Page:The American Cyclopædia (1879) Volume VIII.djvu/91

Rh trough, about 12 ft. long, 20 in. wide at its upper end, and 30 in. at the other. It terminates below with an inclined riddle of punched sheet iron, through which the material is carried by a stream of water entering at the other end, and falls upon a riffle box below. A fresh supply of dirt is continually shovelled in at the head of the trough. This arrangement works faster than the rocker, and is not so liable to become packed with sand; but the sluice, which has now generally superseded it, is capable of washing still greater quantities and with less loss of gold. This is generally a long inclined wooden trough, into which the dirt is shovelled, and through which a rapid stream of water continually flows. The ordinary sluice is a series of rough wooden boxes, each 12 ft. long, 16 by 20 in. wide, and 10 in. to a foot deep. The grade is commonly 10 to 18 in. on each box. False bottoms are employed to retain the gold and prevent the wearing out of the boxes. Sluices are sometimes paved with stones or wooden blocks, in the crevices of which the gold is caught and retained. Riffles are also inserted, and quicksilver is very generally employed to assist in catching the gold. The dirt or gravel containing gold is shovelled into the sluices at the head of the series. Mercury is usually poured, an hour or two after the commencement of sluicing, into the head of the apparatus, and smaller quantities are also introduced at various places along the boxes. When the gold is exceedingly fine, amalgamated copper plates are sometimes set in the sluices, and are considered as effective for saving fine gold as an equal surface of pure mercury, while they are both cheaper and more easily managed. Another arrangement for obtaining fine gold consists in allowing a current carrying suspended gold, sand, &c., to pass over tanned hides, laid with the hairs directed against the course of the stream, or over rough baize or blanket, such as is now manufactured for the purpose in California. The blankets are frequently removed and washed in tanks. Where skins are used, as in Brazil, they may be dried and beaten over a cloth, placed to receive the fallen particles. Sluice washing is generally carried on during the day only; but when water is abundant and cheap, the work may be continued throughout the whole twenty-four hours. The sluices are cleaned up once a week, or more seldom, according to the rate at which gold and amalgam accumulate. The amalgam and mercury taken from the sluice are panned, to separate them from sand, &c., and then strained through buckskin or canvas to remove the liquid quicksilver. The auriferous amalgam is removed from copper plates by first warming and then scraping them. This, together with the solid amalgam from the strainers, is retorted; the quicksilver passing over from the retort is condensed in water and thus recovered; while the gold is left in the form of a light yellow porous mass, called retort gold, and usually constituting 35 to 40 per cent. of the weight of amalgam retorted. The length of the sluices employed in this process is limited only by the cost of their construction and maintenance, and the control of the necessary grade. Ground sluices are natural gullies, answering the purpose of wooden sluices in localities where water is abundant for short periods only, and the construction of permanent sluices would not be judicious. In river mining, the current of a stream is turned aside, and sluices are erected in its bed for washing the dirt there accumulated. In beach mining, as carried on along the northern part of the California coast and the southern part of the Oregon coast, the sands on the seashore are explored, and certain portions of them, which are found to be sufficiently auriferous, are transported to some neighboring stream and washed. The origin of this gold is the natural concentration by tides and currents of a bluff of auriferous sand, which in stormy weather is undermined by the waves. The position of the deposits is frequently changed, and mining must therefore be carried on in a new place every day.—Hill diggings and bank diggings are names which explain themselves. Many deposits of auriferous clay and gravel have been subsequently overlaid by barren alluvium; and the ordinary operations of shovelling or blasting would be too expensive for the removal of such enormous masses of unprofitable material. Tunnels and drifts are frequently employed for the purpose of extracting the richer strata. They are particularly necessary in those deep placers in which the drift materials are united by silicious or calcareous matter, constituting a hard, solid cement. This material is usually mined by drifting, and, if too hard for sluicing, is subjected to a treatment similar to that employed for quartz gold. Water for sluicing operations is frequently brought from great distances through canals, ditches, or flumes, the proprietors of which sell the water to miners at so much the miners' inch, a miners' inch being in most localities the quantity flowing in a given time through an aperture one inch square under a head of six inches.—The celebrated hydraulic process, invented in Placer co., Cal., in 1852, consists in washing down the whole surface and underlying mass of auriferous deposits, preparatory to sluicing. This is effected by streams of water under great hydraulic pressure. The first apparatus of the kind had a head of 40 ft. From a barrel situated this distance above the mining claim the water was drawn through a hose 6 in. in diameter, made of common cowhide and ending in a four-foot tin tube, the nozzle of which was one inch in diameter. From this simple beginning has grown in 20 years one of the most remarkable mechanical industries of mining. Hundreds of miles of ditches, canals, and flumes are now employed in conducting water for these operations from the high streams of the Sierra; canvas and iron hose have replaced the original