VOLCANIC ROCKS 255 



in ail parts of the quadrangle, mostly in the form of tuff or volcanic 

 ashes. As a rule, the tuffs are arranged in layers, like sedimentary rocks, 

 but where masses attain a great thickness, as near Emigrant peak, strati- 

 fication is not always to be observed. More often than otherwise the 

 rhyolites of the quadrangle are largely made up of volcanic glass partly 

 in the porous form known as pumice. At some points the magma became 

 almost completely crystalline, as with number 93 of the table of analyses. 

 At other points, to the south of Eed mountain, the rhyolite crystallized 

 in little spherulites in which are imbedded crystals of sanidine, quartz, 

 brown biotite, and other minerals. The rhyolite of the upper part of 

 Red mountain is red in color and shows flow structure beautifully. It is 

 mostly de vitrified and might appropriately be called meta-rhyolite. For 

 the purpose of distinction, however, that term will here be used only for 

 pre-Tertiary rhyolite. The rhyolite shown at various points on the map 

 near basalt areas is usually pumice, and the frequent association of the 

 two lavas, basalt and pumice, suggests that perhaps both came from the 

 same magma, the lower specific gravity of the rhyolite pumice causing 

 it to separate from the heavier basalt. 



Dacites or acid quartz-andesites are not present in large amount. They 

 form in the rhyolite tuffs layers which are conspicuous by reason of their 

 darker color, and are upon nearer inspection found to be made up largely 

 of a dark glass rich in biotite. These layers are usually less than 50 

 feet in thickness and seem to occur at a rather definite horizon in the 

 tuffs, as if erupted at about the same period. One of these layers may 

 be seen along the road east of Cave spring and others in the rhyolite 

 tuffs southeast of Piper peak. There are beds of pumice in the con- 

 glomerate of the Esmeralda formation east of the south end of Big 

 Smoky valley and in the gravels assigned to the same formation north of 

 the west end of Piper Peak ridge. 



There are beds of rhyolite sandstone overlying the marls east of the 

 Clayton Valley playa. These sandstones, although in places they are 

 composed nearly entirely of rhyolite material, are placed in the Esme- 

 ralda formation, and this is likewise the case with similar beds in the 

 north end of Fish Lake valley. 



XXII— Bull. Geol. Soc. Am,, Vol. 20, 1908 



