The sun which comes down through the cloudless medium of an atmosphere unscreened by the usual curtain of vapor, parches and crisps the thirsty soil, whilst the winds that sweep uninterruptedly over the unbroken expanse, fill the air, during the dry season, with sand and dust. These high barren plains occupy a large portion of the centre of the country between Zacatecas, Durango and Saltillo; and such is in fact the character of large portions of the whole of Mexico, except when the comparatively level nature of the soil permits the small rivulets that filter from the Cordillera through the narrow vallies, to form themselves into rivers which may be used for irrigation. Wherever this is the case nature at once recovers her vigor under the influence of heat and moisture.
These physical features, and consequent diversities of temperature, have caused the division of Mexico, as it rises from the two Oceans, into three regions, or superficial strata, which are called, the tierras calientes, or hot lands; the tierras templadas, or temperate lands; and the tierras frias or cold lands. The tierra caliente covers chiefly that portion of the territory which lies on the borders of the Atlantic and Pacific; yet it is not confined exclusively to the coast, inasmuch as all those parts of Mexico in which there is heat and moisture enough to produce the fruits and maladies of the tropics, are classed under this head. The tierra fria comprises the mountainous districts rising above the level of the capital up to the limit of constant snow; while the tierra templada embraces those milder middle regions not comprehended in the two other sections. Classing them by elevation in feet, we may suppose that the tierras calientes extend to between 3,000 and 4,000 feet above the level of the sea; the tierras templadas to between 4 and 8,000 feet; and that the tierras frias embrace all the remaining portions up to the region of eternal ice.