AUGUSTA MOUNTAINS. 655 



The rliyolite which immediately adjoins the granite body of Granite 

 Point on the west is a white porphyritic rhyohte rich in quartz, which is 

 accompanied by a white, somewhat earthy breccia, carrying small crystals 

 of quartz, sanidin, and black biotite, and including, as breccia material, frag- 

 ments of dark-blue quartzite, rounded crystals of orthoclase-feldspar, and 

 opaque quartz, which evidently result from the decomposition of the gran- 

 ite, and which can easily be distinguished from the small, limpid qunrtz- 

 crystals that belong to the rliyolite itself. 



Along the foot-hills to the north of the granite body is a series of red 

 rhyolites, among which were noticed a porphyritic rhy elite containing large 

 crystals of fresh, glassy sanidin-feldspar, and opaque orthoclastic-looking 

 feldspars, which may also be included fragments, resulting from the decom- 

 position of the granite. A second rliyolite, a little distance from the granite 

 body along the eastern foot-hills, consists mostly of a red porphyritic ground- 

 mass, with comparatively few included crystals of quartz, sanidin, and mica, 

 but no opaque feldspars. From fragments obtained in the debris of the canon 

 next north of the granite body, it is probable that the hornblende-porphyry 

 extends at least as far north as this. 



The main crest of the range between Granite Point and Antimony 

 Canon is made up of a drab rhyolite having the peculiar uneven fracture 

 of the red porphyritic rhyolites. It contains only crystals of sanidin and 

 quartz in a grayish-yellow felsitic groundmass, which, examined microscop- 

 ically, is seen to consist almost wholly of longitudinal bands, showing a 

 good axial fibration. This rhyolite, which occurs horizontally bedded, is 

 underlaid by a bed of dark pearlitic rhyolite, containing crystals of sani- 

 din and quartz, similar to those found between the Desatoya and Shoshone 

 Ranges. 



North of the line of Granite Point and Antimony Canon, there is a 

 broad depression in the range, in which it consists of a series of low ridges, 

 forming a shallow basin, open toward the west. This portion of the range, 

 which in ante-Tertiary times was scarcely elevated above the surrounding 

 plains, is now occupied by extensive flows of rhyolite, which, for some dis- 

 tance north of Shoshone Pass, dip to the northwest, at first with an angle of 

 15°, but gradually shallowing out, and in general conforming in slope with 



