A74 OLIVER C. FARRINGTON 
by retinites, which may come to predominate, as in the hill 
Xicuco between Tula and Mixquiahuala of the state of Hidalgo. 
The violet-colored rhyolites that occur in flows in the Tertiary 
formations of the Acacico near Yahualica, Jalisco, are notable 
for the curious forms (axiolites) which the microfelsitic paste 
presents under the microscope; forms very similar to those 
which Zirkel describes in a rock from the Black Rock Mount- 
ains, Nevada. 
In the central part of the country between the parallels 19° 
and 21’ N. Lat., a notable eruptive zone exists. In this zone 
the appearance of modern eruptives has commenced generally 
with the rhyolites, to which have succeeded andesites of a sec- 
ond eruption, a lesser number of trachytes and the labradorites 
and basalts which form the chief eruptions of the modern vol- 
canoes. 
The andesites of the second epoch always present characters 
by which they can readily be distinguished from those of the 
first. The orthorhombic pyroxene, hypersthene occurs fre- 
quently among the ferro-magnesian elements of the rocks, now 
as a principal and now as an accessory constituent. The micro- 
litic feature so marked in the earlier rocks diminishes little by 
little and the proportion of the amorphous groundmass, always 
devitrified, is increased. 
A grouping of the andesites by regions, subdividing them by 
varieties of structure or the predominating ferro-megnsian 
element, is not practicable on account of the constant change 
which occurs in the nature of the component minerals and 
degree of crystallization. The hornblende-andesites of a micro- 
litic and felsitic magma sometimes containing quartz (dacites) 
appear to have been erupted immediately after the rhyolites in 
the second andesitic period. The dacites are anterior to the 
pyroxene-andesites, likewise of the second epoch, as is shown by 
the frequently observed superposition of the latter and by the 
thick sedimentary deposits which have covered the dacites. 
Alluvium with pebbles of dacite is also found at great depths in 
the interior of several of the valleys. The hornblende-andesites 
