48 SOUTHWESTERN NEVADA AND EASTERN CALIFORNIA. 
Goldfield-Cactus Spring road, presents features of considerable inter- 
est. The groundmass is composed in part of secondary interlocking 
plates of orthoclase micropoikilitically inclosing quartz blebs and in 
part of a secondary microgranitic mosaic of quartz and orthoclase vary- 
ing greatly in grain throughout the rock. Alunite in grains is scat- 
tered through the groundmass. A few orthoclase phenocrysts are 
unaltered, but most of them are changed to an aggregate of alunite 
crystals and quartz granules. The rock appears to have suffered 
through reerystallization, with the introduction of sulphurous or a 
related acid and probably sonic silica. A thin section of the altered 
rhyolite from the Calico Hills, near Shoshone and Skull mountains, 
is somewhat different. The original rock was a glass containing nu- 
merous tabular orthoclase crystals and deeply embayed quartz grains. 
Chalcedonic quart/ has replaced some of the feldspar phenocrysts, 
and with it. forming pseudomorphs after other feldspar phenocrysts 
and occurring scattered throughout the groundmass, are tiny irregu- 
lar specks and granular aggregates of a mineral too small for satis- 
factory determination, probably scapolite. It is evidently contem- 
poraneous with the chalcedonic quartz, and its even distribution 
throughout the rock -hows that the solutions which deposited it must 
have saturated the whole rhyolite mass. The kaolinized rhyolite i> a 
white, chalky, rather incoherent rock. The feldspar phenocrysts 
pass to a white unctuous substance and are finally completely re- 
moved, forming casts, and the biotite changes to a silvery micaceous 
mineral. 
These quartz veins, the fillings of open fissures, and the attendant 
silicification of the country rock are evidently due to ascending heated 
waters which carried, in addition to metallic salts, notable quantities of 
silica, while locally they contained salts of calcium as well as fluorine, 
sulphurous acid and chlorine. The presence of the fluorine is indi- 
cated by the alteration of biotite to muscovite; sulphurous acid is a 
constituent of alunite and chlorine of scapolite. The waters in part, 
then, were of magmatic origin and contained gases typical of waning 
volcanism. These ore deposits, except those in the rhyolite of Stone- 
wall Mount a in and that to the north of Cuprite and some of those in 
the Southern Klondike hills, occur in the early Miocene rhyolite. 
But at Wellington and Wilsons Camp silicification appears to affect 
not alone this rhyolite, but the succeeding formation, the biotite 
andesite. This shows that at these places either the silicification was 
dependent on the magmatic waters derived from andesite or it 
occurred long after the extrusion of the rhyolite, probably through 
waters ascending from warm or hot portions of the rhyolite buried 
at considerable depths. The presence of silicified reefs in the Sieberj 
lake beds in the Silver Peak Range and on Shoshone Mountain i- a 
phenomenon probably of later origin. 
