PROCESSES OF ALTERATION. 
191 
as very fine-grained aggregates. 
Analysis C on page 189 maybe calculated as follows: 
Si0 2 , 57.91; CaC0 3 , 1.50; MgO, 0.33; FeS,, 2.10; Na 2 0, 0.45; K 2 0, 13.35. Com¬ 
pared with analysis B it shows practically the same chemical result as analysis A. 
The great predominance of K.,0 can not be explained simply by the formation of 
sericite, but the rock must contain an abundance of adularia. 
METASOMATIC CHANGES IN BRECCIA. 
The volcanic breccia is far more extensively altered than any of the rocks thus 
far described. Its great porosity, often equivalent to that of a sandstone, fully 
explains this; the various chemically active solutions easily penetrated this rock 
by diffusion and convection. In the central part of the district, where fissures are 
abundant, as on Raven Hill and Gold Hill, it would be very difficult to find any 
entirely unaltered breccia. On the northern slope of Bull Cliff and on Big Bull 
Mountain, where there are few mineral deposits, much of the breccia is practically 
unchanged, except for a consolidation and hardening which in any rock of this kind 
involves some chemical processes. Such breccia is free from pyrite and appears 
as a purplish to brownish rock, the individual fragments of phonolitic rock being 
clearly visible. Unaltered breccia also occurs to the southwest of the Golden Cycle 
mine in a long crosscut ; here it is loose and sandy. Crosscuts north from the Vindi¬ 
cator and Isabella mines also run into unaltered breccia. In some kinds the angular 
fragments are several inches in diameter, while other breccias are very fine grained— 
in fact, look rather like unstratified tuffs. Granitic fragments of all sizes are 
plentiful at almost all places, but are naturally most abundant near the granite 
contacts. The altered breccia is a hard, bluish-gray or greenish-gray rock in which 
the individual fragments are sometimes difficult to observe. The most general 
change appears in the destruction of the dark silicates and their replacement by 
sericite, carbonates, and pyrite. Often a little residual serpentine is also seen. 
Pyrite is the commonest metasomatic mineral, and occurs widely distributed as 
very small crystals. The next mineral in point of abundance is a dolomitic carbonate 
sometimes also calcite, which replaces the cementing groundmass or any of the 
fragments and often shows roughlv rhomboliedral forms. Sericite in moderate 
quantities and some adularia appear in most altered breccias; the former partly, 
very rarely' wholly, replacing the feldspars or the cement, the latter preferring the 
veinlets or the cavities of dissolution. Anhedral fluorite appears in sporadic develop¬ 
ment replacing groundmass, and sometimes contains inclusions of carbonates and 
secondary apatite (specimens 708, 709 C. C.). The porous cement of the breccias 
sometimes allows zinc blende and molybdenite to be deposited with pyrite. Calave- 
rite also is known as a metasomatic mineral, for example, in the cement of a phonolite 
breccia from the W. P. H. mine, where it appears as irregular, soft, silvery-white 
grains. On the whole, the changes are entirety similar to those described in la tit e- 
phonolite. 
A breccia from the Golden Cyxle mine within the sheeted zone of the Legal 
Tender vein, between levels 8 and 9, is gray, hard, and fine grained, with abundant 
small and angular fragments of phonolitic rocks, in which the ortlioclase microlitlis 
are almost the only remaining original constituents. There are also many small 
pieces of granitic feldspars and a cement of the same character as the fragments. 
