As regards the comparative merits of chlorination and cyanization, it may be said the one is the equal of the other. Under certain conditions, chlorine gives a higher percentage of gold; under others the same may be said of cyanide. A description of either process would be out of place, however, in a simple elementary work. Handed down through the centuries, the primitive arrastra is still useful in certain contingencies. It is like a cider mill in its principle, and was probably suggested by recollections of that machine, or else of the Spanish wine-press. A circular, shallow pit, a dozen feet or more in diameter, is first paved with hard, uncut stones of granite, basalt, or other hard rock. This pavement is a foot thick, and beneath it is a bed of puddled clay 6 inches deep. A vertical shaft with an arm, or arms, revolves in the center of the arrastra. Grinding blocks weighing 400, or perhaps even 1,000 pounds, are fastened to the arms by chains or rawhide strips. The forward part of each stone is raised a couple of inches off the floor. Mule, horse, water or steam power may be used, the speed ranging from 4 to 18 turns a minute. Nothing can be simpler, less expensive, or save a greater proportion of the value in the ore than the arrastra. Its limited capacity is its worst fault. An arrastra 10 feet in diameter will treat 500 or 600 pounds of ore at a charge, and handle one ton a day of 24 hours. Ores that were so poor they yielded nothing to the stamp mill have paid well with the arrastra. This humble device may be used to advantage, probably in some of the poorer gold-bearing cemented gravels of the Northwest. The ore should be crushed to pigeon-egg size. Small quantities of mercury, about a tablespoonful to every five tons of gravel, has been found a satisfactory proportion in California.