_ at Potosi, being threads of
pure silver entangled, or wound up together, like burnt lace, and so
fine that it is called _arana_, or spider ore, from its resemblance to
a cobweb.

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