Mesozoio and Cmnozoic, Geology and Paleontology. ' 131 



S. F. Emmons,* geologist of the division west of North Platte, said 

 that Bridger's Pass, which connects the valle_ys of the Upper Sage 

 iiveek and the south fork of the Little Mudd}-, has been eroded out of 

 the soft beds of the Colorado Cretaceous. Along the'northern and western 

 borders of this valley extends a ridge of white massive sandstones 

 of the Fox Hills Group, standing at angles of 10° to 25°, and curving 

 in strike approximately with the shape of the ridge. To the north of 

 tlie gap, they form a continuous ridge about 15 miles in length, showing 

 n blutf face to the southwest toward Bridger's Pass, at the base of 

 which are exposed the clayey beds of the Colorado Group.: A thick- 

 ness of 3,000 to 4,000 feet of heavy-bedded sandstones, mostly white 

 and buff, with a few included beds of shale, and some thin seams of coal, 

 dipping to the northward at an iingle of 10° to 20°, is exposed. 

 ' In going northward from a point on the Little Muddy, about five 

 miles west of the Sulphur Springs, a thickness of between 3,000 and4,000 

 feet of beds of the Laramie Group, dipping northwest at an angle of 

 20°, is crossed. Of these, the lower 2,000 feet are composed of massive 

 white and^^ellow sandstones, in which the shale beds are of subordinate 

 importance. The upper sandstones are stained and striped in red, by 

 iron oxide, and form ridges with considerable clayey valleys between. 

 In the upper 800 feet are several coal seams, and near the top is a 

 prominent bed of bright vermilion color, only a few feet in thickness, of 

 fine-grained, hard, argillaceous material, abounding in well preserved 

 impressions of leaves. This is overlaid b3' a white sandstone, about 

 200 feet in thickness, carrying a coal seam, which in turn is capped by 

 a thin-bedded brown sandstone, which weathers into flags about three 

 inches in thickness; the dip of these upper beds has shallowed to 10°, 

 and to the north the beds of the Laramie Group are practicallj 

 horizontal. 



The exposures of the Fox Hills Group, as seen in Bear Ridge, near 

 the valley of the L^pper Tampa river, show a series of massive, white, 

 fine-grained sandstones of several thousand feet in thickness. 



The Cretaceous of the Uinta Mountain region consists of over 

 10,000 feet of beds of sandstones and clays, carrying coal seams,which 

 are most abundant in the upper part of the series. The Dakota Group 

 consists of about 500 feet of rather thinly-bedded sandstones, with 

 some claj^ beds, having at its base the persistent conglomerate carr}'- 

 ing small pebbles of black chert. The Colorado Group, about' 2,000 

 feet in thickness, is made up mostl}* of claj'S and j'ellow marls, with 



* Geo. Sur. 40th parallel. 



