172 
ANALYSES OF ROCKS, U. S. GEOLOGICAL SURVEY. 
[bull. 168. 
4. MISCELLANEOUS ROCKS. 
A. Obsidian, Obsidian Hill camp, Tewan Mountains. Described by 
Iddings, Tth Ann., p. 292. A rhyolitic obsidian, containing grains of 
iron oxide and a few microscopic feldspars. Resembles that from 
Obsidian Cliff in the Yellowstone National Park. Analysis by L. G. 
Eakins, record No. 851. Sp. gr., 2,352, 23°. 5. 
B. Trachyte (?.), from Los Cerillos. Described by Diller, Bull. 42, 
p. 39. Analysis by F. W. Clarke, record No. 346. Rock composed 
chiefly of orthoclase, with a considerable amount of biotite, epidote, 
pyrite, and limonite : and some amorphous substance. It is the matrix 
or gangue rock of the Los Cerillos turquois. 
Si0 2 .. 
A1A- 
Fe 2 O s - 
FeO . . 
MgO 
CaO . . 
NaA 
K 2 0.. 
Ti0 2 . 
PA. 
FeS 2 . 
MnO 
CuO. 
76.20 
13.17 
.34 
.73 
.19 
.42 
4.31 
4.46 
.33 
trace 
10 
100. 25 
B. 
56.68 
16.62 
6.28 
.79 
.59 
1.03 
11.18 
3.28 
.22 
.73 
2.21 
1.02 
trace 
100. 63 
