90 RECONNAISSANCE OF PART OF WESTERN ARIZONA. 
A-24, A-25, A-27, and A-37 are very similar in appearance. 
The texture of all of them is porphyritic, with phenocrysts of olivine 
and a groundmass made up of laths of feldspar and grains of augite 
and magnetite and glass full of magnetite dust. The amount of 
glass is small and fills spaces between the plagioclases, but these 
spaces are very few and hardly show in the thickness of the section. 
The olivine phenocrysts are rather fresh, being generally altered only 
around the rims. Secondary calcite is common. 
A-37 has more plagioclase than A-24 and the olivine is entirely 
altered to a dirty brownish-black substance. 
A-30 and A-34 are porphyritic, with irregular rounded to angular 
crystals of olivine which are perfectly fresh. Some magnetite may 
be classed with the phenocrysts. The groundmass consists of long, 
narrow laths of labradorite with extinction angles of from 32° to 35°, 
and a less amount of a purplish, very slightly pleochroic augite, 
which fills the interspaces. The augite is not in typical ophitic form, 
but in irregular laths which fill the interstices. Generally it is very 
fresh, although there are patches of serpentine in the groundmass, 
some of which is definitely derived from olivine. Within the augite 
there is a great deal of magnetite in strings and in skeleton crystals, 
in beautiful branching forms. 
DIKE ROCKS. 
OLIVINE DIABASE. 
Cutting the rhyolite at the eastern edge of Black Mesa, in Gold 
Roads Pass at Mud Spring, is a large dike, some 25 feet in thickness, 
of olivine diabase (A-14), apparently connected with a dark sheet 
which overlies the rhyolite. Megascopically the rock is dark in 
color and is rather coarsely granular. Under the microscope it has 
a typical ophitic texture and there is more feldspar than augite. The 
feldspar occurs in twinned laths with a maximum extinction angle 
of 31°, and is labradorite. It is very fresh, although occasionally it 
contains serpentine in cleavage cracks. Fresh augite in less amount 
than the plagioclase fills the interspaces between the feldspar laths. 
It is pale purple in color, and is nonpleochroic or very slightly pleo- 
chroic. It contains a small amount of iron oxide in extremely fine 
particles. Olivine, less in amount than the augite, occurs in irregu- 
lar rounded grains, mostly small, though some are large. Some of 
the olivine is altered to serpentine, chlorite, and red iron oxide 
around the edges and in cracks. Magnetite occurs in still less amount 
and in irregular grains. 
RHYOLITE PORPHYRY. 
At the base of Boundary Cone is a plug, or dike from a plug, of 
rhyolite porphyry (A-10). Megascopically it is a pink, fine-grained 
