University of California Berkeley DEPARTMENT OF THE INTERIOR-U. S. GEOLOGICAL SURVEY CHARLES I). WALCOTT, DIRECTOR GEORGE HOMA1STS ELDRIDG-E EXTRACT FKOM THE SIXTEENTH ANNUAL REPORT OF THE SURVEY, 1894-95 PART IT-PAPERS OK AN' ECONOMIC CHARACTER \\ A s ii i N <; GOVERNMENT PRINTING OFFICE 1895 A GEOLOGICAL RECONNAISSANCE ACROSS IDAHO. GEORGE H. ELDRIDGE. 211 CONTENTS. Page. Prefatory 217 Topography 217 Drainage system of the Snake River 217 Drainage system of the Columbia River 218 Mountains 219 Canyons anil iutermontaue valleys 220 Glacial action 223 The formations 224 Granites and metamorphic rocks 224 Archean 224 Algonkian 225 Unaltered sedimentary rocks 226 Paleozoic 226 Cenozoic 230 Pleistocene 234 Eruptive rocks 234 General structural features 948 Mining districts 250 Gold and silver 250 Bear Creek district 250 Atlanta district 253 Sheep Mountain district 258 Yellow Jacket district 259 Wood River district 264 Silver City district 271 Placers 273 Coal 274 Salmon City Valley 274 Payette Valley 274 Agriculture 275 213 }i A, y\ ' ILLUSTRATIONS. Page PLATE XV. General map of the State of Idaho 216 XVI. Sketch-map of Atlanta district 254 XVII. Skotch-map of Wood Rivet district 264 FIG. 38. Section of Tertiary gravels at Lemhi placer mine 233 39. Sketch-map of Bear Creek district 251 40. Sketch-map of Yellow Jacket district 260 41. Sketch-map of Silver City 272 215 U. S. GEOLOGICAL SURVEY GENERAL MAP OF SIXTEENTH ANNUAL REPORT PART II PL. XV STATE OF IDAHO. A GEOLOGICAL RECONNAISSANCE ACROSS IDAHO. BY GEOKGE H. ELDEIDGE. PREFATORY. The following pages contain an account of a reconnaissance across Idaho on a northeast line through Boise and Salmon City. The work was authorized as a preliminary to the future geologic study of the State. In its prosecution the available maps have been the atlas sheets of the Survey, between the meridians of 114 and 117 west and the parallels of 43 and 44 30' north; the maps of the United States Gen- eral Land Office; and a rough sketch-map of Lemhi County. 1 The general map of the State accompanying this report is a compilation from the above sources. The fossil plant remains referred to in the text have been identified by Mr. Knowlton ; the mollusks, by Messrs. Walcott and Stanton ; the eruptives, by Mr. Cross ; and the chemical analyses have been made in the laboratory of the Survey. TOPOGRAPHY. The broad topographic features of Idaho are the drainage systems of the Snake and Columbia rivers, with a vast arid plain along the former stream; a labyrinthine mass of rugged mountains northward from the plain; and a succession of desert ranges on the divide between the Snake Kiver and the Great Basin, along the southern border of the State. DRAINAGE SYSTEM OF THE SNAKE RIVER. The Snake Plains, which extend completely across the southern end of Idaho, constitute one of the prominent features of the West. The floor is rolling, consisting of sand and lava underlain by Tertiary sedi- ments several hundred feet thick. These have been cut by the river to a depth of 400 to 1,000 feet, the stream being confined between canyon walls, or lying in a valley with bottom-lands on either side. Following are a few of the determined altitudes of the plains. 2 1 Prepared by Mr. Birdseye, Salmon City, Idaho. ' Chiefly from GanueU's Dictionary of Altitudea, Bull. U. S. Geol. Survey No. 70, 1891. 217 218 GEOLOGICAL RECONNAISSANCE ACROSS IDAHO. Altitudes of localities on the Snake Plains. Feet. Slioshone 3,975 Bliss 3, 269 Glenns Ferry 2, 556 Boise 2,768 Kaiupa 2, 489 Caldwell 2, 374 Weiser 2, 125 Feet. East end of Snake Plains, about . . 5, 000 Camas 4,822 Market Lake 4, 781 Eagle Rock 4,714 Blackibot 4, 505 Pocatello 4, 468 American Falls 4, 343 Minidoka 4, 287 These figures show a. gentle gradient of approximately 3,000 feet in 400 miles, or an average of 7 feet to the mile, a little heavier at the eastern end, a little lighter at the western. Additional elevations along Snake River are: Lewiston, about 1,100 feet; Pasco Junction, Wash., 386 feet; Snake Eiver (mouth), 328 feet. These indicate the maintenance, for the balance of the river's course, from Weiser to the junction with the Columbia, of a gradient of approximately 7 feet. The drainage of Idaho, with the exception of the southeast corner and a narrow strip of 130 miles at the north, is entirely through the Snake Kiver. The copious waters of this stream are derived from the Continental Divide in the vicinity of the Yellowstone Park, from the great mass of mountains north, and from the divide between it and the Salt Lake and Humboldt regions to the south, just beyond the border of the State. The river forms a third of the western boundary of the State, receiving along this portion the Owyhee, Boise, Payette, Weiser, Salmon, and Clearwater rivers, all draining large areas of country. Of these, the Clearwater rises in the Bitter Boot Mountains; the Salmon drains an extensive interior area between the Continental Divide and a parallel range, the Sawtooth, in the middle of the State; the Boise and Payette drain from the latter range west; the Weiser occupies a longitudinal valley parallel with the Snake, from which it is separated by a narrow, rugged range, the continuation of the Seven Devils Mountains; and the Owyhee drains the southwestern corner of the State. The Palouse Eiver, also a tributary of the Snake, drains a small scope of territory just north of the Clearwater, within the western edge of the State. DRAINAGE SYSTEM OF THE COLUMBIA RIVER. From the divide north of the Palouse and the Clearwater the drain- age is to the Columbia, through the Spokane Kiver, Clarks Fork, and the Kootenai, the first of these, with its tributaries, heading within the State, in the Bitter Root Mountains, the others crossing it from Mon- tana. Of the larger lakes in this system, Cceur d'Alene has an altitude above sea-level of 2,150 feet; Pend d'Oreille, 2,080 feet. The distribution and relative areas of the several drainage basins are shown on the general map. ELDEIDOE.] DRAINAGE AND MOUNTAINS. 219 MOUNTAINS. The portion of Idaho lying north of the Snake Plains is an intricate region of lofty mountains and deep canyons. The altitude of the moun- tains varies between 6,000 and 12,000 feet, while 3,000 to 4,000 feet is a common depth for the canyons. The mountains lie either " en masse "- nearly devoid of topographic system or in ranges. The development "en masse" is due to early geologic accidents fracturing, faulting, and folding and subsequent modification by erosion, the region being one of nearly structureless granite of almost homogeneous texture. The range form, on the contrary, is the effect of erosion either upon pro- nounced folds in sedimentary beds and foliated granites or upon a complex in which there existed a difference in the texture and hardness of the rocks, decay and disintegration taking place more rapidly in one than in another. The range of first importance in the topography of Idaho is the Continental Divide. This consists of folded metamorphosed beds quartzites and schists the structural and topographic axes coincid- ing. Second in importance are the ranges dividing the chief drainage basins. Among these are the Sawtooth, in the center of the State, nearly parallel with the Continental Divide; two north and south ranges on the western side of the State the Seven Devils between the Snake and Weiser rivers, and a shorter ridge between the latter stream and the North Fork of the Payette; a transverse ridge of irregular trend extending from the southern end of the main Sawtooth Range to the Continental Divide, and separating the Salmon waters from those of the Snake to the south ; a ridge north of and having the same direction as the last, springing from the Sawtooth in the vicinity of Cape Horn, and constituting the watershed of the main and middle forks of the Salmon; and a range from the Sawtooth west, dividing the Boise and Payette drainage. Of these secondary ranges, the two in the western part of the State are unknown to the writer ; the others were crossed once or twice. The Sawtooth and the northern of the transverse ridges are composed chiefly of granite, with local eruptives, while the southern transverse ridge consists in part of granite and in part of more or less metamorphosed .sedimentary beds cut by eruptives and folded. Subordinate still to the foregoing are the ranges sepa- rating the major water courses within the several drainage basins. Particularly noticeable among these are the Salmon and Lost River ranges and others just to their west, in eastern Idaho, largely composed of folded metamorphic rocks; the north and south ranges between the several forks of the Salmon River, springing from the northern of the transverse ridges and consisting chiefly of structureless granite, except at the east, where quartzites prevail ; the ranges between the Boise and Payette basins, also of granite; and finally the Owyhee Range in southwestern Idaho, consisting ot granite and eruptives. 220 GEOLOGICAL RECONNAISSANCE ACROSS IDAHO. The mountains of purest range type occur chiefly in the eastern part of the State, in proximity to the Continental Divide, in a region of uplifted sedimentaries, altered or unaltered quartzites, schists, and limestones. West of this area, in the region of granite, the ''en masse" form prevails; but the range type is still present in the Owyhee, Boise, Trinity, Sawtooth, and transverse ranges, where the granite is strongly foliated or the rocks are of different texture and hardness and erosion has removed one class faster than another. CANYONS AND INTERMONTANE VALLEYS. Throughout the mountain region are many quite impassable canyons, and others hardly less so. The early prospectors, prior to last year (1894), had made a few passable with trails, and recently the Salmon above Salmon City has been opened by a portion of the State wagon- road. The walls of the canyons are precipitous, sheer drops of 1,000 feet being not infrequent, while the confining slopes rise from 2,000 to 3,000 feet higher. The bottoms are filled with debris, the streams being swift and wild. Among the most rugged of the canyons are : the canyon of the South Fork of the Boise ; several along the Salmon ; that of the Middle Fork of the Salmon, with its tributary, Loon Creek; and those of the numerous branches of the Clearwater. Within the mountains, also, are occasionally to be found open valleys from 10 to 20 miles in length by 2 to 7 or 8 in width. They have become the repositories of the products of modern erosion, or of materials derived from the inclosing mountains and laid down in Tertiary times or their early canyons hav3 been filled to depths of several hundred feet with lava-flows, to be recut by later streams to their present levels. Prominent among these valleys are the following : Along the portion of the Salmon traversed, one at the head of the main fork, east of the Sawtooth Eauge. There is here a stretch of nearly 40 miles north and south by C or 8 east and west occupied by material, chiefly crystalline and eruptive, derived from the surrounding mountains. Long and gentle talus slopes border the valley, while its center is a broad flat, cut to a depth of 5 to 30 feet by the river channel. The valley was prob- ably once the site of a glacial lake,- heavy beds of morainal matter being still visible along the base of the Sawtooth. At Challis is another opening. This in itself is 10 or 15 miles in diameter, but tributary valleys to the east and west carry the general topographic depression some 10 to 15 miles farther back. The Salmon enters the valley 7 miles southeast of Challis through a sharp canyon in crystalline and eruptive rocks, and after a northerly course of about 15 miles, along which are rich bottom and bench lands, again enters the mountains in a canyon continuous with the exception of a small open- ing in the vicinity of the Pahsimeri to within 6 or 7 miles of Salmon City. The valley of Antelope Creek, the tributary from the east, is broad and level, the stream small. The valleys to the west are more INTERMONTANE VALLEYS. 221 rolling, but the creeks, again, carry little water. There is a deep deposit of Quaternary gravel in the Challis Valley, but the evidence of glacial action, along the route traversed at least, is not nearly so pronounced as in the upper valley of the Salmon east of the Sawtooth Range. The Pahsinieri Valley appears open for fully 15 miles from its mouth. It is 6 or 7 miles broad, with a narrow strip of bottom land along the center, the remainder being bench land underlain with Quaternary debris resting upon crystallines and eruptives. A third and still broader iutermoutane valley of the Salmon occurs about Salmon City. This is an illustration of the mountain-locked valleys that have become repositories of materials in Tertiary times. The depth of the pre-Tertiary valley is unknown, but the sediments indicate it to have been several hundred feet greater than at present. The extent of the valley is about 20 miles along the Salmon, in a north and south direction, by 10 miles in an east and west direction. To the southeast it is continued, in the Lemhi Valley, fully 30 miles, maintain- ing here a width of 5 to 8 miles. A long spur of the Salmon River Range lies in the forks of the valleys. Both the Salmon and Lemhi valleys present gentle slopes from side to center, locally rendered somewhat uneven by the folds into which the Tertiary beds have been thrown. Along the present stream channels, particularly the Salmon, the Tertiary strata frequently form precipitous bluffs 50 to 80 feet high. The first-bottom lauds are from a half mile to 1 or 2 miles wide, but there is generally a long, evenly sloping bench on one side or the other which is susceptible of cultivation wherever water is available. The configuration of the valley has perhaps changed slightly from time to time, not only by reason of erosion, but also through the agency of dynamic movements, indicated by flexures in the Tertiary strata. Of the other intermontane valleys, that of Lost River was traversed only along the two branches forming its head. The northern of these has its source in the Thousand Springs, near Dickey. The valley is here 3 or 4 miles broad and very level ; just below it opens to a still greater width and becomes continuous with the main valley below the forks. The west or main fork has a narrow bottom, rarely over a half mile wide, confined between rugged hills from 600 to 2,000 feet high. The valley of Wood River from a point about 15 miles above Ketchum to its exit from the mountains maintains a width of bottom of between one-half and 1J miles. The floor is level, and is underlain with a Quaternary wash of varied material. The tributary valleys are usually sharp mountain gorges, their bottoms rarely over one-quarter mile wide. From Hailey to Boise the route of reconnaissance lay along the southern border of the mountain mass of Idaho ; for the eastern half of the distance, in the broad, prairie-like valley of Camas Creek ; for the western half from High Prairie at the head of Camas Creek to the Boise Valley within the foothills of the mountains. The valley of Camas Creek is from 10 to 15 miles wide, extending directly from 222 GEOLOGICAL RECONNAISSANCE ACROSS IDAHO. the base of the mountains on its north to the low range of hills partly sedimentary, partly crystalline or eruptive which divides it from the Snake Plains to the south. The channel of Camas Creek is deep cut, but the lateral streams are rarely depressed more than 10 or 15 feet. Were the water supply sufficient the valley would present some of the most favorable agricultural conditions observed during the reconnaissance. From the Camas Valley westward for a distance of 25 or 30 miles is a region of high, irregularly eroded hills, extending from the mountains prairieward 10 to 15 miles. The drainage of this area is partly into the Boise, partly direct to the Snake River. The divide between the two waters threads its way with irregular trend through the region, passing finally into thegreat lava flat which constitutes apart of the Snake Plains at their western end. From this divide, at the head of Indian Creek, a high range passes northwest and walls the Boise intermontane drainage basin from the general valley without. The Boise River debouches from a rugged defile in the range at a point about 12 miles southeast of Boise City. In the vicinity of Little Camas Prairie, however, the early topo- graphic rim, confining on the south the intermontane drainage of the South Fork of the Boise, has been deeply degraded, and but for the cutting of the stream its recuttiug, even, through the comparatively recent lavas at a more rapid rate than the denudation of the rim, the waters of the South Fork would have found an exit at this point direct to the Snake. In the hill area just described the valley of greatest extent is that of the Little Camas, 7 or 8 miles long and from 1 to 5 miles wide. Meadow lauds occupy its upper, narrower portion, while below it assumes a prairie aspect and is underlain by a broad flow of lava. A ridge 2,000 feet high extends across the southern end of the valley, and continues east ward for several miles as the southern border, also, of the Big Camas Valley. The entire foothill region between the head of Big Camas Creek and the main Boise Valley west of the moun- tains is one of granite, lava, and other erupttves in dike form, and the effect of such varied rock assemblage has been n most irregular devel- opment of topographic features. The lava generally occurs as a thin sheet or a succession of thin sheets upon the granite, and wherever it is present the country becomes rough and difficult to travel. North of the region just described' and within the rim of the Boise drainage basin is a flat, 8 to 10 miles in diameter, known as Smiths Prairie. The floor is wholly of lava, which extends, also, for some dis- tance into the tributary valleys. The surface is locally smooth, rugose, or gently undulating, and in only one or two instances is it relieved by low hills. The general altitude of the surface is about 4,800 feet, a little lower than the small lava prairie a few miles to the southeast at the mouth of Little Camas Creek. Along the southern edge of the prairie the South Fork of the Boise has cut a gorge over 1,000 feet deep, in the precipitous walls of which appear two seemingly distinct lava- flows, one occupying the upper 500 to 700 feet of the gorge, the other 200 to 300 feet at the bottom. The relative ages of these flows were not ELDRIDOE.; GLACIAL ACTION. 223 determined, but either might be the older. The flows may be traced at intervals the entire length of the South Fork, and also along the main stream below, to its debouchment from the mountains. At the mouth of Moores Creek they are joined by others which extend nearly to Idaho City. Usually the lava forms narrow ribbons adhering to the sides of the canyons. These appear to drop in altitude as the can- yon is descended, indicating an ancient lava river down an earlier- eroded gorge, with occasional lava lakes in the openings. Twenty-eight miles north of Boise, at Horseshoe Bend, on the Pay- ette, is a small mountain locked valley carrying deposits of Tertiary age. The length of the valley is 10 to 12 miles, the width 4 or 5 miles, the trend northeast. Along the western edge the Payette flows for a distance of 5 or 6 miles, entering and leaving by a sharp, rugged canyon in the granite of the Boise Kauge. The valley is apparently one of erosion. Its configuration is somewhat irregular, both upon original outlines and from the later encroachment of the heavy talus slopes along the base of the mountains. Moreover, the underlying Tertiary beds here and there project through the Quaternary in buttes or ridges. Gentle folding has taken place here, as in the valley at Salmon City. One of the most peculiar valleys encountered, of which the topo- graphic origin was not determined, is that locally known as " Prairie Basin," about 45 miles southwest of Leesburg. This is a high inter- montane depression, apparently of considerable depth originally, but since filled by heavy deposits of glacial drift, which in still later times has been partially removed through the channels of Big Creek and its tributaries. The valley surface is now in long, rounded ridges and hillocks. Owing to its altitude, to the extent of opening, and to the heavy growth of grass covering its every acre, the "basin" is easily recognized from distant peaks. GLACIAL ACTION. Evidences of early glaciers were observed at many points along the route of reconnaissance, and it is probable that their existence was quite general throughout the mountain region of Idaho. Conspicuous local- ities in the southern half of the State are the Trinity and Sawtooth ranges. In the upper portions of many of the valleys heading in the former are glacial bowlders, grooved rock surfaces, roches moutonnees, and lateral and terminal moraines. North of the Trinity Eauge, and also east of the Sawtooth, are numerous glacial lakes, 1 to 2 miles in diameter, their waters held back by terminal moraines. East of the Sawtooth the mass of debris from the earlier glaciers is enormous, extending 50 or 60 miles along its front and forming a belt from 5 to 8 miles broad. This is cut by the streams of the present day, and now forms a rough, often heavily timbered slope most difficult to traverse. Other observed localities are the ranges west of Salmon City, particu- larly about the head of Panther Creek and the several tributaries of the Yellow Jacket, a branch of Carnas Creek. Prairie Basin has already 224 GEOLOGICAL RECONNAISSANCE ACROSS IDAHO. been noted as the possible site of an ancient glacial lake, the heavy deposit of bowlders having been derived from the surrounding moun- tains. Indeed, nearly all of the valleys traversed in the reconnaissance show, at their heads, more or less evidence of former glaciers. Even now ice action is rife at the higher altitudes, where the snows are exceed- ingly deep and their disappearance is rarely complete from one season to another. THE FORMATIONS. The rocks occurring in the southern half of Idaho embrace granites, gneisses, syenites, schists, quartzites, limestones, calcareous and non- calcareous shales, sandstones, clays, and eruptives in great variety. The granites and syenites, in part at least, are probably of the Archeau age; the schists, Algonkian. The quartzites are distributed from Algou- kian to Carboniferous, while the limestones may include both Silurian and Carboniferous. The age of the great calcareous shale series of the Wood River and neighboring districts is undetermined, but the evidence points to the Carboniferous. It is undoubtedly Paleozoic. The sand- stones and clays encountered are all of Tertiary age, Eocene ( ?) and Neocene. Post- Pliocene gravels are abundant. The eruptives are possi- bly of all ages, from Archean to Recent. The assignment of the several series of rocks, except the sub-Carboniferous and Tertiaries, is provi- sional, being without fossil evidence; lithology and stratigraphy alone form the basis of reference. Fossils might be found at several horizons, but the exigencies of the trip did not permit careful search. GRANITES AND METAMORPHIC ROCKS. ARCHEAN. To this group are provisionally assigned the granite and gneiss, but there are instances where, by reason of included bands of calcareo- micaceous or quartzitic slates, this reference to the Archean instead of the Algonkiau is questionable. Again, it is probable that, in part at least, the granite is of igneous origin. The granite is of wide occurrence, and, in the main, of a single type, with three or four regional modifications. The type rock is gray, mod- erately coarse in texture, and composed of feldspar, quartz, and mica, with few accessory minerals. The feldspar is white, chiefly orthoclase, with perhaps an occasional small amount of plagioclase. The orthoclase is often porphyritically developed, while the color varies locally, but rarely, to a faint pink. The quartz is generally granular. The mica includes both the black and white varieties, the former predominating, but showing considerable variation in amount. It is distributed irreg- ularly throughout the mass of the rock, or is lineally disposed, when it imparts a more or less definite foliation. An accessory mineral is horn- blende, in fine and coarse crystals, but on the line of reconnaissance its presence is local and somewhat rare. It occurred notably in the granite ELDB.DOK.] THE FORMATIONS. 225 of Trinity Range, 8 or 9 miles west of Rocky Bar, and again in con- spicuously large crystals along the lower portion of Napias Creek, 20 to 30 miles west of Salmon City. In the latter locality a further modi- fication of the type granite takes place in the development of the orthoclase crystals to an extraordinary size from 1 to 3 or 4 inches in the direction of the vertical axis. The outlines of the crystals are usually somewhat rounded. On exposed rock surfaces the crystals weather in knobs, but fresh fractures extend through rock and crystals alike, often along cleavage planes of the latter, the rock then present- ing the appearance of a more or less uniformly crystalline granular matrix sharply relieved by the smooth, brilliant faces of the porphy- ritically developed feldspar. To the granite of this description the designation "bird's-eye" may be appropriately applied. A second departure from the type granite occurs in the precipitous walls of the Loon Creek Canyon, 3 or 4 miles below Oro Grande. Here the gray variety is wholly replaced by one of deep pink, derived from orthoclase of this color. A faint greenish tint is sometimes induced by the decomposition of the biotite. The texture of this granite is fine to coarse, and the grain even. The areal extent is unde- termined, but it outcrops at intervals for several miles. A third and local modification of the normal granite takes place in proximity to mineral veins. This consists in the loss of a very large proportion of the mica, the remaining feldspar and quartz constituting a somewhat conspicuous rock, by means of which the course of the vein may readily be traced, and which, in fact, may enter into the composi- tion of the vein itself as "ledge matter." The clearest illustration of this occurrence is to be seen in the mining camps in the vicinity of Rocky Bar and Atlanta. Typical gneiss occurs in some of the spurs of the Sawtooth Range, notably about the drainage system of Upper Redfish Lake. Wherever observed it has the mineral composition of the typical gray granite. In the locality referred to, the foliation is so complete as to create the impression of bedded strata, while the effect upon topographic lines is markedly that of an unaltered stratified rock. The granites and gneisses prevail in the mountains of the western half of the State, while in the eastern they occur as range cores in con- nection with schists, quartzites, or limestones. None of the latter rocks, however, were encountered in the western half of the State. ALGONKIAN. To this is provisionally assigned the great series of micaceous, quartz- itic, and chloritic schists of eastern Idaho. The reference is based merely upon lithological character and a resemblance to other beds in the Cordilleras which have already been so assigned. The series embraces, together with the schists, numerous beds of quartzite, and all have the general field appearance of clastic rocks. Many of the layers 16 GEOL, PT 2 15 226 GEOLOGICAL RECONNAISSANCE ACROSS IDAHO. contain a very considerable amount of carbonate of lime. The series in regions of strong development has a probable thickness of 3,000 to 4,000 feet, and is believed to be unconlbrmable with the granite. In any event there was a time-break prior to the deposition upon the granite of the superincumbent strata, for the same series does not every- where follow in the areas brought under observation. The region of crystalline schists is distinctively the eastern half of the State, although in this portion are found also the Paleozoic meas- ures, as well as the older granites forming cores of many of the ranges. The strongest development of the series observed on the route of recon- naissance was in the Continental Divide east of Salmon City, in the region of Big Creek and its tributaries, and in the range separating the drainage of this stream from that of the main Salmon. The series is also exposed at several points along the Salmon above Salmon City> notably, from a point about 9 miles above the city to one 18 miles above. It is then cut out by eruptives of undetermined extent ; after a partially covered stretch of 10 miles it again appears along the river for a dis- tance of about 2i miles; eruptives again succeed, the schists finally disappearing in a small, disconnected outcrop near the mouth of the Pahsimeri. Schistose rocks occur associated with the slates of Lost and Wood rivers, but it is somewhat doubtful if they belong, in their entirety at least, to the great series of crystalline strata just described. They are here apparently associated more closely with Paleozoic measures. UNALTERED SEDIMENTARY ROCKS. PALEOZOIC. The succession of beds in southern Idaho is difficult of determina- tion, owing, first, to a marked difference in their petrographic charac- ters from recognized formations elsewhere in the Rocky Mountains, this preventing their assignment to a definite position in the scale of formations; secondly, to frequent interruptions of continuity by erup- tives; and in the third place, to folds and faults. Later than the granites, and probably also than the schists described above, is a great body of pink and white quartzites, of at least 1,500 feet maximum thickness. They are heavy bedded, hard, and uniform in texture and composition. Their greatest development is along the Salmon River from a point about 5 or 6 miles below the mouth of the Pahsimeri to within 6 or 7 miles of Challis. At the lower end of this stretch they are thrown into a prominent anticline, the eastern end of which is cut by the sharp gorge of the river, showing them in uninter- rupted succession for 1,000 to 1,500 feet. Numerous other similarly disposed anticlines occur between this point and Challis, all in these measures. In none was Archean observed, and none was capped by higher strata of different nature. A small exposure of mica and quartz- itic schists (Algonkian?) occurs just below the mouth of the Pahsimeri, ELDRIDOE] PALEOZOIC MEASURES. 227 and although of the same strike as the quartzites on either side, they are separated from them by intervals of 3 miles or over occupied by eruptives or a Quaternary wash, and the relations between the two series are thus obscured. On Deer Creek, about 6 miles above its con- fluence with Wod River, is a local body of quartzite, which somewhat resembles those just described along the Salmon. It rests directly upon Archean granite, and is overlain by the dark-blue and black calcareous slates which form so import;uit a feature of the Wood River district. The areal extent of this quartzite was not determined, but it is apparently small. A little south of Deer Creek it disappears with marked rapidity, the slates coming down on the granite. Similar quartzites probably exist in large bodies in the Salmon River Range also. This quartzite is evidently to be classed with the older sedimentary rocks encountered; moreover, it closely resembles the established Cam- brian quartzite of Colorado; on these grounds it is here referred to the Cambrian. In the high range of mountains forming to the south the watershed of Wood River and to the north and northeast that of Salmon and Lost rivers are several thousand feet of quartzites, slates, conglomerates, calcareous shales, and limestones, which it has been impossible in the time available either to segregate into formations or to refer to definite ages. The range itself is the southeast extension of the Sawtooth, and carries some of the loftiest peaks in Idaho, notable among them being Mount Hyndman, 12,000 feet, 10 to 15 miles southeast of Ketchum. The range is an anticline, the core being gray granite, of the same type as that of the main Sawtooth, Trinity, and Boise ranges, and with a width of exposure along the Ketchum-Challis ro:id of between 2 and 3 miles. Resting upon the granite on both the north and south sides of the range is the series of sedimentary beds mentioned above. The stratigraphic succession of the several members of the series is not confidently determined, and, moreover, there is an apparent difference in the succession on the two sides of the range. On the north side, about the heads of the several branches of Lost River, quartzites and slates, dipping northeasterly, follow one another in quick succession. Black limestones are also present, and in some localities, not visited, they are apparently of considerable importance, judging from, bowlders encountered in the valley. It is doubtful, how- ever, if any single layer is more than a few feet thick. For 2 miles above East Fork of Lost River, large bodies of lava occupy both valley and hillsides, interrupting the series of sedimentary beds. Below Bast Fork, however, the quartzites and limestones, or at least quartzites with strongly calcareous layers included, reappear, continuing for 8 or 9 miles down the valley to the point where the road turns from the river northward toward Dickey. Within this distance the quartzites per- haps predominate, but black and probably often calcareous slates are 228 GEOLOGICAL RECONNAISSANCE ACROSS IDAHO. not infrequent. The calcareous nature of some of the quartzite layers is particularly marked a short distance below East Fork. For the lower 2 miles of the distance there occur in the series a number of con- glomerate layers 1 to 10 feet thick. These are composed of a mass of cherty, subangular pebbles, and in the heavier beds, of quartzite and limestone debris in addition, the whole cemented by flue, granular material of the same nature. East of the Dickey road, after its turn northward from the valley of Lost River, what are probably sub-Car- boniferous limestones appear in a prominent hill, a spur of the high range to the south. This limestone, which overlies the quartzite and slate series, outcrops in great bodies in the mountains to the north, forming the western periphery of the Thousand Springs Valley. South of the Sawtooth Range, along Trail Creek and the Ketch urn road, the succession of beds is somewhat different from that on the north side. Dark quartzites prevail immediately above the granite, followed after several hundred feet by a thick zone of light-colored, white and gray quartzites. About 6 miles from the summit the entire series is repeated, though whether in natural succession or by faulting was not ascertained. It is then overlain by several hundred feet of dark-gray and black slates or shales, probably often calcareous, which continue nearly to the mouth of Trail Creek Canyon. At this point the succession is interrupted by eruptives, which extend for a consider- able distance along the Wood River Valley on its eastern side. West of Wood River, opposite Ketchum, is a heavy body of limestone, some- what resembling in its massive character, its mode of weathering, and its general appea ranee the sub-Carboniferous of the Rocky Mountains, but the age was not definitely determined. This limestone is called by the miners the "gray limestone," in distinction from the "blue lime- stone," which occurs as thin beds in the series of shale overlying. The dip is here down the river (southward). The overlying shales are gray, dark-blue, and black, and carry numerous one foot to six-foot lime- stone bauds distributed through them. The thickness of this shaly series is estimated at 5,000 feet. It constitutes the principal ore- bearing series of the Wood River district, though the lower slates and quartzites occasionally carry mineral. On Deer Creek, a tributary of Wood River from the west, an erup- tive again cuts into the sedimentary beds and occupies the hills to the north of the valley for a mile above its mouth. Above this, however, quartzites come in, followed by a few hundred feet of heavy-bedded dark-blue limestones, which bear considerable resemblance, in texture, weathering, and general appearance, to the sub-Carboniferous. Black calcareous slates of the general character of those already described follow the limestones, and are in turn succeeded by granite, just above Warm Springs, 4 miles above the mouth of the creek. This succession of shales by granite is possibly due to faulting. The granite extends up the valley for about 3 miles, when 400 or 500 feet of the heavy-bedded ELDRIDOE.I PALEOZOIC MEASURES. 229 Cambrian-like quartzite succeed, overlain by blue and black slates and limestones similar to those noted farther down the creek. The quartz ites of this second area of sedimentary beds did not appear in the first series, east of the granite at the Warm Springs. The dip of the series below the Warm Springs is doubtful ; west of the granite block, however, it is westward, bending around gradually to the south- west and south at the head of Deer Creek and its tributary from the south, Bed Cloud Gulch. The black shales and limestones of Deer Creek are undoubtedly a part of the general series which constitutes the leading formation of the Wood River district, but their horizon in the latter is undetermined. South of Deer Creek, east of Red Cloud and Narrow Gauge gulches, the blue calcareous slates are in direct con- tact with the granites to the east, the quartzite observed in the valley of Deer Creek having here disappeared. Whether faulting or iion- deposition is the cause of irregularity in this succession of the beds is unknown. It is possible that the foregoing series of rocks from the granite up, perhaps 10,000 to 15,000 feet in all, fall wholly within the Paleozoic- Cambrian and younger systems; but except in the case of the sub-Car- boniferous and overlying shales, neither their age, division lines, nor intersuccession was satisfactorily determined. The heavy bed of white and pink quartzite on Deer Creek belongs, perhaps, to the Cambrian; the dark quartzite series at the head of Trail Creek, and the calcareous slates, quartzites, and conglomerates of Lost River from an unknown member of which the one or two fossils collected have been determined by Mr. C. D. Walcott to be not older than the Trenton are apparently of a post-Cambrian age; the sub-Carboniferous is recognized ; but the over- lying shales are, again, beyond a general reference to the Carboniferous, in doubt. Lithologically they bear a slight resemblance to the Weber of Colorado and Utah. In the entire Wood Eiver district there is much and varied faulting as well as folding, and only work in great detail will effect a solution of the many geological problems presented. The foregoing series doubtless occurs in many ranges to the east and northeast of the localities described, especially in the mountains east of the Thousand Springs Valley and of Lost River. The sub-Carboniferous, which, of the Paleozoic series is most clearly recognized, is, excepting possibly in the Wood River region, repre- sented by the massive blue cave limestone, so characteristic of it throughout the Rocky Mountain region. Its identification is based upon lithological resemblances and the contained fossils, although no collection of fossils was attempted. The thickness is between 100 and 400 feet. The limestone occurs in especial force in the several ranges between the Challis Valley and the main fork of Lost River, but it is said to be present in many localities in eastern Idaho. Its rela- tions to underlying beds were unobserved except in a single locality, in the forks of Lost Eiver, where it apparently succeeds the series of 230 GEOLOGICAL RECONNAISSANCE ACROSS IDAHO. calcareous quartzites, slates, and conglomerates described above. On the divide between Antelope Creek and Lost River it is overlain by drab calcareous shales resembling those in the Wood River district, though possibly less metamorphosed. In the range of mountains forming the southwest side of the Warm Spring and Antelope valleys, just east of the Salmon River, are beds, possibly 500 to 600 feet in total thickness, which are partly quartzite, partly limestone, the latter pinkish-drab, massive, and somewhat resembling the Silurian limestones of the Colorado areas. In the hur- ried examination given them, however, no fossils were found. In this locality, also, on the lower flanks of the range, are numerous large hot-spring deposits, the springs being now extinct. At the eastern end of the range, overlying what from a distance is apparently the sub- Carboniferous limestone, are about 50 feet of red beds of unknown composition. Above these are several hundred feet of calcareous shales, the same as those already mentioned on the divide between Antelope Creek and Lost River. The Little Lost River Range east of the Antelope and Thousand Springs valleys is reported by prospectors as being composed of the sub-Carboniferous limestone and overlying shales, and from a distance this seems to be the case. CENOZOIC. This system is represented in Idaho by the great series of sedimen- tary beds underlying the Snake Valley and by others which occupy certain of the intermontane valleys. Paleobotanic evidence points to the Eocene or Miocene as the age of the intermontane sediments, and molluscan and mammalian remains to the Pliocene as that of the Snake River beds. The materials of the Snake River beds were derived largely from the mountains to the north and south of the valley, though do-ibtless more or less detritus from the region of the headwaters has been commingled with the other sediments for its entire length. The material, in the western half of the State at least, is chiefly of granitic origin, consist- ing of quartz and feldspar grains, often with a kaolin-looking cement, slightly ferruginous. The prevailing rock is a flue to medium grained, friable, gray sandstone, but clays occur, and also conglomerates. The formation was examined only locally, and there are doubtless many variations. Associated with the Snake River beds, and in some instances inter- stratifled with them, are flows of the lava which in one locality or another is such a well-known feature of the Snake Plains. Apparently the outpouring of this rock took place, in part at least, while the sedi- mentary beds were still being deposited. The sediinentaries outcrop along the Snake River in bluffs from one hundred to several hundred feet high, and also occur in bent-lies of considerable elevation next to the mountains both north and south. ELDRIDGE.) CENOZOIC IN SALMON-LEMHI VALLEY. 231 The age of the Snake River beds is probably Pliocene. Melania tay- lori Gabb and Liihasia antiqua Gabb, together with an undetermined vertebra (carniverous), have been found by Mr. Arthur Foote, at Glenns Ferry, on the Snake, in Elmore County, and Prof. O. C. Marsh personally reports Pliocene mammalia from the same series of beds on the north side of the Boise River, a few miles below Boise. 1 The intermontane valley of the Salmon and Lemhi rivers, in the center of which Salmon City is situated, is occupied by a considerable thickness of Tertiary beds, the materials for which were derived from the early formations of the inclosing ranges. These materials are, for the most part, of granitic or quartzitic debris, and the beds are either clays, sandstones, or conglomerates. The clays occupy a large area in the center of the valley. They are light-green and red, forming a conspicuous feature in the landscape. The colors are in two zones, the red at the higher level, nearer the periphery of the valley, and possibly a coloration of later times. The clays are slightly arenaceous in some layers, in others distinctly sandy, carrying even occasional thin bands of sandstone or conglom- erate near the top. This conglomerate is a mass of small, round, or lenticular pebbles of half-inch maximum diameter, chiefly quartzite. Near the same horizon are thin layers of hard, white or brown, homo- geneous clay, which usually has an abundance of minutely divided vegetable matter through it, sometimes sufficient to render it lignitic. Here and there fragmentary stumps of trees are found in a carbosilici- fied condition. The sandstones of this intermontane Tertiary series occur at the base of the measures exposed, just beneath the clay division, and in all are probably 400 or 500 feet thick. They outcrop in bold bluffs along the Salmon. Their material is chiefly quartz. They are light yellow or gray and for the most part massive, though in some layers thin bedded and even shaly. The gray beds are usually the finer in texture, the yellow sometimes approaching a grit. The series carries plant remains, leaves, stems, etc., which locally are in sufficient quantity to form with the shale a lignitic band. In one instance, just below Salmon City, this carbonaceous matter is distributed through 4 or 5 feet of shales, forming a coaly slate, within which is a 6-inch bed of dark-brown, woody lignite. So far as at present exposed, this is not of economic value. Overlying the slate are 10 feet of sandstone, succeeded by 5 or 10 feet more of a very hard, moderately coarse quartzite-conglomerate, which is somewhat ferruginous, and often fractures across matrix and pebbles alike. Within a short distance of this outcrop is another exposure of a similar conglomerate, in appear- ance 30 or 40 feet higher up than the first, but not definitely so deter- mined. This second conglomerate in particular bears a resemblance 'See also various references in Bull. U. S. Geol. Survey No. 84, Correlation papers Neocene, by Ball and Harris, pp. 285 and 286, 1892. 232 GEOLOGICAL RECONNAISSANCE ACROSS IDAHO. to certain beds along the base of the Continental Divide in the vicinity of the Lemhi placer mine on Kirtley Creek. Just beneath this con- glomerate bed, along the Salmon, are some white sandstones, more or less argillaceous and ferruginous, carrying kidney shaped iron concre- tions, and leaf-bearing. Among the forms collected Mr. Kuowltou has recognized Sequoia langstiorfii (Brong.) Ung. Glyptostrobus europceus Brong. Equisetum (?) sp. Dicotyledons: Ficus (?), Quercus (?), etc. Plant stems. Mr. Knowlton adds the following remarks in the letter submitting the results of his examination : Neither of the conifers can be relied upon to prove close questions of age, for they have a considerable vertical range, (ityptotirobus europfeus is, however, absolutely, and Sequoia lanijsdorfii almost exclusively, confined to the Tertiary. Both species have a wide geographical range, and are very abundant forms. The first is abundant in this country in the Fort Union group, Upper Laramie (Fort Union) of Canada, Mackenzie River, Alaska, and Arctic Miocene in general. In Europe it is mainly confined to the Eocene and Miocene. Sequoia laiujsdorjii has been once reported from the true Laramie, but I regard this identification as extremely doubtful. It is very abundant in the Kort Union group, and is also found at the same places as the other. The Dicotyledons are fragmentary but seem to belong to such genera as Ficus, Quercus, etc. The Equisetum and vegetable steins are worthless for stratigraphic purposes. The evidence, incomplete as it is, shows the Tertiary age of these beds, but whether they are Eocene or Miocene it is impossible to say. In the bluffs of Kirtley Creek, about 1 miles below the Lemhi Placer Company's bar, is the following exposure: Feet. At top of bluff, obscured beneath Pleistocene gravel 2( > Argillaceous sands 5 Pure sands, concretions iit base 10 Coarse gravel 10 Bright-red clays, very pure 5 Red clays with pebbles finer than in the gravel above 5 Gravel probably still underlies, but it is covered. The position of the beds is nearly horizontal, with a possible slight dip to west. Their relation to the other beds in the valley could not at the time be determined, but they are quite likely younger than the series of green clays, at one point, indeed, apparently resting upon them or a nearly allied stratum. Moreover, apparently the red clays everywhere occur at a higher altitude than the green. The conglomerates of this series of Tertiary beds outcrop in great force about the periphery of the Salmon City Valley, especially at the west base of the Continental Divide, and are difficult of reference. Being auriferous, they are the most important, from an economic stand- point, of all the beds. They are exposed in the cuts of the Lemhi F.UIIIIDGK 1 CENOZOIC IN SALMON-LEMHI VALLEY. 233 Placer Mining Company on Kirtley Creek, about 2 miles from the base of the range. At this point the section given below was obtained. The gravels of this section, Pleistocene and Tertiary, are made up of quart/ites and schists derived from the neighboring mountains. The two are readily distinguished, the Pleistocene being loose, easily disintegrated, and carrying in the interstices of the bowlders and peb- bles considerable earthy matter; the Tertiary gravels being a compact mass of large pebbles, up to 1 foot HI diameter, in a grit matrix, the whole moderately hard and difficult to hydraulic. The unconformity between beds c and d is evidenced not only by the wavy lines between the two, as shown in the sketch, but also by the fact that the gravel beds c rests successively upon several of the underlying beds. Both series of strata, however, those above and those below the line of unconformity, are alike flexed in the crumpling that has taken place in the region. The beds