104 SOUTHWESTERN NEVADA AND EASTERN CALIFORNIA. 
clase phenocrysts, which are more abundant than those of hornblende 
or biotite, arc usually complex crystals showing albite and, less com- 
monly. Carlsbad and pericline twinning. Zonal growth is rather 
common. The plagioclase is slightly altered to calcite, zoisite, sericiteJ 
and chlorite Biotite is altered to chlorite, calcite. magnetite, titanitej 
and epidote. Areas of chlorite, epidote, calcite, and quartz rudely 
preserve hornblende forms, while a little fresh brown hornblende is 
present. 
Dike- of monzonite porphyry cut the Paleozoic sediments and the 
main mass is probably an eroded stock. The outcrops of monzonite 
porphyry are cut by rather closely -paced joints and in consequence 
break down into angular blocks. Spheroidal weathering is here and 
there poorly developed. 
The monzonite porphyry in place- appears to lie upon the eroded 
edges of the Paleozoic sedimentary rocks and inclusions of mon- 
zonite porphyry occur in the rhyolite on its northern border. This 
rock is rather similar to the biotite-horn blende latite of the Cactus 
Range and more closely resembles the quartz-monzonite porphyry of 
Stonewall. Shoshone, and Skull mountains. These rocks are in a 
broad way contemporaneous and of late Eocene age. 
Earlier rhyolite. — Earlier rhyolite with associated latite and dacitq 
form< most of the mountain range north of an east-west line •_> miles 
north of Kawich. Small areas occur south of this line and in the 
Reveille Valley. The rhyolite south of Rose Spring and on the flanks 
of the range north of this point is unquestionably a (low. The crest, 
of the range north of Rose Spring is composed of compact rhyolite, 
in which How bedding i- absent or very obscure. It is probable that 
the crest in part or as a whole lies upon the fissure from which the 
rock was extruded. Some of the masses of rhyolite in the vicinity of 
Kawich are perhaps dikes intruded in the monzonite porphyry, and 
the poorly exposed rhyolite in the Paleozoic quartzite <>' miles west 
of north of Rose Spring may also be a dike. 
These acidic rocks include many facies. Perhaps the predominant 
type i> a reddish rhyolite with compact flinty groundmass, containing 
abundant medium-sized phenocrysts of glassy orthoclase and slightly 
smoky quartz, with fewer and smaller phenocrysts of black mica, which 
in some instances are absent. The quartz phenocrysts are. as a rule, 
rounded crystals and only a few show the dihexagonal pyramid and 
prism. Other rhyolites are gray or brown in color, while some black 
glasses with the normal phenocrysts also occur. In the more glass! 
types flow lines, spherulites. and perlitic parting are common. The 
microscope shows the groundmass of the rhyolite to be of turbid 
brown glass, in which flow lines and spherulites are well developed 
locally. The phenocrysts are in some instances well formed and in 
others are angular fragments, showing that flow continued while the 
