*T.joux] VOLCANIC ROCKS PIERRE^ BASIN. 503 



quite heavy bedded, and, as seen in great blocks rounded by the action 

 of the weather, it is sometimes coarsely vesicular, recalling the por- 

 phyritic obsidian trachytes which form the basis of the flows. The lami- 

 nated ledges are weathered into the most varied and picturesque monu- 

 mental shapes all along the precipitous canon sides, where the exposures 

 frequently reach a height of 200 or 300 feet, perhaps more, above the 

 bed of the stream. They are overlaid by the thin-bedded, pink and 

 drab trachyte, with disseminated feldspar and sanidine, which in places 

 show a thickness of above 40 feet. In the laminated ledges, at the point 

 of contact with the trachyte, undulations in the surface are observed, 

 like nonconformities or wedge-shaped masses, such as are not infrequent 

 in deposits of purely aqueous deposition. And in the prairie upland 

 masses of dark vesicular basaltic lava are encountered overlying the 

 trachyte, and which, physically, are undistinguishable from the basalts 

 which overspread the lower level of the Snake plain. But the connec- 

 tion between the basalts of the upland and those of the plain is not so 

 apparent, denudation having effected changes in the surface since the 

 flow of these materials which will require careful research in order to dis- 

 cover their true relations. It should also be observed that the basaltic 

 ledges were found in positions indicating that the trachyte had been 

 eroded prior to the flow of the basalt, which latter occurs at levels 

 scooped out of the laminated porphyritic deposits. Erratic-bike masses 

 of steel-gray, intensely hard basalt are met with in the foreland slope 

 west of Station XXXIII, though no regidar ledges were observed in 

 this position. At this point, in the east-facing bluff slopes of one of 

 the foreland benches, the same series of volcanics is shown that was 

 noticed in the vicinity of the confluence of the Xorth Fork and Pierre's 

 Eiver ; that is, the laminated porphyritic trachytes, overlaid by the pink 

 and drab ledges, and in the surface large masses of the tough, dark, 

 bluish-green basalt, which weather by exfoliation ; the ledges having 

 been upraised a thousand feet above their position at the latter locality 

 in a distance of 10 miles. All the above volcanic ledges extend south 

 as far as the upland divide between Caiion and Leigh's Creeks, beyond 

 which the basalt appears not to extend, and the inferior deposits have 

 been much attenuated by erosion. 



Similar pink, rusty-weathered trachytic ledges rise up in great benches 

 around the northern end of the Snake Eiver Eange, attaining about the 

 same height as in the Teton Eange, about 8,000 feet, or between 2,000 

 and 3,000 feet above the general level of Pierre's Basin. Towards the 

 lower level of the Snake Plain, to the north, heavy outliers of pink 

 trachyte are met with, forming low bluffs a hundred feet in height along 

 the edge of the upland, where it is skirted by Fall Eiver, or the Middle 

 Fork of Henry's Fork, below the mouth of Conant Creek. At the latter 

 locality, which was also visited by Professor Bradley, the ledges are 

 much disturbed, dipping from vertical 75° northeast, and at another point 

 60° southwest. The cleavage structure gives the mass a thin-bedded 

 appearance. This rock extends several miles along the stream above, 

 and. rising the grassy uplands between Conant Creek and North Fork 

 of Pierre's Eiver, similar ledges appear in the declivities, generally in 

 narrow outcrops running in a southeasterly direction ; the exposures, 

 however, not showing marked disturbance. A rather high ridge in the 

 midst of this upland is reported by Professor Bradley as marking the " rim 

 of an old broken-down crater, which faces nearly due east. The lava 

 is a laminated mixture of quartz, obsidian, and feldspar; but no masses 

 seemed to be in situ, so that direction of structure could not be deter- 

 mined." 



