636 DESCEIPTIVE GEOLOGY. 



SECTION II. 

 FEOM EEESE EIVER TO OSOBB VALLEY. 



BY S. F. EMMONS. 



Southern Shoshone Range. — In the deep cut made by Reese River 

 through this range, only a few exposures of sedimentary rocks are seen. 

 The hills around are entirely made up of rhyolites, which slope toward 

 the depression of this canon. It is evident that the main erosion of the 

 range had taken place previous to the outflow of rhyolite, and that this 

 pass probably existed prior to that period. In almost all the Nevada 

 ranges, where the oldest sedimentary rocks have been overwhelmed by 

 volcanic flows, subsequent erosion lays bare what were the peaks, and it 

 is on the higher parts of the range that we usually find the underlying 

 rocks, whereas the passes, in spite of modern erosion, are generally occu- 

 pied by the later rocks. From this, it appears that the great erosion is 

 altogether prior to the rhyolite period. This is also evident from the mod- 

 erate thickness of the Quaternary, and the proportionately greater thickness 

 of the fresh-water Tertiaries, which antedate the rhyolite period. While 

 the sharp post-glacial cutting has unquestionably modified the topography 

 to a very great extent, yet the larger blocking out of the peaks and passes 

 and the general shaping of the sedimentary masses were all done before the 

 volcanic period. 



The general structure of the Shoshone Range south of Reese River, as 

 far as the few outcrops of older rocks indicate, is that of an original Archaean 

 island of granite, wrapped around by fine-grained micaceous . slate, which 

 forms the main north and south ridge, to the east of which is a second line 

 of elevation of later origin, represented by Ravenswood Peak and Jacob's 

 Promontory, which is composed of series of quartzites and overlying lime- 

 stones, which have been traversed and partially concealed by later eruptive 

 rocks. 



Archaean granite is exposed along the crest of the ridge for a dis- 



