GOLD-MINING. Gold-mining operations may he divided into two classes, which are commonly called placer mining and quartz mining. The de posits of gold first mined were called placers, and were the river-bed deposits of gold-hearing sand and gravel. These deposits are known as dry placers when they are ancient river-beds from which the water has gone, and as wet placers when they are the beds of existing streams. The process of dry-placer mining consists in excavat ing the gold-bearing sand and separating the gold from it by washing, according to one of the methods described in the succeeding section on Metallurgy. Dry-placer workings are divided into surface placers and deep placers. The most important method of placer mining is the hy draulic method, which was first attempted in a crude way in 1852 in Placer County, Cal., and is now practiced on an immense scale. Briefly described, hydraulic mining consists in directing a powerful stream of water from a pipe against the gold-bearing gravel bank, which breaks down the material and washes it away through sluices, where the separation of metal is accomplished as described below. Wet-placer mining from the beds of existing streams consists either in divert ing the stream and working the dried bed in the open or in dredging the gold-bearing sand and gravel. In the latter case the dredges are gener ally mounted on flat-bottomed boats, which also carry the necessary appliances for extracting the gold. Gold-dredging is extensively practiced
in some of the Western States, particularly along the Snake River in Jdaho and the Feather River in California, and also in New Zealand.
Quartz mining, or the mining of gold in veins, is carried on by underground mining operations which do not differ materially from similar op erations in mining other ores. (See MINING.) Gold-quartz ores are crushed in stamp mills or other forms of crushing machines, and the metal is extracted by one of the metallurgical processes described in a succeeding section. A view of a modern stamp mill and extraction plant is shown on the accompanying plate. In this mill the ore is first crushed to coarse size in gyrating rock breakers, and then passes to the storage bins, from which it is fed to the stamps. After fine crushing the ore is passed over concentrating tables, which remove a portion of the values as concentrates. The remaining material, commonly referred to as 'tailings,' passes through settling tanks, and is then changed into amalgamating pans, where a further portion of the values is recovered. For illustrations of stamp mill, see GRINDING AND CRUSHING MACHINERY.