126 ECONOMIC GEOLOGY OF SILVERTON QUADRANGLE, [bull. 188. 
bon dioxide is absent. The little sulphuric anhydride present is prob- 
ably combined with baryta as barite. 
The mineralogical composition of the altered rock may be roughly 
calculated as follows: 
Mineralogical composition of altered andesitic breccia from White Cloud, mine. 
Mineral. 
Quartz 
Kaolin. 
Pyrite. 
Rutile . 
Per cent. 
78.5 
13.8 
.6 
96.8 
About 1 per cent of water is not accounted for, as there are no alka- 
lies or alumina to combine with it. It may exist as chalcedony or 
opal, or as some hydrated compound of iron. No attempt has been 
made to calculate the small amounts of ferric and ferrous iron, lime, 
phosphoric acid, sulphuric acid, and baryta, which probably corre- 
spond to a little barite, apatite, and other minerals present in minute 
quantities in the rock. The total absence of sericite is rather remark- 
able. The character of the alteration points to acid waters as the 
cause of metamorphism. 
Of the same general character as the foregoing is the metamorphism 
which has transformed andesitic tuffs or breccias of the Silverton series 
into the highly siliceous rock forming the greater part of the pictur- 
esque knob in which the National Belle mine lies, at the village of 
Red Mountain. The resulting rock is nearly white and decidedly 
porous. Examined with a lens, it appears to consist wholly of quartz 
with a little pyrite, and is full of minute cavities sparkling with 
quartz crystals. Locally this porous structure may be so pronounced 
as to give the rock a pumiceous appearance. 
Under the microscope, in ordinary light, the remnants of porphyritic 
structure are easily recognizable. Relatively clear areas having the 
shape of feldspar phenocrysts and dark areas containing much finely 
divided pyrite and other opaque particles, clearly pseudomorphs after 
some phenocrysts — probably ferromagnesian constituents — lie in a 
groundmass somewhat turbid with minute dark particles. With the 
nicols crossed, however, this structure practically vanishes, and the 
whole appears as a finely crystalline granular aggregate of quartz. 
The space formerly occupied by the phenocrysts of feldspar is now 
filled with quartz, somewhat more coarsely crystalline than in the 
groundmass. Frequently the quartz does not quite fill the space, but 
has left a microscopic vug in the center. The quartz making up the 
rock is not pure, being crowded with indeterminable inclusions. i>ut 
