CENTEAL AND EASTEEN NEVADA. 329 



centrated in bands or bunches. It forms the southern foot-hills of Telegraph 

 Peak, underlying the nietamorphic rocks, which are dark-blue siliceous lime- 

 stone-shales, dipping to the north and east at a low angle. At Telegraph 

 Pass it is exposed on the eastern slope of the ridge, where it is traversed by 

 a dike, about 15 feet wide, of white granulitic rock, containing sparse crys- 

 tals of mica, and black crystalline grains, probably of tourmaline, concentrated 

 in bunches throughout the mass; this dike has a northwest strike, which. is 

 the direction of the principal veins of this neighborhood. Granite forms the 

 crest of the ridge, as far south as Mount Prometheus. This peak is chiefly 

 conspicuous as forming the summit of Lander Hill, the spur in which have 

 been found the richest veins of the district; its summit, and the flat-topped 

 ridge at the head of Marshall's Canon, are formed of rhyolite, which covers 

 the eastern slopes of the ridge, and forms the head of the Park basin ; while, 

 on the main ridge south of Marshall's Canon, are found the dark metamor- 

 phic slates of Telegraph Peak, dipping southeast at a low angle with a north- 

 east strike, and separating the granite, on which they rest, from the rhyolite, 

 which forms the saddle connecting this ridge with the hills north of Geneva. 

 The rhyolite of Mount Prometheus is not a very characteristic variety, inasmuch 

 as it contains but little free quartz, while that of the Marshall's Canon ridge is 

 very like a trachyte, or, in some respects, an older porphyry ; but their mode 

 of occurrence, geological relations, and certain mineralogical characteristics 

 favor the assignment to them of a rhyolitic rather than a trachytic origin, 

 while their resemblance to the older porphyries is confined to the compact 

 matrix of a limited local occurrence. 



The mass of Prometheus and its eastern slopes are composed of a brown- 

 ish-purple vesicular rock, in which a microcrystalline feldspathic paste incloses 

 crystals of glassy feldspar, magnesian mica in large quantities, and occasional 

 grains of smoky quartz ; largely disseminated throughout the mass, and lining 

 the cavities, which are frequently as much as an inch in diameter, are spher- 

 ulitic concretions of feldspar, whose occurrence is generally considered to be 

 characteristic of rhyolitic rocks. On the western crest of Prometheus, to- 

 ward the main granite body, this rock passes into a black pearlite, in which 

 the spherulitic structure is still found, though not so prominently developed 

 as in the former, nor with the same concentric structure. The vitreous paste 

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