THE LARAMIE FORMATION AND THE SHOSHONE GROUP 35 



or the other of these conditions exists in many locahties that have 

 been closely studied and may possibly be realized throughout the 

 area in which the Shoshone beds now occur. But there is at least 

 a theoretical possibility that in some places now represented by 

 exposures deposition was continuous from the marine Cretaceous 

 to the Fort Union epoch. In such places Laramie and Shoshone 

 strata come together in conformity or a transitional formation 

 must be created. It should be kept in mind while considering the 

 subject that this situation is not peculiar to the boundary between 

 Laramie and Shoshone beds but may arise in any case where it is 

 desired to separate strata preceding from those formed during or 

 immediately after a great uplift. 



The line to be drawn between Laramie and Shoshone in a series 

 of conformable and consecutive deposits must be determined at 

 some future time, when such a series is definitely known. The 

 criterion for the distinction may be a lithologic or paleontologic 

 change, or as is more probable, some arbitrarily selected feature. At 

 the present time, it seems to me that the difficulty of distinguishing 

 between Laramie and Shoshone beds is either a general theoretical 

 one, or a local and practical one due to a lack of knowledge concern- 

 ing the true relations and paleontologic contents of the strata in 

 some special district. 



The characterization of the difficulty as perhaps largely theoretical 

 rests on the following consideration: The uplift terminating the 

 widespread Laramie sedimentation doubtless caused at first a rela- 

 tively small landmass, or perhaps several of them. Denudation of 

 such land areas must have begun at once; erosion and deposition, 

 either continental or submarine, were, indeed, accompaniments of 

 uplift. But as larger and larger tracts gained some notable elevation 

 erosion must have quickly removed a great part of the most recent 

 deposits, if not all of them. The whole of the Rocky Mountain 

 province may thus have been denuded of the beds which would con- 

 stitute a transition series if preserved. In this case a complete sec- 

 tion must be looked for in outlying areas which were but little ele- 

 vated during the Shoshone epoch. On the Great Plains side it 

 is clear that such beds can be found only in quite restricted districts, 

 owing to Tertiary erosions or a mantle of later deposits. On the 

 west, too, the areas within which transition beds can exist are few. 



