156 
ANALYSES OF EOCKS, U. S. GEOLOGICAL SURVEY. [bull. 168. 
H. Rhyolite (nevadite), from Chalk Mountain. Mainly quartz and 
feldspar, the latter being sanidine and plagioclase. A little biotite, 
magnetite, apatite, and zircon are present. Analysis by Hillebrand. 
P. R. C. 64. 
I. Sanidine from H. Analysis by Hillebrand. P. R. C. 64. 
H. 
Si0 2 _. 
A1A 
Fe 2 3 
FeO.. 
MgO 
CaO. 
Na 2 0. 
K 2 0. 
H 2 0. 
PA 
MnO 
LLO 
74.45 
14.72 
none 
.56 
.37 
.83 
3.97 
4.53 
.66 
.01 
.28 
trace 
100. 38 
65.04 
20.40 
.79 
4.11 
9.74 
.29 
100. 3- 
