<<

, ->i: . ^ m^;^m^ z*?&iti

DESCRIPTION OF THE SUNDANCE QUADRANGLE

By N. H. Barton.

GEOGRAPHY. streams rise in the high plateau, cross represented by a westward-sloping plateau that . the region of crystalline rocks, and flow through descends gradually to the plains. These plains POSITION AND EXTENT OF THE QUADRANGLE. DESCRIPTION OF THE BOCKS. canyons in the flanking regions of the eastern side are underlain by the soft shales of the upper Cre­ The Sundance quadrangle embraces the quarter to Cheyenne River. All around the taceous, which outcrop in low, rolling and In the Sundance quadrangle there are three of a square degree which lies between parallels 44° the limestone plateau slopes outward, but near its wide valleys. classes of rocks metamorphic, sedimentary, and §.:::«^ and 44° 30' north latitude and meridians 104° and base there is a low of Minnekahta limestone . The greater part of the igneous igneous. The first are exposed in small areas at i;l:-|||g-: 104° 30' west longitude. It measures approxi­ with a steep infacing from 40 to 50 feet masses of the quadrangle are marked by promi­ the base of the sedimentary series in the Nigger mately 34 miles from north to south and 25 miles high, surmounted by a bare, rocky incline which nent elevations, the most extensive of which is the Hill uplift and in the Bear Lodge uplift. The from east to west, and its area is 856| square miles. descends several hundred feet into the Red Val­ . This range rises in steep, sedimentary rocks are a of , It comprises the southeast corner of Crook County ley. This minor escarpment and slope is sharply rocky slopes crossed by numerous canyons, and , and shales, ranging in age from Middle and'-the northeast corner of Weston County, Wyo., notched at intervals by canyons, which on each has a central range of high peaks culminating in to , having a thickness of and includes also a narrow area of Pennington and stream form a characteristic narrows or "gate." Warren Peaks, the altitude of which is somewhat about 4500 feet, and presenting the general char­ Lawrence counties, S. Dak. The quadrangle lies The Red Valley. The Red Valley is a wide over 6700 feet. Sundance , another acteristics, thicknesses, and relations given on the mainly in the Black Hills, but a small portion of depression that extends continuously around the igneous mass, just south of the town of Sun- columnar section sheet. The igneous rocks com­ its southwest corner extends out into the Great hills, with long, high limestone slopes Oil the inner dance, is a steep-sided which rises to an prise several varieties which have been intruded Plains. The district is drained by tributaries of side and the steep hogback ridge on the outer. It altitude of over 5800 feet or more than 1000 among the sedimentary rocks at different hori­ Belle Fourche and Cheyenne rivers, the divide is in some places 2 miles wide, though it is much feet above the surrounding valleys. Its steep, zons and are of early Tertiary age. passing diagonally across the southern half of narrower where the strata dip steeply, and is one of rocky sides with long slopes of talus are a METAMOKPHIC EOCKS. the quadrangle. Many features of the Black the most conspicuous features of the region, owing characteristic feature (see fig. 7 on illustration Hills are exhibited in this quadrangle, and, as its in no small degree to the red color of its soil and sheet). Inyankara Mountain is a somewhat simi­ Algonkian System. area is small, a general account of that region will the absence of trees, the main Jbrests of the Black lar feature rising to an altitude of 6313 feet, and In the Nigger Hill uplift there is exposed a be given before the detailed description of the Hills ending at the margin of the limestone slopes. so isolated from high lands that it is a very promi­ small area of pre-Cambrian metamorphic , quadrangle is presented. The larger streams flowing out of the hills gener­ nent landmark. Black , which are com­ similar in most respects to those which outcrop in ally cross it without material deflection, and their posed mainly of igneous rocks, have less marked the central area of the Black Hills. A portion of THE BLACK HILLS. valleys lie between divides which are usually so topographic features, consisting of a series of rocky them appear to be in place, upturned in the center General features. In western low as to give the Red Valley the appearance of knobs, mostly steep sided, that rise considerably of the uplift, but other portions are included in the and eastern a small group of mountains being continuous, but in its middle eastern section above the level of the plateau on the east and younger igneous rocks which apparently completely known as the Black Hills rises several thousand it is extensively choked with deposits. appear very prominent when viewed from the west surround them. These schists are penetrated by feet above the plains. The abundant rainfall and The hogback rim. The hogback range consti­ and north. The Nigger Hill uplift is marked by intrusive rocks of various kinds, including , consequent vegetation and streams make the local­ tuting the outer rim of the hills is usually a single*- a series of irregular high ridges with summits sev­ of Algonkian age, and other rocks. ity an oasis in the semiarid region. \The hills are crested ridge of hard , varying in promi­ eral hundred feet higher than the adjoining pla­ In the Bear Lodge uplift large masses of probable carved from a dome-shaped uplift of the 's nence and in steepness of slope. At the north and teaus. Cement Ridge, a part of its southeast rim Algonkian granites are included in the . , and consist largely of rocks which are older south and locally along the middle western section and its highest portion, rises prominently to an All these igneous rocks are described on a later page. than those forming the surface of the it spreads out into long, sloping plateaus. It altitude of 6600 feet. Lytle Hill, on the north, is -. and which contain valuable . The length nearly always presents a steep face toward the Red another prominent feature of this uplift, its summit of the more elevated area is about 100 miles, and Valley, above which its crest line rises several hun­ reaching 5800 feet above sea level. The Needles Occurrence. The schists of the Nigger Hill its greatest width is 50 miles. The hills rise dred feet, but on the outer side it slopes more or are sharp peaks of igneous occupying a small uplift occur in several irregular areas in the abruptly from the plains, although the flanking less steeply down to the plains that extend far out area east of Lytle Hill. central part of the region. The largest mass ridges are of moderate elevation. The salient from the Black Hills in every direction. The hog­ Valleys and creeks. The Sundance quadrangle extends in a rudely semicircular course from features are an encircling hogback ridge, consti­ back rim is crossed by numerous valleys or can­ is traversed by deep valleys of various kinds, near bench mark 6238 westward around the head tuting the outer rim of the hills; next a contin­ yons, which divide it into level-topped ridges of including numerous canyons. In the Red Valley of Spottedtail Gulch and along the slopes west uous depression, the Red Valley, which extends various lengths. At the southern point of the and in the area west of it the valleys are wide and of that depression to a point north of Welcome. completely around the uplift; then a limestone hills Cheyenne River has cut a tortuous valley shallow. Skull Creek and Oil Creek flow in can­ Other areas occur on the slopes and spurs about plateau with infacing escarpment; and, finally, a through the ridge for several miles, and the Belle yons with walls of considerable height, and the Bear Gulch, especially west and north of that central area of high ridges culminating in the pre­ Fourche does the same at the northern end of the adjoining ridges are cut by deep canyons or place. The masses vary in size from a few feet cipitous crags of Harney Peak, at an altitude of uplift. branches of these streams. Stockade Beaver Creek to half a mile in width and most of them are 7216 feet. Two branches of Cheyenne River flows in a canyon about 500 feet deep and, like inclosed in the younger igneous rocks. At the GEOGRAPHIC FEATURES OF-THE QUADRANGLE. nearly surround the hills and receive many tribu­ Oil Creek, Skull Creek, Turner Creek, and west end of Cement Ridge the schist is in place taries from them. Limestone slopes. This quadrangle presents Creek, runs southward into Beaver Creek, a trib­ under the basal Cambrian sediments, and possibly The ceniral area. The central area of the Black many characteristic features of the Black Hills, utary of Cheyenne River. Soldier Creek flows at some other points the rock is connected with Hills comprises an elevated basin, eroded in crys­ but does not extend into the central region of in a high-walled valley of considerable width and the main mass of schist, which is believed to be the talline schists and , in which scattered granites and schists. Its southeast corner includes empties into Inyankara Creek near Inyankara post- floor under the younger igneous intrusions. rocky ridges and groups of mountains are inter­ a portion of the limestone plateau which, in a office. Beaver Creek and Mason Creek also flow It is probable that the largest body of the schist spersed with park-like valleys. The wider valleys modified form, extends northwestward to the into Inyankara Creek, which joins the Belle was elevated en -masse at the time of the are above the heads of canyons of greater or less region south of the Nigger Hill uplift. In this Fourche northwest of the Sundance quadrangle. intrusion, and does not form an inclusion in the size, which become deeper and steeper sided as extension, however, it is cut by numerous deep Hewston and Miller creeks are part of the same . Its elevated position, the fact that it they extend outward to the northeast, east, and canyons and branch draws, which produce an drainage system. Cold Springs Creek, as well as partly encircles Hill, that it was not noted south. _ irregular, rolling surface in a wide area underlain the drainage eastward for several miles, flows into along Spottedtail Gulch, and that it is not exposed The limestone plateau. The limestone plateau by the Minnelusa formation. The slope of Min­ Creek, which also receives the waters from along Sand Creek (except as shown on the geo­ forms an interior highland belt around the central nekahta limestone is of wide extent in this quad­ Red Canyon and eventually joins the Red water, logic map) would seem to indicate that this is hills, rising considerably above the greater part rangle, but it is deeply incised by valleys and a branch of the Belle Fourche. Sundance and the case and that if it is the floor of the , of the area of crystalline rocks. Its- western por­ canyons. To the east it is terminated by the Rocky Ford creeks are branches of the same drain­ the latter is extremely uneven. Of the other schist tion is much more extensive than its eastern and is canyons of Stockade Beaver Creek and Cold age systems. These two creeks rise on the slopes areas some, perhaps all, are included in the por­ broad and flat, sloping gently downward near its Springs Creek, toward which it presents a high of Bear .Lodge Mountains, pass out of the moun­ phyry. The larger areas along the bottom of Bear outer margin, but being level near its eastern inner escarpment. Much of its surface slopes gently tains in deep canyons, cross the Red Valley in Gulch, however, may represent the irregular base side, which presents a line of cliffs many miles to the west and presents broad expanses, which broad depressions, and cut deep canyonsHhrough on which the porphyry rests. long and often 800 feet above the central valleys. appear very smooth. the limestone and sandstone in the region farther Description. The schists are moderately dark- It attains altitudes slightly more than 7000 feet, Red Valley. The Red Valley is a strongly east. The lower part of the Cold Springs Creek gray rocks, quite fissile when weathered, and con­ locally almost equaling Harney Peak in height, marked feature which extends north and south drainage and the many branch streams which enter sist essentially of and , or of quartz, and forms the main divide of the Black Hills. through the center of the quadrangle. To the the creek from the east flow in steep-walled can­ , and biotite, with a small proportion of The streams which flow down its western slope are south it is interrupted by numerous high ridges, yons, which in Grand Canyon have a depth of , s The biotite, which constitutes between affluents of Beaver Creek to the southwest and of and it is almost completely blocked by Strawberry 600 feet. Sand Creek, into which Cold Springs a third and a half of the bulk of the rock, occurs the Belle Fourche to the northwest. They rise in Mountain, but north of Inyankara Creek it is Creek empties, and its branches also flow in deep in abundant small leaves arranged parallel to the shallow, park-like valleys in the plateau and sink broad, with gentle slopes and low buttes. Near canyons. East of the Nigger Hill uplift and along plane of schistosity, the quartz and occur­ into deep canyons with precipitous walls of lime­ Black Flat and on Sundance Creek it attains, a the east-central margin of the quadrangle are the ring in grains, some equidimensional, but most of stone, often many hundred feet high. The lime­ width of 6 miles. A narrow branch of the Red headwaters of Little Spearfish Creek, a branch of them somewhat flattened. A few small, colorless stone plateau extending southward swings around Valley extends past Sundance far to the northwest. Spearfish Creek, which also belongs to the Red- occur in some portions of the schist. to the eastern side of the hills, where, owing to the Hogback ranee. To the west lies a region of water and Belle Fourche drainage. The streams While it can not be positively stated, on the greater dip of the strata, it narrows to a ridge hav­ rolling hills interspersed with mesas, which are on the north side of the Nigger Hill uplift flow evidence of their mineralogic composition and ing a steep western face. This ridge is intersected by capped by. the Lakota sandstone, usually giving in deep canyons having high, rocky walls and general lithologic character alone, that these mica­ the water gaps of all the larger streams in the south­ rise to steep cliffs. These mesas are outlying por­ narrow, even-crested, separating ridges that slope ceous schists as a whole are metamorphosed pre- eastern and eastern portions of the hills. These tions of the hogback range, which in this area is gently northward. Cambrian sediments, it seems probable that this is the case, rather than that they have been derived rock. In some of these cases the quartzite appears been distinguished, although it may be represented tains. Along Stockade Beaver Creek, where there from ancient igneous rocks. to be at the top of the formation, and at one local­ by 40 or 50 feet of dark-gray sandy limestones in are moderately steep and continuous dips to the SEDIMENTARY ROCKS. ity some of the top green shale appears. In Budy the lower portion of the Pahasapa limestone. west, the formation outcrops in a narrow zone Canyon, on the west side of the uplift, nearly 300 Fossils. In the region farther southeast the for­ Cambrian System. deeply incised by canyons. Farther north the feet of Deadwood beds appear, owing to the igneous mation carries a few fossils of age, dips flatten, and in a low, broad anticline which DEADWOOD FOKMATION. rock taking a lower plane of intrusion for some dis­ believed to represent the horizon of the Chouteau extends from the Nigger Hill uplift through Bald General relations. The Deadwood formation tance. The top member, under the Whitewood limestone. The following forms have been reported: Mountain the outcrop widens to 17 miles in a outcrops in the Nigger Hill, Bear Lodge, and limestone, is 30 feet or more of green shale, as in Leptama rhomboidalis, Chonetes ornatus ?, Pro­ rough, wooded region of ridges and canyons. Sol­ Black Buttes laccolithic uplifts, and doubtless Sheep Mountain; then there are 10 to 40 feet of ducing aff. P. mesialis, Spirifer striatiformis, dier Creek and its branches have cut through the underlies most other portions of the quadrangle. purplish to gray quartzite, 40 feet of thin-bedded Spirifer mysticensis, Spirifer peculiarisf, Camaro- thin of the Minnekahta and Opeche for­ Its rocks consist largely of sandstones, shales, lime­ buff to purplish-gray sandstones and limestones toRchia metallicaf, Pugnax n. sp., Pterinopecten sp., mations into the Minnelusa and afford extensive stones, and limestone . , The beds are not with and a small amount of flat-pebble Platyceras sp. - exposures of all these, and the Minnelusa beds are often exposed in regular order and in complete limestone conglomerate, 200 feet of thin-bedded bared by numerous streams near the Black Buttes PAHASAPA LIMESTONE. sequence from base to summit, owing to the very sandstone and sandy shale of buff and greenish- uplift. Northwest and north of the Nigger Hill irregular manner in which they have been intruded buff color with some reddish layers, and a 10- to General relations. The margin of the extensive uplift there is a broad outcrop belt of the Minne­ by the igneous rocks. Cement Bidge, southwest 30-foot of quartzite lying on igneous rocks. limestone plateau of the northwestern Black Hills lusa, crossed by numerous deep canyons, along of Welcome, presents a complete section ranging About 200 feet farther east a thin mass of quartzite extends into the eastern edge of the Sundance which the formation is bared for many miles, from Algonkian rocks to Whitewood limestone. is included in the porphyry. quadrangle, but within a short distance to the west the intervening ridges being capped by Minne­ Elsewhere the is intruded at various Slack Buttes area. The outcrop in the Black the dip increases rapidly and the Pahasapa lime­ kahta limestone. Bed Canyon and Sundance horizons so as to split the strata into wedge-shaped Buttes is near the north end of the igneous area, stone is soon carried beneath the Minnelusa forma­ Creek and their branches also cut into it deeply. masses from 25 to 200 feet in thickness, and some where the porphyry has been intruded at a lower tion. It is brought up again in nearly complete Strawberry and Green mountains expose a low of these masses have been carried for considerable horizon for a short distance, uplifting upper Dead- circular outcrops about the Nigger Hill and Bear dome of the upper beds of the formation. Inyan­ distances from their original positions. wood beds. They consist of 40 to 50 feet of sand­ Lodge uplifts and in Bald Mountain. Biflepit, kara Mountain brings up the Minnelusa along Thickness and character. The formation is stones and sandy shales overlain by the regular Battlesnake, and Sand Creek canyons expose the its northern side, and the Bear Lodge uplift pre­ about 300 feet thick and consists of a great mass succession of and lime­ top of the formation for some distance. Several sents it in an elliptical outcrop zone which extends of coarse sandstones with local conglomeratic beds stones. detached areas are exposed in Black Buttes, and on entirely around Bear Lodge Mountains, in places at the base, sandstones and shales above, and local Fossils and age. Fossils occur in the Deadwood the northwest flank of Inyankara Mountain there split by igneous intrusions. areas of limestone and limestone breccia near the formation at most localities. The principal hori­ is a small uplifted mass of it. Thickness and local features. The thickness of top, the latter capped or replaced by a thin layer of zons are the lower sandstone and the upper shales, Thickness. The complete thickness of the for­ the formation appears to vary considerably, but in sandstone, which immediately underlies the White- and the forms are regarded as Acadian (Middle mation is exposed on the flanks of the Nigger Hill general it seems to become thinner to the north­ wood limestone. The sandstones consist of quartz Cambrian). The following species, in part from and Bear Lodge uplifts and in the northwest por­ west. In the deep well at Cambria, not far from sand varying from brownish to pale reddish in color, shales on the east slope of the Nigger Hill uplift, tion of the Black Buttes, and the amount is seen the southern margin of the quadrangle, a thickness and are usually only moderately thick-bedded. The have been determined by Mr. Charles D. Walcott: to average about 600 feet. At the east end of of 850 feet was found, consisting of fine-grained basal beds nearly everywhere are conglomerates, the Dicellomus politus Hall; D. pectinoides Whitfield; Cement Bidge the igneous rock cuts out the forma­ gray, buff, and brown sandstones, with a large pro­ pebbles of which, in some districts, are one-half Ptychoparia oweni Hall; Obolus (Lingulella] sim- tion for some distance. portion of calcareous cement. In Stockade Beaver inch in diameter. ilis Walcott; and Obolus (Lingulepis] acuminatus Character. The Pahasapa limestone is a light- Valley in the southeast corner of the quadrangle Shales occur at intervals in the formation. They Conrad. colored rock consisting of nearly pure of the thickness is about 500 feet, comprising 300 are generally dark colored, greenish or dark gray, Ordovician System. lime. It is massively bedded, weathers to a light feet of lower beds of buff and gray sandstones, 30 with a purplish cast, and are fine grained and com­ dove color, is fine grained, and presents no not­ to 40 feet of red sandstones, 50 feet of hard white WHITEWOOD LIMESTONE. pact. Some beds of shale on Little Spearfish Creek able variations in character in different parts. It sandstones, 80 feet of red brecciated sandstones, are bright red. The mineral glauconite, in its char­ Character and outcrops. This formation is a appears to merge into the underlying Englewood and the top bed of hard white sandstone, which is acteristic small bottle-green grains, occurs exten­ hard, buff-colored limestone which outcrops in limestone, but the transition takes place in a few thin for a short distance but gradually thickens to sively in the sandstones and on the pebbles of the Bear Lodge, Nigger Hill, and Black Buttes inches. In its upper portion there is some uncer­ the north. This top sandstone is over 100 feet the limestone conglomerates. This conglomerate is uplifts. Its thickness averages 70 feet, and, as it tainty as to whether there should be included in thick on the slopes north of the Nigger Hill uplift, seen in the valley of Little Spearfish Creek, in is thin and lies beneath a thick mass of limestone, the formation some light-colored limestones from where it is a conspicuous feature and caps the walls Budy Can}ron, northwest of Sundance, and also it is usually hidden by talus from overlying beds. 50 to 60 feet thick, which are separated from the of many deep canyons. Ordinarily it is coarse near the saddle southwest of the Needles. It con­ It extends partly around both of the larger uplifts, main mass by 40 feet of sandy beds. grained and cross-bedded. In Grand Canyon the sists of flat pebbles or twisted and broken thin lay­ but in places the plane of rises Physiographic features. This limestone is a formation consists mainly of three members at the ers of gray to pinkish limestones in a limestone and cuts it out, this being the case at the east end resistant rock, rising in high ridges or giving rise top white sandstone, 125 feet thick; next 50 feet, , a characteristic occurrence in the Cambrian of Cement Bidge, near the Needles, and at the to cliffs and benches; consequently it usually is more or less, of soft sandstones, red in the upper in various portions of the West. heads of Lytle and Beaver creeks. The small area well exposed for * observation in all parts of the part; and a basal series of thinner-bedded, buff Nigger Hill uplift. The Deadwood formation in Black Buttes is cut off at both ends by the por­ region, except on some of the slopes where there sandstones with a few limy layers. On the west appears in a nearly circular outcrop around the phyry. The rock, which is of uniform character is considerable soil covering. It outcrops in many side of Inyankara Mountain the formation appears slopes of the Nigger Hill uplift. The principal throughout, is a hard, dirty, < buff-colored, and bare ledges in the canyons in the Bear Lodge to have a thickness of only 350 feet, consisting plane of laccolithic intrusion is at or near the base mottled limestone, apparently of a fair degree of Mountains and the Nigger Hill uplift, and it mainly of gray and buff sandstone. On the east of the formation, except for a short distance north purity. It appears at intervals from Lytle Hill appears extensively in the canyons of Little slopes of the Bear Lodge Mountains it is about of the Needles and at the east end of Cement Bidge, west and south to Cement Bidge, one conspicuous Spearfish Creek and the branches of Stockade 350 feet thick and on the west side it is about 300 where the igneous rocks rise to a somewhat higher exposure appearing on the east side of Sand Creek. Beaver Creek. In these canyons the rock pre­ feet thick, the upper white sandstone being its most horizon. In portions of the area the formation is In Budy Canyon it outcrops prominently in mot­ sents steep walls of light-gray, massive limestone. characteristic feature. split by the igneous intrusions, but in the central tled gray and pinkish ledges, but is cut out by Bald Mountain is an isolated outcrop of the lime­ /Section of Minnelusa sandstone near the lower end of and western portions of Cement Bidge all the beds the igneous rock a short distance north of the stone, due to a sharp upward flexure of the beds Creek Canyon. appear in regular order. Much of this ridge is canyon. The Whitewood limestone caps the south and the of the overlying Minnelusa and Feet. due to the basal conglomerates, which are known end of Sheep Mountain, where it is brought up by younger formations. Opeche red sandstone. Buff sandstone, in beds of medium thick­ as "cement" by the miners. This rock consists a fault, and rises in cliffs 30 feet high above the Fossils. Fossils occur in most, parts of the ness, with abundant cherty concretions. 10 mainly of quartz pebbles, but at some points near slopes of green shales at the top of the Deadwood Pahasapa limestone, but seldom in large num­ White to buff, massive, coarse-grained sand­ the basal contacts includes also pebbles and bowl­ formation. Here it is a dark, mottled, pink lime­ stone in vertical walls ...... 130 bers. The following are the principal forms which Yellowish to reddish limestone, moderately ders of dark-colored igneous rocks of Algonkian stone about 60 feet thick, containing considerable have been obtained: Syringopora surcularia, Lep- thin bedded...... 9 age. Most of the overlying beds are sandy shales sand. tcena rhomboidalis, Schuchertella incequalis, Chonetes White to reddish, coarse-grained, massive sandstone...... 80 and slabby brown sandstones, and masses of thin Fossils. The Whitewood limestone contains gregariusf, Productus semireticulatus, Spirifer cen- Gray to purplish and reddish impure lime­ rocks, as well as of conglomerate, are included large Maclureas and a few other fossils of Ordo­ tronatus, Spirifer striatus var. madisonensis, and stone, moderately massive ...... 12 White to red, coarse-grained, cross-bedded between some of the igneous bodies in the north­ vician age. Syringothyris carteri. These are of Mississippian sandstone ...... 8 western part of the uplift. The formation is exten­ Carboniferous System. age and represent approximately the fauna of the Gray limestone, fine-grained, moderately sively exposed in the slopes adjoining Beaver Creek Madison limestone of and the Little Horn thin bedded ...... 25 ENGLEWOOD LIMESTONE. White to buff, coarse-grained, cross-bedded, north of Bear Gulch, and in the slopes south and limestone of the . massive sandstone ...... 20 southwest of Lytle Hill. Character and outcrops. The basal member of Grayish, fine-grained, hard sandstone, thin- MINNELUSA SANDSTONE. bedded ...... 10 Sheep Mountain area. The Deadwood forma­ the great series of Carboniferous limestones in the White, massive sandstone...... 22 tion appears prominently on the south slope of Black Hills has been separated as a formation Relations and distribution. This formation con­ Gray, fine-grained, impure limestone, irregu­ Sheep Mountain, brought up above the Minne- known as the Englewood limestone. In its typi­ sists mainly of light-colored sandstones, but some larly bedded ...... 3 White, massive sandstone...... 20 kahta limestone by a local but profound fault. cal development it consists of rocks of pinkish color limestone beds are included and most of the sand­ Pale-gray, fine-grained limestone, thick to About 300 feet of beds are exposed, consisting of and moderately thin bedding, characters which dis­ stones have a calcareous cement in their unweath- thin bedded...... 20 brown sandstones at the base, overlain by alterna­ Massive, white sandstone...... 5 tinguish it from the overlying massive gray lime- ered condition. The upper member is characteristic, Pale-gray, very fine-grained limestone, heav­ tions of thin-bedded sandstones and shale of dirty- Ston.6 of the Pahasapa. Its thickness is about 50 consisting of coarse-grained massive sandstone, ily bedded ...... 75 buff to greenish-buff color, a prominent ledge of feet. :The formation is not so characteristic a fea­ mostly white, and often forming high cliffs. Light-colored sandstone, stained more or less grayish, about ...... 40 brown, "worm-eaten" sandstone, and, at the top, ture in the Sundance quadrangle as in the region Softer, thinner-bedded sandstones, in part of red Pabasapa limestone. 30 feet or more of grayish-green shale, as in the farther east, for its pink color and thin bedding color, lie below. This lower series contains several Deadwood region. are less pronounced. Its zone of outcrop extends beds of limestone and some sandy shales. Total...... 489 Bear Lodge area. In most portions of the Bear around the Nigger Hill and Bear Lodge uplifts Part of the broad belt of outcrops of the Minne­ Age. No fossils were obtained in this formation Lodge Mountains the Deadwood formation is rep­ and it-appears to a limited extent in the northern lusa formation which surrounds the Black Hills in this area, but, from molluscan remains found resented by a thick mass of'gray to pinkish quartz- portion of Black Buttes. About the Needles, at uplift extends across the east side of the Sundance in its upper beds in the southern Black Hills, ite, or conglomerate, into which the igneous rocks the east end of Cement Bidge, and in part of the quadrangle, and the formation also appears at the the age of these beds at least is believed to be have been intruded, and sometimes several such Bear Lodge uplift it is cut out by the porphyry. surface in the laccolithic uplifts marked by Bear Pennsylvania!!, though the lower beds may be masses are found separated by layers of igneous In the Bear Lodge Mountains the formation has not Lodge, Inyankara, Green, and Strawberry moun­ Mississippian. OPECHE FORMATION. Age. The limestone is classed in the west across the western half of the Sundance across the western and southwestern parts of the Character and outcrop. This thin series of in the sense in which this term is used in the Mis­ quadrangle, and varies in width from one-half quadrangle. In this zone it is in some places shales and sandstones lies next above the Min- sissippi Valley, the classification being based on mile to 7 miles. There is an extensive outlier in deeply covered by talus from the cliffs above, but nelusa sandstone and is exposed mainly in slopes fossils which it has yielded in the region west of the Mount Pisgah Range and two very small out­ there are many exposures. Owing to the low dips beneath the escarpment of overlying Minnekahta Hot Springs. liers of lower beds found on the east side of Bear and deep erosion by streams which flow eastward limestone. Its thickness averages from 60 to 80 (?) System. Lodge Mountains, 3 miles northeast of Sundance. across the western slope of the Black Hills, the feet, the amount decreasing gradually toward the In greater part it extends along the slopes which formation has been removed in such manner that northwest. Its outcrop area is a very irregular SPEAEFISH FORMATION. rise to the ridges and mesas capped by Lakota many outliers remain in ridges of irregular outline. zone on the margin of the wide belt of the Minne- Character and outcrop. The , sandstone, where it is often overlain by a con­ The deposits of this formation are distinguished lusa formation in the eastern part of the quad­ known also as the "Red Beds," consists of* red siderable amount of sandstone talus. Exposures from those of the Sundance in most cases by color rangle, and around the uplifts of Bear Lodge, sandy shales with intercalated beds of gypsum are extensive and the formation everywhere pre­ and by the massive texture of the shale. The most Green, Inyankara, and Strawberry mountains. and has a thickness of 500 to 600 feet. Its out­ sents characteristic features. extensive exposures are to be seen in the ridges The material consists of moderately soft, brownish- crop extends from north to south across the Character. The formation consists of shales adjoining Beaver Creek, along Mason Creek near red sandstone, mainly in beds from 1 to 4 inches middle of the Sundance quadrangle in a broad, and sandstones in a series which, in the main, the county line, in Skull Creek Valley north and thick, and red sandy shales. At the top of the treeless, red valley, and usually presents wide, is regular. Its thickness is about 325 feet, and northwest of the Hoi well ranch, in Black Canyon, formation, for the first few feet below the Minne­ bare slopes and high buttes of bright-red with it appears not to vary greatly from place to along Oil Creek, and in the ridges farther east. kahta limestone, there are shales which invariably outcrops of snow-white gypsum in striking con­ place. At the base there are dark shales, although The formation contains much white sandstone in have a deep purple color. The Opeche is exposed trast. The sedimentary material is almost entirely at some localities these are underlain by a local Oil Creek Valley, especially west of the T E ranch. at numerous points, and from its relations to the sandy, red shale, generally thin bedded and with­ thin bed of sandstone. Their thickness averages Age. The contains fossil Minnekahta limestone its identity can always be out any special features except the gypsum, which from 20 to 50 feet, the amount increasing grad­ bones of saurians, which have usually been regarded established. occurs mainly at a horizon about: 120 feet above ually toward the south. Four miles south of Sun- as late in age, but some eminent author­ Age. The age of the Opeche formation has not the base of the formation and appears to extend dance the basal shale is 30 feet thick and lies on a ities now believe that they are earlier , yet been definitely determined, as it has yielded no continuously over the entire region, with a thick­ 3-foot bed of red sandstone at the top of the Spear­ and as its stratigraphic relations in Colorado and fossils. From the fact that the overlying Minne­ ness of 20 to 30 feet in most cases. fish formation. In the small outlier northeast of elsewhere sustain this view in some measure it is kahta limestone contains Permian fossils and there A narrow zone of the formation also extends Sundance the basal bed is 8 feet of light-colored, here assigned to the Cretaceous. are red deposits intercalated in the upper part of past the town of Sundance and along the west side soft, sandy shales and sandstone overlain by 30 LAKOTA SANDSTONE. the Permian of Kansas and eastern Nebraska, it is of the Bear Lodge uplift. A small anticlinal area is feet of green shale capped by the usual buff, mass­ provisionally assigned to that age. exposed southwest of Sundance, and the formation ive sandstone. Next above the lower shales is a Occurrence. The Lakota sandstone is a con­ extends for some distance along Inyankara Greek series of buff, fine-grained sandstones, in part spicuous feature in the western and southwest­ MINNEKAHTA LIMESTONE. and up some of its branches, owing to a low anti­ massively bedded and having a thickness of 30 ern portions of the Sundance quadrangle, rising Character and outcrop. This formation, for­ cline. It is faulted out for a short distance along to 40 feet, constituting a characteristic and per­ in prominent cliffs above slopes of the underly­ merly known as the "Purple limestone," outcrops the west side of Inyankara Mountain. Outlying sistent member of the formation and one of the ing shales, both in the plateau which here repre­ in a broad zone extending north and south across remnants of the Spearfish formation occur at vari­ most prominent features. It usually outcrops as a sents the hogback range and in numerous outlying the Sundance quadrangle and caps the high ridges ous points on the Minnekahta limestone plateaus, distinct bench with precipitous ledges in the gen­ mesas to the east. Owing to the low dip and in the northeast corner of the area. Along the notably about Boyd and north of Inyankara. eral shaly slope. It often gives rise to terraces numerous deep canyons which traverse the sand­ eastern margin of the Red Valley its westerly dip Gypsum.- The outcrop of the gypsum bed and low buttes. The upper part of the sand­ stone its boundary is exceedingly irregular. carries it beneath the Spearfish red beds, but it is extends southward from Rocky Ford, a short stone member merges into sandy shales of buff Character. The rocks consist mainly of gray brought up again by the dome-shaped uplifts of distance west of the foot of the slopes of the to gray color and these into soft, impure sand­ to buff, coarse-grained, massive-bedded sandstone, Bear Lodge, Green, Inyankara, and Strawberry Minnekahta limestone, usually having a width stones of a distinctly reddish tint, having a thick­ usually of considerable hardness and having a mountains and Lime Buttes, a small knoll 4 of several hundred yards, but at some localities ness of 40 to 60 feet. Above are dark-gray and thickness of 150 to 300 feet. It is very thin locally miles south of Sundance Mountain. The rock is where the dips are low it is spread out more greenish-gray shales about 150 feet thick, contain­ near T E ranch, on the west side of Oil Creek Val­ of light-gray color, but has a slight pinkish or widely. It passes around the west side of Green ing thin layers of fossiliferous limestone. ley. Its basal beds are often conglomeratic, and purplish tinge, which suggested the name "Purple Mountain, caps some of the high points west of Relations at top. At the top of the formation thin streaks of conglomerate occur higher in the limestone." It is thin, averaging slightly less Horton, extends down Oil Creek to a point 5 the shales give place to a thin mass of yellowish formation. than 40 feet in thickness, but, owing to its hard­ miles above T E ranch, and thence east and sandstone which lies directly below the Morrison Coal beds. At or near its base it often contains ness and flexibility, it usually forms prominent south. In this region a thick bed of gypsum shales and may possibly represent the Unkpapa coal in lens-shaped deposits, or possibly channels, ridges with presenting nearly the begins at the top of the formation and becomes a sandstone. There is no evidence of of considerable extent. These coal beds vary in entire thickness of the formation, Ordinarily the conspicuous feature in the mountain slopes in range at the top of the formation, but the next succeed­ thickness, reaching a maximum of 9 feet in the cliffs appear to consist of massively bedded rock, 61, township 47, where its thickness is 25 feet in ing deposit begins very abruptly. Absence of the ridge west of Inyankara Mountain, where they but 011 close examination it is seen that the layers most places. One of its most interesting exposures Unkpapa sandstone would indicate a time break consist of mixed coal and carbonaceous shale. are thin and clearly defined by slight differences is at Red Butte, which it caps (see fig. 8 on illustra­ of considerable length between Sundance and Mor­ Owing, to the mantle of talus from the cliffs above in color. On it breaks into slabs tion sheet). In the southeast corner of the quad­ rison deposition. and to the disintegration and burning out of the usually 2 to 3 inches in thickness. The Minne­ rangle there is also a thin layer of gypsum lying Fossils. Fossils are particularly abundant in the coal, it is exceedingly difficult to explore the hori­ kahta limestone generally rises gradually from the on the Minnekahta\ limestone at the base of the slopes west and southwest of Sundance, but are zon without extensive digging. Local coal basins Red Valley and extends far up declivities whose formation. Four miles north by west from Inyan­ plentiful in all the exposures. One of the most have been found in the ridge southeast and south higher ridges expose the Minnelusa formation. In kara Mountain a bed of gypsum appears near the conspicuous and abundant fossils is Selemnites of Holwell ranch, on the ridge west of Inyankara these long slopes it is often cut away by canyons top of the formation for some distance, and in the densus, a cigar-shaped form of heavy, hard car­ Mountain, and west and southwest of Sundance. and valleys, leaving it on ridges and outliers, such vicinity of Sundance it is 8 feet below the top and bonate of lime, smooth on the outside but having Age. From extensive collections of plaints which as the long, detached area extending southeastward 4 feet in thickness. The gypsum forms both a radiated structure within. They occur mostly have been made in the Lakota formation in the from Henderson's ranch, which is cut off by Sol­ ridges and sinks, the latter where there is under­ in sandy layers in the upper shale series and often Black Hills, its age is known to be Lower Cretaceous. dier Creek, and in the areas capping the ridge ground seepage on the slopes, the former on many weather out on the Surface in such numbers as FUSON FOKMATION. near Bentley's ranch, .Manhattan, and north of of the smaller divides. One of the most remark­ to be a notable feature in most of the outcrops. the Nigger Hill uplift. In Canyon Springs Prairie able sinks is on the east side of the main road 3 In the upper shales there also occur the following Character and outcrops. Lying between the and Williams Divide it is spread out widely with miles east of the center of Green Mountain. It is species: massive sandstones of the Lakota and Dakota gentle slopes which are extensively farmed, but 30 feet in diameter and 25 feet or more deep, and Ostrea strigilecula. Tancredia corbuliformis. formations, there is a thin series of shales and elsewhere its surface is mostly rocky and bears has a rim of red shales. Numerous small sinks Avicula inucronata. Tancredia bulbosa. thin-bedded sandstones which have been desig­ were observed southwest of Black Buttes. A cir­ Camptoneetes bellistriatus. Tancredia postica. nated the Fuson formation. The shales are in scattered bushes and occasional cedars or pines. Astarte fragilis. Dosinia jurassica. In the smaller, dome-shaped uplifts the formation cle of gypsum ridges surrounds Green Mountain Trapezium bellefourchensis. Saxicava jurassica. places red or maroon colored, but in others are usually dips steeply away from the center and and the limestone dome south of Sundance Moun­ Pleuromya newtoni. Ammonites cordiformis. gray and buff. They merge into sandstones that Tancredia inornata. presents revetments extending part of the way up tain, and extends partly around Inyankara and are mostly thin bedded and contain shaly part­ the slopes. This is especially noticeable in the Strawberry mountains. A local dome, known as In the northwest portion of the area occasional ings. Possibly in some locations the formation is beautiful low dome of Green Mountain (see fig. 4 Gypsum Buttes, on the northeast side of Black layers of fossiliferous limestone occur in the lower absent, but in nearly all clear exposures it appears on illustration sheet). Flat, is marked by a rim of gypsum ridges with shales. The forms reported are as follows: to be separable from the adjoining beds. Its thick­ Composition. The composition of the Minne­ revetments. It is a miniature reproduction of Pentacrinoides asteristicus. Pseudomonotis curta. ness varies from 10 and 15 feet to 40 feet in excep­ kahta limestone varies somewhat, mainly in the Green Mountain and of the limestone dome just Ostrea strigilecula. Psammobia prematura. tional cases, but it is so often obscured by talus from admixture of carbonate of magnesia, which usually north, but was not uplifted sufficiently for erosion Camptoneetes bellistriatus. Belemnites densus. the sandstones above that good exposures are rare. is present in considerable proportion, and in clay, to expose the underlying limestone. All of these species are of upper and middle On the west side of Oil Creek, near T E ranch, which is a constant ingredient. In some of the Age. The Spearfish deposits are distinctly sep­ Jurassic age. the formation consists of 20 feet of dark-gray shale layers flakes of clay or impure limestone give a arated from the Minnekahta limestone below by The Sundance formation is believed to be equiv­ and sandy shale overlain by 6 feet of thin-bedded mottled appearance to the weathered bedding an abrupt change of materials. No fossils have alent to the Ellis formation of Montana and the buff sandstone. Its outcrop extends across the planes of the rock. been discovered in the Spearfish formation and Yellowstone Park region. southwest corner of the quadrangle and exposures its precise age is unknown. From the fact that Structure. The Minnekahta exhibits more local Cretaceous System. may be seen occasionally along the west side of the variations in the amount and direction of its dips it lies between the Permian below and the marine North Fork of Mason Creek, on Skull Creek, in MORRISON SHALE. than other formations. This is due to its being a Jurassic above, it has been regarded as Triassic in Black Canyon, and under the outliers of Dakota relatively hard bed of homogeneous rock lying age, but it may prove to be Permian. It is sepa­ Character and outcrops. The Morrison shale sandstone along the lower part of Oil Creek. between masses of soft beds, so that it has fre­ rated from the Sundance formation by a planation consists of a sheet of massive shale or clay, usu­ Fossils. The formation has not been examined quently been bent, the plasticity of the inclosing unconformity representing all of earlier Jurassic ally of light-gray or greenish-gray color, merging for fossils in this area, but in the Hay Creek coal beds favoring local flexing and warping. The and probably part at least of Triassic time. into maroon, buff, and purple tints. It includes field, farther north, it constitutes part of "Bed No. thin bedding planes are traversed by small faults Jurassic System. some thin beds of fine-grained gray sandstones and 2 of the Dakota group," from which large num­ and minute crumplings, but considering the large SUNDANCE FORMATION. an occasional layer of limestone. It averages about bers of plants of earlier Cretaceous age were col­ amount of deformation to which the formation has Outcrops. The outcrop of the Sundance forma­ 150 feet in thickness and outcrops below escarp­ lected by Prof. W. P. Jenney, and it is therefore been subjected the flexures are rarely broken. tion extends diagonally from southeast to north­ ments of Lakota sandstone in a zone extending placed in the Lower Cretaceous. Sundance. DAKOTA SANDSTONE. and its upper part grades into the Carlile shales the divide between Beaver and Mason creeks in consequently the rock names here used are pro­ with 6 or 8 feet of passage beds. range 63. These deposits usually lie on well- visional. Outcrop and character. This formation caps the marked terraces that are elevated considerably CARLILE, FORMATION. Algonkian (?) Intrusive Rocks. plateau west of the North Fork of Mason Creek and above the present stream bottoms and consist -SCHIST. west of Skull Creek, and it occurs in outliers also The Carlile formation occupies a. very small area largely of sand, pebbles, and bowlders of various on the ridges between Skull Creek and the head­ in the extreme southwest corner of the quadrangle. kinds. The largest deposit occupies a broad ter­ Basic dikes, metamorphosed to tough, fine­ waters of the branches of Plum Creek southwest of It consists of an alternation of shales and sand­ race level south of Boyd, where it lies mainly on grained, greenish, and usually schistose rocks, T E ranch. Some other outlying areas occur north stones of light-brown color, the latter rising in low a thin mass of Spearfish red shales, and other por­ are common in the mica-schists of the Nigger of Mason Creek, in range 63. Mason Creek crosses ridges on the shale slopes. The thickness of the tions are found north and northwest of Boyd, cap­ Hill uplift. They are composed of green horn­ the formation in a valley which presents numerous entire formation averages about 700 feet, but only ping several ridges. The materials here are blende with minor proportions of orthoclase, exposures. its lower half is found in this quadrangle. and loams containing many pebbles.and bowlders quartz, and a sodic . Biotite, in small The Dakota sandstone is a coarse-grained, buff- Tertiary System. derived from the Minnekahta, Minnelusa, and scales, with apatite, , and granular titan- colored rock, often massively bedded and cross- Pahasapa formations, and, just south of Boyd, ite, the last apparently secondary after ilmenite, bedded, which merges into a thinner-bedded SAND, GRAVEL, AND CONGLOMERATE. they contain many lens-shaped siliceous concre­ are common accessory minerals. variety, especially in its upper portion. Its thick­ General relations. In the Sundance quadrangle tions from the top of the Pahasapa formation. GBANITE AND . ness ordinarily is less than 100 feet, and in the there remain several small outliers of Tertiary About Inyankara there are terraces thinly capped ridges west of Mason and Skull creeks from 40 to deposits, which are believed to belong to the by sands containing pebbles brought from vari­ Granitic rocks of supposed pre-Cambrian age 60 feet are usually found. The massive lower White River group, but whether they represent ous local sources. In the Sundance region. and are present in both the Nigger Hill and Bear member generally forms cliffs of reddish color, the Chadron sand, the Brule clay, or even the farther northeast extensive thin sheets of sands and Lodge uplifts. In the former they cut the mica- often with a characteristic rude columnar structure. higher Arikaree beds of the , is not gravels cap wide terraces. West of Inyankara, on schist as dikes. In the latter they occur only as The Dakota sandstone contains considerable iron, known. The materials comprise impure fuller's both sides of Inyankara Creek, there are well- inclusions, with blocks of schist, in the porphyry to which is due the reddish-brown color of the earth, gravels, sand, and conglomerates. They lie marked terraces with a covering of gravel, sand, of the laccolith. rocks when weathered, and much of which occurs far above the level of recent alluvial deposits and and loam, evidently deposited by a predecessor of also as small concretions of ironstone or sand seem to be much older than any of the Quaternary the present creek. At the gap in the ridge in the NIGGER HILL LACCOLITH. cemented by oxide of iron. beds known in the vicinity of the Black Hills. west-central portion of township 50, range 63, there Relations. The numerous dikes of the Nigger Age. From plants collected in other parts of Local features. On the northeast side of the is a deposit of gravel and sand that was left by a Hill region range from 1 foot to 50 feet in width. the Black Hills the Dakota sandstone is classed as Bear Lodge Mountains there is an extensive stream which flowed southward. This is known Most of them have a northerly or northwesterly Upper Cretaceous. deposit, over 100 feet thick, of Tertiary gravels by the fact that there are numerous remnants of trend that follows the cleavage of the schist. In and conglomerates, interstratified with fine sand, terraces extending in that direction and sloping an included mass of schist, on one of the spurs of GEANEROS SHALE. sandy clays, and impure fuller's earth, reaching down part of the way to the level of Inyankara Cement Ridge southwest of Mineral Hill, seven The southwest corner of the Sundance quadran­ altitudes of 5800 to 6000 feet, and constituting Creek. On the next divide south, where the road dikes were noted within a distance of a little over gle is occupied in greater part by the Graneros a plateau which extends far northward along crosses the ridge, there is an extensive flat covered 400 feet. Occasionally, however, the}7" cut across shale, the lowest division of the Benton group. the extension of the Bear Lodge Mountains. The with gravels and sand, possibly representing the the schist, as, for example, north of Mineral Hill, It consists of fissile shales, mostly of dark color, conglomerate contains numerous pebbles and ancient course of Dry Creek flowing into Inyan­ on Sand Creek, where at one point the schists, having an aggregate thickness of about 1000 feet. bowlders, derived from the adjacent formations, kara Creek. Gravel deposits begin on the ridge which strike N. 5° E., are cut by a trending It extends far up the slopes of the plateau, which including many of the igneous rocks> from the Bear west of Skull Creek at a point 2 miles west- N. 72° E. No well-defined inclusions of either in this part of the Black Hills represents the hog­ Lodge Mountains. This deposit extends south­ southwest of Holwell ranch and extend at inter­ schist or granite were seen in the -syenite back range, reaching nearly to its edge west of ward along the east slope of the mountain east of vals to the southern margin of the quadrangle, (foyaite) of Mineral Hill, although some granite Skull Creek, southeast of Holwell ranch. Within Warren Peaks, at altitudes ranging from 5800 to probably representing a predecessor of the present occurs near the eastern base of this hill, just south about 200 feet of its base the formation contains 6050 feet, and constitutes a distinct bench or ter­ Skull Creek. of Welcome, and, as loose blocks, at a point on the a conspicuous series of harder shales and thin- race, in places over one-half mile in width, abut­ The gravel deposit overlying the supposed Ter­ northeast spur of the same hill north of Welcome. bedded, fine-grained sandstones which weather to a ting against the higher slopes of the mountain. tiary formation west of the North Fork of Mason The granite dikes occasionally show partings paral­ light-gray color and, owing to their hardness, form One of these terraces forms the divide between Creek, in township 49, may possibly belong to the lel to their walls. At one point in the schist area a line of k-nobs and ridges that rise slightly above the headwaters of North Redwater and Beaver Quaternary and be related to the early system of southwest of Mineral Hill, granite cements brec­ the main shale slope. The ridge due to this mem­ creeks. The deposits are fully 100 feet thick the drainage above described, but apparently it cia ted schist. ber is prominent on the west side of Dry and and they dip northward at the rate of about 100 belongs to a still earlier one. It contains frag­ Description. The granites of the Nigger Hill Turner creeks, and it usually bears a sparse feet per mile. A somewhat similar material caps ments of Minnekahta limestone and Belemnites uplift are very light gray, frequently yellowish growth of pine trees, from which is derived the small outlying areas of the Spearfish formation on densus. A small cap of bowlders of early Qua­ when weathered, and range from a moderately local name, Piney Ridge. This member is an the narrow ridges on the northern margin of the ternary age lies on the Sundance formation in the fine-granular rock to one whose individual con­ extension of the Mo wry beds of the Bighorn quadrangle, near the State line, at an altitude of small butte 4 miles southeast of Sundance. stituents reach a diameter of 15 cm. The grain Mountains and adjoining region. 4100 feet. Six miles farther south, just north of Alluvium. The principal alluvial deposits are is variable, a single hand specimen usually show­ In its upper portion the con­ Lytle Hill, small areas of apparently the same for­ in the valleys of the larger streams in the western ing both and fine-grained areas. , tains a thin bed of a deposit known as bentonite, a mation occur at an altitude of 5100 to 5200 feet. half of the quadrangle, but they are so narrow and comprising orthoclase, , and albite, pre­ light-colored, massive, soft rock or hard clay, from Deposits of fuller's earth and overlying bowlders discontinuous that their representation on the map dominate over the quartz. These three species may 3 to 5 feet thick, which is conspicuous in a series occur at the head of the east fork of Hewston is not desirable. The most extensive ones are along be present in various proportions, although albite of slopes and knobs just north of the railroad east Creek, near the -foot of the Bear Lodge Mountains, Sundance Creek for 8 or 10 miles below Sundance, is rarely dominant. Micrographic intergrowths of of Iron Creek. It has been mined to some extent at an altitude of 5800 feet, apparently having a along Beaver Creek from Black Flat to its mouth, feldspar and quartz are rare. In some of the at this place and, owing to the low dip of the beds, thickness of 100 feet. Another deposit extends on Inyankara Creek below Inyankara, on Mason dikes a silvery white mica is fairly abundant, Creek, Skull Creek, and on Oil Creek in the vicin­ is spread out widely so that it is readily accessible. alongo the ridgeo followed by«/ the road east of Mil- though many of them are free from mica. Tour­ It has great capacity for absorbing water and in ler Creek, and slopes upward from an altitude of ity of T E ranch. Soldier, Cold Springs, and maline is a minor constituent of some of the dikes some portions of its area, where the drainage is 5300 feet at the south to 5800 feet at the north. Little Spearfish creeks run in narrow valleys that in the schist area west of Sand Creek. In many imperfect, the bentonite is softened into "soap On the divide in the southeast corner of township contain more or less alluvium and local wash. The of the granite dikes about Tinton , in holes," or deep, miry spots in which cattle are 49, range 64, a deposit consisting of about 50 feet alluvium consists mainly of local materials and varies grains of various sizes, is irregularly disseminated. occasionally lost. of soft, buff sand, with some darker clayey sand, in thickness from thin soils to deposits 20 or 30 feet Pegmatitic fades. While some of the dikes of the At the top of the Graneros formation are gray caps small outlying areas of Graneros shale at an thick in some of the deeper valleys, such as Beaver Nigger Hill region are typical granites, others have shales containing hard, calcareous concretions which altitude of 4750 feet. Doubtless there are small or Inyankara. Many of the creeks cut through the character of , with the coarse texture give rise to knobs and low ridges lying about 1-J- areas of Tertiary deposits in other parts of the their valley floors into the underlying deposit, and and graphic intergrowth of quartz and feldspar miles southwest of the railroad. region, for they lie in out-of-the-way places and all alluvium merges into local talus on the slopes characteristic of these rocks. This texture, however, The Graneros shale contains very few fossils are not always discernible. adjoining the valleys. has been obscured or obliterated by the crushing except fish scales, which are very numerous in the Original extent. These scattered outcrops indi­ of the rocks during the later igneous intrusions, IGNEOUS EOCKS. beds constituting Piney Ridge. cate that the slopes of the Black Hills were origin­ and the grain has been so reduced as to resemble, By W. S. TASTGIBR SMITH. ally covered by an extensive deposit of Tertiary under the microscope, that of fine-grained granite. . materials which have been removed by the wide­ General statement. The igneous rocks of the . All of the granite when exam­ One of the most prominent features in the plains spread erosion of later Tertiary and Quaternary Sundance quadrangle belong to two Avidely sep­ ined under the microscope shows a cataclastic immediately adjoining the Black Hills is a low but times. arated periods of time, the first pre-Cambrian structure due to squeezing. The quartz and feld­ distinct escarpment due to the hard Greenhorn (Algonkian?), the second probably post-Cretaceous spar are deformed or granulated, and are in part Quaternary System. limestone in the middle of the Benton group. It (early ?) To the earlier period belong the cemented by secondary quartz, feldspar, and mus- crosses the southwest corner of the Sundance quad­ The Quaternary formations of the Sundance granites and the metamorphic amphibolites of the covite. In some places the cementing minerals rangle, rising in a ridge facing northwest, about 2 quadrangle comprise recent alluvial deposits along Nigger Hill laccolith, and probably the granite of have partially replaced the original crushed min­ miles southwest of the railroad. the stream valleys and upland gravels and sands the Bear Lodge uplift. The Eocene (?) igneous erals. This is especially noticeable in some of the The limestone is thin-bedded and is characterized that occupy old terraces which are remnants of a rocks form an interesting group of closely related Tinton rocks, in which also cassiterite seems to by a large number of impressions of Inoceramus previous epoch of topographic development. types, all of which have probably been derived by have formed metasomatically at the expense of the labiatus (see fig. 9B on illustration sheet), a fossil Older terrace deposits. The largest of the older differentiation from a rich in sodium. They granulated primary constituents. Cassiterite has which is of infrequent occurrence in the adjoining terrace deposits occurs in the Red Valley between form laccolithic masses and associated minor intru­ been found only in the granite of the central part formations. It contains a considerable amount of Inyankara and the canyon of Stockade Beaver sions. The rock types of the principal of the laccolith, where this rock forms inclusions clay and some sand, and on hardening by expo­ Creek. Other areas occur on the south side of are few, comprising - and syenite-por­ in the porphyry. sure breaks into thin, pale-buff slabs exhibiting Inyankara Creek, on terraces west of Sundance, on phyries and a phonolitic rock which here, as else­ Veins. Occasional quartz veins carrying frag- impressions of the distinctive fossils. Its thick­ the divides between Sundance Creek and Rocky where in the Black Hills, is apparently of later age mental black tourmaline (deep blue in thin sec­ ness averages about 40 feet, including some shaly Ford Creek, on the ridge north of Rocky Ford than the other porphyries. Rocks of identical type tion) are associated with the granite dikes in the beds in its upper portion, and it dips gently to the Creek, on the divide between Skull Creek and but from different laccoliths are distinguishable by schist area west of Sand Creek. One of them southeast. At its base it is distinctly separated Turner Creek, and along an old stream channel slight but characteristic differences. Very few shows under the microscope a small amount of from the black shales of the Graneros formation, extending southward through the depression in chemical analyses have been made of these rocks, feldspar, its manner of occurrence being such as t o suggest the nearly complete replacement of an Tertiary Intrusive Rocks. into the other and clearly belong to one igneous texture is common in the groundmass of

original granitic dike by secondary minerals, largely PETROGRAPHY. mass. some specimens. quartz. Inyankara Mountain. The porphyry of Inyan- The weathered and more typical porphyry of MONZONITE- AND SYENITE-PORPHYRIES. BEAR LODGE MOUNTAINS. kara Mountain has fewer than that of the Bear Lodge Mountains differs considerably General character. The principal rock at each Nigger Hill, the ratio of phenocrysts to ground- from that just described. It is characterized by Relations. In the Bear Lodge Mountains, in of the centers of intrusion shown on the geologic mass ranging from 1.4 to 1.7. forms unusual variability both in grain and in the kind, the northwest corner of the quadrangle, granites map belongs to a group prominent elsewhere in the principal phenocrysts, but there is a minor size, and abundance of its phenocrysts, the varia­ occur within the main igneous area of the uplift the Black Hills and indeed throughout the Rocky amount of orthoclase which often appears as a tions occurring sometimes even within a few yards. and to a slight extent at several points outside of Mountains. In general these rocks are distinctly border about the oligoclase. and segirite- The rock is nearly always . Pheno­ this area. All of the masses shown on the map,, porphyritic and are characterized by many pheno­ augite are the chief ferromagnesian minerals. crysts are sometimes abundant, sometimes rare. notwithstanding the dike-like form of some of crysts of feldspar and in a light-gray, They are associated with a little brown hornblende They consist of feldspar, as the few ferromagnesian them, are probably inclusions in later igneous aphanitic, but microcrystalline groundmass. While showing partial resorption. minerals which were originally present in most of rocks. This interpretation is the more probable soda-lime feldspars predominate among the pheno­ The groundmass of the Inyankara porphyry these rocks have been weathered out. Oligoclase as none of the granitic rocks of other parts of the crysts, potash feldspar is the chief constituent of varies in texture from microgranular to trachytic, is the dominant feldspar, the frequently Black Hills are known to be of post-Cambrian age. the groundmass. Quartz is quite subordinate or the latter being more common near the borders of having rims of orthoclase or sanidine. The Smaller granitic inclusions within the main intru- is absent. The ratio between the and the mass. The groundmass of the central part of groundmass of these rocks, though sometimes sives of the laccolith are common, and the granitic the sodium- feldspars is variable but on the intrusion is coarser grained than that of most granular or trachytic-granular, is typically a fine­ fragments are abundant in the igneous breccias the whole is near unity, and most of the rocks of the Nigger Hill porphyry, and contains in addi­ grained trachytic felt of orthoclase. The chief described on page 8. belong in the monzonite group. Some of them, tion to orthoclase and magnetite a considerable products of the weathering of these porphyries are The inclusions vary greatly in size, ranging from however, would more properly be designated as amount of augite. The Inyankara porphyry muscovite, kaolin, and limonite. microscopic fragments of the different minerals com­ syenite-porphyry, if a sharp distinction were desir­ seems richer in alkalies than the Nigger Hill rock, A coarser-grained syenite-porphyry occurs spo­ posing the granite to the body of granite extending able or practicable. and is a syenite-porphyry rather than a monzonite- radically in small masses within the main laccolith. for more than 3 miles along the eastern side of the Nigger Hill laccolith. The monzonite-porphyry porphyry. This is, so far as known, the most coarsely ­ laccolith, on the headwaters of Beaver and Red- of the Nigger Hill laccolith and associated sheets At three points on the rim of the laccolith, sit­ line porphyry of the quadrangle. The largest water creeks, with a width, east of Warren Peaks, and dikes is a light-gray rock with abundant phe­ uated respectively south, east, and northeast of the occurrence noted was east of the summit road, near of about three-eighths of a mile. This body lies nocrysts of oligoclase- and of common main peak of the mountain, an exceptionally the southern margin of the laccolith, where there roughly parallel to the Cambrian beds just east of hornblende. Biotite phenocrysts are rare. alkalic of porphyry is found. This is appears to be a considerable body of this rock it, and was probably never entirely enveloped by The phenocrysts range from 0.5 to 5.5 mm. in almost free from ferromagnesian minerals and extending for nearly a mile along the ridge. the monzonitic magma by which it was uplifted. length, the average being about 1 mm. They consists essentially of alkali feldspar with a small No satisfactory exposures of syenite-porphyry For the greater part of its course it is overlain by usually constitute about one-fourth of the rock by proportion of magnetite and a little interstitial were seen in the field, and its relation to the typi­ a thin bed of quartzite that forms the base of the volume, the hornblende being generally subordi­ quartz. It contains a few scattered orthoclase cal porphyry is obscure. In most places there Deadwood formation. This bed, however, is sep­ nate to the other phenocrysts in total bulk. and anorthoclase phenocrysts, comparable in size appears to be an abrupt change to the finer- arated from the succession of sedimentary rocks to The groundmass is always holocrystalline and to those of the common facies of this porphyry, grained rock, although southwest of Warren Peaks the east by porphyry sheets. The granite mass consists mainly of orthoclase, with subordinate and is characterized by an abundant generation of toward the margin of the laccolith, one grades into may be an upraised originally intruded beneath magnetite, plagioclase, and quartz, the last being anorthoclase crystals intermediate in size between the other. the Cambrian rocks, or it may be part of the pre- probably in part secondary, and a little apatite these large crystals and the laths and grains of Some specimens of the coarse facies, when Cambrian formation upon which these sediments and . The texture of the groundmass is the groundmass, and averaging about 0.15 mm. in broken, have a pronounced fetid odor, which was were laid down. granular or trachytic. The average diameter of length. noted especially in the rocks near the southern Outside the main laccolith small, scattered inclu­ the groundmass constituents ranges from 0.02 mm. Sundance Mountain. The rock of Sundance margin of the laccolith. Like most of the other sions of fine-grained biotite-granite occur in the pho- to about 0.06 mm. Micropoikilitic and fluidal tex­ Mountain is a syenite-porphyry showing a few rocks of the area, the syenite-porphyry is consider­ nolite on the round knob south of the head of North tures are occasionally developed. small scattered phenocrysts of oligoclase, with only ably weathered and at only one place noted within Miller Creek. Inclusions of medium-grained gran­ The grain of the dike and sill rocks is but an occasional prism of brown or green . the main laccolith did it contain any unaltered ite, up to 18 inches long, are numerous, with frag­ slightly finer than in the porphyry of the largest The mass of the rock is composed essentially of a ferromagnesian minerals, in this case augite. This ments of other rocks in a sill of porphyry near the mass. In two sills near the northern margin of fine-grained aggregate of two species of feldspar. facies differs from the typical porphyry mainly in head of the east fork of Blacktail Creek, about 1J the uplift hornblende is so abundant as to render One, occurring chiefly in numerous laths, is prob­ the coarser grain of the groundmass, although it miles north of the Sundance quadrangle. Granite the rock unusually dark. ably oligoclase. The other, which forms the larger is also somewhat more alkalic than much of the of various types, in blocks and smaller fragments, The hornblende of the porphyry alters into part of the rock, is microgranular and is probably finer-grained porphyry. The phenocrysts, which all more or less rounded, is present in an agglom­ chlorite and calcite with a minor proportion of orthoclase. are usually well differentiated from the ground- erate occupying a shallow saddle at the western and occasionally of a colorless mica. The The rock as a whole is uniform in petrographic mass, consist of orthoclase and sometimes a little base of a knob of a little over 5 miles chief products of the weathering of the feldspars character, but at one point on the northern face of oligoclase, which occasionally occurs as a core to east of north of Sundance. Similar are sericite, calcite, and kaolin. the mountain, a little east of south of Sundance, at the orthoclase. The groundmass, like that of the

have been described by«/ Jaggaroo at the Black Buttes. The monzonite-porphyry of the an altitude of 5400 feet, it is banded with narrow typical porphyry, is trachytic in texture, and in and the Little Buttes, in the Devils Tower Black Buttes is similar to that of Nigger Hill, but stripes of lighter and darker gray. this facies the average diameter of the grains is quadrangle, northwest of the Sundance quadrangle. has certain peculiar features worthy of note. The Bear Lodge Mountains. The principal laccolith between 0.2 and 0.3 mm. The groundmass is com­ Description. The Bear Lodge granites are aggre­ rock is on the whole probably more alkalic than of the Bear Lodge Mountains and most of the asso­ posed of orthoclase, with usually a minor propor­ gates of quartz with one or more species of feldspar. that of Nigger Hill, as indicated by the preva­ ciated smaller masses about the flanks of the uplift tion of albite. . Magnetite, apatite, and are the usual acces­ lence of alkali-feldspar phenocrysts. The Black consist of porphyry which is so much richer in Outside of the main igneous area of the Bear sories. Mica is absent as a rule, or present only Buttes porphyry occurs as a single mass associated orthoclase than that of Nigger Hill that it may Lodge Mountains syenite-porphyry, with relatively as an accessory. At one point, however, near the with minor bodies of phonolite and bostonite and appropriately be termed syenite-porphyry. There coarse-grained groundmass, has been noted in one eastern edge of the large granitic area referred to of itself. Within this mass the rock presents two is, however, much variation among the Bear Lodge of the minor intrusives on Lytie Creek; also as a above, biotite is so abundant as to be a prominent facies, distinguishable by the character of their porphyries, which show both monzonitic and tra­ dike of comparatively fresh rock north of a lac­ constituent. larger feldspar phenocrysts. North of the road chytic facies. colith situated nearly 3 miles beyond the northern Orthoclase or microcline is usually the dominant which crosses the buttes from east to west, the These rocks have a yellowish, reddish, or grayish boundary of the quadrangle. feldspar, with microperthite, albite, and oligoclase larger feldspars are roughly equidimensional, with color, and are in general greatly altered, usually in~ various minor proportions. Very rarely albite a maximum diameter of about 1J cm. South of containing minute cavities due to the decay or NEPHELTNE-SYENITE. ' is the principal feldspar. Quartz is usually abun­ the road the phenocrysts are prismatic or tabular, solution of some of the minerals of the rock. Occurrence. Nepheline-syenite of the foyaite dant. with a maximum length of 3 cm. Of the two the Comparatively fresh porphyry occurs as intrusive type is known to occur within the quadrangle in These granites differ both in grain and in mineral northern facies more nearly resembles the Nigger sheets on the northwest flanks of the laccolith, and the Nigger Hill uplift only. With minor masses v OO composition from those of Nigger Hill. Moreover, Hill rock, the only essential difference being in also within the main igneous mass in the low area of allied rocks it entirely surrounds Mineral Hill, they seldom show the crushing, cementation, and the greater number and different character of the south and southeast of Warren Peaks. inclosing the central core of pseudoleucite-por- metasomatic replacement so common in the rocks phenocrysts of the Black Buttes rock. In addi­ This fresher porphyry, which is not typical of phyry. On the west, south, and north the foyaite of the other uplift. Neither tourmaline nor cas- tion to the large, scattered crystals of orthoclase the laccolith as a whole, resembles some of the zone has an average width of nearly half a mile, siterite has been noted in the granites of the Bear just referred to, smaller phenocrysts of ortho­ porphyry of the Black Buttes. TE'e groundmass, but it is much narrower on the east. On the sum­ Lodge uplift. Though not usually porphyritic clase, oligoclase, and , the last com­ however, is on the whole finer grained, and ferro­ mits of the long northern spurs of Mineral Hill there are sometimes more or less tabular feldspar pletely altered in all surface exposures, occur. magnesian minerals are more abundant, in places foyaite occurs only locally and in subordinate phenocrysts reaching a maximum length of between Oligoclase is the dominant feldspar and some­ even in excess of the feldspar phenocrysts. They amount, the rocks here, south of the mica-schist 3 and 4 cm. The rocks, on the whole, are much times has a narrow border of orthoclase. There comprise one or more of the following: A pale- and monzonite-porphyry, consisting for the most finer grained than those of Nigger Hill, and are is probably also a little anorthoclase present. ' green augite in one instance associated with a part of basic differentiation products of the Mineral more nearly uniform in texture. A porphyritic The large, scattered, tabular phenocrysts of the little segirite-augite, brown to green amphibole, Hill magma. texture is occasionally present, due to the develop­ southern facies are sanidine. Unaltered ferromag- and biotite, the last not found in the Black Buttes Description. The nepheline-syenite is a dark- ment of tabular phenocrysts of feldspar to lengths nesian minerals, comprising augite and segirite- porphyry. Any one of these except the aegirite- gray, fine-granular rock, with generally trachytoid of 1 or 2 inches. augite, derived in part from brown amphibole augite may be the dominant ferromagnesian min­ texture. It is occasionally porphyritic, orthoclase In most of the granite the individual crystal by resorption with occasionally a little unchanged eral, though as a rule the biotite is subordinate occurring in two generations. The chief constit­ grains or andedra reach a maximum length of amphibole, are common as subordinate pheno­ when found with the augite or amphibole. The uents of the rock are orthoclase, nepheline, pyrox­ between one-half and 1 cm., the average diam­ crysts, and are sparingly present in the ground- feldspar is usually oligoclase, occasionally with a ene, and magnetite. Oligoclase is occasionally eter being usually in the neighbornood of 2 mm. mass, which is generally coarser grained-than in narrow orthoclase rim; in one occurrence, how­ present and rarely it is an essential constituent. Rarely the granite is much finer grained, as, for the other facies. It consists chiefly of orthoclase ever, northwest of the main laccolith, the feldspar Nepheliue is generally abundant, but it is occa­ example, toward the north end of the large mass with a very little plagioclase, and has a trachy- is sanidine, with only a small proportion of oligo­ sionally scarce, and the rock passes locally into near the east margin of the laccolith. The gen­ toidal texture. Some marginal phases, however, clase. The groundmass of this porphyry is com­ pulaskite. In some parts of the rock mass the eral fineness of the grain, together with the pov­ have a trachytic groundmass of even finer grain posed chiefly of a very fine-grained trachytic felt nepheline is represented only by alteration prod­ erty in mica, allies these rocks with the than that of the northern facies. of orthoclase, with occasionally subordinate augite, ucts. A second , usually anhedral, rather than with the typical granites. The two general facies just described grade one as in some of the Black Buttes porphyry. Micro­ though occasionally in sharp crystals with clear Sundance. 6 margins and clouded centers, was noted in some of some of the limonite. Occasionally the feld­ Lodge Mountains, Black Buttes, and very subor- Generally only one feldspathoid is present, usually* specimens. It is apparently noselite. Pyroxene spar of the groundmass has a parallel or trachytic dinately in the Nigger Hill uplift. They include the same species that occurs among the phe­ is as a rule the dominant ferromagnesian mineral, arrangement and may be original; whereas, the both nephelinitoid and trachytoid types, together nocrysts. Nepheline in clear, colorless, short, though in one variety or facies biotite is the only granular, feldspar has probably all been formed by with tinguaite and nepheline-syenite-porphyry. hexagonal prisms is occasionally abundant in dark mineral present. The pyroxene is usually a recrystallization of the rock. These rocks do not form large laccolithic masses the groundmass of some of the rocks. a pale-green augite, sometimes accompanied by Other pseudoleucite rocks in bodies too small to within this quadrangle, but occur only as dikes, The tinguaites of the Bear Lodge uplift, as dis­ segirite-augite. In some facies segirite-augite is map occur within the nepheline-syenite mass. sheets, and subordinate laccolithic intrusions tinguished from the just described, are the only pyroxene present. Brown amphibole, One of these is a pseudoleucite-syenite, which is within or on the flanks of the larger uplift. characterized chiefly by their decided green color, sometimes intergrown with augite, biotite, and known in place only at one point, nearly half a Bear Lodge Mountains. Phonolite occurs in due to the abundance of segirite needles contained pleochroic apatite, are common accessory minerals. mile west of south of Welcome, just south of the the Bear Lodge Mountains as dikes or small in the groundmass. They have, on the whole, Titanite and melanite are less common, although fork in the road and near the margin of the foyaite masses within the main laccolith; also as sheets, fewer phenocrysts than the phonolites, and these the latter in two places is an important constituent area. Bowlders of the same rock, however, are minor laccoliths, and dikes on the flanks of the are generally small. In most cases pseudomorphs, of the rock. common in the gravels of Sand Creek for some dis­ uplift. The tinguaite forms dikes or small bodies apparently after nepheline, are the most abundant White, fine-granular, aplitic dikes, consisting tance south of the mouth of Mallory Gulch. The within the main porphyry mass. phenocrysts, though sometimes sanidine, in small essentially of the dominant light-colored constit­ source of this rolled material was not determined. The phonolites are light to dark gray, usually lath-like forms, is dominant. In some varieties no uents of the nepheline-syenite, are in places abun­ It may come from some of the densely wooded with a greenish tinge. In general they are with­ feldspar at all is present. yEgirite-augite some­ dant in the nepheline-syenite area. western slopes along Sand Creek north of Welcome. out noticeable cleavage or parting, though in one times occurs, occasionally with pale augite centers Associated . Angular fragments of The pseudoleucite-syenite is dark gray and is instance they are almost slaty. They are mostly and margins of segirite, as in the phonolites, and pyroxenite are included in the nepheline-syenite characterized by many pseudophenocrysts having of the trachytoid variety, although there are some melanite, though rare, is occasionally an important at several places, particularly near its borders. In the distinctive form of leucite. Some of these are nephelinitoid phonolites, and locally the rock of . Biotite, not seen in the phonolite, some localities the nepheline-syenite, crowded with light gray and small; others are larger, with a the long, narrow mass east of Miller Creek occurs sparingly in one of these rocks. Apatite these inclusions, seems to grade into pyroxenite as maximum diameter of 1.5 cm., and have a reddish approaches nepheline-syenite-porphyry in texture and magnetite are the usual accessory minerals. a basic facies. tinge. Occasionally they show zones due appar­ and composition. The groundmass is usually very fine-grained, The are nearly black, medium- to ently to the arrangement of included pyroxene. These rocks are always porphyritic, and fre­ frequently with pronounced flow structure. It fine-granular rocks, composed of augite, magnetite, As shown by the microscope, the pseudoleucites quently contain numerous coarse, tabular, sanidine consists of a felt of fine 83girite needles, with and apatite, usually with scattered flakes of biotite. are composed of nephelite, or its alteration prod­ crystals having a maximum length of 3 cm. or minute laths of feldspar. In some of these rocks The augite, which is pale green and slightly pleo­ ucts, and orthoclase, and lie in a matrix composed more. The phenocrysts include the following nepheline is abundant in the groundmass; in chroic in , constitutes the bulk of the essentially of pyroxene-nephelite, magnetite, some minerals: Sanidine (or. perhaps soda-orthoclase), others an isotropic feldspathoid, not seen in well- rock, magnetite forming from one-fourth to one- orthoclase, and usually . The pyroxene is anorthoclase, segirite-augite (sometimes with augite defined forms, is moderately common. In one third. Apatite is common and is more abundant segirite-augite, usually with more or less augite. centers or segirite rims or both), one or more feld- rock a very little was noted as a secondary in the coarser-grained varieties of the rock. One Apatite, titanite, zircon, biotite, and rarely a small spathoids, melanite garnet, magnetite, titanite, apa­ product. of these, from near the western base of Mineral flake of museovite occur as accessories. This rock tite, and rarely zircon. Feldspar or a feldspathoid Blade Buttes. The southern and largest mass Hill, contains in places 10 per cent of apatite. differs from the pseudoleucite-porphyry in the is usually dominant, though sometimes aagirite- of phonolitic rocks in the Black Buttes consists of This coarse-grained rock differs from most of the absence of a porphyritic texture, although a por­ augite preponderates. Brownish or brownish-yel­ nepheline-syenite-porphyry. Some of this rock pyroxenites at other points in containing a very phyritic appearance is given to it in some cases by low melanite is occasionally common. Titanite occurs also in the eastern part of the northern little partly decomposed orthoclase and the decom­ the great variation in the size of the pseudoleucites. and apatite are common accessories in most of the area of phonolite. The rest of the mass, however, position products of what was probably nepheline. Pseudoleucite rocks occur within the nepheline- rocks, but magnetite is seldom abundant. with a little intrusion shown on the map between syenite as dikes. These are usually medium- to The are altered to aggregates of the two larger areas, consists of phonolite and a PSEUDOLEUCITE-PORPHYRY. dark-gray porphyritic rocks, with very fine­ analcite, , calcite, mica (museovite?), and rock intermediate between phonolite and mon- Occurrence. The pseudoleucite rocks of the grained to aphanitic groundmass. They are sim­ probably other minerals. The original mineral zonite-porphyry. quadrangle are limited to Mineral Hill and its ilar, on the whole, to the pseudoleucite-porphyry was probably in most cases nepheline, although The Black Buttes phonolites are gray or nearly vicinity. The only body of them represented on of Mineral Hill, although some of them are much the presence of or noselite in some of black rocks, with generally abundant phenocrysts the map is that which, surrounded by nepheline- less weathered. the varieties is inferred from the isotropy of unal­ of moderate size. These consist chiefly of alkali syenite, forms the mass of Mineral Hill. The Relations of pseudoleucite-porphyry and nephe­ tered crystal grains and the outlines of the pseu- feldspar and pyroxene, the former predominating. pseudoleucite-porphyry is usually gray but is line-syenite. Although the nepheline-syenitic area domorphs. That the feldspathoid is in some A feldspathoid which is probably either sodalite occasionally yellowish or reddish. Though the practically surrounds Mineral Hill, nepheline- instances altered nephelite is suggested by the or noselite is always present in subordinate amount rock is sometimes without noticeable phenocrysts, syenite is absent, as has already been stated, in following analysis of a trachytoid phonolite from among the phenocrysts. Among the minor con­ light-gray or nearly white porphyritic pseudo- a part of this annular area, on the northern spur Warren Peaks. In this rock only one feld­ stituents, yellowish-brown melanite is sometimes morphs after leucite (pseudoleucite) up to 5 cm. in of the hill northwest of Welcome, although the spathoid was noted, a brownish mineral showing common, and brown amphibole, not noted in the diameter are common. The porphyry is everywhere rocks here consist chiefly of differentiation prod­ frequent hexagonal sections, which is common as Bear Lodge phonolite, is occasionally present. greatly weathered and generally contains numerous ucts of the nepheline-syenite magma. Mineral a phenocryst. The analysis shows only traces of The feldspar is mainly orthoclase, although sani­ small or minute cavities due to the partial or com­ Hill is asymmetrically placed within the nephe­ chlorine and little sulphuric acid, while the calcu­ dine, soda orthoclase, and a little oligoclase, fre­ plete leaching out of some of the minerals. line-syenite area, which is much narrower on the lated norm of the rock gives 8.5 per cent of neph­ quently mantled or intergrown with orthoclase, In its original condition the rock was appar­ east than on the west. elite. In drawing conclusions, however, the fact are present in some of the rocks. As in the Bear ently fine grained, holocrystalline, and porphyritic, It is possible that the nepheline-syenite and that sodalite and noselite give rise on weathering Lodge rocks, the pyroxene is generally segirite- with phenocrysts of leucite and accessory magnet­ pseudoleucite-porphyry together form a minor lac­ to alteration products free from chlorine and sul­ augite, sometimes with light augite centers. ^Egir- ite and apatite. The groundmass appears to have colith within the larger Nigger Hill uplift, resting, phuric acid must not be overlooked. ite was noted only in one rock from near the center been originally granular, and it, also, probably as does the latter, on an irregular floor of pre-Cam- The following analysis of trachytoid phonolite of the buttes. consisted largely of leucite. Other probable con­ brian rocks. The mutual relations of the rock (pulaskose) from Warren Peaks was * made by In the general texture and composition of the stituents were nepheline, pyroxene, and garnet. masses, however, as well as their general form and George Steiger in the laboratory of the United groundmass these rocks resemble the Bear Lodge1 Owing to their altered condition these rocks sel­ structure, which are shown on the map, indicate States . The ,rock was called phonolites. A second generation of the same dom show any of the minerals originally contained that the nepheline-syenite and pseudoleucite-por­ syenite-porphyry in Bulletin No. 228 of the Sur- feldspathoid found among the phenocrysts usually, in them except occasionally apatite and perhaps phyry form a that is intrusive into the mon- vey (p. 102). occurs as a minor constituent of the groundmass. a little magnetite. The rock now consists essen­ zonite-porphyry of Nigger Hill and they are so Analysis of trachytoid phonolite from Warren Peaks, No other feldspathoid was identified in these rocks, tially of three secondary minerals orthoclase, a represented in the accompanying cross sections of Wyoming. although some of the phonolite from near the Per cent. colorless mica (presumably museovite), and limon- of the uplift (see figs. 1 and 2). It is probable SiO...... 55.14 center of the Black Buttes contains analcite, which ite. Orthoclase, the chief constituent of the also that these two rocks, although they were A13O8 ...... 18.98 was probably derived from nepheline. The accom­ Fe2 O 3 ...... 2.60 rock, occurs generally as a granular mosaic, derived from the same magma, were not simul­ FeO ...... 1.62 panying analysis, however, of one of the phono­ through which the mica is scattered in micro­ taneously intruded, but that they resulted from MgO...... 32 lites (No. 1) from the Black Buttes indicates that scopic scales or aggregates. The form and mode two eruptions that were separated by a compara­ CaO...... 3.96 nepheline, sodalite, and noselite are all present. Na3 O ...... J*:'. 5.38 of occurrence of these aggregates suggest in many tively short interval of time. The strongest evi­ K2 O...... 6.64 The following analyses of phonolitic rocks from cases the replacement of some feldspathoid, possi­ dence in support of this view is the fact that the H2O ...... 63 the Black Buttes were made by George Steiger in H,O+...... 3.70 bly nepheline. This is rendered more probable rocks of the two areas do not grade into each TiO 2 ...... 50 the laboratory of the United States Geological by the fact that this mica is a common alteration other, but, though subject to some local variation CO2 ...... None. Survey. product of nepheline in the rocks of this region. within their own boundaries, retain their distinc­ P2 0 5 ...... 17 The rocks of intermediate character previously SO,...... 10 The limonite, though sometimes associated with tive characters throughout their respective areas. Cl...... Trace. referred to, which have been mapped with the pho­ the mica, occurs plainly in patches, sometimes with Additional evidence is furnished by the asymmet­ S...... 03 nolite on account of close field association with this MnO...... Trace. hexagonal outline. The numerous small cavities ric position of the pseudoleucite-porphyry within BaO...... rock, are exposed near the road running north from in the rock are usually lined with limonite or with the nepheline-syenite area and by the existence of the center of the Black Buttes, where they occur opaline or chalcedonic silica. dikes of pseudoleucite-porphyry within the nephe­ Total.. >.77 along the western margin of the narrow strip of Less O.... .01 The pseudoleucite phenocrysts are aggregates of line-syenite west of Mineral Hill. phonolite, and also near the road which runs orthoclase and museovite which are quite distinct While both of the rock masses, as shown by their 99.76 southwest from the center of the buttes. These in the hand specimen, but differ microscopically mineralogical compositions, are comparatively rich In only one place the long, narrow area east of rocks megascopically resemble some of the phono­ from the rest of the rock only in their freedom in alkalies, the pseudoleucite-porphyry seems rel­ Miller Creek -was any unaltered nepheline seen lite just described, but microscopically are more from limonite (or magnetite) and in a coarser crys­ atively richer in potash, and the nepheline-sye­ among the phenocrysts of the phonolite. closely allied to the monzonite-porphyry. They are tallization of*the feldspar. That pyroxene and nite in soda. The porphyry also appears to have The groundmass of the phonolites is always less alkalic than the phonolite, oligoclase is more feldspar were originally present in small amount a higher content of silica and alumina, but is some­ fine grained, sometimes granular, more often tra­ abundant and is usually the dominant feldspar, in the rock is suggested by the forms of some of what poorer in lime, iron, and magnesia than the chytic, and usually shows flow structure. It is and the segirite-augite contains less of the segirite the cavities, and some weathered pyroxene crys­ nepheline-syenite. composed mainly of alkali-feldspar laths with molecule. No feldspathoids have been recognized tals were identified in one thin section. The aagirite or segirite-augite or both, sometimes augite, in any of these rocks. More or less brown horn­ PHONOLITE. occurrence of original magnetite and garnet in the one or more feldspathoids, frequently a small blende and biotite occur as phenocrysts. The apa­ groundmass of the rock is suggested by the forms Occurrence. Phonolitic rocks occur in the Bear amount of magnetite, and rarely a little garnet. tite of most of these rocks is pleochroic. 7

Analyses of phonolitic rocks from Black Suites, Wyoming. some extent, their complementary composition, line with a considerable amount of calcite. This sory apatite, as is indicated for one of them by the No. 1. No. 2. together with the areal relations of some of them rock gelatinizes readily in hydrochloric acid, and a comparatively large amount of phosphoric acid Per cent. Per cent. to the nepheline-syenite and pseudoleucite-por­ drop of the solution yields on evaporation abun­ shown in its analysis, which is given below. In ...... 57.46 58.08 Al8 O t ...... 18.41 18.38 phyry, are grounds for considering these small dant cubes of sodium chloride. Much altered rock, addition to these minerals, brown amphibole is Pe8 O 8 ...... 2.40 3.02 masses as differentiation products of the same probably , occurs on the northern spur common as a phenocryst in one facies of the FeO...... 1.28 1.42 MgO ...... None. .96 magma which solidified as the nepheline-syenite of Mineral Hill, a little south of the monzonite- vogesite. CaO... 4.02 3.85 and pseudoleucite-porphyry of Mineral Hill. The porphyry contact. The groundmass, which is often crowded with Na2 O. 9.23 6.22 pseudoleucite-porphyry itself, as has been shown, Augite-fourchite. This rock occurs as dikes in dark-colored constituents, consists of augite, mag­ 4.93 5.11 H 8O . .45 .37 may have been a differentiation product of this the monzonite-porphyry on the east side of Beaver netite, a variable amount of biotite, feldspar, altered 1.12 1.55 general magma which probably had an average Creek, near the southern margin of the Nigger Hill nepheline, and, locally, brown hornblende. Ortho­ TiO .42 .58 CO8 ...... None. None. composition near that of the nepheline-syenite. laccolith, and in the small southernmost area of clase and andesine, the former usually dominant, p8o5 ...... 11 .21 Judged by the exposures within the nepheline- schist on the west side of Beaver Creek. It is a are the principal light-colored constituents of the SO...... 50 .07 syenite area this magma was particularly subject dark-gray, dense rock, consisting of numerous groundmass. In the facies analyzed there are Cl...... 23 Trace. 8...... 05 None. to differentiation. This is shown not only by augite prisms and magnetite grains in a brown to occasional crystals of a considerably altered color­ MnO...... 11 .10 the variations of its nepheline content, but espe­ nearly colorless isotropic base containing abundant less mineral which is probably noselite. The min­ BaO...... None. None. eral is roughly hexagonal in outline and contains cially in the varying proportions of its dark augite microlites and occasionally a little spherulitic O v O Total...... 100.72 99.92 minerals, in the common local segregations of feldspar. The rock on the east side of Beaver numerous microscopic inclusions of magnetite cen­ Less O...... 08 pyroxenite, and in the occasional occurrence of Creek contains abundant little spheroids up to trally massed. These rocks gelatinize with acid. 100.64 aplitic veins almost free from dark minerals. half an inch in diameter, which weather out of The following analysis of nepheline-bearing No. 1. Phonolite (laurdalose) from a point near the center In the process of differentiation the iron oxides, the mass. The spheroids differ from the rest of augite-vogesite (>kentallenose), from the summit of the Black Buttes. magnesia, and lime have segregated in one direc­ the rock in containing comparatively little isotropic of the northern spur of Mineral Hill, was made No. 2. Trachytoid phonolite (akerose) from a place just tion, as is seen in the rocks rich in ferromagnesian base. The groundmass consists largely of albite in by George Steiger in the laboratory of the United north of the road near the western margin of the Black O O «/ Buttes igneous area. minerals, and the alumina and alkalies in another, long laths and needles, having a tendency to form States Geological Survey: In Bulletin No. 228 of the United States Geological Survey, as is exemplified in the dike rocks with abundant fan-shaped aggregates, and of orthoclase in small page 102, No. 1 was called -porphyry (essexose), and Analysis of augite-vogesite from Mineral Hill, Wyoming. No. 2, syenite-porphyry (akerose-laurvikose). feldspars or nepheline and scanty ferromagnesian xenomorphic grains. The spheroids are separated Per cent. constituents. The Mineral Hill magma was com­ from the rest of the rock by broad, brownish, glassy Si0 8 ...... 42.95 The nepheline-syenite-porphyry of the large pho­ paratively rich in phosphoric acid, and this was bands. A1 8 O3 ...... 12.44 nolitic mass near the southern margin of the buttes concentrated in the first group of oxides, yielding Another rock, provisionally classed as a fourchite, Fe2O3...... 10.16 FeO...... 5.18 apparently forms a subsidiary laccolith of the at the basic end of the rock series a pyroxenite rich forms a dike cutting the Carboniferous rocks south MgO...... 5.82 Black Buttes uplift and is very different, both in apatite. The rock types produced are the fol­ of the Nigger Hill porphyry area. This dike has CaO ...... 13.11 Na8 0...... 2.10 megascopically and microscopically, from the pho­ lowing: Pyroxenite, , nephelinite, fourchite, a trend 8. 78° E., an exposed length of about K2O ...... 2.29 nolitic rocks previously described. It is light gray, camptonite, vogesite, garnet-nepheline rocks, pseu­ 250 feet, and a width of between 50 and 75 feet. H3 O ...... 91 often with a reddish tinge, and is frequently spot­ doleucite-porphyry, phonolite, monzonite-porphyry, It shows a more or less well-developed, horizontal H2 O+ ...... 1.98 TiO2 ...... 1.34 ted with brownish-red or yellow phenocrysts of syenite-porphyry, syenite (including nepheline- columnar structure. The rock of this dike differs CO...... None. weathered nepheline. The rock shows a marked syenite), and alkali-granite-porphyry. Light- and considerably from the fourchites already described. P3O5...... 1.37 SO3 ...... 15 tendency to split into thin plates. It is holocrys- dark-colored rocks are -about equally abundant. It is dark gray, almost black, very fine grained, and Cl...... 07 talline and somewhat porphyritic, with pheno­ A large part of the leucocratic, or light-colored porphyritic, the phenocrysts, which are small and S ...... None. crysts of nepheline (elseolite), segirite, feldspar, and members of the group occur between Spottedtail scattered, being slightly pleochroic augite, magnet­ MnO...... 29 BaO ...... Noue. sometimes a little magnetite. Nepheline forms the and Sand Creek gulches, not far from the mar­ ite, some apatite, and an occasional andesine feld­ Total...... 100.16 largest phenocrysts, the average diameter of the gins of the nepheline-syenite and pseudoleucite- spar. The matrix in which these minerals occur Less O...... 02 sections being a little less than 1 mm. On weath­ porphyry. On the northern spurs of Mineral is pale gray to nearly colorless, and for the most 100.14 ering it develops a prismatic cleavage and gradu­ Hill the leucocratic rocks are closely associated part is isotropic or nearly so. It consists largely ally alters to colorless or yellowish analcite. The with melanocratic, or dark-colored types, the out­ of clear and almost colorless minute grains, averag­ Garnet-nepheline rocks. At two points on the feldspar is microperthite or cryptoperthite. crops along the ridges within the area colored as ing between 0.02 and 0.03 mm. in diameter. These northwest slopes of Mineral Hill occur rocks char­ The groundmass, which constitutes the bulk of nepheline-syenite on the map showing alternating resemble leucite in form, though having no dis­ acterized by abundant minute garnets. One of the the rock, is coarser grained than in the other pho­ light and dark rocks. The dark rocks, which seem tinct boundaries, and usually contain numerous rocks is very fine grained, and, though dark in nolitic rocks of the Black Buttes. It consists here to be on the whole more abundant, form con­ inclusions of augite and magnetite microlites, color, consists largely of light-colored minerals. chiefly of microperthite and cryptoperthite and has siderable masses in which the light-colored rocks zonally arranged. The optical characters of these The dark minerals are mainly brown garnet and a trachytic, or, less commonly, a granular texture. occur as dikes. Nearly all the rocks of this group grains and chemical tests indicate that the light- biotite, with a minor proportion of aagirite-augite. A grayish, weathered feldspathoid, either sodalite which are noted in the pseudoleucite-porphyry area colored portions of the groundmass probably con­ The light-colored matrix is granular and consists or noselite, occurs in the groundmass in minute occur in the northwestern part. They are chiefly sist in large part of analcite, either as an original of more or less altered nepheline and orthoclase (?). crystals up to 0.1 mm. in diameter, and there is leucocratic, the only distinctly melanocratic rock mineral or as a pseudomorph after leucite. In The other rock, which is porphyritic, with the light occasionally a little asgirite present. seen being pyroxenite, which is found near the addition to the minerals mentioned the ground- and dark minerals nearly evenly divided, is con­ The nepheline-syenite-porphyry of the northern western base of Mineral Hill. mass contains a variable but generally small pro­ siderably altered. The phenocrysts are brown gar­ phonolitic intrusion differs from that just described Among the light-colored rocks of the group are portion of feldspar in untwinned or simply twinned net, augite, magnetite, and a probable feldspathoid in that the nepheline is confined to the groundmass syenite-porphyry and monzonite-porphyry, some laths, or in small micropoikilitic areas. Calcite altered to analcite with apatite as an accessory. and the scattered phenocrysts consist of orthoclase of the latter resembling the laccolith porphyry of occurs in small amount as a secondary mineral. The groundmass consists of the same secondary with subordinate aBgirite-augite. Nigger Hill. It is possible that some of the mon­ Augite-camptonite. Intruded in the pre-Carn- analcite with abundant small garnets, some mag­ zonite-porphyry dikes, especially those outside of brian schists in Bear Gulch, between 300 and netite, very much pyroxene, and a very little BOSTOXITE. Mineral Hill, are derived from the Nigger Hill 400 yards south of the settlement of that name, feldspar, perhaps secondary. Bostonite forms three sharp-pointed hills in the magma, but their general distribution and their are two narrow dikes of what is here called augite- Phonolite. Near the end of the west fork of western part of Black Buttes. The rock is light close association with dikes evidently related genet­ camptonite, though the rocks, strictly speaking, are the northern spur of Mineral Hill occurs a dark- gray, holocrystalline, and porphyritic, with fine­ ically to the nepheline-syenite magma indicate their intermediate in character between camptonite and greenish-gray trachytoid, tephritic phonolite, with grained groundmass, and consists essentially of derivation from the Mineral Hill center. fourchite. They are dark gray and porphyritic, numerous phenocrysts of feldspar-augite, brown feldspar with accessory magnetite, apatite, and Ijolite. The igneous sheet in the Minnelusa with compact groundmass, and the dark minerals amphibole, magnetite, and a much altered isotropic titanite. No ferromagnesian minerals were seen. formation in Grand Canyon, northwest of Mineral are more abundant than the light-colored ones. feldspathoid in a dense groundmass. The feldspars The feldspars include orthoclase, anorthoclase, and Hill, is probably a somewhat altered ijolite. The The phenocrysts are augite and magnetite, with are orthoclase and oligoclase, the former occurring oligoclase and occur in various proportions in the bottom of the sheet is 60 feet'above the floor of accessory apatite and small biotite crystals. In largely as a mantle to the latter. The original different masses. The phenocrysts are orthoclase the valley, and its thickness is 10 or 11 feet. one dike the light-colored part of the groundmass feldspathoid was probably nepheline. Titanite or anorthoclase and lie in a trachytic ground- The rock is dark gray and fine granular except consists of feebly polarizing feldspar and a minor and apatite occur as accessories. The ground- mass composed mainly of the same species. close to the contacts, where it contains minute proportion of isotropic material. In the other the mass consists mainly of weathered feldspar laths light-colored phenocrysts in an aphanitic ground- matrix is mostly isotropic, with little feldspar. with microscopic grains of magnetite. DIKES AND SHEETS. mass. The sedimentary rocks are hardened near The feldspar consists of both orthoclase and oligo­ Monzonite-porphyry and syenite-porphyry. Occurrence. In the Nigger Hill uplift are the intrusion. clase in nearly equal amounts. The isotropic These porphyries are especially common northwest numerous dikes, sheets, and small masses, which The sheet is essentially a hypidiomorphic-gran- mineral is somewhat brownish and is probably and north of Mineral Hill, within the pseudoleucite- occur principally in or near the nepheline-syenite ular aggregate of altered nepheline, segirite-augite, analcite. Both rocks gelatinize with hydrochloric porphyry area. But they also occur on the north­ or pseudoleucite-porphyry, especially on the north­ and melanite, the nepheline being in excess of the acid, and cubes of sodium chloride form on evapo­ ern spur of Mineral Hill north of the pseudoleucite- ern spurs of Mineral Hill, and also in the schist. dark minerals. There is in places a little untwinned ration of the solution. porphyry area, in the schist on Sand Creek below Only two intrusions belonging to this group are feldspar, and apatite and magnetite occur as acces­ Augite-vogesite. Augite-vogesite is by far the the mouth of Spottedtail Gulch, in the schist a known in the monzonite-porphyry of Nigger Hill. sories. Marginal facies of the rock, which are most abundant dark rock on the northe>n spurs of short distance south of the settlement of Bear These rocks are also intruded in the Carboniferous much weathered, contain very many minute por­ Mineral Hill, where it forms a considerable mass. Gulch, and within the Nigger Hill porphyry area. rocks at three places near the southeastern mar­ phyritic crystals of pseudoleucite, many small gar­ It occurs also as a dike, 1 foot wide, in the schist Most of these dikes and sheet rocks do not differ gin of the uplift, near its southwestern margin, and nets, a small amount of magnetite, and in the on the north side of Sand Creek a short distance essentially from the monzonite- and syenite-por­ in Grand Canyon about 5 miles northwest of Min­ rock from\ the lower contact an undetermined below the mouth of Spottedtail Gulch. Vogesite phyries of the Nigger Hill and Bear Lodge lac­ eral Hill. Many of the dikes in the nepheline- feldspathoid (?). is dark gray, often nearly black, porphyritic, holo­ coliths. They are usually light gray, and, like syenite and in the pseudoleucite-porphyry are not Nephelinite. The thin sheet intruded in the crystalline, and usually very fine grained. The most of the light-colored rocks of the quadrangle, shown on the geologic map. Most of the others Pahasapa formation north of the head of Rattle­ dark-colored minerals are as a rule the more abun­ are generally much weathered. They vary in are represented by a single color under the head snake Canyon is a dark-gray, porphyritic nephe­ dant. Phenocrysts are common, and consist chiefly texture from facies similar to the coarser-grained "Dikes and sheets." linite, consisting of numerous phenocrysts of of augite and magnetite, the former predominating, syenite-porphyry of the Bear Lodge Mountains to Character. The rocks of these minor bodies pale-green, slightly pleochroic augite, magnetite, together with accessory apatite, biotite, and, rarely, facies having very fine-grained groundmass and range from extremely basic pyroxenites to light- and apatite in a gray matrix made up mainly of plagioclase feldspars. Most of these rocks, like well-defined phenocrysts. Unlike the Nigger Hill colored aplites. Their peculiar character and, to what are apparently alteration products of nephe­ many others of the group, are fairly rich in acces­ porphyry, most of these rocks contain orthoclase Sundance. 8 phenocrysts, the porphyritic feldspars being usu­ especially the ferromagnesian constituents, are important form taken by the Tertiary igneous While the porphyry has in general elevated the ally orthoclase, or orthoclase and oligoclase. Fer- common both in the matrix and in many of rocks of the Sundance quadrangle is the lacco­ sedimentary rocks without fracturing them, it has romagnesian minerals are abundant in some of the included rock fragments. The matrix usu­ lith. This consists in some instances of a single at two points at least (at the northeast and south­ these, but nearly absent from others. They com­ ally contains abundant microscopic grains of red intrusion of one general rock type, as at Sundance east margins of the laccolith) broken across the prise biotite and hornblende. The groundmass, or yellow iron oxide, sometimes so numerous as to Mountain and perhaps at Inyankara Mountain. Cambrian into the lower part of the Carboniferous. which is trachytic or granular, consists, as in most render it almost opaque. The clearer parts, as seen In other cases, as in the Bear Lodge and Nigger Thus the Needles, near the northeastern margin of of the porphyries of the quadrangle, essentially of with a high power, show a faintly polarizing gran­ Hill uplifts, while the main laccolithic mass con­ the Nigger Hill laccolith, consist of a minor lac­ orthoclase. ular or somewhat felted feldspar aggregate. sists of a single undifferentiated or but slightly colithic intrusion of the porphyry connected with One of the more important of the light-colored The igneous breccias of the Black Buttes por­ differentiated type, it is associated with intrusions the main mass by a cross-cutting body of porphyry. rocks of the group, probably a nepheline-syenite- phyry area (mentioned above) are similar to those of other rocks. Some of the laccoliths include Here the Cambrian rocks have been elevated on the porphyry, forms a broad dike-like mass on the west just described, except that they contain no granitic subsidiary laccolithic masses within the main south and west, while on the east the porphyry has and northwest slopes of Mineral Hill, between 200 material. area or about its borders, as in the case of the broken across them, and locally, at least, across a and 300 feet above Spotted tail Creek. This mass The sheet of phonolite in the Minnelusa forma­ Needles in the Nigger Hill uplift. This is espe­ part of the Pahasapa formation as well. appears to continue unbroken for half a mile or tion north of Sundance and near the northern mar­ cially true of the Bear Lodge uplift. The Black The porphyry occasionally exhibits a well- more. The rock is grayish and more or less yel­ gin of the quadrangle is intruded in what appears Buttes differ somewhat from these in having developed columnar structure, which was noted lowish in color, and is composed largely of a to be a bed of shale belonging to this formation been formed by several intruded masses of at the Needles, as well as at places along Beaver trachytic aggregate of thin, tabular orthoclase and is associated with a breccia consisting of sand­ more nearly equal importance. It is a lacco­ Creek and southeast of the settlement at Bear phenocrysts, elongated parallel to the axis and stone, sand grains, shale, and porphyry, in a small lithic aggregate. The principal laccoliths, as Gulch, where, as at the Needles, there is, in addi­ usually simply twinned. These feldspars have an proportion of cement which is apparently igneous. has appeared from the preceding petrographic tion to the vertical jointing, a series of nearly hori­ average length of between 1 and 2 cm. They Another breccia, also associated with an abun­ descriptions, are those of Nigger Hill, Bear Lodge zontal joints, which occur at intervals several times contain frequent small inclusions, probably altered dance of shale, is found nearly 1^ miles southeast Mountains, Black Buttes, Inyankara Mountain, as great as those between the vertical joints. nepheline, often hexagonal, squarish, or more or of this occurrence, along the northwestern base of and Sundance Mountain. The general geologic The western rim of the Needles consists of a less rounded in form and composed mainly of the small body of phonolite which occurs at this relations of the Tertiary igneous rocks at each massive outcrop of porphyry cut by vertical part­ aggregates of secondary muscovite in microscopic point. The breccia includes shale, limestone, dark- of these eruptive centers or uplifts will now be ings into great outstanding ribs which, toward the flakes and of limonite. In the angular spaces gray, fine-grained quartzite, and various types of briefly reviewed. northern end of the group, form almost as pro- between the feldspars is a considerable amount of interstitial matter of greenish- or yellowish-gray MineralHill color, composed largely of the same altered nephe­ Cd Cmii line (?) and tabular orthoclase. Magnetite arid apatite are occasionally present, and small cav­ ities probably due to the weathering out of some 3OOOfeet above sealevel minor ferromagnesian constituent are moderately common Avithin these areas. PIG. 1. Northwest-southeast section through Nigger Hill laccolith along line G-G on structure-section sheet. As, Mica-schist; ed, Deadwood formation; Cpe, Pahasapa and Englewood limestones; Cml, Minnelusa sandstone; Tm, monzonite- and syenite-porphyries; Tns, nepheline-syenite; Tpl, pseudoleucite-porphyry. Syenite. On the northern spur of Mineral Hill Horizontal and vertical scales: 1 inch=l mile. are five dikes and one larger mass, 400 feet wide and several hundred yards long, of syenitic rocks. granite and , besides mineral fragments from Nigger Hill uplift. The igneous rocks of the nounced a feature as the Needles themselves. The One of the dikes is nepheline-syenite with abun­ the granular rocks. The cement of this agglom­ Nigger Hill uplift consist of pre-Cambrian granite rim has a rather rude, horizontal, platy or colum­ dant altered nepheline, having brown amphibole erate appears to have been weathered out for the and of Tertiary nepheline-syenite (foyaite), pseu­ nar structure, far less pronounced than the vertical and augite as its ferromagnesian constituents. The most part, leaving the loose, fragmental material, doleucite rocks, and porphyry, the last probably partings. others, which are considerably altered, appear to much of which, however, still retains a coating of Eocene. The granite occurs as dikes in the pre- Bear Lodge Mountains. The syenite-porphyry have contained little or no uepheline. the matrix with its abundant smaller inclusions. Cambrian schist. The porphyry, the main igneous of this uplift was intruded at the base and between Alkali-granite-porpliyry. This rock occurs at This matrix does not differ essentially from that of rock of the uplift, forms an extensive laccolith with the beds of the Cambrian, and all the strata form­ the Interocean mine, west of Welcome. It is the other breccias described. The granite of this associated marginal intrusive sheets and minor lac­ ing the cover of the laccolith have been eroded light gray, porphyritic, and fine grained. Phe­ breccia is undoubtedly pre-Cambrian and is like coliths. The nepheline-syenite and pseudoleucite (see section A-A). The phonolites and related nocrysts are numerous, and consist of feldspar that of the Devils Tower , about 25 rocks are possibly somewhat older than the por­ rocks are in most if not all cases the young­ with a minor and sometimes insignificant pro­ miles northwest of this area, described by Jaggar phyry, but apparently together constitute an intru­ est igneous rocks of the uplift. They occur both portion of biotite. Magnetite is never more than in his account of the laccoliths of the Black Hills sive stock cutting the laccolith, and they are so as intrusive sheets and as minor laccoliths about an accessory. The feldspar is sometimes ortho­ (Twenty-first Ann. Kept. U. S. Geol. Survey, pt. 3, shown in the section (figs. 1 and 2). In addi­ the margin of the main uplift and also as dikes clase and microperthite with a minor proportion of p. 263). It resembles the Archean granite of the tion to these larger bodies of igneous rocks there cutting the principal area of igneous rocks. oligoclase. Sometimes the oligoclase is the chief , rather than the granite of other are numerous dikes intruded in the porphyry, Within this area the dikes are somewhat more^ phenocryst, with smaller amounts of orthoclase parts of the Black Hills. Two varieties of shale are syenite, and pseudoleucite rocks and in the larger numerous than has been indicated on the map, and very little microperthite, the last two occurring found in this breccia, one nearly black and occur­ masses of pre-Cambrian schist. Those in the igne­ the omission being due largely to the difficulty of in part as a mantle to the oligoclase. The ground- ring in thin flakes, the other very dark gray and ous rocks are confined chiefly to the Nigger Hill distinguishing these rocks from the trachytes when mass is holocrystalline-granular and consists of breaking into small angular fragments. These laccolith and its immediate vicinity. Two occur­ much weathered. The phonolitic rocks of the area feldspar and quartz. The feldspars are orthoclase shales are probably derived, in part at least, from rences of igneous rocks, one a dike and the other mapped are for the most part fairly fresh. Except­ and microperthite, with sometimes one and some­ the Minnelusa formation. a thin sheet, were noted in the Carboniferous ing the phonolites and the granite, the igneous times the other in excess. The quartz is abundant

and frequently occurs in micropoikilitic areas. Cement Ridge Cd ! --- IGNEOUS BRECCIA. Cml Many of the fissures through which the por­ phyries were intruded must have contained* more or less broken rock. The intrusion itself undoubt­ SOOOfeet above sealevel edly in many instances brecciated the fissure walls, FIG. 2. Southwest-northeast section through Nigger Hill laccolith along line H-H on structure-section sheet. As, Mica-schist; Ag, granite and pegmatite; ed, Deadwood formation; Cpe, Pahasapa and Englewood limestones; Cml, Minnelusa sandstone; Tm, monzonite- and syenite-porphyries. and where there were successive intrusions the brec- Horizontal and vertical scales: 1 inch=l mile. ciation involved previously intruded porphyry as well as older igneous or sedimentary rocks. Such In addition to the occurrences described above, strata also, near the southeast and southwest mar­ rocks of the uplift as a whole are greatly weath­ breccias, of which the intruding rock forms the there is a considerable body of breccia in the igne­ gins of the main uplift. ered, sometimes so much so that it is impossible to matrix, are commonly associated with the intrusive ous area a little over a mile northeast of Sundance. While the post-Cambrian igneous rocks of the determine the original composition of even the eruptions of the northern Black Hills and are found The breccia is found here just northwest of the Nigger Hill region may differ slightly in age, they freshest obtainable specimen from a given locality. most often along the margin of the invading mass. phonolite and forms most of the northwestern por­ probably all belong to the same geologic period and Black Buttes. The Black Buttes laccolith dif­ In the Sundance quadrangle they occur in the Bear tion of the area. It is well exposed in the north­ to the same general period of intrusion. fers from the laccoliths of Inyankara and Sundance Lodge Mountains, in the Black Buttes, and in con­ ern part of the area, close to the phonolite, and The porphyry which forms the Nigger Hill lac­ mountains in that it has apparently been formed nection with several of the minor intrusives on the here the material occurs in layers which vary from colith penetrated the Algonkian rocks and spread by several successive intrusions instead of a single east side of the Bear Lodge uplift. coarse to fine grained, but are without sharp divi­ out along the basal contact of the Cambrian strata, one (see section D-D). It is not a simple but a The breccias of the Bear Lodge laccolith were sion lines. These layers strike N. 40° E. and dip elevating the overlying sedimentary rocks into an compound laccolith, masses of igneous rock being noted on and in the vicinity of the two highest northwest at 38°. They lie parallel to the plane irregular dome. The Cambrian rocks thus consti­ irregularly intruded at various horizons among the peaks of the Warren Peaks group; on the ridge of contact with the phonolite. These breccias are tute over the greater part of the area the basal por­ sedimentary rocks. Owing to this irregularity in and slopes west of the road, near and north of the characterized by their light-gray color and, unlike tion of the original cover of the laccolith, remnants structure the main igneous mass forms a some­ northern boundary of the quadrangle; on the spur those last described, appear to consist wholly of of which still remain. The floor of schist on which what elongated angular area in which erosion south of Beaver Creek, just west of the granite; igneous material (phonolite and syenite-porphyry, the porphyry rests is doubtless very irregular. In has developed a group of peaks rather than a and at several points about 2 miles southeast of with scattered fragments of feldspar, augite, horn­ breaking through these rocks the porphyry tore single dome. the highest of the Warren Peaks. These Bear blende, and magnetite), derived, so far as could be away large masses and carried them to higher There is little evidence as to the relative ages of Lodge breccias contain abundant fragments of determined, from the intrusive rocks which form the levels. These included masses still preserve in a the different rocks of Black Buttes, although it is granite and of various facies of the Bear Lodge southeastern portion of this area. There is com­ general way their original dip and strike. The probable that the intrusion of the southernmost and porphyries, together with numerous fragments of paratively little matrix, often little more than a film porphyry has also intruded the Cambrian rocks largest of the phonolitic masses was subsequent to minerals from the coarser-grained rocks, especially between the closely packed fragments of rock. at a number of points. Following the bed­ that of the monzouite-porphyry and that it has granite, all embedded in a reddish,, yellowish, This matrix is similar in general microscopic char­ ding of these rocks, the porphyry has taken the formed an independent dome on the margin of the brownish, or grayish matrix, which as a rule acter to that of the breccias previously described. form of sheets and minor laccoliths on the flanks larger laccolith. The other phonolitic rocks appear constitutes but a small part of the rock. The of the main uplift. These intrusions in the to be of approximately the same age as the porphyry rock is always greatly weathered, and minute GEOLOGIC RELATIONS. Cambrian occur northwest and southeast of the and to have been formed as differentiation products cavities, due to the leaching out of minerals, General statement. The most striking and main area. of the magma from which the porphyry was derived. 9

The southernmost area of the bostonite probably structure sections on the structure-section sheet. minations of their thickness made where they are the monoclinal structure, which consists of some forms a minor laccolith, while the northern area Their position on the map is on the line of the uplifted. gentle undulations and of steep dome-shaped uplifts occurs as a laccolithic sill. upper edge of the blank space. The vertical and General structuralfeatures. The Sundance quad­ due to the intrusion of laccoliths or lens-shaped Owing to differences in various factors, such as horizontal scales are the same, so that the actual rangle embraces a portion"of the northwest slope of masses of igneous rocks. the mode of intrusion, the level at which it took form and slope of the land and the actual though the Black Hills uplift, with rocks dipping to the In the southeast corner of the quadrangle the place, the height to which the were forced, generalized relations of the rocks are shown. The west in the greater part of the quadrangle and to Pahasapa limestone lies nearly level, a structure the volume of the intrusion, and the resistance of structure where buried is inferred from the position the north in the northeast corner of the quad­ which extends several miles eastward and south­ the various rocks to weathering, each type presents of the strata observed at the surface and from deter­ rangle. There are several local irregularities in eastward in the high plateau on the central portion characteristic erosion forms. The bostonite forms remarkably sharp peaks with steep slopes. The monzonite-porphyry occurs as a group of rugged hills, steep sided but less abrupt than those com­ posed of bostonite and with moderately broad summits. The large southern area of phonolite, though forming the highest part of Black Buttes, has on the whole more moderate slopes and more broadly rounded summits. The smaller phono- litic areas as a rule occupy the valley bottoms. Where exposures are favorable the monzonite- porphyry usually shows well-defined jointing. Inyankara Mountain. The igneous rocks of Inyankara Mountain form its central mass and also its eastern and southern rim. The central part consists of two peaks separated by a shallow saddle. The igneous portion of the rim of the lac­ colith, which stands somewhat lower than the cen­ tral peaks and is joined to the southern of these by a shallow saddle, has a gradual though marked descent toward the north. This ridge is charac­ terized by a narrow tofl flanked on either side by steep slopes. As a rule the outer slope is the steeper, the angle for a considerable distance being about 30°. The igneous rocks are everywhere jointed and break into thin plates, especially along the rim of the laccolith. In addition to the finer jointing the central area has two or more well-developed systems of coarser joints, the width of the divisions ranging from a few inches to several feet. To these joint systems the columnar structure of the central por­ tion is due (see fig. 5 on illustration sheet). On the northern peak two systems were noted, the \\\\\\ \\V\\! strike of one being NV48°E., of the other N. 60° \\\\\\v\\\v> to 75° W. The strike of the two systems seen on \ \V\\\\\\\v\ the southern peak was N. 48° E. and N. 50° W. The dips of these joints, as shown in the resulting columnar structure and as seen from a distance, appear to be quaquaversal, radiating from some point above the summit of the north peak. The strike of the joints of the rim, as usually seen at any given point, is parallel to the direction of the rim at that point, though occasionally it is trans­ verse. It is probable that there are two sets of joints, one parallel and the other transverse to the trend of the rim. These joints dip at a high angle, sometimes toward the center of the laccolith, and sometimes away from it. STRUCTURE. STRUCTURE OF THE BLACK HILLS UPLIFT. The Black Hills uplift, if not eroded, would pre­ sent an irregular dome rising on the northern end of an anticlinal axis extending northward from the Laramie or Front Range of the Rocky Mountains (see fig. 3). It is elongated to the south and north­ west, has steep slopes on the sides, is nearly flat on top, and is subordinately fluted. The greatest ver­ tical displacement of the strata, as indicated by the height at which the granite and schist floor is now found, amounts to about 9000 feet. The minor flutings of the dome are mainly along the eastern side of the uplift, the most notable ones being in the ridge of Minnekahta limestone just west of Hot Springs. Another fluting of considerable prominence occurs 3 miles east of Hot Springs and a smaller one rises a short distance east of Aladdin. These subordinate flexures are characterized by steeper dips on their west side and gentler dips to the east. They merge into the general dome to the north and run out with declining pitch to the south. In the northern Black Hills there are numerous local domes and flexures due mainly to laccolithic igneous intrusions, but no similar fea­ tures are indicated by the structure of the southern portion. Faults are rarely observed and not many have been found which amount to more than a few feet in vertical displacement, these being short breaks due to igneous intrusion.

STRUCTURE OF THE SUNDANCE QUADRANGLE. .. 3. Black Hills uplift represented by contours on the surface of Minnekahta limestone. Where the Minnekahta limestone has been removed by erosion the calculated position of the contours is shown by broken lines. Long dashes indicate areas from which Minnekakta and overlying formations have been eroded ; short dashes, Structure sections. The principal structural fea­ areas from]which all sedimentary rocks nave been removed. Contour interval, 250_feet. B, ; B B, Black Butte; BL, Bear Lodge Mountains; C, Crook Mountain; CP, Crow Peak; D, Deadwood; DT, Devils Tower; E, Elkhorn Ridge; EM, Edgemont; G, Green Mountain; H, Harney Peak; HS, Hot Springs; IN, Inyan­ tures of this quadrangle are illustrated by the six kara Mountain; MB, Little ; N, Nigger Hill; NC, Newcastle; OL, Oelrichs; OW, Old Woman Creek; R, Rapid; S, Sundance; ST, Sturgis. Sundance. 10 of the uplifts. As they approach the State line, dips of 8° to 10° on its flanks. Its strata are sediments derived from its waste. The quartz ponding saline water in lakes, in which accumu­ the beds pitch steeply downward to the west, but eroded to the upper sandstone of the Minnelusa sand and pebbles of coarse sandstones and con­ lated the bright-red sands and sandy muds of the flatten again in the canyon of Stockade Beaver formation only (see fig. 4 on the illustration sheet). glomerates, such as are found in the Lakota for­ Opeche formation. The Minnekahta limestone, Creek. West of this canyon low westerly dips The two low domes of Lime and Gypsum buttes mation, whatever their original source in crystalline which is the next in sequence, was deposited from prevail, giving a wide expanse of Minnekahta are also supposed to be laccolithic, but no igneous rocks, have been repeatedly redistributed by streams sea water, and from its fossils we know with a limestone in the Canyon Springs Prairie district. rock is exposed. and concentrated by wave action on beaches. Red fair degree of certainty that it is a product of the In this area there are various undulations of the In the Black Buttes the structure is somewhat shales and sandstones such as make up the "Red latest Carboniferous or Permian time. It was laid strata, the most notable of which are a low anti­ irregular, there being two domes considerably Beds" usually result directly from the revival of down in thin layers, to a thickness now represented cline which extends southward through Mount broken by cross intrusion. The Pahasapa lime­ erosion on a land surface long exposed to rock by only 40 feet of the limestone, yet the very great Pisgah and a strong general pitch of the beds to stone and a small area of the Deadwood formation decay and oxidation and hence covered by a deep uniformity of this formation over the entire Black the south in the ridges east of T E ranch. A are brought to the surface on the west and the east residual soil. Limestones, on the other hand, if Hills area is an impressive feature, probably indie-1' low local anticline extending east and west crosses sides of the northern dome. The southern dome deposited near the shore, indicate that the land ative of widespread submergence. Oil Creek Valley 1^ miles south of T E ranch. is almost completely inclosed by the Minnekahta was low and that its streams were too sluggish to Red gypsiferous sediments. At the close of the Gentle southwesterly dips prevail in the Black limestone. carry off coarse sediments, the sea receiving only epoch represented by the Minnekahta limestone, Canyon and Skull Creek and from Inyankara Inyankara Mountain is an irregular dome, with fine sediment and substances in solution. The there was a resumption of red-clay deposition and Creek to the southwest corner of the quadrangle. Pahasapa and Minnelusa formations dipping radi­ older formations exposed by the Black Hills uplift the great mass of red shales constituting the Spear­ It is these low dips and the thinness of the forma­ ally from the flanks of the igneous rock on the were laid down from seas which covered a large fish was accumulated. These beds probably were tions that give such irregular boundary lines to the northern side. It is considerably broken and faulted portion of the central-western United States, for laid down in vast salt lakes that resulted from formations in the southern portion of the quadran­ on its southern side, where the igneous rock ascends many of the rocks are continuous over a va"st area. extensive uplift and aridity. The mud accumu­ gle. This is notably the case with the Minnekahta to higher horizons, and, at one point in the western The land surfaces were probably large islands of lated in thin layers to a thickness of 600 feet or limestone in the vicinity of Soldier Creek Valley. part of the ridge, has carried a mass of Pahasapa an archipelago, which was to some degree coexten­ more, as now represented by the formation, which In the region east of Cold Springs Creek the limestone to the level of the Cretaceous strata. sive with the present Rocky Mountain province, is so uniformly of a deep-red tint that this is Minnelusa formation presents a number of gentle Sundance Mountain presents no evidence of but the peripheral shores are not even approxi­ undoubtedly its original color. This color is undulations, composing a general flattened, broad disturbance of the surrounding strata, but prob­ mately determined for any one epoch, and the present not Only throughout the extent of the anticline, sharply accented into a local dome at ably it is a remnant of the lower part of a lac­ relations of land and sea varied greatly from time formation, but also through its entire thickness, Bald Mountain, and rising steeply to the north colith spread out horizontally in Sundance shales to time. Pursuing these general ideas in greater as is shown by deep borings, and therefore is not over the great dome of the Nigger Hill uplift. (see section B-B). The upturned overlying beds detail, one finds that the strata brought to view by due to later or surface oxidation. Either the orig­ West and north of these domes there is a general and the edges of the igneous lens have been the Black Hills uplift record many local variations inal material of the sediments was red, or it was dip to the west and north, which is very gentle removed by erosion. A similar remnant of lac­ in the ancient geography and topography of the colored during deposition by the precipitation of and regular far out into the Red Valley, except colithic rock on a floor of Spearfish formation . iron oxide. At various times, which were not syn­ where it is interrupted by the local uplift of the forms the sharp knob 2 miles southeast of Sheep Cambrian submergence. One of the notable chronous throughout the region, accumulation of Black Buttes. Mountain. events of early North American geologic history clay was interrupted by chemical precipitation of North of Green Mountain there is a broad, shal­ The Bear Lodge intrusion is mainly a dome was the wide expansion of an interior sea over comparatively pure gypsum, free from mechanical low syncline occupied by Spearfish red beds, with somewhat elongated from northwest to southeast, the central-western region. The submergence sediment, in beds ranging in thickness from a few a steep rise on the flanks of the Bear Lodge uplift with the strata dipping away regularly on the sides, reached the Rocky Mountain province during inches to 30 feet. It is believed that these beds to the west. The Green Mountain dome is a local the dips being steepest on the west. Small masses the early Cambrian and for a time the central 'are the products of evaporation during epochs of feature, soon lost to the south, but repeated again of Deadwood sandstone have been included in the portion of the Black Hills remained as one of the little or no rainfall and consequently of tempora­ on a smaller scale in Lime Buttes and Gypsum igneous mass and there has been minor intrusion at islands rising above the waters. From the ancient rily suspended erosion; otherwise it is difficult to Buttes, east of which there is a broad, shallow higher planes also. At its southern end, just north crystalline rocks streams and waves gathered and understand their nearly general purity. The syncline. In Black Flat the dips are very gentle of the town of Sundance, is a small adjoining dome, concentrated sands and pebbles, which were depos­ Spearfish red beds have been supposed to rep­ to the west. South and west of Sundance the dips mainly exhibited in the Minnekahta limestone, ited as a widespread sheet of sandstone and con­ resent the Triassic, but there is no direct evi­ are low for some miles and there is a low anticline out of the sides of which igneous rock has broken glomerate, partly on sea beaches, partly in shallow dence of this, and in part at least they may be which passes near the west line of range 63, crosses along two lines of intrusion. A small branching waters offshore, and partly in estuaries. Num­ Permian. Their deposition appears to have been Inyankara Creek 4 or 5 miles northwest of Inyan­ dome of Minnekahta limestone rises locally east erous exposures of these sediments, containing followed by extensive uplift without local struc­ kara Mountain, and merges into a general mono­ of the main Bear Lodge uplift at the forks of much local material, abut against the irregular tural deformation, but with general planation and cline of southwesterly dips in the vicinity of the Rocky Ford Creek, R. 62 W. At Sheep Moun­ surface of the crystalline rocks which formed the occasional channeling, which represents a period of Crook-Weston county line. West of this anti­ tain the beds uplifted by the Bear Lodge intrusion shore. Subsequently, the altitude being reduced Triassic time of unknown duration, and^was suc­ cline there are gentle westerly dips that extend are steeply upturned and profoundly faulted, evi­ by erosion and the area possibly being lessened ceeded by the deposition of later Jurassic sediments. along the northwest side of the quadrangle, but dently by the intrusion of igneous rock, which has by submergence, the islands yielded the finer- Jurassic sea. In the Black Hills region the steepen considerably along the west slope of the lifted a small block of Deadwood formation nearly grained muds now represented by the shales Jurassic was a period of varying conditions, shal­ Bear Lodge uplift. 1500 feet against Minnekahta and underlying which occur in the upper portion of the Cam­ low and deep marine waters alternating. The Laccolithic uplifts. The most striking struc­ formations. The fault passes along the western, brian in some areas. In many regions the land materials are nearly all fine grained and indicate tural feature of the quadrangle is the presence of southern, and eastern base of the mountain (see surface of crystalline rocks was buried beneath the waters without strong currents. In the southeast­ a number of laccolithic domes. These are nearly fig. 6, on the illustration sheet) and to the north sediments. ern Black Hills region some of the earliest deposits all of very local extent, and vary in size with the gives place rapidly to a steep-^ided flexure. The Ordovician- conditions. From the are thin masses of coarse sandstone, indicating shore mass of igneous rock to which they are due. The igneous mass, which probably underlies the uplifted close of Cambrian to the beginning of Carbon­ conditions, but generally the Spearfish red beds are Nigger Hill uplift is almost a perfect dome in block, does not appear, but there is a small sheet iferous time the Black Hills area presents a scanty overlain by shale which was deposited in moder­ shape, beginning with gentle dips in the Min­ and dike in the Deadwood sandstone near the base , the Ordovician, , and ately deep water. This shale is followed by the nelusa formation. These become steeper on the of the mountain on its south side. Immediately Devonian being absent to the south, and only a ripple-marked sandstone, evidently laid down in flanks of the uplift and apparently materially west of the mountain and fault there is a shallow portion of the Ordovician (the Whitewood) being shallow water and probably the product of a time dimmish over its top, judging by the attitude of syncline containing a small mass of Minnekahta present in the north. This is probably due to the when sedimentation was in excess of submergence, the Deadwood sandstone. Owing to the some­ limestone. fact that during these periods there was in this if not during an arrest of submergence. The red what irregular intrusion of the igneous rock in whole region an extensive but very shallow sea, or color of the upper part of the medial sandy series in GEOLOGIC HISTORY. the lower sedimentary formations of this uplift, land so low as to leave no noticeable evidence of some portions of the Black Hills appears to show there is great irregularity in the structure of the The general sedimentary record. The rocks erosion. Whether it remained land or sea, or a transient return to arid conditions similar to those lower beds and fragments of them are completely appearing at the surface within the limits of alternated from one to the other condition, the under which the Spearfish formation was laid down. inclosed in the igneous rock. In some cases por­ the Sundance quadrangle are mainly of sedimen­ region shows no evidence of having undergone An extensive marine fauna and limestone layers in tions of the formations have been carried above tary origin that is, they were deposited by water. any considerable uplift or depression until early the upper shales of the Sundance formation indi­ the present level and have been removed by ero­ They consist of sandstone, shale, limestone, sand, in Carboniferous time, when there was a decided cate that deeper water followed. After this stage sion, a feature which is particularly marked at loam, and gravels, all presenting more or less vari­ subsidence, which established relatively deep water marine conditions gave place to fresh-water bodies, the eastern end of Cement Ridge, where for some ety in composition and appearance. The principal and marine conditions, not only over the Black probably through widespread uplift. The first miles the porphyry is in contact with the Minne­ materials of which they are composed were orig­ Hills area, but generally throughout the Rocky product was the thick body of fine sand of the lusa sandstone. Elsewhere there is a complete ring inally gravel, sand, or mud derived from the waste Mountain province. Unkpapa sandstone, now a prominent feature in of the Pahasapa limestone around the uplift, with of older rocks, or chemical precipitates from salty Carboniferous sea. Under the marine conditions the eastern portion of the Black Hills, but thinner radial dips ill all directions, steep on the flanks waters. There are also igneous rocks which have that prevailed during early Carboniferous time or absent elsewhere. of the igneous rock but of less amount elsewhere. been intruded in a molten condition, and two of there were laid down calcareous sediments, which Cretaceous seas. During the Cretaceous period The dips on the north side of the laccolith are 10°, the uplifts reveal small areas of the floor of schists are now represented by several hundred feet of deposits of various kinds, but generally uniform on its west side 40° in places, and on its southwest and granites on which the sedimentary rocks lie. nearly pure limestone, the greater part of which is over wide areas, gathered in a great series, begin­ side from 14° to 25°. Their diminution to low These rocks afford a record of physical geography known as the Pahasapa limestone. As no coarse ning with such as are characteristic of shallow angles farther away is shown by the long exten­ from middle Cambrian time to the present. The deposits of this period are found, it is probable that waters along a coastal plain, passing into sedi­ sion of the Pahasapa down Spottedtail and Sand composition, appearance, and relations of strata no crystalline rocks were then exposed above water ments from deep marine waters, and changing creeks and Rattlesnake Canyon. The limestone is indicate in some measure the conditions under in this region, although elsewhere the limestone or toward the end to fresh-water sands and clays nearly flat north of the Needles, where the igneous which they were deposited. Sandstones ripple- some stratigraphic equivalent was deposited imme­ with marsh vegetation. The first deposits now rock cuts across it. marked by waters and cross-bedded by currents diately upon them. In the latter part of the Car­ constitute the Morrison formation, a widespread The regular but sharply uplifted dome of the and shales cracked by drying on mud flats are boniferous the conditions were so changed that fine mantle of sandy shales, which is absent to the Bald Mountain limestone area is apparently due to deposited in shallow water; pure limestones gen­ sand was brought into the region in large amount southeast, although probably originally deposited laccolithic intrusion, but none of the igneous rock erally indicate open seas and scarcity of land- and deposited in thick but regular beds, apparently there to a greater or less thickness and then is yet exposed. Strawberry Mountain is of similar derived sediment. The fossils that the strata with much calcareous precipitate and more or less removed by erosion in consequence of slight but character a steep-sided dome affecting the Spear- .contain may belong to species known to inhabit ferruginous material, as is indicated by the color of widespread uplift. The extent of this degradation fish, Minnekahta, Opeche, and Minnelusa forma­ waters which are fresh, brackish, or salt, warm or many beds of the Minnelusa formation. Minne­ is not known, but it caused a general- erosional tions, but not exhibiting igneous rock. Green cold, muddy or clear. The character of the adja­ lusa deposition is believed to have been followed unconformity at the base of the Lakota sandstone, Mountain is a beautifully symmetrical dome with cent land may be shown by the character of the by an uplift which appears to have resulted in the next succeeding deposit, which is of coastal and possibly estuarian origin. This formation consists south and remain on some of the high buttes varieties of soil do not coincide with the bound­ Mountains. Sites for dams may be found on mainly of coarse sands spread by strong currents to the north. There was probably slow but aries of the rock formations are in the river bot­ many canyons, which are wide enough and have in beds 30 to 40 feet thick, but it includes several continuous uplift during the Loup Fork epoch, toms, in the sand dunes, in the areas of high-level sufficiently low declivity to hold moderate amounts thin partings of clay and local accumulations of and materials were contributed by the higher gravels, in the smaller valleys, and on steep slopes of water. vegetal material. The next deposit was a thin cal­ slopes of the Black Hills at that time, but where soils derived from rocks higher up the slope MINERAL RESOURCES. careous series, represented by the Minnewaste lime­ whether the formations were ever deposited in have washed down and mingled with or covered stone, which was apparently laid down only in a the immediate vicinity is not ascertained. the soils derived from the rocks lying immediately Gold. Small amounts of gold have been local basin in the southern portion of the Black Quaternary uplift and erosion. During the beneath. Soils of this class are known as over- obtained from igneous rocks or the adjoining <-> a ' n Hills and is not present in the Sundance quadran­ early part of the Quaternary period there was placed, and a special map of large scale would be strata in mines about Welcome and in some pros­ gle. This was followed by a thin but widely widespread denudation of the preceding deposits, required to show their distribution. pects in the Bear Lodge Mountains. In the Nig­ extended sheet of clays of the Fuson formation. and many of the old valleys were revived, with Distribution. The arable lands of the Sundance ger Hill uplift gold occurs in the gravels of the After the deposition of these clays there was a much rearrangement of the drainage, which on the quadrangle are irregularly distributed and occur larger streams; in the basal conglomerates of the return to shallow waters and strong currents, as eastern side of the Black Hills was caused mainly on several formations. The most extensive farm Deadwood formation; in veins associated with in Lakota times, and coarse sands of the Dakota by increased tilting to the northeast. This rear­ areas are on the Canyon Springs Prairie and on dikes of igneous rock, especially about Mineral formation were accumulated. At the beginning of rangement has caused several streams, superim­ alluvial deposits in the valleys of Sundance, Hill; and in the pre-Cambrian schists and granite. the Benton there was everywhere in the region a posed upon the Oligocene deposits, to cut across Beaver, Inyankara, Mason, and Skull creeks. Some prospecting has been done in the Carbon­ rapid change of sediment from sand to clay. old divides, in some cases connecting a valley Many areas which have naturally fertile soils are iferous rocks bordering and outside of the Nigger During the great later Cretaceous submergence with its neighbor to the north. Such streams not situated favorably for farming, especially those Hill uplift, apparently with unsatisfactory results. marine conditions prevailed throughout the Ben- flow southeastward in pre-Oligocene valleys for which are at such high altitudes that frosts are too In the igneous area near Welcome there has been ton, Mobrara, and Pierre epochs, and several thou­ some distance and then turn northward into prevalent. Scant rainfall is a great handicap in extensive prospecting, especially in Mineral Hill sand feet of clay were deposited. In Benton time canyons of post-Oligocene age, leaving numerous many parts of the area, especially in localities and Nigger Hill. In the former there are several there were occasional deposits of sand, two of them elevated saddles to mark the southeasterly course where there is not sufficient running water for tunnels 200 feet or more in length, but their in the latter part of the epoch that were general of the old valleys. Some of the offsetting in the irrigation. results were not encouraging. A stamp mill was over the greater part of the Black Hills region, and present drainage has been largely increased by early The soils of the Pahasapa limestone are very erected at Welcome, for the purpose of crushing one earlier that was local and produced the lenses Quaternary erosion and recent stream robbing. fertile, especially on the gentler slopes and in some supposed free-milling from the Inter- of sandstone which are now found in the vicinity There was apparently still further uplift in late the valleys, where there is admixture with more Ocean mine, situated on the west slope of Mineral of Newcastle and elsewhere. Another marked epi­ Quaternary time, for the present valleys, below the or less alluvial material, as in the valley of Cold Hill, but most of the found was not only of sode was that which resulted in the general deposi­ level of the earlier Quaternary high-level deposits, Springs and Little Spearfish creeks, and these val­ low grade, but proved to be refractory. In recent tion of the thin Greenhorn limestone in the middle seem to be cut more deeply than they would be leys are farmed to some extent. On account of developments on the west side of Mineral Hill good of the Bentoii sediments. The shale of the Benton in simply grading their profiles to the level of the high altitude, however, only the more hardy assays have been obtained and it is claimed that was followed by several hundred feet of impure Missouri and Cheyenne rivers. Wide, shallow products, such as oats, can be raised. The Minne­ ore bodies of considerable size are exposed, espe­ chalk, now constituting the , valleys have developed in the soft deposits, and lusa formation produces sandy soils which are rela­ cially in one narrow vein which shows free gold and this in turn by over 1200 feet of , canyons of moderate extent and depth in the tively barren, but in the valleys of Soldier and in its walls. Some of the prospecting near Wel­ deposited under very uniform conditions; but these harder rocks. Erosion has progressed without Cold Springs creeks and in Red and Grand can­ come has been done in the altered syenitic rock latter formations are not now present in the Sun- aggradation in the main, but in some cases, yons, where there is also more or less alluvium, along Sand Creek, which is considerably broken dance quadrangle. The retreat of the Cretaceous with the shifting of channels, there have been there are a number of small farms. The Minne- and seamed and contains pyrite, both in seams^ sea corresponds with the Fox Hills epoch, during accumulations of local deposits on small terraces kahta limestone yields a thin but fertile soil and and scattered through the rock. The pegmatite which sands were spread in an extensive sheet over at various levels. presents broad areas of level lands, which at some about Nigger Hill has been prospected for gold, the clay beds, and resulted in the development of . localities are well suited for farming. Williams though has been the principal mineral looked extensive bodies of brackish or fresh water, which Divide and Canyon Springs Prairie are notable for here. The Deadwood sandstones are gold SOILS. received the sands, clays, and marsh deposits of the examples, especially the latter, which is now occu­ bearing in places in the northern Black Hills, Laramie. Whether these two last-named groups of Derivation. The soils in this region are closely pied by many farms that obtain good returns of especially in the conglomerate at their base known sediments were deposited over the area now occu­ related to the underlying rocks, from which they the more hardy crops. These areas have less frost as "cement" by the miners. At many points in pied by the Black Hills is. not definitely known, are residual products of decay and disintegration than the higher district farther east; they also the Welcome uplift both the "cement" and the but it is possible that they were, as they are except when they are formed as alluvial deposits appear to obtain much more frequent rainfall dur­ rocks above it have been prospected without find­ upturned around two sides of the uplift. in the larger valleys. In the process of disinte­ ing the summer than the adjoining valleys on the ing valuable ores, so far as known. Early Tertiary mountain growth. The Black gration residual soil develops more or less rapidly west, and the soils are more retentive of moisture The gravels of Sand, Bear, and Beaver creeks Hills dome developed early in Tertiary time or on the several rocks of the region according to from both snows and rains. The increased rainfall and their more important tributaries have been possibly in latest Cretaceous time to a moderate the character of the cement holding the particles is probably due to condensation from moisture- mined extensively for gold. The work has been height, and the larger topographic outlines of the together. Siliceous cement dissolves most slowly, bearing breezes which meet the cooler air currents done mainly within the limits of the porphyry region were established before the Oligocene epoch, and rocks in which it is present, such as quartzite in ascending the slopes of the plateau. In the area, but also, considerably outside of it, the the dome being truncated and its larger old valleys and sandstones, are extremely durable and produce vicinity of Boyd part of Canyon Springs Prairie gravels on Sand Creek having been mined for a excavated in part to their present depths. This is but a scanty soil. Calcareous cement, on the other is occupied by a thin layer of Spearfish red beds distance of-about 10 miles below Welcome. indicated by the occurrence in them of White River hand, is more readily dissolved by water contain­ thinly mantled by upland gravels and loams, and At the present time mining on these streams is (Oligocene) deposits, even in some of their deeper ing carbonic acid, and on its removal clay and on such prairies are found the most favorable con­ being carried on at a number of places in a desul­ portions. Where the great mass of eroded mate­ sand remain, to form often a deep soil. If the cal­ ditions for agriculture. The Red Valley has broad tory manner by a few miners, either singly or in rial was carried is not known, for in the lower careous cement is present in small' proportion areas of barren Spearfish deposits, but on some of groups of two or three. The methods used hi lands to the east and south there are no early only, it is often leached out far below the surface, the broader flats there is more or less mingling of washing the gravels are crude and hampered by Eocene deposits nearer than those on the Gulf the rock retaining its form, but becoming soft and wash and alluvium, and good soils result. This the scarcity of water. This lack of sufficient water coast and Mississippi embayment. porous, as in the case of the Minnelusa sandstone. is notably the case in Black Flat on which there is the chief difficulty in handling the gravels and The igneous intrusions probably occurred during If, as on the limestone plateaus, the calcareous are a number of good farms. The Sundance for­ makes the work unprofitable except when it is early Tertiary time and in connection with the gen­ material forms a greater part of the rock, the insol­ mation makes soils that are usually fertile, but it done on a small scale. The gravels have been eral uplift. In portions of the area the igneous uble portions collect on the surface as a mantle, generally lies on dry slopes, where the rainfall in worked for many (since 1875) and appear to rocks cut or uplifted Upper Cretaceous formations varying in thickness with the character of the most seasons is not sufficient for raising crops. be now largely exhausted, though they still yield as young as the Benton, and the fine-grained limestone, being thin where the latter is pure, but The Lakota and Dakota sandstones jfield relatively small returns to the men engaged in working them. deposits of the Niobrara and Pierre in the vicinity often very thick where the rock contains much barren soils and are usually so dry and so situated Coarse-gold nuggets have been occasionally found. indicate that there was no interruption in the sedi­ insoluble matter. The amount of soil remaining topographically that they are not favorable for Probably all the rocks within the porphyry area, mentation until, at the earliest, the later part of Cre­ on the rocks depends on erosion, for where there farming. The soils of the Graneros shale are thin Cambrian, pre-Cambrian, and igneous, contribute taceous time. As fragments of the igneous rocks are slopes the erosion is often sufficient to remove and poor, and they are not farmed at all in the some gold to these placers, though the chief supply occur in the early Oligocene deposits, it is evident the soil as rapidly as it forms, leaving bare rock Sundance quadrangle. very likely comes from the Algonkian schists. In that they were intruded prior to the Oligocene. surfaces. Crystalline schists and granitic rocks Along Sundance, Rocky Ford, Beaver, Inyan­ working these gravels for gold a considerable quan­ Oligocene fresh-water deposits. Oligocene depos­ decompose mostly by hydration of a portion of kara, Mason, Skull, and Oil creeks there are allu­ tity of the heavier and the less common minerals its were laid down by streams and in local lakes the contained feldspar, and the result is usually a vial deposits of considerable extent, often half a of the pegmatite and schists have been obtained, and finally covered the country to a level now far mixture of clay, quartz grains, mica, and other mile wide. There are several farms around Sun- and among those which are listed are magnetite, up the flanks of the Black Hills. Erosion has materials. Shales are disintegrated in consequence dance and down the valley for the next 7 or 8 hematite, cassiterite, columbite, tantalite, wolfram­ removed them from most of the higher regions of changes of temperature, by frost, and by water, miles, and at intervals along Rocky Ford Creek ite, garnet, zircon, and topaz. where they formerly existed, especially along the and thus by softening and washing give rise to soils. and the other creeks above enumerated. The soils In the Bear Lodge uplift there has been much western side of the hills, but in the vicinity of Lead If they are sandy, sandy soils result, and if they of these are usually moderately rich and often con­ prospecting, but no notable amount of gold has small outliers remain at an altitude of over 5200 are composed of relatively pure clay, a very clayey tain and retain moisture in considerable amount. yet been produced. The igneous rock contains feet, and on the north end of the Bear Lodge soil is the product. The character of the soils thus These farms are within reach of irrigation, for they gold, but the amount is very small, even along the Mountains they are seen 1000 feet higher. In derived from the various geologic formations being lie along creeks from which water has been taken few faint zones of mineralization which have been many places on the slopes of the uplift there is known, their distribution may be approximately at some points for use in this way, notably at discovered. In Ruby Canyon the lower sand­ clear evidence of superimposition of drainage due determined from the map showing the area! geol­ the Holwell ranch. Water could be stored at stones and conglomerate of the Deadwood forma­ to a former capping of Oligocene formations. ogy, which thus serves also as a soil map. It must many points in the Sundance region by building tion have been found to be sparingly gold bearing Middle Tertiary mountain growth. Just after be borne in mind that some of the geologic forma­ dams across the canyons, and supplies could be and a small mill was erected for testing them, but the Oligocene epoch the dome was raised several tions present alternations of beds of various mate­ thus preserved for irrigation. The rainfall aver­ the results appear not to be very encouraging. hundred feet higher and was more extensively rials, shales and sandstones, for instance, alternating ages about 14 inches a , and if this supply Tin. As was noted above, cassiterite (stream eroded. No representatives of the succeeding with limestone. These give abrupt transitions in could be suitably preserved and used it would suf­ tin) is one of the constituents of the placer gravels, Loup Fork group the Arikaree and Ogalalla the character of their disintegration products, and fice for extensive irrigation. Large amounts of particularly in Bear Gulch, where it occurs in con­ formations have been discovered in the imme­ soils which differ widely in composition and agri­ rock are available for the building of dams, siderable quantity. In the earlier days of mining diate vicinity of the Black Hills, but they cultural capabilities occur side by side. The only especially along the slopes of the Minnekahta in this region, before the nature and value of the are extensively developed in Pine Ridge to the areas in which the boundaries between different limestone and the flanks of the Bear Lodge mineral were known, it was thrown aside as worth- Sundance. 12 less, or worse than worthless, for on account of its sandstone of the basal bed of the Lakota forma­ Analysis ofbentonite. Inyaiikara two springs of large volume issue from high gravity it interfered with the separation of tion. The bone burns well, but leaves a large Per cent. these beds and enter Inyankara Creek, sustaining Silica...... 63.25 the gold from the gravels. Now that it is known amount of white ash, and it slacks readily. The Alumina...... 12.63 in large part the flow of that stream. to contain tin, it is carefully saved and occasional dip is very gentle to the southwest. About 60 feet Magnesia...... 3.97 UNDERGROUND WATERS. above the main deposit is a smaller one, varying Lime...... 4.12 small shipments of the ore are made to England. Potash...... 3.55 At the time the region was visited a little more from 1^ feet to 3 feet in thickness, composed of a Iron oxide...... 3.70 Very little progress has been made in exploring than 2 tons had been sold at four cents a pound mixture of clay, sand, and coal, too impure for fuel. Water...... 6.91 for underground waters in the Sundance quadran­ Sulphuric acid...... 1.58 and shipped to England. The following analyses of the coals of the main gle, but doubtless some of the porous sandstones The stream tin occurs in small, angular grains, bed have been furnished through the kindness of Limestone. The extensive exposures of Paha- which underlie the greater part of the area at various seldom reaching one-half inch in diameter. The Mr. Bidwell, of the Chicago and Northwestern sapa and Minnekahta limestones afford an horizons can be expected to yield water supplies to grains, though generally of nearly pure cassiterite Railway Company: unlimited supply of rock for burning into lime borings. The Dakota and Lakota sandstone beds (oxide of tin), occasionally have small fragments or other purposes. The purity is sufficient for which cross the southwest corner of the quadrangle of their parent rock adhering to them. The peg- Analyses of coal from Holwell ranch, Wyoming. all ordinary uses, but aside from slight local needs yield a water supply to many wells in various por­ matitic dikes of Nigger Hill and its vicinity are / for lime the formations are of little economic tions of the plain immediately adjoining the Black Sample S imple Sample No. 1, No. 2, No. 8, Hills, and at Jerome, just beyond the margin of the sources of the stream tin, and it is the near­ from from bitumi­ importance. large small nous shale the quadrangle, there is an excellent flowing well ness to their source that accounts for the angular, tunnel. tunnel. parting. WATEE SUPPLY. unrolled character of these gravels, since in general which derives its water from this source at a depth Per cent. Per cent. Per cent. SURFACE WATERS. they have not been transported far enough by the 10.45 11.00 4.77 of 520 feet. At Cambria, a few miles south of the streams to be appreciably rounded.. Volatile combustible matter. . . 39.51 41.16 30.85 The amount of flowing water on the Sundance quadrangle, a well has been sunk into the Pahasapa So far as known only the pegmatite bodies on Fixed carbon ...... 41.87 40.37 25.69 quadrangle is not great, and many of the draws, limestone and at a depth of 2115 feet obtains a large the ridge on which Nigger Hill is situated are tin Ash ...... 8.17 7.47 38.69 valleys, and canyons are dry except when there supply of water of excellent quality which is now bearing, those to the west of this ridge, including Total...... 100.00 100.00 100.00, is rain or the snows are melting. None of the used at Cambria and Newcastle. It is possible or its westerly spurs, containing no cassiterite, or at 3.63 4.03 2.40 streams has a continuous flow from head to mouth, even probable that this water horizon furnishes the least no appreciable quantity. The tin ore occurs and many of them appear at intervals in springs large springs in Sand Creek below the mouth of in irregularly shaped grains of variable size and is In the same quarter section, about 500 feet west, or bottom seeps, which supply waters that flow for Red Canyon and some of the excellent springs in very irregularly distributed through the rock. is another adit, 100 feet long, on the main coal greater or less distances and then are either evap­ the upper valleys of Cold Springs and Stockade The pegmatites are somewhat auriferous and have bed, which is here 6J- feet thick and does not con­ orated or pass underground. Often the water in Beaver creeks. In consideration of the proba­ been mined for both gold and tin. At the time tain the bony deposit near its bottom. The coal a valley will consist of a series of pools. Probably bility that this is a continuous water horizon, its this region was visited the shafts sunk in the is very firm and of excellent quality. It is over­ the most vigorous streams are Stockade Beaver and position is given on the artesian-water map in this granite were closed and the workings were inac­ lain by 3 feet of light-colored sandy clay capped Little Spearfish creeks, which are fed by numerous folio. In the Deadwood sandstones which under­ cessible. The pegmatites of Nigger Hill are by a thick mass of smooth, uniform sandstone, springs near their heads and receive other springs lie the Pahasapa limestone there are prospects for believed to be inclusions in the porphyry, and if which forms a good roof. The floor is. a very at intervals in their flow. They are both of about water supplies also, but probably the horizons would this is true the depth to which they extend is hard sandstone, as in the other adit. Two miles to the same size, and in ordinary seasons carry about lie too deep to be available. Their location is shown limited. To judge from the amount of cassiterite the southwest, on sec. 12, T. 47, R. 63, near the 10 second-feet of water. Sundance Creek has more in the cross sections. The sandstones of the Min­ in the rocks and from their probable limitation in north line of the northwest quarter, is another adit or less flow from Sundance nearly to the north edge nelusa formation are porous at the surface and depth (as inclusions in the porphyry), as well as 100 feet long. The same bed as exposed here is of the quadrangle, where the stream sinks. Sand appear to have a constitution favorable for the from the practical results thus far obtained in about 5 feet 4 inches thick, and it is nearly all Creek in its upper portion has a small flow, which flow of underground waters, but in the well at attempting to work them for tin, it is probable pure coal of more than usual hardness. The roof has been extensively used for working the placer Cambria, where the entire thickness of the for­ that their yield will never be great. If they are at this place is sandstone without the intervening deposits, but sinks near its junction with Cold mation was penetrated, the sand grains of the made to pay at all, it will probably be only when shale. West of Holwell ranch, on the west side Springs Creek and emerges again in extensive rocks were so firmly cemented by carbonate of worked with great economy and on a comparatively of Skull Creek, coal has been exposed at one or springs at its junction with Red Canyon. The lime that the porosity is very slight. There is small scale. two points at the base of the sandstone cliffs, but latter, above Sand Creek, contains little water no evidence of water in the Minnekahta lime­ Copper. Thin veins and seams of copper occur its thickness,and extent have not been ascertained, and that mainly in local pools. Beaver Creek, in stone, although in places it is cavernous and at at various places in the Bear Lodge , and although apparently the deposit in this locality is the Nigger Hill district, is a small flowing stream some localities small springs emerge from it. The at a locality 1 mile north of Warren Peaks these of diminished thickness. for part of its course, but seldom contains more Spearfish red beds have not yielded water and their have been worked to-some extent. Red oxide and On the ridge west of Inyankara Mountain it is than half a second-foot of water. Oil Creek and constitution is not favorable for its storage or flow. malachite are the principal minerals, and a load of reported that the coal deposit near the base of the Skull Creek flow only in parts of their courses The vigorous springs which issue from the forma­ moderately good ore has1 been obtained by care­ Lakota formation has a thickness of 9 feet includ­ and in small volume. The largest amount of tion southeast of Inyankara probably come through fully sorting a large amount of excavated material. ing a number of layers of shale and bone. West water in Skull Creek is found in the vicinity of crevices from the Minnelusa sandstone a few hun­ The ore is in joint planes and some decomposed of Sundance the principal coal opening exposes a Holwell ranch, where it flows nearly all the year dred feet below. The Sundance and Morrison for­ portions of the rock, and though there is a con­ bed somewhat over 4 feet in thickness and of excel­ about 2 second-feet. Soldier Creek has pools at mations consist in part of clay /and fine-grained siderable5 amount in sight, it is too widely scattered lent quality. intervals and is nearly always dry in its lower por­ sediments which do not contain much water. to promise profitable workings. Gypsum. The Spearfish red beds carry deposits tion. Cold Springs Creek has extensive springs The Dakota and Lakota sandstones probably Silver-lead. In the limestone in the central por­ of gypsum a hydrous sulphate of lime through­ near the State line and water flows in it to the contain considerable water and may be depended tion of the Black Buttes there are irregular deposits out their extent, and often the mineral occurs in Crook-Weston county line and sometimes to below upon for an underground water supply/in the of silver-bearing lead ore which have been pros­ thick beds. These are relatively pure, and if they McCready's ranch, but for the remainder of its southeast corner of the quadrangle, as is demon­ pected to some extent. The principal opening is a were nearer to good markets, the deposits would be course it is dry or presents only occasional pools strated by the well at Jerome, referred to above; tunnel on the north side of the road which crosses of value. The only commercial operations so far except in times of freshet. The various canyons and, as the sandstone ridge has considerable eleva­ the buttes from east to west. A small amount of have been at Hot Springs and Sturgis, on the oppo­ east of Cold Spring^ Canyon ordinarily are dry, tion, flowing wells may be expected. galena has been obtained, which is said to assay site side of the Black Hills, where the expense of or only occasionally contain water pools. Inyan­ The well at Jerome begins in Graneros shales fairly high in silver. The ore occurs in limestone taking the product to market has proved a serious kara Creek usually contains water below Inyan­ and extends into Lakota sandstone. The Bur­ along the phonolite contact. hindrance. When the gypsum is calcined at a low kara, but the amount is very small and there are lington and Missouri River Railroad Company Coal. In the basal part of the Lakota - sand­ red heat, to drive off the chemically combined generally some points at which there is no flow. has kindly furnished the following analysis of stone there are local deposits of coal, an extension water, and ground, the product is plaster of Paris. Beaver Creek, in the southwest corner of the quad­ artesian water from this well: northward of the Cambria coal field. The deposits The area of the Spearfish formation is indicated on rangle, does not flow but has a number of water Analysis of water from Jerome, S. Dak. Grains per are not continuous and thicken and thin irregu­ the economic geology map. holes which last far into the summer. Iron and gallon. larly. Owing to the talus of sandstone blocks Bentonite. This mineral is a hydrous Turner creeks contain a small amount of water in Sodhim chloride...... 0.7 Sodium sulphate ...... 20.8 which accumulates on the slopes along the Lakota- of alumina with some other components in small pools. Magnesia sulphate...... 7.0 Morrison outcrop there is great difficulty in explor­ proportions, and is valuable on account of its high There are some notable springs in the Sundance Lime sulphate...... 2.3 ing the coal horizon. On the economic-geology absorbent qualities, as it has the capacity of absorb­ quadrangle, the largest of which is on Sand Creek Magnesia carbonate ...... 5 map is shown the outcrop line of the horizon, but ing three times its weight of water. It occurs in 2 miles below its junction within Red Canyon. TIMBEE. at only a very few points was it found bare of considerable abundance in the Newcastle quad­ From these there issues, within an area of a half The Black Hills Forest Reserve extends into the talus, and even at these localities the coal may rangle and has been mined to some extent three- acre, a large volume of water, estimated at about Sundance quadrangle, including the greater part of have weathered or burned out and the sandstone fourths of a mile east of Pedro switch, a mile west 12 second-feet, apparently derived from an under­ the South Dakota area and parts of range 60 in roof closed down. of that switch, and at a point 3^- miles northwest ground flow rising through the Pahasapa lime­ townships 49 to 52 in Wyoming. In this district Coal has been mined to some extent for local of Osage, on the east side of the railroad track. stone, which underlies a wide region farther south there are heavy bodies of pine timber, some of it use 2 miles southeast of Holwell ranch and at sev­ The material is a light-gray, fine-textured, soft, and into which the upper waters of Sand and of large size. Some pine is also found west to eral localities west of Sundance. Recently some massive stone, but locally it occurs as a light other creeks undoubtedly pass underground. Two Cold Springs Creek Valley and to Black Buttes. openings have been made 3 miles west of Inyan- powdered substance resembling white corn meal, springs of considerable volume issue from the; Scattered bodies of pine and numerous individual kara Mountain, exposing a thick deposit. The in beds lying between thin layers of reddish-brown talus of the northwest corner of Sundance Moun­ trees grow on portions of the plateau and slopes main opening southeast of Hoi well's is on the concretionary material. Near Pedro it is 12 feet tain, which afford a supply for the town of Sun- of Minnekahta limestone and on Lakota sandstone southeast quarter of sec. 31, T. 48, R. 62. An thick and occurs in the steep-dipping transition dance. In the Nigger Hill uplift, in the Bear and portions of the Graneros shale. Strawberry adit has been run about 115 feet, exposing a face beds at the top of the Niobrara formation. Near Lodge Mountains, and in Black Buttes there are and Inyankara mountains bear a small amount of of coal 8^ feet thick, comprising 5 feet of hard, Osage the bed is about 4 feet thick and occurs in numerous small springs. At Buckley's ranch an timber, mostly of small size, and on the slopes of pure coal, 1^ feet of bone merging into cannel coal, the Graneros shale, the dips of which are so low excellent spring issues from the Minnelusa sand­ the Bear Lodge Mountains there is an extensive and, at the base, about 2 feet of very hard, pure that the deposit is exposed over a considerable area. stone. There are several small springs in the pine growth. About Warren Peaks there is a coal, which is particularly valuable for black­ The mineral has been used with success in the man­ Pahasapa limestone in the southeastern part of central area which is treeless, owing, it is said, smith's use. The upper coal contains considerable ufacture of soap, but it has proved most valuable the area, notably along Little Spearfish Creek and to heavy fires which once burned out the timber. sulphur, a mineral which is of infrequent occur­ as a packing for a special kind of horseshoe, and Stockade Beaver Creek. Small springs are found The Red Valley is in greater part treeless, except rence in the lower bed. Over the coal are about 2 as a diluent for certain powerful drugs sold in pow­ at intervals in the Minnekahta limestone and the for a few cotton woods and an occasional pine or feet of sandy shales overlain by hard sandstone dered form. An analysis made by the Wyoming Lakota sandstone. The Spearfish red beds seldom cedar on some of the steeper slopes. which makes an admirable roof. The floor is State. School of Mines is as follows: yield water, but at a point 2 miles southwest of June, 1904.