CRETACEOCJS UPLIFT OF BITTEIi CREEK. 235 



similar species of Ostrea and Corbula to those found on the east side of the 

 fold. Owing to their steeper angle of dip, there is less liability of confound- 

 ing them with the overlying Tertiaries, and no unmistakably fresh-water 

 types have been recognized in them. The springs in this neighborhood, as 

 to the eastward, are largely charged with sulphur. 



To the west of Rock Springs, the sandstone ridges become lower and 

 more infrequent, and are gradually concealed beneath surface debris. The 

 highest outcrop observed was that of a yellow, slightly calcareous sandstone. 

 From these ridges to the base of the cliifs, formed by the beds of the Green 

 River series, to the north of the railroad, the surface is covered by a light- 

 reddish clayey soil, resulting from the decomposition of the upper beds of 

 the Vermillion Creek series, which rest unconformably upon the Laramie 

 beds. The unconformity is most distinctly marked, however, in the beds 

 of the Grreen River series, which dip only 4° to the west, as seen in the 

 bluffs north of Bitter Creek, a short distance west of the sandstone ridges of 

 the Laramie group. To the north of the railroad, our observations extended 

 but a little distance. To the south, the sandstone ridges of the Laramie 

 group curve in strike to the eastward, being partially concealed beneath the 

 horizontal Tertiaries which cover the flanks of Quaking Asp Mountain, and 

 at the head of South Bitter Creek, as already observed, are found with a 

 strike of north 30° west. 



The study of the rocks of this region, while it only serves to confirm 

 the observations on the beds of the Laramie group at other points, which 

 show that they were deposited conformably over the older Cretaceous for- 

 mations, and prior to the great period of plication and uplift in which the 

 Rocky Mountains and the Uinta and Wahsatch Ranges received their main 

 elevation, and that they may therefore be properly regarded as of Creta- 

 ceous age, while the mingling of marine and brackish-water forms in their 

 fauna indicate local shallowings in the seas in which they were deposited, 

 where even fresh- water shells, brought down by rivers, may have been min- 

 gled with the remains of animals which actually lived in their waters, shows 

 also that similar conditions of life existed during the early part of the 

 Eocene Tertiary period, which immediately succeeded it, and that when 

 the deposits of this period were laid in approximate or actual conformity 



