198 BULLETIX UNITED STATES GEOLOGICAL SURVEY. [YoLY. 



their deposition in shallow waters, and tlie thickness attained is from 

 3,000 to 5,000 feet. 



The lower sandstones are similar to those of the Fox Hills Group of 

 the Cretaceous, and the two formations are conformable. As far as noted, 

 the conformability extends from the Carboniferous to the top of the 

 Laramie Group, as exposed in the district under consideration. 



There were some indications in Ham's Fork Caiion that the Laramie 

 Group rested on the upturned edges of the Triassic Eed Beds, but the 

 relations were obscure, and all other occurrences seemed to indicate that 

 the Laramie Group is conformable to the underlying formations. 



It should be stated here that the western shore-line of the Post- 

 Cretaceous has not been identified, and that it is probable that if the 

 peripheral i:)ortions of the Laramie Group could be observed, as Dr. 

 White remarks, some unconformity would be seen between it and the 

 Fox Hills Group. He says: "There must necessarily be some uncon- 

 formity between these groups in the i^eripheral portions of the Laramie, 

 because, as will be shown farther on, the area ui)ou which its waters 

 rested was cut off from the great open sea by the elevation of portions 

 of the bottom upon which the Fox Hills deposits were made."* 



Wherever the Wahsatch Group was seen in superi^osition with the 

 Laramie Group in our district, it was markedly unconformable, and also 

 represented only the u];)iier portion of the group. The regions in which 

 it occurred were near the shore-lines of the Wahsatch Lake, and its con- 

 glomerates were seen resting unconformably on Silurian, Carboniferous, 

 Jura-Trias, and Cretaceous stra;ta, as well as on Post-Cretaceous. A 

 considerable portion of the area that was below the level of the Laramie 

 Sea became land probably before the end of the i^eriod, as the condition 

 of the strata extending from Silurian to the Post-Cretaceous, inclusive, 

 shows that a vast period of erosion intervened between the uplift and 

 the time when the Wahsatch Group, or rather its upper x)ortion, was 

 dex)Osited on the eroded edges of the older beds. This might perhaps 

 be explained by the fact that a subsidence occurred which allowed the 

 Wahsatch Lake to spread gradually over the regions where now we find 

 only the upper beds of the groui^. This erosion, however, was on an enor- 

 mous scale, and the whole of it could scarcely have been effected during 

 the early Wahsatch time. Again : the fossils from the grouj) in our 

 district are all equivalent to those of the Bear Elver Estuary Beds, 

 which Dr. White is incliiied to think are older than the fossils from the 

 group at other locahties. He therefore thinks the upward movement of 

 the Post-Cretaceous may have begun before the complete deposition of 

 the strata composing the group. 



This fact as to the age of the fossils taken in connection with the strati- 

 graphical facts just noted render it very probable that the beds of the 

 group exposed in Western Wyoming and Southeastern Idaho represent 

 only the lower portions of the Laramie Group. • 



* Bulletin of U. S. Geol. and Geograpli. Surv. of the Terr., vol. iv, No. 4, p. 866. 



