848 - DESCRIPTIVE GEOLOGY. 



West of Black Mountain, at the head of Louis Valley, occurs a body 

 of rhyolite, lying between the granite and the main mass of the Virginia 

 Kauge, which has escaped the great flow of basalt. It is a reddish-gray 

 rock, with a microfelsitic groundmass, and the characteristic banded struct- 

 ure of rhyolites. Sanidin and mica are the prevailing constituents, the 

 latter being remarkably well developed in minute flakes. Hornblende 

 appears to be wanting, A thin section of this rock has been illustrated by 

 Professor Zirkel, in Plate VI, fig. 4, accompanying his report, showing the 

 marked manner in which the mica flakes have been broken and twisted. 

 It is interesting to note that on the opposite side of the range, a little north 

 of east, there crops out from beneath the basalt a very similar looking rock, 

 having the banded fluidal structure of the groundmass remarkably well 

 developed; indeed, there are few specimens in the collection that show this 

 structure as well. The rock is more porous than the one from Louis Valley, 

 and there is no macroscopical mica, but instead a few minute black horn- 

 blendes. 



North of the basaltic body, the Virginia Eange terminates in a group 

 of low hills, which border Pyramid Lake on the northwest, and connect 

 with the southern end of the Madelin Mesa. Astor Pass cuts through 

 these hills, connecting Pyramid Lake with Honey Lake Valley of California, 

 and lies below the level of the ancient La Hontan Lake, the calcareous 

 tufas covering the flanks of the hills, and showing conclusively the flow of 

 those alkaline waters westward beyond the boundary of Nevada. 



On the geological map, these hills are colored as trachytes ; it is proba- 

 ble, however, that rhyolites ai'e represented here ; indeed, the entire group 

 belongs to that class of rocks which stand on the border line between these 

 two types of acidic rocks. They are characterized by reddish-brown and 

 ■gray colors, a decidedly crystalline texture, with the individual minerals 

 usually well developed. One of the most striking rocks of the region, and 

 one characteristic of Astor Pass, is found near the entrance oif the pass, and 

 about 4 miles northwest from Pyramid Lake, where it forms broad table- 

 like masses. The prevailing color of its groundmass is brownish gray, in 

 which, forming the greater part of the rock, are porphyritically enclosed 

 crystals of feldspar, mica, hornblende, and quartz. Many of the feldspars 



