192 DESCKIPTIVE GEOLOGY. 



the Wahsatch Range on the west, and extending from the sources of the 

 Green River, in the Wind River Mountains, on the north, to the base of 

 the Uinta Range on the south. The northern portion of this basin, which 

 extends nearly a hundred miles beyond the limits of our exploration, resem- 

 bles so closely in physical features, as well as in geological structure, the 

 Tertiary area represented on the map, that a description of the latter may 

 answer to give a general idea of the whole basin. 



From the western flanks of the Park Range to the outlying ridges of 

 the Wahsatch, a distance of about 150 miles, extends a practically desert 

 region, watered only by a few widely-separated streams and scattered 

 springs of water often too impure to drink, whose surface is almost bare of 

 vegetation, having neither the coarse grass that covers the plains east of the 

 Rocky Mountains nor the abundant growth of desert shrubs which are 

 characteristic of the valley plains of the Great Basin. The topography of 

 the region is eminently suggestive of its geological structure, which is that 

 of a comparatively undisturbed succession of Tertiary strata deposited on 

 the bottom of an enclosed fresh- water sea. Its general appearance is that of 

 a great plain, or succession of plains, which rise in low terrace-like steps 

 from the centre of the basin toward its borders. In the western half, this 

 effect is most striking, the flat-topped ridges rising with so gentle and regu- 

 lar a slope from the line of Black's Fork toward the base of the Uinta 

 Mountains, that the peaks of this range seem like an island ridge rising out 

 of a level plain, when in reality there is a diff"erence of level of 2,000 to 

 3,000 feet between the middle of the basin and its border. On an east and 

 west line, the slope of the basin is still more gentle, its present level rising 

 only about 800 feet from the line of greatest meridional depression along 

 Green River to either shore. The topography of the eastern half of the 

 basin is, to a limited degree, more varied than that of the western half, since, 

 while in the latter, the horizon is always bounded by a perfectly horizontal 

 line, among the prevailing mesa ridges of the former, are some of more 

 irregular outline in the Cretaceous uplift of the region of Bitter Creek, 

 which represents a system of elevation existing previous to the deposition of 

 the sediment of the Tertiary seas. 



The minor streams of the basin, which have cut broad, but compara- 



