The principal ore of silver is the sulphate, although native silver is also freely encountered in some districts. The ores were very generally decomposed to a depth of about 300 feet. Argentiferous galena is plentiful, and silver is freely found in conjunction with copper ores. The _caliches_, a chalk-like substance, easily worked, is another rich form of occurrence of the metal, and there are others less important. Various different methods of separating silver from its ores are used; the prevailing ones being those of smelting, lixiviation, and the _patio_ process, which last has accounted for 90 per cent. of the production. Indeed, the recovery of silver by the _patio_ process has always been one of the most important industries of Spanish-American countries, especially in Mexico, Peru, and Chile. In Mexico it has been employed continuously since the year 1557, when it was invented by Medina at the _hacienda_ Purisima Grande. This was the first application of amalgamation to silver ores, and permitted the treatment of the vast quantities of low-grade ores, which did not pay to smelt. To-day great quantities of ore are still treated by this method. The process is too well known to require much description here. Its main points of advantage are the simplicity--in practice, for its chemistry is complicated in theory--of its methods and appliances. The principal agents employed may be said to be mercury and horseflesh, or rather mule-flesh; the mercury forming an amalgam with the precious metals under the incorporation brought about by the trampling hoofs of the mules. The trampling and incorporation of the _torta_, or charge of pounded ore, mercury, water, salt, copper sulphate, and other constituents, mixed into a paste, was originally performed by barefooted natives, but the practice of using mules for the purpose came from Peru, in 1783, as before mentioned. The _patio_, as its name implies, consists of a paved yard upon which the crushed mineral is treated. This is in some cases of very large capacity, one of the most important in the country, that of the Guadalupe works at Pachuca, which treats nearly a thousand tons of ore a week, being as large as the _plaza_ of a city. Upon this the _torta_ is spread, and bands of a dozen mules, or mules and horses, harnessed together, are driven up and down from morning till afternoon, through the slushy mass. The animals are then bathed to remove the chemicals, but notwithstanding this the work is deleterious, and they last but a few years--the old ones but a few months--as they become poisoned by the copper sulphate. At some of the _haciendas_ of Pachuca six hundred horses are employed in this work, and the total throughout the country is considerable. Constant efforts have been made for the use of mechanical appliances, to take the place of the equine mixer, but these have not been found to give the same efficiency. The process is typical of the country and the