The various deposits of gold may be divided into two classes--"veins" and "placers." The vein mining of gold does not greatly differ from that of similar deposits of metals (see MINERAL DEPOSITS). In the placer or alluvial deposits, the precious metal is found usually in a water-worn condition imbedded in earthy matter, and the method of working all such deposits is based on the disintegration of the earthy matter by the action of a stream of water, which washes away the lighter portions and leaves the denser gold. In alluvial deposits the richest ground is usually found in contact with the "bed rock"; and, when the overlying cover of gravel is very thick, or, as sometimes happens, when the older gravel is covered with a flow of basalt, regular mining by shafts and levels, as in what are known as tunnel-claims, may be required to reach the auriferous ground.
The extraction of gold may be effected by several methods; we may distinguish the following leading types:
1. By simple washing, i.e. dressing auriferous sands, gravels, &c.;
2. By amalgamation, i.e. forming a gold amalgam, afterwards removing the mercury by distillation;
3. By chlorination, i.e. forming the soluble gold chloride and then precipitating the metal;
4. By the cyanide process, i.e. dissolving the gold in potassium cyanide solution, and then precipitating the metal;
5. Electrolytically, generally applied to the solutions obtained in processes (3) and (4).
1. _Extraction of Gold by Washing._--In the early days of gold-washing in California and Australia, when rich alluvial deposits were common at the surface, the most simple appliances sufficed. The most characteristic is the "pan," a circular dish of sheet-iron or "tin," with sloping sides about 13 or 14 in. in diameter. The pan, about two-thirds filled with the "pay dirt" to be washed, is held in the stream or in a hole filled with water. The larger stones having been removed by hand, gyratory motion is given to the pan by a combination of shaking and twisting movements so as to keep its contents suspended in the stream of water, which carries away the bulk of the lighter material, leaving the heavy minerals, together with any gold which may have been present. The washing is repeated until enough of the enriched sand is collected, when the gold is finally recovered by careful washing or "panning out" in a smaller pan. In Mexico and South America, instead of the pan, a wooden dish or trough, known as "batea," is used.