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 Sierra de Guadalupe; to the east the Sierra de Zinguilacan, which ends in an extensive ridge, channeled by deep furrows, which connect the Sierra mentioned with the Sierra Nevada. By means of mountains and ridges forming the Sierra de Xuchitepec, to the southeast of the valley, the Sierra Nevada is connected with that of Ajusco, which is connected to the southwest with that of Las Cruces, which, extending to the northwest, forms the Cordillera de Monte Alto, which is connected, as already stated, with the western arm of the Sierra de los Pitos. In all these chains there are heights of importance such as; in the Sierra Nevada, Popocatepetl, lovely volcano, and Ixtaccihuatl, merely a snow-cap.... Popocatepetl--smoking mountain--is the highest mountain in Mexican territory and measures 5452 m. above sea-level. The ascent of this colossus is full of discomforts, but when these have been endured, the result is surprising. The most suitable road for the ascent is the one which goes from Amecameca to the ranch of Tlamacas, which is situated at 3897 m. altitude and almost at the limit of tree growth; the trees there met with are stunted; the day temperature is 8°, and at night 0 C., in summer. In winter these temperatures are more extreme. Until one thousand metres beyond the ranch some firs are seen, which are the last; to these follows a soil covered with a dark sand, very fine and slippery, over which the horses can scarcely make their way. Here and there upon this sandy zone are tufts of dry grass. These gradually disappear, until, finally, there remains no sign of vegetation. A little later snow begins, at a place called La Cruz, to which a great wooden cross, reared upon a heap of rocks, gives name. At this point, the line of perpetual snow is found, at 4300 m., little more or less, above sea-level.