Ch. 1: Gold and Silver in 1912

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256                                     MINERAL RESOURCES, 1912.
placer material in rockers. Such ground usually embraces only a small acreage, and has been the most productive of any in Arizona.
The total production of gold from dry placers has been variously estimated. The largest amounts are credited to the Yuma County
E lacers and it seems probable that further production of gold can e expected from some of these placers, but whether at a profit to operators and investors or not will depend on thorough under­standing of the conditions and on technical and financial management, not only honest but efficient and economical as well. The early production from many localities is known only by hearsay and the estimate of it rather increases as years go by. It is estimated that between 1860 and 1880 the Yuma County placers produced from $20,000,000 to $42,000,000 in gold. Verification of these figures is lacking and they are probably excessive. Since that time about $224,000 has been produced.
Machinery and equipment.—Various forms of dry-placer mining apparatusx are used, of which the first is a batea or wooden pan, which is the most elementary form of separating the values.
In using this the air blast is supplied by the lungs and the implement is manipu­lated •with rotary and other motions, which gather the heavy material toward the center, while distributing the lighter material toward the margin, whence it is blown away. The air blast is varied in force according to the material to be removed, and, toward the last, the residue of valueless material is separated from the gold by clever manipulation and blown out of the batea. * * * More commonly used is the Mexican dry washer, said to have been introduced by Hungarians about 1850, and extensively used in northern Mexico. This consists of a tray about 1J by 3 feet, with muslin bottom and five cross-riffles resting, at an inclination of about 15°, on a frame, beneath which is a canvas bellows. The latter forces through the muslin an intermittent blast of air, which drives away most of the fine light material. The gravel is fed through a hopper upon the upper end of the tray and is moved slowly down the slope by the jar of the bellows' stroke. As the operation proceeds the gold lodges in the riffles, chiefly behind the two uppermost, and the material of lower specific gravity flows on over the riffles and gradually passes out of the machine at the lower end. * * * The chief advance in the development of the Mexican machine * * * is an air jig, made by Steele, Sutton & Steele, in El Paso. In this the stationary tray is replaced by a revolving table or belt about 3 feet wide with riffles about 8 inches apart, and screened with muslin. This is inclined about 30°, the slope being adjustable. The operation of this machine is identical with that of the Mexican "dry washer," except that the operation is continuous, and as the fine is driven away by the intermittent bellows' blast, which passes through the muslin, the riffled belt raises the concentrate to the top of the machine and, passing over, dumps it into a box at the rear. * * * The CurtiV.s machine, made in Pon-
1 Merrill, F. J. H., Dry concentration of placer gold: Min. and Sci. Press, July 13,1912.
Ch. 1: Gold and Silver in 1912 Page of 93 Ch. 1: Gold and Silver in 1912
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US Geol. Surv. 1912. Gemstones, Metals.
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