Wichita Moimtaijis and Arbiicklc Hills. — Vaiighan. 47 
Report upon the Rocks of Wichita Mountains 
By a. C. Spencer. 
The rocks of the Wichita mountains which were submitted to mt' 
fcr examination have been determined as follows: 
Granites: Nos. i, 9, 7, 15, are hornblende granites. 
No. I, near Johnson's shaft, Hornblende granite, is a perfectly 
granular rock composed of quartz, orthoclase, hornblende and mica, 
with apatite and magnetite as accessory minerals. 
No. 7. Soldier's Spring Mt. — Granite porphyry. Large crys- 
tals of quartz and red feldspar imbedded in a holo-crystalline matrix of 
the same minerals. There is a small amount of green pleochroic horn- 
blende in the section. The borders of the feldspar phenocrysts fre- 
quently consist of intergrown quartz and feldspar. 
No. 15, western foot of Tymatee Mt. — Granite, is similar to No. 
7 in its mineral composition but is not porphyritic. The feldspar and 
qucirtz are frequently intergrown. 
Quartz porphyries. The two specimens of quartz porphyry. No. 17. 
Eoicheast of Mt. Scott, and No. 18, [3 miles east of Hennepin, L T.,] 
are dense red rocks composed of crthoclase and quartz laid in a 
ground-mass of quartz and feldspar with a hypocrystalline or micro- 
poicilitic structure. The red color of these rocks is due to disseminated 
oxide of iron in the feldspar. 
The remainder of the rocks belong to the gabbro family; they com- 
prise typical gabbro, hypersthene gabbro, troctolyte, and anorthosyte. 
Gabbro — Nos. 11, 5, 16. 
No. II, Soldier's Spring Mt. — Gabbro, ophitic structure, is from a 
narrow dike in the granite. Its structure is dififerent from that of the oth- 
er rocks being quite plainly ophitic, otherwise it is similar to the other 
gabbros. It consists of basic labradorite and pyroxene with a large 
amount of ilmenite. Apatite is also present. 
No. 5, S. side of quartz vein. Johnson's prospect shaft. Typical 
gabbro. This rock has been considerably altered but its original com- 
position may be still made out. Basic labradorite and pyroxene are 
the essential constituents. The former has very few inclusions by which 
feature it is dififerent from the feldspars of the rocks to be described. 
The feldspar and pyroxene are intergrown so that unoriented crystals 
of the former are set in large patches of the latter. This poicilitic 
structure may be plainly recognized in hand specimens. 
Accessory minerals are ilmenite and apatite in small amounts. This 
specimen was collected from the walls of a quartz vein and the sec- 
tion studied is threaded by minute stringers of that mineral. 
No. 16, Sec. 13, T. 4 N., R. 17 W.. Middle Otter creek. Gabbro. 
This rock differs from No. 5 only in containing a very much larger 
percentage of ilmenite. 
No. 2, dike in granite near Jolinson's shaft, and No. 6, Iron moun- 
tain. — Troctolyte. These rocks are composed essentially of the same 
l;asic feldspar already described with a large amount of olivine. In 
