ASHLEY CEEEK BASIN. 295 



edges of the Triassic and Jurassic beds. We may therefore conclude that 

 the denudation of this basin has taken place principally since the time of 

 the deposition of the Eocene Tertiary, and that a very considerable amount 

 had already been accomplished previous to the deposition of the Wyoming 

 Conglomerate. 



In the lower portion of the valley of Ashley Creek, and between this 

 creek and Brush Creek, is found a country of low benches, made up, as 

 proved by occasional exposures in some of the cuts, largely of blue and 

 yellow clays, but in which no structure-lines could be observed. On the 

 northern face of Wonsits Ridge, near Green River, were found, however, 

 beds of yellow and white massive sandstone, with more shaly beds between, 

 resembling, in lithological character, the rocks of the Fox Hill group, to 

 which they have been referred, though it was impossible to form a very 

 definite idea of their relative horizon. The beds exposed here are, at the 

 base, several hundred feet of brown, shaly sandstone, overlaid by 100 feet 

 of very white massive sandstone, 50 feet of partially bituminous sandstone, 

 a bed of greenish coarse-grained sandstone, and 50 feet of highly bitumin- 

 ous sandstone, the whole covered unconformably by conglomerate and red- 

 dish gravelly Tertiary beds. This bituminous sandstone is rather a pecu- 

 liar occurrence. On its weathered surface, it can hardly be distinguished 

 from ordinary sandstone, but, in fracture, is perfectly black. The bitumin- 

 ous matter is not very inflammable, but is driven away in a very hot fire, 

 with but little flame. It is probably a variety of asphalt. The specimens 

 obtained from these beds give about 11 per cent, of bituminous matter and 

 85^ per cent, of silica. The sandstone strata have a strike at this point of 

 east 20° south, and dip of 20° southwest, and are seen to form outlying 

 ridges in the White River Valley, on the eastern side of Green River, run- 

 ning generally east and west, some distance to the south of Section Ridge. 

 In their outHne, the S-like curves of the lower beds have been generalized 

 into one sweep around the outer edges of all these curves, while the interval 

 between these and the harder sandstones of the base of the Colorado group, 

 in which all structure has been obliterated by the decomposition of the 

 clay beds, is supposed to be made up mostly of the heavy clayey beds ol 

 the Colorado group. 



