450 REPORT UNITED STATES GEOLOGICAL SURVEY. 



Alpine plants spring- up, which have contributed vegetable matter and 

 transformed the little basins into accumulations of rich loam. 



At a point near the summit reddish gneiss appears dipping about 

 W. X. W. at an angle of 20°. It is overlaid by a grayish-white intensely 

 hard quartzite, which shows at one point an inclination X. 30° E., at 

 angles of 35° to 40°. A few yards south of the summit the white quartz- 

 ite dips 15°, S. 45° E. It is conglomeritic in places, laminated and 

 cross-bedded, pinkish above and below, probably the prevalent color, 

 attaining an exposed thickness of at least 50 feet. The summit of the 

 mountain is a confused mass of angular blocks of this rock, in which a 

 few obscure fucoid-like markings are observable. Intercalated between 

 the quartzite and the gray gneiss occurs a layer of beautiful pink-red, 

 very quartzose gneiss, like that mentioned in the northwest side. There 

 are many varieties of gneissose and schistose rocks in the mountain, in- 

 tersected by granitic and quartz veins, and these deposits have been 

 much broken up and recemented, forming a sort of coarse breccia. From 

 the above wiU readily be inferred the greatly-disturbed condition of the 

 Archaean nucleus of this portion of the range. As. time did not permit 

 carrying our work into the heart of the range, to make up for the de- 

 ficiency as far as possible, a rather detailed notice of the geological ap- 

 pearances presented from this elevated outlook is here appended. 



Station XLIV attains a height of about 4,000 feet above Jackson's 

 Basin, and is the westernmost culminating peak of the range, situate 

 about midway between the Gros Ventre and Hoback's Rivers, command- 

 ing extended views of the surrounding country. From the north, east, 

 round to the south, the Gros Ventre Mountains are spread out in a 

 grand panorama ; the remainder of the circle comprehending the Tetons 

 and the beautiful plain of Jackson's Basin, with, to the north, part of 

 the low, much eroded tertiary hills that occupy the angle north and 

 east of the Buffalo Fork and Snake River. To the northeast, about 

 eight miles distant, the western foreland sweeps up from the level of 

 Jackson's Basin into the shattered sedimentary crown of Station XL VI, 

 which, with its easterly continuation, shuts out the view to the north- 

 ward, save here and there loftier distant mountain crests. Intermediate, 

 perhaps one to two and a half miles from XLIV, a lower but still high 

 ridge shows heavy ledges of Quebec Group limestones, capped by the 

 more massive-bedded limestones, probably of Xiagara age, the base of 

 the seclimentaries being composed of the Primordial quartzites, whose 

 rusty-reddish debris strews the lov> r er declivities descending to the light- 

 gray Archaean floor of the amphitheatre which separates the latter ridge 

 from Station XLIV. 



This ridge is abruptly terminated on one of the tributary branches of 

 the Little Gros Ventre from the south, and just beyond a glimpse is 

 caught of a low crown protruding in the slopes facing Station XL VI, 

 which also appears to be composed of Archaean rocks, the sedimentaries 

 having been denuded over an inconsiderable area. The ground here 

 referred to constitutes one of those great tilted blocks of sedimentary 

 strata, the exact counterpart of the foreland on the west flank of the 

 TCton Range, the slope descending gradually to the level of Jackson's 

 Basin, the strata dipping at a somewhat steeper angle in the same di- 

 rection, so that, in the debouchure of the Little Gros Ventre, the heavy- 

 bedded, rough-weathered magnesian limestones, Xiagara or Carbonifer- 

 ous, appear in uplifted outliers in the foot of the mountain. There is, 

 however, this distinction : that, in the western flank of the Gros Ventre 

 Range, besides the great uplift, the strata were thrown into quite con- 

 spicuous minor folds, which will be noticed more at length further on. 



