48 TJic American Geologist. July, isoo 
No. 2 there is besides, a small amount of pyroxene but ilmenite is pres- 
ent in small amount. In No. 6 there is practically no pyroxene and a 
large amount of the iron mineral. The feldspars of the two specimen;- 
differ in the size of the individual crystals, but appear to be identical in 
composition. In both rocks they are crowded with inclusions. 
Near No. 6, Iron mountain, there is a mass of magnetite which has 
been exploited by prospectors and which is claimed to contain silver 
in appreciable amounts. If, as seems likely, this deposit of magnetite or 
ilmenite has been of igneous origin, the presence of silver in the parent 
magma and its concentration with the iron oxide is worthy of more 
careful study. 
Anorthosyte. Nos. 14, 13, 4, and 8. 
No. 14, About 2j^ miles east of Johnson's shaft. 
No. 13, About 2^ miles east of Johnson's shaft. 
No. 4, Head of Otter Creek. N. side of Otter Creek Mt. 
No. 8, Knoll about 3 miles north of Twin Mt. 
These rocks are made up almost entirely of feldspar of the same 
basic labradorite composition which has been found to characterize 
the gabbro rocks of the Witchita region. The relation of the feldspar 
rocks to those which carry appreciable amounts of ferro-magnesian 
minerals is, however, shown by the occurrence in some instances of 
small grains of pyroxene. In every case the feldspars are crowded 
with inclusions of needle-like and tabular form. 
Throughout the gabbro series the feldspars are twinned according to 
both the albite and orthoclase laws and in some cases the pericline 
twinning is present at the same time. The presence of the two laws 
first mentioned has made the determination of this mineral easy. The 
extinctions indicate a labradorite of basic composition. Furthermore 
the mineral is strongly attacked by hydrochloric acid. 
Hypersthene gabbro. No. 12, West side of creek just north of j\It. 
Sheridan. 
This specimen was collected at some distance from the other gab- 
bro rocks. It dififtrs from the typical gabbros in its structure and in 
character of its pyroxene. The relative amount of ferro-magnesian 
minerals and of feldspar is about equal. The pyroxene is niostly mon- 
oclinic but a part is orthorhombic and slightly pleochroic. Both py- 
roxenes are full of inclusions. The feldspar is similar to that of the 
other gabbros but its inclusions are somewhat different. 
The remarkable variety of these related rocks is of extreme interest 
and they deserve careful study in the field. 
The gabbro in some instances was fotmd in dikes, cutting 
the granite. Specimens of rock, apparent!}' decomposed 
rhyoHtic tufif were found at several places. The quartz porphyry 
forms rounded hills near Ft. Sill, and to these Mr. Comstock 
gave the name Carleton mountains. The rounded topography 
of the quartz porphyry hills is very different from that of the 
jagged granite mountains. 
