sT.Jomr.J PIEEEE'S MOUNTAINS. 429 



gained in the vicinity of Spring Creek, and at Station XLII, the Car- 

 boniferous limestones at this point are found to extend quite to the edge 

 of Pierre's Basin, where they are buried beneath Post-Tertiary accumu- 

 lations. 



In the foot of the mountains, and in low outlying mounds a short 

 distance south of Spring Creek, which issues a copious stream from a 

 spring source in the limestone ridge, quite extensive exposures of drab, 

 cherty limestone with crinoidal columns, overlaid by the reddish-buff sili- 

 ceous beds, were seen, dipping to the southwestward at an angle of about 

 35°. These deposits doubtless belong to the lower portion of the series 

 shown in the section through Station XL, given above. Between the 

 latter locality, which we designated by the name Spring Point, and the 

 debouchure of Low Pass Creek, the Jurassic beds occupy a belt which 

 extends southeasterly over to the ridges southwest of Station XXXIX, 

 in the Teton Pass Mountains. On the southwest border of this belt 

 there are indications of a sharp fold in the Mesozoic strata which form 

 the crest of the before-mentioned spur-ridge rising into Station XL, 

 where, at one point, the heavy conglomerate ledge which overlies the 

 drab Jurassic beds was observed to dip to the northeastward at angles 

 of 30° to 50°. The latter exposure occurs in the crest of the spur-ridge 

 but a short distance northwest of Low Pass Creek, and where the 

 conglomerate is underlaid by chocolate red beds, which are in turn 

 succeeded below by drab limestones, recalling the association of beds 

 observed to the southwest of Station XL. But, higher in the ridge, its 

 crest lies to the southwest of the axis of the fold, which also gradually 

 rises in that direction, so that the southwest flank of the fold exhibits 

 the strata reclining in that direction. It is possible the fold here alluded 

 to may be of very local extent, which would readily explain the north- 

 erly expansion of the Carboniferous deposits appearing in Spring Point, 

 and. which are believed to belong to the ridge of Station XLII, as here- 

 after to be mentioned. 



Soon after crossing Spring Creek, the foot-hills to the northwest 

 show rusty weathered pinkish trachytic ledges, gently dipping toward 

 the basin, or northeasterly — their first exhibition along this side of 

 the basin after leaving the exposures in the debouchure of West Teton 

 Pass Creek. On one of these upraised volcanic points Station XLI 

 was established, at an elevation of about 2,000 feet above the lower 

 level of the basin opposite. But to the southwest the sedimentaries are 

 seen rising from beneath the volcanic border-deposits into a higher 

 mountain ridge which forms an easterly spur prolongation of Station 

 XLII ridge. The volcanics skirt the mountain foot for several miles to 

 the northwestward, when they bear round to the westward, rising high 

 up on the northern terminus of the range where they form the long fore- 

 land bench which descends over gentle grassy slopes into the great 

 plain of the Snake. 



Just within this fringe of volcanics, a rather extensive recess along 

 the upper course of Packsaddle Creek is filled with low, rounded hills, 

 rising into a more prominent ridge along the eastern border, and flanked 

 on the southwest by the more rugged mountain ridge, on the culminat- 

 ing point of which Station XLII was located, at an altitude of 10,080 

 feet above the sea, or nearly 4,000 feet above the bed of Pierre's Basin. 

 A section carried from the volcanic border along a southwest line into 

 the heart of the mountains through Station XLII ridge is shown in the 

 accompanying plate, section A, and exhibits the following stratigraphic 

 elements : 



