560 DESCRIPTIVE (xEOLOGY. 



Washoe, this sequence of hornblendic trachyte, having affinities with 

 andesite, followed by normal sanidin-trachyte. East of Pinto Peak, and 

 a little to the north, in the middle of the trachytic zones are a series of north 

 and south dikes of black vesicular basalt, which have small local outflows 

 covering a group of hills with thin sheets. These basalts are poor in olivine 

 and rich in acidic glass. They are also noticeable for the comparatively 

 large size of the triclinic feldspars. 



The contact between the sanidin-trachytes and the rhyolites lying- 

 farther to the south was not actually observed; but, from the contour of the 

 hills and the peculiar southward slope of the trachyte surface, it seems 

 most probable that it passes under the rhyolite. The volcanic rocks in the 

 region of Pinon Pass are a light earthy rhyolite, having a very trachytic 

 texture, containing, in a micro-crystalline groundmass composed of quartz, 

 sanidin, and biotite, numerous dihexahedral quartz-grains, earthy, half- 

 decomposed orthoelase, and an unusually large projiortion of biotite flakes. 

 With the exception of the biotite, these minerals are not disseminated regu- 

 larly through the mass, but occur in isolated bunches, ten or twelve large 

 quartz-grains appearing together, and, in others, groups of broken crystals of 

 orthoelase. It is one of the richest in biotite of any rhyolite in the collec- 

 tion. But for the presence of quartz, both as a secreted mineral and in the 

 groundmass, its general appearance and texture would unite it with the 

 trachytes. 



About 12 miles farther to the southward, at the base of the range, 

 occurs another rhyolitic outflow, which forms low broken hills in contact 

 with the Devonian limestone. This rock is a half-glassy hyaline variety, of 

 uniform texture, and of a dark-gray color, with large sanidin feldspars 

 imbedded in the glassy groundmass. Like many others of the same class, 

 it withstands atmospheric agencies remarkably well, but erosion-outlines 

 are usually sharp and angular. Between these two bodies of rhyolite are 

 found small isolated patches of basalt, occupying the same relation to the 

 Palseozoic strata as the rhyolitic masses. One occurs on the foot-hills near 

 Fossil Pass, and is a hard, fine-grained, black rock. They appear simply as 

 outlying bodies of the broad field of basalt, which borders the rhyolite 

 east of Piiion Pass, and forms a continuous group of hills from the Pinon 



