YELLOWSTONE NATIONAL PARK SHEETS.

GENERAL DESCRIPTION. features of central Wyoming, and is one of the the amount of precipitation is higher, and the tion of the schists and granites under atmospheric longest and broadest ranges in the State, extend­ mean annual temperature lower. Rain storms agencies. These earlier sediments everywhere BY ARNOLD HAGUE, ing in a series of high alpine ridges well up occur frequently throughout the summer, while consist of coarse quartz grains and fragmentary S---71I GEOLOGIST IN CHARGE. toward the Park. An outlying ridge within the snow is likely to fall at any time between Sep­ material of the underlying rocks. Over this area of the map has been designated Big Game tember and May. Such climatic conditions favor comes somewhat finer and lighter material, carry­ GEOGRAPHY. Ridge, and the culminating point has long been forest development and the growth of luxuriant ing mica and clay, the latter derived from the The area covered by the maps of the Yellow- known as Mount Hancock. This mountain, grasses, together with a varied alpine verdure. decomposition of the feldspars contained in the stone National Park folio is represented upon situated just inside the southern line of the These conditions, with numerous mountain tor­ crystalline rocks. Overlying these latter beds four atlas sheets, known as the Gallatin, Canyon, Park, has an altitude of 10,100 feet above sea- rents, plateaus, lakes, and waterfalls descending the first calcareous sediments were deposited, Lake, and Shoshone sheets, and is embraced level, and commands one of the most picturesque from the table-land to the lowland, add much to associated with quartz and mica and carrying less between the parallels of 44° and 45° north lati­ views of the adjacent country. the scenic attractions of the far-famed Yellow- and less foreign material, until beds of nearly tude and the meridians of 110° and 111°. It is Along the entire eastern side of the Park stone Park. Owing to the elevation of the coun­ pure limestone made their appearance. In these situated in the northwest corner of the State of stretches the Absaroka Range, so called from the try and consequent severity of its climate, the beds occur the first remains of a marine fauna, Wyoming, and includes 3,412 square miles. Indian name of the Crow Nation. At its south­ region is one unfit for agriculture and undesir­ showing a grouping of species characteristic of In 1871 Dr. F. V. Hayden, a United States ern end the range is topographically so closely able for settlement, and as the Park and Forest the middle Cambrian period. From the base of geologist, explored this region, at that time com­ connected with the Wind River Range by the Reservation are under Government supervision, this limestone to the top of the Madison forma­ paratively unknown, accompanied by a corps of Wind River Plateau that any line of separation no person is allowed to reside there permanently tion the great limestone body of the Carbon­ skilled scientific assistants including geologists between them must be drawn arbitrarily. The or without permit from the Secretary of the iferous period in the northern Rocky Mountains and topographical engineers. Their explorations Absarokas trend in a north-south direction for Interior. Good roads traverse the country and the beds consist almost wholly of calcareous were eminently successful, and immediately more than 80 miles, forming an unbroken barrier substantial bridges span the rivers, both being deposits, varying considerably, however, in the attracted the attention of the world. It must along the entire eastern side of the Park. All the built and maintained by the General Govern­ purity of their sedimentation, some of them always redound to the credit of Dr. Hayden that western slope of the mountains and many of the ment. arenaceous, others more or less argillaceous, while he appreciated the exceptional character of the higher peaks come within the Park limits, but GENERAL GEOLOGY. still others are marked by the presence of ferru­ region and the advisability of its forever being the great body of the range lies to the eastward, Archean rocks. The oldest rocks in this region ginous material and belts of cherty and nodular held intact by the General Government. He presenting an uncommonly rugged group of are crystalline granites, gneisses, and schists of limestones. Differences of color, mode of bed­ laid the matter before the Congress of the United peaks and mountain masses, scored by deep various kinds, and, like similar occurrences else­ ding, and a tendency of certain beds to assume a States, and upon his earnest solicitation the Yel- canyons. Near the northeastern corner of the where, are assigned to the Archean age. They shaly structure, render it possible to separate this lowstone National Park was established. Park a confused mass of mountains connects the are thought to constitute the earlier rock forma­ great body of limestone into a series of beds. By far the greater part of the Yellowstone Absarokas with the Snowy Range. The latter tions of the crust of the earth. The granites and Many of them are easily recognized and their Park is situated within the area of the four atlas range, high and alpine in character, as its name gneisses are for the most part coarsely crystalline, geological position determined wherever they are sheets, but a strip of country about 2|- miles in indicates, shuts in the Park on the north, its west­ and the entire series shows the eiiect of meta- exposed. One of these characteristic beds in the width lies to the northward in Montana, and a ern foothills connecting with the outlying eastern morphism by pressure. No exposures of Archean Cambrian has been designated the Mottled lime­ still narrower belt extends along the western side spurs of the Gallatin Range. Only the southern rocks are known in the central region, but they stone, so named from the irregular dark-gray and in Idaho and Montana. The eastern boundary of end of the Snowy Range falls within the borders occur in connection with all the great mountain brown lenticular patches scattered through it. It the Park, as defined by law, is placed 10 miles of the Park reservation. uplifts that encircle the Park plateau. In con­ is a persistent horizon, easily traced far beyond east of the most eastern point of the Yellowstone Across the elevated plateau inclosed by these sequence, they are found only near, the borders of the limits of the Park region. No well-preserved Lake. This eastern line nearly coincides with mountains lies the continental divide, separating the district, where, with the exception of those organic remains have been found in it, but the the meridian of 110°. The southern boundary the waters of the Atlantic from those of the found in the Gallatin Range, they represent out­ fauna found below it has been referred by Mr. lies 10 miles south of the lake, and is shown on Pacific. Entering the Park at the southeast cor­ lying portions of much larger masses. In the C. D. Walcott to the middle Cambrian, while all the maps (Shoshone and Lake sheets). ner, it runs with an irregular course in a north­ Snowy Range they occur along the northern fossils immediately above this horizon in the In the organic act of 1872, defining the pur­ west direction. Following along the top of Two border of the map, and are found on both sides Gallatin Range belong to the upper Cambrian. poses and boundaries of the Park, Congress Ocean Plateau, it skirts the northern escarpment of the Yellowstone for a long distance. Along The Cambrian has an estimated thickness of 860 declared that the reservation was " dedicated and of Flat Mountain, winds among the undulating Big Game Ridge and in the mountain mass of feet. set apart as a public park and pleasure ground low ridges lying between Yellowstone and which Wildcat Peak is one of the culminating The beds assigned to the Silurian, consist of for the benefit and enjoyment of the people." As Shoshone lakes, and thence, with a broad sweep­ points, no Archean rocks occur, but southward, dark-colored, massive limestones, for the most defined by law, the area of the Yellowstone ing curve around the streams running into the in the Wind River Range, as in the Tetons, they part sharply defined lithologically, but in the National Park includes 3,344 square miles. latter lake, crosses the Madison Plateau and form the central body of the mountains. Gallatin Range poorly defined by organic remains. All the country situated between the southern leaves the Park a short distance southwest of the Algorikicm rocks. In the Rocky Mountains As the Cambrian limestone passes into that of line of the Park and the parallel of 44° belongs Upper Geyser Basin. north of the Yellowstone Park there rests uncon- the Silurian without any marked change of to what is known as the Yellowstone Park Forest Four large rivers drain this region. The Snake formably upon the Archean a series of sandstones deposition, it is natural that conditions favorable Reservation, set aside by proclamation of Presi­ carries off all the water on the south and west and slates which have been referred to the to the development of many preexisting species dent Harrison on March 30, 1891, and therefore side of the great divide, and the Yellowstone, Algonkian period, yielding as yet no organic should continue upward into higher horizons, and may be regarded as practically forming a part of Madison, and Gallatin that on the east and north. remains. Within the Park, rocks provisionally that there should be a commingling of both the the grand National reservation. The four atlas The Snake River drains somewhat more than one- assigned to the Algonkian period do not occur Cambrian and the Silurian species. In many locali­ sheets may therefore be regarded as a unit and quarter of the area. Two tributaries of the Snake, associated with the older granites and gneisses. ties only organic forms common to both periods described as a single geographical area. Falls and Bechler rivers, run off on the west side They occupy limited and isolated areas, for the have been obtained in the 160 feet assigned to of the Tetons, flowing into the main channel most part buried beneath lava flows, preventing the Silurian. In the same way, at other localities TOPOGRAPHY. beyond the limits of the map. All other tribu­ their relations with other formations from being the species found near the top are such as have The central portion of the Yellowstone Park taries from this region join the main stream before studied. The beds consist mainly of massive, a wide range and might occur in both Silurian may be described as a broad volcanic plateau reaching Jackson Basin. dense, bluish-white quartzite. They are recognized and Devonian rocks. Again, without any appar­ between 7,200 and 8,300 feet above sea-level, with The Yellowstone has its source among the only in the southern end of the Park, and are ent interruption in the continuity of the oceanic an average elevation of nearly 8,000 feet. Sur­ snows of the Absaroka Range and the Wind best exposed upon the southern slope of Mount sediments other than by occasional more or less rounding this plateau on the south, east, north, River Plateau. It enters the Park at the extreme Sheridan. Their assignment to the Algonkian shaly beds, the limestones for the next 190 feet and northwest rise mountain ranges with culmi­ southeast corner, runs northwesterly for 30 miles period is based largely upon the fact that similar are sharply defined by marine fauna carrying nating peaks and ridges standing from 2,000 to before flowing into the lake, thence across Hay­ rocks are unknown in the Paleozoic series, and no such species as Atrypa reticularis and Pacliy- 4,000 feet above the general level of the inclosed den Valley and through the far-famed Grand sedimentary rocks older than these quartzites phyllum woodmani, characteristic of the Devonian. area. The Gallatin Range shuts in the Park on Canyon of the Yellowstone, and finally leaves the are exposed in this region. Nearly or quite one-half the sediments of the the north and northwest. It is a bold, pictur­ Park at its northern boundary. A large part of Paleozoic and Mesozoic rocks. Upon the Paleozoic sea in this region consist of Carbonifer­ esque range of mountains extending from Mount the central plateau is drained by the Firehole and Archean crystalline bodies, which rose as islands ous limestones. They have been designated the Holmes, at the southern end, far northward into Gibbon rivers, which unite to make the Madison. or continental areas from a broad open sea, there Madison limestone, and present a great thickness Montana, where it presents a sharply defined The latter river, running nearly due west, cuts a was deposited unconformably a great thickness of beds laid down under fairly uniforn conditions. ridge along the west side of Yellowstone Valley. deep gorge through the Madison Plateau, and, of sediments made up of sandstones, limestones, In the uplifted areas they play a conspicuous Electric Peak, situated on the northern boundary after a winding course, pours its waters into the and clays. These overlying sediments present a part in mountain building, forming the summits of the Park, and the highest point of the range, Missouri at Three Forks. Within the Park the conformable series of Paleozoic and Mesozoic of many high peaks. Fossil remains character­ attains an altitude of 11,100 feet. South of Gallatin drains a much smaller region than any strata extending from the base of the middle istic of the lower Carboniferous occur throughout Mount Holmes the volcanic lavas which form the other of the large rivers, being restricted to the Cambrian, the lowest beds exposed, through the the limestone, many of the species showing a Madison Plateau stretch southward beyond the western slope of the Gallatin Range, with a drain­ upper Cambrian, Silurian, Devonian, Carbonifer­ wide vertical range, as might be expected in such limits of the map. The Teton Range upon the age of less than 80 square miles, and, like the Madi­ ous, Juratrias, and Cretaceous, including the uniform beds. In the Park region the fauna of south looms up grandly and forms one of the most son, it empties into the Missouri at Three Forks. Laramie sandstone. They were laid down under the Carboniferous is much the same wherever prominent geographic features of the northern Across the Park Plateau and the Absaroka Range varying physical conditions, partly as coarse obtained, and several species, among them Rocky Mountains. The eastern wall of this the country presents an unbroken mountain mass material in shallow waters as inshore deposits, Chonetes loganensis and Spirifer cent/ronatus, mountain mass rises with unrivaled boldness over 75 miles in width, with an average elevation and in great part as finer sediments carried farther occur at a number of localities. nearly 7,000 feet above Jackson Lake, which lies unsurpassed by any other area of equal extent in away from land and deposited in relatively deeper At the close of the Madison period the lime­ immediately at its base in the open valley of the northern Rocky Mountains. From its position and quieter seas. stone-making epoch of the Paleozoic sea came to Snake River. Northward the ridges of the it offers exceptional physical conditions for gather­ During the Paleozoic age, from the time of the an end. Marked changes took place in the sedi­ Tetons fall away abruptly, and only the outlying ing the moisture-laden clouds, which, passing over middle Cambrian to the end of the Carboniferous, mentation, and siliceous material began to accumu­ bold spurs and foothills come within the limits of the arid region, precipitate large quantities of the accumulation of sediments attained a thick­ late. At first the deposited quartz grains were the forest reservation. To the east of the Tetons, snow and rain upon the cool table-land and ness, as measured in the Gallatin Range, of 3,250 accompanied by calcareous material, which gradu­ across the broad valley of the Snake, stretches adjacent country. The climate is in many ways feet. The basal beds resting directly upon the ally gave way to pure siliceous beds. These have the Gros Ventre and Wind River ranges. The quite unlike that found in the surrounding low old continental land surfaces were formed from been designated the Quadrant quartzites. Near latter forms one of the most prominent orographic country. As is shown by meteorological records, the detrital material derived from the disintegra­ the top impure beds again came in, showing a mingling of calcareous and arenaceous material, came to an end. The entire region was once mentary formation of Tertiary age in the Park Denudation of these early basic breccias has but apparently devoid of life. With the end of more elevated above the sea. The strata were are beds of waterworn conglomerate and friable taken place on a grand scale. Deep canyons the siliceous deposition the great accumulation of plicated and folded. The region became one of sands exposed in the walls of the Grand Canyon were trenched and a vast amount of volcanic sediments deposited in a Paleozoic sea came to a profound dynamic action and a center of moun­ and in the valley of Lamar River, in both cases ejectamenta carried away. With material so close. In a series of conformable beds unfavor­ tain-building on a grand scale. So far as the age overlain by basalt. In the Grand Canyon they varied in texture, erosion has produced many able for the preservation of organic remains, it is of these mountains encircling the Park is con­ are both overlain and underlain by basalt. They curious rock forms peculiar to the interbedded difficult to say just where the line of separation cerned, evidence goes to show that upheaval was were deposited after the outbursts of andesitic breccias, agglomerates, and mud flows which make between the Paleozoic and Mesoic rocks should contemporaneous in all of them and coincident breccia, but before the great flows of rhyolite up this group of lavas. Bowlders and irregular be drawn. In some localities in the northern with dynamic influences which uplifted other which coyered the country were poured out, so masses of dense basic rocks, withstanding erosion Rocky Mountains the well-recognized Quadrant north-and-south ranges stretching across Wyom­ that their geological position with reference to better than the more friable cementing material, quartzites are followed by a limestone carrying ing and Montana. These orographic movements the igneous \rocks is well determined. They are have protected the underlying rocks and have Carboniferous fossils, but in this region such blocked out for the most part the Rocky Moun­ of slight importance in themselves, seldom exceed­ produced the most fantastic and ever-changing characteristic limestones are wanting. The beds tains after the close of the Laramie epoch. There ing 50 feet in thickness, and they cover limited forms of rock sculpture. This rock sculpturing immediately overlying the quartzites constitute is evidence, however, to show that in the region areas. \ is well exposed in the Hoodoo Basin, at the head in general a grouping of thinly bedded cherts of the Park powerful faulting and displacement, Volcanic period. Throughout post-Cretaceous of Lamar River, in the Absaroka Range, just and argillaceous shales by no means easy to cor­ accompanied by continental elevation, prolonged time the Yelldjwstone Park region was character­ east of the Park boundary. These grotesque relate in the different areas, indicating variable the work of mountain-building through the ized by great volcanic energy, enormous volumes forms of erosion are strikingly shown in fig. 1, conditions of deposition within limited areas. The Cretaceous and the greater part of Tertiary time. of eruptive material being poured out during the where the so-called hoodoos stand out boldly on only fossils yet obtained from these cherty beds But such orographic movements must be neces­ Eocene and l^eocene periods, but probably not the brink of a profound canyon wall. . are a few specimens of linguloid shells of little sarily greatly obscured by the accumulation of extending into Pleistocene time, at least not after That the duration of the earlier eruptions of value in determining the age of the strata. These lavas that cover the Park region. For instance, the appearance of glacial ice. Active volcanoes acid and basic breccias and agglomerates con­ cherty and shaly beds are followed by red shales the post-Cretaceous movement which uplifted the surrounded the Park on the east, north, and west, tinued throughout a long period of time is made and sandstones forming a very persistent and Livingston formation northward in Montana can and broke out in the central region. The greater evident by plant remains preserved in volcanic sharply defined horizon south of the Park." They not be recognized as such, although it would, part of the Absaroka Range as seen to-day was ashes and in mud flows associated with the coarser are overlain by a series of limestones, sandstones, ,seem more than probable that it must have built up by vast accumulations of volcanic breccias and more or less compacted lavas. Much marls, and clays, all of them more or less arena­ exerted a powerful influence in all adjacent con­ ejectamenta, and the depressed basin lying of this plant material is in an excellent state of ceous, constituting a group of beds known as tinental areas. between the encircling ranges was filled to its preservation, and it is in these beds that the well- the Ellis formation, with a maximum thickness The Gallatin is the only range formed mainly present elevation: by flows of rhyolite, forming known fossil floras occur. In the acid lavas a of only 200 feet. These indicate constantly pf Paleozoic and Mesozoic rocks that is embraced the present Park Plateau. distinct flora has been discovered in a number of changing conditions of sedimentation, and carry within the area of the map. Structurally the Amongst the oldest igneous rocks in the Park localities upon both sides of the Yellowstone throughout a varied marine fauna of character­ sedimentary beds present a broad syncline com­ are those found in the Gallatin Range, where River. In the escarpments along the west side of istic Jurassic species. Among them may be men­ pressed between two grand Archean bodies. Only they occur as intrusive bodies in the form of Lamar Valley the forest and plant-bearing beds tioned Pleuromya subcompressa, and JRhynchon- the northerly dipping beds of this syncline come massive laccoliths, as intercalated sheets forced are admirably displayed, erosion having cut ella myrina. within the Park, the axis of the fold lying still between sedimentary strata, and as dikes of vary­ numerous lateral ravines in the eruptive material The Ellis formation is followed by decidedly northward beyond Electric Peak. Along the ing widths cutting older rocks. The intercalated of the basic lavas. Many trees are still standing arenaceous deposits, varying from medium-grained southern borders of the Snowy Range the sedi­ sheets frequently pass from one bed to another, in upright positions, others lie horizontal, firmly sandstone to coarse conglomerate. They are mentary rocks lie gently inclined toward the and then resemble normal dikes. All these imbedded in the layers of volcanic muds and known as the Dakota sandstone and conglomer­ Park. In the northern end of the Absaroka intrusive bodies penetrate sedimentary beds of all ashes. The section across the beds exposed by ate. In this region, as elsewhere, the horizon is Range the strata dip uniformly southward and ages from the middle Cambrian to the Laramie. the ravines measures nearly 2,000 feet in thick­ regarded as the base of the Cretaceous period. rapidly pass beneath igneous rocks. Big Game While these early intrusive bodies show consider­ ness, with evidence of a fossil flora from base to In the Park the conglomerates form the most Ridge and the region of country of which Huckle­ able variation in mode of occurrence, they pos­ summit. conspicuous portion of the beds, made up for the berry Mountain is the culminating peak are sess a chemical composition intermediate between Large collections have been made from the most part of smooth waterworn pebbles firmly formed of compressed and folded Mesozoic strata basic and acid magmas, and have been grouped extensive fossil floras found in the Park, and all compacted together. They are evidently shallow- broken up by profound longitudinal faults. Out- together under the designation of andesite-por- the material has been referred to Prof. F. H. water, in-shore deposits. Associated with the |)ursts of igneous rocks have complicated and in phyry. Similar rocks may occur elsewhere in the Knowlton for study and identification. From conglomerates in the Gallatin Range occur a few many instances obscured the structure. In the Park, but if so they are limited in their exposures this rich field of research nearly 150 species have feet of dark-gray limestone, in places filled with Tetons the entire series of conformable strata lies or else are buried beneath later extrusive flows. been determined, of which more than one-half are fresh-water gasteropods. It is evident that the steeply inclined as a broad anticline curving Next in order of succession in the Gallatin Range new to science. The new species are fairly well limestone was deposited in a land-locked area, around the northern end of a massive Archean comes the more acid (or siliceous) rock, dacite- divided between the two groups of rocks, and and as no fresh-water Jurassic fauna has been body which extends far southward into Wyoming. porphyry. It occurs as a laccolithic body younger only a few of them, so far as known, are common recognized in this region it indicates the first Volcanic energy, which later played so import­ than the andesite-porphyry, as shown both at to both. Of the species found in these lavas pre­ appearance of fresh-water life. ant a part in the geological development of the Mount Holmes and at Echo Peak. Similar viously described as occurring elsewhere, only 5 The middle Cretaceous, or Colorado formation, region, was probably intimately connected with masses elsewhere in the Park are too much are common. to both the acid and basic breccias. shows evidences of, a deeper sea throughout a the post-Laramie movement, and followed closely obscured by later rhyolite flows to present any The flora from the acid breccia is so closely long period of alternating ferruginous sandstones upon the elevation of the mountains and the field evidence of their relations to earlier igneous allied to that obtained from the Fort Union beds and clay shales, the latter greatly predominating. accompanying dislocation and compression of masses. Electric Peak, situated in the Gallatin near the mouth of the Yellowstone River in Associated with them are occasional limestone sedimentary beds. The eruptive masses forcing Range, lies partly within the area of the map, Montana, that they are regarded as identical in strata. A marine fauna occurs at different hori­ their way upward followed lines of least resist­ but in great part beyond its border. It was the age, and are consequently referred to the Eocene zons from base to summit. Fragmentary plant ance, along planes of faulting or wherever strain center of volcanic energy continued throughout a period. remains, evidently transported for considerable had been greatest in the crumpled sediments. long period of time. The eruptive material, At several localities along the contact of the distances from neighboring land areas, are by no Eocene rocks. Between the blocking out of the presenting a great variety of igneous rocks, acid and basic breccias between Yanceys Station means uncommon in the shales. In distinction to mountains at the close of the Laramie and the occurs as intrusive bodies penetrating the shale and Crystal Creek there occurs a flora distinct in the Colorado, the overlying Montana formation is pouring out of vast eruptions of igneous rocks and sandstone strata of the Colorado and Montana its grouping of species from that which is found essentially a sandstone, and, considering its great there were deposited certain strata, which in the formations. As the area represented by them is either in the acid breccia below or the basic thickness, was evidently deposited under fairly Park region are limited to the southeast corner small, they have been grouped together under breccia above. The flora from this limited area uniform conditions. This is so marked that on (Lake sheet). Here are found, resting uncon- one color and designated the Electric eruptives. carries 30 species, of which more than one-half Big Game Ridge and in the region of Huckle­ formably upon the tilted and eroded sandstones The early eruptions are probably closely related are new to science, and of those previously berry Mountain it is impossible to differentiate of the Laramie epoch, a series of nearly horizontal to the intrusive masses southward in the Gallatin described many have a wide geological distribu­ Pierre shales from Fox Hill sandstones. The beds which have been designated the Piny on con­ Range, while the later ones are apparently con­ tion. This grouping of trees has been designated conditions governing deposition being similar, it glomerate, named from the mountain where they nected with the extrusive, basic breccias of the Intermediate flora, and is regarded as belong­ is natural to find that life continued much the are best exposed, on the divide between Wol­ Sepulchre Mountain. ing to the base of the Neocene period. The flora same, and as a matter of fact it is found difficult verine and Gravel creeks. Nine-tenths of this Extrusive lavas, or those that have been forced from the basic breccia which is so characteristic­ to separate by their faunas formations elsewhere material consists of waterworn pebbles mingled out and cooled near the surface, cover by far the ally developed at the Fossil Forest is of still later clearly defined. There is a decided mingling of with sand and gravel. Associated with them are greater part of the Park area. From Sepulchre age. It has been named the Lamar flora. a marine fauna throughout the Montana epoch. fragmentary pieces of rock derived from the Mountain eastward along the northern boundary Among other characteristic species found imbed­ The Laramie, essentially a sandstone formation, underlying Mesozoic strata, with occasional well- of the Park, and in the northern end of the ded in the acid breccias at Crescent Hill and along is far less uniform ,in its sedimentation than the rounded pebbles of volcanic origin, together with Absaroka Range, occur areas of acid andesitic Yellowstone River near Hellroaring Creek are underlying Montana. The sandy beds are less granite and gneiss. This material has accumu­ breccias and flows consisting mainly of horn­ the following: Sapindus offinis, Cornus acumi- pure and frequently intercalated .with beds of lated to a thickness of nearly 600 feet, and is blende - andesite and hornblende - mica - andesite. nata, Populus specwsa, Sequoia couttsice. clay and shale. It bears evidence of shallow- clearly a shallow-water, in-shore deposit, as it is They are among the oldest of the extrusive lavas From the Intermediate flora the following may water deposits and constantly changing con­ far too coarse to have been transported to any of this region. They occupy relatively small be mentioned as characteristic: Platanus mon- ditions of land to water. In the Laramie occur great distance. As regards their age, these con­ areas, owing to the accumulation of later eruptive tana, Quercus yanceyi, Laurinoxylon amethysten- the great coal-bearing beds which have proved of glomerates have been provisionally referred to material, and in most instances are exposed only ium, Populoxylon wardii. such importance in the development of the the Eocene period, since they were deposited sub­ by erosion of the overlying lavas. Intimately From the abundant collections from the Fossil country northward in Montana. In this region sequent to the Laramie upheaval and must have associated with them, but of later age, occurs a Forest there may be selected the following species occur seams of bituminous clays and impure preceded the accumulation of volcanic rocks. If vast accumulation of basic breccias, agglomerates, as typical of a much higher horizon: Platanus coals of little economic value, but showing physi­ not older than the volcanic lavas, they would and mud flows. Interbedded in these later guillelmce, Laurus californica, Magnolia specta- cal conditions favorable to its formation similar certainly contain large quantities of the basic breccias occur flows of compact basalt, varying bilis, Planera longifolia, Aralia whitneyi. to those found elsewhere. The Laramie has fur­ breccias of the Absaroka Range and of the acid in extent and of greater or less thickness. This The trunks of two well-preserved silicified trees, nished a varied flora and a characteristic fauna of rhyolites of the Park Plateau. They show no mass of breccias was piled up to a thickness of for the most part laid bare by erosion, but with brackish-water types. All these conditions fore­ evidences of either, but on the contrary are over­ several thousand feet, and forms a greater part of their roots firmly imbedded in coarse breccia, are shadowed marked changes in the geological lain by both in the neighborhood of Pinyon Peak. the northern end of the Absaroka Range, extend­ shown in fig. 2. Near by stands a bold crag of development of continental areas. These gravels are the northern extension of much ing eastward far beyond the limits of the Park. breccia which conveys a good impression of the Post-Laramie movement. With the close of larger areas occurring in the Wind River Moun­ Throughout the greater part of this area all easily eroded volcanic material. These trees, the deposition of the .Laramie sandstone the con­ tains. earlier rock formations were buried beneath these with others in the immediate neighborhood, may formable series of Paleozoic and Mesozoic strata Neocene rocks. The only other strictly sedi­ volcanic accumulations. be easily seen in the bluff which overlooks Lamar Valley near Crystal Creek on Specimen Ridge. eruption of the rhyolite are known, the two prin­ side of the river low knobs of dacite are partially basalt sheets, both near the top and the bottom, At Sunset Peak, in the Snowy Range, beyond cipal sources being the grand volcano of which concealed beneath the flows of rhyolite forming and followed by the more recent basalts, which, as the limits of the Park, occur one or two outflows Mount Washburn is now the culminating peak, the slopes of Huckleberry and Wildcat moun­ far as can be told, brought to a close the volcanic of acid lava closely related to the early basic and the Sheridan volcano, of which Mount Sheri­ tains. At one time this dacite was probably com­ period in the Park country. The section also breccias. They have been designated trachytic dan is the central point. On all sides the long pletely buried beneath the later rhyolite. brings out the relation of the Pleistocene deposits rhyolite from their resemblance to both these slopes of the preexisting ranges were submerged Some thin flows of basalt were forced to the of still later age to the igneous rocks. types of rock. Similar bodies occur along the beneath the rhyolite. It rests against the deeply surface before the completion of the rhyolite The Glacial epocJi. With the last of the Yellowstone Valley within the Park, where their eroded slopes of the Absaroka Range and buries eruptions, flows of the latter rock overlying the basaltic eruptions volcanic energy came to a close. relations to both the older and the younger rocks the outlying low spurs of the Teton Range and former at several localities, notably along the That it lasted well through the Pliocene period are probably much the same, although their geo­ Big Game Ridge. On both the east and west Grand Canyon of the Yellowstone near Tower seems probable, but there is slight evidence of logical relations are not always easily made out. sides of the G-allatin Range the rhyolite encircles Creek. In general the large masses of basaltic its continuation during or after Glacial time. In places they are overlain by basalt, but the the sedimentary beds at about the same level. lava were poured out after the cessation of rhyo- After the dying out of the basalts came the relative age of the latter is not well determined. The rhyolites of the Park have been referred to litic activity. These recent basalt bodies occur glacial ice. The Park and adjacent mountains In the southern end of the Absaroka Kange other the Neocene period, although no plant remains or near the outer edge of the rhyolite plateau, and constitute such a broad mass of high country bodies of similar mineral composition are recog­ invertebrate fossils have as yet been found asso­ are well shown about Bunsen Peak and in Falls favorable to the precipitation of moisture that the nized, but they occupy very limited areas, and ciated with them. Strong evidence as to their River Basin. Occasionally they cut the rhyolite entire plateau was in Glacial time buried beneath only one occurrence is indicated on the map. age, however, t.as been obtained from the Canyon in the form of dikes, but in no single instance is a heavy capping of ice, constituting one of the Resting directly upon the early basic breccia conglomerate exposed in the Grand Canyon of there an extrusive flow of basalt in the central largest of the many glacial centers occurring in comes a series of basalt flows, piled up one upon the Yellowstone just north of Tower Creek. portion of the plateau. the Rocky Mountain region south of the conti­ another till in places they attain a thickness of Here a few vertebrate remains were collected, nental ice sheet. The ice sheet broadened preex­ 1,000 feet. These basalts form the tops of many which, though fragmentary, were sufficient to isting drainage channels, opened new waterways, of the higher summits of the Absaroka Range, enable Prof. O. C. Marsh to determine them as and deepened profound gorges through the rhyo­ appearing as broad, table-like masses with vertical belonging to the skeleton of a fossil horse of \Lacustrme formations. lite lavas. Glacial lakes, terminal moraines, walls. They are remnants of a much larger field. Pliocene time. Overlying this conglomerate is a Glacial drift. kames, and nearly all the phenomena of ice action They may be observed on both sides of the thin flow of basalt, in turn covered by rhyolite, usually seen in high glaciated regions may be Lamar River, where they form the somber escarp­ a portion of the great rhyolite field. In the con­ found here. ments exposed along the west side of Mirror glomerate careful search failed to find any frag­ From the Absaroka Range glaciers were forced Plateau. From the latter plateau they present a ments of rhyolite, proving that none were present down into the Lamar and Yellowstone valleys, continuous body nearly as far southward as the in the region when the deposits were laid down. thence westward over the top of Mount Everts to southern limit of the early basic breccias which As the basalt, conglomerate, and rhyolite appear the Mammoth Hot Springs Basin. On the opposite terminate in the Signal Hills east of Yellowstone in the walls on both sides of the river, it is evi­ side of the Park the ice from the summit of the Lake. In determining the relative age of the dent that they were deposited before the canyon Basalt. Gallatin Range moved eastward across Swan Canyon conglomerate. eruptive masses these early basalt fields play an was cut. Valley, and, passing over the top of Terrace important part, as they overlie the early series of The Grand Canyon of the Yellowstone is a Mountain, joined the ice mass coming from the acid and basic breccias and underlie a somewhat magnificent and picturesque gorge penetrating east. The united ice sheet ploughed its way similar series of eruptives designated late acid deeply into the volcanic rocks of the Park northward to the lower Yellowstone, where the breccias and flows and late basic breccias and Plateau. An excellent picture of the canyon, broad valley may be seen strewn with material flows. The surface relations of these different reproduced from a large photograph, is shown in transported from both the east and west rims of eruptive bodies are well shown from Pelican fig. 3. It presents on a grand scale a remarkable the Park. This glacier has been named the Cone eastward to Lamar Valley (Canyon sheet). illustration of recent canyon-cutting, being later Yellowstone glacier. Another glacier having its The second series of acid and basic breccias than the rhyolites and basalts of Pliocene age. source on the plateau moved southward down the bears the closest resemblance in mode of occur­ The Yellowstone River, leaving Yellowstone Lake broad valley of the Snake, receiving tributaries rence to the early series, and Prof. J. P. Iddings at its broad outlet, flows northward through an from both the Wind River and the Teton ranges. has shown that in mineral composition they are open valley for about 15 miles, then suddenly It has been designated the Snake glacier. much the same. Unlike the early acid breccias, plunges, by two impressive waterfalls, respec­ An instance of the transporting power and mag­ the late acid breccias are not so deeply buried tively 110 and 312 feet in height, into the Grand nitude of these ice fields is shown in the granite beneath more recent lavas, but form the tops of Canyon. All that portion of the canyon repre­ bowlder which rests upon the rhyolite near the many high peaks and cover broad areas along the sented in the picture has been excavated out of brink of the Grand Canyon about 3 miles below summit of the range, extending from Pelican the rhyolite, which, however, still forms the under­ the Falls of the Yellowstone. This massive block Cone southward for 30 miles. lying rock of the river. The walls of the canyon measures 24 feet in length by 20 feet in breadth, The late basic breccias resting upon the more rise with more or less abruptness 800 to 1,100 and stands 18 feet above its base. It came from acid outbursts make up the entire southern por­ feet above the rushing, turbulent stream at the the Snowy Range. Fig. 10 is a reproduction from tion of the Absaroka Range, stretching south­ bottom of the gorge, as clearly indicated in the an excellent photograph of this majestic bowlder, ward far beyond the limits of the map, and form picture. which stands alone, hidden in the midst of the the top of the Wind River Plateau. The illustration brings out strongly the relation coniferous forest, and consequently is rarely seen The successive flows and beds of these later of the canyon to the Park Plateau, the undulating by travelers, although easily accessible. basic breccias are nowhere more expressively surface of the rhyolite stretching far southward In the Teton Range, beyond the limits of the shown than in the magnificent escarpment found in a series of monotonous ridges covered with a map, small glaciers still exist, remnants of a much on both sides of the Upper Yellowstone Valley somber coniferous forest. The vicinity of the larger system, and in the Beartooth Mountains, above the Lake. In places these abrupt walls canyon has been an active center of hydrothermal to the northwest of the Park, neve fields stretch expose a thickness of over 2,000 feet of accumu­ energy throughout a long period of time. Much for miles along the more elevated portions. lated volcanic material. The west wall of Table of the exquisite beauty and impressive grandeur In the Absaroka Range just east of the PaTk, Mountain, with its irregular and turreted edge, is of the region comes from the brilliancy of color­ but within the. forest reservation, a small glacier shown in fig. 11, presenting an excellent illustra­ ing found in the canyon walls. Probably no lies in the amphitheater of the deep canyon of tion of this interesting geological structure, which other area of equal extent in the world affords so Sulphur Creek, one of the principal tributaries of forms a marked physical feature of the Yellow- varied and vivid a bit of natural color. Hot Sunlight Creek. The glacier flows northward, stone Park and the Absaroka Range. springs, steam vents, and solfataras are still found and is protected from the direct rays of the sun GENERALIZED SECTION OF THE IGNEOUS ROCKS OF THE Resting upon the late acid and basic breccias along the river bank, remnants of a much earlier YELLOWSTONE PARK. and the warm winds from the south by bold are found numerous flows of dense hornblende- and more powerful thermal activity. In the accompanying illustration there is given escarpments of volcanic breccia. It has been mica-andesite. As this rock withstands erosion The rhyolite walls from the bank of the river a generalized section, in their order of suc­ named the Sunlight glacier, and, although limited better than the underlying breccias, it frequently to the brink" of the canyon have everywhere cession, of the extrusive lavas of the Yellow- in extent, presents all the phenomena of an active occurs as the summit rock on several of the higher undergone decomposition by solfataric and ther­ stone Park, from the earliest acid breccias to the glacier. and isolated peaks of the range. Although mal agencies. Every product of decomposition top of the recent basalts, as described in detail in Since the close of the Glacial period no geolog­ covering relatively small areas, these occurrences may be observed, from nearly fresh rhyolites to the text. At the base of the section the upturned ical events have brought about any marked note a distinct period in the phases of volcanic rocks completely altered to pure white kaolin. Laramie sandstones are represented in highly changes in the physical features of the Park other phenomena in the Absaroka Range, for, so far as Every shade and tint of red occurs on the canyon inclined ridges, with the Pinyon conglomerate than those produced by the active agencies of can be determined, no breccias followed their out­ walls, from delicate pink and salmon to a vivid resting horizontally upon them. Over the con­ steam and thermal waters. Although eruptions bursts. Indian red, the prevailing color being a deep glomerate comes the earlier series of acid and of lava ceased with the basalt flows, unmistakable Closely related to these andesitic flows are num­ orange. Tints of yellow and green are inter­ basic breccias, followed by the more acid trachytic evidences of underground heat are clearly visible erous dikes penetrating the breccias. In "the mingled with those of the prevailing red. rhyolite and the monotonously bedded early in the active hot springs, geysers, and solf ataras region of Sylvan Pass they are especially abund­ It is this ever-changing condition .of rock tex­ basalt flows. The latter, which spread over a of to-day. These hydrothermal phenomena, ant, representing many types of igneous rocks. ture, brought about by these processes of decorn- large area, mark a distinct period in the volcanic which are in a sense' evidences of the gradual While not necessarily contemporaneous, some of positibn, that has produced the varied and fantastic history of the region, resting directly upon the dying out of volcanic energy, have doubtless been them are known to cut the late basic breccias. forms of rock sculpturing which present so char­ early series of breccias, in turn followed by the active throughout a long period of time. A proof Owing to the small scale of the map, they are acteristic a feature of the Grand Canyon of the later acid and basic breccias. The Electric and of the antiquity of the earlier hot-spring deposits grouped together under the designation Sylvan Yellowstone and which have made it so famous Sylvan intrusives, which in many ways resemble may be advanced, based upon the strongest geo­ intrusives. throughout the world. each other, are represented as penetrating the logical reasoning. After the cessation of the volcanic energy Closely related to the rhyolite, but insignificant breccias, the former those of the older series in At Terrace Mountain, just west of Mammoth which forced to the surface the andesitic and in their occurrence, are several small exposures the neighborhood of Electric Peak, and the latter Hot Springs, the flat summit is overlain by basaltic breccias and flows a long period of ero­ of dacite found on both sides of Snake River, the later series in the Absaroka Range. The deposits of travertine, the beds forming the oldest sion followed. Then, with renewed activity, below the mouth of Lewis River (Shoshone hornblende-mica-andesite flows found capping portions of the Mammoth Spring sediments. immense volumes of rhyolite were poured out, sheet). The interest in them consists in the fact several of the prominent peaks in the Absarokas Lying upon the surface of this travertine on the which converted the depressed basin lying that'they are slightly more basic than the broad are represented as overlying the entire body of top of the mountain are found glacial bowlders between the surrounding ranges into the Park areas of the acid rhyolite, and apparently ante­ breccias. Overlying the andesites come the great brought from the summit of the Gallatin Range, Plateau. Only a few large vents or centers of date them ,in the order of eruption, as on the east accumulations of rhyolite, with the interbedded 15 miles to the westward, which were transported Yellowstone National Park 3. on an ice sheet across Swan Valley and deposited properly to designate them arsenical waters. SEDIMENTARY ROCKS. fossils of middle Cambrian type. The general on the top of the mountain 700 feet above the Arsenic is found in all the geyser waters, in lithological character of the formation is quite intervening valley. These bowlders present amounts varying from .02 to .25 per cent of the BY WALTEB, HARVEY WEED. constant, but the shale and limestone attain some­ ample evidence that the travertine is older than mineral matter in solution. The greater part of what different proportions in the various ranges the glacier. this arsenic is probably present in the waters as Sedimentary rocks appear in relatively limited enclosing the Park. Good exposures occur at the Long-continued currents of thermal waters and soluble sodium arseniate. Coating the overflow areas within the boundaries of the Yellowstone south base of Antler Peak and at several other acid vapors have acted as powerful agents in the channels from the hot pools, and incrusting the Park, being for the most part buried beneath the localities in the Gallatin Range. The beds are decomposition of the igneous rocks. They have rocks surrounding the springs, are frequently lava flows and volcanic accumulations of the also exposed on Berry Creek at the north end of produced an indelible impression upon the surface found arsenical minerals, such as the red and region. They present, however, a prominent fea­ the Teton Range, and in the walls of Slough features of the plateau. Large areas of altered yellow sulphides of arsenic, realgar and orpiment, ture of the geology of the Park, as they form sev­ Creek Canyon in the Absarokas. rhyolite and extinct solfatara indicate the former and scorodite, an arseniate of iron. eral of the mountain ranges which inclose the Gallatin limestone. This limestone is named existence of still greater thermal activity before The number of thermal springs known in the Park Plateau, while the small exposures rising from, its typical occurrence in the Gallatin Range, historical time. Decomposition of rhyolite by Yellowstone Park exceeds 4,000. There are above the rhyolite floor give an indication of their where it forms the upper part of the Cambrian thermal waters and deposition of sediment by about 100 geysers in the Park. Between a geyser former extension over the entire district. series and has a thickness of 110 feet. It is siliceous waters are extremely slow processes, and a hot spring no sharp definition can be drawn, The Paleozoic and Mesozoic rocks of this region essentially a series of limestones, more massively judging from what may be seen going on to-day although a geyser may be defined as a hot spring embrace representations of all the grand divisions bedded than those of the underlying Fathead for­ in the different geyser basins. It is evident that throwing with intermittent action a column of of geologic time from the Cambrian period to the mation, and forms the first prominent limestone to accomplish such changes, even through more water and steam into the air. A hot spring may Cretaceous. Leaving out the old Algonkian rocks, bluff that rises above the Archean areas. The intense action than the present, a long period of boil unceasingly without violent eruptive energy; whose exact relations to the earliest formations of lowest bed is a massive limestone, varying from time was required. This thermal activity is con­ a geyser may lie dormant for years without any the Paleozoic age are not known, the conformable 50 to 100 feet in thickness, characteristically fined almost exclusively to the area of rhyolite explosive action and again break forth with series begins at the base with middle Cambrian mottled black and gray, and forming a readily eruptions, and it is probable that the few hot- renewed force. strata and is capped by beds of the Laramie recognizable and persistent horizon. Above this spring localities that lie beyond its limits are Old Faithful, in the Upper Geyser Basin, may (Cretaceous) epoch. The total thickness of con­ mottled limestone the rocks are more thinly bed­ closely connected with it by subterranean be regarded as typical in many ways of an active formable sedimentary rocks in the Gallatin Range ded, and carry fossils that are of distinctive passages. It is these unrivaled hydrothermal geyser. The regularity of its eruptions, the is nearly 9,000 feet, of which less than one-half upper Cambrian types. Good exposures occur manifestations and their close relations to vol­ violence of its explosions, and the grace and formed during Paleozoic time. This is the most along Soda Butte and Slough creeks in the north­ canic eruptions, which are on a far grander scale beauty of its water column, make it one of the complete section of the conformable series of the western corner of the Park, in the Teton Range, than those of either New Zealand or Iceland, that most admired of geysers. The illustration given Paleozoic and Mesozoic rocks to be found within as well as in the range from which the limestone have made the Yellowstone Park famous through­ in fig. 6 represents Old Faithful in a state of the Park borders. takes its name. out the world. eruption. The interval between eruptions aver­ The occurrence of stratified rocks is largely The thermal waters of the Park may be classed ages 65 minutes. The period of eruption lasts confined to the mountainous regions of the Park. STRATA OF THE SILURIAN PERIOD. under three heads: first, calcareous waters hold­ about 4^ minutes, during which it throws with In the northeastern part of the reservation they The rocks assigned to this period have been ing calcium carbonate in solution; second, sili­ great power into the air a column of water seldom have been deeply covered by the breccias of the recognized in all the Paleozoic areas of the Park; ceous waters usually carrying free acid in solu­ Jess than 95 feet and seldom exceeding 130 feet in Absaroka Range, recent erosion exposing them in and aggregate about 160 feet of more or less tion ; third, siliceous, alkaline waters rich in dis­ height. The Giantess Geyser, also in the Upper the deeper valleys. In the Gallatin Range, within impure and arenaceous limestone. The forma­ solved silica. Geyser Basin, when in action is a far more power­ the Park area the sedimentary rocks dip uni­ tion is named from its occurrence in the Jefferson Calcareous waters, upon reaching the surface, ful geyser than Old Faithful, but plays with less formly to the north and are but slightly dis­ Range west of the Park. have deposited immense quantities of calcium regularity, with intervals of nearly three weeks' turbed by numerous intrusions of igneous rocks. The Jefferson limestone. This is composed of carbonate, the famous Mammoth Hot Springs duration. It has never built up a sinter cone, To the eastward of this range a few exposures layers of dark-blue or black limestone alternating being built up almost entirely of travertine. but, on the contrary, presents a funnel-shaped are seen, notably at Mount Everts, but to the with brown granular limestone; the darker,rocks With one or two exceptions, only siliceous waters crater or chasm that gradually fills to the brim, south no sedimentary rocks outcrop until the possess a strong fetid odor, and are distinctly are found issuing from fissures in the rhyolite. standing for several days with water slightly Birch Hills are reached. Still farther south they crystalline. The brown beds are usually markedly Areas of acid waters may easily be recognized by below the boiling point before explosive action are exposed in the vicinity of Berry Creek, where arenaceous. Fossils are rare. This horizon is the whitened and altered appearance of the rhy­ takes place. The crater measures 35 feet across the great anticlinal uplift of the Teton Range prominent in the mountain ranges of Montana, olite in the region of the vents, and in most its widest expansion, and is surrounded by a thin dies out. In the mountainous area about Mount where the limestone forms bold bluff ledges, and instances by efflorescent incrustations of alum and crust of laminated sinter, which projects over the Hancock the Cretaceous rocks attain their most weathers with a peculiarly rough and pitted sur­ brilliantly colored salts of iron^ and not infre­ sides of the basin. During the most violent erup­ conspicuous development. face. Good exposures occur in the Gallatin quently by the presence of minute crystals of tions the column of water is thrown to a height Range at Antler Peak, and in the Teton area in STRATA OF THE ALGONKIAN PERIOD. yellow sulphur. These acid waters are charac­ of nearly 250 feet. The picture of the Giantess, the gorge of Berry Creek. terized by an astringent taste. Alkaline springs fig. 7, is a good illustration of a dormant geyser. Sheridan quartzite. Beds assigned to the present more of general interest than the acid Illustrations of both active and dormant thermal Algonkian occupy very limited areas in the Park, STRATA OF THE DEVONIAN PERIOD. waters, as it is only in connection with the former springs are well shown in figs. 8 and 9. Chrome and are found at only three localities. They are The rocks of this period have been recognized that the geysers are found. They are the principal Springs, in Hayden Valley, is in a constant state best exposed upon the slopes of Mount Sheridan, in the Park, but are seldom well exposed. The waters of all the geyser basins and most of the of ebullition, boiling and bubbling with great which has furnished the name for the formation. best exposures are in the ranges to the north­ hot-spring areas. Alkaline waters deposit mainly violence to a height of 2 feet or more. The name They are made up wholly of siliceous material, ward, where a typical Devonian fauna has been amorphous silica as siliceous sinter in an endless is suggested by the color of the water, which and for the most part consist of dense, bluish- obtained from the shaly members of the forma­ variety of forms, as shown around the geyser is due to finely disseminated sulphur held in white quartzite, weathering with rounded out­ tion. vents and incrustations upon the surface and suspension by the agitated water. Water slowly lines and breaking with a conchoidal fracture. The Threeforlks limestone. This formation, so edges of the hot pools. These sinters form the flows from the rim of the spring, leaving a thin Bowlders of this rock may be seen strewn over a named from its fine development near the junc­ brilliant white deposits found over large areas in coating of siliceous sinter around the outside of large area, having been transported southward by tion of the three forks of the Missouri River, is all geyser basins. the pool. Black Sand Spring, in the Upper glacial ice. Similar quartzites outcrop near the in this region generally a massive gray limestone While deposits accumulated about the open Geyser Basin, on the contrary, is rarely agitated, shores of the Yellowstone Lake and on Flat carrying cherty beds, with more thinly bedded vents vary greatly in appearance, due to the ever- and presents the appearance of a quiet pool not Mountain, and at both these localities are over­ limestones that are argillaceous and carry typical changing conditions under which they are laid unlike a dormant geyser. A thin crust of sinter lain unconformably by the Ellis limestone. The Devonian fossils. The Devonian has a thickness down, it is always easy to distinguish the traver­ encircles the edge of the pool in a way seldom third area is an obscure exposure west of Lewis of 190 feet. tine and sinters from each other by their mode seen in other than comparatively still waters. River. of occurrence. Fig. 5 is a reproduction from a The spring lies at the base of a gravel bench STRATA OF THE CARBONIFEROUS PERIOD. photograph of Minerva Terrace at the Mammoth largely made up of black obsidian sands derived STRATA OF THE CAMBRIAN PERIOD. The Carboniferous rocks occur in all the moun­ Hot Springs, and illustrates the building up, one from rhyolite rocks. Cambrian rocks occur in several of the.moun­ tain ranges of the Park, and form a number of above another, of a series of terraces and shallow After the disappearance of the glacial ice sheet tain ranges surrounding the Park and form the isolated areas that rise above the rhyolite plateau. basins by the gradual deposition of calcic car­ the only other geological events which impressed prominent terrane at the south end of the Galla­ The series consists of two divisions, lithologically bonate. Minerva Terrace is one of the most themselves upon the physical features of the tin Range. The strata consist of sandstone, shale, distinct, but characterized by a similar fossil beautiful of the far-famed travertine deposits, Park are the lacustrine deposits and the still later and limestone, the latter constituting the upper fauna. The rocks constitute the great limestone and has for a long period of years poured out a alluvium. The lacustrine sediments attain prom­ member. The basal sandstone rests directly upon terrane of the mountains, weathering into rough steady flow of calcareous waters. inence only on the border of that grand sheet of the Archean and includes many fragments of and rugged peaks that form the most striking The Castle Geyser, situated in the Upper water, Yellowstone Lake, which at one period, gneisses and schists of that age. The system is scenery of the ranges to the north and west. Geyser Basin, shown in fig. 4, is a typical example after the melting of the ice, reached its maximum separable into the divisions known as the Flat- Contrasted with the softer Mesozoic beds the of siliceous sinter deposited about the vent of one elevation of 160 feet above the present level of head and the Gallatin formations, the two being Carboniferous rocks rise in strong relief, but the of the most characteristic, if not one of the most the lake. Evidences of such deposits are every­ distinguished by different fossil faunas, a sepa­ separation from the underlying Devonian lime­ powerful, geysers in the region. The terrace where to be seen in broad benches encircling the ration that is also defined by lithological differ­ stone is not so easily recognized. structure is wanting, the deposition being most Lake for the 100 miles of its circuitous shore. ences. I he Madison limestone. This formation is irregular, with a tendency to produce botryoidal These lacustrine deposits reach their maximum Flathead formation. This group of beds, named from the mountain range of the same forms, piled up one upon another in a most con­ elevation at about 8,000 feet above sea-level, the which takes its name from exposures in the old name west of the Park. It consists of 1,600 feet fused mass. present level of Yellowstone Lake being about Indian pass across the Bridger Range, includes of limestone of varying color and composition, After the deposition of the sinter from the 7,840 feet above the sea. The alluvium covers the oldest fossiliferous rocks of the Park and con­ but characterized throughout by a very uniform siliceous waters, nearly all the remaining mineral most of the river valleys and flood plains, and stitutes the base of the Paleozoic section. The fossil fauna of lower Carboniferous types. The constituents, consisting mainly of salts of the partially fills the depressions in the undulating lowest bed is a sandstone or quartzite, usually formation is separable into two divisions. The alkalies, for the most part very soluble in water, surfaces of the rhyolite, as shown by the frequent weathering with a reddish or yellow tinge, and is lower division, embracing 1,200 feet of beds, con­ are carried off by running streams. In some meadow lands dotted over the plateau. It is frequently a conglomerate at the base. The sists of bluish, dark-gray, or dove-colored lime­ instances other mineral substances, such as oxidesi mainly derived from the disintegration of the shales of the Flathead formation lie above the stones, well bedded, and often weathering in of iron and manganese, are deposited around igneous rocks, and consists of gravels, sands, and quartzite, and consist of micaceous and arena­ broad flagstones. These rocks are sometimes orifices of thermal springs, but always in limited! clays. It is in part older than the hot-spring ceous material, changing to an alteration of shale coarsely crystalline, but normally dense and com­ quantities. Next to the silica held in solution by deposits, and partly younger, when it forms a and thinly bedded limestone above. The latter pact, carrying an abundance of fossils. Impure these waters arsenic is the most interesting con­ loose volcanic soil resting directly upon the light- are usually glauconitic and frequently conglomer­ limestones that might be classed as calcareous stituent, and is present in sufficient quantity colored sinter. atic with limestone pebbles. These rocks carry shales also occur, but do not form a prominent feature of the series. The limestones/ are fre- from a coarsely crystalline, quite pure limestone ish sandstones of varying degrees of hardness. terraces about Yellowstone Lake are all included quently cherty, but this is not an essential fea­ carrying an abundance of fossils which are, The beds are singularly uniform in character, and under this designation. The beds are not consol­ ture. however, generally fragmentary to a true sand­ carry fossils at several horizons. Along the idated, and consist of glacial, water worn material The upper part of the Madison limestone dif­ stone, often with conglomerate layers. Owing to northern border of the Park the Fox Hill is a assorted by the lake waters when they stood at fers quite strongly in general character and its resistance to weathering, this sandstone is series of argillaceous sandy shales and interbed­ much higher levels than they do to-day. These appearance from these lower beds. The rocks often well exposed resting on the slopes above ded sandstones. The latter are not persistent beds are formed of material derived from the are cream-colored or white, compact, and quite the gray clays of the underlying series. beds, but form lenticular bodies, rarely over a few adjacent uplands and transported by the out­ massively bedded. They weather in rugged and hundred feet' in length and from 5 to 25 feet in spreading ice sheets of the Glacial period to the craggy blocks. Near the top the limestones STRATA OF THE CRETACEOUS PERIOD. thickness. This feature may be seen in the face shores of the ancient lake. possess a saccharoidal, sandy texture, and are fre­ Cretaceous rocks cover large areas along the of Mount Everts. In the Gallatin Range the Fox Alluvium. The deposits included under this quently brecciated. These granular limestones southern borders of the map. Their aggregate Hill is essentially the same, and usually may be heading embrace the stream gravels, sands, and. carry layers of red or purple earthy magnesian thickness greatly exceeds that of all the Paleozoic distinguished from the underlying Pierre by its clays forming the stream bottoms and terrace material, which often weather as brilliant red strata of this region. The Dakota, Colorado, lighter color and greater resistance to erosion. lands found in all the larger valleys, together streaks. In the Gallatin Range the reddish beds Montana, and Laramie formations, the four sub­ The Laramie. This formation consists of with accumulations of debris forming the allu­ occur irregularly distributed throughout the lime­ divisions of this period, are all recognized, light-colored, grayish-yellow sandstones and inter- vium cones and fans found at the base of steep stone, to which they impart a pink color. Fossils although the distinctions are not always easily bedded shales. The sandstones are often cross mountain slopes. Under this designation there from this horizon are all of lower Carboniferous defined in the Teton and Wind River ranges. bedded, of variable character, and formed essen­ are also found limited areas of diatomaceous types. The Livingston rocks of the post-Laramie epoch, tially of quartzitic material. The shales are earth, generally occurring in connection with The Madison limestone is the great mountain- which are so greatly developed in the region argillaceous, and frequently carry considerable centers of thermal waters. These deposits are forming formation of the sedimentary series, and north of the Park, are not found within its limits. carbonaceous material, in places developing into usually covered by a thin layer of loose soil upon is the most prominent horizon in all the Pale­ The Cretaceous rocks differ materially from the impure coal. In Montana the Laramie contains which grasses have grown up more or less luxuri­ ozoic areas of the Park. The upper valley of the older sedimentary rocks of the Park, consisting valuable seams of coal, the formation being the antly. Gallatiii River is cut out of these rocks, and they of sandstones and earthy shales with impure lime­ great coal-bearing horizon of the northern Rocky may be seen west of Swan Valley, forming the stones, a series whose color, physical characters, Mountains. HOT-SPEESTG DEPOSITS. slopes of Quadrant Mountain. They are also and mode of weathering are all in strong contrast Plant remains occur throughout the formation, Areas indicated upon the map as hot-spring prominent at the northern end of the Absaroka to those of the Paleozoic rocks. and brackish-water fossils have been found at a deposits include calcareous, siliceous, and solfa- Range, in the spurs of the Teton Range, and east The Dakota sandstone. This formation consists number of localities. The Laramie forms the taric areas, both active and extinct. JSTo distinc­ of the fault line that separates the Cretaceous of three members: a basal sandstone that is in summit of Mount Everts and occurs along the tion has been made upon the map between depos­ beds of Big Game Ridge from the sedimentary part a conglomerate, overlain by sandy clays northern boundary of the Park, but attains its its formed by the hot waters and the areas of rocks which lie to the westward of Two Ocean carrying a thin bed of fossiliferous fresh-water greatest prominence in Big Game Ridge. The top decomposed and altered rocks brought about by Plateau. limestone, with a massively bedded quartzite at of the Laramie is probably not exposed in this solfataric vapors. The latter are often difficult 1 Tie Quadrant qwa/rimte. This formation con­ the top. The conglomerate is the characteristic region, but there is a thickness of at least 600 of precise delimitation, since the altered and sists of white, yellowish, and occasionally pink feature of the Dakota, and wherever exposed feet on the summit of Mount Everts, and a still unaltered rocks pass into each other by gradual beds of quartzite, with intercalated beds of drab throughout the Park region is a readily recogniz­ greater development is found to the south. transition. In a few cases solfataric areas which saccharoidal limestones. The quartzite is gener­ able horizon. It consists of well-rounded* vari­ might be indicated on the map have not been STRATA OF THE EOCENE PERIOD. ally compact, occurs in beds from 4 to 25 feet in colored, firmly-cemented, siliceous pebbles, which discriminated as such; as in the Grand Canyon thickness, and weathers in massive blocks. More break with the matrix. This conglomerate forms Pinyon conglomerate. The sedimentary beds below the Falls of the Yellowstone a fine example rarely it breaks into small fragments that form but a part of the basal sandstone, and its position referred to this period occupy limited areas and of bleached and decomposed rhyolite, the result debris slopes, as seen in the Teton Range. The in the beds varies at different localities. The are found only in the southern end of the Park. of solfataric activity, is colored upon the map as total thickness averages 400 feet in the Gallatin fossiliferous limestone is filled with fresh-water They have been designated the Pinyon conglom­ rhyolite. The distribution of these hot-spring Range. In the southwest corner of the Park it fossils, and in the Gallatin Range affords a well- erate from the name of the mountain where they areas upon the map of the Park is confined to is far less prominent than in the Gallatin, but its determined horizon, but one which has not been are best exposed.. They consist of a series of the rhyolitic areas and their immediate borders. resistance to weathering makes it easily recogniz­ recognized in the Teton and Wind River ranges. conglomerate beds with local intercalations of Calcareous deposits. These deposits consist of able, outcropping beneath the soft red clays of The quartzite bed capping the formation is a sandstone, the formation resting unconformably carbonate of lime occurring principally as traver­ the Juratrias. It takes its name from the Quad­ cream-colored, dense rock that breaks into massive upon the upturned Laramie (Cretaceous). The tine at several localities in the Park. They rant Mountains in the Gallatin Range, where it is blocks in weathering. conglomerate is almost wholly made up of peb­ attain their greatest magnitude at the Mammoth well developed and forms a picturesque bluff The Colorado. This formation embraces the bles, sometimes as large as 10 inches in diameter, Hot Springs. Here the hot waters flowing since encircling the mountain. two subdivisions, Benton shales and Niobrara of vari-colored quartzites, carrying occasional pre-Glacial time have built up large accumula­ limestones, aggregating 2,000 feet in thickness in fragments of gneiss, and still more rarely a few tions of travertine, forming the well-known STRATA OF THE JURATRIAS PERIOD. the Gallatin region and exceeding this in the polished pebbles of porphyry. The quartzite terraces. The deposits themselves are pure-white, The rocks of the Juratrias period embrace a southern ranges. These subdivisions are not pebbles are mostly light-colored, but are associ­ but when freshly formed are often brilliantly thickness of several hundred feet of strata whose recognizable in the Park, as the lithological ated with others of widely contrasting shades of colored by the algae that live in hot waters. This upper part is characterized by a typical marine characters are not sufficiently distinct, and the red, purple, and green. They are well rounded plant life is the principal agent in producing the Jurassic fauna. The Juratrias is divided into meager fauna does not warrant separation. The and polished, and show indentations and sheared formation of travertine. two formations: the underlying Teton formation, Benton shales consist of finely laminated, carbona­ surfaces that are the result of pressure and move­ Siliceous sinter. This is the deposit formed and the Ellis formation above. The two are ceous shales, seldom well defined, and carrying ment. The pebbles are cemented by a matrix of by the hot-spring waters carrying silica in solu­ clearly defined upon lithological grounds, the thin beds of impure sandstones. Good exposures coarse sand, which passes in places into lenticular tion. It is a white, opaque mineral occurring in Teton being the probable equivalent of the well- are to be seen on the south face of Electric Peak, beds of sandstone. This sandstone is coarse and a great variety of forms, consisting of amorphous known Red Beds of Wyoming and Colorado. at the base of Mount Everts, along the Snake loosely compacted, and shows strong cross-bed­ silica carrying from 3 to 6 per cent of water and The Teton formation. This formation takes its River near Mount Hancock, and on the flanks of ding. small quantities of impurities. It is this sinter name from the imposing Teton Range. Within the Tetons. Marine fossils and indistinct, vegeta­ that forms the brilliant deposits over large areas the Park it attains its most conspicuous develop­ ble remains characterize the beds. The ISTiobrara STRATA OF THE NEOCENE PERIOD. in all of the geyser basins of the Park. The ment along Snake River and on Red Creek. At consists of calcareous, argillaceous beds carrying The Canyon conglomerate. Rocks of the Neo­ geyser cones and the beaded, coral-like incrusta­ the northern end of the Tetons and in the Gal­ nodules of limestones with occasional thin beds cene period occupy still more restricted areas tions about the springs, as well as the basins of latin Range, and at other localities, the series is of sandstones. The shales are lighter colored than those of the earlier Tertiary deposits. The the springs themselves, are built up of this thinner and the beds are very generally concealed than the Benton, but no line of separation can be Canyon conglomerate is named from its occur­ material. The principal sinter deposits of the by the soft clays into which they so readily established. Moreover, the beds are not well rence in the Grand Canyon of the Yellowstone, Park are found at the JSTorris, Lower and Upper, weather. The formation consists of the basal defined from the Pierre shales above. and consists of thinly bedded conglomerates and and Shoshone Geyser basins, and on the shores sandstone, usually dull-brown in color and more The Montana. This subdivision of the Creta­ gravels, exposed only in stream cuttings along the of Yellowstone and Heart lakes. Siliceous sinter or less calcareous, characterized by rods and rolls ceous includes both the Pierre shales and the Fox Lamar River and the Grand Canyon. The beds is deposited partly by evaporation, but chiefly of white chert, and carrying interbedded gray Hill sandstones. It covers large areas along the are light-colored and are composed of well- through the agency of vegetable (algous) life, limestones containing linguloid shells. Above southern part of the map, and occurs also along rounded pebbles of Archean gneisses and ande- whose vivid colors tint the margins of the pools this basal bed are gray and greenish calcareous the northern border at Electric Peak and Mount sitic material, derived from the underlying brec­ and hot-water streams. Siliceous sinter is not shales, often micaceous and capped by a red Everts. The character of the beds varies widely cias, the latter greatly predominating. At both found at the Mammoth Hot Springs, but is very arenaceous shale that forms a conspicuous part of between the northern and southern regions. In localities the beds are capped by recent basalt. generally present wherever alkaline waters occur. the formation. This series of beds is capped by the region to the south, drained by the Snake It is also deposited sparingly by evaporation from a sandstone, generally bright-yellow, with red. River and its tributaries, the subdivisions of the DEPOSITS OF THE PLEISTOCENE PERIOD. acid waters carrying free hydrochloric acid, form­ weathered surface, this rock delimiting the over­ Montana are poorly defined, the rock consisting Glacial drift. Glacial drift covers large areas ing spine-covered concretionary pebbles about the lying Ellis formation. The thickness is estimated mainly of yellowish sandstones. In the northern within the Park, but it has been indicated upon borders of the springs and along overflowed at over 400 feet in the Snake River region and areas the lower member, Pierre shales, is quite the map only in a few conspicuous instances. channels. 200 feet in the Gallatin Range. clearly defined lithologically from the overlying Gravel, sand, and bowlders of a great variety of Solfataric areas. These areas are common The Ellis formation. This formation consti­ Fox Hill sandstones. The Pierre beds consist of crystalline, igneous, and sedimentary rocks are throughout the Park. At such places hot vapors tutes a series of fossiliferous beds carrying a leaden-gray argillaceous shales carrying bowlder- common throughout the glaciated areas. In Hay- rising through the rocks have largely decomposed Jurassic fauna. The formation consists of two like limestone masses. They are distinctly bed­ den and Pelican valleys extensive morainal and altered them. Where the action is complete divisions: a lower one, limestone, and an upper ded, with well-developed joint planes, producing deposits occur whose clays and loosely cemented the hot water charged with sulphuric acid has one, chiefly sandstone. The formation derives its abrupt cliff faces. The rocks weather readily to sands closely resemble lake beds. The uplands leached out the soluble constituents and left a name from the old military post near Bozeman in plastic clays, and are distinguished by the pres­ about Yellowstone Lake show extensive accumu­ white residue of silica. In places where the the Gallatin Valley. ence of gypsum scattered through them and lations of glacial sands heaped in typical morainal decomposition is not so complete the rocks are The limestones of the Ellis formation form its covering the outcrops with white efflorescence. forms. The bowlder moraine at Junction Valley tinted yellow and red by the oxides of iron and lower part. They are impure, argillaceous, gray Good exposures occur along the Gardiner River is the most striking example of this-variety of their compounds. limestones and shales, and carry marine fossils at and in the bluff face of Mount Everts. In the drift. The glacial heapings of Swan Valley and The Highland Springs, Crater Hill, Norris several horizons. Good exposures occur north of southern areas these clays are not well exposed the sand hills east of the junction of the Firehole Geyser Basin, and other localities furnish strik­ Terrace Mountain and at a number of localities and the formation is essentially a sandstone. and Gibbon rivers are noteworthy occurrences of ing examples of solfataric rock decomposition. in the Gallatin Range. The Fox Hill sandstone, the upper member of drift deposits due to glacial ice. The so-called mud pots and paint pots of the The sandstone of the Ellis formation overlies the Montana, is the predominant stratum of Big Lacustrine deposits. The water-laid and Park belong to this class, as they are formed of the impure limestones just noted. It varies Game Ridge. In this area the rocks are yellow­ assorted sands, gravels, and clays forming the clays derived from decomposed rock. Tellowstone National Park 5. IGNEOUS BOOKS. quite light. They are often distinctly bedded, as Late andesite -flows. The late andesite flows is well developed at Obsidian Cliff, in the walls in Sepulchre Mountain and in the mountains on within the Yellowstone Park constitute some of of the Grand Canyon near Deep Creek, in Madi­ BY JOSEPH PAXSON CODINGS. both sides of Soda Butte Creek, and in many the more prominent peaks in the Absaroka son Canyon, and elsewhere. other localities in the Absaroka Bange. The Bange: Mount Doane, Mount Stevenson, and By far the larger part of the area covered by character of the andesite becomes more basic Colter Peak. They are light-colored, massive INTRUSIVE ROCKS. the maps of the Yellowstone Park folio consists toward the upper portion of the accumulation, rocks with well-developed columnar structure. The intrusive igneous rocks within the Yellow- of igneous rocks. Notwithstanding the broad where it is often basaltic, passing into the thick They are hornblende-mica-andesite, some having stone Park form a great number of small bodies area thus covered, similar rocks extend far layer of basalt sheets directly overlying this brec­ very few ferromagnesian constituents. The light- presenting considerable variation in mode of beyond the limits of the Park into Idaho, Mon­ cia. It carries abundant plant remains in some gray groundmass carries phenocrysts of plagio- occurrence and mineral composition. tana, and Wyoming, reaching from the highest localities, notably those of the Fossil Forest on clase-feldspar, besides those of hornblende and Outside the limits of the Gallatin Bange they peaks to the lowest valleys. These erupted the eastern side of Lamar Valley. It occurs biotite. These andesites appear to have flowed are by no means conspicuous, and, because of masses present marked differences in their field over a much larger area than the early acid brec­ as lava streams over an irregular surface of ande­ their restricted areas and wide petrographical appearance and geological relations, but they all cia. The greater part of the volcanic mountains sitic breccia. In places they are closely connected range, have not been closely discriminated on the fall readily into one or the other of two groups, in the northeastern portion of the Yellowstone with intrusive bodies of similar andesites and map by differences of color. In other words, extrusive and intrusive rocks, the distinction being Park is formed of it. have broken up through the breccia. rocks of different composition but similar in based upon their mode of occurrence. The intru­ Ea/rl/y basalt sheets. Early basalt sheets are JBasalt. Basalt in small sheets or flows occurs mode of occurrence have been grouped together. sive rocks consolidated beneath the surface; the closely associated with the basic breccia just immediately upon the late basic breccia in a few They have been represented in chronological extrusive rocks reached the surface and formed described and constitute a massive capping to the localities and beneath the great sheets of rhyolite groups, and fall into three periods. The first of lava-flows, mud-flows, breccias, and ash-beds. As underlying rocks, in places 1,000 feet in thick­ which form the Park Plateau. The basalt sheets these is subdivided petrographically because of the extrusive rocks are the most widespread they ness. They are lava-flows that have poured one are of no great thickness and are frequently the prominence of the intrusive masses belonging will be first described. over another, with an occasional intercalated columnar. The rock is dark-gray to bluiah-black, to it. The first series embraces porphyries of layer of tuff or breccia. The individual sheets more or less vesicular to compact, with pheno­ both acid and intermediate composition, dacite, EXTRUSIVE BOOKS. vary in thickness from 5 to 25 feet. The basalt crysts of feldspar and divine^ and sometimes of and andesite. The distinctions between the various rocks, is andesitic in habit, consisting of microlitic augite. Often the phenocrysts are quite small and Andesite-porpJiyry. Andesite-porphyry occurs indicated by differences of color, are based partly groundmass with porphyritic crystals. Its com­ scarcely noticeable. Its habit is mostly andesitic in laccoliths and intrusive sheets and dikes in the on petrographical characteristics, partly on geo­ position varies within narrow limits, the more rather than ophitic. It occurs beneath rhyolite sedimentary strata of the Gallatin Mountains. logical ones, including their order of succession. extreme forms being rather alkaline and carrying at the southeast end of Mount Everts, at Osprey The rocks are light-colored, mostly gray, with They might all be treated under the general leucite with alkaline feldspars. These early Falls, and in the neighborhood of Tower Falls lithoidal to finely crystalline groundmass and groupings of andesitic breccias, agglomerates, and basalt sheets form the east wall of Mirror Plateau and elsewhere. It occurs intercalated with rhyo­ numerous small phenocrysts of plagioclase-felds- lava-flows; basaltic breccias, agglomerates, and and cap the high summits of the mountains east lite in the canyon of the Yellowstone near the par, hornblende, and biotite. In some cases bio­ lava-flows; rhyolitic lava-flows. They will be of Soda Butte Creek, as well as other heights of mouth of Deep Creek. Larger bodies of similar tite is wanting. The habit of the rock is andesitic, described, however, so far as possible, in chrono­ the Absaroka Bange. There are occasional sheets basalt occur overlying the rhyolite south and east with phenocrysts varying in amount and size. In logical order, which is also the order followed in of basalt in the large body of early basic breccia of Bunsen Peak and in Falls Biver Basin. But composition they range from that of acid andesite the legend on the atlas sheets. which are not indicated by a special color on the no great body of it is found within the Yellow- to that of intermediate magmas with 61 per cent Andesitic and basaltic breccias or agglomerates map. Their petrographical characters resemble stone *Park limits. of silica. Mineralogically they correspond to are subaerial accumulations of rock fragments, those of the more numerous basalt sheets immedi­ Dacite. Small exposures of dacite occur on hornblende - mica - andesite and hornblende-ande- dust, and mud-flows. In general they consist of ately over this breccia. Columnar structure is both sides of Snake Biver. Those on the west site. angular fragments of andesite and basalt, of vari­ quite generally developed in these flows. side are situated along the river bottom, partially Dacite-porpJiyry. Dacite-porphry occurs in ous sizes, cemented together by finer material of Trachytic rhyolite. This occurs in lava-flows concealed by glacial and alluvial deposits. Those large laccoliths and as smaller intrusions in the the same or of similar composition. In places intimately associated with the oldest basic brec­ on the east side lie several hundred feet up the southern end of the Gallatin Mountains, and in the petrographical character of fragments and cia immediately north of the boundary of the steep slope of the ridge, near the boundary of the two other localities, Bunsen Peak and Birch cement is quite uniform through large masses. Yellowstone Park. Within the Park it forms rhyolite flows with the Montana sandstone. Hills. It is light-gray to white, with aphanitic In most instances the fragments differ consider­ scattered patches of massive lava on both sides ^Rhyolite. Bhyolite constitutes the largest to finely crystalline groundmass and few or many ably in their habit, and somewhat in mineral com­ of the Yellowstone Biver. It resembles the rhyo­ body of igneous rock within the Park. It is phenocrysts. In the Gallatin Mountains the rock position. The breccia for the most part is rudely lite of the region in general appearance, being almost wholly in the form of massive lava-flows has few phenocrysts of biotite and plagioclase, bedded in slightly inclined layers, and there are lithoidal and glassy, with pronounced flow-struc­ without fragmental breccia, except at Mount and in places, if non-porphyritic, having the few contemporaneous sheets of massive lava. In ture. It carries phenocrysts of sanidine and Sheridan. A small deposit of rhyolitic tuff appearance of a felsite-porphyry. At Bunsen places intercalated lava streams are more abund­ plagioclase, but none of quartz. It differs from underlies it in places, as at Mount Everts and Peak it is porphyritic with small phenocrysts of ant, and sometimes predominate over the breccia. the rhyolite of the Park in having biotite in small elsewhere. The rock is quite uniform in com­ feldspar, biotite, and occasionally quartz. At Occasionally the bedding is distinct, especially amount. It is also lower in silica. The lithoidal position, but varies greatly in physical appear­ Birch Hills the phenocrysts are somewhat larger where volcanic dust prevailed. Thus the more varieties are white to yellowish, reddish, gray, ance and habit. It ranges from wholly glassy to and the porphyry-like habit is more pronounced. perfect bedding is usually found farther from and more or less altered. Chemically and miner- a lithoidal or stony texture, and is filled with In the Gallatin Mountains it is younger than the the centers of eruption. Near these centers the alogically the rock is intermediate between phenocrysts of quartz, sanidine, and plagioclase andesite-porphyry. breccia exhibits little or no bedding and the trachyte and rhyolite, with the physical aspect in some cases, but is entirely free from them in Elecfcic intrusives. The Electric intrusives petrographical character is more uniform. In of the latter. The rock frequently carries frag­ others. It generally exhibits marked banding or are situated in the northwest corner of the Park, certain localities there are waterworn and well- ments of andesite. The same kind of rock occurs flow-structure, and often carries spheroidal bodies in the neighborhood of Electric Peak. They stratified beds of volcanic material intercalated in as tuff or breccia associated with the early basalt about the size of pebbles, called spherulites, occur as dikes and larger vertical bodies, and as the roughly bedded breccia. They are plainly in several localities, notably at Two Ocean Pass. besides hollow ones called lithophysse. In color a stock or volcanic neck. They possess a wide the result of water action, but it is impossible to It is characterized by its light color, abundant it may be any tone of gray, from white to black. range in texture and composition, and embrace differentiate them on the map. sanidines with plagioclase, some biotite, and the The lithoidal varieties are usually light-gray, diorite, diorite-porphyry, and andesite-porphyry, Early acid breccia. The early acid breccia absence of quartz. It is mixed with many frag­ bluish, yellowish, or purplish. Bed, orange, and both basic and acid, and also quartz-bearing and flows consist of hornblende-andesites and ments of andesite and basalt. Its exposures are yellow besides black and white occur in the varieties, besides andesites, dacite, and basalt as hornblende-mica-andesites for the most part. They of very limited extent and are not always indi­ glassy varieties. Brecciated masses often pre­ intrusive bodies. A detailed description of these are generally light-colored and variegated, less cated on the map. sent a highly variegated appearance. Glassy rocks has been published as a special paper in often dark-colored. This oldest breccia frequently Late acid breccia. The late acid breccia and forms of rhyolite (obsidian, pitch stone, perlite, the Twelfth Annual Beport of the United States contains fragments of gneiss and schist, besides flows resemble the early ones very closely in and pumice) cover a large part of the area of the Geological Survey, " The eruptive rocks of Elec­ other foreign material that formed the country many particulars. The rocks are light-colored, plateau, forming the upper surface of the tric Peak and Sepulchre Mountain." These rocks rock through which the earliest eruptions were variegated hornblende-mica-andesites and horn- rhyolitic lava-flows and sometimes the bottom are probably in part contemporaneous with, and forced. This breccia is more or less highly blende-andesite, sometimes with intermingled frag­ surface. They often constitute the whole mass in part subsequent to, the early basic breccia. indurated and compacted to a mass which fre­ ments of basic andesite. In general the late acid of small flows. The most noticeable exposure of Sylvan intrusives. The Sylvan intrusives quently breaks with an even fracture traversing breccia is not so highly indurated as the early obsidian is at Obsidian Cliff. Fragments of break through the late basic breccias and earlier rock-fragments and cement alike. It underlies acid breccia. It directly overlies the early basalt black glass are strewn everywhere. Pitchstone rocks in the vicinity of Sylvan Pass in the the early basic breccia, in some cases being clearly sheets on Mirror Plateau and in the mountains Plateau and the plateau west of the Upper Gey­ Absaroka Bange. Here the rocks form dikes delimited from it by strongly contrasted colors south of Lamar Biver. Massive lavas are seldom ser Basin are covered with black and red obsidian. and irregularly shaped bodies. They range from and by a chloritization of the ancient surface of observed, the great body of the formation being Pumice is much less abundant, and has possibly diorite and granite-porphyry to andesite-porphyry the acid breccia. The best localities in which to subaerial breccia or agglomerate. Its extent is been carried away in large quantities from the and andesite, and vary in composition from basic observe this relationship are: the north base of considerable, reaching from Mirror Plateau to surface by erosion. Lithoidal rhyolite forms the to acid, from diorite to granite, from basalt to Sepulchre Mountain, the lower portion of Cache Mountain Creek. great mass of the rock, constituting the interior dacite. They are probably contemporaneous with Creek Valley, and a ravine at the south base of Late basic breccia. The late basic breccia and portions of the lava-flows, where they were of the later outbursts of the late basic breccia and Prospect Peak not far from Tower Creek. In flows consist of pyroxene-andesites and horn- considerable thickness. It is often highly lamin­ the latest andesitic lava-flows. Owing to the places the lower breccia grades into the upper blende-pyroxene-andesites and basaltic andesites, ated parallel to the planes of flow, as at Obsidian small size of these intrusive bodies and to the breccia without a noticeable plane of demarca­ which closely resemble those of the early basic Cliff. Typical exposures of rhyolite may be seen great variability in their composition, they are all tion. Massive contemporaneous flows of acid breccia. This breccia directly overlies the late at- the Golden Gate, in the canyon of Gibbon represented in one color. andesite are seldom observed. The early acid acid breccia just described, grading into it in Biver, and in the lower portion of the Grand Diorite. On Sulphur Creek south of Dunraven andesitic breccia in the several scattered localities some places, and in others being separated from Canyon of the Yellowstone. The brilliantly Peak (Canyon sheet) occurs an obscure exposure where it is exposed in the valley of Tower Creek it by distinct unconformity. The basic breccia colored part of the Grand Canyon exhibits the of fine-grained diorite, composed of plagioclase- and its tributaries consists in part of well-rounded is mostly dark chocolate-brown, and is bedded in effects of alteration and solfataric action within a feldspar and hornblende. Similar, diorites occur and water-laid pebbles of acid andesite. In places almost horizontal layers throughout large areas, great body of lithoidal rhyolite which forms the elsewhere in the region, notably among the Elec­ it carries leaf impressions and fragments of wood. notably in the mountains on both sides of the walls of the canyon. Spherulitic rhyolite is tric Peak and Sylvan Pass intrusives, but they Early basic breccias. The early basic breccias upper Yellowstone Biver and Thoroughfare widespread, and may be seen at Obsidian Cliff, are too far removed to be correlated with It. and flows consist of fragments of pyroxene-ande- Creek. This breccia forms the greater mass of the Natural Bridge, on the west side of the Yel­ The occurrence is of slight importance in itself, site, hornblende - pyroxene - andesite, and basalt the Absaroka Bange in the southern part of the lowstone Lake, at Keppler's Cascade, on the the interest lying in the fact that it is the only associated with lava-flows of the same kinds of Park, thinning out northward where it caps the Firehole Biver, and at many other places. Col­ body of the kind known in the central portion of rock. They are generally dark-colored, mostly summits of the mountains in the vicinity of umnar structure similar to that of basalt is fre­ the Park. chocolate-brown, but sometimes the colors are Mount Chittenden. quent, both in glassy and lithoidal varieties. It May, 1896.