620 THE POPULAR SCIENCE MONTHLY. 



the summer, but at other times the air of the plateau is inconveniently 

 dry. 



A large part ot 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 consti- 

 tutes 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, fre- 

 quently 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 Yeta Madre of Guanajuato and the Veta 

 Grande of Zacatecas, which have been worked for about three hun- 

 dred 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 suj^ply accessible 

 to the railroads. Mining is still conducted by working on the old 

 Mexican plan, and this system has been found, under existing circum- 

 stances, 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 black- 

 ish 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 



