842 DESCRIPTIVE GEOLOGY. 



flinty fracture. These two rocks together, although assigned in one case 

 to propyhte and in the other to andesite, have a strong affinity with the 

 trachytes. 



The andesite zone extends for about 6 miles north -and south, and is 

 accompanied along its entire eastern edge by a belt of dacite. This dacite 

 body averages about a mile in width, and consists of clouded masses of 

 jDurple and bluish-gray rock. In general^ the purple variety occurs in the 

 most easterly beds, and seems to be the earliest of the dacite belt. It pre- 

 sents sometimes a compact, fine-grained purple base, without any minerals 

 recognizable to the naked eye, but under the microscope reveals the presence 

 of triclinic feldspars and quartz grains. The main part, however, of the 

 purple zone is a micro-crystalline base of feldspar and a decomposed brown 

 mineral, probably hornblende. The feldspars themselves show every grade 

 of passage into carbonate of lime. Breaking through this purple dacite are 

 brecciated masses, which are characterized by a larger proportion of mica 

 and quartz than is found in the earlier body. 



An obscure outcrop of gray brecciated propylite was observed, par- 

 tially concealed by dacite, but no connection with any other rocks could be 

 traced. Later than the purple breccias, a great outburst of pale-green dacite 

 took place, in which also the feldspars have suffered considerable decompo- 

 sition, resulting partly in carbonate of lime and partly in a soft kaolinized 

 mass; but, where decomposition has not proceeded too far, they show, under 

 the microscope, distinct striation. The quartz is in some instances crys- 

 tallized in double pyramids, a not infrequent occurrence in dacites, and 

 especially noticeable in the rock from Shoshone Peak, Shoshone Range. 

 This green dacite extends south as far as Sheep Corral Canon, where it is 

 found to overlie a typical greenish-gray propylite already mentioned,, but 

 occupying too small an area to be represented on the geological map. 



Breaking through the purple dacite, and in some instances crossing in 

 dikes through the green dacite, is a long and powerful outburst of rhyolite, 

 which occupies the foot-hills from within a mile of Sheep Corral Canon to 

 within 4 miles of Pyramid Lake. The main mass of the rhyolite in the 

 region of Berkshire Canon comes up through a broad vein, making a dike 

 nearly 400 feet in width, and overflowing the dacites, forms a bold group 



