IGNEOUS ROCKS OF WESTERN ARIZONA. 85 
groundmass, besides the spherulites, a quartz-orthoclase aggregate in 
finger-like interlocking particles. The rock is rather dark with feritic 
material, and there is considerable kaolin and other alteration prod- 
ucts. In the. rays of the spherulites there is often a deposit of red 
hematite or limonite, and cubes of what may have been magnetite 
show a slight red edge in transmitted light, as though altered to 
limonite. 
YOUNGER ANDESITES. 
Definitely in place in a number of localities above the rhyolite is 
an andesite (107, A-18). This is much more limited in extent than 
the earlier flow. A number of other andesites which occur in the 
area either rest directly upon the granite (A-40, A-41) or are in 
masses isolated from the other efTusives, so that no relationship is 
seen (112, A-2, A-42) ; they may belong either to the upper flow, the 
underlying rhyolite having been eroded, or to the earlier flow. In 
the field all of the andesites are of a dark-red to brown color, are often 
vesicular, and are often very hard to distinguish from the overlying 
olivine basalts. 
The : two specimens (107, A-18) from above the rhyolite are dark 
reddish black in color and are vesicular, with more phenocrysts in A- 
18 than in 107. Under the microscope the two rocks are very similar 
in appearance. Both of them are porphyritic-intersertal in texture 
and contain more phenocrysts than groundmass. The phenocrysts 
occur in two generations and consist of plagioclase (andesine to lab- 
radorite), augite, and magnetite. The larger plagioclase crystals 
occur as irregular fragments, often zonal, while the smaller occur in 
perfect laths. There is much less augite than plagioclase. This also 
occurs in two generations, and in each case is less in amount than the 
corresponding generation of plagioclase. Two generations of mag- 
netite, less in amount than the augite, occur in quadratic and irregu- 
lar crystals and grains. Of these the larger crystals are less in amount 
and the grains are smaller than the second generation of augite, and 
the small crystals are very small and are scattered through the ground- 
mass in isometric crystals. They occasionally occur in the augite, 
but very rarely in the feldspar. The groundmass in both specimens 
is isotropic and consists of brown glass full of magnet ite— so much 
in A-18 that it is nearly opaque. 
Specimens A-40 and A-41 , from sheets in the walls of Black Canyon, 
are augite andesite; possibly the latter at one time contained olivine; 
if so, it was an olivine basalt. Megascopicallv they are medium dark 
red-brown in color, with more or less black and white phenocrysts in 
an aphanitic groundmass. Under the microscope they are porphy- 
ritic and sempatic. The phenocrysts of 40 consist of broad, irregular 
laths, and the groundmass is apparently a devitrified glass, full of 
