The intervening ridges are then worked out.
The entire operation of getting the ore into the chutes is called "milling." Placer Mining is a general term applied to deposits of minerals accumulated into workable quantities of economical importance through the natural processes of erosion and concentration. The heavy and not easily decomposed minerals in the parent rock are freed as the rock is broken down, and are sorted by the action of water and concentrated in the lower parts of stream beds. Alluvial, used largely in the British Empire, is applied to placers formed by the mechanical action of moving water, whether of streams, lakes or oceans. Eluvial is a term applied to placers found close to the parent rock, see fig. 2. Erosion has set free the gold which has worked down the slope due to the combined action of gravity and rains. Once a flowing stream is reached the gold may be carried a long distance before it is deposited. A placer deposit may be formed by one or more of a number of different minerals.
Placer deposits are found in present stream beds, in benches above present streams, in elevated channels of ancient streams now cutting across present drainage (as the White Channel placers of the Yukon), in gravels deeply buried (deep leads) beneath present streams or covered by lava flows. They have also been discovered at the mouths of streams flowing into lakes and as beach deposits formed by the breaking, transporting and sort ing action of waves. Some beach deposits have been elevated above the present ocean level. Workable placers have also been formed by the action of wind, which carried away the lighter minerals leaving a residual concentrate of economic importance.
The size of placer gold varies from nuggets of 2,000 oz. troy or more to such fine flakes that 2,000 of them are worth only I cent. Gold coated with silica, oxide of manganese or iron oxide will not be caught by mercury. Small pockets of auriferous gravel are worked by the pan or the rocker. A rocker is a small wooden trough or box mounted on two transverse rockers so the device can be given a side motion. By diligent work an ex perienced panner may handle i cu.yd. of gravel in io hours. Two men with a rocker can handle from 3 to 5 cu.yd. of gravel in io hr. under average conditions. Mechanically operated rockers, driven by a small internal-combustion engine, are used extensively to speed up operations.
Sluicing is the term applied to working placers where the gravel is carried to the head of the sluice by shovelling, in wheel barrows, in small cars, by scrapers, by dragline excavators or by small steam shovels. Large placer deposits are worked by by draulicking, the gravel being loosened from place and washed into the sluice by powerful jets of water from nozzles called hydraulic giants. To be effective the head of water on the giant should not be less than 200 ft., equivalent to a pressure of 86.8 lb. per square inch. In case more head room is required than natural conditions afford, additional lift of gravel may be had through the use of an hydraulic elevator, a steeply inclined pipe having a nozzle placed inside at the bottom. The gravel is washed into the elevator at a point just above the nozzle and is carried up by the force of the jet. Water for the hydraulic giant is measured in terms of the miner's inch, or a flow of 1.5 cu.ft. per minute. The volume of gravel loosened and washed through a sluice by a miner's inch of water used for 24 hr. varies with conditions of the gravel, the slope of the sluice and the pressure of the water. As a rough guide, under average conditions the duty of a miner's inch is from 3 to 6 cu.yd. of gravel per 24 hours.