140 



REPORT UNITED STATES GEOLOGICAL SURVEY. 



ular succession without difficulty. A more intricate question is ^pre- 

 sented in the position occupied by the Sweetwater Group. By Dr. 

 Hayden it is considered as being of the same age as the Wind River 

 Group. This latter he places under the White River series, which is, 

 according to him, probably synchronous with the Bridger. lie places, 

 therefore, the Sweetwater Group between Green River and Bridger. 

 Comstock finds its resemblance greater to Bridger than to Green River, 

 and King places the White River above the Bridger. But little appli- 

 cable information could be obtained from the stratigrapnical relations 

 of these groups within our district. Reviewing their evidence so far as 

 it pertains to this point, we find a few facts which may tend to throw 

 some light upon the subject. Station 21 is located upon Bridger, over- 

 lying Green River. Had the hills composed of Sweetwater beds existed 

 at the time the Bridger was deposited, we would expect to find, from 

 the configuration of the country, Bridger superimposed upon Sweet- 

 water, or evidence of its having been removed. Neither was observed. 

 The Sweetwater Hills most closely approaching the Bridger show no 

 evidence of having been extensively denuded at that locality. The sys- 

 tem of drainage by which the Bridger beds of that immediate region 

 were cut, is the one which must have existed prior to the deposition of 

 the Sweetwater Group. For these reasons I place the latter as chro- 

 nologically succeeding the Bridger. Younger Tertiary beds, which are 

 found in the Sweetwater Valley, Dr. Hayden has shown to be identical 

 with the Pliocene deposits of the Niobrara River. To the same group 

 I count the local Pliocene strata west of South Pass. As the youngest 

 Tertiary Group, we have the Wyoming conglomerates. Although this 

 enumeration of groups comprises but a portion of those forming the 

 entire Tertiary formation of the West, we may regard their relative 

 position as established. Difference of opinion prevails, however, as to 

 the places they should occupy in systematic classification. 



The following table will furnish a synopsis of the formations younger 

 than Cretaceous observed in the Sweetwater district : 



Formation. 



Division. 



Group. 







Sand-dunes, soil, loose drift, and gravel. 







Lake beds, glacial deposits, and old river-drift. 





Tertiary i 



Pliocene \ 



Wyoming conglomerates. 



Mobrara Group. 





Sweetwater Group. 



Bridger Group. 



Green Itiver Group. 





Wasatch Group. 









Laramie Group. 











Fox Hills Group. 







