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662 DESCRIPTIVE GEOLOGY. 



propylite is the oldest, and presents but few outcrops. It is best observed 

 in the basin of Storm Canon, just east of the quartzite body, where several 

 small lateral canons, coming down from Mount Moses, unite, forming an 

 open valley. From this valley, the rhyolites have suffered considerable 

 denudation, leaving low, uncovered ridges between the lesser canons, and 

 irregular bosses of propylite protruding out of the great mass of surround- 

 ing rhyolite. That these propylite bodies are connected and represent an 

 older eruptive mass, now buried, there can be no doubt. They are too 

 small, however, to afford evidence of any considerable erosion of the older 

 rocks before the pouring out of the later rhyolite. 



This propylite is a characteristic rock, and its behavior, both in 

 the field and in the hand-specimen, clearly indicates its eruptive origin. 

 Under the' microscope, the groundmass shows the same habit as the typical 

 propylites of Washoe, and, like them, differs from the more recent rocks 

 by the same resemblances to the older diorites. It has a greenish-gray 

 color, occasionally shading off into yellowish-gray, due in part to the more 

 or less decomposed condition of tire hornblende. It presents a somewhat 

 porphyritic appearance, owing to the feldspars, which are frequently a 

 quarter of an inch in length, lying imbedded in a finer groundmass. This 

 groundmass shows a medium-grained rock, compact, and breaking under 

 the hammer with an angular or hackly fracture. A few monoclinic feldspars 

 are associated with the triclinic forms. Hornblende, which has a fibrous 

 structure, gives the prevailing greenish tint to the rock, and appears to 

 be much decomposed. In • some of the specimens collected, a few small 

 flakes of biotite may be recognized by the unaided eye. Under the micro- 

 scope, Zirkel has detected the presence of apatite, some sections of augite 

 crystals, and, in the secondary products arising from the decomposition 

 of hornblende, calcite, with rhombohedral cleavage, epidote, and a dirty- 

 green substance like viridite. 



A careful chemical analysis of the Storm Canon propylite was made 

 by Mr. E. W. Woodward, who reports the following composition : 



Silica 60.55 60.58 



Alumina 20.55 20.38 



Ferric oxide 2.30 2.63 



