WYOMING FORMATION. 19 



Denver Basin at sonic distance from the foothills can be only a matter of 

 conjecture in the present state of knowledge. It is possible that the lower 

 portions of the series, where the observed thickness is greatest, may corre- 

 spond to what is elsewhere considered Upper Carboniferous on similar 

 grounds of stratigraphical and lithological correspondence, and which, on 

 the Pikes Peak sheet of the Geologic Atlas, lias been designated by Mr. 

 Whitman Cross the Fountain formation. That the upper part of the typical 

 Red Beds are of Triassic age is rendered more than probable by the dis- 

 covery in them at various points in the Cordilleran region of characteristic 

 vertebrate and invertebrate remains, together with typical plants. It lias 

 seemed necessary, therefore, to recognize them as a distinct formation, and 

 the name Wyoming has been assigned to them because of their widespread 

 development in that State. 



In the present Held .Mr. Kldi-id^e has judged besl to divide the formation 

 into an upper ami a lower series on grounds of lithological composition 

 and structure. 



I.owi'.i; WYOMING FORMATION. 



This formation varies in thickness in this field from about 500 to 2,500 

 feet. It consists essentially of coarse sandstones and conglomerates with 

 subordinate red shales, and a few thin beds of limestone, generally compact, 

 with conchoidal fracture, and of light-drab or white color. The base of 

 the formation always consists of waterworn fragments 'if granite, gneiss, 

 and schist, or their constituents, feldspar and quartz, the prevailing red color 

 being largely due to the abundance of red feldspar. In the upper part is a 

 series of white sandstones, made np almost entirely of quartz grains, called 

 from then prevailing color the "creamy" sandstones. 



The variation in thickness of this formation is due mainly to the un- 

 evenness of the floor or sea-bottom upon which it was deposited. Besides 

 the regular slope away from the mountains, or eastward, it also deepened 

 southward. Moreover, in the neighborhood of Grolden a ridge or low prom- 

 ontory extended for some distance from the foothills, upon which in the 

 earlier part of the period there was no sedimentation. It is assumed, how- 

 ever, that the ocean-bottom was continually sinking during this period and 

 the sediments consequently advancing shoreward on the gentler slopes and 



