FORMATIONS AND PHYSICAL HISTORY 



489 



red U'ds which arc thought to be Triassic outcrop interruptedly 

 along the eastern base of the Rocky Mountains from British America 

 to New Mexico. These beds are thin, and contain more or less 

 gypsum and salt. Here and there they contain fossil leaves. 



Farther west, red beds have representation among the surface 

 rocks, and some of them are perhaps Triassic; but in much of the 

 \u>tern interior, undifferentiated Triassic and Permian rest con- 

 formably on Carboniferous (Pennsylvanian). In southwestern 

 Colorado and eastern Utah, the Trias is unconformable on older, 

 deformed, unfossiliferous red beds (presumably Permian), and on 

 strata of Pennsylvanian age. 1 



In the eastern part of the western area, the Triassic system is 

 thin, in places no more than 100 feet. To the west it thickens, 

 reaching 2,000 to 2,500 feet in the Uinta Mountains, beyond which 

 it again becomes thinner. 



The Pacific slope. The Triassic system has here its greatest 

 development in America. In the latitude of Nevada, the Pacific 



Fig. 416. Chugwater (Triassic) Red Beds near Shell, Wyo. (U. S. Geol. Surv.) 



seems to have extended eastward over the site of the Sierras to 

 longitude 117 (approximately). Farther north the shore line has 

 not been located definitely. It probably was irregular, and, in 

 1 Cross and Howe; Bull. G. S. A., Vol. xvi, p. 447. 



