I have already spoken of the large quantity of gold and silver wrought into various articles of elegance and utility for the Incas; though the amount was inconsiderable, in comparison with what could have been afforded by the mineral riches of the land, and with what has since been obtained by the more sagacious and unscrupulous cupidity of the white man. Gold was gathered by the Incas from the deposits of the streams. They extracted the ore also in considerable quantities from the valley of Curimayo, northeast of Caxamarca, as well as from other places; and the silver mines of Porco, in particular, yielded them considerable returns. Yet they did not attempt to penetrate into the bowels of the earth 'by sinking a shaft, but simply excavated a cavern in the steep sides of the mountain, or, at most, opened a horizonal vein of moderate depth. They were equally deficient in the knowledge of the best means of detaching the precious metal from the dross with which it was united, and had no idea of the virtues of quicksilver,--a mineral not rare in Peru, as an amalgam to effect this decomposition.22 Their method of smelting the ore was by means of furnaces built in elevated and exposed situations, where they might be fanned by the strong breezes of the mountains. The subjects of the Incas, in short, with all their patient perseverance, did little more than penetrate below the crust, the outer rind, as it were, formed over those golden caverns which lie hidden in the dark depths of the Andes. Yet what they gleaned from the surface was more than adequate for all their demands. For they were not a commercial people, and had no knowledge of money.23 In this they differed from the ancient Mexicans, who had an established currency of a determinate value. In one respect, however, they were superior to their American rivals, since they made use of weights to determine the quantity of their commodities, a thing wholly unknown to the Aztecs. This fact is ascertained by the discovery of silver balances, adjusted with perfect accuracy, in some of the tombs of the Incas.24