222 DESCRIPTIVE (*EOLOGY. 



patches of conglomerate containing- pebbles of red quartzite, which have 

 also been referred to the Wyoming Conglomerate. 



Brown's Park and thr Yampa Valley.— The valley of Brown's Park 

 is a bay-like depression, from 6 to 8 miles in width, occupying the geological 

 axis of the eastern end of the Uinta Mountains. It is the deepest valley of 

 the Green River Basin, its average elevation being but little over 5,000 feet. 

 Its surface is occujDied by a series of flat Tertiary ridges, intersected by the 

 broad alluvial bottoms of Green River and its tributary creeks. From the 

 eastern end of the valley, these mesa-ridges rise gently to the eastward, at- 

 taining an elevation of about 1,000 feet above the level of Green River, on 

 the divide between it and the Little Snake, and descend as gradually to the 

 east. The continuation of this line of depression, in the valleys of the Little 

 Snake and Yampa Rivers, extends as far as the upper canon of the Yampa 

 River, and includes also the enclosed valley of Lily's Park at the base of the 

 White River divide. The whole of this low region is occupied by a series of 

 Tertiary beds of somewhat different lithological character from any of those 

 hitherto described. The material of which they are composed is extremely 

 fine-grained, very white in color, and contains a comparatively large propor- 

 tion of carbonate of lime. From these Tertiary beds, no fossils were obtained 

 by our parties which could determine their geological horizon. Their strati- 

 graphical relations, as well as the predominance of calcareous beds, ally them 

 most nearly to the Green River group, to which they have consequently 

 been referred. Their extremely local development, their peculiar lithological 

 character, and the fact of the decided non-conformity of their beds with the 

 underlying Tertiaries, along the line of Elk Gap and Godiva Ridge, suggest 

 . the possibility that they may represent a local development of one of the 

 later Tertiary formations. There can, however, be little doubt that the 

 beds of the Green River group have occupied this region, and that, if these 

 beds are later, the latter underlie them. These white Tertiaries occupy, in 

 general, a nearly horizontal position, sloping a few degrees in approximate 

 conformity with the slopes of the valleys which they occupy. 



In the view, given in Plate IV, of Brown's Park, taken from the hills at 

 the mouth of the canon of Lodore, looking across the valley in a northerly 

 direction, the white line of the Tertiary beds can be distinguished on the 



