PYEOXION KAN DKSITE. 239 



piiniice.s and tuffs, wliicli stivtcli eastward Itt'iu'atli the town of" Eureka as 

 far as Richmond Mountain. 



ISimihvr transitions may also be seen in the neighborhood of I'iiUo 

 Peak, altliough on a less extensive scale. It seems impossible for any 

 region to exhiljit a finer display of pumices and tufls than those occupying 

 the basin between County Peak and Richmond Mountain. Here, in tlu^ 

 neighl)orhood of Hornitos Cone, a symmetrical hill of tuff 400 feet in 

 height, these pumices are shown with every possible variation in color and 

 textiu'e, the resvdts of alteration produced by the breaking through of 

 basaltic masses. The color and density have undergone marked changes, 

 but the mineral development remains much the same as in the normal 

 rhyolite. Nothing could surpass the abrupt changes in physical liabit 

 which these rocks undergo. 



Pyroxene-andesite (Augite-andesite).— Uuder pyroxenc-andesito are included 

 all those volcanic rocks whose essential constituents consist of triclinic feld- 

 spai's and pyroxenes, the rock differing fundamentally from the hornblende- 

 andesite in having the hornblende replaced by some form of pyroxene, 

 usually a mingling of both hypersthene and augite. Hypersthene in most 

 rocks of this group surpasses the augite in amount and in certain localities 

 predominates to sucli a degree that the rock might properly be classed as 

 hypersthene-andesite. Whether there exists any large body of extrusive 

 lava ui the Great Basin retaining the andesitic liabit, in which the augite is 

 the prevailing mineral, to the exclusion of hypersthene and without the 

 accompaniment of olivine, is a matter of some doubt. The rock is rich in 

 magnetite, disseminated throughout the mass, the mineral playing a far 

 more important part than in hornblende-andesites. In addition to the essen- 

 tial mineral constituents which make up the rock, both biotite and black- 

 bordered hornblende have been identified in the pyi'oxene-andesites from 

 several localities in considerable quantity. Richmond Mountain is the only 

 body of pyroxene-andesite in the Eureka District. It is well represented 

 in the Cliff Hills just south of the Fish Creek basin, the northern end of 

 which is shown on atlas sheet xii. It covers such an extensive area, j)resent- 

 ing not only an important feature of tlic lavas of this region, but is such a 

 typical rock of many other localities, tliat it iccjuires to be described in detail. 



