13 



extends from the llawliiis uplift west to the Green Eiver, and from the 

 Colorado line north to the bluffs south of the Sweetwater Eiver, it can 

 best be described as follows : In the western portion there is an island- 

 like mass of Cretaceous rocks surrounded by Eocene Tertiary beds. 

 The Cretaceous exposure has been caused by an anticlinal fold whose 

 western limit is some distance north of Salt Wells, and which extends 

 in a southern direction nearly to the State line. The exposed core of 

 this fold belongs to the Fort Pierre group, around which are extensive 

 beds of Fox Hills and Laramie formations. (Ju the east Hank of this 

 fold are the coal mines at Point of Pocks, Hallville, and Black Buttes; 

 on the west are the celebrated coal mines at Rock Springs. To the 

 west of Rock Springs the (ireen River Eocene ( ?) lies conformable 

 ui)on the Larjimie. 



The Green River beds are made uj) of shales that are cut by deep 

 and almost inaccessible canyons. The canyon of the Green River is 

 at some points more than 1,000 feet deep. 



From Black Buttes eastward there is a synclinal basin that is cov- 

 ered with Wasatch and Bridger Eocene, beyond which to the east the 

 Laramie outcrops and extends from the State line north to Green 

 Mountain, forming the eastern bouiulary of the desert. 



The Tertiary rocks covering the basins are quite level, and only form 

 bluffs and escarpments occasionally along the railroad. To the north 

 the country is level for from 30 to 40 miles to where it is hemmed in 

 by high and precipitous bluffs. This rough country extends along the 

 entire northern border. In the northwestern portion the country is 

 very broken, and the castle-like buttes rise to a height of 2,000 to 3,000 

 feet above the level country. 



Oregon Butte is a famous old landmark, and is in reality a mountain 

 made by erosion. 



South of the railroad the country is also broken, but is in no way 

 comi^arable with the northern border. 



Another interesting feature is the eruptive overflows in the Creta- 

 ceous rocks some 12 or 15 miles north and west of Point of Rocks. 

 The main eruptive mass is known as the Leucite Hills, but there are 

 numerous outlying elevations, such as Pilot Butte, Flat Top, and 

 Black Rock Butte. 



SOILS. 



The soil of the Red Desert differs materially in the different parts of 

 the region. It is, however, little more than the geological formations 

 would indicate. Probably all the soils of the region must be charac- 

 terized as saline, but the absolute amount of salts present in any par- 

 ticular locality depends to a great extent upon the conformation of the 

 surface. Through long-continued processes of leaching some forma- 

 tions have lost and others have gained in salt content. Flats and 

 basin-like depressions, receiving the drainage from the slopes, have 

 become more and more heavily impregnated. The rainfall is too limited 



