82 RECONNAISSANCE OF PART OF WESTERN ARIZONA. 
both in small and thin and in large and stout prisms, the latter being 
sometimes irregular in form. The accessories occur within both the 
quartz and the feldspar. 
Farther to the south, in Williams Canyon, the banding of the 
underlying rocks becomes much more marked, giving long-drawn-out 
black and white bands (A-49), for which reason the name of "Banded 
Canyon" is applied to the gorge through which Williams River flows. 
Lee describes the rock as apparently continuous with the granitic 
rocks to the north, although the transition was not followed. Mega- 
scopically the rock is aphanitic, black and white banded, the black 
bands occasionally showing white phenocrysts and rounded inclusions. 
Under the microscope the texture is markedly schistose. The con- 
stituents are irregular rounded grains, very variable in size, of horn- 
blende, orthoclase, some plagioclase, and a little quartz. There is 
much chlorite, and a little epidote and titanite. 
EFFUSIVE ROCKS. 
OLDER ANDESITE. 
Some time during the Tertiary period, after the Paleozoic and 
Mesozoic sediments had been removed from the area and the under- 
lying granite deeply eroded, began a period of volcanic activity. 
The earliest erupted rocks of which traces now remain were andesites, 
a remnant of which occurs at Black Mesa, north of Boundary Cone 
(A-ll). It here appears to underlie and is cut by dikes of rhyolite 
(A-12). This is the only area where any eruptive rock is definitely 
known between the rhyolite and the granite. In several places the 
andesite (A-40, A-41) rests directly upon the granite, but no rhyolite 
occurs above it, and consequently the later flow and the intervening 
rocks may have been eroded. The andesite from Black Mesa is, then, 
as described above, beneath the rhyolite and is cut by it; and since 
within the rhyolite flow breccia from various parts of the region 
andesite fragments occur, it is very probable that this rock is in place 
and is older than the rhyolite. 
Megascopically tins andesite (A-l 1 ) is dark reddish gray in color 
and shows numerous white phenocrysts in a fine aphanitic ground- 
mass. Under the microscope the texture is porphyritic and sem- 
patic. The phenocrysts are fairly regular laths, some of which are 
partially and some completely altered. The partially altered phe- 
nocrysts are plagioclase, which still show indistinctly the original 
twinning, but the extinction angle can not now be determined. They 
have an index of refraction higher than Canada balsam, and are much 
altered to calcite, sericite, and kaolin. Besides these, there are fewer 
chlorite pseudomorphs and still less magnetite. The chlorite pseu- 
domorphs are lath-shaped and are very similar in form and size to 
