256 
ANALYSES OF ROCKS, U. S. GEOLOGICAL SURVEY. [no. 228. 
2. PERIDOTITE, DOUGLAS COUNTY. 
The matrix of the nickel silicate at Riddle. Described by Diller and 
Clarke in Bull. 60, p. 21. 
The rock, which may be classed as saxonite, consists essentially of 
olivine and enstatite, with a little chromite and magnetite. Olivine 
predominates, and the enstatite forms less than one-third of the mass. 
Quartz, serpentine, and genthite are present as alteration products. 
Analyses by F. W. Clarke, record Nos. 811, 811, 792. 
A. The fresh rock. P. R. C. 111. 
B. Separated olivine. 
C. The genthite found in the altered rock. Probably derived from 
olivine. 
A. 
B. 
c. 
SiO-2 ■ 
41.43 
.04 
2.52 
6.25 
43.74 
.55 
42.81 
44.73 
A1 2 3 
} 1. 18 
Fe 2 3 
2.61 
7.20 
45. 12 
none 
FeO 
MgO 
10.56 
CaO 
H 2 OatllO° 
8.87 
H 2 ign 
4.41 
.76 
one 
.10 
.57 
.79 
none 
.26 
6.99 
Civ0 3 
MnO . 
NiO 
27.57 
99.80 
99.36 
99.90 
3. CRATER LAKE. 
Rocks collected by J. S. Diller. All except the last one in the series 
are described by H. B. Patton in P. P., No. 3. Analyses A to N, 
inclusive, by H. N. Stokes, record Nos. 1671, 1953. 
A. Vitrophyric rhyolite, south edge of Llao Rock flow. Lassenose. 
Contains plagioclase, hypersthene, hornblende, and apatite in a glassy 
groundmass crowded with augite microlites. P. R. C. 1314. 
B. Streaked rhyolite, near " Wine Glass " Grotto Cove. Lassenose. 
Contains plagioclase, hypersthene, hornblende, and magnetite, with 
black glass. A few small inclusions of basalt and hypersthene-andesite. 
P. R. C. 1316. 
C. Rhyolite, small dike immediately below Llao Rock. Lassenose. 
Contains plagioclase, hornblende, hypersthene, and magnetite, in a 
glassy groundmass crowded with microlites of feldspar and augite. 
P. R. C 1317. 
