EASTERN EOOT-HILLS. 45 



their steepest sides toward the range, and falHng away gently to the east- 

 ward. Farther to the eastward the Fort Pierre clays produce a second 

 slight depression, or valley, but frequently somewhat broader and less regu- 

 lar, showing low benches and occasionally isolated mounds or hills. 



Organic remains characterize several well-determined horizons in the 

 Colorado group east of the mountains, and in many localities appear very 

 abundant, especially the genus Ostrea, which frequently forms almost entire 

 beds, held firmly together by a little calcareous cement. They show, how- 

 ever, but little variety in species. Many of the fossils in our collections 

 are too poor for specific identification. 



The following forms, which occur in the Colorado group, have been 

 found east of the mountains : 



Inoceramus prohkmaticns. CaUista JDeiveyi. 

 Inoceramus difformis. Anisomyon sexsulcatus. 

 Inoceramus Barrahini. Pinna restituta. 

 Inoceramus incurvus. Scapliites nodosus. 

 Ostrea congesta. Baculites ovatus. 

 Ostrea '■ — f Ammonite's f 



In addition to the above, many of the clay-beds are characterized by 

 numerous small finely preserved fish-scales, lying parallel to the plane of 

 stratification. 



Desceiptive Geology of the Eastern Foot-hills. — In giving the 

 general geological and lithological features of the main formations, which 

 make up the belt of sedimentary beds along the base of the- range, but little 

 has been said of the details in any one locality, and scarcely anything of the 

 structural peculiarities of ridges, showing the relations which they bear to 

 each other, and to the older Archsean rocks. In the following pages some of 

 the more salient features will be pointed out. Beginning with the northern 

 end of *the Laramie Hills, the first occurrence of Palaeozoic and Mesozoic rocks 

 south of the Laramie River is found at the head of Bush Creek, in a small 

 bay-like depression in the Archaean rocks, known as Ped Valley, just 

 where they jut out to the eastward from the main body. Here the Palaeo- 

 zoic series rise somewhat abruptly in a short isolated ridge, while the Trias- 

 sic beds are the only Mesozoic rocks represented. The latter lie nearly 



