HAVALLAH RANGE. 685 



weakness, the rhyolites have been forced out in large massive eruptions in 

 direct contact with the older beds, high up in the range, and extending 

 down to the Quaternary plains below. At the north, they occupy the 

 re-entering angle lying between the granite and the Triassic uplift, falling 

 away in low broken hills to the Humboldt River. Wherever examined, the 

 rock presented much the same general appearance in texture, color, and 

 mineral composition, possessing a groundmass varying from microfelsitic 

 to microgranular, with but little well-developed sanidin, mica, or horn- 

 blende. In color, it varies from light gray to reddish gray, the red tints 

 appearing as if caused by some decomposition product, which the micro- 

 scope shows to be derived from minute grains of ferrite. 



West of Fairbank's Point, on the extreme northern foot-hills, near the 

 Humboldt River, is an isolated outcrop of quite a singular rhyolite. It is a 

 coarse-grained crumbling rock of light pearl-gray color, made up of large 

 crystals of sanidin and cracked quarts;, with scattered hornblende particles 

 in a micro-crystalline groundmass. It may possibly be related to the quartz- 

 bearing trachyte south of Gold Run Creek, a reference which seems all the 

 more probable as it bears a close resemblance to the quartziferous trachyte 

 of White Rock in the Cedar Mountains of Utah. Under the microscope, 

 its groundmass is seen to be entirely crystalline, and made up apparently 

 of quartz and feldspar, containing also microscopical hornblende, mica, 

 and apatite. 



At the southern end of the range, the rhyolite forms a long nan-ow 

 ridge rising in places nearly 2,000 feet above the level of the valley, 

 which, across its broadest portion, scarcely measures four miles. The 

 ridges follow exactly the trend of the main uplift, and a marked instance 

 is afforded of the close relation existing between the older lines of upheaval 

 and the volcanic activity of the Tertiary age; for it is indeed somewhat 

 remarkable that such an immense mass of intrusive material, as is here 

 exposed, should be thrown up and confined within such narrow limits. This 

 rhyolite, wherever visited, scarcely differs in its general habit from those 

 bodies already described in the range; it is, perhaps, more compact, with 

 the fluidal structure in places well shown, and, as a result of this compact- 

 ness, weathering has frequently produced sharper and more angular forms, 



