490 DESCEIPTIVE GEOLOGY. 



in wliicli are white spots of decomposed feldspars, the whole being filled with 

 fine flakes of black mica and small crystalline fragments of quartz. Among 

 the included fragments of undecomposed rock, the original rhyolite is seen 

 to be a porphyritic variety, containing large crystals of greenish sanidin- 

 feldspar, with a small amount of very fine mica and hornblende, and large 

 rounded quartz, in a drab felsitic groundmass. This tufa, or breccia, is 

 immediately overlaid by the flows of dark, compact, half-glassy basalt, which 

 form the summit, under which, on the eastern side, can be distinguished a 

 fcAV westerly-dipping, but nearly horizontal, beds of drab limestone, carry- 

 ing the same fossils as those found in the western ridge. The rocks which 

 constitute the basalt flow of this ridge and the one next Avest are in general 

 dark, compact, fine-grained varieties, in which scarcely any distinct crystals 

 can be distinguished by the naked eye. In general, they present a dull, 

 greasy lustre and a semi-vitreous texture, breaking easily under the hammer 

 into thin, curved fragments, which ring like glass. In these, a few minute 

 feldspar crystals can be detected and some small, sparse grains of olivine. 

 In the granular variety, which is a heavy, dark-gray rock, the color of the 

 former being nearly black, no crystalline ingredients could be detected at 

 all. These flows of basalt are comparatively thin, not more than 50 to 100 

 feet in thi<jkness ; their beds incline with the slope of the hills, and stand in 

 isolated patches on the spurs which they have preserved from erosion. 



The western ridge, which forms the eastern border of Ruby Valley, is 

 made up of heavy beds of the same drab and cream-colored limestones, some 

 of which are rich in fossils. The beds have a dip of 10° to the eastward, 

 and their western base is underlaid by calcareous and argillaceous shales. 

 They probably represent the upper horizon of the Wahsatch limestone as 

 developed in this region. Among the fossils obtained from these beds, the 

 following were determined by Prof. F. B. Meek : 



Prodiidus niuUistriatus. 



Productus semireticulatus. 



Productus Nevadensis. 



Spiriferina pidclira. 



Atliyris suhtilita. 



Athjris Poissyii. 



