Mexico

Volcanic activity must have begun some time during the Cre taceous or at the beginning of the Tertiary era. Many geologists think that the high plateau is a folded region on which enormous quantities of eruptive rocks accumulated, in the first instance andesitic lava; later, after valleys had been cut through it, new eruptions have originated the formation of thick layers of rhyolites and dacites; and, still further, one more period of volcanic ac tivity resulted in the formation of basaltic tables found above all the other rocks.

Climate.—Two causes contribute to make the climate of Mexico among the most varied on earth: one, the great longi tudinal extension of the country, which stretches through 17 parallels of latitude, with the tropic of Cancer crossing it about midway, and the other, the fact that the larger part of the coun try consists of a very high plateau. In order to give an accurate account of the climate of Mexico, we have to resort to the old division into three (even four) vertical zones, tierra caliente, tierra templada and tierra fria, which, although applicable to other countries, such as Guatemala and Colombia, finds its best possible application in Mexico.

The

tierra caliente rises from sea-level to an altitude of about 3,28o ft. and includes the coastal zone, 3o to 4o m. wide, the Yucatan peninsula, all of Tabasco, part of Chiapas, the isthmus of Tehuantepec and a relatively small part of Oaxaca, its mean temperature ranging from 77 to 82° F, rarely falling to less than 60°, but reaching 105°, iio° and even higher on many points of the Pacific coast, especially Guaymas (max. 9° F), Mazatlan, Manzanillo and Acapulco though on the Atlantic coast, even in Yucatan, regular breezes make the heat somewhat less oppressive. The maxima, observed anywhere in Mexico, are not higher, how ever, than those recorded in some parts of Texas and Arizona.

The tierra templada, or subtropical zone, rises to an elevation of from 5,500 to 6,000 ft. and comprises "the greatest portion of Coahuila, Nuevo LeOn, San Luis Potosi, nearly half of Tamau lipas, a small part of Veracruz, nearly the whole of Chiapas and Oaxaca, and a large portion of Guerrero, Jalisco, Sinaloa and Sonora." Its mean annual temperature is 75° F and it has justly been called the best of the three zones, even to Europeans, on account of its lacking the anaemiating influences that are found at higher altitudes. Orizaba and Jalapa, in the State of Vera cruz, are typical stations.

The so-called tierra fria, which ranges from about 5,500 to 9,00o ft., is made of all the higher parts of the plateau, and its average temperature is about 63° F. At such altitudes there is never any real heat, nor are winters cold, although a slight frost during the night is a frequent occurrence. Once in ten years a

few snow flakes may be seen falling on the City of Mexico ; but during the winter there is a great difference in temperature in the same house between the rooms exposed to the sun and the others less favourably situated, and foreigners find it impossible to stay in the latter without artificial heat. Its climate may be compared to that of Tuscany, but the maxima of heat and cold that can be observed in Florence are never experienced on the Mexican high plateau. In Mexico City the coldest months are December and January while the warmest are April and May.

Some climatologists consider a fourth zone, the tierra helada, or frozen land. It is found above the third one and is made of all the mountains and peaks above the high plateau. The term is exaggerated when applied to its lower parts where forests are found and where cereals are cultivated, but it is evident that at a very high altitude polar conditions are encountered. Such zones are, of course, uninhabited except in rare cases, the ranch of Tlamecas on the Popocatepetl, the altitude of which is 12,500 ft. and which is inhabited the whole year round, being one of them. The succession of climates represented by these different zones can be experienced in a few hours by going on the train from Vera cruz to Mexico City. Where the plateau is cut by deep valleys, one can admire the marvelous spectacle offered by the three zones in succession with their wheat-fields, maize, coffee trees, light green patches of sugar-cane and finally the banana tree in its finest development.

Rainfall.

As has been mentioned in another part of this article, the annual rainfall is very heavy in the tierra caliente, south of the tropic of Cancer, while near the border on the west side, there is almost none. In certain parts of southern Campeche, in Tabasco and in Veracruz the annual rainfall reaches 8o, 90 and even ioo in. (Huatusco district). In most of Mexico it is the absence or the presence of the rain that marks the changing seasons. As is the case with many countries of South America, Mexico has only two such seasons, the dry and the rainy. These seasons come at slightly different times according to latitude and altitude. In Mexico City the former which, despite the fact that it is called summer, is characterized by colder weather, dur ing a few months, begins in October and ends in June. The latter corresponds to the rest of the year. Each season shows some short changes for a few days, on regular dates, in connection with astronomical phenomena. The annual rainfall for the whole re public is 59 in. ; it is less than 25 in. in the Valley of Mexico.