It may naturally be supposed that mines, as well as other things, are subject to variation in their productiveness. The mines which, till very lately, yielded most silver, were those of _Oroura_, a small town about eight leagues from Arica. In the year 1712, one was discovered at _Ollachea_ near Cusco, so rich that it yielded 2500 marks of silver of eight ounces each, or 20,000 ounces, out of each _caxon_ or chest, being almost a fifth part of the ore; but it has since declined much, and is now [1720] only reckoned among the ordinary sort. Those of Lipes have had a similar fate. Those at Potosi now yield but little, and are worked at a very heavy expence, owing to their excessive depth. Although the mines here are far diminished in their productiveness, yet the quantity of ore which has been formerly wrought, and has lain many years on the surface, is now thought capable of yielding a second crop; and when I was at Lima, they were actually turning it up, and milling it over again with great success. This is a proof that these minerals generate in the earth like all other inanimate things;[3] and it likewise appears, from all the accounts of the Spaniards, that gold, silver, and other metals are continually growing and forming in the earth. This opinion is verified by experience in the mountain of Potosi, where several mines had fallen in, burying the workmen and their tools; and these being again opened up after some years, many boxes and pieces of wood were discovered, having veins of silver actually running through them.[4]
[Footnote 3: It is merely a proof that the ore had been formerly very imperfectly managed, and still contained enough of silver to pay for extraction with profit, by more expert methods.--E.]
[Footnote 4: This proves only change of place, by solution, infiltration, and deposition not growth, increase, or new production.--E.]
All these mines become the property of their first discoverer, who immediately presents a petition to the magistrates, desiring to have such a piece of ground for his own. This is accordingly granted, and a spot of ground eighty Spanish yards in length by forty in breadth[5] is measured out and appropriated to the discoverer, who chuses what spot he pleases within these bounds, and does with it as he thinks fit. The exact same quantity is then measured off as belonging to the king, and is sold to the best bidder, there being always many who are willing to purchase, what may turn out an inestimable treasure. After this, if any person may incline to work a part of this mine on his own account, he bargains with the proprietor for a particular vein. All that is dug out by any one is his own, subject however to payment of the royal duties; being one-twentieth part for gold, and a fifth for silver; and some proprietors find a good account in letting out their grounds and mills to others.