124 
ECONOMIC GEOLOGY OF S1LVERTON QUADRANGLE, [bull. 132 
Although the altered facies just described appears to indicate only 
slight silicification or actual addition of silica to the rock, this process 
has taken place to a greater extent in the immediate walls of the fis- 
sures and in fragments of country rock included within the vein 
quartz. Such facies show little or no diaspore, but more sericite. 
The secondary quartz in such rocks is often more coarsely granular, 
and in thin section can be distinguished from the quartz which fills 
veinlets only through its association with nests of sericite, kaolinite, 
and a little diaspore. All original structure of the latite disappears 
between crossed nicols. 
Summing up briefly,. the characteristic features of the Polar Star 
metamorphism are (1) its restriction to the immediate vicinity of the 
lode, (2) the removal of at least 2 per cent of the substance of the 
rock, including all of the magnesia and carbon dioxide, nearly all of 
the alkalies and lime, and much of the iron, (3) the addition of silica, 
water, alumina, and sulphur, and (4) the formation of a secondary 
aggregate chiefly of quartz, kaolinite, pyrite, diaspore, and sericite. 
The depth to which the alteration of the character extends is not 
known. The specimens studied evidently came from the deepest 
workings, probably over 500 feet from the surface. 
It is not probable that the metamorphism described can have been 
effected by solutions in which alkaline carbonates were the principal 
constituents. It is more likely that the effective agent was acid 
water. If, however, the water contained sulphuric or hydrochloric 
acids it is, at first thought, rather difficult to understand the forma- 
tion of kaolinite and diaspore. But as these minerals themselves are 
not soluble in acids, they may be capable of forming, under certain 
conditions, even in the presence of such a solvent for alumina as free 
sulphuric acid. Acid waters containing sulphuric acid circulate 
through the rocks of the Red Mountain region at the present day and 
have not prevented the formation of kaolinite nor effected its 
removal. In fact, they have probably in some cases aided directly in 
its formation by serving as solvents from which the alumina has been 
precipitated as kaolinite or diaspore under favorable conditions. The 
formation of diaspore instead of kaolinite in the presence of abun- 
dant silica indicates a very moderate hydration. 
The metasomatic alteration of the rocks of the Red Mountain Range 
has frequently been referred to in this report. It is not limited to 
the immediate vicinity of known ore bodies, but is so widespread and 
noticeable a feature as to become an important problem in the gen- 
eral geology of the region and a serious embarrassment to the geol- 
ogist intent upon unraveling the structure and relationships of the 
various igneous rocks involved. 
As a rule, the metamorphosed rocks, where not superficially stained 
by iron oxide, are nearly white. Frequently traces of original struc- 
ture, such as outlines of feldspar phenocrysts or of breccia fragments. 
