2 58 



E. C. CASE 



they are deeply buried beneath the Triassic and most probably do 

 not appear on the flanks of the Rockies. The change from the 

 Clear Fork to the Double Mountain was a very gradual one but 

 sufficiently profound to render the presence of fossils in the beds 

 or of animals in the region during the time of deposition (the two 

 things are very different) almost impossible. 



The contact between the Triassic and the Double Mountain is 

 marked in several places by sharp unconformities, but the beds are 



Fig. II. — Vertical beds of Triassic age on the south side of the small valley at Las 

 Vegas Hot Springs. 



so irregular above and below that it is impossible to evaluate the 

 unconformity. It is certain that after a period of erosion or expo- 

 sure conditions of sedimentation closely resembling those of the 

 Clear Fork followed the more uniform conditions of the Double 

 Mountain. That this interval was long is suggested by the totally 

 new fauna contained in the beds, but this is not conclusive. The 

 fauna may have been evolving during the Double Mountain time or 

 may have started even earlier and reached this region by migration 

 at the beginning of the Triassic. 



The sediments of the upper part of the Permian were undoubt- 

 edly derived from some not very remote land mass but the outlines 



