86 ECONOMIC GEOLOGY 



points, one point being inserted in each square yard, and driven 

 gradually by a hammer. Each point will thaw from 2 to 5 cu. yd. 

 of gravel per day. 



The washing of the gravel is usually done by sluices. These are 

 long wooden troughs made in 12-f t. lengths, and about 10 in. broad. 

 The bottom is lined with wooden riffles consisting generally of 

 longitudinal bars, by which the gold and heavy minerals are caught. 

 The common sluice head has a fall of 8 in. in the 12 ft. and has 

 a capacity of 120 cu. ft. per minute. 



Water is very scarce in some districts and must be used econom- 

 ically. In some instances the water is conducted for long dis- 

 tances in sluice boxes. In case the valley is wide and the pay 

 streak is on the opposite side of the valley from the stream, the water 

 is raised by centrifugal pumps to a height of 30 or 40 ft. and 

 conveyed across the valley by a long flume. In the final wash-up 

 by which the gold is recovered from the sluice boxes the riffles are re- 

 moved and a copious stream of water sent down the sluice which 

 carries away the fine gravel and leaves the gold and the heavy 

 black sand that accompanies it. When dry, the sand is removed 

 by blowers. 



The placer mining upon creeks and hillsides is somewhat dif- 

 ferent. On a creek a shaft is sunk down to bed rock. Four lat- 

 eral drifts are driven from the shaft along the surface of the bed rock, 

 and opened out in a fan-like manner, to the limits of the claim. 

 The outermost portions are worked first, and the ground is mined 

 toward the shaft, or retreating. During the retreat the rock and 

 the overlying muck are allowed to cave and settle down to the bed 

 rock. Timbering is thus entirely avoided. The frozen grounds 

 require no support, and chambers often 100 ft. square are found 

 covered by an icy roof of muck. 



Amalgamation. For amalgamation, the ore must be free mill- 

 ing, that is, not require roasting before the gold or silver will 

 unite directly with mercury. The ore is first crushed to a size 

 varying from 1 to 2 in. in diameter. It then passes with 

 water to the stamps where it is reduced to an impalpable pulp. 

 It is then carried over plates covered with silver-plated amalga- 

 mated copper. From these plates it passes directly to concen- 

 trating tables or Frue vanners where the sulphides are separated 

 by their higher specific gravity and shipped direct to the smelter. 

 The tailings comprise that portion that goes into the streams as 

 waste. The plates were formerly made of copper, but the copper 



