624 DESCRIPTIVE GEOLOGY. 



traces of bedding- ; and it is possible that they are actual ejections of fine 

 mud. The r/hite rhyolite tufas of this portion of the range are so nearly 

 on a common level, that one is inclined to refer them to lake origin. It is 

 not often that two types of rhyolites so distinct in habit as these two varieties 

 can be observed so easily in the same locality. 



Tlie coarse porphyritic rhyolites which border the Shoshone Range 

 extend eastward across the valley, and skirt the base of Carico and Railroad 

 Peaks. In this way, rhyolitic formations completely encircle the northern 

 end of Carico Valley and the southwestern end of Crescent Valley. Indeed, 

 the two valleys are only separated by rhyolitic flows, which before the 

 eruption formed evidently one long northeast and southwest depression. 

 They are now connected by Rocky Pass, a narrow gap in the rhyolite, 

 through which in wet seasons the drainage runs from Carico Lake into 

 Crescent Valley and thence northward into the Humboldt River. 



Of the lithological character of the rhyolites bordering Carico and 

 Railroad Peaks, little need be said, since they bear so close a resemblance 

 to those already described, carrying in the same rough crumbling ground- 

 mass, broken crystals of sanidin and large grains of quartz. North of 

 Railroad Peak, the rhyolites attain a considerable elevation, reaching an 

 altitude of 1,500 to 2,000 feet above the valley, which in their mode of 

 occurrence present more the outlines of rugged granite forms than is usually 

 seen in rhyolitic masses. Here they form numerous sharp cones and pin- 

 nacles, with precipitous sides, and but little decomposed earthy debris. 



Carico and Railroad Peaks. — These somewhat isolated peaks belong 

 geologically to the same uplift and the same formation, attaining elevations 

 between 8,000 and 8,500 feet above sea-level. They appear in all respects 

 quite similar, even in topographical details and modes of weathering. They 

 form a broad mass, about 14 miles in width, of nearly pure white quartzite, 

 in places having a faint tinge of blue and in others a light brown. It pos- 

 sesses a dull vitreous lustre, a saccharoidal texture, and conchoidal fracture. 

 The quartz-grains are visible to the unaided eye, and appear more or less 

 rounded and with scarcely any base. Under the microscope, the quartz- 

 grains reveal numerous liquid-inclusions with mobile bubbles. A specimen 



