26 GEOLOGY. 



sionally they contain fossil leaves of types which seem to ally the beds 

 with the Trias of the east. In Wyoming, the Triassic beds, because 

 of their high color and unique mode of erosion, are the most conspicuous 

 formations of the State. 1 The Triassic beds of this region are not 

 always readily distinguished from the Permian on the one hand, and 

 from the Jurassic on the other. So difficult is the separation, that 

 the Trias and Juras of this region are often grouped under the name 

 Jura-Trias. Triassic beds have, however, been identified by means 

 of fossils, in the Wind River region of Wyoming, where the fossil-bearing 

 beds are 550 feet above the base of the Red Beds and 250 feet below 

 the top. 2 The upper part of the Red Beds in this region is gypsiferous. 

 Triassic beds have also been recognized in southern Wyoming by their 

 vertebrate fossils. 



Farther west, so far as the country has been carefully studied, 

 Red beds have frequent representation among the surface rocks, but 

 the outcrops are usually confined to narrow belts about the moun- 

 tains where uplift and subsequent erosion have exposed the edges of the 

 strata, or in valleys excavated through younger formations.. Through- 

 out all this region, red sandstones and shales make up a notable part 

 of the Triassic system. Conglomerates are present locally, and gyp- 

 sum is a common accompaniment of the clastic beds over most but 

 not over all of the area. 3 



In southwestern Colorado, and in the adjacent part of New Mexico, 

 some of the Triassic deposits seem to have been made in fresh water. 4 

 The fresh-water beds here and in Texas, and the salt-lake deposits 

 over many other parts of the inland region, suggest that the Triassic 

 sediments of different localities were laid down in separate basins. 

 In much of this western interior region the undifferentiated Triassic 

 and Permian rest conformably on the Carboniferous (Pennsylvanian), 

 though occasionally, as in some parts of Wyoming, they overlap it 

 and rest upon pre-Cambrian formations. Where non-f ossiferous Red 



1 Knight, Bull. 45, Wyoming Exp. Station, p. 133. 



2 Williston and Branson, unpublished data. 



3 For details, see the following folios of the U. S. Geol. Surv.: Ten Mile, Anthracite 

 and Crested Butte, Telluride, Walsenburg, Pike's Peak, La Plata, and Pueblo, Colo.; 

 Fort Benton, Little Belt, Livingston, and Three Forks, Mont.; Yellowstone Park, 

 Wyo.; also Gilbert, 17th Ann. Rept. U. S. Geol. Surv., Pt. II, p. 560; and Knight, 

 Bull. 45, Wyo. Exp. Station. 



4 Dolores formation, Telluride folio, U. S. Geol. Surv. 



