VOLCANIC ROCKS 257 



The rhyolitic areas about the north end of Fish Lake valley contain 

 much pumice and perlite, and rounded fragments of black obsidian. 

 These fragments appear to owe their rounded form to having been blown 

 into the air from the volcano while cooling. There are dark brown 

 layers of dacite in these rhyolitic areas and more or less rhyolitic sand- 

 stone. 



Ehyolite ridge is a very picturesque accumulation of rhyolitic sand- 

 stones and tuffs with one or more layers of darker dacite. The sandstones 

 and tuffs are very regularly bedded, dipping to the west usually at low 

 angles. The rocks are mostly of a light buff color, with some red layers, 

 the stratification being magnificently shown on the nearly perpendicular 

 bluffs of the east side. 



ANDESITE 



Andesite forms large areas in the quadrangle, chiefly in the form of 

 breccias or tuffs. Such is the andesite of the north foothills of the 

 Palmetto mountains, those about Cow camp, and elsewhere. There are, 

 however, massive lavas of coarse grain, which are here placed with the 

 andesites because of the difficulty of separating these rocks from true 

 andesites in the held. These massive lavas differ mineralogically from 

 andesites proper in containing orthoclase in the groundmass. Their 

 chemical composition is indicated by analysis number 27. It will be seen 

 that this rock is low in lime and rich in potash when compared with a 

 typical andesite. The large andesite area northeast of Piper Peak basalt 

 area contains much of this alkali-rich type, which approximates in com- 

 position to the latites of Dr F. L. Eansome. 24 



Partial Analysis of Latite-granophyre, Number 27 

 George Steiger, Analyst 



Silica 64.28 



Lime 3 . 79 



Soda 3.97 



Potash 4.55 



Specimen number 27 is a coarse grained gray lava from the west slope of 

 the Silver Peak range, south of the Cave Spring road. It shows large plagio- 

 clases up to one-half inch or more in diameter. Microscopically, the rock has 

 a microgranular groundmass which gives a Becke reaction for alkaline feld- 

 spar. In this groundmass are imbedded phenocrysts of andesine, labradorite, 

 biotite, and augite. Magnetite and apatite are present as accessories. 



BASALT 



Dark feldspathic lavas containing basic plagioclase or labradorite and 

 the ferro-magnesian silicates, pyroxene, and olivine are called basalt. 



24 Sonora folio and Bulletin no. 89, U. S. Geological Survey. 



