IGNEOUS ROCKS. 51 
entirely filled with quartz and chalcedony. Near its base the trachyte 
in places becomes ferruginous, yielding from 6 to 7 per cent of iron. 
The upper 30 feet of the bed has a somewhat varying character in 
the Antelope Range area, being composed of a main layer of lavender- 
gray trachyte, with thin layers of tuffaceous, scoriaceous, and dense 
trachytes, and has been mapped separately as far as it could be dis- 
tinctly traced. 
In the Swett Hills the early trachyte bed is dark gray and the 
phenocrysts are few. In the Eightmile Hills it is red, with numerous 
plienocrysts of sanidine and plagioclase and fewer of biotite. East of 
the Eightmile Hills and in the Harmony Mountains parts of the bed 
are porous and even scoriaceous, and have a predominance of lime- 
soda feldspar over orthoclase. In Cottonwood Canyon and westward 
it is complicated by additional beds of rhyolite, trachyte, and andesite 
similar to some of the overlying lavas. South of Iron Mountain the 
bed is amygdaloidal. The amygdules are spherical or oblong in 
shape, varying from 5 to 50 millimeters in diameter, and are filled 
with quartz and calcite. 
Early tuffaceous rhyolite. — In the northeast and in the southern 
part of the Antelope Range area the early tuffaceous rhyolite bed is 
of uniform character throughout its vertical extent. It is a light- 
gray, pink, or white porphyritic rock with numerous phenocrysts, 
and small irregular fragments of other volcanic rocks. The pheno- 
crysts are mainly quartz, orthoclase, and plagioclase, with less 
abundant biotite and hornblende. Zonal structure, secondary 
enlargement, and inclusions of long narrow crystals of apatite are 
common. The biotite is largely altered to phlogopite and is fre- 
quently associated with magnetite. Pyroxene appears in the included 
rock fragments, but not in the rhyolite itself. The groundmass is 
amorphous, except for small crystals of quartz and feldspar and 
cavities which are partly or wholly filled with chalcedony or calcite. 
The early rhyolite bed in and near the Antelope Range in the west- 
ern part of the Antelope Range area differs somewhat from the above. 
The succession, beginning at the bottom, is (1) a few feet of gray or 
black rhyolitic pitchstone, with numerous phenocrysts of quartz, 
feldspar, and mica; (2) hard red rhyolite which grades up into 
(3) a light-red rhyolite with numerous fragments of other volcanic 
rocks. (2) and (3) grade laterally into the tuffaceous phase of the 
eastern part of the area, but (1) does not change its character and is 
found in the same position, viz, at the bottom of the early rhyolite 
series, throughout this area as well as in other parts of the quadrangle. 
The minerals in these 3 phases are very much the same as in the 
tuffaceous phase, except that little or no hornblende is present. 
The feldspars are mainly orthoclase, but plagioclase is fairly abundant. 
They are altered to calcite, kaolin, and quartz; the plagioclase more 
