272 EEPOET UNITED STATES GEOLOGICAL SURVEY. 



doubtless in large part also to the condition of the bottom of the seas 

 which covered the area that now constitutes the regions here discussed. 

 It is sufficient in this connection to point out a single fact bearing upon 

 this subject. The strata of the Cretaceous formations of the West are 

 almost all and everywhere sandstones or sandy shales, and therefore the 

 whole sea bottom in those regions must have been almost always and 

 everywhere covered with sand. Our knowledge of the faunse of ex- 

 isting seas tells us that the continued prevalence of this condition could 

 not but have exerted a material effect upon the Cretaceous fauna of this 

 portion of ISTorth America. Notwithstanding these faunal differences in 

 the strata of the different regions, it is believed that they are all 

 respectively synchronous or nearly so ; and being so, it is expected that 

 the classification adopted by Hayden and Meek for those of the Upper 

 Missouri Eiver region may be appropriately applied to the greater part 

 if not all the Cretaceous strata of Western Iliforth America. 



