IGNEOUS ROCKS OF WESTERN ARIZONA. 87 
slightly pleochroic in pink and green tones — hypersthene. Magnetite 
occurs in grains which, in size, grade into the groundmass. There is 
no definite line, and it is in such irregular grains that it may be included 
with either the phenocrysts or the groundmass. Exclusive of the 
femic minerals, the groundmass is made up of xenomorphic areas and 
laths of feldspar which are probably of the same kind as the large 
phenocrysts. The xenomorphic feldspar has an index of refraction 
greater than Canada balsam and is consequently plagioclase, but the 
areas are so small that the kind is indeterminable. Besides the feld- 
spar there is an isotropic substance which occurs in very small anhe- 
drons and which may be glass or a zeolite. The groundmass is full 
of magnetite dust and there is an alteration product in the cleavage 
cracks of some of the feldspars which is anisotropic and has an index 
of refraction less than Canada balsam. It is a zeolite. 
Twenty-five miles west of Congress Junction there occurs a coarse 
breccia formed of large angular blocks cemented with red material. 
Megascopically the specimen (A-2) has a red compact groundmass, 
and there are phenocrysts of hornblende forming about one-eighth 
of the mass. Microscopically the texture is porphyritic and perpatic. 
The phenocrysts are hornblende, much less plagioclase, and still less 
biotite. The hornblende is very peculiar in several ways. It has a 
very low extinction angle, generally appearing as parallel, the largest 
found being only 3f° — that is, c: c is circa 4°. The pleochroism is 
also peculiar. For a it is a greenish yellow, while for c it is a deep 
red-brown. Absorption c>a. The elongation is parallel to jC, and 
therefore it is positive, while the optical character of the mineral is 
negative. Surrounding the mineral there is always a dark rim of 
iron oxide. This is usually black, but occasionally it shows a red 
tone. The cleavage is good, parallel to c in lath-shaped sections, and 
in basal sections the usual amphibole cleavage. The mineral is ba- 
saltic hornblende. The feldspar is generally zonal and is positive in 
character. It is between andesine and labradorite. Actually the 
amount of plagioclase that occurs is much less than the amount of 
hornblende, but the phenocrysts of feldspar have either weathered 
out or been ground out in preparing the thin section. Apparently 
they were originally much more numerous and both longer and broader 
than the hornblende. Biotite occurs in a very few long, thin flakes. 
It has dark rims, and magnetite is included along the cleavage lines. 
Pleochroism is brownish to greenish. Magnetite occurs in irregular 
grains. The groundmass consists chiefly of laths of plagioclase, which 
have an extinction angle of 30° and are consequently labradorite. 
Between the feldspar laths there is a much iron-stained substance 
which is probably glass. Iron oxide occurs in the form of very fine 
dust and in patches, producing the red color of the rock. 
