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DEPARTMENT OF THE INTERIOR
MONOGRAPHS
OF THE
United States Geological Survey
VOLUME XXXII F^RT II
WASHINGTON
GOVERNMENT PRINTING OFFICE
1899
UNITED STATES GEOLOGICAL SURVEY
CHARLKS II. WALCOTT, DIRKCTOK
GEOLOGY
YELLOWSTONE NATIONAL PARK
P^RT II
DESCRIPTIVE GEOLOGY, PETROGRAPHY, AND PALEONTOLOGY
ARNOLD HAGUE, J. P. IDDINGS, W. H. WEED
C. D. WALCOTT, G. H. 6IRTY, T. W. STANTON, AND F. H. KNOWLTON
V.
WASHINGTON
GOVERNMENT PRINTING OFFICE 1899
^510
CONTENTS,
Page.
Letter of transmittal xiii
OUTLINK XV
Chapter I. — Descriptive geology of the Gallatin Mountains, by J. P. IcUlings and W. H.
Weed 1
Introduction 1
The Crags and vicinity 3
The Crags 3
Crowfoot section 6
Mountains south of Panther Creek 9
South End Hills 10
Trilobite Point 11
The Dome 12
Indian Creek laccolith 13
Mount Holmes by sraallth 16
Antler Peak 20
Three River Peak 23
Bighorn Pass 24
Crowfoot Ridge and Gallatin Valley 27
Quadrant Mountain, Bannock Peak, and the valley of the Gallatin Eiver 31
Banuock Peak 31
Quadrant Mountain 33
Little Quadrant Mountain and Fawn Creek Valley 36
Little Quadrant Mountain 36
Fawn Creek Valley 39
Region north of Gallatin River 41
The Fan 45
Electric Peak 50
Western flanks of the Gallatin Range 56
Eastern flank of the Madison Range 57
Chapter II. — The intrlsive rocks of the Gallatin Mountains, Bunsen Peak, and Mount
Everts, by J. P. Iddings 60
Indian Creek laccolith 60
Hornblende-mica-andesite-porphy ry 60
Mount Holmes bysmalith 64
Dacite-porphyry , 64
Bighorn Pass sheet 69
Kersantite 69
Gray Mountain mass and connected sheets 73
Hornblende-mica-andesito porphyry and andesite 73
Hornbleude-andesite-porphy ry and andesite 77
Hornblende-pyroxene-andesite-porphyries and andesites 80
Chemical composition 81
V
VI CONTENTS.
Chapter II — Continued. Page.
Ditterentiated sheet southeast of Electric Peak 82
Gallatin River laccolith 84
Dacite-porphy ry 84
Intrusive sheets in Mount Everts 85
The Bunsen Peak mass 86
Daeite-porphy ry 86
Chapter III. — The igneous rocks of Electric Peak and Sepulchre Mountain, hy J. P.
Iddings 89
Geological sketch of the region 89
The intrusive rocks in Electric Peak 92
The dike rock.s and certain contact forms of the stock 94
The stock rocks and apophyses 97
Varieties in which the dark-colored and light-colored minerals are nearly equal 99
Varieties in which the light-colored minerals are in excess, hut in which quartz is
not excessive 102
Varieties with an excess of light-colored minerals, in which quartz is abundant 103
Quartz-mica-diorite-porphyry 103
General consideration of the mineral and chemical composition of the intrusive rocks
in Electric Peak 105
Mineral composition 105
Chemical composition 115
The volcanic rocks of Sepulchre Mountain 121
The lower breccia 121
The upper breccia 122
The dike rooks 128
General consideration of the mineral and chemical composition of the eruptive rocks
of Sepulchre Mountain 134
Miueral composition 134
Chemical composition 135
The extrusive igneous rocks west and southwest of the Gallatin Mountains 137
Comparison of the rocks from Electric Peak and Sepulchre Mountain 138
Correlation of the rocks on a chemical basis 142
Chapter IV. — Descriptive geology op the northern end of the Teton Range, by J. P.
Iddings and W. H. Weed 149
Introduction 149
Topographic features 151
Crystalline axis aud region east 152
Region west of the crystalline axis 157
Chapter V. — Descriptive geology of Huckleberry Mountain and Big Game Ridge, by
Arnold Hague 165
General features 165
Region of Wildcat Peak and Huckleberry Mountain 168
Region of Snake River gorge 173
Region between Red and Basin creeks 175
Snake River Hot Springs 177
Region of Coulter Creek and Bobcat Ridge 179
Region of Wolverine Creek 181
Region of Pinyon Peak 184
Big Game Ridge '. 188
Chicken Ridge 191
CONTENTS. VII
ClUPTF.n V — Continued. Page.
Outlet t'iiiiyoii 194
Clianui'l Miiuntaiii 196
Flat Mouutaiii 196
West Base of Two Ocean Plateau 197
Two Ocean Plateau 200
ChaI'TKR VI.— (JkoI.OGY of the 80UTHEHN END OF THE Snowy Range, by W. H. Weed 203
(ieueial description 203
Topography 201
Sedimentary rocks 20.5
Buffalo Plateau 206
Lamar Valley 207
Slough Creek 208
Soda Butte Creek 210
Pebble Creek 211
Soda Butte Valley 212
CiiArxER VII.— The dissected volcano of Crandall Basin, Wyoming, by J. P. Iddings.. 21,5
Introduction 215
Geological description 216
General features 216
Early acid breccia 219
Basic breccia and flows 220
Distinctly bedded breccia 221
Chaotic breccia 222
Dikes 224
Extent of erosion 232
Petrography of the rocks of the district 237
Early acid breccia 237
Basic breccia and lava flows 238
Basalt flows 239
Intrusive rocks 240
Outlyiug dikes 240
Granular core and intersecting dikes 246
Mineral and chemical variations of rocks 259
Crystallization 265
Development of pheuocrysts 266
Chapter VIII. — The igneous rocks of the Absaroka Range and Two Ocean Plateau
and op outlying portions of the Yellowstone National Park, by J. P. Iddings 269
Introduction 269
Early acid breccia 270
Early basic breccia and associated basaltic flows 275
Late acid breccia 281
Late basic breccia 296
Dikes and surficial flows 304
Vicinity of Sylvan Pass 304
Dikes south and southeast of Sylvan Pass 311
Massive flows and intrusions of light-colored andesite 314
Trachy tic rhyolite 321
Chapter IX. — Absarokitk-shoshonite-banakite series, by J. P. Iddings 326
Introduction 326
Absarokite 328
VIII CONTENTS.
Chapter IX— Continued. Page.
Shoahonite 339
Banakite 347
Similar rocks in Montana 351
Chapter X.— The khyoi.ites, Ijy J. P. Iddings 356
Introduction 356
Megascopical cliaracters 357
Vicinity of the Mammotli Hot Springs 357
Obsidian Cliff 359
Canyons of Gibbon River and Madison River 366
Madison Plateau north of the Lower Geyser Basin 367
Vicinity of the Lovrer Geyser Basin 369
Upper Geyser Basin 372
Madison Plateau south of the Geyser basins 374
Bechler Canyon 375
Falls River Basin 377
Pitchstone Plateau 379
Red Mountains 381
Vicinity of Yellowstone Lake 382
Natural Bridge 386
North and east of Yello wstoue Lake 387
Vicinity of Yellowstone River 388
Vicinity of the Grand Cauyon of the Yellowstone 389
Northeastern corner of Yellowstone Park 391
Microscopical characters of the rhyolite 393
Phenocrysts 394
Quartz 395
Sanidiue 398
Plagioclase 399
Pyroxene 399
Magnetite and titaniferous iron oxide 400
Zircon ^01
Pseudobrookite *01
AUanite and apatite 402
Groundmass 4"2
Glasses free or almost free from microlites 403
Globulitic glass 406
Microlitic glass 408
Forms of growth of microscopic crystals 410
Lithophysie ^^°
Microgr.anular structure - 4—
Relations of the various microstructures to one another in the rock mass 423
Lamination and banding 41.4
Variations in composition among the rhyolites 427
Intermingled rhyolite and basalt 430
Chapter XL— Recent basalts, by J. P. Iddings 433
Ophitic basalt "136
Basalts related to those with ophitic structure 437
A'ery fiue-grained bas.alts with minute phenocryts - 439
Chapter XIL— Paleozoic fossils 440
Section I.— Cambrian fossils, by C. D. VValcott 440
Section II.— Devonian and Carboniferous fossils, by G. H. Girty 479
CONTENTS. IX
Pago.
Chaptku XIII. — Mksozoic Fds.siLS, by T. W. Stanton 600
Chai'Ter XIV. — Fossil, I'l.oitA, by F. H. Knowlton ''51
Historical sunimary *'51
Enumeration anil description of fossil plants from the Laramie 6.^5
Discussion of Laramie flora ''"3
Enumeration and duscription of fossil plants from the Tertiary 665
Plants, exclusive of fossil wood ''65
Fossil forests .- ^^5
Biological consideration of the Tertiary flora 773
Geological consideration of the Tertiary flora 783
Indkx *^83
ILLUSTRATIONS.
Page.
Plate I. Mountains north of Mount Holmes 4
II. Paleozoic section, Crowfoot Ridge 8
III. Geological cross sections of Gallatin Range 12
IV. Panoramic view of Gallatin Range from Norris Geyser Basin 18
V. Cross sections showing Mount Holmes bysmalith 18
VI. Antler Peak from valley 22
VII. Three River Peak from Gallatin Valley 24
VIII. Bannock Peak from Panther Creek Valley 32
IX . Geological cross sections 50
X. Geological map of Gallatin Range 56
XI. Photomicrographs of audesiteporphyry and. dacite-porphyry 62
XII. View of Echo Peak 68
XIII. Electric Peiik from Sepulchre Mountain 90
XIV. Head of East Gulch of Electric Peak 90
XV. Sepulchre Mountain from its northwest spur 96
XVI. Geological map of Electric Peak and Sepulchre Mountain 96
XVII. Diorites 100
XVIII. Granite and diorite-porphy ry 100
XIX. Photomicrographs of andesite-porphyry and diorite 104
XX. Photomicrographs of diorite and diorite-porphyry 104
XXI. Photomicrographs of diorite-porphyry and dacite 104
XXII. Photomicrographs of pyroxene-andesite and dacite 130
XXIII. Map of the northern end of the Teton Range 150
XXIV. Snake River Hot Springs 178
XXV. Outlet Canyon 194
XXVI. BarouettPeak 204
XXVII. Geological map showing dissected volcano of Crandall Basin, Wyoming 216
XXVIII. Index Peak 218
XXIX. The Thunderer and Mount Norris 222
XXX. Koodoos 222
XXXI. Hurricane Ridge 226
XXXIl. Geological cross sections 232
XXXIII. Photomicrographs of gabbro, diorite, and granitic aplite 250
XXXIV. Photomicrographs of monzouite, diorite, and basalt 250
XXXV. Eagle Peak 296
XXXVI. Photomicrographs of absarokite 332
XXXVII. Photomicrographs of shoshonite and diorite-porphyry 344
XXXVIII. Photomicrographs of banakite, quartz-banakite, and andesite 350
XXXIX. Obsidian Cliff columns 360
XL. Top of columns 360
XLI. Lithophysie 364
XI
XII ILLUSTRATIONS.
Page.
Plate XLII. Fissile litboidal rhyolite, Obsidian Cliff 364
XLIII. Lithophys:^ 364
XLIV. Columnar rhyolite 364
XLV. Columnar Cliff, Madison Canyon 368
XLVI. Banded perlite 370
XLVII. Perlite with spherulites 370
XLVIII. Natural Bridge 386
XLIX. Natural Bridge, vertical plates 386
L. Photomicrographs of rhyolitiu glasses 406
LI. Photomicrographs of rhyolitio glasses 406
LII. Spherulites 410
LIU, Spherulites and feldspar needles 414
LIV. Photomicrographs of micrographic phenocrysts and spherulites 414
LV. Photomicrographs of spherulitic structures 414
LVI. Photomicrographs of spherulitic structures and feldspar microlites 422
LVII. Diagrams of lithophys;e 422
LVIII. Columnar structure 436
LIX. Photomicrographs of basalt '. 436
LX-LXV. Cambrian fossils 468-478
LXVI-LXXI. Devonian and Carboniferous fossils 580-598
LXXII-LXXVI. Mesozoic fossils 642-650
LXXVII-CXXI. Fossil flora 794-882
Fig. 1. Diagram showing variation in silica percentages of rocks from Electric Peak 117
2. Diagram showing molecular variation of the rocks at Electric Peak 119
3. Diagram showing molecular variation of the rocks of Sepulchre Mountain 136
4. Sections of spherulites with projecting prisms of orthoclase and a crescent-shaped belt
free from granulation 413
LETTER OF TRANSMITTAL,
Department of the Interior,
United States Geological Survey,
Washington, D. C, June 30, 1896. Sir: 1 have the honor to transmit herewith the manuscript of Part II of a monograph on the Geology of the Yellowstone National Park. It embraces chapters on the descriptive geology of the mountains surrounding the Park Plateau, by myself and colleagues; elaborate investigations of the petrography of the crystalline rocks, by Prof. J. P. Iddings; reports upon the invertebrate paleontology of the Park and the Absaroka Range, by Messrs. C. D. Walcott, George H. Girty, and T. W. Stanton; and an exhaustive study of the fossil flora of the region, by Mr. F. H. Knowlton Very respectfully,
Arnold Hagie,
Geologist in Charge. Hon. Charles D. Walcott,
Director United States Geological Survey.
XIII
OUTLINM: OF THIS VOLUME.
CiiArTEK I. The (iallatin Mountains, extending IK miles within the boundary of the Yellowstone National Talk, consist of sedimentary strata ranging from the Cambrian, through the Silurian, Devonian, Carboniferous, and Juratrias, to the Laramie of the Cretaceous. These sedimentary rocks have been uplifted by forces acting from the southwest. They dip northeast, and have been folded to a slight extent transverse to the strike. .Subseqnently they have been strongly faulted. The dis- location at the close of the Laramie was accompanied by intrusions of igneous magmas in several large lacoolithio bodies and in numerous sheets, and in the vicinity of Electric Peak by dikes. Erosion has uncovered crystalline schists at the southern and southwestern end of the range, and has laid baie exposures of all the sedimentary and igneons rocks. Finally, glaciation has modified the topography in a striking manner. The structural relations of the sedimentary and igneous rocks are illustrated by a number of geological sections.
Chapter II. Thi.s chapter treats almost exclusively of the intrusive rocks of the Gallatin Moun- tains. They are mainly line-grained and aphanitic masses, in most occurrences porphyritic and andesitic in character. The large bodies differ from one another .somewhat in composition, and vary slightly in texture, in diH'erent parts of the rock bodies. In one intrusive sheet there has been a pro- nounced dlft'erentiation by the settling of phenocrysts of augite.
Chapter III. Electric Peak aud Sepulchre Mountain are described as parts of a Tertiary volcano ■which were faulted across the conduit, the amount of vertical displacement having been more than 5,000 feet. The deeper i)ortion.s of the mountains, consisting of sedimentary strata intersected by dikes, sheets, and the stock or conduit of the volcano, have been brought to the surface, as shown in the mass of Electric Peak. The ejected breccias and lava flows, together with the upper portion of the conduit, constitute Sepulchre Mountain. Lavas which are andesites in the latter mass are diorites and porphyries in the former. Rocks with like chemical composition are found to have different mineral composition according as they are crystallized into phanerocrystalline diorites or into apha- nitic andesites.
Chapter IV. The northern end of the Teton Range extends but a short distance within the Yellow- stone National Park. It consists of a nucleus of crystalline schists aud gneisses overlain by Paleozoic and Mesozoie strata flexed in an anticline witli northward-dipping axis and faulted to a slight extent. Birch Hills, a few miles to the north, are an outlier of the range. Upon greatly eroded strata basic breccias were thrown out, and after these had undergone fresh erosion vast flows of rhyolite covered the country and now form a part of the plateau of the Park, beneath which the northern extremity of the Teton Range is hidden.
Chapter V. The country described in this chapter embraces a mountainous area irregular in outline and of great diversity of form. It is situated in the southern part of the Park and the Yel. lowstone Park Forest Reservation. It consists of a number of ridges trending northwesterly and southeasterly, formed for the most part of Mesozoie rocks. The older sedimentary rocks are exposed but the ridges are essentially made up of sandstones of Cretaceous age. The irregular outline of the mountains is due to the rhyolites of the Park Plateau that abut against the slopes of the upturned beds. The principal physical features of the region are Wildcat Peak and Huckleberry Mountain, Bobcat Ridge, Big Game Ridge, Chicken Ridge, Two Ocean Plateau, and the gorge of Snake River. West of Huckleberry Mountain occur several exposures of dacite surrounded by rhyolite. They are among the few outcrops of dacite known in the Park, and are apparently older than the rhyolite. In the gorge of Snake River the Madison limestones, Teton sandstones, and the Ellis limestones and
XVI OUTLINE OF THIS VOLUME.
shales are well shown. The Snake River hot springs are situated near the contact of the rhyolite with the Carboniferous limestone, the lime of the travertine being derived from the Madison lime- stones. The incrustations around the springs resemble the travertine deposits found at the Mammoth Hot Springs. The characteristic and limited Wolverine flora, of Laramie age, occurs near the base of Pinyon Peak. The conglomerate of Pinyon Peak, a striking physical feature of the region, is described as overlying unconformably the Laramie sandstones, and evidence is given showing that the conglomerate probably belongs to Eocene time, as it iinderlies the basic breccia of the Absaroka Range. The impressive gorge of Outlet Canyon cuts a deep passage completely through Chicken Ridge. The interesting feature of the canyon is that it at one time served as the discharge for the waters of Yellowstone Lake. This sheet of water, which now flows northward and drains to the Atlantic through Yellowstone Canyon, formerly discharged into Snake River and thence to the Pacific. Two Ocean Plateau shuts in the sedimentary ridges on the east. The plateau, which rises 10,000 feet above sea level, forms a part of the Absaroka range and Is made up of similar volcanic breccias
and silts.
Chaptbk VI. The extreme southern end of the Snowy Kange forms the northeast corner of the
Park. The crystalline core of the range forms a broad, plateau-like summit, bordered by sedimentary rocks of Paleozoic age, which along the south slope dip gently away from it toward the Park. The highest peaks, together with extensive areas, are formed of andesitic breccias, but erosion has cut through them and exposed the underlying limestones, showing that the volcanic rocks rest upon a very uneven anil rugged surface. Detailed sections of the Paleozoic sedimentary rocks from the Flat- head formation to the Madison limestone are given, but the igneous rocks are described in other
chapters.
Chapter VII. The Miocene volcano of Crandall Basin built itself upon a ridge of eroded Pale- ozoic rocks which dip toward the southwest from the crystalline schists of the Beartooth Range. Beneath the volcano are remnants of Eocene breccias and lava flows. The volcano consisted of basic andesitic breccias topped by basalt flows and traversed by dikes that radiated from the stock or core which was the conduit beneath the crater. While bedded breccias characterize the outer portions of the volcano, chaotic unbedded breccias form the central portion. Comparison with modern active volcanoes indicates that the Crandall volcano rose to about 13,400 feet above its limestone floor. The phanerocrystalline rocks within the core are gabbros and diorites, approaching monzonites in part, and are chemically like the basalts and andesites of the breccias, dikes, and flows, but diifer from them in mineral composition. They also are parts of the volcano and are properly volcanic rocks.
Chapter VIII. The Absaroka Range consists largely of volcanic breccias, with subordinate amounts of massive flows or intrusive bodies. This chapter presents a petrographic treatment of those io-neous rocks which lie withiu the limits of the Yellowstone Park, and their discussion is confined to an account of their field occurrence and distribution and a systematic description of their mineralogical characteristics and composition. The earliest accumulations occur at the northern end of the range and are made up of early acid breccias found in disconnected remnants beneath early basic breccias. They consist mainly of hornblende-andesite and hornblende-mica-andesite. The early basic breccias are pyroxene-andesite, passing upward into the massive basalt flows. Upon the latter were thrown the late acid breccias, similar in composition and appearance to the early acid breccia. This passes upward into late basic breccia, consisting of basic andesites with less basalt than is associated with the early basic breccia. The late basic breccia forms the southern portion of the range within the Yellowstone Park and also Two Ocean Plateau. At Sylvan Pass and in its vicinity it is traversed by dikes of andesite and a few of diorite. Remnants of surficial flows of massive andesite form the summits of Mount Stevenson, Mount Doane, Colter Peak, and several prominent mountains south of Sylvan Pass.
OUTLINE OK THUS VOLUME. XVII
t'llAPTEl! IX. Certain b.isultic Mini otluT rocks assiiriatfd with the andesitic hrecrias and basalt Hows have a considerable content of iiitlio(Mase in niicrnscci))!!' crystals, and a comparatively high percentage oC potash. They occnr as lava Hows and as dikes in v.ariiiiis localities within the I'ark. According to their ('honiical and niiiicral composition tliey have been classed as absarokites, shoshonitcs, and banakites.
CllAl'TF.R X. The rbVoIites of the I'aiU are almost wholly extrnsivc Lavas of very niiiform compo- sition, bnt having a wide range of color, texture, and mog.ascopic habit. Tlic appcar.ancc of the rhy- olito ill the field, and tlio microscopical cliarnctcristics of pbcnocrysts, spheriilites, litliophys:c, and groniidmass, are dencrilied in detail. The dill'ciont modidcations of cryst.allization, besides the lami- nation and formation of pnniicc, are referable to lieteiogeiieity of the molten magma, especially with reference to the amonnt of vapor contained in it. Examples of intermingled b.asalt and rhyolite are de.scribed, in which the basalt appears to have been inclosed and partly melted by the rhyolite.
CiiAPTEU XI. The recent biisalts overlie the rhyolite in most inst.anccs, but .are found beneath it, and also between older and younger sheets of rhyolite in several localities. These basalts arc distin- guished from those .associated with the early .and Kate basic breccias by being ophitic .and iionpor- pbyritic for the most p.art.
Chapter XII. This chapter describes the Paleozoic fossils known to occur in the Yellowstone Niitional Park and the Absaroka Range. It is divided into two sections, the first treating of the Cam- brian species and the second of the Devonian .and Carboniferous species. Both Flathead and GalKatiu formaticms have yielded a sm.all but characteristic fauna. From the Cambrian 21 species in all have been obtained, several of which are new to science .and described here for the first time. No fossils of undoubted Silurian age have been obtained, altliongh the beds carry imperfect and partially oblit- erated organic forms. The Three Forks limestone has furnished a well-recognized Devonian fauna. From the Madison limestone a varied fauna has been collected, but belonging wholly to the Lower Carboniferous period.
CllAPTKU XIII. The Mesozoic fossils obtained from the Yellowstone N.atioual Park were found in the G.all.atin Range near Electric Peak, Teton Range, in the neighborhood of Wildcat Peak .and Huckleberry Mountain, and from the Cret.accous ridges in the southern end of the Park and Yellow- stone Forest Reserve. The Mesozoic str.ata have yielded 78 species of invertebrates, of which one is from beds supposed to be of Triassic age, 4G are .Turassie, and 31 are Cretaceous. The fossils obtained were mainly from the Ellis form.atiou of the .Jura and the Colorado of the Cretaceous. The .Jnr.assic fossils form much the largest and most prominent part of this collection, and in number of species it compares favorably to the .Jur.assic of other parts of the Rocky Mount.ains.
Chapter XIV. The Mesozoic fo.ssil llora of the Yellowstone National Park is confined to the Laramie sandstones of the Cretaceous .and is found on Mount Everts, near M.ammoth Hot Springs, and at the base of Pinyou Peak near the head of Wolverine Creek. The llora from this Latter locality has been designated the Wolverine Creek llora. The Tertiary llora is very v.aried and possesses great biological interest. It is a rich llora, and on comparing it with the living llora it becomes app.areut th.at great clim.atic ch.anges must have taken place since the do.se of Miocene time to have m.ade these changes in pl.ant life possible. It is found at numerous localities associated with the breccias and silts of the igneous rocks of the Absaroka Range. It is found in the early acid breccias, in the early basic breccias, in the late acid breccias, and in the l.ate basic lireccias, where the muds and silts furnish a soil favorable for a vegetable growth. The most interesting locality as reg.ards number of species and mode of occurrence is the well-known Fossil Forest of .Specimen Ridge. The Terti.ary fossil llora embraces about 150 forms th.at have been distrrbuted among .S3 natural orders. This fossil flora is illustrated by forty-five plates. . MON XXXII, PT II II
GEOLOGY OF THE YELLOWSTONE NATIONAL PARK, PART IL
By ARNOLD HAGUE AND OTHERS.
CHAPTER I. DESCRIPTIVE GEOLOGY OF THE GALLATIN MOUNTAINS.
By Joseph Paxson Iddings aud Walter Harvey Weed.
I^TTRODUCTION.
The Gallatiu Mountains form a range of peaks and ridges extending southward for 63 miles from the vicinity of Bozeman, on the line of the Northern Pacific Railroad, about latitude 45° 40'. The range lies between the Yellowstone and Gallatin rivers and terminates in the neighborhood of Mount Holmes, at about latitude 44° 45'. The southernmost 18 miles of the range lies within the boundary of the Yellowstone National Park aud forms that portion of it described in the present chapter. The northern portion falls within the region described in folios 1 and 24 of the Geologic Atlas of the United States.^
Within the Park boundary the peaks of the main chain reach altitudes of from 10,000 to 10,500 feet, and at Electric Peak 11,100 feet, and stretch from Electric Peak, which is situated directly on the northern boundary line, southward to Mount Holmes. The country has been deeply cut by erosion, and is drained by tributaries of the Yellowstone, Gallatin, and
I Geologic Atlas U. S., folio 1, Livingston, Mout., 1893 ; and folio 24, Three Forks, Mont., 1896. MON XXXII, PT II 1 1
2 GEOLOGY OF THE YELLOWSTOInE NATIONAL PARK.
Madison rivers, the watersheds between which meet one another in Tln-ee River Peak. The special description of the physiographic features of the region, however, inchiding the glaciation, will be found in Part I of this report, where it is treated by Mr. Hague. Without entering into a topo- graphic description of the Gallatin Mountains, it will be in place here to call attention to the fact that the region in question, within the Park boundary, is a block of country delimited on the east and on the west by profound faults trending nearly north and south, the western fault line having a somewhat northeasterly trend. This block, about 7 miles wide, is bounded on the south by a capping of lavas, which borders it also to some extent on the east and on the west. The northern end of the block lies beyond the Park boundary, in the neighborhood of Cinnabar Mountain. The block is wider at the south, and narrows northward. It is a wedge- shaped mass cut diagonally across a synclinal trough, with one long and one very short limb. The latter appears for only a short distance at the northern end, in Cinnabar Mountain. Within the area of the Park the block has the structure of a monocline, dipping northeast across the longer diameter of the block. Minor faults and folds modify the structure some- what and introduce local complications, which will be described in detail.
As a result of the dipping of the block to the northeast, the oldest formations are found at the southern and southwestern ends, and the youngest formations at the northern. The rocks are well exposed, the succession of the strata is clearly made out, and the form and character of the igneous material that has been forced through the sedimentary rocks are readily observed. The study of the igneous bodies and their relations to the geological structure of the block proves that the dynamic history of this particular area was complex, and extended over a long period after the deposition of the coal-bearing Laramie sandstones. In fact, a succession of dislocations must have followed one another through the greater part of the Tertiary period. This will aj^pear from the description which follows.
Erosion has carved deeply the surface of this upturned, fractured, and distorted block, grooving it with valleys and gulches, the eastern system trending northeast and east and draining with the dip of the strata, the western system trending and draining northwest along the general line of the strike and being in all probability controlled by lines of fracture in this direction. The intervening elevations rise abruptly to sharp peaks
THE CRAGS. 3
and ridges, attaining altitudes of from 10,000 to 11,000 feet above sea level, with occasional plateaus, 2,000 feet or more above the valley bottom.
The bold escarpment and barren upper portions of these mountains permit their general structure to be made out with ease, even from a distance. Thus the general structure of the eastern face of the range may be seen from Bunsen Peak or Ten-ace Mountain. The open, park-like character of the valleys and lower slopes of the mountains, the abundance of grass and water, and the multitude of flowers that cover the whole country during the summer season render this one of the most picturesque and delightful of mountain regions, both for the geologist and for the artist.
THE CRAGS AISTP VICINITY.
A description of the geological features of the Gallatin Range naturally begins with an account of the region where the basal and lowest rocks of the series are exposed. These occur in the southern and southwestern part of the range, and a descrijDtion of the range from these peaks northward is, in general, also a description of successively later geological formations. The oldest rocks of the region are crystalline schists, which are mainly gneisses. These rocks form two prominent topographic features of the southern end of the range. The first of these is the group of rugged peaks called The Crags, together with their less elevated spurs to the south, and their prolongation in the ridge trending northwest, parallel to Grayling Creek. Crowfoot Ridge constitutes the second prominent mass of crystal- line schists, while the low rounded hills at the head of Grayling Creek are also formed of these rocks. A few inconspicuous outlying exposures of schist occur to the southwest, where erosion has removed the overlying sheet of rhyolitic lava.
THE CRAGS.
The rocky summits of The Crags and the ridges northwest are very rugged and difficult to traverse on account of the loose ddbris and thick- timbered slopes. But while the southern escarpment of Crowfoot Ridge is equally obstructed, its summit is comparatively open and level topped, showing little erosion since the removal of the sedimentary cover. The lower hills between Grayling and Maple creeks are rounded and smoothed, with every evidence of having been glaciated and considerably worn. Tlnroughout this area of crystalline schists, coarse and fine grained gneisses
4 GEOLOGY OP THE YELLOWSTONE NATIONAL PARK.
alternate with one another, the coarser varieties being generally light- colored mica-gneisses rich in feldspar and quartz. The finer-grained, dark-colored varieties are for the most part mica-gneisses richer in biotite. Mica-schists, sometimes highly garnetiferous, occur in smaller quantities, and amphibolites are also found. The pronounced lamination or schistositv of the whole body of these rocks is quite uniform in its jiosition, the layers standing at high angles or nearly vertical, with a general north-south trend. The microscopical study of these rocks shows them to be normal crystalline schists, having the microstructure of highly metamorphosed rocks and exhibiting no traces of their previous character. Their study in the field was not thorough enough to throw any light on the question of their possible origin.
South and west of The Crags the crystalline schists are directly overlain by volcanic breccia and tuffs of andesites, whose subaerial accumulation is beyond question. These rocks are generally dark colored, and occur in rugged outcrops and rough, angular talus blocks. In general, the easterly slopes are smooth, covered with soil, and less steep than the western sides of the hills. The andesites are variegated in color and are chiefly hornblende-andesite, carrying some pyroxene and a little biotite. The occurrence of these subaerial breccias shows that at the time of their eruption the crystalline schists were exposed sui-face rocks which had undergone extensive erosion, by which they had acquired a pronounced mountainous topography.
On the west and south the schists pass under massive rhyolitic lava, which is part of the great plateau lavas farther south, and whose position with respect to the crystalline schists and andesitic breccias is such as to show that the rhyolitic lava flooded the lower levels of this gneissic region after the andesitic material had been accumulated and had been partly removed by erosion. That the rhyolite overlies the andesitic breccia is clearly shown in the walls of Maple Creek Canyon. The rhyolite also fills the valley bottom between two ridges of andesitic breccia in this vicinity, indicating- the extent to which the andesite had been previously eroded.
There is marked contrast in the scenery and topography of the gneissic areas and of the country formed by the rhyolite, the former being essentially rugged and broken, while the latter is as yet comparatively little affected by erosion, the streams flowing in trenches and canyons cut
THE CKAGS. 5
in the soft rock. Often the boundary between gneiss and rhyolite is defined by small drainage ways woni along the contact.
Along the western side of the gneissic area the rhyolitic lava rises to altitudes of 8,000 to 8,200 feet, while at the head of Maple Creek it rises to 8,700 feet, the level of the divide between this creek and Grayling Creek. From this it seems highly probable that the lava flooded the valley of Grayling Creek at the time of its eruption, and has since been removed by erosion. The absence of au)^ remnant of rhyolite within this valley, as the map represents, is not based on a careful examination of the valley, but expresses our ignorance in respect to its occurrence there.
The marked contrast between the topographic character of the southern side of Crowfoot Ridge and that of the northern side is note- worthy. The southern slopes are almost free from lateral spurs of any size and the ridge is approximately straight. On the north, spurs branch off at short intervals, increasing in size toward the west until they attain the proportions of mountain ridges. The tlu-ee most prominent of these spurs trend north.
This contrast in topographic configuration is to be explained by the position of the former covering of sedimentary rocks, which were removed during the downcutting of the Grayling Creek Valley. This creek, following the well-known habit of streams, formerly cut its channel westward along the strike of the northward-dipping sedimentary rocks, gradually deepening its channel until, reaching the underlying crystalline schists, it was compelled to continue in the same course, deepening and widening the valley, whose straight northerly walls are due to the absence of lateral drainage channels consequent upon the northward dip of the strata. The same configuration is seen in the upper valley of the Gallatin River and the valley of Fawn Creek, where the mountain gorges are cut in sedimentary rocks.
The topography of this vicinity is so closely dependent on the charac- ter and position of the strata immediately overlying the gneiss, and these strata have been tilted, curved, and faulted to such an extent, that it is advisable to postpone the description of this area until the less distributed rocks lying immediately east of the main body of crystalline schists have been described.
The region just mentioned lies east of the main gneissic area, and is
6 GEOLOGY OF THE YELLOWSTONE NATIONAL PAEK,
separated from it by a fault trending a little west of north. This fault crosses the southern end of Crowfoot Ridge, about a mile west of the summit of Three River Peak, and passes southward along the west base of the mountains, between Indian and Maple creeks, disappearing beneath the more recent rhyolitic lavas. The eastern area embraces the most joromi- nent peaks of the southern portion of the Gallatin Mountains, including Mount Holmes and the bare porphyry peaks around the head of Indian Creek, besides Trilobite Point, The Dome, Antler Peak, and Three River Peak, peaks that are directly connected with the mountains north of Panther Creek.
CROWFOOT SECTION.
Before taking up the description of these mountains, involving sedi- mentary rocks, it will be best to give an account of the stratigraphic series. A very carefully measured section was made of the Paleozoic strata exposed on a northern lateral spur of Crowfoot Ridge. This high mountain ridge shows the entire sequence of the Paleozoic sediments of the region, from the crystalline schists to the top of the Carboniferous, exposed in an unbroken succession of apparently conformable beds dipping at an angle of 30° N. The general form and profile of the ridge is shown in PL II, which gives a view of the ridge from the east. The illustration shows the bluffs formed by the harder beds of the series, rising above the slopes into which the shales and thinl)^ bedded strata have weathered down. Two lines of cliffs, formed by the mottled limestone. No. 14 of the section, and the Jefferson limestone. No. 19, are seen in the view. These horizons form characteristic cliffs throughout the range, and are an important aid in reading the structure of the mountains. The section of the sedimentary rocks made at this place has served as a basis of comparison for all the other sections of the Paleozoic rocks made in the Gallatin Range. The beds are well exposed, the crest of the ridge being bare of soil or vegetation and the trend of the ridge being very nearly at right angles to the strike of the strata.
THE GALLATIN MOUNTAINS.
Grotrfoot Ridge section.
Nuinber. ^eet. 33 Quailiaiit qnartzite. Siindstono and ([uartzito, saccharoidal in texture, reddish near the base, with daik-gray and very calcareous layers; passes at top to a white sand- stone 200
d. Limestone, light gray, brecciat(Hl and broken; in places iron stained 100
c. Limestone, light gray, varied with brown baiidsor fiuestripes antl lines; contains
chert, and is in ])lace8 breeciated, the fragments being cemented by calcite 400
b. Limestone; crystalline, very light gray, weathering creamy white; contains
white chert in bands and nodules 30
a. Limestone, crystalline, dark brownish gray, intersected with calcite seams.
Chert bands and nodules are abundant 125
655
Limestone, light gray and gray, weathering gray ; banded with brown ; banded appear- ance on weathered surface ; finely crystalline. Fossils abundant 380
' b. Limestone, crystalline, massive, light gray, with small brownish fossil fragments scat- tered through it. The upper 10 feet is a conglomerate of red, often cellular lime- stone. Fossils iO
a. Limestone, massive, light gray, similar to No. 28 15
31'
31
30
Is
29 Limestone, light gray and brown, very finely crystalline or granular; well bedded, with layers 10 to 20 feet thick of brown or cream-colored limestone. Certain layers are banded with light-colored chert, weathering bufi' or brown; these layers carry
corals and fossil shells 85
28 Limestone, crystalline, light gray, generally massive, but in places more thinly bedded,
and striped with brown. The rock is often magnesian and impure. Fossils 200
27 Limestone, dark gray and butf, very argillaceous, thick and thinly bedded. Fossils from the upper portion and the lower portion of these beds are Lower Carboniferous.
This limestone is very much like the bed beneath it 50
b. Limestone, more massively bedded than the underlying bed and coarser in crystal- lization; a quite pure limestone, full of fossil fragments 15
26 \ a. Limestone, fissile and thinly bedded, impure and argillaceous. The fossils occur
in lower and upper beds 60
I
75
|
r |
|
|
^-a |
|
|
SS |
|
|
hS |
20 1 |
|
^<ti |
|
|
OJH, |
25 Limestone, coarsely crystalline, dark gray, somewhat variable; a little cherty; fossil-
iferous - 80
24 Limestone, cherty; at base very finely crystalline; occurs in massive layers and is pinkish gray. Higher up the beds are cherty, and the upper portion contains many crinoid stems and a few indistinct corals and shells. Weathers red, and is cracked
and breeciated 60
23 Limestone, bnff and red, fissile, near base passing into a more thickly bedded limestone.
The rock is a dense, compact, light limestone, argillaceous and siliceous 30
22 Limestone, crystalline, dense, compact; the upper 20 feet is red and cracked; the beds
beneath .ire massive, with toothed junction surfaces. Eock is decidedly magnesian.. 50 21 Limestone, in alternating beds of thin and fissile buff and massive gray limestones, the l)eds 15 to 20 feet thick, and the limestone much cracked This limestone is
impure, argillaceous, and in some beds quite arenaceous 100
b. Limestone, crystalline, dark gray, thick and thinly bedded, with fetid odor, and
C(mtaining gasteropod shells. The limestone is quite pure 15
a. Limestone, crystalline, brown, crackled, with fetid odor. Slightly arenaceous ;
granular 10
25
GEOLOGY OF THE YELLOWSTONE NATIO]!f AL PAEK.
Number.
19 i
Crowfoot Bidge section — Continued.
Limestone, gray mottled with black, crystalline, in part magnesian, well lieilded, in jilaces crackled, and with much iron oxide and calcite. Strike N.70^ W. Dip 30^ N 70
Limestone, mottled, gray and buff, very ferruginous in places. Beds 5 feet
thick 30
Limestone, white, crystalline, weathers cream, with granular surface and indistinct fossil traces ; is magnesian and very finely crystalline. Forms a cliff, and is iron stained in places 35
Feet.
O
a
18 Limestone, conglomerate, nodular, and shaly layers near the base, overlain by thick and thin beds of densely crystalline limestone alternating with thinner, shaly and fissile strata. Toward the upper part less shaly, denser, with brown layers and layers of
very fossiliferous, crystalline limestone
17 Shale, calcareous, thin; purple, green, and brown
16 Limestone, very argillaceous, buff brown, very fissile . . -
15 Shale, greenish gray, very soft and crumbly
14 Mottled limestone. The upper 2 feet is an arenaceous conglomerate, in which the frag- ments are rounded pebbles of shale and sandstone; the matrix a slightly argillaceous sandstone. Strike N. 70° W. Dip 27^ N. The mottled limestone is a pure, thickly bedded (20 feet) rock, dark gray mottled with browu or black; crystalline, with
granular weathered surface of unchanged color
' 13 Limestone, variously moditied. The lower layers thickly and thinly bedded, much of it coarsely crystalline, with green grains of glanconite and great numbers of trilobite spines. luterbedded with this limestone are layers of dense, gray, fissile and thinly bedded limestone, with yellow bands, and limestone conglomerate. About the middle of the beds there are several thick beds of crystalline limestone containing green "•rains. This is overlain by a conglomerate. The matrix is pure limestone, the pebbles
slightly argillaceous and resembling a mud deposit
12 Shale, very thin, olive green and dark purple
11 Limestone, pure and ferrnginous. In general this limestone is red-brown, but contains masses of dense, dark-colored limestone, which weather in balls with spherical shells.
Balls 3 feet through
f Limestone conglomerate; brown-gray and gray pebbles in buff matrix. Fragments'!
j well rounded I
I Limestone, very similar to No. 9, and taking section to base of long ridge. Over these |
I beds are layers of crystalline limestone, with green grains J
9 Limestone, buff mottled with brownish gray; thinly bedded
8 Limestone, massivelybedded, quite pure; weathers with smooth surface; color, brown. 7 Limestone, pure gray, with dense, dark-gray layers and streaks in a buff, granular j
matrix. Runs up to top of first point shown in PI. II J
6 Limestones, thinly bedded, light and dark gray in color, showing remains of shells and
trilobites
5 Limestones, thinly bedded, with' interbedded micaceous shale, having fossil indications.
Fossils collected from the upper part and from lowest beds
4 Shale, micaceous, green and purplish
3 Sandstones, slightly indurated, red and green, with grains well rounded; quartzose
2 Quartzite and sandstones, cross bedded, and containing well-rounded pebbles of gneiss. 1 Gneiss.
135
40 5
55
100 150
10
50
175
10
60
75
30
100
THE GALLATIN JMOINTAINS. 9
MOUNTAINS SOUTH OF PANTHER CREEK.
The geological structure of the mountains east of the fault already mentioned as crossing Crowfoot Ridge is somewhat complicated by the presence of large bodies of igneous rock that have been forced between, and also across, tlie sedimentary strata. The exact position of these eruptive masses will appear from the description and may) (PI. X). In general, the stratified rocks form a flat arch, the central portion of which is nearly horizontal, and beneath which the crystalline schists are exposed along the east base of the mountain escarpment for a distance of 4 miles. The strata immediately overlying these schists at the northern end dip at a low angle, 5°, toward the northeast, and at the southern end they dip about 3° toward the southwest. This is the simple structure of the eastern portion of the area along a line through Antler Peak, The Dome, Trilobite Peak, and the hills south of Winter Creek, which is shown in the geological cross section, PI. Ill, fig. 1.
The igneous magma which was intruded into the shaly layers of the Flathead formation and was afterwards^ consolidated as the Indian Creek laccolith, was forced upward from some source at the north, uplifting the strata southward and wedging them apart, and being itself separated into two sheets by a thin, wedge-shaped layer of limestone. The sheets, which are nearly horizontal for a considerable distance, become thinner southward and have only a slight thickness where last exposed in this direction. In the ridge north of Indian Creek the sedimentary rocks overlying the intruded body arch over it from east to west in a pronounced manner, which is shown in cross section in PL III, fig. 2, and which will be described later on.
Subsequent to the intrusion of this double sheet of igneous rock, there was another outbreak of molten magma of a slightly different character, which was forced upward directly through the rocks just described. The manner of its intrusion is shown by the nature of the contacts between the second eruptive mass and the surrounding rocks. This body forms a great mass, whose present exposure is 3 miles long and 2 miles wide, embracing the six white peaks surrounding the head of Indian Creek, of which Mount Holmes is the most conspicuous. With this preliminary sketch before us, it is possible to 2)i'0ceed to a more detailed description of the geology of this portion of the country.
10 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK.
SOUTH END HILLS.
Commencing with the most sontheru end of the eastern part of the ridge, where the sedimentary rocks begin to rise above the rhyolite plateau, we find hmestone exposed on the crest of the low ridge 3J miles soiith of Mount Holmes. The bedding is nearly horizontal, the dip being but 3° to 5° SW. throughout tlie greater part of the ridge. The character of the limestone varies from shaly and fissile to massive beds, the highest strata being mottled and banded, dark and light gray, and in places conglomeratic. The lithological characters are like those of the Cambrian formations about 300 to 600 feet above the base of the series as it exists in the section north of Crowfoot Ridge.
At the southwestern extremity of the southern end of this ridge there is a small exposure of coarse-grained gneiss, against which lies a bed of fine-grained granular quartzite, about 50 feet thick, over which is light -gray limestone in apparent conformity. The highly inclined position of these beds, dipping 70° NE., with strike N. 50° W., and the nearly horizontal position of the limestone a short distance east, indicate a fault between these two sets of beds, which probably trends northwest and southeast, with hade to northeast, and with a downtlu-ow of not more than 500 feet. The extension of the fault could not be traced on account of the covering of lava.
The igneous rock intruded between the limestone and shales already mentioned is the thin edge of one of the sheets of the Indian Creek laccolith, and may be called andesite-porphyiy. At the northern end of the ridge in question it is exposed in a cliff 100 feet high, which is about the thickness of the sheet in this place. Limestone is exposed beneath and also above it. The inti'usive sheet can be traced for several miles southward, becoming thinner, until it is but 10 feet thick where last seen, before disappearing beneath the rhyolite of the plateau. Above it, on the highest portion of the ridge, two small dikes of andesite-porphyry traverse the limestone across the axis of the ridge. A short distance to the southwest there is a narrow vertical dike of similar rock, about 3 feet wide, trending southwest and northeast. It rises slightly above the shaly surface of the ground and exhibits two systems of inclined joints, forming rhombic hori- zontal columns. Near the sides of the dike a third, horizontal joint splits
TRILOBITE POINT. 11
the rock into hexagonal cohimns, which are well defined near the outside of the dike, but disap2)ear toward the center. The rock is dark colored and dense near the contact walls, becoming lighter colored and less dense toward the center, and containing irregular vesicular cavities, flattened parallel to the walls of the dike.
Direct connection between the fonnations of this ridge and those of the mountains north is obscured by the rhyolite lava which extends up Winter Creek and across the saddle of the divide to Maple Creek. A close coirespondence, however, between the sections of limestones and the intru- sive sheets is observed on both sides of Christmas Tree Park, and when the dip of the strata in each case and the altitudes at which similar horizons are exposed are taken into account, it appears that the rocks on both sides of the valley were continuous, with low southerly dip, before the valley was eroded and filled with rhyolite, or that a very slight fault has dropped the strata of the ridge just described a few hundred feet. In the diagi-am of cross sections, PI. Ill, fig. 1, the strata are drawn as though not faulted.
TRILOBITE POINT.
At the south and east base of the group of peaks directly connected with Mount Holmes, including Trilobite Point and The Dome, crystalline schists are exposed at altitudes of from 8,000 to 8,500 feet, and in isolated localities at 8,700 feet. These outcrops form a low, rounded bench at the base of the mountains, the upper limit of the gneiss being highest at the south, and lowest in elevation north, in the bottom of Indian Creek. The isolated exposures near the head of Winter Creek are close to the margin of the great intrusive mass of Mount Holmes, and, though at about the same altitude as the other outcrops, show by the dip of the neighboring stratified rocks that their position has been disturbed by the intrusion of this igneous ^ mass. The limestones dij? steeply in various directions, and the beds are largely concealed by drift, the outcrops being small and disconnected, so that the precise stratigraphic structure was not apparent at this locality. The map represents the beds as continuous with those to be described in Trilobite Point, though they are in fact locally disturbed by the intru- sion of the Holmes mass.
The character of the crystalline schists is like that of the area about The Crags — coarse-grained gneisses, mostly micaceous, with subordinate
12 GEOLOGY OF THE YELLOWSTOISI E NATIONAL PARK.
amount of schists and some ancient and metamorphosed intrusive bodies; the more detailed account of which is given in Part I of this monograph.
Along the eastern base of the mountains the gneiss forms prominent exposures, constituting the end of the long southeast spur of Trilobite Point and forming bold escarpments on either side of the valley northeast of Mount Holmes, below the level of the glacially carved lake basins. The gneiss forms a bench extending along the eastern base of The Dome, the exposure having a height of 300 or 400 feet, and occurring across the valley of Indian Creek, where it is last seen to form a low, wooded hill on the northern side of the creek.
Immediately overlying the gneiss around three sides of Trilobite Point is the lower sheet of andesite-porphyry, a light-gray aphanitic rock with porphyritical crystals of feldspar, hornblende, and biotite. The sheet is between 200 and 300 feet thick and occupies the horizon of the Flathead shales, being overlain by 150 to 200 feet of Cambrian, Flathead Hmestone in nearly horizontal beds. Above the limestone is another sheet of andes- ite-porphyry, from 100 to 200 feet thick, which in turn is topped by the Upper Cambrian shale and trilobite-bearing limestone. The upper surface of the uppermost sheet of andesite-porphyry is quite irregularly defined, and the overlying limestone is traversed by small dikes and veins^ of igne- ous rock, that are in part offshoots from the lighter-colored igneous mass of Mount Holmes. The basal (lower) sheet of andesite-porphyry is thicker at the northern side and thimier at the southern side of the mountain.
At the saddle on the ridge connecting Trilobite Point with Mount Holmes, near the contact of the rocks just described with the igneous rock of the latter mountain, the beds of limestone and andesite-porphyry are turned up to an angle of about 45°, dipping eastward, away from the Holmes mass. The strata are greatly fractured and are penetrated by many small bodies of the Holmes rock.
THE DOME. •
The Dome is a mountain siimmit northeast of Mount Holmes and con- nected with it by a low ridge. It is separated from Antler Peak and the northern portion of the range by the wide and deep valley of Indian Creek. The mountain is largely formed of andesite-porphyry, an extension of the Indian Creek laccolith, which rests upon crystalline schists and is capped by Cambrian limestones forming the summit of the peak. A thin belt of
OS GEOLOGICAL SURVEY
MONCGRAPK xzxi: PAP-T :: 7-L :"
MOUNT HOLM ES
ANTLER PEAK
QUADRANT MOUNTAIN
GRAY PEAK
ELECTRIC PEAK
Jftgn
ELECTRIC PEAK
BANNOCK PEAK
ANTLER PE
CROWFOOT ridge:
THREE PI Vf.R PEAK
«gn
Section 11-^
MOUNT HOLMES
BANNOCK PEAK
GRAY PEAK
01-()I,()(iU AJ, SIXTIONS ACKOSS CAI.LAIIN l(AN(il'; ],k(.i;n»
JURAjmAS CARBONIFEftOUS
|
D,r,l |
i |
SILURIAN |
e |
CAMBRIAN of Miles |
NEOCFNf n7)i ' Nei ^ |
1 |
dp |
Aniiiislli- l...,-|0.v,-v a n p ■ |
ARCHEAN |
|
1 |
Sea i |
INDIAN CREEK LACCOLITH. 13
these limestones is also included in the laccolithic mass separating the igne- ous rock into the two sheets, whose southward extension has already been noted at Trilobite Point and the South End Hills.
On the southern side of the mountain the lower intruded sheet rests directly upon the crystalline schists, and as we pass up the southern slope of the mountain, which is precipitous in places, we find a layer of limestone about 300 feet thick, which is not wholly continuous. Above it are several hundi'ed feet of porphyry, and then a bluff wall of 200 feet of limestone that forms the surface of the table-topped portion of the mountain, upon which rises a cone of limestone 400 feet in height. The limestone has a slight dip southward, which brings the porphyry out at a higher altitude on the northern side of the peak. The character of the limestone overlying the porphyry is the same as that of the limestone on the northei-n side of Indian Creek, which occurs in the same position, and which has been iden- tified as Middle Cambrian. As these beds approach the eruptive mass forming the peak southwest of The Dome, they turn up abruptly to a steep angle, dipping away from it toward the northeast at 55°.
The steep northern face of The Dome, below the fiat top, is almost wholly andesite-porphyry to within 300 feet of the bed of the creek; the lower part being limestone, forming steep walls that rise above a bench of g-neiss. A thin belt of shale or limestone occurs about halfway up the slope, inclosed in the porphyry. It does not appear to be continuous horizontally, though quite persistent. The contact between the rocks just described and the inti'usive mass to the west is sharply marked and nearly vertical, and will be described in connection with the occurrence of that rock body.
The limestone underlying the andesite-porphyry at the northern base of The Dome is in nearly horizontal beds, but at the eastern base of the mountain the gneiss rises up and cuts it off. At a higher altitude the lime- stone is exposed with a steep westerly dip, evidently bent and faulted, with a throw of several hundred feet. At about this place the lower sheet of porphyry cuts down to the horizon of the gneiss.
INDIAN CREEK LACCOLITH.
North of the valley of Indian Creek the slopes rise steeply upward to the base of a wall or cliff that extends westward from the eastern face of the mountains to the head of Indian Creek Valley. The lower part of this
14 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK.
great wall is formed of nearlj^ horizontal beds of limestone (Flathead), upon which rests the great mass of intrasive audesite-porphyry whose southei'n extension has already been noted, and over which arches the distinctly bedded limestone that forms the eastern ^jart and the summit of Antler Peak. This is the great intrusion termed by W. H. Holmes the "Indian Creek laccolite,"^ whose mass forms the greater part of Antler Peak, the bold summit north of Indian Creek, and its extension westward to the slopes of Three River Peak. The structure and topogi'aphy of this part of the ridge are clearly shown in the sketch by Mr. Holmes." In the middle of the ridge the overlying strata are absent, but at the western end of the ridge they recur, completing the westward-dipping limb of the arch, as shown in PI. Ill, fig. 2.
This dome-shaped body of intrasive rock is a cross section of the double sheet met with in the mountains south. In Antler Ridge it attains its maximum thickness, and appears as one massive body with a thin layer of shale or limestone inclosed near its middle, which is indicated in Mr. Holmes's sketch. It is, however, not absolutely continuous. No doubt it is the thin edge of the limestone wedge that split the intrusive mass in two as it was forced southward.
An examination of the limestone underlying the laccolith shows that the prominent cliff, 75 to 100 feet high, is formed of the nodular limestones of the Flathead formation corresponding to the lower limestone belt of the Crowfoot section. These beds are more fully noted in the section of the sedimentary rocks of Antler Peak. Within the lowest micaceous shale beneath the laccolith there occurs a layer of white, lithoidal, igneous rock, 50 feet thick, and evidently a horizontal sheet, which is again exposed at about the same horizon 2 miles farther west. Petrographically it resembles the rock of Mount Holmes, of which it is probably an offshoot.
The contact between the lower massive limestone and the bottom of the laccolith is plainly exposed in places. The limestone exhibits little or no metamoi-j^hism, there being only a slight lightening of its color along the immediate contact. The crude columnar jointing of the massive lime- stone may have been the result of baking by the laccolithic mass, but it is not pronounced ; however, it may easily be mistaken at a distance for the
' Twelfth Ann. Rept. U. S. Geol. and Geog. Surv. Terr, (for 1878), Pint II, Washington, 1883. ^ Op. cit., PL XIII, p. 24.
INDIAN OREEK LACCOLITH. 15
well-known jointing' of igneous rock. The limestone and shale inclosed within the body of the laccolith, also show almost no evidence of meta- morphism other than a lio-htening- of their color in innnediate contact with the })orphyry. The inclosed stratum of limestone is 20 to 40 feet thick, and rather persistent. It is nearly horizontal, or arches gradually with the curve of the laccolith dome. Several masses of red and gi'een shale, tilted at high angles, were seen in the igneous rock. These are probably blocks of the heavier shale belt forming the upper part of the Flathead formation, and which is the horizon in which the laccolith appears to have been intruded. There are also small fragments of limestone and gneiss included in the porphyry, caught up in its passage through the lower rocks which were ruptured at the time of its intrusion.
The laccolith sheet thins out eastward under Antler Peak, disappearing near the base of the eastern slope, where the upper and lower limestone strata meet and foi-m the whole of the northeast spur of the mountain, dipping at the low angle of about 6° NE. The apparently gradually increasing dip of the overlying limestoiies as they arch over the laccolith is found on investigation to be irregular, the dips varying along the cliflPs forming the bare southern exposure, increasing from 5° to 10°, and farther west to 20°, then becoming nearly horizontal just before reaching the depression on the ridge. There is, however, a northerly element of the dip which is not noticeable on the southern exposure. Where the dip changes noticeably, the limestone is shattered by innumerable small, vertical faults, close together; it thus behaved as a brittle, not as a plastic, mass at the time of the laccolithic intrusion. The overlying limestone embraces the upper part of the Cambrian formations, including the massive mottled limestone which is the base of the Gallatin limestones, together with the bai'ren strata that represent the Silurian and Devonian, and about 400 feet of the Carboniferous, which forms the summit of Antler Peak.
The laccolith is about 1,200 feet thick at the middle, where it forms a high point projecting into the valley of Indian Creek. Here the limestone capping has been removed by erosion, leaving the slope of the ridge to indicate about the slope of the old plane of contact. The triangular peak northwest of this point shows the overlying limestones dipping at 2° to 3° NW. and extending down the long divide to Bighorn Pass, where they dip over the porphyry for a short distance at 10° and also at 25° NW.,
16 GEOLOGY OF THE YELLOWSTONE NATIONAL PAEK,
the generally low dip recumng again farther north. At Bighorn Pass the laccolith thins out and reaches its western limit.
In Three River Peak the porphyry is also overlain by limestone in nearly horizontal beds with slight westerly dip, the uppermost strata belono'ing' to the Madison limestones of the Carboniferous.
Along the western boundary of the laccolith, between Three River Peak and Bighorn Pass at the head of Gallatin River Valley, the limestone strata dip W. 45°, becoming less inclined farther west, where they encounter a fault trending west of north, which brings them against gneiss and steeply tilted Cambrian beds. Unfortunately the strata bordering the laccolith on the north, along the bottom of Panther Creek Valley, are covered with loose material from the mountain slopes; hence the position of the rocks adjacent to the laccolith on the north was not discovered. In several places the porphyry has broken up through the overlying limestone.
Without entering too minutely into the petrographical character of the laccolith of andesite-porphyry, which will be described in detail in Chapter II, it may be well to mention some of its general characteristics. The andesite-porphyry is a light-gray to whitish aphanitic rock with many small phenocrysts of feldspar and fewer of biotite and hornblende. It forms massive outcrops intersected by joints in all directions, and the rock splits, upon weathering, into sherdy, angi;lar fragments. In only one place was columnar jointing noted — on the southeast spur of The Dome. In the central part of the laccolith the groundmass of the rock has a crystalline texture, though extremely fine grained. Near the margin the rock gi'ows denser and darker colored. The same is true where the sheets become thinner toward the south. The extent of this mass in exposure is shown on the map, and its relation to the suiTOunding rocks is given in the cross sections.
MOUNT HOLMES BYSMALITH.
A great mass of igneous rock, 3 miles long and 2 miles wide, forms Mount Holmes and the ridge north to the summit of the peak west of The Dome, and extends across the head of Indian Creek and constitutes the chain of four peaks west of this creek and south of Three River Peak. This great body of igneous rock breaks up through the sedimentary strata as a
BYSMALITHS. 17
core or plug. It is a type of intrusion for which the name bysmahth has been proposed.^
A laccoHth as defined by Gilbert^ is a body of igneous rock which has insinuated itself between two strata and opened for itself a chamber by lifting all the superior beds. A symmetrical dome-shaped body is the exceptional or ideal form, and, as Cross'' has pointed out, Gilbert's use of the term practically included all thick lenticular masses of intrusive igneous rock occurring at a certain geological horizon in a sedimentary complex. Cross includes under the term laccolith all masses in which the expansion of the body has taken place from a plane approximately parallel to the bedding, and says that numerous causes may affect the regularity of the form. Of these the principal are: (1) Oblique position of the plane of expansion to bedding planes of the sediments; (2) lines of structural weakness in the strata; (3) presence of earlier intrusions; (4) lack of coherence and of pronounced bedding in strata invaded. These factors, we understand, simply modify the form of the laccolith, whose essential characters are those described by Gilbert. They can not in any sense replace the latter.
A laccolith is distinguished from an intrusive sheet of igneous rock, which is an intrusion between strata accompanied by a certain amount of lifting of the superincumbent rock. The difference lies in the thickening of the igneous body into a more or less lenticular mass in the case of a laccolith, over which the strata arch ; whereas the upper and lower surfaces of a sheet are almost parallel to each other. In sheets the lateral dimen- sions are very great as compared with the depth or thickness; in laccoliths the difference between the thickness and the lateral dimensions is much less.
Cross has shown that a certain amount of vei'tical displacement may accompany the arching of the overlying strata, as in the laccolith of Mount Marcellina,^ without changing the general character of the intrusion. But where vertical displacement with faulting is one of the chief characteristics of the intrusion, a distinction from normal laccolithic intrusion should be recognized. In the extreme this would result in the forcing upward of a
' Iddings, J. p., Bysmaliths : Jour. Geol., Vol. VI, 1898.
-Gilbert, G. K., Report on the Geology of the Henry Mountaius, U. S. Geog. and Geol. Surv. Rocky Mountain region (J. W. Powell in charge), 1877, p. 160, PI. V.
^ Cross, Whitman, The laccolithic mountain groups of Colorado, Utah, and Arizona : Fourteenth Ann. Rept. U. S. Geol. Survey (for 1892-93), 1895, p. 236.
■•Loc. cit., p.236. HON XXXII, PT II 2
18 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK.
more or less circular cone or cylinder of strata, having the form of a plug, which might be driven out at the surface of the earth or might terminate in a dome of strata resembling the dome over a laccolith. By this mode of intrusion the vertical dimension of the intruded mass becomes still greater as compared with the lateral dimensions, so that the shape is more that of a plug or core. Such an intruded plug of igneous rock may be termed a hysmalith (/JiSayuai^plug, Az9o?— stone). We have, then, transition from a flat, thin, intrusive sheet to a laccolith with lenticular form, and from this to a bysmalith with much greater depth and considerable vertical displacement.
Examples of bysmaliths are not common as yet. RusselP has called attention to what he considers volcanic plugs in the region of the Black Hills of Dakota, and has suggested their recognition as types of intrusion different from normal laccoliths. A sharp discrimination of the two types may not always be possible, since they grade into each other, as in Mount Marcellina. In the intrusive bodies of Mount Holmes and the Indian Creek laccolith the contrast is sufficiently marked and the two types are well illustrated. Nearly two-thirds of the circumference of the Holmes mass is exposed as a nearly vertical plane of contact crossing almost hori- zontal strata. The western boundary is against gneiss and along a fault plane of considerable magnitude, which probably acted as the conduit throuo-h which the mao^ma was forced. There is no means of knowing what may be the shape of the bottom of this bysmalith. It is possible that it may have risen through the fault fissure until it encountered the sedimentary^ strata resting upon the gneiss, witli its inclosed laccolith. It may have spread laterally along shaly strata near the gneiss and beneath the laccolith; then its movement laterally may have been checked, for the pressure upward became sufficient to rupture the strata and laccolith and to force a mass of these rocks covering an area of over 5 square miles up more than 2,000 feet — probably more than twice that height.
The areal relation of the Mount Holmes bysmalith to the surrounding terranes is shown on the geological map, PI. X. The vertical relations are shown in the profile sections of the Gallatin Mountains, PI. Ill, fig. 3, and PI. IX, fig. 4, and in PI. V, figs. 1, 2, 3, which are profile sections through Echo Peak and Three River Peak, and through The Dome and the peak
1 Russell, I. C, Jour. Geol., Vol. IV, 1896, p. 23.
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U S GEOLOGICAL SURVT^
MONOQRAPHXXXII.PARTIIPL V
(K'OSS SKC.TIOXS SIIOWINO MOUNT HOI.IVIKS I5YSMAT()I,IT1I
I.l-.OKMn
CARBONIFEROUS DEVONIAN
"3J~] ^
NEOCENE
DnuitR- porphyiy.
dp
Scale or Miles
MOUNT HOLMES HYSMALITH. 19
southwest, and tlirougli Mount Holmes and Trilobite Point. These cross tlie contact i)lane between the bysmaUtli and the strata with the inclosed laccolith.
Owing to the crystalline character of the rock constituting the bys- malitli, there is little doubt that it soliditied beneath a covering of strata. The crystals are larger than those forming the mass of the Indian Creek laccolith. A possible reconstruction of the position of the strata after the intrusion is given in PI. V, fig 4, in which all of the formations up to the top of the coal-bearing Laramie are represented — a total thickness of 9,000 feet.
The upper parts of the mountains into which this intrusion has been carved are barren and rocky above 9,000 feet, with comparatively smooth slopes covered with loose fragments of porphyry. The peaks are pointed in some cases and rounded in others, as may be seen from PI. IV. The rock is very uniform in appearance throughout the entire extent of the mass. It is light gray to white, aphanitic to fine crystalline, with a slightly porphyritic structure in part. It shows small flakes of biotite and indistinct phenocrysts of feldspar. It is massive, with irregular joint ci'acks, and weathers into angular blocks and. slabs. Its megascopical chai'acters are quite uniform throughout the greater part of the mass, which varies slightly in grain. But near the margin of the body the rock becomes denser and more aphanitic, showing a broad banding parallel to the walls of contact with the surrounding rocks. These walls are nearly vertical in the moun- tain west of The Dome, on the saddle east of Mount Holmes, and also on that of Echo Peak. In all cases examined, the neighboring limestones dip away at angles of 40° to 55°, and the adjacent andesite-jiorphyry has been crushed an<l dislocated. It is reddened and in places is filled with veins and apophyses from the dacite-porphyry, which is clearly proved to be an intrusion subsequent to that of the Indian Creek laccolith. Its western border in contact with the crystalline schists is obscured by debris, being located at the base of the mountains. These relations are shown in PI. V.
The eruption appears to have taken place along the fault line that lies west of Three River Peak. There seems to be no break in the continuity of the bysmalith mass, and this fact indicates that it was intruded at one time. The vertical displacement of a mass of rock 2^ miles long and 2 miles wide, by what appears to have been a single act, is remarkable. The peti'ographical character of the rock is that of an intrusive, not a surficial, body ; hence.
20 GEOLOGY OF THE YELLOWSTOJ^E NATIONAL PARK.
we may assume that it did not reach the surface of the earth, but was covered by the sedimentary rocks it displaced. The same kind of por- phyry occurs in two small bodies about 3 miles farther north, along the Crowfoot fault line, west of Three River Peak. Hei-e they have broken into Carboniferous limestone, which otherwise in this region is quite free from intrusions of igneous rock.
Still another small intrusion occurs along the northern border of the Indian Creek laccolith, but is confined to the upper horizon of the Cam- brian rocks. It is a dark basic porphyry of an unusual character, with occasional phenocrysts of hornblende, mica, and feldspar. It forms a small sheet, 50 to 75 feet thick, exposed on the divide south of Bighorn Pass and along the south base of Bannock Peak. It was not found in contact with the laccolith, and the relative times of their intrusion were not made out. There is no rock similar to it in the reg-ion explored, except a small sheet in Three River Peak, and nothing approaching it in composition occurs nearer than Electric Peak.
ANTLER PEAK.
The sedimentary rocks overlying the Indian Creek laccolith, as already noted, form the summits of Antler and Three River peaks; stratigraphic sections wei'e made at both these localities. Antler Peak is the prominent summit lying between The Dome and the flat-topped mass of Quadi-aut Mountain. (See PI. VI.) The greater part of the mountain is formed of the intrusive mass of the Indian Creek laccolith, as just described. The sedimentary rocks are best exposed on the southern side of the mountain and at the eastern slopes, where the laccolithic rock passes beneath the limestones.
The gneiss is exposed on the low wooded hill at the southeast base of the peak, indicated on the map by the 8,000-foot contour. This hill and the slopes back of it are covered heavily with drift, which usually conceals the gneiss and the overlying stratified rocks; there being no exposures except near the base of the great limestone ledge which forms such a prominent feature of the valley. The strata were examined where the ledge has been cut through by a small stream channel from the summit, the debris which elsewhere conceals the foot of the wall having here been washed away. The lowest strata exposed were thinly bedded limestones and rather heavy
ANTLER PEAK. 21
iiig' ill was made at this place:
micaceous beds containing' indistinct traces of fossils. The following- section
Indian Creek section.
Crowfoot Nimi-
section. brr. Feet.
Laccolith, ande8ite-)ii>r])liyry.
11 6. Liinestoiic, rather tliiuly bedded, dark bliic-gray with lighter weathered surface. 6
7-10 5. Limosloiies, f'oiuiing the great ledge of the monutaiii side. Many beds are of a
crystalline, fine-grained, dark-gray and dense limestone seamed with calcite.
Weathers light gray, often rusty. At the base is thinly bedded, breaking
readily into small angular pieces. At top, beds are slightly cherty and fossil-
iferous. In ci'ntcr, beds are massive and appear irregularly bedded. Strike
N.Si*^ E. and dip 3'^ N 225
Dacite-porphyry, probably an offshoot of the Holmes bysmalith 25
6 4. Limestone, compact, brown, weathering gray 20
5 3. Slialy beds, micaceous and schistose, with thin bands of limestone 50
2-4 2. Interval, no exposure 300
1 1. Gneiss.
It will be seen from the above section that the andesite-porphvry of the laccolith immediately overlies the limestone No. 6 of this section, which con-esponds to the Flathead limestones of the Crowfoot Ridge section. The laccolith has therefore been intruded in the upper shale belt of the Flathead formation, the shales being 150 feet thick in the Crowfoot section. The occurrence of the laccolithic intrusion is the same at the base of Tliree River Peak, where above the porphyry a part of the shales is found beneath the limestones that form the highest beds of the Flathead forma- tion.
At Antler Peak the laccolith incloses parts of the Flathead limestones, as well as a thin belt of limestone and fragments of the underlying shale. Along the base of steeper slopes toward the northeast, the drift has covered all exposures; even the beds of the great limestone ledge are partially hidden. Over these beds we find a platform where the overlying shale and poi-phyry have been eroded, leaving the limestones underneath intact. That the bench is due to the erosion of the shale seems probable; easilj^ yield- ing to disintegrating agencies, it has been carried away, undermining the porphyry, which has also been swept off by glacial action.
The lowest bed noted above the laccolith is a finely crystalline, light- di-ab limestone, probably the upper beds of the Flathead limestone of the general section. The following succession of strata is exposed on the northeast spur of the mountain from the summit of the peak down to this bed:
22
GEOLOGY OF THE YELLOWSTONE NATIONAL PARK.
Crowfoot aectioD.
31
S
g1
30(1,6 29
26o
25 24
27, 28 < 22
Num- ber.
81
21 20
Antler Peak section.
Limestone, crystalline, moderately dense, brown, weathering grayish, witli rough pitted surface, and breaking readily into small angular pieces. Dip, N. .50 E., 20=
Limestone, less splintery than bed above, light brown, fissile, with fossil layers.
Limestone, finely crystalline, light gray with buff or pink, finely granular, weathered surface. Very fissile, plates warped
Limestone, brown or purplish brown, very fissile and platy. Fossils abundant.
Limestone, massively bedded, light drab and brown, breaking readily into frag- ments. Fossils somewhat abundant. Dip, 5" NE
Limestone, finely crystalline, light and dark gray and gray-brown, weathering bntf, with smooth, finely granular surface. Layers thickly and thinly bedded, with very fissile fossiliferous strata near base
Limestone, mottled, massive, and seamed with calcite. Lenticular arrangement of light-brown in darker mass gives appearance of bedding
Limestone, very fissile and containing fossils
Limestone, massive, purple, containing white fossil fragments
Limestone, alternating layers of very fissile, gray, and fossiliferous limestone and more massive rock
Limestone, coarsely crystalline, without chert; is fossiliferous and seamed with
;alcite
Cherty limestone, forming a prominent ledge shown in Holmes's sketch. Hard, dense, crystalline; contains corals and crinoid stems and much blue chert in bands and nodules. The rock is somewhat seamed with calcite
Feet.
100 40
15
100
50
100
15
5
15
170
50
100
23
22
21
20
19c
19
18
17
16
15 14 13
111
Limestone, somewhat massive and thickly bedded (5 to 10 feet), light gray, with lifht, very rough, and irregular weathered surface. Few fossils and a little light-colored chert 40
Limestone; alternating beds of dense light-drab limestone and brown arena- ceous sandstone, with rough and pitted weathered surface and fetid odor, parts of it resembling No. 16 of this section. Extends up to base of cone top. The rock is an arenaceous limestone, dark brown-gray, weathering brown 130
Limestone, compact, dense, hard, dark gray, weathering very light brown-gray,
with finely granular surface 20
Limestone, dark brown-gray, weathering straw-color and rich brown. Is an
arenaceous limestone 5
Limestone, white, pitted and rotted, with harder mottled places 6
Interval, no exposure 10
Limestone, brown, dense 5
Limestone, light creamy yellow mottled with gray, thinly bedded, breaking into
small cuboidal blocks. Strike, N. 5° W. Dip, 3° W 35
Limestone, finely crystalline, white 30
15-19fc 10 Limestones, grading at top into cherty limestone. 14 9 Mottled limestone, but 30 feet exposed
I-
(11
8 Interval, no exposure.
7 Limestone, finely crystalline, light drab.
160 50
THREE ElVEB PEAK.
23
West of the eastern summit the edges of" the lower beds outcrop, and the Galhitin Hmestone is exposed in the saddle. On the northeast spur of the peak the Flathead limestones are exposed, the dip being- N 30° E., 10°.
THREE RIVER PEAK.
Three River Peak is a shaq) point whose position at the head of the Gallatin River and of branches of the Gardiner and Madison rivers makes the name appropriate. The slopes rise abruptly from the head of Indian Creek Valley on the east, while to the north an almost vertical wall rises above the deep blue waters of Gallatin Lake. The peak occurs on the western side of the Indian Creek laccolith, and the beds composing it, like those forming- the summit of Antler Peak, consist of Paleozoic strata ranging from the Cambrian limestones to those of the Carboniferous. The sedi- mentary rocks are, however, penetrated by several sheets of intrusive rocks which are much decomposed, but represent phases of the Mount Holmes bysmalith. The following section shows the sequence of rocks exposed on the northern spurs of the peak from Indian Creek Pass to the summit: .
Three River Peak section.
Crowfoot Num- seetion. ber.
26a
2.T
.12
in
10
24
n^
«g
03 J
23
21-22
15-20
.U
13
r 3
f
Feet. 75 30 15 25
30 5
35
50 10 17 5 25 Limestone, thinly bedded, dark and light gray, magnesiau 130
Limestones, in a series of beds, not individually noted 160
Limestonfl, thinly bedded 70
Mottled limestone, matrix light brown, mottled with black; due to aggrega- tion of black grains of matrix 40
Gray, blue-gray, brown, and black limestones, with layers of "glaueonitic" limestone, the grains black instead of green. Is fosaillferous. Thickly and thinly bedded, with some yellow argillaceous layers and one conglomerate bed 100
12 Limestone, crystalline, gray, fossiliferous; contains crinoid stems
Limestone, light gray, dense, massive, cherty
Porphyry, fissile, much decomposed, yellow and purple
Limestone, light gray
Porphyry, fissile, decomposed and yellow at base ; upper half massive and
fresher
Limestone, crystalline, light gray, with granular, weathered surface
Limestone, brownish gray, weathering a light brown; massively bedded; with splintery fracture
Porphyry, light colored, nearly white, weathering brown, fissile; thoroughly
decomposed
Limestone, baked by porphyry
Porphyry, fissile, yellow and rusty gray, thoroughly decomposed
Limestone, black and rusty, much baked
Porphyry, dense, compact, looking like quartzite
24 GEOLOGY OF THE YELLOWSTOI^E NATIONAL PARK,
The shores of the Galhxtin Lake, and the small hills adjoining, are formed of the laccolith rock, audesite-porphyry. The western boundary of this rock runs northward along a gully, west of the di-ainage from the lake, and through a small shallow pond to the saddle of Bighorn Pass. To the south the porphyry extends to the base of Three River Peak in the rear of the lake, and forms the saddle in the pass between Indian Creek and the valley of the Gallatin River; from here northward the exposure extends along the western base of the ridge to Bighorn Pass.
The position of the strata seen in the precipitous northern face of tins peak is shown in PI. VII. At the western base of this mountain they dip sharply over the edge of the laccolith, changing from nearly horizontal to 50° or 70° W., and gradually decreasing again westward. Into the axis of this abrupt bend a vertical offshoot from the andesite-porphyry has been intruded, showing that the limestones were ruptured at this place.
The limestone strata of Three River Peak are traversed by dikes of lithoidal igneous rock at various angles. One broad dike, 100 feet thick in places, cuts diagonally across the northern face, appearing on the eastern slope about halfway up to the si;mmit. Another, about 10 feet thick, without phenocrysts, lies horizontally between the strata and might easily be mistaken for a compact sandstone. A narrower dike cuts nearly verticallv throuo-h the western side of the mountain.
At the west base of the peak the gneiss is faulted against the lime- stones by the soutliern extension of the Gallatin fault. The position of the sedimentary beds which abut against the gneiss, as well as their fracturing, shows clear evidence of the presence of the fault. An intrusive body of igneous rock related to the Mount Holmes rock occurs at this locality.
On the saddle between Three River Peak and Echo Peak, near the contact of the Holmes bysmalith with the andesite-porphyry and lime- stone, the latter rocks are seen to have been turned up, so as to dip 40° N., away from the bysmalith, and to be greatly shattei'ed and dislocated, producing slickensides and a pulverizing of the rock along the fracture planes.
BIGHORN PASS.
Bisrhorn Pass is a low divide between the head waters of the Gallatm River and the drainage of Panther Creek, and affords an easy passage from the valleys west of the mountains across the range to the central region of
EIGIJOUN I'ASS. 25
the Park. The pass is cut in the Paleozoic sedimentary rocks, which are sH^htly tihed by an intruded sheet of andesite-i)orphyry that is the northern extension of the Inchan Creek hiccohth. A dark, Liinprophyric rock occurs at the lowest })oint in the pass, where it is seen to form a sheet 50 feet thick intrusive in the Cambrian shales. The hjoh ridgfe extending north from the head of Indian Creek to Bighorn Pass is formed of sedi- mentary beds that overlie the northward extension of the Indian Creek laccolith. Above the andesite-])orpliyry of the laccolith which forms the Indian Creek Pass the green Flathead shales are exposed, overlain by the upper limestone series of the Flathead, which are here 100 feet thick and resemble quite closely the beds of this horizon as developed in Crow- foot Ridge. The summit of the ridge is formed of the Gallatin "mottled limestone," which dips to the northwest and makes a well-defined ledge, with a cliff" face 30 feet high and a rounded but hummocky surface, -the result of glacial planing. Near Bighorn Pass the beds are locally affected by an intrusion of the laccolith, and dip more steeply than the beds north of the pass, there being a difference of 5° to 8°. Two sections of the Paleozoic rocks were measured in this vicinity; the first was made on the ridge running south, the second from Bighorn Pass to the summit of Bannock Peak. These sections show the following sequence of beds, arranged in descending order:
Bighorn Pass section.
Crowfoot Num- section. ber. Feet.
^J
H
"-5
Cherty sandstones .and limestones, brown and gray, chert nodules, witli whitened surface
11
21 Quartzose conglomerate ; matrix a light-gray limestone 20
33 20 White beds of sandstone, quartzite, and limestone, generally saccharoidal 325
32 19 Limestone, crystalline, light colored. The upper portion is a brecciated and nearly pure limestone. The lower beds are dense, finely crystalline, weath- ering white, with granular surface. Magnesian, and containing sparsely disseminated chert. Strike, S. 30^ W.; dip, 10° NW 275
32 18 Limestone, very finely crystalline, with calcite strings, brown, dense, massive;
nearly pure 25
31 17 Limestone, breccia ; the matrix is similar to the bed below ; the fragments resemble the beds above and below. The lower beds are slightly siliceous, light-gray limestones weathering pale yellow 50
31 16 Limestone, crystalline, light brown, with granular weathered surface; mass- ively bedded, and is vertically jointed and contains some chert in bands 75
26
GEOLOGY OF THE YELLOWSTONE NATIONAL PARK.
29
28
Bighorn Pass section — Continued.
Crowfoot Num- sectioD. ber. Feet.
30 15 Cherty limestone. The lower 30 feet ia a brown fossiliferous limestone, over- lain by 70 feet of a moderately coarsely crystalline gray rock, weathering brownish. One bed is a compact brown limestone, full of criuoul stems and fossil fragments, which appear as crystalline masses of calcite on fresh fracture, but on weathered surface show their true nature. The upper
portion of No 15 is cherty and massive 225
14 Limestone; the lower portion light brown, shading to gray; massive, with cherty band near center. The upper part Is rich in coral. This limestone
varies in crystallization, being both fine and coarse 30
13 Limestone, gray, weathering brown, finely crystalline and compact 12
12 Limestone, linely crystalline, white, cherty. Chert is light gray, weathering
rusty or black 5
11 Limestone, blue, finely crystalline, very splintery 6
10 C'hprty limestone, white, finely granular 30
9e Limestone, coarsely crystalline, light drab, with few fossils; runs down to
saddle 25
9b Limestone, finely crystalline, compact, massively bedded, light drab, vertically
jointed. Dip 18-, N. 20^ W 20
9a Limestone, very light gray, coarsely crystalline, containing crinoid stems and
other fossils 50
8 Limestone, snmewhat coarsely crystalline, compact, white; the base concealed
by talus of No. 9 of this section 30
7 Limestone, fine grained, fissile, dark gray, weathering blue; certain layers
are quite fossiliferous 90
27 6 Limestone, dark gray, often brownish gray; cherty and siliceous near base. The upper 50 feet more friable and soft; contains numerous fossils, shells, crinoid stems, and corals 250
5 Limestone ; finely crystalline, with quartzose band near top ; gray, weather- ing creamy. The top is concealed by the talus of No. 6, and the thickness
should be increased about 40 feet iO
4 Limestune, dense, massive, dark blue-gray, impure (argillaceous) 20
3 Limestone, alternating beds of massive, steely gray, arenaceous limestone, weathering brown and containing corals, and fissile, light-gray, and dense limestone ''O
) 2 Limestone, at the base dark blue-gray and very compact, changing to a
browu-giay arenaceous limestone. Dip 15°, N. 10° W 10
) 1 Limestone, light drab, finely crystalline and dense, somewhat cherty. Chert
light and dark gray ; thickly bedded.
The dip of the lower beds of the section is 15°. Five hundred feet above the pass it is 18°, and near the summit of Bannock Peak it is 10°. The direction of the dip also changes from N. 11° W. to NW., becoming N. 41° W. on the summit. The beds appeared to be perfectly conformable throughout.
GALLATIN VALLEY. 27
It will be observed that the thicknesses given in the foregoing section to the beds niindx'red 16 to 10 inclusive differ considenibly from those of the Crowfoot Ridge section The correlation of the beds seems to be correct; the error can not ])e large either in this or in the estimates of thicknesses, as the white Quadrant sandstones above and the arenaceous Jefferson lime- stones form a check on the work. This difference amounts to nearly 400 feet and is believed to occur in the upper beds of the Madison limestones.
West of Quadrant Mountain and Bannock Peak the range consists of a rugged region drained by the Gallatin River. This stream, which heads in Gallatin Lake at the base of Three River Peak, flows through a valley that is one of the most beautiful parts of the park. Broad open meadows, diversified with clusters of pines and spruces, alternate with small patches of forest that cover the broad valley bottom. To the south the slopes rise steeply to the peaks of Crowfoot Ridge, while bold cliffs of white limestone wall in the valley upon the north. The river flows rapidly, in a succession of rapids and clear pebbly reaches, cutting the heavily bedded limestones that form the valley floor.
CROWFOOT RIDGE AND GALLATIN VALLEY.
On the west side of the ridge along which the chief stratigraphic section was studied a branch of Grayling Creek has cut a deep gulch, trending toward the northwest. This follows the outcrop of the Flathead shales, and has the gneiss and steeply dipping basal sandstone on the south side and the massive Paleozoic Ihnestones on the north. About a mile down the gulch a fault crosses the country in a direction east of north, letting down the block of sedimentary rocks and crystalline schists on the west side of the fault, so that the strata dip at a more uniform inclination of 15° to 20°, and also 30°, NE. This throws the basal quartzite at least 800 feet lower down than the west end of the crest of Crowfoot Ridge, and brings the Quadrant quartzite back to the summit of the west spur of Crowfoot Ridge, from which it extends down its northern slope.
At the western base of the end of this mountain ridge the sedimentary rocks are lost sight of beneath a deep accumulation of glacial drift, which obscures the contact between these rocks and the rhyolite lava that has buried them and the underlying crystalline schists, as already pointed out. The north and south ends of the fault just noticed are lost beneath the same drift.
28 GEOLOGY OF THE YELLOWSTOJTE NATIONAL PARK.
East of the high ridge along which the sedimentary section was made, abeady referred to as Crowfoot Ridge, the strata are folded and faulted in a pronounced, though not an extreme, manner. In the short spur between the two branches of the drainage east of the ridge there is a marked bend in the beds of limestone, which in the higher part of the spur di)) steeply and can be traced continuously into the main body of the ridge, but at the lower end of the spur are nearly horizontal. There is thus a short fault line west of the spur, which runs out in the head of the gulch, and probably joins a longer fault which terminates somewhere near the junction of this drainage with the Gallatin River, as shown on the map There is also evidence of horizontal thrust in the telescoping of the limestone layers, which is seen on the east escarpment of the ridge.
The next spur east of the one called Section Ridge is a long low ridge, formed of nearly horizontal beds, with a slight syncline across its middle, the axis of the syncline being about northwest and southeast. At its south- ern end the beds turn up abruptly against the gneiss ridge, and the shaly horizons are eroded down, and do not rise in a high spur as erroneously drawn on the general map. Tliere is a fold or bend in the strata as they come from Section Ridge, the beds curving down towai'd the east so as to permit the strata in the low spur to lie at a low angle. This is probably accompanied hj slight faulting, with north-south trend, situated near the bottom of the drainage. It was not obseiwed, however, in the field.
Between this spur and the next large spur, about a mile east, there is a broad fold in the strata. The beds that dip at a low angle of about 20° to the noi'th and northeast, arch over to an abrupt pitch with steep angle at the east side, near the south end of the east spur. This general arch is compli- cated by minor folds, not indicated on the map. The changes in dip and the differences in hardness of the shales and limestones show themselves in the topography, which is modified by glaciation. The easier degradation of the shaly layers leads to sink holes beneath stronger limestone layers. One has been made in the lowest micaceous shale, with the first massive limestone layers to the north. Farther east a small rock -bound glacial lake occurs on the gneiss at its contact with basal quartzite. North and north- east of this lake the lowest belt of Cambrian limestone forms a bench and a long slope down to the drainage, which flows west of north. Here the general dip of the strata is 30° NE. Near the lower end of the slope just mentioned are four small folds of the strata, with axes trending about
UALLATIN VALLF.Y. 29
N. 10° W. The upper part of tliis drainage is located on the upper shale belt in the Cambrian, l)nt leaves it lower down the slope. The upper shale belt may be traced across country by its influence on the topography, form- ing saddles where it crosses spurs which trend north, and giving rise to lateral drainage channels, feeding larger ones running north, or forming basin-like depressions with sink holes, as already noted.
The head of the gulch cut in the shale belt just mentioned is not shown on the map, but it is (|uite strongly marked, being narrow and deep and trend- ing north, and receiving the drainage of the small pond southeast of the larger lake noted above. The west wall of this gulch is formed of the lowest belt of Flathead limestone, with the lower micaceous shales of the Flathead formation to the west. At the spot where the drainage from the small j^ond falls into the deep gulch, these strata are inverted, dipping 60° or 70° W. ; strike, north and south. Hence the lower beds appear to overlie the upper ones. The gneiss is only a short distance west. The inverted beds can be traced northward into vertical beds, and then into others dipping toward the northeast. To the south the inverted beds continue in the same position until they abut against the gneiss. It is evident that there has been some faulting and displacement of the basal formations for a short distance in the neighborhood of the unconformity just mentioned.
The portion of the high ridge east of the shale gulch and the ponds previously mentioned is in general a syncline with a flat anticlinal fold at its northern end, which is south of the saddle crossed by the fault to be described. The dip of the strata, which are very steep near the gneiss, changes from almost 45° NE. to 15° farther north, flattening to the syncline already mentioned. The axis of the synclinal fold is somewhat west of north, and the same fold may be observed to the southeast of this ridge. Southeast of the southern end of the ridge a drainage channel follows the line of the upper shale belt in a southeast dkection. The southern side of this drainasre is formed of the lowest massive belt of limestone, and south of this parallel gulches have been worn in the lower shale belts. These drain either through cuts across the belt of massive limestone or in sink holes beneath it. Here again the strata are inverted, with a dip of 50° to 80° SW., changing in places to vertical and also to steep dips to the northeast. Near where the gneiss ridge is faulted by the north-south fault, the basal beds of the sedimentary series are inverted, with dip of 20° to 50° SW.,
30 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK.
and form a narrow wedge between this fault and the gneiss. The Gallatin fault, which crosses the west base of Three River Peak, trends in a north- northwest direction, crossing Crowfoot Ridge three-quarters of a mile west of Three River Peak, and crossing the ridge north of it at the saddle one mile north of the gneiss, thence following down the drainage, to die out where it joins the short fault east of Section Ridge. The trend of the fault is nearly jjarallel with that of the Gallatin River, as will be seen on the map. The maximum displacement is about 2,000 feet.
The long, low, flat-topped ridge lying between this fault and Gallatin River consists of nearh' horizontal beds of Carboniferous limestone capped by the white Quadrant quartzite or sandstone occun-ing at the top of the Carboniferous series. The dip of the beds is about 5° NE. From this it is evident that there must be a fault or a fold between this ridge and the higher one east of Gallatin River. A fold exists west of Bighorn Pass, but it was not followed down the valley. On both sides of the low ridge west of Gallatin River are bodies of intrusive igneous rock, related to the dacite- porphyry of the Holmes bysmalith in composition and petrographical character. The rock is lithoidal and holds small mica phenocrysts; it is fissile near the contact with sedimentary rocks, and mas.sive a few feet dis- tant. It crosses the fault line and is found on its western side intnided in the axis of an anticlinal fold in Cambrian rocks. Its intrusion followed or accompanied the faulting. On the eastern side of the flat ridge it apjjears as an intrusive sheet, about 50 feet thick, forced between beds of Carbonif- erous limestone. This exceptional occurrence of igneous rock as an intru- sive sheet in massive Carboniferous limestone is of limited extent and is in the immediate neighborhood of a fault, with which it is dii-ectly connected. Similar rock has been intruded into the west base of Three River Peak, and it may be assumed that the Holmes bysmalith was connected with the same line of faulting. The intrusion of this mass has been shown to have been subsequent to the upheaval that permitted the intrusion of the Indian Creek laccolith; hence it follows that the more steeply upturned position of the ffneiss and Cambrian strata west of this fault was due to a later movement than the general uplifting of the body of the range. This steeper uplift was limited on the east b}^ the fault last mentioned, and by that cutting across the northwest end of Crowfoot Ridge, which faults are probably contempo- raneous and were accompanied by a slight faulting east of Section Ridge.
THE GALLATIN MOUNTAINS. 31
QUADRANT MOUNTAIN, BANNOCK PEAK, AND THE VALLEY OF THE
GALLATIN KIVER.
In the less disturbed eastern portion of the GaHatin Range the Cam- brian and Devonian strata pass northward with a low northeasterly dij), dis- appearing beneath the more massive beds of Carboniferous limestone alone- the base of the mountains north of Panther Creek. The bold southern escarpments of Quadrant Mountain and Bannock Peak exhibit almost the entire section of Carboniferous strata, since they are topped near the summit of the former mountain by Juratrias beds. The nearly horizontal beds form massive cliffs that extend with gentle inclination along the eastern escarp- ment of this mountain, in lines parallel to the slope of its plateau-like top, and that sink beneath the level of the valley as Fawn Creek is approached. They may be plainly made out in Mr. Holmes's panoramic sketch of the Gallatin Range, PI. IV. Their character in Bannock Peak is seen in PI. VIII. From here they extend westward along both sides and the bottom of the valley of the Gallatin River, forming the cliff along its northern side and dipping at a low angle toward the southeast, while on the south they form a high ridge and the mountainous spur of Crowfoot Ridge.
BANNOCK PEAK.
Bannock Peak is a sharp mountain summit north of the head of Panther Creek. Resting upon the more readily eroded beds of the Silurian and Devonian terranes, the massive Madison limestones form the main mass of the mountain and are capped by the resistant beds of the Quadrant quartzite, whose white ledges form a bold escai-pment that encircles the peak.
On the northern side of the mountain a section was made of the strata exposed in the wall of the amphitheater cut between this peak and the broad plateau summit of Quadrant Mountain. This amphitheater, though appar- ently open, as shown on the map, is divided by a spur pi'ojecting southward from the extreme western end of Quadrant Mountain. This section was made from the bed of Panther Creek up the center of the amphitheater to the crest of the ridge dividing this from the amphitheater at the head of Fawn Creek. The beds are exposed in a series of steps or benches, the lowest strata being the arenaceous Jefferson Hmestones, the underlying beds being covered by drift.
The beds dip N. 21° W. at 8°, the determination being made on No. 4 of the section
32
GEOLOGY OF THE YELLOWSTONE NATIONAL PARK.
Crowfoot Num- sectioD . ber.
(o) 33 23
f22
21
32 <
31,
20
as
30a, 6 12 29 11
28 10
28 9 28 8
2 26 1
Bannock Peak section. „ ,
Feet.
White beds, sandstones and quartzites with interbedded limestone 325
Limestone, drab, finely crystalline, hard, forms knoll of saddle 100
Limestone, dense, well bedded, very light gray-brown, compact, weathered
surface, very finely granular 30
Limestone, forming great ledge of amphitheater saddle. At the base this limestone is brecciated, being a gray limestone with brown and blue frag- ments. The limestone is generally splintered, the lower two-thirds well brecciated and splintered and weathering in pinnacles, recesses, etc. This weathering is further facilitated by the occurrence of great seams of caloite, 4 inches thick, and pockets of the same material. In general the rock is
finely crystalline, gray, weathering gray-brown 200
Limestone breccia 10
Limestone, light gray-brown 15
Limestone, very linely crystalline, splintery, dense, light gray 15
Limestone, finely crystalline, brown, weathering same color 20
Limestone, finely crystalline, very light brown, well bedded and banded. No
fossils seen 30
Limestone, coarsely crystalline, massively bedded, splintery, brownish, weath- ering gray-brown ; is fossiliferous, and forms a ledge 50
Interval 125
Limestone, coarsely crystalline, brown-gray, weathering a lighter tint. Con- tains fossils 50
Limestone, white or very light gray, crystalline, compact, massive, not splintered; with small scattered calcite crystals, weathered surface, finely granular. The upper beds are more coarcely crystalline and sometimes
indurated 165
Limestone, brown, banded with darker fossiliferous layers, the rest denser; weathers with buff granular surface, often pink or light red. Breaks into
fragments 2 to 8 inches across 30
Limestone, brown, fissile, contains fossils 6
Limestone, dense, blue-gray mottled with buff-brown, with occasional thin
layers of coarsely crystalline limestone 100
Limestone, fissile, purple-red, fossiliferous, and resembles the limestone of the
saddle of Antler Peak 5
Limestone, brecciated, fragments sparsely scattered, fossils abundant 5
Limestone, crystalline, gray, crumbly, containing fossils, weathers gray and exposes fossils and crinoid stems. Beds 2 to 6 feet thick, and strike jointed. Becomes darker and more finely crystalline near top. Strike, S. 70° W. ;
dip,8°N 80
Limestone, cherty, crystalline, gray-brown, weathers light with finely granular
surface, sometimes pinkish, the chert weathering a rust color 25
Limestone, crystalline, cherty, gray, thinly bedded (2 to 6 inches) ; few crinoid stems; blue chert; fossils more abundant in upper layers. Slope of beds,
70 NW 125
Interval, probably a blue limestone, crystalline, dense 50
Limestone, crystalline, light gray and gray, very arenaceous, somewhat granular, with rough, pitted, and honeycombed weathered surface. This bed underlies any seen in the east face of Bannock Peak 60
a Quadrant quartzite.
THE GALLATIN MOUNTAINS. 33
QUADRANT MOUNTAIN.
The broad sumrnit of Quadrant Mountain is an open and grass-covered area, above wliicli a bold pyramid formed of the red Teton sandstones rises quite abruptly. Snow fields cover the summit in early spring and remain through the summer, nourishing streams that flow in cascades over the walls of the Pocket. The upper slopes, which lie beneath the cliffs of white Quadrant sandstones, are dark with the lichen-covered debris from the overhanging walls, while the slopes beneath are thickly timbered. The sujnmit of the mountain slopes northward with an angle of 3° to 4°, con- forming very nearly to the bed of the rocks. The flatness of the mountain top is clearly due to the resistant natui'e of the Teton limestones, as the overlying clays and sandstones are rapidly removed by erosion. This mountain block is of very simple stratigraphical structure. The beds are slightly flexed, without faulting, and are undisturbed by intrusives. The beds forming it are the Madison limestones, covered by the white Quadrant quartzites overlain by the cherty beds of the Teton formation. The strata forming the mountain are clearly a continuation of those of Antler Peak and Panther Creek Canyon; on the west the bods are seen to be connected with those of Bannock Peak through the saddle of the amphitheater, while on the north the Quadi-ant quartzites form the floor of Fawn Creek Valley and pass under the slopes of Little Quadrant Mountain. Eastward the beds end in a cliff and steep slope along that north-south line which separates the sedimentaries of the range from the lavas of the plateau. The general dip of the beds is a little west of north, about 8°, so that the slope of the summit corresponds approximately to the dip of the beds.
The summit of the mountain has been carefully examined. West of the Pocket the cherty Teton limestone covers the surface, which, when examined closely, is seen to be rough, gullied, and hummocked. The red hill on the summit of the mountain, southeast of the Pocket, is composed of the red Teton sandstones. This point is about 200 feet higher than the surrounding summit. On the east, south, west, and northwest these beds commence at the very foot of the hill ; on the northeast the area extends about one-fifth of a mile farther. The general summit from the red jDoint north is formed of the Teton cherty limestone and its associated lingula-bearing limestone, down to 9,200 feet, where the red beds again cover a portion of PT II 3
34
GEOLOGY OF THE YELLOWSTONE NATIONAL PARK.
the mountain. Eastward the cherty beds have been removed from the bench, and the white beds of Quadrant quartzite are exposed.
On the summit of the 9,100-foot hill a small outcrop of the red and yellow sandstone (No. 38 of section below) is exposed. West of this hill is a deep cut, into which the small drainages of this part of the summit flow. The surface of the area covered by the red beds is generally smooth and grassy; the shales are exposed in cuts, but appear only as detritus on the summit.
On all sides of the mountain the white beds of Quadrant sandstone form an escarpment, often capped by the cherty limestone. This Teton cherty limestone varies greatly in character. Often it is a cherty sandstone with little if any calcareous material in it; again it is a true limestone; and these two extremes grade into each other. The chert seems to be formed of sand, for the transition between the sandstone and the chert is often very gradual. The color is a grayish brown.
|
Crow- foot sec- tion. |
Nuni ber. |
|
|
-39 |
38 |
|
|
38 |
37 |
|
|
37 |
36 |
|
|
o 1' |
36 35 |
35 34 |
|
34 |
33 |
|
|
32 31 30 29 28 27 |
||
|
T |
33 |
26 25 24 23 22 21 20 |
Section of beds exposed at the southeastern end of Quadrant Mountain.
Feet.
Sandstone, red and orange colored, coarsely granular, weathers in blocks 15
Calcareous shale, dark slate colored, fossiliferons 5
Limestone, bright yellow, fissile, with platy debris, grading into yellow and red
spotted calcareous sandstones below 40
Shales, light greenish drab, changing to red and grading into overlying beds 75
Conglomerate of red and gray quartz pebbles in limestone matrix 10
Cherty sandstone, the lower part without chert, is brown; slightly calcareous at
base, becoming more so above 100
Limestone, dense, white, weathering same color 10
Sandstone and limestone in alternating bands 15
Limestone, light gray and dense and pure 12
Quartzite, pure whi te, thin bedded, forms a persistent band along face of mountain . . 6
Quartzite, more thickly bedded, white 65
Limestone, very light gray, somewhat dense, containing very small fragments of
light-colored chert 10
Quartzite, white, calcareous, with intercalated bands of limestone carrying quartzite
fragments 30
Limestone, light gray, with angular fragments of sandstone 5
Quartzite, white, weathering pink and rust color, but appearing black when Been
from a distance, owing to the growth of lichens upon it 130
Limestone, light gray, dense 2
Sandstone, well banded, white, saccharoidal 6
Sandstone, very calcareous, saccharoidal, white and rust color 10
Talus slope of sandstone blocks; also of cherty limestone 100
QUAUKANT MOUNTAIN.
35
Section of beds exposed at the xoutheastern end of Quadrant Mountain — Continued.
Crow- foot nee- tlou.
32
Num- ber.
19
Feet.
18
17
16
15
f 14
13
12
U
31
29^ 3
30
28 28
Liiiii'stoiio breccia. Tbo lower 30 foet ia a white limestone, weathering cream, cnn- taininj; angular fragments of bine, brown, and buff limestone, from one-eighth of an inch to 2 inches in diameter. The matri.'i is coarsely granular, and grades above the first .30 feet into a crnshed and splintered limestone similar to the iniitrix Just mentioned. The outcrop is massive, rough weathering, often pinkish. Kock is slightly clierty. Dip, 12^' N. ; strike, S. 78'^ W
Limestone, finely crystalline, gray, weathering light gray, with granular surface. Generally massive, though bedded at base. No fossils seen; is splintery and weathers rough
Limesti>ne, very light brownish gray, splintered and cemented by calclte in strings, and by blue limestone
Limestone, brecciated at the base; matrix gray, fragments angular, brown and brownish gray ; above this the limestone is massive and gray. Seamed with cal- cite and carries much of that mineral in pockets. It is somewhat cherty at the base
Interval; no exposure
Limestone, finely crystalline and granular, brown, somewhat cherty
Limestone, very fine grained with spai'sely disseminated chert, but varying to a very coarse-grained blue-gray rock. Color in general a light brown-gray. Weathers into irregular warped plates, due to jointing
Limestone, coarsely crystalline, brownish gray, weathering gray with granular surface. Irregularly bedded ; fossiliferous; much broken by vertical jointing. ..
Limestone, finely crystalline, generally thinly bedded, sometimes dark gray, but mostly brown, with granular weathered surface, or a blue-gray more coarsely
crystalline limestone. Dip, 5° N
10b Limestone, similar to No. 10a, but well banded. The fossilii'erovis layers weather- ing blue-gray ; the nonfossiliferous bands denser, granular, weatheriug light
brown , and 1 to 3 inches thick
lOo Limestone, in alternating layers of light gray, finely crystalline and darker, coarsely crystalline and fossiliferous. Dip, 4° NW .1.
Interval ; no exposure
Limestone, finely granular, light brown, weathering same color; fissile, contains remains of fossils. Talus indicates a greater thickness than that given
Interval; no exposure
Limestone, light gray, finely crystalline, with gray and granular, glistening, weath- ered surface
Interval; no exposure
Limestone, coarsely crystalline, dark gray, weathering the same color; fossiliferous fragments
Limestone, forming a well-marked and prominent ledge extending around the mountain; massive, light drab, weathering dark gray and brown with glossy
beaded crust. Contains corals and other fossils
2 Interval ; no exposure
1 Limestone, cherty, crystalline but not granular, compact, massively bedded. The upper part fossiliferous, containing crinoid stems, corals, and spirifers. Dip, about 5° N
135
65
35
50 80 10
75
25
90
10
25 25
5 10
12 65
15
45 60
60
36 GEOLOGY OF THE YELLOWSTONE NATIONAL PAEK.
lilTTLE QUADRANT MOUNTAIN AND FAWN CREEK YALIiEY.
LITTLE QUADRANT MOUNTAIN.
Nortli of Quadrant Mountain is another flat-topped elevation, known as Little Quadrant Mountain, wliich is clearly defined from tlie adjacent moun- tains by the deeply incised valleys of Fawn Creek and the headwaters of the Gardiner. The mountain is carved out of a block of Mesozoic strata, into wliich numerous sheets of andesite-porphyry, offshoots from the Gray Peak intrusion, have been injected. The resistant nature of these intruded sheets has produced the marked terracing which now forms so characteristic a feature of the southern slopes. The following section, made by Mr. Geoi'ge M. Wright, represents the beds exposed on the southern side of the mountain, the lowest strata being the brecciated limestone forming the top of the Madison limestone series, through wliich Fawn Creek has cut a small canyon at the forks of the stream:
Section of beds exposed on south side of Little Quadrant Mountain.
Kum- ber. Feet.
4 Sandstone 20
Interval with no exposure (broad bench about 300 yards wide) 75
Mica-hornblende-andesite-porphyry 50
3 Cherty, arenaceous, and calcareous beds :
Limestone 2
Interval with no exposure 35
Cherty, arenaceous, and calcareous beds 40
Teton limestone.
77
Quadrant quartzite.
2 Sandstone and limestone :
(6) Limestone, white, much of it brecciated and mixed up with
sandstone 20 or 25
(o) Sandstone 175
200
Interval with no exposure r- 100
Madison ( 1 Brecciated limestone, having some layers compact, not brecciated; in walls of limestone, i miniature canyon at junction of branches of Fawn Creek 40
The total thickness of this section is not given, owing to the uncer- tainty of the exact thickness of two of the intervals above mentioned.
Above the beds of the section just noted there is a long steep slope, rising 300 feet or more to a prominent cliff formed of the Ellis sandstones.
LITTLE QUADRANT MOUNTAIN.
37
Another section, made by Mr. Wrij^ht, sliows the sequence and thickness of the beds from this liorizon to the summit of the mountain.
Section of beds exposed on south side of Little Quadrant Mountain.
Num- ber.
Feet.
Colorado .
*
Mica-horDblende-porpbyry 125+
Dakota
Ellis
sandstoDe. |
I
Ellis limestone.
I 11 Carbonaceous sbalos.
Interval with no exposure 100 or 150
10 Sandstone.
Interval with no exposure .50
Additional interval with no exposure 175
Mica-hornblende-porphyiy several feet thick, with sandstone in small exposures below it in slope showing no other exposures. Number of feet given is height
of slope 100
9 Sandstone and conglomerate. These are here overlain by the mica-horn- blende-andesite. The sectiou was again continued at a place about 100 yards
southeast from the last-menC med exposure 25
Interval with no exposure 340
8 Soft gray and drab beds, weathering into light-green shales 10
7 Calcareous sandstoue and limestone 50
Interval with no exposure 75
6 Oolitic limestone 2
I Fossiliferous shales, occurring as follows:
Shalef, gray, soft 15
Interval with uo exposure 60
Shales, gray, clayey, in cut of stream flowing from pass at west end of
Little Quadrant Few.
Limestone 30
At the west end of Little Quadrant Mountain a branch of Fawn Creek has cut back to a low divide separating this mountain from Gray Peak. The lowest beds exposed by this stream are the Ellis limestones, which are exposed in the gulch 25 feet deep near the forks of the stream. The beds dip N. 25° W. at 10°. The strata contain numerous fossils and are quite like the beds described later in the Fan Creek section. In the stream channel above, there is an exposure of very fissile calcareous sandstone in a ledge 5 feet thick, which is overlain by very arenaceous, granular, cross- bedded, gray limestone, containing fossils which are mostly comminuted and broken. This exposure is 20 feet thick, and the bed is overlain by a sandstone containing a few scattered pebbles. The strike is S. 35° W., and the dip is 10° NW. Above this the stream flows over a small exposure of Dakota conglomerate, which is overlain by andesite-porphyry, forming a cliff 75 feet high, over which the stream flows in a succession of cascades This rock, which is an extension of the Gray Peak intrusion, is hornblendic,
38
GEOLOGY OF THE YELLOWSTONE NATIONAL PARK.
generally much decomposed, and shows no megascopic mica. This por- phyry forms a bench north of the stream which extends back to the base of the cliffs where Dakota conglomerates occur. In the stream channel a brownish-gray sandstone speckled with black, and belonging to the Dakota series, is overlain to the east by the andesite-porphyry, forming the two 9,000-foot hills shown on the map. These higher beds, forming the west end of Little Quadrant, are tilted up by the porphyry intrusion. The following section, made by Mr. Wright, shows the sequence and thickness of the beds exposed in ascending the creek, the beds being given in descend- ing order. The thicknesses are estimated and approximate, and are given in feet:
Num- ber.
Section on north side of head of Faion Greek Valley.
Feot.
Dakota ...
Ellis Bauds tone.
Ellis limestone.
Teton limestone. Quadrant quartzite.
50
50+
50
{Mica-hornbleude porphyry ..."|
Limestone > 300 Mica-hornblende-porphyry ...J
7 Sandstone
6 Conglomerate
^ Interval with no exposure
^) Sandstone Few.
Hornblende-porpliyry, in sloping belt across an interval 165
4 Sandstone, very calcareous 15
3 Limestone, very arenaceous 25
Interval with no exposure— steep slope 365
Mica-hornbleuile-porphyry 40+
Interval with no exposure 100
2 Cherty, arenaceous, and calcareous beds 35+
Interval with no exposure 100
1 Sandstone in bed of Fawn Creek.
Total thickness of section 1, 295+
Total thickness (approximate) of intruded porphyry 505
Total thickness of sedimentary rocks and intervals 790
The slopes on the northern side of the west end of the mountain show a succession of beds very similar to that of the section just given. The meas- urement made of the series from the Dakota conglomerate to the summit of the ridge showed the 100 feet of conglomerate and sandstone, which was assigned to the Dakota, overlain by 45 feet of compact gray limestone considerably altered by intrusive sheets of porphyry, which occur both above and below the bed and also spKt it in half. The overlying sand- stone, which forms the summit of the Dakota series, is a pure white, rather
FAWN CltEKK VALLEY. 39
soft and fine-grained, massive bed, whose outcrops often weather a light- brown. The thickness is 50 feet, and it is overhiin by carbonaceous shales which are (juite arenacetms at tho base and become more argillaceous above. These beds form the cliffs extending eastward around the sides of the crescent-like amphitheater cut in the northwestern wall of the mountain. The black shales extend eastward along the crests of the cliffs of the crescent ann)hitheater for nearly a mile, when they break down and the sandstone forms the summit of the cliff. In the northwestern slopes of the mountain, sandstone is exposed in nearly all the lateral gullies and stream channels, the upper sandstone bed of the Dakota being especially prominent and forming a persistent ledge that extends ai'ound the north spur of the mountain and constitutes the wall of the amphitheater cut iu its eastern face. Beneath the cliffs which form the wall of the northwestern part of the mountain an andesite-porphyry sheet has furnished the material for a great morainal accumulation of ang-ular rocks, concealing all expos- ures and rendering travel difficult. The persistent nature of these andesite- porphyry sheets is shown by their occurrence iu so many localities at the same stratigraphic horizon. In the stream channel noi'th of Little Quadrant the sheet of andesite-porphyry occurring between the Dakota limestone and the conglomerate is well exposed at 9,000 feet. In the vicinity of the lakes at the head of the valley the porphyry forms low rounded knolls, whose surfaces are scored and polished by glacial action. The lower slopes east of Little Quadrant have been carefully examined, but the morainal drift obscures all outcrops.
FAWN CREEK VALLEY.
The valley of Fawn Creek shows good exposures of the Carboniferous rocks, overlain by the softer Mesozoic series. In a little gulch near the forking of the creek, the Quadrant quartzite series is well exposed. On the south fork of the stream, just above this junction, a green magnesian bed, whose surface is red from the wash of the weathered outcrop, is also exposed. The overlying bed is a dark pm-plish-red rock spotted with green, highly ferruginous and argillaceous, being a very impure dolomite. This rock is overlain b}^ an outcrop of brecciated limestone, which is believed to repi'esent the highest bed of the Madison limestone series. This brecciated character and the granular weathered surface of these
40 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK.
limestones are persistent features of the upper beds of the IMadison series in this vicinity. These beds are overlain by the white Quadrant quartzites, which are well exposed in the stream channel. In the bottom of the amphi- theater at the head of Fawn Creek, a coarsely crystalline brown Hmestone is well exposed. The rock is fossiliferous, but the fauna presents no features different from those of the underlying limestones. The beds dip 10° N. The amphitheater floor is heaped up in places with great piles of debris, but presents many smooth exposures of a dark, slaty limestone and of the coarsely crystalline rock just mentioned. In the latter there are numerous large sink holes or "swallow"^ holes, in which the waters flowing from the snow banks of the amphitheater walls pass underground, to reappear two miles down the valley as a large stream which forms the headwaters of Fawn Creek. The section of beds exposed in the amphitheater walls to the east has already been given in the account of Quadrant Mountain. Nowhere is the character of the Quadrant quartzite and of the immediately underlying Madison limestones better shown than it is in the walls south of Fawn Creek Valley. The sections which have already been given show the relative thickness of these beds and the development of the impure argillaceous dolomites whose red ledges form so prominent a feature of the rock outcrops.
A comparison of the sections of the Quadrant quartzites made on the walls of Quadrant Mountain is given in the following table. A precise separation of the sandstones from the interbedded limestones is not always possible. Many of the sandstones are very calcareous, and in some cases would perhaps be classed as arenaceous limestones.
I Handbook of Physical Geology, A.. J. Jukes-Brown, p. 87, London, 1884.
REGION NOltTII OF (iALLATIN RIVER. Com})ari8on of sections of Quadrant formation.
41
|
"Pocket." |
Feet. |
"Amphitheater." |
Feet. |
"yiiailninl." southeast corner. |
FePt. |
|
r, q. Sandstone |
15 |
Sandstone with in- |
r. Limestone |
10 |
|
|
p. Limestone |
6 |
terhodded 1 i m e - |
p, (/. Sandstones . ^ |
15 |
|
|
5 |
stone |
265 |
0. Limestones . S |
||
|
n. Limestone |
15 |
n. Limestone |
12 |
||
|
m. Sandstone |
18 |
k-m. Qnartzite |
71 |
||
|
1. Limestone |
2 |
j. Limestone |
10 |
||
|
k. Sandstone, etc . |
148 |
/. Limestone |
4 |
i. Limestone h. Limestone |
30 5 |
|
209 |
|||||
|
e. Sandstone |
3 |
f,f,9- Qnartzite |
130 |
||
|
272 |
283 |
||||
|
d. Limestone |
2 |
d. Limestone .. .. |
2 |
||
|
c. Sandstone |
6 |
c. Sandstone |
6 |
||
|
h. Lime.stone |
10 |
b. Limestone |
10 |
||
|
a. Sandstone |
36 |
a. Talus |
100 |
||
|
326 |
401 |
||||
|
Madison limestones beneath above series. |
On the summit of the ridge west of Fawn Creek amphitheater, the Teton limestone is well exposed, the dip being 15° and the direction N. 26° W. These beds are undoubtedly tilted by their proximity to the Gray Peak bysmalith mass, although the tilting is not uniform, as the red sandstones forming the hill farther north have a dip of but 10° in the direction N. 15° W.
REGION NORTH OF GALLATIN RIVER.
As already pointed out, the escarpment wall on the north side of Gallatin River consists of massive Madison limestones topped by the white sandstone or qnartzite of the Quadrant formation. These beds dip at an angle of about 10° NE. throughout the greater part of the distance, having a more northerly dip in the vicinity of Bannock Peak, and at the western end of their exposure curving over from a northeasterly to a northwesterly dip of 5° to 10°, bending down toward the profound fault plane that bounds them on the west and brings them against subaerial breccias of andesite. Within the limits of this low anticlinal arch the sandstone is in places broken and polished with slickensides accomjDanying slight displacement
42 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK.
of the rock. A well-marked bench occurs along the top of the steep slope or escarpment, its surface in places sloping with the dip of the strata. It is most pronounced when above the harder beds, especially where these are overlain by the more friable sandstone and soft shales of the Juratrias formations.
The bench is well developed on both sides of the drainage channel running west from Fawn Pass. Upon entering the terrane of the Juratrias formations, which are mainly fissile limestones and shales, passing down- ward into the sandstones of possible Triassic age, and passing upward into the friable sandstones underlying the Dakota conglomerate,' we again encounter intruded sheets of igneous rock. These intrusive sheets are nearly conformable with the sedimentary strata, following the shale belts for long distances, and only occasionally breaking up across the strata to follow higher horizons. The first of these intrusive sheets met with above the Carboniferous limestones occurs in the base of the Juratrias shales below the Ellis shale beds, and forms a cliff rising above the bench west of Fawn Pass. The sheet of aiidesite-porphyry is possibly 200 feet thick at this place. It can be traced west and east from this exposure, con- tinuing at nearly the same horizon. The shale in contact with the poi'phyry is more or less baked, and, like the igneous rock, resists erosion better than the altered shales, thus leading to the formation of blutfs or ledges.
Above this porphyry cliff the country slopes gradually and stretches eastward to Fawn Pass, rising abruptly to the triangular peak 1^ miles southwest of Gray Peak. This comj^aratively level country occupies the axis of a flat synclinal arch that dips to the northwest. The strata bend around from the gentle northeast dip through a northwesterly one to a southwest dip of 15° to 20°, in which position they form the triangular peak just mentioned. It is evident, from a study of the region, that this southwesterly dip is due to the intrusion of a large body of igneous rock connected with the bysmalith of Gray Peak. The highest sedimentary rock in the triangular peak is Dakota conglomerate. It occurs again high up on the west spur of Gray Peak, where it dips toward the northeast. The ridge between these points traverses an anticlinal arch of Jurassic beds that bend over the igneous mass already mentioned. The shales include at least two thin sheets of igneous rock, each from 40 to 100 feet thick. One of these, in the mass of the triangular peak, thins out perceptibly toward the
REGION NORTH OF GALLATIN RIVER, 43
southwest. The coiTesponding intrusive sheets beneath the Dakota con- glomerate in the southern slope of Gray Peak thin out towai-d the east, and near the end of the spur one of them breaks upward as a dike-like body across the Dakota conglomerate.
The porphyry forming the axis of this small arch extends south, constitutmg the ridge of Fawn Pass. It extends east down the valley of Fawn Creek as an intrusive sheet near the base of the Juratrias shales, and extends south of Fawn Pass as an intrusive sheet at the same horizon, and has been traced as a ledge along the ridge south and westward to the cliff first described north of Gallatin River. It becomes thinner as it is followed farther from the bysmalith, and it is e\'ident that the intrusive sheets in this vicinity proceeded from the Gray Peak intrusive mass.
The sedimentary beds forming the mountain side south of Gray Peak dip into the mountain toward the north and northeast at a low angle and encounter the igneous rock of the bysmalith which forms the highest portion of the mountain mass from Gray Peak to Joseph Peak, and extends down the east slope to a level of 9,000 feet and down the west side to below this altitude. The igneous rock extends along tlie north face of the ridge west of Gray Peak. From it also proceed sheets of porphyry intruded between the Juratrias strata which are exposed along the south face of Little Quadi-ant Mountain and may be traced around the northern slopes.
On the northern side of the mass the sedimentary beds dip- toward the southeast, into the igneous core Again, as at the southern side of this body, the highest horizon is that of the Dakota conglomerate which is found at the summit of Joseph Peak in contact with the intruded mass. As may be seen from the map and cross sections (Pis. IX and X), there is a quaquaversal arching of the strata, the center of which is located in the head of Fan Creek, northwest of Joseph Peak. From this point the beds dip south, southeast, east, northeast, north, and northwest. In the three valleys heading against the ridge surrounding this arch the beds dip to the east, northeast, and north at angles not far from 10° — in some cases reaching 25°.
At the west end of the ridge north of this part of Fan Creek, the beds arch over to the west and southwest with a dip of 20°, and pitch against the same fault plane noted north of Gallatin River which let down the volcanic breccia. Between the sedimentarv strata, sheets of andesite-
44 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK.
poi-phyry have been intruded exactly as on the southern and eastern sides of the bysmahth. The lowest one exposed, however, is beneath the Juratrias beds, immediately above the Carboniferous limestone. It appears around the head of Fan Creek, thinning- out northward. Within the Ellis shales, beneath the Dakota conglomerate, there are five thin sheets of intrusive rock on the northern slope of Joseph Peak. Three of these have been recognized north of the saddle between Fan Creek and Gardiner River. They grow thinner and less noticeable to the northwest, and may be traced down the east slope of Joseph Peak, where, on account of the position of the beds, they form isolated patches. These sheets vary in thickness from 16 or 20 to 100 feet.
Above the Dakota conglomerate and sandstone, the shales and sand- stones that alternate with one another through a thickness of nearly 3,000 feet, constituting the Colorado and Montana formations, take part in the quaquaversal arching already described — that is, on the northern side. On the south they have been removed by erosion. In the ridge noi'th of Fan Creek they dip to the northwest and north, curving over to a northeasterly dip in the ridge connecting this with Electric Peak, throughout which latter ridge they maintain a generally uniform dip to the northeast, con- tinuing the same attitude beyond the boundary of the Yellowstone Park to the synclinal trough at Horr. In the ridge between the head branches of Gaidiner Kiver, these beds curve from an easterly dip near its west end to a northeasterly one farther down the ridge, and in Little Quadrant Mountain they also maintain a general northeasterly dip, as already noted.
The alternation of shales and sandstone layers seems to have been particularly favorable to the intrusion of sheets of igneous magmas. The fissile shale offered numerous planes of weakness and parting, while the sandstone layers tended to stifi"en the strata and cause the splitting to follow more nearly constant horizons, for though there is some cross fracturing of the sedimentary beds, where the igneous rock may be seen crossing the strata to higher horizons, yet the persistency of the intrasive sheets is one of their marked features. This is observed both upon actual exjjosures over long distances and upon the comparison of geological sections made across the strata by several observers in numerous localities.
In the ridge north of Fan Creek the Colorado shales form the northern slope and steep spurs and a small portion of the western end. Directly
THE FAN. 45
above the Dakota oong-lonierate tlie shales are spht by two thin sheets of aiidesite-porphvry, and also by a massive layer that appears as a small lac- colith constituting- the northwestern end of the ridge. The petrographical character of the sheets is not the same in all cases, but the distinctions are slig-ht and will be discussed in Chapter II. In the high ridge between the head branches of Gardiner Kiver, at least five different sheets of andesite- porphyry were observed, having the general dip of the shales and sandstones, with occasional ruptures across the beds. They were also found crossing the valley to the north and forming part of the ridge leading to Electric Peak, as represented on the map. Their thicknesses are not constant, as may be seen in their exposui'es, but the actual variation is greatly exaggerated in appearance by the positions of the exposures, whether directly across the thickness or more or less parallel to the sheet. This impression is still further increased by the appearance produced by the more persistent talus slopes of the harder porphyry, which often obsciire more easily removable areas of the softer shale. The same sheets occur in the shales in the upper part of Little Quadrant Mountain.
THE FAN.
Fan Creek drains the mountainous area whose various spurs converge to the west at the ribs of the Fan, from which resemblance the region derives its name. The encircling ridge which forms the divide between the waters of Fan Creek and those of the Gallatin and Gardiner drainages culminates in two prominent peaks, one of which. Gray Peak, has already been described. The other, lying to the north, is named Joseph Peak, and occupies a commanding position just west of Little Quadrant Mountain. The southei'n fork of Fan Creek is named Stellaria Creek. At the head of this stream the high ridge which is the southwestern extension of Gray Peak is formed of Mesozoic beds, having a strike of N. 20° E. and a dip of 10° W. The peak is formed of Dakota sandstone resting upon the Jura- trias beds, and is cixt by intrusive sheets of andesite-porphyry from the Gray Peak bysmalith, as already described on page 42.
46 GEOLOGY OF THE YELLOWSTONE NATIONAL TARK.
The following section represents the beds exposed on the northern slopes of this ridge at a point just west of the 9,900-foot peak:
Section at ridge southwest of Gray Peak.
Num- ber. Feet.
3 Dakota conglomerate and sandstone; pebbly in layers, but much of it a fine-grained sand- stone, buff with red and white blotches, and cross bedded 60
2 Shaly beds, very arenaceous and light brown at top; softer and more argillaceous below, where the layers are generally a blue-black and contain some splintery limestone. The
typical rock is a soft argillaceous sandstone, light gray, weathering brown 50
Andesite-porphyry, dark gray, compact; rock occurring irregularly columnar in ledges, with two or three layers of brown altered porphyry with fine concentric weathering 35
1 Limestone, gray with rusty speckling, saccharoidal texture. The upper 8 feet soft and
purple shale, weathering brown 18
Strike, N. 20'^ E. ; dip, 8^-10^^ W.
On the west side of this peak, 100 feet below the summit, the black shales are well exposed and form the crest of the ridge down to the saddle. West of the saddle they give place to the Dakota sandstone, which extends westward to the cliffs indicated upon the map. Farther west the ridge shows a succession of light-gi-ay limestones overlying the red Teton sand- stones. The latter beds form red slopes that extend westward to the andesite-porjihyry hill shown upon the map.
The central ridge of the Fan, lying north of Stellaria Creek and west of Joseph Peak, is a long flat-topped mass with grassy meadows and dense forests of pine. On the western end of this ridge exposures are scarce and must be sought for in the stream channel. A short distance above the forks of Fan Creek, Stellaria Creek has cut a gorge through the intrusive sheet of andesite-porphyry. This rock is also exposed on the south slopes of the ridge to the north for a distance of 2^ miles above the mouth of Stellaria Creek. The rock is generally much decomposed, of a light-buff color, with numerous decomposed acicular hornblende and white plagioclase phenocrysts. The porphyrj^ forms great heaps of tabular dc^bris, often arranged in ridges running apj^roximately east and west and separated from the solid rock by the hollows between the cliffs and these morainal ridges. These hollows are often without outlet, and sometimes hold small ponds. From the junction of the stream eastward the summit of the ridge shows no outcrops of sedimentary rocks, the covering of the porphyry sheet just noticed having been removed by erosion and the summit being now hea\aly mantled with glacial drift, which seems to be at least 100 feet thick in the
JOSEPH PEAK. 47
transverse drainage channel. This transverse drainage channel, which runs northward to join Fan Creek, forms the natural boundary line between the sedinientaries on the east and the andesite-porphyry on the west. The sedimentary rocks are light-gray limestones, having a strike of S. 20° W., and a dip of 10° W., which would carry the beds under the drift forming the summit of the ridge to the west. The higher slopes to the east show good exposm-es of the sedimentary rocks. A western spur of Joseph Peak shows the following section of sedimentary rocks, the lowest bed exposed being part of tlie Quadrant quartzites.
Joseph Peak section.
Num- ber.
Feet.
{5 Ked bed.s, calcareous sandstones, etc.
4 Sandstone and limestones, fissile, gray, weathering brown-gray 25
3 Limestone, gray, compact 10
Teton >
limestones \^ Cherty sandstones and limestones 100
Quadrant
niiart?itps ' ^ Sandstones, quartzites, and interbedded gray limestones 300
> 1 Sandstones, quartzites, and interbedded gray limestones .
At the base of this spur the summit of the ridge is nearly flat and is largely strewn with chert weathered out of the Teton limestones. The beds here apparently dip SE. 10°, Avhich takes them underneath Joseph Peak. The cherty beds of the Teton series are here quite well exposed. The chert occurs in both banded and nodular forms, and is so abundant that the remaining material, which is generally sandstone, forms but a minor feature. In seams and patches, however, the rock is free from chert, and is then much more calcareous and contains fragments of fossils. North of the spur from Joseph Peak the Teton limestone beds dip W. 8°, and strike S. 8° E. Farther north the flat summit of the ridge is formed of a fine-grained andesite-porphyry, whose rust-colored exposures much resem- ble those of the sedimentary rocks. This rock forms the summit of a bold cliff to the north. This cliff", which is about 500 feet high, shows excellent exposures of the Quadrant quartzites resting upon Madison limestones, forming the channel of Fan Creek, and capped just beneath the andesite- porphyry by the cherty beds of the Teton series. This exposure shows a total thickness of 350 feet of Quadrant quartzite and 125 feet of the Teton limestone. The character of the cherty limestones varies from a dove- colored, nearly pure limestone to a granular brown sandstone which is not
48 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK.
calcareous. The talus at the base of the cliffs, like that of the andesitic intrusive of Stellaria Creek, is arranged in morainal ridges, with a depres- sion between these heapings and the base of the cliff. The valley of Fan Creek has been cut in the low anticline, exposing the Madison lime- stones overlain by the Quadrant quartzites, with the softer Mesozoic rocks forming higher slopes to the north. In the small drainage cutting the slopes north of the creek the following section was measured:
Fan Greeh section.
Num- Feet,
ber.
( 15 Dakota conglomerate and sandstone, the latter gray, with white spots,
* °'" (14 Sandstone, rusty yellow, blotched with piuk, slightly calcareous 125
(•13 Limestone, dark gray, crystalline 5
12 Limestone, fissile and shaly, gray 15
lie Arenaceous limestone, gray, crumbly, rusty 1
EDig 116 Sh.iles and crumbly limestones 2
sandatoue. 1 Ha Limestone, crysfcilline, gray, dense, splintery, argillaceous 1
10 Arenaceous limestone, or calcareous sandstone, light brown-gray 15
9 Limestone, grading at top into No. 10. The limestone is pure and full of
fossils 15
8 Limestone, argillaceous, soft, crumbly 10
7 Argillaceous limestones, crumbly, containing fossils, and with interbedded
layers of harder crystalline limestone 140
6 Red shales 20
5 Green and blue shales 30
4 Interval (?)
3 Cherty beds 125
2 AVhite sandstones, etc 300
1 Limestone, crystalline, creamy, with patches of red magnesian limestone 30
Ellis J limestone. |
Teton I shales. |
Teton ) limestone. \ Quadrant ( quartzite. ^ Madison limestone
The summit of the ridge north of Fan Creek, already mentioned on page 43, forms what might be termed the northern rib of the Fan and is capped by Dakota beds, whose persistent nature, combined with that of the intrusive sheets of andesite-porphyry, has left the ridge sharply defined. The Dakota conglomerate is but 20 feet thick and is overlain by buff-colored and pink sandstones similar to those mentioned in the section just given. At the head of Fan Creek a depression in the mountain ridge forms a pass to the headwaters of Gardiner River. The western slopes of this pass are thickly covered with soil and vegetation, and no exposures are seen, but to the east the beds are well exposed where the streams from the snow banks of the ridge have washed the surface of the rocks bare of
Ellis limestone
FAN.PASS. 49
soil. The saddle itself is toniu'd of liiuestoues broken throiiii-h hv juidesite- porphyr\-. The folhtwiug- section shows the series of beds exposed from the pass to the snnnnit of the peak to the north:
Fan I'ass section.
Num- l"r. Feet.
Dakota. S Dakota coiiu'lomerate and saudstono; creamy, pink, broken tlirongli by an-
de.'iite porphyry ^0
( 11) I>iniestone, bard, gray, crystalliuo, weathering brown 10
7« Limestone, granular, arenaceous, light drab 15
Ellis J (! Limestone, more or less arenaceous in certain bauds, fossililerons. Strike,
sandstone. 1 N.45-' W.; dip, 2.")- N 75
I oft Limestone, soft and sandy ; 1q
\ 5a ]>imestone, arenaceous, gray-brown 5
I ib Ked shale, crumbly and soft g
in Sliite ; metamorphosed by audesite-porphyry 2
.Vndesite-jiorphyry-
3 Limestones, soft, crumbly, very argillaceous, with harder crystalline layers;
very fossil iferons 5g
2 Limestone, finely crystalline, weathers brown, somewhat slialy 5
1 Limestone, shaly, broken through by andesite-porphyry 45
To the east of Fan Pass an extension of the anticlinal uplift noted to the west brings up the red shales and sandstones from the upper portion of the Teton formation. From the summit of the mountain peak north of Fan Pass the ledge extends in a northwest direction until it meets the long southwestern spur of Electric Peak. The Dakota conglomerate forms the crest of the ridge as far as the second peak north of Fan Pass, and, as already noted, the same rock extends westward, forming the crest of the encircling ridge The summit of the second peak north of Fan Pass is formed of andesite-porphyry. This rests upon Dakota conglomerate and is overlain by cherty limestone, which is apparently part of the Dakota limestones, but is of different habit and carries light greenish-yellow chert. Between this point and the 10,100-foot peak to the northeast a succession of beds is exposed in which the shales are cut by intrusive sheets of andesite-porphyry. The Dakota limestone, somewhat metamorphosed, but •showing the crystalline marks and the little gasteropod shells so character- istic of this horizon, is overlain by very splintery greenish-yellow shale, weathering brown. This in turn is capped by a baked sandstone about 20 feet thick, which is overlain by the upper quartzite belt of the Dakota series, the bed being here 30 feet thick. The carbonaceous shales of the Benton formation form the ridge from this point eastward to the slopes of
MON XXXII, PT II 4
50 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK.
Electric Peak. The shales are cut by numerous intrusive sheets of andesite-porplivry and b}' a few dikes, one of which cuts the 10,100- foot peak already noted. In the sandstone bed intercalated in the lower part of the Benton shales the little oyster, Ontrea anomioides, occurs abundantly. The beds here have a dip of 10° N., the strike being N. 75° E. The andesitic sheets noted on this ridge ai-e continuous for long distances, their ledges being traceable along the slopes ou either side of the ridge. In general, the andesitic sheets, being less easily eroded than the soft black shales, form the high points and mountainous summits of the ridge, while the saddles are cut in the softer rocks.
ELECTRIC PEAK.
Electric Peak is the highest and most imposing summit of the Gallatin Range. Its apex rises boldly above the adjacent mountains, and the long ridges which form its foundations dominate the country for many miles. The mountain is composed of sedimentary rocks of Cretaceous age, broken through and in part largel}' altered by igneovis rocks. The sedimentary rocks only will be treated here, as these igneous rocks and their occurrence are of such interest that a special chapter is devoted to them.
The sedimentary rocks composing the mountain embrace the most recent strata of the sedimentary series to be found in the Gallatin Range, including a thickness of 4,300 feet of Cretaceous beds, whose uppermost portion is coal-bearing and belongs to the Laramie. Complete sections may be studied at two localities. One is the southeastern spur of the peak, where the beds are sharply upturned against the Gallatin fault; the other is the long northern ridge of the peak which terminates in that mass of upturned and exposed strata known as Cinnabar Mountain. Although the latter locality lies just outside the limits of the Park, the section there exposed is typical for the region, and, combined with the sections already given of the Teton, Ellis, and Dakota formations, it forms a complete sec- tion of the Mesozoic strata of the range.
Section of beds exposed on southeast sjmr of Electric Peak.
Nam- ber.
f 22 Carbonaceous sliale '40
Anilesite-porphy ry ■10
Feet.
s
g ' 5 ' Carbonaceoiis shale 100
«
o Andesite-porpby ry ^0
I Carbouaceous shale 300
.500
9
U S GEOLOGICAL SURVEY
MOHooFAPH XXXII, fart:; ?L ll<
CROWFOOT RIDGE
OUNT HOLMES
EMIGRANT PEAK
THE CRAGS
«gn
SEPULCHRE MOUNTAIN
ELECTRIC PEtt-
GRAY PEAK JOSEPH PEAK
ELECTRIC PEAK
EMIGRANT PEAK
SEPULCHRE MT-
GEOLOOICAL SKCTIONS ACHOSS CALLATLX HANOK
I.KIVKNL)
CRETACEOUS JURATRIAS CARBONIFEROUS CAMBRIAN
ik/h i^' h B H Hi' 'CK
Scale oF Miles
r^y^
ELECTRIC PEAK SECTION.
51
Scctimi of beds c.qjoscd on southeast spur of Electric I'eal- — Continued.
Niini- bor.
21 Cnlcareous saiidstonos, chan;;ing to pure Siindstoues at the bottom
20 S;iii(lsti)iio, mottled and carbonaPrDiis
ly Sandstone, indurated and argiUaeeous
, 18 Sandstones, speckled
Andesite-porpliyry, much decomposed
17 Calcareous sandstone
16 Sandstone, indurated, argillaceous
Andesite-porphy ry
Interval, no exposure
15 Limestone, very impure, argillaceous and arenaceous
Interval, no exposure
14 Indurated sandstone
13 Shales, very dark slate-colored, argillaceous rocks which are poorly exposed
12 Sandstone, indurated to granular quartz ite 5
Interval, no exposure 75
Indurated sandstone 10
11
Lio
f 9
r 6
I 5
W I
i: J
^ V
Sandstones and arenaceous limestones, -with two intruded sheets of andesite-por- phyry aggregating 15 feet in thickness
Interval, no exposure, hut showing debris of dark-colored carbonaceous shales belonging to bed helow
Andesite-porphyry, much decomposed
Carbonaceous, argillaceous shale
Sandstone
Interval, no exposure
Limestone
Interval, no exposure
Conglomerate and sandstone, broken through l)y andesite-porphyry
Arenaceous limestone
Arenaceous limestones, passing downward into sandstones, cross bedded, and chang- ing to limestones near base
Interval, no exposure
4 Sandstone, dark gray, mottled with carbonaceous matter, fine grained and not indu- rated
Interval, no exposure
3 Limestone
Interval, no exposure
2 Quartzite and highly indurated calcareous sandstones
^)
IS L
Limestone, light cream colored; bands of red limestone exposed along north bank
of Gardiner River, 200 feet below top
Limestone, brownish gray, dense
Ft-et.
175 1)1) 20 10 15 30 50 30
10.) 10
100 10
300
(10 160
300
15
100
20
50
5
50
5
45
50
5
100
100
15
200
250
50
Total thicliness of section. Thickness of intrusive sheets
3,47« 148
Total thickness of sedimentary rocks •. 3, 330
52 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK.
The audesite-porphyvies of this section occur as sheets intruded along the planes of bedding- of the sedimentary rocks. With one exception intrusion is regular and does not break across the bedding planes of the shale. The dip and strike of the beds vary in passing upward from Gardi- ner River to the summit of the peak. In many cases this change of dip is apparent in the exposure, but in most cases the outcrop is not suffi- ciently extensive to make the flexure apparent.
The long southwestern spur of Electric Peak, which has already been noted, shows sheet after sheet of andesite-porphyry cutting across the crest of the ridge and intruded in the dark shales of the Colorado formation, the beds dipping NE. about 10°. Three different bodies of andesite- porphyry have broken through one another in the high point southwest of the peak. One of these intrusions may be traced eastward along the southern slope of the mountain for some distance. The western slope of the mountain shows the sedimentary and igneous layers in strong relief, and they may be distinguished at a distance. The inclination of these ledges is about 10° N., corresponding to the greater dip, which is to the northeast. The sheets of igneous rock are seen to follow the bedding planes of the strata for considerable distance. Only one sheet was seen break- ing up across the strata and proceeding along the higher horizon. The direction of this uprising is from sovith toward the north, and this occur- rence, together with observations made on the eastern side of the moun- tain, showing a similar rise from the west to the east and a thinning out of the sheets in the same direction, indicates that these intrusive masses were injected from the southwest— that is, probably from the center occu- pied by the Gray Peak bysmalith. The intrusive sheets vary from a few feet to a hundred or more feet in thickness. The rocks differ slightly in petrographical character and will be described more fully in Chapter II. The peak itself is formed of the soft shales and thinly bedded sandstones belonging to the Colorado and Montana formations. These rocks are penetrated by a number of intrusive dikes of andesite-porphyry, and the sedimentary series is much altered by the great intrusion of igneous rock forming the volcanic core east of the peak. On the summit of the peak the normal sandstones and shales are altered to slates and quartzites, the rocks being much shattered by joints and breaking readily into short, angular ddbris. The beds dip N. 10° to 20°.
ELKCTUIC PEAK,
53
The intrusive sheets extend, in Jiininishin<>' numbers, northward in tlie north ridge of Ek'ctrii' Peak. The}' i-esenibk^ the sheets intrude<l in tlu^ Jurassic shales of Cinnabar Mountain, thoug-h it is ])ossible that tlie latter intrusive rocks were derived from other sources more directly connected with the synclinal folding and faulting of Cinnabar Mountain.
Tlie north ridge of Electric Peak terminates in the low knob called Cinnabar jMountain. This elevation is formed of upturned sedimentary beds, presenting a most excellent and complete section of the stratigraphic series from the Paleozoic to the summit of the Laramie, a section which is here given, as it is typical for the Gallatin region.'
Section of the Mesozoic sedimentary rocks exposed in Cinnabar Mountain and
Electric Peal:
i\
Sandstones alternating with shales and carrying coal .seams, many of ivhieb are workable
|- White, massive, and cross-bedded sandstone, easily disintegrated and crumbling readily I under pressure
Impure sandstone, gray in color, carrying argillaceous material and often calcareous. The outcrops frequently weather into flagstones a few inches thick, forming broken reefs that project above the smoother slopes
Alternating fissile sandstones and impure argillaceous shales which frequently carry lenses of purer sandstone, and near the base contain much bituminous shale, which is soft and crumbly upon weathering
Arenaceous shales and shaly sandstones, generally greenish gray in color, and weathering
into line brown df^bris
' Sandstones, generally forming ledges projecting above the slopes of shale
Gray shales and shaly sandstones
Soft, bituminous, black shale, weathering readily and forming a smooth slope covered with the fine fragments of the leafy shale
Calcareous beds, varying from impure arenaceous limestones to calcareous shale
Black shales, alternating with thin arenaceous beds, and containing strata of blue splintery limestones which are argillaceous, dense in structure, and do not form per- sistent horizons
Quartzite, forming a reef that projects as a wall above the slopes of shale
Black and dark-blue shales, varying from arenaceous, light-gray, impure sandstones to black, laminated, bituminous shales, and carrying the same impure dark-blue lime- stones found above
Sandstone, generally massive, gray in color, weathering with a rusty surface; fissile, granular in texture, and forming a wall projecting above the slope
Sandstone, very fissile, and grading into an arenaceous shale
Impure limestone, passing into argillaceous black shales and arenaceous shales
Dark-colored, bUiish-black, finely laminated shales with occasional interbedded Saud- is stones
Feet. 800
125 240
450
226
38
164
500 40
400 5
350
15 50
265
' See Cinnabar and Bozemau coal fields, by W. H. Weed: Bull. Geol. Soc. Am., Vol. II, 1891, p. 3.52.
|
■ |
||
|
C3 |
||
|
O |
||
|
s |
C3 |
|
|
e |
Q |
|
|
o |
||
|
- |
54 GEOLOGY OF THE YELLOWSTONE NATIONAL PAEK.
Section of the Mesozoic sedimentary rocks exposed in Cinnabar Mountain, etc. — Cont'd.
Feet.
Quartzite rhangiug to sandstone, light gray, dense in texture, and forming a prominent wall 15 to 20 feet wide 5q
Limestones, blue-gray in color but weathering to a creamy buff tint on the surface. The rock is dense in texture and shows the remains of the same gasteropods found in the beds of Little Quadrant Mountain. In the Cinnabar section this intermediate horizon includes a portion of the thickness given to the Dakota conglomerate, but it is char- acterized by red maguesian limestoues which possess many characters similar to those of the volcanic ash beds found farther north above the Dakota conglomerate, and passing into sandy shales which are capped by 10 feet of quite pure limestone 75
Dakota conglomerate and sandstone 175
f Reddish shales and impure limestones 85
Limestoues and calcareous sandstones, occasionally a conglomerate; carries an abun- dance of fossil remains, which are generally fragmentary near the bas<i 75
Red argillaceous shale $
Gray calcareous shales and impure limestones, characterized by an abundance of fossil remains, particularly iu the upper strata. The beds are separated near the center by oolitic limestoues 132
Green and red shales 50
Sandstone, saccharoidal in texture, generally light gray or bufi' in color, but red or browu on weathered surface 50
Red beds, consisting of very fissile sandstones and impure arenaceous clays 75
Limestone, compact in texture, gray iu color, and carrying remains of liugulas 20
■§ * Limestones, dark gray in color, generally fetid, often arenaceous, and frecjuently char- acterized by rod-like masses of chert, which are seen to consist of grains of sand
embedded in the siliceous matrix, the concretions having a white chalky surface 125
[ Quadrant iiuartzites.
The western face of Electric Peak shows a number of sheets of iffiieous rock, varying from 5 to 50 feet in thickness; but no dikes, either vertical or incHned, were observed on this side of the mountain. The western summit consists of a sheet of andesite-porphyry several hundred feet thick. In the eastern spurs of the mountain numerous vertical and inclined dikes cut the ujjturned beds. Those on the southeastern spur trend to the south- west and northeast. They also traverse the eastern summit of Electric Peak. The southeastern spur of the mountain shows steej^ly upturned sedimentary l)eds, which at the base are overthrown and reversed. This spur probably consists of a synclinal fold which was accompanied by faulting, the eastern limb, forming the lower portion of the spur, having a nearly vertical position. The axis of the syucline has a trend to the southwest and northeast. The overthrown beds at the southern extrem- ity of this spur show Madison limestones. Quadrant quartzites, and the regular sequence of overlying Mesozoic strata. The beds strike north, and they dip from 50° to 70° E. The Gardiner River cuts across the
ELKGTHIO PEAK. 55
t'lul of" this spur, tlic rocks being exposed in a small hill on the south- western side of" the stream, the channel having j)robably been deflected at this place by the accumidatiou of glacial drift filling the old valley between Sepulchre Mountain and Little Quadrant. A detailed section of the beds ex})osed on the crest oi this ridge has been given. The strike of the beds varies somewhat in ascending the s})ur, and the dip also changes. The Dakota ledge, which crosses the ridge about 500 feet above the river, has a strike of N. 32° E., showing a considerable change in direction between the ex[)osures here and those in the river bed. The axis of this synclinal fold and the fault plane are found high up on this southeastern spur, where the Colorado shales have a vertical dip. The exact position of the fault plane can not be determined, and the extent of displacement is not known. That the intrusive sheets of igneous rock antedated the folding is clearly shown in the crushing and slight dynamic metamorjjhism observed here. Slickensides are found within the porphyry, and dragging planes are observable between the hard eruptive rock and the soft shales. A coue-in-cone structure of the latter rocks is also observed. On the other hand, the dikes of igneous rock which cut this spur of the mountain show no sign of disturbance attributable to the synclinal folding. They intersect sedimentary beds, and also the inter- calated sheets of porphyry, at various angles. Where the shales and intrusive sheets are on edge, the dikes are often parallel to them and are easily confused with the intrusive sheets, which they closely resemble in petrographical character. These dikes are connected with a large body of igneous rock, mostly a diorite, which occupies a position on the line of faulting and is situated in the deep gulch cut in the eastern summit of Electric Peak. The sedimentary beds in the neighborhood of this mass of igneous rock are extensively metamorphosed.
At the eastern base of Electric Peak a profound fault separates the mountain mass from the complex body of volcanic tuff-breccias and massive igneous rocks to the east. This fault is the northern continuation of the Gallatin fault, which has given rise to the abrupt escarpment faces on the east side of the Gallatin Range.
On account of the special importance of the relations existing between the igneous rocks of Electric Peak and those forming Sepulchre Mountain to the east of the fault, a detailed description of the geology of this locality is given in Chapter III, in which the petrology also will be fully discussed.
56 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. WESTERSr FLANKS OF THE GALLATIN RAKGE.
In the northwestern portion of the Gallatin Range, within the Yellow- stone Park, the western slopes of the mountains are abruptly terminated by a fault, bringing- up the sedimentary beds against subaerial volcanic breccias that probably represent a remnant of the old Electric Peak and Sepulchre Mountain volcanic cone. The fault, where it crosses Fan Creek and along its course down Cinnabar Creek, shows a profound displacement. Its southern continuation has already been noted west of Gray Peak, but it becomes of slight importance in the vicinity of Grayling Creek, west of the end of Crowfoot Ridge. The andesitic breccias which form the high mountain ridges west of this fault are continuous with the high range of the Gallatin which stretches northward along the western side of the Yellowstone River to the vicinity of Bozeman. Within the park region the underlying upturned and irregularly eroded sedimentary beds are not exposed. The position and dip of the strata forming the high ridge on the eastern side of the Gallatin River, just within the northwestern corner of the park, show a monoclinal structure which would bring the Montana shales and sandstones beneath the andesite breccias and against the Fan Creek fault. It is clear that the horizon west of the fault was consid- erably higher than that to the east, for the latter is near the base of the Colorado shales and contains laccolithic sheets, which must have been intruded at considerable depths beneath the surface of the country, and were contemporaneous with those intruded in the upper part of the Colo- rado formation of Electric Peak, which is but 3 miles distant. Another fact that is apparent is that the sedimentary strata were upturned and eroded down to the Carboniferous sandstone, 7 miles to the west, before the volcanic breccias were thrown out over the country.
The only other andesitic tuff-breccia in this vicinity occurs in isolated patches resting directly upon crystalline schists in the neighborhood of The Crags, 5 and 10 miles south of the breccia west of Gray Peak, and from 13 to 17 miles distant from the Electric Peak center of the eruption. From these facts it would appear that the surface of the country at the time when the andesitic tuff-breccias were deposited consisted of crystalline schists in the south, of Carboniferous strata in the west, and of Cretaceous strata in the northeast. This indicates the uplifting of the
U S GEOLOGICAL SURVEY.
MONOGRAPH XXXII.PART II, PL X.
MONTANA "WYOM/Nd" "k
/ anp
"0 50' BOUNDARY
-. ■; 5
GEOLOGICAL MAP
OF
GALLATIN RAN^GE , YELLOWSTONE NATIONAL PARK.
A.Hoen&Co.Litb.Ballir>i
PLEISTOCENE
CRETACEOUS
LEGEND
JURATRIAS
CARBONIFEROUS DEVONIAN SILURIAN
|
Phs |
Pal |
Pgd |
|
Km |
Kc |
M |
|
' Je " |
Jt |
|
' Cq 'i |
Cm |
tfr:
Sj
Hot Springs AUnviran. Glacial Montana Coloraflo Dakota Ellis Totoii Quadrant Madison
formation drift. formatioii-. formatioii. fomiatioiL formation, formation, formation, limestone.
CAMBRIAN
NEOCENE
EOCENE
Thi"eot()rt(s limestone.
ARCHEAN
JetfersoH limestone.
H
Nbst
Nrh
Nel
|
Nebb |
Eeab |
dp |
anp |
-fl^n |
— |
Gallatin PJathead limestone, formation-
Basalt.
Rliyolite. Khmiiir Kersanlite. Earlv basic f^ailv acid UaciUi- Andtisite- intrusives. breccici. breccia. porphyry, porphyi'y.
Graiiilo and s^nciss.
Yaoilts.
^-^-^£-^-S-
Scale 12SOOO
■ f f ?
CONTOUR INTERVAL lOO FEET.
MADISUN liANGE. 57
sedimentary strata and their tilting' iKirthward, followed by extensive erosion at the south prior to the extravasation of tlie andesitic breccia.
We have already jjointed out the connection between the hitrusion of the Holmes bysmalith and the fault traversing the eastern end of Crow- foot Rido-e, noting their probable contemporaneity. The intrusion of the Holmes mass nuxst have been followed by extensive erosion before the cr'S'stalline schists were exposed at the level they now occupy relative to the Holmes mass, after which erosion the andesitic breccias were thrown ujjon them. This separates the eruption of the Holmes bysmalith and that of the andesitic breccias by a very considerable length of time. No definite time relation has been made out, however, between the two great intrusive bodies at the southern end of the Gallatin Range — Indian Creek laccolith and Holmes bysmalith — and the more complex intrusion of Gray Peak and the associated sheets of andesite-porphyry in the northern part of the range. Though separated by only a small distance, there is no structural feature which connects their intrusions in point of time, except the general fact that the}^ are all much older than the eruptions that centered at Electric Peak.
EASTERN FliANK OF THE MADISON^ RANGE.
In the extreme northwestern corner of the Yellowstone Park there is a small area of mountainous country that is part of the eastern flank of the Madison Range. This area lies wholly within the Montana portion of the reservation. The Gallatin River has cut a narrow valley across this tract, exposing folded strata, in which the same sedimentary series seen in the Gallatin Range is developed, the lowest rocks belonging to the Cambrian and the highest being of Colorado Cretaceous age. These strata are flexed about a laccolith of andesite-porphyry.
This mountain area is terminated on the south by the northern end of the rhyolite plateau, whose lavas cover the southeastern flanks of the high mountain east of the Gallatin River and also occur in small isolated patches upon the mountain slopes to the north and west.
Topographically this little tract consists of parts of four distinct moun- tain masses. The largest lies east of the Gallatin and is embraced between that river and Fan Creek. This block and that north of it, and the flat- topped mountain west of the Gallatin River, are all parts of the lacco- lithic uplift, which has been cut through by the river. The east bank
Three Forks and
Jefferson.
58 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK.
of the Gallatin shows an excellent exposure of the contact between the andesite-porphyiy and the Cambrian shales, the latter being altered by contact metaniorphism for a few feet from the andesite-porphyry. The Cambrian shales are overlain l^y limestones in which there are intruded several sheets of andesite-porphyry, and are capped by cliffs of a heavily bedded white limestone of the Madison formation, with basic intrusions, near tlie summit of the mountain. The following partial section shows the series found immediately above the laccolith :
Section east of Gallatin River, below Fan Creel:
Feet. Madison. Limestones carrying cdrals, thickly bediled, of a dense texture, drab or dark-gray
colored, and holding black chert. Limestone shale of pink, red, buff, and pnrplish colors, carrying a few fossils and
underlain by dark-blue (almost black) limestone 200
Andesite-porphyry, poorly exposed 25
Granular, dark-brown and black limestone, sometimes banded, and of Silurian
aspect 15
Thinly bedded and fissile light-gray limestone, dense and not crystalline, impure
and carrying argillaceous matter 5
Uark-colored granular limestone, carrying Obolella 12
Limestone, thinly bedded and with a knotty texture, dark blue in color, of typical
Cambrian aspect, and evidently of shallow-water origin 30
Limestone shale, blue and olive gray in color 5
Limestone, dense in texture and dove colored 5
Limestone, becoming shale ; dip mO"^ to the east 5
Shales, green or olive colored, seldom exposed 15
Limestone, mottled, of typical Cambrian aspect 15
Shale 5
Andesite-porphyry laccolith.
West of the Gallatin River the mountain slopes show andesite-porphyry extendino- up nearly to the summit of the flat-topped mountain, but the stratified rocks are seen both to the north and to the south, forming great cm-ved plates, with dip away from the intrusion in every direction. A stream from the west has cut its valley in the dome, exposing the sedi- mentary rocks on the valley walls.
The mountain opposite the mouth of Fan Creek is composed entirely of Paleozoic strata, which are not affected by the laccolithic uplift, but dip to the east and northeast, away from the axis of the Madison Range. The lower slopes show Cambrian beds, which are overlain by the Silurian rocks, of which the most prominent strata are quartzitic in nature and form heavy, massive beds that cap the summit of the mountain and extend east-
Gallatin.
MADISON RANGE. 59
ward down to tlie valley of the Gallatin Kiver, the dip being about 20°. The summit of the mountain is flat, and shows Carboniferous limestones dijiping northeast at gentle angles. This mountain and the one north of it both show the characteristic flat-to2')ped topography noticed in the lesser marginal peaks of the Madison Range.
The andesite-porphyry of the laccolith is (p^ite like those rocks in the Gallatin already described, and its petrographic description is given in another chapter. A chai'acteristic feature of the exposures seen of this rock is the occurrence of numerous included fragments of gneiss, schist, and hornblende-porphyry. The two patches of rhyolite which occur upon the slopes rest directly upon this andesite-porphyry, showing a thorough dis- section of the laccolithic fold before the outpouring of the rhyolite flows.
CHAPTER 11.
THE INTRUSIVE ROCKS OF THE GALLATIN MOUNTAINS, BUNSEN PEAK, AND MOUNT EVERTS.
By Joseph Paxson Iddings.
Having described the occurrence of the igneous rocks that have been intruded within the sedimentary beds of the Gallatin Mountains, or have been thrown over their surface, so far as their occurrence is related to the history of the dynamic events that brought about the present structure and topography of the range, we may now describe their petrographical characters in relation to the mode of their occurrence, ^vith special reference to the size of the various bodies of rock and then* geological position.
From what has already been shown as to the relative age of the different intrusions, it will be proper to consider them in the following order: Indian Ci'eek laccolith; Holmes bysmalith and connected outliers; Bighorn Pass sheet; Gray Mountain intrusive and connected sheets; Electric Peak stock and dikes, together with the extrusive breccias and intrusive dikes of Sepulchre Mountain; and the breccias west and south of the Gallatin Range. In this connection may also be described the Buusen Peak intrusive and the intrusive sheets in Mount Everts.
INDIAIi^ CREEK LACCOLITH.
HORNBLENDE-MICA-ANDESITE-PORPHYRY.
The rock constituting this laccolith and its two sheet-like apophyses to the south is an intrusive mass, quite uniform in mineral composition throughout its whole extent. It exhibits a limited variability in texture and habit, ranging from those of a compact aphanitic or lithoidal lava to those of a minutely crystalline porphyry-like rock. Its predominant min- eral constituents are lime-soda feldspar, hornblende, and biotite, with a small amount of magnetite and, in the coarser-gi-ained forms, quartz. For this reason it may be called an andesite-porphyry with andesite facies. Its
60
INDIAN CKEEK LACCOLITH.
61
chemical coin]iositinn is rr'wen below. The portion of tlie rock analyzed was the unalt(;reil coarser-g-rained form (55)' occurring in the middle of the laccolith on the north side of Indian Creek.
,1 iKilysis of hornhlende-micaandesite porphyry.
[Analyst, J. E. Wliitfleld. 1
Coustitueut.
sio,
TiOj
AI2O3
FecO:,
FeO
MnO
MgO
CaO
Li;0
NajO
K.O
P2OB
SO-,
H;0
Total
Per cent.
61. 50
None.
17.42
4.66
1.09
Trace.
1.26
5. 33
.03
3.99
1.29
.60
.35
2.44
99.96
The main body of the laccolith, where it is about 1,000 feet thick, is a light-gray rock crowded with small crystals of feldspar, mica, and horn- blende, with a subordinate amount of groundmass, whose component grains are not discernible with the naked eye. The phenocrysts are 1 or 2 mm. in diameter and smaller; occasional ones reach 3 mm. The rock is distinctly massive, cracking with irregular joints into angular or somewhat tabular fragments, and exhibiting columnar jointing hi only one locality, on the southeast slope of The Dome. Under the microscope the most crystalline portion of the laccolith (57), which proved to be the eastern- central part of the mass on the south side of Indian Creek, is seen to consist of the pheno- crysts already named, cemented together by a holocrystalline aggregation of quartz and feldspar with scattered grains of biotite, hornblende, and magnetite (PI. XI, lig. 1). The areas of quartz inclose minute idiomorphic feldspar, in part, if not wholly, lime-soda feldspars, probably oligoclase. The quartz is allotriomorphic and has a micropoikilitic structure, the grains
' Numerals in brackets used in connection with the petrography in this monograph refer to the specimen numbers in the Yellowstone Park collection.
(32 GEOLOGY OF THE YELLOWSTONE XATIO^STAL PARK.
ranging from 0.08 to 0.2 mm. in diameter, the inclosed feldspars being about 0.04 mm. long and 0.008 mm. wide, and upward. Somewhat finer-grained forms were found in the central portions of the mass north of Indian Creek. In these there is a more marked difference between the groundmass and phenocrysts. When seen under the microscope, the microstructure of the groundmass is more evenly granular, the grains averaging about 0.04 mm. in diameter in one case (56), and about 0.024 mm. in another (55). This degree of crystallization corresponds to grade 20 of the table for the rocks of Electric Peak (Table XVII), for the first case, and to grades 11 and 9 of the same table for the last two respectively. The last is shown in PI. XI, fig. 2.
The phenocrysts are not sharply outlined, and have numerous inclu- sions of irregular grains or streaks of quartz and feldspar. The feldspar phenocrysts are all lime-soda feldspar, in part labradorite, in part andesine. They are frequently shattered, with irregular cracks, and are penetrated by irregular streaks of quartz and feldspar, whicli are sometimes granular. In places it looks as though the groundmass of the rock had penetrated cracks in the feldspars before it solidified. The feldspar individuals in one rock section are not all equally fissured, and not always in the same direc- tion, so that the cracking appears to antedate the solidification of the rock. The biotites exhibit very slight dislocation or bending in some cases, which may be referred to the same i:)eriod. The biotite is dark brown, with mod- erate absorption and occasional twinning. The outlines, often idiomorphic, are sometimes very irregular, there being marginal inclosures of quartz and feldspar, and sometimes of magnetite, but not often. The hornblende is green, with moderate pleochroism from strong green to light bi'own. The outlines are quite irregular, and inclusions of the other minerals are fre- quent. There is sometimes a chloritic mineral present in small pseudo- morphs, which may possibly l)e altered pj^roxene. In some instances it is decomposed hornblende. Magnetite is present in microscopic crystals, often idiomorphic; and apatite forms colorless microscopic crystals. Both hornblende and biotite take part in the composition of the groundmass in the more crystalline varieties.
Somewhat finer-grained microcrystalline structures are found in the rock, where it forms the thinner sheets, 100 to 150 feet thick, beneath Trilo- bite Point (72, 73). Here the structure is confused, being partly micro- poildhtic, partly nncrogranular. In the still finer-grained modifications the
U. S. QEOLOaiCAL SURVEY
MONOGRAPH XXXII PART II PL. XI
rA) % 29
rsj X 3 1
<C) X 28
(D) X 45
PHOTOMICROGRAPHS OF ANDESITE-PORPHYRY AND DACITE-PORPHYRY
THE HELIOTYPE PRrNTINO CO.. BOSTON
INDIAN CKEEK LACCOLITH. 63
inicropoikilitic (nuirtz patches fjTow less and less noticeable, and tlic feldspar inicrolites become more [)ronounced, the microstructm-e beiny moi'e like the characteristic t'elt-like or pilotaxitic structure of andesites. This transi- tion occurs as the rock approaches the contact with inclosing rocks, and where the body thins out. It accompanies a darkening of the rock and an iucreasino-ly andesitic habit. The rock 20 feet from the bottom contact near the east edge of the laccolith north of Indian Creek is bluish gray, with prominent feldspars that are decomposed (OO). The hornblendes are altered to chlorite and calcite. Biotite is still fresh. The s'roundmass is holocrystalline and • pilotaxitic. Near the Ijottom contact of the middle poi'tion of the main laccolith body the rock grows darker and denser. At 6 feet from the contact it is darker gray than the main mass. The micro- structure of the groundmass is micropoikilitic, with the minute feldspars maintaining a fluidal arraugen^ent (61). The rock 1 foot from contact is darker colored; its structure is still micropoikilitic, with more minute feld- spars (62), while the rock directly in contact with the limestone is still darker and the microstructure still finer grained and micropoikilitic (63). There is considerable calcite scattered in irregular microscopic aggregates through the groundmass. The hornblendes are decomposed, and there is some secondary quartz. The micropoikilitic structure, however, is not secondary, as seems to be the case in some porphyries,^ since it varies in size of grain according to the distance from the contact plane, and is quite the same as that observed in perfectly fresh andesite-porphyries in other places. Similar modifications occur near the contact of the andesite-porphyry with the inclosed belt of limestone in the central part of the laccolith north of Indian Creek (66). The feldspar and biotite phenocrysts are fresh, while the hornblende is entirely decomposed.
"Where the andesite-porphyry is exposed in contact with the overlying limestone at the northwest base of Three River Peak, the same transition from coarser-grained to finer-grained groundmass is observed (68, 69, 70, 71). The rock nearest the contact is very dark colored, dense, and dis- tinctl)' porphyritic, and under the microscope is found to have the micro- structure of a holocrystalline andesite — that is, the groundmass consists of microlites of feldspar and pyroxene, with scattered grains of magnetite, and
'Williams, G. H., on the use of the terms poikilitic and micropoikilitic in petrography: .Jour. Geol., Vol. I, No. 2, 1893. p. 179. Baseoiu, F., The stnutiires, origin, aud nomenclature of the acid volcanic rocks of South Mountain : Jour. Geol., Vol. I, No. 8, 1893, p. 814.
64 GEOLOGY OF THE YELLOWSTONE NATIONAL PAEK.
has au ill-defined micropoikilitic structure. Among the phenocrysts are a few irregular individuals of quartz.
The closest approach to typical andesitic microstructure occurs in the dike cutting shales in the ridge south of Winter Creek, already described (p. 10). The transition from a groundmass of brown microcryptocrystalline matrix with distinct lath-shaped feldspar microlites and magnetite grains to one that is gray in thin section with larger feldspar laths and a slightly micropoikihtic structure can be observed in one rock section IJ inches long (76). The finest-grain is at the contact with the inclosing rock. The dike is 3 feet wide and the central portion is slightly more crystalline. There is a pronounced fluidal arrangement of the feldspar microlites, more or less parallel to the sides of the dike. The hornblende and biotite are both altered to chlorite, which also fills the centers of the feldspar phenocrysts, leaving a clear marginal zone. Magnetite occurs in phenocrysts and in minute crystals in the groundmass. Whatever ferromaghesian minerals may have been constituents in the groundmass have been chloritized, and there is no evidence that they were present in any considerable amount. In the rock from the horizontal sheet in this ridge of limestone the ground- mass, which is coarser grained, contains abundant microlites of mica and altered hornblendes, with minute magnetites. These minerals are also abundant as phenocrysts. Apatites and long, thin, doubly terminated crystals of zircon occur. Except for this slightly more ferroraagnesian modification of the rock (74), the mineralogical composition of the laccolith is very uniform throughout the whole of its exposure, which covers a dis- tance of 7 miles.
Segregations occur in places. Tliey consist of comparatively coarse- o-rained crystallizations of green hornblende, with brownish tones, marked pleochroism, and orthopinacoidal twinning, besides lime-soda feldspar, in part labradorite, with magnetite, some biotite, and a little quartz and grains of calcite; the whole having a hypidiomorphic granular structure.
MOUNT HOLMES BXS3IALITH.
DACITE-PORPHYRY.
The rock constituting this great body, which embraces the mass of five mountain peaks, and is 3 miles long and 2 miles wide, is very uniform in general appearance through the whole extent of the body. It is grayish white, with few small phenocrysts of feldspar and biotite, and has a fine-
MOUNT HOLMES BYSMALITH.
65
grained to aphanitic texture. It is not markedly porphyritic. The variations in texture occur near the margin of the body, where they bear a definite relation to the contact plane. They are accompanied by a slight change in the chemical composition. The uniformity in the character of the mass indicates that the whole body was one magma, erupted at one time. Its mineral composition is seen with the microscope to be quartz and alkali feldspar with biotite, corresponding to that of biotite-granite. Its chemical composition is shown by the following analyses, one of which represents the main mass of the rock; the other, which is more siliceous, is of rock from near the margin of the bysmalith, at Echo Peak.
Analyses of dacite-porphyry and rliyolite-felsite.
[Analyst, J. '&. Wljitfield.]
|
Constituent. |
(77) Mount Holmes. |
(87) Echo Peak. |
|
SiOii |
69.54 None. 17.95 2.50 .22 None. .50 1.80 Trace. 4.30 1.21 None. .37 1.96 |
74.51 None. 14.83 1.09 Trace. Trace. .47 .81 .02 4.38 2.72 Trace. .24 .92 |
|
TiOj |
||
|
AID, |
||
|
F.,03 |
||
|
FeO |
||
|
MuO |
||
|
MgO |
||
|
CaO |
||
|
Li02 |
||
|
NaO |
||
|
K2O |
||
|
P.O5 |
||
|
SO3 |
||
|
H,0 |
||
|
Total |
||
|
100. 35 |
99.99 |
|
The chemical composition corresponds to that of a granite high in soda. The main mass has a chemical composition just on the border line between soda-rhyolite and dacite, while the marginal portion has the composition of soda-rhyolite. As already noted, its texture is not markedly porphyritic, so that it stands between a distinct jDorphyry and a felsite. It is, consequently, difficult to give it a name that will not be open to criticism. We have called the main mass dacite-porphyry.
In thin section the rock appears as a gray, very fine-grained mass with abundant minute specks of biotite. Under the microscope it is seen that
MON XXXII, PT 11 5
66 GEOLOGY OF THE YELLOWSTONE NATIONAL PAEK.
the microstructure and grain of the rock from the summit and south slope of Mount Hohiies (77, 78), and those of the rock from the White Peaks west of the head of Indian Creek (80, 81, 82), ai-e very uniform, and represent the coarsest-grained forms found. The highest part of the mass as it now exists, the summit of Mount Hohnes, is nearly as coarsely crystallized as any part of the body examined.
The coarsest varieties consist of quartz in allotriomorphic individuals, inclosing nearly idiomorphic crystals of feldspar, with fewer of biotite and magnetite. The structure is thus micropoikilitic, or is almost exactly analogous in the relative proportions and sizes of the crystals to microphitic structure in ophitic basalt. The quartz extinguishes light between crossed nicols throughout small areas, in which are scattered more or less rectangular and lath-shaped feldspars. The quartzes are colorless and have few inclo- sures of liquid with moving bubbles and salt cubes. The feldspars are cloudy and partly altered; hence the minerals are easily distinguished. The structure is shown in PI. XI, fig. 3. The feldspars are partly unstriated, in Carlsbad twins, with low extinction angles and low double refraction; these are probably orthoclase; others are partly striated, in polysynthetic twins, with low extinction angles, and are lime-soda feldspars, probably oligoclase. It appears as thougli the latter predominated. The few phenocrysts are striated lime-soda feldspars. Owing to the low percentage of calcium oxide in the rock, the feldspars must correspond to plagioclase rich in soda. In one instance the feldspar contains inclusions of what appears to be glass, but its exact nature is doubtful. Biotite occiu-s in six-sided plates and as irregular individuals, with brown color and strong absorption. They often ■contain minute magnetite grains. The biotite is sometimes in small aggre- gates with magnetite, which also occurs in scattered crystals. Apatite is present in long slender prisms, but is rare. In some cases it exhibits a distinct blue and brownish-pui-ple pleochroism. Minute zircon prisms are present.
Among the secondary minerals is a little muscovite in radiating tufts. Chlorite, resulting from the alteration of biotite, is occasionally noticed. The decomposition of the feldspar produces a dust-like, indeterminable mineral, white by incident light and yellowish in transmitted light.
In the dacite-porphyry of Echo Peak there are occasional inclosures of what seems to be coarse-grained gneiss. The only specimen examined, however, shows a coarse-grained rock with somewhat gneissic structure,
MOUNT HOLMES BYSMALITH. 67
but composed of the same minerals as the dacite-ijorpliyry: alkah feldspar, quartz, and biotite, with magnetite and zircon. The niicroscopical char- acters of these minerals are like those of the minerals in the porphyry, and it is possible that these coarse-g-rained inclosures may be nothing but coarsely crystalline portions of the dacite magma. The gneissic structure may be the result of irregular differentiation, as in the case of the banded g-abbro of Skye,' described by Geikie and Teall.
The transition from the more coarsely crystalline central ])ortion to the denser and tiner-grained aphanitic marginal portion is shown in the changes in microstructure in specimens (83, 84, 86, 87, 88) collected from Echo Peak and in the contact zone in the gulch between this and Tlu-ee River Peak.
As the constituent minerals become smaller the poikilitic quartz in two cases (84, 86) assumes more of an idiomorphic form, interrupted by small feldspar crystals lying at various angles. The quartz sections appear in nearly rhomljic forms, the direction of extinction being diagonal to the rhombs. The crystals are more or less perfect hexagonal bipyramids, formed by ± R- They sometimes lie in a finer-grained mixture of feldspar and quartz, which, however, does not amount to a groundmass, being in rela- tively small quantity. In other cases the finer-grained modifications of the rock are still micropoikilitic, and have essentially the same structure as the coarser kinds. It would seem as though the idiomorphism of the quartzes occurred in cases where the feldspar was a little more abundant. The rock from which the second analysis (87) was made is minutely micropoikilitic. This aphanitic variety is niottled with small dark sjiots that prove to be chlorite, containing small scales and j^lates of muscovite, which also occui's scattered through the rock in small amount. No biotite is present. Mag- netite occurs in small crystals. The chlorite and muscovite are seen in some cases to be alteration products of biotite, so that in all the specimens examined it may be assumed to have had the same origin.
This more lithoidal or aphanitic form of the rock occurs in broad bands parallel to the plane of contact around the margin of the bysmalith. The banding is recognizable at a distance, and is shown in the photograph (PI. XII) of the north side of Echo Peak. The banding stands at steejj angles, which are more nearly vertical in lower exposures, suggesting a
'Geikie, A.., and Teall, J. J. H., On the banded structure of some Tertiary gabbros in the Isle of Skye: Quart. Jour. Gaol. Soc, Loudon, Vol. L, No. 200, 1894, pp. 645-660.
68 GEOLOGY OP THE YELLOWSTONE NATIONAL PAEK.
dome-like shape to the bysmalith. The rock of the bands is massive, the banding being due to dififerences in the constituents or in the colors and texture, and not to parallel jointing. In the vicinity of Echo Peak, on its north side, the banding jjitches downward at 30° to 35°, passing under the tilted limestone. Very close to the contact with limestone the jjorphyry or felsite is dense and slaty (94), being split into thin plates parallel to the contact plane. These are traversed by numerous irregular joints, which break it into sherdy pieces at right angles to the contact plane. In places it carries quartz phenocrysts, and has the appearance of a quartz-porphyry.
The dense aphanitic variety is fine grained and without phenocrysts. It is holocrystalline, with the small quartzes idiomorphic and the feldspars less so, though many of the small feldspars are idiomoi'phic, and the structure approaches panidiomorphic-granular. The average size of the quartzes is about 0.03 mm.
A very similar modification of this rock forms an intrusive sheet or apophysis from the bysmalith in the limestone and shale beneath the Indian Creek laccolith on the north side of the valley of Indian Creek (95). It resembles the last-described variety in megascopical habit and platy parting and in microstructui'e, but the idiomorphism of the quartz is less pronounced. The thickness of the sheet is not known.
The marginal modification of the bysmalith is well shown in the mountain ridge west of The Dome and north of Mount Holmes. The same broad banding is present, the position of the bands being almost vertical in the southern exposure, where the contact is visible for hundreds of feet. The central, more crystalline form of the rock passes into a more plainly porphyritic zone, and this into an aphanitic zone, Avhich is spotted near its contact with the surrounding rocks. The inclosing rocks are penetrated by narrow dikes of the aphanitic dacite-porphyry. The aphanitic modifica- tion (93) has very much the same microstructure as that near the contact north of Echo Peak, except for abundant small feldspars, which are larger than the constituents of the groundmass. They are only sparingly present in the case of the other locality. Parts of this contact zone are aphanitic, with irregularly stellate or dendritic spots (90). The micro- structure is rather panidiomorphic, with distinct quartz crystals, and the dark-colored spots are biotite and muscovite and alteration products, now mostly iron oxide, probably derived from biotite. The mica, when unal-
BIGHORN PASS SHEET. 69
tered, extends tliroug-h the grouiidinass for some distance as single crystals, inclosing many quartz crystals, in a ])oikilitic manner.
Tlie same kind of" contact zone exists east of Mount Holmes and west of Trilobite Point. The plane of contact is nearly vertical, and from the ai)hanitic marginal zone numerous offshoots penetrate the adjacent rocks. The dikes are white, aphanitic, and exhibit banding and flow structure. Specimens from the contact were studied and found to be very fine grained, with micropoikilitic structure, the quartz individuals being about 0.15 mm. in diameter, and having a skeleton-like form, the outline of each quartz being nearly idiomorphic, but not continuous, as .shown in PI. XI, fig. 4. The feldspar forms minute clouded grains and crystals. There are micro- scopic flakes of muscovite scattered through the rock, and some calcite. In the bysmalith rock immediately in contact with the andesite-porphyrv of the laccolith the microscopic skeleton quartzes are scattered in a micro- cryptocrystalline groundmass. Calcite is abundant in irregular grains. The aphanitic rock penetrates the limestone in sheets that sometimes break into thin crumpled layers. This modification is microgranular, very fine grained, and not poikilitic, and consists of quartz and feldspar in allotrio- morphic grains. The size of the grains varies slightly in alternate layers, producing the lamination. Minute flakes of muscovite are scattered through the rock and intersect one another at all angles. They are more abundant in some layers than in others.
Similar offshoots of microgranular rock occur on the north side of Panther Creek and near the ridge west of the head of Gallatin River. In these bodies, however, biotite is more abundant, and the micropoikilitic structure passes into micrographic structure as the feldspar inclosures assume a more uniform orientation (96, 97, 98).
BIGHORN PASS SHEET. KERSANTITE.
The small obscure body of dark-colored porphyry-like rock which is exposed in the vicinity of Bighorn Pass is characterized by phenocrysts of hornblende and mica, and rarely those of feldspar. In places the horn- blendes are quite large ; in other parts of the mass there are no phenocrysts (124). On the pass the sheet is from 50 to 75 feet thick. On the north side of Three River Peak there is a nearly horizontal intrusive sheet, 10 feet
70
GEOLOGY OF THE YELLOWSTONE NATIONAL PARK,
thick, of dense aphanitic gray and red rock (100), which might be mistaken at first 2-lance for a fine-g-rained sandstone. It is at about the same horizon as the sheet at Bighorn Pass, and proves to be of similar rock. Its resemblance to this rock was not recognized in the field, and though associ- ated with dikes of dacite-porphyry its relation to them was not noted.
Under the microscope the rock from Bighorn Pass is seen to consist of a holocrystalline groundmass of feldspar, mostly plagioclase, with quartz and some orthoclase, and larger crystals of augite, biotite, and occasional hornblende, with abundant magnetite, besides chlorite and calcite. It is not fresh, the augite and hornblende being partly decomposed. Its chemical composition is as follows:
Analysis of J^ersantite from Bighorn Pass.
[Analyst, J. E. Whitfield.]
Constituent.
SiO-,..
TiOi .
Al.Oa.
Fe,0.,
FeC.
MnO.
MgO .
CaC.
SrO.-
BaO..
Li,0 .
NajO.
K:0..
P2O,..
SO3 ..
CI....
CO, ..
H20..
1.
liBss O for CI .
48.73
1.34
11.92
4.79
4.56
.36
5.93
9.24
None.
Trace.
Trace.
2.62
2.47
.32
.34
.11
5.80
1.52
47.73
100. 05 .02
10.07 7.39 4.29 .23 7.66 6.97
3.78 1.22
49.82
14. 50 8.06
5.81 7.69
3.03 3.50
Trace.
Trace.
0.88 4.46
4.42 2.54
99.68
99.37
100. 03
i
1 = Kersantite, Bighorn Pass.
2 = Minette, Eiobelberg, Heidelberg.'
3^Ker8antite, between Falkenstein and Steinbacb Miihle, Fichtelgebirge.^
' From Roth's Tables of chemical analyses, Beitr:ige zur Petrographie der plutonischen Gesteine, 4°, Berlin, 1873, xxvi. •^ Ibid, 1884, xxiv.
BIGHORN PASS SHEET. 71
The high percentage of carbon dioxide, 5.80, corresponds to the abundance of calcito. The comparatively low alumina and relatively high alkalies are noteworthy. The potash is comparatively high for so basic a rock, and accounts for the j)resence of abundant biotite. Magnesia is below the normal ])ercentage for a rock with so little silica. It has entered into the composition of biotite, malacolite, and hornblende. No orthorhombic pyroxene or olivine has been developed. A comparison of this rock with several others somewhat similar in chemical composition will be made later on.
The microscopic feldspars are polysynthetic twins of lime-soda feldspar, with high extinction angles, corresponding to labradorite. They are nearly idiomorphic, rectangular to lath-shaped crystals, of pure substance, and when not obscured by calcite they appear perfectly fresh and not at all crushed. It seems as though the calcite had been derived from other sources — that is, from the pyroxene, or by infiltration from the inclosing limestone. The absence of strain or crushing is significant in connection with the proximity of this thin .sheet to the massive laccolith, and indicates that this lamprophyric rock is the more recent intrusion. In places the form of the feldspar is tabular. And sometimes the rectangular crystals are bounded by a margin of unstriated feldspar with allotriomorphic out- line, and in some cases idiomorphic outline. This feldspar has a lower index of refraction than that of the inclosed feldspar, and is undoubtedly orthoclase. Its mode of occurrence is precisely the same as that of the orthoclase in the groundmass of the basaltic rocks, absarokite and shosho- nite, described in Chapter IX. Grains of quartz constitute the last crystal- lization of the groundmass. It is probable, however, that some of the quartz is secondary, since it occurs in idiomorphic crystals surrounded by calcite. There are many grains and crystals of magnetite and abundant minute hexagonal prisms of colorless apatite. Brown biotite is in part idiomorphic, in part allotriomorphic, with penetrations of plagioclase and inclusions of apatite and magnetite. The monoclinic pyroxene, with large angle of extinction and rather low double refraction, is almost colorless in thin section, and is a diopside or malacolite. It is partly altered along cracks and around the margin, with the formation of calcite and chlorite. It is mostly idiomorphic, in comparatively large crystals, and does not occur in microlites in the groundmass. The crystals have the ordinary form and
72 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK.
distinct prismatic cleavage. Inclusions of magnetite occur. The pyroxene appears to have been an earlier crystallization than the biotite. Hornblende, which is the most prominent constituent in some forms of the rock, is hardly seen in the thin sections prepared. It occurs to only a small extent in microscopic crystals.
In the finer-grained modifications of the rock the porphyritical charac- ter of the pyroxene becomes more pronounced. The groundmass consists of allotriomorphic feldspars, with scattered lath-shaped crystals of jjlagioclase, apparently belonging to the more calcic varieties, together with magnetite or apatite; biotite is partly in microscopic individuals, partly in megascopic ones. There are also microscopic grains and crystals of calcite and patches of chlorite. In these varieties the pyroxene is wholly altered to chlorite or serpentine.
In a marginal modification of the rock, without phenocrysts, the crystal- lization of the groundmass is very fine grained, and the original structure is greatl.y obscured by secondary biotite in microscopic plates, which project into aggregates of quartz. This quartz exhibits peculiar interference phe- nomena, suggesting polysynthetic twinning.
The mineral composition of the rock, as well as the chemical, is unusual. They both correspond somewhat closely to certain kersantites and minettes, analyses of one of each of which are placed in columns by the side of the analysis of this rock for comparison. It is to be remarked that the minette, according to analysis, contains less potash than the kersantite. There is, in fact, nothing in the chemical composition to suggest the crystallization of orthoclase feldspar. But this is equally the case in the rock, leucite- absarokite, from Ishawooa Canyon, whose analysis is given in Chapter IX, and in which the feldspathic constituents are orthoclase and leucite. Several analogous magmas form dikes in the vicinity of the Crandall volcano (Chapter VII). They are somewhat richer in magnesia and potash, and are characterized by olivine, biotite, and orthoclase feldspar. As in their cases, this unusual magma is known only in a small mass. Mineral- ogicaUy it may be classed with kersantites, although it bears a certain resemblance to absarokite.
INTRUSIVE KOCKS OF THE GALLATIN MOUNTAINS. 73
GRAY MOUNTAIN 1HA8S AND CONNECTED SHEETS.
The igneous mass of Gray Mountain and Joseph Peak, with the intru- sive sheets directly connected with it, consists of andesite-porphyry and holocrystaUine andesites, having a considerable range of composition. The greater portion is hornblende-mica-andesite-porphyry ; a considerable part is hornblende-andesite-porphyry, and a small part is hornblende-pyroxene, or pyroxene-audesite-porphyry, while some varieties might be classed as dacite. The numerous bodies examined exhibit a variation in the mineral composi- tion, even within some of the bodies of small size, especially with reference to the relative proportions of phenocrysts of hornblende and biotite. So that hornblende-mica-andesite-porphyries are in some places richer in mica than in others, or richer in hornblende. There is also a variation in the amount of dark-colored minerals ])resent. Some are rich in ferromagnesian silicates ; others poor in them. The latter are richer in feldspar and in ground- mass, and are usually lighter colored. In general, it is found that in the vai'ieties with comparatively few ferromagnesian silicates biotite is in excess of hornblende, but not always. In those richer in these minerals hornblende preponderates over biotite in most cases, but not in all. There is conse- quently a transition in varieties from those rich in hornblende with little or no biotite to those containing biotite with little or no hornblende. The last-named variety, however, does not constitute any considerable body. Only a very few carry quartz phenocrysts, but quartz is a microscopic constituent of the groundraass in all the more crystalline varieties, so that the classification of any of the rocks as dacite must rest upon a chemical basis.
HORNBLENDE-MICA-ANDESITE-PORPHYRY AND ANDESITE.
The main mass of the intrusion is hornblende-mica-andesite-porphyry. It is a light-gray rock, with abundant small phenocrysts of feldspar, horn- blende, and biotite, the groundmass being aphanitic. It is compact, with an even to hackly fracture, cracking into slabs and angular fragments. It resembles the Indian Creek laccolith very closely (146, 147, 170).
The forty-five thin sections representing these hornblende-mica-andesite- porphyries resemble one another in so many respects that their microscopical characteristics may be described collectively. The constituent minerals being alike in nearly all cases, the difference between the various rock bodies lies in the crystallization of the groundmass.
74 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK.
In only a few of the' rocks are all of the rain"erals unaltered, or nearly fresh. In most cases the hornblende is completely decomposed, while biotite is generally unaltered in most of them. The feldspars are unaltered in nearly all the rocks examined. The least altered rocks were found in the Gray Mountain mass (146, 147), in a heavy sheet in the g-ulch on the sovithwest slope of Electric Peak (191), and in the sheet forming the western summit of Electric Peak (197). In these bodies the hornblende is almost entirely fresh.
The hornblende is greenish brown with the usual pleoclu'oism, between dai-k greenish brown and light brown. In some cases a zonal structure is exhibited, the zones being different shades of the same color. In other cases, notably in a segregation of hornblende, the color is chestnut brown to purplish brown, jiassing into greenish brown, and into green at the margin, the zonal arrangement of the colors not being parallel to crystal- lographic forms, but irregular. In some individuals the margin is reddish brown. These tones also occur in phenocrysts in the groundmass that incloses the segregations of hornblende. This particular rock is rich in hornblende and poor in biotite, and appears to be a less siliceous variety. In