Vol. 4] Smith. — Upper Region of Main Walker River. 



25 



the Later Andesite. — This rock occurs as a cap in the north- 

 west corner of the map PI. 1. Specimen No. 131 is representative. 

 The color of the rock is red, both on the weathered and fresh 

 fractured surfaces. There are numerous phenocrysts of feldspar, 

 2 mm. to 4 mm. long, inclosed in a groundmass. The specific 

 gravity of the feldspar was determined by a heavy solution to 

 be 2.685, thus indicating it to be a basic andesine or acid labra- 

 dorite. The evidence obtainable in the slide was not sufficient 

 to be confirmatory. No quartz was detected, and a determination 

 of the silica gave 57 per cent., which clearly indicates the rock 

 to be an andesite. A few large phenocrysts of feldspar in the 

 slide are polysynthetically twinned, and they show zonal growth. 

 Occasionally the centers of the phenocrysts contain numerous 

 inclusions, while the margins are free, and a few crystals of horn- 

 blende of lesser size are present. The groundmass contains innu- 

 merable lath-feldspars and an abnndance of a translucent min- 

 eral of a reddish brown color and of rectangular cross-section. 

 It may be a pseudomorphic replacement of hornblende by limon- 

 ite. There is no parallelism of structure in the groundmass. and 

 no glass. 



The Rhyolite. — This rock is well represented over the region. 

 It weathers to a reddish color, and on a fresh fracture it is some- 

 what lighter. It often contains foreign material, such as frag- 

 ments of the surface over which it flowed. Quartz, glassy feld- 

 spar and some biotite are noticeable in the specimens. Lying 

 usually under this rock is a white one which appears rather tufa- 

 ceous, and on the weathered surface of this latter may be found 

 dihexhedral quartz crystals 3 mm. to 4 mm. in length; also a 

 glassy, idiomorphic feldspar of similar size. These crystals of 

 feldspar and quartz are frequently corroded and often frac- 

 tured. The pyramidal faces of the quartz are well represented, 

 and there is usually a prism face in evidence. Faces correspond- 

 ing to the following are recognized on the feldspar: OP (001), 

 ooP(iio),2P» (201 and 201), Poo (101), P (111), P3, (130) 

 and (130) , copco (010) , and 2P°o (021 and 021 ) . In the sections 

 only one of the phenocrysts could be measured for a positive 

 result. A section normal to the brachypinacoid, co p oo (010) of 

 a crystal possessing lamellar twinning shows the two feldspar 



