476 OLIVER C. FARRINGTON 
groundmass, or even in very small microlites. The andesitic 
eruptions of our volcanoes have produced these vitreous rocks 
charged with pyroxene. Such may be seen in the lavas of the 
volcano Colima and the early eruptions of Popocatepetl. 
While andesites of the vitreous types predominate in the more 
recent eruptions, trachytes of the vitreous type alSo Occuia mune 
lavas of 1870 thrown from the volcano of Ceboruco furnish us a 
good example of this and may be designated as obsidian-like 
pyroxene-trachytes. Vitreous trachytes are likewise found in 
the volcanoes of Popocatepetl and Colima. 
The presence of olivine as an accidental element in these 
rocks gives them to the naked eye the appearance of basalts. 
With these they have sometimes been confounded owing to the 
similarity of color and superficially blistered appearance, com- 
mon to the basaltic lavas. There can be no doubt that they 
pass from one to another by insensible gradations since the 
diminution of oligoclase with the absolute predominance of 
labradorite, brings them to basic types represented by labrado- 
rites and basalts. Such gradations may be actually observed 
in some places in the valley of Mexico. 
After the hornblende-andesites, which seem to have suc- 
ceeded the rhyolites, the eruptions continued not only by emis- 
sion of compact rocks, but also by an enormous quantity of 
broken products that were changed to sediments by watery 
vapors of the same eruptions and by atmospheric agencies 
Thus have originated those thick deposits of andesitic tuffs, 
breccias, etc., which are so abundant in different portions of the 
great central valleys of the country. The more superficial lay- 
ers, having the lightness and fineness of detritus, have indeed 
been confounded at times with zolian products. 
Lastly may be noted the labradorites, that is, basalts con- 
taining no, or only accidental, olivine, and the true basalts which 
occur in the volcanic regions or in rare cases rest upon or break 
through the Cretaceous limestones. 
Labradorites occur in contact with the andesites of the first 
epoch and rhyolites, at various points along the eastern slopes of 
