228 DESCRIPTIVE GEOLOGY. 



and dislocations produced by contraction in the overlying sedimentary beds 

 along this line. 



This uplift is an anticlinal, or rather quaquaversal fold, whose longer 

 axis is approximately north and south ; the beds involved in it having a 

 steeper dip toward the west and south, while to the east and north they 

 slope off so gently as to present no non-conformity of angle with the over- 

 lying Tertiaries. These dips are about 5° to 7° on the east, as shown 

 along the railroad from Black Butte Station to Salt Wells, and 12° to 15° 

 on the west side, as seen along Bitter Creek from Salt Wells Valley to 

 Rock Springs ; while in the canon of Little Bitter Creek, to the south of 

 Quaking Asp Mountain, are outcrops of the sandstones of the Laramie series, 

 which dip at angles of 25° to 35° to the southwest. 



The open valley of Salt Wells in the centre of this fold is covered for the 

 most part by clayey Quaternary soil, and presents no outcrops, with the 

 exception of a little rounded hill of argillaceous beds in the middle of the 

 valley, from which, however, no fossils were obtained. From the lithological 

 character of the few beds seen, and their position relative to the surrounding 

 and overlying sandstones, it is evident that the upper beds of the Colorado 

 series have here been denuded to a probable depth of 500 to 1,000 feet. 

 The existence and relative position of these beds have been deduced rather 

 from the angle of the overlying Fox Hill beds, which form the bluffs sur- 

 rounding the valley, than from actual observation. Toward Quaking Asp 

 Mountain, at the southern end of the valley, the anticlinal fold seems to 

 close together with sharper dips, while, at the north, its beds pass under the 

 Tertiaries at low angles, forming low circling ridges facing inward. 



Capping the bluffs, which face the valley on its western edge, about 

 ^ six miles east of Rock Springs, is a bed of compact close-grained sand- 

 stone, almost approaching the nature of a quartzite, in which were found 

 casts of fragments of Ammonites, and some small bivalves, possibly Cardium 

 and Inoceramus. Although too fragmentary for specific determination, these 

 remains were sufficient to identify the beds as belonging to the Fox Hill 

 group. They have been therefore considered as marking approximately 

 the dividing line between this and the Laramie group. They are enclosed, 

 both below and above, in coarse gray sandstones, of no very distinctive 



