POPOCATEPETL AND IXTACCIHUATL—FARRINGTON. 11g 
those laid bare by the water courses. None of these, so far as I 
observed, showed glacial striz or smoothing. The rounding of their 
solid angles was rather to be attributed to water. Most of the exposed 
rock surfaces, too, were rough and angular, showing none of the 
rounding and scoring which would be expected from glaciation. 
Again, the scored surfaces below Porfirio Diaz Glacier, as has been 
said, gave evidence of having been submitted to a comparatively 
gentle pressure, as if the glacier had nearly spent its force when it 
reached them, rather than having just begun its career. While con- 
fident, therefore, that the glaciers of the mountains had at one time a 
greater extent than now, I can hardly believe that they reached the 
proportion of the great mer de glace which Packard has pictured. 
PETROGRAPHY. 

The prevailing rock of Ixtaccihuatl is a compact amphibole- 
andesite of a gray color. In some areas it is porous and slag-like, 
and here, through oxidation of the iron, may become reddish in 
color. In structure it is porphyritic. The most prominent pheno- 
crysts are white, lath-shaped crystals of plagioclase feldspar, ‘some of 
which are from 6 to 8 mm. length. Under a lens distinct striations 
can be seen on many of the crystals, which mark them as plagio- 
clase. Twins according to the Carlsbad law as well as trillings and 
fourlings, are not uncommon. The amphibole phenocrysts are less 
noticeable, but on close examination they are seen to be quite numer- 
ous. They are black, shining with the peculiar horn-like luster char- 
acteristic of this mineral, lath-shaped, and frequently attain a length 
of 30or4 mm. Distinguished by a dark-green color and sub-resinous 
luster, are also to be seen phenocrysts of pyroxene. These crystals are 
of stout, prismatic habit, showing the unit prism, pinacoids, and 
occasional pyramidal faces. They are more numerous in some por- 
tions of the rock than others, seeming at times to prevail in quantity 
over the amphibole. 
Under the microscope the ground mass resolves into a carpet of 
microlites, showing the well-known hyalopilitic structure. The 
microlites are transparent, needle-like, .o2 to .o6 mm. in length, 
Scattered among these are opaque grains referable to iron oxide. 
The larger part of the ground mass remains dark during a complete 
rotation in parallel light, indicating a glassy base. Certain of the 
larger microlites by their inclined extinction and low interference 
colors are marked as feldspar. Less numerous are light-green grains 

