ELKBEAD MOUNTAINS. 167 



SECTION VII. 

 ELKHEAD MOUNTAINS. 



BY S. F. EMMONS. 



General Description. — To the west of the Park Rang-e, on the borders 

 of the broad Tertiary plains of the Green River Basin, Hes a singularly pic- 

 turesque and beautiful group of high volcanic peaks, known as the Elkhead 

 Mountains. Their steep, rugged slopes are covered for the most part to their 

 very summits with a dense growth of pine forests, while the valleys which are 

 enclosed between them present a pleasing variety of open glades and groves 

 of quaking-asp and pine. The highest peaks, which attain an elevation of 

 over 10,000 feet above sea-level, are arranged somewhat in the form of a 

 cross, of which one bar is formed by the north and south trachytic eleva- 

 tions of Whitehead Peak and Steves Ridge, and the other by the east and 

 west ridge of basalt, of which Anita Peak and Mount Weltha are the cul- 

 minating points. Out of the gently-sloping plains in the northwest angle 

 of this cross rise a number of isolated peaks and dike-like ridges both of 

 trachyte and basalt. 



From the few outcrops of sedimentary rocks exposed along the bases of 

 these peaks, and in some cases high up on their slopes, it is evident that the 

 eruptive rocks broke through and covered a pre-existing line of elevation 

 of Cretaceous, and possibly also Tertiary beds, whose summits, and a portion 

 of whose slopes, have thus been preserved from erosion by their envelope 

 of more resisting volcanic rock. 



From a mineralogical point of view, the eruptive rocks of this region 

 form a remarkably interesting and peculiar group, being characteristically 

 different from any of the widespread groups of volcanic rocks, which cover 

 so large an area in the western portion of the region embraced within our 

 explorations. They consist mainly of quartziferous trachytes and nepheline- 

 basalts. 



The trachytes, which belong essentially to the class of sanidin- 



