THE ERUPTIVE ROCKS OF MEXICO 475 
are distinguished as a general rule from the hornblende-andes- 
ites of theformer epoch by the exclusive existence of gray horn- 
blende with strong dichroism, altered often in the periphery of the 
crystals into ferruginous products. The colors which generally 
predominate are grayish-violet and red. The latter color comes 
from the alteration of the violet by the decomposition of the 
crystals of amphibole into oxides of iron which are disseminated 
in the groundmass. The majority are of trachyto-porphyritic 
aspect. 
Contemporaneous with or perhaps previous to these andes- 
ites should be noted the greater part of the hornblendic or 
micaceous trachytes whose number is, to be sure, limited, espec- 
lally if compared with the number of those which were con- 
sidered trachytes before the application of the microscope. 
The presence of hypersthene in abundance in andesites marks 
the end of the andesitic eruption, since such andesites are seen 
alternating with the basalts of modern outflows. 
Hornblende-hypersthene-andesites are found in abundance 
in many places. Such are localities in the sierra which bounds 
the valley of Mexico’on the west, in the valley of Toluca and 
in some parts of the Sierra Madre of the state of Chihuahua. 
The vitreous types of these rocks and some dacites frequently 
occur. These present a vitreous magma having spherulitic and 
perlitic structures. Andesitic obsidians with amphibole or mica 
are found in dikes as intrusives in the valley of Mexico. 
Passing to the andesites made up wholly or chiefly of hyper- 
sthene, two varieties may be distinguished: first, those having a 
largely microlitic groundmass, and, second, those in which the 
amorphous groundmass predominates (andesitic obsidians). In 
many localities the two aspects of structure are associated and 
the fact that they grade into one another shows that the differ- 
ences arise from variations in the conditions at the time of 
eruption. 
In the andesitic-obsidians the augite appears successively in 
the first and second generations and then under a quasi-crystal 
Jine form, that is, as a simple devitrification of the amorphous 
