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64 mill of this kind is about 2 tons per day of twenty-four hours, but, as it wears rapidly, its capacity is proportionately lessened, until at the end of twelve or eighteen months a new "voladora" is required.

The third is the "arrastra," and consists simply in substituting a large bowlder for the stone wheel of the "trapiche," which, being lashed to the end of the axle, is dragged over the metals, thus milling them by friction.

Thousands of these "arrastras," "volantes," and "valadoras" are scattered about the abandoned works of Araca, the silent witnesses to the vast mining operations of the early Spaniards at that point.

Plata piña.—The ore having been thus milled, is next subjected to the ordinary process of amalgamation and washing, and the metal is then taken up by quicksilver and placed in cast-iron patterns and pressed, thereby giving it form and expelling a large percentage of the quicksilver, when it is removed from the press and placed in a retort, where the remaining quicksilver is expelled, leaving as the finished product a porous mass of pure silver in the shape of a pineapple; hence its Spanish name "plata piña," or silver pineapple.

This is the class of silver used for coining and in the industrial arts, while large quantities are clandestinely taken out of the country, thus escaping the payment of the export duty of 80 cents per marc.

This process was first introduced by the early Spaniards, and was a great advance over the smelting process they found in use among the ancient Indian miners, which consisted simply of small cone-shaped furnaces, called in the Aymara language "guayra," meaning blasts, built upon the tops of the-hills and mountains, where the strong winds rushing through the air funnels in the sides of the furnace fanned the charcoal fires within until the metal was separated from extraneous substances.