Page:Vol 3 History of Mexico by H H Bancroft.djvu/606

586 near the surface, but generally this is not the case, a circumstance which in colonial times, with the prevailing backwardness of drainage and other operations, impeded the search for deeper-lying zones in the veins.

There are essentially two forms under which argentiferous ores occur. Near the surface, where exposed to external influence, the metallic substances are generally in the form of oxides, or combined with iron, chlorine, or bromine, and receive from their reddish color the name colorados. Those at greater depth have usually retained the condition of all primitive sulphuric bases, and are found in connection with pyrites, galena, or blende. The latter two predominate, and a dark color results which has given rise to their designation of black ores, or negros. They give the greatest part of all the silver produced in the country. The average richness of the ores has been frequently overrated, and the occurrence of enormous blocks of native silver considered as frequent, while in reality they are very rare, and never larger than those found in European mines. The average yield is from three to four ounces of silver to the quintal of ore, and the enormous returns of New Spain are due rather to the great abundance of the ore. Gold is obtained chiefly from placers in Sonora and the northern regions. In Oajaca it also occurs in rocks, but the exploitation has not proved very profitable. Elsewhere it is rarely found except in connection with argentiferous ores, in some instances in the proportion of about two ounces to the quintal.

Although mining was chiefly confined to silver, gold, and cinnabar ores, the country by no means lacked other metals. Iron existed at Colima, in