760 DESClilPTIVE GEOLOGY. 



be wanting. Sphserulitic forms, so common in pearlites, are everywhere 

 scattered throughout the gray glass. 



About 4 miles south of Valley Caiion, and rising out of the plain, which 

 is here formed of Humboldt Pliocene, thinly covered with Quaternary, is a 

 mountain-mass GOO feet high, known as Lovelock's Knob. It is essentially 

 a mass of granite, overflowed by rhyolite, which cover the greater part of 

 the hill, and carry here and there fragments of included granite that were 

 evidently picked up on the surface, since they are distinctly rounded and 

 water- worn. These rhyolites present a great variety of colors, of which the 

 more prevalent is a yellowish flesh-tint. There are also pale olive-gray 

 varieties, containing pink brecciated masses ; still others are lavender and 

 reddish-gray. In general, these rhyolites possess a microfelsitic groundmass, 

 with but few well-developed mineral secretions, and these mainly brilliant 

 fragments of sanidin. It is quite remarkable that rocks in which chemical 

 analysis detects scarcely any difl'erences, with identical mineral composition, 

 and apparently the same geological relations, should, in their physical 

 aspects, vary so greatly within such limited areas. Under the microscope, 

 these variations are, in a measure, explained by the great difi'erences in the 

 detailed habit of the groundmasses, which here present many of the charac- 

 teristic structural phenomena of rhyolitic rocks. 



On the northwest side of Lovelock's Knob is a small outburst of black 

 basalt, penetrating both the granite and rhyolite. 



From Granite Point, for about 15 miles in a northwest direction, there 

 is a long sheet of basalt, which has overflowed the rhyolite and covered 

 even the summit of the range. These basalts cap heavy masses of Archaean 

 rock, which occupy the carton-bottom of the pass leading from Lovelock 

 Station to Sage Valley. To the southeast, the basalts flow down to within 

 a mile of the extreme point, in irregular fields, which are more or less 

 eroded, showmg grayish and purplish rhyolites along the western slope of 

 the mountain promontory, and a large field of white rhyolite near the rail- 

 road. On the northeast side of Granite Point promontory, erosion, cutting 

 deeply into the basalt, displays three considerable outcrops of granite, upon 

 whose flanks rest highly metamorphosed slates, in general representing those 

 of French Canon, the beds on the south side of the body dipping to the 



