constitutes the tierra fria (cold land). It is the most populous part of the Republic.
In the high valleys, as those of Toluca and Mexico, the descent of the mercurial column often shows considerable falls of temperature; in winter the column reaches 8° or 10° below 0 C. and frosts are frequent. In general, however, the winters are mild. The mean temperature is from 13° to 14° C.
In many places exceptional conditions have brought the vegetable areas into abrupt juxtaposition; thus, while upon the summit of some ridge, only plants of European character may live and flourish, in the plains surrounding it are seen palms and bananas. From the summit of the great volcanoes, the three superposed zones may be clearly seen, at once.
The rapid communication, which today happily exists, presents to the traveler the marvelous opportunity of passing, in a few hours, through the three distinct regions of which we speak, which in other parts of the globe are separated by thousands of kilometres.
In some places these zones remain clearly distinguished from one another, but this is exceptional, since commonly they crowd upon each other, mingling one with another by imperceptible transitions. It is common to mention some certain place as belonging to one and the other zone, because the line of separation for both runs irregularly in mountainous regions. A zone of reciprocal penetration has been formed, on account of the multiple phenomena of temperature, of winds and of plant groupings. So, too, cañons and slopes are met with, which, by their vegetation, may be considered foci of tierra caliente, included within the fully developed tierra templada.
POPOCATEPETL.
The valley of Mexico lies, then, surrounded by various chains, which are: to the north the Sierra de Pitos and its branches, of which one is the