IGNEOUS ROCKS. 55 
view is strengthened by the fact that the banded appearance of this 
formation in other parts of the district is caused by a difference in 
the character of the ground mass, the lighter bands being porous 
and the darker ones more dense. 
The rocks of the latest trachyte bed over other parts of the district 
are very similar to the one described above with regard to mineral 
composition and texture, except for the lesser abundance of quartz 
in the lighter colored bands. 
Pyroxene andesite agglomerate and breccia. — In the western part of 
the Antelope Range area the pyroxene andesite bed is a greenish- 
gray copper-stained breccia, with little or no difference between 
fragments and groundmass. Toward the east it grades into a coarse 
agglomerate with huge dark bowlder-like fragments, cemented by 
gray tuffaceous material. This agglomerate is very much like the 
hornblende andesite agglomerate, but as a general rule it has larger 
fragments. 
The rock composing the fragments in the breccia and agglomerate 
is a pyroxene andesite. It is dark gray in color and has porphyritic 
texture, the phenocrysts making up perhaps one-tenth of the rock 
and averaging one-eighth inch in diameter. The phenocrysts are 
pyroxene and plagioclase, variety labradorite, the former being the 
more abundant. The groundmass is dense and appears to be com- 
posed of minute crystals. The pyroxene is light green in color, shows 
a slight brownish pleochroism, and has included magnetite fragments. 
Often it occurs in aggregates of 2 or 3 and sometimes shows twinning 
parallel to the orthopiuacoid (100). Plagioclase is not very abun- 
dant, and has suffered considerable alteration to calcite and kaolin. 
It shows zonal structure and, like the pyroxene, occurs in aggregates. 
The groundmass is composed of numerous crystals of plagioclase 
and fewer of orthoclase, pyroxene, and magnetite, separated by 
amorphous areas. 
Biotite dacite. — The biotite dacite has the same texture and mineral 
composition throughout both its vertical and horizontal extent. 
The color, however, varies slightly. In the Antelope Range area the 
main part has a reddish-brown color, while at the base there are 50 
feet or less of a pinkish-gray color. Outside of the Antelope Range 
area the latter makes up the entire formation. 
The biotite dacite is a porphyritic rock with phenocrysts ranging up 
to one-eighth inch in diameter and making up fully half of the rock 
mass. They are mainly feldspar and biotite, subordinately diopside, 
quartz, and brown hornblende. The feldspars are mainly plagio- 
clase, ranging from basic andesine through labradorite to acidic 
bytownite. Orthoclase is also present, but not as abundant. A 
large number of the feldspars show well-developed zonal structure, 
