238 GEOLOGY OF THE EUEEKA DISTE1CT. 



of iron oxide iu the groundmass. It occurs only as a fissure eruption along 

 the Rescue fault, the brilliancy of coloring causing it to stand out promi- 

 nently in contrast with the inclosing dark gray limestones. It is largely 

 composed of glass base in which are porphyritically imbedded exception- 

 ally brilliant grains of dark quartz and tabular crystals of sanidine. The 

 Pinto Peak rhyolite, on the other hand, is characterized by a more crystal- 

 line groundmass, much of it being holocrystalline. It is iu general lighter 

 in color, more varied in tint, carries less iron, and is almost wholly 

 free from ferro-magnesian secretions. In places it disintegrates readily 

 into loose quartz grains and feldspathic fragments. The name is taken 

 from Pinto Peak, a prominent elevation, made up wholly of rhyolitic 

 accumulations, lying between Spring Hill and Carbon Ridge, just east of the 

 Hoosac fault. Similar rhyolites, singularly uniform in composition and 







crystalline structure, extend without a break in their continuity the entire 

 distance from Pinto Peak to Gray Fox Peak. Nearly all the lesser out- 

 bursts of rhyolite scattered over the district and breaking out along fault 

 planes belong to the Pinto Peak variety. It is characteristic of many vol- 

 canic centers in the Great Basin as well as Eureka, and may be considered as 

 a typical rock over large areas. 



Pumice and Tuff. All rocks placed under this head are closely allied to 

 the rhyolites in mineral and chemical composition and belong to the same 

 natural group. The rhyolites and pumices break out under very similar 

 geological conditions and frequently pass from one into the other, even 

 more readily than the transition between the crystalline andesites and 

 glassy pearlites. They cover much larger areas than the corresponding 

 andesitic rocks and in their field occurrence offer such striking contrast to 

 the more compact rhyolite that it is thought best to separate them, more 

 especially as they exhibit definite characteristics sufficient to group them on 

 the map by themselves, and in several localities the erupted material con- 

 sists wholly of pumices and tuffs. Transitions from normal rhyolites into 

 pumices are admirably shown along the base of Purple Mountain. The 

 mass of the mountain rises 400 feet above the valley and is formed of a 

 characteristic rhyolite, while the base spreads out in a great variety of 



