472 OLIVER C. FARRINGTON 
rania del Gigante, belong undoubtedly to a pre-Cretaceous 
epoch. Fragments of these granites, syenites, etc., occur in 
the red conglomerate of Guanajuato. They are undoubtedly 
anterior to the rocks of La Luz, which may be considered to be 
recent Tertiary. 
In the metalliferous regions of Pachuca, Real del Monte, 
and El Chico, in the state of Hidalgo, altered pyroxene-ande- 
sites and dacites of green, dark and light gray, and violet color, 
constitute the predominating eruptive rocks. They are distin- 
guished from the rocks previously cited chiefly by their struc- 
ture, which may be considered as invariably trachyto-porphy- 
ritic; a structure produced by large crystals of labradorite and 
altered remains of crystals of pyroxene. Andesite tuffs like 
those of Guanacevi or rhyolite tuffs like those of Zacatecas 
scarcely occur at all. There are many other points along the 
Sierra Madre where the andesitic greenstones occur, chiefly in 
the states of Chihuahua, Sinaloa, Durango, Jalisco, and the ter- 
ritory of Tepic. From these the regions of Zacatecas, Guana- 
juato, and Pachuca are somewhat distinct from an orographic 
point of view. Considered petrographically, however, they are 
mountain regions which, on account of the order of eruption of 
their igneous rocks, may be regarded as branches of the Sierra 
Madre penetrating toward the interior. 
Trachytes and trachyte-andesites have in some of the locali- 
ties mentioned immediately succeeded the andesites, either as a 
modification of the latter or as a later eruption. Trachytes 
are, however, relatively rare in Mexico, especially .among the 
earlier eruptives. They are more frequent before the second 
period of andesites and in the modern eruptions. 
The rhyolites appeared after the andesites, presenting the 
variations common to rocks of this type. They occur in many 
localities, only the principal ones of which can be indicated. In 
the central part of the Sierra Madre the rhyolites cover great 
areas. Here the structure passes from the micro-granulitic to one 
entirely vitreous giving obsidians and retinites. The rhyolites 
are likewise notably abundant in many places of the Mesa Cen- 
