262 DIFFERENCE BETWEEN PROPYLITE AND ANDESITE. 



are fully 12,000 feet thick, and often have the aspect of trachyte, a 

 condition which varies with the quantity of orthoclase. 



In the Washoe district three zones of fissures occur through which 

 hornblende-andesite was poured out so as sometimes to cut the diorite 

 and propylite, and form table-like masses. The rock poured out from 

 the dykes in Crown Point ravine is TOO feet thick, but the most 

 important spread covers the country near Devil's Gate in Gold Canon. 

 Nearly all the felspar is fresh plagioclase, with rare sanidine in Carls- 

 bad twins. The hornblende is decomposed into a light-green substance, 

 and the rock contains no augite, biotite, quartz, tridymite, or olivine. 

 The ground mass is a dark-grey aggregate of ledge-shaped felspars, 

 felspar microliths, and grains of magnetite, with a yellowish half- 

 glassy base, as in the andesites of the Siebengebirge. Many varieties 

 of andesite occur on the south side of the entrance to Truckee Canon. 

 The ground mass varies in colour from grey to reddish. The felspars 

 often contain glassy or half-glassy grains. When augite, which is 

 greenish, occurs, it is rich in glass inclusions, which are always absent 

 from the brown hornblende, as in the Siebengebirge in Hungary ; but 

 at times no large hornblende crystals are developed. A typical andesite 

 occurs in Berkshire Canon, where the rock contains no augite. Half- 

 glassy andesite is found on the west shore of Pyramid Lake in Nevada. 

 The ground mass is made up of colourless felspar, brownish microliths, 

 magnetite, and a brown glass. Characteristic andesites occur in the 

 Augusta Mountains in Nevada, and one from Augusta Canon resembles 

 the andesite of the Wolkenburg, having hornblende crystals a milli- 

 metre long, usually fractured, sometimes into thirty or forty pieces. 

 Above Tuscarora, andesite has a brownish-grey felsitic ground mass, 

 with a macroscopical plagioclase and green hornblende. Multitudes 

 of black trichites give a wavy fluidal structure to the rock. Some of 

 the andesites in the Washoe Mountains are remarkable for the Iamina3 

 of brown mica, so that they might be termed mica-andesites, especially 

 as the quantity of hornblende is very small. 



Distinction between Propylite and Andesite. The chief con- 

 stituents of propylite and andesite are identical. They are, however, 

 easily distinguished by observation in the field. Zirkel observes that 

 the propylite ground mass is greenish grey, while the andesite ground 

 mass is pure grey or brown ; propylite is rich in minute particles of 

 hornblende, while andesite only contains large crystals of hornblende ; 

 propylite felspars are filled with hornblende dust, while the andesite 

 felspars are free from it. The hornblende in propylite is green, but 

 in andesite it is almost invariably brown, and has a black border never 

 seen in propylite. Propylite hornblende is built up of thin needles, 

 and therefore does not cleave. This is never seen in andesite. 

 Microscopical epidote is common in propylite, but is almost unknown 

 in andesite. A glassy propylitic ground mass is unknown, w r hile 

 andesites sometimes have a half- glassy ground mass. 



