72 DESCRIPTIVE GEOLOGY. 



They are recognized only on the Sybille Creek and its' tributaries, and in 

 the region of the Chugwater and Pebble Creeks; localities where the Palae- 

 ozoic and Mesozoic strata are wanting, the Tertiary formations abutting 

 directly against the Archaean body. They are best developed on the 

 Sybille, where they reach a thickness of 300 or 400 feet, made up largely 

 of Archaean material, from rocks found in the Laramie Hills. The forma- 

 tion is cut through by numerous streams, which give it a bench-like terrace 

 formatiop. That they overlie the Niobrara basin there can be no doubt, 

 and at the same time are older than the loose Quaternary detritus that 

 covers the plains all along the base of the range. Upon the geological 

 map, they have been referred, along with somewhat similar beds on the Big 

 Thompson, although at the latter locality they are not so well developed, 

 to the Wyoming Conglomerate of the Green River basin. 



