516 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



Jura from the Trias. He divided the Jura-Trias beds into the Shinarump, VermiUon Cliff, 

 White Chff, and Flaming Gorge groups, having a total thickness of 3,845 feet. The Trias of 

 the Fortieth Parallel Survey appears to correspond to the Shinarump, Vermihon Chff, and 

 White Cliff groups of Powell. The writer has classed the shales at the base of the Trias with 

 the Permo-Carboniferous. The line of division between the Trias and the Permian is placed 

 at the base of the massive cross-bedded sandstones. 



In the Uinta uplift the Trias, Jura, and Cretaceous strata are largely covered by Tertiary 

 sediments or glacial debris. The principal areas of the outcrop are in the Flaming Gorge 

 Canyon of the Green River in the vicinity of Split Mountain and Ashley and Dry Forks creeks, 

 and on the Duchesne River several miles below the mouth of West Fork, on the south side of 

 the range. The Trias is much thicker in the eastern part of the range. In the western part 

 the clayey beds of the upper part of the Trias were apparently not deposited. The coarse 

 cross-bedded sandstone, 300 to 400 feet thick, is a dark-buff color in the Duchesne region, 

 which varies to lighter color to the eastward. 



The Jura is composed of sandstones, shales, and clay beds, with a prominent Hmestone 

 series having a maximum thickness of 200 to 300 feet, the whole averaging from 600 to 800 

 feet in thickness, as determined by the Fortieth Parallel Survey. The Flaming Gorge group 

 of Powell is considered to represent the Jura. The writer considers the prominent drab hme- 

 stones to form the base of the series, the remainder of the Jura being formed of the overlying 

 sandstones, shales, and clays. 



The areas of outcrop of the Jura in the Uinta uphft correspond to those of the Trias and 

 appear to have a greater thickness of beds in the eastern part of the range. In the Duchesne 

 Valley the Jura is represented by a compact drab and gray hmestone and soft red calcareous 

 shales. The transition from the buff or brown cross-bedded sandstone of the Trias to the 

 Jurassic oohtic and granular limestone is made in less than 10 feet of strata. 



In northern Colorado Triassic rocks have been identified by fossils found on 

 Red Dirt Creek, a tributary of Grand River near the headwaters, northwest of 

 Leadville and south of the fortieth parallel.^*'' Still farther north, in central Wyo- 

 ming, Triassic fossils have been collected near Lander from strata called by Williston*" 

 thePopo Agie beds and described as being from "40 to 80 feet in thickness" and 

 occurring in the red bed?, about 200 feet below their top and 600 feet above their 

 base. The type locality is Popo Agie River, near the east end of the Wind River 

 Range, and thus not far from the Owl Creek and Bighorn ranges, in which Darton 

 has distinguished the supposed Triassic under the name Chugwater formation. 

 (See K 12-13, pp. 530, 531.) 



Regarding the possible absence of Triassic on the eastern side of the Front 

 Range in Colorado, where the red beds are believed to be largely if not wholly 

 Permian, Cross's views have been quoted. (See Chapter XI, pp. 490-492). Willis- 

 ton"^^ refers to the discovery by Marsh ®'^ of Hallopus in Garden Park, near Canon 

 City, and regards the fossil as significant of the Upper Triassic, although the exact 

 horizon can not be identified. 



Fenneman-^^^ describes the Fountain, Lyons, and Lykins formations of the 

 Boulder district, Colorado (north of the Canon City district), as possibly Triassic. 

 The Fountain, however, is of Pottsville age — that is, lower Pennsylvanian. (See 

 Chapter VIII, pp. 369-370.) The Lyons and Lykins are either Triassic or Permian. 

 Fenneman says : 



The rocks in this region, hitherto regarded as of Triassic age and called "Wyoming," 

 embrace three formations — the Fountain, the Lyons, and the Lykins. The term "Red Beds" 

 is popularly used without definite limitations, but the authors of the monograph on the Denver 



