86 KEOONNAISSANCE OF PART OF WESTERN ARTZONA. 
dust and grains of magnetite and hematite. Specimen 41 has pheno- 
crysts of plagioclase, somewhat greater in amount than augite and 
much greater than some red-brown pseudomorphs which may be 
altered biotite or olivine. The groundmass in this specimen is in 
laths and irregular patches, much of which is definitely plagioclase. 
Besides the feldspar, much black and red iron oxide and shreds of 
augite and dark altered biotite (?) occur. 
A-42 is a biotite-augite andesite from a sheet forming Eagle Rock, 
near Little Round Island, and is similar in character to the two pre- 
ceding. The phenocrysts, less in amount than the groundmass, are, 
in the order of their importance, plagioclase, biotite, diopside, and 
magnetite. The plagioclase is andesine to labradorite. The biotite 
has a peculiar deep red-brown color, is pleochroic with t>^=brownish 
red and a==gree,nish yellow, and has much the appearance of basaltic 
hornblende. It has, however, parallel extinction, gives a uniaxial in- 
terference figure, shows parallel cleavage in laths, has the " bird's-eye 
maple" surface, and is optically negative. It always has a dark rim 
of magnetite. The pyroxene is pale green in color, is nonpleochroic, 
has an extinction of about 40°, and has good cleavage — parallel in 
laths and at right angles in basal sections. It is diopside. Both the 
plagioclase and the pyroxene are much altered to calcite. The ground 
mass is yellowish to red, cloudy, and apparently partially devi trifled 
glass full of red ore dust. Most of the anisotropic part of the ground- 
mass is spherulitic or contains innumerable microliter. 
South of Seligman there is a hypersthene andesite (113). This is 
from the lower of two flows, the upper of which is darker and may be 
basalt, though no specimen was examined. Megascopically 113 is 
a dark-gray rock with white patches and occasional calcite fillings in 
cavities. Microscopically the texture is porphyritic and dopatic. The 
phenocrysts are either thin or broad laths, and the groundmass is 
hypautomorphic. The phenocrysts, in the order of their abundance, 
are plagioclase, augite, and hypersthene. The feldspar occurs in both 
large and small laths. The stout crystals are not twinned and are 
always zonal; the narrow crystals are usually twinned and give an 
extinction between 20° and 25°, or that of andesine. In the zonal 
feldspars the index of refraction at the center is higher than at the 
rims; the interference figures are positive, and show a comparatively 
small value for 2E. This excludes all the feldspars but andesine and 
labradorite. Since the rims have a lower index but are still positive 
in character, it shows that the feldspar is either labradorite or andesine 
or both, and that the interior is more basic than the exterior. The 
femic minerals consist of very irregular grains of a greenish pyroxene 
with a high extinction angle — augite; and another pyroxene occur- 
ring in elongated prisms with longitudinal cleavage, having parallel 
extinction, showing high interference colors, optically negative, and 
