500 REPORT UNITED STATES GEOLOGICAL SURVEY. 



a high summit. 7,200 feet, at this end of the highlands. The deposit, 

 which evidently belongs to a heavy bed, is divided into uneven layers 2 to 

 12 inches or more in thickness, which ring under the hammer, and which 

 dip 40° to the northeast, declining in the steep northeast slope of the 

 hill, upon whose flanks rests a heavy mass of basaltic lava. 



In the Caribou Eange, to the northeast of the last above locality, some 

 interesting exhibitions of the trachytic deposits were met with, but al- 

 ways occurring as isolated remnants of a formerly much more exten- 

 sively distributed flow. Near and on the crest of Station XX ridge, a 

 couple of miles north of the upper entrance to Fall Creek Caiion, occur 

 more or less abraded masses of a rusty brown vesicular trachytic lava, 

 with which are associated concretionary nodules of light-drab and black 

 obsidian, fragments of the latter being abundant in the summit of the 

 ridge. This rock both in its denuded outcrop and physical features re- 

 sembles and is apparently identical with the above-mentioned vesicular 

 volcanic ledges west of Blackfoot Peak. It appears to have partially 

 metamorphosed the Jurassic shales with which it came in contact, al- 

 though no satisfactory exposures of the rock were here seen in situ. It 

 muse, however, have overflowed the denuded and tilted Jurassic strata, 

 yet the whole mountain ridge doubtless has been subjected to great ero- 

 sion subsequent to the period of the flow, in which the latter indeed has 

 suffered almost complete destruction. It was traced in the western de- 

 clivity of the range at the head of Porcupine Creek, indicating the prob- 

 able former connection of the at present isolated exposures. Also, in 

 heights about four miles almost due north and northwest of Station XX, 

 in the neighborhood respectively of Stations XXI and XXIV, and at 

 Station XXIII, on either side of the narrow northern summit of the 

 range, considerable plateau-like remnants of the same material are met 

 with, the inclination being gently to the west, northwest, and north, off 

 some of the highest elevations in this part of the range, 7,200 to 7,400 

 feet above tide. At the latter locality, Station XXIII, the bowlder-like 

 masses are underlaid by a heavy deposit of conglomerate, the coarse 

 materials consisting chiefly of more or less abraded fragments of quartzite 

 and limestone, the cementing material resembling a variegated trachytic 

 tuff, which in places greatly predominates, where it assumes inucn the 

 appearance of the ordinary variegated trachytic materials, in color buff, 

 reddish-chocolate mottled, drab to pink, and decomposing into a fine 

 ashy soil. This deposit at one point dips at an angle of 25° to the 

 northward in the direction of the low country in the debouchure of the 

 Snake. 



In the high plateau bench at the northern end of the Caribou Eange, 

 6,800 feet, a considerable extent of the surface is based upon the drab 

 and pink trachytic flow, which has a gentle inclination west of north. 

 This deposit extends round the northern end of the range to the western 

 flank, more or less interrupted by denudation, in which latter region it 

 forms the coping of sloping plateaus which rise up out of the volcanic- 

 floored upland of the Willow Creek Basin on the flank of the range to 

 an elevation of 7,200 feet. Of the same character are the sloping table 

 ridges southwest and west of Stations XXI and XXII, to the south of 

 which they doubtless still occur over considerable areas and were once 

 connected with the mass at the northern extremity of the basin ridges 

 north of Station XVI. 



In the Blackfoot Valley, near our southern border, similar innk-drab 

 trachytes were found resting upon the soft Pliocene lake-beds. At 

 Station XXX such an exposure occurs, dipping in the direction of 

 the valley, northeast, at an angle of from 15° to 20°. At the latter 



