IGNEOUS ROCKS. 53 
a little more abundant. They are very much altered to a while 
clayey material, probably kaolin. The biotite is fresh, dark brown, 
and strongly pleochroic. It has a few inclusions of apatite. Mag- 
netite particles are numerous. The groundmass is cloudy from 
specks of ferrite and shows flow structure, especially around the 
phenocrysts. It appears to be partly amorphous with tiny specks of 
feldspar arranged along flow lines. The phenocrysts are also arranged 
parallel to the lines of flow. 
Hornblende andesite breccia and agglomerate. — In the western and 
northern parts of the Antelope Range area the hornblende andesite 
is a greenish-gray breccia, but toward the south it grades into a coarse 
agglomerate, with dark-gray and black fragments. In the breccia 
the fragments are of dark andesite and rather small, the largest 
being about 10 inches in diameter, and by far the larger part being 
smaller. The material between the fragments is of the same 
character as the fragments but somewhat softer. In the agglom- 
erate the fragments are larger, ranging up to perhaps 2 feet in 
diameter. Most of them are composed of the same material as the 
fragments of the breccia, but other fragments are red and trachytic. 
The intervening material in the agglomerate is light gray and 
tuffaceous and weathers readily, leaving the fragments strewn 
around over the surface like bowlders. These soon become black 
and shiny from desert varnish. 
The hornblende andesite composing the fragments is a dark-gray 
porphyritic rock stained green in many places. The main pheno- 
crysts are narrow crystals of hornblende, sometimes one-fourth inch 
long. A few phenocrysts of feldspar are present. The groundmass 
is dark gray and finely crystalline and makes up about nine teen- 
twentieths of the rock mass. The hornblende has a dark-brown 
color and shows pleochroism. Fragments of magnetite are found 
bordering it and included in it, but generally separated from il by 
an alteration rim of ferrite. The hornblende is altered to ferrite on 
the surface and sometimes well into the interior. 
The ground mass is composed largely of small crystals of ortho- 
clase and plagioclase of the variety labradorite, the latter being a 
little more abundant. Besides the feldspar, pyroxene, hornblende, 
and magnetite occur in the groundmass. The small crystals are 
separated by areas of crypt ocrystalline material. 
Latest trachyte. — The latest trachyte bed is easily recognized 
throughout its extent by a characteristic banded appearance. It is 
composed of several different layers; the full section, which is present 
only in the eastern part of the Antelope Range area, is as follows, 
beginning at the base: (1) a few feet of black trachytic pitchstone 
with phenocrysts mainly sanidine, but with plagioclase, biotite, and 
