478 OLIVER C. FARRINGTON 
basalts occur cutting their way through Mesozoic sediments and 
covering the Tertiary eruptives. In the plain extending to the 
south of the Cordillera of Mazapil and in the portion between 
the mountains of Grufiidora and Ahorcados, Tertiary basalts 
come up through the schistose Cretaceous limestones and along 
the upper surface of the latter marked metamorphism has been 
produced by the contact. 
After the work of eroding waters had filled the valley of 
Mexico and made it habitable by man, a mighty cataclysm 
devastated the southern part of the valley. A flow of basalt 
ten miles in length, accompanied by showers of ashes, came 
from the volcano of Xitli and buried much of the inhabited 
region. Hence in the layers of pumiceous tuffs upon which 
these lavas rest, numerous utensils of primitive industry, human 
bones and bones of other modern vertebrates are found. The 
volcano of Toluca had ceased activity at this time, but Popocat- 
apetl continued to pour forth eruptions of hypersthene-andesites. 
The eruptions of the latter volcano ceased at the beginning of 
this century, and now only a solfataric activity exists. The 
same is true of the Pico de Orizaba. 
About the middle of the last century a new volcano appeared 
in the Mal Pais in the state of Michoacan. Its products were 
black basalts highly charged with olivine. With this last vol- 
canic phenomenon was closed the prolonged inundation of 
basaltic and andesitic lavas which began to make itself felt at 
the end of the Tertiary period. 
OLIVER C. FARRINGTON. 
