178 
ANALYSES OF ROCKS, U. S. GEOLOGICAL SURVEY. 
[no. 228. 
7. ELK MOUNTAINS. 
Analyses made by L. G. Eakins in the Denver laboratory, and hith- 
erto unpublished. Petrographic data supplied by Whitman Cross. 
A. Rhyolite, East Mountain, Crested Butte district. Alsbachose. 
Phenocrysts of orthoclase, oligoclase, quartz, and biotite, in a micro- 
spherulitic and cryptocrystalline groundmass. P. R. C. 514. 
B. Rlryolite, Round Mountain, Crested Butte district. Toscanose. 
Small phenocrysts of quartz, sanidine, biotite, and oligoclase, in a 
groundmass of quartz and orthoclase. P. R. C. 513. 
C. Diorite, Brush Creek, Gunnison County. Tonalose. Contains 
several varieties of plagioclase, with orthoclase, quartz, hornblende, 
biotite, augite, sphene, apatite, and magnetite. P. R. C. 93. 
Si0 2 . 
A1 2 3 . 
Fe 2 3 . 
FeO . . 
MgO . 
CaO . . 
Na 2 . 
K,( ) . 
H 2 0.. 
PA- 
MnO . 
A. 
74.84 
14.05 
.17 
.31 
trace 
1.57 
3.66 
3.14 
2.33 
Sp. gr 
100. 07 
2. 38, 17° 
71.56 
14. 91 
1.47 
1. 04 
.08 
1.98 
3. 78 
4. 94 
.44 
trace 
100. 20 
2. 59, 18° 
62.71 
17.06 
3.79 
2.74 
1.78 
5.51 
3.54 
2. 96 
.24 
none 
trace 
100. 33 
2. 791, 30° 
