Mineralogy and Geology. 135 
sionally met with; in some instances the opal is interfoliated be- 
tween layers of true quartz, and is sufficiently auriferous to repay 
the expenses of treatment. ; : f 
e metallic minerals enclosed in the gangue of auriferous veins 
are ordinary iron pyrites, blende, and galena, and less frequently, 
arsenical pyrites, magnetic and copper pyrites, and cinnabar. 
These sulphids invariably contain gold; and veins in which some 
one or more of them does not occur in considerable amounts, are 
In one of the detrital beds in the vicinity of the village of 
-Voleano in the county of Amador, and elsewhere, ey 
marked quartz veins may be observed cutting through the grave 
0 evidently formed by the action of water holding silica in so- 
ution, 
exceedingly limited; and in order to obtain sections agus 
good examples, even of small size, it is necessary to select s 
and are often so opaque, apparently rendered so by being inter- 
~~ coated with « Galle of clay, that no vacuities can be dis- 
Out of more than fifty sections of veinstone examined, only 
some six or seven were found to contain fluid-cavities of casein of 
