DESCRIPTION OF THE THREE FORKS SHEET.

GEOGRAPHY AND TOPOGRAPHY. against the foothills or lower margin of a some­ of stratified rocks, somewhat complicated by of the earth's crust. Great variations are ob­ what irregular mass which is partly mountainous faulting. They rest upon Archean gneisses served in both their mineral and their physical The square degree represented on the Three and partly plateau-like. This is the Jefferson which form the western spurs or foothills features. Movement and pressure have greatly Forks sheet lies between the meridians 111° and Range, and it can be divided into three sections. and also a very considerable portion of the changed them from their original character, and 112° and the parallels 45° and 46°. The district First is a southern hilly rather than mountainous mountain mass. The overlying stratified beds, they are now highly metamorphosed and con­ is in the southwestern mountainous portion of mass, which rises gradually to a long, narrow, whose erosion has contributed to the fertility of torted. In pre-Cambrian time these rocks formed Montana, and includes 3,354 square miles, dis­ plateau-like ridge extending north and south, and the Gallatin Valley, are sometimes overturned, a land mass which comprised nearly all if not all tributed mainly between three counties of is composed mainly of limestones. This ridge has sometimes in normal position, and form a sharp the area included in the map, and which also the State. Nearly the whole of the eastern half an elevation of about 8,000 feet, and at its north­ crest with peaks whose forms are somewhat doubtless extended far to the southward as well is in Gallatin County. Toward the southeast a ern end culminates in Old Baldy Mountain, 9,672 rounded as seen from the Gallatin Valley. Of as to the east and west. The exact limits of this part of Park County appears, and in the extreme feet high, at the edge of the area of gneisses in the summits, Bridger Peak attains the greatest ancient land it is now impossible to determine. southeast corner the Yellowstone National Park which lies the famous mining region of Alder elevation, reaching 9,106 feet. The middle sec­ It was long subjected to erosive influences, and barely encroaches upon the area. Two hundred Creek and Virginia City. This gneissic area is tion of the range reaches from Spring Hill Pass from the degradation of its surface came the sedi­ square miles of the northwestern corner belong continuous to the northward with the area which to Flathead Pass. The general elevation of this ments which formed the Belt formation, in which to Jefferson County, and the remainder of the forms the second section, surrounding Wards portion of the range is somewhat greater than conglomerates predominate in this district. In western half lies within the limits of Madison Peak, which has an elevation of 10,267 feet. that of the southern section, and its peaks are the lower portion of the formation pebbles and County.- The sheet derives its name from the Between the two sections thus outlined, and form­ more rugged in outline. Ross Peak, composed large angular masses of gneiss are found, showing fact that the three forks of the Missouri River ing the divide between the Madison Valley and mainly of sharply eroded limestones, is the most that the ancient shore was not far distant when the Jefferson, Gallatin, and Madison unite in Alder Guleh, is the third section, a basaltic pla­ prominent point, at the southern end of the sec­ they were laid down. The series, as measured, its northern-central portion, in what is known as teau, the elevation of which averages about 8,000 tion, and stands out conspicuously, as seen from has a thickness of 6,000 to 10,000 feet, and may the " Three Forks " Valley. This valley is import­ feet. This plateau is 12 miles long and 4 miles all parts of the valley. Farther north are several in some places reach even 12,000 feet. While its ant from a historic as well as from a geographic wide, and has a gradual slope to the eastward. other high peaks. The summit of the range is beds were being deposited a gradual subsidence standpoint, as being the first place within the The Madison River, to reach its lower valley, still made up of sedimentary rocks, but the dip of the region was going on, and just before the limits of the sheet to be visited by white men. cuts across the northwestern end of the Madison is not so high as farther south, and here the Flathead quartzite was formed the sea spread Lewis and Clark reached this point in their Range, in a rough canyon which is about 16 gneisses are absent, the underlying beds that over a vast extent of country which prior to that explorations in July, 1805, and named the three miles in length and whose gneissic walls are from form the foothills being the dark-green and steel- time was a land area. The Flathead quartzites large rivers which there unite to form the head 2,000 to 3,000 feet high. gray sandstones, shales, and cherty limestones of represent the beach and shore deposits of the of the Missouri. The general elevation of the The third large valley of the district is the the Algonkian, which attain a great thickness. advancing sea, and were deposited unconformably region is high, the lowest point not being below Three Forks Valley. It is about 5 by 8 miles The underlying gneisses do not come to the sur­ upon both the Archean gneisses to the southward 4,000 feet. About one-half the area is occupied in its immediate extent, but is continuous with face in any part of this section of the range. and upon the Belt formation to the northward. by high mountains. The highest elevation is the lower Madison Valley, which reaches to the The northern section of the Bridger Range Limestone, sandstone, and shale form the Paleo­ attained by Lone Mountain, a sharp porphyritic canyon of the Madison, 18 miles south of the extends from Flathead Pass to where the hills zoic and Mesozoic beds, and the conditions under peak which is 11,194 feet above sea-level and junction of the three rivers. It also merges with gradually decrease in elevation and die out into which they were deposited varied from time to rises nearly 6,000 feet above the Madison Valley, the Jefferson Valley, whose upper limit is 12 the basin of Sixteenmile Creek. The Algonkian time. Shallow seas prevailed sometimes, and at which it overlooks. Most of the high peaks are miles southwest of the "Three Forks." sandstones, shales, and limestones here also form other times the waters were relatively deep. found in the gneissic area that extends diagonally The mountain ranges of this region are the the central core, but they do not appear so prom­ Many of the beds were coarse inshore deposits, across the district, dividing it into a north­ Gallatin, the Madison, the Jefferson, and a part inently as farther south, and the Paleozoic and while others may have been formed at a long eastern and a southwestern section. Although of the Bridger Range. Of these the Madison overlying rocks which curve around the end of distance from the shore in deep waters. During they range in height only from 9,000 to 11,000 Range occupies the largest area. It lies in the the range form the greater part of the mountain the Paleozoic age there were many minor oscilla­ feet, they are very sharp and rugged and rise in southern-central portion, almost entirely in the mass. tions of the surface, and they were probably abrupt pine-clad walls that tower from 5,000 to southern half, between the Gallatin and Madison Taking out the valleys and mountain masses, more frequent during Cambrian time than during 6,000 feet above the streams that cut deep canyons rivers. The Archean gneisses form the bulk of the remainder of the area shown upon the Three the Devonian and Carboniferous periods, the across this area. About three-quarters of the its northern part and also a small portion of the Forks sheet is about equally divided between the latter being represented almost entirely by lime­ district is occupied by valleys, three of which southern end, reaching beyond the limits of the plateaus or terrace-plains and high rolling coun­ stones. In the Juratrias period sandstones and the Gallatin, Madison, and Three Forks are of map. The central portion of the range is com­ try. The most noticeable plateau is the one argillaceous and arenaceous limestones predomi­ considerable extent within its limits, while a posed of an immense laccolite, or mass of volcanic extending northward between the Gallatin and nate. Near the lower part of the Cretaceous a fourth the Jefferson expands widely west of rock, which has pushed up through the strata Madison rivers. It is 12 miles in width and 16 persistent bed crowded with fresh-water shells is the district. and has carried portions of the sedimentary rocks miles long. With a gentle inclination it reaches found wherever the Cretaceous strata are seen. The Gallatin Valley is 24 miles in length, and almost to the summit of the range and has over­ northward from the gneissic foothills of the Toward the close of the Cretaceous period a has an average width of 12 miles. Its average turned, broken, and changed the strata along the Madison Range to the Three Forks Valley. A general elevation of the region began, and soon elevation is 4,700 feet. To the south it is bor­ western and northwestern edges of the range. similar but narrower plateau is seen west of the after the deposition of the Laramie formation dered by gently rolling, grass-covered hills, back Many of the peaks in this part of the range rise Madison River. there was an upward movement, during which of which are the sharp, snow-covered peaks of considerably above 10,000 feet. The drainage of the Three Forks district is the strata were folded and subjected to erosion the Gallatin Mountains. On the east, long, slop­ The Gallatin Range occupies about 200 square mainly by the Gallatin and Madison rivers. Both coincident with considerable and widespread ing terraces lead to the foot of the Bridger Range, miles east of the Gallatin River and south of the streams come from beyond the area of the sheet, volcanic activity. It was then that the Livings- whose rather rounded and bare summits stand out Gallatin Valley. The summit is, in general, having their extreme sources in the Yellowstone ton formation was laid down unconformably against the sky, culminating in Bridger Peak and plateau-like, and is composed largely of volcanic National Park, the northwestern corner of which upon these eroded older beds. The Livingston Ross Peak. On the north a series of compara­ breccias, which dip steeply to the eastward, and forms a small portion of the southeastern corner beds are almost entirely made up of volcanic and tively low ridges separates the valley from the along the western edge reach an elevation of of the Three Forks area. The Gallatin drains igneous material. Volcanic activity continued upper Missouri Valley. A plateau of nearly about 10,000 feet. Above this level a number nearly one-half of the country shown upon the throughout a large part of the Eocene and Neo­ horizontal lake beds on the west divides the of peaks rise several hundred feet higher. The Three Forks sheet. It enters the southeast corner cene periods. The formation of the mountain valley from the Madison Valley. Next in size most prominent of these peaks lie at the northern of this area, emerging from a canyon, and for ranges and their subsequent erosion resulted in to the Gallatin Valley is the upper Madison end of the range; among them is Mount Black- about 18 miles flows through comparatively open many valleys, which became the basins of fresh­ Valley, which occupies a very considerable area more, with an elevation of 10,196 feet. country, with two expansions, known as the water lakes. These were eventually partly filled in the southwestern section of the district. It The Jefferson Range, already described, can be Upper and Lower basins. In this part of its with deposits derived from the adjacent moun­ is about 30 miles in length, with an average naturally divided into three parts. North of course the fall of the river is about 1,000 feet, or tain areas. Finally the lakes were drained by width of 7 miles. It ranges in elevation from Wards Peak, which is only one of a number of something over 50 feet to the mile. From the the gradual cutting away of their outlets. 5,000 feet at the northern or lower end to about similar high mountains still unnamed, there is a Lower Basin the Gallatin plunges into a canyon Sphinx Mountain, in the Madison Range, is 6,000 feet at the southern end or head of the val­ large area of granitic, gold-bearing rocks. The of extreme ruggedness. In the 20 miles of its composed of a thick mass of conglomerates, ley. A remarkable system of terraces occupies southern part of the range consists in reality of canyon course the fall is about 80 feet per mile. probably the remnant of a once extensive forma­ this valley. They are very persistent and uni­ two high ridges, the first or more eastern of which, From the time the river leaves this canyon it tion of Eocene age. The fresh-water lakes form on both sides of the Madison River, form­ as already described, is plateau-like. The west­ flows along the western side of the Gallatin Val­ attained their greatest extent in the Neocene ing three benches, the first of which is about 10 ern ridge, which is the higher, extends south­ ley until it turns westward along the northern period, lasting in all probability until the Pleisto­ feet, the second 30 feet, and the third 100 feet west from Old Baldy Mountain, and is a mono­ edge of the Gallatin-Madison plateau to join the cene period was well advanced. During the above the river, which flows slowly through a cline in which the sedimentary beds are over­ Madison and Jefferson rivers in the Three Forks earlier portion of the lake period there was a nearly level alluvial plain dotted with numerous turned, the inversion being most marked near Valley. time of intense volcanic activity, or perhaps islands and adjacent marshy areas. The terraces the southwest end of the ridge, where it passes The Madison River drains about 1,000 square there were several such periods. Immense bodies end abruptly against the foothills on either side out of the district. miles of the territory included in the Three Forks of volcanic dust were carried by winds from erup­ of the valley. On the east is the Madison Range, Although the greater part of the Bridger quadrilateral, and has a course of nearly 70 miles tive centers lying to the southward, possibly in whose mountains are among the highest in the Range lies within the area of the Livingston within its limits. Its course has been described the Yellowstone Park. They were deposited in district, and the valley is overlooked from this sheet, a small portion crosses the extreme north­ when referring to the valleys of the Madison. the waters of the lakes as white dust beds, and side by three prominent peaks, each composed of eastern edge of the Three Forks district, and The Jefferson River drains about 700 square miles at the same time fell upon the surrounding land rocks very different from either of the others. farther south the larger part of the western in the northwestern part of the district. Almost surfaces. From the latter they were afterwards Wedge Peak, toward the south, is a dark-gray, foothills of the range come within its area. As one-half of its course is in canyon through a washed into the lakes and deposited upon the gneissic summit. Next, to the north, is the seen from the Gallatin Valley, the range is very region of complicated, folded, and faulted strata. pure white volcanic dust, from which they are conspicuous. It divides naturally, from this easily distinguished by their yellowish and rusty Sphinx, 10,800 feet high, carved out of a mass of GENERAL GEOLOGY. red conglomerates. Still farther north is a group point of view, into three sections which are colors. In the upper part of the lake beds the of sharp pinnacles of light-colored porphyritic quite distinct from one another. First is the Crystalline schists and gneisses are the oldest volcanic dust is mingled with debris from the rock, in which the central peak attains an eleva­ section lying between, the Bridger Canyon and rocks found in this region. They are designated shore rocks exposed later on, after erosion of the tion of nearly 11,000 feet. The terraces west of Spring Hill Pass. This southern extension of Archean, which are generally regarded as among dust beds. The dust showers destroyed both the Madison River are narrow, and soon end the range is mainly an overturned monocline the earliest rocks entering into the composition animal and vegetable life, and their remains were carried into the lakes and buried in the upper ites, and gneisses, well shown southwest of Madi­ and more calcareous toward the top, with inter- crowded with Devonian fossils and capped by a part of their deposits. In the central portion of son Valley. They are all highly inclined and are laminated, thin bands of glauconitic limestone. band of yellow laminated sandstone 25 feet thick. the Gallatin-Madison lake-basin, in some of the perfectly conformable to one another, occupying The shales are 290 feet thick, and are so soft These Devonian fossils were found both in the upper beds, large deposits of fossil bones and an area of 30 to 40 square miles in the foothills generally that they are concealed beneath their northern and in the southern portion of the dis­ opalized and silicified wood are found. Both immediately west of the Madison River, a few own debris in most places. However, they form trict, and doubtless occur wherever the beds are bones and wood present all the evidences of hav­ miles north of the southern boundary of the map. characteristic ravines, which can be traced with­ found. A coal-black slate is seen near the upper ing been tossed about by the action of the waves They are folded, but the folds are somewhat out difficulty. The beds are highly fossiliferous part of the shales in the vicinity of the head of the for a considerable time after being washed into obscure, so that it is impossible to estimate at several horizons, all the forms found being of Missouri River. In the Jefferson Canyon this has the lake and before being covered by the deposits. accurately the total thickness, but it is certainly middle Cambrian age. developed into a bed of coal, in which are found The streams that emptied into these lakes occu­ not less than several thousand feet. A limited Gallatin formation. This formation consists limestone nodules containing the same Devonian pied about the same relative positions that they area of these beds occurs on the east side of the essentially of limestones, which occur in three forms that are elsewhere found in the shales. do to-day, and where they entered the lakes Madison Valley at the western edge of the Madi­ bluff-faced exposures, usually separated by beds The shales are near the border line of the Devo­ coarser beds and conglomerates are now found. son Range. Between Cherry Creek and Wigwam of shales. The lower limit of the formation has nian and Carboniferous, and their organic remains As the barriers that held back the waters were Creek, on the west side of the Madison Valley, been drawn at the base of the first well-defined contain a mingling of forms of both, but the pre­ worn away the lakes were gradually drained, and the unchanged beds of the Cambrian rest upon limestone (trilobite limestone), which immediately ponderance of evidence is in favor of their Devo­ erosive agencies soon removed not only all vestiges the upturned edges of this group. follows the fine, green, sandy shales of the upper nian age. of the dust that had been deposited upon the land, Belt formation. The oldest unaltered sedi­ part of the Flathead formation. This limestone CARBONIFEKOUS. but also carried away a large part of the lake beds mentary rocks mapped on the Three Forks is 120 feet thick, massively bedded in the center, themselves. Later on, and possibly contempora­ sheet are found in the northern portion of the and somewhat laminated at the top, and is very After the deposition of the shales and sand­ neously with the later stages of the lakes, occurred region; first in the foothills of the northern por­ persistent, forming a low bluff back of the sharp stones of the upper Devonian, there followed an a period of glaciation which has left its evidence tion of the Bridger Range; then in the hills ridge of the Flathead quartzite, from which it is epoch almost continuously favorable to the for­ in morainal material found in many canyons of north of the Gallatin and East Gallatin rivers; separated by the Flathead shales. The limestone mation of calcareous deposits. Resting upon the nearly all the mountain ranges of the district. and again in the rugged hills north of the Jeffer­ is characteristically fossiliferous, especially at the Three Forks shales is a series of beds 1,600 feet The glaciers were local, and moraines are found son canyon, in the northwestern part of the dis­ base. Following the trilobite limestones, the in thickness, the lower 1,250 feet of which con­ in the Madison Range; in the Jefferson Range, trict. They form a series of beds of littoral for­ Obolella shales occupy a space of about 280 feet. sist entirely of limestones and the upper 350 feet both at the north near Ward Peak and in the mation which, as exposed in the area just The outcrops are very obscure, owing to the soft of an alternation of limestone and quartzites. The south below Old Baldy Mountain; and also in described, reach a measured thickness of from character of the calcareous and sandy beds, the rocks, therefore, are easily divisible on litholog- the Gallatin and Bridger ranges. The drift 2,300 to 6,000 feet. The lower portion of the erosion of which has formed deep ravines parallel ical grounds into two formations, the Madison deposits in the valleys are mainly of local origin, formation consists of an alternation of coarse to the strike of the beds. An undetermined and Quadrant. The former probably represents and on the map the drift has not been distin­ sandstones and conglomerates,, whose arkose char­ species of Obolella is the only fossil recognized as the lower and middle Carboniferous although guished from the later alluvial deposits found acter is evident. They are of somber hue, dark- coming from this horizon. The limestones that the two can not be differentiated and the latter along the courses of the principal streams. The green and steel-gray colors predominating. In come next are massively bedded, forming a char­ is most likely upper Carboniferous. thin covering of drift material found on the the central portion of the series argillites and acteristic topographic feature wherever noted, The Madison formation. This formation con­ plateaus of the Bozeman lake beds and in other siliceous limestones are frequently found, and in and derive their name of " mottled" limestones sists entirely of limestone beds, having a thick­ areas has not been indicated upon the map. The the upper part sandstones again seem to pre­ from the characteristic mottling of yellowish and ness of about 1,250 feet. These can be divided latest volcanic flows were of basalt. They were, dominate. The formation, as far as examined, is dark spots seen in the central portion of the beds. into three portions, the lower 325 feet consisting however, shortly preceded by rhyolitic eruptions. non-fossiliferous. To the northward, beyond the They are 260 feet in thickness, and are followed of laminated limestone, followed by 350 feet of These lava sheets, both basaltic and rhyolitic, limits of the district, these beds attain a thick­ by the Dry Creek shales, a series of brownish- massively bedded limestone, upon which rest 575 now form the summits of tables or mesas found ness of from 10,000 to 12,000 feet. In the north­ yellow, red, and pink, saccharoidal shales and feet of limestone sometimes called the jaspery mainly in the southern portions of the district. ern quarter of the area the Belt formation immedi­ thin-bedded sandstones about 30 feet in thickness, limestones, although the jasper which gives them ately underlies the Flathead quartzite, but to the which separate them from the pebbly limestones. their name is conspicuous only in the upper part DESCRIPTION OF ROCK FORMATIONS. southward it is absent and does not intervene The latter consist of about 145 feet of light- of the series. As a whole, the paleontological More than half of the area included in the between the Archean and the undoubted Cam­ colored laminated limestones in bands of 4, 6, or aspect of the Madison limestones is that of the Three Forks sheet is covered, so far as the sur­ brian, and the formation certainly was not laid 8 feet each in thickness. They are pebbly through­ lower Carboniferous epoch, to which it must be face is concerned, with rocks of sedimentary down on the Archean except within a compara­ out, the lower bands being the coarsest, and con­ at least provisionally referred. origin, while the crystalline rocks occupy an tively narrow strip of the northern edge of taining also glauconite in abundance. The peb­ Quadrant formation. After the deposition of area of approximately 1,000 square miles. the area shown on the map, and even here the bles are of limestone. Fossils are found at the the Madison limestones there seems to have been The remainder of the area is covered with actual contact with the gneisses has not been base, in the middle, and at the top, the forms a marked change in the character of the sedi­ igneous material, the most prominent of which is seen, although the line of junction is probably being apparently the same throughout. Some of ments, due possibly to the prevalence of much the andesitic breccia forming the main part of not far below the base of the sections seen in the them are the same that in other regions occur in shallower seas or to more active erosion of land the Gallatin Range. The next in extent is the Bridger Range and near the Jefferson Canyon. the Cambrian, passing into the base of the Ordo- areas. The lower beds, to which the name of great porphyrite laccolite of the Madison Range, It is possible that further investigation may vician. "red" limestone has been given, are everywhere result in the reference of this formation to the arenaceous and argillaceous, and in many locali­ and then comes the basaltic plateau lying between DEVONO-SILUKIAN. the Madison Valley and Virginia City. lower part of the Cambrian. At present, how­ ties a conglomerate of limestone pebbles lies at Various dikes and sheets occur scattered over ever, it is referred provisionally to the Algon­ The pebbly limestones pass imperceptibly into the very base of the formation. Although no true the map in such small areas that it is impossible kian. a series of black, magnesian limestones which for dolomites are found, these lower limestones are several hundred feet are, so far as examined, all more or less magnesian. The section varies to differentiate them in mapping them. CAMBRIAN. devoid of fossils. It. is possible, as the pebbly considerably at different points, but its thickness AKCHEAN. The rocks of this period begin with a series of limestones have every indication of being on or is between 300 and 400 feet. The red limestone The Archean gneisses possibly include some sandstones and shales, the lower member of which near the border line between the Cambrian and is from 170 to 200 feet thick, and the brilliant beds that may eventually be referred to the is sometimes known as the basal quartzite, which the Silurian, that their upper beds and a part of red color of its debris makes it very conspicuous, Algonkian. The principal areas in which they rests with marked unconformity upon the the overlying black limestones may eventually be while its soft character leads to the development occur are the foothills or western spurs of the Archean throughout three-fourths of the area of referred to the Silurian. There is little doubt of a ravine back of the outcrops of the Madison Bridger Range and the mountain masses at the the district. In the remainder it rests upon the that the sedimentation was continuous, and we limestone. The fossils found in the red limestone northern and southern ends of the Madison Range, Algonkian, where the unconformity, if it exists, need not be surprised if sometime in the future are of upper Carboniferous age. Following the especially in the central portion of the district, is very slight, probably because the subsidence of this now barren interval should somewhere furnish red limestone are from 150 to 180 feet of thin- where a belt of gneissic rocks from 10 to 12 miles the country was very gradual, allowing the beds a mingling of Silurian and Devonian forms. bedded, cherty limestone, alternating with quartz- in width and nearly 30 miles in length extends to overlap with little, if any, discordance. The itic layers, the latter predominating at the top DEVONIAN. from the western limits of the Three Forks sheet beds gradually become more and more calcareous and being capped by a prominent bed of quartz­ to near its southeastern edge. The Madison as we ascend in the section, until limestones pre­ The black limestones referred to under the ite or quartzitic sandstone, which has been taken River, which cuts across this Archean belt a dominate in the upper part. The oldest fossils head of Devono-Silurian have been provisionally as the base of the overlying Mesozoic. little west of its center, shows the best section of found are of Middle Cambrian age, but as none referred to the Jefferson formation, because the JUEATEIAS. its beds, the canyon containing most beautiful were discovered in the very lowest members of fragments of fossils found in the middle and exposures of the highly contorted rocks. Another the section, there is a possibility that the latter upper part, although few and very indistinct, Just above the Carboniferous lies a prominent gneissic area is found north of Virginia City and may be of Lower Cambrian age. The total thick­ were recognized to be the same as those occurring bed of quartzitic sandstone, which may belong to west of the Madison Valley, abutting against the ness of the section referable to the Cambrian is in the Three Forks shales, which are, without that period or may be the equivalent of the red eruptive granite of the Revenue mining region, about 1,300 feet, as seen in the northern part of much doubt, Devonian. The total thickness sandstones which at more southern localities rep­ just north of the Jefferson Range. South of Vir­ the district; in the southern sections the series is referred to the Devonian is about 870 feet. resent the base of the Juratrias. The latter is ginia City and southwest of Old Baldy Mountain, somewhat thinner. Everywhere the strata are Jefferson formation. The Jefferson limestone probably the more correct view, but in the in the southern part of the Jefferson Range, conformable throughout. forms the base of the formation, consisting of absence of paleontologic evidence no attempt has gneissic rocks prevail, which here, as elsewhere Flathead formation. The Flathead formation several hundred feet of black or mud-colored been made to distinguish the quartzite from the in the district, are gold and silver bearing. The consists of the Flathead quartzite, a persistent limestone, which is generally crystalline and mag­ overlying beds, all -being included on the map foothills of the Gallatin Range south of the quartzite or sandstone which lies at the base of nesian from top to bottom. It is well exposed in under the head of the Ellis formation. This Gallatin Valley are also gneissic, and similar the conformable section, with overlying beds of all parts of the Three Forks region, and at most quartzite is from 40 to 60 feet thick, and is rocks show beneath the Bozeman lake beds at softer, shaly sandstone. Its total thickness is localities is non-fossiliferous, only a few forms remarkably persistent throughout the district. the southern end of the plateau, between the only 125 feet, and as it is usually steeply inclined, having been obtained from near the top of the Ellis formation. Above the basal quartzite Gallatin and Madison valleys. Wherever the its outcrop is narrow and its area at the surface formation, and these only in the region north of this formation consists largely of argillaceous sedimentary beds are found in contact with the is relatively but little more than a line, as colored the East Gallatin River. limestone, many of the beds being crowded with Archean they are unconformable. upon the map. It is non-fossiliferous and has The Three Forks shales, which rest conform­ characteristic Jurassic fossils. These fossiliferous calcareous beds are near the base, the upper and SEDIMENTARY ROCKS. been provisionally correlated with the quartzite ably upon the Jefferson, limestone, may be that in other parts of the Rocky Mountain region divided into three parts: the lower or orange- middle beds being more arenaceous and devoid of ALGONKIAN. has been referred to the lower Cambrian. The colored shales, about 50 feet in thickness; a band fossils. The total thickness of the formation is Cherry Creek beds. The Cherry Creek beds Flathead shales are frequently separated from the of grayish-brown, argillaceous limestone, 20 feet between 300 and 400 feet, and it outcrops in all consist of a series of marbles, or crystalline lime­ quartzite by a band of eruptive rock. They are thick; and the upper green, black, and argilla­ portions of the district with the overlying Creta­ stones, and interlaminated mica-schists, quartz^ arenaceous at the base, gradually becoming more ceous shales, some 50 feet in thickness, which are ceous, never, however, occupying any great area. Its occurrence is distinctly marked by the two Living ston formation. The Livingston forma­ try. The beds of pure white dust in the Gal- In the latter region also, at one of the lowest quartzitic sandstones which lie, one at the base tion occupies at the present time comparatively latin-Madison lake have a total thickness of only horizons noted, old hot springs deposits calcare­ and the other just at the top, both being equally little area within the limits of the Three Forks about 20 feet, while above them the beds of rear­ ous tufa were seen. Many of the cones built up persistent. sheet, and nowhere is it likely that the entire ranged dust attain a thickness of over 1,000 feet. by the old springs, with the tubes through which CRETACEOUS. thickness of the formation is shown. The In the upper beds, especially near the edges of the water flowed, are in perfect condition, The Cretaceous rocks found in the northern largest area is probably that in the vicinity of the basins, the dust deposits contain, mingled with although long after their formation they were and northwestern parts of the district occupy an Sphinx Mountain, where the Sphinx conglom­ the pure pumiceous glass, a small proportion of buried beneath an immense accumulation of the erea of about 75 square miles, occurring in what erate rests unconformably upon it. This area is foreign fragments, derived undoubtedly from the lake deposits. The pumiceous dust occurs in may be described as five separate basins, two of about 15 to 20 square miles in extent, and the adjacent shores by the erosion of the underlying beds 2 to 5 feet thick, separated by thin cal­ which are connected across the top of the range deposits are made up of a mass of volcanic rocks exposed after the removal of the dust careous bands. The total thickness is about 20 that lies south of the Jefferson River. Nixon materials indistinctly bedded, mostly andesitic in deposits from the land surfaces. The erosion feet. They consist of sharp, angular fragments of basin, the most eastern one, is connected to the nature, and of a somber hue. At one or two following the draining of the lakes at length pumiceous glass, with occasional gas cavities, and northward with the basin of Sixteen-mile Creek, places conglomerates made up of all sorts of carried away every vestige of the dusts from the show the peculiar curvilinear outlines character­ which is included in the Fort Logan sheet. The volcanic pebbles are seen near the base. This land, and also an enormous mass of the lake istic of such deposits. The petrographic nature basin next to the westward is simply the pro­ generally black mass rests unconformably upon deposits themselves. An effusion of basalt of the material is rhyolitic. longation of a basin1 lying immediately west of the eroded surfaces of the previously deposited occurred subsequent to the deposition of the In the summit of the Madison Bluffs, in a layer the Missouri River, just north of the limits of the Cretaceous formations, contrasting strongly in lake beds. Near Virginia City, lake beds out­ of gray conglomerate sandstone, numerous frag­ sheet. Next is the basin lying between the north color with the Laramie sandstones and the crop beneath the basalt plateau, and basaltic ments of fossil bones were found, which were Boulder Creek and the Jefferson River. This is Dakota conglomerates, with both of which it is flows are seen on the lake beds west of Salesville, identified as the same as those found at other partially covered by Bozeman lake beds, although in contact at different points. Dikes of porphy- in the vicinity of Red Bluff, and in the upper localities in the Pliohippus beds of Marsh. In erosion in many places has reduced the covering rite intersect these volcanic deposits, extending Madison Valley. In the latter locality rhyolite the sandstones below the fossil horizon immense to a very thin sheet. South of the last-named from the laccolitic mass that occupies the center also se^ms to have flowed over a part of the val­ quantities of opalized wood are found, the logs basin is a smaller one, on the Jefferson River, of the range to the northward. ley after an interval of considerable erosion, and all presenting evidences of having been water- where the Cretaceous is faulted partly against a The Livingston beds, in the northwest corner of is now found in palisades along the course of worn. high ridge of Algonkian rocks and partly against the district, present a somewhat different aspect the river just as it enters the southern boundary The lake that occupied the upper Madison a narrow strip of Devonian lying at the base of from those just described, being not so dark in of the district. Valley, although not all included within the the ridge in the western portion of the basin. color and showing more distinct evidences of Bozeman lake beds. There were two principal limits of the Three Forks sheet, was less exten­ The Cretaceous rocks here are continuous across bedding; they are also more nearly conformable lakes within the limits of the Three Forks sheet. sive than the one just described. Its deposits at the mountain range on the south side of the to the underlying Laramie beds with which they The larger of these was the Gallatin Lake, which present cover about 270 square miles; the out­ Jefferson in a narrow, curving strip connecting are in contact. In the vicinity of Jefferson was undoubtedly connected with another large crops are very obscure but are sufficient to show with those of the small basin on the South Canyon they have a dark greenish-gray color and lake that occupied the Jefferson Valley, lying that the character of the beds is similar to that Boulder and Antelope creeks. This is a region apparently contain interbedded hornblende-ande- mainly within the area mapped on the Dillon of those seen in the larger basin. Beds of very of considerable complicated folding and faulting. sites, which seem to have been laid down as lava sheet, directly west of the Three Forks sheet. It pure pumiceous dust were found near the south­ The section as shown in these basins does not flows. Other beds are conglomerates composed was also connected with the lake of the north ern edge of the map. The deposits of this lake attain the thickness seen elsewhere in the region, of all sorts of volcanic material, andesites, how­ Boulder Valley, which extends northward beyond must be from 1,000 to 1,500 feet in thickness. and in many places so much has been eroded and ever, as at other localities apparently predominat­ the limits of the map, and which was itself con­ They have been cut into a beautiful series of carried away that only the lower members of the ing. The period during which they were nected with the Jefferson Lake. The Gallatin terraces, covered with drift of local origin. This, section are seen. In the Mxon basin the Livings- deposited was undoubtedly one of great volcanic Lake also extended a short distance northward at the upper or southern end of the valley, is ton formation does not show at all, and nearly activity. beyond the limits of the map, along the west very coarse, and interferes with the agricultural all of the Laramie has been worn away, only its side of the Bridger Range, and to the eastward, development of this portion of the valley. EOCENE. lower portion showing in the central part of the in the area of the Livingston sheet, west of At the northern edge of the district, in the main basin. The lines between the different Sphinx conglomerate. In the Sphinx Moun­ Bozeman, the town from which the beds have vicinity of the Horseshoe Bend of the Missouri divisions of the Cretaceous are indistinct, and on tain, which reaches an elevation of 10,840 feet in been named on account of the good exposures in River, is a small area of lake beds, the southern the map the Colorado and Montana formations the central part of the Madison Range, is found its vicinity. end of a basin lying mainly on the east side of have been represented as one, separated from the a group of beds which once may have spread The deposits of this lake occupy at present the Missouri. This basin was apparently distinct Dakota below and from the Laramie above. over an extensive area of country, although at only about 700 square miles of the surface of the from the Three Forks basin, and also from the The latter has been outlined because of its present occupying only an area of about 2 square district, and yet at its greatest expansion the lake area on the Missouri around Townsend, some economic importance. The Livingston formation miles. This remnant is between 2,000 and must have had an area nearly twice as great. miles to the northward. is usually distinctly unconformable to the rest of 3,000 feet in thickness, made up of reddish Although it probably never connected with the PLEISTOCENE. the Cretaceous, especially in the southern half of sandstones and coarse conglomerates of limestone Madison Lake to the southward, except by the the district. pebbles and bowlders cemented with a reddish river, it reached to within about 2 or 3 miles of Glacial drift and moraines. Evidences of Dakota formation.- The lowest beds referred sand. The entire mass of the peak is composed it, as shown by the deposits left as remnants in glaciation can be seen in all the mountainous to the Dakota formation consist of sandstones, of these beds, which are horizontal in position Burnt Creek Valley, south of Norris. The small portions of the district, and well-defined moraines which in some places are quartzitic and conglom- and distinctly stratified, the rugged and steep basin in Spanish Creek Valley was also continu­ occur in the vicinity of Wards Peak, in the Galla­ eritic and in one or two localities consist of true slopes of the mountain rendering evident the ous with it through the lower canyon of the Gal­ tin Canyon in the Madison Range, and south of conglomerates. This conglomeritic quartzite is gigantic scale of the erosion which has left it in latin River. Old Baldy Mountain. The best-defined moraine usually a very conspicuous topographical feature this district the only monument of this group of The beds in the main basin have a dip of a few is found on the west side of the south branch of wherever the formation is seen. Its thickness is beds. So far as observed the strata are non- degrees to the eastward or northeastward, causing Indian Creek, north of the Wedge Peak. It is 4 about 50 feet. Following it, and sometimes fossiliferous, the character of the beds precluding the slope noticeable in the surface of the plateau miles in length and rises from 400 to 500 feet separated from it by an intrusive sheet, are shaly the preservation of fossils. They are, therefore, that lies between the Madison and Gallatin rivers. above the valley of the stream. It is composed beds, generally reddish and greenish-gray in color. somewhat arbitrarily referred to the Eocene, but If continued this dip would carry the beds there of gneissic debris from the mountains at the head Next follows a light-bluish limestone, composed they form a group certainly younger than the visible below those exposed in the hills east of of the stream. Along the west side of the foot­ largely of the remains of a fresh-water gastero- Bozeman lake beds and older than the Livingston Bozeman, and make the total thickness of the hills of the Bridger Range, north of Reese Creek, pod, seemingly of one species, although there are formation. deposits more than 2,000 feet. At no one place, there is a considerable area of drift that might however, can a complete or continuous section be be of glacial origin. sometimes associated with it two or three other NEOCENE. fresh-water forms. obtained. The highest occurrence observed was Alluvium and di*ift. In the areas along the Colorado and Montana formations. These two The movements that began during the latter at the contour of 5,700 or 5,800 feet, and at principal streams very recent fluviatile deposits formations consist of about 2,000 feet of beds part of the Cretaceous period ultimately resulted present the lowest exposure is at an elevation of have been distinguished. The three principal that are generally very shaly, although near the not only in the folding of the strata but also in about 4,000 feet. In the hills east of Bozeman areas are in the Gallatin Valley, the upper Madi­ top there are several conspicuous bands of sand­ the formation of the numerous basins or valleys the dip is from 5° to 8°, which would give there son Valley, and the Three Forks Valley, includ­ stone. The beds in the lower portion consist of found throughout the mountainous portions of a thickness of at least 1,000 feet, and an ing the prolongation up the valleys of the Jeffer­ fine black shales and greenish-gray sands and a Montana. Some of these valleys were filled with artesian boring made at Bozeman is sunk 400 feet son and the Madison. A small area occurs on few beds of limestone. The upper beds are water during the early part of the Tertiary deeper, yet the pure dust beds are not reached in upper Willow Creek, and there is an alluvial generally of a light-gray color. The shales period, while others did not become lakes until this section, and they probably occur about one- area on the Jefferson above the canyon extend­ become gradually more and more argillaceous in the Neocene. Toward the end of that period, third of the distance from the base. ing westward from the mouth of the North and ascending order. however, a series of fresh-water lakes had been In the Madison Valley, 12 to 15 miles above South Boulder creeks, and narrow belts along the Laramie formation. From an economic stand­ formed along the courses of nearly every stream Three Forks, a section 800 to 1,000 feet thick is Jefferson River both in the canyon and below it, point the Laramie formation is the most import­ in the western part of Montana. At first these shown, and if these beds are lower, as they appar­ as well as a very small area in the Gallatin River ant division of the Cretaceous, as it usually con­ lakes were probably more numerous and more ently are, than those near Bozeman, from which above the canyon. The larger areas described tains two or three beds of coal, which, however, distinct from one another than during their later they differ, the total thickness would be nearer are undoubtedly underlain by Bozeman lake are not of equal importance in all localities. The stages. The deposits found in the lower part of 2,500 than 2,000 feet. beds. formation consists essentially of light-gray or the basins show by their character that they were On the southern and western sides of the basin IGNEOUS ROCKS. whitish sandstones, with interlaminated argilla­ derived from the land areas of closely adjacent the lowest beds, whenever exposed, are conglom­ A very considerable part of the area mapped ceous beds, some of which are locally much shores. After their deposition there followed a erates of Archean pebbles. There is little doubt in the Three Forks sheet consists of igneous indurated. The two areas in which the strata period during which immense showers of fine that throughout the greater portion of their rocks. They form some of the highest and most are best exposed are in the Nixon basin, north of volcanic dust, evidently wind-carried, fell upon extent the lake beds rest upon an Archean base. rugged peaks of two of the principal mountain the Gallatin Valley, and in the Gallatin basin the surfaces of the lakes and upon the land. In However, at the northern end, where the shores chains of the region, and occur as well in other lying between the Gallatin and Madison ranges. the former they quietly settled to the bottom as were of limestones, white secondary limestones tracts of less topographic prominence. Although The total thickness of the formation is from 800 pure white sediments in which no traces of are the lowest beds of the lake formation. It is varying widely in geological occurrence and to 1,000 feet. In the Nixon basin unios and foreign material are found. These beds occur in only in the central portions that the beds of pure mineralogical character, they can be readily other fresh-water shells are found in connection the central portions of the large valleys, and upon volcanic dust are found, and the best exposures divided into two groups: intrusive, and extrusive with the coal. A fossil-plant horizon occurs just them, spreading out over wider areas, rest undis­ occur in the vicinity of Foster Creek, southeast of or volcanic. The intrusive rocks occur either as above the upper coal bed in nearly every turbed beds of a rusty color, although composed Hillsdale, although they were seen also in some great bodies of massive material which has exposure, especially in the southern portions of of the same pumiceous dust, which was evidently of the ravines that cut into the plateau lying broken through the Archean and later stratified the district. washed into the lakes from the surrounding coun­ south and southwest of Manhattan or Moreland. formations, or as sheets and dikes penetrating Three Forks 3. various formations. The effusive rocks, or surface ordinary tpye, sufficiently unaltered to be readily in texture from fine-grained tuffs resembling sand­ map is the result of movements which have dis­ volcanics,. consist of tuffs, agglomerates, and recognizable in the field. stones to irregular, chaotic mixtures of large turbed the regular successions of strata, resulting breccias, the fragmental deposits of explosive The rocks embraced under peridotite include and small rock fragments. The greater part of in the present structure of the district, which is volcanic activity, together with various lava flows. those rare rock types known as hornblende- the formation consists of thickly bedded agglom­ delineated in both the areal and the. structure picrites, saxonites, wehrlites, and, in one instance erates in which large and small fragments are section sheet. INTRUSIVE BOOKS. at least, pyroxenite. (They are described in a promiscously mingled. These fragments repre­ The movements of the earth's crust have Granite. In the western part of the district, paper by Dr. G. P. Merrill, in Proceedings sent compact, vesicular, and, rarely, scoriaceous occurred in two directions vertically, both just north of the central line, there is a mass of United States National Museum, Vol. XVII rocks of very closely allied types of andesite. upward and downward, and horizontally. Verti­ granite 16 miles long and 4 or 5 miles in width, (1894), pp. 637-673, 1895). The picrites are The formation is generally hard, weathers into cal movements are recognized in the evidences of into which the northerly ridges, spurs, and foot­ sometimes extremely altered into serpentine, picturesque crags, and affords but scanty foothold uplift and subsidence of various areas, and hori­ hills of the Jefferson Range extend. This rock especially in the region west of Meadow Creek. for vegetation. In color it varies from brown zontal movements in the folding and overthrust- breaks up through crystalline schists, but the age Augite-porpTiyrite, syenite, 'etc. The rocks and black exposures through various shades of ing of the sedimentary beds. of its intrusion is not known. It is a very mas­ included under this head are found mainly in the dull-red, brown, and, rarely, green. In petro­ The structure is complicated by eruptions of sive, coarsely crystalline rock, of uniform com­ area between the Gallatin River and the Horse graphic character the breccia is made up of black igneous rocks, which occur in the forms of lacco- position throughout the entire mass, and is of Shoe Bend of the Missouri, and in the vicinity of or dark-gray, more rarely light-colored, rocks, lites, dikes, and surface flows. economic interest, as the ore deposits of Revenue the South Boulder Creek, one of the southern which are classed as andesites. The ground- The beginnings of disturbances in the earth's and Pony occur in it. branches of the Jefferson River. The most inter­ mass is dense and compact and contains crystals crust antedate the earliest records, and the move­ Porphyrite (andesite-porpliyry). The most com­ esting occurrence is found in connection with the of plagioclase and augite, and more rarely of ments have continued down to very recent times, mon form of intrusive rock occurring in the dip faults noted in the basal quartzite of the Flat- mica and hornblende, which are visible to the or perhaps to the present. region is a porphyrite, the name indicating a por­ head formation north of the East Gallatin River, naked eye. Vertical movements. The thickness of the sedi­ phyry in which the feldspars are plagioclase. about 8 miles east of Logan. . Under the one The andesite flows are associated with the mentary rocks deposited in this region during The different masses shown upon the map under color upon the map are included two rocks, which breccias just described. In the area of this sheet pre-Cambrian, Paleozoic, and Mesozoic time was this name vary somewhat in mode of occurrence, in most places are closely associated; the first is they occur at many localities as intercalated lava many thousand feet.. The materials for the although there is little difference in mineral com­ a granular syenitic rock with an upper, more flows in the breccia, but are discriminated only deposits were worn from land areas, which must position. One body occurs in the extreme south­ basic facies, and the other an augite-porphyrite, in the northern part of the Gallatin Range, where have been .raised either somewhat steadily or at eastern corner of the district, where it is in part best displayed in the deep cuts of Cottonwood the outcrops occur capping some of the hills times separated by quiet intervals, the altitudes included within the limits of the Yellowstone Creek north of Logan, and also near the Horse west of the Gallatin River and where they are favorable to erosion being thus restored. The Park, and others also occur in its vicinity, a little Shoe Bend of the Missouri River. The exact seen also beneath the western edges of the seas which received the sediments must therefore to the westward. Near the Jefferson River, in relationship of this porphyritic type with the breccia-capped summit of Mount Blackmore and have deepened by more or less regular subsidence, the northwestern corner of the district, there is syenitic rock has not as yet been definitely ascer­ neighboring peaks. On the eastern side of or the vast volume of debris would have filled another mass, which is noticeable as an intrusion tained. Of all the rocks within the area of the Boulder Creek are other limited areas of andesite them. occurring in the Cambrian formation and split­ sheet, the syenitic rock and its basic phase are of lava flows, occurring in successive sheets, the Unconformities exist which show that areas ting apart its beds. By far the largest body of the greatest interest in the present state of petro­ oldest and lower rock here being a compact previously raised to land surfaces and worn this rock is the "laccolite" in the central part of graphic science. The syenite is a holocrystalline, hornblende-andesite, overlain in turn by a semi- down have subsided, have been crossed by an the Madison Range, forming there an extremely granular, feldspathic rock with accessory black glassy hypersthene-andesite, and this in turn by advancing shore, and later have passed beneath rugged region of sharp and high peaks, only one mica and greatly elongated, green augites and basalt, followed by a small sheet of rhyolite. the sea. of which, however, Lone Mountain (11,194 feet), sporadic sodalite. With it is always associated Rliyolite. Rhyolite occurs in numerous small The movements of the Algonkian period are is named. The porphyry breaks irregularly a black basic rock consisting of a feldspathic remnants of lava flows which rest upon various obscurely, though not doubtfully recorded. Their through sedimentary formations of Cretaceous base and abundant phenocrysts of olivine and rocks and show that a long period of. erosion sequence was certainly prolonged. At the base age, which are broken, crumpled, and overturned augite. As first studied in the obscure outcrops elapsed after the formation of the volcanic of the Algonkian, between the Cherry Creek for­ about its western borders, the soft shales being of the Flathead shales north of the East Gallatin breccias. The rhyolite is the same rock which mation and the Archean, is a probable uncon­ frequently altered by the heat and vapors of the River, the rocks, as described in Bulletin 110 of covers so large a portion of the Yellowstone formity, the significance of which is not yet intrusion into dense and hard, flinty rocks." Areas the Survey, were so decomposed as to greatly Park, and in the Middle Basin and Lower Basin worked out. The complex structure of the of these Cretaceous rock's shown upon the map as obscure their true lithological nature, but the of the Gallatin River the mesa-like hills are Cherry Creek formation, which is indicated in surrounded by the igneous rock are fragments rock as it occurs on Antelope Creek is much possibly the remnants of a former extension of the southern part of Section D, is the record of a and masses broken from the sedimentary beds fresher, and the conditions for study are there the rhyolite flow from the park. In general the prolonged series of events, its highly disturbed and carried upward with the porphyrite. Many more favorable, although the relationship between rhyolite appears to have filled the old valleys, and metamorphosed condition marking a great of the metamorphosed shaly masses are imbedded the two types is less apparent at the latter local­ but the diversion of the drainage has not been epoch of folding and deep erosion prior to the in the porphyrite, and dikes extend from the ity. . There can, however, be no doubt that the marked, and the present remnants are thus left deposition of the Belt formation. main mass intersecting the adjacent sedimentary syenitic and the black basic overlying rock, as tables by the cutting of narrower valleys At the base of the Cambrian is the uncon­ beds. although entirely distinct from a lithological below the surface of the older and broader valley. formity marked by the overlapping of the Flat- In petrographic characters the rocks composing standpoint, are parts of one and the same geolog­ Similar areas are found along the upper course head formation. The subsidence was extensive the various porphyrites of the region are acidic ical body. The lithological peculiarities of the of the Madison River, one of which forms pali­ and gradual. It was one of a series of gentle (siliceous) rocks, of a normal aridesite-porphyry rocks can not be dwelt upon in detail here, and sades on the river as it enters the district. The vertical oscillations which, continuing during type. The rock presents a remarkable uniform­ reference must be made to the paper in the Pro­ rocks vary from glassy to felsitic forms. They Paleozoic time, became more energetic in the are light-colored, sometimes are banded, and Mesozoic. These conditions are apparent in the ity of character, its most noticeable mineral being ceedingso of the National Museum above noted. the black hornblende crystals scattered abund­ show clear glassy crystals of sanidine and quartz, generally calcareous character of the Cambrian, EFFUSIVE KOCKS. antly through its mass. Near the line of- lower and more rarely glistening flakes of mica. Devonian, and Carboniferous formations and in contact with the shales, however, the rock The effusive rocks of the region not only cover Succeeding the outflows of rhyolitic lavas was their moderate thickness. The coarser mechanical becomes somewhat micaceous and carries free considerable areas, but, by reason of their geolog­ a period of volcanic activity, of which the only character of the Cretaceous sediments, and their quartz. At the immediate contact it is a dense ical relations, afford evidence of the succession of representative deposit found within the limits of great thickness also, indicate'the more rapid sub­ or aphanitic rock, with the appearance of porce­ volcanic events which have transpired since the the district appears to be the tuffs and beds of sidence of the sea bottom and the energetic ero­ lain, and breaking with a marked conchoidal post-Cretaceous uplifting of the region. volcanic glass which now constitute the Neocene sion dependent upon a more vigorous uplift of fracture. The earliest of the volcanic rocks are; those lake beds. the land. From the Flathead to the Laramie the Diorite. Near the extreme northern limit of which now form the Livingston beds. Though a Basalt. The basalt outflows were the last of formations dip conformably, a proof that, what­ the map an area of diorite and diorite-porphyry stratified, water-laid formation, it should be'men­ the lavas of the district, and now form conspicu­ ever the irregularities of vertical movement may is shown. These rocks, differing from each tioned here, as the pebbles are chiefly volcanic ous if but limited areas, resting upon older rocks. have been, they did not involve marked tilting of other only in coarseness of grain, constitute what and constitute the only evidence we have of the The most extensive exposure is that west of the the beds. is probably one body. The diorite is a coarsely beginning of a "period of volcanic activity which Gallatin Valley in the vicinity of Virginia City, Horizontal movements. The great series of granular rock in which black hornblendes and with varying intermissions has probably lasted where there is a basalt plateau occupying an area conformable strata topped by the Laramie forma­ pinkish or gray feldspar crystals are seen with up to the present epoch. We have no evidence of about 50 square miles. The rocks vary in tion is closely folded. The strata have been the unaided eye. The porphyry phase of this to show the location of the vent in which these color from dull-gray to nearly black, and in pushed up in arches, many of which have been same rock, mapped as diorite-porphyry, is a dark- rocks had their origin. structure from coarsely vesicular rocks to dense, overturned. This is the effect of horizontal gray, holocrystalline rock thickly sprinkled with After the uplift and the epoch of erosion which compact forms, which often show yellow crystals thrust, which began to act about the close of the dark hornblende crystals, some of which are over succeeded this early period of volcanic activity of olivine large enough to be recognized by the Cretaceous period. The area mapped on the a half inch in length and which often occur in came a time of prolonged and intense volcanic eye. In many places the basalt plateau near Three Forks sheet does not afford sufficient data stellate clusters of radiating crystals. outbursts. The remains of the material ejected Virginia City is underlain by light-colored tuffs for generalizing with reference to forces which Dike rocks. The intrusive rocks which are at this period now constitute the Gallatin Range, and breccias, and at the southern end of the affected the whole Rocky Mountain province, but found in the form of dikes or sheets are common where the andesite breccias cover an area of 150 plateau these tuffs rest upon Archean gneisses these facts are apparent in the map and the struc­ features in many parts of the region and present square miles and form many of the high peaks so and sedimentary rocks, the latter mainly of Car­ ture-section sheet. unusual and quite interesting rock types. Upon conspicuous immediately to .the southward of the boniferous, age. Small isolated areas of basalt, The geologic map being taken as a standard, the sections the map they are shown by two shades of intense city of Bozeman. In the area of the Three Forks the remnants of former extensive flows, occur at were adjusted to it and the dips of strata were modified. In red, the distinction corresponding to two groups sheet these rocks are from 1,500 to 2,000 feet a number of localities, in several instances rest­ the original diagrams the hades of all faults are drawn in a ing upon the Neocene lake beds. In a few vertical position: In redrawing, these were changed to hades one of newer and the other of older rocks the thick, and are seen to rest upon eroded older in the direction of the upthrow, characteristic of overthrusts. first representing dikes of augite-porphyrite, sedimentary beds. instances varieties of the! basalt are found carry­ The author did not see the proof, and in justice to him this These basic breccias and agglomerates, with ing porphyritical quartz crystals, which show explanation and the following corrections are inserted at his syenite, and allied rocks; the second embracing request. . ''» similar intrusions of diabase and peridotite. The other associated lava flows, constitute a geolog­ zonal bands of green augite similar to those of At the northeastern end of Section C there is no flattening rocks of the first class are seen to occur mainly ical unit, because of their mechanical constitution the well-known California and New Mexico of the dip in the Belt formation next to the valley. The sec­ quartz-basalts. . tion line cuts the beds obliquely and therefore shows a greater as intruded sheets in the folded sedimentary and common origin. They are aerial, fragment­ thickness. Where the section line crosses the fault west of ary, volcanic accumulations, and present in many rocks, while the basic diabase and peridotites STRUCTURAL GEOLOGY.* Salesville the hade should be vertical or nearly so, and the occur as dikes cutting the crystalline schists. localities no evidence of having been laid down ends of the sedimentary beds should not be so much turned up against the Archean. In Section D, where the line crosses The latter are seen in greatest abundance in the by water action. The rocks are for the most General consideration. The complex arrange­ the mountain ridge between the Sphinx Mountain and Indian ranges about the northern end of the Madison part, however, rudely bedded and dip eastward ment of rock masses apparent in the Three Forks Creek, the fault line should not cut the Cherry Creek forma­ tion, but should lie between it and the sedimentaries, and the Valley. at low angles. The bedding is sometimes quite *The structure sections were redrawn in the office of the Flathead formation should not come so close to the surface. Diabase and peridotite. The diabase is of the prominent in large exposures. The rocks vary Geological Survey from diagrams furnished by the author. EDITOR. The simple as well as the overturned synclines of the area of the Dillon sheet. East of the Jef- The structure of the region has been modified anthracite coal in the vicinity of the great por- are marked by the areas of Laramie (Cretaceous) erson River this fault involves Carboniferous and by the late eruptions of volcanic rock, especially phyritic laccolite of the Madison Range. The beds, which at the time of the folding were the Juratrias beds, which are faulted against the Belt by the basaltic lava flows near Virginia City and occurrence of Devonian coal on the north side of latest and highest of the formations. But the beds of the Algonkian. West of the South the rhyolites that are part of the great volcanic the Jefferson canyon, several miles above Sapping- arches between the troughs, having been exposed Boulder, for 5 or 6 miles, the fault is obscured by flow of the Yellowstone Park. These are ton, is of geologic interest, though not of very by their elevation to excessive erosion, have been lake beds, and when it is seen again is somewhat described under "Igneous Rocks." great economic importance. The co#l is of poor worn down to the older and even to the oldest changed in direction, involving Carboniferous, quality, owing to its containing many nodules of ECONOMIC GEOLOGY. rocks, the Archean. Juratrias, and Cretaceous strata, which are now limestone, and the area in which it is likely to The Archean areas shown are the axes of exten­ faulted against the Carboniferous, Devonian, and The mineral resources of the district may be be found is of rather limited extent. sive uplifts, modified by faulting, and largely Cambrian rocks. The next fault is only a few enumerated under the head of gold, silver, iron Brick clays. Brick clays are found in the denuded of their former covering of sedimentary miles to the southeast of the one just described, ore, limestone, copper, coal, brick clays, building alluvium of a number of the streams and in some rocks and are the greater, while the folds, which and begins with the Livingston formation faulted stone, polishing materials, and mineral springs. of the beds of the Bozeman lake beds, although form such conspicuous features of the map, are against the Algonkian. This fault is profound Auriferous gravels. Gold-bearing gravels in the latter are'apt to be somewhat sandy. Bricks the lesser features. and soon develops into a double fault. The the district are found in the famous Alder Gulch are burned at present only in the vicinity of Unlike the Appalachian folds, which are strik­ Livingston formation dips sharply to the north­ above Virginia City, which is said to have yielded Bozeman. ingly parallel and continuous, the central lines or ward against a very narrow strip of Devonian nearly one-half of all the gold produced in Polishing materials. In the Bozeman lake axes of the synclines in the Three Forks region shales and limestones, which themselves dip at Montana since the first discovery in the Territory. beds, at many localities throughout the Gallatin lie in various directions, trending south in the about the same angle in the same direction, being The placers are still worked there, as well as in and Madison valleys, beds of fine pumiceous vol­ southwestern corner, southeast in the body of the faulted against the Algonkian conglomerates that some of the adjacent gulches, although the yield canic dust occur which could readily be utilized area, northeast in the northern part, and east near form the lower foothills of the rugged area north seems to be gradually declining. Other placers as polishing material, as similar deposits have the western margin. This is apparently the of the Jefferson River. This double fault may are worked at Washington Bar, on Meadow been utilized in other localities, particularly in result of several independent centers of uplift. be traced in a curving line to the summit of the Creek, on Norwegian Creek, on Cherry Creek, the States of Kansas and Nebraska. The finer To the north the great anticline of the Belt Range range south of the Jefferson River, which it and in the Jefferson Canyon between the mouth portions might be separated from the coarser by has probably affected the structure in the vicinity crosses at right angles to its trend. From the of the South Boulder Creek and the mouth of a process of washing. of the "Three Forks," while to the south the lac- range westward and southwestward into the area Antelope Creek. Building stone. Although good building stone colite of the Madison Range, and eastward that of included in the Dillon sheet the fault involves In some gravels of probable glacial origin, may be obtained from rocks in various localities, the Snowy Mountains, have, perhaps, influenced only the Cretaceous and Carboniferous strata, the which are more or less consolidated, and form such as the granite of the Revenue region and the great folds of the region. The anticlines are former being faulted against the latter. what has been called the Gravel Range, lying on the sandstones of the Flathead formation, in limited in longitudinal extent, rising usually from In the area north of the East Gallatin River, the west side of the divide between Warm nearly every place where the formation is exposed a low arch, often to an overturned fold, and sink­ west of Dry Creek, several interesting transverse Spring Creek and the Madison Valley, southeast little quarrying has been done, mainly on account ing again on the opposite pitch within short dis­ faults are seen, where the Flathead quartzite is of Virginia City, gold is said to exist in consider­ of the lack of a sufficient demand. The Flathead tances. In general the steeper dips are on the apparently thrown down in blocks, and wherever able quantity, although little has been done to formation west of Salesville has been quarried western, northwestern, and northern limbs of the these blocks occur an intrusive sheet of igneous develop any mining operations. and yields a reddish sandstone that has been synclines. In the vicinity of "Three Forks" the rock is found above the quartzite. These faults Gold veins. The principal gold-bearing quartz utilized to a limited extent in Bozeman. There strata are markedly overturned southeastward; are at right angles to the strike of the beds, districts are found in the vicinity of Virginia are many localities, among which one in the and similar effects are seen in the Bridger, Madi­ showing a horizontal displacement of 300 feet City, around Wards Peak, in the Revenue Dis­ vicinity of Antelope Creek, near the Jefferson, son, and southern Jefferson ranges. and a throw of about 100 feet. They seem in trict, near Red Bluff, and in the mountains may be mentioned, where a quartzite or quartz- There principal faults cross the Gallatin Range, most cases to involve only the Flathead quartz­ adjacent to Pony. In all of these, quartz mines itic sandstone occurs in this formation that can and two of them extend across the Madison ite, although one of them was traced through the are worked, gold being the principal product. scarcely be distinguished in color or texture from Range to the western part of the area of this Cambrian as far as the upper part of the Carbon­ ^Silver. Silver is found in connection with the Potsdam sandstone (quartzite) of New York sheet. Their course is about 50° to 55° west of iferous. gold at most of the localities enumerated above, State, and which will probably furnish an equally north. The hade or dip of the fault planes is not The folding of the Cretaceous and pre-Creta- especially in the Red Bluff and Pony districts. good building stone. The Laramie formation also determined, but is drawn on the structure sec­ ceous strata was accompanied or succeeded by Free silver occurs in the vicinity of Cherry Creek, furnishes at many localities a light-gray sand­ tions as though the Archean had been overthrust an uplifting of the region, which provided con­ 8 or 10 miles northeast of Red Bluff, but at stone which resembles the Berea sandstone of southwestward, which is probably not the case. ditions favorable to extensive erosion. The result­ present little or no mining is done there. Ohio. It has been utilized to some extent in The northernmost of these faults springs from ing detritus was deposited unconformably upon Iron ore. Iron ore, mostly hematite, so far as Bozeman, where blocks of large dimensions have the northern limb of a syncline, 10 miles south of the Laramie and earlier strata, constituting part seen, is found at numerous localities throughout been used in a number of buildings. It works Bozeman, and extends parallel to the axis of the of the Livingston formation, which also contains the district, especially in the foothills and moun­ readily when first quarried, but hardens upon fold, crossing the Grallatin Valley where it is large amounts of volcanic material erupted dur­ tains south of the Gallatin Valley, and in the exposure. As the State becomes more thickly covered by the lake beds, and reappearing for a ing this epoch. In the area about Sphinx Moun­ vicinity of Three Forks, although it is nowhere settled there will be greater demand for building few miles west of the valley. In consequence of tain the Livingston beds are twisted and bent, and mined at present. stone, and the demand will undoubtly be met this break the Archean is brought into contact along Jefferson River, as already noted, they are Limestone. The supply of limestone for the from an abundance of material, of which not a with Cambrian, Devonian, and Carboniferous faulted. Compressive movements were, therefore, burning of lime is abundant in all sections of the small portion will be found within the area of strata. The hade of this fault is vertical, or renewed after the deposition of the Livingston. district. Limekilns have been built in the can­ the Three Forks sheet. nearly so. A more extensive fault occurs 6 miles Development of the lake basins. The Bozeman yons south of Bozeman and at many other locali­ Mineral springs. The mineral springs of the farther south, and is parallel to the first. It also lake beds occupy basins which extend across ties near that city, and they furnish a fair quan­ district may all be properly placed under the traverses the northern limb of a syncline parallel other structural features of the area in a manner tity of lime for building purposes. head of warm springs, and include those at to the axis, and brings the Archean in contact so irregular that the development of the basins Copper. Copper is found in connection with the following localities: Ferris Hot Springs on with Paleozoic strata up to the Carboniferous. appears to belong not only to a later but to a silver throughout the Archean area extending the west Gallatin River, which at present is the Its southeastern end is buried beneath the great different movement. Westward from Bozeman from Sterling to Pony and between Cherry and only one that is developed and utilized as a mass of andesitic breccias of the Grallatin Range, these Neocene beds rest upon the Archean, and Elk creeks, east of the Madison River. At resort; the springs east of Red Bluff, which have and northwestward it passes beneath the Boze­ north of the Gallatin and Jefferson rivers they present its economic importance in the district is a temperature of 112° F.; and the springs 5 miles man lake beds west of Madison River. The third extend in undisturbed attitude across the various not very great. south of Pony on the south .branch of Willow fault of the system lies south of Grallatin Peak; upturned formations from Algonkian to Creta­ Goal. Wherever the Laramie formation is Creek, which cover an area of several acres and crossing the Gallatin River at the head of Gral­ ceous. The basin of the Madison Lake covers a exposed, from one to three beds of coal may be have temperatures of 80°, 90°, 100°, 116°, and latin Canyon, it extends almost parallel to the broad anticline in the older rocks. Thus the looked for. It seems to be best developed and 122° F. There is a small warm spring in the strike of Devonian and Carboniferous beds, but unconformity between the Neocene and the occupies larger areas in Nixon Basin and in the lower canyon of the, Jefferson above Willow soon cuts across their ends, and still farther west earlier formations is very strongly marked. Gallatin basins above the lower canyon of the Creek. The temperature of the water is about crosses a broad syncline of Cretaceous strata, The Bozeman lake beds are now the floors of Gallatin River. At no place in either are any 90° F. after which it cuts across the northern end of extensive valleys separating the detached moun­ extensive mining operations carried on at present, The Ferris Hot Springs were formerly known the overturned sedimentaries of the west side of tain ranges, which rise about 6,000 feet above and careful detailed exploration will be necessary as the Mathews Warm Springs. They are situated Madison Range. Northwest of Madison River their bases. As the lake deposits are about 2,000 before the value of the individual beds can be about 7 miles west of Bozeman, and have been this fault is more obscure, but it is probably feet deep, the difference of elevation between the determined. Coal is also found in the vicinity of improved by the erection of a hotel and bath indicated by the line between the Archean bottoms of the lake basins and the summits of the the Jefferson Canyon, in the valleys of Ruby and houses. There are two principal springs, with gneisses and the granite of the Revenue region. peaks is probably 8,000 feet. The relations of Warm Spring creeks, in the southwest corner of temperatures of 114° and 122° F. The water may It constitutes a well-defined feature of the topo­ the relatively undisturbed lacustrine strata to the district; also in the basin of Jackass Creek, be classified as sulphated alkaline-saline; that is, graphic map. the ranges show that the region was a mountain­ east of the Madison Valley, and east of Flathead it contains both alkaline carbonates and sulphates In the vicinity of the Jefferson River, in the ous one before the development of the lakes. It Pass, in the extreme northeastern corner of the as the predominating constituents. It resembles northwest corner of the area of this sheet, are two can not now be stated how high the mountains map. However, the basin of the upper Gallatin is somewhat the water of the Carlsbad Sprudel other interesting faults, although neither is of were before the lakes were formed, nor what the largest coal field of the district, as it includes Springs, although the proportion of solid con­ very great extent within the limits of the district. part of the existing difference of 8,000 feet is due a very considerable portion of the area lying tents is less and the temperature is not quite so The first, or most northern, with a general direc­ to the subsidence of the valleys and what part to between the Madison Mountains and the Gallatin high. tion of about south 60° or 70° west, crosses the the relative uplift of the mountains. In the River, south of the mountains in which Gallatin Jefferson River about a mile or two below the evolution of the existing relief, movements and Peak is the center. There is a possibility that A. C. PEALE, mouth of South Boulder Creek and continues erosion have both operated to accent the topo­ careful and detailed exploration may sometime Geologist. southwestward into at least the eastern portion graphic differences. result in the finding of anthracite or semi- April, 1896. Three Forks 5.