224 DESCRIPTIVE GEOLOGY. 



and fragments, of quartz and feldspar, and minute black particles, whicli 

 may be hornblende or mica. Another light-colored siliceous bed, which is 

 more compact, though less fine-grained than the former, is characterized by 

 a conchoidal fracture and a peculiar silky lustre, and is made up largely of 

 small colorless acicular-shaped crystals. These siliceous beds pass through 

 fine-grained white siliceous limestones into fine calcareous shales, which 

 are extensively developed throughout the formation. With these are found 

 thin beds of carbonate of lime, in which a singular concretionary structure 

 is developed. The lower beds of the formation are of generally coarser ma- 

 terial, and contain numerous gravelly beds which pass into a sort of con- 

 glomerate. This is more particularly the character of the upturned beds 

 already mentioned along the northeastern edge of Brown's Park.^ From the 

 eastern end of Brown's Park, the strata rise gently toward the divide, and de- 

 scend again with a still more gentle slope into the valley of the Little Snake, 

 where the stream-beds are less deeply cut and the exposures more infrequent. 

 Throughout the valleys of the Little Snake and Yampa, the Tertiary 

 beds have been more generally disintegrated than in Brown's Park, and 

 form more rounded ridges, in which but little of the undecomposed rock 

 can be found. The character of the beds is, however, apparently similar 

 to that of those observed in Brown's Park. Along the western base of 

 Yampa Peak, the Tertiaries are concealed beneath considerable accumula- 

 tions of sand. Li the basin east of Yampa Peak, where the streams have 

 cut deeper into these beds, good exposures are found, and the material of 

 which they are composed is seen to be quite similar to that in Brown's Park. 

 They occupy an approximately horizontal position, but slope up slightly 

 toward the flanks of the hills. On the northern face of the White River 

 divide, to the southwest of Yampa Peak, is a considerable thickness of 

 sandy and gravelly beds, having a dip of 3° to the northward, overlying 

 the upturned edges of the Cretaceous strata whicli form this ridge. They 

 here reach a height of 1,500 feet above the valley of the Yampa, and seem 

 to be remnants of higher beds of the series, which have been entirely carried 



^ Since t be above was written, it has been reported th;»t fossils of a Pliocene type 

 have been found in the Tertiary beds of Brown's Park. If this be true, it is probable 

 that these upturned beds may represent a lower unconformable series, corresponding 

 in age to the Green Eiver beds, to which the whole gronp had been referred by us. 



