Humboldt went to inspect the pyramids of the sun and moon at Teotihuacan, and afterwards gave a prolonged study to mines, visiting first Moran and Real del Monte, northeast of the capital, and afterwards Guanajuato. Long before the arrival of the Spaniards, the natives of Mexico were acquainted with the working of subterranean veins to find metal. Cortés says that gold, silver, copper, lead, and tin were all sold in the markets of Tenochtitlan. They either collected grains of native gold in small baskets of slender rushes, or melted the metal into bars, like those now used in trade, represented in Mexican paintings. Humboldt found the methods of mining not advanced from the sixteenth century, without any of the improvements known in his time. The hard work was performed by Indians, the beasts of burden of the mines. They carried out the metal in bags on their backs, going up and down thousands of steps, in long files of fifty or sixty, men of seventy years old, and children of ten or twelve. The mine of Valenciana, in Humboldt's time the most celebrated of Guanajuato, and the richest then known in Mexico, was not much wrought until the end of the eighteenth century, although it had been somewhat worked by the early Indians and the first Spanish settlers. In 1760, a poor man named Obregon, a Spaniard, began to explore a new vein. As he was a worthy man, he found friends willing to advance small sums from time to time to carry on his work. For several years the cost was much greater than the produce, but the pit grew rich as it became deep, and at last yielded quantities of sulphuretted silver. When Obregon, or, as he came to be called, the Count of Valenciana, began to work the vein, goats were browsing over the hill-tops all about the ravine of San Xavier. Ten years after, on the same spot, the climbing streets of Guanajuato sheltered a large population; and at present it is a flourishing city, surrounded by a region all rich in minerals. The produce from the mine at Valenciana has fallen behind that of other later veins, and scarcely covers the outlay.