120 
ECONOMIC GEOLOGY OF SILVERTON QUADRANGLE. [bull.18£ 
accessible, but the metamorpliie processes connected with ore deposi- 
tion may be studied to considerable advantage from surface exposures 
and the material of the dumps. The country rock of the Polar Star 
is a dark-gray rock of andesitic appearance, carrying abundant glis- 
tening phenocrysts of plagioclase up to 7 or 8 millimeters in length. 
In thin section, under the microscope, the plagioclase is seen fee be chiefly 
Labradorite (ab 1 an x ) which is almost perfectly fresh. Phenocrysts 
of a pale augite or diopside are almost wholly altered to calcite and 
chlorite, and some bastite pseudomorphs indicate the former presence 
of phenocrysts of an orthorhombic pyroxene. Quartz, and probably 
some orthoclase, are present in the groundmass. The chemical anal- 
ysis of this rock is given in Column I, page 122. This analysis 
differs from that of a normal andesite in its rather high alkalies and 
relatively high potash. It might be classed as a latite. Owing to its 
somewhat weathered condition and lack of knowledge as to the accu- 
rate chemical constituents of the resulting chloritic and serpentinous 
products, an attempt to calculate the mineralogical composition from 
a chemical and microscopical analysis is not entirely satisfactory. 
Calculating the carbon dioxide as calcite, the soda as labradorite 
(ab x ani), the phosphoric anhydride as apatite, the residual lime as 
diopside, all the potash as orthoclase, and dividing up the other con- 
stituents between chlorite, serpentine, magnetite, etc, the following 
approximate mineralogical composition was obtained. 
Mineralogical composition of andesitic rock (latite?) from Engineer Mountain. 
Constituent. 
Labradorite (abiani) 
Orthoclase (?) , 
Quartz 
Diopside (CaMg(Si0 3 ) 2 ) 
Chlorite (H^CFeMgJo^AlnSiisOgo) 
Calcite - 
Serpentine 
Per 
cent, by 
weight. 
33.9 
22.4 
14 
4.7 
7 
Constituent. 
Kaolinite 
Magnetite , 
Hematite 
Apatite 
Rutile and leucoxene 
Per 
cent, by- 
weight. 
2.6 
2.3 
3.8 
1 
1.4 
98.9 
There still remains about 1 per cent of alumina not accounted for 
in the above calculation. Combined with part of the ferrous iron 
here calculated as magnetite and chlorite, and with some of the silica, 
reckoned as quartz, it may enter into the composition of the pyrox- 
ene, which probably does not correspond exactty to the theoretical 
diopside molecule. The percentage of kaolinite is rather higher than 
the microscopical investigation would indicate. The specific gravity 
as calculated from the foregoing mineralogical composition, ignoring 
porosity, is about 2.8. The foregoing specimen was taken a few hun- 
dred feet west of the mine, and, although fresher rock occurs in the 
vicinity, this was selected for chemical analysis as being undoubtedly 
the particular facies in which the Polar Star lode was formed. 
The dumps of the Polar Star mine are made up largely of a locally 
