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



collected was ever covered with standing water. Nevertheless, Merrill's 

 hypothesis, that the desert varnish is due to a local segregation of the 

 metallic contents of the rocks on which it occurs, seems to best account 

 for the varnish. It is found only on the upper exposed surfaces and on 

 slopes that have been exposed to the elements for a long period, not being 

 observable on the boulders of the newer washes and alluvial fans. 



Pre-Cambrian Complex 

 location and composition of the complex 



Within the limits of the quadrangle, there are two areas which are 

 regarded as pre-Cambrian in age. One of these forms the larger part of 

 Mineral ridge, west of Silver Peak village, and the other, a small area, 

 lies 5 miles northeast of B. M. 4996, in the northeast part of the quad- 

 rangle and just west of an area of the basal dolomite. This dolomite is 

 placed for convenience with the Lower Cambrian. These areas seem to 

 represent the oldest rocks of the district, and this complex distinctly 

 underlies the Lower Cambrian beds. 



The complex is composed of granite-gneiss, quartz-monzonite-gneiss, 

 granite-augen-schists, calcareous augen-schists, and small lenses of 

 hydrous mica-schists. 



THE GNEISSES 



The gneisses vary in composition. Some of them are true granite- 

 gneisses, in which the feldspar is chiefly orthoclase, and such gneisses 

 usually contain muscovite or white mica often with some biotite. A 

 second kind may be called quartz-monzonite-gneiss or a granodiorite- 

 gneiss, since it contains both orthoclase and plagioclase in approximately 

 equal amounts, and in this type the predominating mica is biotite. 



A partial analysis of one of the quartz-monzonite-gneisses (number 

 224) by George Steiger gives the following results : 



Silica 69.34 



Lime 2 . 35 



Soda 4.51 



Potash 3.19 



At some points where these two gneisses occur together, there seems 

 to be evidence that one is intrusive in the other, but the evidence as to 

 the relative age of the two is not consistent at different points, the 

 granite-gneiss apparently being older at one locality and the quartz- 

 monzonite-gneiss at another. Other gneisses, similar in general appear- 

 ance to the above, contain chiefly plagioclase, thus forming a quartz- 



