The veins of mineray, of whatever sort they may be, are generally richer in the middle than towards the edges; and where two veins happen to cross each other, the place where they meet is always very rich.  It is also observed that those which lie north and south are richer than those which lie in any other direction.  Those also which are near to places where mills can be erected, and can consequently be more commodiously wrought, are often preferable to others that are richer, but require more expense in working.  For this reason, at Lipes and Potosi, a chest of ore must yield ten marks or eighty ounces of silver, to pay the charges of working; while those in the province of Tarama only require five merks or forty ounces to defray the expences.  When even very rich, and they happen to sink down so as to be liable to be flooded, the adventurers must have recourse to pumps and machines in order to drain them; or to cocabones or levels dug through the sides of the mountain, which often ruin the owners by the enormous expence they are insensibly drawn into.  At some of the mines, where the methods of separation already described fail, they use other means of extracting the silver from the ore, and from other metals which may be combined with it; as by fire, or strong separating waters; and there the silver is cast into a sort of ingots, called bollos.  But the most general and useful method is that already described.

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]