

250 H. W. TURNER GEOLOGY OF THE SILVER PEAK QUADRANGLE 



The granite rocks of the northeast portion of the quadrangle, north of 

 Clayton valley, are chiefly true granites and granite-porphyry, but there 

 are here also some granite-gneisses. These rocks are composed of alkali- 

 feldspar and quartz, with some biotite and muscovite. The alkali- 

 feldspar varies in its character at different points — orthoclase, microcline, 

 micropegmatite, microperthite, and albite having been detected in the 

 specimens collected. 



The south base of Lone mountain, the summit of which is not on the 

 quadrangle, is made up of a coarse, light gray biotite-granite-gneiss con- 

 taining orthoclase, microcline, micropegmatite, oligoclase, quartz, biotite, 

 iron-oxide, titanite, and apatite. The isolated butte in the detritus lying 

 about three-fifths of a mile south of the south base of Lone mountain 

 is composed of an even grained granite of fine texture containing quartz, 

 microcline, micropegmatite, and orthoclase with a little oligoclase, mus- 

 covite, and chlorite. The large area north of Weep ah, 6 miles in diame- 

 ter, is in part a granite-porphyry composed of phenocrysts of orthoclase, 

 oligoclase, quartz, and biotite in a microgranular quartz-feldspar ground- 

 mass which contains a little iron oxide, apatite, and zircon, but some of 

 the specimens collected from this area are evenly granular rocks with 

 gneissic structure locally developed, composed of orthoclase, microcline, 

 albite, and quartz with a little muscovite, biotite, iron oxide, and apatite. 

 The quartz occurs in aggregates of interlocking grains of smaller size 

 than the feldspar grains. In the southwest portion of this area there are 

 also true granite-gneisses. 



The small area that lies 3% miles northeast of the Cla} r ton Valley 

 crater is composed chiefly of a coarse biotite-granite, but the northwest 

 portion of the mass is a white medium grained quartzite-like rock which 

 the microscope shows to be chiefly quartz and albite or soda-feldspar. 



QUARTZ-MONZONITE 



Granitic rocks in which the alkali and soda-lime feldspars are both pres- 

 ent in abundance are here termed quartz-monzonite. Eocks of this type 

 are very common in the southern portion of the quadrangle. The two 

 large areas, shown on the map in Spurr's report, in the southern part of 

 the Silver Peak range and as extending thence southeast to the Palmetto 

 mountains, differ from the granolites of the other portions of the quad- 

 rangle in containing more plagioclase and biotite, and are therefore better 

 designated by the term quartz-monzonite. They may be differentiated 

 into two types — a coarse variety, often with porphyritic feldspars, and a 

 medium, even-grained variety. The coarse rock forms the larger part of 

 these two areas and is probably the older rock. This type is of rather 



