THE ERUPTIVE ROCKS OF MEXICO 477 
the Sierra Madre. Among these points may be mentioned El 
Parral, in the state of Chihuahua; E] Nayarit, in the territory of 
Tepic; the eastern part of the state of Jalisco; and near the vol- 
cano of Jorullo in the state of Michoacan. The early age of 
these labradorites is indicated by the fact that olivine is rare 
or absent. In the modern labradorites it occurs with more 
frequency. 
These labradorites frequently exhibit a columnar structure, 
the most striking example of which is seen in the famous col- 
umns, 130 feet in length, along the Barranca of Regla in the 
state of Hidalgo. These stand upon Tertiary tuffs, resting in 
their turn upon arenaceous Upper Cretaceous limestones. The 
labradorites are compact, of gray color, and contain scattered 
grains of olivine. Geodes of zeolites occur in the columns, 
chabazite being most common, while thomsonite and arragonite 
are also found. The later series of labradorites and basalts con- 
tinued up to the latest volcanic eruptions. Examples are found 
in the mountains about the valley of Mexico, as in the Pefion de 
los Bafios, where pyroxene-labradorites in thin layers exhibit 
curious undulations. In some lava flows near the volcano of 
Ajusco at the southern part of the valley occur labradorites 
formed simply by a marked diminution of the olivine in the 
basalts which predominate in the region. 
The vitreous hypersthene-andesites which formed the larger 
part of the early volcanic outflows were succeeded by basalts 
erupted through new volcanic foci. These exhibit an abun- 
dance of olivine in a microlitic groundmass of labradorite and 
pyroxene. The outflow of basalts in turn ceased, however, and 
the field was left anew to the hypersthene rocks which charac- 
terize the lavas of the volcanoes now in activity. This alterna- 
tion is illustrated by contrasting the olivine-basalts of Jorullo of 
the middle of the last century with the present outflows of 
Ceboruco and Colima which present good types of trachytes 
and pyroxene-andesites with vitreous groundmasses. 
In the northern region of the country, over the Mesa Cen- 
tral and along various points of the eastern Cordillera Madre, 
