THE ROCKS OF THE WASIIOI'] DISTllICT. 45 



Diagnostic peculiarities. — Some Varieties of this rock, especially a i)ortiou of 

 tlie small patch sliown on the map in square C. 7, greatly resemble Mount 

 Davidson diorite, and indeed the difference under the microscope is chiefly 

 in the species of feldspar. On the hills west of the Fhirida, and in some 

 other localities, it is much like augite-andesite or basalt in apoearance, but 

 the macroscopical resemblance does not answer to any microscopical simi- 

 larity. It sometimes occuis in rounded shapes such as basalt often assumes. 

 In many cases weathered surfaces of this rock can l)e recognized by the 

 crystal outlines which they exhibit. These are often polygons of a variable 

 number of sides, and represent sections of hornblendes crystallized in re- 

 markably short prisms and provided with terminal faces. 



QUARTZ-PORPHYRY. 



General character. — Quartz-porpliyry covers a large area in the southwestern 

 portion of the Washoe District, and extends for miles in the direction of 

 Washoe Lake. It presents a rough surface varying in color from white to 

 a yellowish or reddish gray, and is thickly set with quartz-grains the size 

 of a mustard seed, and smaller. Mica is nearly alwa3\s visible, and horn- 

 blende occasionally. Only in one small area near the granite is the quartz 

 inacroscopically suppressed, and here the rock is finer-grained than else- 

 where. Underground it extends to a considerable distance farther north and 

 east than its northern limit on the surface, and there underlies hornblende- 

 andesite and augite-andesite. 



Composition. — Nouc of tlie rocli is really fresh and, though an earnest 

 search was made, not a single specimen could be found showing the con- 

 stituent minerals in an undecomposed state. Those which enter most largely 

 into the composition of the rock are feldspar, quartz, mica, hornblende, and 

 ores of iron. As accessory minei-als, titanite, apatite, and zircon were 

 observed. The feldspars are not well defined, Init occm- in irregular or 

 rounded grains. They are in part striated, but the larger portion show no 

 trace of polysynthetic structure, and while in some slides so large a quantity 

 of calcite is distributed through the feldspars that the striations might be 



