihart quartzitoa, 

view showing 878 

Belt terrane. Little Belt Mountains, Mon- 
tana, rocks and general features of 279-284 

section of 283-284 

Benton groupof claims, Neihart mining dis- 
trict, Montana, develofiments of. . 436-437 
Benzoin dilleri n.sp., Cascade Kange.. . 46-47,60 

Big Baldy Mountain, amphitheater in 338 

geologic features of 335-341 

granite-porphyry of &I0-512 

petrography of analcite basalt from . . 547-550 

section through 336 

views of 318,336 

Big Park, Montana, geologic section in.. 339-340 
Big Seven mine, Montana, crusted ore 

from 410 

description of 487-439 

paragencbis of minerals in 410-413 

Big Seven vein, Neihart district, Montana, 

diagrams of 411, 438 

Bill Cummins claim, Yogo district, Mon- 
tana, note on 450 

Bishop tunnel, De Lamar mining district, 

Idaho, sketch plan of 166 

Bishop vein, De Lamar mining district. 

Idaho, descrii)tion of 155-156 

Black Butte, Montana, geologic features of. 305 
Black Jack mine, De Lamar district, Idaho, 

features of 136-140 

Black Jack vein, De Lamar district, Idaho, 

diagrams showing features of 139 

Black Jack-Trade Dollar vein, De Lamar 
mining district, Idalio, evidence as 

to mode of ore deposition at 165-166 

Blende, Neihart district, Montana, descrip- 
tion of 406 

Blenkinsop Creek, Montana, intrusive sheet 

on 351 



Page. 
Blossom vein, Florence gold raining dis- 
trict, Idaho, features of 236 

Blue Dick claim, Yogo district, Montana, 

developments of 450 

Blue River mining region, Oregon, notes 

on 31-32 

Bluff Mountain, Montana, geology of 302 

Bohemia mining region of wetiteni Oregon. 1-36 

alteration of country rock in 14-15 

gangue in veins of 17-18 

geology of 10-19 

history of 7 

location and topography of 9-10 

map showing geographic relations of. . 9 

mines and piv^pects of 19-31 

ores in veins of 18-19 

output of 7-8 

rocks of 11-15 

veins of 15-19 

Booneville mine, De Lau)ur district, Idalio, 

features of 134-136 

Booneville, Black Jack, and Trade Dollar 
mines, Florida Mounuiin, Idaho, 

plan of workings of 138 

Booneville, Black Jack, and Trade Dollar 
vein, Florida Mountain, Idaho. 

section following plane of 140 

Bostonite, Little Belt Mountains, Montana, 

occurrence of 531 

petrographic description of 024-525 

Bracket t and Noyes, analyses by 549 

Broadwater mine, Neihart mining district, 

Montana, description of 433-436 

longitudinal section of 430 

plan of underground workings of 4S6 

Broadwater vein, Neihart mining district, 

Montana, cross section of 43S 

face of 435 

Broadwa}' claim, Bohemia region, Oregon, 

features of 26 

Butcherknife Creek, Montana, geologic fu i- 

. tures of 341 

Butcherknife Mountain, Montana, geologic 

features of 341 

C. 

Cala)M)03'a Mountain, Oregon, composition, 

structure, age, fossils of. 9-11, 32-36, 51-52 
California claim, Bohemia region, Oregon, 

features of 23 

I Califomiaclaim, Yogo mining district, Mon- 
tana, developments of 449 

Camas No. 2 vein. Wood River mining dis- 
trict, Idaho, features of 209 

I ('ambrian fossils, Little Belt Mountains, 

t Montana, list of 286-287 

Cambrian rocks, Montana, description of. 284-287 

sections of t84, 32R-3:«). 340 

Carboniferous rocks, Little Belt Mountains, 

Mon tona, description of 289-298 

sec^tions of 294, 2<W, 296-298, 301 

Carpenter Creek, Montana, granite-por- 
phyry of riOl-502 

porphyry of 376-377 

view of mouth of 44S 



INDEX. 



585 



Pag& 

Carter mine, Keihart minlDg diatriot, Mon- 
tana, note on 444 

plan and nection of 443 

Cascade Kaufce, evidence of plant remains 

aA to age of 11,32-36,51-52 

foHsil pUntfl associated with lavaa of . . 37-64 
structure of 32-36 

Caatanea caMtanetefotia (Unger) Kn., from 

Cascade Kangn, note on 40-41 

Castle limestone, Montana, features of 293 

sections of 294.320,362 

Cbaloopyrite. Neibart district, Montana, 

occurrence of 408 

Chamberlain shaleH, Montana, section of. 283-284 

Champion mine, Boliemla region, Oregon, 

description of 26-27 

Charity vein, Warren gold-mining district, 

Idaho, description of 249 

Chamberlain shales, Montana, deticriptiou 

of 282 

Chautauqua tunnel, De Lamar mine, Idaho, 

figure showing form of 130 

Chemical analynes. Sfr Analyses. 

Cliert layers in Cambrian limestone, plate 

showing t78 

Chocolate porphyry, Barker district, Mon- 
tana, description of 349-^51 

Christopher Columbo claim, Yogo mining 

diHtrict, Montana, developments of. 449 

Cinnamomum dilleri n. sp., from CuhcmIo 

Riinge 47-48,60 

Clarence olaim, Bohemia region, Oregon, 

features of 19 

Cleveland Creek, Montana, geology of 

region near 315 

Clendennin Mountain, ^lontant), intrusive 

sheets of 357-358 

Climax claim, Togo district, Montana, 

note on 450 

Climax mine, Wood River district, Idaho, 

description of 200 

Columbia lava, Idaho, topography and gen- 

erwl features of 78, 01-93 

Combination mine. Bohemia region, On*gon, 

deReri])tion of 20 

ComiienAation ^roup of claims. Wood River 

district, Idaho, developments of 200 

Concentration and amalgamation, gold and 

Hilver minei* of Idaho, details of. . 114-115 

Conglomerate of Cambrian limestone peb- 
bles, plate showing t78 

Conglomerate (intrafomiational) of lime- 
stone lenses, plate showing 973 

Contact met amor plilsni, Yogo mining dis- 
trict, Montana, features of 322-323 

Copper deposits of Seven Devils, Idaho, de- 
scription of 249-253 

Coxcomb Butte, Montana, geoh)gic features 

of 305 

Cretaceous rocks, Cascade Range, Oregon. . 35, 36 
Montana, section of 296 

Croesus mine. Wood River mining district, 

Idaho, description of 207-208 

Crown Point vein, I)e Lamar mining dis- 
trict, Idaho, features of 145 



Page. 

C mated ore flrom Montana mines, plate 

showing 410 

Cuddy Mountains, Idaho, general geologic 

features of 88 

Cumberland mine, De Lamar mining dis- 
trict, Idaho, features of 150-151 

Cumberland vein, De Lamar raining dis- 
trict, Idaho, diagram showing IJil 

Cyanide process of gold mining, Idaho, ex 

periments with 116 

D. 

Dacite-porphyry of Bohemia mining re- 
gion, Oregon 11-12 

Dakota mining claim, Xeihart mining dis- 
trict, Montana, developments of. . 482-433 
Davis, W. M. , exploration of Little Belt 

Mountains by 273 

Dawn and Foster claims, Montana, view of 

Mackay mine on 440 

De Lamar, Idaho, plans of mines of Ii4, 128 

hydrothermal alteration of rocks at 178 

De Lamar mine, Idaho, analyses of altered 

rhyolites from 179-184 

diagram showing distribution of ore in 

shoots of • 127 

history, production, and general fea- 
tures of 122-129 

ore from 168 

plan of Chautauqua tunnel in ISO 

table showing proportions of gold and 

silveriu 129 

veins of 1.58-100 

De Lamar mine and mill, Idaho, view of ItS 

De Lamar mining district, Idaho, ores of. 109-172 

placer deposits of 163 

structure of ores of 160-172 

vein system of 136 

De Lamar and Silver City mining districts, 

Idaho, age of ore deposits in 163 

depth of richest ores in 165 

fissure systems of 164 

gangue minerals of 164-165 

general features of ore deposits of.. . 163-166 

history of 108-110 

geology of 116-121 

mineralogy of veins of 166-169 

ores of 164 

De Lamar and Silver City, Idaho, orea from . 174 
process of gold and silver mining at . . 113-116 

pro<luction of 111-113 

Delia claim, Yogo mining district, Mon- 
tana, developments of. 448-449 

Delta claim, Bohemia region, Oregon, fea- 
tures of 25 

Depth of richest ore deposits, gold and sil- 
ver mining districts in Idaho 165 

Devonian rocks, Big Park, Montana, section 

of 33^-340 

Diabase, analysis of 176 

Dikelets (aplltic) of Yogo rocks, Montana, 

figure showing 494 

mode of formation of 566 

Dikes. Barker district, Montana 352-353 

Big Baldy Mountain, Montana 337 



686 



INDEX. 



Bikes, Idaho, in granite 85-86 

Little lielt Mountains, petro^^raphy of. 40g-S2 

Keihart district, Montana 377 

theory of formation of 400 

Wood River district^ I daho 196 

Yogo mining district, Montana. . . 321, 325-327 

Yogo Peak, Montana 310 

Dikes and sheets accompanying laccoliths. 
Little Belt Mountains, Montana, 

occurrence of 390 

Diller, J . S. , paper on Bohemia mining region 

of western Oregon by 1-36 

Dioritc, Neihart district, Montana, oocar- 

rence and character of 37S-375 

petrographic description of 488-493 

plate showing 488 

Diorites, analyses of 251,490 

Diorite-porphyry, figare showing zoned 

plagioclase in 516 

Little Belt Moantains, Montana, compo- 
sition of rocks of 515-517 

Steamboat Mountain, Montana, plate 

showing micro-drawing of 616 

western-central Idaho, dikes of , 86 

Diorite-syenite-porphjTj, Little Belt Moan- 
tains, Montana, analyses of 519 

Dioritic rocks, analyses of 82-83, 219-231 

Dirty Creek, Montana section on 301 

Doelter, C. , analysis by 548-549 

Doniphan, Idaho, figure showing location of 

claims near t08 

Dry Creek shale, Montana, description of. . 286 

sections of 303-384. 368 

Dry Wolf Creek, Montana, geologic fea- 
tures of region near 339-341, 343 

Dry Wolf Creek mining district, Montana, 

ore deposits of 453 

Dunn, Matthew, acknowledgments to 402 

Eakins, L. G., analyses by 466, 467, 484, 487, 490 

Eldridge, G. H., cited on geology of west- 

em-centralldaho 76 

cited on occurrence of iron ores of Lit- 
tle Belt Moantains, Montana '459-460 

cited on occurrence of schistosity in 
granitic rocks of western-central 

Idaho 85 

XUkhom mine, Idaho, production and gen- 
eral features of 209-210 

Elkhom yein, Idaho, vertical section of SIO 

Elk Kldge, Montana, geologic features of . . 321 
Elsie Dora claim, Bohemia region, Oregon, 

features of 25 

Empire claim, view showing apex of vein at . i£0 
Empire State vein, De Lamar mining dis- 
trict, Idaho, features of 145 

Empire vein system, De Lamar mining dis- 
trict, Idaho, description of. 154-155 

Eureka claim. Wood Biver mining district, 

Idaho, notes on 200, 201 

Eureka divide, Montana, dikes of 326-327 

petroin*aphy of analcite-basalts of .. . 546-647 
Eureka E>ine, Running Wolf district, Men- 

tan«,noteson 453 



F. Page. 

Fairview claim, Neihart- mining district, 

Montana, note on 446 

Faulting along veins, Idaho mining dis- 
tricts 144-145 

Feldspathic jiorphyries of Little Belt 
Mountains, petrographic descrip- 
tions of 498-525 

Ficus 7 hesperia n. sp., from Cascade Range . 45, 56 
Ficus sp. cf. F. sordida Lx., from Cascade 

Range 46,60 

Ficus sp.? from Cascade Range 46,58 

Fissure systems of Silver Cfty and De La- 
mar mining districts, Idaho, fea- 
tures of 126,164 

Fissures and veins, Idaho mining districts. 101- 

106 

Neihart district, Mont ana 414, 416 

Flathead sandstone of Montana, descrip- 
tion of 285 

section of 285-364 

Flint mining district, Idaho, view of l£i 

silver mines of .*. 187-188 

Flora. See Plants, fossil. 

Florence, Idaho, map showing principal 

mines at MS4 

Florence gold-mining district, Idaho, loca- 
tion, history, geology, and mineral 

deposits of 232-237 

Florence mine, Montana, crusted ore from. 410 

description of 425-426 

fault in 4S6 

poly basite at 412 

veinin 497 

views of 868,440 

Florida Mountain, Idaho, general plan of. . 184 

gold and silver veins of 134-147 

ores of ; 172 

thermal alteration of rocks at 184-186 

veins of 160-161 

Fossil plants. See Plants, fossil. 

G. 

Galena, Minnie Moore mine, Idaho 213-214 

Neihart district, Montana 406 

Galena Creek, Montana, porphyry dike 

near 359-860 

Gait mine, Montana, shaft house and ore 

bins of 428 

Gait vein, Neihart mining district, Mon- 
tana, description of 427-428 

diag^ram showing relation of porphyry 

dike to 4i7 

Gangue minerals, Bohemia district, Oregon. 17-18 
Neihart district, Montana 408-410 

Garfield claims, De Lamar mining district, 
Idaho, geological features shown in 
developments of 131-132 

Giant Rock, Belt Creek Canyon, Montana, 

view of 292 

Gilbert, G. K., cited on forms of laccolithlc 

intrusions 891, 894 

Girty, G. H., cited on Devonian foesila of 

Little Belt Moantains. Montana. 288-289 

Glaciation in weetem-oentral Idaho 100 



INDEX. 



587 



Page. 
Gneiss, Belt Creek, Montana, view showing 

outcrops of 880 

Gneisses and schists of Little Belt Monn* 

tains, Montana, features of 278- 

270, 371-373 
Gold, Lane County, Oregon, production, 

1888-1895 8 

Neihart district^ Montana, occurrence . . 408 
Gold and silver, De Lamar mine, Idaho, 

table showing proportions of 129 

Owyhee County, Idaho, tables showing 

production of 111-113 

Gold and silver veins of western central 

Idaho «5-256 

classification of 104-106 

Gold Bug vein, Idaho gold mining districts, 

featurcHof 157,236 

Gold Fork placer deposits, Idaho, descrip- 
tion of 242-244 

Gold qnartz veins, Warren gold mining dis- 
trict, Idaho 244-246 

Wood River mining district, Idaho.. 207-20P 

Gold Run Basin, Montana, features of 359 

Golden Chariot mine, De Lamar mining dis- 
trict, Idaho, production and general 

featurenof 149-150 

Golden Chariot shaft, De Lamar mining dis- 

, trict, Idaho, vertical section of 149 

Golden Slipper claim, Bohemia region, Ore- 
gon, features of 25 

Goodenoiigh claim, Warren gold mining 

district, Idaho, developments of. . 347-248 
Granites, western-central Idaho, analyses 

of 81-83 

dikes in 85-86 

hydrothermal alteration of 174 

occurrenceandcharacterof. 60-85. 117-118, 195 

silver lead veins in 206-307 

Granitic rocks, western central Idaho, 

analyses of 219-231 

extent and character of 80-^ 

Granite-porphyry, analyses of 499, 605, 511 

dikeeof 85-86 

occurrence of 351,358-360 

petrographic description of 498-512 

thin section of 608 

Granite syenite-aplite, Montana, petro- 
graphic characters of 496-497 

Granite-Byenit«porphyry, Montana, petro- 
graphic characters of 512-613 

Grendal claim, Neihart miningdistrict,Mon- 

tana, note on 446 

Greyson shale, Montana, description of 282 

section of 283 

Griswold, L. S., aid i-endered by 271 



H. 



Hailey, Idaho, map showing geology of Min- 
eral Hill mining district near 190 

ores from mining districts near S14 

view of 19t 

Hamilton vein, De Lamar mine, figures 

showing Its 

Harley Creek, Montana, ore deposits near. . 440 



Paso. 
Harrison Creek, Montana, geology of region 

near 313,314,315 

Helena claim, Bohemia region, Oregon, fea- 
tures of 29^^ 

Henley, James, acknowledgments to 402 

Henrietta mine, De Lamar mining district,  

Idaho, features of 132-133 

Henrietta '^fein, diagram showing form of. . 133 

Hillebrand, W. F., analyses by 82, 167, 170, 

179-180, 219-231, 466, 467, 473, 478, 
484, 400, 406-497, 499, 605, 511, 514, 
516-517, 523, 531, 544, 548, 680-581 
Hi Tu vein, Florence gold mining district, 

Idaho, features of 236 

Hobson, S. S., acknowledgments to 402 

Hughesville, Mont., syenite intrusive stock 

near 353 

Hunter, M., analysis by 544-545 

Huntington, Oregon, character of rocks 

near 83 

Hurlbnrt, E. B., analysis by 478 

Hydrothermal alteration of rooks, Idaho 

gold and silver mines 177-187 

L 

Ida Elmore shaft, De Lamar mining district, 

Idaho, vertical section of 149 

Ida Elmore vein, De Lamar mining district, 

Idaho, description of ' 149 

Idaho, analyses of granite from 81-83 

fissures and veins of western-central 

portion of 101-106 

geology of western-central part of 75-106 

glaciation in 100 

gold and silver production of mines of 

Owhyee County of 111-113 

gold and silver veins of mining districts 

in 65-256 

granite rocks in western -central 80-85 

map of part of 78 

map showing geology of western-central 

part of 76 

map showing mineral deposits of west- 
ern-central part of 100 

maps showing geology of mining diH- 

tricUin 116,190 

mineral deposita of western c«ntral part 

of 101-106 

ores from mining districts of IGS, 174, il4 

plans and sections of mines and veins 

in 124, 126, lf8, 134, 138, 140, 14S 

Pleistocene deposits in 100-101 

veins and fissures of western-central por- 
tion of 101-106 

viewsin 88,lit,146 192,238 

Idaho Democrat mine, Wood River mining 

district, description of 206-207 

Idahoan claim, Wood River mining district, 

Idaho, notes on 200-201 

Igneous rooks of Little Beit Mountains, 

Montana, description of 313-316 

primary olaasifloation of 463-465 

petrography of 463-581 

Ingersoll No. 2 vein, Neihart mining dis- 
trict, Montana, description of 430-431 



588 



INDEX. 



Pago. 
Ingersoll yein, Keihart mining distriut, 

Montana, desoription of 429-430 

diagram showing workings on 431 

Intraformational conglomerate of limestone 

lenses, plate showing 978 

Intrusive rocks of Little Belt Mountains, 

petrography of acidic feldpathic 

porphyries of 498-525 

Iron ores of Little Belt Monntains, Montana, 

occurence and character of 459-461 

I XL and £ureka mine, Neihart district, 

Montana, description of 437 

J. 

Jannasch, P., analysis by 473 

Jay Gould Extension, silver-lead mine, 

Idaho, section of vein in tl6 

Jefferaun limestone, Montana, corals in . . . 988 

features and fossiln of 287-289 

outcrop of fB88 

sections of 329-330, 339, 363 

John F. SuUivau tunnel, De Lamar mining 

district, Idahu, features of 147 

Judith Plateau, Montana, figore showing 

section across 311 

Judith River, Montana, geology of region 

drained hy 310-316 

igneous rocks of region drained by . . 313-316 { 

ore deposits of Middle Fork of 446 

section on 206-298 

Juglaus sp., from Cascade Range 39-40, 54 i 

Jurassic rocks, Montana, section of 297, 301 

K. 

Kaoliuite. Idaho gold and silver veins . . . 171-172 { 
Kersantite, Barker district, Montana, dikes ' 

of 353 

rocksshowingtransitionof miuetteto 538-539 ! 
Kibbey sandstone, Montana, description of. 295 
King Creek, Montana, geology of region 

near 313,314,315 

intrusive rocks of 515 

ontcrop of Jefferson limestone on 288 

Knowlton, F. H., paper on fossil plants 

associated with lavas of Cascade 

Range by 37-64 

Knott vein, Warren gold mining district, 

Idaho, description of 249 

Knott and other claims, Bohemia region, 

Oregon, features of 27-28 

Kootanie beds, Montana, section of 296 



Laccoliths, Barker Mountain, Montana.. 354-356 ' 
chemical iind mineral composition of 

rocks of 559-560 

diagram showing distribution of S8S 

forms of 391-396, 392, 393 

Otter Mountain, Montana 356-357 [ 

IMtrography of rocks of 559-563, 498-525 

relation of stock ixicks to 400 

Tenderfoot Mountain, Montana 367 

Thunder Mountain, Montana 364-367 

Togo District, Montana 323-325 

327, 331-332, 333-835, 341-343, 387-396 



PagBb 
Lamprophyres, Little Belt Mountains, 

Montana, analysos of 548 

petrographio descriptions of 526-556 

thin sections of 6S8 

Lamprophyric dike rocks, western-central 

Idaho, occurrence of 86 

Lastrea (Goniopteris) flschori Heer, from 

Cascade Range 38, 54 

Laurus similis n. ap., from Cascade 

Range 48-49,6* 

Lava plateau south of Seven Devils, 

Idaho, plate showing 80 

Lavas of Cascade Range, fossil plants asso- 
ciated with 37-64 

Lead-silver ores of Neihart district, Mon- 
tana, description of 405-413 

Lenny, Daniel, acknowledgments to 402 

Lepley claims, De Lamar mining district, 
Idaho, geological features shown in 

developments of 130-131 

Liberty mine, Neihart district, Montana, 

description of 445-446 

Limestone cliffs. Monarch, Montana, view 

of S90 

Belt Creek, Montana, view of 368 

Limestones of western-central Idaho, occur- 
rence and character of . . w 86, 87, 88 

Linck, G., analysis by 531-582 

Lindgren, Waldemar, cited on igneous 
mass of Thunder Mountain, Mon- 
tana 364,365.366 

paper on gold and silver veins of Silver 
City, De Lamar, and other mining 

districts in Idaho by 65,260 

Lion Creek, Montana, section near 339-340 

Lion Creek mining district, Montana, ore 

deposits of , 453 

Little Belt Mountains, Montana, analyses 

of rocks of 578-581 

descriptive geology of 299-381 

diagram showing variation of chemical 

composition of rocks of 571 

drainage of 275 

general geology of 382-400 

general petrology of 558-568 

geographic position uf 271-272 

geologic history of 382-384 

geologic map of S86 

index max) showing location of 27f 

intrusive rocks of 385-400 

laccolithic intrusions of 387-396 

i ron ores of 459-461 

ore deposits of 401-461 

outcrop of Jefferson limestones in S88 

paper by L. V. Pirssou on petrography 

of igneous rocks of 463-581 

paper by W. H. "Weed on geology of. 257-461 

rock formations of 278-298 

section of 393 

settlements in 272-273 

structural features of 276-277, 384-385 

topographic map of f 71 

topography of 273-275 

Little Emma claim, Togo district, Montana, 

developments of 450 



INDEX. 



589 



Page. 

Little Giant mine, Idaho, plan and sections 

of 247, 248 

Little Giant vein, Warren gold mining dis- 
trict, Idaho, description of 246 

Lizzie veins, Neihart mining district, Mon- 
tana, descriptions of -. 432-433 

Lizzie Bullock claim, Bohemia region, Ore- 
gon, features of 25 

M. 

McAssoy, J.. acknowledgmentH to 402 

McKinley mine, Neihart district, Montana, 

note on 446 

Mackey Creek, Montana, ore deposits near 439-440 

Mackey mines, Montana, view of 440 

Madison group of fonnations, Montana, 

rocks and fossils of 289l-298 

Magmas of rocks of Little Belt Mountains, 

absorption of sed imen ts by 577 

diagn*axnmatic discussion of 560-676 

Magnetlt« (skeleton) in olivine of shon- 
kiniteof Yogo Peak, Montana, plate 

showing 476 

Mahogany mine, De Lamar district, Idaho, 

history of 150 

Mammoth mining district, Idaho, silver 

veins of 180 

Manebach twin of orthoclase from granite- 
porphyry of Mount Barker, Mon- 
tana 604 

May and Edna mine, Neihart raining dis- 
trict, Montana, note on 444 

plan ofworkings of. 444 

Mayflower vein, W004I River mining dis- 
trict, Idaho, description of 201-202 

Meagher limestone, Montana, description 

of 285-286 

Melville, W. H,, analysis by 490 

Metamorphic rocks. Little Belt Mountains, 

character of 278-279 

Metamorphism, Bohemia region, Oregon 14-15 

Yogo mining district, Montana 322-323 

Metasomatic changes in rocks, Idaho gold 

and silver mines 177-187, 218-231 

Miargyrite from Idaho, crystallographic fea- 
tures of 168-169 

Mica (white), Neihart district, Montana, 

character of 408 

Mineral composition of rocks of Little Belt 

Mountains, variation of 567-568 

Mineral Hill mining district, Idaho, geolog- 
ical map of 190 

Minerals of deposits of Idaho mining dis- 
trict, table showing 255-256 

Mines, Barker district, Montana 442-446 

Bohemia mining region, Oregon 19-31 

Florence district, Idaho 232-237 

Mackey Creek, Montana 439-440 

Neihart, Montana 423-436 

Running Wolf district, Montana 450-453 

Silver City and De I^nmar districts, 

Idaho 122-166 

Warren district, Idaho 237-249 

Woo<l River district, Idaho 198-211 

Yogo district, Montana 447-450 



Page. 
Minette, Little Belt Mountains, Montana, 

analyses of 631 

intrusive occurrences of. 307-309 

332-333, 351-352, 377 

included masses iu 536-537 

petrographic description of 526-530 

plate showing 438 

thin section of 528 

table showing actual and theoretical 

composition of 572-573 

transition rocks from analcite-basalt 

to 551-556 

Mining and milling, western central Idaho, 

processes of 113-116 

Minnesota mine, Do Lamar mining district, 

Idaho, production of 150 

Minnie Moore mine, Wood River mining 

district, Idaho, description of 198-199 

forms of galena from 213-214 

Miocene eruptions in western central Idaho, 

areas covered by 100 

Miocene rocks of Cascade Range, Oregon.. 33, 

35, 38, 51-52 

Mitchell, W. F., analyses by 467 

Mixes Baldy Peak, Montana, features of in- 
trusive mass at 358-360 

granite-porphyry of 501 

Monarch, Montana, geology of region 

near 360-370 

limestone cliff at 290 

sections at and near 861-364 

Monarch district, Montana, geologic fea- 
tures of 360-370 

Monroe, William, acknowledgments to 402 

Montana, columnar sections of Cambrian 

formations in S84 

columnar section of Carlwniferous for- 
mations in S96 

geology of Little Belt Mountains of. . 257-161 
IMtrography of igneous rocks of Little 

Belt Mountains of 463^681 

Monzonite of Yogo Peak, Montana, petro- 
graphic description of 475-479 

plate showing 470 

plate showing intergrowth of two alkali 

feldspars in 476 

Monzouites, analyses of 478 

Morning Star mine, De Lamar mining dis- 
trict, Idaho, description of 156-157 

Moulton mine, Neihart mining district, 

Montana, description of 428-429, 445 

view of 4S9 

Mount Barker, Montana, granite-porphyry 

of 604-506 

Mount Hillers, Henry Mountains, figure 

showing form of laccolith of S9t 

Mount Holmes, Yellowstone National Park, 

figure showing cross section of S9S 

Mount Marcelllna, figure showing form of 

laccolith of S92 

Mount Taylor, Montana, mines of 451 

Mountain Chief vein, Neihart district, 

Montana, description of 424-425 

Mountainside mine, Runninc: Wolf district, 

Montana, note on 452 



590 



INDEX. 



Page. 
Musick mine, Bohemia region, Oregon, de- 
scription of 20-23 

sections at SI 

Mystery claim, Bohemia region. Oregon, 

features of 24 

N. 

Karrow Gauge mine. Wood River mining 

district, Idaho, description of 205 

Neocene iake of westem-centrai Idaho, 

extent of 96 

Neocene lake beds of westem-centrai Idaho, 
occurrence, character, and fossils 

of 95-06, »7-09 

Neocene lavas, Wood River district, Idaho, 

occurrence and character of 196-197 

Neocene rocks of Cascade Range, Oregon . 33, 35, 86 
Nephelite-minette, Little Belt Mountains, 
Montana, petrographic description 

of 539-^40 

Nephelito-syenite, Montana, i)etrographio 

description of 469-471 

Neihart, Montana, apex of vein at 420 

Archean slopes at S76 

Florence mine at 368 

porphyry benches near §76 

geologic section near 283-284 

intrusive porphyry sheet near. S78 

outcrops of various rocks near f76, 

278, SS8, S68, 378, 380, 420 

Pinto dioritt) of 488-493 

view of 370 

Neihart mining district Montana, Archean 

rocks of 871-373 

claim map of 422 

descriptive geology of 371-381 

diagram showing relations af veins to 
one another and relation of pay ore 

to country rock in 419 

fissure system of 414-416 

geologic history of 380-381 

history of 402-405 

igneous rocks of 373-377 

mai» of 40S 

mode of ore deposition in 420-421 

ores of 405-413 

paragonesis of minerals of 410-412 

secondary enrichment of veins in 421-422 

section across 40o 

setlimcntary Hn-as of 377-380 

value of ores of 413 

veins of 413-422 

Neihart Mountain, Montana, views of S&), 438 

Neihart porphyry, Montana, occurrence and 

character of 375. 376 

Neihart quartzite, Montana, features of 281 

section of 284 

view showing 378 

Newberry, J. S., exploration of Little Belt 

Mountains by 273 

Newland Creek hills, Montana, geology 

of 302-304 

Kewland limestone, Montana, features of. 282 

section of 283 

Noonday mine, Bohemia region, Oregon, 

description of 28-29 

North Star mine, Idaho, description of 211 



O. Page. 

O'Brien Creek reservoir, Montana, view of. 380 

O'Brien vein, Neihart district, Montana, 

description of 424 

Olivine, skeleton magnetite in 476 

thin section of 608 

Ontario vein, De Lamar mining district, 

Idaho, fefitures of 146 

Ophir claim, Bohemia region, Oregon, fea- 
tures of 19 

Ore deposits. Silver City and De Lamar 
mining districts, Idaho, general fea- 
tures, age, and depth of 163-166 

Ores in veins of Bohemia district, Oregon. . 18-19 

Ores of Idaho mining districts, structure 

of 169-174, 274, fi4 

Ores of Neihart district, Montana, plate 

showing 410 

value of 413 

Oregon, Blue River mining region of 31-32 

Bohemia mining region of. 8e4 Bohemia. 
Production of gold in Lane County of, 

1888-1895 8 

Oro Fino vein. Be Lamar mining district, 

Idaho, features of 147-149 

Otter Creek, Montana, shonkinite of 488 

Otter Mountain, Montana, laccolith of 356-357 

Otter shale, Montana, features of 295 

Overland mine. Wood River mining dis- 
trict, Idaho, production of 190 

Owhyee Mountain Range, Idaho, topography 

and geology of 77-107 

Ozark vein, Florence gold mining district, 

Idaho, features of 236-237 

P. 

Packard, R. L., analyses of diorite by 251 

Paddy Valley, Idaho, map and section show- 
ing placer dejMMits at 243,244 

Paine shales, Montana, features and fossils 

of 290-291 

outcrop of 290 

section or 329,339,362,363 

Paleobotany. See Plants, fossil. 

Paragon mine. Neihartminingdistrict, Mon- 
tana, note on 444 

Park shsle, Montana, description of. 286 

section of 368 

Parker mine, Idaho, production of 211 

Pass group of claims, Wood River mining 

dirttrict, Idaho, Argent vein of. . . 204-205 

Pauper vein, De Lamar mining district, 

Idaho, history of 154 

Pay shoots, Neihart district, Montana, de- 
scription of 418-420 

Payette formation, Idaho, fauna and flora 

of 97-99 

occurrence and character of 196-197 

Peek-a-boo claim, Bohemia region, Oregon, 

features of 19 

Pegmatite dikes in granite, western-central 

Idaho, occurrence of 85 

Penlield, S. L., cited on crystallographic 

form of polybasite 407 

quoted ou features of crystals of miar- 

gyrite and pyrostilpnite in 168-169 



INDEX. 



591 



Pajje. 
Petrography, !Bohemia mluixtg region, Ore- 
gon 11-16 

igneouB rocks of Little Belt MoantAina, 

Montana 463-581 

Phyllites oregonianua n. sp., from Cascade 

Range 4»-50,CS 

Phyllites sp., from Cascade lUunge 50,64 

Pilgrim Creek, Montana, geologic features 

of valley of 367-368 

section on , 368 

Pilgrim limestone, Montana, description of. 286 

section of 330,340,364,868 

Pinto diorite, Nelhart district, Montana, 

character and occurrence of 373-375 

exposure of 3S8 

petrographic description of 488-493 

plate showing 488 

Pinus sp., from Cascade Range 39, £4 

Pirsson, L. Y., analyses by 367,484,545 

cited on conditions required for forma- 
tion of laccoliths and intrusive 

stocks - 394 

paper on petrography of igneous rocks 

of LiUe Belt Mountains by 463-581 

quote<l on forms of orthoclase crystals 

from Idaho 167 

Plaoer depoeit:*, Idaho gold mining dis- 
tricts, age and character of 234 

f eatn res of 1 03, 240-244 

Placer mining, western-central Idaho, local- 
ities for 113-114 

Plagioclase, soned, figure Khowing 616 

Plants (fossil) associated with lavas of Cas- 
cade Range, descriptions and plates 

of 37-64 

table showing distribution of 51 

Pleistocene deiM>sit8, western -central Idaho, 

arpas of 197-198 

occurrence and character of lOO-lOl 

Pliocene basalts, Idaho, occurrence of 99-100 

Pluto vein, De Lamar mining district, 

Idaho, fealureti of 145 

Pohlmann, cited on spheroidal structure in 

kersautite 534 

Polj'basite, Neihart district, Montana- 
figures showing 407,411 

occurrence and character of 407, 41 1-412 

Poorman vein, Idaho gold-mining districts, 

features of 151-154,237 

vertical section of 15S 

Poorman vein system, De Lamar mining dis- 
trict, Idaho, features of 151-154 

Populns zaddachi? Heer, from Cascade 

Range : 40,54 

Porphyries of Little Belt Mountains, 
Montana, occurrence and characters 

of 349-35 1, 351-350, 375-377, 498^25 

Porphyry Peak, Montana, geologic features 

of 310 

Porphyry sheet near Xoihart, Montana, 

view of S78 

Potosi mine, De Lamar mining district, 

Idaho, description of 157 

Pratt, J. H., cited on crystallography of 
sapphires of Yogo Gulch, Mon- 
tana .' 554-556 



Page. 

Prichard, W. A., aid rendered by 75 

Prospect Ridge, Montana, geologic features 

of 316 

Pyrargyrite, Neihart district, Montana, 

occurrence and character of 408 

Pyrite, Neihart district, Montana, descrip- 
tion of 406 

Pypomorphite, Xeihart district, Montana, 

occurence of 408 

Pyroatilpnite from Idaho, cryMtallographic 

features of 169 

Q. 
Quadrant group, columnar sections of for- 
mations of g96 

rooks and fossils of 294-298 

Quaker City claim, Yogo district, Montana, 

developments of 448, 449 

Quaker City mine, Idaho, production and 

general features of 210, 211 

Quartz, laminated ("zerhackter. Quarz"), 
Idaho gold and silver veins, occur- 
rence of 170 

Keihart district, Montana, character of. 408 
pseudomorphic, De Lamar and Oro Fine 

ore, Idaho, plate showing 17S 

Qnartzdiorite,Wood River miningdistrict, 

Idaho, occurrence of 195 

Quartz-diorite- porphyry, Little Belt Moun- 
tains, petrographic description of. 518 

western central Idaho, dikes of 86 

Quartz mining, western-central Idaho, 

methods of 114 

Quartz-monzonite, analyses of . . 219-231 

Quartz-porphyry from Yogo Peak, Montana, 

analysis of 523 

petrographic description of 520-524 

Quartz veins, Florence gold-mining district, 

Idaho, description of 235-238 

Quartzite of Neihart district, Montana, oc- 
currence and character of 377 

Queen Esther mine, Neihart district, Mon- 
tana, description of 445-446 

Queen of the Hills mine. Wood River min- 
ing district, Idaho, description of. . . 199 
Queen of the Hills mine, Montana, view of. 4S4 
Queen of the Hills shaft house and ore bins, 

Montana, view of 4g4 

Queen of the Mountains vein, Neihart 
mining district, Montana, descrip- 
tion of 430 

Queen vein, Neihart district, Montana, de- 
scription of 424 

Quercus applegatei n. sp,, from Cascade 

Range 42-43, 64 

Quercus breweri Lx., from Cascade Range, 

notes on 42, 56 

Quercus consimilis Newb., from Cascade 

Range, notes on 44 

Quercus pacifica n. sp., Cascade Range. . 43-44, 54 
Quercus subsinuata n. sp., from Cascade 

Range 41,56 

Quercus ? sp., from Cascade Range 41-42, 54 

R. 

Ransome, F. L., analysis by 484 

Red Cloud mine. Wood River mining dis- 
trict, Idaho, description of 204 



592 



INDEX. 



Page. 
Red Cload vein.^ood River miDing district, 

Idaho, vertical section of S04 

Red Elephant mine, Wood River mining 

district, Idaho, description of 202-203 

section of tl7 

Resciio vein, Warren gold-mininj^ district, 

Idahr., doflcription of 246-247 

Rhyoliic of Idaho gold and Hilver mines, 

alteration ol 177-187 

analyses of altered forms of 170-184 

occurrence and character of 1 lft-120 

Rhyolite- porphyry from Montana, analyses 

of 523 

character of 351,375 

micro-drawing of 516 

petrographic description of 420-524 

Ribbon stmcture, veins of Neihart district, 

Montana, description of 417 

Rioard Mountain, Montana, geologic fea- 
tures of 331-332 

Riverside and other claims, Bohemia region, 

Oregon, features of 30-31 

Rhus mixta Lx., from Cascade R;inge. . . 49, 63, C4 
Robinson, H. H., cited on crystallography of 

barite from Montana 400 

Rock Creek, Montana, exposure of Pinto 

diorito on SS8 

rhyolite-porphyry of i 375 

Rock Creek vein. Neihart district, Montana, 

description of 431 

Rock structure as related to depth, notes 

on 562-563 

Rosenbnaoh, H., analysis by 549 

cited on spheroidal structure in kersan- 

tite 534 

Running Wolf mining district, Montana, 

ore deposits and mines of 450-463 

Running Wolf Creek, Montana, shonkinite 

of 488 

Running Welf Ridge, Montana, geologic 

features of 322-323 

Russell, I. C, cited on features of the Can- 
yon of Snake River 91 

S. 

Sage Creek Mountain, Montana, geologic 

features of 333 

granite-porphyry of 512 

St. Louis mine, I^Teihart district, Montana, 

note on 446 

Salmon River, Idaho, features of 78 

Salmon River Canyon, Idaho, rocks of 87 

view of 86 

Sapphire-bearing diice.Yogo Canyon, Mon- 
tana, figure sliowing section of 4o6 

Sapphiro Coul6e, Togo district, Montana, 

view of 420 

Sapphire luines of Togo, alontana, descrip- 
tion of 454r-459 

map showing location of 454 

views of 450 

Sapphire rock of Togo Gulch, Montana, pe- 
trography of 552-556 

Sapphires irom Togo Gulch, Montana. tl<!ure 

showing markings on 556 



Page. 
Sapphires from Yogo Gulch, Montana, ori* 

gin and character of 653-556 

plate showing forms of 554 

Sawmill Creek, Montana, view showing for- 
est growth on porphyry benches 

near 276 

Schists and gneisses of Little Belt Moun- 
tains, features of 278-279,371-373 

Schmelck, V., analysis by 478 

Schrader, F. C, aid rendered by 75 

Schuchert, Charles, Carboniferous fossils 

identified by 290-291, 292, 296 

cited on fauna of the Wood River forma- 
tion, Idaho 89-90 

Sequoia anguatifolia? Lx., from Cascade 

Range, notes on 89 

Sequoia langsdorfii (Brgt.) Heer, from Cas- 
cade Range, notes on 39 

Sericite, Neihart district, Montana, occur- 
rence and character of 408 

Seven Devils, Idaho, copper deposits of.. 249-253 

plate showing 88 

rocks of 88 

Sheep Creek, Montana, copper mines on. . . 306 

geology of valley of 304-310 

granite-syenite-aplite of 496-497 

minette intruded in limestones of. 307, 572-573 
Sheep Mountain, Montana, geologic features 

of 330-331 

Shonkinite, Yogo Peak, Montana, analyses 

of 484 

character of 318-319 

dikes cutting 319 

petrographic description of 479-488 

plate showing 470 

skeleton magnetite in olivine of 476 

thin section of 486 

Sierra Nevada vein, De Lamar mining dis- 
trict, Idaho, features of 1 45 

Siluro- Devonian rocks and fossils, Montana, 

description of 287-289 

Silver, native, Neihart district, Montana, 

occu rren ce o f 408 

Silver and gold, De Lamar mine, Idaho, pro- 
portions of 129 

Owhyee County, Idaho, production 

of 111-113 

Silver and gold veins of western central 

Idaho, paper on .' 65-256 

Silver City, Idaho, geology of gold and sil- 
ver veins of 107-121 

map sho\vin<; geology of 116 

silver miuen south of 187-189 

view of Iff 

Silver City and De Lamar mining districts, 

Idaho, ago of ore deposits in 165 

depths of richest ores in 165 

fisHuro systems of 164 

gangue minerals of 164-165 

geology of 116-121 

history of 108-110 

mineralogy of veins of r: 166-169 

ores of...'. 163-166,174 

production of 111-113 

processes of gold and silver mining in. 113-116 
Silver Cord shaft, DeLamnr mining district, 

Idaho, features of 154 



INDEX. 



593 



Silver Cord yein, De Lamar mining diatriot, 

Idaho, vertical eectiou of 169 

Silver-lead orea of Neiliart district, Mon* 

tana, description of 406-418 

Silver-lead veins, Idaho mines, character, 

age, and gttneaia of 19S-206, 21&-217 

structure of oreof 214-216 

Sir Walter Soott mine, Running Wolf dis- 
trict, Montana, deeoription of 451-452 

Sluice Box CanyoUf Montana, geolo^io fea- 
tures of 38© 

view of 294 

Smoky Mountain, Montana, geology of. . . 804>306 

Snake River, Idaho, features of 78 

features of canyon of 91-93 

geology of valley of 77 

topography of region north of 77-78 

vegetation and culture in valley of 79 

views of canyon and valley of SO, 90^98 

Snow Creek mines, Keihart mining district, 

Montana, description of 486-439 

South Chariot shaft, De Lamar mining dis- 
trict, Idaho, features of 150 

South Mountain mining district, Idaho, 

mineral deposito of 18a-189 

Spar (ankerite), Montana, analyses of 409 

occurrence and character of 40(M12 

Spokane shale, Montana, description of . . 282>288 
Spring deposits. Silver City and De Lamar 
mining districts, Idaho, occurrence 

of 187 

Squaw Butte, Idaho, Columbia lava at 91 

Star mine, Wood River mining district. 

Idaho, description of 199-200 

Steamboat Mountain, Montana, diorite-por- 

phyry of 516-617 

figure showing soned plagiodaae in 

diorite-porphyry of 516 

geologic features of 333-336 

strata flexed by laccolith of SS5 

Steiger. George, chemical analyses by 82, 107 

Stephanite, Keihart district, Montana, oc- 
currence and character of 406 

Stock rocks of Little Belt Mountains, Mon- 
tana, petrography of 583-568 

relation of laccoliths to 400 

Stokes, H. K., analyses by 179-180, 

409, 467, 478, 484, 509, 510, 580-581 
Storr Peak, Montana, petrograpic desorip- 

tionsof rocksof. 474 

section through 386 

Story claim, Bohemia region, Oregon, fea- 
tures of 25 

Sullivan vein, De Lamar mining district, 

Idaho, features of 145 

Summit vein, De Lamar mining district, 

Idaho, features of 147 

Syenite. Montana, analyses of 466, 467, 468, 478 

petiograpbic character of 863, 466-475 

plate showing 470 

Syenite • diorlte - porphyry, Little Belt 

Mountains, Montana, analyses of. 514. 619 
petrograpbic description of 518-520 

20 GEOL, PT 3 38 



Page. 
Syenite-porphyry, Little Belt Mountains, 

Montana, analyses of 514 

character of 358 

petrograpbic description of 513-515 

T. 

Taylor Peak, Montana, geologic features 

of 341-343 

mineral prospect near 343 

T. C. Power daim, Yogo district, Montana, 

developments of 440-450 

Tenderfoot Creek, Montana, geologic fea- 
tures of valley of 367-360 

Tenderfoot Mountain, Montana, general 

geologic features of 367 

Threeforks shales, Montana, features of . . . 280 

Thunder Mountain, Montana, geologic fea- 
tures of J 364-367 

granite-porphyry of. 506-510 

iron ores of. 461 

section near 318 

sections of 366, S66 

thin section of granite-porphyry of 509 

Tiger Butte, Montana, features of 2/^9-^Q 

granite-porphyry of 510 

Tiger mine, Keihart mining district, Mon- 
tana, description of 445 

Tllllnghast laccolith. Little Belt Mountains, 

Montana, granite porphyry of 512 

Tip Top mine. Wood River mining district, 

Idaho, description of 208 

Tip Top vein, De Lamar mining district, 

Idaho, features of. 145-146 

section of 146 

Trachyte, Little Belt Mountains, Montana, 

petrographic description of 524-526 

occurrence of 361 

Trade Dollar mine, De Lamar district, 

Idaho, description of 140-145 

Trade Dollar mine and mill, Florida Moun- 
tain, Idaho, view of lit 

Trade Dollar vein, De Lamar district, Idaho, 

dlagramsof 142,148,144 

Trade Dollar-Black Jack vein, De Lamar 
mining district, Idaho, mode of ore 
deposition at 166-166 

Trap dikes of Keihart district, Montana, 

occurrence and character of 377 

Triassio rocks, Montana, section of. 297 

Trook and Jennings vein, De Lamar mining 

district, Idaho, features of 155 

Tuffs of Bohemia mining region, Oregon ... 14 

T. W. mine, Keihart mining district, Mon- 
tana, note on 444-445 

U. 

Ulmos oregoniana n. sp., from Cascade 

Rang3 44-45,55 

V. 

Yalencianite firom Idaho, form and compo- 
sition of crystals of 167-168 

plate showing crystals of 170 

Vanadinite, figures showing crystals of 452 



594 



INDEX. 



Page. 
VarioUtic forms of miuette, Little Belt 

Mountains, petrographio characters 

of 532>585 

Vein systems, Wood River mining district, 

Idaho, description of 212 

Yeina of Bohemia mining region, Oregon.. 15-19 
Veins of Xeihart district, Montana, descrip- 

tionof. 418-422 

secondary enrichment of 421-427 

Veins of silyer-lead, Idaho mines, age and 

genesis of 217 

Veins and fissures of western-central Idaho, 

general featores of 101-106 

Vesuvins claim. Bohemia region, Oregon, 

features of 24-2S 

Vogesite, Little Belt Mountains, Montana, 

intrusive sheets cf 352 

petrographio description of 54 1-542 

Volcano Valley, Montana, fault in 806 

Vulte, H. T., analyses hy 545 

W. 

Walcott, C. D., Carboniferous fossils fbom 

Montana identified by 296-296 

formations of Belt terrane, Montana, as 

determined by 281 

reconnaissance of Little Belt Mountains 

by 280 

Wall Street claim, Bohemia region, Oregon, 

features of 24 

War Eagle Mountain, Idaho,' general plan 

of 148 

veins and ores of 147-157, 161-163, 173 

Warren, Idaho, placer deposits near 240-242 

map showing mineral deposits near — t40 

view of SS8 

Warren gold mining district, Idaho, loca- 
tion, history, geology, and mineral 

deposits of. 237-249 

Waverly vein, Florence gold mining dis- 
trict, Idaho, features of 287 

Weatherwax claim, Togo mining district, 

Montana, note on 449 

Weatherwax Creek, Montana, geology of re- 
gion near 812,814 

intrusive rocks of 515 

Webster vein, I>e Lamar mining district, 

Idaho, features of 146-147 

Wee<l, W. H., paper on geology of Little 

Belt Mountains, Montana, by 257-461 

Whippoorwill mine, Keihart mining dis- 
trict, Montana, note on 440 

view of 44« 

White Ghost claim, Bohemia region, Ore- 
gon, features of 23-24 

White Swan claim, Bohemia region, Oregon, 

featuresof 19-20 

White Wings and other claims, Bohemia 

region, Oregon, features of 30 

Williams Mountain, Montana, geologic fea- 
tures of 809-310 

Wolf Butte, Montana, geologic features 

of 341-343 

granite-porphyry of 496-601 



PagSb 
Wolf Creek, Montana, geologic features of 

region near 388 

Wolf porphyry, Montana, characters of. . 858-860 

occurrence of 351 

Woisey Mountain, Montans, geologic fea- 
tures of 310 

Woisey shale, Montana, description of 285 

sectionof 340 

Wood River formation, Idaho, rocks and 

fossils of 89-80,198-196 

Wood River mining district, Idaho, geog- 
raphy, geology, and mines of 190-281 

gold-qnartz veins of 207-209 

precious metal production of 192 

silver-lead veins in granite of 206-207 

vein systems and mineral.deposits of. Sll-318 

Wood River Valley, Idaho, view of iflf 

Woodhnrst iron mines, Montana, descrip- 
tion of 460 

Woodhnrst limestone, Mcmtana, features 

and fossils of 291-S 

sectionof 829,36S 

Woodhnrst Mountain, Montana, intruded 

stock of 817 

Woodhurst-Mortson mine, Running Wolf 

district, Montana, description of. . . 451 

viewof 468 

Wright and Edwards mine, Keihart mining 

district, Montuia, description of. 448-444 
syenite exposed in 854 



T. 



Yankee Girl claim, Running Wolf district, 

Montana, notes on 452-458 

Yogo, Montana, iron ore deposits near 460 

sapphires of 564,568 

sapphire mines of. 454-468 

section near 828-880 

Yogo Creek, Montana, geologic features of 

valley of. 328-331 

minesof 447-450 

Yogo Gulch, Montana, markings on sap- 
phires flrom 666 

petrography of dike rock in 550-551 

petrography of sapphire-bearing rock 

of 552-556 

Yogo intrusive stock. Little Belt Moun- 
tains, Montana, description of 396-399 

Yogo limestone, Montana, description of . . 

sectionsof 839^0,868,: 

Yogo district, Montana, contact meta- 

morphismin 822-828 

descriptions of minesof 447-450 

descriptive geology of 817-385 

map showing location of sapphire mines 

of 464 

relations of intruded and stratified rocks 

in 822 

Sapphire Coul6e in 1 4t0 

sapphire minesof 456 

Yogo Peak, Montana, chemical composition 

of rocks of 565 

composition of magmas of rocks of. . . 669^76 



INDICX. 



595 



Yoga Peak, Montan*, diagram showing 

aplite dikeleu in rocks of 494 

diagram showing section through S18 

diagram showing differentiation of aotd 

and basic rocks at 664 

diagram showing rariationa of minerals 

inrocksof M8 

geologic features of 8]7~322 

granite-porphyry of 502-S04 

igneous rooks of 470 

m ineral components of rooks of M7 



Yogo PealL, Mootana, moleoalar ptop s t U eaa 

of lime and magneaia ia rsoka of. . . 607 
petrographic descriptioa of roeks of. . . 471- 

4m,4»-4»i 
view of Sl» 

Z. 

Zinc bleude, Nelbart dlstriei, Mositaaa, de- 
scription of 406-407 

crystal of 411 

Zoned plagioclase in diorlte-potphyfy biA 



o