POPOCATEPETL AND IXTACCIHUATL—FARRINGTON. — IOI 
The feldspar phenocrysts are idiomorphic and many display 
beautiful zonal structure. Many of them contain magmatic and gas 
inclusions which are arranged in zones. The species of feldspar 
indicated by observation of the angle of extinction on the albite 
lamelle is labradorite, the highest angle of extinction observed being 
2h. 
The hypersthene phenocrysts are quite as numerous as those of 
feldspar, but of smaller size. The crystals are of prismatic habit with 
a (100), 4 (o10), predominant, and m (110), subordinate. Sections 
across the prisms show rectangles with truncated corners in which 
the prismatic and pinacoidal cleavages || to 4 (o10) are well marked. 
The color is green to brownish according as the section is parallel 
with the prism or at right angles toit. The pleochroism is, w, red- 
dish brown; h, reddish yellow; c, light green. 
Beside the individual phenocrysts there are numerous aggregates 
without apparent twinning plane, as well as intergrowths of hyper- 
sthene and feldspar. Magnetite is common in grains though not 
abundant. 
The rock at the edge of the crater is, as might be expected, more 
porous and slaggy than that at La Cruz. Being more subject to 
weathering processes it is largely reddish or chocolate-brown in color. 
It breaks with a coarse-granular rather than a conchoidal fracture. 
The phenocrysts are neither as numerous nor as large as those of 
the rock of La Cruz. This is accounted for, as already mentioned, 
by the fact that this lava was farther from the source of heat and 
had less time for cooling. As in the rock of La Cruz, the phenocrysts 
are lath-shaped feldspars and grains and druses of pyroxene. There 
are numerous grains of pyrite noticeable as well. It is probably of 
secondary origin. The ground mass as a rule is light-brown in color 
and made up of microlites of feldspar and augite not over .o2 mm. in 
length. In portions of the section the ground mass becomes com- 
pletely isotropic and has a marked fluidal arrangement about the 
phenocrysts. The phenocrysts are smaller and less numerous than 
in the lavas of La Cruz, but in other respects do not differ. 
A monoclinic pyroxene, probably augite, is present in large phen- 
ocrysts. It canbe distinguished from the hypersthene by its inclined 
extinction. Twins are occasionally to be seen, the twinning plane 
being the common plane, a (100). 
The microscopic structure of the rock marks it as quite distinct 
from that of La Cruz. There is therefore reason to doubt the view 
of Aguilera and Ordofiez that the rocks belong to the same lava 
flow. 
