:>1<> The American Geologist. May, 1894 
Sierra Nevada are some small extincl rhyoHtic volcanoes perhaps of 
Pleistocene age. They have been described by Prof. I. C. Russell.* 
Rhyolite is especially abundant in the fool hills of Amador and Cala- 
veras counties, forming, when ii caps the points, the level tops of table 
mountains, of which Buena Vista and Valley Springs peaks arc conspic- 
uous examples. No place has been noted in the foot-hills from which 
rhyolitic eruptions issued. The occurrences seem always remnants of 
former extensive areas I' rum other sources. 
At Buena Vista peak and other points, a peculiar feature in the weath- 
ering of the rhyolite was noted. The exposed surfaces are more com- 
pact than the interior, presenting, as a fractured surface, a vitreous ap- 
pearance. This seems to be due to the silica of the rhyolite being 
leached ou1 by atmospheric moisture and deposited nearly in place. 
The same phenomenon was previously noted by Mr. < toodyearf at several 
places. 
Andesite. The following varieties of andesite have been 
noted in the Sierra Nevada: — 
1. Hornblende-mica-andesite. This rock occurs in a mas- 
sive form in two small areas near Grizzly peak, in Plumas 
county, and at other points. The rock is light gray in color, 
of a trachytic aspect, strongly resembling the dacite of the 
Lassen peak region, but shows in thin section no quartz phen- 
ocrysts. The hornblende-mica-andesite appears to be among 
the oldest of the Tertiary volcanic rocks. Analysis No. 198, 
Plumas county. 
2. Hornblende-andesite. There are few areas of andesite 
in which hornblende is the only bisilicate. Analysis No. 12, 
Plumas count} 7 . 
3. Hornblende-pyroxene-andesite. The andesite that for- 
merly covered so much of the range, chiefly in a fragmental 
form, may be designated hornhleiide-pyroxene-andesite, the 
two bisilicates varying a good deal in their relative propor- 
tions. 
There is a reason to believe that much of the andesite-breccia issued 
from fissures near the summit of the range mixed with water, forming 
vast mud flows which poured down the slopes, displacing the rivers. 
Undoubtedly many areas of andesite-breccia have been formed sub- 
aerially by the explosive action of volcai s, the lava being thrown out 
as ashes and fragments, which, falling on the surrounding country, be- 
comes roughly stratified. But this does no1 appear to have been the 
mode of the formation of the bulk of the andesite lull' ami breccia of 
the Sierra Nevada. The evidence thai the material issued from fissures 
* Eighth Ann. Report. U. S. Geol. Survey, p. 37& 
(■See Auriferous Gravels, p. 520. 
