714 DESCRIPTIVE GEOLOGY. 



scarce in the range, and indeed, from here westward to the Sierra Nevada of 

 California, arborescent growth is scanty, even on the more elevated mount- 

 ain-tops, and confined to a few scattered junipers and pines. 



The southern half of the range is much more irregular in outline, vary- 

 ing in height from a few hundred to two thousand feet above the adjoining 

 valleys, the sedimentary beds being less persistent in structure, and broken 

 up by masses of volcanic rocks. 



Geologically, the West Humboldt bears a close analogy to the Pah-Ute 

 and Havallah Ranges, and, like them, is made up of an Archsean nucleus, 

 although here but a small body, around which, and resting unconformably 

 upon it, are Triassic strata of great thickness. These beds consist of quartz- 

 ite overlaid by interstratified beds of limestone, quartzite, and felsitic rocks, 

 which are in turn overlaid by Jurassic slates and shales. The Archsean and 

 Triassic strata are occasionally cut by dikes of intrusive rocks of Mesozoic 

 . age, those observed being chiefly diabase. 



Along the base of the range, and wherever the strata have been weak- 

 ened by displacement, by great flexure, or wherever they plunge rapidly 

 downward, intrusive masses of rhyolite or basalt occur, confusing the local 

 structure, and filling up great breaks in the Mesozoic depressions. Indeed, 

 the range aifords excellent opportunities for studying the relations between 

 points of greatest weakness in the sedimentary uplifts and the outbursts of 

 the Tertiary volcanic rocks. In one or two localities in the extreme foot- 

 hills, the upturned Tertiary strata, referred to the Miocene age, crop out, 

 but they are of little geological interest. 



Northern Region. — The northern half of the West Humboldt Range, 

 containing the oldest rocks and the best exposures of Star Peak Triassic 

 beds, the local name being derived from their great development on the 

 slopes of Star Peak, will be first described. 



On the west side of the range, and rising nearly to the summit of the 

 ridge, is a mass of granite about two miles in width, which is cut by two 

 deep canons, offering good exposures of the body. Throughout this gran- 

 ite, there is a series of structure-planes, striking northeast and dipping to 

 the northwest ; also, a series of occasional, nearly horizontal jointing planes, 

 while in places there is noticeable a rough tendency to conoidal forms. It 



