the summer, but at other times the air of the plateau is inconveniently dry.
A large part of the country is overlaid by the igneous rocks, of which trachyte, feldspar, porphyry, and amygdaloid basalt, are of most frequent occurrence. In the Sierra Madre the metamorphic rocks are common. Limestone is extensively quarried at Orizaba, and constitutes the greater part of the eastern branch of the Cordillera between San Luis Potosi and Monterey. The Cordillera, from Chihuahua on the north to Oaxaca on the south, contains very extensive deposits of gold, silver, iron, copper, and lead; and zinc, mercury, tin, platinum, and coal occur in a few places. The argentiferous veins constitute the principal part of the mineral wealth of the country. The silver occurs generally in the form of sulphides, in gangues of quartz, frequently in the metamorphic clay-slate, but sometimes in porphyry, as at Real del Monte, or in talcose slate, as in some mines at Guanajuato. Among the most remarkable mineral veins of the continent, after the Comstock lode, are the Veta Madre of Guanajuato and the Veta Grande of Zacatecas, which have been worked for about three hundred years.
The next most important deposits are the immense beds of iron, chiefly in the form of the magnetite and hematite ores. The well-known Cerro del Mercado, in the State of Durango, has been estimated to contain sixty million cubic yards of iron-ore, which have a weight of five billion quintals, and give, according to an analysis by Mr. M. H. Borje, of Philadelphia, sixty-six per cent of pure metal. Lead-ores are abundant; copper is mined at various places; oxide of tin is found in veins and alluvial beds at Durango. Mercury occurs as cinnabar in several States; and zinc-ores, with platinum, antimony, cobalt, and nickel, in not large quantities, are found in Chihuahua. The principal coal-beds are in the States of Oaxaca, Vera Cruz, Mexico, Puebla, Nuevo Leon, Tamaulipas, and Sonora. The anthracite-bed recently discovered at Barranca, on the Yaqui River in Sonora, is probably the largest and richest deposit of coal in the republic. Lignite, or brown coal, occurs in many places, but is not used to any great extent. The demand for coal is, so far, much greater than the supply accessible to the railroads. Mining is still conducted by working on the old Mexican plan, and this system has been found, under existing circumstances, to be more economical and profitable than a system in which modern and improved methods are applied.
Some of the oldest mines in Mexico, many of which were worked before the Spanish conquest, are at Pachuca, in the State of Hidalgo. There are about one hundred and fifty of them, seventy-five of which are in the Real del Monte, affording an ore composed mainly of blackish silver sulphides. The ore is worked here, as at Guanajuato, by the patio process, which is illustrated in the accompanying view. It is first crushed by a revolving stone wheel, iron-tired, in a pit, at the