GREGORY.] 
HAYSTACK RHYOLITE. 
155 
shows a more uniform orientation of the feldspars than was observed 
in the other two. The characteristic features of this variety of the 
micropoikilitic structure are, feldspar laths embedded in a base of 
quartz, quartz roughly grouped into areas which extinguish simultane- 
ously, and, within these quartz areas, the feldspar fragments without 
uniform optical orientation. If further study shows this structure to 
be of wider distribution an appropriate variety name might be useful. 
Analysis. — The following analysis (I) of the rock forming the main 
mass of the mountain has been made by Dr. W. F. Hillebrand, of 
the United States Geological Survey: 
Analyses of rhyolites. 
Constituent. 
I. 
II. 
III. 
IV. 
v. 
VI. 
Si0 2 
Per cent. 
75.98 
12.34 
.85 
.93 
.15 
.13 
4.02 
4.44 
.24 
.64 
.17 
.03 
Per cent. 
75.69 
12.26 
Per cent. 
74.90 
13.64 
.66 
.50 
Trace. 
.61 
4.22 
4.64 
.33 
.15 
Per cent. 
75. 20 
12.96 
.37 
.27 
.12 
.29 
2.02 
8.38 
.58 
Per cent. 
75.31 
12.96 
3.05 
Per cent. 
77.28 
11.24 
1.74 
None. 
.21 
Trace. 
3.10 
4.55 
1.16 
Trace. 
A1 2 3 
Fe,Oo 
FeO 
2.93 
MgO 
CaO 
1.13 
3.01 
4.74 
} 
5.11 
3.15 
.61 
.41 
Na 2 
K 2 
H 2 O-105°.... 
H 2 O+105°.... 
Ti0 2 
J 
Zr0 2 
Cr 2 3 
? 
? 
None. 
Trace? 
.07 
Trace? 
Trace? 
.03 
None. 
? 
? 
None. 
V-O, 
NiO . . . 
MnO 
Trace. 
.03 
BaO 
SrO 
Li 2 
Trace. 
P 2 5 
.06 
Trace. 
.05 
.02 
co 2 
CI 
Fl 
FeS 2 
Total 
100. 02 
99.82 
99.65 
100. 22 
100. 65 
99.47 
I. Rhyolite, Haystack Mountain, Maine. Analysis by W. F. Hillebrand, United States Geological 
Survey. 
II. Rhyolite, Rescue Canyon, Nevada. Analysis by It. W. Mahan. Hague, Geology of Eureka dis- 
trict: Mon. United States Geological Survey, Vol. XX, p. 264. 
