196 
ANALYSES OF EOCKS, U. S. GEOLOGICAL SURVEY. [no. 228. 
4. MISCELLANEOUS ROCKS. 
A. Obsidian, Obsidian Hill camp, Tewan Mountains. Liparose. 
Described by iddings, 7th Ann., p. 292. A rhyolitic obsidian, con- 
taining 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. 3-16. 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. 
C. Gabbro-porphyry, Mount McKensie, Los Cerillos Mountains. 
Andose. Received from J. F. Kemp. Described by D. W. Johnson, 
in School of Mines Quarterly, vol. 25, p. 82. Analysis by George 
Steiger, record No. 2027. Contains augite, plagioclase, orthoclase, 
and biotite. P. R. C. 1300. 
Si0 2 . 
FeA 
FeO.. 
MgO . 
CaO.. 
Na 2 0. 
H 2 at 105° .... 
H 2 above 105° 
Ti0 2 
PA 
FeS 2 
MnO 
BaO 
SrO 
CuO 
76.20 
13.17 
.34 
.73 
.19 
.42 
4.31 
4.46 
.33 
trace 
.10 
B. 
100. 25 
56.68 
16.62 
6.28 
.79 
.59 
1.03 
11.18 
3.28 
.22 
.73 
2.21 
1.02 
trace 
100. 63 
C. 
48.21 
17.96 
5.18 
4.47 
4.11 
9.72 
3.68 
2.99 
.21 
1.41 
.84 
.58 
.31 
.07 
trace 
99. 74 
