MEXICO greater part of Mexico lies between the United States frontier and the Isthmus of Tehuantepec, and consists of a plateau rising from an elevation of 4,000 feet in the north to about 8,000 feet in the south. The plateau is bordered on the east by the Sierra Madre Oriental, between which and the Gulf of Mexico there stretches a coastal plain from 10 to 100 miles in width, and on the west by the Sierra Madre Occidental. Between this latter range and the Pacific there is another coastal plain, less fully developed than that on the east.
The geological structure of the country is as yet but imperfectly known. In the eastern mountains, the principal formation is limestone, while, in the western, Archaean rocks, covered in many places by recent volcanic material, predominate. On the plateau itself, the limestone comes to the surface over wide areas, while the remainder of the region consists of debris, either volcanic in origin or derived from the weathering of the surrounding mountains. The coastal plains are also of recent formation. Metamorphic and volcanic action, to both of which the country has been subject in an extraordinary degree, account to a great extent for the richness of its mineral wealth.
As a result of its varied configuration the climate of Mexico presents some striking contrasts. In the low-lying regions tropical conditions prevail, but on the uplands temperature is reduced, and over the greater part of the country the climate ranges from sub tropical to temperate. Three climatic zones are generally recog nised. The terra caliente, which includes all the land from sea level to an altitude of about 3,000 feet, has a mean temperature of about 75° to 80° F., with a small annual range. Between 3,000 feet and 5,000 or 6,000 feet above sea-level lies the terra templada, where the mean temperature is between 62° and 70° F. The annual range here is also small and the region is said to enjoy a perpetual spring. Above 6,000 feet is the Serra Fria, where the mean annual temperature, except in the mountains, varies from 58° to 62° F., and where the range between day and night is usually greater than that between summer and winter. The rainfall, which takes place
between June and October, is very unevenly distributed. On the coastal plains and the seaward slopes of the mountains, south of the twenty-second parallel, it is generally between 40 and 80 inches, except around the Gulf of Campeche, where it exceeds the latter amount. The remainder of the coastal region and the southern part of the plateau have, as a rule, from 20 to 40 inches, while in the north there is never more than 20 inches, and in some places there is less than 10.
Climatic conditions afford the best basis for a division of the country into natural regions. The tierra caliente is suitable for the cultivation of tropical plants of all kinds, although in places where the rainfall is deficient recourse must be had to irrigation. Sugar-cane is extensively grown, and forms a valuable crop, but the methods pursued, both in regard to cultivation and manufacture, are defective. The production is now sufficient to meet the home demand, and in some years to allow of a considerable export. A rubber-producing plant (Castilloa elastica) is found growing wild in the forests and is also cultivated in plantations, the most favour able conditions for which are found south of latitude 20° N., at an elevation of not more than 1,000 feet, and in districts where the rainfall is at least 100 inches. The success of these plantations does not yet appear to be assured. Recent chemical discoveries also seem to have proved the feasibility of extracting rubber from the guayule shrub, which grows extensively on the northern plains. Among other plants of the tierra caliente are vanilla, which thrives best in damp districts, tobacco, for which the sandy coastal plains with their abundance of decaying organic matter are most suitable, cacao, and various kinds of fruit. Mexican tobacco is becoming an important article of export as it is rapidly growing in favour in the United States. Coffee is also cultivated in this region, but it finds its most favourable environment at a greater elevation.