84 RECONNAISSANCE OF PART OF WESTERN ARIZONA. 
between these fragments and the single specimen obtained from the 
flow below the rhyolite at Black Mesa. There may have been, and 
probably were, volcanic eruptions previous to that of the rhyolite, 
of which now no trace remains, and the fragments in the rhyolite flow 
breccia indicate that the andesite was much more extensive than at 
the present time. 
West of Boulder Canyon in Colorado River is a volcanic tuff (A-39). 
It is gray to pink in color, ashlike, gritty, very friable, and fine grained, 
and occurs in horizontal beds. Under the microscope it is tufTaceous 
and is composed chiefly of small angular glass fragments, needle-like 
and in flakes, and contains a few very small and irregular quartz 
grains. 
Compact normal rhyolite (117, A-7, A-12, A-43) occurs within 
the rhyolite series, but no attempt was made to find the relation- 
ship which may exist between these flows and the sheets of 
breccia, flow breccia, and so on, in which it occurs. Megascopically 
these rocks are white or pink and sometimes banded. Under the 
microscope a glassy flow texture is plainly seen in one of the rocks 
(A-43), while in others (117, A-7), although now devitrified, the flow 
texture can be clearly made out. In 117 the original flow lines of 
the groundmass show in polarized light by the fact that there are 
larger and more numerous quartz particles along certain lines, while 
areas between these lines are much finer grained, and very often the 
centers of these areas are almost entirely isotropic. In the specimen 
from Black Mesa (A-12), from the region where the andesite (A-ll) 
underlying the rhyolite was obtained, no flow texture can now be 
made out. It is completely devitrified and is now an aggregate of 
quartz and feldspar. According to Lee, this rhyolite occurs in dike- 
like intrusions and in sheets overlying the andesite, a fact which 
clearly proves the older age of the andesite, provided that the intruded 
and overlying rocks are both like the specimen (A-12). All of the 
sections are perpatic. In some, fragments of orthoclase, a little 
plagioclase, and some quartz occur, and in others the original phe- 
nocrysts are much altered. Magnetite and zircon are accessory, and 
calcite and zeolites secondary minerals. Chlorite, from an original 
femic mineral, occurs in 117. 
Two of the rhyolite specimens are spherulitic (A-l, A-8). Mega- 
scopically these specimens are pink, spherulitic, and full of large 
irregular cavities, occasionally filled with quartz crystals. Under 
the microscope the rock shows very few phenocrysts of orthoclase, 
plagioclase, and biotite, and rather rare magnetite crystals in a spheru- 
litic groundmass. In one specimen (A-8) the entire rock is filled with 
trichites, generally in hairlike bunches. Sometimes margarites occur — 
that is, hairlike strings of globules; and in places microlites appear 
in schools. In the specimen from Wickenburg (A-l) there is, in the 
