bt. jobs.] GROS VENTRE RANGE. 453 



Ventres there are indications of a sharp anticlinal fold, by which the 

 red beds are lifted into high crests north of Hoback's Eiver, but which 

 may prove to belong to the southeastern extension of the before-men- 

 tioned fold west of Station XLIV. 



The relations of the folds and involved sedimentary formations ob- 

 served by Professor Bradley in the Grand Canon of the Snake to similar 

 phenomena and stratigraphical elements met with in the northwestern 

 extension of the Snake Eiver Range have been briefly discussed in a 

 preceding chapter. The relative age of mountain elevation is always a 

 subject of prime importance, and, in a region like the present, it is sure 

 to draw the attention of the observer to every fact which seems to have 

 a bearing, near or remote, toward the solution of a problem which is so 

 constantly before the mind. In this connection, it seems as though we 

 have hi the disturbed late Tertiary deposits within the area of Jackson's 

 Basin at least a hint as to the comparatively modern date of the eleva- 

 tion, or one of its latest phases, of the Gros Ventre Range, and, inferen- 

 tially, the relatively more remote date of the disturbances which resulted 

 in the upheaval of the Snake River Range to the southwest. The evi- 

 dence here observed is almost precisely the same as that furnished by 

 the variegated Tertiary deposits in the lower valley of the Snake on the 

 opposite side of the latter range, which he tilted unconformably, on the 

 border of the opposed Caribou Mountains. 



As has before been mentioned, our knowledge of the geological struc- 

 ture of the eastern portion of the Gros Ventre Range is derived from the 

 observations of Dr. Hayden, in I860.* Dr. Hay den mentions, on the 

 upper waters of the Gros Ventre, extensive developments of lignite- 

 bearing Tertiary deposits, and lower down the valley successively ap- 

 pear Cretaceous, Jura, and Triassic deposits, which recline on the north- 

 ern flank of the range. The Mesozoic formations doubtless pass out into 

 Jackson's Basin, north of the debouchure of the Gros Ventre, where, how- 

 ever, they are covered by late Tertiary beds, and even in the rounded 

 foot-hills on the border of the basin they are so concealed beneath detrital 

 materials as to escape detection. 



The distribution of the late Tertiary deposits in Jackson's Basin has 

 already been alluded to, as also their upraised position on the flank of 

 the mountain north of the point where the Little Gros Ventre leaves the 

 hills. This mountain border exhibits the heavy -bedded, rough-weathered 

 magnesian limestone, so like the Niagara, but no fossils were observed here 

 by which its age might be established with certainty. As we pass north- 

 ward, however, these beds are lost to view in a recess a few miles south of 

 the debouchure of the Gros Ventre, but soon reappear, and finally pass be- 

 neath the Carboniferous deposits across which the stream has eroded a nar- 

 row valley in its passage into the basin. The latter deposits, as exposed in 

 the south angle of the debouchure, are made up of dark-brownish drab, 

 porous, magnesian limestone, dipping 15° to 20°, N. 5° E. It occurs in 

 heavy beds, and would make an excellent building material. It weathers 

 much like the buff magnesian limestones referred to the Niagara, from 

 which it is distinguished both by its darker color and the prevalence of 

 crinoidal columns, a ftpirifer and Hemipronites crenistria, all Carbonifer- 

 ous forms. These magnesian beds are overlaid by dark-gray limestone, 

 also charged with Hemipronites and a small Athyris, &c, and which ap- 

 pear in a low bluff escarpment on the opposite side of the stream a short 

 distance above, dipping in the same direction, and overlaid in turn by 

 light-gray limestones rising up on the northern flank of the Gros Ventre 

 Mountains. These exposures are often covered with a lichen-growth, 



* Reprint, Report U. S. Geol. Survey, 1872, chap. I. 



