642 DESCRIPTIVE GEOLOGY. 



evidently older than the neighboring volcanic rocks. Its weathered sur- 

 face is of a light-gray color, and shows distinctly the included feldspar 

 crystals. The rock from the canon at the north point of the hills is made 

 up of plagioclase-feldspar and yellowish-green augite imbedded in a glassy 

 groundmass, which, under the microscope, is seen to be made up almost 

 entirely of brown glass, with no globulitic secretions, cementing microlites 

 of plagioclase and augite, and grains of magnetite. It contains a little 

 sanidin, which is mostly decomposed, but no olivine or hornblende. The 

 larger plagioclases and the augites contain inclusions of brown glass, Avith 

 rectangular bubbles. The augite-andesite from the southern point of the 

 hills contains more sanidin, and also some little hornblende, which is 

 sometimes found included in the sanidin crystals. A characteristic of 

 these rocks is the fact that the augites occur in well-defined, perfect crys- 

 tals, while the hornblendes are broken, and seem to be an accessory 

 constituent. 



On the western slopes of the hills is a rhyolite, which might at first 

 be taken for an andesite. It has a similar dark-gray groundmass, but 

 contains large sanidin crystals and comparatively few plagioclases, with 

 fresh dark-green hornblendes. Under the microscope, the resemblance to 

 andesite is still maintained in the fact that the groundmass is a dark-gray 

 felt-like aggregation of microlites, but its crystalline ingredients are thor- 

 oughly rhyolitic, consisting of quartz, sanidin, a little plagioclase, and 

 much dark hornblende with its characteristic dark border, together with a 

 few light-green augites containing glass-inclusions. Associated with this 

 rhyolite is a dark-gray sanidin-trachyte, which contains some little plagio- 

 clase, and also some augite, though with a large predominance of horn- 

 blende, in a micro-crystalline groundmass. The summit of the hills is formed 

 of a dark, compact, fine-grained basalt, which has broken through and cov- 

 ered the quartzites. It belongs to the feldspar-basalts, and contains augite 

 and plagioclase and very little olivine. In places, it is slightly vesicular, 

 the cavities being partially filled by carbonate of lime. 



In the low benches west of Reese River, near Jacobs ville, is an expo- 

 sure of from 50 to 100 feet of horizontal Tertiary beds, made up of gravel 

 conglomerates and coarse arenaceous limestones, full of cavities, like the 



