One of the foremost silver-producing countries in the world is Mexico, and its fame extends a long distance into the past. The metal was known to the ancient Aztecs, and was worked by them, with exquisite skill, into numerous ornamental and useful articles, but among the vast mineral treasures of Montezuma, the quantity of silver was small compared with that of gold, and gave little promise of the argentiferous mines of his territories. [Sidenote: THE SPANISH MINERS.] The Spaniards had a keen eye for valuable things, and no sooner did they find what the country contained in mineral wealth, than they proceeded at once to develop it. They opened mines wherever there were indications of silver, and so fast did they progress that it was estimated, at the beginning of this century, that operations were going on in from four thousand to five thousand localities, which might all be included in about three thousand distinct mines. These were scattered along the range of the Cordilleras in eight groups, the principal of which, known as the central group, contains the famous mining districts of Guanajuato, Catoree, Zacatecas, and Sombrerete, and furnished more than half of all the silver produced in Mexico. The great vein, or _Veta Madre_, is referred to elsewhere, and is one of the most remarkable deposits anywhere known. It is contained chiefly in clay slate, and crosses the southern slope of the hills in a northwest and southeast direction, dipping with the slates (the range which it follows) from forty-five to forty eight degrees toward the southwest. The width averages a hundred and fifty feet, and the depth is unknown. It has been traced about twelve miles, but its most productive portion thus far, or rather, the portion operated, is only about a tenth of that distance. The mines of Zacatecas, opened in 1548, are also upon a single vein, called the Veta Grande, averaging in thickness about thirty feet. The formation is of green stone and clay slate, the former the most productive. The veins of Catoree are in limestone, supposed to be of carboniferous age. [Sidenote: THE ORE IN THE MINES.] The greatest proportion of silver in every mining district of Mexico is obtained from the sulphuret of silver, an ore of gray color, disseminated through the quartz matrix in minute particles, and more or less combined with other metals. The other varieties of argentiferous ores are numerous, but comparatively small in quantity; they are the chloride of silver, ruby silver, native silver, argentiferous pyrites, and argentiferous galena.