CHAPTER III. 



GEOLOGY OF THE SWEETWATEE YALLET AND ADJACENT 



KEGIONS. 



ELEVATED AREAS. 



The region to which this chapter is devoted comprises an area con- 

 taining all the Sweetwater drainage except that which enters into the 

 river within the metamorphic rocks. It forms a belt about thirty miles 

 wide, trending east and west. Longitude 107° west forms the eastern 

 terminus, while the western is located a short distance west of the Little 

 Caiion of the Sweetwater. 



Two small ranges of hills occur within this section. The one is that 

 of the Sweetwater Hills with their eastern extension, the Seminoles, 

 while the Granite Hills north of the river form the other. Orograph- 

 ically both of these are very simple, the former being merely an upheaved 

 chain, and the latter a group of rocky islands in a tertiary basin. Be- 

 sides these two small ranges a considerable elevation is reached by the 

 Sweetwater Plateau at its northern termination. 1 regard the western 

 end of the Sweetwater Hills, considering it from the standpoint of a 

 geologist, as being defined by the most westerly outcrops of the granitic 

 hills south of the river. 



Li order to present very briefly some of the most striking character- 

 istics of these hills, a few words may here be said in regard to their 

 structure. We may essentially consider the Sweetwater Hills as a small 

 anticlinal range, forming a continuation of the anticlinal upheaval east 

 of the Wind Eiver Mountains. In their more elevated portions, all met- 

 amorphics have disappeared, excepting those which, in the preceding 

 chapter, have been regarded as the youngest ones. Throughout the 

 entire eastern extension of the hills this character is maintained and is 

 made very apparent by the position of the sedimentary strata. Analyz- 

 ing this structure more carefully we find that in this instance the vsouth- 

 easterly extension of the anticlinal axis coincides with the axis of 

 elevation manifested by the Wind Eiver Eange. We have, therefore, 

 an anticlinal fold augmented by the force of general upheaval. This 

 results in placing the younger formations into more prominent positions 

 on the north side of the hills than on the south. When the maps of the 

 district shall have been prej)ared I will be better able to demonstrate 

 this fact. So far as possible, I will do so in the subjoined pages by pre- 

 senting a series of approximate parallel sections which will furnish some 

 idea of the relative distribution and position of strata. 



DEPRESSED AREAS. 



Of these we have but one in the region under discussion. It is the 

 valley or, geographically speaking, basin of the Sweetwater. Sloping 

 toward the river, both from the north and south, a shallow depression is 

 formed, which is cut into two parts of unequal size by the river. In the 



95 



