846 DESCEIPTIVE GEOLOGY. 



map as covering too broad a field, and that within the territory so colored 

 other volcanic outbursts may be found. They occur, however, along the 

 shore of Pyramid Lake on both sides of Mullen's Gap, rising from 1,200 

 to 1 ,500 feet above the water-level. The only other volcanic rocks recog- 

 nized in the region were rhyolites and basalts, both overlying the quartz- 

 propylite, the former in low hills bordering the gap, and the latter capping 

 the higher summits. 



Greologically the most notable feature of these rhyolites is found in the 

 great variety of structure which they present in such a comparatively lim- 

 ited area. Rhyolites from the same hill or outflow may show great differ- 

 ences in color, texture, and superficial appearance, but, in general, are char- 

 acterized, even over large regions, by much the same structural habit of the 

 groundmass and arrangement of mineral constituents. There are found 

 here in Mullen's Gap rhyolites, having a microfelsitic groundmass rich in 

 secreted minerals, others in which the groundmass is distinctly lithoidal and 

 homogeneous, associated with pearlites, pumicestones, light-colored tufas, 

 and coarse rhyolitic breccias and ash. The microfelsitic varieties, which 

 have apparently broken through the quartz-propylites, have a purplish-gray 

 color, a rough texture, and hackly fracture. Quartz and sanidin are the 

 prevailing mineral secretions. The rock shows a tendency to pass by 

 gradual transitions into the lithoidal varieties, not unlike the rhyolites from 

 the Sou Spring Hills, Pah-Ute Range, which they closely resemble in gen- 

 eral habit. In this connection, it is interesting to note that the rhyolites 

 from Mullen's Gap, which are characterized by a lithoidal groundmass, 

 possess a brilliant Indian-red color, also very similar to rocks described 

 from the Sou Spring Hills, a similarity which is still further shown by the 

 behavior of the well-defined fresh crysta'ls of sanidin and the occasional 

 particles of quartz. It would seem as if the segregated fragments of quartz 

 in the microcrystalline and microfelsitic varieties had become dissolved in 

 the magma, and, uniting with the ferritic material of the groundmass, had 

 formed, during the period of solidification, a homogeneous, porcelain-like, 

 red rock, which presented physically, although not chemically, a totally 

 different product from the rough, purplish-gray types. 



The pumicestones and pearlites are remarkably light and porous, 



