264 DESCRIPTIVE GEOLOGY. 



west bank of Green Rivei', in the foreground, is composed of sandstones of 

 the Cretaceous formation, standing at 90°. The water at the extreme 

 right represents the mouth of Henry's Fork, at its junction with Green Eiver. 

 Owing to the small scale of the map, the structure of this ridge has neces- 

 sarily been much generalized, and the details of its structure are unavoid- 

 ably lost. 



A section of the beds exposed was made from Camp Stevenson to its 

 highest point overlooking the Horseshoe Cafion of Green River (Plate I), 

 and the general character of the beds and their approximate horizons are 

 given descending geologically, but ascending the ridge from Henry's Fork 

 Valley to the summit. 



The outer wall of the ridge shows a thickness of 50 to 100 feet of gray 

 flaggy sandstones, underlaid by a body of 100 to 150 feet of blue-gray 

 clays and shales, carrying small fish-scales, and enclosing a prominent bed 

 of coal, whose outcrops are too decomposed to give a definite thickness. Next 

 the clays are al^ut 150 feet of yellowish-gray sandstones, with carbonaceous 

 seams, which form the base of the Colorado series. 



At the top of the Dakota group are 150 to 200 feet of sandstones, which, 

 in the upper portion, have a brownish color, growing whiter in the middle, 

 and toward the base assuming a reddish hue, due to the oxidation of iron, 

 which in some parts forms a perfect stocJcwerJc of thin seams, standing out in 

 reticulated lines upon the surface. Below these are 75 to 100 feet of 

 striped clay beds, of pinkish and pale-green colors, succeeded by about 

 50 feet of impure sandstone, quite rich in iron oxide, and full of peculiar 

 cavities, from which concretions have dropped out, which separates these 

 from another series of variegated clays, of a great variety of colors, about 

 100 feet in thickness. Some of the clays of these beds are very pure, 

 and the white ones have almost the consistency of a kaolin. At the base of 

 these clays is a gravel conglomerate, characterized by small, rather angular 

 pebbles of black chert, which is the base of the Dakota group. 



The Jurassic formation, which was assumed to include all the beds 

 between the gravel conglomerate and the massive, crossed-bedded, buif 

 sandstone, was estimated to have a thickness of from 600 to 750 feet, where 

 crossed, though the continuity of the beds was broken by some covered 



