AUGUSTA MOUNTAINS. 659 



dark, fresh hornblendes, which are so strongly in contrast with the general 

 decomposed character of the rock, that they would seem to have been 

 mechanically included in the flow. 



Beyond Shoshone Pass, to the north, are white rhyolites, having a 

 green, conchoidal fracture, and containing an unusually large development 

 of quartz crystals, which are frequently smoky, and of fresh, clear sanidin 

 crystals, which often have the bnlliant blue colors peculiar to these rocks. 

 These are succeeded near the pass, at the northern end of Edward's Creek 

 Valley, by red porphyritic rhyolites, which contain mica and hornblende, 

 with rather opaque feldspars and very little free quartz. Under the 

 microscope, the biotites are seen to be penetrated by numerous prisms of 

 apatite. In the neighborhood of Shoshone Springs and the limestone body, 

 similar earthy varieties of pink and green colors are found, associated with 

 which is a gray, pearlitic rhyolite, enclosing fragments of the darker-colored 

 pearlitic rhyolite and crystals of sanidin and quartz. Under the microscope, 

 this rhyolite is seen to be made up of bands of colorless glass, alternating 

 with partially opaque bands and stripes of light yellowish-gray microfelsite, 

 which has an intermediate character between a true glass and a granular 

 crystalline aggregation In this groundmass are seen to be imbedded some 

 broken quartz and feldspar crystals, many of which present the bluish opal- 

 izing colors already described. 



On the spur to the southwest of Shoshone Springs is an isolated out- 

 crop of rather light-colored, very vesicular basalt, which is so much decom- 

 posed that its mineralogical character can hax'dly be distinguished. Along 

 the foot-hills, both to the east and the west of the Soldier's Spring Valley 

 basin, are outcrops of upturned Truckee Miocene. In those bordering 

 Lone Hill Valley, some coal-seams are said to have been discovered since 

 the completion of the field-work, but no rehable data as to their character 

 have been obtained. 



