{109} The _moliendas_, or crushing-mills (_arrastres_, as called at some mines), employed for the purpose of grinding the [Pg183] ores, are somewhat singular machines. A circular (or rather annular) cistern of some twenty or thirty feet in diameter is dug in the earth, and the sides as well as the bottom are lined with hewn stone of the hardest quality. Transversely through an upright post which turns upon its axis in the centre of the plan, passes a shaft of wood, at each end of which are attached by cords one or two grinding-stones with smooth flat surfaces, which are dragged (by mules fastened to the extremities of the shaft) slowly around upon the bottom of the cistern, into which the ore is thrown after being pounded into small pieces. It is here ground, with the addition of water, into an impalpable mortar, by the constant friction of the dragging stones against the sides and bottom of the cistern. A suitable quantity of quicksilver is perfectly mixed with the mortar; to which are added some muriates, sulphates, and other chemical substances, to facilitate the amalgamation. The compound is then piled up in small heaps, and not disturbed again until this process is supposed to be complete, when it is transferred to the washing-machine. Those I have observed are very simple, consisting of a kind of stone tub, into which a stream of water is made to flow constantly, so as to carry off all the lighter matter, which is kept stirred up by an upright studded with pegs, that revolves in the centre, while the amalgamated metals sink {110} to the bottom. Most of the quicksilver is then pressed out, and the silver submitted to a burning process, by which the remaining portion of mercury is expelled. The silver which is taken from the furnace, generally contains an intermixture of gold, averaging from ten to thirty per cent.; but what is extracted by amalgamation is mostly separated in the washing. While in a liquid state, the gold, from its greater specific gravity, mostly settles to the bottom: yet it usually retains a considerable alloy of silver. The [Pg184] compound is distinguished by the name of _oroche_. The main portion of the silver generally retains too little gold to make it worth separating.