G58 DESCRIPTIVE GEOLOGY. 



tlie two feldspars, apatite and titanite, together with a compact mineral 

 without cleavage, which ZirkeP regards as tourmaline. 



Of the almost infinite variety of rhyoliteis which make up this broad 

 expanse of low hills, only a few of the more typical forms will be described. 

 In crossing from Antimony Canon to Shoshone Pass, at right angles to the 

 strike of the flows, the following are the more prominent beds: The ridge 

 north of Antimony Canon consists of a light-reddish, very porous, por- 

 phyritic rhyolite, which contains some few brecciated fragments of red 

 decomposed rh^^olite. The rock itself has evidently been exposed to 

 solfataric action, and contains, in well-preserved crystals, only mica and 

 quartz. Of the cavities which abound in the mass, many retain the form 

 of the original feldspar crystals which once filled them. 



On the north of the ridge is a bed of white rhyolite, having a compact 

 hornstone-like groundmass, and containing fresh crystals of sanidin-feldspar 

 and quartz, together with occasional plagioclase crystals, but no mica. 

 This rhyolite also contains included fragments of other rhyolites. Above 

 this is a prominent bed of brilliant emerald-green rhyolite, which also hag 

 included breccia fragments, and is remarkably rich in quartz. Its ground- 

 mass has a compact hornstone-like texture, and under the microscope is 

 seen to be made up partially of fibrous sphserulites, and of axially fibrous 

 strings, which are surrounded by an imperfectly crystalline material. 



Above this gi-een rhyolite are various earthy rhyolites of delicate pink 

 and yellowish-brown colors, which present no crystalline development, and 

 which are succeeded by red porphyritic diyolites rich in crystals of sanidin 

 and mica, and containing comparatively little quartz, which have a pecu- 

 liarly trachytic fracture. Interstratified with these red porphyritic rhyolites, 

 as in the region between the New Pass and Shoshone Ranges, are beds of 

 dark, nearly black, pearlitic rhyolites, which have porphyritically-imbedded 

 crystals of quartz and sanidin, and whose groundmass is largely made up 

 of brown glass. Above the red porphyritic rhyolites are a series of earthy 

 rhyolites, which present a similar succession of color to those already passed, 

 namely, green below, succeeded by pink and yellowish-brown varieties. 

 This green rhyolite, however, contains many crystals of sanidin, and some 



^ Microscopical Petrography, vol. vi, 87. 



