480 DESCRIPTIVE GEOLOGY. 



quite opaque. It is seen to contain, besides the sanidin and hornblende, 

 an apparently large amount of plagioclase, with a little pale, yellowish 

 augite. 



At the very mouth of Spring Canon, under the rhyolite flows, is an 

 exposure of andesite, which also forms an isolated butte to the south of 

 Last Chance Spring. Both these rocks have been classed by Zirkel among 

 the augite-andesites. The rock at the mouth of the canon has a dark-gray, 

 compact groundmass, in which are crystals of black, fresh-looking horn- 

 blende and plagioclase-feldspar, with occasional black micas. Under the 

 microscope, the groundmass is seen to have the glassy texture peculiar to 

 the augite-andesites. There are also detected a few sanidin-feldspars and 

 dark yellow augites, which are somewhat dichroitic. No olivine is found. 

 This rock has all the external habit and somewhat greasy lustre of an 

 andesite; that from the butte, however, is more like a basalt in its external 

 appearance. It is a dark, compact rock, with a conchoidal fracture, in 

 which but few macroscopical crystals can be detected. It has a somewhat 

 resinous lustre, and, under the microscope, is seen to be rich in plagioclase- 

 feldspars and yellow-brown augites, both of which contain glass-inclusions, 

 while the groundmass shows the characteristic interwoven network of micro- 

 lites. 



North of Spring Canon, the volcanic rocks lie next the granite bod)^, 

 falling off gradually toward the Desert, with a rough, broken surface of hills 

 and ridges quite characteristic of rhyolite regions. Of the only two volcanic 

 rocks observed here, andesites and rhyolites, the latter occupy by far the 

 greater area; the former, indeed, only reach the surface in one or two 

 isolated hills, where erosion has apparently worn away the softer, super- 

 imposed rhyolites, leaving the compact andesite in bold outcrops. There 

 are but few localities, if any, within our belt of exploration, where the rela- 

 tion between the two can be better observed than in the Wachoe Mount- 

 ains. Here the contact between the two may be distinctly seen, the rhyo- 

 lite flowing over and concealing the older basic eruptions. 



A specimen of these andesites, in the collection from the northern end 

 of the Wachoe Mountains, has some marked peculiarities. It possesses a 



