608 



BULLETIN UNITED STATES GEOLOGICAL SUEVEY. 



Following is a serial list of the groups of strata that have hitherto 

 been recognized and published, in the two regions respectively. As to 

 the geological age of a part of these groups, there has been, among 

 geologists, a difference of opinion. It seems clear that we have in those 

 regions an almost or entirely unbroken series, both stratigraphical and 

 paleontological, from earlier Cretaceous to at least Middle Tertiary. 

 There is, therefore, no well-defined line of separation between the strata 

 of these two great periods, but a certain portion of these strata form a 

 transitional series from those of one to those of the other period. This 

 fact is attempted to be shown in the following method of tabulating the 

 •groups :— 



THE GREEN EIVER REGION. 



^ 



Cenozoic, or Tertiary . . <J 



Post-Cretaceous 



Later Mesozoic, or Cre- 

 taceous. - i 



Earlier Mesozoic 



Brown's Park Group. 



Bridger Group. 



Green Eiver Group. 



Wahsatch Group. 



Laramie Group. 



Fox Hills Group. 



Colorado Group. 



Dakota Group. 



Jurassic. 



