CLARKE.] 
IGNEOUS AND CRYSTALLINE ROCKS. 
161 
9. SAN JUAN REGION. 
Rocks collected by Whitman Cross, who supplies the petrographic 
data. Hitherto unpublished. 
A. Rhyolitic vitrophyre, near Del Norte, Rio Grande County. 
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. 598. 
B. Rhyolite, Summit district, Rio Grande County. Large pheno- 
crysts 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. 164. 
C. Augite-diorite, Sultan Mountain, San Juan County. Contains 
plagioclase, orthoclase, quartz, augite, biotite, magnetite, and apatite. 
Some chlorite and epidote as alteration products. Analysis by L. G. 
Eakins in the Denver laboratory. Sp. gr. 2.751, 14°. 
D. Augite-diorite, Stony Mountain, Ouray County. Analysis by 
Eakins in the Denver laboratory. Sp. gr. 2.891, 13.5°. P. R. C. 199. 
SiO, 
Fe 2 3 
FeO.. 
MgO. 
CaO.. 
H 2 0. 
P 2 5 - 
MnO 
68.61 
16. 43 
.73 
1.52 
.05 
1.79 
2.82 
4.65 
3.35 
99. 95 
B. 
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 
i>. 
52.05 
17.96 
4.09 
6.33 
5. 03 
8.64 
2.99 
1.61 
.97 
.31 
.43 
100. 41 
E. Diorite, La Plata Mountains. Contains augite, hornblende, 
lagioclase, and orthoclase in large amount, with biotite, quartz, sphene, 
patite, and magnetite as subordinate constituents. Also secondary 
hlorite, muscovite, and calcite. Analysis by W. F. Hillebrand, 
ecord No. 1640. Sp. gr. 2.79, 21°. 
F. Diorite-porphyry, La Plata Mountains. Contains phenocrysts 
f hornblende, plagioclase, occasional quartz, sphene, apatite, and mag- 
etite, in a groundmass of orthoclase, plagioclase, and quartz. Also 
icondary epidote, chlorite, and calcite. Analysis by Hillebrand, 
icord No. 1636. Sp. gr. 2.677, 24°. A trace of sulphur is present. 
Bull. 168 11 
