100 FIELD CoOLUMBIAN MusEUM—GEOLOGY, VOL. 1. 
to the peak, they would prefer to connect it with a somewhat similar 
elevation on the southeast and regard the two as the remains of 
an earlier crater corresponding to the Monte Somma of modern 
Vesuvius. Not having been able to visit the Pico del Fraile myself 
I am unable to give any opinion on the question at issue, but the 
problem is an interesting one for further study. 
DETERMINATIONS OF THE ALTITUDE OF EL PICO DEL FRAILE. 




METERS.| FEET. 
5,004 16,418 | Sonneschmidt, 1770. 
5,050.1 16,570 | Sonntag, 1857. 
5,142 16,870 | Gerolt, 1834. 

PETROGRAPHY. 
The lava outcropping at La Cruz and seen in scattered fragments 
about this portion of the mountain is a compact hypersthene-andesite 
of grayish-black color. Owing to the glassy nature of the feldspars, 
the general lack of conspicuity of the phenocrysts and the conchoidal 
fracture of the rock, it has a strong resemblance to obsidian. No 
true obsidian is, however, known in the region. Upon weathering, 
certain areas take on a reddish tone. 
Megascopic examination shows the rock to possess a porphy- 
ritic structure; the white or light-colored phenocrysts are feldspar, 
the dark-green grains, pyroxene. The feldspar phenocrysts are 
small compared with those of the andesite of Ixtaccihuatl; they rarely 
exceed 2 mm. in length. They do not have well formed crystal 
outlines, but so far as these appear they show the habit of the 
feldspars to be tabular, Distinct crystal outlines can rarely be deter- 
mined for the pyroxene, the mineral occurring as coarse grains or 
lining druses. 
Under the microscope the. ground mass is seen to have the 
typical hyalopilitic or felt-like structure. It is made up chiefly of 
minute microlites averaging .o2 mm. in length, and trichites with 
-some margarites. The microlites are probably feldspar laths but their 
extremely small size prevents accurate determination. Quite a large 
proportion of the base is isotropic or glassy. There are indications of 
.a gathering of the microlites into bundles to give a variolitic or sub- 
-spherulitic structure. Opaque grains numerous in the ground mass 
are presumably iron oxide. 
