Geological Notes on the Sierra- Nevada. — Turner. 309 
ANALYSES OF QuAKTZ-PORPBTTRIES AM) POBPHYRITES. 
Quartz-porpliyrite. Quartz-porpli yry. Quartz-porphyry . 
No. 55:5, No. 323, No. 15a, 
Calaveras county. Plumas county. Plumas county. 
Silica 71.19 73.25 75.06 
Lime 2.87 2.23 .38 
Potassa 1.82 3.79 t.27 
Soda 4.24 2.69 3.78 
EFFUSIVE [GNEOUS ROCKS. 
Rhyolite, The rhyolitic rocks of the Sierra Nevada are 
chiefly white or pink in color and seem to have issued as Hows 
from various sources, chiefly near the crest of the range. 
These rocks usually occur overlying the Neocene auriferous 
river gravels. At Valley Springs peak in Calaveras county, 
and at other points, they overlie the lone formation. The 
rhyolite flows evidently followed the old river channels to a 
remarkable extent. The exact nature of these Hows is not 
determined. They have been considered as tuffs or mud 
Mows, but in thin sections they frequently show trains of 
spherulites and other evidences of How structure. This is the 
ease with some of the rhyolite near Mokelumne hill, which 
has been spoken of as a tuff. Occasionally, as at Valley 
Springs peak, the rhyolite contains pebbles of foreign rock, 
but fragments of foreign material are, so far as known to the 
writer, rare in the rhyolite. True tuffs unquestionably occur 
and some of them may have reached their present position as 
mud flows. 
The rhyolite occurs in massive form as dikes in the granite and 
schists. One.of these (No. loo Plumas county) is remarkable as con- 
taining large amounts of gold in little quartz seams which pi' nd rate the 
dike in all directions. This dike which lii's about one mile easl of 
Onion valley, a1 the headwaters of Poorman creek, is much decomposed 
in the auriferous portion. It is a white rock with abundant foils of 
brown mica, sanidine and plagioclase, and a few rounded quartzes in a 
microcrystalline groundmass. 
'flu' rhyolite of the Sierra Nevada appears to be all of about the same 
age. 'flir flows overlie the Neocene auriferous river gravels and under- 
lie i In' andesite-breccias, ami may be early Pliocene* in age. 
To ihr south of Mono lake and jusl easl of the escarpmenl of the 
* It has been suggested thai the clays of the lone formation have" re- 
sulted from tin' decomposition of rhyolite. In this case, since the lone 
formation appears to be in pari Miocene in age, the rhyolite Hows must 
have commenced earlier than tin- Pliocene. 
