180 
ANALYSES OF ROCKS, U. S. GEOLOGICAL SURVEY. [no. 228. 
9. SAN JUAN REGION. 
Rocks A to O collected by Whitman Cross, who supplies the petro- 
graphic data. Hitherto unpublished, except when otherwise stated. 
A. Rhyolitic vitrophyre, near Del Norte, Rio Grande County. Tos- 
canose. Reported by Cross as containing phenocrysts of oligoclase, 
quartz, biotite, and augite in a dark, fresh, glassy groundmass, the 
latter being predominant. Analysis made by Eakins in the Denver 
laboratory. Sp. gr., 2.423, 14°. P. R. C. 164. 
B. Rhyolite, Summit district, Rio Grande County. Toscanose. 
Large phenocrysts of sanidine, with smaller ones of oligoclase and 
biotite, in a groundmass of orthoclase, quartz, oligoclase, biotite, and 
magnetite. Analysis by Eakins, made in the Denver laboratory. Sp. 
gr., 2.489, 14°. P. R. C. 8. 
C. Quartz-monzonite, Sultan Mountain, San Juan County. Amiat- 
ose. Contains plagioclase, orthoclase, quartz, augite, biotite, magnet- 
ite, and apatite. Some chlorite and epidote as alteration products. 
Analysis b}^ L. G. Eakins in the Denver laboratory. Sp. gr. , 2.751, 14°. 
Described by Cross in 2lst Ann., part 2. P. R. C. 204. 
D. Augite-diorite, Stony Mountain, Ouray County. Ilessose. 
Analysis by Eakins in the Denver laboratory. Sp. gr., 2.891, 13.5°. 
P. R. C. 199. 
Si0 2 .. 
A1A- 
FeA 
FeO . . 
MgO . 
CaO_. 
Na 2 0. 
K 2 0.. 
H 2 0.. 
PA-- 
MnO . 
68.61 
16.43 
.73 
1.52 
.05 
1.79 
2.82 
4.65 
3.35 
99.95 
p.. 
68.85 
17.01 
1.78 
.65 
trace 
1.62 
3.44 
5.11 
1.79 
trace 
100. 25 
c. 
63.91 
17.07 
4.39 
1.51 
.81 
4.47 
3.48 
3.74 
.33 
.21 
99.92 
52. 05 
17.96 
4.09 
6. 33 
5.03 
8.64 
2.99 
1. 61 
.97 
.31 
.43 
100.41 
E. Monzonite, La Plata Mountains. Akerose. Contains augite, 
hornblende, plagioclase, and orthoclase in large amount, Avith biotite, 
quartz, sphene, apatite, and magnetite as subordinate constituents. 
Also secondary chlorite, muscovite, and calcite. Analysis by W. F 
Hillebrand, record No. 1640. Sp. gr., 2.79, 21°. Described by Cross 
in 21st Ann., part 2. P. R. C. 1284. 
F. Diorite-porphyry, Deadwood Gulch, La Plata Mountains. Aker- 
ose. Contains phenocrysts of hornblende, plagioclase, occasional 
quartz, sphene, apatite, and magnetite in a groundmass of orthoclase, 
